Objective: In the ENGOT-EN6-NSGO/GOG3031/RUBY trial, dostarlimab+carboplatin-paclitaxel demonstrated significant improvement in progression free survival and a positive trend in overall survival compared with placebo+carboplatin-paclitaxel, with manageable toxicity, in patients with primary advanced or recurrent endometrial cancer. Here we report on patient-reported outcomes in the mismatch repair-deficient/microsatellite instability-high population, a secondary endpoint in the trial., Methods: Patients were randomized 1:1 to dostarlimab+carboplatin-paclitaxel or placebo+carboplatin-paclitaxel every 3 weeks for 6 cycles followed by dostarlimab or placebo monotherapy every 6 weeks for ≤3 years or until disease progression. Patient-reported outcomes, assessed with the European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire Core 30 and Endometrial Cancer Module, were prespecified secondary endpoints. A mixed model for repeated measures analysis, a prespecified exploratory analysis, was conducted to generate least-squares means to compare between-treatment differences while adjusting for correlations across multiple time points within a patient and controlling for the baseline value. Results are provided with 2-sided, nominal p values., Results: Of 494 patients enrolled, 118 were mismatch repair-deficient/microsatellite instability-high. In this population, mean change from baseline to end of treatment showed visual improvements in global quality of life (QoL), emotional and social function, pain, and back/pelvis pain for dostarlimab+carboplatin-paclitaxel. Meaningful differences (least-squares mean [standard error]) favoring the dostarlimab arm were reported for change from baseline to end of treatment for QoL (14.7 [5.45]; p=0.01), role function (12.7 [5.92]); p=0.03), emotional function (14.3 [4.92]; p<0.01), social function (13.5 [5.43]; p=0.01), and fatigue (-13.3 [5.84]; p=0.03)., Conclusions: Patients with mismatch repair-deficient/microsatellite instability-high primary advanced or recurrent endometrial cancer receiving dostarlimab+carboplatin-paclitaxel demonstrated improvements in several QoL domains over patients receiving placebo+carboplatin-paclitaxel. The observed improvements in progression free survival and overall survival while improving or maintaining QoL further supports dostarlimab+carboplatin-paclitaxel as a standard of care in this setting., Trial Registration: ClinicalTrials.gov NCT03981796., Competing Interests: Competing interests: GV reports consulting fees from GSK; honoraria from AstraZeneca, GSK, and MSD; travel support from AstraZeneca and PharmaMar; participation in advisory boards for AstraZeneca, Eisai, GSK, and MSD. MAP reports consulting fees from GSK, Tesaro, Merck, Eisai, SeaGen, Clovis Oncology, and AstraZeneca. SH reports consulting fees from AstraZeneca, Eisai, GSK, and MSD and honoraria from AstraZeneca and GSK. EMM reports advisory board meeting fees from AstraZeneca, GSK, and Tempus; honoraria from OncLive and Opinions in Gyn Malignancies; and support for attending meetings from Opinions in Gyn Malignancies and OncLive. ZN reports honoraria from Sofmedica, AstraZeneca, and MSD; support for attending meetings from Sofmedica and Preglem; participation on a data safety monitoring board or advisory board for AstraZeneca and Richter Gedeon; stock options from Richter Gedeon; and receipt of equipment, materials, drugs, medical writing, gifts, or other services from AstraZeneca. RH reports honoraria from GSK, AstraZeneca, Clovis Oncology, Eisai, and Merck. DD reports receiving consulting fees from AstraZeneca, GSK/Tesaro, Roche, Eisai Germany, and MSD Oncology; honoraria for advisory roles from Roche, AstraZeneca, GSK/Tesaro, Intuitive Surgical, KLS Martin, MSD, PharmaMar, and Seagen; and travel support from AstraZeneca. TM reports honoraria from Immunogen. LG reports institutional grants from Alkermes, AstraZeneca, Clovis Oncology, Esperas, IMV, ImmunoGen, Karyopharm, MSD, Mersana, Novocure, OncoQuest Pharmaceuticals, Pfizer, Roche, and Tesaro; consulting fees from Merck; honoraria from