scPharmaceuticals (SCPH) 10-K 21 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Mar. 19, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	SCPH
Entity Registrant Name	SCPHARMACEUTICALS INC.
Entity Central Index Key	0001604950
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		18,580,430
Entity Public Float			$ 48,716,169

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 89,478	$ 118,298
Prepaid expenses	1,757	823
VAT receivable	479	655
Other current assets	179	107
Total current assets	91,893	119,883
Restricted cash	182	182
Property and equipment, net	164	203
Right-of-use lease assets - operating (Type B), net	1,506	1,773
Deposits and other assets	10	7
Total assets	93,755	122,048
Current liabilities
Accounts payable	587	1,591
Accrued expenses	2,922	3,063
Term loan, short term	2,811	314
Current portion of lease obligation - operating (Type B)	353	242
Other current liabilities		1
Total current liabilities	6,673	5,211
Term loan, long term	6,826	9,105
Long term lease obligation - operating (Type B)	1,353	1,683
Other liabilities	159	52
Total liabilities	15,011	16,051
Commitments and contingencies (Note 13)
Stockholders’ Equity
Preferred stock, $0.0001 par value; 10,000,000 shares authorized and no shares issued and outstanding
Common stock; $0.0001 par value; 150,000,000 shares authorized at December 31, 2018; 18,534,240 and 18,569,289 shares issued and outstanding at December 31, 2017 and December 31, 2018, respectively	2	2
Additional paid-in capital	175,201	173,011
Accumulated deficit	(96,459)	(67,016)
Total stockholders’ equity	78,744	105,997
Total liabilities and stockholders’ equity	$ 93,755	$ 122,048

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 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	$ 0.0001
Common stock, shares authorized	150,000,000
Common stock, shares issued	18,569,289	18,534,240
Common stock, shares outstanding	18,569,289	18,534,240

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses:
Research and development	$ 15,948	$ 14,331	$ 11,856
General and administrative	13,719	9,105	6,054
Total operating expenses	29,667	23,436	17,910
Loss from operations	(29,667)	(23,436)	(17,910)
Other income (expense)	(56)	75	38
Interest income	1,712	341	7
Interest expense	(1,432)	(797)	(6,512)
Net loss and comprehensive loss	$ (29,443)	$ (23,817)	$ (24,377)
Net loss per share, basic and diluted	$ (1.59)	$ (8.04)	$ (25.01)
Weighted—average common shares outstanding, basic and diluted	18,556,126	2,962,859	974,660

Consolidated Statements of Conv

Consolidated Statements of Convertible Preferred Stock and Stockholders' (Deficit) Equity - USD ($) $ in Thousands	Total	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Beginning Balance at Dec. 31, 2015	$ (18,240)	$ 18,073			$ 582	$ (18,822)
Beginning Balance, Shares at Dec. 31, 2015		17,565,679		825,577
Net loss	(24,377)					(24,377)
Beneficial conversion features on convertible notes payable	4,653				4,653
Conversion of convertible notes payable to convertible preferred stock		$ 8,429	$ 5,963
Conversion of convertible notes payable to convertible preferred stock, Shares		8,183,792	5,962,784
Issuance of stock, net of underwriting discounts, commissions and offering costs			$ 40,638
Issuance of stock, net of underwriting discounts, commissions and offering costs, Shares			41,000,000
Issuance of common stock upon exercise of stock options	1				1
Issuance of common stock upon exercise of stock options, Shares				8,900
Vesting of restricted stock	13				13
Vesting of restricted stock, Shares				235,610
Stock-based compensation	876				876
Ending Balance at Dec. 31, 2016	(37,074)	$ 26,502	$ 46,601		6,125	(43,199)
Ending Balance, Shares at Dec. 31, 2016		25,749,471	46,962,784	1,070,087
Net loss	(23,817)					(23,817)
Issuance of stock, net of underwriting discounts, commissions and offering costs	92,710			$ 1	92,709
Issuance of stock, net of underwriting discounts, commissions and offering costs, Shares				7,294,968
Automatic conversion of preferred stock	73,125	$ (26,502)	$ (46,601)	$ 1	73,124
Automatic conversion of preferred stock, Shares		(25,749,471)	(46,962,784)	10,126,771
Issuance of common stock upon exercise of stock options	$ 101				101
Issuance of common stock upon exercise of stock options, Shares	39,722			39,636
Vesting of restricted stock	$ 5				5
Vesting of restricted stock, Shares				2,778
Stock-based compensation	947				947
Ending Balance at Dec. 31, 2017	105,997			$ 2	173,011	(67,016)
Ending Balance, Shares at Dec. 31, 2017				18,534,240
Net loss	(29,443)					(29,443)
Offering costs	(4)				(4)
Issuance of common stock upon exercise of stock options	$ 58				58
Issuance of common stock upon exercise of stock options, Shares	34,561			34,561
Vesting of restricted stock	$ 1				1
Vesting of restricted stock, Shares				488
Stock-based compensation	2,135				2,135
Ending Balance at Dec. 31, 2018	$ 78,744			$ 2	$ 175,201	$ (96,459)
Ending Balance, Shares at Dec. 31, 2018				18,569,289

Consolidated Statements of Co_2

Consolidated Statements of Convertible Preferred Stock and Stockholders' (Deficit) Equity (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)
Series B Convertible Preferred Stock
Offering costs	$ 8
Common Stock
Offering costs	2,300
Underwriting discounts and commissions	$ 7,100

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (29,443,000)	$ (23,817,000)	$ (24,377,000)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation expense	38,000	17,000	5,000
Amortization expense - right-of-use leased assets - operating (Type B)	294,000	150,000	91,000
Stock-based compensation	2,135,000	947,000	876,000
Non-cash interest expense	374,000	198,000	6,512,000
Changes in operating assets and liabilities
Prepaid expenses and other assets	(833,000)	(1,128,000)	(214,000)
Accounts payable, accrued expenses and other liabilities	(1,377,000)	951,000	1,652,000
Net cash flows used in operating activities	(28,812,000)	(22,682,000)	(15,455,000)
Cash flows from investing activities
Purchases of property and equipment		(194,000)	(9,000)
Net cash flows used in investing activities		(194,000)	(9,000)
Cash flows from financing activities
Proceeds from common stock offering, net of underwriter discounts and offering costs	(4,000)	92,710,000
Proceeds from term loan, net of costs	(62,000)	9,273,000
Proceeds from issuance of convertible notes			12,600,000
Costs related to issuance of convertible notes			(66,000)
Proceeds from the early exercise of stock options		1,000
Proceeds from the exercise of vested stock options	58,000	101,000	1,000
Purchase of restricted stock		(3,000)
Net cash flows provided by (used in) financing activities	(8,000)	102,074,000	53,173,000
Net increase (decrease) in cash	(28,820,000)	79,198,000	37,709,000
Cash, cash equivalents and restricted cash, beginning of year	118,480,000	39,282,000	1,573,000
Cash, cash equivalents and restricted cash, end of year	89,660,000	118,480,000	39,282,000
Supplemental cash flow information
Interest paid	1,059,000	599,000
Taxes paid	298,000	14,000
Supplemental disclosure of non-cash activities
Conversion of convertible preferred stock into common stock		73,102,000
Conversion of convertible notes into convertible preferred stock, including accrued interest			14,392,000
Beneficial conversion feature of convertible notes			4,653,000
Vesting of restricted stock	(1,000)	(5,000)	(13,000)
Acquisition of right-of-use leased assets - operating (Type B), net of disposal	$ 26,000	1,603,000
Series B Convertible Preferred Stock
Cash flows from financing activities
Proceeds from issuance of stock			41,000,000
Costs related to issuance of stock		$ (8,000)	$ (362,000)

Description of Business and Bas

Description of Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of Business and Basis of Presentation	1. Description of Business and Basis of Presentation Description of Business scPharmaceuticals LLC was formed as a Limited Liability Company under the laws of the State of Delaware on February 19, 2013. On March 24, 2014, scPharmaceuticals LLC was converted to a Delaware Corporation and changed its name to scPharmaceuticals Inc. (“the Company”). The Company is a pharmaceutical company focused on developing and commercializing products that have the potential to transform the way therapy is delivered, advance patient care and reduce healthcare costs. The Company’s proprietary platform is designed to enable the subcutaneous administration of therapies that have previously been limited to intravenous, or IV, delivery. The Company’s headquarters and primary place of business is Burlington, Massachusetts. Basis of Presentation The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States (“U.S. GAAP”) and have been prepared on a basis which assumes that the Company will continue as a going concern, which contemplates the realization of assets and the satisfaction of liabilities and commitments in the normal course of business. The consolidated financial statements reflect the operations of the Company and its wholly-owned subsidiary, scPharmaceuticals Securities Corporation. All significant intercompany balances and transactions have been eliminated in consolidation. At December 31, 2018, the Company had cash, cash equivalents and restricted cash of $89.7 million and working capital of $85.2 million. During the year ended December 31, 2018, the Company incurred a net loss totaling $29.4 million and used cash in operating activities totaling $28.8 million. The Company expects to continue to incur losses and use cash in operating activities in 2019. In November 2017, the Company completed an initial public offering ("IPO"), in which the Company issued and sold 7,294,968 shares of common stock at a public offering price of $14.00 per share, resulting in net proceeds of $92.7 million after deducting underwriting discounts and commissions and offering costs. Prior to the IPO, the Company funded its operations primarily through convertible notes and the sale of equity in private placements. The Company believes that, based on its current development plans and activities, its cash balance of $89.5 million as of December 31, 2018 will be sufficient to satisfy its liquidity requirements for more than one year from the issuance date of these consolidated financial statements.

