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 untreate d acute lymphocytic leukemia (ALL) and 31 patients with relapsed ALL u sing a previously described fluorescent MTX analog (PT430) displacemen t assay (Blood 80:1158, 1992). Only 13% of untreated patients were con sidered to have impaired MTX transport, whereas more than 70% of relap sed patients had evidence of impaired MTX transport. To further charac terize the basis for this defect, Northern analyses for the reduced fo late carrier (RFC) were performed on the RNA available from the leukem ic blasts of 24 patients in whom MTX transport had been measured. Six of nine samples with impaired MTX transport had decreased RFC expressi on (one had no detectable RFC expression), white three had no decrease in RFC expression. None of 15 samples with normal MTX transport had d ecreased RFC expression. A reverse-transcriptase polymerase chain reac tion (RT-PCR) assay was developed to quantitate RFC mRNA expression mo re accurately. Decreased RFC expression was demonstrated in six of the nine samples with impaired MTX transport, confirming the results obta ined by Northern blot. These data indicate decreased RFC expression as sociated with impaired MTX transport is observed in relapsed ALL follo wing treatment with MTX-containing therapy. (C) 1997 by The American S ociety of Hematology.