NEW DOSE-TIME RELATIONSHIPS OF FOLATE ANTAGONISTS TO SUSTAIN INHIBITION OF HUMAN-LYMPHOBLASTS AND LEUKEMIC-CELLS INVITRO

  • 1 January 1979
    • journal article
    • research article
    • Vol. 39  (9) , 3612-3618
Abstract
In this study, 3 methods are utilized to analyze toxicity produced by methotrexate and the lipid-soluble antifolate, 2,4-diamino-5-(3'',4''-dichlorophenyl)-6-methylpyrimidine, in human lymphoblasts (WIL-2) and leukemic [MRC-5] cells. These methods detect increasingly severe metabolic damage: inhibition of deoxyuridine incorporation into DNA, the reversibility of inhibition of deoxyuridine incorporation by supplementation with formyltetrahydrofolate as Ca2+ leucovorin, and the ability of cells to form clones in soft agarose. The critical dose and exposure time for establishing and maintaining the metabolic toxicity of methotrexate is examined in detail. If an initial loading dose of methotrexate is of high enough concentration or is maintained for a sufficient period to achieve > 98% inhibition of deoxyuridine incorporation, this inhibition can be sustained by low concentrations of 2,4-diamino-5-(3'',4''-dichlorophenyl)-6-methylpyrimidine or methotrexate. Concentrations of methotrexate or 2,4-diamino-5-(3'',4''-dichlorophenyl)-6-methylpyrimidine that equal or exceed 0.1 .mu.M are sufficient for maintenance of inhibition by an initial loading dose of methotrexate but escape from inhibition that occurs if lower levels of drug are used. The possible implications of these observations for in vivo protocols are discussed.