Day One buys Mersana (NASDAQ: MRSN); Bain Capital funds exit stake

Rhea-AI Filing Summary

Bain Capital Life Sciences-affiliated funds have fully exited their stake in Mersana Therapeutics, Inc. following its sale to Day One Biopharmaceuticals. This Schedule 13D amendment reports that Bain Capital Life Sciences Fund II, L.P., BCIP Life Sciences Associates, LP and BCLS II Investco, LP now beneficially own 0 shares of Mersana common stock, representing 0% of the class.

The change follows a tender offer and merger under which Day One’s subsidiary acquired all outstanding Mersana shares for $25.00 per share in cash plus one non-tradeable contingent value right per share, providing potential contingent milestone payments of up to an additional $30.25 per share in cash. The tender offer was accepted and the merger closed on January 6, 2026, with Mersana becoming a wholly owned subsidiary of Day One and its Bain-affiliated board representative, Dr. Andrew Hack, resigning at the merger’s effective time.

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Equity transactions & M&A analyst

Mersana is now wholly owned by Day One, and Bain’s funds report owning 0%.

The filing shows the completion of Day One Biopharmaceuticals’ acquisition of Mersana Therapeutics. A tender offer was launched at $25.00 per share in cash plus a contingent value right (CVR) worth up to an additional $30.25 per share in potential milestone payments. On January 6, 2026, Day One’s acquisition vehicle accepted the tendered shares and merged with Mersana, making Mersana a wholly owned subsidiary.

For the Bain Capital Life Sciences entities, all previously held Mersana common shares were cancelled and converted into the right to receive the stated offer consideration, leaving them with 0% beneficial ownership after the merger. Governance influence tied to their stake also ended, as their representative, Dr. Andrew Hack, resigned from Mersana’s board at the merger’s effective time. Subsequent company filings may provide more detail on CVR milestone mechanics and actual payments, if achieved.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 8)

Mersana Therapeutics, Inc.

(Name of Issuer)

Common Stock, par value $0.0001 per share

(Title of Class of Securities)

59045L205

(CUSIP Number)

Bain Capital Life Sciences Inv
200 Clarendon Street,
Boston, MA, 02116
617-516-2000

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

01/06/2026

(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.

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 ("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).

SCHEDULE 13D

CUSIP No. 59045L205

1

Name of reporting person

Bain Capital Life Sciences Fund II, L.P.

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

WC

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

CAYMAN ISLANDS

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

11

Aggregate amount beneficially owned by each reporting person

0.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

0 %

14

Type of Reporting Person (See Instructions)

PN

SCHEDULE 13D

CUSIP No. 59045L205

1

Name of reporting person

BCIP Life Sciences Associates, LP

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

WC

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

11

Aggregate amount beneficially owned by each reporting person

0.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

0 %

14

Type of Reporting Person (See Instructions)

PN

SCHEDULE 13D

CUSIP No. 59045L205

1

Name of reporting person

BCLS II Investco, LP

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

WC

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

11

Aggregate amount beneficially owned by each reporting person

0.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

0 %

14

Type of Reporting Person (See Instructions)

