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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2018	Aug. 06, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2018
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	Accelerated Filer
Entity Common Stock, Shares Outstanding		23,355,013

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 32,204	$ 35,168
Restricted cash	101	106
Short-term investments	66,181	94,806
Prepaid expenses and other current assets	6,210	3,362
Total current assets	104,696	133,442
Long-term investments		3,246
Property and equipment, net	415	267
Other assets	259	231
Total assets	105,370	137,186
Current liabilities:
Accounts payable	5,297	2,232
Accrued expenses and other current liabilities	12,484	11,993
Current portion of deferred revenue	1,517	1,573
Total current liabilities	19,298	15,798
Long-term liabilities:
Deferred revenue, less current portion	15,409	16,759
Other long-term liabilities	311	310
Total long-term liabilities	15,720	17,069
Total liabilities	35,018	32,867
Commitments
Stockholders’ equity:
Preferred stock, $0.001 par value, 10,000,000 shares authorized; 0 shares outstanding at June 30, 2018 and December 31, 2017
Common stock, $0.0001 par value, 100,000,000 shares authorized; 22,705,794 and 24,390,033 shares issued and outstanding at June 30, 2018 and December 31, 2017, respectively	2	2
Additional paid-in capital	473,686	470,571
Accumulated other comprehensive loss	(89)	(143)
Accumulated deficit	(403,247)	(366,111)
Total stockholders’ equity	70,352	104,319
Total liabilities and stockholders’ equity	$ 105,370	$ 137,186

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
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	100,000,000
Common stock, shares issued	22,705,794	24,390,033
Common stock, shares outstanding	22,705,794	24,390,033

Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue:
Total revenues	$ 379	$ 305	$ 758	$ 610
Type of revenue [extensible list]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Total revenues	$ 379	$ 305	$ 758	$ 610
Operating expenses:
Research and development	14,851	9,862	30,190	19,414
General and administrative	4,479	4,285	9,270	8,215
Total operating expenses	19,330	14,147	39,460	27,629
Loss from operations	(18,951)	(13,842)	(38,702)	(27,019)
Other income (expense):
Interest income, net	459	290	934	549
Other income (expense)	104	(87)	(17)	(140)
Total other income (expense)	563	203	917	409
Net loss	(18,388)	(13,639)	(37,785)	(26,610)
Other comprehensive (loss) income:
Unrealized (losses) gains on marketable securities	129	38	54	(20)
Comprehensive loss	(18,259)	(13,601)	(37,731)	(26,630)
Net loss attributable to common stockholders	$ (18,388)	$ (13,639)	$ (37,785)	$ (26,610)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.74)	$ (0.70)	$ (1.54)	$ (1.41)
Weighted-average number of common shares used to compute net loss per share attributable to common stockholders—basic and diluted	24,705,441	19,497,581	24,592,483	18,868,089

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (37,785)	$ (26,610)
Adjustments to reconcile net loss to net cash from operating activities:
Depreciation, amortization and accretion	(137)	79
Stock-based compensation	2,992	2,825
Other	(2)	8
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(2,847)	(1,082)
Accounts payable	3,067	800
Deferred revenue	(758)	(610)
Accrued expenses and other liabilities	451	122
Net cash used in operating activities	(35,019)	(24,468)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(173)
Purchases of short-term investments	(34,149)	(87,989)
Proceeds from sales and maturities of short-term investments	66,250	62,447
Net cash provided by (used in) investing activities	31,928	(25,542)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock in follow-on public stock offering, net		48,836
Proceeds from stock option exercises	26	269
Proceeds from Employee Stock Purchase Plan	97
Other	(1)	(1)
Net cash provided by financing activities	122	49,104
NET DECREASE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(2,969)	(906)
CASH, CASH EQUIVALENTS AND RESTRICTED CASH—beginning of period	35,389	24,110
CASH, CASH EQUIVALENTS AND RESTRICTED CASH —end of period	32,420	23,204
SUPPLEMENTAL DISCLOSURES OF NONCASH INVESTING AND FINANCING ACTIVITIES:
Property and equipment purchases included in accounts payable and accrued expenses		41
Issuance costs included in accounts payable and accrued expenses	$ 32	161
Vesting of restricted stock		$ 59

Nature of Business	6 Months Ended
Nature of Business	Jun. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Business	1. Nature of Business Syndax Pharmaceuticals, Inc. (“we”, “us”, “our”, or the “Company”) is a clinical stage biopharmaceutical company developing an innovative pipeline of cancer therapies. We were incorporated in Delaware in 2005. We base our operations in Waltham, Massachusetts and we operate in one segment.

Basis of Presentation	6 Months Ended
Basis of Presentation	Jun. 30, 2018
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. 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, 2018, and the results of operations and comprehensive loss for the three and six months ended June 30, 2018 and 2017, and cash flows for the six months ended June 30, 2018 and 2017. The results for the three and six months ended June 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018, 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, 2017, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K that was filed with the Securities and Exchange Commission (“SEC”) on March 8, 2018. 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.

Summary of Significant Accounting Policies

6 Months Ended

Jun. 30, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

3. Summary of Significant Accounting Policies

Significant Accounting Policies

The Company’s significant accounting policies, which are disclosed in the audited consolidated financial statements for the year ended December 31, 2017 and the notes thereto are included in the Company’s Annual Report on Form 10-K that was filed with the SEC on March 8, 2018. Certain amounts reported in the previous year have been recast as a result of the retrospective adoption of new accounting standards in the first quarter of 2018. Refer to Recently Issued and Adopted Accounting Pronouncements and Note 4 "Revenue from Contracts with Customers" for further discussion.

Revenue Recognition

The Company adopted Accounting Standards Codification Rule 606, or ASC 606, Revenue from Contracts with Customers, on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The reported results for 2018 reflect the application of ASC 606 guidance while the reported results for 2017 were prepared under the guidance of ASC 605, Revenue Recognition (ASC 605). For the Company’s accounting policy for revenue recognition under ASC 605, refer to Item 8 of the Annual Report on Form 10-K for the year ended December 31, 2017. As of January 1, 2018, the Company had only one contract within the scope of ASC 606, a license agreement with Kyowa Hakko Kirin Co., Ltd. (“KHK”) (the “KHK License Agreement”), under which the Company granted KHK an exclusive license to develop and commercialize entinostat in Japan and Korea. The KHK License Agreement is discussed further in Footnote 6.

The Company enters into license agreements for the development and commercialization of its product candidates. License agreements may include non-refundable upfront payments, contingent payments based on the occurrence of specified events under the Company’s license arrangements, partial or complete reimbursement of research and development expenses, and license fees and royalties on sales if they are successfully approved and commercialized. The Company’s performance obligations under the license agreements may include the transfer of intellectual property rights in the form of licenses, obligations to provide research and development services and related materials and participation on certain development and/or commercialization committees.

Revenue is recognized when, or as, performance obligations are satisfied, which occurs when control of the promised products or services is transferred to customers. Revenue is measured as the amount of consideration the Company expects to receive in exchange for transferring products or services to a customer (“transaction price”). To the extent that the transaction price includes variable consideration, the Company estimates the amount of variable consideration that should be included in the transaction price utilizing the most likely amount method. Variable consideration is included in the transaction price if, in the Company’s judgment, it is probable that a significant future reversal of cumulative revenue under the contract will not occur. Estimates of variable consideration and determination of whether to include estimated amounts in the transaction price are based largely on an assessment of the Company’s anticipated performance and all information (historical, current and forecasted) that is reasonably available.

The Company assesses the promises to determine if they are distinct performance obligations. Once the performance obligations are determined, the transaction price is allocated based on a relative standalone selling price basis. Milestone payments and royalties are typically considered variable consideration at the outset of the contract and are recognized in the transaction price either upon occurrence or when the constraint of a probable reversal is no longer applicable.

