CORRELATION OF MULTIDRUG-RESISTANCE (MDR1) PROTEIN EXPRESSION WITH FUNCTIONAL DYE DRUG EFFLUX IN ACUTE MYELOID-LEUKEMIA BY MULTIPARAMETER FLOW-CYTOMETRY - IDENTIFICATION OF DISCORDANT MDR(-)/EFFLUX(+) AND MDR1(+)/EFFLUX(-) CASES/

Resistance to chemotherapy is a major factor limiting successful treat ment of acute myeloid leukemia (AML); one of the best characterized dr ug resistance mechanisms is extrusion of drugs by the energy-dependent multidrug resistance (MDR1) transport protein. Expression of MDR1 is common in AML and has been linked to lower remission induction rates a nd decreased remission durations. Because MDR1 efflux function may be modified by drugs such as cyclosporin A, accurate identification of MD R1(+)/efflux(+) AML cases will be critical to identify patients who ma y benefit from therapies that contain such MDR1 modulators. We have op timized single and multiparameter flow cytometric assays to detect eff lux of drugs or fluorescent dyes by previously cryopreserved AML blast s. These assays allowed precise identification of efflux by leukemic b lasts, and correlation with CD34 and MDR1 expression. We subsequently studied a series of 60 previously untreated AML cases. Functional effl ux was identified in 39 cases and was significantly correlated with MD R1 expression (P = .0002). However, discrepant cases were identified; 10 cases were efflux(+) without significant MDR1 expression, whereas 6 MDR1(+) cases were efflux(-). There was also a highly significant cor relation of efflux with CD34;31 (79%) of the 39 efflux(+) cases were C D34(+) in comparison with only 5 (24%) of the 21 efflux(-) cases (P < .0001). Multivariate analysis showed that efflux was significantly ass ociated with independent effects of both CD34 (P = .0011) and MDR1 exp ression (P = .034); the majority of efflux(+) cases were CD34(+), wher eas 5 of the 6 MDR1(+) efflux(-) cases lacked CD34 expression. Cyclosp orin A blocked efflux in all but 2 cases regardless of MDR1 expression . Functional efflux in AML is frequently detected without the classic MDR1(+) phenotype indicating that alternate non-MDR1-mediated efflux m echanisms may be important. Efflux assays may better identify patients who would benefit from therapies that include efflux modulators. (C) 1995 by The American Society of Hematology.