A continuous fluorescence assay for the study of P-glycoprotein-mediated drug efflux using inside-out membrane vesicles

A fluorimetric procedure for assaying the transport activity of P-glycoprotein (P-gp) using a membrane vesicle model has been developed. In this assay methylene blue is incorporated into inside-out vesicles prepared from human acute lymphoblastic leukemic cells resistant to 100 ng. ml-1 vinblastine (VBL100) and their sensitive controls. The fluorescence of a fluorescent derivative of colchicine (fluorescein-colchicine) is quenched as the probe is transported across the vesicle membrane. The fluorescein-colchicine transport was found to be dependent on the presence of P-glycoprotein, required ATP, and was inhibited by vanadate and the reversal agent, verapamil, in a dose-dependent manner. Furthermore, the transport was competed against by the P-gp substrates, vinblastine and methotrexate. The transport of fluorescein-colchicine by P-gp was found to be cooperative (n = 1. 23). The assay is rapid, requires small amounts of sample, and removes the need for the radioactive procedures used in the past. The assay should find use in characterizing the transport kinetics of P-gp, for examining and optimizing combinations of chemotherapeutics, and for examining the effects of reversal agents and substrates which potentially compete for transport with the fluorescent substrate probe. Other possible applications include examining P-gp-mediated transport properties of purified P-gp in reconstituted systems.