Licenses of intellectual property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. For licenses that are bundled with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, up-front fees. Arrangements containing licenses to the Company’s intellectual property typically provide for a know-how transfer period. These arrangements may or may not also include rights to future updates of that intellectual property and related know-how. Revenues from non-refundable, up-front fees allocated to the licenses are recognized as the license is transferred to the customer and the customer is able to use and benefit from the license. This generally takes place over the related know-how transfer period, or if applicable, over the term of transfer of future updates to the intellectual property.

Development Milestone Payments: At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. The transaction price is then allocated to each performance obligation on a relative stand-alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect license fees and earnings in the period of adjustment. For development milestones related to the KHK Agreement, the Company does not take a substantive role or control the research, development or commercialization of any products generated by KHK. Therefore, the Company is not able to reasonably estimate when, if at all, any development milestone payments may be payable to the Company. As such, the development milestone payments associated with the KHK Agreement involve a substantial degree of uncertainty and risk that they may never be received.

Commercial Milestone Payments and Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of commercial sales, and the license is deemed to be the predominant item to which the royalties or commercial milestones relate, the Company will recognize revenue at the later of when the related sales occur or when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date no commercial milestone payments or royalties have been achieved.

When no performance obligations are required of the Company, or following the completion of the performance obligation period, such amounts are recognized as revenue upon transfer of control of the goods or services to the customer. Generally, all amounts received or due other than sales-based milestones and royalties are classified as license fees. Sales-based milestones and royalties will be recognized as royalty revenue at the later of when the related sales occur or when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied).

Deferred revenue arises from amounts received in advance of the culmination of the earnings process and is recognized as revenue in future periods as performance obligations are satisfied. Deferred revenue expected to be recognized within the next twelve months is classified as a current liability. Upfront payment contract liabilities resulting from the Company’s license agreements do not represent a financing component as the payment is not financing the transfer of goods or services, and the technology underlying the licenses granted reflects research and development expenses already incurred by the Company.

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.

Recently Issued and Adopted Accounting Pronouncements

In June 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2018-07, Compensation-Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”). ASU 2018-07 is intended to simplify several aspects of the accounting for nonemployee share-based payment transactions by expanding the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. This ASU is effective for public reporting companies for interim and annual periods beginning after December 15, 2018, with early adoption permitted but no earlier than an entity’s adoption date of Topic 606. The Company is in the process of evaluating the effect of the new guidance on the Company’s consolidated financial statements and related disclosures.

In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”). ASU 2017-09 provides guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting. An entity should account for the effects of a modification unless all of the following are met: (1) the fair value (or calculated value or intrinsic value, if such an alternative measurement method is used) of the modified award is the same as the fair value (or calculated value or intrinsic value, if such an alternative measurement method is used) of the original award immediately before the original award is modified. If the modification does not affect any of the inputs to the valuation technique that the entity uses to value the award, the entity is not required to estimate the value immediately before and after the modification; (2) the vesting conditions of the modified award are the same as the vesting conditions of the original award immediately before the original award is modified; (3) the classification of the modified award as an equity instrument or a liability instrument is the same as the classification of the original award immediately before the original award is modified. The Company adopted ASU 2017-09 on January 1, 2018, and it did not have a material impact on its condensed consolidated balance sheet, condensed statement of comprehensive loss or condensed statement of cash flows. As part of the adoption of this guidance, the Company adopted a policy to account for the effects of a modification unless certain exclusions are met.

In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (“ASU 2016-18”). ASU 2016-18 requires that restricted cash and restricted cash equivalents be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The statement of cash flows must also explain the change during the period in the total of cash, cash equivalents, and restricted cash or restricted cash equivalents. The Company adopted ASU 2016-18 on January 1, 2018, utilizing the retrospective transition method and it did not have a material impact on its condensed statement of cash flows. As part of the adoption of this guidance, the Company included restricted cash with cash and cash equivalents in the condensed statement of cash flows for both first quarter of 2018 and 2017. The following table provides a reconciliation of the cash, cash equivalents, and restricted cash balances as of June 30, 2018 and December 31, 2017, as shown above:

	June 30, 2018		December 31, 2017
	(In thousands)
Cash and cash equivalents	$	32,204	$	35,168
Restricted cash included in current and noncurrent assets		216		221
Cash, cash equivalents and restricted cash	$	32,420	$	35,389

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) (“ASU 2016-02”). Under ASU 2016-02, lessees will be required to recognize, for all leases of 12 months or more, a liability to make lease payments and a right-of-use asset representing the right to use the underlying asset for the lease term. Additionally, the guidance requires improved disclosures to help users of financial statements better understand the nature of an entity’s leasing activities. This ASU is effective for public reporting companies for interim and annual periods beginning after December 15, 2018, with early adoption permitted, and must be adopted using a modified retrospective approach. The standard will be effective for the Company on January 1, 2019. The Company is currently assessing the impact of the standard, including optional practical expedients that the Company may elect upon adoption and is progressing with an implementation plan. The implementation plan includes identifying the Company’s lease population, updating the Company’s lease database and identifying changes to processes and controls. The Company is currently assessing the impact that this standard will have on its consolidated financial statements.

In May 2014, the Financial Accounting Standards Board (FASB) issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”). The Company adopted ASU 2014-09 and its related amendments (collectively known as ASC 606) effective on January 1, 2018 using the modified retrospective method. See Note 4 "Revenue from Contracts with Customers" for the required disclosures related to the impact of adopting this standard and a discussion of the Company's updated policies related to revenue recognition.

Revenue from Contracts with Customers

6 Months Ended

Jun. 30, 2018

Revenue From Contract With Customer [Abstract]

Revenue from Contracts with Customers

4. Revenue from Contracts with Customers

Financial Statement Impact of Adopting ASC 606

On January 1, 2018, the Company adopted ASC 606 applying the modified retrospective method, which only impacted the accounting for the KHK License Agreement. As a result of applying the modified retrospective method to adopt the new revenue guidance, the following adjustments were made to accounts on the consolidated balance sheet as of January 1, 2018:

	As Reported at December 31, 2017			Adjustments Due to ASC 606			Balance at January 1, 2018
LIABILITIES AND STOCKHOLDERS’ EQUITY
Current liabilities:
Current portion of deferred revenue		1,573			(56	)		1,517
Total current liabilities		15,798			(56	)		15,742
Long-term liabilities:
Deferred revenue, less current portion		16,759			(593	)		16,166
Total long-term liabilities		17,069			(593	)		16,476
Total liabilities		32,867			(649	)		32,218
Stockholders’ equity:
Accumulated deficit		(366,111	)		649			(365,462	)
Total stockholders’ equity		104,319			649			104,968
Total liabilities and stockholders’ equity	$	137,186			—		$	137,186

Impact of New Revenue Guidance on Financial Statement Line Items

Results for reporting periods beginning after January 1, 2018 were presented under Topic 606, while prior period amounts were not adjusted and reported under the accounting standards in effect for the prior periods. The following tables show the impact on the reported condensed consolidated balance sheet for the six months ended June 30, 2018, and the statement of income for the three and six months ended June 30, 2018, for pro-forma amounts had the previous guidance been in effect (in thousands):

Financial Statement Line Item *	Increase (Decrease)
Condensed Consolidated Statement of Income	Three months ended June 30, 2018			Six months ended June 30, 2018
License fee		(14	)		(28	)
Net loss		(14	)		(28	)
Comprehensive loss		(14	)		(28	)

Condensed Consolidated Balance Sheet **				June 30, 2018
Current portion of deferred revenue					(56	)
Deferred revenue, less current portion					(565	)
Accumulated deficit					621

*	Excludes line items that were not affected by the Company’s adoption of ASC 606. The adoption had no impact to cash provided by or used in net operating, investing or financing activities in the Condensed Consolidated Statement of Cash Flows.

