Background: SG is an antibody-drug conjugate composed of an anti-trophoblast cell-surface antigen-2 (Trop-2) antibody coupled to a topoisomerase-I inhibitor, SN-38, via a hydrolyzable linker. In Cohort 1 of the phase 2 TROPHY-U-01 trial, SG monotherapy resulted in a 27% objective response rate (ORR) and median overall survival (OS) of 10.9 months with an overall manageable toxicity profile in pts with locally advanced or mUC who previously received platinum-based therapy and a checkpoint inhibitor (CPI; Tagawa S, et al. J Clin Oncol. 2021). These results led to accelerated FDA approval of SG in this population. The CPI ZIM is a fully human IgG4 monoclonal antibody targeting PD-1. CPIs are active in mUC, including avelumab that is FDA-approved as switch maintenance therapy for locally advanced or mUC that has not progressed with first-line platinum-based chemotherapy. Cohort 5 of the TROPHY-U-01 trial will evaluate the safety, tolerability, and efficacy of SG + ZIM vs ZIM alone vs avelumab as switch maintenance in pts with mUC who have received gemcitabine (GEM)/cisplatin without progressive disease (PD). Methods: TROPHY-U-01 (NCT03547973) is a multicohort, open-label, global, phase 2 trial. Cohort 5 includes pts who have not progressed after completion of 4-6 cycles of GEM/cisplatin. Key eligibility requirements include ≥18 y; ECOG PS 0-1; available tissue for biomarker testing; adequate blood counts without transfusion or growth factor within 2 weeks of study drug initiation; creatinine clearance ≥30 mL/min. A safety lead-in of 6-8 pts will be conducted, where pts will be treated with SG 10 mg/kg IV on D1 and D8 of a 21-D cycle plus ZIM 360 mg IV q3wk on a 21-D cycle. On safety lead-in, if SG + ZIM is deemed safe, pts will be randomized 1:1:1. Those in Arm 1 will receive SG 10 mg/kg IV on D1 and D8 of a 21-D cycle followed by ZIM 360 mg IV, q3wk (D1 of a 21-D cycle). Pts in Arm 2 will receive avelumab 800 mg IV q2wk (D1 of a 14-D cycle). Pts in Arm 3 will receive ZIM 360 mg IV q3wk. All pts will continue treatment until PD, unacceptable toxicity, or loss of clinical benefit. The primary endpoint is progression-free survival based on central review by RECIST 1.1 criteria. Secondary endpoints include OS (all arms), safety/tolerability of SG in combination with ZIM (Arm 1). Cohort 5 aims to enroll an estimated 158 pts. A sample size of 50 pts per randomized arm is determined based on clinical consideration for exploring the activity of anticancer agents. Clinical trial information: NCT03547973.