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 thin transporter. The steady-state accumulation of [H-3]-vinblastine in P-gp expressing CH'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 [H-3]-vinblastine binding to P-gp in CH(r)B30 cell membranes was displaced by SR33557 with an IC50 of 8.3+/-4.5 nM. 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.