Kineta (KANT) 10-K 8 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 28, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
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 Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Common Stock, Shares Outstanding		51,040,615
Entity Public Float			$ 92,932,233

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 28,810	$ 29,724
Short-term investments	89,569	44,738
Restricted cash		294
Prepaids and other current assets	2,481	1,377
Total current assets	120,860	76,133
Operating lease, right-of-use asset	13,849	472
Property and equipment, net	605	424
Other assets		33
Restricted cash, net of current portion	828	1,656
Total assets	136,142	78,718
Current liabilities:
Accounts payable	1,884	2,098
Accrued expenses	5,661	6,120
Deferred revenue		1,108
Operating lease liabilities	1,056	559
Total current liabilities	8,601	9,885
Derivative liability	3	25
Operating lease liabilities, net of current portion	13,196
Total liabilities	21,800	9,910
Commitments and contingencies (Note 14)
Stockholders’ equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized; no shares issued and outstanding as of December 31, 2018 and 2017
Common stock, $0.001 par value; 125,000,000 shares authorized; 50,808,422 and 34,416,088 shares issued and outstanding as of December 31, 2018 and 2017, respectively	51	35
Additional paid-in capital	391,825	285,583
Accumulated other comprehensive income (loss)	1	(2)
Accumulated deficit	(277,535)	(216,808)
Total stockholders’ equity	114,342	68,808
Total liabilities and stockholders’ equity	$ 136,142	$ 78,718

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
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	50,808,422	34,416,088
Common stock, shares outstanding	50,808,422	34,416,088

Statements of Operations

Statements of Operations - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Revenue									$ 2,840	$ 5,341
Operating expenses:
Research and development	$ 13,717	$ 15,591	$ 12,604	$ 8,400	$ 12,282	$ 12,894	$ 15,370	$ 13,108	50,312	53,654
General and administrative	3,780	4,150	3,957	3,823	2,847	2,741	2,902	3,170	15,710	11,660
Total operating expenses	17,497	19,741	16,561	12,223	15,129	15,635	18,272	16,278	66,022	65,314
Loss from operations	(17,497)	(18,686)	(15,718)	(11,281)	(13,507)	(14,084)	(17,125)	(15,257)	(63,182)	(59,973)
Interest income	342	171	194	165	126	155	169	191	872	641
Other income (expense), net	255	87	46	90	(43)	(25)	(2)	(30)	478	(100)
Net loss	$ (16,900)	$ (18,428)	$ (15,478)	$ (11,026)	$ (13,424)	$ (13,954)	$ (16,958)	$ (15,096)	$ (61,832)	$ (59,432)
Net loss per share—basic and diluted	$ (0.36)	$ (0.50)	$ (0.43)	$ (0.32)	$ (0.51)	$ (0.56)	$ (0.68)	$ (0.60)	$ (1.61)	$ (2.34)
Weighted average common shares outstanding—basic and diluted	46,687,910	36,694,957	36,009,109	34,474,004	26,465,521	25,093,344	25,040,131	25,020,337	38,495,103	25,407,943

Statements of Comprehensive Los

Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (61,832)	$ (59,432)
Other comprehensive loss:
Unrealized gain on investments	3	20
Comprehensive loss	$ (61,829)	$ (59,412)

Statements of Stockholders' Equ

Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	ATM Program [Member]	Common Stock [Member]	Common Stock [Member]ATM Program [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]ATM Program [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2016	$ 81,456		$ 26		$ 238,902		$ (22)	$ (157,450)
Beginning balance, shares at Dec. 31, 2016			25,000,734
Exercise of stock options	31				31
Exercise of stock options, shares			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 for payment of consulting services	767				767
Issuance of common stock for payment of consulting services, shares			188,462
Issuance of common stock upon completion of public offering, net of offering costs, shares			9,200,000
Issuance of common stock upon completion of public offering, net of offering costs	42,884		$ 9		42,875
Impact of adoption of ASC | ASC 842 [Member]	74							74
Other comprehensive income	20						20
Net loss	(59,432)							(59,432)
Ending balance at Dec. 31, 2017	$ 68,808		$ 35		285,583		(2)	(216,808)
Ending balance, shares at Dec. 31, 2017	34,416,088		34,416,088
Exercise of stock options	$ 21				21
Exercise of stock options, shares	8,775		8,775
Stock-based compensation expense	$ 3,526				3,526
Issuance of common stock pursuant to employee stock purchase plan	55				55
Issuance of common stock pursuant to employee stock purchase plan, shares			18,903
Issuance of common stock for payment of consulting services	889				889
Issuance of common stock for payment of consulting services, shares			239,490
Issuance of common stock upon completion of public offering, net of offering costs, shares			12,650,000	3,475,166
Issuance of common stock upon completion of public offering, net of offering costs	80,126	$ 21,641	$ 13	$ 3	80,113	$ 21,638
Impact of adoption of ASC | ASC 606 [Member]	1,105							1,105
Other comprehensive income	3						3
Net loss	(61,832)							(61,832)
Ending balance at Dec. 31, 2018	$ 114,342		$ 51		$ 391,825		$ 1	$ (277,535)
Ending balance, shares at Dec. 31, 2018	50,808,422		50,808,422

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (61,832)	$ (59,432)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	1,778	1,402	$ 1,400
Amortization (accretion) of short-term investments	(438)	178
Stock-based compensation expense	3,526	2,965
Stock issued for consulting services	889	767
Change in fair value of derivative liability	(22)	(66)
Loss on disposal of property and equipment	41
Changes in operating assets and liabilities:
Accounts receivable		668
Prepaids and other current assets	(1,104)	2,682
Other assets	33	35
Accounts payable	(204)	67
Accrued expenses	(239)	1,481
Deferred revenue	(3)	(1,848)
Operating lease liabilities	(1,269)	(1,294)
Net cash used in operating activities	(58,844)	(52,395)
Cash flows from investing activities:
Purchases of short-term investments	(111,240)	(61,220)
Proceeds received from maturities of short-term investments	66,850	83,221
Purchases of property and equipment	(415)	(118)
Net cash provided by (used in) investing activities	(44,805)	21,883
Cash flows from financing activities:
Proceeds from issuance of common stock	21,703
Proceeds from exercise of stock options	21	31
Deferred offering costs	(431)	(35)
Net cash provided by financing activities	101,613	43,279
Net increase (decrease) in cash, cash equivalents and restricted cash	(2,036)	12,767
Cash, cash equivalents and restricted cash at beginning of year	31,674	18,907
Cash, cash equivalents and restricted cash at end of year	29,638	31,674	$ 18,907
At-the-market Common Stock Offering Program Sales Agreement [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock	80,265	43,240
Addition of Operating Lease Right-Of-Use Asset [Member]
Supplemental disclosure of non-cash investing and financing activities:
Capital expenditures incurred but not yet paid	14,962
Accounts Payable [Member]
Supplemental disclosure of non-cash investing and financing activities:
Capital expenditures incurred but not yet paid	230	321
Employee Stock Purchase Plan [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 55	$ 43

