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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2018	Mar. 01, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	ASNS
Entity Registrant Name	ARSANIS, INC.
Entity Central Index Key	0001501697
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		14,643,737
Entity Public Float			$ 27,036,480

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 30,754	$ 76,793
Grant and incentive receivables	2,859	1,608
Restricted cash	101	0
Prepaid expenses and other current assets	1,366	1,129
Total current assets	35,080	79,530
Property and equipment, net	285	421
Restricted cash	539	355
Other assets	100	948
Total assets	36,004	81,254
Current liabilities:
Accounts payable	545	1,893
Accrued expenses	4,116	5,779
Unearned income	737	694
Loans payable, net of discount		2,314
Total current liabilities	5,398	10,680
Loan payable, net of discount and current portion	7,894	9,922
Unearned income	1,107	1,936
Other long-term liabilities	5	9
Total liabilities	14,404	22,547
Commitments and contingencies
Stockholders’ equity:
Common stock	15	15
Additional paid-in capital	156,630	150,830
Accumulated other comprehensive income	241	127
Accumulated deficit	(135,286)	(92,265)
Total stockholders' equity	21,600	58,707
Total liabilities and stockholders’ equity	$ 36,004	$ 81,254

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	14,572,246	14,294,383
Common stock, shares outstanding	14,572,246	14,294,383

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue:
Revenue	$ 3,500
Type of Revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Operating expenses:
Research and development	$ 30,979	$ 28,128	$ 17,831
General and administrative	18,406	8,005	6,515
Total operating expenses	49,385	36,133	24,346
Loss from operations	(45,885)	(36,133)	(24,346)
Other income (expense):
Grant and incentive income	3,179	3,868	2,390
Interest expense	(1,039)	(2,079)	(2,515)
Interest income	809	214
Change in fair value of warrant liability		(31)	39
Change in fair value of derivative liability		762	1,388
Loss on extinguishment of debt		(462)	(35)
Other income (expense), net	(85)	(16)	104
Total other income (expense), net	2,864	2,256	1,371
Net loss	(43,021)	(33,877)	(22,975)
Accretion of redeemable convertible preferred stock to redemption value		(44)	(25)
Net loss attributable to common stockholders	$ (43,021)	$ (33,921)	$ (23,000)
Net loss per share attributable to common stockholders—basic and diluted	$ (3.01)	$ (16.45)	$ (44.79)
Weighted average common shares outstanding—basic and diluted	14,307,934	2,061,845	513,527

CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (43,021)	$ (33,877)	$ (22,975)
Other comprehensive loss:
Foreign currency translation gain (loss)	114	(707)	116
Comprehensive loss	$ (42,907)	$ (34,584)	$ (22,859)

- Definition

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

+ References + Details

CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Initial Public Offering	Private Placement	Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock Initial Public Offering	Redeemable Convertible Preferred Stock Series C Preferred Stock	Redeemable Convertible Preferred Stock Series D Convertible Redeemable Preferred Stock	Common Stock	Common Stock Initial Public Offering	Common Stock Private Placement	Additional Paid-in Capital	Additional Paid-in Capital Initial Public Offering	Additional Paid-in Capital Private Placement	Accumulated Other Comprehensive Income	Accumulated Deficit
Beginning balance at Dec. 31, 2015	$ (34,322)							$ 1			$ 372			$ 718	$ (35,413)
Beginning balance (in shares) at Dec. 31, 2015								513,900
Beginning balance at Dec. 31, 2015				$ 29,948
Beginning balance (in shares) at Dec. 31, 2015				5,076,970
Issuance of stock, net of issuance costs						$ 5,413
Issuance of stock, net of issuance costs (in shares)						569,946
Conversion of convertible preferred stock into common stock upon closing of initial public offering						$ 4,452
Conversion of convertible preferred stock into common stock upon closing of initial public offering (in shares)						461,396
Accretion of redeemable convertible preferred stock to redemption value	(25)			$ 25							(25)
Stock-based compensation expense	644										644
Foreign currency translation adjustment	116													116
Net loss	(22,975)														(22,975)
Ending balance at Dec. 31, 2016	(56,562)							$ 1			991			834	(58,388)
Ending balance (in shares) at Dec. 31, 2016								513,900
Ending balance at Dec. 31, 2016				$ 39,838
Ending balance (in shares) at Dec. 31, 2016				6,108,312
Issuance of stock, net of issuance costs							$ 39,845
Issuance of stock, net of issuance costs (in shares)							12,340,380		4,600,000	2,000,000
Conversion of convertible preferred stock into common stock upon closing of initial public offering		$ 90,829			$ (90,829)		$ 11,102		$ 7			$ 90,822
Conversion of convertible preferred stock into common stock upon closing of initial public offering (in shares)					(21,869,096)		3,420,404		7,180,483
Issuance of stock, net of issuance costs		$ 39,484	$ 18,600						$ 5	$ 2		$ 39,479	$ 18,598
Accretion of redeemable convertible preferred stock to redemption value	(44)			$ 44							(44)
Conversion of convertible preferred stock warrants into common stock warrants	78										78
Stock-based compensation expense	906										906
Foreign currency translation adjustment	(707)													(707)
Net loss	(33,877)														(33,877)
Ending balance at Dec. 31, 2017	$ 58,707							$ 15			150,830			127	(92,265)
Ending balance (in shares) at Dec. 31, 2017	14,294,383							14,294,383
Exercise of common stock options	$ 177										177
Exercise of common stock options (in shares)	27,226							27,226
Issuance of common stock under employee stock purchase plan	$ 3										3
Issuance of common stock under employee stock purchase plan (in shares)								637
Issuance of restricted common stock, (in Share)								250,000
Stock-based compensation expense	5,620										5,620
Foreign currency translation adjustment	114													114
Net loss	(43,021)														(43,021)
Ending balance at Dec. 31, 2018	$ 21,600							$ 15			$ 156,630			$ 241	$ (135,286)
Ending balance (in shares) at Dec. 31, 2018	14,572,246							14,572,246
Ending balance (in shares) at Dec. 31, 2018	0

CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS EQUITY (DEFICIT) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Initial Public Offering
Issuance costs	$ 6,516
Private Placement
Issuance costs	1,400
Series C Preferred Stock
Issuance costs		$ 87
Series D Convertible Redeemable Preferred Stock
Issuance costs	$ 208

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (43,021)	$ (33,877)	$ (22,975)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	5,620	906	644
Depreciation and amortization expense	155	195	285
Non-cash interest expense	804	1,773	2,307
Non-cash rent expense		(23)	9
Loss on extinguishment of debt		462	35
Change in fair value of warrant liability		31	(39)
Change in fair value of derivative liability		(762)	(1,388)
Changes in operating assets and liabilities:
Grant and incentive receivables	(1,374)	(64)	152
Prepaid expenses and other assets	612	284	(2,219)
Accounts payable	(1,339)	182	1,264
Accrued expenses	(1,677)	3,290	521
Unearned income	(683)	(268)	(235)
Net cash used in operating activities	(40,903)	(27,871)	(21,639)
Cash flows from investing activities:
Purchases of property and equipment	(34)	(42)	(73)
Net cash used in investing activities	(34)	(42)	(73)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock		40,053	5,500
Proceeds from issuance of loans payable			7,000
Proceeds from issuance of convertible promissory notes		4,935	5,500
Proceeds from issuance of loans under funding agreements		694	514
Proceeds from issuance of common stock in initial public offering		46,000
Proceeds from issuance of common stock in private placement		20,000
Proceeds from exercise of stock options	179
Repayments of loans payable	(4,667)	(2,333)	(250)
Payments of issuance costs of convertible promissory notes		(17)
Payments of issuance costs of redeemable convertible preferred stock		(197)	(87)
Payments of issuance costs of loans payable			(30)
Net cash provided by (used in) financing activities	(4,531)	101,262	18,147
Effect of exchange rate changes on cash	(286)	370	(103)
Net increase (decrease) in cash, cash equivalents and restricted cash	(45,754)	73,719	(3,668)
Cash, cash equivalents and restricted cash at beginning of period	77,148	3,429	7,097
Cash, cash equivalents and restricted cash at end of period	31,394	77,148	3,429
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses			2
Initial public offering costs included in accounts payable and accrued expenses		43
Issuance of redeemable convertible preferred stock upon extinguishment of convertible promissory notes		11,102	4,452
Derivative liability in connection with issuance of convertible promissory notes		403	3,929
Extinguishment of convertible promissory notes		8,405	2,677
Extinguishment of derivative liability in connection with extinguishment of convertible promissory notes		2,234	1,741
Issuance of warrants in connection with issuance of loans payable			60
Conversion of redeemable convertible preferred stock warrants into common stock warrants		78
Conversion of redeemable convertible preferred stock into common stock		90,829
Accretion of redeemable convertible preferred stock to redemption value		44	$ 25
Initial Public Offering
Cash flows from financing activities:
Payments of stock issuance cost	$ (43)	(6,473)
Private Placement
Cash flows from financing activities:
Payments of stock issuance cost		$ (1,400)

Nature of the Business

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Nature of the Business

1.	Nature of the Business

Arsanis, Inc. (the “Company”) was incorporated under the laws of the State of Delaware on August 2, 2010, and is headquartered in Waltham, Massachusetts. The Company is a clinical-stage biopharmaceutical company that has historically focused on applying monoclonal antibody, or mAb, immunotherapies to address serious infectious diseases. The Company possesses a deep understanding of the pathogenesis of infection, paired with access to what the Company believes to be some of the most advanced mAb discovery techniques and platforms available today. The Company’s pipeline is comprised of mAbs targeting multiple serious bacterial and viral pathogens, including Staphylococcus aureus (“S. aureus”) and respiratory syncytial virus (“RSV”).

On June 28, 2018, the Company announced the discontinuation of its Phase 2 clinical trial of ASN100 for the prevention of S. aureus pneumonia in high-risk, mechanically ventilated patients following the completion of a planned interim analysis of unblinded trial data for 118 patients by an independent data review committee, or DRC. Based on the results of this analysis, the DRC determined that the trial was futile, meaning that it was not likely to meet its primary end-point upon completion, and recommended that trial enrollment be discontinued. During the third quarter of 2018, the Company completed follow-up visits on patients dosed in the trial per the study protocol, and during the fourth quarter of 2018, the Company completed its analysis of the complete dataset from the 154 patients that were enrolled in the trial. The Company is currently exploring potential collaborations or out-licensing opportunities for the potential continued development of ASN100.

Following its discontinuation of ASN100 clinical development, in August 2018 the Company announced that it was considering strategic options that may potentially result in changes to its business strategy and future operations and the Company’s board of directors approved a reduction in workforce to reduce operating costs and better align its workforce with the needs of its business following its discontinuation of the clinical development of ASN100. As part of the planned reduction in workforce, the Company eliminated 28 positions across the company, representing approximately 65% of its workforce, through March 1, 2019.

On November 26, 2018, the Company and X4 Pharmaceuticals, Inc. (“X4”) have entered into an Agreement and Plan of Merger, as amended (the “Merger Agreement”), pursuant to which a wholly owned subsidiary of the Company will merge with and into X4, with X4 continuing as a wholly owned subsidiary of the Company and the surviving corporation of the merger (the “Merger”). The Company expects to devote significant time and resources to completion of the Merger. However, there can be no assurance that such activities will result in the completion of the Merger. Further, the completion of the Merger ultimately may not deliver the anticipated benefits or enhance shareholder value.

The Company has devoted substantially all of its resources to building its business to support discovery, research and development activities for its programs. The Company does not have any products approved for sale and has not generated any revenue from product sales.

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 government regulations and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including 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 product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales.

Reverse Stock Split

On November 3, 2017, the Company effected a one-for-3.4130 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s redeemable convertible preferred stock. Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the preferred stock conversion ratios.

Initial Public Offering

On November 20, 2017, the Company completed an initial public offering (“IPO”) of its common stock, and issued and sold 4,000,000 common shares at a price to the public of $10.00 per share. Concurrent to the IPO, (i) the Company issued an additional 600,000 common shares at a price of $10.00 per share pursuant to the exercise of the underwriters’ over-allotment option and (ii) New Enterprise Associates 16, L.P., or NEA, purchased 2,000,000 shares of the Company’s common stock at the initial per share public offering price of $10.00 in a private placement. The aggregate net proceeds to the Company from the IPO, inclusive of the over-allotment exercise, and the private placement were $58.1 million after deducting underwriting discounts and commissions and offering expenses payable by the Company. Upon the closing of the IPO, all of the outstanding redeemable convertible preferred stock of the Company automatically converted into 7,180,483 shares of the Company’s common stock.

Going Concern

In accordance with Accounting Standards Update (“ASU”) 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 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 the consolidated financial statements are issued. As of December 31, 2018, the Company had an accumulated deficit of $135.3 million. During the year ended December 31, 2018, the Company incurred a net loss of $43.0 million and used $40.9 million of cash in operations. The Company expects to continue to generate operating losses for the foreseeable future. Based on its current operating plan, the Company expects that its cash and cash equivalents of $30.8 million as of December 31, 2018, will be sufficient to fund its operating expenses, capital expenditure requirements and debt service payments for at least 12 months from the issuance date 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. The Company will seek additional funding through public or private financings, debt financing, license agreements or government grants. Although the Company has been successful in raising capital in the past, there is no assurance that it will be successful in obtaining such additional financing on terms acceptable to the Company, if at all. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

Basis of Presentation

The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standard Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”).

Use of Estimates

The preparation of 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 financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, the accrual for research and development expenses and the valuation of common stock and stock options. Estimates are periodically reviewed in light of 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 consolidated financial statements include the accounts of Arsanis, Inc. and its wholly owned subsidiary, Arsanis Biosciences GmbH, which was incorporated in Vienna, Austria. All significant intercompany accounts and transactions have been eliminated.

Foreign Currency and Currency Translation

The functional currency for Arsanis Biosciences GmbH, is the Euro. Assets and liabilities of Arsanis Biosciences GmbH are translated into United States dollars at the exchange rate in effect on the balance sheet date. Income items 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 statements of redeemable convertible preferred stock and stockholders’ equity (deficit) as a component of accumulated other comprehensive income. Adjustments that arise from exchange rate changes on transactions denominated in a currency other than the local currency are included in other income (expense), net in the consolidated statements of operations as incurred.

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with original final maturities of 90 days or less from the date of purchase to be cash equivalents. The Company’s cash equivalents consist of money market funds. Cash equivalents are reported at fair value.

Restricted Cash

The Company maintains letters of credit for the benefit of the landlords in connection with the Company’s office, laboratory, parking and storage space leases in Waltham, MA and Vienna, Austria and another letter of credit in connection with the Company’s corporate credit cards. As of December 31, 2018, short term restricted cash consisted of a letter of credit related to the Company’s lease of its corporate facility at 890 Winter Street, Waltham, MA and a letter of credit related to the Company’s corporate credit cards of approximately $51,000 and $50,000, respectively. The Company had no short term restricted cash as of December 31, 2017.

As of December 31, 2018, long term restricted cash consisted of letters of credit related to the Company’s leases of office, laboratory, parking and storage space lease in Vienna, Austria, which expires on April 30, 2021, and its new corporate headquarters at 950 Winter Street, Waltham, MA, which expires on December 31, 2023, of approximately $0.3 million and $0.3 million, respectively. As of December 31, 2017, long term restricted cash consisted of a letter of credit related to the Company’s lease on prior corporate headquarters, which expired on January 31, 2019, of approximately $0.4 million.

Concentrations of Credit Risk

Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. Periodically, the Company maintains deposits in accredited financial institutions in excess of federally insured limits. The Company deposits its cash in financial institutions that it believes have high credit quality and has not experienced any losses on such accounts and does not believe it is exposed to any unusual credit risk beyond the normal credit risk associated with commercial banking relationships.

Property and Equipment

Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the estimated useful lives of the assets. As of December 31, 2018 and 2017, the Company’s property and equipment consisted of laboratory and office equipment, computer equipment and software, furniture and fixtures and leasehold improvements. Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the estimated useful lives of the assets as follows:

		Estimated Useful Life
Laboratory and office equipment		3 to 10 years
Furniture and fixtures		3 to 10 years
Computer equipment and software		1 to 5 years
Leasehold improvements		Shorter of lease term or 10 years

Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred.

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 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, determined based on discounted cash flows. To date, the Company has not recognized any impairment losses on long-lived assets.

Fair Value Measurements

Certain assets 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.

The carrying values of cash equivalents, other current assets, accounts payable, and accrued expenses approximate their fair values due to the short-term nature of these assets and liabilities. The carrying value of the Company’s loans received under the funding agreements with Österreichische Forschungsförderungsgesellschaft mbH (“FFG”) approximates their fair value because the Company records imputed interest expense based on rates that approximate market rates of interest as of the issuance date of each FFG loan.

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 singular current focus is on applying mAb therapies to address serious infectious diseases.

Revenue Recognition

Effective January 1, 2018, the Company adopted Accounting Standards Codification (“ASC”), Topic 606, Revenue from Contracts with Customers (“Topic 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 did not have any arrangements that were in the scope of ASC 606 on January 1, 2018 and thus there was no impact to the consolidated financial statements as a result of the adoption. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as collaboration arrangements and leases.

Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration to which it is entitled in exchange for the goods or services it transfers to the customer.

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

The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, an entity is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct.

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

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

If an arrangement includes development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received.

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

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

License Revenue

The Company enters into out-licensing agreements that are within the scope of Topic 606. The terms of such out-license agreements include the license of functional intellectual property, given the functionality of the intellectual property is not expected to change substantially as a result of the licensor’s ongoing activities, and typically include payment of one or more of the following: non-refundable up-front license fees and exclusivity fees related to customer options. Nonrefundable up-front license fees are recognized as revenue at a point in time when the licensed intellectual property is made available for the customer’s use and benefit, which is generally at the inception of the arrangement. Exclusivity fees are recognized as revenue when the performance obligation to which the customer option has been allocated has been satisfied.

