0001628280-20-003484 | 10-K | XBRL Viewer

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Cover - USD ($)	12 Months Ended
Cover - USD ($)	Dec. 31, 2019	Mar. 06, 2020	Jun. 30, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
Document Transition Report	false
Entity File Number	001-38295
Entity Registrant Name	X4 PHARMACEUTICALS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	27-3181608
Entity Address, Address Line One	955 Massachusetts Avenue
Entity Address, Address Line Two	4th Floor
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	857
Local Phone Number	529-8300
Title of 12(b) Security	Common Stock
Trading Symbol	XFOR
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Public Float			$ 183,715,365
Documents Incorporated by Reference	Portions of the registrant’s definitive proxy statement, or the 2020 Proxy Statement, for its 2020 Annual Meeting of Stockholders, which the registrant intends to file pursuant to Regulation 14A with the Securities and Exchange Commission not later than 120 days after the registrant's fiscal year ended December 31, 2019, are incorporated by reference into Part III of this Annual Report on Form 10-K.
Entity Common Stock, Shares Outstanding		16,134,495
Entity Central Index Key	0001501697
Amendment Flag	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 126,184	$ 8,134
Research and development incentive receivable	1,998	0
Prepaid expenses and other current assets	1,096	1,205
Total current assets	129,278	9,339
Property and equipment, net	403	241
Goodwill	27,109	0
Right-of-use assets	1,959
Other assets	1,949	364
Assets, Total	160,698	9,944
Current liabilities:
Accounts payable	2,088	2,969
Accrued expenses	6,461	3,251
Current portion of lease liability	898
Current portion of long-term debt, net of discount	0	1,687
Liabilities, Current, Total	9,447	7,907
Preferred stock warrant liability	0	4,947
Long-term debt, including accretion, net of discount and current portion	20,097	8,145
Deferred rent		417
Lease liabilities	1,918
Other liabilities	16	205
Total liabilities	31,478	21,621
Commitments and contingencies (Note 10)
Convertible preferred stock, $0.001 par value; 10,000,000 and 59,413,523 shares authorized as of December 31, 2019 and December 31, 2018, respectively; 0 and 40,079,567 shares issued and outstanding as of December 31, 2019 and December 31, 2018, respectively		64,675
Stockholders’ equity (deficit):
Common Stock	16	0
Additional paid-in capital	261,367	2,151
Accumulated other comprehensive income (loss)	(119)	0
Accumulated deficit	(132,044)	(79,237)
Total stockholders’ equity (deficit)	129,220	(77,086)
Total liabilities, convertible preferred stock, redeemable common stock and stockholders’ equity (deficit)	160,698	9,944
Convertible Preferred Stock
Current liabilities:
Convertible preferred stock, $0.001 par value; 10,000,000 and 59,413,523 shares authorized as of December 31, 2019 and December 31, 2018, respectively; 0 and 40,079,567 shares issued and outstanding as of December 31, 2019 and December 31, 2018, respectively	0	64,675
Redeemable Common Stock
Current liabilities:
Convertible preferred stock, $0.001 par value; 10,000,000 and 59,413,523 shares authorized as of December 31, 2019 and December 31, 2018, respectively; 0 and 40,079,567 shares issued and outstanding as of December 31, 2019 and December 31, 2018, respectively	$ 0	$ 734

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Preferred stock, authorized (in shares)		49,413,523
Preferred stock issued (in shares)		40,079,567
Preferred Stock Outstanding (in shares)		40,079,567
Common stock par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, outstanding (in shares)	16,128,862	351,652
Common stock, authorized (in shares)	33,333,333	11,070,776
Common stock, issued (in shares)	16,128,862	351,652
Convertible Preferred Stock
Preferred stock par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, authorized (in shares)	10,000,000	59,413,523
Preferred stock issued (in shares)	0	40,079,567
Preferred Stock Outstanding (in shares)	0	40,079,567
Redeemable Common Stock
Preferred Stock Outstanding (in shares)	0	107,371
Common stock par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, outstanding (in shares)	0	107,371
Common stock, issued (in shares)	0	107,371

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Expenses [Abstract]
Research and development	$ 30,163	$ 20,346	$ 17,066
General and administrative	17,640	8,739	5,181
Loss on transfer of nonfinancial assets	3,900	0	0
Total operating expenses	51,703	29,085	22,247
Loss from operations	(51,703)	(29,085)	(22,247)
Nonoperating Income (Expense) [Abstract]
Interest income	1,197	236	64
Interest expense	(2,147)	(720)	(490)
Change in fair value of preferred stock warrant liability	(288)	(3,398)	1,360
Change in fair value of derivative liability	183	(89)	(94)
Loss on preferred stock repurchase liability	0	0	(587)
Loss on extinguishment of debt	(566)	(229)	0
Other income	517	0	0
Total other income (expense), net	(1,104)	(4,200)	253
Net loss	(52,807)	(33,285)	(21,994)
Accruing dividends on Series A convertible preferred stock	(592)	(3,000)	(3,000)
Adjustment to accumulated deficit in connection with repurchase of Series Seed convertible preferred stock	0	(22)	0
Net loss attributable to common stockholders	$ (53,399)	$ (36,307)	$ (24,994)
Net loss per share attributable to common stockholders - basic and diluted (in dollars per share)	$ (4.63)	$ (79.15)	$ (54.58)
Weighted average shares of common stock outstanding - basic and diluted (in shares)	11,530	459	458
Net loss	$ (52,807)	$ (33,285)	$ (21,994)
Currency translation adjustments	(119)	0	0
Total comprehensive loss	$ (52,926)	$ (33,285)	$ (21,994)

Consolidated Statements of Convertible Preferred Stock, Redeemable Common Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Convertible Preferred Stock	Redeemable Common Stock
Beginning balance, convertible preferred stock (in share) at Dec. 31, 2016						22,061,973	107,371
Beginning balance, convertible preferred stock at Dec. 31, 2016						$ 34,307	$ 734
Increase (Decrease) in Temporary Equity [Roll Forward]
Repurchase of Series Seed convertible preferred stock, net of issuance costs of $11	$ 0
Issuance cost of convertible preferred stock, net of issuance cost (in shares)						15,956,995
Issuance costs of convertible preferred stock, net of issuance costs	0					$ 26,596
Conversion of redeemable common stock into common stock	0
Ending balance, convertible preferred stock (in shares) at Dec. 31, 2017						38,018,968	107,371
Ending balance, convertible preferred stock at Dec. 31, 2017						$ 60,903	$ 734
Beginning balance (in shares) at Dec. 31, 2016		345,856
Beginning balance at Dec. 31, 2016	(23,052)	$ 0	$ 884	$ 0	$ (23,936)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		4,751
Exercise of stock options	9		9
Stock-based compensation expense	492		492
Net loss	(21,994)				(21,994)
Ending balance (in shares) at Dec. 31, 2017		350,607
Ending balance at Dec. 31, 2017	(44,545)	$ 0	1,385	0	(45,930)
Increase (Decrease) in Temporary Equity [Roll Forward]
Repurchase of Series Seed convertible preferred stock, net of issuance cost of $11,000 (in shares)						(598,975)
Repurchase of Series Seed convertible preferred stock, net of issuance costs of $11	(22)				(22)	$ (517)
Issuance cost of convertible preferred stock, net of issuance cost (in shares)						2,659,574
Issuance costs of convertible preferred stock, net of issuance costs	0					$ 4,289
Conversion of redeemable common stock into common stock	$ 0
Ending balance, convertible preferred stock (in shares) at Dec. 31, 2018	40,079,567					40,079,567	107,371
Ending balance, convertible preferred stock at Dec. 31, 2018	$ 64,675					$ 64,675	$ 734
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		1,045
Exercise of stock options	7		7
Stock-based compensation expense	759		759
Net loss	(33,285)				(33,285)
Ending balance (in shares) at Dec. 31, 2018		351,652
Ending balance at Dec. 31, 2018	(77,086)	$ 0	2,151	0	(79,237)
Increase (Decrease) in Temporary Equity [Roll Forward]
Repurchase of Series Seed convertible preferred stock, net of issuance costs of $11	0
Conversion of redeemable common stock into common stock (in shares)		107,364					(107,371)
Conversion of redeemable common stock into common stock	734		734				$ (734)
Conversion of convertible preferred shares into common stock (in shares)		3,808,430				(40,079,567)
Conversion of convertible preferred shares into common stock	64,675	$ 4	64,671			$ (64,675)
Ending balance, convertible preferred stock (in shares) at Dec. 31, 2019						0	0
Ending balance, convertible preferred stock at Dec. 31, 2019						$ 0	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exchange of common stock in connection with Merger (in shares)		2,440,582
Exchange of common stock in connection with Merger	45,541	$ 2	45,539
Fair value of replacement equity awards	817		817
Reclassification of warrant liability to permanent equity	5,235		5,235
Issuance of common stock and prefunded warrants for the purchase of common stock, net of issuance costs of $11.5 million (in shares)		9,336,667
Issuance of common stock and prefunded warrants for the purchase of common stock, net of issuance costs of $11.4 million	$ 139,388	$ 9	139,379
Exercise of stock options (in shares)	51,617	50,321
Exercise of stock options	$ 345	$ 1	344
Exercise of warrants (in shares)		33,846
Exercise of warrants	447		447
Stock-based compensation expense	2,050		2,050
Foreign currency translation adjustment	(119)			(119)
Net loss	(52,807)				(52,807)
Ending balance (in shares) at Dec. 31, 2019		16,128,862
Ending balance at Dec. 31, 2019	$ 129,220	$ 16	$ 261,367	$ (119)	$ (132,044)

Consolidated Statements of Convertible Preferred Stock, Redeemable Common Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Convertible Preferred Shares
Issuance costs		$ 539	$ 2,600
Preferred Stock Issuance Costs		$ 11
Common Stock
Issuance costs	$ 11,400

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (52,807)	$ (33,285)	$ (21,994)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,050	759	492
Depreciation and amortization expense	103	103	71
Loss on transfer of nonfinancial assets	3,900	0	0
Non-cash lease expense	569	0	0
Accretion of debt discount	695	152	157
Loss on extinguishment of debt	566	229	0
Loss on preferred stock repurchase liability	0	0	587
Change in fair value of preferred stock warrant and derivative liability	105	3,487	(1,266)
Changes in operating assets and liabilities:
Prepaid expenses, other current assets and research & development incentive receivable	109	279	402
Accounts payable	(2,752)	1,307	(1,169)
Accrued expenses	160	1,549	1,409
Lease liabilities	(753)	0	0
Net cash used in operating activities	(48,055)	(25,420)	(21,311)
Cash flows from investing activities:
Cash, cash equivalents and restricted cash acquired in connection with the Merger	26,406	0	0
Proceeds from transfer of non-financial assets	1,000	0	0
Purchase of property, equipment and intangible assets	(174)	0	(378)
Net cash provided by (used in) investing activities	27,232	0	(378)
Cash flows from financing activities:
Proceeds from exercise of stock options and warrants	792	7	9
Proceeds from borrowings under loan and security agreements, net of issuance costs	9,849	9,908	6,000
Proceeds from issuance of Series B convertible preferred stock, net of issuance costs	0	4,461	27,413
Repurchase of Series Seed convertible preferred stock	0	(1,126)	0
Repayments of borrowings under loan and security agreement	(9,368)	(6,380)	0
Proceeds from sale of common stock and warrants, net of issuance costs	139,388	0	0
Net cash provided by financing activities	140,661	6,870	33,422
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(250)	0	0
Net increase (decrease) in cash, cash equivalents and restricted cash	119,588	(18,550)	11,733
Cash, cash equivalents and restricted cash at beginning of period	8,498	27,048	15,315
Cash, cash equivalents and restricted cash at end of period	128,086	8,498	27,048
Supplemental disclosure of cash flow information
Cash paid for interest	1,284	540	282
Supplemental disclosure of non-cash investing and financing activities:
Purchase of property, equipment and intangible assets included in accounts payable	40	0	0
Issuance costs related to sale of common stock and warrants, not yet paid	775
Conversion of convertible preferred stock into common stock	64,675	0	0
Conversion of redeemable common stock into common stock	734	0	0
Conversion of convertible preferred stock warrants into common stock warrants	5,235	0	0
Fair value of net assets acquired in the Merger in exchange for common shares, excluding cash acquired	19,952	0	0
Initial fair value of derivative liability in connection with loan and security agreement	0	18	0
Issuance of warrants in connection with Series B convertible preferred stock financing	0	172	817
Issuance of warrants in connection with loan and security agreement	$ 0	$ 132	$ 0

Nature of the Business and Basis of Presentation	12 Months Ended
Nature of the Business and Basis of Presentation	Dec. 31, 2019
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	Nature of the Business and Basis of Presentation X4 Pharmaceuticals, Inc. (formerly Arsanis, Inc.), together with its subsidiaries (the “Company”), is a clinical-stage biotechnology company focused on the research, development and commercialization of novel therapeutics for the treatment of rare diseases. The Company’s lead product candidate, mavorixafor (X4P-001), is a potential first-in-class, once-daily, oral inhibitor of CXCR4 and is currently in a Phase 3 clinical trial for the treatment of WHIM syndrome, a rare, inherited, primary immunodeficiency disease caused by genetic mutations in the CXCR4 receptor gene. The Company is headquartered in Cambridge, Massachusetts. The Company is subject to risks and uncertainties common to pre-commercialization companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with governmental regulations and the ability to secure additional capital to fund operations. Drug candidates currently under development will require extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel, and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. Merger with Arsanis On November 26, 2018, Arsanis, Inc., a publicly held Delaware corporation (“Arsanis”), Artemis AC Corp., a Delaware corporation and a wholly-owned subsidiary of Arsanis (“Merger Sub”), and X4 Therapeutics, Inc. (“X4”) entered into an Agreement and Plan of Merger, as amended on December 20, 2018 and March 8, 2019 (the “Merger Agreement”), pursuant to which the Merger Sub merged with and into X4, with X4 surviving the merger as a wholly-owned subsidiary of Arsanis. The transactions described in the foregoing sentence may be referred to in these consolidated financial statements as “the Merger.” The transaction was accounted for as a reverse merger in accordance with accounting principles generally accepted in the United States of America ("GAAP"). Under this method of accounting, X4 was deemed to be the accounting acquirer for financial reporting purposes. This determination was primarily based on the facts that, immediately following the Merger: (i) the Company’s stockholders own a substantial majority of the voting rights in the combined organization, (ii) the Company designated a majority of the members of the initial board of directors of the combined organization and (iii) the Company’s senior management hold all key positions in the senior management of the combined organization. Accordingly, for accounting purposes, the business combination was treated as the equivalent of X4 issuing stock to acquire the net assets of Arsanis. As a result, as of the closing date of the Merger, the net assets of Arsanis were recorded at their acquisition-date fair values in the financial statements of the Company and the reported operating results prior to the business combination are those of the Company. In addition, transaction costs incurred by the Company in connection with the business combination have been expensed as incurred. On March 13, 2019, Arsanis, X4 and Merger Sub completed the Merger pursuant to the terms of the Merger Agreement. Pursuant to the terms of the Merger Agreement, each outstanding share of X4’s common stock and preferred stock was exchanged for 0.5702 shares of Arsanis’s common stock (the “Exchange Ratio”). In addition, all outstanding options exercisable for common stock and warrants exercisable for convertible preferred stock of X4 became options and warrants exercisable for the same number of shares of common stock of Arsanis multiplied by the Exchange Ratio. In connection with the Merger, X4 changed its name to X4 Therapeutics, Inc. Following the closing of the Merger, X4 Therapeutics, Inc. became a wholly-owned subsidiary of the Company, which changed its name to X4 Pharmaceuticals, Inc. As used herein, the words “the Company” refers to, for periods following the Merger, X4 Pharmaceuticals, Inc. (formerly Arsanis, Inc.), together with is direct and indirect subsidiaries, and for periods prior to the Merger, X4 Therapeutics, Inc. (formerly X4 Pharmaceuticals, Inc.), and its direct and indirect subsidiaries, as applicable. Immediately following the Merger, stockholders of X4 owned approximately 64% of the combined organization’s outstanding common stock. On March 14, 2019, the combined organization’s common stock began trading on The Nasdaq Capital Market under the ticker symbol “XFOR.” Principles of Consolidation— The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, including X4 Pharmaceuticals (Austria) GmbH, which is incorporated in Vienna, Austria and was formerly named Arsanis Biosciences GmbH (“X4 GmbH”), and X4 Therapeutics, Inc. All significant intercompany accounts and transactions have been eliminated. Reverse Stock Split— On March 13, 2019, immediately following the closing of the Merger, the Company effected a 1-for-6 reverse stock split of its common stock (the “Reverse Stock Split”). Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect the Reverse Stock Split. No fractional shares were issued in connection with the Reverse Stock Split. Unless otherwise noted, all references to common stock share and per share amounts have also been adjusted to reflect the exchange ratio of 0.5702. Going Concern Assessment— In accordance with Accounting Standards Update (“ASU”) No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40), the Company has evaluated whether there are certain conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the consolidated financial statements are issued. As of March 12, 2020, the issuance date of these consolidated financial statements, the Company expects that its cash and cash equivalents will be sufficient to fund its forecasted operating expenses, capital expenditure requirements and debt service payments for at least the next twelve months from the issuance date of these financial statements. Since its inception, the Company has incurred significant operating losses and negative cash flows from operations. The Company has not yet commercialized any products and does not expect to generate revenue from the commercial sale of any products for several years, if at all. The Company expects that its research and development and general and administrative expenses will continue to increase and, as a result, will need additional capital to fund its future operations, which it may raise through a combination of equity offerings, debt financings, other third-party funding, marketing and distribution arrangements and other collaborations, strategic alliances and licensing arrangements. The Company has funded its operations to date primarily with proceeds from sales of common stock, warrants and prefunded warrants for the purchase of shares of preferred stock and shares of common stock, sales of preferred stock, proceeds from the issuance of convertible debt and borrowings under loan and security agreements. In 2019, the Company completed a merger with Arsanis and acquired its $26.4 million of cash, cash equivalents and restricted cash. In addition, in 2019, the Company raised $139.4 million, net of offering costs, through public offerings of common stock, prefunded warrants to purchase shares of common stock, and accompanying warrants to purchase shares of common stock. In June 2019, the Company received $9.8 million in net proceeds as a result of refinancing its loan agreement (the “Hercules Loan Agreement”) with Hercules Capital, Inc. (“Hercules”), under which, subject to approval by Hercules, an additional $5.0 million is available for borrowing through December 15, 2020 and an additional term loan of $10.0 million is available through June 15, 2022. Principal payments under the Hercules Loan Agreement commence in February 2022. If the Company is unable to obtain future funding when needed, the Company may be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or pre-commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations. There is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Summary of Significant Accounting PoliciesUse of Estimates— The preparation of the Company’s consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses, the valuation of intangible assets acquired in business combinations, the valuations of common stock prior to the Merger, the valuation of stock options, preferred stock warrants (and the resulting preferred stock warrant liability), derivative instruments (and the resulting derivative liabilities), valuation of lease liabilities and the constraint of variable consideration from revenue transactions. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates when there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates.

Foreign Currency and Currency Translation— For the Company's subsidiaries that transact in a functional currency other than the U.S. dollar, assets and liabilities are translated at current exchange rates as of the balance sheet date while income and expenses are translated at the average exchanges rates for the period. Adjustments resulting from the translation of the financial statements of the Company's foreign operations into U.S. dollars are excluded from the determination of net loss and are recorded in accumulated other comprehensive loss, a component of stockholders' equity (deficit).

Concentrations of Credit Risk and Significant Suppliers— Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash and cash equivalents and research and development incentive receivables. The Company generally maintains cash balances in various operating accounts at financial institutions that management believes to be of high credit quality in amounts that may exceed federally insured limits. The Company has not experienced losses related to its cash and cash equivalents.

The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. The Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in these manufacturing services or in the supply of active pharmaceutical ingredients and formulated drugs.

Cash and Cash Equivalents— The Company considers all highly liquid investments with maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents consisted of money market funds as of December 31, 2019 and 2018.

Restricted Cash—


(in thousands)	As of December 31, 2019		As of December 31, 2018
Letter of credit security: Cambridge lease	$	264	$	264
Letter of credit security: Waltham lease	250		—
Letter of credit security: Vienna Austria lease	94		—
Letter of credit security: Allston lease	1,144		—
Corporate credit card collateral	150		100
Total restricted cash (non-current)	$	1,902	$	364

In connection with the Company’s lease agreement for its facilities in Massachusetts and Austria, the Company maintains letters of credit, which are secured by restricted cash, for the benefit of the landlord. In addition, as of December 31, 2019, and 2018, the Company was required to maintain a separate cash balance of $150 thousand and $100 thousand, respectively, to collateralize corporate credit cards with a bank.

In accordance with the Company’s Amended and Restated Loan Agreement with Hercules, as further described in Note 8, the Company is required to maintain cash in an account accessible by the lender in an amount not less than 125% of the outstanding loan balance, or if the Company’s consolidated cash is lower than this amount, all of the Company’s cash other than $2.5 million. As of December 31, 2019, the Company maintained $25.0 million in an account accessible by the lender in accordance with the terms of Amended and Restated Loan Agreement.

