Epizyme (EPZM) 10-Q 6 Aug 15 2015 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2015	Jul. 31, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2015
Document Fiscal Year Focus	2,015
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		41,329,700

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Current Assets:
Cash and cash equivalents	$ 236,695	$ 190,095
Accounts receivable	723	2,075
Prepaid expenses and other current assets	2,011	2,840
Total current assets	239,429	195,010
Property and equipment, net	4,856	3,620
Restricted cash and other assets	709	573
Total Assets	244,994	199,203
Current Liabilities:
Accounts payable	3,022	8,300
Accrued expenses	9,476	7,043
Current portion of capital lease obligation	534
Current portion of deferred revenue	174	1,702
Total current liabilities	13,206	17,045
Capital lease obligation, net of current portion	1,017
Deferred revenue, net of current portion	21,449	21,449
Other long-term liabilities	$ 416	$ 427
Commitments and contingencies
Stockholders' Equity:
Preferred stock, $0.0001 par value; 5,000,000 shares authorized; 0 shares issued and outstanding
Common stock, $0.0001 par value; 125,000,000 shares authorized; 41,240,338 shares and 34,426,012 shares issued and outstanding, respectively	$ 4	$ 3
Additional paid-in capital	407,072	271,364
Accumulated deficit	(198,170)	(111,085)
Total stockholders' equity	208,906	160,282
Total Liabilities and Stockholders' Equity	$ 244,994	$ 199,203

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	41,240,338	34,426,012
Common stock, shares outstanding	41,240,338	34,426,012

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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Collaboration revenue	$ 736	$ 9,494	$ 1,647	$ 22,885
Operating expenses:
Research and development	20,551	17,499	77,602	32,846
General and administrative	5,970	5,306	11,207	10,262
Total operating expenses	26,521	22,805	88,809	43,108
Loss from operations	(25,785)	(13,311)	(87,162)	(20,223)
Other income, net:
Interest income, net	6	26	37	42
Other income	20	12	40	24
Other income, net	26	38	77	66
Loss before income taxes	(25,759)	(13,273)	(87,085)	(20,157)
Income tax expense		113		113
Net loss	$ (25,759)	$ (13,386)	$ (87,085)	$ (20,270)
Loss per share allocable to common stockholders:
Basic and Diluted	$ (0.63)	$ (0.40)	$ (2.29)	$ (0.63)
Weighted average shares outstanding:
Basic and Diluted	41,087	33,156	38,056	32,064
Comprehensive loss	$ (25,759)	$ (13,386)	$ (87,085)	$ (20,270)

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (87,085)	$ (20,270)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Acquired in-process research and development	40,000
Depreciation and amortization	653	362
Stock-based compensation	4,910	3,095
Loss on disposal of property and equipment	6
Changes in operating assets and liabilities:
Accounts receivable	1,352	31,476
Prepaid expenses and other current assets	829	(235)
Accounts payable	(5,212)	(142)
Accrued expenses	2,433	(410)
Deferred revenue	(1,528)	(9,387)
Restricted cash and other assets	(136)	169
Other long-term liabilities	(11)	8
Net cash (used in) provided by operating activities	(43,789)	4,666
CASH FLOWS FROM INVESTING ACTIVITIES:
Acquisition of in-process research and development	(40,000)
Purchases of property and equipment	(229)	(824)
Net cash used in investing activities	(40,229)	(824)
CASH FLOWS FROM FINANCING ACTIVITIES:
Payment under capital lease obligation	(181)
Proceeds from public offering, net of commissions	130,712	101,283
Proceeds from stock options exercised	215	1,334
Excess tax benefit from stock option plan		28
Issuance of shares under employee stock purchase plan	239	201
Payment of public offering costs	(367)	(649)
Proceeds from reimbursement of public offering costs		269
Net cash provided by financing activities	130,618	102,466
Net increase in cash and cash equivalents	46,600	106,308
Cash and cash equivalents, beginning of period	190,095	123,564
Cash and cash equivalents, end of period	236,695	229,872
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Purchases of property and equipment unpaid at period end	15	113
Equipment acquired under capital lease	1,732
Income taxes paid	$ 2	$ 241

