Immunogen (IMGN) 10-Q 2 Nov 23 2023 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2023	Oct. 25, 2023
Document and Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2023
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		266,264,274
Entity Central Index Key	0000855654
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q3
Amendment Flag	false

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($)	Sep. 30, 2023	Dec. 31, 2022
ASSETS
Cash and cash equivalents	$ 605,535,000	$ 275,138,000
Accounts receivable	130,694,000	12,596,000
Unbilled receivable	3,026,000	1,531,000
Non-cash royalty receivable	3,438,000	3,851,000
Inventory	5,495,000
Prepaid and other current assets	18,712,000	11,005,000
Total current assets	766,900,000	304,121,000
Property and equipment, net of accumulated depreciation	4,431,000	4,377,000
Operating lease right-of-use assets	8,338,000	10,231,000
Long-term inventory	28,273,000	16,196,000
Other assets	14,159,000	14,011,000
Total assets	822,101,000	348,936,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Accounts payable	24,853,000	45,353,000
Accrued compensation	15,155,000	11,111,000
Other accrued liabilities	43,571,000	38,783,000
Current portion of liability related to the sale of future royalties, net of deferred financing costs of $128 and $162, respectively	9,437,000	8,659,000
Current portion of operating lease liability	4,458,000	4,096,000
Current portion of deferred revenue	37,186,000	13,856,000
Total current liabilities	134,660,000	121,858,000
Senior secured term loan, net	72,113,000
Deferred revenue, net of current portion	26,718,000	36,355,000
Operating lease liability, net of current portion	7,759,000	11,148,000
Liability related to the sale of future royalties, net of current portion and deferred financing costs of $111 and $205, respectively	16,455,000	23,449,000
Other long-term liabilities	2,800,000	300,000
Total liabilities	260,505,000	193,110,000
Commitments and contingencies (Note K)
Shareholders' equity:
Preferred stock, $.01 par value; authorized 5,000 shares; 22 and 0 shares issued and outstanding as of each of September 30, 2023 and December 31, 2022, respectively
Common stock, $.01 par value; authorized 600,000 shares; 265,842 and 220,046 shares issued and outstanding as of September 30, 2023 and December 31, 2022, respectively	2,435,000	2,260,000
Additional paid-in capital	2,267,747,000	1,847,638,000
Accumulated deficit	(1,708,586,000)	(1,694,072,000)
Total shareholders' equity	561,596,000	155,826,000
Total liabilities and shareholders' equity	$ 822,101,000	$ 348,936,000

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
CONSOLIDATED BALANCE SHEETS
Sale of future royalties, current portion and deferred financing costs	$ 128	$ 162
Sale of future royalties, noncurrent portion and deferred financing costs	$ 111	$ 205
Preferred stock, par value (in dollars per share)	$ 0.01
Preferred stock, authorized shares	5,000	5,000
Preferred stock, shares issued	22	0
Preferred stock, shares outstanding	22	0
Common stock, par value (in dollars per share)	$ 0.01
Common stock, authorized shares	600,000	600,000
Common stock, issued shares	265,842	226,046
Common stock, outstanding shares	265,842	226,046

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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenues:
Total revenues	$ 113,425	$ 15,375	$ 246,447	$ 67,615
Cost and operating expenses:
Cost of sales	2,155		3,690
Research and development	47,570	59,181	149,267	154,885
Selling, general and administrative	37,744	33,623	114,116	74,064
Total cost and operating expenses	87,469	92,804	267,073	228,949
Income (loss) from operations	25,956	(77,429)	(20,626)	(161,334)
Interest income	7,383	1,539	14,775	2,183
Interest expense on term loan	(2,539)		(5,857)
Non-cash interest expense on liability related to the sale of future royalties and term loan	(1,054)	(867)	(2,986)	(3,194)
Other (expense) income, net	(164)	(998)	(109)	(1,576)
Net loss before income taxes	29,582	(77,755)	(14,803)	(163,921)
Income tax benefit	(1,166)		(289)
Net loss	$ 30,748	$ (77,755)	$ (14,514)	$ (163,921)
Net income (loss) per common share - basic (Note B)	$ 0.10	$ (0.31)	$ (0.05)	$ (0.65)
Net income (loss) per common share - diluted (Note B)	$ 0.10	$ (0.31)	$ (0.05)	$ (0.65)
Basic weighted average common shares outstanding (in shares)	273,341	253,511	265,265	253,371
Diluted weighted average common shares outstanding (in shares)	287,590	253,511	265,265	253,371
License and milestone fees
Revenues:
Total revenues	$ 51	$ 7,382	$ 15,122	$ 45,247
Non-cash royalty revenue related to the sale of future royalties
Revenues:
Total revenues	7,355	$ 7,993	17,936	21,537
Research and development support
Revenues:
Total revenues	855		1,310	$ 831
Product revenue, net
Revenues:
Total revenues	$ 105,164		$ 212,079

CONSOLIDATED STATEMENTS OF SHAR

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Preferred Stock Series A Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Balance at Dec. 31, 2021		$ 2,204	$ 1,794,525	$ (1,471,143)	$ 325,586
Balance (in shares) at Dec. 31, 2021		220,361
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss				(24,145)	(24,145)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan		$ 1	619		620
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)		173
Restricted stock units vested (in shares)		2
Stock option and restricted stock compensation expense			4,196		4,196
Directors' deferred share unit compensation			211		211
Balance at Mar. 31, 2022		$ 2,205	1,799,551	(1,495,288)	306,468
Balance (in shares) at Mar. 31, 2022		220,536
Balance at Dec. 31, 2021		$ 2,204	1,794,525	(1,471,143)	325,586
Balance (in shares) at Dec. 31, 2021		220,361
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss					(163,921)
Balance at Sep. 30, 2022		$ 2,208	1,810,863	(1,635,064)	178,007
Balance (in shares) at Sep. 30, 2022		220,751
Balance at Mar. 31, 2022		$ 2,205	1,799,551	(1,495,288)	306,468
Balance (in shares) at Mar. 31, 2022		220,536
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss				(62,021)	(62,021)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan		$ 1	410		411
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)		108
Stock option and restricted stock compensation expense			4,760		4,760
Directors' deferred share unit compensation			213		213
Balance at Jun. 30, 2022		$ 2,206	1,804,934	(1,557,309)	249,831
Balance (in shares) at Jun. 30, 2022		220,644
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss				(77,755)	(77,755)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan		$ 2	447		449
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)		107
Stock option and restricted stock compensation expense			5,336		5,336
Directors' deferred share unit compensation			146		146
Balance at Sep. 30, 2022		$ 2,208	1,810,863	(1,635,064)	178,007
Balance (in shares) at Sep. 30, 2022		220,751
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss				(59,008)	(59,008)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan		$ 1	423		424
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)		103
Issuance of common stock, net of issuance costs		$ 51	25,596		25,647
Issuance of common stock, net of issuance costs (in shares)		5,167
Restricted stock units vested (in shares)		25
Stock option and restricted stock compensation expense			10,610		10,610
Directors' deferred share unit compensation			146		146
Balance at Dec. 31, 2022		$ 2,260	1,847,638	(1,694,072)	$ 155,826
Balance (in shares) at Dec. 31, 2022		226,046			226,046
Increase (Decrease) in Shareholders' Equity (Deficit)
Issuance of Series A Preferred Stock in exchange for common stock
Issuance of Series A Preferred Stock in exchange for common stock (in shares)					0
Net loss				(41,014)	$ (41,014)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan		$ 1	38		39
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)		16
Stock option and restricted stock compensation expense			6,916		6,916
Directors' deferred share unit compensation			151		151
Directors' common stock compensation		8
Balance at Mar. 31, 2023		$ 2,261	1,854,743	(1,735,086)	121,918
Balance (in shares) at Mar. 31, 2023		226,070
Balance at Dec. 31, 2022		$ 2,260	1,847,638	(1,694,072)	$ 155,826
Balance (in shares) at Dec. 31, 2022		226,046			226,046
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss					$ (14,514)
Balance at Sep. 30, 2023	$ 22	$ 2,435	2,267,747	(1,708,586)	$ 561,596
Balance (in shares) at Sep. 30, 2023		265,842			265,842
Balance at Mar. 31, 2023		$ 2,261	1,854,743	(1,735,086)	$ 121,918
Balance (in shares) at Mar. 31, 2023		226,070
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss				(4,248)	(4,248)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan		$ 32	14,874		14,906
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)		3,234
Issuance of common stock, net of issuance costs		$ 299	350,534		350,833
Issuance of common stock, net of issuance costs (in shares)		29,900
Issuance of common stock pursuant to pre-funded warrant exchange		11,357
Issuance of Series A Preferred Stock in exchange for common stock (common stock)		(21,853)
Stock option and restricted stock compensation expense			7,281		7,281
Directors' deferred share unit compensation			152		152
Directors' common stock compensation		4
Balance at Jun. 30, 2023	$ 22	$ 2,374	2,227,802	(1,739,334)	490,842
Balance (in shares) at Jun. 30, 2023		248,712
Increase (Decrease) in Shareholders' Equity (Deficit)
Issuance of Series A Preferred Stock in exchange for common stock		$ (218)	218
Issuance of Series A Preferred Stock in exchange for common stock (in shares)	22
Net loss				30,748	30,748
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan		$ 61	32,105		32,166
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)		6,136
Issuance of common stock, net of issuance costs			(34)		(34)
Issuance of common stock pursuant to pre-funded warrant exchange		10,992
Stock option and restricted stock compensation expense			7,807		7,807
Directors' deferred share unit compensation			67		67
Directors' common stock compensation		2
Balance at Sep. 30, 2023	$ 22	$ 2,435	$ 2,267,747	$ (1,708,586)	$ 561,596
Balance (in shares) at Sep. 30, 2023		265,842			265,842
Increase (Decrease) in Shareholders' Equity (Deficit)
Issuance of Series A Preferred Stock in exchange for common stock
Issuance of Series A Preferred Stock in exchange for common stock (in shares)					22

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Cash flows from operating activities:
Net loss	$ (14,514)	$ (163,921)
Adjustments to reconcile net loss to net cash used for operating activities:
Non-cash royalty revenue related to sale of future royalties	(8,494)	(9,027)
Non-cash interest expense on liability related to sale of future royalties and convertible senior notes	2,691	3,194
Non-cash interest expense on amortization of debt discount and issuance costs	295
Depreciation and amortization	1,298	1,355
Stock and deferred share unit compensation	22,374	14,862
Change in operating assets and liabilities:
Accounts receivable	(118,098)	4,425
Unbilled receivable	(1,495)	1,649
Inventory	(17,572)
Contract asset		3,000
Prepaid and other current assets	(7,707)	(9,392)
Operating lease right-of-use assets	1,893	1,583
Other assets	(148)	(4,389)
Accounts payable	(20,395)	1,689
Accrued compensation	4,044	3,152
Other accrued liabilities	7,472	23,057
Deferred revenue	13,693	(38,257)
Operating lease liability	(3,027)	(2,583)
Net cash used for operating activities	(137,690)	(169,603)
Cash flows from investing activities:
Purchases of property and equipment	(1,641)	(1,116)
Net cash used for investing activities	(1,641)	(1,116)
Cash flows from financing activities:
Proceeds from issuance of common stock under stock plans	47,111	1,480
Proceeds from term loan, net of $3,182 of issuance costs	71,818
Proceeds from common stock issuance, net of $526 of transaction costs	350,799
Net cash provided by financing activities	469,728	1,480
Net change in cash and cash equivalents	330,397	(169,239)
Cash and cash equivalents, beginning of period	275,138	478,750
Cash and cash equivalents, end of period	605,535	$ 309,511
Supplemental cash flow information:
Cash paid during the year for interest	4,857
Cash paid during the year for taxes	$ 1,361

