Immunogen (IMGN) 10-Q 29 Oct 21 2021 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Oct. 26, 2021
Document and Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2021
Entity File Number	0-17999
Entity Registrant Name	ImmunoGen, Inc.
Entity Incorporation, State or Country Code	MA
Entity Tax Identification Number	04-2726691
Entity Address, Address Line One	830 Winter Street
Entity Address, City or Town	Waltham
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02451
City Area Code	781
Local Phone Number	895-0600
Title of 12(b) Security	Common Stock
Trading Symbol	IMGN
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		202,619,488
Entity Central Index Key	0000855654
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Amendment Flag	false

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
ASSETS
Cash and cash equivalents	$ 245,761	$ 293,856
Accounts receivable	221	35
Unbilled receivables	4,706	11
Contract assets	2,500
Non-cash royalty receivable	3,369	22,451
Prepaid and other current assets	14,330	7,901
Total current assets	270,887	324,254
Property and equipment, net of accumulated depreciation	4,636	5,760
Operating lease right-of-use assets	12,745	14,072
Other assets	8,535	10,986
Total assets	296,803	355,072
LIABILITIES AND SHAREHOLDERS' EQUITY
Accounts payable	10,520	9,538
Accrued compensation	4,718	4,620
Other accrued liabilities	34,718	29,320
Convertible 4.5% senior notes, net of deferred financing costs of $0 and $7, respectively		2,093
Current portion of liability related to the sale of future royalties, net of deferred financing costs of $206 and $319, respectively	4,945	44,357
Current portion of operating lease liability	3,358	3,146
Current portion of deferred revenue	53,526	29,249
Total current liabilities	111,785	122,323
Deferred revenue, net of current portion	52,479	80,860
Operating lease liability, net of current portion	16,045	18,651
Liability related to the sale of future royalties, net of current portion and deferred financing costs of $427 and $584, respectively	37,322	41,082
Other long-term liabilities	2,079	2,586
Total liabilities	219,710	265,502
Commitments and contingencies (Note I)
Shareholders' deficit:
Preferred stock, $.01 par value; authorized 5,000 shares; no shares issued and outstanding as of each of September 30, 2021 and December 31, 2020
Common stock, $.01 par value; authorized 300,000 shares; issued and outstanding 202,443 and 194,998 shares as of September 30, 2021 and December 31, 2020, respectively	2,024	1,950
Additional paid-in capital	1,509,040	1,419,460
Accumulated deficit	(1,433,971)	(1,331,840)
Total shareholders' equity	77,093	89,570
Total liabilities and shareholders' equity	$ 296,803	$ 355,072

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
CONSOLIDATED BALANCE SHEETS
Current portion of deferred financing costs	$ 0	$ 7
Interest rate (as a percent)	4.50%	4.50%
Sale of future royalties, current portion and deferred financing costs	$ 206	$ 319
Sale of future royalties, noncurrent portion and deferred financing costs	$ 427	$ 584
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, authorized shares	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, authorized shares	300,000,000	300,000,000
Common stock, issued shares	202,443,000	194,998,000
Common stock, outstanding shares	202,443,000	194,998,000

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues:
Total revenues	$ 9,210	$ 18,189	$ 41,864	$ 46,501
Operating expenses:
Research and development	33,147	24,685	102,149	75,014
General and administrative	10,297	10,231	30,234	28,862
Restructuring charges				1,524
Total operating expenses	43,444	34,916	132,383	105,400
Loss from operations	(34,234)	(16,727)	(90,519)	(58,899)
Investment income, net	11	11	35	719
Non-cash interest expense on liability related to the sale of future royalties and convertible senior notes	(2,751)	(5,645)	(10,952)	(17,428)
Interest expense on convertible senior notes		(24)	(47)	(71)
Other (expense) income, net	(365)	11	(648)	(81)
Net loss	$ (37,339)	$ (22,374)	$ (102,131)	$ (75,760)
Basic and diluted net loss per common share	$ (0.18)	$ (0.13)	$ (0.51)	$ (0.44)
Basic and diluted weighted-average common shares outstanding	204,844	174,508	201,212	172,215
Total comprehensive loss	$ (37,339)	$ (22,374)	$ (102,131)	$ (75,760)
Non-cash royalty revenue related to the sale of future royalties
Revenues:
Total revenues	6,533	18,087	38,768	45,159
License and milestone fees
Revenues:
Total revenues	$ 2,677	97	3,086	1,325
Research and development support
Revenues:
Total revenues		$ 5	$ 10	$ 17

CONSOLIDATED STATEMENTS OF SHAR

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Balance at Dec. 31, 2019	$ 1,501	$ 1,209,846	$ (1,287,468)	$ (76,121)
Balance (in shares) at Dec. 31, 2019	150,136
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(29,088)	(29,088)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 1	239		240
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	86
Issuance of common stock, net of issuance costs	$ 245	97,499		97,744
Issuance of common stock, net of issuance costs (in shares)	24,524
Stock option and restricted stock compensation expense		3,122		3,122
Restricted stock units vested (in shares)	2
Restricted stock award, net of forfeitures	$ (4)	4
Restricted stock award, net of forfeitures (in shares)	(487)
Balance at Mar. 31, 2020	$ 1,743	1,310,710	(1,316,556)	(4,103)
Balance (in shares) at Mar. 31, 2020	174,261
Balance at Dec. 31, 2019	$ 1,501	1,209,846	(1,287,468)	(76,121)
Balance (in shares) at Dec. 31, 2019	150,136
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss				(75,760)
Balance at Sep. 30, 2020	$ 1,746	1,318,591	(1,363,228)	(42,891)
Balance (in shares) at Sep. 30, 2020	174,585
Balance at Mar. 31, 2020	$ 1,743	1,310,710	(1,316,556)	(4,103)
Balance (in shares) at Mar. 31, 2020	174,261
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(24,298)	(24,298)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 1	424		425
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	122
Stock option and restricted stock compensation expense		3,409		3,409
Adjustment of issuance costs		(1)		(1)
Restricted stock units vested	$ 1	(1)
Restricted stock units vested (in shares)	157
Directors' deferred share unit compensation		45		45
Balance at Jun. 30, 2020	$ 1,745	1,314,586	(1,340,854)	(24,523)
Balance (in shares) at Jun. 30, 2020	174,540
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(22,374)	(22,374)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 1	127		128
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	45
Stock option and restricted stock compensation expense		3,729		3,729
Directors' deferred share unit compensation		149		149
Balance at Sep. 30, 2020	$ 1,746	1,318,591	(1,363,228)	(42,891)
Balance (in shares) at Sep. 30, 2020	174,585
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			31,388	31,388
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 2	676		678
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	205
Issuance of common stock, net of issuance costs	$ 200	96,328		96,528
Issuance of common stock, net of issuance costs (in shares)	19,972
Stock option and restricted stock compensation expense		3,718		3,718
Restricted stock units vested	$ 2	(2)
Restricted stock units vested (in shares)	236
Directors' deferred share unit compensation		149		149
Balance at Dec. 31, 2020	$ 1,950	1,419,460	(1,331,840)	$ 89,570
Balance (in shares) at Dec. 31, 2020	194,998			194,998
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(34,051)	$ (34,051)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 4	1,282		1,286
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	397
Issuance of common stock, net of issuance costs	$ 45	33,447		33,492
Issuance of common stock, net of issuance costs (in shares)	4,544
Stock option and restricted stock compensation expense		3,674		3,674
Restricted stock units vested (in shares)	2
Directors' deferred share unit compensation		149		149
Balance at Mar. 31, 2021	$ 1,999	1,458,012	(1,365,891)	94,120
Balance (in shares) at Mar. 31, 2021	199,941
Balance at Dec. 31, 2020	$ 1,950	1,419,460	(1,331,840)	$ 89,570
Balance (in shares) at Dec. 31, 2020	194,998			194,998
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss				$ (102,131)
Balance at Sep. 30, 2021	$ 2,024	1,509,040	(1,433,971)	$ 77,093
Balance (in shares) at Sep. 30, 2021	202,443			202,443
Balance at Mar. 31, 2021	$ 1,999	1,458,012	(1,365,891)	$ 94,120
Balance (in shares) at Mar. 31, 2021	199,941
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(30,741)	(30,741)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 1	377		378
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	75
Stock option and restricted stock compensation expense		3,598		3,598
Conversion of debt	$ 3	997		1,000
Conversion of debt (in shares)	239
Common stock issuance costs		(34)		(34)
Directors' deferred share unit compensation		144		144
Balance at Jun. 30, 2021	$ 2,003	1,463,094	(1,396,632)	68,465
Balance (in shares) at Jun. 30, 2021	200,255
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(37,339)	(37,339)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 1	367		368
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	95
Issuance of common stock, net of issuance costs	$ 21	12,336		12,357
Issuance of common stock, net of issuance costs (in shares)	2,150
Issuance of pre-funded warrant, net of issuance costs		29,765		29,765
Stock option and restricted stock compensation expense		3,298		3,298
Directors' deferred share units converted		179		179
Restricted stock award, net of forfeitures	$ (1)	1
Restricted stock award, net of forfeitures (in shares)	(57)
Balance at Sep. 30, 2021	$ 2,024	$ 1,509,040	$ (1,433,971)	$ 77,093
Balance (in shares) at Sep. 30, 2021	202,443			202,443

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net loss	$ (102,131)	$ (75,760)
Adjustments to reconcile net loss to net cash used for operating activities:
Non-cash royalty revenue related to sale of future royalties	(35,035)	(45,159)
Non-cash interest expense on liability related to sale of future royalties and convertible senior notes	10,952	17,428
Depreciation and amortization	1,555	1,569
Gain on sale/disposal of fixed assets and impairment charges		(691)
Stock and deferred share unit compensation	11,042	10,454
Change in operating assets and liabilities:
Accounts receivable	(186)	2,450
Unbilled receivable	(4,695)	996
Contract asset	(2,500)	3,631
Prepaid and other current assets	(6,429)	(2,450)
Operating lease right-of-use assets	1,327	1,110
Other assets	2,451	(4,786)
Accounts payable	1,354	(1,028)
Accrued compensation	98	(4,651)
Other accrued liabilities	5,153	8,829
Deferred revenue	(4,104)	3,094
Operating lease liability	(2,394)	(2,191)
Net cash used for operating activities	(123,542)	(87,155)
Cash flows from investing activities:
Purchases of property and equipment	(1,065)	(815)
Proceeds from sale of equipment		1,426
Net cash (used for) provided by investing activities	(1,065)	611
Cash flows from financing activities:
Payments upon settlement of convertible senior notes	(1,100)
Proceeds from issuance of common stock under stock plans	2,032	793
Proceeds from warrant issuance, net of $181 of transaction costs	29,765
Proceeds from common stock issuance, net of $143 and $230 of transaction costs, respectively	45,815	97,743
Net cash provided by financing activities	76,512	98,536
Net change in cash and cash equivalents	(48,095)	11,992
Cash and cash equivalents, beginning of period	293,856	176,225
Cash and cash equivalents, end of period	$ 245,761	$ 188,217

