Kineta (KANT) 10-Q 6 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 02, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
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 Common Stock, Shares Outstanding		49,158,422
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true

Condensed Balance Sheets

Condensed Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 19,712	$ 29,724
Short-term investments	26,692	44,738
Restricted cash		294
Prepaids and other current assets	1,930	1,377
Total current assets	48,334	76,133
Operating lease, right-of-use asset	14,147	472
Property and equipment, net	648	424
Other assets	6	33
Restricted cash, net of current portion	828	1,656
Total assets	63,963	78,718
Current liabilities:
Accounts payable	2,145	2,098
Accrued expenses	8,406	6,120
Deferred revenue		1,108
Operating lease liabilities	1,033	559
Total current liabilities	11,584	9,885
Derivative liability	3	25
Operating lease liabilities, net of current portion	13,469
Total liabilities	25,056	9,910
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized as of September 30, 2018 and December 31, 2017; no shares issued and outstanding as of September 30, 2018 and December 31, 2017
Common stock, $0.001 par value; 125,000,000 shares authorized as of September 30, 2018 and December 31, 2017; 36,696,948 and 34,416,088 shares issued and outstanding as of September 30, 2018 and December 31, 2017, respectively	37	35
Additional paid-in capital	299,512	285,583
Accumulated other comprehensive loss	(7)	(2)
Accumulated deficit	(260,635)	(216,808)
Total stockholders’ equity	38,907	68,808
Total liabilities and stockholders’ equity	$ 63,963	$ 78,718

Condensed Balance Sheets (Paren

Condensed Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 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	36,696,948	34,416,088
Common stock, shares outstanding	36,696,948	34,416,088

Condensed Statements of Operati

Condensed Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Revenue	$ 1,055	$ 1,551	$ 2,840	$ 3,719
Operating expenses:
Research and development	15,591	12,894	36,595	41,372
General and administrative	4,150	2,741	11,931	8,813
Total operating expenses	19,741	15,635	48,526	50,185
Loss from operations	(18,686)	(14,084)	(45,686)	(46,466)
Interest income	171	155	530	515
Other income (expense), net	87	(25)	224	(57)
Net loss	$ (18,428)	$ (13,954)	$ (44,932)	$ (46,008)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.50)	$ (0.56)	$ (1.26)	$ (1.84)
Weighted average common shares outstanding—basic and diluted	36,694,957	25,093,344	35,734,159	25,051,536

Condensed Statements of Compreh

Condensed Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (18,428)	$ (13,954)	$ (44,932)	$ (46,008)
Other comprehensive loss:
Unrealized gain (loss) on investments	5	20	(5)	15
Comprehensive loss	$ (18,423)	$ (13,934)	$ (44,937)	$ (45,993)

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net loss	$ (44,932)	$ (46,008)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	1,428	1,001
Premium (discount) on short-term investments	(205)	42
Stock-based compensation expense	2,697	2,228
Stock issued for consulting services	704	555
Change in fair value of derivative liability	(22)	(88)
Loss on disposal of property and equipment	41
Changes in operating assets and liabilities:
Accounts receivable		(411)
Prepaids and other current assets	(553)	2,221
Other assets	27	27
Accounts payable	47	514
Accrued expenses	2,286	1,527
Deferred revenue	(3)	(1,202)
Operating lease liabilities	(1,019)	(963)
Net cash used in operating activities	(39,504)	(40,557)
Cash flows from investing activities:
Purchases of short-term investments	(30,604)	(23,727)
Proceeds received from maturities of short-term investments	48,850	65,563
Purchases of property and equipment	(406)	(117)
Net cash provided by investing activities	17,840	41,719
Cash flows from financing activities:
Proceeds from exercise of stock options	21	30
Deferred offering costs	(62)
Net cash provided by financing activities	10,530	73
Net increase (decrease) in cash, cash equivalents and restricted cash	(11,134)	1,235
Cash, cash equivalents and restricted cash at beginning of period	31,674	18,907
Cash, cash equivalents and restricted cash at end of period	20,540	20,142
At-the-market Common Stock Offering Program Sales Agreement [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock	10,546
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		12
Employee Stock Purchase Plan [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 25	$ 43

