# EDGAR Filing Document

**Accession Number:** 0001442836
**File Stem:** 0001104659-25-055240
**Filing Date:** 2025-6
**Character Count:** 20526
**Document Hash:** 8461ae9fbf2dba8c6dd0c08837a933b2
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001104659-25-055240.hdr.sgml**: 20250602

**ACCESSION NUMBER**: 0001104659-25-055240

**CONFORMED SUBMISSION TYPE**: 8-K

**PUBLIC DOCUMENT COUNT**: 13

**CONFORMED PERIOD OF REPORT**: 20250602

**ITEM INFORMATION**: Regulation FD Disclosure

**ITEM INFORMATION**: Other Events

**ITEM INFORMATION**: Financial Statements and Exhibits

**FILED AS OF DATE**: 20250602

**DATE AS OF CHANGE**: 20250602

**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:** 8-K
- **SEC ACT:** 1934 Act
- **SEC FILE NUMBER:** 001-38129
- **FILM NUMBER:** 251013201

**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

?xml version='1.0' encoding='ASCII'?

**UNITED STATES**

**SECURITIES AND EXCHANGE COMMISSION**

**Washington, D.C. 20549**

**FORM 8-K**

**CURRENT REPORT**

**Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934**

Date of Report (Date of earliest event reported): **June 2, 2025**

**MERSANA THERAPEUTICS, INC.**

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

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| | | |
|:---|:---|:---|
| **Delaware** | **001-38129** | **04-3562403** |
| **(State or other jurisdiction <br> of incorporation)** | **(Commission**<br> **File Number)**  | **(IRS Employer**<br> **Identification No.)**  |

---

---

| | |
|:---|:---|
| **840 Memorial Drive** **<br> Cambridge**, Massachusetts** | **02139** |
| **(Address of Principal Executive Offices)** | **(Zip Code)** |

---

Registrant's telephone number, including area code: **(617) 498-0020**

**Not Applicable**

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (<u>see</u> General Instruction A.2. below):

◻ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

◻ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

◻ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

◻ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

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| | | |
|:---|:---|:---|
| **<u>Title of each class</u>** | **<u>Trading Symbol(s)</u>** | **<u>Name of each exchange on which registered</u>** |
| Common Stock, $0.0001 par value | MRSN | The Nasdaq Stock Market LLC |

---

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company ◻

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 13(a) of the Exchange Act. ◻

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| | |
|:---|:---|
| **Item 7.01** | **Regulation FD Disclosure.** |

---

On June 2, 2025, Mersana Therapeutics, Inc. (the "Company") issued a press release titled "Mersana Therapeutics Reports Additional Positive Interim Phase 1 Clinical Data for Emi-Le in Oral Presentation at 2025 ASCO Annual Meeting." A copy of this press release is being furnished as Exhibit 99.1 to this Current Report on Form 8-K (the "Report").

Also on June 2, 2025, the Company posted an updated corporate presentation on the Company's website. To access the presentation, investors should visit the "Events & Presentations" page under the "Investors & Media" section of the Company's website at ir.mersana.com.

The information furnished in this Item 7.01, including Exhibit 99.1, shall not be deemed to be "filed" for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), or otherwise subject to the liabilities of that section, nor shall such information be deemed to be incorporated by reference in any filing by the Company with the Securities and Exchange Commission ("SEC") under the Securities Act of 1933, as amended, or the Exchange Act, regardless of any general incorporation language contained in such a filing, except as expressly set forth by specific reference in such a filing.

---

| | |
|:---|:---|
| **Item 8.01.** | **Other Events.** |

---

On June 2, 2025, the Company announced the presentation in an oral session at the American Society of Clinical Oncology 2025 Annual Meeting ("ASCO") of additional positive interim clinical data from its Phase 1 clinical trial of Emi-Le, its B7-H4-directed Dolasynthen antibody-drug conjugate ("ADC"). The presentation focused on data as of a March 8, 2025 data cut date from the dose escalation and backfill cohorts of the Company's Phase 1 clinical trial of Emi-Le in patients with triple-negative breast cancer ("TNBC"); hormone-receptor-positive, human epidermal growth factor receptor 2-negative breast cancer; ovarian cancer; endometrial cancer and adenoid cystic carcinoma type 1 ("ACC-1").

The following clinical activity data from evaluable patients (those with measurable disease at baseline and at least one post-baseline scan) as of March 8, 2025 were presented at ASCO:

&nbsp;&nbsp;&nbsp;&nbsp;· 31% confirmed objective response rate ("ORR") (8/26) across all enrolled tumor types with B7-H4 high tumor expression
(defined as a tumor proportion score of 70% or higher) receiving intermediate Emi-Le doses (38.1 milligrams per meter squared ("mg/m<sup>2</sup>")
to 67.4 mg/m<sup>2</sup> per cycle).

