Immunogen (IMGN) 10-Q 28 Apr 23 2023 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2023	Apr. 21, 2023
Document and Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Mar. 31, 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		226,070,419
Entity Central Index Key	0000855654
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q1
Amendment Flag	false

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($)	Mar. 31, 2023	Dec. 31, 2022
ASSETS
Cash and cash equivalents	$ 201,249,000	$ 275,138,000
Accounts receivable	27,342,000	12,596,000
Unbilled receivable	1,249,000	1,531,000
Non-cash royalty receivable	2,455,000	3,851,000
Inventory	614,000
Prepaid and other current assets	10,955,000	11,005,000
Total current assets	243,864,000	304,121,000
Property and equipment, net of accumulated depreciation	4,067,000	4,377,000
Operating lease right-of-use assets	9,627,000	10,231,000
Long-term inventory	16,291,000	16,196,000
Other assets	14,496,000	14,011,000
Total assets	288,345,000	348,936,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Accounts payable	32,260,000	45,353,000
Accrued compensation	11,780,000	11,111,000
Other accrued liabilities	30,918,000	38,783,000
Current portion of liability related to the sale of future royalties, net of deferred financing costs of $150 and $162, respectively	9,014,000	8,659,000
Current portion of operating lease liability	4,213,000	4,096,000
Current portion of deferred revenue	13,444,000	13,856,000
Total current liabilities	101,629,000	121,858,000
Deferred revenue, net of current portion	34,055,000	36,355,000
Operating lease liability, net of current portion	10,049,000	11,148,000
Liability related to the sale of future royalties, net of current portion and deferred financing costs of $172 and $205, respectively	20,394,000	23,449,000
Other long-term liabilities	300,000	300,000
Total liabilities	166,427,000	193,110,000
Commitments and contingencies (Note L)
Shareholders' equity:
Preferred stock, $.01 par value; authorized 5,000 shares; no shares issued and outstanding as of each of March 31, 2023 and December 31, 2022, respectively
Common stock, $.01 par value; authorized 600,000 shares; 226,070 and 220,046 shares issued and outstanding as of March 31, 2023 and December 31, 2022, respectively	2,261,000	2,260,000
Additional paid-in capital	1,854,743,000	1,847,638,000
Accumulated deficit	(1,735,086,000)	(1,694,072,000)
Total shareholders' equity	121,918,000	155,826,000
Total liabilities and shareholders' equity	$ 288,345,000	$ 348,936,000

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2023	Dec. 31, 2022
CONSOLIDATED BALANCE SHEETS
Sale of future royalties, current portion and deferred financing costs	$ 150	$ 162
Sale of future royalties, noncurrent portion and deferred financing costs	$ 172	$ 205
Preferred stock, par value (in dollars per share)	$ 0.01
Preferred stock, authorized shares	5,000	5,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.01
Common stock, authorized shares	600,000	600,000
Common stock, issued shares	226,070	226,046
Common stock, outstanding shares	226,070	226,046

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Jun. 30, 2022	Jun. 30, 2021
Revenues:
Total revenues	$ 49,869	$ 38,078
Cost and operating expenses:
Cost of sales	626
Research and development	51,620	44,282
Selling, general and administrative	40,016	16,648
Total cost and operating expenses	92,262	60,930
Loss from operations	(42,393)	(22,852)
Investment income, net	2,169	54
Non-cash interest expense on liability related to the sale of future royalties and convertible senior notes	(853)	(1,249)
Other (expense) income, net	63	(98)
Net loss	$ (41,014)	$ (24,145)
Basic net loss per common share	$ (0.16)	$ (0.10)
Diluted net loss per common share			$ (0.34)	$ (0.32)
Basic weighted average common shares outstanding (in shares)	258,848	253,263
Diluted weighted average common shares outstanding (in shares)			253,263	199,365
Total comprehensive loss	$ (41,014)	$ (24,145)
License and milestone fees
Revenues:
Total revenues	15,031	30,892
Non-cash royalty revenue related to the sale of future royalties
Revenues:
Total revenues	4,839	6,428
Research and development support
Revenues:
Total revenues	455	$ 758
Product revenue, net
Revenues:
Total revenues	$ 29,544

CONSOLIDATED STATEMENTS OF SHAR

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	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
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)
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	$ 8	151		151
Balance at Mar. 31, 2023	$ 2,261	$ 1,854,743	$ (1,735,086)	$ 121,918
Balance (in shares) at Mar. 31, 2023	226,070			226,070