Overview and Basis of Presentat

Overview and Basis of Presentation	6 Months Ended
	Jun. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Overview and Basis of Presentation	1. Overview and Basis of Presentation 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 cancer patients. The Company has built a proprietary product platform that it uses to create small molecule inhibitors of a 96-member class of enzymes known as histone methyltransferases (HMTs). Genetic alterations can result in changes to the activity of HMTs, making them oncogenic. The Company’s therapeutic strategy is to inhibit oncogenic HMTs to treat the underlying causes of the associated cancers. 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 for the fiscal year ended December 31, 2014 (the “Annual Report”). The unaudited condensed consolidated financial statements include the accounts of Epizyme and its subsidiary. 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 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, 2015 and 2014 are referred to as the second quarter of 2015 and 2014, 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. In March 2015, the Company conducted a public offering of its common stock, selling 6,000,000 shares at a price of $20.75 per share. The Company received net proceeds before expenses from the sale of these 6,000,000 shares of $117.0 million after deducting underwriting discounts and commissions paid by the Company. In April 2015, the Company issued and sold an additional 701,448 shares in connection with the March 2015 public offering at a price of $20.75 per share pursuant to the underwriters’ option to purchase additional shares that the Company granted in connection with such public offering. The Company received net proceeds before expenses from the sale of these 701,448 shares of $13.7 million after deducting underwriting discounts and commissions paid by the Company.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies In the six months ended June 30, 2015, the Company updated its accounting policy regarding property and equipment as a result of property and equipment acquired pursuant to a capital lease. Property and Equipment The Company records property and equipment at cost. Property and equipment acquired under a capital lease is recorded at the lesser of the present value of the minimum lease payments under the capital lease or the fair value of the leased property at lease inception. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives: Asset Category Useful Lives Laboratory equipment 5 - 20 years Office furniture and equipment 3 - 10 years or term of respective lease, if shorter Leasehold improvements 3 - 10 years or term of respective lease, if shorter Amortization of capital lease assets is included in depreciation expense. The Company capitalizes expenditures for new property and equipment and improvements to existing facilities and charges the cost of maintenance to expense. The Company eliminates the cost of property retired or otherwise disposed of, along with the corresponding accumulated depreciation, from the related accounts, and the resulting gain or loss is reflected in the results of operations. Additionally, the Company updated its accounting policies as a result of the amended and restated collaboration and license agreement the Company executed with Eisai Co., Ltd. (“Eisai”), pursuant to which the Company recorded the reacquisition of worldwide rights, excluding Japan, to its EZH2 program, including tazemetostat (also known as EPZ-6438), as an acquisition of in-process research and development. Acquired In-Process Research and Development The Company records upfront payments that relate to the acquisition of a development-stage product candidate as research and development expense in the period in which they are incurred, provided that the acquired development-stage product candidate did not also include processes or activities that would constitute a business, the product candidate has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use. There have been no other material changes to the significant accounting policies previously disclosed in the Company’s Annual Report. Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers Revenue Recognition In August 2014, the FASB issued ASU No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern Presentation of Financial Statements—Going Concern In April 2015, the FASB issued ASU No. 2015-05, Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement Internal-Use Software In June 2015, the FASB issued ASU No. 2015-10, Technical Corrections and Improvements

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2015
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The Company classifies fair value based measurements using a three-level hierarchy that prioritizes the inputs used to measure fair value. This hierarchy requires entities to maximize the use of observable inputs and minimize the use of unobservable inputs. The three levels of inputs used to measure fair value are as follows: Level 1, quoted market prices in active markets for identical assets or liabilities; Level 2, observable inputs other than quoted market prices included in Level 1 such as quoted market prices for markets that are not active or other inputs that are observable or can be corroborated by observable market data; and 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, including estimates and assumptions developed by the Company, reflective of those that a market participant would use, as inputs to certain pricing models, discounted cash flow methodologies and similar techniques that use significant unobservable inputs. The Company’s financial instruments as of June 30, 2015 and December 31, 2014 consisted primarily of cash and cash equivalents, accounts receivable and accounts payable. As of June 30, 2015 and December 31, 2014, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of June 30, 2015 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 196,924 $ 196,924 $ — $ — Total $ 196,924 $ 196,924 $ — $ — Fair Value as of December 31, 2014 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 184,257 $ 184,257 $ — $ — Total $ 184,257 $ 184,257 $ — $ —

Accrued Expenses

Accrued Expenses	6 Months Ended
	Jun. 30, 2015
Payables and Accruals [Abstract]
Accrued Expenses	4. Accrued Expenses Accrued expenses consisted of the following: June 30, December 31, (In thousands) Employee compensation and benefits $ 2,068 $ 2,623 Research and development and professional expenses 7,408 4,420 Accrued expenses $ 9,476 $ 7,043

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2015
Income Tax Disclosure [Abstract]
Income Taxes	5. 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, 2015 due to the expected loss before income taxes to be incurred for the year ending December 31, 2015, as well as the Company’s continued maintenance of a full valuation allowance against its net deferred tax assets. The Company recorded $0.1 million of income tax expense in the three and six months ended June 30, 2014 due to provision-to-return adjustments identified related to the year ended December 31, 2013. The Company did not record a federal or state income tax provision or benefit for the three or six months ended June 30, 2014 related to the year ending December 31, 2014, due to the then expected loss before income taxes to be incurred for the year ending December 31, 2014, 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, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies Commitments In the first quarter of 2015, the Company acquired computer equipment pursuant to a capital lease. Future minimum equipment lease payments under this capital lease, net of imputed interest, as of June 30, 2015 are as follows: Future minimum lease payments in year ending December 31: (In thousands) 2015 $ 332 2016 665 2017 665 2018 111 Total future minimum lease payments 1,773 Less: Amount representing imputed interest on equipment lease (222 ) Capital lease obligation $ 1,551 The Company also entered into an agreement in June 2015 to lease approximately 4,000 square feet of office space in Durham, North Carolina through July 2017. Total future minimum lease payments under this office lease agreement are approximately $0.2 million. In connection with the amended and restated collaboration and license agreement that the Company executed with Eisai in March 2015, the Company and Eisai entered into an amended and restated letter agreement related to their December 2012 companion diagnostic agreement with Roche Molecular Systems (“Roche”). Upon the execution of the amended and restated letter agreement with Eisai, the Company assumed responsibility for up to $15.5 million of the remaining development costs under the agreement with Roche. Eisai continues to be responsible for up to $1.0 million of the remaining Japan-specific development costs under the agreement with Roche. Contingencies In connection with the execution of the amended and restated collaboration and license agreement with Eisai, the Company agreed to pay Eisai up to a total of $20.0 million upon the achievement of specified clinical development milestones and up to a total of $50.0 million upon the achievement of specified regulatory milestones. In addition, the Company may be required to pay Eisai royalties at a percentage in the mid-teens on worldwide net sales of any EZH2 product, excluding net sales in Japan.

