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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2016	Mar. 07, 2017	Jun. 30, 2016
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	FY
Trading Symbol	SNDX
Entity Registrant Name	SYNDAX PHARMACEUTICALS INC
Entity Central Index Key	0001395937
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		18,244,423
Entity Public Float			$ 109.6

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 23,844	$ 23,179
Restricted cash	151	54
Short-term investments	81,486	63,310
Prepaid expenses and other current assets	3,029	1,464
Total current assets	108,510	88,007
Property and equipment, net	260	88
Other assets	243	1,808
Total assets	109,013	89,903
Current liabilities:
Accounts payable	2,375	1,452
Accrued expenses and other current liabilities	6,771	2,175
Current portion of deferred revenue	1,220	1,220
Total current liabilities	10,366	4,847
Long-term liabilities:
Common stock warrant liability		2,848
Deferred revenue, less current portion	14,220	15,440
Other long-term liabilities	288	70
Total long-term liabilities	14,508	18,358
Total liabilities	24,874	23,205
Commitments (Note 16)
Convertible preferred stock (Note 12)		319,113
Stockholders' equity (deficit):
Preferred stock, value	0	0
Common stock, $0.0001 par value, 100,000,000 and 20,800,000 shares authorized; 18,215,181 and 85,440 shares outstanding at December 31, 2016 and December 31, 2015, respectively	2	1
Additional paid-in capital	389,374
Accumulated other comprehensive income	56	28
Accumulated deficit	(305,293)	(259,675)
Total stockholders' equity (deficit)	84,139	(252,415)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	$ 109,013	89,903
Series A Convertible Preferred Stock [Member]
Stockholders' equity (deficit):
Preferred stock, value		7,231
Total stockholders' equity (deficit)		$ 7,231

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2016	Dec. 31, 2015
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	100,000,000	20,800,000
Common stock, shares outstanding	18,215,181	85,440
Series A Convertible Preferred Stock [Member]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	0	3,512,194
Preferred stock, shares issued	0	700,435
Preferred stock, shares outstanding	0	700,435

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues:
License fees	$ 1,220	$ 627
Total revenues	1,220	627
Operating expenses:
Research and development	31,665	9,549	$ 10,175
General and administrative	13,321	11,591	11,157
Total operating expenses	44,986	21,140	21,332
Loss from operations	(43,766)	(20,513)	(21,332)
Other (expense) income:
Interest income (expense), net	956	(1,414)	(289)
Change in fair value of common stock warrant liability	(1,703)	(2,155)	1,789
Other income (expense), net	41	(37)	4
Total other (expense) income	(706)	(3,606)	1,504
Net loss	(44,472)	(24,119)	(19,828)
Net loss attributable to common stockholders	$ (47,070)	$ (103,845)	$ (26,357)
Net loss per share attributable to common stockholders-basic and diluted	$ (3.22)	$ (1,519.27)	$ (453.02)
Weighted-average common shares outstanding-basic and diluted	14,619,716	68,352	58,181

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net loss	$ (44,472)	$ (24,119)	$ (19,828)
Other comprehensive loss:
Unrealized gains on marketable securities, net of tax	28	28
Comprehensive loss	$ (44,444)	$ (24,091)	$ (19,828)

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member]	Series B-1 [Member]	Series B-1 [Member] KHK Agreement [Member]	Series C-1 Convertible Preferred Stock in June 2015 [Member]	Series C-1 [Member]	Series C-1 Convertible Preferred Stock in August 2015 [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2013			$ 140,324
Beginning balance at Dec. 31, 2013	$ (128,475)	$ 7,231							$ 1			$ (135,707)
Beginning balance, Shares at Dec. 31, 2013			6,105,956
Beginning balance, Shares at Dec. 31, 2013		700,435							56,573
Exercise of stock options, Value	$ 6									$ 6
Exercise of stock options, Shares	0								1,944
Accretion for convertible preferred stock dividends	$ (6,529)									(2,263)		(4,266)
Accretion for convertible preferred stock dividends			$ 6,529
Stock-based compensation expense	2,257									2,257
Net loss	(19,828)											(19,828)
Ending balance at Dec. 31, 2014	(152,569)	$ 7,231							$ 1			(159,801)
Ending balance at Dec. 31, 2014			$ 146,853
Ending balance, Shares at Dec. 31, 2014		700,435							58,517
Ending balance, Shares at Dec. 31, 2014			6,105,956
Issuance of convertible preferred stock, Value					$ 7,713	$ 18,526		$ 61,084
Accretion of convertible preferred stock to redemption value	(69,715)									(123)		(69,592)
Issuance of convertible preferred stock, Shares					536,049	1,340,113		4,377,902
Conversion of 2014 Notes into Series C-1, Value							$ 5,211
Conversion of 2014 Notes into Series C-1, Shares							372,446
Exercise of stock options, Value	$ 75									75
Accretion of convertible preferred stock to redemption value			$ 69,715
Exercise of stock options, Shares	41,607								24,876
Accretion for convertible preferred stock dividends	$ (10,011)									(3,848)		(6,163)
Accretion for convertible preferred stock dividends			10,011
Vesting of restricted stock, Value	14									14
Vesting of restricted stock, Shares									2,047
Stock-based compensation expense	3,882									3,882
Unrealized gain on short-term investments	28										$ 28
Net loss	(24,119)											(24,119)
Ending balance at Dec. 31, 2015	(252,415)	$ 7,231							$ 1		28	(259,675)
Ending balance at Dec. 31, 2015	319,113		$ 319,113	$ 96,348			$ 152,015
Ending balance, Shares at Dec. 31, 2015		700,435							85,440
Ending balance, Shares at Dec. 31, 2015			12,732,466	3,568,020			6,090,461
Exercise of stock options, Value	$ 2,058									2,058
Exercise of stock options, Shares	441,573								441,573
Accretion for convertible preferred stock dividends	$ (2,598)									(1,452)		(1,146)
Accretion for convertible preferred stock dividends			$ 2,598
Proceeds from initial public offering, net of offering costs of $7,186, Value	50,527									50,527
Proceeds from initial public offering, net of offering costs of $7,186, Shares									4,809,475
Conversion of preferred stock into common stock, Value	321,711	$ (7,231)	$ (321,711)						$ 1	328,941
Conversion of preferred stock into common stock, Shares		(700,435)	(12,732,466)						12,872,551
Reclassification of common stock warrant liability	4,551									4,551
Vesting of restricted stock, Value	42									42
Vesting of restricted stock, Shares									6,142
Stock-based compensation expense	4,708									4,708
Unrealized gain on short-term investments	28										28
Repurchase of fractional shares resulting from reverse stock splits	(1)									(1)
Net loss	(44,472)											(44,472)
Ending balance at Dec. 31, 2016	$ 84,139								$ 2	$ 389,374	$ 56	$ (305,293)
Ending balance, Shares at Dec. 31, 2016									18,215,181
Ending balance, Shares at Dec. 31, 2016			0

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Aug. 31, 2015	Jun. 30, 2015	Dec. 31, 2016	Dec. 31, 2015
Preferred stock, par value			$ 0.001	$ 0.001
Common stock, par value			0.0001	0.0001
Series A Convertible Preferred Stock [Member]
Preferred stock, par value			0.001	0.001
Convertible Preferred Stock [Member]
Preferred stock, par value			$ 0.001	$ 0.001
Series C-1 [Member]
Issuance of Series C-1 convertible preferred stock, net of issuance costs	$ 200	$ 200
Offering costs			$ 7,186

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (44,472)	$ (24,119)	$ (19,828)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	89	21	15
Amortization of debt discount and debt issuance costs		762	40
Amortization and accretion of investments	(126)	303	46
Stock-based compensation	4,708	3,882	2,257
Change in fair value of warrants	1,703	2,155	(1,789)
Write-off of deferred costs associated with postponed initial public offering			4,319
Other	25	8	(4)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,629)	(1,205)	64
Accounts payable	923	1,045	(726)
Deferred revenue	(1,220)	16,660
Accrued expenses and other liabilities	4,842	(1,940)	1,213
Net cash used in operating activities	(35,157)	(2,428)	(14,393)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(261)	(49)	(4)
Change in restricted cash	(97)	(118)	(1)
Purchases of short-term investments	(158,319)	(102,008)	(3,393)
Proceeds from sales and maturities of short-term investments	140,297	40,506	5,286
Net cash (used in) provided by investing activities	(18,380)	(61,669)	1,888
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock in initial public stock offering, net	52,148
Proceeds from issuance of convertible preferred stock, net		87,323
Proceeds from issuance of debt			14,000
Proceeds from exercise of stock options	2,058	191	6
Deferred issuance costs		(1,245)	(1,594)
Payments on term loans		(9,000)
Other	(4)	(2)	(2)
Net cash provided by financing activities	54,202	77,267	12,410
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	665	13,170	(95)
CASH AND CASH EQUIVALENTS-beginning of year	23,179	10,009	10,104
CASH AND CASH EQUIVALENTS-end of year	$ 23,844	$ 23,179	$ 10,009

Nature of Business	12 Months Ended
Nature of Business	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Business	1. Nature of Business Syndax Pharmaceuticals, Inc. (the Company) is a clinical stage biopharmaceutical company developing an innovative pipeline of combination therapies in multiple cancer indications. The Company’s lead product candidate, entinostat, is currently being evaluated in a Phase 3 clinical trial for advanced hormone receptor positive, human epidermal growth factor receptor 2 negative breast cancer. Entinostat was granted Breakthrough Therapy designation by the U.S. Food and Drug Administration following positive results from the Company’s Phase 2b clinical trial, ENCORE 301. The Company is developing entinostat, which has direct effects on both cancer cells and immune regulatory cells, and SNDX-6352, a monoclonal antibody that targets the colony stimulating factor-1 receptor to enhance the body’s immune response on tumors that have shown sensitivity to immunotherapy. The Company is evaluating entinostat as a combination therapeutic in Phase 1b/2 clinical trials with Merck & Co., Inc. for non-small cell lung cancer and melanoma; with Genentech, Inc. for triple negative breast cancer; and with Merck KGaA, Darmstadt, Germany, and Pfizer Inc. for ovarian cancer. We acquired the exclusive rights to SNDX-6352 in July 2016 and are evaluating SNDX-6352 in a single ascending dose Phase 1 clinical trial. The Company plans to continue to leverage the technical and business expertise of our management team and scientific collaborators to license, acquire and develop additional cancer therapies to expand our pipeline. In March 2016, the Company completed its initial public offering (“IPO”) whereby it sold 4,809,475 shares of common stock at the initial public offering price of $12.00 per share, which included 409,475 shares issued pursuant to the underwriters’ partial exercise of their over-allotment option to purchase additional shares of common stock. The aggregate net proceeds received by the Company from the offering were $50.5 million, net of underwriting discounts and commissions of $4.0 million and offering expenses of $3.1 million. Upon the closing of the IPO, all outstanding shares of the Company’s outstanding convertible preferred stock converted into 12,872,551 shares of common stock; and the Company’s outstanding warrant liability to purchase 357,840 shares of the Company’s common stock valued at $4.6 million was reclassified to additional paid-in capital. The shares trade on the Nasdaq Global Select Market under the symbol “SNDX.” In connection with the closing of the Company’s IPO, the Company filed an amended and restated certificate of incorporation and adopted amended and restated bylaws, both of which were approved by the Company’s board of directors and stockholders on September 28, 2015 and February 24, 2016, respectively. Pursuant to the amended and restated certificate of incorporation, the Company is now authorized to issue 100,000,000 shares of common stock and 10,000,000 shares of preferred stock. Since its inception, the Company has devoted its efforts principally to research and development and raising capital. The Company is subject to risks common to companies in the development stage, including, but not limited to, successful development of therapeutics, obtaining additional funding, protection of proprietary therapeutics, compliance with government regulations, fluctuations in operating results, dependence on key personnel and collaborative partners, and risks associated with industry changes. The Company’s long-term success is dependent upon its ability to successfully develop and market its product candidates, expand its oncology drug pipeline, earn revenue, obtain additional capital when needed, and ultimately, achieve profitable operations. The Company anticipates that it will be several years before either entinostat or SNDX-6352 is approved, if ever, and the Company begins to generate revenue from sales of entinostat and SNDX-6352. Accordingly, management expects to incur substantial losses on the ongoing development of entinostat and SNDX-6352 and does not expect to achieve positive cash flow from operations for the foreseeable future, if ever. As a result, the Company will continue to require additional capital to move forward with its business plan. While certain amounts of this additional capital were raised in the past, there can be no assurance that funds necessary beyond these amounts will be available in amounts or on terms sufficient to ensure ongoing operations. The Company’s management believes that the cash, cash equivalents and short-term investments balances as of December 31, 2016 should enable the Company to maintain its planned operations for at least the next 12 months. The Company’s ability to fund all of its planned operations internally beyond that date, including the completion of its ongoing and planned clinical trial activities, may be substantially dependent upon whether the Company can obtain sufficient funding on terms acceptable to the Company. Proceeds from additional capital transactions would allow the Company to accelerate and/or expand its planned research and development activities. In the event that sufficient funds were not available, the Company may be required to delay or reduce expenditures to conserve cash, which could involve scaling back or curtailing development and general and administrative activities.

Basis of Presentation	12 Months Ended
Basis of Presentation	Dec. 31, 2016
Accounting Policies [Abstract]
Basis of Presentation	2. Basis of Presentation The Company has prepared the accompanying consolidated financial statements in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). On February 24, 2016, the Company effected a 1-for-1.25 reverse stock split of the Company’s outstanding common stock and convertible preferred stock; and on June 3, 2014, the Company effected a 1-for-12.3 reverse stock split of the Company’s outstanding common stock and convertible preferred stock. Stockholders entitled to fractional shares as a result of the reverse stock splits received a cash payment in lieu of receiving fractional shares. All of the Company’s historical share and per share information shown in the accompanying financial statements and related notes have been retroactively adjusted to give effect to these reverse stock splits. In 2011, the Company established a wholly owned subsidiary in the United Kingdom. There have been no activities for this entity to date. In 2014, the Company established a wholly owned U.S. subsidiary, Syndax Securities Corporation. The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.

