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We investigated the effect of an endothelin-1 antagonist on the development of breast cancer bone metastases. ET-1 is an angiogenic factor that plays a key role in the progression of breast metastases to bone. Murine mammary carcinoma 4T1 cells were injected in a skin-fold chamber implanted with bone explants on immunocompetent CB6 mice. Mice were treated with or without an antagonist of both ET-1 receptors. The progression of the vascularization within the chamber was monitored over time by intravital microscopy (IVM). The tumor growth and the development of bone metastases were assessed by histological studies. In addition, expression of endothelin-associated mRNA and proteins were evaluated in both tumor and bone metastases. Results indicate that the use of IVM in this model allowed for the continuous monitoring of the change in vascularization associated with the development of bone metastases. Additionally, treatment with the ET-1 antagonist was associated with reduced tumor mass and tumor bone metastases. Also, ET-1 antagonist treatment resulted in vessels with significantly smaller diameters (-40%) compared to those of control mice. The endothelin pathway was affected with significant changes in both mRNA and protein expressions in both tumor and bone metastases. Specifically, ETRA expression decreased whereas ETRB significantly increased in the tumor mass of treated animals compared to control mice (p0.05). Thus, our data support the hypothesis that both vascular regulators ET-1 and iNOS play key roles in the angiogenesis associated with the development of breast metastases to bone. (Supported by a grant from the Susan G. Komen Breast Cancer Foundation)

[Proc Amer Assoc Cancer Res, Volume 47, 2006]