Fluorescence studies of drug binding and translocation by membrane transporters

Resistance to multiple drugs is a serious limitation to chemotherapy treatment of human cancers. In addition, many clinically useful drugs show limited uptake in the intestine and cannot gain access to the brain. Three multidrug efflux pumps of the ABC superfamily (P-glycoprotein/ABCB1, MRP1/ABCC1, and BCRP/ABGG2) are responsible for most drug transport out of mammalian cells. P-glycoprotein is the best characterized of the ABC drug transporters. However, the lipophilic nature of its substrates has made it difficult to directly quantitate drug binding to the protein by classical biochemical methods, and the measurement of drug transport rates has also proved challenging. In recent years, fluorescence spectroscopic approaches have proved very useful in overcoming these problems. This chapter focuses on the use of fluorescence tools to quantitate the affinity of binding of various drugs to purified P-glycoprotein and to measure its drug transport activity in reconstituted proteoliposomes in real time. The ability of various drugs to inhibit P-glycoprotein mediated transport can also be assessed using this approach.