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Background: PD-1/PD-L1 immune checkpoint inhibitors (ICIs) have demonstrated activity in the post-platinum and platinum-ineligible settings for advanced urothelial carcinoma (aUC). As only around 50% of patients with aUC can tolerate platinum-containing treatment, treatments combining first-line ICIs with non-platinum drugs are urgently needed. Therefore, we assessed the safety and efficacy of the anti-PD-L1 monoclonal antibody Socazolimab in combination with nab-paclitaxel as first-line therapy in aUC (NCT04603846)., Methods: This was a multi-center, single-arm, phase Ib study that enrolled patients with treatment-naive aUC. Patients received Socazolimab (5 mg/kg) and nab-paclitaxel (260 mg/m2) Q3w. The primary endpoint was safety and tolerability of the combination regimen. Second endpoints were the objective response rate (ORR) and progression-free survival., Results: Between September, 2020 and September, 2021, 20 patients with urothelial carcinoma were enrolled, arising from renal pelvis (5), bladder (8), and ureter (7). After a median follow-up of 17 months, the median number of treatment cycles was 12. No patients had dose limiting toxicity. All patients had treatment-related adverse events (TRAEs), most of which were grade 1 or 2. The common TRAEs (≥20%) were peripheral neurotoxicity, alopecia, rash, increased ALT, weight loss, weakness, pruritus, increased AST, increased γGT, increased ALP, neutropenia, emesis, and anorexia. Nine patients (45%) developed grade 3 TRAEs including peripheral neurotoxicity (30.0%), increased ALT (10.0%), and increased γGT (5.0%). Two patients (10%) discontinued treatment because of grade 3 mouth ulcer (n = 1) and grade 2 lung fibrosis (n = 1). No grade 4-5 TRAEs were observed. Among the 17 patients who had received at least one tumor assessment, ORR was 58.8% (95% CI, 32.9%-81.6%) and the median progression-free survival was 8.3 months (95% CI, 5.2-19.5). The median duration of response was 13.3 months (95% CI, 2.0-20.1), and the overall survival was 19.5 months (95% CI, 11.2-not reached)., Conclusion: Socazolimab combined with nab-paclitaxel has shown good safety and promising antitumor activity as first-line therapy in patients with advanced urothelial carcinoma., (© The Author(s) 2024. Published by Oxford University Press.)