Role of amino acid depletion in combined treatment of neoplastic cells with methotrexate and L-asparaginase.

The effect of pretreatment with L-asparaginase on the cytotoxicity of methotrexate (MTX) was studied in L5178Y murine leukemic cells and L-929 cells grown in vitro. The results were correlated with the effects of L-asparaginase on DNA synthesis. At low concentrations of the enzyme, pretreatment of cells enhanced the effect of MTX; at higher concentrations, it prevented the effect of MTX. Elimination of the MTX action was demonstrated also in L5178Y cells growing in the medium lacking L-asparagine. L-Asparagine readdition resulted in enhancement of MTX cytotoxicity. Pulse-labeling experiments demonstrated that in every case elimination of MTX action was related to a diminution of the fraction of cells in the phase of DNA synthesis. In contrast, enhancement of the effect of MTX was related to an increase in the fraction of cells in the DNA synthesis phase. A similar parallelism of effects occurred in L-929 cells when pretreatment with L-asparaginase was followed by washing the cells and culturing them in medium without the enzyme.