Collaborations

Collaborations	6 Months Ended
	Jun. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborations	7. Collaborations Eisai In April 2011, the Company entered into a collaboration and license agreement with Eisai under which the Company granted Eisai an exclusive worldwide license to its small molecule HMT inhibitors directed to the EZH2 HMT, including the Company’s product candidate tazemetostat, while retaining an opt-in right to co-develop, co-commercialize and share profits with Eisai as to licensed products in the United States (the “original agreement”). Additionally, as part of the research collaboration, the Company provided research and development services related to the licensed compounds through December 31, 2014. The Company recognized $2.0 million and $3.6 million of collaboration revenue in the three and six months ended June 30, 2014, respectively, under the original agreement. As of December 31, 2014, the Company had completed its performance obligations under the original agreement. Accordingly, the Company had no remaining deferred revenue as of December 31, 2014 related to the original agreement. In March 2015, the Company entered into an amended and restated collaboration and license agreement with Eisai, under which the Company reacquired worldwide rights, excluding Japan, to its EZH2 program, including tazemetostat. Under the amended and restated collaboration and license agreement, the Company is responsible for global development, manufacturing and commercialization outside of Japan of tazemetostat and any other EZH2 product candidates, with Eisai retaining development and commercialization rights in Japan, as well as a right to elect to manufacture tazemetostat and any other EZH2 product candidates in Japan. Under the original agreement, Eisai was solely responsible for funding all research, development and commercialization costs for EZH2 compounds. Under the amended and restated collaboration and license agreement, the Company is solely responsible for funding global development, manufacturing and commercialization costs for EZH2 compounds outside of Japan, including up to $15.5 million of the remaining development costs due under a Roche companion diagnostic agreement, and Eisai is solely responsible for funding Japan-specific development and commercialization costs for EZH2 compounds. The Company recorded the reacquisition of worldwide rights, excluding Japan, to the EZH2 program, including tazemetostat, under the amended and restated collaboration and license agreement with Eisai as an acquisition of an in-process research and development asset. As this asset was acquired without corresponding processes or activities that would constitute a business, has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use, the Company recorded the $40.0 million upfront payment made to Eisai in March 2015 as research and development expense in the condensed consolidated statements of operations and comprehensive loss for the three months ended March 31, 2015. The Company has also agreed to pay Eisai up to $20.0 million in clinical development milestone payments, up to $50.0 million in regulatory milestone payments and royalties at a percentage in the mid-teens on worldwide net sales of any EZH2 product, excluding net sales in Japan. The Company is eligible to receive from Eisai royalties at a percentage in the mid-teens on net sales of any EZH2 product in Japan. Celgene In April 2012, the Company entered into a collaboration and license agreement with Celgene Corporation and Celgene International Sàrl, an affiliate of Celgene Corporation (Celgene Corporation and its affiliated entities are collectively referred to as “Celgene”), to discover, develop and commercialize, in all countries other than the United States, small molecule HMT inhibitors targeting the DOT1L HMT, including pinometostat (also known as EPZ-5676), and any HMT targets from its product platform, other than the EZH2 HMT including tazemetostat and targets covered by its collaboration with Glaxo Group Limited, an affiliate of GlaxoSmithKline (“GSK”), which the Company refers to as the available targets. On July 8, 2015, the Company entered into an amendment and restatement of its collaboration and license agreement with Celgene Corporation and Celgene RIVOT Ltd., an affiliate of Celgene Corporation. Refer to Note 11, Subsequent Events 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 basis, to exclusively license, for all countries other than the United States, rights to small molecule HMT inhibitors targeting any HMT targets, other than the EZH2 HMT including tazemetostat and targets covered by the Company’s collaboration and license agreement dated January 8, 2011 with GSK. Under the original agreement, Celgene’s option was exercisable during an option period that would have expired on July 9, 2015. 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 $6.7 million of global development co-funding through June 30, 2015. The Company was also eligible to receive $35.0 million in an additional clinical development milestone payment and up to $100.0 million in regulatory milestone payments related to DOT1L as well as up to $65.0 million in payments, including a combination of clinical development milestone payments and an option exercise fee for each available target as to which Celgene had the right to exercise its option during an initial option period that ended in July 2015 (each a “selected target”), and up to $100.0 million in regulatory milestone payments for each selected target. As to DOT1L and each selected target, the Company retained all product rights in the United States and was eligible to receive royalties for each target at defined percentages ranging from the mid-single digits to the mid-teens on net product sales outside of the United States subject to reduction in specified circumstances The Company is 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 will equally co-fund global development and each party will solely fund territory-specific development costs for its territory. Collaboration Revenue Through June 30, 2015, in addition to amounts allocated to Celgene’s purchase of shares of the Company’s series C redeemable convertible preferred stock, the Company had recorded a total of $99.8 million in cash and accounts receivable under the Celgene agreement, including the $3.0 million implied premium on Celgene’s purchase of shares of the Company’s series C redeemable convertible preferred stock. Through June 30, 2015, the Company has recognized $71.5 million of collaboration revenue related to this agreement, including $0.1 million and $0.2 million in the three and six months ended June 30, 2015, respectively, and $1.2 million and $2.9 million in the three and six months ended June 30, 2014, respectively, and $6.7 million of global development co-funding as a reduction to research and development expense, including $0.4 million and $0.9 million in the three and six months ended June 30, 2015, respectively, and $0.9 million and $1.3 million in the three and six months ended June 30, 2014, respectively, in the condensed consolidated statements of operations and comprehensive loss. As of June 30, 2015 and December 31, 2014, the Company had deferred revenue of $21.6 million and $21.7 million, respectively, related to this agreement. 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 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, $15.0 million of preclinical research and development milestone payments and $9.0 million for research and development services. The Company is eligible to receive up to $18.0 million in additional preclinical research and development milestone payments, up to $109.0 million in clinical development milestone payments, up to $275.0 million in regulatory milestone payments and up to $218.0 million in sales-based milestone payments. In addition, GSK is required to pay the Company royalties, at percentages from the mid-single digits to the low double-digits, on a licensed product-by-licensed product basis, on worldwide net product sales, subject to reduction in specified circumstances. 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 GSK. Due to the varying stages of development of each licensed target, the Company is not able to determine the next milestone that might be achieved under this agreement, if any. GSK became solely responsible for development and commercialization for each licensed target in the collaboration when the research term ended on January 8, 2015. Collaboration Revenue Through June 30, 2015, the Company received a total of $53.0 million in cash under the GSK agreement, which the Company recognized as collaboration revenue in the condensed consolidated statements of operations and comprehensive loss, including $0.6 million and $1.4 million in the three and six months ended June 30, 2015, respectively, and $6.3 million and $16.4 million in the three and six months ended June 30, 2014, respectively, including a $1.0 million preclinical research and development milestone achieved and recognized as collaboration revenue in the three months ended June 30, 2014 and an additional $2.0 million preclinical research and development milestone achieved and recognized as collaboration revenue in the six months ended June 30, 2014. As of December 31, 2014, the Company had deferred revenue of $1.4 million related to this agreement, which was fully recognized as collaboration revenue by June 30, 2015. Companion Diagnostics Roche In December 2012, Eisai and the Company entered into an agreement with Roche under which Eisai and the Company agreed to fund Roche’s development of a companion diagnostic to identify patients who possess certain point mutations in EZH2. At the same time, Eisai and the Company entered into a letter agreement pursuant to which Eisai agreed to be responsible for the development costs under the Roche agreement. In October 2013, this agreement was amended to include additional point mutations in EZH2. Under the terms of the amended agreement, Roche is to be paid up to a total of $21.5 million to develop and to make commercially available the companion diagnostic. In connection with the March 2015 execution of the amended and restated collaboration and license agreement with Eisai, the Company and Eisai entered into an amended and restated letter agreement, pursuant to which the Company agreed to be responsible for up to $15.5 million of the remaining development costs under the agreement with Roche. Eisai continues to be responsible for up to $1.0 million of the remaining Japan-specific development costs under the agreement with Roche.