Summary of Significant Accounting Policies	12 Months Ended
Summary of Significant Accounting Policies	Dec. 31, 2016
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	3. Summary of Significant Accounting Policies *Use of Estimates* The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of costs and expenses during the reporting period. The Company bases estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. The Company evaluates its estimates and assumptions on an ongoing basis. The Company’s actual results may differ from these estimates under different assumptions or conditions. *Cash Equivalents* Cash equivalents include all highly liquid investments maturing within 90 days or less from the date of purchase. Cash equivalents include money market funds, corporate debt securities, U.S. government agency notes, and overnight deposits. *Restricted Cash* The Company classifies as restricted cash all cash pledged as collateral to secure long-term obligations and all cash whose use is otherwise limited by contractual provisions. Amounts are reported as non-current unless restrictions are expected to be released in the next 12 months. *Short-Term Investments* Short-term investments include marketable securities with maturities of less than one year or where management’s intent is to use the investments to fund current operations or to make them available for current operations. All investments in marketable securities are classified as available-for-sale and are reported at fair value with unrealized gains and losses excluded from earnings and reported net of tax in accumulated other comprehensive income, which is a component of stockholders’ equity (deficit). Unrealized losses that are determined to be other-than-temporary, based on current and expected market conditions, are recognized in earnings. Declines in fair value determined to be credit related are charged to earnings. The cost of marketable securities sold is determined by the specific identification method. Investments with remaining maturities or that are due within one year from the balance sheet date are classified as current. *Segment Reporting* Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision making group, in making decisions regarding resource allocation and assessing performance. The Company has one operating segment. *Other Assets* Other assets consist of deferred issuance costs, long-term security deposits, and noncurrent restricted cash. Deferred issuance costs consist primarily of direct incremental legal and accounting fees relating to the Company’s IPO. As of December 31, 2016 and 2015, the Company had capitalized deferred IPO issuance costs of $0 and $1.6 million, respectively. Upon the closing of the IPO, these costs were reclassified to additional paid-in capital as a reduction of the IPO proceeds. In September 2014, the Company determined that it was likely its IPO would be postponed for a period in excess of 90 days. As a result, in accordance with the Securities and Exchange Commission guidance in Staff Accounting Bulletin Topic 5-A, Expenses of Offering, the Company expensed as general and administrative expenses previously deferred IPO costs of $4.3 million. *Concentrations of Credit Risk* Cash and cash equivalents, restricted cash, and short-term investments are financial instruments that potentially subject the Company to concentrations of credit risk. Substantially all of the Company’s cash, cash equivalents, and short-term investments were deposited in accounts at two financial institutions, and at times, such deposits may exceed federally insured limits. The Company has not experienced any losses in such accounts, and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. The Company’s available-for-sale investments primarily consist of U.S. Treasury securities, U.S. government agency securities, corporate debt securities, certificates of deposit and overnight deposits and potentially subject the Company to concentrations of crdit risk. *Property and Equipment* Property and equipment are recorded at cost. Depreciation is recorded using the straight-line method over the estimated useful lives of the assets (three to five years). Assets under capital leases are amortized over the shorter of their useful lives or lease term using the straight-line method. Major replacements and improvements are capitalized, while general repairs and maintenance are expensed as incurred. *Impairment of Long-Lived Assets* Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. When such events occur, the Company compares the carrying amounts of the assets to their undiscounted expected future cash flows. If this comparison indicates that there is impairment, the amount of impairment is calculated as the difference between the carrying value and fair value. To date, no such impairments have been recognized. *Revenue Recognition* The Company enters into license agreements for the development and commercialization of its product candidates, entinostat and SNDX-6352. License agreements may include non-refundable upfront payments, contingent payments based on the occurrence of specified events under the Company’s license arrangements, partial or complete reimbursement of research and development expenses, license fees and royalties on sales of entinostat if they are successfully approved and commercialized. The Company’s performance obligations under the license agreements may include the transfer of intellectual property rights in the form of licenses, obligations to provide research and development services and related materials and participation on certain development and/or commercialization committees with the collaboration partners. Revenue is recognized when (i) persuasive evidence of an arrangement exists, (ii) transfer of technology has been completed, services have been performed or products have been delivered, (iii) the fee is fixed and determinable, and (iv) collection is reasonably assured. For revenue agreements with multiple-elements, the Company identifies the deliverables included within the agreement and evaluates which deliverables represent separate units of accounting based on the achievement of certain criteria including whether the deliverable has stand-alone value. Upfront payments received in connection with licenses of the Company’s technology rights are deferred if facts and circumstances dictate that the license does not have stand-alone value and are recognized as license revenue over the estimated period of performance that is generally consistent with the terms of the research and development obligations contained in the specific license agreement. The Company periodically reviews its estimated periods of performance based on the progress under each arrangement and accounts for the impact of any changes in estimated periods of performance on a prospective basis. At the inception of each agreement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. The Company evaluates factors such as the scientific, regulatory, commercial and other risks that must be overcome to achieve the respective milestone, the level of effort and investment required to achieve the respective milestone and whether the milestone consideration is reasonable relative to all deliverables and payment terms in the arrangement in making this assessment. Non-refundable payments that are contingent upon achievement of a substantive milestone are recognized in their entirety in the period in which the milestone is achieved, assuming all other revenue recognition criteria are met. Other contingent payments in which a portion of the milestone consideration is refundable or adjusts based on future performance or non-performance (e.g., through a penalty or claw-back provision) are not considered to relate solely to past performance, and therefore, not considered substantive. Amounts that are not recognized as revenue due to the uncertainty as to whether they will be retained or because they are expected to be refunded are recorded as a liability. The Company recognizes non-substantive milestone payments over the remaining estimated period of performance once the milestone is achieved. Contingent payments associated with the achievement of specific objectives in certain contracts that are not considered substantive because the Company does not contribute effort to the achievement of such milestones are recognized as revenue upon achievement of the objective, as long as there are no undelivered elements remaining and no continuing performance obligations by the Company, assuming all other revenue recognition criteria are met. *Research and Development* Research and development costs are expensed as incurred. Research and development expenses include payroll and personnel expenses, consulting costs, external contract research and development expenses, and allocated overhead, including rent, equipment depreciation, and utilities. Research and development costs that are paid in advance of performance are capitalized as a prepaid expense and amortized over the service period as the services are provided. The Company expense upfront license payments related to acquired technologies that have not yet reached technological feasibility and have no alternative future use. In instances where the Company enters into cost-sharing arrangements, all research and development costs reimbursed by the collaborators are accounted for as reductions to research and development expense. During the year ended December 31, 2016, the Company incurred $0.5 million in external costs related to cost-sharing collaborations, of which $0.3 million has been recorded as a reduction to research and development expense. There were no cost-sharing arrangements in 2015 or 2014. *Clinical Trial Costs* Clinical trial costs are a component of research and development expenses. The Company accrues and expenses clinical trial activities performed by third parties based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, or other information provided to us by our vendors. *Income Taxes* The Company records deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax bases of assets and liabilities and for loss and credit carryforwards using enacted tax rates expected to be in effect in the years in which the differences reverse. A valuation allowance is provided to reduce the net deferred tax assets to the amount that will more likely than not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more-likely-than-not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. *Guarantees and Indemnifications* As permitted under Delaware law, the Company indemnifies its officers, directors, and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. The Company has standard indemnification arrangements under office leases (as described in Note 16) 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 Company’s lease. Through December 31, 2016, the Company had not experienced any losses related to these indemnification obligations and no claims 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 were established. *Stock-Based Compensation* The Company accounts for all stock option awards granted to employees and non-employees using a fair value method. Stock-based compensation is measured at the grant date fair value of employee stock option grants and is recognized over the requisite service period of the awards (usually the vesting period) on a straight-line basis, net of estimated forfeitures. Stock option awards to non-employees are subject to periodic revaluation over their vesting terms. *Convertible Preferred Stock* Upon closing of the IPO, all of the outstanding shares of the Company’s outstanding convertible preferred stock converted into shares of the common stock. Prior to the IPO, the Company had classified certain series of convertible preferred stock as temporary equity in the consolidated balance sheets due to certain change in control events that were outside of the Company’s control, including liquidation, sale, or transfer of control of the Company, as holders of the convertible preferred stock could cause redemption of the shares in these situations. The carrying value of the convertible preferred stock was presented at its maximum redemption value. As of December 31, 2015, the Series A preferred stock had no liquidation preference and was presented in permanent equity. *Debt Discount* The Company has recorded the fair value of the derivative liability related to its term loans as debt discount, which is presented in the consolidated balance sheets as an offset to the carrying value amount of the debt. Debt discount is amortized to interest expense using the effective interest rate method or a method that approximates the effective interest rate method over the expected period that the debt is expected to be outstanding. In October 2015, the Company prepaid the outstanding balance on the term loans and wrote off the unamortized debt discount related to the term loans. *Derivative Liabilities* The Company records potential payments that would be made to lenders upon certain triggering events as derivative financial liabilities. The derivative liability is initially valued at fair value using a probability-weighted expected return model. Gains and losses arising from changes in fair value are recognized in other income (expense) in the consolidated statement of operations at each period end while such liabilities are outstanding. *Common Stock Warrants* The Company has recorded common stock warrants issued with license agreements as derivative financial liabilities, as the terms of the warrants are not fixed due to potential adjustments in the exercise price and/or the number of shares issuable under the warrants. The common stock warrants are initially recorded at fair value, with gains and losses arising from changes in fair value recognized in other income (expense) in the consolidated statement of operations at each period end while such instruments are outstanding. The warrant liabilities were valued using a Black-Scholes option-pricing model. Upon the closing of the IPO, the Company’s outstanding warrant liability was reclassified to additional paid-in capital. *Recently Issued and Adopted Accounting Pronouncements* In November 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) ASU 2016-18, “Statement of Cash Flows (Topic 230): Restricted Cash” (“ASU 2016-18”). ASU 2016-18 requires that restricted cash and restricted cash equivalents be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The statement of cash flows must also explain the change during the period in the total of cash, cash equivalents, and restricted cash or restricted cash equivalents. For public business entities, this ASU is effective for fiscal years beginning after December 15, 2017. The Company does not expect the adoption of this standard to have a material impact on its consolidated financial statements. In March 2016, the FASB issued ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting (“ASU 2016-09”). ASU 2016-09 simplifies several aspects related to the accounting for share-based payment transactions, including the accounting for income taxes, forfeitures, statutory tax withholding requirements and classification on the statement of cash flows. ASU 2016-09 will be effective for the Company on January 1, 2017. The Company does not expect the adoption of this standard to have a material impact on its consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, “Leases (Topic 842).” Under ASU 2016-02, lessees will be required to recognize, for all leases of 12 months or more, a liability to make lease payments and a right-of-use asset representing the right to use the underlying asset for the lease term. Additionally, the guidance requires improved disclosures to help users of financial statements better understand the nature of an entity’s leasing activities. This ASU is effective for public reporting companies for interim and annual periods beginning after December 15, 2018, with early adoption permitted, and must be adopted using a modified retrospective approach. The standard will be effective for the Company on January 1, 2019. The Company is in the process of evaluating the effect of the new guidance on the Company’s consolidated financial statements and related disclosures. In August 2014, FASB issued ASU 2014-15, Presentation of Financial Statements-Going Concern (“ASU 2014-15”). ASU 2014-15 provides guidance on management’s responsibility in evaluating whether there are conditions or events that raise substantial doubt about a company’s ability to continue as a going concern within one year from the date the financial statements are issued, and about related footnote disclosures. ASU 2014-15 is effective for the annual period ending after December 15, 2016, and for annual periods and interim periods thereafter. The Company adopted ASU 2014-15 for the year ended December 31, 2016, and the adoption of this standard did not impact the Company’s consolidated financial statements and related disclosures. In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”). ASU 2014-09 supersedes the revenue recognition requirements of FASB Accounting Standards Codification (“ASC”) Topic 605, Revenue Recognition and most industry-specific guidance throughout the Accounting Standards Codification, resulting in the creation of FASB ASC Topic 606, Revenue from Contracts with Customers. ASU 2014-09 requires entities to recognize revenue in a way that depicts the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled to in exchange for those goods or services. Adoption will be permitted using either a retrospective or modified retrospective approach. In July 2015, FASB voted to delay the effective date of the standard by one year to the first quarter of 2018 to provide companies sufficient time to implement the standard. Early adoption will be permitted, but not before the first quarter of 2017. The Company is currently evaluating the method by which it will implement this standard and the impact of the adoption of this standard on the Company’s consolidated financial statements.

Net Loss per Share Attributable to Common Stockholders

12 Months Ended

Dec. 31, 2016

Earnings Per Share [Abstract]

Net Loss per Share Attributable to Common Stockholders

4. Net Loss per Share Attributable to Common Stockholders

Basic net loss attributable to common stockholders per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common shares outstanding for the period. Because the Company has reported a net loss for the three years ended December 31, 2016, 2015, and 2014, diluted net loss per common share is the same as basic net loss per common share for those periods. The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company (in thousands, except per share data):


	Years Ended December 31,
	2016			2015			2014
Numerator—basic and diluted:
Net loss	$	(44,472	)	$	(24,119	)	$	(19,828	)
Accretion of convertible preferred stock dividends		(2,598	)		(10,011	)		(6,529	)
Accretion of convertible preferred stock to redemption value		—			(69,715	)		—

Net loss attributable to common stockholders—basic and diluted	$	(47,070	)	$	(103,845	)	$	(26,357	)

Net loss per share—basic and diluted	$	(3.22	)	$	(1,519.27	)	$	(453.02	)

Denominator—basic and diluted:
Weighted-average common shares used to compute net loss per share—basic and diluted		14,619,716			68,352			58,181

The following potentially dilutive securities have been excluded from the computation of diluted weighted-average shares outstanding because such securities have an antidilutive impact due to losses reported (in common stock equivalent shares):


	December 31,
	2016		2015		2014
Convertible preferred stock		—		12,872,551		6,246,041
Options to purchase common stock		2,560,737		2,606,195		831,148
Common stock warrant		357,840		277,486		127,099
Convertible notes and related accrued interest		—		—		363,294
Restricted stock subject to future vesting		8,542		14,684		—

Significant Agreements

12 Months Ended

Dec. 31, 2016

Equity [Abstract]

Significant Agreements

5. Significant Agreements

UCB Biopharma Sprl

In July 2016, the Company entered into a license agreement (the “UCB License Agreement”) with UCB Biopharma Sprl (“UCB”), under which UCB granted to the Company a worldwide, sublicenseable, exclusive license to UCB6352, which the Company refers to as SNDX-6352, an IND-ready anti-CSF-1R monoclonal antibody. The Company made a nonrefundable upfront payment of $5.0 million to UCB in the third quarter of 2016. Additionally, subject to the achievement of certain milestone events, the Company may be required to pay UCB up to $119.5 million in one-time development and regulatory milestone payments over the term of the UCB License Agreement. In the event that the Company or any of its affiliates or sublicensees commercializes SNDX-6352, the Company will also be obligated to pay UCB low double-digit royalties on sales, subject to reduction in certain circumstances, as well as up to an aggregate of $250.0 million in potential one-time, sales-based milestone payments based on achievement of certain annual sales thresholds. Under certain circumstances, the Company may be required to share a percentage of non-royalty income from sublicensees, subject to certain deductions, with UCB. The Company will be solely responsible for the development and commercialization of SNDX-6352, except that UCB is performing a limited set of transitional chemistry, manufacturing and control tasks related to SNDX-6352. Each party may terminate the UCB License Agreement for the other party’s uncured material breach or insolvency; and the Company may terminate the UCB License Agreement at will at any time upon advance written notice to UCB. UCB may terminate the UCB License Agreement if the Company or any of its affiliates or sublicensees institutes a legal challenge to the validity, enforceability, or patentability of the licensed patent rights. Unless terminated earlier in accordance with its terms, the UCB License Agreement will continue on a country-by-country and product-by-product basis until the later of: (i) the expiration of all of the licensed patent rights in such country; (ii) the expiration of all regulatory exclusivity applicable to the product in such country; and (iii) 10 years from the date of the first commercial sale of the product in such country.

As of the date of the UCB License Agreement, the asset acquired had no alternative future use nor had it reached a stage of technological feasibility. As the processes or activities that were acquired along with the license do not constitute a “business,” the transaction has been accounted for as an asset acquisition. As a result of these findings, the upfront payment of $5.0 million has been recorded as research and development expense in the consolidated statements of operations.

Kyowa Hakko Kirin Co., Ltd.

On December 19, 2014 (the “Effective Date”), the Company entered into a license agreement (the “KHK License Agreement”) with Kyowa Hakko Kirin Co., Ltd. (“KHK”), under which the Company granted KHK an exclusive license to develop and commercialize entinostat in Japan and Korea. Under the terms of the KHK License Agreement, the Company will be responsible for the manufacture and supply of the products during the development activities. In addition to the license and manufacturing obligations, the Company is obligated to provide KHK access to know-how and regulatory information the Company may develop over the life of the entinostat patent. Lastly, to the extent additional intellectual property is developed during the term of the agreement, KHK will receive the right to the intellectual property when and if available. KHK will conduct the development, regulatory approval filings, and commercialization activities of entinostat in Japan and Korea. KHK paid the Company $25.0 million upfront, which included a $7.5 million equity investment of 536,049 shares of Series B-1 convertible preferred stock and a $17.5 million non-refundable cash payment. In addition, to the extent certain development and commercial milestones are achieved, KHK will be required to pay the Company up to $75.0 million in milestone payments over the term of the license agreement. The term of the agreement commenced on the Effective Date and, unless earlier terminated in accordance with the terms of the agreement, will continue on a country-by-country and product-by-product basis, until the later of: (i) the date all valid claims of the last effective patent among the Company’s patents expires or is abandoned, withheld, or is otherwise invalidated in such country; and (ii) 15 years from the date of the first commercial sale of a product in the Japan or Korea.

The purchase of the Series B-1 and the up-front payment of the license fee were accounted for separately. The Company allocated the amount of consideration related to Series B-1 equal to the fair value of the Series B-1 shares on the Effective Date based on a share price of $14.39 per share, which resulted in $7.7 million of proceeds allocated to the Series B-1 and the remaining consideration of $17.3 million allocated to the up-front license fee. The fair value of the Series B-1 of $14.39 per share was based on a contemporaneous valuation. The Company received $7.5 million and issued the Series B-1 in January 2015 and received the remaining $17.5 million in February 2015. On the date of issuance, the Company recorded accretion of $5.4 million to record the Series B-1 at its redemption value.

The Company has concluded that this agreement is within the scope of ASC 605-25, Revenue Recognition, Multiple-Element Arrangements. Pursuant to this guidance, the Company identified the following deliverables: (i) licenses, (ii) clinical supply and manufacturing obligations, (iii) rights to access and use materials and data, and (iv) rights to additional intellectual property. All other potential deliverables included in the arrangement have been deemed either contingent or inconsequential or perfunctory, individually and in the aggregate. Moreover, the Company has evaluated all deliverables included in the KHK License Agreement and determined that there are two units of accounting in connection with its obligations at inception under the KHK License Agreement: (i) license unit of accounting and (ii) rights to additional intellectual property. The first three deliverables identified above comprise the license unit of accounting. The Company concluded that the stand-alone selling price for the rights to additional intellectual property unit of account is immaterial. As such, the entire $17.3 million allocated to the upfront payment was allocated to the license unit of accounting.