Government Contracts, Grant Agreements and Incentive Programs

The Company recognizes proceeds received from the FFG grants, research and development incentives from the Austrian government and the grant agreements with the Bill & Melinda Gates Foundation (the “Gates Foundation”) as other income, rather than as revenue, in the consolidated statements of operations because the corresponding agreements contain no specified performance obligations other than to conduct research on a particular program or in a particular field and contain no obligations to deliver specified products or technology.

Income from grants and incentives is recognized in the period during which the related qualifying expenses are incurred, provided that the conditions under which the grants or incentives were provided have been met. For grants under the funding agreements with FFG and for proceeds under the research and development incentive program from the Austrian government, the Company recognizes grant and incentive income in an amount equal to the qualifying expenses incurred in each period multiplied by the applicable reimbursement percentage. For grants received under the grant agreements with the Gates Foundation, the Company recognizes grant income in an amount equal to the qualifying expenses incurred in each period, up to the amount previously funded by the Gates Foundation.

Grant funding that has been received by the Company in advance of incurring qualifying expenses is recorded in the consolidated balance sheet as unearned income. Grant and incentive income recognized upon incurring qualifying expenses in advance of receipt of grant funding or proceeds from research and development incentives is recorded in the consolidated balance sheet as grant and incentive receivables.

Loans the Company has received under the funding agreements with FFG bear interest at rates that are below market rates of interest. The Company accounts for the imputed benefit arising from the difference between a market rate of interest and the rate of interest charged by FFG as additional grant funding from FFG, and records interest expense for the FFG loans at a market rate of interest. On the date that FFG loan proceeds are received, the Company recognizes the portion of the loan proceeds allocated to grant funding as a discount to the carrying value of the loan and as unearned income, which is subsequently recognized as additional grant income over the term of the funding agreement.

Research and Development Costs

The Company expenses all costs incurred in performing research and development activities. Research and development expenses consist of costs incurred in performing research and development activities, including salaries, stock-based compensation and benefits, facilities costs, depreciation, third-party license fees, and external costs of outside vendors engaged to conduct clinical development activities and clinical trials as well as to manufacture clinical trial materials. Non-refundable prepayments for goods or services that will be used or rendered for future research and development activities are recorded as prepaid expenses. Such amounts are recognized as an expense as the goods are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered or the services rendered.

Research Contract Costs and Accruals

The Company has entered into various research and development-related contracts with companies both inside and outside of the United States. These agreements are cancelable, and related costs are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or clinical trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.

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.

Stock-Based Compensation

The Company measures 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 over the requisite service period, which is generally the vesting period of the respective award. Forfeitures are accounted for as they occur. Generally, 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.

For stock-based awards granted to consultants and non-employees, compensation expense is recognized over the period during which services are rendered by such consultants and non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is 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 fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option pricing model. Prior to November 20, 2017, the Company had been a private company and lacked company-specific historical and implied volatility information for its stock. Therefore, it estimates its expected stock price volatility based on the historical volatility of publicly traded peer companies and expects to continue to do so until such time as 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-employees 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 is based on 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.

Comprehensive Loss

Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the year ended December 31, 2018, comprehensive loss included $0.1 million of foreign currency translation gain adjustments. For the years ended December 31, 2017 and 2016, comprehensive loss included $0.7 million of foreign currency translation loss adjustments and $0.1 million of foreign currency translation gain 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 on the basis of the differences 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

The Company follows the two-class method when computing net income (loss) per share as the Company has issued shares during the years ended December 31, 2017 and 2016 that meet 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. 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 shares of common stock outstanding for the period, including potential dilutive common shares. For purpose of this calculation, options to purchase common stock, unvested restricted stock, redeemable convertible preferred stock, warrants to purchase common stock and warrants to purchase redeemable convertible preferred stock are considered potential dilutive common shares.

The Company’s redeemable convertible preferred stock contractually entitled the holders of such shares to participate in dividends but contractually did not 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, 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.

Recently Adopted Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”), which supersedes most existing revenue recognition guidance under GAAP. The FASB also issued several amendments and updates to the new revenue standard (collectively, “Topic 606”). The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those goods or services. The standard defines a five-step process to achieve this principle, and will require companies to use more judgment and make more estimates than under the current guidance. These judgments and estimates include identifying performance obligations in the customer contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts. In March 2016, the FASB issued ASU No. 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (“ASU 2016-08”), which further clarifies the implementation guidance on principal versus agent considerations in ASU 2014-09. In April 2016, the FASB issued ASU No. 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing, clarifying the implementation guidance on identifying performance obligations and licensing. Specifically, the amendments in this update reduce the cost and complexity of identifying promised goods or services and improve the guidance for determining whether promises are separately identifiable. The amendments in this update also provide implementation guidance on determining whether an entity’s promise to grant a license provides a customer with either a right to use the entity’s intellectual property (which is satisfied at a point in time) or a right to access the entity’s intellectual property (which is satisfied over time). In May 2016, the FASB issued ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients (“ASU 2016-12”), which clarifies the objective of the collectability criterion, presentation of taxes collected from customers, non-cash consideration, contract modifications at transition, completed contracts at transition and how guidance in ASU 2014-09 is retrospectively applied. ASU 2016-08, ASU 2016-10 and ASU 2016-12 have the same effective dates and transition requirements as ASU 2014-09. The Company adopted this guidance, effective January 1, 2018, and its adoption had no impact on the Company’s financial position, results of operations or cash flows as the Company did not have any arrangements that were in the scope of ASC 606 on January 1, 2018.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), 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, respectively. 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 will be accounted for similar to existing guidance for operating leases today. ASU 2016-02 (Accounting Standards Codification (“ASC”) (Topic 842) supersedes the previous leases standard, ASC 840, Leases. The standard is effective for public entities for annual periods beginning after December 15, 2018 including interim periods within those fiscal years. In July 2018, the FASB issued ASU 2018-11, Leases, Targeted Improvements, (“ASU 2018-11”), which contains certain amendments to ASU 2016-02 intended to provide relief in implementing the new standard. ASU 2018-11 provides registrants with an option to use either (1) its effective date or (2) the beginning of the earliest comparative period presented in the financial statements as its date of initial application. The Company adopted the new leasing standard on 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 Company is in the process of completing its review of its existing lease agreements under ASC 842 and does not expect the adoption of the new leasing standard to have a material impact on its financial position, results of operations or cash flows.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”), to address diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. The Company adopted the standard retrospectively to all periods presented on the required effective date of January 1, 2018, and its adoption had no impact on the Company’s financial position, results of operations or cash flows.

In October 2016, the FASB issued ASU No. 2016-16, Income Taxes (Topic 740): Intra-Entity Transfer of Assets Other than Inventory (“ASU 2016-16”), which requires the recognition of the income tax consequences of an intra-entity transfer of an asset, other than inventory, when the transfer occurs. The Company adopted this guidance, effective January 1, 2018, and its adoption had no impact on the Company’s financial position, results of operations or cash flows.

In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (“ASU 2016-18”), which requires restricted cash to be presented with cash and cash equivalents on the statement of cash flows and disclosure of how the statement of cash flows reconciles to the balance sheet if restricted cash is shown separately from cash and cash equivalents on the balance sheet. The Company adopted this standard on January 1, 2018. The adoption of ASU 2016-18 resulted in the Company's cash, cash equivalents and restricted cash being included in the beginning and ending amounts for the periods shown on the statement of cash flows and was applied retroactively and reflected in the balances presented for any prior periods. The adoption of this guidance did not have a significant impact on the Company’s consolidated financial statements and related disclosures.

The restricted cash as of December 31, 2018 and 2017 is held as letters of credit for the benefit of the landlords in connection with the Company’s office leases and corporate credit cards.

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheet that sum to the total of the same such amounts shown in the statement of cash flows.

	December 31,		December 31,
	2018		2017
Cash and cash equivalents	$	30,754	$	76,793
Restricted cash – current		101		—
Restricted cash – non-current		539		355
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$	31,394	$	77,148

In January 2017, FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business (“ASU 2017-01”). The amendments in this update clarify the definition of a business with the objective of adding guidance to assist entities with evaluating whether transactions should be accounted for as acquisitions or disposals of assets or businesses. The definition of a business affects many areas of accounting including acquisitions, disposals, goodwill and consolidation. The Company adopted this guidance, effective January 1, 2018, and its adoption had no impact on the Company’s financial position, results of operations or cash flows.

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 new standard will be effective beginning January 1, 2019 and early adoption is permitted for public entities. 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 May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”), which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. The Company adopted this guidance, effective January 1, 2018, 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 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features 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. ASU 2017-11 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. 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 March 2018, the FASB issued ASU 2018-05, Income Taxes (Topic 740) - Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 ("ASU 2018-05"). This standard amends ASC 740, Income Taxes (“ASC 740”) to provide guidance on accounting for the tax effects of the Tax Cuts and Jobs Act pursuant to Staff Accounting Bulletin No. 118, which allows companies to complete the accounting under ASC 740 within a one-year measurement period from the Tax Cuts and Jobs Act (“Tax Act”) enactment date. This standard was effective upon issuance. The Company previously provided a provisional estimate of the effect of the Tax Act in its results of operations, cash flows and consolidated financial statements. In the fourth quarter of 2018, the Company completed its analysis to determine the effect of the Tax Act and concluded no adjustments were necessary.

In June 2018, the FASB issued ASU 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 transactions for acquiring goods and services from nonemployees. The new standard specifies that Topic 718 applies to all share-based payment transactions in which a grantor acquires goods or services to be used or consumed in a grantor’s own operations by issuing share-based payment awards. The amendments also clarify that Topic 718 does not apply to share-based payments used to effectively provide (1) financing to the issuer or (2) awards granted in conjunction with selling goods or services to customers as part of a contract accounted for under Topic 606, Revenue from Contracts with Customers. ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. The Company adopted this guidance, effective January 1, 2019, and its adoption had no material impact on the Company’s financial position, results of operations or cash flows.

Recently Issued Accounting Pronouncements

In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement, (“ASU 2018-13”). The new standard removes certain disclosures, modifies certain disclosures and adds additional disclosures related to fair value measurement. The new standard will be effective beginning January 1, 2020 and early adoption is permitted. The Company is currently evaluating the impact that the adoption of ASU 2018-13 will have 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.

Fair Value Measurements

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

3.	Fair Value Measurements

The following tables present information about the Company’s financial assets and liabilities that have been measured at fair value at December 31, 2018 and 2017, and indicates the fair value hierarchy of the valuation inputs utilized to determine such fair value (in thousands):

	Fair Value Measurements
	as of December 31, 2018 Using:
	Level 1		Level 3		Total
Assets:
Cash equivalents - Money Market Funds	$	29,836	$	—	$	29,836
	$	29,836	$	—	$	29,836

	Fair Value Measurements
	as of December 31, 2017 Using:
	Level 1		Level 3		Total
Assets:
Cash equivalents - Money Market Funds	$	70,891	$	—	$	70,891
	$	70,891	$	—	$	70,891

There were no changes to the valuation methods during the years ended December 31, 2018 and 2017. There were no transfers within the fair value hierarchy during the years ended December 31, 2018 and 2017.

- 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

Prepaid Expenses and Other Current Assets

12 Months Ended

Dec. 31, 2018

Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]

Prepaid Expenses and Other Current Assets

4.	Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following (in thousands):

	December 31,
	2018		2017
Prepaid clinical trial costs	$	—	$	257
Prepaid directors' and officers' and other corporate insurance		1,007		524
Prepaid other		359		348
	$	1,366	$	1,129

Property and Equipment, Net

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Property and Equipment, Net

5.	Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):

	December 31,
	2018			2017
Laboratory and office equipment	$	1,676		$	1,739
Furniture and fixtures		402			419
Leasehold improvements		298			297
Computer equipment and software		180			189
		2,556			2,644
Less: Accumulated depreciation and amortization		(2,271	)		(2,223	)
	$	285		$	421

Depreciation and amortization expense for the years ended December 31, 2018, 2017 and 2016 was $0.2 million, $0.2 million and $0.3 million, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

Accrued Expenses

6.	Accrued Expenses

Accrued expenses consisted of the following (in thousands):

	December 31,
	2018		2017
Accrued clinical trial costs	$	294	$	2,317
Accrued compensation and benefits		2,522		2,454
Accrued professional fees		1,156		510
Accrued other		144		498
	$	4,116	$	5,779

Accrued Clinical Trial Costs

In connection with the Company’s decision to discontinue the Phase 2 clinical trial for ASN100, spending and activities related to ASN100 significantly declined and the Company does not expect to incur material costs for this program in 2019.

Restructuring Costs

On August 10, 2018, the Company’s board of directors approved a reduction in workforce to reduce operating costs and better align the Company’s workforce with the needs of its business following the Company’s discontinuation of the clinical development of ASN100. As part of this reduction in workforce, the Company eliminated 28 positions across the company, representing approximately 65% of its workforce, through March 1, 2019.

Between August 2018 and November 2018, the Company’s board of directors approved employee retention arrangements to incentivize certain employees to remain with the Company through early 2019.

The Company currently estimates that it will incur total expenses relating to the reduction in workforce of approximately $5.9 million, which is comprised of employee severance and retention payments. The Company records these charges in accordance with ASC 420, Exit or Disposal Cost Obligations. During the year ended December 31, 2018, the Company recorded severance and retention expense of approximately $3.5 million related to the reduction in workforce. The Company expects to record the remaining $2.4 million of severance and retention expense during the first quarter of 2019.

The following table summarizes the Company’s restructuring activities for the year ended December 31, 2018, which is included in accrued compensation and benefits as a component of accrued expenses on the Company's consolidated balance sheet as of December 31, 2018 (in thousands):

	December 31,
	2018
Employee severance benefits	$	1,648
Employee retention benefits		1,886
Payments		(1,539	)
Total	$	1,995

Collaboration, License and Funding Arrangements

12 Months Ended

Dec. 31, 2018

Collaboration [Abstract]

Collaboration, License and Funding Arrangements

7.	Collaboration, License and Funding Arrangements

Adimab Collaboration Agreement

In May 2011, the Company entered into a collaboration agreement with Adimab, LLC (“Adimab”), a related party (see Note 17) (as amended, and together with certain applicable option exercise letters the Company has sent to Adimab, the “Adimab Collaboration Agreement”). Under the Adimab Collaboration Agreement, the Company and Adimab were required to use reasonable efforts to conduct certain research, which was funded by the Company, to discover and optimize antibodies directed against targets selected by the Company. With respect to each target that was the subject of the research, the Company had an exclusive option to obtain, with respect to a specified number of antibodies directed against such target and discovered or optimized by Adimab, (i) ownership of certain patent rights relating to such antibodies and (ii) exclusive and non-exclusive licenses in a specified field, with the right to grant sublicenses, under certain patent rights and know-how.

Under the Adimab Collaboration Agreement, for each target for which the Company has exercised an option, the Company is required to use commercially reasonable efforts to develop and commercialize at least one product in major markets. If the Company does not fulfill these diligence obligations, Adimab may consider it a material breach, allowing Adimab to terminate the Adimab Collaboration Agreement with respect to such target and all associated products.

The Company is obligated to pay Adimab royalties at a mid single-digit percentage of net sales made by the Company or its affiliates of products based on antibodies for which the Company exercised its option, or products that use or are based on any antibody discovered or optimized under the agreement, any derivative or modified version of any such antibody, or any sequence information as to any such antibody. In addition, if the Company sells or licenses to any third party, or otherwise grants rights to any third party to, any of the products for which the Company is obligated to pay Adimab royalties, either alone or as part of a package including specified patents not directed to these antibodies, the Company is obligated to pay Adimab either (i) the same royalties on net sales of such products by such third party or (ii) a percentage, ranging from the low double digits to a maximum of less than 30%, of the payments the Company receives from such third parties that are attributable to such grant of rights. In April 2017, the Company entered into a letter agreement with the Gates Foundation, pursuant to which the Company licensed to the Gates Foundation certain rights under its ASN100 program. The Company has no payment obligations under the Adimab Collaboration Agreement with respect to sales of certain antibody products if they are sold at cost in developing countries under its letter agreement with the Gates Foundation. However, if such products are sold in developing countries for an amount that exceeds cost, then the amount of such excess over cost will be subject to the royalty payment obligations described above.

If the Company (or one of its affiliates with rights under the agreement) undergoes a change in control and, at the time of such change in control, the Company has not sold or licensed to third parties all of its rights in antibodies for which the Company is obligated to pay Adimab royalties under the agreement, then the Company is obligated to either (i) pay Adimab a percentage, in the mid double digits, of the payments it receives from that change in control that are reasonably attributable to those rights and certain patents arising from the collaboration or (ii) require the Company’s acquirer and all of its future third-party collaborators to pay to Adimab the royalties at a mid single-digit percentage of net sales based on those rights. If the Company grants rights to a third party under certain patents that are not directed to the antibodies for which the Company is obligated to pay Adimab royalties (as described above), the Company is also obligated to pay Adimab, in place of royalties or a percentage of payments received from the third party, a lump sum in the high six digits.

The Adimab Collaboration Agreement will expire on a country-by-country basis twelve years after the first commercial sale in such country of the last product for which the Company is obligated to pay Adimab royalties in such country under the Adimab Collaboration Agreement. The Company has the right to terminate the Adimab Collaboration Agreement for any reason by providing Adimab with a specified amount of prior written notice. Adimab has the right to terminate the Adimab Collaboration Agreement if the Company materially breaches the agreement and fails to cure such breach within a specified cure period, including for its failure to use commercially reasonable efforts to develop and commercialize at least one product directed at a target for which the Company has exercised an option in major markets. If Adimab terminates the Adimab Collaboration Agreement for the Company’s breach, or if the Company terminates the agreement for convenience, then the Company must transfer or license to Adimab certain rights and assets relating to targets and antibodies for which the Company has exercised its option. Adimab is then obligated to make payments to the Company with respect to these targets and antibodies that are similar to the payments the Company is required to make to Adimab during the term of the agreement. Certain of the Company’s payment obligations relating to specified products and patents arising from the agreement survive expiration or termination of the agreement.