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Warrant
Transaction costs	$ 181
Common Stock
Transaction costs	$ 143	$ 230

Nature of Business and Plan of

Nature of Business and Plan of Operations	9 Months Ended
	Sep. 30, 2021
Nature of Business and Plan of Operations
Nature of Business and Plan of Operations	A. Nature of Business and Plan of Operations ​ ImmunoGen, Inc. (the Company) was incorporated in Massachusetts in 1981 and is focused on the development of antibody-drug conjugates (ADCs). The Company has generally incurred operating losses and negative cash flows from operations since inception, incurred a net loss of $102.1 million during the nine months ended September 30, 2021, and has an accumulated deficit of approximately $1.4 billion as of September 30, 2021. The Company has primarily funded these losses through payments received from its collaborations and equity, convertible debt, and other financings. To date, the Company has had no product revenue and management expects to continue to incur operating expenses related to research and development and potential commercialization of its portfolio over the next several years. As of September 30, 2021, the Company had $245.8 million of cash and cash equivalents on hand. The Company anticipates that its current capital resources will enable it to meet its operational expenses and capital expenditures for more than twelve months after the date these financial statements were issued. The Company may raise additional funds through equity, debt, or other financings, or generate revenues from collaborators through a combination of upfront license payments, milestone payments, royalty payments, and research funding. There can be no assurance that the Company will be able to obtain additional equity, debt, or other financing or generate revenues from collaborators on terms acceptable to the Company or at all. The failure of the Company to obtain sufficient funds on acceptable terms when needed could have a material adverse effect on the Company’s business, results of operations, and financial condition and require the Company to defer or limit some or all of its research, development, and/or clinical projects. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, the development by its competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, manufacturing and marketing limitations, complexities associated with managing collaboration arrangements, third-party reimbursements, and compliance with governmental regulations.

Basis of Presentation and Signi

Basis of Presentation and Significant Accounting Policies	9 Months Ended
	Sep. 30, 2021
Basis of Presentation and Significant Accounting Policies
Basis of Presentation and Significant Accounting Policies	B. Basis of Presentation and Significant Accounting Policies Basis of Presentation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany transactions and balances have been eliminated. The consolidated financial statements include all of the adjustments, consisting only of normal recurring adjustments, which management considers necessary for a fair presentation of the Company’s financial position in accordance with accounting principles generally accepted in the U.S. for interim financial information. The December 31, 2020 consolidated balance sheet presented for comparative purposes was derived from the Company’s audited financial statements, and certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. The preparation of interim financial statements requires the use of management’s estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the interim financial statements and the reported amounts of revenues and expenditures during the reported periods. The results of the interim periods are not necessarily indicative of the results for the entire year. Accordingly, the interim financial statements should be read in conjunction with the audited financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020 filed with the SEC on March 1, 2021. Significant Accounting Policies Common Stock Warrants ​ The Company accounts for common stock warrants as either equity-classified or liability-classified instruments based on an assessment of the warrant’s specific terms and applicable authoritative guidance in Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) 480, Distinguishing Liabilities from Equity Derivatives and Hedging 815, including whether the warrants are indexed to the Company’s own common stock and whether the warrant holders could potentially require “net cash settlement” in a circumstance outside of the Company’s control, among other conditions for equity classification. This assessment, which requires the use of professional judgment, is conducted at the time of warrant issuance and as of each subsequent quarterly period end date while the warrants are outstanding. ​ For issued or modified warrants that meet all of the criteria for equity classification, the warrants are required to be recorded as a component of additional paid-in capital at the time of issuance. For issued or modified warrants that do not meet all the criteria for equity classification, the warrants are required to be recorded at their initial fair value on the date of issuance and remeasured each balance sheet date thereafter. Changes in the estimated fair value of the liability-classified warrants are recognized as a non-cash gain or loss in the accompanying consolidated statements of operations and comprehensive loss. ​ Other than the accounting for warrants noted above, the significant accounting policies used in preparation of these condensed consolidated financial statements for the three and nine months ended September 30, 2021 are consistent with those discussed in Note B to the consolidated financial statements included in the Company’s 2020 Annual Report on Form 10-K, except as described under Recently Adopted Accounting Pronouncements ​ Revenue Recognition Transaction Price Allocated to Future Performance Obligations Deferred revenue under ASC 606, Revenue from Contracts with Customers 13 61 Contract Balances from Contracts with Customers The following tables present changes in the Company’s contract assets and contract liabilities during the nine months ended September 30, 2021 and 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Nine months ended September 30, 2021 ​ December 31, 2020 Additions ​ Deductions ​ Impact of Netting ​ September 30, 2021 Contract asset ​ $ — ​ $ 2,500 ​ $ — ​ $ — ​ $ 2,500 Contract liabilities (deferred revenue) ​ $ 110,109 ​ $ 25 ​ $ (4,129) ​ $ — ​ $ 106,005 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Nine months ended September 30, 2020 ​ December 31, 2019 ​ Additions ​ Deductions ​ Impact of Netting ​ September 30, 2020 Contract asset ​ $ 3,631 ​ $ — ​ $ (8,000) ​ $ 4,369 ​ $ — Contract liabilities (deferred revenue) ​ $ 127,432 ​ $ 50 ​ $ (1,325) ​ $ 4,369 ​ $ 130,526 ​ The Company recognized the following revenues as a result of changes in contract asset and contract liability balances in the respective periods (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ ​ September 30, ​ September 30, ​ ​ ​ 2021 ​ 2020 ​ 2021 ​ 2020 ​ ​ Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 3,292 ​ $ 72 ​ $ 4,129 ​ $ 1,300 ​ ​ ​ During the nine months ended September 30, 2021, the Company recorded a contract asset of $2.5 million for a probable development milestone pursuant to its license agreement with Viridian Therapeutics, Inc. (Viridian), which was subsequently achieved in October 2021. The Company also recorded $0.2 million as license and milestone fee revenue for delivery of certain materials to Viridian that had been previously deferred, and $0.2 million of license and milestone fee revenue related to numerous collaborators’ rights to technological improvements that had been previously deferred. Additionally, the Company recorded $3.7 million of previously deferred non-cash royalty revenue related to the sale of rights to Kadcyla royalties, further details of which can be found in Note E, “Liability Related to Sale of Future Royalties.” During the nine months ended September 30, 2020, the Company recorded $0.2 million as license and milestone fee revenue for delivery of certain materials to CytomX that had been previously deferred, and $1.1 million of license and milestone fee revenue related to numerous collaborators’ rights to technological improvements that had been previously deferred, which included $0.9 million related to the termination of a license agreement with Takeda. Additionally, a contract asset of $3.6 million, net of $4.4 million of related contract liabilities, was recorded for two probable milestones in 2019 pursuant to license agreements with CytomX and Novartis, which were subsequently achieved and paid during the nine months ended September 30, 2020. The timing of revenue recognition, billings, and cash collections results in billed receivables, unbilled receivables, contract assets, and contract liabilities on the consolidated balance sheets. When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded (under the caption deferred revenue). Contract liabilities are recognized as revenue after control of the products or services is transferred to the customer and all revenue recognition criteria have been met. Financial Instruments and Concentration of Credit Risk Cash and cash equivalents are primarily maintained with three financial institutions in the U.S. Deposits with banks may exceed the amount of insurance provided on such deposits. Generally, these deposits may be redeemed upon demand and, therefore, bear minimal risk. The Company’s cash equivalents consist of money market funds with underlying investments primarily being U.S. Government-issued securities and high quality, short-term commercial paper. Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash, cash equivalents, and marketable securities. The Company held no marketable securities as of September 30, 2021 and December 31, 2020. The Company’s investment policy, approved by the Board of Directors, limits the amount it may invest in any one type of investment, thereby reducing credit risk concentrations. Cash and Cash Equivalents The Company considers all highly liquid financial instruments with maturities of three months or less when purchased to be cash equivalents. As of September 30, 2021 and December 31, 2020, the Company held $245.8 million and $293.9 million, respectively, in cash and money market funds, which were classified as cash and cash equivalents. Non-cash Investing and Financing Activities During the nine months ended September 30, 2021, $1.0 million of outstanding convertible 4.5% senior notes were converted into 238,777 shares of the Company’s common stock. There was no similar activity during the nine months ended September 30, 2020. The Company had $0.1 million and $0.7 million of accrued capital expenditures as of September 30, 2021 or December 31, 2020, respectively, which have been treated as a non-cash investing activity and, accordingly, are not reflected in the consolidated statement of cash flows. Fair Value of Financial Instruments Fair value is defined under ASC 820, Fair Value Measurements and Disclosures ● Level 1 - Quoted prices in active markets for identical assets or liabilities. ● Level 2 - Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. ● Level 3 – Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. As of September 30, 2021 and December 31, 2020, the Company held certain assets that are required to be measured at fair value on a recurring basis. The fair value of the Company’s cash equivalents is based on quoted prices from active markets (Level 1 inputs). The carrying amounts reflected in the consolidated balance sheets for accounts receivable, unbilled receivables, prepaid and other current assets, accounts payable, accrued compensation, and other accrued liabilities approximate fair value due to their short-term nature. The gross carrying amount and estimated fair value of the convertible 4.5% senior notes was $2.1 million and $4.3 million, respectively, as of December 31, 2020. The fair value of the convertible notes was influenced by interest rates, the Company’s stock price, stock price volatility, and by prices observed in trading activity for the convertible notes. However, because there were no trades involving the convertible notes since September 2019, the fair value as of December 31, 2020 used Level 3 inputs. During the nine months ended September 30, 2021, Computation of Net Loss per Common Share Basic and diluted net loss per share is calculated based upon the weighted-average number of shares of common stock outstanding during the period. Shares of the Company’s common stock underlying pre-funded warrants are included in the calculation of basic and diluted earnings per share. During periods of income, participating securities are allocated a proportional share of income determined by dividing total weighted-average participating securities by the sum of the total weighted-average shares of common stock and participating securities (the two-class method). Shares of the Company’s restricted stock participate in any dividends that may be declared by the Company and are therefore considered to be participating securities. Participating securities have the effect of diluting both basic and diluted earnings per share during periods of income. During periods of loss, no loss is allocated to participating securities since they have no contractual obligation to share in the losses of the Company. Diluted loss per share is computed after giving consideration to the dilutive effect of stock options, convertible notes, and restricted stock that are outstanding during the period, except where such non-participating securities would be anti-dilutive. The Company’s common stock equivalents, as calculated in accordance with the treasury-stock method for options and unvested restricted stock and the if-converted method for the convertible notes, are shown in the following table (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ September 30, ​ September 30, ​ 2021 2020 2021 2020 Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock/units at end of period ​ 20,862 ​ 19,181 ​ 20,862 ​ 19,181 Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock/units ​ 2,116 929 ​ 2,743 ​ 1,112 Common stock equivalents under if-converted method for convertible notes ​ - ​ 501 ​ - ​ 501 ​ The Company’s common stock equivalents have not been included in the net loss per share calculation because their effect is anti-dilutive due to the Company’s net loss position. Stock-Based Compensation As of June 30, 2021, the Company was authorized to grant future awards under three employee share-based compensation plans, which are the ImmunoGen, Inc. Amended and Restated 2018 Employee, Director and Consultant Equity Incentive Plan (the 2018 Plan), the Employee Stock Purchase Plan (the ESPP), and the ImmunoGen Inducement Equity Incentive Plan (the Inducement Plan). At the annual meeting of shareholders on June 16, 2021, the 2018 Plan was amended to provide for the issuance of stock grants, the grant of options, and the grant of stock-based awards for up to an additional 6,600,000 shares of the Company’s common stock, as well as up to 22,392,986 shares of common stock, which represent the number of shares of common stock remaining under the 2018 Plan as of March 31, 2021, and awards previously granted under the 2018 Plan and the Company’s former stock-based plans, including the ImmunoGen, Inc. 2016 and 2006 Employee, Director and Consultant Equity Incentive Plans, that forfeit, expire, or cancel without delivery of shares of common stock or which resulted in the forfeiture of shares of common stock back to the Company subsequent to March 31, 2021. The Inducement Plan was approved by the Board of Directors in December 2019, and pursuant to subsequent amendments, provides for the issuance of non-qualified option grants for up to 3,500,000 shares of the Company’s common stock. Options awarded under the two plans are granted with an exercise price equal to the market price of the Company’s stock at the date of grant. Options vest at various periods of up to four years and may be exercised within ten years of the date of grant under each of these plans. The stock-based awards are accounted for under ASC 718, Compensation—Stock Compensation ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ 2021 ​ 2020 ​ 2021 ​ 2020 Dividend ​ None ​ None ​ None ​ None Volatility ​ 82.7% ​ 88.3% ​ 85.2% ​ 85.0% Risk-free interest rate ​ 0.95% ​ 0.36% ​ 0.68% ​ 1.23% Expected life (years) ​ 6.0 ​ 6.0 ​ 6.0 ​ 6.0 ​ Using the Black-Scholes option-pricing model, the weighted-average grant date fair values of options granted during the three months ended September 30, 2021 and 2020 were $3.99 and $4.56 per share, respectively, and $5.34 and $4.57 for options granted during the nine months ended September 30, 2021 and 2020, respectively. A summary of option activity under the Company’s equity plans for the nine months ended September 30, 2021 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ Number ​ Average ​ ​ ​ of Stock ​ Exercise ​ ​ ​ Options ​ Price ​ Outstanding at December 31, 2020 ​ 18,398 ​ $ 6.10 ​ Granted ​ 4,380 ​ ​ 7.53 ​ Exercised ​ (503) ​ ​ 3.34 ​ Forfeited/Canceled ​ (1,415) ​ ​ 8.06 ​ Outstanding at September 30, 2021 ​ 20,860 ​ ​ 6.34 ​ ​ In 2020, the Company issued 2.6 million performance-based stock options to certain employees, all of which remain outstanding as of September 30, 2021, that will vest upon the achievement of specified performance goals. As of September 30, 2021, the Company determined it was not probable that any of these performance goals will be achieved and, therefore, no expense had been recorded to date. In October 2021, upon approval by the Compensation Committee of the Company’s Board of Directors, certain terms of the performance-based stock option award agreements were modified. Pursuant to ASC 718, the Company determined the modification to be a Type IV (improbable-to-improbable) modification, revalued the modified awards as of the modification date, and determined the modified performance goals were not probable of being achieved. The modified fair value of the performance-based stock options that could be expensed in future periods is $10.5 million. A summary of restricted stock and restricted stock unit activity under the Company’s equity plans for the nine months ended September 30, 2021 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted- ​ ​ Restricted ​ Average Grant ​ ​ Stock Shares ​ Date Fair Value Unvested at December 31, 2020 ​ 61 ​ $ 2.47 Vested (2) ​ ​ 2.53 Forfeited ​ (57) ​ ​ 2.47 Unvested at September 30, 2021 ​ 2 ​ $ 2.53 ​ In June 2018, the Company's Board of Directors, with shareholder approval, adopted the ESPP. Following the automatic share increase on January 1, 2021, pursuant to the ESPP’s “evergreen” provision, an aggregate of 2,000,000 shares of common stock have been reserved for issuance under the ESPP. On June 30, 2021 and 2020, approximately 64,000, and 78,000 shares, respectively, were issued to participating employees at a fair value of $2.14 and $1.86 per share, respectively. The fair value of each ESPP award is estimated on the first day of the offering period using the Black-Scholes option-pricing model. The Company recognizes share-based compensation expense equal to the fair value of the ESPP awards on a straight-line basis over the offering period. Stock compensation expense related to stock options and restricted stock awards granted under the stock plans and the ESPP was $3.3 million and $10.6 million during the three and nine months ended September 30, 2021, respectively, compared to stock compensation expense of $3.7 million and $10.3 million for the three and nine months ended September 30, 2020, respectively. As of September 30, 2021, the estimated fair value of unvested employee awards, exclusive of performance awards, was $28.3 million. The weighted-average remaining vesting period for these awards is approximately three years . Segment Information During all periods presented, the Company continued to operate in one reportable business segment under the management approach of ASC 280, Segment Reporting During the three and nine months ended September 30, 2021, 71% and 93%, respectively, of revenues were from Roche, consisting primarily of non-cash royalty revenue, compared to 99% and 97% of revenue from Roche in the three and nine months ended September 30, 2020, respectively. During the three months ended September 30, 2021, 28% of revenues were from Viridian. There were no other customers of the Company that generated significant revenues in the three or nine months ended September 30, 2021 and 2020. Recently Adopted Accounting Pronouncements In December 2019, the Financial Accounting Standards Board issued Accounting Standards Update (ASU) 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes No other recently issued or effective ASUs had, or are expected to have, a material effect on the Company's results of operations, financial condition, or liquidity.

