# EDGAR Filing Document

**Accession Number:** 0001773187
**File Stem:** 0001193125-25-280672
**Filing Date:** 2025-11
**Character Count:** 13079
**Document Hash:** 01cf65775b6dd449cf1d7e6eece108ca
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001193125-25-280672.hdr.sgml**: 20251113

**ACCESSION NUMBER**: 0001193125-25-280672

**CONFORMED SUBMISSION TYPE**: SCHEDULE 13D/A

**PUBLIC DOCUMENT COUNT**: 1

**FILED AS OF DATE**: 20251113

**DATE AS OF CHANGE**: 20251113

**SUBJECT COMPANY**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** Mersana Therapeutics, Inc.
- **CENTRAL INDEX KEY:** 0001442836
- **STANDARD INDUSTRIAL CLASSIFICATION:** PHARMACEUTICAL PREPARATIONS [2834]
- **ORGANIZATION NAME:** 03 Life Sciences
- **EIN:** 000000000
- **STATE OF INCORPORATION:** DE
- **FISCAL YEAR END:** 1231

**FILING VALUES:**
- **FORM TYPE:** SCHEDULE 13D/A
- **SEC ACT:** 1934 Act
- **SEC FILE NUMBER:** 005-90033
- **FILM NUMBER:** 251480316

**BUSINESS ADDRESS:**
- **STREET 1:** 840 MEMORIAL DRIVE
- **CITY:** CAMBRIDGE
- **STATE:** MA
- **ZIP:** 02139
- **BUSINESS PHONE:** 617-498-0020

**MAIL ADDRESS:**
- **STREET 1:** 840 MEMORIAL DRIVE
- **CITY:** CAMBRIDGE
- **STATE:** MA
- **ZIP:** 02139

**FORMER COMPANY:**
- **FORMER CONFORMED NAME:** MERSANA THERAPEUTICS INC
- **DATE OF NAME CHANGE:** 20080813
**FILED BY**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** Bain Capital Life Sciences Fund II, L.P.
- **CENTRAL INDEX KEY:** 0001773187

**ORGANIZATION NAME:**
- **EIN:** 000000000
- **STATE OF INCORPORATION:** E9
- **FISCAL YEAR END:** 1231

**FILING VALUES:**
- **FORM TYPE:** SCHEDULE 13D/A

**BUSINESS ADDRESS:**
- **STREET 1:** 200 CLARENDON STREET
- **CITY:** BOSTON
- **STATE:** MA
- **ZIP:** 02116
- **BUSINESS PHONE:** 6175162000

**MAIL ADDRESS:**
- **STREET 1:** 200 CLARENDON STREET
- **CITY:** BOSTON
- **STATE:** MA
- **ZIP:** 02116

### UNITED STATES SECURITIES AND EXCHANGE COMMISSION
**Washington, D.C. 20549**

## SCHEDULE 13D

### Under the Securities Exchange Act of 1934

**(Amendment No. 7)**

**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**<br>200 Clarendon Street<br>Boston MA 02116<br>617-516-2000

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

**11/12/2025**

*(Date of Event Which Requires Filing of this Statement)*

| **CUSIP No.** | **59045L205** |

---

| | | | |
|:--|:--|:--|:--|
| 1 | Name of reporting person<br>**Bain Capital Life Sciences Fund II, L.P.** | Name of reporting person<br>**Bain Capital Life Sciences Fund II, L.P.** | |
| 2 | Check the appropriate box if a member of a Group (See Instructions)<br>[x] (a)<br>[ ] (b) | Check the appropriate box if a member of a Group (See Instructions)<br>[x] (a)<br>[ ] (b) | |
| 3 | SEC use only | SEC use only | |
| 4 | Source of funds (See Instructions)<br>**WC** | Source of funds (See Instructions)<br>**WC** | |
| 5 | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | |
| 6 | Citizenship or place of organization<br>**CAYMAN ISLANDS** | Citizenship or place of organization<br>**CAYMAN ISLANDS** | |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 7 | Sole Voting Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 8 | Shared Voting Power<br>**131776.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 9 | Sole Dispositive Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 10 | Shared Dispositive Power<br>**131776.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**131776.00** | Aggregate amount beneficially owned by each reporting person<br>**131776.00** | |
| 12 | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | |
| 13 | Percent of class represented by amount in Row (11)<br>**2.6%** | Percent of class represented by amount in Row (11)<br>**2.6%** | |
| 14 | Type of Reporting Person (See Instructions)<br>**PN** | Type of Reporting Person (See Instructions)<br>**PN** | |

