Leucovorin enhances cytotoxicity of trimetrexate/fluorouracil, but not methotrexate/fluorouracil, in CCRF-CEM cells.

Background: Increased response rates in studies of patients with colon cancer have indicated that the cytotoxic effects of fluorouracil (5-FU) are potentiated by leucovorin (LV) and by methotrexate (MTX). However, preliminary studies using a sequential combination of MTX, LV, and 5-FU showed no additional potentiation.

Purpose: We hypothesized that the lack of additional cell kill with this combination could be due to competition of LV with MTX for cellular uptake and reduced folate polyglutamylation. We have tested this possibility by comparing the cytotoxicity of drug combinations containing MTX with that of drug combinations containing trimetrexate (TMTX), an antifolate that does not compete with LV for uptake or polyglutamylation.

Methods: Human lymphocytic leukemia CCRF-CEM cells were exposed to MTX or TMTX for 24 hours and to 5-FU during the last 4 hours of antifolate exposure. LV was administered 30 minutes before 5-FU.

Results: After 20 hours of exposure to TMTX or MTX, intracellular levels of phosphoribosyl pyrophosphate were elevated to a similar degree, and these levels did not decrease after a 30-minute exposure to LV. No additional cell kill was observed when LV was added to the MTX/5-FU combination, but cytotoxicity was enhanced when LV was added to the TMTX/5-FU combination.

Conclusions: This study supports the hypothesis that the lack of additional cell kill when high-dose LV is added to the MTX/5-FU combination may be due to competition of MTX with LV for cellular uptake and/or competition of MTX or its polyglutamates with polyglutamylation of reduced folates. Inasmuch as TMTX does not compete with LV and reduced folates for uptake and polyglutamylation, the synergy obtained with the combination of TMTX plus 5-FU and high-dose LV further supports this hypothesis.