Rein Therapeutics (RNTX) 10-Q 7 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 01, 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	ALRN
Entity Registrant Name	AILERON THERAPEUTICS INC
Entity Central Index Key	1,420,565
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		14,748,475

Condensed Balance Sheets

Condensed Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 7,716	$ 11,863
Investments	20,229	38,889
Prepaid expenses and other current assets	2,083	1,000
Restricted cash	25	88
Total current assets	30,053	51,840
Property and equipment, net	6,832	154
Restricted cash, non-current	568
Other assets	680	694
Total assets	38,133	52,688
Current liabilities:
Accounts payable	2,994	1,600
Accrued expenses and other current liabilities	4,111	3,291
Total current liabilities	7,105	4,891
Construction financing liability	4,997
Total liabilities	12,102	4,891
Commitments and contingencies (Note 9)
Stockholders’ equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized at September 30, 2018 and December 31, 2017; no shares issued and outstanding at September 30, 2018 and December 31, 2017
Common stock, $0.001 par value; 150,000,000 shares authorized at September 30, 2018 and December 31, 2017; 14,737,402 and 14,723,818 shares issued and outstanding at September 30, 2018 and December 31, 2017, respectively	15	15
Additional paid-in capital	187,478	184,761
Accumulated other comprehensive loss	(3)	(33)
Accumulated deficit	(161,459)	(136,946)
Total stockholders’ equity	26,031	47,797
Total liabilities and stockholders’ equity	$ 38,133	$ 52,688

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	150,000,000	150,000,000
Common stock, shares issued	14,737,402	14,723,818
Common stock, shares outstanding	14,737,402	14,723,818

Condensed Statements of Operati

Condensed Statements of Operations and Comprehensive Loss - 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	$ 0	$ 0	$ 0	$ 0
Operating expenses:
Research and development	4,321	3,825	14,487	9,928
General and administrative	3,177	2,601	10,433	6,039
Total operating expenses	7,498	6,426	24,920	15,967
Loss from operations	(7,498)	(6,426)	(24,920)	(15,967)
Interest income, net	64	167	407	228
Net loss	(7,434)	(6,259)	(24,513)	(15,739)
Accretion of redeemable convertible preferred stock to redemption value				(41)
Net loss attributable to common stockholders	$ (7,434)	$ (6,259)	$ (24,513)	$ (15,780)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.50)	$ (0.45)	$ (1.66)	$ (3.16)
Weighted average common shares outstanding—basic and diluted	14,737,402	13,939,950	14,735,660	4,990,535
Comprehensive loss:
Net loss	$ (7,434)	$ (6,259)	$ (24,513)	$ (15,739)
Other comprehensive gain (loss):
Unrealized gain (loss) on investments, net of tax of $0	9	(6)	30	(6)
Total other comprehensive gain (loss)	9	(6)	30	(6)
Total comprehensive loss	$ (7,425)	$ (6,265)	$ (24,483)	$ (15,745)

Condensed Statements of Opera_2

Condensed Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Unrealized gain (loss) on investments, tax	$ 0	$ 0	$ 0	$ 0

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	$ (24,513)	$ (15,739)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,689	1,084
Depreciation and amortization expense	169	85
Net amortization of premiums and discounts on investments	(184)	(88)
Change in deferred rent	(11)	(11)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(315)	(909)
Other assets	15	33
Accounts payable	(44)	(233)
Accrued expenses and other current liabilities	931	1,085
Net cash used in operating activities	(21,263)	(14,693)
Cash flows from investing activities:
Purchases of property and equipment	(2,491)	(74)
Purchases of investments	(26,231)	(52,130)
Proceeds from sales or maturities of investments	45,105	8,775
Net cash provided by (used in) investing activities	16,383	(43,429)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		471
Proceeds from exercise of stock options	28	100
Increase in construction financing liability	1,210
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts		52,313
Payments of initial public offering costs		(2,049)
Net cash provided by financing activities	1,238	50,835
Net decrease in cash, cash equivalents and restricted cash	(3,642)	(7,287)
Cash, cash equivalents and restricted cash at beginning of period	11,951	20,715
Cash, cash equivalents and restricted cash at end of period	8,309	13,428
Supplemental disclosure of non-cash financing activities:
Accretion of redeemable convertible preferred stock to redemption value		41
Deferred offering costs included in accounts payable and accrued expenses		255
Capitalization of construction-in-progress related to facility lease obligation	6,616
Fixed asset addition included in accounts payable and accrued expenses	$ 1,437
Conversion of convertible preferred stock into common stock upon listing of the Company's common stock on the Nasdaq		$ 130,412

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Aileron Therapeutics, Inc. (“Aileron” or the “Company”) is a clinical-stage biopharmaceutical company that is focused on developing and commercializing a novel class of cell-permeating alpha-helical peptides to address intracellular targets in oncology and other therapeutic areas. The Company’s lead product candidate, ALRN-6924, targets the tumor suppressor p53 for the treatment of a wide variety of cancers. ALRN-6924 is a cell-permeating peptide that disrupts the interaction of the two primary p53 suppressor proteins, MDMX and MDM2, with p53, thereby reactivating tumor suppression in p53 wild-type cancers. ALRN-6924 was in multiple clinical trials as of September 30, 2018. The Company is subject to risks common to companies in the biotechnology industry, including but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. 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 infrastructure, and extensive compliance-reporting capabilities. The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary governmental regulatory approval or that any approved products will be commercially viable. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its key employees and consultants. The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Reverse Stock Split On June 16, 2017, in connection with its initial public offering of its common stock (“IPO”), the Company effected a one-for-9.937 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s redeemable convertible preferred stock (see Note 6). Accordingly, all common share and per share amounts for all periods presented in the accompanying financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and the associated adjustment of the preferred stock conversion ratios. Initial Public Offering On June 28, 2017, the Company’s registration statement on Form S-1 relating to its IPO was declared effective by the Securities and Exchange Commission (“SEC”). In the IPO, which closed on July 5, 2017, the Company issued and sold 3,750,000 shares of common stock at a public offering price of $15.00 per share for net proceeds of $50,009 after deducting underwriting discounts and commissions of $3,937 and offering expenses of $2,304. Upon the closing of the IPO, all 106,114,520 shares of redeemable convertible preferred stock then outstanding converted into an aggregate of 10,509,774 common shares. Liquidity In accordance with Accounting Standards Update (“ASU”) No. 2014-15, Disclosures of Uncertainties about an Entity’s Ability to Continue as a Going Concern The Company’s interim financial statements have been prepared on a going concern basis, which contemplates the continuity of operations, realization of assets and the satisfaction of liabilities in the ordinary course of business. Through September 30, 2018, the Company has funded its operations with net proceeds of $50,009 from its IPO, $131,211 from sales of preferred stock and $34,910 from a collaboration agreement. As of September 30, 2018, the Company had cash, cash equivalents and investments of $27,945. The Company has incurred losses and negative cash flows from operations and had an accumulated deficit of $161,459 as of September 30, 2018. The Company expects to continue to generate losses for the foreseeable future. The Company believes that, based on its current operating plan, its cash, cash equivalents and investments of $27,945 as of September 30, 2018, will enable it to fund its operating expenses and capital expenditure requirements into the third quarter of 2019. Accordingly, there is substantial doubt about the Company’s ability to continue as a going concern as the Company does not believe that its cash, cash equivalents and investments will be sufficient to fund operations for at least twelve months from the date of issuance of these interim financial statements. The Company plans to address this condition by raising additional capital to finance its operations, however, these plans cannot be considered probable, as defined in accounting standards ASU No. 2014-15 (subtopic 205-40). Although the Company has been successful in raising capital in the past, there is no assurance that it will be successful in obtaining such additional financing and therefore it is not considered probable that the Company’s plans to raise additional capital will alleviate the substantial doubt regarding its ability to continue as a going concern. To execute its business plans, the Company will need substantial funding to support its continuing operations and pursue its growth strategy. Until such time as the Company can generate significant revenue from product sales, if ever, it expects to finance its operations through the sale of common stock in public offering and/or private placements, through debt financings or from other capital sources, including collaborations with other companies or other strategic transactions. The Company may not be able to obtain financing on acceptable terms or at all. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion plans or commercialization efforts, which could adversely affect its business prospects.

