Kineta (KANT) 10-Q 14 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Nov. 09, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
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	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		25,216,088

Condensed Balance Sheets

Condensed Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 18,192	$ 18,613
Short-term investments	25,034	66,897
Restricted cash	294
Accounts receivable	1,079	668
Prepaids and other current assets	1,838	4,059
Total current assets	46,437	90,237
Property and equipment, net	12,582	541
Other assets	41	68
Restricted cash, net of current portion	1,656	294
Total assets	60,716	91,140
Current liabilities:
Accounts payable	2,535	2,021
Accrued expenses	5,855	4,328
Deferred revenue	1,754	2,204
Deferred rent	139	201
Other liabilities	756
Total current liabilities	11,039	8,754
Deferred revenue, net of current portion		752
Deferred rent, net of current portion		87
Derivative liability	3	91
Other liabilities, net of current portion	11,355
Total liabilities	22,397	9,684
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized as of September 30, 2017 and December 31, 2016, respectively; no shares issued and outstanding as of September 30, 2017 and December 31, 2016
Common stock, $0.001 par value; 125,000,000 shares authorized as of September 30, 2017 and December 31, 2016, respectively; 25,112,724 and 25,000,734 shares issued and outstanding as of September 30, 2017 and December 31, 2016, respectively	26	26
Additional paid-in capital	241,758	238,902
Accumulated other comprehensive loss	(7)	(22)
Accumulated deficit	(203,458)	(157,450)
Total stockholders’ equity	38,319	81,456
Total liabilities and stockholders’ equity	$ 60,716	$ 91,140

Condensed Balance Sheets (Paren

Condensed Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	25,112,724	25,000,734
Common stock, shares outstanding	25,112,724	25,000,734

Condensed Statements of Operati

Condensed Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Revenue	$ 1,551	$ 1,715	$ 3,719	$ 4,324
Operating expenses:
Research and development	12,894	9,218	41,372	23,498
General and administrative	2,741	3,266	8,813	8,682
Total operating expenses	15,635	12,484	50,185	32,180
Loss from operations	(14,084)	(10,769)	(46,466)	(27,856)
Interest income	155	36	515	56
Other expense, net	(25)	(38)	(57)	(15)
Net loss	(13,954)	(10,771)	(46,008)	(27,815)
Accruing dividends on preferred stock				(1,378)
Net loss attributable to common stockholders	$ (13,954)	$ (10,771)	$ (46,008)	$ (29,193)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.56)	$ (0.54)	$ (1.84)	$ (1.75)
Weighted average common shares outstanding—basic and diluted	25,093,344	20,073,685	25,051,536	16,672,368

Condensed Statements of Compreh

Condensed Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (13,954)	$ (10,771)	$ (46,008)	$ (27,815)
Other comprehensive loss:
Unrealized gain (loss) on investments	20	(9)	15	(9)
Comprehensive loss	$ (13,934)	$ (10,780)	$ (45,993)	$ (27,824)

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net loss	$ (46,008)	$ (27,815)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	187	248
Premium on short-term investments	(140)	(124)
Amortization of premium on short-term investments	182	2
Non-cash rent expense	(149)	(134)
Stock-based compensation expense	2,228	1,245
Stock issued for consulting services	555	130
Change in fair value of derivative liability	(88)	140
Change in fair value of preferred stock warrant liability		(82)
Gain on disposal of property and equipment		(13)
Changes in operating assets and liabilities:
Accounts receivable	(411)	473
Prepaids and other current assets	2,221	(2,200)
Other assets	27	63
Accounts payable	514	390
Accrued expenses	1,527	1,849
Deferred revenue	(1,202)	(1,992)
Net cash used in operating activities	(40,557)	(27,820)
Cash flows from investing activities:
Change in restricted cash	(1,656)
Purchases of short-term investments	(23,727)	(56,299)
Proceeds received from maturities of short-term investments	65,563
Purchases of property and equipment	(117)	(32)
Proceeds received from disposal of property and equipment		13
Net cash provided by (used in) investing activities	40,063	(56,318)
Cash flows from financing activities:
Proceeds from issuance of common stock upon completion of initial public offering, net of commissions and underwriting discounts		46,500
Proceeds from issuance of common stock		70,265
Proceeds from exercise of stock options	30	144
Net cash provided by financing activities	73	114,122
Net increase (decrease) in cash and cash equivalents	(421)	29,984
Cash and cash equivalents at beginning of period	18,613	13,844
Cash and cash equivalents at end of period	18,192	43,828
Supplemental disclosure of non-cash investing and financing activities:
Conversion of convertible preferred stock into common stock		112,292
Issuance of common stock to settle accrued Series A preferred stock dividends		3
Issuance of common stock for partial payment of accrued bonus		62
Conversion of preferred stock warrants into common stock warrants		28
Deferred offering costs included in accounts payable and accrued expenses		524
Amounts capitalized under build-to-suit lease transaction	12,111
Initial Public Offering [Member]
Cash flows from financing activities:
Payments of public offering costs		(2,515)
Follow-on Public Offering [Member]
Cash flows from financing activities:
Payments of public offering costs		$ (272)
Employee Stock Purchase Plan [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 43

