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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2019	Aug. 02, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Trading Symbol	XFOR
Entity Registrant Name	X4 PHARMACEUTICALS, INC.
Entity Central Index Key	0001501697
Entity Current Reporting Status	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		12,430,434
Entity File Number	001-38295
Entity Tax Identification Number	273181608
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	Massachusetts
Entity Address, Postal Zip Code	02139
City Area Code	(857)
Local Phone Number	529-8300

CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 90,180	$ 8,134
Restricted cash	3,163
Research and development incentive receivable	1,708
Prepaid expenses and other current assets	1,598	1,205
Total current assets	96,649	9,339
Property and equipment, net	258	241
Intangible assets	4,900
Goodwill	27,109
Right-of-use assets	2,187
Restricted cash	2,298	364
Other assets	106
Total assets	133,507	9,944
Current liabilities:
Accounts payable	3,099	2,969
Accrued expenses	5,094	3,251
Current portion of lease liability	860
Current portion of long-term debt, net of discount	4,151	1,687
Total current liabilities	13,204	7,907
Preferred stock warrant liability		4,947
Long-term debt, including accretion, net of discount and current portion	21,748	8,145
Deferred rent		417
Lease liability	2,369
Other liabilities	18	205
Total liabilities	37,339	21,621
Commitments and contingencies (Note 9)
Convertible preferred stock (Series Seed, A and B), $0.001 par value; 10,000,000 and 59,413,523 shares authorized as of June 30, 2019 and December 31, 2018, respectively; 0 and 40,079,567 shares issued and outstanding as of June 30, 2019 and December 31, 2018, respectively		64,675
Common stock	13
Stockholders’ equity (deficit):
Common stock	13
Additional paid-in capital	199,690	2,151
Accumulated other comprehensive income	(42)
Accumulated deficit	(103,493)	(79,237)
Total stockholders’ equity (deficit)	96,168	(77,086)
Total liabilities, convertible preferred stock, redeemable common stock and stockholders’ equity (deficit)	$ 133,507	9,944
Convertible Preferred Stock [Member]
Current liabilities:
Convertible preferred stock (Series Seed, A and B), $0.001 par value; 10,000,000 and 59,413,523 shares authorized as of June 30, 2019 and December 31, 2018, respectively; 0 and 40,079,567 shares issued and outstanding as of June 30, 2019 and December 31, 2018, respectively		64,675
Redeemable Common Stock [Member]
Current liabilities:
Common stock		734
Stockholders’ equity (deficit):
Common stock		734
Total stockholders’ equity (deficit)		$ 734

CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
Convertible preferred stock, shares authorized		49,413,523
Convertible preferred stock, shares issued		40,079,567
Convertible preferred stock, shares outstanding		40,079,567
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	33,333,333	11,070,776
Common stock, shares issued	12,429,057	351,652
Common stock, shares outstanding	12,429,057	351,652
Convertible Preferred Stock [Member]
Convertible preferred stock, par or stated value per share	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized	10,000,000	59,413,523
Convertible preferred stock, shares issued	0	40,079,567
Convertible preferred stock, shares outstanding	0	40,079,567
Redeemable Common Stock [Member]
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares issued	0	107,364
Common stock, shares outstanding	0	107,364

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating expenses:
Research and development	$ 8,854	$ 4,755	$ 14,509	$ 9,499
General and administrative	4,560	1,621	9,343	2,987
Total operating expenses	13,414	6,376	23,852	12,486
Loss from operations	(13,414)	(6,376)	(23,852)	(12,486)
Other income (expense):
Interest income	394	67	463	136
Interest expense	(512)	(167)	(911)	(336)
Change in fair value of preferred stock warrant liability		283	(288)	(309)
Change in fair value of derivative liability		159	183	(406)
Other income (expense)	149		149
Total other income (expense), net	31	342	(404)	(915)
Net loss	(13,383)	(6,034)	(24,256)	(13,401)
Accruing dividends on Series A convertible preferred stock		(748)	(592)	(1,488)
Adjustment to accumulated deficit in connection with repurchase of Series Seed convertible preferred stock		(22)		(22)
Net loss attributable to common stockholders	$ (13,383)	$ (6,804)	$ (24,848)	$ (14,911)
Net loss per share attributable to common stockholders—basic and diluted	$ (1.02)	$ (14.83)	$ (3.32)	$ (32.53)
Weighted average common shares outstanding—basic and diluted	13,177,235	458,718	7,479,178	458,346

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (13,383)	$ (6,034)	$ (24,256)	$ (13,401)
Other comprehensive loss
Currency translation adjustments	(65)		(42)
Total comprehensive loss	$ (13,448)	$ (6,034)	$ (24,298)	$ (13,401)

- Definition

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

+ References + Details

CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK, REDEEMABLE COMMON STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Unaudited) - USD ($) $ in Thousands	Total	Convertible Preferred Shares [Member]	Redeemable Common Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2017	$ (44,545)		$ 734		$ 1,385		$ (45,930)
Convertible preferred shares, beginning balance, shares at Dec. 31, 2017		38,018,968
Convertible preferred shares, beginning balance at Dec. 31, 2017		$ 60,903
Beginning balance, shares at Dec. 31, 2017			107,364	350,607
Repurchase of Series Seed convertible preferred stock, net of issuance costs of $1	(22)	$ (517)					(22)
Repurchase of Series Seed convertible preferred stock, shares		(598,975)
Stock-based compensation expense	128				128
Net loss	(7,367)						(7,367)
Ending balance at Mar. 31, 2018	(51,806)		$ 734		1,513		(53,319)
Convertible preferred shares, ending balance, shares at Mar. 31, 2018		37,419,993
Convertible preferred shares, ending balance at Mar. 31, 2018		$ 60,386
Ending balance, shares at Mar. 31, 2018			107,364	350,607
Beginning balance at Dec. 31, 2017	(44,545)		$ 734		1,385		(45,930)
Convertible preferred shares, beginning balance, shares at Dec. 31, 2017		38,018,968
Convertible preferred shares, beginning balance at Dec. 31, 2017		$ 60,903
Beginning balance, shares at Dec. 31, 2017			107,364	350,607
Repurchase of Series Seed convertible preferred stock, net of issuance costs of $1	(22)
Net loss	(13,401)
Ending balance at Jun. 30, 2018	(57,688)		$ 734		1,665		(59,353)
Convertible preferred shares, ending balance, shares at Jun. 30, 2018		37,419,993
Convertible preferred shares, ending balance at Jun. 30, 2018		$ 60,386
Ending balance, shares at Jun. 30, 2018			107,364	351,652
Beginning balance at Mar. 31, 2018	(51,806)		$ 734		1,513		(53,319)
Convertible preferred shares, beginning balance, shares at Mar. 31, 2018		37,419,993
Convertible preferred shares, beginning balance at Mar. 31, 2018		$ 60,386
Beginning balance, shares at Mar. 31, 2018			107,364	350,607
Repurchase of Series Seed convertible preferred stock, net of issuance costs of $1	(22)
Exercise of stock options	7				7
Exercise of stock options, shares				1,045
Stock-based compensation expense	145				145
Net loss	(6,034)						(6,034)
Ending balance at Jun. 30, 2018	(57,688)		$ 734		1,665		(59,353)
Convertible preferred shares, ending balance, shares at Jun. 30, 2018		37,419,993
Convertible preferred shares, ending balance at Jun. 30, 2018		$ 60,386
Ending balance, shares at Jun. 30, 2018			107,364	351,652
Beginning balance at Dec. 31, 2018	$ (77,086)		$ 734		2,151		(79,237)
Convertible preferred shares, beginning balance, shares at Dec. 31, 2018	40,079,567	40,079,567
Convertible preferred shares, beginning balance at Dec. 31, 2018	$ 64,675	$ 64,675
Beginning balance, shares at Dec. 31, 2018			107,364	351,652
Conversion of redeemable common stock into common stock	734		$ (734)	$ 1	733
Conversion of redeemable common stock into common stock, shares			(107,364)	107,364
Conversion of convertible preferred shares into common stock	64,675	$ (64,675)		$ 4	64,671
Conversion of convertible preferred shares into common stock, shares		(40,079,567)		3,808,430
Exchange of common stock in connection with Merger	45,541			$ 2	45,539
Exchange of common stock in connection with Merger, shares				2,440,582
Fair value of replacement equity awards	817				817
Reclassification of warrant liability to permanent equity	5,235				5,235
Exercise of stock options	113				113
Exercise of stock options, shares				16,483
Stock-based compensation expense	262				262
Currency translation adjustments	23					$ 23
Net loss	(10,873)						(10,873)
Ending balance at Mar. 31, 2019	29,441			$ 7	119,521	23	(90,110)
Ending balance, shares at Mar. 31, 2019				6,724,511
Beginning balance at Dec. 31, 2018	$ (77,086)		$ 734		2,151		(79,237)
Convertible preferred shares, beginning balance, shares at Dec. 31, 2018	40,079,567	40,079,567
Convertible preferred shares, beginning balance at Dec. 31, 2018	$ 64,675	$ 64,675
Beginning balance, shares at Dec. 31, 2018			107,364	351,652
Conversion of redeemable common stock into common stock	$ 734
Exercise of stock options, shares	17,183
Net loss	$ (24,256)
Ending balance at Jun. 30, 2019	96,168			$ 13	199,690	(42)	(103,493)
Ending balance, shares at Jun. 30, 2019				12,429,057
Beginning balance at Mar. 31, 2019	29,441			$ 7	119,521	23	(90,110)
Beginning balance, shares at Mar. 31, 2019				6,724,511
Issuance of common stock and prefunded warrants for the purchase of common stock, net of issuance costs of $931	79,297			$ 6	79,291
Issuance of common stock and prefunded warrants for the purchase of common stock, net of issuance costs of $931, shares				5,670,000
Exercise of stock options	5				5
Exercise of stock options, shares				700
Exercise of warrants	440				440
Exercise of warrants, shares				33,846
Stock-based compensation expense	433				433
Currency translation adjustments	(65)					(65)
Net loss	(13,383)						(13,383)
Ending balance at Jun. 30, 2019	$ 96,168			$ 13	$ 199,690	$ (42)	$ (103,493)
Ending balance, shares at Jun. 30, 2019				12,429,057

CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK, REDEEMABLE COMMON STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2019	Mar. 31, 2018
Convertible Preferred Shares [Member]
Net of issuance costs		$ 1
Common Stock [Member]
Issuance costs	$ 931

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (24,256)	$ (13,401)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	695	273
Depreciation expense	44	51
Non-cash lease expense	244
Non-cash interest expense	369	59
Change in fair value of preferred stock warrant liability	288	309
Change in fair value of derivative liability	(183)	406
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	13	490
Accounts payable	(1,702)	(605)
Accrued expenses	(1,157)	453
Lease liabilities	(397)
Net cash used in operating activities	(26,042)	(11,965)
Cash flows from investing activities:
Cash, cash equivalents and restricted cash acquired in connection with the Merger	26,406
Acquisition of property, plant and equipment	(10)
Net cash provided by investing activities	26,396
Cash flows from financing activities:
Proceeds from exercise of stock options and warrants	565	7
Proceeds from borrowings under loan and security agreements, net of issuance costs	9,849
Repurchase of Series Seed convertible preferred stock		(1,160)
Repayments of borrowings under loan and security agreement	(2,914)	(1,000)
Sale of common stock, pre-funded warrants and Class A warrants, net of issuance costs	79,291
Net cash provided by (used in) financing activities	86,791	(2,153)
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(2)
Net increase (decrease) in cash, cash equivalents and restricted cash	87,143	(14,118)
Cash, cash equivalents and restricted cash at beginning of period	8,498	27,048
Cash, cash equivalents and restricted cash at end of period	95,641	$ 12,930
Supplemental disclosure of non-cash investing and financing activities:
Issuance costs not yet paid	354
Conversion of convertible preferred stock into common stock	64,675
Conversion of redeemable common stock into common stock	734
Conversion of convertible preferred stock warrants into common stock warrants	5,235
Fair value of net assets acquired in the Merger	$ 46,358

Nature of the Business and Basis of Presentation

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Nature of the Business and Basis of Presentation

1.	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 Phase 3 development for 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 early-stage 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 condensed consolidated financial statements as “the Merger.”

The transaction was accounted for as a reverse merger in accordance with 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 will be those of the Company. In addition, transaction costs incurred by the Company in connection with the business combination will be 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 63.7% 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.”

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 condensed 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 August 6, 2019, the issuance date of these condensed 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 of these financial statements. The future viability of the Company beyond that point is dependent on its ability to raise additional capital to finance its operations.