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 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, the accrual of research and development expenses and the valuation of common stock and stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates. Unaudited Interim Financial Information The accompanying unaudited condensed financial statements as of September 30, 2018 and for the nine months ended September 30, 2018 and 2017 have been prepared by the Company, pursuant to the rules and regulations of 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 financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017 that was filed with the SEC on April 2, 2018. The unaudited interim condensed financial statements have been prepared on the same basis as the audited annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of September 30, 2018, the results of its operations for the three and nine months ended September 30, 2018 and 2017 and its cash flows for the nine months ended September 30, 2018 and 2017. The financial data and other information disclosed in these notes related to the three and nine months ended September 30, 2018 and 2017 are unaudited. The results for the nine months ended September 30, 2018 are not necessarily indicative of results to be expected for the year ending December 31, 2018, any other interim periods, or any future year or period. The accompanying balance sheet as of December 31, 2017 has been derived from the Company’s audited financial statements for the year ended December 31, 2017 previously filed with the SEC. Cash Equivalents The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents. Cash equivalents, which consist of money market accounts, corporate notes and commercial paper, are stated at fair value. Restricted Cash As of September 30, 2018, current restricted cash of $25 consisted entirely of cash deposited in a separate restricted bank account as a security deposit for the Company’s corporate credit cards. As of December 31, 2017, current restricted cash consisted of $25 of cash deposited in a separate restricted bank account as a security deposit for the Company’s corporate credit cards and $63 of cash deposited in a separate restricted bank account as a security deposit for the lease of the Company’s facility. As of September 30, 2018, non-current restricted cash consisted of $568 of cash deposited in a separate restricted bank account as a security deposit for Investments The Company classifies its available-for-sale investments as current assets on the balance sheet if they mature within one year from the balance sheet date. The Company classifies all of its investments as available-for-sale securities. The Company’s investments are measured and reported at fair value using quoted prices in active markets for similar securities or using other inputs that are observable or can be corroborated by observable market data. Unrealized gains and losses on available-for-sale securities are reported as accumulated other comprehensive income (loss), which is a separate component of stockholders’ equity (deficit). The cost of securities sold is determined on a specific identification basis, and realized gains and losses are included in other income (expense) within the statements of operations and comprehensive loss. The Company evaluates its investments with unrealized losses for other-than-temporary impairment. When assessing investments for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary”, the Company reduces the investment to fair value through a charge to the statements of operations and comprehensive loss. No such adjustments were necessary during the periods presented. 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 cash equivalents and investments are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair value due to the short-term nature of these liabilities. Net Income (Loss) per Share The Company follows the two-class method when computing net income (loss) per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net income (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. Basic net income (loss) per share attributable to common stockholders is computed by dividing the net income (loss) attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) attributable to common stockholders is computed by adjusting income (loss) per share attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares. For purpose of this calculation, outstanding options to purchase common stock and shares of redeemable convertible preferred stock are considered potential dilutive common shares. The Company’s redeemable convertible preferred stock contractually entitled the holders of such shares to participate in dividends but contractually did not require the holders of such stock to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. Recently Adopted Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (the “FASB”) issued ASU No. 2014-09, Revenue from Contracts with Customers Revenue from Contracts with Customers , Revenue from Contract with Customers: Principal versus Agent Considerations Revenue from Contracts with Customers: Identifying Performance Obligations and Licensing Revenue from Contracts with Customers: Narrow-Scope Improvements and Practical Expedients. The adoption of these standards did not have an impact on the Company’s financial position, results of operations or cash flows as the Company does not currently have any revenue generating arrangements. In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments Compensation – Stock Compensation Scope of Modification Accounting Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases Leases In June 2018, the FASB issued ASU No. 2018-07, Compensation — Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting Compensation — Stock Compensation Equity — Equity-Based Payments to Nonemployees Revenue from Contracts with Customers

Fair Value of Financial Assets

Fair Value of Financial Assets	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets	3. Fair Value of Financial Assets The following tables present information about the Company’s assets that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: Fair Value Measurements as of September 30, 2018 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 5,725 $ — $ — $ 5,725 Investments: Corporate notes — 8,089 — 8,089 Commercial paper — 12,140 — 12,140 $ 5,725 $ 20,229 $ — $ 25,954 Fair Value Measurements as of December 31, 2017 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 10,509 $ — $ — $ 10,509 Investments: Corporate notes — 25,710 — 25,710 Commercial paper — 13,179 — 13,179 $ 10,509 $ 38,889 $ — $ 49,398 As of September 30, 2018 and December 31, 2017, the Company’s cash equivalents and investments were invested in money market funds, corporate notes and commercial paper and were valued based on Level 1 and Level 2 inputs. In determining the fair value of its corporate notes and commercial paper at each date presented above, the Company relied on quoted prices for similar securities in active markets or using other inputs that are observable or can be corroborated by observable market data. The Company’s cash equivalents have original maturities of less than 90 days from the date of purchase. All available-for-sale investments have contractual maturities of less than one year. During the nine months ended September 30, 2018 and the year ended December 31, 2017, there were no transfers between Level 1, Level 2 and Level 3.