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheets to the sum to the total of amounts shown in the Company’s consolidated statement of cash flows as of December 31, 2019, 2018 and 2017:


(in thousands)	December 31, 2019		December 31, 2018		December 31, 2017		December 31, 2016
Cash and cash equivalents	$	126,184	$	8,134	$	26,684	$	15,160
Restricted cash, non-current (included within other assets)	1,902		364		364		155
Total cash, cash equivalents and restricted cash	$	128,086	$	8,498	$	27,048	$	15,315

Property and Equipment— Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset, as follows:


			Estimated Useful Life
Office furniture			3 years
Computer equipment			3 years
Laboratory equipment			3 to 10 years
Leasehold improvements			Shorter of lease term or 10 years

Estimated useful lives are periodically assessed to determine if changes are appropriate. Maintenance and repairs are charged to expense as incurred. When assets are retired or otherwise disposed of, the cost of these assets and related accumulated depreciation or amortization are eliminated from the consolidated balance sheet and any resulting gains or losses are included in the consolidated statements of operations and comprehensive loss in the period of disposal. Costs for capital assets not yet placed into service are capitalized as construction-in-progress and depreciated once placed into service.

Right-of-Use Assets and Leases— Effective January 1, 2019, the Company adopted Accounting Standards Codification (“ASC”), Topic 842, Leases (“ASC 842”), using the modified retrospective approach through a cumulative-effect adjustment and utilizing the effective date as its date of initial application, with prior periods unchanged and presented in accordance with the guidance in Topic 840, Leases (“ASC 840”).

At the inception of an arrangement, the Company determines whether the arrangement contains a lease based on the unique facts and circumstances present. Leases with a non-cancellable term greater than one year are recognized on the balance sheet as right-of-use assets with associated current and non-current lease liabilities. The Company has elected not to recognize on the balance sheet leases with terms of one year or less. Options to renew a lease are not included in the Company’s initial lease term assessment unless there is reasonable certainty that the Company will renew the lease. If a lease is cancellable without penalty, the Company excludes from the lease term periods following the cancellation notice period unless it is reasonably certain that the Company will not cancel the lease.

Operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected remaining lease term. Certain adjustments to the right-of-use operating asset may be required for items such as incentives received or accrued rent. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rates, which are the rates it incurs to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. The Company has referenced the effective rate of its Hercules borrowings, as adjusted for differences terms, to determine calculate its incremental borrowing rate for each of its operating leases.

In accordance with the guidance in ASC 842, components of a lease are split into lease components and non-lease components. A policy election is available pursuant to which an entity may elect to not separate lease and non-lease components. Rather, each lease component and the related non-lease components are accounted for together as a single component. For new and amended leases beginning in 2019 and after, the Company has elected to account for the lease and non-lease components as a combined lease component for its office and laboratory building leases.

Impairment of Long-Lived Assets— Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized in loss from operations when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value. To date, the Company has not recorded any material impairment losses on long-lived assets.

Goodwill— Business combinations are accounted for under the acquisition method. The total purchase price of an acquisition is allocated to the underlying identifiable net assets, based on their respective estimated fair values as of the acquisition date. Determining the fair value of assets acquired and liabilities assumed requires management’s judgment and often involves the use of significant estimates and assumptions, including assumptions with respect to future cash inflows and outflows, probabilities of success, discount rates, and asset lives, among other items. Assets acquired and liabilities assumed are recorded at their estimated fair values. The excess of the purchase price over the estimated fair values of the net assets acquired is recorded as goodwill.

Goodwill is tested quantitatively for impairment at the reporting unit level annually in the fourth quarter, or more frequently when events or changes in circumstances indicate that the asset might be impaired. Examples of such events or circumstances include, but are not limited to, a significant adverse change in legal or business climate, an adverse regulatory action or unanticipated competition.

The Company has determined that it operates in a single operating segment and has a single reporting unit. To perform its quantitative test, the Company compares the fair value of the reporting unit to its carrying value. If the fair value of the reporting unit exceeds the carrying value of its net assets, goodwill is not impaired, and no further testing is required. If the fair value of the reporting unit is less than the carrying value, the Company measures the amount of impairment loss, if any, as the excess of the carrying value over the fair value of the reporting unit. The Company determined that goodwill was not impaired as of December 31, 2019 base on its quantitative test.

Intangible Assets— In connection with the Merger, the Company acquired certain in-process research and development ("IPR&D") assets, which were classified as indefinite-lived intangible assets. Acquired IPR&D represents the fair value assigned to research and development assets that the Company acquires and have not been completed at the acquisition date. The fair value of IPR&D acquired in a business combination is recorded on the Company’s consolidated balance sheets at the acquisition-date fair value and is determined by estimating the costs to develop the technology into commercially viable products, estimating the resulting revenue from the projects, and discounting the projected net cash flows to present value. IPR&D is not amortized, but rather is reviewed for impairment on an annual basis or more frequently if indicators of impairment are present, until the project is completed, abandoned or transferred to a third party.

The projected discounted cash flow models used to estimate the Company’s IPR&D reflect significant assumptions regarding the estimates a market participant would make in order to evaluate a drug development asset, including the following:

•Probability of successfully completing clinical trials and obtaining regulatory approval;

•Market size, market growth projections, and market share;

•Estimates of future cash flows from potential milestone payments and royalties related to out-licensed product sales; and

•A discount rate reflecting the Company's weighted average cost of capital and specific risk inherent in the underlying assets.

During the year ended December 31, 2019, the Company entered into an out-licensing arrangement with a third party that transferred the rights to develop and commercialize one of the programs underlying an IPR&D intangible asset. In addition, the Company entered into amended out-licensing option agreements with a third party who had previously entered into an option agreement with Arsanis to license the rights to develop and commercialize two other programs underlying the IPR&D intangible assets. Following the amendment to these option agreements, the options were exercised by the third party and the in-process research and development programs were out-licensed to the third party. As of December 31, 2019, all programs underlying IPR&D intangible assets acquired in the Merger were transferred to these third parties and the Company has no continuing involvement in any ongoing research and development activities associated with the programs. As a result of the transfer of the IPR&D projects to third parties, the Company derecognized the IPR&D intangibles asset through a charge to "loss on transfer of nonfinancial assets" during 2019. (See Note 16)

Deferred Rent— The Company’s lease agreements include payment escalations and lease incentives (including a leasehold improvement tenant allowance). For periods prior to January 1, 2019, these payments were accrued or deferred as appropriate such that rent expense was recognized on a straight-line basis over the respective lease terms. Effective January 1, 2019, upon the adoption of ASC 842, deferred rent was reclassified as a reduction to the applicable right-of-use asset as further described in Note 9.

Fair Value Measurements— Certain assets and liabilities of the Company are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:

•Level 1—Quoted prices in active markets for identical assets or liabilities.

•Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.

•Level 3—Unobservable inputs that are supported by little or no market activity that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.

Prior to the Merger, the Company’s preferred stock warrant liability, derivative liability and preferred stock repurchase liability were carried at fair value, determined according to Level 3 inputs in the fair value hierarchy described above (see Note 5). The Company’s cash equivalents, consisting of money market funds invested in U.S. Treasury securities, are carried at fair value, determined based on Level 1 and Level 2 inputs in the fair value hierarchy described above. The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. The carrying value of the Company’s outstanding loan and security agreement with Hercules approximates its fair value at December 31, 2019 because the debt bears interest at a variable market rate and the Company’s credit risk has not materially changed since the inception of the agreement.

Segment Information— The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions. The Company’s focus is on the research, development and commercialization of novel therapeutics for the treatment of rare diseases.

Revenue Recognition— Effective January 1, 2018, the Company adopted ASC Topic 606, Revenue from Contracts with Customers (“ASC 606”), as amended, using the modified retrospective transition method. The modified retrospective method requires that the cumulative effect of initially applying ASC 606 be recognized as an adjustment to the opening balance of retained earnings or accumulated deficit of the annual period that includes the date of initial application. The Company had no arrangements that were in the scope of ASC 606 on January 1, 2018 and thus there was no impact to the consolidated financial statements as a result of the adoption. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as collaboration arrangements and leases.

The Company’s revenues are generated primarily through research, development and commercialization agreements. The terms of these agreements may contain multiple promised goods and services, which may include (i) licenses, or options to obtain licenses, to the Company’s technology, and (ii) in certain cases, services in connection with the manufacturing of preclinical and clinical materials. Payments to the Company under these arrangements typically include one or more of the following: non-refundable, upfront license fees; option exercise fees; milestone payments; payments for clinical and commercial product supply, and royalties on future product sales. To date, all revenue from these agreements has been fully constrained and, therefore, no revenue has been recognized on the consolidated statements of operations.

The Company analyzes its arrangements to assess whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities and are therefore within the scope of ASC Topic 808, Collaborative Arrangements (“ASC 808”). This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For collaboration arrangements that are deemed to be within the scope of ASC 808, the Company first determines which elements of the collaboration are deemed to be within the scope of ASC 808 and those that are more reflective of a vendor-customer relationship and, therefore, within the scope of ASC 606. The Company’s policy is generally to recognize amounts received from collaborators in connection with joint operating activities that are within the scope of ASC 808 as a reduction in research and development expense. To date, there have been no transactions within the scope of ASC 808.

Under ASC 606, the Company recognizes revenue when its customers obtain control of promised goods or services, in an amount that reflects the consideration which the Company determines it expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that the Company determines are within the scope of ASC 606, the Company performs the following five steps: (1) identify the contract(s) with a customer; (2) identify the performance obligation(s) in the contract; (3) determine the transaction price; (4) allocate the transaction price to the performance obligation(s) in the contract; and (5) recognize revenue when (or as) the Company satisfies its performance obligation(s).

As part of the accounting for these arrangements, the Company must make significant judgments, including identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and; allocating the transaction price to each performance obligation.

Once a contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within the contract and determines those that are performance obligations. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations.

The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that:

i.the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and

ii.the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract).

In assessing whether a promised good or service is distinct, the Company considers factors such as the research, manufacturing and commercialization capabilities of the customer and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, an entity is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct.

The transaction price is then determined and allocated to the identified performance obligations in proportion to their standalone selling prices (“SSP”) on a relative SSP basis. SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. Determining the SSP for performance obligations requires significant judgment. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations.

If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using either the expected value method or the most-likely-amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price reflects the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis in the period of adjustment.

In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the licensees and the transfer of the promised goods or services to the licensees will be one year or less. The Company assesses each of its revenue generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in any of its arrangements.

The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied at a point in time or over time, and if over time based on the use of an output or input method.

At the inception of each arrangement that includes non-refundable payments for contingent milestones, including preclinical research and development, clinical development and regulatory, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most-likely-amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of the achievement of contingent milestones and the likelihood of a significant reversal of such milestone revenue, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect licensing revenue in the period of adjustment. This quarterly assessment may result in the recognition of revenue related to a contingent milestone payment before the milestone event has been achieved.

Research and Development Programs— Proceeds under the research and development incentive program from the Austrian government are recognized as other income in an amount equal to the qualifying expenses incurred in each period multiplied by the applicable reimbursement percentage. Incentive income recognized upon incurring qualifying expenses in advance of receipt of proceeds from research and development incentives is recorded in the consolidated balance sheet as research and development incentive receivable.

Research and Development Costs— Costs associated with internal research and development and external research and development services, including drug development and preclinical studies, are expensed as incurred. Research and development expenses include costs for salaries, employee benefits, subcontractors, facility-related expenses, depreciation and amortization, stock-based compensation, third-party license fees, laboratory supplies, and external costs of outside vendors engaged to conduct discovery, preclinical and clinical development activities and clinical trials as well as to manufacture clinical trial materials, and other costs. The Company recognizes external research and development costs based on an evaluation of the progress to completion of specific tasks using information provided to the Company by its service providers.

Nonrefundable advance payments for services to be received in the future for use in research and development activities are recorded as prepaid expenses. Such prepaid expenses are recognized as an expense when the related services have been performed, or when it is no longer expected that the goods will be delivered or the services rendered.

Patent Costs— All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.

Debt Issuance Costs— Debt issuance costs consist of payments made to secure commitments under certain debt financing arrangements. These amounts are recognized as interest expense over the period of the financing arrangement using the effective interest method. If the financing arrangement is canceled or forfeited, or if the utility of the arrangement to the Company is otherwise compromised, these costs are recognized as interest expense immediately. The Company’s consolidated financial statements present debt issuance costs related to a recognized debt liability as a direct reduction from the carrying amount of that debt liability.

Stock-Based Compensation— The Company measures all stock-based awards granted to employees and directors based on the fair value on the date of the grant and recognizes compensation expense for those awards, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. The Company issues stock-based awards with service-based vesting conditions and records the expense for these awards using the straight-line method. The Company has not issued any stock-based awards with performance-based vesting conditions.

Effective January 1, 2019, the Company adopted ASU No. 2018-7, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-7”), which expands the scope of Topic 718 to include share-based payment awards to nonemployees. As a result, stock-based awards granted to non-employees are accounted for in the same manner as awards granted to employees and directors as described above. The impact of adopting this new guidance did not have a material impact on the Company’s consolidated financial statements. Prior to the adoption of ASU 2018-7, for stock-based awards granted to non-employee consultants, compensation expense was recognized over the period

during which services were rendered by such nonemployee consultants until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards was remeasured using the then-current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model.

The Company classifies stock-based compensation expense in its consolidated statement of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.

The Company recognizes compensation expense for only the portion of awards that are expected to vest. In developing a forfeiture rate estimate, the Company has considered its historical experience to estimate pre-vesting forfeitures for service-based awards. The impact of a forfeiture rate adjustment is recognized in full in the period of adjustment, and if the actual forfeiture rate is materially different from the Company’s estimate, the Company may be required to record adjustments to stock-based compensation expense in future periods.

The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. Prior to March 13, 2019, the Company had been a private company and lacked company-specific historical and implied volatility information for its common stock. Therefore, the Company estimates its expected common stock price volatility based on the historical volatility of publicly traded peer companies and expects to continue to do so until it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employee consultants is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield considers the fact that the Company has never paid cash dividends on common stock and does not expect to pay any cash dividends in the foreseeable future.

Preferred Stock Warrant Liability— Prior to the Merger with Arsanis, the Company classified warrants for the purchase of shares of its convertible preferred stock (see Note 11) as a liability on its consolidated balance sheets as these warrants are freestanding financial instruments that may have required the Company to transfer assets upon exercise. The warrant liability, which consisted of warrants for the purchase of Series A and Series B convertible preferred stock, was initially recorded at fair value upon the date of issuance of each warrant and was subsequently remeasured to fair value at each reporting date. Changes in the fair value of the preferred stock warrant liability are recognized as a component of other income (expense), net in the consolidated statement of operations and comprehensive loss. Concurrent with the closing of the Merger, all X4 preferred stock was converted to common stock and the X4 preferred stock warrants converted to warrants for the purchase of Arsanis common stock. The Company assessed the features of the warrants and determined that they qualify for classification as permanent equity. Accordingly, the Company remeasured the warrants to fair value upon the closing of the Merger and reclassified the resulting warrant liability to additional paid-in capital.

Derivative Liabilities: Genzyme Contingent Payment— The Company’s license agreement with Genzyme Corporation (“Genzyme”) (see Note 4) contains a contingent payment obligation that required the Company to make a cash payment to Genzyme upon a change of control event of the Company. The contingent payment obligation met the definition of a derivative instrument as the contingent payment obligation was not clearly and closely related to its host instrument and was a cash-settled liability. Accordingly, the Company classified this derivative as a liability within other liabilities (non-current) on its consolidated balance sheet. The derivative liability was initially recorded at fair value on the date of entering into the license agreement and was subsequently remeasured to fair value at each reporting date. Changes in the fair value of this derivative liability were recognized as a component of other income (expense), net in the consolidated statement of operations and comprehensive loss. The Merger with Arsanis (see Note 1) qualified as a change of control event, as defined in the license agreement, but resulted in no payment being due to Genzyme under the license agreement. As a result, on March 13, 2019, the closing date of the Merger with Arsanis, the derivative liability was remeasured to fair value, which was $0, and subsequent changes in fair value will no longer be recognized in the consolidated statements of operations because the contingent payment obligation to Genzyme expired at that time.

Derivative Liabilities: Hercules Loan Redemption Feature— The Company’s Hercules loan (see Note 8) contains a redemption feature that, upon an event of default, provides Hercules the option to accelerate and demand repayment of the debt, including a prepayment premium. The redemption feature meets the definition of a derivative instrument as the repayment of the debt contains a substantial premium, resulting in the redemption feature not being clearly and closely related to its host instrument. Accordingly, the Company classifies this derivative as a liability within other liabilities (non-current) on its consolidated balance

sheets. The derivative liability was initially recorded at fair value on the date of the Hercules Loan Agreement and is subsequently remeasured to fair value at each reporting date. Changes in the fair value of this derivative liability, which is included in other liabilities, are recognized as a component of other income (expense), net in the consolidated statement of operations and comprehensive loss. Changes in the fair value of this derivative liability will continue to be recognized until all amounts outstanding under the Hercules Loan Agreement are repaid or until the Hercules Loan Agreement is terminated. During the year ended December 31, 2019, there were no changes to the fair value of this liability.

Comprehensive Loss— Comprehensive loss includes net loss as well as foreign currency translation adjustments. For the year ended December 31, 2019, comprehensive loss includes $119 thousand of foreign currency translation adjustments.

Income Taxes— The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies.

The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.

Net Loss per Share— For periods prior to the Merger with Arsanis on March 13, 2019, the Company followed the two-class method when computing net loss per share as the Company had issued shares that met the definition of participating securities. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed.

Basic net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Basic shares outstanding includes the weighted average effect of the Company’s outstanding prefunded warrants, the exercise of which requires little or no consideration for the delivery of shares of common stock. Diluted net loss attributable to common stockholders is computed by adjusting net loss attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period, including potential dilutive shares of common stock. For purpose of this calculation, outstanding stock options, convertible preferred stock and warrants to purchase shares of convertible preferred stock or common stock are considered potential dilutive shares of common stock.

The Company’s convertible preferred stock contractually entitled the holders of such shares to participate in dividends but did not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, such losses are not allocated to such participating securities. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive shares of common stock are not assumed to have been issued if their effect is anti-dilutive. The Company reported a net loss attributable to common stockholders for the years ended December 31, 2019, 2018 and 2017.

Recently Adopted Accounting Pronouncements

In February 2016, the Financial Accounting Standards Board ("FASB") issued ASU No. 2016-2, Leases (Topic 842) (“ASU 2016-2” or “ASC 842”)), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less may be accounted for similar to prior guidance for operating leases. The Company adopted the new lease standard as of the required effective date of January 1, 2019 using a modified retrospective transition approach applied to leases existing as of, or entered into after, January 1, 2019. The adoption had no impact on accumulated deficit. The new lease standard provides a number of optional practical expedients in transition. The Company applied the package of practical expedients to leases that commenced prior to the effective date, whereby it will elect to not reassess the following: (i) whether any expired or existing contracts contain leases; (ii) the lease classification for any expired or existing leases; and (iii) initial direct costs for any existing leases. The Company also elected the short-term lease recognition exemption for all leases that qualify, where a right-of-use asset or lease liability will not be recognized for short-term leases. Upon the adoption of ASC 842, the Company recorded $2.5 million of operating lease liabilities and $2.0 million of operating lease right-of-use assets on its consolidated balance sheets. The adoption did not have a material impact on the Company’s consolidated statement of operations and comprehensive loss or statement of cash flows.

In January 2017, the FASB issued ASU No. 2017-4, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. The new standard simplifies the subsequent measurement of goodwill by eliminating the second step of the goodwill impairment test. This ASU will be applied prospectively and is effective for annual or interim goodwill impairment tests in fiscal years beginning after December 15, 2019 with early adoption permitted. The Company adopted this guidance on January 1, 2019 and applied it to its annual impairment test for the year ending December 31, 2019. The adoption of this guidance did not have an impact on the Company's consolidated financial statements.

In April 2017, the FASB issued ASU 2017-8, Receivables – Nonrefundable Fees and Other Costs (“Subtopic 310-20”). The new standard amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period for the premium to the earliest call date. Subtopic 310-20 calls for a modified retrospective application under which a cumulative-effect adjustment will be made to retained earnings as of the beginning of the first reporting period in which the guidance is adopted. The Company adopted this guidance, effective January 1, 2019, and the adoption of this guidance did not have an impact on the Company's consolidated financial statements and related disclosures.

In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) (Part I) Accounting for Certain Financial Instruments with Down Round Features (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception (“ASU 2017-11”). Part I applies to entities that issue financial instruments such as warrants, convertible debt or convertible preferred stock that contain down-round features. Part II replaces the indefinite deferral for certain mandatorily redeemable noncontrolling interests and mandatorily redeemable financial instruments of nonpublic entities contained within ASC Topic 480 with a scope exception and does not impact the accounting for these mandatorily redeemable instruments. For public entities, ASU 2017-11 is effective for annual reporting periods beginning after December 15, 2018, including interim periods within those fiscal years. The Company adopted ASU 2017-11 as of the required effective date of January 1, 2019. The provisions of the new guidance related to accounting for financial instruments with down round features was taken into account by the Company in its evaluation of whether the Class B warrants issued in the November 2019 sale of common stock required liability classification. See Note 12.

