Abstract C113: Discovery of Best-in-Class FGFR3 small molecule inhibitors with high isoform selectivity and activity against gatekeeper mutations

Abstract FGFR3 genomic alterations, with S249C being the most prevalent, are established oncogenic drivers in 10-60% of all bladder cancers depending on the disease stage. Erdafitinib, a pan-FGFR inhibitor, has been approved for the second line treatment of advanced or metastatic urothelial carcinoma with susceptible FGFR3 genetic alterations. However, the emergence of resistance together with dose-limiting toxicities driven by off-target inhibition of FGFR1/2/4, limit the overall response rate to approximately 35%. These limitations often lead to treatment discontinuation and prevent the use of Erdafitinib in earlier lines of treatment, less advanced stages of the disease or combination regimens. Thus, there is a clear need to develop a highly potent and selective FGFR3 small molecule inhibitor to fully unlock the therapeutic potential of this target. At Onco3R Therapeutics, our patient centric approach, integrating deep translational science with rational, structure-based and AI-augmented drug design has led to the identification of novel series of highly potent, isoform-selective small molecule FGFR3 inhibitors. These series exhibit a best-in-class potency and selectivity profile to mitigate off-target toxicities while preserving activity against acquired FGFR3 resistance (gatekeeper) mutations. Our lead series inhibit FGFR3 activity with single-digit nanomolar potencies in cells exogenously expressing FGFR3 S249C or the double resistant FGFR3 S249C/V555M mutant. Importantly, our lead molecules exhibit a unique selectivity window against other FGFR isoforms, with >66-fold, >16-fold and >22-fold selectivity over FGFR1, FGFR2 and FGFR4, respectively. Potency was further confirmed in cancer cell lines endogenously addicted to oncogenic FGFR3 alterations, including RT112 (FGFR3-TACC3 fusion), UMUC14 (S294C mutation), and KMS11 (Y373C mutation). Kinome-wide selectivity profiling demonstrated high selectivity for FGFR3. Furthermore, compounds from our lead series possess a favourable in vitro ADME and toxicology profile and have demonstrated in vivo efficacy in a bladder cancer cell-derived xenograft (CDX) model. Here, we identified unique FGFR3 selective inhibitor series with a best-in-class potency and selectivity profile. Thorough characterization and optimization efforts are ongoing to identify clinical development candidates with the ultimate goal of achieving greater efficacy and enhanced safety, providing meaningful benefits to patients diagnosed with FGFR3-altered cancer. Citation Format: Sandrine Vendeville, Lijs Beke, Ellen Lanckacker, Caroline Joannesse, Petra Vinken, Line Oste, Stéphane de Cesco, Shaun Martin, Sara Musch, David Moreno Delgado, Godelieve Lammens, Bart Stoops, Pieter Peeters, Pierre Raboisson, Francois Gonzalvez. Discovery of Best-in-Class FGFR3 small molecule inhibitors with high isoform selectivity and activity against gatekeeper mutations [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr C113.