Single nucleotide polymorphisms in multidrug resistance associated protein 2 (MRP2/ABCC2): its impact on drug disposition

Associations of ABCB1, ABCC2, and ABCG2 polymorphisms with irinotecan-pharmacokinetics and clinical outcome in patients with advanced non-small cell lung cancer

BACKGROUND

The authors investigated whether ABCB1, ABCC2, and ABCG2 genetic polymorphisms affect pharmacokinetics (PK) of irinotecan and treatment outcome of patients with advanced nonsmall cell lung cancer (NSCLC).

METHODS

Blood samples from 107 NSCLC patients treated with irinotecan and cisplatin chemotherapy were used for genotyping ABCB1 (1236C > T, 2677G > T/A, 3435C > T), ABCC2 (-24C > T, 1249G > A, 3972C > T), and ABCG2 (34G > A, 421C > A) polymorphisms. Genotypes were correlated with irinotecan-PK, toxicity, tumor response, and survival.

RESULTS

Among 8 polymorphisms, 3435TT and 2677TT were associated with AUC(SN-38G) and CL(SN-38G). When haplotypes are assigned, 2677TT/3435TT carriers showed significantly lower AUC(SN-38G) (P = .006), whereas 2677GG/3435CC carriers showed significantly higher AUC(SN-38) (P = .039). These findings suggest that 2677TT and 3435TT variants are associated with higher efflux activity. In toxicity, the 2677G/T or A was associated with grade 4 neutropenia. The 2677GG carriers showed significantly lower absolute neutrophil count during the 1(st) cycle (P = .012) as well as entire course of chemotherapy (P = .042). The 3435TT was associated with higher frequency of grade 3 diarrhea (P = .047). In tumor response, ABCC2 -24TT and 3972TT genotypes were associated with higher response rates (P = .031 and P = .048, [corrected] respectively) and longer progression-free survival (P = .010 and P = .019, [corrected] respectively), which was sustained in haplotype analysis.

CONCLUSIONS

Specific polymorphisms of ABCB1 and ABCC2 can influence disposition and tumor response to irinotecan by regulating transporter activity. These findings may help to individualize irinotecan-based chemotherapy in patients with advanced NSCLC.

Individualization of mycophenolate mofetil dose in renal transplant recipients

The immunosuppressive agent mycophenolate mofetil has been successfully used over the past 10 years to prevent acute allograft rejection after renal transplantation. It has mainly been administered as a fixed dose of mycophenolate mofetil 1000 mg b.i.d. The pharmacokinetics of mycophenolic acid, the active moiety of the prodrug mycophenolate mofetil, show large between-patient variability, and exposure to mycophenolic acid correlates with the risk for acute rejection. This suggests that already excellent clinical results can be further improved by mycophenolate mofetil dose individualization. This review discusses different arguments in favour of individualization of mycophenolate mofetil dose, as well as strategies for managing mycophenolate mofetil therapy individualization, including pharmacokinetic and pharmacodynamic monitoring and dose individualization based on pharmacogenetic information. It is expected that pharmacokinetic monitoring of mycophenolic acid will offer the most effective and feasible tool for mycophenolate mofetil dose individualization.

MRP2 haplotypes confer differential susceptibility to toxic liver injury

OBJECTIVES

Multidrug resistance protein 2 (MRP2, ABCC2) plays an important role in the biliary clearance of a wide variety of endogenous and exogenous toxic compounds. Therefore, polymorphisms and mutations in the MRP2 gene may affect individual susceptibility to hepatotoxic reactions.

METHODS

Associations between genetic variations of MRP2 and toxic hepatitis were investigated using integrated population genetic analysis and functional molecular studies.

RESULTS

Using a gene scanning method, 12 polymorphisms and mutations were found in the MRP2 gene in a Korean population. Individual variation at these sites was analyzed by conventional DNA screening in 110 control subjects and 94 patients with toxic hepatitis induced mostly by herbal remedies. When haplotypes were identified, over 85% of haploid genes belonged to the five most common haplotypes. Among these, a haplotype containing the g.-1774delG polymorphism showed a strong association with cholestatic or mixed-type hepatitis, and a haplotype containing the g.-1549G>A, g.-24C>T, c.334-49C>T, and c.3972C>T variations was associated with hepatocellular-type hepatitis. A comprehensive functional study of these sites revealed that genetic variations in the promoter of this gene are primarily responsible for the susceptibility to toxic liver injuries. The g.-1774delG variation and the combined variation of g.-1549G>A and g.-24C>T decreased MRP2 promoter activity by 36 and 39%, respectively. In addition, the promoter carrying the g.-1774delG allele showed a defect in the bile acid-induced induction of promoter activity.

CONCLUSIONS

These results suggest that genetic variations of MRP2 are an important predisposing factor for herbal-induced or drug-induced toxic liver injuries.

The effect of lopinavir/ritonavir on the renal clearance of tenofovir in HIV-infected patients

We determined the effects of lopinavir/ritonavir on tenofovir renal clearance. Human immunodeficiency virus-infected subjects taking tenofovir disoproxil fumarate (TDF) were matched on age, race, and gender and were enrolled into one of the following two groups: group 1: subjects taking TDF plus lopinavir/ritonavir plus other nucleoside reverse transcriptase inhibitors (NRTIs); group 2: subjects taking TDF plus NRTIs and/or non-NRTIs but no protease inhibitors. Twenty-four-hour blood and urine collections were carried out in subjects for tenofovir quantification. Drug transporter genotype associations with tenofovir pharmacokinetics were examined. In 30 subjects, median (range) tenofovir apparent oral clearance, renal clearance, and fraction excreted in urine were 34.6 l/h (20.6-89.5), 11.3 l/h (6.2-22.6), and 0.33 (0.23-0.5), respectively. After adjusting for renal function, tenofovir renal clearance was 17.5% slower (P=0.04) in subjects taking lopinavir/ritonavir versus those not taking a protease inhibitor, consistent with a renal interaction between these drugs. Future studies should clarify the exact mechanism and whether there is an increased risk of nephrotoxicity.

Pharmacogenetics/genomics of membrane transporters in cancer chemotherapy

Inter-individual variability in drug response and the emergence of adverse drug reactions are main causes of treatment failure in cancer therapy. Recently, membrane transporters have been recognized as an important determinant of drug disposition, thereby affecting chemosensitivity and -resistance. Genetic factors contribute to inter-individual variability in drug transport and targeting. Therefore, pharmacogenetic studies of membrane transporters can lead to new approaches for optimizing cancer therapy. This review discusses genetic variations in efflux transporters of the ATP-binding cassette (ABC) family such as ABCB1 (MDR1, P-glycoprotein), ABCC1 (MRP1), ABCC2 (MRP2) and ABCG2 (BCRP), and uptake transporters of the solute carrier (SLC) family such as SLC19A1 (RFC1) and SLCO1B1 (SLC21A6), and their relevance to cancer chemotherapy. Furthermore, a pharmacogenomic approach is outlined, which using correlations between the growth inhibitory potency of anticancer drugs and transporter gene expression in multiple human cancer cell lines, has shown promise for determining the relevant transporters for any given drugs and predicting anticancer drug response.

