Allena Pharmaceuticals (ALNAQ) 10-K 27 Mar 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Mar. 01, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	ALNA
Entity Registrant Name	ALLENA PHARMACEUTICALS, INC.
Entity Central Index Key	1,624,658
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		20,695,556
Entity Public Float			$ 0

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 94,494	$ 25,250
Short-term investments		23,505
Prepaid expenses and other current assets	1,539	520
Total current assets	96,033	49,275
Property and equipment, net	127	169
Other assets	89	35
Total assets	96,249	49,479
Current liabilities:
Accounts payable	1,724	1,464
Loan payable, net of discount	3,870	176
Accrued expenses	1,949	1,610
Total current liabilities	7,543	3,250
Loan payable, net of current portion and discount	5,516	9,409
Warrants for the purchase of shares subject to redemption		267
Other liabilities	320	103
Total liabilities	13,379	13,029
Commitments and contingencies (Note 13)
Stockholders’ equity (deficit):
Common stock, $0.001 par value; 125,000,000 shares authorized at December 31, 2017, 75,000,000 shares authorized at December 31, 2016; 20,694,658 and 1,341,385 shares issued and outstanding at December 31, 2017 and 2016, respectively	20	1
Additional paid-in capital	164,807	1,031
Accumulated other comprehensive loss		(2)
Accumulated deficit	(81,957)	(60,307)
Total stockholders’ equity (deficit)	82,870	(59,277)
Total liabilities, convertible preferred stock and stockholders’ equity (deficit)	$ 96,249	49,479
Series A Convertible Preferred Stock
Current liabilities:
Convertible preferred stock		17,967
Series B Convertible Preferred Stock
Current liabilities:
Convertible preferred stock		24,931
Series C Convertible Preferred Stock
Current liabilities:
Convertible preferred stock		$ 52,829

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	125,000,000	75,000,000
Common stock, shares issued	20,694,658	1,341,385
Common stock, shares outstanding	20,694,658	1,341,385
Series A Convertible Preferred Stock
Convertible preferred stock, par value	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized	0	18,510,200
Convertible preferred stock, shares issued	0	18,367,344
Convertible preferred stock, shares outstanding	0	18,367,344
Series B Convertible Preferred Stock
Convertible preferred stock, par value	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized	0	19,481,270
Convertible preferred stock, shares issued	0	19,841,270
Convertible preferred stock, shares outstanding	0	19,841,270
Series C Convertible Preferred Stock
Convertible preferred stock, par value	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized	0	20,037,736
Convertible preferred stock, shares issued	0	20,000,000
Convertible preferred stock, shares outstanding	0	20,000,000
Undesignated Preferred Stock
Convertible preferred stock, par value	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized	5,000,000	0
Convertible preferred stock, shares issued	0	0
Convertible preferred stock, shares outstanding	0	0

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Research and development	$ 15,519	$ 20,103	$ 11,540
General and administrative	5,431	4,083	2,365
Total operating expenses	20,950	24,186	13,905
Other income (expense):
Interest income (expense), net	(443)	(200)	(335)
Other income (expense), net	(257)	(121)	(7)
Other income (expense), net	(700)	(321)	(342)
Net loss	$ (21,650)	$ (24,507)	$ (14,247)
Net loss per share attributable to common stockholders — basic and diluted	$ (4.80)	$ (18.35)	$ (11.35)
Weighted-average common shares outstanding — basic and diluted	4,520,337	1,339,254	1,258,123
Comprehensive loss:
Net loss	$ (21,650)	$ (24,507)	$ (14,247)
Other comprehensive income (loss):
Unrealized loss on investments		(2)
Total other comprehensive loss		(2)
Comprehensive loss	$ (21,650)	$ (24,509)	$ (14,247)

Consolidated Statements of Conv

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Series C Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at Dec. 31, 2014		$ 17,948	$ 24,891
Balance, Shares at Dec. 31, 2014		18,367,344	19,841,270
Balance at Dec. 31, 2014	$ (20,902)				$ 1	$ 650		$ (21,553)
Balance, Shares at Dec. 31, 2014					1,135,653
Issuance of Series C convertible preferred stock, net of issuance costs of $218				$ 52,782
Issuance of Series C convertible preferred stock, net of issuance costs of $218, Shares				20,000,000
Exercise of common stock options	7					7
Exercise of common stock options, shares					17,394
Vesting of restricted common stock	3					3
Vesting of restricted common stock, shares					162,893
Accretion of convertible preferred stock to redemption value		$ 11	$ 22	$ 4
Accretion of convertible preferred stock to redemption value	(37)					(37)
Stock-based compensation	207					207
Net loss	(14,247)							(14,247)
Balance at Dec. 31, 2015		$ 17,959	$ 24,913	$ 52,786
Balance, Shares at Dec. 31, 2015		18,367,344	19,841,270	20,000,000
Balance at Dec. 31, 2015	(34,969)				$ 1	830		(35,800)
Balance, Shares at Dec. 31, 2015					1,315,940
Exercise of common stock options	19				$ 0	19
Exercise of common stock options, shares					24,770
Vesting of restricted common stock, shares					675
Accretion of convertible preferred stock to redemption value		$ 8	$ 18	$ 43
Accretion of convertible preferred stock to redemption value	(69)					(69)
Stock-based compensation	251					251
Change in unrealized gain (loss) on available for sale investments	(2)						$ (2)
Net loss	(24,507)							(24,507)
Balance at Dec. 31, 2016		$ 17,967	$ 24,931	$ 52,829
Balance, Shares at Dec. 31, 2016		18,367,344	19,841,270	20,000,000
Balance at Dec. 31, 2016	$ (59,277)				$ 1	1,031	(2)	(60,307)
Balance, Shares at Dec. 31, 2016	1,341,385				1,341,385
Issuance of common stock, net of issuance costs	$ 66,999				$ 5	66,994
Issuance of common stock, net of issuance costs, shares					5,350,302
Exercise of common stock options	28					28
Exercise of common stock options, shares					33,061
Exercise of common stock warrants, shares					24,401
Accretion of convertible preferred stock to redemption value		$ 7	$ 15	$ 36
Accretion of convertible preferred stock to redemption value	(58)					(58)
Conversion of preferred stock into common stock		$ (17,974)	$ (24,946)	$ (52,865)
Conversion of preferred stock into common stock, shares		(18,367,344)	(19,841,270)	(20,000,000)
Conversion of preferred stock into common stock	95,785				$ 14	95,771
Conversion of preferred stock into common stock, shares					13,945,509
Conversion of warrant liability to equity	524					524
Stock-based compensation	517					517
Change in unrealized gain (loss) on available for sale investments	2						$ 2
Net loss	(21,650)							(21,650)
Balance, Shares at Dec. 31, 2017		0	0	0
Balance at Dec. 31, 2017	$ 82,870				$ 20	$ 164,807		$ (81,957)
Balance, Shares at Dec. 31, 2017	20,694,658				20,694,658

Consolidated Statements of Con6

Consolidated Statements of Convertible Preferred Stock and Stockholders' (Deficit) Equity (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2015USD ($)
Series C Convertible Preferred Stock
Stock issuance cost	$ 218

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (21,650,000)	$ (24,507,000)	$ (14,247,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	517,000	251,000	207,000
Depreciation expense	73,000	46,000	98,000
Non-cash interest expense	351,000	204,000	167,000
Amortization of premium on investments	33,000	153,000
Loss on disposal of equipment			7,000
Change in fair value of warrant liability	257,000	132,000	36,000
Changes in assets and liabilities:
Prepaid expenses and other current assets	(984,000)	343,000	(639,000)
Other assets	(89,000)
Accounts payable	199,000	(525,000)	1,001,000
Accrued expenses	228,000	689,000	219,000
Other liabilities		(180,000)	(24,000)
Net cash used in operating activities	(21,065,000)	(23,394,000)	(13,175,000)
Cash flows from investing activities:
Purchases of property and equipment	(57,000)	(102,000)
Purchases of investments	(1,247,000)	(53,210,000)
Maturities of investments	24,721,000	29,550,000
Sale of property and equipment			5,000
Net cash provided by (used in) investing activities	23,417,000	(23,762,000)	5,000
Cash flows from financing activities:
Proceeds from the issuance of common stock, net of issuance costs	67,197,000
Proceeds from exercise of stock options	28,000	19,000	7,000
Proceeds from loan payable		10,000,000	3,250,000
Repayment of loan payable	(333,000)	(6,256,000)	(744,000)
Debt issuance costs paid		(368,000)
Net cash provided by financing activities	66,892,000	3,395,000	55,295,000
Net increase (decrease) in cash and cash equivalents	69,244,000	(43,761,000)	42,125,000
Cash and cash equivalents, beginning of period	25,250,000	69,011,000	26,886,000
Cash and cash equivalents, end of period	94,494,000	25,250,000	69,011,000
Supplemental disclosure of non-cash activities:
Cash paid for interest	452,000	472,000	193,000
Property and equipment purchases included in accounts payable	11,000	38,000	9,000
Issuance of warrants in connection with loan payable		67,000	7,000
Accretion of convertible preferred stock to redemption value	58,000	$ 69,000	37,000
Initial public offering costs included in accounts payable and accrued expenses	$ 199,000
Series C Convertible Preferred Stock
Cash flows from financing activities:
Proceeds from the sale of convertible preferred stock, net of issuance costs			$ 52,782,000

Organization and Basis of Prese

Organization and Basis of Presentation	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization and Basis of Presentation	1. Organization and Basis of Presentation Allena Pharmaceuticals, Inc. (the “Company”) is a clinical stage company focused on developing non-absorbed oral enzyme therapeutics to treat metabolic conditions including hyperoxaluria and hyperuricemia. The Company was incorporated under the laws of the State of Delaware on June 24, 2011. The Company’s headquarters are in Newton, Massachusetts. On November 6, 2017, the Company completed an initial public offering (“IPO”), in which the Company issued and sold 5,333,333 shares of its common stock at a public offering price of $14.00 per share, for aggregate gross proceeds of $74.7 million. The underwriters partially exercised their over-allotment option December 1, 2017, and purchased an additional 16,969 shares of our common stock for aggregate gross proceeds of $0.2 million. As a result of the IPO, the Company received approximately $67.0 million in net proceeds after deducting $7.9 million of underwriting discounts and commissions and offering costs. Upon the closing of the IPO, all of the outstanding shares of convertible preferred stock automatically converted into 13,945,509 shares of common stock. Subsequent to the closing of the IPO, there were no shares of preferred stock outstanding. In connection with the IPO, the Company amended and restated its certificate of incorporation to change the authorized capital stock to 125,000,000 shares designated as common stock and 5,000,000 shares designated as preferred stock, all with a par value of $0.001 per share. In connection with preparing for its IPO, the Company effected a 1-for-4.174 reverse stock split of the Company’s common stock. The reverse stock split became effective on October 23, 2017. The par value and authorized shares of common stock and convertible preferred stock were not adjusted as a result of the reverse stock split. All share and per share amounts in the financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. The financial statements have also been retroactively adjusted to reflect adjustments to the conversion ratio for each series of convertible preferred stock effected in connection with the reverse stock split. The Company is subject to risks common to companies in the biotechnology industry, including but not limited to, risks of failure of preclinical studies and clinical trials, the need to obtain marketing approval for any drug product candidate that it may identify and develop, the need to successfully commercialize and gain market acceptance of its product candidates, dependence on key personnel, protection of proprietary technology, compliance with government regulations, development by competitors of technological innovations, reliance on third party manufacturers, ability to transition from pilot-scale manufacturing to large-scale production of products and the need to obtain adequate additional financing to fund the development of its product candidates. The Company has an accumulated deficit of $82.0 million at December 31, 2017, and will require substantial additional capital to fund operations. The future success of the Company is dependent on its ability to identify and develop its product candidates and ultimately upon its ability to attain profitable operations. At December 31, 2017, the Company had $94.5 million of cash and cash equivalents. The Company believes its cash and cash equivalents as of December 31, 2017 will be sufficient to fund the Company's operating plan for a period of at least one year from the issuance date of the consolidated financial statements. Thereafter, the Company will be required to obtain additional funding in the future to achieve its operating plan. There can be no assurances, however, that the current operating plan will be achieved or that additional funding will be available on terms acceptable to the Company, or at all. Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standard Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”).