**	Balance sheet line item amounts include the cumulative-effect adjustment recorded on December 31, 2017.

Impact to KHK License Agreement Revenue

Under ASC 606, the Company determined that the performance obligations associated with the KHK License Agreement include (i) the combined license, rights to access and use materials and data, and rights to additional intellectual property, and (ii) the clinical supply obligation. All other goods or services promised to KHK are immaterial in the context of the agreement. Under ASC 606, the identification of the clinical supply obligation as a distinct performance obligation separate and apart from the license performance obligation resulted in a change in the performance period. The start of the performance period under ASC 606 was determined to be the contract inception date, December 19, 2014, as opposed to the initial delivery of the clinical trial materials in June 2015. The clinical supply was identified as a separate performance obligation under ASC 606 as (i) the Company is not providing a significant service of integration whereby the clinical supply and other promises are inputs into a combined output, (ii) the clinical supply does not significantly modify or customize the other promises nor is it significantly modified or customized by them, and (iii) the clinical supply is not highly interdependent or highly interrelated with the other promises in the agreement as KHK could choose not to purchase the clinical supply from the Company without significantly affecting the other promised goods or services. The Company further concluded that the clinical supply represented an immaterial performance obligation and therefore the entire $17.3 million allocated to the upfront payment was allocated to the combined license and will be recognized ratably over the performance period, representing contract inception though 2029. In 2017, KHK achieved a development milestone, and was required to pay the Company $5.0 million. The Company is recognizing the development milestone consideration over the performance period coinciding with the license to intellectual property. As the Company determined that its performance obligations associated with the KHK Agreement at contract inception were not distinct and represented a single performance obligation, and that the obligations for goods and services provided would be completed over the performance period of the agreement, any payments received by the Company from KHK, including the upfront payment and progress-dependent development and regulatory milestone payments, are recognized as revenue using a time-based proportional performance model over the contract term (December 2014 through 2029) of the collaboration, within license fees. To date no commercial milestone payments or royalties have been achieved.

Contract liabilities consisted of deferred revenue, as presented on the consolidated balance sheet, as of June 30, 2018. Deferred revenue related to the KHK License Agreement was $16.9 million as of June 30, 2018 and will be recognized over the remainder of the contract term. The Company recognized license fees revenue of $0.4 million during the three months ended June 30, 2018 that were included in the deferred revenue balance as of January 1, 2018.

Net Loss per Share Attributable to Common Stockholders

6 Months Ended

Jun. 30, 2018

Earnings Per Share [Abstract]

Net Loss per Share Attributable to Common Stockholders

5. 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,
	2018			2017			2018			2017
	(In thousands, except share and per share data)						(In thousands, except share and per share data)
Numerator—basic and diluted:
Net loss	$	(18,388	)	$	(13,639	)	$	(37,785	)	$	(26,610	)
Net loss attributable to common stockholders—basic and diluted	$	(18,388	)	$	(13,639	)	$	(37,785	)	$	(26,610	)
Net loss per share attributable to common stockholders—basic and diluted	$	(0.74	)	$	(0.70	)	$	(1.54	)	$	(1.41	)
Denominator—basic and diluted:
Weighted-average common shares used to compute net loss per share—basic and diluted		24,705,441			19,497,581			24,592,483			18,868,089

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,
	2018		2017
Options to purchase common stock		4,232,590		3,281,644
Common stock warrant		—		357,840
Employee Stock Purchase Plan		15,804		—

