ATP-dependent transport of a novel thromboxane A2 receptor antagonist, [2-(4-chlorophenylsulfonylaminomethyl)indan-5-yl]acetate (Z-335) and its xenobiotic taurine conjugate (Z-335-Tau) by rat bile canalicular membrane vesicles

ABCC2/Abcc2: a multispecific transporter with dominant excretory functions

ABCC2/Abcc2 (MRP2/Mrp2) is expressed at major physiological barriers, such as the canalicular membrane of liver cells, kidney proximal tubule epithelial cells, enterocytes of the small and large intestine, and syncytiotrophoblast of the placenta. ABCC2/Abcc2 always localizes in the apical membranes. Although ABCC2/Abcc2 transports a variety of amphiphilic anions that belong to different classes of molecules, such as endogenous compounds (e.g., bilirubin-glucuronides), drugs, toxic chemicals, nutraceuticals, and their conjugates, it displays a preference for phase II conjugates. Phenotypically, the most obvious consequence of mutations in ABCC2 that lead to Dubin-Johnson syndrome is conjugate hyperbilirubinemia. ABCC2/Abcc2 harbors multiple binding sites and displays complex transport kinetics.

The apical conjugate efflux pump ABCC2 (MRP2)

ABCC2 is a member of the multidrug resistance protein subfamily localized exclusively to the apical membrane domain of polarized cells, such as hepatocytes, renal proximal tubule epithelia, and intestinal epithelia. This localization supports the function of ABCC2 in the terminal excretion and detoxification of endogenous and xenobiotic organic anions, particularly in the unidirectional efflux of substances conjugated with glutathione, glucuronate, or sulfate, as exemplified by leukotriene C(4), bilirubin glucuronosides, and some steroid sulfates. The hepatic ABCC2 pump contributes to the driving forces of bile flow. Acquired or hereditary deficiency of ABCC2, the latter known as Dubin-Johnson syndrome in humans, causes an increased concentration of bilirubin glucuronosides in blood because of their efflux from hepatocytes via the basolateral ABCC3, which compensates for the deficiency in ABCC2-mediated apical efflux. In this article we provide an overview on the molecular characteristics of ABCC2 and its expression in various tissues and species. We discuss the transcriptional and posttranscriptional regulation of ABCC2 and review approaches to the functional analysis providing information on its substrate specificity. A comprehensive list of sequence variants in the human ABCC2 gene summarizes predicted and proven functional consequences, including variants leading to Dubin-Johnson syndrome.

P-GLYCOPROTEIN PLAYS A MAJOR ROLE IN THE EFFLUX OF FEXOFENADINE IN THE SMALL INTESTINE AND BLOOD-BRAIN BARRIER, BUT ONLY A LIMITED ROLE IN ITS BILIARY EXCRETION

Fexofenadine is a selective, nonsedating H(1)-receptor antagonist approved for symptoms of allergic conditions, which is mainly excreted into feces via biliary excretion. The purpose of this study is to investigate its pharmacokinetics in mice and rats to determine the role of P-glycoprotein (P-gp) in its biliary excretion. In mice, biliary excretion clearance (17 ml/min/kg) accounted for almost 60% of the total body clearance (30 ml/min/kg). Comparing the pharmacokinetics after intravenous and oral administration indicated that the bioavailability of fexofenadine was at most 2% in mice. Knockout of Mdr1a/1b P-gp did not affect the biliary excretion clearance with regard to both plasma and liver concentrations, whereas the absence of P-gp caused a 6-fold increase in the plasma concentration after oral administration. In addition, the steady-state brain-to-plasma concentration ratio of fexofenadine was approximately 3-fold higher in Mdr1a/1b P-gp knockout mice than in wild-type mice. Together, these results show that P-glycoprotein plays an important role in efflux transport in the brain and small intestine but only a limited role in biliary excretion in mice. In addition, there was no difference in the biliary excretion between normal and hereditarily multidrug resistance-associated protein 2 (Mrp2)-deficient mutant rats (Eisai hyperbilirubinemic rats) and between wild-type and breast cancer resistance protein (Bcrp) knockout mice. These results suggest that the biliary excretion of fexofenadine is mediated by unknown transporters distinct from P-gp, Mrp2, and Bcrp.

Pharmacogenomics of MDR and MRP subfamilies

Drug-metabolizing enzymes, drug transporters and drug targets play significant roles as determinants of drug efficacy and toxicity. Their genetic polymorphisms often affect the expression and function of their products and are expected to become surrogate markers to predict the response to drugs in individual patients. With the sequencing of the human genome, it has been estimated that approximately 500–1200 genes code for drug transporters and, recently, there have been significant and rapid advances in the research on the relationships between genetic polymorphisms of drug transporters and interindividual variation of drug disposition. At present, the clinical studies of multi-drug resistance protein 1 (MDR1, P-glycoprotein, ABCB1), which belongs to the ATP-binding cassette (ABC) superfamily, are the most comprehensive among the ABC transporters, but clinical investigations on other drug transporters are currently being performed around the world. MDR1 can be said to be the most important drug transporter, since clinical reports have suggested that it regulates the disposition of various types of clinically important drugs, but in vitro investigations or animal experiments have strongly suggested that the members of the multi-drug resistance-associated protein (MRP) subfamily can also become key molecules for pharmacotherapy. In addition to those, breast cancer resistance protein (BCRP, ABCG2), another ABC transporter, is well known as a key molecule of multi-drug resistance to several anticancer agents. However, this review focuses on the latest information on the pharmacogenetics of the MDR and MRP subfamilies, and its impact on pharmacotherapy is discussed.