If the Company is unable to obtain funding, the Company will 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. Although management continues to pursue these plans, 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

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

Unaudited Interim Financial Statements--- The condensed balance sheet at December 31, 2018 was derived from audited financial statements but does not include all disclosures required by GAAP. The accompanying condensed consolidated financial statements are unaudited. The accompanying unaudited interim consolidated financial statements have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”) for interim financial statements. Accordingly, certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited condensed financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto for the year ended December 31, 2018. In the opinion of management, all adjustments, consisting only of normal recurring adjustments as necessary, for the fair statement of the Company’s condensed financial position, condensed results of its operations and cash flows have been made. The results of operations for the three and six months ended June 30, 2019 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2019.

Use of Estimates--- The preparation of the Company’s condensed 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 liability), and the preferred stock repurchase liability, and valuation of lease liabilities. 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.

Principles of Consolidation--- The condensed 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.

Foreign Currency and Currency Translation--- Management has determined that the functional currency for the Company’s wholly owned foreign subsidiary, X4 GmbH, is the euro. Management’s assessment considered the currency environment in which the entity operates, including inflows of cash from research and development incentive programs and outflows of cash for operating expenditure. Accordingly, the assets and liabilities of X4 GmbH are translated from the euro into U.S. dollars at the exchange rate in effect on the balance sheet date and income and expenses are translated at the average exchange rate in effect during the period. Unrealized translation gains and losses are recorded as a cumulative translation adjustment, which is included in the consolidated balance sheet as a component of accumulated other comprehensive income (loss). Adjustments that arise from exchange rate changes on transactions denominated in a currency other than the euro are included in other income (expense), net in the consolidated statements of operations as incurred.

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 June 30, 2019 and December 31, 2018.

Restricted Cash and Compensating Balance Arrangements---

	As of June 30, 2019		As of December 31, 2018
Letter of credit security: Cambridge Lease (a)	$	264	$	264
Letter of credit security: Waltham Lease (b)		250		—
Letter of credit security: Vienna Austria Lease (c)		95		—
Corporate credit card collateral (d)		150		100
Compensating balance arrangement (e)		1,539
Total restricted cash (non-current)	$	2,298	$	364

(a)	In connection with the Company’s lease agreement for its facility in Cambridge, Massachusetts, the Company maintains a letter of credit of $264, which is secured by restricted cash, for the benefit of the landlord.

(b)	The Company maintains a letter of credit, which is secured by a restricted cash amount of $250 as of June 30, 2019, for the benefit of the landlord in connection with a leased facility in Waltham, Massachusetts.

(c)	In connection with the Company’s lease agreement for its laboratory and office facility in Vienna, Austria, the Company maintains a letter of credit of $95, which is secured by restricted cash, for the benefit of the landlord.

(d)	As of June 30, 2019 and December 31, 2018, the Company was required to maintain a separate cash balance of $150 and $100, respectively, to collateralize corporate credit cards with a bank.

(e)

In accordance with the Company’s loan agreement with Österreichische Forschungsförderungsgesellschaft mbH (“FFG”), as amended (see Note 7), the Company must maintain a cash balance of at least 70% of the outstanding principal of the FFG loan in a local Austrian bank account. As of June 30, 2019, non-current restricted cash includes 70% of the non-current portion of the FFG loan. In addition, 70% of the current portion of the FFG loan, or $3,163, is classified as current restricted cash on the Company’s consolidated balance sheet as of June 30, 2019.

In accordance with the Company’s Amended and Restated Loan Agreement with Hercules Capital, Inc. (“Hercules”) as further described in Note 7, 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 $2,500. As of June 30, 2019, the Company maintained $25,000 in an account accessible by the lender in accordance with the terms of Amended and Restated Loan Agreement. The underlying cash subject to these compensating balance agreements is classified within cash and cash equivalents on the consolidated balance sheet.

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheets to the sum to the total of amounts shown in the Company’s condensed consolidated statement of cash flows as of June 30, 2019, December 31, 2018, June 30, 2018 and December 31, 2017:

	June 30, 2019		December 31, 2018		June 30, 2018		December 31, 2017
Cash and cash equivalents	$	90,180	$	8,134	$	12,566	$	26,684
Restricted cash, current portion		3,163		—		—		—
Restricted cash, non-current portion		2,298		364		364		364
Total cash, cash equivalents and restricted cash	$	95,641	$	8,498	$	12,930	$	27,048

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
Software		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, lease liabilities and long-term 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.

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 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 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.

The Company assesses qualitative factors to determine whether the existence of events or circumstances would indicate that it is more likely than not that the fair value of the reporting unit was less than its carrying amount. If after assessing the totality of events or circumstances, the Company were to determine that it is more likely than not that the fair value of the reporting unit is less than its carrying amount, then the Company would perform a quantitative impairment 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 the 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 has determined there were no indicators of goodwill impairment as of June 30, 2019.

Intangible Assets--- The Company acquired certain in-process research and development assets (“IPR&D”), which are 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. If the Company determines that IPR&D becomes impaired or is abandoned, the carrying value is written down to its fair value with the related impairment charge recognized in the Company’s consolidated statement of operations in the period in which the impairment occurs. Upon successful completion of each project and launch of the product, the Company would make a separate determination of the estimated useful life of the IPR&D intangible asset and the related amortization will be recorded as an expense prospectively over its estimated useful life.

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 regarding the timing of and the expected costs to advance the Company’s clinical programs to commercialization;

•

Estimates of future cash flows from potential product sales; and

•

A discount rate.

Additionally, to the extent the Company acquires other indefinite-lived intangible assets through its business combinations, these assets are reviewed for impairment on an annual basis or more frequently if indicators of impairment are present. If the Company determines that the asset becomes impaired, the carrying value is written down to its fair value with the related impairment charge recognized in its consolidated statements of operations in the period in which the impairment occurs.

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 8.

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 4). 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 2 inputs in the fair value hierarchy described above (see Note 4). 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 (the “Hercules Loan Agreement”) with Hercules approximates its fair value at June 30, 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. The carrying value of the Company’s loans under the funding agreements with FFG were recorded at fair value on the opening balance sheet of Arsanis as of the date of the Merger and approximates the fair value of the loans at June 30, 2019. (See Note 3).

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”), 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 condensed 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. Prior to completion of the Merger, Arsanis entered into a single out-licensing agreement with Janssen Pharmaceuticals, Inc. (see Note 13) that was within the scope of ASC 606. The revenue associated with this arrangement is not reflected in the consolidated financial statements of the Company as it occurred prior to the Merger.

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 goods or 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 goods have been delivered or the related services have been performed, or when it is no longer expected that the goods will be delivered or the services rendered.

Upfront payments, milestone payments and annual maintenance fees under license agreements are expensed in the period in which they are incurred.

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 only 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. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”), which expands the scope of Topic 718 to include share-based payment awards to nonemployees. As a result, stock-based awards granted to consultants and non-employees are accounted for in the same manner as awards granted to employees and directors as described above. The impact of adoption this new guidance did not have a material impact on the Company’s consolidated financial statements. Prior to the adoption of ASU 2018-07, for stock-based awards granted to non-employee consultants, compensation expense was recognized over the period during which services were rendered by such non-employee 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 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 will be 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 10) 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 warrant liability are recognized as a component of other income (expense), net in the consolidated statement of operations. 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 (see Note 10).

Derivative Liabilities: Genzyme Continent Payment---The Company’s license agreement with Genzyme Corporation (“Genzyme”) (see Note 13) 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 condensed 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 condensed consolidated statement of operations. 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 7) 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 condensed consolidated balance sheet. The derivative liability was initially recorded at fair value on the date of entering into the Hercules Loan Agreement and is subsequently remeasured to fair value at each reporting date. Changes in the fair value of this derivative liability are recognized as a component of other income (expense), net in the condensed 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.

Comprehensive Loss--- Comprehensive loss includes net loss as well as foreign currency translation adjustments. For the three and six months ended June 30, 2019, comprehensive loss includes $65 and $42 of foreign currency translation adjustments, respectively.

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 Income (Loss) per Share---For periods prior to the Merger with Arsanis on March 13, 2019, the Company follows the two-class method when computing net income (loss) per share as the Company had issued shares that met the definition of participating securities. The two-class method determines net income (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 income (loss) per share attributable to common stockholders is computed by dividing the net income (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 pre-funded warrants issued in April 2019, the exercise of which requires little or no consideration for the delivery of shares of common stock. Diluted net income (loss) attributable to common stockholders is computed by adjusting net income (loss) attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares. For purpose of this calculation, outstanding stock options, convertible preferred stock and warrants to purchase shares of convertible preferred stock are considered potential dilutive common shares.

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 common shares 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 three and six months ended June 30, 2019 and 2018.

Recently Adopted Accounting Pronouncements

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02” 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 leasing standard as of the required effective date of January 1, 2019 using a modified retrospective transition approach to be 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. The adoption did not have a material impact on the Company’s statement of operations, statement of comprehensive loss or statement of cash flows.

In January 2017, the FASB issued ASU No. 2017-04, 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 has adopted this guidance on January 1, 2019 and will apply it to its annual impairment test, and any interim impairment tests during the year ending December 31, 2019.

In April 2017, the FASB issued ASU 2017-08, 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 its adoption had no impact on the Company’s financial position, results of operations or cash flows.

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 adoption of ASU 2017-11 had no impact on the Company’s financial position, results of operations or cash flows.

In June 2018, the FASB issued ASU 2018-07, 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 2018-07 on January 1, 2019 and the adoption of this standard had no impact on the Company’s financial position, results of operations or cash flows.

Recently Issued Accounting Pronouncements

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 guidance is effective beginning January 1, 2020. The Company does not expect the adoption of this standard to have a material impact on its 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. However, the Company will be required to additionally disclose the changes in unrealized gains and losses included in other comprehensive income 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 fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The amendments relating to additional disclosure requirements will be applied prospectively for only the most recent interim or annual period presented in the initial year of adoption. All other amendments will be applied retrospectively to all periods presented upon their effective date. The Company does not believe that the adoption of ASU 2018-13 will have a material impact on its consolidated financial statements.

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 early adoption is permitted. The amendments in this update should be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. The Company is currently evaluating the impact that the adoption of ASU 2018-15 will have on its consolidated financial statements.

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. Early adoption, including adoption in any period, is permitted. The new guidance is required to be applied retrospectively as of the date of adoption of the new revenue guidance under ASC 606. The Company is evaluating the impact, if any, of this guidance on its consolidated financial statements and related disclosures and the timing of adoption.

In April 2019, the FASB issued ASU 2019-04, Codification Improvements to Topic 326 Financial Instruments- Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825 (“ASU 2019-04”). ASU 2014-04 clarifies the accounting treatment for the measurement of credit losses under ASC 236 and provides further clarification of ASU 2017-12, Derivative and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities and ASU 2016-01, Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities. ASU 2019-04 is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. The Company is evaluating the impact, if any, of this new standard but does not anticipate that it will have a material impact on its consolidated financial statements and related disclosures.

Merger Accounting

6 Months Ended

Jun. 30, 2019

Business Combinations [Abstract]

Merger Accounting

3.	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 31.3% 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 68.7% 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:

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:

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 $27,109 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 Arsanis common shares following the announcement of the Merger with X4. During the quarter ended June 30, 2019, goodwill was adjusted by $298 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 June 30, 2019, and there were no indicators of impairment as of June 30, 2019.

The Company incurred costs directly related to the Merger of approximately $1 million for the six months ended June 30, 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 costs to develop the project into commercially viable products, estimating the resulting revenue from the projects, and discounting the projected net cash flows to present value using a weighted average discount rate of 19.5%. 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, in July 2019, the Company entered into outlicensing arrangements with two third parties and, as a result, transferred the rights to develop and commercialize the programs underlying the IPR&D intangible assets. Accordingly, the Company considered whether these subsequent transactions indicated that the fair value of the IPR&D intangible assets was lower than their carrying amounts and thus impaired. The Company concluded that the carrying amount continued to reflect the fair value of the IPR&D assets based on assumptions that market participants would use in pricing the assets and assuming the highest and best use of the assets.

The preliminary allocation of the purchase price for the Merger was based on estimates of the fair value of the net assets acquired and is subject to adjustment upon finalization of the valuation of the acquired intangible assets, property, plant and equipment, right-of-use assets, lease obligations, fair value of debt and any related deferred taxes. Measurements of these items inherently require significant estimates and assumptions.

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

	Three Months Ended June 30,						Six Months Ended June 30,
							2019			2018
							(unaudited)
Revenue	$	—		$	—		$	—		$	—
Net loss	$	(12,904	)	$	(18,616	)	$	(29,162	)	$	(38,851	)

The above unaudited pro forma information was determined based on the historical GAAP results of the Company and Arsanis. The unaudited 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 unaudited 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 $2,663 from the six months ended June 30, 2019 to the six months ended June 30, 2018. There were no material pro forma adjustments for the three month periods ended June 30, 2019 or June 30, 2018.

