Breast cancer bone metastasis and current small therapeutics

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Abstract
Patients with advanced breast cancer frequently develop metastasis to bone. Bone metastasis results in intractable pain and a high risk of fractures due to tumor-driven bone loss (osteolysis), which is caused by increased osteoclast activity. Osteolysis releases bone-bound growth factors including transforming growth factor beta (TGF-β). The widely accepted model of osteolytic bone metastasis in breast cancer is based on the hypothesis that the TGF-β released during osteolytic lesion development stimulates tumor cell parathyroid hormone related protein (PTHrP), causing stromal cells to secrete receptor activator of NFκB ligand (RANKL), thus increasing osteoclast differentiation. Elevated osteoclast numbers results in increased bone resorption, leading to more TGF-β being released from bone. This interaction between tumor cells and the bone microenvironment results in a vicious cycle of bone destruction and tumor growth. Bisphosphonates are commonly prescribed small molecule therapeutics that target tumor-driven osteoclastic activity in osteolytic breast cancers. In addition to bisphosphonate therapies, steroidal and non-steroidal antiestrogen and adjuvant therapies with aromatase inhibitors are additional small molecule therapies that may add to the arsenal for treatment of osteolytic breast cancer. This review focuses on a brief discussion of tumor-driven osteolysis and the effects of small molecule therapies in reducing osteolytic tumor progression.