Investments

Investments	9 Months Ended
	Sep. 30, 2018
Investments Debt And Equity Securities [Abstract]
Investments	4. Investments As of September 30, 2018 and December 31, 2017, the fair value of available-for-sale investments by type of security was as follows: September 30, 2018 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value Investments: Corporate notes $ 8,091 $ — $ (2 ) $ 8,089 Commercial paper 12,141 — (1 ) 12,140 $ 20,232 $ — $ (3 ) $ 20,229 December 31, 2017 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value Investments: Corporate notes $ 25,733 $ — $ (23 ) $ 25,710 Commercial paper 13,189 — (10 ) 13,179 $ 38,922 $ — $ (33 ) $ 38,889

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2018
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	5. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following: September 30, 2018 December 31, 2017 External research and development services $ 1,678 $ 1,284 Payroll and payroll-related costs 975 1,120 Accrued severance 547 — Professional fees 560 536 Other 351 351 $ 4,111 $ 3,291

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Redeemable Convertible Preferred Stock	6. Redeemable Convertible Preferred Stock Prior to the closing of the Company’s IPO in July 2017, the Company had shares of redeemable convertible preferred stock outstanding, including shares of Series A, Series A-1, Series B, Series C-1 and Series C-2 redeemable convertible preferred stock (collectively, the “Junior Preferred Stock”) and Series D, Series D-1, Series E, Series E-1, Series E-2, Series E-3 and Series F redeemable convertible preferred stock (collectively, the “Senior Preferred Stock” and, together with the Junior Preferred Stock, the “Redeemable Preferred Stock”). The Redeemable Preferred Stock was classified outside of stockholders’ equity (deficit) because the shares contain redemption features that are not solely within the control of the Company. In February 2017, the Company issued 483,501 shares of Series F redeemable convertible preferred stock (the “Series F preferred stock”) at a price of $1.36 per share, resulting in proceeds of $626, net of issuance costs of $32. Pursuant to the amended Series F preferred stock purchase agreement, holders of 4,411,765 shares of Series E-1 preferred stock that participated in the February 2017 closing elected to convert their shares of Series E-1 preferred stock into 4,411,765 shares of Series E-3 preferred stock. The Company determined that the conversion of shares of preferred stock that occurred in February 2017 represented modifications of these securities for accounting purposes; however, the modifications did not result in the recognition of a deemed dividend for accounting purposes because the modifications did not result in a transfer of value from common stockholders to preferred stockholders. Pursuant to the terms of the amended Series F preferred stock purchase agreement, if the second tranche closing did not occur prior to the closing of the Company’s initial public offering of common stock, then, immediately prior to such closing, the purchasers of the Series F preferred stock would be required to purchase a number of shares of the Company’s common stock equal to $11,516 divided by the price per share paid by the public in the initial public offering in a concurrent private offering. This requirement to purchase shares immediately prior to the closing of the Company’s initial public offering could be waived in whole or in part by the Company’s board of directors. On June 15, 2017, the Company’s board of directors waived in whole, effective immediately prior to the closing of the Company’s IPO, the requirement of the purchasers of Series F preferred stock to purchase shares of the Company’s common stock in a concurrent private offering in connection with the Company’s initial public offering. Upon the closing of the Company’s IPO on July 5, 2017, all shares of the Redeemable Preferred Stock converted into an aggregate of 10,509,774 shares of common stock. As of September 30, 2018, there were no shares of Redeemable Preferred Stock authorized, issued or outstanding.