Significant Agreements	6 Months Ended
Significant Agreements	Jun. 30, 2018
Equity [Abstract]
Significant Agreements	6. Significant Agreements Vitae Pharmaceuticals, Inc. In October 2017, the Company entered into a license agreement (the “Allergan License Agreement”) with Vitae Pharmaceuticals, Inc., a subsidiary of Allergan plc (“Allergan”), under which Allergan granted the Company an exclusive, sublicensable, worldwide license to a portfolio of preclinical, orally available, small molecule inhibitors of the interaction of Menin with the Mixed Lineage Leukemia (“MLL”) protein (the “Menin Assets”). The Company made a nonrefundable upfront payment of $5.0 million to Allergan in the fourth quarter of 2017. Additionally, subject to the achievement of certain milestone events, the Company may be required to pay Allergan up to $99 million in one-time development and regulatory milestone payments over the term of the Allergan License Agreement. In the event that the Company or any of its affiliates or sublicensees commercializes the Menin Assets, the Company will also be obligated to pay Allergan low single to low double-digit royalties on sales, subject to reduction in certain circumstances, as well as up to an aggregate of $70.0 million in potential one-time, sales-based milestone payments based on achievement of certain annual sales thresholds. Under certain circumstances, the Company may be required to share a percentage of non-royalty income from sublicensees, subject to certain deductions, with Allergan. The Company is solely responsible for the development and commercialization of the Menin Assets. Each party may terminate the Allergan License Agreement for the other party’s uncured material breach or insolvency; and the Company may terminate the Allergan License Agreement at will at any time upon advance written notice to Allergan. Allergan may terminate the Allergan License Agreement if the Company or any of its affiliates or sublicensees institutes a legal challenge to the validity, enforceability, or patentability of the licensed patent rights. Unless terminated earlier in accordance with its terms, the Allergan License Agreement will continue on a country-by-country and product-by-product basis until the later of: (i) the expiration of all of the licensed patent rights in such country; (ii) the expiration of all regulatory exclusivity applicable to the product in such country; and (iii) 10 years from the date of the first commercial sale of the product in such country. UCB Biopharma Sprl In 2016, the Company entered into a license agreement (the “UCB License Agreement”) with UCB Biopharma Sprl (“UCB”), under which UCB granted to the Company a worldwide, sublicenseable, exclusive license to UCB6352, which the Company refers to as SNDX-6352, an investigational new drug (“IND”) ready anti-CSF-1R monoclonal antibody. The Company made a nonrefundable upfront payment of $5.0 million to UCB in 2016. Additionally, subject to the achievement of certain milestone events, the Company may be required to pay UCB up to $119.5 million in one-time development and regulatory milestone payments over the term of the UCB License Agreement. In the event that the Company or any of its affiliates or sublicensees commercializes SNDX-6352, the Company will also be obligated to pay UCB low double-digit royalties on sales, subject to reduction in certain circumstances, as well as up to an aggregate of $250.0 million in potential one-time, sales-based milestone payments based on achievement of certain annual sales thresholds. Under certain circumstances, the Company may be required to share a percentage of non-royalty income from sublicensees, subject to certain deductions, with UCB. The Company is solely responsible for the development and commercialization of SNDX-6352, except that UCB is performing a limited set of transitional chemistry, manufacturing and control tasks related to SNDX-6352. Each party may terminate the UCB License Agreement for the other party’s uncured material breach or insolvency; and the Company may terminate the UCB License Agreement at will at any time upon advance written notice to UCB. UCB may terminate the UCB License Agreement if the Company or any of its affiliates or sublicensees institutes a legal challenge to the validity, enforceability, or patentability of the licensed patent rights. Unless terminated earlier in accordance with its terms, the UCB License Agreement will continue on a country-by-country and product-by-product basis until the later of: (i) the expiration of all of the licensed patent rights in such country; (ii) the expiration of all regulatory exclusivity applicable to the product in such country; and (iii) 10 years from the date of the first commercial sale of the product in such country. As of the date of the UCB License Agreement, the asset acquired had no alternative future use nor had it reached a stage of technological feasibility. As the processes or activities that were acquired along with the license do not constitute a “business,” the transaction has been accounted for as an asset acquisition. As a result of these findings, in 2016, the upfront payment of $5.0 million has been recorded as research and development expense in the condensed consolidated statement of comprehensive loss. Kyowa Hakko Kirin Co., Ltd. On December 19, 2014 (the “Effective Date”), the Company entered into a license agreement (the “KHK License Agreement”) with Kyowa Hakko Kirin Co., Ltd. (“KHK”), under which the Company granted KHK an exclusive license to develop and commercialize entinostat in Japan and Korea. Under the terms of the KHK License Agreement, the Company will be responsible for the manufacture and supply of the products during the development activities. In addition to the license and manufacturing obligations, the Company is obligated to provide KHK access to know-how and regulatory information the Company may develop over the life of the entinostat patent. Lastly, to the extent additional intellectual property is developed during the term of the agreement, KHK will receive the right to the intellectual property when and if available. KHK will conduct the development, regulatory approval filings, and commercialization activities of entinostat in Japan and Korea. KHK paid the Company $25.0 million upfront, which included a $7.5 million equity investment and a $17.5 million non-refundable cash payment. In addition, to the extent certain development and commercial milestones are achieved, KHK will be required to pay the Company up to $75.0 million in milestone payments over the term of the license agreement. The term of the agreement commenced on the Effective Date and, unless earlier terminated in accordance with the terms of the agreement, will continue on a country-by-country and product-by-product basis, until the later of: (i) the date all valid claims of the last effective patent among the Company’s patents expires or is abandoned, withheld, or is otherwise invalidated in such country; and (ii) 15 years from the date of the first commercial sale of a product in the Japan or Korea. The equity purchase and the up-front payment of the license fee were accounted for separately. The Company allocated the amount of consideration equal to the fair value of the shares on the Effective Date, which resulted in $7.7 million of proceeds allocated to the equity purchase and the remaining consideration of $17.3 million allocated to the up-front license fee. In October 2017, the Company announced that KHK enrolled the first Japanese patient into a local pivotal study of entinostat for the treatment of hormone receptor positive, human epidermal growth factor receptor 2 negative breast cancer. In accordance with the terms of the license agreement, KHK paid the Company a $5.0 million milestone payment which the Company received in December 2017. Please refer to Note 4, Revenue from Contracts, for further discussion related to the accounting for the milestone. In October 2016, the Company entered into a clinical trial co-funding agreement with KHK under which the Company expanded its clinical trial agreement with Eastern Cooperative Oncology Group (the “ECOG Agreement”) to include enrollments from sites in Korea. Eastern Cooperative Oncology Group In March 2014, the Company entered into the ECOG Agreement with Eastern Cooperative Oncology Group, a contracting entity for the Eastern Cooperative Oncology Group—American College of Radiology Imaging Network Cancer Research Group (“ECOG-ACRIN”), that describes the parties’ obligations with respect to the NCI-sponsored pivotal Phase 3 clinical trial of entinostat. Under the terms of the ECOG Agreement, ECOG-ACRIN will perform this clinical trial in accordance with the clinical trial protocol and a mutually agreed scope of work. The Company is providing a fixed level of financial support for the clinical trial through an upfront payment of $0.7 million and a series of payments of up to $1.0 million each that are comprised of milestone payments through the completion of enrollment and time-based payments through the completion of patient monitoring post-enrollment. In addition, the Company is obligated to supply entinostat and placebo to ECOG-ACRIN for use in the clinical trial. During the second quarter of 2016, the ECOG Agreement was amended to provide additional study activities and the contractual obligation increased by $0.8 million. During the first quarter of 2017, the ECOG Agreement was amended to expand the study to include enrollments from sites in Korea and to provide additional study activities and the contractual obligation increased by $2.0 million. As of June 30, 2018, the Company’s aggregate payment obligations under this agreement were approximately $24.3 million; and its remaining payment obligations are approximately $11.9 million over an estimated period of approximately four 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 U.S. Food and Drug Administration (“FDA”) or National Cancer Institute (“NCI”) withdraws the authorization to perform the clinical trial in the United States. The parties may jointly terminate the ECOG Agreement if the parties agree that safety-related issues support termination of the clinical trial. The Company records the appropriate clinical trial expenses in its financial statements by matching those expenses with the period in which the services and efforts are expended. The Company accounts for these expenses according to the progress of the clinical trial as measured by patient enrollment and the timing of various aspects of the clinical trial. The Company determines accrual estimates through financial models, taking into account discussion with applicable personnel and ECOG-ACRIN as to the progress or state of consummation of the clinical trial or the services completed. Bayer Pharma AG (formerly known as Bayer Schering Pharma AG) In March 2007, the Company entered into a license agreement (the “Bayer Agreement”) with Bayer Schering Pharma AG (“Bayer”) for a worldwide, exclusive license to develop and commercialize entinostat and any other products containing the same active ingredient. Under the terms of the Bayer Agreement, the Company paid a nonrefundable 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 was exercised or 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, requiring 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 was exercisable at an exercise price of $1.54 per share and would have expired 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 using current assumptions just prior to the reclassification. On March 1, 2018, Bayer notified the Company of its election to exercise the warrant utilizing the net exercise feature contained therein, resulting in the Company’s issuance to Bayer of 299,215 shares of the Company’s common stock for no net cash proceeds.

Fair Value Measurements

6 Months Ended

Jun. 30, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

7. Fair Value Measurements

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

Level 1—

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

Level 2—

Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs for which all significant inputs are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

Level 3—

Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

During the 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)
June 30, 2018
Assets:
Cash and cash equivalents	$	32,204	$	27,458	$	4,746	$	—
Short-term investments		66,181		—		66,181		—
Total assets	$	98,385	$	27,458	$	70,927	$	—
December 31, 2017
Assets:
Cash and cash equivalents	$	35,168	$	24,972	$	10,196	$	—
Short-term investments		94,806	—			94,806		—
Long-term investments		3,246	—			3,246		—
Total assets	$	133,220	$	24,972	$	108,248	$	—

Cash equivalents of $27.5 million and $25.0 million as of June 30, 2018 and December 31, 2017, respectively, consisted of overnight investments and 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 $4.7 million and $10.2 million as of June 30, 2018 and December 31, 2017, respectively, consisted of highly rated corporate bonds and commercial paper and are classified within Level 2 of the fair value hierarchy because pricing inputs are other than quoted prices in active markets, which are either directly or indirectly observable as of the reporting date, and fair value is determined through the use of models or other valuation methodologies. Short-term investments of $66.2 million and $94.8 million as of June 30, 2018 and December 31, 2017, respectively, and long-term investments of $3.2 million as of December 31, 2017 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, 2018, 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 six months ended June 30, 2018 and 2017. 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, 2018, 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)
June 30, 2018
Commercial paper	$	28,817	$	5	$	(4	)	$	28,818
Corporate bonds		42,199		—		(90	)	$	42,109
	$	71,016	$	5	$	(94	)	$	70,927
December 31, 2017
Commercial paper	$	36,567	$	—	$	(40	)	$	36,527
Corporate bonds		71,824		—		(103	)		71,721
	$	108,391	$	—	$	(143	)	$	108,248

Prepaid Expenses and Other Current Assets

6 Months Ended

Jun. 30, 2018

Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]

Prepaid Expenses and Other Current Assets

8. Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:

	June 30, 2018		December 31, 2017
	(In thousands)
Short-term deposits	$	1,405	$	1,286
Prepaid clinical supplies		2,194		220
Interest receivable on investments		252		377
Reimbursable costs		1,635		1,029
Prepaid insurance		439		192
Other		285		258
Total prepaid expenses and other current assets	$	6,210	$	3,362

Accrued Expenses and Other Current Liabilities

6 Months Ended

Jun. 30, 2018

Payables And Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

9. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:

