Mersana Therapeutics, Inc. (Nasdaq: MRSN), a clinical-stage biopharmaceutical company focused on discovering and developing a pipeline of antibody drug conjugates (ADCs) targeting cancers in areas of high unmet medical need, today announced that it intends to offer and sell shares of its common stock in an underwritten public offering of 5 million shares of its common stock. All shares being offered are to be sold by Mersana. As part of the offering, Mersana intends to grant the underwriters a 30-day option to purchase up to an additional 15% of the shares of common stock offered in the public offering, at the public offering price less underwriting discounts and commissions.

Cowen and SVB Leerink are acting as joint bookrunning managers for the offering. The offering is subject to market and other conditions, and there can be no assurance as to whether or when the offering may be completed, or as to the actual size or terms of the offering.

Mersana intends to use the net proceeds from the offering to support clinical development of XMT-1536 and XMT-1592, to progress Mersana’s next ADC product candidates into Phase 1 clinical development, to progress Mersana’s early platform development and the balance to fund working capital, capital expenditures and other general corporate purposes.

The shares of common stock described above are being offered by Mersana pursuant to an effective shelf registration statement on Form S-3 (File No. 333-238140), which was declared effective by the Securities and Exchange Commission (SEC) on May 15, 2020. The offering will be made only by means of a preliminary prospectus supplement and accompanying prospectus, copies of which may be obtained, when available, from Cowen and Company, LLC, c/o Broadridge Financial Solutions, 1155 Long Island Avenue, Edgewood, New York 11717 Attention: Prospectus Department, by telephone at (833) 297-2926, or by email at [email protected], and SVB Leerink LLC, Attention: Syndicate Department, One Federal Street, 37th Floor, Boston, Massachusetts 02110, by telephone at 800-808-7525, ext. 6218 or by email at [email protected]. Electronic copies of the preliminary prospectus supplement and accompanying prospectus will also be available on the website of the SEC at www.sec.gov.

This press release does not constitute an offer to sell or a solicitation of an offer to buy the securities in the offering, nor shall there be any sale of these securities in any jurisdiction in which an offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of such jurisdiction.