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Sep. 30, 2021
Cash flows from operating activities:
Net loss	$ (41,014)	$ (24,145)
Adjustments to reconcile net loss to net cash used for operating activities:
Non-cash royalty revenue related to sale of future royalties	(2,157)	(2,364)
Non-cash interest expense on liability related to sale of future royalties and convertible senior notes	853	1,249
Depreciation and amortization	448	473
Stock and deferred share unit compensation	7,067	4,407
Change in operating assets and liabilities:
Accounts receivable	(14,746)	3,277
Unbilled receivable	282	(1,298)
Inventory	(709)
Prepaid and other current assets	50	(1,485)
Operating lease right-of-use assets	604	504
Other assets	(485)	39
Accounts payable	(12,975)	(2,308)
Accrued compensation	670	(2,061)
Other accrued liabilities	(7,913)	5,023
Deferred revenue	(2,712)	(21,957)
Operating lease liability	(982)	(756)
Net cash used for operating activities	(73,719)	(41,402)
Cash flows from investing activities:
Purchases of property and equipment	(209)	(307)
Net cash used for investing activities	(209)	(307)
Cash flows from financing activities:
Proceeds from issuance of common stock under stock plans	39	620
Net cash provided by financing activities	39	620
Net change in cash and cash equivalents	(73,889)	(41,089)
Cash and cash equivalents, beginning of period		275,138	$ 478,750
Cash and cash equivalents, end of period	$ 201,249	$ 437,661