Nature of the Business

Nature of the Business	9 Months Ended
	Sep. 30, 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 developing small molecule therapeutics to treat cystic fibrosis (“CF”) and other diseases caused by dysfunctional protein processing. The Company focuses on identifying therapies that restore protein function. CF is a disease caused by defects in the cystic fibrosis transmembrane conductance regulator (“CFTR”) protein and insufficient CFTR protein function. The Company’s lead product candidates, PTI-428, an amplifier, PTI-801, a third-generation corrector, and PTI-808, a potentiator, as well as a dual combination consisting of PTI-801 and PTI-808, and a triple combination consisting of PTI-428, PT-801, and PTI-808 are in clinical development. The Company’s other drug candidates are in the preclinical development and discovery phases. 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 October 2018, the Company completed a follow-on public offering whereby the Company sold 11,000,000 shares of common stock at a price to the public of $6.75 per share. The aggregate net proceeds received by the Company from the offering were approximately $69.5 million, after deducting underwriting discounts and commissions and estimated offering expenses payable by the Company. On November 6, 2018, the Company issued and sold an additional 1,650,000 shares of its common stock at the offering price of $6.75 per share as a result of the full exercise by the underwriters of their option to purchase additional shares. The net proceeds from this issuance were $10.5 million, after deducting underwriting discounts and commissions. 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, having an aggregate offering price 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. In October 2018, the Company sold 1,364,348 shares of its common stock for net proceeds of approximately $11.2 million under the ATM program. As of October 19, 2018, the Company had sold an aggregate of 3,475,166 shares of its common stock for total net proceeds of approximately $21.7 million under the ATM program and $27.6 million of common stock remained available for sale. In connection with its follow-on offering in October 2018, the Company has suspended sales under the ATM program. In accordance with ASC 205-40, Going Concern . As of September 30, 2018, the Company had an accumulated deficit of $260.6 million. During the nine months ended September 30, 2018, the Company incurred losses of $44.9 million and used $39.5 million of cash in operations. $46.4 million and the net proceeds from its follow-on offering of approximately $80.0 million, after deducting underwriting discounts and commissions and estimated offering expenses, and net proceeds from the ATM program of approximately $11.2 million during the fourth quarter of 2018 will be sufficient to fund its operating expenses and capital requirements into early 2020.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
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, 2017 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 September 30, 2018 and for the three and nine months ended September 30, 2018 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, 2017 included in the Company’s Annual Report on Form 10-K filed with the SEC on March 14, 2018. 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 September 30, 2018, results of its operations for the three and nine months ended September 30, 2018, and cash flows for the nine months ended September 30, 2018 have been made. The results of operations for the three and nine months ended September 30, 2018 are not necessarily indicative of the results of operations to be expected for the year ending December 31, 2018. 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, 2017 and the notes thereto are included in the Company’s Annual Report on Form 10-K that was filed with the SEC on March 14, 2018. Certain amounts reported in the previous year have been recast as a result of the retrospective adoption of new accounting standards in the first quarter of 2018. Refer to Note 2 - Recently Issued and Adopted Accounting Pronouncements, Collaboration Agreement Leases, 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 modified retrospective transition method. Under this method, results for reporting periods beginning after January 1, 2018 are presented under ASC 606, while prior period amounts are not adjusted and continue to be reported in accordance 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 are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. 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 - Collaboration Agreement Recently Issued and Adopted Accounting Pronouncements ASU No. 2014-09, Revenue from Contracts with Customers In May 2014, the FASB issued ASU No. 2014-09, which amends the guidance for accounting for revenue from contracts with customers. This ASU supersedes the revenue recognition requirements in ASC Topic 605, Revenue Recognition Revenue from Contracts with Customers As a result of adopting ASC 606 on January 1, 2018, the Company has recorded a cumulative-effect decrease to opening accumulated deficit of $1.1 million as of January 1, 2018 and a corresponding decrease to deferred revenue. The remaining impacts related to ASC 606 adoption were as follows (in thousands): September 30, 2018 As Reported under ASC 606 As Determined under ASC 605 Decrease upon Adoption of ASC 606 Balance Sheet: Deferred revenue, current $ — $ 617 $ (617 ) Income Statement: Revenue for the three months ended September 30, 2018 1,055 1,751 (696 ) Revenue for the nine months ended September 30, 2018 2,840 4,691 (1,851 ) 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 Pharma Inc. 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 milestone payments were being recognized by the Company as revenue on a straight-line basis over the 3½ 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. Under ASC 606, the Company is recognizing the revenue allocated to the one performance obligation measuring progress using the proportional performance method. 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 non-current, 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 Impacts to Previously Reported Results The impact of the adoption of the new leasing standard on the December 31, 2017 balance sheet 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 (“ASU 2016-18”). ASU 2016-18 requires a statement of cash flows to explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of- period total amounts shown on the statement of cash flows. This resulted in the $0.8 million and $2.0 million of restricted cash for the nine months ended September 30, 2018 and September 30, 2017, respectively, being displayed within the overall change in the cash balance on the statement of cash flows. 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 2018-07”). The new standard simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. The new standard will be effective beginning January 1, 2019 and early adoption is permitted. The Company is currently evaluating the impact that the adoption of ASU 2018-07 will have on its results of operations. 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	9 Months Ended
	Sep. 30, 2018
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 September 30, 2018 and December 31, 2017 (in thousands): September 30, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S government-sponsored enterprise securities $ 5,235 $ — $ (2 ) $ 5,233 U.S. treasury securities 21,464 — (5 ) 21,459 $ 26,699 $ — $ (7 ) $ 26,692 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 three and nine months ended September 30, 2018 and 2017 2017

