MEMBRANE TRANSPORTERS AND DRUG RESPONSE: INTRODUCTION

MEMBRANE TRANSPORTERS AND DRUG RESPONSE - Kathleen M. Giacomini and Yuichi Sugiyama MEMBRANE TRANSPORTERS AND DRUG RESPONSE: INTRODUCTION Transporters are membrane proteins that are present in all organisms. These proteins control the influx of essential nutrients and ions and the efflux of cellular waste, environmental toxins, and other xenobiotics. Consistent with their critical roles in cellular homeostasis, approximately 2000 genes in the human genome ~7 of the total number of genes code for transporters or transporter-related proteins. The functions of membrane transporters may be facilitated (equilibrative, not requiring energy) or active (requiring energy). In considering the transport of drugs, pharmacologists generally focus on transporters from two major superfamilies, ABC (ATP binding cassette) and SLC (solute carrier) transporters. Most ABC proteins are primary active transporters, which rely on ATP hydrolysis to actively pump their substrates across membranes. There are 49 known genes for ABC proteins that can be grouped into seven subclasses or families (ABCA to ABCG) (Borst and Elferink, 2002). Among the best recognized transporters in the ABC superfamily are P-glycoprotein (P-gp, encoded by ABCB1, also termed MDR1) and the cystic fibrosis transmembrane regulator (CFTR, encoded by ABCC7). The SLC superfamily includes genes that encode facilitated transporters and ion-coupled secondary active transporters that reside in various cell membranes. Forty-three SLC families with approximately 300 transporters have been identified in the human genome (Hediger, 2004). Many serve as drug targets or in drug absorption and disposition. Widely recognized SLC transporters include the serotonin and dopamine transporters (SERT, encoded by SLC6A4; DAT, encoded by SLC6A3). Drug-transporting proteins operate in pharmacokinetic and pharmacodynamic pathways, including pathways involved in both therapeutic and adverse effects (Figure 2-1). MEMBRANE TRANSPORTERS IN THERAPEUTIC DRUG RESPONSES Pharmacokinetics. Transporters that are important in pharmacokinetics generally are located in intestinal, renal, and hepatic epithelia. They function in the selective absorption and elimination of endogenous substances and xenobiotics, including drugs (Dresser et al., 2001; Kim, 2002). Transporters work in concert with drug-metabolizing enzymes to eliminate drugs and their metabolites (Figure 2-2). In addition, transporters in various cell types mediate tissue-specific drug distribution (drug targeting); conversely, transporters also may serve as protective barriers to particular organs and cell types. For example, P-glycoprotein in the blood-brain barrier protects the central nervous system (CNS) from a variety of structurally diverse compounds through its efflux mechanisms. Many of the transporters that are relevant to drug response control the tissue distribution as well as the absorption and elimination of drugs. Pharmacodynamics: Transporters As Drug Targets. Membrane transporters are the targets of many clinically used drugs. For example, neurotransmitter transporters are the targets for drugs used in the treatment of neuropsychiatric disorders (Amara and Sonders, 1998; Inoue et al., 2002). SERT (SLC6A4) is a target for a major class of antidepressant drugs, the serotonin selective reuptake inhibitors (SSRIs). Other neurotransmitter reuptake transporters serve as drug targets for the tricyclic antidepressants, various amphetamines (including amphetaminelike drugs used in the treatment of attention deficit disorder in children), and anticonvulsants (Amara and Sonders, 1998; Jones et al., 1998; Elliott and Beveridge, 2005). These transporters also may be involved in the pathogenesis of neuropsychiatric disorders, including Alzheimer's and Parkinson's diseases (Shigeri et al., 2004). Transporters that are nonneuronal also may be potential drug targets, e.g., cholesterol transporters in cardiovascular disease, nucleoside transporters in cancers, glucose transporters in metabolic syndromes, and Na+-H+ antiporters in hypertension (Damaraju et al., 2003; Pascual et al., 2004; Rader, 2003; Rosskopf et al., 1993). Drug Resistance. Membrane transporters play a critical role in the development of resistance to anticancer drugs, antiviral agents, and anticonvulsants. For example, P-glycoprotein is overexpressed in tumor cells after exposure to cytotoxic anticancer agents (Gottesman et al., 1996; Lin and Yamazaki, 2003; Leslie et al., 2005). P-glycoprotein pumps out the anticancer drugs, rendering cells resistant to their cytotoxic effects. Other transporters, including breast cancer resistance protein (BCRP), the organic anion transporters, and several nucleoside transporters, also have been implicated in resistance to anticancer drugs (Clarke et al., 2002; Suzuki et al., 2001). The overexpression of multidrug-resistance protein 4 (MRP4) is associated with resistance to antiviral nucleoside analogs (Schuetz et al., 1999). MEMBRANE TRANSPORTERS AND ADVERSE DRUG RESPONSES Through import and export mechanisms, transporters ultimately control the exposure of cells to chemical carcinogens, environmental toxins, and drugs. Thus, transporters play critical roles in the cellular toxicities of these agents. Transporter-mediated adverse drug responses generally can be classified into three categories, as shown in Figure 2-3. Transporters in the liver and kidney affect the exposure of drugs in the toxicological target organs. Transporters expressed in the liver and kidney, as well as metabolic enzymes, are key determinants of drug exposure in the