Significant Accounting

Significant Accounting	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Significant Accounting Policies	2. Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant items subject to such estimates and assumptions include accruals related to development costs and clinical activities, valuation of stock options used for the calculation of stock-based compensation prior to the Company’s IPO, valuation of common and preferred stock used in the determination of the beneficial conversion feature of convertible notes and preferred stock, and the establishment of the tax valuation allowance. Actual results could differ from those estimates. Foreign Currency Transactions The functional currency of the Company is the U.S. dollar. Accordingly, gains and losses resulting from translating transactions denominated in currencies and balances of assets and liabilities outstanding at the balance sheet date, other than U.S. dollars, are included in net loss in the Statements of Operations and Comprehensive Loss. Cash, Cash Equivalents and Restricted Cash Cash, cash equivalents and restricted cash consists of bank deposits, certificates of deposit and money market accounts with financial institutions. Cash equivalents are carried at cost which approximates fair value due to their short-term nature and which the Company believes do not have a material exposure to credit risk. The Company considers all highly liquid investments with maturities of three months or less from the date of purchase to be cash equivalents. The Company places its cash and cash equivalents with institutions with high credit quality. However, at certain times such cash and cash equivalents may be in excess of Federal Deposit Insurance Corporation and Securities Investor Protection Corporation insurance limits. The Company has not experienced any losses with respect to these accounts. As of December 31, 2018, the Company classified $182,000 as restricted cash related to a letter of credit issued as a security deposit in connection with the Company’s lease of its corporate office facilities (Note 13). Cash, cash equivalents and restricted cash consists of the following: December 31, 2017 December 31, 2018 Cash and cash equivalents $ 118,298 $ 89,478 Restricted cash 182 182 Cash, cash equivalents and restricted cash $ 118,480 $ 89,660 Research and Development Costs Research and development costs are expensed as incurred. Nonrefundable advance payments, if any, for goods or services used in research and development are initially recorded as an asset and then recognized as an expense as the related goods are delivered or services are performed. Research and development expenses include contract services, consulting, salaries, materials and supplies and overhead. Income Taxes The Company accounts for income taxes in accordance with the ASC 740, Income Taxes. The Company provides reserves for potential payments of tax to various tax authorities related to uncertain tax positions. The tax benefits recorded are based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings or positions is “more likely than not” to be realized following resolution of any uncertainty related to the tax benefit, assuming that the matter in question will be raised by the tax authorities. Potential interest and penalties associated with such uncertain tax positions are recorded as a component of income tax expense. At December 31, 2018, the Company had no such accruals. Stock-Based Compensation Stock-based compensation expense is recognized based on the grant-date fair value using the Black-Scholes valuation model. The Company recognizes compensation expense only for those stock-based awards expected to vest after considering expected forfeitures. Cumulative compensation expense is at least equal to the compensation expense for vested awards. Stock-based compensation is recognized on a straight-line basis over the service period of each award. Stock compensation costs have not been capitalized by the Company. The Company accounts for stock-based awards issued to non-employees by recognizing compensation expense based on the fair value of such awards when the services are completed over the vesting period of the award. Segments 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 (“CODM”) in making decisions regarding resource allocation and assessing performance. The Company’s chief executive officer is the CODM, and he uses consolidated financial information in determining how to allocate resources and assess performance. The Company has determined that it operates in one segment. All of the Company’s assets are located in the United States. Change in Accounting Principle In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”). ASU 2016-02 is intended to improve financial reporting of leasing transactions by requiring organizations that lease assets to recognize assets and liabilities for the rights and obligations created by leases that extend more than twelve months from the balance sheet date. This accounting update also requires additional disclosures surrounding the amount, timing, and uncertainty of cash flows arising from leases. ASU 2016-02 is effective for financial statements issued for annual and interim periods beginning after December 15, 2018 for public business entities. Early adoption is permitted. The Company elected to early adopt ASU 2016-02 as of January 1, 2018 with retrospective application to January 1, 2016, the beginning of the earliest period to be presented in the Annual Report on Form 10-K for the year ended December 31, 2018. The Company has elected the package of practical expedients permitted in ASC Topic 842. Accordingly, the Company accounted for its existing operating leases as operating leases under the new guidance, without reassessing (a) whether the contracts contain a lease under ASC Topic 842, (b) whether classification of the operating leases would be different in accordance with ASC Topic 842, or (c) whether the unamortized initial direct costs before transition adjustments (as of December 31, 2015) would have met the definition of initial direct costs in ASC Topic 842 at lease commencement. The Company does not allocate the consideration between lease and non-lease components. As a result of the adoption of the new lease accounting guidance, the Company recognized on January 1, 2016 (a) a lease liability of approximately $409,000, which represents the present value of the remaining lease payments, as of the date of adoption, of approximately $540,000, discounted using the Company’s incremental borrowing rate of 9.63%, and (b) a right-of-use asset of approximately $396,000 which represents the lease liability of $409,000 adjusted for accrued rent of approximately $13,000. Adoption of the standard requires the Company to restate certain previously reported results, including the recognition of additional ROU assets and lease obligations for operating leases. This standard did not have a material impact on the Company’s consolidated balance sheets or cash flows from operations and had no impact on the Company’s operating results. The most significant impact was the recognition of ROU assets and lease obligations for operating leases. Recently Issued Accounting Standards In May 2014, the FASB and the International Accounting Standards Board jointly issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASC 606”), which supersedes the revenue recognition requirements in ASC 605 and most industry-specific guidance. The new standard requires that an entity recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods and services. The update also requires additional disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. ASC 606 is effective for public entities for annual and interim periods within those annual periods beginning after December 15, 2017. The Company has adopted ASC 606 as of January 1, 2018 and there was no impact to the Company’s financial statements. The future impact of ASC 606 will be dependent on the nature of the Company’s future revenue contracts and arrangements, if any. In July 2018, the FASB issued ASU No. 2018-10, Codification Improvements to Topic 842, Leases (“ASU 2018-10”). ASU 2018-10 is intended to address questions on the application of ASU No. 2016-02 and to clarify its guidance. ASU 2018-10 is effective for financial statements issued for annual and interim periods beginning after December 15, 2018 for public business entities. For entities who have early adopted ASU No. 2016-02, the guidance is effective upon the issuance of ASU 2018-10. The Company adopted ASU 2018-10 in July 2018 and there was no impact to the Company’s financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820) (“ASU 2018-13”). ASU 2018-13 modifies fair value disclosure requirements, specifically around level transfers and valuation of Level 3 assets and liabilities. ASU 2018-13 is effective for financial statements issued for annual and interim periods beginning after December 15, 2019 for all entities. Early adoption of all or part of ASU No. 2018-13 is permitted. The Company does not expect ASU 2018-13 to have a material impact on its financial statements.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Loss per Share	3. Net Loss per Share Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period without consideration of dilutive common stock equivalents. Diluted net loss per share is the same as basic net loss per common share, since the effects of potentially dilutive securities are anti-dilutive. Dilutive common stock equivalents are comprised of convertible preferred stock, unexercised stock options outstanding under the Company’s equity plan and unvested restricted stock. The following table sets forth the computation of basic and diluted net loss per share of common stock (in thousands, except shares and per share data): FOR THE YEAR ENDED DECEMBER 31, 2016 DECEMBER 31, 2017 DECEMBER 31, 2018 Net loss and comprehensive loss $ (24,377 ) $ (23,817 ) $ (29,443 ) Weighted—average common shares outstanding, basic and diluted 974,660 2,962,859 18,556,126 Net loss per share, basic and diluted $ (25.01 ) $ (8.04 ) $ (1.59 ) The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because their inclusion would be anti-dilutive (in common stock equivalent shares): FOR THE YEAR ENDED DECEMBER 31, 2016 DECEMBER 31, 2017 DECEMBER 31, 2018 Convertible preferred stock, on an as-converted basis 10,126,771 — — Stock options to purchase common stock 444,410 1,195,495 1,588,306 Unvested restricted stock 5,053 488 — 10,576,234 1,195,983 1,588,306

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Property and Equipment	4. Property and Equipment Purchased property and equipment consist of the following as of December 31, (dollars in thousands): ESTIMATED USEFUL LIFE 2017 2018 Office equipment 5 years $ 10 $ 10 Office furniture 7 years 116 116 Computer equipment 3 years 8 8 Leasehold improvements Life of lease 95 95 229 229 Less: Accumulated depreciation (26 ) (65 ) Property and equipment, net $ 203 $ 164 Depreciation expense for the years ended December 31, 2016, 2017 and 2018 was $5,000, $17,000 and $38,000, respectively. Leased property and equipment consist of the following as of December 31, (dollars in thousands): ESTIMATED USEFUL LIFE 2017 2018 Right-of-use lease assets - operating (Type B) Lease term $ 2,014 $ 2,041 Less: Accumulated amortization (241 ) (535 ) Right-of-use lease assets - operating (Type B), net $ 1,773 $ 1,506 Amortization expense for the years ended December 31, 2016, 2017 and 2018 was $91,000, $150,000, and $294,000, respectively.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2018
Accruals [Abstract]
Accrued Expenses	5. Accrued Expenses Accrued expenses at December 31 consist of (in thousands): 2017 2018 Contract research and development $ 1,610 $ 1,492 Employee compensation and related costs 871 860 Consulting and professional service fees 287 356 State taxes 192 165 Financing related costs 90 — Other 13 49 Total accrued expenses $ 3,063 $ 2,922

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	6. Income Taxes The Company accounts for income taxes in accordance with ASC 740, Income Taxes Accordingly, the Company had no net income tax provision or benefit during the years ended December 31, 2017 and 2018. Components of the net deferred tax asset at December 31, 2017 and 2018 are as follows (in thousands): 2017 2018 Federal net operating loss carryforwards $ 3,672 $ 6,189 State net operating loss carryforwards 1,000 1,670 Research and development tax credits 1,087 1,824 Accrued liabilities 278 257 Stock-based compensation — 510 Depreciation and amortization 10 10 Capitalized research and development costs 10,452 14,227 Other 153 20 16,652 24,707 Valuation allowance (16,652 ) (24,707 ) Net deferred tax asset $ — $ — At December 31, 2018, the Company had available federal net operating loss carryforwards of $17.5 million, which expire at various dates through 2037, and $12.0 million, which may be carried forward indefinitely. At December 31, 2018, the Company had available state net operating loss carryforwards of $26.4 million, which expire at various dates through 2038, and $0.1 million, which may be carried forward indefinitely. In assessing the realizability of net deferred tax assets, management considers whether it is more likely than not that the net deferred tax assets will be realized. The ultimate realization of net deferred tax assets is dependent upon the generation of future taxable income during the periods in which temporary differences representing future deductible amounts become deductible. Management has established a full valuation allowance against the net deferred tax assets at December 31, 2017 and 2018 since it is more likely than not that these future tax benefits will not be realized. During 2018, the valuation allowance increased by $8.1 million. At December 31, 2018, the Company had federal and state research and development credit carryforwards of $1.5 million and $0.4 million, respectively. The net credit carryforwards may be used to offset future income taxes and expire at various dates through 2038. Changes in the Company’s ownership, as defined in the U.S. Internal Revenue Code, may limit the Company’s ability to utilize the tax credit and net operating loss carryforwards. On December 22, 2017, the United States enacted new tax reform (“Tax Cuts and Jobs Act”). The Tax Cuts and Jobs Act contains provisions with separate effective dates but is generally effective for taxable years beginning after December 31, 2017. Beginning with the year ending December 31, 2018, the corporate statutory rates on U.S. earnings was reduced from 34% to 21%. In the fourth quarter of 2017, the Company recognized a provisional charge of $6.9 million for the revaluation of the Company’s deferred tax assets at the lower enacted corporate tax rate. This charge was offset by a corresponding reduction in the valuation allowance. U.S. GAAP requires companies to recognize the effect of tax law changes in the period of enactment. The Company’s assessment of the remeasurement of deferred tax assets at the lower enacted corporate tax rate is now complete. A reconciliation of income tax (expense) benefit at the statutory federal income tax rate and income taxes as reflected in the consolidated financial statements at December 31, 2016, 2017 and 2018 are as follows: 2016 2017 2018 Federal income tax at statutory rate 34.00 % 34.00 % 21.00 % State income tax, net of federal benefit 3.46 % 4.85 % 5.45 % Research and development credits 1.46 % 2.07 % 2.35 % Book compensation related to stock options (0.93 )% (0.16 )% (0.61 )% Change in income tax rate (0.18 )% — (0.38 )% Effect on Tax Cuts & Job Acts rate reduction — (29.15 )% — Non-cash interest (9.08 )% — — Other (0.40 )% (0.51 )% (0.45 )% Increase in valuation allowance (28.33 )% (11.10 )% (27.36 )% Effective tax rate — % — % — % The Company files tax returns in the United States, Massachusetts and other states. The tax years 2015 through 2018 remain open to examination by major taxing jurisdictions to which the Company is subject, which are primarily the United States federal and Massachusetts, as carryforward attributes generated in years past may still be adjusted upon examination by the Internal Revenue Service or state tax authorities if they have or will be used in a future period. The Company is currently not under examination by the Internal Revenue Service or any other jurisdictions for any tax years. The Company recognizes both accrued interest and penalties related to unrecognized benefits in income tax expense. The Company has not recorded any interest or penalties on any unrecognized tax benefits since its inception. A reconciliation of the beginning and ending amount of uncertain tax benefits is as follows (in thousands): 2016 2017 2018 Beginning uncertain tax benefits $ 104 $ 178 $ 284 Prior year - increases — — 46 Current year - decreases — (19 ) — Current year - increases 74 125 146 Ending uncertain tax benefits $ 178 $ 284 $ 476