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	9 Months Ended
	Sep. 30, 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 September 30, 2018 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 17,340 $ — $ — $ 17,340 Short-term investments: U.S. government-sponsored enterprise securities — 5,233 — 5,233 U.S. treasury securities — 21,459 — 21,459 $ 17,340 $ 26,692 $ — $ 44,032 Liabilities: Derivative liability $ — $ — $ 3 $ 3 $ — $ — $ 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 $ — $ — $ 25 $ 25 During the periods ended September 30, 2018 and December 31, 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 change of control provision in the Company’s Cystic Fibrosis Foundation Therapeutics, Inc. (“CFFT”) 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. As of September 30, 2018 and December 31, 2017, the Company determined the per share common stock price available based on the closing price of its common stock on the NASDAQ Global Market as of September 28, 2018 and December 29, 2017, respectively. The Company determined the expected term of the instrument to be 1.50 years and 2.00 years as of September 30, 2018 and December 31, 2017, respectively. The Company estimated its expected stock volatility to be 80.0% as of September 30, 2018 and December 31, 2017, respectively, based on the historical volatility of publicly traded peer companies for terms matching the expected term of the instrument for each respective period. The risk-free interest rate was determined to be 2.40% and 1.87% as of September 30, 2018 and December 31, 2017, respectively, by reference to the U.S. Treasury yield curve for terms matching the expected term of the instrument for each respective period. Changes in the values of the derivative liability are summarized below (in thousands): Derivative Liability Fair value at December 31, 2017 $ 25 Change in fair value (22 ) Fair value at September 30, 2018 $ 3

Prepaids and Other Current Asse

Prepaids and Other Current Assets	9 Months Ended
	Sep. 30, 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): September 30, December 31, 2018 2017 Prepaid clinical, manufacturing and scientific expenses $ 911 $ 568 Prepaid insurance expenses 347 55 Other prepaid expenses and other current assets 672 754 $ 1,930 $ 1,377

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2018
Payables And Accruals [Abstract]
Accrued Expenses	6. Accrued Expenses Accrued expenses consisted of the following (in thousands): September 30, December 31, 2018 2017 Accrued payroll and related expenses $ 2,003 $ 1,542 Accrued research and development expenses 5,510 3,930 Accrued professional fees 647 556 Accrued other 246 92 $ 8,406 $ 6,120

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2018
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 September 30, 2018, the total number of shares reserved under the 2016 Plan and 2008 Plan was 4,343,120 and the Company had 971,898 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 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 and (iii) an amount determined by the Company’s board of directors or the compensation committee of the board of directors. During the nine months ended September 30, 2018, 6,114 shares of common stock were issued pursuant to the 2016 ESPP. As of September 30, 2018, the total number of shares reserved under the 2016 ESPP was 399,328 shares. The Company recognized less than $0.1 million of stock-based compensation during the three and nine months ended September 30, 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 September 30, 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. 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 will recognize 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 September 30, 2018, 163,425 had been granted with a total expense of $0.1 million and $ .4 million recognized for the three and nine months ended September 30, 2018. 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 September 30, For the Nine Months Ended September 30, 2018 2017 2018 2017 Research and development $ 419 $ 412 $ 1,195 $ 1,070 General and administrative 606 636 2,206 1,713 $ 1,025 $ 1,048 $ 3,401 $ 2,783

