Interaction of D, L-erythro- and D,L-threo-gamma-fluoromethotrexate with human leukemia cell dihydrofolate reductase.

Gamma-fluoromethotrexate (FMTX) is a poorly glutamylated mimic of the anti-cancer drug methotrexate (MTX) which is useful in studies of the roles of MTX poly-gamma-glutamates. A second chiral center occurs at C-4 of the 4-fluoroglutamate used to synthesize FMTX and, as a consequence, FMTX occurs as both D,L-erythro and D,L-threo diastereomers. The interaction of both diastereomers with intracellular dihydrofolate reductase has been examined in the human leukemia cell line CCRF-CEM, using a centrifugal column technique. Measurements of the rate at which radiolabel was displaced from [3H]MTX-saturated dihydrofolate reductase following suspension of the cells in unlabeled drug indicated that MTX and the erythro isomer of FMTX gave essentially the same rate of displacement; the rate of displacement by the threo isomer of FMTX was slower, but the interpretation of these data was ambiguous since the rate of transport of threo-FMTX may have been limiting. In reciprocal experiments in which dihydrofolate reductase was saturated with [3H]erythro-FMTX, the erythro isomer and MTX again behaved equivalently in terms of displacement. When dihydrofolate reductase was saturated with [3H]threo-FMTX, the radiolabel was clearly displaced at a much faster rate than either other radiolabel regardless of whether the displacing agent was MTX or the isomer. These results indicate a distinct stereospecificity for interaction of inhibitor with dihydrofolate reductase in which the threo isomer has a faster off-rate. Of the two FMTX diastereomers, the erythro isomer thus most closely mimics the properties of MTX.