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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2016	Aug. 08, 2016
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q2
Trading Symbol	SNDX
Entity Registrant Name	SYNDAX PHARMACEUTICALS INC
Entity Central Index Key	0001395937
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		17,782,150

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 14,441	$ 23,179
Restricted cash	54	54
Short-term investments	111,016	63,310
Prepaid expenses and other current assets	1,868	1,464
Total current assets	127,379	88,007
Property and equipment, net	304	88
Other assets	173	1,808
Total assets	127,856	89,903
Current liabilities:
Accounts payable	1,588	1,452
Accrued expenses and other current liabilities	3,745	2,175
Current portion of deferred revenue	1,220	1,220
Total current liabilities	6,553	4,847
Long-term liabilities:
Common stock warrant liability		2,848
Deferred revenue, less current portion	14,830	15,440
Other long-term liabilities	232	70
Total long-term liabilities	15,062	18,358
Total liabilities	21,615	23,205
Commitments
Convertible preferred stock		319,113
Stockholders' equity (deficit):
Preferred stock, value
Common stock, $0.0001 par value, 100,000,000 and 20,800,000 shares authorized; 17,770,538 and 85,440 shares outstanding at June 30, 2016 and December 31, 2015, respectively	2	1
Additional paid-in capital	385,556
Accumulated other comprehensive income	192	28
Accumulated deficit	(279,509)	(259,675)
Total stockholders' equity (deficit)	106,241	(252,415)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	$ 127,856	89,903
Series A Convertible Preferred Stock [Member]
Stockholders' equity (deficit):
Preferred stock, value		7,231
Total stockholders' equity (deficit)		$ 7,231

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2016	Dec. 31, 2015
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
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	17,770,538	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

Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Revenue:
License fees	$ 305	$ 17	$ 610	$ 17
Total revenues	305	17	610	17
Operating expenses:
Research and development	6,131	2,271	10,917	3,994
General and administrative	2,808	3,288	7,080	5,999
Total operating expenses	8,939	5,559	17,997	9,993
Loss from operations	(8,634)	(5,542)	(17,387)	(9,976)
Other income (expense):
Interest income (expense), net	276	(322)	420	(661)
Change in fair value of common stock warrant liability		(346)	(1,703)	(478)
Other expense		(4)	(18)	(10)
Total other income (expense)	276	(672)	(1,301)	(1,149)
Net loss	(8,358)	(6,214)	(18,688)	(11,125)
Other comprehensive (loss) income:
Unrealized (losses) gains on marketable securities	(36)	(11)	164	(8)
Comprehensive loss	(8,394)	(6,225)	(18,524)	(11,133)
Net loss attributable to common stockholders	$ (8,358)	$ (26,338)	$ (21,286)	$ (38,410)
Net loss per share attributable to common stockholders-basic and diluted	$ (0.47)	$ (440.52)	$ (1.91)	$ (649.30)
Weighted-average number of common shares used to compute net loss per share attributable to common stockholders-basic and diluted	17,769,514	59,788	11,155,525	59,156

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (18,688)	$ (11,125)
Adjustments to reconcile net loss to net cash from operating activities:
Depreciation, amortization and accretion	57	267
Stock-based compensation	2,969	1,424
Change in fair value of warrants and derivative	1,703	478
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(394)	(454)
Accounts payable	(54)	350
Deferred revenue	(610)	17,270
Accrued expenses and other liabilities	1,707	(1,170)
Net cash (used in) provided by operating activities	(13,293)	7,049
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(206)	(5)
Purchases of short-term investments	(109,932)	(42,038)
Proceeds from sales and maturities of short-term investments	62,377	5,345
Net cash used in investing activities	(47,761)	(36,698)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock in initial public stock offering, net	52,318
Proceeds from issuance of convertible preferred stock		26,378
Proceeds from stock option exercises		12
Other	(2)	(1)
Net cash provided by financing activities	52,316	26,389
NET DECREASE IN CASH AND CASH EQUIVALENTS	(8,738)	(3,260)
CASH AND CASH EQUIVALENTS-beginning of period	23,179	10,009
CASH AND CASH EQUIVALENTS-end of period	$ 14,441	$ 6,749

