Inclusion Criteria:

          -  Age ≥18 years at the time of screening and female.

          -  Histologically confirmed diagnosis of epithelial endometrial carcinoma. All
             histologies, including carcinosarcomas, will be allowed. Sarcomas will not be allowed.

          -  Patient must have endometrial cancer in one of the following categories:

               1. Newly diagnosed Stage III disease (measurable disease per RECIST 1.1 following
                  surgery or diagnostic biopsy),

               2. Newly diagnosed Stage IV disease (with or without disease following surgery or
                  diagnostic biopsy)

               3. Recurrence of disease where the potential for cure by surgery alone or in
                  combination is poor.

          -  Naïve to first line systemic anti-cancer treatment. For patients with recurrent
             disease only, prior chemotherapy is allowed only if it was administered in the
             adjuvant setting and there is at least 12 months from date of last dose of
             chemotherapy administered to date of subsequent relapse

          -  FPPE tumor sample must be available for MMR evaluation.

          -  Has Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 within 7
             days of starting study treatment.

        Exclusion Criteria:

          -  History of leptomeningeal carcinomatosis.

          -  Brain metastases or spinal cord compression.

          -  Prior treatment with PARP inhibitors.

          -  Prior immune checkpoint inhibitors or prior treatment with an agent directed to a

        stimulatory or co-inhibitor T-cell receptor other than anti-PD-1, anti-PD-L1, or anti-PD-L2
        agent.