Epizyme (EPZM) 10-Q 7 Aug 17 2017 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2017	Aug. 01, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q2
Trading Symbol	EPZM
Entity Registrant Name	EPIZYME, INC.
Entity Central Index Key	1,571,498
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		58,466,534

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Current Assets:
Cash and cash equivalents	$ 88,529	$ 77,895
Marketable securities	104,475	164,297
Accounts receivable	25	23
Prepaid expenses and other current assets	8,631	6,457
Total current assets	201,660	248,672
Property and equipment, net	2,985	3,124
Restricted cash and other assets	665	645
Total Assets	205,310	252,441
Current Liabilities:
Accounts payable	9,594	4,994
Accrued expenses	16,143	16,007
Current portion of capital lease obligation	427	620
Other current liabilities	15
Total current liabilities	26,179	21,621
Capital lease obligation, net of current portion		110
Deferred revenue, net of current portion	28,809	28,809
Other long-term liabilities	282	201
Commitments and contingencies
Stockholders' Equity:
Common stock $0.0001 par value; 125,000 shares authorized; 58,433 shares and 58,050 shares issued and outstanding, respectively	6	6
Additional paid-in capital	564,428	555,473
Accumulated other comprehensive loss	(60)	(106)
Accumulated deficit	(414,334)	(353,673)
Total stockholders' equity	150,040	201,700
Total Liabilities and Stockholders' Equity	$ 205,310	$ 252,441

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	58,433,000	58,050,000
Common stock, shares outstanding	58,433,000	58,050,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Collaboration revenue	$ 10,000	$ 473	$ 10,000	$ 945
Operating expenses:
Research and development	27,292	21,450	51,987	39,190
General and administrative	11,170	7,424	19,439	13,270
Total operating expenses	38,462	28,874	71,426	52,460
Loss from operations	(28,462)	(28,401)	(61,426)	(51,515)
Other income, net:
Interest income, net	428	404	866	624
Other income	10	16	14	31
Other income, net	438	420	880	655
Net loss	(28,024)	(27,981)	(60,546)	(50,860)
Other comprehensive income (loss):
Unrealized gain on available for sale securities	34	25	46	25
Comprehensive loss	$ (27,990)	$ (27,956)	$ (60,500)	$ (50,835)
Loss per share allocable to common stockholders:
Basic	$ (0.48)	$ (0.49)	$ (1.04)	$ (0.90)
Diluted	$ (0.48)	$ (0.49)	$ (1.04)	$ (0.90)
Weighted average shares outstanding:
Basic	58,377	57,352	58,298	56,250
Diluted	58,377	57,352	58,298	56,250

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows $ in Thousands	6 Months Ended
	Jun. 30, 2017USD ($)	Jun. 30, 2016USD ($)
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (60,546)	$ (50,860)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	824	791
Stock-based compensation	5,778	5,126
Amortization of discount on investments	(69)	(27)
Changes in operating assets and liabilities:
Accounts receivable	(2)	202
Prepaid expenses and other current assets	(1,947)	(3,095)
Accounts payable	4,600	(980)
Accrued expenses	138	(393)
Deferred revenue		(944)
Restricted cash and other assets	(20)	116
Other liabilities	96	(86)
Net cash used in operating activities	(51,148)	(50,150)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of available for sale securities	(80,846)	(199,441)
Maturities of available for sale securities	140,552
Purchases of property and equipment	(683)	(319)
Net cash provided by (used in) investing activities	59,023	(199,760)
CASH FLOWS FROM FINANCING ACTIVITIES:
Payment under capital lease obligation	(303)	(274)
Proceeds from public offering, net of commissions	1,587	130,067
Proceeds from stock options exercised	1,128	1,498
Issuance of shares under employee stock purchase plan	347	150
Payment of public offering costs		(374)
Net cash provided by financing activities	2,759	131,067
Net increase (decrease) in cash and cash equivalents	10,634	(118,843)
Cash and cash equivalents, beginning of period	77,895	208,323
Cash and cash equivalents, end of period	88,529	89,480
SUPPLEMENTAL CASH FLOW INFORMATION:
Purchases of property and equipment unpaid at period end	0	$ 0
Unrealized gain on investments	46
Cumulative catch up related to the adoption of ASU 2016-09 (Note 2)	$ 115