Nature of Business	6 Months Ended
Nature of Business	Jun. 30, 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 focused on developing an innovative pipeline of combination therapies in multiple cancer indications. The Company was incorporated under the laws of the State of Delaware on October 11, 2005 (date of inception) and is headquartered in Waltham, Massachusetts. Our lead product candidate, entinostat, which was granted Breakthrough Therapy designation by the U.S. Food and Drug Administration (“FDA”) following positive results from the Company’s Phase 2b clinical trial, ENCORE 301, is currently being evaluated in a Phase 3 clinical trial for advanced hormone receptor positive breast cancer. The Company is developing entinostat, which has direct effects on both cancer cells and immune regulatory cells, to potentially enhance the body’s immune response on tumors that have shown sensitivity to immunotherapy. Entinostat is also being evaluated as a combination therapy in Phase 1b/2 clinical trials with (i) Merck & Co., Inc. (“Merck”) for patients with non-small cell lung cancer and melanoma, (ii) Genentech, Inc. for patients with triple negative breast cancer (“TNBC”), and (iii) Pfizer Inc. and Merck KGaA, Darmstadt, Germany, for patients with ovarian cancer. On July 1, 2016, the Company entered into a license agreement with UCB Biopharma Sprl (“UCB”) under which UCB granted to the Company a worldwide, sublicenseable, exclusive license to UCB6352, which the Company will refer to as SNDX-6352, an IND-ready anti-CSF-1R monoclonal antibody. Concurrently with the development of entinostat, the Company is also developing SNDX-6352 to potentially enhance the body’s immune response on tumors that have shown sensitivity to immunotherapy. The Company believes that SNDX-6352 has the potential to be used to treat a variety of cancer indications in combination with other oncology agents, including immune checkpoint inhibitors, radiation, chemotherapy and entinostat. We expect to begin the Phase 1 single ascending dose study with SNDX-6352 in healthy volunteers to determine safety and pharmacokinetics during the fourth quarter of 2016. 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 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 are 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 June 30, 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	6 Months Ended
Basis of Presentation	Jun. 30, 2016
Accounting Policies [Abstract]
Basis of Presentation	2. Basis of Presentation The Company has prepared the accompanying condensed consolidated financial statements in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. Certain amounts have been reclassified to conform to the current period presentation. The interim unaudited condensed financial statements have been prepared on the same basis as the annual audited financial statements; and in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of June 30, 2016, and the results of operations, comprehensive loss, and cash flows for the three and six months ended June 30, 2016 and 2015. The results for the three and six months ended June 30, 2016 are not necessarily indicative of the results to be expected for the year ending December 31, 2016, any other interim periods, or any future year or period. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2015, and the notes thereto, which are included in the Company’s Prospectus that forms a part of the Company’s Registration Statement on Form S-1 (File No. 333-208861), which was filed with the Securities and Exchange Commission (the “SEC”) pursuant to Rule 424 on March 3, 2016 (the “Prospectus”). 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 condensed consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. 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. Stockholders entitled to fractional shares as a result of the reverse stock split 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 this reverse stock split.

Summary of Significant Accounting Policies	6 Months Ended
Summary of Significant Accounting Policies	Jun. 30, 2016
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	3. Summary of Significant Accounting Policies Significant Accounting Policies—There have been no significant or material changes to the Company’s significant accounting policies during the three and six months ended June 30, 2016, from the Company’s policies disclosed in the audited consolidated financial statements for the year ended December 31, 2015 and the notes thereto, which are included in the Prospectus. Use of Estimates – The preparation of condensed 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 condensed 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. 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 IPO and issuance of debt. As of December 31, 2015, the Company had capitalized deferred IPO issuance costs of $1.6 million. Upon the closing of the IPO, these costs were reclassified to additional paid-in capital as a reduction of the IPO proceeds. Recently Issued Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”). ASU 2014-09 supersedes the revenue recognition requirements of the FASB’s Accounting Standards Codification (“ASC”) Topic 605, Revenue Recognition, and most industry-specific guidance throughout the ASC, 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. In March 2016, the FASB issued ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net), (“ASU 2016-08”), which amends ASU 2014-09 to clarify the implementation guidance on principal versus agent considerations (gross versus net revenue presentation). ASU 2016-08 does not change the core principle of the guidance stated in ASU 2014-09, instead, the amendments in this ASU are intended to improve the operability and understandability of the implementation guidance on principal versus agent considerations and whether an entity reports revenue on a gross or net basis. Adoption of both ASU 2016-08 and 2014-09 will be permitted using either a retrospective or modified retrospective approach. In July 2015, the 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. The collective guidance will be effective for the Company on January 1, 2018, with early adoption permitted but not before January 1, 2017. The Company is currently evaluating the method by which it will implement these standards and the impact of the adoption of these standards on the Company’s 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 Company is in the process of evaluating the effect of the new guidance on the Company’s consolidated financial statements and disclosures. 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 with early adoption permitted. The Company is currently evaluating the impact that the adoption of ASU 2016-09 will have on the Company’s consolidated financial statements.

Net Loss per Share Attributable to Common Stockholders

6 Months Ended

Jun. 30, 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 per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common shares outstanding for the period. Because the Company has reported a net loss for all periods presented, 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:


	Three Months Ended June 30,						Six Months Ended June 30,
	2016			2015			2016			2015
	(In thousands, except share and per share data)
Numerator—basic and diluted:
Net loss	$	(8,358	)	$	(6,214	)	$	(18,688	)	$	(11,125	)
Accretion of convertible preferred stock dividends		—			(1,929	)		(2,598	)		(3,678	)
Accretion of convertible preferred stock to redemption value		—			(18,195	)		—			(23,607	)

Net loss attributable to common stockholders—basic and diluted	$	(8,358	)	$	(26,338	)	$	(21,286	)	$	(38,410	)

Net loss per share attributable to common stockholders—basic and diluted	$	(0.47	)	$	(440.52	)	$	(1.91	)	$	(649.30	)

Denominator—basic and diluted:
Weighted-average common shares used to compute net loss per share—basic and diluted		17,769,514			59,788			11,155,525			59,156

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):


	June 30,
	2016		2015
Convertible preferred stock		—		8,494,649
Options to purchase common stock		2,715,428		1,787,581
Common stock warrants		357,840		184,257
Restricted stock subject to future vesting		11,612		—

Significant Agreements

6 Months Ended

Jun. 30, 2016

Equity [Abstract]

Significant Agreements

5. Significant Agreements

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. To the extent additional intellectual property is developed during the term of the agreement, KHK will receive a license 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. Unless terminated earlier in accordance with the terms of the agreement, it 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 upfront 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 upfront 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 will be 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. For the three and six months ended June 30, 2016, the Company recognized $0.3 million and $0.6 million, respectively, of revenue associated with the KHK License Agreement. As of June 30, 2016, there was $16.1 million of deferred revenue related to the KHK License Agreement, which is classified as current or long-term in the condensed consolidated balance sheets.

