Drug Transporters: Recent Advances and Therapeutic Applications (2009)

Clinical Pharmacology & Therapeutics (2010) 87 1, 3–7. doi:10.1038/clpt.2009.239 “False facts are highly injurious to the progress of science, for they often endure long; but false views, if supported by some evidence, do little harm, for everyone takes a salutary pleasure in proving their falseness.” —Charles Darwin, The Descent of Man (1872) Membrane-localized transport proteins have evolved to catalyze migration or expulsion of an endogenous or exogenous molecule across biological membranes. Such evolution has been necessary because membrane lipid bilayers would otherwise present virtually impenetrable barriers to most (hydrophilic) molecules or, alternatively, permit excessive intracellular accumulation. Hence, it became possible to regulate the composition of intracellular and extracellular fluids with the advent of membrane transport proteins (“transporters”). Moreover, such regulation was made progressively more sophisticated as more types of proteins evolved to transport a wide array of molecular species.1 Contemporary bioinformatics analyses have provided an emerging picture of how the majority of membrane transporters probably evolved, and such studies have allowed postulation of the specific pathways taken for their appearance.2 The ancestral precursors of most membrane transporters were simple peptide channels, and the pathways most frequently taken were evidently tandem intragenic duplications that gave rise to larger helical bundles with the potential to form discrete stereospecific intramembrane substrate binding sites. They could also be constrained for coupling to other transport processes and, through conformational coupling, were subject to control by a superimposed primary energy-yielding process such as adenosine triphosphate (ATP) hydrolysis. Until recently, the number of identified membrane transporters in mammalian species was relatively modest, but in the postgenomic era this number is now rapidly increasing; based on homology/motif searching and semiautomated curation, the TransportDB website currently lists as many as 1,022 transporters for Homo sapiens alone (http://www.membranetransport.org). Although most of these transporters have unknown physiological function, several have gained interest within the scientific community at an ever-increasing rate, partly because it is recognized that drugs may “hitchhike” on these proteins that normally act on endogenous substrates. Although the concept of a transporter-mediated movement of certain drugs into and out of cells had already been proposed by the 1950s,3 it was not until the human membrane transporter ABCB1 (also known as P-glycoprotein, MDR1) was cloned in 1987 that the drug transporter field truly took off.4 Since then, the physiological and pharmacological roles of several of these proteins have been studied in a wide range of therapeutic areas, including oncology, cardiovascular diseases, infectious diseases, and diabetes. The primary focus of many initial studies was the evaluation of the impact of membrane transporters on the development of cellular resistance to drugs and on their role in the absorption and disposition of xenobiotics. The vast majority of transporter research has been conducted on the ATP-binding cassette (ABC) transporters, a major superfamily of membrane transporters that consists of at least 48 proteins that bind and hydrolyze ATP so as to drive the efflux of various compounds out of the cell. Although the role of ABC transporters in drug resistance has historically been a major focus of research, recent years have seen a greater emphasis on studies devoted to the so-called solute carriers, which comprise more than 300 proteins organized into 47 distinct families and which are involved in the uptake of molecules into cells. These transporters do not require ATP to function and are commonly classified as passive transporters, ion-coupled transporters, or exchangers. One contributor to the apparent historical lack of interest in solute carrier research is the long-held belief that most xenobiotics are transported across membranes via passive diffusion at a rate related to their hydrophobicity.5 Not only is this accepted paradigm itself poorly supported by experimental data, but evidence accumulated over the past decade now supports the notion that solute carrier-mediated migration of drugs across the highly organized lipid bilayers of biological membranes is the predominant mechanism of cellular uptake, even for hydrophobic agents.1 The two most commonly studied solute carrier families are the SLCO family of genes that encode organic anion-transporting polypeptides (OATPs) and the SLC22A family of genes that encode organic cation transporters (OCTs) and organic anion transporters (OATs). The first solute carrier to be cloned was the rodent Oatp1a1 transporter,6 with the rodent Oct1 and Oat1 cloned shortly thereafter.7,8,9 Whereas the early in vitro and in vivo studies were capable of functionally characterizing various solute carriers, it was the actual cloning of these proteins in the early 1990s that really allowed for the massive surge in understanding and exploiting them in a therapeutic context. In this issue of Clinical Pharmacology & Therapeutics, Hagenbuch provides a historical perspective on the early studies on hepatic and renal solute carriers and the subsequent boom in studies evaluating their role in regulating the pharmacokinetics and pharmacodynamics of drugs.10 One line of evidence supporting the importance of solute carrier-mediated cellular uptake of drugs is that there are now many cases in which drugs are known to be taken up into cells via defined transporters expressed in specific tissues or organs. An example of this was the identification of fexofenadine as a substrate for OATP1A2, a solute carrier that is localized in enterocytes and may account for the absorption of fexofenadine after oral administration.11 Another line of reasoning demonstrating that drugs can be transported across membranes into cells by solute carriers is the notion that many drugs specifically accumulate in particular tissues, that their degree of accumulation is often greater than any possible number of intracellular binding sites, and that this process can be affected by chemical inhibitors of solute carriers. For example, a recent study reported in this journal indicated that the ability of the anticancer drug cisplatin to specifically accumulate in renal proximal tubular cells and subsequently cause severe, dose-limiting nephrotoxicity is determined by a tissue-specific