Overview

Overview	6 Months Ended
	Jun. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Overview	1. Overview Epizyme, Inc. (collectively referred to with its wholly owned, controlled subsidiary, Epizyme Securities Corporation, as “Epizyme” or the “Company”) is a clinical stage biopharmaceutical company that discovers, develops and plans to commercialize novel epigenetic therapies for patients with cancer and other diseases. The Company’s lead product candidate, tazemetostat, is a potent and selective inhibitor of EZH2, an enzyme that plays an important role in various cancers. The Company owns the global development and commercialization rights to tazemetostat outside of Japan. Eisai Co. Ltd (“Eisai”) holds the rights to develop and commercialize tazemetostat in Japan, and holds a limited right of first negotiation for the rest of Asia. The Company has additional programs in development, including pinometostat, a clinical program that is subject to a collaboration with Celgene Corporation and Celgene RIVOT Ltd., an affiliate of Celgene Corporation (“Celgene”) (refer to Note 8, Collaborations Collaborations Through June 30, 2017, the Company has raised an aggregate of $740.3 million to fund its operations, of which $217.8 million was non-equity The Company commenced active operations in early 2008. Since its inception, the Company has generated an accumulated deficit of $414.3 million through June 30, 2017, and will require substantial additional capital to fund its research and development. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, risks of failure of clinical trials and preclinical studies, the need to obtain additional financing to fund the future development of tazemetostat and the rest of its pipeline, the need to obtain marketing approval for its product candidates, the need to successfully commercialize and gain market acceptance of its product candidates, dependence on key personnel, protection of proprietary technology, compliance with government regulations, development by competitors of technological innovations and ability to transition from clinical-stage manufacturing to commercial-stage production of products.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The condensed consolidated financial statements of the Company included herein have been prepared, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these condensed consolidated financial statements should be read in conjunction with the financial statements and notes thereto included in the Company’s Annual Report on Form 10-K The unaudited condensed consolidated financial statements include the accounts of Epizyme, Inc. and its wholly owned, controlled subsidiary, Epizyme Securities Corporation. All intercompany transactions and balances of subsidiaries have been eliminated in consolidation. In the opinion of management, the information furnished reflects all adjustments, all of which are of a normal and recurring nature, necessary for a fair presentation of the results for the reported interim periods. The Company considers events or transactions that occur after the balance sheet date but before the consolidated financial statements are issued to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The three months ended June 30, 2017 and 2016 are referred to as the second quarter of 2017 and 2016, respectively. The results of operations for interim periods are not necessarily indicative of results to be expected for the full year or any other interim period. Significant Accounting Policies There have been no material changes to the Company’s significant accounting policies during the six months ended June 30, 2017, as compared to the significant accounting policies disclosed in Note 2, Summary of Significant Accounting Policies Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue From Contracts With Customers 2014-09 Revenue Recognition 2016-10 2016-12, ASU 2014-09. 2014-09 2014-09, In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) right-of-use In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments zero-coupon In November 2016, the FASB issued ASU 2016-18, Restricted Cash 2016-18, 2018-18 2016-18 In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting Going Concern At each reporting period, the Company evaluates whether there are conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued. The Company is required to make certain additional disclosures if it concludes substantial doubt exists and it is not alleviated by the Company’s plans or when its plans alleviate substantial doubt about the Company’s ability to continue as a going concern. The Company’s evaluation entails analyzing prospective operating budgets and forecasts for expectations of the Company’s cash needs, and comparing those needs to the current cash, cash equivalent and marketable security balances. After considering the Company’s current research and development plans and the timing expectations related to the progress of its programs, and after considering its existing cash, cash equivalents and marketable securities as of June 30, 2017, the Company did not identify conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the date these financial statements were issued. Share-Based Payment As of January 1, 2017, the Company adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting paid-in-capital

Marketable Securities

Marketable Securities	6 Months Ended
	Jun. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	3. Marketable Securities The following table summarizes the available for sale securities held at June 30, 2017 (in thousands): Description Amortized Cost Unrealized Unrealized Fair Value Commercial paper $ 43,570 $ — $ (17 ) $ 43,553 Corporate notes 57,973 — (43 ) 57,930 U.S. government agency securities and U.S. Treasuries 2,992 — 2,992 Total $ 104,535 $ — $ (60 ) $ 104,475 The following table summarizes the available for sale securities held at December 31, 2016 (in thousands): Description Amortized Cost Unrealized Unrealized Fair Value Commercial paper $ 68,407 $ — $ (32 ) $ 68,375 Corporate notes 70,489 — (81 ) 70,408 U.S. government agency securities and U.S. Treasuries 25,507 7 — 25,514 Total $ 164,403 $ 7 $ (113 ) $ 164,297 The amortized cost of available for sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At June 30, 2017, the balance in the Company’s accumulated other comprehensive loss was composed solely of activity related to the Company’s available for sale marketable securities. There were no realized gains or losses recognized on the sale or maturity of available-for-sale The aggregate fair value of available for sale securities held by the Company in an unrealized loss position for less than twelve months as of June 30, 2017 was $88.4 million. The aggregate unrealized loss for those securities in an unrealized loss position for less than twelve months as of June 30, 2017 was $0.1 million. The Company determined that there was no material change in the credit risk of any of its investments. As a result, the Company determined it did not hold any investments with any other-than-temporary impairment as of June 30, 2017. The weighted average maturity of the Company’s portfolio was approximately two months at June 30, 2017.

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. Fair Value Measurements The Company’s financial instruments as of June 30, 2017 and December 31, 2016 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. As of June 30, 2017 and December 31, 2016, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of June 30, 2017 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 63,592 $ 52,054 $ 11,538 $ — Marketable securities: Commercial paper 43,553 — 43,553 — Corporate notes 57,930 — 57,930 — U.S. government agency securities and treasuries 2,992 — 2,992 — Total $ 168,067 $ 52,054 $ 116,013 $ — Fair Value as of December 31, 2016 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 62,854 $ 59,862 $ 2,992 $ — Marketable securities: Commercial paper 68,375 — 68,375 — Corporate notes 70,408 — 70,408 — U.S. government agency securities and treasuries 25,514 — 25,514 — Total $ 227,151 $ 59,862 $ 167,289 $ — Cash equivalents and marketable securities have been initially valued at the transaction price and subsequently valued, at the end of each reporting period, utilizing third-party pricing services or other market observable data. The Company measures its cash equivalents at fair value on a recurring basis. The cash equivalents that the Company classifies within Level 1 of the fair value hierarchy are classified within Level 1 because they are valued using observable inputs that reflect quoted prices for identical assets in active markets. The Company measures its marketable securities at fair value on a recurring basis and classifies those instruments and some cash equivalents within Level 2 of the fair value hierarchy. The pricing services used by management utilize industry standard valuation models, including both income and market based approaches and observable market inputs to determine the fair value of marketable securities and some cash equivalents.