In June 2018, the FASB issued ASU 2018-7, which superseded Subtopic 505-50, Equity—Equity-Based Payments to Non-Employees and amends ASC 718 to include share-based payments issued to non-employees for goods or services. Consequently, the accounting for share-based payments to non-employees and employees will be substantially aligned. The Company adopted ASU 2018-7 on January 1, 2019 and the adoption of this guidance did not have an impact on the Company's consolidated financial statements and related disclosures.

Recently Issued Accounting Pronouncements

In November 2018, the FASB issued ASU 2018-18, Clarifying the Interaction between Topic 808 and Topic 606 (“ASU 2018-18”). ASU 2018-18 clarifies the interaction between the accounting guidance for collaborative arrangements and revenue from contracts with customers. The amendments become effective January 1, 2020 and were adopted by the Company retrospectively as of January 1, 2018, the date the Company adopted the new revenue guidance under ASC 606. The adoption of this guidance did not have a material impact on the Company's consolidated financial statements and related disclosures.

In June 2016, the FASB issued ASU 2016-13, Credit Losses (Topic 326) ("ASU 2016-13"). ASU 2016-13 requires that financial assets measured at amortized cost, such as trade receivables, be presented net of expected credit losses, which may be estimated based on relevant information such as historical experience, current conditions, and future expectation for each pool of similar financial asset. The new guidance requires enhanced disclosures related to trade receivables and associated credit losses. The Company adopted this guidance on January 1, 2020. The adoption of ASU 2016-13 is not expected to have a material impact on the Company's consolidated financial statements and related disclosures.

In August 2018, the FASB issued ASU 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract, (“ASU 2018-15”). The amendments in this update align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The accounting for the service element of a hosting arrangement that is a service contract is not affected by the amendments in this update. The new standard will be effective beginning January 1, 2020 and was adopted by the Company on that date. The adoption of ASU 2018-15 is not expected to have a material impact on the Company's consolidated financial statements and related disclosures.

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”), which removes, adds and modifies certain disclosure requirements for fair value measurements in Topic 820. The Company will no longer be required to disclose the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy as well as the valuation processes of Level 3 fair value measurements. The Company will be required to provide additional disclosure related to the changes in unrealized gains and losses included in other comprehensive loss for recurring Level 3 fair value measurements and the range and weighted average of assumptions used to develop significant unobservable inputs for Level 3 fair value measurements. ASU 2018-13 is effective for the Company on January 1, 2020 and was adopted on that date. The adoption of ASU 2018-13 is not expected to have an impact on the Company's consolidated financial statements and related disclosures.

In December 2019, the FASB issued ASU 2019-12, Simplifying the Accounting for Income Taxes ("ASU 2019-12"). ASU 2019-12 simplifies the accounting for income taxes, including the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. ASU 2019-12 is effective for fiscal years beginning after December 15, 2020, including interim periods within those fiscal years. The Company does not anticipate that the adoption of ASU 2019-12 will have a material impact on its consolidated financial statements and related disclosures.

Merger Accounting

12 Months Ended

Dec. 31, 2019

Business Combinations [Abstract]

Merger Accounting

On March 13, 2019, the Company completed its merger with Arsanis. Based on the Exchange Ratio of 0.5702 , immediately following the Merger, former Arsanis stockholders, Arsanis option holders and other persons holding securities or other rights directly or indirectly convertible, exercisable or exchangeable for Arsanis common stock owned approximately 32.9% of the outstanding capital stock of the combined organization on a fully diluted basis, and former X4 stockholders, holders of options or warrants to acquire X4 capital stock and other persons holding securities and other rights directly or indirectly convertible, exercisable or exchangeable for X4 capital stock owned approximately 67.1% of the outstanding capital stock of the combined organization on a fully diluted basis. At the closing of the Merger, all shares of X4 common stock and X4 preferred stock then outstanding were exchanged for Arsanis common stock.

In addition, pursuant to the terms of the Merger Agreement, the Company, for accounting purposes, assumed all outstanding stock options to purchase shares of Arsanis common stock at the closing of the Merger. At the closing of the Merger, such stock options became options to purchase an aggregate of 271,230 shares of the Company’s common stock after giving effect to the Reverse Stock Split.

The total purchase price paid in the Merger has been allocated to the tangible and intangible assets acquired and liabilities assumed of Arsanis based on their fair values as of the completion of the Merger, with the excess allocated to goodwill. The following summarizes the preliminary estimate of the purchase price paid in the Merger (in thousands, except share and per share amounts):


Number of shares of the combined organization owned by Arsanis stockholders ⁽¹⁾	2,440,582
Multiplied by the fair value per share of Arsanis common stock ⁽²⁾	$	18.66
Fair value of consideration issued it effect the Merger	$	45,541
Fair value of replacement awards held by former employees, board of directors and consultants of Arsanis that were vested as of the Merger.	817
Purchase price:	$	46,358

________________________

⁽¹⁾ The number of shares of 2,440,582 represents the historical 14,643,737 shares of Arsanis common stock outstanding immediately prior to the closing of the Merger, adjusted for the Reverse Stock Split.

⁽²⁾ Based on the last reported sale price of Arsanis common stock on the Nasdaq Global Market on March 13, 2019, the closing date of the Merger, and gives effect to the Reverse Stock Split.

The following summarizes the allocation of the purchase price to the net tangible and intangible assets acquired (in thousands):


Cash, cash equivalents and restricted cash	$	26,406
Other current assets	2,147
Property and equipment, net	68
IPR&D indefinite-lived intangible assets	4,900
Other assets, non-current	486
Current liabilities	(5,205)
Loans payable	(8,713)
Other liabilities, non-current	(840)
Goodwill	27,109
Purchase price	$	46,358

The goodwill of $27.1 million is not tax deductible and represents the excess of the consideration paid over the fair value of assets acquired and liabilities assumed. Goodwill is mainly attributable to the enhanced value of the combined company, as reflected in the increase in market value of the shares of Arsanis common stock following the announcement of the Merger with X4. During the quarter ended June 30, 2019, goodwill was adjusted by $298 thousand to reflect a change in the allocation of purchase price to the fair value of an acquired operating lease as of March 13, 2019. There have been no other changes in the value of goodwill from the acquisition date to December 31, 2019, and goodwill was not impaired as of December 31, 2019 based on management's annual quantitative impairment test.

The Company incurred costs directly related to the Merger of approximately $1 million, which were recorded in general and administrative expenses, for the twelve months ended December 31, 2019.

The in-process research and development intangible assets represent the acquisition-date fair value of three development programs acquired from Arsanis, which the Company refers to as the ASN200, ASN300 and ASN500 programs. The fair value of the IPR&D intangible assets was based on assumptions that market participants would use in pricing the assets, based on the most advantageous market for the assets assuming highest and best use. The fair value was determined by estimating the resulting revenue from out-license arrangements related to the projects, and discounting the projected net cash flows to present value using a weighted average discount rate of 20%. These IPR&D intangible assets are not amortized, but rather are reviewed for impairment on an annual basis or more frequently if indicators of impairment are present, until the project is completed, abandoned or transferred to a third party. As further described in Note 16, during the year ended December 31, 2019, the Company entered into arrangements with two third parties that transferred the rights to develop and commercialize the programs underlying the IPR&D intangible assets. Accordingly, the Company derecognized the IPR&D intangible assets through a charge to "loss on transfer of nonfinancial assets" during the year ended December 31, 2019.

The following supplemental pro forma information presents the Company’s financial results as if the acquisition of Arsanis had occurred on January 1, 2018 (unaudited)


	Year Ended December 31,
(in thousands)	2019		2018

Revenue	$	—	$	3,500
Net loss	$	(57,820)	$	(76,248)

The above unaudited pro forma information was determined based on the historical GAAP results of the Company and Arsanis. The pro forma consolidated results are not necessarily indicative of what the Company’s consolidated results of operations would have been if the acquisition was completed on January 1, 2018. The pro forma consolidated net loss includes pro forma adjustments primarily relating to the reclassification of transaction costs and severance payments directly related to the closing of the Merger of $2.7 million from the twelve months ended December 31, 2019 to the twelve months ended December 31, 2018.

License, Collaboration, and Funding Agreements	1 Months Ended
License, Collaboration, and Funding Agreements	Aug. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
License, Collaboration, and Funding Agreements	License, Collaboration, and Funding Agreements Genzyme Agreement In July 2014, the Company entered into a license agreement (the “Genzyme Agreement”) with Genzyme pursuant to which the Company was granted an exclusive license to certain patents and intellectual property owned or controlled by Genzyme related to the CXCR4 receptor to develop and commercialize products containing licensed compounds (including but not limited to mavorixafor) for all therapeutic, prophylactic and diagnostic uses, with the exception of autologous and allogenic human stem cell therapy. Under the terms of the Genzyme Agreement, the Company is obligated to use commercially reasonable efforts to develop and commercialize licensed products for use in the field in the United States and at least one other major market country. The Company has the right to grant sublicenses of the licensed rights that cover mavorixafor to third parties. In exchange for these rights, in August 2014, the Company made an upfront payment of $50 thousand to Genzyme. The Company accounted for the acquisition of technology as an asset acquisition because it did not meet the definition of a business. The Company recorded the upfront payment as research and development expense in the consolidated statement of operations and comprehensive loss because the acquired technology represented in-process research and development and had no alternative future use. In August 2015, as a result of the closing of the Company’s Series A preferred stock financing, the Company made an additional cash payment of $300 thousand to Genzyme and issued to Genzyme 107,371 shares of its common stock, as adjusted for the 1-for-6 Reverse Stock Split and Exchange Ratio, each as required by the Genzyme Agreement. The $300 thousand payment and the $734 thousand fair value of the 107,371 shares of common stock issued to Genzyme were recorded as research and development expense in the consolidated statements of operations and comprehensive loss for the year ended December 31, 2015. Prior to the Merger with Arsanis, Genzyme had the right to require the Company to repurchase all, but not less than all, of these shares of common stock at any time during the term of the Genzyme Agreement for a price of $0.01 per share. Due to this redemption feature, the shares of common stock issued to Genzyme were classified outside of stockholders’ deficit on the consolidated balance sheets as of December 31, 2018. On March 13, 2019, the closing date of the Merger with Arsanis, these redeemable shares of common stock were exchanged for shares of common stock and, as a result, the fair value of the shares was reclassified to permanent equity. Under the Genzyme Agreement, the Company is obligated to pay Genzyme milestone payments in the aggregate amount of up to $25.0 million, contingent upon the achievement by the Company of certain clinical-stage regulatory and sales milestones with respect to licensed products. The Company is also obligated to pay Genzyme tiered royalties based on net sales of licensed products that the Company commercializes under the agreement. The obligation to pay royalties for each licensed product expires on a country-by-country basis on the latest of (i) the expiration of licensed patent rights that cover that licensed product in that country, (ii) the expiration of regulatory exclusivity in that country and (iii) ten years after the first commercial sale of such licensed product in that country. Royalty rates are subject to reduction under the agreement in specified circumstances, including in any country if the Company is required to obtain a license from any third party to the extent the Company’s patent rights might infringe the third party’s patent rights, if a licensed product is not covered by a valid claim in that country or if sales of generic products reach certain thresholds in that country. If the Company enters into a sublicense under the Genzyme Agreement, the Company will be obligated to pay Genzyme a percentage of certain upfront fees, maintenance fees, milestone payments and royalty payments paid to the Company by the sublicensee. Under the Genzyme Agreement, the Company will itself manufacture and supply, or enter into manufacturing or supply agreements with Genzyme or third parties to manufacture and supply, clinical and commercial supplies of licensed compounds and each licensed product. The Company is also responsible for all costs related to the filing, prosecution and maintenance of the licensed patent rights. The Genzyme Agreement will remain in effect until the expiration of the royalty term in all countries for all licensed products. The Genzyme Agreement may be terminated by either party with at least 90 days’ notice in the event of material breach by the other party that remains uncured for 90 days, by either party for insolvency or bankruptcy of the other party, immediately by Genzyme if the Company challenges the licensed patents, or immediately by the Company if a material safety issue arises. During the twelve months ended December 31, 2019, 2018 and 2017, the Company did not incur any payment obligations to Genzyme under the Genzyme Agreement. Georgetown Agreement In December 2016, the Company entered into a license agreement (the “Georgetown Agreement”) with Georgetown University (“Georgetown”) pursuant to which the Company obtained an exclusive, worldwide license to make, have made, use, sell, offer for sale and import of products covered by patent rights co-owned by Georgetown. The rights licensed to the Company are for all therapeutic, prophylactic and diagnostic uses in all disease indications in humans and animals. Under the terms of the Georgetown Agreement, the Company paid a one-time only, upfront fee of $50 thousand and the Company may be required to make milestone payments of up to an aggregate of $800 thousand related to commercial sales of a product. The Company recorded the upfront payment as research and development expense in the consolidated statement of operations and comprehensive loss because the acquired technology represented in-process research and development and had no alternative future use. Under the Georgetown Agreement, the Company is solely responsible for all development and commercialization activities and costs in its respective territories. The Company is also responsible for all costs related to the filing, prosecution and maintenance of the licensed patent rights. The term of the Georgetown Agreement will continue until the expiration of the last valid claim within the patent rights covering the product. Georgetown may terminate the agreement in the event (i) the Company fails to pay any amount and fails to cure such failure within 30 days after receipt of notice, (ii) the Company defaults in its obligation to obtain and maintain insurance and fails to remedy such breach within 45 days after receipt of notice, or (iii) the Company declares insolvency or bankruptcy. The Company may terminate the Georgetown Agreement at any time upon at least 60 days’ written notice. During the twelve months ended December 31, 2019, 2018 and 2017, the Company did not incur any payment obligations to Georgetown under the Georgetown Agreement and no milestone payments were made or due under the Georgetown Agreement. Beth Israel Deaconess Medical Center Agreement In December 2016, the Company entered into a license agreement (the “BIDMC Agreement”) with Beth Israel Deaconess Medical Center (“BIDMC”), pursuant to which the Company obtained an exclusive, worldwide license to make, have made, use, sell, offer for sale and import products covered by patent rights co-owned by BIDMC. The rights licensed to the Company are for all fields of use. Under the terms of the BIDMC Agreement, the Company paid a one-time, upfront fee of $20 thousand and the Company is responsible for all future patent prosecution costs. The Company recorded the upfront payment as research and development expense in the consolidated statement of operations because the acquired technology represented in-process research and development and had no alternative future use. The term of the BIDMC Agreement will continue until the expiration of the last valid claim within the patent rights covering the licensed products. BIDMC may terminate the agreement in the event (i) the Company fails to pay any amount and fails to cure such failure within 15 days after receipt of notice, (ii) the Company is in material breach of any material provision of the BIDMC Agreement and fails to remedy such breach within 60 days after receipt of notice, or (iii) the Company declares insolvency or bankruptcy. The Company may terminate the BIDMC Agreement at any time upon at least 90 days’ written notice. The Company did not incur any payment obligations under the BIDMC Agreement during the twelve months ended December 31, 2019, 2018 and 2017. Research and Development Incentive Program The Company's wholly-owned subsidiary X4 GmbH, which was acquired in March 2019 in the Merger, participates in a research and development incentive program provided by the Austrian government whereby the Company is entitled to reimbursement by the Austrian government for a percentage of qualifying research and development expenses incurred by the Company’s subsidiary in Austria. Under the program, the reimbursement rate for qualifying research and development expenses incurred by the Company through its subsidiary in Austria is 14% for the current year. The Company recognizes incentive income from Austrian research and development incentives when qualifying expenses have been incurred, there is reasonable assurance that the payment will be received, and the consideration can be reliably measured. Management has assessed the Company’s research and development activities and expenditures to determine which activities and expenditures are likely to be eligible under the research and development incentive program described above. At each reporting date, management estimates the reimbursable incentive income available to the Company based on available information at the time. As of December 31, 2019, the amount due under the program was $2.0 million, which is included in research and development incentive receivable in the consolidated balance sheet and of which $1.5 million was collected in the first quarter of 2020. During the year ended December 31, 2019, the Company recorded $446 thousand of income related to the program on the consolidated statement of operations and comprehensive loss within "other income". Abbisko Agreement In July 2019, the Company entered into a license agreement (the “Abbisko Agreement”) with Abbisko Therapeutics Co., Ltd. (“Abbisko”). Under the terms of the Abbisko Agreement, the Company granted Abbisko the exclusive right to develop, manufacture and commercialize mavorixafor in mainland China, Taiwan, Hong Kong and Macau (referred to in the Abbisko Agreement as the “Abbisko Territory”). The agreement provides Abbisko with the exclusive rights in the Abbisko Territory to develop and commercialize mavorixafor in combination with checkpoint inhibitors or other agents in multiple oncology indications. Pancreatic cancer, ovarian cancer and triple negative breast cancer will be explored initially. The Company retains the full rest-of-world rights to develop and commercialize mavorixafor outside of Greater China for all indications and the ability to utilize data generated pursuant to the Abbisko collaboration for rest-of-world development. In addition, Abbisko has the right of first refusal if the Company determines to pursue additional products in the Abbisko Territory. In accordance with the Abbisko Agreement, the Company has agreed to enter into separate agreements whereby the Company would provide Abbisko with a clinical supply and, if the product is commercialized in the Abbisko Territory, a commercial supply of the licensed compound. Pursuant to the Abbisko Agreement, upon the closing of a qualified financing (as defined in the Abbisko Agreement), Abbisko will make a one-time, non-refundable, non-creditable financial milestone payment in the low single-digit millions to the Company. The Company is also eligible to receive potential development and regulatory milestone payments, which vary based on the number of indications developed, and potential commercial milestone payments based on annual net sales of mavorixafor-based licensed products. Assuming mavorixafor is developed by Abbisko in six indications, the Company would be entitled to milestone payments of up to $208.0 million, which will vary based on the ultimate sales, if any, of the approved licensed products. In addition, upon commercialization of mavorixafor in the Abbisko Territory, the Company is eligible to receive a tiered royalty, with a percentage range in the low double-digits, on net sales of approved licensed products. Abbisko is obligated to use commercially reasonable efforts to develop and commercialize mavorixafor in the Abbisko Territory. Abbisko has responsibility for all activities and costs associated with the further development, manufacture and commercialization of mavorixafor in the Abbisko Licensed Territory. The Company evaluated the Abbisko Agreement under ASC 606 and determined the Abbisko Agreement contained a single performance obligation related to the exclusive license to develop and commercialize mavorixafor and the transfer of know-how that was satisfied at the inception of the arrangement. The transaction price related to the transfer of the license and know-how was fully constrained and the Company ascribed no transaction price to the development, regulatory and commercial milestones under the "most-likely amount" method. As part of its evaluation, the Company considered multiple factors: (i) the ability of Abbisko to achieve a qualified financing (as defined in the Abbisko Agreement) is dependent on many factors outside of its control (ii) regulatory approvals are outside of the control of Abbisko, and (iii) certain development and regulatory milestones are contingent upon the success of future clinical trials, if any, which is out of the control of Abbisko. Any consideration related to the initial transfer of the license and know-how will be recognized when it is probable that Abbisko will achieve the related financial milestone. Any consideration related to development and regulatory milestones will be recognized when the corresponding milestones are achieved. Any consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as these amounts have been determined to relate predominantly to the license granted to Abbisko and therefore are recognized at the later of when the performance obligation is satisfied or the related sales occur. The Company determined that the future sale of clinical and commercial supply are optional goods that will be subject to the customer's future purchasing decisions and do not represent performance obligations in the Abbisko Agreement. The Company concluded that the amount to be charged for the clinical supply will be reflective of market value and, therefore, the Abbisko Agreement does not provide a discount on such supply that would be accounted for as material right at the outset of the contract. In arriving at these conclusions, the Company considered the complexity of the manufacturing process for the licensed compound and the potential ability for Abbisko to obtain the compound directly from other manufactures in the future. The Company expects that it will recognize revenue at a point in time when such clinical supply (and commercial supply, if applicable) is delivered to Abbisko in the future. The Company will re-evaluate the transaction price, including its estimated variable consideration for milestones included in the transaction price and all constrained amounts, in each reporting period and as uncertain events are resolved or other changes in circumstances occur.

Fair Value of Financial Assets and Liabilities

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets and Liabilities

The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy used to determine such fair values:


	Fair Value Measurements as of December 31, 2019 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	23,638	$	39,999	$	—	$	63,637
	$	23,638	$	39,999	$	—	$	63,637

Liabilities: none


	Fair Value Measurements as of December 31, 2018 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market fund	$	—	$	8,134	$	—	$	8,134
	$	—	$	8,134	$	—	$	8,134
Liabilities:
Preferred stock warrant liability	$	—	$	—	$	4,947	$	4,947
Derivative liability	—		—		201		201
	$	—	$	—	$	5,148	$	5,148

As of December 31, 2019 and December 31, 2018, there were no transfers between Level 1, Level 2 and Level 3.

The Company’s cash equivalents consisted of money market funds invested in U.S. Treasury securities. The money market funds were valued based on reported market pricing for the identical asset or by using inputs observable in active markets for similar securities, which represents a Level 2 measurement in the fair value hierarchy.

