Epizyme (EPZM) 10-Q 6 May 21 2021 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2021	Apr. 30, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
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		101,975,604
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	Mar. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 73,711	$ 168,215
Marketable securities	225,232	205,391
Accounts receivable, net	9,764	3,105
Inventory	14,817	10,461
Prepaid expenses and other current assets	16,893	17,921
Total current assets	340,417	405,093
Property and equipment, net	2,035	2,152
Operating lease assets	16,272	17,305
Intangible assets, net	45,964	47,002
Restricted cash and other assets	2,023	2,021
Total assets	406,711	473,573
Current liabilities:
Accounts payable	5,357	10,163
Accrued expenses	23,853	28,572
Current portion of operating lease obligation	4,796	4,665
Deferred revenue	5,000
Total current liabilities	39,006	43,400
Operating lease obligation, net of current portion	14,150	15,409
Related party long-term debt, net of debt discount	215,858	215,670
Other long-term liabilities	20	21
Related party liability related to sale of future royalties	14,646	14,176
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; 150,000 shares authorized; 101,969 shares and 101,627 shares issued and outstanding, respectively	10	10
Additional paid-in capital	1,145,875	1,137,470
Accumulated other comprehensive income	6	3
Accumulated deficit	(1,058,987)	(988,713)
Total stockholders’ equity	123,031	184,897
Total liabilities and stockholders’ equity	$ 406,711	$ 473,573

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	338,000	338,000
Preferred stock, shares outstanding	338,000	338,000
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	101,969,000	101,627,000
Common stock, shares outstanding	101,969,000	101,627,000
Common stock conversion ratio	10.00%	10.00%
Common stock upon conversion	3,378,000	3,378,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenue:
Total revenue	$ 7,631	$ 1,354
Operating expenses:
Cost of revenue	2,853	614
Research and development	32,704	25,163
Selling, general and administrative	36,411	26,927
Total operating expenses	71,968	52,704
Operating loss	(64,337)	(51,350)
Other (expense) income, net:
Interest (expense) income, net	(5,476)	756
Other income (expense), net	9	(48)
Related party non-cash interest expense related to sale of future royalties	(470)	(295)
Other (expense) income, net	(5,937)	413
Net loss	(70,274)	(50,937)
Other comprehensive income (loss):
Unrealized gain (loss) on available-for-sale securities	3	(94)
Comprehensive loss	$ (70,271)	$ (51,031)
Net loss per share attributable to common stockholders:
Basic	$ (0.69)	$ (0.51)
Diluted	$ (0.69)	$ (0.51)
Weighted-average common shares outstanding used in net loss per share attributable to common stockholders:
Basic	101,790	99,616
Diluted	101,790	99,616
Product [Member]
Revenue:
Total revenue	$ 6,191	$ 1,284
Collaboration and Other Revenue [Member]
Revenue:
Collaboration and other revenue	$ 1,440	$ 70

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (70,274)	$ (50,937)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,282	512
Stock-based compensation	7,015	6,510
Amortization of discount (premium) on investments	363	(270)
Amortization of debt discount	188	83
Loss on disposal of property and equipment		19
Non-cash interest expense associated with the sale of future royalties	470	295
Changes in operating assets and liabilities:
Accounts receivable	(6,659)	918
Inventory	(4,356)	(1,703)
Prepaid expenses and other current assets	1,028	(7,104)
Accounts payable	(4,816)	(2,716)
Accrued expenses	(4,719)	(4,489)
Deferred revenue	5,000
Operating lease assets	1,033	733
Operating lease liabilities	(1,128)	(155)
Other assets and liabilities	(2)	36
Net cash used in operating activities	(75,575)	(58,268)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of available-for-sale securities	(120,589)	(13,999)
Maturities of available-for-sale securities	100,389	58,578
Purchase of intangible asset		(25,000)
Purchases of property and equipment	(119)	(63)
Net cash (used in) provided by investing activities	(20,319)	19,516
CASH FLOWS FROM FINANCING ACTIVITIES:
Payment of offering costs		(79)
Proceeds from the issuance of debt		25,000
Payment of debt issuance costs		(90)
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	199	3,140
Issuance of shares under employee stock purchase plan	1,191	646
Net cash provided by financing activities	1,390	78,532
Net (decrease) increase in cash, cash equivalents and restricted cash	(94,504)	39,780
Cash, cash equivalents and restricted cash, beginning of period	169,724	140,991
Cash, cash equivalents and restricted cash, end of period	75,220	180,771
SUPPLEMENTAL CASH FLOW INFORMATION:
Interest paid	5,368	914
Property and equipment included in accounts payable or accruals	$ 10	$ 22

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Put Option [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 (net of commissions and offering costs), Value		$ 49,915							$ 49,915
Issuance of common stock (net of commissions and offering costs), 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 gain (loss) 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, 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 gain (loss) 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

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Stockholders' Equity (Parenthetical) $ in Thousands	3 Months Ended
	Mar. 31, 2020USD ($)
Put Option [Member]
Financing Cost	$ 85