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) $ in Thousands	9 Months Ended
	Sep. 30, 2023 USD ($)
Term loan issuance costs	$ 3,182
Common Stock
Common stock issuance transaction costs	$ 526

Nature of Business and Plan of

Nature of Business and Plan of Operations	9 Months Ended
	Sep. 30, 2023
Nature of Business and Plan of Operations
Nature of Business and Plan of Operations	IMMUNOGEN, INC. NOTES TO CONSOLIDATED FINANCIAL STATEMENTS September 30, 2023 ​ A. Nature of Business and Plan of Operations ​ ImmunoGen, Inc. (the Company) was incorporated in Massachusetts in 1981 and is focused on the development and commercialization of antibody-drug conjugates (ADCs). On November 14, 2022, the U.S. Food and Drug Administration (FDA) granted accelerated approval for ELAHERE ® The Company has generally incurred operating losses and negative cash flows from operations since inception, incurred a net loss of $14.5 million during the nine months ended September 30, 2023, and had an accumulated deficit of approximately $1.7 billion as of September 30, 2023. To date, the Company has funded these losses through payments received from its collaborations, equity, convertible debt, and other financings, such as royalty financing transactions and a term loan facility, and, more recently, through At September 30, 2023, the Company had $605.5 million of cash and cash equivalents on hand. The Company currently believes that its existing capital resources will be sufficient to fund its operating expenses and capital expenditures for more than twelve months after the date these financial statements were issued. The Company expects to generate additional funds through a combination of commercial sales of ELAHERE and revenues from collaborations, including upfront license payments, milestone payments, royalty payments, and research funding, to support its planned operating activities ; however, 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, challenges entering into new collaborations, complexities associated with managing collaboration arrangements, third-party reimbursements, and compliance with governmental regulations. ​

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2023
Summary of 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, 2022 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, 2022 filed with the SEC on March 1, 2023. Significant Accounting Policies There were no changes to significant accounting policies used in preparation of these condensed consolidated financial statements for the three and nine months ended September 30, 2023, from those discussed in Note B to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2022. ​ Revenue Recognition Transaction Price Allocated to Future Performance Obligations Deferred revenue under Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC), 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, 2023 and 2022 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ December 31, 2022 Additions ​ Deductions ​ Impact of Netting ​ September 30, 2023 Contract liabilities (deferred revenue) ​ $ 50,211 ​ $ 23,227 ​ $ (9,534) ​ $ — ​ $ 63,904 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ December 31, 2021 ​ Additions ​ Deductions ​ Impact of Netting ​ September 30, 2022 Contract asset ​ $ 3,000 ​ $ — ​ $ (3,000) ​ $ — ​ $ — Contract liabilities (deferred revenue) ​ $ 92,068 ​ $ 5,704 ​ $ (43,961) ​ $ — ​ $ 53,811 ​ 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, ​ ​ 2023 ​ 2022 ​ 2023 ​ 2022 Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 3,929 ​ $ 7,337 ​ $ 9,534 ​ $ 43,961 ​ 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. During the nine months ended September 30, 2023, a $23.2 million upfront payment received pursuant to a collaboration and license agreement with Takeda Pharmaceutical Company Limited (Takeda) was recorded as deferred revenue and none of this amount was recognized as revenue during the nine months ended September 30, 2023. Additionally, the Company received an upfront payment of $15.0 million pursuant to a multi-target license and option agreement executed with Vertex Pharmaceuticals Incorporated (Vertex) which was recorded as license and milestone fee revenue in the nine months ended September 30, 2023. Further details of these agreements can be found in Note C, “Collaboration and License Agreements.” During the nine months ended September 30, 2023, the Company also recognized $9.4 million of previously deferred non-cash royalty revenue related to the sale of rights to KADCYLA During the nine months ended September 30, 2022, pursuant to the Company’s license agreement with Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd. (Huadong), upon delivery of clinical materials in the nine months ended September 30, 2022, the Company recognized as license and milestone fee revenue the remaining $28.5 million of the deferred revenue balance as of December 31, 2021, related to the $45.0 million of upfront and development milestone payments previously received. Additionally, pursuant to a license agreement executed with Eli Lilly and Company (Lilly), during the nine months ended September 30, 2022, the Company received upfront payments of $19.5 million, of which $13.8 million was recognized as license and milestone fee revenue and the remainder deferred. The Company also recognized $12.5 million of previously deferred non-cash royalty revenue related to the sale of rights to KADCYLA $2.9 million of license and milestone fee revenue related to numerous collaborators’ rights to technological improvements that had been previously deferred, which includes $2.8 million related to Novartis Institutes for BioMedical Research, Inc.’s (Novartis) termination of certain of the license agreements between the Company and Novartis in August 2022. 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, the Company does not believe it is exposed to significant 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, 2023 and December 31, 2022. 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, 2023 and December 31, 2022, the Company held $605.5 million and $275.1 million, respectively, in cash and money market funds, which were classified as cash and cash equivalents. Non-cash Investing and Financing Activities The Company had $0.3 million of accrued capital expenditures as of December 31, 2022, which has been treated as a non-cash investing activity and, accordingly, is not reflected in the consolidated statement of cash flows. There were no accrued capital expenditures as of September 30, 2023. 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, 2023 and December 31, 2022, 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, non-cash royalty receivable, prepaid and other current assets, accounts payable, accrued compensation, and other accrued liabilities approximate fair value due to their short-term nature. As of September 30, 2023, the estimated fair value and gross carrying amount of the term loan was $79.7 million and $75.0 million, respectively. The Company’s disclosed fair value of the term loan falls into the Level 2 category within the fair value level hierarchy and the fair value was determined using quoted prices for similar liabilities in active markets, as well as inputs that are observable for the liability (other than quoted prices), such as interest rates that are observable at commonly quoted intervals. Accounts Receivable Accounts receivable arise from product sales and amounts due from the Company’s collaboration partners. The amount from product sales represents amounts due from specialty distributors and specialty pharmacy providers in the U.S. The Company monitors economic conditions and the financial performance and credit worthiness of its counterparties to identify facts or circumstances that may indicate that its receivables are at risk of collection. The Company provides reserves against accounts receivable for estimated losses that may result from a customer’s inability to pay based on the composition of its accounts receivable, considering past events, current economic conditions, and reasonable and supportable forecasts about the future economic conditions. The contractual life of accounts receivable is generally short-term. Amounts determined to be uncollectible are charged or written off against the reserve. For the three and nine months ended September 30, 2023 and 2022, the Company did not record any expected credit losses related to outstanding accounts receivable. Inventory Inventories are stated at the lower of cost or estimated net realizable value with cost based on the first-in first-out method. Inventory that can be used in either the production of clinical or commercial products is expensed as research and development costs when identified for use in clinical trials. The Company classifies its inventory costs as long-term when it expects to utilize the inventory beyond its normal operating cycle based on forecasted levels of sales. Prior to the regulatory approval of its drug candidates, the Company incurs expenses for the manufacture of drug product to support clinical development that could potentially be available to support the commercial launch of those drugs. Until the date at which regulatory approval has been received or is otherwise considered probable, the Company records all such costs as research and development expenses. The Company performs an assessment of the recoverability of capitalized inventories during each reporting period and writes down any excess and obsolete inventory to its net realizable value in the period in which the impairment is first identified. Such impairment charges, should they occur, are recorded as a component of cost of sales in the consolidated statements of operations and comprehensive loss. The determination of whether inventory costs will be realizable requires the use of estimates by management. If actual market conditions are less favorable than projected by management, additional write-downs of inventory may be required. There were no expenses recorded for excess inventory or other impairments during the three and nine months ended September 30, 2023. There was no inventory held by the Company during the three and nine months ended September 30, 2022. Debt issuance costs and debt discount Debt issuance costs and debt discounts are presented on the accompanying consolidated balance sheets as a direct reduction from the carrying value of the debt and are amortized to interest expense over the term of the related debt using the effective interest method. See Note G, “Senior Secured Term Loan” for further discussion related to long-term debt. 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, par value $.01 per share, underlying pre-funded warrants are included in the calculation of basic and diluted earnings per share. Shares of the Company’s Series A Convertible Preferred 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 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 common shares and participating securities (the two-class method). 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 and restricted stock units that are outstanding during the period, except where such non-participating securities would be antidilutive. The dilutive effect of participating securities is calculated using the more dilutive of either (i) the treasury stock method (for stock options and restricted stock units) and “if-converted” method (for Series A Convertible Preferred Stock) or (ii) the two-class method assuming the Series A Convertible Preferred Stock is not converted and applying the treasury stock method (for stock options and restricted stock units). The following table sets forth the computation of basic and diluted earnings per share for the three months ended September 30, 2023. There was a net loss in all other periods presented, and as such, no loss was allocated to participating securities pursuant to the two class method for those periods. ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ September 30, ​ ​ 2023 Numerator: ​ ​ ​ Net income ​ $ 30,748 Allocation of earnings to participating securities ​ ​ (2,276) Numerator for basic EPS — income available to common stockholders (A) ​ $ 28,472 Effect of dilutive securities: ​ ​ ​ Add back allocation of earnings to participating securities ​ $ 2,276 Reallocation of earnings to participating securities considering potentially dilutive securities ​ ​ (2,171) Numerator for diluted EPS — income available to common stockholders (C) ​ $ 28,577 Denominator: ​ ​ ​ Denominator for basic EPS — weighted average shares (B) ​ ​ 273,341 Effect of dilutive securities: ​ ​ ​ Common stock equivalents ​ ​ 14,249 Denominator for diluted EPS — adjusted weighted average shares (D) ​ ​ 287,590 ​ ​ ​ ​ Basic EPS (A / B) ​ $ 0.10 Diluted EPS (C / D) ​ $ 0.10 The Company’s common stock equivalents, as calculated in accordance with the treasury-stock method for options and unvested restricted stock units and the if-converted method for the Series A Convertible Preferred Stock, are shown in the following table (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ September 30, ​ September 30, ​ 2023 2022 2023 2022 Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock units at end of period ​ 29,927 ​ 31,479 ​ 29,927 ​ 31,479 Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock units ​ 14,249 2,246 ​ 10,671 ​ 1,437 Common stock equivalents under if-converted method for Series A Convertible Preferred Stock ​ 21,853 ​ — ​ 21,853 ​ — ​ Stock-Based Compensation As of September 30, 2023, 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 15, 2022, 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 13,000,000 shares of the Company’s common stock, as well as up to 28,742,013 shares of common stock, which represent the number of shares of common stock remaining under the 2018 Plan as of April 1, 2022, 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 April 1, 2022. 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 13,500,000 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, ​ 2023 ​ 2022 ​ 2023 ​ 2022 Dividend ​ None ​ None ​ None ​ None Volatility ​ 88.4% ​ 83.3% ​ 83.4% ​ 83.2% Risk-free interest rate ​ 4.52% ​ 3.44% ​ 3.78% ​ 2.48% Expected life (years) ​ 5.6 ​ 5.6 ​ 5.7 ​ 5.9 ​ Using the Black-Scholes option-pricing model, the weighted-average grant date fair values of options granted during the three months ended September 30, 2023 and 2022 were $11.52 and $3.62 per share, respectively, and $4.85 and $3.58 for options granted during the nine months ended September 30, 2023 and 2022, respectively. A summary of option activity under the Company’s equity plans for the nine months ended September 30, 2023 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ Number ​ Average ​ ​ of Stock ​ Exercise ​ ​ Options ​ Price Outstanding at December 31, 2022 ​ 33,126 ​ $ 5.76 Granted ​ 6,316 ​ ​ 6.71 Exercised ​ (9,121) ​ ​ 5.05 Forfeited/Canceled ​ (2,653) ​ ​ 6.30 Outstanding at September 30, 2023 ​ 27,668 ​ $ 6.15 ​ In 2020, the Company issued 2.6 million performance-based stock options to certain employees with vesting conditioned upon the achievement of specified performance goals. In 2022, 75% of the 2.6 million performance-based stock options vested upon achievement of specified performance goals and 12.5% were forfeited. There was no stock-based compensation recorded during the three or nine months ended September 30, 2023 related to these stock options. The fair value of the remaining unvested performance-based stock options that could be expensed in future periods is $1.3 million. A summary of restricted stock unit activity under the Company’s equity plans for the nine months ended September 30, 2023 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted- ​ ​ Restricted ​ Average Grant ​ ​ Stock Shares ​ Date Fair Value Unvested at December 31, 2022 ​ 138 ​ $ 5.45 Granted ​ 2,380 ​ ​ 6.32 Forfeited ​ (259) ​ ​ 4.66 Unvested at September 30, 2023 ​ 2,259 ​ $ 7.11 ​ In June 2018, the Company's Board of Directors, with shareholder approval, adopted the Employee Stock Purchase Plan (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. ESPP purchase periods are six months and begin on January 1 and July 1 of each year, with purchase dates occurring on the final business day of the given purchase period. 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 unit awards granted under the stock plans and the ESPP was $7.8 million and $22.0 million during the three and nine months ended September 30, 2023, respectively, compared to $5.3 million and $14.3 million for the three and nine months ended September 30, 2022, respectively. The increase in stock compensation expense is primarily due to significant growth in personnel in the second half of 2022. As of September 30, 2023, the estimated fair value of unvested employee awards was $72.0 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 months ended September 30, 2023, 93% of revenues were generated from net U.S. sales of ELAHERE to four specialty distributors and specialty pharmacy providers, and 7% of revenues were generated from an agreement with Roche, compared to 52%, 30% and 18% of revenues from Roche, Lilly and Novartis, respectively, during the three months ended September 30, 2022. During the nine months ended September 30, 2023, 86% of revenues were generated from net U.S. sales of ELAHERE to four specialty distributors and specialty pharmacy providers, and 7% and 6% of revenues were generated from agreements with Roche and Vertex, respectively, compared to 43%, 32% and 20% from agreements with Huadong, Roche, and Lilly, respectively, during the nine months ended September 30, 2022. There were no other customers of the Company that generated significant revenues in the three and nine months ended September 30, 2023 and 2022. Recently Adopted Accounting Pronouncements There were no recently issued or effective FASB Accounting Standards Updates (ASUs) that had, or are expected to have, a material effect on the Company's results of operations, financial condition, or liquidity.