Fair Value of Financial Assets and Liabilities

6 Months Ended

Jun. 30, 2019

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets and Liabilities

4.	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 June 30, 2019 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	—	$	89,545	$	—	$	89,545
	$	—	$	89,545	$	—	$	89,545
Liabilities: None

	Fair Value Measurements as of December 31, 2018 Using:
	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 June 30, 2019 and December 31, 2018, there were no transfers between Level 1, Level 2 and Level 3.

The Company’s cash equivalents consisted of a money market fund invested in U.S. Treasury securities. The money market fund was valued 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 Notes 10), (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 7) 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 7). 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 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 June 30, 2019 as all such warrants were converted to warrants for the purchase of common stock upon the Merger. The Company had been a private company prior to the Merger and lacked company-specific historical and implied volatility information of its stock. Therefore, it 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 Company’s July 2014 license agreement with Genzyme (see Note 13) 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. The Merger with Arsanis (see Note 1) qualified as a change of control event, as defined in the license agreement, but results 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, this 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 and comprehensive loss 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 7) 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 June 30, 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:

	Preferred Stock Warrant Liability			Derivative Liability
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 as of June 30, 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

6 Months Ended

Jun. 30, 2019

Property Plant And Equipment [Abstract]

Property and Equipment, Net

5.	Property and Equipment, Net

Property and equipment, net consisted of the following:

	June 30, 2019			December 31, 2018
Leasehold improvements	$	336		$	299
Furniture and fixtures		105			53
Computer equipment		52			56
Software		9			9
Lab equipment		61			—
		563			417
Less: Accumulated depreciation and amortization		(305	)		(176	)
	$	258		$	241

Depreciation and amortization expense related to property and equipment was $44 and $51 for the six months ended June 30, 2019 and 2018, respectively.

Accrued Expenses

6 Months Ended

Jun. 30, 2019

Text Block [Abstract]

Accrued Expenses

6.Accrued Expenses

Accrued expenses consisted of the following:

	June 30, 2019		December 31, 2018
Accrued employee compensation and benefits	$	1,374		924
Accrued external research and development expenses		2,561		754
Accrued professional fees		879		1,324
Other		280		249
	$	5,094	$	3,251

Long-Term Debt

6 Months Ended

Jun. 30, 2019

Debt Disclosure [Abstract]

Long-Term Debt

7.	Long-Term Debt

Long-term debt consisted of the following:

	June 30, 2019			December 31, 2018
Principal amount of long-term debt	$	26,717		$	10,000
Less: Current portion of long-term debt		(4,519	)		(1,687	)
Long-term debt, net of current portion		22,198			8,313
Debt discount, net of accretion		(672	)		(226	)
Cumulative accretion of final payment due at maturity		222			58
Long-term debt, including accretion, net of current portion	$	21,748		$	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 $6,000, which was borrowed by the Company in June 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, including unpaid principal of $4,333, a final payment of $270, a prepayment premium of $87 and accrued interest of $23. As of October 19, 2018, the date of repayment of all borrowings under the SVB Loan Agreement, the 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 $13,000, consisting of (i) a term loan of up to $8,000, which was available upon entering into the agreement, (ii) subject to specified financing conditions, an additional term loan of up to $2,000, 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 $2,000 described above, an additional term loan of up to $3,000, available for borrowing until March 31, 2019. In October 2018, the Company borrowed $8,000 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 $2,000 provided under the Hercules Loan Agreement, as amended by the First Amendment. In March 2019, the conditions necessary for borrowing the remaining $3,000 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 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 and $282, respectively. Upon the closing of the Merger, the warrants were converted to warrants for common stock and are no longer adjusted to fair value.

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 “June 2019 Loan Agreement”) with Hercules. The June 2019 Loan Agreement provides for aggregate maximum borrowings of $35,000, of which $10,000 was previously outstanding. The Company agreed to borrow $20,000 as of the closing date on June 27, 2019, including $10,000 in new borrowings and $10,000 rolled over from the previous agreement. An additional $5,000 is available for borrowing through December 15, 2020 and, subject to approval by Hercules, an additional term loan of $10,000 is available through June 15, 2022. Borrowings under the June 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 June 2019 Loan Agreement, and until such event is no longer continuing, the interest rate applicable to borrowings under the June 2019 Loan Agreement would be increased by 4.0%.

Borrowings under the June 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 June 2019 Loan Agreement provides for payments by the Company to Hercules of (i) $795,000 payable upon the earlier of November 1, 2021 or the repayment in full of all obligations under the June 2019 Loan Agreement, and (ii) 4.0% of the aggregate principal drawn under the June 2019 Loan Agreement payable upon the earlier of maturity or the repayment in full of all obligations under such agreement.

Borrowings under the June 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 June 2019 Loan Agreement also contains a covenant that requires the Company to repay its FFG Loan Agreement on or prior to December 31, 2019. The FFG Loan Agreement was not modified as a result of this covenant. The Company was in compliance with all covenants under the June 2019 Loan Agreement as of June 30, 2019 and the FFG principal amounts are classified on the consolidated balance sheet in accordance with their contractual maturity dates.

The Company’s obligations under the June 2019 Loan Agreement are subject to acceleration upon occurrence of specified events of default, including payment default, insolvency and a material adverse change in the Company’s business, operations or financial or other conditions. The Company concluded that the redemption feature meets the definition of an embedded derivative instrument as the repayment of the debt contains a substantial premium, resulting in the redemption feature not being clearly and closely related to the host instrument. The Company recorded the issuance-date fair value of the derivative liability of $18 as a component of debt issuance costs.

In connection with entering into the Hercules Loan Agreement and the First Amendment, the Company had deferred $180 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 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 June 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 June 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 June 2019 Loan Agreement of $382 and $0 during the three months ended June 30, 2019 and 2018, respectively, and $739 and $0 during six months ended June 30, 2019 and 2018, respectively. Interest expense includes $101 and $215 for the three and six months ended June 30, 2019, respectively, related to the accretion of the debt discount and the final payment. As of June 30, 2019 the unamortized debt discount was $104. The annual effective interest rate on the 2019 June Loan Agreement as of June 30, 2019 is 11.0%. There were no principal payments due or paid under the Hercules Loan Agreement, as amended by the First Amendment, during the six months ended June 30, 2019. Principal payments begin in January 2022.

FFG Loan Agreement

Between September 2011 and March 2017, Arsanis GmbH entered into a series of funding agreements with Österreichische Forschungsförderungsgesellschaft mbH (“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 2.0% per annum. Giving effect to the Settlement Agreement (as defined below), the loans matured at various dates between March 31, 2019 and March 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 $2,914 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 was in compliance with all covenants under the FFG Loan Agreement as of June 30, 2019 and the principal amounts of the FFG loans are classified as either current of non-current on the consolidated balance sheet in accordance with the contractual maturity dates within the FFG Loan Agreement. As noted above, the June 2019 Loan Agreement with Hercules includes a covenant that requires the Company to repay the FFG loans in full on or prior to June 30, 2019.

As of June 30, 2019, future principal payments and the final payment due under the Company’s loan agreements were as follows:

Year Ending December 31,	Hercules		FFG		Total
2019	$	—	$	—	$	—
2020		—		4,519		4,519
2021		—		2,198		2,198
2022		11,897		—		11,897
2023		8,103		—		8,103
Long-term debt	$	20,000	$	6,717	$	26,717

Leases

6 Months Ended

Jun. 30, 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 $2,026 and lease liabilities of $2,538, of which $1,925 was classified as non-current and $613 as current. The difference between the value of the right-of-use asset and the lease liabilities related to $512 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, and Waltham, Massachusetts, which is the former headquarters of Arsanis. The Company plans to sublease its Waltham, Massachusetts facility. 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 thirteen thousand square feet in Cambridge, Massachusetts (“Cambridge Lease”) which expires on July 31, 2022. The Cambridge lease includes an annual rent escalation clause and the Company has the option to extend the lease for one period of five additional years. Base rent is approximately $816 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 six thousand 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 $262 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 has ceased using the leased space and is actively seeking to obtain a subtenant. As a result, the Company has adjusted the value of the right-of-use asset to its estimated fair value, which represents management’s best estimate of sublease income that could be obtained for the space, less costs to obtain a 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 four hundred 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 two years. The lease is cancellable by the Company upon three months’ notice with no penalty. The annual base rent is approximately $155. 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.

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 three and six months ended June 30, 2019 were as follows:

Lease Cost	For the Three months ended June 30, 2019		For the Six months ended June 30, 2019
Fixed operating lease cost	$	198	$	385
Short-term lease costs		38		46
Total lease expense	$	236	$	431
Other information
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$	266	$	467
Leased assets obtained in exchange for new operating lease liabilities (1)			$	388
Weighted-average remaining lease term—operating leases			3.5 years
Weighted-average discount rate—operating leases				9.0	%

⁽¹⁾	Acquired in Merger with Arsanis

Maturities of lease liabilities due under these lease agreements as of June 30, 2019 are as follows:

Maturity of lease liabilities	Operating Leases
2019 (remainder of 2019)	$	551
2020		1,085
2021		1,098
2022		754
2023		263
Thereafter		—
Total lease payments		3,751
Less: interest		(522	)
Total operating lease liabilities as of June 30, 2019	$	3,229

The Company adopted ASU 2016-02 on January 1, 2019 as noted above, and as required, the following disclosure is provided for periods prior to adoption. Future annual minimum lease payments due under the Company’s operating leases as of December 31, 2018 were as follows:

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

Commitment and Contingencies

6 Months Ended

Jun. 30, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitment and Contingencies

9.	Commitment and Contingencies

Sponsored 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 over a three-year 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 three and six months ended June 30, 2019, the Company incurred $41 and $83, respectively, of research and development expenses related to its payment obligations to the university under the agreement. As of June 30, 2019, the Company had non-cancelable purchase commitments under this agreement totaling $139, with $83 committed in 2019 and $56 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 $1,000 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 June 30, 2019 or December 31, 2018.

License Agreements

In February 2017, Arsanis entered into an option and license agreement with Adimab, LLC (“Adimab”), under which the Company is obligated to make contingent and non-contingent payments should the Company exercise its option to obtain rights to certain RSV antibodies, which are being utilized in what the Company refers to as the “ASN500” program. The option fee includes an up-front payment of approximately $250 and potential milestone payments of up to approximately $25,000, 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 by the Company, its affiliates, licensees or sublicensees of products based on certain RSV antibodies during the applicable term for such product in that country. In July 2019, as further described in Note 16, the Company entered into an outlicensing arrangement (the “ASN500 Agreement”) with a third party whereby the Company transferred intellectual property related to the ASN500 program through an exclusive, worldwide license. In accordance with the ASN500 Agreement, the third party agreed to fund the exercise of the Adimab license agreement and is required to honor any future payments 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

6 Months Ended

Jun. 30, 2019

Derivative Instruments And Hedging Activities Disclosure [Abstract]

Preferred and Common Stock Warrants

10.	Preferred and Common Stock Warrants

Prior to the Merger with Arsanis (see Note 1), the Company had issued warrants for the purchase of its preferred stock and had classified its 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. 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 $5,235 at March 13, 2019, with $288 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 a public offering that closed on April 16, 2019, the Company issued 3,900,000 Class A warrants, which have an exercise price of $13.20 per warrant and are convertible into shares of the Company’s common stock. These warrants expire on April 15, 2024 and were immediately exercisable. In addition, the Company issued 2,130,000 pre- funded warrants for proceeds of $10.999 per share. Each of the pre-funded warrants is convertible into one common share, 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:

	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		$	1.94		4.23
Converted to warrants for the purchase of common stock and adjusted for the Exchange Ratio and Reverse Stock Split		(4,657,350	)
Issued		6,035,000			12.43
Exercised		(33,846	)		13.20
Cancelled		—			—
Outstanding and exercisable as of June 30, 2019		6,490,204		$	13.03		3.15

As of June 30, 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
		6,490,204

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

Warrant Name

Issuance Date

Number of

Shares of

Preferred

Stock Issuable

Exercise

Price

Exercisable

for

Classification

Expiration Date

Series A warrants

August 14, 2015

854,785

2.07

Series A

Liability

August 14, 2020

Series A warrants

August 21, 2015

732,453

2.07

Series A

Liability

August 21, 2020

SVB warrants

October 25, 2016

54,256

1.88

Series A

Liability

October 24, 2026

Series B warrants

November 1, 2017

1,374,435

1.88

Series B

Liability

October 31, 2020

Series B warrants

November 17, 2017

88,845

1.88

Series B

Liability

November 16, 2020

Series B warrants

December 4, 2017

59,576

1.88

Series B

Liability

December 3, 2020

Series B warrants

December 28, 2017

72,875

1.88

Series B

Liability

December 27, 2020

Series B warrants

December 28, 2017

1,219,815

1.88

Series B

Liability

December 28, 2027

Series B warrants

September 12, 2018

265,957

1.88

Series B

Liability

September 12, 2021

Series B warrants

September 12, 2018

212,765

1.88

Series B

Liability

September 12, 2028

Series B warrants

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

6 Months Ended

Jun. 30, 2019

Federal Home Loan Banks [Abstract]

Common Stock, Redeemable Common Stock, and Convertible Preferred Stock

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

Common Stock

As of June 30, 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, pre-funded 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 warrants and the pre-funded warrants were issued at a price of $10.999 per pre-funded warrant and accompanying Class A warrants. The Class A warrants have an exercise price of $13.20, will expire five years from the date of issuance, and are immediately exercisable with certain restrictions. The gross proceeds from the offering were $85.8 million before deducting underwriting discounts and offering expenses. The offering closed on April 16, 2019.