The Company

The Company	3 Months Ended
	Mar. 31, 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 March 31, 2021, in addition to revenues from product sales, the Company has raised an aggregate of $1,527.4 million to fund its operations. This includes $243.8 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), or the Lenders, $839.5 million from the sale of common stock and series A convertible preferred stock in the Company’s public 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 March 31, 2021, the Company had $298.9 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 two FL indications 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 $1,059.0 million through March 31, 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 its product candidates, 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 of products.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 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 March 31, 2021 and 2020 are referred to as the first 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 months ended March 31, 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 and it is not alleviated by the Company’s plans or when its plans alleviate substantial doubt about the Company’s ability to continue as a going concern. The Company’s evaluation entails analyzing prospective operating budgets and forecasts for expectations of the Company’s cash needs, and comparing those needs to its available cash, cash equivalents and marketable securities. The analysis for the first quarter of 2021 included consideration of the Company’s current cash needs, including its research and development plans, commercialization activities associated with the ongoing launch of TAZVERIK in the ES and FL indications, and its existing debt service obligations. The Company also evaluated its forecasted product revenues from sales of TAZVERIK. Such estimates of future sales contain significant judgement as TAZVERIK was recently launched and there is little or no history with which to base such estimates. The Company expects its available cash, cash equivalents and marketable securities will be sufficient to fund current planned operations and capital expenditure requirements for at least the next twelve months from the filing date of this Quarterly Report on Form 10-Q with the SEC. As a result, the Company concluded that it did not identify conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the date these financial statements were issued. 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. Recently Adopted Accounting Pronouncements Revenue Recognition – Collaboration Revenue In November 2018, the FASB, issued ASU 2018-18, Collaborative Arrangements, or ASC 808, 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 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. Cost of Revenues The cost of revenues primarily consists of costs related to the sales of TAZVERIK and sales of tazemetostat API and drug product Cost of revenues for the included approximately $0.8 million related to sales of tazemetostat drug product. 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 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. 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. 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 2021 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 March 31, 2021 (in thousands): March 31, 2021 Estimated useful life (years) In-licensed rights $ 50,000 12.2 Less: accumulated amortization (4,036 ) Total intangible asset, net $ 45,964 The Company recorded approximately $1.0 million and $0.3 million in amortization expense related to intangible assets, using the straight-line methodology, during the three months ended March 31, 2021 and 2020, respectively. Estimated future amortization expense for intangible assets for the remainder of the year ended December 31, 2021 is $3.1 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 consolidated balance sheet. 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.

Product Revenue Net

Product Revenue Net	3 Months Ended
	Mar. 31, 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 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 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 consolidated balance sheet. 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 three months ended March 31, 2021: Chargebacks, Discounts, and Government and Other Fees Rebates Returns Total (In thousands) Balance, January 1, 2021 $ 133 $ 428 $ 67 $ 628 Provision 399 610 — 1,009 Payments or credits (323 ) (572 ) — (895 ) Balance, March 31, 2021 $ 209 $ 466 $ 67 $ 742 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 months ended March 31, 2021 and 2020, net product revenue was primarily accounted from four individual customers. Revenue earned from each customer as a percentage of net product revenue is as follows: Three Months Ended March 31, 2021 2020 Customer 1 42 % 59 % Customer 2 14 % 3 % Customer 3 25 % 13 % Customer 4 19 % 25 % As of March 31, 2021 and December 31, 2020, the four individual customers were accounted for as a percentage of accounts receivable as follows: March 31, 2021 December 31, 2020 Customer 1 22 % 21 % Customer 2 14 % 14 % Customer 3 33 % 29 % Customer 4 31 % 36 % No other customer accounted for more than 10 percent of net product revenue or accounts receivable.

Cash

Cash	3 Months Ended
	Mar. 31, 2021
Cash And Cash Equivalents [Abstract]
Cash	4. Cash A reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheets that sum to the total of the same such amounts shown in the condensed consolidated statements of cash flows, is as follows: As of March 31, 2021 2020 (In thousands) Cash and cash equivalents $ 73,711 $ 179,262 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 $ 75,220 $ 180,771 The $1.5 $0.5 $1.0 .

Marketable Securities

Marketable Securities	3 Months Ended
	Mar. 31, 2021
Investments Debt And Equity Securities [Abstract]
Marketable Securities	5. Marketable Securities The following table summarizes the available-for-sale securities held at March 31, 2021 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 132,118 $ 6 $ (8 ) $ 132,116 Corporate notes 13,067 — (5 ) 13,062 U.S. government agency securities and U.S. Treasuries 80,041 13 — 80,054 Total $ 225,226 $ 19 $ (13 ) $ 225,232 The following table summarizes the available-for-sale securities held at December 31, 2020 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses 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 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 March 31, 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 three months ended March 31, 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 March 31, 2021 was $61.0 million, which consisted of 10 commercial paper securities and 5 corporate notes securities. The aggregate unrealized loss for those securities in an unrealized loss position for less than twelve months as of March 31, 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 determined that there was no material change in the credit risk of any of its investments. As a result, the Company determined it did not hold any investments that were impaired as of March 31, 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 March 31, 2021.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements	6. Fair Value Measurements The Company’s financial instruments as of March 31, 2021 and December 31, 2020 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. As of March 31, 2021 and December 31, 2020, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of March 31, 2021 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 68,366 $ 60,517 $ 7,849 $ — Marketable securities: Commercial paper 132,116 — 132,116 — Corporate notes 13,062 — 13,062 — U.S. government agency securities and treasuries 80,054 — 80,054 — Total $ 293,598 $ 60,517 $ 233,081 $ — 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	3 Months Ended
	Mar. 31, 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 March 31, 2021 and December 31, 2020: March 31, 2021 December 31, 2020 (In thousands) Raw materials $ 1,307 $ 1,068 Work in process 12,659 8,564 Finished goods 851 829 Total $ 14,817 $ 10,461 As of March 31, 2021 the Company has not capitalized inventory costs related to its other drug development programs.