Collaboration and License Agree

Collaboration and License Agreements	9 Months Ended
	Sep. 30, 2023
Collaboration and License Agreements
Collaboration and License Agreements	C. Collaboration and License Agreements The Company has numerous collaboration and license agreements with third parties. These agreements typically provide the licensee with rights to use the Company’s ADC platform technology with the licensee’s antibodies or related targeting vehicles to a defined target to develop products. The licensee is generally responsible for the development, clinical testing, manufacturing, registration, and commercialization of any resulting product candidate. As part of these agreements, the Company is generally entitled to receive upfront fees, potential milestone payments, royalties on the sales of any resulting products, and research and development funding based on activities performed at our collaborative partner’s request. See below for details regarding the Company’s collaboration and license agreements with activity in the financial statement periods presented. Takeda On August 25, 2023, the Company entered into a collaboration and license agreement with Takeda. The collaboration and license agreement grants Takeda an exclusive, royalty-bearing right to develop and commercialize ELAHERE (mirvetuximab soravtansine-gynx) (the Licensed Product) in Japan. Under the terms of the collaboration and license agreement, the Company received a non-refundable upfront payment of $23.2 million, with the potential for up to ¥19.9 billion ( The Company evaluated the agreement and determined it was within the scope of ASC 606. The Company determined the promised goods and services included the license to intellectual property and know-how and the clinical supply of the Licensed Product to Takeda for a specified period. The Company concluded that the license to intellectual property and know-how is not distinct from the clinical supply of the Licensed Product because the clinical supply is essential to the use of the license and an alternative source of clinical supply is not readily available in the marketplace. Accordingly, these two promised goods and services are considered a single combined performance obligation. The Company determined there were no options in the agreement that represented material rights. The transaction price was determined to consist of the upfront payment of $23.2 million and estimated payments to be received for clinical supply of the Licensed Product. Future regulatory milestones have been fully constrained. Any consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as these amounts have been determined to relate predominantly to the license granted to Takeda. The Company re-evaluates the transaction price, including its estimated variable consideration included in the transaction price and all constrained amounts, at each reporting period and as uncertain events are resolved or other changes in circumstances occur. The Company determined that revenue related to the agreement would be recognized as the clinical supply of the Licensed Product is delivered to Takeda, estimated to be completed over approximately 1.5 years. The Company has estimated the total clinical supply to be delivered during this time and will reassess the percentage of clinical supply that has been delivered on an ongoing basis. If a change in estimate is determined to be necessary, the Company will adjust revenue using a cumulative catch-up method. No revenue related to this agreement has been recognized in the three months ended September 30, 2023. Vertex (each, an Option and, collectively, the Options) before the end of the research term In addition, upon exercise of each Option by Vertex, the Company will be eligible to receive up to approximately $337.0 million per target in potential option exercise fees and milestone payments based on the achievement of pre-specified development, regulatory, and sales-based milestones. With respect to each target that Vertex exercises an Option, the Company will also be eligible to receive tiered royalties, on a product-by-product basis, as a percentage of worldwide annual net sales by Vertex, its affiliates and sublicensees, based on certain net sales thresholds. Vertex is responsible for all costs related to the research and development of the compounds during the research term and commercialization of any ensuing products. The Company evaluated the agreement and determined it was within the scope of ASC 606. The Company determined the promised goods and services included a license to use the Company’s intellectual property and know-how to research, manufacture, and evaluate products related to each of the initial research targets selected by Vertex during the research term. The Company determined that the agreement has a single performance obligation for these promised goods and services. The Options to obtain exclusive development and commercialization licenses and the right to select additional research targets during the research term do not represent a material right as the fees associated with each option are at or above the standalone selling price. Accordingly, upon exercise, these Options will be accounted for as a separate arrangement. The transaction price related to the single performance obligation was determined to consist of the upfront payment of $15.0 million. The transfer of intellectual property and know-how to Vertex to allow Vertex to derive benefit from the license over the research term was completed during the three months ended March 31, 2023. As such, the Company’s performance obligation was satisfied, and the Company recognized $15.0 million of license and milestone fee revenue during the nine months ended September 30, 2023. Lilly In February 2022, the Company entered into a license agreement with Lilly, pursuant to which the Company granted Lilly worldwide exclusive rights to research, develop, and commercialize antibody-drug conjugates based on the Company’s novel camptothecin technology. Under the terms of the license agreement, the Company received a non-refundable upfront payment of $13.0 million, reflecting initial targets selected by Lilly. During 2022, pursuant to the terms of the agreement, Lilly selected additional targets for which the Company received an additional $13.0 million in non-refundable payments. The transfer of intellectual property and know-how to Lilly to allow for Lilly to derive benefit from the initial and additional target licenses was completed during the three months ended March 31, 2022. As such, during 2022 the Company recognized $18.4 million of license and milestone fee revenue related to the portion of the transaction price allocated to the initial and additional target licenses, of which $13.8 million was recorded during the nine months ended September 30, 2022. The $7.6 million allocated to the material rights to obtain licenses to replacement targets is included in long-term deferred revenue as of September 30, 2023 and will be recognized when the right is either exercised or expires. ​ Huadong In October 2020, the Company entered into a collaboration and license agreement with Huadong. The collaboration and license agreement grants Huadong an exclusive, royalty-bearing, and sublicensable right to develop and commercialize ELAHERE (the Licensed Product) in the People’s Republic of China, Hong Kong, Macau, and Taiwan (collectively, Greater China). The Company retains exclusive rights to the Licensed Product outside of Greater China. Under the terms of the collaboration and license agreement, the Company received a non-refundable upfront payment of $40.0 million with the potential for approximately $265.0 million in development, regulatory, and sales-based milestone payments. In addition, the Company is entitled to receive tiered royalties ranging from low double digits to high teens as a percentage of commercial net sales of the licensed product, if approved, by Huadong in Greater China, subject to adjustment in specified circumstances. To date, the Company has received $15.0 million in milestone payments. The Company determined that revenue related to the agreement would be recognized as the clinical supply of the Licensed Product is delivered to Huadong, estimated to be completed over approximately two years . Accordingly, based on clinical supply delivered to Huadong during the nine months ended September 30, 2022, the Company recorded the remaining $28.5 million of deferred revenue as of December 31, 2021 related to $45.0 million of upfront and development milestone payments previously received. Roche In 2000, the Company granted Genentech, now a unit of Roche, an exclusive development and commercialization 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, $17.9 million and $21.5 million of non-cash royalties on net sales of KADCYLA were recognized and included in non-cash royalty revenue for the nine months ended September 30, 2023 and 2022, respectively. The Company sold its rights to receive royalty payments on the net sales of KADCYLA through two separate transactions in 2015 and 2019. Following the 2019 transaction, OMERS, the defined benefit pension plan for municipal employees in the Province of Ontario, Canada, is entitled to receive all of these royalties. Novartis The Company previously granted Novartis exclusive development and commercialization licenses to the Company’s maytansinoid and IGN ADC technology for use with antibodies to specified targets under a now-expired right-to-test agreement established in 2010. In August 2022, Novartis terminated certain of the remaining development and commercialization licenses. The Company had $2.8 million of deferred revenue associated with the terminated licenses related to the portion of the transaction price previously allocated to rights to future technological improvements. In consideration that no technological improvements would be provided to Novartis and, therefore, no unsatisfied obligations remained related to such licenses, the $2.8 million was recorded as revenue and is included in license and milestone fees for the three and nine months ended September 30, 2022. With respect to the remaining license, $0.7 million of deferred revenue related to the portion of the transaction price previously allocated to rights to future technological improvements continues to be amortized over the remaining estimated term of the license agreement, and we are entitled to receive up to a total of $199.5 million in potential milestone payments, of which $5 million has been received to date, plus royalties on the commercial sales of any resulting products. For additional information related to these agreements, as well as the Company’s other collaboration and license agreements, please read Note C, “Collaboration and License Agreements,” to the audited financial statements included within the Company’s Annual Report on Form 10-K for the year ended December 31, 2022 filed with the SEC on March 1, 2023.

