504 Background: Patients with advanced metastatic urothelial carcinoma (UC) who experience disease progression after standard therapy have limited treatment options. Phase I studies of toripalimab in subjects with heavily pretreated metastatic UC have demonstrated an acceptable safety profile and promising clinical activity. Here we report the preliminary safety and efficacy result of toripalimab in a Phase II clinical study in Chinese patients with refractory/metastatic UC (Clinical trial ID: NCT03113266). Methods: Metastatic UC patients received toripalimab, also known as JS001, 3 mg/kg Q2W until disease progression or unacceptable toxicity. All patients with measurable disease were assessed for clinical response every 8 weeks according to RECISTv1.1. Tumor PD-L1 expression and tumor mutational burden was measured for correlation with clinical response. Results: From May 2017 to Sep 2019, 151 patients were enrolled from 15 participating centers. Patient enrollment was completed for this study. The median age was 62.0 years with 66.9% of the population being male. By the cut-off date of 21 Oct 2019, common treatment related AEs were mostly Grade 1 or 2, including anemia, blood triglycerides increased, fever, protein urine present and blood glucose increased. Among 128 efficacy evaluable patients, 4 complete responses, 25 partial responses, and 35 stable diseases were observed, for an objective response rate (ORR) of 22.7% and a disease control rate of 50.0%. 62.1% (18/29) of responses were ongoing at the cut-off date. PD-L1 expression results were obtained from 121 efficacy evaluable subjects. PD-L1+ patients (n = 39, 32.2%) had significant better ORR than PD-L1- patients (n = 82), 38.5% versus 13.4%. Conclusions: Toripalimab has demonstrated encouraging clinical activity in chemo-refractory UC patients and a manageable safety profile. Toripalimab elicited a favorable 38.5% ORR in PD-L1+ patients. Nevertheless PD-L1- patients also achieved a 13.4% ORR, including one complete response. Patients will be continuously monitored for additional safety and efficacy readouts (DOR, PFS and OS). Clinical trial information: NCT03113266.