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;o 44% confirmed ORR (7/16) in the subset of patients with ≤4 prior lines of therapy.

&nbsp;&nbsp;&nbsp;&nbsp;· 56% ORR (5/9, including one unconfirmed response as of the March 8, 2025 data cut date that was subsequently confirmed) in ACC-1
regardless of dose and B7-H4 expression. Among all ACC-1 patients who were enrolled in the Phase 1 clinical trial, the median progression
free survival ("PFS") had not yet been reached as of the March 8, 2025 data cut date. ACC-1 is a rare and aggressive
cancer that most frequently originates in the salivary glands. With no approved therapies, median PFS and median overall survival for
patients with ACC-1 are reported to be 2-3 months and 2-3 years, respectively.<sup>1</sup>

Furthermore, the Company observed a 50% ORR (2/4) in evaluable endometrial cancer patients with B7-H4 high tumor expression receiving intermediate doses of Emi-Le.

The Company previously reported that as of the March 8, 2025 data cut date, among evaluable patients with TNBC who received intermediate doses of Emi-Le, ORR, PFS and preliminary overall survival ("OS") data include the following:

---

| | | |
|:---|:---|:---|
| | &nbsp;&nbsp;Patients with B7-H4 high<br> TNBC receiving ≤4 prior<br> treatment lines in advanced/<br> metastatic setting (n=7) | &nbsp;&nbsp;Patients with B7-H4 low<br> TNBC receiving ≤4 prior<br> treatment lines in advanced/<br> metastatic setting (n=11) |
| &nbsp;&nbsp;**Received ≥1 Prior Topoisomerase-1 Inhibitor ("Topo-1") ADC** | &nbsp;&nbsp;100% (7/7) | &nbsp;&nbsp;73% (8/11) |
| &nbsp;&nbsp;**ORR** | &nbsp;&nbsp;29% (2/7) | &nbsp;&nbsp;0% (0/11) |
| &nbsp;&nbsp;**Median PFS** | &nbsp;&nbsp;16.0 weeks | &nbsp;&nbsp;6.4 weeks |
| &nbsp;&nbsp;**Median OS** | &nbsp;&nbsp;Not reached | &nbsp;&nbsp;5.7 months |

---

Additionally, as of the March 8, 2025 data cut date, the dose escalation and backfill cohorts of the Company's Phase 1 clinical trial of Emi-Le had enrolled a total of 141 patients. The enrolled patient population was heavily pretreated, with patients receiving up to 15 and a median of 4 prior lines of therapy. The Company retrospectively analyzed tumor samples from patients in the trial for B7-H4 expression. As of the data cut date, the Company had determined B7-H4 tumor expression for 116 patients. Among the 116 patients with known B7-H4 tumor expression, approximately 41% had a tumor proportion score of 70% or higher, which the Company has preliminarily characterized as "B7-H4 High."

Emi-Le was observed to be generally well tolerated. As of the March 8, 2025 data cut date, the most common treatment-related adverse events ("TRAEs") of any grade across the entire patient population were transient aspartate aminotransferase ("AST") increase (44% of patients), generally asymptomatic and reversible proteinuria (40%), low-grade fatigue (33%) and generally low-grade nausea (31%). The only Grade 3 TRAEs in ≥5% or more of all patients were AST increase (17%), proteinuria (14%) and anaemia (6%). Across the entire enrolled patient population, TRAEs leading to discontinuation, dose reduction and dose delay were observed in 3.5%, 16.3% and 23.4% of patients, respectively. No dose-limiting treatment-related neutropenia, neuropathy, ocular toxicity, interstitial lung disease or thrombocytopenia were reported as of the March 8, 2025 data cut date, which the Company believes differentiates Emi-Le from many other approved and clinical-stage ADCs.

The Company is currently conducting the dose expansion portion of its Phase 1 clinical trial of Emi-Le in patients with TNBC who have received one to four prior treatment lines, including at least one prior topo-1 ADC. These patients are being enrolled in two dose expansion cohorts: the "Dose A" cohort, in which patients are receiving 67.4 mg/m<sup>2</sup> of Emi-Le administered every four weeks ("Q4W"), and the "Dose B" cohort, in which patients are receiving 80 mg/m<sup>2</sup> of Emi-Le Q4W with a loading dose of 44.5 mg/m<sup>2</sup> of Emi-Le on days 1 and 8 of the first four-week cycle.