Product Revenue Reserves and Al

Product Revenue Reserves and Allowances	9 Months Ended
	Sep. 30, 2023
Product Revenue Reserves and Allowances
Product Revenue Reserves and Allowances	D. Product Revenue Reserves and Allowances In November 2022, the FDA granted accelerated approval for ELAHERE for the treatment of adult patients with FRα positive, platinum-resistant epithelial ovarian, fallopian tube, or primary peritoneal cancer, who have received one to three prior systemic treatment regimens. The Company recorded net product revenue of $105.2 million and $212.1 million from U.S. sales of ELAHERE during the three and nine months ended September 30, 2023, respectively. ​ The following table summarizes activity in each of the product revenue reserve and allowance categories as of September 30, 2023 and 2022, respectively. (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ September 30, ​ 2023 2022 Beginning balance at January 1 ​ $ 313 ​ $ — Provision related to sales in the current period ​ ​ 36,012 ​ ​ — Credits and payments made ​ ​ (28,223) ​ ​ — Ending balance at September 30 ​ $ 8,102 ​ $ — ​

Inventory

Inventory	9 Months Ended
	Sep. 30, 2023
Inventory
Inventory	E. Inventory Capitalized inventory consists of the following at September 30, 2023 and December 31, 2022 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ December 31, ​ 2023 2022 Raw materials ​ $ 28,555 ​ $ 15,952 Work in process ​ ​ 2,839 ​ ​ — Finished goods ​ ​ 2,374 ​ ​ 244 Total inventory ​ $ 33,768 ​ $ 16,196 ​

Liability Related to Sale of Fu

Liability Related to Sale of Future Royalties	9 Months Ended
	Sep. 30, 2023
Liability Related to Sale of Future Royalties
Liability Related to Sale of Future Royalties	F. Liability Related to Sale of Future Royalties In 2015, Immunity Royalty Holdings, L.P. (IRH) purchased the right to receive 100% of the royalty payments on commercial sales of KADCYLA 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 reached 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 consummation of the transaction, the Company received cash proceeds of $200.0 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 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 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 is being 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, $9.4 million and $12.5 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, 2023 and 2022, respectively. 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, 2023 (in thousands): ​ ​ ​ ​ ​ ​ ​ Nine Months Ended ​ ​ September 30, 2023 Liability related to sale of future royalties, net — beginning balance ​ $ 32,108 Proceeds from sale of future royalties, net ​ — KADCYLA royalty payments received and paid ​ (8,907) Non-cash interest expense recognized ​ ​ 2,691 Liability related to sale of future royalties, net — ending balance ​ $ 25,892 ​ The Company receives royalty reports and royalty payments related to sales of KADCYLA from Roche one

Debt, Long-Term

Debt, Long-Term	9 Months Ended
	Sep. 30, 2023
Debt Disclosure [Abstract]
Debt, Long-Term	G. Senior Secured Term Loan Net proceeds from the initial tranche of the term loan, after deducting the lenders fees and transaction costs of $3.2 million, were $71.8 million. The loan agreement permits voluntary prepayment at any time, subject to a prepayment premium. The loan agreement also includes a make-whole premium in the event of a voluntary prepayment, a prepayment due to a change in control or acceleration following an Event of Default (as defined in the loan agreement) on or prior to the three-year anniversary of the closing date, in each case in an amount equal to foregone interest from the date of prepayment through the three-year anniversary of the closing date. A change of control also triggers a mandatory prepayment of the term loan. The loan agreement contains affirmative and negative covenants customary for transactions of this type and includes certain customary events of default. The Company was in compliance with all such covenants at September 30, 2023. The term loan is secured by a perfected security interest on substantially all of the Company’s assets, excluding certain products and related intellectual property and contracts that are not related to ELAHERE. The Company assessed all terms and features of the loan agreement in order to identify any potential embedded features that would require bifurcation. As part of this analysis, the Company assessed the economic characteristics and risks of the loan agreement, including put and call features. The Company determined that all features of the loan agreement were either clearly and closely associated with a debt host and did not require bifurcation as a derivative liability, or the fair value of the feature was immaterial to the Company's financial statements. The Company reassesses the features on a quarterly basis to determine if they require separate accounting. The following table presents the carrying value of the Company’s term loan balance as of September 30, 2023 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2023 Principal loan balance ​ $ 75,000 Debt discount and issuance costs, unamortized ​ ​ (2,887) Term loan, net ​ $ 72,113 ​ During the three and nine months ended September 30, 2023, the Company recognized interest expense related to the term loan of $2.5 million and $4.9 million, respectively. Additionally, given the Company’s current capital and expected sales of ELAHERE, the Company determined the likelihood of drawing the second tranche of $50.0 million to be remote, and as such, recorded a $1.0 million facility fee that is owed to the lender regardless of whether the additional funding is drawn as interest expense for the nine months ended September 30, 2023.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2023
Income Taxes
Income Taxes	H. Income Taxes The liability method is used in the Company’s accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on differences between financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that are expected to be in effect when the differences are expected to reverse . The realization of deferred income tax assets is dependent on the generation of sufficient taxable income during future periods in which temporary differences are expected to reverse. Where the realization of such assets does not meet the more likely than not criterion, the Company applies a valuation allowance against the deferred income tax asset under consideration. The valuation allowance is reviewed periodically and if the assessment of the more likely than not criterion changes, the valuation allowance is adjusted accordingly. As of September 30, 2023, the Company has a full valuation allowance applied against its deferred tax assets. As part of the Tax Cuts and Jobs Act of 2017 (2017 Tax Act), beginning with the 2022 tax year, the Company is required to capitalize research and development expenses, as defined under Internal Revenue Code Section 174. For expenses that are incurred for research and development in the U.S., the amounts will be amortized over five years, and expenses that are incurred for research and experimentation outside the U.S. will be amortized over 15 years. Pursuant to additional Section 174 guidance issued by the Internal Revenue Service prior to finalizing the Company’s 2022 tax return but subsequent to the Company preparing its 2022 year-end tax provision, the Company recorded a $1.2 million favorable tax adjustment during the three months ended September 30, 2023. Partially offsetting this benefit, during the nine months ended September 30, 2023, the Company recorded income tax expense of $0.8 million as a provision for calendar 2023.

Capital Stock

Capital Stock	9 Months Ended
	Sep. 30, 2023
Capital Stock
Capital Stock	I. Capital Stock Pre-Funded Warrants Pursuant to transactions completed in 2021, the Company issued pre-funded warrants to purchase up to an aggregate of 21,434,782 and 11,363,636 shares of the Company’s common stock to RA Capital Healthcare Fund, L.P. (RA Capital) Related Party Disclosures The pre-funded warrants’ 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 shareholders 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 warrants also include 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 holder’s rights under the pre-funded warrants 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 holder to the Company. The Company assessed the pre-funded warrants 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 warrants are freestanding instruments that do not meet the definition of a liability pursuant to ASC 480 and do not meet the definition of a derivative pursuant to ASC 815. The pre-funded warrants are indexed to the Company’s common stock and meet 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 warrants are freestanding equity-linked financial instruments that meet the criteria for equity classification under ASC 480 and ASC 815. Accordingly, the pre-funded warrants were classified as equity and accounted for as a component of additional paid-in capital at the time of issuance and at each subsequent balance sheet date. The Company also determined that the pre-funded warrants should be included in the determination of basic and diluted earnings per share in accordance with ASC 260, Earnings per Share During the nine months ended September 30, 2023, Redmile completed a cashless exercise in full of its outstanding pre-funded warrant to purchase 11,357,272 shares of the Company’s common stock and RA Capital exercised 11,000,000 of its 21,434,872 pre-funded warrants outstanding, resulting in the issuance of 10,992,330 shares of the Company’s common stock. Series A Convertible Preferred Stock On May 1, 2023, the Company entered into an exchange agreement with RA Capital Each share of the Series A Preferred Stock is convertible into 1,000 shares of the Company’s common stock at the option of the holder at any time until the tenth anniversary of the issuance of the Series A Preferred Stock, at which time the Series A Preferred Stock will automatically convert to the Company’s common stock In addition, the Company has the right to request the conversion of the Series A Preferred Stock into the Company’s common stock in certain circumstances. The conversion of the Series A Preferred Stock into common stock is Shares of Series A Preferred Stock will have no voting rights, except as required by law and except that the affirmative vote of the holders of the then outstanding Series A Preferred Stock will be required to amend the terms of the Series A Preferred Stock, increase the number of authorized shares of Series A Preferred Stock, or enter into an agreement with respect to any of the foregoing The holders of the Series A Preferred Stock are entitled to receive a nominal preference of $0.001 per share of Series A Preferred Stock upon the liquidation, dissolution, or winding up of the Company (the Liquidation Preference) before any payments are made or any assets are distributed to holders of the Company’s common stock. However, if the amount payable to holders of the Company’s common stock upon the Company’s liquidation, dissolution, or winding up is greater than the Liquidation Preference on a per share basis, then the holders of the Series A Preferred Stock will instead receive, on a per-share and as-converted basis, the same assets that are distributed to holders of the Company’s common stock. In the event of certain fundamental transactions, including a merger, holders of the Series A Preferred Stock will automatically receive, as consideration for the Series A Preferred Stock, the same kind and amount of securities, cash, or property as the holders of the Series A Preferred Stock would have been entitled to receive had the holders of the Series A Preferred Stock instead held the Company’s common stock immediately prior to the occurrence of the fundamental transaction, subject to certain exceptions. The Company evaluated the Series A Preferred Stock for liability or equity classification under ASC 480, “ ,” and determined that equity treatment was appropriate because the Preferred Stock did not meet the definition of a liability under ASC 480. The Series A Preferred Stock is not redeemable for cash or other assets on a fixed or determinable date or at the option of the holder. Additionally, as noted above, upon the liquidation of the Company or in the event of a fundamental transaction, such as a merger or acquisition, the holders of the Series A Preferred Stock will receive the same assets that are distributed to the holders of the Company’s common stock. As such, the Company recorded the Series A Preferred Stock as permanent equity. Compensation Policy for Non-Employee Directors Pursuant to the Compensation Policy for Non-Employee Directors, as amended, non-employee directors are granted restricted stock units (RSUs) upon initial election to the Board of Directors and annually thereafter. Initial and annual RSUs vest annually 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 RSUs awarded is fixed per the policy on the date of the award. All unvested RSUs will automatically vest immediately prior to the occurrence of a change of control or in the event a director ceases to serve as a member of the Board due to death or disability. Directors can elect to defer or re-defer RSU and/or deferred share unit (DSU) awards under the Company’s 2004 Non-Employee Director Compensation and Deferred Share Unit Plan, as amended. The directors received a total of approximately 105,000 RSUs in June 2023. Prior to 2023, non-employee directors were granted DSUs with similar vesting to the RSUs. 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 approximately 157,000 and 322,000 options in 2023 and 2022, respectively. Compensation expense related to stock options and RSUs for the three and nine months ended September 30, 2023 and 2022 is included in the amounts discussed in the “Stock-Based Compensation” section of Note B above. In addition, pursuant to the Compensation Policy for Non-Employee Directors, as amended, the Company may issue the Company’s common stock in lieu of cash to pay fees earned by the Company’s directors at each director’s election. The directors received a total of 14,112 shares of the Company’s common stock in lieu of cash in 2023. Prior to 2023, directors could not elect to receive the Company’s common stock in lieu of cash.

