Kineta (KANT) 10-K 14 Mar 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Mar. 12, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	PTI
Entity Registrant Name	PROTEOSTASIS THERAPEUTICS, INC.
Entity Central Index Key	1,445,283
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Common Stock, Shares Outstanding		34,482,574
Entity Public Float			$ 85,266,450

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 29,724	$ 18,613
Short-term investments	44,738	66,897
Restricted cash	294
Accounts receivable		668
Prepaids and other current assets	1,377	4,059
Total current assets	76,133	90,237
Property and equipment, net	16,567	541
Other assets	33	68
Restricted cash	1,656	294
Total assets	94,389	91,140
Current liabilities:
Accounts payable	2,098	2,021
Accrued expenses	6,120	4,328
Deferred revenue	1,108	2,204
Deferred rent	87	201
Other liabilities	902
Total current liabilities	10,315	8,754
Deferred revenue, net of current portion		752
Deferred rent, net of current portion		87
Derivative liability	25	91
Other liabilities, net of current portion	15,315
Total liabilities	25,655	9,684
Commitments and contingencies (Note 17)
Stockholders’ equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized as of December 31, 2017 and 2016, respectively; no shares issued and outstanding as of December 31, 2017 and 2016, respectively
Common stock, $0.001 par value; 125,000,000 shares authorized as of December 31, 2017 and 2016, respectively; 34,416,088 and 25,000,734 shares issued and outstanding as of December 31, 2017 and 2016, respectively	35	26
Additional paid-in capital	285,583	238,902
Accumulated other comprehensive loss	(2)	(22)
Accumulated deficit	(216,882)	(157,450)
Total stockholders’ equity	68,734	81,456
Total liabilities and stockholders’ equity	$ 94,389	$ 91,140

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	34,416,088	25,000,734
Common stock, shares outstanding	34,416,088	25,000,734

Statements of Operations

Statements of Operations - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue	$ 1,622	$ 1,551	$ 1,147	$ 1,021	$ 4,060	$ 1,715	$ 1,451	$ 1,158	$ 5,341	$ 8,384	$ 4,312
Operating expenses:
Research and development	12,282	12,894	15,370	13,108	10,461	9,218	7,404	6,876	53,654	33,959	22,524
General and administrative	2,847	2,741	2,902	3,170	3,198	3,266	3,115	2,301	11,660	11,880	6,322
Total operating expenses	15,129	15,635	18,272	16,278	13,659	12,484	10,519	9,177	65,314	45,839	28,846
Loss from operations	(13,507)	(14,084)	(17,125)	(15,257)	(9,599)	(10,769)	(9,068)	(8,019)	(59,973)	(37,455)	(24,534)
Interest income	126	155	169	191	190	36	20		641	246
Interest expense											(599)
Other income (expense), net	(43)	(25)	(2)	(30)	(8)	(38)	(5)	28	(100)	(23)	93
Net loss	(13,424)	(13,954)	(16,958)	(15,096)	(9,417)	(10,771)	(9,053)	(7,991)	(59,432)	(37,232)	(25,040)
Accruing dividends on preferred stock								(1,378)		(1,378)	(9,724)
Net loss attributable to common stockholders	$ (13,424)	$ (13,954)	$ (16,958)	$ (15,096)	$ (9,417)	$ (10,771)	$ (9,053)	$ (9,369)	$ (59,432)	$ (38,610)	$ (23,309)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.51)	$ (0.56)	$ (0.68)	$ (0.60)	$ (0.38)	$ (0.54)	$ (0.47)	$ (0.87)	$ (2.34)	$ (2.06)	$ (42.14)
Weighted average common shares outstanding—basic and diluted	26,465,521	25,093,344	25,040,131	25,020,337	24,975,010	20,073,685	19,139,183	10,766,722	25,407,943	18,759,369	553,162
Series A Preferred Stock [Member]
Operating expenses:
Modifications of preferred stock											$ 11,481
Series B Preferred Stock [Member]
Operating expenses:
Modifications of preferred stock											$ (26)

Statements of Comprehensive Los

Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (59,432)	$ (37,232)	$ (25,040)
Other comprehensive loss:
Unrealized gain (loss) on investments	20	(22)
Comprehensive loss	$ (59,412)	$ (37,254)	$ (25,040)

Statements of Convertible Prefe

Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Convertible Preferred Stock [Member]	Series B Convertible Preferred Stock [Member]	Initial Public Offering [Member]	Follow-on Public Offering [Member]	Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member]Series A Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member]Series B Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member]Series A Convertible Preferred Stock [Member]	Common Stock [Member]Initial Public Offering [Member]	Common Stock [Member]Follow-on Public Offering [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]Series A Convertible Preferred Stock [Member]	Additional Paid-in Capital [Member]Series B Convertible Preferred Stock [Member]	Additional Paid-in Capital [Member]Initial Public Offering [Member]	Additional Paid-in Capital [Member]Follow-on Public Offering [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2014	$ (95,084)					$ 86,859			$ 1				$ 93						$ (95,178)
Beginning balance, shares at Dec. 31, 2014						75,463,000			520,305
Exercise of stock options	95												95
Exercise of stock options, shares									50,832
Stock-based compensation expense	472												472
Issuance of common stock upon completion of public offering, net of offering costs								$ 21,090
Issuance of common stock upon completion of public offering, net of offering costs, shares								17,107,303
Conversion of convertible preferred stock to common stock								$ 15,798
Conversion of convertible preferred stock to common stock, shares								12,284,466
Modifications of preferred stock		$ 11,481	$ (26)				$ (11,481)	$ 26						$ 11,481	$ (26)
Net loss	(25,040)																		(25,040)
Ending balance at Dec. 31, 2015	(108,102)					$ 112,292			$ 1				12,115						(120,218)
Ending balance, shares at Dec. 31, 2015						104,854,769			571,137
Exercise of stock options	268												268
Exercise of stock options, shares									101,816
Stock-based compensation expense	1,805												1,805
Issuance of common stock upon completion of public offering, net of offering costs				$ 42,529	$ 69,469						$ 6	$ 6				$ 42,523	$ 69,463
Issuance of common stock upon completion of public offering, net of offering costs, shares											6,250,000	5,750,000
Conversion of convertible preferred stock to common stock	112,292					$ (112,292)			$ 10				112,282
Conversion of convertible preferred stock to common stock, shares						(104,854,769)			9,699,600
Issuance of common stock to settle accruing dividends										$ 3				$ (3)
Issuance of common stock to settle accruing dividends, Shares										2,590,742
Issuance of common stock for partial payment of accrued bonus	62												62
Issuance of common stock for partial payment of accrued bonus, shares									10,218
Conversion of preferred stock warrant to common stock warrant	28												28
Issuance of common stock for payment of consulting services	359												359
Issuance of common stock for payment of consulting services, shares									27,221
Other comprehensive loss	(22)																	$ (22)
Net loss	(37,232)																		(37,232)
Ending balance at Dec. 31, 2016	$ 81,456								$ 26				238,902					(22)	(157,450)
Ending balance, shares at Dec. 31, 2016	25,000,734								25,000,734
Exercise of stock options	$ 31												31
Exercise of stock options, shares	11,714								11,714
Stock-based compensation expense	$ 2,965												2,965
Issuance of common stock pursuant to employee stock purchase plan	43												43
Issuance of common stock pursuant to employee stock purchase plan, shares									10,829
Issuance of common stock upon exercise of common stock warrant, shares									4,349
Issuance of common stock upon completion of public offering, net of offering costs					$ 42,884							$ 9					$ 42,875
Issuance of common stock upon completion of public offering, net of offering costs, shares												9,200,000
Issuance of common stock for payment of consulting services	767												767
Issuance of common stock for payment of consulting services, shares									188,462
Other comprehensive loss	20																	20
Net loss	(59,432)																		(59,432)
Ending balance at Dec. 31, 2017	$ 68,734								$ 35				$ 285,583					$ (2)	$ (216,882)
Ending balance, shares at Dec. 31, 2017	34,416,088								34,416,088

Statements of Convertible Pref7

Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($)	Feb. 11, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Net of issuance cost	$ 4,000,000	$ 800,000
Series B Convertible Preferred Stock [Member]
Net of issuance cost			$ 900,000	$ 910,000

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (59,432)	$ (37,232)	$ (25,040)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	235	306	272
Premium on short-term investments	(27)	(184)
Amortization of premium on short-term investments	205	77
Non-cash rent expense	(127)	(181)	(63)
Non-cash interest expense			599
Stock-based compensation expense	2,965	1,805	472
Stock issued for consulting services	767	359
Change in fair value of derivative liability	(66)	89	(63)
Change in fair value of preferred stock warrant liability		(82)	(10)
Gain on disposal of property and equipment		(13)
Changes in operating assets and liabilities:
Accounts receivable	668	250	506
Prepaids and other current assets	2,682	(3,871)	190
Other assets	35	76	182
Accounts payable	67	(289)	361
Accrued expenses	1,481	2,350	498
Deferred revenue	(1,848)	(5,385)	2,681
Net cash used in operating activities	(52,395)	(41,925)	(19,415)
Cash flows from investing activities:
Change in restricted cash	(1,656)
Purchases of short-term investments	(61,220)	(69,812)
Proceeds received from maturities of short-term investments	83,221	3,000
Purchases of property and equipment	(118)	(229)	(263)
Proceeds received from sale of property and equipment		13
Net cash provided by (used in) investing activities	20,227	(67,028)	(263)
Cash flows from financing activities:
Proceeds from issuance of common stock upon completion of initial public offering, net of commissions and underwriting discounts		46,500
Proceeds from issuance of common stock	43,240	70,265
Proceeds from issuance of convertible preferred stock, net of issuance costs			21,090
Proceeds from exercise of stock options	31	268	95
Proceeds from issuance of convertible promissory notes			5,000
Payments of public offering costs	(35)	(3,311)	(1,456)
Net cash provided by financing activities	43,279	113,722	24,729
Net increase in cash and cash equivalents	11,111	4,769	5,051
Cash and cash equivalents at beginning of year	18,613	13,844	8,793
Cash and cash equivalents at end of year	29,724	18,613	13,844
Supplemental disclosure of non-cash investing and financing activities:
Conversion of convertible promissory notes and accrued interest into Series B convertible preferred stock			15,798
Conversion of convertible preferred stock into common stock		112,292
Issuance of common stock to settle accrued Series A preferred stock dividends		3
Issuance of common stock for partial payment of accrued bonus		62
Conversion of preferred stock warrants to common stock warrants		28
Deferred offering costs included in accounts payable and accrued expenses	321		1,288
Series A Preferred Stock [Member]
Supplemental disclosure of non-cash investing and financing activities:
Modifications of preferred stock			(11,481)
Series B Preferred Stock [Member]
Supplemental disclosure of non-cash investing and financing activities:
Modifications of preferred stock			$ 26
Employee Stock Purchase Plan [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock	43
Build-To-Suit Lease Transaction [Member]
Supplemental disclosure of non-cash investing and financing activities:
Capital expenditures incurred but not yet paid	$ 16,143
Accounts Payable or Accrued Expenses [Member]
Supplemental disclosure of non-cash investing and financing activities:
Capital expenditures incurred but not yet paid		$ 60

Nature of the Business

Nature of the Business	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business	1. Nature of the Business Proteostasis Therapeutics, Inc. (the “Company”) was incorporated in Delaware on December 13, 2006. The Company is an innovative biopharmaceutical company committed to the discovery and development of novel therapeutics that treat diseases caused by an imbalance in the proteostasis network, a set of pathways that control protein biosynthesis, folding, trafficking and clearance. The Company’s initial therapeutic focus is on cystic fibrosis, which is caused by defects in the cystic fibrosis transmembrane conductance regulator (“CFTR”) protein and insufficient CFTR protein function. The Company’s lead product candidates, PTI-428, an amplifier, PTI-801, a third generation corrector, and PTI-808, a potentiator, as well a dual combination consisting of PTI-801 and PTI-808, and a triple combination consisting of PTI-428, PTI-801, and PTI-808 are in clinical development. The Company’s other drug candidates are in the preclinical development and discovery phase. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. In accordance with ASC 205-40, Going Concern, the Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date the financial statements are issued. As of December 31, 2017, the Company had an accumulated deficit of $216.9 million. During the year ended December 31, 2017, the Company incurred a loss of $59.4 million and used $52.4 million of cash in operations. The Company expects to continue to generate operating losses in the foreseeable future. The Company currently expects that its cash, cash equivalents and short-term investments of $74.5 million will be sufficient to fund its operating expenses and capital requirements, based upon its current operating plan, into early 2019. In October 2017, the Company reduced headcount through the elimination of 13 positions. As a result of this action, which was completed in the fourth quarter of 2017, the Company incurred one-time severance and related costs of $0.2 million in the fourth quarter of 2017. Reverse Stock Split On January 19, 2016, the Company effected a 1-for-10.8102 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s Preferred Stock (see Note 10). Accordingly, all share and per share amounts for all periods presented in the accompanying financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the preferred stock conversion ratios. Public Offerings In February 2016, the Company issued and sold 6,250,000 shares of its common stock in its initial public offering (“IPO”) at a public offering price of $8.00 per share, for net proceeds of $42.5 million after deducting underwriting discounts and commissions of $3.5 million and other offering expenses of $4.0 million. Upon the closing of the IPO, all shares of convertible preferred stock then outstanding converted into an aggregate of 9,699,600 shares of common stock, and the Company issued 2,590,742 shares of common stock as payment of $36.0 million of accruing dividends due to holders of Series A preferred stock. In addition, the Company’s convertible preferred stock warrant outstanding at the close of the IPO converted to a warrant to purchase common stock. In connection with the completion of the IPO, the Company amended its certificate of incorporation to authorize the future issuance of up to 5,000,000 shares of undesignated preferred stock, par value $0.001 per share. As of December 31, 2017 and 2016, there were no shares of preferred stock issued or outstanding. In September 2016, the Company issued and sold 5,750,000 shares of its common stock in a public offering at a public offering price of $13.00 per share, for net proceeds of $69.5 million after deducting underwriter discounts and commissions of $4.5 million and other offering expenses of $0.8 million. The foregoing includes the exercise by the underwriters of their options to purchase an additional 750,000 shares of the Company’s common stock within 30 days following the date of the final prospectus for the offering. In December 2017, the Company issued and sold 9,200,000 shares of its common stock in a public offering at a public offering price of $5.00 per share, for net proceeds of $42.9 million after deducting underwriting discounts and commissions of $2.8 million and other offering expenses of $0.3 million. The foregoing includes the exercise by the underwriters of their option to purchase an additional 1,200,000 shares of the Company’s common stock within 30 days following the date of the final prospectus for the offering

