Figure 1: Tumour cells colonize bone tissue and secrete growth factors that stimulate differentiation and proliferation of osteoclasts (OCL). The latter induce bone resorption that is the cause of the skeletal related events (SREs). ZOL accumulates prevalently in the bone tissue that is lacking phosphate salts due to the resorption mediated by OCL. Therefore, ZOL accumulates near OCL and is actively phagocyted by OCL where it induces growth inhibition and apoptosis. ZOL encapsulated in liposomes (Lipo-ZOL) accumulates in tissues with fenestrated vessels that are typical of tumour tissues and lacks the high affinity for bone tissue of free ZOL. Therefore, it preferentially accumulates in tumour tissues with consequent increased anti-cancer potential.