RAF265, a dual BRAF and VEGFR2 inhibitor, prevents osteoclast formation and resorption. Therapeutic implications

Introduction The RAS/RAF/MEK/ERK signaling pathway plays an important role in osteoclast (OC) differentiation and survival mediated by macrophage-colony stimulating factor (M-CSF). Also, vascular endothelial growth factor (VEGF) may greatly influence OC formation and resorption through VEGFR1 and VEGFR2. RAF265 is a novel, orally bioavailable dual inhibitor of RAF kinase and VEGFR2. Methods Effect of RAF265 on osteoclastogenesis from peripheral blood mononuclear cells (PBMCs) and OC resorption on calcium-coated wells was assessed by appropriate in vitro assays. Immunoblotting, real-time RT-PCR and flow cytometry were used to evaluate RAF265 mechanism of action. Results RAF265 significantly impaired in vitro differentiation of PBMCs to OCs induced by receptor activator of NF-kB ligand (RANKL) and M-CSF (IC50 ≅ 160 nM). In parallel, RAF265 exerted a potent inhibition of OC resorptive capacity (IC50 ≅ 20 nM). RAF265 treatment led to ERK inhibition and diminished expression of c-fos and NFATc1 (nuclear factor of activated T cells, calcineurin-dependent 1), which would likely account for inhibition of osteoclastogenesis. The reduced gene expression of aVb3 integrin, CCR1, cathepsin K, carbonic anhydrase II, matrix metalloproteinase 9, urokinase and tissue-type plasminogen activators, vacuolar H+-ATPase subunit (ATP6V1A) and Rab7 GTPase would probably mediate RAF265 hindered resorption. RAF265 inhibitory effect on VEGFR2 (noticeable at 10–50 nM) was also found to be implicated in the potent inhibition of this agent on OC function. Conclusions We have found a new therapeutic application for RAF265 as an inhibitory agent of osteoclastogenesis and OC function, which might be useful for the treatment of skeletal disorders associated with increased bone resorption.