Nature of the Business

Nature of the Business	9 Months Ended
	Sep. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business	1. Nature of the Business Proteostasis Therapeutics, Inc. (the “Company”) was incorporated in Delaware on December 13, 2006. The Company is 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, PTI-801 and PTI-808 are in early clinical development, with plans for double and triple combinations, and the Company’s other drug candidates are in the preclinical development and discovery phases. As of December 31, 2016, the Company adopted the provisions of Financial Statement Account Standards Board (FASB) Accounting Standard Codification (ASC) Topic 205-40, Presentation of Financial Statements – Going Concern requires management to 1) identify and disclose if there are initial conditions indicating substantial doubt about the Company’s ability to continue as a going concern within twelve months of the issuance date of the financial statements, 2) disclose the principal conditions that gave rise to substantial doubt, 3) disclose management’s evaluation of the significance of those conditions in relation to the Company’s ability to meet its obligations and 4) disclose management’s plans that are intended to mitigate the adverse conditions. In accordance with the accounting standard, when considering management’s plans to mitigate the conditions giving rise to substantial doubt, management can only consider those plans which are probable to be successfully implemented The Company has incurred losses from operations since its inception. As of September 30, 2017, the Company had an accumulated deficit of $203.5 million. During the three and nine months ended September 30, 2017, the Company incurred losses of $14.0 million and $46.0 million and during the nine months ended September 30, 2017 the Company used $40.6 million of cash in operations. The Company expects to continue to generate operating losses in the foreseeable future. The Company currently expects that its cash, cash equivalents and short-term investments of $43.2 million will be sufficient to fund its operating expenses and capital requirements, based upon its current operating plan, through the second quarter of 2018. As of September 30, 2017, management has further assessed this risk and, in accordance with the requirements of ASC 205-40, determined that there are initial conditions indicating that there is substantial doubt about the Company’s ability to continue as a going concern within twelve months of the issuance date of these condensed financial statements. These indicators are the Company’s accumulated deficit and the forecasted cash expenditures. As such, under the requirements of ASC 205-40, management may not consider the potential for future capital raises in its assessment of the Company’s ability to meet its obligations for the next twelve months. If the Company is unable to obtain funding, the Company would be forced to delay, reduce or eliminate its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations. The ability to reduce spending at a level that mitigates the factors described above, is not considered probable, as defined in the accounting standards; as such, under the requirements of ASC 205-40, the full extent to which management may extend the Company’s funds through these actions may not be considered in management’s assessment of the Company’s ability to continue as a going concern for the next twelve months. Thus, in accordance with the requirements of ASC 205-40, management has concluded that substantial doubt exists about the Company’s ability to continue as a going concern for twelve months from the date these condensed financial statements are issued. The condensed financial statements have been prepared assuming that the Company will continue as a going concern, which contemplates continuity of operations, the realization of assets and the satisfaction of liabilities and commitments in the normal course of business .