PN

SCHEDULE 13D

Item 1.	Security and Issuer
(a)	Title of Class of Securities: Common Stock, par value $0.0001 per share
(b)	Name of Issuer: Mersana Therapeutics, Inc.
(c)	Address of Issuer's Principal Executive Offices: 840 Memorial Drive, Cambridge, MASSACHUSETTS , 02139.
Item 1 Comment: This Amendment No. 8 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, Amendment No. 4 filed on April 14, 2022, Amendment No. 5 filed on May 11, 2022, Amendment No. 6 filed on November 15, 2024 and Amendment No. 7 filed on November 13, 2025 (the "Initial Statement" and, as further amended by this Amendment No. 8, the "Schedule 13D"). Capitalized terms used but not defined herein shall have the meanings ascribed to them in the Initial Statement.
Item 4.	Purpose of Transaction
	Item 4 of the Initial Statement is hereby amended and supplemented to add the following: As previously disclosed, on November 12, 2025 the Issuer entered into an Agreement and Plan of Merger (the "Merger Agreement") by and among the Issuer, Day One Biopharmaceuticals, Inc. ("Parent"), and Emerald Merger Sub, Inc., a direct, wholly owned subsidiary of Parent ("Purchaser"), pursuant to which, on December 5, 2025, Purchaser commenced a tender offer (the "Offer") to acquire all of the issued and outstanding shares of the Issuer for (i) $25.00 per Share, net to the stockholder in cash without interest and less any applicable tax withholding, plus (ii) one non-tradeable contingent value right per share, which represents the right to receive certain contingent milestone payments of up to an aggregate of $30.25 per share in cash, without interest and less any applicable tax withholding (collectively, the "Offer Price"). On January 6, 2026, Purchaser irrevocably accepted for payment the shares of the Issuer's Common Stock that were validly tendered and not validly withdrawn pursuant to the Offer and, following the consummation of the Offer, pursuant to the terms and conditions of the Merger Agreement, on January 6, 2026, Purchaser was merged with and into the Issuer (the "Merger"), with the Issuer surviving the Merger as a direct wholly-owned subsidiary of Parent. Dr. Hack resigned as a director of the Issuer effective as of the effective time of the Merger (the "Effective Time").
Item 5.	Interest in Securities of the Issuer
(a)	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. Following the Consummation of the Merger on January 6, 2026, the Rerporting Persons ceased to hold any shares of Common Stock.
(b)	See Item 5(a) hereof.
(c)	At the Effective Time, pursuant to the terms of the Merger Agreement, all shares of Common Stock held by the Reporting Persons were cancelled and converted into the right to receive the Offer Price, without interest, subject to any applicable withholding tax.
(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)	See Item 5(a) hereof.

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.

Bain Capital Life Sciences Fund II, L.P.

Signature:	/s/ Andrew Hack
Name/Title:	Andrew Hack, Partner of Bain Capital Life Sciences Investors, LLC
Date:	01/08/2026

BCIP Life Sciences Associates, LP

Signature:	/s/ Andrew Hack
Name/Title:	Andrew Hack, Authorized Signatory of Boylston Coinvestors, LLC
Date:	01/08/2026

BCLS II Investco, LP

Signature:	/s/ Andrew Hack
Name/Title:	Andrew Hack, Partner of Bain Capital Life Sciences Investors, LLC
Date:	01/08/2026

FAQ

What does this Schedule 13D/A filing report for Mersana (MRSN)?

It reports that Bain Capital Life Sciences Fund II, L.P., BCIP Life Sciences Associates, LP and BCLS II Investco, LP now beneficially own 0 shares of Mersana common stock, representing 0% of the class after Mersana’s sale to Day One Biopharmaceuticals.

What were Mersana (MRSN) shareholders offered in the Day One acquisition?

Shareholders were offered $25.00 per share in cash, plus one non-tradeable contingent value right per share providing potential contingent milestone payments of up to an additional $30.25 per share in cash.

When did the Mersana–Day One merger close?

The tender offer was accepted and the merger was consummated on January 6, 2026, after which Mersana became a direct wholly owned subsidiary of Day One Biopharmaceuticals, Inc..

Why do Bain Capital Life Sciences entities report 0% ownership of Mersana (MRSN)?

At the merger’s effective time, all Mersana common shares held by the Bain Capital Life Sciences entities were cancelled and converted into the right to receive the cash plus CVR offer price, leaving them with no remaining beneficial ownership.

What happened to Bain’s board representative at Mersana after the merger?

Dr. Andrew Hack, a Bain Capital Life Sciences partner and director of Mersana, resigned as a director effective as of the merger’s effective time on January 6, 2026.

Which entities are listed as reporting persons in this Mersana (MRSN) Schedule 13D/A?

The reporting persons are Bain Capital Life Sciences Fund II, L.P., BCIP Life Sciences Associates, LP and BCLS II Investco, LP, each reporting 0 shares and 0% beneficial ownership of Mersana common stock.