Evaluation of X-Methyl Folate Preparations as Possible Folic Acid Antagonists

Crude X-methyl folate and two purified fractions (purified X-methyl folate and 9-methyl folate) were evaluated as possible folic acid antagonists. Antifolate activity was measured by the deoxyuridine suppression assay, direct measurement of dihydrofolate reductase and morphological changes characteristic of folate deficiency. Cytotoxicity was evaluated by cell growth curves. These drugs were studied in freshly obtained human normal bone marrow and leukemia cells as well as in three established cell lines HL-60 (human promyelocytic leukemia), CEM (human T lymphocytes), and L1210 (murine lymphoid leukemia). Purified X-methyl folate and 9-methyl folate at concentrations up to 2×10-5 M failed to induce cell toxicity or inhibit deoxyuridine suppression of 3H-thymidine into DNA. Dihydrofolate reductase was not inhibited by 9-methyl folate but was partially inhibited by purified X-methyl folate. Crude X-methyl folate (2×10-1M) is a weak folate antagonist as compared to methotrexate since it requires a 1,000-fold higher concentration to inhibit cell growth and dihydrofolate redutcase activity. In addition, the crude X-methyl folate contains a folate-like fraction since it will correct the abnormal deoxyuridine suppression in folate-depleted cells. None of the drugs tested were able to induce myeloid differentiation in the HL-60 cells. The data suggest that 9-methyl folate would not be active as a folate antagonist antitumor agent and that purified and crude X-methyl folate in large concentrations will inhibit folate metabolism. Additional studies are needed to remove the folate-like activity in the crude material in order to further establish the antitumor effects of the crude X-methyl folate.