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Unaudited Interim Financial Information The condensed balance sheet at December 31, 2016 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, 2017 and for the three and nine months ended September 30, 2017 are unaudited. The accompanying unaudited interim financial statements 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. However, 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, 2016 included in the Company’s Annual Report on Form 10-K filed with the SEC on March 30, 2017. In the opinion of management, all adjustments, consisting only of normal recurring adjustments as necessary, for the fair statement of the Company’s condensed financial position as of September 30, 2017 and condensed results of its operations and cash flows for the three and nine months ended September 30, 2017 have been made. The results of operations for the three and nine months ended September 30, 2017 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2017. 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. Restricted Cash As of September 30, 2017 and December 31, 2016, restricted cash consisted of a certificate of deposit in the amount of $0.3 million collateralizing a letter of credit issued as a security deposit in connection with the Company’s lease of its corporate facilities at 200 Technology Square, Cambridge, MA. On September 19, 2017, the Company entered into a lease agreement for a new corporate headquarters at 80 Guest Street, Brighton, MA (see Note 11). As of September 30, 2017, restricted cash, net of current portion, consisted of a money market account in the amount of $1.7 million collateralizing a letter of credit issued as a security deposit for this lease agreement. Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less at the date of purchase to be cash equivalents. As of September 30, 2017 and December 31, 2016, the Company’s cash equivalents consisted of money market funds. Short-term Investments Short-term investments represent holdings of available-for-sale marketable securities in accordance with the Company’s investment policy and cash management strategy. Short-term investments mature within one-year from the balance sheet date. Investments in marketable securities are recorded at fair value, with any unrealized gains and losses, net of taxes, reported as a component of stockholders’ equity until realized or until a determination is made that an other-than-temporary decline in market value has occurred. The cost of marketable securities sold is determined based on the specific identification method and any realized gains or losses on the sale of investments are reflected as a component of other expense, net. Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s derivative liability, short-term investments and cash equivalents are carried at fair value determined according to the fair value hierarchy described above (see Note 4). The carrying value of cash and cash equivalents, accounts receivable, accounts payable and accrued expenses approximate their fair value due to the short-term nature of these assets and liabilities. Net Loss per Share In February 2016, upon the closing of the IPO, all of the outstanding shares of the Company’s redeemable convertible preferred stock automatically converted into 9,699,600 shares of the Company’s common stock. Prior to this conversion, the Company followed the two-class method when computing net loss per share as the Company had issued shares that met the definition of participating securities. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. The Company’s redeemable convertible preferred stock contractually entitled the holders of such shares to participate in dividends, but did not contractually require the holders of such shares to participate in losses of the Company. Accordingly, the two-class method did not apply for periods in which the Company reported a net loss or a net loss attributable to common stockholders resulting from dividends or accretion related to its redeemable convertible preferred stock. Basic net income (loss) per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of common shares, including potential dilutive common shares assuming the dilutive effect of outstanding stock options and unvested restricted common shares, as determined using the treasury stock method. For periods in which the Company has reported net losses, diluted net loss per common share attributable to common stockholders is the same as basic net loss per common share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. For any period in which the Company has reported net income, basic net income per common share attributable to common stockholders is adjusted for certain amounts to calculate diluted net loss per common share attributable to common stockholders, since dilutive common shares are assumed to have been issued if their effect is dilutive and not to have been issued if their effect is anti-dilutive. Recently Issued Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes existing revenue recognition guidance under GAAP. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The standard defines a five-step process to achieve this principle, and will require companies to use more judgment and make more estimates than under the current guidance. The Company expects that these judgments and estimates will include identifying performance obligations in the customer contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date , which delays the effective date of ASU 2014-09 such that the standard is effective for public entities for annual periods beginning after December 15, 2017 and for interim periods within those fiscal years. Early adoption of the standard is permitted for annual periods beginning after December 15, 2016. In April 2016, the FASB issued ASU 2016-10, Identifying Performance Obligations and Licensing . The new standard clarifies two aspects of ASU 2014-09, Revenue from Contracts with Customers (Topic 606) : identifying performance obligations and the licensing implementation guidance. These new standards will become effective for the Company on January 1, 2018. The Company only has one contract (see Note 8) within the scope of Topic 606 and intends to use the modified retrospective method for adoption. Since the inception of the contract the Company has recognized $8.1 million in revenue. The Company is still assessing the impact that the adoption of these new standards will have on its financial statements and related disclosures. In February 2016, the FASB issued ASU No. 2016-02, Leases . In March 2016, the FASB issued ASU No. 2016-09, Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting . As a result, the Company has made an accounting policy election to account for forfeitures as they occur. The adoption of ASU 2016‑09 also requires excess tax benefits and tax deficiencies be recorded in the income statement as opposed to additional paid‑in capital when the awards vest or are settled. This had no material impact on the condensed financial statements as of and for the three and nine months ended September 30, 2017 In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments . In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (Topic 719): Scope of Modification Accounting

Short Term Investments

Short Term Investments	9 Months Ended
	Sep. 30, 2017
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, 2017 and December 31, 2016 (in thousands): September 30, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S government-sponsored enterprise securities $ 12,519 $ — $ (3 ) $ 12,516 U.S. treasury securities 12,522 — (4 ) 12,518 $ 25,041 $ — $ (7 ) $ 25,034 December 31, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S government-sponsored enterprise securities $ 53,384 $ — $ (20 ) $ 53,364 U.S. treasury securities 13,535 — (2 ) 13,533 $ 66,919 $ — $ (22 ) $ 66,897 The Company did not have any realized gains or losses on its short-term investments for the three and nine months ended September 30, 2017 and 2016 2016

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	4. Fair Value of Financial Assets and Liabilities The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (in thousands): Fair Value Measurements as of September 30, 2017 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 14,370 $ — $ — $ 14,370 Short-term investments: U.S. government-sponsored enterprise securities — 12,516 — 12,516 U.S. treasury securities — 12,518 — 12,518 $ 14,370 $ 25,034 $ — $ 39,404 Liabilities: Derivative liability $ — $ — $ 3 $ 3 $ — $ — $ 3 $ 3 Fair Value Measurements as of December 31, 2016 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 15,440 $ — $ — $ 15,440 Short-term investments: U.S. government-sponsored enterprise securities — 53,365 — 53,365 U.S. treasury securities — 13,532 — 13,532 $ 15,440 $ 66,897 $ — $ 82,337 Liabilities: Derivative liability $ — $ — $ 91 $ 91 $ — $ — $ 91 $ 91 During the periods ended September 30, 2017 and December 31, 2016, 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 our 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, 2017 and December 31, 2016, the Company determined the per share common stock price available based on the closing price of its common stock on the NASDAQ Global Market as of September 29, 2017 and December 30, 2016, respectively. The Company determined the expected term of the instrument to be 2.25 years and 2.50 years as of September 30, 2017 and December 31, 2016, respectively. The Company estimated its expected stock volatility to be 80.0% and 81.1% as of September 30, 2017 and December 31, 2016, respectively, based on the historical volatility of publicly traded peer companies for terms matching the expected term of the instrument for each respective period. The risk-free interest rate was determined to be 1.51% and 1.33% as of September 30, 2017 and December 31, 2016, 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, 2016 $ 91 Change in fair value (88 ) Fair value at September 30, 2017 $ 3