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Principles of Consolidation The consolidated financial statements include the accounts of Allena Pharmaceuticals, Inc. and its wholly owned subsidiaries Allena Pharmaceuticals Security Corporation (“Security Corporation”), which was incorporated in December 2014, and Allena Pharmaceuticals Ireland Limited, which was incorporated in March 2017. All intercompany transactions and balances have been eliminated. Use of Estimates The preparation of the Company's consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, expenses and related disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of expenses during the reporting period. On an ongoing basis, the Company's management evaluates its estimates, which include but are not limited to management’s judgments of accrued expenses, fair value of common stock prior to the closing of the Company’s IPO, valuation of share-based awards, fair value of warrants, prior to the conversion into warrants for the purchase of common stock upon the closing of the IPO, and income taxes. Actual results could differ from those estimates. The Company utilized significant estimates and assumptions in determining the fair value of its common stock. The Company utilized various valuation methodologies in accordance with the framework of the American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision-maker in deciding how to allocate resources and assess performance. The Company's chief operating decision-maker, the Company's chief executive officer, views the Company's operations and manages its business as a single operating segment, which is the business of discovering and developing non-absorbed oral enzyme therapeutics. Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. The Company’s only element of other comprehensive income (loss) is unrealized gains and losses on available-for-sale investments. Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less to be cash equivalents. Cash and cash equivalents include cash held in banks and amounts held in money market funds. Cash equivalents are stated at cost, which approximates market value. Cash and cash equivalents consist of the following at December 31, 2017 and 2016 (in thousands): December 31, 2017 2016 Cash and cash equivalents: Cash $ 749 $ 948 Money market funds 93,745 24,302 $ 94,494 $ 25,250 Investments The Company invests available cash primarily in U.S. and foreign corporate debt securities and United States government treasury securities. The Company classifies its investments as available-for-sale. Available-for-sale investments are carried at fair value with unrealized gains and losses included in stockholders’ (deficit) equity. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense. All investments are classified as current assets as they have contractual maturities of less than one year and are available to meet working capital needs and to fund current operations. The cost of securities sold is determined on a specific identification basis, and realized gains and losses are included in interest income (expense) within the statement 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 statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented. Concentration of Credit Risk and Off-Balance Sheet Risk Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and investments. The Company maintains all of its cash, cash equivalents and investments balances at a single accredited financial institution, in amounts that exceed federally insured limits. The Company generally invests its excess cash in money market funds, corporate notes and United States government securities that are subject to minimal credit and market risk. Management has established guidelines relative to credit ratings and maturities intended to safeguard principal balances and maintain liquidity. The investment portfolio is maintained in accordance with the Company’s investment policy, which defines allowable investments, specifies credit quality standards and limits the credit exposure of any single issuer. The Company has no significant off-balance sheet risk such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Significant Suppliers The Company is dependent on third-party manufacturers to supply products for research and development activities of its programs, including preclinical and clinical testing. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs. Fair Value of Financial Instruments Fair value is defined as the price that would be received upon sale of an asset or paid to transfer a liability between market participants at measurement dates. ASC Topic 820, Fair Value Measurement Level 1 inputs: Quoted prices in active markets for identical assets or liabilities. Level 2 inputs: Inputs other than quoted prices included within Level 1 that are either directly or indirectly observable, such as quoted market prices, interest rates and yield curves. Level 3 inputs: Unobservable inputs developed using estimates or assumptions developed by the Company, which reflect those that a market participant would use in pricing the asset or liability. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument's level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Property and Equipment Property and equipment consists of laboratory equipment, computer equipment, software and leasehold improvements recorded at cost. These amounts are depreciated using the straight-line method over the estimated useful lives of the assets as follows: Laboratory equipment 4 years Computer equipment 3 years Software 5 years Leasehold improvements Shorter of useful life or remaining term of related lease Upon retirement or sale, the cost of the assets disposed of and the related accumulated depreciation are eliminated from the balance sheet and related gains or losses are reflected in the statement of operations and comprehensive loss. Repairs and maintenance costs are expensed as incurred and costs of significant improvements are capitalized. Impairment of Long-Lived Assets The Company continually evaluates long-lived assets for potential impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparing the book values of the assets to the expected future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book values of the assets exceed their fair value. The Company did not recognize any impairment losses for the years ended December 31, 2017, 2016 and 2015. Warrants to Purchase Shares Subject to Redemption Prior to the closing of the IPO, the Company accounted for warrant instruments that either conditionally or unconditionally obligate the issuer to transfer assets as liabilities regardless of the timing of the redemption feature or price, even though the underlying shares may be classified as equity. These warrants were subject to revaluation at each balance sheet date, and any changes in fair value were recorded as a component of other income (expense) in the statements of operations and comprehensive loss. Upon the closing of the IPO, the warrants met the criteria to be classified in stockholders’ equity and the fair value of the warrant liability as of the IPO date was reclassified to stockholders’ equity (deficit). Research and Development The Company expenses all costs incurred in performing research and development activities. Research and development expenses include salaries and benefits, materials and supplies, preclinical and clinical trial expenses, manufacturing expenses, stock-based compensation expense, depreciation of equipment, contract services and other outside expenses. Costs of certain development activities, such as manufacturing, are recognized based on an evaluation of the progress to completion of specific tasks. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the consolidated financial statements as prepaid or accrued research and development costs. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred and capitalized. The capitalized amounts are expensed as the related goods are delivered or the services are performed. Patent Costs The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the accompanying consolidated statements of operations and comprehensive loss. Accounting for Stock-Based Compensation The Company accounts for its stock-based compensation in accordance with ASC Topic 718, Compensation—Stock Compensation Equity-Based Payments to Non-Employees The Black-Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the expected term of the award, (c) the risk-free interest rate and (d) expected dividends. Due to the lack of a public market for the Company’s common stock and a lack of company-specific historical and implied volatility data, the Company has based its computation of expected volatility on the historical volatility of a representative group of public companies with similar characteristics to the Company, including stage of product development and life science industry focus. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The Company uses the simplified method as prescribed by the SEC Staff Accounting Bulletin No. 107, Share-Based Payment In the first quarter of 2017, the Company made an accounting policy election to recognize forfeitures as they occur upon adoption of guidance per ASU No. 2016-09, Compensation – Stock Compensation The Company expenses the fair value of its share-based compensation awards to employees on a straight-line basis over the requisite service period, which is generally the vesting period. Stock-based compensation awards to non-employees are adjusted through stock-based compensation expense at each reporting period end to reflect the current fair value of such awards and are expensed on a straight-line basis. Income Taxes The Company accounts for income taxes using the liability method in accordance with ASC Topic 740, Income Taxes The Company recognizes the effect of income tax positions only if those positions are more likely than not of being sustained. Recognized income tax positions are measured at the largest amount that is greater than 50% likely of being realized. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs. Net Loss Per Share The Company has reported losses since inception and has computed basic net loss per share attributable to common stockholders by dividing net loss attributable to common stockholders by the weighted-average number of common shares outstanding for the period, without consideration for potentially dilutive securities. The Company has computed diluted net loss per common share after giving consideration to all potentially dilutive common shares, including options to purchase common stock, restricted common stock, convertible preferred stock and warrants to purchase convertible preferred stock, outstanding during the period determined using the treasury-stock and if-converted methods, except where the effect of including such securities would be anti-dilutive. Because the Company has reported net losses since inception, these potential common shares have been anti-dilutive and basic and diluted loss per share have been the same. Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share data): Years Ended December 31, 2017 2016 2015 Numerator: Net loss $ (21,650 ) $ (24,507 ) $ (14,247 ) Accretion of convertible preferred stock (58 ) (69 ) (37 ) Net loss attributable to common stockholders $ (21,708 ) $ (24,576 ) $ (14,284 ) Denominator: Weighted-average common shares – basic and diluted 4,520,337 1,339,254 1,258,123 Net loss per share attributable to common stockholders –basic and diluted $ (4.80 ) $ (18.35 ) $ (11.35 ) The following table sets forth the potentially dilutive securities that have been excluded from the calculation of diluted net loss per share because to include them would be anti-dilutive (in common stock equivalent shares): Years Ended December 31, 2017 2016 2015 Series A convertible preferred stock — 4,400,410 4,400,410 Series B convertible preferred stock — 4,753,536 4,753,536 Series C convertible preferred stock — 4,791,563 4,791,563 Warrants 9,040 43,265 34,225 Stock options 1,508,124 1,348,845 759,167 Restricted stock — — 675 Total 1,517,164 15,337,619 14,739,576 Loss Contingencies In accordance with ASC 450, Contingencies Guarantees The Company has identified the guarantees described below as disclosable, in accordance with ASC 460, Guarantees As permitted under Delaware law, the Company indemnifies its officers and directors for certain events or occurrences while the officer or director is, or was, serving at the Company’s request in such capacity. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ and officers’ insurance coverage that should limit its exposure and enable it to recover a portion of any future amounts paid. The Company is a party to a number of agreements entered into in the ordinary course of business that contain typical provisions that obligate the Company to indemnify the other parties to such agreements upon the occurrence of certain events. Such indemnification obligations are usually in effect from the date of execution of the applicable agreement for a period equal to the applicable statute of limitations. The aggregate maximum potential future liability of the Company under such indemnification provisions is uncertain. The Company leases office space under several noncancelable operating leases. The Company has standard indemnification arrangements under these leases that require it to indemnify the landlord against all costs, expenses, fines, suits, claims, demands, liabilities, and actions directly resulting from any breach, violation, or nonperformance of any covenant or condition of the respective lease. As of December 31, 2017, 2016 and 2015, the Company had not experienced any losses related to these indemnification obligations, and no material claims with respect thereto were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves have been established. Recently Adopted Accounting Pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its consolidated financial position or results of operations upon adoption. In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements—Going Concern In April 2015, the FASB issued ASU No. 2015-03, Simplifying the Presentation of Debt Issuance Costs In November 2015, the FASB issued ASU No. 2015-17, Balance Sheet Classification of Deferred Taxes In March 2016, the FASB issued ASU No. 2016-09, Compensation-Stock Compensation Compensation-Stock Compensation. Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments In May 2017, the FASB issued ASU No. 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting

Investments

Investments	12 Months Ended
	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Investments	3. Investments The Company did not have any investments as of December 31, 2017. The following table summarizes the Company’s investments as of December 31, 2016 (in thousands): Amortized Gross Unrealized Estimated Cost Gains Losses Fair Value Available-for-sale securities: U.S. treasury securities $ 13,258 $ 3 $ (2 ) $ 13,259 U.S. corporate debt securities 7,397 — (3 ) 7,394 Foreign corporate debt securities 2,852 — — 2,852 Total available-for-sale securities $ 23,507 $ 3 $ (5 ) $ 23,505 The aggregate fair value of investments with unrealized losses was approximately $16.5 million at December 31, 2016. At December 31, 2016, 10 investments were in an unrealized loss position. All such investments have been in an unrealized loss position for less than a year and these losses are considered temporary. The Company has the ability and intent to hold these investments until a recovery of their amortized cost.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. Fair Value Measurements The following tables present information about the Company’s financial assets and liabilities that have been measured at fair value at December 31, 2017 and 2016 and indicates the fair value hierarchy of the valuation inputs utilized to determine such fair value (in thousands): Description December 31, 2017 Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds, included in cash and cash equivalents $ 93,745 $ 93,745 $ — $ — Total assets $ 93,745 $ 93,745 $ — $ — Liabilities: Warrants for the purchase of shares subject to redemption $ — $ — $ — $ — Total liabilities $ — $ — $ — $ — Description December 31, 2016 Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds, included in cash and cash equivalents $ 24,302 $ 24,302 $ — $ — Investments: U.S. treasury securities 13,259 13,259 — — U.S corporate debt securities 7,394 — 7,394 Foreign corporate debt securities 2,852 — 2,852 — Total assets $ 47,807 $ 37,561 $ 10,246 $ — Liabilities: Warrants for the purchase of shares subject to redemption $ 267 $ — $ — $ 267 Total liabilities $ 267 $ — $ — $ 267 At December 31, 2017 and 2016, all of the Company’s cash equivalents were comprised of money market funds. At December 31, 2016, items classified as Level 2 within the valuation hierarchy consist of U.S. and foreign corporate debt securities. The Company estimates the fair values of these investments by taking into consideration valuations obtained from third-party pricing sources. These pricing sources utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include market pricing based on real-time trade data for the same or similar securities, issuer credit spreads, benchmark yields, and other observable inputs. The Company validates the prices provided by its third-party pricing sources by understanding the models used, obtaining market values from other pricing sources and analyzing pricing data in certain instances. There have been no changes to the valuation methods used during the years ended December 31, 2017 and 2016. There were no transfers within the fair value hierarchy during the years ended December 31, 2017 and 2016. The carrying amounts reflected in the consolidated balance sheets for cash and cash equivalents, prepaid expenses and other current assets, accounts payable and accrued expenses approximate their carrying values. The Company believes the terms of the loan payable reflect current market conditions for an instrument with similar terms and maturity, therefore the carrying value of the Company’s debt approximates its fair value based on Level 3 of the fair value hierarchy. Warrants to Purchase Shares Subject to Redemption In connection with entering into a Loan and Security Agreement (“Loan Agreement”) in August 2014 (Note 7), the Company issued a warrant for the purchase of 71,428 shares of Series A preferred stock (“Series A Warrant”) when the Loan Agreement was executed. In connection with the first advance under the Loan Agreement, the Series A Warrant became exercisable for an additional 38,265 shares. In connection with a second advance under the Loan Agreement in March 2015, the Series A Warrant became exercisable for an additional 33,163 shares. The Company amended the Loan Agreement in May 2016. In connection with the first advance under the amended Loan Agreement in May 2016, the Company issued a warrant for the purchase of up to 28,302 shares of Series C preferred stock (“Series C Warrant”, together with the Series A Warrant, the “Warrants”). In connection with the second advance under the amended Loan Agreement in December 2016, the Series C Warrant became exercisable for an additional 9,434 shares. The estimated fair value of the Warrants was determined using the Black-Scholes option-pricing model. A significant input to the fair value of the warrants is the fair value of the Series A and C Preferred Stock which was determined based upon the Company’s common stock valuations. The fair value of the Warrants was remeasured at each reporting date through the date of the IPO using then-current assumptions with changes in fair value charged to other income (expense) on the statements of operations and comprehensive loss. As of November 6, 2017 and December 31, 2016, the Warrants were valued at $0.5 million and $0.3 million, respectively. The following assumptions were used in valuing the Warrants: November 6, 2017 December 31, 2016 Risk-free interest rate 2.1% - 2.2% 2.3% - 2.5% Expected dividend yield —% —% Expected term (in years) 6.8 - 8.5 7.6 - 9.3 Expected volatility 84% - 86% 90% The following table sets forth a summary of changes in the fair value of the Warrants, which represented a recurring measurement classified within Level 3 of the fair value hierarchy, wherein fair value was estimated using significant unobservable inputs (in thousands, except share data): Balance at December 31, 2015 $ 68 Initial fair value of Series C Warrant for the purchase of 28,302 shares 45 Initial fair value of Series C Warrant for the purchase of 9,434 shares 22 Change in fair value of Warrants included in other income (expense) 132 Balance at December 31, 2016 267 Change in fair value of Warrants through the date of the IPO included in other income (expense) 257 Fair value of Warrants reclassified to stockholders' equity (deficit) upon conversion to warrants for the conversion of common stock (524 ) Balance at December 31, 2017 $ — An entity may choose to measure many financial instruments and certain other items at fair value at specified election dates. Subsequent unrealized gains and losses on items for which the fair value option has been elected will be reported in earnings. The Company did not elect to measure any financial instruments or other items at fair value.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment includes the following at December 31, 2017 and 2016 (in thousands): December 31, 2017 2016 Property and equipment: Laboratory equipment $ 263 $ 232 Computer equipment 6 6 Software 39 39 308 277 Less: Accumulated depreciation (181 ) (108 ) Property and equipment, net $ 127 $ 169 The Company recognized $73,000, $46,000 and $0.1 million of depreciation expense for the years ended December 31, 2017, 2016 and 2015, respectively. In September 2016, the Company wrote-off $0.1 million of fully depreciated leasehold improvements upon moving out of a laboratory facility.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued Expenses	6. Accrued Expenses Accrued expenses consist of the following (in thousands): December 31, 2017 2016 Payroll and employee-related expenses $ 1,132 $ 914 Professional fees 420 172 Third-party research and development expenses 135 413 Loan interest 40 28 IPO-related costs 111 — Other 111 83 Total accrued expenses $ 1,949 $ 1,610