Nature of the Business

Nature of the Business	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business	1. Nature of the Business Proteostasis Therapeutics, Inc. (the “Company”) was incorporated in Delaware on December 13, 2006. The Company is a clinical stage biopharmaceutical company committed to the discovery and development of novel therapeutics to treat cystic fibrosis (“CF”) and other diseases caused by an imbalance in the proteostasis network, a set of pathways that control protein biosynthesis, folding, trafficking and clearance. The Company focuses on identifying therapies that restore protein function. CF is a disease caused by defects in the function or abundance of cystic fibrosis transmembrane conductance regulator (“CFTR”). The Company’s CF focused pipeline consists of novel CFTR modulators including correctors, potentiators and amplifiers. Upon discovery of amplifiers, a novel class of CFTR modulators, the Company has exploited its novel mechanism of action as a drug screening tool and has subsequently identified correctors and potentiators to be developed as part of combination therapies. Investigational agents representative of all three classes of CFTR modulators are currently in clinical development and include PTI-801 a third generation CFTR corrector, PTI-808, a CFTR potentiator, and . The Company is pursuing proprietary dual combination of PTI-801 and PTI-808, and triple combination of PTI-801, PTI-808 and PTI-428 as product opportunities. Additionally, the Company believes that both PTI-801 and PTI-428 have potential as add-on therapy to current and future standard-of-care CFTR modulator treatments. The Company is developing and, if approved, intends to commercialize their own therapies, including add-on and proprietary combination therapies for CF patients who have at least one F508del mutation, representing the majority of the patient population. 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 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. Public Offerings In the fourth quarter 2018, the Company completed a follow-on public offering whereby the Company sold 12,650,000 shares of common stock at a public offering price of $6.75 per share, which included 1,650,000 shares issued pursuant to the exercise by the underwriters of their option to purchase additional shares of common stock. The Company received net proceeds of approximately $80.1 million, after deducting underwriting discounts and commissions and other offering expenses. In March 2018, the Company entered into a sales agreement with Leerink Partners LLC (“Leerink”) with respect to an at-the-market (“ATM”) offering program under which the Company may issue and sell, from time to time at its sole discretion, shares of its common stock, resulting in aggregate gross offering proceeds of up to $50.0 million. Leerink is not required to sell any specific amount but acts as the Company’s sales agent on a commercially reasonable efforts basis consistent with its normal trading and sales practices. Common stock will be sold at prevailing market prices at the time of the sale; and as a result, prices may vary. The Company will pay Leerink up to 3% of the gross proceeds from any common stock sold through the sales agreement. As of December 31, 2018, the Company had sold an aggregate of 3,475,166 shares of its common stock for total net proceeds of approximately $21.6 million under the ATM program. At December 31, 2018, $27.6 million of common stock remains available for sale under the ATM. In December 2017, the Company completed a follow-on public offering whereby the Company sold 9,200,000 shares of common stock at a public offering price $42.9 million after deducting underwriting discounts and commissions and other offering expenses.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The 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, stock-based compensation, determining the value of the operating lease right of use asset and the valuation of the derivative liability. 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, and short-term investments. 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 only customer in 2018 was Astellas Pharma Inc. (“Astellas”) and as of December 31, 2018, the Astellas Agreement (as defined below) had completed and all amounts were paid in full. 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, 2018 and 2017, the Company’s cash equivalents consisted of money market funds, U.S. government-sponsored securities and U.S. treasury securities. Restricted Cash As of December 31, 2018 and 2017, long-term restricted cash represents cash pledged as collateral to secure the Company’s long-term Boston lease obligation. As of December 31, 2017, short-term restricted cash represents cash pledged as collateral to secure the Company’s Cambridge lease obligation, which expired in May 2018, resulting in a decrease of $0.3 million in short-term restricted cash. Amounts are reported as noncurrent unless the restrictions are expected to be released in the next 12 months. Long-term restricted cash decreased by $0.8 million from December 31, 2017 as the Company was entitled to reduce the amount of the letter of credit and the corresponding restricted cash upon meeting certain conditions noted within the lease agreement. Short-Term Investments Short-term investments represent holdings of available-for-sale debt 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 income (expense), net, in the accompanying statement of operations. 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 for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. In the event that such cash flows are not expected to be sufficient to recover the carrying amount of the assets, the assets are written-down to their fair values. 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 payable and accrued expenses approximate their fair value due to the short-term nature of these 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 is collaboration revenue earned from activities in the United States. Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers Under ASC 605, the Company recognized revenue for each unit of accounting when all of the following criteria were met: persuasive evidence of an arrangement existed; delivery had occurred or services had been rendered; the seller’s price to the buyer was fixed or determinable; and collectability was reasonably assured. The Company was recognizing revenue related to collaboration agreements over the expected research and development period using the proportional performance method. 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. The Company enters into licensing agreements which are within the scope of ASC 606, under which it may exclusively license rights to research, develop, manufacture and commercialize its product candidates to third parties. The terms of these arrangements typically include payment to the Company of one or more of the following: nonrefundable, upfront license fees; reimbursement of certain costs; customer option exercise fees; development, regulatory and commercial milestone payments; and royalties on net sales of licensed products. As part of the accounting for these arrangements, the Company must use significant judgment to determine: a) the number of performance obligations based on the determination under step (ii) above; b) the transaction price under step (iii) above; and c) the stand-alone selling price for each performance obligation identified in the contract for the allocation of transaction price in step (iv) above. The Company uses judgment to determine whether milestones or other variable consideration should be included in the transaction price as described further below. The transaction price is allocated to each performance obligation on a relative stand-alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. In developing the stand-alone price for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the stand-alone selling price for performance obligations by evaluating whether changes in the key assumptions used to determine the stand-alone selling prices will have a significant effect on the allocation of transaction price between multiple performance obligations. The Company recognizes a contract asset or liability for the difference between the Company’s performance (i.e., the goods or services transferred to the customer) and the customer’s performance (i.e., the consideration paid by, and unconditionally due from, the customer). Exclusive Licenses If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, the Company recognizes revenue from nonrefundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. In assessing whether a promise or performance obligation is distinct from the other promises, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from a promise for its intended purpose without the receipt of the remaining promise, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the research and development and licensing agreement. Such a change could have a material impact on the amount of revenue the Company records in future periods. Research and Development Services The promises under the Company’s collaboration and license agreements generally include research and development services to be performed by the Company on behalf of the collaboration partner. As the provision of research and development services is a part of the Company’s central operations, when the Company is principally responsible for the performance of these services under the agreements, the Company recognizes revenue on a gross basis for research and development services in accordance with the ASC 606 framework described above. Customer Options If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. The Company allocates the transaction price to material rights based on the relative stand-alone selling price, which is determined based on the identified discount and the probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised. Milestone Payments At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones or other variable consideration should be included in the transaction price. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making Royalties For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. For a complete discussion of accounting for collaboration revenues, see Note 10, “Significant Agreements.” 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, Inc. (“CFF”) (see Note 4). 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. 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. Research and development costs that are paid in advance of performance are capitalized as prepaid expenses and expensed over the period that the goods or services are provided 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. Stock-Based Compensation The Company measures all stock-based awards granted to employees based on the fair value on the date of the grant and recognizes compensation expense of those awards over the requisite service period, which is usually the vesting period of the respective award. Generally, the Company issues stock options and restricted stock awards to employees with service-based vesting conditions and records the expense for these awards using the straight-line method. The Company measures stock-based awards granted to nonemployees 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. 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 nonemployees for which satisfaction of the performance conditions is not solely within the control of the holder until the performance conditions have been met. The fair value of each stock option is estimated on the date of grant using the Black-Scholes option-pricing model. Due to the lack of complete company-specific historical and implied volatility data for the full expected term of the stock-based awards, the Company 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 it has adequate historical data regarding the volatility of its own traded stock price. The expected term of stock options granted to employees is determined utilizing the “simplified” method, whereby the expected term equals the average of the vesting term and the original contractual term of the option. The expected term of stock options granted to nonemployees is equal to the contractual term of the option. 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. The expected dividend yield is based upon the fact the Company has never paid cash dividends on common stock and does not expect to pay any cash dividends in the foreseeable future. The Company accounts for forfeitures as they occur and classifies stock-based compensation expense in its statement of operations in the same manner that the award recipients’ payroll costs are classified or in which the award recipients’ service payments are classified. Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. 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 and diluted net loss per share have been calculated by dividing net loss by the weighted average number of common shares outstanding during the year. Diluted net income per share is calculated by dividing the net income attributable to common stockholders by the weighted-average number of common equivalent shares outstanding for the period, including any dilutive effect from outstanding stock options, unvested restricted stock, restricted stock units, and employee stock purchase plan stock using the treasury stock method. Basic net loss is based solely on the number of common shares outstanding during the year. Since the Company is reporting a net loss for all periods presented, all common stock equivalents are considered anti-dilutive and are excluded from the calculation of diluted net loss per share. Recently Adopted Accounting Pronouncements ASU No. 2014-09, Revenue from Contracts with Customers In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 606, which supersedes the revenue recognition requirements in ASC Topic 605, Revenue Recognition 2016, the FASB issued and aspects of identification performance obligations and licensing 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 modified retrospective method for adoption. As of January 1, 2018, t On January 1, 2018, the Company recorded a cumulative-effect decrease to the opening accumulated deficit of $1.1 million and a corresponding decrease to deferred revenue due to the adoption of ASC 606. As of December 31, 2018, the impact of the adoption of ASC 606 on the Company’s financial statement was as follows (in thousands): December 31, 2018 As Reported under ASC 606 Balance without Adoption of ASC 606 Effect of Change Higher/(Lower) Balance Sheet: Deferred revenue, current $ — $ — $ — Income Statement: Revenue for the 12 months ended December 31, 2018 $ 2,840 $ 3,945 $ (1,105 ) The most significant change related to the Company’s determination of transaction price at inception and each reporting period as well as the revenue recognition pattern for the Astellas Agreement (as defined below) with Astellas as well as treatment of variable consideration in the form of milestone payments. Under ASC 605, the research funding support payments, reimbursement of third-party costs, and over the 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. ASU No. 2016-02, Leases In February 2016, the FASB issued ASU No. 2016-02, Leases Due to the Company commencing a new lease in Boston, Massachusetts, in January 2018, the Company elected to early adopt the standard effective January 1, 2018, as permitted by the guidance, in order to enhance overall transparency within financial reporting. The Company has implemented the standard using the required modified retrospective approach and has also elected to utilize the package of practical expedients. The expedients used by the Company are as follows: (1) allowing an entity to not reassess the lease classification for any expired or existing leases, (2) allowing an entity to not reassess the treatment of initial direct costs as they related to existing leases, and (3) allowing an entity to not reassess whether expired or existing contracts are or contain leases. The company also elected the practical expedient to use hindsight in determining the appropriate lease term and in assessing impairment of its right-of- use assets. In using the modified retrospective approach, the Company is required to recognize and measure leases existing at, or entered into after, the beginning of the earliest comparative period presented. During 2017, the Company was deemed the accounting owner of the construction project for the build-to-suit lease in Boston, Massachusetts, because of the Company’s involvement in the build-out of the space. Under the new standard, the Company is no longer considered the accounting owner of the leased space due to (1) not having the right to obtain or control the leased premises during the construction period, (2) having no right of payment for the partially constructed assets, and the leased premises are not of a specialized nature and, thus, could be potentially leased to another tenant, and (3) not legally owning or controlling the land on which the property improvements will be constructed. As such, upon transition, the existing construction-in-progress balance within property and equipment, and the corresponding build-to-suit facility lease financing obligation balance within other liabilities, current and noncurrent, have been derecognized. Prior period results have been restated to this effect through to the earliest period presented. The adoption of this standard has had a material impact on the Company’s financial position but did not significantly affect the Company’s results of operations. See Impacts on Previously Reported Results below. Impact to Previously Reported Results The impact of the adoption of the new leasing standard on the balance sheet as of December 31, 2017, is as follows (in thousands): December 31, 2017 As Previously Reported New Lease Standard Adjustment As restated Operating lease right-of-use assets, net $ — $ 472 $ 472 Property and equipment, net 16,567 (16,143 ) 424 Deferred rent, current 87 (87 ) — Operating lease liabilities, current — 559 559 Other liabilities, current 902 (902 ) — Other liabilities, net of current 15,315 (15,315 ) — Accumulated deficit (216,882 ) 74 (216,808 ) ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments ASU 2016-18, Statement of Cash Flows (Topic 230) – Restricted Cash In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230) - Restricted Cash new standard was for on January 1, 2018. This resulted in $0.8 $2.0 ASU 2017-09, Compensation – Stock Compensation (Topic 719): Scope of Modification Accounting In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (Topic 719): Scope of Modification Accounting Recently Issued Accounting Pronouncements ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting In June 2018, the FASB issued ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting ASU No. 2018-13, Fair Value Measurement (Topic 820): Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued ASU No. 2018-13, Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement

Short-Term Investments

Short-Term Investments	12 Months Ended
	Dec. 31, 2018
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, 2018 and 2017 (in thousands) : December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S government-sponsored enterprise securities $ 41,478 $ 2 $ (3 ) $ 41,477 U.S. treasury securities 48,090 3 (1 ) $ 48,092 $ 89,568 $ 5 $ (4 ) $ 89,569 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 The Company did not have any realized gains or losses on its short-term investments for the years ended December 31, 2018 and 2017. There were no other-than-temporary impairments recognized for the years ended December 31, 2018 and 2017.