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	7. Stock-Based Compensation Stock Options The Company’s 2014 Stock Incentive Plan (the “2014 Stock Plan”) terminated in November 2017 effective upon the completion of the Company’s initial public offering. No additional options will be granted under the 2014 Stock Plan. At December 31, 2018, there were 946,609 options outstanding under the 2014 Stock Plan. In October 2017, the board of directors approved the 2017 Stock Option and Incentive Plan (or the “2017 Stock Plan”) which became effective in November 2017, upon the closing of the initial public offering. The 2017 Stock Plan will expire in October 2027. Under the 2017 Stock Plan, the Company may grant incentive stock options, non-statutory stock options, restricted stock awards and other stock-based awards. There were 1,430,000 shares of the Company’s common stock initially reserved for issuance under the 2017 Stock Plan. In addition, the number of shares of common stock that may be issued under the 2017 Stock Plan will automatically increase on each January 1, beginning on January 1, 2018 and ending on January 1, 2027, by a number of shares equal to 4% of the Company’s shares of common stock outstanding on the immediately preceding December 31, subject to limitation. On January 1, 2018 and January 1, 2019, the number of shares issuable under the 2017 Stock Plan increased by 741,389 and 742,772 shares, respectively. Total stock-based compensation expense, including the effect of forfeitures, recorded in research and development and general and administrative expenses, respectively, for employees, directors and non-employees during the years ended December 31, 2016, 2017 and 2018 is as follows (in thousands): 2016 2017 2018 Research and development $ 175 $ 159 $ 567 General and administrative 701 788 1,568 Total $ 876 $ 947 $ 2,135 At December 31, 2018, there were 1,686,893 options available for issuance and 641,697 options outstanding under the 2017 Stock Plan. Options granted under the 2017 Plan have a term of ten years. Vesting of options under the 2017 Stock Plan is determined by the compensation committee of the board of directors, but is generally a four-year term. The fair value of options at date of grant was estimated using the Black-Scholes option-pricing model with the following assumptions: 2016 2017 2018 Risk-free interest rate 1.08%—1.58% 1.89%—2.24% 2.42%—2.86% Expected dividend yield 0% 0% 0% Expected life 6.0—6.4 years 5.0—7.0 years 5.5—7.0 years Expected volatility 86%—93% 78%—88% 76%—86% Weighted-average grant date fair value $ 6.51 $ 3.62 $ 7.57 Due to the lack of a public market for the trading of the Company’s common stock prior to its initial public offering and the lack of company specific historical volatility, volatility was estimated using historical volatilities of similar companies. The expected life of the awards is estimated based on the simplified method, which calculates the expected life based upon the midpoint of the term of the award and the vesting period. The Company uses the simplified method because it does not have sufficient option exercise data to provide a reasonable basis upon which to estimate the expected term. The Company has no history of paying dividends nor does management expect to pay dividends over the contractual terms of these options. The risk-free interest rates are based on the United States Treasury yield curve in effect at the time of grant, with maturities approximating the expected life of the stock options. The following table summarizes information about stock option activity during 2017 and 2018 (in thousands, except share and per share data): NUMBER OF SHARES WEIGHTED- AVERAGE EXERCISE PRICE WEIGHTED- AVERAGE REMAINING CONTRACTUAL TERM AGGREGATE INTRINSIC VALUE Outstanding, December 31, 2016 444,410 $ 6.99 Granted 888,497 4.82 Exercised (39,722 ) 2.57 Forfeited (97,690 ) 8.88 Outstanding, December 31, 2017 1,195,495 $ 5.38 Granted 939,296 10.54 Exercised (34,561 ) 1.69 Forfeited (511,924 ) 10.85 Outstanding, December 31, 2018 1,588,306 $ 6.75 8.44 $ 86 Vested and exercisable, December 31, 2018 572,376 $ 5.23 7.71 $ 86 Vested and expected to vest, December 31, 2018 1,391,103 $ 6.71 8.37 $ 86 During 2016, 2017 and 2018, the Company received $1,000, $102,000 and $58,000, respectively, upon exercise of stock options. The intrinsic value of the options exercised in 2016, 2017 and 2018 was $64,000, $330,000 and $270,000, respectively. Among those options exercised, 86 were exercised prior to vesting in 2017 pursuant to the 2014 Stock Plan. Options exercised prior to vesting are held under restricted stock agreements and will vest according to the provisions under the original stock option agreements. The cash received upon early exercise of options, $1,000 in 2017, was recorded as a deposit liability on the Company’s consolidated balance sheet and was recorded as common stock and additional paid in capital as the shares vested in 2018. The Company repurchased 1,871 unvested restricted stock shares in 2017 for $3,000. The deposit liability as of December 31, 2017 was $1,000. Unrecognized compensation expense related to unvested awards as of December 31, 2018 was $3.5 million, net of forfeitures, and will be recognized over the remaining vesting periods of the underlying awards. The weighted-average period over which such compensation is expected to be recognized is 2.62 years. During the year ended December 31, 2018, the Company extended the exercise period for 102,661 vested options with a weighted average exercise price of $7.33 pursuant to separation agreements. The Company recorded incremental stock-based compensation expense of $117,000. Restricted Stock At the time of the Company’s conversion from a Limited Liability Company to a Delaware Corporation in 2014, the Company imposed restrictions on 821,512 shares of common stock owned by a founder (“2014 Restricted Stock Awards”). The terms of the restrictions allowed for 50% of the shares to vest immediately, with the remainder of the shares vesting over 3 years. The initial vesting of the shares was deemed to be non-substantive for accounting purposes, as there was no service required for the lapse of the restrictions. The fair value of the common stock at the time of the restrictions was $1.66. In May 2016, the Company terminated its right to repurchase the remaining unvested shares of the 2014 Restricted Stock Awards, thereby causing all unvested shares to become vested and any unrecognized compensation to be accelerated. During the year ended December 31, 2016, $0.4 million was recognized as compensation expense for the vesting of the 2014 Restricted Stock Awards.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	8. Fair Value of Financial Instruments The Financial Accounting Standards Board (“FASB”) Accounting Standard Codification (“ASC”) Topic, Fair Value Measurements and Disclosures (“ASC 820”), Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly. Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and observable. To the extent that 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 Company does not have any recurring fair value measurements as of December 31, 2018. The carrying values of the Company’s cash, cash equivalents and restricted cash, prepaid expenses, VAT receivable and deposits approximate their fair values due to their short-term nature. The carrying value of the Company’s loan payable was considered a reasonable estimate of fair value because the Company’s interest rate is near current market rates for instruments with similar characteristics. The Company’s cash equivalents are classified within Level 1 of the fair value hierarchy. The following table summarizes the Company’s money market funds as of December 31, 2018 (in thousands): TOTAL Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash equivalents: Cash equivalents $ 75,306 $ 75,306 $ — $ — Total cash equivalents $ 75,306 $ 75,306 $ — $ —

Term Loan

Term Loan	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Term Loan	9. Term Loan In May 2017, the Company entered into a loan and security agreement, or 2017 Loan Agreement, with Solar Capital Ltd. and Silicon Valley Bank for $10.0 million. The 2017 Loan Agreement has a maturity date of May 1, 2021. In connection with the 2017 Loan Agreement, the Company entered into an exit fee agreement which provides for an aggregate payment of 4% of the loan commitment, or $400,000, to the lenders upon the occurrence of an exit event, including an initial public offering. The Company concluded that the exit payment obligation met the definition of a derivative that was required to be accounted for as a separate unit of accounting. The Company recorded the issuance-date fair value of the payment obligation of $392,000 as a derivative liability in the Company’s consolidated balance sheet. The Company paid the fee in November 2017 in conjunction with the Company’s IPO. Debt issuance costs for the 2017 Loan Agreement amounting to $727,000, including the exit payment obligation, were recorded as a debt discount in 2017. In November 2018 and December 2018, the Company entered into the First Amendment to the Loan and Security Agreement and the Second Amendment to the Loan and Security Agreement, respectively, collectively the “Amendments”. In connection with the Amendments, the Company incurred additional debt issuance costs of $62,000. The interest rate under the 2017 Loan Agreement is LIBOR plus 8.45%. The initial interest-only period was until November 30, 2018, followed by a 30-month principal and interest period. The First Amendment to the Loan and Security Agreement extended the interest-only period, which currently runs through May 2019. If and when certain conditions are met, the interest-only period may be extended to August or November 2019. As of December 31, 2018, unpaid borrowings under the 2017 Loan Agreement totaled $10.0 million. For the years ended December 31, 2017 and 2018, the Company recorded $146,000 and $280,000, respectively, related to the amortization of debt discount associated with the 2017 Loan Agreement. The 2017 Loan Agreement allows the Company to voluntarily prepay all (but not less than all) of the outstanding principal at any time. A prepayment premium of 1% would be assessed on the outstanding principal. A final payment fee of $250,000 was due upon the earlier to occur of the maturity date or prepayment of such borrowings. The final payment fee was increased to $325,000 in the First Amendment to the Loan and Security Agreement. In an event of default under the 2017 Loan Agreement, the interest rate will be increased by 5% and the balance under the loan may become immediately due and payable at the option of the lenders. The 2017 Loan Agreement includes restrictions on, among other things, the Company’s ability to incur additional indebtedness, change the name or location of the Company’s business, merge with or acquire other entities, pay dividends or make other distributions to holders of its capital stock, make certain investments, engage in transactions with affiliates, create liens, sell assets or pay subordinated debt. Total term loan and unamortized debt discount balances are as follows (in thousands): DECEMBER 31, 2017 DECEMBER 31, 2018 Face value $ 10,000 $ 10,000 Less: discount (581 ) (363 ) Total $ 9,419 $ 9,637 Less: current portion (314 ) (2,811 ) Long-term portion $ 9,105 $ 6,826 As of December 31, 2018, future principal payments due under the 2017 Loan Agreement are as follows (in thousands): Year ended: December 31, 2019 $ 2,917 December 31, 2020 5,000 December 31, 2021 2,083 Total minimum principal payments $ 10,000