The Company evaluated the Class A and pre-funded 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 warrants nor pre-funded warrants meet the definition of a liability instrument. Neither the Class A warrants nor the pre-funded warrants are mandatorily redeemable and they do not embody an obligation for the Company to repurchase them in circumstances that are outside of the Company’s control that could require the transfer of assets. The funds received in the equity offering related to these warrants has been classified as permanent equity as the warrants are freestanding financial instruments that are legally detachable and separately exercisable from the common stock with which they were issued, are immediately exercisable, do not embody an obligation for the Company to repurchase its shares and permit the holders to receive a fixed number of common stock upon exercise. Furthermore, the delivery of common stock to the warrant holder upon exercise of the warrant is within the Company’s control.

Redeemable Common Stock

Pursuant to the requirements of the July 2014 license agreement with Genzyme (see Note 13), in August 2015, the Company issued to Genzyme for no additional consideration 107,371, as adjusted for the Reverse Stock Split and Exchange Ratio, 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 $0.01 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 June 30, 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 June 30, 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	$	1,444		143,630
Series A preferred stock		22,000,000		19,946,862		32,480		47,624		1,895,610
Series B preferred stock		25,100,000		18,616,569		30,885		34,999		1,769,190
		49,413,523		40,079,567	$	64,675	$	84,067		3,808,430

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

Stock-Based Compensation

6 Months Ended

Jun. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

12.	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 “2017 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.

As of June 30, 2019, 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. 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% 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.

2017 Employee Stock Purchase Plan

In 2017, the board of directors and shareholders of Arsanis adopted the 2017 ESPP. The 2017 ESPP 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 six months ended June 30, 2019, no shares of common stock were issued under the 2017 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). As of June 30, 2019, the total number of shares of common stock that may be issued under the 2019 Plan is 150,000 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.

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:

	Three Months Ended June 30,						Six Months Ended June 30,
	2019			2018			2019			2018
Risk-free interest rate		2.0	%		2.6	%		2.1	%		2.6	%
Expected term (in years)		6.05			5.67			6.00		5.74
Expected volatility		88.0	%		87.4	%		88.9	%		86.7	%
Expected dividend yield		0	%		0	%		0	%		0	%

Stock Options

The following table summarizes the Company’s stock option activity for the six months ended June 30, 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		377,906			16.21
Exercised		(17,183	)		6.68
Forfeited		(156,352	)		51.98
Outstanding as of June 30, 2019		1,273,532		$	21.97		7.65	$	7,168
Exercisable as of June 30, 2019		489,033		$	21.62	7.02
Vested and expected to vest as of June 30, 2019		1,097,545		$	17.69	8.34

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 six months ended June 30, 2019 was $130. The weighted average grant-date fair value per share of stock options granted during the six months ended June 30, 2019 was $11.92.

Restricted Stock Units

During the three months ended June 30, 2019, the Company granted 116,689 restricted stock units to employees. The restricted stock units vest 25% annually on the grant anniversary over four years and had a grant date fair value of $1,721, which will be recognized as stock-based compensation expense, net of estimated forfeitures, over the vesting period.

Stock-Based Compensation

Effective January 1, 2019, the Company adopted ASU 2018-07 and no longer remeasures the fair value of equity awards granted to non-employees at each reporting period end (see Note 2).

As of June 30, 2019, total unrecognized compensation expense related to unvested stock options and restricted stock units was $6,529, which is expected to be recognized over a weighted average period of 3.4 years.

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

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development expense	$	214	$	51	$	298	$	89
General and administrative expense		219		94		397		184
Total stock-based compensation		433		145		695		273

License, Collaboration, and Funding Agreements

6 Months Ended

Jun. 30, 2019

Organization Consolidation And Presentation Of Financial Statements Abstract

License, Collaboration, and Funding Agreements

13.	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 X4P-001) 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 X4P-001 to third parties.

In exchange for these rights, in August 2014, the Company made an upfront payment of $50 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 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 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 (see Note 11), each as required by the Genzyme Agreement. The $300 payment and the $734 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 common shares were exchanged for common shares 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 $25,000, contingent upon the achievement by the Company of certain late-stage regulatory and sales milestones with respect to licensed products. In addition, the Company may be required to make a one-time milestone payment to Genzyme upon the consummation by the Company of a change of control transaction, in an amount equal to 5.5% of the consideration paid to equity holders of the Company, other than Genzyme, in connection with such change of control transaction, after deducting outstanding debt obligations of the Company and the aggregate cash investments made by equity holders into the Company prior to the closing of the change of control transaction. The Merger with Arsanis qualifies as a change of control transaction, as defined in the license agreement, but resulted in no payment being due to Genzyme under the license agreement.

The Company concluded that this contingent payment obligation meets the definition of a derivative instrument as the contingent payment obligation is not clearly and closely related to its host instrument and is a cash-settled liability (see Note 2). Accordingly, the Company classifies this derivative as a liability within other liabilities (non-current) on its consolidated balance sheet (see Note 2), and changes in the fair value of the derivative liability are recognized as a component of other income (expense), net in the consolidated statement of operations and comprehensive loss (see Note 4). On March 13, 2019, the closing date of the Merger with Arsanis, this 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 and comprehensive loss because the contingent payment obligation expired at that time.

Under the Genzyme Agreement, the Company is 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 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. During the six months ended June 30, 2019, the Company did not enter into any third-party manufacturing or supply agreements in connection with the Genzyme Agreement. 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 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 six months ended June 30, 2019, 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 and the Company may be required to make milestone payments of up to an aggregate of $800 related to commercial sales of a product. 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.

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 agreement at any time upon at least 60 days’ written notice.

During the six months ended June 30, 2019, 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 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 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 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 six months ended June 30, 2019.

Research and Development Incentive Program

The Company 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 June 30, 2019, the amount due under the program is $1.7 million, which amounts were included in grant and incentive receivables in the consolidated balance sheet. During the three and six months ended June 30, 2019, the Company recorded $136 of income related to the program within the condensed consolidated statement of operations as “other income (expense)”.

Janssen License and Option Agreement

On December 12, 2018, Arsanis entered into a patent license and option agreement with Janssen Pharmaceuticals, Inc. (“Janssen”), (the “Janssen License and Option Agreement”). Pursuant to the Janssen License and Option Agreement, Arsanis granted to Janssen (i) a non-exclusive license to specified patents in Arsanis’s portfolio related to the ASN200 program, and (ii) an option for Janssen to acquire these patents in the future if specified conditions are met. Janssen agreed to pay Arsanis $3.5 million within 15 business days after the December 12, 2018 effective date of the Janssen License and Option Agreement, in addition to a future $0.5 million payment in the event Janssen exercises its option to acquire the relevant patents. Arsanis received the $3.5 million and recognized this amount as revenue. Such revenue is not reflected in the consolidated financial statements of the Company as it occurred prior to the Merger. Janssen’s option to the relevant patents will be recognized as revenue in full in the period in which the option exercise occurs.

Income Taxes

6 Months Ended

Jun. 30, 2019

Income Tax Disclosure [Abstract]

Income Taxes

14.	Income Taxes

The Company did not record a federal or state income tax benefit for its losses for the three and six months ended June 30, 2019 and 2018 due to the conclusion that a full valuation allowance is required against the Company’s deferred tax assets.

Net Loss per Share

6 Months Ended

Jun. 30, 2019

Earnings Per Share [Abstract]

Net Loss per Share

15.	Net Loss per Share

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

	Three Months Ended June 30,						Six Months Ended June 30,
	2019			2018			2019			2018
Numerator:
Net loss	$	(13,383	)	$	(6,034	)	$	(24,256	)	$	(13,401	)
Accruing dividends on Series A convertible preferred stock		—			(748	)		(592	)		(1,488	)
Adjustment to accumulated deficit in connection with repurchase of Series Seed convertible preferred stock		—			(22	)		—			(22	)
Net loss attributable to common stockholders	$	(13,383	)	$	(6,804	)	$	(24,848	)	$	(14,911	)
Denominator:
Weighted average common shares outstanding—basic and diluted		13,177,235			458,718			7,479,178			458,346
Net loss per share attributable to common stockholders— basic and diluted	$	(1.02	)	$	(14.83	)	$	(3.32	)	$	(32.53	)

The Company has included 107,371 shares of redeemable common stock in its computation of basic and diluted weighted average common shares outstanding for the six months ended June 30, 2019 and the three and six months ended June 30, 2018 as this class of stock participates in losses similarly to other common stockholders. Basic and diluted weighted average common shares outstanding for the three and six months ended June 30, 2019 also includes the weighted average effect of 2,130,000 pre-funded warrants for the purchase of common shares, which were issued in April 2019 and for which the remaining unfunded exercise price is less than $0.01 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 three and six months ended June 30, 2018 and included outstanding stock options and warrants to purchase common stock for the three and six months ended June 30, 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 common shares, 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:

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Options to purchase common stock		1,273,532		532,081		1,273,532		532,081
Convertible preferred stock (as converted to common stock)		—		3,556,147		—		3,556,147
Warrants to purchase common stock (excluding prefunded warrants, which are included in basic shares outstanding)		4,360,204		423,539		4,360,204		423,539
		5,633,736		4,511,767		5,633,736		4,511,767

Subsequent Events

6 Months Ended

Jun. 30, 2019

Subsequent Events [Abstract]

Subsequent Events

16.	Subsequent Events

On July 17, 2019, the Company announced that it has entered into an agreement with Abbisko Therapeutics (“Abbisko”) to develop and commercialize the Company’s product candidate, mavorixafor, in combination with checkpoint inhibitors or other agents in Greater China for oncology indications. The agreement provides Abbisko with the exclusive rights in China, Taiwan, Hong Kong and Macau to develop and commercialize mavorixafor in combination with checkpoint inhibitors or other agents in 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 and data generated pursuant to the Abbisko collaboration for rest-of-world development. The Company is assessing the financial impact of this arrangement and will account for this arrangement beginning in the third quarter of 2019.

In July 2019, the Company entered into an outlicensing arrangement with Evotec International GmbH (“Evotec”) whereby the Company transferred intellectual property and other rights related to its ASN500 program through an exclusive, worldwide license in return for an upfront fee, future payments based on achievement of clinical and regulatory milestones, and royalties based on net sales of the resulting licensed product. In addition, in July 2019, Bravos Bioscience LLC (“Bravos”), an entity that had entered into option agreements with Arsanis in 2018 to license certain intellectual property and other rights related to two preclinical stage programs that Arsanis referred to as the ASN200 and ASN300 programs, exercised amended versions of the option agreements. In accordance with the amended option agreements, Bravos obtained exclusive, sublicensable, rights to the ASN200 and ASN300 programs in return for an upfront fee and future payments to the Company, which are based on a percentage of future transaction fees, as defined, obtained by Bravos related to the programs. The Company is assessing the financial impact of these arrangements will account for these arrangements beginning in the third quarter of 2019.

Summary of Significant Accounting Policies (Policies)

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Unaudited Interim Financial Statements

Use of Estimates

Principles of Consolidation

Foreign Currency and Currency Translation

Concentrations of Credit Risk and Significant Suppliers

Cash and Cash Equivalents

Restricted Cash and Compensating Balance Arrangements

Restricted Cash and Compensating Balance Arrangements---

	As of June 30, 2019		As of December 31, 2018
Letter of credit security: Cambridge Lease (a)	$	264	$	264
Letter of credit security: Waltham Lease (b)		250		—
Letter of credit security: Vienna Austria Lease (c)		95		—
Corporate credit card collateral (d)		150		100
Compensating balance arrangement (e)		1,539
Total restricted cash (non-current)	$	2,298	$	364

(a)	In connection with the Company’s lease agreement for its facility in Cambridge, Massachusetts, the Company maintains a letter of credit of $264, which is secured by restricted cash, for the benefit of the landlord.

