Mersana Therapeutics Stock Drops Following Phase 1 Trial Data Release

On Friday, Mersana Therapeutics Inc. (MRSN) released initial data from the Phase 1 dose escalation and backfill cohorts for emiltatug ledadotin (Emi-Le). The drug was generally well tolerated, with no Grade 4 or 5 treatment-related adverse events reported. At intermediate doses (38.1 mg/m2 to 67.4 mg/m2), the confirmed objective response rate (ORR) among evaluable patients was 23% across all B7-H4 high tumors and 23% with B7-H4 high triple-negative breast cancer (TNBC). These patients had previously been treated with at least one topo-1 ADC.

In comparison, the ORR with standard-of-care single-agent chemotherapy in relapsed/refractory TNBC was approximately 5%, with progression-free survival of approximately seven weeks in the ASCENT Phase 3 trial of sacituzumab govitecan. Encouraged by these results, Mersana has advanced a dose of 67.4 mg/m2 every four weeks into an expansion cohort in patients with TNBC who have received one to four prior treatment lines.

At high doses above 76 mg/m2, the confirmed ORR among evaluable patients was 22% across all B7-H4 high tumors, with 78% of patients experiencing a ≥30% tumor reduction in target lesions. However, objective responses in multiple evaluable patients with B7-H4 high tumors were not confirmed after protocol-mandated dose delays for proteinuria. Mersana is implementing proteinuria mitigation efforts and continues to examine higher doses in dose escalation and backfill cohorts to identify a second dose for the expansion portion of the trial.

Concurrently, the FDA granted an additional Fast Track designation to Mersana Therapeutics’ XMT-1660 for advanced or metastatic breast cancer in patients with HER2 low or HER2-negative disease, including TNBC, who have received a prior topoisomerase-1 inhibitor ADC. Hormone-receptor positive patients should also have received or be ineligible for endocrine therapy. The World Health Organization has approved emiltatug ledadotin (abbreviated as Emi-Le) as XMT-1660’s international nonproprietary name.

Despite these developments, Mersana Therapeutics' stock price dropped by 36.6% to $0.8245 at last check on Friday.