Multinational phase 3 trial of pembrolizumab or placebo plus adjuvant chemo(radio)therapy for high-risk endometrial cancer: results in the dMMR subgroup

Mismatch repair-deficient (dMMR) endometrial cancer has poor outcomes when meeting high-risk criteria and has limited treatment options in the adjuvant setting. The multinational phase 3 KEYNOTE-B21 trial evaluated pembrolizumab or placebo plus adjuvant chemotherapy with or without radiotherapy for newly diagnosed, high-risk endometrial cancer after surgery with curative intention. Interim analysis of results in the dMMR subgroup of the trial has now been published. In the intention-to-treat population, 141 patients in the pembrolizumab group and 140 patients in the placebo groups had dMMR tumors. At the interim analysis, median DFS was not reached in either group. Two-year DFS rates were 92.4% (95% CI 84.4-96.4) in the pembroizumab group and 80.2% (70.8-86.9) in the placebo group (HR 0.31; 0.14-0.69). No new safety signals occurred. The authors conclude that addition of pembrolizumab to adjuvant chemotherapy with or without radiotherapy after curative-intent surgery improved DFS among newly diagnosed high-risk endometrial cancer patients with dMMR tumors.