Effects of Nonionic Detergents on P-Glycoprotein Drug Binding and Reversal of Multidrug Resistance1

Multidrug-resistant cells are thought to maintain low intracellular cytotoxic drug concentration though the active efflux of drugs across the cell membrane. It is presently believed that P-glycoprotein mediates this en ergy-dependent drug efflux by interacting directly with various lipophilic compounds. In this report, we have used [-'HJazidopine in a photoaffinity labeling assay to study the effect of detergents and denaturing agents on P-glycoprotein drug binding in intact cells. Nonionic detergents such as Triton X-IOO or Nonidet P-40 at very low concentrations were found to completely abolish azidopine photolabeling to P-glycoprotein and are able to reverse the multidrug resistance phenotype. In contrast, high concen trations of the denaturing agent urea or the zwitterionic detergent l-[(3cholamidopropyDdimethylaminol-l-propanesulfonate did not inhibit azidopine photolabeling to P-glycoprotein. A comparison between verapamil and Triton X-100 revealed that the latter was more effective in inhibiting azidopine photolabeling to P-glycoprotein while verapamil was more effective in potentiating |'II|vinblastine accumulation in drug-resis tant cells. Drug transport studies showed that [-'HJTriton X-100 accumu lated in both drug-sensitive and -resistant cells, and its accumulation was not modulated by excess vinblastine, verapamil, or colchicine. Taken to gether, these findings suggest that low concentrations of Triton X-100 reverse the multidrug resistance phenotype by inhibiting P-glycoprotein drug binding. In addition, it is also suggested that the site(s) of P-glyco protein drug binding is localized to sequences found within the lipid bilayer.