Loan and Security Agreement

Loan and Security Agreement	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Loan and Security Agreement	7. Loan and Security Agreement In August 2014, the Company entered into a Loan Agreement with Silicon Valley Bank (“SVB”) to borrow up to $7.0 million. Upon entering into the Loan Agreement, SVB advanced $3.8 million to the Company. In March 2015, SVB advanced the remaining $3.2 million available under the Loan Agreement. In May 2016, the Loan Agreement was amended (“Amended Loan Agreement”) to borrow up to $10.0 million with a portion of the proceeds to be used to pay down the outstanding balance of the original $7.0 million of advances. At the time of the Amended Loan Agreement, SVB advanced a gross amount of $7.5 million to the Company. Net proceeds received by the Company were $1.6 million after deducting $5.3 million for repayment of the original advances and $0.6 million for final interest due upon maturity or prepayment of the original advances. The Amended Loan Agreement was accounted for as a debt modification pursuant to ASC 450-70, Modifications or Extinguishments The borrowings are secured by a lien on all Company assets, excluding intellectual property. The May 2016 and December 2016 advances have a floating per annum interest rate of the greater of 4.0% or 0.5% above the prime rate. The interest rate on the borrowings at December 31, 2016 and December 31, 2017 was 4.25% and 5.0%, respectively. Beginning in May 2016, payments were interest only for a period of 12 months. In December 2016, the interest only period was extended to 18 months. Upon the expiration of the interest only period, amounts borrowed will be repaid over 30 equal monthly payments of principal and interest. At its option, the Company may prepay all, but not less than all, of the outstanding borrowings subject to a prepayment premium as defined in the Amended Loan Agreement. The Company is also required to make a final payment equal to 8.25% of the total borrowings which is due on the earliest of the loan maturity date, an acceleration of the loan as defined in the Amended Loan Agreement or at the time of prepayment. The final payment is being accreted to interest expense through the maturity date of the loan. The minimum aggregate future loan and interest payments at December 31, 2017 are as follows (in thousands): Years Ending December 31, 2018 $ 4,377 2019 4,185 2020 2,512 Total minimum payments 11,074 Less: Amount representing interest (1,407 ) Less: Discount (281 ) Less: Current portion (3,870 ) Loan payable, net of current portion $ 5,516 The Company issued a Series A Warrant to SVB to purchase 71,428 shares of Series A convertible preferred stock (“Series A Preferred Stock”) at $0.98 per share upon executing the Loan Agreement. In addition, the Company issued an additional warrant to SVB to purchase a number of shares of Series A Preferred Stock equal to 1.0% of each additional loan advance amount. In connection with the initial advance and the advance in March 2015, the Company issued additional Series A Warrants to purchase up to 38,265 and 33,163 shares of Series A Preferred Stock at $0.98 per share, respectively. The Series A Warrants expire on August 17, 2024. Under the terms of the Amended Loan Agreement, the Company issued a Series C Warrant to SVB to purchase a number of shares of Series C convertible preferred stock (“Series C Preferred Stock”) at $2.65 per share equal to 1.0% of each loan advance amount. In connection with the initial advance and the advance in December 2016, the Company issued Series C Warrants to purchase up to 28,302 and 9,434 shares of Series C Preferred Stock at $2.65 per share, respectively. The Series C Warrants expire on May 1, 2026. Upon the closing of the IPO, the Series A Warrants converted to common stock warrants to purchase up to 34,225 shares of common stock at an exercise price of $4.09 per share. The Series C Warrants converted to common stock warrants to purchase up to 9,040 shares of common stock at an exercise price of $11.06 per share. On December 12, 2017, SVB exercised its warrants to purchase 34,225 shares of common stock using the cashless exercise option permitted per the terms of the warrant agreement. The Company issued 24,401 shares of common stock to SVB as settlement of the warrant. At December 31, 2017, warrants to purchase up to 9,040 shares of common stock at an exercise price of $11.06 remained outstanding. The Company recorded the fair value of the Warrants at issuance as a reduction to the loan payable balance. The discount is being accreted to interest expense over the period that the loan will be outstanding. The offsetting credit was recorded as warrants to purchase shares subject to redemption in the long-term liabilities section on the consolidated balance sheets. The fair value of the Warrants was remeasured at each reporting period through the closing of the IPO and changes in fair value The Amended Loan Agreement contains negative covenants restricting the Company’s activities, including limitations on dispositions, mergers or acquisitions, incurring indebtedness or liens, paying dividends or making investments and certain other business transactions. There are no financial covenants associated with the Amended Loan Agreement. The obligations under the Amended Loan Agreement are subject to acceleration upon the occurrence of specified events of default, including a material adverse change in the Company’s business, operations or financial or other condition. The Company has determined that the risk of subjective acceleration under the material adverse events clause is remote and therefore has classified the outstanding principal in current and long-term liabilities based on scheduled principal payments. The Loan Agreement was previously amended in December 2014 in conjunction with the formation of the Security Corporation, requiring the Company to maintain a balance of cash and cash equivalents in the Company’s operating, depository and securities accounts in an amount not less than 125% of the outstanding debt balance so long as the Company maintains a cash balance in the Security Corporation. At December 31, 2017 and 2016, the Company was required to have and had a balance of not less than $12.1 million and $12.5 million, respectively, included in cash and cash equivalents, representing 125% of the outstanding debt balances at December 31, 2017 and 2016 of $9.7 million and $10.0 million, respectively, in the Company’s operating account. The Company evaluated the Loan Agreement and Amended Loan Agreement for embedded features that require bifurcation, noting that the contingent interest feature and the incremental interest upon an event of default were required to be bifurcated, but were concluded to be de minimus in value at inception and at December 31, 2017 and 2016.

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2017
Temporary Equity Disclosure [Abstract]
Convertible Preferred Stock	8. Convertible Preferred Stock Series A Convertible Preferred Stock From September 2011 to May 2014, the Company issued a total of 18,367,344 shares of Series A Preferred Stock at $0.98 per share for total net proceeds of $17.9 million. Series B Convertible Preferred Stock In November 2014, the Company issued 19,841,270 shares of the Series B Convertible Preferred Stock (“Series B Preferred Stock”) at $1.26 per share for net proceeds of $24.9 million. Series C Convertible Preferred Stock In November 2015, the Company issued 20,000,000 shares of the Series C Preferred Stock (collectively with the Series A Preferred Stock and Series B Preferred Stock, the “Preferred Stock”) at $2.65 per share for net proceeds of $52.8 million. Conversion to Common Stock Upon the closing of the IPO, all of the outstanding shares of convertible preferred stock automatically converted into 13,945,509 shares of common stock. Subsequent to the closing of the IPO, there were no shares of preferred stock outstanding. In connection with the IPO, the Company amended and restated its certificate of incorporation to change the authorized capital stock to 125,000,000 shares designated as common stock and 5,000,000 shares designated as preferred stock, all with a par value of $0.001 per share.

Stockholders_ Equity (Deficit)

Stockholders’ Equity (Deficit)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Stockholders’ Equity (Deficit)	9. Stockholders’ Equity (Deficit) Common Stock The holders of common stock are entitled to one vote for each share held. Common stockholders are not entitled to receive dividends, unless declared by the Board of Directors. The Company has reserved for future issuances the following shares of common stock as of December 31, 2017 and 2016: December 31, 2017 December 31, 2016 Series A convertible preferred stock — 4,400,410 Series B convertible preferred stock — 4,753,536 Series C convertible preferred stock — 4,791,563 Warrants 9,040 43,265 Stock options 3,538,345 2,130,560 Employee stock purchase plan 206,284 — Total 3,753,669 16,119,334 Restricted Common Stock In 2011 and 2012, the Company issued a total of 1,211,035 shares of restricted common stock to founders, employees, directors and consultants. These shares of common stock vested over four years. If any of these individuals ceased to be employed or to provide services to the Company prior to vesting, the Company had the right to repurchase any unvested shares of common stock at the price paid by the holder. 675 shares of restricted stock with a weighted average grant date fair value of $0.42 per share remained unvested at December 31, 2015. All of these shares vested during the year ended December 31, 2016. The fair value of restricted stock awards that vested during the year ended December 31, 2016, based on estimated fair values of the stock underlying the restricted stock awards on the day of vesting was $800. There were no shares of unvested restricted stock awards outstanding at December 31, 2017 and 2016.

