UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): July 1, 2025

IGM Biosciences, Inc.

(Exact name of Registrant as Specified in Its Charter)

Registrant’s Telephone Number, Including Area Code: (650) 965-7873

Not Applicable

(Former Name or Former Address, if Changed Since Last Report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§ 230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§ 240.12b-2 of this chapter).

Emerging growth company ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

Agreement and Plan of Merger

On July 1, 2025, IGM Biosciences, Inc. (the “Company”) entered into an Agreement and Plan of Merger (the “Merger Agreement”) with Concentra Biosciences, LLC, a Delaware limited liability company (“Concentra”), and Concentra Merger Sub V, Inc., a Delaware corporation and a wholly owned subsidiary of Concentra (“Merger Sub”). The Merger Agreement provides for, among other things: (i) the acquisition of all of the Company’s outstanding shares of common stock, par value $0.01 per share (the “Common Stock”), by Concentra through a cash tender offer (the “Offer”), for a price per share of the Common Stock of (A) $1.247 in cash (the “Cash Amount”), subject to applicable tax withholding and without interest; plus (B) one contingent value right (a “CVR”) (such amount being the “CVR Amount” and the Cash Amount plus the CVR Amount, collectively being the “Offer Price”) and (ii) the merger of Merger Sub with and into the Company (the “Merger”) with the Company surviving the Merger.

The Company’s Board of Directors (the “Board”), has unanimously: (i) determined that the Offer, the Merger and the other transactions contemplated by the Merger Agreement and the CVR Agreement (as defined below) (collectively, the “Transactions”) are fair to, and in the best interest of, the Company and its stockholders; (ii) approved and declared advisable the Merger and the execution, delivery and performance by the Company of the Merger Agreement and the consummation of the Transactions; (iii) resolved that the Merger Agreement and the Merger shall be governed by and effected under Section 251(h) of the Delaware General Corporation Law (the “DGCL”) and that the Merger shall be consummated as soon as practicable following the Offer Closing Time (as defined in the Merger Agreement); and (iv) recommended that the Company’s stockholders accept the Offer and tender their shares of Common Stock pursuant to the Offer. Under the Merger Agreement, Concentra is required to commence the Offer as promptly as practicable, and in any event no later than ten (10) business days after the date of the Merger Agreement.

Pursuant to the terms of the Merger Agreement, as of immediately prior to the effective time of the Merger (the “Effective Time”), by virtue of the Merger and without any action on the part of the holders of Common Stock: (i) each outstanding share of Common Stock, other than any shares of Common Stock held in the treasury of the Company, owned by Concentra, Merger Sub or any other subsidiary of Concentra, or by any stockholders of the Company who are entitled to and who properly exercise appraisal rights under Delaware law, will be converted into the right to receive the Offer Price without interest, less any applicable withholding taxes; (ii) (A) each option to purchase shares of Common Stock from the Company (“Company Stock Options,” and each, a “Company Stock Option”) that is then outstanding but not then vested or exercisable shall become immediately vested and exercisable in full, (B) each Company Stock Option that has an exercise price per share that is less than the Cash Amount (each, an “In-the-Money Option”) that is then outstanding will be cancelled and, in exchange therefor, the holder of such cancelled In-the-Money Option will be entitled to receive in consideration of the cancellation of such In-the-Money Option (x) an amount in cash without interest, less any applicable tax withholding, equal to the product obtained by multiplying (1) the excess of the Cash Amount over the exercise price per share of Common Stock underlying such In-the-Money Option by (2) the number of shares of Common Stock underlying such In-the-Money Option as of immediately prior to the Effective Time and (y) one CVR for each share of Common Stock underlying such In-the-Money Option, and (C) each Company Stock Option that has an exercise price per share that is equal to or greater than the Cash Amount that is then outstanding will be cancelled for no consideration; and (iii) the vesting for each restricted stock unit of the Company (“Company Restricted Stock Units,” and each, a “Company Restricted Stock Unit”) shall be accelerated and each Company Restricted Stock Unit that is then outstanding will be cancelled and, in exchange therefor, the holder of such cancelled Company Restricted Stock Unit will be entitled to receive in consideration of the cancellation of such Company Restricted Stock Unit (A) an amount in cash without interest, less any applicable tax withholding, equal to the Cash Amount and (B) one CVR. In addition, at the Effective Time, each pre-funded warrant (each, a “Company Pre-Funded Warrant”) to purchase Common Stock sold pursuant to that certain Underwriting Agreement, dated as of December 8, 2020, by and among the Company and the representatives of the several underwriters named therein, shall be canceled and the holder thereof shall be entitled to receive: (x) an amount in cash without interest, less any applicable tax withholding, equal to the product obtained by multiplying (A) the excess of the Cash Amount over the exercise price per share of Common Stock underlying such Company Pre-Funded Warrant by (B) the number of shares of the Common Stock underlying such Company Pre-Funded Warrant; and (ii) one CVR for each share of Common Stock underlying such Company Pre-Funded Warrant.