Leases

Leases	9 Months Ended
	Sep. 30, 2023
Leases
Leases	J. Leases The Company currently has one There have been no material changes in lease obligations from those disclosed in Note K, “Leases,” to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2022 filed with the SEC on March 1, 2023.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2023
Commitments and Contingencies
Commitments and Contingencies	K. Commitments and Contingencies Manufacturing Commitments As of September 30, 2023, the Company had noncancelable obligations under several agreements related to in-process and future manufacturing of antibody, drug substance, linker, and cytotoxic agents required for supply of the Company’s product candidates totaling $45.3 million. Additionally, pursuant to commercial agreements for future production of antibody, the Company’s noncancelable commitments total $47.3 million at September 30, 2023. Litigation ​ The Company is not a party to any material litigation.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2023
Related Party Transactions
Related Party Transactions	L. Related Party Transactions entered into an exchange agreement with RA Capital pursuant to which RA Capital agreed to exchange 21,853,000 shares of the Company’s common stock for 21,853 shares of newly designated Series A Convertible Preferred Stock. No cash was exchanged related to the transaction. Further details of the agreement can be found in Note I, “Capital Stock.” Stuart A. Arbuckle serves as the chief operating officer at Vertex and has served as a member of the Company’s board of directors since 2018. In February 2023, the Company entered into a multi-target license and option agreement with Vertex, pursuant to which the Company granted Vertex rights to the Company’s ADC technology to research and evaluate ADCs to specified targets, further details of which can be found in Note C, “Collaboration and License Agreements.” The Company’s chief executive officer has served as a director on the board of directors of Ergomed PLC since June 2021. In 2022, the Company executed agreements with Ergomed Clinical Research, Inc. and PrimeVigilance USA, Inc., subsidiaries of Ergomed PLC, for clinical trial and pharmacovigilance-related services. Ergomed Clinical Research, Inc. and PrimeVigilance USA, Inc. are each considered related parties pursuant to ASC 850, Related Party Disclosures

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2023
Subsequent Events
Subsequent Events	M. Subsequent Events The Company has evaluated all events or transactions that occurred after September 30, 2023, up through the date the Company issued these financial statements. The Company did not have any material subsequent events.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended	9 Months Ended
	Jun. 30, 2023	Sep. 30, 2023
Summary of 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, 2022 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, 2022 filed with the SEC on March 1, 2023.
Significant Accounting Policies		Significant Accounting Policies There were no changes to significant accounting policies used in preparation of these condensed consolidated financial statements for the three and nine months ended September 30, 2023, from those discussed in Note B to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2022.
Revenue Recognition		Revenue Recognition Transaction Price Allocated to Future Performance Obligations Deferred revenue under Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC), 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, 2023 and 2022 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ December 31, 2022 Additions ​ Deductions ​ Impact of Netting ​ September 30, 2023 Contract liabilities (deferred revenue) ​ $ 50,211 ​ $ 23,227 ​ $ (9,534) ​ $ — ​ $ 63,904 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ December 31, 2021 ​ Additions ​ Deductions ​ Impact of Netting ​ September 30, 2022 Contract asset ​ $ 3,000 ​ $ — ​ $ (3,000) ​ $ — ​ $ — Contract liabilities (deferred revenue) ​ $ 92,068 ​ $ 5,704 ​ $ (43,961) ​ $ — ​ $ 53,811 ​ 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, ​ ​ 2023 ​ 2022 ​ 2023 ​ 2022 Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 3,929 ​ $ 7,337 ​ $ 9,534 ​ $ 43,961 ​ 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. During the nine months ended September 30, 2023, a $23.2 million upfront payment received pursuant to a collaboration and license agreement with Takeda Pharmaceutical Company Limited (Takeda) was recorded as deferred revenue and none of this amount was recognized as revenue during the nine months ended September 30, 2023. Additionally, the Company received an upfront payment of $15.0 million pursuant to a multi-target license and option agreement executed with Vertex Pharmaceuticals Incorporated (Vertex) which was recorded as license and milestone fee revenue in the nine months ended September 30, 2023. Further details of these agreements can be found in Note C, “Collaboration and License Agreements.” During the nine months ended September 30, 2023, the Company also recognized $9.4 million of previously deferred non-cash royalty revenue related to the sale of rights to KADCYLA During the nine months ended September 30, 2022, pursuant to the Company’s license agreement with Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd. (Huadong), upon delivery of clinical materials in the nine months ended September 30, 2022, the Company recognized as license and milestone fee revenue the remaining $28.5 million of the deferred revenue balance as of December 31, 2021, related to the $45.0 million of upfront and development milestone payments previously received. Additionally, pursuant to a license agreement executed with Eli Lilly and Company (Lilly), during the nine months ended September 30, 2022, the Company received upfront payments of $19.5 million, of which $13.8 million was recognized as license and milestone fee revenue and the remainder deferred. The Company also recognized $12.5 million of previously deferred non-cash royalty revenue related to the sale of rights to KADCYLA $2.9 million of license and milestone fee revenue related to numerous collaborators’ rights to technological improvements that had been previously deferred, which includes $2.8 million related to Novartis Institutes for BioMedical Research, Inc.’s (Novartis) termination of certain of the license agreements between the Company and Novartis in August 2022.
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, the Company does not believe it is exposed to significant 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, 2023 and December 31, 2022. 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, 2023 and December 31, 2022, the Company held $605.5 million and $275.1 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 The Company had $0.3 million of accrued capital expenditures as of December 31, 2022, which has been treated as a non-cash investing activity and, accordingly, is not reflected in the consolidated statement of cash flows. There were no accrued capital expenditures as of September 30, 2023.
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, 2023 and December 31, 2022, 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, non-cash royalty receivable, prepaid and other current assets, accounts payable, accrued compensation, and other accrued liabilities approximate fair value due to their short-term nature. As of September 30, 2023, the estimated fair value and gross carrying amount of the term loan was $79.7 million and $75.0 million, respectively. The Company’s disclosed fair value of the term loan falls into the Level 2 category within the fair value level hierarchy and the fair value was determined using quoted prices for similar liabilities in active markets, as well as inputs that are observable for the liability (other than quoted prices), such as interest rates that are observable at commonly quoted intervals.
Accounts Receivable		Accounts Receivable Accounts receivable arise from product sales and amounts due from the Company’s collaboration partners. The amount from product sales represents amounts due from specialty distributors and specialty pharmacy providers in the U.S. The Company monitors economic conditions and the financial performance and credit worthiness of its counterparties to identify facts or circumstances that may indicate that its receivables are at risk of collection. The Company provides reserves against accounts receivable for estimated losses that may result from a customer’s inability to pay based on the composition of its accounts receivable, considering past events, current economic conditions, and reasonable and supportable forecasts about the future economic conditions. The contractual life of accounts receivable is generally short-term. Amounts determined to be uncollectible are charged or written off against the reserve. For the three and nine months ended September 30, 2023 and 2022, the Company did not record any expected credit losses related to outstanding accounts receivable.
Inventory		Inventory Inventories are stated at the lower of cost or estimated net realizable value with cost based on the first-in first-out method. Inventory that can be used in either the production of clinical or commercial products is expensed as research and development costs when identified for use in clinical trials. The Company classifies its inventory costs as long-term when it expects to utilize the inventory beyond its normal operating cycle based on forecasted levels of sales. Prior to the regulatory approval of its drug candidates, the Company incurs expenses for the manufacture of drug product to support clinical development that could potentially be available to support the commercial launch of those drugs. Until the date at which regulatory approval has been received or is otherwise considered probable, the Company records all such costs as research and development expenses. The Company performs an assessment of the recoverability of capitalized inventories during each reporting period and writes down any excess and obsolete inventory to its net realizable value in the period in which the impairment is first identified. Such impairment charges, should they occur, are recorded as a component of cost of sales in the consolidated statements of operations and comprehensive loss. The determination of whether inventory costs will be realizable requires the use of estimates by management. If actual market conditions are less favorable than projected by management, additional write-downs of inventory may be required. There were no expenses recorded for excess inventory or other impairments during the three and nine months ended September 30, 2023. There was no inventory held by the Company during the three and nine months ended September 30, 2022.
Debt issuance costs and debt discount		Debt issuance costs and debt discount Debt issuance costs and debt discounts are presented on the accompanying consolidated balance sheets as a direct reduction from the carrying value of the debt and are amortized to interest expense over the term of the related debt using the effective interest method. See Note G, “Senior Secured Term Loan” for further discussion related to long-term debt.
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, par value $.01 per share, underlying pre-funded warrants are included in the calculation of basic and diluted earnings per share. Shares of the Company’s Series A Convertible Preferred 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 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 common shares and participating securities (the two-class method). 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 and restricted stock units that are outstanding during the period, except where such non-participating securities would be antidilutive. The dilutive effect of participating securities is calculated using the more dilutive of either (i) the treasury stock method (for stock options and restricted stock units) and “if-converted” method (for Series A Convertible Preferred Stock) or (ii) the two-class method assuming the Series A Convertible Preferred Stock is not converted and applying the treasury stock method (for stock options and restricted stock units). The following table sets forth the computation of basic and diluted earnings per share for the three months ended September 30, 2023. There was a net loss in all other periods presented, and as such, no loss was allocated to participating securities pursuant to the two class method for those periods. ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ September 30, ​ ​ 2023 Numerator: ​ ​ ​ Net income ​ $ 30,748 Allocation of earnings to participating securities ​ ​ (2,276) Numerator for basic EPS — income available to common stockholders (A) ​ $ 28,472 Effect of dilutive securities: ​ ​ ​ Add back allocation of earnings to participating securities ​ $ 2,276 Reallocation of earnings to participating securities considering potentially dilutive securities ​ ​ (2,171) Numerator for diluted EPS — income available to common stockholders (C) ​ $ 28,577 Denominator: ​ ​ ​ Denominator for basic EPS — weighted average shares (B) ​ ​ 273,341 Effect of dilutive securities: ​ ​ ​ Common stock equivalents ​ ​ 14,249 Denominator for diluted EPS — adjusted weighted average shares (D) ​ ​ 287,590 ​ ​ ​ ​ Basic EPS (A / B) ​ $ 0.10 Diluted EPS (C / D) ​ $ 0.10 The Company’s common stock equivalents, as calculated in accordance with the treasury-stock method for options and unvested restricted stock units and the if-converted method for the Series A Convertible Preferred Stock, are shown in the following table (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ September 30, ​ September 30, ​ 2023 2022 2023 2022 Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock units at end of period ​ 29,927 ​ 31,479 ​ 29,927 ​ 31,479 Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock units ​ 14,249 2,246 ​ 10,671 ​ 1,437 Common stock equivalents under if-converted method for Series A Convertible Preferred Stock ​ 21,853 ​ — ​ 21,853 ​ —
Stock-Based Compensation		Stock-Based Compensation As of September 30, 2023, 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 15, 2022, 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 13,000,000 shares of the Company’s common stock, as well as up to 28,742,013 shares of common stock, which represent the number of shares of common stock remaining under the 2018 Plan as of April 1, 2022, 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 April 1, 2022. 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 13,500,000 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, ​ 2023 ​ 2022 ​ 2023 ​ 2022 Dividend ​ None ​ None ​ None ​ None Volatility ​ 88.4% ​ 83.3% ​ 83.4% ​ 83.2% Risk-free interest rate ​ 4.52% ​ 3.44% ​ 3.78% ​ 2.48% Expected life (years) ​ 5.6 ​ 5.6 ​ 5.7 ​ 5.9 ​ Using the Black-Scholes option-pricing model, the weighted-average grant date fair values of options granted during the three months ended September 30, 2023 and 2022 were $11.52 and $3.62 per share, respectively, and $4.85 and $3.58 for options granted during the nine months ended September 30, 2023 and 2022, respectively. A summary of option activity under the Company’s equity plans for the nine months ended September 30, 2023 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ Number ​ Average ​ ​ of Stock ​ Exercise ​ ​ Options ​ Price Outstanding at December 31, 2022 ​ 33,126 ​ $ 5.76 Granted ​ 6,316 ​ ​ 6.71 Exercised ​ (9,121) ​ ​ 5.05 Forfeited/Canceled ​ (2,653) ​ ​ 6.30 Outstanding at September 30, 2023 ​ 27,668 ​ $ 6.15 ​ In 2020, the Company issued 2.6 million performance-based stock options to certain employees with vesting conditioned upon the achievement of specified performance goals. In 2022, 75% of the 2.6 million performance-based stock options vested upon achievement of specified performance goals and 12.5% were forfeited. There was no stock-based compensation recorded during the three or nine months ended September 30, 2023 related to these stock options. The fair value of the remaining unvested performance-based stock options that could be expensed in future periods is $1.3 million. A summary of restricted stock unit activity under the Company’s equity plans for the nine months ended September 30, 2023 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted- ​ ​ Restricted ​ Average Grant ​ ​ Stock Shares ​ Date Fair Value Unvested at December 31, 2022 ​ 138 ​ $ 5.45 Granted ​ 2,380 ​ ​ 6.32 Forfeited ​ (259) ​ ​ 4.66 Unvested at September 30, 2023 ​ 2,259 ​ $ 7.11 ​ In June 2018, the Company's Board of Directors, with shareholder approval, adopted the Employee Stock Purchase Plan (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. ESPP purchase periods are six months and begin on January 1 and July 1 of each year, with purchase dates occurring on the final business day of the given purchase period. 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 unit awards granted under the stock plans and the ESPP was $7.8 million and $22.0 million during the three and nine months ended September 30, 2023, respectively, compared to $5.3 million and $14.3 million for the three and nine months ended September 30, 2022, respectively. The increase in stock compensation expense is primarily due to significant growth in personnel in the second half of 2022. As of September 30, 2023, the estimated fair value of unvested employee awards was $72.0 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 months ended September 30, 2023, 93% of revenues were generated from net U.S. sales of ELAHERE to four specialty distributors and specialty pharmacy providers, and 7% of revenues were generated from an agreement with Roche, compared to 52%, 30% and 18% of revenues from Roche, Lilly and Novartis, respectively, during the three months ended September 30, 2022. During the nine months ended September 30, 2023, 86% of revenues were generated from net U.S. sales of ELAHERE to four specialty distributors and specialty pharmacy providers, and 7% and 6% of revenues were generated from agreements with Roche and Vertex, respectively, compared to 43%, 32% and 20% from agreements with Huadong, Roche, and Lilly, respectively, during the nine months ended September 30, 2022. There were no other customers of the Company that generated significant revenues in the three and nine months ended September 30, 2023 and 2022.
Pending Accounting Pronouncements	Recently Adopted Accounting Pronouncements There were no recently issued or effective FASB Accounting Standards Updates (ASUs) that had, or are expected to have, a material effect on the Company's results of operations, financial condition, or liquidity.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2023
Summary of 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, 2023 and 2022 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ December 31, 2022 Additions ​ Deductions ​ Impact of Netting ​ September 30, 2023 Contract liabilities (deferred revenue) ​ $ 50,211 ​ $ 23,227 ​ $ (9,534) ​ $ — ​ $ 63,904 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ December 31, 2021 ​ Additions ​ Deductions ​ Impact of Netting ​ September 30, 2022 Contract asset ​ $ 3,000 ​ $ — ​ $ (3,000) ​ $ — ​ $ — Contract liabilities (deferred revenue) ​ $ 92,068 ​ $ 5,704 ​ $ (43,961) ​ $ — ​ $ 53,811 ​ 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, ​ ​ 2023 ​ 2022 ​ 2023 ​ 2022 Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 3,929 ​ $ 7,337 ​ $ 9,534 ​ $ 43,961
Schedule of common stock equivalents, as calculated in accordance with the treasury-stock method	The following table sets forth the computation of basic and diluted earnings per share for the three months ended September 30, 2023. There was a net loss in all other periods presented, and as such, no loss was allocated to participating securities pursuant to the two class method for those periods. ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ September 30, ​ ​ 2023 Numerator: ​ ​ ​ Net income ​ $ 30,748 Allocation of earnings to participating securities ​ ​ (2,276) Numerator for basic EPS — income available to common stockholders (A) ​ $ 28,472 Effect of dilutive securities: ​ ​ ​ Add back allocation of earnings to participating securities ​ $ 2,276 Reallocation of earnings to participating securities considering potentially dilutive securities ​ ​ (2,171) Numerator for diluted EPS — income available to common stockholders (C) ​ $ 28,577 Denominator: ​ ​ ​ Denominator for basic EPS — weighted average shares (B) ​ ​ 273,341 Effect of dilutive securities: ​ ​ ​ Common stock equivalents ​ ​ 14,249 Denominator for diluted EPS — adjusted weighted average shares (D) ​ ​ 287,590 ​ ​ ​ ​ Basic EPS (A / B) ​ $ 0.10 Diluted EPS (C / D) ​ $ 0.10 The Company’s common stock equivalents, as calculated in accordance with the treasury-stock method for options and unvested restricted stock units and the if-converted method for the Series A Convertible Preferred Stock, are shown in the following table (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ September 30, ​ September 30, ​ 2023 2022 2023 2022 Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock units at end of period ​ 29,927 ​ 31,479 ​ 29,927 ​ 31,479 Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock units ​ 14,249 2,246 ​ 10,671 ​ 1,437 Common stock equivalents under if-converted method for Series A Convertible Preferred Stock ​ 21,853 ​ — ​ 21,853 ​ —
Schedule of risk-free rate of the stock options based on US Treasury rate	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ 2023 ​ 2022 ​ 2023 ​ 2022 Dividend ​ None ​ None ​ None ​ None Volatility ​ 88.4% ​ 83.3% ​ 83.4% ​ 83.2% Risk-free interest rate ​ 4.52% ​ 3.44% ​ 3.78% ​ 2.48% Expected life (years) ​ 5.6 ​ 5.6 ​ 5.7 ​ 5.9
Summary of stock option activity	A summary of option activity under the Company’s equity plans for the nine months ended September 30, 2023 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ Number ​ Average ​ ​ of Stock ​ Exercise ​ ​ Options ​ Price Outstanding at December 31, 2022 ​ 33,126 ​ $ 5.76 Granted ​ 6,316 ​ ​ 6.71 Exercised ​ (9,121) ​ ​ 5.05 Forfeited/Canceled ​ (2,653) ​ ​ 6.30 Outstanding at September 30, 2023 ​ 27,668 ​ $ 6.15
Summary of restricted stock activity	A summary of restricted stock unit activity under the Company’s equity plans for the nine months ended September 30, 2023 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted- ​ ​ Restricted ​ Average Grant ​ ​ Stock Shares ​ Date Fair Value Unvested at December 31, 2022 ​ 138 ​ $ 5.45 Granted ​ 2,380 ​ ​ 6.32 Forfeited ​ (259) ​ ​ 4.66 Unvested at September 30, 2023 ​ 2,259 ​ $ 7.11