Nature of Business and Plan of

Nature of Business and Plan of Operations	3 Months Ended
	Mar. 31, 2023
Nature of Business and Plan of Operations
Nature of Business and Plan of Operations	IMMUNOGEN, INC. NOTES TO CONSOLIDATED FINANCIAL STATEMENTS March 31, 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 $41.0 million during the three months ended March 31, 2023, and had an accumulated deficit of approximately $1.7 billion as of March 31, 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, a term loan facility, and commercial sales of ELAHERE. Management expects to continue to generate substantial operating losses for at least the near term as the Company incurs significant operating expenses related to research and development and selling and marketing of ELAHERE. At March 31, 2023, the Company had $201.2 million of cash and cash equivalents on hand. In April 2023, the Company executed a loan agreement with BioPharma Credit PLC, BPCR Limited Partnership, and BioPharma Credit Investments V (Master) LP, which are funds managed by Pharmakon Advisors, LP (collectively, “Pharmakon”), that provides for up to a $175.0 million senior secured term loan consisting of two tranches that each mature on April 6, 2028. The initial tranche of $75.0 million was drawn upon execution of the loan agreement. The second tranche of $50.0 million will be available at the Company’s option upon the achievement of positive top-line data from the Company’s confirmatory MIRASOL trial and a net sales threshold for ELAHERE. This tranche may be increased to $100.0 million upon mutual agreement of the parties. In consideration of the cash received pursuant to the term loan facility, the Company’s current capital resources, and anticipated sales of ELAHERE based on sales to date, the Company has concluded that the factors which previously raised substantial doubt about its ability to continue as a going concern no longer exist as of the issuance date of these financial statements. The Company currently believes that its existing capital resources and cash from anticipated sales of ELAHERE will be sufficient to fund its operational 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, equity or other financings, such as royalty financing transactions, additional debt pursuant to the current term loan facility, 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	3 Months Ended
	Mar. 31, 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 The significant accounting policies used in preparation of these condensed consolidated financial statements for the three months ended March 31, 2023 are consistent with 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 three months ended March 31, 2023 and 2022 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ December 31, 2022 Additions ​ Deductions ​ Impact of Netting ​ March 31, 2023 Contract liabilities (deferred revenue) ​ $ 50,211 ​ $ — ​ $ (2,712) ​ $ — ​ $ 47,499 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ December 31, 2021 ​ Additions ​ Deductions ​ Impact of Netting ​ March 31, 2022 Contract asset ​ $ 3,000 ​ $ — ​ $ — ​ $ — ​ $ 3,000 Contract liabilities (deferred revenue) ​ $ 92,068 ​ $ 3,803 ​ $ (25,760) ​ $ — ​ $ 70,111 ​ 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 ​ ​ March 31, ​ ​ 2023 ​ 2022 Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 2,712 ​ $ 25,760 ​ 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 three months ended March 31, 2023, 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 current period, further details of which can be found in Note C, “Collaboration and License Agreements.” The Company also recognized $2.7 million of previously deferred non-cash royalty revenue related to the sale of rights to KADCYLA During the three months ended March 31, 2022, pursuant to the Company’s license agreement with Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd. (Huadong), upon delivery of clinical materials, the Company recognized as license and milestone fee revenue $21.6 million of the $28.5 million remaining 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 three months ended March 31, 2022, the Company received an upfront payment of $13.0 million, of which $9.2 million was recognized as license and milestone fee revenue and the remainder deferred, further details of which can be found in Note C, “Collaboration and License Agreements.” The Company also recognized $4.1 million of previously deferred non-cash royalty revenue related to the sale of rights to KADCYLA ® royalties, further details of which can be found in Note F, “Liability Related to Sale of Future Royalties,” and recognized $0.1 million of license and milestone fee revenue related to numerous collaborators’ rights to technological improvements that had been previously deferred. 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 March 31, 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 March 31, 2023, and December 31, 2022, the Company held $201.2 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 $0.2 million and $0.3 million of accrued capital expenditures as of March 31, 2023 and December 31, 2022, respectively, which have been treated as a non-cash investing activity and, accordingly, are not reflected in the consolidated statement of cash flows. Fair Value of Financial Instruments Fair value is defined under ASC 820, Fair Value Measurements and Disclosures ● Level 1 - Quoted prices in active markets for identical assets or liabilities. ● Level 2 - Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. ● Level 3 – Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. As of March 31, 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. 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 months ended March 31, 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. Inventory used in clinical trials is also expensed as research and development expense, when selected for such use. 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 months ended March 31, 2023. There was no inventory held by the Company during the three months ended March 31, 2022. Computation of Net Loss per Common Share Basic and diluted net loss per share is calculated based upon the weighted average number of shares of common stock outstanding during the period. Shares of the Company’s common stock underlying pre-funded warrants are included in the calculation of basic and diluted earnings per share. During periods of income, participating securities are allocated a proportional share of income determined by dividing total weighted-average participating securities by the sum of the total weighted average common shares and participating securities (the two-class method). Shares of the Company’s restricted stock participate in any dividends that may be declared by the Company and are therefore considered to be participating securities. Participating securities have the effect of diluting both basic and diluted earnings per share during periods of income. During periods of loss, no loss is allocated to participating securities since they have no contractual obligation to share in the losses of the Company. Diluted loss per share is computed after giving consideration to the dilutive effect of stock options, convertible notes, and restricted stock that are outstanding during the period, except where such non-participating securities would be anti-dilutive. The Company’s common stock equivalents, as calculated in accordance with the treasury-stock method for options and unvested restricted stock are shown in the following table (in thousands): ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ March 31, ​ 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 ​ 39,064 ​ 27,012 Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock/units ​ 1,098 ​ 1,981 ​ The Company’s common stock equivalents have not been included in the net loss per share calculation because their effect is anti-dilutive due to the Company’s net loss position. Stock-Based Compensation As of March 31, 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 March 31, ​ 2023 ​ 2022 Dividend None ​ None Volatility 82.3% ​ 83.0% Risk-free interest rate 3.65% ​ 1.81% Expected life (years) 5.6 ​ 6.0 ​ Using the Black-Scholes option-pricing model, the weighted-average grant date fair values of options granted during the three months ended March 31, 2023 and 2022 were $3.26 and $3.78 per share, respectively. A summary of option activity under the Company’s equity plans for the three months ended March 31, 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 ​ 4,757 ​ ​ 4.62 Exercised ​ (16) ​ ​ 2.31 Forfeited/Canceled ​ (763) ​ ​ 5.60 Outstanding at March 31, 2023 ​ 37,104 ​ $ 5.61 ​ 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 months ended March 31, 2023 and 2022 related to these 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 three months ended March 31, 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 ​ 1,824 ​ ​ 4.65 Forfeited ​ (2) ​ ​ 4.66 Unvested at March 31, 2023 ​ 1,960 ​ $ 4.71 ​ 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 $6.9 million and $4.2 million during the three months ended March 31, 2023 and 2022, respectively. As of March 31, 2023, the estimated fair value of unvested employee awards was $72.4 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 March 31, 2023, 59% of revenues were generated from net U.S. sales of ELAHERE to four specialty distributors and specialty pharmacy providers, and 30% and 10% of revenues were generated from agreements with Vertex and Roche, respectively, compared to 59%, 24% and 17% of revenue from agreements with Huadong, Lilly, and Roche, respectively, during the three months ended March 31, 2022. There were no other customers of the Company that generated significant revenues in the three months ended March 31, 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	3 Months Ended
	Mar. 31, 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. 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 three months ended March 31, 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 $9.2 million was recorded during the three months ended March 31, 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 March 31, 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 tired royalties ranging from low double digits to high teens as a percentage of commercial 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 three months ended March 31, 2022, the Company recorded $21.6 million of 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, $4.8 million and $6.4 million of non-cash royalties on net sales of KADCYLA were recognized and included in non-cash royalty revenue for the three months ended March 31, 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. 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	3 Months Ended
	Mar. 31, 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 $29.5 million from U.S. sales of ELAHERE during the three months ended March 31, 2023. The following table summarizes activity in each of the product revenue reserve and allowance categories for the three months ended March 31, 2023 and 2022 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ March 31, ​ 2023 2022 Beginning balance at January 1 ​ $ 313 ​ $ — Provision related to sales in the current period ​ ​ 4,144 ​ ​ — Credits and payments made ​ ​ (1,533) ​ ​ — Ending balance at March 31 ​ $ 2,924 ​ $ — ​

Inventory

Inventory	3 Months Ended
	Mar. 31, 2023
Inventory
Inventory	E. Inventory Capitalized inventory consists of the following at March 31, 2023 and December 31, 2022 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, ​ December 31, ​ 2023 2022 Raw materials ​ $ 15,983 ​ $ 15,952 Work in process ​ ​ 639 ​ ​ — Finished goods ​ ​ 283 ​ ​ 244 Total Inventory ​ $ 16,905 ​ $ 16,196 ​

Liability Related to Sale of Fu

Liability Related to Sale of Future Royalties	3 Months Ended
	Mar. 31, 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 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, $2.7 million and $4.1 million of revenue related to the residual rights was recorded and is included in non-cash royalty revenue for the three months ended March 31, 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 three-month period ended March 31, 2023 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ March 31, 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 ​ (3,553) ​ Non-cash interest expense recognized ​ ​ 853 ​ Liability related to sale of future royalties, net — ending balance ​ $ 29,408 ​ ​ The Company receives royalty reports and royalty payments related to sales of KADCYLA from Roche one

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2023
Income Taxes
Income Taxes	G. 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. As part of the Tax Cuts and Jobs Act of 2017 (TCJA), 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. As of March 31, 2023, the Company determined a provision for income tax was not required for the calendar year ended December 31, 2023.

