The multi‐drug resistance reversal agent SR33557 and modulation of vinca alkaloid binding to P‐glycoprotein by an allosteric interaction

1 The interaction of the indolizin sulfone SR33557 with the multidrug resistance P‐glycoprotein (P‐gp), was used to explore the nature of drug binding site(s) on this transporter. The steady‐state accumulation of [ 3 H]‐vinblastine in P‐gp expressing CH r B30 cells was increased by SR33557 with greater potency than verapamil. Furthermore, SR33557 potentiated the affinity of verapamil to modulate vinblastine transport when added simultaneously. 2 Verapamil elicited a 1.5 to 2.5 fold stimulation of basal ATPase activity in CH r B30 membranes, whereas SR33557 and vinblastine inhibited activity, but only at relatively high concentrations. However, SR33557 and vinblastine decreased the V max but not the K m for verapamil stimulation of ATPase activity. This is indicative of a non‐competitive interaction, most likely at distinct sites. 3 The specific [ 3 H]‐vinblastine binding to P‐gp in CH r B30 cell membranes was displaced by SR33557 with an IC 50 of 8.3±4.5 n M . Moreover, SR33557 caused a 3 fold increase in the dissociation rate of vinblastine binding to P‐gp indicating a negative allosteric effect on the vinca alkaloid acceptor site. 4 These results demonstrate that SR33557 interacts with a site on P‐gp which is distinct from, but allosterically linked to the vinca alkaloid site. The apparent broad substrate specificity displayed by P‐gp may be explained by a multiple drug binding site model.British Journal of Pharmacology (1997) 122 , 765–771; doi: 10.1038/sj.bjp.0701429