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, revenue recognition, the accrual for research and development expenses, including clinical trials, and the valuation of common stock, preferred stock warrant liability, derivative liability and build-to-suit lease obligation. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates. Concentrations of Credit Risk and Significant Customers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, short-term investments and accounts receivable. The Company mitigates its risk with respect to cash, cash equivalents and short-term investments by maintaining its balances at two accredited financial institutions, in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company’s accounts receivable balances are due from the counterparty to its collaboration agreements (see Note 13) that the Company believes to be creditworthy. As of December 31, 2016, accounts receivable consisted of amounts due from one such counterparty. Deferred Offering Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering. As of December 31, 2015, the Company had recorded $2.7 million of deferred offering costs in contemplation of the Company’s IPO, which was completed on February 11, 2016. In conjunction with the completion of the IPO, the Company reclassified $4.0 million of deferred financing costs to additional paid-in capital as a reduction of the proceeds from the IPO. In September 2016, the Company reclassified $0.8 million of deferred financing costs to additional paid-in capital as a reduction of the proceeds from the follow-on public offering. In December 2017, the Company reclassified less than $0.4 million of deferred financing costs to additional paid-in capital as a reduction of the proceeds from its public offering, of which $0.3 million is included within accrued expenses and accounts payable as of December 31, 2017. Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less at the date of purchase to be cash equivalents. As of December 31, 2017, the Company’s cash equivalents consisted of money market funds, U.S. government-sponsored securities and U.S. treasury securities. The Company’s cash equivalents as of December 31, 2016 consisted of money market funds. Restricted Cash As of December 31, 2017and 2016, short term restricted cash consisted of a certificate of deposit collateralizing a letter of credit issued as a security deposit in connection with the Company’s lease of its corporate facilities at 200 Technology Square, Cambridge, MA. On September 19, 2017, the Company entered into a lease agreement for a new corporate headquarters at 80 Guest Street, Brighton, MA (see Note 16). As of December 31, 2017, long term restricted cash consisted of a money market account in the amount of $1.7 million collateralizing a letter of credit issued as a security deposit for this lease agreement. Short-Term Investments Short-term investments represent holdings of available-for-sale marketable securities in accordance with the Company’s investment policy and cash management strategy. Short-term investments mature within one-year from the balance sheet date. Investments in marketable securities are recorded at fair value, with any unrealized gains and losses, net of taxes, reported as a component of stockholders’ equity until realized or until a determination is made that an other-than-temporary decline in market value has occurred. The cost of marketable securities sold is determined based on the specific identification method and any realized gains or losses on the sale of investments are reflected as a component of other expense, net. Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the estimated useful life of each asset. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred. Impairment of Long-Lived Assets Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses on long-lived assets. Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s investments and its derivative liability are carried at fair value determined according to the fair value hierarchy described above (see Note 4). The carrying value of accounts receivable, accounts payable and accrued expenses approximate their fair value due to the short-term nature of these assets and liabilities. Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is developing therapeutics to treat protein conformational diseases. All of the Company’s tangible assets are held in the United States. To date, all of the Company’s revenue has been generated in the United States. Revenue Recognition The Company recognizes revenue in accordance with Accounting Standards Codification (“ASC”) Topic 605, Revenue Recognition The Company records as deferred revenue any amounts received prior to satisfying the revenue recognition criteria. Deferred revenue not expected to be recognized within the next twelve months is reported as non-current deferred revenue. Collaborative Research and License Agreements The terms of these agreements contain multiple deliverables, which may include licenses and research and development activities. The terms of these agreements may also include nonrefundable upfront license fees, payments for research and development activities, reimbursement of certain third-party costs, payments based upon the achievement of specified milestones, and royalty payments based on product sales derived from the collaboration. The Company evaluates multiple-element arrangements based on the guidance in ASC Topic 605-25, Revenue Recognition—Multiple-Element Arrangements (“ASC 605-25”) The consideration received under the arrangement that is fixed or determinable is then allocated among the separate units of accounting based on the relative selling prices of the separate units of accounting. The Company determines the selling price of a unit of accounting within each arrangement following the hierarchy of evidence prescribed by ASC 605-25. Accordingly, the Company determines the estimated selling price for units of accounting within each arrangement using vendor-specific objective evidence (“VSOE”), if available; third-party evidence (“TPE”) of selling price if VSOE is not available; or best estimate of selling price (“BESP”), if neither VSOE nor TPE is available. The Company typically uses BESP to estimate the selling price as it generally does not have VSOE or TPE of selling price for its units of accounting. Determining the BESP for a unit of accounting requires significant judgment. In developing the BESP for a unit of accounting, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the BESP for units of accounting by evaluating whether changes in the key assumptions used to determine the BESP will have a significant effect on the allocation of arrangement consideration between multiple units of accounting. The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria in ASC 605 are satisfied for that particular unit of accounting. In the event that a deliverable does not represent a separate unit of accounting, the Company recognizes revenue from the combined unit of accounting over the contractual or estimated performance period for the undelivered items, which is typically the term of the Company’s research and development obligations. If there is no discernible pattern of performance or objectively measurable performance measures do not exist, then the Company recognizes revenue under the arrangement on a straight-line basis over the period the Company is expected to complete its performance obligations. Conversely, if the pattern of performance over which the service is provided to the customer can be determined and objectively measurable performance measures exist, then the Company recognizes revenue under the arrangement using the proportional performance method. Revenue recognized is limited to the lesser of the cumulative amount of payments received or the cumulative amount of revenue earned, as determined using the straight-line method or proportional performance method, as applicable, as of the period ending date. At the inception of an arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether: (1) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item as a result of a specific outcome resulting from the Company’s performance to achieve the milestone, (2) the consideration relates solely to past performance, and (3) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluates factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the particular milestone and the level of effort and investment required to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether a milestone satisfies all of the criteria required to conclude that a milestone is substantive. The Company will recognize revenue in its entirety upon successful accomplishment of any substantive milestones, assuming all other revenue recognition criteria are met. Milestones that are not considered substantive are recognized as earned if there are no remaining performance obligations or over the remaining period of performance, with a cumulative catch-up for the elapsed portion of the research term, assuming all other revenue recognition criteria are met. Research Grant Contracts Under these contracts, the Company is typically compensated for specific research or development activities. The Company recognizes revenue as the activities specified under the research grant contracts are performed and all of the revenue recognition criteria in ASC 605 are satisfied. Adoption of ASC 606 The Company will adopt new revenue accounting guidance in the first quarter of 2018, which it expects to impact this policy, the amount and timing of our future revenue recognition, and the amount of revenue previously recognized in our published financial statements. For further discussion, see Recent Accounting Pronouncements Embedded Derivatives Embedded derivatives that are required to be bifurcated from the underlying host instrument are accounted for and valued as a separate financial instrument. An embedded derivative exists associated with an alternate payment option upon change of control within the research, development and commercialization agreement with Cystic Fibrosis Foundation Therapeutics, Inc. (see Note 13). The embedded derivative has been bifurcated and is classified as a liability on the balance sheet and separately accounted for at its fair value. The derivative liability is marked-to-market every reporting period. Changes in fair value of the derivative liability are recognized as a component of other income (expense), net in the accompanying statement of operations and comprehensive loss. Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries, stock-based compensation and benefits, facilities costs, depreciation, third-party license fees, and external costs of outside vendors engaged to conduct preclinical development activities and clinical trials. Research and development expenses include the Company’s costs of performing services in connection with its collaboration agreements and research grant. Nonrefundable prepayments for goods or services that will be used or rendered for future research and development activities are deferred and capitalized. Such amounts are recognized as an expense as the goods are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered or the services rendered. Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies both inside and outside of the United States. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs. Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Preferred Stock Warrant Liability Through February 2016, the Company classified a warrant to purchase shares of its Series A preferred stock as a liability on its balance sheets as this warrant was a free-standing financial instrument could have required the Company to transfer assets upon exercise. The warrant was initially recorded at fair value on date of grant and subsequently remeasured to fair value at each balance sheet date. Changes in fair value of the warrant were recognized as a component of other income (expense), net in the statement of operations. Following the reverse stock split of the common stock of the Company and the completion of the Company’s IPO in February 2016, the warrant became a warrant to purchase 14,800 shares of common stock. At that time, the warrant was reclassified to a component of stockholder’s equity and is no longer subject to remeasurement. The Company used the Black-Scholes option-pricing model, which incorporates assumptions and estimates, to value the preferred stock warrant. The Company has assessed these assumptions and estimates on a quarterly basis as additional information impacting the assumptions is obtained. Estimates and assumptions impacting the fair value measurement include the fair value per share of the underlying Series A preferred stock, the remaining contractual term of the warrant, risk-free interest rate, expected dividend yield and expected volatility of the price of the underlying preferred stock. The Company determined the fair value per share of the underlying preferred stock by taking into consideration the most recent sales of its convertible preferred stock, results obtained from third-party valuations and additional factors that are deemed relevant. Up until February 2016, the Company was a private company and therefore lacks company-specific historical and implied volatility information of its stock. Therefore, it estimates its expected stock volatility based on the historical volatility of publicly traded peer companies for a term equal to the remaining contractual term of the warrant. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the remaining contractual term of the warrant. Expected dividend yield is determined considering that the underlying Series A preferred stock is entitled to dividends of 8.0% per year, whether or not declared. Stock-Based Compensation The Company measures all stock options and other stock-based awards granted to employees and directors based on the fair value on the date of the grant and recognizes compensation expense of those awards, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues stock options and restricted stock awards to employees with only service- based vesting conditions and records the expense for these awards using the straight-line method. The Company measures stock-based awards granted to consultants and non-employees based on the fair value of the award on the date on which the related service is complete. Compensation expense is recognized over the period during which services are rendered by such consultants and non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is remeasured using the then-current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model. The Company does not recognize compensation expense for awards with performance-based vesting conditions granted to consultants and non-employees for which satisfaction of the performance conditions is not solely within the control of the holder until the performance conditions have been met. The Company classifies stock-based compensation expense in its statement of operations in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified. Prior to January 1, 2017, the Company recognized compensation expense for only the portion of awards that are expected to vest. In developing a forfeiture rate estimate, the Company has considered its historical experience to estimate pre- vesting forfeitures for service-based awards. Effective January 1, 2017, the company adopted ASU No. 2016-09 on a modified retrospective basis. As a result, the Company has made an accounting policy election to account for forfeitures as they occur The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option- pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends on common stock and does not expect to pay any cash dividends in the foreseeable future. Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2017 and 2016, comprehensive loss included less than $0.1 million of unrealized gains and less than $0.1 million of unrealized losses on short-term investments, respectively. There was no difference between net loss and comprehensive loss for the year ended December 31, 2015. Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. Net Loss per Share Basic net loss per share is computed using the weighted average number of common shares outstanding during the period. Diluted net loss per share is computed using the sum of the weighted average number of common shares outstanding during the period and, if dilutive, the weighted average number of potential shares of common stock, including the assumed exercise of stock options and unvested restricted stock. Net loss per share attributable to common stockholders is calculated using the two-class method, which is an earnings allocation formula that determines net loss per share for the holders of the Company’s common shares and participating securities. The Company’s convertible preferred stock contains participation rights in any dividend paid by the Company and are deemed to be participating securities. Net loss attributable to common stockholders and participating preferred shares are allocated to each share on an as-converted basis as if all of the earnings for the period had been distributed. The participating securities do not include a contractual obligation to share in losses of the Company and are not included in the calculation of net loss per share in the periods that have a net loss. Diluted net loss per share is computed using the more dilutive of (a) the two-class method or (b) the if- converted method. The Company allocates earnings first to preferred stockholders based on dividend rights and then to common and preferred stockholders based on ownership interests. The weighted average number of common shares included in the computation of diluted net loss gives effect to all potentially dilutive common equivalent shares, including outstanding stock options, warrants, preferred stock and the potential issuance of stock upon the conversion of the Company’s convertible notes. Common stock equivalent shares are excluded from the computation of diluted net loss per share if their effect is antidilutive. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. The Company reported a net loss attributable to common stockholders for the years ended December 31, 2017, 2016 and 2015. Recently Issued Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes existing revenue recognition guidance under GAAP. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The standard defines a five-step process to achieve this principle, and will require companies to use more judgment and make more estimates than under the current guidance. The Company expects that these judgments and estimates will include identifying performance obligations in the customer contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date , which delays the effective date of ASU 2014-09 such that the standard is effective for public entities for annual periods beginning after December 15, 2017 and for interim periods within those fiscal years. In April 2016, the FASB issued ASU 2016-10, Identifying Performance Obligations and Licensing , which clarifies two aspects of ASU 2014-09, Revenue from Contracts with Customers (Topic 606) : identifying performance obligations and the licensing implementation guidance. These new standards will become effective for the Company on January 1, 2018. Companies have the option of applying this new guidance retrospectively to each prior reporting period presented (the full retrospective method) or retrospectively with the cumulative effect of initially applying this update recognized at the date of initial application (the modified retrospective method). The Company only has one contract (see Note 13) within the scope of Topic 606 and intends to use the modified retrospective method for adoption. Since the inception of the contract, the Company has recognized $9.7 million in revenue. The Company is assessing, but has not yet completed its assessment of the impact of the adoption of this standard on its consolidated financial statements and related disclosures. Currently, the Company anticipates a potential impact on the revenue recognition method used to recognize revenue under its agreement with Astellas and the accounting for research funding payments based on a specified rate for time incurred as well as the recognition of milestone revenue prior to achievement. The expected impact is further described below. Estimated impacts from the adoption of this standard could differ upon the final adoption and implementation of the standard. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. ASC 606 also impacts certain other areas, such as the accounting for costs to obtain or fulfill a contract. With respect to its collaboration with Astellas, the Company currently expects the one performance obligation identified under the provisions of ASC 606 will be consistent with the one unit of accounting identified under the provisions of ASC 605. However, as previously described, it currently expects that the pattern of revenue recognition under step (v) above will differ from the pattern of revenue recognition under ASC 605. Any

Short-Term Investments

Short-Term Investments	12 Months Ended
	Dec. 31, 2017
Cash Cash Equivalents And Short Term Investments [Abstract]
Short-Term Investments	3. Short-Term Investments The following is a summary of the Company’s short-term investments as of December 31, 2017 and 2016 (in thousands) : December 31, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S government-sponsored enterprise securities $ 9,817 $ — $ (1 ) $ 9,816 U.S. treasury securities 34,923 1 (2 ) $ 34,922 $ 44,740 $ 1 $ (3 ) $ 44,738 December 31, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S government-sponsored enterprise securities $ 53,384 $ — $ (20 ) $ 53,364 U.S. treasury securities 13,535 — (2 ) 13,533 $ 66,919 $ — $ (22 ) $ 66,897 The Company did not have any realized gains or losses on its short-term investments for the years ended December 31, 2017 and 2016. There were no other-than-temporary impairments recognized for the years ended December 31, 2017 and 2016.

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	4. Fair Value of Financial Assets and Liabilities The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (in thousands): Fair Value Measurements as of December 30, 2017 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 13,871 $ — $ — $ 13,871 U.S. government-sponsored enterprise securities — 8,960 — 8,960 U.S. treasury securities — 3,497 — 3,497 Short-term investments: U.S. government-sponsored enterprise securities — 9,816 — 9,816 U.S. treasury securities — 34,922 — 34,922 $ 13,871 $ 57,195 $ — $ 71,066 Liabilities: Derivative liability $ — $ — $ 25 $ 25 $ — $ — $ 25 $ 25 Fair Value Measurements as of December 30, 2016 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 15,440 $ — $ — $ 15,440 Short-term investments: U.S. government-sponsored enterprise securities — 53,365 — 53,365 U.S. treasury securities — 13,532 — 13,532 $ 15,440 $ 66,897 $ — $ 82,337 Liabilities: Derivative liability $ — $ — $ 91 $ 91 $ — $ — $ 91 $ 91 During the years ended December 31, 2017, 2016 and 2015, there were no transfers between Level 1, Level 2 and Level 3. The fair value of the derivative liability (see Note 13) is based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of the derivative instrument was determined using the Monte-Carlo simulation analysis. In determining the fair value of the derivative liability, the inputs impacting fair value include the fair value of the Company’s common stock, expected term of the derivative instrument, expected volatility of the common stock price, risk-free interest rate, expected sales-based milestone payments, discount rate, probability of a change of control event, and the probability that the counterparty would elect to accept the alternative cash payment in lieu of its right to the future sales-based milestone payments. As of December 31, 2017 and 2016, the Company determined the per share common stock price available based on the closing price of its common stock on The NASDAQ Global Market as of December 29, 2017 and December 30, 2016, respectively. The Company determined the expected term of the instrument to be 2.00 years and 2.50 years as of December 31, 2017 and 2016, respectively. The Company estimated its expected stock volatility to be 80.0% and 81.1% as of December 31, 2017 and 2016, respectively, based on the historical volatility of publicly traded peer companies for terms matching the expected term of the instrument for each respective period. The risk-free interest rate was determined to be 1.87% and 1.33% for the years ended December 31, 2017 and 2016, respectively, by reference to the U.S. Treasury yield curve for terms matching the expected term of the instrument for each respective period. The Company estimated the expected sales-based milestone payments based on four times the maximum research funding allowable under the CFFT collaboration agreement (see Note 13) plus the expected achievement of certain milestones, which totaled $28.5 million for each of the years ended December 31, 2017 and 2016. The Company estimated the discount rate in the calculation of the present value of the expected future milestone payments to be 25% for the years ended December 31, 2017 and 2016, respectively, based on expected returns of alternative investments of a similar type. Changes in the values of the preferred stock warrant liability and the derivative liability are summarized below (in thousands): Preferred Warrant Liability Derivative Liability Fair value at December 31, 2014 $ 120 $ 65 Change in fair value (10 ) (63 ) Fair value at December 31, 2015 110 2 Change in fair value (82 ) 89 Conversion to common stock warrant (28 ) — Fair value at December 31, 2016 — 91 Change in fair value — (66 ) Fair value at December 31, 2017 $ — $ 25

Prepaids and Other Current Asse

Prepaids and Other Current Assets	12 Months Ended
	Dec. 31, 2017
Prepaid Expense And Other Assets Current [Abstract]
Prepaids and Other Current Assets	5. Prepaids and Other Current Assets Prepaids and other current assets consisted of the following (in thousands): December 31, 2017 2016 Prepaid clinical, manufacturing and scientific expenses $ 568 $ 1,390 Prepaid insurance expenses 55 104 Other prepaid expenses and other current assets 754 2,565 $ 1,377 $ 4,059

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Property and Equipment, Net	6. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): Useful Life December 31, (Years) 2017 2016 Laboratory equipment 5 years $ 3,299 $ 3,192 Furniture and fixtures 5 years 158 147 Leasehold improvements Shorter of the lease term or 5 years 382 382 Computer and office equipment 3 – 5 years 143 143 Construction-in-progress 16,143 — 20,125 3,864 Less: Accumulated depreciation and amortization (3,558 ) (3,323 ) $ 16,567 $ 541 Amounts included within construction-in-progress represent the Company’s build-to-suit lease, see Note 16. Depreciation and amortization expense was $0.2 million for the year ended December 31, 2017 and $0.3 million for each of the years ended December 31, 2016 and 2015, respectively.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Accrued Expenses	7. Accrued Expenses Accrued expenses consisted of the following (in thousands): December 31, 2017 2016 Accrued payroll and related expenses $ 1,542 $ 1,813 Accrued research and development expenses 3,930 1,612 Accrued professional fees 556 383 Accrued other 92 520 $ 6,120 $ 4,328

Convertible Promissory Notes

Convertible Promissory Notes	12 Months Ended
	Dec. 31, 2017
Restructuring And Related Activities [Abstract]
Convertible Promissory Notes	8. Convertible Promissory Notes In July 2014, the Company entered into an agreement to issue $10.0 million of convertible promissory notes. The Company received proceeds from the issuance of convertible promissory notes aggregating $10.0 million for the year ended December 31, 2014. In July 2015, the Company issued an additional $5.0 million of convertible promissory notes, increasing the aggregate principal amount of convertible promissory notes to $15.0 million. On September 2, 2015, in connection with the Company’s issuance of Series B preferred stock, the principal amount of all outstanding convertible promissory notes, aggregating $15.0 million, and accrued interest thereon, aggregating $0.8 million, were automatically converted into 12,284,466 shares of Series B preferred stock at a price of $1.286 per share.

Preferred Stock Warrant Liabili

Preferred Stock Warrant Liability	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Preferred Stock Warrant Liability	9. Preferred Stock Warrant Liability In July 2008, the Company issued a preferred stock warrant to an investor in connection with the issuance of Series A preferred stock that was immediately exercisable for the purchase of 160,000 shares of Series A preferred stock at an exercise price of $1.00 per share, over a term of ten years from issuance. The fair value of the warrant on the date of grant of $0.1 million was recorded as a reduction to the initial carrying amount of the Series A preferred stock. The Company remeasured the fair value of the liability for this preferred stock warrant at each reporting date from its grant date through February 2016, with any adjustments being recorded as a component of other income (expense), net in the Company’s statement of operations and comprehensive loss. The Company recorded gains of less than $0.1 million each year for the years ended December 31, 2016 and 2015, respectively, to reflect the change in fair value of this preferred stock warrant. Upon the closing of the Company’s IPO on February 11, 2016, all outstanding shares of the Series A preferred stock were automatically converted to common stock and the preferred stock warrant liability was remeasured at fair value and was reclassified to additional paid-in capital. The following assumptions and inputs were used in determining the fair value of the preferred stock warrant liability valued using the Black-Scholes option-pricing model: As of February 10, 2016 Expected term (in years) 2.25 Expected volatility 73.24 % Risk-free interest rate 0.74 % Expected dividend yield 8.0 % The warrant for the purchase of common stock was exercised in February 2017 and the Company issued 4,349 shares of common stock in a net settlement of the warrant exercise.