	June 30, 2018		December 31, 2017
	(In thousands)
Accrued professional fees	$	776	$	265
Accrued compensation and related costs		1,794		2,393
Accrued clinical costs		9,749		9,177
Other		165		158
Total prepaid expenses and other current assets	$	12,484	$	11,993

Stock-Based Compensation

6 Months Ended

Jun. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

10. Stock-Based Compensation

In January 2018, the number of shares of common stock available for issuance under the 2015 Omnibus Incentive Plan (“2015 Plan”), was increased by 975,601 shares due to the automatic annual provision to increase shares available under the 2015 Plan. As of June 30, 2018, the total number of shares of common stock available for issuance under the 2015 Plan was 1,443,394. The Company recognized stock-based compensation expense related to the issuance of stock option awards to employees and non-employees and related to the 2015 Employee Stock Purchase Plan (“ESPP”) in the condensed consolidated statements of comprehensive loss as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
	(In thousands)				(In thousands)
Research and development	$	516	$	347	$	968	$	630
General and administrative		1,054		1,101		2,024		2,195
Total	$	1,570	$	1,448	$	2,992	$	2,825

Compensation expense by type of award in the three and six months ended June 30, 2018 and 2017 was as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
	(In thousands)				(In thousands)
Stock options	$	1,540	$	1,448	$	2,934	$	2,825
Employee Stock Purchase Plan		30		—		58		—
Total	$	1,570	$	1,448	$	2,992	$	2,825

During the three and six months ended June 30, 2018, the Company granted 158,400 and 973,400 stock options to certain executives and employees. The grant date fair value of these options was $6.3 million, or $6.43 per share on a weighted-average basis, and will be recognized as compensation expense over the requisite service period of three to four years.

There were 7,850 options exercised during the three and six months ended June 30, 2018, resulting in total proceeds of $26,000. The intrinsic value of the options exercised was $91,000. In accordance with the Company’s policy, the shares were issued from a pool of shares reserved for issuance under the 2007 and 2015 Plans.

As of June 30, 2018, there was $11.9 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 2015 and 2007 Plans, which is expected to be recognized over a weighted-average remaining service period of 2.6 years. Stock compensation costs have not been capitalized by the Company.

Employee Stock Purchase Plan	6 Months Ended
Employee Stock Purchase Plan	Jun. 30, 2018
Postemployment Benefits [Abstract]
Employee Stock Purchase Plan	11. Employee Stock Purchase Plan In January 2018, the number of shares of common stock available for issuance under the ESPP, was increased by 243,900 shares as a result of the automatic increase provision of the ESPP. As of June 30, 2018, the total number of shares of common stock available for issuance under the ESPP was 667,441. The first offering period commenced on August 1, 2017 and the first purchase took place on January 31, 2018. The Company issued 8,696 shares during the first six months of 2018. The ESPP is considered a compensatory plan with the related compensation cost expensed over the six-month offering period. The compensation expense related to the ESPP for the three and six months ended June 30, 2018 was approximately $30,000 and $58,000, respectively. There was no compensation expense related to the ESPP recorded in the three and six months ended June 30, 2017.

Stockholders' Equity

6 Months Ended

Jun. 30, 2018

Equity [Abstract]

Stockholders' Equity

12. Stockholders’ Equity

The following table presents the changes in stockholders’ equity for the six months ended June 30, 2018:

	Common Stock $0.0001 Par Value					Additional Paid-In Capital			Accumulated Other Comprehensive Income / (Loss)			Accumulated Deficit			Total Stockholders’ Equity
	Shares			Amount
Balance as of December 31, 2017		24,390,033		$	2	$	470,571		$	(143	)	$	(366,111	)	$	104,319
Stock purchase under ESPP		8,696			—		—			—			—			—
Stock-based compensation expense		—			—		2,992			—			—			2,992
Stock issuance due to warrant exercise, cashless		299,215			—		—			—			—			—
Proceeds from exercise stock options		7,850			—		26			—			—			26
Unrealized gains on short-term investments		—			—		—			54			—			54
Employee withholdings ESPP		—			—		97			—			—			97
Cumulative effect adjustment of adoption ASU 2014-09		—			—		—			—			649			649
Retirement of common stock in exchange for common stock warrant¹		(2,000,000	)		—		(16,780	)		—			—			(16,780	)
Issuance of common stock warrant in exchange for retirement of common stock¹		—			—		16,780			—			—			16,780
Net loss		—			—		—			—			(37,785	)		(37,785	)
Balance as of June 30, 2018		22,705,794		$	2	$	473,686		$	(89	)	$	(403,247	)	$	70,352

¹On June 18, 2018, the Company signed an exchange agreement with Biotechnology Value Fund and certain affiliated funds (“BVF”) under which BVF exchanged 2,000,000 shares of common stock for 2,000,000 warrant shares. The Company recorded the issuance of the warrant shares and the retirement of the common stock at fair value within additional paid-in capital. BVF can exercise the warrant shares at an exercise price per share equal to $0.0001 per share and the warrant shares expire 20 years from issuance. Per the terms of the warrant agreement, the outstanding warrants to purchase shares of common stock may not be exercised if the holder's ownership of the Company's common stock would exceed 9.99 percent following such exercise.

In April 2017, the Company entered into a sales agreement with Cowen and Company, LLC (“Cowen”) under which the Company may issue and sell shares of our common stock having aggregate sales proceeds of up to $50.0 million from time to time through Cowen, acting as agent, in a series of one or more at-the-market (“ATM”) equity offerings. Cowen is not required to sell any specific amount, but acts as our sales agent using commercially reasonable efforts consistent with its normal trading and sales practices. In the third quarter of 2018, through August 6, 2018, the Company sold 633,231 shares of common stock under the at-the-market (ATM) program for net proceeds of $4.4 million. As of August 6, 2018, we have $43.6 million of common stock available for sale under the ATM program.

Income Taxes	6 Months Ended
Income Taxes	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	13. 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. In the first quarter of 2018 we completed our evaluation of the accounting for the tax effects of enactment of the Tax Cuts and Jobs Act of 2017 (the “Act”). The only impact of the Act was the remeasurement of the Company’s deferred tax assets and liabilities, which was recorded in fiscal 2017 as a result of the reduction in U.S. corporate tax rates from 35% to 21%. As of December 31, 2017, the Company determined it had no accumulated unrepatriated foreign earnings, and therefore had recorded no liability for the repatriation transition tax. No changes have been made to the estimates recorded in fiscal 2017.

Related-Party Transactions	6 Months Ended
Related-Party Transactions	Jun. 30, 2018
Related Party Transactions [Abstract]
Related-Party Transactions	14. Related-Party Transactions The Company’s chief executive officer and member of the board of directors is also a managing director at MPM Asset Management, LLC, which holds an investment in the Company’s common stock.

Subsequent Events	6 Months Ended
Subsequent Events	Jun. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	15. Subsequent Events The Company considers events and transactions that occur after the balance sheet date but prior to the issuance of the financial statements to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The Company has completed an evaluation of all subsequent events through the date of this filing of this Quarterly Report on Form 10-Q.

Basis of Presentation (Policies)

6 Months Ended

Jun. 30, 2018

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.