Stock-Based Compensation

Stock-Based Compensation	6 Months Ended
	Jun. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	8. Stock-Based Compensation Total stock-based compensation expense related to stock options, restricted stock units and the employee stock purchase plan was $2.5 million and $1.6 million for the three months ended June 30, 2015 and 2014, respectively, and $4.9 million and $3.1 million for the six months ended June 30, 2015 and 2014, respectively. Stock-based compensation expense is classified in the condensed consolidated statements of operations and comprehensive loss as follows: Three Months Ended June 30, Six Months Ended June 30, 2015 2014 2015 2014 (In thousands) Research and development $ 1,437 $ 815 $ 2,795 $ 1,496 General and administrative 1,057 817 2,115 1,599 Total $ 2,494 $ 1,632 $ 4,910 $ 3,095 Stock Options The weighted-average fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $12.46 and $17.15 per option for those options granted during the three months ended June 30, 2015 and 2014, respectively, and $14.68 and $22.53 per option for those options granted during the six months ended June 30, 2015 and 2014, respectively. Key assumptions used to apply this pricing model were as follows: Six Months Ended June 30, 2015 2014 Risk-free interest rate 1.5 % 1.6 % Expected life of options 6.0 years 6.0 years Expected volatility of underlying stock 84.2 % 93.4 % Expected dividend yield 0.0 % 0.0 % The following is a summary of stock option activity for the six months ended June 30, 2015: Number of Weighted Weighted Aggregate (In years) (In thousands) Outstanding at December 31, 2014 2,959,506 $ 10.66 Granted 763,557 20.65 Exercised (100,931 ) 2.13 Forfeited or expired (273,385 ) 16.78 Outstanding at June 30, 2015 3,348,747 $ 12.70 7.1 $ 41,909 Exercisable at June 30, 2015 1,649,769 $ 5.47 5.4 $ 31,791 As of June 30, 2015, there was $20.6 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 2.7 years. Restricted Stock Units The following is a summary of restricted stock unit activity for the six months ended June 30, 2015: Number of Weighted Average Outstanding at December 31, 2014 — $ — Granted 37,313 18.49 Outstanding at June 30, 2015 37,313 $ 18.49 As of June 30, 2015, there was $1.1 million of unrecognized compensation cost related to restricted stock units that are expected to vest, including $0.6 million of unrecognized compensation cost related to restricted stock units to be issued in the first quarter of 2016, pursuant to a February 2015 employment agreement with the Company’s chief financial officer, for which service is currently being provided. These costs are expected to be recognized over a weighted average remaining vesting period of 3.6 years.

Loss Per Share

Loss Per Share	6 Months Ended
	Jun. 30, 2015
Earnings Per Share [Abstract]
Loss Per Share	9. 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, 2015 2014 2015 2014 (In thousands except per share data) Net loss $ (25,759 ) $ (13,386 ) $ (87,085 ) $ (20,270 ) Weighted average shares outstanding 41,087 33,156 38,056 32,064 Basic and diluted loss per share allocable to common stockholders $ (0.63 ) $ (0.40 ) $ (2.29 ) $ (0.63 ) In February 2014, the Company issued 3,673,901 shares of common stock in connection with a public offering. In March 2015, the Company issued 6,000,000 shares of common stock in connection with a public offering. In April 2015, the Company issued and sold an additional 701,448 shares of common stock in connection with the March 2015 public offering at a price of $20.75 per share pursuant to the underwriters’ option to purchase additional shares that the Company granted in connection with such public offering. The issuance of these shares contributed to a significant increase in the Company’s shares outstanding, to 41,240,338 shares as of June 30, 2015, and in the weighted average shares outstanding for the three and six months ended June 30, 2015 when compared to the comparable prior year periods and is expected to continue to impact the year-over-year comparability of the Company’s (loss) earnings per share calculations through 2015. 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, 2015 2014 2015 2014 (In thousands) Stock options 3,349 4,074 3,349 4,074 Unvested restricted stock units 37 — 37 — Shares issuable under employee stock purchase plan 7 7 7 7 3,393 4,081 3,393 4,081

Related Party Transactions

Related Party Transactions	6 Months Ended
	Jun. 30, 2015
Related Party Transactions [Abstract]
Related Party Transactions	10. Related Party Transactions The Company’s collaboration partner Celgene has made a series of equity investments in the Company, owning 3,674,640 shares of common stock representing 8.2% of the Company’s fully diluted equity and 8.9% of the voting interests of the Company as of June 30, 2015. Refer to Note 7, Collaborations Subsequent Event Under the Celgene collaboration agreement, the Company recognized $0.1 million and $0.2 million of collaboration revenue in the three and six months ended June 30, 2015, respectively, and $1.2 million and $2.9 million of collaboration revenue in the three and six months ended June 30, 2014, respectively. As of June 30, 2015 and December 31, 2014, the Company recorded $21.6 million and $21.7 million of deferred revenue related to the Celgene collaboration arrangement, respectively. Additionally, in the three and six months ended June 30, 2015, the Company recorded $0.4 million and $0.9 million, respectively, and $0.9 million and $1.3 million in the three and six months ended June 30, 2014, respectively, in global development co-funding from Celgene. As of June 30, 2015 and December 31, 2014, the Company recorded accounts receivable of $0.5 million and $1.1 million, respectively, related to this collaboration arrangement.

Subsequent Event

Subsequent Event	6 Months Ended
	Jun. 30, 2015
Subsequent Events [Abstract]
Subsequent Event	11. Subsequent Event On July 8, 2015, the Company entered into an amendment and restatement of its collaboration and license agreement dated April 2, 2012 with Celgene. Refer to Note 7, Collaborations 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 DOT1L, including pinometostat, and an option, on a target-by-target basis, to exclusively license, for all countries other than the United States, rights to small molecule HMT inhibitors targeting any other HMT targets, excluding the EZH2 HMT including tazemetostat and targets covered by the Company’s collaboration and license agreement with GSK. Under the original agreement, Celgene’s option was exercisable during an option period that would have expired on July 9, 2015. Under the amended and restated collaboration and license agreement: • Celgene retains its exclusive license to small molecule HMT inhibitors targeting DOT1L, including pinometostat, • Celgene’s other option rights have been narrowed to 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 have been expanded to include the United States, with the exclusive license to the third Option Target continuing to be for all countries other than the United States, • Celgene’s option period has been extended for each of the Option Targets and 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 development milestone-based license payment, • 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 development milestone-based license payment, and • The Company’s research and development obligations with respect to each Option Target under the amended agreement have been 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 rights, the Company’s research and development obligations have been expanded to include the completion of a Phase 1 clinical trial as to each Option Target following Celgene’s exercise of its option at IND filing. Under the amended 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. 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 or royalty payments from Celgene. The amended 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 right, for the DOT1L target and each of the Option Targets, the Company is required to conduct and solely fund development costs of the Phase 1 clinical trials for HMT inhibitors directed to such targets, including for pinometostat. After the completion of Phase 1 development, as to DOT1L and the Option Target for which the Company retains U.S. rights, Celgene and the Company will equally co-fund global development and each party will solely fund territory-specific development costs for its respective territory; and, as to the other two Option Targets, after the completion of Phase 1 development, Celgene will solely fund all development costs on a worldwide basis.

