Tumor cell responses to inhibition of thymidylate synthase

Whether inhibition of thymidylate synthase is lethal to a population of tumor cells depends upon three factors: 1) the dependence of the cells upon de novo synthesis of thymidine nucleotides; 2) the length of time enzyme is inhibited and the requirement for thymidine nucleotides during this period; and 3) the biochemical responses of the cells to the initial inhibition of enzyme, many of which interfere with maintenance of thymidylate synthase in an inhibited state. Following inhibition of thymidylate synthase, deoxyuridylate accumulates, as does the cellular content of thymidylate synthase. In addition, the initially formed enzyme-inhibitor complexes dissociate. These biochemical sequelae alter the effectiveness of the blockade of thymidylate synthase in a time-dependent, continuously-changing manner. Whether cell kill occurs depends on whether the dynamic balance of these factors allows a sufficiently low enzymatic activity to be maintained for a long enough period of time. An analysis of this interaction of factors leads us to the conclusions that efficient tumor cell kill with fluoropyrimidines is best attained by combination with reduced folate cofactors and inhibitors of deoxypyrimidine biosynthesis. Each of these agents modifies the response of tumor cells with the result that the fluorodeoxyuridylate-induced inhibition of thymidylate synthase is maintained. This analysis also suggests that folate analogs inhibitory to thymidylate synthase are more compatible than pyrimidine analogs with inhibition of thymidylate synthase as an approach to cancer chemotherapy.

Folinic acid augmentation of the effects of fluoropyrimidines on murine and human leukemic cells

The effects of the fluoropyrimidines on leukemic cells of mouse and human origin have been studied in the presence of folinic acid. This reduced folate enhanced the cytotoxicity and the growth inhibitory potency of 5-fluorouracil (5-FUra) and of 5-fluoro-2'-deoxyuridine (FUdR) against all cell lines examined. The human leukemic cell lines used (two T- and two B-cells) were affected by these fluoropyrimidines only at substantially higher concentrations than were found to be inhibitory to mouse L1210 cells; however, the enhancement of the activity of the fluoropyrimidines occurred over the same range of folinic acid concentrations in mouse and human cells. Whereas the total intracellular folate pool increased continuously with every increment of folinic acid added to the medium, the enhancement of the potency of the fluoropyrimidines was limited. Augmentation of the effects of FUdR exceeded that of 5-fluorouracil in the human leukemic cells studied. The cytotoxicity of the fluoropyrimidines (as defined by cloning efficiency) was enhanced to a greater extent than was growth inhibition so that an impressive lethal synergism was noted; for instance, exposure of L1210 cells to nontoxic concentrations of 5-fluorouracil or FUdR in the presence of folinic acid resulted in a 98 or 99.9% cell kill, respectively. In contrast to previous predictions, the fluoropyrimidines were more inhibitory to mouse leukemic cells containing folate pools that were suboptimal for growth than for folate-replete cells. Growth rate experiments showed that cells exposed to moderate concentrations of FUdR were initially inhibited but recovered with time, whereas in cells exposed to both FUdR and folinic acid, the initial growth inhibitory effects were sustained. We conclude that folinic acid stabilizes the effects of the fluoropyrimidines on thymidylate synthase of both mouse and human leukemic cell populations and that this enhancement is reflected in both inhibition of the growth of and the lethality to these cells. We suggest that only doses of the fluoropyrimidines that are capable of initially inhibiting thymidylate synthase to a high degree will be synergistic with excess reduced folates.