Mersana Therapeutics' Emiltatug Ledadotin (XMT-1660) has received Fast Track designation from the U.S. Food and Drug Administration (FDA) for the treatment of advanced or metastatic breast cancer. The designation specifically targets 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 antibody-drug conjugate (ADC). This action expands the potential reach of Emiltatug Ledadotin, building on its existing Fast Track status for advanced or metastatic recurrent TNBC. Simultaneously, the World Health Organization (WHO) has approved 'emiltatug ledadotin' (abbreviated as Emi-Le) as the international nonproprietary name (INN) for XMT-1660. These developments highlight the growing recognition and importance of Emiltatug Ledadotin in addressing unmet needs in breast cancer therapy.
Clinical Data and Tolerability
Data from an ongoing Phase 1 clinical trial (NCT05377996) supports the FDA's decision. The trial, which enrolled 130 heavily pre-treated patients, demonstrated an objective response rate (ORR) of 23% among evaluable patients who received intermediate doses of Emiltatug Ledadotin. This response was observed across all patients with B7-H4 high tumors and in those with B7-H4 high TNBC, all of whom had prior treatment with a topoisomerase-1 ADC.
The treatment was generally well-tolerated, with no grade 4 or 5 treatment-related adverse events (TRAEs) reported. Common TRAEs included transient aspartate aminotransferase (AST) increase (38%), proteinuria (31%), nausea (29%), and fatigue (28%).
Comparison with Existing Therapies
Erika Hamilton, MD, director of Breast Cancer Research at Sarah Cannon Research Institute, noted the encouraging tolerability and clinical activity of Emiltatug Ledadotin. She emphasized that all TNBC patients who responded had previously been treated with a topoisomerase-1 ADC, suggesting that Emiltatug Ledadotin may address a significant need in this population.
In comparison, the Phase 3 ASCENT trial of sacituzumab govitecan, another topo-1 ADC, showed an ORR of 5% and progression-free survival of 7 weeks in relapsed/refractory TNBC, indicating the potential for Emiltatug Ledadotin to offer improved outcomes.
Implications of Fast Track Designation
The FDA's Fast Track program is designed to accelerate the development and review of drugs that treat serious conditions and fill unmet medical needs. This designation provides Mersana Therapeutics with opportunities for more frequent interactions with the FDA, potentially leading to accelerated approval, priority review, or rolling review of a Biologics License Application (BLA).
Targeting B7-H4 in Breast Cancer
Emiltatug Ledadotin is an antibody-drug conjugate (ADC) that targets B7-H4, a protein overexpressed in various tumors, including breast cancer. Studies suggest that B7-H4 expression in tumors is associated with reduced immune cell infiltration. Preclinical data indicate that ADCs targeting B7-H4 can elicit an immune response, potentially overcoming this immunosuppressive effect.
Management Commentary
Martin Huber, M.D., President and Chief Executive Officer of Mersana Therapeutics, stated that topoisomerase-1 inhibitor ADCs are becoming standard care for metastatic TNBC and hormone-receptor positive breast cancer. He added that patients who progress after these treatments are exceedingly difficult to treat, making this population a primary focus for Mersana. Huber expressed excitement about the additional Fast Track designation and the initial clinical data from the ongoing Phase 1 trial.