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Convertible Preferred Stock	10. Convertible Preferred Stock As of December 31, 2015, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue 110,057,398 shares of $0.001 par value preferred stock. The Company has issued Series A and Series B preferred stock (collectively “Preferred Stock”). The Preferred Stock is classified outside of stockholders’ equity (deficit) because the shares contain certain redemption features that are not solely within the control of the Company. In January 2014, the Company issued 5,000,000 shares of Series A preferred stock at an issuance price of $1.00 per share for proceeds of $4.9 million, net of issuance costs of $0.1 million. In September 2014, the Company modified the Conversion Price of the Series A preferred stock from $10.8102 per share to $3.6030397 per share. This amendment to the Series A Conversion Price was accounted for as a modification of preferred stock based on a quantitative assessment of the change in the fair value that resulted from the modification of the conversion price on the modification date. The increase in the fair value of Series A preferred stock measured immediately before the modification and immediately after the modification, equal to $6.0 million, was recorded as a deemed dividend from holders of common stock to the holders of preferred stock, resulting in a decrease to additional paid-in capital of $1.0 million and an increase to accumulated deficit of $5.0 million as well as a corresponding increase of $6.0 million to the carrying value of the Series A preferred stock. In May 2015, the Company modified the Conversion Price of the Series A preferred stock from $3.6030397 per share to $10.8102 per share. This amendment to the Series A Conversion Price was accounted for as a modification of preferred stock based on a quantitative assessment of the change in the fair value that resulted from the modification of the conversion price on the modification date. The decrease in the fair value of Series A preferred stock measured immediately before the modification and immediately after the modification, equal to $10.6 million, was recorded as a deemed dividend from holders of preferred stock to the holders of common stock, resulting in an increase to additional paid-in capital of $10.6 million and a corresponding decrease of $10.6 million to the carrying value of the Series A preferred stock. In September 2015, the Company issued 17,107,303 shares of Series B preferred stock at an issuance price of $1.286 per share for proceeds of $21.1 million, net of issuance costs of $0.9 million. Concurrent with the sale of Series B preferred stock, all outstanding convertible promissory notes, aggregating $15.0 million, and accrued interest thereon, aggregating $0.8 million, were automatically converted into 12,284,466 shares of Series B preferred stock at a price of $1.286 per share. In September 2015, in connection with the Series B preferred stock financing, (1) the right of the holders of Series A preferred stock to receive accruing dividends, whether or not declared, at a rate of 8.0% per year of the Original Issue Price per share (as described below) ceased as of August 31, 2015, at which time such cumulative accruing dividends totaled $36.0 million, and (2) the right of holders of Series A preferred stock to receive a cash payment of accruing dividends upon the automatic conversion of the Series A preferred stock into common stock was modified such that all previously accrued but unpaid dividends on Series A preferred stock will be paid in shares of common stock, at a price of $13.9019172 per share, upon such conversion of the Series A preferred stock. If as of January 1, 2016, the Series A preferred stock has not automatically converted into shares of common stock according to its terms, the holders of the Series A preferred stock will be entitled to receive dividends at the rate of 8.0% per year of the respective Original Issue Price per share, commencing as of the respective original issuance date of the Series A preferred stock. These amendments to the Series A preferred stock dividend rights were accounted for as a modification of preferred stock based on a quantitative assessment of the change in the fair value that resulted from the modification of the dividend rights on the modification date. The decrease in the fair value of Series A preferred stock measured immediately before the modification and immediately after the modification, equal to $0.2 million, was recorded as a deemed dividend from holders of preferred stock to the holders of common stock, resulting in an increase to additional paid-in capital of $0.2 million and a corresponding decrease of $0.2 to the carrying value of the Series A preferred stock. On December 17, 2015, the right of holders of Series A and Series B preferred stock to become entitled to accruing dividends was further modified such that if, as of April 1, 2016, the Series A and Series B preferred stock has not automatically converted into shares of common stock according to its terms, the holders of the Series A and Series B preferred stock will be entitled to receive dividends at the rate of 8.0% per year of the respective Original Issue Price per share, commencing as of the respective original issuance date of the Series A and Series B preferred stock. The decrease in the fair value of Series A preferred stock measured immediately before the modification and immediately after the modification, equal to $0.8 million, was recorded as a deemed dividend from holders of preferred stock to the holders of common stock resulting in an increase to additional paid-in capital of $0.8 million and a corresponding decrease of $0.8 million to the carrying value of the Series A preferred stock. The increase in the fair value of Series B preferred stock measured immediately before the modification and immediately after the modification, equal to less than $0.1 million, was recorded as a deemed dividend from holders of common stock to holders of preferred stock the resulting in an decrease to additional paid-in capital of less than $0.1 million and a corresponding increase of less than $0.1 million to the carrying value of the Series B preferred stock. In connection with the completion of the Company’s IPO in February 2016, all outstanding shares of convertible preferred stock were converted to common stock. As of December 31, 2015, Preferred Stock consisted of the following: Preferred Shares Authorized Preferred Shares Issued and Outstanding Carrying Value Liquidation Preference Common Stock Issuable Upon Conversion Series A preferred stock 76,000,000 75,463,000 $ 75,378 $ 111,594 6,980,712 Series B preferred stock 34,057,398 29,391,769 36,914 37,798 2,718,888 110,057,398 104,854,769 $ 112,292 $ 149,392 9,699,600 The holders of the Preferred Stock had the following rights and preferences: Dividends At the time of their issuance, the holders of Series A preferred stock were entitled to receive dividends in preference to any dividend on common stock at the rate of 8.0% per year of the Original Issue Price. Such dividends accrued daily, compounded annually, were cumulative and were payable, whether or not declared, upon any liquidation event or upon the conversion of the Series A preferred stock into common stock. In connection with the Series B preferred stock financing in September 2015, the dividends on the Series A preferred stock ceased accruing as of August 31, 2015. In addition, the right of holders of Series A preferred stock to receive a cash payment of accruing dividends upon the automatic conversion of the Series A preferred stock into common stock was modified such that all previously accrued but unpaid dividends on Series A preferred stock will be paid in shares of common stock, at a price of $13.9019172 per share, upon such conversion. If, as of April 1, 2016, the Series A preferred stock has not automatically converted into shares of common stock according to its terms, the holders of the Series A and Series B preferred stock will be entitled to receive dividends at the rate of 8.0% per year of the respective Original Issue Price per share, commencing as of the respective original issuance date of the Series A and Series B preferred stock. Such dividends will accrue daily, compounding annually, and will be cumulative and payable when and if declared by the Company’s board of directors. In addition, such dividends will be payable, whether or not declared, upon any liquidation event or conversion of Series A or Series B preferred stock into common stock. The Original Issue Price is $1.00 per share for Series A preferred stock and $1.286 per share for Series B preferred stock. In February 2016, upon completion of the IPO, the Company issued 2,590,742 shares of common stock in full payment of $36.0 million of accrued dividends. Conversion Each share of Preferred Stock is convertible into common stock at the option of the stockholder at any time after the date of issuance. In addition, each share of Preferred Stock will be automatically converted into shares of common stock, at the applicable conversion ratio then in effect, upon a qualified public offering with net proceeds of at least $50.0 million that results in the Company’s common stock being listed on the NASDAQ or NYSE exchange. In addition, each share of Series A preferred stock will be automatically converted into shares of common stock upon the date specified by vote or written consent of the holders of at least two-thirds of the then outstanding shares of Series A preferred stock, and each share of Series B preferred stock will be automatically converted into shares of common stock upon the date specified by vote or written consent of the holders of at least the majority of the then outstanding shares of Series B preferred stock. The conversion ratio of each series of Preferred Stock is determined by dividing the Original Issue Price per share of each series of preferred stock by the Conversion Price of each series. The initial Conversion Price for Series A preferred stock was $10.8102 per share, subject to appropriate adjustment in the event of any stock dividend, stock split, combination or recapitalization affecting the Series A preferred stock. On September 30, 2014, the Conversion Price for Series A preferred stock was amended to be $3.6030397 per share. In May 2015, the Conversion Price for Series A preferred stock was amended to be $10.8102 per share. In February 2016, upon completion of the Company’s IPO, all outstanding shares of convertible preferred stock were converted into 9,699,600 shares of common stock and the issuance of 2,590,742 shares of common stock as payment of $36.0 million in accruing dividends on the Series A preferred stock.

Common and Preferred Stock

Common and Preferred Stock	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Common and Preferred Stock	11. Common and Preferred Stock As of December 31, 2017 and 2016, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue 125,000,000 shares of $0.001 par value common stock. On February 11, 2016, the Company completed its IPO, which resulted in the sale of 6,250,000 shares of its common stock at a public offering price of $8.00 per share. Upon closing of the IPO, all outstanding shares of convertible preferred stock were converted into 9,699,600 shares of common stock and the Company issued 2,590,742 shares of common stock as a payment of $36.0 million on accruing dividends due to holders of Series A preferred stock. Additionally, the Company’s convertible sock warrant outstanding at the close of the IPO converted to a warrant to purchase common stock. In conjunction with the IPO and the amended and restated certificate of incorporation, the Company is authorized to issue 5,000,000 shares of preferred stock, $0.001 par value, all of which is undesignated. In September 2016, the Company issued and sold 5,750,000 shares of its common stock in a public offering at a public offering price of $13.00 per share, for net proceeds of $69.5 million after deducting underwriter discounts and commissions of $4.5 million and other offering expenses of $0.8 million. The foregoing includes the exercise by the underwriters of their options to purchase an additional 750,000 shares of our common stock within 30 days following the date of the final prospectus for the offering. In December 2017, the Company issued and sold 9,200,000 shares of its common stock in a public offering at a public offering price of $5.00 per share, for net proceeds of $42.9 million after deducting underwriter discounts and commissions of $2.8 million and other offering expenses of $0.3 million. The foregoing includes the exercise by the underwriters of their options to purchase an additional 1,200,000 shares of our common stock within 30 days following the date of the final prospectus for the offering. Each share of common stock entitles the holder to one vote for the election of directors and on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to receive dividends, as may be declared by the board of directors, if any, subject to the preferential dividend rights of the preferred stockholders. Through December 31, 2017, no dividends have been declared. As of December 31, 2017, the Company had reserved 3,741,251 shares of common stock for the exercise of outstanding stock options under the Company’s 2008 Plan and 2016 Plan and the number of shares remaining for grant under the Company’s 2016 Plan (see Note 12) and the number of shares available under the Company’s 2016 Employee Stock Purchase Plan (the “2016 ESPP”).

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	12. Stock-Based Compensation 2008 Equity Incentive Plan The Company’s 2008 Equity Incentive Plan, as amended, (the “2008 Plan”) provides for the Company to sell or issue common stock or restricted common stock, or to grant incentive stock options or nonqualified stock options for the purchase of common stock, to employees, members of the board of directors and consultants of the Company. The 2008 Plan is administered by the board of directors, or at the discretion of the board of directors, by a committee of the board. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of stock option may not be greater than ten years. Stock options granted to employees and directors under the 2008 plan typically vest over four years. Stock options granted to non-employees under the 2008 plan typically vest over periods ranging from six months to four years, depending on the period during which the services are being provided. On February 9, 2016, the Company’s 2016 Stock Option and Incentive Plan (the “2016 Plan”) became effective and no further stock options or other awards will be made under the 2008 Plan. S hares of common stock underlying any awards that are forfeited, canceled, repurchased or are otherwise terminated by the Company under the 2016 Plan and the 2008 Plan will be added back to the shares of common stock available for issuance under the 2016 Plan. 2016 Stock Option and Incentive Plan On February 3, 2016, the Company’s stockholders approved the 2016 Stock Option and Incentive Plan (the “2016 Plan”), which became effective on February 9, 2016. The 2016 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards and other stock-based awards. The number of shares initially reserved for issuance under the 2016 Plan is 1,581,839 shares. The number of shares of common stock that may be issued under the 2016 Plan will automatically increase on each January 1, beginning on January 1, 2017, by the lesser of 3% of the shares of the Company’s common stock outstanding on the immediately preceding December 31 or an amount determined by the Company’s board of directors or the compensation committee of the board of directors. Stock options granted under the 2016 Plan vest based on the grantee’s continued service with the Company during a specified period following the grant. Awards granted to employees generally vest ratably over four years, with a 25% cliff vesting at the one-year anniversary for new employee awards. Stock options granted to directors generally vest ratably over three years. All awards are exercisable for a period of ten years from the grant date. 2016 Employee Stock Purchase Plan On February 3, 2016, the Company’s stockholders approved the 2016 Employee Stock Purchase Plan (the “2016 ESPP”), which became effective in connection with the completion of the Company’s IPO. A total of 138,757 shares of common stock were reserved for issuance under this plan. In addition, the number of shares of common stock that may be issued under the 2016 ESPP will automatically increase on each January 1, beginning on January 1, 2017 and ending on January 1, 2026, by the least of (i) 138,757 shares of common stock, (ii) 1% of the Company’s shares of common stock outstanding on the immediately preceding December 31 and (iii) an amount determined by the Company’s board of directors or the compensation committee of the board of directors. No shares were issued under this plan during the year ended December 31, 2016. During the year ended December 31, 2017, 10,829 shares were issued under this plan. Stock option grants and shares to non-employees Prior to 2013, the Company issued options to purchase 203,964 shares of common stock to non-employees, primarily members of the Company’s scientific advisory board, that vest upon the achievement of specified development and clinical milestones. As of December 31, 2017 and 2016, options for the purchase of 83,250 shares held by non-employees remained unvested, pending achievement of the specified milestones, and had an aggregate fair value of $0.3 million per year. During the years ended December 31, 2017, 2016 and 2015, the Company did not grant any options to non-employees under this plan. On October 28, 2015, the Company made a one-time grant of options to a non-employee to purchase 9,250 shares of our common stock with an exercise price of $2.38 per share which were granted outside of the 2008 Stock Option Plan. The options were fully vested upon the grant date as such the Company calculated the fair value of the options on the date of the grant. The Company calculated fair value at $12.23 per option utilizing the Black-Scholes option pricing model with the following inputs used to determine the fair value (i) Risk-free interest rate of 0.0 %; (ii) Expected term (in years) of 0.08 years; (iii) Expected volatility of 60.28 % and (iv) Expected dividend yield of 0.0 %. The Company recognized $0.1 million of stock compensation expense within general and administrative expenses for the year ended December 31, 2015. The Company received cash proceeds of less than $0.1 million for the option exercise during the year ended December 31, 2015. The aggregate intrinsic value of the stock option exercised during the year ended December 31, 2015 was $0.1 million. Stock-based compensation expense for the three months ended June 30, 2015 was reduced by $0.5 million for the cumulative correction of immaterial errors associated with the recognition of stock-based compensation for certain stock options with performance-based vesting conditions. Of this amount, $0.2 million related to years prior to 2015 and $0.3 million related to the three months ended March 31, 2015. Based upon its evaluation of relevant factors, the Company concluded that the uncorrected errors in its previously issued financial statements for any of the periods affected are immaterial and that the impact of recording the cumulative correction during the six months ended June 30, 2015 is not material to the Company’s results for the year ending December 31, 2015. Stock Option Valuation The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis: Year Ended December 31, 2017 2016 2015 Risk-free interest rates 2.00 % 1.35 % 1.68 % Expected term (in years) 5.96 6.09 5.99 Expected volatility 75.18 % 73.19 % 56.86 % Expected dividend yield — — — Stock Options The following table summarizes the Company’s stock option activity since December 31, 2016 (in thousands except share and per share amounts): Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (Years) Outstanding at December 31, 2016 2,030,517 $ 7.29 8.42 $ 10,510 Granted 1,201,789 6.88 Exercised (11,714 ) 2.68 Forfeited (382,034 ) 10.59 Outstanding at December 31, 2017 2,838,558 $ 6.69 8.10 $ 3,213 Vested and expected to vest at December 31, 2017 2,755,308 $ 6.82 8.25 $ 2,926 Options exercisable at December 31, 2017 1,054,350 $ 6.29 7.18 $ 1,404 The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the year ended December 31, 2017 and 2016 was less than $0.1 million and $0.4 million, respectively. The Company received cash proceeds from the exercise of stock options of less than $0.1 million, $0.3 million and $0.1 million during the years ended December 31, 2017, 2016 and 2015. The weighted average grant-date fair value of stock options granted during the years ended December 31, 2017, 2016 and 2015 was $4.57, $5.47 and $7.13, respectively. The total fair value of options vested during the years ended December 31, 2017, 2016 and 2015 was $2.5 million, $1.1 million and $0.5 million, respectively. Stock-based Compensation Stock-based compensation expense was classified in the statements of operations and comprehensive loss as follows: Year Ended December 31, 2017 2016 2015 Research and development $ 1,326 $ 556 $ 72 General and administrative 2,406 1,608 400 $ 3,732 $ 2,164 $ 472 As of December 31, 2017, total unrecognized compensation cost related to the unvested stock-based awards was $6.6 million, which is expected to be recognized over weighted average period of 2.57 years.

