Objective: Retifanlimab is a humanized immunoglobulin G4 monoclonal antibody against programmed death 1 being investigated in several solid tumor types. We report final results from patients with recurrent microsatellite instability-high (MSI-H)/mismatch repair deficient (dMMR) endometrial cancer treated with retifanlimab in a POD1UM-101 expansion cohort., Methods: Eligible patients (≥18 years; histologically proven/unresectable/recurrent, MSI-H/dMMR endometrial cancer; checkpoint inhibitor naive) received retifanlimab 500 mg intravenously every 4 weeks for ≤2 years. Primary endpoint was safety/tolerability., Results: At data cutoff (May 17, 2023), 76 patients had received at least one retifanlimab dose. Median (range) age was 67 (49-88) years; 88.2% of patients had recurrent metastatic disease and 80.3% had visceral metastases. Seventy-five patients (98.7%) had received at least one prior systemic therapy. Median retifanlimab exposure was 10.0 (0.03-25.9) months; 23 patients completed treatment. 38 patients (50.0%) had grade ≥3 treatment-emergent adverse events (TEAEs), most commonly anemia (n = 10 [13.2%]). 63 patients (82.9%) had treatment-related AEs (TRAEs; grade ≥3, n = 14 [18.4%]); most common was fatigue (n = 14 [18.4%]). Two patients had TEAEs that led to death; no TRAEs were fatal. 39 patients had objective responses (51.3%; 95% CI, 39.6-63.0%); 19 patients (25.0%) had complete response and 20 (26.3%) had partial response. Median progression-free survival was 12.2 months; 30 patients (76.9%) had duration of response (DOR) ≥12 months. Median DOR was not reached after median follow-up time of 26.0 months., Conclusions: Retifanlimab was generally well tolerated and demonstrated encouraging anti-tumor activity in patients with pre-treated recurrent MSI-H/dMMR endometrial cancer., Competing Interests: Declaration of competing interest Patricia Pautier received travel funding from Amgen, AstraZeneca, GlaxoSmithKline, MSD, Novartis, PharmaMar, and Tesaro; had an advisory role with AstraZeneca, Genmab, MSD, Onxeo, PharmaMar, and Roche; and received honoraria from AstraZeneca, Eisai, MSD, and PharmaMar. Domenica Lorusso had consultancy roles with Novartis and PharmaMar; had advisory and invited speaker roles with AstraZeneca, Clovis Oncology, Genmab, GlaxoSmithKline, ImmunoGen, MSD, and Seagen; received institutional support from Clovis Oncology, Corcept, Genmab, ImmunoGen, and MSD; and received grants for academic trials from Clovis Oncology, GlaxoSmithKline, and MSD. Christine Gennigens received a grant from AstraZeneca, GlaxoSmithKline; received consulting fees from Ispen, GlaxoSmithKline, and MSD; received honoraria for lectures from AstraZeneca, Bristol Myers Squibb, GlaxoSmithKline, Ipsen, MSD, Pfizer, and PharmaMar; received support for meetings and/or travel from GlaxoSmithKline, Ipsen, MSD, Pfizer, and PharmaMar; and participated in a data safety monitoring board/advisory board for AstraZeneca, Bristol Myers Squibb, Eisai, GlaxoSmithKline, Ipsen, and MSD. Laurence Gladieff participated in an advisory board for AstraZeneca, Clovis Oncology, GlaxoSmithKline, and MSD; received honoraria from AstraZeneca, GlaxoSmithKline, MSD, PharmaMar, and Roche; and received congress funding from GlaxoSmithKline, PharmaMar, Roche, and Viatris. Jill Bowman, Chuan Tian, and Mark Cornfeld have employment and stock ownership at Incyte Corporation. Toon Van Gorp received consulting fees from AstraZeneca, BionTech, Eisai, GlaxoSmithKline, ImmunoGen, Incyte, Karyopharm, MSD, OncXerna, Seagen, Tubulis, and Zentalis; had corporate-sponsored research from Amgen, AstraZeneca, and Roche; and received honoraria for lectures from GlaxoSmithKline, ImmunoGen, and MSD. All payments were via the institution. Dominique Berton and Anna Kryzhanivska have no conflict of interest related to this manuscript., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)