Concentra’s obligation to accept shares of Common Stock tendered in the Offer is subject to conditions, including: (i) that the number of shares of voting Common Stock validly tendered (and not properly withdrawn) prior to the expiration of the Offer (excluding shares tendered pursuant to guaranteed delivery procedures that have not yet been “received” by the “depository,” as such terms are defined by Section 251(h) of the DGCL) that, when considered together with all other shares of the voting Common Stock (if any) owned by Concentra and its “affiliates” (as defined in Section 251(h)(6)(a) of the DGCL), equals at least one share more than 50% of all shares of voting Common Stock then issued and outstanding as of the expiration of the Offer; (ii) the absence of any Legal Restraint (as defined in the Merger Agreement) in effect preventing or prohibiting the consummation of the Offer, the Merger or any of the other transactions contemplated by the Merger Agreement or the CVR Agreement; (iii) the accuracy of the representations and warranties made by the Company in the Merger Agreement, subject to specified materiality qualifications and exceptions; (iv) compliance by the Company with its covenants under the Merger Agreement in all material respects; and (v) the Closing Net Cash (as defined in the Merger Agreement) shall be no less than $82 million. The obligations of Concentra and Merger Sub to consummate the Offer and the Merger under the Merger Agreement are not subject to a financing condition.

Following the completion of the Offer, upon the terms and subject to the conditions of the Merger Agreement, Merger Sub will merge with and into the Company, with the Company surviving as a wholly owned subsidiary of Concentra, pursuant to the procedure provided for under Section 251(h) of the DGCL, without any additional Company stockholder approvals. The Merger will be effected as soon as practicable following the time of purchase by Concentra of shares of Common Stock validly tendered and not withdrawn in the Offer.

The Merger Agreement contains representations and warranties from both the Company, on the one hand, and Concentra and Merger Sub, on the other hand, customary for a transaction of this nature. The Merger Agreement also contains customary covenants and agreements, including with respect to the operations of the business of the Company between the date of the Merger Agreement and the closing of the Merger.

The Merger Agreement contains customary termination rights for both Concentra and Merger Sub, on the one hand, and the Company, on the other hand, including, among others, for failure to consummate the Offer on or before September 29, 2025. If the Merger Agreement is terminated under certain circumstances specified in the Merger Agreement, including in connection with the Company’s entry into an agreement with respect to a superior proposal, the Company will be required to pay Concentra a termination fee of $2.425 million. If Concentra terminates the Merger Agreement due to the Company having Closing Net Cash of less than $82.0 million, the Company will be required to reimburse expenses incurred by Concentra up to a maximum amount of $0.5 million.