Stock Incentive Plans

Stock Incentive Plans	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Incentive Plans	10. Stock Incentive Plans Stock Option Plans On October 16, 2017, the Company’s stockholders approved the 2017 Stock Option and Incentive Plan (“2017 Plan”), which became effective on October 31, 2017. As of the effective date of the 2017 Plan, no further grants will be made under the 2011 Plan. The 2017 Plan initially provides for the grant of awards for 2,038,021 shares of common stock. In addition to the shares available for grant under the 2017 Plan, any awards outstanding under the 2011 Plan as of the October 31, 2017 that are cancelled, forfeited or otherwise terminated without being exercised, the number of shares underlying such awards will be available for future grant under the 2017 Plan. The 2017 Plan also provides that an additional number of shares will automatically be added to the shares authorized for issuance under the 2017 Plan on January 1 of each year. The number of shares added each year will be equal to the lesser of: (i) 4% of the outstanding shares on the immediately preceding December 31 or (ii) such amount as determined by the Compensation Committee of the registrant’s Board of Directors. On January 1, 2018, the shares available for grant under the 2017 Plan was automatically increased by 827,786 shares. All of the Company’s employees, officers, directors, consultants and advisors are eligible to be granted options, restricted stock units (“RSUs”), and other share-based awards under the terms of the 2017 Plan. As of December 31, 2017, 2,030,221 shares of common stock were available for future grant under the 2017 Plan. All stock option grants are nonstatutory stock options except option grants to employees (including officers and directors) intended to qualify as incentive stock options under the Internal Revenue Code of 1986, as amended. Incentive stock options may not be granted at less than the fair market value of the Company’s common stock on the date of grant. Nonqualified stock options may be granted at an exercise price established by the Board of Directors at its sole discretion (which has not been less than fair market value on the date of grant) and the vesting periods may vary. Vesting periods are generally four years and are determined by the Board of Directors or a delegated subcommittee. Stock options become exercisable as they vest. Options granted under the 2017 and 2011 Plans expire no more than 10 years from the date of grant. Stock-based compensation expense included in the Company’s statements of operations and comprehensive loss is as follows (in thousands): Years Ended December 31, 2017 2016 2015 Research and development $ 147 $ 68 $ 74 General and administrative 370 183 133 Total $ 517 $ 251 $ 207 The fair value of each stock option granted to employees and directors was estimated on the date of grant using the Black-Scholes option-pricing model, with the following range of assumptions for the years ended December 31, 2017, 2016 and 2015: Years Ended December 31, 2017 2016 2015 Risk-free interest rate 1.9% – 2.3% 1.3% – 1.7% 1.8% – 1.9% Expected dividend yield —% —% —% Expected term (in years) 5.6 – 6.3 5.4 – 6.4 6.25 Expected volatility 81% 77% 85% The expense related to awards granted to employees and directors for their service on the Board of Directors was $0.5 million, $0.2 million, and $0.2 million for the years ended December 31, 2017, 2016 and 2015, respectively. The fair value of each stock option granted to non-employees was estimated on the date of grant using the Black-Scholes option-pricing model, with the following range of assumptions for the years ended December 31, 2016 and 2015: Years Ended December 31, 2016 2015 Risk-free interest rate 1.9% – 2.4% 2.4 % Expected dividend yield —% — % Expected term (in years) 8.9 – 10.0 10.0 Expected volatility 89% 85 % We did not grant any stock options to non-employees during the year ended December 31, 2017. The expense related to awards granted to non-employees was $65,000, $24,000 and $28,000 for the years ended December 31, 2017 and 2016 and 2015, respectively. A summary of the stock option activity under the 2017 and 2011 Plans is as follows: Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Life (in years) Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2016 1,348,845 $ 1.29 8.6 $ 3,642 Granted 209,797 5.50 Exercised (33,061 ) 0.86 Cancelled (17,457 ) 1.87 Outstanding at December 31, 2017 1,508,124 $ 1.89 7.9 $ 12,328 Exercisable at December 31, 2017 787,375 $ 1.24 7.4 $ 6,945 Vested and expected to vest at December 31, 2017 1,508,124 $ 1.89 7.9 $ 12,328 The weighted-average fair value of options granted to employees and directors for their service on the Board of Directors during the years ended December 31, 2017, 2016 and 2015 was $6.16 ,$1.13 and $0.83 per share, respectively. The weighted-average fair value of options granted to non-employees during the year ended December 31, 2016 and 2015 was $1.38 and $1.00 per share, respectively. There were no options granted to non-employees during the year ended December 31, 2017. 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 total intrinsic value of options exercised during the years ended December 31, 2017 and 2016 was $276,000 and $21,000, respectively. As of December 31, 2017, total unrecognized stock-based compensation expense relating to unvested stock options was $1.8 million. This amount is expected to be recognized over a weighted-average period of 2.4 years.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes New Tax Legislation On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act (“TCJA”). This legislation reduced the U.S. corporate tax rate from the current rate of 34% to 21% for tax years beginning after December 31, 2017. As a result of the enacted law, the Company was required to revalue deferred tax assets and liabilities existing as of December 31, 2017 from the 34% federal rate in effect through the end of 2017, to the new 21% rate. The Company has recognized the impact of the Tax Reform Act in these consolidated financial statements and related disclosures. Due to the complexities involved in accounting for the enactment of the Tax Reform Act, the SEC staff issued Staff Accounting Bulletin No. 118 ("SAB 118"), which allows a registrant to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. In accordance with SAB 118, the Company recorded provisional amounts reflecting the impact of the Tax Reform Act in these consolidated financial statements and related disclosures. The impact of the remeasurement of the Company’s U.S. deferred tax assets and liabilities to 21% resulted in the reduction of deferred tax assets of approximately $9.3 million, which is offset by a full valuation allowance. There was no impact to the Company’s income statement due to the reduction in the U.S. corporate tax rate. Our preliminary estimate of the TCJA and the remeasurement of our deferred tax assets and liabilities is subject to the finalization of management’s analysis related to certain matters, such as developing interpretations of the provisions of the TCJA, changes to certain estimates and the filing of our tax returns. U.S. Treasury regulations, administrative interpretations or court decisions interpreting the TJCA may require further adjustments and changes in our estimates. The final determination of the TCJA and the remeasurement of the Company’s deferred assets and liabilities will be completed as additional information becomes available, but no later than one year from the enactment of the TCJA. Income Taxes The Company records a provision or benefit for income taxes on pre-tax income or loss based on its estimated effective tax rate for the year. During the year ended December 31, 2017, the Company recorded a net loss of approximately $21.7 million and, since it maintains a full valuation allowance on its deferred tax assets, the Company did not record an income tax benefit for the year ended December 31, 2017. Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. The significant components of the Company’s deferred tax assets are comprised of the following (in thousands): December 31, 2017 2016 Deferred tax assets: Net operating loss carryforwards $ 21,190 $ 23,028 Research and development credits 4,732 3,418 Licenses 24 38 Other 370 118 Total gross deferred tax asset 26,316 26,602 Less: Valuation allowance (26,316 ) (26,602 ) Net deferred tax asset $ — $ — ASC 740 requires a valuation allowance to reduce the deferred tax assets reported if, based on the weight of the evidence, it is more likely than not that some portion or all of the deferred tax assets will not be realized. After consideration of all the evidence, both positive and negative, the Company has recorded a valuation allowance against its deferred tax assets at December 31, 2017 and 2016, respectively, because the Company's management has determined that it is more likely than not that the Company will not recognize the benefits of its federal and state deferred tax assets primarily due to its cumulative loss position and, as a result, a valuation allowance of $26.3 million and $26.6 million has been established at December 31, 2017 and 2016, respectively. The valuation allowance decreased by approximately $0.3 million in the year ended December 31, 2017 due primarily to the revaluation of the deferred tax assets at a 21% Federal tax rate and increased by approximately $6.3 million during the year ended December 31, 2016 due primarily to the generation of net operating losses. The Company adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting on January 1, 2017, however the Company did not have any NOL carryforwards associated with deductible stock option exercises as of December 31, 2016 and therefore the adoption of ASU 2016-09 has no impact on the Company’s operations, financial position or cash flows. A reconciliation of income tax expense computed at the statutory federal income tax rate to income taxes reflected in the financial statements is as follows: 2017 2016 2015 Income tax computed at federal statutory tax rate 34.0 % 34.0 % 34.0 % Permanent differences (2.4 )% (0.5 )% (0.4 )% State taxes, net of federal benefit 4.8 % 5.1 % (0.1 )% Research and development and other tax credits 5.0 % 6.1 % 5.5 % Other — % — % 0.1 % Federal rate change (42.7 )% — % — % Change in deferred tax asset valuation allowance 1.3 % (44.7 )% (39.1 )% — % — % — % As of December 31, 2017 and 2016, the Company had U.S. federal net operating loss carryforwards of approximately $77.9 million and $58.8 million, respectively, which may be available to offset future income tax liabilities and expire at various dates through 2037. As of December 31, 2017 and 2016, the Company also had U.S. state net operating loss carryforwards of approximately $76.5 million and $57.5 million, respectively, which may be available to offset future income tax liabilities and expire at various dates through 2037. As of December 31, 2017 and 2016, the Company had federal research and development tax credit carryforwards of approximately $3.3 million and $2.4 million, respectively, available to reduce future tax liabilities which expire at various dates through 2037. As of December 31, 2017 and 2016, the Company had state research and development tax credit carryforwards of approximately $1.9 million and $1.6 million, respectively, available to reduce future tax liabilities which expire at various dates through 2032. The Company has generated research credits but has not conducted a study to document the qualified activity. This study may result in an adjustment to the Company’s research and development credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s research and development credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the deferred tax asset established for the research and development credit carryforwards and the valuation allowance. Under the provisions of the Internal Revenue Code, the net operating loss and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating loss and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three-year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has completed several financings since its inception which resulted in a change in control as defined by Sections 382 and 383 of the Internal Revenue Code. Ownership changes, as defined in the Internal Revenue Code, including those resulting from the issuance of common stock in connection with our public offerings, may limit the amount of net operating loss and tax credit carryforwards that can be utilized to offset future taxable income or tax liability. The amount of the limitation is determined in accordance with Section 382 of the Internal Revenue Code. We have performed an analysis of ownership changes through December 31, 2017. Based on this analysis, we do not believe that any of our tax attributes will expire unutilized due to Section 382 limitations. The Company will recognize interest and penalties related to uncertain tax positions in income tax expense. As of December 31, 2017 and 2016, the Company had no accrued interest or penalties related to uncertain tax positions and no such amounts have been recognized in the Company’s statements of operations and comprehensive loss.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	12. Related Party Transactions From September 2015 to June 2017, the Company received consulting and management services from one of its investors, Third Rock Ventures LLP (“Third Rock Ventures”). The Company paid Third Rock Ventures $2,000, $69,000 and $19,000 for these services during the years ended December 31, 2017, 2016 and 2015, respectively. No amounts were payable to Third Rock Ventures at December 31, 2017 and 2016.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	13. Commitments and Contingencies In August 2011 and October 2013, the Company and an independent third party entered into operating leases for approximately 6,055 square feet of office space in Newton, MA (“Newton Lease”) and approximately 3,170 square feet of laboratory space in Natick, MA, respectively. Base rent for the office space during the initial rent period was approximately $0.1 million per year and increased annually. Base rent for the lab space was approximately $59,000 annually. Rent expense, inclusive of the escalating rent payments and free rent period, was recognized on a straight-line basis over the term of each lease agreement. The Company and the independent third party were each jointly and severally liable for the obligations under both leases. In October 2016, the Newton lease was amended to extend the term one year to May 2018 and, effective June 1, 2017 removed the independent third party from the lease and all related obligations of the lease. In December 2017, the Newton lease was amended again to increase the amount of office space to approximately 7,795 square feet and extend the term to December 31, 2020. The amended lease for laboratory space expired October 31, 2017. In March 2015, the Company and the independent third party entered into a sublease agreement for approximately 5,385 square feet of office space in Newton, MA that expired in February 2017. In August 2016, the Company entered into an operating lease for approximately 3,890 square feet of laboratory space in Sudbury, MA. This lease was to expire in August 2017. In February 2017, the Company amended this lease to extend the term to February 2019 and increase the amount of rentable space to approximately 5,133 square feet, with an option to lease another 2,029 square feet. Base rent for this space is approximately $0.1 million annually. Rent expense includes the Company’s allocated portion of rental obligations under the leases. The Company recorded $0.4 million, $0.2 million and $0.2 million, of rent expense for the years ended December 31, 2017, 2016 and 2015, respectively. The minimum aggregate future operating lease commitments at December 31, 2017 are as follows (in thousands): December 31, 2017 2018 $ 458 2019 379 2020 362 $ 1,199

License Agreement

License Agreement	12 Months Ended
	Dec. 31, 2017
License Agreement [Abstract]
License Agreement	14. License Agreement In March 2012, the Company entered into an exclusive license agreement (“License Agreement”) with Althea Technologies, Inc. (“Althea”) for certain intellectual property. The Company reimbursed Althea for patent related fees and costs incurred by Althea totaling $0.1 million in the aggregate and issued a total of 88,186 shares of common stock to Althea. Under the terms of the License Agreement, the Company agreed to pay annual license maintenance fees, milestone payments and royalties as a percentage of net sales. Annual license maintenance fees are creditable against royalties earned during the same calendar year and are not material to the financial statements. Milestone payments are triggered upon the achievement of specified development, regulatory and commercialization milestones and are not creditable against royalties. Actual amounts due under the License Agreement will vary depending on the number of products developed, the type and development path of the products, and other related factors. Milestone payments could total up to $56.0 million. The Company may terminate the agreement at any time with 60 days prior written notice.

