UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): July 3, 2017

MERSANA THERAPEUTICS, INC.

(Exact name of registrant as specified in its charter)

(Registrant’s telephone number, including area code): (617) 498-0020

Not Applicable

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

o Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

o Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

o Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

o Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company x

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. x

Item 5.03 Amendments to Articles of Incorporation or Bylaws; Change in Fiscal Year.

On July 3, 2017, and in connection with the consummation of Mersana Therapeutics, Inc.’s (the “Company”) initial public offering of its common stock (the “IPO”), the Company filed a restated certificate of incorporation (the “Restated Certificate”) with the Secretary of State of the State of Delaware. The Restated Certificate became effective on July 3, 2017. The Company’s board of directors and stockholders previously approved the Restated Certificate to be filed in connection with the consummation of the IPO. The Restated Certificate restates the Company’s certificate of incorporation in its entirety to, among other things: (i) authorize 175 million shares of common stock; (ii) eliminate all references to previously-existing series of preferred stock; (iii) authorize 25 million shares of undesignated preferred stock that may be issued from time to time by the Company’s board of directors in one or more series; and (iv) eliminate the ability of the Company’s stockholders to take action by written consent in lieu of a meeting.

The foregoing description of the Restated Certificate is qualified by reference to the Restated Certificate, a copy of which is attached hereto as Exhibit 3.1 and is incorporated herein by reference.

Also on July 3, 2017, and in connection with the consummation of the IPO, the amended and restated bylaws of the Company (the “Amended and Restated Bylaws”), previously approved by the Company’s board of directors and stockholders to become effective immediately upon the consummation of the IPO, became effective. The Amended and Restated Bylaws amend and restate the Company’s bylaws in their entirety to, among other things: (i) establish procedures relating to the presentation of stockholder proposals at stockholder meetings; (ii) establish procedures relating to the nomination of directors; (iii) modify the indemnification provisions for the Company’s directors and officers; and (iv) conform to the amended provisions of the Restated Certificate.

The foregoing description of the Amended and Restated Bylaws is qualified by reference to the Amended and Restated Bylaws, a copy of which is attached hereto as Exhibit 3.2 and is incorporated herein by reference.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Date: July 10, 2017

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EXHIBIT INDEX

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