The foregoing description of the Merger Agreement does not purport to be complete and is qualified in its entirety by reference to the Merger Agreement, a copy of which is filed as Exhibit 2.1 hereto and is incorporated herein by reference. The Merger Agreement and the foregoing description thereof has been included to provide investors and security holders with information regarding its terms. It is not intended to provide any other factual information about the Company, Concentra, Merger Sub or their respective subsidiaries and affiliates. The Merger Agreement contains representations and warranties by the Company, on the one hand, and Concentra and Merger Sub, on the other hand, made solely for the benefit of the other. The assertions embodied in those representations and warranties are subject to qualifications and limitations agreed to by the respective parties in negotiating the terms of the Merger Agreement, including information in confidential disclosure schedules delivered in connection with the signing of the Merger Agreement. Moreover, certain representations and warranties in the Merger Agreement were made as of a specified date, may be subject to a contractual standard of materiality different from what might be viewed as material to investors, or may have been used for the purpose of allocating risk between the Company, on the one hand, and Concentra and Merger Sub, on the other hand, rather than establishing matters as facts. Accordingly, the representations and warranties in the Merger Agreement should not be relied on by any persons as characterizations of the actual state of facts about the Company, Concentra, Merger Sub or their respective subsidiaries or affiliates at the time they were made or otherwise. In addition, information concerning the subject matter of the representations and warranties may change after the date of the Merger Agreement, which subsequent information may or may not be fully reflected in the Company’s public disclosures.

Limited Guaranty

Concurrently with the execution of the Merger Agreement and the CVR Agreement, and as a condition and inducement to the Company’s willingness to enter into the Merger Agreement, Tang Capital Partners, LP, a Delaware limited partnership, has delivered to the Company a duly executed limited guaranty, dated as of the date of the Merger Agreement, in favor of the Company, in respect of certain of Concentra and the Merger Sub’s obligations arising under, or in connection with, the Merger Agreement and CVR Agreement. Certain obligations under the Limited Guaranty are subject to a cap of $78.0 million, plus certain enforcement costs, under the Merger Agreement and an amount equivalent to the CVR Proceeds (as defined in the CVR Agreement), plus certain enforcement costs, under the CVR Agreement.

Contingent Value Rights Agreement

At or prior to the time at which Concentra first irrevocably accepts for purchase the shares of Common Stock tendered in the Offer, Concentra and Merger Sub expect to enter into a Contingent Value Rights Agreement (the “CVR Agreement”) with a rights agent (“Rights Agent”) and a representative, agent and attorney-in-fact of the holders of CVRs. Each CVR will represent a contractual right to receive contingent cash payments equal to: (i) 100% of the amount by which Closing Net Cash (as finally determined pursuant to the Merger Agreement) exceeds $82.0 million, adjusted for any claims Concentra reasonably determines to be valid five (5) business days prior to the Merger Closing Date (as defined in the Merger Agreement) and that are not accounted for in such Closing Net Cash; and (ii) 80% of the Net Proceeds (as defined in the CVR Agreement), if any, from any sale, transfer, license or other disposition or grant of rights (each, a “Disposition”) by Concentra or any of its affiliates, including the Company after the Merger, of all or any part of (a) the Company’s product candidates known as (A) imvotamab, an IgM-based CD20 X CD3 bispecific antibody T cell engager; (B) IGM-2644, an IgM-based CD38 X CD3 bispecific T cell engager; (C) aplitabart (or IGM-8444), a Death Receptor 5 agonist; and

(D) IGM-7354, a targeted IL-15/IL-15R IgM antibody; (E) IGM-8447, an IgM-based Death Receptor 5 agonist; and (F) IGM-2537, an IgM-based CD123 X CD3 bispecific antibody T cell engager (the “CVR Product”) and (b) patents and patent applications owned or controlled by the Company immediately prior to the closing of the Merger and set forth on Schedule 1 of the CVR Agreement, together with all patents issuing thereon or claiming priority thereto, that occurs within the period beginning on the Merger Closing Date and ending on the first (1^st) anniversary following the Merger Closing Date (the “Disposition Period”). In the event that no Dispositions occur by the first (1^st) anniversary of the Merger Closing Date, holders of the CVRs will not receive any payment pursuant to the CVR Agreement with respect to a Disposition. Concentra shall, and shall cause the Company after the Merger, to use commercially reasonable efforts to, among other things, during the Disposition Period: (i) enter into one or more Disposition Agreements (as defined in the CVR Agreement) as soon as practicable following the Effective Time; (ii) retain an employee or consultant of Concentra or Merger Sub for the purpose of maintaining and preserving the CVR Products and Company Patents and seeking, negotiating and executing Disposition Agreements; (iii) maintain the CVRs (including fees and expenses related to the Rights Agent and the Representative (each as defined in the CVR Agreement)); (iv) maintain and prosecute the intellectual property relating to the CVR Products and the intellectual property set forth on Schedule 1 to the CVR Agreement; and (v) continue the CMC Activities (as defined in the Merger Agreement).

