Angiopoietin-2 blocking antibodies improve tumor growth inhibition and survival in mice treated with low doses of radiation

Radiation induced tumor cell death is strongly dependent on oxygen. As abnormal tumor vasculature promotes tumor hypoxia, drugs that induce vascular normalization, such as the anti-vascular endothelial growth factor (VEGF) antibody, have been tested as radiation sensitizers in preclinical and clinical settings. The insufficient benefit obtained with anti-VEGF therapy prompted us to test if antibodies blocking the endothelial growth factor angiopoietin-2 (Ang2) could improve the effect of radiation in mouse tumor allografts and human tumor xenografts. Mouse or human tumor cells were injected subcutaneously in isogenic immunocompetent or immunodeficient (NSG) mice, respectively, and tumors were allowed to form. The mice were injected with anti-Ang2 or control antibodies every three or four days starting three days before 3x2 Gy or 4x0.5 Gy whole-body radiation, followed by analysis of tumor growth, histology, vasculature, hypoxia and necrosis. Combination treatment with anti-Ang2 and radiation improved tumor growth inhibition and extended the survival of mice with melanoma or colorectal carcinoma allografts. A similar anti-Ang2 plus radiation response was also obtained in immunodeficient mice implanted with a human colorectal carcinoma xenograft, indicating that the adaptive immune response was not essential for the effect. Histological and immunohistochemical analysis of the tumors showed that the combination treatment decreased tumor vasculature, and increased tumor hypoxia and tumor necrosis in comparison with control tumors and tumors treated with the monotherapies. Our results suggest that a combination of Ang2 blocking antibodies and radiation increases tumor growth inhibition and extends the survival of tumor-bearing mice. Significance: These findings offer a preclinical rationale for further testing of the use of Ang2 blocking antibodies in combination with radiation to improve the overall outcome of cancer treatment