circulating blood (Mizuno et al., 2003) (Figure 2-3, top panel). For example, after oral administration of an HMG-CoA reductase inhibitor (e.g., pravastatin), the efficient first-pass hepatic uptake of the drug by the organic anion-transporting polypeptide OATP1B1 maximizes the effects of such drugs on hepatic HMG-CoA reductase. Uptake by OATP1B1 also minimizes the escape of these drugs into the systemic circulation, where they can cause adverse responses such as skeletal muscle myopathy. Transporters in the liver and kidney, which control the total clearance of drugs, thus have an influence on the plasma concentration profiles and subsequent exposure to the toxicological target. Transporters in toxicological target organs or at barriers to such organs affect drug exposure by the target organs. Transporters expressed in tissues that may be targets for drug toxicity (e.g., brain) or in barriers to such tissues [e.g., the blood-brain barrier (BBB)] can tightly control local drug concentrations and thus control the exposure of these tissues to the drug (Figure 2-3, middle panel). For example, to restrict the penetration of compounds into the brain, endothelial cells in the BBB are closely linked by tight junctions, and some efflux transporters are expressed on the blood-facing (luminal) side. The importance of the ABC transporter multidrug-resistance protein (ABCB1, MDR1; P-glycoprotein, P-gp) in the BBB has been demonstrated in mdr1a knockout mice (Schinkel et al., 1994). The brain concentrations of many P-glycoprotein substrates, such asdigoxin, used in the treatment of heart failure (see Chapters 33 and 34), and cyclosporin A (see Chapter 52), an immunosuppressant, are increased dramatically in mdr1a(-/-) mice, whereas their plasma concentrations are not changed significantly. Another example of transporter control of drug exposure can be seen in the interactions of loperamide and quinidine. Loperamide is a peripheral opioid used in the treatment of diarrhea and is a substrate of P-glycoprotein. Coadministration of loperamide and the potent P-glycoprotein inhibitor quinidine results in significant respiratory depression, an adverse response to the loperamide (Sadeque et al., 2000). Because plasma concentrations of loperamide are not changed in the presence of quinidine, it has been suggested that quinidine inhibits P-glycoprotein in the BBB, resulting in an increased exposure of the CNS to loperamide and bringing about the respiratory depression. Inhibition of P-glycoprotein-mediated efflux in the BBB thus would cause an increase in the concentration of substrates in the CNS and potentiate adverse effects. Drug-induced toxicity sometimes is caused by the concentrative tissue distribution mediated by influx transporters. For example, biguanides (e.g., metformin and phenformin), widely used as oral hypoglycemic agents for the treatment of type II diabetes mellitus, can produce lactic acidosis, a lethal side effect. Phenformin was withdrawn from the market for this reason. Biguanides are substrates of the organic cation transporter OCT1, which is highly expressed in the liver. After oral administration of metformin, the distribution of the drug to the liver in oct1(-/-) mice is markedly reduced compared with the distribution in wild-type mice. Moreover, plasma lactic acid concentrations induced by metformin are reduced in oct1(-/-) mice compared with wild-type mice, although the plasma concentrations of metformin are similar in the wild-type and knockout mice. These results indicate that the OCT1-mediated hepatic uptake of biguanides plays an important role in lactic acidosis (Wang et al., 2003). The organic anion transporter 1 (OAT1) provides another example of transporter-related toxicity. OAT1 is expressed mainly in the kidney and is responsible for the renal tubular secretion of anionic compounds. Some reports have indicated that substrates of OAT1, such as cephaloridine, a b-lactam antibiotic, sometimes cause nephrotoxicity. In vitro experiments suggest that cephaloridine is a substrate of OAT1 and that OAT1-expressing cells are more susceptible to cephaloridine toxicity than control cells. Transporters for endogenous ligands may be modulated by drugs and thereby exert adverse effects (Figure 2-3, bottom panel). For example, bile acids are taken up mainly by Na+-taurocholate cotransporting polypeptide (NTCP) (Hagenbuch et al., 1991) and excreted into the bile by the bile salt export pump (BSEP, ABCB11) (Gerloff et al., 1998). Bilirubin is taken up by OATP1B1 and conjugated with glucuronic acid, and bilirubin glucuronide is excreted by the multidrug-resistance-associated protein (MRP2, ABCC2). Inhibition of these transporters by drugs may cause cholestasis or hyperbilirubinemia. Troglitazone, a thiazolidinedione insulin-sensitizing drug used for the treatment of type II diabetes mellitus, was withdrawn from the market because it caused hepatotoxicity. The mechanism for this troglitazone-induced hepatotoxicity remains unclear. One hypothesis is that troglitazone and its sulfate conjugate induced cholestasis. Troglitazone sulfate potently inhibits the efflux of taurocholate (Ki = 0.2 mM) mediated by the ABC transporter BSEP. These findings suggest that troglitazone sulfate induces cholestasis by inhibition of BSEP function. BSEP-mediated transport is also inhibited by other drugs, including cyclosporin A and the antibiotics rifamycin and rifampicin (Stieger et al., 2000). Thus, uptake and efflux transporters determine the plasma and tissue concentrations of endogenous compounds and xenobiotics and thereby can influence the systemic or site-specific toxicity of drugs.