Valuation of Preferred Stock Warrant Liabilities— The preferred stock warrant liability in the table above consists of the fair values of (i) warrants to purchase shares of Series A convertible preferred stock that were issued in 2015 and shares of Series B convertible preferred stock that were issued in 2017 and 2018 in connection with the Company’s Series A and Series B convertible preferred stock financings, respectively (see Note 11), (ii) warrants to purchase shares of Series A convertible

preferred stock that were issued in 2016 in connection with the Company’s entering into a loan and security agreement with Silicon Valley Bank (see Note 8) and (iii) warrants to purchase shares of Series B convertible preferred stock that were issued or were issuable in 2018 in connection with the Company’s entering into the Hercules Loan Agreement (see Note 8). The liability associated with the warrants was recorded at fair value on the dates the warrants were issued and exercisable and was subsequently remeasured to fair value at each reporting date through December 31, 2018. Upon the closing of the Merger on March 13, 2019, all X4 preferred stock warrants were converted to warrants for Company’s common stock and, as a result, the warrants were adjusted to fair value and reclassified to permanent equity. The aggregate fair value of the warrant liability was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy.

The Company used various valuation methods, including the Monte Carlo method, the option-pricing method and the hybrid method (which is a combination of an option-pricing method and a probability-weighted expected return method), all of which incorporate assumptions and estimates, to value the preferred stock warrants. Estimates and assumptions impacting the fair value measurement include the fair value per share of the underlying shares of the Company’s Series A and Series B convertible preferred stock, risk free interest rate, expected dividend yield, expected volatility of the price of the underlying preferred stock, and either the remaining contractual term of the warrants (except for warrants that would be automatically exercised upon an initial public offering, in which case the remaining estimated term to automatic exercise was used). The most significant assumption in the Monte Carlo method, the option-pricing method and the hybrid method impacting the fair value of the preferred stock warrants is the fair value of the Company’s convertible preferred stock as of each remeasurement date. The Company determines the fair value per share of the underlying preferred stock by taking into consideration the most recent sales of its convertible preferred stock, results obtained from third-party valuations and additional factors that are deemed relevant. As of December 31, 2018, the fair value of the Series A convertible preferred stock was $1.70 per share and the fair value of the Series B convertible preferred stock was $1.86 per share. There were no warrants for the purchase of convertible preferred shares as of December 31, 2019 as all such warrants were converted to warrants for the purchase of common stock upon the Merger. The Company was a private company prior to the Merger and lacked company-specific historical and implied volatility information of its stock. Therefore, the Company had estimated its expected stock volatility based on the historical volatility of publicly traded peer companies for a term equal to the estimated remaining term of the warrants. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the estimated remaining term of the warrants. The Company estimated a 0% expected dividend yield as the Company has never paid or declared dividends and does not intend to do so in the foreseeable future.

Valuation of Derivative Liability— The fair value of the derivative liability recognized in connection with the Genzyme Agreement (see Note 4) was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of this derivative liability is reported within other liabilities on the consolidated balance sheets. The fair value of this derivative liability was estimated by the Company at each reporting date based, in part, on the results of third-party valuations, which were prepared using the option-pricing method or the hybrid method, each of which considered as inputs the type, timing and probability of occurrence of a change of control event, the potential amount of the payment under potential exit scenarios, the fair value per share of the underlying common stock and the risk-adjusted discount rate. As of December 31, 2018, the fair value of this derivative liability was $183 thousand. The Merger (see Note 1) qualified as a change of control event, as defined in the Genzyme Agreement, but results in no payment being due to Genzyme under the Genzyme Agreement. As a result, on March 13, 2019, the closing date of the Merger with Arsanis, this derivative liability was remeasured to fair value, which was zero, and subsequent changes in fair value will no longer be recognized in the consolidated statements of operations because the contingent payment obligation to Genzyme expired at that time.

The fair value of the derivative liability recognized in connection with the Hercules Loan Agreement (see Note 8) was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of this derivative liability is reported within other liabilities on the consolidated balance sheets. The fair value of this derivative liability was estimated by the Company at each reporting date based, in part, on the results of third-party valuations, which were prepared based on a discounted cash flow model that considered the timing and probability of occurrence of a redemption upon an event of default, the potential amount of prepayment upon an event of default and the risk-adjusted discount rate. As of December 31, 2019 and December 31, 2018, the fair value of this derivative liability was immaterial.

The following table provides a roll-forward of the aggregate fair values of the Company’s warrant liability and derivative liability, for which fair values are determined using Level 3 inputs:


(in thousands)	Preferred Stock Warrant Liability		Derivative Liability
Balance as of December 31, 2017	$	1,245	$	94
Issuance of warrants to purchase shares of Series B convertible preferred stock	304		—
Initial fair value of derivative liability in connection with the Hercules Loan Agreement	—		18
Change in fair value	3,398		89
Balance as of December 31, 2018	4,947		201
Change in fair value	288		(183)
Conversion of convertible preferred stock warrant into common stock warrant in connection with Merger	(5,235)		—
Balance at December 31, 2019	$	—	$	18

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Property and Equipment, Net

12 Months Ended

Dec. 31, 2019

Property, Plant and Equipment [Abstract]

Property and Equipment, Net

Property and equipment, net consisted of the following:


(in thousands)	December 31, 2019		December 31, 2018
Leasehold improvements	$	299	$	299
Furniture and fixtures	139		53
Computer equipment	37		56
Software	33		9
Lab equipment	159		—
	667		417
Less: Accumulated depreciation and amortization	(264)		(176)
	$	403	$	241

Depreciation and amortization expense related to property and equipment was approximately $103 thousand for each of the twelve months ended December 31, 2019 and 2018 and $71 thousand for the year ended December 31, 2017.

Accrued Expenses

12 Months Ended

Dec. 31, 2019

Other Income and Expenses [Abstract]

Accrued Expenses

Accrued expenses consisted of the following


(in thousands)	December 31, 2019		December 31, 2018
Accrued employee compensation and benefits	$	2,916	$	924
Accrued external research and development expenses	1,977		754
Accrued professional fees	1,347		1,324
Other	221		249
	$	6,461	$	3,251

Long-Term Debt

12 Months Ended

Dec. 31, 2019

Debt Disclosure [Abstract]

Long-Term Debt

Long-Term DebtLong-term debt consisted of the following:


(in thousands)	December 31, 2019		December 31, 2018
Principal amount of long-term debt	$	20,000	$	10,000
Less: Current portion of long-term debt	—		(1,687)
Long-term debt, net of current portion	20,000		8,313
Debt discount, net of accretion	(317)		(226)
Cumulative accretion of final payment due at maturity	414		58
Long-term debt, including accretion, net of discount and current portion	$	20,097	$	8,145

SVB Loan Agreement

In October 2016, the Company entered into a loan and security agreement with Silicon Valley Bank (“SVB”), which the Company refers to as the SVB Loan Agreement, pursuant to which SVB made certain term loans available to the Company. The SVB Loan Agreement provided for a term loan of up to $6.0 million, which was borrowed by the Company in September 2017. Borrowings under the SVB Loan Agreement bore interest at a variable rate equal to 5.5% plus the greater of (i) 3.5% or (ii) The Wall Street Journal prime rate. In October 2018, in connection with entering into the Hercules Loan Agreement, the Company terminated the SVB Loan Agreement and repaid all amounts due under the SVB Loan Agreement of $4.7 million, including outstanding principal, prepayment premiums and accrued interest. The Company accounted for the termination of the SVB Loan Agreement as an extinguishment and recognized a loss of $229 thousand, which was calculated as the difference between the reacquisition price of the borrowings under the SVB Loan Agreement and the carrying value of the debt at the time of the extinguishment.

The Company recognized aggregate interest expense under the SVB Loan Agreement of $484 thousand and $490 thousand for the years ended December 31, 2018 and 2017, respectively, reflecting an effective interest rate of approximately 11.8% and 12.0%, respectively. As of October 19, 2018, the date of repayment of all borrowings under the SVB Loan Agreement, the contractual interest rate applicable to borrowings under the SVB Loan Agreement was 10.75%.

Hercules Loan Agreements

Hercules Loan Agreement and First Amendment

In October 2018, the Company entered into the Hercules Loan Agreement. The Hercules Loan Agreement provided for aggregate borrowings of up to $13.0 million, consisting of (i) a term loan of up to $8.0 million, which was available upon entering into the agreement, (ii) subject to specified financing conditions, an additional term loan of up to $2.0 million, available for borrowing from January 1, 2019 to March 31, 2019, and (iii) subject to specified financing conditions and the receipt of the second tranche term loan in the amount of $2.0 million described above, an additional term loan of up to $3.0 million, available for borrowing until March 31, 2019. In October 2018, the Company borrowed $8.0 million under the Hercules Loan Agreement. In December 2018, the Company entered into the First Amendment to the Hercules Loan Agreement (the “First Amendment”), which amended the available borrowing dates of the second tranche from between January 1, 2019 and March 31, 2019 to between December 11, 2018 and December 14, 2018 and amended the term loan maturity date to November 1, 2021. In December 2018, the Company borrowed the additional $2.0 million provided under the Hercules Loan Agreement, as amended by the First Amendment. In March 2019, the conditions necessary for borrowing the remaining $3.0 million under the Hercules Loan Agreement, as amended by the First Amendment, were not met and the borrowing capacity expired at that time.

In connection with entering into the Hercules Loan Agreement in October 2018, the Company issued to Hercules warrants for the purchase of 210,638 shares of Series B convertible preferred stock at an exercise price of $1.88 per share (which were subsequently converted to warrants for the purchase of 20,016 shares of common stock at an exercise price of $19.78 per share following the Merger). These warrants were immediately exercisable and expire in October 2028. In addition, in connection with entering into the First Amendment in December 2018, the Company agreed to issue to Hercules warrants for the purchase of a specified number of shares of convertible preferred stock at an aggregate exercise price of $99. On March 18, 2019, as a result of the closing of the Merger with Arsanis on March 13, 2019, the Company issued to Hercules warrants for the purchase of 5,000 shares of common stock of the combined organization at an exercise price of $19.80 per share, each of which reflected the share Exchange Ratio of 1-for-0.5702 applied in the Merger as well as the Reverse Stock Split effected by the combined organization on March 13, 2019. On October 19, 2018 and December 11, 2018, the dates the Company entered into the Hercules Loan Agreement and the First Amendment, respectively, the Company recorded the aggregate initial fair value of the warrants of $132 thousand as a preferred stock warrant liability, with a corresponding amount recorded as a debt discount on the Company’s

consolidated balance sheet. As of March 13, 2019, and December 31, 2018, the fair value of the warrants were $326 thousand and $282 thousand, respectively. Upon the closing of the Merger, the warrants were converted to warrants for common stock and are no longer adjusted to fair value.

2019 Amended and Restated Loan Agreement

In June 2019, the Company refinanced the Hercules Loan Agreement, as amended, and entered into an Amended and Restated Loan and Security Agreement (the “2019 Loan Agreement”) with Hercules. The 2019 Loan Agreement provides for aggregate maximum borrowings of $35.0 million, of which $10.0 million was previously outstanding. The Company agreed to borrow $20.0 million as of the closing date on June 27, 2019, including $10.0 million in new borrowings and $10.0 million rolled over from the previous agreement. An additional $5.0 million is available for borrowing through December 15, 2020 and, subject to approval by Hercules, and an additional term loan of $10.0 million is available through June 15, 2022. Borrowings under the 2019 Loan Agreement bear interest at a variable rate equal to the greater of (i) 8.75% or (ii) 8.75% plus The Wall Street Journal prime rate minus 6.0%. In an event of default, as defined in the 2019 Loan Agreement, and until such event is no longer continuing, the interest rate applicable to borrowings under the 2019 Loan Agreement would be increased by 4.0%.

Borrowings under the 2019 Loan Agreement are repayable in monthly interest-only payments through January 1, 2022, and in equal monthly payments of principal and accrued interest from February 1, 2022 until the maturity date of the loan, which is July 1, 2023. At the Company’s option, the Company may prepay all, but not less than all, of the outstanding borrowings, subject to a prepayment premium of up to 2.0% of the principal amount outstanding as of the date of repayment. In addition, the 2019 Loan Agreement provides for payments by the Company to Hercules of (i) $0.8 million payable upon the earlier of November 1, 2021 or the repayment in full of all obligations under the 2019 Loan Agreement, and (ii) 4.0% of the aggregate principal drawn under the 2019 Loan Agreement payable upon the earlier of maturity or the repayment in full of all obligations under such agreement.

Borrowings under the 2019 Loan Agreement are collateralized by substantially all of the Company’s personal property and other assets except for the Company’s intellectual property (but including rights to payment and proceeds from the sale, licensing or disposition of the intellectual property). Under the 2019 Loan Agreement, the Company has agreed to affirmative and negative covenants to which it will remain subject until maturity or repayment of the loan in full. The covenants include (a) maintaining a minimum liquidity amount of the lesser of (i) 125% of the aggregate principal amount of outstanding borrowings under the June 2019 Loan Agreement and (ii) 100% of the Company and its consolidated subsidiaries’ cash and cash equivalents (other than up to $2.5 million which may be held by an excluded subsidiary, as defined) in an account in which Hercules has a first priority security interest, as well as (b) restrictions on the Company’s ability to incur additional indebtedness, pay dividends, encumber its intellectual property, or engage in certain fundamental business transactions, such as mergers or acquisitions of other businesses. The 2019 Loan Agreement also contains a covenant that requires the Company to repay its loan with Österreichische Forschungsförderungsgesellschaft mbH ("FFG") on or prior to December 31, 2019. The FFG Loan Agreement was settled during the three months ended September 30, 2019 as discussed further below.

In connection with entering into the Hercules Loan Agreement and the First Amendment, the Company had deferred $180 thousand associated with upfront (1) fees associated with entering into the agreement, (2) the fair value of the warrants issued and (3) the fair value of an embedded derivative associated with the accelerated redemption feature. The $180 thousand is classified as a debt discount, which is reflected as a reduction of the carrying value of long-term debt on the Company’s consolidated balance sheet and is being amortized to interest expense over the term of the loan using the effective interest method. As a result of the June 2019 refinancing and entering into the 2019 Loan Agreement, the Company considered whether the previous debt was either extinguished or modified based on the difference in the cash flows of the previous and new debt. The Company determined that Hercules Loan Agreement, as amended by the First Amendment, was modified. Accordingly, the unamortized debt discount of the previous debt and newly incurred fees paid to the lender related to the 2019 Loan Agreement will be amortized to interest expense over the life of the new debt arrangement using the effective interest method.

The Company recognized aggregate interest expense under the Hercules Loan Agreement, as amended by the First Amendment, and the 2019 Loan Agreement of $1.8 million and $236 thousand during the twelve months ended December 31, 2019 and 2018, respectively. Interest expense includes $61 thousand and $355 thousand related to the accretion of the debt discount and the final payment for the year ended December 31, 2019, respectively. As of December 31, 2019, the unamortized debt discount was $317 thousand. The annual effective interest rate on the 2019 Loan Agreement is 10.9%. There were no principal payments due or paid under the Hercules Loan Agreement, as amended by the First Amendment, during the twelve months ended December 31, 2019. Principal payments begin in January 2022.

FFG Loan Agreements

Between September 2011 and March 2017, Arsanis GmbH entered into a series of funding agreements with FFG that provided for loans and grants to fund qualifying research and development expenditures of X4 GmbH on a project-by-project basis, as approved by FFG. Amounts due under the FFG loans bear interest at rates ranging from 0.75% to 2.00% per annum. Giving effect to the Settlement Agreement (as defined below), the loans matured at various dates between March 31, 2019 and March 31, 2021. Interest on amounts due under the loans is payable semi-annually in arrears, with all principal and remaining accrued interest due upon maturity.

On March 8, 2019, Arsanis, Merger Sub, X4 and Arsanis GmbH entered into a Settlement Agreement with FFG (the “Settlement Agreement”) in respect to allegations by FFG in February 2019 that Arsanis and Arsanis GmbH breached certain reporting, performance and other obligations in connection with grants and loans made by FFG to Arsanis GmbH between September 2011 and March 2017 to fund qualifying research and development expenditures. Pursuant to the terms of the Settlement Agreement, in exchange for FFG’s waiver of all claims against Arsanis and Arsanis GmbH (except for its claims for repayment of the loans and regular interest but including its waiver of claims for repayment of grants and interest exceeding regular interest), subject to compliance by Arsanis and Arsanis GmbH with the terms of the Settlement Agreement, Arsanis GmbH agreed to repay the outstanding loan principal (plus regular interest accrued thereon) on an accelerated payment schedule of three years instead of five years, with the final accelerated installment due and payable on June 30, 2021. The parties also agreed, among other things, that (i) the portion of such loans to be repaid in 2019 will be $2.9 million on the first business day following March 31, 2019, and such amount was paid on April 1, 2019, and (ii) until all of the loans have been repaid and subject to other terms specified in the Settlement Agreement, commencing April 30, 2019, a minimum cash balance equal to 70% of the then-outstanding principal amount of the loans will be maintained at X4 Pharmaceuticals (Austria) GmbH in an account held with an Austrian bank.

The Company recognized interest expense under the FFG agreement of $316 thousand for the year ended December 31, 2019. In September 2019, the Company repaid all amounts due under the FFG loans and recognized a loss on extinguishment of debt of $566 thousand, which was calculated as the difference between the reacquisition price of the borrowings under the FFG Loan Agreement and the carrying value of the debt at the time of the extinguishment.

As of December 31, 2019, future principal payments and the final payment due under the Company’s loan agreements were as follows (in thousands):


Year Ending December 31	Total
2020	$	—
2021	—
2022	11,897
2023	8,103
Long-term debt	$	20,000

Leases

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

Leases

Effective January 1, 2019, the Company adopted ASC 842 using the modified retrospective approach through a cumulative-effect adjustment and utilizing the effective date as its date of initial application, with prior periods unchanged and presented in accordance with the previous lease accounting guidance. Upon adoption, the Company recorded right-of-use assets of $2.0 million and lease liabilities of $2.5 million, of which $1.9 million was classified as non-current and $613 thousand as current. The difference between the value of the right-of-use assets and the lease liabilities related to $512 thousand of net deferred, accrued and prepaid rent that was reclassified against the right-of-use asset upon adoption of ASC 842 on January 1, 2019. The Company has lease agreements for its facilities in Cambridge, Massachusetts, which is the Company’s global headquarters; Vienna, Austria, which is the Company’s research and development center; Waltham, Massachusetts, which is the former headquarters of Arsanis that the Company has sublet to a third party; and Allston, Massachusetts, as further discussed below. There are no restrictions or financial covenants associated with any of the lease agreements.

Cambridge Lease— In August 2017, the Company entered into a non-cancellable operating lease agreement for office space of approximately 13,000 square feet in Cambridge, Massachusetts (“Cambridge Lease”) which expires on July 31, 2022. The Cambridge Lease includes an annual fixed rent escalation clause and such rent escalations were included in the calculation of the right-of-use asset. The Company has the option to extend the lease for one period of 5 additional years. Base rent is approximately $832 thousand annually and the monthly rent expense is being recognized on a straight-line basis over the term of the lease as the Company amortizes the associated operating lease right-of-use asset. In addition to the base rent, the Company is also responsible

for its share of operating expenses, electricity and real estate taxes, in accordance with the terms of lease. These costs are not included in the determination of the leases’ right-of-use operating assets or lease operating liabilities.

Waltham Lease— On March 13, 2019, as part of its Merger with Arsanis, the Company acquired a non-cancellable operating lease for approximately 6,000 square feet of office space in Waltham, Massachusetts (“Waltham Lease”). The Waltham Lease, as amended, commenced on January 1, 2019, and expires approximately 5 years from the commencement date. The base rent is approximately $263 thousand annually. In addition to the base rent, the Company is also responsible for its share of operating expenses, electricity and real estate taxes, which costs are not included in the determination of the leases’ right-of-use assets or lease liabilities. The Company is subleasing the space to a third party for the duration of the lease. As a result, the Company has adjusted the value of the right-of-use asset to its estimated fair value, which represents future sublease income less costs to obtain sublease. The right-of-use asset is being amortized to rent expense over the 5 year term of the lease.

Vienna Austria Lease— On March 13, 2019, as part of its Merger with Arsanis, the Company acquired an operating lease for approximately 400 square meters of laboratory and office space in Vienna, Austria, (the “Vienna Austria Lease”) which commenced on March 1, 2019, as amended, for a term of 2 years. The lease is cancellable by the Company upon three months’ notice with no penalty. The annual base rent is approximately $154 thousand. The Company has classified this lease as a short-term lease as it is not reasonably certain that the Company with not terminate the lease within one year and, accordingly, has not recorded a right-of-use asset. Accordingly, rent expense is recorded on a straight-line basis as incurred over the term of the lease.

Allston Lease— On November 11, 2019, the Company entered into a lease agreement for approximately 28,000 square feet of office space currently under construction in a building located in Allston, Massachusetts. The office space is expected to replace the Company’s current headquarters located in Cambridge, Massachusetts. The Company intends to move into the premises in mid-2020 upon the completion of construction. Monthly rent payments under the lease are expected to commence in May, 2020, reflecting a 180-day rent-free period following the execution of the Lease, subject to the timely completion of construction of the premises. Base rental payments will be approximately $1.0 million annually, plus certain operating expenses. The term of the lease will continue until November 2026, unless earlier terminated in accordance with the terms of the lease. The Company has the right to sublease the premises, subject to landlord consent. The Company also has the right to renew the lease for an additional five years at the then prevailing effective market rental rate. The Company is required to provide the landlord with a $1.1 million security deposit in the form of a letter of credit, which is classified as restricted cash.

