Defective transport is a common mechanism of acquired methotrexate resistance in acute lymphocytic leukemia and is associated with decreased reduced folate carrier expression.
Methotrexate (MTX) transport was examined in 27 patients with untreated acute lymphocytic leukemia (ALL) and 31 patients with relapsed ALL using a previously described fluorescent MTX analog (PT430) displacement assay (Blood 80:1158, 1992). Only 13% of untreated patients were considered to have impaired MTX transport, whereas more than 70% of relapsed patients had evidence of impaired MTX transport. To further characterize the basis for this defect, Northern analyses for the reduced folate carrier (RFC) were performed on the RNA available from the leukemic blasts of 24 patients in whom MTX transport had been measured. Six of nine samples with impaired MTX transport had decreased RFC expression (one had no detectable RFC expression), while three had no decrease in RFC expression. None of 15 samples with normal MTX transport had decreased RFC expression. A reverse-transcriptase polymerase chain reaction (RT-PCR) assay was developed to quantitate RFC mRNA expression more accurately. Decreased RFC expression was demonstrated in six of the nine samples with impaired MTX transport, confirming the results obtained by Northern blot. These data indicate decreased RFC expression associated with impaired MTX transport is observed in relapsed ALL following treatment with MTX-containing therapy.
Gorlick
R
R
Program of Molecular Pharmacology and Therapeutics, Memorial Sloan-Kettering Cancer Center, New York 10021, USA.
Goker
E
E
Trippett
T
T
Steinherz
P
P
Elisseyeff
Y
Y
Mazumdar
M
M
Flintoff
W F
WF
Bertino
J R
JR
D000293:Adolescent; D000328:Adult; D000368:Aged; D001692:Biological Transport; D001752:Blast Crisis / Q000188:drug therapy / Q000378:metabolism; D002352:Carrier Proteins / Q000096:biosynthesis*; D002648:Child; D002675:Child, Preschool; D004351:Drug Resistance; D005452:Fluoresceins / Q000378:metabolism; D005492:Folic Acid / Q000096:biosynthesis*; D006801:Humans; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D008727:Methotrexate / Q000031:analogs & derivatives / Q000378:metabolism*; D008875:Middle Aged; D054198:Precursor Cell Lymphoblastic Leukemia-Lymphoma / Q000188:drug therapy* / Q000378:metabolism*; D058978:Reduced Folate Carrier Protein
D002352:Carrier Proteins; D005452:Fluoresceins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D058978:Reduced Folate Carrier Protein; C548699:SLC19A1 protein, human; C120043:SLC19A2 protein, human; C036539:fluoresceinated methotrexate; D005492:Folic Acid; D008727:Methotrexate