On September 17, 2020 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, reported updated interim safety, tolerability and efficacy data for the ovarian cancer cohort of the ongoing expansion portion of the Phase 1 study evaluating XMT-1536, its first-in-class ADC candidate targeting NaPi2b, as part of the 2020 European Society of Medical Oncology (ESMO) (Free ESMO Whitepaper) Virtual Congress (Press release, Mersana Therapeutics, SEP 17, 2020, View Source [SID1234565275]). The Company will host a conference call and webcast today at 8:00 a.m. ET, during which investigator Erika Hamilton, MD, Director of the Breast Cancer and Gynecologic Cancer Research Program from the Sarah Cannon Research Institute at Tennessee Oncology, and members of the Mersana executive team will present and discuss these data.

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"These data further support the continued development of XMT-1536, our first-in-class Dolaflexin ADC targeting NaPi2b, which has recently been granted FDA Fast Track Designation. We are eager to advance XMT-1536 into registration-enabling studies based on its observed antitumor activity and favorable safety profile in an ovarian cancer population with a very poor prognosis and limited treatment options," said Anna Protopapas, President and Chief Executive Officer of Mersana Therapeutics. "Additionally, we look forward to presenting more comprehensive and mature results from the ongoing expansion cohort around the end of this year."

This interim analysis focused on the ovarian cancer cohort of the Phase 1 expansion study, including heavily pre-treated patients with platinum-resistant or refractory ovarian cancer, fallopian tube or primary peritoneal cancer who have received up to three lines of prior therapy, and in some cases four lines of prior therapy regardless of platinum status. With a data cutoff of August 18, 2020 these data include 47 patients. These data include additional follow up on the 27 ovarian cancer patients previously presented at the American Society of Clinical Oncology (ASCO) (Free ASCO Whitepaper) virtual program in May of 2020 as well as 20 new patients who entered the study between May 1, 2020 and August 18, 2020.

Key findings include:

Safety profile consistent with previously reported expansion data and no new safety signals observed
oThe most frequently reported treatment-related adverse events (TRAEs) were generally Grade 1-2 fatigue, nausea, decreased appetite, vomiting and transient AST elevation without associated changes in bilirubin or cases of Hy’s law.
oThere were no reported cases of severe neutropenia, peripheral neuropathy or ocular toxicity.

•Continued, significant anti-tumor activity in platinum-resistant and platinum-refractory ovarian cancer and in ovarian cancer previously treated with bevacizumab, PARP inhibitors, or both
oOf the 29 patients that were evaluable for response, 2/29 (7%) achieved confirmed complete responses (CRs) and 8/29 (28%) achieved confirmed partial responses (PRs) for an objective response rate (ORR) of 34%. Additionally, 13/29 (45%) patients achieved stable disease (SD); the disease control rate (DCR) was 23/29 (79%).
•70% of responses were observed within two cycles and 100% of responses were observed within four cycles. Responses appeared to deepen over time, including responses in patients receiving reduced dose levels.
•The majority of responders had prior treatment with bevacizumab, PARP inhibitors, or both. Both patients with confirmed CRs had prior treatment with bevacizumab and PARP inhibitors.
•Reduction in tumor volume was observed in the majority of patients achieving a best response of stable disease.

•Data continue to support a NaPi2b biomarker-based patient selection strategy based on depth, time on study and quality of response
o50% of patients with higher NaPi2b expression are ongoing in the study while only 33% of patients with lower Napi2b expression are ongoing in study. Median duration of response was not yet reached in the 7 patients with ovarian cancer with higher NaPi2b expression.
oThe Company expects to define the patient selection strategy based on the total data set from patients treated with XMT-1536.
Conference Call Details
Mersana Therapeutics will host a conference call and webcast today at 8:00 a.m. ET to discuss these data. To access the call, please dial 877-303-9226 (domestic) or 409-981-0870 (international) and provide the Conference ID 5731456. A live webcast of the presentation will be available on the Investors & Media section of the Mersana website at www.mersana.com.