Supplemental Balance Sheet Info

Supplemental Balance Sheet Information	3 Months Ended
	Mar. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Supplemental Balance Sheet Information	8. Supplemental Balance Sheet Information Accrued expenses consisted of the following: March 31, 2021 December 31, 2020 (In thousands) Employee compensation and benefits $ 6,229 $ 11,921 Research and development expenses 10,762 10,664 Professional services and other 6,862 5,987 Accrued expenses $ 23,853 $ 28,572

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes The Company did not record a federal or state income tax provision or benefit for the three months ended March 31, 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	3 Months Ended
	Mar. 31, 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 Commitments and Contingencies

Leases

Leases	3 Months Ended
	Mar. 31, 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 are 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. Under the current terms of the Technology Square Lease, the Company does not have any further right to extend the term beyond November 30, 2022. 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 condensed consolidated balance sheet. In applying the ASU 2016-02, Leases , or ASC 842, 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 consolidated balance sheet. 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 months ended March 31, 2021 and 2020: Three Months Ended March 31, 2021 2020 Lease cost Operating lease cost $ 1,515 $ 1,550 Variable lease cost 480 422 Total lease cost $ 1,995 $ 1,972 Other information Operating cash flows used for operating leases $ 1,605 $ 981 Weighted average remaining lease term 4.7 years 5.2 years Weighted average discount rate 9.81 % 9.63 % Future minimum lease payments under the Company’s non-cancelable operating leases as of March 31, 2021, are as follows: (In thousands) 2021 $ 4,832 2022 6,256 2023 2,984 2024 3,053 Thereafter 6,966 Total lease payments $ 24,091 Less: imputed interest (5,145 ) Total operating lease liabilities at March 31, 2021 $ 18,946

Collaborations

Collaborations	3 Months Ended
	Mar. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations	12. Collaborations 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 March 31, 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 March 31, 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. 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 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. The Company has deferred $5.0 million of revenue allocated to the Company’s waiver of its exclusive right to supply of tazemetostat drug substance as of March 31, 2021, which will be recognized in April 2021 upon delivery of the Company’s waiver to the drug substance manufacture. During the three months ended March 31, 2021, the Company recognized $1.3 million related to the delivery of tazemetostat drug product in other revenue. During the three months ended March 31, 2020, Eisai purchased $0.4 million 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 March 31, 2021 and December 31, 2020, the Company had accounts receivable of $5.2 million and less than $0.1 million, respectively, due from Eisai. During the three months ended March 31, 2021, the Company recorded $0.9 million related to the worldwide royalties due under the Eisai License Agreement in cost of revenue based on U.S. sales of TAZVERIK and as of March 31, 2021, $0.9 million in royalties were payable under the Eisai License Agreement. During the three months ended March 31, 2020, the Company recorded $0.2 million in cost of revenue related to the worldwide royalties due under the Eisai License Agreement based on U.S. sales of TAZVERIK and as of March 31, 2020, $0.2 million in royalties were payable under the Eisai License Agreement. F or additional information regarding 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 March 31, 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 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 perio d expired unexercise d, 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 $ 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 Boehrin ger Ingelheim to provide research funding of $ 0.1 million . The additional research activities were completed prior to the end of 2020. Collaboration Revenue Through March 31, 2021, the Company has recognized $26.0 million in total collaboration revenue since the inception of this collaboration. During the three months ended March 31, 2021, the Company did not recognize collaboration revenue under its agreement with Boehringer Ingelheim. During the three months ended March 31, 2020, the Company recognized $0.1 million in collaboration revenue under its agreement with Boehringer Ingelheim. As of March 31, 2021 and December 31, 2020, the Company did not have any deferred revenue or accounts receivable related to this agreement.

Sale of Future Royalties

Sale of Future Royalties	3 Months Ended
	Mar. 31, 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 On November 18, 2020, we borrowed the Tranche D Loan Long-Term Debt the Amended and Restated Loan Agreement 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 March 31, 2021 and December 31, 2020, RPI and its affiliates owned 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. As of March 31, 2021, the Company’s assessment of the estimated future royalty payments to RPI resulted in a current effective interest rate of approximately 13.3%. The following table shows the activity of the Royalty Obligation since the transaction inception through March 31, 2021: As of March 31, 2021 (In thousands) Proceeds from sale of future royalties $ 12,601 Non-cash interest expense recognized 2,045 Liability related to the sale of future royalties - ending balance $ 14,646 During the three months ended March 31, 2021 and 2020, no non-cash royalties from net sales of tazemetostat in Japan were recorded and the Company recorded $0.5 million and $0.3 million, respectively, of related non-cash interest expense.

Long-Term Debt

Long-Term Debt	3 Months Ended
	Mar. 31, 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 March 31, 2021 (in thousands): As of March 31, 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 (4,142 ) Less current portion — Long-term debt, net of current portion $ 215,858 For the three months ended March 31, 2021 and 2020, interest expense related to the Company's Loan Agreement was approximately $5.4 million and $0.7 million, respectively. The total carrying value of debt is classified as long-term on the condensed consolidated balance sheet as of March 31, 2021.

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity	3 Months Ended
	Mar. 31, 2021
Equity [Abstract]
Stockholders' (Deficit) Equity	15. 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 “Charter Amendment”). At the Company’s 2020 Annual Meeting of Stockholders, the stockholders of the Company approved the 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, a proposed 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 number of authorized shares of preferred stock would not be affected by the proposed amendment. 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. 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. The issuance of these shares contributed to significant increases in the Company’s shares of common stock outstanding as of March 31, 2021 and 2020 and in the weighted average shares outstanding for the three months ended March 31, 2021 and 2020 when compared to the comparable prior year periods. Convertible Preferred Stock On March 6, 2019, the Company entered into an Underwriting Agreement, (the “Preferred Stock Agreement”), that related to the public offering of 350,000 shares of Series A Convertible Preferred Stock, par value $0.0001 per share (“Series A Preferred Stock”), for a purchase price to the public of $115.00 per share. All of the Series A Preferred Stock was sold by the Company for net proceeds of $37.4 million. Upon issuance, each share of Series A Preferred Stock included an embedded beneficial conversion feature because the market price of the Company’s common stock on the date of issuance of the Series A Preferred Stock at $12.34 per share as compared to an effective conversion price of the Series A Preferred Stock of $11.50 per share. As a result, the Company recorded the intrinsic value of the beneficial conversion feature of $2.9 million as a discount on the Series A Preferred Stock at issuance. Because the Series A Preferred Stock is immediately convertible upon issuance and does not include mandatory redemption provisions, the discount on the Series A Preferred Stock was immediately accreted. The Company evaluated the Series A Preferred Stock for liability or equity classification in accordance with the provisions of ASC 480, Distinguishing Liabilities from Equity 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 fund s 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 were classified as equity and recorded at their relative fair value of $8.4 million to additional paid-in-capital on the consolidated balance sheets. The Common Stock Warrant remain outstanding as of March 31, 2021.