Stock-Based Awards

Stock-Based Awards	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Awards	7. Stock-Based Awards 2017 Stock Incentive Plan The Company’s 2017 Stock Incentive Plan (the “2017 Plan”) was approved by the Company’s stockholders on June 16, 2017 and became effective on June 28, 2017. Under the 2017 Plan, the Company may grant incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, awards of restricted stock units and other stock-based awards. The Company’s employees, officers, directors, consultants and advisors are eligible to receive awards under the 2017 Plan; however, incentive stock options may only be granted to employees. The 2017 Plan is administered by the board of directors or, at the discretion of the board of directors, by a committee of the board. The number of shares of common stock covered by options and the date those options become exercisable, type of options to be granted, exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or its committee if so delegated. Stock options granted under the 2017 Plan with service-based vesting conditions generally vest over four years and may not have a duration in excess of ten years, although options have been granted with vesting terms of less than four years. The total number of shares of common stock that may be issued under the 2017 Plan was 2,442,249 as of September 30, 2018, of which 1,357,008 shares remained available for grant. The Company initially reserved During the nine months ended September 30, 2018, pursuant to the terms of the 2017 Plan, the Company granted options to employees and directors to purchase 555,414 shares of common stock at a weighted average exercise price of $5.34 per share. Shares that are expired, terminated, surrendered or canceled without having been fully exercised will be available for future awards. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for the grant of awards. The exercise price for stock options granted may not be less than the fair market value of the common stock as of the date of grant. 2017 Employee Stock Purchase Plan On June 16, 2017, the Company’s stockholders approved the 2017 Employee Stock Purchase Plan (the “2017 ESPP”), which became effective on June 28, 2017. A total of 150,000 shares of common stock were initially reserved for issuance under this plan. The number of shares of common stock that may be issued under the 2017 ESPP will automatically increase on each January 1, beginning with the fiscal year ending December 31, 2018 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2027, equal to the least of (i) 622,408 shares, (ii) 1% of the outstanding shares of common stock on such date and (iii) an amount determined by the Company’s board of directors. On January 1, 2018, the Company’s board of directors determined not to increase the number of shares of common stock that may be issued under the 2017 ESPP. 2016 Stock Incentive Plan The Company’s 2016 Stock Incentive Plan (the “2016 Plan”) provided for the Company to grant incentive stock options or nonqualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. The 2016 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated. Stock options granted under the 2016 Plan with service-based vesting conditions vest over four years and expire after ten years. In connection with the IPO, the board of directors determined to grant no further awards under the 2016 Plan. No stock options or other awards have been made under the 2016 Plan since the adoption of the 2017 Plan. Shares that are expired, terminated, surrendered or canceled without having been fully exercised will be available for future awards under the 2017 Plan. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for the grant of awards under the 2017 Plan. 2006 Stock Incentive Plan The Company’s 2006 Stock Incentive Plan, as amended, (the “2006 Plan”) provided for the Company to grant incentive stock options or nonqualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. The 2006 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated. Stock options granted under the 2006 Plan with service-based vesting conditions generally vest over four years and expire after ten years, although options have been granted with vesting terms of less than four years. The 2006 Plan expired in 2016. Since its expiration no further awards have been granted under the 2006 Plan. Shares that are expired, terminated, surrendered or canceled without having been fully exercised will be available for future awards under the 2017 Plan. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for the grant of awards under the 2017 Plan. Stock Option Valuation The assumptions that the Company used to determine the grant-date fair value of the stock options granted to employees and directors during the nine months ended September 30, 2018 and 2017 were as follows, presented on a weighted average basis: Nine Months Ended September 30, 2018 Nine Months Ended September 30, 2017 Risk-free interest rate 2.77 % 2.15 % Expected term (in years) 6.2 6.1 Expected volatility 76.0 % 80.6 % Expected dividend yield 0 % 0 % Stock Options The following table summarizes the Company’s stock option activity since January 1, 2018: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding at December 31, 2017 2,096,233 $ 8.02 8.0 $ 7,332 Granted 555,414 5.34 Exercised (13,584 ) 2.03 Forfeited (455,423 ) 9.53 Outstanding at September 30, 2018 2,182,640 $ 7.06 6.4 $ 241 Options exercisable at September 30, 2018 1,187,074 $ 6.23 4.0 $ 241 Options vested and expected to vest at September 30, 2018 2,146,086 $ 7.05 6.4 $ 241 Options exercisable at December 31, 2017 784,190 $ 4.71 6.0 $ 4,667 Options vested and expected to vest at December 31, 2017 2,030,629 $ 7.96 7.9 $ 7,198 The weighted average grant-date fair value of stock options granted during the nine months ended September 30, 2018 and 2017 was $3.65 and $7.51, respectively. The aggregate fair value of stock options that vested during the nine months ended September 30, 2018 and 2017 was $2,633 and $810, respectively. The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the nine months ended September 30, 2018 and 2017 was $93 and $256, respectively. Stock-Based Compensation The Company recorded stock-based compensation expense related to stock options in the following expense categories of its statements of operations and comprehensive loss: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development expenses $ 226 $ 191 $ 736 $ 396 General and administrative expenses 362 425 1,953 688 $ 588 $ 616 $ 2,689 $ 1,084 As of September 30, 2018, the Company had an aggregate of $5,281 of unrecognized stock-based compensation expense, which it expects to recognize over a weighted average period of 2.8 years. In May 2018, the Company modified certain equity awards in connection with a separation agreement with its former Chief Executive Officer. The modification included acceleration of vesting of stock options to purchase 80,822 shares of common stock and an extension of the post-termination exercise period for vested options from 90 days to up to two years. In connection with this modification, the Company recorded an incremental compensation charge of $612 during the nine months ended September 30, 2018.

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Net Loss per Share	8. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows : Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Numerator: Net loss $ (7,434 ) $ (6,259 ) $ (24,513 ) $ (15,739 ) Accretion of redeemable convertible preferred stock to redemption value — — — (41 ) Net loss attributable to common stockholders $ (7,434 ) $ (6,259 ) $ (24,513 ) $ (15,780 ) Denominator: Weighted average common shares outstanding—basic and diluted 14,737,402 13,939,950 14,735,660 4,990,535 Net loss per share attributable to common stockholders—basic and diluted $ (0.50 ) $ (0.45 ) $ (1.66 ) $ (3.16 ) The Company’s potential dilutive securities, which include stock options and redeemable convertible preferred stock, have been excluded from the computation of diluted net loss per share attributable to common stockholders whenever the effect of including them would be to reduce the net loss per share. In periods where there is a net loss, 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 following potential shares of common stock, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three and Nine Months Ended September 30, 2018 2017 Stock options to purchase common stock 2,182,640 2,055,264 2,182,640 2,055,264

