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 1, 2019

MERSANA THERAPEUTICS, INC.

(Exact name of registrant as specified in its charter)

(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:

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

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

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

o 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:

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 x

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. x

Item 8.01                                           Other Events.

On June 1, 2019, Mersana Therapeutics, Inc. (the “Company”) issued a press release providing interim Phase 1 data for XMT-1536.  The Company’s press release is attached as Exhibit 99.1 to this current report and is incorporated herein by reference.

Item 9.01                                           Financial Statements and Exhibits.

(d) Exhibits.

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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.

Date: June 4, 2019

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Exhibit 99.1

Mersana Presents Interim Phase 1 Data for XMT-1536 at the 2019 American Society of Clinical Oncology Annual Meeting

June 1, 2019

Encouraging early signs of clinical efficacy

Well tolerated with favorable safety profile in heavily pre-treated patient population

CAMBRIDGE, Mass., June 01, 2019 (GLOBE NEWSWIRE) — Mersana Therapeutics, Inc. (Nasdaq: MRSN), a clinical-stage biopharmaceutical company focused on discovering and developing a pipeline of antibody drug conjugates (ADCs) targeting cancers in areas of high unmet medical need, today presented new interim efficacy and safety data from its ongoing Phase 1 dose-escalation study evaluating XMT-1536, its first-in-class ADC candidate targeting NaPi2b, in patients with ovarian cancer, non-small cell lung (NSCLC) adenocarcinoma and other tumor types. The data will be presented in a poster session and poster discussion today at the 2019 American Society of Clinical Oncology (ASCO) Annual Meeting being held from May 31 — June 4, 2019 in Chicago, IL.

“We are encouraged by the early signs of efficacy coupled with the favorable safety and tolerability profile and promising treatment duration we have seen to date in this ongoing Phase 1 study with XMT-1536 in heavily pre-treated, advanced cancer patients,” said Anna Protopapas, President and CEO, Mersana Therapeutics. “We look forward to moving into the dose expansion phase of the study which will focus on platinum-resistant ovarian cancer and NSCLC adenocarcinoma patient populations.”

In a poster titled “Phase 1 dose escalation study of XMT-1536, a novel NaPi2b-targeting antibody-drug conjugate (ADC), in patients (pts) with solid tumors likely to express NaPi2b,” Mersana demonstrated that as of May 10, 2019, XMT-1536 is well tolerated at doses up to 30 mg/m2, with observation of objective responses at 20 mg/m2 and higher. Of the 37 patients enrolled, tumor types included 22 ovarian, fallopian tube or primary peritoneal cancer, four NSCLC, eight endometrial, two papillary renal, and one salivary duct cancer. Patients were heavily pre-treated, with a median of four prior lines of treatment (range 1-13) for all patients and a median of five lines of prior treatment in ovarian cancer patients (range 1-11). Interim results included:

·       The most common treatment-related adverse events (TRAEs) were Grade 1-2 nausea, fatigue, and headache, and the most frequent Grade 3 TRAE was transient AST elevation.

·       In patients with tumor types selected for the planned expansion phase (platinum-resistant ovarian cancer and NSCLC adenocarcinoma) treated at >20 mg/m2 (N=18), three (17%) achieved partial responses (PRs) and eight (44%) achieved stable disease (SD) for a disease control rate (DCR) of 11/18 (61%), and a treatment duration lasting beyond 16 weeks in 9 patients (50%).

·       In ovarian cancer patients treated at >30 mg/m2 (N=7), two (28%) achieved partial responses (PRs) and three (43%) achieved stable disease (SDs) for a disease control rate (DCR) of 5/7 (71%), and three of these patients (43%) were treated on study for more than 16 weeks.

The Company continues to evaluate patients in the dose escalation portion of the study in the once-every-four-week dose level of 36 mg/m2. Upon completion of the 36 mg/m2 evaluation, the Company expects to choose either the 30 mg/m2 or 36 mg/m2 every four weeks as the go forward dose for the dose expansion phase of the study. Mersana is planning to begin dosing patients in the dose expansion phase in third quarter of 2019.

About Mersana Therapeutics

Mersana Therapeutics is a clinical-stage biopharmaceutical company using its differentiated and proprietary ADC platforms and its modular Synthemer scaffold to develop highly targeted drugs with increased tolerability and expanded opportunities to deliver meaningful clinical benefit to patients. Mersana’s lead product candidate, XMT-1536, is in a Phase 1 clinical trial in patients with tumors expressing NaPi2b, including ovarian cancer, NSCLC adenocarcinoma, and other cancers. In addition, multiple partners are using Mersana’s platform to advance their ADC pipelines.

Investor Contacts
Sarah Carmody

scarmody@mersana.com

617-844-8577

Stern Investor Relations, Inc.
Christina Tartaglia

christina@sternir.com

212-362-1200

Media Contact
Paul Kidwell

pkidwell@mersana.com

617-680-1088

Source: Mersana Therapeutics, Inc.