Epizyme (EPZM) 10-Q 9 Nov 21 2021 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Nov. 04, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Trading Symbol	EPZM
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common stock, $0.0001 par value
Entity Registrant Name	EPIZYME, INC.
Entity Central Index Key	0001571498
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Document Quarterly Report	true
Document Transition Report	false
Entity Common Stock Shares Outstanding		103,895,022
Entity Shell Company	false
Entity File Number	001-35945
Entity Tax Identification Number	26-1349956
Entity Address, Address Line One	400 Technology Square, 4th Floor
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Incorporation, State or Country Code	DE
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	229-5872

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 124,517	$ 168,215
Marketable securities	96,776	205,391
Accounts receivable, net	3,332	3,105
Inventory	1,521	10,461
Prepaid expenses and other current assets	21,455	17,921
Total current assets	247,601	405,093
Property and equipment, net	1,739	2,152
Operating lease assets	21,141	17,305
Intangible assets, net	43,887	47,002
Restricted cash and other assets	17,903	2,021
Total assets	332,271	473,573
Current liabilities:
Accounts payable	9,862	10,163
Accrued expenses	30,922	28,572
Current portion of operating lease obligation	3,951	4,665
Total current liabilities	44,735	43,400
Operating lease obligation, net of current portion	19,741	15,409
Related party long-term debt, net of debt discount	216,254	215,670
Other long-term liabilities	0	21
Deferred revenue	11,950	0
Related party liability related to sale of future royalties	15,581	14,176
Warrants to purchase common stock	8,630	0
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.0001 par value; 5,000 shares authorized; 338 shares issued and outstanding (equivalent to 3,378 shares of common stock upon conversion at a 10:1 ratio)	36,127	36,127
Common stock, $0.0001 par value; 225,000 shares and 150,000 shares authorized, respectively; 103,662 shares and 101,627 shares issued and outstanding, respectively	10	10
Additional paid-in capital	1,168,430	1,137,470
Accumulated other comprehensive income (loss)	(3)	3
Accumulated deficit	(1,189,184)	(988,713)
Total stockholders’ equity	15,380	184,897
Total liabilities and stockholders’ equity	$ 332,271	$ 473,573

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares shares in Thousands	Sep. 30, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000	5,000
Preferred stock, shares issued	338	338
Preferred stock, shares outstanding	338	338
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	225,000	150,000
Common stock, shares issued	103,662	101,627
Common stock, shares outstanding	103,662	101,627
Common stock conversion ratio	10.00%	10.00%
Common stock upon conversion	3,378	3,378

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenue:
Total revenue	$ 5,201	$ 3,566	$ 25,844	$ 7,387
Operating expenses:
Cost of revenue	1,999	1,608	7,345	3,244
Research and development	34,549	25,738	102,110	77,253
Selling, general and administrative	32,793	30,575	103,095	90,161
Total operating expenses	69,341	57,921	212,550	170,658
Operating loss	(64,140)	(54,355)	(186,706)	(163,271)
Other (expense) income, net:
Interest expense , net	(5,645)	(1,364)	(16,703)	(1,177)
Other expense, net	(24)	(42)	(70)	(105)
Change in fair value of warrants to purchase common stock	4,420		4,420
Related party non-cash interest expense related to sale of future royalties	(445)	(312)	(1,412)	(908)
Other expense, net	(1,694)	(1,718)	(13,765)	(2,190)
Net loss	(65,834)	(56,073)	(200,471)	(165,461)
Other comprehensive income (loss):
Unrealized (loss) gain on available-for-sale securities	(1)	(340)	(6)	101
Comprehensive loss	$ (65,835)	$ (56,413)	$ (200,477)	$ (165,360)
Net loss per share attributable to common stockholders:
Basic	$ (0.64)	$ (0.55)	$ (1.96)	$ (1.64)
Diluted	$ (0.64)	$ (0.55)	$ (1.96)	$ (1.64)
Weighted-average common shares outstanding used in net loss per share attributable to common stockholders:
Basic	102,513	101,512	102,123	100,747
Diluted	102,513	101,512	102,123	100,747
Product [Member]
Revenue:
Total revenue	$ 5,186	$ 3,445	$ 19,364	$ 6,963
Collaboration and Other Revenue [Member]
Revenue:
Collaboration and other revenue	$ 15	$ 121	$ 6,480	$ 424

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (200,471)	$ (165,461)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	3,915	2,674
Share-based compensation expense	19,903	21,188
Amortization of discount (premium) on investments	923	(262)
Amortization of debt discount	584	260
Change in fair value of warrant liability	(4,420)
Loss on disposal of property and equipment		19
Non-cash interest expense associated with the sale of future royalties	1,412	908
Changes in operating assets and liabilities:
Accounts receivable	(227)	(2,044)
Inventory, current and noncurrent	(6,483)	(8,157)
Prepaid expenses and other current assets	(3,267)	(2,987)
Accounts payable	(308)	(1,296)
Accrued expenses	2,300	263
Deferred revenue	11,950	7
Operating lease assets	(3,836)	2,791
Operating lease liabilities	3,618	(1,099)
Other assets and liabilities	(480)	(25)
Net cash used in operating activities	(174,887)	(153,221)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of available-for-sale securities	(203,895)	(156,938)
Maturities of available-for-sale securities	311,581	212,421
Purchase of intangible asset		(50,000)
Purchases of property and equipment	(387)	(594)
Net cash (used in) provided by investing activities	107,299	4,889
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from the issuance of shares under employee stock purchase plan	8,261
Payment of offering costs	(217)	(79)
Proceeds from the issuance of debt		45,000
Proceeds from Issuance of Warrants	13,050
Payment of debt issuance costs		(93)
Proceeds from the issuance of common stock in connection with the exercise of the Put Option, net of financing costs		49,915
Proceeds from stock options exercised	916	6,275
Proceeds from the issuance of shares under employee stock purchase plan	1,880	1,254
Net cash provided by financing activities	23,890	102,272
Net decrease in cash, cash equivalents and restricted cash	(43,698)	(46,060)
Cash, cash equivalents and restricted cash, beginning of period	169,724	140,991
Cash, cash equivalents and restricted cash, end of period	126,026	94,931
SUPPLEMENTAL CASH FLOW INFORMATION:
Interest paid	16,266	3,635
Unpaid offering costs	50
Property and equipment included in accounts payable or accruals		$ 42

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Put Option [Member]	Series A Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member]Put Option [Member]	Common Stock [Member]Series A Convertible Preferred Stock [Member]	Preferred Stock [Member]	Preferred Stock [Member]Series A Convertible Preferred Stock [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member]Put Option [Member]	Additional Paid-In Capital [Member]Series A Convertible Preferred Stock [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning Balance, Value at Dec. 31, 2019	$ 331,137			$ 10			$ 37,432		$ 1,050,695			$ (757,019)	$ 19
Beginning Balance, Shares at Dec. 31, 2019				97,783,476			350,000
Issuance of common stock, Value		$ 49,915								$ 49,915
Issuance of common stock, Shares					2,500,000
Issuance of Series A Convertible Preferred Stock, net of commissions and beneficial conversion charge, Value								$ (1,305)			$ 1,305
Issuance of Series A Convertible Preferred Stock, net of commissions and beneficial conversion charge, Shares						122,000		(12,200)
Exercise of stock options and vesting of restricted stock units, Value	3,140								3,140
Exercise of stock options and vesting of restricted stock units, Shares				579,919
Stock-based compensation	6,475								6,475
Issuance of shares under employee stock purchase plan, Value	646								646
Issuance of shares under employee stock purchase plan, Shares				60,576
Issuance of shares of common stock in lieu of board fees	35								35
Issuance of shares of common stock in lieu of board fees, Shares				1,404
Unrealized (loss) gain on available-for-sale securities	(94)												(94)
Net loss	(50,937)											(50,937)
Ending Balance, Value at Mar. 31, 2020	340,317			$ 10			$ 36,127		1,112,211			(807,956)	(75)
Ending Balance, Shares at Mar. 31, 2020				101,047,375			337,800
Beginning Balance, Value at Dec. 31, 2019	331,137			$ 10			$ 37,432		1,050,695			(757,019)	19
Beginning Balance, Shares at Dec. 31, 2019				97,783,476			350,000
Unrealized (loss) gain on available-for-sale securities	101
Net loss	(165,461)
Ending Balance, Value at Sep. 30, 2020	244,409			$ 10			$ 36,127		1,130,632			(922,480)	120
Ending Balance, Shares at Sep. 30, 2020				101,576,378			337,800
Beginning Balance, Value at Mar. 31, 2020	340,317			$ 10			$ 36,127		1,112,211			(807,956)	(75)
Beginning Balance, Shares at Mar. 31, 2020				101,047,375			337,800
Exercise of stock options and vesting of restricted stock units, Value	2,670								2,670
Exercise of stock options and vesting of restricted stock units, Shares				414,150
Stock-based compensation	8,257								8,257
Issuance of shares of common stock in lieu of board fees	35								35
Issuance of shares of common stock in lieu of board fees, Shares				2,229
Unrealized (loss) gain on available-for-sale securities	535												535
Net loss	(58,451)											(58,451)
Ending Balance, Value at Jun. 30, 2020	293,363			$ 10			$ 36,127		1,123,173			(866,407)	460
Ending Balance, Shares at Jun. 30, 2020				101,463,754			337,800
Exercise of stock options and vesting of restricted stock units, Value	465								465
Exercise of stock options and vesting of restricted stock units, Shares				55,417
Stock-based compensation	6,352								6,352
Issuance of shares under employee stock purchase plan, Value	608								608
Issuance of shares under employee stock purchase plan, Shares				55,055
Issuance of shares of common stock in lieu of board fees	34								34
Issuance of shares of common stock in lieu of board fees, Shares				2,152
Unrealized (loss) gain on available-for-sale securities	(340)												(340)
Net loss	(56,073)											(56,073)
Ending Balance, Value at Sep. 30, 2020	244,409			$ 10			$ 36,127		1,130,632			(922,480)	120
Ending Balance, Shares at Sep. 30, 2020				101,576,378			337,800
Beginning Balance, Value at Dec. 31, 2020	184,897			$ 10			$ 36,127		1,137,470			(988,713)	3
Beginning Balance, Shares at Dec. 31, 2020				101,627,070			337,800
Exercise of stock options and vesting of restricted stock units, Value	199								199
Exercise of stock options and vesting of restricted stock units, Shares				188,000
Stock-based compensation	6,943								6,943
Issuance of shares under employee stock purchase plan, Value	1,191								1,191
Issuance of shares under employee stock purchase plan, Shares				146,049
Issuance of shares of common stock in lieu of board fees	72								72
Issuance of shares of common stock in lieu of board fees, Shares				7,632
Unrealized (loss) gain on available-for-sale securities	3												3
Net loss	(70,274)											(70,274)
Ending Balance, Value at Mar. 31, 2021	123,031			$ 10			$ 36,127		1,145,875			(1,058,987)	6
Ending Balance, Shares at Mar. 31, 2021				101,968,751			337,800
Beginning Balance, Value at Dec. 31, 2020	184,897			$ 10			$ 36,127		1,137,470			(988,713)	3
Beginning Balance, Shares at Dec. 31, 2020				101,627,070			337,800
Unrealized (loss) gain on available-for-sale securities	(6)
Net loss	(200,471)
Ending Balance, Value at Sep. 30, 2021	15,380			$ 10			$ 36,127		1,168,430			(1,189,184)	(3)
Ending Balance, Shares at Sep. 30, 2021				103,661,881			337,800
Beginning Balance, Value at Mar. 31, 2021	123,031			$ 10			$ 36,127		1,145,875			(1,058,987)	6
Beginning Balance, Shares at Mar. 31, 2021				101,968,751			337,800
Issuance of common stock, Value	1,534								1,534
Issuance of common stock, Shares				182,866
Exercise of stock options and vesting of restricted stock units, Value	717								717
Exercise of stock options and vesting of restricted stock units, Shares				93,237
Stock-based compensation	6,679								6,679
Issuance of shares of common stock in lieu of board fees	39								39
Issuance of shares of common stock in lieu of board fees, Shares				4,663
Unrealized (loss) gain on available-for-sale securities	(8)												(8)
Net loss	(64,363)											(64,363)
Ending Balance, Value at Jun. 30, 2021	67,629			$ 10			$ 36,127		1,154,844			(1,123,350)	(2)
Ending Balance, Shares at Jun. 30, 2021				102,249,517			337,800
Issuance of common stock, Value	6,727								6,727
Issuance of common stock, Shares				1,247,599
Stock-based compensation	6,132								6,132
Issuance of shares under employee stock purchase plan, Value	689								689
Issuance of shares under employee stock purchase plan, Shares				157,195
Issuance of shares of common stock in lieu of board fees	38								38
Issuance of shares of common stock in lieu of board fees, Shares				7,570
Unrealized (loss) gain on available-for-sale securities	(1)												(1)
Net loss	(65,834)											(65,834)
Ending Balance, Value at Sep. 30, 2021	$ 15,380			$ 10			$ 36,127		$ 1,168,430			$ (1,189,184)	$ (3)
Ending Balance, Shares at Sep. 30, 2021				103,661,881			337,800

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2020
Commissions And Offering Costs	$ 217	$ 49
Put Option [Member]
Financing Cost			$ 85

