Mersana Therapeutics, Inc. (MRSN)

Mersana Therapeutics was a clinical-stage biopharmaceutical company based in Cambridge, Massachusetts, that developed antibody-drug conjugates (ADCs) for cancer. The company was founded in 2001 and built its research around proprietary technologies for attaching cytotoxic payloads to antibodies. Its stated goal was to improve the tolerability and potency of ADCs compared with earlier approaches.

The company's earlier platform, Dolaflexin, used a polymer scaffold intended to carry a high number of payload molecules per antibody. Its first clinical candidate from that platform, upifitamab rilsodotin (also known as XMT-1536), targeted NaPi2b in ovarian cancer and other tumors. Mersana later developed Dolasynthen, a platform that allows the drug-to-antibody ratio to be controlled more precisely, and Immunosynthen, which was designed to deliver a STING agonist payload to tumor-targeted cells.

Mersana completed its initial public offering in 2017 and listed its common stock on the Nasdaq Global Select Market under the symbol MRSN. Over the years it signed collaborations with large drug developers, including Takeda and Merck KGaA, which provided upfront payments and research funding. Those agreements were an important part of how the company financed its early-stage programs.

The company's fortunes changed in 2023 and 2024. Development of upifitamab rilsodotin was discontinued after clinical results did not support further investment, and Mersana restructured its operations and reduced its workforce. It refocused on Emiltatug ledadotin (Emi-Le), an ADC directed at B7-H4, along with earlier pipeline programs, and it continued to rely on equity financing to fund operations.

On November 14, 2025, Mersana announced a definitive agreement to be acquired by Day One Biopharmaceuticals, a commercial-stage biopharmaceutical company listed on Nasdaq under the symbol DAWN. Under the agreement, Day One launched a tender offer to buy all outstanding Mersana shares for $25.00 per share in cash. Each shareholder would also receive one non-transferable contingent value right entitling the holder to milestone-based cash payments totaling up to $30.25 per share, which would bring the maximum total consideration to $55.25 per share.

The tender offer expired on January 5, 2026, and the merger closed on January 6, 2026, using the short-form procedure available under Section 251(h) of the Delaware General Corporation Law. Mersana became a wholly owned subsidiary of Day One, and its shares ceased trading on Nasdaq before the market opened on January 6. An SEC Form 25 was filed to remove the common stock from listing and registration on the exchange.

Because the transaction was a cash acquisition by a listed company, Mersana's shareholders no longer hold a public equity stake in the business. The contingent value rights are not listed and cannot be traded. The ticker MRSN is no longer in use, and the company's programs, including Emi-Le, are now part of Day One's pipeline.