For the Allston lease, the Company will participate in the construction of the office space and will incur construction costs to prepare the office space for its use, which will be partially reimbursed by the landlord. The Company has concluded that these construction costs generate and enhance the landlord's assets and, as such, costs that are not reimbursed will be classified as prepaid rent and then reclassified to the right-of-use asset on the lease commencement date, which is expected to occur once the landlord's asset is completed and available for additional leasehold improvements funded by the Company. Upon the lease commencement date, which had not yet occurred as of December 31, 2019, the Company will recognize the lease liability, which will reflect the future rent payments for the term of the lease discounted at the Company's collateralized borrowing rate, and the right-of-use asset, which will be measured as the lease liability plus the prepaid rent incurred to date.

As the Company’s leases do not provide an implicit rate, the Company estimated the incremental borrowing rate in calculating the present value of the lease payments. The Company utilizes its incremental borrowing rates, which are the rates incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment.

The components of lease expense for the year ended December 31, 2019 were as follows (dollars in thousands):


Lease Cost	For the Year Ended December 31, 2019
Fixed operating lease cost	$	827
Short-term lease costs	122
Total lease expense	$	949
Other information
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$	1,007
Leased assets obtained in exchange for new operating lease liabilities⁽¹⁾	$	484
Weighted-average remaining lease term—operating leases	3.0 years
Weighted-average discount rate—operating leases	9.0		%
Sublease income	$	14

___________________________________________

⁽¹⁾Acquired in the Merger

Maturities of lease liabilities due under these lease agreements as of December 31, 2019 are as follows (in thousands):


Maturity of lease liabilities	Operating Leases
2020	$	1,124
2021	1,098
2022	754
2023	263
Total lease payments	3,239
Less: interest	(423)
Total operating lease liabilities as of December 31, 2019	$	2,816

The above table does not include future lease payments related to the Company's Allston lease for which a lease liability has not yet been recorded. The lease liability will be recorded upon the commencement date of the lease. Future lease payments upon the commencement date of the lease are expected to be approximately $0.6 million in 2020, $1.0 million in 2021, $1.0 million in 2022, $1.1 million in 2023, $1.1 million in 2024, $1.1 million in 2025 and $1.1 million in 2026. In addition, the Company expects to incur unreimbursed expenditures of $3.1 million related to the construction of the office space.

As of December 31, 2018, minimum future lease payments under non-cancellable operating leases for each of the following five years and a total thereafter were as follows (in thousands):


Year Ending December 31,	Operating Leases
2019	$	810
2020	823
2021	835
2022	492
Total	$	2,960

Commitment and Contingencies	12 Months Ended
Commitment and Contingencies	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitment and ContingenciesSponsored Research Agreement Commitments— In April 2017, the Company entered into a sponsored research agreement with a university pursuant to which the Company and the university are conducting a research program related to understanding the mechanisms of failed long-term adaptive immunity in WHIM patients. Under the terms of the agreement, the Company agreed to provide funding for the research program of up to $499 thousand over a three years period. The agreement will remain in effect for three years, unless earlier terminated. The Company may terminate the agreement at any time upon at least 60 days’ prior written notice. For the years ended December 31, 2019, 2018, and 2017, the Company incurred $166 thousand, $166 thousand and $111 thousand, respectively, of research and development expenses related to its payment obligations to the university under the agreement. As of December 31, 2019, the Company had non-cancelable purchase commitments under this agreement totaling $55 thousand committed in 2020. In May 2019, the Company amended its agreement with a clinical research organization (“CRO”) pursuant to which the Company and the CRO are conducting a global Phase 3 clinical trial of mavorixafor for the treatment of WHIM syndrome. The Company may terminate the agreement by providing 30 days’ notice and if such termination occurred, the Company would incur early termination fees of up to $0.6 million based on a percentage of committed resources of the CRO as of the termination. Indemnification Agreements— In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its executive officers that will require the Company to, among other things, indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnification obligations. The Company is not currently aware of any indemnification claims and has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2019 or December 31, 2018. License Agreements— In February 2017, Arsanis entered into an option and license agreement with Adimab to obtain rights to certain RSV antibodies, which are being used in the “ASN500” program. The Company exercised this option for an up-front payment of $250 thousand. In July 2019, as further described in Note 16, the Company transferred the intellectual property related to the ASN500 program through an exclusive, worldwide license with a third party. The Company remains obligated to make payments to Adimab based on future clinical and regulatory milestones of up to approximately $25.0 million, as well as royalty payments on a product-by-product and country-by-country basis of a mid-single-digit percentage based on net sales of products based on certain RSV antibodies during the applicable term for such product in that country. The third party is required to fund any future payments that may become due to Adimab. Legal Proceedings— The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to such legal proceedings.

Preferred and Common Stock Warrants

12 Months Ended

Dec. 31, 2019

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Preferred and Common Stock Warrants

Prior to the Merger (see Note 1), the Company issued warrants for the purchase of its preferred stock and had classified these preferred stock warrants as a liability on its consolidated balance sheet as the warrants were deemed to be freestanding financial instruments that may have required the Company to transfer assets upon exercise. The liability associated with each of these warrants was initially recorded at fair value upon the issuance date of each warrant and was subsequently remeasured to fair value as a component of other income (expense), net in the consolidated statement of operations and comprehensive loss. Upon the closing of the Merger, pursuant to the Merger Agreement, all of the outstanding X4 preferred stock was converted to Arsanis common stock and the X4 preferred stock warrants converted to warrants for the purchase of Arsanis common stock. The Company assessed the features of the warrants and determined that they qualify for classification as permanent equity upon the closing of the Merger. Accordingly, the Company remeasured the warrants to fair value upon the closing of the Merger, which was $5.2 million at March 13, 2019, with $288 thousand of expense recorded during the three months ended March 31, 2019. Upon the closing of the Merger, the warrant liability was reclassified to additional paid-in capital.

In connection with its issuance of common stock in public offerings that closed on April 16, 2019 and November 29, 2019, the Company issued 3,900,000 Class A warrants, which are exercisable for shares of the Company's common stock, and 5,416,667

Class B warrants, which are exercisable for shares of the Company's common stock or prefunded warrants to purchase shares of the Company's common. The Class A warrants have an exercise price of $13.20 per warrant, expire on April 15, 2024 and were immediately exercisable upon issuance. The Class B warrants were immediately exercisable upon issuance, have an exercise price of $15.00 per warrant and expire on a date that is the earlier of (a) the date that is 30 calendar days from the date on which the Company issues a press release announcing top-line data from its Phase 3 clinical trial of mavorixafor for the treatment of patients with WHIM syndrome (or, if such date is not a business day, the next business day) and (b) November 28, 2024. In addition, in connection with the April 16, 2019 and November 29, 2019 offerings, the Company issued 2,130,000 and 1,750,000 prefunded warrants, respectively, for proceeds of $10.999 and $11.999 per share, respectively. Each of the prefunded warrants is exercisable into one share of the Company's common stock, has a remaining exercise price of $0.001 per share and was immediately exercisable upon issuance.

The following table provides a roll forward of outstanding warrants for the twelve month period ended December 31, 2019:


	Number of warrants	Weighted Average Exercise Price		Weighted Average Contractual Term (Years)
Outstanding and exercisable warrants to purchase preferred shares as of December 31, 2018	5,146,400	$	—	4.23
Conversion of warrants to purchase preferred shares to warrants for the purchase of common stock and adjusted for the Exchange Ratio and Reverse Stock Split	(4,657,350)
Issuance of warrants for the purchase of common stock or prefunded warrants to purchase common stock	13,201,667	13.43
Exercised	(33,846)	13.20
Outstanding and exercisable as of December 31, 2019	13,656,871	$	13.68	4.59

As of December 31, 2019, the Company’s outstanding warrants to purchase shares of common stock consisted of the following:


Issuance Date	Number of Shares of Common Stock Issuable	Exercise Price		Classification	Expiration Date
August 14, 2015	81,228	$	21.78	Equity	August 14, 2020
August 21, 2015	69,603	$	21.78	Equity	August 21, 2020
October 25, 2016	5,155	$	19.78	Equity	October 24, 2026
November 1, 2017	130,609	$	19.78	Equity	October 31, 2020
November 17, 2017	8,442	$	19.78	Equity	November 16, 2020
December 4, 2017	5,661	$	19.78	Equity	December 3, 2020
December 28, 2017	6,925	$	19.78	Equity	December 27, 2020
December 28, 2017	115,916	$	19.78	Equity	December 28, 2027
September 12, 2018	25,275	$	19.78	Equity	September 12, 2021
September 12, 2018	20,220	$	19.78	Equity	September 12, 2028
October 19, 2018	20,016	$	19.78	Equity	October 19, 2028
March 13, 2019	5,000	$	19.80	Equity	March 12, 2029
April 16, 2019	3,866,154	$	13.20	Equity	April 15, 2024
April 16, 2019	2,130,000	$	11.00	Equity	n/a
November 29, 2019	5,416,667	$	15.00	Equity	November 28, 2024
November 29, 2019	1,750,000	$	12.00	Equity	n/a
	13,656,871

As of December 31, 2018, the Company’s outstanding warrants to purchase shares of preferred stock (which converted into warrants to purchase common stock upon close of the Merger) consisted of the following (not adjusted for the Reverse Stock Split or Exchange Ratio):


December 31, 2018
Issuance Date	Number of Shares of Preferred Stock Issuable	Exercise Price		Exercisable for	Classification	Expiration Date
August 14, 2015	854,785	$	2.07	Series A	Liability	August 14, 2020
August 21, 2015	732,453	$	2.07	Series A	Liability	August 21, 2020
October 25, 2016	54,256	$	1.88	Series A	Liability	October 24, 2026
November 1, 2017	1,374,435	$	1.88	Series B	Liability	October 31, 2020
November 17, 2017	88,845	$	1.88	Series B	Liability	November 16, 2020
December 4, 2017	59,576	$	1.88	Series B	Liability	December 3, 2020
December 28, 2017	72,875	$	1.88	Series B	Liability	December 27, 2020
December 28, 2017	1,219,815	$	1.88	Series B	Liability	December 28, 2027
September 12, 2018	265,957	$	1.88	Series B	Liability	September 12, 2021
September 12, 2018	212,765	$	1.88	Series B	Liability	September 12, 2028
October 19, 2018	210,638	$	1.88	Series B	Liability	October 19, 2028
	5,146,400

Common Stock, Redeemable Common Stock and Convertible Preferred Stock (converted to Common Stock)

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Common Stock, Redeemable Common Stock and Convertible Preferred Stock (converted to Common Stock)

Common Stock, Redeemable Common Stock, and Convertible Preferred Stock (converted to Common Stock)

Common Stock— As of December 31, 2019 and December 31, 2018, the Company’s Certificate of Incorporation, as amended and restated, authorized the Company to issue 33,333,333 shares and 11,070,776, shares, respectively, of $0.001 par value common stock. The voting, dividend and liquidation rights of the holders of the Company’s common stock are subject to and qualified by the rights, powers and preferences of the holders of the preferred stock. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to receive dividends, as may be declared by the board of directors, if any. No cash dividends have been declared or paid to date.

On April 12, 2019, the Company entered into an underwriting agreement with Cowen and Company, LLC and Stifel, Nicolaus & Company, Incorporated, as representatives of the several underwriters named therein pursuant to which it sold 5,670,000 shares of common stock and, in lieu of common stock, prefunded warrants to purchase 2,130,000 shares of common stock, and accompanying Class A warrants to purchase 3,900,000 shares of its common stock. The common stock was issued at a price to the public of $11.00 per share and accompanying Class A warrant and the prefunded warrants were issued at a price of $10.999 per prefunded warrant and accompanying Class A warrant. The Class A warrants have an exercise price of $13.20, will expire 5 years from the date of issuance, and are immediately exercisable with certain restrictions. The gross proceeds from the offering, which closed on April 16, 2019, were $85.8 million before deducting underwriting discounts and offering expenses.

On November 26, 2019, the Company entered into an underwriting agreement with Cowen and Company, LLC and Stifel, Nicolaus & Company, Incorporated, as representatives of the several underwriters named therein pursuant to which it sold 3,666,667 shares of common stock and, in lieu of common stock, prefunded warrants to purchase 1,750,000 shares of common stock, and accompanying Class B warrants to purchase 5,416,667 shares of its common stock or prefunded warrants to purchase shares of common stock. The common stock was issued at a price to the public of $12.00 per share and accompanying Class B warrant and the prefunded warrants were issued at a price of $11.999 per prefunded warrant and accompanying Class B warrant. The Class B warrants have an exercise price of $15.00 per warrant, which includes a down-round contingent price adjustment feature; expire on a date that is the earlier of (a) the date that is 30 calendar days from the date on which the Company issues a press release announcing top-line data from its Phase 3 clinical trial of mavorixafor for the treatment of patients with WHIM syndrome (or, if such date is not a business day, the next business day) and (b) November 28, 2024; and were immediately exercisable upon issuance. The gross proceeds from the offering, which closed on November 29, 2019, were $65.0 million before deducting underwriting discounts and offering expenses.

For each of the common stock offerings, the Company evaluated the Class A, Class B and prefunded warrants for liability or equity classification in accordance with the provisions of ASC 480, Distinguishing Liabilities from Equity, and ASC 815-40, Derivatives and Hedging, and determined that equity treatment was appropriate because neither the Class A, Class B or prefunded warrants meet the definition of a liability.

Redeemable Common Stock— Pursuant to the requirements of the July 2014 license agreement with Genzyme (see Note 4), in August 2015, the Company issued to Genzyme for no additional consideration 107,371 shares of common stock, which had an aggregate fair value of $734 on the date of issuance. Genzyme had the right to require the Company to repurchase all, but not less than all, of these shares of common stock at any time during the term of the license agreement for a price of $0.001 per share. Because of this redemption feature, the shares of common stock issued to Genzyme were classified outside of stockholders’ deficit on the consolidated balance sheets. As a result of the Merger, these shares were exchanged for common stock.

Convertible Preferred Stock (converted to Common Stock)— The Company has issued Series Seed convertible preferred stock (the “Series Seed preferred stock”), Series A convertible preferred stock (the “Series A preferred stock”) and Series B convertible preferred stock (the “Series B preferred stock”). As of December 31, 2019 and December 31, 2018, the Company’s Certificate of Incorporation, as amended and restated, authorized the Company to issue a total of 10,000,000 shares and 59,413,523 shares, respectively, of preferred stock, with a par value of $0.001 per share.

The holders of Preferred Stock have liquidation rights in the event of a deemed liquidation that, in certain situations, are not solely within the control of the Company. Therefore, the Preferred Stock is classified outside of stockholders’ deficit on the consolidated balance sheet.

As of December 31, 2019, there was no preferred stock outstanding. As of December 31, 2018, the preferred stock consisted of the following:


	December 31, 2018
	Preferred Stock Designated	Preferred Stock Issued and Outstanding	Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion ⁽¹⁾
Series Seed preferred stock	2,313,523	1,516,136	$	1,310,000	$	1,444,000	143,630
Series A preferred stock	22,000,000	19,946,862	32,480,000		47,624,000		1,895,610
Series B preferred stock	25,100,000	18,616,569	30,885,000		34,999,000		1,769,190
	49,413,523	40,079,567	$	64,675,000	$	84,067,000	3,808,430

___________________________________

⁽¹⁾ Adjusted to reflect Reverse Stock Split and Exchange Ratio.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2019

Share-based Payment Arrangement [Abstract]

Stock-Based Compensation

Summary of Plans— Upon completion of the Merger with Arsanis on March 13, 2019, X4’s 2015 Employee, Director and Consultant Equity Incentive Plan, as amended (the “2015 Plan”), Arsanis’ 2017 Equity Incentive Plan (the “2017 Plan”) and Arsanis’ 2017 Employee Stock Purchase Plan (the “ESPP”) were assumed by the Company. In June 2019, the Company adopted the 2019 Inducement Equity Incentive Plan (the “2019 Plan”). These plans are administered by the Board of Directors or, at the discretion of the Board of Directors, by a committee of the Board of Directors. The exercise prices, vesting and other restrictions are determined at the discretion of the Board of Directors, or its committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of the stock option may not be greater than ten years. Incentive stock options granted to employees and restricted stock awards granted to employees, officers, members of the board of directors, advisors, and consultants of the Company typically vest over four years. Non-statutory options granted to employees, officers, members of the Board of Directors, advisors, and consultants of the Company typically vest over three or four years. Shares that are expired, terminated, surrendered or canceled under the Plans without having been fully exercised will be available for future awards. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for the grant of awards.

2015 Employee, Director and Consultant Equity Incentive Plan— In 2015, the Board of Directors and shareholders of X4 adopted the 2015 Plan, which provided for the Company to grant incentive stock options or nonqualified stock options, restricted

stock awards and other stock-based awards to employees, directors and consultants of the Company. Each stock option outstanding under the 2015 Plan at the effective time of the Merger was automatically converted into a stock option exercisable for a number of shares of the Company’s common stock calculated based on the Exchange Ratio and the exercise price per share of such outstanding stock option.

The total number of shares of common stock that may be issued under the 2015 Plan is 969,340 shares, adjusted for the Exchange Ratio and Reverse Stock Split. Shares that are expired, forfeited, canceled or otherwise terminated without having been fully exercised will be available for future grant under the 2015 Plan. As of December 31, 2019, approximately 100,000 shares were available for future issuance under the 2015 Plan. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for future grants.

2017 Equity Incentive Plan— In 2017, the Board of Directors and shareholders of Arsanis adopted the 2017 Plan, which provided for the Company to grant incentive stock options, non-qualified options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. Incentive stock options may be granted only to the Company’s employees, including officers and directors who are also employees. Awards other than incentive stock options may be granted to employees, officers, members of the board of directors, advisors and consultants of the Company. The number of shares of common stock reserved for issuance under this plan will automatically increase on January 1 of each year, through January 1, 2027, in an amount equal to the lowest of 170,915 shares of the Company’s common stock (as adjusted for the Reverse Stock Split), 4.0% of the number of shares of the Company’s common stock outstanding on January 1 of each year and an amount determined by the Company’s Board of Directors. As of December 31, 2019, approximately 20,000 shares were available for future issuance under the 2017 Plan.

Employee Stock Purchase Plan— In 2017, the Board of Directors and shareholders of Arsanis adopted the ESPP, which provides participating employees with the opportunity to purchase shares of the Company’s common stock at defined purchase prices over six-month offering periods. For the twelve months ended December 31, 2019, no shares of common stock were issued under the ESPP.

2019 Inducement Equity Incentive Plan— On June 17, 2019, the Board of Directors approved the adoption of the 2019 Plan, which provides for the Company to grant nonqualified stock options, restricted stock awards and other stock-based awards to new employees of the Company. Awards issued from the 2019 Plan are intended to be material inducements to each employee’s acceptance of employment with the Company in accordance with Nasdaq Listing Rule 5635(c)(4). The total number of shares of common stock that may be issued under the 2019 Plan is 400,000 shares. Shares that are expired, forfeited, canceled or otherwise terminated without having been fully exercised will be available for future grant under the 2019 Plan. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for future grants. As of December 31, 2019, approximately 250,000 shares were available for future issuance under the 2019 Plan.

Stock Option Valuation— The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted to employees, directors and non- employees:


	Year Ended December 31,
	2019		2018		2017
Risk-free interest rate	2.0	%	2.8	%	2.1	%
Expected term (in years)	5.99		5.94		6.06
Expected volatility	88.5	%	86.0	%	77.3	%
Expected dividend yield	0	%	0	%	0	%

Stock Options

The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2019:


	Number of Shares	Weighted Average Exercise Price		Weighted Average Contractual Term (Years)	Aggregate Intrinsic Value
Outstanding as of December 31, 2018	797,931	$	8.29	8.42	$	6,486
Assumed as part of Merger with Arsanis	271,230	62.60
Granted	589,110	14.69
Exercised	(51,617)	7.05
Forfeited	(309,625)	31.55
Outstanding as of December 31, 2019	1,297,029	$	17.05	8.41	$	1,286
Exercisable as of December 31, 2019	495,740	$	21.66	7.21	$	952
Vested and expected to vest as of December 31, 2019	1,122,542	$	17.15	8.32	$	1,245

The aggregate intrinsic value of options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the twelve months ended December 31, 2019 was $363 thousand. The intrinsic value of options exercised during the years ended December 31, 2018 and 2017 was not significant. The weighted average grant-date fair value per share of stock options granted during the years ended December 31, 2019, 2018 and 2017 was $10.78, $7.05, and $4.74, respectively.

Restricted Stock Units— During the twelve months ended December 31, 2019, the Company granted 116,689 restricted stock units to employees at a weighted average grant date fair value of $14.75 per share. The restricted stock units vest 25% annually on the grant anniversary over four years and had a grant date fair value of $1.7 million, which will be recognized as stock-based compensation expense, net of estimated forfeitures, over the vesting period.

The following table summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2019:


			Number of Shares
Unvested at December 31, 2018			—
Granted			116,689
Vested			—
Forfeited			(14,916)
Unvested at December 31, 2019			101,773

Stock-Based Compensation— Effective January 1, 2019, the Company adopted ASU 2018-7 and no longer remeasures the fair value of equity awards granted to non-employees at each reporting period end (see Note 2).