The right to the contingent payments contemplated by the CVR Agreement is a contractual right only and will not be transferable, except in the limited circumstances specified in the CVR Agreement. The CVRs will not be evidenced by a certificate or any other instrument and will not be registered with the U.S. Securities and Exchange Commission (the “SEC”). The CVRs will not have any voting or dividend rights and will not represent any equity or ownership interest in Concentra, any constituent corporation party to the Merger or any of their respective affiliates.

The foregoing description of the CVR Agreement does not purport to be complete and is qualified in its entirety by reference to the Form of CVR Agreement, a copy of which is filed as Exhibit 10.1 hereto and is incorporated herein by reference.

The CVR Agreement and the foregoing description thereof has been included to provide investors and security holders with information regarding its terms. It is not intended to provide any other factual information about the Company, Concentra, Merger Sub or their respective subsidiaries and affiliates. The CVR Agreement contains representations and warranties by the Company, on the one hand, and Concentra and Merger Sub, on the other hand, made solely for the benefit of the other. The assertions embodied in those representations and warranties are subject to qualifications and limitations agreed to by the respective parties in negotiating the terms of the CVR Agreement, including information in confidential disclosure schedules delivered in connection with the signing of the Merger Agreement. Moreover, certain representations and warranties in the CVR Agreement were made as of a specified date, may be subject to a contractual standard of materiality different from what might be viewed as material to investors, or may have been used for the purpose of allocating risk between the Company, on the one hand, and Concentra and Merger Sub, on the other hand, rather than establishing matters as facts. Accordingly, the representations and warranties in the CVR Agreement should not be relied on by any persons as characterizations of the actual state of facts about the Company, Concentra, Merger Sub or their respective subsidiaries or affiliates at the time they were made or otherwise. In addition, information concerning the subject matter of the representations and warranties may change after the date of the CVR Agreement, which subsequent information may or may not be fully reflected in the Company’s public disclosures.

Departures of Section 16 Officers

In connection with the transactions contemplated by the Merger Agreement, on July 1, 2025, (1) the employment of Misbah Tahir, the Company’s Chief Financial Officer, has been terminated effective as of August 1, 2025; (2) the employment of Steven Weber, the Company’s Senior Vice President, Corporate Controller and Principal Accounting Officer, has been terminated effective as of July 11, 2025; and (3) the employment of Lisa L. Decker, the Company’s Chief Business Officer, has been terminated effective as of August 1, 2025. Mr. Tahir, Mr. Weber and Dr. Decker may continue to assist the Company as consultants after each of their last day of employment with the Company in order to support the transition of their responsibilities.

In addition, it is expected that the employment of Mary Beth Harler, the Company’s Chief Executive Officer, will be terminated effective as of the Effective Time.

On July 1, 2025, the Company issued a press release announcing the signing of the Merger Agreement, a copy of which is attached hereto as Exhibit 99.1 and incorporated by reference herein.

The information contained in this Item 7.01, including Exhibit 99.1 attached hereto, is being furnished and shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liabilities of that section, nor shall it be deemed incorporated by reference into any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as expressly set forth by specific reference in such filing.

Important Additional Information and Where to Find It

The Offer described in this communication has not yet commenced, and this communication is neither a recommendation, nor an offer to purchase nor a solicitation of an offer to sell any shares of the common stock of the Company or any other securities, nor is it a substitute for the tender offer materials that Concentra and Merger Sub will file with the SEC on commencement of the Offer. On the commencement date of the Offer, Concentra and Merger Sub will file with the SEC a tender offer statement on Schedule TO, including an offer to purchase, a letter of transmittal and related documents, and the Company will file with the SEC a Solicitation/Recommendation Statement on Schedule 14D-9. The Offer to purchase the outstanding shares of Common Stock will only be made pursuant to the offer to purchase, the letter of transmittal and related documents filed as a part of the Schedule TO.

