title: Direct demonstration of high affinity interactions of immunosuppressant drugs with the drug binding site of the human P-glycoprotein.
abstract: The interactions between the human P-glycoprotein (Pgp) and two different types of immunosuppressant drugs known to modulate multidrug resistance in tumor cells have been directly investigated using our newly developed drug-stimulated ATPase assay for Pgp function. The macrolides FK506 and FK520 stimulate the Pgp-ATPase activity with affinities in the 100 nM range, nearly 10 times higher than that of verapamil, a well known Pgp substrate. On the other hand, the cyclic peptides cyclosporin A and dihydrocyclosporin C do not stimulate the Pgp-ATPase activity at all. They do, however, act as potent competitive inhibitors of verapamil-stimulated Pgp-ATPase activity, with affinity constants in the 20-25 nM range. Thus, although these two classes of immunosuppressant drugs affect the Pgp in different ways, they both probably interact with high affinity at the transported drug binding site(s) of the Pgp, which would explain their ability to resensitize multidrug-resistant cells to the killing action of certain antitumor drugs. Possible implications of these findings for Pgp function, cancer chemotherapy, and immunosuppression are discussed.
authors.affiliation: Department of Pharmacology, University of North Carolina, Chapel Hill 27599.
authors.forename: U S
authors.initials: US
authors.lastname: Rao
authors.forename: G A
authors.initials: GA
authors.lastname: Scarborough
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000251:Adenosine Triphosphatases / Q000037:antagonists & inhibitors / Q000378:metabolism; D001665:Binding Sites; D002352:Carrier Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D003524:Cyclosporins / Q000494:pharmacology; D006801:Humans; D007166:Immunosuppressive Agents / Q000378:metabolism*; D066298:In Vitro Techniques; D008562:Membrane Glycoproteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011994:Recombinant Proteins; D016559:Tacrolimus / Q000031:analogs & derivatives / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D002352:Carrier Proteins; D003524:Cyclosporins; D007166:Immunosuppressive Agents; D008562:Membrane Glycoproteins; D011994:Recombinant Proteins; C058028:immunomycin; D014700:Verapamil; D000251:Adenosine Triphosphatases; D016559:Tacrolimus