During the years ended December 31, 2018, 2017 and 2016, the Company recognized research and development expense of $0, $0, and less than $0.1 million, respectively, under the Adimab Collaboration Agreement.

Adimab Option and License Agreement

In February 2017, the Company entered into an option and license agreement with Adimab, a related party (the “Adimab Option Agreement”). Under the Adimab Option Agreement, Adimab has provided to the Company certain proprietary antibodies against respiratory syncytial virus (“RSV antibodies”) for its evaluation during a specified option period and has granted the Company an exclusive, non-sublicensable license in a specified field under certain Adimab patent rights and know-how during the option period. Under the Adimab Option Agreement, the Company has an exclusive option, exercisable during the option period upon payment of an option fee to Adimab, to require Adimab to assign to the Company all rights in up to a specified number of RSV antibodies selected by the Company and certain patent rights owned by Adimab that cover these antibodies, and to obtain from Adimab a non-exclusive license in a specified field, with the right to grant sublicenses, under certain other patent rights and know-how owned by Adimab.

If the Company exercises its option under the Adimab Option Agreement, the Company is required to use commercially reasonable efforts to develop and commercialize at least one product based on a licensed RSV antibody in major markets and is obligated to pay Adimab an option fee of $0.3 million and make future milestone payments upon the achievement of specified clinical and regulatory milestones in the aggregate amount of up to $24.4 million. The Company is obligated to pay Adimab royalties at a mid single-digit percentage of net sales of products based on the initial RSV antibodies (including modified or derivative forms of those antibodies created by or for the Company) by the Company or any of its affiliates, licensees or sublicensees, regardless of whether these products practice any of the assigned or licensed patents or know-how. If the Company materially breaches these diligence obligations, Adimab will have the right to terminate the Adimab Option Agreement.

The Company has no payment obligations under the Adimab Option Agreement with respect to sales of products based on licensed RSV antibodies to the extent they are sold at cost in developing countries under the February 2017 Gates Foundation grant agreement, as amended and restated in August 2018, and the August 2018 Gates Foundation grant agreement (which are described in further detail below). However, if such products are sold in developing countries for an amount that exceeds cost, then the amount of such excess will be subject to the royalty payment obligations described in the preceding paragraph.

If the Company does not exercise its option, the Adimab Option Agreement will expire on the Company’s achievement of specified preclinical milestones under the grant agreements with the Gates Foundation, but in any event no later than mid-2019. If the Company does exercise its option, the Adimab Option Agreement will expire on the last-to-expire royalty term (defined, on a product-by-product and country-by-country basis, as the period ending on the later of twelve years after the first commercial sale of such product in such country and the expiration of the last of a specified set of patents and patent applications covering such product in such country) for any and all products for which the Company is obligated to pay Adimab royalties under the Adimab Option Agreement. The Company has the right to terminate the Adimab Option Agreement for any reason by providing Adimab with a specified amount of prior written notice. Adimab has the right to terminate the Adimab Option Agreement if the Company materially breaches the agreement and fails to cure such breach within a specified cure period, including for the Company’s failure to use commercially reasonable efforts to develop and commercialize at least one product based on a licensed RSV antibody in major markets. If Adimab terminates the Adimab Option Agreement for the Company’s breach, if the Company terminates the agreement for convenience or if the agreement expires before the Company exercises its option, then the Company must return or destroy certain know-how, including all initial RSV antibodies, and all modified or derivative forms of those antibodies, in its possession other than those for which the Company has made all payments required under the Adimab Option Agreement, assign certain patents covering certain RSV antibodies to Adimab, grant Adimab a non-exclusive, royalty-free license under certain other patents, and grant Adimab a time-limited right of first negotiation to obtain an exclusive license to certain patents and know-how and the transfer and assignment of certain regulatory filings and approvals and other related assets related to products based on licensed RSV antibodies. Certain of the Company’s payment obligations relating to specified products and patents arising from the agreement survive expiration or termination of the agreement.

During the years ended December 31, 2018 and 2017, the Company recognized research and development expense of $0.1 million and $0.1 million, respectively, in connection with the Adimab Option Agreement, which consisted of reimbursement for services performed by Adimab.

February 2017 Grant Agreement and August 2018 Amended and Restated Gates Foundation Grant Agreement

In February 2017, the Company entered into a grant agreement with the Gates Foundation, a related party, under which the Gates Foundation agreed to provide the Company up to $9.3 million to conduct preclinical development of monoclonal antibodies for the prevention of RSV infection in newborns (the “RSV project”).

In connection with this grant agreement, the Company has granted to the Gates Foundation a non-exclusive, perpetual, royalty-free, fully paid up, sublicensable license to make, use, sell, offer to sell, import, distribute, copy, modify, create derivative works, publicly perform and display the funded developments and, to the extent incorporated into a funded development or required to use a funded development, any other technology created outside of the RSV project that was used as part of the RSV project, for the benefit of people in developing countries. This license survives any expiration or termination of the grant agreement.

The grant agreement expires on October 31, 2019. The Gates Foundation can modify, suspend or discontinue any payment under the grant agreement, or terminate the grant agreement, if it is not reasonably satisfied with the Company’s progress on the RSV project; if there are significant changes to the Company’s leadership or other factors that the Gates Foundation reasonably believes may threaten the RSV project’s success; if the Company undergoes a change in control; if there is a change in the Company’s tax status; if the RSV project is no longer aligned with the Gates Foundation’s programmatic strategy; or if the Company fails to comply with the grant agreement. Any grant funds that have not been used for, or committed to, the RSV project upon the expiration or termination of the grant agreement must be returned to the Gates Foundation or otherwise used as directed by the Gates Foundation.

In March 2017, the Company received a payment of $1.6 million from the Gates Foundation under the grant agreement. The payment received from the Gates Foundation under the grant agreement was classified as restricted cash (current) in the consolidated balance sheet due to restrictions on the use of the funds imposed by the agreement (see Note 2). Such funds received from the Gates Foundation were no longer classified as restricted cash once the Company incurred qualifying expenses under the grant agreement and the restrictions no longer applied.

In August 2018, the Company entered into an amended and restated grant agreement which replaces the February 2017 grant agreement in its entirety. The amended and restated grant agreement includes amendments to conform to current Gates Foundation audit, reporting, and other administrative requirements, as well as to make the perpetual Gates Foundation license grant described below irrevocable.

During the years ended December 31, 2018 and 2017, the Company recognized grant income of $0 and $1.6 million, respectively, under the grant agreement with the Gates Foundation upon incurring qualifying expenses. As of December 31, 2018 and 2017, there were no amounts recorded as unearned income under the grant agreement with the Gates Foundation.

August 2018 Gates Foundation Grant Agreement

In August 2018, the Company entered into an additional grant agreement with the Gates Foundation pursuant to which the Gates Foundation granted to the Company up to $1.1 million to conduct preclinical development activities for the RSV project that were not included in the February 2017 grant agreement, as amended and restated in August 2018. In return, the Company has agreed to conduct the RSV project in a manner that ensures that the knowledge and information gained from the project will be promptly and broadly disseminated, and that the products, technologies, materials, processes and other intellectual property resulting from the RSV project (collectively referred to as the funded developments) will be made available and accessible at an affordable price to people most in need within developing countries. These obligations survive any expiration or termination of the grant agreement.

To this end, the Company has granted to the Gates Foundation a non-exclusive, perpetual, irrevocable, royalty-free, fully paid up, sublicensable license to make, use, sell, offer to sell, import, distribute, copy, modify, create derivative works, publicly perform and display the funded developments and, to the extent incorporated into a funded development or required to use a funded development, any other technology created outside of the RSV project that was used as part of the RSV project, for the benefit of people in developing countries. The Company has also agreed to seek prompt publication of data and results developed under the RSV project under “open access” terms and conditions. This license and these publication obligations survive any expiration or termination of the grant agreements.

During the year ended December 31, 2018, the Company recognized grant income of $1.1 million, under the August 2018 grant agreement with the Gates Foundation upon incurring qualifying expenses. Accordingly, unearned income under the August 2018 grant agreement with the Gates Foundation was $0 as of December 31, 2018. The August 2018 grant agreement expired during the year ended December 31, 2018.

Gates Foundation Letter Agreement and Investment

In April 2017, the Company entered into a letter agreement with the Gates Foundation. In connection with the letter agreement, the Gates Foundation purchased 2,464,799 shares of the Company’s Series D redeemable convertible preferred stock (the “Series D preferred stock”), which converted into 722,179 shares of the Company’s common stock in connection with the IPO after giving effect to a one-for-3.4130 reverse-stock-split. The Company committed to use the proceeds of $8.0 million from the investment by the Gates Foundation solely to advance the development of a specified monoclonal antibody program that involves the monoclonal antibodies ASN-1, ASN-2 and ASN-3 and the Company’s product candidate, ASN100. Under the letter agreement, in addition to the initial project funded by the Gates Foundation with its initial investment, the Company also agreed to conduct up to four additional projects to be proposed and to be funded by the Gates Foundation.

The letter agreement contains certain global access obligations as well as requirements relating to the Company’s use of the funds received from the Gates Foundation investment. In the event that the Company fails to comply with these obligations or requirements or any related U.S. legal obligations set forth in the letter agreement, the Gates Foundation will have the right, after expiration of a specified cure period, to require the Company to redeem all of the shares owned by the Gates Foundation or to locate a third party that will purchase such shares. For any redemption or purchase resulting from such default, the shares of the Company’s stock held by the Gates Foundation will be redeemed at an amount equal to the greater of the original purchase price (plus specified interest) or the fair market value of such stock on the date of such redemption. The term of the letter agreement continues in perpetuity.

In connection with this letter agreement, the Company has granted to the Gates Foundation and/or Gates Foundation-supported entities certain licenses, including a non-exclusive, non-terminable, royalty-free (except as required under the Adimab Collaboration Agreement), sublicensable license to products, technologies, materials, processes and other intellectual property developed using funds provided by the Gates Foundation or a Gates Foundation-supported entity, or developed in connection with the Company’s conduct of any funded project or additional funded project, as well as all of the Company’s background intellectual property, to utilize and exploit products and services directed at pathogens or other targets subject to any funded project or additional funded project.

The proceeds received from the Gates Foundation in connection with the Company’s sale and issuance of Series D preferred stock were incurred on qualifying expenses under the letter agreement during the year ended December 31, 2017.

During the years ended December 31, 2018 and 2017, the Company incurred qualifying expenses of $0 and $8.0 million, respectively, under the letter agreement with the Gates Foundation.

Funding Agreements with FFG

Between September 2011 and March 2017, the Company entered into a series of funding agreements with FFG that provided for loans and grants to fund between 50% and 70% of qualifying research and development expenditures of the Company’s subsidiary in Austria on a project-by-project basis, as approved by FFG.

FFG Grants

For grants under the funding agreements with FFG, the Company recognized grant income of $0 million, $0.2 million and $0.6 million during the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018 and 2017, the Company recorded grant receivables from FFG of $0.1 million and $0.1 million, respectively, for qualifying expenses incurred that were reimbursable under the funding agreements. As of December 31, 2018 and 2017, there were no amounts recorded as unearned income in connection with the FFG grants.

FFG Loans

Loans under the funding agreements with FFG bear interest at rates that are below market rates of interest. The Company accounts for the imputed benefit arising from the difference between a market rate of interest and the rate of interest charged by FFG as additional grant funding from FFG. On the date that FFG loan proceeds are received, the Company recognizes the portion of the loan proceeds allocated to grant funding as a discount to the carrying value of the loan and as unearned income, which is recognized as additional grant income over the term of the funding agreement.

The Company recognized grant income of $0.7 million, $0.6 million and $0.4 million during the years ended December 31, 2018, 2017, and 2016, respectively, related to the recognition of the unearned income recorded for the imputed benefit of FFG loans at below-market interest rates. Unearned income (current) related to the imputed benefit of FFG loans at below-market interest rates was $0.7 million and $0.7 million as of December 31, 2018 and 2017, respectively, and unearned income (non-current) related to such benefit was $1.1 million and $1.9 million as of December 31, 2018 and 2017, respectively.

Research and Development Incentive

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 was 14%, 12% and 12% for the years ended December 31, 2018, 2017 and 2016, respectively.

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.

The Company recognized incentive income of $1.4 million, $1.4 million and $1.4 million during the years ended December 31, 2018, 2017 and 2016, respectively, in connection with the Austrian research and development incentive program. As of December 31, 2018 and 2017, the Company recorded receivables for amounts due under the program of $2.7 million and $1.5 million, respectively, which amounts were included in grant and incentive receivables in the consolidated balance sheet.

Loans Payable

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Loans Payable

8.	Loans Payable

The aggregate principal amount of debt outstanding as of December 31, 2018 and 2017 consisted of the following (in thousands):

	December 31,
	2018		2017
Term loans under 2012 Loan Agreement	$	—	$	4,667
FFG Loans		9,738		10,225
	$	9,738	$	14,892

Current and non-current debt obligations reflected in the consolidated balance sheets as of December 31, 2018 and 2017 consisted of the following (in thousands):

	December 31,
	2018			2017
Current liabilities:
Term loans under 2012 Loan Agreement	$	—		$	2,333
FFG loans		—			—
Unamortized debt discount		—			(19	)
Loans payable, net of discount		—			2,314
Non-current liabilities:
Term loans under 2012 Loan Agreement	$	—		$	2,334
FFG loans		9,738			10,225
Unamortized debt discount		(1,844	)		(2,637	)
Loans payable, net of discount and current portion		7,894			9,922
Total loans payable, net of discount	$	7,894		$	12,236

2012 Loan Agreement

On December 7, 2012, the Company entered into a loan and security agreement (the “2012 Loan Agreement”) with Silicon Valley Bank (“SVB”), and borrowed an aggregate of $2.5 million in two separate tranches: $0.5 million in December 2012 and $2.0 million in February 2013.

On February 19, 2016, the Company entered into the First Amendment to the 2012 Loan Agreement (the “First Amendment”). The First Amendment provided for two additional advances not to exceed, in the aggregate, $3.5 million, with each advance being for a minimum of $0.5 million, and total borrowings under the 2012 Loan Agreement not to exceed $7.0 million. The Company borrowed the full $7.0 million available in two separate tranches: $3.5 million in February 2016, and $3.5 million in August 2016. Following the August 2016 borrowing, no additional amounts remained available for borrowing under the 2012 Loan Agreement.

In connection with the 2012 Loan Agreement, between December 2012 and August 2016, the Company issued to SVB warrants to purchase an aggregate of 11,013 shares of Series A-2 redeemable convertible preferred stock (the “Series A-2 preferred stock”) at an exercise price of $4.54 per share and warrants to purchase an aggregate of 14,502 shares of Series B redeemable convertible preferred stock (the “Series B preferred stock”) at an exercise price of $7.24 per share. The warrants became exercisable in connection with the Company’s borrowings under the 2012 Loan Agreement and are fully exercisable. The warrant to purchase shares of Series A-2 preferred stock expires on December 6, 2022, and the warrant to purchase shares of Series B preferred stock expires on February 18, 2026.

Upon the closing of the IPO in November 2017, the Series A-2 warrants converted into common stock warrants to purchase up to 3,940 shares of common stock at an exercise price of $12.70 per share. The Series B warrants converted into common stock warrants to purchase up to 6,474 shares of common stock at an exercise price of $16.22 per share. At December 31, 2018, these warrants to purchase up to 3,940 shares of common stock at an exercise price of $12.70 and 6,474 shares of common stock at an exercise price of $16.22 remained outstanding.

Borrowings under the 2012 Loan Agreement bore interest at a rate per annum equal to the greater of 3.25% and The Wall Street Journal prime rate, in each case minus 0.25%; provided, however, that in an event of default, as defined in the 2012 Loan Agreement, the interest rate applicable to borrowings under the agreement would be increased by 4.0%.

The Company was required to make monthly interest-only payments through December 1, 2016 and was required to make equal monthly payments of principal as well as accrued interest from January 1, 2017 through December 1, 2019, when all unpaid principal and interest would become due and payable. The Company had the right to voluntarily prepay all, but not less than all, of the outstanding principal at any time prior to the maturity date, subject to a prepayment fee, which ranged from 1% to 2% of the outstanding principal if paid prior to February 19, 2018, which was the second anniversary of the First Amendment effective date. The prepayment fee was 0% subsequent to the second anniversary of the First Amendment effective date. A final payment of $0.4 million was due upon the earlier to occur of the maturity of the loan or the prepayment of all outstanding principal.

Borrowings under the 2012 Loan Agreement were collateralized by a pledge of substantially all of the Company’s assets other than intellectual property, including 65% of the outstanding capital stock of the Company’s subsidiary in Austria. The 2012 Loan Agreement contained customary affirmative and negative covenants, including restrictions on the Company’s ability to pay dividends and encumber the Company’s intellectual property, but did not contain any financial covenants.

The Company was in compliance with all covenants under the 2012 Loan Agreement as of December 31, 2017. In March 2018, the Company entered into an Option and License Agreement with BB100, LLC, a subsidiary of Bravos Biosciences, LLC, under which BB100, LLC secured an exclusive, worldwide preclinical development license, and an option to a clinical development and commercialization license, to mAbs targeting E. coli that were discovered by the Company in its ASN200 program. In June 2018, the Company entered into an Option and License Agreement with BB200, LLC, a portfolio company of Bravos Biosciences, LLC, under which BB200, LLC secured an exclusive, worldwide preclinical development license, and an option to a clinical development and commercialization license to selected mAbs targeting K. pneumoniae that were discovered by the Company in its ASN300 program, including lead preclinical development candidate, ASN-5. As a result of entering into such Option and License Agreements without obtaining prior written consent of SVB, and subsequently delivering compliance certificates under the 2012 Loan Agreement that did not disclose these violations, the Company became in default under the 2012 Loan Agreement. On August 8, 2018, the Company and SVB entered into a Forbearance Agreement (the “Forbearance Agreement”), pursuant to which SVB agreed to forbear from exercising its rights and remedies with respect to such default until the earlier to occur of (i) another event of default under the 2012 Loan Agreement or (ii) October 31, 2018.