Capital Stock

Capital Stock	3 Months Ended
	Mar. 31, 2023
Capital Stock
Capital Stock	H. 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 and Redmile Group, LLC, respectively. The per share exercise price of the pre-funded warrants is $0.01. RA Capital and Redmile Group, LLC are each considered related parties pursuant to ASC 850, 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 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 awards under the Company’s 2004 Non-Employee Director Compensation and Deferred Share Unit Plan, as amended. 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 321,622 and 352,000 options in 2022 and 2021, respectively, and the related compensation expense for the three months ended March 31, 2023 and 2022 is included in the amounts discussed in the “Stock-Based Compensation” section of Note B above.

Leases

Leases	3 Months Ended
	Mar. 31, 2023
Leases
Leases	I. 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	3 Months Ended
	Mar. 31, 2023
Commitments and Contingencies
Commitments and Contingencies	J. Commitments and Contingencies Manufacturing Commitments As of March 31, 2023, the Company had noncancelable obligations under several agreements related to in-process and future manufacturing of antibody, drug substance, and cytotoxic agents required for supply of the Company’s product candidates totaling $22.3 million. Additionally, pursuant to commercial agreements for future production of antibody, our noncancelable commitments total $46.7 million at March 31, 2023. Litigation ​ The Company is not a party to any material litigation.

