Cartesian Therapeutics (RNAC) 10-K 12 Mar 20 2019 FY Annual report Financial data

Cover page

Cover page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Mar. 06, 2020	Jun. 30, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
Document Transition Report	false
Entity File Number	001-37798
Entity Registrant Name	Selecta Biosciences, Inc
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-1622110
Entity Address, Address Line One	480 Arsenal Way
Entity Address, City or Town	Watertown
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02472
City Area Code	617
Local Phone Number	923-1400
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Trading Symbol	SELB
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Smaller Reporting Company	true
Entity Emerging Growth Company	true
Entity Extended Transition Period	true
Entity Shell Company	false
Entity Public Float			$ 58.5
Entity Common Stock, Shares Outstanding		86,420,195
Documents Incorporated by Reference	Portions of the registrant’s definitive Proxy Statement relating to its 2020 Annual Meeting of Stockholders to be filed with the Securities and Exchange Commission are incorporated by reference into Part III of this Annual Report on Form 10-K.
Amendment Flag	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Central Index Key	0001453687
Current Fiscal Year End Date	--12-31

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 89,893	$ 37,403
Restricted cash	279	0
Accounts receivable	5,000	0
Prepaid expenses and other current assets	1,495	4,673
Total current assets	96,667	42,076
Property and equipment, net	1,222	2,127
Right-of-use asset, net	301
Long-term restricted cash	1,379	279
Total assets	99,569	44,482
Current liabilities:
Accounts payable	500	1,100
Accrued expenses	13,492	11,700
Loan payable	18,905	21,385
Lease liability	372
Deferred revenue	1,674	959
Total current liabilities	34,943	35,144
Non‑current liabilities:
Deferred revenue	14,680	13,818
Warrant liabilities	41,549	0
Other long‑term liabilities	0	938
Total liabilities	91,172	49,900
Commitments and contingencies (Note 17)
Stockholders’ equity (deficit):
Preferred stock, $0.0001 par value; 10,000,000 shares authorized; no shares issued and outstanding at December 31, 2019 and December 31, 2018, respectively	0	0
Common stock, $0.0001 par value; 200,000,000 shares authorized; 86,325,547 and 22,471,776 shares issued and outstanding as of December 31, 2019 and December 31, 2018, respectively	9	3
Additional paid-in capital	348,664	279,539
Accumulated deficit	(335,753)	(280,403)
Accumulated other comprehensive loss	(4,523)	(4,557)
Total stockholders’ equity (deficit)	8,397	(5,418)
Total liabilities and stockholders’ equity (deficit)	$ 99,569	$ 44,482

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	86,325,547	22,471,776
Common stock, shares outstanding	86,325,547	22,471,776

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Grant and collaboration revenue	$ 6,677	$ 903	$ 207
Operating expenses:
Research and development	42,743	47,687	45,165
General and administrative	16,389	18,238	18,826
Total operating expenses	59,132	65,925	63,991
Loss from operations	(52,455)	(65,022)	(63,784)
Investment income	834	1,050	617
Loss on extinguishment of debt	0	0	(673)
Foreign currency transaction (loss), net	(47)	120	(123)
Interest expense	(1,519)	(1,494)	(1,206)
Change in fair value of warrant liabilities	(857)	0	0
Other (expense), net	(1,306)	10	(152)
Net loss	(55,350)	(65,336)	(65,321)
Other comprehensive loss:
Foreign currency translation adjustment	34	(153)	78
Unrealized gain on securities	0	16	20
Total comprehensive loss	$ (55,316)	$ (65,473)	$ (65,223)
Net loss per share:
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (1.22)	$ (2.92)	$ (3.20)
Weighted average common shares outstanding:
Basic and diluted (in shares)	45,548,511	22,389,286	20,425,050

Consolidated Statements of Chan

Consolidated Statements of Changes in Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Total	Common stock	Additional paid-In Capital	Stock option receivable	Accumulated deficit	Accumulated other comprehensive loss
Beginning balance (in shares) at Dec. 31, 2016		18,438,742
Beginning balance at Dec. 31, 2016	$ 54,957	$ 1	$ 211,125	$ (75)	$ (151,576)	$ (4,518)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)		16,263
Issuance of common stock under Employee Stock Purchase Plan	180		180
Issuance of common stock upon exercise of options (in shares)		269,842
Issuance of common stock upon exercise of options	705		630	75
Stock‑based compensation expense	4,080		4,080
Issuance of common stock, license agreement (in shares)		529,616
Issuance of common stock, license agreement	10,000		10,000
Issuance of common stock through private placement, net (in shares)		3,088,791
Issuance of common stock through private placement, net of issuance costs of $37	47,115	$ 2	47,113
Currency translation adjustment	78					78
Unrealized gains (losses) on securities	20					20
Net loss	(65,321)				(65,321)
Ending balance (in shares) at Dec. 31, 2017		22,343,254
Ending balance at Dec. 31, 2017	51,814	$ 3	273,128	0	(216,897)	(4,420)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)		24,738
Issuance of common stock under Employee Stock Purchase Plan	196		196
Issuance of common stock upon exercise of options (in shares)		103,784
Issuance of common stock upon exercise of options	501		501
Stock‑based compensation expense	5,714		5,714
Currency translation adjustment	(153)					(153)
Unrealized gains (losses) on securities	16					16
Net loss	$ (65,336)				(65,336)
Ending balance (in shares) at Dec. 31, 2018	22,471,776	22,471,776
Ending balance at Dec. 31, 2018	$ (5,418)	$ 3	279,539	0	(280,403)	(4,557)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)		17,205
Issuance of common stock under Employee Stock Purchase Plan	28		28
Issuance of common stock upon exercise of options (in shares)		125,600
Issuance of common stock upon exercise of options	150		150
Stock‑based compensation expense	5,161		5,161
Issuance of common stock through private placement, net (in shares)		3,178,174
Issuance of common stock through private placement, net of issuance costs of $37	5,715		5,715
Issuance of common stock, pre-funded warrants and warrants through private placement, net of issuance costs (in shares)		37,634,883
Issuance of common stock, pre-funded warrants and warrants through private placement, net of issuance costs	26,129	$ 4	26,125
Currency translation adjustment	34
Unrealized gains (losses) on securities	0
Net loss	(55,350)
Issuance of common stock, net of issuance costs (in shares)		22,188,706
Issuance of common stock, net of issuance costs	30,942	$ 2	30,940
Issuance of common stock through at-the-market offering, net (in shares)		615,453
Issuance of common stock through at-the-market offering, net	1,006		1,006
Issuance of restricted stock units (in shares)		93,750
Issuance of restricted stock units	$ 0
Ending balance (in shares) at Dec. 31, 2019	86,325,547	86,325,547
Ending balance at Dec. 31, 2019	$ 8,397	$ 9	$ 348,664	$ 0	$ (335,753)	$ (4,523)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net loss	$ (55,350)	$ (65,336)	$ (65,321)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	726	975	750
Amortization of premiums (accretion of discounts) on investments	(154)	(101)	233
Amortization of right-of-use assets	1,301	0	0
Loss (gain) on disposal of property and equipment	104	(81)	36
Stock‑based compensation expense	5,161	5,714	4,081
Non‑cash interest expense	402	449	390
Warrant liabilities revaluation	857	0	0
Loss on extinguishment of debt	0	0	673
Net realized losses on investments	(1)	0	0
Changes in operating assets and liabilities:
Accounts receivable	(5,000)	0	152
Prepaid expenses, deposits and other assets	3,179	(2,631)	1,225
Accounts payable	(600)	(516)	(2,327)
Deferred revenue	337	(101)	2,432
Accrued expenses and other liabilities	(2,397)	2,467	5,650
Net cash used in operating activities	(51,435)	(59,161)	(52,026)
Cash flows from investing activities
Receipts from the maturity of short-term investments	16,350	41,655	60,158
Purchases of short-term investments	(18,188)	(15,598)	(61,527)
Sale of short term investments	1,992	0	0
Purchases of property and equipment	(47)	(884)	(733)
Proceeds from the sale of property and equipment	122	99	4
Net cash provided by (used in) investing activities	229	25,272	(2,098)
Cash flows from financing activities
Proceeds from issuance of long-term debt, net of expenses	0	0	20,957
Repayments of principal on outstanding debt	(2,800)	0	(12,934)
Net proceeds from issuance of common stock	30,942	0	10,000
Proceeds from exercise of stock options	150	501	706
Proceeds from issuance of common stock under Employee Stock Purchase Plan	28	196	180
Net cash provided by financing activities	105,041	697	66,023
Effect of exchange rate changes on cash	34	(153)	78
Net change in cash, cash equivalents, and restricted cash	53,869	(33,345)	11,977
Cash, cash equivalents, and restricted cash at beginning of period	37,682	71,027	59,050
Cash, cash equivalents, and restricted cash at end of period	91,551	37,682	71,027
Supplement cash flow information
Cash paid for interest	1,223	1,134	905
Noncash investing and financing activities
Purchase of property and equipment not yet paid	0	145	103
Issuance costs in connection with common stock, common warrants, and pre-funded warrants in accrued liabilities	4,381	0	0
Unrealized gain on marketable securities	0	16	20
At-The-Market Offering
Cash flows from financing activities
Net proceeds from issuance of common stock	1,006	0	0
Private Placement
Cash flows from financing activities
Net proceeds from issuance of common stock	5,715	0	47,114
Note Warrant
Cash flows from financing activities
Net proceeds from issuance of common stock	$ 70,000	$ 0	$ 0

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	Nature of the Business and Basis of Presentation Selecta Biosciences, Inc. (the “Company”) was incorporated in Delaware on December 10, 2007, and is based in Watertown, Massachusetts. The Company is a clinical-stage biotechnology company focused on unlocking the full potential of biologic therapies based on its immune tolerance technology (ImmTOR™) platform. The Company plans to combine ImmTOR with a range of biologic therapies for rare and serious diseases that require new treatment options due to high immunogenicity of existing therapies. Since inception, the Company has devoted its efforts principally to research and development of its technology and product candidates, recruiting management and technical staff, acquiring operating assets, and raising capital. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, or maintained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. The accompanying financial statements have been prepared on a basis that assumes the Company is a going concern, and does not include any adjustments to reflect the possible future effects on the recoverability and classification of assets or the amounts and classifications of liabilities that may result from any uncertainty related to its ability to continue as a going concern. Liquidity and Management's Plan The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain profitable operations. The Company is subject to a number of risks similar to other early-stage life science companies, including, but not limited to, successful development of its product candidates, raising additional capital with favorable terms, protection of proprietary technology and market acceptance of any approved future products. The successful development of product candidates requires substantial working capital which may not be available to the Company on favorable terms or at all. To date, the Company has financed its operations primarily through the initial public offering of its common stock, a private placement of its common stock, issuances of common and preferred stock, debt, research grants and research collaborations. The Company currently has no source of product revenue, and it does not expect to generate product revenue for the foreseeable future. To date, all of the Company's revenue has been collaboration and grant revenue. The Company has devoted substantially all of its financial resources and efforts to developing its ImmTOR platform, identifying potential product candidates and conducting preclinical studies and its clinical trials. The Company is in the early stages of development of its product candidates, and it has not completed development of any ImmTOR-enabled therapies. As of December 31, 2019 , the Company’s cash, cash equivalents and restricted cash were $91.6 million , of which $1.7 million was restricted cash related to lease commitments and $0.4 million was held by its Russian subsidiary designated solely for use in its operations. The Company has incurred losses and negative cash flows from operating activities since inception. As of December 31, 2019 and December 31, 2018 , the Company had an accumulated deficit of $335.8 million and $280.4 million , respectively. The Company anticipates operating losses to continue for the foreseeable future due to, among other things, costs related to research, development of its product candidates, conducting preclinical studies and clinical trials, and its administrative organization. The Company will require substantial additional financing to fund its operations and to continue to execute its strategy, and the Company will pursue a range of options to secure additional capital. These conditions raise substantial doubt about the Company's ability to continue as a going concern within one year after the date that the financial statements are issued. Management is actively exploring licenses and other strategic collaborations that have the potential to provide non-dilutive capital and accelerate the development of new or existing product candidates incorporating the Company’s ImmTOR platform. Additionally, the Company may seek to fund its operations through issuances of equity and other securities. If the Company enters into strategic collaborations and alliances, which may include existing collaboration partners, the Company may have to relinquish valuable rights to its technologies or product candidates, or grant licenses on terms that are not favorable to the Company. To the extent that the Company raises additional capital through the sale of equity, the ownership interest of its existing shareholders will be diluted and other preferences may be necessary that adversely affect the rights of existing shareholders. The Company requires additional external sources of capital to complete the planned Phase 3 clinical program for SEL-212. If the Company is unable to raise sufficient capital through strategic collaborations and the sale of equity or other securities, it intends to curtail expenses contemplated by the current operating plan, and the Company may be required to delay, limit, reduce or terminate its product development efforts or grant rights to develop and market product candidates that it would otherwise prefer to develop and market itself. Because of the uncertainty in securing additional capital and the insufficient amount of capital resources at December 31, 2019 , management has concluded that substantial doubt exists with respect to the Company's ability to continue as a going concern within one year after the date of the filing of this Annual Report on Form 10-K . All amounts due under the 2017 Term Loan (see Note 9) have been classified as a current liability as of December 31, 2019 due to the considerations discussed above and the assessment that the material adverse change clause under the 2017 Term Loan is not within the Company's control. The Company has not been notified of an event of default by the Lender as of the date of the filing of this Annual Report on Form 10-K . Guarantees and Indemnifications As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through December 31, 2019 , the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta RUS, LLC (“Selecta (RUS)”), a Russian limited liability corporation, and Selecta Biosciences Security Corporation, a Massachusetts Security Corporation. All significant intercompany accounts and transactions have been eliminated. Foreign Currency The functional currency of Selecta (RUS) is the Russian ruble. Assets and liabilities of Selecta (RUS) are translated at period-end exchange rates, while revenues and expenses are translated at average exchange rates for the period. Translation gains and losses are reflected in accumulated other comprehensive loss within stockholders’ equity (deficit). Foreign currency transaction gains or losses are reflected in the consolidated statements of operations and comprehensive loss. Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition, accounting for stock-based compensation, the valuation of its warrant liabilities and estimating accrued research and development expenses. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates. Segment Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, the Company’s Chief Executive Officer, in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment, the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases. Cash Equivalents, Short-term Investments and Restricted Cash Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Investments consist of securities with remaining maturities greater than 90 days when purchased. The Company classifies these marketable securities and records them at fair value in the accompanying consolidated balance sheets. Investments with less than one year until maturity are classified as short term, while investments with maturities greater than one year are classified as long term. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment. Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. During the year ended December 31, 2019 , there were de minimis realized losses on sales of investments, and no investments were adjusted for other than temporary declines in fair value. As of December 31, 2019 , the Company had restricted cash balances relating to secured letters of credit in connection with its current Headquarters Lease and New Headquarters Lease (as defined in Note 8). 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 consolidated statement of cash flows: Year Ended December 31, 2019 2018 2017 Cash and cash equivalents $ 89,893 $ 37,403 $ 70,622 Restricted cash 279 — 76 Long-term restricted cash 1,379 279 329 Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows $ 91,551 $ 37,682 $ 71,027 Concentrations of Credit Risk and Off‑Balance Sheet Risk Financial instruments that potentially subject the Company to concentration of credit risk consist primarily of cash, cash equivalents, short-term deposits and investments, and accounts receivable. Cash and cash equivalents are deposited with federally insured financial institutions in the United States and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially creditworthy and, accordingly, minimal risk exists with respect to those balances. Generally, these deposits may be redeemed upon demand and therefore bear minimal interest rate risk. As an integral part of operating its Russian subsidiary, the Company also maintains cash in Russian bank accounts in denominations of both Russian rubles and U.S. dollars. As of December 31, 2019 , the Company maintained approximately $0.4 million in Russian bank accounts, all of which was held in U.S. dollars. The Company did not have any off-balance sheet arrangements as of December 31, 2019 and December 31, 2018 . Fair Value of Financial Instruments The Company’s financial instruments consist mainly of cash equivalents, restricted cash, accounts payable, loans payable, and common warrants. The carrying amounts of cash equivalents, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. At December 31, 2019 , the carrying amount of the Company's loan payable approximates its estimated fair value due to the short-term nature of the instrument. Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A three‑level hierarchy is used to prioritize the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements), and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below: Level 1 —Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. Level 2 —Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term of the asset or liability. Level 3 —Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date. To the extent that a valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair value of warrant liabilities were determined using Level 3 inputs. Fair value is a market‑based measure considered from the perspective of a market participant rather than an entity‑specific measure. Therefore, even when market assumptions are not readily available, the Company’s own assumptions are set to reflect those that market participants would use in pricing the asset or liability at the measurement date. The Company uses prices and inputs that are current as of the measurement date, including during periods of market dislocation. In periods of market dislocation, the observability of prices and inputs may change for many instruments. This condition could cause an instrument to be reclassified within levels in the fair value hierarchy. There were no transfers within the fair value hierarchy during the years ended December 31, 2019 or December 31, 2018 . Property and Equipment Property and equipment are recorded at cost and depreciated using the straight‑line method over the estimated useful lives of the respective assets, generally seven years for furniture and fixtures, five years for laboratory equipment, software and office equipment and three years for computer equipment. Leasehold improvements are amortized over their useful life or the life of the lease, whichever is shorter. Major additions and betterments are capitalized. Maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operations as incurred. Costs incurred for construction in progress are recorded as assets and are not amortized until the construction is substantially complete and the assets are ready for their intended use. Impairment of Long‑Lived Assets The Company reviews long‑lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. In order to determine if assets have been impaired, assets are tested at the lowest level for which identifiable independent cash flows are available, which is at the entity level ("asset group"). An impairment loss is recognized when the sum of projected undiscounted cash flows is less than the carrying value of the asset group. The measurement of the impairment loss to be recognized is based on the difference between the fair value and the carrying value of the asset group. Based on management's evaluation, the fair value of the asset group, measured as the market capitalization of the Company exceeds its carrying value, and for this reason the Company did no t recognize any material impairment losses during the years ended December 31, 2019 and 2018 . Debt Issuance Costs Debt issuance costs and fees paid to lenders are classified as a debt discount and are recorded as a direct deduction from the face amount of the related debt. Issuance costs paid to third parties that are the direct result of the debt issuance are capitalized as a direct deduction from the face amount of the related debt. Debt issuance costs are amortized over the term of the related debt using the interest method and recorded as interest expense. Costs and fees paid to third parties are expensed as incurred. Accumulated Other Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in the equity of a business entity during a period from transactions and other events and circumstances from non‑owner sources. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive income (loss) consists of: (i) all components of net loss and (ii) all components of comprehensive loss other than net loss, referred to as other comprehensive loss. Other comprehensive loss is comprised of foreign currency translation adjustments and the unrealized gains and losses recognized through net income. The components of accumulated other comprehensive income (loss), net of tax, were as follows (in thousands): Foreign currency translation adjustment Unrealized gains (losses) on marketable securities Accumulated other comprehensive income (loss) Balance at December 31, 2016 $ (4,482 ) $ (36 ) $ (4,518 ) Other comprehensive income during the year 78 20 98 Balance at December 31, 2017 $ (4,404 ) $ (16 ) $ (4,420 ) Other comprehensive income (loss) during the year (153 ) 16 (137 ) Balance at December 31, 2018 $ (4,557 ) $ — $ (4,557 ) Other comprehensive income during the year $ 34 $ — $ 34 Balance at December 31, 2019 $ (4,523 ) $ — $ (4,523 ) Revenue Recognition Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. Pursuant to ASC Topic 606, Revenue from Contracts with Customers (ASC 606) , a customer is a party that has contracted with an entity to obtain goods or services that are an output of the entity’s ordinary activities in exchange for consideration. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract, including whether they are distinct in the context of the contract; (iii) determine the transaction price, including the constraint on variable consideration; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. If a promised good or service is not distinct, it is combined with other performance obligations. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. For example, certain performance obligations associated with Spark and AskBio (see Note 12) will be satisfied over time, and revenue will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term. Collaboration and Grant Revenue: The Company currently generates its revenue through grants, collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. Grants and license agreements with customers are accounted for in accordance with ASC 606. The Company analyzes collaboration arrangements by first assessing whether they are within the scope of ASC Topic 808, Collaborative Arrangements (ASC 808 ), and evaluates whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards that are dependent on the commercial success of such activities. Collaboration agreements with customers that are not within the scope of ASC 808 are accounted for in accordance with ASC 606. To the extent the collaboration agreement is within the scope of ASC 808, the Company also assesses whether any aspects of the agreement are within the scope of other accounting literature (specifically ASC 606). The Company early adopted ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 , which provides guidance on evaluating certain transactions between collaborative arrangement participants. If the Company concludes that some or all aspects of the agreement are distinct and represent a transaction with a customer, the Company accounts for those aspects of the arrangement within the scope of ASC 606. The Company recognizes the shared costs incurred that are not within the scope of other accounting literature as a component of the related expense in the period incurred by analogy to ASC Topic 730, Research and Development (ASC 730) , and records reimbursements from counterparties as an offset to the related costs. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements in accordance with ASC 606, the Company performs the five steps above. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. The terms of the Company’s arrangements typically include one or more of the following: (i) up-front fees; (ii) milestone payments related to the achievement of development, regulatory, or commercial goals; (iii) royalties on net sales of licensed products; (iv) reimbursements or cost-sharing of research and development (R&D) expenses; and (v) profit/loss sharing arising from co-promotion arrangements. Licenses of Intellectual Property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. If not distinct, the license is combined with other performance obligations in the contract. For licenses that are combined with other performance obligations, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Optional licenses are evaluated to determine if they are issued at a discount, and therefore, represent material rights and accounted for as separate performance obligations. Milestone Payments: At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. The Company also evaluates the milestone to determine whether they 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 to be allocated, otherwise, such amounts are constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. Manufacturing Supply Services: Arrangements that include a promise for future supply of drug substance or drug product for either clinical development or commercial supply at the customer’s discretion are evaluated to determine if they are distinct and optional. For optional services that are distinct, the Company assesses if they are priced at a discount, and therefore, provide a material right to the licensee to be accounted for as separate performance obligations. Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied) in accordance with the royalty recognition constraint. Research and Development Costs Costs incurred in the research and development of the Company’s products are expensed as incurred. Research and development expenses include costs incurred in performing research and development activities, including salaries and benefits, facilities cost, overhead costs, contract services, supplies and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made. Clinical Trial Costs Clinical trial expenses are a significant component of research and development expenses, and the Company outsources a significant portion of these costs to third parties. Third party clinical trial expenses include patient costs, clinical research organization costs and costs for data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through costs. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as a prepaid asset or accrued clinical trial cost. These third party agreements are generally cancelable, and related costs are recorded as research and development expenses as incurred. Non-refundable advance clinical payments for goods or services that will be used or rendered for future R&D activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. The Company also records accruals for estimated ongoing clinical research and development costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical clinical accrual estimates made by the Company have not been materially different from the actual costs. Income Taxes The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more-likely-than-not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more-likely-than-not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more‑likely‑than‑not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred interest and penalties related to uncertain tax positions. Warrants The Company determines the accounting classification of warrants that are issued, as either liability or equity, by first assessing whether the warrants meet liability classification in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity , and then in accordance with ASC 815-40, Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock . Under ASC 480, warrants are considered liability classified if the warrants are mandatorily redeemable, obligate the issuer to settle the warrants or the underlying shares by paying cash or other assets, or must or may require settlement by issuing variable number of shares. If warrants do not meet liability classification under ASC 480-10, the Company assesses the requirements under ASC 815-40, which states that contracts that require or may require the issuer to settle the contract for cash are liabilities recorded at fair value, irrespective of the likelihood of the transaction occurring that triggers the net cash settlement feature. If the warrants do not require liability classification under ASC 815-40, in order to conclude equity classification, the Company assesses whether the warrants are indexed to its common stock and whether the warrants are classified as equity under ASC 815-40 or other applicable GAAP. After all relevant assessments are made, the Company concludes whether the warrants are classified as liability or equity. Liability classified warrants are required to be accounted for at fair value both on the date of issuance and on subsequent accounting period ending dates, with all changes in fair value after the issuance date recorded in the statements of operations as a gain or loss. Equity classified warrants are accounted for at fair value on the issuance date with no changes in fair value recognized after the issuance date. Stock‑Based Compensation The Company accounts for all stock‑based compensation granted to employees and non‑employees using a fair value method. Stock‑based compensation is measured at the grant date fair value and is recognized over the requisite service period of the awards, usually the vesting period, on a straight‑line basis, net of estimated forfeitures. The Company reduces recorded stock‑based compensation for estimated forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were adjusted. Stock‑based compensation expense recognized in the consolidated financial statements is based on awards that are ultimately expected to vest. Net Loss Per Share The Company has reported losses since inception and has computed basic net loss per share by dividing net loss by the weighted average number of common shares and pre-funded warrants outstanding for the period. The Company has computed diluted net loss per common share after considering all potentially dilutive common shares, including stock options, convertible preferred stock, and warrants outstanding during the period except where the effect of including such securities would be antidilutive. Because the Company has reported net losses since inception, these potential common shares have been anti‑dilutive and basic and diluted loss per share have been the same. Contingent Liabilities The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies . A provision is recorded when it is both probable that a liability has been incurred and the amount of the loss can be reasonably estimated. With respect to legal matters, provisions are reviewed and adjusted to reflect the impact of negotiations, estimated settlements, legal rulings, advice of legal counsel and other information and events pertaining to a particular matter. As of December 31, 2019 and December 31, 2018 , the Company was not a party to any litigation that could have a material adverse effect on the Company’s business, financial position, results of operations or cash flows. Leases Under ASC Topic 842, Leases (ASC 842) , which was adopted January 1, 2019, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present. Most leases with a term greater than one year are recognized on the balance sheet as right-of-use assets, lease liabilities and, if applicable, long-term lease liabilities. The Company elected not to recognize leases with a term less than one year on its balance sheet. Operating lease right-of-use (ROU) assets and their corresponding lease liabilities are recorded based on the present value of lease payments over the expected remaining lease term. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rates, which are the rates incurred to borrow on a collateralized basis over a similar term, an amount equal to the lease payments in a similar economic environment. In accordance with the guidance in ASC 842, components of a lease should be split into three categories: lease components (e.g. land, building, etc.), non-lease components (e.g. common area maintenance, consumables, etc.), and non-components (e.g. property taxes, insurance, etc.) Then the fixed and in-substance fixed contract consideration (including any related to non-components) must be allocated based on fair values to the lease components and non-lease components. Although separation of lease and non-lease components is required, the Company elected the practical expedient to not separate lease and non-lease components. The lease component results in an operating right-of-use asset being recorded on the balance sheet and amortized on a straight-line basis as lease expense. Right-of-use assets and operating lease liabilities are remeasured upon certain modifications to leases using the present value of remaining lease payments and estimated incremental borrowing rate upon lease modification. See Note 8 for details. Under prior guidance, rent expense and lease incentives from operating leases were recognized on a straight‑line basis over the lease term. The difference between rent expense recognized and rental payments was recorded as deferred rent in the accompanying consolidated balance sheets. Recent Accounting Pronouncements Recently Adopted In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update No. (“ASU”) 2016-02, Leases (Topic 842). ASU 2016-02 requires lessees to recognize most leases on their balance sheet as a right-of-use asset and a lease liability. Leases are classified as either operating or finance based on criteria similar to current lease accounting, with the classification affecting the pattern and classification of expense recognition in the statement of operations. Subsequently, in July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements (ASU 2018-11) , which includes certain amendments to ASU 2016-02 intended to provide relief in implementing the new standard. Among these amendments is the option to not restate comparative periods presented in the financial statements. The Company has elected this transition approach, using a cumulative-effect adjustment on the effective date of the standard, with comparative periods presented in accordance with the existing guidance in ASC 840. The Company adopted the new standard as of the required effective date of January 1, 2019 resulting in the recognition of a net additional lease liability and right-of-use asset. The standard did not impact the Company's consolidated net loss. See Note 8 for details. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the