Significant Accounting Policies

Revenue Recognition

Use of Estimates

Recently Issued and Adopted Accounting Pronouncements

	June 30, 2018		December 31, 2017
	(In thousands)
Cash and cash equivalents	$	32,204	$	35,168
Restricted cash included in current and noncurrent assets		216		221
Cash, cash equivalents and restricted cash	$	32,420	$	35,389

Summary of Significant Accounting Policies (Tables)

6 Months Ended

Jun. 30, 2018

Accounting Policies [Abstract]

Reconciliation of Cash, Cash Equivalents and Restricted Cash Balances

The following table provides a reconciliation of the cash, cash equivalents, and restricted cash balances as of June 30, 2018 and December 31, 2017, as shown above:

	June 30, 2018		December 31, 2017
	(In thousands)
Cash and cash equivalents	$	32,204	$	35,168
Restricted cash included in current and noncurrent assets		216		221
Cash, cash equivalents and restricted cash	$	32,420	$	35,389

Revenue from Contracts with Customers (Tables)

6 Months Ended

Jun. 30, 2018

Accounting Standards Update 2014-09 [Member]

Schedule of Financial Statement Impact of Adopting ASC 606

	As Reported at December 31, 2017			Adjustments Due to ASC 606			Balance at January 1, 2018
LIABILITIES AND STOCKHOLDERS’ EQUITY
Current liabilities:
Current portion of deferred revenue		1,573			(56	)		1,517
Total current liabilities		15,798			(56	)		15,742
Long-term liabilities:
Deferred revenue, less current portion		16,759			(593	)		16,166
Total long-term liabilities		17,069			(593	)		16,476
Total liabilities		32,867			(649	)		32,218
Stockholders’ equity:
Accumulated deficit		(366,111	)		649			(365,462	)
Total stockholders’ equity		104,319			649			104,968
Total liabilities and stockholders’ equity	$	137,186			—		$	137,186

Financial Statement Line Item *	Increase (Decrease)
Condensed Consolidated Statement of Income	Three months ended June 30, 2018			Six months ended June 30, 2018
License fee		(14	)		(28	)
Net loss		(14	)		(28	)
Comprehensive loss		(14	)		(28	)

Condensed Consolidated Balance Sheet **				June 30, 2018
Current portion of deferred revenue					(56	)
Deferred revenue, less current portion					(565	)
Accumulated deficit					621

*	Excludes line items that were not affected by the Company’s adoption of ASC 606. The adoption had no impact to cash provided by or used in net operating, investing or financing activities in the Condensed Consolidated Statement of Cash Flows.

**	Balance sheet line item amounts include the cumulative-effect adjustment recorded on December 31, 2017.

Net Loss per Share Attributable to Common Stockholders (Tables)

6 Months Ended

Jun. 30, 2018

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,
	2018			2017			2018			2017
	(In thousands, except share and per share data)						(In thousands, except share and per share data)
Numerator—basic and diluted:
Net loss	$	(18,388	)	$	(13,639	)	$	(37,785	)	$	(26,610	)
Net loss attributable to common stockholders—basic and diluted	$	(18,388	)	$	(13,639	)	$	(37,785	)	$	(26,610	)
Net loss per share attributable to common stockholders—basic and diluted	$	(0.74	)	$	(0.70	)	$	(1.54	)	$	(1.41	)
Denominator—basic and diluted:
Weighted-average common shares used to compute net loss per share—basic and diluted		24,705,441			19,497,581			24,592,483			18,868,089

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

	June 30,
	2018		2017
Options to purchase common stock		4,232,590		3,281,644
Common stock warrant		—		357,840
Employee Stock Purchase Plan		15,804		—

Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2018

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)
June 30, 2018
Assets:
Cash and cash equivalents	$	32,204	$	27,458	$	4,746	$	—
Short-term investments		66,181		—		66,181		—
Total assets	$	98,385	$	27,458	$	70,927	$	—
December 31, 2017
Assets:
Cash and cash equivalents	$	35,168	$	24,972	$	10,196	$	—
Short-term investments		94,806	—			94,806		—
Long-term investments		3,246	—			3,246		—
Total assets	$	133,220	$	24,972	$	108,248	$	—

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)
June 30, 2018
Commercial paper	$	28,817	$	5	$	(4	)	$	28,818
Corporate bonds		42,199		—		(90	)	$	42,109
	$	71,016	$	5	$	(94	)	$	70,927
December 31, 2017
Commercial paper	$	36,567	$	—	$	(40	)	$	36,527
Corporate bonds		71,824		—		(103	)		71,721
	$	108,391	$	—	$	(143	)	$	108,248

Prepaid Expenses and Other Current Assets (Tables)

6 Months Ended

Jun. 30, 2018

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, 2018		December 31, 2017
	(In thousands)
Short-term deposits	$	1,405	$	1,286
Prepaid clinical supplies		2,194		220
Interest receivable on investments		252		377
Reimbursable costs		1,635		1,029
Prepaid insurance		439		192
Other		285		258
Total prepaid expenses and other current assets	$	6,210	$	3,362

Accrued Expenses and Other Current Liabilities (Tables)

6 Months Ended

Jun. 30, 2018

Payables And Accruals [Abstract]

Schedule of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:

	June 30, 2018		December 31, 2017
	(In thousands)
Accrued professional fees	$	776	$	265
Accrued compensation and related costs		1,794		2,393
Accrued clinical costs		9,749		9,177
Other		165		158
Total prepaid expenses and other current assets	$	12,484	$	11,993

Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Share Based Compensation Expense Related to Issuance of Stock Option Awards to Employees and Non Employees and Related to the 2015 Employee Stock Purchase Plan (“ESPP”)

The Company recognized stock-based compensation expense related to the issuance of stock option awards to employees and non-employees and related to the 2015 Employee Stock Purchase Plan (“ESPP”) in the condensed consolidated statements of comprehensive loss as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
	(In thousands)				(In thousands)
Research and development	$	516	$	347	$	968	$	630
General and administrative		1,054		1,101		2,024		2,195
Total	$	1,570	$	1,448	$	2,992	$	2,825

Compensation Expense by Type of Award

Compensation expense by type of award in the three and six months ended June 30, 2018 and 2017 was as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
	(In thousands)				(In thousands)
Stock options	$	1,540	$	1,448	$	2,934	$	2,825
Employee Stock Purchase Plan		30		—		58		—
Total	$	1,570	$	1,448	$	2,992	$	2,825

Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2018

Equity [Abstract]

Summary of Changes in Stockholders' Equity

The following table presents the changes in stockholders’ equity for the six months ended June 30, 2018:

	Common Stock $0.0001 Par Value					Additional Paid-In Capital			Accumulated Other Comprehensive Income / (Loss)			Accumulated Deficit			Total Stockholders’ Equity
	Shares			Amount
Balance as of December 31, 2017		24,390,033		$	2	$	470,571		$	(143	)	$	(366,111	)	$	104,319
Stock purchase under ESPP		8,696			—		—			—			—			—
Stock-based compensation expense		—			—		2,992			—			—			2,992
Stock issuance due to warrant exercise, cashless		299,215			—		—			—			—			—
Proceeds from exercise stock options		7,850			—		26			—			—			26
Unrealized gains on short-term investments		—			—		—			54			—			54
Employee withholdings ESPP		—			—		97			—			—			97
Cumulative effect adjustment of adoption ASU 2014-09		—			—		—			—			649			649
Retirement of common stock in exchange for common stock warrant¹		(2,000,000	)		—		(16,780	)		—			—			(16,780	)
Issuance of common stock warrant in exchange for retirement of common stock¹		—			—		16,780			—			—			16,780
Net loss		—			—		—			—			(37,785	)		(37,785	)
Balance as of June 30, 2018		22,705,794		$	2	$	473,686		$	(89	)	$	(403,247	)	$	70,352

¹On June 18, 2018, the Company signed an exchange agreement with Biotechnology Value Fund and certain affiliated funds (“BVF”) under which BVF exchanged 2,000,000 shares of common stock for 2,000,000 warrant shares. The Company recorded the issuance of the warrant shares and the retirement of the common stock at fair value within additional paid-in capital. BVF can exercise the warrant shares at an exercise price per share equal to $0.0001 per share and the warrant shares expire 20 years from issuance. Per the terms of the warrant agreement, the outstanding warrants to purchase shares of common stock may not be exercised if the holder's ownership of the Company's common stock would exceed 9.99 percent following such exercise.