On October 31, 2018, the Company voluntarily remitted payment on its outstanding obligations under the 2012 Loan Agreement with SVB. Total outstanding obligations paid to SVB under the 2012 Loan Agreement on October 31, 2018 consisted of $2.7 million of principal, $0.4 million of final payment and less than $0.1 million of interest. All obligations under the 2012 Loan Agreement were satisfied by the Company on October 31, 2018.

The Company recognized interest expense under the 2012 Loan Agreement, as amended, of $0.3 million, $0.4 million and $0.3 million during the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018 and 2017, the unamortized debt discount was $0 and less than $0.1 million, respectively.

FFG Loans

In connection with the funding agreements with FFG, the Company received loans from FFG. Loans from FFG were made on a project-by-project basis and had an aggregate principal amount outstanding of $9.7 million and $10.2 million as of December 31, 2018 and 2017, respectively. Amounts due under the FFG loans bear interest at rates ranging from 0.75% to 2.0% per annum. As of December 31, 2018, before giving effect to the Settlement Agreement (as defined below), the loans matured at various dates between June 2020 and March 2023. Interest on amounts due under the loans is payable semi-annually in arrears, with all principal and remaining accrued interest due upon maturity.

In addition, the Company has recorded a discount to the carrying value of each FFG loan for the portion of the loan proceeds allocated to grant funding, which is being amortized to interest expense over the term of the loan using the effective interest method. As of December 31, 2018 and 2017, the unamortized debt discount related to FFG loans was $1.8 million and $2.6 million, respectively.

The Company recognized interest expense of $0.8 million, $0.7 million and $0.5 million during the years ended December 31, 2018, 2017 and 2016, respectively, related to the FFG loans. There were no principal payments due or paid under the FFG loans during the years ended December 31, 2018, 2017 and 2016.

In the event that the underlying program research results in a scientific or technical failure, the principal then outstanding under any loan may be forgiven by FFG and converted to non-repayable grant funding on a project-by-project basis. The FFG loans are not secured by any of the Company’s assets.

As of December 31, 2018, the aggregate minimum future principal payments due in connection with the FFG loans are summarized as follows (in thousands):

Year Ending December 31,
2019	$	—
2020		4,948
2021		3,931
2022		859
2023		—
	$	9,738

FFG Settlement

On February 4, 2019, the Company and its wholly owned subsidiary, Arsanis Biosciences GmbH, received letters from counsel to FFG (see Note 20) alleging that they breached reporting, performance and other obligations in connection with the grants and loans made by FFG to Arsanis Biosciences GmbH between September 2011 and March 2017 to fund qualifying research and development expenditures (collectively, the “Subsidies”).

The letters demanded the immediate repayment of all Subsidies, totaling approximately EUR 18.1 million ($20.4 million, based on an exchange rate of US$1.12 per EUR 1.00 on March 7, 2019), on or before February 19, 2019. The repayment amount consisted of approximately EUR 7.2 million ($8.1 million) for the reimbursement of grants previously received by the Company, approximately EUR 8.5 million ($9.6 million) for the repayment of outstanding loan principal and approximately EUR 2.4 million ($2.7 million) for assessed interest (collectively, the “FFG Demands”). FFG reserved all rights and remedies in connection with the Subsidies.

The Company is party to a series of Patronatserklärung (“parent-company support letters”) for the benefit of Arsanis Biosciences GmbH under Austrian law, pursuant to which the Company has agreed to take certain steps including, in some instances, to maintain the ability of Arsanis Biosciences GmbH to pay its debts and liabilities, or to enable Arsanis Biosciences GmbH to meet its obligations and to prevent Arsanis Biosciences GmbH from becoming insolvent, subject to the terms and conditions set forth in the parent-company support letters.

On March 8, 2019, the Company, Arsanis Biosciences GmbH, X4 and Artemis AC Corp. (“Merger Sub”), a wholly owned subsidiary of the Company, entered into a settlement agreement (the “Settlement Agreement”) with FFG in respect of the FFG Demands (see Note 20).

As the Company believes it was not in breach of its reporting, performance and other obligations as set forth by FFG, which resulted in FFG’s demand for the immediate repayment of all Subsidies, the Company did not present such repayment amounts as current on the consolidated balance sheet for the year ended December 31, 2018.

Convertible Promissory Notes

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Convertible Promissory Notes

9.	Convertible Promissory Notes

2015 Notes

In December 2015, the Company issued an aggregate of $4.0 million of convertible promissory notes, or the 2015 Notes. The 2015 Notes accrued interest at a rate of 0.56% per annum, with a maturity date of December 16, 2016, unless earlier converted under the terms of the 2015 Notes. All principal and interest accrued under the 2015 Notes, totaling $4.0 million, was converted into 461,396 shares of Series C redeemable convertible preferred stock (the “Series C preferred stock”) at a price equal to 90% of the $9.65 per share in April 2016.

2016 Notes

In April 2016, the Company issued an aggregate of $5.5 million of convertible promissory notes, or the 2016 Notes, which accrued interest at a rate of 0.7% per annum and had a maturity date of October 12, 2017, unless earlier converted under the terms of the 2016 Notes. All principal and interest accrued under the 2016 Notes, totaling $5.5 million, was automatically converted into 1,896,297 shares of Series D preferred stock at a price equal to 90% of $3.2457 per share in April 2017.

2017 Notes

In January 2017, the Company issued an aggregate of $4.9 million of convertible promissory notes, or the 2017 Notes. The 2017 Notes accrued interest at a rate of 0.96% per annum, with a maturity date of October 12, 2017, unless earlier converted under the terms of the 2017 Notes. All principal and interest accrued under the 2017 Notes, totaling $4.9 million, was automatically converted into 1,524,107 shares of Series D preferred stock at a price equal to $3.2457 per share in April 2017.

There were no outstanding convertible promissory notes as of December 31, 2018 and 2017.

Redeemable Convertible Preferred Stock

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Redeemable Convertible Preferred Stock

10.	Redeemable Convertible Preferred Stock

Series A-1 Redeemable Convertible Preferred Stock

In September 2010, the Company issued and sold 200,001 shares of Series A-1 redeemable convertible preferred stock (the “Series A-1 preferred stock” and, collectively with the Series A-2 preferred stock, the Series B preferred stock, the Series C preferred stock and the Series D preferred stock, the “Preferred Stock”) at a price of $2.00 per share, for proceeds of $0.4 million, net of issuance costs of $27,000.

Series A-2 Redeemable Convertible Preferred Stock

In January 2011, the Company issued and sold 2,114,538 shares of Series A-2 preferred stock, at a price of $4.54 per share, for proceeds of $9.6 million, net of issuance costs of $8,000.

Series B Redeemable Convertible Preferred Stock

In July 2013, the Company issued and sold 1,795,580 shares of Series B preferred stock, at a price of $7.24 per share, for proceeds of $12.9 million, net of issuance costs of $68,000.

In May 2015, the Company issued and sold an additional 966,851 shares of Series B preferred stock, at a price of $7.24 per share, for proceeds of $7.0 million, net of issuance costs of $12,000.

Series C Redeemable Convertible Preferred Stock

In April 2016, the Company issued and sold 569,946 shares of Series C preferred stock, at a price of $9.65 per share, for proceeds of $5.4 million, net of issuance costs of $0.1 million. In addition, in connection with the issuance and sale of the Company’s Series C preferred stock, all outstanding principal and accrued interest under the 2015 Notes was automatically converted into an aggregate of 461,396 shares of Series C preferred stock.

Series D Redeemable Convertible Preferred Stock

In April 2017, the Company issued and sold 10,799,880 shares of Series D preferred stock, at a price of $3.2457 per share, for proceeds of $34.8 million, net of issuance costs of $0.2 million. In addition, in connection with the Company’s issuance and sale of Series D preferred stock, all of the outstanding principal and accrued interest under the 2016 Notes and 2017 Notes were automatically converted into an aggregate of 1,896,297 shares and 1,524,107 shares, respectively, of Series D preferred stock.

In addition, the Series D preferred stock purchase agreement provided that Company would use commercially reasonable efforts to sell up to an additional 1,540,508 shares of Series D preferred stock at a price of $3.2457 per share within 180 days of the initial Series D preferred stock closing. The Company concluded that its commitment to use commercially reasonable efforts to sell such shares did not represent a future tranche right of the holders of the Series D preferred stock and that no separate accounting was required related to such commitment.

In September 2017, the Company issued and sold 1,540,500 shares of Series D preferred stock, at a price of $3.2457 per share, for cash proceeds of $5.0 million, with no issuance costs.

Conversion to Common Stock

Upon the closing of the IPO, all of the outstanding shares of redeemable convertible preferred stock automatically converted into 7,180,483 shares of common stock. Subsequent to the closing of the IPO, there were no shares of preferred stock outstanding.

Common Stock

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Common Stock

11.	Common Stock

As of December 31, 2018 and 2017, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue 200,000,000 shares of common stock and 10,000,000 shares of preferred stock, all with a par value of $0.001 per share.

As of December 31, 2018, each share of common stock entitles the holder to one vote on all matters submitted to the stockholders for a vote. The holders of common stock are entitled to remove directors of the Company for cause by vote of at least 75% of the votes that all of the Company’s stockholders would be entitled to cast in an annual election of the directors. In addition, the Company’s bylaws establish an advance notice procedure for stockholder proposals to be brought before an annual meeting of stockholders, including proposed nominations of candidates for election to the Company’s board of directors. Stockholders at an annual meeting may only consider proposals or nominations specified in the notice of meeting or brought before the meeting by or at the direction of the Company’s board of directors, or by a stockholder of record on the record date for the meeting who is entitled to vote at the meeting and who has delivered timely written notice in proper form to the Company’s secretary of the stockholder’s intention to bring such business before the meeting. Common stockholders are also entitled to receive dividends, as may be declared by the board of directors, if any. Through December 31, 2018, no dividends have been declared or paid.

As of December 31, 2018 and 2017, the Company had reserved 2,731,203 shares and 2,187,252 shares of common stock, respectively, for the exercise of outstanding stock options, the number of shares remaining available for grant under the Company’s 2017 Equity Incentive Plan and 2017 Employee Stock Purchase Plan and the exercise of outstanding warrants to purchase shares of common stock.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

12.	Stock-Based Compensation

2017 Equity Incentive Plan

The Company’s 2017 Equity Incentive Plan (the “2017 Plan”) provides for the grant by the Company of 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. Following the adoption of the 2017 Plan, no further grants will be made under the Company’s 2010 Special Stock Incentive Plan (“Special Plan”) and 2011 Stock Incentive Plan (“2011 Plan”).

Upon its adoption, the number of shares of the Company’s common stock initially reserved for issuance under the 2017 Plan was the sum of 585,994 shares, plus the number of shares of the Company’s common stock available for issuance under the Special Plan and the 2011 Plan immediately prior to the effectiveness of the 2017 Plan. In addition, the number of shares of the Company’s common stock subject to outstanding awards under the Special Plan and 2011 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right will be available for future grant under the 2017 Plan. 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 1,025,490 shares of the Company’s common stock, 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.

The 2017 Plan is administered by the board of directors. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, except that the exercise price per share of incentive stock options may not be less than 100% of the fair market value of the common stock on the date of grant and the term of stock options may not be greater than 10 years. Stock options awarded under the 2017 Plan expire 10 years after the grant date, unless the board of directors sets a shorter term. Vesting periods for awards under the 2017 Plan are determined at the discretion of the board of directors. 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 2017 Plan 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.

2017 Employee Stock Purchase Plan

The Company’s 2017 Employee Stock Purchase Plan (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. A total of 219,748 shares were initially reserved for issuance under the 2017 ESPP. The number of shares of common stock reserved for issuance under this plan will automatically increase on January 1 of each year, beginning on January 1, 2019 and continuing through January 1, 2029, in an amount equal to the lowest of 512,745 shares of the Company’s common stock, 2% 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. During the year ended December 31, 2018, 637 shares of common stock were issued under the 2017 ESPP.

Stock Option Valuation

The assumptions that the Company used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted to employees and directors were as follows, presented on a weighted average basis:

	Year Ended December 31,
	2018			2017			2016
Risk-free interest rate		2.72	%		1.96	%		1.26	%
Expected term (in years)		6.00			6.02			5.80
Expected volatility		77.4	%		75.7	%		75.3	%
Expected dividend yield		0	%		0	%		0	%

The Company recorded stock-based compensation expense for options granted to employees and directors of $5.6 million, $0.9 million and $0.6 million during the years ended December 31, 2018, 2017, and 2016, respectively.

The assumptions that the Company used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted to non-employees in 2016 were as follows, presented on a weighted average basis:

	Year Ended December 31,
	2016
Risk-free interest rate		2.08	%
Expected term (in years)		8.82
Expected volatility		77.8	%
Expected dividend yield		0	%

The Company did not grant any stock options to non-employees during the years ended December 31, 2018 and 2017. The Company recorded stock-based compensation expense for options granted to non-employees of $13,000, $7,000 and $15,000 during the years ended December 31, 2018, 2017 and 2016, respectively.

Stock Options

The following table summarizes the Company’s stock option activity since December 31, 2017 (in thousands, except share and per share amounts):

						Weighted
				Weighted		Average
				Average		Remaining		Aggregate
	Number of			Exercise		Contractual		Intrinsic
	Shares			Price		Term		Value
						(in years)
Outstanding as of December 31, 2017		1,403,119		$	6.26		8.81	$	9,128
Granted		829,500			17.19
Exercised		(27,226	)		6.51
Forfeited		(262,003	)		12.11
Outstanding as of December 31, 2018		1,943,390		$	10.13		8.00	$	49
Options exercisable as of December 31, 2018		1,053,481		$	8.26		7.27	$	49
Options unvested as of December 31, 2018		889,909		$	12.35		8.86	$	—

The aggregate intrinsic value of stock 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 stock options that had exercise prices lower than the fair value of the Company’s common stock.

The weighted average grant-date fair value per share of stock options granted during the years ended December 31, 2018, 2017 and 2016 was $11.77, $3.62 and $6.02, respectively.

The total fair value of options vested during the year ended December 31, 2018 was $4.5 million, of which approximately $2.5 million relates to the modification of awards of the Company’s former President and Chief Executive Officer and Chief Scientific Officer. The total fair value of options vested during the years ended December 31, 2017 and 2016 was $0.8 million and $0.5 million, respectively.

Restricted Common Stock

The Company has granted restricted common stock with time-based vesting conditions. The exercise price of the restricted stock awards is determined by the board of directors. Unvested shares of restricted common stock may not be sold or transferred by the holder. These restrictions lapse according to the time-based vesting conditions of each award. The Company has the option to repurchase the restricted stock awards at the original purchase price if the grantee terminates its working relationship with the Company prior to the stock becoming vested. The following table summarizes the Company’s restricted common stock activity:

			Weighted Average
	Number of		Grant Date
	Shares		Fair Value
Unvested restricted common stock as of December 31, 2017		—		—
Issued		250,000	$	4.12
Vested		—		—
Unvested restricted common stock as of December 31, 2018		250,000	$	4.12

The total expense related to employee and non-employee restricted stock for the years ended December 31, 2018, 2017, and 2016 was approximately $25,000, $0 and $1,000, respectively.

As of December 31, 2018, the Company had unrecognized stock‑based compensation expense related to its employee and non‑employee unvested restricted stock awards of $1.0 million which is expected to be recognized over a remaining weighted average vesting period of 3.9 years.

Stock-Based Compensation Expense

Stock-based compensation expense related to stock option awards, restricted common stock awards and the 2017 ESPP are classified in the consolidated statements of operations as follows (in thousands):

	Year Ended December 31,
	2018		2017		2016
Research and development expenses	$	1,079	$	329	$	294
General and administrative expenses		4,541		577		350
	$	5,620	$	906	$	644

As of December 31, 2018 and 2017, total unrecognized compensation cost related to the unvested stock option awards, restricted common stock awards and the 2017 ESPP was $7.2 million and $4.0 million, respectively, which is expected to be recognized over weighted average periods of 2.78 years and 2.76 years, respectively.

License Revenue

12 Months Ended

Dec. 31, 2018

Revenue From Contract With Customer [Abstract]

License Revenue

13.	License Revenue

Janssen License and Option Agreement

On December 12, 2018, the Company 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, the Company granted to Janssen (i) a non-exclusive license to specified patents in the Company’s portfolio related to the ASN200 E. coli program, and (ii) an option for Janssen to acquire these patents in the future if specified conditions are met. Janssen agreed to pay the Company $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. The Company received the $3.5 million payment from Janssen in December 2018.

The Company assessed this arrangement in accordance with Topic 606 and concluded that at the date of contract inception, only one performance obligation, consisting of delivery of the license which was satisfied at contract inception, was identified. Accordingly, the entire nonrefundable license fee of $3.5 million was recognized as revenue upon contract execution given there were no other unsatisfied performance obligations in the arrangement as the time of contract inception. Because the single performance obligation was previously satisfied, Janssen’s option to the relevant patents will be recognized as revenue in full in the period in which the option exercise occurs. Janssen’s option to acquire such patents are not considered probable of being achieved until specified conditions are met. As the specified conditions are not within the Company’s control or the licensee’s control, this option is constrained and excluded from the transaction price until such time the conditions are met and Janssen exercises its option to acquire the relevant patents.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

14.	Income Taxes

During the years ended December 31, 2018, 2017 and 2016, the Company recorded no income tax benefits for the net operating losses incurred and research and development tax credits earned in each year or interim period, due to its uncertainty of realizing a benefit from those items. The Company’s losses before income taxes were generated in the United States and Austria.