Summary of Significant Accoun17

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2015
Accounting Policies [Abstract]
Property and Equipment	Property and Equipment The Company records property and equipment at cost. Property and equipment acquired under a capital lease is recorded at the lesser of the present value of the minimum lease payments under the capital lease or the fair value of the leased property at lease inception. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives: Asset Category Useful Lives Laboratory equipment 5 - 20 years Office furniture and equipment 3 - 10 years or term of respective lease, if shorter Leasehold improvements 3 - 10 years or term of respective lease, if shorter Amortization of capital lease assets is included in depreciation expense. The Company capitalizes expenditures for new property and equipment and improvements to existing facilities and charges the cost of maintenance to expense. The Company eliminates the cost of property retired or otherwise disposed of, along with the corresponding accumulated depreciation, from the related accounts, and the resulting gain or loss is reflected in the results of operations. Additionally, the Company updated its accounting policies as a result of the amended and restated collaboration and license agreement the Company executed with Eisai Co., Ltd. (“Eisai”), pursuant to which the Company recorded the reacquisition of worldwide rights, excluding Japan, to its EZH2 program, including tazemetostat (also known as EPZ-6438), as an acquisition of in-process research and development.
Acquired In-Process Research and Development	Acquired In-Process Research and Development The Company records upfront payments that relate to the acquisition of a development-stage product candidate as research and development expense in the period in which they are incurred, provided that the acquired development-stage product candidate did not also include processes or activities that would constitute a business, the product candidate has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use. There have been no other material changes to the significant accounting policies previously disclosed in the Company’s Annual Report.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers Revenue Recognition In August 2014, the FASB issued ASU No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern Presentation of Financial Statements—Going Concern In April 2015, the FASB issued ASU No. 2015-05, Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement Internal-Use Software In June 2015, the FASB issued ASU No. 2015-10, Technical Corrections and Improvements

Summary of Significant Accoun18

Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2015
Accounting Policies [Abstract]
Useful Lives for Property, Plant and Equipment	The Company records property and equipment at cost. Property and equipment acquired under a capital lease is recorded at the lesser of the present value of the minimum lease payments under the capital lease or the fair value of the leased property at lease inception. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives: Asset Category Useful Lives Laboratory equipment 5 - 20 years Office furniture and equipment 3 - 10 years or term of respective lease, if shorter Leasehold improvements 3 - 10 years or term of respective lease, if shorter

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2015
Fair Value Disclosures [Abstract]
Summary of Company's Financial Assets Recognized at Fair Value	As of June 30, 2015 and December 31, 2014, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of June 30, 2015 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 196,924 $ 196,924 $ — $ — Total $ 196,924 $ 196,924 $ — $ — Fair Value as of December 31, 2014 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 184,257 $ 184,257 $ — $ — Total $ 184,257 $ 184,257 $ — $ —

Accrued Expenses (Tables)

Accrued Expenses (Tables)	6 Months Ended
	Jun. 30, 2015
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following: June 30, December 31, (In thousands) Employee compensation and benefits $ 2,068 $ 2,623 Research and development and professional expenses 7,408 4,420 Accrued expenses $ 9,476 $ 7,043

Commitments and Contingencies (

Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments	Future minimum equipment lease payments under this capital lease, net of imputed interest, as of June 30, 2015 are as follows: Future minimum lease payments in year ending December 31: (In thousands) 2015 $ 332 2016 665 2017 665 2018 111 Total future minimum lease payments 1,773 Less: Amount representing imputed interest on equipment lease (222 ) Capital lease obligation $ 1,551

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	6 Months Ended
	Jun. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Stock-Based Compensation Expense	Stock-based compensation expense is classified in the condensed consolidated statements of operations and comprehensive loss as follows: Three Months Ended June 30, Six Months Ended June 30, 2015 2014 2015 2014 (In thousands) Research and development $ 1,437 $ 815 $ 2,795 $ 1,496 General and administrative 1,057 817 2,115 1,599 Total $ 2,494 $ 1,632 $ 4,910 $ 3,095
Assumptions Used to Applying Pricing Model	Key assumptions used to apply this pricing model were as follows: Six Months Ended June 30, 2015 2014 Risk-free interest rate 1.5 % 1.6 % Expected life of options 6.0 years 6.0 years Expected volatility of underlying stock 84.2 % 93.4 % Expected dividend yield 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, 2015: Number of Weighted Weighted Aggregate (In years) (In thousands) Outstanding at December 31, 2014 2,959,506 $ 10.66 Granted 763,557 20.65 Exercised (100,931 ) 2.13 Forfeited or expired (273,385 ) 16.78 Outstanding at June 30, 2015 3,348,747 $ 12.70 7.1 $ 41,909 Exercisable at June 30, 2015 1,649,769 $ 5.47 5.4 $ 31,791
Summary of Restricted Stock Activity	The following is a summary of restricted stock unit activity for the six months ended June 30, 2015: Number of Weighted Average Outstanding at December 31, 2014 — $ — Granted 37,313 18.49 Outstanding at June 30, 2015 37,313 $ 18.49

Loss Per Share (Tables)

