SEC Form SC 13G/A filed by ImmunoGen Inc. (Amendment)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 13G/A

Under the Securities Exchange Act of 1934

(Amendment No. 3)*

ImmunoGen, Inc.

(Name of Issuer)

Common Stock, $0.01 par value per share

(Title of Class of Securities)

45253H101

(CUSIP Number)

December 31, 2023

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

¨ Rule 13d-1(b)

x Rule 13d-1(c)

¨ Rule 13d-1(d)

*The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No. 45253H101

CUSIP No. 45253H101

CUSIP No. 45253H101

CUSIP No. 45253H101

ImmunoGen, Inc. (the “Issuer”)

830 Winter Street, Waltham, MA 02451

The names of the persons filing this report (collectively, the “Reporting Persons”) are:

RA Capital Management, L.P. (“RA Capital”)

Peter Kolchinsky

Rajeev Shah

RA Capital Healthcare Fund, L.P. (the “Fund”)

The address of the principal business office of each of the Reporting Persons is:

c/o RA Capital Management, L.P., 200 Berkeley Street, 18^th Floor, Boston MA 02116

RA Capital and the Fund are Delaware limited partnerships. Dr. Kolchinsky and Mr. Shah are United States citizens.

Common stock, $0.01 par value per share (“Common Stock”)

Not applicable.

The Fund directly holds (i) 23,750,601 shares of Common Stock and (ii) 21,853,000 Pre-funded warrants (“Pre-Funded Warrants”), through which it has the right to acquire 21,853,000 shares of Common Stock subject to a Beneficial Ownership Blocker (as defined below).

The shares reported herein for the Reporting Persons represent (i) 23,750,601 shares of Common Stock held directly, and (ii) 3,165,426 shares of Common Stock that the Reporting Persons beneficially own based on the right to acquire, upon the exercise of the Pre-Funded Warrants. The Pre-Funded Warrants are subject to a beneficial ownership blocker (“Beneficial Ownership Blocker”), which precludes the exercise of the Pre-Funded Warrants to the extent that, following exercise, the Reporting Persons, together with its affiliates and other attribution parties, would own more than 9.99% of the Common Stock outstanding. The Reporting Persons are currently prohibited from exercising the Pre-Funded Warrants to the extent that such exercise would result in beneficial ownership of more than 26,916,027 shares of Common Stock.

The information required by this item with respect to each Reporting Person is set forth in Rows 5 through 9 and 11 of the cover page to this Schedule 13G/A. The beneficial ownership percentages reported are based on (i) 266,264,274 outstanding shares of Common Stock, as reported in the Issuer’s Form 10-Q filed with the Securities and Exchange Commission on November 2, 2023, plus (ii) 3,165,426 shares of Common Stock of which the Reporting Person may acquire beneficial ownership upon the exercise of Pre-Funded Warrants, as limited by the Beneficial Ownership Blocker.

RA Capital Healthcare Fund GP, LLC is the general partner of the Fund. The general partner of RA Capital is RA Capital Management GP, LLC, of which Dr. Kolchinsky and Mr. Shah are the controlling persons. RA Capital serves as investment adviser for the Fund and may be deemed a beneficial owner, for purposes of Section 13(d) of the Securities Exchange Act of 1934 (the “Act”), of any securities of the Issuer held by the Fund. The Fund has delegated to RA Capital the sole power to vote and the sole power to dispose of all securities held in the Fund’s portfolio, including the shares of the Issuer’s Common Stock reported herein. Because the Fund has divested voting and investment power over the reported securities it holds and may not revoke that delegation on less than 61 days’ notice, the Fund disclaims beneficial ownership of the securities it holds for purposes of Section 13(d) of the Act. As managers of RA Capital, Dr. Kolchinsky and Mr. Shah may be deemed beneficial owners, for purposes of Section 13(d) of the Act, of any securities of the Issuer beneficially owned by RA Capital. RA Capital, Dr. Kolchinsky, and Mr. Shah disclaim beneficial ownership of the securities reported in this Schedule 13G/A other than for the purpose of determining their obligations under Section 13(d) of the Act, and the filing of this Schedule 13G/A shall not be deemed an admission that either RA Capital, Dr. Kolchinsky, or Mr. Shah is the beneficial owner of such securities for any other purpose.

If this statement is being filed to report the fact that as of the date hereof the Reporting Persons have ceased to be the beneficial owner of more than five percent of the class of securities, check the following ¨.

Not applicable.

Not applicable.

Not applicable.

Not applicable.

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under § 240.14a–11.

Exhibit List

Exhibit 1: Joint Filing Agreement

SIGNATURE

After reasonable inquiry and to the best of its knowledge and belief, each of the undersigned certifies that the information set forth in this statement is true, complete and correct.

Date:      February 14, 2024