UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

SCHEDULE 13D

(Rule 13d-101)

Information to be Included in Statements Filed Pursuant

to § 240.13d-1(a) and Amendments Thereto Filed

Pursuant to § 240.13d-2(a)

Under the Securities Exchange Act of 1934

(Amendment No. 1)

MERSANA THERAPEUTICS, INC.

(Name of Issuer)

Common Stock, $0.0001

par value per share

(Title of Class of Securities)

59045L106

(CUSIP Number)

Bain Capital Life Sciences Investors, LLC

200 Clarendon Street

Boston, MA 02116

617-516-2000

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

January 8, 2021

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  ☐

Note. Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See § 240.13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934, as amended (“Act”), or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

This Amendment No. 1 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 (the “Initial Statement” and, as amended by this Amendment No. 1, the “Schedule 13D”). Capitalized terms used but not defined herein shall have the meanings ascribed in the Initial Statement.

Item 3. Source and Amount of Funds or Other Consideration

Item 3 of the Initial Statement is hereby and supplemented to add the following:

On January 8, 2021, each of BCLS II and BCIPLS purchased 89,143 shares and 10,857 shares, respectively, of Common Stock at a weighted average purchase price of $18.3863 per share in open market transactions. On January 11, 2021, each of BCLS II and BCIPLS purchased 66,857 shares and 8,143 shares, respectively, of Common Stock at a weighted average purchase price of $18.5193 per share in open market transactions. On January 12, 2021, each of BCLS II and BCIPLS purchased 66,857 and 8,143 shares, respectively, of Common Stock at a weighted average purchase price of $18.1595 per share in open market transactions. Each of the Reporting Persons 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 4.8% of the outstanding shares of Common Stock, and BCIPLS holds 401,244 shares of Common Stock, representing approximately 0.6% 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 3,695,667 shares of Common Stock, representing approximately 5.4% of the outstanding shares of Common Stock. The percentage of the outstanding shares of Common Stock held by the Reporting Persons is based on 68,497,425 shares of Common Stock outstanding as of November 4, 2020, as reported by the Issuer in its Quarterly Report on Form 10-Q for the quarter ended September 30, 2020.

(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.

SIGNATURES

After reasonable inquiry and to the best of its knowledge and belief, the undersigned certifies that the information in this statement is true, complete and correct.