Abstract 1172: Targeting c-Met and VEGFR2 in the stromal compartment of prostate cancer bone metastasis

Advanced-stage prostate cancer patients commonly develop bone metastasis, accounting for significant morbidity and mortality. Metastatic prostate cancer cells characteristically induce osteoblastic reactions, for which no targeted therapies are currently available. Recent clinical and experimental data showed that dual inhibition of c-Met and VEGF receptor (VEGFR)-2 kinase activity reduced prostate cancer growth in bone with indications for suppressing osteoblastic activities, while the specific functions of c-Met and VEGFR2 in osteoblasts remain unclear. In our in vitro studies using three types of osteoblasts (MC3T3-E1 subclone 4, hFOB1.19 and murine calvarial osteoblasts), hepatocyte growth factor (HGF, a ligand for c-Met) and VEGFA increased expression of receptor activator of nuclear factor κB ligand (RANKL) and macrophage colony stimulating factor (M-CSF), two essential factors for osteoclastogenesis. In addition, siRNA-mediated knockdown of c-Met or VEGFR2 in osteoblasts suppressed HGF- or VEGFA-dependent gene expression, respectively. We demonstrate that insulin growth factor (IGF) increased RANKL and M-CSF expression in osteoblasts via c-Met transactivation. Furthermore, the conditioned media from IGF-, HGF-, or VEGFA-treated osteoblasts promoted in vitro osteoclastogenesis that was suppressed by inhibition of c-Met and VEGFR2 in osteoblasts. Subsequently, we examined whether cabozantinib, a dual kinase inhibitor of c-Met and VEGFR2 currently in clinical trials for metastatic prostate cancer, could suppress prostate cancer bone metastasis via inhibition of osteoblasts. To examine the effects of cabozantinib specifically in osteoblasts, cabozantinib-resistant PC-3 prostate cancer cells were generated and implanted in the tibiae of male nude mice. Cabozantinib (60mg/kg) or control diluent was administered via daily oral gavage for 3 weeks. In agreement with our in vitro data, cabozantinib suppressed tumor growth in bone (determined by in vivo bioluminescence) and tumor-induced osteolysis (determined by X-ray). Tumor cells were isolated and in vitro cultured to demonstrate that cabozantinib resistance was maintained during 3-week in vivo growth. Osteoblasts and osteoclasts numbers were reduced in the bones of cabozantinib-treated mice, and these histological changes were accompanied by significantly lower levels of RANKL and M-CSF levels in bone. Collectively, inhibition of c-Met and VEGFR2 in osteoblasts reduces RANKL and M-CSF expression, associated with decreased osteoclastogenesis and tumor-induced osteolysis. Therefore, we conclude that c-Met and VEGFR2 are promising therapeutic targets in the stromal compartment of prostate cancer bone metastasis, suggesting hat the effects of cabozantinib on skeletal-related events of prostate cancer are, at least in part, mediated by suppression of osteoblast function. Citation Format: Changki Lee, Preston Campbell, Young Mi Whang, Jamey D. Young, Florent Elefteriou, Serk In Park. Targeting c-Met and VEGFR2 in the stromal compartment of prostate cancer bone metastasis. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1172. doi:10.1158/1538-7445.AM2014-1172