• Has a histologically-confirmed diagnosis of endometrial carcinoma or carcinosarcoma.
• Has radiographically evaluable disease, either measurable or nonmeasurable per RECIST 1.1, as assessed by BICR.
• Has received prior platinum-based chemotherapy and anti-programmed cell death 1 protein (PD-1)/anti- programmed cell death ligand 1 (PD-L1) therapy, either separately or in combination.

• Has neuroendocrine tumors or endometrial sarcoma, including stromal sarcoma, leiomyosarcoma, adenosarcoma, or other types of pure sarcomas.
• Has a history of documented severe dry eye syndrome, severe Meibomian gland disease and/or blepharitis, or corneal disease that prevents/delays corneal healing.
• Has active inflammatory bowel disease requiring immunosuppressive medication or previous history of inflammatory bowel disease.
• Has had a recurrence of endometrial carcinoma or carcinosarcoma more than 180 days after completing platinum-based therapy administered in the curative-intent or adjuvant setting without any additional platinum-based therapy received in the metastatic or recurrent setting.
• Has received more than 3 prior lines of therapy for endometrial carcinoma or carcinosarcoma.

Experimental: MK-2870 Participants will receive 4 mg/kg of MK-2870 via intravenous (IV) infusion on Day 1 of each 14-day cycle. Additionally, participants receive diphenhydramine (or equivalent), an H2 antagonist of investigator’s choice, acetaminophen (or equivalent), and dexamethasone (or equivalent) per each drug’s product label prior to the first 4 infusions of MK-2870. At subsequent infusions, the H2 antagonist and dexamethasone are optional, at the discretion of the investigator.

Active Comparator: Chemotherapy Participants will receive 60 mg/m^2 of doxorubicin by IV infusion on Day 1 of each 21-day cycle; or 80 mg/m^2 of paclitaxel by IV infusion on Days 1, 8, and 15 of each 28-day cycle.

A Phase 3, Randomized, Active-controlled, Open-label, Multicenter Study to Compare the Efficacy and Safety of MK-2870 Monotherapy Versus Treatment of Physician’s Choice in Participants With Endometrial Cancer Who Have Received Prior Platinum-based Chemotherapy and Immunotherapy (MK-2870-005/ENGOT-en23/GOG-3095)

The study supports a patient reimbursement program for qualifying expenses.

The primary objectives of this study are to compare MK-2870 to Treatment of Physician’s Choice (TPC) with respect to progression-free survival (PFS) per RECIST 1.1, as assessed by blinded independent central review (BICR), and overall survival (OS). The primary hypotheses are that MK-2870 is superior to TPC with respect to PFS per RECIST 1.1, as assessed by BICR, and that MK-2870 is superior to TPC with respect to OS.