Employee Benefit Plan

Employee Benefit Plan	12 Months Ended
	Dec. 31, 2017
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]
Employee Benefit Plan	15. Employee Benefit Plan Effective January 2012, employees of the Company are eligible to participate in the Company’s 401(k) retirement plan (“401(k) Plan”). Participants may contribute up to 100% of their annual compensation to the 401(k) Plan, subject to statutory limitations. The 401(k) Plan does not allow the Company to make matching contributions.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	16. Selected Quarterly Financial Data (Unaudited) The following table contains quarterly financial information for 2017 and 2016. The Company believes that the following information reflects all normal recurring adjustments necessary for the fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. 2017 First Quarter Second Quarter Third Quarter Fourth Quarter Total (in thousands, except per share amounts) Total operating expenses $ 5,543 $ 4,474 $ 4,362 $ 6,571 $ 20,950 Loss from operations (5,543 ) (4,474 ) (4,362 ) (6,571 ) (20,950 ) Net loss (5,671 ) (4,632 ) (4,670 ) (6,677 ) (21,650 ) Net loss attributable to common stockholders - basic and diluted $ (4.24 ) $ (3.46 ) $ (3.49 ) $ (0.48 ) $ (4.80 ) 2016 First Quarter Second Quarter Third Quarter Fourth Quarter Total (in thousands, except per share amounts) Total operating expenses $ 5,041 $ 7,041 $ 6,102 $ 6,002 $ 24,186 Loss from operations (5,041 ) (7,041 ) (6,102 ) (6,002 ) (24,186 ) Net loss (5,113 ) (7,039 ) (6,136 ) (6,219 ) (24,507 ) Net loss attributable to common stockholders - basic and diluted $ (3.85 ) $ (5.26 ) $ (4.59 ) $ (4.65 ) $ (18.35 )

Summary of Significant Accoun24

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standard Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”).
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the accounts of Allena Pharmaceuticals, Inc. and its wholly owned subsidiaries Allena Pharmaceuticals Security Corporation (“Security Corporation”), which was incorporated in December 2014, and Allena Pharmaceuticals Ireland Limited, which was incorporated in March 2017. All intercompany transactions and balances have been eliminated.
Use of Estimates	Use of Estimates The preparation of the Company's consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, expenses and related disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of expenses during the reporting period. On an ongoing basis, the Company's management evaluates its estimates, which include but are not limited to management’s judgments of accrued expenses, fair value of common stock prior to the closing of the Company’s IPO, valuation of share-based awards, fair value of warrants, prior to the conversion into warrants for the purchase of common stock upon the closing of the IPO, and income taxes. Actual results could differ from those estimates. The Company utilized significant estimates and assumptions in determining the fair value of its common stock. The Company utilized various valuation methodologies in accordance with the framework of the American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation
Segment Information	Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision-maker in deciding how to allocate resources and assess performance. The Company's chief operating decision-maker, the Company's chief executive officer, views the Company's operations and manages its business as a single operating segment, which is the business of discovering and developing non-absorbed oral enzyme therapeutics.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. The Company’s only element of other comprehensive income (loss) is unrealized gains and losses on available-for-sale investments.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less to be cash equivalents. Cash and cash equivalents include cash held in banks and amounts held in money market funds. Cash equivalents are stated at cost, which approximates market value. Cash and cash equivalents consist of the following at December 31, 2017 and 2016 (in thousands): December 31, 2017 2016 Cash and cash equivalents: Cash $ 749 $ 948 Money market funds 93,745 24,302 $ 94,494 $ 25,250
Investments	Investments The Company invests available cash primarily in U.S. and foreign corporate debt securities and United States government treasury securities. The Company classifies its investments as available-for-sale. Available-for-sale investments are carried at fair value with unrealized gains and losses included in stockholders’ (deficit) equity. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense. All investments are classified as current assets as they have contractual maturities of less than one year and are available to meet working capital needs and to fund current operations. The cost of securities sold is determined on a specific identification basis, and realized gains and losses are included in interest income (expense) within the statement 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 statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented.
Concentration of Credit Risk and Off-Balance Sheet Risk	Concentration of Credit Risk and Off-Balance Sheet Risk Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and investments. The Company maintains all of its cash, cash equivalents and investments balances at a single accredited financial institution, in amounts that exceed federally insured limits. The Company generally invests its excess cash in money market funds, corporate notes and United States government securities that are subject to minimal credit and market risk. Management has established guidelines relative to credit ratings and maturities intended to safeguard principal balances and maintain liquidity. The investment portfolio is maintained in accordance with the Company’s investment policy, which defines allowable investments, specifies credit quality standards and limits the credit exposure of any single issuer. The Company has no significant off-balance sheet risk such as foreign exchange contracts, option contracts, or other foreign hedging arrangements.
Significant Suppliers	Significant Suppliers The Company is dependent on third-party manufacturers to supply products for research and development activities of its programs, including preclinical and clinical testing. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Fair value is defined as the price that would be received upon sale of an asset or paid to transfer a liability between market participants at measurement dates. ASC Topic 820, Fair Value Measurement Level 1 inputs: Quoted prices in active markets for identical assets or liabilities. Level 2 inputs: Inputs other than quoted prices included within Level 1 that are either directly or indirectly observable, such as quoted market prices, interest rates and yield curves. Level 3 inputs: Unobservable inputs developed using estimates or assumptions developed by the Company, which reflect those that a market participant would use in pricing the asset or liability. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument's level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.
Property and Equipment	Property and Equipment Property and equipment consists of laboratory equipment, computer equipment, software and leasehold improvements recorded at cost. These amounts are depreciated using the straight-line method over the estimated useful lives of the assets as follows: Laboratory equipment 4 years Computer equipment 3 years Software 5 years Leasehold improvements Shorter of useful life or remaining term of related lease Upon retirement or sale, the cost of the assets disposed of and the related accumulated depreciation are eliminated from the balance sheet and related gains or losses are reflected in the statement of operations and comprehensive loss. Repairs and maintenance costs are expensed as incurred and costs of significant improvements are capitalized.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company continually evaluates long-lived assets for potential impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparing the book values of the assets to the expected future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book values of the assets exceed their fair value. The Company did not recognize any impairment losses for the years ended December 31, 2017, 2016 and 2015.
Warrants to Purchase Shares Subject to Redemption	Warrants to Purchase Shares Subject to Redemption Prior to the closing of the IPO, the Company accounted for warrant instruments that either conditionally or unconditionally obligate the issuer to transfer assets as liabilities regardless of the timing of the redemption feature or price, even though the underlying shares may be classified as equity. These warrants were subject to revaluation at each balance sheet date, and any changes in fair value were recorded as a component of other income (expense) in the statements of operations and comprehensive loss. Upon the closing of the IPO, the warrants met the criteria to be classified in stockholders’ equity and the fair value of the warrant liability as of the IPO date was reclassified to stockholders’ equity (deficit).
Research and Development	Research and Development The Company expenses all costs incurred in performing research and development activities. Research and development expenses include salaries and benefits, materials and supplies, preclinical and clinical trial expenses, manufacturing expenses, stock-based compensation expense, depreciation of equipment, contract services and other outside expenses. Costs of certain development activities, such as manufacturing, are recognized based on an evaluation of the progress to completion of specific tasks. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the consolidated financial statements as prepaid or accrued research and development costs. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred and capitalized. The capitalized amounts are expensed as the related goods are delivered or the services are performed.
Patent Costs	Patent Costs The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the accompanying consolidated statements of operations and comprehensive loss.
Accounting for Stock-Based Compensation	Accounting for Stock-Based Compensation The Company accounts for its stock-based compensation in accordance with ASC Topic 718, Compensation—Stock Compensation Equity-Based Payments to Non-Employees The Black-Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the expected term of the award, (c) the risk-free interest rate and (d) expected dividends. Due to the lack of a public market for the Company’s common stock and a lack of company-specific historical and implied volatility data, the Company has based its computation of expected volatility on the historical volatility of a representative group of public companies with similar characteristics to the Company, including stage of product development and life science industry focus. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The Company uses the simplified method as prescribed by the SEC Staff Accounting Bulletin No. 107, Share-Based Payment In the first quarter of 2017, the Company made an accounting policy election to recognize forfeitures as they occur upon adoption of guidance per ASU No. 2016-09, Compensation – Stock Compensation The Company expenses the fair value of its share-based compensation awards to employees on a straight-line basis over the requisite service period, which is generally the vesting period. Stock-based compensation awards to non-employees are adjusted through stock-based compensation expense at each reporting period end to reflect the current fair value of such awards and are expensed on a straight-line basis.
Income Taxes	Income Taxes The Company accounts for income taxes using the liability method in accordance with ASC Topic 740, Income Taxes The Company recognizes the effect of income tax positions only if those positions are more likely than not of being sustained. Recognized income tax positions are measured at the largest amount that is greater than 50% likely of being realized. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs.
Net Loss Per Share	Net Loss Per Share The Company has reported losses since inception and has computed basic net loss per share attributable to common stockholders by dividing net loss attributable to common stockholders by the weighted-average number of common shares outstanding for the period, without consideration for potentially dilutive securities. The Company has computed diluted net loss per common share after giving consideration to all potentially dilutive common shares, including options to purchase common stock, restricted common stock, convertible preferred stock and warrants to purchase convertible preferred stock, outstanding during the period determined using the treasury-stock and if-converted methods, except where the effect of including such securities would be anti-dilutive. Because the Company has reported net losses since inception, these potential common shares have been anti-dilutive and basic and diluted loss per share have been the same. Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share data): Years Ended December 31, 2017 2016 2015 Numerator: Net loss $ (21,650 ) $ (24,507 ) $ (14,247 ) Accretion of convertible preferred stock (58 ) (69 ) (37 ) Net loss attributable to common stockholders $ (21,708 ) $ (24,576 ) $ (14,284 ) Denominator: Weighted-average common shares – basic and diluted 4,520,337 1,339,254 1,258,123 Net loss per share attributable to common stockholders –basic and diluted $ (4.80 ) $ (18.35 ) $ (11.35 ) The following table sets forth the potentially dilutive securities that have been excluded from the calculation of diluted net loss per share because to include them would be anti-dilutive (in common stock equivalent shares): Years Ended December 31, 2017 2016 2015 Series A convertible preferred stock — 4,400,410 4,400,410 Series B convertible preferred stock — 4,753,536 4,753,536 Series C convertible preferred stock — 4,791,563 4,791,563 Warrants 9,040 43,265 34,225 Stock options 1,508,124 1,348,845 759,167 Restricted stock — — 675 Total 1,517,164 15,337,619 14,739,576
Loss Contingencies	Loss Contingencies In accordance with ASC 450, Contingencies
Guarantees	Guarantees The Company has identified the guarantees described below as disclosable, in accordance with ASC 460, Guarantees As permitted under Delaware law, the Company indemnifies its officers and directors for certain events or occurrences while the officer or director is, or was, serving at the Company’s request in such capacity. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ and officers’ insurance coverage that should limit its exposure and enable it to recover a portion of any future amounts paid. The Company is a party to a number of agreements entered into in the ordinary course of business that contain typical provisions that obligate the Company to indemnify the other parties to such agreements upon the occurrence of certain events. Such indemnification obligations are usually in effect from the date of execution of the applicable agreement for a period equal to the applicable statute of limitations. The aggregate maximum potential future liability of the Company under such indemnification provisions is uncertain. The Company leases office space under several noncancelable operating leases. The Company has standard indemnification arrangements under these leases that require it to indemnify the landlord against all costs, expenses, fines, suits, claims, demands, liabilities, and actions directly resulting from any breach, violation, or nonperformance of any covenant or condition of the respective lease. As of December 31, 2017, 2016 and 2015, the Company had not experienced any losses related to these indemnification obligations, and no material claims with respect thereto were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves have been established.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its consolidated financial position or results of operations upon adoption. In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements—Going Concern In April 2015, the FASB issued ASU No. 2015-03, Simplifying the Presentation of Debt Issuance Costs In November 2015, the FASB issued ASU No. 2015-17, Balance Sheet Classification of Deferred Taxes In March 2016, the FASB issued ASU No. 2016-09, Compensation-Stock Compensation Compensation-Stock Compensation.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments In May 2017, the FASB issued ASU No. 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting

Summary of Significant Accoun25

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Cash and Cash Equivalents	Cash and cash equivalents consist of the following at December 31, 2017 and 2016 (in thousands): December 31, 2017 2016 Cash and cash equivalents: Cash $ 749 $ 948 Money market funds 93,745 24,302 $ 94,494 $ 25,250
Summary of Estimated Useful Lives of Assets	These amounts are depreciated using the straight-line method over the estimated useful lives of the assets as follows: Laboratory equipment 4 years Computer equipment 3 years Software 5 years Leasehold improvements Shorter of useful life or remaining term of related lease
Summary 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 data): Years Ended December 31, 2017 2016 2015 Numerator: Net loss $ (21,650 ) $ (24,507 ) $ (14,247 ) Accretion of convertible preferred stock (58 ) (69 ) (37 ) Net loss attributable to common stockholders $ (21,708 ) $ (24,576 ) $ (14,284 ) Denominator: Weighted-average common shares – basic and diluted 4,520,337 1,339,254 1,258,123 Net loss per share attributable to common stockholders –basic and diluted $ (4.80 ) $ (18.35 ) $ (11.35 )
Summary of Anti-dilutive Securities Excluded from Calculation of Diluted Net Loss per Share	The following table sets forth the potentially dilutive securities that have been excluded from the calculation of diluted net loss per share because to include them would be anti-dilutive (in common stock equivalent shares): Years Ended December 31, 2017 2016 2015 Series A convertible preferred stock — 4,400,410 4,400,410 Series B convertible preferred stock — 4,753,536 4,753,536 Series C convertible preferred stock — 4,791,563 4,791,563 Warrants 9,040 43,265 34,225 Stock options 1,508,124 1,348,845 759,167 Restricted stock — — 675 Total 1,517,164 15,337,619 14,739,576