Eastern Cooperative Oncology Group – In March 2014, the Company entered into a clinical trial agreement (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 National Cancer Institute (“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 $0.8 million. As of June 30, 2016, the Company’s aggregate payment obligations under this agreement were approximately $21.4 million; and the Company’s remaining payment obligation is approximately $16.0 million over an estimated period of approximately five years.

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 AG (formerly known as 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 upfront 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
			(In thousands, except share and per share data)
June 30, 2015		184,257	$	1.54	$	7.20		74	%		2.06	%		0.0	%		7.00	$	1,171
March 8, 2016		357,840	$	1.54	$	13.55		69	%		1.82	%		0.0	%		10.00	$	4,551

Fair Value Measurements

6 Months Ended

Jun. 30, 2016

Fair Value Disclosures [Abstract]

Fair Value Measurements

6. 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 periods 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 for any of periods presented.

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


	Fair Value Measurements Using
			Quoted		Significant
			Prices		Other		Significant
	Total		in Active		Observable		Unobservable
	Carrying		Markets		Inputs		Inputs
	Value		(Level 1)		(Level 2)		(Level 3)
	(In thousands)
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

June 30, 2016
Assets:
Cash equivalents	$	13,641	$	12,938	$	703	$	—
Short-term investments		111,016		—		111,016		—

Total assets	$	124,657	$	12,938	$	111,719	$	—

Cash equivalents of $12.9 million as of June 30, 2016 and $9.2 million as of December 31, 2015 consisted of money market funds 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 $0.7 million as of June 30, 2016 and $13.9 million as of December 31, 2015 consisted of 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.

Short-term investments of $111.0 million as of June 30, 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 June 30, 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 and six months ended June 30, 2016 and 2015; and 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 June 30, 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:


	Amortized		Unrealized		Unrealized
	Cost		Gains		Losses			Fair Value
	(In thousands)
December 31, 2015
Commercial paper	$	28,980	$	48	$	—		$	29,028
Corporate bonds		34,302		—		(20	)		34,282

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

June 30, 2016
Commercial paper	$	67,987	$	191	$	—		$	68,178
Corporate bonds		42,837		13		(12	)		42,838

	$	110,824	$	204	$	(12	)	$	111,016

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


	Common Stock Warrant Liability			Derivative Liability
	(In thousands)
Balance — December 31, 2014	$	693		$	126
Change in fair value		478			8

Balance—June 30, 2015	$	1,171		$	134


	Common Stock Warrant Liability			Derivative Liability
	(In thousands)
Balance — December 31, 2015	$	2,848		$	133
Change in fair value		1,703			17
Reclassification		(4,551	)		(150	)

Balance—June 30, 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 warrants 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 is 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 in June 2018 is included in other long-term liabilities.

Prepaid Expenses and Other Current Assets

6 Months Ended

Jun. 30, 2016

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

Prepaid Expenses and Other Current Assets

7. Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:


	June 30, 2016		December 31, 2015
	(In thousands)
Short-term deposits	$	437	$	805
Prepaid insurance		435		34
Interest receivable on short-term investments		270		258
Prepaid clinical supplies		538		210
Other		188		157

Total prepaid expenses and other current assets	$	1,868	$	1,464

Accrued Expenses and Other Current Liabilities

6 Months Ended

Jun. 30, 2016

Payables and Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

8. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:


	June 30, 2016		December 31, 2015
	(In thousands)
Accrued clinical costs	$	2,407	$	574
Accrued professional fees		383		561
Accrued compensation and related costs		736		698
Other		219		342

Total accrued expenses	$	3,745	$	2,175

Convertible Preferred Stock

6 Months Ended

Jun. 30, 2016

Equity [Abstract]

Convertible Preferred Stock

9. Convertible Preferred Stock

Convertible preferred stock consisted of the following as of December 31, 2015:


	Preferred Shares Designated		Issuance Date	Preferred Shares Issued and Outstanding		Liquidation Preference		Carrying Value
			(In thousands, except share and per share data)
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

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 June 30, 2016, the Company does not have any convertible preferred stock issued or outstanding.

Stock-Based Compensation

6 Months Ended

Jun. 30, 2016

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

Stock-Based Compensation

10. 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”). The 2015 Plan 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. 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. As of June 30, 2016, there were 1,752,111 shares available for issuance under the 2015 Plan.

The Company recognized stock-based compensation expense related to the issuance of stock option awards to employees and non-employees in the condensed consolidated statements of comprehensive loss as follows:


	Three Months Ended June 30,				Six Months Ended June 30,
	2016		2015		2016		2015
	(In thousands)
Research and development	$	256	$	229	$	479	$	293
General and administrative		523		871		2,490		1,131

Total	$	779	$	1,100	$	2,969	$	1,424

During the six months ended June 30, 2016, the Company granted options to purchase 186,180 shares of its common stock to certain executives and employees. The grant date fair value of these options was $1.4 million, or $7.41 per share on a weighted-average basis, and will be recognized as compensation expense over the requisite service period of three to four years.

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.

As of June 30, 2016, there was $7.2 million of unrecognized compensation cost related to employee and non-employee unvested stock options and unvested restricted stock share-based compensation arrangements granted under the 2007 Plan, which is expected to be recognized over a weighted-average remaining service period of 2.7 years. Stock compensation costs have not been capitalized by the Company.

- 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	6 Months Ended
Income Taxes	Jun. 30, 2016
Income Tax Disclosure [Abstract]
Income Taxes	11. 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.