The arrangement consideration allocated to the license unit of accounting will be recognized as revenue ratably over the Company’s expected services period (currently expected to be through 2029) commencing on the date of the first delivery of the clinical trial materials. In June 2015, the Company began delivering clinical materials to KHK and commenced recognizing revenue from the upfront consideration of $17.3 million. During the years ended December 31, 2016 and 2015, the Company recognized $1.2 million and $0.6 million, respectively, of revenue associated with the KHK License Agreement. As of December 31, 2016, there was $15.4 million of deferred revenue related to the KHK License Agreement, which is classified as current or long-term in the consolidated balance sheets.

In October 2016, the Company entered into a clinical trial co-funding agreement with KHK under which the Company will expand its clinical trial agreement with Eastern Cooperative Oncology Group (the “ECOG Agreement”) to include enrollments from sites in Korea.

Eastern Cooperative Oncology Group

In March 2014, the Company entered into the “ECOG Agreement with Eastern Cooperative Oncology Group, a contracting entity for the Eastern Cooperative Oncology Group—American College of Radiology Imaging Network Cancer Research Group (“ECOG-ACRIN”), that describes the parties’ obligations with respect to the NCI-sponsored pivotal Phase 3 clinical trial of entinostat. Under the terms of the ECOG Agreement, ECOG-ACRIN will perform this clinical trial in accordance with the clinical trial protocol and a mutually agreed scope of work. The Company will provide a fixed level of financial support for the clinical trial through an upfront payment of $695,000 and a series of payments of up to $1.0 million each that are comprised of milestone payments through the completion of enrollment and time-based payments through the completion of patient monitoring post-enrollment. In addition, the Company is obligated to supply entinostat and placebo to ECOG-ACRIN for use in the clinical trial. During the second quarter of 2016, the ECOG Agreement was amended to provide additional study activities and the contractual obligation increased by $0.8 million. As of December 31, 2016, the Company’s aggregate payment obligations under this agreement were approximately $21.4 million; and as of December 31, 2016, the Company’s remaining payment obligations are approximately $12.9 million over an estimated period of approximately four years. During the first quarter of 2017, the ECOG Agreement was amended to expand the study to include enrollments from sites in Korea and the contractual obligation increased by $0.5 million.

Data and inventions from the Phase 3 clinical trial are owned by ECOG-ACRIN. The Company has access to the data generated in the clinical trial, both directly from ECOG-ACRIN under the ECOG Agreement as well as from the NCI. Additionally, ECOG-ACRIN has granted the Company a non-exclusive royalty-free license to any inventions or discoveries that are derived from entinostat as a result of its use during the clinical trial, along with a first right to negotiate an exclusive license to any of these inventions or discoveries. Either party may terminate the ECOG Agreement in the event of an uncured material breach by the other party or if the FDA or NCI withdraws the authorization to perform the clinical trial in the United States. The parties may jointly terminate the ECOG Agreement if the parties agree that safety-related issues support termination of the clinical trial.

The Company records the appropriate clinical trial expenses in its financial statements by matching those expenses with the period in which the services and efforts are expended. The Company accounts for these expenses according to the progress of the clinical trial as measured by patient enrollment and the timing of various aspects of the clinical trial. The Company determines accrual estimates through financial models, taking into account discussion with applicable personnel and ECOG-ACRIN as to the progress or state of consummation of the clinical trial or the services completed.

Bayer Pharma AG (formerly known as Bayer Schering Pharma AG)

In March 2007, the Company entered into a license agreement (the “Bayer Agreement”) with Bayer Schering Pharma AG (“Bayer”) for a worldwide, exclusive license to develop and commercialize entinostat and any other products containing the same active ingredient. Under the terms of the Bayer Agreement, the Company paid a nonrefundable up-front license fee of $2.0 million and is responsible for the development and marketing of entinostat. The Company recorded the $2.0 million license fee as research and development expense during the year ended December 31, 2007, as it had no alternative future use. The Company will pay Bayer royalties on a sliding scale based on net sales, if any, and make future milestone payments to Bayer of up to $150.0 million in the event that certain specified development and regulatory goals and sales levels are achieved. In June 2014, a development milestone was achieved, and the Company recorded $2.0 million of research and development expense, which has been fully paid.

In connection with the Bayer Agreement, the Company issued to Bayer a warrant to purchase the number of shares of the Company’s common stock equal to 1.75% of the shares of common stock outstanding on a fully diluted basis as of the earlier of the date the warrant is exercised or the closing of the IPO. Upon the closing of the IPO, the total number of shares of the Company’s common stock issuable upon exercise of the warrant was set at 357,840. Prior to the closing of the IPO, the warrant contained anti-dilution protection to maintain Bayer’s potential ownership at 1.75% of the shares of common stock outstanding on a fully diluted basis, which requires that the actual number of shares of common stock issuable pursuant to the warrant be increased or decreased for any changes in the fully diluted shares of common stock outstanding. The warrant is exercisable at an exercise price of $1.54 per share and expires upon the earlier of the 10-year anniversary of the closing of the IPO or the date of the consummation of a disposition transaction. The warrant was classified as a long-term liability and recorded at fair value with the changes in the fair value recorded in other expense. The Company used the Black-Scholes option-pricing model to determine the fair value of the warrant. Upon the closing of the IPO, the anti-dilution protection for the warrant expired, resulting in the reclassification of the warrant liability to additional paid-in capital. The warrant was re-measured on March 8, 2016, using current assumptions just prior to the reclassification.

The total shares exercisable under the warrant, the fair value associated with the warrant and the Black-Scholes option-pricing model assumptions used to value the shares of common stock issuable pursuant to the warrant were as follows:


	Total Shares of Common Stock Issuable Under the Warrant		Average Exercise Price		Fair Value of Common Stock		Estimated Volatility			Risk-Free Interest Rate			Estimated Dividend Yield			Estimated Remaining Contractual Life (in years)		Fair Value of Warrant Liability
March 8, 2016		357,840	$	1.54	$	13.55		69	%		1.82	%		0.0	%		10.00	$	4,551
December 31, 2015		277,486	$	1.54	$	11.13		73	%		2.15	%		0.0	%		8.06	$	2,848

Property and Equipment, net

12 Months Ended

Dec. 31, 2016

Property, Plant and Equipment [Abstract]

Property and Equipment, net

6. Property and Equipment, net

Property and equipment, net, consisted of the following (in thousands):


	December 31,
	2016			2015
Office and computer equipment	$	117		$	117
Furniture and fixtures		195			74
Office equipment under capital lease		13			13
Leasehold improvements		215			48
Construction in progress		—			28

Total property and equipment		540			280
Accumulated depreciation		(280	)		(192	)

Property and equipment, net	$	260		$	88

Fair Value Measurements

12 Months Ended

Dec. 31, 2016

Fair Value Disclosures [Abstract]

Fair Value Measurements

7. Fair Value Measurements

The carrying amounts of cash and cash equivalents, restricted cash, accounts payable, and accrued expenses approximated their estimated fair values due to the short-term nature of these financial instruments. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value are performed in a manner to maximize the use of observable inputs and minimize the use of unobservable inputs. The accounting standard describes a fair value hierarchy based on three levels of inputs, of which the first two are considered observable and the last unobservable, that may be used to measure fair value, which are the following:


Level 1—		Quoted prices in active markets that are accessible at the market date for identical unrestricted assets or liabilities.

Level 2—		Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs for which all significant inputs are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.
Level 3—		Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

During the years presented, the Company has not changed the manner in which it values assets and liabilities that are measured at fair value using Level 3 inputs. The Company recognizes transfers between levels of the fair value hierarchy as of the end of the reporting period. There were no transfers within the hierarchy during the years ended December 31, 2016, 2015 and 2014.

A summary of the assets and liabilities carried at fair value in accordance with the hierarchy defined above is as follows (in thousands):


			Fair Value Measurements Using
	Total Carrying Value		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
December 31, 2016
Assets:
Cash equivalents	$	23,844	$	17,089	$	6,755	$	—
Short-term investments		81,486		—		81,486		—

Total assets	$	105,330	$	17,089	$	88,241	$	—

December 31, 2015
Assets:
Cash equivalents	$	23,154	$	9,208	$	13,946	$	—
Short-term investments		63,310		—		63,310		—

Total assets	$	86,464	$	9,208	$	77,256	$	—

Liabilities:
Derivative liability	$	133	$	—	$	—	$	133
Common stock warrant liability		2,848		—		—		2,848

Total liabilities	$	2,981	$	—	$	—	$	2,981

Cash equivalents of $17.1 million as of December 31, 2016 and $9.2 million as of December 31, 2015 consisted of overnight investments, money market funds and highly rated corporate bonds and are classified within Level 1 of the fair value hierarchy because they are valued using quoted market prices in active markets. Cash equivalents of $6.8 million as of December 31, 2016 and $13.9 million as of December 31, 2015 consisted of highly rated corporate bonds and commercial paper and are classified within Level 2 of the fair value hierarchy because pricing inputs are other than quoted prices in active markets, which are either directly or indirectly observable as of the reporting date; and fair value is determined through the use of models or other valuation methodologies.

Short-term investments of $81.5 million as of December 31, 2016 and $63.3 million as of December 31, 2015 consisted of commercial paper and highly rated corporate bonds and are classified within Level 2 of the fair value hierarchy because pricing inputs are other than quoted prices in active markets, which are either directly or indirectly observable as of the reporting date; and fair value is determined through the use of models or other valuation methodologies.

The short-term investments are classified as available-for-sale securities. As of December 31, 2016, the remaining contractual maturities of the available-for-sale securities were less than one year, and the balance in the Company’s accumulated other comprehensive income was comprised solely of activity related to the Company’s available-for-sale securities. There were no realized gains or losses recognized on the sale or maturity of available-for-sale securities during the three years ended December 31, 2016. As a result, the Company did not reclassify any amounts out of accumulated other comprehensive income for the same periods. The Company has a limited number of available-for-sale securities in insignificant loss positions as of December 31, 2016, which the Company does not intend to sell and has concluded it will not be required to sell before recovery of the amortized cost for the investment at maturity. The following table summarizes the available-for-sale securities (in thousands):


	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2016
Commercial paper	$	30,125	$	61	$	—		$	30,186
Corporate bonds		51,305		6		(11	)		51,300

	$	81,430	$	67	$	(11	)	$	81,486

December 31, 2015
Commercial paper	$	28,980	$	48	$	—		$	29,028
Corporate bonds		34,302		—		(20	)		34,282

	$	63,282	$	48	$	(20	)	$	63,310

A roll-forward of the recurring fair value measurements of the common stock warrant liability and the derivative liability categorized with Level 3 inputs are as follows (in thousands):


	Common Stock Warrant Liability			Derivative Liability
Balance—January 1, 2014	$	2,482		$	—
Initial fair value of derivative		—			130
Change in fair value		(1,789	)		(4	)

Balance—December 31, 2014		693			126
Change in fair value		2,155			7

Balance—December 31, 2015		2,848			133
Change in fair value		1,703			17
Reclassification		(4,551	)		(150	)

Balance—December 31, 2016	$	—		$	—

The common stock warrant liability was recorded at fair value determined by using the Black-Scholes option-pricing model. This method of valuation involves using inputs such as the fair value of the Company’s common stock, stock price volatility, the contractual term of the warrant, risk-free interest rates, and dividend yields. Due to the nature of these inputs, the valuation of the warrant was considered a Level 3 measurement. Upon the closing of the IPO, the warrant was reclassified to additional paid-in capital. See Note 5 for further discussion of the accounting for the Bayer common stock warrant as well as for a summary of the significant inputs and assumptions used to determine the fair value of the warrant.

The derivative liability related to the contingent success fee owed under the term loans. Upon the completion of an IPO or upon the occurrence of certain change of control or liquidation events, the Company was required to pay a $0.2 million success fee. The Company had recorded the success fee as a derivative financial liability. The initial fair value of the derivative of $0.1 million had been recorded as a debt discount. The term loans were paid in full in October 2015; however, the liability for the success fee survived the repayment of the term loans. Upon completion of the IPO this is no longer accounted for as a derivative, and the success fee of $0.2 million that is payable by June 2018 is included in other long-term liabilities.

Prepaid Expenses and Other Current Assets

12 Months Ended

Dec. 31, 2016

Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]

Prepaid Expenses and Other Current Assets

8. Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following (in thousands):


	December 31,
	2016		2015
Short-term deposits	$	1,630	$	805
Prepaid clinical supplies		461		210
Interest receivable on short-term investments		306		258
Reimbursable costs		262		2
Prepaid insurance		168		54
Other		202		135

Total prepaid expenses and other current assets	$	3,029	$	1,464

Accrued Expenses and Other Current Liabilities

12 Months Ended

Dec. 31, 2016

Payables and Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

9. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):


	December 31,
	2016		2015
Accrued professional fees	$	247	$	561
Accrued compensation and related costs		1,641		698
Accrued clinical costs		4,493		574
Derivative liability		—		133
Other		390		209

Total accrued expenses	$	6,771	$	2,175

Convertible Notes	12 Months Ended
Convertible Notes	Dec. 31, 2016
Debt Disclosure [Abstract]
Convertible Notes	10. Convertible Notes In September 2014, the Company entered into a bridge loan financing with various investors, in which it issued convertible unsecured promissory notes for an aggregate principal amount of $5.0 million (the “2014 Notes”), in two closings. The first closing occurred in September 2014 and $4.9 million was received, and the balance of $0.1 million was received in October 2014. The 2014 Notes accrued interest at 6% per annum and had a maturity date of September 30, 2015 (the “Maturity Date”). The 2014 Notes were convertible upon the occurrence of the certain events during the period that the loans are outstanding. In June 2015, in conjunction with the Series C-1 financing, the outstanding principal of $5.0 million of the 2014 Notes and the related accrued interest of $0.2 million were converted into 372,446 shares of Series C-1 at $14.00 per share, which was the same price paid by Series C-1 investors.

Long-Term Debt	12 Months Ended
Long-Term Debt	Dec. 31, 2016
Debt Disclosure [Abstract]
Long-Term Debt	11. Long-Term Debt *Solar Capital, Ltd, Term Loan* In June 2014, the Company entered into a loan and security agreement with Solar Capital Ltd. (“Solar”), as collateral agent and lender, consisting of a $15.0 million senior secured term loan facility. The loan was secured by substantially all of the Company’s existing and after-acquired assets except its intellectual property, but including right of payment with respect to any such intellectual property and all proceeds from the disposition of any such intellectual property. The intellectual property of the Company was subject to a negative pledge. In September and December 2014, the Company amended the term loan facility. The term loan facility had a maturity date of June 13, 2018. In September 2014, the initial term loan (the “Term A Loan”), in the aggregate principal amount of $5.0 million was funded; and in December 2014, a second term loan (the “Term C Loan”) in the aggregate principal amount of $4.0 million was funded with the following post-closing conditions: pursuant to the KHK License Agreement, the Company was required to receive $7.5 million in net equity proceeds no later than January 9, 2015 and was required to receive $17.5 million in license-related proceeds no later than February 13, 2015, or return the $4.0 million of proceeds from Term Loan C to Solar. The Company achieved the post-closing conditions. Interest accrued at a floating rate per annum equal to LIBOR plus 8.8%, payable monthly in arrears. The Company was required to make interest-only payments on any term loans funded under the term loan facility until July 1, 2015. Beginning on July 1, 2015, it was required to make payments of principal plus accrued interest in equal monthly installments until the maturity date. In addition, the Company was required to pay a final fee equal to 4% of the amount of term loans funded that was due on the earlier of the maturity date of the term loan facility or upon the occurrence of certain change of control or liquidity events. The Company accrued the final fee of $0.4 million on the outstanding term loans through interest expense using the effective-interest method over the period that the debt was outstanding. The Company incurred $0.3 million of debt issuance costs for this term loan facility, which were amortized as interest expense over the period that the related debt was outstanding. The Company had the option to prepay the term loans provided it paid a prepayment fee equal to 2% of the outstanding principal if paid prior to the one-year anniversary and 1% of the outstanding principal if paid after the one-year anniversary of the funding. In October 2015, the Company prepaid the outstanding balance on the term loans of $8.3 million plus accrued interest of $0.1 million and a final fee and prepayment penalty of $0.4 million. In conjunction with this prepayment, the Company recorded an expense of $0.3 million for the final fee and prepayment penalty and wrote off $0.3 million of unamortized debt discount and deferred issuance costs related to the term loans. Upon the completion of an IPO or upon the occurrence of certain change of control or liquidity events, the Company is required to pay a $0.2 million success fee that will be due on the earlier of the maturity date of the term loan facility or upon the occurrence of certain change of control or liquidity events. The Company initially recorded the success fee as a derivative financial liability. The initial fair value of the derivative of $0.1 million was recorded as a debt discount. The term loans were paid in full in October 2015; however, the liability for the success fee survived the repayment of the term loans. Upon completion of the IPO, the success fee is no longer considered a derivative liability. The success fee of $0.2 million is payable by June 2018 and included in other long-term liabilities.