The Company

The Company	9 Months Ended
	Sep. 30, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
The Company	1. The Company Epizyme, Inc. (collectively referred to with its wholly owned, controlled subsidiary, Epizyme Securities Corporation, as “Epizyme” or the “Company”) is a commercial-stage biopharmaceutical company that is committed to rewriting treatment for people with cancer through the discovery, development, and commercialization of novel epigenetic medicines. The Company aspires to change the standard of care for patients and physicians by developing targeted medicines with fundamentally new mechanisms of action directed at specific causes of hematological malignancies and solid tumors. Through September 30, 2021, in addition to revenues from product sales, the Company has raised an aggregate of $ 1,560.6 million to fund its operations. This includes $ 268.8 million of non-equity funding through its collaboration agreements, $ 368.1 million of funding, consisting of $ 150.0 million in equity funding received through agreements with RPI Finance Trust, or RPI, and $ 218.1 million in debt financing received through a loan agreement with BioPharma Credit Investments V (Master) LP and BPCR Limited Partnership (as transferee of BioPharma Credit Investments V (Master) LP’s interest as a lender) (the "Lenders"), $ 847.7 million from the sale of common stock and series A convertible preferred stock in the Company’s public offerings and at-the-market offerings and $ 76.0 million from the sale of redeemable convertible preferred stock in private financings prior to the Company’s initial public offering in May 2013 . As of September 30, 2021, the Company had $ 221.3 million in cash, cash equivalents and marketable securities. In 2020, the Company’s EZH2 inhibitor, tazemetostat, was approved in the United States as TAZVERIK for the treatment of epithelioid sarcoma, or ES, and follicular lymphoma, or FL. Commercial sales of TAZVERIK for the treatment of ES commenced in the first quarter of 2020 and commercial sales of TAZVERIK for the treatment of FL commenced near the end of the second quarter of 2020. The Company commenced active operations in early 2008. Since its inception, the Company has generated an accumulated deficit of $ 1,189.2 million through September 30, 2021 and will require substantial additional capital to fund its research, development, and commercialization efforts. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, risks of failure of commercialization, clinical trials and preclinical studies, the need to obtain additional financing to fund the future development and commercialization 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 TAZVERIK and of any product candidates that may be approved in the future, the impact of the COVID-19 pandemic on the Company’s business, results of operations, and financial condition, 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, marketing, and sale of products. Operating Cost Reduction On August 9, 2021, the Company announced a cross-functional reduction of approximately 11 % of its then current workforce under a cost reduction plan. Affected employees were offered separation benefits, including severance payments along with temporary healthcare coverage assistance. The severance and termination-related costs total approximately $ 2.0 million, $ 1.6 million of which were recorded as selling general and administrative expenses and $ 0.4 million of which was recorded as research and development expenses in the third quarter of 2021. The Company expects that payments of these costs will be made through August 2022.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation and Principles of Consolidation 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, or the 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, 2020, or the Annual Report. 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 condensed 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 September 30, 2021 and 2020 are referred to as the third quarter of 2021 and 2020 , 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. Use of Estimates The preparation of these condensed consolidated financial statements in accordance with accounting principles generally accepted in the United States requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities, as of the date of the condensed consolidated financial statements, and the reported amounts of revenue and expenses during the reporting period. Actual results and outcomes may differ materially from management’s estimates, judgments and assumptions. Significant Accounting Policies The significant accounting policies used in preparation of these condensed consolidated financial statements for the three and nine months ended September 30, 2021 are consistent with those discussed in Note 2 to the consolidated financial statements in the Annual Report and are updated below as necessary. 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 about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued and such doubt is not alleviated by the Company’s plans or when the Company's 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 its available cash, cash equivalents and marketable securities. The Company has recurring losses and expects to have recurring losses for the foreseeable future with the continued commercialization of TAZVERIK in ES and FL, the development of tazemetostat in other indications, and the development of the Company’s other product candidates. In addition, the Company has experienced and continues to experience challenges in the continued commercialization of TAZVERIK resulting from the ongoing COVID-19 pandemic, which the Company believes has had an adverse impact on TAZVERIK revenues. In response to the challenges that the Company has continued to face since the Company commenced its launch of TAZVERIK in FL in June 2020, the Company implemented an operational cost reduction plan that was announced on August 9, 2021 and continues to evaluate its costs on an on-going basis with the intent to streamline such costs. The analysis of the Company’s ability to continue as a going concern for the third quarter of 2021 included consideration of the Company’s current cash needs, including its research and development plans, commercialization activities associated with the continued commercialization of TAZVERIK in the ES and FL indications, its existing debt service obligations, anticipated cost savings resulting from the August 2021 operational cost reduction plan, including ongoing efforts to eliminate costs not related to the Company’s strategic focus. The analysis included forecasted product revenues from sales of TAZVERIK. Such estimates of future sales contain significant judgment as TAZVERIK was first launched in the first half of 2020 and there is little history with which to base such estimates. In addition, the Company’s ongoing efforts to eliminate costs not related to the Company’s strategic focus contains uncertainties as to whether the Company can attain such benefits. Based on the analysis, the Company concluded that its available cash, cash equivalents and marketable securities as of September 30, 2021 will be sufficient to fund current planned operations and capital expenditure requirements and pay our debt service obligations as they become due into the fourth quarter of 2022 which is at least 12 months from the filing date of this Quarterly Report on Form 10-Q with the SEC. The Company’s current operating plan is based on assumptions that may prove to be wrong, and the Company could use its capital resources sooner than it expects, in which case the Company would evaluate further reductions in its expenses or obtaining additional financing sooner than it otherwise would, which additional financing may not be available or may only be available on terms that are not acceptable to the Company. Recently Adopted Accounting Pronouncements Revenue Recognition – Collaboration Revenue In November 2018, the FASB, issued ASU 2018-18, Collaborative Arrangements, or ASC 808, which clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue when the collaborative arrangement participant is a customer in the context of a unit of account and precludes recognizing as revenue consideration received from a collaborative arrangement participant if the participant is not a customer. The new standard is effective in the first quarter of fiscal 2021. The Company adopted ASC 808 effective in the first quarter of fiscal 2021 and the Company’s adoption of this standard did not have a material effect on the Company’s condensed consolidated statements of operations and comprehensive loss or condensed consolidated statements of cash flows. Income Taxes In December 2019, the Financial Accounting Standards Board, or the FASB, issued ASU 2019-12, Income Taxes , or ASC 740, which simplifies the accounting for income taxes. The new standard is effective in the first quarter of fiscal 2021. The Company adopted ASC 740 effective in the first quarter of fiscal 2021 and the Company’s adoption of this standard did not have a material effect on the Company’s condensed consolidated statements of operations and comprehensive loss or condensed consolidated statements of cash flows. Revenue Recognition The Company recognizes revenue when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the Company expects to receive in exchange for those goods or services. To determine revenue recognition, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. For a further discussion of accounting for net product revenue see Note 3, Product Revenue, Net . Other Revenue Other revenue consists of revenue from the sales of tazemetostat active pharmaceutical ingredient (API) and drug product to the Company’s licensees or collaborators. The Company recognizes revenue on tazemetostat API and drug product when control has transferred under the terms of each agreement. Cost of Revenues Cost of revenues primarily consists of costs related to the sales of TAZVERIK and sales of tazemetostat API and drug product to the Company’s licensees or collaborators. These costs include materials, labor, manufacturing overhead, amortization of milestone payments, and royalties payable on net sales of TAZVERIK. Cost of revenues for the nine months ended September 30, 2021 included approximately $ 0.8 million related to sales of tazemetostat drug product. There were no sales of tazemetostat drug product during the three months ended September 30, 2021 or during the three and nine months ended September 30, 2020. Accounts Receivable The Company extends credit to customers based on its evaluation of the customer’s financial condition. The Company records receivables for all billings when amounts are due under standard terms. Accounts receivable are stated at amounts due net of applicable prompt pay discounts and other contractual adjustments as well as an allowance for doubtful accounts. The Company assesses the need for an allowance for doubtful accounts by considering a number of factors, including the length of time trade accounts receivable are past due, the customer’s ability to pay its obligation and the condition of the general economy and the industry as a whole. The Company will write off accounts receivable when the Company determines that they are uncollectible. In general, the Company has experienced no significant collection issues with its customers. Inventory The Company outsources the manufacturing of TAZVERIK and uses contract manufacturers to produce the raw and intermediate materials used in the production of TAZVERIK as well as the finished product. The Company currently has one supplier qualified for each step in the manufacturing process and is in the process of qualifying additional suppliers. Inventory is composed of raw materials, intermediate materials, which are classified as work-in-process, and finished goods, which are goods that are available for sale. The Company states inventory at the lower of cost or net realizable value with the cost based on the first-in, first-out method. Inventory is classified as long-term when it is expected to be utilized beyond the Company’s normal operating cycle and is included in restricted cash and other assets on the Company's condensed consolidated balance sheets. If the Company identifies excess, obsolete or unsalable items, it writes down its inventory to its net realizable value in the period in which the impairment is identified. These adjustments are recorded based upon various factors related to the product, including the level of product manufactured by the Company, the level of product in the distribution channel, current and projected demand, the expected shelf-life of the product and firm inventory purchase commitments. Shipping and handling costs incurred for inventory purchases are included in inventory costs and costs incurred for product shipments are recorded as incurred in cost of revenue. Prior to receiving its first approval from the U.S. Food and Drug Administration, or FDA, on January 23, 2020 to sell TAZVERIK, the Company expensed all costs incurred related to the manufacture of TAZVERIK as research and development expense because of the inherent risks associated with the development of a product candidate, the uncertainty about the regulatory approval process and the lack of history for the Company of regulatory approval of drug candidates. Intangible Assets, Net Intangible assets consist of capitalized milestone payments made to third parties under an in-license of patent rights upon receiving regulatory approval of TAZVERIK. The finite-lived intangible assets are being amortized on a straight-line basis over the expected time period the Company will benefit from the in-licensed rights, which is generally the patent life. Intangible assets are recorded at cost at the time of their acquisition and are stated in the Company’s condensed consolidated balance sheets net of accumulated amortization and impairments, if applicable. The amortization expense is recognized as cost of revenue in the Company’s condensed consolidated statement of operations and comprehensive loss. During the first quarter of 2020 the Company paid a $ 25.0 million milestone payment under its agreement with Eisai, Co., Ltd., or Eisai, upon regulatory approval of tazemetostat for ES. During the second quarter of 2020 the Company paid a $ 25.0 million milestone payment under its agreement with Eisai upon regulatory approval of tazemetostat for FL. Both regulatory milestones have been capitalized as intangible assets. The following table presents intangible assets as of September 30, 2021 (in thousands): September 30, 2021 Estimated useful In-licensed rights $ 50,000 12.2 Less: accumulated amortization ( 6,113 ) Total intangible asset, net $ 43,887 The Company recorded approximately $ 1.0 million and $ 3.1 million in amortization expense related to intangible assets, using the straight-line methodology, during the three and nine months ended September 30, 2021, respectively. The Company recorded approximatel y $ 1.0 million and $ 2.0 million in amortization expense related to intangible assets, using the straight-line methodology, during the three and nine months ended September 30, 2020, respectively. Estimated future amortization expense for intangible assets for the remainder of the year ended December 31, 2021 is $ 1.0 million and approximately $ 4.2 million per year thereafter. The Company assesses its intangible assets for impairment if indicators are present or changes in circumstance suggest that impairment may exist. Events that could result in an impairment, or trigger an interim impairment assessment, include the receipt of additional clinical or nonclinical data regarding one of the Company’s drug candidates or a potentially competitive drug candidate, changes in the clinical development program for a drug candidate, or new information regarding potential sales for the drug. If impairment indicators are present or changes in circumstance suggest that impairment may exist, the Company performs a recoverability test by comparing the sum of the estimated undiscounted cash flows of each intangible asset to its carrying value on the Company's condensed consolidated balance sheets. If the undiscounted cash flows used in the recoverability test are less than the carrying value, the Company would determine the fair value of the intangible asset and recognize an impairment loss if the carrying value of the intangible asset exceeds its fair value. During the three months ended June 30, 2021, the Company concluded the lower than anticipated current and projected future revenue, due to the impact of the COVID-19 pandemic as well as other factors, was an indicator that impairment may exist related to its finite-lived intangible assets. As a result, the Company performed a recoverability test and determined that the finite-lived intangible assets were recoverable. The Company’s quantitative assessment considered significant assumptions related to estimates of future TAZVERIK sales, offset by direct costs to derive the sales. The estimates of future TAZVERIK sales and associated costs include estimates of significant growth, however, these estimates are uncertain as the product was first launched in the first half of 2020 and due to the uncertainties associated with the ongoing COVID-19 pandemic. Given the limited history of sales and the inherent difficulty in making a long-range forecast, such estimates contain significant uncertainty. If the assumptions regarding forecasted revenue or the costs to derive such revenues prove to be inaccurate, the Company may be required to perform future impairment analyses and record an impairment charge for its intangible assets in future periods.

Product Revenue Net

Product Revenue Net	9 Months Ended
	Sep. 30, 2021
TAZVERIK [Member]
Concentration Risk [Line Items]
Product Revenue Net	3. Product Revenue, Net The Company sells TAZVERIK in the United States principally to a limited number of specialty pharmacies, which dispense the product directly to patients, and specialty distributors, which in turn sell the product to hospital pharmacies and community practice pharmacies (collectively, healthcare providers) for the treatment of patients. The specialty pharmacies and specialty distributors are referred to as the Company’s customers. Product revenue is recognized by the Company in an amount that reflects the consideration which the Company expects to receive in exchange for those goods or services when the customer obtains control of the product, which occurs at a point in time, typically when the product is received by the Company’s customers. The Company provides a right of return to its customers for unopened product for a limited time before and after its expiration date, which right of return lapses upon shipment to a patient. Healthcare providers to whom specialty distributors sell TAZVERIK hold limited inventory that is designated for patients, and the Company monitors inventory levels in the distribution channel to limit the risk of return. Reserves for Variable Consideration Revenues from product sales are recorded as product revenue at the net sales price (transaction price), which includes estimates of variable consideration for which reserves are established and which result from discounts, returns, chargebacks, rebates, co-pay assistance and other allowances that are offered within contracts between the Company and its customers, health care providers, payors and other indirect customers relating to the Company’s product sales. These reserves are based on the amounts earned or to be claimed on the related sales and are classified as reductions of accounts receivable (if the amount is payable to the customer) or a current liability (if the amount is payable to a party other than a customer). Where appropriate, these estimates take into consideration a range of possible outcomes that are probability-weighted for relevant factors such as the Company’s historical experience, current contractual and statutory requirements, specific known market events and trends, industry data and forecasted customer buying and payment patterns. Overall, these reserves reflect the Company’s best estimates of the amount of consideration to which the Company is entitled based on the terms of the contract(s). The amount of variable consideration that is included in the transaction price may be constrained, and is included in the net sales price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. Actual amounts of consideration ultimately received may differ from the Company’s estimates. If actual results in the future vary from the Company’s estimates, the Company will adjust these estimates, which would affect net product revenue and earnings in the period such variances become known. Trade Discounts and Allowances : The Company generally provides customers with discounts that include incentive fees that are explicitly stated in customer contracts and are recorded as a reduction of revenue in the period the related product revenue is recognized. In addition, the Company receives sales order management, data and distribution services from certain customers. To the extent the services received are distinct from the Company’s sale of products to the customer, these payments are classified in selling, general and administrative expenses in the Company's condensed consolidated statements of operations and comprehensive loss. Product Returns : Consistent with industry practice, the Company generally offers customers a limited right of return based on the product’s expiration date for product that has been purchased from the Company, which lapses upon shipment to a patient. The Company estimates the amount of product sales that may be returned by customers and records this estimate as a reduction of revenue in the period in which the related product revenue is recognized. The Company currently estimates product return liabilities using available industry data and the Company’s own historical sales information, including its visibility into the product remaining in the distribution channel. Provider Chargebacks and Discounts : Chargebacks for fees and discounts to healthcare providers represent the estimated obligations resulting from contractual commitments to sell products to qualified healthcare providers at prices lower than the list prices charged to customers who directly purchase the product from the Company. Customers charge the Company for the difference between what they pay for the product and the ultimate selling price to the qualified healthcare providers. These reserves are established in the same period that the related revenue is recognized, resulting in a reduction of product revenue and accounts receivable. Chargeback amounts are generally determined at the time of resale to the qualified healthcare provider by customers, and the Company generally issues credits for such amounts within a few weeks of the customer’s notification to the Company of the resale. Reserves for chargebacks consist of credits that the Company expects to issue for units that remain in the distribution channel at each reporting period end that the Company expects will be sold to qualified healthcare providers, and chargebacks that customers have claimed but for which the Company has not yet issued a credit. Government Rebates: The Company is subject to discount obligations under state Medicaid programs and Medicare. The Company estimates its Medicaid and Medicare rebates based upon a range of possible outcomes that are probability-weighted for the estimated payor mix. These reserves are recorded in the same period in which the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability that is included in accrued expenses on the Company’s condensed consolidated balance sheets. For Medicare, the Company also estimates the number of patients in the prescription drug coverage gap for whom the Company will owe an additional liability under the Medicare Part D program. The Company’s liability for these rebates consists of invoices received for claims from prior quarters that have not been paid or for which an invoice has not yet been received, estimates of claims for the current quarter, and estimated future claims that will be made for product that has been recognized as revenue, but remains in the distribution channel inventories at period end. Payor Rebates: The Company may contract with various private payor organizations, primarily insurance companies and pharmacy benefit managers, for the payment of rebates with respect to utilization of the Company’s products. The Company estimates these rebates and records such estimates in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability. Other Incentives/Patient Assistance Programs: The Company also offers voluntary patient assistance programs such as co-pay assistance. Co-pay assistance programs are intended to provide financial assistance to qualified commercially insured patients with prescription drug co-payments required by payors. The calculation of the accrual for co-pay assistance is based on an estimate of claims and the cost per claim that the Company expects to receive associated with product that has been recognized as revenue, but remains in the distribution channel inventories at period end. The following table summarizes activity in each of the above product revenue allowances and reserve categories for the nine months ended September 30, 2021: Chargebacks, Government Fees Rebates Returns Total (In thousands) Balance, January 1, 2021 $ 133 $ 428 $ 67 $ 628 Provision 1,344 2,152 252 3,748 Payments or credits ( 1,294 ) ( 2,110 ) ( 199 ) ( 3,603 ) Balance, September, 2021 $ 183 $ 470 $ 120 $ 773 Concentration of Credit Risk Financial instruments which potentially subject the Company to concentrations of credit risk consist of accounts receivable from customers and cash held at financial institutions. The Company believes that such customers and financial institutions are of high credit quality. For the three and nine months ended September 30, 2021 and 2020, net product revenue was primarily generated from four individual customers. Revenue earned from each customer as a percentage of net product revenue is as follows: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Customer 1 27 % 42 % 34 % 48 % Customer 2 19 % 10 % 13 % 12 % Customer 3 19 % 18 % 32 % 15 % Customer 4 35 % 23 % 21 % 18 % As of September 30, 2021 and December 31, 2020, the four individual customers represented as a percentage of accounts receivable as follows: September 30, December 31, Customer 1 15 % 21 % Customer 2 17 % 14 % Customer 3 15 % 29 % Customer 4 53 % 36 % No other customer represented more than 10 percent of net product revenue or accounts receivable.

Cash

Cash	9 Months Ended
	Sep. 30, 2021
Cash And Cash Equivalents [Abstract]
Cash	4. Cash A reconciliation of cash, cash equivalents, and restricted cash reported within the Company's condensed consolidated balance sheets that sum to the total of the same such amounts shown in the Company's condensed consolidated statements of cash flows, is as follows: As of September 30, 2021 2020 (In thousands) Cash and cash equivalents $ 124,517 $ 93,422 Restricted cash, as part of other assets 1,509 1,509 Total cash, cash equivalents, and restricted cash shown in the condensed consolidated statements of cash flows $ 126,026 $ 94,931 The $ 1.5 million in restricted cash as of both September 30, 2021 and September 30, 2020 is comprised of $ 0.5 million in a letter of credit as a security deposit for the Company’s office and laboratory lease at Technology Square in Cambridge, Massachusetts and $ 1.0 million in a letter of credit as a security deposit for the Company’s office lease at Hampshire Street in Cambridge, Massachusetts. The Company has recorded cash held to secure these letters of credit as restricted cash in restricted cash and other assets on its condensed consolidated balance sheets. The restricted cash is classified as non-current based on the related lease terms.

Marketable Securities

Marketable Securities	9 Months Ended
	Sep. 30, 2021
Investments Debt And Equity Securities [Abstract]
Marketable Securities	5. Marketable Securities The following table summarizes the available-for-sale securities held at September 30, 2021 (in thousands): Description Amortized Fair Value Commercial paper $ 72,733 $ 4 $ ( 2 ) $ 72,735 Corporate notes 7,022 — ( 4 ) 7,018 U.S. government agency securities and U.S. Treasuries 17,023 — — 17,023 Total $ 96,778 $ 4 $ ( 6 ) $ 96,776 The following table summarizes the available-for-sale securities held at December 31, 2020 (in thousands): Description Amortized Fair Value Commercial paper $ 158,907 $ 14 $ ( 8 ) $ 158,913 Corporate notes 33,437 3 ( 7 ) 33,433 U.S. government agency securities and U.S. Treasuries 13,044 1 — 13,045 Total $ 205,388 $ 18 $ ( 15 ) $ 205,391 Certain short-term debt securities with original maturities of less than 90 days are included in cash and cash equivalents within the Company's condensed consolidated balance sheets and are not included in the tables above. The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At September 30, 2021 , 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 securities during the nine months ended September 30, 2021, and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the same period. 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 September 30, 2021 was $ 33.5 million, which consisted of 13 commercial paper securities and 1 c orporate note security. The aggregate unrealized loss for those securities in an unrealized loss position for less than twelve months as of September 30, 2021 was less than $ 0.1 million. The Company does not intend to sell and it is unlikely that the Company will be required to sell the above investments before recovery of their amortized cost bases, which may be maturity. The Company has determined that there has been no material change in the credit risk of any of its investments. As a result, the Company determined it did not hold any investments that were impaired as of September 30, 2021 . The Company reviews its portfolio of available-for-sale debt securities, using both quantitative and qualitative factors, to determine if declines in fair value below cost have resulted from a credit-related loss or other factors. If the decline in fair value is due to credit-related factors, a loss is recognized in net income, whereas if the decline in fair value is not due to credit-related factors, the loss is recorded in other comprehensive income (loss). The weighted-average maturity of the Company’s portfolio was approximately four months at September 30, 2021 .