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Summary of Investments	The following table summarizes the Company’s investments as of December 31, 2016 (in thousands): Amortized Gross Unrealized Estimated Cost Gains Losses Fair Value Available-for-sale securities: U.S. treasury securities $ 13,258 $ 3 $ (2 ) $ 13,259 U.S. corporate debt securities 7,397 — (3 ) 7,394 Foreign corporate debt securities 2,852 — — 2,852 Total available-for-sale securities $ 23,507 $ 3 $ (5 ) $ 23,505

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Summary of Financial Assets and Liabilities Measured at Fair Value	The following tables present information about the Company’s financial assets and liabilities that have been measured at fair value at December 31, 2017 and 2016 and indicates the fair value hierarchy of the valuation inputs utilized to determine such fair value (in thousands): Description December 31, 2017 Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds, included in cash and cash equivalents $ 93,745 $ 93,745 $ — $ — Total assets $ 93,745 $ 93,745 $ — $ — Liabilities: Warrants for the purchase of shares subject to redemption $ — $ — $ — $ — Total liabilities $ — $ — $ — $ — Description December 31, 2016 Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds, included in cash and cash equivalents $ 24,302 $ 24,302 $ — $ — Investments: U.S. treasury securities 13,259 13,259 — — U.S corporate debt securities 7,394 — 7,394 Foreign corporate debt securities 2,852 — 2,852 — Total assets $ 47,807 $ 37,561 $ 10,246 $ — Liabilities: Warrants for the purchase of shares subject to redemption $ 267 $ — $ — $ 267 Total liabilities $ 267 $ — $ — $ 267
Schedule of Assumptions Used in Valuing Warrants	The following assumptions were used in valuing the Warrants: November 6, 2017 December 31, 2016 Risk-free interest rate 2.1% - 2.2% 2.3% - 2.5% Expected dividend yield —% —% Expected term (in years) 6.8 - 8.5 7.6 - 9.3 Expected volatility 84% - 86% 90%
Summary of Changes in Fair Value of Warrants, Represented a Recurring Measurement Classified Within Level 3 of Fair Value Hierarchy, using Significant Unobservable Inputs	The following table sets forth a summary of changes in the fair value of the Warrants, which represented a recurring measurement classified within Level 3 of the fair value hierarchy, wherein fair value was estimated using significant unobservable inputs (in thousands, except share data): Balance at December 31, 2015 $ 68 Initial fair value of Series C Warrant for the purchase of 28,302 shares 45 Initial fair value of Series C Warrant for the purchase of 9,434 shares 22 Change in fair value of Warrants included in other income (expense) 132 Balance at December 31, 2016 267 Change in fair value of Warrants through the date of the IPO included in other income (expense) 257 Fair value of Warrants reclassified to stockholders' equity (deficit) upon conversion to warrants for the conversion of common stock (524 ) Balance at December 31, 2017 $ —

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Summary of Property and Equipment, Net	Property and equipment includes the following at December 31, 2017 and 2016 (in thousands): December 31, 2017 2016 Property and equipment: Laboratory equipment $ 263 $ 232 Computer equipment 6 6 Software 39 39 308 277 Less: Accumulated depreciation (181 ) (108 ) Property and equipment, net $ 127 $ 169

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following (in thousands): December 31, 2017 2016 Payroll and employee-related expenses $ 1,132 $ 914 Professional fees 420 172 Third-party research and development expenses 135 413 Loan interest 40 28 IPO-related costs 111 — Other 111 83 Total accrued expenses $ 1,949 $ 1,610

Loan and Security Agreement (Ta

Loan and Security Agreement (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Summary of Minimum Aggregate Future Loan and Interest Payments	The minimum aggregate future loan and interest payments at December 31, 2017 are as follows (in thousands): Years Ending December 31, 2018 $ 4,377 2019 4,185 2020 2,512 Total minimum payments 11,074 Less: Amount representing interest (1,407 ) Less: Discount (281 ) Less: Current portion (3,870 ) Loan payable, net of current portion $ 5,516

Stockholders_ Equity (Deficit)

Stockholders’ Equity (Deficit) (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Summary of Shares of Common Stock Reserved	The Company has reserved for future issuances the following shares of common stock as of December 31, 2017 and 2016: December 31, 2017 December 31, 2016 Series A convertible preferred stock — 4,400,410 Series B convertible preferred stock — 4,753,536 Series C convertible preferred stock — 4,791,563 Warrants 9,040 43,265 Stock options 3,538,345 2,130,560 Employee stock purchase plan 206,284 — Total 3,753,669 16,119,334

Stock Incentive Plans (Tables)

Stock Incentive Plans (Tables)	12 Months Ended
	Dec. 31, 2017
Summary of Stock-based Compensation Expense	Stock-based compensation expense included in the Company’s statements of operations and comprehensive loss is as follows (in thousands): Years Ended December 31, 2017 2016 2015 Research and development $ 147 $ 68 $ 74 General and administrative 370 183 133 Total $ 517 $ 251 $ 207
Summary of Stock Option Activity	A summary of the stock option activity under the 2017 and 2011 Plans is as follows: Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Life (in years) Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2016 1,348,845 $ 1.29 8.6 $ 3,642 Granted 209,797 5.50 Exercised (33,061 ) 0.86 Cancelled (17,457 ) 1.87 Outstanding at December 31, 2017 1,508,124 $ 1.89 7.9 $ 12,328 Exercisable at December 31, 2017 787,375 $ 1.24 7.4 $ 6,945 Vested and expected to vest at December 31, 2017 1,508,124 $ 1.89 7.9 $ 12,328
Employees and Directors
Schedule of Assumptions Used in Black-Scholes Option Pricing Model to Estimate Fair Value of Stock Options	The fair value of each stock option granted to employees and directors was estimated on the date of grant using the Black-Scholes option-pricing model, with the following range of assumptions for the years ended December 31, 2017, 2016 and 2015: Years Ended December 31, 2017 2016 2015 Risk-free interest rate 1.9% – 2.3% 1.3% – 1.7% 1.8% – 1.9% Expected dividend yield —% —% —% Expected term (in years) 5.6 – 6.3 5.4 – 6.4 6.25 Expected volatility 81% 77% 85%
Non-Employees
Schedule of Assumptions Used in Black-Scholes Option Pricing Model to Estimate Fair Value of Stock Options	The fair value of each stock option granted to non-employees was estimated on the date of grant using the Black-Scholes option-pricing model, with the following range of assumptions for the years ended December 31, 2016 and 2015: Years Ended December 31, 2016 2015 Risk-free interest rate 1.9% – 2.4% 2.4 % Expected dividend yield —% — % Expected term (in years) 8.9 – 10.0 10.0 Expected volatility 89% 85 %

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Summary of Deferred Tax Assets	The significant components of the Company’s deferred tax assets are comprised of the following (in thousands): December 31, 2017 2016 Deferred tax assets: Net operating loss carryforwards $ 21,190 $ 23,028 Research and development credits 4,732 3,418 Licenses 24 38 Other 370 118 Total gross deferred tax asset 26,316 26,602 Less: Valuation allowance (26,316 ) (26,602 ) Net deferred tax asset $ — $ —
Reconciliation of Income Taxes Computed at Statutory Federal Income Tax Rate to Income Taxes Reflected In Financial Statements	A reconciliation of income tax expense computed at the statutory federal income tax rate to income taxes reflected in the financial statements is as follows: 2017 2016 2015 Income tax computed at federal statutory tax rate 34.0 % 34.0 % 34.0 % Permanent differences (2.4 )% (0.5 )% (0.4 )% State taxes, net of federal benefit 4.8 % 5.1 % (0.1 )% Research and development and other tax credits 5.0 % 6.1 % 5.5 % Other — % — % 0.1 % Federal rate change (42.7 )% — % — % Change in deferred tax asset valuation allowance 1.3 % (44.7 )% (39.1 )% — % — % — %

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Commitments for Operating Leases	The minimum aggregate future operating lease commitments at December 31, 2017 are as follows (in thousands): December 31, 2017 2018 $ 458 2019 379 2020 362 $ 1,199

Selected Quarterly Financial 35

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	The following table contains quarterly financial information for 2017 and 2016. The Company believes that the following information reflects all normal recurring adjustments necessary for the fair statement of the information for the periods presented. 2017 First Quarter Second Quarter Third Quarter Fourth Quarter Total (in thousands, except per share amounts) Total operating expenses $ 5,543 $ 4,474 $ 4,362 $ 6,571 $ 20,950 Loss from operations (5,543 ) (4,474 ) (4,362 ) (6,571 ) (20,950 ) Net loss (5,671 ) (4,632 ) (4,670 ) (6,677 ) (21,650 ) Net loss attributable to common stockholders - basic and diluted $ (4.24 ) $ (3.46 ) $ (3.49 ) $ (0.48 ) $ (4.80 ) 2016 First Quarter Second Quarter Third Quarter Fourth Quarter Total (in thousands, except per share amounts) Total operating expenses $ 5,041 $ 7,041 $ 6,102 $ 6,002 $ 24,186 Loss from operations (5,041 ) (7,041 ) (6,102 ) (6,002 ) (24,186 ) Net loss (5,113 ) (7,039 ) (6,136 ) (6,219 ) (24,507 ) Net loss attributable to common stockholders - basic and diluted $ (3.85 ) $ (5.26 ) $ (4.59 ) $ (4.65 ) $ (18.35 )

Organization and Basis of Pre36

Organization and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands	Dec. 01, 2017USD ($)shares	Nov. 06, 2017USD ($)$ / sharesshares	Oct. 23, 2017	Dec. 31, 2017USD ($)$ / sharesshares	Nov. 07, 2017shares	Dec. 31, 2016USD ($)$ / sharesshares	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Class of Stock [Line Items]
Net proceeds after deducting underwriting discounts and commissions and offering costs | $				$ 67,197
Convertible preferred stock, shares outstanding					0
Common stock, shares authorized				125,000,000		75,000,000
Common stock, par value | $ / shares				$ 0.001		$ 0.001
Accumulated deficit | $				$ 81,957		$ 60,307
Cash and cash equivalents | $				$ 94,494		$ 25,250	$ 69,011	$ 26,886
Common Stock
Class of Stock [Line Items]
Common stock, shares issued and sold				5,350,302
Reverse stock split ratio			0.2396
Reverse stock split effective date			Oct. 23, 2017
Initial Public Offering
Class of Stock [Line Items]
Common stock, shares issued and sold		5,333,333
Issuance price per shares | $ / shares		$ 14
Gross proceeds from issuance of common stock | $		$ 74,700
Net proceeds after deducting underwriting discounts and commissions and offering costs | $		67,000
Underwriting discounts and commissions and offering costs | $		$ 7,900
Convertible preferred stock converted into common stock		13,945,509		13,945,509
Common stock, shares authorized		125,000,000		125,000,000
Preferred stock, shares authorized		5,000,000
Common stock, par value | $ / shares		$ 0.001
Preferred stock, par value | $ / shares		$ 0.001
Over-allotment Option
Class of Stock [Line Items]
Common stock, shares issued and sold	16,969
Gross proceeds from issuance of common stock | $	$ 200

Summary of Significant Accoun37

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2017USD ($)
Summary Of Significant Accounting Policies [Line Items]
Entity incorporated date	Dec. 31, 2014
Income tax positions recognized, minimum percentage	50.00%
ASU No. 2016-09
Summary Of Significant Accounting Policies [Line Items]
Excess tax benefit associated with stock option exercises	$ 0
Deferred tax asset	$ 0
Allena Pharmaceuticals Security Corporation
Summary Of Significant Accounting Policies [Line Items]
Entity incorporated date	Dec. 31, 2014
Allena Pharmaceuticals Ireland Limited
Summary Of Significant Accounting Policies [Line Items]
Entity incorporated date	Mar. 31, 2017

Summary of Significant Accoun38

Summary of Significant Accounting Policies - Summary of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash and cash equivalents:
Cash	$ 749	$ 948
Money market funds	93,745	24,302
Cash and Cash Equivalents, at Carrying Value	$ 94,494	$ 25,250	$ 69,011	$ 26,886

Summary of Significant Accoun39

Summary of Significant Accounting Policies - Summary of Estimated Useful Lives of Assets (Details)	12 Months Ended
	Dec. 31, 2017
Laboratory Equipment
Property, Plant and Equipment [Line Items]
Laboratory equipment	4 years
Computer Equipment
Property, Plant and Equipment [Line Items]
Laboratory equipment	3 years
Software
Property, Plant and Equipment [Line Items]
Laboratory equipment	5 years
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Leasehold improvements	Shorter of useful life or remaining term of related lease