Regulation of transporter expression in mouse liver, kidney, and intestine during extrahepatic cholestasis

It is hypothesized that during cholestasis, the liver, kidney, and intestine alter gene expression to prevent BA accumulation; enhance urinary excretion of BA; and decrease BA absorption, respectively. To test this hypothesis, mice were subjected to either sham or bile-duct ligation (BDL) surgery and liver, kidney, duodenum, ileum, and serum samples were collected at 1, 3, 7, and 14 days after surgery. Serum total BA concentrations were 1-5 mumol/l in sham-operated mice and were elevated at 1, 3, 7, and 14 days after BDL, respectively. BDL decreased liver Ntcp, Oatp1a1, 1a5, and 1b2 mRNA expression and increased Bsep, Oatp1a4, and Mrp1-5 mRNA levels. In kidney, BDL decreased Oatp1a1 and increased Mrp1-5 mRNA levels. In intestine, BDL increased Mrp3 and Ibat mRNA levels in ileum. BDL increased Mrp1, 3, 4, and 5 protein expression in mouse liver. These data indicate that the compensatory regulation of transporters in liver, kidney, and intestine is unable to fully compensate for the loss of hepatic BA excretion because serum BA concentration remained elevated after 14 days of BDL. Additionally, hepatic and renal Oatp and Mrp genes are regulated similarly during extrahepatic cholestasis, and may suggest that transporter expression is regulated not to remove bile constituents from the body, but instead to remove bile constituents from tissues.

Role of pharmacogenetics of ATP-binding cassette transporters in the pharmacokinetics of drugs

Interindividual differences of drug response are an important cause of treatment failures and adverse drug reactions. The identification of polymorphisms explaining distinct phenotypes of drug metabolizing enzymes contributed in part to the understanding of individual variations of drug plasma levels. However, bioavailability also depends on a major extent from the expression and activity of drug transport across biomembranes. In particular efflux transporters of the ATP-binding cassette (ABC) family such as ABCB1 (P-glycoprotein, P-gp), the ABCC (multidrug resistance-related protein, MRP) family and ABCG2 (breast cancer resistance protein, BCRP) have been identified as major determinants of chemoresistance in tumor cells. They are expressed in the apical membranes of many barrier tissue such as the intestine, liver, blood-brain barrier, kidney, placenta, testis and in lymphocytes, thus contributing to plasma, liquor, but also intracellular drug disposition. Since expression and function exhibit a broad variability, it was hypothesized that hereditary variances in the genes of membrane transporters could explain at least in part interindividual differences of pharmacokinetics and clinical outcome of a variety of drugs. This review focuses on the functional significance of single nucleotide polymorphisms (SNP) of ABCB1, ABCC1, ABCC2, and ABCG2 in in vitro systems, in vivo tissues and drug disposition, as well as on the clinical outcome of major indications.

Gene expression analysis for predicting gemcitabine sensitivity in pancreatic cancer patients

BACKGROUND/AIMS

Current in vitro drug sensitivity tests have limitations and disadvantages. This study investigated the use of gene expression data to predict the sensitivity of pancreatic cancers to gemcitabine.

MATERIALS AND METHODS

Cancer cells isolated from 14 pancreatic cancer patients were tested in vitro for gemcitabine sensitivity using the collagen droplet drug sensitivity test (CD-DST). On the basis of this test, 9 of the 14 cancers were identified as either gemcitabine-sensitive or gemcitabine-resistant. Total RNA was extracted from each of those nine cancers and used as a template to synthesize Cy3-labeled cDNA. Pancreatic RNA extracted from six normal individuals was used as a control. Labeled probes were hybridized to an Atlas Glass Human 1.0 Microarray chip, after which the chips were washed and scanned, and the data were analyzed using Microsoft Excel-embedded software. The expression profiles of selected genes were confirmed using real-time PCR analysis.

RESULTS

Statistical analysis of the microarray data showed that four genes were differentially expressed in gemcitabine-sensitive cancers: microsomal glutathione S-transferase 1 (GSTT1), topoisomerase II alpha (TOP2A), caspase 3, and ATP-binding cassette and subfamily C member 2 (ABCC2). More than 20 other genes were additionally identified as possible candidate genes associated with drug resistance.

CONCLUSIONS

Expression of drug resistance-related genes appeared to predict whether a cancer was gemcitabine-sensitive or -resistant. Further study will enable a drug resistance scoring system to be established on the basis of gene expression. Such a system will allow more efficient application of chemotherapy.

Human pharmacogenomic variations and their implications for antifungal efficacy

Pharmacogenomics is defined as the study of the impacts of heritable traits on pharmacology and toxicology. Candidate genes with potential pharmacogenomic importance include drug transporters involved in absorption and excretion, phase I enzymes (e.g., cytochrome P450-dependent mixed-function oxidases) and phase II enzymes (e.g., glucuronosyltransferases) contributing to metabolism, and those molecules (e.g., albumin, A1-acid glycoprotein, and lipoproteins) involved in the distribution of antifungal compounds. By using the tools of population genetics to define interindividual differences in drug absorption, distribution, metabolism, and excretion, pharmacogenomic models for genetic variations in antifungal pharmacokinetics can be derived. Pharmacogenomic factors may become especially important in the treatment of immunocompromised patients or those with persistent or refractory mycoses that cannot be explained by elevated MICs and where rational dosage optimization of the antifungal agent may be particularly critical. Pharmacogenomics has the potential to shift the paradigm of therapy and to improve the selection of antifungal compounds and adjustment of dosage based upon individual variations in drug absorption, metabolism, and excretion.

Multidrug Resistance Protein 2 Genetic Polymorphisms Influence Mycophenolic Acid Exposure in Renal Allograft Recipients

BACKGROUND

Mycophenolic acid (MPA) is glucuronidated by uridine diphosphate-glucuronosyltransferases (UGTs) to its pharmacologically inactive 7-O-glucuronide metabolite (MPAG). MPAG is excreted into the bile via the multidrug resistance-associated protein 2 (MRP2/ABCC2), which is essential for enterohepatic (re)circulation (EHC) of MPA(G).

METHODS

The objective of this study was to determine the relationship between single nucleotide polymorphisms (SNPs) in the MRP2 (G-1549A, G-1023A, A-1019G, C-24, G1249A, C3972T and G4544A) and UGT1A9 (C-2152T, T-275AandT98C) genes and MPA pharmacokinetics in 95 renal allograft recipients at days 7, 42, 90, and 360 after transplantation. In addition to mycophenolate mofetil, all patients received tacrolimus and corticosteroids as immunosuppression.

