Structural modifications that alter the P-glycoprotein efflux properties of compounds.

Retrospective analysis of marketed drugs has revealed that very low prevalence of P-glycoprotein (P-gp) efflux and moderate to high passive permeability are two key properties that distinguish drugs which engage targets in the central nervous system (CNS) from those that act predominantly in the periphery. Quantification of the efflux properties that differentiate CNS drugs has been determined using in vitro assays for human and rodent P-gp, and revealed in vivo by comparing brain drug exposures in P-gp (mdr1a/b(−/−)) knockout mice against those observed in wild type mice. Recognition by P-gp not only dramatically affects the distribution properties of compounds into bodily compartments including the CNS, testis, and placenta but also confers resistance to certain cancer chemotherapeutics. This perspective is focused on structural modifications and strategies that can be applied during compound optimization in order to modulate the P-gp efflux properties of small molecules with a particular emphasis on implications for CNS penetration. The topic was last reviewed in 2006 by Raub who noted at the time a surprising paucity of published examples where P-gp efflux had been purposefully circumvented. Greater awareness of the implications of efflux coupled with advances in the quality and throughput of in vitro permeability and efflux assays, greater access to genetic mouse models that lack expression of P-gp, and the development of tool compounds that inhibit efflux transporters has led to an improved understanding of how structural properties influence efflux recognition. However, the precise molecular interactions that confer P-gp efflux remain relatively poorly defined. The promiscuous nature of P-gp renders the rational circumvention of efflux, while simultaneously optimizing compounds for efficacy, safety, and pharmacokinetic properties, an extremely challenging undertaking. The sequence of the human genome has revealed that 500− 1200 genes encode for transport proteins. Energy-dependent active transport proteins are utilized to varying degrees by all cells to traffic a wide variety of substrates including ions, small molecules, and macromolecules against concentration gradients spanning extraand intracellular biological membranes. Active uptake proteins control the translocation of molecules required for healthy cellular function, whereas efflux transporters serve to expel endogenous and exogenous molecules that are recognized as being potentially harmful. Eukaryotic transmembrane transport proteins have been categorized into several subclasses, the two largest being the solute carrier class (SLC) with 348 members identified, and the ATP-binding cassette (ABC) family containing 7 subfamilies (A−G) and 48 identified members. Three ABC proteins; P-gp (encoded by ABCB1 (MDR1)), multidrug resistance protein 1 (MRP1 encoded by ABCC1), and breast cancer resistance protein (BCRP encoded by ABCG2) confer resistance to tumor cells and consequently are among the most extensively studied of the ABC family members. In addition to their role in cancer chemotherapy resistance, P-gp, MRPs, and BCRP also have a significant influence on the absorption, clearance, and penetration of a vast array of small molecules into specific cellular and tissue compartments. P-gp was the first human ABC transporter to be cloned and has been the most extensively studied. P-gp is a glycosylated membrane protein of 170 kDa, and is broadly expressed but particularly enriched in the intestines, liver, kidneys, and at the blood−brain barrier (BBB) and blood−cerebrospinal fluid barrier (BCSFB). P-gp is a highly permissive transporter, recognizing and effluxing a vast diversity of small molecules and peptides. At least 15 additional efflux transporters including members from the organic anion transporter (OAT), multidrug resistance-associated protein (MRP), multidrug resistance protein (MDR), and organic anion transporting polypeptide (OATP) families have been detected at the BBB at the mRNA level. However, P-gp has assumed prominence primarily due to the fact that it has been confirmed experimentally as possessing the broadest substrate specificity.