GSK reported positive early results from its Phase I BEHOLD-1 trial of mocertatug rezetecan (Mo-Rez), an antibody-drug conjugate targeting B7-H4, in platinum-resistant ovarian cancer and recurrent endometrial cancer. The company said response rates were 62% in PROC patients and 67% in patients with advanced or recurrent endometrial cancer, with a safety profile described as favorable. GSK said the program is supported by five planned pivotal Phase III trials later in 2026 across ovarian and endometrial cancer settings. The company said Mo-Rez induced meaningful antitumor activity in the dataset, and highlighted the need for options in PROC given widespread platinum resistance. GSK disclosed that over 60% of PROC patients and over 50% of endometrial cancer patients experienced severe adverse events, while overall treatment discontinuation was low (4%). The data were not yet peer-reviewed.