Loss before the provision for income taxes for the years ended December 31, 2018, 2017 and 2016 consisted of the following (in thousands):

	Year Ended December 31,
	2018			2017			2016
United States	$	(32,022	)	$	(24,618	)	$	(12,969	)
Foreign (Austria)		(10,999	)		(9,259	)		(10,006	)
	$	(43,021	)	$	(33,877	)	$	(22,975	)

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

	Year Ended December 31,
	2018				2017				2016
U.S. federal statutory income tax rate		(21.0	)	%		(35.0	)	%		(35.0	)	%
State income taxes, net of federal benefit		(4.5	)			(3.5	)			(2.8	)
Foreign rate differential		(1.0	)			1.6				3.2
Research and development tax credits		(0.5	)			(0.4	)			(1.0	)
Nondeductible expenses		0.1				1.2				0.7
Uncertain tax position reserves		0.2				0.2				0.5
Stock-based compensation		0.7				0.5				0.3
Change in rates due to tax reform		—				16.5				—
Change in deferred tax asset valuation allowance		26.0				18.9				34.1
Effective income tax rate		—		%		—		%		—		%

Net deferred tax assets as of December 31, 2018, 2017 and 2016 consisted of the following (in thousands):

	December 31,
	2018			2017			2016
Net operating loss carryforwards	$	26,519		$	20,216		$	13,134
Research and development tax credit carryforwards		493			317			264
Start-up costs		7,755			4,350			3,917
Accrued expenses and other		1,460			850			689
Total deferred tax assets		36,227			25,733			18,004
Valuation allowance		(36,227	)		(25,733	)		(18,004	)
Net deferred tax assets	$	—		$	—		$	—

As of December 31, 2018, the Company had U.S. federal and state net operating loss carryforwards of $38.8 million (of which $14.5 million was generated in 2018 and has an indefinite life) and $34.9 million, respectively, which may be available to offset future income tax liabilities and begin to expire in 2031 and 2036, respectively. In addition, as of December 31, 2018, the Company had foreign net operating loss carryforwards of $64.7 million, which do not expire. As of December 31, 2018, the Company also had U.S. federal and state research and development tax credit carryforwards of $0.4 million and $0.1 million, respectively, which begin to expire in 2032 and 2031, respectively. As of December 31, 2018, uncertain tax position reserves recorded were $0.2 million for U.S. federal research and development tax credits and less than $0.1 million for state research and development tax credits.

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

The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of its deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2018, 2017 and 2016. Management reevaluates the positive and negative evidence at each reporting period.

Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2018, 2017 and 2016 related primarily to the increases in net operating loss carryforwards and research and development tax credit carryforwards and were as follows (in thousands):

	Year Ended December 31,
	2018			2017			2016
Valuation allowance at beginning of year	$	(25,733	)	$	(18,004	)	$	(10,367	)
Increases recorded to income tax provision		(10,494	)		(7,729	)		(7,637	)
Valuation allowance at end of year	$	(36,227	)	$	(25,733	)	$	(18,004	)

Changes in unrecognized tax benefits consisted of the following (in thousands):

	Year Ended December 31,
	2018		2017		2016
Unrecognized tax benefits at beginning of year	$	192	$	126	$	21
Increases for tax positions taken in current year		94		66		105
Unrecognized tax benefits at end of year	$	286	$	192	$	126

The Company’s U.S. federal and state income tax returns are generally subject to tax examinations for the tax years ended December 31, 2015 through December 31, 2017. There are currently no pending income tax examinations. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service and state tax authorities to the extent utilized in a future period. The Company’s policy is to record interest and penalties related to income taxes as part of its income tax provision.

On December 22, 2017, the United States enacted the Tax Cuts and Jobs Act ("Tax Reform Legislation"), which made significant changes to U.S. federal income tax law. The Company expects that certain aspects of the Tax Reform Legislation will positively impact the Company’s future after-tax earnings in the U.S., primarily due to the lower federal statutory tax rate. Set forth below is a discussion of certain provisions of the Tax Reform Legislation and the Company’s assessment of the effect of such provisions on the Company’s results of operations, cash flows and consolidated financial statements.

Beginning January 1, 2018, the Company’s U.S. income is taxed at a 21 percent federal corporate rate. Further, the Company is required to recognize the effect of this rate change on its deferred tax assets and liabilities and deferred tax asset valuation allowances in the period the tax rate change is enacted. The Company does not expect any material non-cash impact from this rate change, with adjustments to deferred tax balances offset by adjustments to deferred tax valuation allowances. Further, the Tax Reform Legislation provides for a one-time “deemed repatriation” of accumulated foreign earnings for the year ended December 31, 2017. The Company does not expect to pay U.S. federal cash taxes on the deemed repatriation due to an accumulated deficit in foreign earnings for tax purposes. The Company does not expect that its future foreign earnings will be subject to U.S. federal income tax.

On December 22, 2017, Staff Accounting Bulletin No. 118 ("SAB 118") was issued, which allowed the Company to record provisional amounts during a measurement period not to exceed beyond one year of the enactment date. As a result, the Company previously provided a provisional estimate of the effect of the Tax Act in its results of operations, cash flows and consolidated financial statements. In the fourth quarter of 2018, the Company completed its analysis to determine the effect of the Tax Act and concluded no adjustments were necessary.

Net Loss per Share

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss per Share

15.	Net Loss per Share

Net Loss per Share Attributable to Common Stockholders

Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts):

	Year Ended December 31,
	2018			2017			2016
Numerator:
Net loss	$	(43,021	)	$	(33,877	)	$	(22,975	)
Accretion of redeemable convertible preferred stock to redemption value		—			(44	)		(25	)
Net loss attributable to common stockholders	$	(43,021	)	$	(33,921	)	$	(23,000	)
Denominator:
Weighted average common shares outstanding—basic and diluted		14,307,934			2,061,845			513,527
Net loss per share attributable to common stockholders— basic and diluted	$	(3.01	)	$	(16.45	)	$	(44.79	)

The Company’s potentially dilutive securities, which include stock options, warrants to purchase shares of Preferred Stock and common stock, unvested restricted stock, convertible promissory notes and Preferred Stock, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. 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, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:

	Year Ended December 31,
	2018		2017		2016
Options to purchase common stock		1,943,390		1,403,119		548,903
Restricted common stock		250,000		—		—
Redeemable convertible preferred stock (as converted to common stock)		—		—		1,789,704
Warrants to purchase common stock		10,414		10,414		—
Warrants to purchase redeemable convertible preferred stock (as converted to common stock)		—		—		7,475
		2,203,804		1,413,533		2,346,082

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

16.	Commitments and Contingencies

Lease Agreements

In November 2010, the Company entered into a lease agreement for office, laboratory, parking and storage space in Vienna, Austria (“Vienna Lease”), which expires on April 30, 2021. The Company has the option to extend the lease agreement for an additional year. The Vienna Lease includes a rent escalation clause based on an inflation index. The Company provided a security deposit of approximately $0.3 million during the year ended December 31, 2010, which is included as a component of restricted cash on the Company's consolidated balance sheet as of December 31, 2018.

In July 2015, the Company entered into a lease agreement for an animal-use facility in Vienna, Austria (“Animal-use Lease”). The lease initially had a one-year noncancelable term, which expired in June 2016, after which the lease became cancelable by either party upon six months’ prior written notice. Base rent for the Animal-use Lease is approximately $0.4 million annually, accordingly, rent expense is being recognized on a straight-line basis over the lease term.

On August 31, 2018 and in accordance with the terms of the Animal-use Lease, the Company provided the landlord with written notice that the lease agreement will terminate no later than February 28, 2019. As of March 1, 2019, the Company had vacated the animal-use facility and the Company’s Animal-use Lease agreement with the landlord was terminated.

In November 2015, the Company entered into a lease agreement for office and laboratory space in Waltham, MA (“Waltham Lease”), which expired on January 31, 2019. The Waltham Lease includes a rent escalation clause, and accordingly, rent expense is being recognized on a straight-line basis over the lease term.

In June 2018, the Company entered into a lease agreement for office space in Waltham, MA (“Lease Agreement”) with BP Bay Colony LLC (the “Lessor”). The Company amended the Lease Agreement in August 2018 (“Amended Lease Agreement”). Under the terms of the Amended Lease Agreement, the Company will relocate its Waltham, MA office to a new location with 5,711 square feet of office space as compared to the 10,290 square feet premises in the original Lease Agreement. The term of the Amended Lease Agreement commences on January 1, 2019 (the “Commencement Date”) and expires approximately five years from the Commencement Date. The Company has the option to extend the term for one additional five-year period upon the Company’s written notice to the Lessor at least nine months and no more than 12 months in advance of the extension. Base rent is approximately $0.3 million annually, accordingly, rent expense will be recognized on a straight-line basis over the lease term. 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 the Amended Lease Agreement. The Company provided a security deposit of approximately $0.3 million during the year ended December 31, 2018, which is included as a component of restricted cash on the Company's consolidated balance sheet as of December 31, 2018.

The Company recognizes rent expense over the respective lease period and has recorded deferred rent for rent expense incurred but not yet paid.

The Company recorded rent expense of $1.3 million, $1.2 million and $1.2 million during the years ended December 31, 2018, 2017, and 2016, respectively.

The following table summarizes the future minimum lease payments due under the Company’s operating leases as of December 31, 2018 (in thousands):

Year Ending December 31,
2019	$	884
2020		789
2021		438
2022		263
2023		263
	$	2,637

License Agreements

The Company entered into the Adimab Option Agreement in February 2017 under which it is obligated to make contingent and non-contingent payments should the Company exercise its option to obtain rights to certain RSV antibodies (see Note 5). If the Company chose to exercise its option, it would be obligated to pay Adimab an option fee of $0.3 million and make clinical and regulatory milestone payments of up to $24.4 million, as well as royalty payments on a product-by-product and country-by-country basis of a mid single-digit percentage based on net sales 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. The Company may choose to exercise its option under the terms of the Adimab Option Agreement at any time on or before August 31, 2019. As of December 31, 2018 and 2017, the Company had not exercised its option under the Adimab Option Agreement.

Manufacturing Commitments

In July 2016, the Company entered into an agreement with Boehringer Ingelheim International GmbH (“BI”), a contract manufacturing organization, for the manufacture and supply of ASN100 drug product for the Company’s completed Phase 1 and discontinued Phase 2 clinical trials. In March 2016, the Company entered into an agreement with Cytovance Biologics, Inc. (“Cytovance”) for process development and the manufacture and supply of ASN100 drug product. Under such agreements, the Company is obligated to pay BI and Cytovance development and manufacturing milestones, in addition to reimbursement of certain material production-related costs. Additionally, the Company is required to make prepayments for process development services and manufacture and delivery of ASN100 material. The terms of these agreements require future delivery and formal acceptance of the clinical material upon delivery from BI and Cytovance. Formal acceptance includes validation of the clinical material and that established specifications have been met and good manufacturing practices, or GMP, standards have been followed during the manufacture of the material. It is only after acceptance that title and risks and rewards of ownership pass to the Company and at that time advance payments will be applied to the purchase of clinical materials required to be produced under the agreements. The purchase of the clinical material will, at the point of delivery, be charged to research and development expense. The Company’s policy is to expense research and development costs as incurred (i.e., as services are provided by the Company’s vendors or as qualifying materials are delivered).

As of December 31, 2018, BI and Cytovance have completed the development activities and manufacturing of ASN100 drug product, accordingly, the Company expensed all advance payments previously made to BI and Cytovance and accrued for any final payments owed. All commitments and obligations under the manufacture and supply agreements pertaining to the workorders with BI and Cytovance for the delivery of the clinical materials were met by BI, Cytovance and the Company as of December 31, 2018.

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 that will require the Company, among other things, to 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 indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2018, 2017 or 2016.

Legal Proceedings

The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities.

Related Party Transactions

12 Months Ended

Dec. 31, 2018

Related Party Transactions

17.	Related Party Transactions

Agreements with Adimab, LLC

In May 2011, the Company entered into the Adimab Collaboration Agreement with Adimab. The chairman of the Company’s board of directors is a co-founder of Adimab and currently serves as Adimab’s Chief Executive Officer. During the years ended December 31, 2018, 2017 and 2016, the Company made payments to Adimab of $0, $0 and $0.1 million, respectively, under the Adimab Collaboration Agreement. During the years ended December 31, 2018, 2017 and 2016, the Company recognized research and development expense of $0, $0 and $8,000, respectively, in connection with the Adimab Collaboration Agreement. As of December 31, 2018 and 2017, no amounts were due to Adimab under the Adimab Collaboration Agreement.

In February 2017, the Company entered into the Adimab Option Agreement with Adimab. During the years ended December 31, 2018 and 2017, the Company made payments to Adimab of $0.1 million and $0.1 million, respectively, under the Adimab Option Agreement. The Company recognized $0.1 million and $0.1 million during the years ended December 31, 2018 and 2017, respectively, of research and development expense under the Adimab Option Agreement. As of December 31, 2018 and 2017, the Company owed $0 and $21,000 to Adimab under the Adimab Option Agreement.

Services and Facilities Agreement with EveliQure Biotechnologies GmbH

The Company’s wholly owned subsidiary, Arsanis Biosciences GmbH, leases office and lab space in Vienna, Austria from a third party. In February 2015, Arsanis Biosciences GmbH entered into a services and facilities agreement with EveliQure Biotechnologies GmbH (“EveliQure”) under which the Company provided certain laboratory services and sublet office and lab space to EveliQure. Tamas Henics, the husband of Eszter Nagy, the Company’s former Chief Scientific Officer, serves as Chief Scientific Officer at EveliQure.

On June 28, 2018 and in accordance with the terms of this agreement with EveliQure, the Company provided EveliQure with written notice that the services and facilities agreement will terminate and EveliQure will vacate the sublet space no later than December 31, 2018. As of December 31, 2018, the Company had vacated the office and lab space and the Company’s services and facilities agreement with EveliQure was terminated.

During the years ended December 31, 2018, 2017 and 2016, the Company received payments from EveliQure under the agreement of $0.2 million, less than $0.1 million and less than $0.1 million, respectively. During the years ended December 31, 2018, 2017 and 2016, the Company recognized other income under the agreement of $0.1 million, $0.1 million and less than $0.1 million, respectively. As of December 31, 2018 and 2017, amounts due from EveliQure total less than $0.1 million and $0.1 million, respectively.

401(k) Savings Plan

12 Months Ended

Dec. 31, 2018

Compensation And Retirement Disclosure [Abstract]

401(k) Savings Plan

18.	401(k) Savings Plan

The Company established a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. This plan covers substantially all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. Matching contributions to the plan may be made at the discretion of the Company’s board of directors. The Company made no contributions to the plan during the years ended December 31, 2018, 2017 and 2016.

Selected Quarterly Financial Information (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Information (Unaudited)

19.	Selected Quarterly Financial Information (Unaudited)

Selected quarterly results from operations for the years ended December 31, 2018 and 2017 are as follows:

	2018 Quarter Ended
	March 31			June 30			September 30			December 31
Revenue	$	—		$	—		$	—		$	3,500
Operating Expenses		10,950			12,616			12,847			12,972
Net Loss		(10,630	)		(12,133	)		(10,900	)		(9,358	)
Basic and diluted net loss per common share	$	(0.74	)	$	(0.85	)	$	(0.76	)	$	(0.65	)

	2017 Quarter Ended
	March 31			June 30			September 30			December 31
Operating Expenses	$	5,827		$	5,644		$	13,056		$	11,606
Net Loss		(5,385	)		(5,705	)		(11,600	)		(11,187	)
Basic and diluted net loss per common share	$	(10.49	)	$	(11.13	)	$	(22.60	)	$	(1.68	)

Subsequent Events

12 Months Ended

Dec. 31, 2018

Subsequent Events [Abstract]

Subsequent Events

20.	Subsequent Events

FFG Settlement

On February 4, 2019, the Company and its wholly owned subsidiary, Arsanis Biosciences GmbH, received letters from counsel to FFG (see Notes 2, 7 and 8) alleging that they breached reporting, performance and other obligations in connection with the grants and loans made by FFG to Arsanis Biosciences GmbH between September 2011 and March 2017 to fund qualifying research and development expenditures (collectively, the “Subsidies”).

Pursuant to the terms of the Settlement Agreement, in exchange for FFG’s waiver of all claims against the Company and Arsanis Biosciences GmbH except for its claims for repayment of the loans and regular interest, including its waiver of claims for repayment of grants and interest exceeding regular interest, subject to compliance by the Company and Arsanis Biosciences GmbH with the terms of the Settlement Agreement, Arsanis Biosciences GmbH has agreed to repay the outstanding loan principal equal to EUR 8,505,204 ($9.5 million, based on an exchange rate of US$1.12 per EUR 1.00 on March 7, 2019) (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 have also agreed that (i) the portion of such loans to be repaid in 2019 is EUR 2,596,320 ($2.9 million, based on an exchange rate of US$1.12 per EUR 1.00 on March 7, 2019) (the “2019 Payment”) and such payment will be made on March 31, 2019, (ii) until all of the loans have been repaid and subject to other terms specified in the Settlement Agreement, a minimum cash balance equal to 70% of the then-outstanding principal amount of the loans will be maintained at Arsanis Biosciences GmbH in an account held with an Austrian bank (the “minimum cash requirement”), (iii) at least until December 31, 2021 and subject to other terms specified in the Settlement Agreement, Arsanis Biosciences GmbH will maintain a physical premises in Austria with a minimum of eight full-time equivalent employees, retain ownership rights to intellectual property (“IP”) which generated or may be generated (if any) from or in relation to the projects that are subject to the subsidy agreements with FFG in Austria, and to the extent that it licenses such IP to any third party it will receive arm’s length compensation as consideration and (iv) Arsanis Biosciences GmbH will comply with specified quarterly financial reporting obligations. The Company agreed (i) to procure the timely transfer of sufficient funds to Arsanis Biosciences GmbH to ensure the minimum cash requirement is met, (ii) to use its best efforts to enable Arsanis Biosciences GmbH to comply with specified obligations and (iii) to refrain from any instructions and measures that might endanger compliance with the specified obligations. X4 and Merger Sub agreed (i) to use commercially reasonable efforts to enable Arsanis Biosciences GmbH and the Company to comply with their above-mentioned obligations and (ii) to refrain from any instructions and measures that might endanger compliance with such specified obligations. If the Company or Arsanis Biosciences GmbH breaches specified obligations under the Settlement Agreement (and fails to cure such breach during any applicable grace period), FFG is entitled to accelerate the repayment of any outstanding loans. In addition, subject to the fulfillment of Arsanis Biosciences GmbH’s obligations and commitments under the Settlement Agreement, FFG has agreed that effective as of December 31, 2021, it will release the parties, as applicable, from all obligations and claims arising in relation to the subsidies and their commitments provided under the Settlement Agreement and under other documents in favor of Arsanis Biosciences GmbH, as in effect as of the date of the Settlement Agreement.

