Cytotoxic synergism of methioninase in combination with 5-fluorouracil and folinic acid.

Abstract Potentiation of the cytotoxic activity of 5-fluorouracil (FUra) by folinic acid (5-HCO-H 4 folate) is due to elevation of the methylene tetrahydrofolate (CH 2 -H 4 folate) level, which increases the stability of the ternary complex of thymidylate synthase (TS), fluorodeoxyuridine monophosphate, and CH 2 -H 4 folate that inactivates the TS. Methionine deprivation results in the production of tetrahydrofolate (H 4 folate) and, subsequently, CH 2 -H 4 folate from methyl tetrahydrofolate, as a consequence of the induction of methionine synthesis. We hypothesized that the efficacy of FUra could be augmented by the combination of high-concentration 5-HCO-H 4 folate and recombinant methioninase (rMETase), a methionine-cleaving enzyme. Studies in vitro were performed with the cell line CCRF-CEM. Cytotoxic synergism of FUra + rMETase and FUra + 5-HCO-H 4 folate + rMETase was demonstrated with the combination index throughout a broad concentration range of FUra and rMETase. A subcytotoxic concentration of rMETase reduced the IC 50 of FUra by a factor of 3.6, and by a factor of 7.5, in the absence and in the presence of 5-HCO-H 4 folate, respectively. 5-HCO-H 4 folate increased the intracellular concentrations of CH 2 -H 4 folate and H 4 folate from their baseline levels. Concentrations of folates were not changed by exposure to rMETase. Levels of free TS in cells treated with FUra + 5-HCO-H 4 folate and with FUra + rMETase were lower than those in cells exposed to FUra alone. The decrease of TS was still more pronounced in cells treated with FUra + 5-HCO-H 4 folate + rMETase. The synergism described in this study will be a basis for further exploration of combinations of fluoropyrimidines, folates, and rMETase.