Convertible Preferred Stock

12 Months Ended

Dec. 31, 2016

Equity [Abstract]

Convertible Preferred Stock

12. Convertible Preferred Stock

Upon the closing of the IPO, all of the outstanding shares of the Company’s convertible preferred stock were converted into 12,872,551 shares of its common stock. As of December 31, 2016, the Company does not have any convertible preferred stock issued or outstanding. In connection with the closing of the Company’s IPO, the Company filed an amended and restated certificate of incorporation and adopted amended and restated bylaws; and pursuant to the amended and restated certificate of incorporation, the Company is now authorized to issue 10,000,000 shares of preferred stock in one or more series and to fix the powers, designations, preferences and relative participating option or other rights thereof, including dividend rights, conversion rights, voting rights, redemption terms, liquidation preferences and the number of shares constituting any series, without any further vote or action by the Company’s shareholders.

Convertible preferred stock consisted of the following (in thousands, except share data) as of December 31, 2015:


	Preferred Shares Designated		Issuance Date	Preferred Shares Issued and Outstanding		Liquidation Preference		Carrying Value
Series A-1		3,160,975	March 2013		2,801,745	$	48,032	$	65,666
Series B		3,382,113	March and August 2013		272,240		2,933		5,084
Series B-1		3,965,411	March, April, July, August and November 2013 and January 2015		3,568,020		96,348		96,348
Series C-1		6,090,481	June and August 2015		6,090,461		152,015		152,015

Total								$	319,113

Series A		3,512,194	March and August 2013		700,435	$	—	$	7,231

In August 2015, the Company issued 4,377,902 additional shares of Series C-1 for $61.1 million in cash, net of offering costs of $0.2 million. On the date of issuance, the Company recorded accretion of $46.1 million to record the convertible preferred stock at its redemption value.

In June 2015, the Company issued 1,712,559 shares of Series C-1 for $18.5 million in cash, net of offering costs of $0.2 million, and the conversion of the outstanding principal on the 2014 Notes of $5.0 million and related accrued interest of $0.2 million. On the date of issuance, the Company recorded accretion of $18.2 million to record the convertible preferred stock at its redemption value.

In January 2015, in accordance with the terms of the KHK License Agreement, the Company issued 536,049 shares of Series B-1 at an issuance price of $14.39 per share, the fair value of the Series B-1 based on the results of a contemporaneous valuation. On the date of issuance, the Company recorded accretion of $5.4 million to record the convertible preferred stock at its redemption value.

Common Stock

12 Months Ended

Dec. 31, 2016

Text Block [Abstract]

Common Stock

13. Common Stock

In connection with the closing of the Company’s IPO, the Company filed an amended and restated certificate of incorporation and adopted amended and restated bylaws; and pursuant to the amended and restated certificate of incorporation, the Company is now authorized to issue 100,000,000 shares of common stock. The holders of each share of common stock are entitled to one vote per share held and are entitled to receive dividends, if and when declared by the Board, and to share ratably in the Company’s assets available for distribution to stockholders, in the event of liquidation.

The Company has reserved for future issuance the following shares of common stock related to the potential warrant exercise, future vesting of restricted stock, exercise of stock options, and the employee stock purchase plan:


	December 31, 2016
Common stock issuable under Bayer warrant		357,840
Options to purchase common stock		4,025,966
Restricted stock subject to future vesting		8,542
Employee Stock Purchase Plan		250,000

Total		4,642,348

Stock-Based Compensation

12 Months Ended

Dec. 31, 2016

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-Based Compensation

14. Stock-Based Compensation

In September 2015, the Company’s board of directors adopted its 2015 Omnibus Incentive Plan (“2015 Plan”), which was subsequently approved by its stockholders and became effective upon the closing of the IPO on March 8, 2016. The 2015 Plan replaces the 2007 Stock Plan (“2007 Plan”) and allows for the granting of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock, unrestricted stock, stock units, dividend equivalent rights, performance awards, annual incentive awards, and other equity-based awards to the Company’s executives and other employees, non-employee members of the board of directors, and consultants of the Company. Any options or awards outstanding under the Company’s 2007 Plan remain outstanding and effective. Any shares of common stock related to awards outstanding under the 2007 Plan that thereafter terminate by expiration, forfeiture, cancellation or otherwise without the issuance of such shares will be added to, and included in, the 2015 Plan reserve amount. The Company initially reserved 1,750,000 shares of its common stock for the issuance of awards under the 2015 Plan. As of December 31, 2016, there were 1,465,229 shares available for issuance under the 2015 Plan.

The 2015 Plan provides that the number of shares reserved and available for issuance under the 2015 Plan will automatically increase each January 1, beginning on January 1, 2017, by 4% of the outstanding number of shares of common stock on the immediately preceding December 31 or such lesser number of shares as determined by the Company’s board of directors. On January 1, 2017, the shares available for issuance under the 2015 Plan were increased to 4,754,914.

Stock Options

The Company recognized stock-based compensation expense related to the issuance of stock option awards to employees and non-employees in the consolidated statements of operations as follows (in thousands):


	Years Ended December 31,
	2016		2015		2014
Research and development	$	919	$	846	$	527
General and administrative		3,789		3,036		1,730

Total	$	4,708	$	3,882	$	2,257

As of December 31, 2016, there was $7.8 million of unrecognized compensation cost related to employee and non-employee unvested stock options and unvested restricted stock granted under the 2007 and 2015 Plans, which is expected to be recognized over a weighted-average remaining service period of 2.5 years. Stock compensation costs have not been capitalized by the Company.

The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model with the weighted-average assumptions noted in the table below. Expected volatility for the Company’s common stock was determined based on an average of the historical volatility of a peer group of similar public companies. The Company estimated the expected term of its employee service-based stock options using the “simplified” method, whereby, the expected term equals the average of the vesting term and the original contractual term of the option. The contractual life of the option was used for the estimated life of the non-employee grants. The assumed dividend yield is based upon the Company’s expectation of not paying dividends in the foreseeable future. The risk-free interest rate for periods within the expected life of the option is based upon the U.S. Treasury yield curve in effect at the time of grant. The accounting guidance for stock-based compensation requires forfeitures to be estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The grant date fair values of options issued to employees and non-employees were estimated using the Black-Scholes option-pricing model with the following assumptions:


	Years Ended December 31,
	2016			2015			2014
Expected term (in years)		5.85			5.89			5.96
Volatility rate		71.40	%		69.92	%		70.36	%
Risk-free interest rate		1.41	%		1.73	%		1.91	%
Expected dividend yield		0.00	%		0.00	%		0.00	%

In determining the exercise prices for options granted, the Board has considered the fair value of the common stock as of each grant date. Prior to our IPO, the fair value of the common stock underlying the stock options has been determined by the Board at each award grant date based upon a variety of factors, including the results obtained from an independent third-party valuation, the Company’s financial position and historical financial performance, the status of technological developments within the Company’s products, the composition and ability of the current clinical and management team, an evaluation or benchmark of the Company’s competition, the current business climate in the marketplace, the illiquid nature of the common stock, arm’s-length sales of the Company’s capital stock (including convertible preferred stock), the effect of the rights and preferences of the preferred stockholders, and the prospects of a liquidity event, among others.

A summary of employee and non-employee option activity under the Company’s equity award plans is presented below (in thousands, except share data):


	Number of Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding—January 1, 2016		2,606,195		$	7.56		8.3	$	9,463
Granted		478,530		$	12.64
Exercised		(441,573	)	$	4.66
Canceled or forfeited		(82,415	)	$	8.87

Outstanding—December 31, 2016		2,560,737		$	8.97		8.4	$	503

Exercisable—December 31, 2016		2,234,249		$	8.33		8.2	$	503

Options vested, exercisable or expected to vest—December 31, 2016		2,503,957		$	8.94		8.5	$	504

The weighted-average grant date fair value of options granted during the years ended December 31, 2016, 2015 and 2014, was $7.94, $5.74 and $8.92, respectively. The fair value is being expensed over the vesting period of the options (usually three to four years) on a straight-line basis as the services are being provided.

There were 441,573 options exercised for the year ended December 31, 2016, resulting in total proceeds of $2.1 million; and 41,607 options exercised for the year ended December 31, 2015, resulting in total proceeds of $0.2 million, including 16,731 shares subject to repurchase by the Company. No options were exercised in 2014. The intrinsic value of options exercised during the years ended December 31, 2016 and 2015 was $3.8 million, and $0.2 million, respectively. In accordance with the Company’s policy, the shares were issued from a pool of shares reserved for issuance under the 2007 and 2015 Plans.

Upon the closing of the IPO in March 2016, the Company recorded $0.7 million of additional stock compensation expense related to certain options granted to two of the Company’s executives.

The 2007 Plan allowed employees to exercise unvested options in exchange for restricted common stock. Such arrangements permitted the Company to subsequently repurchase such shares at the exercise price if the vesting conditions were not satisfied. Such an exercise is not substantive for accounting purposes; therefore, the payment received by the Company for the exercise price is recognized as an early exercise liability on the Company’s consolidated balance sheet and will be transferred to common stock and additional paid-in capital as such shares vest. The Company issued 16,731 shares of restricted common stock upon early exercise of stock options during the year ended December 31, 2015, with total proceeds of $0.1 million on the exercise date. As of December 31, 2016, 8,542 unvested shares were legally issued and outstanding and subject to repurchase by the Company. In connection with these unvested shares, the Company has recorded a liability of $0.1 million, of which $42,000 is included in accrued expenses and other current liabilities and $17,000 is included in other long-term liabilities in the Company’s consolidated balance sheet.

Employee Stock Purchase Plan

In September 2015, the Company’s Board adopted the Employee Stock Purchase Plan (the “ESPP”), which was subsequently approved by the Company’s stockholders in February 2016 and became effective upon the closing of our IPO on March 8, 2016. The ESPP authorizes the initial issuance of up to a total of 250,000 shares of common stock to the Company’s employees. No offering periods have been approved at this time; and as of the December 31, 2016, no shares of common stock have been purchased under the ESPP.

Effective January 1, 2017, and continuing until the expiration of the ESPP, which is the earlier of (a) ten years after the date of adoption of the ESPP or (b) such time as all shares of common stock that may be made available for purchase under the ESPP have been issued, the number of common shares available for purchase by the Company’s employees will automatically increase annually on January 1, beginning January 1, 2017, in an amount equal to the lesser of (i) 1% of the total number of issued and outstanding common stock as December 31 of the immediately preceding year or (ii) 250,000 shares of our common stock or such lesser number of shares as determined by the Company’s board of directors. On January 1, 2017, the shares of common stock reserved for issuance under the ESPP was increased to 432,237.

Employee Benefit Plan

The Company has a Section 401(k) defined contribution savings plan for its employees. The plan covers substantially all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pretax basis, subject to legal limitations. Company contributions to the plan may be made at the discretion of the Board. For the years ended December 31, 2016, 2015 and 2014, the Company made $72,000, $33,000 and $0 contributions to the plan, respectively.

- Definition

The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details.

+ References + Details

Income Taxes

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Income Taxes

15. Income Taxes

The Company has not recorded any net tax provision for the periods presented due to the losses incurred and the need for a full valuation allowance on net deferred tax assets. The difference between the income tax expense at the U.S. federal statutory rate and the recorded provision is primarily due to the valuation allowance provided on all deferred tax assets. The Company’s loss before income tax for the periods presented was generated entirely in the United States.

The significant components of the Company’s deferred tax are as follows (in thousands):


	Years Ended December 31,
	2016			2015
Deferred tax assets (liabilities):
Net operating loss carryforwards	$	13,744		$	14,598
Research and development credits		1,739			1,452
Capitalized start-up and research and development costs		47,048			37,641
Deferred revenue		5,420			—
Depreciation and amortization		(10,530	)		(8,433	)
Accruals		604			332
Other temporary differences		2,298			1,769

Deferred tax assets before valuation allowance		60,323			47,359
Valuation allowances		(60,323	)		(47,359	)

Net deferred tax assets	$	—		$	—

The Company has provided a valuation allowance for the full amount of the net deferred tax assets as the realization of the deferred tax assets is not determined to be more likely than not. The valuation allowance increased by $13.0 million and $6.6 million in 2016 and 2015, respectively, due to the increase in deferred tax assets, primarily due to increases in capitalized start-up and research and development costs.

As of December 31, 2016, the Company had approximately $36.0 million and $27.5 million in federal and state Net Operating Losses (“NOLs”), respectively, which expire at various dates through 2036. As of December 31, 2016, the Company had federal and state research credits of $1.1 million and $0.9 million, respectively, which begin to expire in 2021.

Realization of future tax benefits is dependent on many factors, including the Company’s ability to generate taxable income within the net operating loss carryforward period. Under the Internal Revenue Code provisions, certain substantial changes in the Company’s ownership, including the sale of the Company or significant changes in ownership due to sales of equity, have limited and may limit in the future, the amount of net operating loss carryforwards which could be used annually to offset future taxable income. The Company completed an analysis through March 8, 2016 and determined that on March 30, 2007 and August 21, 2015, ownership changes had occurred. The Company may also experience ownership changes in the future as a result of subsequent shifts in our stock ownership, some of which may be outside the Company’s control. As a result, the Company’s ability to use our pre-change NOLs to offset U.S. federal taxable income may be subject to limitations, which could potentially result in increased future tax liability to the Company. In addition, at the state level, there may be periods during which the use of NOLs is suspended or otherwise limited, which could accelerate or permanently increase state taxes owed.

As of December 31, 2016 and 2015, the Company had uncertain tax positions of $0.2 million related to capitalized research and development costs and research and development credits, which reduce the deferred tax assets with a corresponding decrease to the valuation allowance. The Company has elected to recognize interest and penalties related to income tax matters as a component of income tax expense, of which no interest or penalties were recorded for the years ended December 31, 2016, 2015 and 2014. The Company expects none of the unrecognized tax benefits to decrease within the next 12 months related to expired statutes or settlement with the taxing authorities. Due to the Company’s valuation allowance as of December 31, 2016, none of the Company’s unrecognized tax benefits, if recognized, would affect the effective tax rate.

A reconciliation of the Company’s unrecognized tax benefits is as follows (in thousands):


	Years Ended December 31,
	2016		2015		2014
Unrecognized tax benefit—beginning of year	$	241	$	241	$	680
Decreases related to prior period positions		—		—		(439	)

Unrecognized tax benefit—end of year	$	241	$	241	$	241

The Company files tax returns in the United States, Massachusetts, California, South Carolina, New Jersey and New York. All tax years since inception (October 11, 2005) remain open to examination by major tax jurisdictions to which the Company is subject, as carryforward attributes generated in years past may still be adjusted upon examination by the Internal Revenue Service or state tax authorities if they have or will be used in a future period. The Company is currently not under examination by the Internal Revenue Service or any other jurisdictions for any tax years.

Commitments

12 Months Ended

Dec. 31, 2016

Commitments and Contingencies Disclosure [Abstract]

Commitments

16. Commitments

License Agreements

NovaMedica—In August 2013, in connection with the third tranche of its Series B-1 financing, the Company entered into a Technology Transfer Agreement (the “Tech Transfer Agreement”) with Domain Russia Investments Limited (“DRI”). Pursuant to the Tech Transfer Agreement, in exchange for nominal payment, the Company assigned to DRI certain patent applications and granted to DRI a license to develop and commercialize entinostat in certain Eastern European countries (the “Covered Territory”). The Company concurrently entered into a sublicense agreement with DRI (the “DRI Sublicense”) and a sublicense agreement (the “NovaMedica Sublicense”) with NovaMedica LLC (“NovaMedica”), which is jointly owned by Rusnano Medinvest LLC and DRI. Pursuant to the DRI Sublicense, the Company granted to DRI an exclusive sublicense to develop, manufacture and commercialize entinostat in the Russian Federation. Pursuant to the NovaMedica Sublicense, the Company granted to NovaMedica an exclusive sublicense to develop, manufacture and commercialize entinostat in the rest of the Covered Territory. Immediately thereafter, the Company, DRI and NovaMedica executed an assignment and assumption agreement, pursuant to which the assigned patents and all of DRI’s rights and obligations under the Tech Transfer Agreement and the DRI Sublicense were transferred to NovaMedica. Under the Tech Transfer Agreement, in certain cases, the Company is required to assist NovaMedica, and NovaMedica is required to reimburse the Company for any out-of-pocket expenses incurred in providing this assistance, including travel-related expenses.