Marketable Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	Marketable Securities As of December 31, 2019 , and December 31, 2018 , the Company did not have marketable securities.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share The Company has reported a net loss for the years ended December 31, 2019 , 2018 , and 2017. For this reason basic and diluted net loss per share are the same for all periods presented. Since the shares underlying the 8,342,128 pre-funded warrants are issuable for little or no consideration, they are considered outstanding for both basic and diluted earnings per share. The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per‑share data): Year Ended December 31, 2019 2018 2017 Numerator: Net loss attributable to common stockholders $ (55,350 ) $ (65,336 ) $ (65,321 ) Denominator: Weighted‑average common shares and pre-funded warrants outstanding—basic and diluted 45,548,511 22,389,286 20,425,050 Net loss per share attributable to common stockholders —basic and diluted $ (1.22 ) $ (2.92 ) $ (3.20 ) All potential dilutive common shares have been excluded from the computation of the diluted net loss per share for all periods presented, as the effect would have been anti-dilutive. Potential dilutive common share equivalents consist of the following: Year Ended December 31, 2019 2018 2017 Stock options to purchase common stock 6,796,669 4,093,979 2,657,187 Unvested restricted stock units 181,250 175,000 — Stock warrants to purchase common stock 23,084,120 95,619 176,432 Total 30,062,039 4,364,598 2,833,619

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Assets and Liabilities Measured at Fair Value on a Recurring Basis The tables below present information about the Company’s financial assets and liabilities that are measured and carried at fair value as of December 31, 2019 and December 31, 2018 , and indicate the level within the fair value hierarchy where each measurement is classified. Below is a summary of assets and liabilities measured at fair value on a recurring basis (in thousands): December 31, 2019 Total (Level 1) (Level 2) (Level 3) Assets: Money market funds $ 50,401 $ 50,401 $ — $ — Total $ 50,401 $ 50,401 $ — $ — Liabilities: Warrant liabilities $ 41,549 $ — $ — $ 41,549 Total $ 41,549 $ — $ — $ 41,549 December 31, 2018 Total (Level 1) (Level 2) (Level 3) Assets: Money market funds $ 10,123 $ 10,123 $ — $ — Total $ 10,123 $ 10,123 $ — $ — At each of December 31, 2019 and December 31, 2018 , the money market funds were classified as cash and cash equivalent on the accompanying consolidated balance sheet as they mature within 90 days from the date of purchase. Assumptions Used in Determining Fair Value of Common Warrants In December 2019, we issued common warrants in connection with our private placement of common shares. Pursuant to the terms of the common warrants, we could be required to settle the common warrants in cash in the event of certain acquisitions of the Company and, as a result, the common warrants are required to be measured at fair value and reported as a liability on the balance sheet. We recorded the fair value of the common warrants upon issuance using the Black-Scholes valuation model and are required to revalue the common warrants at each reporting date with any changes in fair value recorded on our statement of operations. The valuation of the common warrants is considered under Level 3 of the fair value hierarchy due to the need to use assumptions in the valuation that are both significant to the fair value measurement and unobservable. The significant unobservable inputs used in the fair value measurement of the warrant liabilities were the volatility rate and the estimated term of the warrants. Generally, increases (decreases) in the fair value of the underlying stock and estimated term would result in a directionally similar impact to the fair value measurement. The change in the fair value of the Level 3 warrant liability is reflected in the statement of operations for the year ended December 31, 2019 . The estimated fair value of warrants is determined using Level 3 inputs inherent in the Black Scholes simulation valuation. Estimated fair value of the underlying stock . The Company estimates the fair value of the common stock based on the closing stock price at the end of each reporting period. Risk-free interest rate . The risk-free interest rate is based on the U.S. Treasury at the valuation date commensurate with the expected remaining life assumption. Dividend rate . The dividend rate is based on the historical rate, which the Company anticipates will remain at zero. Expected life . The expected life of the warrants is assumed to be equivalent to their remaining contractual term which expires on December 23, 2024. Volatility . The Company estimates stock price volatility based on the Company’s historical volatility and the historical volatility of peer companies for a period of time commensurate with the expected remaining life of the warrants. A summary of the Black Scholes pricing model assumptions used to record the fair value of the warrants is as follows: Issued on December 23, Year Ended December 31, 2019 2019 Risk-free interest rate 1.75 % 1.69 % Dividend yield — — Expected life (in years) 5.00 4.98 Expected volatility 87.29 % 87.74 % Changes in Level 3 Liabilities Measured at Fair Value on a Recurring Basis The following table reflects the change in the Company’s Level 3 warrant liabilities, (see Note 10), for the year ended December 31, 2019 (in thousands): Warrant liabilities Fair value as of December 31, 2018 $ — Warrants issued in connection with December 2019 private placement 40,692 Change in fair value 857 Fair value as of December 31, 2019 $ 41,549

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment consists of the following (in thousands): December 31, December 31, 2019 2018 Laboratory equipment $ 4,836 $ 5,379 Computer equipment and software 515 561 Leasehold improvements 278 278 Furniture and fixtures 237 247 Office equipment 135 135 Construction in process 2 79 Total property and equipment 6,003 6,679 Less accumulated depreciation (4,781 ) (4,552 ) Property and equipment, net $ 1,222 $ 2,127 Depreciation expense was $0.7 million , $1.0 million and $0.8 million for the years ended December 31, 2019 , 2018 , and 2017, respectively. The Company recorded accelerated depreciation costs of $0.04 million in the reported property and equipment for the year ended December 31, 2019 relating to the upcoming new corporate headquarters move in 2020.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2019
Payables and Accruals [Abstract]
Accrued Expenses	Accrued Expenses Accrued expenses consist of the following (in thousands): December 31, December 31, 2019 2018 Payroll and employee related expenses $ 2,235 $ 2,497 Current portion of deferred rent and lease incentive — 117 Collaboration and licensing 1,050 1,222 Accrued patent fees 487 736 Accrued external research and development costs 4,379 5,344 Accrued professional and consulting services 446 994 Accrued grant refund — 175 Accrued interest 82 106 Issuance costs, December financing 4,381 — Other 432 509 Accrued expenses $ 13,492 $ 11,700

Leases

Leases	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Leases	Leases On January 1, 2019, the Company adopted ASC 842 using the modified retrospective approach. The Company recorded operating lease assets (right-of-use assets) of $1.6 million and operating lease liabilities of $1.8 million and reversed a lease liability of $0.2 million related to straight-line rent and incentives. There was no impact to accumulated deficit upon adoption of ASC 842. The underlying assets of the Company’s leases are primarily office space. The Company determines if an arrangement qualifies as a lease at its inception. As a practical expedient permitted under ASC 842, the Company has elected to account for the lease and non-lease components as a single lease component for all leases of which it is the lessee. Lease payments, which may include lease and non-lease components, are included in the measurement of the Company’s lease liabilities to the extent that such payments are either fixed amounts or variable amounts that depend on a rate or index as stipulated in the lease contract. When the Company cannot readily determine the rate implicit in the lease, the Company determines its incremental borrowing rate by using the rate of interest that it would have to pay to borrow on a collateralized basis over a similar term, an amount equal to the lease payments in a similar economic environment. On January 1, 2019, the discount rate used on existing operating leases at adoption, which had remaining lease terms of 15 months , was 10.0% . For new or renewed leases starting in 2020, the discount rate is determined based on the Company’s incremental borrowing rate adjusted for the lease term including any reasonably certain renewal periods. The Company enters into lease agreements with terms generally ranging from 2 - 8 years. Some of the Company’s lease agreements include Company options to either extend and/or early terminate the lease, the costs of which are included in its operating lease liabilities to the extent that such options are reasonably certain of being exercised. Leases with renewal options allow the Company to extend the lease term typically between 1 and 5 years. When determining the lease term, renewal options reasonably certain of being exercised are included in the lease term. When determining if a renewal option is reasonably certain of being exercised, the Company considers several economic factors, including but not limited to, the significance of leasehold improvements incurred on the property, whether the asset is difficult to replace, underlying contractual obligations, or specific characteristics unique to that particular lease that would make it reasonably certain that the Company would exercise such option. Renewal and termination options were generally not included in the lease term for the Company’s existing operating leases. Leases with an initial term of 12 months or less are not recorded on the balance sheet; the Company recognizes lease expense for these leases on a straight-line basis over the lease term. For lease agreements entered into or reassessed after the adoption of ASC 842, lease and non-lease components are combined. The Company has a non‑cancellable operating lease for its laboratory and office space located at 480 Arsenal Way, Watertown, Massachusetts ("Headquarters Lease"). As part of the Headquarters Lease agreement, the landlord provided the Company a tenant improvement allowance of up to $0.7 million , which the Company fully utilized during 2012. The leasehold improvements are capitalized as a component of property and equipment. In connection with the Headquarters Lease, the Company secured a letter of credit for $0.3 million which renews automatically each year and is classified in restricted cash. In August 2016, the Company signed an amendment to the Headquarters Lease, which extends the term through March 31, 2020. In October 2017, the Company entered into a lease for approximately 5,100 square feet of additional office space located at 75 North Beacon Street, Watertown, Massachusetts (the “75 North Beacon Lease”). On January 11, 2019, the Company vacated 75 North Beacon Street, Watertown, MA and consolidated all employees at its corporate headquarters at 480 Arsenal Way, Watertown, MA. The right-of-use asset carrying amount of $0.2 million attributable to the 75 North Beacon Lease was written down to zero during the first quarter of 2019. The Company has a month‑to‑month facility agreement for its Moscow, Russia office. Rent expense is recognized as incurred. As of December 31, 2018 and prior to the adoption of ASC 842, the aggregate future minimum lease payments related to leases are as follows (in thousands): Year ending December 31, 2019 1,482 2020 375 Total minimum lease payments $ 1,857 In July 2019, the Company entered into a lease for 25,078 square feet of laboratory and office space located at 65 Grove Street, Watertown, Massachusetts (the “New Headquarters Lease”). The Company estimates that it will incur $0.8 million in non-reimbursable lessee-paid construction costs for lessor assets. None of these costs were incurred as of December 31, 2019. The lease begins in March 2020, consistent with when the Company takes control of the office space and the expected lease term is 8 years, therefore the right-of-use asset and lease liability is not recorded on the balance sheet as of December 31, 2019 . Rent payments are expected to occur in May 2020, and the base rent for the first year is $0.2 million per month. The total minimum rental commitments for the New Headquarters Lease are $15.8 million . In connection with the New Headquarters Lease, the Company secured a letter of credit from Silicon Valley Bank for $1.4 million which renews automatically each year. The Company's total minimum rental commitments for the New Headquarters Lease as of December 31, 2019 are as follows (in thousands): December 31, 2019 2020 $ 1,191 2021 1,811 2022 1,865 2023 1,921 2024 1,979 Thereafter 7,027 Total New Headquarters Lease commitment $ 15,794 The right-of-use asset and lease liability has not been recorded as of December 31, 2019 as lease commencement will occur in 2020. Rent expense for the years ended December 31, 2019 , 2018 and 2017 was $2.1 million, $2.0 million and $1.9 million respectively. For the year ended December 31, 2019 , the components of lease costs were as follows (in thousands): Year Ended December 31, 2019 Operating lease expense $ 1,365 Variable lease expense 828 Short-term lease expense 16 Total lease expense $ 2,209 The maturity of the Company's operating lease liabilities as of December 31, 2019 were as follows (in thousands): December 31, Operating leases: 2019 2020 375 Total future minimum lease payments $ 375 Less imputed interest 3 Total operating lease liabilities $ 372 Included in the condensed consolidated balance sheet: Current operating lease liabilities $ 372 Non-current operating lease liabilities — Total operating lease liabilities $ 372 The following information represents supplemental disclosure for the statement of cash flows related to operating leases (in thousands): Year Ended Operating leases: 2019 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 1,482 The changes in the Company’s right-of-use asset and lease liability for the year ended December 31, 2019 are reflected in the changes in prepaid expenses, deposits and other assets and accrued expenses and other liabilities, respectively, in the consolidated statements of cash flows. The following summarizes additional information related to operating leases: December 31, Operating leases: 2019 Weighted-average remaining lease term 0.3 years Weighted-average discount rate 10 %

