Alteration of Fluorouracil Metabolism in Human Colon Cancer Cells by Dipyridamole with a Selective Increase in Fluorodeoxyuridine Monophosphate Levels

The nucleoside transport inhibitor dipyridamole can increase the cytotoxicity of 5-fluorouracil in a human colon cancer cell line (HCT 116) without affecting the total amount of fluorouracil incorporated into the acid soluble and insoluble fractions (J. L. Grem and P. H. Fischer, Cancer Res., 45: 2967–2972, 1985). We now report that dipyridamole altered the pattern of fluorouracil metabolism and provided a selective increase in intracellular fluorodeoxyuridine monophosphate (FdUMP) levels. At 2 and 4 h after exposure to fluorouracil and dipyridamole, FdUMP levels were approximately 5-fold higher in the presence of dipyridamole. The ratio of FdUMP to fluorouridine triphosphate at 4 h was substantially increased in the presence of dipyridamole (0.4 ± 0.05) compared to fluorouracil alone (0.08 ± 0.03). In cells preloaded with fluorodeoxyuridine (FdUrd), dipyridamole potently inhibited the efflux of FdUrd, leading to an increased retention of intracellular FdUMP. One h following removal of H]FdUrd, the FdUMP levels were increased 8-fold in the presence of dipyridamole, and the half-life of intracellular FdUMP was increased from 24 to 78 min. We have previously shown that the addition of sufficient thymidine (25 μ can prevent the augmentation of fluorouracil toxicity produced by dipyridamole. In these studies, the addition of 25 μM thymidine reduced the FdUMP levels to less thanhalf of those measured in the presence of fluorouracil plus dipyridamole for the first 8 h of exposure, and reduced the FdUMP levels to 6% of the FdUMP levels seen with fluorouracil and dipyridamole after 24 h of exposure. Thymidineprevented the enhanced intracellular retention of FdUMP produced by dipyridamole in cells preloaded with FdUrd. In addition, thymidine inhibited the accumulation of FdUMP in cells exposed to FdUrd. In cancer cells which significantly catabolize FdUMP, the ability of dipyridamole to block the efflux of FdUrd may provide an effective means of selectively increasing FdUMP levels and enhancing the toxicity of fluorouracil. Furthermore, dipyridamole blocked the efflux of deoxyuridine and prolonged the intracellular half-life of deoxyuridine monophosphate. In cells prelabeled with dUrd, transfer of tritium to FdUrd and FdUMP occurred in cells exposed to fluorouracil and dipyridamole. These data suggest that blockade of nucleoside efflux can enhance the availability of deoxyribose–1–phosphate donors for the synthesis of FdUrd. Thus, dipyridamole’s ability to inhibit nucleoside transport can perturb the metabolism of a nucleobase, fluorouracil.