• Age 18 years or older

• ECOG performance status 0-1

• Endometrial cancer: all histologies (submission of pathology report is required for all patients who underwent surgery outside of MSK)

• MMR-D or MSI-H (submission of report\[s\] required for. patients who underwent testing outside of MSK

‣ Patients with hypermethylation of MMR gene promoters (MLH1, MSH2, MSH6 or PMS2) will be included

⁃ POLE-mutated endometrial cancers will be included

⁃ Patients with stage IIIB and IIIC2 are not eligible

• Must have undergone a complete surgical staging and have stage III/IVA disease.

‣ Patients with measurable disease (RECIST 1.1) after surgery are NOT eligible.

⁃ PatienSurgery completed between 3 weeks and 12 weeks (inclusive) before cycle 1 day 1 of therapy, and must have adequately recovered from surgery and any complications of surgery.ts with isolated tumor cells in lymph nodes found on surgery will be counted as clinical stage III disease

• Surgery completed between 3 weeks and 12 weeks (inclusive) before cycle 1 day 1 of therapy, and must have adequately recovered from surgery and any complications of surgery.

• Has a negative serum pregnancy test within 7 days prior to taking study treatment if of childbearing potential, and agrees use an adequate method of contraception from screening through 150 days after the last dose of study treatment, or is of nonchildbearing potential. Nonchildbearing potential is defined as follows (by other than medical reasons):

• °≥45 years of age and has not had menses for \>1 year

⁃ Patients who have been amenorrhoeic for \<2 years without history of a hysterectomy and oophorectomy must have a follicle stimulating hormone value in the postmenopausal range upon screening evaluation

⁃ Post-hysterectomy, post-bilateral oophorectomy, or post-tubal ligation. Documented hysterectomy or oophorectomy must be confirmed with medical records of the actual procedure or confirmed by an ultrasound. Tubal ligation must be confirmed with medical records of the actual procedure, otherwise the patient must be willing to use an adequate birth control method throughout the study, starting with the screening visit through 150 days after the last dose of study treatment. See list of acceptable birth control methods. Information must be captured appropriately within the site's source documents. Note: Abstinence is acceptable if this is the established and preferred contraception for the patient.

• Participant must agree to not breastfeed during the study or for 150 days after the last dose of study treatment.

• Demonstrate adequate organ function as defined below. All screening labs should be performed within 14 days of treatment initiation.

‣ Absolute neutrophil count (ANC) ≥1,500 /mcL

⁃ Platelets ≥100,000 / mcL

⁃ Hemoglobin ≥9 g/dL

⁃ Creatinine Clearance GFR ≥ 45

⁃ Serum total bilirubin ≤ 1.5 X ULN (patients with known Gilbert's disease Serum total bilirubin who have bilirubin level ≤ 3 x ULN may be enrolled)

⁃ AST and ALT 3 ULN OR ≤ 5 X ULN for subjects with liver metastases

⁃ TSH within normal limits. If TSH is not within normal range despite no symptoms of thyroid dysfunction, normal free T4 level is required.