KINETICS OF ANTHRACYCLINE EFFLUX FROM MULTIDRUG-RESISTANCE PROTEIN-EXPRESSING CANCER-CELLS COMPARED WITH P-GLYCOPROTEIN-EXPRESSING CANCER-CELLS

The multidrug resistance protein (MRP) has been shown to mediate ATP-d ependent efflux of anticancer agents of diverse structure, such as dau norubicin (DNR), vincristine and etoposide. Thus, this protein does co nfer a multidrug resistant phenotype to cancer cells, similar to P-gly coprotein (Pgp). The substrate specificity of both transporter protein s is partly overlapping but is otherwise very distinct; because MRP is a multiple organic anion transporter, it transports certain glutathio ne conjugates and may be partly dependent on intracellular glutathione levels for the transport of anthracyclines. We have studied the trans port kinetics of a series of anthracyclines in MRP and Pgp that overex press tumor cell lines to obtain information on the substrate specific ity of these proteins. The anthracyclines have modifications in the su gar moiety. The mean active efflux coefficient k(a),, used to characte rize the efficiency of the active efflux, was very similar for DNR and one of its 4'-deoxy-derivatives (eso-DNR) for MRP and Pgp [10-20 x 10 (-10)/sec/(cells/ml)l. The permanently neutral derivatives 3'-deamino- 3'-hydroxy-doxorubicin (OH-DOX) and 3'-deamino-3'-hydroxy-daunorubicin (OH-DNR) were effluxed by both proteins but had a lower k(a), [2 x 10 (-10) and 6 x 10(-10)/sec/(cells/ml) (OH-DOX) and 2 x 10(-10) and 5 x 10(-10)/sec/(cells/ml) (OH-DNR)] for MRP and Pgp. Two anthracyclines, the doxorubicin derivative pirarubicin and 2'-bromo-4'-epi-DNR seemed to have a slightly higher k(a), value for Pgp than for MRP. The appare nt Michaelis-Menten constants (K-m,) and maximal efflux rates (V-M) fo r the active transport were within a narrow range for both transporter s, except for OH-DOX and OH-DNR, which had a lower V-M, in the case of MRP-mediated transport, suggesting a role of the amino group in the i nteraction with glutathione. Determination of the Hill coefficient (n( H)) of the MRP-mediated efflux gave most values close to 2, which sugg ests cooperativity of the transport of anthracyclines as reported befo re for Pgp. In conclusion, the transport kinetics of anthracyclines by MRP and Pgp are very similar.