Study of First-line Pembrolizumab (MK-3475) With Lenvatinib (MK-7902/E7080) in Urothelial Carcinoma Cisplatin-ineligible Participants Whose Tumors Express Programmed Cell Death-Ligand 1 and in Participants Ineligible for Platinum-containing Chemotherapy (MK-7902-011/E7080-G000-317/ LEAP-011)

The purpose of this study is to evaluate the efficacy and safety of lenvatinib (MK-7902/E7080) in combination with pembrolizumab (MK-3475) in the treatment of cisplatin-ineligible participants with a Programmed Cell Death-Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥10, or in participants ineligible for any platinum-containing chemotherapy regardless of CPS, with advanced/unresectable or metastatic urothelial carcinoma (UC). The primary hypotheses for this study are that: 1. Pembrolizumab + lenvatinib is superior to pembrolizumab + placebo with respect to Progression-free Survival (PFS) per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1) by blinded independent central review (BICR), and 2. Pembrolizumab + lenvatinib is superior to pembrolizumab + placebo with respect to Overall Survival (OS). Based on recommendation of the external Data Monitoring Committee (eDMC), Amendment 3 (effective: September \[Sep\]-24-2021) was implemented to unblind the study and discontinue lenvatinib and placebo treatment. The eDMC was then disbanded. With Amendment 4 (effective: December-5-2022) second course pembrolizumab will no longer be offered. Any participant receiving second course pembrolizumab treatment prior to initiation of Amendment 4 will be able to complete treatment as planned. Study participation will end after the final administration of pembrolizumab. Participants who either complete 35 administrations of pembrolizumab or discontinue pembrolizumab will discontinue from the study following the safety follow-up visit. AEs and spontaneously reported pregnancies will be reported and followed per protocol. The overall study ends when the last participant completes the last study-related contact or visit, withdraws from the study, or is lost to follow-up.