Cytotoxicity of floxuridine and 5-fluorouracil in human T-lymphoblast leukemia cells: Enhancement by leucovorin

The inhibitory effects of leucovorin (LV) combined with 5-fluorouracil (FUra) or floxuridine (FdUrd) on growth of human T-Lymphoblast leukemia cells (CCRF-CEM) were determined as a function of time, dose, and sequence of exposure. Exposure of CCRF-CEM cells in exponential growth to LV (1-100 μM) for 4 hours and to FUra (100 μM) or FdUrd (0.5 μM) during the last 2 hours resulted in synergistic inhibitory effects on cell growth. Synergism was dependent on LV dose (100 > 10 > 1 μM) and did not occur at 0.1 μM. No clear dependence of synergy on sequence was observed with FUra and LV combinations. With LV and FdUrd combinations, synergism was dependent on sequence of exposure (LV + FdUrd and LV → FdUrd were synergistic, but FdUrd → LV was not). Thymidine (0.1 μM), added after drug treatment, substantially rescued CCRF-CEM cells from LV → FUra cytotoxicity. Concomitant hypoxanthine (100 μM) only partially protected CCRF-CEM cells from the toxicity of this combination. These results are consistent with the hypothesis that the mechanism by which LV potentiates fluoropyrimidine cytotoxicity is the enhancement of complex formation between thymidylate synthase and 5-fluorodeoxyuridylate, presumably as a consequence of an increase of intracellular levels of 5,10-methylenetetrahydrofolate generated from LV. Also, enhanced stability of this complex in the presence of high levels of the folate coenzyme may contribute to the synergy observed. These data also provide a rationale for use of FUra and especially FdUrd and LV in the treatment of lymphoid malignancies in man.