Convertible Notes

Convertible Notes	12 Months Ended
	Dec. 31, 2018
Convertible Longterm Notes Payable Current And Noncurrent [Abstract]
Convertible Notes	10. Convertible Notes On January 21, 2016, the Company executed a Convertible Note Purchase Agreement (“the January 2016 Convertible Note Purchase Agreement”) under which the Company was authorized to issue $7.5 million in convertible promissory notes in an initial closing, subsequent closing and a second closing tied to the achievement of development milestones. The initial closing occurred on January 21, 2016 whereby the Company issued $3.0 million of convertible notes pursuant to the January 2016 Convertible Note Purchase Agreement. In January and February 2016, the Company issued $0.7 million of additional convertible notes as part of the subsequent closing to the January 2016 Convertible Note Purchase Agreement. The Company achieved the development milestones in May 2016 and, as such, was able to draw on the second closing of the January 2016 Convertible Note Purchase Agreement. In May and June 2016, the Company issued $3.7 million of additional convertible notes as part of the second closing to the January 2016 Convertible Note Purchase Agreement. On July 1, 2016, the Company executed the First Amendment to Note Purchase Agreement which replaced the principal amount of notes to which Company could issue with $8.2 million. On July 7, 2016, the Company issued $0.6 million of additional convertible notes under the First Amendment to Note Purchase Agreement. Principal and interest on all note issuances under the January 2016 Convertible Note Purchase Agreement, which accrued at a rate of 8% per annum, was due and payable upon the earlier of written demand by holders of the requisite noteholders any time on or after January 21, 2017, unless earlier converted upon automatic conversion at (i) a “Qualified Equity Financing” where such transaction results in the Company having raised gross proceeds of at least $15.0 million or (ii) a “Qualified IPO” where the Company sells shares of common stock to investors at a price per share equal to at least $21.55 and gross proceeds to the Company of at least $35.0 million, or upon optional conversion at (i) a “Non-Qualified Equity Financing” or (ii) a “Non-Qualified IPO” or upon the sale of the Company. Certain of the conversion features of the notes under the January 2016 Convertible Note Purchase Agreement allowed holders to convert principal and interest on each issuance into shares of the Company at a discount. The conversion price was equal to eighty percent (80%) of the per share price at which shares of equity financing securities or common stock are to be sold. Based on the terms of the notes under the January 2016 Convertible Note Purchase Agreement and the Company’s assessment that conversion of the notes prior to maturity in a “Qualified Equity Financing” was the predominant feature, the Company determined that the notes were share-settled debt, and as such accreted the notes over their term, to the value of the preferred stock into which the notes would be converted, $9.9 million, recognizing accretion to the redemption value through the date the convertible notes were converted as interest expense. Upon maturity, the noteholders had the option to convert any outstanding principal and accrued but unpaid interest into shares of Series A convertible preferred stock at a purchase price equal to $1.00. This embedded conversion feature met the definition of a beneficial conversion feature and was recognized by allocating a portion of the proceeds equal to the intrinsic value of the beneficial conversion feature measured at the commitment date, or $4.7 million, to additional paid in capital. The accretion of the beneficial conversion feature was recognized through the date the convertible notes were converted as interest expense. The Company allocated $38,000 in transaction costs as a discount to the notes. On August 22, 2016, the Company executed the Second Amendment to Note Purchase Agreement and Election to Convert. This amendment added a conversion option to convert the outstanding principal and accrued but unpaid interest into Series A convertible preferred stock at a purchase price equal to $1.00. At that time, the convertible notes, plus accrued interest, converted into 8,183,792 shares of Series A convertible preferred stock pursuant to the new redemption feature (Note 11). The amendment was treated as an extinguishment of debt which required the carrying value of the debt to be derecognized and the fair value of the debt to be recognized as new debt. At the amendment date, the carrying value of the debt included principal of $8.0 million, and accrued interest, accretion to the redemption value, and accretion of the beneficial conversion feature of $0.2 million, $0.9 million, and $2.1 million, respectively. Additionally, the unamortized beneficial conversion feature of $2.6 million was allocated to the carrying value of the debt and the fair value of the new debt was established at $8.4 million. The intrinsic value of the beneficial conversion feature was measured at the amendment date, $0.3 million, and recorded as a reduction in additional paid in capital. The loss on extinguishment of $1.8 million was recorded as interest expense. On August 22, 2016, the Company executed a Note Purchase Agreement (“the August 2016 Note Purchase Agreement”) under which the Company was authorized to issue $10.0 million in convertible promissory notes in an initial closing, second closing, third closing, and subsequent closings. The second closing was subject to the second closing development milestone. The third closing was subject to the third closing development milestone. The subsequent closings had to occur on or before the occurrence of the third closing. The initial closing occurred on August 22, 2016 whereby the Company issued $4.0 million of convertible notes pursuant to the August 2016 Note Purchase Agreement. In September 2016, the Company issued $0.7 million of additional convertible notes as part of the subsequent closing to the August 2016 Note Purchase Agreement. Principal and interest on all note issuances under the August 2016 Note Purchase Agreement, which accrued at a rate of 8% per annum, was due and payable upon the earlier of written demand by holders of the requisite noteholders any time on or after August 22, 2017, unless sooner accelerated upon automatic conversion at (i) a “Qualified Equity Financing” where such transaction results in the Company having raised gross proceeds of at least $25.0 million or (ii) a “Qualified IPO” where the Company sells shares of common stock to investors at a price per share equal to at least $21.55 and gross proceeds to the Company of at least $40.0 million, or upon optional conversion at (i) a “Non-Qualified Equity Financing” or (ii) a “Non-Qualified IPO” or upon the sale of the Company. Certain of the conversion features of the notes under the August 2016 Note Purchase Agreement allowed holders to convert principal and interest on each issuance into shares of the Company at a discount. For the Qualified Equity Financing, the conversion price was equal to ninety percent (90%), if converted before the 60 th Based on the terms of the notes under the August 2016 Convertible Note Purchase Agreement and the Company’s assessment that conversion of the notes prior to maturity in a “Qualified Equity Financing” was the predominant feature, the Company determined that the notes were share-settled debt, and as such accreted the notes over their term, to the value of the preferred stock into which the notes would be converted, $5.8 million, recognizing accretion to the redemption value through the date the convertible notes were converted as interest expense. Upon maturity, the noteholders have the option to convert any outstanding principal and accrued but unpaid interest into shares of Series A convertible preferred stock at a purchase price equal to $1.00. This embedded conversion feature meets the definition of a beneficial conversion feature and was recognized by allocating a portion of the proceeds equal to the intrinsic value of the beneficial conversion feature measured at the commitment date, or $0.2 million, to additional paid in capital. The accretion of the beneficial conversion feature was recognized through the date the convertible notes were converted as interest expense. The Company allocated $28,000 in transaction costs as a discount to the notes. On December 22, 2016, pursuant to the August 2016 Note Purchase Agreement and in connection with the issuance of Series B convertible preferred stock (Note 11), the convertible notes and accrued interest converted into 5,962,784 shares of Series B convertible preferred stock. The redemption was treated as an extinguishment of debt which required the carrying value of the debt to be derecognized and the fair value of the debt to be recognized as new debt. At the redemption date, the carrying value of the debt included principal of $4.7 million, and accrued interest, accretion to the redemption value, and accretion of the beneficial conversion feature of $120,000, $356,000, and $60,000, respectively. Additionally, the unamortized portion of the beneficial conversion feature of $126,000 was allocated to the carrying value of the debt and the fair value of the new debt was established at $6.0 million. The beneficial conversion feature at the conversion date was determined to be out of the money and, as such, was derecognized. The loss on extinguishment of $1.0 million was recorded as interest expense.