<sup>1</sup> Sources: Ferrarotto et al., Clin Can Res. 2020; de Sousa Clin Can Res. 2023; Hanna et al., Cancer Res Commun. 2023; Ferrarotto et al., Abs 903P, ESMO 2023

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| | |
|:---|:---|
| **Item 9.01.** | **Financial Statements and Exhibits.** |

---

(d) Exhibits

---

| | |
|:---|:---|
| **Exhibit No.** | **Description** |
| [99.1](tm2516680d1_ex99-1.htm) | [Press Release issued by the Company on June 2, 2025.](tm2516680d1_ex99-1.htm) |
| 104 | Cover Page Interactive Data File (embedded within the Inline XBRL document). |

---

**SIGNATURES**

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

---

| | | |
|:---|:---|:---|
|  | **MERSANA THERAPEUTICS, INC.** | **MERSANA THERAPEUTICS, INC.** |
| Date: June 2, 2025 | By: | /s/ Brian DeSchuytner |
|  |  | Brian DeSchuytner |
|  |  | Senior Vice President, Chief Operating Officer and Chief Financial Officer |

---

## Exhibit 99.1

**Exhibit 99.1**

**Mersana Therapeutics Reports Additional Positive Interim Phase 1 Clinical Data<br> for Emi-Le in Oral Presentation at 2025 ASCO Annual Meeting**

&nbsp;&nbsp;&nbsp;&nbsp;· *31% confirmed ORR (8 responses in 26 evaluable patients) across B7-H4 high tumors at intermediate doses* 

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;o *44% confirmed ORR (7 responses in 16 evaluable patients) in the subset of patients with ≤4 prior lines of therapy* 

**CAMBRIDGE, Mass., June 2, 2025** – Mersana Therapeutics, Inc. (NASDAQ: MRSN), a clinical-stage biopharmaceutical company focused on the development of antibody-drug conjugates (ADCs) targeting cancers in areas of high unmet medical need, today announced additional interim Phase 1 clinical data for emiltatug ledadotin (Emi-Le; XMT-1660), the company's B7-H4-directed Dolasynthen ADC. These data are being presented in an oral session today at the American Society of Clinical Oncology (ASCO) 2025 Annual Meeting in Chicago, Illinois.

The presentation will focus on Emi-Le's Phase 1 dose escalation and backfill cohorts as of a March 8, 2025 data cut-off in patients with triple-negative breast cancer (TNBC); hormone-receptor-positive, human epidermal growth factor receptor 2-negative breast cancer; ovarian cancer; endometrial cancer and adenoid cystic carcinoma type 1 (ACC-1).

"We are excited to share additional interim data in an oral presentation at ASCO," said Martin Huber, M.D., President and Chief Executive Officer of Mersana Therapeutics. "The results from our dose escalation and backfill cohorts led us to focus our initial expansion work on patients with TNBC who have previously been treated with a topoisomerase-1 inhibitor ADC, a population with high unmet need that we believe Emi-Le may be uniquely suited to address within the B7-H4 ADC field. We also continue to be encouraged by the activity observed in other B7-H4-expressing tumor types, demonstrating Emi-Le's broader development potential. On behalf of the Mersana team, we would like to express our gratitude to the patients and investigators who have participated in this clinical trial."

Safety and tolerability as of the March 8, 2025 data cut-off were consistent with initial data previously reported in January 2025, and no new safety signals were observed. Additionally, the following clinical activity data are being presented from evaluable patients (those with measurable disease at baseline and at least one post-baseline scan):

&nbsp;&nbsp;&nbsp;&nbsp;· 31% confirmed objective response rate (ORR) (8/26) across all enrolled tumor types with B7-H4 high tumor expression (defined as a
tumor proportion score of 70% or higher) receiving intermediate Emi-Le doses (38.1 milligrams per meter squared (mg/m<sup>2</sup>) to
67.4 mg/m<sup>2</sup> per cycle).

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;o 44% confirmed ORR (7/16) in the subset of patients with ≤4 prior lines of therapy.

&nbsp;&nbsp;&nbsp;&nbsp;· 56% ORR (5/9, including one unconfirmed response as of the March 8, 2025 data cut-off that was subsequently confirmed) in ACC-1
regardless of dose and B7-H4 expression. Among all ACC-1 patients who were enrolled in the Phase 1 clinical trial, the median progression
free survival (PFS) had not yet been reached as of the March 8, 2025 data cut-off. ACC-1 is a rare and aggressive cancer that most
frequently originates in the salivary glands. With no approved therapies, median PFS and median overall survival for patients with ACC-1
are reported to be 2-3 months and 2-3 years, respectively. <sup>1</sup>

<sup>1</sup> Sources: Ferrarotto et al., Clin Can Res. 2020; de Sousa Clin Can Res. 2023; Hanna et al., Cancer Res Commun. 2023; Ferrarotto et al., Abs 903P, ESMO 2023

"The clinical activity observed for emiltatug ledadotin across all enrolled tumor types is encouraging, and its safety and tolerability profile appears differentiated from many other ADCs," said Antonio Giordano, MD, Ph.D., Assistant Professor of Medicine, Harvard Medical School, Dana-Farber Cancer Institute. "The objective responses were particularly notable in patients with late-stage triple-negative breast cancer who were previously treated with topoisomerase-1 inhibitors and in patients with ACC-1, both of which are aggressive and difficult-to-treat cancers with high unmet need."