Debt

Debt	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Debt	Debt 2017 Term Loan On September 12, 2017, the Company entered into a term loan facility of up to $21.0 million (the “ 2017 Term Loan ”) with Silicon Valley Bank, a California corporation (“SVB”). The 2017 Term Loan is governed by a loan and security agreement, dated September 12, 2017, between the Company and SVB (the “Loan Agreement”). The 2017 Term Loan was funded in full on September 13, 2017 (the “Funding Date”). On the Funding Date, the Company entered into a payoff letter with SVB, pursuant to which SVB utilized $10.0 million of the 2017 Term Loan to pay off all outstanding obligations under the 2015 Term Loan. The Company recognized a loss on extinguishment of debt in the amount of $0.7 million during the three months ended September 30, 2017. The Company incurred less than $0.1 million in debt issuance costs in connection with the closing of the 2017 Term Loan. Debt issuance costs are presented in the consolidated balance sheet as a direct deduction from the associated liability and amortized to interest expense over the term of the related debt. The 2017 Term Loan will mature on February 1, 2022. Each advance under the 2017 Term Loan accrues interest at a floating per annum rate equal to one-half of one percent above the prime rate (as published in the money rates section of The Wall Street Journal). The 2017 Term Loan provided for interest-only payments monthly until August 31, 2019. On September 1, 2019, the Company began making amortization payments on the Term Loan, which will continue to be payable monthly in equal installments of principal and variable interest to fully amortize the outstanding principal over the remaining term of the loan. The monthly interest is subject to recalculation upon a change in the prime rate. The Company may prepay the 2017 Term Loan in full but not in part provided that the Company (i) provides five business days’ prior written notice to SVB, (ii) pays on the date of such prepayment for all outstanding principal plus accrued and unpaid interest, 1% if prepaid after the second anniversary. Amounts outstanding during an event of default are payable upon SVB’s demand and shall accrue interest at an additional rate of 4.0% per annum of the past due amount outstanding. The events of default under the Loan Agreement include, but are not limited to, the Company’s failure to make any payments of principal or interest under the Loan Agreement or other transaction documents, the Company’s breach or default in the performance of any covenant under the Loan Agreement or other transaction documents, the occurrence of a material adverse effect, the Company making a false or misleading representation or warranty in any material respect under the Loan Agreement, the Company’s insolvency or bankruptcy, any attachment or judgment on the Company’s assets in excess of approximately $0.3 million , or the occurrence of any default under any agreement or obligation of the Company involving indebtedness in excess of approximately $0.3 million . If an event of default occurs, SVB is entitled to take enforcement action, including acceleration of amounts due under the Loan Agreement. The 2017 Term Loan is secured by a lien on substantially all of the assets of the Company, other than intellectual property, provided that such lien on substantially all assets includes any rights to payments and proceeds from the sale, licensing or disposition of intellectual property. The Company has also granted SVB a negative pledge with respect to its intellectual property. The 2017 Term Loan does not include any financial covenants. The 2017 Term Loan requires a final payment fee of 5% on the aggregate principal amounts borrowed upon repayment at maturity, on a prepayment date, or upon default. The final payment fee totaling $1.1 million is recorded as a loan discount. Under the 2017 Term Loan , the Company is not required to maintain a minimum cash balance. All deposits in operating, depository and securities accounts are required to be maintained with SVB in an amount equal to the lessor of (i) 100% of the Company's cash balance or (ii) 105% of the dollar amount of the then outstanding obligations. In addition, the 2017 Term Loan contains a subjective acceleration clause whereby in an event of default, an immediate acceleration of repayment occurs if there is a material impairment of the lenders’ lien or the value of the collateral, a material adverse change in the business condition or operations, or a material uncertainty exists that any portion of the loan may not be repaid. The Company assessed all terms and features of the 2017 Term Loan in order to identify any potential embedded features that would require bifurcation. As part of this analysis, the Company assessed the economic characteristics and risks of the 2017 Term Loan , including any put and call features. The Company determined that all features of the 2017 Term Loan were clearly and closely associated with the debt host and did not require bifurcation as a derivative liability, or the fair value of the embedded feature was immaterial to the Company's consolidated financial statements. The Company reassesses the identified features on a quarterly basis to determine if they require bifurcation. As of December 31, 2019 and December 31, 2018 , the outstanding principal balance under the 2017 Term Loan was $18.2 million and $21.0 million , respectively. Future minimum principal and interest payments on the 2017 Term Loan as of December 31, 2019 are as follows (in thousands): 2020 9,084 2021 8,716 2022 2,461 Total minimum debt payments $ 20,261 Less: Amount representing interest (1,011 ) Less: Debt discount and deferred charges (345 ) Less: Current portion of loan payable (18,905 ) Loan payable, net of current portion $ — All amounts due under the 2017 Term Loan have been classified as a current liability as of December 31, 2019 due to the considerations discussed in Note 1 and the assessment that the material adverse change clause under the 2017 Term Loan is not within the Company's control. The Company has not been notified of an event of default by SVB as of the date of the filing of this Annual Report on Form 10-K . During the years ended December 31, 2019 , 2018 and 2017, the Company recognized $1.5 million , $1.5 million and $1.2 million respectively of interest expense related to the 2017 Term Loan.

Equity

Equity	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Equity	Equity December 2019 Financing On December 18, 2019, the Company entered into a private purchase agreement (the "2019 Purchase Agreement"), and closed the Offering on December 23, 2019. Pursuant to the 2019 Purchase Agreement, the Company sold an aggregate of 37,634,883 shares of its common stock at a purchase price of $ 1.46 per share, warrants to purchase an aggregate of 22,988,501 shares of common stock at a purchase price of $ 0.125 per share underlying each common warrant, and pre-funded warrants to purchase an aggregate of 8,342,128 shares of common stock at a purchase price of $ 1.46 per share, all with five year terms. The exercise price of the pre-funded warrants was $0.0001 per share and the exercise price for the common warrants is $ 1.46 per share. In the event of a certain sale of the Company, the terms of the common warrants require us to make a payment to such common warrant holders based on a Black-Scholes valuation (using variables as specified in the warrants). This provision does not apply to the pre-funded warrants. Therefore, we are required to account for the common warrants as liabilities and record them at fair value, while the pre-funded warrants met the criteria to be classified as permanent equity. We recorded the fair value of the common warrants of $ 40.7 million upon issuance using the Black-Scholes valuation model. The common warrants were revalued as of December 31, 2019 at $ 41.5 million; the change in fair value of $ 0.9 million was recorded in our statement of operations for the year ended December 31, 2019 . Issuance costs were allocated between the equity component with an offset to additional paid-in capital and the liability component recorded as expense on a relative fair value basis. Total net proceeds from the equity offering was $ 65.6 million, after deducting transaction costs and commissions of $4.4 million that were accrued at December 31, 2019 . Pursuant to the Registration Rights Agreement, the Company agreed to prepare and file a registration statement with the Securities and Exchange Commission (the “SEC”) within 45 days after the closing of the Offering for purposes of registering the resale of the Shares, shares of Common Stock issuable upon exercise of the Warrants, and any shares of Common Stock issued as a dividend or other distribution with respect to the Shares or shares of Common Stock issuable upon exercise of the Warrants. If the Company did not file such registration statement by the 45-day filing deadline, the Company would have been required to make pro-rata payments to each investor in an amount equal to 1% of the aggregate amount paid pursuant to the stock purchase agreement entered into by such investor for each 30-day period or pro-rata portion thereof following the filing deadline. The Company filed a registration statement on Form S-3 on January 29, 2020, which became effective on February 6, 2020, so no such payments were required. The Company agreed, among other things, to indemnify the Investors, their officers, directors, members, employees and agents, successors and assigns under the registration statement from certain liabilities and to pay all fees and expenses (excluding any legal fees of the selling holder(s), and any underwriting discounts and selling commissions) incident to the Company’s obligations under the Registration Rights Agreement. August 2019 Financing On August 19, 2019, the Company sold 3,178,174 shares of its common stock pursuant to a stock purchase agreement (the "2019 Private Placement") to individual investors, including certain executive officers and members of the board of directors of the Company for aggregate net proceeds of approximately $ 5.7 million, after deducting transaction costs, at a purchase price equal to $ 1.81 per share, which was equal to the most recent consolidated closing bid price on the Nasdaq Global Market on August 19, 2019. The shares in the 2019 Private Placement were issued as “restricted securities” (as defined in Rule 144 of the Securities Act) and carry no registration rights that require or permit the filing of any registration statement. January 2019 Financing On January 25, 2019 , the Company completed an underwritten public offering (the “2019 Follow-On”) of 20,000,000 shares of its common stock at a public offering price of $1.50 per share. On January 29, 2019 , an additional 2,188,706 shares were sold at a public offering price of $1.50 per share pursuant to the underwriters’ exercise of an over allotment option. The total net proceeds from the offering were $ 30.9 million , after deducting underwriting discounts, transaction costs and commissions. 2019 “At-the-Market” Offerings Concurrent with the filing of the shelf registration statement, the Company entered into a sales agreement (the “Sales Agreement”) with Jefferies LLC, as sales agent, pursuant to which the Company may, from time to time, issue and sell common stock with an aggregate value of up to $50 million in an "at-the-market" offering. Sales of common stock, if any, pursuant to the Sales Agreement, may be made in sales deemed to be an “at the market offering” as defined in Rule 415(a) of the Securities Act, including sales made directly through the Nasdaq Global Market or on any other existing trading market for the Company’s common stock. The Company intends to use the proceeds from the offering for working capital and other general corporate purposes. The Company may suspend or terminate the Sales Agreement at any time. During the year ended December 31, 2019, the Company sold 615,453 shares of its common stock pursuant to the Sales Agreement at an average price of approximately $ 1.84 per share for aggregate net proceeds of $1.0 million , after deducting commissions and other transaction costs. August 2017 Shelf Registration Statement On August 10, 2017 , the Company filed a universal shelf registration statement on Form S-3 with the SEC to sell an aggregate amount of up to $200.0 million of certain of our securities. The shelf registration statement was declared effective by the SEC on August 28, 2017 . June 2017 Financing On June 26, 2017, the Company entered into a securities purchase agreement (the "Institutional Purchase Agreement") with a select group of institutional investors (the “Institutional Investors”) and a securities purchase agreement with Timothy A. Springer, Ph.D., a member of the board of directors (the "Springer Purchase Agreement") for a private placement of the Company's securities (the "2017 PIPE"). The closing of the 2017 PIPE occurred on June 27, 2017. Pursuant to the Institutional Purchase Agreement, the Company sold an aggregate of 2,750,000 shares of its common stock at a purchase price equal to $16.00 per share. Pursuant to the Springer Purchase Agreement, the Company sold to Dr. Springer an aggregate of 338,791 shares of common stock at a purchase price equal to $17.71 per share, which was equal to the most recent consolidated closing bid price on the Nasdaq Global Market on June 23, 2017, and warrants to purchase up to 79,130 shares of common stock (“Warrant Shares”), exercisable at $17.71 per Warrant Share, and with a term of five years . The purchase price for each warrant was equal to $0.125 for each Warrant Share, consistent with Nasdaq Global Market requirements for an “at the market” offering. Under the terms of the Common Stock Purchase Warrant, the warrants can be settled in unregistered shares. The Warrant Shares qualify for equity classification. The fair value of the allocated proceeds was determined on the relative fair value basis. After deducting for placement agent fees and offering expenses, the aggregate net proceeds from the 2017 PIPE were approximately $47.1 million . On June 27, 2017, in connection with the 2017 PIPE, the Company entered into a Registration Rights Agreement (the “Registration Rights Agreement”) with the Institutional Investors and Dr. Springer. Pursuant to the Registration Rights Agreement, the Company agreed to prepare and file a registration statement with the SEC within 20 days after the closing of the 2017 PIPE for purposes of registering the resale of the shares of common stock issued and sold in the 2017 PIPE (the “Shares”), the Warrant Shares, and any shares of common stock issued as a dividend or other distribution with respect to the Shares or Warrant Shares. The 2017 PIPE registration statement was declared effective by the SEC on July 21, 2017. The Company agreed to indemnify the Institutional Investors and Dr. Springer, their officers, directors, members, employees and agents, successors and assigns under the registration statement from certain liabilities and to pay all fees and expenses (excluding any legal fees of the selling holder(s), and any underwriting discounts and selling commissions) incident to the Company’s obligations under the Registration Rights Agreement. Warrants The following table summarizes warrant activity for the years ended December 31, 2019, 2018 and 2017 as follows: Number of Warrants Equity classified Liability classified Total Weighted average exercise price Outstanding at December 31, 2017 176,432 — 176,432 $ 17.32 Issuance — — — — Exercises — — — — Expirations/ cancellations (80,813 ) — (80,813 ) 17.55 Outstanding at December 31, 2018 95,619 — 95,619 17.12 Issuance 8,342,128 22,988,501 31,330,629 1.07 Exercises — — — — Expirations/ cancellations — — — — Outstanding at December 31, 2019 8,437,747 22,988,501 31,426,248 $ 1.12 Common Stock As of December 31, 2019 , the Company had 200,000,000 shares of common stock authorized for issuance, $0.0001 par value per share, with 86,325,547 shares issued and outstanding. The voting, dividend and liquidation rights of the common stockholders are subject to and qualified by the rights, powers and preferences of the preferred stock. The common stock has the following characteristics: Voting The common stockholders are entitled to one vote for each share of common stock held with respect to all matters voted on by the stockholders of the Company. Dividends The common stockholders are entitled to receive dividends, if and when declared by the Board of Directors. Through December 31, 2019 , no dividends have been declared or paid on common stock. Liquidation Upon liquidation of the Company, the common stockholders are entitled to receive all assets of the Company available for distribution to such stockholders. Reserved Shares The Company has authorized shares of common stock for future issuance as follows: Period ending December 31, 2019 December 31, 2018 Exercise of common and pre-funded warrants 31,426,248 95,619 Shares available for future stock incentive awards 1,765,018 1,586,925 Unvested restricted stock units 181,250 175,000 Outstanding common stock options 6,796,669 4,093,979 Total 40,169,185 5,951,523

Stock Incentive Plans

Stock Incentive Plans	12 Months Ended
	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Stock Incentive Plans	Stock Incentive Plans Stock Options The Company maintains the 2008 Stock Incentive Plan (the “2008 Plan”) for employees, consultants, advisors, and directors. The 2008 Plan provided for the granting of incentive and non‑qualified stock option and restricted stock awards as determined by the Board. At inception of the 2008 Plan, a total of 2,213,412 shares of common stock were authorized for grants under the 2008 Plan. The Company ceased granting awards under the 2008 Plan upon the effectiveness of the 2016 Plan (as defined below); however, awards issued under the 2008 Plan remain subject to the terms of the 2008 Plan and the applicable 2008 Plan agreement. Shares subject to awards that were granted under the 2008 Plan and that expire, lapse or terminate following the effectiveness of the 2016 Plan become available under the 2016 Plan as shares available for future grants. All unvested stock options granted under the 2008 Plan may be exercised into restricted stock subject to forfeiture upon termination prior to vesting. On June 7, 2016, the Company’s stockholders approved the 2016 Incentive Award Plan (the “2016 Plan”), which became effective June 21, 2016. The 2016 Plan provides for the granting of incentive and non‑qualified stock option, restricted stock and other stock and cash-based awards as determined by the Board. Shares subject to awards that are granted under the 2016 Plan and that expire, lapse or terminate are available for future grants under the 2016 Plan. At inception of the 2016 Plan, a total of 1,210,256 shares of common stock were authorized for future issuance under the 2016 Plan. The number of shares of common stock that may be issued under the 2016 Plan automatically increases on the first day of each calendar year, beginning in 2017 and ending in and including 2026, by an amount equal to the lesser of: (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) such smaller number of shares as is determined by the Board. During the year ended December 31, 2019 and 2018 , the number of shares of common stock that may be issued under the 2016 Plan was increased by 898,871 shares and 893,730 shares, respectively. As of December 31, 2019 , 267,612 shares remain available for future issuance under the 2016 Plan. The 2008 Plan and 2016 Plan provide that the exercise price of incentive stock options cannot be less than 100% of the fair market value of the Company's common stock on the grant date for participants who own 10% or less of the total combined voting power of the Company, and not less than 110% for participants who own more than 10% of the Company’s voting power. Options and restricted stock awards granted under the 2008 Plan and 2016 Plan vest over periods as determined by the Board, which are generally four years and, for options, with terms that generally expire ten years from the grant date. The Company’s 2018 Employment Inducement Incentive Award Plan (the “Inducement Incentive Award Plan”), which was adopted by the Board on September 25, 2018 without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules ("Rule 5635(c)(4)"), provides for the grant of equity-based awards in the form of non-qualified stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards and other stock or cash based awards. In accordance with Rule 5635(c)(4), awards under the Inducement Incentive Award Plan may only be made to a newly hired employee who has not previously been a member of the Board, or an employee who is being rehired following a bona fide period of non-employment by the Company, as a material inducement to the employee’s entering into employment with the Company. The Company reserved 1,175,000 shares of its common stock for issuance under the Inducement Incentive Award Plan. On March 25, 2019, the Board approved the amendment and restatement of the Inducement Incentive Award Plan to reserve an additional 2,000,000 shares of the Company’s common stock for issuance thereunder. As of December 31, 2019 , there are 750,000 shares available for future grant under the Inducement Incentive Award Plan. The fair value of each option award was estimated on the grant date using the Black‑Scholes option pricing model. Expected volatilities are based on historical volatilities from guideline companies because the Company's common stock has not traded for a period that is at least equal to the expected term of its stock option awards. The Company uses the “simplified” method to estimate the expected life of options granted and are expected to be outstanding. The risk‑free interest rate used is the rate for a U.S. Treasury zero coupon issue with a remaining life consistent with the options expected life on the grant date. The Company has not paid and does not expect to pay in the foreseeable future, any cash dividends. Forfeitures are estimated at the time of grant and are adjusted, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company has estimated a forfeiture rate of 10% based on historical attrition trends. The Company records stock‑based compensation expense only on awards that are expected to vest. The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: Year Ended December 31, 2019 2018 2017 Risk-free interest rate 1.86 % 2.85 % 2.03 % Dividend yield — — — Expected term 5.94 6.06 5.90 Expected volatility 87.66 % 85.17 % 84.52 % Weighted-average fair value of common stock $ 2.01 $ 8.45 $ 15.32 The weighted average grant date fair value of stock options granted to employees during the years ended December 31, 2019 , 2018 and 2017 was $1.47 , $6.17 , and $10.97 respectively. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2019 , 2018 and 2017 was $0.1 million , $0.6 million , and $2.6 million respectively. As of December 31, 2019 and December 31, 2018, total unrecognized compensation expense related to unvested employee stock options was $9.8 million and $9.8 million , respectively, which is expected to be recognized over a weighted average period of 2.5 years and 3.1 years, respectively. Effective November 20, 2019, the Company amended the terms of the options granted to a non-employee director in connection with his resignation from the board of directors, whereby he was entitled to receive immediate vesting of his outstanding, unvested stock options ( 141,328 shares), provided that the options will become exercisable in accordance with their original vesting schedules irrespective of his termination of service, and an extension of the right to exercise each of his vested stock options ( 234,772 shares) until the final expiration date of the applicable option, resulting in a modification. The subsequent stock-based compensation amount recognized as of December 31, 2019 was $0.1 million. The estimated grant date fair values of non-employee stock option awards granted under the 2016 Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: Year Ended December 31, 2019 2018 2017 Risk-free interest rate 1.92 % 2.77 % — % Dividend yield — — — Expected life (in years) 5.33 5.81 0.00 Expected volatility 88.60 % 85.86 % — % The weighted average grant date fair value of stock options granted to non-employees during the years ended December 31, 2019 and 2018 was $1.32 and $6.78 , respectively. As of December 31, 2019 and 2018 total unrecognized compensation expense related to unvested non‑employee stock options was less than $0.1 million and $1.1 million , respectively, which is expected to be recognized over a weighted average period of 0.4 years and 2.2 years, respectively. The following table summarizes the activity under the 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan: Weighted‑average remaining Aggregate Number of Weighted-average contractual term intrinsic value options exercise price ($) (in years) (in thousands) Employee awards Outstanding at December 31, 2018 3,681,575 $ 9.49 7.77 $ 300 Granted 3,876,255 $ 2.01 Exercised (125,600 ) $ 1.20 Forfeited (1,108,634 ) $ 10.38 Outstanding at December 31, 2019 6,323,596 $ 4.91 8.71 $ 1,716 Vested at December 31, 2019 1,485,228 $ 10.02 6.73 $ 90 Vested and expected to vest at December 31, 2019 5,843,053 $ 5.10 8.65 $ 1,500 Non‑employee awards Outstanding at December 31, 2018 412,404 $ 6.44 6.42 $ 28 Granted 73,489 $ 1.86 Exercised — $ — Forfeited (12,820 ) $ 0.47 Outstanding at December 31, 2019 473,073 $ 5.89 6.23 $ 38 Vested at December 31, 2019 330,528 $ 5.61 5.06 $ 21 Vested and expected to vest at December 31, 2019 473,073 $ 5.89 6.23 $ 38 Restricted Stock Units During the first quarter of 2019, the Company awarded 100,000 restricted stock units under the Inducement Incentive Award Plan, of which 50,000 were determined to be granted and 50,000 were reserved for issuance consistent with ASC Topic 718, Compensation-Stock Compensation (ASC 718) . The 50,000 granted restricted stock units had a fair value of $ 2.29 per share based on the closing price of the Company’s common stock on the date of grant. These restricted stock units were valued at approximately $ 0.1 million , and will vest on the date an applicable performance condition is achieved on or prior to December 31, 2020. If the performance condition is not satisfied on or prior to December 31, 2020, the restricted stock units will be forfeited for no consideration. The 50,000 reserved restricted stock units did not have defined performance criteria until August 6, 2019, at which time, they were deemed both granted and vested. These restricted stock units had a fair value of $ 1.65 per share. These restricted stock units were valued at approximately $ 0.1 million and fully expensed as of December 31, 2019 . Unrecognized compensation expense for the restricted stock units was $0.8 million as of December 31, 2019 , which is expected to be recognized over a weighted average period of 2.8 years . The following table summarizes the status of the Company’s restricted stock units: Number of shares Weighted average fair value ($) Unvested at December 31, 2018 175,000 $ 6.03 Granted 100,000 1.97 Vested 93,750 1.65 Forfeited — — Unvested at December 31, 2019 181,250 $ 5.00 Employee Stock Purchase Plan On June 7, 2016, the Company’s stockholders approved the 2016 Employee Stock Purchase Plan (the “ESPP”), which became effective June 21, 2016. The ESPP is intended to qualify as an "employee stock purchase plan" under Section 423 of the Internal Revenue Code of 1986 with the purpose of providing employees with an opportunity to purchase the Company's common stock through accumulated payroll deductions. Under the ESPP, the Company has set two six-month offering periods during each calendar year, one beginning March 1 st and the other beginning September 1 st of each calendar year, during which employees may elect to have up to 25% of their eligible compensation deducted on each payday on an after-tax basis for use in purchasing the Company's common stock on the last trading day of each offering period, subject to limits imposed by the Internal Revenue Code. The purchase price of the shares may not be less than 85% of the fair market value on the first or last trading day of the offering period, whichever is lower. The first ESPP offering period began on March 1, 2017. At inception of the ESPP, a total of 173,076 shares of common stock were authorized and reserved for future issuance under the ESPP. The number of shares of common stock that may be issued under the ESPP will automatically increase on the first day of each calendar year, beginning in 2017 and ending in and including 2026, by an amount equal to the lesser of: (i) 1% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) such smaller number of shares as is determined by the Company’s Board of Directors. During the year ended December 31, 2019 and 2018 , the number of shares of common stock that may be issued under the ESPP was increased by 224,717 shares and 223,432 shares, respectively. During the year ended December 31, 2019 , the Company issued 17,205 shares of common stock under the ESPP. As of December 31, 2019 , 747,406 shares remain available for future issuance under the ESPP. For each of the years ended December 31, 2019 and 2018 , the Company recognized less than $0.1 million of stock-based compensation expense under the ESPP. The Company recorded stock-based compensation expense related to stock option awards, restricted stock units and the ESPP in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Year Ended December 31, 2019 2018 2017 Research and development $ 2,079 $ 2,453 $ 1,779 General and administrative 3,082 3,261 2,302 Total stock-based compensation expense $ 5,161 $ 5,714 $ 4,081

