Mersana Therapeutics, Inc. announced that update focuses on the ovarian cancer expansion cohort of the XMT-1536 Phase 1 study which is enrolling heavily pre-treated patients with ovarian cancer who have received up to four prior lines of therapy. With a data cutoff of December 3, 2020, these data include 72 patients evaluable for safety and 47 patients evaluable for RECIST response. Adverse event profile consistent with previously reported expansion data: The most frequently reported treatment-related adverse events (TRAEs) were generally Grade 1-2 fatigue, nausea, transient AST elevation without associated changes in bilirubin or cases of Hy’s law, and transient thrombocytopenia; 31% of patients experienced a dose reduction, delay, or discontinuation due to a treatment-related adverse event; Serious adverse events occurred in 39% of patients regardless of relatedness with the most common related SAEs occurring in more than 2 patients of pyrexia (4), vomiting (3), abdominal pain (2), pneumonitis (2); There were no reported cases of severe neutropenia, peripheral neuropathy or ocular toxicity. Anti-tumor activity in platinum-resistant and platinum-refractory ovarian cancer previously treated with bevacizumab, PARP inhibitors, or both Activity observed in higher NaPi2b expressing population: 31 patients with higher NaPi2b expression were evaluable for response, with 2 achieving confirmed complete responses (CRs) and 8 achieving confirmed partial responses (PRs) for an ORR of 32% (10/31). Additionally, 13 patients achieved stable disease (SD) for a disease control rate (DCR) of 74% (23/31); The median duration of response was estimated at 5 months in patients with higher NaPi2b expression; A trend toward higher response rate as well as deeper and more durable responses in patients with higher NaPi2b expression supports the continued development of a NaPi2b diagnostic assay. Activity observed in overall population, regardless of NaPi2b expression: Among all 47 patients evaluable for response, 3 additional patients achieved PRs for an ORR of 28% (13/47). 6 additional patients achieved SD for a DCR of 68% (32/47); 69% of responses were observed by the first scan; 67% of patients showed reduction in target lesions.