Related-Party Transactions	6 Months Ended
Related-Party Transactions	Jun. 30, 2016
Related Party Transactions [Abstract]
Related-Party Transactions	12. 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 Company’s Series C-1 convertible preferred stock financing (“Series C-1 financing”), the Company issued 1,130,740 shares of Series C-1 convertible preferred stock for total gross proceeds, including the conversion of $5.0 million in convertible unsecured promissory notes (“2014 Notes”) and a 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. In September and October 2014, the Company issued $5.0 million of the 2014 Notes to stockholders of the Company. The 2014 Notes were converted into 465,563 shares of Series C-1 convertible preferred stock in June 2015, in connection with the Series C-1 financing. Interest expense related to the 2014 Notes held by these stockholders was $0.1 million for three and six months ended June 30, 2015, respectively. There have been no related-party transactions recorded during the three and six months ended June 30, 2016.

Stockholders' Equity (Deficit)

6 Months Ended

Jun. 30, 2016

Equity [Abstract]

Stockholders' Equity (Deficit)

13. Stockholders’ Equity (Deficit)

The following table presents the changes in convertible preferred stock and stockholders’ equity (deficit) for the six months ended June 30, 2016:


	Convertible Preferred Stock $0.001 Par Value						Series A Convertible Preferred Stock $0.001 Par Value						Common Stock $0.0001 Par Value				Additional Paid-In Capital			Accumulated Other Comprehensive Income		Accumulated Deficit			Total Stockholders’ Equity (Deficit)
	Shares			Amount			Shares			Amount			Shares		Amount		Additional Paid-In Capital			Accumulated Other Comprehensive Income		Accumulated Deficit			Total Stockholders’ Equity (Deficit)
										(In thousands, except share and per share data)
Balance as of December 31, 2015		12,732,466		$	319,113			700,435		$	7,231			85,440	$	1	$	—		$	28	$	(259,675	)	$	(252,415)
Accretion for convertible preferred stock dividends		—			2,598			—			—			—		—		(1,452	)		—		(1,146	)		(2,598	)
Proceeds from IPO, net of offering costs of $7,186		—			—			—			—			4,809,475		—		50,527			—		—			50,527
Conversion of preferred stock into common stock		(12,732,466	)		(321,711	)		(700,435	)		(7,231	)		12,872,551		1		328,941			—		—			321,711
Reclassification of common stock warrant liability		—			—			—			—			—		—		4,551			—		—			4,551
Vesting of restricted stock		—			—			—			—			3,072		—		21			—		—			21
Stock-based compensation expense		—			—			—			—			—		—		2,969			—		—			2,969
Unrealized gain on short-term investments		—			—			—			—			—		—		—			164		—			164
Repurchase of fractional shares resulting from reverse stock splits		—			—			—			—			—				(1	)		—		—			(1	)
Net loss		—			—			—			—			—		—		—			—		(18,688	)		(18,688	)

Balance as of June 30, 2016		—		$	—			—		$	—			17,770,538	$	2	$	385,556		$	192	$	(279,509	)	$	106,241

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 4.E)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
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-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Name Accounting Standards Codification
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-Name Accounting Standards Codification
-Topic 235
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-Paragraph 1
-Subparagraph (SX 210.4-08(e))
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Reference 6: http://www.xbrl.org/2003/role/presentationRef
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-SubTopic 10
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-Subparagraph (SX 210.5-02(31))
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Reference 7: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 505
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Reference 8: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 210
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(30))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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Reference 10: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(d))
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Reference 12: http://www.xbrl.org/2003/role/presentationRef
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Reference 13: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Preferred Stock
-URI http://asc.fasb.org/extlink&oid=6521494

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-Name Accounting Standards Codification
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+ Details

Supplemental Cash Flow Information

6 Months Ended

Jun. 30, 2016

Supplemental Cash Flow Elements [Abstract]

Supplemental Cash Flow Information

14. Supplemental Cash Flow Information

Supplemental disclosures of cash flow information are as follows:


	Six Months Ended June 30,
	2016		2015
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Interest paid	$	2	$	407
SUPPLEMENTAL DISCLOSURES OF NONCASH INVESTING AND FINANCING ACTIVITIES:
Property and equipment purchases included in accounts payable and accrued expenses	$	51	$	—
Accretion of convertible preferred stock to redemption value	$	—	$	23,607
Accretion of dividends on convertible preferred stock	$	2,598	$	3,678
Conversion of convertible notes and accrued interest into Series C-1 convertible preferred stock	$	—	$	5,211
Issuance costs included in accounts payable and accrued expenses	$	170	$	139
Vesting of restricted stock	$	21	$	—
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	$	—