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	12 Months Ended
	Dec. 31, 2018
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 31, 2018 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 26,806 $ — $ — $ 26,806 U.S. treasury securities — 500 — 500 Short-term investments: U.S. government-sponsored enterprise securities — 41,477 — 41,477 U.S. treasury securities — 48,092 — 48,092 $ 26,806 $ 90,069 $ — $ 116,875 Liabilities: Derivative liability $ — $ — $ 3 $ 3 Fair Value Measurements as of December 31, 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 During the years ended December 31, 2018 and 2017, there were no transfers between Level 1, Level 2 and Level 3. Derivative Liability The derivative liability relates to a cash settlement option associated with the changes of control provision in the Company’s Cystic Fibrosis Foundation, Inc. (“CFF”) agreement, which meets the definition of an embedded derivative.

Prepaids and Other Current Asse

Prepaids and Other Current Assets	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 Prepaid clinical, manufacturing and scientific expenses $ 1,660 $ 568 Prepaid insurance expenses 108 55 Other prepaid expenses and other current assets 713 754 $ 2,481 $ 1,377

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2018
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) 2018 2017 Laboratory equipment 5 years $ 2,715 $ 3,299 Furniture and fixtures 5 years 368 157 Computer and office equipment 3 – 5 years 209 169 Construction-in-progress 8 — 3,300 3,625 Less: Accumulated depreciation and amortization (2,695 ) (3,201 ) $ 605 $ 424 Depreciation and amortization expense was $1.8 million and $1.4 million for the years ended December 31, 2018 and 2017, respectively. As part of the move from the Cambridge office to the Boston office, the Company retired $1.1 million of fixed assets with a net book value of less than $0.1 million.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued Expenses	7. Accrued Expenses Accrued expenses consisted of the following (in thousands): December 31, 2018 2017 Accrued payroll and related expenses $ 2,746 $ 1,542 Accrued research and development expenses 1,965 3,930 Accrued professional fees 799 556 Accrued other 151 92 $ 5,661 $ 6,120

Common and Preferred Stock

Common and Preferred Stock	12 Months Ended
	Dec. 31, 2018
Text Block [Abstract]
Common and Preferred Stock	8. Common and Preferred Stock As of December 31, 2018, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue 125,000,000 shares of The Company has reserved for future issuance the following number of shares of common stock: Years Ended December 31, 2018 2017 2016 and 2008 Stock option incentive plan 4,258,783 3,474,566 2016 Employee stock purchase plan 386,539 266,685 4,645,322 3,741,251

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	9 . Stock-Based Compensation 2016 Stock Option and Incentive Plan On February 3, 2016, the Company’s stockholders approved 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. The shares 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 Equity Incentive Plan, as amended (the “2008 Plan”) will be added back to the shares of common stock available for issuance under the 2016 Plan. As of December 31, 2018, the total number of shares reserved under the 2016 Plan and 2008 Plan was 4,258,783 and the Company had 936,682 shares available for future issuance under the 2016 Plan. 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 on February 9, 2016. 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. During the years ended December 31, 2018 and 2017, 18,903 and 10,829 shares were issued under this plan, respectively. As of December 31, 2018, the total number of shares reserved under the 2016 ESPP was 386,539 shares. The Company recognized less than $0.1 million of stock-based compensation during 2018 related to this plan. Stock Option Grants and Shares to Nonemployees Prior to 2013, the Company issued options to purchase 203,964 shares of common stock to nonemployees, primarily members of the Company’s scientific advisory board, that vest upon the achievement of specified development and clinical milestones. As of December 31, 2018, options for the purchase of 83,250 shares held by nonemployees remained unvested, pending achievement of the specified milestones, and had an aggregate fair value of $0.1 million. During the years ended December 31, 2018 and 2017, the Company did not grant any options to nonemployees under this plan. Bonus Restricted Stock Units (RSUs) On February 1, 2018, the Company’s board approved executive bonuses for the year ended December 31, 2017, and elected payment to be made through a grant of an equivalent number of RSUs based on the February 1, 2018, closing share price of the Company’s common stock. The requisite service period for the awards is from February 1, 2018, to February 1, 2019 (the vesting period). The Company recognizes employee stock-based compensation expense for the bonus RSU grants on a straight-line basis over the vesting period of the awards. The grants do not meet the criteria for liability classification as there is a fixed number of shares to be issued, and there is no variability in the number of shares which have been granted. As of December 31, 2018, there are 163,425 RSUs are remaining to vest. Total expense recognized for the year ended December 31, 2018 was $0.5 million. A summary of the Company’s restricted stock unit activity and related information is as follows: Number of Shares Weighted Average Grant Date Fair Value Unvested balance at December 31, 2017 — $ — Granted 197,265 3.20 Vested — — Forfeited (33,840 ) 3.20 Unvested balance at December 31, 2018 163,425 $ 3.20 Stock Option Valuation The fair value of each option awards was estimated on the date of grant using the Black Scholes option-pricing model with the following weighted-average assumptions: Years Ended December 31, 2018 2017 Risk-free interest rates 2.68 % 2.00 % Expected term (in years) 6.00 5.96 Expected volatility 78.47 % 75.18 % Expected dividend yield — — Stock Options The following table summarizes the Company’s stock option activity (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, 2017 2,838,558 $ 6.69 8.10 $ 3,213 Granted 1,004,025 3.22 Exercised (8,775 ) 2.43 Forfeited (675,132 ) 5.14 Outstanding at December 31, 2018 3,158,676 $ 5.93 7.53 $ 471 Vested and expected to vest at December 31, 2018 3,075,426 $ 6.03 7.67 $ 399 Options exercisable at December 31, 2018 1,697,979 $ 6.44 6.92 $ 280 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 years ended December 31, 2018 and 2017 was less than The Company received cash proceeds from the exercise of stock options of less than $0.1 million during the years ended December 31, 2018 and 2017. The weighted average grant-date fair value of stock options granted during the years ended December 31, 2018 and 2017, was $3.22 and $4.57, respectively. The total fair value of options vested during the years ended December 31, 2018 and 2017, was $3.5 million and $2.5 million, respectively. Stock-based Compensation Stock-based compensation expense was classified in the statements of operations as follows (in thousands): Years Ended December 31, 2018 2017 Research and development $ 1,571 $ 1,326 General and administrative 2,844 2,406 $ 4,415 $ 3,732 As of December 31, 2018, total unrecognized compensation cost related to the unvested stock-based awards was $4.3 million, which is expected to be recognized over a weighted average period of 2.02 years.