Collaboration, Research Grant a

Collaboration, Research Grant and License Agreements	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaboration, Research Grant and License Agreements	13. Collaboration, Research Grant and License Agreements Astellas Pharma Inc. In November 2014, the Company entered into a worldwide Collaborative Research, Development, Commercialization and License Agreement (the “Astellas Agreement”) with Astellas Pharma Inc. (“Astellas”). The focus of the Astellas Agreement is to identify, develop and commercialize therapeutic candidates relating to the Unfolded Protein Response (“UPR”) pathway. Financial Terms Under terms of the Astellas Agreement, Astellas purchased from the Company convertible promissory notes totaling $5.0 million with terms consistent with those of other investors that purchased convertible promissory notes issued during 2014 (see Note 8). In addition, the Company will be eligible to receive research funding support, based on the establishment of an annual research budget, and future research, development and sales milestone payments of up to $398.5 million, as well as tiered royalty payments ranging in the mid single-digit to low double-digit percentages of net sales, as defined in the agreement. Under the agreement, the companies will conduct research during the initial research term, which is three and a half years, to identify lead compounds for clinical development. Astellas will reimburse the Company at a specified rate for time incurred as well as certain agreed upon third-party costs incurred by the Company, based on the annual budget. The Company received payments for the years ended December 31, 2017 and 2016 of approximately $3.5 million and $2.5 million, respectively. The Company determined that the deliverables under the agreement include (i) the research license, (ii) the research services to be provided over the research term, which is three and a half years, and (iii) the Company’s participation in the Joint Research Committee (the “Committee”) to be provided over the initial three and a half year research term of the agreement. The Company concluded that the research license and the involvement in the Committee did not have standalone value to Astellas and, therefore, are not separable from the research services. Therefore, the research license, research services and participation in the Committee have been combined and accounted for as a single unit of accounting. Accordingly, the research funding support payments and any reimbursement of third-party costs are being recognized by the Company as revenue over the three and a half year research term of the agreement, which commenced in January 2015, with a cumulative catch-up for the elapsed portion of the research term being recognized at the time any such payments are earned. The Company concluded at the outset of the arrangement that none of the future milestone payments included in the arrangement qualified as substantive milestones. Non-substantive milestone payments totaling $1.8 million and $0.8 million earned during the years ended December 31, 2016 and 2015, respectively, are being recognized, along with the other arrangement consideration, over the three and half year research term of the agreement, with a cumulative catch-up for the elapsed portion of the research term. Revenue recognized under the Astellas Agreement during the years ended December 31, 2017 and 2016 totaled $5.3 million and $3.2 million, respectively. As of December 31, 2017 and 2016, deferred revenue related to the Astellas Agreement totaled $1.1 million and $3.2 million, respectively. Term and Termination The term of the Astellas Agreement commenced in November 2014 and will continue in full force and effect, unless terminated under the conditions described below, until expiration of all applicable royalty terms with respect to all licensed products in all countries in the territory defined as per the agreement. The agreement will automatically terminate at the end of the three and a half year research term, in the second quarter of 2018, if Astellas has not designated at least one development compound, unless mutually agreed to be extended. Astellas has the unilateral right to terminate the agreement on a project-by-project basis by providing written notice to the Company. Reciprocal termination rights under the agreement include termination for breach and termination for bankruptcy. Biogen In December 2013, the Company entered into a Collaborative Research, Development, Commercialization and License Agreement (the “Biogen Agreement”) with Biogen New Ventures, formerly Biogen Idec New Ventures Inc. (“Biogen”). The focus of the Biogen Agreement is to research, develop and commercialize licensed products to attack toxic proteins implicated in the development of Alzheimer’s and Parkinson’s diseases. The Biogen Agreement was terminated in December 2016. Financial Terms Under the terms of the agreement, Biogen agreed to pay a nonrefundable upfront fee to the Company of $2.5 million and to purchase $5.0 million of its Series A preferred stock under existing terms purchased on January 3, 2014. Research funding payments due to the Company guaranteed over the first two years of the agreement and totaled $4.0 million. In addition, third-party costs incurred by both parties are shared at the same ratio with corresponding payments made between the parties on a quarterly basis. The Company determined that the deliverables under the agreement include (i) the research license, (ii) the research services to be provided over the four-year research term of the agreement, and (iii) the Company’s participation in the Joint Research Committee (the “Committee”) to be provided over the initial four-year research term of the agreement. The Company concluded that the research license and the involvement in the Committee did not have standalone value to Biogen and, therefore, are not separable from the research services. Therefore, the research license, research services and participation in the Committee have been combined and accounted for as a single unit of accounting. Accordingly, the upfront fee, research payments and any reimbursement of third-party costs are being recognized by the Company as revenue over the four-year research term of the agreement, which commenced in December 2013, with a cumulative catch-up for the elapsed research term being recognized at the time any such payments are earned. The Company concluded at the outset of the arrangement that none of the future milestone payments included in the arrangement qualified as substantive milestones. A $2.0 million non-substantive milestone payment earned during the year ended December 31, 2014 was being recognized, along with the other arrangement consideration, over the four-year research term of the agreement, with a cumulative catch-up for the elapsed portion of the research term. In December 2016, Biogen exercised its right to terminate the agreement and the Company recognized all remaining deferred revenue upon termination. Revenue recognized under the Biogen Agreement during the years ended December 31, 2016 and 2015 totaled $3.0 million and $2.4 million, respectively. Cystic Fibrosis Foundation Therapeutics, Inc. In March 2012, the Company entered into a Research, Development and Commercialization Agreement (the “CFFT Agreement”) with Cystic Fibrosis Foundation Therapeutics, Inc. (“CFFT”). Under terms of the CFFT Agreement, which was subsequently amended in May 2013 and January 2014, CFFT agreed to provide up to $5.7 million (the “Award’) in research funding to the Company over two non-consecutive one-year periods from March 2012 to March 2013 and from January 2014 to December 2014. The Company did not recognize any revenue under the CFFT Agreement during the years ended December 31, 2017, 2016 and 2015. Under the CFFT agreement, the Company has agreed to make future sales-based milestone payments (which the agreement refers to as royalties) to CFFT of up to $34.2 million upon achieving specified commercialization milestones with respect to the first of any product developed utilizing any compound covered under the collaboration agreement. The Company has also agreed to pay to CFFT royalties of a mid single-digit percentage, up to an aggregate of $22.8 million, on any amounts received by the Company from the sale, license or transfer to a third party of rights in the technology developed as a result of this collaboration. Any such royalty payments shall be credited against the first three sales-based milestone payments owed by the Company through the second anniversary of the first commercial sale of a product developed as a result of this collaboration. As of December 31, 2017, 2016 and 2015, the Company had not developed a commercial product in connection with this collaboration, and it had not sold, licensed or transferred rights in the technology resulting from this collaboration. In lieu of the milestone and royalty payments described above, in the event of a change of control of the Company, CFFT may elect to accept a one-time payment equal to the consideration CFFT would have received if it had owned (a) 268,265 shares of the Company’s common stock if the change of control occurs prior to the selection by the Company of a compound intended for product approval, or (b) 444,025 shares of the Company’s common stock if the change of control occurs after the selection by the Company of a compound intended for product approval. This alternative payment option upon a change of control would be cash settled in the event of a change of control and meets the definition of an embedded derivative. The Company estimated the fair value of this liability and concluded that the liability was immaterial as of the inception date of the CFFT Agreement. The Company estimated the fair value of this derivative liability to be less than $0.1 million and $0.1 million as of December 31, 2017 and 2016, respectively (see Note 4). The CFFT Agreement will expire when there are no longer any payment obligations, unless terminated earlier. Each party may terminate for an uncured material breach of any material covenants or obligations or if any representation or warranty is materially untrue as of the date made and uncured after 30 days from notice. CFFT may also terminate if a case or proceeding under the bankruptcy laws is filed against the Company and not dismissed within 60 days, or if the Company files for insolvency, reorganization, receivership, dissolution or liquidation. Presidents and Fellows of Harvard College Licensing Agreement The Company has acquired certain exclusive and nonexclusive rights to use, research, develop and offer for sale certain products and patents under a licensing agreement, as amended in December 2013, with Presidents and Fellows of Harvard College (the “Harvard Agreement”). The Harvard Agreement was terminated in October 2017. The surviving rights obligate the Company to make payments to the licensor for milestones and royalties. Due to the termination of the Harvard Agreement, the future development, clinical and commercialization and sales milestone payments under the licensing agreement are up to $2.375 million. Under the surviving provisions of the licensing agreement, the Company will also owe low single-digit royalties on sales of commercial products, if any, developed using the licensed technologies for 10 years following the first commercial sale. As of December 31, 2017, 2016 and 2015, the Company had not developed a commercial product using the licensed technologies and no pre-commercialization milestones had been achieved. The Company recorded research and development expenses of $0.1 million during each of the years ended December 31, 2017, 2016 and 2015, respectively, for licensing fees due under the Harvard Agreement.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	14. Income Taxes During the years ended December 31, 2017, 2016 and 2015, the Company recorded no income tax benefits for the net operating losses incurred in each year or interim period, due to its uncertainty of realizing a benefit from those items. All of the Company’s losses before income taxes were generated in the United States. A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: December 31, 2017 2016 2015 Federal statutory income tax rate (34.0 )% (34.0 )% (34.0 )% State income taxes, net of federal benefit (5.6 ) (5.1 ) (5.0 ) Research and development tax credit carryforwards (3.2 ) (4.3 ) (4.4 ) Expiration of state net operating loss carryforwards — — — Non-deductible expenses 0.7 0.5 1.2 Change in deferred tax asset valuation allowance 2.0 42.9 42.2 Effect of U.S. Tax Reform 40.1 — — Effective income tax rate (0.0 )% (0.0 )% (0.0 )% Net deferred tax assets as of December 31, 2017 and 2016 consisted of the following (in thousands): December 31, 2017 2016 Net operating loss carryforwards 52,861 $ 53,723 Research and development tax credit carryforwards 9,232 6,633 Accrued expenses and other 1,583 2,152 Total deferred tax assets before valuation allowance 63,676 62,508 Valuation allowance (63,660 ) (62,442 ) Net deferred tax assets 16 66 Depreciation (16 ) (66 ) Net deferred tax assets $ — $ — On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act (“TCJA”) tax reform legislation. The legislation resulted in significant changes to the U.S. corporate income tax system, including reducing the U.S. corporate tax rate from a top marginal rate of 35% down to a flat rate of 21% effective for tax years beginning after December 31, 2017, elimination, reduction or limitation of certain domestic deductions and credits, limitation of the deduction for NOLs to 80% of current year taxable income, elimination of NOL carrybacks, immediate deductions for certain new investments instead of deductions for depreciation expense over time, and modification or repeal of many business deductions and credits, including the orphan drug tax credit. As a result of the enacted tax law, the Company has revalued the deferred tax assets and liabilities at the new rate. The revaluation resulted in a reduction in deferred tax assets of $24.0 million with a corresponding reduction in the valuation allowance and therefore no net effect on the 2017 tax provision affecting the Company’s statement of operations. As of December 31, 2017, the Company had federal and state net operating loss carryforwards (“NOLs”) of $196.9 million and $182.1 million, respectively, which may be available to offset future taxable income and federal and state NOLs begin to expire in 2026 and 2030, respectively. As of December 31, 2017, the Company also had federal and state research and development tax credit carryforwards of $6.8 million and $3.1 million, respectively, which may be available to offset future income tax liabilities. Federal and state research and development tax credit carryforwards begin to expire in 2027 and 2026, respectively. Utilization of the net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of net operating loss and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. The Company has not currently completed an evaluation of ownership changes through December 31, 2017 to assess whether utilization of the Company’s net operating loss or research and development credit carryforwards would be subject to an annual limitation under Section 382. To the extent an ownership change occurs in the future, the net operating loss and credit carryforwards may be subject to limitation. Further, until a study is completed and any limitation is known, no amounts are being presented as an uncertain tax position. The Company has not yet conducted a study of its research and development credit carryforwards. This study may result in an increase or decrease to the Company’s credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s credits, and if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. As a result, there would be no impact to the statements of operations and comprehensive loss or statements of cash flows if an adjustment were required. The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets, which are comprised principally of net operating losses and research and development tax credit carryforwards. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2017 and 2016. Management reevaluates the positive and negative evidence at each reporting period. Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2017 and 2016 related primarily to the increases in net operating loss carryforwards and research and development tax credit carryforwards and were as follows (in thousands): Year Ended December 2017 2016 Valuation allowance at beginning of year $ (62,442 ) $ (46,476 ) Increases recorded to income tax provision (1,218 ) (15,966 ) Valuation allowance at end of year $ (63,660 ) $ (62,442 ) The Company has not yet recorded any amounts for unrecognized tax benefits, interest or penalties historically through December 31, 2017. The Company files income tax returns in the U.S. federal and state jurisdictions in which it operates. The Company’s income tax returns are generally subject to tax examinations for the tax years ended December 31, 2014 to the present. There are currently no pending income tax examinations. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service and state tax authorities to the extent utilized in a future period.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Net Loss per Share	15. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands except share and per share amounts): Year Ended December 31, 2017 2016 2015 Numerator: Net loss $ (59,432 ) $ (37,232 ) $ (25,040 ) Modifications of Series A preferred stock — — 11,481 Modifications of Series B preferred stock — — (26 ) Accruing dividends on preferred stock — (1,378 ) (9,724 ) Net loss attributable to common stockholders $ (59,432 ) $ (38,610 ) $ (23,309 ) Denominator: Weighted average number of common shares outstanding—basic and diluted 25,407,943 18,759,369 553,162 Net loss per share attributable to common stockholders—basic and diluted $ (2.34 ) $ (2.06 ) $ (42.14 ) The Company’s potential dilutive securities, which include stock options, convertible preferred stock and a warrant to purchase preferred stock, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: December 31, 2017 2016 2015 Convertible preferred stock (as converted to common stock) — — 9,699,600 Payment of accruing dividends on Series A preferred stock in shares of common stock upon conversion of Series A preferred stock — — 2,590,742 Options to purchase common stock 2,838,558 2,030,517 979,334 Warrant for the purchase of convertible preferred stock (as converted to common stock) — 14,800 14,800 2,838,558 2,045,317 13,284,476

Build-to-suit Lease

Build-to-suit Lease	12 Months Ended
	Dec. 31, 2017
Leases [Abstract]
Build-to-suit Lease	16. Build-to-suit Lease On September 19, 2017, the Company entered into a lease agreement for its new headquarters, consisting of approximately 30,000 square feet of laboratory and office space located in Brighton, Massachusetts. The rent commencement date will be the earlier of (i) the first date on which tenant has commenced occupancy for the conduct of business in the premises, or (ii) April 15, 2018, subject to extension for certain delays. The lease will extend for a lease term from the rent commencement date and then for ten years starting with the first day of the month following the month in which the commencement date falls, if the rent commencement date does not fall on the first day of a month. The Company is entitled to one seven-year option to extend. Annual rent under the lease, exclusive of operating expenses and real estate taxes, will be approximately $1.7 million in the first year after taking occupancy, with annual increases of 2.75% each year thereafter. Total expected rental payments through the initial lease term are approximately $18.8 million. The Company will be entitled to an improvement allowance of approximately $4.8 million for certain permitted costs related to the design and construction of Company improvements on the premises. The lease contains customary provisions allowing the landlord to terminate the lease if the Company fails to remedy a breach of any of its obligations within specified time periods, or upon bankruptcy or insolvency of the Company. The Company is not the legal owner of the leased space. However, in accordance with ASC 840, Leases, the Company is deemed to be the owner of the leased space during the construction period because of the Company’s involvement with the build-out of the space. As a result, the Company has capitalized approximately $16.1 million as construction-in-progress within property, plant and equipment equal to the estimated fair value of its leased portion of the premises. Also, the Company has recognized a corresponding build-to-suit facility lease financing obligation with a current portion of $0.9 million classified as other liabilities and $15.3 million classified as other non-current liabilities on its balance sheet as of December 31, 2017. The Company’s expected costs to complete construction is estimated to be $0.7 million as of December 31, 2017. The construction is expected to be completed in the second quarter of 2018, upon which time the Company will assess and determine if the assets and corresponding liability should be de-recognized. As of December 31, 2017, minimum rental commitments under this lease for each of the next five fiscal years and total thereafter are as follows: 2018 $ 1,173 2019 1,686 2020 1,733 2021 1,780 2022 1,830 Thereafter 10,636 Total $ 18,838

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	17. Commitments and Contingencies Lease In March 2009, the Company entered into a lease agreement for office and laboratory space, which, as amended, has a term expiring on May 31, 2018. Monthly lease payments, inclusive of non-rent shared tenant occupancy costs, total less than $0.2 million. Monthly lease payments include base rent charges of $0.1 million, which are subject to an annual increase of 1.4%. The Company recognizes rent expense on a straight-line basis over the lease period and has recorded deferred rent for rent expense incurred but not yet paid. The Company issued an unconditional and irrevocable standby letter of credit in the amount of $0.3 million as a security deposit pursuant to the lease agreement. The irrevocable standby letter of credit is secured by a certificate of deposit, renews annually automatically and expires on May 31, 2018. The Company recorded the certificate of deposit purchase as restricted cash in its financial statements. The Company recorded rent expense of $1.7 million during each of the years ended December 31, 2017 and 2015 and $1.6 million during the year ended December 31, 2016, respectively. The following table summarizes the future minimum lease payments due under the operating lease as of December 31, 2017 (in thousands): 2018 $ 563 Total $ 563 Also see Note 16 for the Company’s build-to-suit lease and related lease commitments. Dr. Stelios Papadopoulos In May 2016, the Company entered into a Letter Agreement with Dr. Stelios Papadopoulos with an effective date of July 1, 2016 and a term of 36 months. Under the terms of the Letter Agreement, Dr. Papadopoulos will provide certain consulting and advisory services to the Company as and when requested. The Company will pay a quarterly retainer of $0.2 million to Dr. Papadopoulos for the duration of the Letter Agreement with aggregate fees totaling $2.5 million over the term of the Letter Agreement. The retainer may be paid in cash, common stock of the Company or a combination of the two at the discretion of the Company. Any common stock issued in settlement of payments due under this agreement will be valued at the average closing price of the stock for 20 trading days, as listed on the NASDAQ, ending three trading days prior to the issuance of the shares. Additionally, under the terms of the Letter Agreement, if the Company consummates a merger and acquisition (“M&A”) transaction, as defined in the Letter Agreement, with another party during the term of the Agreement or the 12-month period following the expiration of the Letter Agreement, Dr. Papadopoulos will be entitled to a M&A transaction fee equal to 1% of the value of the transaction, as defined in the Letter Agreement. During the years ended December 31, 2017 and 2016, the Company issued 188,462 and 27,221 shares under this consulting agreement for $0.8 million and $0.4 million, respectively, which is recorded within general and administrative expense in the accompanying statements of operations. The following table summarizes the future retainer payments due under the Letter Agreement as of December 31, 2017 (in thousands): 2018 $ 840 2019 420 Total $ 1,260 Collaboration and License Agreements The Company has entered into collaboration and license agreements under which it is obligated to make contingent payments (see Note 13). Research Commitments During the year ended December 31, 2015, the Company entered into research and development agreements with various vendors to provide chemists, research scientists and testing services to assist with its research and development efforts. Legal Proceedings The Company is not currently a party to any material legal proceedings. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to its legal proceedings. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company is not aware of any claims under indemnification arrangements, and it has not accrued any liabilities related to such obligations in its financial statements as of December 31, 2017 or 2016.