Eddingpharm—In April 2013, the Company entered into a License and Development Agreement (the “Eddingpharm License Agreement”) and a Series B-1 purchase agreement (the “Eddingpharm Purchase Agreement”) with Eddingpharm International Company Limited (“Eddingpharm”). Under the terms of the Eddingpharm License Agreement, Eddingpharm, in exchange for rights to develop and commercialize entinostat in China and certain other Asian countries, purchased $5.0 million of Series B-1 and agreed to make certain contingent milestone and royalty payments based on revenue targets. In certain cases, the Company is required to assist Eddingpharm, and Eddingpharm is required to reimburse the Company for any out-of-pocket expenses incurred in providing this assistance, including reimbursement for person-hours above a certain cap.

Lease Commitments

In September 2016, the Company entered into a new five-year operating lease for approximately 12,207 square feet of office space in Waltham, Massachusetts, with a lease commencement date of March 1, 2017, and an option to extend the lease term once for an additional three years. The Company also has an option to cancel the lease after three years with the termination fee consisting of $55,000. The Company’s existing Waltham lease expires in April 2017 and will relocate to the new space on March 1, 2017. The lease has monthly lease payments of $25,000 the first 12 months with annual rent escalations thereafter and provides a rent abatement of $0.2 million for the first year. The Company will record the lease abatement as deferred rent and will amortize these amounts on a straight-line basis as a reduction of rent expense over the lease term. The Company paid the landlord a security deposit of $0.1 million in 2016, which will be returned without interest at the end of the lease. The Company recorded the security deposit as a long-term deposit on its consolidated balance sheet.

In December 2015, the Company entered into a new 62-month building lease for approximately 4,039 square feet of space in New York, New York, which commenced on January 1, 2016. The lease has monthly lease payments of $18,000 the first 12 months with an annual rent escalation each year thereafter and provides a rent abatement of $18,000 per month for the first two months. The Company also has an option to cancel the lease after three years with the termination fee consisting of three months of rent. The Company recorded the lease abatement as deferred rent and will amortize these amounts on a straight-line basis as a reduction of rent expense over the lease term. In accordance with the lease, in December 2015, the Company entered into a cash-collateralized irrevocable standby letter of credit in the amount of $0.1 million naming the landlord as beneficiary.

In December 2013, the Company entered into a 40-month lease for approximately 4,712 square feet of office space in Waltham, Massachusetts. The Company also leases office equipment, which is accounted for as a capital lease and included in property and equipment at cost.

Future annual minimum lease payments as of December 31, 2016, are as follows (in thousands):


	Operating Leases		Capital Lease Obligations
For the years ended December 31,
2017	$	396	$	4
2018		524		3
2019		569		—
2020		588		—
2021		395		—
2022 and thereafter		59		—

Total minimum lease payments	$	2,531		7

Less amounts representing interest				1

Present value of net minimum lease payments			$	6

Rent expense recognized under all operating leases, including additional rent charges for utilities, maintenance, and real estate taxes, is calculated on a straight-line basis and amounted to $0.4 million, $0.1 million and $0.1 million for the years ended December 31, 2016, 2015, and 2014, respectively.

Supplemental Cash Flow Information

12 Months Ended

Dec. 31, 2016

Supplemental Cash Flow Elements [Abstract]

Supplemental Cash Flow Information

17. Supplemental Cash Flow Information


	Years Ended December 31,
	2016		2015		2014
	(In thousands)
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Interest paid	$	2	$	688	$	170
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING ACTIVITIES:
Accretion of convertible preferred stock to redemption value	$	—	$	69,715	$	—
Accretion of dividends on convertible preferred stock	$	2,598	$	10,011	$	6,529
Conversion of convertible notes and accrued interest into Series C-1 convertible preferred stock	$	—	$	5,211	$	—
Term loan proceeds allocated to derivative liability	$	—	$	—	$	130
Issuance costs included in accounts payable and accrued expenses	$	—	$	376	$	614
Property and equipment purchases included in accrued expenses	$	—	$	27	$	3
Vesting of restricted stock	$	42	$	14	$	—
Reclassification of common stock warrant liability to additional paid-in capital	$	4,551	$	—	$	—
Conversion of preferred stock to common stock upon closing of initial public offering	$	328,941	$	—	$	—

Related-Party Transactions	12 Months Ended
Related-Party Transactions	Dec. 31, 2016
Related Party Transactions [Abstract]
Related-Party Transactions	18. Related-Party Transactions In June 2015, the Company hired a Chief Executive Officer who was also appointed as a member of the Board. This individual is also a managing director at MPM Asset Management, LLC, which holds an investment in the Company’s common stock. In June 2015, in conjunction with the Series C-1 financing, the Company issued 1,130,740 shares of Series C-1 convertible preferred stock for total gross proceeds, including 2014 Notes conversion and cash purchase price, of $12.7 million to existing stockholders of the Company; and in August 2015 in conjunction with the Series C-1 financing, the Company issued 474,628 shares of Series C-1 convertible preferred stock for total gross proceeds of $5.3 million to an existing stockholder of the Company. See Note 12 for further discussion. In September 2014, the Company issued $5.0 million of 2014 Notes to stockholders of the Company. In June 2015, in conjunction with the Series C-1 financing, the outstanding principal of $5.0 million and the related accrued interest of $0.2 million on the 2014 Notes were converted into 465,563 shares of Series C-1. As of December 31, 2015, no amount of principal or related accrued interest on the 2014 Notes was outstanding. As of December 31, 2014, an aggregate of $5.0 million of principal was outstanding under the 2014 Notes and $0.1 million of related accrued interest were held by stockholders of the Company. Interest expense related to the 2014 Notes held by these stockholders was $0.3 million and $0.1 million for the years ended December 31, 2015 and 2014, respectively.

Quarterly Financial Information (Unaudited)

12 Months Ended

Dec. 31, 2016

Quarterly Financial Information Disclosure [Abstract]

Quarterly Financial Information (Unaudited)

19. Quarterly financial information (unaudited)

The following table contains quarterly financial information for 2016 and 2015. The Company believes that the following information reflects all normal recurring adjustments necessary for a 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.


	Three Months Ended
	2016
In thousands, except per share data	March 31			June 30			September 30			December 31
License fees	$	305		$	305		$	305		$	305

Operating expenses:
Research and development⁽¹⁾		4,786			6,131			12,274			8,474
General and administrative		4,272			2,808			3,269			2,972

Total expenses		9,058			8,939			15,543			11,446

Loss from operations		(8,753	)		(8,634	)		(15,238	)		(11,141	)
Other (expense) income		(1,577	)		276			269			326

Net loss	$	(10,330	)	$	(8,358	)	$	(14,969	)	$	(10,815	)

Net loss per share attributable to common stockholders—basic and diluted	$	(2.85	)	$	(0.47	)	$	(0.84	)	$	(0.59	)

Weighted-average shares—basic and diluted		4,541,536			17,769,514			17,899,481			18,193,027


	Three Months Ended
	2015
In thousands, except per share data	March 31			June 30			September 30			December 31
License fees	$	—		$	17		$	305		$	305

Operating expenses:
Research and development		1,723			2,271			2,968			2,587
General and administrative		2,711			3,288			3,195			2,397

Total expenses		4,434			5,559			6,163			4,984

Loss from operations		(4,434	)		(5,542	)		(5,858	)		(4,679	)
Other (expense) income		(477	)		(672	)		(1,873	)		(584	)

Net loss	$	(4,911	)	$	(6,214	)	$	(7,731	)	$	(5,263	)

Net loss per share attributable to common stockholders—basic and diluted	$	(206.30	)	$	(440.52	)	$	(790.85	)	$	(105.57	)

Weighted-average shares—basic and diluted		58,517			59,788			71,639			83,157

⁽¹⁾	Research and development expenses for three months ended September 30, 2016 included the $5.0 million upfront payment related to the UCB License Agreement.

Basis of Presentation (Policies)	12 Months Ended
Basis of Presentation (Policies)	Dec. 31, 2016
Accounting Policies [Abstract]
Reverse Stock Split	On February 24, 2016, the Company effected a 1-for-1.25 reverse stock split of the Company’s outstanding common stock and convertible preferred stock; and on June 3, 2014, the Company effected a 1-for-12.3 reverse stock split of the Company’s outstanding common stock and convertible preferred stock. Stockholders entitled to fractional shares as a result of the reverse stock splits received a cash payment in lieu of receiving fractional shares. All of the Company’s historical share and per share information shown in the accompanying financial statements and related notes have been retroactively adjusted to give effect to these reverse stock splits.
Principles of Consolidation	In 2011, the Company established a wholly owned subsidiary in the United Kingdom. There have been no activities for this entity to date. In 2014, the Company established a wholly owned U.S. subsidiary, Syndax Securities Corporation. The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
Use of Estimates	*Use of Estimates* The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of costs and expenses during the reporting period. The Company bases estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. The Company evaluates its estimates and assumptions on an ongoing basis. The Company’s actual results may differ from these estimates under different assumptions or conditions.
Cash Equivalents	*Cash Equivalents* Cash equivalents include all highly liquid investments maturing within 90 days or less from the date of purchase. Cash equivalents include money market funds, corporate debt securities, U.S. government agency notes, and overnight deposits.
Restricted Cash	*Restricted Cash* The Company classifies as restricted cash all cash pledged as collateral to secure long-term obligations and all cash whose use is otherwise limited by contractual provisions. Amounts are reported as non-current unless restrictions are expected to be released in the next 12 months.
Short-Term Investments	*Short-Term Investments* Short-term investments include marketable securities with maturities of less than one year or where management’s intent is to use the investments to fund current operations or to make them available for current operations. All investments in marketable securities are classified as available-for-sale and are reported at fair value with unrealized gains and losses excluded from earnings and reported net of tax in accumulated other comprehensive income, which is a component of stockholders’ equity (deficit). Unrealized losses that are determined to be other-than-temporary, based on current and expected market conditions, are recognized in earnings. Declines in fair value determined to be credit related are charged to earnings. The cost of marketable securities sold is determined by the specific identification method. Investments with remaining maturities or that are due within one year from the balance sheet date are classified as current.
Segment Reporting	*Segment Reporting* Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision making group, in making decisions regarding resource allocation and assessing performance. The Company has one operating segment.
Other Assets	*Other Assets* Other assets consist of deferred issuance costs, long-term security deposits, and noncurrent restricted cash. Deferred issuance costs consist primarily of direct incremental legal and accounting fees relating to the Company’s IPO. As of December 31, 2016 and 2015, the Company had capitalized deferred IPO issuance costs of $0 and $1.6 million, respectively. Upon the closing of the IPO, these costs were reclassified to additional paid-in capital as a reduction of the IPO proceeds. In September 2014, the Company determined that it was likely its IPO would be postponed for a period in excess of 90 days. As a result, in accordance with the Securities and Exchange Commission guidance in Staff Accounting Bulletin Topic 5-A, Expenses of Offering, the Company expensed as general and administrative expenses previously deferred IPO costs of $4.3 million.
Concentrations of Credit Risk	*Concentrations of Credit Risk* Cash and cash equivalents, restricted cash, and short-term investments are financial instruments that potentially subject the Company to concentrations of credit risk. Substantially all of the Company’s cash, cash equivalents, and short-term investments were deposited in accounts at two financial institutions, and at times, such deposits may exceed federally insured limits. The Company has not experienced any losses in such accounts, and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. The Company’s available-for-sale investments primarily consist of U.S. Treasury securities, U.S. government agency securities, corporate debt securities, certificates of deposit and overnight deposits and potentially subject the Company to concentrations of crdit risk.
Property and Equipment	*Property and Equipment* Property and equipment are recorded at cost. Depreciation is recorded using the straight-line method over the estimated useful lives of the assets (three to five years). Assets under capital leases are amortized over the shorter of their useful lives or lease term using the straight-line method. Major replacements and improvements are capitalized, while general repairs and maintenance are expensed as incurred.
Impairment of Long-Lived Assets	*Impairment of Long-Lived Assets* Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. When such events occur, the Company compares the carrying amounts of the assets to their undiscounted expected future cash flows. If this comparison indicates that there is impairment, the amount of impairment is calculated as the difference between the carrying value and fair value. To date, no such impairments have been recognized.
Revenue Recognition	*Revenue Recognition* The Company enters into license agreements for the development and commercialization of its product candidates, entinostat and SNDX-6352. License agreements may include non-refundable upfront payments, contingent payments based on the occurrence of specified events under the Company’s license arrangements, partial or complete reimbursement of research and development expenses, license fees and royalties on sales of entinostat if they are successfully approved and commercialized. The Company’s performance obligations under the license agreements may include the transfer of intellectual property rights in the form of licenses, obligations to provide research and development services and related materials and participation on certain development and/or commercialization committees with the collaboration partners. Revenue is recognized when (i) persuasive evidence of an arrangement exists, (ii) transfer of technology has been completed, services have been performed or products have been delivered, (iii) the fee is fixed and determinable, and (iv) collection is reasonably assured. For revenue agreements with multiple-elements, the Company identifies the deliverables included within the agreement and evaluates which deliverables represent separate units of accounting based on the achievement of certain criteria including whether the deliverable has stand-alone value. Upfront payments received in connection with licenses of the Company’s technology rights are deferred if facts and circumstances dictate that the license does not have stand-alone value and are recognized as license revenue over the estimated period of performance that is generally consistent with the terms of the research and development obligations contained in the specific license agreement. The Company periodically reviews its estimated periods of performance based on the progress under each arrangement and accounts for the impact of any changes in estimated periods of performance on a prospective basis. At the inception of each agreement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. The Company evaluates factors such as the scientific, regulatory, commercial and other risks that must be overcome to achieve the respective milestone, the level of effort and investment required to achieve the respective milestone and whether the milestone consideration is reasonable relative to all deliverables and payment terms in the arrangement in making this assessment. Non-refundable payments that are contingent upon achievement of a substantive milestone are recognized in their entirety in the period in which the milestone is achieved, assuming all other revenue recognition criteria are met. Other contingent payments in which a portion of the milestone consideration is refundable or adjusts based on future performance or non-performance (e.g., through a penalty or claw-back provision) are not considered to relate solely to past performance, and therefore, not considered substantive. Amounts that are not recognized as revenue due to the uncertainty as to whether they will be retained or because they are expected to be refunded are recorded as a liability. The Company recognizes non-substantive milestone payments over the remaining estimated period of performance once the milestone is achieved. Contingent payments associated with the achievement of specific objectives in certain contracts that are not considered substantive because the Company does not contribute effort to the achievement of such milestones are recognized as revenue upon achievement of the objective, as long as there are no undelivered elements remaining and no continuing performance obligations by the Company, assuming all other revenue recognition criteria are met.
Research and Development	*Research and Development* Research and development costs are expensed as incurred. Research and development expenses include payroll and personnel expenses, consulting costs, external contract research and development expenses, and allocated overhead, including rent, equipment depreciation, and utilities. Research and development costs that are paid in advance of performance are capitalized as a prepaid expense and amortized over the service period as the services are provided. The Company expense upfront license payments related to acquired technologies that have not yet reached technological feasibility and have no alternative future use. In instances where the Company enters into cost-sharing arrangements, all research and development costs reimbursed by the collaborators are accounted for as reductions to research and development expense. During the year ended December 31, 2016, the Company incurred $0.5 million in external costs related to cost-sharing collaborations, of which $0.3 million has been recorded as a reduction to research and development expense. There were no cost-sharing arrangements in 2015 or 2014.
Clinical Trial Costs	*Clinical Trial Costs* Clinical trial costs are a component of research and development expenses. The Company accrues and expenses clinical trial activities performed by third parties based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, or other information provided to us by our vendors.
Income Taxes	*Income Taxes* The Company records deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax bases of assets and liabilities and for loss and credit carryforwards using enacted tax rates expected to be in effect in the years in which the differences reverse. A valuation allowance is provided to reduce the net deferred tax assets to the amount that will more likely than not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more-likely-than-not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes.
Guarantees and Indemnifications	*Guarantees and Indemnifications* As permitted under Delaware law, the Company indemnifies its officers, directors, and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. The Company has standard indemnification arrangements under office leases (as described in Note 16) 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 Company’s lease. Through December 31, 2016, the Company had not experienced any losses related to these indemnification obligations and no claims 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 were established.
Stock-Based Compensation	*Stock-Based Compensation* The Company accounts for all stock option awards granted to employees and non-employees using a fair value method. Stock-based compensation is measured at the grant date fair value of employee stock option grants and is recognized over the requisite service period of the awards (usually the vesting period) on a straight-line basis, net of estimated forfeitures. Stock option awards to non-employees are subject to periodic revaluation over their vesting terms.
Convertible Preferred Stock	*Convertible Preferred Stock* Upon closing of the IPO, all of the outstanding shares of the Company’s outstanding convertible preferred stock converted into shares of the common stock. Prior to the IPO, the Company had classified certain series of convertible preferred stock as temporary equity in the consolidated balance sheets due to certain change in control events that were outside of the Company’s control, including liquidation, sale, or transfer of control of the Company, as holders of the convertible preferred stock could cause redemption of the shares in these situations. The carrying value of the convertible preferred stock was presented at its maximum redemption value. As of December 31, 2015, the Series A preferred stock had no liquidation preference and was presented in permanent equity.
Debt Discount	*Debt Discount* The Company has recorded the fair value of the derivative liability related to its term loans as debt discount, which is presented in the consolidated balance sheets as an offset to the carrying value amount of the debt. Debt discount is amortized to interest expense using the effective interest rate method or a method that approximates the effective interest rate method over the expected period that the debt is expected to be outstanding. In October 2015, the Company prepaid the outstanding balance on the term loans and wrote off the unamortized debt discount related to the term loans.
Derivative Liabilities	*Derivative Liabilities* The Company records potential payments that would be made to lenders upon certain triggering events as derivative financial liabilities. The derivative liability is initially valued at fair value using a probability-weighted expected return model. Gains and losses arising from changes in fair value are recognized in other income (expense) in the consolidated statement of operations at each period end while such liabilities are outstanding.
Common Stock Warrants	*Common Stock Warrants* The Company has recorded common stock warrants issued with license agreements as derivative financial liabilities, as the terms of the warrants are not fixed due to potential adjustments in the exercise price and/or the number of shares issuable under the warrants. The common stock warrants are initially recorded at fair value, with gains and losses arising from changes in fair value recognized in other income (expense) in the consolidated statement of operations at each period end while such instruments are outstanding. The warrant liabilities were valued using a Black-Scholes option-pricing model. Upon the closing of the IPO, the Company’s outstanding warrant liability was reclassified to additional paid-in capital.
Recently Issued and Adopted Accounting Pronouncements	*Recently Issued and Adopted Accounting Pronouncements* In November 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) ASU 2016-18, “Statement of Cash Flows (Topic 230): Restricted Cash” (“ASU 2016-18”). ASU 2016-18 requires that restricted cash and restricted cash equivalents be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The statement of cash flows must also explain the change during the period in the total of cash, cash equivalents, and restricted cash or restricted cash equivalents. For public business entities, this ASU is effective for fiscal years beginning after December 15, 2017. The Company does not expect the adoption of this standard to have a material impact on its consolidated financial statements. In March 2016, the FASB issued ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting (“ASU 2016-09”). ASU 2016-09 simplifies several aspects related to the accounting for share-based payment transactions, including the accounting for income taxes, forfeitures, statutory tax withholding requirements and classification on the statement of cash flows. ASU 2016-09 will be effective for the Company on January 1, 2017. The Company does not expect the adoption of this standard to have a material impact on its consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, “Leases (Topic 842).” Under ASU 2016-02, lessees will be required to recognize, for all leases of 12 months or more, a liability to make lease payments and a right-of-use asset representing the right to use the underlying asset for the lease term. Additionally, the guidance requires improved disclosures to help users of financial statements better understand the nature of an entity’s leasing activities. This ASU is effective for public reporting companies for interim and annual periods beginning after December 15, 2018, with early adoption permitted, and must be adopted using a modified retrospective approach. The standard will be effective for the Company on January 1, 2019. The Company is in the process of evaluating the effect of the new guidance on the Company’s consolidated financial statements and related disclosures. In August 2014, FASB issued ASU 2014-15, Presentation of Financial Statements-Going Concern (“ASU 2014-15”). ASU 2014-15 provides guidance on management’s responsibility in evaluating whether there are conditions or events that raise substantial doubt about a company’s ability to continue as a going concern within one year from the date the financial statements are issued, and about related footnote disclosures. ASU 2014-15 is effective for the annual period ending after December 15, 2016, and for annual periods and interim periods thereafter. The Company adopted ASU 2014-15 for the year ended December 31, 2016, and the adoption of this standard did not impact the Company’s consolidated financial statements and related disclosures. In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”). ASU 2014-09 supersedes the revenue recognition requirements of FASB Accounting Standards Codification (“ASC”) Topic 605, Revenue Recognition and most industry-specific guidance throughout the Accounting Standards Codification, resulting in the creation of FASB ASC Topic 606, Revenue from Contracts with Customers. ASU 2014-09 requires entities to recognize revenue in a way that depicts the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled to in exchange for those goods or services. Adoption will be permitted using either a retrospective or modified retrospective approach. In July 2015, FASB voted to delay the effective date of the standard by one year to the first quarter of 2018 to provide companies sufficient time to implement the standard. Early adoption will be permitted, but not before the first quarter of 2017. The Company is currently evaluating the method by which it will implement this standard and the impact of the adoption of this standard on the Company’s consolidated financial statements.

