Idarubicin DNA intercalation is reduced by MRP1 and not Pgp

Currently available data regarding the substrate specificity of the multi-d rug resistance (MDR) mechanisms P-glycoprotein (Pgp) and MDR-associated pro tein (MRP1) for idarubicin are inconclusive. A multiparameter flow cytometr y method was developed which allows simultaneous quantitative measurement o f total cellular fluorescence and the amount of anthracyclines intercalated into the DNR Anthracycline DNA intercalation was measured by fluorescence resonance energy transfer (FRET) between Hoechst 33342 and anthracyclines. Daunorubicin and idarubicin accumulation were studied and compared in estab lished cell lines expressing Pgp and MRP1. The data demonstrate that daunor ubicin DNA intercalation is affected by both Pgp and MRP1 whereas idarubici n DNA intercalation is affected only by MRP1. MRP1 and Pgp function could b e blocked completely by 5 mu m PAK 104P, while higher concentrations of ver apamil, PSC 833 and cyclosporin A were necessary to attain complete blockin g of MRP1 compared to Pgp. Daunorubicin DNA intercalation correlates better with cell survival and is more sensitive at physiological MDR expression a s observed in hematopoietic progenitors than daunorubicin levels measured b y total cellular fluorescence. In conclusion, idarubicin DNA intercalation is reduced by MRP1 but not by Pgp. PAK-104P is an effective modulator for b oth Pgp and MRP1 and may further improve idarubicin efficacy.