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2018
Temporary Equity Disclosure [Abstract]
Convertible Preferred Stock	11. Convertible Preferred Stock On March 24, 2014, the Company entered into a Series A Preferred Stock Purchase Agreement (“Series A Preferred SPA”). Per the Series A Preferred SPA, the Company agreed to sell to the Purchasers, for cash, an aggregate of up to 16,000,000 shares of Series A convertible preferred stock, par value $0.0001 per share, for the purchase price of $1.00 per share over two closings, an Initial Closing and a Milestone Closing. In addition, pursuant to the convertible note agreements, the Convertible Notes converted into 1,315,679 shares of Series A convertible preferred stock at the Initial Closing. The Initial Closing occurred on the date of the Series A Preferred SPA and the Milestone Closing was to occur on the 15 th In conjunction with the execution of the Series A Preferred SPA, the Company received $8.0 million from the sale of 8,000,000 shares of Series A convertible preferred stock, par value $0.0001, at a price of $1.00 per share at the Initial Closing. Costs associated with the financing were $0.2 million resulting in net cash received of $7.8 million. In October 2014, the Series A Preferred SPA was amended to include an additional investment of $250,000. The Company received $250,000 from the sale of 250,000 shares of Series A convertible preferred stock, par value $0.0001, at a price of $1.00 per share on October 14, 2014. On April 8, 2015, the Company received $8.0 million from the sale of 8,000,000 shares of Series A convertible preferred stock, par value $0.0001, at a price of $1.00 per share at the Milestone Closing of the Series A Preferred SPA. Costs associated with the financing were $7,000 resulting in net cash received of $7,993,000. On August 22, 2016, pursuant to the January 2016 Convertible Note Purchase Agreement, the Company issued 8,183,792 shares of Series A convertible preferred stock upon conversion of the underlying convertible notes (Note 10). On December 22, 2016, the Company entered into a Series B Preferred Stock Purchase Agreement (“Series B Preferred SPA”). Per the Series B Preferred SPA, the Company agreed to sell to the Purchasers, for cash, an aggregate of up to 46,962,784 shares of Series B convertible preferred stock, par value $0.0001 per share, for the purchase price of $1.00 per share. As part of the Series B Preferred SPA, pursuant to the August 2016 Note Purchase Agreement, the underlying convertible notes converted into 5,962,784 shares of Series B convertible preferred stock (Note 10). In conjunction with the execution of the Series B Preferred SPA, the Company received $41.0 million from the sale of 41,000,000 shares of Series B convertible preferred stock, par value $0.0001, at a price of $1.00 per share. Costs associated with the financing were $0.4 million resulting in net cash received of $40.6 million. On November 16, 2017, immediately prior to the closing of the IPO, the Company’s Series A convertible preferred stock and the Series B convertible preferred stock, collectively “Convertible Preferred Stock”, converted into 10,126,771 shares of common stock. Prior to their conversion into shares of common stock, the Convertible Preferred stock had the following characteristics: Dividends Holders of each share of Series A convertible preferred stock were entitled to receive non-cumulative cash dividends, prior and in preference to any declaration or payment of any dividend on shares of common stock at the rate of six percent of the Series A original issue price, payable only when, as and if declared by the Board of Directors. Through December 31, 2017, holders of each share of Series B convertible preferred stock were entitled to receive non-cumulative cash dividends, prior and in preference to any declaration or payment of any dividend on shares of common stock at the rate of six percent of the Series B original issue price, payable only when, as and if declared by the Board of Directors. From and after January 1, 2018, dividends at the rate per annum of six percent were to accrue on each share of Series B Preferred Stock and would become payable at the election of the Board of Directors in cash. In no event should the value of the Series B Preferred Stock dividend exceed twenty percent of the Series B original issue price on a cumulative basis. Voting Rights Holders of each share of Convertible Preferred Stock were entitled to that number of votes equal to the number of whole shares of common stock into which a holder’s shares of Convertible Preferred Stock could be converted as of the record date of any vote. Conversion Rights Shares of Convertible Preferred Stock were convertible, at the option of the holder, into shares of the Company’s common stock at a conversion value determined by dividing the Series A original issue price or the Series B original issue price, as the case may be, by the applicable conversion price. The Series A conversion price of $7.180193 (which reflects the stock split described in Note 12) and the Series B conversion price of $7.180193 (which reflects the stock split described in Note 12) are collectively referred to as the “Conversion Price”. All outstanding shares of Convertible Preferred Stock automatically converted into common stock upon the closing of the Company’s IPO. Liquidation Preference In the event of any liquidation, dissolution or winding-up of the Company, which would include the sale of the Company, the Series B convertible preferred stockholders were entitled to be paid, before any distribution or payment could be made upon the holders of Series A convertible preferred stock or common stock, an amount per share equal to the Series B original issue price, plus any Series B original dividend declared but unpaid thereon, plus any unpaid Series B accruing dividends accrued thereon, beginning in January 2018, plus any other dividends declared but unpaid thereon. Any assets remaining following the preferential distribution to the holders of Series B convertible preferred stock would have been available for distribution to the holders of Series A convertible preferred stock in an amount per share equal to the Series A original issue price, plus any dividends declared but unpaid thereon. In the event that assets remained after all the preferential amounts required to be paid to the holders of shares of preferred stock are paid, the remaining assets would have been distributed among the holders of the shares of preferred stock and common stock, ratably among all holders of preferred stock and common stock pro-rata based on the number of shares held by each holder, treating the preferred stock as if they had been converted to common stock prior to such liquidation, dissolution, or winding-up. The maximum amount distributed to holders of Series B convertible preferred stock would have been the greater of three times the Series B original issue price or the amount the holders would have received if all shares of Series B convertible preferred stock had been converted into common stock immediately prior to such liquidation, dissolution or winding-up of the Company. The maximum amount distributed to holders of Series A convertible preferred stock would have been the greater of three times the Series A original issue price or the amount the holders would have received if all shares of Series A convertible preferred stock had been converted into common stock immediately prior to such liquidation, dissolution or winding-up of the Company. Because a majority of voting power of the outstanding common stock could be obtained without the Company’s approval, redemption of the Convertible Preferred Stock could have been triggered. This required the Company to record the Convertible Preferred Stock in temporary equity between liabilities and equity in the balance sheet. Convertible Preferred Stock was recorded net of issuance costs. Registration Rights The holders of shares of Convertible Preferred Stock were entitled to certain demand registration rights with respect to these securities, as set forth in the investors’ rights agreement between the Company and the holders of these securities. These registration rights would require the Company to use its commercially reasonable efforts to register the shares of the Company’s common stock underlying the Convertible Preferred Stock under the Securities Act of 1933, subject to certain conditions and limitations. The cost of registration would be incurred by the Company.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Stockholders' Equity	12. Stockholders’ Equity Stock Split On November 6, 2017, the Company effectuated a 1-for-7.180193 reverse stock split of its outstanding common stock, which was approved by the Company’s board of directors on October 27, 2017 and by the Company’s stockholders on November 6, 2017. The reverse stock split resulted in an adjustment to the preferred stock conversion prices to reflect a proportional decrease in the number of shares of common stock to be issued upon conversion. The accompanying consolidated financial statements and notes to the consolidated financial statements give retroactive effect to the reverse stock split for all periods presented. The shares of common stock retained a par value of $0.0001 per share. Accordingly, the stockholders’ (deficit) equity reflects the reverse stock split by reclassifying from common stock to additional paid-in capital in an amount equal to the par value of the decreased shares resulting from the reverse stock split. Common Stock The Company issued 39,722 and 34,561 additional shares in 2017 and 2018, respectively, as a result of restricted stock issuances and stock option exercises. In addition, the Company repurchased 1,871 shares of unvested restricted stock in 2017. There were 18,534,240 and 18,569,289 shares outstanding at December 31, 2017 and 2018, respectively. Voting, dividend and liquidation rights of the holders of the common stock are subject to the Company’s articles of incorporation, corporate bylaws and underlying shareholder agreements. Reserved Shares The Company has reserved 1,588,306 shares of common stock for the exercise of outstanding options to purchase common stock.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	13. Commitments and Contingencies Operating Leases The Company entered into noncancelable operating leases for office facilities located in Lexington, Massachusetts and Burlington, Massachusetts through December 31, 2022 and November 30, 2022, respectively. Rent expense under the operating leases totaled $0.2 million, $0.3 million and $0.5 million for the years ended December 31, 2016, 2017 and 2018, respectively. Certain leases provide for increases in future minimum annual rental payments as defined in the lease agreements. The leases generally also include real estate taxes and common area maintenance (“CAM”) charges in the annual rental payments. Pursuant to the terms of its lease agreement for the Company’s headquarters in Burlington, Massachusetts, the Company obtained a letter of credit in the amount of approximately $182,000 as security on the lease obligation. The letter of credit is listed as restricted cash on the Company’s consolidated balance sheets. Short-term leases are leases having a term of twelve months or less. The Company recognizes short-term leases on a straight-line basis and does not record a related lease asset or liability for such leases. The following is a maturity analysis of the annual undiscounted cash flows reconciled to the carrying value of the operating lease liabilities as of December 31, 2018 (in thousands): Year ended: December 31, 2019 $ 513 December 31, 2020 528 December 31, 2021 537 December 31, 2022 496 Total minimum lease payments 2,074 Less imputed interest (368 ) Total $ 1,706 2017 2018 Lease cost: Operating lease cost $ 290 $ 483 Short-term lease cost 3 8 Sublease income — (39 ) Total lease cost $ 293 $ 452 Other information Cash paid for amounts included in the measurement of liabilities $ 92 $ 429 Operating cash flows from operating leases $ 141 $ 49 Weighted-average remaining lease term - operating leases 4.9 years 3.9 years Weighted-average discount rate - operating leases 10.1 % 10.1 % In February 2018, the Company signed a sublease agreement for its facility located in Lexington, Massachusetts. The sublease commenced on April 1, 2018 and has an initial term of three years with an extension term through December 2022.

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
401(k) Savings Plan	14. 401(k) Savings Plan In July 2014, the Company established a defined contribution savings plan under Section 401(k) of the Internal Revenue Code covering all of its employees. Employees may make contributions by withholding a percentage of their salary. The plan includes an employer match equal to 100% on the first 3% of deferred compensation and an additional 50% on the next 2% of deferred compensation. During the years ended December 31, 2016, 2017 and 2018, the Company has recognized compensation expense of $122,000, $165,000 and $225,000, respectively, for the employer match contribution.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the financial statements to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. On January 9, 2019, the Company held a Type C meeting with the FDA to discuss the dose delivery validation protocol. At the January 2019 Type C meeting, the FDA informed the Company that the FDA did not agree that the Company’s proposed dose validation study was adequate, and an agreement was not reached during the January 2019 meeting. In January 2019, the Company entered into a Development Agreement (the “Development Agreement”) with West Pharmaceutical Services, Inc. (“West”). Under the terms of the Development Agreement, the Company paid to West a one-time upfront payment of approximately $1.7 million upon signing of the agreement. In addition, the Company has committed to pay West for development and validation milestones, as well as exclusivity fees for treating congestion with loop diuretics. In January 2019, the Company implemented a restructuring plan to reduce its workforce by approximately 43%, to 13 employees. The Company currently estimates that it will record a charge in 2019 of approximately $1.4 million, consisting of severance, benefits and outplacement services.

Significant Accounting (Policie

Significant Accounting (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant items subject to such estimates and assumptions include accruals related to development costs and clinical activities, valuation of stock options used for the calculation of stock-based compensation prior to the Company’s IPO, valuation of common and preferred stock used in the determination of the beneficial conversion feature of convertible notes and preferred stock, and the establishment of the tax valuation allowance. Actual results could differ from those estimates.
Foreign Currency Transactions	Foreign Currency Transactions The functional currency of the Company is the U.S. dollar. Accordingly, gains and losses resulting from translating transactions denominated in currencies and balances of assets and liabilities outstanding at the balance sheet date, other than U.S. dollars, are included in net loss in the Statements of Operations and Comprehensive Loss.
Cash Cash Equivalents and Restricted Cash	Cash, Cash Equivalents and Restricted Cash Cash, cash equivalents and restricted cash consists of bank deposits, certificates of deposit and money market accounts with financial institutions. Cash equivalents are carried at cost which approximates fair value due to their short-term nature and which the Company believes do not have a material exposure to credit risk. The Company considers all highly liquid investments with maturities of three months or less from the date of purchase to be cash equivalents. The Company places its cash and cash equivalents with institutions with high credit quality. However, at certain times such cash and cash equivalents may be in excess of Federal Deposit Insurance Corporation and Securities Investor Protection Corporation insurance limits. The Company has not experienced any losses with respect to these accounts. As of December 31, 2018, the Company classified $182,000 as restricted cash related to a letter of credit issued as a security deposit in connection with the Company’s lease of its corporate office facilities (Note 13). Cash, cash equivalents and restricted cash consists of the following: December 31, 2017 December 31, 2018 Cash and cash equivalents $ 118,298 $ 89,478 Restricted cash 182 182 Cash, cash equivalents and restricted cash $ 118,480 $ 89,660
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Nonrefundable advance payments, if any, for goods or services used in research and development are initially recorded as an asset and then recognized as an expense as the related goods are delivered or services are performed. Research and development expenses include contract services, consulting, salaries, materials and supplies and overhead.
Income Taxes	Income Taxes The Company accounts for income taxes in accordance with the ASC 740, Income Taxes. The Company provides reserves for potential payments of tax to various tax authorities related to uncertain tax positions. The tax benefits recorded are based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings or positions is “more likely than not” to be realized following resolution of any uncertainty related to the tax benefit, assuming that the matter in question will be raised by the tax authorities. Potential interest and penalties associated with such uncertain tax positions are recorded as a component of income tax expense. At December 31, 2018, the Company had no such accruals.
Stock-Based Compensation	Stock-Based Compensation Stock-based compensation expense is recognized based on the grant-date fair value using the Black-Scholes valuation model. The Company recognizes compensation expense only for those stock-based awards expected to vest after considering expected forfeitures. Cumulative compensation expense is at least equal to the compensation expense for vested awards. Stock-based compensation is recognized on a straight-line basis over the service period of each award. Stock compensation costs have not been capitalized by the Company. The Company accounts for stock-based awards issued to non-employees by recognizing compensation expense based on the fair value of such awards when the services are completed over the vesting period of the award.
Segments	Segments 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 (“CODM”) in making decisions regarding resource allocation and assessing performance. The Company’s chief executive officer is the CODM, and he uses consolidated financial information in determining how to allocate resources and assess performance. The Company has determined that it operates in one segment. All of the Company’s assets are located in the United States.
Change in Accounting Principle	Change in Accounting Principle In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”). ASU 2016-02 is intended to improve financial reporting of leasing transactions by requiring organizations that lease assets to recognize assets and liabilities for the rights and obligations created by leases that extend more than twelve months from the balance sheet date. This accounting update also requires additional disclosures surrounding the amount, timing, and uncertainty of cash flows arising from leases. ASU 2016-02 is effective for financial statements issued for annual and interim periods beginning after December 15, 2018 for public business entities. Early adoption is permitted. The Company elected to early adopt ASU 2016-02 as of January 1, 2018 with retrospective application to January 1, 2016, the beginning of the earliest period to be presented in the Annual Report on Form 10-K for the year ended December 31, 2018. The Company has elected the package of practical expedients permitted in ASC Topic 842. Accordingly, the Company accounted for its existing operating leases as operating leases under the new guidance, without reassessing (a) whether the contracts contain a lease under ASC Topic 842, (b) whether classification of the operating leases would be different in accordance with ASC Topic 842, or (c) whether the unamortized initial direct costs before transition adjustments (as of December 31, 2015) would have met the definition of initial direct costs in ASC Topic 842 at lease commencement. The Company does not allocate the consideration between lease and non-lease components. As a result of the adoption of the new lease accounting guidance, the Company recognized on January 1, 2016 (a) a lease liability of approximately $409,000, which represents the present value of the remaining lease payments, as of the date of adoption, of approximately $540,000, discounted using the Company’s incremental borrowing rate of 9.63%, and (b) a right-of-use asset of approximately $396,000 which represents the lease liability of $409,000 adjusted for accrued rent of approximately $13,000. Adoption of the standard requires the Company to restate certain previously reported results, including the recognition of additional ROU assets and lease obligations for operating leases. This standard did not have a material impact on the Company’s consolidated balance sheets or cash flows from operations and had no impact on the Company’s operating results. The most significant impact was the recognition of ROU assets and lease obligations for operating leases.
Recently Issued Accounting Standards	Recently Issued Accounting Standards In May 2014, the FASB and the International Accounting Standards Board jointly issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASC 606”), which supersedes the revenue recognition requirements in ASC 605 and most industry-specific guidance. The new standard requires that an entity recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods and services. The update also requires additional disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. ASC 606 is effective for public entities for annual and interim periods within those annual periods beginning after December 15, 2017. The Company has adopted ASC 606 as of January 1, 2018 and there was no impact to the Company’s financial statements. The future impact of ASC 606 will be dependent on the nature of the Company’s future revenue contracts and arrangements, if any. In July 2018, the FASB issued ASU No. 2018-10, Codification Improvements to Topic 842, Leases (“ASU 2018-10”). ASU 2018-10 is intended to address questions on the application of ASU No. 2016-02 and to clarify its guidance. ASU 2018-10 is effective for financial statements issued for annual and interim periods beginning after December 15, 2018 for public business entities. For entities who have early adopted ASU No. 2016-02, the guidance is effective upon the issuance of ASU 2018-10. The Company adopted ASU 2018-10 in July 2018 and there was no impact to the Company’s financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820) (“ASU 2018-13”). ASU 2018-13 modifies fair value disclosure requirements, specifically around level transfers and valuation of Level 3 assets and liabilities. ASU 2018-13 is effective for financial statements issued for annual and interim periods beginning after December 15, 2019 for all entities. Early adoption of all or part of ASU No. 2018-13 is permitted. The Company does not expect ASU 2018-13 to have a material impact on its financial statements.

