Reversal of methotrexate cytotoxicity to human bone marrow cells and leukemic K562 cells by leucovorin: methotrexate polyglutamates formation as a possible important factor.
Methotrexate (MTX) is metabolized intracellularly to MTX-polyglutamates (MTX-PGs), which markedly inhibit several folate-dependent enzymes. Polyglutamation defect, therefore, is one of the important factors in drug resistance. In this study, reversal of MTX cytotoxicity by l-leucovorin (l-LV) was investigated using normal human bone marrow granulocyte progenitor cells (G-CFCs), and MTX-sensitive and -resistant leukemic K562 cell lines; the latter showed diminished polyglutamation. Cytotoxicity of 10(-7)M MTX to G-CFCs was completely reversed by an equimolar concentration of l-LV, but with higher MTX concentrations, relatively more l-LV was required. The reversal of MTX cytotoxicity by l-LV was more effective against bone marrow cells than MTX-sensitive K562 cells; this reversal seemed to be correlated to the total intracellular MTX levels as well as MTX-PG formation (low in bone marrow cells and high in K562 cells). When MTX-sensitive and -resistant K562 cells were incubated with MTX under conditions in which the total intracellular MTX levels of both cells were similar, successful reversal of MTX toxicity by l-LV was demonstrated in MTX-resistant cells, but not in MTX-sensitive cells, suggesting that an increase of MTX-PG formation in MTX-sensitive cells may explain the failure of l-LV to overcome MTX cytotoxicity. In addition to competitive reversal of MTX cytotoxicity by LV, noncompetitive reversal relating to variable formation of MTX-PGs is suggested to be another important factor in the mechanism of the reversal of MTX cytotoxicity by LV.
Department of Pediatrics, Kanazawa University School of Medicine.
S
S
Koizumi
Y
Y
Ueno
I
I
Ohno
T
T
Ichihara
Y
Y
Tamaru
H
H
Matsukawa
D000971:Antineoplastic Combined Chemotherapy Protocols / Q000627:therapeutic use; D001853:Bone Marrow / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D004351:Drug Resistance; D006098:Granulocytes / Q000187:drug effects; D006801:Humans; D066298:In Vitro Techniques; D002955:Leucovorin / Q000494:pharmacology*; D004915:Leukemia, Erythroblastic, Acute / Q000188:drug therapy*; D008727:Methotrexate / Q000009:adverse effects* / Q000031:analogs & derivatives / Q000378:metabolism; D011099:Polyglutamic Acid / Q000031:analogs & derivatives / Q000378:metabolism
D011099:Polyglutamic Acid; C014085:methotrexate polyglutamate; D002955:Leucovorin; D008727:Methotrexate