Significant Agreements

Significant Agreements	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Significant Agreements	1 0 . Significant Agreements Genentech In December 2018, the Company entered into a Technology Transfer and License Agreement (the “Genentech Agreement”) with Genentech, Inc. (Genentech) under which the Company granted Genentech an exclusive worldwide license for technology and materials relating to potential therapeutic small molecule modulators of an undisclosed target within the proteostasis network. The rights do not include CFTR modulators and are unrelated to the Company’s investigational medicines or other ongoing research programs in cystic fibrosis. In connection with the terms of the Genentech Agreement, the Company is entitled to a nonrefundable cash payment of $5.0 million following the successful completion of the technology and materials transfer to Genentech and future milestone payments in the aggregate of approximately $96.0 million upon the achievement of specified development, regulatory and commercial milestones. In addition, Genentech is obligated to pay the Company tiered royalties in the low single-digits based on net sales of products covered by the licenses granted under the Genentech Agreement. There are no cancellation, termination or refund provisions in the Genentech Agreement that contain material financial consequences to the Company. Unless earlier terminated, the Genentech Agreement continues in full force and effect until the passing or expiration of all royalty payment obligations. Reciprocal termination rights under the agreement include termination for breach and termination for bankruptcy. Genentech may terminate the Genentech Agreement in its entirety for convenience upon thirty days’ notice to the Company. The Company evaluated the Genentech Agreement in accordance with the provisions of ASC 606. The Company’s obligations under the Genentech Agreement are the following promises: (i) exclusive license to certain intellectual property associated with a specific target and (ii) technology and materials transfer related to the intellectual property underlying the licensed technology. The Company determined that the exclusive license is not distinct from the associated technology and materials transfer because the customer cannot benefit from or utilize the license without the technology and materials transfer. Specifically, the Company concluded that the exclusive license is not capable of being distinct from the associated technology and materials transfer because Genentech does not have the knowledge or expertise to fully exploit potential product candidates without the accompanying technology and materials provided pursuant to the transfer and no other third party can perform the transfer due to the Company’s proprietary knowledge and specialized expertise with respect to the licensed intellectual property. Accordingly, the Company concluded that these promises should be combined into a single performance obligation (the “License and Transfer Performance Obligation”). As of December 31, 2018, the Company has measured the transaction price solely in reference to the $5.0 million payment due upon receipt of notice from Genentech regarding the satisfactory completion of the technology and materials transfer. None of the variable consideration payable under the arrangement has been included in the transaction price as of December 31, 2018. Through December 31, 2018, the Company has not achieved any research, development, regulatory or commercial milestones or earned any royalties under the Genentech Agreement. The Company utilizes the most likely amount method to estimate the amount of research, development and regulatory milestone payments to be received. As part of the evaluation for the research and development milestone payments, the Company considers several factors, including the stage of research and development of the compounds included in the arrangement, the risk associated with the remaining research and development work required to achieve the milestone and the Company’s level of involvement in the research and development activities. All research and development milestone payments have been excluded from the transaction price through December 31, 2018 due to the uncertainty of achieving the associated research outcome and the uncertainty of initiating the specified phase of clinical research. Regulatory milestone payments are triggered upon the first commercial sale following receipt of regulatory approval from the FDA or other global regulatory authorities; therefore, such amounts will be excluded from the transaction price until the associated regulatory approval is received. The commercial milestone payments and royalties are subject to the royalty recognition constraint whereby such amounts will be recognized as revenue upon the later of: (i) when the related sales occur or (ii) when the performance obligation to which some or all of the payment has been allocated has been satisfied, or partially satisfied, because the exclusive license is deemed to be the sole or predominant item to which the payments relate. The Company will update its assessment of the estimated transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur. The Company did not adjust the promised amount of consideration for the effects of a significant financing component because the Company expects that the period between when the promised goods and services are transferred and when the customer pays for those goods and services will be one year or less. There were no changes in the transaction price during the year ended December 31, 2018. The Company allocated the transaction price of $5.0 million to the sole identified performance obligation in its entirety. Amounts allocated to the combined performance obligation comprised of the exclusive license and the associated technology and materials transfer will be recognized as revenue at a point in time based on the date on which Genentech provides notice regarding the satisfactory completion of the technology and materials transfer. Upon the successful execution of the technology and materials transfer, control is considered to be transferred to both the exclusive license and the technology and materials transfer promises due primarily to the payment contingency and timing of the conveyance of risks and rewards of ownership. The Company did not recognize any revenue associated with the Genentech Agreement through December 31, 2018 because the technology and materials transfer had not yet been completed. In February 2019, the Company completed the technology and materials transfer and received the upfront payment of $5.0 million. As of December 31, 2018, the Company did not have any receivables or deferred revenue related to the Genentech Agreement because neither had any payments under the arrangement become due nor had the underlying performance obligation been transferred. Astellas Pharma Inc. In November 2014, the Company entered into the Collaborative Research, Development, Commercialization and License Agreement (the “Astellas Agreement”) with Astellas. The focus of the Astellas Agreement was to identify, develop and commercialize therapeutic candidates relating to the Unfolded Protein Response (“UPR”) pathway. The Astellas Agreement ended in December 2018. 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. In addition, the Company was 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 conducted research during the initial research term, which was 3 ½ years, to identify lead compounds for clinical development. At the end of the research term, Astellas, in its sole discretion, could designate a development compound and make a milestone payment to the Company. The Company had the right, but not the obligation, to co-develop the compound. If the Company did not exercise its option to co-develop the compound, Astellas had an exclusive right to the compound and sole right and responsibility for the development of the compound. Term and Termination The term of the Astellas Agreement commenced in November 2014 and would 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 was set to automatically terminate at the end of the 3 ½ year research term, in the second quarter of 2018, if Astellas had not designated at least one development compound, unless mutually agreed to be extended. In April 2018, the Company and Astellas entered into Amendment No. 6 (“Amendment No. 6”) to the Astellas Agreement. Amendment No. 6 was effective beginning in April 2018 and extended the research term for the initial project under the Astellas Agreement to December 2018. As of December 31, 2018, the Astellas Agreement was complete; and all of the Company’s performance obligations under the contract had been satisfied. Astellas had 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 included termination for breach and termination for bankruptcy. Accounting Analysis The Company assessed this arrangement in accordance with ASC 606 and concluded that the contract counterparty, Astellas, is a customer. The Company identified the following material promises as of the most recent amendment in April 2018: (1) the research license; (2) the research services to be provided over the research term; and (3) participation in the Joint Research Committee (the “Committee”) to be provided over the research term of the agreement. The Company determined that the license and research services were not distinct from one another, as the license has limited value without the performance of the research and development activities. Participation in the Committee to oversee the research activities was determined to be quantitatively and qualitatively immaterial and therefore is excluded from performance obligations. As such, the Company determined that these promises should be combined into a single performance obligation. The Company evaluated the Astellas option right to designate a development compound, as described above, to determine whether it provides Astellas with a material right. The Company concluded that the option was not issued at a significant and incremental discount and Astellas has only the right to pursue negotiations for additional projects, and therefore does not provide a material right. As such, they are excluded from performance obligations at the inception of the arrangement. Under the Astellas Agreement, in order to evaluate the appropriate transaction price as of the adoption of ASC 606, the Company determined that the payments received to date for research funding support and reimbursement of third-party costs, the estimated payments that will be received for research funding support and reimbursement of third-party costs over the remaining research term, and the milestone payments received to date represent the transaction price, which was allocated to the single performance obligation. The transaction price as of the adoption of ASC 606 and December 31, 2018 was $13.8 million. The option exercise fee that may be received is excluded from the transaction price until the customer option is exercised. Future potential milestone payments were excluded from the transaction price, as all milestone amounts were fully constrained due to Astellas having only the right to pursue negotiations for additional projects. Revenue associated with the performance obligation was recognized as the research and development services were provided using an input method, according to the costs incurred as related to the research and development activities and the costs expected to be incurred in the future to satisfy the performance obligation. The transfer of control occurred over this period and in management’s judgment, was the best measure of progress towards satisfying the performance obligation. As of December 31, 2018, the research and development services related to this performance obligation were complete. The Company recognized revenue of $2.8 million and $5.3 million for the years ended December 31, 2018 and 2017, respectively. Amounts recorded as deferred revenue under the Astellas Agreement totaled $1.1 million as of December 31, 2017. 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 December 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 $2.4 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 ten years following the first commercial sale. As of December 31, 2018 and 2017, the Company had not developed a commercial product using the licensed technologies and no pre-commercialization milestones had been achieved. Cystic Fibrosis Foundation, Inc. In March 2012, the Company entered into a Research, Development and Commercialization Agreement (the “CFF Agreement”) with CFF. Under terms of the CFF Agreement, which was subsequently amended in May 2013 and January 2014, CFF 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 CFF Agreement during the years ended December 31, 2018 and 2017. Under the CFF agreement, the Company has agreed to make future sales-based milestone payments, which the agreement refers to as royalties, to CFF 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 CFF 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, 2018 and 2017, 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, CFF may elect to accept a one-time payment equal to the consideration CFF 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 CFF Agreement. The Company estimated the fair value of this derivative liability to be less than $0.1 million as of December 31, 2018 and 2017 (see Note 4). The CFF 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. CFF 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.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	1 1 . Income Taxes During the years ended December 31, 2018 and 2017, 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 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, 2018 2017 Federal statutory income tax rate (21.0 )% (34.0 )% State income taxes, net of federal benefit (6.1 ) (5.6 ) Research and development tax credit carryforwards (3.9 ) (3.2 ) Return to provision (0.6 ) — Nondeductible expenses 0.4 0.7 Change in deferred tax asset valuation allowance 31.2 2.0 Effect of U.S. Tax Reform — 40.1 Effective income tax rate (0.0 )% (0.0 )% Net deferred tax assets as of December 31, 2018 and 2017 consisted of the following (in thousands): December 31, 2018 2017 Net operating loss carryforwards 68,219 $ 52,861 Research and development tax credit carryforwards 12,799 9,232 Accrued expenses and other 2,066 1,583 Total deferred tax assets before valuation allowance 83,084 63,676 Valuation allowance (82,965 ) (63,660 ) Net deferred tax assets 119 16 Depreciation (119 ) (16 ) Net deferred tax assets $ — $ — On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act 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% 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 revalued the deferred tax assets and liabilities in 2017 at the new rate. The revaluation resulted in a reduction in deferred tax assets as of December 31, 2017 of $24.0 million with a corresponding reduction in the valuation allowance. Therefore, there was no net effect on the 2017 tax provision affecting the Company’s statement of operations. As of December 31, 2018, the Company had federal and state net operating loss carryforwards (“NOLs”) of $252.6 million and $240.1 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, 2018, the Company also had federal and state research and development tax credit carryforwards of $10.0 million and $3.6 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, 2018 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, 2018 and 2017. Management reevaluates the positive and negative evidence at each reporting period. Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2018 and 2017 related primarily to the increases in net operating loss carryforwards and research and development tax credit carryforwards and were as follows (in thousands): Years Ended December 31, 2018 2017 Valuation allowance at beginning of year $ (63,660 ) $ (62,442 ) Increases recorded to income tax provision (19,305 ) (1,218 ) Valuation allowance at end of year $ (82,965 ) $ (63,660 ) The Company has not yet recorded any amounts for unrecognized tax benefits, interest or penalties historically through December 31, 2018. 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, 2015 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, 2018
Earnings Per Share [Abstract]
Net Loss per Share	1 2 . Net Loss per Share Basic and diluted net loss per share was calculated as follows (in thousands except share and per share amounts): Year Ended December 31, 2018 2017 Numerator: Net loss $ (61,832 ) $ (59,432 ) Denominator: Weighted average number of common shares outstanding—basic and diluted 38,495,103 25,407,943 Net loss per share—basic and diluted $ (1.61 ) $ (2.34 ) The following common stock equivalents have been excluded from the computation of diluted weighted-average shares outstanding, because such securities had an antidilutive impact: December 31, 2018 2017 Options to purchase common stock 3,158,676 2,838,558 Restricted stock units 163,425 — 3,322,101 2,838,558

Leases

Leases	12 Months Ended
	Dec. 31, 2018
Leases [Abstract]
Leases	1 3 . Leases The Company has operating leases of office and laboratory space. In September 2017, the Company entered into a lease agreement for its new corporate headquarters, consisting of approximately 30,000 square feet of laboratory and office space located in Boston, Massachusetts. The new lease commenced in January 2018, and rent payments began in April 2018. This lease has a ten-year initial term with an option to extend for seven additional years. The Company’s prior lease in Cambridge, Massachusetts, expired in May 2018. The lessee has the right to terminate the lease in the event of the inability to use the space due to substantial damage while the lessor has the right to terminate the lease for tenant’s default of lease financial obligations. Per the terms of the lease agreement, the Company does not have any residual value guarantees. In calculating the present value of the lease payments, the Company has utilized its incremental borrowing rate based on electing the original lease term to account for each lease component and its associated non-lease components as a single lease component. It has allocated all the contract consideration across lease components only. This may result in the initial and subsequent measurement of the balances of the right-of-use asset and lease liability for leases being greater than if the policy election was not applied. The Company’s real estate leases in Cambridge and Boston are net leases, as their non-lease components (i.e. common area maintenance) are paid separately from rent based on actual costs incurred. Therefore, the non-lease components were not included in the right-of-use asset and liability and are reflected as an expense in the period incurred. As of December 31, 2018 and 2017, assets under operating lease were $13.8 million and $0.5 million, respectively. The elements of lease expense were as follows (in thousands): For the Years Ended December 31, 2018 2017 Lease cost Operating lease cost $ 2,232 $ 1,143 Variable lease cost (1) 366 — Total lease cost $ 2,598 $ 1,143 Other information Operating cash flows used for operating leases $ 1,634 $ 1,344 Operating lease liabilities arising from obtaining right-of-use assets $ 14,252 $ — Weighted-average remaining lease term 9.34 years 0.34 years Weighted-average discount rate 4.50 % 4.06 % (1) As of December 31, 2018, minimum rental commitments under this lease are as follows (in thousands): 2019 $ 1,686 2020 1,733 2021 1,780 2022 1,829 2023 1,880 Thereafter 8,757 Total lease payments 17,665 Less: imputed interest (3,413 ) Total $ 14,252

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	1 4 . Commitments and Contingencies 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, 2018 and 2017, under this consulting agreement, the Company issued 239,490 and 188,462 shares, respectively, for $0.9 million and $0.8 million, respectively, which is recorded within general and administrative expense in the accompanying statements of operations. Future retainer payments due under the Letter Agreement total $0.4 million as of December 31, 2018. 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 due to 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, 2018 or 2017. Purchase Commitments Purchase commitments represent non-cancelable contractual commitments associated with certain clinical trial activities within the Company’s clinical research organization.

