The growth-inhibitory effect of fluoropyrimidines combined with a short-term exposure to leucovorin and the pattern of polyglutamylation of folates were compared between parental CCRF-CEM cells and a cell line with impaired ability to form polyglutamates (CCRF-CEM/P). The combination of leucovorin with 5-fluorouracil or 5-fluorodeoxyuridine increased the growth inhibition of CCRF-CEM cells compared to the fluoropyrimidine alone in the parent cell line but not in CCRF-CEM/P cells. In addition, leucovorin produced a significant increase in the inhibition of intracellular thymidylate synthase activity caused by 5-fluorouracil or 5-fluorodeoxyuridine as compared to these drugs alone in CCRF-CEM cells, but no increase in inhibition over that produced by the single drugs alone was observed in CCRF-CEM/P cells. Although levels of 5,10-methylene tetrahydrofolate after leucovorin administration were similar in both cell lines, polyglutamylation of this coenzyme was decreased in the CCRF-CEM/P cell line. The inability of CCRF-CEM/P cells to form significant levels of polyglutamates of N5,N10-methylenetetrahydrofolate, may be responsible for the lack of enhanced cell kill observed when a short exposure to leucovorin is used with fluoropyrimidines.

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Supported by the USPHS (Grants CA 08010 and CA 22754).

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