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	16. Stock-Based Compensation 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 $7.0 million and $6.5 million for the three months ended March 31, 2021 and March 31, 2020, respectively. Stock-based compensation expense is classified in the condensed consolidated statements of operations and comprehensive loss as follows: Three Months Ended March 31, 2021 2020 (In thousands) Research and development $ 2,230 $ 2,162 General and administrative 4,785 4,348 Total $ 7,015 $ 6,510 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 $6.75 and $13.37 per option for those options granted during the three months ended March 31, 2021 and 2020, respectively. Key assumptions used to apply this pricing model were as follows: Three Months Ended March 31, 2021 2020 Risk-free interest rate 0.4 % 1.3 % Expected life of options 6.00 years 5.98 years Expected volatility of underlying stock 70.5 % 70.3 % Expected dividend yield 0.0 % 0.0 % The following is a summary of stock option activity for the three months ended March 31, 2021: Number of Options Weighted Average Exercise Price per Share Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (In thousands) (In years) (In thousands) Outstanding at December 31, 2020 10,225 $ 14.77 Granted 2,509 10.94 Exercised (22 ) 9.15 Forfeited (396 ) 15.51 Outstanding at March 31, 2021 12,316 $ 13.97 7.81 $ 304 Exercisable at March 31, 2021 5,054 $ 14.29 6.58 $ 186 As of March 31, 2021, there was $57.0 million of unrecognized compensation cost related to stock options that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 2.79 years. Restricted Stock Units During the three months ended March 31, 2021, 971,078 restricted stock units (“RSUs”) were granted to executives and employees. The awards were service-based. Assuming all service conditions are achieved, 25% of the RSUs would vest annually for four years. Number of Service Based RSU Shares (in thousands) Weighted Average Grant Date Fair Value Outstanding at December 31, 2020 668 $ 17.56 Granted 971 10.99 Vested (166 ) 17.60 Forfeited (49 ) 14.20 Outstanding at March 31, 2021 1,424 $ 13.20 Compensation expense totaling $1.1 million and $0.5 million was recognized for the service-based RSUs for the three months ended March 31, 2021 and 2020, respectively. Compensation expense totaling As of March 31, 2021, there was $17.2 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 3.39 years. There was no unrecognized compensation cost as of March 31, 2021, related to performance-based RSUs, as all of the performance conditions have been achieved. During 2019, the Company granted 604,000 RSUs to executives and employees, which contained performance conditions, 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.

Loss Per Share

Loss Per Share	3 Months Ended
	Mar. 31, 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 March 31, 2021 2020 (In thousands except per share data) Net loss $ (70,274 ) $ (50,937 ) Weighted average shares outstanding 101,790 99,616 Basic and diluted loss per share allocable to common stockholders $ (0.69 ) $ (0.51 ) The following common stock equivalents were excluded from the calculation of diluted loss per share allocable to common stockholders because their inclusion would have been anti-dilutive: Three Months Ended March 31, 2021 2020 (In thousands) Stock options 12,316 9,624 Restricted stock units 1,424 839 Shares issuable under employee stock purchase plan 31 9 Series A Preferred Stock (if converted) 3,378 3,378 Warrants 2,500 2,500 19,649 16,350

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 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 March 31, 2021 and 2020 are referred to as the first 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 months ended March 31, 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 and it is not alleviated by the Company’s plans or when its plans alleviate substantial doubt about the Company’s ability to continue as a going concern. The Company’s evaluation entails analyzing prospective operating budgets and forecasts for expectations of the Company’s cash needs, and comparing those needs to its available cash, cash equivalents and marketable securities. The analysis for the first quarter of 2021 included consideration of the Company’s current cash needs, including its research and development plans, commercialization activities associated with the ongoing launch of TAZVERIK in the ES and FL indications, and its existing debt service obligations. The Company also evaluated its forecasted product revenues from sales of TAZVERIK. Such estimates of future sales contain significant judgement as TAZVERIK was recently launched and there is little or no history with which to base such estimates. The Company expects its available cash, cash equivalents and marketable securities will be sufficient to fund current planned operations and capital expenditure requirements for at least the next twelve months from the filing date of this Quarterly Report on Form 10-Q with the SEC. As a result, the Company concluded that it did not identify conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the date these financial statements were issued. 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.
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, 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 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.
Cost of Revenues	Cost of Revenues The cost of revenues primarily consists of costs related to the sales of TAZVERIK and sales of tazemetostat API and drug product Cost of revenues for the included approximately $0.8 million related to sales of tazemetostat drug product.
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 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. 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. 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 2021 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 March 31, 2021 (in thousands): March 31, 2021 Estimated useful life (years) In-licensed rights $ 50,000 12.2 Less: accumulated amortization (4,036 ) Total intangible asset, net $ 45,964 The Company recorded approximately $1.0 million and $0.3 million in amortization expense related to intangible assets, using the straight-line methodology, during the three months ended March 31, 2021 and 2020, respectively. Estimated future amortization expense for intangible assets for the remainder of the year ended December 31, 2021 is $3.1 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 consolidated balance sheet. 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.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Summary of Intangible Assets	The following table presents intangible assets as of March 31, 2021 (in thousands): March 31, 2021 Estimated useful life (years) In-licensed rights $ 50,000 12.2 Less: accumulated amortization (4,036 ) Total intangible asset, net $ 45,964

