On Friday, Mersana Therapeutics Inc. (NASDAQ:MRSN) released initial data from the Phase 1 dose escalation and backfill cohorts for emiltatug ledadotin.

Emi-Le was generally well tolerated, with no Grade 4 or 5 treatment-related adverse events reported.

At intermediate doses in the trial (38.1 mg/m2 to 67.4 mg/m2), the confirmed objective response rate among evaluable patients was 23% (6 of 26 patients) across all B7-H4 high tumors and 23% (3 of 13 patients) with B7-H4 high TNBC, all of whom had previously been treated with at least one topo-1 ADC.

In the ASCENT Phase 3 trial of sacituzumab govitecan, the ORR with standard-of-care single-agent chemotherapy in relapsed/refractory TNBC was approximately 5%, with progression-free survival of approximately seven weeks.

Based on these encouraging Emi-Le data at intermediate doses, 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% (2 of 9) across all B7-H4 high tumors.

Additionally, 78% (7 of 9 patients) had ≥30% tumor reduction in target lesions.

At these high dose levels, 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.

The company also announced that the World Health Organization has approved emiltatug ledadotin (abbreviated as Emi-Le) as XMT-1660’s international nonproprietary name.

The new Fast Track designation is for advanced or metastatic breast cancer in patients with human epidermal growth factor receptor 2 (HER2) low (IHC 1+ or IHC 2+/ISH–) or HER2-negative (IHC 0) disease, including triple-negative breast cancer (TNBC), who have received a prior topoisomerase-1 inhibitor ADC.

Additionally, hormone-receptor positive patients should also have received or be ineligible for endocrine therapy.

Price Action: MRSN stock is down 36.6% at $0.8245 at last check Friday.

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