Kineta (KANT) 10-Q 8 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	May 03, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	PTI
Entity Registrant Name	PROTEOSTASIS THERAPEUTICS, INC.
Entity Central Index Key	0001445283
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		51,099,307
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true

Condensed Balance Sheets

Condensed Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 31,584	$ 28,810
Short-term investments	73,747	89,569
Prepaids and other current assets	3,134	2,481
Total current assets	108,465	120,860
Operating lease, right-of-use asset	13,549	13,849
Property and equipment, net	563	605
Other assets	88
Restricted cash	828	828
Total assets	123,493	136,142
Current liabilities:
Accounts payable	2,004	1,884
Accrued expenses	6,519	5,661
Operating lease liabilities	1,080	1,056
Total current liabilities	9,603	8,601
Derivative liability	3	3
Operating lease liabilities, net of current portion	12,920	13,196
Total liabilities	22,526	21,800
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized; no shares issued and outstanding as of March 31, 2019 and December 31, 2018
Common stock, $0.001 par value; 125,000,000 shares authorized; 51,043,158 and 50,808,422 shares issued and outstanding as of March 31, 2019 and December 31, 2018, respectively	51	51
Additional paid-in capital	392,851	391,825
Accumulated other comprehensive income	18	1
Accumulated deficit	(291,953)	(277,535)
Total stockholders’ equity	100,967	114,342
Total liabilities and stockholders’ equity	$ 123,493	$ 136,142

Condensed Balance Sheets (Paren

Condensed Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
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	51,043,158	50,808,422
Common stock, shares outstanding	51,043,158	50,808,422

Condensed Statements of Operati

Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Revenue	$ 5,000	$ 942
Operating expenses:
Research and development	16,148	8,400
General and administrative	3,943	3,823
Total operating expenses	20,091	12,223
Loss from operations	(15,091)	(11,281)
Interest income	357	165
Other income, net	316	90
Net loss	$ (14,418)	$ (11,026)
Net loss per share—basic and diluted	$ (0.28)	$ (0.32)
Weighted average common shares outstanding—basic and diluted	50,976,907	34,474,004
Other comprehensive income (loss):
Unrealized gain (loss) on investments	$ 17	$ (26)
Comprehensive loss	$ (14,401)	$ (11,052)

Statements of Stockholders' Equ

Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2017	$ 68,808	$ 35	$ 285,583	$ (2)	$ (216,808)
Beginning balance, shares at Dec. 31, 2017		34,416,088
Exercise of stock options	21		21
Exercise of stock options, shares		8,775
Stock-based compensation expense	914		914
Issuance of common stock for payment of consulting services	25		25
Issuance of common stock for payment of consulting services, shares		51,597
Issuance of common stock pursuant to employee stock purchase plan	308		308
Issuance of common stock pursuant to employee stock purchase plan, shares		6,114
Impact of adoption of ASC 606 | ASC 606 [Member]	1,105				1,105
Other comprehensive income (loss)	(26)			(26)
Net loss	(11,026)				(11,026)
Ending balance at Mar. 31, 2018	60,129	$ 35	286,851	(28)	(226,729)
Ending balance, shares at Mar. 31, 2018		34,482,574
Beginning balance at Dec. 31, 2018	$ 114,342	$ 51	391,825	1	(277,535)
Beginning balance, shares at Dec. 31, 2018	50,808,422	50,808,422
Exercise of stock options	$ 3		3
Exercise of stock options, shares	2,543	2,543
Stock-based compensation expense	$ 801		801
Issuance of common stock for payment of consulting services	169		169
Issuance of common stock for payment of consulting services, shares		22,615
Issuance of common stock pursuant to employee stock purchase plan	$ 53		53
Issuance of common stock pursuant to employee stock purchase plan, shares	2,543	46,153
Vesting of restricted stock units, shares		163,425
Other comprehensive income (loss)	$ 17			17
Net loss	(14,418)				(14,418)
Ending balance at Mar. 31, 2019	$ 100,967	$ 51	$ 392,851	$ 18	$ (291,953)
Ending balance, shares at Mar. 31, 2019	51,043,158	51,043,158