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	9. Commitments and Contingencies Operating Lease 490 Arsenal Way On April 4, 2018, a lease agreement (the “Lease”) entered into between the Company and 480 Arsenal Group LLC became effective. The Lease is for approximately 18,768 square feet of office and laboratory space in Watertown, Massachusetts. The Lease has an initial term of eight years and provides the Company with an option to extend the Lease term for one additional five-year period. The future minimum rent commitment for the initial eight-year term is approximately $8,771. In addition to rent, the Lease requires the Company to pay additional amounts for taxes, insurance, maintenance and other operating expenses. The Company was required to provide a $568 security deposit, which the Company provided in the form of a letter of credit in the favor of the landlord which is included as non-current restricted cash on the balance sheet as of September 30, 2018. 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 be capitalized within property, plant and equipment, net. Rental payments made under the Lease will be allocated to imputed ground rent, 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 $4,997 as of September 30, 2018. 281 Albany Street In February 2010, the Company entered into an operating lease agreement for office and laboratory space in Cambridge, Massachusetts, which, as amended, expired in August 2018. Upon entering into the agreement, the Company was required to maintain a security deposit which was recorded as restricted cash on the Company’s balance sheet. The agreement requires no future minimum lease payments for the year ending December 31, 2018. The Company recognizes rent expense on a straight-line basis over the lease period and has recorded deferred rent for rent expense incurred but not yet paid. Rental expense under operating leases totaled $220 and $127 for the three months ended September 30, 2018 and 2017, respectively and $559 and $367 for the nine months ended September 30, 2018 and 2017, respectively. Intellectual Property Licenses Harvard and Dana-Farber Agreement In August 2006, the Company entered into an exclusive license agreement with President and Fellows of Harvard College (“Harvard”) and Dana-Farber Cancer Institute (“DFCI”). The agreement granted the Company an exclusive worldwide license, with the right to sublicense, under specified patents and patent applications to develop, obtain regulatory approval for and commercialize specified product candidates based on cell-permeating peptides. Under the agreement, the Company is obligated to use commercially reasonable efforts to develop and commercialize one or more licensed products and to achieve specified milestone events by specified dates. In connection with entering into the agreement, the Company paid an upfront license fee and issued to Harvard and DFCI shares of its common stock. In February 2010, the agreement was amended and restated (the “Harvard/DFCI agreement”) under which additional patent rights were added to the scope of the license agreement and the annual license maintenance fees were increased. Under the Harvard/DFCI agreement, the Company is obligated to make aggregate milestones payments of up to $7,700 per licensed therapeutic product upon the Company’s achievement of specified clinical, regulatory and sales milestones with respect to such product and up to $700 per licensed diagnostic product upon the Company’s achievement of specified regulatory and sales milestones with respect to such product. In addition, the Company is obligated to pay royalties of low single-digit percentages on annual net sales of licensed products sold by the Company, its affiliates or its sublicensees. The royalties are payable on a product-by-product and country-by-country basis and may be reduced in specified circumstances. In addition, the agreement obligates the Company to pay a percentage, up to the mid-twenties, of fees received by the Company in connection with its sublicense of the licensed products. In accordance with the terms of the agreement, the Company’s sublicense payment obligations may be subject to specified reductions. The Harvard/DFCI agreement requires the Company to pay annual license maintenance fees of $145 each year. Any payments made in connection with the annual license maintenance fees will be credited against any royalties due. The Company As of September 30, 2018, the Company had not developed a commercial product using the licensed technologies and no royalties under the agreement had been paid or were due. Under the Harvard/DFCI agreement, the Company is responsible for all patent expenses related to the prosecution and maintenance of the licensed patents and applications in-licensed under the agreement as well as cost reimbursement of amounts incurred for all documented patent-related expenses. The agreement will expire on a product-by-product and country-by-country basis upon the last to expire of any valid patent claim pertaining to licensed products covered under the agreement. Umicore Agreement In December 2006, the Company entered into a license agreement with Materia, Inc. (“Materia”), under which it was granted a non-exclusive worldwide license, with the right to sublicense, under specified patent and patent applications to utilize Materia’s catalysts to develop, obtain regulatory approval for and commercialize specified peptides owned or controlled by Materia and the right to manufacture specified compositions owned or controlled by Materia. In February 2017, Materia assigned the license agreement (the “Umicore agreement”) to Umicore Precious Metals Chemistry USA, LLC (“Umicore”), and Umicore agreed to continue to supply the Company under the agreement. Under the Umicore agreement, the Company is obligated to make aggregate milestone payments to Umicore of up to $6,400 upon the Company’s achievement of specified clinical, regulatory and sales milestones with respect to each licensed product. In addition, the Company is obligated to pay tiered royalties ranging in the low single-digit percentages on annual net sales of licensed products sold by the Company or its sublicensees. The royalties are payable on a product-by-product and country-by-country basis, and may be reduced in specified circumstances. The Umicore agreement requires the Company to pay annual license fees of $50. The Company did not incur any license fees during the three and nine months ended September 30, 2018 and 2017, respectively. In addition, the Company did not make any milestone payments during the three and nine months ended September 30, 2018 and 2017. During the nine months ended September 30, 2018, no milestones were achieved and no liabilities for additional milestone payments were recorded in the Company’s financial statements. The agreement expires upon the expiration of the Company’s obligation to pay royalties in each territory covered under the agreement. Scripps Agreement In October 2010, the Company entered into a patent license agreement (the “Scripps agreement”) with The Scripps Research Institute (“Scripps”) under which it was granted a license, with the right to sublicense, for the exclusive worldwide rights to utilize Scripps’ “Click” chemistry for therapeutics and non-exclusive worldwide rights for diagnostics with the Company’s stabilized peptide and protein technology platforms. Under the agreement, the Company is obligated to make aggregate milestone payments to Scripps of up to $1,900 for each licensed peptide product and up to $950 for each licensed protein product upon achieving of specified clinical, regulatory and commercial milestones. In addition, the Company is obligated to pay tiered royalties ranging in the low single-digit percentages on annual net sales of licensed products sold by the Company or its sublicensees. The royalties are payable on a product-by-product and country-by-country basis. The Scripps agreement requires the Company to pay annual license fees of $50. The Company did not incur any license fees during the three and nine months ended September 30, 2018 and 2017, respectively. As of September 30, 2018, no milestones had been achieved and no liabilities for milestone payments had been recorded in the Company’s financial statements. As of September 30, 2018, the Company had not developed a commercial product using the licensed technologies and no royalties under the agreement had been paid or were due. The agreement expires upon expiration of the last of any patent rights covered under the agreement. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it had not accrued any liabilities related to such obligations in its financial statements as of September 30, 2018 or December 31, 2017.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	10. Income Taxes The Company did not provide for any income taxes for the three and nine months ended September 30, 2018 and 2017. The Company has evaluated the positive and negative evidence bearing upon the realizability of its U.S. net deferred tax assets. As required by the provisions of ASC 740, Income Taxes , management has determined that it is more-likely-than-not that the Company will not utilize the benefits of federal and state U.S. net deferred tax assets for financial reporting purposes. Accordingly, the net deferred tax assets are subject to a valuation allowance at September 30, 2018 and December 31, 2017.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
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, the accrual of research and development expenses and the valuation of common stock and stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.
Unaudited Interim Financial Information	Unaudited Interim Financial Information The accompanying unaudited condensed financial statements as of September 30, 2018 and for the nine months ended September 30, 2018 and 2017 have been prepared by the Company, pursuant to the rules and regulations of 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 financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017 that was filed with the SEC on April 2, 2018. The unaudited interim condensed financial statements have been prepared on the same basis as the audited annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of September 30, 2018, the results of its operations for the three and nine months ended September 30, 2018 and 2017 and its cash flows for the nine months ended September 30, 2018 and 2017. The financial data and other information disclosed in these notes related to the three and nine months ended September 30, 2018 and 2017 are unaudited. The results for the nine months ended September 30, 2018 are not necessarily indicative of results to be expected for the year ending December 31, 2018, any other interim periods, or any future year or period. The accompanying balance sheet as of December 31, 2017 has been derived from the Company’s audited financial statements for the year ended December 31, 2017 previously filed with the SEC.
Cash Equivalents	Cash Equivalents The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents. Cash equivalents, which consist of money market accounts, corporate notes and commercial paper, are stated at fair value.
Restricted Cash	Restricted Cash As of September 30, 2018, current restricted cash of $25 consisted entirely of cash deposited in a separate restricted bank account as a security deposit for the Company’s corporate credit cards. As of December 31, 2017, current restricted cash consisted of $25 of cash deposited in a separate restricted bank account as a security deposit for the Company’s corporate credit cards and $63 of cash deposited in a separate restricted bank account as a security deposit for the lease of the Company’s facility. As of September 30, 2018, non-current restricted cash consisted of $568 of cash deposited in a separate restricted bank account as a security deposit for
Investments	Investments The Company classifies its available-for-sale investments as current assets on the balance sheet if they mature within one year from the balance sheet date. The Company classifies all of its investments as available-for-sale securities. The Company’s investments are measured and reported at fair value using quoted prices in active markets for similar securities or using other inputs that are observable or can be corroborated by observable market data. Unrealized gains and losses on available-for-sale securities are reported as accumulated other comprehensive income (loss), which is a separate component of stockholders’ equity (deficit). The cost of securities sold is determined on a specific identification basis, and realized gains and losses are included in other income (expense) within the statements of operations and comprehensive loss. The Company evaluates its investments with unrealized losses for other-than-temporary impairment. When assessing investments for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary”, the Company reduces the investment to fair value through a charge to the statements of operations and comprehensive loss. No such adjustments were necessary during the periods presented.
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 cash equivalents and investments are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair value due to the short-term nature of these liabilities.
Net Income (Loss) per Share	Net Income (Loss) per Share The Company follows the two-class method when computing net income (loss) per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net income (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. Basic net income (loss) per share attributable to common stockholders is computed by dividing the net income (loss) attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) attributable to common stockholders is computed by adjusting income (loss) per share attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares. For purpose of this calculation, outstanding options to purchase common stock and shares of redeemable convertible preferred stock are considered potential dilutive common shares. The Company’s redeemable convertible preferred stock contractually entitled the holders of such shares to participate in dividends but contractually did not require the holders of such stock to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (the “FASB”) issued ASU No. 2014-09, Revenue from Contracts with Customers Revenue from Contracts with Customers , Revenue from Contract with Customers: Principal versus Agent Considerations Revenue from Contracts with Customers: Identifying Performance Obligations and Licensing Revenue from Contracts with Customers: Narrow-Scope Improvements and Practical Expedients. The adoption of these standards did not have an impact on the Company’s financial position, results of operations or cash flows as the Company does not currently have any revenue generating arrangements. In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments Compensation – Stock Compensation Scope of Modification Accounting
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases Leases In June 2018, the FASB issued ASU No. 2018-07, Compensation — Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting Compensation — Stock Compensation Equity — Equity-Based Payments to Nonemployees Revenue from Contracts with Customers