Nature of Business - Additional Information (Detail)	6 Months Ended
Nature of Business - Additional Information (Detail)	Jun. 30, 2018 Segment
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Number of operating segments	1
Date of incorporation	Oct. 11, 2005
State of incorporation	Delaware

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash Balances (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 32,204	$ 35,168
Restricted cash included in current and noncurrent assets	216	221
Cash, cash equivalents and restricted cash	$ 32,420	$ 35,389	$ 23,204	$ 24,110

Revenue from Contracts with Customers - Schedule of Impact on Consolidated Balance Sheet of Adopting ASC 606 (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Current liabilities:
Current portion of deferred revenue	$ 1,517		$ 1,573
Total current liabilities	19,298		15,798
Long-term liabilities:
Deferred revenue, less current portion	15,409		16,759
Total long-term liabilities	15,720		17,069
Total liabilities	35,018		32,867
Stockholders’ equity:
Accumulated deficit	(403,247)		(366,111)
Total stockholders’ equity	70,352		104,319
Total liabilities and stockholders’ equity	$ 105,370		$ 137,186
Accounting Standards Update 2014-09 [Member]
Current liabilities:
Current portion of deferred revenue		$ 1,517
Total current liabilities		15,742
Long-term liabilities:
Deferred revenue, less current portion		16,166
Total long-term liabilities		16,476
Total liabilities		32,218
Stockholders’ equity:
Accumulated deficit		(365,462)
Total stockholders’ equity		104,968
Total liabilities and stockholders’ equity		137,186
Accounting Standards Update 2014-09 [Member] \| Adjustments Due to ASC 606 [Member]
Current liabilities:
Current portion of deferred revenue		(56)
Total current liabilities		(56)
Long-term liabilities:
Deferred revenue, less current portion		(593)
Total long-term liabilities		(593)
Total liabilities		(649)
Stockholders’ equity:
Accumulated deficit		649
Total stockholders’ equity		$ 649

Revenue from Contracts with Customers - Schedule of Impact of New Revenue Guidance on Financial Statement Line Items (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Condensed Consolidated Statement of Income
License fee	$ 379	$ 305	$ 758	$ 610
Type of revenue [extensible list]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Net loss	$ (18,388)	$ (13,639)	$ (37,785)	$ (26,610)
Comprehensive loss	(18,259)	$ (13,601)	(37,731)	$ (26,630)
Condensed Consolidated Balance Sheet
Current portion of deferred revenue	1,517		1,517			$ 1,573
Deferred revenue, less current portion	15,409		15,409			16,759
Accumulated deficit	(403,247)		(403,247)			$ (366,111)
Accounting Standards Update 2014-09 [Member]
Condensed Consolidated Balance Sheet
Current portion of deferred revenue					$ 1,517
Deferred revenue, less current portion					16,166
Accumulated deficit					$ (365,462)
Accounting Standards Update 2014-09 [Member] \| Pro Forma had the Previous Accounting Been in Effect [Member]
Condensed Consolidated Statement of Income
License fee	$ (14)		$ (28)
Type of revenue [extensible list]	us-gaap:LicenseMember		us-gaap:LicenseMember
Net loss	$ (14)		$ (28)
Comprehensive loss	(14)		(28)
Condensed Consolidated Balance Sheet
Current portion of deferred revenue	(56)		(56)
Deferred revenue, less current portion	(565)		(565)
Accumulated deficit	$ 621		$ 621

Revenue from Contract with Customer - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Dec. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Licenses fees revenue		$ 379	$ 305	$ 758	$ 610
Type of revenue [extensible list]		us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Kyowa Hakko Kirin Co., Ltd. [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Milestone payment received	$ 5,000
License Agreement [Member] \| Kyowa Hakko Kirin Co., Ltd. [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Upfront payment allocation to performance obligation		$ 17,300		$ 17,300
Performance obligation period		2029		2029
Milestone payment received						$ 5,000
Deferred revenue		$ 16,900		$ 16,900
Licenses fees revenue				$ 400
Type of revenue [extensible list]				us-gaap:LicenseMember

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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator—basic and diluted:
Net loss	$ (18,388)	$ (13,639)	$ (37,785)	$ (26,610)
Net loss attributable to common stockholders—basic and diluted	$ (18,388)	$ (13,639)	$ (37,785)	$ (26,610)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.74)	$ (0.70)	$ (1.54)	$ (1.41)
Denominator—basic and diluted:
Weighted-average number of common shares used to compute net loss per share attributable to common stockholders—basic and diluted	24,705,441	19,497,581	24,592,483	18,868,089

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

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, 2018	Jun. 30, 2017
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	4,232,590	3,281,644
Common Stock Warrant [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per common share		357,840
Employee Stock Purchase Plan [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	15,804

Significant Agreements - Additional Information (Detail) - USD ($)		1 Months Ended							3 Months Ended					6 Months Ended		12 Months Ended
	Mar. 01, 2018	Dec. 31, 2017	Oct. 31, 2017	Dec. 19, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2007	Mar. 31, 2007	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development expense									$ 14,851,000		$ 9,862,000			$ 30,190,000	$ 19,414,000
Proceeds from public stock offerings															$ 48,836,000
Common Stock [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Issuance of common stock upon utilizing net exercise feature of warrant														299,215
Kyowa Hakko Kirin Co., Ltd. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment received		$ 5,000,000
Allergan License Agreement \| Vitae Pharmaceuticals Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront milestone payable										$ 5,000,000
Potential milestone payments to be made			$ 99,000,000
Aggregate potential milestone payable			$ 70,000,000
License expiration year			10 years
UCB License Agreement [Member] \| UCB Biopharma [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront milestone payable																	$ 5,000,000
Potential milestone payments to be made																	119,500,000
Aggregate potential milestone payable																	$ 250,000,000
License expiration year																	10 years
Research and development expense																	$ 5,000,000
License Agreement [Member] \| Kyowa Hakko Kirin Co., Ltd. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement upfront payment received				$ 25,000,000
Period of termination after first commercial sale of first licensed product				15 years
Up-front payment allocated to equity purchase				$ 7,700,000
Up-front payment allocated to license fee				17,300,000
Milestone payment received																$ 5,000,000
License Agreement [Member] \| Kyowa Hakko Kirin Co., Ltd. [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment receivable upon achievement of development and commercial milestone				$ 75,000,000
License Agreement [Member] \| Bayer Pharma AG [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development expense					$ 2,000,000		$ 2,000,000
Up-front license fee paid								$ 2,000,000
Percentage of shares issuable upon conversion of warrant														1.75%
Warrant exercisable									$ 1.54					$ 1.54
Warrant expiration period														10 years
Proceeds from public stock offerings	$ 0
License Agreement [Member] \| Bayer Pharma AG [Member] \| Common Stock [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Issuance of common stock upon utilizing net exercise feature of warrant	299,215
License Agreement [Member] \| Bayer Pharma AG [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate payment obligation								$ 150,000,000
Clinical Trial [Member] \| Eastern Cooperative Oncology Group [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront milestone payable						$ 700,000
Milestone payment payable						$ 1,000,000
Increase in contractual obligation												$ 2,000,000	$ 800,000
Aggregate payment obligation									$ 24,300,000					$ 24,300,000
Remaining contractual obligation									$ 11,900,000					$ 11,900,000
Period of contractual obligation						4 years

Fair Value Measurements - Schedule of Fair Values of Financial Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	$ 66,181	$ 94,806
Long-term investments		3,246
Carrying Value [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	32,204	35,168
Short-term investments	66,181	94,806
Long-term investments		3,246
Total assets	98,385	133,220
Quoted Prices in Active Markets Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	27,458	24,972
Total assets	27,458	24,972
Significant Other Observable Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	4,746	10,196
Short-term investments	66,181	94,806
Long-term investments		3,246
Total assets	$ 70,927	$ 108,248

