United States

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

MERSANA THERAPEUTICS, INC.

(Name of Issuer)

Common Stock, par value $0.0001 per share

(Title of Class of Securities)

59045L106

(CUSIP Number)

September 10, 2021

(Date of Event Which Requires Filing This Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

☐ Rule 13d-1(b)

☒ Rule 13d-1(c)

☐ Rule 13d-1(d)

The information required in 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 (“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).

2 of 7

CUSIP No. 59045L106

3 of 7

1(a) Name of Issuer: Mersana Therapeutics, Inc.

1(b) Address of Issuer’s Principal Executive Offices:

840 Memorial Drive, Cambridge, MA 02139, United States of America

2(a) Name of Person Filing: BB Biotech AG (“BB Biotech”) on behalf of its wholly-owned subsidiary, Biotech Target N.V. (“Biotech Target”)

2(b) Address of Principal Business Office or, if none, Residence:

BB Biotech AG: Schwertstrasse 6, CH-8200 Schaffhausen, Switzerland

Biotech Target N.V.: Ara Hill Top Building, Unit A-5, Pletterijweg Oost 1, Curaçao

2(c) Citizenship: BB Biotech AG: Switzerland

Biotech Target N.V.: Curacao

2(d) Title of Class of Securities Common Stock, par value $0.0001 per share

2(e) CUSIP Number 59045L106

If this statement is filed pursuant to Rule 13d-1(b), or 13d-2(b) or (c), check whether the person filing is a:

a. [ ] Broker or Dealer registered under Section 15 of the Act.

b. [ ] Bank as defined in Section 3(a)(6) of the Act.

c. [ ] Insurance company as defined in Section 3(a)(19) of the Act.

d. [ ] Investment company registered under section 8 of the Investment Company Act of 1940.

e. [ ] An investment adviser in accordance with Rule 13d-1(b)(1)(ii)(E);

f. [ ] An employee benefit plan or endowment fund in accordance with Rule 13d-1(b)(1)(ii)(F);

g. [ ] A parent holding company or control person in accordance with Rule 13d-1(b)(1)(ii)(G);

h. [ ] A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);

i. [ ] A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940;

j. [ ] Group, in accordance with Rule 13d-1(b)(1)(ii)(J).

4 of 7

Item 4. Ownership

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.

(a) Amount beneficially owned: 4,135,000

(b) Percent of class: 5.8%

(c) Number of shares as to which the person has:

(i) Sole power to vote or to direct the vote 0

(ii) Shared power to vote or to direct the vote 4,135,000

(iii) Sole power to dispose or to direct the disposition of 0

(iv) Shared power to dispose or to direct the disposition of 4,135,000

Item 5. Ownership of Five Percent or Less of a Class

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following: [     ]

Item 6. Ownership of More than Five Percent on Behalf of Another Person.

N/A

Item 7. Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company or Control Person.

This statement is filed jointly by BB Biotech and Biotech Target, Biotech Target is a wholly-owned subsidiary of BB Biotech.

Item 8. Identification and Classification of Members of the Group

N/A

Item 9. Notice of Dissolution of Group

N/A

5 of 7

Item 10. Certification

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

SIGNATURES

After reasonable inquiry and the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

BB Biotech AG

Biotech Target N.V.

6 of 7

Exhibit A

Joint Filing Statement

We, the undersigned, hereby express our agreement that the attached Schedule 13G (including all amendments thereto) is filed on behalf of each of the undersigned.

7 of 7