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements	6. Fair Value Measurements The Company’s financial instruments as of September 30, 2021 and December 31, 2020 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. As of September 30, 2021 and December 31, 2020, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of September 30, 2021 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 97,136 $ 83,412 $ 13,724 $ — Marketable securities: Commercial paper 72,735 — 72,735 — Corporate notes 7,018 — 7,018 — U.S. government agency securities and treasuries 17,023 — 17,023 — Total $ 193,912 $ 83,412 $ 110,500 $ — Fair Value as of December 31, 2020 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 163,264 $ 113,505 $ 49,759 $ — Marketable securities: Commercial paper 158,913 — 158,913 — Corporate notes 33,433 — 33,433 — U.S. government agency securities and treasuries 13,045 — 13,045 — Total $ 368,655 $ 113,505 $ 255,150 $ — 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, which approximates the net asset value per share. The Company classifies some of its cash equivalents within Level 1 of the fair value hierarchy 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 those cash equivalents classified within Level 2 of the fair value hierarchy.

Inventory

Inventory	9 Months Ended
	Sep. 30, 2021
Inventory Disclosure [Abstract]
Inventory	7. Inventory All of the Company’s inventories relate to the manufacturing of TAZVERIK. The following table sets forth the Company’s inventories as of September 30, 2021 and December 31, 2020: September 30, December 31, (In thousands) Raw materials $ 3,116 $ 1,068 Work in process 11,591 8,564 Finished goods 2,237 829 Total $ 16,944 $ 10,461 Balance sheet classification Inventory $ 1,521 $ 10,461 Restricted cash and other assets 15,423 — Total $ 16,944 $ 10,461 The Company’s active pharmaceutical ingredient has a long shelf life and the Company’s finished drug product has a three-year expiry, however the realizability of the inventory is subject to forecasted future sales of TAZVERIK. The Company’s forecasted sales currently support the realizability of the Company’s inventory but are uncertain and could change in the future, which would require the Company to write down the value of such inventory. Due to the revisions to the Company’s forecast of future TAZVERIK sales during the quarter ended June 30, 2021, the Company classified a portion of its inventory as long-term. As of September 30, 2021 the Company has not capitalized inventory costs related to its other drug development programs.

Supplemental Balance Sheet Info

Supplemental Balance Sheet Information	9 Months Ended
	Sep. 30, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Supplemental Balance Sheet Information	8. Supplemental Balance Sheet Information Accrued expenses consisted of the following: September 30, December 31, (In thousands) Employee compensation and benefits $ 10,578 $ 11,921 Research and development expenses 18,804 10,664 Professional services and other 1,540 5,987 Accrued expenses $ 30,922 $ 28,572

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes The Company did no t record a federal or state income tax provision or benefit for the three months ended September 30, 2021 and 2020 due to the expected and known loss before income taxes to be incurred, or incurred, as applicable, for the years ended December 31, 2021 and 2020 , 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	9 Months Ended
	Sep. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	10. Commitments and Contingencies There have been no significant changes to the Company’s commitments and contingencies, other than the minimum lease payments as disclosed in Note 11, Leases , in the three and nine months ended September 30, 2021, as compared to those disclosed in Note 9, Commitments and Contingencies , included in the Company's Annual Report.

Leases

Leases	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Leases	11. Leases The Company enters into lease arrangements for its facilities as well as certain equipment. A summary of the arrangements is as follows: Operating Leases 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, or the Technology Square Lease, with ARE-TECH Square, LLC, a Delaware limited liability company. In May 2017, the Company exercised its option to extend the term of the Technology Square Lease to November 30, 2022. Under the Technology Square Lease as amended, the Company 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 of approximately $ 33,000 and annual increases of approximately $ 9,000 on December 1 of each subsequent year until the last increase, which will occur on December 1, 2021. On August 11, 2021, the Company, entered into a fifth amendment to the Technology Square Lease (the “Fifth Amendment”) with ARE-TECH Square, LLC. Under the Fifth Amendment, the Company extended the term of the Technology Square Lease through November 30, 2024 . Under the Fifth Amendment, the Company will continue to pay the Landlord the current monthly base rent amount contemplated by the Technology Square Lease through November 30, 2022, with an increase commencing on December 1, 2022 and adjusting the monthly base rent amount to approximately $ 377,000 and an increase commencing on December 1, 2023 and adjusting the monthly base rent amount to approximately $ 388,000 through November 30, 2024. In addition, under the Fifth Amendment, the Landlord agreed to provide the Company with a tenant improvement allowance of up to approximately $ 430,000 if requested by the Company by August 11, 2022, subject to specified terms and conditions. In accordance with ASU 2016-02, Leases, or ASC 842, the Company accounted for the Fifth Amendment as a lease modification and remeasured the operating lease liability, resulting in an additional $ 7.0 million operating lease liability and right of use asset. Under the current terms of the Technology Square Lease, the Company does not have any further right to extend the term beyond November 30, 2024. The Company has a $ 0.5 million letter of credit as a security deposit for the Technology Square Lease and has recorded cash held to secure this letter of credit as restricted cash and other assets on the Company's condensed consolidated balance sheets. On August 16, 2019, the Company entered into a lease, or the Hampshire Street Lease, with BMR-Hampshire LLC, or BMR. The Hampshire Street Lease is for 33,525 rentable square feet of office space in Cambridge, Massachusetts. The Hampshire Street Lease commenced as of December 1, 2019 . The Hampshire Street Lease has an initial term of seven years and four months from the commencement date and provides the Company with an option to extend the lease term for one additional five-year period . After a four-month period during which base rent was not payable, the Hampshire Street Lease provides for monthly rent payments starting at approximately $ 0.2 million and increasing 2.5 % per year. In the event that the Company exercises its option to extend the lease term, the Hampshire Street Lease provides for monthly rent payments during the additional five-year period at the greater of the base rent rate at the end of the initial term or the then-current market rent. The Company has a $ 1.0 million letter of credit in favor of BMR as a security deposit for the Hampshire Street Lease and has recorded cash held to secure this letter of credit as restricted cash and other assets on the Company's condensed consolidated balance sheets. In applying ASC 842, the Company determined the classification of the Hampshire Street Lease to be operating and recorded a lease liability and a right-of-use asset as of December 31, 2019. The Company is required to pay certain variable costs to its landlords in addition to fixed rent. These costs include common area maintenance, real estate taxes, and parking and are included in lease expense. The following table contains a summary of the lease costs recognized under ASC 842 and other information pertaining to the Company’s operating leases for the three and nine months ended September 30, 2021 and 2020: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Lease cost Operating lease cost $ 1,603 $ 1,554 $ 4,634 $ 4,660 Variable lease cost 498 405 1,545 1,351 Total lease cost $ 2,101 $ 1,959 $ 6,179 $ 6,011 Other information Operating cash flows used for operating leases $ 1,601 $ 1,000 $ 4,826 $ 2,980 Weighted average remaining lease term 4.4 years 4.9 years 4.4 years 4.9 years Weighted average discount rate 9.73 % 9.73 % 9.73 % 9.73 % Future minimum lease payments under the Company’s non-cancelable operating leases as of September 30, 2021, are as follows: (In thousands) 2021 $ 1,610 2022 6,202 2023 7,517 2024 7,322 Thereafter 6,966 Total lease payments $ 29,617 Less: imputed interest ( 5,925 ) Total operating lease liabilities at September 30, 2021 $ 23,692

Collaborations and Licensing Ag

Collaborations and Licensing Agreements	9 Months Ended
	Sep. 30, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations and Licensing Agreements	12. Collaborations and Licensing Agreements HutchMed On August 7, 2021 , the Company entered into a license agreement (the “HutchMed License Agreement”) with Hutchison China MediTech Investment Limited (“HutchMed”) for the development, manufacture and commercialization of tazemetostat, either as a monotherapy or as a part of combinations with other therapies, including HutchMed proprietary compounds, agreed by the parties under the HutchMed License Agreement (“Licensed Products”) for the treatment of epithelioid sarcoma, follicular lymphoma, diffuse large B-cell lymphoma in humans, and any additional indications agreed by the parties in accordance with the terms of the HutchMed License Agreement (the “Field”) in mainland China, Taiwan, Hong Kong and Macau (each, a “Jurisdiction”, and collectively, the “Territory”). Agreement Structure The Company has granted HutchMed licenses under patent rights and know-how controlled by the Company to enable HutchMed to develop and commercialize Licensed Products in the Field in the Territory. The licenses granted to HutchMed are co-exclusive with the Company with respect to the development of Licensed Products in the Field in the Territory and exclusive with respect to the commercialization of Licensed Products in the Field in the Territory. The Company also granted HutchMed a license under patent rights and know-how controlled by the Company to enable HutchMed to manufacture tazemetostat drug substance and drug product for the purpose of developing and commercializing Licensed Products in the Field in the Territory. The Company retains development and commercialization rights with respect to Licensed Products in the rest of the world outside of the Territory except for Japan. During the term of the HutchMed License Agreement, each party and its affiliates is prohibited from developing or commercializing in the Field in the Territory any other compound or product that inhibits, modulates or degrades EZH1, EZH2, or any other member of the polycomb repressive complex 2, including the EED protein, provided that, subject to limitations specified in the HutchMed License Agreement, HutchMed may develop, without the use of know-how or patent rights licensed by Epizyme, its existing preclinical compound that inhibits EZH1 and EZH2. The Company has agreed to conduct a technology transfer of manufacturing technology to HutchMed to enable HutchMed to manufacture clinical and commercial quantities of tazemetostat drug substance and drug product to carry out its obligations and exercise its rights under the HutchMed License Agreement. Subject to the execution of a clinical supply agreement or commercial supply agreement, as applicable, and until the completion of the technology transfer to HutchMed, the Company has agreed to manufacture and supply HutchMed with tazemetostat drug substance and drug product in sufficient quantities for HutchMed’s development or commercialization activities for Licensed Products in the Field in the Territory. HutchMed has agreed to use commercially reasonable efforts to carry out development activities in the Territory as agreed by the parties and to seek to obtain and maintain regulatory approval of the Licensed Products in the Territory. HutchMed agreed to use commercially reasonable efforts to commercialize Licensed Products in the Field in the Territory. HutchMed is responsible for all costs it incurs in developing, obtaining regulatory approval of, and commercializing Licensed Products in the Field in the Territory, including costs incurred by HutchMed in conducting clinical trials that only include clinical sites in the Territory. For global studies conducted by the Company that HutchMed elects to participate in by conducting any such study in the Territory, HutchMed will be responsible for enrolling and treating in the Territory 20 % of the total number of study patients of such global study and will be responsible for costs for those patients enrolled and treated in such trials. HutchMed will also be responsible for 20 % of the costs of such global studies that are not specific to any territory and the Company will be responsible for all other costs of such global studies. HutchMed has agreed to participate in the Company’s EZH-301 and EZH-302 global studies, however under certain circumstances where the EZH-302 global study is not considered a confirmatory trial for regulatory approval in China, the Company shall be responsible for the costs of the trial in the Territory. Pursuant to the HutchMed License Agreement, the Company received a nonrefundable upfront payment of $ 25.0 million in September 2021. The Company is also entitled to milestone payments of up to $ 110.0 million in the aggregate for achievement of specified development and regulatory milestones with respect to Licensed Products in the Territory, and up to $ 175.0 million in the aggregate for achievement of specified sales milestones in the Territory with respect to the Licensed Products. The Company will also be entitled to receive tiered royalties, ranging from a mid-teens percentage to a low twenties percentage based on HutchMed’s cumulative annual net sales, if any, of Licensed Products in the Territory. Royalties are payable for each Licensed Product commencing on the first commercial sale of the applicable Licensed Product and ending, on a Jurisdiction-by-Jurisdiction basis, on the latest of expiration of specified patent coverage, expiration of specified regulatory exclusivity or a specified period following the first commercial sale in such Jurisdiction and may be reduced in various circumstances. Under the HutchMed License Agreement, the Company issued a warrant to HutchMed (the “HutchMed Warrant”) , exercisable at any time prior to August 7, 2025 for up to 5,653,000 shares of the Company’s common stock at an exercise price of $ 11.50 per share. On September 21, 2021 the Company filed with the SEC a registration statement on Form S-3 registering for resale the shares of the Company’s common stock issuable upon exercise of the HutchMed Warrant in accordance with the HutchMed Warrant. Such registration statement on Form S-3 was declared effective by the SEC on September 29, 2021. Unless earlier terminated, the HutchMed License Agreement will expire upon the expiration of the last royalty term for the last Licensed Product in the Field in the Territory. HutchMed may terminate the HutchMed License Agreement in its entirety for any or no reason upon 12 months’ prior written notice to the Company. Either party may, subject to specified cure periods, terminate the HutchMed License Agreement in the event of the other party’s uncured material breach, and under specified circumstances relating to the other party’s insolvency or if the other party or its affiliates challenges the validity, patentability, or enforceability of patent rights that are owned by or licensed to such party or its affiliates and that are subject to the licenses granted in the HutchMed License Agreement. License Revenue The Company evaluated the terms of the HutchMed License Agreement and first determined that the HutchMed Warrant should be accounted for pursuant to ASC 815, Derivatives and Hedging, with the HutchMed Warrant's fair value of approximately $ 13.0 million (Note 15) at execution considered outside of the revenue arrangement. The Company identified the following performance obligations at the inception of the HutchMed License Agreement: (1) exclusive license with rights to develop, manufacture and commercialize tazemetostat in the Territory, (2) research and development services related to global trials, and (3) a material right related to the Company’s obligation to provide clinical supply of tazemetostat. In addition, the Company may also provide certain technology transfer services related to providing HutchMed with the capability to manufacture tazemetostat, for which the Company will receive reimbursement that approximates stand-alone selling price. The Company evaluated the HutchMed License Agreement under ASC 606, Revenue from Contracts with Customers . Based on that evaluation, the $ 12.0 million of the up-front fee remaining after allocation to the HutchMed Warrant and the reimbursement to be received for its research and development services constituted the amount of the consideration to be included in the transaction price. In addition, should the EZH-302 global trial not be deemed a confirmatory trial for purposes of regulatory approval in China, the Company shall be responsible for reimbursing HutchMed for the costs of the portion of the EZH-302 global trial that will be performed in China. The Company concluded that this repayment provision represented variable consideration under the arrangement. Due to the uncertainty of potential repayment, which is based solely on the decision of a regulatory authority, the Company could not assert that it was probable that a significant reversal of revenue would not occur. As a result, the Company determined that the transaction price should be fully constrained. The Company will evaluate the application of the constraint on a quarterly basis and should the contingency be resolved without future payment to HutchMed for the EZH-302 global trial, the full upfront fee and any reimbursement of research and development services will be included in the transaction price. In addition, should the estimated payment to HutchMed for the EZH-302 global trial be determined to be less than the cumulative up-front fee and research and development reimbursement payments, such excess will be included in the transaction price. None of the development or regulatory milestones have been included in the transaction price, as all such milestone amounts were fully constrained. As part of the Company's evaluation of the constraint, the Company considered numerous factors, including that receipt of the milestones is outside the control of the Company and contingent upon success in future clinical trials and the licensee’s efforts. Any consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as these amounts have been determined to relate predominantly to the license granted to HutchMed and therefore are recognized at the later of when the performance obligation is satisfied or the related sales occur. The Company delivered the license during the third quarter of 2021 and expects that based on the estimated standalone selling price of the license, that the majority of the consideration in the arrangement will be allocated to the license performance obligation, once such consideration is no longer constrained. As the Company performs research and development services, it will recognize revenue as such services are performed, upon the transaction price no longer being fully constrained. GSK In January 2011, the Company entered into a collaboration and license agreement with Glaxo Group Limited (an affiliate of GlaxoSmithKline plc), or 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. Subsequent to a GSK strategic portfolio prioritization, the Company received notice in October 2017 that GSK terminated the agreement with respect to the third target, effective December 31, 2017, which returned all rights to that target to the Company. The two other targets, PRMT5 and PRMT1, continue to be subject to the agreement and were not impacted by the termination with respect 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 received and recognized as collaboration revenue totaling $ 89.0 million, consisting of upfront payments, fixed research funding, research and development services and preclinical and research and development milestone payments. As of September 30, 2021 , for the two remaining targets, the Company is eligible to receive up to $ 50.0 million in clinical development milestone payments, up to $ 197.0 million in regulatory milestone payments and up to $ 128.0 million in sales-based milestone payments. As a result of the termination of the agreement as it relates to the third target, the Company will receive no additional payments related to that target. 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. 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 September 30, 2021 , the Company has earned a total of $ 89.0 million in total collaboration revenue since inception of the GSK agreement, which the Company recognized as collaboration revenue in the condensed consolidated statements of operations and comprehensive loss. The Company did no t recognize any collaboration revenue under the agreement in the three and nine months ended September 30, 2021 and 2020, respectively. The Company did not have any deferred revenue related to this agreement as of September 30, 2021 and 2020, respectively. Any future revenues pursuant to this arrangement will relate to any milestone payments and royalties received under the agreement with respect to the two remaining targets. All remaining milestone payments as of September 30, 2021 have been deemed not probable and therefore have not been recognized as revenue. 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. As of December 31, 2014, the Company had completed its performance obligations under the original agreement. In March 2015, the Company entered into an amended and restated collaboration and license agreement with Eisai (the “Eisai License Agreement”), under which the Company reacquired worldwide rights, excluding Japan, to its EZH2 program, including tazemetostat. Under the Eisai 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, and a right of first negotiation for the rest of Asia. Eisai waived its right of first negotiation for the rest of Asia in 2018. Under the original collaboration and license agreement, Eisai was solely responsible for funding all research, development and commercialization costs for EZH2 compounds. Under the Eisai License Agreement, the Company is solely responsible for funding global development, manufacturing and commercialization costs for EZH2 compounds outside of Japan, including the remaining development costs due under a companion diagnostics agreement with Roche Molecular Systems, Inc., or Roche Molecular, which was amended to assign all of Roche Molecular’s rights and obligations under the companion diagnostics agreement to Roche Sequencing, effective January 1, 2020. 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 Eisai License Agreement, 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, had not achieved regulatory approval for marketing and, absent obtaining such approval, had 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. The Company also agreed to pay Eisai up to $ 20.0 million in clinical development milestones, and 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. Pursuant to the Eisai License Agreement, the Company has paid total milestone payments of $ 70.0 million, $ 50.0 million of which related to regulatory approval of tazemetostat for ES and FL, which were capitalized as intangible assets on the Company’s condensed consolidated balance sheets. In March 2021, the Company and Eisai entered into a supply agreement providing for the manufacture and supply to Eisai of tazemetostat drug product. Under the terms of the supply agreement, the Company also agreed to waive its right of exclusive supply of tazemetostat drug substance from the Company’s drug substance manufacturer. During the nine months ended September 30, 2021, the Company recognized $ 6.3 million related to the Company’s waiver of its exclusive right to supply of tazemetostat drug substance from the Company’s drug substance manufacturer and delivery of tazemetostat drug product in collaboration and other revenue. No such revenue was recognized in the three months ended September 30, 2021. During the three and nine months ended September 30, 2020, Eisai purchased $ 3.7 and $ 4.4 million, respectively, of drug product from the Company at cost to facilitate development within Japan under the Eisai License Agreement which was recognized as a reduction to research and development expense. As of September 30, 2021 and December 31, 2020 , the Company had accounts receivable of less than $ 0.1 million for both periods, due from Eisai. During the three and nine months ended September 30, 2021, the Company recorded $ 0.8 and $ 2.7 million, re spectively, related to the worldwide royalties due under the Eisai License Agreement in cost of revenue based on U.S. sales of TAZVERIK. During the three and nine months ended September 30, 2020, the Company r ecorded $ 0.5 and $ 1.0 million , respectively, in cost of revenue related to the worldwide royalties due under the Eisai License Agreement based on U.S. sales of TAZVERI K. As of September 30, 2021 and 2020, $ 0.8 million and $ 0.5 million, respectively, in royalties were payable under the Eisai License Agreement. F or additional information regardin g certain of the Eisai royalties, see Note 13, Sale of Future Royalties . Roche In December 2012, Eisai and the Company entered into a companion diagnostics 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 and Eisai entered into a second amendment to the companion diagnostics agreement with Roche Molecular. The agreement was further amended in March 2018. Under the amended agreement, the Company was responsible for remaining development costs of $ 10.4 million due under the agreement as of March 2018 and Eisai agreed to reimburse the Company $ 0.9 million of this amount related to a regulatory milestone for Japan. In July 2019, the Company entered into a fourth amendment to the companion diagnostics agreement. Under the amended agreement, the Company and Roche Molecular agreed to divide a $ 1.0 million regulatory milestone for the United States into two separate milestone payments, of which $ 0.5 million was paid by the Company as part of the signed amendment, and the remaining $ 0.5 million was paid by the Company in December 2019 upon the satisfaction of certain conditions set forth in the fourth amendment to the companion diagnostics agreement. As part of this fourth amendment, Roche Molecular also assigned all of its rights and obligations under the companion diagnostics agreement to Roche Sequencing due to a reorganization at Roche group, and this assignment became effective as of January 1, 2020. As of September 30, 2021 , the Company is responsible for the remaining development costs of $ 1.0 million due under the agreement. The $ 0.9 million that Eisai agreed to reimburse the Company related to a regulatory milestone for Japan was achieved as of June 30, 2020 and payment received in the fourth quarter of 2020. In addition, the Company paid $ 1.0 million for the achievement of a development milestone in the fourth quarter of 2020. Under the agreement with Roche Sequencing, Roche Sequencing is obligated to use commercially reasonable efforts to develop and to make commercially available the companion diagnostic. Roche Sequencing has exclusive rights to commercialize the companion diagnostic. On June 18, 2020 the FDA approved the companion diagnostic that is intended to identify follicular lymphoma patients with an EZH2 mutation for treatment with tazemetostat. The agreement with Roche Sequencing will expire when the Company and Eisai are no longer developing or commercializing tazemetostat. The Company and Eisai may terminate the agreement by giving Roche Sequencing 90 days’ written notice if the Company and Eisai discontinue development and commercialization of tazemetostat or determine, in conjunction with Roche Sequencing, that the companion diagnostic is not needed for use with tazemetostat. Any party may also terminate the agreement in the event of a material breach by any 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 Sequencing may become entitled to specified termination fees. Boehringer Ingelheim In November 2018, the Company entered into a collaboration and license agreement with Boehringer Ingelheim International GmbH (“Boehringer Ingelheim”) to discover, research, develop and commercialize small molecule compounds that are inhibitors of an undisclosed histone acetyltransferase, or HAT, target and an undisclosed helicase target, along with associated predictive biomarkers (the “Target Projects”). Under the terms of the agreement, the Company granted to Boehringer Ingelheim an exclusive, worldwide license to the undisclosed target inhibitors technology. The agreement also included reciprocal licenses to utilize each other’s know-how, patents and technologies for activities under the agreement. Further, each party was granted the license to develop, manufacture, commercialize and otherwise exploit any compound or product that successfully achieves start of lead optimization (“SoLO”). The Company was also obligated to provide R&D services through SoLO approval for both Target Projects, and to serve on the Joint Steering Committee (“JSC”) throughout the term of the contract. The parties were to jointly research and develop the shared helicase target program and will share commercialization activities within the United States. Boehringer Ingelheim had agreed to assume responsibility for commercialization outside of the United States. On December 21, 2020, the Company received written notice from Boehringer Ingelheim that it elected to terminate the Collaboration Agreement without cause, and in accordance with the terms of the Collaboration Agreement. The termination became effective on January 31, 2021. The Target Project for the helicase target and the reciprocal licenses terminated as of this date. The Company is entitled to pursue the HAT target and helicase target programs in all fields worldwide without further obligation to Boehringer Ingelheim. Agreement Structure Under the terms of the agreement, the Company received a $ 15.0 million upfront payment and $ 5.0 million in research funding for the costs to be incurred by the Company in connection with its research activities, payable quarterly in four equal installments during 2019. At its discretion, Boehringer Ingelheim had the option to extend the research period by up to one year , subject to the Company’s agreement to the specified research activities and additional research funding. During the third quarter of 2019, Boehringer Ingelheim’s option to extend the research period expired unexercised, and therefore the research period ended on December 31, 2019. In March 2020, the Company and Boehringer Ingelheim amended the agreement to extend the research period for the shared program targeting enzymes within helicase families with Boehringer Ingelheim providing research funding of $ 0.4 million. Additionally, in March 2020, the Company received notice of termination for the program targeting enzymes with HAT families, which program termination became effective in June 2020. In September 2020, the Company and Boehringer Ingelheim further amended the agreement to extend the research period for the shared program targeting enzymes within helicase families with Boehringer Ingelheim to provide research funding of $ 0.1 million. The additional research activities were completed prior to the end of 2020. Collaboration Revenue Through September 30, 2021 , the Company has recognized $ 26.0 million in total collaboration revenue since the inception of this collaboration. During the three and nine months ended September 30, 2021, the Company did no t recognize collaboration revenue under its agreement with Boehringer Ingelheim. During the three and nine months ended September 30, 2020, the Company recognized $ 0.1 and $ 0.4 million, respectively, in collaboration revenue under its agreement with Boehringer Ingelheim. The Company will not receive any additional revenue under the terms of its agreement with Boehringer Ingelheim. As of September 30, 2021 and December 31, 2020 , the Company did no t have any deferred revenue or accounts receivable related to this agreement.