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2018
Postemployment Benefits [Abstract]
401(k) Savings Plan	1 5 . 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. As of December 31, 2018, no contributions have been made to the plan by the Company.

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (Unaudited)	1 6 . 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 except for share and per share amounts). Three Months Ended March 31, 2018 June 30, 2018 September 30, 2018 December 31, 2018 Revenue $ 942 $ 843 $ 1,055 $ — Operating expenses: Research and development 8,400 12,604 15,591 13,717 General and administrative 3,823 3,957 4,150 3,780 Total operating expenses 12,223 16,561 19,741 17,497 Loss from operations (11,281 ) (15,718 ) (18,686 ) (17,497 ) Interest income 165 194 171 342 Other income (expense), net 90 46 87 255 Net loss $ (11,026 ) $ (15,478 ) $ (18,428 ) $ (16,900 ) Net loss per share—basic and diluted $ (0.32 ) $ (0.43 ) $ (0.50 ) $ (0.36 ) Weighted average common shares outstanding—basic and diluted 34,474,004 36,009,109 36,694,957 46,687,910 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 Other income (expense), net (30 ) (2 ) (25 ) (43 ) Net loss $ (15,096 ) $ (16,958 ) $ (13,954 ) $ (13,424 ) Net loss per share—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

Nature of the Business (Policie

Nature of the Business (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Public Offerings	Public Offerings In the fourth quarter 2018, the Company completed a follow-on public offering whereby the Company sold 12,650,000 shares of common stock at a public offering price of $6.75 per share, which included 1,650,000 shares issued pursuant to the exercise by the underwriters of their option to purchase additional shares of common stock. The Company received net proceeds of approximately $80.1 million, after deducting underwriting discounts and commissions and other offering expenses. In March 2018, the Company entered into a sales agreement with Leerink Partners LLC (“Leerink”) with respect to an at-the-market (“ATM”) offering program under which the Company may issue and sell, from time to time at its sole discretion, shares of its common stock, resulting in aggregate gross offering proceeds of up to $50.0 million. Leerink is not required to sell any specific amount but acts as the Company’s sales agent on a commercially reasonable efforts basis consistent with its normal trading and sales practices. Common stock will be sold at prevailing market prices at the time of the sale; and as a result, prices may vary. The Company will pay Leerink up to 3% of the gross proceeds from any common stock sold through the sales agreement. As of December 31, 2018, the Company had sold an aggregate of 3,475,166 shares of its common stock for total net proceeds of approximately $21.6 million under the ATM program. At December 31, 2018, $27.6 million of common stock remains available for sale under the ATM. In December 2017, the Company completed a follow-on public offering whereby the Company sold 9,200,000 shares of common stock at a public offering price $42.9 million after deducting underwriting discounts and commissions and other offering expenses.
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, stock-based compensation, determining the value of the operating lease right of use asset and the valuation of the derivative liability. 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, and short-term investments. 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 only customer in 2018 was Astellas Pharma Inc. (“Astellas”) and as of December 31, 2018, the Astellas Agreement (as defined below) had completed and all amounts were paid in full.
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, 2018 and 2017, the Company’s cash equivalents consisted of money market funds, U.S. government-sponsored securities and U.S. treasury securities.
Restricted Cash	Restricted Cash As of December 31, 2018 and 2017, long-term restricted cash represents cash pledged as collateral to secure the Company’s long-term Boston lease obligation. As of December 31, 2017, short-term restricted cash represents cash pledged as collateral to secure the Company’s Cambridge lease obligation, which expired in May 2018, resulting in a decrease of $0.3 million in short-term restricted cash. Amounts are reported as noncurrent unless the restrictions are expected to be released in the next 12 months. Long-term restricted cash decreased by $0.8 million from December 31, 2017 as the Company was entitled to reduce the amount of the letter of credit and the corresponding restricted cash upon meeting certain conditions noted within the lease agreement.
Short-Term Investments	Short-Term Investments Short-term investments represent holdings of available-for-sale debt 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 income (expense), net, in the accompanying statement of operations.
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 for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. In the event that such cash flows are not expected to be sufficient to recover the carrying amount of the assets, the assets are written-down to their fair values.
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 payable and accrued expenses approximate their fair value due to the short-term nature of these 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 is collaboration revenue earned from activities in the United States.
Revenue Recognition	Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers Under ASC 605, the Company recognized revenue for each unit of accounting when all of the following criteria were met: persuasive evidence of an arrangement existed; delivery had occurred or services had been rendered; the seller’s price to the buyer was fixed or determinable; and collectability was reasonably assured. The Company was recognizing revenue related to collaboration agreements over the expected research and development period using the proportional performance method. 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. The Company enters into licensing agreements which are within the scope of ASC 606, under which it may exclusively license rights to research, develop, manufacture and commercialize its product candidates to third parties. The terms of these arrangements typically include payment to the Company of one or more of the following: nonrefundable, upfront license fees; reimbursement of certain costs; customer option exercise fees; development, regulatory and commercial milestone payments; and royalties on net sales of licensed products. As part of the accounting for these arrangements, the Company must use significant judgment to determine: a) the number of performance obligations based on the determination under step (ii) above; b) the transaction price under step (iii) above; and c) the stand-alone selling price for each performance obligation identified in the contract for the allocation of transaction price in step (iv) above. The Company uses judgment to determine whether milestones or other variable consideration should be included in the transaction price as described further below. The transaction price is allocated to each performance obligation on a relative stand-alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. In developing the stand-alone price for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the stand-alone selling price for performance obligations by evaluating whether changes in the key assumptions used to determine the stand-alone selling prices will have a significant effect on the allocation of transaction price between multiple performance obligations. The Company recognizes a contract asset or liability for the difference between the Company’s performance (i.e., the goods or services transferred to the customer) and the customer’s performance (i.e., the consideration paid by, and unconditionally due from, the customer). Exclusive Licenses If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, the Company recognizes revenue from nonrefundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. In assessing whether a promise or performance obligation is distinct from the other promises, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from a promise for its intended purpose without the receipt of the remaining promise, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the research and development and licensing agreement. Such a change could have a material impact on the amount of revenue the Company records in future periods. Research and Development Services The promises under the Company’s collaboration and license agreements generally include research and development services to be performed by the Company on behalf of the collaboration partner. As the provision of research and development services is a part of the Company’s central operations, when the Company is principally responsible for the performance of these services under the agreements, the Company recognizes revenue on a gross basis for research and development services in accordance with the ASC 606 framework described above. Customer Options If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. The Company allocates the transaction price to material rights based on the relative stand-alone selling price, which is determined based on the identified discount and the probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised. Milestone Payments At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones or other variable consideration should be included in the transaction price. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making Royalties For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. For a complete discussion of accounting for collaboration revenues, see Note 10, “Significant Agreements.”
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, Inc. (“CFF”) (see Note 4). 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.
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. Research and development costs that are paid in advance of performance are capitalized as prepaid expenses and expensed over the period that the goods or services are provided 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.
Stock-Based Compensation	Stock-Based Compensation The Company measures all stock-based awards granted to employees based on the fair value on the date of the grant and recognizes compensation expense of those awards over the requisite service period, which is usually the vesting period of the respective award. Generally, the Company issues stock options and restricted stock awards to employees with service-based vesting conditions and records the expense for these awards using the straight-line method. The Company measures stock-based awards granted to nonemployees 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. 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 nonemployees for which satisfaction of the performance conditions is not solely within the control of the holder until the performance conditions have been met. The fair value of each stock option is estimated on the date of grant using the Black-Scholes option-pricing model. Due to the lack of complete company-specific historical and implied volatility data for the full expected term of the stock-based awards, the Company 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 it has adequate historical data regarding the volatility of its own traded stock price. The expected term of stock options granted to employees is determined utilizing the “simplified” method, whereby the expected term equals the average of the vesting term and the original contractual term of the option. The expected term of stock options granted to nonemployees is equal to the contractual term of the option. 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. The expected dividend yield is based upon the fact the Company has never paid cash dividends on common stock and does not expect to pay any cash dividends in the foreseeable future. The Company accounts for forfeitures as they occur and classifies stock-based compensation expense in its statement of operations in the same manner that the award recipients’ payroll costs are classified or in which the award recipients’ service payments are classified.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders.
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 and diluted net loss per share have been calculated by dividing net loss by the weighted average number of common shares outstanding during the year. Diluted net income per share is calculated by dividing the net income attributable to common stockholders by the weighted-average number of common equivalent shares outstanding for the period, including any dilutive effect from outstanding stock options, unvested restricted stock, restricted stock units, and employee stock purchase plan stock using the treasury stock method. Basic net loss is based solely on the number of common shares outstanding during the year. Since the Company is reporting a net loss for all periods presented, all common stock equivalents are considered anti-dilutive and are excluded from the calculation of diluted net loss per share.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements ASU No. 2014-09, Revenue from Contracts with Customers In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 606, which supersedes the revenue recognition requirements in ASC Topic 605, Revenue Recognition 2016, the FASB issued and aspects of identification performance obligations and licensing 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 modified retrospective method for adoption. As of January 1, 2018, t On January 1, 2018, the Company recorded a cumulative-effect decrease to the opening accumulated deficit of $1.1 million and a corresponding decrease to deferred revenue due to the adoption of ASC 606. As of December 31, 2018, the impact of the adoption of ASC 606 on the Company’s financial statement was as follows (in thousands): December 31, 2018 As Reported under ASC 606 Balance without Adoption of ASC 606 Effect of Change Higher/(Lower) Balance Sheet: Deferred revenue, current $ — $ — $ — Income Statement: Revenue for the 12 months ended December 31, 2018 $ 2,840 $ 3,945 $ (1,105 ) The most significant change related to the Company’s determination of transaction price at inception and each reporting period as well as the revenue recognition pattern for the Astellas Agreement (as defined below) with Astellas as well as treatment of variable consideration in the form of milestone payments. Under ASC 605, the research funding support payments, reimbursement of third-party costs, and over the 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. ASU No. 2016-02, Leases In February 2016, the FASB issued ASU No. 2016-02, Leases Due to the Company commencing a new lease in Boston, Massachusetts, in January 2018, the Company elected to early adopt the standard effective January 1, 2018, as permitted by the guidance, in order to enhance overall transparency within financial reporting. The Company has implemented the standard using the required modified retrospective approach and has also elected to utilize the package of practical expedients. The expedients used by the Company are as follows: (1) allowing an entity to not reassess the lease classification for any expired or existing leases, (2) allowing an entity to not reassess the treatment of initial direct costs as they related to existing leases, and (3) allowing an entity to not reassess whether expired or existing contracts are or contain leases. The company also elected the practical expedient to use hindsight in determining the appropriate lease term and in assessing impairment of its right-of- use assets. In using the modified retrospective approach, the Company is required to recognize and measure leases existing at, or entered into after, the beginning of the earliest comparative period presented. During 2017, the Company was deemed the accounting owner of the construction project for the build-to-suit lease in Boston, Massachusetts, because of the Company’s involvement in the build-out of the space. Under the new standard, the Company is no longer considered the accounting owner of the leased space due to (1) not having the right to obtain or control the leased premises during the construction period, (2) having no right of payment for the partially constructed assets, and the leased premises are not of a specialized nature and, thus, could be potentially leased to another tenant, and (3) not legally owning or controlling the land on which the property improvements will be constructed. As such, upon transition, the existing construction-in-progress balance within property and equipment, and the corresponding build-to-suit facility lease financing obligation balance within other liabilities, current and noncurrent, have been derecognized. Prior period results have been restated to this effect through to the earliest period presented. The adoption of this standard has had a material impact on the Company’s financial position but did not significantly affect the Company’s results of operations. See Impacts on Previously Reported Results below. Impact to Previously Reported Results The impact of the adoption of the new leasing standard on the balance sheet as of December 31, 2017, is as follows (in thousands): December 31, 2017 As Previously Reported New Lease Standard Adjustment As restated Operating lease right-of-use assets, net $ — $ 472 $ 472 Property and equipment, net 16,567 (16,143 ) 424 Deferred rent, current 87 (87 ) — Operating lease liabilities, current — 559 559 Other liabilities, current 902 (902 ) — Other liabilities, net of current 15,315 (15,315 ) — Accumulated deficit (216,882 ) 74 (216,808 ) ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments ASU 2016-18, Statement of Cash Flows (Topic 230) – Restricted Cash In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230) - Restricted Cash new standard was for on January 1, 2018. This resulted in $0.8 $2.0 ASU 2017-09, Compensation – Stock Compensation (Topic 719): Scope of Modification Accounting In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (Topic 719): Scope of Modification Accounting Recently Issued Accounting Pronouncements ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting In June 2018, the FASB issued ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting ASU No. 2018-13, Fair Value Measurement (Topic 820): Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued ASU No. 2018-13, Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Significant Accounting Policies [Line Items]
Schedule of Impact of Adoption of New Leasing Standard	The impact of the adoption of the new leasing standard on the balance sheet as of December 31, 2017, is as follows (in thousands): December 31, 2017 As Previously Reported New Lease Standard Adjustment As restated Operating lease right-of-use assets, net $ — $ 472 $ 472 Property and equipment, net 16,567 (16,143 ) 424 Deferred rent, current 87 (87 ) — Operating lease liabilities, current — 559 559 Other liabilities, current 902 (902 ) — Other liabilities, net of current 15,315 (15,315 ) — Accumulated deficit (216,882 ) 74 (216,808 )
ASC 606 [Member]
Significant Accounting Policies [Line Items]
Schedule of Impacts Related to ASC 606 Adoption	As of December 31, 2018, the impact of the adoption of ASC 606 on the Company’s financial statement was as follows (in thousands): December 31, 2018 As Reported under ASC 606 Balance without Adoption of ASC 606 Effect of Change Higher/(Lower) Balance Sheet: Deferred revenue, current $ — $ — $ — Income Statement: Revenue for the 12 months ended December 31, 2018 $ 2,840 $ 3,945 $ (1,105 )