Prepaids and Other Current Asse

Prepaids and Other Current Assets	9 Months Ended
	Sep. 30, 2017
Prepaid Expense And Other Assets Current [Abstract]
Prepaids and Other Current Assets	5. Prepaids and Other Current Assets Prepaids and other current assets consisted of the following (in thousands): September 30, December 31, 2017 2016 Prepaid clinical, manufacturing and scientific expenses $ 1,197 $ 1,390 Prepaid insurance expenses 218 104 Other prepaid expenses and other current assets 423 2,565 $ 1,838 $ 4,059

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2017
Text Block [Abstract]
Accrued Expenses	6. Accrued Expenses Accrued expenses consisted of the following (in thousands): September 30, December 31, 2017 2016 Accrued payroll and related expenses $ 1,293 $ 1,813 Accrued research and development expenses 4,086 1,612 Accrued professional fees 399 383 Accrued other 77 520 $ 5,855 $ 4,328

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2017
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 is 1,581,839 shares. The number of shares of common stock that may be issued under the 2016 Plan will automatically increase on each January 1, beginning on January 1, 2017, by the lesser of 3% of the shares of the Company’s common stock outstanding on the immediately preceding December 31 or an amount determined by the Company’s board of directors or the compensation committee of the board of directors. The shares of common stock underlying any awards that are forfeited, canceled, repurchased or are otherwise terminated by the Company under the 2016 Plan and the 2008 Plan will be added back to the shares of common stock available for issuance under the 2016 Plan. As of September 30, 2017, the total number of shares reserved under the 2016 Plan and 2008 Plan was 3,577,930 and the Company had 631,077 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 least of (i) 138,757 shares of common stock, (ii) 1% of the Company’s shares of common stock outstanding on the immediately preceding December 31 and (iii) an amount determined by the Company’s board of directors or the compensation committee of the board of directors. During the nine months ended September 30, 2017, 10,829 shares of common stock were issued pursuant to the 2016 ESPP. As of September 30, 2017, the total number of shares reserved under the 2016 ESPP was 266,685 shares. The Company recognized less than $0.1 million of stock-based compensation during each of the three and nine month periods ended September 30, 2017. Stock Option Grants and Shares to Non-employees Prior to 2013, the Company issued options to purchase 203,964 shares of common stock to non-employees, primarily members of the Company’s scientific advisory board, that vest upon the achievement of specified development and clinical milestones. As of September 30, 2017, options for the purchase of 83,250 shares held by non-employees remained unvested, pending achievement of the specified milestones, and had an aggregate fair value of $0.1 million. Stock-Based Compensation Stock-based compensation expense, including shares issued to a non-employee for consulting services, was classified in the statements of operations as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Research and development $ 412 $ 119 $ 1,070 $ 351 General and administrative 636 451 1,713 1,024 $ 1,048 $ 570 $ 2,783 $ 1,375

Collaboration Agreement

Collaboration Agreement	9 Months Ended
	Sep. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaboration Agreement	8. Collaboration Agreement Astellas Under the Collaborative Research, Development, Commercialization and License Agreement (as further amended, the “Astellas Agreement”) with Astellas Pharma Inc. (“Astellas”), entered into in November 2014, the Company recognized revenue of $1.6 million and $1.0 million for the three months ended September 30, 2017 and 2016, respectively, and $3.7 million and $2.2 million for the nine months ended September 30, 2017 and 2016, respectively. The Company recognizes revenue from all upfront payments, research funding payments, non-substantive milestone payments and reimbursements of third-party costs under this arrangement, together as a single unit, over the services being recognized at the time any non-substantive milestone payment or other consideration is earned. Amounts recorded as deferred revenue under the Astellas Agreement totaled $1.8 million and $3.0 million as of September 30, 2017 and December 31, 2016, respectively. Biogen Under the now-terminated collaboration agreement between the Company and Biogen New Ventures, formerly Biogen Idec New Ventures Inc. (“Biogen”), the Company recognized revenue of $0.7 million and $2.1 million for the three and nine months ended September 30, 2016. The Company did not have any deferred revenue under the Biogen agreement as of September 30, 2017 and December 31, 2016 and will not recognize any additional revenue under this terminated agreement in the future.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2017
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, 2017 and 2016 due to the conclusion that a full valuation allowance is required against the Company’s deferred tax assets.

