The highly selective and potent reversible-covalent FGFR4 inhibitor, BB102, induces targeted-protein degradation through lysosomal autophagy

Introduction: Fibroblast growth factor 19 (FGF19) overexpression contributes to the tumorigenesis of certain forms of hepatocellular carcinoma (HCC) and fibroblast growth factor receptor 4 (FGFR4) mutations have been found in rhabdomyosarcoma, breast, colorectal, and gastric cancer. Thus, FGFR4 is considered as a novel target to treat cancer with hyperactivated FGF19/FGFR4 signaling. We previously reported BB102 was a highly selective and potent reversible-covalent FGFR4 inhibitor. Here we demonstrated for the first time that BB102 not only inhibited FGFR4 kinase activity but also induced FGFR4 degradation through lysosomal autophagy. Methods: BB102 was developed through structure-based drug design, and optimized by SAR analysis and medicinal chemistry iteration. Biochemical and cell-based assays were applied in evaluation of BB102 inhibitory activity. Parallel artificial membrane permeability assay (PAMPA) and permeability in MDCK-MDR1 cell line were conducted. The degradation of FGFR4 protein induced by BB102 was demonstrated in both tumor cell lines and tumors from xenograft models in mice. Results: BB102 inhibited FGFR4 kinase activity with IC 50 of 2.5 nM and displayed >400-fold selectivity against the other 206 kinases tested. BB102 blocked the phosphorylation of the downstream protein ERK with IC 50 of 2.4 nM. BB102 exhibited strong inhibitory effects on the cell proliferation with the high expression of both FGF19 and FGFR4, or FGFR4-mutated tumor cells and suppressed tumor growth in mouse models. PAMPA and permeability in MDCK-MDR1 cell line had shown BB102 was permeable to blood-brain barrier (BBB) and BB102 was found in the rat brain tissues. BB102 clearly induced FGFR4 protein degradation in FGFR4 over-expressed HuH-7, Hep3B, and SJCRH30 cancer cell lines, which was blocked by Bafilomycin A1, an inhibitor of lysosomal autophagy. BB102 induced degradation of FGFR4 only in the tumors but not in the normal lungs and livers from xenograft mouse models. Conclusion: BB102 is a degrader of FGFR4 with highly selective and potent reversible-covalent FGFR4 kinase inhibition. BB102 has remarkable antitumor activity in mice bearing HCC tumor xenografts. It is permeable to BBB. It has shown safe profiles in the phase Ia study and preliminary promising efficacy in the phase Ib study.