The 2025 ASCO Annual Meeting data presentation can be accessed on the Publications section of the Mersana website at <u>www.mersana.com</u>.

**<u>About Emi-Le</u>**

Emi-Le is a B7-H4-directed Dolasynthen ADC with a precise, target-optimized drug-to-antibody ratio (DAR 6) and a proprietary auristatin payload with controlled bystander effect. This candidate is being investigated in Mersana's ongoing Phase 1 clinical trial. The dose expansion portion of the Phase 1 clinical trial is enrolling patients with triple-negative breast cancer (TNBC) who have received one to four prior treatment lines, including at least one topoisomerase-1 inhibitor (topo-1) ADC.

The U.S. Food and Drug Administration has granted two Fast Track designations to Emi-Le for the treatment of 1) adult patients with advanced or metastatic triple-negative breast cancer, and 2) advanced or metastatic HER2 low / HER2 negative breast cancer post-topo-1 ADC (including TNBC and certain HR+ breast cancers). For more information about Mersana's ongoing Phase 1 trial of Emi-Le, please visit clinicaltrials.gov (NCT05377996).

**<u>About Mersana Therapeutics</u>**

Mersana Therapeutics is a clinical-stage biopharmaceutical company focused on the development of novel antibody-drug conjugates (ADCs) and driven by the knowledge that patients are waiting for new treatment options. The company has developed proprietary cytotoxic (Dolasynthen) and immunostimulatory (Immunosynthen) ADC platforms that have generated a pipeline of wholly-owned and partnered product candidates with the potential to treat a range of cancers. Its pipeline includes Emi-Le (emiltatug ledadotin; XMT-1660), a Dolasynthen ADC targeting B7-H4, and XMT-2056, an Immunosynthen ADC targeting a novel epitope of human epidermal growth factor receptor 2 (HER2). Mersana routinely posts information that may be useful to investors on the "Investors & Media" section of its website at www.mersana.com.

**<u>Forward-Looking Statements</u>**

This press release contains "forward-looking" statements and information within the meaning of the Private Securities Litigation Reform Act of 1995. These statements may be identified by words such as "aims," "anticipates," "believes," "could," "estimates," "expects," "forecasts," "goal," "intends," "may," "plans," "possible," "potential," "seeks," "will" and variations of these words or similar expressions, although not all forward-looking statements contain these words. Forward-looking statements in this press release include, but are not limited to, statements concerning the potential clinical benefits of and opportunity for Emi-Le and the development and potential of Mersana's product candidates, platforms, technology and pipeline of ADC candidates. Mersana may not actually achieve the plans, intentions or expectations disclosed in these forward-looking statements, and you should not place undue reliance on these forward-looking statements. Actual results or events could differ materially from the plans, intentions and expectations disclosed in these forward-looking statements as a result of various factors, including, among other things, uncertainties inherent in research and development, in the advancement, progression and completion of clinical trials and in the clinical development of Mersana's product candidates, including Emi-Le; the risk that Mersana may face delays in patient enrollment in its Phase 1 clinical trial of Emi-Le; the risk that outcomes of preclinical studies may not be predictive of clinical trial results; the risk that initial or interim results from a clinical trial may not be predictive of the final results of the trial or the results of future trials; the risk that clinical trial data may not support regulatory applications or approvals; the risk that Mersana may not realize the intended benefits of its platforms, technology and collaborations;; and other important factors, any of which could cause Mersana's actual results to differ from those contained in the forward-looking statements, that are described in greater detail in the section entitled "Risk Factors" in Mersana's Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission ("SEC") on May 15, 2025, as well as in other filings Mersana may make with the SEC in the future. Any forward-looking statements contained in this press release speak only as of the date hereof, and Mersana expressly disclaims any obligation to update any forward-looking statements contained herein, whether because of any new information, future events, changed circumstances or otherwise, except as otherwise required by law.

**###**

**Contact:**

Jason Fredette

617-498-0020

<u>jason.fredette@mersana.com</u>