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (14,418)	$ (11,026)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	351	563
Accretion of short-term investments	(317)	(55)
Stock-based compensation expense	801	914
Stock issued for consulting services	169	308
Change in fair value of derivative liability		(12)
Changes in operating assets and liabilities:
Prepaids and other current assets	(653)	(605)
Other assets	(88)	9
Accounts payable	120	(876)
Accrued expenses	858	(1,058)
Deferred revenue		(3)
Operating lease liabilities	(252)	(193)
Net cash used in operating activities	(13,429)	(12,034)
Cash flows from investing activities:
Purchases of short-term investments	(20,844)	(7,957)
Proceeds received from maturities of short-term investments	37,000	8,850
Purchases of property and equipment	(9)
Net cash provided by investing activities	16,147	893
Cash flows from financing activities:
Proceeds from exercise of stock options	3	21
Net cash provided by financing activities	56	46
Net increase (decrease) in cash, cash equivalents and restricted cash	2,774	(11,095)
Cash, cash equivalents and restricted cash at beginning of period	29,638	31,674
Cash, cash equivalents and restricted cash at end of period	32,412	20,579
Supplemental disclosure of non-cash investing and financing activities:
Deferred offering costs included in accrued expenses		160
Addition of Operating Lease Right-Of-Use Asset [Member]
Supplemental disclosure of non-cash investing and financing activities:
Addition of operating lease, right-of-use asset		15,304
Employee Stock Purchase Plan [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 53	$ 25

Nature of the Business

Nature of the Business	3 Months Ended
	Mar. 31, 2019
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 PTI-428, a CFTR amplifier. 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. The Company is developing and, if approved, intends to commercialize their own 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 . The Company has incurred losses and negative cash flows from operations since its inception. As of March 31, 2019, the Company had an accumulated deficit of $292.0 million. During the three months ended March 31, 2019, the Company incurred losses of $14.4 million and used $13.4 million of cash in operations. $105.3 million will be sufficient to fund its operating expenses and capital requirements, based upon its current operating plan, for at least 12 months from the date that these financial statements are issued. 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, in an aggregate offering amount of up to $50.0 million. As of March 31, 2019, 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. As of March 31, 2019, $27.6 million of common stock remained available for sale under the ATM. On March 28, 2019 the Company delivered a notice of termination of the ATM offering program to Leerink, effective as of April 7, 2019. In the fourth quarter of 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 $80.1 million, after deducting underwriting discounts and commissions and other offering expenses.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Unaudited Interim Financial Information The condensed balance sheet as of December 31, 2018 was derived from audited financial statements but does not include all disclosures required by accounting principles generally accepted in the United States of America (“GAAP”). The accompanying condensed financial statements as of March 31, 2019 and for the three months ended March 31, 2019 are unaudited and have been prepared by the Company, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. The Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited condensed financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto for the year ended December 31, 2018 included in the Company’s Annual Report on Form 10-K filed with the SEC on March 8, 2019. In the opinion of management, all adjustments, consisting only of normal recurring adjustments as necessary, for the fair statement of the Company’s financial position as of March 31, 2019, results of its operations for the three months ended March 31, 2019, and cash flows for the three months ended March 31, 2019 have been made. The results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results of operations to be expected for the year ending December 31, 2019. Summary of Significant Accounting Policies The Company’s significant accounting policies, which are disclosed in the audited consolidated financial statements for the year ended December 31, 2018 and the notes thereto are included in the Company’s Annual Report on Form 10-K that was filed with the SEC on March 8, 2019. 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 and the valuation of common stock, and 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. Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers . 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 records any amounts received prior to satisfying the revenue recognition criteria as deferred revenue. Amounts recognized as revenue, but not yet received or invoiced are recorded as current assets. 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 standalone 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 this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. 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 8 - Significant Agreements Recently Issued and Adopted Accounting Pronouncements ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting In June 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting Recently Issued Accounting Pronouncements 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	3 Months Ended
	Mar. 31, 2019
Cash Cash Equivalents And Short Term Investments [Abstract]
Short-Term Investments	3. Short-Term Investments The following table summarizes the Company’s short-term investments as of March 31, 2019 and December 31, 2018 (in thousands): March 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S government-sponsored enterprise securities $ 44,311 $ 14 $ — $ 44,325 U.S. treasury securities 29,418 4 — 29,422 $ 73,729 $ 18 $ — $ 73,747 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 The Company did not have any realized gains or losses on its short-term investments for the three months ended March 31, 2019 and 2018 2018

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	3 Months Ended
	Mar. 31, 2019
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 March 31, 2019 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 30,758 $ — $ — $ 30,758 Short-term investments: U.S. government-sponsored enterprise securities — 44,325 — 44,325 U.S. treasury securities — 29,422 — 29,422 $ 30,758 $ 73,747 $ — $ 104,505 Liabilities: Derivative liability $ — $ — $ 3 $ 3 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 During the periods ended March 31, 2019 and December 31, 2018, there were no transfers between Level 1, Level 2 and Level 3. The derivative liability relates to a cash settlement option associated with the change of control provision in the Company’s Cystic Fibrosis Foundation, Inc. (“CFF”) agreement, which meets the definition of a derivative. The fair value of the derivative liability is based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of the derivative instrument was determined using the Monte-Carlo simulation analysis. In determining the fair value of the derivative liability, the inputs impacting fair value include the fair value of the Company’s common stock, expected term of the derivative instrument, expected volatility of the common stock price, risk-free interest rate, expected sales-based milestone payments, discount rate, probability of a change of control event, and the probability that the counterparty would elect to accept the alternative cash payment in lieu of its right to the future sales-based milestone payments. The fair value of the derivative liability was not material at March 31, 2019 and December 31, 2018.

Prepaids and Other Current Asse

Prepaids and Other Current Assets	3 Months Ended
	Mar. 31, 2019
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): March 31, December 31, 2019 2018 Prepaid clinical, manufacturing and scientific expenses $ 1,430 $ 1,660 Prepaid insurance expenses 929 108 Other prepaid expenses and other current assets 775 713 $ 3,134 $ 2,481

Accrued Expenses

Accrued Expenses	3 Months Ended
	Mar. 31, 2019
Payables And Accruals [Abstract]
Accrued Expenses	6. Accrued Expenses Accrued expenses consisted of the following (in thousands): March 31, December 31, 2019 2018 Accrued payroll and related expenses $ 1,137 $ 2,746 Accrued research and development expenses 4,677 1,965 Accrued professional fees 654 799 Accrued other 51 151 $ 6,519 $ 5,661

