Background: First-line cisplatin-based chemotherapy (70 mg/m²) is the standard of care for LA/mUC patients (pts). However, about 50% will be ineligible for Cisplatin according to Galsky´s criteria. Moreover, a significant proportion of cisplatin-fit pts will receive carboplatin based on physician criteria. s-GC represents a feasible alternative in such situations, and could improve response rate compared to carboplatin regimens. Atezo is a programmed death-ligand 1 (PD-L1) inhibitor that is approved as first line treatment for cisplatin-ineligible LA/mUC pts with PD-L1 expression ≥5% (Ventana SP142). We present the study design of a phase II single arm trial of Atezo +s-GC in previously untreated pts with LA/mUC (NCT04602078). Methods: This single arm, open-label, multicenter study evaluates the efficacy and safety of Atezo +s-GC in previously untreated pts with LA/mUC. 66 pts will be enrolled and receive s-GC x 6 cycles (Cisplatin 35mg/m2 + Gemcitabine 1000mg/m2 on days 1 and 8 Q3W) and Atezo (1200 mg IV Q3W), followed by Atezo (1200 mg IV Q3W) until disease progression, toxicity or absence of clinical benefit. Eligibility criteria include histologically confirmed unresectable LA/mUC, measurable disease per RECIST 1.1 and adequate organ and marrow. Pts must be unfit for full cisplatin dose based on: age > 70 years, PS ECOG 0-2, creatinine Clearance >30 and <60 mL/min per Cockroft-Gault formula or by 24-hour urine collection. Other reasons for cisplatin ineligibility as considered by investigator, including those uncovered by Galsky´s criteria, will be allowed, prior discussion with PI. Exclusion criteria include prior systemic therapy for LA/mUC (adjuvant/neoadjuvant allowed if finished > 12 months prior to inclusion), prior autoimmune disease and uncontrolled significant illnesses. The primary endpoint is ORR per RECIST 1.1 assessed by investigator; the secondary endpoints are DoR, OS, PFS and safety. Biomarker analysis, including PD-L1 expression and microbiome relationship, will be an exploratory objective. The first two patients were enrolled in February 2021. Clinical trial information: NCT04602078