Product Revenue Reserves and _2

Product Revenue Reserves and Allowances (Tables)	9 Months Ended
	Sep. 30, 2023
Product Revenue Reserves and Allowances
Schedule of product revenue allowance and reserve categories	​ The following table summarizes activity in each of the product revenue reserve and allowance categories as of September 30, 2023 and 2022, respectively. (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ September 30, ​ 2023 2022 Beginning balance at January 1 ​ $ 313 ​ $ — Provision related to sales in the current period ​ ​ 36,012 ​ ​ — Credits and payments made ​ ​ (28,223) ​ ​ — Ending balance at September 30 ​ $ 8,102 ​ $ — ​

Inventory (Tables)

Inventory (Tables)	9 Months Ended
	Sep. 30, 2023
Inventory
Schedule of capitalized inventory	Capitalized inventory consists of the following at September 30, 2023 and December 31, 2022 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ December 31, ​ 2023 2022 Raw materials ​ $ 28,555 ​ $ 15,952 Work in process ​ ​ 2,839 ​ ​ — Finished goods ​ ​ 2,374 ​ ​ 244 Total inventory ​ $ 33,768 ​ $ 16,196 ​

Liability Related to Sale of _2

Liability Related to Sale of Future Royalties (Tables)	9 Months Ended
	Sep. 30, 2023
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, 2023 (in thousands): ​ ​ ​ ​ ​ ​ ​ Nine Months Ended ​ ​ September 30, 2023 Liability related to sale of future royalties, net — beginning balance ​ $ 32,108 Proceeds from sale of future royalties, net ​ — KADCYLA royalty payments received and paid ​ (8,907) Non-cash interest expense recognized ​ ​ 2,691 Liability related to sale of future royalties, net — ending balance ​ $ 25,892 ​

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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Apr. 06, 2023	Dec. 31, 2022
Net loss			$ (14,514)	$ (163,921)
Accumulated deficit	$ (1,708,586)		(1,708,586)			$ (1,694,072)
Total revenues	113,425	$ 15,375	246,447	$ 67,615
Cash and cash equivalents	605,535		605,535			$ 275,138
Senior Secured Term Loan, Tranche One [Member]
Senior secured term loan					$ 75,000
Secured Debt [Member]
Senior secured term loan					175,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche One [Member]
Senior secured term loan	75,000		75,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche Two [Member] | Minimum
Senior secured term loan	$ 50,000		$ 50,000		50,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche Two [Member] | Maximum
Senior secured term loan					$ 100,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Significant Account Policies (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Apr. 06, 2023
Senior Secured Term Loan, Tranche One [Member]
Debt Instrument [Line Items]
Senior secured term loan		$ 75,000
Secured Debt [Member]
Debt Instrument [Line Items]
Senior secured term loan		175,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche One [Member]
Debt Instrument [Line Items]
Senior secured term loan	$ 75,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche Two [Member] | Minimum
Debt Instrument [Line Items]
Senior secured term loan	$ 50,000	50,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche Two [Member] | Maximum
Debt Instrument [Line Items]
Senior secured term loan		$ 100,000

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Revenue Recognition (Details)	1 Months Ended
	Aug. 31, 2023
Takeda
Summary of Significant Accounting Policies
Term of agreement	1 year 6 months

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Performance Obligations (Details) $ in Millions	Sep. 30, 2023 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 63.9
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	0.58%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	0.41%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2032-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	0.01%
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-07-01
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]: 2027-07-01
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]: 2032-07-01
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]: 2027-07-01
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]: 2032-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	120 months

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Contract Balances (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Changes in the Company's contract assets and contract liabilities
Contract asset, Beginning balance		$ 3,000
Contract asset, Additions		0
Contract asset, Deductions		3,000
Contract liabilities:
Contract liabilities (deferred revenue), Beginning balance	$ 50,211	92,068
Contract liabilities (deferred revenue), Additions	23,227	5,704
Contract liabilities (deferred revenue), Deductions	(9,534)	(43,961)
Contract liabilities (deferred revenue), Ending balance	$ 63,904	$ 53,811

Summary of Significant Accoun_8

Summary of 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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenue recognized in the period from:
Amounts included in contract liabilities at the beginning of the period	$ 3,929	$ 7,337	$ 9,534	$ 43,961

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Contract Balances from Contracts with Customers - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Aug. 31, 2023	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Feb. 28, 2023	Dec. 31, 2022	Feb. 28, 2022	Dec. 31, 2021
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract with customer liability		$ 63,904	$ 53,811	$ 63,904	$ 53,811		$ 50,211		$ 92,068
Revenue from contract with customer		113,425	15,375	246,447	67,615
Revenue recognized, previously deferred		3,929	7,337	9,534	43,961
Current portion of deferred revenue		37,186		37,186			13,856
Deferred revenue, net of current portion		26,718		26,718			$ 36,355
License and milestone fees
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from contract with customer		51	7,382	15,122	45,247
Future Technological Improvements
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from contract with customer				100
Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd | License and milestone fees
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue recognized, previously deferred					28,500
Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd | Upfront payment
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
License agreement upfront payment receivable			45,000		45,000
Lilly
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
License agreement upfront payment receivable								$ 13,000
Lilly | License and milestone fees
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from contract with customer					13,800
Lilly | Upfront payment
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
License agreement upfront payment receivable			$ 19,500		19,500
KADCYLA | License and milestone fees
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue recognized, previously deferred					2,900
KADCYLA | Royalty revenue
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net proceeds from sale of residual rights to receive royalty payments				9,400
Revenue recognized, previously deferred					12,500
Novartis | License and milestone fees
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue recognized, previously deferred					$ 2,800
Vertex | License and milestone fees
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from contract with customer				15,000
Vertex | Upfront payment
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
License agreement upfront payment receivable						$ 15,000
Revenue from contract with customer				15,000
Takeda
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from contract with customer		0
Takeda | Upfront payment
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from contract with customer	$ 23,200
Deferred revenue, net of current portion		$ 23,200		$ 23,200

Summary of Significant Accou_10

Summary of Significant Accounting Policies - Financial Instruments and Concentration of Credit Risk (Details)	9 Months Ended
	Sep. 30, 2023 USD ($) item	Dec. 31, 2022 USD ($)
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

Summary of Significant Accou_11

Summary of Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Summary of Significant Accounting Policies
Cash and cash equivalents	$ 605,535	$ 275,138

