Discovery of INCB126503 as a Potent and Selective FGFR2/3 Inhibitor.

Fibroblast growth factor receptors (FGFRs) are well-established oncology targets, with aberrant FGFR2 and FGFR3 activation implicated in multiple tumor types, including cholangiocarcinoma and urothelial carcinoma. Currently approved FGFR2/3-targeted therapies rely on pan-FGFR small-molecule kinase inhibitors, which often lead to off-target toxicities due to unintended inhibition of FGFR1 and FGFR4, as well as acquired resistance driven by gatekeeper mutations. Herein, we report the discovery of INCB126503, a highly potent, orally bioavailable FGFR2/3 inhibitor with excellent isoform selectivity and equipotent activity against gatekeeper mutants. INCB126503 effectively suppresses FGFR signaling in vivo without inducing hyperphosphatemia and demonstrates significant antitumor efficacy in xenograft models harboring FGFR3 genetic alterations.