(b)	The Company maintains a letter of credit, which is secured by a restricted cash amount of $250 as of June 30, 2019, for the benefit of the landlord in connection with a leased facility in Waltham, Massachusetts.

(c)	In connection with the Company’s lease agreement for its laboratory and office facility in Vienna, Austria, the Company maintains a letter of credit of $95, which is secured by restricted cash, for the benefit of the landlord.

(d)	As of June 30, 2019 and December 31, 2018, the Company was required to maintain a separate cash balance of $150 and $100, respectively, to collateralize corporate credit cards with a bank.

(e)

	June 30, 2019		December 31, 2018		June 30, 2018		December 31, 2017
Cash and cash equivalents	$	90,180	$	8,134	$	12,566	$	26,684
Restricted cash, current portion		3,163		—		—		—
Restricted cash, non-current portion		2,298		364		364		364
Total cash, cash equivalents and restricted cash	$	95,641	$	8,498	$	12,930	$	27,048

Property and Equipment

		Estimated Useful Life
Office furniture		3 years
Computer equipment		3 years
Software		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 regarding the timing of and the expected costs to advance the Company’s clinical programs to commercialization;

•

Estimates of future cash flows from potential product sales; and

•

A discount rate.

Deferred Rent

Fair Value Measurements

•

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

•

Segment Information

Revenue Recognition

Research and Development Programs

Research and Development Costs

Upfront payments, milestone payments and annual maintenance fees under license agreements are expensed in the period in which they are incurred.

Patent Costs

Debt Issuance Costs

Stock-Based Compensation

Preferred Stock Warrant Liability

Derivative Liabilities

Comprehensive Loss

Income Taxes

Net Income (Loss) per Share

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Schedule of Compensating Balance Arrangements

	As of June 30, 2019		As of December 31, 2018
Letter of credit security: Cambridge Lease (a)	$	264	$	264
Letter of credit security: Waltham Lease (b)		250		—
Letter of credit security: Vienna Austria Lease (c)		95		—
Corporate credit card collateral (d)		150		100
Compensating balance arrangement (e)		1,539
Total restricted cash (non-current)	$	2,298	$	364

(a)	In connection with the Company’s lease agreement for its facility in Cambridge, Massachusetts, the Company maintains a letter of credit of $264, which is secured by restricted cash, for the benefit of the landlord.

(b)	The Company maintains a letter of credit, which is secured by a restricted cash amount of $250 as of June 30, 2019, for the benefit of the landlord in connection with a leased facility in Waltham, Massachusetts.

(c)	In connection with the Company’s lease agreement for its laboratory and office facility in Vienna, Austria, the Company maintains a letter of credit of $95, which is secured by restricted cash, for the benefit of the landlord.

(d)	As of June 30, 2019 and December 31, 2018, the Company was required to maintain a separate cash balance of $150 and $100, respectively, to collateralize corporate credit cards with a bank.

(e)

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

	June 30, 2019		December 31, 2018		June 30, 2018		December 31, 2017
Cash and cash equivalents	$	90,180	$	8,134	$	12,566	$	26,684
Restricted cash, current portion		3,163		—		—		—
Restricted cash, non-current portion		2,298		364		364		364
Total cash, cash equivalents and restricted cash	$	95,641	$	8,498	$	12,930	$	27,048

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
Software		3 years
Laboratory equipment		3 to 10 years
Leasehold improvements		Shorter of lease term or 10 years

Merger Accounting (Tables)

6 Months Ended

Jun. 30, 2019

Business Combinations [Abstract]

Schedule of Business Acquisitions by Acquisition, Contingent Consideration

The following summarizes the preliminary estimate of the purchase price paid in the Merger:

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.

Schedule of Recognized Identified Assets Acquired and Liabilities Assumed

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

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

Business Acquisition, Pro Forma Information

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

	Three Months Ended June 30,						Six Months Ended June 30,
							2019			2018
							(unaudited)
Revenue	$	—		$	—		$	—		$	—
Net loss	$	(12,904	)	$	(18,616	)	$	(29,162	)	$	(38,851	)

Fair Value of Financial Assets and Liabilities (Tables)

6 Months Ended

Jun. 30, 2019

Fair Value Disclosures [Abstract]

Schedule of Assets and Liabilities Measured at Fair Value

	Fair Value Measurements as of June 30, 2019 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	—	$	89,545	$	—	$	89,545
	$	—	$	89,545	$	—	$	89,545
Liabilities: None

	Fair Value Measurements as of December 31, 2018 Using:
	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

Summary of Aggregate Fair Values of Warrant Liability and Derivative Liability

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:

	Preferred Stock Warrant Liability			Derivative Liability
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 as of June 30, 2019	$	—		$	18

Property and Equipment, Net (Tables)

6 Months Ended

Jun. 30, 2019

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment, net consisted of the following:

	June 30, 2019			December 31, 2018
Leasehold improvements	$	336		$	299
Furniture and fixtures		105			53
Computer equipment		52			56
Software		9			9
Lab equipment		61			—
		563			417
Less: Accumulated depreciation and amortization		(305	)		(176	)
	$	258		$	241

Accrued Expenses (Tables)

6 Months Ended

Jun. 30, 2019

Text Block [Abstract]

Schedule of Accrued Expenses

Accrued expenses consisted of the following:

	June 30, 2019		December 31, 2018
Accrued employee compensation and benefits	$	1,374		924
Accrued external research and development expenses		2,561		754
Accrued professional fees		879		1,324
Other		280		249
	$	5,094	$	3,251

Long-Term Debt (Tables)

6 Months Ended

Jun. 30, 2019

Debt Disclosure [Abstract]

Summary of Long Term Debt

Long-term debt consisted of the following:

	June 30, 2019			December 31, 2018
Principal amount of long-term debt	$	26,717		$	10,000
Less: Current portion of long-term debt		(4,519	)		(1,687	)
Long-term debt, net of current portion		22,198			8,313
Debt discount, net of accretion		(672	)		(226	)
Cumulative accretion of final payment due at maturity		222			58
Long-term debt, including accretion, net of current portion	$	21,748		$	8,145

Schedule of Future Principal Payments and the Final Payments Due

As of June 30, 2019, future principal payments and the final payment due under the Company’s loan agreements were as follows:

Year Ending December 31,	Hercules		FFG		Total
2019	$	—	$	—	$	—
2020		—		4,519		4,519
2021		—		2,198		2,198
2022		11,897		—		11,897
2023		8,103		—		8,103
Long-term debt	$	20,000	$	6,717	$	26,717

Leases (Tables)

6 Months Ended

Jun. 30, 2019

Leases [Abstract]

Schedule of Components of Lease Expense

The components of lease expense for the three and six months ended June 30, 2019 were as follows:

Lease Cost	For the Three months ended June 30, 2019		For the Six months ended June 30, 2019
Fixed operating lease cost	$	198	$	385
Short-term lease costs		38		46
Total lease expense	$	236	$	431
Other information
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$	266	$	467
Leased assets obtained in exchange for new operating lease liabilities (1)			$	388
Weighted-average remaining lease term—operating leases			3.5 years
Weighted-average discount rate—operating leases				9.0	%

⁽¹⁾	Acquired in Merger with Arsanis

Schedule of Maturities of Lease Liabilities

Maturities of lease liabilities due under these lease agreements as of June 30, 2019 are as follows:

Maturity of lease liabilities	Operating Leases
2019 (remainder of 2019)	$	551
2020		1,085
2021		1,098
2022		754
2023		263
Thereafter		—
Total lease payments		3,751
Less: interest		(522	)
Total operating lease liabilities as of June 30, 2019	$	3,229

Summary of Future Minimum Lease Payments Due under Operating Leases

Future annual minimum lease payments due under the Company’s operating leases as of December 31, 2018 were as follows:

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

Preferred and Common Stock Warrants (Tables)

6 Months Ended

Jun. 30, 2019

Derivative Instruments And Hedging Activities Disclosure [Abstract]

Schedule of Outstanding Warrants

The following table provides a roll forward of outstanding warrants:

	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		$	1.94		4.23
Converted to warrants for the purchase of common stock and adjusted for the Exchange Ratio and Reverse Stock Split		(4,657,350	)
Issued		6,035,000			12.43
Exercised		(33,846	)		13.20
Cancelled		—			—
Outstanding and exercisable as of June 30, 2019		6,490,204		$	13.03		3.15

Schedule of Warrant to Purchase Redeemable Convertible Preferred Stock

As of June 30, 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
		6,490,204

December 31, 2018

Warrant Name

Issuance Date

Number of

Shares of

Preferred

Stock Issuable

Exercise

Price

Exercisable

for

Classification

Expiration Date

Series A warrants

August 14, 2015

854,785

2.07

Series A

Liability

August 14, 2020

Series A warrants

August 21, 2015

732,453

2.07

Series A

Liability

August 21, 2020

SVB warrants

October 25, 2016

54,256

1.88

Series A

Liability

October 24, 2026

Series B warrants

November 1, 2017

1,374,435

1.88

Series B

Liability

October 31, 2020

Series B warrants

November 17, 2017

88,845

1.88

Series B

Liability

November 16, 2020

Series B warrants

December 4, 2017

59,576

1.88

Series B

Liability

December 3, 2020

Series B warrants

December 28, 2017

72,875

1.88

Series B

Liability

December 27, 2020

Series B warrants

December 28, 2017

1,219,815

1.88

Series B

Liability

December 28, 2027

Series B warrants

September 12, 2018

265,957

1.88

Series B

Liability

September 12, 2021

Series B warrants

September 12, 2018

212,765

1.88

Series B

Liability

September 12, 2028

Series B warrants

October 19, 2018

210,638

1.88

Series B

Liability

October 19, 2028

5,146,400

- Definition

Tabular disclosure of warrants or rights issued. Warrants and rights outstanding are derivative securities that give the holder the right to purchase securities (usually equity) from the issuer at a specific price within a certain time frame. Warrants are often included in a new debt issue to entice investors by a higher return potential. The main difference between warrants and call options is that warrants are issued and guaranteed by the company, whereas options are exchange instruments and are not issued by the company. Also, the lifetime of a warrant is often measured in years, while the lifetime of a typical option is measured in months. Disclose the title of issue of securities called for by warrants and rights outstanding, the aggregate amount of securities called for by warrants and rights outstanding, the date from which the warrants or rights are exercisable, and the price at which the warrant or right is exercisable.

+ References + Details

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

6 Months Ended

Jun. 30, 2019

Federal Home Loan Banks [Abstract]

Preferred Stock

As of June 30, 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	$	1,444		143,630
Series A preferred stock		22,000,000		19,946,862		32,480		47,624		1,895,610
Series B preferred stock		25,100,000		18,616,569		30,885		34,999		1,769,190
		49,413,523		40,079,567	$	64,675	$	84,067		3,808,430

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

Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock Option Valuation

	Three Months Ended June 30,						Six Months Ended June 30,
	2019			2018			2019			2018
Risk-free interest rate		2.0	%		2.6	%		2.1	%		2.6	%
Expected term (in years)		6.05			5.67			6.00		5.74
Expected volatility		88.0	%		87.4	%		88.9	%		86.7	%
Expected dividend yield		0	%		0	%		0	%		0	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the six months ended June 30, 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		377,906			16.21
Exercised		(17,183	)		6.68
Forfeited		(156,352	)		51.98
Outstanding as of June 30, 2019		1,273,532		$	21.97		7.65	$	7,168
Exercisable as of June 30, 2019		489,033		$	21.62	7.02
Vested and expected to vest as of June 30, 2019		1,097,545		$	17.69	8.34

Summary of Stock-Based Compensation Expense Classification

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

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development expense	$	214	$	51	$	298	$	89
General and administrative expense		219		94		397		184
Total stock-based compensation		433		145		695		273

Net Loss per Share (Tables)

6 Months Ended

Jun. 30, 2019

Earnings Per Share [Abstract]

Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders

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

	Three Months Ended June 30,						Six Months Ended June 30,
	2019			2018			2019			2018
Numerator:
Net loss	$	(13,383	)	$	(6,034	)	$	(24,256	)	$	(13,401	)
Accruing dividends on Series A convertible preferred stock		—			(748	)		(592	)		(1,488	)
Adjustment to accumulated deficit in connection with repurchase of Series Seed convertible preferred stock		—			(22	)		—			(22	)
Net loss attributable to common stockholders	$	(13,383	)	$	(6,804	)	$	(24,848	)	$	(14,911	)
Denominator:
Weighted average common shares outstanding—basic and diluted		13,177,235			458,718			7,479,178			458,346
Net loss per share attributable to common stockholders— basic and diluted	$	(1.02	)	$	(14.83	)	$	(3.32	)	$	(32.53	)