Sale of Future Royalties

Sale of Future Royalties	9 Months Ended
	Sep. 30, 2021
Deferred Revenue Disclosure [Abstract]
Sale of Future Royalties	13. Sale of Future Royalties On November 4, 2019, the Company entered into a loan agreement with BioPharma Credit PLC, or the Collateral Agent, and the Lenders , providing for up to $ 70.0 million in secured term loans to be advanced in up to three tranches, or the Loan Agreement. As of June 30, 2021, the Company had borrowed an aggregate principal amount under the first tranche of $ 25.0 million (the “Tranche A Note Payable”), the second tranche of $ 25.0 million (the “Tranche B Note Payable”), and the third tranche of $ 20.0 million (the “Tranche C Note Payable”) under the Loan Agreement. On November 3, 2020, the Company, the Collateral Agent and the Lenders amended and restated the Loan Agreement, (as amended and restated, the “Amended and Restated Loan Agreement”), to provide for, among other things, an additional secured term loan of $ 150.0 million, or the Tranche D Loan. On November 18, 2020, the Company borrowed the Tranche D Loan (see Note 14, Long-Term Debt ). Under the Amended and Restated Loan Agreement the Company has the right to request up to an additional $ 150.0 million in secured term loans, subject to the approval of the Lenders, provided that the Company has not prepaid any outstanding term loans at the time of the Company’s request and such request is made before November 18, 2021. On November 4, 2019, the Company also executed a purchase agreement (the “RPI Purchase Agreement”) with RPI. Pursuant to the RPI Purchase Agreement, the Company agreed to sell to RPI 6,666,667 shares of its common stock, a warrant to purchase up to 2,500,000 shares of common stock at an exercise price of $ 20.00 per share (the “Common Stock Warrant”), and all of the Company’s rights to receive royalties from Eisai with respect to net sales by Eisai of tazemetostat products in Japan pursuant to the Eisai License Agreement and any successor arrangement for Japan sales (the “Japan Royalty”, and collectively, the “Transaction”). In consideration for the sale of shares of common stock, the Common Stock Warrant and the Japan Royalty, RPI paid the Company $ 100.0 million upon the closing of the RPI Purchase Agreement. In addition, RPI agreed, in connection with RPI’s acquisition from Eisai of the right to receive royalties from the Company under the Eisai License Agreement, to reduce the Company’s royalty obligation by low single digits upon the achievement of specified annual net sales levels over $ 1.5 billion. In addition, under the RPI Purchase Agreement, the Company has the right to sell, and RPI has the obligation to purchase, subject to certain conditions, including a maximum purchase price of $ 20.00 per share, $ 50.0 million of shares of common stock at the Company’s option for an 18-month period from the date of execution of the RPI Purchase Agreement (the “Put Option”). In February 2020, the Company sold 2.5 million shares of its common stock to RPI, for an aggregate of $ 50.0 million in proceeds pursuant to the Put Option. Additionally, under the terms of the RPI Purchase Agreement, the founder and chief executive officer of RP Management, an affiliate of RPI, and a co-founder of Pharmakon Advisors LP, an affiliate of the Lenders, was elected as a director of the Company. As of September 30, 2021 and December 31, 2020 , RPI and its affiliates owned approximately 9.0 % and 9.0 % of the Company’s common stock, respectively. The Company accounted for the Loan Agreement and RPI Purchase Agreement as a single arrangement as RPI and the Lenders are related parties and the agreements were negotiated together. The aggregate proceeds of $ 125.0 million were allocated on a relative fair value basis, which approximated their respective actual fair values, to the four units of accounting pursuant to the transaction as follows: (1) $ 79.0 million to the common stock issued to RPI based on the closing price of the Company’s common stock on the date of the transaction, (2) $ 8.4 million to the Common Stock Warrant to purchase shares of common stock, based on the Black-Scholes option pricing model, (3) $ 12.6 million to the liability related to the sale of future royalties based on a discounted cash flow model and (4) $ 25.0 million to the Tranche A Note Payable based on the terms of the Loan Agreement. Transaction costs of $ 2.0 million were allocated directly to the units of accounting it relates to. Although the Company sold all of its rights to receive the Japan Royalty, under the terms of the RPI Agreement, the Company continues to own all tazemetostat intellectual property rights and at execution had significant continuing involvement in the generation of these royalties. Due to the Company’s continuing involvement, the Company will continue to account for any royalties due as revenue and recorded the proceeds from this transaction as a liability (“Royalty Obligation”) that will be accreted using the effective interest method over the estimated life of the RPI Purchase Agreement. As royalties are remitted to RPI from Eisai, the balance of the Royalty Obligation will be effectively repaid over the life of the Eisai License Agreement. In order to determine the accretion of the Royalty Obligation, the Company is required to estimate the total amount of future royalty payments to RPI over the life of the Eisai License Agreement. The $ 12.6 million recorded at execution will be accreted to the total of these royalty payments as interest expense over the life of the Royalty Obligation. At execution, the Company’s estimate of this total interest expense resulted in an effective annual interest rate of approximately 9.01 %. This estimate contains significant assumptions that impact both the amount recorded at execution and the interest expense that will be recognized over the royalty period. The Company periodically assesses the estimated royalty payments to RPI from Eisai and to the extent the amount or timing of such payments is materially different than the original estimates, an adjustment will be recorded prospectively to increase or decrease interest expense. There are a number of factors that could materially affect the amount and timing of royalty payments to RPI from Eisai, and correspondingly, the amount of interest expense recorded by the Company, most of which are not within the Company’s control. Such factors include, but are not limited to, delays or discontinuation of development of tazemetostat in Japan, regulatory approval, changing standards of care, the introduction of competing products, manufacturing or other delays, generic competition, intellectual property matters, adverse events that result in governmental health authority imposed restrictions on the use of the drug products, significant changes in foreign exchange rates as the royalties remitted to RPI are made in U.S. dollars (USD) while the underlying Japan sales of tazemetostat will be made in currencies other than USD, and other events or circumstances that are not currently foreseen as tazemetostat is still under development in Japan and subject to regulatory approval. Changes to any of these factors could result in increases or decreases to both royalty revenues and interest expense. On June 23, 2021, Eisai announced that it had obtained manufacturing and marketing approval for the EZH2 inhibitor “Tazverik® Tablets 200 mg” (tazemetostat hydrobromide) in Japan with the indication of relapsed or refractory EZH2 gene mutation-positive follicular lymphoma (only when standard treatment is not applicable), which caused the Company to reassess the estimated future royalty payments to RPI. As of September 30, 2021, the Company’s assessment of the estimated future royalty payments to RPI resulted in a current effective interest rate of approximately 11.4 %. The following table shows the activity of the Royalty Obligation since the transaction inception through September 30, 2021: As of September 30, 2021 (In thousands) Proceeds from sale of future royalties $ 12,601 Non-cash royalty revenue ( 7 ) Non-cash interest expense recognized 2,987 Liability related to the sale of future royalties - ending balance $ 15,581 During the three and nine months ended September 30, 2021 , the Company recorded non-cash royalties from net sales of tazemetostat in Japan of less than $ 0.1 million. During the three and nine months ended September 30, 2020, the Company did no t record non-cash royalties from net sales of tazemetostat in Japan. During the three and nine months ended September 30, 2021 and 2020, the Company recor ded $ 0.4 million and $ 1.4 million, respectively, and $ 0.3 million and $ 0.9 million, respectively, of related non-cash interest expense.

Long-Term Debt

Long-Term Debt	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Long-Term Debt	14. Long-Term Debt On November 4, 2019, the Company entered into the Loan Agreement, which provided for up to $ 70.0 million in secured term loans to be advanced in up to three tranches. The Company borrowed $70.0 million in the aggregate under the three tranches pursuant to the Loan Agreement. With the FDA’s June 2020 approval of tazemetostat for the treatment of FL in the United States, the Company also had the right, but not the obligation, to request up to an additional $ 300.0 million in secured term loans, subject to the approval of the Lenders, provided the Company has not prepaid any outstanding term loans at the time of such request and such request is made before November 18, 2021. On November 3, 2020, the Company entered into the Amended and Restated Loan Agreement with the Lenders. The Amended and Restated Loan Agreement provides for, among other things, an additional secured term loan of $ 150.0 million, or the Tranche D Loan. On November 3, 2020, the Company also delivered written notice to the Lenders to draw down the Tranche D Loan, which was funded on November 18, 2020. The Company paid a commitment fee of 2.00 % of the original $ 70.0 million committed facility amount in November 2019 and 2 % of the $ 150.0 million Tranche D Loan in November 2020, as well as expenses incurred by the Lender in executing the agreements. The interest rate for the Tranche D Loan will be determined by reference to a Eurodollar rate plus 7.75 % above such Eurodollar rate. The Eurodollar rate will have a 2.00 % floor. The Tranche D Loan will be due in eight equal quarterly principal payments commencing on the 51st month anniversary of the date on which the Lenders fund the Tranche D Loan. All unpaid principal and interest under the Tranche D Loan will be due and payable on the 72nd month anniversary of the date on which the Lenders funded the Tranche D Loan. The Amended and Restated Loan Agreement also amended the payment period principal and interest for the first three tranches of term loans. Under the original terms, the Company was required to make interest only payments on the outstanding obligation through February 28, 2023, and thereafter eight quarterly payments of principal and interest. Under the amended and restated terms, the Company is required to make interest only payments on the $ 70.0 million outstanding obligation through November 2023, and thereafter four quarterly payments of principal and interest. All unpaid principal and interest on the $ 70.0 million borrowed under the original Loan Agreement is due and payable in November 2024, the 60th month anniversary of the date on which the Lenders funded the first tranche of term loans. The interest rates for the existing tranches of term loans remain unchanged and will continue to be determined by reference to a Eurodollar rate plus 7.75% above such Eurodollar rate. The Eurodollar rate will have a 2.00% floor. Under the Amended and Restated Loan Agreement the Company has the right to request from the Lenders, subject to the Lenders’ agreement to lend additional amounts to the Company, up to an additional $ 150.0 million, provided that the Company has not prepaid any outstanding term loans at the time of the Company’s request and such request is made before November 18, 2021. Each of the four term loans may be prepaid before maturity in whole or in part, however there is a $ 50.0 million minimum prepayment for any prepayment of the loans. If the Company prepays any tranche of term loans, in whole or in part, during the first 36 months from the date on which the Lenders funded such tranche of term loans, then the Company must pay a prepayment premium equal to the greater of (x) a make-whole amount equal to the interest that would have accrued on the principal amount to be prepaid and (y) a premium equal to 0.03 multiplied by the principal amount to be prepaid. If the Company prepays a tranche of term loan, in whole or in part, between the 36th month and 48th month from the date on which the Lenders funded such tranche of term loans, then the Company must pay a prepayment premium equal to 0.02 multiplied by the principal amount to be prepaid. If the Company prepays a tranche of term loans, in whole or in part, between the 48th month and 60th month from the date on which the Lenders funded such tranche of term loans, then the Company must pay a prepayment premium equal to 0.01 multiplied by the principal amount to be prepaid. The Amended and Restated Loan Agreement was accounted for as a debt modification based on a comparison of the present value of the cash flows under the terms of the debt immediately before and after the effective date of The Amended and Restated Loan Agreement, which resulted in a change of less than 10%. As a result, issuance costs paid to the Lenders in connection with The Amended and Restated Loan Agreement were recorded as a reduction of the carrying amount of the debt liability and unamortized issuance costs as of the date of the modification are amortized to interest expense over the repayment term of The Amended and Restated Loan Agreement. The obligations under the Amended and Restated Loan Agreement, including the Company’s payment obligations in respect of the Tranche D Loan are secured by the first priority security interest in and a lien on substantially all of the assets of the Company, subject to certain exceptions, that the Company granted to the Lenders in connection with the first tranche of term loans under the Loan Agreement. The Amended and Restated Loan Agreement contains certain customary representations and warranties, affirmative and negative covenants and events of default applicable to the Company and its subsidiaries. If an event of default occurs and is continuing, the Collateral Agent may, among other things, accelerate the loans and foreclose on the collateral. The Company has determined that the risk of subjective acceleration under the material adverse events clause is not probable and therefore has classified the outstanding principal in non-current liabilities based on scheduled principal payments. The Company has the following minimum aggregate future loan payments at September 30, 2021 (in thousands): As of September 30, 2021 (In thousands) 2021 $ — 2022 — 2023 — 2024 70,000 2025 75,000 2026 75,000 Total minimum payments 220,000 Less amounts representing interest and discount ( 3,746 ) Less current portion — Long-term debt, net of current portion $ 216,254 For the three and nine months ended September 30, 2021 and 2020, interest expense related to the Company's Amended and Restated Loan Agreement was approximately $ 5.5 million and $ 16.3 million, respectively, and $ 1.3 million and $ 3.4 million, respectively. The total carrying value of debt is classified as long-term on the Company's condensed consolidated balance sheet as of September 30, 2021 and December 31, 2020.

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Stockholders' (Deficit) Equity	15. Stockholders’ (Deficit) Equity Common Stock On March 24, 2020, the Company’s board of directors adopted, subject to stockholder approval, an amendment to the Company’s Restated Certificate of Incorporation to increase the number of authorized shares of common stock, $ 0.0001 par value per share, from 125,000,000 to 150,000,000 (the “2020 Charter Amendment”). At the Company’s 2020 Annual Meeting of Stockholders, the stockholders of the Company approved the 2020 Charter Amendment, which was filed with the Secretary of State of the State of Delaware on May 29, 2020. On April 8, 2021, the Company’s board of directors adopted, subject to stockholder approval, an amendment to the Company’s Restated Certificate of Incorporation to increase the number of authorized shares of common stock from 150,000,000 to 225,000,000 (the "2021 Charter Amendment"). At the Company’s 2021 Annual Meeting of Stockholders, the stockholders of the Company approved the 2021 Charter Amendment, which was filed with the Secretary of State of the State of Delaware on June 11, 2021. The number of authorized shares of preferred stock was not affected by these Charter Amendments. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to dividends when and if declared by the board of directors. 2021 At-the-Market Offering Program On May 6, 2021, the Company entered into an Open Market Sale Agreement SM (“ATM Sale Agreement”), with Jefferies LLC (“Jefferies”) to sell, from time to time, shares of the Company's common stock having an aggregate offering price of up to $ 200.0 million through an “at the market offering” as defined in Rule 415 under the Securities Act of 1933, as amended, under which Jefferies would act as sales agent (the "ATM Offering"). The shares that may be sold under the ATM Sale Agreement, if any, are issued and sold pursuant to the Company’s shelf registration statement on Form S-3 that was declared effective by the Securities and Exchange Commission on May 13, 2021. The Company agreed to compensate Jefferies at a fixed commission rate equal to 3.0 % of the gross sales proceeds of such shares. During the three months ended June 30, 2021, the Company sold a total of 182,866 shares of the Company’s common stock under the ATM Sale Agreement, at a volume weighted average gross selling price of approximately $ 8.53 per share for net proceeds of approximately $ 1.5 million. During the three months ended June 30, 2021, in addition to sales commissions to Jefferies of approximately $ 48 thousand, which have been accounted for as an offset to additional paid-in capital, the Company incurred approximately $ 0.3 million of legal, accounting and other costs to establish and activate the ATM program. During the three months ended September 30, 2021, the Company sold a total of 1,247,599 shares of the Company’s common stock under the ATM Sale Agreement, at a volume weighted average gross selling price of approximately $ 5.45 per share for net proceeds of approximately $ 6.7 million. During the three months ended September 30, 2021, the company incurred commissions, accounting and legal costs totaling $ 0.2 million, which have been accounted for as an offset to additional paid-in capital. From the initiation of the ATM Offering through September 30, 2021, the Company has issued and sold 1,430,465 shares under the ATM Offering, resulting in aggregate net proceeds of $ 8.3 million after deducting issuance costs of $ 0.3 million. RPI Put Option In February 2020, the Company sold 2,500,000 shares of its common stock in connection with the exercise of its Put Option to sell shares of its common stock for an aggregate of $ 49.9 million in net proceeds after deducting financing costs of $ 0.1 million (see Note 13, Sale of Future Royalties ). Convertible Preferred Stock The Company has 337,800 shares of Series A Convertible Preferred Stock outstanding as of September 30, 2021 and as of December 31, 2020. Voting Rights Shares of Series A Preferred Stock will generally have no voting rights except as required by law and except that the consent of the holders of a majority of the outstanding shares of Series A Preferred Stock will be required to amend the terms of the Series A Preferred Stock or take certain other actions with respect to the Series A Preferred Stock. Dividends Shares of Series A Preferred Stock will be entitled to receive dividends equal to (on an as-if-converted-to-common stock basis), and in the same form and manner as, dividends actually paid on shares of the Company’s common stock. Liquidation Rights Subject to the prior and superior rights of the holders of any senior securities of the Company, upon liquidation, dissolution or winding up of the Company, whether voluntary or involuntary, each holder of shares of Series A Preferred Stock shall be entitled to receive, in preference to any distributions of any of the assets or surplus funds of the Company to the holders of common stock, an amount equal to $ 0.001 per share of Series A Preferred Stock, plus an additional amount equal to any dividends declared but unpaid on such shares, before any payments shall be made or any assets distributed to holders of any class of common stock. If, upon any such liquidation, dissolution or winding up of the Company, the assets of the Company shall be insufficient to pay the holders of shares of the Series A Preferred Stock the amount required under the preceding sentence, then all remaining assets of the Company shall be distributed ratably to holders of the shares of the Series A Preferred Stock in proportion to the respective amounts which would otherwise be payable in respect of the shares held by them upon such distribution if all amounts payable on or with respect to such shares were paid in full. Conversion Each share of Series A Preferred Stock shall be convertible, at any time and from time to time from and after the issuance date, at the option of the holder thereof, into a number of shares of common stock equal to 10 shares of common stock, provided that the holder will be prohibited from converting Series A Preferred Stock into shares of the Company’s common stock if, as a result of such conversion, the holder, together with its affiliates and attribution parties, would own more than 9.99 % of the total number of shares of common stock then issued and outstanding. The holder can change this requirement to a higher or lower percentage, not to exceed 9.99% of the number of shares of common stock outstanding, upon 61 days’ notice to the Company. In February 2020, 12,200 shares of Series A Preferred Stock were converted to 122,000 shares of common stock. Redemption The Company is not obligated to redeem or repurchase any shares of Series A Preferred Stock. Shares of Series A Preferred Stock are not entitled to any redemption rights or mandatory sinking fund or analogous fund provisions. Warrants In November 2019, the Company issued the Common Stock Warrant for the purchase of up to 2,500,000 shares of Common Stock at an exercise price of $ 20.00 per share to RPI pursuant to the RPI Purchase Agreement (for additional information see Note 13, Sale of Future Royalties ), which was classified as equity and recorded at its relative fair value of $ 8.4 million to additional paid-in capital on the Company's condensed consolidated balance sheets. The Common Stock Warrant remains outstanding as of September 30, 2021. In August 2021, the Company issued the HutchMed Warrant to HutchMed under the HutchMed License Agreement, exercisable at any time prior to August 7, 2025 for up to 5,653,000 shares of the Company’s common stock at an exercise price of $ 11.50 per share. Under the HutchMed Warrant, the number of shares issuable under the warrant is reduced from 5,653,000 to 2,826,500 in the event that the HutchMed License Agreement is terminated by HutchMed at any time and for any reason or no; terminated by the Company for HutchMed's material breach of the HutchMed License Agreement; terminated by the Company in connection with a challenge by HutchMed of any of the patent rights licensed to HutchMed under the HutchMed License Agreement; or terminated by the Company in connection with an event of bankruptcy of HutchMed, in each case as more fully described in the HutchMed License Agreement. Due to this provision in the HutchMed Warrant, the Company concluded that the warrant does not meet the exception from derivative accounting pursuant to ASC 815, Derivatives and Hedging, which requires that the warrant be accounted for as a derivative. Accordingly, the Company recorded a warrant liability in the amount of approximately $ 13.0 million upon issuance of the HutchMed Warrant. The fair value of the HutchMed Warrant was determined using a Black-Scholes and Monte Carlo pricing model. The HutchMed Warrant is subject to revaluation at each balance sheet date and any changes in fair value are recorded as a non-cash gain or (loss) in the Company's condensed consolidated statement of operations and comprehensive loss as a component of other income (expense), net until the earlier of the exercise or expiration of the HutchMed Warrant or upon the completion of a liquidation event. Upon exercise, the HutchMed Warrant is subject to revaluation just prior to the date of the warrant exercise and any changes in fair value are recorded as a non-cash gain or (loss). The Company recorded non-cash gains of approximately $ 4.4 million during the three and nine months ended September 30, 2021 in its condensed consolidated statement of operations and comprehensive loss attributable to the decreases in the fair value of the warrant liability that resulted from a reduction in the Company's stock price as of September 30, 2021. The following table rolls forward the fair value of the HutchMed Warrant liability, the fair value of which is determined by Level 3 inputs at inception on August 7, 2021, and as of September 30, 2021: (In thousands) Fair value at inception $ 13,050 Decrease in fair value ( 4,420 ) Fair value at September 30, 2021 $ 8,630 The key assumptions used to value the HutchMed Warrant were as follows: Inception As of September 30, 2021 Risk-free interest rate 0.6 % 0.72 % Expected term (in years) 4.0 years 3.85 years Expected volatility of underlying stock 70.0 % 70.0 % Expected dividend yield - - Stock price $ 6.47 $ 5.12

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	16. Stock-Based Compensation The Company maintains one stock incentive plan, the 2013 Stock Incentive Plan, as well as the 2013 Employee Stock Purchase Plan. In addition, during the three and nine months ended September 30, 2021, the Company granted options to purchase an aggregate of 80,000 shares of Epizyme common stock and 53,333 restricted stock units (RSUs) to two new employees as equity inducement awards outside of the Company's 2013 Stock Incentive Plan and material to the employees’ acceptance of employment with the Company. These equity awards were approved in accordance with Nasdaq Listing Rule 5635(c)(4), and these equity awards remained outstanding as of September 30, 2021. These options have an exercise price of $ 5.31 per share, which is equal to the closing price of Epizyme common stock on August 16, 2021 , the grant date of these options. These inducement awards are included in stock-based compensation expense and the following tables. Total stock-based compensation expense related to stock options, restricted stock units, shares issued under the employee stock purchase plan, and shares granted to non-employee directors in lieu of board fees was $ 6.2 million and $ 6.4 million for the three months ended September 30, 2021 and September 30, 2020 , respectively, and $ 19.9 million and $ 21.2 million for the nine months ended September 30, 2021 and September 30, 2020, respectively. Stock-based compensation expense is classified in the condensed consolidated statements of operations and comprehensive loss as follows: Three Months Ended Nine Months Ended 2021 2020 2021 2020 (In thousands) (In thousands) Research and development $ 2,042 $ 2,079 $ 6,295 $ 7,045 General and administrative 4,128 4,307 13,608 14,143 Total $ 6,170 $ 6,386 $ 19,903 $ 21,188 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 $ 4.93 and $ 8.56 per option for those options granted during the three months ended September 30, 2021 and 2020 , respectively, and $ 3.27 and $ 11.94 for those options granted during the nine months ended September 30, 2021 and 2020, respectively. Key assumptions used to apply this pricing model were as follows: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Risk-free interest rate 0.4 % 0.3 % 0.4 % 1.0 % Expected life of options 5.83 years 6.0 years 5.89 years 5.99 years Expected volatility of underlying stock 69.6 % 72.1 % 70.1 % 70.9 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 % The following is a summary of stock option activity for the nine months ended September 30, 2021: Number of Weighted Weighted Aggregate (In thousands) (In years) (In thousands) Outstanding at December 31, 2020 10,225 $ 14.77 Granted 5,943 8.06 Exercised ( 104 ) 8.83 Forfeited ( 2,458 ) 14.39 Outstanding at September 30, 2021 13,606 $ 11.95 7.66 $ 10 Exercisable at September 30, 2021 5,697 $ 14.40 5.77 $ 10 As of September 30, 2021, there wa s $ 43.5 million of unrecognized compensation cost related to stock options that are expected to vest. These costs are expected to be recognized over a weig hte d average remaining vesting period of 2.47 y ears. Restricted Stock Units During the nine months ended September 30, 2021, 2,186,595 restricted stock units (“RSUs”) were granted to executives and employees and 72,540 RSUs were granted to non-employee directors. The awards were service-based. Assuming all service conditions are achieved, the executive and employee RSUs will vest as to 50 %, 33 %, and 25 %, respectively, of the shares of Company common stock underlying the RSUs on an annual basis over a two , three or four year period of time from the grant dates, respectively, and the non-employee director RSUs will vest as to 100 % of the shares of Company common stock underlying the RSUs in full on the earlier of the first anniversary of the grant date and the date of the succeeding annual meeting of stockholders. Number Weighted Outstanding at December 31, 2020 668 $ 17.56 Granted 2,259 7.86 Vested ( 177 ) 17.35 Forfeited ( 444 ) 13.13 Outstanding at September 30, 2021 2,306 $ 8.92 Compensation expense totaling $ 1.1 million and $ 3.3 million was recognized for the service-based RSUs for the three and nine months ended September 30, 2021, respectively. Compensation expense totaling $ 0.7 million and $ 2.0 million was recognized for the service-based RSUs for the three and nine months ended September 30, 2020, respectively. As of September 30, 2021 , there was $ 16.3 million of unrecognized compensation cost related to service-based RSUs that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 2.42 years. During 2019, the Company granted 604,000 RSUs to executives and employees, which contained performance conditions, and 20 % of the RSUs vested on June 30, 2019, 25 % of the RSUs vested on January 23, 2020, 20 % of the RSUs vested on March 24, 2020, and 30 % of the RSUs vested on June 25, 2020 in connection with achievement of the final performance milestone. There was no unrecognized compensation cost as of September 30, 2021, related to performance-based RSUs. Compensation expense totaling $ 0.0 million and $ 3.5 million was recognized for the performance-based RSUs for the three and nine months ended September 30, 2020, respectively. There was no compensation expense recognized for the performance-based RSUs for the three and nine months ended September 30, 2021.