Revenue Arrangements

Revenue Arrangements	12 Months Ended
	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Revenue Arrangements	Revenue Arrangements The Company adopted ASC 606 on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The reported results for 2018 reflect the application of ASC 606 guidance while the reported results for 2017 were prepared under the guidance of ASC 605, Revenue Recognition (ASC 605), which is also referred to herein as "legacy GAAP" or the "previous guidance". The adoption of ASC 606 represents a change in accounting principle that will more closely align revenue recognition with the delivery of the Company's services and will provide financial statement readers with enhanced disclosures. Financial Statement Impact of Adopting ASC 606 The cumulative effect of applying the new guidance to all contracts with customers that were not completed as of January 1, 2018, was recorded as an adjustment to accumulated deficit as of the adoption date. As a result of applying the modified retrospective method to adopt the new revenue guidance, the Company recorded an adjustment to long term deferred revenue and accumulated deficit totaling $1.8 million . In connection with the adoption of ASC 606, the Company identified three collaboration/grant arrangements that required analysis to quantify the impact of adoption to its opening accumulated deficit balance as of January 1, 2018: Spark Therapeutics, Inc., Skolkovo Foundation and National Institutes of Health. Asklepios Biopharmaceutical, Inc. License Agreement for Pompe Disease On December 17, 2019, the Company and AskBio entered into a License Agreement, referred to as the AskBio License Agreement. Pursuant to the AskBio License Agreement, AskBio has exercised its option to exclusively license the Company’s intellectual property rights covering the Company’s ImmTOR platform to research, develop, and commercialize certain AAV gene therapy products utilizing ImmTOR, and targeting the GAA gene, or derivatives thereof, to treat Pompe Disease. Pursuant to the AskBio License Agreement and ancillary documents, AskBio agreed to pay to the Company upfront fees of an aggregate of $7.0 million . Assuming successful development and commercialization, the Company could receive up to an additional $237.0 million in development, regulatory, and sales milestone payments. If commercialized, the Company would be eligible to receive tiered royalties on global net sales at percentages ranging from mid-to-high single digits. Under the terms of the agreement, the Company will be eligible to receive these royalties commencing on the first commercial sale of the licensed product until the expiration of the later of (i) ten years after the first commercial sale and (ii) expiration of the last to expire valid claim on patents covering the licensed product. Pursuant to the AskBio License Agreement, the Company will supply AskBio with its ImmTOR platform ("Supply Obligation") and AskBio will be responsible for all preclinical, clinical and commercial manufacture and supply of Licensed Products (other than ImmTOR) and carry out all other activities related to the research, development, and commercialization of Licensed Products at its sole expense, including all regulatory activities related thereto. The Company determined that the AskBio License was not capable of being distinct from the Supply Obligation. The Company has concluded that AskBio cannot derive benefit from the license without the simultaneous transfer of the patent protected ImmTOR supply. Therefore, the License Obligation and Supply Obligation represent the only promise in the arrangement and are combined as a single performance obligation (the “AskBio License and Supply Obligation”). In determining the transaction price, the Company concluded that the future development milestones, regulatory milestones, sales milestones, and sales royalties all represent variable consideration. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of such milestones is outside the control of the Company. Consideration related to sales-based milestones as well as royalties on net sales upon commercialization by AskBio, will be recognized when the related sales occur, as they were determined to relate predominantly to the intellectual property granted to AskBio and, therefore, have also been excluded from the transaction price in accordance with the royalty recognition constraint. As of December 31, 2019, all milestones were constrained. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved, or as other changes in circumstances occur. The total initial transaction price of the contract on the effective date is $7.0 million , which is comprised of the $2.0 million initial up-front payment upon agreement of terms, and $5.0 million initial up-front execution fee. The $5.0 million execution fee became billable on the effective date and is recorded on the balance sheet as a receivable and a corresponding contract liability as of December 31, 2019. As of December 31, 2019, the Company recorded $1.7 million as a short-term contract liability and $5.3 million as a long-term contract liability representing deferred revenue associated with this agreement. Spark Therapeutics, Inc. Spark License Agreement In December 2016, the Company entered into a License and Option Agreement (“Spark License Agreement”) with Spark Therapeutics, Inc. (“Spark”) pursuant to which the Company and Spark agreed to collaborate on the development of gene therapies for certain targets utilizing the ImmTOR platform. The Spark License Agreement provides Spark with certain exclusive, worldwide, royalty bearing licenses to the Company’s intellectual property, allowing Spark to develop and commercialize gene therapies in combination with ImmTOR for an initial identified target. In addition to an upfront cash payment of $10.0 million under the Spark License Agreement, additional payments of an aggregate of $5.0 million in two payments of $2.5 million each were paid within twelve months of December 2, 2016 (“Contract Date”). The first of the two additional payments was scheduled to be made on or before May 31, 2017 (the “May 2017 License Payment”) (see “Spark Letter Agreement” below) and the second was made on October 31, 2017. Spark may also exercise options to research, develop and commercialize gene therapies utilizing the ImmTOR platform for up to four additional targets. The Company was eligible to receive a variable fee up to $2.0 million for each additional target option elected, dependent on the incidence of the applicable indication. The election period in which Spark could have exercised additional targets under the Spark License Agreement was a term of three years from the Contract Date, which expired on December 1, 2019. Assuming successful development and commercialization, the Company could receive up to an additional $65.0 million in development and regulatory milestone payments and $365.0 million in commercialization milestone payments for each indication. If commercialized, the Company would be eligible to receive tiered royalties on global net sales at percentages ranging from mid-single to low-double digits, all of which apply on a target-by-target basis. Under the terms of the agreement, the Company will be eligible to receive these royalties commencing on the first commercial sale of the licensed product and terminating upon the later of (i) ten years after the first commercial sale, (ii) expiration of the last to expire valid claim on patents covering the jointly invented field specific improvements, or (iii) the expiration of regulatory exclusivity in the applicable country for the licensed product. The Spark License Agreement may be terminated by Spark for convenience upon ninety days ’ notice. Either party may terminate the Spark License Agreement on a target-by-target basis for material breach with respect to such target. In December 2016, the Company also entered into a Share Purchase Agreement (the “Spark Purchase Agreement”) with Spark. Pursuant to the Spark Purchase Agreement, the Company sold 197,238 shares of the Company’s common stock to Spark for gross proceeds of $5.0 million , or $25.35 per share of common stock, at an initial closing (the “Initial Closing”). The purchase price per share represents an amount equal to 115% of the average daily volume weighted average price (“VWAP”) of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the Contract Date. Beyond the Initial Closing, the Spark Purchase Agreement contemplated potential future sales of shares by the Company to Spark as follows: • First Acquisition Right. During the period beginning on May 1, 2017 and ending on June 1, 2017, Spark had the right (the “First Acquisition Right”) to purchase a number of shares of common stock equal to an aggregate price of $5.0 million . See "Spark Letter Agreement" below. • Second Acquisition Right. During the period beginning on October 1, 2017 and ending on November 1, 2017, Spark had the right (the “Second Acquisition Right”) to purchase a number of shares of common stock equal to an aggregate price of $5.0 million . On October 31, 2017 Spark exercised this right and purchased 205,254 shares of common stock from the Company for $5.0 million , or $24.36 per share of common stock. The purchase price per share represents an amount equal to 115.0% of the average daily VWAP of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the Second Acquisition Right notification date. The First Acquisition Rights and Second Acquisition Rights are collectively referred to herein as the “Acquisition Rights”. Under the Spark Purchase Agreement, Spark agreed not to dispose of any of the shares acquired at either the Initial Closing or the from the subsequent Acquisition Rights that it may acquire until January 1, 2018 and, thereafter, transfers are contractually subject to volume limitations applicable to an “affiliate” under Rule 144 of the Securities Act. In connection with the Spark License Agreement and Spark Purchase Agreement, the Company has made contractual payments defined in the MIT license agreement (see Note 14) totaling $2.2 million for the MIT sub-license provided to Spark, and $0.4 million relative to the calculated premium paid by Spark for the equity investments made under the Spark Purchase Agreement. The terms of the Spark Purchase Agreement and the Spark License Agreement were negotiated at the same time between the parties and the terms of the Spark Purchase Agreement are referenced in the Spark License Agreement in multiple sections. The pricing and terms of the agreements are unique and must be considered in contemplation with each other. There are provisions within the Spark License Agreement that link to the Spark Purchase Agreement related to provisions that constitute a material breach of the license agreement. Therefore, the Company concluded that the two agreements must be combined and evaluated as a single agreement. While the Spark Purchase Agreement and the Spark License agreement are considered to be a single agreement, the Company determined that the purchase of common stock and future acquisition rights are not within the scope of ASC 606. The Company determined that the initial purchase of common stock combined with the embedded future stock Acquisition Rights had a fair value of $2.7 million and this amount was recorded in equity as of the effective date. The remaining $2.3 million of cash received in exchange for the stock and acquisition rights is included in allocable consideration, as this represents the premium paid by Spark on the purchase of common stock, and should be allocated to the remaining performance obligations. The Company identified the following components of the agreement: (1) certain exclusive, worldwide, royalty bearing licenses to the Company’s intellectual property and a license to conduct certain research activities under the collaboration, (the "Spark License"), (2) options to research, develop and commercialize gene therapies utilizing the ImmTOR platform for up to four additional target therapy options, (the "Option Obligation"), (3) manufactured supply of ImmTOR, (the "Supply Obligation") at a discount. In exchange, the Company received an upfront payment of $15.0 million and is eligible to receive additional payments of up to $35.0 million based on the achievement by Spark of future specified development milestones, up to $30.0 million based on the achievement by Spark of future specified regulatory milestones, up to $110.0 million based on the achievement by Spark of future specified commercial milestones, and up to $255.0 million based on the achievement by Spark of future specified sales milestones. The Company will also be eligible to receive tiered royalty payments that reach low double-digits based on future net sales for the duration of the royalty term. The Company determined that the Spark License and Supply Obligation represent a single promise and performance obligation (the “Combined License and Supply Obligation”). This is because Spark cannot derive benefit from the license without the simultaneous transfer of the patent protected ImmTOR supply. The Company also determined that the Target Options, which includes the related Supply Obligation, provides the customer with a material right and is considered a performance obligation in the arrangement since it was priced at an incremental discount. Therefore, the Company determined that the Spark agreement contains five distinct performance obligations: the Combined License and Supply Obligation, and the four separate Target Options. In determining the transaction price, the Company considered the future development milestones, regulatory milestones, commercial milestones, sales milestone, and sales royalties all represent variable consideration. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of such milestones is outside the control of the Company. Separately, any consideration related to sales-based milestones as well as royalties on net sales upon commercialization by Spark, will be recognized when the related sales occur as they were determined to relate predominantly to the intellectual property granted to Spark and, therefore, have also been excluded from the transaction price in accordance with the royalty recognition constraint. As of December 31, 2019 , all future milestones are constrained. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved, or as other changes in circumstances occur. The Company determined that the up-front payment of $12.3 million ( $15.0 million , less fair value of the equity totaling $2.7 million as discussed above) was included in the transaction price and was allocated to the performance obligations based on the Company’s best estimate of their relative stand-alone selling prices. The Company allocated $7.1 million to the Combined License and Supply Obligation and $5.2 million to the discount on the Target Options ( $1.3 million for each option) using the relative standalone selling price method to each obligation. The standalone selling price for the Combined License and Supply Obligation was determined using a discounted cash flow model. The standalone selling price for the Target Options were determined based on the fair value of the license minus the strike price of the option (the probability of exercise was included in the valuation) as well as the estimated discount of the Supply Obligation. The estimated proceeds expected to be received from the sale of the Supply Obligation were also included in the transaction price for the Combined License and Supply Obligation. The total consideration allocated to the Combined License and Supply Obligation will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term which was initially estimated to be approximately four years . On December 1, 2019, the term for Spark to exercise additional target options expired; the Company recognized $6.7 million in revenue from deferred revenue as originally allocated. In addition, during the year ended December 31, 2019 , there were two deliveries resulting in less than $0.1 million of revenue recognized. No revenue related to the Spark License Agreement was recognized during the year ended December 31, 2018. As of December 31, 2019 , there was a contract liability of $9.2 million representing deferred revenue presented as non-current associated with this agreement. As of December 31, 2018 , there was $14.7 million of deferred revenue related to this agreement. Spark Letter Agreement On June 6, 2017, the Company and Spark entered into a letter agreement (the “Letter Agreement”), pursuant to which the parties agreed that Spark would make the May 2017 License Payment by June 6, 2017. The May 2017 License Payment was received, and recorded as a liability as of June 30, 2017, of which some or all may potentially constitute the reimbursement described below. The parties also agreed that Spark would be deemed to have delivered notice on May 31, 2017 exercising its right to purchase the shares pursuant to the First Acquisition Right. The Letter Agreement further outlines a cost reimbursement arrangement, pursuant to which the Company agreed to reimburse Spark for all costs and expenses, including the cost of materials provided by the Company, associated with the preclinical research and toxicology studies being performed by Spark for any licensed products for a specified amount of time (the “Reimbursement Period”), in an amount not to exceed $2.5 million . Consistent with the First Acquisition Right, Spark purchased 324,362 shares of common stock pursuant to the Spark Purchase Agreement, as amended by the Letter Agreement, for an aggregate purchase price of $5.0 million , or $15.41 per share of common stock. The purchase price per share represents an amount equal to 115.0% of the average daily VWAP of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the First Acquisition Right notification date. At the initial contract assessment, the Company allocated $2.7 million to equity (representing the fair value of the initial purchase of common stock combined with the embedded future stock Acquisition Rights). Upon exercise of the First Acquisition Right, the Company recorded the purchase amount to stockholders ’ equity (deficit). The Company determined that the Letter Agreement resulted in a modification to the original agreement. The amount received totaling $2.5 million and the reimbursements pursuant to the Letter Agreement totaling $2.5 million were both included in the transaction price, and a liability was recorded for the amount expected to be repaid. As repayments were made, the underlying liability was reduced. To the extent that an amount was expected to be applied towards the clinical supply obligation, the analysis of variable consideration was updated accordingly. On October 31, 2017, Spark paid the Company a $2.5 million milestone payment pursuant to the Spark License Agreement, which was included in the transaction price and allocated to the performance obligations using the relative standalone selling price. In addition, Spark exercised the Second Acquisition Right set forth in Section 2.4 of the Spark Purchase Agreement and purchased 205,254 shares of common stock from the Company for $5.0 million , or $24.36 per share of common stock. The purchase price per share represents an amount equal to 115.0% of the average daily VWAP of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the Second Acquisition Right notification date. On June 5, 2019, the term of the Reimbursement Period under the Letter Agreement expired. During the year ended December 31, 2019 , the Company updated its estimate of variable consideration included in the transaction price to include $1.2 million of unpaid reimbursements to Spark. Skolkovo Foundation The Company has received grant funding from the Russia-based Development Fund of New Technologies Development and Commercialization Center ("Skolkovo"). From grant inception through December 31, 2019 , the Company received $2.0 million from Skolkovo. Based on the guidance in ASC 606, the Company concluded that the entire $2.0 million of grant funds received from Skolkovo is variable consideration. Although the Company believes it has an enforceable right to the amounts received, there is risk that an audit could result in the Company needing to refund certain amounts back to Skolkovo, resulting in variability in the transaction price. The Company utilized the “expected value” approach in determining the amount that can be recognized. The Company estimated that it will be entitled to revenue of $1.8 million from the Skolkovo grant, and recorded this amount. The remainder of $0.2 million was recorded as a contract liability. During the year ended December 31, 2018, the Company made a decision to cease work relating to the Skolkovo grant. As a result, Skolkovo performed a formal review of project expenses incurred by the Company. Skolkovo concluded that the Company should (i) return unused grant funds to Skolkovo in the amount of less than $0.1 million and (ii) reimburse $0.1 million of costs deemed to have been overspent relative to the cost share requirement stipulated in the grant. As of December 31, 2019 , a contract liability of $0.1 million remains on the balance sheet and will not be recognized as revenue until the expiration of the three-year audit period, expected April 2021, or sooner, if resolution is reached with Skolkovo or there is a change in the estimate. Transaction Price Allocated to Future Performance Obligations Remaining performance obligations represent the transaction price of contracts for which work has not been performed (or has been partially performed). As of December 31, 2019 , the aggregate amount of the transaction price allocated to remaining performance obligations was $9.2 million . The Company does not expect to recognize revenue on the remaining performance obligations over the next 12 months. Contract Balances from Contracts with Customers ( AskBio, Spark and Skolkovo Foundation ) The following table presents changes in the Company’s contract liabilities during the year ended December 31, 2019 (in thousands): Balance at Balance at beginning of period Additions Deductions end of period Year Ended December 31, 2019 Contract liabilities: Deferred revenue $ 14,777 $ 8,254 $ (6,677 ) $ 16,354 Other liabilities (1) 2,126 — (2,126 ) — Total contract liabilities $ 16,903 $ 8,254 $ (8,803 ) $ 16,354 (1) On June 5, 2019, the term of the Reimbursement Period under the Letter Agreement expired. During the year ended December 31, 2019 , the Company updated its estimate of variable consideration included in the transaction price to include $1.2 million of unpaid reimbursements to Spark.