Summary of Significant Accou_12

Summary of Significant Accounting Policies - Non-cash Investing and Financing Activities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Sep. 30, 2023
Summary of Significant Accounting Policies
Accrued capital expenditures	$ 0.3
Gross carrying amount		$ 75

Summary of Significant Accou_13

Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Details) $ / shares in Units, $ in Millions	Sep. 30, 2023 USD ($) $ / shares
Summary of Significant Accounting Policies
Estimated fair value	$ 79.7
Gross carrying amount	$ 75
Common stock, par value (in dollars per share) | $ / shares	$ 0.01

Summary of Significant Accou_14

Summary of Significant Accounting Policies - Accounts Receivable (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Summary of Significant Accounting Policies
Expected credit losses	$ 8,102	$ 313

Summary of Significant Accou_15

Summary of Significant Accounting Policies - Inventory (Details) - USD ($)	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Summary of Significant Accounting Policies
Inventory	$ 5,495,000	$ 0
Inventory impairment	$ 0

Summary of Significant Accou_16

Summary of Significant Accounting Policies - Computation of Net Loss per Common Share (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
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	29,927	31,479	29,927	31,479
Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock/units	14,249	2,246	10,671	1,437
Common stock equivalents under if-converted method for Series A Convertible Preferred Stock	21,853		21,853

Basis of Presentation and Signi

Basis of Presentation and Significant Accounting Policies - Schedule of two class method (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended							9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Earnings Per Share [Abstract]
Net loss	$ 30,748	$ (4,248)	$ (41,014)	$ (59,008)	$ (77,755)	$ (62,021)	$ (24,145)	$ (14,514)	$ (163,921)
Allocation of earnings to participating securities	(2,276)
Numerator for basic EPS - income available to common stockholders (A)	28,472
Add back allocation of earnings to participating securities	2,276
Reallocation of earnings to participating securities considering potentially dilutive securities	(2,171)
Numerator for diluted EPS - income available to common stockholders (C)	$ 28,577
Denominator for basic EPS - weighted average shares (B)	273,341				253,511			265,265	253,371
Common stock equivalents	14,249				2,246			10,671	1,437
Denominator for diluted EPS - adjusted weighted average shares (D)	287,590				253,511			265,265	253,371
Basic EPS (A/B)	$ 0.10				$ (0.31)			$ (0.05)	$ (0.65)
Diluted EPS (C/D)	$ 0.10				$ (0.31)			$ (0.05)	$ (0.65)

Summary of Significant Accou_17

Summary of Significant Accounting Policies - Segment Information (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023 item	Sep. 30, 2022	Sep. 30, 2023 item segment	Sep. 30, 2022	Dec. 31, 2022 segment
Segment Information
Number of operating segments | segment			1		1
Other customers | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized	0%	0%	0%	0%
Specialty distributors and pharmacy providers | Revenue | Customer concentration
Segment Information
Number of operating segments | item	4		4
Percentages of revenue recognized	93%		86%
Roche | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized	7%	52%	7%	32%
Huadong | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized				43%
Lilly | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized	30%			20%
Novartis | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized	18%
Vertex Pharmaceuticals Incorporated [Member] | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized			6%

Summary of Significant Accou_18

Summary of Significant Accounting Policies - Stock-Based Compensation (Details) $ / shares in Units, $ in Thousands	3 Months Ended							9 Months Ended		12 Months Ended
	Sep. 30, 2023 USD ($) $ / shares shares	Jun. 30, 2023 USD ($) shares	Mar. 31, 2023 USD ($) $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Sep. 30, 2022 USD ($) $ / shares	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Sep. 30, 2023 USD ($) plan $ / shares shares	Sep. 30, 2022 USD ($) $ / shares	Dec. 31, 2022 $ / shares shares	Dec. 31, 2020 shares	Jun. 30, 2018 shares
Stock-Based Compensation
Number of employee share-based compensation plans | plan								3
Share-based Compensation Arrangement by Share-based Payment Award		0
Weighted-average assumptions used to estimate the fair value of each stock option
Dividend (as a percent)								0%
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Directors' deferred share unit compensation | $	$ 67	$ 152	$ 151	$ 146	$ 146	$ 213	$ 211
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
Stock options and restricted stock awards
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Stock compensation expense | $	7,800				$ 5,300			$ 22,000	$ 14,300
Estimated fair value that could be expensed | $	$ 72,000							$ 72,000
Stock options
Weighted-average assumptions used to estimate the fair value of each stock option
Dividend (as a percent)	0%				0%			0%	0%
Volatility (as a percent)	88.40%				83.30%			83.40%	83.20%
Risk-free interest rate (as a percent)	4.52%				3.44%			3.78%	2.48%
Expected life	5 years 7 months 6 days				5 years 7 months 6 days			5 years 8 months 12 days	5 years 10 months 24 days
Weighted-average grant date fair value (in dollars per share) | $ / shares	$ 11.52				$ 3.62			$ 4.85	$ 3.58
Number of Stock Options
Outstanding at the beginning of the period (in shares)			33,126,000					33,126,000
Granted (in shares)								6,316,000
Exercised (in shares)								(9,121,000)
Forfeited/Canceled (in shares)								(2,653,000)
Outstanding at the end of the period (in shares)	27,668,000			33,126,000				27,668,000		33,126,000
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares			$ 5.76					$ 5.76
Granted (in dollars per share) | $ / shares								6.71
Exercised (in dollars per share) | $ / shares								5.05
Forfeited/Canceled (in dollars per share) | $ / shares								6.30
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 6.15			$ 5.76				$ 6.15		$ 5.76
Performance shares
Stock-Based Compensation
Vesting percentage										75%
Forfeited percentage										12.50%
Number of Stock Options
Granted (in shares)										2,600,000	2,600,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Estimated fair value that could be expensed | $	$ 1,300							$ 1,300
Restricted stock
Number of Restricted Stock Shares
Unvested at the beginning of the period (in shares)			138,000					138,000
Granted (in shares)								2,380,000
Forfeited (in shares)								(259,000)
Unvested at the end of the period (in shares)	2,259,000			138,000				2,259,000		138,000
Weighted-Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share) | $ / shares			$ 5.45					$ 5.45
Granted (in dollars per share) | $ / shares								6.32
Forfeited (in dollars per share) | $ / shares								4.66
Unvested at the end of the period (in dollars per share) | $ / shares	$ 7.11			$ 5.45				$ 7.11		$ 5.45
Immunogen Inc Restated Stock Option Plan
Stock-Based Compensation
Common stock authorized for issuance (in shares)	28,742,013							28,742,013
2018 Plan
Stock-Based Compensation
Common stock authorized for issuance (in shares)	13,000,000							13,000,000
Inducement Plan
Stock-Based Compensation
Number of employee share-based compensation plans | plan								2
Common stock authorized for issuance (in shares)	13,500,000							13,500,000
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

Collaboration and License Agr_2

Collaboration and License Agreements - Roche (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Collaborative Agreements disclosures
Revenue from contract with customer	$ 113,425	$ 15,375	$ 246,447	$ 67,615
Non-cash royalty revenue related to sale of future royalties			(8,494)	(9,027)
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer	$ 51	$ 7,382	$ 15,122	45,247
Roche
Collaborative Agreements disclosures
Period in arrears to receive royalty reports and payments related to sales of kadcyla			3 months
Roche | KADCYLA
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties			$ (17,900)	$ (21,500)

Collaboration and License Agr_3

Collaboration and License Agreements - Novartis (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Aug. 31, 2022	Dec. 31, 2021
Collaborative Agreements disclosures
Revenue from contract with customer	$ 113,425	$ 15,375	$ 246,447	$ 67,615
Contract with customer liability	63,904	53,811	63,904	53,811	$ 50,211		$ 92,068
Revenue recognized, previously deferred	3,929	7,337	9,534	43,961
Milestone payments plus royalties on the commercial sales | Novartis
Collaborative Agreements disclosures
Potential milestone payment	199,500		199,500
Revenue recognized, previously deferred			5,000
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer	51	$ 7,382	15,122	45,247
License and milestone fees | Novartis
Collaborative Agreements disclosures
Deferred revenue	700		700
Revenue recognized, previously deferred				2,800
Research and development support
Collaborative Agreements disclosures
Revenue from contract with customer	$ 855		$ 1,310	831
Research and development support | Novartis
Collaborative Agreements disclosures
Revenue from contract with customer				$ 2,800
Deferred revenue						$ 2,800

Collaboration and License Agr_4

Collaboration and License Agreements - Viridian (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Collaborative Agreements disclosures
Revenue from contract with customer	$ 113,425	$ 15,375	$ 246,447	$ 67,615
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer	$ 51	$ 7,382	$ 15,122	$ 45,247

Collaboration and License Agr_5

Collaboration and License Agreements - Huadong (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 31, 2020	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2021
Collaborative Agreements disclosures
Revenue from contract with customer		$ 113,425	$ 15,375	$ 246,447	$ 67,615
Revenue recognized, previously deferred		3,929	7,337	9,534	43,961
Upfront payment | Huadong
Collaborative Agreements disclosures
Revenue from contract with customer	$ 40,000
Upfront payment | Huadong | Development milestones
Collaborative Agreements disclosures
Revenue recognized, previously deferred						$ 45,000
Deferred revenue			28,500		28,500
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer		$ 51	$ 7,382	15,122	$ 45,247
License and milestone fees | Huadong | Development milestones
Collaborative Agreements disclosures
Revenue from contract with customer				$ 15,000
License and milestone fees | Huadong | Sales milestones
Collaborative Agreements disclosures
Potential milestone payment	$ 265,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-07-01 | License and milestone fees | Huadong | Development milestones
Collaborative Agreements disclosures
Remaining performance obligation, expected timing of satisfaction		2 years		2 years

Collaboration and License Agr_6

Collaboration and License Agreements - Takeda (Details) $ in Thousands, ¥ in Billions	1 Months Ended	3 Months Ended		9 Months Ended
	Aug. 31, 2023 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Aug. 25, 2023 JPY (¥)
Collaborative Agreements disclosures
Revenue from contract with customer		$ 113,425	$ 15,375	$ 246,447	$ 67,615
Takeda
Collaborative Agreements disclosures
Term of agreement	1 year 6 months
Revenue from contract with customer		0
Takeda | Sales milestones
Collaborative Agreements disclosures
Potential milestone payment	$ 135,000					¥ 19.9
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer		51	$ 7,382	15,122	45,247
Upfront payment | Takeda
Collaborative Agreements disclosures
Revenue from contract with customer	$ 23,200
Research and development support
Collaborative Agreements disclosures
Revenue from contract with customer		$ 855		$ 1,310	$ 831

Collaboration and License Agr_7

Collaboration and License Agreements - Magenta (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer	$ 113,425	$ 15,375	$ 246,447	$ 67,615
License and milestone fees
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer	$ 51	$ 7,382	$ 15,122	$ 45,247

Collaboration and License Agr_8

Collaboration and License Agreements - Lilly (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Feb. 28, 2022
Collaborative Agreements disclosures
Aggregate amount of transaction price allocated to remaining performance obligations	$ 63,900		$ 63,900
Revenue from contract with customer	113,425	$ 15,375	246,447	$ 67,615
Amount of obligation included in long-term deferred revenue	26,718		26,718		$ 36,355
Lilly
Collaborative Agreements disclosures
License agreement upfront payment receivable						$ 13,000
License agreement additional payment receivable						19,500
License agreement, target selection fees and development, regulatory and commercial milestone payments receivable						$ 1,700,000
Lilly | Upfront payment
Collaborative Agreements disclosures
License agreement upfront payment receivable		$ 19,500		19,500
Lilly | Initial targets
Collaborative Agreements disclosures
Revenue from contract with customer				$ 13,800	18,400
Lilly | Additional targets
Collaborative Agreements disclosures
License agreement additional payment receivable					$ 13,000
Lilly | Material rights to obtain licenses to replacement targets
Collaborative Agreements disclosures
Amount of obligation included in long-term deferred revenue	$ 7,600		$ 7,600