Agreements

Agreements	9 Months Ended
	Sep. 30, 2021
Agreements
Agreements	C. Agreements Significant Collaborative Agreements Roche In May 2000, the Company granted Genentech, now a member of the Roche Group, an exclusive license to use the Company’s maytansinoid ADC technology. Pursuant to this agreement, Roche developed and received marketing approval for its HER2-targeting ADC, Kadcyla, in the U.S., Japan, the European Union, and numerous other countries. In accordance with the Company’s revenue recognition policy, $38.8 million and $45.2 million of non-cash royalties on net sales of Kadcyla were recorded and included in non-cash royalty revenue for the nine months ended September 30, 2021 and 2020, respectively. Kadcyla sales occurring after January 1, 2015 were covered by a royalty purchase agreement whereby the associated cash, except for a residual tail, was initially remitted to Immunity Royalty Holdings, L.P. (IRH). In January 2019, the Company sold its residual tail to OMERS, the defined benefit pension plan for municipal employees in the Province of Ontario, Canada, for a net payment of $65.2 million, as discussed further in Note E. Simultaneously, OMERS purchased IRH’s right to the royalties the Company previously sold as described above, therefore obtaining the rights to 100% of the royalties on the commercial sales of Kadcyla received from that date on. Viridian In October 2020, the Company entered into a license agreement with Viridian pursuant to which the Company granted Viridian the exclusive right to develop and commercialize an insulin-like growth factor-1 receptor (IGF-1R) antibody for all non-oncology indications that do not use radiopharmaceuticals in exchange for an upfront payment, with the potential to receive up to a total of $143.0 million in milestone payments plus royalties on the commercial sales of any resulting product. The total milestones are categorized as follows: development and regulatory milestones—$48.0 million; and sales milestones—$95.0 million. During the quarter ended September 30, 2021, a development milestone became probable of being achieved, which resulted in $2.5 million that was allocated to the delivered license being recorded as revenue and included in license and milestone fees for the three and nine months ended September 30, 2021. The development milestone was subsequently achieved in October 2021. For additional information related to this agreement, as well as the Company’s other significant collaborative agreements, please read Note C, “Agreements - Significant Collaborative Agreements,” to the audited financial statements included within the Company’s Annual Report on Form 10-K for the year ended December 31, 2020 filed with the SEC on March 1, 2021.

Convertible 4.5% Senior Notes

Convertible 4.5% Senior Notes	9 Months Ended
	Sep. 30, 2021
Convertible 4.5% Senior Notes
Convertible 4.5% Senior Notes	D. Convertible 4.5% Senior Notes In 2016, the Company issued convertible notes with an aggregate principal amount of $100.0 million, of which $2.1 million remained outstanding as of December 31, 2020. In June 2021, $1.0 million of outstanding convertible notes were converted into 238,777 shares of the Company’s common stock and the remaining $1.1 million outstanding was repaid in full by a cash payment upon maturity on July 1, 2021. The convertible notes were senior unsecured obligations with an interest rate of 4.5% per year, paid semi-annually in arrears on January 1 and July 1 of each year, commencing on January 1, 2017. The Company recorded $47,000 and $71,000 of interest expense for the nine months ended September 30, 2021 and 2020, respectively. The Company analyzed the terms of the convertible notes and determined that under current accounting guidance the notes were entirely accounted for as debt, and none of the terms of the notes required separate accounting.

