Inostamycin, an Inhibitor of P‐Glycoprotein Function, Interacts Specifically with Phosphatidylethanolamine

The mechanism of inostamycin action was further studied. When multidrug‐resistant KB‐C4 cells were preincubated with inostamycin for 30 min, the accumulation of [(3)H]vinblastine was increased for as long as 48 h thereafter. Inostamycin inhibited azidopine binding to P‐glycoprotein, even after KB plasma membranes had been preincubated with inostamycin and washed. Carbon 14‐labeled inostamycin bound to KB plasma membranes irreversibly, but the binding capacity did not parallel the amount of P‐glycoprotein in three KB cell lines. Inostamycin was found to interact specifically with purified phosphatidylethanolamine. These results suggest that inostamycin can inhibit P‐glycoprotein irreversibly by binding to plasma membranes irreversibly through phosphatidylethanolamine.