Collaboration Agreement

Collaboration Agreement	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaboration Agreement	8. Collaboration Agreement 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 is to identify, develop and commercialize therapeutic candidates relating to the Unfolded Protein Response (“UPR”) pathway. Financial Terms Under terms of the Astellas Agreement, Astellas purchased from the Company convertible promissory notes totaling $5.0 million with terms consistent with those of other investors that purchased convertible promissory notes issued during 2014. In addition, the Company will be eligible to receive research funding support, based on the establishment of an annual research budget, and future research, development and sales milestone payments of up to $398.5 million, as well as tiered royalty payments ranging in the mid-single-digit to low double-digit percentages of net sales, as defined in the agreement. Under the agreement, the companies will conduct research during the initial research term, which is approximately 3½ years, to identify lead compounds for clinical development. The Company will provide Astellas with a report of the actual expenses incurred within 30 days after the end of the quarter, which Astellas will provide payment to the Company within 30 days of receiving of the report. At the end of the research term, Astellas, in its sole discretion, may designate a development compound and make a milestone payment to the Company. The Company has the right, but not the obligation, to co-develop the compound. If the Company does not exercise its option to co-develop the compound, Astellas will have 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 will continue in full force and effect, unless terminated under the conditions described below, until expiration of all applicable royalty terms with respect to all licensed products in all countries in the territory defined as per the agreement. The agreement 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 is effective beginning in April 2018 and extends the research term for the initial project under the Astellas Agreement to December 2018. As of September 30, 2018, all of the Company’s performance obligations under the contract have been satisfied. Astellas has the unilateral right to terminate the agreement on a project-by-project basis by providing written notice to the Company. Reciprocal termination rights under the agreement include termination for breach and termination for bankruptcy. 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 September 30, 2018 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. The Company will reevaluate the transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur, and, if necessary, adjust its estimate of the transaction price. Revenue associated with the performance obligation is being 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 occurs over this time period and, in management’s judgment, is the best measure of progress towards satisfying the performance obligation. Any amounts received that had not yet been recognized as revenue are recorded in deferred revenue on the Company’s condensed balance sheet. As of September 30, 2018, the research and development services related to this performance obligation are complete. The Company recognized revenue of $1.1 million and $1.6 million for the three months ended September 30, 2018 and 2017, respectively, and $2.8 million and $3.7 million for the nine months ended September 30, 2018 and 2017, respectively. There was $0.1 million recorded as a contract asset under the Astellas Agreement as of September 30, 2018 . Amounts recorded as deferred revenue under the Astellas Agreement totaled $1.1 million as of December 31, 2017

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
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 and nine months ended September 30, 2018 and 2017 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	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Net Loss per Share	10. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): For the Three Months Ended September 30, For the Nine Months Ended September 30, 2018 2017 2018 2017 Numerator: Net loss $ (18,428 ) $ (13,954 ) $ (44,932 ) $ (46,008 ) Denominator: Weighted average number of common shares outstanding—basic 36,694,957 25,093,344 35,734,159 25,051,536 Net loss per share attributable to common stockholders —basic and diluted $ (0.50 ) $ (0.56 ) $ (1.26 ) $ (1.84 ) The following potentially dilutive securities outstanding have been excluded from the computation of diluted weighted-average shares outstanding, because such securities had an antidilutive impact due to the losses reported : September 30, 2018 2017 Options to purchase common stock 3,207,797 2,946,853 Restricted stock units 163,425 — Potentially dilutive securities outstanding 3,371,222 2,946,853

Leases

Leases	9 Months Ended
	Sep. 30, 2018
Leases [Abstract]
Leases	11. Leases The Company has operating leases of office and laboratory space. In September 2017, the Company entered into a new lease agreement for office and laboratory space in Boston, Massachusetts, which is its new corporate headquarters. 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 elected to utilize its incremental borrowing rate based on the original lease term and not the remaining lease term. The Company has elected to account for each lease component and its associated non-lease components as a single lease component and has allocated all of 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 September 30, 2018 and December 31, 2017, assets under operating lease were $14.1 million and $0.5 million, respectively. The elements of lease expense were as follows (in thousands): For the Three Months Ended September 30, 2018 2017 Lease cost Operating lease cost $ 447 $ 286 Variable lease cost (1) 154 — Total lease cost $ 601 $ 286 For the Nine Months Ended September 30, 2018 2017 Lease Cost Operating lease cost $ 1,769 $ 857 Variable lease cost (1) 463 — Total Lease Cost $ 2,232 $ 857 Other information Operating cash flows used for operating leases $ 1,220 $ 1,006 Operating lease liabilities arising from obtaining right-of-use assets $ 14,502 $ — Weighted-average remaining lease term 9.59 years 0.59 years Weighted-average discount rate 4.50 % 4.06 % (1) The variable lease costs for the three and nine months ended September 30, 2018 include adjustments to the initial direct costs for the Boston lease along with common area maintenance charges. Future lease payments under no ncancelable leases as of September 30, 2018 (in thousands): Future Operating Lease Payments 2018 $ 414 2019 1,686 2020 1,733 2021 1,780 2022 1,829 Thereafter 10,637 Total lease payments 18,079 Less: imputed interest (3,577 ) Total operating lease liabilities $ 14,502