Loss Per Share

Loss Per Share	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Loss Per Share	17. Loss Per Share Basic and diluted loss per share allocable to common stockholders are computed as follows: Three Months Ended Nine Months Ended 2021 2020 2021 2020 (In thousands except per share data) (In thousands except per share data) Net loss $ ( 65,834 ) $ ( 56,073 ) $ ( 200,471 ) $ ( 165,461 ) Weighted average shares outstanding 102,513 101,512 102,123 100,747 Basic and diluted loss per share allocable to common stockholders $ ( 0.64 ) $ ( 0.55 ) $ ( 1.96 ) $ ( 1.64 ) 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: For the Three and Nine Months Ended As of September 30, 2021 2020 (In thousands) Stock options 13,606 10,148 Restricted stock units 2,306 663 Shares issuable under employee stock purchase plan 47 32 Series A Preferred Stock (if converted) 3,378 3,378 Warrants 8,153 2,500 27,490 16,721

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2021
Subsequent Events [Abstract]
Subsequent Events	18. Subsequent Events On October 1, 2021, the Company granted options to purchase an aggregate of 125,866 shares of Epizyme common stock and 25,289 restricted stock units (RSUs) to a new employee as equity inducement awards outside of the Company's 2013 Stock Incentive Plan and material to the employee’s acceptance of employment with the Company. These options have an exercise price of $ 5.19 per share, which is equal to the closing price of Epizyme common stock on October 1, 2021 , the grant date of these options. On November 1, 2021, the Company granted options to purchase an aggregate of 42,500 shares of Epizyme common stock and 28,333 restricted stock units (RSUs) to a new employee as equity inducement awards outside of the Company's 2013 Stock Incentive Plan and material to the employee’s acceptance of employment with the Company. These options have an exercise price of $ 4.44 per share, which is equal to the closing price of Epizyme common stock on November 1, 2021 , the grant date of these options. These equity awards were approved in accordance with Nasdaq Listing Rule 5635(c)(4), and these equity awards remain outstanding as of the date of the filing of this Quarterly Report on Form 10-Q.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation 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, or the 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, 2020, or the Annual Report. 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 condensed 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 September 30, 2021 and 2020 are referred to as the third quarter of 2021 and 2020 , 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.
Use of Estimates	Use of Estimates The preparation of these condensed consolidated financial statements in accordance with accounting principles generally accepted in the United States requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities, as of the date of the condensed consolidated financial statements, and the reported amounts of revenue and expenses during the reporting period. Actual results and outcomes may differ materially from management’s estimates, judgments and assumptions.
Significant Accounting Policies	Significant Accounting Policies The significant accounting policies used in preparation of these condensed consolidated financial statements for the three and nine months ended September 30, 2021 are consistent with those discussed in Note 2 to the consolidated financial statements in the Annual Report and are updated below as necessary.
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 about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued and such doubt is not alleviated by the Company’s plans or when the Company's 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 its available cash, cash equivalents and marketable securities. The Company has recurring losses and expects to have recurring losses for the foreseeable future with the continued commercialization of TAZVERIK in ES and FL, the development of tazemetostat in other indications, and the development of the Company’s other product candidates. In addition, the Company has experienced and continues to experience challenges in the continued commercialization of TAZVERIK resulting from the ongoing COVID-19 pandemic, which the Company believes has had an adverse impact on TAZVERIK revenues. In response to the challenges that the Company has continued to face since the Company commenced its launch of TAZVERIK in FL in June 2020, the Company implemented an operational cost reduction plan that was announced on August 9, 2021 and continues to evaluate its costs on an on-going basis with the intent to streamline such costs. The analysis of the Company’s ability to continue as a going concern for the third quarter of 2021 included consideration of the Company’s current cash needs, including its research and development plans, commercialization activities associated with the continued commercialization of TAZVERIK in the ES and FL indications, its existing debt service obligations, anticipated cost savings resulting from the August 2021 operational cost reduction plan, including ongoing efforts to eliminate costs not related to the Company’s strategic focus. The analysis included forecasted product revenues from sales of TAZVERIK. Such estimates of future sales contain significant judgment as TAZVERIK was first launched in the first half of 2020 and there is little history with which to base such estimates. In addition, the Company’s ongoing efforts to eliminate costs not related to the Company’s strategic focus contains uncertainties as to whether the Company can attain such benefits. Based on the analysis, the Company concluded that its available cash, cash equivalents and marketable securities as of September 30, 2021 will be sufficient to fund current planned operations and capital expenditure requirements and pay our debt service obligations as they become due into the fourth quarter of 2022 which is at least 12 months from the filing date of this Quarterly Report on Form 10-Q with the SEC. The Company’s current operating plan is based on assumptions that may prove to be wrong, and the Company could use its capital resources sooner than it expects, in which case the Company would evaluate further reductions in its expenses or obtaining additional financing sooner than it otherwise would, which additional financing may not be available or may only be available on terms that are not acceptable to the Company.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements Revenue Recognition – Collaboration Revenue In November 2018, the FASB, issued ASU 2018-18, Collaborative Arrangements, or ASC 808, which clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue when the collaborative arrangement participant is a customer in the context of a unit of account and precludes recognizing as revenue consideration received from a collaborative arrangement participant if the participant is not a customer. The new standard is effective in the first quarter of fiscal 2021. The Company adopted ASC 808 effective in the first quarter of fiscal 2021 and the Company’s adoption of this standard did not have a material effect on the Company’s condensed consolidated statements of operations and comprehensive loss or condensed consolidated statements of cash flows. Income Taxes In December 2019, the Financial Accounting Standards Board, or the FASB, issued ASU 2019-12, Income Taxes , or ASC 740, which simplifies the accounting for income taxes. The new standard is effective in the first quarter of fiscal 2021. The Company adopted ASC 740 effective in the first quarter of fiscal 2021 and the Company’s adoption of this standard did not have a material effect on the Company’s condensed consolidated statements of operations and comprehensive loss or condensed consolidated statements of cash flows.
Revenue Recognition	Revenue Recognition The Company recognizes revenue when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the Company expects to receive in exchange for those goods or services. To determine revenue recognition, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. For a further discussion of accounting for net product revenue see Note 3, Product Revenue, Net .
Other Revenue	Other Revenue Other revenue consists of revenue from the sales of tazemetostat active pharmaceutical ingredient (API) and drug product to the Company’s licensees or collaborators. The Company recognizes revenue on tazemetostat API and drug product when control has transferred under the terms of each agreement.
Cost of Revenues	Cost of Revenues Cost of revenues primarily consists of costs related to the sales of TAZVERIK and sales of tazemetostat API and drug product to the Company’s licensees or collaborators. These costs include materials, labor, manufacturing overhead, amortization of milestone payments, and royalties payable on net sales of TAZVERIK. Cost of revenues for the nine months ended September 30, 2021 included approximately $ 0.8 million related to sales of tazemetostat drug product. There were no sales of tazemetostat drug product during the three months ended September 30, 2021 or during the three and nine months ended September 30, 2020.
Accounts Receivable	Accounts Receivable The Company extends credit to customers based on its evaluation of the customer’s financial condition. The Company records receivables for all billings when amounts are due under standard terms. Accounts receivable are stated at amounts due net of applicable prompt pay discounts and other contractual adjustments as well as an allowance for doubtful accounts. The Company assesses the need for an allowance for doubtful accounts by considering a number of factors, including the length of time trade accounts receivable are past due, the customer’s ability to pay its obligation and the condition of the general economy and the industry as a whole. The Company will write off accounts receivable when the Company determines that they are uncollectible. In general, the Company has experienced no significant collection issues with its customers.
Inventory	Inventory The Company outsources the manufacturing of TAZVERIK and uses contract manufacturers to produce the raw and intermediate materials used in the production of TAZVERIK as well as the finished product. The Company currently has one supplier qualified for each step in the manufacturing process and is in the process of qualifying additional suppliers. Inventory is composed of raw materials, intermediate materials, which are classified as work-in-process, and finished goods, which are goods that are available for sale. The Company states inventory at the lower of cost or net realizable value with the cost based on the first-in, first-out method. Inventory is classified as long-term when it is expected to be utilized beyond the Company’s normal operating cycle and is included in restricted cash and other assets on the Company's condensed consolidated balance sheets. If the Company identifies excess, obsolete or unsalable items, it writes down its inventory to its net realizable value in the period in which the impairment is identified. These adjustments are recorded based upon various factors related to the product, including the level of product manufactured by the Company, the level of product in the distribution channel, current and projected demand, the expected shelf-life of the product and firm inventory purchase commitments. Shipping and handling costs incurred for inventory purchases are included in inventory costs and costs incurred for product shipments are recorded as incurred in cost of revenue. Prior to receiving its first approval from the U.S. Food and Drug Administration, or FDA, on January 23, 2020 to sell TAZVERIK, the Company expensed all costs incurred related to the manufacture of TAZVERIK as research and development expense because of the inherent risks associated with the development of a product candidate, the uncertainty about the regulatory approval process and the lack of history for the Company of regulatory approval of drug candidates.
Intangible Assets, Net	Intangible Assets, Net Intangible assets consist of capitalized milestone payments made to third parties under an in-license of patent rights upon receiving regulatory approval of TAZVERIK. The finite-lived intangible assets are being amortized on a straight-line basis over the expected time period the Company will benefit from the in-licensed rights, which is generally the patent life. Intangible assets are recorded at cost at the time of their acquisition and are stated in the Company’s condensed consolidated balance sheets net of accumulated amortization and impairments, if applicable. The amortization expense is recognized as cost of revenue in the Company’s condensed consolidated statement of operations and comprehensive loss. During the first quarter of 2020 the Company paid a $ 25.0 million milestone payment under its agreement with Eisai, Co., Ltd., or Eisai, upon regulatory approval of tazemetostat for ES. During the second quarter of 2020 the Company paid a $ 25.0 million milestone payment under its agreement with Eisai upon regulatory approval of tazemetostat for FL. Both regulatory milestones have been capitalized as intangible assets. The following table presents intangible assets as of September 30, 2021 (in thousands): September 30, 2021 Estimated useful In-licensed rights $ 50,000 12.2 Less: accumulated amortization ( 6,113 ) Total intangible asset, net $ 43,887 The Company recorded approximately $ 1.0 million and $ 3.1 million in amortization expense related to intangible assets, using the straight-line methodology, during the three and nine months ended September 30, 2021, respectively. The Company recorded approximatel y $ 1.0 million and $ 2.0 million in amortization expense related to intangible assets, using the straight-line methodology, during the three and nine months ended September 30, 2020, respectively. Estimated future amortization expense for intangible assets for the remainder of the year ended December 31, 2021 is $ 1.0 million and approximately $ 4.2 million per year thereafter. The Company assesses its intangible assets for impairment if indicators are present or changes in circumstance suggest that impairment may exist. Events that could result in an impairment, or trigger an interim impairment assessment, include the receipt of additional clinical or nonclinical data regarding one of the Company’s drug candidates or a potentially competitive drug candidate, changes in the clinical development program for a drug candidate, or new information regarding potential sales for the drug. If impairment indicators are present or changes in circumstance suggest that impairment may exist, the Company performs a recoverability test by comparing the sum of the estimated undiscounted cash flows of each intangible asset to its carrying value on the Company's condensed consolidated balance sheets. If the undiscounted cash flows used in the recoverability test are less than the carrying value, the Company would determine the fair value of the intangible asset and recognize an impairment loss if the carrying value of the intangible asset exceeds its fair value. During the three months ended June 30, 2021, the Company concluded the lower than anticipated current and projected future revenue, due to the impact of the COVID-19 pandemic as well as other factors, was an indicator that impairment may exist related to its finite-lived intangible assets. As a result, the Company performed a recoverability test and determined that the finite-lived intangible assets were recoverable. The Company’s quantitative assessment considered significant assumptions related to estimates of future TAZVERIK sales, offset by direct costs to derive the sales. The estimates of future TAZVERIK sales and associated costs include estimates of significant growth, however, these estimates are uncertain as the product was first launched in the first half of 2020 and due to the uncertainties associated with the ongoing COVID-19 pandemic. Given the limited history of sales and the inherent difficulty in making a long-range forecast, such estimates contain significant uncertainty. If the assumptions regarding forecasted revenue or the costs to derive such revenues prove to be inaccurate, the Company may be required to perform future impairment analyses and record an impairment charge for its intangible assets in future periods.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Summary of Intangible Assets	The following table presents intangible assets as of September 30, 2021 (in thousands): September 30, 2021 Estimated useful In-licensed rights $ 50,000 12.2 Less: accumulated amortization ( 6,113 ) Total intangible asset, net $ 43,887

Product Revenue Net (Tables)

Product Revenue Net (Tables)	9 Months Ended
	Sep. 30, 2021
Net Product Revenue [Member]
Concentration Risk [Line Items]
Schedules of Concentration of Risk Percentage	Revenue earned from each customer as a percentage of net product revenue is as follows: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Customer 1 27 % 42 % 34 % 48 % Customer 2 19 % 10 % 13 % 12 % Customer 3 19 % 18 % 32 % 15 % Customer 4 35 % 23 % 21 % 18 %
Accounts Receivable [Member]
Concentration Risk [Line Items]
Schedules of Concentration of Risk Percentage	As of September 30, 2021 and December 31, 2020, the four individual customers represented as a percentage of accounts receivable as follows: September 30, December 31, Customer 1 15 % 21 % Customer 2 17 % 14 % Customer 3 15 % 29 % Customer 4 53 % 36 %
TAZVERIK [Member]
Concentration Risk [Line Items]
Summary of Product Revenue Allowance and Reserve Categories	The following table summarizes activity in each of the above product revenue allowances and reserve categories for the nine months ended September 30, 2021: Chargebacks, Government Fees Rebates Returns Total (In thousands) Balance, January 1, 2021 $ 133 $ 428 $ 67 $ 628 Provision 1,344 2,152 252 3,748 Payments or credits ( 1,294 ) ( 2,110 ) ( 199 ) ( 3,603 ) Balance, September, 2021 $ 183 $ 470 $ 120 $ 773

Cash (Tables)

Cash (Tables)	9 Months Ended
	Sep. 30, 2021
Cash And Cash Equivalents [Abstract]
Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash	A reconciliation of cash, cash equivalents, and restricted cash reported within the Company's condensed consolidated balance sheets that sum to the total of the same such amounts shown in the Company's condensed consolidated statements of cash flows, is as follows: As of September 30, 2021 2020 (In thousands) Cash and cash equivalents $ 124,517 $ 93,422 Restricted cash, as part of other assets 1,509 1,509 Total cash, cash equivalents, and restricted cash shown in the condensed consolidated statements of cash flows $ 126,026 $ 94,931

Marketable Securities (Tables)

Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2021
Investments Debt And Equity Securities [Abstract]
Summary of Available-for-Sale Securities Held	The following table summarizes the available-for-sale securities held at September 30, 2021 (in thousands): Description Amortized Fair Value Commercial paper $ 72,733 $ 4 $ ( 2 ) $ 72,735 Corporate notes 7,022 — ( 4 ) 7,018 U.S. government agency securities and U.S. Treasuries 17,023 — — 17,023 Total $ 96,778 $ 4 $ ( 6 ) $ 96,776 The following table summarizes the available-for-sale securities held at December 31, 2020 (in thousands): Description Amortized Fair Value Commercial paper $ 158,907 $ 14 $ ( 8 ) $ 158,913 Corporate notes 33,437 3 ( 7 ) 33,433 U.S. government agency securities and U.S. Treasuries 13,044 1 — 13,045 Total $ 205,388 $ 18 $ ( 15 ) $ 205,391

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Summary of Company's Financial Assets Recognized at Fair Value	As of September 30, 2021 and December 31, 2020, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of September 30, 2021 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 97,136 $ 83,412 $ 13,724 $ — Marketable securities: Commercial paper 72,735 — 72,735 — Corporate notes 7,018 — 7,018 — U.S. government agency securities and treasuries 17,023 — 17,023 — Total $ 193,912 $ 83,412 $ 110,500 $ — Fair Value as of December 31, 2020 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 163,264 $ 113,505 $ 49,759 $ — Marketable securities: Commercial paper 158,913 — 158,913 — Corporate notes 33,433 — 33,433 — U.S. government agency securities and treasuries 13,045 — 13,045 — Total $ 368,655 $ 113,505 $ 255,150 $ —

Inventory (Tables)

Inventory (Tables)	9 Months Ended
	Sep. 30, 2021
Inventory Disclosure [Abstract]
Summary of Inventory	All of the Company’s inventories relate to the manufacturing of TAZVERIK. The following table sets forth the Company’s inventories as of September 30, 2021 and December 31, 2020: September 30, December 31, (In thousands) Raw materials $ 3,116 $ 1,068 Work in process 11,591 8,564 Finished goods 2,237 829 Total $ 16,944 $ 10,461 Balance sheet classification Inventory $ 1,521 $ 10,461 Restricted cash and other assets 15,423 — Total $ 16,944 $ 10,461

Supplemental Balance Sheet In_2

Supplemental Balance Sheet Information (Tables)	9 Months Ended
	Sep. 30, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following: September 30, December 31, (In thousands) Employee compensation and benefits $ 10,578 $ 11,921 Research and development expenses 18,804 10,664 Professional services and other 1,540 5,987 Accrued expenses $ 30,922 $ 28,572

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Summary of Lease Costs and Company's Operating Leases	The following table contains a summary of the lease costs recognized under ASC 842 and other information pertaining to the Company’s operating leases for the three and nine months ended September 30, 2021 and 2020: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Lease cost Operating lease cost $ 1,603 $ 1,554 $ 4,634 $ 4,660 Variable lease cost 498 405 1,545 1,351 Total lease cost $ 2,101 $ 1,959 $ 6,179 $ 6,011 Other information Operating cash flows used for operating leases $ 1,601 $ 1,000 $ 4,826 $ 2,980 Weighted average remaining lease term 4.4 years 4.9 years 4.4 years 4.9 years Weighted average discount rate 9.73 % 9.73 % 9.73 % 9.73 %
Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases	Future minimum lease payments under the Company’s non-cancelable operating leases as of September 30, 2021, are as follows: (In thousands) 2021 $ 1,610 2022 6,202 2023 7,517 2024 7,322 Thereafter 6,966 Total lease payments $ 29,617 Less: imputed interest ( 5,925 ) Total operating lease liabilities at September 30, 2021 $ 23,692

Sale of Future Royalties (Table

Sale of Future Royalties (Tables)	9 Months Ended
	Sep. 30, 2021
Deferred Revenue Disclosure [Abstract]
Schedule of Activity of Royalty Obligation	The following table shows the activity of the Royalty Obligation since the transaction inception through September 30, 2021: As of September 30, 2021 (In thousands) Proceeds from sale of future royalties $ 12,601 Non-cash royalty revenue ( 7 ) Non-cash interest expense recognized 2,987 Liability related to the sale of future royalties - ending balance $ 15,581

Long Term Debt (Tables)

