Drug resistance remains a major problem in the treatment of patients with acute myeloid leukemia (AML). Expression of the MDR1 gene in leukemic cells was shown previously to be associated with worse clinical outcome of the patients. The multidrug resistance-associated protein (MRP) has been shown recently to be another protein causing the multidrug resistance phenotype in cell lines, but its impact on clinical outcome in patients with AML remains to be proven. To determine the clinical significance of MRP in patients with de novo AML, we have studied the MRP expression in leukemic cells and its association with both response to induction chemotherapy and survival of the patients. MRP gene expression was determined by immuno-cytochemistry (n = 80) by means of the monoclonal antibodies QCRL-1 and QCRL-3. MRP expression was low, intermediate, and high in 19, 55, and 26% of the patients, respectively. High MRP expression was independent of age and sex of the patients, WBC count, and percentage of blasts. However, high MRP expression was more frequent in the FAB M5 subtype as compared to the other subtypes. MRP expression had no impact on clinical outcome. The complete remission rates were 65, 68, and 63% for patients with low, intermediate, and high expression, respectively. Overall survival was also independent of MRP expression. In contrast, patients with P-glycoprotein-positive AML had lower complete remission rates and shorter durations of survival. These data indicate that MRP is expressed in patients with de novo AML but, in contrast to P-glycoprotein, does not predict for outcome of induction chemotherapy or survival.

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