Osteosarcoma is the most frequent malignant primary bone tumors. Despite recent improvements in multimodal therapy the problem of non-response to mono-chemotherapy remains. Therefore, novel multi-drug combinations targeting various molecular pathways are needed to decrease the emergence of resistance phenomenon and to potentiate the treatment efficacy. In this context, the effects of RAD001, a new orally available mTOR inhibitor was investigated in vitro and in vivo on osteosarcoma proliferation, both alone and in combination with Zoledronic acid (ZOL). The in vitro effects of ZOL and RAD001 were analyzed on human (MG63), rat (OSRGa) and mouse (POS-1 and MOS-J) osteosarcoma cell lines in terms of cell proliferation (XTT assay, manual cell counting, time-lapse microscopy), cell cycle analysis (flow cytometry analysis) and apoptosis (caspase 1, 3 and 8 activity). RAD001 and ZOL inhibit MG63, OSRGA and POS-1 osteosarcoma cell proliferation in a dose- and time-dependent manner without any modification of cell cycle distribution. In contrast, MOS-J cells are resistant to ZOL and RAD001. In all cell lines assessed, combination of RAD001 and ZOL exerts synergistic effect on the inhibition of cell proliferation and induces a significant decrease of P-mTOR, P-4EBP1 and Ras expression with no accumulation of IPP and ApppI. This drug combination has been then investigated in a mouse osteosarcoma model induced by i.m. inoculation of MOS-J cells in C57BL/6J mice. Clinical relevant doses of RAD001 (5 mg/kg) and ZOL (100 μg/kg) alone have no effect on tumor growth in contrast to combination of both drugs which decreases osteosarcoma progression. ZOL (alone or in combination) strongly increases bone formation. The combination of RAD001 with ZOL improves tissue repair as shown by important area of fibrosis into the residual tumor mass. The present work demonstrates the in vitro and in vivo synergistic effect of mTOR (RAD001) and mevalonate (ZOL) pathway inhibitors and suggests that ZOL potentiates RAD001 activity through Ras molecular pathway.