Product Revenue Net (Tables)

Product Revenue Net (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2021 2020 Customer 1 42 % 59 % Customer 2 14 % 3 % Customer 3 25 % 13 % Customer 4 19 % 25 %
Accounts Receivable [Member]
Concentration Risk [Line Items]
Schedules of Concentration of Risk Percentage	As of March 31, 2021 and December 31, 2020, the four individual customers were accounted for as a percentage of accounts receivable as follows: March 31, 2021 December 31, 2020 Customer 1 22 % 21 % Customer 2 14 % 14 % Customer 3 33 % 29 % Customer 4 31 % 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 three months ended March 31, 2021: Chargebacks, Discounts, and Government and Other Fees Rebates Returns Total (In thousands) Balance, January 1, 2021 $ 133 $ 428 $ 67 $ 628 Provision 399 610 — 1,009 Payments or credits (323 ) (572 ) — (895 ) Balance, March 31, 2021 $ 209 $ 466 $ 67 $ 742

Cash (Tables)

Cash (Tables)	3 Months Ended
	Mar. 31, 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 condensed consolidated balance sheets that sum to the total of the same such amounts shown in the condensed consolidated statements of cash flows, is as follows: As of March 31, 2021 2020 (In thousands) Cash and cash equivalents $ 73,711 $ 179,262 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 $ 75,220 $ 180,771

Marketable Securities (Tables)

Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2021 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 132,118 $ 6 $ (8 ) $ 132,116 Corporate notes 13,067 — (5 ) 13,062 U.S. government agency securities and U.S. Treasuries 80,041 13 — 80,054 Total $ 225,226 $ 19 $ (13 ) $ 225,232 The following table summarizes the available-for-sale securities held at December 31, 2020 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses 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)	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Summary of Company's Financial Assets Recognized at Fair Value	As of March 31, 2021 and December 31, 2020, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of March 31, 2021 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 68,366 $ 60,517 $ 7,849 $ — Marketable securities: Commercial paper 132,116 — 132,116 — Corporate notes 13,062 — 13,062 — U.S. government agency securities and treasuries 80,054 — 80,054 — Total $ 293,598 $ 60,517 $ 233,081 $ — 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)	3 Months Ended
	Mar. 31, 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 March 31, 2021 and December 31, 2020: March 31, 2021 December 31, 2020 (In thousands) Raw materials $ 1,307 $ 1,068 Work in process 12,659 8,564 Finished goods 851 829 Total $ 14,817 $ 10,461

Supplemental Balance Sheet In_2

Supplemental Balance Sheet Information (Tables)	3 Months Ended
	Mar. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following: March 31, 2021 December 31, 2020 (In thousands) Employee compensation and benefits $ 6,229 $ 11,921 Research and development expenses 10,762 10,664 Professional services and other 6,862 5,987 Accrued expenses $ 23,853 $ 28,572

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 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 months ended March 31, 2021 and 2020: Three Months Ended March 31, 2021 2020 Lease cost Operating lease cost $ 1,515 $ 1,550 Variable lease cost 480 422 Total lease cost $ 1,995 $ 1,972 Other information Operating cash flows used for operating leases $ 1,605 $ 981 Weighted average remaining lease term 4.7 years 5.2 years Weighted average discount rate 9.81 % 9.63 %
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 March 31, 2021, are as follows: (In thousands) 2021 $ 4,832 2022 6,256 2023 2,984 2024 3,053 Thereafter 6,966 Total lease payments $ 24,091 Less: imputed interest (5,145 ) Total operating lease liabilities at March 31, 2021 $ 18,946

Sale of Future Royalties (Table

Sale of Future Royalties (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2021: As of March 31, 2021 (In thousands) Proceeds from sale of future royalties $ 12,601 Non-cash interest expense recognized 2,045 Liability related to the sale of future royalties - ending balance $ 14,646

Long Term Debt (Tables)

Long Term Debt (Tables)	3 Months Ended
	Mar. 31, 2021
Debt Disclosure [Abstract]
Schedule of Minimum Aggregate Future Loan Payments	The Company has the following minimum aggregate future loan payments at March 31, 2021 (in thousands): As of March 31, 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 (4,142 ) Less current portion — Long-term debt, net of current portion $ 215,858

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2021 2020 (In thousands) Research and development $ 2,230 $ 2,162 General and administrative 4,785 4,348 Total $ 7,015 $ 6,510
Assumptions Used in Applying Pricing Model	Key assumptions used to apply this pricing model were as follows: Three Months Ended March 31, 2021 2020 Risk-free interest rate 0.4 % 1.3 % Expected life of options 6.00 years 5.98 years Expected volatility of underlying stock 70.5 % 70.3 % Expected dividend yield 0.0 % 0.0 %
Summary of Stock Option Activity	The following is a summary of stock option activity for the three months ended March 31, 2021: Number of Options Weighted Average Exercise Price per Share Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (In thousands) (In years) (In thousands) Outstanding at December 31, 2020 10,225 $ 14.77 Granted 2,509 10.94 Exercised (22 ) 9.15 Forfeited (396 ) 15.51 Outstanding at March 31, 2021 12,316 $ 13.97 7.81 $ 304 Exercisable at March 31, 2021 5,054 $ 14.29 6.58 $ 186
Summary of Service Based Restricted Stock Units	During the three months ended March 31, 2021, 971,078 restricted stock units (“RSUs”) were granted to executives and employees. The awards were service-based. Assuming all service conditions are achieved, 25% of the RSUs would vest annually for four years. Number of Service Based RSU Shares (in thousands) Weighted Average Grant Date Fair Value Outstanding at December 31, 2020 668 $ 17.56 Granted 971 10.99 Vested (166 ) 17.60 Forfeited (49 ) 14.20 Outstanding at March 31, 2021 1,424 $ 13.20