Significant Accounting (Tables)

Significant Accounting (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of Cash, Cash Equivalents and Restricted Cash	As of December 31, 2018, the Company classified $182,000 as restricted cash related to a letter of credit issued as a security deposit in connection with the Company’s lease of its corporate office facilities (Note 13). Cash, cash equivalents and restricted cash consists of the following: December 31, 2017 December 31, 2018 Cash and cash equivalents $ 118,298 $ 89,478 Restricted cash 182 182 Cash, cash equivalents and restricted cash $ 118,480 $ 89,660

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss Per Share of Common Stock	The following table sets forth the computation of basic and diluted net loss per share of common stock (in thousands, except shares and per share data): FOR THE YEAR ENDED DECEMBER 31, 2016 DECEMBER 31, 2017 DECEMBER 31, 2018 Net loss and comprehensive loss $ (24,377 ) $ (23,817 ) $ (29,443 ) Weighted—average common shares outstanding, basic and diluted 974,660 2,962,859 18,556,126 Net loss per share, basic and diluted $ (25.01 ) $ (8.04 ) $ (1.59 )
Schedule of Antidilutive Securities Excluded from Computation of Net Loss Per Share	The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because their inclusion would be anti-dilutive (in common stock equivalent shares): FOR THE YEAR ENDED DECEMBER 31, 2016 DECEMBER 31, 2017 DECEMBER 31, 2018 Convertible preferred stock, on an as-converted basis 10,126,771 — — Stock options to purchase common stock 444,410 1,195,495 1,588,306 Unvested restricted stock 5,053 488 — 10,576,234 1,195,983 1,588,306

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Schedule of Purchased Property and Equipment	Purchased property and equipment consist of the following as of December 31, (dollars in thousands): ESTIMATED USEFUL LIFE 2017 2018 Office equipment 5 years $ 10 $ 10 Office furniture 7 years 116 116 Computer equipment 3 years 8 8 Leasehold improvements Life of lease 95 95 229 229 Less: Accumulated depreciation (26 ) (65 ) Property and equipment, net $ 203 $ 164
Schedule of Leased Property and Equipment	Leased property and equipment consist of the following as of December 31, (dollars in thousands): ESTIMATED USEFUL LIFE 2017 2018 Right-of-use lease assets - operating (Type B) Lease term $ 2,014 $ 2,041 Less: Accumulated amortization (241 ) (535 ) Right-of-use lease assets - operating (Type B), net $ 1,773 $ 1,506

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2018
Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses at December 31 consist of (in thousands): 2017 2018 Contract research and development $ 1,610 $ 1,492 Employee compensation and related costs 871 860 Consulting and professional service fees 287 356 State taxes 192 165 Financing related costs 90 — Other 13 49 Total accrued expenses $ 3,063 $ 2,922

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Components of Net Deferred Tax Asset	. Components of the net deferred tax asset at December 31, 2017 and 2018 are as follows (in thousands): 2017 2018 Federal net operating loss carryforwards $ 3,672 $ 6,189 State net operating loss carryforwards 1,000 1,670 Research and development tax credits 1,087 1,824 Accrued liabilities 278 257 Stock-based compensation — 510 Depreciation and amortization 10 10 Capitalized research and development costs 10,452 14,227 Other 153 20 16,652 24,707 Valuation allowance (16,652 ) (24,707 ) Net deferred tax asset $ — $ —
Schedule of Reconciliation of Income Tax (Expense) Benefit	A reconciliation of income tax (expense) benefit at the statutory federal income tax rate and income taxes as reflected in the consolidated financial statements at December 31, 2016, 2017 and 2018 are as follows: 2016 2017 2018 Federal income tax at statutory rate 34.00 % 34.00 % 21.00 % State income tax, net of federal benefit 3.46 % 4.85 % 5.45 % Research and development credits 1.46 % 2.07 % 2.35 % Book compensation related to stock options (0.93 )% (0.16 )% (0.61 )% Change in income tax rate (0.18 )% — (0.38 )% Effect on Tax Cuts & Job Acts rate reduction — (29.15 )% — Non-cash interest (9.08 )% — — Other (0.40 )% (0.51 )% (0.45 )% Increase in valuation allowance (28.33 )% (11.10 )% (27.36 )% Effective tax rate — % — % — %
Schedule of Reconciliation of Uncertain Tax Benefits	A reconciliation of the beginning and ending amount of uncertain tax benefits is as follows (in thousands): 2016 2017 2018 Beginning uncertain tax benefits $ 104 $ 178 $ 284 Prior year - increases — — 46 Current year - decreases — (19 ) — Current year - increases 74 125 146 Ending uncertain tax benefits $ 178 $ 284 $ 476

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock-Based Compensation Expense	Total stock-based compensation expense, including the effect of forfeitures, recorded in research and development and general and administrative expenses, respectively, for employees, directors and non-employees during the years ended December 31, 2016, 2017 and 2018 is as follows (in thousands): 2016 2017 2018 Research and development $ 175 $ 159 $ 567 General and administrative 701 788 1,568 Total $ 876 $ 947 $ 2,135
Schedule of Estimated Fair Value of Options Valuation Assumptions	The fair value of options at date of grant was estimated using the Black-Scholes option-pricing model with the following assumptions: 2016 2017 2018 Risk-free interest rate 1.08%—1.58% 1.89%—2.24% 2.42%—2.86% Expected dividend yield 0% 0% 0% Expected life 6.0—6.4 years 5.0—7.0 years 5.5—7.0 years Expected volatility 86%—93% 78%—88% 76%—86% Weighted-average grant date fair value $ 6.51 $ 3.62 $ 7.57
Summary of Information about Stock Option Activity	The following table summarizes information about stock option activity during 2017 and 2018 (in thousands, except share and per share data): NUMBER OF SHARES WEIGHTED- AVERAGE EXERCISE PRICE WEIGHTED- AVERAGE REMAINING CONTRACTUAL TERM AGGREGATE INTRINSIC VALUE Outstanding, December 31, 2016 444,410 $ 6.99 Granted 888,497 4.82 Exercised (39,722 ) 2.57 Forfeited (97,690 ) 8.88 Outstanding, December 31, 2017 1,195,495 $ 5.38 Granted 939,296 10.54 Exercised (34,561 ) 1.69 Forfeited (511,924 ) 10.85 Outstanding, December 31, 2018 1,588,306 $ 6.75 8.44 $ 86 Vested and exercisable, December 31, 2018 572,376 $ 5.23 7.71 $ 86 Vested and expected to vest, December 31, 2018 1,391,103 $ 6.71 8.37 $ 86

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Summary of Money Market Funds	The Company’s cash equivalents are classified within Level 1 of the fair value hierarchy. The following table summarizes the Company’s money market funds as of December 31, 2018 (in thousands): TOTAL Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash equivalents: Cash equivalents $ 75,306 $ 75,306 $ — $ — Total cash equivalents $ 75,306 $ 75,306 $ — $ —

Term Loan (Tables)

Term Loan (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of Total Term Loan and Unamortized Debt Discount	Total term loan and unamortized debt discount balances are as follows (in thousands): DECEMBER 31, 2017 DECEMBER 31, 2018 Face value $ 10,000 $ 10,000 Less: discount (581 ) (363 ) Total $ 9,419 $ 9,637 Less: current portion (314 ) (2,811 ) Long-term portion $ 9,105 $ 6,826
Schedule of Future Principal Payments Due	As of December 31, 2018, future principal payments due under the 2017 Loan Agreement are as follows (in thousands): Year ended: December 31, 2019 $ 2,917 December 31, 2020 5,000 December 31, 2021 2,083 Total minimum principal payments $ 10,000

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Schedule of Maturity Analysis of Annual Undiscounted Cash Flows of Operating Lease Liabilities	The following is a maturity analysis of the annual undiscounted cash flows reconciled to the carrying value of the operating lease liabilities as of December 31, 2018 (in thousands): Year ended: December 31, 2019 $ 513 December 31, 2020 528 December 31, 2021 537 December 31, 2022 496 Total minimum lease payments 2,074 Less imputed interest (368 ) Total $ 1,706
Schedule of Lease Cost and Other Information of Operating Lease Liabilities	2017 2018 Lease cost: Operating lease cost $ 290 $ 483 Short-term lease cost 3 8 Sublease income — (39 ) Total lease cost $ 293 $ 452 Other information Cash paid for amounts included in the measurement of liabilities $ 92 $ 429 Operating cash flows from operating leases $ 141 $ 49 Weighted-average remaining lease term - operating leases 4.9 years 3.9 years Weighted-average discount rate - operating leases 10.1 % 10.1 %