Liability Related to Sale of Fu

Liability Related to Sale of Future Royalties	9 Months Ended
	Sep. 30, 2021
Liability Related to Sale of Future Royalties
Liability Related to Sale of Future Royalties	E. Liability Related to Sale of Future Royalties In 2015, IRH purchased the right to receive 100% of the royalty payments on commercial sales of Kadcyla subsequent to December 31, 2014 arising under the Company’s development and commercialization license with Genentech, until IRH had received aggregate royalties equal to $235.0 million or $260.0 million, depending on when the aggregate royalties received by IRH reach a specified milestone. Once the applicable threshold was met, the Company would thereafter have received 85% and IRH would have received 15% of the Kadcyla royalties for the remaining royalty term. At the consummation of the transaction, the Company received cash proceeds of $200 million. As part of this sale, the Company incurred $5.9 million of transaction costs, which are presented net of the liability in the accompanying consolidated balance sheet and are being amortized to interest expense over the estimated life of the royalty purchase agreement. Although the Company sold its rights to receive royalties from the sales of Kadcyla, as a result of its then ongoing involvement in the cash flows related to these royalties, the Company continues to account for these royalties as revenue and recorded the $200.0 million in proceeds from this transaction as a liability related to sale of future royalties (Royalty Obligation) that is being amortized using the interest method over the estimated life of the royalty purchase agreement. In January 2019, the Company sold its residual rights to receive royalty payments on commercial sales of Kadcyla to OMERS, the defined benefit pension plan for municipal employees in the Province of Ontario, Canada, for a payment of $65.2 million (amount is net of $1.5 million in broker fees). Simultaneously, OMERS purchased IRH’s right to the royalties the Company previously sold to IRH as described above, therefore obtaining the rights to 100% of the royalties received from that date on. Because the Company will not be involved with the cash flows related to the residual royalties, the $65.2 million of net proceeds received from the sale of its residual rights to receive royalty payments was recorded as deferred revenue and will be amortized as the royalty revenue related to the residual rights is earned using the units of revenue approach. During the second quarter of 2021, the aggregate royalty threshold was met and, in accordance with the Company’s revenue recognition policy, $3.7 million of revenue related to the residual rights was recorded and is included in non-cash royalty revenue for the nine months ended September 30, 2021. Additionally, the purchase of IRH’s interest by OMERS did not result in an extinguishment or modification of the original instrument and, accordingly, the Company continues to account for the remaining obligation as a liability as outlined above. The following table shows the activity within the liability account during the nine-month period ended September 30, 2021 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended ​ ​ September 30, 2021 ​ Liability related to sale of future royalties, net — beginning balance ​ $ 85,439 ​ Proceeds from sale of future royalties, net ​ — ​ Kadcyla royalty payments received and paid ​ (54,117) ​ Non-cash interest expense recognized ​ ​ 10,945 ​ Liability related to sale of future royalties, net — ending balance ​ $ 42,267 ​ ​ The Company receives royalty reports and royalty payments related to sales of Kadcyla from Roche one 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 are paid in U.S. dollars (USD) while significant portions of the underlying sales of Kadcyla are made in currencies other than USD, and other events or circumstances that could result in reduced royalty payments from Kadcyla, all of which would result in a reduction of non-cash royalty revenues and the non-cash interest expense over the life of the Royalty Obligation. Conversely, if sales of Kadcyla are more than expected, the non-cash royalty revenues and the non-cash interest expense recorded by the Company would be greater over the term of the Royalty Obligation. ​

Capital Stock

Capital Stock	9 Months Ended
	Sep. 30, 2021
Capital Stock
Capital Stock	F. Capital Stock Pre-Funded Warrant On August 11, 2021, the Company entered into a Securities Purchase Agreement (SPA) with RA Capital Healthcare Fund, L.P. (the Investor), pursuant to which the Company agreed to sell to the Investor a pre-funded warrant (the Pre-Funded Warrant) to purchase up to an aggregate of 5,434,782 shares of the Company’s common stock, par value $0.01 per share (common stock), for $5.51 per share of common stock underlying the Pre-Funded Warrant, which, together with the per share exercise price, is equal to $5.52. The private placement resulted in aggregate gross proceeds of $29.9 million, before $0.2 million of transaction costs. The issuance and sale of the Pre-Funded Warrant under the SPA and the shares of common stock issuable upon exercise of the Pre-Funded Warrant were registered pursuant to the Company’s shelf registration statement on Form S-3 (File No. 333-251502). The Pre-Funded Warrant’s fundamental transaction provision does not provide the warrant holders with the option to settle any unexercised warrants for cash in the event of any fundamental transactions; rather, in all fundamental transaction scenarios, the warrant holder will only be entitled to receive from the Company or any successor entity the same type or form of consideration (and in the same proportion) that is being offered and paid to the stockholders of the Company in connection with the fundamental transaction, whether that consideration be in the form of cash, stock or any combination thereof. The Pre-Funded Warrant also includes a separate provision whereby the exercisability of the warrants may be limited if, upon exercise, the warrant holder or any of its affiliates would beneficially own more than 9.99% of the Company’s common stock. This threshold is subject to the Investor’s rights under the Pre-Funded Warrant to increase or decrease such percentage to any other percentage not in excess of 19.99% upon at least 61 days’ prior notice from the Investor to the Company ​ The Company has assessed the Pre-Funded Warrant for appropriate equity or liability classification pursuant to the Company’s accounting policy described in Note B, “Summary of Significant Accounting Policies.” During this assessment, the Company determined the Pre-Funded Warrant is a freestanding instrument that does not meet the definition of a liability pursuant to ASC 480 and does not meet the definition of a derivative pursuant to ASC 815. The Pre-Funded Warrant is indexed to the Company’s common stock and meets all other conditions for equity classification under ASC 480 and ASC 815. Based on the results of this assessment, the Company concluded that the Pre-Funded Warrant is a freestanding equity-linked financial instrument that meets the criteria for equity classification under ASC 480 and ASC 815. Accordingly, the Pre-Funded Warrant is classified as equity and is accounted for as a component of additional paid-in capital at the time of issuance. The Company also determined that the Pre-Funded Warrant should be included in the determination of basic and diluted earnings per share in accordance with ASC 260, Earnings per Share ​ Compensation Policy for Non-Employee Directors Pursuant to the Compensation Policy for Non-Employee Directors, as amended, non-employee directors are granted deferred share units upon initial election to the Board of Directors and annually thereafter. Initial awards and annual retainers vest quarterly over approximately three years and one year from the date of grant, respectively, contingent upon the individual remaining a director of ImmunoGen as of each vesting date. The number of deferred share units awarded is fixed per the policy on the date of the award. All unvested deferred share units will automatically vest immediately prior to the occurrence of a change of control. The redemption amount of deferred share units issued will be paid in shares of common stock of the Company on the date a director ceases to be a member of the Board of Directors. Pursuant to the Compensation Policy for Non-Employee Directors, as amended, non-employee directors also receive stock option awards upon initial election to the Board of Directors and annually thereafter. The directors received a total of 308,000 and 300,000 options during the nine months ended September 2021 and 2020, respectively, and the related compensation expense for the three and nine months ended September 30, 2021 and 2020 is included in the amounts discussed in the “Stock-Based Compensation” section of Note B above.

Restructuring Charges

Restructuring Charges	9 Months Ended
	Sep. 30, 2021
Restructuring Charges
Restructuring Charges	G. Restructuring Charges During the nine months ended September 30, 2020, the Company recorded a $(0.1) million adjustment to severance charges and $1.6 million in incremental benefits related to the 2019 corporate restructuring. A summary of payments made against the corporate restructuring charge related to the employee terminations in 2021 is as follows: ​ ​ ​ ​ ​ ​ Employee ​ ​ Termination ​ Benefits Costs Balance at December 31, 2020 ​ $ 784 Payments during the period ​ ​ (278) Balance at September 30, 2021 ​ $ 506 ​ In addition to the termination benefits and other related charges, the Company has subleased laboratory and office space at 830 Winter Street in Waltham, Massachusetts no longer used in the business. The decision to vacate part of its corporate office resulted in a change in asset groupings and also represented an impairment indicator. The Company determined and continues to believe that the right-of-use asset and leasehold improvements are recoverable based on expected sublease income, and therefore, no impairment has been recorded.

