This Amendment No. 5 to Schedule 13D relates to the Common Stock of Mersana Therapeutics, Inc. (the
“Issuer”) 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,
Amendment No. 3 filed on February 8, 2022 and Amendment No. 4 filed on April 14, 2022 (the “Initial Statement” and, as further amended by this Amendment No. 5, the “Schedule 13D”). Capitalized terms used
but not defined herein shall have the meanings ascribed to them in the Initial Statement.
On May 9, 2022, the Issuer filed its Quarterly Report on
Form 10-Q for the quarter ended March 31, 2022 with the Securities and Exchange Commission, which disclosed that the Issuer’s total number of outstanding shares of Common Stock had increased to
96,991,298. This Amendment No. 5 is being filed to report a decrease of more than one percent in the percentage of outstanding shares of Common Stock that the Reporting Persons may be deemed to beneficially own, which resulted solely from the
increase in the number of shares of Common Stock reported as outstanding by the Issuer.
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 3.4% of the outstanding shares of Common
Stock, BCIPLS holds 401,244 shares of Common Stock, representing approximately 0.4% of the outstanding shares of Common Stock, and BCLS II Investco holds 4,041,733 shares of Common Stock, representing approximately 4.2% 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 8.0%
of the outstanding shares of Common Stock. The percentage of the outstanding shares of Common Stock held by the Reporting Persons is based on 96,991,298 shares of Common Stock outstanding as of May 5, 2022, as reported by the Issuer in its
Quarterly Report on Form 10-Q for the quarter ended March 31, 2022.
(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.