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Unaudited Interim Financial Information	Unaudited Interim Financial Information The condensed balance sheet as of December 31, 2017 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 September 30, 2018 and for the three and nine months ended September 30, 2018 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, 2017 included in the Company’s Annual Report on Form 10-K filed with the SEC on March 14, 2018. 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 September 30, 2018, results of its operations for the three and nine months ended September 30, 2018, and cash flows for the nine months ended September 30, 2018 have been made. The results of operations for the three and nine months ended September 30, 2018 are not necessarily indicative of the results of operations to be expected for the year ending December 31, 2018.
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, 2017 and the notes thereto are included in the Company’s Annual Report on Form 10-K that was filed with the SEC on March 14, 2018. Certain amounts reported in the previous year have been recast as a result of the retrospective adoption of new accounting standards in the first quarter of 2018. Refer to Note 2 - Recently Issued and Adopted Accounting Pronouncements, Collaboration Agreement Leases,
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 modified retrospective transition method. Under this method, results for reporting periods beginning after January 1, 2018 are presented under ASC 606, while prior period amounts are not adjusted and continue to be reported in accordance 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 are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. 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 - Collaboration Agreement
Recently Issued and Adopted Accounting Pronouncements	Recently Issued and Adopted Accounting Pronouncements ASU No. 2014-09, Revenue from Contracts with Customers In May 2014, the FASB issued ASU No. 2014-09, which amends the guidance for accounting for revenue from contracts with customers. This ASU supersedes the revenue recognition requirements in ASC Topic 605, Revenue Recognition Revenue from Contracts with Customers As a result of adopting ASC 606 on January 1, 2018, the Company has recorded a cumulative-effect decrease to opening accumulated deficit of $1.1 million as of January 1, 2018 and a corresponding decrease to deferred revenue. The remaining impacts related to ASC 606 adoption were as follows (in thousands): September 30, 2018 As Reported under ASC 606 As Determined under ASC 605 Decrease upon Adoption of ASC 606 Balance Sheet: Deferred revenue, current $ — $ 617 $ (617 ) Income Statement: Revenue for the three months ended September 30, 2018 1,055 1,751 (696 ) Revenue for the nine months ended September 30, 2018 2,840 4,691 (1,851 ) 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 Pharma Inc. 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 milestone payments were being recognized by the Company as revenue on a straight-line basis over the 3½ 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. Under ASC 606, the Company is recognizing the revenue allocated to the one performance obligation measuring progress using the proportional performance method. 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 non-current, 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 Impacts to Previously Reported Results The impact of the adoption of the new leasing standard on the December 31, 2017 balance sheet 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 (“ASU 2016-18”). ASU 2016-18 requires a statement of cash flows to explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of- period total amounts shown on the statement of cash flows. This resulted in the $0.8 million and $2.0 million of restricted cash for the nine months ended September 30, 2018 and September 30, 2017, respectively, being displayed within the overall change in the cash balance on the statement of cash flows. 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 2018-07”). The new standard simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. The new standard will be effective beginning January 1, 2019 and early adoption is permitted. The Company is currently evaluating the impact that the adoption of ASU 2018-07 will have on its results of operations. 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	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 Therapeutics, Inc. (“CFFT”) 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. As of September 30, 2018 and December 31, 2017, the Company determined the per share common stock price available based on the closing price of its common stock on the NASDAQ Global Market as of September 28, 2018 and December 29, 2017, respectively. The Company determined the expected term of the instrument to be 1.50 years and 2.00 years as of September 30, 2018 and December 31, 2017, respectively. The Company estimated its expected stock volatility to be 80.0% as of September 30, 2018 and December 31, 2017, respectively, based on the historical volatility of publicly traded peer companies for terms matching the expected term of the instrument for each respective period. The risk-free interest rate was determined to be 2.40% and 1.87% as of September 30, 2018 and December 31, 2017, respectively, by reference to the U.S. Treasury yield curve for terms matching the expected term of the instrument for each respective period. Changes in the values of the derivative liability are summarized below (in thousands): Derivative Liability Fair value at December 31, 2017 $ 25 Change in fair value (22 ) Fair value at September 30, 2018 $ 3

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 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 December 31, 2017 balance sheet 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	The remaining impacts related to ASC 606 adoption were as follows (in thousands): September 30, 2018 As Reported under ASC 606 As Determined under ASC 605 Decrease upon Adoption of ASC 606 Balance Sheet: Deferred revenue, current $ — $ 617 $ (617 ) Income Statement: Revenue for the three months ended September 30, 2018 1,055 1,751 (696 ) Revenue for the nine months ended September 30, 2018 2,840 4,691 (1,851 )