Fair Value of Financial Assets

Fair Value of Financial Assets (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Summary of Assets Measured at Fair Value on Recurring Basis	The following tables present information about the Company’s assets that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: Fair Value Measurements as of September 30, 2018 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 5,725 $ — $ — $ 5,725 Investments: Corporate notes — 8,089 — 8,089 Commercial paper — 12,140 — 12,140 $ 5,725 $ 20,229 $ — $ 25,954 Fair Value Measurements as of December 31, 2017 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 10,509 $ — $ — $ 10,509 Investments: Corporate notes — 25,710 — 25,710 Commercial paper — 13,179 — 13,179 $ 10,509 $ 38,889 $ — $ 49,398

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2018
Investments Debt And Equity Securities [Abstract]
Summary of Fair Value of Available for Sale Investments	As of September 30, 2018 and December 31, 2017, the fair value of available-for-sale investments by type of security was as follows: September 30, 2018 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value Investments: Corporate notes $ 8,091 $ — $ (2 ) $ 8,089 Commercial paper 12,141 — (1 ) 12,140 $ 20,232 $ — $ (3 ) $ 20,229 December 31, 2017 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value Investments: Corporate notes $ 25,733 $ — $ (23 ) $ 25,710 Commercial paper 13,189 — (10 ) 13,179 $ 38,922 $ — $ (33 ) $ 38,889

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2018
Payables And Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: September 30, 2018 December 31, 2017 External research and development services $ 1,678 $ 1,284 Payroll and payroll-related costs 975 1,120 Accrued severance 547 — Professional fees 560 536 Other 351 351 $ 4,111 $ 3,291

Stock-Based Awards (Tables)

Stock-Based Awards (Tables)	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Valuation Assumptions of Stock Options Granted	The assumptions that the Company used to determine the grant-date fair value of the stock options granted to employees and directors during the nine months ended September 30, 2018 and 2017 were as follows, presented on a weighted average basis: Nine Months Ended September 30, 2018 Nine Months Ended September 30, 2017 Risk-free interest rate 2.77 % 2.15 % Expected term (in years) 6.2 6.1 Expected volatility 76.0 % 80.6 % Expected dividend yield 0 % 0 %
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity since January 1, 2018: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding at December 31, 2017 2,096,233 $ 8.02 8.0 $ 7,332 Granted 555,414 5.34 Exercised (13,584 ) 2.03 Forfeited (455,423 ) 9.53 Outstanding at September 30, 2018 2,182,640 $ 7.06 6.4 $ 241 Options exercisable at September 30, 2018 1,187,074 $ 6.23 4.0 $ 241 Options vested and expected to vest at September 30, 2018 2,146,086 $ 7.05 6.4 $ 241 Options exercisable at December 31, 2017 784,190 $ 4.71 6.0 $ 4,667 Options vested and expected to vest at December 31, 2017 2,030,629 $ 7.96 7.9 $ 7,198
Summary of Stock Based Compensation Expense	The Company recorded stock-based compensation expense related to stock options in the following expense categories of its statements of operations and comprehensive loss: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development expenses $ 226 $ 191 $ 736 $ 396 General and administrative expenses 362 425 1,953 688 $ 588 $ 616 $ 2,689 $ 1,084

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
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 : Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Numerator: Net loss $ (7,434 ) $ (6,259 ) $ (24,513 ) $ (15,739 ) Accretion of redeemable convertible preferred stock to redemption value — — — (41 ) Net loss attributable to common stockholders $ (7,434 ) $ (6,259 ) $ (24,513 ) $ (15,780 ) Denominator: Weighted average common shares outstanding—basic and diluted 14,737,402 13,939,950 14,735,660 4,990,535 Net loss per share attributable to common stockholders—basic and diluted $ (0.50 ) $ (0.45 ) $ (1.66 ) $ (3.16 )
Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share	The following potential shares of common stock, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three and Nine Months Ended September 30, 2018 2017 Stock options to purchase common stock 2,182,640 2,055,264 2,182,640 2,055,264