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

The Company excluded the following potential common shares, 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:

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Options to purchase common stock		1,273,532		532,081		1,273,532		532,081
Convertible preferred stock (as converted to common stock)		—		3,556,147		—		3,556,147
Warrants to purchase common stock (excluding prefunded warrants, which are included in basic shares outstanding)		4,360,204		423,539		4,360,204		423,539
		5,633,736		4,511,767		5,633,736		4,511,767

Nature of the Business and Basis of Presentation - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2019	Mar. 13, 2019
Schedule Of Accounting Policies [Line Items]
Percentage of common stock owned		63.70%
Reverse stock split, description	1-for-6
Arsanis [Member]
Schedule Of Accounting Policies [Line Items]
Business acquisition share exchange ratio	0.5702

Summary of Significant Accounting Policies - Schedule of Compensating Balance Arrangements (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Total restricted cash (non-current)	$ 2,298	$ 364	$ 364	$ 364
Corporate Credit Card Collateral [Member]
Total restricted cash (non-current)	150	100
Compensating Balance Arrangement [Member]
Total restricted cash (non-current)	1,539
Letter of Credit [Member] \| Cambridge Lease Agreement [Member]
Total restricted cash (non-current)	264	$ 264
Letter of Credit [Member] \| Waltham Lease [Member]
Total restricted cash (non-current)	250
Letter of Credit [Member] \| Vienna Austria Lease [Member]
Total restricted cash (non-current)	$ 95

Summary of Significant Accounting Policies - Schedule of Compensating Balance Arrangements (Detail) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 08, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Total restricted cash (non-current)	$ 2,298		$ 364	$ 364	$ 364
Restricted Cash, Current	$ 3,163
FFG Loan Agreement [Member]
Percentage of minimum cash balance		70.00%
Percentage of non-current portion of loan	70.00%
Percentage of current portion of loan	70.00%
Restricted Cash, Current	$ 3,163
Corporate Credit Card Collateral [Member]
Total restricted cash (non-current)	150		100
Letter of Credit [Member] \| Cambridge Lease Agreement [Member]
Total restricted cash (non-current)	264		$ 264
Letter of Credit [Member] \| Waltham Lease [Member]
Total restricted cash (non-current)	250
Letter of Credit [Member] \| Vienna Austria Lease [Member]
Total restricted cash (non-current)	95
Letter of Credit [Member] \| Vienna Austria Lease [Member] \| Laboratory And Office Facility Leases [Member]
Total restricted cash (non-current)	$ 95

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Mar. 13, 2019	Jan. 01, 2019
Restricted Cash And Cash Equivalents Items [Line Items]
Minimum liquidity as a percentage of debt	125.00%	125.00%
Maximum liquidity other than cash and cash equivalents	$ 2,500	$ 2,500
Debt instrument covenant liquidity in cash	25,000	25,000
Fair value of derivative liability	0	0	$ 5,235
Comprehensive loss included of foreign currency translation gain adjustments	(65)	(42)
Operating lease liabilities	3,229	3,229
Right-of-use assets	2,187	2,187
Accounting Standards Update 2016-02 [Member]
Restricted Cash And Cash Equivalents Items [Line Items]
Operating lease liabilities	2,500	2,500		$ 2,538
Right-of-use assets	$ 2,000	$ 2,000		$ 2,026

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 90,180	$ 8,134	$ 12,566	$ 26,684
Restricted Cash, Current	3,163
Restricted cash, non-current portion	2,298	364	364	364
Total cash, cash equivalents and restricted cash	$ 95,641	$ 8,498	$ 12,930	$ 27,048

Summary of Significant Accounting Policies - Schedule of Property and Equipment (Detail)	6 Months Ended
	Jun. 30, 2019
Furniture and Fixtures [Member]
Estimated useful life	3 years
Computer Equipment [Member]
Estimated useful life	3 years
Software Development [Member]
Estimated useful life	3 years
Laboratory Equipment [Member] \| Minimum [Member]
Estimated useful life	3 years
Laboratory Equipment [Member] \| Maximum [Member]
Estimated useful life	10 years
Leasehold Improvements [Member]
Estimated useful life	10 years
Estimated useful life	Shorter of lease term or 10 years

Merger Accounting - Additional Information (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($) Program shares	Jun. 30, 2018 USD ($)	Mar. 13, 2019
Business Acquisition [Line Items]
Business acquisition, equity interest			63.70%
Goodwill	$ 27,109
Goodwill, Purchase Accounting Adjustments	298
Business combination, acquisition related costs	$ 1,000
Number of development programs acquired \| Program	3
Weighted average discount rate	19.50%
Pro Forma [Member]
Business Acquisition [Line Items]
Business combination, acquisition related costs	$ 2,663	$ 2,663
Arsanis Security Holders [Member]
Business Acquisition [Line Items]
Business acquisition, equity interest	31.30%
XFour Security Holders [Member]
Business Acquisition [Line Items]
Business acquisition, equity interest	68.70%
Arsanis [Member]
Business Acquisition [Line Items]
Business acquisition, date of acquisition	Mar. 13, 2019
Business acquisition share exchange ratio	0.5702
Business combination options to purchase of common stock \| shares	271,230
Goodwill	$ 27,109

Merger Accounting - Schedule of Business Acquisitions by Acquisition, Contingent Consideration (Detail) - USD ($)
$ / shares in Units, $ in Thousands

3 Months Ended

6 Months Ended

Mar. 31, 2019

Jun. 30, 2019

Acquisition Date [Line Items]

Fair value of consideration issued it effect the Merger

$ 45,541

Arsanis [Member]

Acquisition Date [Line Items]

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.

Merger Accounting - Schedule of Business Acquisitions by Acquisition, Contingent Consideration (Parenthetical) (Detail)	6 Months Ended
	Jun. 30, 2019 shares
Acquisition Date [Line Items]
Stock issued during period, shares, reverse stock splits	2,440,582
Arsanis [Member]
Acquisition Date [Line Items]
Stock issued during period, shares, reverse stock splits	14,643,737

Merger Accounting - Schedule of Recognized Identified Assets Acquired and Liabilities Assumed (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($)
Acquisition Date [Line Items]
Goodwill	$ 27,109
Arsanis [Member]
Acquisition Date [Line Items]
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

Merger Accounting - Business Acquisition, Pro Forma Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Business Combinations [Abstract]
Net loss	$ (12,904)	$ (18,616)	$ (29,162)	$ (38,851)

Fair Value of Financial Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Cash equivalents	$ 89,545	$ 8,134
Liabilities:
Derivative liability		5,148
Preferred Stock Warrant Liability [Member]
Liabilities:
Derivative liability		4,947
Derivative Liability [Member]
Liabilities:
Derivative liability		201
Money Market Funds [Member]
Assets:
Cash equivalents	89,545	8,134
Level 2 [Member]
Assets:
Cash equivalents	89,545	8,134
Level 2 [Member] \| Money Market Funds [Member]
Assets:
Cash equivalents	$ 89,545	8,134
Level 3 [Member]
Liabilities:
Derivative liability		5,148
Level 3 [Member] \| Preferred Stock Warrant Liability [Member]
Liabilities:
Derivative liability		4,947
Level 3 [Member] \| Derivative Liability [Member]
Liabilities:
Derivative liability		$ 201

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Transfers between fair value levels	$ 0	$ 0
Genzyme Agreement [Member]
Fair value of derivative liability	$ 0	$ 183
Measurement Input, Expected Dividend Rate [Member]
Expected dividend yield	0.00%
Arsanis [Member] \| Genzyme Agreement [Member]
Fair value of derivative liability	$ 0
Series A Convertible Preferred Stock [Member] \| Arsanis And Merger Sub [Member]
Preferred stock Fair value per share		$ 1.70
Series B Convertible Preferred Stock [Member] \| Arsanis And Merger Sub [Member]
Preferred stock Fair value per share		$ 1.86
Convertible Preferred Stock [Member]
Number of warrants for purchase of convertible preferred shares	0

Fair Value of Financial Assets and Liabilities - Summary of Aggregate Fair Values of Warrant Liability and Derivative Liability (Detail) - Level 3 [Member] $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($)
Preferred Stock Warrant Liability [Member]
Fair Value Assets Measured On Recurring And Nonrecurring Basis [Line Items]
Balance as of December 31, 2018	$ 4,947
Change in fair value	288
Conversion of convertible preferred stock warrant into common stock warrant in connection with Merger	(5,235)
Derivative Liability [Member]
Fair Value Assets Measured On Recurring And Nonrecurring Basis [Line Items]
Balance as of December 31, 2018	201
Change in fair value	(183)
Balance as of June 30, 2019	$ 18

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	$ 563	$ 417
Less: Accumulated depreciation and amortization	(305)	(176)
Property and Equipment, Net	258	241
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	336	299
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	105	53
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	52	56
Software [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	9	$ 9
Lab Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	$ 61

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 44	$ 51

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued employee compensation and benefits	$ 1,374	$ 924
Accrued external research and development expenses	2,561	754
Accrued professional fees	879	1,324
Other	280	249
Total accrued expenses	$ 5,094	$ 3,251

Long-Term Debt - Summary of Long Term Debt (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Long Term Debt [Abstract]
Principal amount of long-term debt	$ 26,717	$ 10,000
Less: Current portion of long-term debt	(4,519)	(1,687)
Long-term debt, net of current portion	22,198	8,313
Debt discount, net of accretion	(672)	(226)
Cumulative accretion of final payment due at maturity	222	58
Long-term debt, including accretion, net of current portion	$ 21,748	$ 8,145

Long Term Debt - SVB Loan Agreement - Additional Information (Detail) - SVB Loan Agreement [Member] - USD ($) $ in Thousands	1 Months Ended		6 Months Ended
	Oct. 31, 2018	Oct. 31, 2016	Jun. 30, 2019	Oct. 19, 2018	Jun. 30, 2017
Schedule Of Long Term Debt [Line Items]
Debt instrument variable percentage		5.50%
Debt instrument description			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.
Repayment of outstanding loan principal	$ 4,333
Repayments portion of total borrowing	270
Debt instrument prepayment premium	87
Debt instrument accrued interest	$ 23
Debt instrument stated percentage				10.75%
Minimum [Member]
Schedule Of Long Term Debt [Line Items]
Debt instrument variable percentage		3.50%
Term Loan A [Member]
Schedule Of Long Term Debt [Line Items]
Maximum term loan borrowing capacity					$ 6,000