Short-Term Investments (Tables)

Short-Term Investments (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 and 2017 (in thousands) : December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S government-sponsored enterprise securities $ 41,478 $ 2 $ (3 ) $ 41,477 U.S. treasury securities 48,090 3 (1 ) $ 48,092 $ 89,568 $ 5 $ (4 ) $ 89,569 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

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
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 31, 2018 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 26,806 $ — $ — $ 26,806 U.S. treasury securities — 500 — 500 Short-term investments: U.S. government-sponsored enterprise securities — 41,477 — 41,477 U.S. treasury securities — 48,092 — 48,092 $ 26,806 $ 90,069 $ — $ 116,875 Liabilities: Derivative liability $ — $ — $ 3 $ 3 Fair Value Measurements as of December 31, 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

Prepaids and Other Current As_2

Prepaids and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 Prepaid clinical, manufacturing and scientific expenses $ 1,660 $ 568 Prepaid insurance expenses 108 55 Other prepaid expenses and other current assets 713 754 $ 2,481 $ 1,377

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2018
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) 2018 2017 Laboratory equipment 5 years $ 2,715 $ 3,299 Furniture and fixtures 5 years 368 157 Computer and office equipment 3 – 5 years 209 169 Construction-in-progress 8 — 3,300 3,625 Less: Accumulated depreciation and amortization (2,695 ) (3,201 ) $ 605 $ 424

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2018
Payables And Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following (in thousands): December 31, 2018 2017 Accrued payroll and related expenses $ 2,746 $ 1,542 Accrued research and development expenses 1,965 3,930 Accrued professional fees 799 556 Accrued other 151 92 $ 5,661 $ 6,120

Common and Preferred Stock (Tab

Common and Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Text Block [Abstract]
Summary of Number of Shares of Common Stock Reserved for Future Issuance	The Company has reserved for future issuance the following number of shares of common stock: Years Ended December 31, 2018 2017 2016 and 2008 Stock option incentive plan 4,258,783 3,474,566 2016 Employee stock purchase plan 386,539 266,685 4,645,322 3,741,251

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Restricted Stock Unit Activity and Related Information	A summary of the Company’s restricted stock unit activity and related information is as follows: Number of Shares Weighted Average Grant Date Fair Value Unvested balance at December 31, 2017 — $ — Granted 197,265 3.20 Vested — — Forfeited (33,840 ) 3.20 Unvested balance at December 31, 2018 163,425 $ 3.20
Schedule of Share-based Payment Award Stock Options Valuation Assumptions	The fair value of each option awards was estimated on the date of grant using the Black Scholes option-pricing model with the following weighted-average assumptions: Years Ended December 31, 2018 2017 Risk-free interest rates 2.68 % 2.00 % Expected term (in years) 6.00 5.96 Expected volatility 78.47 % 75.18 % Expected dividend yield — —
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity (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, 2017 2,838,558 $ 6.69 8.10 $ 3,213 Granted 1,004,025 3.22 Exercised (8,775 ) 2.43 Forfeited (675,132 ) 5.14 Outstanding at December 31, 2018 3,158,676 $ 5.93 7.53 $ 471 Vested and expected to vest at December 31, 2018 3,075,426 $ 6.03 7.67 $ 399 Options exercisable at December 31, 2018 1,697,979 $ 6.44 6.92 $ 280
Summary of Stock-Based Compensation Expense	Stock-based compensation expense was classified in the statements of operations as follows (in thousands): Years Ended December 31, 2018 2017 Research and development $ 1,571 $ 1,326 General and administrative 2,844 2,406 $ 4,415 $ 3,732

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 Federal statutory income tax rate (21.0 )% (34.0 )% State income taxes, net of federal benefit (6.1 ) (5.6 ) Research and development tax credit carryforwards (3.9 ) (3.2 ) Return to provision (0.6 ) — Nondeductible expenses 0.4 0.7 Change in deferred tax asset valuation allowance 31.2 2.0 Effect of U.S. Tax Reform — 40.1 Effective income tax rate (0.0 )% (0.0 )%
Schedule of Net Deferred Tax Assets	Net deferred tax assets as of December 31, 2018 and 2017 consisted of the following (in thousands): December 31, 2018 2017 Net operating loss carryforwards 68,219 $ 52,861 Research and development tax credit carryforwards 12,799 9,232 Accrued expenses and other 2,066 1,583 Total deferred tax assets before valuation allowance 83,084 63,676 Valuation allowance (82,965 ) (63,660 ) Net deferred tax assets 119 16 Depreciation (119 ) (16 ) 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, 2018 and 2017 related primarily to the increases in net operating loss carryforwards and research and development tax credit carryforwards and were as follows (in thousands): Years Ended December 31, 2018 2017 Valuation allowance at beginning of year $ (63,660 ) $ (62,442 ) Increases recorded to income tax provision (19,305 ) (1,218 ) Valuation allowance at end of year $ (82,965 ) $ (63,660 )

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders	Basic and diluted net loss per share was calculated as follows (in thousands except share and per share amounts): Year Ended December 31, 2018 2017 Numerator: Net loss $ (61,832 ) $ (59,432 ) Denominator: Weighted average number of common shares outstanding—basic and diluted 38,495,103 25,407,943 Net loss per share—basic and diluted $ (1.61 ) $ (2.34 )
Schedule of Antidilutive Securities Excluded from Computation of Diluted Weighted-average Shares Outstanding	, because such securities had an antidilutive impact: December 31, 2018 2017 Options to purchase common stock 3,158,676 2,838,558 Restricted stock units 163,425 — 3,322,101 2,838,558