Net Loss per Share Attributable to Common Stockholders (Tables)

12 Months Ended

Dec. 31, 2016

Earnings Per Share [Abstract]

Computation of Basic and Diluted Net Loss per Share Attributable to Common Stockholders

The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company (in thousands, except per share data):


	Years Ended December 31,
	2016			2015			2014
Numerator—basic and diluted:
Net loss	$	(44,472	)	$	(24,119	)	$	(19,828	)
Accretion of convertible preferred stock dividends		(2,598	)		(10,011	)		(6,529	)
Accretion of convertible preferred stock to redemption value		—			(69,715	)		—

Net loss attributable to common stockholders—basic and diluted	$	(47,070	)	$	(103,845	)	$	(26,357	)

Net loss per share—basic and diluted	$	(3.22	)	$	(1,519.27	)	$	(453.02	)

Denominator—basic and diluted:
Weighted-average common shares used to compute net loss per share—basic and diluted		14,619,716			68,352			58,181

Potential Dilutive Securities Excluded from Computation of Diluted Net Loss per Common Share


	December 31,
	2016		2015		2014
Convertible preferred stock		—		12,872,551		6,246,041
Options to purchase common stock		2,560,737		2,606,195		831,148
Common stock warrant		357,840		277,486		127,099
Convertible notes and related accrued interest		—		—		363,294
Restricted stock subject to future vesting		8,542		14,684		—

Significant Agreements (Tables)

12 Months Ended

Dec. 31, 2016

Equity [Abstract]

Schedule of Assumptions Used to Value Shares of Common Stock Issuable Pursuant to Warrant


	Total Shares of Common Stock Issuable Under the Warrant		Average Exercise Price		Fair Value of Common Stock		Estimated Volatility			Risk-Free Interest Rate			Estimated Dividend Yield			Estimated Remaining Contractual Life (in years)		Fair Value of Warrant Liability
March 8, 2016		357,840	$	1.54	$	13.55		69	%		1.82	%		0.0	%		10.00	$	4,551
December 31, 2015		277,486	$	1.54	$	11.13		73	%		2.15	%		0.0	%		8.06	$	2,848

Property and Equipment, net (Tables)

12 Months Ended

Dec. 31, 2016

Property, Plant and Equipment [Abstract]

Property and Equipment, Net

Property and equipment, net, consisted of the following (in thousands):


	December 31,
	2016			2015
Office and computer equipment	$	117		$	117
Furniture and fixtures		195			74
Office equipment under capital lease		13			13
Leasehold improvements		215			48
Construction in progress		—			28

Total property and equipment		540			280
Accumulated depreciation		(280	)		(192	)

Property and equipment, net	$	260		$	88

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2016

Fair Value Disclosures [Abstract]

Schedule of Fair Values of Financial Assets and Liabilities Measured at Fair Value

A summary of the assets and liabilities carried at fair value in accordance with the hierarchy defined above is as follows (in thousands):


			Fair Value Measurements Using
	Total Carrying Value		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
December 31, 2016
Assets:
Cash equivalents	$	23,844	$	17,089	$	6,755	$	—
Short-term investments		81,486		—		81,486		—

Total assets	$	105,330	$	17,089	$	88,241	$	—

December 31, 2015
Assets:
Cash equivalents	$	23,154	$	9,208	$	13,946	$	—
Short-term investments		63,310		—		63,310		—

Total assets	$	86,464	$	9,208	$	77,256	$	—

Liabilities:
Derivative liability	$	133	$	—	$	—	$	133
Common stock warrant liability		2,848		—		—		2,848

Total liabilities	$	2,981	$	—	$	—	$	2,981

Summary of Available-for-Sale Securities

The following table summarizes the available-for-sale securities (in thousands):


	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2016
Commercial paper	$	30,125	$	61	$	—		$	30,186
Corporate bonds		51,305		6		(11	)		51,300

	$	81,430	$	67	$	(11	)	$	81,486

December 31, 2015
Commercial paper	$	28,980	$	48	$	—		$	29,028
Corporate bonds		34,302		—		(20	)		34,282

	$	63,282	$	48	$	(20	)	$	63,310

Summary of Changes in Fair Value of Common Stock Warrant Liability and Derivative Liability Inputs

A roll-forward of the recurring fair value measurements of the common stock warrant liability and the derivative liability categorized with Level 3 inputs are as follows (in thousands):


	Common Stock Warrant Liability			Derivative Liability
Balance—January 1, 2014	$	2,482		$	—
Initial fair value of derivative		—			130
Change in fair value		(1,789	)		(4	)

Balance—December 31, 2014		693			126
Change in fair value		2,155			7

Balance—December 31, 2015		2,848			133
Change in fair value		1,703			17
Reclassification		(4,551	)		(150	)

Balance—December 31, 2016	$	—		$	—

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Prepaid Expenses and Other Current Assets (Tables)

12 Months Ended

Dec. 31, 2016

Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]

Schedule of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following (in thousands):


	December 31,
	2016		2015
Short-term deposits	$	1,630	$	805
Prepaid clinical supplies		461		210
Interest receivable on short-term investments		306		258
Reimbursable costs		262		2
Prepaid insurance		168		54
Other		202		135

Total prepaid expenses and other current assets	$	3,029	$	1,464

Accrued Expenses and Other Current Liabilities (Tables)

12 Months Ended

Dec. 31, 2016

Payables and Accruals [Abstract]

Schedule of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):


	December 31,
	2016		2015
Accrued professional fees	$	247	$	561
Accrued compensation and related costs		1,641		698
Accrued clinical costs		4,493		574
Derivative liability		—		133
Other		390		209

Total accrued expenses	$	6,771	$	2,175

Convertible Preferred Stock (Tables)

12 Months Ended

Dec. 31, 2016

Equity [Abstract]

Schedule of Convertible Preferred Stock

Convertible preferred stock consisted of the following (in thousands, except share data) as of December 31, 2015:


	Preferred Shares Designated		Issuance Date	Preferred Shares Issued and Outstanding		Liquidation Preference		Carrying Value
Series A-1		3,160,975	March 2013		2,801,745	$	48,032	$	65,666
Series B		3,382,113	March and August 2013		272,240		2,933		5,084
Series B-1		3,965,411	March, April, July, August and November 2013 and January 2015		3,568,020		96,348		96,348
Series C-1		6,090,481	June and August 2015		6,090,461		152,015		152,015

Total								$	319,113

Series A		3,512,194	March and August 2013		700,435	$	—	$	7,231

Common Stock (Tables)

12 Months Ended

Dec. 31, 2016

Text Block [Abstract]

Schedule of Common Stock Reserved for Future Issuance


	December 31, 2016
Common stock issuable under Bayer warrant		357,840
Options to purchase common stock		4,025,966
Restricted stock subject to future vesting		8,542
Employee Stock Purchase Plan		250,000

Total		4,642,348

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2016

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Share Based Compensation Expense Related to Issuance of Stock Option Awards to Employees and Non Employees


	Years Ended December 31,
	2016		2015		2014
Research and development	$	919	$	846	$	527
General and administrative		3,789		3,036		1,730

Total	$	4,708	$	3,882	$	2,257

Grant Date Fair Values of Options Issued to Employees and Non-employees Estimated Using Black-Scholes Option Pricing Model

The grant date fair values of options issued to employees and non-employees were estimated using the Black-Scholes option-pricing model with the following assumptions:


	Years Ended December 31,
	2016			2015			2014
Expected term (in years)		5.85			5.89			5.96
Volatility rate		71.40	%		69.92	%		70.36	%
Risk-free interest rate		1.41	%		1.73	%		1.91	%
Expected dividend yield		0.00	%		0.00	%		0.00	%

Summary of Employee and Non-Employee Option Activity

A summary of employee and non-employee option activity under the Company’s equity award plans is presented below (in thousands, except share data):


	Number of Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding—January 1, 2016		2,606,195		$	7.56		8.3	$	9,463
Granted		478,530		$	12.64
Exercised		(441,573	)	$	4.66
Canceled or forfeited		(82,415	)	$	8.87

Outstanding—December 31, 2016		2,560,737		$	8.97		8.4	$	503

Exercisable—December 31, 2016		2,234,249		$	8.33		8.2	$	503

Options vested, exercisable or expected to vest—December 31, 2016		2,503,957		$	8.94		8.5	$	504

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Components of Deferred Tax

The significant components of the Company’s deferred tax are as follows (in thousands):


	Years Ended December 31,
	2016			2015
Deferred tax assets (liabilities):
Net operating loss carryforwards	$	13,744		$	14,598
Research and development credits		1,739			1,452
Capitalized start-up and research and development costs		47,048			37,641
Deferred revenue		5,420			—
Depreciation and amortization		(10,530	)		(8,433	)
Accruals		604			332
Other temporary differences		2,298			1,769

Deferred tax assets before valuation allowance		60,323			47,359
Valuation allowances		(60,323	)		(47,359	)

Net deferred tax assets	$	—		$	—

Summary of Reconciliation of Company's Unrecognized Tax Benefits

A reconciliation of the Company’s unrecognized tax benefits is as follows (in thousands):


	Years Ended December 31,
	2016		2015		2014
Unrecognized tax benefit—beginning of year	$	241	$	241	$	680
Decreases related to prior period positions		—		—		(439	)

Unrecognized tax benefit—end of year	$	241	$	241	$	241

Commitments (Tables)

12 Months Ended

Dec. 31, 2016

Commitments and Contingencies Disclosure [Abstract]

Schedule of Future Annual Minimum Lease Payments

Future annual minimum lease payments as of December 31, 2016, are as follows (in thousands):


	Operating Leases		Capital Lease Obligations
For the years ended December 31,
2017	$	396	$	4
2018		524		3
2019		569		—
2020		588		—
2021		395		—
2022 and thereafter		59		—

Total minimum lease payments	$	2,531		7

Less amounts representing interest				1

Present value of net minimum lease payments			$	6

Supplemental Cash Flow Information (Tables)

12 Months Ended

Dec. 31, 2016

Supplemental Cash Flow Elements [Abstract]

Supplemental Disclosures of Cash Flow Information


	Years Ended December 31,
	2016		2015		2014
	(In thousands)
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Interest paid	$	2	$	688	$	170
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING ACTIVITIES:
Accretion of convertible preferred stock to redemption value	$	—	$	69,715	$	—
Accretion of dividends on convertible preferred stock	$	2,598	$	10,011	$	6,529
Conversion of convertible notes and accrued interest into Series C-1 convertible preferred stock	$	—	$	5,211	$	—
Term loan proceeds allocated to derivative liability	$	—	$	—	$	130
Issuance costs included in accounts payable and accrued expenses	$	—	$	376	$	614
Property and equipment purchases included in accrued expenses	$	—	$	27	$	3
Vesting of restricted stock	$	42	$	14	$	—
Reclassification of common stock warrant liability to additional paid-in capital	$	4,551	$	—	$	—
Conversion of preferred stock to common stock upon closing of initial public offering	$	328,941	$	—	$	—

Quarterly Financial Information (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2016

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Financial Information


	Three Months Ended
	2016
In thousands, except per share data	March 31			June 30			September 30			December 31
License fees	$	305		$	305		$	305		$	305

Operating expenses:
Research and development⁽¹⁾		4,786			6,131			12,274			8,474
General and administrative		4,272			2,808			3,269			2,972

Total expenses		9,058			8,939			15,543			11,446

Loss from operations		(8,753	)		(8,634	)		(15,238	)		(11,141	)
Other (expense) income		(1,577	)		276			269			326

Net loss	$	(10,330	)	$	(8,358	)	$	(14,969	)	$	(10,815	)

Net loss per share attributable to common stockholders—basic and diluted	$	(2.85	)	$	(0.47	)	$	(0.84	)	$	(0.59	)

Weighted-average shares—basic and diluted		4,541,536			17,769,514			17,899,481			18,193,027


	Three Months Ended
	2015
In thousands, except per share data	March 31			June 30			September 30			December 31
License fees	$	—		$	17		$	305		$	305

Operating expenses:
Research and development		1,723			2,271			2,968			2,587
General and administrative		2,711			3,288			3,195			2,397

Total expenses		4,434			5,559			6,163			4,984

Loss from operations		(4,434	)		(5,542	)		(5,858	)		(4,679	)
Other (expense) income		(477	)		(672	)		(1,873	)		(584	)

Net loss	$	(4,911	)	$	(6,214	)	$	(7,731	)	$	(5,263	)

Net loss per share attributable to common stockholders—basic and diluted	$	(206.30	)	$	(440.52	)	$	(790.85	)	$	(105.57	)

Weighted-average shares—basic and diluted		58,517			59,788			71,639			83,157

⁽¹⁾	Research and development expenses for three months ended September 30, 2016 included the $5.0 million upfront payment related to the UCB License Agreement.