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2023
Related Party Transactions
Related Party Transactions	K. Related Party Transactions The Company’s chief executive officer has served as a director on the board of directors of Ergomed PLC since June 2021. During the three months ended March 31, 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	3 Months Ended
	Mar. 31, 2023
Subsequent Events
Subsequent Events	L. Subsequent Events The Company has evaluated all events or transactions that occurred after March 31, 2023, up through the date the Company issued these financial statements. On April 6, 2023, the Company entered into an agreement with BioPharma Credit PLC as collateral agent, BPCR Limited Partnership, and BioPharma Credit Investments V (Master) LP, which are funds managed by Pharmakon Advisors, LP (collectively, Pharmakon), as lenders and the guarantors party to the agreement. The loan agreement provides for up to a $175.0 million senior secured term loan consisting of two tranches that each mature on April 6, 2028. The initial tranche of $75.0 million was drawn upon execution of the loan agreement. The second tranche of $50.0 million will be available at the Company’s option upon the achievement of positive top-line data from the Company’s confirmatory MIRASOL trial and a net sales threshold for ELAHERE. This tranche may be increased to $100.0 million upon mutual agreement of the parties. The term loan bears interest at a rate based upon the secured overnight financing rate (SOFR), subject to a SOFR floor of 2.75% per annum, plus 8.00% per annum. Payments will be interest-only for the first 36 months with an extension of 12 months if certain conditions are met, after which ratable principal payments will commence for the remainder of the term. The loan agreement contains customary affirmative and negative covenants for transactions of this type and includes certain customary events of default. If an event of default occurs and is continuing, the Company may be required to repay all amounts outstanding under the loan agreement. 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 did not have any other material subsequent events.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 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 The significant accounting policies used in preparation of these condensed consolidated financial statements for the three months ended March 31, 2023 are consistent with 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 three months ended March 31, 2023 and 2022 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ December 31, 2022 Additions ​ Deductions ​ Impact of Netting ​ March 31, 2023 Contract liabilities (deferred revenue) ​ $ 50,211 ​ $ — ​ $ (2,712) ​ $ — ​ $ 47,499 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ December 31, 2021 ​ Additions ​ Deductions ​ Impact of Netting ​ March 31, 2022 Contract asset ​ $ 3,000 ​ $ — ​ $ — ​ $ — ​ $ 3,000 Contract liabilities (deferred revenue) ​ $ 92,068 ​ $ 3,803 ​ $ (25,760) ​ $ — ​ $ 70,111 ​ 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 ​ ​ March 31, ​ ​ 2023 ​ 2022 Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 2,712 ​ $ 25,760 ​ 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 three months ended March 31, 2023, 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 current period, further details of which can be found in Note C, “Collaboration and License Agreements.” The Company also recognized $2.7 million of previously deferred non-cash royalty revenue related to the sale of rights to KADCYLA During the three months ended March 31, 2022, pursuant to the Company’s license agreement with Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd. (Huadong), upon delivery of clinical materials, the Company recognized as license and milestone fee revenue $21.6 million of the $28.5 million remaining 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 three months ended March 31, 2022, the Company received an upfront payment of $13.0 million, of which $9.2 million was recognized as license and milestone fee revenue and the remainder deferred, further details of which can be found in Note C, “Collaboration and License Agreements.” The Company also recognized $4.1 million of previously deferred non-cash royalty revenue related to the sale of rights to KADCYLA ® royalties, further details of which can be found in Note F, “Liability Related to Sale of Future Royalties,” and recognized $0.1 million of license and milestone fee revenue related to numerous collaborators’ rights to technological improvements that had been previously deferred.
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 March 31, 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 March 31, 2023, and December 31, 2022, the Company held $201.2 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 $0.2 million and $0.3 million of accrued capital expenditures as of March 31, 2023 and December 31, 2022, respectively, which have been treated as a non-cash investing activity and, accordingly, are not reflected in the consolidated statement of cash flows.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Fair value is defined under ASC 820, Fair Value Measurements and Disclosures ● Level 1 - Quoted prices in active markets for identical assets or liabilities. ● Level 2 - Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. ● Level 3 – Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. As of March 31, 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.
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 months ended March 31, 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. Inventory used in clinical trials is also expensed as research and development expense, when selected for such use. 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 months ended March 31, 2023. There was no inventory held by the Company during the three months ended March 31, 2022.
Computation of Net Loss per Common Share	Computation of Net Loss per Common Share Basic and diluted net loss per share is calculated based upon the weighted average number of shares of common stock outstanding during the period. Shares of the Company’s common stock underlying pre-funded warrants are included in the calculation of basic and diluted earnings per share. During periods of income, participating securities are allocated a proportional share of income determined by dividing total weighted-average participating securities by the sum of the total weighted average common shares and participating securities (the two-class method). Shares of the Company’s restricted stock participate in any dividends that may be declared by the Company and are therefore considered to be participating securities. Participating securities have the effect of diluting both basic and diluted earnings per share during periods of income. During periods of loss, no loss is allocated to participating securities since they have no contractual obligation to share in the losses of the Company. Diluted loss per share is computed after giving consideration to the dilutive effect of stock options, convertible notes, and restricted stock that are outstanding during the period, except where such non-participating securities would be anti-dilutive. The Company’s common stock equivalents, as calculated in accordance with the treasury-stock method for options and unvested restricted stock are shown in the following table (in thousands): ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ March 31, ​ 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 ​ 39,064 ​ 27,012 Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock/units ​ 1,098 ​ 1,981 ​ The Company’s common stock equivalents have not been included in the net loss per share calculation because their effect is anti-dilutive due to the Company’s net loss position.
Stock-Based Compensation	Stock-Based Compensation As of March 31, 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 March 31, ​ 2023 ​ 2022 Dividend None ​ None Volatility 82.3% ​ 83.0% Risk-free interest rate 3.65% ​ 1.81% Expected life (years) 5.6 ​ 6.0 ​ Using the Black-Scholes option-pricing model, the weighted-average grant date fair values of options granted during the three months ended March 31, 2023 and 2022 were $3.26 and $3.78 per share, respectively. A summary of option activity under the Company’s equity plans for the three months ended March 31, 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 ​ 4,757 ​ ​ 4.62 Exercised ​ (16) ​ ​ 2.31 Forfeited/Canceled ​ (763) ​ ​ 5.60 Outstanding at March 31, 2023 ​ 37,104 ​ $ 5.61 ​ 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 months ended March 31, 2023 and 2022 related to these 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 three months ended March 31, 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 ​ 1,824 ​ ​ 4.65 Forfeited ​ (2) ​ ​ 4.66 Unvested at March 31, 2023 ​ 1,960 ​ $ 4.71 ​ 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 $6.9 million and $4.2 million during the three months ended March 31, 2023 and 2022, respectively. As of March 31, 2023, the estimated fair value of unvested employee awards was $72.4 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 March 31, 2023, 59% of revenues were generated from net U.S. sales of ELAHERE to four specialty distributors and specialty pharmacy providers, and 30% and 10% of revenues were generated from agreements with Vertex and Roche, respectively, compared to 59%, 24% and 17% of revenue from agreements with Huadong, Lilly, and Roche, respectively, during the three months ended March 31, 2022. There were no other customers of the Company that generated significant revenues in the three months ended March 31, 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)	3 Months Ended
	Mar. 31, 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 three months ended March 31, 2023 and 2022 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ December 31, 2022 Additions ​ Deductions ​ Impact of Netting ​ March 31, 2023 Contract liabilities (deferred revenue) ​ $ 50,211 ​ $ — ​ $ (2,712) ​ $ — ​ $ 47,499 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ December 31, 2021 ​ Additions ​ Deductions ​ Impact of Netting ​ March 31, 2022 Contract asset ​ $ 3,000 ​ $ — ​ $ — ​ $ — ​ $ 3,000 Contract liabilities (deferred revenue) ​ $ 92,068 ​ $ 3,803 ​ $ (25,760) ​ $ — ​ $ 70,111 ​ 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 ​ ​ March 31, ​ ​ 2023 ​ 2022 Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 2,712 ​ $ 25,760
Schedule of common stock equivalents, as calculated in accordance with the treasury-stock method	The Company’s common stock equivalents, as calculated in accordance with the treasury-stock method for options and unvested restricted stock are shown in the following table (in thousands): ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ March 31, ​ 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 ​ 39,064 ​ 27,012 Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock/units ​ 1,098 ​ 1,981
Schedule of risk-free rate of the stock options based on US Treasury rate	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, ​ 2023 ​ 2022 Dividend None ​ None Volatility 82.3% ​ 83.0% Risk-free interest rate 3.65% ​ 1.81% Expected life (years) 5.6 ​ 6.0
Summary of stock option activity	A summary of option activity under the Company’s equity plans for the three months ended March 31, 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 ​ 4,757 ​ ​ 4.62 Exercised ​ (16) ​ ​ 2.31 Forfeited/Canceled ​ (763) ​ ​ 5.60 Outstanding at March 31, 2023 ​ 37,104 ​ $ 5.61
Summary of restricted stock activity	A summary of restricted stock unit activity under the Company’s equity plans for the three months ended March 31, 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 ​ 1,824 ​ ​ 4.65 Forfeited ​ (2) ​ ​ 4.66 Unvested at March 31, 2023 ​ 1,960 ​ $ 4.71 ​