As of December 31, 2019, total unrecognized compensation expense related to unvested stock options and restricted stock units was $6.4 million, which is expected to be recognized over a weighted average period of 3.1 years.

Stock-based compensation expense was classified in the consolidated statements of operations as follows:


	Year Ended December 31,
	2019		2018		2017
Research and development expense	$	909	$	258	$	126
General and administrative expense	1,141		501		366
Total stock-based compensation	$	2,050	$	759	$	492

Income Taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Income Taxes

During the years ended December 31, 2019, 2018, and 2017, the Company recorded no income tax benefits for the net operating losses incurred and research and development credits generated due to the uncertainty of realizing a benefit from those items. The Company's losses before income taxes were generated in the United States and Austria.

Loss before the provision for income taxes for the years ended December 31, 2019, 2018 and 2017 consisted of the following:


	Year Ended December 31,
(in thousands)	2019		2018		2017
United States	$	(52,314)	$	(33,285)	$	(21,994)
Foreign (Austria)	(493)		—		—
	$	(52,807)	$	(33,285)	$	(21,994)

A reconciliation of the U.S. federal statutory income tax rate to the Company's effective income tax rate is as follows:


	Year Ended December 31,
	2019		2018		2017
U.S. federal statutory income tax rate	(21.0)	%	(21.0)	%	(34.0)	%
State income taxes, net of federal benefit	(5.8)		(6.2)		(8.4)
Foreign rate differential	(0.1)		—		—
Research and development tax credits	(1.1)		(3.7)		(1.8)
Change in fair value of preferred stock warrant liability	0.1		2.4		(2.2)
Other permanent differences	1.4		0.9		0.5
Remeasurement of deferred taxes due to tax reform	—		—		27.8
Change in deferred tax asset valuation allowance	26.8		27.8		18.1
Other	(0.3)		(0.2)		—
Effective income tax rate	—	%	—	%	—	%

Net deferred tax assets as of December 31, 2019 and 2018 consisted of the following:


	December 31,
(in thousands)	2019		2018
Net operating loss carryforwards	$	65,487	$	15,482
Research and development tax credit carryforwards	3,654		2,710
Capitalized research and development expenses	2,626		2,837
Capitalized license fees	—		221
Accrual-to-cash basis conversion	—		1,367
Lease liabilities	765		—
Other	1,413		180
Total deferred tax assets	73,945		22,797
Valuation allowance	(73,410)		(22,797)
Deferred tax assets, net of valuation allowance	$	535	$	—
Right of use assets	535		—
Total deferred tax liabilities	$	535	$	—
Total deferred tax assets, net	$	—	$	—

As of December 31, 2019, the Company had U.S. federal and state net operating loss carryforwards of $181.8 million and $177.4 million, respectively, which may be available to offset future taxable income and begin to expire in 2031 and 2035, respectively, and of which $127.7 million related to U.S federal income taxes do not expire but are limited in their usage to an annual deduction equal to 80% of annual taxable income. In addition, as of December 31, 2019 the Company had foreign net operating loss carryforward of $64.4 million, which do not expire. U.S. federal and state net operating loss carryforwards include $75 million and $70 million, respectively, of carryforwards acquired in the Merger with Arsanis, the utilization of which may be subject to limitations as described below. As of December 31, 2019, the Company also had U.S. federal and state research and development tax credit carryforwards of $3.1 million and $0.6 million, respectively, which may be available to offset future tax liabilities and each begin to expire in 2032 and 2030, respectively. As of December 31, 2019, uncertain tax position reserves recorded were $0.2 million for U.S. federal and state research and development tax credits.

Utilization of the U.S. net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain shareholders or public groups in the stock of a corporation by more than 50% over a three-year period. The Company has not conducted a study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception due to the significant complexity and cost associated with such a study. If the Company has experienced a change of control, as defined by Section 382, at any time since inception, utilization of the U.S. net operating loss carryforwards or research and development tax credit carryforwards would be subject to an annual limitation under Section 382, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, and then could be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the net operating loss carryforwards or research and development tax credit carryforwards before utilization.

Each period, the Company evaluates the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of its deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2019, 2018 and 2017.

Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2019, 2018 and 2017 related primarily to the increases in net operating loss carryforwards and research and development tax credit carryforwards and were as follows:


	Year Ended December 31,
(in thousands)	2019		2018		2017
Valuation allowance, beginning of year	$	(22,797)	$	(13,546)	$	(9,738)
Increases recorded to income tax provision	(14,485)		(9,251)		(3,808)
Acquisition of business	(36,128)		—		—
Valuation allowance, end of year	$	(73,410)	$	(22,797)	$	(13,546)

The Company’s U.S. federal and state income tax returns are generally subject to tax examinations for the tax years ended December 31, 2016 through December 31, 2019. There are currently no pending income tax examinations. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service and state tax authorities to the extent utilized in a future period. The Company’s policy is to record interest and penalties related to income taxes as part of its income tax provision.

Net Loss per Share

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Net Loss Per Share

Net Loss per Share

Basic and diluted net loss per share attributable to common stockholders was calculated as follow:


	Year Ended December 31,
(in thousands, except per share data)	2019		2018		2017
Numerator:
Net loss	$	(52,807)	$	(33,285)	$	(21,994)
Accruing dividends on Series A convertible preferred stock	(592)		(3,000)		(3,000)
Adjustment to accumulated deficit in connection with repurchase of Series Seed convertible preferred stock	—		(22)		—
Net loss attributable to common stockholders	$	(53,399)	$	(36,307)	$	(24,994)
Denominator:
Weighted average shares of common stock—basic and diluted	11,530		459		458
Net loss per share attributable to common stockholders— basic and diluted	$	(4.63)	$	(79.15)	$	(54.58)

The Company has included 107,371 shares of redeemable common stock in its computation of basic and diluted weighted average shares of common stock outstanding for the years ended December 31, 2019, 2018 and 2017 as this class of stock participates in losses similarly to other common stockholders. Basic and diluted weighted average shares of common stock outstanding for the year ended December 31, 2019 also includes the weighted average effect of 2,130,000 and 1,750,000 prefunded warrants for the purchase of shares of common stock, which were issued in April 2019 and November 2019, respectively, and for which the remaining unfunded exercise price is less than $0.001 per share.

The Company’s potentially dilutive securities included outstanding stock options, convertible preferred stock, and warrants to purchase shares of convertible preferred stock for the year ended December 31, 2018 and included outstanding stock options and warrants to purchase common stock for the year ended December 31, 2019. These potentially dilutive securities have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share, and thus they are considered “anti-dilutive.” Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential shares of common stock, presented based on amounts outstanding at each period end and adjusted for the Exchange Ratio and Reverse Stock Split, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:


	Year Ended December 31,
	2019	2018	2017
Options to purchase common stock and unvested restricted stock units	1,398,802	798,311	474,282
Convertible preferred stock (as converted to common stock)	—	3,808,894	3,613,069
Warrants to purchase common stock (excluding prefunded warrants, which are included in basic shares outstanding)	9,776,871	489,079	423,567
	11,175,673	5,096,284	4,510,918

Loss on Transfer of Nonfinancial Assets	12 Months Ended
Loss on Transfer of Nonfinancial Assets	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Loss on Transfer of Nonfinancial Assets	Loss on Transfer of Nonfinancial AssetsDuring the year ended December 31, 2019, the Company entered into contractual arrangements with two third parties that transferred the rights to develop and commercialize the programs underlying IPR&D intangible assets acquired in the Merger. As of December 31, 2019, all programs underlying IPR&D intangible assets acquired in the Merger were transferred to these third parties and the Company has no continuing involvement in any ongoing research and development activities associated with the programs. The Company concluded that these third parties are "non-customers" as the underlying development programs transferred to these third parties are focused on potential drug candidates that were not aligned with the Company's strategic focus and, therefore, are not an output of the Company's ordinary activities. Accordingly, the Company accounted for these transactions under ASC Topic 610-20, Gains and Losses from the Derecognition of Nonfinancial Assets ("ASC 610-20"). As a result of the transfer of control of the IPR&D projects to third parties, the Company derecognized the IPR&D intangible assets through a charge to "loss on transfer of nonfinancial assets" in 2019. The $3.9 million loss includes the carrying value of the IPR&D of $4.9 million, partially offset by $1.0 million of non-refundable, upfront fees received from the third parties. In accordance with the contractual arrangements with these third parties, the Company is entitled to future fees that are contingent on the success of the third parties in advancing the IPR&D project toward commercialization. These future fees, if any, will be recognized in future periods as gains on the transfer of nonfinancial assets in accordance with ASC 610-20, which generally provides that the fees would be recognized when it is probable that a future reversal of the fees that would be material to cumulative fees recognized to date would not occur.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Use of Estimates

Use of Estimates— The preparation of the Company’s consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses, the valuation of intangible assets acquired in business combinations, the valuations of common stock prior to the Merger, the valuation of stock options, preferred stock warrants (and the resulting preferred stock warrant liability), derivative instruments (and the resulting derivative liabilities), valuation of lease liabilities and the constraint of variable consideration from revenue transactions. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates when there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates.

Foreign Currency and Currency Translation

Concentrations of Credit Risk and Significant Suppliers

Cash and Cash Equivalents

Property and Equipment


			Estimated Useful Life
Office furniture			3 years
Computer equipment			3 years
Laboratory equipment			3 to 10 years
Leasehold improvements			Shorter of lease term or 10 years

Right-of-Use Assets and Leases

Impairment of Long-Lived Assets

Goodwill

Intangible Assets

•Probability of successfully completing clinical trials and obtaining regulatory approval;

•Market size, market growth projections, and market share;

•Estimates of future cash flows from potential milestone payments and royalties related to out-licensed product sales; and

•A discount rate reflecting the Company's weighted average cost of capital and specific risk inherent in the underlying assets.

Deferred Rent

Fair Value Measurement

•Level 1—Quoted prices in active markets for identical assets or liabilities.

Segment Information

Revenue Recognition

Research and Development Programs

Research and Development Cost

Patent Cost

Debt Issuance Costs

Stock-Based Compensation

Preferred Stock Warrant Liability

Derivatives

Comprehensive Loss

Comprehensive Loss— Comprehensive loss includes net loss as well as foreign currency translation adjustments.

Income Taxes

Net Loss per Share

Recently Adopted and Issued Accounting Pronouncements

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Schedule of Restricted Cash

Restricted Cash—


(in thousands)	As of December 31, 2019		As of December 31, 2018
Letter of credit security: Cambridge lease	$	264	$	264
Letter of credit security: Waltham lease	250		—
Letter of credit security: Vienna Austria lease	94		—
Letter of credit security: Allston lease	1,144		—
Corporate credit card collateral	150		100
Total restricted cash (non-current)	$	1,902	$	364

Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash to Cash Flows


(in thousands)	December 31, 2019		December 31, 2018		December 31, 2017		December 31, 2016
Cash and cash equivalents	$	126,184	$	8,134	$	26,684	$	15,160
Restricted cash, non-current (included within other assets)	1,902		364		364		155
Total cash, cash equivalents and restricted cash	$	128,086	$	8,498	$	27,048	$	15,315

Property and Equipment Useful Life

Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset, as follows:


			Estimated Useful Life
Office furniture			3 years
Computer equipment			3 years
Laboratory equipment			3 to 10 years
Leasehold improvements			Shorter of lease term or 10 years

Merger Accounting (Tables)

12 Months Ended

Dec. 31, 2019

Business Combinations [Abstract]

Schedule of Preliminary Estimates of Purchase Price Paid

The following summarizes the preliminary estimate of the purchase price paid in the Merger (in thousands, except share and per share amounts):


Number of shares of the combined organization owned by Arsanis stockholders ⁽¹⁾	2,440,582
Multiplied by the fair value per share of Arsanis common stock ⁽²⁾	$	18.66
Fair value of consideration issued it effect the Merger	$	45,541
Fair value of replacement awards held by former employees, board of directors and consultants of Arsanis that were vested as of the Merger.	817
Purchase price:	$	46,358

________________________

⁽²⁾ Based on the last reported sale price of Arsanis common stock on the Nasdaq Global Market on March 13, 2019, the closing date of the Merger, and gives effect to the Reverse Stock Split.

Summary of Purchase Price Allocation

The following summarizes the allocation of the purchase price to the net tangible and intangible assets acquired (in thousands):


Cash, cash equivalents and restricted cash	$	26,406
Other current assets	2,147
Property and equipment, net	68
IPR&D indefinite-lived intangible assets	4,900
Other assets, non-current	486
Current liabilities	(5,205)
Loans payable	(8,713)
Other liabilities, non-current	(840)
Goodwill	27,109
Purchase price	$	46,358

Supplemental Pro Forma Information

The following supplemental pro forma information presents the Company’s financial results as if the acquisition of Arsanis had occurred on January 1, 2018 (unaudited)


	Year Ended December 31,
(in thousands)	2019		2018

Revenue	$	—	$	3,500
Net loss	$	(57,820)	$	(76,248)

Fair Value of Financial Assets and Liabilities (Tables)

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value, Assets Measured on Recurring Basis


	Fair Value Measurements as of December 31, 2019 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	23,638	$	39,999	$	—	$	63,637
	$	23,638	$	39,999	$	—	$	63,637

Liabilities: none


	Fair Value Measurements as of December 31, 2018 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market fund	$	—	$	8,134	$	—	$	8,134
	$	—	$	8,134	$	—	$	8,134
Liabilities:
Preferred stock warrant liability	$	—	$	—	$	4,947	$	4,947
Derivative liability	—		—		201		201
	$	—	$	—	$	5,148	$	5,148

Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation

The following table provides a roll-forward of the aggregate fair values of the Company’s warrant liability and derivative liability, for which fair values are determined using Level 3 inputs:


(in thousands)	Preferred Stock Warrant Liability		Derivative Liability
Balance as of December 31, 2017	$	1,245	$	94
Issuance of warrants to purchase shares of Series B convertible preferred stock	304		—
Initial fair value of derivative liability in connection with the Hercules Loan Agreement	—		18
Change in fair value	3,398		89
Balance as of December 31, 2018	4,947		201
Change in fair value	288		(183)
Conversion of convertible preferred stock warrant into common stock warrant in connection with Merger	(5,235)		—
Balance at December 31, 2019	$	—	$	18

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2019

Property, Plant and Equipment [Abstract]

Property, Plant and Equipment

Property and equipment, net consisted of the following:


(in thousands)	December 31, 2019		December 31, 2018
Leasehold improvements	$	299	$	299
Furniture and fixtures	139		53
Computer equipment	37		56
Software	33		9
Lab equipment	159		—
	667		417
Less: Accumulated depreciation and amortization	(264)		(176)
	$	403	$	241

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2019

Other Income and Expenses [Abstract]

Schedule of Accrued Expenses

Accrued expenses consisted of the following


(in thousands)	December 31, 2019		December 31, 2018
Accrued employee compensation and benefits	$	2,916	$	924
Accrued external research and development expenses	1,977		754
Accrued professional fees	1,347		1,324
Other	221		249
	$	6,461	$	3,251

Long-Term Debt (Tables)

12 Months Ended

Dec. 31, 2019

Debt Disclosure [Abstract]

Schedule of Debt

Long-term debt consisted of the following:


(in thousands)	December 31, 2019		December 31, 2018
Principal amount of long-term debt	$	20,000	$	10,000
Less: Current portion of long-term debt	—		(1,687)
Long-term debt, net of current portion	20,000		8,313
Debt discount, net of accretion	(317)		(226)
Cumulative accretion of final payment due at maturity	414		58
Long-term debt, including accretion, net of discount and current portion	$	20,097	$	8,145

Schedule of Maturities of Long-term Debt

As of December 31, 2019, future principal payments and the final payment due under the Company’s loan agreements were as follows (in thousands):


Year Ending December 31	Total
2020	$	—
2021	—
2022	11,897
2023	8,103
Long-term debt	$	20,000

Leases (Tables)

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

Lease, Cost

The components of lease expense for the year ended December 31, 2019 were as follows (dollars in thousands):


Lease Cost	For the Year Ended December 31, 2019
Fixed operating lease cost	$	827
Short-term lease costs	122
Total lease expense	$	949
Other information
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$	1,007
Leased assets obtained in exchange for new operating lease liabilities⁽¹⁾	$	484
Weighted-average remaining lease term—operating leases	3.0 years
Weighted-average discount rate—operating leases	9.0		%
Sublease income	$	14

___________________________________________

⁽¹⁾Acquired in the Merger

Lessee, Operating Lease, Liability, Maturity

Maturities of lease liabilities due under these lease agreements as of December 31, 2019 are as follows (in thousands):


Maturity of lease liabilities	Operating Leases
2020	$	1,124
2021	1,098
2022	754
2023	263
Total lease payments	3,239
Less: interest	(423)
Total operating lease liabilities as of December 31, 2019	$	2,816

Schedule of Future Minimum Rental Payments for Operating Leases

As of December 31, 2018, minimum future lease payments under non-cancellable operating leases for each of the following five years and a total thereafter were as follows (in thousands):


Year Ending December 31,	Operating Leases
2019	$	810
2020	823
2021	835
2022	492
Total	$	2,960

Preferred and Common Stock Warrants (Tables)

12 Months Ended

Dec. 31, 2019

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Schedule of Outstanding Warrants Table

The following table provides a roll forward of outstanding warrants for the twelve month period ended December 31, 2019:


	Number of warrants	Weighted Average Exercise Price		Weighted Average Contractual Term (Years)
Outstanding and exercisable warrants to purchase preferred shares as of December 31, 2018	5,146,400	$	—	4.23
Conversion of warrants to purchase preferred shares to warrants for the purchase of common stock and adjusted for the Exchange Ratio and Reverse Stock Split	(4,657,350)
Issuance of warrants for the purchase of common stock or prefunded warrants to purchase common stock	13,201,667	13.43
Exercised	(33,846)	13.20
Outstanding and exercisable as of December 31, 2019	13,656,871	$	13.68	4.59

Schedule of Stockholders' Equity Note, Warrants or Rights

As of December 31, 2019, the Company’s outstanding warrants to purchase shares of common stock consisted of the following:


Issuance Date	Number of Shares of Common Stock Issuable	Exercise Price		Classification	Expiration Date
August 14, 2015	81,228	$	21.78	Equity	August 14, 2020
August 21, 2015	69,603	$	21.78	Equity	August 21, 2020
October 25, 2016	5,155	$	19.78	Equity	October 24, 2026
November 1, 2017	130,609	$	19.78	Equity	October 31, 2020
November 17, 2017	8,442	$	19.78	Equity	November 16, 2020
December 4, 2017	5,661	$	19.78	Equity	December 3, 2020
December 28, 2017	6,925	$	19.78	Equity	December 27, 2020
December 28, 2017	115,916	$	19.78	Equity	December 28, 2027
September 12, 2018	25,275	$	19.78	Equity	September 12, 2021
September 12, 2018	20,220	$	19.78	Equity	September 12, 2028
October 19, 2018	20,016	$	19.78	Equity	October 19, 2028
March 13, 2019	5,000	$	19.80	Equity	March 12, 2029
April 16, 2019	3,866,154	$	13.20	Equity	April 15, 2024
April 16, 2019	2,130,000	$	11.00	Equity	n/a
November 29, 2019	5,416,667	$	15.00	Equity	November 28, 2024
November 29, 2019	1,750,000	$	12.00	Equity	n/a
	13,656,871


December 31, 2018
Issuance Date	Number of Shares of Preferred Stock Issuable	Exercise Price		Exercisable for	Classification	Expiration Date
August 14, 2015	854,785	$	2.07	Series A	Liability	August 14, 2020
August 21, 2015	732,453	$	2.07	Series A	Liability	August 21, 2020
October 25, 2016	54,256	$	1.88	Series A	Liability	October 24, 2026
November 1, 2017	1,374,435	$	1.88	Series B	Liability	October 31, 2020
November 17, 2017	88,845	$	1.88	Series B	Liability	November 16, 2020
December 4, 2017	59,576	$	1.88	Series B	Liability	December 3, 2020
December 28, 2017	72,875	$	1.88	Series B	Liability	December 27, 2020
December 28, 2017	1,219,815	$	1.88	Series B	Liability	December 28, 2027
September 12, 2018	265,957	$	1.88	Series B	Liability	September 12, 2021
September 12, 2018	212,765	$	1.88	Series B	Liability	September 12, 2028
October 19, 2018	210,638	$	1.88	Series B	Liability	October 19, 2028
	5,146,400

Common Stock, Redeemable Common Stock and Convertible Preferred Stock (converted to Common Stock) (Tables)

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Preferred Stock

As of December 31, 2018, the preferred stock consisted of the following:


	December 31, 2018
	Preferred Stock Designated	Preferred Stock Issued and Outstanding	Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion ⁽¹⁾
Series Seed preferred stock	2,313,523	1,516,136	$	1,310,000	$	1,444,000	143,630
Series A preferred stock	22,000,000	19,946,862	32,480,000		47,624,000		1,895,610
Series B preferred stock	25,100,000	18,616,569	30,885,000		34,999,000		1,769,190
	49,413,523	40,079,567	$	64,675,000	$	84,067,000	3,808,430

___________________________________

⁽¹⁾ Adjusted to reflect Reverse Stock Split and Exchange Ratio.

