• Written informed consent and any locally-required authorization (e.g., HIPAA in the USA) obtained from the subject prior to performing any protocol-related procedures, including screening evaluations.

• Female participants age ≥ 18 years at the time of signing informed consent.

• Must have histologically confirmed diagnosis of advanced or recurrent endometrioid endometrial cancer that is deemed non-curable with either surgery or radiation therapy. Mixed endometrioid patient will be allowed if the endometrioid component is greater than 50% of the tumor and does not include serous or carcinosarcoma. Non-endometrioid endometrial cancer must have a confirmed Wnt-activating mutation (CTNNB1, RNF-43, APC, AXIN1/2, RSPO2/3, and ZNRF3).

• Patients may have received up to 2 prior systemic therapies for recurrent disease. Note: Chemotherapy given in conjunction with radiation or as part of primary therapy does not count as prior systemic therapy for recurrence. Hormonal therapy does not count toward prior therapy.

• Must consent to allow for a pre-treatment tumor biopsy. Tumor material from biopsies done before the screening period are acceptable if the biopsy was performed within 3 months prior to the planned treatment start and no other systemic cancer therapy was administered in the interim. If a biopsy is performed as part of the study and the specimen is considered non-diagnostic or does not have enough tissue (occurs less than 10% of the time), archival tissue can be used to determine the study cohort and the patient can still participate in the trial.

• Must not have received/progressed on treatment with an anti-PD-1/L1 mAb administered either as monotherapy or in combination with other checkpoint inhibitors or other therapies.

• Patients must be off all other anti-tumor therapies (including immunologic agents) for at least four weeks prior to start of treatment. Patients on hormonal agents require a washout for 10 days

• Patients must be off all other anti-tumor therapies (including immunologic agents) for at least four weeks prior to study registration. Patients on hormonal agents require a washout for 10 days

• Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2. Evaluation of ECOG is to be performed within 7 days prior to the first dose of study intervention.

⁃ Women of childbearing potential (WoCBP) must be permanently or surgically sterilized (undergone a total hysterectomy, bilateral lubal tigation, or bilateral oophorectomy) or are postmenopausal for greater than 12 months. (If uncertain of amenorrhea for 12 months, a pregnancy test will be done to confirm pregnancy status.) If ovaries are present and were not previously menopausal at the time of hysterectomy, should have a serum estradiol \<10 pm/mL to confirm ovarian senescence.

⁃ Adequate hematological organ function laboratory values are defined below:

∙ Absolute neutrophil count (ANC) ≥1500/µL

‣ Platelets ≥100 000/µL

‣ Hemoglobin ≥9.0 g/dL or ≥5.6 mmol/La

⁃ Adequate renal organ function laboratory values are defined below:

⁃ • Creatinine OR measured or calculated creatinine clearance (GFR can also be used in place of creatinine or CrCl) of ≤1.5 × ULN OR

⁃ ≥30 mL/min with creatinine levels \>1.5 × institutional ULN

⁃ Adequate hepatic organ function laboratory values are defined below:

∙ Total bilirubin ≤1.5 ×ULN OR direct bilirubin ≤ULN for participants with total bilirubin levels \>1.5 × ULN

‣ AST (SGOT) and ALT (SGPT) ≤2.5 × ULN unless liver metastasis are present, in which case it must be ≤5 × ULN

⁃ Adequate coagulation function laboratory values of international normalized ratio (INR) OR prothrombin time (PT) activated partial thromboplastin time (aPTT) of ≤1.5 × ULN receiving anticoagulant therapy as long as PT or aPTT is within therapeutic range of intended use of anticoagulants.

⁃ Criteria must be met without erythropoietin dependency and without packed red blood cell (pRBC) transfusion within last 2 weeks.

⁃ Creatinine clearance (CrCl) should be calculated per institutional standard. Laboratory value requirements should be adapted according to local regulations and guidelines for the administration of specific chemotherapies.