Nature of Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Nature of Business [Line Items]
Proceeds from public stock offering, net of issuance costs		$ 52,148
Reclassification of common stock warrant liability to additional paid-in capital		$ 4,551
Common stock, shares authorized		100,000,000	20,800,000
Preferred stock, shares authorized		10,000,000	0
Initial Public Offering [Member]
Nature of Business [Line Items]
Common stock, shares issued	4,809,475
Sale of stock, price per share	$ 12.00
Proceeds from public stock offering, net of issuance costs	$ 50,500
Underwriting discounts and commissions	4,000
Offering expenses	$ 3,100
Convertible preferred stock converted in to common stock	12,872,551	12,872,551
Shares of Common Stock Issuable Under the Warrant	357,840
Reclassification of common stock warrant liability to additional paid-in capital	$ 4,600
Common stock, shares authorized		100,000,000
Preferred stock, shares authorized		10,000,000
Over-Allotment Option [Member]
Nature of Business [Line Items]
Common stock, new shares issued	409,475

Basis of Presentation - Additional Information (Detail)	Feb. 24, 2016	Jun. 03, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Stock split, conversion ratio	0.800	0.081

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Claim Segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments \| Segment	1
Capitalized deferred issuance costs				$ 4,300,000
Series A [Member]
Summary Of Significant Accounting Policies [Line Items]
Preferred stock, liquidation preference		$ 0
Guarantees and Indemnifications [Member]
Summary Of Significant Accounting Policies [Line Items]
Indemnification obligations, claims outstanding \| Claim	0
Indemnification obligations claims, reserves established	$ 0
Cost-Sharing Collaborations [Member]
Summary Of Significant Accounting Policies [Line Items]
External costs related to cost-sharing collaborations	500,000	0	$ 0
Cost-sharing collaborations recorded as a reduction to research and development expense	$ (300,000)
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives	P3Y
Percentage of income tax contingency realized	50.00%
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives	P5Y
Initial Public Offering [Member]
Summary Of Significant Accounting Policies [Line Items]
Capitalized deferred issuance costs	$ 0	$ 1,600,000

Net Loss per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator-basic and diluted:
Net loss	$ (10,815)	$ (14,969)	$ (8,358)	$ (10,330)	$ (5,263)	$ (7,731)	$ (6,214)	$ (4,911)	$ (44,472)	$ (24,119)	$ (19,828)
Accretion of convertible preferred stock dividends									(2,598)	(10,011)	(6,529)
Accretion of convertible preferred stock to redemption value										(69,715)
Net loss attributable to common stockholders-basic and diluted									$ (47,070)	$ (103,845)	$ (26,357)
Net loss per share-basic and diluted	$ (0.59)	$ (0.84)	$ (0.47)	$ (2.85)	$ (105.57)	$ (790.85)	$ (440.52)	$ (206.30)	$ (3.22)	$ (1,519.27)	$ (453.02)
Denominator-basic and diluted:
Weighted-average common shares used to compute net loss per share-basic and diluted	18,193,027	17,899,481	17,769,514	4,541,536	83,157	71,639	59,788	58,517	14,619,716	68,352	58,181

Net Loss per Share Attributable to Common Stockholders - Potential Dilutive Securities Excluded from Computation of Diluted Net Loss per Common Share (Detail) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per common share		12,872,551	6,246,041
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per common share	2,560,737	2,606,195	831,148
Common Stock Warrant [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per common share	357,840	277,486	127,099
Convertible Notes and Related Accrued Interest [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per common share			363,294
Restricted Stock Subject to Future Vesting [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per common share	8,542	14,684

Significant Agreements - Additional Information (Detail) - USD ($)		1 Months Ended						3 Months Ended									12 Months Ended
	Dec. 19, 2014	Jul. 31, 2016	Feb. 28, 2015	Jan. 31, 2015	Jun. 30, 2014	Mar. 31, 2014	Mar. 31, 2007	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2007
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense									$ 8,474,000	$ 12,274,000	$ 6,131,000	$ 4,786,000	$ 2,587,000	$ 2,968,000	$ 2,271,000	$ 1,723,000	$ 31,665,000	$ 9,549,000	$ 10,175,000
Up-front payment allocated to preferred stock value									0				$ 0				0	0
Proceeds received from preferred stock issuance																		87,323,000
License Agreement [Member] \| Kyowa Hakko Kirin Co., Ltd. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License agreement upfront payment received	$ 25,000,000
Period of termination after first commercial sale of first licensed product	15 years
Up-front payment allocated to license fee	$ 17,300,000
Up-front license fee received			$ 17,500,000
Revenue recognized during period																	1,200,000	$ 600,000
Deferred revenue from license agreement									$ 15,400,000								$ 15,400,000
License Agreement [Member] \| Kyowa Hakko Kirin Co., Ltd. [Member] \| Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment receivable upon achievement of development and commercial milestone	$ 75,000,000
License Agreement [Member] \| Kyowa Hakko Kirin Co., Ltd. [Member] \| Series B-1 [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Preferred stock issued, shares	536,049
Preferred share price per share				$ 14.39
Up-front payment allocated to preferred stock value	$ 7,700,000
Proceeds received from preferred stock issuance				$ 7,500,000
Accretion to redemption value				$ 5,400,000
License Agreement [Member] \| Bayer Pharma AG [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense					$ 2,000,000															$ 2,000,000
Up-front license fee paid							$ 2,000,000
Percentage of shares issuable upon conversion of warrant																	1.75%
Warrant expiration period																	10 years
Warrant exercisable									$ 1.54								$ 1.54
Number of shares of common stock issuable upon exercise of the warrant																	357,840
License Agreement [Member] \| Bayer Pharma AG [Member] \| Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate payment obligation							$ 150,000,000
Clinical Trial [Member] \| Eastern Cooperative Oncology Group [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront milestone payable						$ 695,000
Milestone payment payable						$ 1,000,000
Aggregate payment obligation									$ 21,400,000								$ 21,400,000
Remaining contractual obligation									$ 12,900,000								$ 12,900,000
Period of contractual obligation						4 years
Increase in contractual obligation											$ 800,000
Clinical Trial [Member] \| Eastern Cooperative Oncology Group [Member] \| Scenario, Forecast [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase in contractual obligation								$ 500,000
UCB License Agreement [Member] \| UCB Biopharma [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront milestone payable										$ 5,000,000
Potential milestone payments to be made		$ 119,500,000
Aggregate potential milestone payable		$ 250,000,000
License expiration year		10 years
Research and development expense		$ 5,000,000

Significant Agreements - Schedule of Assumptions Used to Value Shares of Common Stock Issuable Pursuant to Warrant (Detail) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Mar. 08, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Warrant or Right [Line Items]
Estimated Volatility			71.40%	69.92%	70.36%
Risk-free interest rate			1.41%	1.73%	1.91%
Estimated Dividend Yield			0.00%	0.00%	0.00%
Estimated Remaining Contractual Life (in years)			5 years 10 months 6 days	5 years 10 months 21 days	5 years 11 months 16 days
Common Stock Warrant [Member]
Class of Warrant or Right [Line Items]
Revaluation Date of Warrant	Mar. 08, 2016	Dec. 31, 2015
Total Shares of Common Stock Issuable Under the Warrant	357,840	277,486		277,486
Average Exercise Price	$ 1.54	$ 1.54		$ 1.54
Fair Value of Common Stock	$ 13.55	$ 11.13		$ 11.13
Estimated Volatility	69.00%	73.00%
Risk-free interest rate	1.82%	2.15%
Estimated Dividend Yield	0.00%	0.00%
Estimated Remaining Contractual Life (in years)	10 years	8 years 22 days
Fair Value of Warrant Liability	$ 4,551	$ 2,848		$ 2,848

Property and Equipment, Net - Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 540	$ 280
Accumulated depreciation	(280)	(192)
Property and equipment, net	260	88
Office and Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	117	117
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	195	74
Office Equipment Under Capital Lease [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	13	13
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 215	48
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		$ 28

Fair Value Measurements - Schedule of Fair Values of Financial Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	$ 81,486	$ 63,310
Common stock warrant liability		2,848
Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	23,844	23,154
Short-term investments	81,486	63,310
Total assets	105,330	86,464
Derivative liability		133
Common stock warrant liability		2,848
Total liabilities		2,981
Quoted Prices in Active Markets Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	17,089	9,208
Total assets	17,089	9,208
Significant Other Observable Inputs Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	6,755	13,946
Short-term investments	81,486	63,310
Total assets	$ 88,241	77,256
Significant Unobservable Inputs Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability		133
Common stock warrant liability		2,848
Total liabilities		$ 2,981

Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Oct. 31, 2015
Fair Value Measurements [Line Items]
Available for sale debt securities fair value	$ 81,486,000	$ 63,310,000
Realized gains or losses recognized on the sale or maturity of available-for-sale securities	0
Term Loan [Member]
Fair Value Measurements [Line Items]
Initial fair value of derivative recorded as debt discount			$ 100,000
Initial Public Offering [Member] \| Term Loan [Member] \| Success Fee Payable [Member]
Fair Value Measurements [Line Items]
Success fee payable included in other long-term liabilities	200,000
Quoted Prices in Active Markets Level 1 [Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	17,089,000	9,208,000
Quoted Prices in Active Markets Level 1 [Member] \| Overnight Investments, Money Market Funds and Highly Rated Corporate Bonds Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	17,100,000	9,200,000
Significant Other Observable Inputs Level 2 [Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	6,755,000	13,946,000
Significant Other Observable Inputs Level 2 [Member] \| Corporate Bonds and Commercial Paper [Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	6,800,000	13,900,000
Available for sale debt securities fair value	$ 81,500,000	$ 63,300,000

Fair Value Measurements - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 81,430	$ 63,282
Unrealized Gains	67	48
Unrealized Losses	(11)	(20)
Fair Value	81,486	63,310
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,125	28,980
Unrealized Gains	61	48
Fair Value	30,186	29,028
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	51,305	34,302
Unrealized Gains	6
Unrealized Losses	(11)	(20)
Fair Value	$ 51,300	$ 34,282

Fair Value Measurements - Summary of Changes in Fair Value of Common Stock Warrant Liability and Derivative Liability Inputs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 2,848
Change in fair value	(1,703)	$ (2,155)	$ 1,789
Reclassification	4,551
Ending Balance		2,848
Significant Unobservable Inputs Level 3 [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	2,848
Ending Balance		2,848
Significant Unobservable Inputs Level 3 [Member] \| Derivative Liability [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	133	126
Initial fair value of derivative			130
Change in fair value	17	7	(4)
Reclassification	(150)
Ending Balance		133	126
Common Stock Warrant Liability [Member] \| Significant Unobservable Inputs Level 3 [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	2,848	693	2,482
Change in fair value	1,703	2,155	(1,789)
Reclassification	$ (4,551)
Ending Balance		$ 2,848	$ 693

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Short-term deposits	$ 1,630	$ 805
Prepaid clinical supplies	461	210
Interest receivable on short-term investments	306	258
Reimbursable costs	262	2
Prepaid insurance	168	54
Other	202	135
Total prepaid expenses and other current assets	$ 3,029	$ 1,464

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Payables and Accruals [Abstract]
Accrued professional fees	$ 247	$ 561
Accrued compensation and related costs	1,641	698
Accrued clinical costs	4,493	574
Derivative liability		133
Other	390	209
Total accrued expenses	$ 6,771	$ 2,175

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References + Details

Convertible Notes - Additional Information (Detail) - USD ($)	1 Months Ended
	Jun. 30, 2015	Sep. 30, 2014	Oct. 30, 2014
Debt Instrument [Line Items]
Debt instrument, aggregate principal amount		$ 5,000,000
Convertible Notes Payable [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate		6.00%
Debt instrument, maturity date		Sep. 30, 2015
Principal outstanding	$ 5,000,000
Accrued interest	200,000
Convertible Notes Payable [Member] \| Series C-1 [Member]
Debt Instrument [Line Items]
Debt instrument, aggregate principal amount	$ 5,000,000
Debt instrument, number of shares converted	372,446
Debt instrument, conversion price per share	$ 14.00
Convertible Notes Payable [Member] \| First Closing [Member]
Debt Instrument [Line Items]
Debt instrument, aggregate principal amount		$ 4,900,000
Convertible Notes Payable [Member] \| Second Closing [Member]
Debt Instrument [Line Items]
Debt instrument, aggregate principal amount			$ 100,000

Long-Term Debt - Additional Information (Detail) - USD ($)	1 Months Ended			12 Months Ended
	Oct. 31, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2014
Debt Instrument [Line Items]
Debt instrument, aggregate principal amount							$ 5,000,000
Term loan, accrued interest repaid				$ 2,000	$ 688,000	$ 170,000
Solar Capital Ltd [Member] \| Senior Secured Term Loan Facility [Member]
Debt Instrument [Line Items]
Credit facility, maximum borrowing capacity			$ 15,000,000
Debt instrument maturity date			Jun. 13, 2018
Net equity proceeds		$ 7,500,000
License-related proceeds		17,500,000				17,500,000
Term loan repaid	$ 8,300,000
Description of interest rate on term loan	LIBOR plus 8.8%
Interest rate on term loan	8.80%
Effective fee on term loan	4.00%
Term loan, fee and prepayment penalty	$ 400,000
Debt issuance cost incurred	$ 300,000
Prepayment fee percentage, if paid prior to the one-year anniversary	2.00%
Prepayment fee percentage, if paid after the one-year anniversary	1.00%
Term loan, accrued interest repaid	$ 100,000
Fee and prepayment penalty expenses recorded	300,000
Unamortized debt discount and deferred issuance costs written off	300,000
Success fee liability	200,000
Fair value of derivative	$ 100,000
Solar Capital Ltd [Member] \| Senior Secured Term Loan Facility [Member] \| Term A Loan [Member]
Debt Instrument [Line Items]
Debt instrument, aggregate principal amount							$ 5,000,000
Solar Capital Ltd [Member] \| Senior Secured Term Loan Facility [Member] \| Term C Loan [Member]
Debt Instrument [Line Items]
Debt instrument, aggregate principal amount		4,000,000				$ 4,000,000
Term loan repaid		$ 4,000,000

Convertible Preferred Stock - Additional Information (Detail) - USD ($)	1 Months Ended			12 Months Ended
	Aug. 31, 2015	Jun. 30, 2015	Jan. 31, 2015	Dec. 31, 2015	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Temporary Equity [Line Items]
Preferred stock, shares authorized				0	10,000,000
Cash received from issuance of Convertible preferred stock				$ 87,323,000
Debt instrument, aggregate principal amount								$ 5,000,000
Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Convertible preferred stock issued					0
Convertible preferred stock outstanding				12,732,466	0		6,105,956		6,105,956
Convertible preferred stock, accretion value				$ 69,715,000
Series C-1 [Member]
Temporary Equity [Line Items]
Convertible preferred stock issued	4,377,902	1,712,559		6,090,461
Convertible preferred stock outstanding				6,090,461
Cash received from issuance of Convertible preferred stock	$ 61,100,000	$ 18,500,000
Offering cost	200,000	200,000
Convertible preferred stock, accretion value	$ 46,100,000	18,200,000
Series C-1 [Member] \| Convertible Notes Payable [Member]
Temporary Equity [Line Items]
Debt instrument, aggregate principal amount		5,000,000
Accrued interest		$ 200,000
Series B-1 [Member]
Temporary Equity [Line Items]
Convertible preferred stock issued			536,049	3,568,020
Convertible preferred stock outstanding				3,568,020
Convertible preferred stock, accretion value			$ 5,400,000
Issuance price of shares			$ 14.39
Initial Public Offering [Member]
Temporary Equity [Line Items]
Conversion of convertible preferred stock to shares of common stock					12,872,551	12,872,551
Preferred stock, shares authorized					10,000,000

