Dual targeting of FGFR3 and the ERBB receptor family or AXL enhances the efficacy of FGFR inhibitors in FGFR3 fusion-driven bladder cancer

Abstract BackgroundMutations and fusions in Fibroblast Growth Factor Receptor 3 (FGFR3) occur in 10-20% of metastatic urothelial carcinomas, and can confer sensitivity to FGFR inhibitors such as BGJ398 and erdafitinib. However, responses to these agents are often short-lived due to the development of acquired resistance. The objective of this study was to identify mechanisms of acquired resistance to FGFR inhibitors in two previously uncharacterized bladder cancer cell lines harbouring FGFR3 fusions, and assess rational combination therapies to enhance their activity. MethodsAcquired resistance to FGFR inhibitors was generated in two FGFR3 fusion harbouring cell lines, SW780 (FGFR3-BAIAP2L1 fusion) and RT4 (FGFR3-TACC3 fusion), by either long-term exposure to increasing concentrations of BGJ398 or sustained exposure to high concentrations of drug. Alterations in levels of key cell signalling regulators was assessed in resistant lines by phospho-RTK arrays and immunoblotting. Synergy between BGJ398 and alternate targeted therapies was explored using cell viability and apoptosis assays.ResultsAcquired resistance to BGJ398 in SW780 and RT4 cells was associated with increased expression of members of ERBB family of receptor tyrosine kinases and pAXL. Combination treatment of resistant cells with an FGFR inhibitor and either a pan-ERBB or an AXL inhibitor overcame this resistance. We also noted rapid reactivation of pERK in parental FGFR3 fusion-driven lines within 4-72 hours of BGJ398 treatment, with concomitant increase in pERBB3. Up-front combination treatment with BGJ398 and the pan-ERBB inhibitor AZD8931 delayed the reactivation of pERK, and induced a synergistic inhibition of cell viability and a concomitant increase in apoptosis.ConclusionsWe identify increased activation of AXL and ERBB family receptors as mechanisms of resistance to FGFR inhibition. Our findings suggest that upfront combination treatment with FGFR and pan-ERBB inhibitors warrants further investigation for FGFR3-fusion harbouring bladder cancers.