RESULTS

At day seven after transplantation, in the absence of the MRP2 C-24T SNP, mild liver dysfunction was associated with significantly lower MPA dose-interval exposure and higher MPA oral clearance, while liver dysfunction did not affect MPA pharmacokinetics in patients with the MRP2 C-24T variant. A similar effect is noted for the C-3972T variant, which is in linkage disequilibrium with C-24T. At later time points after transplantation the MRP2 C-24T SNP was associated with significantly higher dose-corrected MPA trough levels. Patients with the MRP2 C-24T variant had significantly more diarrhea in the first year after transplantation.

CONCLUSIONS

The MRP2 C-24T and C-3972T polymorphisms protect renal transplant recipients from a decrease in MPA exposure associated with mild liver dysfunction. Furthermore, this study suggests that the C-24T SNP is associated with a lower oral clearance of MPA in steady-state conditions.

Multidrug resistance protein 2 is an important determinant of paclitaxel pharmacokinetics

PURPOSE

P-glycoprotein (P-gp; ABCB1) efficiently transports lipophilic amphipathic drugs, including the widely used anticancer drug paclitaxel (Taxol). We found previously that human multidrug resistance protein 2 (MRP2; ABCC2) also transports paclitaxel in vitro, and although we expected that paclitaxel pharmacokinetics would be dominated by P-gp, the effect of Mrp2 was tested in vivo.

EXPERIMENTAL DESIGN

We generated and characterized Mdr1a/1b/Mrp2(-/-) mice, allowing assessment of the distinct roles of Mrp2 and Mdr1a/1b P-gp in paclitaxel pharmacokinetics.

RESULTS

Surprisingly, the effect of Mrp2 on i.v. administration of paclitaxel was as great as that of P-gp. The area under plasma concentration-time curve (AUC)i.v. in both Mrp2(-/-) and Mdr1a/1b(-/-) mice was 1.3-fold higher than in wild-type mice, and in Mdr1a/1b/Mrp2(-/-) mice, a 1.7-fold increase was found. In spite of this similar effect, Mrp2 and P-gp had mostly complementary functions in paclitaxel elimination. Mrp2 dominated the hepatobiliary excretion, which was reduced by 80% in Mrp2(-/-) mice. In contrast, P-gp dominated the direct intestinal excretion, with a minor role for Mrp2. The AUCoral of paclitaxel was 8.5-fold increased by Mdr1a/1b deficiency but not affected by Mrp2 deficiency. However, in the absence of Mdr1a/1b P-gp, additional Mrp2 deficiency increased the AUCoral another 1.7-fold.

CONCLUSIONS

Thus far, Mrp2 was thought to mainly affect organic anionic drugs in vivo. Our data show that Mrp2 can also be a major determinant of the pharmacokinetic behavior of highly lipophilic anticancer drugs, even in the presence of other efficient transporters. Variation in MRP2 activity might thus directly affect the effective exposure to paclitaxel, on i.v. administration, but also on oral administration, especially when P-gp activity is inhibited.

Pharmacogenetic differences and drug-drug interactions in immunosuppressive therapy

With the advent of new immunosuppressants and formulations, the elucidation of molecular targets and the evolution of therapeutic drug monitoring, the field of organ transplantation has witnessed significant reductions in acute rejection rates, prolonged graft survival and improved patient outcome. Nonetheless, challenges persist in the use of immunosuppressive medications. Marked interindividual variability remains in drug concentrations and drug response. As medications with narrow therapeutic indices, variations in immunosuppressant concentrations can result in acute toxicity or transplant rejection. Recent studies have begun to identify factors that contribute to this variability with the promise of tailoring immunosuppressive regimens to the individual patient. These advances have uncovered differences in genetic composition in drug-metabolising enzymes, drug transporters and drug targets. This review focuses on commonly used maintenance immunosuppressants (including cyclosporin, mycophenolate mofetil, tacrolimus, sirolimus, everolimus, azathioprine and corticosteroids), examines current studies on pharmacogenetic differences in drug-metabolising enzymes, drug transporters and drug targets and addresses common drug-drug interactions with immunosuppressant therapies. The potential role of drug-metabolising enzymes in contributing to these drug-drug interactions is briefly considered.

Functional analysis of mouse and monkey multidrug resistance-associated protein 2 (Mrp2)

We investigated the intrinsic transport activity of mouse and monkey Mrp2 and compared it with that of rat and dog Mrp2 reported previously. Mrp2 cDNAs were isolated from BALB/c and Macaca fascicularis liver, respectively, and vesicle transport studies were performed using recombinant Mrp2s expressed in insect Sf9 cells. ATP-dependent transport of [3H]leukotriene C4 (LTC4), [3H]17beta-estradiol 17-(beta-D-glucuronide) (E217betaG), [3H]bromosulfophthalein (BSP), and [3H]cholecystokinin octapeptide (CCK-8) were readily detected for all Mrp2s. A species difference in the intrinsic transport activity was apparent for LTC4 (monkey > mouse, dog > rat) and BSP (rat, dog, monkey > mouse). In addition to the difference in the transport activity, complex kinetic profiles were also evident in CCK-8, where a cooperative transport site was observed. Moreover, the transport of [3H]E217betaG by mouse and monkey Mrp2 was quite different from that of rat and dog Mrp2 in that 1) there was practically only nonsaturable uptake for [3H]E217betaG and 2) 4-methylumbelliferon glucuronide (Mrp2 modulator) showed a concentration-dependent stimulatory effect on the transport of [3H]E217betaG in mouse and monkey Mrp2, whereas rat and dog transport activity was inhibited by the modulator. In conclusion, although the substrate specificity is similar, the intrinsic transport activity differs from one species to another. This is due not only to the difference in the Km and Vmax values, but also the qualitatively different mode of substrate and modulator recognition exhibited by different species.

Structure, function, expression, genomic organization, and single nucleotide polymorphisms of human ABCB1 (MDR1), ABCC (MRP), and ABCG2 (BCRP) efflux transporters

The ATP-binding cassette (ABC) transporters constitute a large family of membrane proteins, which transport a variety of compounds through the membrane against a concentration gradient at the cost of ATP hydrolysis. Substrates of the ABC transporters include lipids, bile acids, xenobiotics, and peptides for antigen presentation. As they transport exogenous and endogenous compounds, they reduce the body load of potentially harmful substances. One by-product of such protective function is that they also eliminate various useful drugs from the body, causing drug resistance. This review is a brief summary of the structure, function, and expression of the important drug resistance-conferring members belonging to three subfamilies of the human ABC family; these are ABCB1 (MDR1/P-glycoprotein of subfamily ABCB), subfamily ABCC (MRPs), and ABCG2 (BCRP of subfamily ABCG), which are expressed in various organs. In the text, the transporter symbol that carries the subfamily name (such as ABCB1, ABCC1, etc.) is used interchangeably with the corresponding original names, such as MDR1P-glycoprotein, MRP1, etc., respectively. Both nomenclatures are maintained in the text because both are still used in the transporter literature. This helps readers relate various names that they encounter in the literature. It now appears that P-glycoprotein, MRP1, MRP2, and BCRP can explain the phenomenon of multidrug resistance in all cell lines analyzed thus far. Also discussed are the gene structure, regulation of expression, and various polymorphisms in these genes. Because genetic polymorphism is thought to underlie interindividual differences, including their response to drugs and other xenobiotics, the importance of polymorphism in these genes is also discussed.

