Cabozantinib targets bone microenvironment modulating human osteoclast and osteoblast functions

Cabozantinib, a c-MET and vascular endothelial growth factor receptor 2 inhibitor, demonstrated to prolong progression free survival and improve skeletal diseaserelated endpoints in castration-resistant prostate cancer and in metastatic renal carcinoma. Our purpose is to investigate the direct effect of cabozantinib on bone microenvironment using a total human model of primary osteoclasts and osteoblasts. Osteoclasts were differentiated from monocytes isolated from healthy donors; osteoblasts were derived from human mesenchymal stem cells obtained from bone fragments of orthopedic surgery patients. Osteoclast activity was evaluated by tartrate resistant acid phosphatase (TRAP) staining and bone resorption assays and osteoblast differentiation was detected by alkaline phosphatase and alizarin red staining. Our results show that non-cytotoxic doses of cabozantinib significantly inhibit osteoclast differentiation (p= 0.0145) and bone resorption activity (p= 0.0252). Moreover, cabozantinib down-modulates the expression of osteoclast marker genes, TRAP (p= 0.006), CATHEPSIN K (p= 0.004) and Receptor Activator of Nuclear Factor k B (RANK) (p= 0.001). Cabozantinib treatment has no effect on osteoblast viability or differentiation, but increases osteoprotegerin mRNA (p= 0.015) and protein levels (p= 0.004) and down-modulates Receptor Activator of Nuclear Factor k B Ligand (RANKL) at both mRNA (p< 0.001) and protein levels (p= 0.043). Direct cell-to-cell contact between cabozantinib pre-treated osteoblasts and untreated osteoclasts confirmed the indirect anti-resorptive effect of cabozantinib. We demonstrate that cabozantinib inhibits osteoclast functions "directly" and "indirectly" reducing the RANKL/osteoprotegerin ratio in osteoblasts.