Related-Party Transactions

Related-Party Transactions	12 Months Ended
	Dec. 31, 2019
Related Party Transactions [Abstract]
Related-Party Transactions	Related‑Party Transactions Financing During the year ended December 31, 2019, the Company completed the 2019 Purchase Agreement, 2019 Private Placement, and the 2019 Follow-On. The following tables set forth the number of shares of common stock, common warrants, and pre-funded warrants purchased by executive officers and members of the board of directors of the Company (and related parties thereto), as described in Note 10. 2019 Purchase Agreement Name Shares of common Common warrants Total TAS Partners, LLC 3,940,887 1,970,443 $ 6,000,000 Carrie S. Cox 65,681 32,840 $ 99,999 Timothy Barabe 65,681 32,840 $ 99,999 Scott D. Myers 39,409 19,704 $ 60,000 2019 Private Placement Name Shares of common Total Timothy A. Springer, Ph.D. 1,600,000 $ 2,896,000 TAS Partners, LLC (affiliate of Timothy A. Springer, Ph.D.) 1,100,000 $ 1,991,000 Elona Kogan, J.D. 82,872 $ 149,998 Patrick Zenner 55,248 $ 99,999 Takashi Kei Kishimoto, Ph.D. 50,000 $ 90,500 Carsten Brunn, Ph.D. 41,436 $ 74,999 Scott D. Myers 41,436 $ 74,999 Stephen Smolinski 13,812 $ 25,000 2019 Follow-On Name Shares of common Total Timothy A. Springer, Ph.D. 4,000,000 $ 6,000,000 Entities affiliated with Polaris 666,666 $ 999,999 Chafen Lu (Timothy A. Springer’s wife) 66,666 $ 99,999 Jed Springer (Timothy A. Springer’s brother) 1,000 $ 1,500 Consulting Services During 2018, the Company entered into an amended consulting agreement, (the "Consulting Agreement") with Dr. Omid Farokhzad, a member of its Board of Directors. The term of the amendment to the Consulting Agreement was April 1, 2018 to December 31, 2019, which extended the original consulting term for an additional nine months from March 31, 2019. Compensation included a $85,000 payment for the period beginning January 1, 2019 and ending December 31, 2019. The $85,000 was paid quarterly across the contract term in arrears beginning March 31, 2019. Included within this agreement, a stock option award of 75,000 shares was granted, with a weighted average grant date fair value of $4.35 . Effective November 20, 2019, Dr. Farokhzad resigned as a member of its Board of Directors. In addition, the Company incurred expenses for consulting services provided by its founders totaling $ 0.5 million, $0.2 million and $0.2 million during the years ended December 31, 2019 , 2018 and 2017, respectively. The agreements associated with the above consulting services provided by its founders were terminated effective December 31, 2019. The Company entered into new agreements with its founders to serve roles on its Scientific Advisory Board, effective January 1, 2020 to December 31, 2021, under which they will be paid quarterly for their services.

Collaboration Agreements

Collaboration Agreements	12 Months Ended
	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Collaboration Agreements	Collaboration Agreements Feasibility Study and License Agreement On August 6, 2019, the Company entered into a Feasibility Study and License Agreement with AskBio, which is referred to as the AskBio Collaboration Agreement. Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to license intellectual property rights to each other as part of a collaboration to research, develop, and commercialize certain adeno-associated virus (“AAV”) gene therapy products utilizing the Company’s ImmTOR platform to enable re-dosing of such AAV gene therapy products to treat serious rare and orphan genetic diseases for which there is a significant unmet medical need. Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to conduct proof of concept studies to potentially validate the use of ImmTOR in conjunction with AAV for the treatment of methylmalonic acidemia (MMA), based on the Company’s product candidate SEL-302, to mitigate the formation of neutralizing anti-AAV capsid antibodies (the POC Studies). If the POC Studies are successful, or the parties otherwise elect to do so, the parties will proceed with a collaboration to pursue the development and commercialization of AAV gene therapy product candidates utilizing ImmTOR for the treatment of certain agreed serious rare and orphan genetic diseases. If the POC Studies fail to demonstrate a proof of concept, and the parties do not mutually agree in writing to proceed with the collaboration, the AskBio Collaboration Agreement will expire. The Company and AskBio will share responsibility for the research, development and commercialization of products developed under this collaboration. The parties will also share research, development and commercialization costs equally for all collaboration products, but with a right of either party to opt out of certain products, and thereby no longer be required to share costs for such products. Each party will receive a percentage of net profits for each product sold under the collaboration equal to the percentage of shared costs borne by such party in the development of such product. Pursuant to the AskBio Collaboration Agreement, AskBio is responsible for manufacturing the AAV capsids and AAV vectors and the Company is responsible for manufacturing ImmTOR. The AskBio Collaboration Agreement is considered to be within the scope of ASC 808, as both parties are active participants and exposed to the risks and rewards of the collaborative activity. The Company evaluated the terms of the AskBio Collaboration Agreement and have identified the following promises in the arrangement (1) conducting research and development activities to develop and commercialize products under the collaboration, (the “R&D Services”), (2) granting a non-exclusive, non-transferable, royalty-free, fully paid up, worldwide license to certain intellectual property of the Company, (the “IP Rights”) for the purpose of performing the POC Studies, (the “Research License”), (3) granting an exclusive, nontransferable, worldwide license to the IP Rights for use in certain indications (the” Collaboration License”), (4) providing manufactured supply of preclinical and clinical ImmTOR, (the “Manufactured Supply”), (5) participation on identified steering committees responsible for the oversight of the collaboration, (the “JSC Participation”), and (6) granting an exclusive option to obtain a license under the IP Rights to research, develop and commercialize Licensed Products. The Company determined that the R&D Services, Research License, Collaboration License, Manufactured Supply, and JSC Participation were not capable of being distinct, and therefore must be combined into a single performance obligation. Therefore, promises (1) through (5) identified above were combined into a single performance obligation. Furthermore, the Company evaluated the Option Agreement and determined that it does not provide AskBio with a material right under ASC 606 as the option was not priced at a discount (see discussion of the Option exercise in Note 12). The Company noted that AskBio did not meet the definition of a customer within the scope of ASC 606 for any distinct performance obligations as the Company concluded that such items were not an output of the Company’s ordinary activities. As such, the Company determined that the entire arrangement would be accounted for within the scope of ASC 808. In accordance with ASC 808, collaboration expenses are recognized within R&D expense and selling, general and administrative expense on our condensed consolidated statements of operations. For the year ended December 31, 2019 , the Company did not recognize any collaboration expense or record a receivable under the AskBio Collaboration Agreement in which actual costs incurred by both parties are an approximated 50% cost share. Under certain collaborative arrangements, the Company is entitled to reimbursement of certain R&D expense. Activities under collaborative arrangements for which the Company is entitled to reimbursement are considered to be collaborative activities under the scope of ASC 808. For these units of account, the Company does not analogize to ASC 606 or recognize revenue. Rather, the Company analogizes to the guidance in ASC 730, which requires that reimbursements from counterparties be recognized as an offset to the related costs. In accordance with ASC 730, the Company records reimbursement payments received from collaboration partners as reductions to R&D expense. Massachusetts Institute of Technology On December 13, 2019, the Company entered into the Fourth Amendment (the MIT Amendment) to the Exclusive Patent License Agreement by and between the Company and the Massachusetts Institute of Technology (MIT) (the MIT Agreement). Pursuant to the MIT Amendment, a provision of the MIT Agreement under which the Company was obligated to initiate a Phase 3 clinical trial for a licensed product by a specified date in the fourth quarter of 2019 is tolled until the earlier of (i) a specified date in the second quarter of 2020 or (ii) the effective date of a written amendment to the MIT Agreement. Further, pursuant to the MIT Amendment, the parties agreed to negotiate in good faith to enter into a future amendment to the MIT Agreement after the Company provides MIT with an amended diligence plan. On November 25, 2008, the Company entered into an Exclusive Patent License agreement with MIT, which is referred to as the Exclusive Patent License. The Company received an exclusive royalty‑bearing license to utilize patents held by MIT in exchange for upfront consideration and annual license maintenance fees. Such fees are expensed as incurred and have not been material to any period presented. As of December 31, 2019 , and in connection with the execution of the Spark License Agreement, the Company has made contractual payments pursuant to the Exclusive Patent License totaling $2.2 million for the sublicense granted to Spark, and $0.4 million relative to the calculated premium paid by Spark for the equity investments made under the Spark Purchase Agreement. The Company made no additional payments during the year ended December 31, 2019 . Shenyang Sunshine Pharmaceutical Co., Ltd In May 2014, the Company entered into a license agreement with Shenyang Sunshine Pharmaceutical Co., Ltd. (“3SBio”), which is referred to as the 3SBio License. The Company has paid to 3SBio an aggregate of $3.0 million in upfront and milestone-based payments under the 3SBio License as of December 31, 2019 . We are required to make future payments to 3SBio contingent upon the occurrence of events related to the achievement of clinical and regulatory approval milestones of up to an aggregate of $21.0 million for products containing our ImmTOR platform, and up to an aggregate of $41.5 million for products without our ImmTOR platform. Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. In May 2016, the Company entered into a license agreement with the Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. (collectively, “MEE”), which is referred to as the MEE License. On September 16, 2019, in accordance with the terms of the MEE License, the Company notified MEE of its intention to terminate the MEE License, effective December 15, 2019. In connection with the termination, the Company has accrued less than $ 0.1 million for final license fees as of December 31, 2019 . Through December 31, 2019 , the Company paid a total of $0.4 million in license fees due under the MEE License.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company provides for income taxes under ASC 740. Under ASC 740, the Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax bases of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. For the years ended December 31, 2019 , 2018 and 2017 , the Company did not record a current or deferred income tax expense or benefit. The following table reconciles the federal statutory income rate to the Company's effective income tax rate: Year Ended December 31, 2019 2018 2017 Statutory U.S. federal rate 21.0 % 21.0 % 34.0 % State income taxes - net of federal benefit 6.3 % 7.0 % 5.8 % Permanent items (2.1 )% (0.5 )% (0.1 )% Research tax credits/other 1.1 % 1.6 % 0.5 % Change in enacted rates — % — % (35.8 )% Valuation allowance, net (26.3 )% (29.1 )% (6.7 )% Other — % — % 2.3 % Effective income tax rate — % — % — % The tax effects of temporary differences that give rise to the Company's net deferred tax assets are as follows (in thousands): Year Ended December 31, 2019 2018 Deferred Tax Assets Net operating loss carryforwards $ 71,270 $ 59,126 Research and development credits 6,586 5,780 Stock-based compensations expense 2,516 1,946 Deferred rent and other expenses 812 458 Deferred revenue 4,468 4,037 Operating lease liability 102 — Patent costs/amortization 5,111 4,881 Gross deferred tax assets 90,865 76,228 Deferred Tax Liabilities Depreciation $ (57 ) $ (66 ) Operating lease right-of-use asset (82 ) — Gross deferred tax liabilities (139 ) (66 ) Net deferred tax assets before valuation allowance 90,726 76,162 Valuation allowance (90,726 ) (76,162 ) Net deferred tax assets $ — $ — The Company has provided a full valuation allowance against its net deferred tax assets, as the Company believes that it is more likely than not that the deferred tax assets will not be realized. Realization of future tax benefits is dependent on many factors, including the Company’s ability to generate taxable income within the net operating loss carryforward period. The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets and concluded that it is more likely than not that the Company will not realize the benefit of its deferred tax assets. The valuation allowance increased by $14.6 million and $18.5 million for the years ended December 31, 2019 and 2018, respectively, primarily as a result of an increase in net operating loss. In 2014, the Company's Russian subsidiary was granted a 10 year tax holiday in Russia. The Company's foreign operations continue to benefit from the tax holiday, which is set to expire on December 31, 2023, however the Company is in the process of closing down operations in Russia and does not expect any tax liability. At December 31, 2019, the Company had federal and state net operating loss carryforwards of $262.2 million and $256.5 million , respectively, which will expire at various times through 2037 . Of the federal net operating loss carryforwards, $99.7 million can be carried forward indefinitely. The Company also has federal and state research and development tax credit carryforwards of $4.2 million and $3.0 million , respectively, available to reduce future tax liabilities, which will expire at various times through 2039 . Utilization of the net operating loss and research and development credit carryforwards may be subject to a substantial annual limitation under Section 382 and 383 of the Internal Revenue Code due to ownership change limitations that have occurred previously, or that could occur in the future. These ownership changes may limit the amount of net operating loss and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. As of December 31, 2019, the Company completed a Section 382 study, noting that an ownership change occurred during 2017. However, the Company has determined that all net operating losses would be available in the future. As a result, the deferred tax assets related to the federal and Massachusetts net operating losses and credit carryforwards are not currently limited. The Company applies ASC 740, Income Taxes to uncertain tax positions. As of the adoption date on January 1, 2010 and through December 31, 2019, the Company had no unrecognized tax benefits or related interest and penalties accrued. The Company has not, as of yet, conducted a study of its research and development credit carryforwards. This study may result in an adjustment to the Company’s research and development credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s research and development credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. As a result, there would be no impact to the consolidated balance sheets, statements of operations and comprehensive loss, or cash flows if an adjustment was required. Interest and penalty charges, if any, related to unrecognized tax benefits would be classified as income tax expense in the accompanying statement of operations. As of December 31, 2019, the Company had no accrued interest related to uncertain tax positions. The statute of limitations for assessment by the Internal Revenue Service and Massachusetts tax authorities is open for tax years since inception. The Company files income tax returns in the United States and Massachusetts. There are currently no federal, state or foreign audits in progress.

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2019
Retirement Benefits [Abstract]
Defined Contribution Plan	Defined Contribution Plan The Company maintains a defined contribution plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pretax basis. The 401(k) Plan provides for matching contributions on a portion of participant contributions pursuant to the 401(k) Plan’s matching formula. All matching contributions vest ratably over 4 years and participant contributions vest immediately. Contributions by the Company totaled $0.1 million , $0.2 million and $0.2 million during each of the years ended December 31, 2019 , 2018 and 2017.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies As of December 31, 2019 , the Company had operating lease agreements for offices in Watertown, MA. See Note 8 for additional information regarding the Company's leases. Other As permitted under Delaware law, the Company indemnifies its directors for certain events or occurrences while the director is, or was, serving at the Company’s request in such capacity. The term of the indemnification is for the director’s lifetime. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ insurance coverage that limits its exposure and enables it to recover a portion of any future amounts paid. The Company also has indemnification arrangements under certain of its facility leases that require it to indemnify the landlord against certain costs, expenses, fines, suits, claims, demands, liabilities, and actions directly resulting from certain breaches, violations, or non‑performance of any covenant or condition of the Company’s lease. The term of the indemnification is for the term of the related lease agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. To date, the Company had not experienced any material losses related to any of its indemnification obligations, and no material claims with respect thereto were outstanding. The Company is a party in various other contractual disputes and potential claims arising in the ordinary course of business. The Company does not believe that the resolution of these matters will have a material adverse effect the Company's business, financial position, results of operations or cash flows.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent Events	<div style="font-family:Times New Roman;font-size:10pt;"><div style="line-height:120%;padding-bottom:8px;padding-top:8px;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;font-weight:bold;">Subsequent Events</font></div><div style="line-height:120%;padding-bottom:8px;padding-top:8px;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">one month lease extension- immaterial</font></div><div style="line-height:120%;padding-bottom:8px;padding-top:8px;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;font-style:italic;">Any warrants exercised?</font></div><div style="line-height:120%;padding-bottom:8px;padding-top:8px;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;font-style:italic;">MIT Amendment?</font></div></div>

Selected Quarterly Financial Da

Selected Quarterly Financial Data	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data	Selected Quarterly Financial Data (Unaudited) The following table summarizes unaudited quarterly financial data for the years ended December 31, 2019 and 2018 (in thousands, except per share data). Three Months Ended (unaudited) March 31, June 30, September 30, December 31, 2019 2019 2019 2019 Grant and collaboration revenue $ 10 $ 13 $ — $ 6,654 Operating expenses $ 11,866 $ 16,248 $ 11,794 $ 19,224 Net loss $ (12,074 ) $ (16,394 ) $ (11,994 ) $ (14,888 ) Net loss attributable to common stockholders $ (12,074 ) $ (16,394 ) $ (11,994 ) $ (14,888 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.31 ) $ (0.37 ) $ (0.26 ) $ (0.28 ) Three Months Ended (unaudited) March 31, June 30, September 30, December 31, 2018 2018 2018 2018 Grant and collaboration revenue $ — $ — $ — $ 903 Operating expenses $ 15,813 $ 18,769 $ 15,941 $ 15,402 Net loss $ (15,888 ) $ (18,796 ) $ (16,001 ) $ (14,651 ) Net loss attributable to common stockholders $ (15,888 ) $ (18,796 ) $ (16,001 ) $ (14,651 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.71 ) $ (0.84 ) $ (0.71 ) $ (0.65 )

