Several FGFR inhibitors are approved or in clinical development for the treatment of FGFR-driven urothelial cancer, and molecular mechanisms of resistance leading to patient relapses have not been fully explored. We identified 21 FGFR-driven urothelial cancer patients treated with selective FGFR inhibitors and analyzed post-progression tissue and/or circulating tumor DNA (ctDNA). We detected single mutations in the FGFR tyrosine kinase domain in seven (33%) patients (FGFR3 N540K, V553L/M, V555L/M, E587Q; FGFR2 L551F) and multiple mutations in one (5%) case (FGFR3 N540K, V555L, L608V). Using Ba/F3 cells, we defined their spectrum of resistance/sensitivity to multiple selective FGFR inhibitors. Eleven (52%) patients harbored alterations in the PI3K-mTOR pathway (n = 5 TSC1/2, n = 5 PIK3CA, n = 1 NF2, n = 1 PTEN). In patient-derived models, erdafitinib was synergic with pictilisib in the presence of PIK3CA E545K, while erdafitinib-gefitinib combination was able to overcome bypass resistance mediated by EGFR activation.

<p>IC50 values of Ba/F3 cells harboring FGFR3:TACC3 mutants exposed to eight selective FGFR inhibitors.</p>

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