Modulation of 5-fluoro-2'-deoxyuridine response by folinic acid in human colonic tumor cell lines: the role of thymidylate synthase.

Recent investigations have revealed a significant increase in cytotoxic response to (5-fluoropyrimidine, FP) agents in the presence of the folate folinic acid (CF). It has been suggested that CF provides a source of intracellular reduced folates which, in turn, enhances the inhibition of the cellular target thymidylate synthase (TS) by the FP metabolite 5-fluoro-2'-deoxyuridylate (FdUMP). The extent of variation in the response to FP-CF combinations is unknown but it is an important consideration in view of the utilization of these combinations for the therapy of colorectal carcinoma. In the present study, variation in the response to 5-fluoro-2'-deoxyuridine (FdUrd)-CF combinations was observed between two human colorectal tumor cell lines, RCA and C. The response of both cell lines to FdUrd increased with increasing CF, but the effect was more pronounced in cell line RCA. RCA was 4-fold less responsive than cell line C to FdUrd at low CF concentrations, whereas both cell lines exhibited similar sensitivity at high CF concentrations. RCA accumulated lower levels of TS folate cosubstrates after CF than did C; however, this was not the sole mechanism accounting for the differential response to FdUrd-CF. The two cell lines responded differently to equivalent intracellular levels of 5,10-methylenetetrahydrofolate (CH2H4PteGlu) derivatives, the folate ligands involved in tight-binding inhibition of TS by FdUMP. The differential response to CH2H4PteGlu was not due to lack of folate polyglutamation; the predominant CH2H4PteGlu derivative in both cells was the hexaglutamate form. The difference in response to CH2H4PteGlu was associated with a reduction in the affinity of the RCA TS for CH2H4PteGlu, relative to the C enzyme. Thus, a cell line has been identified that responds poorly to FdUrd at physiological levels of CF and that contains a variant TS enzyme.