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Filing Details

Accession Number:: 0000899140-19-000289
Form Type:: 13G Filing
Publication Date:: 2019-03-11 16:18:22

Filed By:: Sarissa Capital Management
Company:: Mersana Therapeutics Inc. (NASDAQ:MRSN)
Filing Date:: 2019-03-11
SEC Url:: 13G Filing

Ownership Summary

Please notice the below summary table is generated without human intervention and may contain errors. Please refer to the complete filing displayed below for exact figures.

Name	Sole Voting Power	Shared Voting Power	Sole Dispositive Power	Shared Dispositive Power	Aggregate Amount Owned Power	Percent of Class
Sarissa Capital Management	0	3,750,000	0	3,750,000	3,750,000	7.9%
Alexander J. Denner, Ph.D	0	3,750,000	0	3,750,000	3,750,000	7.9%
Sarissa Capital Offshore Master Fund	2,531,000	0	2,531,000	0	2,531,000	5.3%
Sarissa Capital Catapult Fund	761,000	0	761,000	0	761,000	1.6%
Sarissa Capital Hawkeye Fund	458,000	0	458,000	0	458,000	1.0%

Filing

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No. )*

Mersana Therapeutics, Inc.

(Name of Issuer)

Common Stock, par value $0.0001 per share

(Title of Class of Securities)

59045L106

(CUSIP Number)

March 1, 2019

(Date of Event Which Requires Filing of 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 remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

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 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).

CUSIP No. 59045L106				13G	Page 2 of 11 Pages

1.	NAMES OF REPORTING PERSONS Sarissa Capital Management LP
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a) ☐ (b) ☐
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 0
		6.	SHARED VOTING POWER 3,750,000
		7.	SOLE DISPOSITIVE POWER 0
		8.	SHARED DISPOSITIVE POWER 3,750,000
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 3,750,000
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 7.9%(1)
12.	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) PN

(1)	Calculations are based upon 47,684,164 Shares outstanding as of March 7, 2019, as set forth in the Form 10-K of the Issuer filed with the U.S. Securities and Exchange Commission on March 8, 2019.

CUSIP No. 59045L106				13G	Page 3 of 11 Pages

1.	NAMES OF REPORTING PERSONS Alexander J. Denner, Ph.D.
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a) ☐ (b) ☐
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION United States
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 0
		6.	SHARED VOTING POWER 3,750,000
		7.	SOLE DISPOSITIVE POWER 0
		8.	SHARED DISPOSITIVE POWER 3,750,000
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 3,750,000
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 7.9%(1)
12.	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) IN

(1)	Calculations are based upon 47,684,164 Shares outstanding as of March 7, 2019, as set forth in the Form 10-K of the Issuer filed with the U.S. Securities and Exchange Commission on March 8, 2019.

CUSIP No. 59045L106				13G	Page 4 of 11 Pages

1.	NAMES OF REPORTING PERSONS Sarissa Capital Offshore Master Fund LP
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a) ☐ (b) ☐
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION Cayman Islands
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 2,531,000
		6.	SHARED VOTING POWER 0
		7.	SOLE DISPOSITIVE POWER 2,531,000
		8.	SHARED DISPOSITIVE POWER 0
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,531,000
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 5.3%(1)
12.	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) PN

(1)	Calculations are based upon 47,684,164 Shares outstanding as of March 7, 2019, as set forth in the Form 10-K of the Issuer filed with the U.S. Securities and Exchange Commission on March 8, 2019.

CUSIP No. 59045L106				13G	Page 5 of 11 Pages

1.	NAMES OF REPORTING PERSONS Sarissa Capital Catapult Fund LLC
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a) ☐ (b) ☐
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 761,000
		6.	SHARED VOTING POWER 0
		7.	SOLE DISPOSITIVE POWER 761,000
		8.	SHARED DISPOSITIVE POWER 0
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 761,000
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 1.6%(1)
12.	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) OO

(1)	Calculations are based upon 47,684,164 Shares outstanding as of March 7, 2019, as set forth in the Form 10-K of the Issuer filed with the U.S. Securities and Exchange Commission on March 8, 2019.

CUSIP No. 59045L106				13G	Page 6 of 11 Pages

1.	NAMES OF REPORTING PERSONS Sarissa Capital Hawkeye Fund LP
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a) ☐ (b) ☐
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 458,000
		6.	SHARED VOTING POWER 0
		7.	SOLE DISPOSITIVE POWER 458,000
		8.	SHARED DISPOSITIVE POWER 0
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 458,000
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 1.0%(1)
12.	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) PN

(1)	Calculations are based upon 47,684,164 Shares outstanding as of March 7, 2019, as set forth in the Form 10-K of the Issuer filed with the U.S. Securities and Exchange Commission on March 8, 2019.