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	7. Stock-Based Compensation 2016 Stock Option and Incentive Plan On February 3, 2016, the Company’s stockholders approved the 2016 Stock Option and Incentive Plan (the “2016 Plan”), which became effective on February 9, 2016. The 2016 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards and other stock-based awards. The number of shares initially reserved for issuance under the 2016 Plan was 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 March 31, 2019, the total number of shares reserved under the 2016 Plan and 2008 Plan was 5,431,027 and the Company had 866,600 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 connection with the completion of the Company’s initial public offering. A total of 138,757 shares of common stock were initially 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 lesser 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 or (iii) an amount determined by the Company’s board of directors or the compensation committee of the board of directors. During the three months ended March 31, 2019, 46,153 shares of common stock were issued pursuant to the 2016 ESPP. As of March 31, 2019, the total number of shares reserved under the 2016 ESPP was 479,143 shares. The Company recognized less than $0.1 million of stock-based compensation during the three months ended March 31, 2019 related to the 2016 ESPP. 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 grants did not meet the criteria for liability classification as there was a fixed number of shares to be issued and there is no variability in the number of shares which had been granted. The requisite service period for the awards was from February 1, 2018 to February 1, 2019 (the vesting period). The Company recognized employee stock-based compensation expense for the bonus RSU grants on a straight-line basis over the vesting period of the awards. On February 1, 2019, the outstanding RSUs fully vested and were converted to 163,425 shares of common stock. As of March 31, 2019, there were no RSUs outstanding and less than $0.1 million of expense was recognized for the three months ended March 31, 2019. As of March 31, 2018, there were no RSUs vested and $ .1 Number of Shares Weighted Average Grant Date Fair Value Unvested balance at December 31, 2018 163,425 $ 3.20 Granted — — Vested (163,425 ) 3.20 Forfeited — — Unvested balance at March 31, 2019 — $ — Stock-Based Compensation Stock-based compensation expense, including shares issued to a nonemployee for consulting services, was classified in the statements of operations as follows (in thousands): For the Three Months Ended March 31, 2019 2018 Research and development $ 370 $ 363 General and administrative 600 859 $ 970 $ 1,222 The following table summarizes the Company’s stock option activity for the three months ended March 31, 2019 (in thousands except share and per share amounts): Number of Shares Weighted Average Exercise Price Per Share Outstanding at December 31, 2018 3,158,676 $ 5.93 Granted 1,426,500 4.23 Exercised (2,543 ) 1.09 Forfeited (18,206 ) 10.77 Outstanding at March 31, 2019 4,564,427 $ 5.38 Exercisable at March 31, 2019 1,974,359 $ 6.24 Vested and expected to vest at March 31, 2019 4,481,177 $ 5.44 The grant date fair value of options granted during the period was $4.3 million, or $3.03 per share on a weighted-average basis and will be recognized as compensation expense over the requisite service period of four years. There were 2,543 shares of common stock issued upon the exercise of outstanding stock options for the three months ended March 31, 2019, resulting in total proceeds of less than $0.1 million. In accordance with Company policy, the shares were issued from the shares reserved for issuance under the stock plans described above. The intrinsic value of options exercised for the three months ended March 31, 2019 was less than $0.1 million. As of March 31, 2019, there was $7.8 million of unrecognized compensation cost related to employee and nonemployee unvested stock options granted under the 2016 Plan, which is expected to be recognized over a weighted-average remaining service period of 2.76 years. Stock compensation costs have not been capitalized by the Company. 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 March 31, 2019, options for the purchase of 83,250 shares held by nonemployees remained unvested, pending achievement of the specified milestones.

Significant Agreements

Significant Agreements	3 Months Ended
	Mar. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Significant Agreements	8. 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 March 31, 2019, 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 March 31, 2019. Through March 31, 2019, 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. None of the variable consideration payable has been included in the transaction price through March 31, 2019. 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. As all performance obligations have been satisfied the Company will recognize royalty revenue at the date the sales 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 for the three months ended March 31, 2019. 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. In February 2019, the Company completed the technology and materials transfer and received the upfront payment of $5.0 million. The Company recognized revenue of $5.0 million as of March 31, 2019 as the technology and materials transfer has been completed. As of March 31, 2019, 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 satisfied. Astellas In November 2014, the Company entered into the Collaborative Research, Development, Commercialization and License Agreement (the “Astellas Agreement”) with Astellas Pharma Inc. (“Astellas”). The focus of the Astellas Agreement was to identify, develop and commercialize therapeutic candidates relating to the Unfolded Protein Response (“UPR”) pathway. 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 approximately 3½ 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 . As of December 31, 2018, the Astellas Agreement was complete and all of the Company’s performance obligations under the contract have been satisfied. 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. 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 $0.9 million of revenue related to the Astellas agreement for the three months ended March 31, 2018.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes The Company did not record a federal or state income tax benefit for its losses for the three months ended March 31, 2019 and 2018 due to the conclusion that a full valuation allowance is required against the Company’s deferred tax assets. All of the Company’s losses before income taxes were generated in the United States.

Net Loss per Share

Net Loss per Share	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Net Loss per Share	10. Net Loss per Share Basic and diluted net loss per share was calculated as follows (in thousands, except share and per share amounts): For the Three Months Ended March 31, 2019 2018 Numerator: Net loss $ (14,418 ) $ (11,026 ) Denominator: Weighted average number of common shares outstanding—basic and diluted 50,976,907 34,474,004 Net loss per share—basic and diluted $ (0.28 ) $ (0.32 ) The following common stock equivalents have been excluded from the computation of diluted weighted-average shares outstanding, because such securities had an antidilutive impact : March 31, 2019 2018 Options to purchase common stock 4,564,427 3,548,271 Restricted stock units — 197,265 4,564,427 3,745,536

