Fibroblast growth factor receptor 3 (FGFR3), peroxisome proliferator-activated receptor gamma (PPARg), and outcomes with nivolumab (nivo) in metastatic urothelial cancer (UC).

511Background: Prior studies have correlated FGFR3 mutations (mut) and increased PPARg expression with decreased T-cell infiltration raising the possibility that these might represent mediators of PD-1/PD-L1 blockade resistance in luminal UC and targets for combination strategies. The causality of these associations remains unclear and the clinical relevance in PD-1/PD-L1 blockade treated patients has been underexplored. Methods: Archival tumor specimens from patients enrolled on CheckMate 275, a phase 2 trial of nivo in patients with platinum-resistant metastatic UC, underwent whole exome sequencing (WES) and gene expression profiling (HTG EdgeSeq). CD8 expression was determined by immunohistochemistry. Results: WES and gene expression data was available for 139 and 217 patients, respectively; the baseline characteristics and outcomes of both cohorts were similar to the full CheckMate 275 cohort. FGFR3mut were detected in 15 patients (11%) and correlated with higher FGFR3 gene expression. Neither FGFR3mu...