New insights into the P-glycoprotein-mediated effluxes of rhodamines.

Multidrug resistance (MDR) in tumour cells is often caused by the overexpression of the plasma drug transporter P-glycoprotein (P-gp). This protein is an active efflux pump for chemotherapeutic drugs, natural products and hydrophobic peptides. Despite the advances of recent years, we still have an unclear view of the molecular mechanism by which P-gp transports such a wide diversity of compounds across the membrane. Measurement of the kinetic characteristics of substrate transport is a powerful approach to enhancing our understanding of their function and mechanism. The aim of the present study was to further characterize the transport of several rhodamine analogues, either positively charged or zwitterionic. We took advantage of the intrinsic fluorescence of rhodamines and performed a flow-cytometric analysis of dye accumulation in the wild-type drug sensitive K562 that do not express P-gp and its MDR subline that display high levels of MDR. The measurements were made in real time using intact cells. The kinetic parameter, ka = VM/km, which is a measure of the efficiency of the P-gp-mediated efflux of a substrate was similar for almost all the rhodamine analogues tested. In addition these values were compared with those determined previously for the P-gp-mediated efflux of anthracycline. Our conclusion is that the compounds of these two classes of molecules, anthracyclines and rhodamines, are substrates of P-gp and that their pumping rates at limiting low substrate concentration are similar. The findings presented here are the first to show quantitative information about the kinetic parameters for P-gp-mediated efflux of rhodamine analogues in intact cells.