Summary of Significant Accoun40

Summary of Significant Accounting Policies- Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss	$ (6,677)	$ (4,670)	$ (4,632)	$ (5,671)	$ (6,219)	$ (6,136)	$ (7,039)	$ (5,113)	$ (21,650)	$ (24,507)	$ (14,247)
Accretion of convertible preferred stock									(58)	(69)	(37)
Net loss attributable to common stockholders									$ (21,708)	$ (24,576)	$ (14,284)
Denominator:
Weighted-average common shares – basic and diluted									4,520,337	1,339,254	1,258,123
Net loss per share attributable to common stockholders — basic and diluted	$ (0.48)	$ (3.49)	$ (3.46)	$ (4.24)	$ (4.65)	$ (4.59)	$ (5.26)	$ (3.85)	$ (4.80)	$ (18.35)	$ (11.35)

Summary of Significant Accoun41

Summary of Significant Accounting Policies - Summary of Anti-dilutive Securities Excluded from Calculation of Diluted Net Loss per Share (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the calculation of loss per share	1,517,164	15,337,619	14,739,576
Series A Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the calculation of loss per share		4,400,410	4,400,410
Series B Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the calculation of loss per share		4,753,536	4,753,536
Series C Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the calculation of loss per share		4,791,563	4,791,563
Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the calculation of loss per share	9,040	43,265	34,225
Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the calculation of loss per share	1,508,124	1,348,845	759,167
Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the calculation of loss per share			675

Investments - Additional Inform

Investments - Additional Information (Detail)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)Investment
Investments Debt And Equity Securities [Abstract]
Investments	$ 0
Aggregate fair value of investments in unrealized losses		$ 16,500,000
Number of investments in unrealized loss position less than a year | Investment		10

Investments - Summary of Invest

Investments - Summary of Investments (Details) $ in Thousands	Dec. 31, 2016USD ($)
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	$ 23,507
Available-for-sale securities, Gross Unrealized Gains	3
Available-for-sale securities, Gross Unrealized Losses	(5)
Available-for-sale securities, Estimated Fair Value	23,505
U.S. Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	13,258
Available-for-sale securities, Gross Unrealized Gains	3
Available-for-sale securities, Gross Unrealized Losses	(2)
Available-for-sale securities, Estimated Fair Value	13,259
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	7,397
Available-for-sale securities, Gross Unrealized Losses	(3)
Available-for-sale securities, Estimated Fair Value	7,394
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	2,852
Available-for-sale securities, Estimated Fair Value	$ 2,852

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	$ 93,745	$ 47,807
Total liabilities		267
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, debt securities		13,259
U.S. Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, debt securities		7,394
Foreign Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, debt securities		2,852
Warrants for purchase of shares subject to redemption
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative liability, fair value disclosure		267
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	93,745	37,561
Level 1 | U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, debt securities		13,259
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets		10,246
Level 2 | U.S. Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, debt securities		7,394
Level 2 | Foreign Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, debt securities		2,852
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities		267
Level 3 | Warrants for purchase of shares subject to redemption
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative liability, fair value disclosure		267
Money market funds, included in cash and cash equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value disclosure	93,745	24,302
Money market funds, included in cash and cash equivalents | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value disclosure	$ 93,745	$ 24,302

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	May 31, 2016	Aug. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Nov. 06, 2017	Mar. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Change in valuation methods or transfers			$ 0	$ 0
Transfers within fair value hierarchy			$ 0	0
Warrants exercisable shares			9,040
Level 3 | Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of warrants				$ 300,000	$ 500,000
Level 3 | Loan Agreement | Series A Convertible Preferred Stock | Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Issue of warrants for purchase of preferred stock		71,428
Warrants exercisable shares		38,265				33,163
Level 3 | Loan Agreement | Series C Convertible Preferred Stock | Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Issue of warrants for purchase of preferred stock	28,302
Warrants exercisable shares				9,434

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Assumptions Used in Valuing Warrants (Detail) - Warrants	Nov. 06, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Expected volatility		90.00%
Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Risk-free interest rate	2.10%	2.30%
Expected term (in years)	6 years 9 months 18 days	7 years 7 months 6 days
Expected volatility	84.00%
Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Risk-free interest rate	2.20%	2.50%
Expected term (in years)	8 years 6 months	9 years 3 months 19 days
Expected volatility	86.00%

Fair Value Measurements - Sum47

Fair Value Measurements - Summary of Changes in Fair Value of Warrants, Represented a Recurring Measurement Classified Within Level 3 of Fair Value Hierarchy, using Significant Unobservable Inputs (Detail) - Warrants - Level 3 - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance	$ 300
Ending balance		$ 300
Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance	267	68
Change in fair value of Warrants included in other income (expense)	257	132
Fair value of Warrants reclassified to stockholders' equity (deficit) upon conversion to warrants for the conversion of common stock	$ (524)
Ending balance		267
Recurring | Series C Convertible Preferred Stock One
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Initial fair value of Series A Warrant for the purchase of shares		45
Recurring | Series C Convertible Preferred Stock Two
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Initial fair value of Series A Warrant for the purchase of shares		$ 22

Fair Value Measurements - Sum48

Fair Value Measurements - Summary of Changes in Fair Value of Warrants, Represented a Recurring Measurement Classified Within Level 3 of Fair Value Hierarchy, using Significant Unobservable Inputs (Parenthetical) (Detail) - shares	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Warrants exercisable shares	9,040
Warrants | Recurring | Series C Convertible Preferred Stock One | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Warrants exercisable shares		28,302
Warrants | Recurring | Series C Convertible Preferred Stock Two | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Warrants exercisable shares		9,434

Property and Equipment, Net - S

Property and Equipment, Net - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property and equipment:
Property and equipment, gross	$ 308	$ 277
Less: Accumulated depreciation	(181)	(108)
Property and equipment, net	127	169
Laboratory Equipment
Property and equipment:
Property and equipment, gross	263	232
Computer Equipment
Property and equipment:
Property and equipment, gross	6	6
Software
Property and equipment:
Property and equipment, gross	$ 39	$ 39

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Depreciation expense		$ 73,000	$ 46,000	$ 98,000
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Leasehold improvements written off	$ 100,000

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Payroll and employee-related expenses	$ 1,132	$ 914
Professional fees	420	172
Third-party research and development expenses	135	413
Loan interest	40	28
IPO-related costs	111
Other	111	83
Total accrued expenses	$ 1,949	$ 1,610

Loan and Security Agreement - A

Loan and Security Agreement - Additional Information (Details)	1 Months Ended				12 Months Ended
	Dec. 31, 2016USD ($)$ / sharesshares	May 31, 2016USD ($)	Mar. 31, 2015USD ($)$ / sharesshares	Aug. 31, 2014USD ($)$ / sharesshares	Dec. 31, 2017USD ($)MonthlyPayment$ / sharesshares	Dec. 31, 2016USD ($)$ / sharesshares	Dec. 31, 2015USD ($)	Dec. 12, 2017shares	Nov. 06, 2017$ / sharesshares	Dec. 31, 2014USD ($)
Debt Instrument [Line Items]
Payment for final interest due upon maturity or prepayment of the original advances | $					$ 452,000	$ 472,000	$ 193,000
Warrants exercisable shares					9,040
Exercise price of warrants | $ / shares					$ 11.06
Cash and cash equivalents | $	$ 25,250,000				$ 94,494,000	$ 25,250,000	$ 69,011,000			$ 26,886,000
Series A Warrant
Debt Instrument [Line Items]
Warrants exercisable shares									34,225
Exercise price of warrants | $ / shares									$ 4.09
Series C Warrant
Debt Instrument [Line Items]
Warrants exercisable shares									9,040
Exercise price of warrants | $ / shares									$ 11.06
SVB
Debt Instrument [Line Items]
Warrants exercisable shares								34,225
SVB | Warrants
Debt Instrument [Line Items]
Number of common stock issued as settlement of warrant					24,401
Loan Agreement | SVB
Debt Instrument [Line Items]
Loan agreement, maximum borrowing capacity | $		$ 10,000,000		$ 7,000,000
Proceeds from loan agreement | $	$ 2,500,000	7,500,000	$ 3,200,000	$ 3,800,000
Net proceeds from loan agreement | $		1,600,000
Repayment under loan agreement | $		5,300,000
Payment for final interest due upon maturity or prepayment of the original advances | $		$ 600,000
Loan agreement, stated interest rate		4.00%
Loan agreement, interest rate above prime rate		0.50%
Loan agreement, effective interest rate	4.25%				5.00%	4.25%
Loan agreement, interest only payment period		12 months
Loan agreement, extended interest only payment period	18 months
Loan agreement, number of monthly payments | MonthlyPayment					30
Loan agreement, frequency of payments					monthly
Percentage of final payment based on total borrowings					8.25%
Percentage of cash and cash equivalent of outstanding debt balance	125.00%				125.00%	125.00%
Cash in operating depository and securities account | $	$ 10,000,000				$ 9,700,000	$ 10,000,000
Loan Agreement | SVB | Minimum
Debt Instrument [Line Items]
Cash and cash equivalents | $	$ 12,500,000				$ 12,100,000	$ 12,500,000
Loan Agreement | SVB | Series A Convertible Preferred Stock
Debt Instrument [Line Items]
Warrants exercisable shares			33,163
Share price | $ / shares			$ 0.98
Loan Agreement | SVB | Series C Convertible Preferred Stock
Debt Instrument [Line Items]
Warrants exercisable shares	9,434					9,434
Share price | $ / shares	$ 2.65					$ 2.65
Loan Agreement | SVB | Series A Warrant
Debt Instrument [Line Items]
Warrants exercisable shares			38,265
Exercise price of warrants | $ / shares			$ 0.98
Loan Agreement | SVB | Series A Warrant | Series A Convertible Preferred Stock
Debt Instrument [Line Items]
Warrants exercisable shares				71,428
Share price | $ / shares				$ 0.98
Percentage of loan advance amount for which additional warrants were issued to purchase preferred stock				1.00%
Warrant expiration date					Aug. 17, 2024
Loan Agreement | SVB | Series C Warrant
Debt Instrument [Line Items]
Warrants exercisable shares	28,302					28,302
Exercise price of warrants | $ / shares	$ 2.65					$ 2.65
Loan Agreement | SVB | Series C Warrant | Series C Convertible Preferred Stock
Debt Instrument [Line Items]
Share price | $ / shares				$ 2.65
Percentage of loan advance amount for which additional warrants were issued to purchase preferred stock				1.00%
Warrant expiration date					May 1, 2026

Loan and Security Agreement - S

Loan and Security Agreement - Summary of Minimum Aggregate Future Loan and Interest Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
2,018	$ 4,377
2,019	4,185
2,020	2,512
Total minimum payments	11,074
Less: Amount representing interest	(1,407)
Less: Discount	(281)
Less: Current portion	(3,870)	$ (176)
Loan payable, net of current portion	$ 5,516	$ 9,409

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended	33 Months Ended
	Nov. 30, 2015	Nov. 30, 2014	Dec. 31, 2015	May 31, 2014	Dec. 31, 2017	Nov. 06, 2017	Dec. 31, 2016
Temporary Equity [Line Items]
Common stock, shares authorized					125,000,000		75,000,000
Initial Public Offering
Temporary Equity [Line Items]
Issuance price per shares						$ 14
Convertible preferred stock converted into common stock					13,945,509	13,945,509
Preferred stock, shares outstanding					0
Common stock, shares authorized					125,000,000	125,000,000
Convertible preferred stock, shares authorized					5,000,000
Convertible preferred stock, par value					$ 0.001
Series A Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares issued				18,367,344	0		18,367,344
Issuance price per shares				$ 0.98
Proceeds from the sale of convertible preferred stock, net of issuance costs				$ 17,900
Convertible preferred stock, shares authorized					0		18,510,200
Convertible preferred stock, par value					$ 0.001		$ 0.001
Series B Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares issued		19,841,270			0		19,841,270
Issuance price per shares		$ 1.26
Proceeds from the sale of convertible preferred stock, net of issuance costs		$ 24,900
Convertible preferred stock, shares authorized					0		19,481,270
Convertible preferred stock, par value					$ 0.001		$ 0.001
Series C Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares issued	20,000,000				0		20,000,000
Issuance price per shares	$ 2.65
Proceeds from the sale of convertible preferred stock, net of issuance costs	$ 52,800		$ 52,782
Convertible preferred stock, shares authorized					0		20,037,736
Convertible preferred stock, par value					$ 0.001		$ 0.001

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit) - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, voting rights	one vote for each share held
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock issued, shares			1,211,035	1,211,035
Common stock vesting period			4 years	4 years
Shares, Unvested	0	0			675
Weighted average grant date fair value, Unvested					$ 0.42
Estimated fair values of stock underlying restricted stock awards		$ 800