Long Term Debt (Tables)	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Schedule of Minimum Aggregate Future Loan Payments	The Company has the following minimum aggregate future loan payments at September 30, 2021 (in thousands): As of September 30, 2021 (In thousands) 2021 $ — 2022 — 2023 — 2024 70,000 2025 75,000 2026 75,000 Total minimum payments 220,000 Less amounts representing interest and discount ( 3,746 ) Less current portion — Long-term debt, net of current portion $ 216,254

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity (Tables)	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Summary of Change in Fair Value of Warrant Liability	The following table rolls forward the fair value of the HutchMed Warrant liability, the fair value of which is determined by Level 3 inputs at inception on August 7, 2021, and as of September 30, 2021: (In thousands) Fair value at inception $ 13,050 Decrease in fair value ( 4,420 ) Fair value at September 30, 2021 $ 8,630
Summary of Key Assumptions Used to Value Warrant	The key assumptions used to value the HutchMed Warrant were as follows: Inception As of September 30, 2021 Risk-free interest rate 0.6 % 0.72 % Expected term (in years) 4.0 years 3.85 years Expected volatility of underlying stock 70.0 % 70.0 % Expected dividend yield - - Stock price $ 6.47 $ 5.12

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2021
Disclosure Of Compensation Related Costs Sharebased 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 Nine Months Ended 2021 2020 2021 2020 (In thousands) (In thousands) Research and development $ 2,042 $ 2,079 $ 6,295 $ 7,045 General and administrative 4,128 4,307 13,608 14,143 Total $ 6,170 $ 6,386 $ 19,903 $ 21,188
Assumptions Used in Applying Pricing Model	Key assumptions used to apply this pricing model were as follows: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Risk-free interest rate 0.4 % 0.3 % 0.4 % 1.0 % Expected life of options 5.83 years 6.0 years 5.89 years 5.99 years Expected volatility of underlying stock 69.6 % 72.1 % 70.1 % 70.9 % 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 nine months ended September 30, 2021: Number of Weighted Weighted Aggregate (In thousands) (In years) (In thousands) Outstanding at December 31, 2020 10,225 $ 14.77 Granted 5,943 8.06 Exercised ( 104 ) 8.83 Forfeited ( 2,458 ) 14.39 Outstanding at September 30, 2021 13,606 $ 11.95 7.66 $ 10 Exercisable at September 30, 2021 5,697 $ 14.40 5.77 $ 10
Summary of Service Based Restricted Stock Units	During the nine months ended September 30, 2021, 2,186,595 restricted stock units (“RSUs”) were granted to executives and employees and 72,540 RSUs were granted to non-employee directors. The awards were service-based. Assuming all service conditions are achieved, the executive and employee RSUs will vest as to 50 %, 33 %, and 25 %, respectively, of the shares of Company common stock underlying the RSUs on an annual basis over a two , three or four year period of time from the grant dates, respectively, and the non-employee director RSUs will vest as to 100 % of the shares of Company common stock underlying the RSUs in full on the earlier of the first anniversary of the grant date and the date of the succeeding annual meeting of stockholders. Number Weighted Outstanding at December 31, 2020 668 $ 17.56 Granted 2,259 7.86 Vested ( 177 ) 17.35 Forfeited ( 444 ) 13.13 Outstanding at September 30, 2021 2,306 $ 8.92

Loss Per Share (Tables)

Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2021
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 Nine Months Ended 2021 2020 2021 2020 (In thousands except per share data) (In thousands except per share data) Net loss $ ( 65,834 ) $ ( 56,073 ) $ ( 200,471 ) $ ( 165,461 ) Weighted average shares outstanding 102,513 101,512 102,123 100,747 Basic and diluted loss per share allocable to common stockholders $ ( 0.64 ) $ ( 0.55 ) $ ( 1.96 ) $ ( 1.64 )
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: For the Three and Nine Months Ended As of September 30, 2021 2020 (In thousands) Stock options 13,606 10,148 Restricted stock units 2,306 663 Shares issuable under employee stock purchase plan 47 32 Series A Preferred Stock (if converted) 3,378 3,378 Warrants 8,153 2,500 27,490 16,721

The Company - Additional Inform

The Company - Additional Information (Detail) - USD ($) $ in Thousands	Aug. 09, 2021	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Basis Of Presentation [Line Items]
Proceeds from the issuance of shares under employee stock purchase plan					$ 8,261
Issuance of common stock, Value		$ 6,727	$ 1,534
Proceeds from sale of redeemable convertible preferred stock					$ 76,000
Initial public offering completion date					May 2013
Cash, cash equivalents, and marketable securities		221,300			$ 221,300
Accumulated deficit		(1,189,184)			(1,189,184)		$ (988,713)
Selling, general and administrative		32,793		$ 30,575	103,095	90,161
Research and development		34,549		$ 25,738	102,110	$ 77,253
RPI Finance Trust (RPI) and BioPharma Credit Investments V (Master) LP and BioPharma Credit PLC (the Lenders) [Member]
Basis Of Presentation [Line Items]
Fund received from collaborators					368,100
RPI Finance Trust [Member]
Basis Of Presentation [Line Items]
Proceeds from the issuance of shares under employee stock purchase plan					150,000
Royalty Pharma and Pharmakon Advisors [Member]
Basis Of Presentation [Line Items]
Aggregate fund, amount					1,560,600
Non-equity funding through collaboration agreement					268,800
BioPharma Credit Investments V Master LP And BioPharma Credit PLC [Member]
Basis Of Presentation [Line Items]
Debt financing received through loan agreement					218,100
Operating Cost Reduction Plan [Member]
Basis Of Presentation [Line Items]
Percentage of cross-functional reduction of current workforce	11.00%
Severance and termination costs	$ 2,000
Selling, general and administrative		1,600
Research and development		$ 400
IPO [Member]
Basis Of Presentation [Line Items]
Issuance of common stock, Value					$ 847,700

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Accounting Policies [Line Items]
Cost of revenue	$ 1,999	$ 1,608			$ 7,345	$ 3,244
Amortization of Intangible Assets	1,000	1,000			3,100	2,000
Estimated future amortization of intangible assets					1,000
Estimated future amortization of intangible assets thereafter					4,200
Tazemetostat Drug Pruducts [Member]
Accounting Policies [Line Items]
Cost of revenue	$ 0	$ 0			$ 800	$ 0
Eisai [Member]
Accounting Policies [Line Items]
Milestone payments			$ 25,000	$ 25,000

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary of Intangible Assets (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
In-licensed rights	$ 50,000
Less: accumulated amortization	(6,113)
Total intangible asset, net	$ 43,887	$ 47,002
Estimated useful life (years)	12 years 2 months 12 days

Product Revenue Net - Summary o

Product Revenue Net - Summary of Product Revenue Allowance and Reserve Categories (Detail) - TAZVERIK [Member] $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)
Valuation And Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 628
Provision	3,748
Payments or credits	(3,603)
Ending Balance	773
Chargebacks, Discounts, and Fees [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Beginning Balance	133
Provision	1,344
Payments or credits	(1,294)
Ending Balance	183
Government and Other Rebates [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Beginning Balance	428
Provision	2,152
Payments or credits	(2,110)
Ending Balance	470
Returns [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Beginning Balance	67
Provision	252
Payments or credits	(199)
Ending Balance	$ 120

Product Revenue Net - Additiona

Product Revenue Net - Additional Information (Detail) - Customer	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accounts Receivable [Member] | Credit Concentration Risk [Member]
Product Information [Line Items]
Number of customers	4	4	4	4

Product Revenue Net - Schedule

Product Revenue Net - Schedule Of Each Customer As a Percentage Of Net Product Revenue (Detail) - Net Product Revenue [Member] - Customer Concentration Risk [Member]	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Customer 1 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	27.00%	42.00%	34.00%	48.00%
Customer 2 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	19.00%	10.00%	13.00%	12.00%
Customer 3 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	19.00%	18.00%	32.00%	15.00%
Customer 4 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	35.00%	23.00%	21.00%	18.00%

Product Revenue Net - Schedul_2

Product Revenue Net - Schedule Of Each Customer As a Percentage Of Accounts Receivable (Detail) - Accounts Receivable [Member] - Credit Concentration Risk [Member]	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Customer 1 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	15.00%	21.00%
Customer 2 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	17.00%	14.00%
Customer 3 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	15.00%	29.00%
Customer 4 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	53.00%	36.00%

Cash - Summary of Reconciliatio

Cash - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 124,517	$ 168,215	$ 93,422
Restricted cash, as part of other assets	1,509		1,509
Total cash, cash equivalents, and restricted cash shown in the condensed consolidated statements of cash flows	$ 126,026		$ 94,931

Cash - Additional Information (

Cash - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash And Cash Equivalents [Line Items]
Restricted Cash	$ 1.5	$ 1.5
Security deposit	0.5
Letter of Credit [Member] | Technology Square [Member]
Cash And Cash Equivalents [Line Items]
Security deposit	0.5	0.5
Letter of Credit [Member] | Hampshire Street [Member]
Cash And Cash Equivalents [Line Items]
Security deposit	$ 1	$ 1

Marketable Securities - Summary

Marketable Securities - Summary of Available-for-Sale Securities Held (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	$ 96,778	$ 205,388
Available-For-Sale Securities Held, Gross Unrealized Gains	4	18
Available-For-Sale Securities Held, Gross Unrealized Losses	(6)	(15)
Available-For-Sale Securities Held, Fair Value	96,776	205,391
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	72,733	158,907
Available-For-Sale Securities Held, Gross Unrealized Gains	4	14
Available-For-Sale Securities Held, Gross Unrealized Losses	(2)	(8)
Available-For-Sale Securities Held, Fair Value	72,735	158,913
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	7,022	33,437
Available-For-Sale Securities Held, Gross Unrealized Gains		3
Available-For-Sale Securities Held, Gross Unrealized Losses	(4)	(7)
Available-For-Sale Securities Held, Fair Value	7,018	33,433
U.S. Government Agency Securities and U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	17,023	13,044
Available-For-Sale Securities Held, Gross Unrealized Gains		1
Available-For-Sale Securities Held, Fair Value	$ 17,023	$ 13,045

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)Security
Schedule of Available-for-sale Securities [Line Items]
Realized gains (losses) recognized on sale or maturity of marketable equity securities	$ 0
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	33,500
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate loss	$ 100
Weighted average maturity period	4 months
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in unrealized loss position | Security	13
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in unrealized loss position | Security	1

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Company's Financial Assets Recognized at Fair Value (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	$ 97,136	$ 163,264
Total	193,912	368,655
U.S. Government Agency Securities and U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	17,023	13,045
Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	72,735	158,913
Corporate Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	7,018	33,433
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	83,412	113,505
Total	83,412	113,505
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	13,724	49,759
Total	110,500	255,150
Level 2 [Member] | U.S. Government Agency Securities and U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	17,023	13,045
Level 2 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	72,735	158,913
Level 2 [Member] | Corporate Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	$ 7,018	$ 33,433

Inventory - Summary of Inventor

Inventory - Summary of Inventory (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Raw materials	$ 3,116	$ 1,068
Work in process	11,591	8,564
Finished goods	2,237	829
Total	16,944	10,461
Inventory
Total	1,521	10,461
Restricted Cash And Other Assets
Total	$ 15,423

Supplemental Balance Sheet In_3

Supplemental Balance Sheet Information - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Accrued Liabilities Current [Abstract]
Employee compensation and benefits	$ 10,578	$ 11,921
Research and development expenses	18,804	10,664
Professional services and other	1,540	5,987
Accrued expenses	$ 30,922	$ 28,572

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Federal income tax provision or benefit	$ 0	$ 0
State income tax provision or benefit	$ 0	$ 0

Leases - Additional Information

Leases - Additional Information (Detail) - USD ($)	Aug. 11, 2021	Aug. 16, 2019	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Lessee Lease Description [Line Items]
Monthly base rent	$ 377,000,000		$ 200,000
Increase in base rent	388,000		33,000
Annual increase of base rent			9,000
Security deposit			500,000
Tenant Allowance	430,000
Operating Lease Right Of Use Asset	$ 7,000,000		21,141,000	$ 17,305,000
Hampshire Street [Member] | Letter of Credit [Member]
Lessee Lease Description [Line Items]
Security deposit			$ 1,000,000		$ 1,000,000
Operating lease extend description	the Company extended the term of the Technology Square Lease through November 30, 2024
BMR-Hampshire LLC [Member] | Cambridge [Member]
Lessee Lease Description [Line Items]
Operating lease description		the Company entered into a lease, or the Hampshire Street Lease, with BMR-Hampshire LLC, or BMR. The Hampshire Street Lease is for 33,525 rentable square feet of office space in Cambridge, Massachusetts. The Hampshire Street Lease commenced as of December 1, 2019
Operating lease extend description		the Company with an option to extend the lease term for one additional five-year period
Operating lease rent expense		$ 200,000
Operating lease rent expense incremental percentage		2.50%

Leases - Summary of Lease Costs

Leases - Summary of Lease Costs and Company's Operating Leases (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Leases [Abstract]
Operating lease cost	$ 1,603	$ 1,554	$ 4,634	$ 4,660
Variable lease cost	498	405	1,545	1,351
Total lease cost	2,101	1,959	6,179	6,011
Operating cash flows used for operating leases	$ 1,601	$ 1,000	$ 4,826	$ 2,980
Weighted average remaining lease term	4 years 4 months 24 days	4 years 10 months 24 days	4 years 4 months 24 days	4 years 10 months 24 days
Weighted average discount rate	9.73%	9.73%	9.73%	9.73%

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Sep. 30, 2021USD ($)
Leases [Abstract]
2021	$ 1,610
2022	6,202
2023	7,517
2024	7,322
Thereafter	6,966
Total lease payments	29,617
Less: imputed interest	(5,925)
Total operating lease liabilities at September 30, 2021	$ 23,692

Collaborations and Licensing _2

Collaborations and Licensing Agreements - Additional Information (Details) $ / shares in Units, $ in Millions	Aug. 07, 2021USD ($)$ / sharesshares	Jan. 01, 2011Optiontarget	Sep. 30, 2021USD ($)	Nov. 30, 2018USD ($)Installment	Mar. 31, 2015USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2020USD ($)	Mar. 31, 2020USD ($)	Jul. 31, 2019USD ($)	Mar. 31, 2018USD ($)
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Regulatory milestone payments obligation												$ 0.9
Reimbursements receivable of regulatory milestone												1
Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Regulatory milestone payments obligation														$ 1
Notice period in days								90 days
HutchMed License Agreement [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
License agreement date	Aug. 7, 2021
Percentage cost incurred for global studies and total number of patients enrolled In the territory	20.00%
Percentage cost incurred for global studies not specific to any territory	20.00%
Development and regulatory milestone payment	$ 110
Royalties payable for licensed product, description	Royalties are payable for each Licensed Product commencing on the first commercial sale of the applicable Licensed Product and ending, on a Jurisdiction-by-Jurisdiction basis, on the latest of expiration of specified patent coverage, expiration of specified regulatory exclusivity or a specified period following the first commercial sale in such Jurisdiction and may be reduced in various circumstances.
Clinical license agreement expiration terms	HutchMed License Agreement will expire upon the expiration of the last royalty term for the last Licensed Product in the Field in the Territory. HutchMed may terminate the HutchMed License Agreement in its entirety for any or no reason upon 12 months’ prior written notice to the Company.
Upfront milestone			$ 13
Nonrefundable upfront payment received	$ 25
HutchMed License Agreement [Member] | License Revenue [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Total revenue			12
HutchMed License Agreement [Member] | Common Stock [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Date prior to which warrants are exercisable	Aug. 7, 2025
Number of securities into which the class of warrant or right may be converted | shares	5,653,000
Exercise price of warrants | $ / shares	$ 11.50
HutchMed License Agreement [Member] | Maximum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Sales milestone payment	$ 175
GSK [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Number of option targets | Optiontarget		3
Clinical development milestone payment			50			$ 50		$ 50
Additional milestone payments			197			197		197
Sales-based milestone payments			128			128		128
Additional payments received								0
Cash and accounts receivable											$ 89
Deferred revenue			0			0	$ 0	0	$ 0
GSK [Member] | Collaboration Revenue [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Nonrefundable upfront payment received										$ 89
Eisai [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment made					$ 40
Clinical development milestone payments obligation			20			20		20
Regulatory milestone payments obligation			50			50		50
Total milestone payments			70			70		70
Research and development reduction						3.7		4.4
Obligation related to World Wide Royalty						0.8	0.5	2.7	1
Other Receivables			0.1			0.1		0.1				0.1
Royalties payable								0.8	0.5
Eisai [Member] | Tazemetostat Drug Pruducts [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Total milestone payments			50			50		50
Total revenue						0		6.3
Royalty Pharma and Pharmakon Advisors [Member] | Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Remaining unpaid milestone payments			1			1		1							$ 10.4
Reimbursements receivable of development costs			0.9			0.9		0.9
Signed Amendment [Member] | Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Regulatory milestone payments obligation														0.5
Fourth Amendment [Member] | Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Regulatory milestone payments obligation														$ 0.5
Boehringer Ingelheim [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront Payment Received				$ 15
Deferred revenue			$ 0			0		0				$ 0
Total revenue				26		$ 0	0.1	$ 0	0.4
Company received upfront payment				15
Research funding for costs to be incurred				$ 5			$ 0.1		$ 0.1				$ 0.4
Research funding costs, payment frequency				quarterly
Research funding costs, payable installments | Installment				4
Maximum extension term of research period				1 year