Supplemental Balance Sheet Info

Supplemental Balance Sheet Information	6 Months Ended
	Jun. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Supplemental Balance Sheet Information	5. Supplemental Balance Sheet Information Accrued expenses consisted of the following: June 30, December 31, (In thousands) Employee compensation and benefits $ 3,165 $ 4,100 Research and development expenses 9,200 10,925 Professional services and other 3,778 982 Accrued expenses $ 16,143 $ 16,007

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Income Taxes	6. Income Taxes The Company did not record a federal or state income tax provision or benefit for the three and six months ended June 30, 2017 and 2016 due to the expected loss before income taxes to be incurred for the years ended December 31, 2017 and 2016, as well as the Company’s continued maintenance of a full valuation allowance against its net deferred tax assets.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies There have been no significant changes to the Company’s commitments and contingencies in the three and six months ended June 30, 2017, as compared to those disclosed in Note 7, Commitments and Contingencies A $1.5 million payment obligation was incurred and paid in the six months ended June 30, 2017 upon the achievement of a milestone under the companion diagnostic agreement with Roche Molecular Systems, Inc. (“Roche Molecular”). Lease The Company leases office and laboratory space at Technology Square in Cambridge, Massachusetts under a Lease Agreement, dated as of June 15, 2012, as amended (the “Lease”) with ARE-TECH In May 2017, the Company entered into a Third Amendment to Lease (the “Third Amendment”) with the Landlord, and a Fourth Amendment to Lease with the Landlord (the “Fourth Amendment,” and, together with the Third Amendment, the “Amendments”). The Amendments each amend the Lease. Under the Amendments, the Company extended the term of the Lease at the Company’s headquarters in Cambridge, Massachusetts to November 30, 2022, subject to the Company’s right to terminate the Lease effective as of December 31, 2018, by giving written notice to the Landlord by December 31, 2017 and paying an early termination fee. Under the Lease, the Company has agreed to pay a monthly base rent of approximately $0.2 million for the period commencing December 1, 2017 through May 31, 2018, with an increase on June 1, 2018 and annual increases December 1 of each subsequent year until December 1, 2021. Under the Lease as amended by the Amendments, the Company is responsible for aggregate minimum rent payments of $18.6 million, of which approximately $0.5 million was paid prior to June 30, 2017. The remaining future minimum rent payments from July 1, 2017 through November 30, 2022 are as follows (in thousands): July 1, 2017 - December 31, 2017 $ 1,384 2018 3,074 2019 3,335 2020 3,435 2021 3,538 2022 3,332 Total $ 18,098