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2017
Postemployment Benefits [Abstract]
401(k) Savings Plan	18. 401(k) Savings Plan The Company established a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. This plan covers substantially all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. Company contributions to the plan may be made at the discretion of the Company’s board of directors. To date, no contributions have been made to the plan by the Company.

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (Unaudited)	19. Quarterly Financial Data (Unaudited) The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information (in thousands). Three Months Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 Revenue $ 1,021 $ 1,147 $ 1,551 $ 1,622 Operating expenses: Research and development 13,108 15,370 12,894 12,282 General and administrative 3,170 2,902 2,741 2,847 Total operating expenses 16,278 18,272 15,635 15,129 Loss from operations (15,257 ) (17,125 ) (14,084 ) (13,507 ) Interest income 191 169 155 126 Interest expense — — — — Other income (expense), net (30 ) (2 ) (25 ) (43 ) Net loss (15,096 ) (16,958 ) (13,954 ) (13,424 ) Modification of Series A preferred stock — — — — Modification of Series B preferred stock — — — — Accruing dividends on Series A preferred stock — — — — Net loss attributable to common stockholders $ (15,096 ) $ (16,958 ) $ (13,954 ) $ (13,424 ) Net loss per share attributable to common stockholders—basic and diluted $ (0.60 ) $ (0.68 ) $ (0.56 ) $ (0.51 ) Weighted average common shares outstanding—basic and diluted 25,020,337 25,040,131 25,093,344 26,465,521 Three Months Ended March 31, 2016 June 30, 2016 September 30, 2016 December 31, 2016 Revenue $ 1,158 $ 1,451 $ 1,715 $ 4,060 Operating expenses: Research and development 6,876 7,404 9,218 10,461 General and administrative 2,301 3,115 3,266 3,198 Total operating expenses 9,177 10,519 12,484 13,659 Loss from operations (8,019 ) (9,068 ) (10,769 ) (9,599 ) Interest income — 20 36 190 Interest expense — — — — Other income (expense), net 28 (5 ) (38 ) (8 ) Net loss and comprehensive loss (7,991 ) (9,053 ) (10,771 ) (9,417 ) Modification of Series A preferred stock — — — — Modification of Series B preferred stock — — — — Accruing dividends on Series A preferred stock (1,378 ) — — — Net loss attributable to common stockholders $ (9,369 ) $ (9,053 ) $ (10,771 ) $ (9,417 ) Net loss per share attributable to common stockholders—basic and diluted $ (0.87 ) $ (0.47 ) $ (0.54 ) $ (0.38 ) Weighted average common shares outstanding—basic and diluted 10,766,722 19,139,183 20,073,685 24,975,010

Nature of the Business (Policie

Nature of the Business (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Reverse Stock Split	Reverse Stock Split On January 19, 2016, the Company effected a 1-for-10.8102 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s Preferred Stock (see Note 10). Accordingly, all share and per share amounts for all periods presented in the accompanying financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the preferred stock conversion ratios.
Public Offerings	Public Offerings In February 2016, the Company issued and sold 6,250,000 shares of its common stock in its initial public offering (“IPO”) at a public offering price of $8.00 per share, for net proceeds of $42.5 million after deducting underwriting discounts and commissions of $3.5 million and other offering expenses of $4.0 million. Upon the closing of the IPO, all shares of convertible preferred stock then outstanding converted into an aggregate of 9,699,600 shares of common stock, and the Company issued 2,590,742 shares of common stock as payment of $36.0 million of accruing dividends due to holders of Series A preferred stock. In addition, the Company’s convertible preferred stock warrant outstanding at the close of the IPO converted to a warrant to purchase common stock. In connection with the completion of the IPO, the Company amended its certificate of incorporation to authorize the future issuance of up to 5,000,000 shares of undesignated preferred stock, par value $0.001 per share. As of December 31, 2017 and 2016, there were no shares of preferred stock issued or outstanding. In September 2016, the Company issued and sold 5,750,000 shares of its common stock in a public offering at a public offering price of $13.00 per share, for net proceeds of $69.5 million after deducting underwriter discounts and commissions of $4.5 million and other offering expenses of $0.8 million. The foregoing includes the exercise by the underwriters of their options to purchase an additional 750,000 shares of the Company’s common stock within 30 days following the date of the final prospectus for the offering. In December 2017, the Company issued and sold 9,200,000 shares of its common stock in a public offering at a public offering price of $5.00 per share, for net proceeds of $42.9 million after deducting underwriting discounts and commissions of $2.8 million and other offering expenses of $0.3 million. The foregoing includes the exercise by the underwriters of their option to purchase an additional 1,200,000 shares of the Company’s common stock within 30 days following the date of the final prospectus for the offering
Basis of Presentation	Basis of Presentation The financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”).
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, revenue recognition, the accrual for research and development expenses, including clinical trials, and the valuation of common stock, preferred stock warrant liability, derivative liability and build-to-suit lease obligation. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates.
Concentrations of Credit Risk and Significant Customers	Concentrations of Credit Risk and Significant Customers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, short-term investments and accounts receivable. The Company mitigates its risk with respect to cash, cash equivalents and short-term investments by maintaining its balances at two accredited financial institutions, in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company’s accounts receivable balances are due from the counterparty to its collaboration agreements (see Note 13) that the Company believes to be creditworthy. As of December 31, 2016, accounts receivable consisted of amounts due from one such counterparty.
Deferred Offering Costs	Deferred Offering Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering. As of December 31, 2015, the Company had recorded $2.7 million of deferred offering costs in contemplation of the Company’s IPO, which was completed on February 11, 2016. In conjunction with the completion of the IPO, the Company reclassified $4.0 million of deferred financing costs to additional paid-in capital as a reduction of the proceeds from the IPO. In September 2016, the Company reclassified $0.8 million of deferred financing costs to additional paid-in capital as a reduction of the proceeds from the follow-on public offering. In December 2017, the Company reclassified less than $0.4 million of deferred financing costs to additional paid-in capital as a reduction of the proceeds from its public offering, of which $0.3 million is included within accrued expenses and accounts payable as of December 31, 2017.
Cash Equivalents	Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less at the date of purchase to be cash equivalents. As of December 31, 2017, the Company’s cash equivalents consisted of money market funds, U.S. government-sponsored securities and U.S. treasury securities. The Company’s cash equivalents as of December 31, 2016 consisted of money market funds.
Restricted Cash	Restricted Cash As of December 31, 2017and 2016, short term restricted cash consisted of a certificate of deposit collateralizing a letter of credit issued as a security deposit in connection with the Company’s lease of its corporate facilities at 200 Technology Square, Cambridge, MA. On September 19, 2017, the Company entered into a lease agreement for a new corporate headquarters at 80 Guest Street, Brighton, MA (see Note 16). As of December 31, 2017, long term restricted cash consisted of a money market account in the amount of $1.7 million collateralizing a letter of credit issued as a security deposit for this lease agreement.
Short-Term Investments	Short-Term Investments Short-term investments represent holdings of available-for-sale marketable securities in accordance with the Company’s investment policy and cash management strategy. Short-term investments mature within one-year from the balance sheet date. Investments in marketable securities are recorded at fair value, with any unrealized gains and losses, net of taxes, reported as a component of stockholders’ equity until realized or until a determination is made that an other-than-temporary decline in market value has occurred. The cost of marketable securities sold is determined based on the specific identification method and any realized gains or losses on the sale of investments are reflected as a component of other expense, net.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the estimated useful life of each asset. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses on long-lived assets.
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s investments and its derivative liability are carried at fair value determined according to the fair value hierarchy described above (see Note 4). The carrying value of accounts receivable, accounts payable and accrued expenses approximate their fair value due to the short-term nature of these assets and liabilities.
Segment Information	Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is developing therapeutics to treat protein conformational diseases. All of the Company’s tangible assets are held in the United States. To date, all of the Company’s revenue has been generated in the United States.
Revenue Recognition	Revenue Recognition The Company recognizes revenue in accordance with Accounting Standards Codification (“ASC”) Topic 605, Revenue Recognition The Company records as deferred revenue any amounts received prior to satisfying the revenue recognition criteria. Deferred revenue not expected to be recognized within the next twelve months is reported as non-current deferred revenue.
Collaborative Research and License Agreements	Collaborative Research and License Agreements The terms of these agreements contain multiple deliverables, which may include licenses and research and development activities. The terms of these agreements may also include nonrefundable upfront license fees, payments for research and development activities, reimbursement of certain third-party costs, payments based upon the achievement of specified milestones, and royalty payments based on product sales derived from the collaboration. The Company evaluates multiple-element arrangements based on the guidance in ASC Topic 605-25, Revenue Recognition—Multiple-Element Arrangements (“ASC 605-25”) The consideration received under the arrangement that is fixed or determinable is then allocated among the separate units of accounting based on the relative selling prices of the separate units of accounting. The Company determines the selling price of a unit of accounting within each arrangement following the hierarchy of evidence prescribed by ASC 605-25. Accordingly, the Company determines the estimated selling price for units of accounting within each arrangement using vendor-specific objective evidence (“VSOE”), if available; third-party evidence (“TPE”) of selling price if VSOE is not available; or best estimate of selling price (“BESP”), if neither VSOE nor TPE is available. The Company typically uses BESP to estimate the selling price as it generally does not have VSOE or TPE of selling price for its units of accounting. Determining the BESP for a unit of accounting requires significant judgment. In developing the BESP for a unit of accounting, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the BESP for units of accounting by evaluating whether changes in the key assumptions used to determine the BESP will have a significant effect on the allocation of arrangement consideration between multiple units of accounting. The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria in ASC 605 are satisfied for that particular unit of accounting. In the event that a deliverable does not represent a separate unit of accounting, the Company recognizes revenue from the combined unit of accounting over the contractual or estimated performance period for the undelivered items, which is typically the term of the Company’s research and development obligations. If there is no discernible pattern of performance or objectively measurable performance measures do not exist, then the Company recognizes revenue under the arrangement on a straight-line basis over the period the Company is expected to complete its performance obligations. Conversely, if the pattern of performance over which the service is provided to the customer can be determined and objectively measurable performance measures exist, then the Company recognizes revenue under the arrangement using the proportional performance method. Revenue recognized is limited to the lesser of the cumulative amount of payments received or the cumulative amount of revenue earned, as determined using the straight-line method or proportional performance method, as applicable, as of the period ending date. At the inception of an arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether: (1) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item as a result of a specific outcome resulting from the Company’s performance to achieve the milestone, (2) the consideration relates solely to past performance, and (3) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluates factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the particular milestone and the level of effort and investment required to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether a milestone satisfies all of the criteria required to conclude that a milestone is substantive. The Company will recognize revenue in its entirety upon successful accomplishment of any substantive milestones, assuming all other revenue recognition criteria are met. Milestones that are not considered substantive are recognized as earned if there are no remaining performance obligations or over the remaining period of performance, with a cumulative catch-up for the elapsed portion of the research term, assuming all other revenue recognition criteria are met.
Research Grant Contracts	Research Grant Contracts Under these contracts, the Company is typically compensated for specific research or development activities. The Company recognizes revenue as the activities specified under the research grant contracts are performed and all of the revenue recognition criteria in ASC 605 are satisfied.
Adoption of ASC 606	Adoption of ASC 606 The Company will adopt new revenue accounting guidance in the first quarter of 2018, which it expects to impact this policy, the amount and timing of our future revenue recognition, and the amount of revenue previously recognized in our published financial statements. For further discussion, see Recent Accounting Pronouncements
Embedded Derivatives	Embedded Derivatives Embedded derivatives that are required to be bifurcated from the underlying host instrument are accounted for and valued as a separate financial instrument. An embedded derivative exists associated with an alternate payment option upon change of control within the research, development and commercialization agreement with Cystic Fibrosis Foundation Therapeutics, Inc. (see Note 13). The embedded derivative has been bifurcated and is classified as a liability on the balance sheet and separately accounted for at its fair value. The derivative liability is marked-to-market every reporting period. Changes in fair value of the derivative liability are recognized as a component of other income (expense), net in the accompanying statement of operations and comprehensive loss.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries, stock-based compensation and benefits, facilities costs, depreciation, third-party license fees, and external costs of outside vendors engaged to conduct preclinical development activities and clinical trials. Research and development expenses include the Company’s costs of performing services in connection with its collaboration agreements and research grant. Nonrefundable prepayments for goods or services that will be used or rendered for future research and development activities are deferred and capitalized. Such amounts are recognized as an expense as the goods are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered or the services rendered.
Research Contract Costs and Accruals	Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies both inside and outside of the United States. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.
Patent Costs	Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Preferred Stock Warrant Liability	Preferred Stock Warrant Liability Through February 2016, the Company classified a warrant to purchase shares of its Series A preferred stock as a liability on its balance sheets as this warrant was a free-standing financial instrument could have required the Company to transfer assets upon exercise. The warrant was initially recorded at fair value on date of grant and subsequently remeasured to fair value at each balance sheet date. Changes in fair value of the warrant were recognized as a component of other income (expense), net in the statement of operations. Following the reverse stock split of the common stock of the Company and the completion of the Company’s IPO in February 2016, the warrant became a warrant to purchase 14,800 shares of common stock. At that time, the warrant was reclassified to a component of stockholder’s equity and is no longer subject to remeasurement. The Company used the Black-Scholes option-pricing model, which incorporates assumptions and estimates, to value the preferred stock warrant. The Company has assessed these assumptions and estimates on a quarterly basis as additional information impacting the assumptions is obtained. Estimates and assumptions impacting the fair value measurement include the fair value per share of the underlying Series A preferred stock, the remaining contractual term of the warrant, risk-free interest rate, expected dividend yield and expected volatility of the price of the underlying preferred stock. The Company determined the fair value per share of the underlying preferred stock by taking into consideration the most recent sales of its convertible preferred stock, results obtained from third-party valuations and additional factors that are deemed relevant. Up until February 2016, the Company was a private company and therefore lacks company-specific historical and implied volatility information of its stock. Therefore, it estimates its expected stock volatility based on the historical volatility of publicly traded peer companies for a term equal to the remaining contractual term of the warrant. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the remaining contractual term of the warrant. Expected dividend yield is determined considering that the underlying Series A preferred stock is entitled to dividends of 8.0% per year, whether or not declared.
Stock-Based Compensation	Stock-Based Compensation The Company measures all stock options and other stock-based awards granted to employees and directors based on the fair value on the date of the grant and recognizes compensation expense of those awards, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues stock options and restricted stock awards to employees with only service- based vesting conditions and records the expense for these awards using the straight-line method. The Company measures stock-based awards granted to consultants and non-employees based on the fair value of the award on the date on which the related service is complete. Compensation expense is recognized over the period during which services are rendered by such consultants and non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is remeasured using the then-current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model. The Company does not recognize compensation expense for awards with performance-based vesting conditions granted to consultants and non-employees for which satisfaction of the performance conditions is not solely within the control of the holder until the performance conditions have been met. The Company classifies stock-based compensation expense in its statement of operations in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified. Prior to January 1, 2017, the Company recognized compensation expense for only the portion of awards that are expected to vest. In developing a forfeiture rate estimate, the Company has considered its historical experience to estimate pre- vesting forfeitures for service-based awards. Effective January 1, 2017, the company adopted ASU No. 2016-09 on a modified retrospective basis. As a result, the Company has made an accounting policy election to account for forfeitures as they occur The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option- pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends on common stock and does not expect to pay any cash dividends in the foreseeable future.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2017 and 2016, comprehensive loss included less than $0.1 million of unrealized gains and less than $0.1 million of unrealized losses on short-term investments, respectively. There was no difference between net loss and comprehensive loss for the year ended December 31, 2015.
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.
Net Loss per Share	Net Loss per Share Basic net loss per share is computed using the weighted average number of common shares outstanding during the period. Diluted net loss per share is computed using the sum of the weighted average number of common shares outstanding during the period and, if dilutive, the weighted average number of potential shares of common stock, including the assumed exercise of stock options and unvested restricted stock. Net loss per share attributable to common stockholders is calculated using the two-class method, which is an earnings allocation formula that determines net loss per share for the holders of the Company’s common shares and participating securities. The Company’s convertible preferred stock contains participation rights in any dividend paid by the Company and are deemed to be participating securities. Net loss attributable to common stockholders and participating preferred shares are allocated to each share on an as-converted basis as if all of the earnings for the period had been distributed. The participating securities do not include a contractual obligation to share in losses of the Company and are not included in the calculation of net loss per share in the periods that have a net loss. Diluted net loss per share is computed using the more dilutive of (a) the two-class method or (b) the if- converted method. The Company allocates earnings first to preferred stockholders based on dividend rights and then to common and preferred stockholders based on ownership interests. The weighted average number of common shares included in the computation of diluted net loss gives effect to all potentially dilutive common equivalent shares, including outstanding stock options, warrants, preferred stock and the potential issuance of stock upon the conversion of the Company’s convertible notes. Common stock equivalent shares are excluded from the computation of diluted net loss per share if their effect is antidilutive. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. The Company reported a net loss attributable to common stockholders for the years ended December 31, 2017, 2016 and 2015.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes existing revenue recognition guidance under GAAP. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The standard defines a five-step process to achieve this principle, and will require companies to use more judgment and make more estimates than under the current guidance. The Company expects that these judgments and estimates will include identifying performance obligations in the customer contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date , which delays the effective date of ASU 2014-09 such that the standard is effective for public entities for annual periods beginning after December 15, 2017 and for interim periods within those fiscal years. In April 2016, the FASB issued ASU 2016-10, Identifying Performance Obligations and Licensing , which clarifies two aspects of ASU 2014-09, Revenue from Contracts with Customers (Topic 606) : identifying performance obligations and the licensing implementation guidance. These new standards will become effective for the Company on January 1, 2018. Companies have the option of applying this new guidance retrospectively to each prior reporting period presented (the full retrospective method) or retrospectively with the cumulative effect of initially applying this update recognized at the date of initial application (the modified retrospective method). The Company only has one contract (see Note 13) within the scope of Topic 606 and intends to use the modified retrospective method for adoption. Since the inception of the contract, the Company has recognized $9.7 million in revenue. The Company is assessing, but has not yet completed its assessment of the impact of the adoption of this standard on its consolidated financial statements and related disclosures. Currently, the Company anticipates a potential impact on the revenue recognition method used to recognize revenue under its agreement with Astellas and the accounting for research funding payments based on a specified rate for time incurred as well as the recognition of milestone revenue prior to achievement. The expected impact is further described below. Estimated impacts from the adoption of this standard could differ upon the final adoption and implementation of the standard. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. ASC 606 also impacts certain other areas, such as the accounting for costs to obtain or fulfill a contract. With respect to its collaboration with Astellas, the Company currently expects the one performance obligation identified under the provisions of ASC 606 will be consistent with the one unit of accounting identified under the provisions of ASC 605. However, as previously described, it currently expects that the pattern of revenue recognition under step (v) above will differ from the pattern of revenue recognition under ASC 605. Any change in the timing or pattern of revenue recognition will have a corresponding change to the Company’s deferred revenue balance. The Company expects the accounting for research funding payments and over the three and a half year research term of the agreement, which commenced in January 2015, with a cumulative catch-up for the elapsed portion of the research term being recognized at the time any such payments are earned. ASC 606 requires more robust disclosures than required by previous guidance, including disclosures related to disaggregation of revenue into appropriate categories, performance obligations, the judgments made in revenue recognition determinations, adjustments to revenue which relate to activities from previous quarters or years, any significant reversals of revenue, and costs to obtain or fulfill contracts. In February 2016, the FASB issued ASU No. 2016-02, Leases. In March 2016, the FASB issued ASU No. 2016-09, Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230) - Restricted Cash In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (Topic 719): Scope of Modification Accounting
Build-to-suit Lease	The Company is not the legal owner of the leased space. However, in accordance with ASC 840, Leases, the Company is deemed to be the owner of the leased space during the construction period because of the Company’s involvement with the build-out of the space. As a result, the Company has capitalized approximately $16.1 million as construction-in-progress within property, plant and equipment equal to the estimated fair value of its leased portion of the premises. Also, the Company has recognized a corresponding build-to-suit facility lease financing obligation with a current portion of $0.9 million classified as other liabilities and $15.3 million classified as other non-current liabilities on its balance sheet as of December 31, 2017. The Company’s expected costs to complete construction is estimated to be $0.7 million as of December 31, 2017. The construction is expected to be completed in the second quarter of 2018, upon which time the Company will assess and determine if the assets and corresponding liability should be de-recognized.

