INCREASED CYTOTOXICITY OF 5'-DEOXY-5-FLUOROURIDINE BY PROLONGED CULTURE WITH FOLINIC ACID

We investigated the cytotoxic potentiation by folinic acid (FA) of two fluoropyrimidines, 5-fluouracil (FUra) and 5'-deoxy-5-flurouracil (5'-dFUrd), against two human neoplastic cell lines (HeLa and KSE-2 cells). The concentrations of fluoropyrimidine (2-1O mu M) and FA (1 mu M) in media were based on clinically achievable levels, and the duration of culture was relatively long (6 days). The cytotoxic activity of fluorouridine and FA was evaluated by both clonogenic efficiency and thymidylate synthase (TS) inhibition late of the cells. After combined treatment with 2 mu M 5'-dFUrd and 1 mu M FA the clonogenic efficiencies of the two cell lines were significantly lower than those after treatment with 5'-dFUrd alone (p<0.01). Moreover, the TS inhibition rates of these two human cell lines after combined treatment with 10 mu M 5'-dFUrd and 1 mu M FA were significantly higher than those after treatment with 5'-dFUrd alone (p<0.01). However; when these two cell lines were exposed to FUra, concomitant treatment with FA did not enhance cytotoxicity. These differences in the cytotoxic properties of FUra and 5'-dFUrd in response to the addition of FA ascribed to the fact that during 2-3 days and 5-6 days after treatment, intracellular concentrations of 5'-dFUrd produced by combined treatment with 10 mu M 5'-dFUrd and 1 mu M FA were higher than those associated with 10 mu M 5'-dFUrd alone. During these periods, cellular accumulation of 5'-dFUrd achieved by combined treatment with 5'-dFUrd and FA was also higher than that of FUra during combined treatment with FUra plus FA and that of 5'-dFUrd during treatment with 5'-dFUrd alone. There was no statistically significant difference in intracellular FUra concentrations between combined treatment with FUra plus FA and treatment with FUra alone. Prolonged treatment with 5'-dFUrd and FA in clinically feasible concentrations was thus more effective than the combination of FUra and FA in two human neoplastic cell lines.