Enhancing the radiation response of tumors but not early or late responding normal tissues using a vascular disrupting agent.

INTRODUCTION: Vascular disrupting agents (VDAs) damage tumor vasculature and enhance tumor radiation response. In this pre-clinical study, we combined radiation with the leading VDA in clinical development, combretastatin A-4 phosphate (CA4P), and compared the effects seen in tumors and relevant normal tissues.

MATERIAL AND METHODS: Radiation was applied locally to tissues in CDF1 mice to produce full radiation dose-response curves. CA4P (250 mg/kg) was intraperitoneally (i.p.) injected within 30 minutes after irradiating. Response of 200 mm3 foot implanted C3H mammary carcinomas was assessed using percent tumor control at 90 days. Normal tissue effects were evaluated using early responding skin (development of moist desquamation in the foot at 11-30 days), and late responding bladder (50% reduction in reservoir function estimated by cystometry up to 9 months after treatment), and lung (20% increase in ventilation rate measured by plethysmography within 9 months). A Chi-squared test was used for statistical comparisons (significance level of p < .05).

RESULTS: The radiation dose controlling 50% of irradiated tumors was 52 Gy. This significantly decreased to 45 Gy with CA4P. The radiation doses inducing a change in skin, bladder and lung response in 50% of mice were 31 Gy, 14 Gy and 12 Gy, respectively. CA4P had no significant effect on the radiation response of any of these normal tissues.

CONCLUSIONS: VDAs significantly enhance tumor radiation response, but had absolutely no effect on the radiation response of early or late responding normal tissues.