Collaborations

Collaborations	6 Months Ended
	Jun. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborations	8. Collaborations Celgene In April 2012, the Company entered into a collaboration and license agreement with Celgene. On July 8, 2015, the Company entered into an amendment and restatement of the collaboration and license agreement with Celgene. Original Agreement Structure Under the original agreement, the Company granted Celgene an exclusive license, for all countries other than the United States, to small molecule HMT inhibitors targeting the DOT1L HMT, including pinometostat, and an option, on a target-by-target Under the original agreement, the Company received a $65.0 million upfront payment and $25.0 million from the sale of its series C redeemable convertible preferred stock to an affiliate of Celgene, of which $3.0 million was considered a premium and included as collaboration arrangement consideration for a total upfront payment of $68.0 million. In addition, the Company has received a $25.0 million clinical development milestone payment and $7.0 million of global development co-funding mid-single mid-teens The Company was obligated to conduct and solely fund research and development costs of the Phase 1 clinical trials for pinometostat. For all remaining DOT1L program development costs, Celgene and the Company were to equally co-fund Amended and Restated Agreement Structure Under the amended and restated collaboration and license agreement: • Celgene retained its exclusive license to small molecule HMT inhibitors targeting DOT1L, including pinometostat, • Celgene’s other option rights were narrowed to small molecule HMT inhibitors targeting three predefined targets (the “Option Targets”), • The exclusive licenses to HMT inhibitors targeting two of the Option Targets that Celgene may acquire were expanded to include the United States, with the exclusive license to HMT inhibitors targeting the third Option Target continuing to be for all countries other than the United States, • Celgene’s option period was extended for each of the Option Targets and Celgene’s option is exercisable at the time of the Company’s investigational new drug application (“IND”) filing for an HMT inhibitor targeting the applicable Option Target, upon the payment by Celgene at such time of a pre-specified • Celgene’s license may be maintained beyond the end of Phase 1 clinical development for each of the Option Targets, upon payment by Celgene at such time of a pre-specified • The Company’s research and development obligations with respect to each Option Target under the amended and restated agreement were extended for at least an additional three years, subject to Celgene exercising its option with respect to such Option Target at IND filing. Subject to the Company’s opt-out Under the amended and restated agreement, the Company received a $10.0 million upfront payment in exchange for the Company’s extension of Celgene’s option rights to the Option Targets and the Company’s research and development obligations. In addition, the Company is eligible to earn an aggregate of up to $75.0 million in development milestones and license payments, up to $365.0 million in regulatory milestone payments and up to $170.0 million in sales milestone payments related to the three Option Targets. The Company is also eligible to receive royalties on each of the Option Targets as specified in the amended and restated agreement. The Company is also eligible to earn $35.0 million in an additional clinical development milestone payment and up to $100.0 million in regulatory milestone payments related to DOT1L. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, the Company may not receive any additional milestone payments or royalty payments from Celgene. Due to the varying stages of development of each target, the Company is not able to determine the next milestone that might be earned, if any. The amended and restated agreement eliminated the right of first negotiation that the Company had granted to Celgene under the original agreement with respect to business combination transactions that the Company may desire to pursue with third parties. The Company is primarily responsible for the research strategy under the collaboration. During each applicable option period the Company is required to use commercially reasonable efforts to carry out a mutually agreed-upon research plan for each Option Target. Subject to the Company’s opt-out co-fund Collaboration Revenue Through June 30, 2017, the Company has recognized $74.3 million of total collaboration revenue since the inception of the collaboration, including $0.5 million and $0.9 million in the three and six months ended June 30, 2016, respectively. No revenue from the collaboration was earned in the three and six months ended June 30, 2017. The Company recognized total global development co-funding co-funding co-funding GSK In January 2011, the Company entered into a collaboration and license agreement with GSK, to discover, develop and commercialize novel small molecule HMT inhibitors directed to available targets from the Company’s platform. Under the terms of the agreement, the Company granted GSK exclusive worldwide license rights to HMT inhibitors directed to three targets. Additionally, as part of the research collaboration, the Company agreed to provide research and development services related to the licensed targets pursuant to agreed upon research plans during a research term that ended January 8, 2015. In March 2014, the Company and GSK amended certain terms of this agreement for the third licensed target, revising the license terms with respect to candidate compounds and amending the corresponding financial terms, including reallocating milestone payments and increasing royalty rates as to the third target. The Company substantially completed all research obligations under this agreement by the end of the first quarter of 2015 and completed the transfer of the remaining data and materials for these programs to GSK in the second quarter of 2015. Agreement Structure Under the agreement, the Company has recorded a $20.0 million upfront payment, a $3.0 million payment upon the execution of the March 2014 agreement amendment, $6.0 million of fixed research funding, $9.0 million for research and development services and $31.0 million of preclinical research and development milestone payments, which includes a $10.0 million milestone payment earned in May 2017 upon GSK’s initiation of good laboratory practices toxicology studies for a first-in-class mid-single product-by-licensed Collaboration Revenue Through June 30, 2017, the Company has earned a total of $69.0 million under the GSK agreement, which the Company recognized as collaboration revenue in the condensed consolidated statements of operations and comprehensive loss. The Company recognized $10.0 million of collaboration revenue in the three months ended June 30, 2017 related to achievement of the milestone described in the preceding paragraph. The Company did not have any deferred revenue related to this agreement as of June 30, 2017 or December 31, 2016 and any future revenues will relate to any milestone payments and royalties received under the agreement, if any. Roche Molecular In December 2012, Eisai and the Company entered into an agreement with Roche Molecular under which Eisai and the Company engaged Roche Molecular to develop a companion diagnostic to identify patients who possess certain activating mutations of EZH2. In October 2013, this agreement was amended to include additional mutations in EZH2. The development costs due under the amended agreement with Roche Molecular were the responsibility of Eisai until the execution of the amended and restated collaboration and license agreement with Eisai in March 2015, at which time the Company assumed responsibility for the remaining development costs due under the agreement. In December 2015, the Company entered into a second amendment to the companion diagnostic agreement with Roche Molecular. As of June 30, 2017, the Company is responsible for the remaining development costs of $10.5 million due under the agreement. The Company expects the remaining development costs under the agreement to be incurred and paid through 2019. Under the agreement with Roche Molecular, Roche Molecular is obligated to use commercially reasonable efforts to develop and to make commercially available the companion diagnostic. Roche Molecular has exclusive rights to commercialize the companion diagnostic. The agreement with Roche Molecular will expire when the Company is no longer developing or commercializing tazemetostat. The Company may terminate the agreement by giving Roche Molecular 90 days’ written notice if the Company discontinues development and commercialization of tazemetostat or determines, in conjunction with Roche Molecular, that the companion diagnostic is not needed for use with tazemetostat. Either the Company or Roche Molecular may also terminate the agreement in the event of a material breach by the other party, in the event of material changes in circumstances that are contrary to key assumptions specified in the agreement or in the event of specified bankruptcy or similar circumstances. Under specified termination circumstances, Roche Molecular may become entitled to specified termination fees.

