This Amendment No. 4 to Schedule 13D relates to the Common Stock of Mersana Therapeutics, Inc. and
amends the initial statement on Schedule 13D filed by the Reporting Persons on April 17, 2020, as amended by Amendment No. 1 filed on January 12, 2021, Amendment No. 2 filed on January 3, 2022 and Amendment No. 3 filed
on February 8, 2022 (the “Initial Statement” and, as further amended by this Amendment No. 4, the “Schedule 13D”). Capitalized terms used but not defined herein shall have the meanings ascribed to them in the Initial
Statement.
Item 3. Source and Amount of Funds or Other Consideration
Item 3 of the Initial Statement is hereby amended and supplemented to add the following:
On April 12, 2022, BCLS II Investco purchased 598,086 shares of Common Stock at a purchase price of $4.1798 per share. On April 13, 2022, BCLS II
Investco purchased an additional 617,284 shares of Common Stock at a purchase price of $4.05 per share. The shares of Common Stock were purchased through the Issuer’s ATM facility. Cowen and Company, LLC acted as sales agent for the ATM
facility. BCLS II Investco used its own working capital to acquire the shares of Common Stock.
Item 5. Interest in Securities of the Issuer
Item 5 of the Initial Statement is hereby amended and restated as follows:
The information set forth in Items 2 and 3 and on the cover pages of this Schedule 13D is incorporated by reference in its entirety into this Item 5.
(a) – (c) As of the date hereof, BCLS II holds 3,294,423 shares of Common Stock, representing approximately 3.9% of the outstanding shares of Common
Stock, BCIPLS holds 401,244 shares of Common Stock, representing approximately 0.5% of the outstanding shares of Common Stock, and BCLS II Investco holds 4,041,733 shares of Common Stock, representing approximately 4.8% of the outstanding shares of
Common Stock. As a result of the foregoing and the relationships described in Item 2(a) of this Schedule 13D, the Reporting Persons may be deemed to beneficially own in the aggregate 7,737,400 shares of Common Stock, representing approximately 9.1%
of the outstanding shares of Common Stock. The percentage of the outstanding shares of Common Stock held by the Reporting Persons is based on (i) 83,389,806 shares of Common Stock outstanding as of February 25, 2022, as reported by the Issuer
in its Annual Report on Form 10-K for the year ended December 31, 2021 and (ii) the issuance and sale to BCLS II Investco of an aggregate of 1,215,370 shares of Common Stock under the ATM
facility.
(d) Except as otherwise described in this Item 5, no one other than the Reporting Persons has the right to receive, or the power to direct the
receipt of, dividends from, or the proceeds from the sale of, any of the Common Stock beneficially owned by the Reporting Persons as described in this Item 5.
(e) Not applicable.