INTERACTION OF CYCLOSPORINE DERIVATIVES WITH THE ATPASE ACTIVITY OF HUMAN P-GLYCOPROTEIN

1 P-glycoprotein, a 170-180 kDa membrane glycoprotein that mediates mu ltidrug resistance, hydrolyses ATP to efflux a broad spectrum of hydro phobic agents. In this study, we analysed the effects of three MDR rev ersing agents, verapamil, cyclosporin A and [3'-keto-Bmt(1)]-[Val(2)]- cyclosporin (PSC 833), on the adenosine triphosphatase (ATPase) activi ty of human P-glycoprotein. 2 P-glycoprotein was immunoprecipitated wi th a monoclonal antibody (MRK-16) and the P-glycoprotein-MRK-1-Protein A-Sepharose complexes obtained were subjected to a coupled enzyme ATP ase assay. 3 While verapamil activated the ATPase, the cyclosporin der ivatives inhibited both the substrate-stimulated and the basal P-glyco protein ATPase. No significant difference was observed between PSC 833 and cyclosporin A on the inhibition of basal P-glycoprotein ATPase ac tivity. PSC 833 was more potent than cyclosporin A for the substrate-s timulated activity. 4 Kinetic analysis indicated a competitive inhibit ion of verapamil-stimulated ATPase by PSC 833. 5 The binding of 8-azid o-[alpha-P-32]-ATP to P-glycoprotein was not altered by the cyclospori n derivatives, verapamil, vinblastine and doxorubicin, suggesting that the modulation by these agents of P-glycoprotein ATPase cannot be att ributed to an effect on ATP binding to P-glycoprotein. 6 The interacti on of the cyclosporin derivatives with ATPase of P-glycoprotein might present an alternative and/or additional mechanism of action for the m odulation of P-glycoprotein function.