Screening for P-Glycoprotein (Pgp) Substrates and Inhibitors

P-glycoprotein (P-gp), the product of the human ABCB1 gene and often called MDR1, is the best understood membrane protein known to be involved in the active transport of drugs across biological membranes. In addition to mediating or limiting the absorption, distribution, excretion, and toxicity of many drugs, P-gp is the potential locus of a number of pharmacokinetic drug-drug interactions when two drugs, one a substrate and the other a substrate or inhibitor of the transporter, are co-administered. This last point is the reason for the interest of regulatory authorities around the world, several of which (most notably the U.S. FDA and the EMA) now require that all new molecular entities (NMEs) be evaluated as P-gp substrates and inhibitors. This chapter will cover model test systems, including in vitro assays for human P-gp such as cell-based (over-expressing and knockdown cells) and subcellular (membrane vesicle) approaches, as well as in vivo animal models. The chapter will conclude with examples of two cell-based systems, MDR1-MDCK (over-expressing) and Caco-2 (parental and P-gp knockdown cell lines).