Loss Per Share (Tables)

Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2021 2020 (In thousands except per share data) Net loss $ (70,274 ) $ (50,937 ) Weighted average shares outstanding 101,790 99,616 Basic and diluted loss per share allocable to common stockholders $ (0.69 ) $ (0.51 )
Common Stock Equivalents Excluded from Calculation of Diluted Loss Per Share Attributable to Common Stockholders	The following common stock equivalents were excluded from the calculation of diluted loss per share allocable to common stockholders because their inclusion would have been anti-dilutive: Three Months Ended March 31, 2021 2020 (In thousands) Stock options 12,316 9,624 Restricted stock units 1,424 839 Shares issuable under employee stock purchase plan 31 9 Series A Preferred Stock (if converted) 3,378 3,378 Warrants 2,500 2,500 19,649 16,350

The Company - Additional Inform

The Company - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Basis Of Presentation [Line Items]
Proceeds from sale of redeemable convertible preferred stock	$ 76,000
Initial public offering completion date	May 2013
Cash, cash equivalents, and marketable securities	$ 298,900
Accumulated deficit	(1,058,987)	$ (988,713)
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 issuance of common stock, net of commissions	150,000
Royalty Pharma and Pharmakon Advisors [Member]
Basis Of Presentation [Line Items]
Aggregate fund, amount	1,527,400
Non-equity funding through collaboration agreement	243,800
BioPharma Credit Investments V Master LP And BioPharma Credit PLC [Member]
Basis Of Presentation [Line Items]
Debt financing received through loan agreement	218,100
IPO [Member]
Basis Of Presentation [Line Items]
Issuance of common stock (net of commissions and offering costs), Value	$ 839,500

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Jun. 30, 2021
Accounting Policies [Line Items]
Cost of revenue	$ 2,853	$ 614
Amortization of Intangible Assets	1,000	300
Estimated future amortization of intangible assets	3,100
Estimated future amortization of intangible assets thereafter	4,200
Eisai [Member]
Accounting Policies [Line Items]
Milestone payments		$ 25,000
Eisai [Member] | Scenario Forecast [Member]
Accounting Policies [Line Items]
Milestone payments			$ 25,000
Tazemetostat Drug Pruducts [Member]
Accounting Policies [Line Items]
Cost of revenue	$ 800

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary of Intangible Assets (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
In-licensed rights	$ 50,000
Less: accumulated amortization	(4,036)
Total intangible asset, net	$ 45,964	$ 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	3 Months Ended
	Mar. 31, 2021USD ($)
Valuation And Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 628
Provision	1,009
Payments or credits	(895)
Ending Balance	742
Chargebacks, Discounts, and Fees [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Beginning Balance	133
Provision	399
Payments or credits	(323)
Ending Balance	209
Government and Other Rebates [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Beginning Balance	428
Provision	610
Payments or credits	(572)
Ending Balance	466
Returns [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Beginning Balance	67
Ending Balance	$ 67

Product Revenue Net - Additiona

Product Revenue Net - Additional Information (Detail) - Customer	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accounts Receivable [Member] | Credit Concentration Risk [Member]
Product Information [Line Items]
Number of customers	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
	Mar. 31, 2021	Mar. 31, 2020
Customer 1 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	42.00%	59.00%
Customer 2 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	14.00%	3.00%
Customer 3 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	25.00%	13.00%
Customer 4 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	19.00%	25.00%

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]	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Customer 1 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	22.00%	21.00%
Customer 2 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	14.00%	14.00%
Customer 3 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	33.00%	29.00%
Customer 4 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	31.00%	36.00%

Cash - Summary of Reconciliatio

Cash - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 73,711	$ 168,215	$ 179,262
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	$ 75,220		$ 180,771

Cash - Additional Information (

Cash - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 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	Mar. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	$ 225,226	$ 205,388
Available-For-Sale Securities Held, Gross Unrealized Gains	19	18
Available-For-Sale Securities Held, Gross Unrealized Losses	(13)	(15)
Available-For-Sale Securities Held, Fair Value	225,232	205,391
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	132,118	158,907
Available-For-Sale Securities Held, Gross Unrealized Gains	6	14
Available-For-Sale Securities Held, Gross Unrealized Losses	(8)	(8)
Available-For-Sale Securities Held, Fair Value	132,116	158,913
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	13,067	33,437
Available-For-Sale Securities Held, Gross Unrealized Gains		3
Available-For-Sale Securities Held, Gross Unrealized Losses	(5)	(7)
Available-For-Sale Securities Held, Fair Value	13,062	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	80,041	13,044
Available-For-Sale Securities Held, Gross Unrealized Gains	13	1
Available-For-Sale Securities Held, Fair Value	$ 80,054	$ 13,045

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail)	3 Months Ended
	Mar. 31, 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	61,000,000
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate loss	$ 100,000
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	10
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in unrealized loss position | Security	5

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	Mar. 31, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	$ 68,366	$ 163,264
Total	293,598	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	80,054	13,045
Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	132,116	158,913
Corporate Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	13,062	33,433
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	60,517	113,505
Total	60,517	113,505
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	7,849	49,759
Total	233,081	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	80,054	13,045
Level 2 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	132,116	158,913
Level 2 [Member] | Corporate Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	$ 13,062	$ 33,433

Inventory - Summary of Inventor

Inventory - Summary of Inventory (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Inventory Disclosure [Abstract]
Raw materials	$ 1,307	$ 1,068
Work in process	12,659	8,564
Finished goods	851	829
Total	$ 14,817	$ 10,461