Product Revenue Reserves and _2

Product Revenue Reserves and Allowances (Tables)	3 Months Ended
	Mar. 31, 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 for the three months ended March 31, 2023 and 2022 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ March 31, ​ 2023 2022 Beginning balance at January 1 ​ $ 313 ​ $ — Provision related to sales in the current period ​ ​ 4,144 ​ ​ — Credits and payments made ​ ​ (1,533) ​ ​ — Ending balance at March 31 ​ $ 2,924 ​ $ —

Inventory (Tables)

Inventory (Tables)	3 Months Ended
	Mar. 31, 2023
Inventory
Schedule of capitalized inventory	Capitalized inventory consists of the following at March 31, 2023 and December 31, 2022 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, ​ December 31, ​ 2023 2022 Raw materials ​ $ 15,983 ​ $ 15,952 Work in process ​ ​ 639 ​ ​ — Finished goods ​ ​ 283 ​ ​ 244 Total Inventory ​ $ 16,905 ​ $ 16,196

Liability Related to Sale of _2

Liability Related to Sale of Future Royalties (Tables)	3 Months Ended
	Mar. 31, 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 three-month period ended March 31, 2023 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ March 31, 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 ​ (3,553) ​ Non-cash interest expense recognized ​ ​ 853 ​ Liability related to sale of future royalties, net — ending balance ​ $ 29,408 ​

Nature of Business and Plan o_2

Nature of Business and Plan of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Apr. 30, 2023	Apr. 06, 2023	Dec. 31, 2022
Net loss	$ (41,014)	$ (24,145)
Accumulated deficit	(1,735,086)				$ (1,694,072)
Total revenues	49,869	$ 38,078
Cash and cash equivalents	$ 201,249				$ 275,138
Secured Debt [Member]
Senior secured term loan				$ 175,000
Secured Debt [Member] | Subsequent event
Senior secured term loan			$ 175,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche One [Member]
Senior secured term loan				75,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche One [Member] | Subsequent event
Senior secured term loan			75,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche Two [Member] | Minimum
Senior secured term loan				50,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche Two [Member] | Minimum | Subsequent event
Senior secured term loan			50,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche Two [Member] | Maximum
Senior secured term loan				$ 100,000
Secured Debt [Member] | Senior Secured Term Loan, Tranche Two [Member] | Maximum | Subsequent event
Senior secured term loan			$ 100,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Performance Obligations (Details) $ in Millions	Mar. 31, 2023 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 47.5
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	0.28%
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	0.70%
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	0.02%
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_5

Summary of Significant Accounting Policies - Contract Balances (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Changes in the Company's contract assets and contract liabilities
Contract asset, Beginning balance		$ 3,000
Contract asset, Additions		0
Contract asset, Ending balance		3,000
Contract liabilities:
Contract liabilities (deferred revenue), Beginning balance	$ 50,211	92,068
Contract liabilities (deferred revenue), Additions		3,803
Contract liabilities (deferred revenue), Deductions	(2,712)	(25,760)
Contract liabilities (deferred revenue), Ending balance	$ 47,499	$ 70,111

Summary of Significant Accoun_6

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
	Mar. 31, 2023	Mar. 31, 2022
Revenue recognized in the period from:
Amounts included in contract liabilities at the beginning of the period	$ 2,712	$ 25,760

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Contract Balances from Contracts with Customers - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 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	$ 47,499	$ 70,111		$ 50,211		$ 92,068
Revenue from contract with customer	49,869	38,078
Revenue recognized, previously deferred	2,712	25,760
Current portion of deferred revenue	13,444			$ 13,856
License and milestone fees
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from contract with customer	15,031	30,892
Future Technological Improvements
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue recognized, previously deferred		100
Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd | License and milestone fees
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
License agreement upfront payment receivable						45,000
Revenue recognized, previously deferred		21,600
Current portion of deferred revenue						$ 28,500
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		9,200
Lilly | Upfront payment
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract with customer liability		13,000
KADCYLA | Royalty revenue
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net proceeds from sale of residual rights to receive royalty payments	2,700
Revenue recognized, previously deferred		$ 4,100
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