Stock-Based Compensation

Stock-Based Compensation	6 Months Ended
	Jun. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	9. Stock-Based Compensation Total stock-based compensation expense related to stock options, restricted stock units and the employee stock purchase plan was $3.0 million and $2.8 million for the three months ended June 30, 2017 and 2016, respectively, and $5.8 million and $5.1 million for the six months ended June 30, 2017 and 2016, respectively. Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows: Three Months Ended June 30, Six Months Ended June 30, 2017 2016 2017 2016 (In thousands) Research and development $ 1,509 $ 1,366 $ 2,901 $ 2,613 General and administrative 1,512 1,387 2,877 2,513 Total $ 3,021 $ 2,753 $ 5,778 $ 5,126 Stock Options The weighted-average grant date fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $10.13 and $7.27 per option for those options granted during the three months ended June 30, 2017 and 2016, respectively, and $8.54 and $6.43 per option for those options granted during the six months ended June 30, 2017 and 2016, respectively. Key assumptions used to apply this pricing model were as follows: Three Months Ended June 30 Six Months Ended June 30 2017 2016 2017 2016 Risk-free interest rate 1.8 % 1.3 % 1.8 % 1.2 % Expected life of options 6 years 6 years 6 years 6 years Expected volatility of underlying stock 74.4 % 79.0 % 74.5 % 78.9 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 % The following is a summary of stock option activity for the six months ended June 30, 2017: Number of Weighted Weighted Aggregate (In thousands) (In years) (In thousands) Outstanding at December 31, 2016 4,059 $ 14.32 Granted 1,860 12.99 Exercised (181 ) 6.24 Forfeited or expired (631 ) 12.62 Outstanding at June 30, 2017 5,107 $ 14.33 8.19 $ 16,501 Exercisable at June 30, 2017 1,763 $ 16.74 6.55 $ 6,191 As of June 30, 2017, there was $27.7 million of unrecognized compensation cost related to stock options that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 2.9 years. Restricted Stock Units The following is a summary of restricted stock unit activity for the six months ended June 30, 2017: Number of Weighted Outstanding at December 31, 2016 64 $ 12.20 Granted — — Vested (15 ) 12.20 Outstanding at June 30, 2017 49 $ 12.20 As of June 30, 2017, there was $0.6 million of unrecognized compensation cost related to restricted stock units that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 1.6 years.

Loss Per Share

Loss Per Share	6 Months Ended
	Jun. 30, 2017
Earnings Per Share [Abstract]
Loss Per Share	10. Loss Per Share Basic and diluted loss per share allocable to common stockholders are computed as follows: Three Months Ended June 30, Six Months Ended June 30, 2017 2016 2017 2016 (In thousands except per share data) Net loss $ (28,024 ) $ (27,981 ) $ (60,546 ) $ (50,860 ) Weighted average shares outstanding 58,377 57,352 58,298 56,250 Basic and diluted loss per share allocable to common stockholders $ (0.48 ) $ (0.49 ) $ (1.04 ) $ (0.90 ) The following common stock equivalents were excluded from the calculation of diluted loss per share allocable to common stockholders because their inclusion would have been anti-dilutive: Three Months Ended June 30, Six Months Ended June 30, 2017 2016 2017 2016 (In thousands) Stock options 5,107 3,927 5,107 3,927 Unvested restricted stock units 49 79 49 79 Shares issuable under employee stock purchase plan 32 24 32 24 5,188 4,030 5,188 4,030

Related Party Transactions

Related Party Transactions	6 Months Ended
	Jun. 30, 2017
Related Party Transactions [Abstract]
Related Party Transactions	11. Related Party Transactions Celgene has made a series of equity investments in the Company, owning 3,674,640 shares of common stock representing 6.3% of the Company’s outstanding common stock as of June 30, 2017. Refer to Note 8, Collaborations,

Summary of Significant Accoun17

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The condensed consolidated financial statements of the Company included herein have been prepared, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these condensed consolidated financial statements should be read in conjunction with the financial statements and notes thereto included in the Company’s Annual Report on Form 10-K The unaudited condensed consolidated financial statements include the accounts of Epizyme, Inc. and its wholly owned, controlled subsidiary, Epizyme Securities Corporation. All intercompany transactions and balances of subsidiaries have been eliminated in consolidation. In the opinion of management, the information furnished reflects all adjustments, all of which are of a normal and recurring nature, necessary for a fair presentation of the results for the reported interim periods. The Company considers events or transactions that occur after the balance sheet date but before the consolidated financial statements are issued to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The three months ended June 30, 2017 and 2016 are referred to as the second quarter of 2017 and 2016, respectively. The results of operations for interim periods are not necessarily indicative of results to be expected for the full year or any other interim period.
Significant Accounting Policies	Significant Accounting Policies There have been no material changes to the Company’s significant accounting policies during the six months ended June 30, 2017, as compared to the significant accounting policies disclosed in Note 2, Summary of Significant Accounting Policies
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue From Contracts With Customers 2014-09 Revenue Recognition 2016-10 2016-12, ASU 2014-09. 2014-09 2014-09, In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) right-of-use In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments zero-coupon In November 2016, the FASB issued ASU 2016-18, Restricted Cash 2016-18, 2018-18 2016-18 In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting
Going Concern	Going Concern At each reporting period, the Company evaluates whether there are conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued. The Company is required to make certain additional disclosures if it concludes substantial doubt exists and it is not alleviated by the Company’s plans or when its plans alleviate substantial doubt about the Company’s ability to continue as a going concern. The Company’s evaluation entails analyzing prospective operating budgets and forecasts for expectations of the Company’s cash needs, and comparing those needs to the current cash, cash equivalent and marketable security balances. After considering the Company’s current research and development plans and the timing expectations related to the progress of its programs, and after considering its existing cash, cash equivalents and marketable securities as of June 30, 2017, the Company did not identify conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the date these financial statements were issued.
Share-Based Payment	Share-Based Payment As of January 1, 2017, the Company adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting paid-in-capital

Marketable Securities (Tables)