Alkermes, AstraZeneca, Eisai, Eisai-Merck, and GSK. BA reports honoraria from AstraZeneca, Eisai, GSK, MSD, Norartis, and Roche; support for attending meetings from AstraZeneca, GSK, and Roche; participation on a data safety monitoring board or advisory board for Eisai, GSK, MSD, Roche, and Sanofi Aventis. RG reports participation on a data safety monitoring board or advisory board for Pionyr Pharmaceuticals and receipt of equipment, materials, drugs, medical writing, gifts, or other services from Bausch+Lomb. NC reports advisory board fees for Aadii, GSK, Kartos, Novita Pharmaceuticals, Tarveda Therapeutics, Toray, Umoja, and Zentalis. BJM reports consulting fees from Agenus, Akeso Biopharma, Amgen, Aravive, Bayer, Elevar, EMD Merck, Genmab/Seagen, GOG Foundation, Gradalis, ImmunoGen, Iovance, Karyopharm, Macrogenics, Mersana, Myriad, Novartis, Novocure, Pfizer, Puma, Regeneron, Sorrento, US Oncology Research, and VBL and speakers’ bureau honoraria from AstraZeneca, Clovis Oncology, Eisai, Merck, Roche/Genentech, and Tesaro/GSK. TJH reports personal consulting fees from Aadi, AstraZeneca, Caris, Clovis Oncology, Eisai, Epsilogen, Genentech, GSK, Immunogen, J&J, Merck, Mersana, and Seagen; participation on a data safety monitoring board or advisory board for Corcept; and leadership role on the GOG Foundation Board and President of GOG Partners. AS reports royalties as an UptoDate reviewer. BP reports institutional grant support from AstraZeneca, Celsion, Clovis Oncology, Eisai, Genentech/Roche, Karyopharm, Merck, Mersana, SeaGen, Sutro Biopharma, Takeda Pharmaceuticals, Tesaro/GSK, Toray, and VBL Therapeutics; consulting fees from AstraZeneca, Atossa, Clovis Oncology, Deciphera, Elevar Therapeutics, I-Mab Biopharma, Merck, Mersana, Sutro Biopharma, Tesaro/GSK, and Toray; support for attending meetings from GOG Foundation; advisory board fees from Arquer Diagnostics, AstraZeneca, Atossa, Clovis Oncology, Deciphera, Eisai, Elevar Therapeutics, GOG Foundation, I-Mab Biopharma, Lilly, Merck, Mersana, Seagen, Sutro Biopharma, Tesaro/GSK, Toray, and VBL Therapeutics; and noncompensated leadership fees from NYOB Society Secretary, SGO Clinical Practice Committee Chair, and SGO COVID-19 Taskforce Co-Chair. AAS reports support paid to her institution from GSK for the present IGCS abstract; institutional grant support from AbbVie, Aravive, AstraZeneca, Clovis Oncology, Eisai, Ellipses, I-Mab Biopharma, Immunogen, Merck, Oncoquest/Canaria Bio, Roche/Genentech, Seagen, TapImmune, Tesaro/GSK, and VBL Therapeutics; honoraria from @Point of Care Clinical Care Options Curio Science, Peerview, Bio ASCEND, RTP, GOG Foundation (Highlight reel), and GOG Foundation Symposium; patent issued for 'blood based biomarkers in ovarian cancer'; noncompensated participation on a data safety monitoring board/advisory board from AstraZeneca, Clovis Oncology, Gilead, Immunogen, Imvax, Merck, Mersana, Natera, Onconova, and OncoQuest; uncompensated leadership roles with SGO, AAOGF, and NRG and compensated role from GOG; receipt of medical writing support from AstraZeneca; and uncompensated Clinical Trial Steering Committees for the AXLerate trial (Aravive), AtTEnd trial (Hoffman-LaRoche), Oval Trial (VBL Therapeutics), FLORA-5 trial (CanariaBio), and QPT-ORE-004 (CanariaBio). DC reports consultant fees from AstraZeneca and GSK and honoraria from AstraZeneca, GSK, Seagen/Genmab, and Immunogen. MRM reports consulting fees from AstraZeneca, Biocad, GSK, Karyopharm, Merck, Roche, and Zai Lab; speakers’ bureau fees from AstraZeneca and GSK; research funding (to institution) from Apexigen, AstraZeneca, Deciphera (trial chair), GSK, and Ultimovacs; and personal financial interest in Karyopharm (stocks/shares, member of board of directors). DB reports institutional grant fees from GSK; fees for being a member of GOG Partners Investigational Council; and medical director/owner of Trials365. AMT, KPP, EF, OZ, LML, IP, and SS have nothing to disclose. OM, VV, and JG are employees of GSK., (© IGCS and ESGO 2024. Re-use permitted under CC BY-NC. No commercial re-use. Published by BMJ.)