Differential effect of the combination methotrexate/(dl)-5-methyltetrahydrofolate on lymphoid malignant and normal bone marrow cells.

It has been reported that high doses of (dl)-5-methyltetrahydrofolate (mTHF) inhibit the growth of leukemic cell lines. We studied the effect of methotrexate (MTX) and mTHF in combination on the lymphoid cell lines BALM 3, CEM, MOLT 4 and P3HR1 by evaluating the clonogenic cell reduction in a limiting dilution assay. Cells were treated with 0.1 mM MTX for 4 h and 1 mM mTHF for 48 h. The growth of the cell lines was inhibited by 1 mM mTHF, with a clonogenic cell reduction of 3.5 log. MTX, following by mTHF after a delay of 8-24 h, caused a substantial killing of lymphoid leukemia cells, greater than the effect of either drug alone. The clonogenic cell reduction caused by the combination MTX-mTHF ranged from about 3.5 to 4.5 log, depending on the cell line studied. Normal hemopoiesis, as evaluated by primitive hematopoietic precursor colonies and granulocyte-macrophage colony-forming units (CFU-GM) was only slightly affected by the MTX-mTHF treatment at a dose and schedule which caused up to 4.5 log reduction of the leukemic cell lines. mTHF-MTX treatment could be useful in vitro for purging bone marrow cells in autologous bone marrow transplantation for acute leukemia.