Description of Business and B_2

Description of Business and Basis of Presentation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Nov. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Description Of Business And Basis Of Presentation [Line Items]
Cash, cash equivalents and restricted cash		$ 89,660	$ 118,480	$ 39,282	$ 1,573
Working capital		85,200
Net loss		29,443	23,817	24,377
Net cash flows used in operating activities		28,812	$ 22,682	$ 15,455
Cash balance		$ 89,500
IPO
Description Of Business And Basis Of Presentation [Line Items]
Common stock issued and sold	7,294,968
Public offering price of common stock	$ 14
Proceeds from initial offering after deducting underwriting discounts and commissions	$ 92,700

Significant Accounting Policies

Significant Accounting Policies - Additional Information (Details)	Jan. 01, 2016USD ($)	Dec. 31, 2018USD ($)Segment	Dec. 31, 2017USD ($)	Jan. 21, 2016USD ($)
Schedule Of Accounting Policies [Line Items]
Restricted cash		$ 182,000	$ 182,000
Potential interest and penalties associated with uncertain tax positions, accruals		$ 0
Number of operating segments | Segment		1
Lease liability		$ 1,706,000
Remaining lease payments		49,000	141,000
Right-of-use asset		1,506,000	$ 1,773,000
Accounting Standards Update 2016-02
Schedule Of Accounting Policies [Line Items]
Lease liability				$ 409,000
Remaining lease payments	$ 540,000
Incremental borrowing rate	9.63%
Right-of-use asset				396,000
Accrued rent				$ 13,000
Letter of Credit
Schedule Of Accounting Policies [Line Items]
Restricted cash		$ 182,000

Significant Accounting Polici_2

Significant Accounting Policies - Schedule of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 89,478	$ 118,298
Restricted cash	182	182
Cash, cash equivalents and restricted cash	$ 89,660	$ 118,480	$ 39,282	$ 1,573

Net Loss per Share - Computatio

Net Loss per Share - Computation of Basic and Diluted Net Loss Per Share of Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net loss and comprehensive loss	$ (29,443)	$ (23,817)	$ (24,377)
Weighted—average common shares outstanding, basic and diluted	18,556,126	2,962,859	974,660
Net loss per share, basic and diluted	$ (1.59)	$ (8.04)	$ (25.01)

Net Loss per Share - Schedule o

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Computation of Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	1,588,306	1,195,983	10,576,234
Convertible Preferred Stock, on an if-converted Basis
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share			10,126,771
Stock Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	1,588,306	1,195,495	444,410
Unvested Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share		488	5,053

Property and Equipment - Schedu

Property and Equipment - Schedule of Purchased Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
ESTIMATED USEFUL LIFE	Lease term
Property and equipment, gross	$ 229	$ 229
Less: Accumulated depreciation	(65)	(26)
Property and equipment, net	$ 164	203
Office Equipment
Property Plant And Equipment [Line Items]
ESTIMATED USEFUL LIFE	5 years
Property and equipment, gross	$ 10	10
Office Furniture
Property Plant And Equipment [Line Items]
ESTIMATED USEFUL LIFE	7 years
Property and equipment, gross	$ 116	116
Computer Equipment
Property Plant And Equipment [Line Items]
ESTIMATED USEFUL LIFE	3 years
Property and equipment, gross	$ 8	8
Leasehold Improvements
Property Plant And Equipment [Line Items]
ESTIMATED USEFUL LIFE	Life of lease
Property and equipment, gross	$ 95	$ 95

Property and Equipment - Additi

Property and Equipment - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation expense	$ 38,000	$ 17,000	$ 5,000
Amortization expense	$ 91,000	$ 150,000	$ 294,000

Property and Equipment - Sche_2

Property and Equipment - Schedule of Leased Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Right-of-use lease assets - operating (Type B)	$ 2,041	$ 2,014
Less: Accumulated amortization	(535)	(241)
Right-of-use lease assets - operating (Type B), net	$ 1,506	$ 1,773
ESTIMATED USEFUL LIFE	Lease term

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accruals [Abstract]
Contract research and development	$ 1,492	$ 1,610
Employee compensation and related costs	860	871
Consulting and professional service fees	356	287
State taxes	165	192
Financing related costs		90
Other	49	13
Total accrued expenses	$ 2,922	$ 3,063

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Net income tax provision or benefit	$ 0	$ 0
Federal net operating loss carryforwards expiration year	2037
State net operating loss carryforwards expiration year	2038
Increase in valuation allowance	$ 8,100,000
Research and development credit carryforwards	$ 1,824,000	$ 1,087,000
Federal and state research and development credit carryforwards expiration year	2038
Federal income tax at statutory rate	21.00%	34.00%	34.00%
Provisional charge for the revaluation of the company’s deferred tax assets	$ 6,900,000
Federal
Income Taxes [Line Items]
Net operating loss carryforwards	12,000,000
Research and development credit carryforwards	1,500,000
Federal | 2037
Income Taxes [Line Items]
Net operating loss carryforwards	17,500,000
State
Income Taxes [Line Items]
Net operating loss carryforwards	100,000
Research and development credit carryforwards	400,000
State | 2038
Income Taxes [Line Items]
Net operating loss carryforwards	$ 26,400,000

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Net Deferred Tax Asset (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal net operating loss carryforwards	$ 6,189	$ 3,672
State net operating loss carryforwards	1,670	1,000
Research and development tax credits	1,824	1,087
Accrued liabilities	257	278
Stock-based compensation	510
Depreciation and amortization	10	10
Capitalized research and development costs	14,227	10,452
Other	20	153
Gross deferred tax asset	24,707	16,652
Valuation allowance	$ (24,707)	$ (16,652)

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Income Tax (Expense) Benefit (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal income tax at statutory rate	21.00%	34.00%	34.00%
State income tax, net of federal benefit	5.45%	4.85%	3.46%
Research and development credits	2.35%	2.07%	1.46%
Book compensation related to stock options	(0.61%)	(0.16%)	(0.93%)
Change in income tax rate	(0.38%)		(0.18%)
Effect on tax cuts & job acts rate reduction		(29.15%)
Non-cash interest			(9.08%)
Other	(0.45%)	(0.51%)	(0.40%)
Increase in valuation allowance	(27.36%)	(11.10%)	(28.33%)

Income Taxes - Schedule of Re_2

Income Taxes - Schedule of Reconciliation of Uncertain Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Beginning uncertain tax benefits	$ 284	$ 178	$ 104
Prior year - increases	46
Current year - decreases		(19)
Current year - increases	146	125	74
Ending uncertain tax benefits	$ 476	$ 284	$ 178

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($)	Jan. 01, 2019	Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014	Nov. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional options granted			939,296	888,497
Stock option, number of shares outstanding			1,588,306	1,195,495	444,410
Cash received upon exercise of stock options			$ 58,000	$ 102,000	$ 1,000
Intrinsic value of options exercised			270,000	330,000	64,000
Cash received upon early exercise of options				$ 1,000
Repurchase of unvested restricted stock, shares				1,871
Repurchase of unvested restricted stock, value				$ 3,000
Deposit liability				1,000
Unrecognized compensation expense, net of forfeitures			$ 3,500,000
Recognition period (in years)			2 years 7 months 13 days
Number of options vested			102,661
Weighted average exercise price for vested options			$ 7.33
Incremental stock-based compensation expense			$ 117,000
Compensation expense recognized			$ 2,135,000	$ 947,000	876,000
2014 Restricted Stock Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option vesting term						3 years
Number of common stock restricted						821,512
Percentage of shares to vest						50.00%
Common stock fair value						$ 1.66
Compensation expense recognized					$ 400,000
2014 Stock Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option and incentive plan expiration date			Nov. 30, 2017
Additional options granted			0
Stock option, number of shares outstanding			946,609
Stock option exercised prior to vesting				86
2017 Stock Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option and incentive plan expiration date			Oct. 31, 2027
Stock option, number of shares outstanding			641,697
Stock option and incentive plan effective date			Nov. 30, 2017
Number of common stock initially reserved for issuance							1,430,000
Percentage of shares issued on common stock outstanding			4.00%
Shares issuable increased during the period		741,389
Stock options, available for issuance			1,686,893
Term of option granted			10 years
Stock option vesting term			4 years
2017 Stock Plan | Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issuable increased during the period	742,772

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Total Stock Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 2,135	$ 947	$ 876
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	567	159	175
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 1,568	$ 788	$ 701

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Estimated Fair Value of Options Valuation Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	2.42%	1.89%	1.08%
Risk-free interest rate, maximum	2.86%	2.24%	1.58%
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility, minimum	76.00%	78.00%	86.00%
Expected volatility, maximum	86.00%	88.00%	93.00%
Weighted-average grant date fair value	$ 7.57	$ 3.62	$ 6.51
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected life	5 years 6 months	5 years	6 years
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected life	7 years	7 years	6 years 4 months 24 days

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Information about Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
NUMBER OF SHARES
Number of shares outstanding, beginning balance	1,195,495	444,410
Additional options granted	939,296	888,497
Number of shares exercised	(34,561)	(39,722)
Number of shares forfeited	(511,924)	(97,690)
Number of shares outstanding, ending balance	1,588,306	1,195,495
Vested and exercisable, ending balance	572,376
Vested and expected to vest, ending balance	1,391,103
WEIGHTED-AVERAGE EXERCISE PRICE
Weighted average exercise prices, outstanding beginning balance	$ 5.38	$ 6.99
Weighted average exercise prices, granted	10.54	4.82
Weighted average exercise prices, exercised	1.69	2.57
Weighted average exercise prices, forfeited	10.85	8.88
Weighted average exercise prices, outstanding ending balance	6.75	$ 5.38
Weighted average exercise prices, vested and exercisable ending balance	5.23
Weighted average exercise prices, vested and expected to vest ending balance	$ 6.71
WEIGHTED-AVERAGE REMAINING CONTRACTUAL TERM
Weighted average remaining contractual term, outstanding	8 years 5 months 8 days
Weighted average remaining contractual term, vested and exercisable	7 years 8 months 15 days
Weighted average remaining contractual term, vested and expected to vest	8 years 4 months 13 days
AGGREGATE INTRINSIC VALUE
Aggregate intrinsic value, outstanding ending balance	$ 86
Aggregate intrinsic value, vested and exercisable ending balance	86
Aggregate intrinsic value, vested and expected to vest ending balance	$ 86

Fair Value of Financial Instr_3

Fair Value of Financial Instruments - Summary of Money Market Funds (Details) - Fair Value, Measurements, Nonrecurring $ in Thousands	Dec. 31, 2018USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Cash equivalents	$ 75,306
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Cash equivalents	75,306
Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Cash equivalents	75,306
Quoted Prices in Active Markets (Level 1) [Member] | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Cash equivalents	$ 75,306

Term Loan - Additional Informat

Term Loan - Additional Information (Details) - Solar Capital Ltd. and Silicon Valley Bank - USD ($)	1 Months Ended		12 Months Ended
	Nov. 30, 2018	May 31, 2017	Dec. 31, 2018	Dec. 31, 2017
2017 Loan Agreement
Debt Instrument [Line Items]
Loan and security agreement, amount		$ 10,000,000
Loan agreement, maturity date		May 1, 2021
Exit fee, percentage		4.00%
Exit fee		$ 400,000
Derivative liability recorded against exit fee obligation		392,000
Debt issuance costs including exit payment obligation		$ 727,000
Unpaid borrowings			$ 10,000,000	$ 10,000,000
Amortization of debt discount			$ 280,000	146,000
Prepayment premium thereafter			1.00%
Final payment fee			$ 250,000
Amortization of final payment fee			$ 94,000	$ 52,000
Increase in interest rate on default			5.00%
2017 Loan Agreement | Libor
Debt Instrument [Line Items]
Basis spread on variable base rate		8.45%	10.82888%
First and Second Amendment to Loan and Security Agreement
Debt Instrument [Line Items]
Debt issuance costs			$ 62,000
First Amendment to Loan and Security Agreement
Debt Instrument [Line Items]
Final payment fee	$ 325,000