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2018
Lessee Disclosure [Abstract]
Elements of Lease Expense	The elements of lease expense were as follows (in thousands): For the Years Ended December 31, 2018 2017 Lease cost Operating lease cost $ 2,232 $ 1,143 Variable lease cost (1) 366 — Total lease cost $ 2,598 $ 1,143 Other information Operating cash flows used for operating leases $ 1,634 $ 1,344 Operating lease liabilities arising from obtaining right-of-use assets $ 14,252 $ — Weighted-average remaining lease term 9.34 years 0.34 years Weighted-average discount rate 4.50 % 4.06 % (1)
Summary of Future Minimum Lease Payments	As of December 31, 2018, minimum rental commitments under this lease are as follows (in thousands): 2019 $ 1,686 2020 1,733 2021 1,780 2022 1,829 2023 1,880 Thereafter 8,757 Total lease payments 17,665 Less: imputed interest (3,413 ) Total $ 14,252

Quarterly Financial Data (Una_2

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
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 except for share and per share amounts). Three Months Ended March 31, 2018 June 30, 2018 September 30, 2018 December 31, 2018 Revenue $ 942 $ 843 $ 1,055 $ — Operating expenses: Research and development 8,400 12,604 15,591 13,717 General and administrative 3,823 3,957 4,150 3,780 Total operating expenses 12,223 16,561 19,741 17,497 Loss from operations (11,281 ) (15,718 ) (18,686 ) (17,497 ) Interest income 165 194 171 342 Other income (expense), net 90 46 87 255 Net loss $ (11,026 ) $ (15,478 ) $ (18,428 ) $ (16,900 ) Net loss per share—basic and diluted $ (0.32 ) $ (0.43 ) $ (0.50 ) $ (0.36 ) Weighted average common shares outstanding—basic and diluted 34,474,004 36,009,109 36,694,957 46,687,910 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 Other income (expense), net (30 ) (2 ) (25 ) (43 ) Net loss $ (15,096 ) $ (16,958 ) $ (13,954 ) $ (13,424 ) Net loss per share—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

Nature of the Business - Additi

Nature of the Business - Additional Information (Detail)	Oct. 31, 2017Position	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)$ / shares	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / shares
Nature of Business [Line Items]
Accumulated deficit			$ (216,808,000)	$ (277,535,000)				$ (216,808,000)				$ (277,535,000)	$ (216,808,000)
Cash, cash equivalents and short-term investments				118,400,000								118,400,000
Net loss				$ (16,900,000)	$ (18,428,000)	$ (15,478,000)	$ (11,026,000)	(13,424,000)	$ (13,954,000)	$ (16,958,000)	$ (15,096,000)	(61,832,000)	$ (59,432,000)
Number of positions eliminated | Position	13
One-time severance and related costs								$ 200,000
Net proceeds from sales of common stock												$ 21,703,000
Follow-on Public Offering [Member]
Nature of Business [Line Items]
Number of shares issued and sold | shares			9,200,000	12,650,000								1,650,000
Common stock price per share | $ / shares			$ 5	$ 6.75				$ 5				$ 6.75	$ 5
Net proceeds from sales of common stock			$ 42,900,000	$ 80,100,000
ATM Program [Member] | Leerink Partners LLC [Member]
Nature of Business [Line Items]
Number of shares issued and sold | shares												3,475,166
Net proceeds from sales of common stock												$ 21,600,000
Common stock aggregate offering price		$ 50,000,000
Percentage of gross proceeds from common stock sold through sales agreement		3.00%
Common stock remained available for sale, value				$ 27,600,000								$ 27,600,000
Over Allotment [Member]
Nature of Business [Line Items]
Number of shares issued and sold | shares			1,200,000

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)FinancialInstitution	Dec. 31, 2017USD ($)	Jan. 01, 2018USD ($)
Significant Accounting Policies [Line Items]
Number of accredited financial institutions | FinancialInstitution	2
Short-term restricted cash decreased		$ 300
Long-term restricted cash decreased		800
Accumulated deficit	$ (277,535)	(216,808)
ASU 2016-18 [Member]
Significant Accounting Policies [Line Items]
Change in restricted cash	$ 800	$ 2,000
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | ASC 606 [Member]
Significant Accounting Policies [Line Items]
Accumulated deficit			$ (1,100)

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Schedule of Impacts Related to ASC 606 Adoption (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Balance Sheet:
Deferred revenue, current		$ 1,108
Income Statement:
Revenue	$ 2,840	$ 5,341
ASC 606 [Member]
Income Statement:
Revenue	2,840
ASC 606 [Member] | Balance Without Adoption of ASC 606 [Member]
Income Statement:
Revenue	3,945
ASC 606 [Member] | Effect of Change Higher/(Lower) [Member]
Income Statement:
Revenue	$ (1,105)

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Impact of Adoption of New Leasing Standard (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Operating lease right-of-use assets, net	$ 13,849	$ 472
Property and equipment, net	605	424
Operating lease liabilities, current	1,056	559
Accumulated deficit	$ (277,535)	(216,808)
As Previously Reported [Member] | ASU 2016-02 [Member]
Significant Accounting Policies [Line Items]
Property and equipment, net		16,567
Deferred rent, current		87
Other liabilities, current		902
Other liabilities, net of current		15,315
Accumulated deficit		(216,882)
New Lease Standard Adjustment [Member] | ASU 2016-02 [Member]
Significant Accounting Policies [Line Items]
Operating lease right-of-use assets, net		472
Property and equipment, net		(16,143)
Deferred rent, current		(87)
Operating lease liabilities, current		559
Other liabilities, current		(902)
Other liabilities, net of current		(15,315)
Accumulated deficit		$ 74

Short-Term Investments - Summar

Short-Term Investments - Summary of Short-Term Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 89,568	$ 44,740
Gross Unrealized Gains	5	1
Gross Unrealized Losses	(4)	(3)
Fair Value	89,569	44,738
U.S Government-Sponsored Enterprise Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	41,478	9,817
Gross Unrealized Gains	2
Gross Unrealized Losses	(3)	(1)
Fair Value	41,477	9,816
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48,090	34,923
Gross Unrealized Gains	3	1
Gross Unrealized Losses	(1)	(2)
Fair Value	$ 48,092	$ 34,922

Short-Term Investments - Additi

Short-Term Investments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
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 Asset_3

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, 2018	Dec. 31, 2017
Liabilities:
Derivative liability	$ 3	$ 25
Fair Value, Measurements, Recurring [Member]
Assets:
Total assets	116,875	71,066
Liabilities:
Derivative liability	3	25
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	26,806	13,871
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Assets:
Cash equivalents	500	3,497
Short-term investments	48,092	34,922
Fair Value, Measurements, Recurring [Member] | U.S Government-Sponsored Enterprise Securities [Member]
Assets:
Cash equivalents		8,960
Short-term investments	41,477	9,816
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Total assets	26,806	13,871
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	26,806	13,871
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Total assets	90,069	57,195
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury Securities [Member]
Assets:
Cash equivalents	500	3,497
Short-term investments	48,092	34,922
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S Government-Sponsored Enterprise Securities [Member]
Assets:
Cash equivalents		8,960
Short-term investments	41,477	9,816
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Liabilities:
Derivative liability	$ 3	$ 25

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Transfer of assets from level 1 to level 2	$ 0	$ 0
Transfer of assets from level 2 to level 1	0	0
Transfer of liabilities from level 1 to level 2	0	0
Transfer of liabilities from level 2 to level 1	$ 0	0
Fair value determination model	Monte-Carlo simulation analysis
Fair value of derivative liability	$ 0	$ 0

Prepaids and Other Current As_3

Prepaids and Other Current Assets - Schedule of Prepaids and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Prepaid Expense And Other Assets Current [Abstract]
Prepaid clinical, manufacturing and scientific expenses	$ 1,660	$ 568
Prepaid insurance expenses	108	55
Other prepaid expenses and other current assets	713	754
Prepaids and other current assets	$ 2,481	$ 1,377

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	$ 3,300	$ 3,625
Less: Accumulated depreciation and amortization	(2,695)	(3,201)
Property and Equipment, Net	$ 605	424
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Useful Life (years)	5 years
Property and Equipment, Gross	$ 2,715	3,299
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Useful Life (years)	5 years
Property and Equipment, Gross	$ 368	157
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, Gross	$ 209	$ 169
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	$ 8

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 1,778	$ 1,402	$ 1,400
Property and equipment, net	605	$ 424
Boston Office [Member]
Property, Plant and Equipment [Line Items]
Fixed assets retired	1,100
Boston Office [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, net	$ 100

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued payroll and related expenses	$ 2,746	$ 1,542
Accrued research and development expenses	1,965	3,930
Accrued professional fees	799	556
Accrued other	151	92
Total accrued expenses	$ 5,661	$ 6,120

Common and Preferred Stock - Ad

Common and Preferred Stock - Additional Information (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Common stock, shares authorized	125,000,000	125,000,000
Common stock, par value	$ 0.001	$ 0.001
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.
Preferred stock, authorize for future issuance	5,000,000	5,000,000
Preferred stock, par value	$ 0.001	$ 0.001

Common and Preferred Stock - Su

Common and Preferred Stock - Summary of Number of Shares of Common Stock Reserved for Future Issuance (Detail) - shares	Dec. 31, 2018	Dec. 31, 2017	Feb. 03, 2016
Class Of Stock [Line Items]
2016 and 2008 Stock option incentive plan	4,645,322	3,741,251
2016 and 2008 Stock Option Incentive Plan [Member]
Class Of Stock [Line Items]
2016 and 2008 Stock option incentive plan	4,258,783	3,474,566
2016 Employee Stock Purchase Plan [Member}
Class Of Stock [Line Items]
2016 and 2008 Stock option incentive plan	386,539	266,685	138,757

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	Feb. 01, 2018	Feb. 03, 2016	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance			4,645,322	3,741,251
Stock-based compensation			$ 4,415,000	$ 3,732,000
Fair value of option			$ 3,500,000	2,500,000
Stock option grants			1,004,025
Proceeds from stock options exercised			$ 21,000	$ 31,000
Weighted average exercise price			$ 3.22	$ 4.57
Unrecognized compensation cost related to the unvested stock-based awards			$ 4,300,000
Compensation cost not yet recognized, period for recognition			2 years 7 days
Bonus Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation			$ 500,000
RSUs remaining to vest			163,425
Nonemployees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options unvested			83,250
Fair value of option			$ 100,000
Stock option grants			0	0
Nonemployees [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]
Aggregate intrinsic value of the stock option exercised			$ 100,000	$ 100,000
Proceeds from stock options exercised			$ 100,000	$ 100,000
Maximum [Member] | Bonus Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards requisite service period date	Feb. 1, 2019
Minimum [Member] | Bonus Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards requisite service period date	Feb. 1, 2018
2016 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance		1,581,839	936,682
Common stock reserved for issuance, percentage of number of shares of common stock outstanding		3.00%
2016 Plan and 2008 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance			4,258,783
2016 ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance		138,757	386,539	266,685
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			18,903	10,829
2016 ESPP [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation			$ 100,000

