# EDGAR Filing Document

**Accession Number:** 0001442836
**File Stem:** 0001104659-26-001303
**Filing Date:** 2026-1
**Character Count:** 10307
**Document Hash:** 3d5704ec28f540c1e45e98eadb28a8af
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001104659-26-001303.hdr.sgml**: 20260106

**ACCESSION NUMBER**: 0001104659-26-001303

**CONFORMED SUBMISSION TYPE**: S-8 POS

**PUBLIC DOCUMENT COUNT**: 1

**FILED AS OF DATE**: 20260106

**DATE AS OF CHANGE**: 20260106

**EFFECTIVENESS DATE**: 20260106

**FILER**: 

**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:** S-8 POS
- **SEC ACT:** 1933 Act
- **SEC FILE NUMBER:** 333-230159
- **FILM NUMBER:** 26511573

**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

**As filed with the Securities and Exchange Commission on January 6, 2026**

**Registration No. 333-219388**

**Registration No. 333-222845**

**Registration No. 333-230159**

**Registration No. 333-236775**

**Registration No. 333-255975**

**Registration No. 333-263085**

**Registration No. 333-270110**

**Registration No. 333-276492**

**Registration No. 333-284273**

**UNITED STATES**

**SECURITIES AND EXCHANGE COMMISSION**

**Washington, D.C. 20549**

**POST-EFFECTIVE AMENDMENT NO. 1 TO FORM S-8 REGISTRATION STATEMENT NO. 333-219388**

**POST-EFFECTIVE AMENDMENT NO. 1 TO FORM S-8 REGISTRATION STATEMENT NO. 333-222845**

**POST-EFFECTIVE AMENDMENT NO. 1 TO FORM S-8 REGISTRATION STATEMENT NO. 333-230159**

**POST-EFFECTIVE AMENDMENT NO. 1 TO FORM S-8 REGISTRATION STATEMENT NO. 333-236775**

**POST-EFFECTIVE AMENDMENT NO. 1 TO FORM S-8 REGISTRATION STATEMENT NO. 333-255975**

**POST-EFFECTIVE AMENDMENT NO. 1 TO FORM S-8 REGISTRATION STATEMENT NO. 333-263085**

**POST-EFFECTIVE AMENDMENT NO. 1 TO FORM S-8 REGISTRATION STATEMENT NO. 333-270110**

**POST-EFFECTIVE AMENDMENT NO. 1 TO FORM S-8 REGISTRATION STATEMENT NO. 333-276492**

**POST-EFFECTIVE AMENDMENT NO. 1 TO FORM S-8 REGISTRATION STATEMENT NO. 333-284273**

***Under***

***The Securities Act of 1933***

**MERSANA THERAPEUTICS, INC.**

**(Exact name of registrant as specified in its charter)**

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| | |
|:---|:---|
| **Delaware** | **04-3562403** |
| **(State or other jurisdiction of<br> incorporation or organization)** | **(I.R.S. Employer<br> Identification No.)** |
| **840 Memorial Drive**<br> **Cambridge, Massachusetts** | **02139** |
| **(Address of Principal Executive Offices)** | **(Zip Code)** |

---

**Mersana Therapeutics, Inc. 2007 Stock Incentive Plan, as amended**

**Mersana Therapeutics, Inc. 2017 Stock Incentive Plan**

**Mersana Therapeutics, Inc. 2017 Employee Stock Purchase Plan**

**Inducement Stock Option Awards**

**Mersana Therapeutics, Inc. 2022 Inducement Stock Incentive Plan** 

**(Full title of the plans)**

**Jeremy Bender, Ph.D., M.B.A.**

**Chief Executive Officer and President**

**Day One Biopharmaceuticals, Inc.**

**1800 Sierra Point Parkway, Suite 200**

**Brisbane, California 94005**

**(650) 484-0899**

**(Name, address, and telephone number, including area code, of agent for service)**

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***Copy to:***

**Robert A. Freedman**

**David Michaels**

**Julia Forbess**

**Fenwick & West LLP**

**401 Union Street, Floor 5**

**Seattle, Washington 98101**

**(206) 389-4510**

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Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of "large accelerated filer," "accelerated filer," "smaller reporting company," and "emerging growth company" in Rule 12b-2 of the Exchange Act.

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| | | | |
|:---|:---|:---|:---|
| Large accelerated filer | ¨ | Accelerated filer | ¨ |
| Non-accelerated filer | x | Smaller reporting company | x |
|  |  | Emerging growth company | ¨ |

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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 7(a)(2)(B) of the Securities Act. ◻

**DEREGISTRATION OF SECURITIES**

These Post-Effective Amendments (these "<u>Post-Effective Amendments</u>") filed by Mersana Therapeutics, Inc. (the "<u>Registrant</u>") relate to the following registration statements on Form S-8 (together, the "<u>Registration Statements</u>"):

&nbsp;&nbsp;&nbsp;&nbsp;1. Registration Statement No. 333-219388, registering (i) 125,665 shares of common stock, $0.0001 par value per share, of the
Registrant (" <u>Common Stock</u> ") issuable under the Registrant's 2007 Stock Incentive Plan, as amended (the " <u>2007 Plan</u> "), (ii) 90,200 shares of Common Stock issuable under the Registrant's 2017 Stock Incentive Plan (the " <u>2017 Plan</u> ") and (iii) 9,000 shares of Common Stock issuable under the Registrant's 2017 Employee Stock Purchase Plan (the
 " <u>2017 ESPP</u> "), which was filed with the U.S. Securities and Exchange Commission (the " <u>SEC</u> ") on July 21,
2017. &nbsp;&nbsp;&nbsp;&nbsp;2. Registration Statement No. 333-222845, registering 36,424 shares of Common Stock issuable under the 2017 Plan, which was filed
with the SEC on February 2, 2018.

