The BEHOLD-1 Phase I clinical trial yielded highly positive efficacy data for mocertatug rezetecan, abbreviated as Mo-Rez. It is a cutting-edge antibody-drug conjugate (ADC) designed to precisely strike the B7-H4 antigen. This specific immune checkpoint is widely expressed in ovarian and endometrial tumors but is found in negligible amounts in healthy tissues, allowing for highly targeted and precise treatment.
Administering the highest tested monotherapy doses led to a confirmed objective response rate (cORR) of 62% in patients with platinum-resistant ovarian cancer (PROC) and a striking 67% in those with recurrent or advanced endometrial cancer (EC). These results are particularly valuable given that patients suffering from these specific cancer types currently face very limited and largely ineffective treatment options.
This high efficacy did not come at the cost of severe side effects. Data presented by GSK shows that at the highest tested doses, only a fraction of patients had to discontinue treatment due to treatment-related adverse events – 0% for ovarian cancer patients and 4% for those with endometrial cancer. Crucially, patient responses to Mo-Rez were observed across varying levels of B7-H4 antigen expression, further validating it as a prime molecular target.
The Phase I success is accelerating the drug’s development timeline. GSK representatives have already announced that the promising clinical and safety profile allows them to launch five pivotal, global Phase III clinical trials before the end of 2026.