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2017
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): Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Numerator: Net loss $ (13,954 ) $ (10,771 ) $ (46,008 ) $ (27,815 ) Accruing dividends on preferred stock — — — (1,378 ) Net loss attributable to common stockholders-basic and diluted $ (13,954 ) $ (10,771 ) $ (46,008 ) $ (29,193 ) Denominator: Weighted average number of common shares outstanding—basic 25,093,344 20,073,685 25,051,536 16,672,368 Net loss per share attributable to common stockholders—basic and diluted $ (0.56 ) $ (0.54 ) $ (1.84 ) $ (1.75 ) The Company’s potential dilutive securities, which include stock options and a warrant to purchase common stock, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company has reported a net loss for all periods presented. Therefore, diluted net loss per common share is the same as basic net loss per common share. The following potentially dilutive securities outstanding, prior to the use of the treasury stock method or if-converted method, 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, 2017 2016 Options to purchase common stock 2,946,853 1,731,823 Warrant for the purchase of convertible preferred stock (as converted to common stock) — 14,800 2,946,853 1,746,623 The warrant for the purchase of common stock was exercised in February 2016. The Company issued 4,349 shares in a net issuance transaction.

Build-to-Suit Lease

Build-to-Suit Lease	9 Months Ended
	Sep. 30, 2017
Leases [Abstract]
Build-to-Suit Lease	11. Build-to-Suit Lease On September 19, 2017, the Company entered into a lease agreement for its new headquarters, consisting of approximately 30,000 square feet of laboratory and office space located in Brighton, Massachusetts. The lease commencement date will be on the earlier of (i) the first date on which tenant commenced occupancy for the conduct of business in the premises, or (ii) April 15, 2018, subject to extension for certain delays. The lease will extend for a lease term from the commencement date and then for ten years starting with the first day of the month following the month in which the commencement date falls, unless terminated earlier. The Company is entitled to one seven-year option to extend. Annual rent under the lease, exclusive of operating expenses and real estate taxes, will be approximately $1.7 million in the first year, with annual increases of 2.75% each year thereafter. Total expected rental payments through the initial lease term are approximately $18.8 million. The Company will be entitled to an improvement allowance of approximately $4.8 million for certain permitted costs related to the design and construction of Company improvements to the premises. The lease contains customary provisions allowing the landlord to terminate the lease if the Company fails to remedy a breach of any of its obligations within specified time periods, or upon bankruptcy or insolvency of the Company The Company is not the legal owner of the leased space. However, in accordance with ASC 840, Leases Additionally, construction costs incurred as part of the build-out and tenant improvements will also be capitalized within property, plant and equipment, net. Rental payments made under the lease will be allocated to interest expense and the build-to-suit facility lease obligation, based on the implicit rate of the build-to-suit facility lease obligation. The build-to-suit facility lease obligation was approximately $12.1 million as of September 30, 2017 .