Leases

Leases	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
Leases	11. Leases The Company has an operating lease for office and laboratory space for its corporate headquarters in Boston, Massachusetts. The 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 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 lease in Boston is considered a net lease, as the 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. The Company’s prior lease in Cambridge, Massachusetts expired in May 2018. For the three months ended March 31, 2019 and 2018, assets under operating lease were $13.5 million and $15.3 million, respectively. The elements of lease expense were as follows (in thousands): For the Three Months Ended March 31, 2019 2018 Lease cost Operating lease cost $ 462 $ 660 Variable lease cost (1) 137 146 Total lease cost $ 599 $ 806 Other information Operating cash flows used for operating leases $ 414 $ 338 Operating lease liabilities arising from obtaining right-of-use assets $ 14,000 $ 15,670 Weighted-average remaining lease term 9 years 10 years Weighted-average discount rate 4.50 % 4.50 % (1) The variable lease costs for the three months ended March 31, 2019 and 2018 Future lease payments under no ncancelable leases as of March 31, 2019 (in thousands): Future Operating Lease Payments 2019 $ 1,272 2020 1,733 2021 1,780 2022 1,829 2023 1,880 Thereafter 8,757 Total lease payments 17,251 Less: imputed interest (3,251 ) Total operating lease liabilities $ 14,000

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Unaudited Interim Financial Information	Unaudited Interim Financial Information The condensed balance sheet as of December 31, 2018 was derived from audited financial statements but does not include all disclosures required by accounting principles generally accepted in the United States of America (“GAAP”). The accompanying condensed financial statements as of March 31, 2019 and for the three months ended March 31, 2019 are unaudited and have been prepared by the Company, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. The Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited condensed financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto for the year ended December 31, 2018 included in the Company’s Annual Report on Form 10-K filed with the SEC on March 8, 2019. In the opinion of management, all adjustments, consisting only of normal recurring adjustments as necessary, for the fair statement of the Company’s financial position as of March 31, 2019, results of its operations for the three months ended March 31, 2019, and cash flows for the three months ended March 31, 2019 have been made. The results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results of operations to be expected for the year ending December 31, 2019.
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies The Company’s significant accounting policies, which are disclosed in the audited consolidated financial statements for the year ended December 31, 2018 and the notes thereto are included in the Company’s Annual Report on Form 10-K that was filed with the SEC on March 8, 2019.
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 and the valuation of common stock, and 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.
Revenue Recognition	Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers . 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 records any amounts received prior to satisfying the revenue recognition criteria as deferred revenue. Amounts recognized as revenue, but not yet received or invoiced are recorded as current assets. 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 standalone 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 this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. 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 8 - Significant Agreements
Recently Issued and Adopted Accounting Pronouncements	Recently Issued and Adopted Accounting Pronouncements ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting In June 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting Recently Issued Accounting Pronouncements 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
Derivative Liability	The derivative liability relates to a cash settlement option associated with the change of control provision in the Company’s Cystic Fibrosis Foundation, Inc. (“CFF”) agreement, which meets the definition of a derivative. The fair value of the derivative liability is based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of the derivative instrument was determined using the Monte-Carlo simulation analysis. In determining the fair value of the derivative liability, the inputs impacting fair value include the fair value of the Company’s common stock, expected term of the derivative instrument, expected volatility of the common stock price, risk-free interest rate, expected sales-based milestone payments, discount rate, probability of a change of control event, and the probability that the counterparty would elect to accept the alternative cash payment in lieu of its right to the future sales-based milestone payments. The fair value of the derivative liability was not material at March 31, 2019 and December 31, 2018.