Long Term Debt - Hercules Loan Agreement - Additional Information (Detail) $ / shares in Units, $ in Thousands				1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 18, 2019 $ / shares shares	Mar. 13, 2019 USD ($)	Dec. 31, 2018 USD ($)	Oct. 31, 2018 USD ($) $ / shares shares	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 27, 2019 USD ($)	Dec. 11, 2018 USD ($)	Oct. 19, 2018 USD ($)
Schedule Of Long Term Debt [Line Items]
Fair value of derivative liability		$ 5,235			$ 0		$ 0
Minimum liquidity as a percentage of debt					125.00%		125.00%
Maximum liquidity other than cash and cash equivalents					$ 2,500		$ 2,500
Unamortized debt discount			$ 226		672		$ 672
Arsanis [Member]
Schedule Of Long Term Debt [Line Items]
Business acquisition share exchange ratio							0.5702
Hercules Loan Agreement [Member]
Schedule Of Long Term Debt [Line Items]
Line of credit facility, maximum borrowing capacity				$ 13,000
Proceeds from lines of credit				$ 8,000
Fair value of derivative liability		$ 326	282							$ 132	$ 132
Interest Expense, Debt					382	$ 0	$ 739	$ 0
Amount of interest expense including accretion of debt discount.					101		215
Unamortized debt discount					$ 104		$ 104
Effective interest rate of loan					11.00%		11.00%
Principal payments							$ 0
Hercules Loan Agreement [Member] \| Arsanis [Member]
Schedule Of Long Term Debt [Line Items]
Warrant exercise price \| $ / shares	$ 19.80			$ 99
Number of warrants for purchase of convertible preferred shares \| shares	5,000
Business acquisition share exchange ratio	0.5702
Hercules Loan Agreement [Member] \| Series B Redeemable Convertible Preferred Stock [Member]
Schedule Of Long Term Debt [Line Items]
Warrant issued to purchase stock \| shares				210,638
Warrant exercise price \| $ / shares				$ 1.88
Class of warrant expiration period				2028-10
Hercules Loan Agreement [Member] \| Common Stock [Member]
Schedule Of Long Term Debt [Line Items]
Warrant exercise price \| $ / shares				$ 19.78
Number of warrants for purchase of convertible preferred shares \| shares				20,016
Hercules Loan Agreement [Member] \| Additional Term Loan One [Member]
Schedule Of Long Term Debt [Line Items]
Line of credit facility, maximum borrowing capacity				$ 2,000
Proceeds from lines of credit			$ 2,000	2,000
Line of credit facility, expiration date			Nov. 01, 2021
Hercules Loan Agreement [Member] \| Additional Term Loan Two [Member]
Schedule Of Long Term Debt [Line Items]
Line of credit facility, maximum borrowing capacity				3,000
Line of credit facility, expiration date		Mar. 01, 2019
Hercules Loan Agreement [Member] \| Term Loan [Member]
Schedule Of Long Term Debt [Line Items]
Line of credit facility, maximum borrowing capacity				$ 8,000
June 2019 Loan Agreement [Member]
Schedule Of Long Term Debt [Line Items]
Line of credit facility, maximum borrowing capacity					$ 35,000		$ 35,000
Line of credit facility, expiration date							Jul. 01, 2023
Line of credit facility, outstanding					$ 10,000		$ 10,000
Line of credit facility, current borrowing capacity									$ 20,000
Line of credit interest rate, Description							Borrowings under the June 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 June 2019 Loan Agreement, and until such event is no longer continuing, the interest rate applicable to borrowings under the June 2019 Loan Agreement would be increased by 4.0%
Long-term Debt, Maturities repayment terms							Borrowings under the June 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 June 2019 Loan Agreement provides for payments by the Company to Hercules of (i) $795,000 payable upon the earlier of November 1, 2021 or the repayment in full of all obligations under the June 2019 Loan Agreement, and (ii) 4.0% of the aggregate principal drawn under the June 2019 Loan Agreement payable upon the earlier of maturity or the repayment in full of all obligations under such agreement
Debt instrument variable percentage							8.75%
Interest rate increase percentage							4.00%
Line of credit facility periodic payment							$ 795
Line of credit principal amount drawn percentage							4.00%
Minimum liquidity as a percentage of debt					125.00%		125.00%
Minimum liquidity as a percentage of debt					100.00%		100.00%
Maximum liquidity other than cash and cash equivalents					$ 2,500		$ 2,500
Fair value of derivative liability					$ 18		18
Upfront payments							180
Amortization of debt discount							$ 180
June 2019 Loan Agreement [Member] \| First Tranche [Member]
Schedule Of Long Term Debt [Line Items]
Term loan repayment period							Jan. 01, 2022
June 2019 Loan Agreement [Member] \| Second Tranche [Member]
Schedule Of Long Term Debt [Line Items]
Term loan repayment period							Feb. 01, 2022
June 2019 Loan Agreement [Member] \| Prime Rate [Member]
Schedule Of Long Term Debt [Line Items]
Line of credit facility interest rate							6.00%
June 2019 Loan Agreement [Member] \| Minimum [Member]
Schedule Of Long Term Debt [Line Items]
Debt instrument variable percentage							8.75%
June 2019 Loan Agreement [Member] \| Maximum [Member]
Schedule Of Long Term Debt [Line Items]
Debt instrument pre payment premium percentage					2.00%		2.00%
June 2019 Loan Agreement [Member] \| Additional Term Loan One [Member]
Schedule Of Long Term Debt [Line Items]
Line of credit facility, additional borrowing capacity					$ 5,000		$ 5,000
June 2019 Loan Agreement [Member] \| Additional Term Loan Two [Member]
Schedule Of Long Term Debt [Line Items]
Line of credit facility, additional borrowing capacity					$ 10,000		$ 10,000
June 2019 Loan Agreement [Member] \| New Borrowings [Member]
Schedule Of Long Term Debt [Line Items]
Line of credit facility, current borrowing capacity									10,000
June 2019 Loan Agreement [Member] \| Previous Agreement [Member]
Schedule Of Long Term Debt [Line Items]
Line of credit facility, current borrowing capacity									$ 10,000

Long Term Debt - FFG Loan Agreement - Additional Information (Detail) - FFG Loan Agreement [Member] - USD ($) $ in Thousands		6 Months Ended
	Mar. 08, 2019	Jun. 30, 2019
Schedule Of Long Term Debt [Line Items]
Debt instrument, maturity start date		Mar. 31, 2019
Debt instrument, maturity end date		Mar. 31, 2021
Repayment of outstanding loan principal		$ 2,914
Number of years accelerated payment	3 years	5 years
Percentage of minimum cash balance	70.00%
Minimum [Member]
Schedule Of Long Term Debt [Line Items]
Debt instrument, interest rate		0.75%
Maximum [Member]
Schedule Of Long Term Debt [Line Items]
Debt instrument, interest rate		2.00%

Long-Term Debt - Schedule of Future Principal Payments and the Final Payments Due (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Maturities [Line Items]
2019	$ 0
2020	4,519
2021	2,198
2022	11,897
2023	8,103
Principal amount of long-term debt	26,717	$ 10,000
Hercules Loan Agreement [Member]
Debt Maturities [Line Items]
2019	0
2022	11,897
2023	8,103
Principal amount of long-term debt	20,000
FFG Loan Agreement [Member]
Debt Maturities [Line Items]
2019	0
2020	4,519
2021	2,198
Principal amount of long-term debt	$ 6,717

Leases - Additional Information (Detail) $ in Thousands			6 Months Ended
Leases - Additional Information (Detail) $ in Thousands	Mar. 13, 2019 USD ($) ft² m²	Aug. 31, 2017 USD ($) ft²	Jun. 30, 2019 USD ($)	Jan. 01, 2019 USD ($)
Lessee Lease Description [Line Items]
Right-of-use assets			$ 2,187
Operating lease liabilities			3,229
Lease liability, net of current portion			2,369
Current portion of lease liability			$ 860
Cambridge Lease Agreement [Member]
Lessee Lease Description [Line Items]
Lease expiration date		Jul. 31, 2022
Current office space under lease agreement \| ft²		13,000
Option to extend the lease agreement			The Company has the option to extend the lease for one period of five additional years.
Lease, renewal term of contract		5 years
Base rent		$ 816
Waltham Lease [Member]
Lessee Lease Description [Line Items]
Current office space under lease agreement \| ft²	6,000
Base rent	$ 262
Agreement commencement date	Jan. 01, 2019
Operating lease expiration	5 years
Vienna Lease [Member]
Lessee Lease Description [Line Items]
Current office space under lease agreement \| m²	400
Base rent	$ 155
Agreement commencement date	Mar. 01, 2019
Lease, term of contract	2 years
Period for written notice for cancellable	3 months 19 days
Accounting Standards Update 2016-02 [Member]
Lessee Lease Description [Line Items]
Right-of-use assets			$ 2,000	$ 2,026
Operating lease liabilities			$ 2,500	2,538
Lease liability, net of current portion				1,925
Current portion of lease liability				613
Net deferred, accrued and prepaid rent				$ 512

Leases - Schedule of Components of Lease Expense (Detail)
$ in Thousands

3 Months Ended

6 Months Ended

Jun. 30, 2019

USD ($)

Jun. 30, 2019

USD ($)

Leases [Abstract]

Fixed operating lease cost

$ 198

$ 385

Short-term lease costs

Total lease expense

236

431

Operating cash flows from operating leases

$ 266

467

Leased assets obtained in exchange for new operating lease liabilities

$ 388

^[1]

Weighted-average remaining lease term-operating leases

3 years 6 months

Weighted-average discount rate-operating leases

9.00%

[1]	Acquired in Merger with Arsanis

Leases - Schedule of Maturities of Lease Liabilities (Detail) $ in Thousands	Jun. 30, 2019 USD ($)
Leases [Abstract]
2019 (remainder of 2019)	$ 551
2020	1,085
2021	1,098
2022	754
2023	263
Thereafter	0
Total lease payments	3,751
Less: interest	(522)
Total operating lease liabilities as of June 30, 2019	$ 3,229

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

Commitment and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended			3 Months Ended		6 Months Ended
	May 31, 2019	Apr. 30, 2017	Feb. 28, 2017	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Contingencies And Commitments [Line Items]
Research and development expense				$ 8,854,000	$ 4,755,000	$ 14,509,000	$ 9,499,000
Sponsored Research Agreement [Member]
Contingencies And Commitments [Line Items]
Purchase commitment, amount		$ 499,000
Purchase commitment, period		3 years
Research and development expense				41,000		83,000
Non-cancelable purchase commitments amount				139,000		139,000
Non-cancelable purchase commitments amount, 2019				83,000		83,000
Non-cancelable purchase commitments amount, 2020				56,000		56,000
Termination notice period	30 days
Maximum termination fees	$ 1,000,000
Indemnification Agreements [Member]
Contingencies And Commitments [Line Items]
Accrued liabilities				$ 0		$ 0		$ 0
Adimab Option and License Agreement [Member]
Contingencies And Commitments [Line Items]
Potential milestone payments			$ 25,000,000
Option fee including up front payment			$ 250,000

Preferred and Common Stock Warrants - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended	6 Months Ended
	Apr. 16, 2019	Mar. 31, 2019	Jun. 30, 2019	Apr. 12, 2019	Mar. 13, 2019
Fair value of warrant purchase			$ 0		$ 5,235
Gain (loss) on change in fair value of warrant		$ 288
Number of warrants, Issued			6,035,000
Class A Warrant [Member]
Number of warrants, Issued	3,900,000
Warrant exercise price	$ 13.20			$ 13.20
Warrants expiration date	Apr. 15, 2024
Pre Funded Warrant [Member]
Number of warrants, Issued	2,130,000
Warrant exercise price	$ 0.001			$ 10.999
Conversion of warrant, description	Each of the pre-funded warrants is convertible into one common share, has a remaining exercise price of $0.001 per share and was immediately exercisable upon issuance.

Preferred and Common Stock Warrants - Schedule of Outstanding Warrants (Detail) - $ / shares	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Number of warrants, Outstanding and exercisable warrants, Beginning Balance	5,146,400
Number of warrants, Converted to warrants for the purchase of common stock and adjusted for the Exchange Ratio and Reverse Stock Split	(4,657,350)
Number of warrants, Issued	6,035,000
Number of warrants, Exercised	(33,846)
Number of warrants, Outstanding and exercisable warrants, Ending Balance	6,490,204	5,146,400
Weighted Average Exercise Price, Outstanding and exercisable warrants, Beginning Balance	$ 1.94
Weighted Average Exercise Price, Issued	12.43
Weighted Average Exercise Price, Exercised	13.20
Weighted Average Exercise Price, Outstanding and exercisable warrants, Ending Balance	$ 13.03	$ 1.94
Weighted Average Contractual Term, Outstanding and exercisable warrants	3 years 1 month 24 days	4 years 2 months 23 days