Leases

Leases	9 Months Ended
	Sep. 30, 2021
Leases
Leases	H. Leases The Company currently has a The Company’s operating lease liabilities related to its real estate lease agreements were calculated using a collateralized incremental borrowing rate. The weighted average discount rate for the operating lease liability is approximately 11%. A 100 basis point change in the incremental borrowing rate would result in less than a $1 million impact to the ROU assets and liabilities recorded. Lease expense for operating lease payments is recognized on a straight-line basis over the lease term, which was $3.0 million in each of the nine-month periods ended September 30, 2021 and 2020 and is included in operating expenses in the consolidated statement of operations. Cash paid against operating lease liabilities was $4.1 million in each of the nine-month periods ended September 30, 2021 and 2020. As of September 30, 2021, the Company’s ROU asset and lease liability for operating leases totaled $12.7 million and $19.4 million, respectively, and the weighted-average remaining term of the operating leases is 4.5 years ​ The maturities of operating lease liabilities discussed above are as follows (in thousands): ​ ​ ​ ​ 2021 (three months remaining) $ 1,265 2022 ​ 5,389 2023 ​ 5,510 2024 ​ 5,470 2025 ​ 5,490 Thereafter ​ 1,376 Total lease payments ​ ​ 24,500 Less imputed interest ​ ​ (5,097) Total lease liabilities ​ $ 19,403 ​ In addition to the amounts in the table above, the Company is also responsible for variable operating expenses and real estate taxes that are expected to approximate $3.1 million per year through March 2026. Sublease Income In 2020, the Company executed four agreements to sublease a total of approximately 65,000 square feet of the Company’s leased space at 830 Winter Street, Waltham, Massachusetts through March 2026. During the nine months ended September 30, 2021 and 2020, the Company recorded $3.7 million and $1.6 million of sublease income, respectively, inclusive of the sublessees’ proportionate share of operating expenses and real estate taxes for the period. Two of the four sublease agreements include an early termination option after certain periods of time for an agreed-upon fee. Assuming no early termination option is exercised, the Company is entitled to receive $14.0 million in minimum rental payments over the remaining term of the subleases, which is not included in the operating lease liability table above. The sublessees are also responsible for their proportionate share of variable operating expenses and real estate taxes.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies
Commitments and Contingencies	I. Commitments and Contingencies Manufacturing Commitments As of September 30, 2021, the Company had noncancelable obligations under several agreements related to in-process and future manufacturing of antibody, drug substance, and cytotoxic agents required for supply of the Company’s product candidates totaling $3.1 million. Additionally, pursuant to commercial agreements for future production of antibody, our noncancelable commitments total $33.4 million at September 30, 2021. Litigation ​ The Company is not a party to any material litigation.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2021
Subsequent Events
Subsequent Events	J. Subsequent Events Debiopharm In May 2017, Debiopharm International S.A. (Debiopharm) acquired the Company’s IMGN529 program, a clinical-stage anti-CD37 ADC for the treatment of patients with B-cell malignancies, such as non-Hodgkin lymphomas (NHL). Under the terms of the Exclusive License and Asset Purchase agreement, the Company received a $25.0 million upfront payment for specified assets related to IMGN529 and an exclusive license to additional intellectual property necessary or useful for Debiopharm to develop and commercialize IMGN529. The Company also received a $5.0 million milestone payment upon transfer of ImmunoGen technologies related to the program. In October 2021, the Company and Debiopharm amended their agreement, pursuant to which the Company is entitled to receive a percentage of all payments generated from future sublicenses of naratuximab emtansine, including upfront fees, milestones, and royalties, up to an aggregate of $30.0 million and in lieu of a potential $25.0 million development milestone payment under the original agreement. ​ ​ License Commitment In October 2021, as a result of a dispute regarding terms of a 2012 license agreement with a contract manufacturing vendor, the Company and vendor amended their agreement to replace certain annual fees and potential royalties payable by the Company on future sales of mirvetuximab with capped development and sales-based milestone payments totaling $18.0 million, of which $3.0 million was recorded as research and development expense during the nine months ended September 30, 2021. ​ The Company has evaluated all other events or transactions that occurred after September 30, 2021, up through the date the Company issued these financial statements. The Company did not have any other material recognized or unrecognized subsequent events during this period.

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
Basis of Presentation and Significant Accounting Policies
Basis of Presentation	Basis of Presentation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany transactions and balances have been eliminated. The consolidated financial statements include all of the adjustments, consisting only of normal recurring adjustments, which management considers necessary for a fair presentation of the Company’s financial position in accordance with accounting principles generally accepted in the U.S. for interim financial information. The December 31, 2020 consolidated balance sheet presented for comparative purposes was derived from the Company’s audited financial statements, and certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. The preparation of interim financial statements requires the use of management’s estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the interim financial statements and the reported amounts of revenues and expenditures during the reported periods. The results of the interim periods are not necessarily indicative of the results for the entire year. Accordingly, the interim financial statements should be read in conjunction with the audited financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020 filed with the SEC on March 1, 2021.
Significant Accounting Policies	Significant Accounting Policies Common Stock Warrants ​ The Company accounts for common stock warrants as either equity-classified or liability-classified instruments based on an assessment of the warrant’s specific terms and applicable authoritative guidance in Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) 480, Distinguishing Liabilities from Equity Derivatives and Hedging 815, including whether the warrants are indexed to the Company’s own common stock and whether the warrant holders could potentially require “net cash settlement” in a circumstance outside of the Company’s control, among other conditions for equity classification. This assessment, which requires the use of professional judgment, is conducted at the time of warrant issuance and as of each subsequent quarterly period end date while the warrants are outstanding. ​ For issued or modified warrants that meet all of the criteria for equity classification, the warrants are required to be recorded as a component of additional paid-in capital at the time of issuance. For issued or modified warrants that do not meet all the criteria for equity classification, the warrants are required to be recorded at their initial fair value on the date of issuance and remeasured each balance sheet date thereafter. Changes in the estimated fair value of the liability-classified warrants are recognized as a non-cash gain or loss in the accompanying consolidated statements of operations and comprehensive loss. ​ Other than the accounting for warrants noted above, the significant accounting policies used in preparation of these condensed consolidated financial statements for the three and nine months ended September 30, 2021 are consistent with those discussed in Note B to the consolidated financial statements included in the Company’s 2020 Annual Report on Form 10-K, except as described under Recently Adopted Accounting Pronouncements
Revenue Recognition	Revenue Recognition Transaction Price Allocated to Future Performance Obligations Deferred revenue under ASC 606, Revenue from Contracts with Customers 13 61 Contract Balances from Contracts with Customers The following tables present changes in the Company’s contract assets and contract liabilities during the nine months ended September 30, 2021 and 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Nine months ended September 30, 2021 ​ December 31, 2020 Additions ​ Deductions ​ Impact of Netting ​ September 30, 2021 Contract asset ​ $ — ​ $ 2,500 ​ $ — ​ $ — ​ $ 2,500 Contract liabilities (deferred revenue) ​ $ 110,109 ​ $ 25 ​ $ (4,129) ​ $ — ​ $ 106,005 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Nine months ended September 30, 2020 ​ December 31, 2019 ​ Additions ​ Deductions ​ Impact of Netting ​ September 30, 2020 Contract asset ​ $ 3,631 ​ $ — ​ $ (8,000) ​ $ 4,369 ​ $ — Contract liabilities (deferred revenue) ​ $ 127,432 ​ $ 50 ​ $ (1,325) ​ $ 4,369 ​ $ 130,526 ​ The Company recognized the following revenues as a result of changes in contract asset and contract liability balances in the respective periods (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ ​ September 30, ​ September 30, ​ ​ ​ 2021 ​ 2020 ​ 2021 ​ 2020 ​ ​ Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 3,292 ​ $ 72 ​ $ 4,129 ​ $ 1,300 ​ ​ ​ During the nine months ended September 30, 2021, the Company recorded a contract asset of $2.5 million for a probable development milestone pursuant to its license agreement with Viridian Therapeutics, Inc. (Viridian), which was subsequently achieved in October 2021. The Company also recorded $0.2 million as license and milestone fee revenue for delivery of certain materials to Viridian that had been previously deferred, and $0.2 million of license and milestone fee revenue related to numerous collaborators’ rights to technological improvements that had been previously deferred. Additionally, the Company recorded $3.7 million of previously deferred non-cash royalty revenue related to the sale of rights to Kadcyla royalties, further details of which can be found in Note E, “Liability Related to Sale of Future Royalties.” During the nine months ended September 30, 2020, the Company recorded $0.2 million as license and milestone fee revenue for delivery of certain materials to CytomX that had been previously deferred, and $1.1 million of license and milestone fee revenue related to numerous collaborators’ rights to technological improvements that had been previously deferred, which included $0.9 million related to the termination of a license agreement with Takeda. Additionally, a contract asset of $3.6 million, net of $4.4 million of related contract liabilities, was recorded for two probable milestones in 2019 pursuant to license agreements with CytomX and Novartis, which were subsequently achieved and paid during the nine months ended September 30, 2020. The timing of revenue recognition, billings, and cash collections results in billed receivables, unbilled receivables, contract assets, and contract liabilities on the consolidated balance sheets. When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded (under the caption deferred revenue). Contract liabilities are recognized as revenue after control of the products or services is transferred to the customer and all revenue recognition criteria have been met.
Financial Instruments and Concentration of Credit Risk	Financial Instruments and Concentration of Credit Risk Cash and cash equivalents are primarily maintained with three financial institutions in the U.S. Deposits with banks may exceed the amount of insurance provided on such deposits. Generally, these deposits may be redeemed upon demand and, therefore, bear minimal risk. The Company’s cash equivalents consist of money market funds with underlying investments primarily being U.S. Government-issued securities and high quality, short-term commercial paper. Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash, cash equivalents, and marketable securities. The Company held no marketable securities as of September 30, 2021 and December 31, 2020. The Company’s investment policy, approved by the Board of Directors, limits the amount it may invest in any one type of investment, thereby reducing credit risk concentrations.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid financial instruments with maturities of three months or less when purchased to be cash equivalents. As of September 30, 2021 and December 31, 2020, the Company held $245.8 million and $293.9 million, respectively, in cash and money market funds, which were classified as cash and cash equivalents.
Non-cash Investing and Financing Activities	Non-cash Investing and Financing Activities During the nine months ended September 30, 2021, $1.0 million of outstanding convertible 4.5% senior notes were converted into 238,777 shares of the Company’s common stock. There was no similar activity during the nine months ended September 30, 2020. The Company had $0.1 million and $0.7 million of accrued capital expenditures as of September 30, 2021 or December 31, 2020, respectively, which have been treated as a non-cash investing activity and, accordingly, are not reflected in the consolidated statement of cash flows.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Fair value is defined under ASC 820, Fair Value Measurements and Disclosures ● Level 1 - Quoted prices in active markets for identical assets or liabilities. ● Level 2 - Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. ● Level 3 – Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. As of September 30, 2021 and December 31, 2020, the Company held certain assets that are required to be measured at fair value on a recurring basis. The fair value of the Company’s cash equivalents is based on quoted prices from active markets (Level 1 inputs). The carrying amounts reflected in the consolidated balance sheets for accounts receivable, unbilled receivables, prepaid and other current assets, accounts payable, accrued compensation, and other accrued liabilities approximate fair value due to their short-term nature. The gross carrying amount and estimated fair value of the convertible 4.5% senior notes was $2.1 million and $4.3 million, respectively, as of December 31, 2020. The fair value of the convertible notes was influenced by interest rates, the Company’s stock price, stock price volatility, and by prices observed in trading activity for the convertible notes. However, because there were no trades involving the convertible notes since September 2019, the fair value as of December 31, 2020 used Level 3 inputs. During the nine months ended September 30, 2021,
Computation of Net Loss per Common Share	Computation of Net Loss per Common Share Basic and diluted net loss per share is calculated based upon the weighted-average number of shares of common stock outstanding during the period. Shares of the Company’s common stock underlying pre-funded warrants are included in the calculation of basic and diluted earnings per share. During periods of income, participating securities are allocated a proportional share of income determined by dividing total weighted-average participating securities by the sum of the total weighted-average shares of common stock and participating securities (the two-class method). Shares of the Company’s restricted stock participate in any dividends that may be declared by the Company and are therefore considered to be participating securities. Participating securities have the effect of diluting both basic and diluted earnings per share during periods of income. During periods of loss, no loss is allocated to participating securities since they have no contractual obligation to share in the losses of the Company. Diluted loss per share is computed after giving consideration to the dilutive effect of stock options, convertible notes, and restricted stock that are outstanding during the period, except where such non-participating securities would be anti-dilutive. The Company’s common stock equivalents, as calculated in accordance with the treasury-stock method for options and unvested restricted stock and the if-converted method for the convertible notes, are shown in the following table (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ September 30, ​ September 30, ​ 2021 2020 2021 2020 Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock/units at end of period ​ 20,862 ​ 19,181 ​ 20,862 ​ 19,181 Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock/units ​ 2,116 929 ​ 2,743 ​ 1,112 Common stock equivalents under if-converted method for convertible notes ​ - ​ 501 ​ - ​ 501 ​ The Company’s common stock equivalents have not been included in the net loss per share calculation because their effect is anti-dilutive due to the Company’s net loss position.
Stock-Based Compensation	Stock-Based Compensation As of June 30, 2021, the Company was authorized to grant future awards under three employee share-based compensation plans, which are the ImmunoGen, Inc. Amended and Restated 2018 Employee, Director and Consultant Equity Incentive Plan (the 2018 Plan), the Employee Stock Purchase Plan (the ESPP), and the ImmunoGen Inducement Equity Incentive Plan (the Inducement Plan). At the annual meeting of shareholders on June 16, 2021, the 2018 Plan was amended to provide for the issuance of stock grants, the grant of options, and the grant of stock-based awards for up to an additional 6,600,000 shares of the Company’s common stock, as well as up to 22,392,986 shares of common stock, which represent the number of shares of common stock remaining under the 2018 Plan as of March 31, 2021, and awards previously granted under the 2018 Plan and the Company’s former stock-based plans, including the ImmunoGen, Inc. 2016 and 2006 Employee, Director and Consultant Equity Incentive Plans, that forfeit, expire, or cancel without delivery of shares of common stock or which resulted in the forfeiture of shares of common stock back to the Company subsequent to March 31, 2021. The Inducement Plan was approved by the Board of Directors in December 2019, and pursuant to subsequent amendments, provides for the issuance of non-qualified option grants for up to 3,500,000 shares of the Company’s common stock. Options awarded under the two plans are granted with an exercise price equal to the market price of the Company’s stock at the date of grant. Options vest at various periods of up to four years and may be exercised within ten years of the date of grant under each of these plans. The stock-based awards are accounted for under ASC 718, Compensation—Stock Compensation ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ 2021 ​ 2020 ​ 2021 ​ 2020 Dividend ​ None ​ None ​ None ​ None Volatility ​ 82.7% ​ 88.3% ​ 85.2% ​ 85.0% Risk-free interest rate ​ 0.95% ​ 0.36% ​ 0.68% ​ 1.23% Expected life (years) ​ 6.0 ​ 6.0 ​ 6.0 ​ 6.0 ​ Using the Black-Scholes option-pricing model, the weighted-average grant date fair values of options granted during the three months ended September 30, 2021 and 2020 were $3.99 and $4.56 per share, respectively, and $5.34 and $4.57 for options granted during the nine months ended September 30, 2021 and 2020, respectively. A summary of option activity under the Company’s equity plans for the nine months ended September 30, 2021 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ Number ​ Average ​ ​ ​ of Stock ​ Exercise ​ ​ ​ Options ​ Price ​ Outstanding at December 31, 2020 ​ 18,398 ​ $ 6.10 ​ Granted ​ 4,380 ​ ​ 7.53 ​ Exercised ​ (503) ​ ​ 3.34 ​ Forfeited/Canceled ​ (1,415) ​ ​ 8.06 ​ Outstanding at September 30, 2021 ​ 20,860 ​ ​ 6.34 ​ ​ In 2020, the Company issued 2.6 million performance-based stock options to certain employees, all of which remain outstanding as of September 30, 2021, that will vest upon the achievement of specified performance goals. As of September 30, 2021, the Company determined it was not probable that any of these performance goals will be achieved and, therefore, no expense had been recorded to date. In October 2021, upon approval by the Compensation Committee of the Company’s Board of Directors, certain terms of the performance-based stock option award agreements were modified. Pursuant to ASC 718, the Company determined the modification to be a Type IV (improbable-to-improbable) modification, revalued the modified awards as of the modification date, and determined the modified performance goals were not probable of being achieved. The modified fair value of the performance-based stock options that could be expensed in future periods is $10.5 million. A summary of restricted stock and restricted stock unit activity under the Company’s equity plans for the nine months ended September 30, 2021 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted- ​ ​ Restricted ​ Average Grant ​ ​ Stock Shares ​ Date Fair Value Unvested at December 31, 2020 ​ 61 ​ $ 2.47 Vested (2) ​ ​ 2.53 Forfeited ​ (57) ​ ​ 2.47 Unvested at September 30, 2021 ​ 2 ​ $ 2.53 ​ In June 2018, the Company's Board of Directors, with shareholder approval, adopted the ESPP. Following the automatic share increase on January 1, 2021, pursuant to the ESPP’s “evergreen” provision, an aggregate of 2,000,000 shares of common stock have been reserved for issuance under the ESPP. On June 30, 2021 and 2020, approximately 64,000, and 78,000 shares, respectively, were issued to participating employees at a fair value of $2.14 and $1.86 per share, respectively. The fair value of each ESPP award is estimated on the first day of the offering period using the Black-Scholes option-pricing model. The Company recognizes share-based compensation expense equal to the fair value of the ESPP awards on a straight-line basis over the offering period. Stock compensation expense related to stock options and restricted stock awards granted under the stock plans and the ESPP was $3.3 million and $10.6 million during the three and nine months ended September 30, 2021, respectively, compared to stock compensation expense of $3.7 million and $10.3 million for the three and nine months ended September 30, 2020, respectively. As of September 30, 2021, the estimated fair value of unvested employee awards, exclusive of performance awards, was $28.3 million. The weighted-average remaining vesting period for these awards is approximately three years .
Segment Information	Segment Information During all periods presented, the Company continued to operate in one reportable business segment under the management approach of ASC 280, Segment Reporting During the three and nine months ended September 30, 2021, 71% and 93%, respectively, of revenues were from Roche, consisting primarily of non-cash royalty revenue, compared to 99% and 97% of revenue from Roche in the three and nine months ended September 30, 2020, respectively. During the three months ended September 30, 2021, 28% of revenues were from Viridian. There were no other customers of the Company that generated significant revenues in the three or nine months ended September 30, 2021 and 2020.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In December 2019, the Financial Accounting Standards Board issued Accounting Standards Update (ASU) 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes No other recently issued or effective ASUs had, or are expected to have, a material effect on the Company's results of operations, financial condition, or liquidity.