Supplemental Balance Sheet In_3

Supplemental Balance Sheet Information - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Accrued Liabilities Current [Abstract]
Employee compensation and benefits	$ 6,229	$ 11,921
Research and development expenses	10,762	10,664
Professional services and other	6,862	5,987
Accrued expenses	$ 23,853	$ 28,572

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 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. 16, 2019	Mar. 31, 2021	Mar. 31, 2020
Lessee Lease Description [Line Items]
Operating Lease, existence of option to terminate		true
Monthly base rent		$ 200,000
Increase in base rent		33,000
Annual increase of base rent		9,000
Security deposit		500,000
Hampshire Street [Member] | Letter of Credit [Member]
Lessee Lease Description [Line Items]
Security deposit		$ 1,000,000	$ 1,000,000
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 initial term from commencement date	7 years 4 months
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
	Mar. 31, 2021	Mar. 31, 2020
Leases [Abstract]
Operating lease cost	$ 1,515	$ 1,550
Variable lease cost	480	422
Total lease cost	1,995	1,972
Operating cash flows used for operating leases	$ 1,605	$ 981
Weighted average remaining lease term	4 years 8 months 12 days	5 years 2 months 12 days
Weighted average discount rate	9.81%	9.63%

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Mar. 31, 2021USD ($)
Leases [Abstract]
2021	$ 4,832
2022	6,256
2023	2,984
2024	3,053
Thereafter	6,966
Total lease payments	24,091
Less: imputed interest	(5,145)
Total operating lease liabilities at March 31, 2021	$ 18,946

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)

Jan. 01, 2011OptionTarget

Nov. 30, 2018USD ($)Installment

Mar. 31, 2015USD ($)

Mar. 31, 2021USD ($)

Mar. 31, 2020USD ($)

Dec. 31, 2013USD ($)

Dec. 31, 2019USD ($)

Dec. 31, 2020USD ($)

Sep. 30, 2020USD ($)

Jun. 30, 2020USD ($)

Jul. 31, 2019USD ($)

Mar. 31, 2018USD ($)

Revenue Recognition, Multiple-deliverable Arrangements [Line Items]

Regulatory milestone payments obligation

$ 900,000

Reimbursements receivable of regulatory milestone

1,000,000

Roche [Member]

Revenue Recognition, Multiple-deliverable Arrangements [Line Items]

Regulatory milestone payments obligation

$ 1,000,000

Notice period in days

90 days

GSK [Member]

Revenue Recognition, Multiple-deliverable Arrangements [Line Items]

Number of option targets | OptionTarget

3

Clinical development milestone payment

$ 50,000,000

Additional milestone payments

197,000,000

Sales-based milestone payments

128,000,000

Additional payments received

0

Cash and accounts receivable

$ 89,000,000

Deferred revenue

0

$ 0

GSK [Member] | Collaboration Revenue [Member]

Revenue Recognition, Multiple-deliverable Arrangements [Line Items]

Upfront payment received

$ 89,000,000

Eisai [Member]

Revenue Recognition, Multiple-deliverable Arrangements [Line Items]

Upfront payment made

$ 40,000,000

Clinical development milestone payments obligation

20,000,000

Regulatory milestone payments obligation

50,000,000

Total milestone payments

70,000,000

Research and development reduction

400,000

Obligation related to World Wide Royalty

900,000

200,000

Royalties payable

900,000

200,000

Other Receivables

5,200,000

100,000

Eisai [Member] | Tazemetostat Drug Pruducts [Member]

Revenue Recognition, Multiple-deliverable Arrangements [Line Items]

Deferred revenue

5,000,000

Total milestone payments

50,000,000

Total revenue

1,300,000

Royalty Pharma and Pharmakon Advisors [Member] | Roche [Member]

Revenue Recognition, Multiple-deliverable Arrangements [Line Items]

Remaining unpaid milestone payments

1,000,000

$ 10,400,000

Reimbursements receivable of development costs

900,000

Signed Amendment [Member] | Roche [Member]

Revenue Recognition, Multiple-deliverable Arrangements [Line Items]

Regulatory milestone payments obligation

500,000

Fourth Amendment [Member] | Roche [Member]

Revenue Recognition, Multiple-deliverable Arrangements [Line Items]

Regulatory milestone payments obligation

$ 500,000

Boehringer Ingelheim [Member]

Revenue Recognition, Multiple-deliverable Arrangements [Line Items]

Deferred revenue

0

$ 0

Total revenue

$ 26,000,000

$ 0

$ 100,000

Company received upfront payment

15,000,000

Research funding for costs to be incurred

$ 5,000,000

$ 100,000

$ 400,000

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	Mar. 31, 2021USD ($)shares	Mar. 31, 2020USD ($)	Dec. 31, 2020shares	Nov. 03, 2020USD ($)
Line Of Credit Facility [Line Items]
Common stock, shares issued | shares				101,969,000		101,627,000
Royalty Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Proceeds from sale of future royalties				$ 12,601,000
RPI and Affiliates [Member]
Line Of Credit Facility [Line Items]
Percentage of ownership interest related parties				9.00%		9.00%
Tazemetostat [Member] | Royalty Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Non cash royalty revenue				$ 0	$ 0
Non cash interest expense				500,000	$ 300,000
RPI Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Common stock, shares issued | shares	6,666,667
Warrants to purchase shares of common stock exercise price | $ / 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				13.30%
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] | 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
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
Loan Agreement [Member] | Maximum [Member]
Line Of Credit Facility [Line Items]
Right to request for additional loan amount				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
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
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 ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Deferred Revenue Arrangement [Line Items]
Related party liability related to sale of future royalties	$ 14,646	$ 14,176
Royalty Purchase Agreement [Member]
Deferred Revenue Arrangement [Line Items]
Proceeds from sale of future royalties	12,601
Non-cash interest expense recognized	2,045
Related party liability related to sale of future royalties	$ 14,646