Preferred and Common Stock Warrants - Summary of Outstanding Warrants to Purchase Shares of Common Stock (Detail) - $ / shares	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Class of Warrant or Right [Line Items]
Number of Shares of common Stock Issuable	6,490,204	5,146,400
Issuance On August 14, 2015 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Aug. 14, 2015
Number of Shares of common Stock Issuable	81,228
Exercise Price	$ 21.78
Expiration Date	Aug. 14, 2020
Issuance On August 14, 2015 [Member] \| Series A Preferred Stock Warrants [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Aug. 14, 2015
Number of Shares of common Stock Issuable		854,785
Exercise Price		$ 2.07
Expiration Date		Aug. 14, 2020
Issuance On August 21, 2015 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Aug. 21, 2015
Number of Shares of common Stock Issuable	69,603
Exercise Price	$ 21.78
Expiration Date	Aug. 21, 2020
Issuance On August 21, 2015 [Member] \| Series A Preferred Stock Warrants [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Aug. 21, 2015
Number of Shares of common Stock Issuable		732,453
Exercise Price		$ 2.07
Expiration Date		Aug. 21, 2020
Issuance On October 25, 2016 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Oct. 25, 2016	Oct. 25, 2016
Number of Shares of common Stock Issuable	5,155	54,256
Exercise Price	$ 19.78	$ 1.88
Expiration Date	Oct. 24, 2026	Oct. 24, 2026
Issuance On November 1, 2017 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Nov. 01, 2017
Number of Shares of common Stock Issuable	130,609
Exercise Price	$ 19.78
Expiration Date	Oct. 31, 2020
Issuance On November 1, 2017 [Member] \| Series B Preferred Stock Warrants [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Nov. 01, 2017
Number of Shares of common Stock Issuable		1,374,435
Exercise Price		$ 1.88
Expiration Date		Oct. 31, 2020
Issuance On November 17, 2017 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Nov. 17, 2017
Number of Shares of common Stock Issuable	8,442
Exercise Price	$ 19.78
Expiration Date	Nov. 16, 2020
Issuance On November 17, 2017 [Member] \| Series B Preferred Stock Warrants [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Nov. 17, 2017
Number of Shares of common Stock Issuable		88,845
Exercise Price		$ 1.88
Expiration Date		Nov. 16, 2020
Issuance On December 4, 2017 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Dec. 04, 2017
Number of Shares of common Stock Issuable	5,661
Exercise Price	$ 19.78
Expiration Date	Dec. 03, 2020
Issuance On December 4, 2017 [Member] \| Series B Preferred Stock Warrants [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Dec. 04, 2017
Number of Shares of common Stock Issuable		59,576
Exercise Price		$ 1.88
Expiration Date		Dec. 03, 2020
Issuance On December 28, 2017 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Dec. 28, 2017
Number of Shares of common Stock Issuable	6,925
Exercise Price	$ 19.78
Expiration Date	Dec. 27, 2020
Issuance On December 28, 2017 [Member] \| Series B Preferred Stock Warrants [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Dec. 28, 2017
Number of Shares of common Stock Issuable		72,875
Exercise Price		$ 1.88
Expiration Date		Dec. 27, 2020
Issuance On December 28, 2017 One [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Dec. 28, 2017
Number of Shares of common Stock Issuable	115,916
Exercise Price	$ 19.78
Expiration Date	Dec. 28, 2027
Issuance On December 28, 2017 One [Member] \| Series B Preferred Stock Warrants [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Dec. 28, 2017
Number of Shares of common Stock Issuable		1,219,815
Exercise Price		$ 1.88
Expiration Date		Dec. 28, 2027
Issuance On September 12, 2018 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Sep. 12, 2018
Number of Shares of common Stock Issuable	25,275
Exercise Price	$ 19.78
Expiration Date	Sep. 12, 2021
Issuance On September 12, 2018 [Member] \| Series B Preferred Stock Warrants [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Sep. 12, 2018
Number of Shares of common Stock Issuable		265,957
Exercise Price		$ 1.88
Expiration Date		Sep. 12, 2021
Issuance On September 12, 2018 One [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Sep. 12, 2018
Number of Shares of common Stock Issuable	20,220
Exercise Price	$ 19.78
Expiration Date	Sep. 12, 2028
Issuance On September 12, 2018 One [Member] \| Series B Preferred Stock Warrants [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Sep. 12, 2018
Number of Shares of common Stock Issuable		212,765
Exercise Price		$ 1.88
Expiration Date		Sep. 12, 2028
Issuance On October 19, 2018 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Oct. 19, 2018
Number of Shares of common Stock Issuable	20,016
Exercise Price	$ 19.78
Expiration Date	Oct. 19, 2028
Issuance On October 19, 2018 [Member] \| Series B Preferred Stock Warrants [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Oct. 19, 2018
Number of Shares of common Stock Issuable		210,638
Exercise Price		$ 1.88
Expiration Date		Oct. 19, 2028
Issuance On March 13, 2019 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Mar. 13, 2019
Number of Shares of common Stock Issuable	5,000
Exercise Price	$ 19.80
Expiration Date	Mar. 12, 2029
Issuance On April 16, 2019 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Apr. 16, 2019
Number of Shares of common Stock Issuable	3,866,154
Exercise Price	$ 13.20
Expiration Date	Apr. 15, 2024
Issuance On April 16, 2019 One [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Apr. 16, 2019
Number of Shares of common Stock Issuable	2,130,000
Exercise Price	$ 11.00

Common Stock, Redeemable Common Stock, and Convertible Preferred Stock - Additional Information (Detail) - USD ($)		1 Months Ended		6 Months Ended	12 Months Ended
	Apr. 12, 2019	Aug. 31, 2015	Aug. 31, 2014	Jun. 30, 2019	Dec. 31, 2018	Apr. 16, 2019
Common stock, authorized shares				33,333,333	11,070,776
Common stock, par value				$ 0.001	$ 0.001
Common stock, voting right				Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders.
Dividends on common stock declared or paid				$ 0	$ 0
Stock issued during period, shares, new issues	5,670,000
Sale of stock, price per share	$ 11.00
Proceeds from issuance common stock before underwriters discounts and commissions and offering expenses	$ 85,800,000
Convertible preferred stock, shares authorized					49,413,523
Preferred stock outstanding				$ 0
Series Seed Preferred Stock [Member]
Convertible preferred stock, shares authorized				10,000,000	59,413,523
Convertible preferred stock, par or stated value per share				$ 0.001	$ 0.001
Genzyme Agreement [Member]
Stock issued during period, shares, new issues			107,371
Sale of stock, price per share			$ 0.01
Stock issued during period		107,371
Common stock aggregate fair value		$ 734,000
Common stock repurchase price per share		$ 0.01
Pre Funded Warrant [Member]
Class of warrant or right, warrants to purchase of common stock	2,130,000
Warrant exercise price	$ 10.999					$ 0.001
Class A Warrant [Member]
Class of warrant or right, warrants to purchase of common stock	3,900,000
Warrant exercise price	$ 13.20					$ 13.20
Class of warrants or rights expiration period	5 years

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

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	6 Months Ended
	Jan. 31, 2017	Jun. 30, 2019	Jun. 30, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation, description			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.
Vesting rights, descriptions			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.
Number of share options exercisable, aggregate intrinsic Value		$ 130	$ 130
Options granted, weighted average grant date fair value			$ 11.92
Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation, restricted stock units granted		116,689
Restricted stock units vested, percentage		25.00%
Shares based compensation, vested period		4 years
Share based compensation, grant date fair value		$ 1,721
Unrecognized compensation cost of stock based awards		$ 6,529	$ 6,529
Unrecognized compensation cost of stock based awards, recognition period			3 years 4 months 24 days
2017 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock issued			0
2015 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Ordinary share issued		969,340	969,340
2017 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Ordinary share issued		170,915	170,915
Common stock reserved for future issuance, description	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% 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.
Percentage of number of common stock outstanding			4.00%
2019 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Ordinary share issued		150,000	150,000

Stock-Based Compensation - Schedule of Assumptions Used in Black-Scholes Option-Pricing Model to Determine Grant-date Fair Value of Stock Options Granted (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Sharebased Compensation Arrangement By Sharebased Payment Award Stock Options Shares Outstanding Weighted Average Exercise Price And Additional Disclosures [Abstract]
Risk-free interest rate	2.00%	2.60%	2.10%	2.60%
Expected term (in years)	6 years 18 days	5 years 8 months 1 day	6 years	5 years 8 months 26 days
Expected volatility	88.00%	87.40%	88.90%	86.70%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of shares outstanding, Beginning balance	797,931
Number of shares, Assumed as part of merger with Arsanis	271,230
Number of shares, Granted	377,906
Number of shares, Exercised	(17,183)
Number of shares, Forfeited	(156,352)
Number of shares Outstanding, Ending balance	1,273,532	797,931
Number of shares Options, Exercisable	489,033
Number of shares Options, Vested and expected to vest	1,097,545
Weighted average exercise price, Beginning balance	$ 8.29
Weighted average exercise price, Assumed as part of merger with Arsanis	62.60
Weighted average exercise price, Granted	16.21
Weighted average exercise price, Exercised	6.68
Weighted average exercise price, Forfeited	51.98
Weighted average exercise price, Ending balance	21.97	$ 8.29
Weighted average exercise price, Exercisable	21.62
Weighted average exercise price, Vested and expected to vest	$ 17.69
Weighted average contractual term outstanding	7 years 7 months 24 days	8 years 5 months 1 day
Weighted average contractual term outstanding, Exercisable	7 years 7 days
Weighted average contractual term outstanding, Vested and expected to vest	8 years 4 months 2 days
Aggregate intrinsic value, Beginning balance	$ 6,486
Aggregate intrinsic value, Ending balance	$ 7,168	$ 6,486

Stock-Based Compensation - Summary of Stock-Based Compensation Expense Classification (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 433	$ 145	$ 695	$ 273
Research and Development Expense [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	214	51	298	89
General and Administrative Expense [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 219	$ 94	$ 397	$ 184

License, Collaboration, and Funding Agreements - Additional Information (Detail) - USD ($)		1 Months Ended		3 Months Ended	6 Months Ended		12 Months Ended
	Apr. 12, 2019	Dec. 31, 2016	Aug. 31, 2014	Jun. 30, 2019	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Collaboration License And Funding Arrangements [Line Items]
Stock issued during period, shares, new issues	5,670,000
Reverse stock split, description					1-for-6
Shares issued price per share	$ 11.00
Other income				$ 149,000	$ 149,000
AUSTRIA [Member]
Collaboration License And Funding Arrangements [Line Items]
Grant income					14.00%
Genzyme Agreement [Member]
Collaboration License And Funding Arrangements [Line Items]
Upfront payments			$ 50,000
Cash payment			$ 300,000
Stock issued during period, shares, new issues			107,371
Reverse stock split, description			adjusted for the 1 for 6 Reverse Stock Split and Exchange Ratio
Research and Development Expense			$ 734,000
Shares issued price per share			$ 0.01
Future milestone payments						$ 25,000,000
Consideration paid to equity shareholders						5.50%
Derivative Liability				0	$ 0		$ 183,000
License agreement termination description					The 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.
Payment on obligation under license agreement					$ 0
Georgetown Agreement [Member]
Collaboration License And Funding Arrangements [Line Items]
License agreement termination description					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 agreement at any time upon at least 60 days’ written notice.
Payment on obligation under license agreement					$ 0
One time upfront payment		$ 50,000
Milestone payments		800,000
Milestone payment obligation				0	$ 0
BIDMC Agreement [Member]
Collaboration License And Funding Arrangements [Line Items]
License agreement termination description					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 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 agreement at any time upon at least 90 days’ written notice.
Payment on obligation under license agreement					$ 0
One time upfront payment		$ 20,000
Research and Development Incentive [Member]
Collaboration License And Funding Arrangements [Line Items]
Grant receivable				1,700,000	1,700,000
Other income				$ 136,000	$ 136,000
Janssen License and Option Agreement [Member] \| Janssen Pharmaceuticals Incorporation [Member]
Collaboration License And Funding Arrangements [Line Items]
Payables from related parties							3,500,000
Payment received from related parties							$ 3,500,000
License and options agreement description							Pursuant to the Janssen License and Option Agreement, Arsanis granted to Janssen (i) a non-exclusive license to specified patents in Arsanis’s portfolio related to the ASN200 program, and (ii) an option for Janssen to acquire these patents in the future if specified conditions are met. Janssen agreed to pay Arsanis $3.5 million within 15 business days after the December 12, 2018 effective date of the Janssen License and Option Agreement, in addition to a future $0.5 million payment in the event Janssen exercises its option to acquire the relevant patents. Arsanis received the $3.5 million and recognized this amount as revenue.
Janssen License and Option Agreement [Member] \| Janssen Pharmaceuticals Incorporation [Member] \| Patents [Member]
Collaboration License And Funding Arrangements [Line Items]
Option to acquire the relevant patents							$ 500,000

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Federal income tax benefit	$ 0	$ 0	$ 0	$ 0
State income tax benefit	$ 0	$ 0	$ 0	$ 0

Net Loss per Share - Summary of Basic and Diluted Net loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net loss	$ (13,383)	$ (10,873)	$ (6,034)	$ (7,367)	$ (24,256)	$ (13,401)
Accruing dividends on Series A convertible preferred stock			(748)		(592)	(1,488)
Adjustment to accumulated deficit in connection with repurchase of Series Seed convertible preferred stock			(22)	$ (22)		(22)
Net loss attributable to common stockholders	$ (13,383)		$ (6,804)		$ (24,848)	$ (14,911)
Weighted average common shares outstanding—basic and diluted	13,177,235		458,718		7,479,178	458,346
Net loss per share attributable to common stockholders—basic and diluted	$ (1.02)		$ (14.83)		$ (3.32)	$ (32.53)

- 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 (Detail) - $ / shares	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Apr. 16, 2019	Apr. 12, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted Average Common Shares Basic and Diluted	13,177,235	458,718	7,479,178	458,346
Pre Funded Warrant [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted Average Common Shares Basic and Diluted	2,130,000		2,130,000
Exercise Price					$ 0.001	$ 10.999
Pre Funded Warrant [Member] \| Maximum [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Exercise Price	$ 0.01		$ 0.01
Redeemable Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted Average Common Shares Basic and Diluted		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 (Detail) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	5,633,736	4,511,767	5,633,736	4,511,767
Employee Stock Option [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	1,273,532	532,081	1,273,532	532,081
Warrant [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	4,360,204	423,539	4,360,204	423,539
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share		3,556,147		3,556,147

Subsequent Events - Additional Information (Detail)	1 Months Ended
Subsequent Events - Additional Information (Detail)	Jul. 31, 2019 Program
Subsequent Event [Member]
Subsequent Event [Line Items]
Number of additional programs under preclinical stages	2