Basis of Presentation and Sig_3

Basis of Presentation and Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2021
Basis of Presentation and Significant Accounting Policies
Contract assets and contract liabilities	The following tables present changes in the Company’s contract assets and contract liabilities during the nine months ended September 30, 2021 and 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Nine months ended September 30, 2021 ​ December 31, 2020 Additions ​ Deductions ​ Impact of Netting ​ September 30, 2021 Contract asset ​ $ — ​ $ 2,500 ​ $ — ​ $ — ​ $ 2,500 Contract liabilities (deferred revenue) ​ $ 110,109 ​ $ 25 ​ $ (4,129) ​ $ — ​ $ 106,005 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Nine months ended September 30, 2020 ​ December 31, 2019 ​ Additions ​ Deductions ​ Impact of Netting ​ September 30, 2020 Contract asset ​ $ 3,631 ​ $ — ​ $ (8,000) ​ $ 4,369 ​ $ — Contract liabilities (deferred revenue) ​ $ 127,432 ​ $ 50 ​ $ (1,325) ​ $ 4,369 ​ $ 130,526 ​ The Company recognized the following revenues as a result of changes in contract asset and contract liability balances in the respective periods (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ ​ September 30, ​ September 30, ​ ​ ​ 2021 ​ 2020 ​ 2021 ​ 2020 ​ ​ Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 3,292 ​ $ 72 ​ $ 4,129 ​ $ 1,300 ​ ​
Schedule of common stock equivalents, as calculated in accordance with the treasury-stock method	The Company’s common stock equivalents, as calculated in accordance with the treasury-stock method for options and unvested restricted stock and the if-converted method for the convertible notes, are shown in the following table (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ September 30, ​ September 30, ​ 2021 2020 2021 2020 Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock/units at end of period ​ 20,862 ​ 19,181 ​ 20,862 ​ 19,181 Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock/units ​ 2,116 929 ​ 2,743 ​ 1,112 Common stock equivalents under if-converted method for convertible notes ​ - ​ 501 ​ - ​ 501
Schedule of risk-free rate of the stock options based on US Treasury rate	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ 2021 ​ 2020 ​ 2021 ​ 2020 Dividend ​ None ​ None ​ None ​ None Volatility ​ 82.7% ​ 88.3% ​ 85.2% ​ 85.0% Risk-free interest rate ​ 0.95% ​ 0.36% ​ 0.68% ​ 1.23% Expected life (years) ​ 6.0 ​ 6.0 ​ 6.0 ​ 6.0
Summary of stock option activity	A summary of option activity under the Company’s equity plans for the nine months ended September 30, 2021 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ Number ​ Average ​ ​ ​ of Stock ​ Exercise ​ ​ ​ Options ​ Price ​ Outstanding at December 31, 2020 ​ 18,398 ​ $ 6.10 ​ Granted ​ 4,380 ​ ​ 7.53 ​ Exercised ​ (503) ​ ​ 3.34 ​ Forfeited/Canceled ​ (1,415) ​ ​ 8.06 ​ Outstanding at September 30, 2021 ​ 20,860 ​ ​ 6.34 ​
Summary of restricted stock activity	A summary of restricted stock and restricted stock unit activity under the Company’s equity plans for the nine months ended September 30, 2021 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted- ​ ​ Restricted ​ Average Grant ​ ​ Stock Shares ​ Date Fair Value Unvested at December 31, 2020 ​ 61 ​ $ 2.47 Vested (2) ​ ​ 2.53 Forfeited ​ (57) ​ ​ 2.47 Unvested at September 30, 2021 ​ 2 ​ $ 2.53