Sale of Future Royalties - Addi

Sale of Future Royalties - Additional Information (Details)	Nov. 04, 2019USD ($)Tranche$ / sharesshares	Feb. 29, 2020USD ($)shares	Nov. 30, 2019$ / sharesshares	Sep. 30, 2021USD ($)shares	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)shares	Sep. 30, 2020USD ($)	Jun. 30, 2021USD ($)	Dec. 31, 2020shares	Nov. 03, 2020USD ($)
Line Of Credit Facility [Line Items]
Common stock, shares issued | shares				103,662,000		103,662,000			101,627,000
Proceeds from issuance of common stock, net of commissions						$ 8,261,000
Royalty Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Proceeds from sale of future royalties						12,601,000
Non cash royalty revenue					$ 0	(7,000)	0
Non cash interest expense				$ 400,000	$ 1,400,000	$ 300,000	$ 900,000
RPI and Affiliates [Member]
Line Of Credit Facility [Line Items]
Percentage of ownership interest related parties				9.00%		9.00%			9.00%
RPI Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Common stock, shares issued | shares	6,666,667
Warrants To Exercise Common Stock | $ / shares	$ 20		$ 20
Proceed from agreement	$ 100,000,000
Obligation to purchase common stock	50,000,000
Shares option to sell exercised by underwriters | shares		2,500,000
Proceeds from issuance of common stock, net of commissions		$ 50,000,000
Proceeds from sale of future royalties						$ 12,600,000
Effective annual interest Rate						9.01%
RPI Purchase Agreement [Member] | RPI and Affiliates [Member]
Line Of Credit Facility [Line Items]
Payments for Royalties						11.40%
RPI Purchase Agreement [Member] | Achievement of Specified Annual Net Sales [Member]
Line Of Credit Facility [Line Items]
Potential milestone payments	$ 1,500,000,000
Maximum [Member] | Royalty Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Non cash royalty revenue				$ 100,000		$ 100,000
Maximum [Member] | RPI Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Warrants to purchase shares of common stock | shares	2,500,000		2,500,000
Obligation to purchase share price | $ / shares	$ 20
Loan Agreement [Member]
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 70,000,000
Number of tranches | Tranche	3
Debt instrument, covenant description						Under the Amended and Restated Loan Agreement the Company has the right to request up to an additional $150.0 million in secured term loans, subject to the approval of the Lenders, provided that the Company has not prepaid any outstanding term loans at the time of the Company’s request and such request is made before November 18, 2021.
Loan Agreement [Member] | RPI Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Common stock, shares issued | shares				79,000,000		79,000,000
Warrants to purchase shares of common stock | shares						8,400,000
Proceed from agreement						$ 125,000,000
Proceeds from sale of future royalties						12,600,000
Transaction costs				$ 2,000,000		2,000,000
Loan Agreement [Member] | Maximum [Member]
Line Of Credit Facility [Line Items]
Right to request for additional loan amount				150,000,000		150,000,000
First Tranche [Member] | Loan Agreement [Member]
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 25,000,000
Aggregate principal amount								$ 25,000,000
First Tranche [Member] | Loan Agreement [Member] | RPI Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Aggregate principal amount				$ 25,000,000		$ 25,000,000
Second Tranche [Member] | Loan Agreement [Member]
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	25,000,000
Aggregate principal amount								25,000,000
Third Tranche [Member] | Loan Agreement [Member]
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 20,000,000
Aggregate principal amount								$ 20,000,000
Fourth Tranche [Member] | Amended And Restated Loan Agreement [Member]
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity										$ 150,000,000

Sale of Future Royalties - Sche

Sale of Future Royalties - Schedule of Activity of Royalty Obligation (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Deferred Revenue Arrangement [Line Items]
Related party liability related to sale of future royalties		$ 15,581,000		$ 14,176,000
Royalty Purchase Agreement [Member]
Deferred Revenue Arrangement [Line Items]
Proceeds from sale of future royalties		12,601,000
Non cash royalty revenue	$ 0	(7,000)	$ 0
Non-cash interest expense recognized		2,987,000
Related party liability related to sale of future royalties		$ 15,581,000

Long-Term Debt - Additional Inf

Long-Term Debt - Additional Information (Detail)	Nov. 03, 2020USD ($)Installment	Nov. 30, 2020USD ($)	Nov. 30, 2019USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Nov. 04, 2019USD ($)
Tranche Term Loan [Member]
Debt Instrument [Line Items]
Interest rate of loan	7.75%
Number of quarterly payments of term loan | Installment	8
Description of prepayment of tranche term loan						Each of the four term loans may be prepaid before maturity in whole or in part, however there is a $50.0 million minimum prepayment for any prepayment of the loans. If the Company prepays any tranche of term loans, in whole or in part, during the first 36 months from the date on which the Lenders funded such tranche of term loans, then the Company must pay a prepayment premium equal to the greater of (x) a make-whole amount equal to the interest that would have accrued on the principal amount to be prepaid and (y) a premium equal to 0.03 multiplied by the principal amount to be prepaid. If the Company prepays a tranche of term loan, in whole or in part, between the 36th month and 48th month from the date on which the Lenders funded such tranche of term loans, then the Company must pay a prepayment premium equal to 0.02 multiplied by the principal amount to be prepaid. If the Company prepays a tranche of term loans, in whole or in part, between the 48th month and 60th month from the date on which the Lenders funded such tranche of term loans, then the Company must pay a prepayment premium equal to 0.01 multiplied by the principal amount to be prepaid.
Tranche Term Loan [Member] | Interest Rate Floor [Member]
Debt Instrument [Line Items]
Interest rate of loan	2.00%
First Tranche [Member] | BioPharma Credit Investments V (Master) LP [Member]
Debt Instrument [Line Items]
Interest Expense Credit Facilities				$ 5,500,000	$ 16,300,000	$ 1,300,000	$ 3,400,000
Minimum [Member] | Tranche Term Loan [Member]
Debt Instrument [Line Items]
Prepayment of loan	$ 50,000,000
Loan Agreement [Member]
Debt Instrument [Line Items]
Aggregate principal amount								$ 70,000,000
Percentage of commitment fee paid		2.00%	2.00%
Commitment fee		$ 150,000,000	$ 70,000,000
Loan Agreement [Member] | Maximum [Member]
Debt Instrument [Line Items]
Right to request for additional loan amount								$ 300,000,000
Amended And Restated Loan Agreement [Member]
Debt Instrument [Line Items]
Aggregate principal amount	$ 150,000,000
Number of quarterly payments of term loan | Installment	4
Debt instrument outstanding interest only payment	$ 70,000,000
Principal And Interest Due	$ 70,000,000
Amended And Restated Loan Agreement [Member] | Tranche Term Loan [Member]
Debt Instrument [Line Items]
Number of quarterly payments of term loan | Installment	8
Amended And Restated Loan Agreement [Member] | Maximum [Member]
Debt Instrument [Line Items]
Right to request for additional loan amount	$ 150,000,000

Long-Term Debt - Schedule of Mi

Long-Term Debt - Schedule of Minimum Aggregate Future Loan Payments (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Long-term Debt, Fiscal Year Maturity [Abstract]
2024	$ 70,000
2025	75,000
2026	75,000
Total minimum payments	220,000
Less amounts representing interest and discount	(3,746)
Long-term debt, net of current portion	$ 216,254	$ 215,670

Stockholders' (Deficit) Equit_2

Stockholders' (Deficit) Equity - Additional Information (Detail) - USD ($)	May 06, 2021	Nov. 04, 2019	Sep. 30, 2021	Aug. 31, 2021	Feb. 29, 2020	Nov. 30, 2019	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Apr. 08, 2021	Dec. 31, 2020
Equity [Line Items]
Common stock, shares authorized			225,000,000				225,000,000			225,000,000			150,000,000
Common stock, par value			$ 0.0001				$ 0.0001			$ 0.0001			$ 0.0001
Number of votes for each share of common stock										Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to dividends when and if declared by the board of directors.
Proceeds from issuance of common stock, net of commissions										$ 8,261,000
Issuance of common stock, Value							$ 6,727,000	$ 1,534,000
Preferred stock, shares outstanding			338,000				338,000			338,000			338,000
Non cash gains due to adjustment of warrant liability										$ 4,400,000
Issuance costs										$ 217,000	$ 79,000
HutchMed Warrants [Member]
Equity [Line Items]
Warrants to purchase shares of common stock				5,653,000
Warrants to exercise common stock				$ 11.50
Number of shares purchasable reduced upon condition				2,826,500
Warrant liability				$ 13,000,000
Non cash gains due to adjustment of warrant liability							$ 4,400,000
RPI Purchase Agreement [Member]
Equity [Line Items]
Proceeds from issuance of common stock, net of commissions					$ 50,000,000
Warrants to exercise common stock		$ 20				$ 20
RPI Purchase Agreement [Member] | Maximum [Member]
Equity [Line Items]
Warrants to purchase shares of common stock		2,500,000				2,500,000
Series A Convertible Preferred Stock [Member]
Equity [Line Items]
Preferred stock, shares outstanding			337,800				337,800			337,800			337,800
Preferred stock, voting rights										Shares of Series A Preferred Stock will generally have no voting rights except as required by law and except that the consent of the holders of a majority of the outstanding shares of Series A Preferred Stock will be required to amend the terms of the Series A Preferred Stock or take certain other actions with respect to the Series A Preferred Stock.
Liquidation preference per share			$ 0.001				$ 0.001			$ 0.001
Percentage of conversion restriction upon holder of convertible preferred stock shares										9.99%
Conversion of stock notice period										61 days
Number of common shares convertible for each share of convertible preferred stock			10				10			10
Preferred Stock [Member]
Equity [Line Items]
Issuance of Series A Convertible Preferred Stock, net of commissions and beneficial conversion charge, Shares					12,200
ATM Sale Agreement [Member]
Equity [Line Items]
Common stock shares issued							1,247,599	182,866		1,430,465
Sale of common stock, price per share			$ 5.45				$ 5.45	$ 8.53		$ 5.45
Sale of stock, consideration received on transaction							$ 6,700,000	$ 1,500,000
Proceeds from issuance of common stock, net of commissions			$ 8,300,000
Additional paid in capital, common stock								48,000
Cost of establishment							$ 200,000	$ 300,000
Issuance costs										$ 300,000
Jefferies LLC [Member] | ATM Sale Agreement [Member]
Equity [Line Items]
Company common stock aggregate offering price	$ 200,000,000
Commission proceeds from sale	3.00%
Common Stock [Member]
Equity [Line Items]
Common stock, shares authorized			150,000,000				150,000,000			150,000,000		225,000,000	125,000,000
Common stock, par value			$ 0.0001				$ 0.0001			$ 0.0001
Issuance of common stock upon exercise of stock options, Share					2,500,000		1,247,599	182,866
Issuance of common stock, Value					$ 49,900
Proceeds from financing Costs after deducting					$ 100
Issuance of Series A Convertible Preferred Stock, net of commissions and beneficial conversion charge, Shares					122,000
Common Stock [Member] | Series A Convertible Preferred Stock [Member]
Equity [Line Items]
Issuance of Series A Convertible Preferred Stock, net of commissions and beneficial conversion charge, Shares									122,000
Additional Paid-In Capital [Member]
Equity [Line Items]
Issuance of common stock, Value							$ 6,727,000	$ 1,534,000
Additional Paid-In Capital [Member] | RPI Purchase Agreement [Member]
Equity [Line Items]
Proceeds from sale of equity securities fair value						$ 8,400

Stockholders (Deficit) Equity -

Stockholders (Deficit) Equity - Schedule of Fair Value of Warrant Liability (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021
Equity [Abstract]
Fair value at Inception	$ 13,050
Decrease in fair value	(4,420)	$ (4,420)
Fair value at September 30, 2021	$ 8,630	$ 8,630

Stockholders (Deficit) Equity_2

Stockholders (Deficit) Equity - Summary of Key Assumptions Used to Value Warrant (Details) - HutchMed Warrant [Member] - $ / shares	Aug. 07, 2021	Sep. 30, 2021
Equity [Line Items]
Risk-free interest rate	0.60%	0.72%
Expected term (in years)	4 years	3 years 10 months 6 days
Expected volatility of underlying stock	70.00%	70.00%
Expected dividend yield	0.00%	0.00%
Stock price	$ 6.47	$ 5.12

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2019	Aug. 16, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted Average Exercise Price per Share, Exercisable	$ 14.40		$ 14.40			$ 5.31
Grant date of option			Aug. 16, 2021
Stock-based compensation expense related to stock options, restricted stock, shares issued and shares granted to non-employee directors	$ 6,170	$ 6,386	$ 19,903	$ 21,188
Weighted-average fair value of options granted	$ 4.93	$ 8.56	$ 3.27	$ 11.94
Unrecognized compensation cost	$ 43,500		$ 43,500
Expected weighted average period for recognition of compensation cost			2 years 5 months 19 days
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate number of stock granted to purchase	80,000		80,000
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate number of stock granted to purchase	53,333		53,333
Stock-based compensation expense related to stock options, restricted stock, shares issued and shares granted to non-employee directors	$ 6,200	$ 6,400	$ 19,900	$ 21,200
Service Based RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected weighted average period for recognition of compensation cost			2 years 5 months 1 day
Restricted stock units outstanding			2,259,000
Compensation expense was recognized	1,100	700	$ 3,300	2,000
Unrecognized compensation cost	16,300		$ 16,300
Service Based RSU [Member] | Executives and employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units outstanding			2,186,595
Service Based RSU [Member] | Non-Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units outstanding			72,540
Vesting percentage			100.00%
Service Based RSU [Member] | Executive and Employee [Member] | Vesting Period 1 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage			50.00%
Vesting period			2 years
Service Based RSU [Member] | Executive and Employee [Member] | Vesting Period 2 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage			33.00%
Vesting period			3 years
Service Based RSU [Member] | Executive and Employee [Member] | Vesting Period 3 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage			25.00%
Vesting period			4 years
Performance Based RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units outstanding					604,000
Compensation expense was recognized	0	$ 0	$ 0	$ 3,500
Unrecognized compensation cost	$ 0		$ 0
Performance Based RSU [Member] | Vest on June 30, 2019 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage					20.00%
Performance Based RSU [Member] | Vest on January 23, 2020 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage					25.00%
Performance Based RSU [Member] | Vest on March 24, 2020 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage					20.00%
Performance Based RSU [Member] | Vest on June 30, 2020 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage					30.00%

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 6,170	$ 6,386	$ 19,903	$ 21,188
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	2,042	2,079	6,295	7,045
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 4,128	$ 4,307	$ 13,608	$ 14,143

Stock-Based Compensation - Assu

Stock-Based Compensation - Assumptions Used in Applying Pricing Model (Detail) - Employee Stock Option [Member]	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.40%	0.30%	0.40%	1.00%
Expected life of options	5 years 9 months 29 days	6 years	5 years 10 months 20 days	5 years 11 months 26 days
Expected volatility of underlying stock	69.60%	72.10%	70.10%	70.90%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2021	Aug. 16, 2021
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of Options, Outstanding, Beginning balance	10,225
Number of Options, Granted	5,943
Number of Options, Exercised	(104)
Number of Options, Forfeited	(2,458)
Number of Options, Outstanding, Ending balance	13,606
Number of Options, Exercisable	5,697
Weighted Average Exercise Price per Share, Outstanding, Beginning balance	$ 14.77
Weighted Average Exercise Price per Share, Granted	8.06
Weighted Average Exercise Price per Share, Exercised	8.83
Weighted Average Exercise Price per Share, Forfeited	14.39
Weighted Average Exercise Price per Share, Outstanding, Ending balance	11.95
Weighted Average Exercise Price per Share, Exercisable	$ 14.40	$ 5.31
Weighted Average Remaining Contractual Term (In Years), Outstanding	7 years 7 months 28 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	5 years 9 months 7 days
Aggregate Intrinsic Value, Outstanding	$ 10
Aggregate Intrinsic Value, Exercisable	$ 10

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Service Based Restricted Stock Units (Detail) - Service Based RSU [Member] shares in Thousands	9 Months Ended
	Sep. 30, 2021$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Outstanding Shares, Beginning Balance | shares	668
Number of Shares, Granted | shares	2,259
Number of Shares, Vested | shares	(177)
Number of Shares, Forfeited | shares	(444)
Number of Outstanding Shares, Ending Balance | shares	2,306
Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 17.56
Weighted Average Grant Date Fair Value, Granted | $ / shares	7.86
Weighted Average Grant Date Fair Value, Vested | $ / shares	17.35
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	13.13
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 8.92

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						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net loss	$ (65,834)	$ (64,363)	$ (70,274)	$ (56,073)	$ (58,451)	$ (50,937)	$ (200,471)	$ (165,461)
Weighted average shares outstanding	102,513			101,512			102,123	100,747
Basic and diluted loss per share allocable to common stockholders	$ (0.64)			$ (0.55)			$ (1.96)	$ (1.64)

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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	27,490	16,721	27,490	16,721
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	13,606	10,148	13,606	10,148
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	2,306	663	2,306	663
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	47	32	47	32
Series A Preferred Stock (if converted) [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	3,378	3,378	3,378	3,378
Warrants [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	8,153	2,500	8,153	2,500

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - $ / shares	Nov. 01, 2021	Oct. 01, 2021	Sep. 30, 2021	Sep. 30, 2021	Aug. 16, 2021
Subsequent Event [Line Items]
Weighted Average Exercise Price per Share, Exercisable			$ 14.40	$ 14.40	$ 5.31
Grant date of option				Aug. 16, 2021
Common Stock [Member]
Subsequent Event [Line Items]
Aggregate number of stock granted to purchase			80,000	80,000
Subsequent Event [Member]
Subsequent Event [Line Items]
Grant date of option	Nov. 1, 2021	Oct. 1, 2021
Subsequent Event [Member] | Common Stock [Member]
Subsequent Event [Line Items]
Weighted Average Exercise Price per Share, Exercisable	$ 4.44	$ 5.19
Subsequent Event [Member] | 2013 Stock Incentive Plan [Member] | Common Stock [Member]
Subsequent Event [Line Items]
Aggregate number of stock granted to purchase	42,500	125,866
Subsequent Event [Member] | Restricted Stock Units [Member] | 2013 Stock Incentive Plan [Member]
Subsequent Event [Line Items]
Aggregate number of stock granted to purchase	28,333	25,289