Short-Term Investments (Tables

Short-Term Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Cash Cash Equivalents And Short Term Investments [Abstract]
Summary of Short-Term Investments	The following is a summary of the Company’s short-term investments as of December 31, 2017 and 2016 (in thousands) : December 31, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S government-sponsored enterprise securities $ 9,817 $ — $ (1 ) $ 9,816 U.S. treasury securities 34,923 1 (2 ) $ 34,922 $ 44,740 $ 1 $ (3 ) $ 44,738 December 31, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S government-sponsored enterprise securities $ 53,384 $ — $ (20 ) $ 53,364 U.S. treasury securities 13,535 — (2 ) 13,533 $ 66,919 $ — $ (22 ) $ 66,897

Fair Value of Financial Asset30

Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (in thousands): Fair Value Measurements as of December 30, 2017 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 13,871 $ — $ — $ 13,871 U.S. government-sponsored enterprise securities — 8,960 — 8,960 U.S. treasury securities — 3,497 — 3,497 Short-term investments: U.S. government-sponsored enterprise securities — 9,816 — 9,816 U.S. treasury securities — 34,922 — 34,922 $ 13,871 $ 57,195 $ — $ 71,066 Liabilities: Derivative liability $ — $ — $ 25 $ 25 $ — $ — $ 25 $ 25 Fair Value Measurements as of December 30, 2016 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 15,440 $ — $ — $ 15,440 Short-term investments: U.S. government-sponsored enterprise securities — 53,365 — 53,365 U.S. treasury securities — 13,532 — 13,532 $ 15,440 $ 66,897 $ — $ 82,337 Liabilities: Derivative liability $ — $ — $ 91 $ 91 $ — $ — $ 91 $ 91
Summary of Changes in Values of Preferred Stock Warrant Liability and Derivative Liability	Changes in the values of the preferred stock warrant liability and the derivative liability are summarized below (in thousands): Preferred Warrant Liability Derivative Liability Fair value at December 31, 2014 $ 120 $ 65 Change in fair value (10 ) (63 ) Fair value at December 31, 2015 110 2 Change in fair value (82 ) 89 Conversion to common stock warrant (28 ) — Fair value at December 31, 2016 — 91 Change in fair value — (66 ) Fair value at December 31, 2017 $ — $ 25

Prepaids and Other Current As31

Prepaids and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2017
Prepaid Expense And Other Assets Current [Abstract]
Schedule of Prepaids and Other Current Assets	Prepaids and other current assets consisted of the following (in thousands): December 31, 2017 2016 Prepaid clinical, manufacturing and scientific expenses $ 568 $ 1,390 Prepaid insurance expenses 55 104 Other prepaid expenses and other current assets 754 2,565 $ 1,377 $ 4,059

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): Useful Life December 31, (Years) 2017 2016 Laboratory equipment 5 years $ 3,299 $ 3,192 Furniture and fixtures 5 years 158 147 Leasehold improvements Shorter of the lease term or 5 years 382 382 Computer and office equipment 3 – 5 years 143 143 Construction-in-progress 16,143 — 20,125 3,864 Less: Accumulated depreciation and amortization (3,558 ) (3,323 ) $ 16,567 $ 541

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following (in thousands): December 31, 2017 2016 Accrued payroll and related expenses $ 1,542 $ 1,813 Accrued research and development expenses 3,930 1,612 Accrued professional fees 556 383 Accrued other 92 520 $ 6,120 $ 4,328

Preferred Stock Warrant Liabi34

Preferred Stock Warrant Liability (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Assumptions and Inputs were Used in Determining Fair Value of Preferred Stock Warrant Liability	The following assumptions and inputs were used in determining the fair value of the preferred stock warrant liability valued using the Black-Scholes option-pricing model: As of February 10, 2016 Expected term (in years) 2.25 Expected volatility 73.24 % Risk-free interest rate 0.74 % Expected dividend yield 8.0 %

Convertible Preferred Stock (Ta

Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Preferred Stock	As of December 31, 2015, Preferred Stock consisted of the following: Preferred Shares Authorized Preferred Shares Issued and Outstanding Carrying Value Liquidation Preference Common Stock Issuable Upon Conversion Series A preferred stock 76,000,000 75,463,000 $ 75,378 $ 111,594 6,980,712 Series B preferred stock 34,057,398 29,391,769 36,914 37,798 2,718,888 110,057,398 104,854,769 $ 112,292 $ 149,392 9,699,600

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Share-based Payment Award Stock Options Valuation Assumptions	The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis: Year Ended December 31, 2017 2016 2015 Risk-free interest rates 2.00 % 1.35 % 1.68 % Expected term (in years) 5.96 6.09 5.99 Expected volatility 75.18 % 73.19 % 56.86 % Expected dividend yield — — —
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity since December 31, 2016 (in thousands except share and per share amounts): Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (Years) Outstanding at December 31, 2016 2,030,517 $ 7.29 8.42 $ 10,510 Granted 1,201,789 6.88 Exercised (11,714 ) 2.68 Forfeited (382,034 ) 10.59 Outstanding at December 31, 2017 2,838,558 $ 6.69 8.10 $ 3,213 Vested and expected to vest at December 31, 2017 2,755,308 $ 6.82 8.25 $ 2,926 Options exercisable at December 31, 2017 1,054,350 $ 6.29 7.18 $ 1,404
Summary of Stock-Based Compensation Expense	Stock-based compensation expense was classified in the statements of operations and comprehensive loss as follows: Year Ended December 31, 2017 2016 2015 Research and development $ 1,326 $ 556 $ 72 General and administrative 2,406 1,608 400 $ 3,732 $ 2,164 $ 472

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation	A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: December 31, 2017 2016 2015 Federal statutory income tax rate (34.0 )% (34.0 )% (34.0 )% State income taxes, net of federal benefit (5.6 ) (5.1 ) (5.0 ) Research and development tax credit carryforwards (3.2 ) (4.3 ) (4.4 ) Expiration of state net operating loss carryforwards — — — Non-deductible expenses 0.7 0.5 1.2 Change in deferred tax asset valuation allowance 2.0 42.9 42.2 Effect of U.S. Tax Reform 40.1 — — Effective income tax rate (0.0 )% (0.0 )% (0.0 )%
Schedule of Net Deferred Tax Assets	Net deferred tax assets as of December 31, 2017 and 2016 consisted of the following (in thousands): December 31, 2017 2016 Net operating loss carryforwards 52,861 $ 53,723 Research and development tax credit carryforwards 9,232 6,633 Accrued expenses and other 1,583 2,152 Total deferred tax assets before valuation allowance 63,676 62,508 Valuation allowance (63,660 ) (62,442 ) Net deferred tax assets 16 66 Depreciation (16 ) (66 ) Net deferred tax assets $ — $ —
Changes in Valuation Allowance for Deferred Tax Assets	Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2017 and 2016 related primarily to the increases in net operating loss carryforwards and research and development tax credit carryforwards and were as follows (in thousands): Year Ended December 2017 2016 Valuation allowance at beginning of year $ (62,442 ) $ (46,476 ) Increases recorded to income tax provision (1,218 ) (15,966 ) Valuation allowance at end of year $ (63,660 ) $ (62,442 )

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands except share and per share amounts): Year Ended December 31, 2017 2016 2015 Numerator: Net loss $ (59,432 ) $ (37,232 ) $ (25,040 ) Modifications of Series A preferred stock — — 11,481 Modifications of Series B preferred stock — — (26 ) Accruing dividends on preferred stock — (1,378 ) (9,724 ) Net loss attributable to common stockholders $ (59,432 ) $ (38,610 ) $ (23,309 ) Denominator: Weighted average number of common shares outstanding—basic and diluted 25,407,943 18,759,369 553,162 Net loss per share attributable to common stockholders—basic and diluted $ (2.34 ) $ (2.06 ) $ (42.14 )
Schedule of Antidilutive Securities Excluded from Computation of Diluted Net Loss Per Share	The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: December 31, 2017 2016 2015 Convertible preferred stock (as converted to common stock) — — 9,699,600 Payment of accruing dividends on Series A preferred stock in shares of common stock upon conversion of Series A preferred stock — — 2,590,742 Options to purchase common stock 2,838,558 2,030,517 979,334 Warrant for the purchase of convertible preferred stock (as converted to common stock) — 14,800 14,800 2,838,558 2,045,317 13,284,476

Build-to-suit Lease (Tables)

Build-to-suit Lease (Tables)	12 Months Ended
	Dec. 31, 2017
Summary of Future Minimum Lease Payments	The following table summarizes the future minimum lease payments due under the operating lease as of December 31, 2017 (in thousands): 2018 $ 563 Total $ 563
Build-to-suit Lease [Member]
Summary of Future Minimum Lease Payments	As of December 31, 2017, minimum rental commitments under this lease for each of the next five fiscal years and total thereafter are as follows: 2018 $ 1,173 2019 1,686 2020 1,733 2021 1,780 2022 1,830 Thereafter 10,636 Total $ 18,838

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Lease Payments	The following table summarizes the future minimum lease payments due under the operating lease as of December 31, 2017 (in thousands): 2018 $ 563 Total $ 563
Summary of Future Retainer Payment Due Under Letter Agreement	The following table summarizes the future retainer payments due under the Letter Agreement as of December 31, 2017 (in thousands): 2018 $ 840 2019 420 Total $ 1,260

Quarterly Financial Data (Una41

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Summary of Quarterly Financial Information	The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information (in thousands). Three Months Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 Revenue $ 1,021 $ 1,147 $ 1,551 $ 1,622 Operating expenses: Research and development 13,108 15,370 12,894 12,282 General and administrative 3,170 2,902 2,741 2,847 Total operating expenses 16,278 18,272 15,635 15,129 Loss from operations (15,257 ) (17,125 ) (14,084 ) (13,507 ) Interest income 191 169 155 126 Interest expense — — — — Other income (expense), net (30 ) (2 ) (25 ) (43 ) Net loss (15,096 ) (16,958 ) (13,954 ) (13,424 ) Modification of Series A preferred stock — — — — Modification of Series B preferred stock — — — — Accruing dividends on Series A preferred stock — — — — Net loss attributable to common stockholders $ (15,096 ) $ (16,958 ) $ (13,954 ) $ (13,424 ) Net loss per share attributable to common stockholders—basic and diluted $ (0.60 ) $ (0.68 ) $ (0.56 ) $ (0.51 ) Weighted average common shares outstanding—basic and diluted 25,020,337 25,040,131 25,093,344 26,465,521 Three Months Ended March 31, 2016 June 30, 2016 September 30, 2016 December 31, 2016 Revenue $ 1,158 $ 1,451 $ 1,715 $ 4,060 Operating expenses: Research and development 6,876 7,404 9,218 10,461 General and administrative 2,301 3,115 3,266 3,198 Total operating expenses 9,177 10,519 12,484 13,659 Loss from operations (8,019 ) (9,068 ) (10,769 ) (9,599 ) Interest income — 20 36 190 Interest expense — — — — Other income (expense), net 28 (5 ) (38 ) (8 ) Net loss and comprehensive loss (7,991 ) (9,053 ) (10,771 ) (9,417 ) Modification of Series A preferred stock — — — — Modification of Series B preferred stock — — — — Accruing dividends on Series A preferred stock (1,378 ) — — — Net loss attributable to common stockholders $ (9,369 ) $ (9,053 ) $ (10,771 ) $ (9,417 ) Net loss per share attributable to common stockholders—basic and diluted $ (0.87 ) $ (0.47 ) $ (0.54 ) $ (0.38 ) Weighted average common shares outstanding—basic and diluted 10,766,722 19,139,183 20,073,685 24,975,010

Nature of the Business - Additi

Nature of the Business - Additional Information (Detail)	Oct. 31, 2017Position	Feb. 11, 2016USD ($)$ / sharesshares	Jan. 19, 2016	Dec. 31, 2017USD ($)$ / sharesshares	Sep. 30, 2016USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)$ / sharesshares	Sep. 30, 2016USD ($)$ / shares	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)$ / sharesshares	Dec. 31, 2015USD ($)$ / shares	Sep. 30, 2015USD ($)	Jan. 31, 2014$ / sharesshares
Nature Of Business [Line Items]
Accumulated deficit				$ (216,882,000)		$ (216,882,000)				$ (157,450,000)				$ (216,882,000)	$ (157,450,000)
Cash, cash equivalents and short-term investments				$ 74,500,000		74,500,000								74,500,000
Net loss						(13,424,000)	$ (13,954,000)	$ (16,958,000)	$ (15,096,000)	$ (9,417,000)	$ (10,771,000)	$ (9,053,000)	$ (7,991,000)	(59,432,000)	(37,232,000)	$ (25,040,000)
Cash in operations														$ 52,395,000	41,925,000	$ 19,415,000
Number of positions eliminated | Position	13
One-time severance and related costs						$ 200,000
Reverse stock split description														On January 19, 2016, the Company effected a 1-for-10.8102 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s Preferred Stock
Reverse stock split ratio			10.8102
Reverse stock split effective date														Jan. 19, 2016
Proceeds from IPO, net of underwriting discounts, commissions, and other offering expenses															$ 46,500,000
Preferred stock, authorize for future issuance | shares				5,000,000		5,000,000				5,000,000				5,000,000	5,000,000
Preferred stock, par value | $ / shares				$ 0.001		$ 0.001				$ 0.001				$ 0.001	$ 0.001	$ 0.001
Preferred stock, shares issued | shares				0		0				0				0	0
Preferred stock, shares outstanding | shares				0		0				0				0	0
Proceeds from stock issuance, net of underwriting discounts, commissions, and other offering expenses														$ 43,240,000	$ 70,265,000
Minimum [Member]
Nature Of Business [Line Items]
Proceeds from IPO, net of underwriting discounts, commissions, and other offering expenses														$ 50,000,000
IPO [Member]
Nature Of Business [Line Items]
Common stock issued and sold | shares		6,250,000
Common stock price per share | $ / shares		$ 8
Proceeds from IPO, net of underwriting discounts, commissions, and other offering expenses		$ 42,500,000
Underwriting discounts and commissions		3,500,000
Other offering expenses		$ 4,000,000
Number shares issued in share conversion | shares		9,699,600
Number shares issued as payment for accrued dividend | shares		2,590,742
Preferred stock, authorize for future issuance | shares		5,000,000		5,000,000		5,000,000								5,000,000
Preferred stock, par value | $ / shares		$ 0.001		$ 0.001		$ 0.001								$ 0.001
Preferred stock, shares issued | shares				0		0				0				0	0
Preferred stock, shares outstanding | shares				0		0				0				0	0
Follow-on Public Offering [Member]
Nature Of Business [Line Items]
Common stock issued and sold | shares				9,200,000	5,750,000
Common stock price per share | $ / shares				$ 5	$ 13	$ 5					$ 13			$ 5
Underwriting discounts and commissions				$ 2,800,000	$ 4,500,000
Other offering expenses				300,000	800,000
Proceeds from stock issuance, net of underwriting discounts, commissions, and other offering expenses				$ 42,900,000	$ 69,500,000
Over Allotment [Member]
Nature Of Business [Line Items]
Common stock issued and sold | shares				1,200,000	750,000
Over Allotment [Member] | Maximum [Member]
Nature Of Business [Line Items]
Period granted for purchase of additional shares				30 days	30 days
Series A Preferred Stock [Member]
Nature Of Business [Line Items]
Common stock price per share | $ / shares																		$ 1
Accrued dividends		$ 36,000,000															$ 36,000,000
Preferred stock, shares issued | shares																		5,000,000