Stockholders' Equity (Deficit56

Stockholders' Equity (Deficit) - Summary of Shares of Common Stock Reserved (Details) - shares	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Shares of common stock reserved for future issuances	3,753,669	16,119,334
Series A Convertible Preferred Stock
Class of Stock [Line Items]
Shares of common stock reserved for future issuances		4,400,410
Series B Convertible Preferred Stock
Class of Stock [Line Items]
Shares of common stock reserved for future issuances		4,753,536
Series C Convertible Preferred Stock
Class of Stock [Line Items]
Shares of common stock reserved for future issuances		4,791,563
Warrants
Class of Stock [Line Items]
Shares of common stock reserved for future issuances	9,040	43,265
Stock Options
Class of Stock [Line Items]
Shares of common stock reserved for future issuances	3,538,345	2,130,560
Employee Stock Purchase Plan
Class of Stock [Line Items]
Shares of common stock reserved for future issuances	206,284

Stock Incentive Plans - Additio

Stock Incentive Plans - Additional Information (Details) - USD ($)	Jan. 01, 2018	Oct. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expense related to awards granted			$ 517,000	$ 251,000	$ 207,000
Total intrinsic value of options exercised			276,000	$ 21,000
Unrecognized stock-based compensation expense related to unvested employee stock options			$ 1,800,000
Unrecognized stock-based compensation expense, weighted average period for recognition			2 years 4 months 24 days
Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average fair value of options granted			$ 6.16	$ 1.13	$ 0.83
Non-Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of stock options granted			0
Expense related to awards granted			$ 65,000	$ 24,000	$ 28,000
Weighted-average fair value of options granted				$ 1.38	$ 1
2011 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of stock options granted			209,797
2017 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of stock options granted			209,797
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based award, vesting period			4 years
Stock Options | Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expense related to awards granted			$ 500,000	$ 200,000	$ 200,000
Stock Options | 2011 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of stock options granted		0
Stock Options | 2011 Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based award, expiration period			10 years
Stock Options | 2017 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option grant effective date		Oct. 31, 2017
Share-based compensation arrangement by share-based payment award, description			On October 16, 2017, the Company’s stockholders approved the 2017 Stock Option and Incentive Plan (“2017 Plan”), which became effective on October 31, 2017. As of the effective date of the 2017 Plan, no further grants will be made under the 2011 Plan. The 2017 Plan initially provides for the grant of awards for 2,038,021 shares of common stock. In addition to the shares available for grant under the 2017 Plan, any awards outstanding under the 2011 Plan as of the October 31, 2017 that are cancelled, forfeited or otherwise terminated without being exercised, the number of shares underlying such awards will be available for future grant under the 2017 Plan. The 2017 Plan also provides that an additional number of shares will automatically be added to the shares authorized for issuance under the 2017 Plan on January 1 of each year. The number of shares added each year will be equal to the lesser of: (i) 4% of the outstanding shares on the immediately preceding December 31 or (ii) such amount as determined by the Compensation Committee of the registrant’s Board of Directors. On January 1, 2018, the shares available for grant under the 2017 Plan was automatically increased by 827,786 shares.
Number of shares authorized for grant		2,038,021
Percentage of additional shares added on outstanding shares		4.00%
Shares available for future grant			2,030,221
Stock Options | 2017 Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based award, expiration period			10 years
Stock Options | 2017 Plan | Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares available for grant	827,786

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 517	$ 251	$ 207
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	147	68	74
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 370	$ 183	$ 133

Stock Incentive Plans - Schedul

Stock Incentive Plans - Schedule of Assumptions Used in Black-Scholes Option Pricing Model to Estimate Fair Value of Stock Options (Details) - Stock Options	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	1.90%	1.30%	1.80%
Risk-free interest rate, maximum	2.30%	1.70%	1.90%
Expected term (in years)			6 years 3 months
Expected volatility			85.00%
Expected volatility, minimum	81.00%	77.00%
Expected volatility, maximum	87.00%	84.00%
Non-Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum		1.90%
Risk-free interest rate, maximum		2.40%
Expected term (in years)			10 years
Expected volatility			85.00%
Expected volatility, minimum		89.00%
Expected volatility, maximum		96.00%
Risk-free interest rate			2.40%
Minimum | Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 7 months 6 days	5 years 4 months 24 days
Minimum | Non-Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)		8 years 10 months 24 days
Maximum | Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 3 months 18 days	6 years 4 months 24 days
Maximum | Non-Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)		10 years

Stock Incentive Plans - Summa60

Stock Incentive Plans - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
2017 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Outstanding, Beginning balance	1,348,845
Shares, Granted	209,797
Shares, Exercised	(33,061)
Shares, Cancelled	(17,457)
Shares, Outstanding, Ending balance	1,508,124	1,348,845
Shares, Exercisable	787,375
Shares, Vested and expected to vest	1,508,124
Weighted-Average Exercise Price, Outstanding, Beginning balance	$ 1.29
Weighted-Average Exercise Price, Granted	5.50
Weighted-Average Exercise Price, Exercised	0.86
Weighted-Average Exercise Price, Cancelled	1.87
Weighted-Average Exercise Price, Outstanding, Ending balance	1.89	$ 1.29
Weighted-Average Exercise Price, Exercisable	1.24
Weighted-Average Exercise Price, Vested and expected to vest	$ 1.89
Weighted-Average Remaining Contractual Life (in years), Outstanding	7 years 10 months 24 days	8 years 7 months 6 days
Weighted-Average Remaining Contractual Life (in years), Exercisable	7 years 4 months 24 days
Weighted-Average Remaining Contractual Life (in years), Vested and expected to vest	7 years 10 months 24 days
Aggregate Intrinsic Value, Outstanding	$ 12,328	$ 3,642
Aggregate Intrinsic Value, Exercisable	6,945
Aggregate Intrinsic Value, Vested and expected to vest	$ 12,328
2011 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Outstanding, Beginning balance	1,348,845
Shares, Granted	209,797
Shares, Exercised	(33,061)
Shares, Cancelled	(17,457)
Shares, Outstanding, Ending balance	1,508,124	1,348,845
Shares, Exercisable	787,375
Shares, Vested and expected to vest	1,508,124
Weighted-Average Exercise Price, Outstanding, Beginning balance	$ 1.29
Weighted-Average Exercise Price, Granted	5.50
Weighted-Average Exercise Price, Exercised	0.86
Weighted-Average Exercise Price, Cancelled	1.87
Weighted-Average Exercise Price, Outstanding, Ending balance	1.89	$ 1.29
Weighted-Average Exercise Price, Exercisable	1.24
Weighted-Average Exercise Price, Vested and expected to vest	$ 1.89
Weighted-Average Remaining Contractual Life (in years), Outstanding	7 years 10 months 24 days	8 years 7 months 6 days
Weighted-Average Remaining Contractual Life (in years), Exercisable	7 years 4 months 24 days
Weighted-Average Remaining Contractual Life (in years), Vested and expected to vest	7 years 10 months 24 days
Aggregate Intrinsic Value, Outstanding	$ 12,328	$ 3,642
Aggregate Intrinsic Value, Exercisable	6,945
Aggregate Intrinsic Value, Vested and expected to vest	$ 12,328

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Federal statutory tax rate										34.00%	34.00%	34.00%
Reduction in deferred tax assets										$ 9,300,000
Net loss	$ (6,677,000)	$ (4,670,000)	$ (4,632,000)	$ (5,671,000)	$ (6,219,000)	$ (6,136,000)	$ (7,039,000)	$ (5,113,000)		(21,650,000)	$ (24,507,000)	$ (14,247,000)
Income tax benefit										0
Valuation allowance	26,316,000				26,602,000					26,316,000	26,602,000
Valuation allowances period increase (decrease)										(300,000)	6,300,000
Accrued interest and penalties related to uncertain tax positions	0				0					0	0
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	77,900,000				58,800,000					$ 77,900,000	58,800,000
Operating loss carryforwards expiration period										2,037
Federal | Research and Development
Operating Loss Carryforwards [Line Items]
Tax credit carryforward	3,300,000				2,400,000					$ 3,300,000	2,400,000
Tax credit carryforward expiration period										2,037
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	76,500,000				57,500,000					$ 76,500,000	57,500,000
Operating loss carryforwards expiration period										2,037
State | Research and Development
Operating Loss Carryforwards [Line Items]
Tax credit carryforward	$ 1,900,000				1,600,000					$ 1,900,000	1,600,000
Tax credit carryforward expiration period										2,032
ASU No. 2016-09
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards associate with deductible stock option exercises					$ 0						$ 0
Scenario Plan
Operating Loss Carryforwards [Line Items]
Federal statutory tax rate									21.00%

Income Taxes - Summary of Defer

Income Taxes - Summary of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 21,190	$ 23,028
Research and development credits	4,732	3,418
Licenses	24	38
Other	370	118
Total gross deferred tax asset	26,316	26,602
Less: Valuation allowance	$ (26,316)	$ (26,602)

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Income Taxes Computed at Statutory Federal Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income tax computed at federal statutory tax rate	34.00%	34.00%	34.00%
Permanent differences	(2.40%)	(0.50%)	(0.40%)
State taxes, net of federal benefit	4.80%	5.10%	(0.10%)
Research and development and other tax credits	5.00%	6.10%	5.50%
Other			0.10%
Federal rate change	(42.70%)
Change in deferred tax asset valuation allowance	1.30%	(44.70%)	(39.10%)

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - Third Rock Ventures LLP - Consulting and Management Services - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Services received from related party	$ 2,000	$ 69,000	$ 19,000
Due to related party	$ 0	$ 0

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	Jun. 01, 2017	Dec. 31, 2017ft²	Feb. 28, 2017USD ($)ft²	Oct. 31, 2016	Aug. 31, 2016ft²	Mar. 31, 2015ft²	Oct. 31, 2013USD ($)ft²	Aug. 31, 2011USD ($)ft²	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Operating Leased Assets [Line Items]
Operating leases, rent expense, net | $									$ 400,000	$ 200,000	$ 200,000
Newton, MA
Operating Leased Assets [Line Items]
Rentable office space under operating lease								6,055
Base rent for office space | $								$ 100,000
Newton, MA | Sublease Agreement
Operating Leased Assets [Line Items]
Rentable office space under operating lease						5,385
Operating lease expiration period						2017-02
Natick, MA
Operating Leased Assets [Line Items]
Rentable laboratory space under operating lease							3,170
Base rent for lab space | $							$ 59,000
Sudbury, MA
Operating Leased Assets [Line Items]
Rentable laboratory space under operating lease			5,133		3,890
Base rent for lab space | $			$ 100,000
Operating lease extended period			2019-02
Operating lease expiration period					2017-08
Option to lease other, rentable laboratory space under operating lease			2,029
Lease Amendment One | Newton, MA
Operating Leased Assets [Line Items]
Operating lease extended period				2016-10
Operating lease extended term				1 year
Lease Amendment Two | Newton, MA
Operating Leased Assets [Line Items]
Rentable office space under operating lease		7,795
Operating lease extended period		2020-12		2018-05
Operating Lease Effective Period Month And Year	2,017
Operating lease amendment period		2017-12
Lease Amendment Three | Newton, MA
Operating Leased Assets [Line Items]
Operating lease expiration period		2017-10

Commitments and Contingencies66

Commitments and Contingencies - Schedule of Future Minimum Lease Commitments for Operating Leases (Details) $ in Thousands	Dec. 31, 2017USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2,018	$ 458
2,019	379
2,020	362
Total future operating lease commitment	$ 1,199

License Agreement - Additional

License Agreement - Additional Information (Details) - Althea Technologies, Inc	12 Months Ended
	Dec. 31, 2017USD ($)shares
License Agreement [Line Items]
Contract termination period	60 days
Maximum | License Agreement
License Agreement [Line Items]
Potential milestone payable	$ 56,000,000
Patents | License Agreement
License Agreement [Line Items]
Fees and costs incurred	$ 100,000
Issuance of common stock, net of issuance costs, shares | shares	88,186

Employee Benefit Plan - Additio

Employee Benefit Plan - Additional Information (Details) - 401(k) Plan	12 Months Ended
	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum annual contribution per employee	100.00%
Individual contribution, description	Participants may contribute up to 100% of their annual compensation to the 401(k) Plan, subject to statutory limitations. The 401(k) Plan does not allow the Company to make matching contributions

Selected Quarterly Financial 69

Selected Quarterly Financial Data (Unaudited) - Schedule of Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Total operating expenses	$ 6,571	$ 4,362	$ 4,474	$ 5,543	$ 6,002	$ 6,102	$ 7,041	$ 5,041	$ 20,950	$ 24,186	$ 13,905
Loss from operations	(6,571)	(4,362)	(4,474)	(5,543)	(6,002)	(6,102)	(7,041)	(5,041)	(20,950)	(24,186)
Net loss	$ (6,677)	$ (4,670)	$ (4,632)	$ (5,671)	$ (6,219)	$ (6,136)	$ (7,039)	$ (5,113)	$ (21,650)	$ (24,507)	$ (14,247)
Net loss attributable to common stockholders - basic and diluted	$ (0.48)	$ (3.49)	$ (3.46)	$ (4.24)	$ (4.65)	$ (4.59)	$ (5.26)	$ (3.85)	$ (4.80)	$ (18.35)	$ (11.35)