Short-Term Investments (Tables)

Short-Term Investments (Tables)	3 Months Ended
	Mar. 31, 2019
Cash Cash Equivalents And Short Term Investments [Abstract]
Summary of Short-Term Investments	The following table summarizes the Company’s short-term investments as of March 31, 2019 and December 31, 2018 (in thousands): March 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S government-sponsored enterprise securities $ 44,311 $ 14 $ — $ 44,325 U.S. treasury securities 29,418 4 — 29,422 $ 73,729 $ 18 $ — $ 73,747 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

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	3 Months Ended
	Mar. 31, 2019
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 March 31, 2019 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 30,758 $ — $ — $ 30,758 Short-term investments: U.S. government-sponsored enterprise securities — 44,325 — 44,325 U.S. treasury securities — 29,422 — 29,422 $ 30,758 $ 73,747 $ — $ 104,505 Liabilities: Derivative liability $ — $ — $ 3 $ 3 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

Prepaids and Other Current As_2

Prepaids and Other Current Assets (Tables)	3 Months Ended
	Mar. 31, 2019
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): March 31, December 31, 2019 2018 Prepaid clinical, manufacturing and scientific expenses $ 1,430 $ 1,660 Prepaid insurance expenses 929 108 Other prepaid expenses and other current assets 775 713 $ 3,134 $ 2,481

Accrued Expenses (Tables)

Accrued Expenses (Tables)	3 Months Ended
	Mar. 31, 2019
Payables And Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following (in thousands): March 31, December 31, 2019 2018 Accrued payroll and related expenses $ 1,137 $ 2,746 Accrued research and development expenses 4,677 1,965 Accrued professional fees 654 799 Accrued other 51 151 $ 6,519 $ 5,661

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Restricted Stock Unit Activity and Related Information	Number of Shares Weighted Average Grant Date Fair Value Unvested balance at December 31, 2018 163,425 $ 3.20 Granted — — Vested (163,425 ) 3.20 Forfeited — — Unvested balance at March 31, 2019 — $ —
Summary of Stock-Based Compensation Expense, Including Shares Issued to Nonemployee for Consulting Services	Stock-based compensation expense, including shares issued to a nonemployee for consulting services, was classified in the statements of operations as follows (in thousands): For the Three Months Ended March 31, 2019 2018 Research and development $ 370 $ 363 General and administrative 600 859 $ 970 $ 1,222
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity for the three months ended March 31, 2019 (in thousands except share and per share amounts): Number of Shares Weighted Average Exercise Price Per Share Outstanding at December 31, 2018 3,158,676 $ 5.93 Granted 1,426,500 4.23 Exercised (2,543 ) 1.09 Forfeited (18,206 ) 10.77 Outstanding at March 31, 2019 4,564,427 $ 5.38 Exercisable at March 31, 2019 1,974,359 $ 6.24 Vested and expected to vest at March 31, 2019 4,481,177 $ 5.44

Net Loss per Share (Tables)

Net Loss per Share (Tables)	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Net Loss per Share	Basic and diluted net loss per share was calculated as follows (in thousands, except share and per share amounts): For the Three Months Ended March 31, 2019 2018 Numerator: Net loss $ (14,418 ) $ (11,026 ) Denominator: Weighted average number of common shares outstanding—basic and diluted 50,976,907 34,474,004 Net loss per share—basic and diluted $ (0.28 ) $ (0.32 )
Schedule of Antidilutive Securities Excluded from Computation of Diluted Weighted-average Shares Outstanding	The following common stock equivalents have been excluded from the computation of diluted weighted-average shares outstanding, because such securities had an antidilutive impact : March 31, 2019 2018 Options to purchase common stock 4,564,427 3,548,271 Restricted stock units — 197,265 4,564,427 3,745,536

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2019
Lessee Disclosure [Abstract]
Elements of Lease Expense	The elements of lease expense were as follows (in thousands): For the Three Months Ended March 31, 2019 2018 Lease cost Operating lease cost $ 462 $ 660 Variable lease cost (1) 137 146 Total lease cost $ 599 $ 806 Other information Operating cash flows used for operating leases $ 414 $ 338 Operating lease liabilities arising from obtaining right-of-use assets $ 14,000 $ 15,670 Weighted-average remaining lease term 9 years 10 years Weighted-average discount rate 4.50 % 4.50 % (1) The variable lease costs for the three months ended March 31, 2019 and 2018
Schedule of Future Lease Payments under Noncancelable Leases	Future lease payments under no ncancelable leases as of March 31, 2019 (in thousands): Future Operating Lease Payments 2019 $ 1,272 2020 1,733 2021 1,780 2022 1,829 2023 1,880 Thereafter 8,757 Total lease payments 17,251 Less: imputed interest (3,251 ) Total operating lease liabilities $ 14,000

