Mersana Therapeutics Provides Business Update and Announces Third Quarter 2025 Financial Results (Form SC14D9C)

Mersana Therapeutics Provides Business Update and Announces
Third Quarter 2025 Financial Results

CAMBRIDGE, Mass., November 14, 2025 - Mersana Therapeutics, Inc. (NASDAQ: MRSN), a clinical-stage biopharmaceutical company focused on the development of antibody-drug conjugates (ADCs) targeting cancers in areas of high unmet medical need, today provided a business update and reported financial results for the third quarter ended September 30, 2025.

Acquisition by Day One Biopharmaceuticals, Inc.

Yesterday, Mersana announced that it has entered into a definitive merger agreement with Day One Biopharmaceutics, Inc. (Day One), pursuant to which Day One would acquire Mersana, through a tender offer followed by a second step merger, for upfront consideration of $25.00 per share in cash plus up to an aggregate of $30.25 per share in cash potentially payable under contingent value rights (CVRs) triggered upon the achievement of certain clinical development, regulatory and commercial milestones related to Emi-Le, the company's B7-H4-directed Dolasynthen ADC, and upon the achievement of a certain milestone pursuant to an existing Mersana collaboration to be issued in the proposed acquisition, representing a total equity value of approximately $129 million at closing and representing a total deal value of up to approximately $285 million. Closing of the transaction is subject to the satisfaction of customary closing conditions, including that a majority of Mersana's shares of common stock are validly tendered in the tender offer and not validly withdrawn and the receipt of certain U.S. regulatory approvals, and is expected to occur by the end of January 2026.

Emiltatug Ledadotin (Emi-Le; XMT-1660)

In the ongoing Phase 1 clinical trial of Emi-Le, Mersana continues to follow patients in its two dose expansion cohorts in patients with triple-negative breast cancer, or TNBC, who have received one to four prior treatment lines in the locally advanced or metastatic setting, including at least one prior topoisomerase-1 inhibitor ADC.

Mersana is also evaluating Emi-Le in patients with adenoid cystic carcinoma type 1 (ACC-1), a population with very high unmet need, in the backfill cohorts of the dose escalation portion of the ongoing Phase 1 clinical trial. In June 2025, at ASCO, Mersana presented interim clinical data from nine evaluable patients with ACC-1. As of October 1, 2025, Mersana has enrolled a substantially greater number of ACC-1 patients in backfill cohorts than was presented at ASCO. The data from ACC-1 patients enrolled in this trial continues to mature as Mersana enrolls additional patients and patients remain on treatment for longer periods. Mersana has continued to be encouraged by the responses of the patients in these backfill cohorts.

Emi-Le continues to be generally well-tolerated, and its observed safety profile remains consistent with previously disclosed data.

XMT-2056

Mersana's Phase 1 clinical trial of XMT-2056, the company's lead Immunosynthen ADC candidate targeting a novel HER2 epitope, is ongoing. In the third quarter of 2025, Mersana achieved and received a $15 million development milestone under its agreement with GSK plc (GSK), which has an exclusive global license option to co-develop and commercialize XMT-2056.

Collaborations

Mersana continues to support its collaborations with both Janssen Biotech, Inc. (Johnson & Johnson, Dolasynthen research collaboration) and Merck KGaA, Darmstadt, Germany (Immunosynthen research collaboration).

In the third quarter of 2025, Johnson & Johnson received clearance from the U.S. Food and Drug Administration (FDA) of an investigational new drug application for a Dolasynthen ADC developed under Mersana's 2022 collaboration and license agreement with Johnson & Johnson. An $8.0 million development milestone is associated with the further progress of this first-in-human clinical trial pursuant to the agreement with Johnson & Johnson.

Third Quarter 2025 Financial Results and Recent Updates

Conference Call

Mersana's previously scheduled conference call to discuss business updates and its financial results for the third quarter of 2025 has been cancelled.

About Mersana Therapeutics

Mersana Therapeutics is a clinical-stage biopharmaceutical company focused on the development of novel antibody-drug conjugates (ADCs) and driven by the knowledge that patients are waiting for new treatment options. The company has developed proprietary cytotoxic (Dolasynthen) and immunostimulatory (Immunosynthen) ADC platforms that have generated a pipeline of wholly-owned and partnered product candidates with the potential to treat a range of cancers. Its pipeline includes Emi-Le (emiltatug ledadotin; XMT-1660), a Dolasynthen ADC targeting B7-H4, and XMT-2056, an Immunosynthen ADC targeting a novel epitope of human epidermal growth factor receptor 2 (HER2). Mersana routinely posts information that may be useful to investors on the "Investors & Media" section of its website at www.mersana.com.

Mersana Therapeutics, Inc.

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Mersana Therapeutics, Inc.

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Important Information for Investors and Stockholders and Where to Find It

The tender offer referenced in this press release has not yet commenced. This document is for informational purposes only and is neither an offer to purchase nor a solicitation of an offer to sell shares, nor is it a substitute for the tender offer materials that Day One and its subsidiary ("Merger Sub") will file with the Securities and Exchange Commission ("SEC"). The solicitation and offer to buy outstanding shares of common stock of Mersana will only be made pursuant to the tender offer materials that Day One and Merger Sub intend to file with the SEC. At the time the tender offer is commenced, Day One and Merger Sub will file tender offer materials on Schedule TO and Mersana will file a Solicitation/Recommendation Statement on Schedule 14D-9 with the SEC with respect to the tender offer. MERSANA'S STOCKHOLDERS ARE URGED TO CAREFULLY READ THE TENDER OFFER MATERIALS (INCLUDING AN OFFER TO PURCHASE, A RELATED LETTER OF TRANSMITTAL AND CERTAIN OTHER TENDER OFFER DOCUMENTS), AND THE SOLICITATION/RECOMMENDATION STATEMENT ON SCHEDULE 14D-9 (AS EACH MAY BE AMENDED OR SUPPLEMENTED FROM TIME TO TIME), WHEN THEY BECOME AVAILABLE, BECAUSE THEY WILL EACH CONTAIN IMPORTANT INFORMATION ABOUT THE TENDER OFFER AND PROPOSED MERGER THAT MERSANA'S STOCKHOLDERS SHOULD CONSIDER BEFORE MAKING ANY DECISION REGARDING TENDERING THEIR SHARES. The Schedule TO, including the Offer to Purchase, the related Letter of Transmittal and certain other tender offer documents and the Solicitation/Recommendation Statement on Schedule 14d-9, will be made available to all of Mersana's stockholders at no expense to them and will also be made available for free at the SEC's website at www.sec.gov. Additional copies of the tender offer materials filed by Mersana may be obtained for free at Mersana's website at https://www.mersana.com/ or by contacting Mersana's Secretary at [email protected]. Additional copies of the tender offer materials filed by Day One and Merger Sub may be obtained for free under the "SEC Filings" section of Day One's website at https://ir.dayonebio.com/financials-and-filings/sec-filings. In addition to the Offer to Purchase, the related Letter of Transmittal and certain other tender offer documents and the Solicitation/Recommendation Statement on Schedule 14D-9, Mersana and Day One each file annual, quarterly and current reports and other information with the SEC, which are available to the public over the internet at the SEC's website at http://www.sec.gov.