Summary of Significant Accoun43

Summary of Significant Accounting Policies - Additional Information (Detail)	Feb. 11, 2016USD ($)	Dec. 17, 2015	Dec. 31, 2017USD ($)FinancialInstitution	Sep. 30, 2016USD ($)	Sep. 30, 2015	Jan. 31, 2014USD ($)	Dec. 31, 2017USD ($)FinancialInstitution	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2017USD ($)FinancialInstitution	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2017USD ($)FinancialInstitution	Feb. 28, 2016shares
Significant Accounting Policies [Line Items]
Number of accredited financial institutions | FinancialInstitution			2				2								2			2
Deferred offering costs																	$ 2,700,000
Reclassification of deferred offering costs to additional paid-in capital	$ 4,000,000			$ 800,000
Deferred offering costs included within accrued expenses and accounts payable															$ 321,000		1,288,000
Long term restricted cash			$ 1,656,000				$ 1,656,000				$ 294,000				1,656,000	$ 294,000		$ 1,656,000
Unrealized gains (loss) on short-term investments included in comprehensive loss															20,000	(22,000)
Revenue recognized							1,622,000	$ 1,551,000	$ 1,147,000	$ 1,021,000	$ 4,060,000	$ 1,715,000	$ 1,451,000	$ 1,158,000	$ 5,341,000	8,384,000	$ 4,312,000
ASU 2014-09 [Member]
Significant Accounting Policies [Line Items]
Revenue recognized																		9,700,000
Warrants to Purchase Convertible Preferred Stock [Member]
Significant Accounting Policies [Line Items]
Number of warrants to purchase | shares																			14,800
Series A Preferred Stock [Member]
Significant Accounting Policies [Line Items]
Reclassification of deferred offering costs to additional paid-in capital						$ 100,000
Dividends percentage		8.00%			8.00%										8.00%
Money Market Funds [Member]
Significant Accounting Policies [Line Items]
Long term restricted cash			1,700,000				$ 1,700,000								$ 1,700,000			$ 1,700,000
Maximum [Member]
Significant Accounting Policies [Line Items]
Reclassification of deferred offering costs to additional paid-in capital			$ 400,000
Maximum [Member] | Short-term Investments [Member]
Significant Accounting Policies [Line Items]
Unrealized gains (loss) on short-term investments included in comprehensive loss															$ 100,000	$ (100,000)

Short-Term Investments - Summar

Short-Term Investments - Summary of Short-Term Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 44,740	$ 66,919
Gross Unrealized Gains	1
Gross Unrealized Losses	(3)	(22)
Fair Value	44,738	66,897
U.S Government-Sponsored Enterprise Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,817	53,384
Gross Unrealized Losses	(1)	(20)
Fair Value	9,816	53,364
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	34,923	13,535
Gross Unrealized Gains	1
Gross Unrealized Losses	(2)	(2)
Fair Value	$ 34,922	$ 13,533

Short-Term Investments - Additi

Short-Term Investments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Realized gains (losses) on short-term investments	$ 0	$ 0
Other-than-temporary impairments recognized	$ 0	$ 0

Fair Value of Financial Asset46

Fair Value of Financial Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liabilities:
Derivative liability	$ 25	$ 91	$ 2	$ 65
Fair Value, Measurements, Recurring [Member]
Assets:
Total assets	71,066	82,337
Liabilities:
Derivative liability	25	91
Total liabilities	25	91
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	13,871	15,440
Fair Value, Measurements, Recurring [Member] | U.S Government-Sponsored Enterprise Securities [Member]
Assets:
Cash equivalents	8,960
Short-term investments	9,816	53,365
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Assets:
Cash equivalents	3,497
Short-term investments	34,922	13,532
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Total assets	13,871	15,440
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	13,871	15,440
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Total assets	57,195	66,897
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S Government-Sponsored Enterprise Securities [Member]
Assets:
Cash equivalents	8,960
Short-term investments	9,816	53,365
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury Securities [Member]
Assets:
Cash equivalents	3,497
Short-term investments	34,922	13,532
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Liabilities:
Derivative liability	25	91
Total liabilities	$ 25	$ 91

Fair Value of Financial Asset47

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 30, 2016	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Transfer of assets from level 1 to level 2	$ 0		$ 0	$ 0
Transfer of assets from level 2 to level 1	0		0	0
Transfer of liabilities from level 1 to level 2	0		0	0
Transfer of liabilities from level 2 to level 1	$ 0		$ 0	$ 0
Fair value determination model	Monte-Carlo simulation analysis
Expected term	2 years	2 years 6 months
Expected volatility	80.00%	81.10%
Risk-free interest rate	1.87%	1.33%
Expected sales-based milestone payments	$ 28,500,000	$ 28,500,000
Percentage of expected future milestone payments	25.00%	25.00%

Fair Value of Financial Asset48

Fair Value of Financial Assets and Liabilities - Summary of Changes in Values of Preferred Stock Warrant Liability and Derivative Liability (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]
Preferred Stock Warrant Liability, Fair value, Beginning Balance		$ 110	$ 120
Preferred Stock Warrant Liability, Change in fair value		(82)	(10)
Preferred Stock Warrant Liability, Conversion to common stock warrant		(28)
Preferred Stock Warrant Liability, Fair value, Ending Balance			110
Derivative Liability, Fair value, Beginning Balance	$ 91	2	65
Derivative Liability, Change in fair value	(66)	89	(63)
Derivative Liability, Fair value, Ending Balance	$ 25	$ 91	$ 2

Prepaids and Other Current As49

Prepaids and Other Current Assets - Schedule of Prepaids and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Prepaid Expense And Other Assets Current [Abstract]
Prepaid clinical, manufacturing and scientific expenses	$ 568	$ 1,390
Prepaid insurance expenses	55	104
Other prepaid expenses and other current assets	754	2,565
Prepaids and other current assets	$ 1,377	$ 4,059

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	$ 20,125	$ 3,864
Less: Accumulated depreciation and amortization	(3,558)	(3,323)
Property and Equipment, Net	$ 16,567	541
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Useful Life (years)	5 years
Property and Equipment, Gross	$ 3,299	3,192
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Useful Life (years)	5 years
Property and Equipment, Gross	$ 158	147
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Useful Life (years)	Shorter of the lease term or 5 years
Property and Equipment, Gross	$ 382	382
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	$ 143	$ 143
Computer and Office Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Useful Life (years)	3 years
Computer and Office Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Useful Life (years)	5 years
Construction-in-Progress [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	$ 16,143

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Depreciation and amortization	$ 235	$ 306	$ 272

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued payroll and related expenses	$ 1,542	$ 1,813
Accrued research and development expenses	3,930	1,612
Accrued professional fees	556	383
Accrued other	92	520
Total accrued expenses	$ 6,120	$ 4,328

Convertible Promissory Notes -

Convertible Promissory Notes - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Sep. 02, 2015	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Jul. 31, 2015	Jul. 31, 2014
Line of Credit Facility [Line Items]
Proceeds from issuance of convertible promissory notes			$ 5,000	$ 10,000
Series B Preferred Stock [Member]
Line of Credit Facility [Line Items]
Conversion of notes payable to preferred stock		$ 15,000
Convertible Notes Payable [Member]
Line of Credit Facility [Line Items]
Promissory note agreement amount					$ 5,000	$ 10,000
Principal amount of notes outstanding					$ 15,000
Convertible Notes Payable [Member] | Series B Preferred Stock [Member]
Line of Credit Facility [Line Items]
Conversion of notes payable to preferred stock	$ 15,000
Conversion of notes payable to preferred stock, accrued interest	$ 800
Stock issued during period upon conversion of debt, shares	12,284,466
Convertible notes, conversion price per share	$ 1.286

Preferred Stock Warrant Liabi54

Preferred Stock Warrant Liability - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Jul. 31, 2008	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 31, 2014
Class of Warrant or Right [Line Items]
Gains (losses) on fair value of warrants			$ (82,000)	$ (10,000)
Common Stock [Member]
Class of Warrant or Right [Line Items]
Shares of common stock issued in net settlement of warrant exercise		4,349
Series A Preferred Stock [Member]
Class of Warrant or Right [Line Items]
Exercise price					$ 1
Fair value of warrants		$ 100,000
Series A Preferred Stock [Member] | Maximum [Member]
Class of Warrant or Right [Line Items]
Gains (losses) on fair value of warrants			$ 100,000	$ 100,000
Series A Preferred Stock [Member] | Warrant [Member]
Class of Warrant or Right [Line Items]
Exercised shares	160,000
Exercise price	$ 1
Shares exercised period	10 years

Preferred Stock Warrant Liabi55

Preferred Stock Warrant Liability - Assumptions and Inputs were Used in Determining Fair Value of Preferred Stock Warrant Liability (Detail)	Feb. 10, 2016	Dec. 31, 2017	Dec. 31, 2016
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Expected term (in years)		2 years	2 years 6 months
Expected volatility		80.00%	81.10%
Risk-free interest rate		1.87%	1.33%
Preferred Stock Warrant Liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Expected term (in years)	2 years 3 months
Expected volatility	73.24%
Risk-free interest rate	0.74%
Expected dividend yield	8.00%

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Detail) - USD ($)	Feb. 11, 2016	Dec. 17, 2015	Dec. 31, 2017	Sep. 30, 2016	Sep. 30, 2015	May 31, 2015	Sep. 30, 2014	Jan. 31, 2014	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 30, 2014
Class of Stock [Line Items]
Preferred Shares Authorized												110,057,398
Preferred stock, par value			$ 0.001							$ 0.001	$ 0.001	$ 0.001
Stock issued during the period, share			0							0	0
Issuance cost	$ 4,000,000			$ 800,000
Preferred dividend description										In September 2015, in connection with the Series B preferred stock financing, (1) the right of the holders of Series A preferred stock to receive accruing dividends, whether or not declared, at a rate of 8.0% per year of the Original Issue Price per share (as described below) ceased as of August 31, 2015, at which time such cumulative accruing dividends totaled $36.0 million, and (2) the right of holders of Series A preferred stock to receive a cash payment of accruing dividends upon the automatic conversion of the Series A preferred stock into common stock was modified such that all previously accrued but unpaid dividends on Series A preferred stock will be paid in shares of common stock, at a price of $13.9019172 per share, upon such conversion of the Series A preferred stock. If as of January 1, 2016, the Series A preferred stock has not automatically converted into shares of common stock according to its terms, the holders of the Series A preferred stock will be entitled to receive dividends at the rate of 8.0% per year of the respective Original Issue Price per share, commencing as of the respective original issuance date of the Series A preferred stock. These amendments to the Series A preferred stock dividend rights were accounted for as a modification of preferred stock based on a quantitative assessment of the change in the fair value that resulted from the modification of the dividend rights on the modification date. The decrease in the fair value of Series A preferred stock measured immediately before the modification and immediately after the modification, equal to $0.2 million, was recorded as a deemed dividend from holders of preferred stock to the holders of common stock, resulting in an increase to additional paid-in capital of $0.2 million and a corresponding decrease of $0.2 to the carrying value of the Series A preferred stock.
Modifications of preferred stock									$ 1,378,000		$ 1,378,000	$ 9,724,000
Proceeds of IPO qualified for conversion of preferred stock											$ 46,500,000
IPO [Member]
Class of Stock [Line Items]
Preferred stock, par value	$ 0.001		$ 0.001							$ 0.001
Stock issued during the period, share			0							0	0
Preferred stock, issuance price	$ 8
Stock issued during the period, share	6,250,000
Number shares issued as payment for accrued dividend	2,590,742
Proceeds of IPO qualified for conversion of preferred stock	$ 42,500,000
Number shares issued in share conversion	9,699,600
Maximum [Member]
Class of Stock [Line Items]
Issuance cost			$ 400,000
Minimum [Member]
Class of Stock [Line Items]
Proceeds of IPO qualified for conversion of preferred stock										$ 50,000,000
Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Conversion of convertible preferred stock, Shares											(104,854,769)
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Shares Authorized												76,000,000
Stock issued during the period, share								5,000,000
Preferred stock, issuance price								$ 1
Proceeds from issuance of preferred stock value								$ 4,900,000
Issuance cost								$ 100,000
Conversion price per share						$ 10.8102	$ 3.6030397						$ 10.8102
Modifications of preferred stock												$ 11,481,000
Preferred stock percentage		8.00%			8.00%					8.00%
Cumulative accrued dividends	$ 36,000,000				$ 36,000,000
Dividends per share					$ 13.9019172
Series A Preferred Stock [Member] | Dividend [Member]
Class of Stock [Line Items]
Modifications of preferred stock						$ 10,600,000	$ 6,000,000
Series A Preferred Stock [Member] | Additional Paid-in Capital [Member]
Class of Stock [Line Items]
Modifications of preferred stock						10,600,000	1,000,000
Modifications of preferred stock		$ (800,000)										11,481,000
Modifications of preferred stock					$ 200,000
Series A Preferred Stock [Member] | Accumulated Deficit [Member]
Class of Stock [Line Items]
Modifications of preferred stock							$ 5,000,000
Series A Preferred Stock [Member] | Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Modifications of preferred stock		800,000				$ 10,600,000						$ (11,481,000)
Modifications of preferred stock					$ 200,000
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Shares Authorized												34,057,398
Preferred stock, issuance price					$ 1.286
Proceeds from issuance of preferred stock value					$ 21,100,000
Issuance cost					$ 900,000							$ 910,000
Modifications of preferred stock												(26,000)
Stock issued during the period, share					17,107,303
Aggregate promissory notes converted					$ 15,000,000
Aggregate accrued interest					$ 800,000
Conversion of convertible preferred stock, Shares					12,284,466
Share price					$ 1.286
Preferred stock percentage					8.00%
Series B Preferred Stock [Member] | Additional Paid-in Capital [Member]
Class of Stock [Line Items]
Modifications of preferred stock												(26,000)
Series B Preferred Stock [Member] | Additional Paid-in Capital [Member] | Maximum [Member]
Class of Stock [Line Items]
Modifications of preferred stock		(100,000)
Series B Preferred Stock [Member] | Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Modifications of preferred stock												$ 26,000
Stock issued during the period, share												17,107,303
Conversion of convertible preferred stock, Shares												12,284,466
Series B Preferred Stock [Member] | Convertible Preferred Stock [Member] | Maximum [Member]
Class of Stock [Line Items]
Modifications of preferred stock		$ 100,000

Convertible Preferred Stock (De

Convertible Preferred Stock (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2015USD ($)shares
Temporary Equity [Line Items]
Preferred Shares Authorized	110,057,398
Preferred Shares Issued	104,854,769
Preferred Shares Outstanding	104,854,769
Carrying Value | $	$ 112,292
Liquidation Preference | $	$ 149,392
Common Stock Issuable Upon Conversion	9,699,600
Series A Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	76,000,000
Preferred Shares Issued	75,463,000
Preferred Shares Outstanding	75,463,000
Carrying Value | $	$ 75,378
Liquidation Preference | $	$ 111,594
Common Stock Issuable Upon Conversion	6,980,712
Series B Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	34,057,398
Preferred Shares Issued	29,391,769
Preferred Shares Outstanding	29,391,769
Carrying Value | $	$ 36,914
Liquidation Preference | $	$ 37,798
Common Stock Issuable Upon Conversion	2,718,888