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Share-based Payment Arrangement [Abstract]

Stock Option Valuation

The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted to employees, directors and non- employees:


	Year Ended December 31,
	2019		2018		2017
Risk-free interest rate	2.0	%	2.8	%	2.1	%
Expected term (in years)	5.99		5.94		6.06
Expected volatility	88.5	%	86.0	%	77.3	%
Expected dividend yield	0	%	0	%	0	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2019:


	Number of Shares	Weighted Average Exercise Price		Weighted Average Contractual Term (Years)	Aggregate Intrinsic Value
Outstanding as of December 31, 2018	797,931	$	8.29	8.42	$	6,486
Assumed as part of Merger with Arsanis	271,230	62.60
Granted	589,110	14.69
Exercised	(51,617)	7.05
Forfeited	(309,625)	31.55
Outstanding as of December 31, 2019	1,297,029	$	17.05	8.41	$	1,286
Exercisable as of December 31, 2019	495,740	$	21.66	7.21	$	952
Vested and expected to vest as of December 31, 2019	1,122,542	$	17.15	8.32	$	1,245

Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity

The following table summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2019:


			Number of Shares
Unvested at December 31, 2018			—
Granted			116,689
Vested			—
Forfeited			(14,916)
Unvested at December 31, 2019			101,773

Summary of Stock-Based Compensation Expense Classification

Stock-based compensation expense was classified in the consolidated statements of operations as follows:


	Year Ended December 31,
	2019		2018		2017
Research and development expense	$	909	$	258	$	126
General and administrative expense	1,141		501		366
Total stock-based compensation	$	2,050	$	759	$	492

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Schedule of Income before Income Tax, Domestic and Foreign

Loss before the provision for income taxes for the years ended December 31, 2019, 2018 and 2017 consisted of the following:


	Year Ended December 31,
(in thousands)	2019		2018		2017
United States	$	(52,314)	$	(33,285)	$	(21,994)
Foreign (Austria)	(493)		—		—
	$	(52,807)	$	(33,285)	$	(21,994)

Schedule of Effective Income Tax Rate Reconciliation

A reconciliation of the U.S. federal statutory income tax rate to the Company's effective income tax rate is as follows:


	Year Ended December 31,
	2019		2018		2017
U.S. federal statutory income tax rate	(21.0)	%	(21.0)	%	(34.0)	%
State income taxes, net of federal benefit	(5.8)		(6.2)		(8.4)
Foreign rate differential	(0.1)		—		—
Research and development tax credits	(1.1)		(3.7)		(1.8)
Change in fair value of preferred stock warrant liability	0.1		2.4		(2.2)
Other permanent differences	1.4		0.9		0.5
Remeasurement of deferred taxes due to tax reform	—		—		27.8
Change in deferred tax asset valuation allowance	26.8		27.8		18.1
Other	(0.3)		(0.2)		—
Effective income tax rate	—	%	—	%	—	%

Schedule of Deferred Tax Assets and Liabilities

Net deferred tax assets as of December 31, 2019 and 2018 consisted of the following:


	December 31,
(in thousands)	2019		2018
Net operating loss carryforwards	$	65,487	$	15,482
Research and development tax credit carryforwards	3,654		2,710
Capitalized research and development expenses	2,626		2,837
Capitalized license fees	—		221
Accrual-to-cash basis conversion	—		1,367
Lease liabilities	765		—
Other	1,413		180
Total deferred tax assets	73,945		22,797
Valuation allowance	(73,410)		(22,797)
Deferred tax assets, net of valuation allowance	$	535	$	—
Right of use assets	535		—
Total deferred tax liabilities	$	535	$	—
Total deferred tax assets, net	$	—	$	—

Summary of Valuation Allowance


	Year Ended December 31,
(in thousands)	2019		2018		2017
Valuation allowance, beginning of year	$	(22,797)	$	(13,546)	$	(9,738)
Increases recorded to income tax provision	(14,485)		(9,251)		(3,808)
Acquisition of business	(36,128)		—		—
Valuation allowance, end of year	$	(73,410)	$	(22,797)	$	(13,546)

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Schedule of Earnings Per Share, Basic and Diluted

Basic and diluted net loss per share attributable to common stockholders was calculated as follow:


	Year Ended December 31,
(in thousands, except per share data)	2019		2018		2017
Numerator:
Net loss	$	(52,807)	$	(33,285)	$	(21,994)
Accruing dividends on Series A convertible preferred stock	(592)		(3,000)		(3,000)
Adjustment to accumulated deficit in connection with repurchase of Series Seed convertible preferred stock	—		(22)		—
Net loss attributable to common stockholders	$	(53,399)	$	(36,307)	$	(24,994)
Denominator:
Weighted average shares of common stock—basic and diluted	11,530		459		458
Net loss per share attributable to common stockholders— basic and diluted	$	(4.63)	$	(79.15)	$	(54.58)

Schedule of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss per Share Attributable to Common Stockholders

The Company excluded the following potential shares of common stock, presented based on amounts outstanding at each period end and adjusted for the Exchange Ratio and Reverse Stock Split, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:


	Year Ended December 31,
	2019	2018	2017
Options to purchase common stock and unvested restricted stock units	1,398,802	798,311	474,282
Convertible preferred stock (as converted to common stock)	—	3,808,894	3,613,069
Warrants to purchase common stock (excluding prefunded warrants, which are included in basic shares outstanding)	9,776,871	489,079	423,567
	11,175,673	5,096,284	4,510,918

Nature of the Business and Basis of Presentation (Details)			12 Months Ended
	Mar. 18, 2019	Mar. 13, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2019 USD ($)
Restructuring Cost and Reserve [Line Items]
Percentage of voting interest			64.00%
Stock split, conversion ratio		0.1667
Proceeds from sale of common stock and warrants, net of issuance costs			$ 139,388,000	$ 0	$ 0
Proceeds from borrowings under loan and security agreements, net of issuance costs			9,849,000	$ 9,908,000	$ 6,000,000
Arsanis
Restructuring Cost and Reserve [Line Items]
Exchange ratio		0.5702
Cash, cash equivalents and restricted cash		$ 26,406,000	26,400,000
Hercules Loan Agreements
Restructuring Cost and Reserve [Line Items]
Proceeds from borrowings under loan and security agreements, net of issuance costs			$ 9,800,000
Hercules Loan Agreements \| Arsanis
Restructuring Cost and Reserve [Line Items]
Exchange ratio	0.5702
Hercules Amended and Restated Loan Agreement \| Additional Term Loan One
Restructuring Cost and Reserve [Line Items]
Additional borrowing capacity						$ 5,000,000.0
Hercules Amended and Restated Loan Agreement \| Additional Term Loan Two
Restructuring Cost and Reserve [Line Items]
Additional borrowing capacity						$ 10,000,000.0

Summary of Significant Accounting Policies - Schedule of Restricted Cash (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	$ 1,902,000	$ 364,000	$ 364,000	$ 155,000
Debt covenant, cash requirement percentage	125.00%
Debt covenant, minimum consolidated cash amount	$ 2,500,000
Debt covenant, amount to be maintained	25,000,000.0
Letter of credit security: Cambridge lease \| Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	264,000	264,000
Letter of credit security: Waltham lease \| Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	250,000	0
Letter of credit security: Vienna Austria lease \| Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	94,000	0
Letter of credit security: Allston lease \| Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	1,144,000	0
Corporate credit card collateral
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	$ 150,000	$ 100,000

Summary of Significant Accounting Policies - Schedule of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 126,184	$ 8,134	$ 26,684	$ 15,160
Other assets	1,902	364	364	155
Total cash, cash equivalents and restricted cash	$ 128,086	$ 8,498	$ 27,048	$ 15,315

Summary of Significant Accounting Policies - Schedule of Property, Plant and Equipment (Details)	12 Months Ended
	Dec. 31, 2019
Office furniture
Property, Plant and Equipment [Line Items]
Useful life	3 years
Computer equipment
Property, Plant and Equipment [Line Items]
Useful life	3 years
Laboratory equipment \| Minimum
Property, Plant and Equipment [Line Items]
Useful life	3 years
Laboratory equipment \| Maximum
Property, Plant and Equipment [Line Items]
Useful life	10 years
Leasehold improvements \| Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives	10 years

Summary of Significant Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) reporting_unit	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2019 USD ($)	Mar. 13, 2019 USD ($)	Jan. 01, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of reporting units \| reporting_unit	1
Fair value of warrants				$ 288,000	$ 5,200,000
Currency translation adjustments	$ (119,000)	$ 0	$ 0
Operating lease, liability	2,816,000
Right-of-use assets	$ 1,959,000
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, liability						$ 2,500,000
Right-of-use assets						$ 2,000,000.0
Arsanis
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fair value of warrants					$ 0

Merger Accounting - Additional Information (Details)		3 Months Ended	12 Months Ended
Merger Accounting - Additional Information (Details)	Mar. 13, 2019 USD ($) shares	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) development_plan	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Percentage of voting interest			64.00%
Stock options to purchase after merger (in shares) \| shares	271,230
Goodwill	$ 27,100,000		$ 27,109,000	$ 0
Goodwill adjustment		$ 298,000
Goodwill, impairment loss			0
Cost related to merger			$ 1,000,000
Development programs \| development_plan			3
Weighted average discount rate			20.00%
Pro Forma
Business Acquisition [Line Items]
Cost related to merger			$ 2,700,000	$ 2,700,000
Arsanis Security Holders
Business Acquisition [Line Items]
Percentage of voting interest	32.90%
Xfour Security Holders
Business Acquisition [Line Items]
Percentage of voting interest	67.10%
Arsanis
Business Acquisition [Line Items]
Exchange ratio	0.5702
Goodwill	$ 27,109,000

Merger Accounting - Preliminary Purchase Price Paid (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Mar. 13, 2019	Mar. 12, 2019	Dec. 31, 2019
Business Acquisition [Line Items]
Fair value of consideration issued it effect the Merger			$ 45,541
Arsanis
Business Acquisition [Line Items]
Number of shares of the combined organization owned by Arsanis stockholders (in shares)	2,440,582
Multiplied by the fair value per share of Arsanis common stock (in dollars per share)	$ 18.66
Fair value of consideration issued it effect the Merger	$ 45,541
Fair value of replacement awards held by former employees, board of directors and consultants of Arsanis that were vested as of the Merger.	817
Purchase price	$ 46,358
Common stock outstanding (in shares)		14,643,737

Merger Accounting - Summary of Purchase Price Allocation (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Mar. 13, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill	$ 27,109	$ 27,100	$ 0
Arsanis
Business Acquisition [Line Items]
Cash, cash equivalents and restricted cash	$ 26,400	26,406
Other current assets		2,147
Property and equipment, net		68
IPR&D indefinite-lived intangible assets		4,900
Other assets, non-current		486
Current liabilities		(5,205)
Loans payable		(8,713)
Other liabilities, non-current		(840)
Goodwill		27,109
Purchase price		$ 46,358

Merger Accounting - Supplemental Pro Forma Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Business Combinations [Abstract]
Revenue	$ 0	$ 3,500
Net loss	$ (57,820)	$ (76,248)

License, Collaboration, and Funding Agreements - Genzyme Agreement (Details) - USD ($)			1 Months Ended		12 Months Ended
	Nov. 26, 2019	Apr. 12, 2019	Aug. 31, 2015	Aug. 31, 2014	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock issued (in shares)	3,666,667	5,670,000
Stock issued						$ 0	$ 0
Sale price (in dollars per share)	$ 12.00	$ 11.00
Genzyme Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront payments				$ 50,000
Payments to acquire process			$ 300,000
Stock issued (in shares)			107,371
Stock issued			$ 734,000
Sale price (in dollars per share)				$ 0.01
Milestone payments					$ 25,000,000.0
Payment on obligation under license agreement					$ 0	$ 0	$ 0

License, Collaboration, and Funding Agreements - Georgetown Agreement (Details) - Georgetown - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
One time upfront payment	$ 50,000
Payments made	$ 800,000
Termination period	60 days
Payment on obligation under license agreement		$ 0	$ 0	$ 0
Payment obligation		$ 0	$ 0	$ 0

License, Collaboration, and Funding Agreements - Beth Israel Deaconess Medical Center Agreements (Details) - Bidmc Agreement - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
One time upfront payment	$ 20,000
Payment on obligation under license agreement		$ 0	$ 0	$ 0

License, Collaboration, and Funding Agreements - Research and Development Incentive Program (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Other income		$ 517	$ 0	$ 0
Research and Development Incentive Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Amount due under incentive receivable plan		2,000
Other income		$ 446
MTS Austria
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Reimbursement rate for research and development		0.14
Subsequent Event \| Research and Development Incentive Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Other income	$ 1,500

Fair Value of Financial Assets and Liabilities - Measured on a Recurring Basis (Details) - Fair Value, Recurring - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents—money market funds	$ 63,637	$ 8,134
Liabilities		5,148
Preferred stock warrant liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		4,947
Derivative Liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		201
Level 1
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents—money market funds	23,638	0
Liabilities		0
Level 1 \| Preferred stock warrant liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		0
Level 1 \| Derivative Liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		0
Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents—money market funds	39,999	8,134
Liabilities		0
Level 2 \| Preferred stock warrant liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		0
Level 2 \| Derivative Liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		0
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents—money market funds	$ 0	0
Liabilities		5,148
Level 3 \| Preferred stock warrant liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		4,947
Level 3 \| Derivative Liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		$ 201

Fair Value of Financial Assets and Liabilities - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Genzyme Agreement
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivative liability		$ 183,000
Measurement Input, Expected Dividend Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Preferred stock, dividend rate	0.00%
Convertible Preferred Stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants available to purchase (in shares)	0
Arsanis and Merger Sub \| Class A Convertible Preferred Stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Preferred stock, fair value (in dollars per share)		$ 1.70
Arsanis and Merger Sub \| Class B Convertible Preferred Stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Preferred stock, fair value (in dollars per share)	$ 1.86
Arsanis \| Genzyme Agreement
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivative liability		$ 0

Fair Value of Financial Assets and Liabilities -Aggregate Fair Value of Warrant and Derivative Liabilities (Details) - Level 3 - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Preferred Stock Warrant Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 4,947	$ 1,245
Issuance of warrants to purchase shares of Series B convertible preferred stock		304
Initial fair value of derivative liability in connection with the Hercules Loan Agreement		0
Change in fair value	288	3,398
Conversion of convertible preferred stock warrant into common stock warrant in connection with Merger	(5,235)
Ending balance	0	4,947
Derivative Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	201	94
Issuance of warrants to purchase shares of Series B convertible preferred stock		0
Initial fair value of derivative liability in connection with the Hercules Loan Agreement		18
Change in fair value	(183)	89
Conversion of convertible preferred stock warrant into common stock warrant in connection with Merger	0
Ending balance	$ 18	$ 201

Property and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 667	$ 417
Less: Accumulated depreciation and amortization	(264)	(176)
Property, plant and equipment, net	403	241
Depreciation and amortization expense	103	103	$ 71
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	299	299
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	139	53
Computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	37	56
Software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	33	9
Lab equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 159	$ 0

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Other Income and Expenses [Abstract]
Accrued employee compensation and benefits	$ 2,916	$ 924
Accrued external research and development expenses	1,977	754
Accrued professional fees	1,347	1,324
Other	221	249
Accrued expenses	$ 6,461	$ 3,251

Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
Long-term debt	$ 20,000	$ 10,000
Less: Current portion of long-term debt	0	(1,687)
Long-term debt, net of current portion	20,000	8,313
Debt discount, net of accretion	(317)	(226)
Cumulative accretion of final payment due at maturity	414	58
Long-term debt, including accretion, net of discount and current portion	$ 20,097	$ 8,145

Long-Term Debt - SVB Loan Agreement (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Unamortized discount		$ 317	$ 226
Loss on extinguishment of debt		$ (566)	(229)	$ 0
SVB Loan Agreement
Line of Credit Facility [Line Items]
Variable interest rate	5.50%
Repayments of debt	$ 4,700
Interest expense			$ 484	$ 490
Effective interest rate			11.80%	12.00%
Interest rate applicable to borrowings	10.75%
SVB Loan Agreement \| Minimum
Line of Credit Facility [Line Items]
Variable interest rate	3.50%
Term Loan A \| SVB Loan Agreement
Line of Credit Facility [Line Items]
Unamortized discount	$ 6,000

Long-Term Debt - Hercules Loan Agreements (Details)			1 Months Ended		3 Months Ended
Long-Term Debt - Hercules Loan Agreements (Details)	Mar. 18, 2019 $ / shares shares	Mar. 13, 2019 USD ($)	Dec. 31, 2018 USD ($)	Oct. 31, 2018 USD ($) $ / shares shares	Mar. 31, 2019 USD ($)	Dec. 11, 2018 USD ($)
Line of Credit Facility [Line Items]
Fair value of warrants		$ 5,200,000			$ 288,000
Arsanis
Line of Credit Facility [Line Items]
Exchange ratio		0.5702
Fair value of warrants		$ 0
Hercules Loan Agreements
Line of Credit Facility [Line Items]
Maximum borrowing capacity				$ 13,000,000.0
Proceeds from line of credit				$ 8,000,000.0
Fair value of warrants		$ 326,000	$ 282,000			$ 132,000
Hercules Loan Agreements \| Arsanis
Line of Credit Facility [Line Items]
Convertible preferred stock exercise price (in dollars per share) \| $ / shares	$ 19.80			$ 99
Warrants available to purchase (in shares) \| shares	5,000
Exchange ratio	0.5702
Hercules Loan Agreements \| Series B Redeemable Convertible Preferred Stock
Line of Credit Facility [Line Items]
Number of warrants issued (in shares) \| shares				210,638
Convertible preferred stock exercise price (in dollars per share) \| $ / shares				$ 1.88
Hercules Loan Agreements \| Common Stock
Line of Credit Facility [Line Items]
Convertible preferred stock exercise price (in dollars per share) \| $ / shares				$ 19.78
Warrants available to purchase (in shares) \| shares				20,016
Hercules Loan Agreements \| Additional Term Loan One
Line of Credit Facility [Line Items]
Maximum borrowing capacity				$ 2,000,000.0	3,000,000.0
Proceeds from line of credit			$ 2,000,000.0		2,000,000.0
Remaining borrowing capacity					$ 3,000,000.0
Hercules Loan Agreements \| Term Loan
Line of Credit Facility [Line Items]
Maximum borrowing capacity				$ 8,000,000.0

Long-Term Debt - 2019 Amended and Restated Loan Agreement (Details)	1 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 27, 2019 USD ($)	Mar. 31, 2019 USD ($)	Oct. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Debt covenant, cash requirement percentage		125.00%
Debt covenant, minimum consolidated cash amount		$ 2,500,000
Unamortized discount		317,000	$ 226,000
Hercules Amended and Restated Loan Agreement
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 35,000,000.0
Credit outstanding	$ 10,000,000.0
Current borrowing capacity				$ 20,000,000.0
Variable interest rate	8.75%
Increase in interest rate	4.00%
Periodic payment	$ 800,000
Principal percentage	0.040
Debt covenant, cash requirement percentage	125.00%
Debt covenants, liquidity percentage	100.00%
Debt covenant, minimum consolidated cash amount	$ 2,500,000
Upfront payments	180,000
Debt discount amortization	$ 180,000
Hercules Amended and Restated Loan Agreement \| Prime Rate
Line of Credit Facility [Line Items]
Interest rate	6.00%
Hercules Amended and Restated Loan Agreement \| Minimum
Line of Credit Facility [Line Items]
Variable interest rate	8.75%
Payment premium percentage	0.020
Hercules Amended and Restated Loan Agreement \| New Borrowings
Line of Credit Facility [Line Items]
Current borrowing capacity				10,000,000.0
Hercules Amended and Restated Loan Agreement \| Previous Borrowings
Line of Credit Facility [Line Items]
Current borrowing capacity				$ 10,000,000.0
Hercules Amended and Restated Loan Agreement \| Additional Term Loan One
Line of Credit Facility [Line Items]
Additional borrowing capacity	$ 5,000,000.0
Hercules Amended and Restated Loan Agreement \| Additional Term Loan Two
Line of Credit Facility [Line Items]
Additional borrowing capacity	$ 10,000,000.0
Hercules Loan Agreements
Line of Credit Facility [Line Items]
Maximum borrowing capacity						$ 13,000,000.0
Interest expense		1,800,000	236,000
Accretion expense		61,000	$ 355,000
Unamortized discount		$ 317,000
Effective interest rate		10.90%
Principal payments		$ 0
Hercules Loan Agreements \| Additional Term Loan One
Line of Credit Facility [Line Items]
Maximum borrowing capacity					$ 3,000,000.0	$ 2,000,000.0

Long-Term Debt - FFG Loan Agreements (Details) - USD ($) $ in Thousands			1 Months Ended	12 Months Ended
	Apr. 01, 2019	Mar. 08, 2019	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Apr. 30, 2019
Line of Credit Facility [Line Items]
Loss on extinguishment of debt				$ (566)	$ (229)	$ 0
FFG Loan Agreements
Line of Credit Facility [Line Items]
Accelerate payment schedule		3 years
Previous payment schedule		5 years
Repayments of debt	$ 2,900		$ 566
Minimum cash balance, percentage							70.00%
Interest expense				$ 316
Loss on extinguishment of debt			$ 566
Minimum \| FFG Loan Agreements
Line of Credit Facility [Line Items]
Interest rate applicable to borrowings				0.75%
Maximum \| FFG Loan Agreements
Line of Credit Facility [Line Items]
Interest rate applicable to borrowings				2.00%

