Most patients present with disease that appears to be confined to the breast. However, a significant proportion of women will go on to develop metastatic breast cancer with bone being the most frequent site of distant relapse. The bone microenvironment provides a fertile soil for circulating metastatic cells as resorbing bone releases growth factors that are thought to attract circulating cancer cells to the bone surface and facilitate their growth and proliferation. These infiltrating tumour cells in turn produce bone cell-activating factors, which lead to increased bone resorption and further release of growth factors in a ‘vicious cycle’. The third-generation bisphosphonate zoledronic acid is a potent inhibitor of osteoclast-mediated bone resorption. It also has effects on numerous processes in the metastatic cascade, with in vitro and animal studies showing synergistic antitumour effects with a range of cytotoxic and endocrine treatments. The addition of zoledronic acid to adjuvant therapy could therefore be a therapeutic strategy of potential importance.

To investigate this, the AZURE trial was designed to determine whether adjuvant zoledronic acid improves the disease-free and bone metastasis-free survival of women with stage II/III breast cancer. Recruitment of 3300 subjects was completed in January 2006. First efficacy data are expected in 2008. There is a potential for synergistic action with chemotherapy to manifest as enhanced toxicity. Reassuringly, initial safety data show that zoledronic acid is well tolerated and can be safely combined with adjuvant chemotherapy without any increase in myelotoxicity or effect on dose intensity.