Mechanisms of action of bisphosphonates on tumor cells and prospects for use in the treatment of malignant osteolysis

Abstract

Malignant osteolysis is a common complication of many cancers, most notably breast cancer, prostate cancer, and multiple myeloma. Hypercalcemia, pain, and fractures are the main manifestations. Malignant osteolysis can be fatal or cause a rapid deterioration in quality of life. The underlying mechanism in tumor cem-mediated activation of osteoclasts, whose function is normally to resorb bone. It follows that pharmacological agents capable of inhibiting osteoclast activity, including bisphosphonates, are likely to be useful in the treatment of malignant osteolysis. Also, experimental evidence suggest that bisphosphonates act on the tumor cells themselves, either by inhibiting mechanisms involved in the development of bone metastasis (tumor invasion, adhesion of tumor cells to the bone matrix) or by inducing apoptosis of tumor cells. Many clinical trials have found bisphophonates to be effective in the treatment of complications due to malignant osteolysis. Based on these studies, bisphosphonates are now indicated to treat hypercalcemia and to prevent skeletal complications of metastatic breast cancer and myeloma, in a dosage of 1600 mg.d orally for clodronate or 90 mg every four weeks intravenously for pamidronate. Osteoclast inhibition is clearly the mechanism underlying the efficacy of bisphosphonates in these clinical trials. Recent clinical trials found that prophylactic bisphosphonates therapy in patients with nonmetastasic breast cancer decreased the incidence of bone metastases, thus supporting a direct effect of biphosphonates on tumor cells. However, conflicting experimental and clinical data have been reported, so that it remains uncertain whether bisphosphonates have anti-tumor effects in vivo in humans. Nevertheless, biphosphonates now have an undisputed place in the therapeutic armamentarium for cancer.