mTOR Inhibitors (Rapamycin and Its Derivatives) and Nitrogen Containing Bisphosphonates: Bi-functional Compounds for the Treatment of Bone Tumours

N-BP, rapamycin and its derivatives have been originally developed respectively as anti-resorptive and anti-fungal agents. In fact, in vitro and in vivo experiments demonstrated that these compounds are multi-functional molecules exerting their effects on tumour cell growth and bone remodelling. The major challenge in treating cancer relates to mutations in key genes such as p53, Rb or proteins affecting caspase signalling carried by many tumour cells. Whether nitrogen containing bisphosphonates (N-BP) are potent bone inhibitors, they also inhibit tumour cell proliferation and increase atypical apoptosis of bone tumour cells regardless of the p53 and Rb status. N-BP may be then considered as effective therapeutic agents in clinical trials of bone tumours. Rapamycin and its derivatives inhibit mTOR dependent mRNA translation both in osteoclasts and tumour cells. Cellular physiological mechanisms regulated by mTOR integrate many environmental parameters including growth factors, hormones, cytokines, amino acids, energy availability and cellular stresses that are coupled with cell cycle progression and cell growth. Rapamycin and its derivatives as well as N-BP must be considered as bi-(multi) functional molecules affecting simultaneously bone and tumour metabolisms. The present survey describes these two molecular families and discusses their therapeutic interests for primary bone tumours and bone metastases.

Keywords: Rapamycin, bisphosphonate, bone, osteolysis, bone remodelling, primary bone tumours, metastases.