Marketable Securities (Tables)	6 Months Ended
	Jun. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Summary of Available-for-Sale Securities Held	The following table summarizes the available for sale securities held at June 30, 2017 (in thousands): Description Amortized Cost Unrealized Unrealized Fair Value Commercial paper $ 43,570 $ — $ (17 ) $ 43,553 Corporate notes 57,973 — (43 ) 57,930 U.S. government agency securities and U.S. Treasuries 2,992 — 2,992 Total $ 104,535 $ — $ (60 ) $ 104,475 The following table summarizes the available for sale securities held at December 31, 2016 (in thousands): Description Amortized Cost Unrealized Unrealized Fair Value Commercial paper $ 68,407 $ — $ (32 ) $ 68,375 Corporate notes 70,489 — (81 ) 70,408 U.S. government agency securities and U.S. Treasuries 25,507 7 — 25,514 Total $ 164,403 $ 7 $ (113 ) $ 164,297

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2017
Fair Value Disclosures [Abstract]
Summary of Company's Financial Assets Recognized at Fair Value	As of June 30, 2017 and December 31, 2016, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of June 30, 2017 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 63,592 $ 52,054 $ 11,538 $ — Marketable securities: Commercial paper 43,553 — 43,553 — Corporate notes 57,930 — 57,930 — U.S. government agency securities and treasuries 2,992 — 2,992 — Total $ 168,067 $ 52,054 $ 116,013 $ — Fair Value as of December 31, 2016 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 62,854 $ 59,862 $ 2,992 $ — Marketable securities: Commercial paper 68,375 — 68,375 — Corporate notes 70,408 — 70,408 — U.S. government agency securities and treasuries 25,514 — 25,514 — Total $ 227,151 $ 59,862 $ 167,289 $ —

Supplemental Balance Sheet In20

Supplemental Balance Sheet Information (Tables)	6 Months Ended
	Jun. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following: June 30, December 31, (In thousands) Employee compensation and benefits $ 3,165 $ 4,100 Research and development expenses 9,200 10,925 Professional services and other 3,778 982 Accrued expenses $ 16,143 $ 16,007

Commitments and Contingencies (

Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Schedule of Remaining Future Minimum Rent Payments	The remaining future minimum rent payments from July 1, 2017 through November 30, 2022 are as follows (in thousands): July 1, 2017 - December 31, 2017 $ 1,384 2018 3,074 2019 3,335 2020 3,435 2021 3,538 2022 3,332 Total $ 18,098

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	6 Months Ended
	Jun. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Stock-Based Compensation Expense	Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows: Three Months Ended June 30, Six Months Ended June 30, 2017 2016 2017 2016 (In thousands) Research and development $ 1,509 $ 1,366 $ 2,901 $ 2,613 General and administrative 1,512 1,387 2,877 2,513 Total $ 3,021 $ 2,753 $ 5,778 $ 5,126
Assumptions Used in Applying Pricing Model	Key assumptions used to apply this pricing model were as follows: Three Months Ended June 30 Six Months Ended June 30 2017 2016 2017 2016 Risk-free interest rate 1.8 % 1.3 % 1.8 % 1.2 % Expected life of options 6 years 6 years 6 years 6 years Expected volatility of underlying stock 74.4 % 79.0 % 74.5 % 78.9 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 %
Summary of Stock Option Activity	The following is a summary of stock option activity for the six months ended June 30, 2017: Number of Weighted Weighted Aggregate (In thousands) (In years) (In thousands) Outstanding at December 31, 2016 4,059 $ 14.32 Granted 1,860 12.99 Exercised (181 ) 6.24 Forfeited or expired (631 ) 12.62 Outstanding at June 30, 2017 5,107 $ 14.33 8.19 $ 16,501 Exercisable at June 30, 2017 1,763 $ 16.74 6.55 $ 6,191
Summary of Restricted Stock Activity	The following is a summary of restricted stock unit activity for the six months ended June 30, 2017: Number of Weighted Outstanding at December 31, 2016 64 $ 12.20 Granted — — Vested (15 ) 12.20 Outstanding at June 30, 2017 49 $ 12.20

Loss Per Share (Tables)

Loss Per Share (Tables)	6 Months Ended
	Jun. 30, 2017
Earnings Per Share [Abstract]
Basic and Diluted Loss per Share	Basic and diluted loss per share allocable to common stockholders are computed as follows: Three Months Ended June 30, Six Months Ended June 30, 2017 2016 2017 2016 (In thousands except per share data) Net loss $ (28,024 ) $ (27,981 ) $ (60,546 ) $ (50,860 ) Weighted average shares outstanding 58,377 57,352 58,298 56,250 Basic and diluted loss per share allocable to common stockholders $ (0.48 ) $ (0.49 ) $ (1.04 ) $ (0.90 )
Common Stock Equivalents Excluded from Calculation of Diluted Loss per Share Attributable to Common Stockholders	The following common stock equivalents were excluded from the calculation of diluted loss per share allocable to common stockholders because their inclusion would have been anti-dilutive: Three Months Ended June 30, Six Months Ended June 30, 2017 2016 2017 2016 (In thousands) Stock options 5,107 3,927 5,107 3,927 Unvested restricted stock units 49 79 49 79 Shares issuable under employee stock purchase plan 32 24 32 24 5,188 4,030 5,188 4,030

Overview - Additional Informati

Overview - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Basis Of Presentation [Line Items]
Proceeds from sale of redeemable convertible preferred stock	$ 76,000
Initial public offering completion date	2013-05
Cash, cash equivalents, and marketable securities	$ 193,000
Accumulated deficit	(414,334)	$ (353,673)
Collaborative Arrangement [Member]
Basis Of Presentation [Line Items]
Aggregate fund, amount	740,300
Non-equity funding through collaboration agreement	217,800
IPO [Member]
Basis Of Presentation [Line Items]
Sale of common stock in public offering	446,500
Proceeds from sale of redeemable convertible preferred stock	$ 1,600