Summary of Significant Accoun18

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Unaudited Interim Financial Information	Unaudited Interim Financial Information The condensed balance sheet at December 31, 2016 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, 2017 and for the three and nine months ended September 30, 2017 are unaudited. The accompanying unaudited interim financial statements 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. However, 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, 2016 included in the Company’s Annual Report on Form 10-K filed with the SEC on March 30, 2017. In the opinion of management, all adjustments, consisting only of normal recurring adjustments as necessary, for the fair statement of the Company’s condensed financial position as of September 30, 2017 and condensed results of its operations and cash flows for the three and nine months ended September 30, 2017 have been made. The results of operations for the three and nine months ended September 30, 2017 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2017.
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.
Restricted Cash	Restricted Cash As of September 30, 2017 and December 31, 2016, restricted cash consisted of a certificate of deposit in the amount of $0.3 million collateralizing a letter of credit issued as a security deposit in connection with the Company’s lease of its corporate facilities at 200 Technology Square, Cambridge, MA. On September 19, 2017, the Company entered into a lease agreement for a new corporate headquarters at 80 Guest Street, Brighton, MA (see Note 11). As of September 30, 2017, restricted cash, net of current portion, consisted of a money market account in the amount of $1.7 million collateralizing a letter of credit issued as a security deposit for this lease agreement.
Cash Equivalents	Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less at the date of purchase to be cash equivalents. As of September 30, 2017 and December 31, 2016, the Company’s cash equivalents consisted of money market funds.
Short-Term Investments	Short-term Investments Short-term investments represent holdings of available-for-sale marketable securities in accordance with the Company’s investment policy and cash management strategy. Short-term investments mature within one-year from the balance sheet date. Investments in marketable securities are recorded at fair value, with any unrealized gains and losses, net of taxes, reported as a component of stockholders’ equity until realized or until a determination is made that an other-than-temporary decline in market value has occurred. The cost of marketable securities sold is determined based on the specific identification method and any realized gains or losses on the sale of investments are reflected as a component of other expense, net.
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s derivative liability, short-term investments and cash equivalents are carried at fair value determined according to the fair value hierarchy described above (see Note 4). The carrying value of cash and cash equivalents, accounts receivable, accounts payable and accrued expenses approximate their fair value due to the short-term nature of these assets and liabilities.
Net Loss per Share	Net Loss per Share In February 2016, upon the closing of the IPO, all of the outstanding shares of the Company’s redeemable convertible preferred stock automatically converted into 9,699,600 shares of the Company’s common stock. Prior to this conversion, the Company followed the two-class method when computing net loss per share as the Company had issued shares that met the definition of participating securities. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. The Company’s redeemable convertible preferred stock contractually entitled the holders of such shares to participate in dividends, but did not contractually require the holders of such shares to participate in losses of the Company. Accordingly, the two-class method did not apply for periods in which the Company reported a net loss or a net loss attributable to common stockholders resulting from dividends or accretion related to its redeemable convertible preferred stock. Basic net income (loss) per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of common shares, including potential dilutive common shares assuming the dilutive effect of outstanding stock options and unvested restricted common shares, as determined using the treasury stock method. For periods in which the Company has reported net losses, diluted net loss per common share attributable to common stockholders is the same as basic net loss per common share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. For any period in which the Company has reported net income, basic net income per common share attributable to common stockholders is adjusted for certain amounts to calculate diluted net loss per common share attributable to common stockholders, since dilutive common shares are assumed to have been issued if their effect is dilutive and not to have been issued if their effect is anti-dilutive.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes existing revenue recognition guidance under GAAP. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The standard defines a five-step process to achieve this principle, and will require companies to use more judgment and make more estimates than under the current guidance. The Company expects that these judgments and estimates will include identifying performance obligations in the customer contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date , which delays the effective date of ASU 2014-09 such that the standard is effective for public entities for annual periods beginning after December 15, 2017 and for interim periods within those fiscal years. Early adoption of the standard is permitted for annual periods beginning after December 15, 2016. In April 2016, the FASB issued ASU 2016-10, Identifying Performance Obligations and Licensing . The new standard clarifies two aspects of ASU 2014-09, Revenue from Contracts with Customers (Topic 606) : identifying performance obligations and the licensing implementation guidance. These new standards will become effective for the Company on January 1, 2018. The Company only has one contract (see Note 8) within the scope of Topic 606 and intends to use the modified retrospective method for adoption. Since the inception of the contract the Company has recognized $8.1 million in revenue. The Company is still assessing the impact that the adoption of these new standards will have on its financial statements and related disclosures. In February 2016, the FASB issued ASU No. 2016-02, Leases . In March 2016, the FASB issued ASU No. 2016-09, Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting . As a result, the Company has made an accounting policy election to account for forfeitures as they occur. The adoption of ASU 2016‑09 also requires excess tax benefits and tax deficiencies be recorded in the income statement as opposed to additional paid‑in capital when the awards vest or are settled. This had no material impact on the condensed financial statements as of and for the three and nine months ended September 30, 2017 In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments . In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (Topic 719): Scope of Modification Accounting
Derivative Liability	Derivative Liability The derivative liability relates to a cash settlement option associated with the change of control provision in our 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, 2017 and December 31, 2016, the Company determined the per share common stock price available based on the closing price of its common stock on the NASDAQ Global Market as of September 29, 2017 and December 30, 2016, respectively. The Company determined the expected term of the instrument to be 2.25 years and 2.50 years as of September 30, 2017 and December 31, 2016, respectively. The Company estimated its expected stock volatility to be 80.0% and 81.1% as of September 30, 2017 and December 31, 2016, respectively, based on the historical volatility of publicly traded peer companies for terms matching the expected term of the instrument for each respective period. The risk-free interest rate was determined to be 1.51% and 1.33% as of September 30, 2017 and December 31, 2016, 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, 2016 $ 91 Change in fair value (88 ) Fair value at September 30, 2017 $ 3

Short Term Investments (Tables)

Short Term Investments (Tables)	9 Months Ended
	Sep. 30, 2017
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, 2017 and December 31, 2016 (in thousands): September 30, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S government-sponsored enterprise securities $ 12,519 $ — $ (3 ) $ 12,516 U.S. treasury securities 12,522 — (4 ) 12,518 $ 25,041 $ — $ (7 ) $ 25,034 December 31, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S government-sponsored enterprise securities $ 53,384 $ — $ (20 ) $ 53,364 U.S. treasury securities 13,535 — (2 ) 13,533 $ 66,919 $ — $ (22 ) $ 66,897