Long-Term Debt - Additional Inf

Long-Term Debt - Additional Information (Detail)	Nov. 03, 2020USD ($)Installment	Nov. 30, 2020USD ($)	Nov. 30, 2019USD ($)	Mar. 31, 2021USD ($)	Mar. 31, 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] | Loan Agreement [Member] | BioPharma Credit Investments V (Master) LP [Member]
Debt Instrument [Line Items]
Interest Expense Credit Facilities				$ 5,400,000	$ 700,000
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 intrument 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	Mar. 31, 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	(4,142)
Long-term debt, net of current portion	$ 215,858	$ 215,670

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Nov. 04, 2019	Mar. 06, 2019	Feb. 29, 2020	Nov. 30, 2019	Mar. 31, 2021	Mar. 31, 2020	Apr. 08, 2021	Dec. 31, 2020	Dec. 31, 2019
Equity [Line Items]
Common stock, shares authorized					150,000,000			150,000,000
Common stock, par value					$ 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.
Preferred stock, shares issued					338,000			338,000
Preferred stock, par value					$ 0.0001			$ 0.0001
RPI Purchase Agreement [Member]
Equity [Line Items]
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 issued		350,000
Purchase price per share		$ 115
Net proceeds from sale of shares		$ 37.4
Preferred stock, par value		$ 0.0001
Common stock price per share					$ 12.34
Intrinsic value of beneficial conversion feature, discount on share					$ 2.9
Convertible preferred Stock, Conversion Price Per Share					$ 11.50
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
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
Preferred Stock [Member]
Equity [Line Items]
Issuance of Series A Convertible Preferred Stock, net of commissions and beneficial conversion charge, Shares			12,200
Common Stock [Member]
Equity [Line Items]
Issuance of Series A Convertible Preferred Stock, net of commissions and beneficial conversion charge, Shares			122,000
Common Stock [Member]
Equity [Line Items]
Common stock, shares authorized					150,000,000				125,000,000
Common stock, par value					$ 0.0001
Issuance of common stock (net of commissions and offering costs), Value			$ 49.9
Issuance of common stock upon exercise of stock options, Share			2,500,000
Proceeds from financing Costs after deducting			$ 0.1
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
Common Stock [Member] | Subsequent Event [Member]
Equity [Line Items]
Common stock, shares authorized							225,000,000
Additional Paid-In Capital [Member] | RPI Purchase Agreement [Member]
Equity [Line Items]
Proceeds from sale of equity securities fair value				$ 8.4

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense related to stock options, restricted stock, shares issued and shares granted to non-employee directors	$ 7,015	$ 6,510
Weighted-average fair value of options granted	$ 6.75	$ 13.37
Unrecognized compensation cost	$ 57,000
Expected weighted average period for recognition of compensation cost	2 years 9 months 14 days
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense related to stock options, restricted stock, shares issued and shares granted to non-employee directors	$ 7,000	$ 6,500
Service Based RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected weighted average period for recognition of compensation cost	3 years 4 months 20 days
Restricted stock units outstanding	971,000
Vesting percentage	25.00%
Vesting period	4 years
Compensation expense was recognized	$ 1,100	500
Unrecognized compensation cost	$ 17,200
Service Based RSU [Member] | Executives and employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units outstanding	971,078
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	$ 1,400
Unrecognized compensation cost	$ 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
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 7,015	$ 6,510
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	2,230	2,162
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 4,785	$ 4,348

Stock-Based Compensation - Assu

Stock-Based Compensation - Assumptions Used in Applying Pricing Model (Detail) - Employee Stock Option [Member]	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.40%	1.30%
Expected life of options	6 years	5 years 11 months 23 days
Expected volatility of underlying stock	70.50%	70.30%
Expected dividend yield	0.00%	0.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of Options, Outstanding, Beginning balance | shares	10,225,000
Number of Options, Granted | shares	2,509,000
Number of Options, Exercised | shares	(22,000)
Number of Options, Forfited | shares	(396,000)
Number of Options, Outstanding, Ending balance | shares	12,316,000
Number of Options, Exercisable | shares	5,054,000
Weighted Average Exercise Price per Share, Outstanding, Beginning balance | $ / shares	$ 14.77
Weighted Average Exercise Price per Share, Granted | $ / shares	10.94
Weighted Average Exercise Price per Share, Exercised | $ / shares	9.15
Weighted Average Exercise Price per Share, Forfited | $ / shares	15.51
Weighted Average Exercise Price per Share, Outstanding, Ending balance | $ / shares	13.97
Weighted Average Exercise Price per Share, Exercisable | $ / shares	$ 14.29
Weighted Average Remaining Contractual Term (In Years), Outstanding	7 years 9 months 21 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	6 years 6 months 29 days
Aggregate Intrinsic Value, Outstanding | $	$ 304
Aggregate Intrinsic Value, Exercisable | $	$ 186

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Service Based Restricted Stock Units (Detail) - Service Based RSU [Member] shares in Thousands	3 Months Ended
	Mar. 31, 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	971
Number of Shares, Vested | shares	(166)
Number of Shares, Forfeited | shares	(49)
Number of Outstanding Shares, Ending Balance | shares	1,424
Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 17.56
Weighted Average Grant Date Fair Value, Granted | $ / shares	10.99
Weighted Average Grant Date Fair Value, Vested | $ / shares	17.60
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	14.20
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 13.20

Loss Per Share - Schedule of Ba

Loss Per Share - Schedule of Basic and Diluted Loss Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Net loss	$ (70,274)	$ (50,937)
Weighted average shares outstanding	101,790	99,616
Basic and diluted loss per share allocable to common stockholders	$ (0.69)	$ (0.51)

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
	Mar. 31, 2021	Mar. 31, 2020
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	19,649	16,350
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	12,316	9,624
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	1,424	839
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	31	9
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
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	2,500	2,500