Effects of nonionic detergents on P-glycoprotein drug binding and reversal of multidrug resistance.

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 energy-dependent drug efflux by interacting directly with various lipophilic compounds. In this report, we have used [3H]azidopine 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-100 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 concentrations of the denaturing agent urea or the zwitterionic detergent 1-[(3-cholamidopropyl)dimethylamino]-1-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 [3H]vinblastine accumulation in drug-resistant cells. Drug transport studies showed that [3H]Triton X-100 accumulated in both drug-sensitive and -resistant cells, and its accumulation was not modulated by excess vinblastine, verapamil, or colchicine. Taken together, 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-glycoprotein drug binding is localized to sequences found within the lipid bilayer.