MODULATION OF P-GLYCOPROTEIN ACTIVITY BY ESTRAMUSTINE IS LIMITED BY BINDING TO PLASMA-PROTEINS

Background. Estramustine previously has been shown to interact with P- glycoprotein and to restore intracellular accumulation of vinblastine and paclitaxel in cells overexpressing this drug transporter. However, the ability of estramustine to potentiate the cytotoxicities of sever al drugs was less than that expected, To resolve this apparent discord ance, the authors examined the effects of serum on the actions of estr amustine. Methods. The cytotoxicities of anticancer drugs with or with out estramustine or verapamil toward MCF-7 breast carcinoma cells and a P-glycoprotein-overexpressing subline MCF-7/ADR were determined usin g the sulforhodamine-binding assay. The extent of intracellular accumu lation of [H-3]vinblastine and [H-3]paclitaxel was determined for each using standard methods, and the binding of radiolabeled drugs to plas ma proteins was characterized by equilibrium dialysis. Results. Withou t serum, the sensitivities of MCF-7/ADR cells to several P-glycoprotei n-transported drugs were increased by estramustine and verapamil. Conv ersely, when the cells were treated with a 10% serum, the cytotoxiciti es of these drugs were increased by verapamil, but not by estramustine . Without serum, intracellular accumulation of [H-3]vinblastine and [H -3]paclitaxel by MCF-7/ADR cells was increased markedly by verapamil a nd estramustine; however, serum suppressed the effects of estramustine much more strongly than those of verapamil, Equilibrium dialysis expe riments demonstrated that [H-3]estramustine binds to plasma proteins, predominantly albumin, whereas [H-3]paclitaxel binds to albumin and al pha(1)-acid-glycoprotein, and [H-3]vinblastine binds predominantly to alpha(1)-acid-glycoprotein. Conclusion. Although estramustine can bind to P-glycoprotein, its effectiveness as a reversing agent in vivo lik ely is limited by binding to plasma proteins.