Tender Offer Statement (Form SC TO-T)

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UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

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SCHEDULE TO

Tender Offer Statement under Section 14(d)(1) or 13(e)(1)
of the Securities Exchange Act of 1934

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MERSANA THERAPEUTICS, INC.

(Name of Subject Company (issuer))

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EMERALD MERGER SUB, INC.

(Offeror)​

a direct wholly owned subsidiary of

DAY ONE BIOPHARMACEUTICALS, INC.

(Parent of Offeror)
(Names of Filing Persons (identifying status as offeror, issuer or other person))

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Common Stock, $0.0001 par value per share

(Title of Class of Securities)​

59045L 106

(CUSIP Number of Class of Securities)​

Adam Dubow
General Counsel
Day One Biopharmaceuticals, Inc.
1800 Sierra Point Parkway, Suite 200
Brisbane, California 94005
Telephone: (650) 484-0899

(Name, address, and telephone numbers of person authorized to receive notices and communications on behalf of filing persons)

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Copy to:

Robert A. Freedman
David Michaels
Julia Forbess
Fenwick & West LLP
401 Union Street, Floor 5
Seattle, WA
Telephone: (206) 389-4510

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Check the box if the filing relates solely to preliminary communications made before the commencement of a tender offer.

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Check the appropriate boxes below to designate any transactions to which the statement relates:

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Third-party tender offer subject to Rule 14d-1.

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Issuer tender offer subject to Rule 13e-4.

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Going-private transaction subject to Rule 13e-3.

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Amendment to Schedule 13D under Rule 13d-2.

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Check the following box if the filing is a final amendment reporting the results of the tender offer: ☐

If applicable, check the appropriate box(es) below to designate the appropriate rule provision(s) relied upon:

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Rule 13e-4(i) (Cross-Border Issuer Tender Offer)

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Rule 14d-1(d) (Cross-Border Third-Party Tender Offer)

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Items 1 through 9 and Item 11.

This Tender Offer Statement on Schedule TO (together with any amendments and supplements hereto, the "Schedule TO") relates to the offer by Emerald Merger Sub, Inc., a Delaware corporation and a direct wholly owned subsidiary of Day One Biopharmaceuticals, Inc., a Delaware corporation ("Day One"), to purchase all of the issued and outstanding shares of common stock, par value $0.0001 per share (the "Shares"), of Mersana Therapeutics, Inc., a Delaware corporation ("Mersana"), which constitute all of the issued and outstanding shares of capital stock of Mersana, at a purchase price of (i) $25.00 per Share, net to the stockholder in cash, without interest and less any applicable tax withholding, plus (ii) one non-tradable contingent value right (each a "CVR") per Share, representing the right to receive certain contingent payments of up to an aggregate amount of $30.25 per CVR in cash, without interest, less any applicable withholding taxes, upon the achievement of certain specified milestones on or prior to the applicable milestone outside dates in accordance with the terms and conditions set forth in the Contingent Value Rights Agreement, to be entered into with a rights agent mutually agreeable to Day One and Mersana, upon the terms and subject to the conditions set forth in the Offer to Purchase, dated December 5, 2025 (as it may be amended or supplemented from time to time, the "Offer to Purchase"), and in the related Letter of Transmittal (as it may be amended or supplemented from time to time, the "Letter of Transmittal"), copies of which are attached hereto as Exhibits (a)(1)(A) and (a)(1)(B), respectively.

All information contained in the Offer to Purchase (including Schedule I thereto) and the related Letter of Transmittal is hereby expressly incorporated herein by reference in response to Items 1 through 9 and Item 11 of this Schedule TO, except as otherwise set forth below.

Item 10. Financial Statements.

Not applicable.

Item 12. Exhibits.

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*

Filed herewith.

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SIGNATURES

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

Date: December 5, 2025

EMERALD MERGER SUB, INC.

/s/ Charles N. York II

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Name: Charles N. York II, M.B.A.
Title:  Chief Executive Officer and President

DAY ONE BIOPHARMACEUTICALS, INC.

/s/ Jeremy Bender

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Name: Jeremy Bender, Ph.D., M.B.A.
Title:  Chief Executive Officer and President