Collaboration and License Agr_9

Collaboration and License Agreements - Vertex (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Feb. 28, 2023	Dec. 31, 2022
Collaborative Agreements disclosures
Aggregate amount of transaction price allocated to remaining performance obligations	$ 63,900		$ 63,900
Revenue from contract with customer	113,425	$ 15,375	246,447	$ 67,615
Amount of obligation included in long-term deferred revenue	26,718		26,718			$ 36,355
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer	$ 51	$ 7,382	15,122	$ 45,247
Vertex | Upfront payment
Collaborative Agreements disclosures
License agreement upfront payment receivable					$ 15,000
License agreement, target selection fees and development, regulatory and commercial milestone payments receivable					$ 337,000
Revenue from contract with customer			15,000
Vertex | License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer			$ 15,000

Product Revenue Reserves and _3

Product Revenue Reserves and Allowances (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Product Revenue Reserves and Allowances
Beginning balance at January 1			$ 313
Provision related to sales in the current period			36,012
Credits and payments made			(28,223)
Ending balance at December 31	$ 8,102		8,102
Revenue from contract with customer	113,425	$ 15,375	246,447	$ 67,615
Product revenue, net
Product Revenue Reserves and Allowances
Revenue from contract with customer	$ 105,164		$ 212,079

Inventory (Details)

Inventory (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Inventory
Raw materials	$ 28,555	$ 15,952
Work in process	2,839
Finished goods	2,374	244
Total Inventory	$ 33,768	$ 16,196

Liability Related to Sale of _3

Liability Related to Sale of Future Royalties (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended		12 Months Ended
	Jan. 31, 2019	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2015
Liability Related to Sale of Future Royalties
Non-cash royalty revenue related to the sale of future royalties		$ 8,494	$ 9,027
Change in liability related to sale of future royalties
Liability related to sale of future royalties, net - beginning balance		32,108
KADCYLA royalty payments received and paid		(8,907)
Non-cash interest expense recognized		2,691
Liability related to sale of future royalties, net - ending balance		25,892
Kadcyla
Liability Related to Sale of Future Royalties
Percentage of royalty payments if applicable threshold is met				85%
IRH | Kadcyla
Liability Related to Sale of Future Royalties
Percentage of royalty payments				100%
Percentage of royalty payments if applicable threshold is met				15%
Transaction costs for royalty agreements				$ 5,900
Change in liability related to sale of future royalties
Proceeds from sale of future royalties, net		$ 200,000		200,000
Effective annual interest rate		10.50%
Current effective interest rate		12.2
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%
Non-cash royalty revenue related to the sale of future royalties	$ 65,200		12,500
Contingent broker fees	1,500
Net proceeds from sale of residual rights to receive royalty payments	$ 65,200	$ 9,400
Roche
Liability Related to Sale of Future Royalties
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		$ 17,900	$ 21,500

Senior Secured Term Loan (Detai

Senior Secured Term Loan (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended
	Apr. 06, 2023	Sep. 30, 2023	Sep. 30, 2023
Debt Instrument [Line Items]
Lenders fee and deal expenses	$ 3,200
Net proceeds from the term loan	$ 71,800
Interest expense		$ 2,500	$ 4,900
Facility fee			1,000
SOFR
Debt Instrument [Line Items]
Interest rate (as a percent)	2.75%
Plus per annum	8%
Senior Secured Term Loan, Tranche One [Member]
Debt Instrument [Line Items]
Senior secured term loan	$ 75,000
Secured Debt [Member]
Debt Instrument [Line Items]
Senior secured term loan	175,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche One [Member]
Debt Instrument [Line Items]
Senior secured term loan		75,000	75,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche Two [Member] | Minimum
Debt Instrument [Line Items]
Senior secured term loan	50,000	$ 50,000	$ 50,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche Two [Member] | Maximum
Debt Instrument [Line Items]
Senior secured term loan	$ 100,000

Senior Secured Term Loan - Carr

Senior Secured Term Loan - Carrying value of term loan (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Apr. 06, 2023
Debt Instrument [Line Items]
Debt discount and issuance costs, unamortized	$ (2,887)
Term loan, net	72,113
Senior Secured Term Loan, Tranche One [Member]
Debt Instrument [Line Items]
Senior secured term loan		$ 75,000
Secured Debt [Member]
Debt Instrument [Line Items]
Senior secured term loan		$ 175,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche One [Member]
Debt Instrument [Line Items]
Senior secured term loan	$ 75,000

Income Taxes - Loss before prov

Income Taxes - Loss before provision for income taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Reconciliation of the expected statutory tax benefit to the actual income taxes
Loss before income tax expense	$ 29,582	$ (77,755)	$ (14,803)	$ (163,921)

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2023
Income Tax Uncertainties
Capitalize research and development amortization period		5 years
Capitalize research and experimentation expenses, amortization period		15 years
Income tax expense	$ (1.2)	$ 0.8

Capital Stock (Details)

Capital Stock (Details) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended	9 Months Ended
	May 01, 2023 $ / shares shares	Jun. 30, 2023 shares	Sep. 30, 2023 $ / shares shares	Sep. 30, 2023 USD ($) $ / shares shares	Sep. 30, 2022 USD ($) shares	Dec. 31, 2022 shares	Dec. 31, 2021 $ / shares shares	Aug. 11, 2021	Jun. 30, 2018 shares
Stock-based compensation disclosure
Proceeds from issuance of common stock under stock plans | $				$ 47,111	$ 1,480
Common stock, issued shares			265,842,000	265,842,000		226,046,000
Stock received in exchange			21,853,000	21,853,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.01		$ 0.01	$ 0.01
Conversion of stock ratio				1,000
Conversion Blocker			9.99	9.99
Maximum amount of increase or decrease to Conversion Blocker			19.99	19.99
Liquidation preference per share | $ / shares			$ 0.001	$ 0.001
ESPP
Stock-based compensation disclosure
Aggregate number of common shares reserved for future issuance									2,000,000
Series A Convertible Preferred Stock
Stock-based compensation disclosure
Stock exchanged	21,853
Common Stock
Stock-based compensation disclosure
Stock received in exchange	21,853,000
Pre-Funded Warrants
Stock-based compensation disclosure
Warrant exercise price | $ / shares							$ 0.01
Threshold percentage of common stock owned that limits the number of warrants exercised								9.99
Maximum percentage of increase or decrease to amount of common stock owned								19.99
RA Capital Healthcare Fund, L.P. | Pre-Funded Warrants
Stock-based compensation disclosure
Warrants exercised (in shares)				11,000,000
RA Capital Healthcare Fund, L.P. | Pre-Funded Warrants | Common Stock
Stock-based compensation disclosure
Common stock, issued shares			10,992,330	10,992,330
Redmile Group LLC | Pre-Funded Warrants
Stock-based compensation disclosure
Warrants exercised (in shares)		11,357,272
Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares)				6,316,000
Options exercised (in shares)				9,121,000
Securities Purchase Agreement | RA Capital Healthcare Fund, L.P. | Pre-Funded Warrants
Stock-based compensation disclosure
Pre-Funded warrants issued to purchase shares			21,434,872	21,434,872			21,434,782
Securities Purchase Agreement | Redmile Group LLC | Pre-Funded Warrants
Stock-based compensation disclosure
Pre-Funded warrants issued to purchase shares							11,363,636
Compensation Policy for Non-Employee Directors | Common Stock
Stock-based compensation disclosure
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period				14,112
Compensation Policy for Non-Employee Directors | Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares)				157,000	322,000
Compensation Policy for Non-Employee Directors | Restricted stock units
Stock-based compensation disclosure
Stock options granted to directors (in shares)		105,000

Leases (Details)

Leases (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2023 USD ($) ft² lease	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Lessee, Lease, Description [Line Items]
Number of real estate leases | lease	1
Right-of-use assets | $	$ 8,338		$ 10,231
Sublease income | $	$ 2,300	$ 2,400
830 Winter Street, Waltham, MA
Lessee, Lease, Description [Line Items]
Area of space leased | ft²	120,000
Number of executed sub-lease spaces | lease	4
Area of executed sublease space | ft²	65,000

Commitments and Contingencies (

Commitments and Contingencies (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Collaborations and Manufacturing Commitments
Research and development	$ 47,570	$ 59,181	$ 149,267	$ 154,885
In-process and future manufacturing of antibody, drug substance, and cytotoxic agents
Collaborations and Manufacturing Commitments
Noncancelable obligations under several agreements	45,300		45,300
Minimum
Collaborations and Manufacturing Commitments
Manufacturing commitment	$ 47,300		$ 47,300

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended
	May 31, 2023	Sep. 30, 2023	Sep. 30, 2023	Sep. 30, 2022	May 01, 2023
Related Party Transaction
Stock received in exchange		21,853,000	21,853,000
Preferred stock, par value (in dollars per share)		$ 0.01	$ 0.01		$ 0.01
Common stock, par value (in dollars per share)		$ 0.01	$ 0.01
Related Party [Member] | RA Capital Healthcare Fund, L.P.
Related Party Transaction
Stock received in exchange	21,853,000
Stock exchanged	21,853
Related Party [Member] | Ergomed PLC [Member]
Related Party Transaction
Payments to related party			$ 4.8	$ 3.9
Related Party [Member] | PrimeVigilance USA, Inc [Member]
Related Party Transaction
Payments to related party			$ 1.1	$ 0.2

Subsequent Events (Details)

Subsequent Events (Details) - USD ($) $ in Thousands	Apr. 06, 2023	Sep. 30, 2023
SOFR
Subsequent Event [Line Items]
Interest rate (as a percent)	2.75%
Plus per annum	8%
Senior Secured Term Loan, Tranche One [Member]
Subsequent Event [Line Items]
Senior secured term loan	$ 75,000
Secured Debt [Member]
Subsequent Event [Line Items]
Senior secured term loan	175,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche One [Member]
Subsequent Event [Line Items]
Senior secured term loan		$ 75,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche Two [Member] | Minimum
Subsequent Event [Line Items]
Senior secured term loan	50,000	$ 50,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche Two [Member] | Maximum
Subsequent Event [Line Items]
Senior secured term loan	$ 100,000

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Thousands	3 Months Ended							9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Pay vs Performance Disclosure
Net Income (Loss)	$ 30,748	$ (4,248)	$ (41,014)	$ (59,008)	$ (77,755)	$ (62,021)	$ (24,145)	$ (14,514)	$ (163,921)

Insider Trading Arrangements

Insider Trading Arrangements - shares				3 Months Ended
	Aug. 10, 2023	Aug. 04, 2023	Jun. 08, 2023	Sep. 30, 2023
Rule10b5-1 Trading Plan | Anna Berkenblit
Trading Arrangements, by Individual
Name				Dr. Anna Berkenblit
Title				Senior Vice President, Chief Medical Officer
Adoption Date			June 8, 2023
Rule 10b5-1 Arrangement Terminated		true
Termination Date				August 4, 2023
Rule10b5-1 Trading Plan | Stacy A Coen
Trading Arrangements, by Individual
Name				Ms. Stacy A. Coen
Title				Senior Vice President, Chief Business Officer
Rule 10b5-1 Arrangement Adopted				true
Adoption Date	August 10, 2023
Aggregate Available				219,185
Separate Rule10b5-1 Trading Plan | Anna Berkenblit
Trading Arrangements, by Individual
Name				Dr. Anna Berkenblit
Adoption Date			June 8, 2023
Rule 10b5-1 Arrangement Terminated		true
Termination Date		August 4, 2023
Aggregate Available				1,172,876