Common and Preferred Stock - Ad

Common and Preferred Stock - Additional Information (Detail) - USD ($)	Feb. 11, 2016	Dec. 31, 2017	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jan. 31, 2014
Class of Stock [Line Items]
Common stock, shares authorized		125,000,000		125,000,000	125,000,000
Common stock, par value		$ 0.001		$ 0.001	$ 0.001
Preferred stock, authorize for future issuance		5,000,000		5,000,000	5,000,000
Preferred stock, par value		$ 0.001		$ 0.001	$ 0.001	$ 0.001
Proceeds from stock issuance, net of underwriting discounts, commissions, and other offering expenses				$ 43,240,000	$ 70,265,000
Voting description				Each share of common stock entitles the holder to one vote for the election of directors and on all matters submitted to a vote of the Company’s stockholders.
Common dividends declared				$ 0
2008 Plan and 2016 Plan [Member]
Class of Stock [Line Items]
Common stock, reserved for exercise of outstanding stock options		3,741,251		3,741,251
IPO [Member]
Class of Stock [Line Items]
Common stock issued and sold	6,250,000
Common stock price per share	$ 8
Number shares issued in share conversion	9,699,600
Number shares issued as payment for accrued dividend	2,590,742
Preferred stock, authorize for future issuance	5,000,000	5,000,000		5,000,000
Preferred stock, par value	$ 0.001	$ 0.001		$ 0.001
Underwriting discounts and commissions	$ 3,500,000
Other offering expenses	4,000,000
Follow-on Public Offering [Member]
Class of Stock [Line Items]
Common stock issued and sold		9,200,000	5,750,000
Common stock price per share		$ 5	$ 13	$ 5
Proceeds from stock issuance, net of underwriting discounts, commissions, and other offering expenses		$ 42,900,000	$ 69,500,000
Underwriting discounts and commissions		2,800,000	4,500,000
Other offering expenses		$ 300,000	$ 800,000
Over Allotment [Member]
Class of Stock [Line Items]
Common stock issued and sold		1,200,000	750,000
Over Allotment [Member] | Maximum [Member]
Class of Stock [Line Items]
Period granted for purchase of additional shares		30 days	30 days
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Common stock price per share								$ 1
Accrued dividends	$ 36,000,000						$ 36,000,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	Feb. 03, 2016	Oct. 28, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of option					$ 2,500,000	$ 1,100,000	$ 500,000
Stock option grants					1,201,789
Weighted average exercise price					$ 4.57	$ 5.47	$ 7.13
Risk-free interest rate					2.00%	1.35%	1.68%
Expected terms					5 years 11 months 15 days	6 years 1 month 2 days	5 years 11 months 27 days
Expected volatility rate					75.18%	73.19%	56.86%
Expected dividend rate					0.00%	0.00%	0.00%
Proceeds from stock options exercised					$ 31,000	$ 268,000	$ 95,000
Aggregate intrinsic value of the stock option exercised						$ 400,000
Reduction in stock-based compensation expense			$ 500,000
Unrecognized compensation cost related to the unvested stock-based awards					$ 6,600,000
Compensation cost not yet recognized, period for recognition					2 years 6 months 25 days
Prior to 2015 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reduction in stock-based compensation expense				$ 200,000
Adjustments For Immaterial Errors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reduction in stock-based compensation expense				$ 300,000
Non-Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options unvested					83,250	83,250
Fair value of option					$ 300,000	$ 300,000
Stock option grants					0	0	0
Non-Employees [Member] | Prior to 2013 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option issued to purchase common stock					203,964
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from stock options exercised					$ 100,000
Aggregate intrinsic value of the stock option exercised					$ 100,000
2008 Plan [Member] | Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option vesting period					4 years
2008 Plan [Member] | Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option vesting period					4 years
2008 Plan [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of exercise price of fair value per share on grant date					100.00%
2008 Plan [Member] | Minimum [Member] | Non-Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option vesting period					6 months
2008 Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option granted, exercisable period					10 years
2008 Plan [Member] | Maximum [Member] | Non-Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option vesting period					4 years
2016 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option vesting period	10 years
Common stock reserved for issuance	1,581,839
Common stock reserved for issuance, percentage of number of shares of common stock outstanding	3.00%
2016 Plan [Member] | Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option vesting period	4 years
2016 Plan [Member] | Employees [Member] | One-Year Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option vesting percentage	25.00%
2016 Plan [Member] | Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option vesting period	3 years
2016 ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance	138,757
Common stock reserved for issuance, percentage of number of shares of common stock outstanding	1.00%
Common stock available for issuance					138,757
Number of shares issued					10,829	0
One Time Stock Option Grant to non-employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized		9,250
Weighted average exercise price		$ 2.38
Average grant date fair value		$ 12.23
Risk-free interest rate		0.00%
Expected terms		29 days
Expected volatility rate		60.28%
Expected dividend rate		0.00%
Stock or unit option plan expense							$ 100,000
Aggregate intrinsic value of the stock option exercised							100,000
One Time Stock Option Grant to non-employee [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from stock options exercised							$ 100,000

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Share-based Payment Award Stock Options Valuation Assumptions (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price And Additional Disclosures [Abstract]
Risk-free interest rates	2.00%	1.35%	1.68%
Expected term (in years)	5 years 11 months 15 days	6 years 1 month 2 days	5 years 11 months 27 days
Expected volatility	75.18%	73.19%	56.86%
Expected dividend yield	0.00%	0.00%	0.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Number of Shares
Outstanding at December 31, 2016	2,030,517
Granted	1,201,789
Exercised	(11,714)
Forfeited	(382,034)
Outstanding at December 31, 2017	2,838,558	2,030,517
Vested and expected to vest at December 31, 2017	2,755,308
Options exercisable at December 31, 2017	1,054,350
Weighted Average Exercise Price
Outstanding at December 31, 2016	$ 7.29
Granted	6.88
Exercised	2.68
Forfeited	10.59
Outstanding at December 31, 2017	6.69	$ 7.29
Vested and expected to vest at December 31, 2017	6.82
Options exercisable at December 31, 2017	$ 6.29
Weighted Average Remaining Contractual Term, Outstanding	8 years 1 month 6 days	8 years 5 months 1 day
Weighted Average Remaining Contractual Term, Vested and expected to vest at December 31, 2017	8 years 3 months
Weighted Average Remaining Contractual Term, Options exercisable at December 31, 2017	7 years 2 months 4 days
Aggregate Intrinsic Value, Outstanding	$ 3,213	$ 10,510
Aggregate Intrinsic Value, Vested and expected to vest at December 31, 2017	2,926
Aggregate Intrinsic Value, Options exercisable at December 31, 2017	$ 1,404

Stock-Based Compensation - Su62

Stock-Based Compensation - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 3,732	$ 2,164	$ 472
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	1,326	556	72
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 2,406	$ 1,608	$ 400

Collaboration, Research Grant63

Collaboration, Research Grant and License Agreements - Additional Information (Detail) - USD ($)	Jan. 31, 2014	Jan. 03, 2014	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue			$ 1,622,000	$ 1,551,000	$ 1,147,000	$ 1,021,000	$ 4,060,000	$ 1,715,000	$ 1,451,000	$ 1,158,000	$ 5,341,000	$ 8,384,000	$ 4,312,000
Research and development			12,282,000	$ 12,894,000	$ 15,370,000	$ 13,108,000	10,461,000	$ 9,218,000	$ 7,404,000	$ 6,876,000	$ 53,654,000	33,959,000	22,524,000
Astellas Collaboration Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Convertible promissory notes														$ 5,000,000
Initial research term											3 years 6 months
Proceeds from collaborator											$ 3,500,000	2,500,000
Description deliverables under agreement											(i) the research license, (ii) the research services to be provided over the research term, which is three and a half years, and (iii) the Company’s participation in the Joint Research Committee (the “Committee”) to be provided over the initial three and a half year research term of the agreement.
Earned non-substantive milestone payments												1,800,000	800,000
Revenue											$ 5,300,000	3,200,000
Deferred revenue			1,100,000				3,200,000				1,100,000	3,200,000
Astellas Collaboration Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development and sales milestone payments											$ 398,500,000
Biogen License Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from collaborator		$ 2,500,000
Description deliverables under agreement											(i) the research license, (ii) the research services to be provided over the four-year research term of the agreement, and (iii) the Company’s participation in the Joint Research Committee (the “Committee”) to be provided over the initial four-year research term of the agreement.
Earned non-substantive milestone payments														$ 2,000,000
Revenue												3,000,000	2,400,000
Agreement termination date											Dec. 31, 2016
Purchase of preferred stock under existing terms purchased		$ 5,000,000
Research funding payments guaranteed period		2 years
Research funding payments		$ 4,000,000
CFFT [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue											$ 0	0	0
Funds received for research, development and commercialization agreement	$ 5,700,000
Description of funds received in different periods											Under terms of the CFFT Agreement, which was subsequently amended in May 2013 and January 2014, CFFT agreed to provide up to $5.7 million (the “Award’) in research funding to the Company over two non-consecutive one-year periods from March 2012 to March 2013 and from January 2014 to December 2014.
Milestone and royalty payments description											(a) 268,265 shares of the Company’s common stock if the change of control occurs prior to the selection by the Company of a compound intended for product approval, or (b) 444,025 shares of the Company’s common stock if the change of control occurs after the selection by the Company of a compound intended for product approval. This alternative payment option upon a change of control would be cash settled in the event of a change of control and meets the definition of an embedded derivative.
Estimated fair value of derivative liability							$ 100,000					100,000
Payment obligations			$ 0								$ 0
Agreement termination period description											Each party may terminate for an uncured material breach of any material covenants or obligations or if any representation or warranty is materially untrue as of the date made and uncured after 30 days from notice. CFFT may also terminate if a case or proceeding under the bankruptcy laws is filed against the Company and not dismissed within 60 days, or if the Company files for insolvency, reorganization, receivership, dissolution or liquidation.
CFFT [Member] | Occurs Prior to Selection [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock			268,265								268,265
CFFT [Member] | Occurs After Selection [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock			444,025								444,025
CFFT [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payments for Royalties											$ 34,200,000
Royalty Expense											22,800,000
Estimated fair value of derivative liability			$ 100,000								$ 100,000
Harvard Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License agreement termination period															2017-10
License agreement term											10 years
Research and development											$ 100,000	$ 100,000	$ 100,000
Harvard Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future payments for clinical and commercialization sale milestone															$ 2,375,000

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Line Items]
Provision for income taxes		$ 0	$ 0	$ 0
U.S. corporate tax rate		34.00%	34.00%	34.00%
Limitation of net operating loss deduction as percentage of current year taxable income		80.00%
Reduction in deferred tax assets		$ 24,000,000
Federal net operating loss carry-forwards		196,900,000
State net operating loss carry-forwards		182,100,000
Amounts for unrecognized tax benefits, interest or penalties		$ 0
Federal [Member]
Income Tax Disclosure [Line Items]
Operating loss carry-forwards begin to expire year		2,026	2,026
Research and development tax credit carry-forwards		$ 6,800,000
Tax credit carry-forwards begin to expire year		2,027	2,027
State [Member]
Income Tax Disclosure [Line Items]
Operating loss carry-forwards begin to expire year		2,030	2,030
Research and development tax credit carry-forwards		$ 3,100,000
Tax credit carry-forwards begin to expire year		2,026	2,026
Maximum [Member]
Income Tax Disclosure [Line Items]
U.S. corporate tax rate		35.00%
Scenario, Plan [Member]
Income Tax Disclosure [Line Items]
U.S. corporate tax rate	21.00%

Income Taxes - Schedule of Effe

Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	(34.00%)	(34.00%)	(34.00%)
State income taxes, net of federal benefit	(5.60%)	(5.10%)	(5.00%)
Research and development tax credit carryforwards	(3.20%)	(4.30%)	(4.40%)
Non-deductible expenses	0.70%	0.50%	1.20%
Change in deferred tax asset valuation allowance	2.00%	42.90%	42.20%
Effect of U.S. Tax Reform	40.10%
Effective income tax rate	(0.00%)	(0.00%)	(0.00%)

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 52,861	$ 53,723
Research and development tax credit carryforwards	9,232	6,633
Accrued expenses and other	1,583	2,152
Total deferred tax assets before valuation allowance	63,676	62,508
Valuation allowance	(63,660)	(62,442)	$ (46,476)
Net deferred tax assets	16	66
Depreciation	$ (16)	$ (66)

Income Taxes - Changes in Valua

Income Taxes - Changes in Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Valuation allowance at beginning of year	$ (62,442)	$ (46,476)
Increases recorded to income tax provision	(1,218)	(15,966)
Valuation allowance at end of year	$ (63,660)	$ (62,442)

Net Loss per Share - Schedule o

Net Loss per Share - Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss	$ (13,424)	$ (13,954)	$ (16,958)	$ (15,096)	$ (9,417)	$ (10,771)	$ (9,053)	$ (7,991)	$ (59,432)	$ (37,232)	$ (25,040)
Accruing dividends on preferred stock								(1,378)		(1,378)	(9,724)
Net loss attributable to common stockholders	$ (13,424)	$ (13,954)	$ (16,958)	$ (15,096)	$ (9,417)	$ (10,771)	$ (9,053)	$ (9,369)	$ (59,432)	$ (38,610)	$ (23,309)
Denominator:
Weighted average common shares outstanding—basic and diluted	26,465,521	25,093,344	25,040,131	25,020,337	24,975,010	20,073,685	19,139,183	10,766,722	25,407,943	18,759,369	553,162
Net loss per share attributable to common stockholders—basic and diluted	$ (0.51)	$ (0.56)	$ (0.68)	$ (0.60)	$ (0.38)	$ (0.54)	$ (0.47)	$ (0.87)	$ (2.34)	$ (2.06)	$ (42.14)
Series A Preferred Stock [Member]
Numerator:
Modifications of preferred stock											$ 11,481
Series B Preferred Stock [Member]
Numerator:
Modifications of preferred stock											$ (26)

Net Loss per Share - Schedule69

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding	2,838,558	2,045,317	13,284,476
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding			9,699,600
Series A Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding			2,590,742
Warrants to Purchase Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding		14,800	14,800
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding	2,838,558	2,030,517	979,334

Build-to-suit Lease - Additiona

Build-to-suit Lease - Additional Information (Detail) $ in Thousands	Sep. 19, 2017USD ($)ft²	Mar. 31, 2009	Dec. 31, 2017USD ($)
Lessee Lease Description [Line Items]
Operating lease, annual rent			$ 563
Annual increase lease payment		1.40%
Total expected rental payments through expiration of lease			563
Build-To-Suit Lease Transaction [Member]
Lessee Lease Description [Line Items]
Operating lease, annual rent			1,173
Total expected rental payments through expiration of lease			18,838
Ice Box, LLC [Member] | Build-To-Suit Lease Transaction [Member]
Lessee Lease Description [Line Items]
Operating lease, area of property leased | ft²	30,000
Operating lease, term	10 years
Operating lease, annual rent	$ 1,700
Operating lease, renewal term	7 years
Operating lease term description	The lease will extend for a lease term from the rent commencement date and then for ten years starting with the first day of the month following the month in which the commencement date falls, if the rent commencement date does not fall on the first day of a month. The Company is entitled to one seven-year option to extend.
Annual increase lease payment	2.75%
Total expected rental payments through expiration of lease	$ 18,800
Improvement allowance receivable	$ 4,800
Build-to-suit asset capitalized			16,100
Expected costs to complete construction			700
Ice Box, LLC [Member] | Build-To-Suit Lease Transaction [Member] | Other Liabilities [Member]
Lessee Lease Description [Line Items]
Build-to-suit facility lease financing obligation			900
Ice Box, LLC [Member] | Build-To-Suit Lease Transaction [Member] | Other Non-current Liabilities [Member]
Lessee Lease Description [Line Items]
Build-to-suit facility lease financing obligation			$ 15,300

Build-to-suit Lease - Schedule

Build-to-suit Lease - Schedule of Minimum Rental Commitments Under Lease (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Lessee Lease Description [Line Items]
2,018	$ 563
Total	563
Build-to-suit Lease [Member]
Lessee Lease Description [Line Items]
2,018	1,173
2,019	1,686
2,020	1,733
2,021	1,780
2,022	1,830
Thereafter	10,636
Total	$ 18,838

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	May 31, 2016	Mar. 31, 2009	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Contingencies And Commitments [Line Items]
Lease termination date			May 31, 2018
Monthly base rent charges		$ 100,000
Annual increase lease payment		1.40%
Standby letter of credit		$ 300,000
Rent expense			$ 1,700,000	$ 1,600,000	$ 1,700,000
Issuance of stock under consulting agreement, value			$ 767,000	$ 359,000
Common Stock [Member]
Contingencies And Commitments [Line Items]
Issuance of stock under consulting agreement, shares			188,462	27,221
Letter Agreement with Dr. Stelios Papadopoulos [Member]
Contingencies And Commitments [Line Items]
Letter agreement effective date			Jul. 1, 2016
Letter agreement term	36 months
Quarterly retainer payable	$ 200,000
Aggregate retainer fee payable	$ 2,500,000		$ 1,260,000
Percentage of M&A transaction fee payable	1.00%
Issuance of stock under consulting agreement, value			$ 800,000	$ 400,000
Letter Agreement with Dr. Stelios Papadopoulos [Member] | Common Stock [Member]
Contingencies And Commitments [Line Items]
Issuance of stock under consulting agreement, shares			188,462	27,221
Maximum [Member]
Contingencies And Commitments [Line Items]
Non-rent shared tenant occupancy costs		$ 200,000

Commitments and Contingencies73

Commitments and Contingencies - Summary of Future Minimum Lease Payments Under Operating Lease (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,018	$ 563
Total	$ 563

Commitments and Contingencies74

Commitments and Contingencies - Summary of Future Retainer Payment Due Under Letter Agreement (Detail) - Letter Agreement with Dr. Stelios Papadopoulos [Member] - USD ($) $ in Thousands	Dec. 31, 2017	May 31, 2016
Contingencies And Commitments [Line Items]
2,018	$ 840
2,019	420
Total	$ 1,260	$ 2,500

Quarterly Financial Data (Una75

Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Data [Line Items]
Revenue	$ 1,622	$ 1,551	$ 1,147	$ 1,021	$ 4,060	$ 1,715	$ 1,451	$ 1,158	$ 5,341	$ 8,384	$ 4,312
Operating expenses:
Research and development	12,282	12,894	15,370	13,108	10,461	9,218	7,404	6,876	53,654	33,959	22,524
General and administrative	2,847	2,741	2,902	3,170	3,198	3,266	3,115	2,301	11,660	11,880	6,322
Total operating expenses	15,129	15,635	18,272	16,278	13,659	12,484	10,519	9,177	65,314	45,839	28,846
Loss from operations	(13,507)	(14,084)	(17,125)	(15,257)	(9,599)	(10,769)	(9,068)	(8,019)	(59,973)	(37,455)	(24,534)
Interest income	126	155	169	191	190	36	20		641	246
Interest expense											(599)
Other income (expense), net	(43)	(25)	(2)	(30)	(8)	(38)	(5)	28	(100)	(23)	93
Net loss	(13,424)	(13,954)	(16,958)	(15,096)	(9,417)	(10,771)	(9,053)	(7,991)	(59,432)	(37,232)	(25,040)
Accruing dividends on Series A preferred stock								(1,378)		(1,378)	(9,724)
Net loss attributable to common stockholders	$ (13,424)	$ (13,954)	$ (16,958)	$ (15,096)	$ (9,417)	$ (10,771)	$ (9,053)	$ (9,369)	$ (59,432)	$ (38,610)	$ (23,309)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.51)	$ (0.56)	$ (0.68)	$ (0.60)	$ (0.38)	$ (0.54)	$ (0.47)	$ (0.87)	$ (2.34)	$ (2.06)	$ (42.14)
Weighted average common shares outstanding—basic and diluted	26,465,521	25,093,344	25,040,131	25,020,337	24,975,010	20,073,685	19,139,183	10,766,722	25,407,943	18,759,369	553,162
Series A Preferred Stock [Member]
Operating expenses:
Modifications of preferred stock											$ 11,481
Series B Preferred Stock [Member]
Operating expenses:
Modifications of preferred stock											$ (26)