Cytotoxicity of Trimetrexate against Antifolate‐resistant Human T‐Cell Leukemia Cell Lines Developed in Oxidized or Reduced Folate

Cytotoxicity of trimetrexate (TMQ), a lipophilic dihydrofolate reductase inhibitor, was examined in antifolate‐resistant human T‐cell leukemia cell lines developed in oxidized or reduced folate. An approximately 60‐fold methotrexate (MTX)‐resistant subline was developed in oxidized folate (pter‐oylglutamic acid: PGA) (CCRF‐CEM/MTX(60)‐PGA) from human T‐cell leukemia cell line CCRF‐CEM; this line exhibited impaired membrane transport of the drug. Further enhancement of MTX resistance resulted in selection of an approximately 5000‐fold MTX‐resistant subline (CCRF‐CEM/ MTX(5000)‐PGA), which showed increased dihydrofolate reductase activity due to gene amplification in addition to further impairment of MTX transport. An approximately 140‐fold MTX‐resistant subline, and then a 1500‐fold MTX‐resistant subline were developed in reduced folate (10 nM leucovorin) (CCRF‐CEM/MTXm‐LV and CCRF‐CEM/MTX(140)‐LV); they exhibited increased dihydrofolate reductase due to gene amplification accompanied by increased intracellular drug accumulation of MTX. While CCRF‐CEM/MTX(140)‐LV and CCRF‐CEM/MTX(1500)‐LV cells showed cross‐resistance to TMQ, CCRF‐CEM/MTX(60)‐PGA and CCRF‐CEM/MTX(5000)‐PGA cells were at least as sensitive to TMQ as the parent cells. TMQ was more potent against approximately 200‐fold N(10)‐propargyl‐5,8‐dideazafolic‐acid (CB3717)‐resistant human T‐cell leukemia MOLT‐3 sublines developed in PGA (MOLT‐3/CB3717(200)‐PGA) or leucovorin (MOLT‐3/CB3717(200)‐LV), as compared to the parent cells; MOLT‐3/CB3717(200)‐PGA and MOLT‐3/CB3717(200)‐LV cells were resistant to CB3717 by virtue of impaired transport, only the former possessing gene amplification of thymidylate synthase. The Cytotoxicity of TMQ in both MOLT‐3/CB3717(200)‐PGA and MOLT‐3/CB3717(200)‐LV cells was reduced by addition of leucovorin in a dose‐dependent manner, suggesting intracellular folate deficiency as a cause of TMQ sensitivity. These results demonstrate that TMQ overcomes transport‐impaired antifolate resistance, irrespective of gene amplification of dihydrofolate reductase or thymidylate synthase. Types of folate used during the development of antifolate resistance seem to be important in relation to the mechanism of TMQ responsiveness as well as that of antifolate resistance.