Bone is one of the most frequent sites for metastasis in breast cancer patients often resulting in significant clinical morbidity and mortality. Bisphosphonates are currently the standard of care for breast cancer patients with bone metastasis. We have shown previously that doxycycline, a member of the tetracycline family of antibiotics, reduces total tumour burden in an experimental bone metastasis mouse model of human breast cancer. In this study, we combined doxycycline treatment together with zoledronic acid, the most potent bisphosphonate. Drug administration started 3 days before the injection of the MDA-MB-231 cells. When mice were administered zoledronic acid alone, the total tumour burden decreased by 43% compared to placebo treatment. Administration of a combination of zoledronic acid and doxycycline resulted in a 74% decrease in total tumour burden compared to untreated mice. In doxycycline-and zoledronate-treated mice bone formation was significantly enhanced as determined by increased numbers of osteoblasts, osteoid surface and volume, whereas a decrease in bone resorption was also observed. Doxycycline greatly reduced tumour burden and could also compensate for the increased bone resorption. The addition of zoledronate to the regimen further decreased tumour burden, caused an extensive decrease in bone-associated soft tissue tumour burden (93%), and sustained the bone volume, which could result in a smaller fracture risk. Treatment with zoledronic acid in combination with doxycycline may be very beneficial for breast cancer patients at risk for osteolytic bone metastasis.

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1102 Background: The benefits of bisphosphonates (BPs) in reducing or delaying skeletal related events (SREs) in patients with BM have been attributed to their potent osteoclast (OC) inhibiting effect. However, despite the use of modern systemic anti-cancer therapy including potent BPs, many patients with BM continue to suffer from the consequences of their bone disease. An improved understanding of the basic mechanisms of bone destruction would allow further appropriate targeted treatment strategies to be developed. Methods: Archival paraffin embedded BM specimens from patients with MBC were examined for expression of OCs, receptor activator of nuclear factor kappa B (RANK), RANK Ligand (RANKL) and Osteoprotegerin (OPG). Histological specimens were also available for primary breast cancer, lymph node (LN) metastasis, normal breast and bone tissues for comparison. Results: BM specimens were available for 20 BP naïve pts and 2 pts treated with BP. OCs were significantly increased in the BM of the BP naive group compared to controls. There were no OCs seen in the BP treated group. RANK was expressed on tumor cells (TCs) in the both bone and nodal metastases but not on the primary cancer cells. It is also expressed on the OCs which were present in both BM and normal bone. While RANKL was absent in TCs, it was strongly expressed in all stromal cells (SCs) in all specimens and in osteoblasts. The OPG, while present in TCs of the BM and LN metastases, is not detected in the primary cancer. Conclusion: The mechanism of bone destruction in MBC are not fully understood and are clearly multifactorial. OCs may not be the singular obligatory factor for osteolysis in BM. Further investigation of various inhibitors of the RANK/RANKL/OPG pathways, may allow novel treatment strategies to be developed. No significant financial relationships to disclose.