Liability Related to Sale of _2

Liability Related to Sale of Future Royalties (Tables)	9 Months Ended
	Sep. 30, 2021
Liability Related to Sale of Future Royalties
Schedule of Liability account during the period from the inception of the royalty transaction	The following table shows the activity within the liability account during the nine-month period ended September 30, 2021 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended ​ ​ September 30, 2021 ​ Liability related to sale of future royalties, net — beginning balance ​ $ 85,439 ​ Proceeds from sale of future royalties, net ​ — ​ Kadcyla royalty payments received and paid ​ (54,117) ​ Non-cash interest expense recognized ​ ​ 10,945 ​ Liability related to sale of future royalties, net — ending balance ​ $ 42,267 ​

Restructuring Charges (Tables)

Restructuring Charges (Tables)	9 Months Ended
	Sep. 30, 2021
Restructuring Charges
Summary of payments made against the restructuring charge related to the employee terminations	A summary of payments made against the corporate restructuring charge related to the employee terminations in 2021 is as follows: ​ ​ ​ ​ ​ ​ Employee ​ ​ Termination ​ Benefits Costs Balance at December 31, 2020 ​ $ 784 Payments during the period ​ ​ (278) Balance at September 30, 2021 ​ $ 506

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2021
Leases
Schedule of maturities of operating lease liabilities	The maturities of operating lease liabilities discussed above are as follows (in thousands): ​ ​ ​ ​ 2021 (three months remaining) $ 1,265 2022 ​ 5,389 2023 ​ 5,510 2024 ​ 5,470 2025 ​ 5,490 Thereafter ​ 1,376 Total lease payments ​ ​ 24,500 Less imputed interest ​ ​ (5,097) Total lease liabilities ​ $ 19,403

Nature of Business and Plan o_2

Nature of Business and Plan of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Net loss			$ (102,131)	$ (75,760)
Accumulated deficit	$ (1,433,971)		(1,433,971)		$ (1,331,840)
Total revenues	9,210	$ 18,189	41,864	46,501
Cash and cash equivalents	$ 245,761	$ 188,217	$ 245,761	$ 188,217	$ 293,856	$ 176,225
Number of months Capital resources meets capital expenditures			12 months
Product
Total revenues			$ 0

Basis of Presentation and Sig_4

Basis of Presentation and Significant Accounting Policies - Performance Obligations (Details) $ in Millions	Sep. 30, 2021USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 106
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	0.50%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	0.38%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2031-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	0.12%
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	12 months
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	13 months
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2031-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	61 months
Maximum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	60 months
Maximum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2031-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	120 months

Basis of Presentation and Sig_5

Basis of Presentation and Significant Accounting Policies - Contract Balances (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Changes in the Company's contract assets and contract liabilities
Contract asset, Beginning balance		$ 3,631
Contract asset, Additions	$ 2,500	0
Contract asset, Deductions		8,000
Contract Asset, Impact Of Netting.		4,369
Contract asset, Ending balance	2,500
Contract liabilities:
Contract liabilities (deferred revenue), Beginning balance	110,109	127,432
Contract liabilities (deferred revenue), Additions	25	50
Contract liabilities (deferred revenue), Deductions	(4,129)	(1,325)
Contract Liabilities (deferred revenue), Impact Of Netting		4,369
Contract liabilities (deferred revenue), Ending balance	$ 106,005	$ 130,526

Basis of Presentation and Sig_6

Basis of Presentation and Significant Accounting Policies - Revenues Recognized as a Result of Changes in Contract Asset and Liability Balances (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenue recognized in the period from:
Amounts included in contract liabilities at the beginning of the period	$ 3,292	$ 72	$ 4,129	$ 1,300

Basis of Presentation and Sig_7

Basis of Presentation and Significant Accounting Policies - Contract Balances from Contracts with Customers - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Revenue recognized in the period from:
Revenue recognized, previously deferred	$ 3,292	$ 72	$ 4,129	$ 1,300
Revenue from contract with customer	9,210	18,189	41,864	46,501
Contract assets	2,500		2,500			$ 3,631
Contract liability	53,526		53,526		$ 29,249
License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer	2,677	97	3,086	1,325
Kadcyla
Revenue recognized in the period from:
Net proceeds from sale of residual rights to receive royalty payments			3,700
CytomX and Novartis | License and milestone fees
Revenue recognized in the period from:
Revenue recognized, previously deferred				200
CytomX and Novartis | Technological Improvements
Revenue recognized in the period from:
Revenue recognized, previously deferred				1,100
CytomX and Novartis | Probable Milestone
Revenue recognized in the period from:
Contract assets		3,600		3,600
Contract liability		4,400		4,400
Takeda | Technological Improvements
Revenue recognized in the period from:
Deferred Revenue related to termination of license agreement		$ 900		$ 900
Viridian | License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer	2,500		200
Viridian | Technological Improvements
Revenue recognized in the period from:
Revenue recognized, previously deferred			200
Viridian | Probable Milestone
Revenue recognized in the period from:
Contract assets	$ 2,500		$ 2,500

Basis of Presentation and Sig_8

Basis of Presentation and Significant Accounting Policies - Financial Instruments and Concentration of Credit Risk (Details)	9 Months Ended
	Sep. 30, 2021USD ($)item	Dec. 31, 2020USD ($)
Financial Instruments and Concentration of Credit Risk
Number of financial institutions in the U.S. in which cash and cash equivalents are primarily maintained | item	3
Marketable securities held by entity | $	$ 0	$ 0

Basis of Presentation and Sig_9

Basis of Presentation and Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Basis of Presentation and Significant Accounting Policies
Cash and cash equivalents	$ 245,761	$ 293,856	$ 188,217	$ 176,225

Basis of Presentation and Si_10

Basis of Presentation and Significant Accounting Policies - Non-cash Investing and Financing Activities (Details) - USD ($)	1 Months Ended	9 Months Ended		12 Months Ended
	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Jan. 01, 2017
Debt Instrument [Line Items]
Interest rate (as a percent)		4.50%		4.50%
Accrued capital expenditures		$ 100,000		$ 700,000
Convertible 4.5% Senior Notes
Debt Instrument [Line Items]
Debt converted	$ 1,000,000	$ 1,000,000	$ 0
Interest rate (as a percent)		4.50%		4.50%	4.50%
Shares issued with debt conversion (in shares)	238,777	238,777

Basis of Presentation and Si_11

Basis of Presentation and Significant Accounting Policies - Fair Value of Financial Instruments (Details) $ in Thousands	Jul. 01, 2021USD ($)	Jun. 30, 2021USD ($)	Jun. 30, 2021USD ($)	Sep. 30, 2021USD ($)shares	Dec. 31, 2020USD ($)item	Jan. 01, 2017
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)				4.50%	4.50%
Convertible debt amount			$ 1,000
Convertible debt paid in cash	$ 1,100			$ 1,100
Number of trades | item					0
Convertible 4.5% Senior Notes
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)				4.50%	4.50%	4.50%
Gross carrying amount					$ 2,100
Convertible debt amount				$ 1,000
Convertible debt, number of common shares issued | shares				238,777
Convertible debt paid in cash		$ 1,100
Significant Unobservable Inputs (Level 3) | Convertible 4.5% Senior Notes
Fair value hierarchy for the Company's financial assets measured at fair value
Convertible debt fair value					$ 4,300

Basis of Presentation and Si_12

Basis of Presentation and Significant Accounting Policies - Computation of Net Loss per Common Share (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Computation of Net Loss per Common Share
Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock/units at end of period	20,862	19,181	20,862	19,181
Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock/units	2,116	929	2,743	1,112
Common stock equivalents under if-converted method for convertible notes (in shares)		501		501

Basis of Presentation and Si_13

Basis of Presentation and Significant Accounting Policies - Stock-Based Compensation (Details)	3 Months Ended		6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021USD ($)$ / sharesshares	Sep. 30, 2020USD ($)$ / shares	Jun. 30, 2021plan$ / sharesshares	Jun. 30, 2020$ / sharesshares	Sep. 30, 2021USD ($)plan$ / sharesshares	Sep. 30, 2020USD ($)$ / shares	Dec. 31, 2020$ / sharesshares	Jun. 16, 2021shares	Mar. 31, 2021shares	Jun. 30, 2018shares
Stock-Based Compensation
Number of employee share-based compensation plans | plan			3
Weighted-average assumptions used to estimate the fair value of each stock option
Dividend (as a percent)	0.00%	0.00%			0.00%	0.00%
ESPP
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Aggregate number of common shares reserved for future issuance										2,000,000
Shares issued to participating employees			64,000	78,000
Awarded (in dollars per share) | $ / shares			$ 2.14	$ 1.86
Stock options and restricted stock awards
Stock-Based Compensation
Vesting period					3 years
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Allocated Share-based Compensation Expense | $	$ 3,300,000	$ 3,700,000			$ 10,600,000	$ 10,300,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $	$ 28,300,000				$ 28,300,000
Stock options
Weighted-average assumptions used to estimate the fair value of each stock option
Dividend (as a percent)	0.00%	0.00%			0.00%	0.00%
Volatility (as a percent)	82.70%	88.30%			85.20%	85.00%
Risk-free interest rate (as a percent)	0.95%	0.36%			0.68%	1.23%
Expected life	6 years	6 years			6 years	6 years
Weighted-average grant date fair value (in dollars per share) | $ / shares	$ 3.99	$ 4.56			$ 5.34	$ 4.57
Number of Stock Options
Outstanding at the beginning of the period (in shares)			18,398,000		18,398,000
Granted (in shares)					4,380,000
Exercised (in shares)					(503,000)
Forfeited/Canceled (in shares)					(1,415,000)
Outstanding at the end of the period (in shares)	20,860,000				20,860,000		18,398,000
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares			$ 6.10		$ 6.10
Granted (in dollars per share) | $ / shares					7.53
Exercised (in dollars per share) | $ / shares					3.34
Forfeited/Canceled (in dollars per share) | $ / shares					8.06
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 6.34				$ 6.34		$ 6.10
Performance shares
Number of Stock Options
Granted (in shares)							2,600,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Allocated Share-based Compensation Expense | $					$ 0
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $	$ 10,500,000				$ 10,500,000
Restricted stock
Number of Restricted Stock Shares
Unvested at the beginning of the period (in shares)			61,000		61,000
Vested (in shares)					(2,000)
Forfeited (in shares)					(57,000)
Unvested at the end of the period (in shares)	2,000				2,000		61,000
Weighted-Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share) | $ / shares			$ 2.47		$ 2.47
Vested (in dollars per share) | $ / shares					2.53
Forfeited (in dollars per share) | $ / shares					2.47
Unvested at the end of the period (in dollars per share) | $ / shares	$ 2.53				$ 2.53		$ 2.47
2018 Plan
Stock-Based Compensation
Common stock authorized for issuance (in shares)								6,600,000	22,392,986
Inducement Plan
Stock-Based Compensation
Number of employee share-based compensation plans | plan					2
Common stock authorized for issuance (in shares)	3,500,000				3,500,000
2018 Plan and Inducement Plan
Stock-Based Compensation
Exercise period					10 years
2018 Plan and Inducement Plan | Maximum
Stock-Based Compensation
Vesting period					4 years