Nature of the Business - Additi

Nature of the Business - Additional Information (Detail) - USD ($)	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2018
Nature of Business [Line Items]
Accumulated deficit	$ (291,953,000)		$ (291,953,000)	$ (277,535,000)		$ (277,535,000)
Cash, cash equivalents and short-term investments	105,300,000		105,300,000
Net loss			(14,418,000)		$ (11,026,000)
Cash in operations			$ 13,429,000		$ 12,034,000
Follow-on Public Offering [Member]
Nature of Business [Line Items]
Number of shares issued and sold				12,650,000		1,650,000
Common stock price per share				$ 6.75		$ 6.75
Net proceeds from sales of common stock				$ 80,100,000
ATM Program [Member] | Leerink Partners LLC [Member]
Nature of Business [Line Items]
Number of shares issued and sold			3,475,166
Net proceeds from sales of common stock	21,600,000
Common stock aggregate offering price		$ 50,000,000
Common stock remained available for sale, value	$ 27,600,000		$ 27,600,000

Short-Term Investments - Summar

Short-Term Investments - Summary of Short-Term Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 73,729	$ 89,568
Gross Unrealized Gains	18	5
Gross Unrealized Losses		(4)
Fair Value	73,747	89,569
U.S Government-Sponsored Enterprise Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	44,311	41,478
Gross Unrealized Gains	14	2
Gross Unrealized Losses		(3)
Fair Value	44,325	41,477
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,418	48,090
Gross Unrealized Gains	4	3
Gross Unrealized Losses		(1)
Fair Value	$ 29,422	$ 48,092

Short-Term Investments - Additi

Short-Term Investments - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
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	Mar. 31, 2019	Dec. 31, 2018
Liabilities:
Derivative liability	$ 3	$ 3
Fair Value, Measurements, Recurring [Member]
Assets:
Total assets	104,505	116,875
Liabilities:
Derivative liability	3	3
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	30,758	26,806
Fair Value, Measurements, Recurring [Member] | U.S Government-Sponsored Enterprise Securities [Member]
Assets:
Short-term investments	44,325	41,477
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Assets:
Cash equivalents		500
Short-term investments	29,422	48,092
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Total assets	30,758	26,806
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	30,758	26,806
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Total assets	73,747	90,069
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S Government-Sponsored Enterprise Securities [Member]
Assets:
Short-term investments	44,325	41,477
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury Securities [Member]
Assets:
Cash equivalents		500
Short-term investments	29,422	48,092
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Liabilities:
Derivative liability	$ 3	$ 3

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
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	Mar. 31, 2019	Dec. 31, 2018
Prepaid Expense And Other Assets Current [Abstract]
Prepaid clinical, manufacturing and scientific expenses	$ 1,430	$ 1,660
Prepaid insurance expenses	929	108
Other prepaid expenses and other current assets	775	713
Prepaids and other current assets	$ 3,134	$ 2,481

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued payroll and related expenses	$ 1,137	$ 2,746
Accrued research and development expenses	4,677	1,965
Accrued professional fees	654	799
Accrued other	51	151
Total accrued expenses	$ 6,519	$ 5,661