Fair Value of Financial Asset20

Fair Value of Financial Assets and Liabilities (Tables)	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (in thousands): Fair Value Measurements as of September 30, 2017 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 14,370 $ — $ — $ 14,370 Short-term investments: U.S. government-sponsored enterprise securities — 12,516 — 12,516 U.S. treasury securities — 12,518 — 12,518 $ 14,370 $ 25,034 $ — $ 39,404 Liabilities: Derivative liability $ — $ — $ 3 $ 3 $ — $ — $ 3 $ 3 Fair Value Measurements as of December 31, 2016 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 15,440 $ — $ — $ 15,440 Short-term investments: U.S. government-sponsored enterprise securities — 53,365 — 53,365 U.S. treasury securities — 13,532 — 13,532 $ 15,440 $ 66,897 $ — $ 82,337 Liabilities: Derivative liability $ — $ — $ 91 $ 91 $ — $ — $ 91 $ 91
Summary of Changes in Values of Derivative Liability	Changes in the values of the derivative liability are summarized below (in thousands): Derivative Liability Fair value at December 31, 2016 $ 91 Change in fair value (88 ) Fair value at September 30, 2017 $ 3

Prepaids and Other Current As21

Prepaids and Other Current Assets (Tables)	9 Months Ended
	Sep. 30, 2017
Prepaid Expense And Other Assets Current [Abstract]
Schedule of Prepaids and Other Current Assets	Prepaids and other current assets consisted of the following (in thousands): September 30, December 31, 2017 2016 Prepaid clinical, manufacturing and scientific expenses $ 1,197 $ 1,390 Prepaid insurance expenses 218 104 Other prepaid expenses and other current assets 423 2,565 $ 1,838 $ 4,059

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2017
Text Block [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following (in thousands): September 30, December 31, 2017 2016 Accrued payroll and related expenses $ 1,293 $ 1,813 Accrued research and development expenses 4,086 1,612 Accrued professional fees 399 383 Accrued other 77 520 $ 5,855 $ 4,328

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock-Based Compensation Expense, Including Shares Issued to Non-Employee for Consulting Services	Stock-based compensation expense, including shares issued to a non-employee for consulting services, was classified in the statements of operations as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Research and development $ 412 $ 119 $ 1,070 $ 351 General and administrative 636 451 1,713 1,024 $ 1,048 $ 570 $ 2,783 $ 1,375

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Numerator: Net loss $ (13,954 ) $ (10,771 ) $ (46,008 ) $ (27,815 ) Accruing dividends on preferred stock — — — (1,378 ) Net loss attributable to common stockholders-basic and diluted $ (13,954 ) $ (10,771 ) $ (46,008 ) $ (29,193 ) Denominator: Weighted average number of common shares outstanding—basic 25,093,344 20,073,685 25,051,536 16,672,368 Net loss per share attributable to common stockholders—basic and diluted $ (0.56 ) $ (0.54 ) $ (1.84 ) $ (1.75 )
Schedule of Antidilutive Securities Excluded from Computation of Diluted Weighted-average Shares Outstanding	The following potentially dilutive securities outstanding, prior to the use of the treasury stock method or if-converted method, 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, 2017 2016 Options to purchase common stock 2,946,853 1,731,823 Warrant for the purchase of convertible preferred stock (as converted to common stock) — 14,800 2,946,853 1,746,623

Nature of the Business - Additi

Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ (203,458)		$ (203,458)		$ (157,450)
Cash, cash equivalents and short-term investments	43,200		43,200
Net loss	$ (13,954)	$ (10,771)	(46,008)	$ (27,815)
Cash in operations			$ 40,557	$ 27,820

Summary of Significant Accoun26

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 29, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Significant Accounting Policies [Line Items]
Restricted cash		$ 294		$ 294
Restricted cash, net of current portion		1,656		1,656		$ 294
Number of common shares issued on conversion of convertible preferred stock	9,699,600
Revenue recognized		1,551	$ 1,715	3,719	$ 4,324
ASU 2014-09 [Member]
Significant Accounting Policies [Line Items]
Revenue recognized				8,100
Certificate of Deposit [Member]
Significant Accounting Policies [Line Items]
Restricted cash		300		300		$ 300
Money Market Funds [Member]
Significant Accounting Policies [Line Items]
Restricted cash, net of current portion		$ 1,700		$ 1,700

Short Term Investments - Summar

Short Term Investments - Summary of Short Term Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 25,041	$ 66,919
Gross Unrealized Losses	(7)	(22)
Fair Value	25,034	66,897
U.S Government-Sponsored Enterprise Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,519	53,384
Gross Unrealized Losses	(3)	(20)
Fair Value	12,516	53,364
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,522	13,535
Gross Unrealized Losses	(4)	(2)
Fair Value	$ 12,518	$ 13,533

Short Term Investments - Additi

Short Term Investments - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
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 Asset29

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, 2017	Dec. 31, 2016
Liabilities:
Derivative liability	$ 3	$ 91
Fair Value, Measurements, Recurring [Member]
Assets:
Total assets	39,404	82,337
Liabilities:
Derivative liability	3	91
Total liabilities	3	91
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	14,370	15,440
Fair Value, Measurements, Recurring [Member] | U.S Government-Sponsored Enterprise Securities [Member]
Assets:
Short-term investments	12,516	53,365
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Assets:
Short-term investments	12,518	13,532
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Total assets	14,370	15,440
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	14,370	15,440
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Total assets	25,034	66,897
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S Government-Sponsored Enterprise Securities [Member]
Assets:
Short-term investments	12,516	53,365
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury Securities [Member]
Assets:
Short-term investments	12,518	13,532
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Liabilities:
Derivative liability	3	91
Total liabilities	$ 3	$ 91