&nbsp;&nbsp;&nbsp;&nbsp;3. Registration Statement No. 333-230159, registering (i) 37,175 shares of Common Stock issuable under the 2017 Plan and (ii) 9,293
shares of Common Stock issuable under the 2017 ESPP, which was filed with the SEC on March 8, 2019.

&nbsp;&nbsp;&nbsp;&nbsp;4. Registration Statement No. 333-236775, registering (i) 72,620 shares of Common Stock issuable under the 2017 Plan and (ii) 18,000
shares of Common Stock issuable under the 2017 ESPP, which was filed with the SEC on February 28, 2020.

&nbsp;&nbsp;&nbsp;&nbsp;5. Registration Statement No. 333-255975, registering (i) 110,146 shares of Common Stock issuable under the 2017 Plan and (ii) an
aggregate of 21,300 shares of Common Stock issuable under Inducement Stock Options granted to certain employees of the Registrant on August 31,
2020, November 30, 2020, January 19, 2021 and April 26, 2021, which was filed with the SEC on May 10, 2021.

&nbsp;&nbsp;&nbsp;&nbsp;6. Registration Statement No. 333-263085, registering (i) 117,934 shares of Common Stock issuable under the 2017 Plan, (ii) 80,000
shares of Common Stock issuable under the Registrant's 2022 Inducement Stock Incentive Plan and (iii) an aggregate of 9,000
shares of Common Stock issuable under Inducement Stock Options granted to certain employees of the Registrant on August 16, 2021
and October 25, 2021, which was filed with the SEC on February 28, 2022.

&nbsp;&nbsp;&nbsp;&nbsp;7. Registration Statement No. 333-270110, registering (i) 168,231 shares of Common Stock issuable under the 2017 Plan and (ii) 18,000
shares of Common Stock issuable under the 2017 ESPP, which was filed with the SEC on February 28, 2023.

&nbsp;&nbsp;&nbsp;&nbsp;8. Registration Statement No. 333-276492, registering (i) 193,138 shares of Common Stock issuable under the 2017 Plan and (ii) 18,000
shares of Common Stock issuable under the 2017 ESPP, which was filed with the SEC on January 12, 2024.

&nbsp;&nbsp;&nbsp;&nbsp;9. Registration Statement No. 333-284273, registering (i) 185,039 shares of Common Stock issuable under the 2017 Plan and (ii) 18,000
shares of Common Stock issuable under the 2017 ESPP, which was filed with the SEC on January 14, 2025.

All share numbers set forth above reflect a 1-for-25 reverse stock split of Common Stock, that became effective on July 25, 2025.

On January 6, 2026, pursuant to the Agreement and Plan of Merger, dated as of November 12, 2025 (the "<u>Merger Agreement</u>"), by and among the Registrant, Day One Biopharmaceuticals, Inc., a Delaware corporation ("<u>Parent</u>") and Emerald Merger Sub, Inc., a Delaware corporation and a direct wholly-owned subsidiary of Parent ("<u>Purchaser</u>"), Purchaser merged with and into the Registrant, with the Registrant surviving as a direct wholly-owned subsidiary of Parent.

As a result of the transactions contemplated by the Merger Agreement, the Registrant has terminated any and all offerings and sales of securities pursuant to the Registration Statements. In accordance with the undertakings made by the Registrant in the Registration Statements to remove from registration, by means of a post-effective amendment, any of the securities of the Registrant registered under the Registration Statements which remain unsold at the termination of the offerings, the Registrant hereby terminates the effectiveness of the Registration Statements and removes from registration all of the securities that remain unsold under the Registration Statements as of the date hereof, if any. The Registrant is filing these Post-Effective Amendments to withdraw and remove from registration all securities registered pursuant to the Registration Statements that remain unsold as of the date hereof. The Registration Statements are hereby amended, as appropriate, to reflect the deregistration of such securities. After giving effect to these Post-Effective Amendments, there will be no remaining securities registered by the Registrant pursuant to the Registration Statements.

**SIGNATURE**

Pursuant to the requirements of the Securities Act of 1933, as amended, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and has duly caused these Post-Effective Amendments to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Brisbane, State of California, on January 6, 2026.

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| | |
|:---|:---|
| **MERSANA THERAPEUTICS, INC.** | **MERSANA THERAPEUTICS, INC.** |
| By: | /s/ Charles N. York II |
| Name: | Charles N. York II, M.B.A. |
| Title: | Chief Executive Officer |

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Pursuant to Rule 478 under the Securities Act of 1933, as amended, no other person is required to sign these Post-Effective Amendments.