Short-Term Investments (Tables)

Short-Term Investments (Tables)	9 Months Ended
	Sep. 30, 2018
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 September 30, 2018 and December 31, 2017 (in thousands): September 30, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S government-sponsored enterprise securities $ 5,235 $ — $ (2 ) $ 5,233 U.S. treasury securities 21,464 — (5 ) 21,459 $ 26,699 $ — $ (7 ) $ 26,692 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)	9 Months Ended
	Sep. 30, 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 September 30, 2018 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 17,340 $ — $ — $ 17,340 Short-term investments: U.S. government-sponsored enterprise securities — 5,233 — 5,233 U.S. treasury securities — 21,459 — 21,459 $ 17,340 $ 26,692 $ — $ 44,032 Liabilities: Derivative liability $ — $ — $ 3 $ 3 $ — $ — $ 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 $ — $ — $ 25 $ 25
Summary of Changes in Values of Derivative Liability	Changes in the values of the derivative liability are summarized below (in thousands): Derivative Liability Fair value at December 31, 2017 $ 25 Change in fair value (22 ) Fair value at September 30, 2018 $ 3

Prepaids and Other Current As_2

Prepaids and Other Current Assets (Tables)	9 Months Ended
	Sep. 30, 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): September 30, December 31, 2018 2017 Prepaid clinical, manufacturing and scientific expenses $ 911 $ 568 Prepaid insurance expenses 347 55 Other prepaid expenses and other current assets 672 754 $ 1,930 $ 1,377

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2018
Payables And Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following (in thousands): September 30, December 31, 2018 2017 Accrued payroll and related expenses $ 2,003 $ 1,542 Accrued research and development expenses 5,510 3,930 Accrued professional fees 647 556 Accrued other 246 92 $ 8,406 $ 6,120

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
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 September 30, For the Nine Months Ended September 30, 2018 2017 2018 2017 Research and development $ 419 $ 412 $ 1,195 $ 1,070 General and administrative 606 636 2,206 1,713 $ 1,025 $ 1,048 $ 3,401 $ 2,783

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 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 attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): For the Three Months Ended September 30, For the Nine Months Ended September 30, 2018 2017 2018 2017 Numerator: Net loss $ (18,428 ) $ (13,954 ) $ (44,932 ) $ (46,008 ) Denominator: Weighted average number of common shares outstanding—basic 36,694,957 25,093,344 35,734,159 25,051,536 Net loss per share attributable to common stockholders —basic and diluted $ (0.50 ) $ (0.56 ) $ (1.26 ) $ (1.84 )
Schedule of Antidilutive Securities Excluded from Computation of Diluted Weighted-average Shares Outstanding	The following potentially dilutive securities outstanding have been excluded from the computation of diluted weighted-average shares outstanding, because such securities had an antidilutive impact due to the losses reported : September 30, 2018 2017 Options to purchase common stock 3,207,797 2,946,853 Restricted stock units 163,425 — Potentially dilutive securities outstanding 3,371,222 2,946,853

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2018
Lessee Disclosure [Abstract]
Elements of Lease Expense	The elements of lease expense were as follows (in thousands): For the Three Months Ended September 30, 2018 2017 Lease cost Operating lease cost $ 447 $ 286 Variable lease cost (1) 154 — Total lease cost $ 601 $ 286 For the Nine Months Ended September 30, 2018 2017 Lease Cost Operating lease cost $ 1,769 $ 857 Variable lease cost (1) 463 — Total Lease Cost $ 2,232 $ 857 Other information Operating cash flows used for operating leases $ 1,220 $ 1,006 Operating lease liabilities arising from obtaining right-of-use assets $ 14,502 $ — Weighted-average remaining lease term 9.59 years 0.59 years Weighted-average discount rate 4.50 % 4.06 % (1) The variable lease costs for the three and nine months ended September 30, 2018 include adjustments to the initial direct costs for the Boston lease along with common area maintenance charges.
Schedule of Future Lease Payments under Noncancelable Leases	Future lease payments under no ncancelable leases as of September 30, 2018 (in thousands): Future Operating Lease Payments 2018 $ 414 2019 1,686 2020 1,733 2021 1,780 2022 1,829 Thereafter 10,637 Total lease payments 18,079 Less: imputed interest (3,577 ) Total operating lease liabilities $ 14,502

