The effect of reduced derivatives of folic acid on toxicity and antileukemic effect of methotrexate in mice

Abstract Mice inoculated with leukemia L1210 were treated with combinations of methotrexate (amethopterin) and one of the following folic acid derivatives: (a) folic acid, (b) dihydrofolic acid, (c) tetrahydrofolic acid, (d) 10-formyltetrahydrofolic acid, (e) 5-formyltetrahydrofolic acid (citrovorum factor), and (f) prefolic A (5-methyltetrahydrofolic acid). Daily treatment was started 3 days after leukemic inoculation. Treatment with methotrexate (MTX) alone, or in combination with folic acid, resulted in a considerable prolongation of lifetime. All other treatments resulted in leukemic death of the animals at the same time as the untreated controls; i.e. the antileukemic effect of methotrexate was blocked by the metabolite. Delayed administration of ctirovorum factor and prefolic A, after large doses of MTX, showed that both compounds were effective in reducing toxicity but had little or no effect on the antileukemic activity when given 12 to 24 hr after MTX. At 48 hr after administration of MTX neither compound protected against the toxicity of the drug. In view of the extreme sensitivity of dihydrofolic reductase to inhibition by MTX in vitro , it was of particular interest that dihydrofolic acid was able to bring about extensive reversal of the antileukemic effect and toxicity of this substance. A study of folic and dihydrofolic reductase activity in the livers of mice which had received combinations of MTX and folic acid derivatives showed that there was no difference in the extent of inhibition, regardless of whether the combination was toxic or nontoxic to the animals.