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta RUS, LLC (“Selecta (RUS)”), a Russian limited liability corporation, and Selecta Biosciences Security Corporation, a Massachusetts Security Corporation. All significant intercompany accounts and transactions have been eliminated.
Foreign Currency	Foreign Currency
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition, accounting for stock-based compensation, the valuation of its warrant liabilities and estimating accrued research and development expenses. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates.
Segment Information	Segment Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, the Company’s Chief Executive Officer, in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment, the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases.
Cash Equivalents, Short-term Investments and Restricted Cash	Cash Equivalents, Short-term Investments and Restricted Cash Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Investments consist of securities with remaining maturities greater than 90 days when purchased. The Company classifies these marketable securities and records them at fair value in the accompanying consolidated balance sheets. Investments with less than one year until maturity are classified as short term, while investments with maturities greater than one year are classified as long term. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment.
Concentrations of Credit Risk and Off-Balance Sheet Risk	Concentrations of Credit Risk and Off‑Balance Sheet Risk
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s financial instruments consist mainly of cash equivalents, restricted cash, accounts payable, loans payable, and common warrants. The carrying amounts of cash equivalents, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. At December 31, 2019 , the carrying amount of the Company's loan payable approximates its estimated fair value due to the short-term nature of the instrument. Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A three‑level hierarchy is used to prioritize the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements), and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below: Level 1 —Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. Level 2 —Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term of the asset or liability. Level 3 —Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date. To the extent that a valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair value of warrant liabilities were determined using Level 3 inputs.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost and depreciated using the straight‑line method over the estimated useful lives of the respective assets, generally seven years for furniture and fixtures, five years for laboratory equipment, software and office equipment and three years for computer equipment. Leasehold improvements are amortized over their useful life or the life of the lease, whichever is shorter. Major additions and betterments are capitalized. Maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operations as incurred. Costs incurred for construction in progress are recorded as assets and are not amortized until the construction is substantially complete and the assets are ready for their intended use.
Impairment of Long-Lived Assets	Impairment of Long‑Lived Assets
Debt Issuance Costs	Debt Issuance Costs Debt issuance costs and fees paid to lenders are classified as a debt discount and are recorded as a direct deduction from the face amount of the related debt. Issuance costs paid to third parties that are the direct result of the debt issuance are capitalized as a direct deduction from the face amount of the related debt. Debt issuance costs are amortized over the term of the related debt using the interest method and recorded as interest expense. Costs and fees paid to third parties are expensed as incurred.
Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in the equity of a business entity during a period from transactions and other events and circumstances from non‑owner sources. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive income (loss) consists of: (i) all components of net loss and (ii) all components of comprehensive loss other than net loss, referred to as other comprehensive loss. Other comprehensive loss is comprised of foreign currency translation adjustments and the unrealized gains and losses recognized through net income.
Revenue Recognition	Revenue Recognition Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. Pursuant to ASC Topic 606, Revenue from Contracts with Customers (ASC 606) , a customer is a party that has contracted with an entity to obtain goods or services that are an output of the entity’s ordinary activities in exchange for consideration. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract, including whether they are distinct in the context of the contract; (iii) determine the transaction price, including the constraint on variable consideration; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. If a promised good or service is not distinct, it is combined with other performance obligations. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. For example, certain performance obligations associated with Spark and AskBio (see Note 12) will be satisfied over time, and revenue will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term. Collaboration and Grant Revenue: The Company currently generates its revenue through grants, collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. Grants and license agreements with customers are accounted for in accordance with ASC 606. The Company analyzes collaboration arrangements by first assessing whether they are within the scope of ASC Topic 808, Collaborative Arrangements (ASC 808 ), and evaluates whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards that are dependent on the commercial success of such activities. Collaboration agreements with customers that are not within the scope of ASC 808 are accounted for in accordance with ASC 606. To the extent the collaboration agreement is within the scope of ASC 808, the Company also assesses whether any aspects of the agreement are within the scope of other accounting literature (specifically ASC 606). The Company early adopted ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 , which provides guidance on evaluating certain transactions between collaborative arrangement participants. If the Company concludes that some or all aspects of the agreement are distinct and represent a transaction with a customer, the Company accounts for those aspects of the arrangement within the scope of ASC 606. The Company recognizes the shared costs incurred that are not within the scope of other accounting literature as a component of the related expense in the period incurred by analogy to ASC Topic 730, Research and Development (ASC 730) , and records reimbursements from counterparties as an offset to the related costs. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements in accordance with ASC 606, the Company performs the five steps above. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. The terms of the Company’s arrangements typically include one or more of the following: (i) up-front fees; (ii) milestone payments related to the achievement of development, regulatory, or commercial goals; (iii) royalties on net sales of licensed products; (iv) reimbursements or cost-sharing of research and development (R&D) expenses; and (v) profit/loss sharing arising from co-promotion arrangements. Licenses of Intellectual Property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. If not distinct, the license is combined with other performance obligations in the contract. For licenses that are combined with other performance obligations, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Optional licenses are evaluated to determine if they are issued at a discount, and therefore, represent material rights and accounted for as separate performance obligations. Milestone Payments: At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. The Company also evaluates the milestone to determine whether they 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 to be allocated, otherwise, such amounts are constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. Manufacturing Supply Services: Arrangements that include a promise for future supply of drug substance or drug product for either clinical development or commercial supply at the customer’s discretion are evaluated to determine if they are distinct and optional. For optional services that are distinct, the Company assesses if they are priced at a discount, and therefore, provide a material right to the licensee to be accounted for as separate performance obligations. Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied) in accordance with the royalty recognition constraint.
Research and Development Costs	Research and Development Costs Costs incurred in the research and development of the Company’s products are expensed as incurred. Research and development expenses include costs incurred in performing research and development activities, including salaries and benefits, facilities cost, overhead costs, contract services, supplies and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made.
Clinical Trial Costs	Clinical Trial Costs Clinical trial expenses are a significant component of research and development expenses, and the Company outsources a significant portion of these costs to third parties. Third party clinical trial expenses include patient costs, clinical research organization costs and costs for data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through costs. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as a prepaid asset or accrued clinical trial cost. These third party agreements are generally cancelable, and related costs are recorded as research and development expenses as incurred. Non-refundable advance clinical payments for goods or services that will be used or rendered for future R&D activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. The Company also records accruals for estimated ongoing clinical research and development costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical clinical accrual estimates made by the Company have not been materially different from the actual costs.
Income Taxes	Income Taxes The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more-likely-than-not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more-likely-than-not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more‑likely‑than‑not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred interest and penalties related to uncertain tax positions.
Warrants	Warrants The Company determines the accounting classification of warrants that are issued, as either liability or equity, by first assessing whether the warrants meet liability classification in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity , and then in accordance with ASC 815-40, Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock . Under ASC 480, warrants are considered liability classified if the warrants are mandatorily redeemable, obligate the issuer to settle the warrants or the underlying shares by paying cash or other assets, or must or may require settlement by issuing variable number of shares. If warrants do not meet liability classification under ASC 480-10, the Company assesses the requirements under ASC 815-40, which states that contracts that require or may require the issuer to settle the contract for cash are liabilities recorded at fair value, irrespective of the likelihood of the transaction occurring that triggers the net cash settlement feature. If the warrants do not require liability classification under ASC 815-40, in order to conclude equity classification, the Company assesses whether the warrants are indexed to its common stock and whether the warrants are classified as equity under ASC 815-40 or other applicable GAAP. After all relevant assessments are made, the Company concludes whether the warrants are classified as liability or equity. Liability classified warrants are required to be accounted for at fair value both on the date of issuance and on subsequent accounting period ending dates, with all changes in fair value after the issuance date recorded in the statements of operations as a gain or loss. Equity classified warrants are accounted for at fair value on the issuance date with no changes in fair value recognized after the issuance date.
Stock-Based Compensation	Stock‑Based Compensation The Company accounts for all stock‑based compensation granted to employees and non‑employees using a fair value method. Stock‑based compensation is measured at the grant date fair value and is recognized over the requisite service period of the awards, usually the vesting period, on a straight‑line basis, net of estimated forfeitures. The Company reduces recorded stock‑based compensation for estimated forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were adjusted. Stock‑based compensation expense recognized in the consolidated financial statements is based on awards that are ultimately expected to vest.
Net Loss Per Share	Net Loss Per Share The Company has reported losses since inception and has computed basic net loss per share by dividing net loss by the weighted average number of common shares and pre-funded warrants outstanding for the period. The Company has computed diluted net loss per common share after considering all potentially dilutive common shares, including stock options, convertible preferred stock, and warrants outstanding during the period except where the effect of including such securities would be antidilutive. Because the Company has reported net losses since inception, these potential common shares have been anti‑dilutive and basic and diluted loss per share have been the same.
Contingent Liabilities	Contingent Liabilities The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies . A provision is recorded when it is both probable that a liability has been incurred and the amount of the loss can be reasonably estimated. With respect to legal matters, provisions are reviewed and adjusted to reflect the impact of negotiations, estimated settlements, legal rulings, advice of legal counsel and other information and events pertaining to a particular matter. As of December 31, 2019 and December 31, 2018 , the Company was not a party to any litigation that could have a material adverse effect on the Company’s business, financial position, results of operations or cash flows.
Leases	Leases Under ASC Topic 842, Leases (ASC 842) , which was adopted January 1, 2019, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present. Most leases with a term greater than one year are recognized on the balance sheet as right-of-use assets, lease liabilities and, if applicable, long-term lease liabilities. The Company elected not to recognize leases with a term less than one year on its balance sheet. Operating lease right-of-use (ROU) assets and their corresponding lease liabilities are recorded based on the present value of lease payments over the expected remaining lease term. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rates, which are the rates incurred to borrow on a collateralized basis over a similar term, an amount equal to the lease payments in a similar economic environment. In accordance with the guidance in ASC 842, components of a lease should be split into three categories: lease components (e.g. land, building, etc.), non-lease components (e.g. common area maintenance, consumables, etc.), and non-components (e.g. property taxes, insurance, etc.) Then the fixed and in-substance fixed contract consideration (including any related to non-components) must be allocated based on fair values to the lease components and non-lease components. Although separation of lease and non-lease components is required, the Company elected the practical expedient to not separate lease and non-lease components. The lease component results in an operating right-of-use asset being recorded on the balance sheet and amortized on a straight-line basis as lease expense. Right-of-use assets and operating lease liabilities are remeasured upon certain modifications to leases using the present value of remaining lease payments and estimated incremental borrowing rate upon lease modification. See Note 8 for details. Under prior guidance, rent expense and lease incentives from operating leases were recognized on a straight‑line basis over the lease term. The difference between rent expense recognized and rental payments was recorded as deferred rent in the accompanying consolidated balance sheets.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update No. (“ASU”) 2016-02, Leases (Topic 842). ASU 2016-02 requires lessees to recognize most leases on their balance sheet as a right-of-use asset and a lease liability. Leases are classified as either operating or finance based on criteria similar to current lease accounting, with the classification affecting the pattern and classification of expense recognition in the statement of operations. Subsequently, in July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements (ASU 2018-11) , which includes certain amendments to ASU 2016-02 intended to provide relief in implementing the new standard. Among these amendments is the option to not restate comparative periods presented in the financial statements. The Company has elected this transition approach, using a cumulative-effect adjustment on the effective date of the standard, with comparative periods presented in accordance with the existing guidance in ASC 840. The Company adopted the new standard as of the required effective date of January 1, 2019 resulting in the recognition of a net additional lease liability and right-of-use asset. The standard did not impact the Company's consolidated net loss. See Note 8 for details. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 (ASU 2018-18), which provides guidance on whether certain transactions between collaborative arrangement participants should be accounted for with revenue under Topic 606. The new standard is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years with early adoption permitted. The Company early adopted the new standard effective September 30, 2019, and there was no impact on its consolidated financial statements. See Note 14 for details. Not Yet Adopted In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740) - Simplifying the Accounting for Income Taxes. ASU 2019-12 simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740. This ASU is effective for public entities for fiscal years beginning after December 15, 2020. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures. In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments . Subsequently, in November 2018, the FASB issued ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments-Credit Losses . ASU 2016-13 requires entities to measure all expected credit losses for most financial assets held at the reporting date based on an expected loss model which includes historical experience, current conditions, and reasonable and supportable forecasts. ASU 2016-13 also requires enhanced disclosures to help financial statement users better understand significant estimates and judgments used in estimating credit losses. This ASU is effective for smaller reporting companies for fiscal years beginning after December 15, 2022, with early adoption permitted. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Changes to the Disclosure Requirements for Fair Value Measurement (ASU 2018-13) which changes the fair value measurement disclosure requirements of ASC 820. Entities will no longer be required to disclose the amount of, and reasons for, transfers between Level 1 and Level 2 of the fair value hierarchy, the policy of timing of transfers between levels of the fair value hierarchy and the valuation processes for Level 3 fair value measurements. This ASU is effective for public entities for fiscal years beginning after December 15, 2019, with early adoption permitted. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Schedule of Cash and Cash Equivalents	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 consolidated statement of cash flows: Year Ended December 31, 2019 2018 2017 Cash and cash equivalents $ 89,893 $ 37,403 $ 70,622 Restricted cash 279 — 76 Long-term restricted cash 1,379 279 329 Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows $ 91,551 $ 37,682 $ 71,027
Schedule of Restricted Cash and Cash Equivalents	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 consolidated statement of cash flows: Year Ended December 31, 2019 2018 2017 Cash and cash equivalents $ 89,893 $ 37,403 $ 70,622 Restricted cash 279 — 76 Long-term restricted cash 1,379 279 329 Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows $ 91,551 $ 37,682 $ 71,027
Components of Accumulated Other Comprehensive Income (Loss), Net of Tax	The components of accumulated other comprehensive income (loss), net of tax, were as follows (in thousands): Foreign currency translation adjustment Unrealized gains (losses) on marketable securities Accumulated other comprehensive income (loss) Balance at December 31, 2016 $ (4,482 ) $ (36 ) $ (4,518 ) Other comprehensive income during the year 78 20 98 Balance at December 31, 2017 $ (4,404 ) $ (16 ) $ (4,420 ) Other comprehensive income (loss) during the year (153 ) 16 (137 ) Balance at December 31, 2018 $ (4,557 ) $ — $ (4,557 ) Other comprehensive income during the year $ 34 $ — $ 34 Balance at December 31, 2019 $ (4,523 ) $ — $ (4,523 )

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss Per Share	The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per‑share data): Year Ended December 31, 2019 2018 2017 Numerator: Net loss attributable to common stockholders $ (55,350 ) $ (65,336 ) $ (65,321 ) Denominator: Weighted‑average common shares and pre-funded warrants outstanding—basic and diluted 45,548,511 22,389,286 20,425,050 Net loss per share attributable to common stockholders —basic and diluted $ (1.22 ) $ (2.92 ) $ (3.20 )
Schedule of Potential Common Shares Issuable Upon Conversion of Warrants	Potential dilutive common share equivalents consist of the following: Year Ended December 31, 2019 2018 2017 Stock options to purchase common stock 6,796,669 4,093,979 2,657,187 Unvested restricted stock units 181,250 175,000 — Stock warrants to purchase common stock 23,084,120 95,619 176,432 Total 30,062,039 4,364,598 2,833,619

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Summary of Assets Measured at Fair Value on a Recurring Basis	Below is a summary of assets and liabilities measured at fair value on a recurring basis (in thousands): December 31, 2019 Total (Level 1) (Level 2) (Level 3) Assets: Money market funds $ 50,401 $ 50,401 $ — $ — Total $ 50,401 $ 50,401 $ — $ — Liabilities: Warrant liabilities $ 41,549 $ — $ — $ 41,549 Total $ 41,549 $ — $ — $ 41,549 December 31, 2018 Total (Level 1) (Level 2) (Level 3) Assets: Money market funds $ 10,123 $ 10,123 $ — $ — Total $ 10,123 $ 10,123 $ — $ —
Schedule of Fair Value Measurement Inputs and Valuation Techniques	A summary of the Black Scholes pricing model assumptions used to record the fair value of the warrants is as follows: Issued on December 23, Year Ended December 31, 2019 2019 Risk-free interest rate 1.75 % 1.69 % Dividend yield — — Expected life (in years) 5.00 4.98 Expected volatility 87.29 % 87.74 %
Schedule Of Changes In The Warrant Liabilities	The following table reflects the change in the Company’s Level 3 warrant liabilities, (see Note 10), for the year ended December 31, 2019 (in thousands): Warrant liabilities Fair value as of December 31, 2018 $ — Warrants issued in connection with December 2019 private placement 40,692 Change in fair value 857 Fair value as of December 31, 2019 $ 41,549

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consists of the following (in thousands): December 31, December 31, 2019 2018 Laboratory equipment $ 4,836 $ 5,379 Computer equipment and software 515 561 Leasehold improvements 278 278 Furniture and fixtures 237 247 Office equipment 135 135 Construction in process 2 79 Total property and equipment 6,003 6,679 Less accumulated depreciation (4,781 ) (4,552 ) Property and equipment, net $ 1,222 $ 2,127

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2019
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following (in thousands): December 31, December 31, 2019 2018 Payroll and employee related expenses $ 2,235 $ 2,497 Current portion of deferred rent and lease incentive — 117 Collaboration and licensing 1,050 1,222 Accrued patent fees 487 736 Accrued external research and development costs 4,379 5,344 Accrued professional and consulting services 446 994 Accrued grant refund — 175 Accrued interest 82 106 Issuance costs, December financing 4,381 — Other 432 509 Accrued expenses $ 13,492 $ 11,700

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Schedule of Future Minimum Lease Payments	As of December 31, 2018 and prior to the adoption of ASC 842, the aggregate future minimum lease payments related to leases are as follows (in thousands): Year ending December 31, 2019 1,482 2020 375 Total minimum lease payments $ 1,857
Schedule of Minimum Rental Commitments	The Company's total minimum rental commitments for the New Headquarters Lease as of December 31, 2019 are as follows (in thousands): December 31, 2019 2020 $ 1,191 2021 1,811 2022 1,865 2023 1,921 2024 1,979 Thereafter 7,027 Total New Headquarters Lease commitment $ 15,794
Schedule of Lease, Cost	The following summarizes additional information related to operating leases: December 31, Operating leases: 2019 Weighted-average remaining lease term 0.3 years Weighted-average discount rate 10 % The following information represents supplemental disclosure for the statement of cash flows related to operating leases (in thousands): Year Ended Operating leases: 2019 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 1,482 For the year ended December 31, 2019 , the components of lease costs were as follows (in thousands): Year Ended December 31, 2019 Operating lease expense $ 1,365 Variable lease expense 828 Short-term lease expense 16 Total lease expense $ 2,209
Schedule of Lessee, Operating Lease, Liability, Maturity	The maturity of the Company's operating lease liabilities as of December 31, 2019 were as follows (in thousands): December 31, Operating leases: 2019 2020 375 Total future minimum lease payments $ 375 Less imputed interest 3 Total operating lease liabilities $ 372 Included in the condensed consolidated balance sheet: Current operating lease liabilities $ 372 Non-current operating lease liabilities — Total operating lease liabilities $ 372

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Schedule of Future Minimum Payments on the Term Loans	Future minimum principal and interest payments on the 2017 Term Loan as of December 31, 2019 are as follows (in thousands): 2020 9,084 2021 8,716 2022 2,461 Total minimum debt payments $ 20,261 Less: Amount representing interest (1,011 ) Less: Debt discount and deferred charges (345 ) Less: Current portion of loan payable (18,905 ) Loan payable, net of current portion $ —

Equity (Tables)

Equity (Tables)	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Schedule of Stockholders' Equity Note, Warrants or Rights	The following table summarizes warrant activity for the years ended December 31, 2019, 2018 and 2017 as follows: Number of Warrants Equity classified Liability classified Total Weighted average exercise price Outstanding at December 31, 2017 176,432 — 176,432 $ 17.32 Issuance — — — — Exercises — — — — Expirations/ cancellations (80,813 ) — (80,813 ) 17.55 Outstanding at December 31, 2018 95,619 — 95,619 17.12 Issuance 8,342,128 22,988,501 31,330,629 1.07 Exercises — — — — Expirations/ cancellations — — — — Outstanding at December 31, 2019 8,437,747 22,988,501 31,426,248 $ 1.12
Schedule of Authorized Shares of Common Stock for Future Issuance	The Company has authorized shares of common stock for future issuance as follows: Period ending December 31, 2019 December 31, 2018 Exercise of common and pre-funded warrants 31,426,248 95,619 Shares available for future stock incentive awards 1,765,018 1,586,925 Unvested restricted stock units 181,250 175,000 Outstanding common stock options 6,796,669 4,093,979 Total 40,169,185 5,951,523

Stock Incentive Plans (Tables)

Stock Incentive Plans (Tables)	12 Months Ended
	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of Weighted Average Assumptions Used	The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: Year Ended December 31, 2019 2018 2017 Risk-free interest rate 1.86 % 2.85 % 2.03 % Dividend yield — — — Expected term 5.94 6.06 5.90 Expected volatility 87.66 % 85.17 % 84.52 % Weighted-average fair value of common stock $ 2.01 $ 8.45 $ 15.32
Summary of Activity Under the 2008 Plan and the 2016 Plan	The following table summarizes the activity under the 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan: Weighted‑average remaining Aggregate Number of Weighted-average contractual term intrinsic value options exercise price ($) (in years) (in thousands) Employee awards Outstanding at December 31, 2018 3,681,575 $ 9.49 7.77 $ 300 Granted 3,876,255 $ 2.01 Exercised (125,600 ) $ 1.20 Forfeited (1,108,634 ) $ 10.38 Outstanding at December 31, 2019 6,323,596 $ 4.91 8.71 $ 1,716 Vested at December 31, 2019 1,485,228 $ 10.02 6.73 $ 90 Vested and expected to vest at December 31, 2019 5,843,053 $ 5.10 8.65 $ 1,500 Non‑employee awards Outstanding at December 31, 2018 412,404 $ 6.44 6.42 $ 28 Granted 73,489 $ 1.86 Exercised — $ — Forfeited (12,820 ) $ 0.47 Outstanding at December 31, 2019 473,073 $ 5.89 6.23 $ 38 Vested at December 31, 2019 330,528 $ 5.61 5.06 $ 21 Vested and expected to vest at December 31, 2019 473,073 $ 5.89 6.23 $ 38
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award	The following table summarizes the status of the Company’s restricted stock units: Number of shares Weighted average fair value ($) Unvested at December 31, 2018 175,000 $ 6.03 Granted 100,000 1.97 Vested 93,750 1.65 Forfeited — — Unvested at December 31, 2019 181,250 $ 5.00
Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock	The Company recorded stock-based compensation expense related to stock option awards, restricted stock units and the ESPP in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Year Ended December 31, 2019 2018 2017 Research and development $ 2,079 $ 2,453 $ 1,779 General and administrative 3,082 3,261 2,302 Total stock-based compensation expense $ 5,161 $ 5,714 $ 4,081
Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of Weighted Average Assumptions Used	The estimated grant date fair values of non-employee stock option awards granted under the 2016 Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: Year Ended December 31, 2019 2018 2017 Risk-free interest rate 1.92 % 2.77 % — % Dividend yield — — — Expected life (in years) 5.33 5.81 0.00 Expected volatility 88.60 % 85.86 % — %

Revenue Arrangements (Tables)

Revenue Arrangements (Tables)	12 Months Ended
	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Schedule of Changes in Contract Liabilities	The following table presents changes in the Company’s contract liabilities during the year ended December 31, 2019 (in thousands): Balance at Balance at beginning of period Additions Deductions end of period Year Ended December 31, 2019 Contract liabilities: Deferred revenue $ 14,777 $ 8,254 $ (6,677 ) $ 16,354 Other liabilities (1) 2,126 — (2,126 ) — Total contract liabilities $ 16,903 $ 8,254 $ (8,803 ) $ 16,354 (1) On June 5, 2019, the term of the Reimbursement Period under the Letter Agreement expired. During the year ended December 31, 2019 , the Company updated its estimate of variable consideration included in the transaction price to include $1.2 million of unpaid reimbursements to Spark.