Fair Value of Financial Asset30

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Transfer of assets from level 1 to level 2	$ 0	$ 0
Transfer of assets from level 2 to level 1	0	0
Transfer of liabilities from level 1 to level 2	0	0
Transfer of liabilities from level 2 to level 1	$ 0	$ 0
Fair value determination model	Monte-Carlo simulation analysis
Expected term	2 years 3 months	2 years 6 months
Expected volatility	80.00%	81.10%
Risk-free interest rate	1.51%	1.33%

Fair Value of Financial Asset31

Fair Value of Financial Assets and Liabilities - Summary of Changes in Values of Derivative Liability (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2017USD ($)
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]
Derivative Liability, Fair value, Beginning Balance	$ 91
Derivative Liability, Change in fair value	(88)
Derivative Liability, Fair value, Ending Balance	$ 3

Prepaids and Other Current As32

Prepaids and Other Current Assets - Schedule of Prepaids and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Prepaid Expense And Other Assets Current [Abstract]
Prepaid clinical, manufacturing and scientific expenses	$ 1,197	$ 1,390
Prepaid insurance expenses	218	104
Other prepaid expenses and other current assets	423	2,565
Prepaids and other current assets	$ 1,838	$ 4,059

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued payroll and related expenses	$ 1,293	$ 1,813
Accrued research and development expenses	4,086	1,612
Accrued professional fees	399	383
Accrued other	77	520
Total accrued expenses	$ 5,855	$ 4,328

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	Feb. 03, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation		$ 1,048,000	$ 570,000	$ 2,783,000	$ 1,375,000
Non-Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options unvested		83,250		83,250
Fair value of option				$ 100,000
Non-Employees [Member] | Prior to 2013 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option issued to purchase common stock				203,964
2016 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance	1,581,839	631,077		631,077
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		3,577,930		3,577,930
2016 ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance	138,757	266,685		266,685
Common stock reserved for issuance, percentage of number of shares of common stock outstanding	1.00%
Common stock available for issuance		138,757		138,757
Common stock, shares issued				10,829
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 Non-Employee for Consulting Services (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 1,048	$ 570	$ 2,783	$ 1,375
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	412	119	1,070	351
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 636	$ 451	$ 1,713	$ 1,024

Collaboration Agreement - Addit

Collaboration Agreement - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue		$ 1,551,000	$ 1,715,000	$ 3,719,000	$ 4,324,000
Astellas Collaboration Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue		1,600,000	1,000,000	3,700,000	2,200,000
Initial research term	3 years 6 months
Deferred revenue		1,800,000		1,800,000		$ 3,000,000
Biogen License Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue			$ 700,000		$ 2,100,000
Deferred revenue		$ 0		$ 0		$ 0

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net loss	$ (13,954)	$ (10,771)	$ (46,008)	$ (27,815)
Accruing dividends on preferred stock				(1,378)
Net loss attributable to common stockholders-basic and diluted	$ (13,954)	$ (10,771)	$ (46,008)	$ (29,193)
Denominator:
Weighted average number of common shares outstanding—basic	25,093,344	20,073,685	25,051,536	16,672,368
Net loss per share attributable to common stockholders—basic and diluted	$ (0.56)	$ (0.54)	$ (1.84)	$ (1.75)

Net Loss per Share - Schedule39

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Computation of Diluted Weighted-average Shares Outstanding (Detail) - shares	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted weighted-average shares outstanding	2,946,853	1,746,623
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted weighted-average shares outstanding	2,946,853	1,731,823
Warrants to Purchase Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted weighted-average shares outstanding		14,800

Net Loss per Share - Additional

Net Loss per Share - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2017shares
Earnings Per Share [Abstract]
Shares issued in net issuance transaction	4,349

Build-to-Suit Lease - Additiona

Build-to-Suit Lease - Additional Information (Detail) $ in Millions	Sep. 19, 2017USD ($)ft²	Sep. 30, 2017USD ($)
Lessee Lease Description [Line Items]
Build-to-suit asset capitalized		$ 12.1
Build-to-suit facility lease obligation		$ 12.1
Ice Box, LLC [Member]
Lessee Lease Description [Line Items]
Operating lease, area of property leased | ft²	30,000
Operating lease, term	10 years
Operating lease, annual rent	$ 1.7
Operating lease, renewal term	7 years
Operating lease term description		The lease will extend for a lease term from the commencement date and then for ten years starting with the first day of the month following the month in which the commencement date falls, unless terminated earlier. The Company is entitled to one seven-year option to extend.
Annual increase lease payment	2.75%
Total expected rental payments through expiration of lease	$ 18.8
Improvement allowance receivable	$ 4.8