Mersana Therapeutics (MRSN) Announces Positive Interim Data from Expansion Portion of the XMT-1536 Phase 1 Study

May 27, 2020 6:10 AM EDT

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Mersana Therapeutics, Inc. (NASDAQ: MRSN) today reported interim safety, tolerability and efficacy data from the ongoing expansion portion of the Phase 1 study evaluating XMT-1536, its first-in-class ADC candidate targeting NaPi2b, in patients with ovarian cancer and non-small cell lung (NSCLC) adenocarcinoma. The Company will host a conference call and webcast today, Wednesday, May 27, 2020, at 8:00 a.m. ET during which investigator Debra L. Richardson, MD, Associate Professor of Gynecologic Oncology at the Stephenson Cancer Center at the University of Oklahoma Health Sciences Center and the Sarah Cannon Research Institute and members of the Mersana executive team will present and discuss these data. These data will also be presented in a poster session at the American Society of Clinical Oncology 2020 Virtual Scientific Program on Friday, May 29, 2020 starting at 8:00 a.m. ET.

“These data demonstrate not only that XMT-1536, our first-in-class Dolaflexin ADC targeting NaPi2b, can deliver confirmed complete responses, partial responses and durable stable disease in platinum-resistant ovarian cancer, but also that these responses can deepen over time in a patient population with poor prognosis and limited treatment options,” said Anna Protopapas, President and Chief Executive Officer of Mersana Therapeutics. “XMT-1536 continues to demonstrate that it is generally well tolerated, without the dose-limiting toxicities of other ADC platforms such as severe neutropenia, neuropathy and ocular toxicity. These are encouraging signals as we look forward to reporting more mature data in the second half of the year and continuing to advance XMT-1536 for both platinum-resistant ovarian cancer and NSCLC adenocarcinoma patients.”

The expansion portion of the Phase 1 study is enrolling patients with platinum-resistant 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 as well as patients with NSCLC adenocarcinoma who had received prior treatment with platinum-based therapy and immunotherapy or targeted agents. With a data cutoff of May 1, 2020 these data include 34 patients total, 27 with ovarian cancer and seven with NSCLC adenocarcinoma. Patients with ovarian cancer had a median of three prior lines of treatment (range 1-5), and patients with NSCLC adenocarcinoma had a median of two lines of therapy (range 1-3). Fifteen of the patients were dosed at 36 mg/m2, and 19 patients were dosed at 43 mg/m2 every four weeks. Key findings include:

Safety profile consistent with previously reported dose escalation data and no new safety signals observed.
The most frequently (≥20%) reported treatment-related adverse events (TRAEs) were Grade 1-2 fatigue, nausea, vomiting, pyrexia, decreased appetite, diarrhea and fever and transient AST elevation without associated changes in bilirubin or cases of Hy’s law.
There were no reported cases of severe neutropenia, peripheral neuropathy or ocular toxicity.
Promising antitumor activity observed in platinum-resistant ovarian cancer.
Of the 20 patients that were evaluable for response, 2/20 (10%) achieved confirmed complete responses (CRs) and 5/20 (25%) achieved confirmed partial responses (PRs) for an objective response rate (ORR) of 35%. Additionally, 1/20 (5%) patients achieved an unconfirmed partial response for which a confirmatory scan was pending at the time of the data cutoff, and 8/20 (40%) patients achieved stable disease (SD); the disease control rate (DCR) was 16/20 (80%).
The majority of responders had prior treatment with bevacizumab, PARP inhibitors, or both. Both patients with confirmed complete responses had prior treatment with bevacizumab and PARP inhibitors.
Data continue to support a NaPi2b biomarker-based patient selection strategy.
An emerging biomarker-response relationship continues to be observed. For consistency, these data were bifurcated using the same expression level as used in the dose escalation portion of the study. More data are needed to define the patient selection strategy.
Among those patients with higher NaPi2b expression, two (2/14) patients achieved a CR, and two (2/14) achieved a PR.
Two (2/2) patients with NaPi2b expression not yet determined at the time of data cutoff achieved confirmed PRs.
One (1/4) patient with lower NaPi2b expression (H-score of 90) achieved a confirmed PR.
The Company expects to define the patient selection strategy based on the total data set from patients treated with XMT-1536.

Response - Ovarian Cancer N=20*AllHigher NaPi2b oLower NaPi2b ooNaPi2b Not Yet Determined
CR2 (10%)2 (14%)0 (0%)0 (0%)
PR5 (25%)2 (14%)1 (25%)2 (100%)
uPR**1 (5%)1 (7%)0 (0%)0 (0%)
SD8 (40%)7 (50%)1 (25%)0 (0%)
PD4 (20%)2 (14%)2 (50%)0 (0%)

*7 patients not evaluable: 1 withdrew consent (Lower NaPi2b Expression); 1 with unrelated SAE leading to discontinuation and death (Lower NaPi2b Expression); 5 have not yet received a scan**uPR=1 patient with unconfirmed PR; confirmatory scan pending at the time of data cut° Higher NaPi2b Expression: defined in dose escalation as at / above lowest H-score at which response observed (≥110)°° Lower NaPi2b Expression: defined in dose escalation as below the lowest H-score at which response observed (<110)

More data are needed to assess antitumor activity of XMT-1536 in NSCLC adenocarcinoma patients.
At the time of data cutoff, four out of seven NSCLC adenocarcinoma patients were evaluable for response, and 2/4 (50%) patients had achieved SD as best response.

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