Basis of Presentation (Policies)	6 Months Ended
Basis of Presentation (Policies)	Jun. 30, 2016
Accounting Policies [Abstract]
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 condensed consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
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. Stockholders entitled to fractional shares as a result of the reverse stock split 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 this reverse stock split.
Significant Accounting Policies	Significant Accounting Policies—There have been no significant or material changes to the Company’s significant accounting policies during the three and six months ended June 30, 2016, from the Company’s policies disclosed in the audited consolidated financial statements for the year ended December 31, 2015 and the notes thereto, which are included in the Prospectus.
Use of Estimates	Use of Estimates – The preparation of condensed 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 condensed 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.
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 IPO and issuance of debt. As of December 31, 2015, the Company had capitalized deferred IPO issuance costs of $1.6 million. Upon the closing of the IPO, these costs were reclassified to additional paid-in capital as a reduction of the IPO proceeds.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”). ASU 2014-09 supersedes the revenue recognition requirements of the FASB’s Accounting Standards Codification (“ASC”) Topic 605, Revenue Recognition, and most industry-specific guidance throughout the ASC, 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. In March 2016, the FASB issued ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net), (“ASU 2016-08”), which amends ASU 2014-09 to clarify the implementation guidance on principal versus agent considerations (gross versus net revenue presentation). ASU 2016-08 does not change the core principle of the guidance stated in ASU 2014-09, instead, the amendments in this ASU are intended to improve the operability and understandability of the implementation guidance on principal versus agent considerations and whether an entity reports revenue on a gross or net basis. Adoption of both ASU 2016-08 and 2014-09 will be permitted using either a retrospective or modified retrospective approach. In July 2015, the 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. The collective guidance will be effective for the Company on January 1, 2018, with early adoption permitted but not before January 1, 2017. The Company is currently evaluating the method by which it will implement these standards and the impact of the adoption of these standards on the Company’s 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 Company is in the process of evaluating the effect of the new guidance on the Company’s consolidated financial statements and disclosures. 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 with early adoption permitted. The Company is currently evaluating the impact that the adoption of ASU 2016-09 will have on the Company’s consolidated financial statements.
Basis of Presentation	The Company has prepared the accompanying condensed consolidated financial statements in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. Certain amounts have been reclassified to conform to the current period presentation. The interim unaudited condensed financial statements have been prepared on the same basis as the annual audited financial statements; and in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of June 30, 2016, and the results of operations, comprehensive loss, and cash flows for the three and six months ended June 30, 2016 and 2015. The results for the three and six months ended June 30, 2016 are not necessarily indicative of the results to be expected for the year ending December 31, 2016, any other interim periods, or any future year or period. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2015, and the notes thereto, which are included in the Company’s Prospectus that forms a part of the Company’s Registration Statement on Form S-1 (File No. 333-208861), which was filed with the Securities and Exchange Commission (the “SEC”) pursuant to Rule 424 on March 3, 2016 (the “Prospectus”).

Net Loss per Share Attributable to Common Stockholders (Tables)

6 Months Ended

Jun. 30, 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:


	Three Months Ended June 30,						Six Months Ended June 30,
	2016			2015			2016			2015
	(In thousands, except share and per share data)
Numerator—basic and diluted:
Net loss	$	(8,358	)	$	(6,214	)	$	(18,688	)	$	(11,125	)
Accretion of convertible preferred stock dividends		—			(1,929	)		(2,598	)		(3,678	)
Accretion of convertible preferred stock to redemption value		—			(18,195	)		—			(23,607	)

Net loss attributable to common stockholders—basic and diluted	$	(8,358	)	$	(26,338	)	$	(21,286	)	$	(38,410	)

Net loss per share attributable to common stockholders—basic and diluted	$	(0.47	)	$	(440.52	)	$	(1.91	)	$	(649.30	)

Denominator—basic and diluted:
Weighted-average common shares used to compute net loss per share—basic and diluted		17,769,514			59,788			11,155,525			59,156

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


	June 30,
	2016		2015
Convertible preferred stock		—		8,494,649
Options to purchase common stock		2,715,428		1,787,581
Common stock warrants		357,840		184,257
Restricted stock subject to future vesting		11,612		—

Significant Agreements (Tables)

6 Months Ended

Jun. 30, 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
			(In thousands, except share and per share data)
June 30, 2015		184,257	$	1.54	$	7.20		74	%		2.06	%		0.0	%		7.00	$	1,171
March 8, 2016		357,840	$	1.54	$	13.55		69	%		1.82	%		0.0	%		10.00	$	4,551

- Definition

Tabular disclosure of warrants or rights issued. Warrants and rights outstanding are derivative securities that give the holder the right to purchase securities (usually equity) from the issuer at a specific price within a certain time frame. Warrants are often included in a new debt issue to entice investors by a higher return potential. The main difference between warrants and call options is that warrants are issued and guaranteed by the company, whereas options are exchange instruments and are not issued by the company. Also, the lifetime of a warrant is often measured in years, while the lifetime of a typical option is measured in months. Disclose the title of issue of securities called for by warrants and rights outstanding, the aggregate amount of securities called for by warrants and rights outstanding, the date from which the warrants or rights are exercisable, and the price at which the warrant or right is exercisable.

+ References + Details

Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 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:


	Fair Value Measurements Using
			Quoted		Significant
			Prices		Other		Significant
	Total		in Active		Observable		Unobservable
	Carrying		Markets		Inputs		Inputs
	Value		(Level 1)		(Level 2)		(Level 3)
	(In thousands)
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

June 30, 2016
Assets:
Cash equivalents	$	13,641	$	12,938	$	703	$	—
Short-term investments		111,016		—		111,016		—

Total assets	$	124,657	$	12,938	$	111,719	$	—

Summary of Available-for-Sale Securities

The following table summarizes the available-for-sale securities:


	Amortized		Unrealized		Unrealized
	Cost		Gains		Losses			Fair Value
	(In thousands)
December 31, 2015
Commercial paper	$	28,980	$	48	$	—		$	29,028
Corporate bonds		34,302		—		(20	)		34,282

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

June 30, 2016
Commercial paper	$	67,987	$	191	$	—		$	68,178
Corporate bonds		42,837		13		(12	)		42,838

	$	110,824	$	204	$	(12	)	$	111,016

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 is as follows:


	Common Stock Warrant Liability			Derivative Liability
	(In thousands)
Balance — December 31, 2014	$	693		$	126
Change in fair value		478			8

Balance—June 30, 2015	$	1,171		$	134


	Common Stock Warrant Liability			Derivative Liability
	(In thousands)
Balance — December 31, 2015	$	2,848		$	133
Change in fair value		1,703			17
Reclassification		(4,551	)		(150	)

Balance—June 30, 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)

6 Months Ended

Jun. 30, 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:


	June 30, 2016		December 31, 2015
	(In thousands)
Short-term deposits	$	437	$	805
Prepaid insurance		435		34
Interest receivable on short-term investments		270		258
Prepaid clinical supplies		538		210
Other		188		157

Total prepaid expenses and other current assets	$	1,868	$	1,464

Accrued Expenses and Other Current Liabilities (Tables)

6 Months Ended

Jun. 30, 2016

Payables and Accruals [Abstract]

Schedule of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:


	June 30, 2016		December 31, 2015
	(In thousands)
Accrued clinical costs	$	2,407	$	574
Accrued professional fees		383		561
Accrued compensation and related costs		736		698
Other		219		342

Total accrued expenses	$	3,745	$	2,175

Convertible Preferred Stock (Tables)

6 Months Ended

Jun. 30, 2016

Equity [Abstract]

Schedule of Convertible Preferred Stock

Convertible preferred stock consisted of the following as of December 31, 2015:


	Preferred Shares Designated		Issuance Date	Preferred Shares Issued and Outstanding		Liquidation Preference		Carrying Value
			(In thousands, except share and per share data)
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

Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 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


	Three Months Ended June 30,				Six Months Ended June 30,
	2016		2015		2016		2015
	(In thousands)
Research and development	$	256	$	229	$	479	$	293
General and administrative		523		871		2,490		1,131

Total	$	779	$	1,100	$	2,969	$	1,424

Stockholders' Equity (Deficit) (Tables)

6 Months Ended

Jun. 30, 2016

Equity [Abstract]

Summary of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit)

The following table presents the changes in convertible preferred stock and stockholders’ equity (deficit) for the six months ended June 30, 2016:


	Convertible Preferred Stock $0.001 Par Value						Series A Convertible Preferred Stock $0.001 Par Value						Common Stock $0.0001 Par Value				Additional Paid-In Capital			Accumulated Other Comprehensive Income		Accumulated Deficit			Total Stockholders’ Equity (Deficit)
	Shares			Amount			Shares			Amount			Shares		Amount		Additional Paid-In Capital			Accumulated Other Comprehensive Income		Accumulated Deficit			Total Stockholders’ Equity (Deficit)
										(In thousands, except share and per share data)
Balance as of December 31, 2015		12,732,466		$	319,113			700,435		$	7,231			85,440	$	1	$	—		$	28	$	(259,675	)	$	(252,415)
Accretion for convertible preferred stock dividends		—			2,598			—			—			—		—		(1,452	)		—		(1,146	)		(2,598	)
Proceeds from IPO, net of offering costs of $7,186		—			—			—			—			4,809,475		—		50,527			—		—			50,527
Conversion of preferred stock into common stock		(12,732,466	)		(321,711	)		(700,435	)		(7,231	)		12,872,551		1		328,941			—		—			321,711
Reclassification of common stock warrant liability		—			—			—			—			—		—		4,551			—		—			4,551
Vesting of restricted stock		—			—			—			—			3,072		—		21			—		—			21
Stock-based compensation expense		—			—			—			—			—		—		2,969			—		—			2,969
Unrealized gain on short-term investments		—			—			—			—			—		—		—			164		—			164
Repurchase of fractional shares resulting from reverse stock splits		—			—			—			—			—				(1	)		—		—			(1	)
Net loss		—			—			—			—			—		—		—			—		(18,688	)		(18,688	)

Balance as of June 30, 2016		—		$	—			—		$	—			17,770,538	$	2	$	385,556		$	192	$	(279,509	)	$	106,241

Supplemental Cash Flow Information (Tables)

6 Months Ended

Jun. 30, 2016

Supplemental Cash Flow Elements [Abstract]

Supplemental Disclosures of Cash Flow Information

Supplemental disclosures of cash flow information are as follows:


	Six Months Ended June 30,
	2016		2015
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Interest paid	$	2	$	407
SUPPLEMENTAL DISCLOSURES OF NONCASH INVESTING AND FINANCING ACTIVITIES:
Property and equipment purchases included in accounts payable and accrued expenses	$	51	$	—
Accretion of convertible preferred stock to redemption value	$	—	$	23,607
Accretion of dividends on convertible preferred stock	$	2,598	$	3,678
Conversion of convertible notes and accrued interest into Series C-1 convertible preferred stock	$	—	$	5,211
Issuance costs included in accounts payable and accrued expenses	$	170	$	139
Vesting of restricted stock	$	21	$	—
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	$	—

Nature of Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	6 Months Ended
	Mar. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Nature of Business [Line Items]
Entity incorporation date		Oct. 11, 2005
Proceeds from public stock offering, net of issuance costs		$ 52,318
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	10,000,000
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
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Stock split, conversion ratio	0.8

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	Dec. 31, 2015 USD ($)
Initial Public Offering [Member]
Summary Of Significant Accounting Policies [Line Items]
Capitalized deferred issuance costs	$ 1.6

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		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerator-basic and diluted:
Net loss	$ (8,358)	$ (6,214)	$ (18,688)	$ (11,125)
Accretion of convertible preferred stock dividends		(1,929)	(2,598)	(3,678)
Accretion of convertible preferred stock to redemption value		(18,195)		(23,607)
Net loss attributable to common stockholders-basic and diluted	$ (8,358)	$ (26,338)	$ (21,286)	$ (38,410)
Net loss per share attributable to common stockholders-basic and diluted	$ (0.47)	$ (440.52)	$ (1.91)	$ (649.30)
Denominator-basic and diluted:
Weighted-average common shares used to compute net loss per share-basic and diluted	17,769,514	59,788	11,155,525	59,156