Related-Party Transactions - (T

Related-Party Transactions - (Tables)	12 Months Ended
	Dec. 31, 2019
Related Party Transaction [Line Items]
Schedule of Related Party Transactions	2019 Follow-On Name Shares of common Total Timothy A. Springer, Ph.D. 4,000,000 $ 6,000,000 Entities affiliated with Polaris 666,666 $ 999,999 Chafen Lu (Timothy A. Springer’s wife) 66,666 $ 99,999 Jed Springer (Timothy A. Springer’s brother) 1,000 $ 1,500
December 2019 Financing
Related Party Transaction [Line Items]
Schedule of Related Party Transactions	2019 Purchase Agreement Name Shares of common Common warrants Total TAS Partners, LLC 3,940,887 1,970,443 $ 6,000,000 Carrie S. Cox 65,681 32,840 $ 99,999 Timothy Barabe 65,681 32,840 $ 99,999 Scott D. Myers 39,409 19,704 $ 60,000
Private Placement
Related Party Transaction [Line Items]
Schedule of Related Party Transactions	2019 Private Placement Name Shares of common Total Timothy A. Springer, Ph.D. 1,600,000 $ 2,896,000 TAS Partners, LLC (affiliate of Timothy A. Springer, Ph.D.) 1,100,000 $ 1,991,000 Elona Kogan, J.D. 82,872 $ 149,998 Patrick Zenner 55,248 $ 99,999 Takashi Kei Kishimoto, Ph.D. 50,000 $ 90,500 Carsten Brunn, Ph.D. 41,436 $ 74,999 Scott D. Myers 41,436 $ 74,999 Stephen Smolinski 13,812 $ 25,000

Income Taxes - (Tables)

Income Taxes - (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation	The following table reconciles the federal statutory income rate to the Company's effective income tax rate: Year Ended December 31, 2019 2018 2017 Statutory U.S. federal rate 21.0 % 21.0 % 34.0 % State income taxes - net of federal benefit 6.3 % 7.0 % 5.8 % Permanent items (2.1 )% (0.5 )% (0.1 )% Research tax credits/other 1.1 % 1.6 % 0.5 % Change in enacted rates — % — % (35.8 )% Valuation allowance, net (26.3 )% (29.1 )% (6.7 )% Other — % — % 2.3 % Effective income tax rate — % — % — %
Schedule of Deferred Tax Assets and Liabilities	The tax effects of temporary differences that give rise to the Company's net deferred tax assets are as follows (in thousands): Year Ended December 31, 2019 2018 Deferred Tax Assets Net operating loss carryforwards $ 71,270 $ 59,126 Research and development credits 6,586 5,780 Stock-based compensations expense 2,516 1,946 Deferred rent and other expenses 812 458 Deferred revenue 4,468 4,037 Operating lease liability 102 — Patent costs/amortization 5,111 4,881 Gross deferred tax assets 90,865 76,228 Deferred Tax Liabilities Depreciation $ (57 ) $ (66 ) Operating lease right-of-use asset (82 ) — Gross deferred tax liabilities (139 ) (66 ) Net deferred tax assets before valuation allowance 90,726 76,162 Valuation allowance (90,726 ) (76,162 ) Net deferred tax assets $ — $ —

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	The following table summarizes unaudited quarterly financial data for the years ended December 31, 2019 and 2018 (in thousands, except per share data). Three Months Ended (unaudited) March 31, June 30, September 30, December 31, 2019 2019 2019 2019 Grant and collaboration revenue $ 10 $ 13 $ — $ 6,654 Operating expenses $ 11,866 $ 16,248 $ 11,794 $ 19,224 Net loss $ (12,074 ) $ (16,394 ) $ (11,994 ) $ (14,888 ) Net loss attributable to common stockholders $ (12,074 ) $ (16,394 ) $ (11,994 ) $ (14,888 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.31 ) $ (0.37 ) $ (0.26 ) $ (0.28 ) Three Months Ended (unaudited) March 31, June 30, September 30, December 31, 2018 2018 2018 2018 Grant and collaboration revenue $ — $ — $ — $ 903 Operating expenses $ 15,813 $ 18,769 $ 15,941 $ 15,402 Net loss $ (15,888 ) $ (18,796 ) $ (16,001 ) $ (14,651 ) Net loss attributable to common stockholders $ (15,888 ) $ (18,796 ) $ (16,001 ) $ (14,651 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.71 ) $ (0.84 ) $ (0.71 ) $ (0.65 )

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Liquidity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit	$ 335,753	$ 280,403
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents, and restricted cash	91,551	$ 37,682	$ 71,027	$ 59,050
Restricted cash and cash equivalents	1,700
Russian subsidiary | Unrestricted cash
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents, and restricted cash	$ 400

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Segment Information (Details)	12 Months Ended
	Dec. 31, 2019segment
Accounting Policies [Abstract]
Number of operating segments	1

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 89,893	$ 37,403	$ 70,622
Restricted cash	279	0	76
Long-term restricted cash	1,379	279	329
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$ 91,551	$ 37,682	$ 71,027	$ 59,050

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Concentrations of Credit Risk and Off-Balance Sheet Risk (Details) $ in Millions	Dec. 31, 2019USD ($)
Russian subsidiary
Concentration Risk [Line Items]
Cash maintained in Russian bank accounts	$ 0.4

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2019
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Laboratory equipment, software and office equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Computer equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Impairment of Long‑Lived Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Impairment of intangible assets, finite-lived	$ 0	$ 0

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Components of Accumulated Other Comprehensive Income (Loss), Net of Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax
Beginning balance	$ (5,418)	$ 51,814	$ 54,957
Ending balance	8,397	(5,418)	51,814
Foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax
Beginning balance	(4,557)	(4,404)	(4,482)
Other comprehensive income during the year	34	(153)	78
Ending balance	(4,523)	(4,557)	(4,404)
Unrealized gains (losses) on marketable securities
AOCI Attributable to Parent, Net of Tax
Beginning balance	0	(16)	(36)
Other comprehensive income during the year	0	16	20
Ending balance	0	0	(16)
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax
Beginning balance	(4,557)	(4,420)	(4,518)
Other comprehensive income during the year	34	(137)	98
Ending balance	$ (4,523)	$ (4,557)	$ (4,420)

Net Loss Per Share - Narrative

Net Loss Per Share - Narrative (Details)	12 Months Ended
	Dec. 31, 2019shares
Warrant | December 2019 Financing
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Aggregate shares of common stock at a purchase price (in shares)	8,342,128

Net Loss Per Share - Computatio

Net Loss Per Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss attributable to common stockholders	$ (14,888)	$ (11,994)	$ (16,394)	$ (12,074)	$ (14,651)	$ (16,001)	$ (18,796)	$ (15,888)	$ (55,350)	$ (65,336)	$ (65,321)
Denominator:
Weighted‑average common shares outstanding—basic and diluted (in shares)									45,548,511	22,389,286	20,425,050
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.28)	$ (0.26)	$ (0.37)	$ (0.31)	$ (0.65)	$ (0.71)	$ (0.84)	$ (0.71)	$ (1.22)	$ (2.92)	$ (3.20)

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Potential common shares
Total (in shares)	30,062,039	4,364,598	2,833,619
Stock options to purchase common stock
Potential common shares
Total (in shares)	6,796,669	4,093,979	2,657,187
Unvested restricted stock units
Potential common shares
Total (in shares)	181,250	175,000	0
Stock warrants to purchase common stock
Potential common shares
Total (in shares)	23,084,120	95,619	176,432

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Liabilities:
Warrant liabilities	$ 41,549	$ 0
Recurring
Assets:
Total	50,401	10,123
Liabilities:
Warrant liabilities	41,549
Total	41,549
Recurring | (Level 1)
Assets:
Total	50,401	10,123
Liabilities:
Warrant liabilities	0
Total	0
Recurring | (Level 2)
Assets:
Total	0	0
Liabilities:
Warrant liabilities	0
Total	0
Recurring | (Level 3)
Assets:
Total	0	0
Liabilities:
Warrant liabilities	41,549
Total	41,549
Recurring | Money market funds
Assets:
Cash and cash equivalents, fair value disclosure	50,401	10,123
Recurring | Money market funds | (Level 1)
Assets:
Cash and cash equivalents, fair value disclosure	50,401	10,123
Recurring | Money market funds | (Level 2)
Assets:
Cash and cash equivalents, fair value disclosure	0	0
Recurring | Money market funds | (Level 3)
Assets:
Cash and cash equivalents, fair value disclosure	$ 0	$ 0

Fair Value Measurements - Assum

Fair Value Measurements - Assumptions used to record the fair value of the warrants (Details) - Valuation Technique, Option Pricing Model	Dec. 31, 2019	Dec. 31, 2018
Risk-free interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.0175	0.0169
Expected life (in years)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, term	5 years	4 years 11 months 23 days
Expected volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.8729	0.8774

Fair Value Measurements - Chang

Fair Value Measurements - Change in the warrant liabilities (Details) - Warrant $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value as of December 31, 2018	$ 0
Warrants issued in connection with December 2019 private placement	40,692
Change in fair value	857
Fair value as of December 31, 2019	$ 41,549

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 6,003	$ 6,679
Less accumulated depreciation	(4,781)	(4,552)
Property and equipment, net	1,222	2,127
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	4,836	5,379
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	515	561
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	278	278
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	237	247
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	135	135
Construction in process
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 2	$ 79

Property and Equipment - Narrat

Property and Equipment - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 726	$ 975	$ 750
Accelerated depreciation expense	$ 40

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Payroll and employee related expenses	$ 2,235	$ 2,497
Current portion of deferred rent and lease incentive	0	117
Collaboration and licensing	1,050	1,222
Accrued patent fees	487	736
Accrued external research and development costs	4,379	5,344
Accrued professional and consulting services	446	994
Accrued grant refund	0	175
Accrued interest	82	106
Issuance costs, December financing	4,381	0
Other	432	509
Accrued expenses	$ 13,492	$ 11,700

Leases - Narrative (Details)

Leases - Narrative (Details) $ in Thousands	Jan. 01, 2019USD ($)	Jul. 31, 2019USD ($)ft²	Mar. 31, 2019USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Oct. 31, 2017ft²	Dec. 31, 2012USD ($)
Operating Leased Assets [Line Items]
Right of use asset, net				$ 301
Lease liabilities				372
Operating lease, remaining lease term	15 months
Discount rate	10.00%
Lessee-paid construction costs				2,209
Operating lease, expense				2,100	$ 2,000	$ 1,900
operating lease payments due				$ 375
75 North Beacon, Watertown, MA
Operating Leased Assets [Line Items]
Right of use asset, net	$ 200		$ 0
Area of additional office space leased | ft²							5,100
65 Grove Street, Watertown,MA
Operating Leased Assets [Line Items]
Term of contract				8 years
Area of additional office space leased | ft²		25,078
Lessee-paid construction costs		$ 800
Operating lease, expense			$ 200
operating lease payments due		15,800
Letter of credit
Operating Leased Assets [Line Items]
Restricted cash		$ 1,400		$ 300
Minimum
Operating Leased Assets [Line Items]
Term of contract				2 years
Renewal term				1 year
Maximum
Operating Leased Assets [Line Items]
Term of contract				8 years
Renewal term				5 years
Maximum | Laboratory and office space
Operating Leased Assets [Line Items]
Leasehold improvements gross								$ 700
Accounting Standards Update 2016-02
Operating Leased Assets [Line Items]
Right of use asset, net	1,600
Lease liabilities	1,800
Deferred rent and incentives	$ 200

Leases - Lease Commitments (Det

Leases - Lease Commitments (Details) - 65 Grove Street, Watertown,MA $ in Thousands	Dec. 31, 2019USD ($)
Lessee, Lease, Description [Line Items]
2020	$ 1,191
2021	1,811
2022	1,865
2023	1,921
2024	1,979
Thereafter	7,027
Total New Headquarters Lease commitment	$ 15,794

Leases - Components of lease co

Leases - Components of lease costs (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Leases [Abstract]
Operating lease expense	$ 1,365
Variable lease expense	828
Short-term lease expense	16
Total lease expense	$ 2,209

Leases - Operating Lease, Liabi

Leases - Operating Lease, Liability, Maturity (Details) $ in Thousands	Dec. 31, 2019USD ($)
Leases [Abstract]
2020	$ 375
Total future minimum lease payments	375
Less imputed interest	3
Total operating lease liabilities	372
Current operating lease liabilities	372
Non-current operating lease liabilities	$ 0

Leases - Other Information (Det

Leases - Other Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Leases [Abstract]
Operating cash flows from operating leases	$ 1,482

Leases - Lease Term and Discoun

Leases - Lease Term and Discount Rate (Details)	Dec. 31, 2019
Leases [Abstract]
Operating lease, weighted average remaining lease term	9 days
Operating lease, weighted average discount rate, percent	10.00%

Leases - Aggregate future minim

Leases - Aggregate future minimum lease payments related to leases (Details) $ in Thousands	Dec. 31, 2018USD ($)
Leases [Abstract]
2019	$ 1,482
2020	375
Total minimum lease payments	$ 1,857

Debt - Term Loans (Narrative) (

Debt - Term Loans (Narrative) (Details) - USD ($) $ in Thousands	Sep. 12, 2017	Dec. 31, 2019	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Loss on extinguishment of debt					$ 0	$ 0	$ 673
Minimum assets trigger					300
Minimum amount in excess of indebtedness trigger					300
Interest expense related to the Term Loan					1,519	1,494	$ 1,206
2017 Term Loans
Line of Credit Facility [Line Items]
Term loan facility	$ 21,000	$ 18,200			$ 18,200	$ 21,000
Loss on extinguishment of debt				$ 700
Debt issuance costs, line of credit arrangements (less than)	$ 100
Interest rate per annum					4.00%
Final payment fee (as a percent)					5.00%
Final payment fee		1,100			$ 1,100
Debt instrument, basis spread on variable rate					100.00%
Interest expense related to the Term Loan		$ 1,500	$ 1,500	$ 1,200
2017 Term Loans | Base Rate
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	0.05%
2017 Term Loans | Outstanding Balances
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate					105.00%
2017 Term Loans | Prepaid after second anniversary
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)		1.00%			1.00%
Term Loans
Line of Credit Facility [Line Items]
Repayments of debt	$ 10,000

Debt - Schedule of Future Minim

Debt - Schedule of Future Minimum Payments on the Term Loans (Details) - 2017 Term Loans $ in Thousands	Dec. 31, 2019USD ($)
Year ended December 31,
2020	$ 9,084
2021	8,716
2022	2,461
Total minimum debt payments	20,261
Less: Amount representing interest	(1,011)
Less: Debt discount and deferred charges	(345)
Less: Current portion of loan payable	(18,905)
Loan payable, net of current portion	$ 0

Equity - Narrative (Details)

Equity - Narrative (Details)	Dec. 18, 2019$ / sharesshares	Aug. 19, 2019USD ($)$ / sharesshares	Jan. 29, 2019shares	Jan. 25, 2019USD ($)$ / sharesshares	Aug. 10, 2017USD ($)	Jun. 27, 2017USD ($)$ / sharesshares	Dec. 31, 2019USD ($)vote$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares							86,325,547
Class of warrant or right, outstanding (in shares) | shares							31,426,248,000	95,619,000	176,432,000
Warrant liabilities | $							$ 41,549,000	$ 0
Shares issued, value | $							$ 30,942,000	$ 0	$ 10,000,000
Exercise price (in dollars per share) | $ / shares							$ 1,120	$ 17,120	$ 17,320
Common stock, par value (in dollars per share) | $ / shares							$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares) | shares							200,000,000	200,000,000
Number of votes per share that common stockholders are entitled to | vote							1
Dividends declared or paid on common stock | $							$ 0
December 2019 Financing
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares	37,634,883
Share price (in dollars per share) | $ / shares	$ 1.46						$ 0.125
Class of warrant or right, outstanding (in shares) | shares							22,988,501
Warrant Issued | $							$ 40,700,000
Warrant liabilities | $							41,500,000
Warrant, Down Round Feature, (Increase) Decrease in Equity, Amount | $							900,000
Sale common stock | $							65,600,000
Issuance costs | $							$ 4,400,000
Exercise price (in dollars per share) | $ / shares							$ 1.46
Private Placement
Subsidiary, Sale of Stock [Line Items]
Shares issued, value | $							$ 5,715,000	$ 0	$ 47,114,000
Private Placement | Director
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares		3,178,174
Share price (in dollars per share) | $ / shares		$ 1.81
Proceeds from issuance of common stock, net | $		$ 5,700,000
Public Stock Offering
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares				20,000,000
Share price (in dollars per share) | $ / shares				$ 1.50
Sale common stock | $				$ 30,900,000
Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares			2,188,706
At-The-Market Offering
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares							615,453
Share price (in dollars per share) | $ / shares							$ 1.84
Sale common stock | $							$ 50,000,000
Shares issued, value | $							$ 1,006,000	$ 0	$ 0
Public securities offering, amount authorized | $					$ 200,000,000
2017 PIPE
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares						2,750,000
Share price (in dollars per share) | $ / shares						$ 16
Shares issued, value | $						$ 47,100,000
Common stock, par value (in dollars per share) | $ / shares							$ 0.0001
Common stock, shares authorized (in shares) | shares							200,000,000
Springer Purchase Agreement
Subsidiary, Sale of Stock [Line Items]
Share price (in dollars per share) | $ / shares						$ 17.71
Springer Purchase Agreement | Common stock
Subsidiary, Sale of Stock [Line Items]
Stock issued (in shares) | shares						338,791
Warrant
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares						79,130
Exercise price (in dollars per share) | $ / shares						$ 17.71
Warrants and rights outstanding, term						5 years
Share price (in dollars per share) | $ / shares						$ 0.125
Warrant
Subsidiary, Sale of Stock [Line Items]
Class of warrant or right, outstanding (in shares) | shares							8,437,747,000	95,619,000	176,432,000
Warrant | December 2019 Financing
Subsidiary, Sale of Stock [Line Items]
Aggregate shares of common stock at a purchase price (in shares) | shares							8,342,128

Equity - Schedule of Authorized

Equity - Schedule of Authorized Shares of Common Stock for Future Issuance (Details) - shares	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Total (in shares)	40,169,185		5,951,523
Shares available for future stock incentive awards
Class of Stock [Line Items]
Total (in shares)	1,765,018		1,586,925
Unvested restricted stock units
Class of Stock [Line Items]
Total (in shares)	181,250	50,000	175,000
Outstanding common stock options
Class of Stock [Line Items]
Total (in shares)	6,796,669		4,093,979
Warrant
Class of Stock [Line Items]
Total (in shares)	31,426,248		95,619

Equity - Warranty Activity (Det

Equity - Warranty Activity (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Class of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, beginning balance (in shares)	95,619	176,432
Issuance (in shares)	31,330,629	0
Exercises (in shares)	0	0
Expirations/ cancellations (in shares)	0	(80,813)
Class of warrant or right, outstanding, ending balance (in shares)	31,426,248	95,619
Exercise price (in dollars per share)	$ 1,120	$ 17,120	$ 17,320
Exercise price, expirations / cancellations (in dollars per share)		$ 17,550
Exercise price, issuance (in dollars per share)	$ 1,070
Warrant
Class of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, beginning balance (in shares)	95,619	176,432
Issuance (in shares)	8,342,128	0
Exercises (in shares)	0	0
Expirations/ cancellations (in shares)	0	(80,813)
Class of warrant or right, outstanding, ending balance (in shares)	8,437,747	95,619
Warrant
Class of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, beginning balance (in shares)	0	0
Issuance (in shares)	22,988,501	0
Exercises (in shares)	0	0
Expirations/ cancellations (in shares)	0	0
Class of warrant or right, outstanding, ending balance (in shares)	22,988,501	0

Stock Incentive Plans - Narrati

Stock Incentive Plans - Narrative (Details) $ / shares in Units, $ in Thousands	Jun. 21, 2016shares	Mar. 31, 2019$ / sharesshares	Dec. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / shares	Nov. 20, 2019shares	Mar. 25, 2019shares	Sep. 25, 2018shares	Dec. 31, 2008shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)			40,169,185	5,951,523
Shares issued, value | $			$ 30,942
Employee stock option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)			1,765,018	1,586,925
Estimated forfeitures rate			10.00%
Weighted average grant date fair value of stock options (in dollars per share) | $ / shares			$ 1.47	$ 6.17	$ 10.97
Aggregate intrinsic value of stock options exercised | $			$ 100	$ 600	$ 2,600
Unrecognized compensation expense related to unvested employee stock options | $			$ 9,800	$ 9,800
Weighted average period for recognition			2 years 6 months	3 years 1 month 6 days
Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value of stock options (in dollars per share) | $ / shares			$ 1.32	$ 6.78
Vested and expected to vest (in shares)						141,328
Vested and expected to vest, exercisable (in shares)						234,772
share-based payment, plan modification incremental cost | $			$ 100
Unrecognized compensation expense related to unvested non-employee stock options | $			$ 100	$ 1,100
Unrecognized compensation expense related to unvested non-employee stock options, period			4 months 24 days	2 years 2 months 12 days
Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		50,000	181,250	175,000
Grants in period (in shares)		50,000	100,000
Reserved for issuance (in shares)			50,000
Vested in period (in dollars per share) | $ / shares			$ 1.65
Grants in period (in dollars per share) | $ / shares		$ 2.29	$ 1.97
Shares issued, value | $			$ 100
The 2008 Plan and the 2016 Plan | Employee stock option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock upon exercise of options (in shares)			125,600
Aggregate intrinsic value of stock options exercised | $			$ 1,500
Vested and expected to vest (in shares)			5,843,053
The 2008 Plan and the 2016 Plan | Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock upon exercise of options (in shares)			0
Aggregate intrinsic value of stock options exercised | $			$ 38
Vested and expected to vest (in shares)			473,073
2008 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)									2,213,412
2008 Plan | Employee stock option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term			4 years
Award expiration term			10 years
2016 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)			1,210,256
Automatic increase in the number of shares that may be issued (as a percent)			0.04
Number of shares authorized, increase (in shares)			898,871	893,730
Common stock authorized and reserved for future issuance (in shares)			267,612
2016 Plan | Employee stock option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term			4 years
Award expiration term			10 years
2016 Plan | Employee stock option grants | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)			10.00%
2016 Plan | Employee stock option grants | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)			10.00%
2016 Plan | Employee stock option grants | Tranche One | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (not less than)			100.00%
2016 Plan | Employee stock option grants | Tranche Two | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (not less than)			110.00%
Employment Inducement Incentive Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)			750,000
Common stock authorized and reserved for future issuance (in shares)							2,000,000	1,175,000
Employment Inducement Incentive Award Plan | Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period (in shares)		100,000
Weighted average period for recognition			2 years 9 months 18 days
Unrecognized compensation expense | $			$ 800
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Automatic increase in the number of shares that may be issued (as a percent)	0.01
Number of shares authorized, increase (in shares)			224,717	223,432
Common stock authorized and reserved for future issuance (in shares)	173,076		747,406
Exercise price of incentive stock options (not less than)	85.00%