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands	Jul. 05, 2017USD ($)$ / sharesshares	Jun. 16, 2017	Sep. 30, 2018USD ($)shares	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)
Class of Stock [Line Items]
Offering expenses				$ 2,049
Redeemable convertible preferred stock, outstanding (in shares) | shares			0
Proceeds from collaboration agreement			$ 34,910
Cash, cash equivalents and investments			27,945
Accumulated deficit			(161,459)		$ (136,946)
Preferred Stock [Member]
Class of Stock [Line Items]
Proceeds from sales of stock			$ 131,211
IPO [Member] | Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Redeemable convertible preferred stock, outstanding (in shares) | shares	106,114,520
Common Stock [Member] | IPO [Member]
Class of Stock [Line Items]
Reverse stock split description			one-for-9.937 reverse stock split
Reverse stock split ratio		0.100634
Stock issued and sold | shares	3,750,000
Share price | $ / shares	$ 15
Net proceeds from issuance of initial public offering	$ 50,009
Underwriting discounts and commissions incurred	3,937
Offering expenses	$ 2,304
Number of common shares issued on conversion of redeemable convertible preferred stock | shares	10,509,774

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Summary Of Significant Accounting Policies [Line Items]
Restricted cash, current	$ 25,000	$ 88,000
Restricted cash, non-current	568,000
Security Deposit Related to Credit Card Accounts [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash, current	25,000	25,000
Security Deposit Related to Lease Facility [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash, current		63,000
Restricted cash, non-current	$ 568,000	$ 0

Fair Value of Financial Asset_2

Fair Value of Financial Assets - Summary of Assets Measured at Fair Value on Recurring Basis (Detail) - Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total assets	$ 25,954	$ 49,398
Corporate Notes [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments	8,089	25,710
Commercial Paper [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments	12,140	13,179
Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total assets	5,725	10,509
Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total assets	20,229	38,889
Level 2 [Member] | Corporate Notes [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments	8,089	25,710
Level 2 [Member] | Commercial Paper [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments	12,140	13,179
Money Market Funds [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash equivalents	5,725	10,509
Money Market Funds [Member] | Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash equivalents	$ 5,725	$ 10,509

Fair Value of Financial Asset_3

Fair Value of Financial Assets - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Available-for-sale investments, maximum contractual maturity period	1 year
Fair value, assets transfers from Level 1 to Level 2	$ 0	$ 0
Fair value, assets transfers from Level 2 to Level 1	0	0
Fair Value, assets transfers into (out of) Level 3	$ 0	$ 0

Investments - Summary of Fair V

Investments - Summary of Fair Value of Available for Sale Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	$ 20,232	$ 38,922
Investments, Gross Unrealized Loss	(3)	(33)
Investments, Fair Value	20,229	38,889
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	8,091	25,733
Investments, Gross Unrealized Loss	(2)	(23)
Investments, Fair Value	8,089	25,710
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	12,141	13,189
Investments, Gross Unrealized Loss	(1)	(10)
Investments, Fair Value	$ 12,140	$ 13,179

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
External research and development services	$ 1,678	$ 1,284
Payroll and payroll-related costs	975	1,120
Accrued severance	547
Professional fees	560	536
Other	351	351
Total accrued expenses and other current liabilities	$ 4,111	$ 3,291

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jul. 05, 2017	Feb. 28, 2017	Sep. 30, 2018	Sep. 30, 2017
Temporary Equity [Line Items]
Preferred stock issuance costs, paid				$ 2,049
Redeemable convertible preferred stock, authorized (in shares)			0
Redeemable convertible preferred stock, issued (in shares)			0
Redeemable convertible preferred stock, outstanding (in shares)			0
IPO [Member]
Temporary Equity [Line Items]
Initial public offering closing date			Jul. 5, 2017
IPO [Member] | Common Stock [Member]
Temporary Equity [Line Items]
Preferred stock shares issued, price per share	$ 15
Preferred stock issuance costs, paid	$ 2,304
Number of common shares issued on conversion of redeemable convertible preferred stock	10,509,774
Series F Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock shares issued		483,501
Preferred stock shares issued, price per share		$ 1.36
Proceeds from issuance of redeemable convertible preferred stock		$ 626
Preferred stock issuance costs, paid		32
Stock purchase agreement, aggregate purchase price		$ 11,516
Series E-1 Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock shares converted		4,411,765
Series E-3 Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock shares issued on conversion		4,411,765

Stock-Based Awards - Additional

Stock-Based Awards - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jun. 16, 2017	May 31, 2018	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, weighted average exercise price per share				$ 5.34
Weighted average grant-date fair value of stock options				$ 3.65	$ 7.51
Aggregate fair value of stock options vested				$ 2,633	$ 810
Aggregate intrinsic value of stock options exercised				93	$ 256
Aggregate unrecognized stock-based compensation expense			$ 5,281	$ 5,281
Unrecognized stock-based compensation expense, weighted average period expects for recognition				2 years 9 months 18 days
Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Acceleration of vesting of stock options to purchase shares of common stock		80,822
Incremental compensation charge			$ 612	$ 612
2017 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period				4 years
Number of shares, available for grant			2,442,249	2,442,249
Number of shares remained available for grant			1,357,008	1,357,008
Stock reserved for future issuance			1,244,816	1,244,816		588,953
Maximum annual increase in common stock reserved for future issuance				1,244,816
Percentage of common stock shares outstanding				4.00%
Options to purchase				555,414
Common stock, weighted average exercise price per share				$ 5.34
2016 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period				4 years
Stock option expiration period				10 years
2016 Stock Incentive Plan [Member] | Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock reserved for future issuance			424,601	424,601
2017 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock reserved for future issuance	150,000					0
Maximum annual increase in common stock reserved for future issuance	622,408
Percentage of common stock shares outstanding	1.00%
Stock incentive plan description				On June 16, 2017, the Company’s stockholders approved the 2017 Employee Stock Purchase Plan (the “2017 ESPP”), which became effective on June 28, 2017. A total of 150,000 shares of common stock were initially reserved for issuance under this plan. The number of shares of common stock that may be issued under the 2017 ESPP will automatically increase on each January 1, beginning with the fiscal year ending December 31, 2018 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2027, equal to the least of (i) 622,408 shares, (ii) 1% of the outstanding shares of common stock on such date and (iii) an amount determined by the Company’s board of directors. On January 1, 2018, the Company’s board of directors determined not to increase the number of shares of common stock that may be issued under the 2017 ESPP.
2006 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period				4 years
Stock option expiration period				10 years
Stock incentive plan termination year				2,016
Maximum [Member] | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period		2 years
Maximum [Member] | 2017 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period				4 years
Stock option expiration period				10 years
Maximum [Member] | 2006 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period				4 years
Minimum [Member] | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period		90 days

Stock-Based Awards - Valuation

Stock-Based Awards - Valuation Assumptions of Stock Options Granted (Detail)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Risk-free interest rate	2.77%	2.15%
Expected term (in years)	6 years 2 months 12 days	6 years 1 month 6 days
Expected volatility	76.00%	80.60%
Expected dividend yield	0.00%	0.00%