Transmembrane transport of endo- and xenobiotics by mammalian ATP-binding cassette multidrug resistance proteins

Multidrug Resistance Proteins (MRPs), together with the cystic fibrosis conductance regulator (CFTR/ABCC7) and the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) comprise the 13 members of the human "C" branch of the ATP binding cassette (ABC) superfamily. All C branch proteins share conserved structural features in their nucleotide binding domains (NBDs) that distinguish them from other ABC proteins. The MRPs can be further divided into two subfamilies "long" (MRP1, -2, -3, -6, and -7) and "short" (MRP4, -5, -8, -9, and -10). The short MRPs have a typical ABC transporter structure with two polytropic membrane spanning domains (MSDs) and two NBDs, while the long MRPs have an additional NH2-terminal MSD. In vitro, the MRPs can collectively confer resistance to natural product drugs and their conjugated metabolites, platinum compounds, folate antimetabolites, nucleoside and nucleotide analogs, arsenical and antimonial oxyanions, peptide-based agents, and, under certain circumstances, alkylating agents. The MRPs are also primary active transporters of other structurally diverse compounds, including glutathione, glucuronide, and sulfate conjugates of a large number of xeno- and endobiotics. In vivo, several MRPs are major contributors to the distribution and elimination of a wide range of both anticancer and non-anticancer drugs and metabolites. In this review, we describe what is known of the structure of the MRPs and the mechanisms by which they recognize and transport their diverse substrates. We also summarize knowledge of their possible physiological functions and evidence that they may be involved in the clinical drug resistance of various forms of cancer.

Pharmacogenetic characteristics of indinavir, zidovudine, and lamivudine therapy in HIV-infected adults: a pilot study

OBJECTIVE

The aim of the study was to investigate relationships among indinavir, lamivudine-triphosphate, and zidovudine-triphosphate pharmacokinetics and pharmacodynamics with polymorphisms in CYP3A5, MDR1, MRP2, MRP4, BCRP, and UGT1A1 genes.

STUDY DESIGN

Retrospective pilot investigation among 33 subjects who participated in a randomized pharmacological study of indinavir, lamivudine, and zidovudine. Subjects were defined as genetic variant carriers or not. Relationships were investigated with multivariable regression. Indinavir clearance was adjusted for African American race; triphosphates for sex; and HIV-response for study arm, drug exposure, and baseline HIV-RNA.

RESULTS

Genetically determined CYP3A5 expressors had 44% faster indinavir oral clearance versus nonexpressors (P = 0.002). MRP2-24C/T variant carriers had 24% faster indinavir oral clearance (P = 0.05). Lamivudine-triphosphate concentrations were elevated 20% in MRP4 T4131G variant carriers (P = 0.004). A trend for elevated zidovudine-triphosphates was observed in MRP4 G3724A variant carriers (P = 0.06). The log10 changes in HIV-RNA from baseline to week 52 were -3.7 for MDR1 2677 TT, -3.2 for GT, and -2.2 for GG (P < 0.05). Bilirubin increases were 2-fold higher in UGT1A1 [TA]7/[TA]7 genotypes. No relationships were identified with BCRP.

DISCUSSION

Novel relationships were identified among genetic variants in drug transporters and indinavir, lamivudine-triphosphate, and zidovudine-triphosphate concentrations. CYP3A5 expression was associated with faster indinavir oral clearance. These pilot data provide a scientific basis for more rational utilization of antiretroviral drugs.

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.

Function of multidrug resistance-associated protein 2 in acute hepatic failure rats

The function of multidrug resistance-associated protein 2 (Mrp2) in the intestine and liver, as well as intestinal Mrp2 expression, was analyzed in CCl(4)-induced acute hepatic failure rats with hyperbilirubinemia. The plasma level of bilirubin glucuronides, endogenous Mrp2-substrates, was 26 microM at 24 h after CCl(4) treatment. Mrp2 protein levels in jejunum decreased to 41% of control level. Mrp2-mediated efflux of 2,4-dinitrophenyl-S-glutathione (DNP-GSH), an Mrp2-substrate, in jejunum decreased to 31% of control in vitro, and was almost completely suppressed in vivo to the same level as that in the presence of probenecid, an Mrp2-inhibitor. Biliary excretion of DNP-GSH was suppressed to the same level as that inhibited by intravenous probenecid. The suppression of Mrp2 and the increased plasma bilirubin glucuronides recovered within 24 h thereafter. These results suggest that hyperbilirubinemia in disease states may be related to the systemic suppression of Mrp2 function.

Transporters and drug discovery: why, when, and how

Drug transporters are now increasingly recognized as important determinants of variable drug disposition and response. In addition, transporter associated problems appear to be occurring with greater frequency during the drug discovery and development process. What has not been clear is whether drug transporter related issues are a truly new problem, or whether such issues had existed all along, but were previously unrecognized or ignored. In this review, a brief overview of key drug transporters will be outlined. In addition, a commentary on specific issues of relevance to pharmaceutical sciences in terms of the role and relevance of drug transporters to the drug discovery and development process is provided.

Effect of culture conditions on the expression and function of Bsep, Mrp2, and Mdr1a/b in sandwich-cultured rat hepatocytes

Rat hepatocytes cultured in a sandwich configuration form functional canalicular networks. The influence of extracellular matrix configuration, medium composition, and confluency on the expression and function of Bsep, Mrp2, and Mdr1a/b in sandwich-cultured (SC) rat hepatocytes was examined. Primary rat hepatocytes were: (1) maintained in various extracellular matrix sandwich configurations, (2) cultured in Dulbecco's modified Eagle's medium (DMEM), Modified Chee's medium (MCM) or Williams' E medium (WME), and/or (3) plated at decreasing cell density. Bsep, Mrp2, and Mrdr1a/b expression in day 4 SC rat hepatocytes was assessed by Western blot; function was measured by accumulation of taurocholate, 5(and 6)-carboxy-2',7'-dichlorofluorescein, and rhodamine 123, respectively, in canalicular networks. In general, the extracellular matrix conditions examined resulted in similar protein expression and function. Function of Bsep, Mrp2, and Mdr1a/b was higher in SC rat hepatocytes maintained in DMEM or WME. Mrp2 and Mdr1a/b expression, representative of total cellular content, did not always correlate directly with function, which should be reflective of canalicular membrane expression. Mrp2 expression decreased significantly as cell density decreased in SC hepatocytes. Low plating density in Biocoat plates resulted in poor canalicular network formation and reduced function of Mrp2 and Mdr1a/b. Expression and/or function of Mrp2 and Mdr1a/b in rat hepatocytes cultured in a sandwich configuration may be influenced by plating density and media type.