Stock Incentive Plans - Schedul

Stock Incentive Plans - Schedule of Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.86%	2.85%	2.03%
Dividend yield	0.00%	0.00%	0.00%
Expected term	5 years 11 months 8 days	6 years 21 days	5 years 10 months 24 days
Expected volatility	87.66%	85.17%	84.52%
Weighted-average fair value of common stock (in dollars per share)	$ 2.01	$ 8.45	$ 15.32
Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.92%	2.77%	0.00%
Dividend yield	0.00%	0.00%	0.00%
Expected term	5 years 3 months 29 days	5 years 9 months 21 days	0 days
Expected volatility	88.60%	85.86%	0.00%

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Activity Under the 2008 Plan and the 2016 Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Nov. 20, 2019	Dec. 31, 2017
Employee awards
Aggregate intrinsic value
Vested and expected to vest	$ 100	$ 600		$ 2,600
Non‑employee awards
Number of Options
Vested and expected to vest (in shares)			141,328
Weighted-average exercise price
Granted (in dollars per share)	$ 4.35
The 2008 Plan and the 2016 Plan | Employee awards
Number of Options
Beginning balance (in shares)	3,681,575
Granted (in shares)	3,876,255
Exercised (in shares)	(125,600)
Forfeited (in shares)	(1,108,634)
Ending balance (in shares)	6,323,596	3,681,575
Vested (in shares)	1,485,228
Vested and expected to vest (in shares)	5,843,053
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 9.49
Granted (in dollars per share)	2.01
Exercised (in dollars per share)	1.20
Forfeited (in dollars per share)	10.38
Ending balance (in dollars per share)	4.91	$ 9.49
Vested (in dollars per share)	10.02
Vested and expected to vest (in dollars per share)	$ 5.10
Weighted-average remaining contractual term
Outstanding, term	8 years 8 months 15 days	7 years 9 months 7 days
Vested, term	6 years 8 months 23 days
Vested and expected to vest, term	8 years 7 months 24 days
Aggregate intrinsic value
Outstanding	$ 1,716	$ 300
Vested	90
Vested and expected to vest	$ 1,500
The 2008 Plan and the 2016 Plan | Non‑employee awards
Number of Options
Beginning balance (in shares)	412,404
Granted (in shares)	73,489	0
Exercised (in shares)	0
Forfeited (in shares)	(12,820)
Ending balance (in shares)	473,073	412,404
Vested (in shares)	330,528
Vested and expected to vest (in shares)	473,073
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 6.44
Granted (in dollars per share)	1.86
Exercised (in dollars per share)	0
Forfeited (in dollars per share)	0.47
Ending balance (in dollars per share)	5.89	$ 6.44
Vested (in dollars per share)	5.61
Vested and expected to vest (in dollars per share)	$ 5.89
Weighted-average remaining contractual term
Outstanding, term	6 years 2 months 23 days	6 years 5 months 1 day
Vested, term	5 years 21 days
Vested and expected to vest, term	6 years 2 months 23 days
Aggregate intrinsic value
Outstanding	$ 38	$ 28
Vested	21
Vested and expected to vest	$ 38

Stock Incentive Plans - Restric

Stock Incentive Plans - Restricted Stock Units (Details) - Unvested restricted stock units - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Shares outstanding at December 31, 2018	175,000	175,000
Grants in period (in shares)	50,000	100,000
Vested in period (in shares)		93,750
Forfeited in period (in shares)		0
Shares outstanding at March 31, 2019		181,250
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Shares outstanding at December 31, 2018 (in dollars per share)	$ 6.03	$ 6.03
Grants in period (in dollars per share)	$ 2.29	1.97
Vested in period (in dollars per share)		1.65
Forfeited in period (in dollars per share)		0
Shares outstanding at March 31, 2019 (in dollars per share)		$ 5

Stock Incentive Plans - Employe

Stock Incentive Plans - Employee Stock Purchase Plan (Narrative) (Details) $ in Millions	Jun. 21, 2016shares	Jun. 20, 2016	Dec. 31, 2019USD ($)offering_periodshares	Dec. 31, 2018USD ($)shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)			40,169,185	5,951,523
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of offering periods under the ESPP | offering_period			2
Term of each offering period		6 months
Eligible compensation withheld for use (up to) (as a percent)	25.00%
Purchase price of shares (not less than) (as a percent)	85.00%
Common stock authorized and reserved for future issuance (in shares)	173,076		747,406
Number of shares of common stock outstanding (as a percent)	0.01
Number of shares authorized, increase (in shares)			224,717	223,432
Shares of common stock issued to employees under the ESPP (in shares)			17,205
Stock-based compensation expense | $			$ 0.1	$ 0.1

Stock Incentive Plans - Sched_2

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Details) - Stock Options And Restricted Common Stock - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 5,161	$ 5,714	$ 4,081
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	2,079	2,453	1,779
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 3,082	$ 3,261	$ 2,302

Revenue Arrangements - Narrativ

Revenue Arrangements - Narrative (Details)	Dec. 17, 2019USD ($)	Jun. 05, 2019USD ($)	Oct. 31, 2017USD ($)$ / sharesshares	Jun. 06, 2017USD ($)$ / sharesshares	Dec. 02, 2016USD ($)obligationcontracttarget	Dec. 31, 2016USD ($)shares	Dec. 31, 2019USD ($)agreement	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2019USD ($)agreementshares	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)$ / shares	Mar. 31, 2018USD ($)	Jan. 01, 2018USD ($)
Disaggregation of Revenue [Line Items]
Accumulated deficit							$ (335,753,000)				$ (280,403,000)				$ (335,753,000)	$ (280,403,000)
Long-term contract liability							14,680,000				13,818,000				14,680,000	13,818,000
Short-term contract liability							1,674,000				959,000				$ 1,674,000	959,000
Number of additional targets | target					4
Sale of stock, number of shares issued in transaction (in shares) | shares															86,325,547
Deferred revenue related to agreement							16,354,000				16,903,000				$ 16,354,000	16,903,000
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights																	$ 10,000,000
Remaining performance obligation							9,200,000								9,200,000
Revenue recognized															6,677,000
Adoption of new accounting principle																			$ 1,830,000
Grant and collaboration revenue							6,654,000	$ 0	$ 13,000	$ 10,000	903,000	$ 0	$ 0	$ 0	6,677,000	903,000	$ 207,000
Spark Letter Agreement
Disaggregation of Revenue [Line Items]
Sale common stock			$ 2,500,000
Premium of average daily VWAP (as a percent)			115.00%
Deferred revenue related to agreement			$ 2,500,000
Aggregate purchase price				$ 5,000,000
Number of shares authorized to be issued pursuant to the Stock Purchase Agreement (in shares) | shares			205,254	324,362
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights					$ 2,700,000
Aggregate purchase price per share (in dollar per share) | $ / shares			$ 24.36	$ 15.41
Reimbursement invoices		$ 1,200,000
License Agreement For Pompe Disease [Member]
Disaggregation of Revenue [Line Items]
Long-term contract liability							5,300,000								5,300,000
Upfront cash payment	$ 7,000,000
Initial Up-front cash payment	2,000,000
Initial Upfront execution fee payment	5,000,000
Short-term contract liability							1,700,000								1,700,000
Sales milestone payments	$ 237,000,000
First Acquisition Right
Disaggregation of Revenue [Line Items]
Aggregate purchase price			$ 5,000,000
Skolkovo
Disaggregation of Revenue [Line Items]
Deferred revenue related to agreement							100,000								100,000				200,000
Payments received																		$ 2,000,000
Grants from entities, unused grant funds																100,000
Overspent costs																100,000
Second Acquisition Right
Disaggregation of Revenue [Line Items]
Share price (in dollars per share) | $ / shares			$ 24.36
Premium of average daily VWAP (as a percent)			115.00%
Aggregate price of shares Spark will have the right to purchase			$ 5,000,000
Spark License Agreement
Disaggregation of Revenue [Line Items]
Upfront cash payment					10,000,000
Aggregate additional payments due within twelve months					$ 5,000,000
Term loan number of payments | contract					2
Two payments due within 12 months of contract date					$ 2,500,000
Number of additional targets | target					4
Variable fee the Company is eligible to receive for each additional target option elected					$ 2,000,000
Election period to exercise additional targets					3 years
Additional development and regulatory milestone payments					$ 65,000,000
Commercialization milestone payments					365,000,000
Sales milestone payments					$ 255,000,000
Period after first commercial sale when the Company is eligible to receive royalties					10 years
Period for prior written notice to terminate license					90 days
Sale of stock, number of shares issued in transaction (in shares) | shares						197,238
Sale common stock						$ 5,000,000
Share price (in dollars per share) | $ / shares																	$ 25.35
Premium of average daily VWAP (as a percent)																	115.00%
Deferred revenue related to agreement							$ 9,200,000				$ 14,700,000				$ 9,200,000	14,700,000
Number of agreements combined and evaluated into a single agreement | agreement							2								2
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights					$ 2,700,000
Remaining performance obligation					12,300,000		$ 2,300,000								$ 2,300,000
Performance obligation per each option					1,300,000
Spark License Agreement | MIT
Disaggregation of Revenue [Line Items]
License and option agreement, additional sub-license payment received liability															2,200,000
License and option agreement, payments made relative to calculated premium paid for initial equity investment made under the purchase agreement															400,000
Spark License Agreement | Consideration for Promises
Disaggregation of Revenue [Line Items]
Upfront cash payment					15,000,000
Spark License Agreement | Development Milestones
Disaggregation of Revenue [Line Items]
Additional development and regulatory milestone payments					35,000,000
Spark License Agreement | Regulatory Milestones
Disaggregation of Revenue [Line Items]
Additional development and regulatory milestone payments					30,000,000
Spark License Agreement | Commercial Milestones
Disaggregation of Revenue [Line Items]
Commercialization milestone payments					$ 110,000,000
Spark License Agreement | License and Supply Obligation
Disaggregation of Revenue [Line Items]
Number of obligations | obligation					5
Remaining performance obligation					$ 7,100,000
Spark License Agreement | Discount on Option Obligation
Disaggregation of Revenue [Line Items]
Number of obligations | obligation					4
Remaining performance obligation					$ 5,200,000
Accumulated deficit
Disaggregation of Revenue [Line Items]
Adoption of new accounting principle																			1,830,000
Accumulated deficit | ASU 2014-09 | Skolkovo
Disaggregation of Revenue [Line Items]
Adoption of new accounting principle																			1,800,000
Grant Revenue
Disaggregation of Revenue [Line Items]
Grant and collaboration revenue															$ 100,000	$ 0
Adjustments due to ASU 2014-09 | ASU 2014-09
Disaggregation of Revenue [Line Items]
Accumulated deficit																			1,800,000
Long-term contract liability																			$ (1,800,000)

Revenue Arrangements - Transact

Revenue Arrangements - Transaction Price Allocated to Future Performance Obligation (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 02, 2016
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 9.2
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period	12 months
Spark License Agreement
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 2.3	$ 12.3
Spark License Agreement | License and Supply Obligation
Revenue from Contract with Customer [Abstract]
Remaining performance obligation		$ 7.1
Spark License Agreement | License and Supply Obligation | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2016-12-03
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period		4 years

Revenue Arrangements - Schedule

Revenue Arrangements - Schedule of Changes in Contract Liabilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Contract liabilities:
Deferred revenue, beginning of period	$ 14,777
Other liabilities, beginning of period	2,126
Contract liabilities, beginning of period	16,903
Deferred revenue, additions	8,254
Other liabilities, additions	0
Contract liabilities, additions	8,254
Deferred revenue, deductions	(6,677)
Other liabilities, deductions	(2,126)
Contract liabilities, deductions	(8,803)
Deferred revenue, end of period	16,354
Other liabilities, end of period	0
Contract liabilities, end of period	$ 16,354

Related-Party Transactions - Na

Related-Party Transactions - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Director | Quarterly payment
Related Party Transaction [Line Items]
Related party transaction, amounts of transaction	$ 85,000
Founders
Related Party Transaction [Line Items]
Consulting services expenses	$ 500,000	$ 200,000	$ 200,000
Non-employee stock options
Related Party Transaction [Line Items]
Options granted (in shares)	75,000
Granted (in dollars per share)	$ 4.35

Related-Party Transactions - Nu

Related-Party Transactions - Number of Shares of Common Stock Purchased (Details) - USD ($)	Dec. 18, 2019	Aug. 19, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Stock issued (in shares)			86,325,547
Shares issued, value			$ 30,942,000	$ 0	$ 10,000,000
Founders
Related Party Transaction [Line Items]
Consulting services expenses			$ 500,000	200,000	200,000
Timothy A. Springer, Ph.D. | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			4,000,000
Shares issued, value			$ 6,000,000
Entities affiliated with Polaris
Related Party Transaction [Line Items]
Stock issued (in shares)			666,666
Shares issued, value			$ 999,999
Chafen Lu (Timothy A. Springer’s wife)
Related Party Transaction [Line Items]
Stock issued (in shares)			66,666
Shares issued, value			$ 99,999
Jed Springer (Timothy A. Springer’s brother)
Related Party Transaction [Line Items]
Stock issued (in shares)			1,000
Shares issued, value			$ 1,500
December 2019 Financing
Related Party Transaction [Line Items]
Stock issued (in shares)	37,634,883
December 2019 Financing | TAS Partners, LLC (affiliate of Timothy A. Springer, Ph.D.)
Related Party Transaction [Line Items]
Stock issued (in shares)			3,940,887
Shares issued, value			$ 6,000,000
December 2019 Financing | Carrie S. Cox | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			65,681
Shares issued, value			$ 99,999
December 2019 Financing | Timothy Barabe | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			65,681
Shares issued, value			$ 99,999
December 2019 Financing | Scott D. Myers | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			39,409
Shares issued, value			$ 60,000
Private Placement
Related Party Transaction [Line Items]
Shares issued, value			$ 5,715,000	$ 0	$ 47,114,000
Private Placement | Director
Related Party Transaction [Line Items]
Stock issued (in shares)		3,178,174
Private Placement | TAS Partners, LLC (affiliate of Timothy A. Springer, Ph.D.) | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			1,100,000
Shares issued, value			$ 1,991,000
Private Placement | Scott D. Myers | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			41,436
Shares issued, value			$ 74,999
Private Placement | Timothy A. Springer, Ph.D. | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			1,600,000
Shares issued, value			$ 2,896,000
Private Placement | Elona Kogan, J.D. | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			82,872
Shares issued, value			$ 149,998
Private Placement | Patrick Zenner | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			55,248
Shares issued, value			$ 99,999
Private Placement | Takashi Kei Kishimoto, Ph.D. | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			50,000
Shares issued, value			$ 90,500
Private Placement | Carsten Brunn, Ph.D. | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			41,436
Shares issued, value			$ 74,999
Private Placement | Stephen Smolinski | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			13,812
Shares issued, value			$ 25,000
Warrant | December 2019 Financing | TAS Partners, LLC (affiliate of Timothy A. Springer, Ph.D.)
Related Party Transaction [Line Items]
Stock issued (in shares)			1,970,443
Warrant | December 2019 Financing | Carrie S. Cox | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			32,840
Warrant | December 2019 Financing | Timothy Barabe | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			32,840
Warrant | December 2019 Financing | Scott D. Myers | Director
Related Party Transaction [Line Items]
Stock issued (in shares)			19,704

Collaboration Agreements - MIT

Collaboration Agreements - MIT (Narrative) (Details) - Spark License Agreement	12 Months Ended
	Dec. 31, 2019USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contractual payments defined in the Exclusive Patent License agreement	$ 0
MIT
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contractual payments defined in the Exclusive Patent License agreement	2,200,000
License and option agreement, payments made relative to calculated premium paid for initial equity investment made under the purchase agreement	$ 400,000

Collaboration Agreements - Shen

Collaboration Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Narrative) (Details) - 3SBio License - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	May 31, 2014
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate amount of upfront and milestone-based payments			$ 3
Aggregate amount for future payments upon achievement of clinical and regulatory approval milestones for products containing ImmTOR platform	$ 21
Aggregate amount for future payments upon achievement of clinical and regulatory approval milestones for products without ImmTOR Platform		$ 41.5

Collaboration Agreements - Mass

Collaboration Agreements - Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
MEE
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
License fees due under agreement	$ 0.4

Income Taxes (Details)

Income Taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2014	Dec. 31, 2019
Income Tax Holiday [Line Items]
Unrecognized tax benefits				$ 0
Net operating loss carryforwards	$ 59,126,000			71,270,000
Valuation allowance, deferred tax asset, increase (decrease), amount	14,600,000	$ 18,500,000
Income tax holiday term			10 years
Research and development credits	5,780,000			6,586,000
Accrued interest related to uncertain tax positions				$ 0
State and Local Jurisdiction
Income Tax Holiday [Line Items]
Operating loss carryforwards	256,500,000
Research and development credits	3,000,000
Domestic Tax Authority
Income Tax Holiday [Line Items]
Operating loss carryforwards	262,200,000
Operating loss carryforwards, carried forward indefinitely	99,700,000
Research and development credits	$ 4,200,000

Income Taxes Reconciliation of

Income Taxes Reconciliation of Income Taxes (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Statutory U.S. federal rate	21.00%	21.00%	34.00%
State income taxes - net of federal benefit	6.30%	7.00%	5.80%
Permanent items	(2.10%)	(0.50%)	(0.10%)
Research tax credits/other	1.10%	1.60%	0.50%
Change in enacted rates	0.00%	0.00%	(35.80%)
Valuation allowance, net	(26.30%)	(29.10%)	(6.70%)
Other	0.00%	0.00%	2.30%
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes Deferred Tax Asset

Income Taxes Deferred Tax Assets and Deferred Tax Liabilities (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Deferred Tax Assets
Net operating loss carryforwards	$ 71,270,000	$ 59,126,000
Research and development credits	6,586,000	5,780,000
Stock-based compensations expense	2,516,000	1,946,000
Deferred rent and other expenses	812,000	458,000
Deferred revenue	4,468,000	4,037,000
Operating lease liability	102,000
Patent costs/amortization	5,111,000	4,881,000
Gross deferred tax assets	90,865,000	76,228,000
Deferred Tax Liabilities
Depreciation	(57,000)	(66,000)
Operating lease right-of-use asset	(82,000)
Gross deferred tax liabilities	(139,000)	(66,000)
Depreciation	(57,000)	(66,000)
Gross deferred tax liabilities	(139,000)	(66,000)
Net deferred tax assets before valuation allowance	(90,726,000)	(76,162,000)
Valuation allowance	(90,726,000)	(76,162,000)
Net deferred tax assets	$ 0	$ 0

Defined Contribution Plan - Nar

Defined Contribution Plan - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Vesting period	4 years
Employer contribution made	$ 0.1	$ 0.2	$ 0.2

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Grant and collaboration revenue	$ 6,654	$ 0	$ 13	$ 10	$ 903	$ 0	$ 0	$ 0	$ 6,677	$ 903	$ 207
Operating expenses	19,224	11,794	16,248	11,866	15,402	15,941	18,769	15,813	59,132	65,925	63,991
Net loss	(14,888)	(11,994)	(16,394)	(12,074)	(14,651)	(16,001)	(18,796)	(15,888)	(55,350)	(65,336)	(65,321)
Net (loss) income attributable to common stockholders - basic & diluted	$ (14,888)	$ (11,994)	$ (16,394)	$ (12,074)	$ (14,651)	$ (16,001)	$ (18,796)	$ (15,888)	$ (55,350)	$ (65,336)	$ (65,321)
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.28)	$ (0.26)	$ (0.37)	$ (0.31)	$ (0.65)	$ (0.71)	$ (0.84)	$ (0.71)	$ (1.22)	$ (2.92)	$ (3.20)