Fair Value Measurements - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Fair Value Measurements [Line Items]
Available for sale debt securities fair value	$ 70,927,000		$ 108,248,000
Realized gains or losses recognized on the sale or maturity of available-for-sale securities	0	$ 0
Quoted Prices in Active Markets Level 1 [Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	27,458,000		24,972,000
Quoted Prices in Active Markets Level 1 [Member] \| Overnight Investments, Money Market Funds and Highly Rated Corporate Bonds Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	27,500,000		25,000,000
Significant Other Observable Inputs Level 2 [Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	4,746,000		10,196,000
Significant Other Observable Inputs Level 2 [Member] \| Corporate Bonds and Commercial Paper [Member]
Fair Value Measurements [Line Items]
Cash equivalents fair value	4,700,000		10,200,000
Significant Other Observable Inputs Level 2 [Member] \| Corporate Bonds and Commercial Paper [Member] \| Short-term Investments [Member]
Fair Value Measurements [Line Items]
Available for sale debt securities fair value	$ 66,200,000		94,800,000
Significant Other Observable Inputs Level 2 [Member] \| Corporate Bonds and Commercial Paper [Member] \| Long-term Investments [Member]
Fair Value Measurements [Line Items]
Available for sale debt securities fair value			$ 3,200,000

Fair Value Measurements - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 71,016	$ 108,391
Unrealized Gains	5
Unrealized Losses	(94)	(143)
Fair Value	70,927	108,248
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	28,817	36,567
Unrealized Gains	5
Unrealized Losses	(4)	(40)
Fair Value	28,818	36,527
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	42,199	71,824
Unrealized Losses	(90)	(103)
Fair Value	$ 42,109	$ 71,721

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Short-term deposits	$ 1,405	$ 1,286
Prepaid clinical supplies	2,194	220
Interest receivable on investments	252	377
Reimbursable costs	1,635	1,029
Prepaid insurance	439	192
Other	285	258
Total prepaid expenses and other current assets	$ 6,210	$ 3,362

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued professional fees	$ 776	$ 265
Accrued compensation and related costs	1,794	2,393
Accrued clinical costs	9,749	9,177
Other	165	158
Total prepaid expenses and other current assets	$ 12,484	$ 11,993

Stock-Based Compensation - Additional Information (Detail) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jan. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options exercised, shares	7,850	7,850
Proceeds from stock option exercises		$ 26,000	$ 269,000
Intrinsic value of options exercised		$ 91,000
2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for issuance				975,601
2015 Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for issuance	1,443,394	1,443,394
2007 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted to certain executives and employees	158,400	973,400
Options grant date fair value amount		$ 6,300,000
Weighted average grant date fair value of options		$ 6.43
2015 and 2007 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to stock option	$ 11,900,000	$ 11,900,000
Weighted average period to recognize compensation expense		2 years 7 months 6 days

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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense, total	$ 1,570	$ 1,448	$ 2,992	$ 2,825
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense, total	516	347	968	630
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense, total	$ 1,054	$ 1,101	$ 2,024	$ 2,195

Stock-Based Compensation - Compensation Expense by Type of Award (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense, total	$ 1,570,000	$ 1,448,000	$ 2,992,000	$ 2,825,000
Options to Purchase Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense, total	1,540,000	1,448,000	2,934,000	2,825,000
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense, total	$ 30,000	$ 0	$ 58,000	$ 0

Employee Stock Purchase Plan - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Employee Benefit Plans [Line Items]
Compensation expense		$ 1,570,000	$ 1,448,000	$ 2,992,000	$ 2,825,000
Employee Stock Purchase Plan [Member]
Employee Benefit Plans [Line Items]
Common stock available for issuance	243,900
Number of shares issued under plan				8,696
Compensation expense		$ 30,000	$ 0	$ 58,000	$ 0
Employee Stock Purchase Plan [Member] \| Maximum [Member]
Employee Benefit Plans [Line Items]
Common stock available for issuance		667,441		667,441

Stockholders' Equity - Summary of Changes in Stockholders' Equity (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Class of Stock [Line Items]
Beginning balance			$ 104,319
Stock-based compensation expense			2,992
Proceeds from exercise stock options, value			$ 26
Proceeds from exercise stock options, shares	7,850		7,850
Unrealized gains on short-term investments	$ 129	$ 38	$ 54	$ (20)
Employee withholdings ESPP			97
Cumulative effect adjustment of adoption ASU 2014-09			649
Retirement of common stock in exchange for common stock warrant			(16,780)
Issuance of common stock warrant in exchange for retirement of common stock			16,780
Net loss	(18,388)	$ (13,639)	(37,785)	$ (26,610)
Ending balance	70,352		70,352
Common Stock [Member]
Class of Stock [Line Items]
Beginning balance			$ 2
Beginning balance, Shares			24,390,033
Stock purchase under ESPP, Shares			8,696
Stock issuance due to warrant exercise, cashless, Shares			299,215
Proceeds from exercise stock options, shares			7,850
Retirement of common stock in exchange for common stock warrant, shares			(2,000,000)
Ending balance	$ 2		$ 2
Ending balance, Shares	22,705,794		22,705,794
Additional Paid-In Capital [Member]
Class of Stock [Line Items]
Beginning balance			$ 470,571
Stock-based compensation expense			2,992
Proceeds from exercise stock options, value			26
Employee withholdings ESPP			97
Retirement of common stock in exchange for common stock warrant			(16,780)
Issuance of common stock warrant in exchange for retirement of common stock			16,780
Ending balance	$ 473,686		473,686
Accumulated Other Comprehensive Income / (Loss) [Member]
Class of Stock [Line Items]
Beginning balance			(143)
Unrealized gains on short-term investments			54
Ending balance	(89)		(89)
Accumulated Deficit [Member]
Class of Stock [Line Items]
Beginning balance			(366,111)
Cumulative effect adjustment of adoption ASU 2014-09			649
Net loss			(37,785)
Ending balance	$ (403,247)		$ (403,247)

Stockholders' Equity - Summary of Changes in Stockholders' Equity (Parenthetical) (Detail) - $ / shares	Jun. 18, 2018	Jun. 30, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common stock, par value	$ 0.0001	$ 0.0001	$ 0.0001
Biotechnology Value Fund, L.P. [Member]
Class of Stock [Line Items]
Number of common stock converted to warrants	2,000,000
Number of common stock warrants issued	2,000,000
Warrant exercise price per share	$ 0.0001
Warrant expiration period	20 years
Maximum percentage of common stock can be held upon exercise of warrant	9.99%

Stockholders' Equity - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		6 Months Ended
	Aug. 06, 2018	Apr. 30, 2017	Jun. 30, 2017
Class of Stock [Line Items]
Proceeds from issuance of common stock in follow-on public stock offering, net			$ 48,836
At The Market Equity Offering Sales Agreement [Member] \| Subsequent Event [Member]
Class of Stock [Line Items]
Proceeds from issuance of common stock in follow-on public stock offering, net	$ 4,400
Common stock, new shares issued	633,231
Common stock available for sale	$ 43,600
Cowen and Company LLC [Member] \| At The Market Equity Offering Sales Agreement [Member]
Class of Stock [Line Items]
Proceeds from issuance of common stock in follow-on public stock offering, net		$ 50,000

Income Taxes - Additional Information (Detail) - USD ($)	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Net tax provision	$ 0	$ 0
Federal income tax rate	21.00%		35.00%
Tax Cuts and Jobs Act of 2017, accounting complete, true or false	true
Accumulated unrepatriated foreign earnings			$ 0
Tax Cuts and Jobs Act of 2017, liability for repatriation transition tax			$ 0