INVESTORS AND SECURITY HOLDERS ARE URGED TO READ THE TENDER OFFER MATERIALS (INCLUDING THE OFFER TO PURCHASE, A LETTER OF TRANSMITTAL AND RELATED DOCUMENTS) AND THE SOLICITATION/RECOMMENDATION STATEMENT ON SCHEDULE 14D-9 REGARDING THE OFFER, AS THEY MAY BE AMENDED OR SUPPLEMENTED FROM TIME TO TIME, WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION THAT INVESTORS AND SECURITY HOLDERS SHOULD CONSIDER BEFORE MAKING ANY DECISION REGARDING TENDERING THEIR SHARES, INCLUDING THE TERMS AND CONDITIONS OF THE OFFER.

Investors and security holders may obtain a free copy of these statements (when available) and other documents filed with the SEC at the website maintained by the SEC at www.sec.gov or by directing such requests to the information agent for the Offer, which will be named in the tender offer statement. Investors and security holders may also obtain, at no charge, the documents filed or furnished to the SEC by the Company under the “SEC Filings” subsection of the Company’s website at https://investor.igmbio.com/. The information contained in, or that can be accessed through, the Company’s website is not a part of, or incorporated by reference herein. In addition to the Offer to Purchase, the related Letter of Transmittal and certain other tender offer documents, as well as the Solicitation/Recommendation Statement, the Company files annual, quarterly, and current reports, proxy statements and other information with the SEC. You may read any reports, statements or other information filed by the Company with the SEC for free on the SEC’s website at www.sec.gov.

Cautionary Note Regarding Forward-Looking Statements

This Current Report on Form 8-K contains “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995, including, but not limited to, statements regarding the Company’s beliefs and expectations and statements about the Offer, the Merger and the other Transactions, the ability to complete the transactions contemplated by the Merger Agreement, including the ability to satisfy the conditions to the consummation of the Offer contemplated thereby and the other conditions set forth in the Merger Agreement, the timing of the Transactions, the potential effects of the proposed Transactions on the Company and the potential payment of proceeds to the Company’s stockholders, if any, pursuant to the CVR Agreement. These statements may be identified by their use of forward-looking terminology including, but not limited to, “anticipate,” “believe,” “continue,” “could,” “estimate,” “expect,” “goal,” “intend,” “may,” “might,” “plan,” “potential,” “predict,” “project,” “should,” “target,” “will,” and “would,” and similar words expressions are intended to identify forward-looking statements. Forward-looking statements are neither historical facts nor assurances of future performance and involve risks and uncertainties that could cause actual results to differ

materially from those projected, expressed or implied by such forward-looking statements. These risks and uncertainties include, but are not limited to: the possibility that various closing conditions set forth in the Merger Agreement may not be satisfied or waived, including uncertainties as to the percentage of the Company’s stockholders tendering their shares in the Offer; the possibility that competing offers will be made; the risk that the Transactions may not be completed in a timely manner, or at all, which may adversely affect the Company’s business and the price of its common stock; significant costs associated with the proposed Transactions; the risk that any stockholder litigation in connection with the Transactions may result in significant costs of defense, indemnification and liability; the risk that activities related to the CVR Agreement may not result in any value to the Company’s stockholders; and other risks and uncertainties discussed in the Company’s most recent Annual Report on Form 10-K filed with the SEC on March 6, 2025 and the Amendment No. 1 to Annual Report on Form 10-K filed with the SEC on April 30, 2025 as well as in the Company’s subsequent filings with the SEC. As a result of such risks and uncertainties, the Company’s actual results may differ materially from any future results, performance or achievements discussed in or implied by the forward-looking statements contained herein. There can be no assurance that the proposed Transactions will in fact be consummated. The Company cautions investors not to unduly rely on any forward-looking statements.

The forward-looking statements contained in this Current Report on Form 8-K are made as of the date hereof, and the Company undertakes no obligation to update any forward-looking statements, whether as a result of future events, new information or otherwise, except as expressly required by law. All forward-looking statements in this document are qualified in their entirety by this cautionary statement.

(d) Exhibits

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.