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Financial Instruments and Concentration of Credit Risk (Details)	3 Months Ended
	Mar. 31, 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 Accoun_9

Summary of Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Summary of Significant Accounting Policies
Cash and cash equivalents	$ 201,249	$ 275,138

Summary of Significant Accou_10

Summary of Significant Accounting Policies - Non-cash Investing and Financing Activities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2023	Dec. 31, 2022
Summary of Significant Accounting Policies
Accrued capital expenditures	$ 0.2	$ 0.3

Summary of Significant Accou_11

Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Details)	Mar. 31, 2023 $ / shares
Summary of Significant Accounting Policies
Common stock, par value (in dollars per share)	$ 0.01

Summary of Significant Accou_12

Summary of Significant Accounting Policies - Accounts Receivable (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Summary of Significant Accounting Policies
Expected credit losses	$ 2,924	$ 313

Summary of Significant Accou_13

Summary of Significant Accounting Policies - Inventory (Details) - USD ($)	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Summary of Significant Accounting Policies
Inventory	$ 614,000	$ 0
Inventory impairment	$ 0

Summary of Significant Accou_14

Summary of Significant Accounting Policies - Computation of Net Loss per Common Share (Details) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 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	39,064	27,012
Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock/units	1,098	1,981

Summary of Significant Accou_15

Summary of Significant Accounting Policies - Stock-Based Compensation (Details) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended					12 Months Ended
	Dec. 31, 2019 plan shares	Mar. 31, 2023 USD ($) plan $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Sep. 30, 2022 USD ($)	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($) $ / shares	Dec. 31, 2022 $ / shares shares	Dec. 31, 2020 shares	Jun. 15, 2022 shares	Apr. 01, 2022 shares	Jun. 30, 2018 shares
Stock-Based Compensation
Number of employee share-based compensation plans | plan		3
Weighted-average assumptions used to estimate the fair value of each stock option
Dividend (as a percent)		0%
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Directors' deferred share unit compensation | $		$ 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 | $		6,900				$ 4,200
Estimated fair value that could be expensed | $		$ 72,400
Stock options
Stock-Based Compensation
Common stock authorized for issuance (in shares)		0
Weighted-average assumptions used to estimate the fair value of each stock option
Dividend (as a percent)		0%				0%
Volatility (as a percent)		82.30%				83%
Risk-free interest rate (as a percent)		3.65%				1.81%
Expected life		5 years 7 months 6 days				6 years
Weighted-average grant date fair value (in dollars per share) | $ / shares		$ 3.26				$ 3.78
Number of Stock Options
Outstanding at the beginning of the period (in shares)		33,126,000
Granted (in shares)		4,757,000
Exercised (in shares)		(16,000)
Forfeited/Canceled (in shares)		(763,000)
Outstanding at the end of the period (in shares)		37,104,000	33,126,000				33,126,000
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares		$ 5.76
Granted (in dollars per share) | $ / shares		4.62
Exercised (in dollars per share) | $ / shares		2.31
Forfeited/Canceled (in dollars per share) | $ / shares		5.60
Outstanding at the end of the period (in dollars per share) | $ / shares		$ 5.61	$ 5.76				$ 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
Restricted stock
Number of Restricted Stock Shares
Unvested at the beginning of the period (in shares)		138,000
Granted (in shares)		1,824,000
Forfeited (in shares)		(2,000)
Unvested at the end of the period (in shares)		1,960,000	138,000				138,000
Weighted-Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share) | $ / shares		$ 5.45
Granted (in dollars per share) | $ / shares		4.65
Forfeited (in dollars per share) | $ / shares		4.66
Unvested at the end of the period (in dollars per share) | $ / shares		$ 4.71	$ 5.45				$ 5.45
Immunogen Inc Restated Stock Option Plan
Stock-Based Compensation
Common stock authorized for issuance (in shares)										28,742,013
2018 Plan
Stock-Based Compensation
Common stock authorized for issuance (in shares)									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
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

Summary of Significant Accou_16

Summary of Significant Accounting Policies - Segment Information (Details) - segment	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Information
Number of operating segments	1
Other customers | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized	0%	0%
Specialty distributors and pharmacy providers | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized	59%
Roche | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized		17%
Huadong | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized		59%
Lilly | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized	10%	24%
Vertex Pharmaceuticals Incorporated [Member] | Revenue | Customer concentration
Segment Information
Percentages of revenue recognized	30%

Collaboration and License Agr_2

Collaboration and License Agreements - Roche (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Sep. 30, 2022	Sep. 30, 2021
Collaborative Agreements disclosures
Revenue from contract with customer	$ 49,869	$ 38,078
Non-cash royalty revenue related to sale of future royalties	(2,157)	(2,364)
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer	$ 15,031	$ 30,892
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			$ (4,800)	$ (6,400)