Net Loss per Share Attributable to Common Stockholders - Potential Dilutive Securities Excluded from Computation of Diluted Net Loss per Common Share (Detail) - shares	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
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		8,494,649
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	2,715,428	1,787,581
Common Stock Warrants [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	184,257
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	11,612

Significant Agreements - Additional Information (Detail) - USD ($)		1 Months Ended					3 Months Ended		6 Months Ended		12 Months Ended
	Dec. 19, 2014	Feb. 28, 2015	Jan. 31, 2015	Jun. 30, 2014	Mar. 31, 2014	Mar. 31, 2007	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2007	Dec. 31, 2015
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment allocated to preferred stock value
Proceeds received from preferred stock issuance										$ 26,378,000
Research and development expense							6,131,000	$ 2,271,000	10,917,000	$ 3,994,000
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
Upfront payment allocated to license fee	$ 17,300,000
Upfront license fee received		$ 17,500,000
Revenue recognized during period							300,000		600,000
Deferred revenue from license agreement							$ 16,100,000		$ 16,100,000
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
Upfront 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]
Upfront license fee paid						$ 2,000,000
Research and development expense				$ 2,000,000							$ 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							16,000,000		$ 16,000,000
Period of contractual obligation					5 years
Increase in contractual obligation							$ 800,000

Significant Agreements - Schedule of Assumptions Used to Value Shares of Common Stock Issuable Pursuant to Warrant (Detail) - Common Stock Warrants [Member] - USD ($) $ / shares in Units, $ in Thousands	Mar. 08, 2016	Jun. 30, 2015
Class of Warrant or Right [Line Items]
Revaluation Date of Warrant	Mar. 08, 2016	Jun. 30, 2015
Total Shares of Common Stock Issuable Under the Warrant	357,840	184,257
Average Exercise Price	$ 1.54	$ 1.54
Fair Value of Common Stock	$ 13.55	$ 7.20
Estimated Volatility	69.00%	74.00%
Risk-Free Interest Rate	1.82%	2.06%
Estimated Dividend Yield	0.00%	0.00%
Estimated Remaining Contractual Life (in years)	10 years	7 years
Fair Value of Warrant Liability	$ 4,551	$ 1,171

Fair Value Measurements - Schedule of Fair Values of Financial Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	$ 111,016	$ 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	13,641	23,154
Short-term investments	111,016	63,310
Total assets	124,657	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	12,938	9,208
Total assets	12,938	9,208
Significant Other Observable Inputs Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	703	13,946
Short-term investments	111,016	63,310
Total assets	$ 111,719	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 ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	Oct. 31, 2015
Fair Value Measurements [Line Items]
Available for sale debt securities fair value	$ 111,016,000		$ 111,016,000		$ 63,310,000
Realized gains or losses recognized on the sale or maturity of available-for-sale securities	0	$ 0	0	$ 0
Corporate Bonds [Member]
Fair Value Measurements [Line Items]
Available for sale debt securities fair value	42,838,000		42,838,000		34,282,000
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		200,000
Quoted Prices in Active Markets Level 1 [Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	12,938,000		12,938,000		9,208,000
Quoted Prices in Active Markets Level 1 [Member] \| Money Market Funds [Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	12,900,000		12,900,000		9,200,000
Significant Other Observable Inputs Level 2 [Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	703,000		703,000		13,946,000
Significant Other Observable Inputs Level 2 [Member] \| Corporate Bonds [Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	700,000		700,000		13,900,000
Significant Other Observable Inputs Level 2 [Member] \| Corporate Bonds And Commercial Paper [Member]
Fair Value Measurements [Line Items]
Available for sale debt securities fair value	$ 111,000,000		$ 111,000,000		$ 63,300,000

Fair Value Measurements - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 110,824	$ 63,282
Unrealized Gains	204	48
Unrealized Losses	(12)	(20)
Fair Value	111,016	63,310
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	67,987	28,980
Unrealized Gains	191	48
Fair Value	68,178	29,028
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	42,837	34,302
Unrealized Gains	13
Unrealized Losses	(12)	(20)
Fair Value	$ 42,838	$ 34,282

Fair Value Measurements - Summary of Changes in Fair Value of Common Stock Warrant Liability and Derivative Liability Inputs (Detail) - Significant Unobservable Inputs Level 3 [Member] - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Derivative Liability [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 133	$ 126
Change in fair value	17	8
Reclassification	(150)
Ending Balance		134
Common Stock Warrant Liability [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	2,848	693
Change in fair value	1,703	478
Reclassification	$ (4,551)
Ending Balance		$ 1,171

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Short-term deposits	$ 437	$ 805
Prepaid insurance	435	34
Interest receivable on short-term investments	270	258
Prepaid clinical supplies	538	210
Other	188	157
Total prepaid expenses and other current assets	$ 1,868	$ 1,464

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Payables and Accruals [Abstract]
Accrued clinical costs	$ 2,407	$ 574
Accrued professional fees	383	561
Accrued compensation and related costs	736	698
Other	219	342
Total accrued expenses	$ 3,745	$ 2,175

Convertible Preferred Stock - Schedule of Convertible Preferred Stock (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Temporary Equity [Line Items]
Carrying Value		$ 319,113
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
Liquidation preference		$ 48,032
Carrying Value		$ 65,666
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
Liquidation preference		$ 2,933
Carrying Value		$ 5,084
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
Preferred Shares Outstanding		3,568,020
Liquidation preference		$ 96,348
Carrying Value		$ 96,348
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
Preferred Shares Outstanding		6,090,461
Liquidation preference		$ 152,015
Carrying Value		$ 152,015
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

