Leucovorin enhancement of the effects of the fluoropyrimidines on thymidylate synthase.

Exposure of tumor cells to reduced folates before or with the fluoropyrimidines, 5-fluorouracil or 5-fluoro-2'deoxyuridine, results in a substantial increase in the activity of these drugs. Available evidence suggests that the mechanism of this synergism is a kinetic stabilization of complex formed between thymidylate synthase and fluorodeoxyuridylate that also involves a mole of the cofactor for the thymidylate synthase reaction, 5,10-methylenetetrahydrofolate. This effect results in an extended time of depletion of thymidine nucleotides with a resultant increased level of cell death. The biochemical aspects of this interaction are discussed and related to the therapeutics of this combination.