Aim: Bisphosphonates are used as an adjuvant treatment in breast cancer bone metastasis patients, often simultaneously with chemotherapeutic agents. Interestingly, their sequential combination has been reported to have synergistic anti-tumor effects on bone metastases in preclinical models. We studied the effects of doxorubicin (DOX) and zoledronic acid (ZOL) and their combination on established bone metastases in the MDA-MB-231(SA)GFP bone metastasis model.

Methods: Tumor burden and osteolytic bone lesions were quantitated by fluorescence imaging and radiography, respectively. The mice were randomized in four groups receiving vehicle, DOX, ZOL or both DOX and ZOL in a sequential combination on day 14. Serum marker of osteoclast number was followed weekly, and blood ionized calcium was measured at sacrifice. Bone and tumor area, apoptosis and proliferation of tumor cells were analyzed from histological sections.

Results: ZOL prevented hypercalcemia and osteolytic lesion progression, whereas DOX induced apoptosis in the MDA-MB-231(SA)GFP cells. However, neither of the treatments alone nor in sequential combination were able to reduce tumor burden in bone. Furthermore, no additive effects on tumor cell apoptosis were observed in the combination group.

Conclusion: No additive effects in combination of DOX and ZOL were observed in this aggressive model of breast cancer bone metastasis.