Collaboration and License Agr_3

Collaboration and License Agreements - Viridian (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Collaborative Agreements disclosures
Revenue from contract with customer	$ 49,869	$ 38,078
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer	$ 15,031	$ 30,892

Collaboration and License Agr_4

Collaboration and License Agreements - Huadong (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Oct. 31, 2020	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2021
Collaborative Agreements disclosures
Revenue from contract with customer		$ 49,869	$ 38,078
Revenue recognized, previously deferred		2,712	25,760
Upfront payment | Huadong
Collaborative Agreements disclosures
Revenue from contract with customer	$ 40,000
Revenue recognized, previously deferred			21,600
Upfront payment | Huadong | Development milestones
Collaborative Agreements disclosures
Revenue recognized, previously deferred				$ 45,000
Deferred revenue			28,500
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer		$ 15,031	$ 30,892
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

Collaboration and License Agr_5

Collaboration and License Agreements - Magenta (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer	$ 49,869	$ 38,078
License and milestone fees
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer	$ 15,031	$ 30,892

Collaboration and License Agr_6

Collaboration and License Agreements - Lilly (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022	Feb. 28, 2022
Collaborative Agreements disclosures
Aggregate amount of transaction price allocated to remaining performance obligations	$ 47,500
Revenue from contract with customer	49,869	$ 38,078
Amount of obligation included in long-term deferred revenue	34,055		$ 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 | Initial targets
Collaborative Agreements disclosures
Revenue from contract with customer	9,200		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

Collaboration and License Agr_7

Collaboration and License Agreements - Vertex (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Feb. 28, 2023	Dec. 31, 2022
Collaborative Agreements disclosures
Aggregate amount of transaction price allocated to remaining performance obligations	$ 47,500
Revenue from contract with customer	49,869	$ 38,078
Amount of obligation included in long-term deferred revenue	34,055			$ 36,355
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer	15,031	$ 30,892
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
	Mar. 31, 2023	Mar. 31, 2022
Product Revenue Reserves and Allowances
Beginning balance at January 1	$ 313
Provision related to sales in the current period	4,144
Credits and payments made	(1,533)
Ending balance at December 31	2,924
Revenue from contract with customer	49,869	$ 38,078
Product revenue, net
Product Revenue Reserves and Allowances
Revenue from contract with customer	$ 29,544

Inventory (Details)

Inventory (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Inventory
Raw materials	$ 15,983	$ 15,952
Work in process	639
Finished goods	283	244
Total Inventory	$ 16,905	$ 16,196

Liability Related to Sale of _3

Liability Related to Sale of Future Royalties (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 31, 2019	Mar. 31, 2023	Mar. 31, 2022	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2015
Liability Related to Sale of Future Royalties
Non-cash royalty revenue related to the sale of future royalties		$ 2,157	$ 2,364
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
Liability related to sale of future royalties, net - beginning balance		32,108
Proceeds from sale of future royalties, net		200,000				200,000
KADCYLA royalty payments received and paid		(3,553)
Non-cash interest expense recognized		853
Liability related to sale of future royalties, net - ending balance		$ 29,408
Effective annual interest rate		10.50%
Current effective interest rate		10
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		$ 4,100
Contingent broker fees	1,500
Net proceeds from sale of residual rights to receive royalty payments	$ 65,200	$ 2,700
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				$ 4,800	$ 6,400

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details)	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Income Tax Uncertainties
Capitalize research and development amortization period		5 years
Capitalize research and experimentation expenses, amortization period	15 years

Capital Stock (Details)

Capital Stock (Details) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2023 USD ($) shares	Mar. 31, 2022 USD ($)	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 | $	$ 39	$ 620
ESPP
Stock-based compensation disclosure
Aggregate number of common shares reserved for future issuance						2,000,000
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 upon at least 61 days prior notice from the investor to the Company					19.99
Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares)	4,757,000
Options exercised (in shares)	16,000
Securities Purchase Agreement | RA Capital Healthcare Fund, L.P. | Pre-Funded Warrants
Stock-based compensation disclosure
Pre-Funded warrants issued to purchase shares				21,434,782
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 | Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares)			321,622	352,000

Leases (Details)

Leases (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2023 USD ($) ft² lease	Mar. 31, 2022 USD ($)	Dec. 31, 2022 USD ($)
Lessee, Lease, Description [Line Items]
Number of real estate leases | lease	1
Right-of-use assets | $	$ 9,627		$ 10,231
Sublease income | $	$ 800	$ 1,200
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
	Mar. 31, 2023	Mar. 31, 2022
Collaborations and Manufacturing Commitments
Research and development	$ 51,620	$ 44,282
In-process and future manufacturing of antibody, drug substance, and cytotoxic agents
Collaborations and Manufacturing Commitments
Noncancelable obligations under several agreements	22,300
Minimum
Collaborations and Manufacturing Commitments
Manufacturing commitment	$ 46,700

Related Party Transactions (Det

Related Party Transactions (Details) $ in Millions	3 Months Ended
	Mar. 31, 2023 USD ($)
Related Party Transactions
Payments to related party	$ 1.3

Subsequent Events (Details)

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