Convertible Preferred Stock - Additional Information (Detail) - shares	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Convertible preferred stock issued	0
Convertible preferred stock outstanding	0		12,732,466
Initial Public Offering [Member]
Temporary Equity [Line Items]
Conversion of convertible preferred stock to shares of common stock	12,872,551	12,872,551

Stock-Based Compensation - Additional Information (Detail) $ / shares in Units, $ in Millions	1 Months Ended	6 Months Ended
	Mar. 31, 2016 USD ($) Executives	Jun. 30, 2016 USD ($) $ / shares shares
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 \| $	$ 0.7
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		1,752,111
Stock option granted to certain executives and employees \| shares		186,180
Options grant date fair value amount \| $		$ 1.4
Weighted average grant date fair value of options \| $ / shares		$ 7.41
2015 Plan [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period to recognize compensation expense		3 years
2015 Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period to recognize compensation expense		4 years
2007 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period to recognize compensation expense		2 years 8 months 12 days
Unrecognized compensation expense related to stock option \| $		$ 7.2

Stock-Based Compensation - Share Based Compensation Expense Related to Issuance of Stock Option Awards to Employees and Non Employees (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense, total	$ 779	$ 1,100	$ 2,969	$ 1,424
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense, total	256	229	479	293
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense, total	$ 523	$ 871	$ 2,490	$ 1,131

Income Taxes - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Income Tax Disclosure [Abstract]
Net tax provision	$ 0	$ 0

Related-Party Transactions - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended	6 Months Ended
	Aug. 31, 2015	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2015	Oct. 31, 2014	Sep. 30, 2014
Related Party Transaction [Line Items]
Gross proceeds from issuance of convertible preferred stock				$ 26,378,000
Convertible Unsecured Promissory Notes, 2014 Notes [Member]
Related Party Transaction [Line Items]
Debt, principal amount					$ 5,000,000	$ 5,000,000
Interest expense			$ 100,000	100,000
Series C-1 [Member]
Related Party Transaction [Line Items]
Convertible preferred stock, new shares issued	474,628	1,130,740
Gross proceeds from issuance of convertible preferred stock	$ 5,300,000	$ 12,700,000
Series C-1 [Member] \| Convertible Unsecured Promissory Notes, 2014 Notes [Member]
Related Party Transaction [Line Items]
Debt, principal amount		$ 5,000,000	$ 5,000,000	$ 5,000,000
Outstanding principal and related interest converted into shares		465,563

Stockholders' Equity (Deficit) - Summary of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Class of Stock [Line Items]
Beginning balance			$ (252,415)
Beginning balance			319,113
Accretion for convertible preferred stock dividends		$ (1,929)	(2,598)	$ (3,678)
Proceeds from IPO, net of offering costs of $7,186, Value			50,527
Conversion of preferred stock into common stock, Value			321,711
Reclassification of common stock warrant liability			4,551
Vesting of restricted stock, Value			21
Stock-based compensation expense			2,969
Unrealized gain on short-term investments			164
Repurchase of fractional shares resulting from reverse stock splits			(1)
Net loss	$ (8,358)	$ (6,214)	(18,688)	$ (11,125)
Ending balance	106,241		106,241
Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Beginning balance			$ 319,113
Beginning balance, Shares			12,732,466
Accretion for convertible preferred stock dividends			$ 2,598
Conversion of preferred stock into common stock, Value			$ (321,711)
Conversion of preferred stock into common stock, Shares			(12,732,466)
Series A Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Beginning balance			$ 7,231
Beginning balance, Shares			700,435
Conversion of preferred stock into common stock, Value			$ (7,231)
Conversion of preferred stock into common stock, Shares			(700,435)
Common Stock [Member]
Class of Stock [Line Items]
Beginning balance			$ 1
Beginning balance, Shares			85,440
Proceeds from IPO, net of offering costs of $7,186, Shares			4,809,475
Conversion of preferred stock into common stock, Value			$ 1
Conversion of preferred stock into common stock, Shares			12,872,551
Vesting of restricted stock, Shares			3,072
Ending balance	$ 2		$ 2
Ending balance, Shares	17,770,538		17,770,538
Additional Paid-In Capital [Member]
Class of Stock [Line Items]
Accretion for convertible preferred stock dividends			$ (1,452)
Proceeds from IPO, net of offering costs of $7,186, Value			50,527
Conversion of preferred stock into common stock, Value			328,941
Reclassification of common stock warrant liability			4,551
Vesting of restricted stock, Value			21
Stock-based compensation expense			2,969
Repurchase of fractional shares resulting from reverse stock splits			(1)
Ending balance	$ 385,556		385,556
Accumulated Other Comprehensive Income [Member]
Class of Stock [Line Items]
Beginning balance			28
Unrealized gain on short-term investments			164
Ending balance	192		192
Accumulated Deficit [Member]
Class of Stock [Line Items]
Beginning balance			(259,675)
Accretion for convertible preferred stock dividends			(1,146)
Net loss			(18,688)
Ending balance	$ (279,509)		$ (279,509)

Stockholders' Equity (Deficit) - Summary of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($)
Equity [Abstract]
Offering costs	$ 7,186

Supplemental Cash Flow Information - Supplemental disclosures of cash flow (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
SUPPLEMENTAL DISCLOSURES OF NONCASH INVESTING AND FINANCING ACTIVITIES:
Property and equipment purchases included in accounts payable and accrued expenses		$ 51
Accretion of convertible preferred stock to redemption value	$ 18,195		$ 23,607
Accretion of dividends on convertible preferred stock	$ 1,929	2,598	3,678
Conversion of convertible notes and accrued interest into Series C-1 convertible preferred stock			5,211
Issuance costs included in accounts payable and accrued expenses		170	139
Vesting of restricted stock		21
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	$ 407