---

| **CUSIP No.** | **59045L205** |

---

| | | | |
|:--|:--|:--|:--|
| 1 | Name of reporting person<br>**BCIP Life Sciences Associates, LP** | Name of reporting person<br>**BCIP Life Sciences Associates, LP** | |
| 2 | Check the appropriate box if a member of a Group (See Instructions)<br>[x] (a)<br>[ ] (b) | Check the appropriate box if a member of a Group (See Instructions)<br>[x] (a)<br>[ ] (b) | |
| 3 | SEC use only | SEC use only | |
| 4 | Source of funds (See Instructions)<br>**WC** | Source of funds (See Instructions)<br>**WC** | |
| 5 | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | |
| 6 | Citizenship or place of organization<br>**DELAWARE** | Citizenship or place of organization<br>**DELAWARE** | |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 7 | Sole Voting Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 8 | Shared Voting Power<br>**16049.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 9 | Sole Dispositive Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 10 | Shared Dispositive Power<br>**16049.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**16049.00** | Aggregate amount beneficially owned by each reporting person<br>**16049.00** | |
| 12 | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | |
| 13 | Percent of class represented by amount in Row (11)<br>**0.3%** | Percent of class represented by amount in Row (11)<br>**0.3%** | |
| 14 | Type of Reporting Person (See Instructions)<br>**PN** | Type of Reporting Person (See Instructions)<br>**PN** | |

---

| **CUSIP No.** | **59045L205** |

---

| | | | |
|:--|:--|:--|:--|
| 1 | Name of reporting person<br>**BCLS II Investco, LP** | Name of reporting person<br>**BCLS II Investco, LP** | |
| 2 | Check the appropriate box if a member of a Group (See Instructions)<br>[x] (a)<br>[ ] (b) | Check the appropriate box if a member of a Group (See Instructions)<br>[x] (a)<br>[ ] (b) | |
| 3 | SEC use only | SEC use only | |
| 4 | Source of funds (See Instructions)<br>**WC** | Source of funds (See Instructions)<br>**WC** | |
| 5 | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | |
| 6 | Citizenship or place of organization<br>**DELAWARE** | Citizenship or place of organization<br>**DELAWARE** | |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 7 | Sole Voting Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 8 | Shared Voting Power<br>**198720.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 9 | Sole Dispositive Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 10 | Shared Dispositive Power<br>**198720.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**198720.00** | Aggregate amount beneficially owned by each reporting person<br>**198720.00** | |
| 12 | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | |
| 13 | Percent of class represented by amount in Row (11)<br>**4.0%** | Percent of class represented by amount in Row (11)<br>**4.0%** | |
| 14 | Type of Reporting Person (See Instructions)<br>**PN** | Type of Reporting Person (See Instructions)<br>**PN** | |

---

**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, MA, 02139

This Amendment No. 7 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 and Amendment No. 6 filed on November 15, 2024 (the "Initial Statement" and, as further amended by this Amendment No. 7, 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 disclosed by the Issuer in its Current Report on Form 8-K, filed with the Securities and Exchange Commission on November 13, 2025, 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 ("Merger Sub"), pursuant to which Merger Sub will commence a tender offer (the "Offer") to acquire all of the issued and outstanding shares of the Issuer.

In connection with the entry into the Merger Agreement, the Reporting Persons entered into a Tender and Support Agreement (the "Support Agreement") with Parent and Merger Sub, dated as of November 12, 2025, pursuant to which the Reporting Persons agreed, among other things, to  (i) to tender all of the shares of Common Stock held by the Reporting Persons in the Offer, subject to certain exceptions (including the valid termination of the Merger Agreement), (ii) to, if applicable, vote all of the Reporting Perons's shares of Common Stock in favor of the Merger, and (iii) to certain other restrictions on its ability to take actions with respect to the Company and its Common Stock.

The foregoing summary of the Support Agreement does not purport to be complete and is qualified in its entirety by reference to the Support Agreement, which is filed as an exhibit hereto and incorporated by reference herein.

**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.

As of the date hereof, (i) BCLS II holds 131,776 shares of Common Stock, representing approximately 2.6% of the outstanding shares of Common Stock, (ii) BCIPLS holds 16,049 shares of Common Stock, representing approximately 0.3% of the outstanding shares of Common Stock, and (iii) BCLS II Investco holds 198,720 shares of Common Stock, representing approximately 4.0% 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 collectively beneficially own an aggregate of 346,545 shares of Common Stock, representing approximately 6.9% of the outstanding shares of Common Stock. The percentage of the outstanding shares of Common Stock held by the Reporting Persons is based on 4,990,692 shares of Common Stock outstanding as of August 8, 2025, as reported by the Issuer in its Quarterly Report on Form 10-Q for the quarter ended June 30, 2025.

**(b)**
See Item 5(a) hereof.

**(c)**
Not applicable.

### 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.

**Reporting Person:** 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:** 11/13/2025

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.

**Reporting Person:** BCIP Life Sciences Associates, LP

**Signature:** /s/ Andrew Hack

**Name/Title:** Andrew Hack, Authorized Signatory of Boylston Coinvestors, LLC

**Date:** 11/13/2025

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.

**Reporting Person:** BCLS II Investco, LP

**Signature:** /s/ Andrew Hack

**Name/Title:** Andrew Hack, Partner of Bain Capital Life Sciences Investors, LLC

**Date:** 11/13/2025