A Study to Compare Sacituzumab Tirumotecan (MK-2870) in Combination With Pembrolizumab (MK-3475) Versus Pembrolizumab Alone as Treatment in Participants With Mismatch Repair Proficient Endometrial Cancer (MK-2870-033/TroFuse-033/GOG-3119/ENGOT-en29)

All participants undergo an initial Induction Phase of six cycles, each cycle consisting of pembrolizumab + carboplatin or cisplatin + paclitaxel or docetaxel. Each cycle is three weeks. Participants whose cancer does not progress enter the Maintenance Treatment Phase and are then randomly assigned to pembrolizumab + sac-TMT or pembrolizumab monotherapy. Participants whose cancer does progress will have the possibility to enter the Subsequent Treatment Phase and are then randomly assigned to pembrolizumab + sac-TMT or sac-TMT monotherapy.

During the Induction Phase, participants receive pembrolizumab 200 mg, carboplatin area under the curve (AUC) 5 (mg/mL/min) or cisplatin 75 mg/m\^2, and paclitaxel 175 mg/m\^2 or docetaxel 75 mg/m\^2 on Day 1 of each 3-week cycle for 6 cycles (up to approximately 4 months).

During the Maintenance Treatment Phase, participants receive sac-TMT 4 mg/kg on Days 1, 15, and 29 of each 6-week cycle and pembrolizumab 400 mg on Day 1 of each 6-week cycle for up to 14 cycles (up to approximately 19 months).

Biological: Pembrolizumab · Drug: Carboplatin · Drug: Paclitaxel · Drug: Docetaxel · Biological: Sacituzumab Tirumotecan · Drug: Cisplatin

During the Induction Phase, participants receive pembrolizumab 200 mg, carboplatin AUC 5 (mg/mL/min) or cisplatin 75 mg/m\^2, and paclitaxel 175 mg/m\^2 or docetaxel 75 mg/m\^2 on Day 1 of each 3-week cycle for 6 cycles (up to approximately 4 months).

During the Maintenance Treatment Phase, participants receive pembrolizumab 400 mg on Day 1 of each 6-week cycle for up to 14 cycles (up to approximately 19 months).

Biological: Pembrolizumab · Drug: Carboplatin · Drug: Paclitaxel · Drug: Docetaxel · Drug: Cisplatin

During the Induction Phase, participants receive pembrolizumab 200 mg, carboplatin AUC 5 (mg/mL/min) or cisplatin 75 mg/m\^2, and paclitaxel 175 mg/m\^2 or docetaxel 75 mg/m\^2 on Day 1 of each 3-week cycle for 6 cycles (up to approximately 4 months).

During the Subsequent Treatment Phase, participants receive sac-TMT 4 mg/kg on Days 1, 15, and 29 of each 6-week cycle until discontinuation criteria is met and pembrolizumab 400 mg on Day 1 of each 6-week cycle for up to 14 cycles (up to approximately 19 months).

Biological: Pembrolizumab · Drug: Carboplatin · Drug: Paclitaxel · Drug: Docetaxel · Biological: Sacituzumab Tirumotecan · Drug: Cisplatin

During the Induction Phase, participants receive pembrolizumab 200 mg, carboplatin AUC 5 (mg/mL/min) or cisplatin 75 mg/m\^2, and paclitaxel 175 mg/m\^2 or docetaxel 75 mg/m\^2 on Day 1 of each 3-week cycle for 6 cycles (up to approximately 4 months).

During the Subsequent Treatment Phase, participants receive sac-TMT 4 mg/kg on Days 1, 15, and 29 of each 6-week cycle until discontinuation criteria is met.

Drug: Carboplatin · Drug: Paclitaxel · Drug: Docetaxel · Biological: Sacituzumab Tirumotecan · Drug: Cisplatin

Intravenous (IV) Infusion

During the Induction Phase, participants receive carboplatin AUC 5 (mg/mL/min) on Day 1 of each 3-week cycle for 6 cycles (up to approximately 4 months) via IV infusion.

