FUNCTIONAL MULTIDRUG-RESISTANCE PHENOTYPE ASSOCIATED WITH COMBINED OVEREXPRESSION OF PGP MDR1 AND MRP TOGETHER WITH 1-BETA-D-ARABINOFURANOSYLCYTOSINE SENSITIVITY MAY PREDICT CLINICAL-RESPONSE IN ACUTE MYELOID-LEUKEMIA/

Overexpression of P-glycoprotein (Pgp) or MDR1 mRNA has been shown to be a negative prognostic factor for clinical outcome in acute myeloid leukemia (AML), However, resistance to chemotherapy also occurs in the absence of Pgp overexpression, Therefore, besides Pgp expression, we have assessed the expression of MRP, a novel drug transporter gene, al ong with the functional multidrug-resistant (MDR) phenotype of leukemi c cells, These MDR parameters are correlated with clinical outcome in individual patients, We found functional changes in fresh leukemic cel ls from de novo or relapsed patients similar to those reported for tum or cell lines with the MDR phenotype, These changes were reduced drug accumulation as assessed with radiolabeled doxorubicin (factor 1.6), d aunomycin (factor 1.13), and vincristine (factor 1.6) in patients who were refractory to the combination treatment of 1-beta-D-arabinofurano sylcytosine (ara-C) and daunomycin or mitoxantrone as opposed to patie nts who had complete responses, Also, the intracellular distribution o f doxorubicin fluorescence (nuclear/cytoplasmic ratio), as assessed wi th laser scan microscopy, was reduced 1.4-fold in blasts from refracto ry patients, Based on historically known clinical response to single-a gent daunomycin or ara-C in the group of responding de novo AML patien ts, we have set a threshold level such that a defined part of the samp les that had the highest drug accumulation or nuclear to cytoplasmic r atios were above this threshold value, This allowed discrimination bet ween patients responding to daunomycin from those who were refractory to this drug, By using this threshold level, in the refractory group c linical resistance corresponded with high sensitivity with a resistant phenotype, A similar threshold was set for the data of the in vitro a ra-C sensitivity test. By combining both assays for all individual pat ients, clinical refractoriness as well as sensitivity could be predict ed with high accuracy, There appeared to be no stringent relationship between the functional MDR phenotype with expression of either Pgp (fl uorescence activated cell sorting analysis) or MRP mRNA (RNase protect ion), However, by combining both parameters the functional MDR phenoty pe correlated with the overexpression of either one or both of the par ameters in 94% of the samples studied, It is concluded that this combi ned overexpression in conjunction with functional changes for MDR drug s and ara-C reveal a correlation of MDR phenotype with clinical resist ance to combination chemotherapy in AML patients and hereby may adequa tely predict clinical MDR in individual AML patients.