The effects of leucovorin and dipyridamole on fluoropyrimidine-induced radiosensitization.

The biomodulators leucovorin and dipyridamole potentiate the cytotoxicity of 5-fluorodeoxyuridine (FdUrd) and 5-fluorouracil (5-FU), respectively. It was hypothesized that these biomodulators would increase fluoropyrimidine-mediated radiosensitization. This hypothesis was tested using cultured HT29 human colon cancer cells. As was predicted, leucovorin increased both FdUrd-mediated cytotoxicity and radiosensitization. The increase in radiation sensitivity was associated with a decrease in the repair of radiation-induced DNA double strand breaks (DSB's). Dipyridamole potentiated the cytotoxicity produced by 5-FU. However, dipyridamole appeared to confer slight protection from irradiation, thus decreasing 5-FU-mediated radiosensitization. This demonstrates that the simple fact that a biomodulator can increase fluoropyrimidine-induced cytotoxicity does not guarantee a corresponding increase in radiation sensitivity. Clinical trials combining fluoropyrimidines and their biomodulators will need to take these potentially complex interactions into account.