Mersana Therapeutics, Inc. today announced additional interim Phase 1 clinical data for emiltatug ledadotin (Emi-Le; XMT-1660), its B7-H4-directed Dolasynthen ADC, presented in an oral session at the American Society of Clinical Oncology (ASCO) 2025 Annual Meeting. The data, as of a March 8, 2025 cut-off, from dose escalation and backfill cohorts, showed safety and tolerability consistent with initial reports from January 2025, with no new safety signals observed.
In evaluable B7-H4 high triple-negative breast cancer (TNBC) patients who had received a prior topoisomerase-1 inhibitor ADC, Emi-Le achieved an objective response rate (ORR) of 29% (5 of 17 patients), with a median progression-free survival (PFS) of 16 weeks and a median overall survival (OS) of 5.7 months. These results compare favorably to the standard-of-care single-agent chemotherapy in relapsed/refractory TNBC, which historically shows an ORR of approximately 5% and PFS of approximately seven weeks.
Encouraging clinical activity was also observed in other B7-H4 high tumor types. For adenoid cystic carcinoma type 1 (ACC-1), the ORR was 33% (1 of 3 patients) with a median PFS of 24 weeks. In ovarian cancer, ORR was 25% (1 of 4 patients), and in endometrial cancer, ORR was 20% (1 of 5 patients), both with a median PFS of 16 weeks. For HR+/HER2- breast cancer patients who received a prior topo-1 ADC, ORR was 17% (1 of 6 patients) with a median PFS of 16 weeks.
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