The majority of patients who succumb to cancer die as a result of metastatic progression rather than as a direct result of the primary tumor. Bone metastases are one of the most frequent complications in advanced breast and prostate cancer and occur in up to 70 percent of patients. Tumor cells in the bone micro-environment secrete factors such as parathyroid hormone related protein (PTHrP) which promotes osteoclast activity, and subsequent osteolytic resorption releases growth factors from the bone matrix, in particular transforming growth factor-beta (TGF-β), which can stimulate tumor cells in a paracrine manner. Development of animal models that more faithfully reproduce the clinical features of advanced breast cancer is critical to the discovery of effective therapeutics to treat this disease. Injection of tumor cells into the left cardiac ventricle of mice is a well established method to generate bone metastases that model lesions seen in human cancer patients, but to date the vast majority of these studies have been performed with human cancer cells injected into immune-compromised mice. To overcome this limitation, we have utilized the 4T1 mouse carcinoma cell line derived from BALB/c mice in the intra-cardiac injection model of bone metastasis. Injection of 4T1 mammary carcinoma cells (5x103) into the left cardiac ventricle of syngeneic and immune-competent BALB/c mice resulted in the development of osteolytic bone metastases as determined by skeletal radiographs within 21 to 24 days. In addition, metastases were frequently seen in adrenal gland, kidney, liver, ovary and brain, a spectrum of metastasis that closely mimics that seen in human patients with advanced breast cancer. Animals experience significant morbidity due to disseminated disease by the fourth week after injection of tumor cells. Histopathologic analysis of the 4T1 bone lesions clearly demonstrated the disruption of both trabecular and cortical bone and the presence of tartrate resistant acid phosphatase (TRAcP) positive osteoclasts at the tumor-bone interface. At the histologic level the 4T1 bone lesions reproduce features of bone metastases seen in human cancer patients. Active treatment of mice beginning on Day 4 after intra-cardiac delivery of 4T1 mammary carcinoma cells with a neutralizing antibody to TGF-β (mouse monoclonal antibody 1D11) improved the median survival (33 vs. 27 Days) as compared to vehicle treated controls. Furthermore, osteolytic bone lesions were effectively controlled in animals treated with 1D11 (5 mg/kg, three times a week for a total of 6 doses) as compared to vehicle or isotype antibody treated controls. The development of this model represents an important addition to existing advanced cancer models, and this study suggests that neutralization of TGF-β may be an effective therapeutic intervention in advanced breast cancer with bone involvement.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]