Nature of the Business - Additi

Nature of the Business - Additional Information (Detail) - USD ($)	Nov. 06, 2018	Oct. 19, 2018	Oct. 31, 2018	Mar. 31, 2018	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Nature of Business [Line Items]
Accumulated deficit						$ (260,635,000)		$ (260,635,000)		$ (216,808,000)
Cash, cash equivalents and short-term investments						46,400,000		46,400,000
Net loss						$ (18,428,000)	$ (13,954,000)	(44,932,000)	$ (46,008,000)
Cash in operations								$ 39,504,000	$ 40,557,000
Follow-on Public Offering [Member] | Scenario Forecast [Member]
Nature of Business [Line Items]
Net proceeds from sales of common stock					$ 80,000,000
Subsequent Event [Member] | Follow-on Public Offering [Member]
Nature of Business [Line Items]
Number of shares issued and sold	1,650,000		11,000,000
Common stock price per share	$ 6.75		$ 6.75
Net proceeds from sales of common stock	$ 10,500,000		$ 69,500,000
Leerink Partners LLC [Member] | ATM Program [Member]
Nature of Business [Line Items]
Common stock aggregate offering price				$ 50,000,000
Percentage of gross proceeds from common stock sold through sales agreement				3.00%
Leerink Partners LLC [Member] | ATM Program [Member] | Scenario Forecast [Member]
Nature of Business [Line Items]
Net proceeds from sales of common stock					$ 11,200,000
Leerink Partners LLC [Member] | Subsequent Event [Member] | ATM Program [Member]
Nature of Business [Line Items]
Number of shares issued and sold		3,475,166	1,364,348
Net proceeds from sales of common stock		$ 21,700,000	$ 11,200,000
Common stock remained available for sale, value			$ 27,600,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Accumulated deficit	$ (260,635)			$ (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_5

Summary of Significant Accounting Policies - Schedule of Impacts Related to ASC 606 Adoption (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Balance Sheet:
Deferred revenue, current					$ 1,108
Income Statement:
Revenue	$ 1,055	$ 1,551	$ 2,840	$ 3,719
ASC 606 [Member]
Income Statement:
Revenue	1,055		2,840
ASC 606 [Member] | As Determined Under ASC 605 [Member]
Balance Sheet:
Deferred revenue, current	617		617
Income Statement:
Revenue	1,751		4,691
ASC 606 [Member] | Decrease Upon Adoption of ASC 606 [Member]
Balance Sheet:
Deferred revenue, current	(617)		(617)
Income Statement:
Revenue	$ (696)		$ (1,851)

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Schedule of Impact of Adoption of New Leasing Standard (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Operating lease right-of-use assets, net	$ 14,147	$ 472
Property and equipment, net	648	424
Operating lease liabilities, current	1,033	559
Accumulated deficit	$ (260,635)	(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	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 26,699	$ 44,740
Gross Unrealized Gains		1
Gross Unrealized Losses	(7)	(3)
Fair Value	26,692	44,738
U.S Government-Sponsored Enterprise Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,235	9,817
Gross Unrealized Losses	(2)	(1)
Fair Value	5,233	9,816
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,464	34,923
Gross Unrealized Gains		1
Gross Unrealized Losses	(5)	(2)
Fair Value	$ 21,459	$ 34,922

Short-Term Investments - Additi

Short-Term Investments - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Realized gains (losses) on short-term investments	$ 0	$ 0	$ 0	$ 0
Other-than-temporary impairments recognized	$ 0	$ 0	$ 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	Sep. 30, 2018	Dec. 31, 2017
Liabilities:
Derivative liability	$ 3	$ 25
Fair Value, Measurements, Recurring [Member]
Assets:
Total assets	44,032	71,066
Liabilities:
Derivative liability	3	25
Total liabilities	3	25
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	17,340	13,871
Fair Value, Measurements, Recurring [Member] | U.S Government-Sponsored Enterprise Securities [Member]
Assets:
Cash equivalents		8,960
Short-term investments	5,233	9,816
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Assets:
Cash equivalents		3,497
Short-term investments	21,459	34,922
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Total assets	17,340	13,871
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	17,340	13,871
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Total assets	26,692	57,195
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S Government-Sponsored Enterprise Securities [Member]
Assets:
Cash equivalents		8,960
Short-term investments	5,233	9,816
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury Securities [Member]
Assets:
Cash equivalents		3,497
Short-term investments	21,459	34,922
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Liabilities:
Derivative liability	3	25
Total liabilities	$ 3	$ 25

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
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
Expected Term of Instrument [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Expected term	1 year 6 months	2 years
Expected Stock Volatility [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative liability, measurement input	80	80
Risk-free Interest Rate [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative liability, measurement input	2.40	1.87