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Restricted Stock Unit Activity and Related Information (Detail) - Bonus Restricted Stock Units (RSUs) [Member]	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Number of Shares
Granted | shares	197,265
Forfeited | shares	(33,840)
Unvested balance at December 31, 2018 | shares	163,425
Weighted Average Grant Date Fair Value
Granted | $ / shares	$ 3.20
Forfeited | $ / shares	3.20
Unvested balance at December 31, 2018 | $ / shares	$ 3.20

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Share-based Payment Award Stock Options Valuation Assumptions (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price And Additional Disclosures [Abstract]
Risk-free interest rates	2.68%	2.00%
Expected term (in years)	6 years	5 years 11 months 15 days
Expected volatility	78.47%	75.18%
Expected dividend yield	0.00%	0.00%

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Number of Shares
Outstanding at December 31, 2017	2,838,558
Granted	1,004,025
Exercised	(8,775)
Forfeited	(675,132)
Outstanding at December 31, 2018	3,158,676	2,838,558
Vested and expected to vest at December 31, 2018	3,075,426
Options exercisable at December 31, 2018	1,697,979
Weighted Average Exercise Price
Outstanding at December 31, 2017	$ 6.69
Granted	3.22
Exercised	2.43
Forfeited	5.14
Outstanding at December 31, 2018	5.93	$ 6.69
Vested and expected to vest at December 31, 2018	6.03
Options exercisable at December 31, 2018	$ 6.44
Weighted Average Remaining Contractual Term, Outstanding	7 years 6 months 10 days	8 years 1 month 6 days
Weighted Average Remaining Contractual Term, Vested and expected to vest at December 31, 2018	7 years 8 months 1 day
Weighted Average Remaining Contractual Term, Options exercisable at December 31, 2018	6 years 11 months 1 day
Aggregate Intrinsic Value, Outstanding	$ 471	$ 3,213
Aggregate Intrinsic Value, Vested and expected to vest at December 31, 2018	399
Aggregate Intrinsic Value, Options exercisable at December 31, 2018	$ 280

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 4,415	$ 3,732
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	1,571	1,326
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 2,844	$ 2,406

Significant Agreements - Additi

Significant Agreements - Additional Information (Detail) - USD ($)	Apr. 23, 2018	Jan. 31, 2014	Feb. 28, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2013	Dec. 31, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue				$ 2,840,000	$ 5,341,000
Deferred revenue					1,108,000
Estimated fair value of derivative liability				0	0
ASC 606 [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue				2,840,000
Genentech [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Nonrefundable cash payment				5,000,000
Future milestone payment				$ 96,000,000
Genentech [Member] | Subsequent Event [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment received			$ 5,000,000
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
Extended research term	2018-12
Description of material promises under agreement				(1) the research license; (2) the research services to be provided over the research term; and (3) participation in the Joint Research Committee (the “Committee”) to be provided over the research term of the agreement.
Revenue				$ 2,800,000	5,300,000
Deferred revenue					1,100,000
Astellas Collaboration Agreement [Member] | ASC 606 [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction price				13,800,000
Astellas Collaboration Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development and sales milestone payments				398,500,000
Harvard Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License agreement termination period						2017-12
License agreement term						10 years
Harvard Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future payments for clinical and commercialization sale milestone						$ 2,400,000
CFF [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue				$ 0	0
Funds received for research, development and commercialization agreement		$ 5,700,000
Description of funds received in different periods				Under terms of the CFF Agreement, which was subsequently amended in May 2013 and January 2014, CFF 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.
Payment obligations				$ 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. CFF 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.
CFF [Member] | Occurs Prior to Selection [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock				268,265
CFF [Member] | Occurs After Selection [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock				444,025
CFF [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

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Provision for income taxes	$ 0	$ 0
U.S. corporate tax rate	21.00%	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	$ 252,600,000
State net operating loss carry-forwards	240,100,000
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
Research and development tax credit carry-forwards	$ 10,000,000
Tax credit carry-forwards begin to expire year	2,027
State [Member]
Income Tax Disclosure [Line Items]
Operating loss carry-forwards begin to expire year	2,030
Research and development tax credit carry-forwards	$ 3,600,000
Tax credit carry-forwards begin to expire year	2,026
Maximum [Member]
Income Tax Disclosure [Line Items]
U.S. corporate tax rate		35.00%

Income Taxes - Schedule of Effe

Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	(21.00%)	(34.00%)
State income taxes, net of federal benefit	(6.10%)	(5.60%)
Research and development tax credit carryforwards	(3.90%)	(3.20%)
Return to provision	(0.60%)
Nondeductible expenses	0.40%	0.70%
Change in deferred tax asset valuation allowance	31.20%	2.00%
Effect of U.S. Tax Reform		40.10%
Effective income tax rate	(0.00%)	(0.00%)

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 68,219	$ 52,861
Research and development tax credit carryforwards	12,799	9,232
Accrued expenses and other	2,066	1,583
Total deferred tax assets before valuation allowance	83,084	63,676
Valuation allowance	(82,965)	(63,660)	$ (62,442)
Net deferred tax assets	119	16
Depreciation	$ (119)	$ (16)

Income Taxes - Changes in Valua

Income Taxes - Changes in Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Valuation allowance at beginning of year	$ (63,660)	$ (62,442)
Increases recorded to income tax provision	(19,305)	(1,218)
Valuation allowance at end of year	$ (82,965)	$ (63,660)

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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ (16,900)	$ (18,428)	$ (15,478)	$ (11,026)	$ (13,424)	$ (13,954)	$ (16,958)	$ (15,096)	$ (61,832)	$ (59,432)
Denominator:
Weighted average common shares outstanding—basic and diluted	46,687,910	36,694,957	36,009,109	34,474,004	26,465,521	25,093,344	25,040,131	25,020,337	38,495,103	25,407,943
Net loss per share—basic and diluted	$ (0.36)	$ (0.50)	$ (0.43)	$ (0.32)	$ (0.51)	$ (0.56)	$ (0.68)	$ (0.60)	$ (1.61)	$ (2.34)

Net Loss per Share - Schedule_2

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Computation of Diluted Weighted-average Shares Outstanding (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding	3,322,101	2,838,558
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding	3,158,676	2,838,558
Bonus Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding	163,425

Leases - Additional Information

Leases - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)	Jan. 19, 2018	Dec. 31, 2017USD ($)	Sep. 30, 2017ft²
Leases [Abstract]
Operating lease, area of property leased | ft²				30,000
Operating lease expiration date	2018-05
Operating lease, initial term		10 years
Operating lease, option to extend additional term		7 years
Operating lease assets | $	$ 13,849		$ 472

Leases - Elements of Lease Expe

Leases - Elements of Lease Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Lease cost
Operating lease cost	$ 2,232	$ 1,143
Variable lease cost	366
Total lease cost	2,598	1,143
Other information
Operating cash flows used for operating leases	1,634	$ 1,344
Operating lease liabilities arising from obtaining right-of-use assets	$ 14,252
Weighted-average remaining lease term	9 years 4 months 2 days	4 months 2 days
Weighted-average discount rate	4.50%	4.06%

Leases - Schedule of Minimum Re

Leases - Schedule of Minimum Rental Commitments Under Lease (Detail) - Build-to-Suit Lease [Member] $ in Thousands	Dec. 31, 2018USD ($)
Lessee Lease Description [Line Items]
2,019	$ 1,686
2,020	1,733
2,021	1,780
2,022	1,829
2,023	1,880
Thereafter	8,757
Total lease payments	17,665
Less: imputed interest	(3,413)
Total	$ 14,252

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	May 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Contingencies And Commitments [Line Items]
Issuance of stock under consulting agreement, value		$ 889	$ 767
Common Stock [Member]
Contingencies And Commitments [Line Items]
Issuance of stock under consulting agreement, shares		239,490	188,462
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
Aggregate retainer fee payable	$ 2,500	$ 400
Percentage of M&A transaction fee payable	1.00%
Issuance of stock under consulting agreement, value		$ 900	$ 800
Letter Agreement with Dr. Stelios Papadopoulos [Member] | Common Stock [Member]
Contingencies And Commitments [Line Items]
Issuance of stock under consulting agreement, shares		239,490	188,462

Quarterly Financial Data (Una_3

Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenue		$ 1,055	$ 843	$ 942	$ 1,622	$ 1,551	$ 1,147	$ 1,021
Operating expenses:
Research and development	$ 13,717	15,591	12,604	8,400	12,282	12,894	15,370	13,108	$ 50,312	$ 53,654
General and administrative	3,780	4,150	3,957	3,823	2,847	2,741	2,902	3,170	15,710	11,660
Total operating expenses	17,497	19,741	16,561	12,223	15,129	15,635	18,272	16,278	66,022	65,314
Loss from operations	(17,497)	(18,686)	(15,718)	(11,281)	(13,507)	(14,084)	(17,125)	(15,257)	(63,182)	(59,973)
Interest income	342	171	194	165	126	155	169	191	872	641
Other income (expense), net	255	87	46	90	(43)	(25)	(2)	(30)	478	(100)
Net loss	$ (16,900)	$ (18,428)	$ (15,478)	$ (11,026)	$ (13,424)	$ (13,954)	$ (16,958)	$ (15,096)	$ (61,832)	$ (59,432)
Net loss per share—basic and diluted	$ (0.36)	$ (0.50)	$ (0.43)	$ (0.32)	$ (0.51)	$ (0.56)	$ (0.68)	$ (0.60)	$ (1.61)	$ (2.34)
Weighted average common shares outstanding—basic and diluted	46,687,910	36,694,957	36,009,109	34,474,004	26,465,521	25,093,344	25,040,131	25,020,337	38,495,103	25,407,943