Integrin α_vβ₃ antagonist Cilengitide enhances efficacy of radiotherapy in endothelial cell and non-small-cell lung cancer models

Purpose: Integrins α_vβ₃ and α_vβ₅ are important in tumor growth and angiogenesis and have been recently explored as targets for cancer therapy. Radiotherapy also inhibits tumor growth and affects vasculature. We explored the combination of integrin antagonist Cilengitide (EMD 121974) and ionizing radiation. Methods and Materials: Levels of α_vβ₃ were determined for human umbilical vein endothelial cells (HUVEC), as well as H157 and H460 human non-small-cell lung cancer cells, using FACS analysis and immunofluorescence imaging. Clonogenic assays, Western immunoblots probed for cleaved caspase 3, and Annexin-V probing were used to evaluate cell survival and apoptosis. A cell detachment assay and matrigel assay were used to further examine the effects of treatment. Results: Human umbilical vein endothelial cells had the highest α_vβ₃ level, followed by H157, and H460. Interestingly, we found that 5 Gy irradiation induced expression of α_vβ₃ in all cell lines. Clonogenic assays showed a radiosensitizing effect with Cilengitide, and calculation of the dose enhancement ratio showed that the effect was highest in HUVECs (1.38), followed by H157 (1.19), and H460 (1.10), corresponding to the levels of target expression. There was an increase in apoptotic cells after combination treatment with Cilengitide and radiation, and there was an increase in detached cells after treatment with Cilengitide. Additionally, there was decreased endothelial tubule formation after combination treatment. Conclusions: We conclude that radiation induces expression of α_vβ₃ integrin in endothelial and non-small-cell lung cancer models, and that integrin antagonist Cilengitide is a radiosensitizer in proportion to the levels of target integrin expression.