Loss Per Share (Tables)	6 Months Ended
	Jun. 30, 2015
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, 2015 2014 2015 2014 (In thousands except per share data) Net loss $ (25,759 ) $ (13,386 ) $ (87,085 ) $ (20,270 ) Weighted average shares outstanding 41,087 33,156 38,056 32,064 Basic and diluted loss per share allocable to common stockholders $ (0.63 ) $ (0.40 ) $ (2.29 ) $ (0.63 )
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, 2015 2014 2015 2014 (In thousands) Stock options 3,349 4,074 3,349 4,074 Unvested restricted stock units 37 — 37 — Shares issuable under employee stock purchase plan 7 7 7 7 3,393 4,081 3,393 4,081

Overview and Basis of Present24

Overview and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			6 Months Ended
	Apr. 30, 2015	Mar. 31, 2015	Feb. 28, 2014	Jun. 30, 2015	Jun. 30, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Common stock issued during period	701,448	6,000,000	3,673,901
Common stock price per share	$ 20.75	$ 20.75
Net proceeds from public offering	$ 13,700	$ 117,000		$ 130,712	$ 101,283

Summary of Significant Accoun25

Summary of Significant Accounting Policies - Estimated Useful Lives of Assets Acquired (Detail)	6 Months Ended
	Jun. 30, 2015
Laboratory Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	5 years
Laboratory Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	20 years
Office Furniture and Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	3 years
Office Furniture and Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	10 years
Leasehold Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	3 years
Leasehold Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	10 years

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Company's Financial Assets Recognized at Fair Value (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 196,924	$ 184,257
Total	196,924	184,257
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	196,924	184,257
Total	196,924	184,257
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0	0
Total	0	0
Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0	0
Total	$ 0	$ 0

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Accrued Liabilities, Current [Abstract]
Employee compensation and benefits	$ 2,068	$ 2,623
Research and development and professional expenses	7,408	4,420
Accrued expenses	$ 9,476	$ 7,043

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Tax Disclosure [Abstract]
Federal income tax provision or benefit	$ 0	$ 0	$ 0	$ 0
State income tax provision or benefit	$ 0	0	$ 0	0
Income tax expense		$ 100,000		$ 100,000

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Detail) $ in Thousands	Jun. 30, 2015USD ($)
Contractual Obligation, Fiscal Year Maturity Schedule [Abstract]
2,015	$ 332
2,016	665
2,017	665
2,018	111
Total future minimum lease payments	1,773
Less: Amount representing imputed interest on equipment lease	(222)
Capital lease obligation	$ 1,551

Commitments and Contingencies30

Commitments and Contingencies - Additional Information (Detail) - Jun. 30, 2015 $ in Millions	USD ($)ft²
Operating Lease [Member] | North Carolina [Member]
Other Commitments [Line Items]
Lease expiration date	Jul. 31, 2017
Total future minimum lease payments	$ 0.2
Rental space | ft²	4,000
Roche [Member]
Other Commitments [Line Items]
Remaining development costs	$ 15.5
Remaining development costs	1
Eisai [Member]
Other Commitments [Line Items]
Clinical development milestone payments	20
Regulatory milestone payments	$ 50

Collaborations - Additional Inf

Collaborations - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended		36 Months Ended	39 Months Ended	54 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue			$ 736,000	$ 9,494,000	$ 1,647,000	$ 22,885,000
Upfront payment made					40,000,000
Eisai [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue				2,000,000		3,600,000
Deferred revenue										$ 0
Upfront payment made	$ 40,000,000
Clinical development milestone payments obligation			20,000,000		20,000,000			$ 20,000,000	$ 20,000,000
Regulatory milestone payments obligation			50,000,000		50,000,000			50,000,000	50,000,000
Celgene [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue			100,000	1,200,000	200,000	2,900,000		71,500,000
Deferred revenue			21,600,000		21,600,000			21,600,000	21,600,000	21,700,000
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			400,000	900,000	900,000	1,300,000		6,700,000
Cash and accounts receivable								99,800,000
Convertible preferred stock issued, premium value								3,000,000
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
GSK [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue			600,000	6,300,000	1,400,000	16,400,000
Deferred revenue										$ 1,400,000
Upfront payment recorded		$ 3,000,000					$ 20,000,000
Clinical development milestone payment			109,000,000		109,000,000			109,000,000	109,000,000
Additional milestone payments			275,000,000		275,000,000			275,000,000	275,000,000
Cash and accounts receivable									53,000,000
Fixed research funding received									6,000,000
Milestone payments received									15,000,000
Research and development services									9,000,000
Additional substantive preclinical research and development milestone payments			18,000,000		18,000,000			18,000,000	18,000,000
Sales-based milestone payments			218,000,000		218,000,000			218,000,000	218,000,000
Preclinical research and development milestone achieved				$ 1,000,000		$ 2,000,000
Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Remaining unpaid milestone payments			15,500,000		15,500,000			15,500,000	15,500,000
Total development milestone payments			21,500,000		21,500,000			21,500,000	21,500,000
Remaining unpaid milestone payments			$ 1,000,000		$ 1,000,000			$ 1,000,000	$ 1,000,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense related to stock options and restricted stock	$ 2,494	$ 1,632	$ 4,910	$ 3,095
Weighted-average fair value of options granted	$ 12.46	$ 17.15	$ 14.68	$ 22.53
Unrecognized compensation cost related to stock options	$ 20,600		$ 20,600
Expected weighted average period for recognition of compensation cost			2 years 8 months 12 days
Unvested Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected weighted average period for recognition of compensation cost			3 years 7 months 6 days
Unrecognized compensation cost related to restricted stock units	1,100		$ 1,100
Unrecognized compensation cost related to restricted stock units to be issued	$ 600		$ 600