Long-Term Debt - Future Principal Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
2020	$ 0
2021	0
2022	11,897
2023	8,103
Long-term debt	$ 20,000	$ 10,000

Leases (Details) $ in Thousands			1 Months Ended	12 Months Ended
Leases (Details) $ in Thousands	Nov. 11, 2019 USD ($) ft²	Mar. 13, 2019 USD ($) ft²	Aug. 31, 2017 USD ($) ft²	Dec. 31, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jan. 01, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Right-of-use assets				$ 1,959
Operating lease, liability				2,816
Lease liabilities				1,918
Current portion of lease liability				898
Right-of-use asset, amortization term		5 years
Notice period for cancellation		3 months
Future Unreimbursed Expenditures Expected To Be Incurred				$ 3,100
Cambridge Lease
Lessee, Lease, Description [Line Items]
Office space area \| ft²			13,000
Base rent			$ 832
Renewal term			5 years
Waltham Lease
Lessee, Lease, Description [Line Items]
Office space area \| ft²		6,000
Base rent		$ 263
Term of contract		5 years
Vienna Lease Arrangement
Lessee, Lease, Description [Line Items]
Office space area \| ft²		400
Base rent		$ 154
Term of contract		2 years
Allston Lease
Lessee, Lease, Description [Line Items]
Office space area \| ft²	28,000
Renewal term	5 years
Rent-free period	180 days
Lease not yet commenced, amount	$ 1,000
Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Right-of-use assets						$ 2,000
Operating lease, liability						2,500
Lease liabilities						1,900
Current portion of lease liability						613
Net deferred rent						$ (512)
Forecast \| Allston Lease
Lessee, Lease, Description [Line Items]
Security deposit liability					$ 1,100

Leases - Schedule of Components of Lease Expense (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Lease Cost
Fixed operating lease cost	$ 827
Short-term lease costs	122
Total lease expense	949
Other information
Operating cash flows from operating leases	1,007
Leased assets obtained in exchange for new operating lease liabilities	$ 484
Weighted-average remaining lease term—operating leases	3 years
Weighted-average discount rate—operating leases	9.00%
Sublease income	$ 14

Leases - Schedule of Maturities of Lease Liabilities (Detail) $ in Thousands	Dec. 31, 2019 USD ($)
Leases [Abstract]
2020	$ 1,124
2021	1,098
2022	754
2023	263
Total lease payments	3,239
Less: interest	(423)
Total operating lease liabilities as of December 31, 2019	$ 2,816

Leases - Schedule of Lease Liability Not Yet Commenced (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
2021			$ 1,098
2022			754
2023			$ 263
Allston Lease \| Forecast
Lessee, Lease, Description [Line Items]
2020		$ 600
2021	$ 1,000
2022	1,000
2023	1,100
2024	1,100
2025	1,100
2026	$ 1,100

Leases - Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Leases [Abstract]
2019	$ 810
2020	823
2021	835
2022	492
Total	$ 2,960

Commitment and Contingencies (Details) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	May 31, 2019	Apr. 30, 2017	Feb. 28, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Funding commitment, termination period				60 days
Research and development				$ 30,163	$ 20,346	$ 17,066
Sponsored Research Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum funding amount		$ 499
Funding commitment period		3 years
Research and development				166	$ 166	$ 111
Non-cancelable purchase commitments				55
Termination notice period	30 days
Maximum termination fees	$ 600
Adimab Option Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Option Fee			$ 250
Milestone payments			$ 25,000
Abbisko Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential milestone payments receivable				$ 208,000

Preferred and Common Stock Warrants - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands					12 Months Ended
	Nov. 29, 2019	Nov. 26, 2019	Apr. 16, 2019	Apr. 12, 2019	Dec. 31, 2019	Mar. 31, 2019	Mar. 13, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Fair value of warrants						$ 288	$ 5,200
Warrants issued (in shares)					13,201,667
Warrant expiration period		30 days		5 years
Common stock par value (in dollars per share)					$ 0.001			$ 0.001
Class A Warrant
Class of Stock [Line Items]
Warrants issued (in shares)			3,900,000
Convertible preferred stock exercise price (in dollars per share)			$ 13.20	$ 13.20
Class B Warrants
Class of Stock [Line Items]
Warrants issued (in shares)	5,416,667
Convertible preferred stock exercise price (in dollars per share)	$ 15.00	$ 15.00
Pre Funded Warrant
Class of Stock [Line Items]
Warrants issued (in shares)	1,750,000		2,130,000
Convertible preferred stock exercise price (in dollars per share)	$ 11.999	$ 11.999	$ 10.999	$ 10.999
Warrants available to purchase (in shares)			1
Common stock par value (in dollars per share)			$ 0.001

Preferred and Common Stock Warrants - Schedule of Outstanding Warrants (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number of warrants
Outstanding and exercisable warrants to purchase preferred shares, beginning balance (in shares)	5,146,400
Conversion of warrants to purchase preferred shares to warrants for the purchase of common stock and adjusted for the Exchange Ratio and Reverse Stock Split (in shares)	(4,657,350)
Warrants issued (in shares)	13,201,667
Exercised (in shares)	(33,846)
Outstanding and exerciseable, Number of warrants, ending balance (in shares)	13,656,871	5,146,400
Weighted Average Exercise Price
Outstanding and exercisable warrants to purchase preferred shares, beginning balance (in dollars per share)	$ 0
Issuance of warrants for the purchase of common stock (in dollars per share)	13.43
Exercisable (in dollars per share)	13.20
Outstanding and exercisable, ending balance (in dollars per share)	$ 13.68	$ 0
Weighted Average Contractual Term (Years)	4 years 7 months 2 days	4 years 2 months 23 days

Preferred and Common Stock Warrants - Summary of Outstanding Warrants to Purchase Shares of Common Stock (Details) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	13,656,871	5,146,400
Issuance On August 14, 2015
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	81,228
Exercise Price (in dollars per share)	$ 21.78
Issuance On August 14, 2015 \| Series A Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)		854,785
Exercise Price (in dollars per share)		$ 2.07
Issuance On August 21, 2015
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	69,603
Exercise Price (in dollars per share)	$ 21.78
Issuance On August 21, 2015 \| Series A Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)		732,453
Exercise Price (in dollars per share)		$ 2.07
Issuance On October 25, 2016
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	5,155
Exercise Price (in dollars per share)	$ 19.78
Issuance On October 25, 2016 \| Series B Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)		54,256
Exercise Price (in dollars per share)		$ 1.88
Issuance On November 1, 2017
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	130,609
Exercise Price (in dollars per share)	$ 19.78
Issuance On November 1, 2017 \| Series B Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)		1,374,435
Exercise Price (in dollars per share)		$ 1.88
Issuance On November 17, 2017
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	8,442
Exercise Price (in dollars per share)	$ 19.78
Issuance On November 17, 2017 \| Series B Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)		88,845
Exercise Price (in dollars per share)		$ 1.88
Issuance On December 4, 2017
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	5,661
Exercise Price (in dollars per share)	$ 19.78
Issuance On December 4, 2017 \| Series B Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)		59,576
Exercise Price (in dollars per share)		$ 1.88
Issuance On December 28, 2017
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	6,925
Exercise Price (in dollars per share)	$ 19.78
Issuance On December 28, 2017 \| Series B Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)		72,875
Exercise Price (in dollars per share)		$ 1.88
Issuance On December 28, 2017
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	115,916
Exercise Price (in dollars per share)	$ 19.78
Issuance On December 28, 2017 \| Series B Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)		1,219,815
Exercise Price (in dollars per share)		$ 1.88
Issuance On September 12, 2018
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	25,275
Exercise Price (in dollars per share)	$ 19.78
Issuance On September 12, 2018 \| Series B Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)		265,957
Exercise Price (in dollars per share)		$ 1.88
Issuance On September 12, 2018
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	20,220
Exercise Price (in dollars per share)	$ 19.78
Issuance On September 12, 2018 \| Series B Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)		212,765
Exercise Price (in dollars per share)		$ 1.88
Issuance On October 19, 2018
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	20,016
Exercise Price (in dollars per share)	$ 19.78
Issuance On October 19, 2018 \| Series B Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)		210,638
Exercise Price (in dollars per share)		$ 1.88
Issuance On March 13, 2019
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	5,000
Exercise Price (in dollars per share)	$ 19.80
Issuance On April 16, 2019
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	3,866,154
Exercise Price (in dollars per share)	$ 13.20
Issuance On April 16, 2019
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	2,130,000
Exercise Price (in dollars per share)	$ 11.00
Issuance on November 29, 2019
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	5,416,667
Exercise Price (in dollars per share)	$ 15.00
Issuance on November 29, 2019
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	1,750,000
Exercise Price (in dollars per share)	$ 12.00

Common Stock, Redeemable Common Stock and Convertible Preferred Stock (converted to Common Stock) - Additional Information (Details) - USD ($)			1 Months Ended	12 Months Ended
	Nov. 26, 2019	Apr. 12, 2019	Aug. 31, 2015	Dec. 31, 2019	Nov. 29, 2019	Apr. 16, 2019	Dec. 31, 2018	Aug. 31, 2014
Class of Stock [Line Items]
Common stock, authorized (in shares)				33,333,333			11,070,776
Common stock par value (in dollars per share)				$ 0.001			$ 0.001
Cash dividends				$ 0
Stock issued (in shares)	3,666,667	5,670,000
Sale price (in dollars per share)	$ 12.00	$ 11.00
Warrant expiration period	30 days	5 years
Proceeds from common stock issuance	$ 65,000,000.0	$ 85,800,000
Fair value of additional common stock issued			$ 734,000
Preferred stock, authorized (in shares)							49,413,523
Preferred stock outstanding				$ 0
Series Seed Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock, authorized (in shares)				10,000,000			59,413,523
Preferred stock par value (in dollars per share)				$ 0.001			$ 0.001
Genzyme Agreement
Class of Stock [Line Items]
Stock issued (in shares)			107,371
Sale price (in dollars per share)								$ 0.01
Shares issued for additional consideration (in shares)			107,371
Common stock purchase price (in dollars per share)			$ 0.001
Pre Funded Warrant
Class of Stock [Line Items]
Common stock par value (in dollars per share)						$ 0.001
Number of warrants issued (in shares)	1,750,000	2,130,000
Convertible preferred stock exercise price (in dollars per share)	$ 11.999	$ 10.999			$ 11.999	10.999
Class A Warrant
Class of Stock [Line Items]
Number of warrants issued (in shares)		3,900,000
Convertible preferred stock exercise price (in dollars per share)		$ 13.20				$ 13.20
Class B Warrants
Class of Stock [Line Items]
Number of warrants issued (in shares)	5,416,667
Convertible preferred stock exercise price (in dollars per share)	$ 15.00				$ 15.00

Common Stock, Redeemable Common Stock and Convertible Preferred Stock (converted to Common Stock) - Schedule of Preferred Stock (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) shares
Temporary Equity [Line Items]
Preferred Stock Designated (in shares)	49,413,523
Preferred Stock Outstanding (in shares)	40,079,567
Preferred Stock Issued (in shares)	40,079,567
Carrying Value \| $	$ 64,675
Liquidation Preference \| $	$ 84,067
Common Stock Issuable Upon Conversion (in shares)	3,808,430
Series Seed preferred stock
Temporary Equity [Line Items]
Preferred Stock Designated (in shares)	2,313,523
Preferred Stock Outstanding (in shares)	1,516,136
Preferred Stock Issued (in shares)	1,516,136
Carrying Value \| $	$ 1,310
Liquidation Preference \| $	$ 1,444
Common Stock Issuable Upon Conversion (in shares)	143,630
Series A preferred stock
Temporary Equity [Line Items]
Preferred Stock Designated (in shares)	22,000,000
Preferred Stock Outstanding (in shares)	19,946,862
Preferred Stock Issued (in shares)	19,946,862
Carrying Value \| $	$ 32,480
Liquidation Preference \| $	$ 47,624
Common Stock Issuable Upon Conversion (in shares)	1,895,610
Series B preferred stock
Temporary Equity [Line Items]
Preferred Stock Designated (in shares)	25,100,000
Preferred Stock Outstanding (in shares)	18,616,569
Preferred Stock Issued (in shares)	18,616,569
Carrying Value \| $	$ 30,885
Liquidation Preference \| $	$ 34,999
Common Stock Issuable Upon Conversion (in shares)	1,769,190

Stock-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum percentage of fair value of common stock	100.00%
Exercises in period, intrinsic value	$ 363
Options granted, weighted average grant date fair value (in dollars per share)	$ 10.78	$ 7.05	$ 4.74
Incentive Stock Options and Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares based compensation, vested period (in years)	4 years
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares based compensation, vested period (in years)	4 years
Share based compensation, restricted stock units granted (in shares)	116,689
Share based compensation, restricted stock units granted (in dollars per share)	$ 14.75
Restricted stock unites vested, percentage	25.00%
Share based compensation, grant date fair value	$ 1,700
Unrecognized compensation cost of stock based awards	$ 6,400
Unrecognized compensation cost of stock based awards, recognition period	3 years 1 month 6 days
Two Thousand Fifteen Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, authorized (in shares)	969,340
Common stock, reserved for future issuance (in shares)	100,000
Two Thousand Seventeen Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, authorized (in shares)	170,915
Common stock, reserved for future issuance (in shares)	20,000,000
Aggregate intrinsic value, options exercised	4.00%
Two Thousand Nineteen Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, authorized (in shares)	400,000
Common stock, reserved for future issuance (in shares)	250,000
Minimum \| Non-Statutory Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares based compensation, vested period (in years)	3 years
Maximum \| Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period (in years)	10 years
Maximum \| Non-Statutory Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares based compensation, vested period (in years)	4 years

Stock-Based Compensation - Stock Option Valuation (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Payment Arrangement [Abstract]
Risk-free interest rate	2.00%	2.80%	2.10%
Expected term (in years)	5 years 11 months 26 days	5 years 11 months 8 days	6 years 21 days
Expected volatility	88.50%	86.00%	77.30%
Expected dividend yield	0.00%	0.00%	0.00%

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number of Shares
Outstanding, beginning balance (in shares)	797,931
Assumed as part of Merger with Arsanis (in shares)	271,230
Granted, (in shares)	589,110
Exercised (in shares)	(51,617)
Forfeited (in shares)	(309,625)
Outstanding, ending balance (in shares)	1,297,029	797,931
Exercisable, ending balance (in shares)	495,740
Vested and expected to vest, ending balance (in shares)	1,122,542
Weighted Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 8.29
Assumed as part of Merger with Arsanis (in dollars per share)	62.60
Granted (in dollars per share)	14.69
Exercised (in dollars per share)	7.05
Forfeited (in dollars per share)	31.55
Outstanding, ending balance (in dollars per share)	17.05	$ 8.29
Exercisable (in dollars per share)	21.66
Vested and expected to vest (in dollars per share)	$ 17.15
Weighted Average Contractual Term (Years)
Outstanding	8 years 4 months 28 days	8 years 5 months 1 day
Exercisable	7 years 2 months 15 days
Vested and expected to vest	8 years 3 months 25 days
Aggregate Intrinsic Value
Outstanding beginning balance	$ 6,486
Outstanding ending balance	1,286	$ 6,486
Exercisable	952
Vested and expected to vest	$ 1,245

Stock-Based Compensation - Restricted Stock Units (Details) - Restricted Stock Units (RSUs)	12 Months Ended
	Dec. 31, 2019 shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested beginning balance (in shares)	0
Granted (in shares)	116,689
Vested (in shares)	0
Forfeited (in shares)	(14,916)
Nonvested ending balance (in shares)	101,773

Stock-Based Compensation - Summary of Stock-Based Compensation Expense Classification (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 2,050	$ 759	$ 492
Research and development expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	909	258	126
General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 1,141	$ 501	$ 366

Income Taxes - Schedule of Income Loss Before Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
United States	$ (52,314)	$ (33,285)	$ (21,994)
Foreign (Austria)	(493)	0	0
Income (loss) before income tax expense (benefit)	$ (52,807)	$ (33,285)	$ (21,994)

Income Taxes - Reconciliation of Effective Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax rate	(21.00%)	(21.00%)	(34.00%)
State income taxes, net of federal benefit	(5.80%)	(6.20%)	(8.40%)
Foreign rate differential	(0.10%)	0.00%	0.00%
Research and development tax credits	(1.10%)	(3.70%)	(1.80%)
Change in fair value of preferred stock warrant liability	0.10%	2.40%	(2.20%)
Other permanent differences	1.40%	0.90%	0.50%
Remeasurement of deferred taxes due to tax reform	0	0	0.278
Change in deferred tax asset valuation allowance	26.80%	27.80%	18.10%
Other	(0.30%)	(0.20%)	0.00%
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Deferred Income Tax Asset (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Components of Deferred Tax Assets [Abstract]
Net operating loss carryforwards	$ 65,487	$ 15,482
Research and development tax credit carryforwards	3,654	2,710
Capitalized research and development expenses	2,626	2,837
Capitalized license fees	0	221
Accrual-to-cash basis conversion	0	1,367
Lease liabilities	765	0
Other	1,413	180
Total deferred tax assets	73,945	22,797
Valuation allowance	(73,410)	(22,797)
Deferred tax assets, net of valuation allowance	535	0
Right of use assets	535	0
Total deferred tax liabilities	535	0
Total deferred tax assets, net	$ 0	$ 0

Income Taxes - Narrative (Details) $ in Millions	Dec. 31, 2019 USD ($)
Income Tax Contingency [Line Items]
Unrecognized tax benefits	$ 0.2
Domestic Tax Authority
Income Tax Contingency [Line Items]
Operating loss carryforwards	181.8
Operating loss carryforwards, not subject to expiration	127.7
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Operating loss carryforwards	177.4
Foreign Tax Authority
Income Tax Contingency [Line Items]
Operating loss carryforwards	64.4
Research Tax Credit Carryforward \| Domestic Tax Authority
Income Tax Contingency [Line Items]
Tax credit carryforward, amount	3.1
Research Tax Credit Carryforward \| State and Local Jurisdiction
Income Tax Contingency [Line Items]
Tax credit carryforward, amount	0.6
Merger, Arsanis \| Domestic Tax Authority
Income Tax Contingency [Line Items]
Operating loss carryforwards	75.0
Merger, Arsanis \| State and Local Jurisdiction
Income Tax Contingency [Line Items]
Operating loss carryforwards	$ 70.0

Income Taxes - Deferred Tax Valuation Allowance (Details) - SEC Schedule, 12-09, Valuation Allowance, Deferred Tax Asset [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Valuation allowance, beginning of year	$ (22,797)	$ (13,546)	$ (9,738)
Increases recorded to income tax provision	(14,485)	(9,251)	(3,808)
Acquisition of business	(36,128)	0	0
Valuation allowance, end of year	$ (73,410)	$ (22,797)	$ (13,546)

Net Loss per Share - Summary of Basic and Diluted Net loss per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net loss	$ (52,807)	$ (33,285)	$ (21,994)
Accruing dividends on Series A convertible preferred stock	592	3,000	3,000
Adjustment to accumulated deficit in connection with repurchase of Series Seed convertible preferred stock	0	22	0
Net loss attributable to common stockholders	$ (53,399)	$ (36,307)	$ (24,994)
Weighted average shares of common stock outstanding - basic and diluted (in shares)	11,530	459	458
Net loss per share attributable to common stockholders - basic and diluted (in dollars per share)	$ (4.63)	$ (79.15)	$ (54.58)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Net Loss per Share - Additional Information (Details) - $ / shares		1 Months Ended	12 Months Ended
	Apr. 16, 2019	Nov. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Nov. 29, 2019	Nov. 26, 2019	Apr. 12, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted average shares of common stock outstanding - basic and diluted (in shares)			11,530,000	459,000	458,000
Pre Funded Warrant
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted average shares of common stock outstanding - basic and diluted (in shares)	2,130,000	1,750,000
Convertible preferred stock exercise price (in dollars per share)	$ 10.999					$ 11.999	$ 11.999	$ 10.999
Minimum \| Pre Funded Warrant
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Convertible preferred stock exercise price (in dollars per share)			$ 0.001
Redeemable Common Stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted average shares of common stock outstanding - basic and diluted (in shares)			107,371	107,371	107,371

Net Loss per Share - Schedule of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss per Share Attributable to Common Stockholders (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded form computation of diluted net loss per share (in shares)	11,175,673	5,096,284	4,510,918
Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded form computation of diluted net loss per share (in shares)	0	3,808,894	3,613,069
Employee Stock and Restricted Stock Units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded form computation of diluted net loss per share (in shares)	1,398,802	798,311	474,282
Warrant
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded form computation of diluted net loss per share (in shares)	9,776,871	489,079	423,567

Loss on Transfer of Nonfinancial Assets - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Indefinite-lived Intangible Assets [Line Items]
Loss on transfer of nonfinancial assets	$ 3,900	$ 0	$ 0
Proceeds from transfer of non-financial assets	1,000	$ 0	$ 0
In Process Research and Development
Indefinite-lived Intangible Assets [Line Items]
Carrying value of IPR&D	$ 4,900