Merger Agreement Amendment

In connection with the Settlement Agreement, on March 8, 2019, the Company, Merger Sub and X4 entered into a Second Amendment to Agreement and Plan of Merger (the “Merger Agreement Amendment”) to the Merger Agreement.

Pursuant to the Merger Agreement Amendment, the Company and X4 have agreed to amend the terms of their previously announced Merger Agreement to reflect the Company’s agreement that 1/3rd of the 2019 Payment, which equals EUR 865,440 (approximately $968,600, based on an exchange rate of US$1.12 per EUR 1.00 on March 7, 2019) (the “Arsanis accelerated payment amount”), would be deducted from the Company’s “net cash” at closing (as defined in the Merger Agreement) and as a result would increase the exchange ratios for the X4 common stock and X4 preferred stock in the merger. Specifically, the Merger Agreement currently excludes the approximately EUR 8.5 million principal amount of FFG loans to Arsanis Biosciences GmbH from the deduction for unpaid indebtedness that otherwise reduces the Company’s “net cash” at closing. The Merger Agreement Amendment provides that this excluded amount (and thus “net cash”) be reduced by the Arsanis accelerated payment amount, to approximately EUR 7.6 million.

Lease Termination Agreement and Sublease Agreement

On February 26, 2019, Arsanis Biosciences GmbH entered into a lease termination agreement (the “Termination Agreement”) effective as of February 28, 2019, with Wüstenrot Marxbox GmbH & Co OG (the “Landlord”) to terminate the lease dated November 26, 2010, by and between the Landlord (as successor-in-interest to Marxbox Bauprojekt GmbH & Co. OG) and Arsanis Biosciences GmbH for its facility in Vienna, Austria (the “Lease”). The Termination Agreement was conditioned upon a new tenant, Hookipa Biotech GmbH (“Hookipa”), entering into a binding agreement to lease a portion of the premises subject to the Lease. Hookipa entered into its lease effective as of March 1, 2019.

In consideration for the termination of the Lease, Arsanis Biosciences GmbH agreed to make a lump sum payment to the Landlord in the amount of EUR 45,000 (approximately $50,600, based on an exchange rate of US$1.12 per EUR 1.00 on March 7, 2019), plus 20% value-added tax, in full satisfaction of its obligations under the Lease.

In connection with the Lease Termination Agreement, Arsanis Biosciences GmbH also entered into a sublease agreement with Hookipa, dated as of February 25, 2019, pursuant to which Arsanis Biosciences GmbH agreed to sublease approximately 400 square meters of the premises subject to the original Lease from Hookipa with a term beginning on March 1, 2019 (the “Sublease”), to be used for laboratory and office space. The Sublease has a term of two years and is terminable by Arsanis Biosciences GmbH upon three months’ notice. The monthly rent for the Sublease is approximately EUR 11,400 ($12,800, based on an exchange rate of US$1.12 per EUR 1.00 on March 7, 2019). In addition, Arsanis Biosciences GmbH agreed to sell certain laboratory equipment and office furniture to Hookipa.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Principles of Consolidation

Foreign Currency and Currency Translation

Cash and Cash Equivalents

Restricted Cash

Concentrations of Credit Risk

Property and Equipment

		Estimated Useful Life
Laboratory and office equipment		3 to 10 years
Furniture and fixtures		3 to 10 years
Computer equipment and software		1 to 5 years
Leasehold improvements		Shorter of lease term or 10 years

Impairment of Long-Lived Assets

Fair Value Measurements

•

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

•

Segment Information

Revenue Recognition

License Revenue

Government Contracts, Grant Agreements and Incentive Programs

Research and Development Costs

Research Contract Costs and Accruals

Patent Costs

Stock-Based Compensation

Comprehensive Loss

Income Taxes

Net Income (Loss) per Share

Recently Adopted Accounting Pronouncements

The restricted cash as of December 31, 2018 and 2017 is held as letters of credit for the benefit of the landlords in connection with the Company’s office leases and corporate credit cards.

	December 31,		December 31,
	2018		2017
Cash and cash equivalents	$	30,754	$	76,793
Restricted cash – current		101		—
Restricted cash – non-current		539		355
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$	31,394	$	77,148

Recently Issued Accounting Pronouncements

FFG Loans

Research and Development Incentive

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Schedule of Property and Equipment

Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the estimated useful lives of the assets as follows:

		Estimated Useful Life
Laboratory and office equipment		3 to 10 years
Furniture and fixtures		3 to 10 years
Computer equipment and software		1 to 5 years
Leasehold improvements		Shorter of lease term or 10 years

Property and equipment, net consisted of the following (in thousands):

	December 31,
	2018			2017
Laboratory and office equipment	$	1,676		$	1,739
Furniture and fixtures		402			419
Leasehold improvements		298			297
Computer equipment and software		180			189
		2,556			2,644
Less: Accumulated depreciation and amortization		(2,271	)		(2,223	)
	$	285		$	421

Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash

	December 31,		December 31,
	2018		2017
Cash and cash equivalents	$	30,754	$	76,793
Restricted cash – current		101		—
Restricted cash – non-current		539		355
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$	31,394	$	77,148

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Schedule of Assets and Liabilities Measured at Fair Value

	Fair Value Measurements
	as of December 31, 2018 Using:
	Level 1		Level 3		Total
Assets:
Cash equivalents - Money Market Funds	$	29,836	$	—	$	29,836
	$	29,836	$	—	$	29,836

	Fair Value Measurements
	as of December 31, 2017 Using:
	Level 1		Level 3		Total
Assets:
Cash equivalents - Money Market Funds	$	70,891	$	—	$	70,891
	$	70,891	$	—	$	70,891

Prepaid Expenses and Other Current Assets (Tables)

12 Months Ended

Dec. 31, 2018

Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]

Schedule of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following (in thousands):

	December 31,
	2018		2017
Prepaid clinical trial costs	$	—	$	257
Prepaid directors' and officers' and other corporate insurance		1,007		524
Prepaid other		359		348
	$	1,366	$	1,129

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the estimated useful lives of the assets as follows:

		Estimated Useful Life
Laboratory and office equipment		3 to 10 years
Furniture and fixtures		3 to 10 years
Computer equipment and software		1 to 5 years
Leasehold improvements		Shorter of lease term or 10 years

Property and equipment, net consisted of the following (in thousands):

	December 31,
	2018			2017
Laboratory and office equipment	$	1,676		$	1,739
Furniture and fixtures		402			419
Leasehold improvements		298			297
Computer equipment and software		180			189
		2,556			2,644
Less: Accumulated depreciation and amortization		(2,271	)		(2,223	)
	$	285		$	421

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consisted of the following (in thousands):

	December 31,
	2018		2017
Accrued clinical trial costs	$	294	$	2,317
Accrued compensation and benefits		2,522		2,454
Accrued professional fees		1,156		510
Accrued other		144		498
	$	4,116	$	5,779

Schedule of Restructuring Activities Included in Accrued Compensation and Benefits as Component of Accrued Expenses on Consolidated Balance Sheet

	December 31,
	2018
Employee severance benefits	$	1,648
Employee retention benefits		1,886
Payments		(1,539	)
Total	$	1,995

Loans Payable (Tables)

12 Months Ended

Dec. 31, 2018

Debt Instrument [Line Items]

Schedule of Aggregate Principal Amount of Debt Outstanding

The aggregate principal amount of debt outstanding as of December 31, 2018 and 2017 consisted of the following (in thousands):

	December 31,
	2018		2017
Term loans under 2012 Loan Agreement	$	—	$	4,667
FFG Loans		9,738		10,225
	$	9,738	$	14,892

Schedule of Current and Non-current Debt obligations

Current and non-current debt obligations reflected in the consolidated balance sheets as of December 31, 2018 and 2017 consisted of the following (in thousands):

	December 31,
	2018			2017
Current liabilities:
Term loans under 2012 Loan Agreement	$	—		$	2,333
FFG loans		—			—
Unamortized debt discount		—			(19	)
Loans payable, net of discount		—			2,314
Non-current liabilities:
Term loans under 2012 Loan Agreement	$	—		$	2,334
FFG loans		9,738			10,225
Unamortized debt discount		(1,844	)		(2,637	)
Loans payable, net of discount and current portion		7,894			9,922
Total loans payable, net of discount	$	7,894		$	12,236

FFG Loans

Debt Instrument [Line Items]

Future Minimum Payment

As of December 31, 2018, the aggregate minimum future principal payments due in connection with the FFG loans are summarized as follows (in thousands):

Year Ending December 31,
2019	$	—
2020		4,948
2021		3,931
2022		859
2023		—
	$	9,738

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 505
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 235
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-Paragraph 1
-Subparagraph (SX 210.4-08.(e),(f))
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Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 9: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
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-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

+ Details

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock Option Valuation

	Year Ended December 31,
	2018			2017			2016
Risk-free interest rate		2.72	%		1.96	%		1.26	%
Expected term (in years)		6.00			6.02			5.80
Expected volatility		77.4	%		75.7	%		75.3	%
Expected dividend yield		0	%		0	%		0	%

	Year Ended December 31,
	2016
Risk-free interest rate		2.08	%
Expected term (in years)		8.82
Expected volatility		77.8	%
Expected dividend yield		0	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity since December 31, 2017 (in thousands, except share and per share amounts):

						Weighted
				Weighted		Average
				Average		Remaining		Aggregate
	Number of			Exercise		Contractual		Intrinsic
	Shares			Price		Term		Value
						(in years)
Outstanding as of December 31, 2017		1,403,119		$	6.26		8.81	$	9,128
Granted		829,500			17.19
Exercised		(27,226	)		6.51
Forfeited		(262,003	)		12.11
Outstanding as of December 31, 2018		1,943,390		$	10.13		8.00	$	49
Options exercisable as of December 31, 2018		1,053,481		$	8.26		7.27	$	49
Options unvested as of December 31, 2018		889,909		$	12.35		8.86	$	—

Summary of Restricted Common Stock Activity

The following table summarizes the Company’s restricted common stock activity:

			Weighted Average
	Number of		Grant Date
	Shares		Fair Value
Unvested restricted common stock as of December 31, 2017		—		—
Issued		250,000	$	4.12
Vested		—		—
Unvested restricted common stock as of December 31, 2018		250,000	$	4.12

Summary of Stock-Based Compensation Expense Classification

	Year Ended December 31,
	2018		2017		2016
Research and development expenses	$	1,079	$	329	$	294
General and administrative expenses		4,541		577		350
	$	5,620	$	906	$	644

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Loss Before Provision for Income Taxes

Loss before the provision for income taxes for the years ended December 31, 2018, 2017 and 2016 consisted of the following (in thousands):

	Year Ended December 31,
	2018			2017			2016
United States	$	(32,022	)	$	(24,618	)	$	(12,969	)
Foreign (Austria)		(10,999	)		(9,259	)		(10,006	)
	$	(43,021	)	$	(33,877	)	$	(22,975	)

Reconciliation of U.S. Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate

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

	Year Ended December 31,
	2018				2017				2016
U.S. federal statutory income tax rate		(21.0	)	%		(35.0	)	%		(35.0	)	%
State income taxes, net of federal benefit		(4.5	)			(3.5	)			(2.8	)
Foreign rate differential		(1.0	)			1.6				3.2
Research and development tax credits		(0.5	)			(0.4	)			(1.0	)
Nondeductible expenses		0.1				1.2				0.7
Uncertain tax position reserves		0.2				0.2				0.5
Stock-based compensation		0.7				0.5				0.3
Change in rates due to tax reform		—				16.5				—
Change in deferred tax asset valuation allowance		26.0				18.9				34.1
Effective income tax rate		—		%		—		%		—		%

Net Deferred Tax Assets

Net deferred tax assets as of December 31, 2018, 2017 and 2016 consisted of the following (in thousands):

	December 31,
	2018			2017			2016
Net operating loss carryforwards	$	26,519		$	20,216		$	13,134
Research and development tax credit carryforwards		493			317			264
Start-up costs		7,755			4,350			3,917
Accrued expenses and other		1,460			850			689
Total deferred tax assets		36,227			25,733			18,004
Valuation allowance		(36,227	)		(25,733	)		(18,004	)
Net deferred tax assets	$	—		$	—		$	—

Changes in Valuation Allowance for Deferred Tax Assets

	Year Ended December 31,
	2018			2017			2016
Valuation allowance at beginning of year	$	(25,733	)	$	(18,004	)	$	(10,367	)
Increases recorded to income tax provision		(10,494	)		(7,729	)		(7,637	)
Valuation allowance at end of year	$	(36,227	)	$	(25,733	)	$	(18,004	)

Changes in Unrecognized Tax Benefits

Changes in unrecognized tax benefits consisted of the following (in thousands):

	Year Ended December 31,
	2018		2017		2016
Unrecognized tax benefits at beginning of year	$	192	$	126	$	21
Increases for tax positions taken in current year		94		66		105
Unrecognized tax benefits at end of year	$	286	$	192	$	126

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2018

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 follows (in thousands, except share and per share amounts):

	Year Ended December 31,
	2018			2017			2016
Numerator:
Net loss	$	(43,021	)	$	(33,877	)	$	(22,975	)
Accretion of redeemable convertible preferred stock to redemption value		—			(44	)		(25	)
Net loss attributable to common stockholders	$	(43,021	)	$	(33,921	)	$	(23,000	)
Denominator:
Weighted average common shares outstanding—basic and diluted		14,307,934			2,061,845			513,527
Net loss per share attributable to common stockholders— basic and diluted	$	(3.01	)	$	(16.45	)	$	(44.79	)

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

	Year Ended December 31,
	2018		2017		2016
Options to purchase common stock		1,943,390		1,403,119		548,903
Restricted common stock		250,000		—		—
Redeemable convertible preferred stock (as converted to common stock)		—		—		1,789,704
Warrants to purchase common stock		10,414		10,414		—
Warrants to purchase redeemable convertible preferred stock (as converted to common stock)		—		—		7,475
		2,203,804		1,413,533		2,346,082

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Summary of Future Minimum Lease Payments Due under Operating Leases

The following table summarizes the future minimum lease payments due under the Company’s operating leases as of December 31, 2018 (in thousands):

Year Ending December 31,
2019	$	884
2020		789
2021		438
2022		263
2023		263
	$	2,637

Selected Quarterly Financial Information (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Results from Operations

Selected quarterly results from operations for the years ended December 31, 2018 and 2017 are as follows:

	2018 Quarter Ended
	March 31			June 30			September 30			December 31
Revenue	$	—		$	—		$	—		$	3,500
Operating Expenses		10,950			12,616			12,847			12,972
Net Loss		(10,630	)		(12,133	)		(10,900	)		(9,358	)
Basic and diluted net loss per common share	$	(0.74	)	$	(0.85	)	$	(0.76	)	$	(0.65	)

	2017 Quarter Ended
	March 31			June 30			September 30			December 31
Operating Expenses	$	5,827		$	5,644		$	13,056		$	11,606
Net Loss		(5,385	)		(5,705	)		(11,600	)		(11,187	)
Basic and diluted net loss per common share	$	(10.49	)	$	(11.13	)	$	(22.60	)	$	(1.68	)

Nature of the Business - Additional Information (Detail) $ / shares in Units, $ in Thousands		3 Months Ended								7 Months Ended	12 Months Ended
	Nov. 20, 2017 USD ($) $ / shares shares	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 01, 2019	Dec. 31, 2018 USD ($) Employee	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of eliminated positions \| Employee											28
Percentage of eliminated positions										65.00%
Reverse stock split, description											one-for-3.4130
Net proceeds from offerings	$ 58,100
Conversion of convertible preferred stock into common stock upon closing of initial public offering (in shares) \| shares	7,180,483
Accumulated deficit		$ (135,286)				$ (92,265)					$ (135,286)	$ (92,265)
Net loss		(9,358)	$ (10,900)	$ (12,133)	$ (10,630)	(11,187)	$ (11,600)	$ (5,705)	$ (5,385)		(43,021)	(33,877)	$ (22,975)
Cash and cash equivalents		30,754				$ 76,793					30,754	$ 76,793
ASU 2014-15
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Accumulated deficit		(135,300)									(135,300)
Net loss											(43,000)
Cash used in operations											40,900
Cash and cash equivalents		$ 30,800									$ 30,800
Initial Public Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Shares issued \| shares	4,000,000
Shares issued, price per share \| $ / shares	$ 10.00
Over-Allotment Option
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Shares issued \| shares	600,000
Shares issued, price per share \| $ / shares	$ 10.00
Private Placement \| New Enterprise Associates 16, L.P.
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Shares issued \| shares	2,000,000
Shares issued, price per share \| $ / shares	$ 10.00

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Nov. 30, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Significant Accounting Policies [Line Items]
Restricted cash		$ 101	$ 0
Lease expiration date			Jan. 31, 2019
Restricted cash – non-current		539	$ 355
Foreign currency translation gain (loss)		$ 114	(707)	$ 116
Revenue from Contracts with Customers (“Topic 606”)
Significant Accounting Policies [Line Items]
Revenue, Practical Expedient, Financing Component		true
Waltham Lease
Significant Accounting Policies [Line Items]
Lease expiration date	Jan. 31, 2019	Dec. 31, 2023
Restricted cash – non-current		$ 300
Office Laboratory
Significant Accounting Policies [Line Items]
Lease expiration date		Apr. 30, 2021
Restricted cash – non-current		$ 300
Corporate Credit Card \| Letter of Credit
Significant Accounting Policies [Line Items]
Restricted cash		50
Corporate Credit Card \| Waltham Lease
Significant Accounting Policies [Line Items]
Restricted cash		$ 51
Company's Office Leases
Significant Accounting Policies [Line Items]
Restricted cash – non-current			$ 400