Polymorphisms of MRP1 (ABCC1) and related ATP-dependent drug transporters

Genetic variations in drug metabolizing enzymes and targets are established determinants of adverse drug reactions and interactions, but less is known about the role of genetic polymorphisms in membrane transport proteins. MRP1 (ABCC1) is one of 13 polytopic membrane proteins that comprise the 'C' subfamily of the ATP-binding cassette (ABC) superfamily of transport proteins. MRP1 and related ABCC family members, including MRP2, 3, 4 and 5 (ABCC2, 3, 4 and 5), each have a distinctive pattern of tissue expression and substrate specificity. Together, these five transporters play important roles in the disposition and elimination of drugs and other organic anions, and in maintenance of blood-tissue barriers, as confirmed by enhanced chemosensitivity of respective knockout mice. Moreover, Mrp2 (Abcc2) deficient animals display mild conjugated hyperbilirubinemia, corresponding to a human condition known as Dubin-Johnson syndrome (DJS). Naturally occurring mutations in MRP/ABCC-related drug transporters have been reported, some of which are non-synonymous single nucleotide polymorphisms. The consequences of the resulting amino acid changes can sometimes be predicted from in vitro site-directed mutagenesis studies or from knowledge of mutations of analogous (conserved) residues in ABCC proteins that cause DJS, Pseudoxanthoma elasticum (ABCC6), cystic fibrosis (CFTR/ABCC7) or persistent hyperinsulinemic hypoglycemia of infancy (SUR1/ABCC8). Continual updating of databases of sequence variants and haplotype analysis, together with in vitro biochemical validation assays and pharmacological studies in knockout animals, should make it possible to determine how genetic variation in the MRP-related transporters contributes to the range of responses to drugs and chemicals observed in different human populations.

MRP2 and 3 in health and disease

MRPs are membrane proteins transporting organic anions at the expense of ATP hydrolysis. MRP2 is known to be a major transporter of organic anions from the liver into bile. We discuss recent results showing allosteric control of human but not rat MRP2. MRP3 has been considered a major player in bile salt metabolism, but our recent results with Mrp3 KO mice do not support this. Instead, we have found a role for MRP3 in the cellular export of drug-glucuronide conjugates. We discuss problems in extrapolating results obtained for murine MRPs.

Implications of genetic polymorphisms in drug transporters for pharmacotherapy

Drug transporters are increasingly recognized as a key determinant of drug disposition and response. It is now widely appreciated that expression of the ATP-dependent efflux transporter, MDR1 (ABCB1, P-glycoprotein), in organs such as the gastrointestinal tract, liver and kidney significantly alters the extent of drug absorption and excretion. Moreover, expression of MDR1 at the level of the blood-brain barrier limits the entry of many drugs into the central nervous system. Given such an important role of MDR1 in the drug disposition process, it is not surprising to see increasing focus on the role of single nucleotide polymorphisms (SNPs) in this transporter as a potential determinant of interindividual variability in drug disposition and pharmacological response. However, drug transport is often the result of the concerted action of efflux and uptake pumps located both in the basolateral and apical membranes of epithelial cells. A growing list of membrane-spanning proteins involved in the in- or outward transport of a large variety of drugs has been recognized and characterized over the past few years in almost all tissues, including organic anion and cation transporters (OAT, OCT, solute carrier family SLC22A), organic anion transport proteins (OATP, solute carrier family SLCO, formerly SLC21A), and MRPs (ABCCs), other members of the ATP-binding cassette family. We are just beginning to appreciate their role for drug delivery and disposition and the contribution of genetic polymorphisms in these transport proteins to interindividual variability in the efficacy and safety for pharmacotherapy. This review summarizes the consequences of inherited differences in drug transport for pharmacotherapy. With the main focus on ABCB1, an update of recent advances is given and clinically relevant examples are used to illustrate how heritable differential drug transport can help to explain individual variability in drug response. The pharmacogenetics of other transporters is briefly introduced.

OATP1B1, OATP1B3, AND MRP2 ARE INVOLVED IN HEPATOBILIARY TRANSPORT OF OLMESARTAN, A NOVEL ANGIOTENSIN II BLOCKER

Hepatic uptake and biliary excretion of olmesartan, a new angiotensin II blocker, were investigated in vitro using human hepatocytes, cells expressing uptake transporters and canalicular membrane vesicles, and in vivo using Eisai hyperbilirubinemic rats (EHBR), inherited multidrug resistance-associated protein (mrp2)-deficient rats. The uptake by human hepatocytes reached saturation with a Michaelis constant (K(m)) of 29.3 +/- 9.9 microM. Both Na(+)-dependent and Na(+)-independent uptake of olmesartan by human hepatocytes were observed. The uptake by Na(+)-independent human liver-specific organic anion transporters OATP1B1 and OATP1B3 expressed in Xenopus laevis oocytes was also saturable, with K(m) values of 42.6 +/- 28.6 and 71.8 +/- 21.6 microM, respectively. The Na(+)-dependent taurocholate-cotransporting polypeptide expressed in HEK 293 cells did not transport olmesartan. The cumulative biliary excretion in EHBR was one-sixth compared with that in Sprague-Dawley rats. ATP-dependent uptake of olmesartan was observed in both human canalicular membrane vesicles (hCMVs) and MRP2-expressing vesicles. An MRP inhibitor, MK-571 ([[[3-[2-(7-chloro-2-quinolinyl)ethenyl]phenyl][3-(dimethylamino)-3-oxopropyl]thio]methyl]thio]-propanoic acid) completely inhibited the uptake of olmesartan by hCMVs. In conclusion, the hepatic uptake and biliary excretion of olmesartan are mediated by transporters in humans. OATP1B1 and OATP1B3 are involved in hepatic uptake, at least in part, and MRP2 plays a dominant role in the biliary excretion.

Knocking down transport: applications of RNA interference in the study of drug transport proteins

RNA interference (RNAi) is a gene silencing process mediated by double-stranded RNA (dsRNA). The silencing process is comprised of an initiation step, in which small interfering RNA (siRNA) is introduced to the cell, and an effector step, which involves degrading mRNA molecules of the target gene. RNA interference has been observed in most organisms from plants to vertebrates. As a gene silencing approach, RNAi has proven to be extremely useful in characterizing gene function and developing new tools in cancer therapy and drug delivery. The development of RNAi-related technologies is an emerging area in biomedical research. In this review, recent progress in the application of RNAi to the study of transport proteins is summarized and evaluated; the advantages, disadvantages and future directions of RNAi technology are discussed.