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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 2,494	$ 1,632	$ 4,910	$ 3,095
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	1,437	815	2,795	1,496
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 1,057	$ 817	$ 2,115	$ 1,599

Stock-Based Compensation - Assu

Stock-Based Compensation - Assumptions Used to Applying Pricing Model (Detail) - Employee Stock Option [Member]	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.50%	1.60%
Expected life of options	6 years	6 years
Expected volatility of underlying stock	84.20%	93.40%
Expected dividend yield	0.00%	0.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - Jun. 30, 2015 - USD ($) $ / shares in Units, $ in Thousands	Total
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Options, Outstanding, Beginning balance	2,959,506
Number of Options, Granted	763,557
Number of Options, Exercised	(100,931)
Number of Options, Forfeited or expired	(273,385)
Number of Options, Outstanding, Ending balance	3,348,747
Number of Options, Exercisable	1,649,769
Weighted Average Exercise Price per Share, Outstanding, Beginning balance	$ 10.66
Weighted Average Exercise Price per Share, Granted	20.65
Weighted Average Exercise Price per Share, Exercised	2.13
Weighted Average Exercise Price per Share, Forfeited or expired	16.78
Weighted Average Exercise Price per Share, Outstanding, Ending balance	12.70
Weighted Average Exercise Price per Share, Exercisable	$ 5.47
Weighted Average Remaining Contractual Term (In Years), Outstanding	7 years 1 month 6 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	5 years 4 months 24 days
Aggregate Intrinsic Value, Outstanding	$ 41,909
Aggregate Intrinsic Value, Exercisable	$ 31,791

Stock-Based Compensation - Su36

Stock-Based Compensation - Summary of Restricted Stock Activity (Detail) - 6 months ended Jun. 30, 2015 - Unvested Restricted Stock Units [Member] - $ / shares	Total
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Outstanding, Beginning balance	0
Number of Units, Granted	37,313
Number of Units, Outstanding, Ending balance	37,313
Weighted Average Grant Date Fair Value per Units, Beginning balance	$ 0
Weighted Average Grant Date Fair Value per Units, Granted	18.49
Weighted Average Grant Date Fair Value per Units, Ending balance	$ 18.49

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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share [Abstract]
Net loss	$ (25,759)	$ (13,386)	$ (87,085)	$ (20,270)
Weighted average shares outstanding	41,087	33,156	38,056	32,064
Basic and diluted loss per share allocable to common stockholders	$ (0.63)	$ (0.40)	$ (2.29)	$ (0.63)

Loss Per Share - Additional Inf

Loss Per Share - Additional Information (Detail) - $ / shares	1 Months Ended
	Apr. 30, 2015	Mar. 31, 2015	Feb. 28, 2014	Jun. 30, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Common stock issued during period	701,448	6,000,000	3,673,901
Common stock, shares outstanding				41,240,338	34,426,012
Common stock price per share	$ 20.75	$ 20.75

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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	3,393	4,081	3,393	4,081
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	3,349	4,074	3,349	4,074
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	37		37
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	7	7	7	7

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Related Party Transaction [Line Items]
Equity investments in common stock	41,240,338		41,240,338		34,426,012
Beneficial Owner [Member]
Related Party Transaction [Line Items]
Equity investments in common stock	3,674,640		3,674,640
Diluted equity	8.20%		8.20%
Percentage of voting interests	8.90%		8.90%
Collaboration revenue related to agreement	$ 0.1	$ 1.2	$ 0.2	$ 2.9
Deferred revenue	21.6		21.6		$ 21.7
Global development co-funding from Celgene	0.4	$ 0.9	0.9	$ 1.3
Accounts receivable related to collaboration arrangement	$ 0.5		$ 0.5		$ 1.1

Subsequent Event - Additional I

Subsequent Event - Additional Information (Detail) - Celgene [Member]	Jul. 08, 2015USD ($)OptionTargets	Jun. 30, 2015	Jun. 30, 2015USD ($)
Subsequent Event [Line Items]
Agreement option expiry date		Jul. 9, 2015
Company received upfront payment			$ 65,000,000
Subsequent Event [Member]
Subsequent Event [Line Items]
Number of option targets | OptionTargets	3
Subsequent Event [Member] | Minimum [Member]
Subsequent Event [Line Items]
Option targets under amended agreement extended period	3 years
Subsequent Event [Member] | Worldwide [Member]
Subsequent Event [Line Items]
Number of option targets | OptionTargets	2
Subsequent Event [Member] | United States [Member]
Subsequent Event [Line Items]
Number of option targets | OptionTargets	1
Subsequent Event [Member] | Option Targets [Member]
Subsequent Event [Line Items]
Company received upfront payment	$ 10,000,000
Subsequent Event [Member] | Option Targets [Member] | Maximum [Member]
Subsequent Event [Line Items]
Development milestone and license payments under agreement	75,000,000
Additional milestone payments	365,000,000
Sales-based milestone payments	$ 170,000,000