Basis of Presentation and Si_14

Basis of Presentation and Significant Accounting Policies - Segment Information (Details) - segment	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Information
Number of operating segments	1	1	1	1
Other customers | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized	0.00%	0.00%	0.00%	0.00%
Viridian | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized	28.00%
Royalty revenue | Roche | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized	71.00%	99.00%	93.00%	97.00%

Agreements - Roche (Details)

Agreements - Roche (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended
	Jan. 31, 2019	Sep. 30, 2021	Sep. 30, 2020
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties		$ 35,035	$ 45,159
Roche
Collaborative Agreements disclosures
Percentage of royalty payments	100.00%
Roche | Kadcyla
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties		$ 38,800	$ 45,200
OMERS | Kadcyla
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties	$ 65,200
Percentage of royalty payments	100.00%

Agreements - Viridian (Details)

Agreements - Viridian (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Collaborative Agreements disclosures
Revenue from contract with customer		$ 9,210	$ 18,189	$ 41,864	$ 46,501
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer		2,677	$ 97	3,086	$ 1,325
Viridian | Development milestones
Collaborative Agreements disclosures
Revenue from contract with customer	$ 48,000
Viridian | Milestone payments
Collaborative Agreements disclosures
Revenue from contract with customer	95,000
Viridian | License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer		$ 2,500		$ 200
Maximum | Viridian | Development and regulatory milestones
Collaborative Agreements disclosures
Potential milestone payment	$ 143,000

Convertible 4.5% Senior Notes (

Convertible 4.5% Senior Notes (Details) - USD ($)	Jul. 01, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Jan. 01, 2017	Dec. 31, 2016
Convertible debt
Convertible debt paid in cash	$ 1,100,000			$ 1,100,000
Interest rate (as a percent)				4.50%		4.50%
Interest expense			$ 24,000	$ 47,000	$ 71,000
Convertible 4.5% Senior Notes
Convertible debt
Principal amount of debt								$ 100,000,000
Convertible debt outstanding						$ 2,100,000
Shares issued with debt conversion (in shares)		238,777		238,777
Convertible debt paid in cash		$ 1,100,000
Debt converted		$ 1,000,000		$ 1,000,000	0
Interest rate (as a percent)				4.50%		4.50%	4.50%
Interest expense				$ 47,000	$ 71,000

Liability Related to Sale of _3

Liability Related to Sale of Future Royalties (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended	9 Months Ended		12 Months Ended
	Jan. 31, 2019	May 31, 2000	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2015
Liability Related to Sale of Future Royalties
Non-cash royalty revenue related to the sale of future royalties				$ 35,035	$ 45,159
Kadcyla
Liability Related to Sale of Future Royalties
Percentage of royalty payments if applicable threshold is met						85.00%
Net proceeds from sale of residual rights to receive royalty payments				3,700
IRH | Kadcyla
Liability Related to Sale of Future Royalties
Percentage of royalty payments						100.00%
Percentage of royalty payments if applicable threshold is met						15.00%
Proceeds from sale of future royalties, net				200,000		$ 200,000
Transaction costs for royalty agreements						5,900
Change in liability related to sale of future royalties
Liability related to sale of future royalties, net - beginning balance				85,439
Proceeds from sale of future royalties, net				200,000		200,000
Royalty payments received and paid				(54,117)
Non-cash interest expense recognized				10,945
Liability related to sale of future royalties, net - ending balance				$ 42,267
Effective annual interest rate				10.50%
Current effective interest rate				19.1
IRH | Kadcyla | Maximum
Liability Related to Sale of Future Royalties
Royalties threshold						260,000
IRH | Kadcyla | Minimum
Liability Related to Sale of Future Royalties
Royalties threshold						$ 235,000
OMERS | Kadcyla
Liability Related to Sale of Future Royalties
Percentage of royalty payments	100.00%
Non-cash royalty revenue related to the sale of future royalties	$ 65,200
Contingent broker fees	1,500
Net proceeds from sale of residual rights to receive royalty payments	$ 65,200		$ 3,700
Roche
Liability Related to Sale of Future Royalties
Percentage of royalty payments	100.00%
Period in arrears to receive royalty reports and payments related to sales of Kadcyla		3 months
Roche | Kadcyla
Liability Related to Sale of Future Royalties
Non-cash royalty revenue related to the sale of future royalties				$ 38,800	$ 45,200

Capital Stock (Details)

Capital Stock (Details) $ / shares in Units, $ in Millions	Aug. 11, 2021USD ($)$ / sharesshares	Dec. 09, 2016	Sep. 30, 2021$ / sharesshares	Sep. 30, 2020shares	Dec. 31, 2020$ / shares
Stock-based compensation disclosure
Common stock, par value (in dollars per share)			$ 0.01		$ 0.01
Pre-Funded Warrants
Stock-based compensation disclosure
Threshold percentage of common stock owned that limits the number of warrants exercised	9.99
Maximum percentage upon at least 61 days prior notice from the investor to the Company	19.99
Securities Purchase Agreement | RA Capital Healthcare Fund, L.P. | Pre-Funded Warrants
Stock-based compensation disclosure
Pre-Funded warrants issued to purchase shares | shares	5,434,782
Common stock, par value (in dollars per share)	$ 0.01
Exercise price	5.51
Warrant exercise price	$ 5.52
Aggregate gross proceeds | $	$ 29.9
Transaction costs | $	$ 0.2
Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares) | shares			4,380,000
Compensation Policy for Non-Employee Directors | Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares) | shares			308,000	300,000
Non-employee directors-initial grant | Deferred share units
Stock-based compensation disclosure
Vesting period		3 years
Non-employee directors-first anniversary | Deferred share units
Stock-based compensation disclosure
Vesting period		1 year

Restructuring Charges (Details)

Restructuring Charges (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Workforce reduction
Restructuring charge
Balance at beginning of the period	$ 784
Additional charges/adjustments during the period		$ (100)
Payments during the period	(278)
Balance at end of the period	$ 506
Incremental retention benefits | 2019 Corporate Restructuring
Restructuring
Severance costs, charges incurred		$ 1,600

Leases - Operating Leases (Deta

Leases - Operating Leases (Details) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2021USD ($)ft²itemlease	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)ft²leaseagreement
Lessee, Lease, Description [Line Items]
Number of real estate leases | lease	1
Right-of-use assets	$ 12,745		$ 14,072
Lease liabilities	$ 19,403
Weighted-average discount rate	11.00%
Lease expense for operating lease payments	$ 3,000	$ 3,000
Cash paid against operating lease liabilities	$ 4,100	4,100
Weighted average remaining term of the operating leases	4 years 6 months
Sublease income	$ 3,700	$ 1,600
Maximum
Lessee, Lease, Description [Line Items]
100 basis point change effect on ROU asset	$ 1,000
CRP/King 830 Winter L.L.C.
Lessee, Lease, Description [Line Items]
Area of space leased | ft²	120,000
Number of additional terms for which lease agreement can be extended | item	2
Operating lease term extension period	5 years
Number of executed sub-lease spaces | lease			4
Area of executed sublease space | ft²			65,000
PDM 930 Unit, LLC
Lessee, Lease, Description [Line Items]
Area of executed sublease space | ft²	10,281
Winter Street 930 Waltham MA
Lessee, Lease, Description [Line Items]
Number of executed sub-lease spaces | agreement			4
Area of executed sublease space | ft²			65,000
Minimum rental payments over the remaining term of the sublease	$ 14,000

Leases - Maturities of Operatin

Leases - Maturities of Operating Lease Liabilities (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)
Leases
2021 (three months remaining)	$ 1,265
2022	5,389
2023	5,510
2024	5,470
2025	5,490
Thereafter	1,376
Total lease payments	24,500
Less imputed interest	(5,097)
Total lease liabilities	19,403
Variable operating costs and real estate taxes	$ 3,100

Commitments and Contingencies (

Commitments and Contingencies (Details) $ in Millions	Sep. 30, 2021USD ($)
In-process and future manufacturing of antibody, drug substance, and cytotoxic agents
Collaborations and Manufacturing Commitments
Noncancelable obligations under several agreements	$ 3.1
Minimum
Collaborations and Manufacturing Commitments
Manufacturing commitment	$ 33.4

Subsequent Events (Details)

Subsequent Events (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Oct. 31, 2021	May 31, 2017	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Subsequent Event [Line Items]
Revenue from contract with customer			$ 9,210	$ 18,189	$ 41,864	$ 46,501
Research and development			$ 33,147	$ 24,685	102,149	$ 75,014
2012 license agreement with a contract manufacturing vendor
Subsequent Event [Line Items]
Research and development					$ 3,000
Debiopharm | Transfer of ImmunoGen technologies
Subsequent Event [Line Items]
Potential milestone payment		$ 5,000
Debiopharm | Upfront payment | IMGN529 program
Subsequent Event [Line Items]
Revenue from contract with customer		$ 25,000
Subsequent event | 2012 license agreement with a contract manufacturing vendor
Subsequent Event [Line Items]
Specific development and sales-based milestone payments	$ 18,000
Subsequent event | Debiopharm | Upfront Fees, Milestones, and Royalties | IMGN529 program | Maximum
Subsequent Event [Line Items]
Potential milestone payment	$ 30,000