The folate antagonists, in particular amethopterin (methotrexate), continue to be useful drugs for the treatment of certain neoplastic diseases in man. The effectiveness of amethopterin in preventing DNA synthesis and cellular replication appears to be a consequence of inhibition of the enzyme dihydrofolate reductase. This premise is supported by in vivo studies which show that in patients treated with amethopterin, the conversion of folate to reduced forms is blocked, as well as in vitro studies which demonstrate that amethopterin inhibits this enzyme activity at remarkably low concentrations. The administration of amethopterin to subjects with leukemia as well as to subjects without hematologic disease results in increased dihydrofolate reductase activity in leukocytes and erythrocytes. This “induction” of dihydrofolate reductase activity parallels the amethopterin concentration in blood cells. The binding of the folate antagonist to the reductase enzyme is tightest at pH 5.9. Because of the in vitro assay conditions (dilution of the enzyme and inhibitor and assay at pH 8.3), this increase in enzyme activity observed in vitro may not represent a true increase of enzyme activity, since most of the increased enzyme activity may be bound by amethopterin within the cell.
A knowledge of the ability of leukemic cells to transport and retain amethopterin may be of value in the prediction of responsiveness to therapy with this agent. However, other factors that deserve further investigation are cited that may also influence responsiveness.
Studies referred to in this article were made possible by grants from the National Institutes of Health and from the American Cancer Society.