Reduced drug accumulation as the mechanism of extreme clinical resistance to methotrexate in the human T-cell leukemia xenograft, LALW-2.
The mechanisms were examined that underlie the extreme resistance to methotrexate (MTX) by near diploid leukemic T-cells (LALW-2) exposed to the drug only during the course of therapy administered to the patient of origin. Despite the LALW-2 cells being highly resistant to MTX (inhibitory dose for 50% of cells, more than 10(-3) mol/l), southern blot analysis did not show any amplification of the dihydrofolate reductase gene, nor was there any evidence, by comparison with drug-sensitive CCRF-CEM cells, that the gene was overexpressed. Kinetic analysis of dihydrofolate reductase activity in the presence of MTX provided no basis for attributing resistance in LALW-2 cells to a change in enzyme structure. By contrast, studies of MTX accumulation revealed that the LALW-2 cells accumulated significantly less drug than either CCRF-CEM cells or a MTX-resistant CCRF-CEM subline with a characterized transport defect. These data suggest that extreme MTX resistance in LALW-2 cells is mediated by reduced drug accumulation in the absence of any effect on the target enzyme.
Norris
M D
MD
Children's Leukaemia and Cancer Research Unit, Prince of Wales Children's Hospital, Sydney, New South Wales, Australia.
Haber
M
M
Kavallaris
M
M
Brian
M J
MJ
Lutze
L H
LH
White
L
L
Stewart
B W
BW
D000818:Animals; D002455:Cell Division / Q000187:drug effects; D004351:Drug Resistance / Q000235:genetics / Q000502:physiology; D005784:Gene Amplification / Q000235:genetics; D015870:Gene Expression / Q000235:genetics; D006801:Humans; D015458:Leukemia, T-Cell / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D008727:Methotrexate / Q000493:pharmacokinetics* / Q000633:toxicity; D051379:Mice; D008807:Mice, Inbred BALB C; D008819:Mice, Nude; D009368:Neoplasm Transplantation; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics / Q000378:metabolism; D014183:Transplantation, Heterologous; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism
D013762:Tetrahydrofolate Dehydrogenase; D008727:Methotrexate