Background: Recent studies have suggested the predictive value of liquid biopsies for immune checkpoint inhibitors. NCT03113266 is a multicenter phase II trial to evaluate the safety and efficacy of toripalimab (anti-PD-1) in metastatic urothelial carcinoma (mUC). Here we report the initial circulating tumor DNA (ctDNA) analysis of genomic alterations from a single-institution biomarker cohort. Methods: Twenty-seven mUC patients receiving toripalimab (3 mg/kg Q2W) at Ren Ji Hospital were enrolled and consented to Institutional Review Board-approved protocols permitting biomaterial collection and genetic sequencing. Serial plasma specimens were obtained at baseline and every two cycles. The 600-gene panel (PredicineATLAS) liquid biopsy assay was applied to assess somatic variants and blood tumor mutational burden (bTMB). Results: The ctDNA assays were performed successfully for 100% of baseline samples (n = 27) with average read depth of 24,389 (range 14,000-31,700). A total of 571 non-synonymous mutations were identified, demonstrating prevalent aberrations in TP53 (63%), TERT promoter (30%), KDM2D (26%), PPM1D (26%), and KDM6A (26%). In 5 patients, FGFR3 variants were detected, including 6 missense sites and 4 FGFR3-TACC3 fusion events. Copy number gain (FGFR1, ERBB2) and loss (PTEN, BRCA2, CDKN2A) were pinpointed. TMB estimation revealed one case with an exceptionally high bTMB (62.6 mutations/Mb) and genomic features of microsatellite instability (MSI). Concordance with tumor-based genotyping and ctDNA kinetics during toripalimab treatment are being determined. Conclusions: Prospective ctDNA analysis using the PredicineATLAS liquid biopsy assay is feasible and represents a minimally invasive approach to detecting cancer-specific genetic landscape and potentially guiding personalized therapeutic decisions in mUC patients. Clinical trial information: NCT03113266. Research Sponsor: Shanghai Junshi BioSciences; Huidu Shanghai Medical Sciences Ltd