overexpression in the kidney of certain OCTs.12 This type of information has supported the idea of possibly exploiting tissue-specific transporter distribution for preventing drug-induced toxicity. Indeed, concomitant use of a known inhibitor of renal OATs, probenecid, with the antiviral agent cidofovir has now been recommended in order to decrease the incidence and severity of cidofovir-induced nephrotoxicity.13,14 Likewise, as discussed by Niemi,15 a wealth of knowledge has accumulated within the past few years pointing to a critical role of several polymorphic transporters in the disposition, toxicity, and activity of statins (or HMG-CoA reductase inhibitors), a class of drugs that lower cholesterol levels in people with or at risk of cardiovascular disease. There are sometimes complications in translating preclinical findings to the clinic, and these can be explained in part by differences in tissue localization and/or substrate recognition of transporters between humans and animals, although future studies could use humanized mice to possibly improve animal-to-human translational findings.10 The use of animal models deficient in certain transporters has historically provided invaluable information on the normal physiological function of these proteins. For example, mice deficient in ABCB1, a transporter involved in the extrusion of structurally diverse toxic substances from cells, are phenotypically normal but are extremely sensitive to certain neurotoxins.16 These studies led to the characterization of an important role of ABCB1 in the transport of numerous prescription drugs across the blood-brain barrier. In this issue of the Journal, the Point/Counterpoint pieces by Marchi et al. and Cascorbi, respectively, provide an example of how this transporter, which was assumed to be an important target responsible for drug-refractory epilepsy on the basis of preclinical studies, may not be as critical in humans as formerly held.17,18 Investigations into how ABC transporters and solute carriers impact drug distribution, toxicity, efficacy, and drug resistance have become the focus of many efforts to now rapidly expand knowledge in this field. The increasing awareness of the importance of these transporters is emphasized by the recent development of the International Transporter Consortium—described in this issue by Huang et al.—which is made up of scientists from academia, industry, and government who have an interest in studying and developing new tools for understanding the role of transporters in drug discovery, development, and regulatory approval.19 Moreover, a summary of the accomplishments of the National Institutes of Health–funded Pharmacogenomics of Membrane Transporters (PMT) project, described by Kroetz et al., provides ample evidence of how public funding in transporter research can improve multidisciplinary and translational research and benefit a wide variety of therapeutic areas.20 The PMT project has significantly contributed to the discovery of inherited variability in transporter genes, including single-nucleotide polymorphisms, many of which can lead to changes in protein expression and function and affect the pharmacokinetic and pharmacodynamic profiles of substrate drugs. Current translational research has also begun to utilize transporters as a way of allowing formerly unavailable administration routes of substrate drugs. For example, the expression of several ABC transporters on the apical surface of epithelial cells of the gastrointestinal tract is known to influence intestinal drug absorption and limit oral bioavailability of many drugs, including several important anticancer agents such as the taxanes docetaxel and paclitaxel, that could historically be administered only by intravenous infusion. As described by Koolen et al., the usage of chemical inhibitors of these transporters has provided a means for administering taxanes orally in the clinic, which is a more practical and convenient method that will allow treatment of patients in the future on an outpatient basis.21 Another important area of current transporter research is evolving around the discovery and development of agents that directly target specific transporters. Last year, two publications in CPT highlighted the utility of dapagliflozin, an inhibitor of the glucose transporter SGLT2 (SLC5A2), as a non-insulin based therapeutic strategy to reduce the re-uptake of glucose from the urine in patients with type 2 diabetes.22,23 In addition, the potential use of inhibitors for the Niemann–Pick C1-like 1 (NPC1L1) transporter, such as ezetimibe, in treating hypercholesterolemia by preventing the intestinal absorption of cholesterol from diet presents an interesting approach to treating this condition, as discussed by Betters and Yu.24 These authors propose a three-pronged drug approach in the treatment of hypercholesterolemia, including inhibition of intestinal absorption (with ezetimibe), inhibition of biosynthesis (with statins), and increased excretion of cholesterol (with an activator of ABC transporters). Finally, in the field of parasitic protozoa treatment, the concept of parasite-specific transporters as targets for drug delivery or drug targeting offers some very exciting opportunities, as highlighted by Landfear.24,25 Drugs can be designed to be specific for parasitic transporters, thus limiting or reducing toxicities in humans. Furthermore, drugs can be developed to inhibit parasitic transporters that are necessary for the delivery of nutrients to the parasite. Although more research must be conducted to confirm a critical role of transporters in drug effects and determine how to utilize transporters to improve treatment, reduce interindividual pharmacokinetic variability, and minimize toxicity, the interest and involvement of all areas of research (academy, industry, and government) give hope that this field will lead to even greater beneficial findings. The research conducted in the past several decades has led to a greater understanding of what happens to a drug once it enters the body, as well as the mechanisms underlying drug transport. Given the sheer magnitude of the number of transporters in humans identified thus far, it is not difficult to imagine that the work done so far represents, at best, the tip of the iceberg. Given the role of membrane transporters for drugs, future studies that can clarify discrepancies and further identify possible targets for therapeutics in humans have the potential to advance the quality of life and improve treatment outcomes of a wide variety of diseases. The authors declared no conflict of interest.