Binding site(s) on P-glycoprotein for a newly synthesized photoaffinity analog of agosterol A.

Agosterol A (AG-A) is a novel agent that reverses P-glycoprotein (P-gp) and multidrug resistance protein-1 (MRP1)-meditated multidrug resistance (MDR). We have synthesized [125I]11-azidophenyl agosterol A (azidoAG-A), a photoaffinity analog of AG-A, and characterized its binding to P-gp in membrane vesicles prepared from multidrug-resistant P-gp-overexpressing KB-C2 cells. The photoanalog photolabeled intact P-gp and both the N- and C-terminal fragments of P-gp. [125I]AzidoAG-A is transported by P-gp and the intracellular accumulation of both [125I]azidoAG-A and [3H]AG-A in KB-C2 cells was lower than that in the parental drug-sensitive KB-3-1 cells. [125I]AzidoAG-A bound to the drug binding site(s) on P-gp because photoaffinity labeling of P-gp was inhibited by a variety of known P-gp substrates, including anticancer, reversing, and anti-human immunodeficiency virus (HIV) agents. The binding of [125I]azidoAG-A to P-gp differs from the binding of other photolabeled probes such as iodoaryl-azidoprazosin (IAAP) to P-gp and from the binding of [125I]azidoAG-A to MRP1 based on the differing effects of flupentixol and glutathione (GSH) on their binding. Thus, [125I]azidoAG-A will be a useful tool to elucidate the structure and function of P-gp because it directly binds to the drug binding site(s) on P-gp, is transported by P-gp, and exhibits different P-gp binding characteristics than IAAP.