Summary of Significant Accounting Policies - Schedule of Estimated Useful Lives of Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Laboratory and Office Equipment \| Minimum
Property, Plant and Equipment [Line Items]
Property and Equipment, Estimated Useful Life	3 years	3 years
Laboratory and Office Equipment \| Maximum
Property, Plant and Equipment [Line Items]
Property and Equipment, Estimated Useful Life	10 years	10 years
Furniture and Fixtures \| Minimum
Property, Plant and Equipment [Line Items]
Property and Equipment, Estimated Useful Life	3 years	3 years
Furniture and Fixtures \| Maximum
Property, Plant and Equipment [Line Items]
Property and Equipment, Estimated Useful Life	10 years	10 years
Computer Equipment and Software \| Minimum
Property, Plant and Equipment [Line Items]
Property and Equipment, Estimated Useful Life	1 year	1 year
Computer Equipment and Software \| Maximum
Property, Plant and Equipment [Line Items]
Property and Equipment, Estimated Useful Life	5 years	5 years
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property and Equipment, Estimated Useful Lives	Shorter of lease term or 10 years	Shorter of lease term or 10 years

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash and cash equivalents	$ 30,754	$ 76,793
Restricted cash	101	0
Restricted cash – non-current	539	355
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$ 31,394	$ 77,148	$ 3,429	$ 7,097

Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets
Cash equivalents	$ 29,836	$ 70,891
Level 1
Assets
Cash equivalents	29,836	70,891
Money Market Funds
Assets
Cash equivalents	29,836	70,891
Money Market Funds \| Level 1
Assets
Cash equivalents	$ 29,836	$ 70,891

Fair Value Measurements - Additional Information (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Transfers between fair value levels	$ 0	$ 0

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Prepaid Expense And Other Assets Current [Abstract]
Prepaid clinical trial costs		$ 257
Prepaid directors' and officers' and other corporate insurance	$ 1,007	524
Prepaid other	359	348
Prepaid expense and other current assets	$ 1,366	$ 1,129

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	$ 2,556	$ 2,644
Less: Accumulated depreciation and amortization	(2,271)	(2,223)
Property and Equipment, Net	285	421
Laboratory and Office Equipment
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	1,676	1,739
Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	402	419
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	298	297
Computer Equipment and Software
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	$ 180	$ 189

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 155	$ 195	$ 285

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued clinical trial costs	$ 294	$ 2,317
Accrued compensation and benefits	2,522	2,454
Accrued professional fees	1,156	510
Accrued other	144	498
Total accrued expenses	$ 4,116	$ 5,779

Accrued Expenses - Additional Information (Detail) $ in Millions	3 Months Ended	7 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 01, 2019	Dec. 31, 2018 USD ($) Employee
Accrued Expenses [Line Items]
Number of eliminated positions \| Employee			28
Percentage of eliminated positions		65.00%
Estimated total expenses relating to reduction in workforce			$ 5.9
Severance and retention expense			$ 3.5
Scenario, Forecast [Member]
Accrued Expenses [Line Items]
Severance and retention expense	$ 2.4

Accrued Expenses - Schedule of Restructuring Activities Included in Accrued Compensation and Benefits as Component of Accrued Expenses on Consolidated Balance Sheet (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Payables And Accruals [Abstract]
Employee severance benefits	$ 1,648
Employee retention benefits	1,886
Payments	(1,539)
Total	$ 1,995

Collaboration, License and Funding Arrangements - Additional Information (Detail)		1 Months Ended				12 Months Ended
	Nov. 20, 2017 shares	Sep. 30, 2017 shares	Apr. 30, 2017 USD ($) shares	Mar. 31, 2017 USD ($)	Feb. 28, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Aug. 31, 2018 USD ($)
Collaboration License And Funding Arrangements [Line Items]
Research and development expense						$ 30,979,000	$ 28,128,000	$ 17,831,000
Grant income						$ 3,179,000	3,868,000	$ 2,390,000
Conversion of redeemable convertible preferred stock into common stock \| shares	7,180,483
Reverse stock split, description						one-for-3.4130
Grant receivable						$ 2,859,000	1,608,000
Unearned income						737,000	694,000
Unearned income						$ 1,107,000	$ 1,936,000
Austria
Collaboration License And Funding Arrangements [Line Items]
Grant income						14.00%	12.00%	12.00%
Series D Redeemable Convertible Preferred Stock
Collaboration License And Funding Arrangements [Line Items]
Shares issued \| shares		1,540,500	10,799,880
February 2017 Grant Agreement \| Gates Foundation
Collaboration License And Funding Arrangements [Line Items]
Grant receivable					$ 9,300,000
Proceeds from related party				$ 1,600,000
Grant income						$ 0	$ 1,600,000
Unearned income						0	0
August 2018 Grant Agreement \| Gates Foundation
Collaboration License And Funding Arrangements [Line Items]
Grant receivable									$ 1,100,000
Grant income						1,100,000
Unearned income						0
Grant Agreement
Collaboration License And Funding Arrangements [Line Items]
Qualifying expenses related to letter agreement						$ 0	8,000,000
Grant Agreement \| Gates Foundation
Collaboration License And Funding Arrangements [Line Items]
Conversion of redeemable convertible preferred stock into common stock \| shares			722,179
Reverse stock split, description						one-for-3.4130 reverse-stock-split
Reverse stock split, conversion ratio			0.2930
Grant Agreement \| Gates Foundation \| Series D Redeemable Convertible Preferred Stock
Collaboration License And Funding Arrangements [Line Items]
Shares issued \| shares			2,464,799
Shares purchased			$ 8,000,000
FFG Grants
Collaboration License And Funding Arrangements [Line Items]
Grant income						$ 0	200,000	$ 600,000
Unearned income						0	0
Grant receivable						100,000	100,000
FFG Loans
Collaboration License And Funding Arrangements [Line Items]
Grant income						700,000	600,000	400,000
Unearned income						700,000	700,000
Unearned income						1,100,000	1,900,000
Research and Development Incentive
Collaboration License And Funding Arrangements [Line Items]
Grant income						1,400,000	1,400,000	1,400,000
Grant receivable						2,700,000	1,500,000
Adimab, LLC \| Adimab Option Agreement
Collaboration License And Funding Arrangements [Line Items]
Collaboration payment obligations						0
Research and development expense						100,000	100,000
Option fee					300,000
Future milestone payments					$ 24,400,000
Adimab, LLC \| Adimab Collaboration Agreement
Collaboration License And Funding Arrangements [Line Items]
Research and development expense						$ 0	$ 0	8,000
Minimum \| Funding Agreements with FFG
Collaboration License And Funding Arrangements [Line Items]
Percentage of loans and grants to fund						50.00%
Maximum \| Funding Agreements with FFG
Collaboration License And Funding Arrangements [Line Items]
Percentage of loans and grants to fund						70.00%
Maximum \| Adimab, LLC \| Adimab Collaboration Agreement
Collaboration License And Funding Arrangements [Line Items]
Research and development expense								$ 100,000

Loans Payable - Schedule of Aggregate Principal Amount of Debt Outstanding (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Principal amount of debt outstanding	$ 9,738	$ 14,892
Term Loans under 2012 Loan Agreement
Debt Instrument [Line Items]
Principal amount of debt outstanding		4,667
FFG Loans
Debt Instrument [Line Items]
Principal amount of debt outstanding	$ 9,738	$ 10,225

Loans Payable - Schedule of Current and Non-current Debt obligations (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current liabilities:
Loans payable, net of discount		$ 2,314
Unamortized debt discount		(19)
Non-current liabilities:
Unamortized debt discount	$ (1,844)	(2,637)
Loan payable, net of discount and current portion	7,894	9,922
Total loans payable, net of discount	7,894	12,236
Term Loans under 2012 Loan Agreement
Current liabilities:
Loans payable, net of discount		2,333
Non-current liabilities:
Loans payable, non-current		2,334
FFG Loans
Non-current liabilities:
Loans payable, non-current	$ 9,738	$ 10,225

Loans Payable - Additional Information (Detail)								1 Months Ended		12 Months Ended
Loans Payable - Additional Information (Detail)	Mar. 08, 2019 USD ($)	Mar. 08, 2019 EUR (€)	Feb. 19, 2019 USD ($)	Feb. 19, 2019 EUR (€)	Feb. 18, 2018	Feb. 29, 2016 USD ($)	Dec. 07, 2012 USD ($)	Oct. 31, 2018 USD ($)	Aug. 31, 2016 USD ($)	Dec. 31, 2018 USD ($) Tranche $ / shares shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 08, 2019 EUR (€)	Mar. 07, 2019 $ / €	Feb. 19, 2019 EUR (€)	Nov. 20, 2017 $ / shares shares
Debt Instrument [Line Items]
Tranches \| Tranche										2
Prepayment fee percentage to outstanding principal amount										0.00%
Final payment of outstanding obligations										$ 400,000
Aggregate principal loan amount outstanding										$ 9,738,000	$ 14,892,000
Subsequent Event
Debt Instrument [Line Items]
Aggregate principal loan amount outstanding	$ 9,500,000												€ 8,505,204
Exchange rate per EUR 1.00 \| $ / €														1.12
Repayment of outstanding loan principal	$ 2,900,000	€ 2,596,320
Exercise Price One \| Common Stock
Debt Instrument [Line Items]
Warrants issued to purchase stock \| shares										3,940
Warrants exercise price \| $ / shares										$ 12.70
Exercise Price One \| Series A-2 Warrants \| Common Stock
Debt Instrument [Line Items]
Warrants exercise price \| $ / shares																$ 12.70
Exercise Price Two \| Common Stock
Debt Instrument [Line Items]
Warrants issued to purchase stock \| shares										6,474
Warrants exercise price \| $ / shares										$ 16.22
Exercise Price Two \| Series B Preferred Stock \| Common Stock
Debt Instrument [Line Items]
Warrants exercise price \| $ / shares																$ 16.22
Series A-2 Warrants
Debt Instrument [Line Items]
Warrants exercise price \| $ / shares										4.54
Series B Preferred Stock
Debt Instrument [Line Items]
Warrants exercise price \| $ / shares										$ 7.24
Minimum
Debt Instrument [Line Items]
Prepayment fee percentage to outstanding principal amount					1.00%
Maximum
Debt Instrument [Line Items]
Prepayment fee percentage to outstanding principal amount					2.00%
Maximum \| Exercise Price One \| Series A-2 Warrants \| Common Stock
Debt Instrument [Line Items]
Warrants issued to purchase stock \| shares																3,940
Maximum \| Exercise Price Two \| Series B Preferred Stock \| Common Stock
Debt Instrument [Line Items]
Warrants issued to purchase stock \| shares																6,474
2012 Loan Agreement
Debt Instrument [Line Items]
Proceeds from borrowings						$ 3,500,000	$ 2,500,000		$ 3,500,000
Maximum borrowing capacity										$ 7,000,000
Additional amounts available to be borrowed										$ 0
Percentage minus Wall Street Journal prime rate							0.25%
Interest rate increase percentage							4.00%
Outstanding capital stock pledged as collateral										65.00%
2012 Loan Agreement \| Series A-2 Warrants
Debt Instrument [Line Items]
Warrants issued to purchase stock \| shares										11,013
2012 Loan Agreement \| Series B Preferred Stock
Debt Instrument [Line Items]
Warrants issued to purchase stock \| shares										14,502
2012 Loan Agreement \| Minimum
Debt Instrument [Line Items]
Advance amount										$ 500,000
Interest rate of loan							3.25%
2012 Term Loan A Advance
Debt Instrument [Line Items]
Proceeds from borrowings							$ 500,000
2012 Term Loan B Advance
Debt Instrument [Line Items]
Proceeds from borrowings							$ 2,000,000
Term Loans under 2012 Loan Agreement
Debt Instrument [Line Items]
Final payment of outstanding obligations								$ 400,000
Outstanding obligations paid of principal amount								2,700,000
Interest expense recognized										300,000	400,000	$ 300,000
Unamortized debt discount										0
Aggregate principal loan amount outstanding											4,667,000
Term Loans under 2012 Loan Agreement \| Maximum
Debt Instrument [Line Items]
Interest expense recognized								$ 100,000
Unamortized debt discount											100,000
FFG Loans
Debt Instrument [Line Items]
Interest expense recognized										800,000	700,000	500,000
Unamortized debt discount										1,800,000	2,600,000
Aggregate principal loan amount outstanding										$ 9,738,000	10,225,000
Loan amount maturity start date										Jun. 30, 2020
Loan amount maturity end date										Mar. 31, 2023
Aggregate principal payments										$ 0	$ 0	$ 0
FFG Loans \| Bank Demand Letters \| Subsequent Event
Debt Instrument [Line Items]
Repayment of grants, outstanding loan principal and assessed interest			$ 20,400,000	€ 18,100,000
Exchange rate per EUR 1.00 \| $ / €														1.12
Reimbursement of grants			8,100,000	7,200,000
Repayment of outstanding loan principal			9,600,000	€ 8,500,000
Repayment of assessed interest			$ 2,700,000												€ 2,400,000
FFG Loans \| Minimum
Debt Instrument [Line Items]
Interest rate of loan										0.75%
FFG Loans \| Maximum
Debt Instrument [Line Items]
Interest rate of loan										2.00%

Loans payable - Future Minimum Payment (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Maturities Of Long Term Debt [Abstract]
2020	$ 4,948
2021	3,931
2022	859
Long-term Debt, Total	$ 9,738

Convertible Promissory Notes - Additional Information (Detail) - USD ($)	1 Months Ended
	Apr. 30, 2017	Jan. 31, 2017	Apr. 30, 2016	Dec. 31, 2015	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Convertible promissory notes outstanding					$ 0	$ 0
2015 Notes
Short-term Debt [Line Items]
Aggregate principal amount				$ 4,000,000
Bore interest rate				0.56%
Debt instrument, Maturity date				Dec. 16, 2016
2015 Notes \| Series C Preferred Stock
Short-term Debt [Line Items]
Debt conversion amount			$ 4,000,000
Shares issued upon conversion of notes			461,396
Conversion percentage per share			90.00%
Shares issued, price per share			$ 9.65
2016 Notes
Short-term Debt [Line Items]
Aggregate principal amount			$ 5,500,000
Bore interest rate			0.70%
Debt instrument, Maturity date			Oct. 12, 2017
2016 Notes \| Series D Preferred Stock
Short-term Debt [Line Items]
Debt conversion amount	$ 5,500,000
Shares issued upon conversion of notes	1,896,297
Conversion percentage per share	90.00%
Shares issued, price per share	$ 3.2457
2017 Notes
Short-term Debt [Line Items]
Aggregate principal amount		$ 4,900,000
Bore interest rate		0.96%
Debt instrument, Maturity date		Oct. 12, 2017
2017 Notes \| Series D Preferred Stock
Short-term Debt [Line Items]
Debt conversion amount	$ 4,900,000
Shares issued upon conversion of notes	1,524,107
Shares issued, price per share	$ 3.2457

Redeemable Convertible Preferred Stock - Additional Information (Detail) - USD ($)		1 Months Ended							12 Months Ended
	Nov. 20, 2017	Sep. 30, 2017	Apr. 30, 2017	Apr. 30, 2016	May 31, 2015	Jul. 31, 2013	Jan. 31, 2011	Sep. 30, 2010	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
Temporary Equity [Line Items]
Proceeds from issuance of preferred stock									$ 40,053,000	$ 5,500,000
Conversion of redeemable convertible preferred stock into common stock	7,180,483
Preferred Shares Outstanding											0
Series A-1 Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Additional shares to be issued								200,001
Additional shares to be issued, price per share								$ 2.00
Proceeds from issuance of preferred stock								$ 400,000
Issuance costs								$ 27,000
Series A-2 Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Additional shares to be issued							2,114,538
Additional shares to be issued, price per share							$ 4.54
Proceeds from issuance of preferred stock							$ 9,600,000
Issuance costs							$ 8,000
Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Additional shares to be issued					966,851	1,795,580
Additional shares to be issued, price per share					$ 7.24	$ 7.24
Proceeds from issuance of preferred stock					$ 7,000,000	$ 12,900,000
Issuance costs					$ 12,000	$ 68,000
Series C Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Additional shares to be issued				569,946
Additional shares to be issued, price per share				$ 9.65
Proceeds from issuance of preferred stock				$ 5,400,000
Issuance costs				$ 100,000
Series C Redeemable Convertible Preferred Stock \| Two Thousand Fifteen Notes
Temporary Equity [Line Items]
Shares issued upon conversion of notes				461,396
Series D Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Additional shares to be issued		1,540,500	10,799,880
Additional shares to be issued, price per share		$ 3.2457	$ 3.2457
Proceeds from issuance of preferred stock		$ 5,000,000	$ 34,800,000
Issuance costs		$ 0	$ 200,000
Series D Redeemable Convertible Preferred Stock \| Preferred Stock Purchase Agreement
Temporary Equity [Line Items]
Additional shares to be issued			1,540,508
Additional shares to be issued, price per share			$ 3.2457
Series D Redeemable Convertible Preferred Stock \| 2016 Notes
Temporary Equity [Line Items]
Shares issued upon conversion of notes			1,896,297
Series D Redeemable Convertible Preferred Stock \| 2017 Notes
Temporary Equity [Line Items]
Shares issued upon conversion of notes			1,524,107

Common Stock - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Common stock, authorized shares	200,000,000	200,000,000
Common stock, par value	$ 0.001	$ 0.001
Preferred stock, authorized shares	10,000,000	10,000,000
Preferred 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 the stockholders for a vote. The holders of common stock are entitled to remove directors of the Company for cause by vote of at least 75% of the votes that all of the Company’s stockholders would be entitled to cast in an annual election of the directors
Dividends on common stock declared or paid	$ 0
Common stock, voting rights percentage	75.00%
Common stock reserved for future issuance	2,731,203	2,187,252