Summary of Significant Accoun25

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2017USD ($)
Accounting Policies [Abstract]
Change in amount of valuation allowance and deferred tax asset	$ 25.7
Amount charged to retained earnings	$ 0.1

Marketable Securities - Summary

Marketable Securities - Summary of Available-for-Sale Securities Held (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	$ 104,535	$ 164,403
Available-For-Sale Securities Held, Unrealized Gains		7
Available-For-Sale Securities Held, Unrealized Losses	(60)	(113)
Available-For-Sale Securities Held, Fair Value	104,475	164,297
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	43,570	68,407
Available-For-Sale Securities Held, Unrealized Losses	(17)	(32)
Available-For-Sale Securities Held, Fair Value	43,553	68,375
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	57,973	70,489
Available-For-Sale Securities Held, Unrealized Losses	(43)	(81)
Available-For-Sale Securities Held, Fair Value	57,930	70,408
U.S. Government Agency Securities and Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	2,992	25,507
Available-For-Sale Securities Held, Unrealized Gains		7
Available-For-Sale Securities Held, Fair Value	$ 2,992	$ 25,514

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail)	3 Months Ended	6 Months Ended
	Jun. 30, 2017USD ($)	Jun. 30, 2017USD ($)
Investments, Debt and Equity Securities [Abstract]
Realized gains (losses) recognized on sale of marketable equity securities	$ 0	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 88,400,000	88,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		$ 100,000
Weighted average maturity period		2 months

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Company's Financial Assets Recognized at Fair Value (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 63,592	$ 62,854
Total	168,067	227,151
Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	43,553	68,375
Corporate Notes [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	57,930	70,408
U.S. Government Agency Securities and Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	2,992	25,514
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	52,054	59,862
Total	52,054	59,862
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	11,538	2,992
Total	116,013	167,289
Level 2 [Member] | Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	43,553	68,375
Level 2 [Member] | Corporate Notes [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	57,930	70,408
Level 2 [Member] | U.S. Government Agency Securities and Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	$ 2,992	$ 25,514

Supplemental Balance Sheet In29

Supplemental Balance Sheet Information - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Accrued Liabilities, Current [Abstract]
Employee compensation and benefits	$ 3,165	$ 4,100
Research and development expenses	9,200	10,925
Professional services and other	3,778	982
Accrued expenses	$ 16,143	$ 16,007

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Tax Disclosure [Abstract]
Federal income tax provision or benefit	$ 0	$ 0	$ 0	$ 0
State income tax provision or benefit	$ 0	$ 0	$ 0	$ 0

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	Jun. 29, 2017	Jun. 30, 2017
Other Commitments [Line Items]
Monthly base rent		$ 0.2
Aggregate minimum rent payments		$ 18.6
Rent paid	$ 0.5
Initial Term [Member]
Other Commitments [Line Items]
Lease expiration date		May 31, 2018
Extended Term [Member]
Other Commitments [Line Items]
Lease expiration date		Nov. 30, 2022
Roche [Member]
Other Commitments [Line Items]
Payment obligation incurred and paid		$ 1.5

Commitments And Contingencies32

Commitments And Contingencies - Schedule of Remaining Future Minimum Rent Payments (Detail) $ in Thousands	Jun. 30, 2017USD ($)
Commitments and Contingencies Disclosure [Abstract]
July 1, 2017 - December 31, 2017	$ 1,384
2,018	3,074
2,019	3,335
2,020	3,435
2,021	3,538
2,022	3,332
Total	$ 18,098