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	Feb. 01, 2019	Feb. 01, 2018	Feb. 03, 2016	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued				2,543
Stock-based compensation				$ 970,000	$ 1,222,000
Stock option grants				1,426,500
Conversion of stock, shares converted	163,425
RSUs outstanding				4,564,427		3,158,676
Grant date fair value of options				$ 4,300,000
Weighted average grant date fair value				$ 3.03
Requisite service period				4 years
Issuance of common stock pursuant to employee stock purchase plan, shares				2,543
Bonus Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation					$ 100,000
RSUs vested				163,425	0
RSUs outstanding				0
Nonemployees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option grants				0
Number of options unvested				83,250
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]
Proceeds from issuance of common stock				$ 100,000
Intrinsic value of options exercised				100,000
Maximum [Member] | Bonus Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation				$ 100,000
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	866,600
Common stock reserved for issuance, percentage of number of shares of common stock outstanding			3.00%
Unrecognized compensation cost related to the unvested stock-based awards				$ 7,800,000
Compensation cost not yet recognized, period for recognition				2 years 9 months 3 days
2016 Plan and 2008 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance				5,431,027
2016 ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance			138,757				479,143
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				46,153
Issuance of common stock pursuant to employee stock purchase plan, shares				46,153
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] - $ / shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Number of Shares
Unvested balance at December 31, 2018	163,425
Vested	(163,425)	0
Weighted Average Grant Date Fair Value
Unvested balance at December 31, 2018	$ 3.20
Vested	$ 3.20

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock-Based Compensation Expense, Including Shares Issued to Nonemployee for Consulting Services (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 970	$ 1,222
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	370	363
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 600	$ 859

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Stock Option Activity (Detail)	3 Months Ended
	Mar. 31, 2019$ / sharesshares
Number of Shares
Outstanding at December 31, 2018 | shares	3,158,676
Granted | shares	1,426,500
Exercised | shares	(2,543)
Forfeited | shares	(18,206)
Outstanding at March 31, 2019 | shares	4,564,427
Exercisable at March 31, 2019 | shares	1,974,359
Vested and expected to vest at March 31, 2019 | shares	4,481,177
Weighted Average Exercise Price Per Share
Outstanding at December 31, 2018 | $ / shares	$ 5.93
Granted | $ / shares	4.23
Exercised | $ / shares	1.09
Forfeited | $ / shares	10.77
Outstanding at March 31, 2019 | $ / shares	5.38
Exercisable at March 31, 2019 | $ / shares	6.24
Vested and expected to vest at March 31, 2019 | $ / shares	$ 5.44

Significant Agreements - Additi

Significant Agreements - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended
	Feb. 28, 2019	Nov. 30, 2014	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2014
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue			$ 5,000	$ 942
Genentech [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Nonrefundable cash payment			5,000		$ 5,000
Future milestone payment					$ 96,000
Revenue			$ 5,000
Upfront payment received	$ 5,000
Astellas Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue				$ 900
Convertible promissory notes						$ 5,000
Initial research term		3 years 6 months
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.
Astellas Collaboration Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development and sales milestone payments		$ 398,500

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Federal income tax benefit	$ 0	$ 0
State income tax benefit	$ 0	$ 0

Net Loss per Share - Schedule o

Net Loss per Share - Schedule of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net loss	$ (14,418)	$ (11,026)
Denominator:
Weighted average common shares outstanding—basic and diluted	50,976,907	34,474,004
Net loss per share—basic and diluted	$ (0.28)	$ (0.32)

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	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding	4,564,427	3,745,536
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding	4,564,427	3,548,271
Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding		197,265

Leases - Additional Information

Leases - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Jan. 19, 2018
Leases [Abstract]
Operating lease expiration date	2018-05
Operating lease, initial term				10 years
Operating lease, option to extend additional term				7 years
Operating lease assets	$ 13,549	$ 13,849	$ 15,300

Leases - Elements of Lease Expe

Leases - Elements of Lease Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Lease cost
Operating lease cost	$ 462	$ 660
Variable lease cost	137	146
Total lease cost	599	806
Other information
Operating cash flows used for operating leases	414	338
Operating lease liabilities arising from obtaining right-of-use assets	$ 14,000	$ 15,670
Weighted-average remaining lease term	9 years	10 years
Weighted-average discount rate	4.50%	4.50%

Leases - Schedule of Future Lea

Leases - Schedule of Future Lease Payments under Noncancelable Leases (Detail) $ in Thousands	Mar. 31, 2019USD ($)
Leases [Abstract]
2019	$ 1,272
2020	1,733
2021	1,780
2022	1,829
2023	1,880
Thereafter	8,757
Total lease payments	17,251
Less: imputed interest	(3,251)
Total operating lease liabilities	$ 14,000