Modifying the osteoblastic niche with zoledronic acid in vivo—Potential implications for breast cancer bone metastasis

INTRODUCTION: Bone metastasis is the most common complication of advanced breast cancer. The associated cancer-induced bone disease is treated with bone-sparing agents like zoledronic acid. Clinical trials have shown that zoledronic acid also reduces breast cancer recurrence in bone; potentially by modifying the bone microenvironment surrounding disseminated tumour cells. We have characterised the early effects of zoledronic acid on key cell types of the metastatic niche in vivo, and investigated how these modify the location of breast tumour cells homing to bone. METHODS: Female mice were treated with a single, clinically achievable dose of zoledronic acid (100 μg/kg) or PBS. Bone integrity, osteoclast and osteoblast activity and number/mm trabecular bone on 1, 3, 5 and 10 days after treatment were assessed using μCT, ELISA (TRAP, PINP) and bone histomorphometry, respectively. The effect of zoledronic acid on osteoblasts was validated in genetically engineered mice with GFP-positive osteoblastic cells. The effects on growth plate cartilage were visualised by toluidine blue staining. For tumour studies, mice were injected i.c. with DID-labelled MDA-MB-231-NW1-luc2 breast cancer cells 5 days after zoledronic acid treatment, followed by assessment of tumour cell homing to bone and soft tissues by multiphoton microscopy, flow cytometry and ex vivo cultures. RESULTS: As early as 3 days after treatment, animals receiving zoledronic acid had significantly increased trabecular bone volume vs. control. This rapid bone effect was reflected in a significant reduction in osteoclast and osteoblast number/mm trabecular bone and reduced bone marker serum levels (day 3–5). These results were confirmed in mice expressing GFP in osteoblastic linage cells. Pre-treatment with zoledronic acid caused accumulation of an extra-cellular matrix in the growth plate associated with a trend towards preferential [1] homing of tumour cells to osteoblast-rich areas of bone, but without affecting the total number of tumour cells. The number of circulating tumour cells was reduced in ZOL treated animals. CONCLUSION: A single dose of zoledronic acid caused significant changes in the bone area suggested to contain the metastatic niche. Tumour cells arriving in this modified bone microenvironment appeared to preferentially locate to osteoblast-rich areas, supporting that osteoblasts may be key components of the bone metastasis niche and therefore a potential therapeutic target in breast cancer.