Term Loan - Schedule of Total T

Term Loan - Schedule of Total Term Loan and Unamortized Debt Discount (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Less: current portion	$ (2,811)	$ (314)
Long-term portion	6,826	9,105
2017 Loan Agreement | Solar Capital Ltd. and Silicon Valley Bank
Debt Instrument [Line Items]
Face value	10,000	10,000
Less: discount	(363)	(581)
Total	9,637	9,419
Less: current portion	(2,811)	(314)
Long-term portion	$ 6,826	$ 9,105

Term Loan - Schedule of Future

Term Loan - Schedule of Future Principal Payments Due (Details) - 2017 Loan Agreement - Solar Capital Ltd. and Silicon Valley Bank - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
December 31, 2019	$ 2,917
December 31, 2020	5,000
December 31, 2021	2,083
Total minimum principal payments	$ 10,000	$ 10,000

Convertible Notes - Additional

Convertible Notes - Additional Information (Details) - USD ($)	Dec. 22, 2016	Oct. 20, 2016	Oct. 19, 2016	Aug. 22, 2016	Jul. 07, 2016	Jan. 21, 2016	Sep. 30, 2016	Feb. 29, 2016	Jun. 30, 2016	Dec. 31, 2016	Jul. 01, 2016
Convertible Notes [Line Items]
Proceeds from issuance of convertible notes										$ 12,600,000
Beneficial conversion feature of convertible notes										4,653,000
Reduction in additional paid in capital, due to conversion feature										4,653,000
January 2016 Convertible Note Purchase Agreement
Convertible Notes [Line Items]
Debt instrument, authorized amount to issue						$ 7,500,000					$ 8,200,000
Proceeds from issuance of convertible notes					$ 600,000
Debt instrument, interest rate percentage						8.00%
Debt instrument, maturity date						Jan. 21, 2017
Minimum gross proceeds from qualified equity financing required for conversion						$ 15,000,000
Minimum gross proceeds from qualified initial public offering required for conversion						$ 35,000,000
Conversion price as a percentage of common stock to be sold						80.00%
Amount of accretion to redemption value recognized				$ 900,000		$ 9,900,000
Beneficial conversion feature of convertible notes				2,100,000		$ 4,700,000
Debt Instrument transaction costs allocated as discount										38,000
Convertible debt, carrying value				8,000,000
Debt instrument, accrued interest				200,000
Unamortized beneficial conversion feature				2,600,000
Convertible debt, fair value of new debt				8,400,000
Reduction in additional paid in capital, due to conversion feature				300,000
January 2016 Convertible Note Purchase Agreement | Interest Expense
Convertible Notes [Line Items]
Loss on extinguishment of debt				$ 1,800,000
January 2016 Convertible Note Purchase Agreement | Series A Convertible Preferred Stock
Convertible Notes [Line Items]
Debt instrument, conversion price				$ 1		$ 1
Convertible notes converted into convertible preferred stock				8,183,792
January 2016 Convertible Note Purchase Agreement | Minimum
Convertible Notes [Line Items]
Common stock sale price per share						$ 21.55
January 2016 Convertible Note Purchase Agreement | Initial Closing
Convertible Notes [Line Items]
Proceeds from issuance of convertible notes						$ 3,000,000
January 2016 Convertible Note Purchase Agreement | Subsequent Closing
Convertible Notes [Line Items]
Proceeds from issuance of convertible notes								$ 700,000
January 2016 Convertible Note Purchase Agreement | Second Closing
Convertible Notes [Line Items]
Proceeds from issuance of convertible notes									$ 3,700,000
August 22, 2016 Convertible Promissory Notes
Convertible Notes [Line Items]
Debt instrument, authorized amount to issue				$ 10,000,000
Debt instrument, interest rate percentage				8.00%
Debt instrument, maturity date				Aug. 22, 2017
Minimum gross proceeds from qualified equity financing required for conversion				$ 25,000,000
Minimum gross proceeds from qualified initial public offering required for conversion				$ 40,000,000
Conversion price as a percentage of common stock to be sold		80.00%	90.00%	80.00%
Amount of accretion to redemption value recognized	$ 356,000			$ 5,800,000
Beneficial conversion feature of convertible notes	60,000			$ 200,000
Debt Instrument transaction costs allocated as discount										$ 28,000
Convertible debt, carrying value	4,700,000
Debt instrument, accrued interest	120,000
Unamortized beneficial conversion feature	126,000
Convertible debt, fair value of new debt	6,000,000
August 22, 2016 Convertible Promissory Notes | Interest Expense
Convertible Notes [Line Items]
Loss on extinguishment of debt	$ 1,000,000
August 22, 2016 Convertible Promissory Notes | Series A Convertible Preferred Stock
Convertible Notes [Line Items]
Debt instrument, conversion price				$ 1
August 22, 2016 Convertible Promissory Notes | Series B Convertible Preferred Stock
Convertible Notes [Line Items]
Convertible notes converted into convertible preferred stock	5,962,784
August 22, 2016 Convertible Promissory Notes | Minimum
Convertible Notes [Line Items]
Common stock sale price per share				$ 21.55
August 22, 2016 Convertible Promissory Notes | Initial Closing
Convertible Notes [Line Items]
Proceeds from issuance of convertible notes				$ 4,000,000
August 22, 2016 Convertible Promissory Notes | Subsequent Closing
Convertible Notes [Line Items]
Proceeds from issuance of convertible notes							$ 700,000

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Details) - USD ($)	Nov. 16, 2017	Dec. 22, 2016	Aug. 22, 2016	Apr. 08, 2015	Oct. 14, 2014	Mar. 24, 2014	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common Stock
Temporary Equity [Line Items]
Convertible preferred stock, issuance costs								$ 2,300,000
Automatic conversion of preferred stock, Shares	10,126,771							10,126,771
Series A Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, reserved for future issuance, shares						16,000,000
Convertible preferred stock, par value				$ 0.0001	$ 0.0001	$ 0.0001
Convertible preferred stock, purchase price				$ 1	$ 1	$ 1
Convertible notes converted into convertible preferred stock						1,315,679
Convertible preferred stock, value new issues				$ 8,000,000		$ 8,000,000
Convertible preferred stock, shares new issues				8,000,000	250,000	8,000,000
Convertible preferred stock, issuance costs				$ 7,000		$ 200,000
Convertible preferred stock, net cash received				$ 7,993,000	$ 250,000	$ 7,800,000
Convertible preferred stock, value of additional shares reserved for future issuance					$ 250,000
Automatic conversion of preferred stock, Shares			8,183,792					(25,749,471)
Convertible preferred stock, dividend rate	6.00%
Convertible preferred stock, conversion price							$ 7.180193
Series B Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, reserved for future issuance, shares		46,962,784
Convertible preferred stock, par value		$ 0.0001
Convertible preferred stock, purchase price		$ 1
Convertible preferred stock, value new issues		$ 41,000,000
Convertible preferred stock, shares new issues		41,000,000
Convertible preferred stock, issuance costs		$ 400,000						$ 8,000	$ 362,000
Convertible preferred stock, net cash received		$ 40,600,000
Automatic conversion of preferred stock, Shares			5,962,784					(46,962,784)
Convertible preferred stock, dividend rate								6.00%
Convertible preferred stock, conversion price							$ 7.180193
Series B Convertible Preferred Stock | Maximum
Temporary Equity [Line Items]
Convertible preferred stock, dividend rate							20.00%

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Reverse stock split	1-for-7.180193
Common stock, par value	$ 0.0001
Issuance of common stock upon exercise of stock options, Shares	34,561	39,722
Repurchase of unvested restricted stock during period, shares		1,871
Common stock, shares outstanding	18,569,289	18,534,240
Reserved shares of common stock for exercise of outstanding options to purchase common stock	1,588,306	1,195,495	444,410

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Feb. 28, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commitments And Contingencies [Line Items]
Operating leases rent expense		$ 500,000	$ 300,000	$ 200,000
Restricted cash		182,000	182,000
Right-of-use asset		1,506,000	1,773,000
Short-term lease liability		353,000	$ 242,000
Operating sublease term	3 years
Short-term Leases Recognized on Straight-line Basis
Commitments And Contingencies [Line Items]
Right-of-use asset		0
Short-term lease liability		0
Letter of Credit
Commitments And Contingencies [Line Items]
Restricted cash		$ 182,000
Lexington, Massachusetts
Commitments And Contingencies [Line Items]
Operating leases expiration date		Dec. 31, 2022
Burlington, Massachusetts
Commitments And Contingencies [Line Items]
Operating leases expiration date		Nov. 30, 2022
Burlington, Massachusetts | Letter of Credit | Lease Agreement Require Standby Letter of Credit
Commitments And Contingencies [Line Items]
Restricted cash		$ 182,000

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Maturity Analysis of Annual Undiscounted Cash Flows of Operating Lease Liabilities (Details) $ in Thousands	Dec. 31, 2018USD ($)
Year ended:
December 31, 2019	$ 513
December 31, 2020	528
December 31, 2021	537
December 31, 2022	496
Total minimum lease payments	2,074
Less imputed interest	(368)
Lease liability	$ 1,706

Commitments and Contingencies_3

Commitments and Contingencies - Schedule of Lease Cost and Other Information of Operating Lease Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Lease cost:
Operating lease cost	$ 483	$ 290
Short-term lease cost	8	3
Sublease income	(39)	0
Total lease cost	452	293
Other information
Cash paid for amounts included in the measurement of liabilities	429	92
Operating cash flows from operating leases	$ 49	$ 141
Weighted-average remaining lease term - operating leases	3 years 10 months 24 days	4 years 10 months 24 days
Weighted-average discount rate - operating leases	10.10%	10.10%

401(k) Savings Plan - Additiona

401(k) Savings Plan - Additional Information (Details) - USD ($)	Jul. 31, 2014	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Defined Benefit Plan, Tax Status [Extensible List]		us-gaap:QualifiedPlanMember	us-gaap:QualifiedPlanMember	us-gaap:QualifiedPlanMember
Compensation expense for employer match contribution		$ 225,000	$ 165,000	$ 122,000
First 3% of Deferred Compensation
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution percentage	100.00%
Employer matching contribution percentage of employees contributions of deferred compensation	3.00%
Defined Benefit Plan, Tax Status [Extensible List]	us-gaap:QualifiedPlanMember
Next 2% of Deferred Compensation
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution percentage	50.00%
Employer matching contribution percentage of employees contributions of deferred compensation	2.00%
Defined Benefit Plan, Tax Status [Extensible List]	us-gaap:QualifiedPlanMember

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2019USD ($)Employee	Dec. 31, 2019USD ($)
Scenario, Forecast
Subsequent Event [Line Items]
Restructuring charges		$ 1.4
Subsequent Event
Subsequent Event [Line Items]
Approximate percentage of workforce reductions	43.00%
Number of positions remaining | Employee	13
West Pharmaceutical Services Inc | Development Agreement | Subsequent Event
Subsequent Event [Line Items]
One-time upfront payment	$ 1.7