Metabolism and transport of oxazaphosphorines and the clinical implications

The oxazaphosphorines including cyclophosphamide (CPA), ifosfamide (IFO), and trofosfamide represent an important group of therapeutic agents due to their substantial antitumor and immuno-modulating activity. CPA is widely used as an anticancer drug, an immunosuppressant, and for the mobilization of hematopoetic progenitor cells from the bone marrow into peripheral blood prior to bone marrow transplantation for aplastic anemia, leukemia, and other malignancies. New oxazaphosphorines derivatives have been developed in an attempt to improve selectivity and response with reduced toxicity. These derivatives include mafosfamide (NSC 345842), glufosfamide (D19575, beta-D-glucosylisophosphoramide mustard), NSC 612567 (aldophosphamide perhydrothiazine), and NSC 613060 (aldophosphamide thiazolidine). This review highlights the metabolism and transport of these oxazaphosphorines (mainly CPA and IFO, as these two oxazaphosphorine drugs are the most widely used alkylating agents) and the clinical implications. Both CPA and IFO are prodrugs that require activation by hepatic cytochrome P450 (CYP)-catalyzed 4-hydroxylation, yielding cytotoxic nitrogen mustards capable of reacting with DNA molecules to form crosslinks and lead to cell apoptosis and/or necrosis. Such prodrug activation can be enhanced within tumor cells by the CYP-based gene directed-enzyme prodrug therapy (GDEPT) approach. However, those newly synthesized oxazaphosphorine derivatives such as glufosfamide, NSC 612567 and NSC 613060, do not need hepatic activation. They are activated through other enzymatic and/or non-enzymatic pathways. For example, both NSC 612567 and NSC 613060 can be activated by plain phosphodiesterase (PDEs) in plasma and other tissues or by the high-affinity nuclear 3'-5' exonucleases associated with DNA polymerases, such as DNA polymerases and epsilon. The alternative CYP-catalyzed inactivation pathway by N-dechloroethylation generates the neurotoxic and nephrotoxic byproduct chloroacetaldehyde (CAA). Various aldehyde dehydrogenases (ALDHs) and glutathione S-transferases (GSTs) are involved in the detoxification of oxazaphosphorine metabolites. The metabolism of oxazaphosphorines is auto-inducible, with the activation of the orphan nuclear receptor pregnane X receptor (PXR) being the major mechanism. Oxazaphosphorine metabolism is affected by a number of factors associated with the drugs (e.g., dosage, route of administration, chirality, and drug combination) and patients (e.g., age, gender, renal and hepatic function). Several drug transporters, such as breast cancer resistance protein (BCRP), multidrug resistance associated proteins (MRP1, MRP2, and MRP4) are involved in the active uptake and efflux of parental oxazaphosphorines, their cytotoxic mustards and conjugates in hepatocytes and tumor cells. Oxazaphosphorine metabolism and transport have a major impact on pharmacokinetic variability, pharmacokinetic-pharmacodynamic relationship, toxicity, resistance, and drug interactions since the drug-metabolizing enzymes and drug transporters involved are key determinants of the pharmacokinetics and pharmacodynamics of oxazaphosphorines. A better understanding of the factors that affect the metabolism and transport of oxazaphosphorines is important for their optional use in cancer chemotherapy.

Characterization of mice lacking the multidrug resistance protein MRP2 (ABCC2)

The multidrug resistance protein Mrp2 is an ATP-binding cassette (ABC) transporter mainly expressed in liver, kidney, and intestine. One of the physiological roles of Mrp2 is to transport bilirubin glucuronides from the liver into the bile. Current in vivo models to study Mrp2 are the transporter-deficient and Eisai hyperbilirubinemic rat strains. Previous reports showed hyperbilirubinemia and induction of Mrp3 in the hepatocyte sinusoidal membrane in the mutant rats. In addition, differences in liver cytochrome P450 and UGT1a levels between wild-type and mutant rats were detected. To study whether these compensatory mechanisms were specific to rats, we characterized Mrp2(-/-) mice. Functional absence of Mrp2 in the knockout mice was demonstrated by showing increased levels of bilirubin and bilirubin glucuronides in serum and urine, a reduction in biliary excretion of bilirubin glucuronides and total glutathione, and a reduction in the biliary excretion of the Mrp2 substrate dibromosulfophthalein. To identify possible compensatory mechanisms in Mrp2(-/-) mice, the expression levels of 98 phase I, phase II, and transporter genes were compared in liver, kidney, and intestine of male and female Mrp2(-/-) and control mice. Unlike in Mrp2 mutant rats, no induction of Mrp3 in Mrp2(-/-) mice was detected. However, Mrp4 mRNA and protein in liver and kidney were increased approximately 6- and 2-fold, respectively. Phenotypic analysis of major cytochrome P450-mediated activities in liver microsomes did not show differences between wild-type and Mrp2(-/-) mice. In conclusion, Mrp2(-/-) mice are a new valuable tool to study the role of Mrp2 in drug disposition.

Expression and function of efflux drug transporters in the intestine

A variety of drug transporters expressed in the body control the fate of drugs by affecting absorption, distribution, and elimination processes. In the small intestine, transporters mediate the influx and efflux of endogenous or exogenous substances. In clinical pharmacotherapy, ATP-dependent efflux transporters (ATP-binding cassette [ABC] transporters) expressed on the apical membrane of the intestinal epithelial cells determine oral bioavailability, intestinal efflux clearance, and the site of drug-drug interaction of certain drugs. The expression and functional activity of efflux transporters exhibit marked interindividual variation and are relatively easily modulated by factors such as therapeutic drugs and daily foods and beverages. In this article, we will summarize the recent findings regarding the intestinal efflux transporters, especially P-glycoprotein (P-gp or human multidrug resistance gene [MDR] 1), multidrug resistance-associated protein 2 (MRP2), and breast cancer resistance protein (BCRP).

Single nucleotide polymorphisms in ABCC2 and ABCB1 genes and their clinical impact in physiology and drug response

Among the ABC proteins, some members including ABCB1, ABCC1, ABCC2 and ABCG2 are believed to contribute to multidrug resistance of cancer chemotherapy. In addition, the broad substrate-specificity and apical localization of the ABCB1 and ABCC2 in mucosal epithelium of intestine and hepatocyte give them a protective role against xenobiotics. The inter-individual variations in activity and expression levels of ABCB1 and ABCC2, thus, might affect on drug response and response to toxic substrates. In this review, I focus on (1) physiological and toxicological relevance of ABCB1 and ABCC2, and on (2) genetic variations of ABCB1 and ABCC2 genes and their association with biochemical function, expression level and tumor incidence.

Role of efflux pumps and metabolising enzymes in drug delivery

The impact of efflux pumps and metabolic enzymes on the therapeutic activity of various drugs has been well established. The presence of efflux pumps on various tissues and tumours has been shown to regulate the intracellular concentration needed to achieve therapeutic activity. The notable members of efflux proteins include P-glycoprotein, multi-drug resistance protein and breast cancer resistance protein. These efflux pumps play a pivotal role not only in extruding xenobiotics but also in maintaining the body's homeostasis by their ubiquitous presence and ability to coordinate among themselves. In this review, the role of efflux pumps in drug delivery and the importance of their tissue distribution is discussed in detail. To improve pharmacokinetic parameters of substrates, various strategies that modulate the activity of efflux proteins are also described. Drug metabolising enzymes mainly include the cytochrome P450 family of enzymes. Extensive drug metabolism due to the this family of enzymes is the leading cause of therapeutic inactivity. Therefore, the role of metabolising enzymes in drug delivery and disposition is extensively discussed in this review. The synergistic relationship between metabolising enzymes and efflux proteins is also described in detail. In summary, this review emphasises the urgent need to make changes in drug discovery and drug delivery as efflux pumps and metabolising enzymes play an important role in drug delivery and disposition.

