The role of bisphosphonates in breast and prostate cancers

Bisphosphonate drugs are a group of pyrophosphate analogues which bind avidly to hydroxyapatite bone mineral surfaces and their major action is to inhibit osteoclast activity and thus bone resorption. In oncology, their role in metastatic bone disease is well established, but there is increasing interest in their potential role in preventing and treating cancer-induced bone loss and their possible anti-tumour effects. Metastatic bone disease is associated with a variety of skeletal complications, including pathologic fractures, bone pain, impaired mobility, spinal cord compression and hypercalcaemia. Intravenous bisphosphonates, particularly zoledronic acid, in conjunction with rehydration, are now established as the treatment of choice for hypercalcaemia. For treatment of bone pain, it has also been shown that bisphosphonates can be an effective supplementary approach to radiotherapy. In breast cancer and myeloma, bisphosphonates have now become part of standard therapy to treat and prevent skeletal-related events (SRE) and, until recently, treatment was largely with intravenous pamidronate or oral clodronate. However, large, randomised, multicentre trials using intravenous administration of the highly potent bisphosphonate zoledronic acid every 3-4 weeks have recently demonstrated a reduction of 20% in the risk of developing an SRE compared with pamidronate for patients with breast cancer. Moreover, these trials have demonstrated, for the first time, that a bisphosphonate significantly reduces the occurrence of skeletal events in hormone-refractory prostate cancer and in non-small cell lung cancer and a range of other solid tumours. Investigations into the potential of the relatively low potency bisphosphonate, clodronate, for the prevention of bone metastases in breast cancer have produced conflicting data. Further large, randomised studies with clodronate and zoledronic acid are planned and until the results are available it is not possible to identify a definite adjuvant role for bisphosphonates. Evidence is accumulating in vitro that bisphosphonates are also able to directly affect tumour cells, in addition to their effects on osteoclasts, with zoledronic acid being particularly potent. Over recent decades there has been a significant improvement in cure rates and survival times in certain cancers and the use of chemotherapy and hormone therapy has expanded greatly, leading to increasing numbers of long-term survivors who have received these treatments. Management of treatment-induced bone loss is therefore assuming a greater importance and bisphosphonates represent an attractive treatment option in such patients. Several placebo-controlled trials using oral clodronate, oral risedronate, intravenous pamidronate and intravenous zoledronic acid have all now demonstrated benefits in reducing the loss in bone mineral density.