Fair Value of Financial Asset_5

Fair Value of Financial Assets and Liabilities - Summary of Changes in Values of Derivative Liability (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018USD ($)
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]
Derivative Liability, Fair value, Beginning Balance	$ 25
Derivative Liability, Change in fair value	(22)
Derivative Liability, Fair value, Ending Balance	$ 3

Prepaids and Other Current As_3

Prepaids and Other Current Assets - Schedule of Prepaids and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Prepaid Expense And Other Assets Current [Abstract]
Prepaid clinical, manufacturing and scientific expenses	$ 911	$ 568
Prepaid insurance expenses	347	55
Other prepaid expenses and other current assets	672	754
Prepaids and other current assets	$ 1,930	$ 1,377

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued payroll and related expenses	$ 2,003	$ 1,542
Accrued research and development expenses	5,510	3,930
Accrued professional fees	647	556
Accrued other	246	92
Total accrued expenses	$ 8,406	$ 6,120

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	Feb. 01, 2018	Feb. 03, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation			$ 1,025,000	$ 1,048,000	$ 3,401,000	$ 2,783,000
Bonus Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation			$ 100,000		$ 400,000
RSUs granted					163,425
Nonemployees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options unvested			83,250		83,250
Fair value of option					$ 100,000
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] | 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	971,898		971,898
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			4,343,120		4,343,120
2016 ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance		138,757	399,328		399,328
Common stock reserved for issuance, percentage of number of shares of common stock outstanding		1.00%
Common stock available for issuance			138,757		138,757
Number of shares issued					6,114
2016 ESPP [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation			$ 100,000		$ 100,000

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock-Based Compensation Expense, Including Shares Issued to Nonemployee for Consulting Services (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 1,025	$ 1,048	$ 3,401	$ 2,783
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	419	412	1,195	1,070
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 606	$ 636	$ 2,206	$ 1,713

Collaboration Agreement - Addit

Collaboration Agreement - Additional Information (Detail) - USD ($) $ in Thousands	Apr. 23, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue		$ 1,055	$ 1,551	$ 2,840	$ 3,719
Deferred revenue						$ 1,108
ASC 606 [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue		1,055		$ 2,840
Astellas Collaboration Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Convertible promissory notes							$ 5,000
Initial research term				3 years 6 months
Maximum period to submit actual expenses report after quarter end				30 days
Maximum period to receive payments for expenses after submit of report				30 days
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		1,100	$ 1,600	$ 2,800	$ 3,700
Deferred revenue						$ 1,100
Astellas Collaboration Agreement [Member] | ASC 606 [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction price				13,700
Astellas Collaboration Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development and sales milestone payments				398,500
Deferred revenue recorded as contract asset		$ 100		$ 100

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Federal income tax benefit	$ 0	$ 0	$ 0	$ 0
State income tax benefit	$ 0	$ 0	$ 0	$ 0

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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net loss	$ (18,428)	$ (13,954)	$ (44,932)	$ (46,008)
Denominator:
Weighted average number of common shares outstanding—basic	36,694,957	25,093,344	35,734,159	25,051,536
Net loss per share attributable to common stockholders—basic and diluted	$ (0.50)	$ (0.56)	$ (1.26)	$ (1.84)

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	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding	3,371,222	2,946,853
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding	3,207,797	2,946,853
Restricted Stock Units [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) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Jan. 19, 2018	Dec. 31, 2017
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	$ 14,147		$ 472

Leases - Elements of Lease Expe

Leases - Elements of Lease Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Lease cost
Operating lease cost	$ 447	$ 286	$ 1,769	$ 857
Variable lease cost	154		463
Total lease cost	$ 601	$ 286	2,232	857
Other information
Operating cash flows used for operating leases			1,220	$ 1,006
Operating lease liabilities arising from obtaining right-of-use assets			$ 14,502
Weighted-average remaining lease term	9 years 7 months 2 days	7 months 2 days	9 years 7 months 2 days	7 months 2 days
Weighted-average discount rate	4.50%	4.06%	4.50%	4.06%

Leases - Schedule of Future Lea

Leases - Schedule of Future Lease Payments under Noncancelable Leases (Detail) $ in Thousands	Sep. 30, 2018USD ($)
Leases [Abstract]
2,018	$ 414
2,019	1,686
2,020	1,733
2,021	1,780
2,022	1,829
Thereafter	10,637
Total lease payments	18,079
Less: imputed interest	(3,577)
Total operating lease liabilities	$ 14,502