Multiple drug resistance, antimutagenesis and anticarcinogenesis

Many cells are protected from excess levels of exogenous chemicals, including mutagens and carcinogens as well as pharmaceutical agents, by being actively extruded through the action of one or more of a series of ATP-binding cassette drug transporter proteins. Those known to be important in humans are the multidrug resistance proteins (P-glycoproteins, encoded by the mdr1 and 3 genes), multidrug-resistance-associated proteins (MRP1-7) and the breast cancer resistance protein (BCRP). These proteins have overlapping but distinct cellular locations and substrate specificities, and jointly govern the likelihood of penetration or distribution of a given mutagen or carcinogen into various tissues including the brain, testis, ovaries and fetus. Thus, they can affect the absorption, distribution and excretion of mutagens and carcinogens, as well as of their metabolites and conjugates, in most cases acting to prevent or reduce mutagenesis or carcinogenesis. However, because ABC transporters may limit the success of chemotherapy, there has been a considerable effort by the pharmaceutical industry to develop inhibitors of this transport process, and these are increasing in use. In general, the mutagenicity of many chemicals may be increased at the cellular levels by the action of these inhibitors, while the altered absorption characteristics favour greater uptake into the body. Thus, in many cases, such inhibitors may counter the antimutagenic and anticarcinogenic effect of the multidrug resistance mechanisms. There are exceptions, however. An increasing number of single nucleotide polymorphisms in multidrug resistance genes are being identified in humans, and may account for many of the significant differences in inter-individual susceptibility to exogenous and endogenous mutagenic and carcinogenic insults.

Pharmacogenetics for individualized cancer chemotherapy

The same doses of medication cause considerable heterogeneity in efficacy and toxicity across human populations. Genetic factors are thought to represent important determinants of drug efficacy and toxicity. Pharmacogenetics focuses on the prediction of the response of tumor and normal tissue to standard therapy by genetic profiling and, thereby, to select the most appropriate medication at optimal doses for each individual patient. In the present review, we discuss the relevance of single nucleotide polymorphisms (SNP) in genes, whose gene products act upstream of the actual drug target sites, that is, drug transporters and drug metabolizing phase I and II enzymes, or downstream of them, that is, apoptosis-regulating genes and chemokines. SNPs in relevant genes, which encode for proteins that interact with anticancer drugs, were also considered, that is, enzymes of DNA biosynthesis and metabolism, DNA repair enzymes, and proteins of the mitotic spindle. A significant body of evidence supports the concept of predicting drug efficacy and toxicity by SNP genotyping. As the efficacy of cancer chemotherapy, as well as the drug-related toxicity in normal tissues is multifactorial in nature, sophisticated approaches such as genome-wide linkage analyses and integrate drug pathway profiling may improve the predictive power compared with genotyping of single genes. The implementation of pharmacogenetics into clinical routine diagnostics including genotype-based recommendations for treatment decisions and risk assessment for practitioners represents a challenge for the future.

Influence of ABCB1, ABCC1, ABCC2, and ABCG2 haplotypes on the cellular exposure of nelfinavir in vivo

OBJECTIVES

The human immunodeficiency virus protease inhibitor nelfinavir is substrate of polyspecific drug transporters encoded by ABCB1 (P-glycoprotein), ABCC1 (MRP1) and ABCC2 (MRP2), and an inhibitor of BCRP, encoded by ABCG2. Genetic polymorphism in these genes may be associated with changes in transport function.

METHODS

A comprehensive evaluation of single nucleotide polymorphisms (39 SNPs in ABCB1, 7 in ABCC1, 27 in ABCC2, and 16 in ABCG2), and inferred haplotypes was done to assess possible associations of genetic variants with cellular exposure of nelfinavir in vivo. Analysis used peripheral mononuclear cells from individuals receiving nelfinavir (n=28). Key results were re-examined in a larger sample size (n=129) contributing data on plasma drug levels.

RESULTS AND CONCLUSIONS

There was no significant association between cellular nelfinavir area under the curve (AUC) and SNPs or haplotypes at ABCC1, ABCC2, ABCG2. There was an association with cellular exposure for two loci in strong linkage disequilibrium: ABCB1 3435C>T; AUCTT>AUCCT>AUCCC (ratio 2.1, 1.4, 1, Ptrend=0.01), and intron 26 +80T>C; AUCCC> AUCCT > AUCTT (ratio 2.4, 1.3, 1, Ptrend=0.006). Haplotypic analysis using tagging SNPs did not improve the single SNP association values.

Predicting drug response and toxicity based on gene polymorphisms

The sequencing of the human genome has allowed the identification of thousands of gene polymorphisms, most often single nucleotide polymorphims (SNP), which may play an important role in the expression level and activity of the corresponding proteins. When these polymorphisms occur at the level of drug metabolising enzymes or transporters, the disposition of the drug may be altered and, consequently, its efficacy may be compromised or its toxicity enhanced. Polymorphisms can also occur at the level of proteins directly involved in drug action, either when the protein is the target of the drug or when the protein is involved in the repair of drug-induced lesions. There again, these polymorphisms may lead to alterations in drug efficacy and/or toxicity. The identification of functional polymorphisms in patients undergoing chemotherapy may help the clinician prescribe the optimal drug combination or schedule and predict with more accuracy the response to these prescriptions. We have recorded in this review the polymorphisms that have been identified up till now in genes involved in anticancer drug activity. Some of them appear especially important in predicting drug toxicity and should be determined in routine before drug administration; this is the case of the most common variations of thiopurine methyltransferase for 6-mercaptopurine and of dihydropyrimidine dehydrogenase for fluorouracil. Other appear determinant for drug response, such as the common SNPs found in glutathione S-transferase P1 or xereoderma pigmentosum group D enzyme for the activity of oxaliplatin. However, confusion factors may exist between the role of gene polymorphisms in cancer risk or overall prognosis and their role in drug response.

