Pathophysiology of bone metastases: how this knowledge may lead to therapeutic intervention

Bone metastases are common in many advanced cancers and are a clinically relevant source of skeletal morbidity. The bone mineral matrix contains numerous growth factors that are released during normal bone remodeling, providing a fertile microenvironment for tumor cell colonization and proliferation. Tumor cells then release a variety of growth factors that promote bone resorption and increase the risk of skeletal complications. Bisphosphonates are potent inhibitors of osteoclast activity that have demonstrated efficacy in the treatment of bone metastases. Bisphosphonates bind avidly to the bone matrix, are released during bone resorption, and are subsequently internalized by osteoclasts, where they interfere with biochemical pathways and induce osteoclast apoptosis. Bisphosphonates also antagonize osteoclastogenesis and promote the differentiation of osteoblasts. As a result, bisphosphonates inhibit tumor-induced osteolysis and reduce skeletal morbidity. Furthermore, preclinical studies suggest that bisphosphonates possess antitumor activity and can inhibit proliferation and induce apoptosis of tumor cell lines. In addition, zoledronic acid, a new-generation bisphosphonate, appears to inhibit tumor cell invasion of the extracellular matrix. These data suggest that zoledronic acid and other bisphosphonates may play a role in the reduction of skeletal tumor burden and the prevention of bone metastasis.