On the rational development of a new drug: the example of the fluorinated pyrimidines.

The specific rationale for the development of the fluorinated pyrimidines and the predictions of their mode of action have been described. The mode of action of 5-FU involves incorporation into RNA and metabolic activation to 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP), which inhibits the essential enzyme for DNA synthesis, thymidylate synthetase. I believe that the latter effect is responsible for the major chemotherapeutic activity. Our studies on the mechanism of the inhibition of that enzyme have led to the demonstration that a ternary covalent complex of enzyme, inhibitor, and cofactor is formed that results in essentially irreversible inhibition. The nature of this ternary complex has enabled us to devise ultrasensitive competitive ligand binding assays for thymidylate synthetase, FdUMP, and dUMP, the substrate of the enzyme. These determinants will be studied in needle biopsy specimens obtained from cancer patients undergoing chemotherapy with 5-FU in a prospective clinical investigation.