Item 1(a).

Name of Issuer:

Mersana Therapeutics, Inc. (the “Issuer”)

Item 1(b).

Address of Issuer’s Principal Executive Offices:

840 Memorial Drive, Cambridge, Massachusetts 02139

Item 2(a).

Name of Person Filing:

The persons filing this statement are Sarissa Capital Management LP, a Delaware limited partnership (“Sarissa Capital”), Sarissa Capital Offshore Master Fund LP, a Cayman Islands exempted limited partnership (“Sarissa Offshore”), Sarissa Capital Catapult Fund LLC, a Delaware limited liability company (“Sarissa Catapult”), Sarissa Capital Hawkeye Fund LP, a Delaware limited partnership (“Sarissa Hawkeye” and, together with Sarissa Offshore and Sarissa Catapult, the “Sarissa Funds”), and Alexander J. Denner, Ph.D., a citizen of the United States of America (collectively, the “Reporting Persons”).

Item 2(b).

Address of Principal Business Office or, if None, Residence:

The principal business address of (i) each of Sarissa Capital, Sarissa Catapult, Sarissa Hawkeye and Dr. Denner is c/o Sarissa Capital Management LP, 660 Steamboat Road, Greenwich, CT 06830 and (ii) Sarissa Offshore is c/o Ogier Fiduciary Services (Cayman) Limited, 89 Nexus Way, Camana Bay, Grand Cayman KY1-9007, Cayman Islands.

Item 2(c).

Citizenship:

See Item 2(a).

Item 2(d).

Title of Class of Securities:

Common Stock, $0.0001 par value per share (“Shares”).

Item 2(e).

CUSIP Number:

59045L106

Item 3.

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

Not applicable.

Item 4.

Ownership.

The information required by Items 4(a)-(c) is set forth in Rows 5-11 of the cover pages hereto for each Reporting Person and is incorporated herein by reference for each such Reporting Person.

Sarissa Offshore has sole voting power and sole dispositive power with regard to 2,531,000 Shares. Sarissa Catapult has sole voting power and sole dispositive power with regard to 761,000 Shares. Sarissa Hawkeye has sole voting power and sole dispositive power with regard to 458,000 Shares. Sarissa Capital, as the investment advisor to the Sarissa Funds, may be deemed to have the shared power to vote or direct the vote of (and the shared power to dispose or direct the disposition of) the 3,750,000 Shares held directly by the Sarissa Funds. By virtue of his position as the Chief Investment Officer of Sarissa Capital and by virtue of his control of the ultimate general partner of each of Sarissa Capital, Sarissa Offshore and Sarissa Hawkeye and of the managing member of Sarissa Catapult, Dr. Denner may be deemed to have the shared power to vote or direct the vote of (and the shared power to dispose or direct the disposition of) the 3,750,000 Shares held directly by the Sarissa Funds.

Item 5.

Ownership of Five Percent or Less of a Class.

Not applicable.

Item 6.

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

Not applicable.

Item 7.

Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company.

Not applicable.

Item 8.

Identification and Classification of Members of the Group.

The Reporting Persons are making this single, joint filing because they may be deemed to constitute a “group” within the meaning of Section 13(d)(3) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”). The agreement among the Reporting Persons to file jointly in accordance with Rule 13d-1(k) of the Exchange Act is attached hereto as Exhibit 99.1. The Reporting Persons disclaim membership in a group and this report shall not be deemed an admission by any of the Reporting Persons that they are or may be members of a “group” for purposes of Rule 13d-5 or for any other purpose.

Item 9.

Notice of Dissolution of Group.

Not applicable.

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, other than activities solely in connection with a nomination under § 240.14a-11.

SIGNATURE

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

Dated: March 11, 2019

SARISSA CAPITAL MANAGEMENT LP

By:	/s/ Mark DiPaolo
	Name: Mark DiPaolo
	Title: General Counsel

SARISSA CAPITAL OFFSHORE MASTER FUND LP

By: Sarissa Capital Offshore Fund GP LLC, its general partner

By:	/s/ Mark DiPaolo
	Name: Mark DiPaolo
	Title: Authorized Person

SARISSA CAPITAL CATAPULT FUND LLC

By: Sarissa Capital Management LP, its managing member

By:	/s/ Mark DiPaolo
	Name: Mark DiPaolo
	Title: General Counsel

SARISSA CAPITAL HAWKEYE FUND LP

By: Sarissa Capital Fund GP LP, its general partner

By:	/s/ Mark DiPaolo
	Name: Mark DiPaolo
	Title: Authorized Person

/s/Alexander J. Denner
Alexander J. Denner

[Signature Page to Schedule 13G]

EXHIBIT INDEX



Exhibit 99.1		Joint Filing Agreement, as required by Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended