Two Part (Phase1b/ Phase3) Trial of Subcutaneous Dostarlimab Plus Carboplatin-Paclitaxel in Participants With Primary Advanced or Recurrent Endometrial Cancer

This is an open-label study, meaning both the researchers and participants will know which treatment is being given. The study has two parts. In Part 1, researchers will identify the most suitable SC dose. In Part 2, participants will be randomly assigned by chance to receive dostarlimab as either an SC injection or an IV infusion. All participants will also receive carboplatin and paclitaxel. Treatment may continue for up to 3 years, with participants closely monitored throughout the study.

Inclusion Criteria: * Participant is at least 18 years of age (or the legal age of consent in the jurisdiction in which the trial is taking place) at the time of signing the Informed Consent Form (ICF). * Participant has primary Stage III or Stage IV disease or first recurrent EC. * Participant must have a documented Mismatch repair (MMR)/Microsatellite Instability (MSI) status. * Participant is capable of giving signed informed consent, including compliance with the requirements and restrictions listed in the ICF and in the protocol. * Participant has an Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1, or 2 for Part 1 and an ECOG performance status of 0 or 1 for Part 2. * Participant has adequate organ function. Exclusion Criteria: * Participant has history of or active autoimmune disease that has required systemic treatments within the 2 years prior to screening. * Participant has a history of severe allergic and/or anaphylactic reactions to chimeric, human or humanized antibodies, fusion proteins, or known hypersensitivity to carboplatin, paclitaxel or dostarlimab components or excipients. * Participant has received prior therapy with anti-programmed death protein 1 (PD-1), anti-programmed cell death ligand 1 (PD-L1), or anti-PD-L2 agent. * Participant has known uncontrolled brain or central nervous system (CNS) metastases.