The effect of dipyridamole (DP), an inhibitor of nucleoside transport, on the uptake and toxicity of 5-fluorouracil (FUra) was examined in a human colon cancer cell line (HCT 116). DP substantially increased the cytotoxicity of FUra in cell growth experiments and in viability assays measuring colony formation. The augmentation by DP was dose and time dependent. Several possible mechanisms by which DP enhanced FUra toxicity were investigated. DP did not alter the uptake of FUra into the acid-soluble and -insoluble fractions of HCT 116 cells. While DP did not affect the uptake of FUra, it did inhibit the transport of the nucleoside analogues, fluorouridine and fluorodeoxyuridine, of FUra. Although DP effectively inhibited the uptake of thymidine and uridine in a dose-dependent manner, several lines of evidence suggested that inhibition of nucleoside salvage was not the critical effect. (a) The toxicity of FUra was not prevented by thymidine, uridine, or the combination of thymidine and uridine. (b) Thymidine triphosphate pools, decreased by 50% during the initial 8 h of exposure to FUra, were not further depleted by the addition of DP. The shrinkage in deoxythymidine triphosphate pools produced by FUra was prevented by concomitant exposure to thymidine; however, this did not translate into protection from FUra lethality. (c) The use of dialyzed serum, which greatly diminished the availability of nucleic acid precursors, did not increase the toxicity of FUra. (d) DP increased the cytotoxicity of FUra as effectively in experiments utilizing dialyzed serum as when nondialyzed serum was used. Surprisingly, however, the addition of sufficient thymidine to overcome the DP block did prevent the augmentation of FUra toxicity produced by DP. DP may provide a novel means of enhancing the cytotoxicity of FUra.
Supported in part by a grant from Boehringer-Ingleheim, Inc., and NIH Grant CA 14520.