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Resistance of bcr‐abl‐positive acute lymphoblastic leukemia to daunorubicin is not mediated by mdr1 Gene Expression

In vitro resistance to anthracyclines is thought to be a poor prognosis in achieving long‐term remission in patients with acute lymphoblastic leukemia (ALL). Expression of a multidrug resistance gene (mdr1) that codes for 170 Kd transmembrane glycoprotein is responsible for conferring resistance to malignant cells to anthracyclines. The t(9:22) translocation, resulting in bcr‐abl fusion gene, is commonly found in B‐lineage ALL and is known to be a poor prognostic factor for long‐term remission. To investigate whether resistance to anthracyclines contributes to poor prognosis in bcr‐abl‐positive ALL, we studied daunorubicin sensitivity by an in vitro colorimetric methyl tetrazolium (MTT) assay in B‐lineage ALL patients who were bcr‐abl‐positive and compared them with the B‐lineage, age‐matched bcr‐abl‐negative group. We also looked for and compared the presence of mdr1 gene expression in these two groups of patients by RT‐PCR. Of the 46 patients included in the study, 16 (34.7%) were positive for the bcr‐abl fusion gene. mdr1 gene expression was seen in 14 of these 46 patients (30.4%). However, the expression of the mdr1 gene was relatively lower in the bcr‐abl‐positive group (3 out of 16, 18.7%) compared to the bcr‐abl‐negative group (11 out of 30, 36.6%). The median LD50 of daunorubicin (concentration lethal to 50% of the leukemic blasts) differed significantly between bcr‐abl‐positive and ‐negative patients (P = 0.018). This in vitro study suggests that bcr‐abl‐positive ALL is relatively resistant to daunorubicin, but this resistance is not mediated through mdr1 gene expression. Am. J. Hematol. 71:172–176, 2002. © 2002 Wiley‐Liss, Inc.