Rapamycin-induced tumor vasculature remodeling in rhabdomyosarcoma xenografts increases the effectiveness of adjuvant ionizing radiation

Purpose: Rapamycin inhibits vascular endothelial growth factor expression. Vascular endothelial growth factor is a tumor-elaborated protein that stimulates neovascularization. This inhibition can cause transient " normalization" of the generally dysfunctional tumor vasculature, resulting in improved tumor perfusion and oxygenation. We hypothesized that this may potentiate the antitumor effects of adjuvant ionizing radiation. Methods: Mice bearing orthotopic Rh30 alveolar rhabdomyosarcomas were treated with rapamycin (5 mg/kg intraperitoneally daily ×5). Tumors were then evaluated for changes in intratumoral oxygenation, perfusion, vessel permeability, and microvessel density. Additional tumor-bearing mice were treated with 5 doses of rapamycin, irradiation (4 Gy), or 5 doses of rapamycin with irradiation administered on the first or sixth day of rapamycin treatment. Results: Although tumor vessel permeability changed only minimally, microvessel density decreased (3153 ± 932 vs 20,477 ± 3717.9 pixels per high-power field), whereas intratumoral oxygenation increased significantly (0.0385 ± 0.0141 vs 0.0043 ± 0.0023 mm Hg/mm ³) after 5 doses of rapamycin. Contrast-enhanced ultrasound demonstrated a significantly increased rate of change of signal intensity after 5 days of rapamycin, suggesting improved intratumoral perfusion. Tumor volume 14 days after treatment was smallest in mice treated with the combination of rapamycin given before irradiation. Conclusion: Combination therapy with rapamycin given before irradiation to normalize the tumor vasculature, thereby improving tumor oxygenation, increased the sensitivity of alveolar rhabdomyosarcoma xenografts to adjuvant irradiation.