Convertible Preferred Stock - Schedule of Convertible Preferred Stock (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Aug. 31, 2015	Jun. 30, 2015	Jan. 31, 2015
Temporary Equity [Line Items]
Preferred Shares Designated	10,000,000	0
Carrying Value		$ 319,113,000
Series A- 1 [Member]
Temporary Equity [Line Items]
Preferred Shares Designated		3,160,975
Issuance Date	2013-03
Preferred Shares Issued		2,801,745
Preferred Shares Outstanding		2,801,745
Carrying Value		$ 65,666,000
Liquidation preference		$ 48,032,000
Series B [Member]
Temporary Equity [Line Items]
Preferred Shares Designated		3,382,113
Issuance Date	March and August 2013
Preferred Shares Issued		272,240
Preferred Shares Outstanding		272,240
Carrying Value		$ 5,084,000
Liquidation preference		$ 2,933,000
Series B-1 [Member]
Temporary Equity [Line Items]
Preferred Shares Designated		3,965,411
Issuance Date	March, April, July, August and November 2013 and January 2015
Preferred Shares Issued		3,568,020			536,049
Preferred Shares Outstanding		3,568,020
Carrying Value		$ 96,348,000
Liquidation preference		$ 96,348,000
Series C-1 [Member]
Temporary Equity [Line Items]
Preferred Shares Designated		6,090,481
Issuance Date	June and August 2015
Preferred Shares Issued		6,090,461	4,377,902	1,712,559
Preferred Shares Outstanding		6,090,461
Carrying Value		$ 152,015,000
Liquidation preference		$ 152,015,000
Series A [Member]
Temporary Equity [Line Items]
Preferred Shares Designated		3,512,194
Issuance Date	March and August 2013
Preferred Shares Issued		700,435
Preferred Shares Outstanding		700,435
Carrying Value		$ 7,231,000
Liquidation preference		$ 0

Common Stock - Additional Information (Detail) - shares	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Common stock, shares authorized	100,000,000	20,800,000
Initial Public Offering [Member]
Class of Stock [Line Items]
Common stock, shares authorized	100,000,000

Common Stock - Schedule of Common Stock Reserved for Future Issuance (Detail)	Dec. 31, 2016 shares
Class of Stock [Line Items]
Common stock reserved for future issuance	4,642,348
Options to Purchase Common Stock [Member]
Class of Stock [Line Items]
Common stock reserved for future issuance	4,025,966
Restricted Stock Subject to Future Vesting [Member]
Class of Stock [Line Items]
Common stock reserved for future issuance	8,542
Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Common stock reserved for future issuance	250,000
Common Stock Issuable Under Bayer Warrant [Member]
Class of Stock [Line Items]
Common stock reserved for future issuance	357,840

Stock-Based Compensation - Additional Information (Detail)		1 Months Ended		12 Months Ended
	Jan. 01, 2017 shares	Mar. 31, 2016 USD ($) Executives	Sep. 30, 2015 shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for issuance \| shares				4,642,348
Weighted-average grant date fair value of options granted \| $ / shares				$ 12.64	$ 5.74	$ 8.92
Stock options exercised, shares \| shares				441,573	41,607	0
Stock options exercised				$ 2,058,000	$ 191,000	$ 6,000
Shares repurchase \| shares				16,731
Intrinsic value of options exercised				$ 3,800,000	200,000
Total liabilities				24,874,000	23,205,000
Accrued expenses and other current liabilities				6,771,000	2,175,000
Other long-term liabilities				$ 288,000	70,000
Stock options to purchase shares of common stock \| shares				478,530
Contributions by the company				$ 72,000	33,000	$ 0
Initial Public Offering [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of company executives \| Executives		2
Additional stock compensation expense		$ 700,000
Restricted Stock Subject to Future Vesting [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for issuance \| shares				8,542
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				3 years
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				4 years
Maximum [Member] \| Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for issuance \| shares	4,754,914
2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance \| shares				1,750,000
Common stock outstanding				4.00%
Shares available for issuance \| shares				1,465,229
Unrecognized compensation expense related to stock option				$ 7,800,000
Weighted average period to recognize compensation expense				2 years 6 months
2007 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to stock option				$ 7,800,000
Weighted average period to recognize compensation expense				2 years 6 months
Total liabilities				$ 100,000
Accrued expenses and other current liabilities				42,000
Other long-term liabilities				$ 17,000
2007 Plan [Member] \| Restricted Stock Subject to Future Vesting [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options exercised					100,000
Shares repurchase \| shares				8,542
Issuance of stock under restricted common stock					$ 16,731
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options to purchase shares of common stock \| shares				0
Employee stock purchase plan description				Effective January 1, 2017, and continuing until the expiration of the ESPP, which is the earlier of (a) ten years after the date of adoption of the ESPP or (b) such time as all shares of common stock that may be made available for purchase under the ESPP have been issued,
Employee Stock Purchase Plan [Member] \| Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock outstanding	1.00%
Shares authorized for issuance \| shares	432,237
Employee Stock Purchase Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance \| shares			250,000
Employee Stock Purchase Plan [Member] \| Maximum [Member] \| Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance \| shares	250,000

Stock-Based Compensation - Share Based Compensation Expense Related to Issuance of Stock Option Awards to Employees and Non Employees (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense, total	$ 4,708	$ 3,882	$ 2,257
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense, total	919	846	527
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense, total	$ 3,789	$ 3,036	$ 1,730

Stock-Based Compensation - Grant Date Fair Values of Options Issued to Employees and Non-employees Estimated Using Black-Scholes Option Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value Assumptions and Methodology for Assets and Liabilities [Abstract]
Expected term (in years)	5 years 10 months 6 days	5 years 10 months 21 days	5 years 11 months 16 days
Volatility rate	71.40%	69.92%	70.36%
Risk-free interest rate	1.41%	1.73%	1.91%
Expected dividend yield	0.00%	0.00%	0.00%

Stock-Based Compensation - Summary of Company's Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Arrangements with Employees and Nonemployees [Abstract]
Options outstanding, Beginning balance	2,606,195
Options granted	478,530
Options exercised	(441,573)	(41,607)	0
Options cancelled or forfeited	(82,415)
Options outstanding, Ending balance	2,560,737	2,606,195
Options exercisable, Ending balance	2,234,249
Options vested, exercisable or expected to vest, Ending balance	2,503,957
Options outstanding, Weighted Average Exercise Price, Ending balance	$ 7.56
Options granted, Weighted Average Exercise Price	12.64	$ 5.74	$ 8.92
Options exercised, Weighted Average Exercise Price	4.66
Options canceled or forfeited, Weighted Average Exercise Price	8.87
Options outstanding, Weighted Average Exercise Price, Ending balance	8.97	$ 7.56
Options exercisable, Weighted Average Exercise Price, Ending balance	8.33
Options vested, exercisable or expected to vest, Weighted Average Exercise Price, Ending balance	$ 8.94
Options outstanding, Weighted Average Remaining Contractual Term	8 years 4 months 24 days	8 years 3 months 18 days
Options exercisable, Weighted Average Remaining Contractual Term, Ending balance	8 years 2 months 12 days
Options vested, exercisable or expected to vest, Weighted Average Remaining Contractual Term, Ending balance	8 years 6 months
Options outstanding, Aggregate Intrinsic Value	$ 503	$ 9,463
Options exercisable, Aggregate Intrinsic Value	503
Options vested, exercisable or expected to vest, Aggregate Intrinsic Value	$ 504

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Line Items]
Net tax provision	$ 0	$ 0
Increase in valuation allowance	$ 13,000,000	6,600,000
Valuation allowance for deferred tax assets, description	The Company has provided a valuation allowance for the full amount of the net deferred tax assets as the realization of the deferred tax assets is not determined to be more likely than not. The valuation allowance increased by $13.0 million and $6.6 million in 2016 and 2015, respectively, due to the increase in deferred tax assets, primarily due to increases in capitalized start-up and research and development costs.
Federal net operating losses	$ 36,000,000
State net operating losses	$ 27,500,000
Operating loss carryforward expiration period	2036
Federal [Member]
Income Taxes [Line Items]
Deferred tax assets research credits	$ 1,100,000
State [Member]
Income Taxes [Line Items]
Deferred tax assets research credits	900,000
Internal Revenue Service [Member]
Income Taxes [Line Items]
Uncertain tax positions	200,000	200,000
Interest or penalties recorded during the period	0	$ 0	$ 0
Unrecognized income tax benefits that would affect effective tax rate	$ 0

Income Taxes - Components of Deferred Tax (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets (liabilities):
Net operating loss carryforwards	$ 13,744	$ 14,598
Research and development credits	1,739	1,452
Capitalized start-up and research and development costs	47,048	37,641
Deferred revenue	5,420
Depreciation and amortization	(10,530)	(8,433)
Accruals	604	332
Other temporary differences	2,298	1,769
Deferred tax assets before valuation allowance	60,323	47,359
Valuation allowances	(60,323)	(47,359)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Summary of Reconciliation of Company's Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefit-beginning of year	$ 241	$ 241	$ 680
Decreases related to prior period positions	0	0	(439)
Unrecognized tax benefit-end of year	$ 241	$ 241	$ 241

Commitments - Additional Information (Detail)	1 Months Ended			12 Months Ended
Commitments - Additional Information (Detail)	Sep. 30, 2016 USD ($) ft²	Dec. 31, 2015 USD ($) ft²	Dec. 31, 2013 ft²	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) ft²	Dec. 31, 2014 USD ($)	Dec. 31, 2013 ft²	Apr. 30, 2013 USD ($)
Commitment And Contingencies [Line Items]
Operating lease, rent expense				$ 400,000	$ 100,000	$ 100,000
Waltham, Massachusetts [Member]
Commitment And Contingencies [Line Items]
Term of contract	5 years		40 months
Area of property leased \| ft²	12,207		4,712				4,712
Lease commenced date				Mar. 01, 2017
Description of lease arrangements				In September 2016, the Company entered into a new five-year operating lease for approximately 12,207 square feet of office space in Waltham, Massachusetts, with a lease commencement date of March 1, 2017, and an option to extend the lease term once by three years. The Company also has an option to cancel the lease after three years with the termination fee consisting of $55,000. The Company’s existing Waltham lease expires in April 2017 and will relocate to the new space on March 1, 2017. The lease has monthly lease payments of $25,000 the first 12 months with annual rent escalations thereafter and provides a rent abatement of $0.2 million for the first year.			In December 2013, the Company entered into a 40-month lease for approximately 4,712 square feet of office space in Waltham, Massachusetts.
Lease extension period	3 years
Lease termination fee	$ 55,000
Lease expiration date				Apr. 30, 2017
Monthly lease payments				$ 25,000
Rent abatement	$ 200,000
Security deposit				$ 100,000
New York [Member]
Commitment And Contingencies [Line Items]
Term of contract		62 months
Area of property leased \| ft²		4,039			4,039
Lease commenced date				Jan. 01, 2016
Description of lease arrangements				In December 2015, the Company entered into a new 62-month building lease for approximately 4,039 square feet of space in New York, New York, which commenced on January 1, 2016. The lease has monthly lease payments of $18,000 the first 12 months with an annual rent escalation each year thereafter and provides a rent abatement of $18,000 per month for the first two months. The Company also has an option to cancel the lease after three years with the termination fee consisting of three months of rent.
Monthly lease payments		$ 18,000
Monthly rent abatement		18,000			$ 18,000
Cash-collateralized irrevocable standby letter of credit		$ 100,000			$ 100,000
Eddingpharm Purchase Agreement [Member]
Commitment And Contingencies [Line Items]
Purchase agreement obligation								$ 5,000,000

Commitments - Schedule of Future Annual Minimum Lease Payments (Detail) $ in Thousands	Dec. 31, 2016 USD ($)
Leases [Abstract]
Operating Leases, 2017	$ 396
Operating Leases, 2018	524
Operating Leases, 2019	569
Operating Leases, 2020	588
Operating Leases, 2021	395
Operating Leases, 2022 and thereafter	59
Operating Leases, Total minimum lease payments	2,531
Capital Lease Obligations, 2017	4,000,000
Capital Lease Obligations, 2018	3,000,000
Capital Lease Obligations, 2019	0
Capital Lease Obligations, 2020	0
Capital Lease Obligations, 2021	0
Capital Lease Obligations, 2022 and thereafter	0
Capital Lease Obligations, Total minimum lease payments	7,000,000
Capital Lease Obligations, Total minimum lease payments	7,000,000
Capital Lease Obligations, Less amounts representing interest	1,000,000
Capital Lease Obligations, Present value of net minimum lease payments	$ 6,000,000

Supplemental Cash Flow Information - Supplemental disclosures of cash flow (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING ACTIVITIES:
Accretion of convertible preferred stock to redemption value		$ 69,715
Accretion of dividends on convertible preferred stock	$ 2,598	10,011	$ 6,529
Conversion of convertible notes and accrued interest into Series C-1 convertible preferred stock		5,211
Term loan proceeds allocated to derivative liability			130
Issuance costs included in accounts payable and accrued expenses		376	614
Property and equipment purchases included in accrued expenses		27	3
Vesting of restricted stock	42	14
Reclassification of common stock warrant liability to additional paid-in capital	4,551
Conversion of preferred stock to common stock upon closing of initial public offering	328,941
Interest paid	$ 2	$ 688	$ 170

Related-Party Transactions - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Aug. 31, 2015	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2014
Related Party Transaction [Line Items]
Debt, principal amount					$ 5,000,000
Series C-1 [Member]
Related Party Transaction [Line Items]
Preferred Shares Issued	4,377,902	1,712,559	6,090,461
Series C-1 [Member] \| Stockholders [Member]
Related Party Transaction [Line Items]
Preferred Shares Issued	474,628	1,130,740
Total gross proceeds	$ 5,300,000
Convertible Unsecured Promissory Notes, 2014 Notes [Member]
Related Party Transaction [Line Items]
Debt, principal amount					$ 5,000,000
Principal outstanding			$ 0	$ 5,000,000
Accrued interest			0	100,000
Interest expense			$ 300,000	$ 100,000
Convertible Unsecured Promissory Notes, 2014 Notes [Member] \| Series C-1 [Member]
Related Party Transaction [Line Items]
Total gross proceeds		$ 12,700,000
Principal outstanding		5,000,000
Accrued interest		$ 200,000
Outstanding principal and related interest converted into shares		465,563

Quarterly Financial Information (Unaudited) - Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
License fees	$ 305	$ 305	$ 305	$ 305	$ 305	$ 305	$ 17		$ 1,220	$ 627
Operating expenses:
Research and development	8,474	12,274	6,131	4,786	2,587	2,968	2,271	$ 1,723	31,665	9,549	$ 10,175
General and administrative	2,972	3,269	2,808	4,272	2,397	3,195	3,288	2,711	13,321	11,591	11,157
Total expenses	11,446	15,543	8,939	9,058	4,984	6,163	5,559	4,434	44,986	21,140	21,332
Loss from operations	(11,141)	(15,238)	(8,634)	(8,753)	(4,679)	(5,858)	(5,542)	(4,434)	(43,766)	(20,513)	(21,332)
Other (expense) income	326	269	276	(1,577)	(584)	(1,873)	(672)	(477)	(706)	(3,606)	1,504
Net loss	$ (10,815)	$ (14,969)	$ (8,358)	$ (10,330)	$ (5,263)	$ (7,731)	$ (6,214)	$ (4,911)	$ (44,472)	$ (24,119)	$ (19,828)
Net loss per share attributable to common stockholders-basic and diluted	$ (0.59)	$ (0.84)	$ (0.47)	$ (2.85)	$ (105.57)	$ (790.85)	$ (440.52)	$ (206.30)	$ (3.22)	$ (1,519.27)	$ (453.02)
Weighted-average shares-basic and diluted	18,193,027	17,899,481	17,769,514	4,541,536	83,157	71,639	59,788	58,517	14,619,716	68,352	58,181

Quarterly Financial Information (Unaudited) - Schedule of Quarterly Financial Information (Parenthetical) (Detail) $ in Millions	3 Months Ended
	Sep. 30, 2016 USD ($)
Research and Development Expense [Member] \| UCB License Agreement [Member]
Quarterly Financial Information [Line Items]
Upfront payment	$ 5.0