Stock-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation arrangement, number of options granted to purchase shares		829,500
Share based compensation, description		The 2017 Plan is administered by the board of directors. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, except that the exercise price per share of incentive stock options may not be less than 100% of the fair market value of the common stock on the date of grant and the term of stock options may not be greater than 10 years. Stock options awarded under the 2017 Plan expire 10 years after the grant date, unless the board of directors sets a shorter term.
Vesting rights, descriptions		Vesting periods for awards under the 2017 Plan are determined at the discretion of the board of directors. 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.
Common stock reserved for future issuance		2,731,203	2,187,252
Share based compensation		$ 5,620	$ 906	$ 644
Options granted, weighted average grant date fair value		$ 11.77	$ 3.62	$ 6.02
Options vested during the period		$ 4,500	$ 800	$ 500
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation		25	$ 0	1
Unrecognized stock‑based compensation expense to unvested restricted stock awards		$ 1,000
Unrecognized compensation cost of stock based awards, recognition period		3 years 10 months 24 days
Restricted Stock \| 2017 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation cost of stock based awards, recognition period		2 years 9 months 10 days	2 years 9 months 3 days
Unrecognized compensation cost of stock based awards		$ 7,200	$ 4,000
2017 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of number of common stock outstanding		2.00%
Common stock reserved for future issuance		219,748
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, beginning on January 1, 2019 and continuing through January 1, 2029, in an amount equal to the lowest of 512,745 shares of the Company’s common stock, 2% 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.
Ordinary share issued		512,745
Shares of common stock issued		637
Employees And Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation		$ 5,600	$ 900	600
Non Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation arrangement, number of options granted to purchase shares		0	0
Share based compensation		$ 13	$ 7	$ 15
Former President and Chief Executive Officer and Chief Scientific Officer
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vested during the period		$ 2,500
2010 Special Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation arrangement, number of options granted to purchase shares		0
2011 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation arrangement, number of options granted to purchase shares		0
2017 Equity Incentive Plan \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation arrangement, number of options granted to purchase shares		585,994
2017 Equity Incentive Plan \| Common Stock \| Board of Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of number of common stock outstanding	4.00%
2017 Equity Incentive Plan \| Common Stock \| Maximum \| Board of Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation arrangement, number of additional shares authorized	1,025,490

Stock-Based Compensation - Schedule of Assumptions Used in Black-Scholes Option-Pricing Model to Determine Grant-date Fair Value of Stock Options Granted (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employees And Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	2.72%	1.96%	1.26%
Expected term (in years)	6 years	6 years 7 days	5 years 9 months 18 days
Expected volatility	77.40%	75.70%	75.30%
Expected dividend yield	0.00%	0.00%	0.00%
Non Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate			2.08%
Expected term (in years)			8 years 9 months 25 days
Expected volatility			77.80%
Expected dividend yield			0.00%

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares Outstanding, Beginning Balance	1,403,119
Number of Shares Granted	829,500
Number of Shares Exercised	(27,226)
Number of Shares Forfeited	(262,003)
Number of Shares Outstanding, Ending Balance	1,943,390	1,403,119
Number of Share Options exercisable	1,053,481
Number of Share Options unvested	889,909
Number of Shares Outstanding, Weighted Average Exercise Price, Beginning Balance	$ 6.26
Number of Shares Granted, Weighted Average Exercise Price	17.19
Number of Shares Exercised, Weighted Average Exercise Price	6.51
Number of Shares Forfeited, Weighted Average Exercise Price	12.11
Number of Shares Outstanding, Weighted Average Exercise Price, Ending Balance	10.13	$ 6.26
Number of Share Options exercisable, Weighted Average Exercise Price	8.26
Number Share Options unvested, Weighted Average Exercise Price	$ 12.35
Number of Shares Outstanding, Weighted Average Remaining Contractual Term at End of Period	8 years	8 years 9 months 21 days
Number of Share Options exercisable, Weighted Average Remaining Contractual Term	7 years 3 months 7 days
Number of Share Options unvested, Weighted Average Remaining Contractual Term	8 years 10 months 9 days
Number of Share Options Outstanding, Aggregate Intrinsic Value Ending Balance	$ 49	$ 9,128
Number of Share Options exercisable, Aggregate Intrinsic Value	$ 49

Stock-Based Compensation - Summary of Restricted Common Stock Activity (Details) - Restricted Stock	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares Unvested Issued \| shares	250,000
Number of Shares Unvested Outstanding, Ending Balance \| shares	250,000
Weighted Average Grant Date Fair Value Unvested Issued \| $ / shares	$ 4.12
Weighted Average Grant Date Fair Value Unvested Outstanding, Ending Balance \| $ / shares	$ 4.12

Stock-Based Compensation - Summary of Stock-Based Compensation Expense Classification (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 5,620	$ 906	$ 644
Research and Development Expenses
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	1,079	329	294
General and Administrative Expenses
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 4,541	$ 577	$ 350

License Revenue - Additional Information (Detail) $ in Thousands		1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 12, 2018 USD ($) Obligation	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payment received from related parties			$ 3,500	$ 3,500
Type of Revenue [Extensible List]				us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Janssen Pharmaceuticals, Inc \| Janssen License and Option Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payables from related parties	$ 3,500
License and options agreement description				Pursuant to the Janssen License and Option Agreement, the Company granted to Janssen (i) a non-exclusive license to specified patents in the Company’s portfolio related to the ASN200 E. coli program, and (ii) an option for Janssen to acquire these patents in the future if specified conditions are met. Janssen agreed to pay the Company $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. The Company received the $3.5 million payment from Janssen in December 2018.
Payment received from related parties		$ 3,500
Type of Revenue [Extensible List]		us-gaap:LicenseMember
Number of performance obligation \| Obligation	1
Proceeds from non refundable license fees Received	$ 3,500
Janssen Pharmaceuticals, Inc \| Janssen License and Option Agreement \| Patents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option to acquire the relevant patents	$ 500

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Income tax benefits for net operating losses incurred	$ 0	$ 0	$ 0
Income tax benefits for research and development tax credits earned	0	0	0
Uncertain tax position reserves	$ 286,000	$ 192,000	$ 126,000	$ 21,000
Federal corporate rate	21.00%	35.00%	35.00%
Maximum
Income Tax Contingency [Line Items]
Provisional amounts measurement period	1 year
Domestic Tax Authority [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 38,800,000
Net operating losses with indefinite life	$ 14,500,000
Net operating loss carryforwards, expiration	begin to expire in 2031
Domestic Tax Authority [Member] \| Earliest Tax Year [Member]
Income Tax Contingency [Line Items]
Income tax returns subject to tax examinations, tax years	2015
Domestic Tax Authority [Member] \| Latest Tax Year [Member]
Income Tax Contingency [Line Items]
Income tax returns subject to tax examinations, tax years	2017
Domestic Tax Authority [Member] \| Research Tax Credit Carryforward [Member]
Income Tax Contingency [Line Items]
Tax credit carryforwards	$ 400,000
Uncertain tax position reserves	$ 200,000
Tax credit carryforwards, expiration	begin to expire in 2032
State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 34,900,000
Net operating loss carryforwards, expiration	begin to expire in 2036
State and Local Jurisdiction [Member] \| Earliest Tax Year [Member]
Income Tax Contingency [Line Items]
Income tax returns subject to tax examinations, tax years	2015
State and Local Jurisdiction [Member] \| Latest Tax Year [Member]
Income Tax Contingency [Line Items]
Income tax returns subject to tax examinations, tax years	2017
State and Local Jurisdiction [Member] \| Research Tax Credit Carryforward [Member]
Income Tax Contingency [Line Items]
Tax credit carryforwards	$ 100,000
Tax credit carryforwards, expiration	begin to expire in 2031
State and Local Jurisdiction [Member] \| Research Tax Credit Carryforward [Member] \| Maximum
Income Tax Contingency [Line Items]
Uncertain tax position reserves	$ 100,000
Foreign Tax Authority [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 64,700,000

Income Taxes - Loss Before Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments [Abstract]
United States	$ (32,022)	$ (24,618)	$ (12,969)
Foreign (Austria)	(10,999)	(9,259)	(10,006)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest, Total	$ (43,021)	$ (33,877)	$ (22,975)

Income Taxes - Reconciliation of U.S. Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]
U.S. federal statutory income tax rate	(21.00%)	(35.00%)	(35.00%)
State income taxes, net of federal benefit	(4.50%)	(3.50%)	(2.80%)
Foreign rate differential	(1.00%)	1.60%	3.20%
Research and development tax credits	(0.50%)	(0.40%)	(1.00%)
Nondeductible expenses	0.10%	1.20%	0.70%
Uncertain tax position reserves	0.20%	0.20%	0.50%
Stock-based compensation	0.70%	0.50%	0.30%
Change in rates due to tax reform		16.50%
Change in deferred tax asset valuation allowance	26.00%	18.90%	34.10%
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components Of Deferred Tax Assets And Liabilities [Abstract]
Net operating loss carryforwards	$ 26,519	$ 20,216	$ 13,134
Research and development tax credit carryforwards	493	317	264
Start-up costs	7,755	4,350	3,917
Accrued expenses and other	1,460	850	689
Total deferred tax assets	36,227	25,733	18,004
Valuation allowance	(36,227)	(25,733)	(18,004)	$ (10,367)
Net deferred tax assets	$ 0	$ 0	$ 0

Income Taxes - Changes in Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Valuation Allowance [Abstract]
Valuation allowance at beginning of year	$ (25,733)	$ (18,004)	$ (10,367)
Increases recorded to income tax provision	(10,494)	(7,729)	(7,637)
Valuation allowance at end of year	$ (36,227)	$ (25,733)	$ (18,004)

Income Taxes - Changes in Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Uncertainties [Abstract]
Unrecognized tax benefits at beginning of year	$ 192	$ 126	$ 21
Increases for tax positions taken in current year	94	66	105
Unrecognized tax benefits at end of year	$ 286	$ 192	$ 126

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								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Net loss	$ (9,358)	$ (10,900)	$ (12,133)	$ (10,630)	$ (11,187)	$ (11,600)	$ (5,705)	$ (5,385)	$ (43,021)	$ (33,877)	$ (22,975)
Accretion of redeemable convertible preferred stock to redemption value										(44)	(25)
Net loss attributable to common stockholders									$ (43,021)	$ (33,921)	$ (23,000)
Weighted average common shares outstanding—basic and diluted									14,307,934	2,061,845	513,527
Net loss per share attributable to common stockholders—basic and diluted	$ (0.65)	$ (0.76)	$ (0.85)	$ (0.74)	$ (1.68)	$ (22.60)	$ (11.13)	$ (10.49)	$ (3.01)	$ (16.45)	$ (44.79)

- 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 - Schedule of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	2,203,804	1,413,533	2,346,082
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	1,943,390	1,403,119	548,903
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	10,414	10,414
Restricted Common 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	250,000
Redeemable Convertible Preferred Stock
Antidilutive Securities Excluded From Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share			1,789,704
Warrants to Purchase Redeemable Convertible Preferred Stock (as Converted to Common Stock)
Antidilutive Securities Excluded From Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share			7,475

Commitments and Contingencies - Additional Information (Detail) $ in Millions	1 Months Ended				12 Months Ended
	Aug. 31, 2018 USD ($) ft²	Nov. 30, 2015	Jul. 31, 2015 USD ($)	Nov. 30, 2010	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Lessee, Lease, Description [Line Items]
Lease expiration date						Jan. 31, 2019
Current office space under lease agreement \| ft²	5,711
Original office space under lease agreement \| ft²	10,290
Lease, term of contract	5 years
Lease agreement extension description					The Company has the option to extend the term for one additional five-year period upon the Company’s written notice to the Lessor at least nine months and no more than 12 months in advance of the extension.
Lease, renewal term of contract	5 years
Base rent	$ 0.3
Recorded rent expense					$ 1.3	$ 1.2	$ 1.2
Adimab Option Agreement
Lessee, Lease, Description [Line Items]
Milestone payments					24.4
Option fee					0.3
Minimum
Lessee, Lease, Description [Line Items]
Period of written notice for extension	9 months
Maximum
Lessee, Lease, Description [Line Items]
Period of written notice for extension	12 months
Restricted Cash
Lessee, Lease, Description [Line Items]
Security Deposit					$ 0.3
Vienna Lease
Lessee, Lease, Description [Line Items]
Lease expiration date				Apr. 30, 2021
Option to extend the lease agreement					The Company has the option to extend the lease agreement for an additional year.
Animal-use Lease
Lessee, Lease, Description [Line Items]
Lease expiration date	Feb. 28, 2019
Description of term of lease					The lease initially had a one-year noncancelable term, which expired in June 2016, after which the lease became cancelable by either party upon six months’ prior written notice.
Base rent			$ 0.4
Waltham Lease
Lessee, Lease, Description [Line Items]
Lease expiration date		Jan. 31, 2019			Dec. 31, 2023
Lease Agreement for Office Space in Waltham MA
Lessee, Lease, Description [Line Items]
Lease description					In June 2018, the Company entered into a lease agreement for office space in Waltham, MA (“Lease Agreement”) with BP Bay Colony LLC (the “Lessor”). The Company amended the Lease Agreement in August 2018 (“Amended Lease Agreement”).

Commitments and Contingencies - Summary of Future Minimum Lease Payments Due Under Operating Leases (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2019	$ 884
2020	789
2021	438
2022	263
2023	263
Future minimum lease payments due	$ 2,637

Related Party Transactions - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Research and development	$ 30,979,000	$ 28,128,000	$ 17,831,000
Adimab, LLC \| Adimab Collaboration Agreement
Related Party Transaction [Line Items]
Payments to related party	0	0	100,000
Research and development	0	0	8,000
Owed to related party	0	0
Adimab, LLC \| Adimab Collaboration Agreement \| Maximum
Related Party Transaction [Line Items]
Research and development			100,000
Adimab, LLC \| Adimab Option Agreement
Related Party Transaction [Line Items]
Payments to related party	100,000	100,000
Research and development	100,000	100,000
Owed to related party	0	21,000
EveliQure Biotechnologies GmbH \| Services and Facilities Agreement \| Maximum
Related Party Transaction [Line Items]
Payments received from related party	200,000	100,000	100,000
Other income from related party	100,000	100,000	$ 100,000
Due from related party	$ 100,000	$ 100,000

401(k) Savings Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Compensation And Retirement Disclosure [Abstract]
Contributions made to the plan	$ 0	$ 0	$ 0

Selected Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 3,500								$ 3,500
Operating Expenses	12,972	$ 12,847	$ 12,616	$ 10,950	$ 11,606	$ 13,056	$ 5,644	$ 5,827	49,385	$ 36,133	$ 24,346
Net loss	$ (9,358)	$ (10,900)	$ (12,133)	$ (10,630)	$ (11,187)	$ (11,600)	$ (5,705)	$ (5,385)	$ (43,021)	$ (33,877)	$ (22,975)
Basic and diluted net loss per common share	$ (0.65)	$ (0.76)	$ (0.85)	$ (0.74)	$ (1.68)	$ (22.60)	$ (11.13)	$ (10.49)	$ (3.01)	$ (16.45)	$ (44.79)

- Definition

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

+ References + Details

Subsequent Events - Additional Information (Detail)									12 Months Ended
Subsequent Events - Additional Information (Detail)	Mar. 08, 2019 USD ($)	Mar. 08, 2019 EUR (€)	Feb. 28, 2019 USD ($)	Feb. 28, 2019 EUR (€)	Feb. 26, 2019 USD ($)	Feb. 26, 2019 EUR (€)	Feb. 19, 2019 USD ($)	Feb. 19, 2019 EUR (€)	Dec. 31, 2018 USD ($)	Mar. 08, 2019 EUR (€)	Mar. 07, 2019 $ / €	Feb. 19, 2019 EUR (€)	Dec. 31, 2017 USD ($)
Subsequent Event [Line Items]
Principal amount of debt outstanding \| $									$ 9,738,000				$ 14,892,000
Number of years accelerated payment term									5 years
FFG Loans
Subsequent Event [Line Items]
Principal amount of debt outstanding \| $									$ 9,738,000				$ 10,225,000
Subsequent Event
Subsequent Event [Line Items]
Exchange rate per EUR 1.00											1.12
Repayment of outstanding loan principal	$ 2,900,000	€ 2,596,320
Principal amount of debt outstanding	$ 9,500,000									€ 8,505,204
Number of years accelerated payment term	3 years	3 years
Percentage of minimum cash balance	70.00%									70.00%
Subsequent Event \| Lease Termination Agreement
Subsequent Event [Line Items]
Exchange rate per EUR 1.00											1.12
Lease agreement termination date					Nov. 26, 2010	Nov. 26, 2010
Lump Sum amount payment to landlord			$ 50,600	€ 45,000
Percentage of value added tax			20.00%	20.00%
Sublease agreement date					Feb. 25, 2019	Feb. 25, 2019
Sublease term					2 years	2 years
Sublease monthly rent					$ 12,800	€ 11,400
Subsequent Event \| X4
Subsequent Event [Line Items]
Exchange rate per EUR 1.00											1.12
One third of business combination payment	$ 968,600	€ 865,440
Cash payment to acquire business \| €		7,600,000
Bank Demand Letters \| Subsequent Event \| FFG Loans
Subsequent Event [Line Items]
Repayment of grants, outstanding loan principal and assessed interest							$ 20,400,000	€ 18,100,000
Exchange rate per EUR 1.00											1.12
Reimbursement of grants							8,100,000	7,200,000
Repayment of outstanding loan principal							9,600,000	€ 8,500,000
Repayment of assessed interest							$ 2,700,000					€ 2,400,000
Bank Demand Letters \| Subsequent Event \| FFG Loans \| X4
Subsequent Event [Line Items]
Repayment of outstanding loan principal \| €		€ 8,500,000