During the Induction Phase, participants receive paclitaxel 175 mg/m\^2 on Day 1 of each 3-week cycle for 6 cycles (up to approximately 4 months) via IV infusion.

During the Induction Phase, participants may receive docetaxel (in place of paclitaxel) 75 mg/m\^2 on Day 1 of each 3-week cycle for 6 cycles (up to approximately 4 months) via IV infusion.

IV Infusion

During the Induction Phase, participants may receive cisplatin (in place of carboplatin) 75 mg/m\^2 on Day 1 of each 3-week cycle for 6 cycles (up to approximately 4 months) via IV infusion

Time frame: Up to approximately 44 months

PFS is defined as the time from randomization to the first documented progressive disease (PD) or death due to any cause, whichever occurs first as assessed by Response Criteria in Solid Tumors Version 1.1 (RECIST 1.1) as evaluated by the blinded independent central review (BICR). PD is defined as ≥20% increase in the sum of diameters of target lesions. In addition to the relative increase of 20%, the sum must also demonstrate an absolute increase of ≥5 mm. The appearance of one or more new lesions is also considered PD.

Time frame: Up to approximately 54 months

OS is defined as time from randomization to death due to any cause.

Time frame: Up to approximately 54 months

PFS2 is defined as the time from randomization to the documented subsequent objective disease progression after initiation of new anticancer therapy or death due to any cause, whichever occurs first. PFS2 as assessed by the investigator will be presented.

Time frame: Up to approximately 27 months

An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention.

Time frame: Up to approximately 24 months

An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention.

Time frame: Baseline and up to approximately 24 months

EORTC QLQ-C30 is a 30-item scale to assess the overall quality of life of cancer patients. Participant responses to the questions "How would you rate your overall health during the past week?" and "How would you rate your overall quality of life during the past week?" are scored on a 7- point scale (1= Very poor to 7=Excellent). Using linear transformation, raw scores are standardized, so that scores range from 0 to 100. A higher score indicates a better overall health status. The change from baseline in EORTC QLQ-C30 Global Health Status/Quality of Life score (Items 29 and 30) will be presented.

Time frame: Baseline and up to approximately 24 months

EORTC QLQ-C30 is a 30-item scale to assess the overall quality of life of cancer patients. The physical functioning score is based on participant responses to questions scored on a 4-point scale (1 = 'Not at All' to 4 = 'Very Much'). Higher scores indicate better physical functioning. The change from baseline in EORTC QLQ-C30 Physical Functioning (Items 1-5) combined score will be presented.

Time frame: Baseline and up to approximately 24 months

EORTC QLQ-C30 is a 30-item scale to assess the overall quality of life of cancer patients. The role functioning score is based on participant responses to questions scored on a 4-point scale (1 = 'Not at All' to 4 = 'Very Much'). Higher scores indicate better role functioning. The change from baseline in EORTC QLQ-C30 Role Functioning (Items 6 and 7) combined score will be presented.

Time frame: Baseline and up to approximately 24 months

The EORTC QLQ-EN24 is a 24-item questionnaire developed to be used in conjunction with the EORTC-QLQ-C30 to assess the quality of life of endometrial cancer patients. Participant responses are scored on a 4-point scale (1=not at all to 4=very much). A higher score reflects greater symptom severity. The change from baseline in EORTC QLQ-EN24 back and pelvis pain score will be presented.

A Phase 3 Randomized, Open-label, Multicenter Study to Compare the Efficacy and Safety of Sacituzumab Tirumotecan (Sac-TMT, MK-2870) in Combination With Pembrolizumab Versus Pembrolizumab Alone as First-line Maintenance Treatment in Participants With Mismatch Repair Proficient Endometrial Cancer (TroFuse-033/GOG-3119/ENGOT-en29)