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)	Jul. 08, 2015OptionTargets	May 31, 2017USD ($)	Mar. 31, 2014USD ($)	Jun. 30, 2017USD ($)	Jun. 30, 2016USD ($)	Jun. 30, 2017USD ($)	Jun. 30, 2016USD ($)	Dec. 31, 2013USD ($)	Jun. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Dec. 31, 2016USD ($)
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue				$ 10,000,000	$ 473,000	$ 10,000,000	$ 945,000
Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Notice period in days						90 days
DOT1L [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Clinical development milestone payment				35,000,000		$ 35,000,000			$ 35,000,000	$ 35,000,000
DOT1L [Member] | Maximum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Additional milestone payments				100,000,000		100,000,000			100,000,000	100,000,000
Celgene [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Company received upfront payment									65,000,000
Proceeds from redeemable convertible preferred stock									25,000,000
Upfront payment recorded									68,000,000
Clinical development milestone achieved									25,000,000
Global development co-funding				0	100,000	100,000	100,000		7,000,000
Number of option targets | OptionTargets	3
Collaboration revenue				0	$ 500,000	0	$ 900,000		74,300,000
Deferred revenue				28,800,000		28,800,000			28,800,000	28,800,000	$ 28,800,000
Celgene [Member] | Minimum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Option targets under amended and restated agreement extended period	3 years
Celgene [Member] | United States [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Number of option targets | OptionTargets	2
Celgene [Member] | Non-US [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Number of option targets | OptionTargets	1
Celgene [Member] | DOT1L [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Clinical development milestone payment				35,000,000		35,000,000			35,000,000	35,000,000
Additional milestone payments				100,000,000		100,000,000			100,000,000	100,000,000
Celgene [Member] | Available Targets [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Additional milestone payments				100,000,000		100,000,000			100,000,000	100,000,000
Additional payments				65,000,000		65,000,000			65,000,000	65,000,000
Celgene [Member] | Option Targets [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Company received upfront payment						10,000,000
Celgene [Member] | Option Targets [Member] | Maximum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Additional milestone payments				365,000,000		365,000,000			365,000,000	365,000,000
Development milestone and license payments associated with the Option Targets						75,000,000
Sales-based milestone payments				170,000,000		170,000,000			170,000,000	170,000,000
GSK [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment recorded			$ 3,000,000					$ 20,000,000
Clinical development milestone payment				103,000,000		103,000,000			103,000,000	103,000,000
Additional milestone payments				278,000,000		278,000,000			278,000,000	278,000,000
Sales-based milestone payments				218,000,000		218,000,000			218,000,000	218,000,000
Collaboration revenue				10,000,000
Deferred revenue				0		0			0	0	$ 0
Fixed research funding received										6,000,000
Milestone payments received										31,000,000
Research and development services										9,000,000
Additional substantive preclinical research and development milestone payments				8,000,000		8,000,000			8,000,000	8,000,000
Milestone payments received		$ 10,000,000
Cash and accounts receivable										69,000,000
Collaborative Arrangement [Member] | Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Remaining unpaid milestone payments				$ 10,500,000		$ 10,500,000			$ 10,500,000	$ 10,500,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense related to stock options and restricted stock	$ 3,021	$ 2,753	$ 5,778	$ 5,126
Weighted-average fair value of options granted	$ 10.13	$ 7.27	$ 8.54	$ 6.43
Unrecognized compensation cost related to restricted stock	$ 27,700		$ 27,700
Expected weighted average remaining vesting period for recognized cost			2 years 10 months 25 days
Unvested Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost related to restricted stock	$ 600		$ 600
Expected weighted average remaining vesting period for recognized cost			1 year 7 months 6 days

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 3,021	$ 2,753	$ 5,778	$ 5,126
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	1,509	1,366	2,901	2,613
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 1,512	$ 1,387	$ 2,877	$ 2,513

Stock-Based Compensation - Assu

Stock-Based Compensation - Assumptions Used in Applying Pricing Model (Detail) - Employee Stock Option [Member]	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.80%	1.30%	1.80%	1.20%
Expected life of options	6 years	6 years	6 years	6 years
Expected volatility of underlying stock	74.40%	79.00%	74.50%	78.90%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended
	Jun. 30, 2017USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Options, Outstanding, Beginning balance | shares	4,059
Number of Options, Granted | shares	1,860
Number of Options, Exercised | shares	(181)
Number of Options, Forfeited or expired | shares	(631)
Number of Options, Outstanding, Ending balance | shares	5,107
Number of Options, Exercisable | shares	1,763
Weighted Average Exercise Price per Share, Outstanding, Beginning balance | $ / shares	$ 14.32
Weighted Average Exercise Price per Share, Granted | $ / shares	12.99
Weighted Average Exercise Price per Share, Exercised | $ / shares	6.24
Weighted Average Exercise Price per Share, Forfeited or expired | $ / shares	12.62
Weighted Average Exercise Price per Share, Outstanding, Ending balance | $ / shares	14.33
Weighted Average Exercise Price per Share, Exercisable | $ / shares	$ 16.74
Weighted Average Remaining Contractual Term (In Years), Outstanding	8 years 2 months 8 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	6 years 6 months 18 days
Aggregate Intrinsic Value, Outstanding | $	$ 16,501
Aggregate Intrinsic Value, Exercisable | $	$ 6,191

Stock-Based Compensation - Su38

Stock-Based Compensation - Summary of Restricted Stock Activity (Detail) - Unvested Restricted Stock Units [Member] shares in Thousands	6 Months Ended
	Jun. 30, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Outstanding, Beginning balance | shares	64
Number of Units, Granted | shares	0
Number of Units, Vested | shares	(15)
Number of Units, Outstanding, Ending balance | shares	49
Weighted Average Grant Date Fair Value per Unit, Beginning balance | $ / shares	$ 12.20
Weighted Average Grant Date Fair Value per Unit, Granted | $ / shares	0
Weighted Average Grant Date Fair Value per Unit, Vested | $ / shares	12.20
Weighted Average Grant Date Fair Value per Unit, Ending balance | $ / shares	$ 12.20

Loss Per Share - Schedule of Ba

Loss Per Share - Schedule of Basic and Diluted Loss per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Net loss	$ (28,024)	$ (27,981)	$ (60,546)	$ (50,860)
Weighted average shares outstanding	58,377	57,352	58,298	56,250
Basic and diluted loss per share allocable to common stockholders	$ (0.48)	$ (0.49)	$ (1.04)	$ (0.90)

Loss Per Share - Common Stock E

Loss Per Share - Common Stock Equivalents from Calculation of Diluted Loss per Share Attributable to Common Stockholders (Detail) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	5,188	4,030	5,188	4,030
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	5,107	3,927	5,107	3,927
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	49	79	49	79
Shares Issuable Under Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	32	24	32	24

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - shares	Jun. 30, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Equity investments in common stock	58,433,000	58,050,000
Beneficial Owner [Member]
Related Party Transaction [Line Items]
Equity investments in common stock	3,674,640
Equity investments in outstanding common stock percentage	6.30%