• Patients must have pathologically confirmed endometrial cancer
• Patients with advanced endometrial cancer
• Stage III with residual disease
• Stage IV
• Recurrent endometrial cancer after adjuvant therapy only
• Patients can have primary or planned interval surgery
• MMR-D on immunohistochemistry OR MSI-H using any commercially available test
• Patients with treated brain metastases are eligible if follow up brain imaging after CNS directed therapy shows no evidence of progression
• Eligible for standard of care chemotherapy with immune checkpoint inhibitor per treating investigator with no clinical contraindications
• For up to 10 patients allowed to enroll after C1D1 of standard of care treatments, they need to be on treatment with no clinical evidence of disease progression
• Patients may have received prior radiation therapy for treatment of endometrial cancer including pelvic radiation, extended field pelvic/para aortic radiation, intravaginal brachytherapy, and/or palliative radiation therapy
• Patients may have received prior hormonal therapy for treatment of endometrial cancer
• Patients may not have received prior therapy with anti-PD-1, anti-PD-L1, anti-CTLA-4 antibodies or similar agents except for up to 10 patients allowed after C1D1 of standard care treatments
• Age 18 years or older
• Not pregnant and not nursing

History of severe allergic reactions to study medication Currently pregnant or breastfeeding Recent participation in another clinical trial within the last 30 days Presence of uncontrolled medical conditions that could affect safety