Enhanced 5-fluorouracil nucleotide formation after methotrexate administration: explanation for drug synergism.

Exposure of L1210 leukemia cells first to 0.1 to 100 micromolar methotrexate and then to 10 micromolar 5-fluorouracil produces a synergistic effect on the number of cells killed in culture. Methotrexate dose-related increases occur in the concentrations of intracellular 5-fluorouracil ribonucleotides and 5-fluoro-2'-deoxyuridylate and in the incorporation of 5-fluorouracil into RNA. These increases are correlated with increased concentrations of intracellular phosphoribosylpyrophosphate. It is proposed that the enhanced formation of ribonucleotides of 5-fluorouracil and the subsequent incorporation of these compounds into RNA in methotrexate-treated cells may account for synergism between these agents.