Stock-Based Awards - Summary of

Stock-Based Awards - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Beginning balance	2,096,233
Number of Shares, Granted	555,414
Number of Shares, Exercised	(13,584)
Number of Shares, Forfeited	(455,423)
Number of Shares, Ending balance	2,182,640	2,096,233
Number of Shares, Options exercisable	1,187,074	784,190
Number of Shares, Options vested and expected to vest	2,146,086	2,030,629
Weighted Average Exercise Price, Beginning balance	$ 8.02
Weighted Average Exercise Price, Granted	5.34
Weighted Average Exercise Price, Exercised	2.03
Weighted Average Exercise Price, Forfeited	9.53
Weighted Average Exercise Price, Ending balance	7.06	$ 8.02
Weighted Average Exercise Price, Options exercisable	6.23	4.71
Weighted Average Exercise Price, Options vested and expected to vest	$ 7.05	$ 7.96
Weighted-Average Remaining Contractual Term, Outstanding	6 years 4 months 24 days	8 years
Weighted Average Remaining Contractual Term, Options exercisable	4 years	6 years
Weighted Average Remaining Contractual Term, Options vested and expected to vest	6 years 4 months 24 days	7 years 10 months 24 days
Aggregate Intrinsic Value	$ 241	$ 7,332
Aggregate Intrinsic Value, Options exercisable	241	4,667
Aggregate Intrinsic Value, Options vested and expected to vest	$ 241	$ 7,198

Stock-Based Awards - Summary _2

Stock-Based Awards - Summary of Stock Based Compensation Expense (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]
Stock-based compensation expense	$ 588	$ 616	$ 2,689	$ 1,084
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	226	191	736	396
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 362	$ 425	$ 1,953	$ 688

Net Loss per Share - Basic and

Net Loss per Share - 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	$ (7,434)	$ (6,259)	$ (24,513)	$ (15,739)
Accretion of redeemable convertible preferred stock to redemption value				(41)
Net loss attributable to common stockholders	$ (7,434)	$ (6,259)	$ (24,513)	$ (15,780)
Denominator:
Weighted average common shares outstanding—basic and diluted	14,737,402	13,939,950	14,735,660	4,990,535
Net loss per share attributable to common stockholders—basic and diluted	$ (0.50)	$ (0.45)	$ (1.66)	$ (3.16)

Net Loss per Share - Summary of

Net Loss per Share - Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	2,182,640	2,055,264	2,182,640	2,055,264
Stock Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	2,182,640	2,055,264	2,182,640	2,055,264

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended	96 Months Ended	105 Months Ended
	Oct. 31, 2010USD ($)	Feb. 28, 2010USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)Milestone	Sep. 30, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2018USD ($)	Apr. 04, 2018USD ($)ft²	Dec. 31, 2006USD ($)
Other Commitments [Line Items]
Property and equipment, net			$ 6,832,000		$ 6,832,000			$ 154,000	$ 6,832,000
Harvard and Dana-Farber Agreement [Member]
Other Commitments [Line Items]
Milestone payments			0	$ 0	$ 0	$ 0
Milestones achieved | Milestone					0
Additional liabilities for milestone payments			0		$ 0				0
Non-refundable cash payment								$ 4,428,000	4,573,000
Payments for royalties					0
Harvard and Dana-Farber Agreement [Member] | Therapeutic Product [Member]
Other Commitments [Line Items]
Aggregate milestones payments		$ 7,700,000
Harvard and Dana-Farber Agreement [Member] | Diagnostic Product [Member]
Other Commitments [Line Items]
Aggregate milestones payments		700,000
Harvard and Dana-Farber Agreement [Member] | License and Maintenance [Member]
Other Commitments [Line Items]
Annual license maintenance fees		$ 145,000
Harvard and Dana-Farber Agreement [Member] | License [Member]
Other Commitments [Line Items]
Annual license maintenance fees					145,000	145,000
Umicore Agreement [Member]
Other Commitments [Line Items]
Aggregate milestones payments											$ 6,400,000
Milestone payments			0	0	$ 0	0
Milestones achieved | Milestone					0
Additional liabilities for milestone payments			0		$ 0				0
Umicore Agreement [Member] | License and Maintenance [Member]
Other Commitments [Line Items]
Annual license maintenance fees							$ 50,000
Umicore Agreement [Member] | License [Member]
Other Commitments [Line Items]
Annual license maintenance fees			0	0	$ 0	0
Scripps Agreement [Member]
Other Commitments [Line Items]
Milestones achieved | Milestone					0
Additional liabilities for milestone payments			0		$ 0				0
Payments for royalties					0
Scripps Agreement [Member] | License and Maintenance [Member]
Other Commitments [Line Items]
Annual license maintenance fees	$ 50,000
Scripps Agreement [Member] | License [Member]
Other Commitments [Line Items]
Annual license maintenance fees			0	0	0	0
Scripps Agreement [Member] | Peptide Product [Member]
Other Commitments [Line Items]
Aggregate milestones payments	1,900,000
Scripps Agreement [Member] | Protein Product [Member]
Other Commitments [Line Items]
Aggregate milestones payments	$ 950,000
Letter of Credit [Member] | Non-current Restricted Cash [Member]
Other Commitments [Line Items]
Security deposit provided to landlord			568,000		568,000				568,000
Cambridge, Massachusetts [Member]
Other Commitments [Line Items]
Rental expense under operating leases			220,000	$ 127,000	559,000	$ 367,000
Build-to-suit Facility [Member]
Other Commitments [Line Items]
Property and equipment, net			6,616,000		6,616,000				6,616,000
Lease obligation			4,997,000		$ 4,997,000				4,997,000
Lease Agreements [Member] | Office and Laboratory Space [Member] | Watertown, Massachusetts [Member] | 480 Arsenal Group LLC [Member]
Other Commitments [Line Items]
Area of land | ft²										18,768
Lease initial term										8 years
Lease option to extend, description					The Lease has an initial term of eight years and provides the Company with an option to extend the Lease term for one additional five-year period.
Lease extension term										5 years
Future minimum rent commitment										$ 8,771,000
Operating Lease Agreement [Member] | Cambridge, Massachusetts [Member]
Other Commitments [Line Items]
Operating lease expiration		2018-08
Future minimum lease payments, 2018			$ 0		$ 0				$ 0

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]
Income tax expense (benefit)	$ 0	$ 0	$ 0	$ 0