Apical/Basolateral Surface Expression of Drug Transporters and its Role in Vectorial Drug Transport

It is well known that transporter proteins play a key role in governing drug absorption, distribution, and elimination in the body, and, accordingly, they are now considered as causes of drug-drug interactions and interindividual differences in pharmacokinetic profiles. Polarized tissues directly involved in drug disposition (intestine, kidney, and liver) and restricted distribution to naive sanctuaries (blood-tissue barriers) asymmetrically express a variety of drug transporters on the apical and basolateral sides, resulting in vectorial drug transport. For example, the organic anion transporting polypeptide (OATP) family on the sinusoidal (basolateral) membrane and multidrug resistance-associated protein 2 (MRP2/ABCC2) on the apical bile canalicular membrane of hepatocytes take up and excrete organic anionic compounds from blood to bile. Such vectorial transcellular transport is fundamentally attributable to the asymmetrical distribution of transporter molecules in polarized cells. Besides the apical/basolateral sorting direction, distribution of the transporter protein between the membrane surface (active site) and the intracellular fraction (inactive site) is of practical importance for the quantitative evaluation of drug transport processes. The most characterized drug transporter associated with this issue is MRP2 on the hepatocyte canalicular (apical) membrane, and it is linked to a genetic disease. Dubin-Johnson syndrome is sometimes caused by impaired canalicular surface expression of MRP2 by a single amino acid substitution. Moreover, single nucleotide polymorphisms in OATP-C/SLC21A6 (SLCO1B1) also affect membrane surface expression, and actually lead to the altered pharmacokinetic profile of pravastatin in healthy subjects. In this review article, the asymmetrical transporter distribution and altered surface expression in polarized tissues are discussed.

Factors affecting delivery of antiviral drugs to the brain

Although the CNS is in part protected from peripheral insults by the blood-brain barrier and the blood-cerebrospinal fluid barrier, a number of human viruses gain access to the brain, replicate within this organ, or sustain latent infection. The efficacy of antiviral drugs towards the cerebral viral load is often limited as both blood-brain interfaces impede their cerebral distribution. For polar compounds, the major factor restricting their entry lies in the tight junctions that occlude the paracellular pathway across these barriers. For compounds with more favourable lipid solubility properties, CNS penetration will be function of a number of physicochemical factors that include the degree of lipophilicity, size and ability to bind to protein or red blood cells, as well as other factors inherent to the vascular and choroidal systems, such as the local cerebral blood flow and the surface area available for exchange. In addition, influx and efflux transport systems, or metabolic processes active in both capillary endothelial cells and choroid plexus epithelial cells, can greatly change the bioavailability of a drug in one or several compartments of the CNS. The relative importance of these various factors with respect to the CNS delivery of the different classes of antiviral drugs is illustrated and discussed.

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.

Treatment of Hyperbilirubinemia in Eisai Hyperbilirubinemic Rat by Transfecting Human MRP2/ABCC2 Gene

PURPOSE

Multidrug resistance-associated protein 2 (MRP2/ABCC2) is predominantly expressed in the liver canalicular membrane and plays an important role in the biliary excretion of organic anions including glucuronide and glutathione conjugates. The purpose of this study is to construct a new evaluation system for human MRP2 by expressing human MRP2 in Eisai hyperbilirubinemic rat (EHBR) liver, the rat Mrp2 function of which is hereditarily defective.

METHODS

In order to express human MRP2 in liver, we used the Tet-off adenovirus expression system. After 72 h infection, we evaluated the protein expression and localization in the liver and the transport activity of [(3)H]E(2)17ssG and [(3)H]DNP-SG by preparing canalicular membrane vesicles (CMVs). We also evaluated the biliary excretion and plasma concentration of DBSP after bolus administration and the plasma concentration of endogenous direct and indirect bilirubin.

RESULTS

The localization of human MRP2 in EHBR liver was found to be at the bile canalicular membrane. Clear ATP-dependent uptake of [(3)H]E(2)17ssG and [(3)H]DNP-SG into CMVs was observed by using the CMVs prepared from the liver where human MRP2 was transfected. Furthermore, the blood to bile clearance of DBSP increased approximately 3-fold after expression of human MRP2. In addition, the plasma direct bilirubin level in EHBR was reduced by the expression of human MRP2.

CONCLUSIONS

These results suggest that this evaluation system for human MRP2 may be useful for evaluating the function of human MRP2.

Eisai hyperbilirubinemic rat (EHBR) as an animal model affording high drug-exposure in toxicity studies on organic anions

The Eisai hyperbilirubinemic rat (EHBR) should be a useful animal model for studies on the toxicity of organic anions which are substrates of multidrug resistance-associated protein 2 (Mrp2), since the systemic exposure to these compounds is expected to be increased in EHBR. In this study, we tested the value of EHBR for this purpose, using pravastatin (PV) and methotrexate (MTX) as model compounds. In the case of a single oral dose of PV (200 mg/kg), C(max) in plasma was 4.0-fold higher and AUC(0-infinity) was 3.6-fold larger than those of normal Sprague-Dawley rats (SDR), respectively. When multiple doses of PV were given to EHBR without co-administration of any other compound, drug-induced skeletal muscle toxicity (myopathy/rhabdomyolysis) and increased creatine phosphokinase (CPK) level were observed, whereas a control experiment using SDR did not show any toxic change. When a single dose of MTX (0.6 mg/kg) was given to EHBR orally, C(max) was 1.7-fold higher and AUC(0-infinity) was 1.6-fold larger than those of SDR, respectively. When multiple doses of MTX were given to EHBR, the changes in bone marrow, spleen and intestines were more severe than those in SDR. These findings support the view that EHBR would be a valuable animal model for toxicity studies on organic anion compounds which are substrates of Mrp2.

High plasma pravastatin concentrations are associated with single nucleotide polymorphisms and haplotypes of organic anion transporting polypeptide-C (OATP-C, SLCO1B1)

This study aimed to characterize possible relationships between polymorphisms in the drug transporter genes organic anion transporting polypeptide-C (OATP-C, SLCO1B1), OATP-B (SLCO2B1), multidrug resistance-associated protein 2 (MRP2, ABCC2) and multidrug resistance transporter (MDR1, ABCB1) and the pharmacokinetics of pravastatin. We studied 41 healthy Caucasian volunteers who had previously participated in pharmacokinetic studies with pravastatin. Six volunteers had a very high pravastatin AUC value and were defined as outliers according to statistical criteria. The OATP-C gene was sequenced completely in all subjects, and they were also genotyped for selected single nucleotide polymorphisms (SNP) in the OATP-B, MDR1 and MRP2 genes. Of the six outliers, five were heterozygous for the OATP-C 521T>C (Val174Ala) SNP (allele frequency 42%) and three were heterozygous for a new SNP in the promoter region of OATP-C (-11187G>A, allele frequency 25%). Among the remaining 35 subjects, two were homozygous and six were heterozygous carriers of the 521T>C SNP (allele frequency 14%, P = 0.0384 versus outliers) and three were heterozygous carriers of the -11187G>A SNP (allele frequency 4%, P = 0.0380 versus outliers). In subjects with the -11187GA or 521TC genotype, the mean pravastatin AUC0-12 was 98% (P = 0.0061) or 106% (P = 0.0034) higher, respectively, compared to subjects with the reference genotype. These results were substantiated by haplotype analysis. In heterozygous carriers of *15B (containing the 388A>G and 521T>C variants), the mean pravastatin AUC0-12 was 93% (P = 0.024) higher compared to non-carriers and, in heterozygous carriers of *17 (containing the -11187G>A, 388A>G and 521T>C variants), it was 130% (P = 0.0053) higher compared to non-carriers. No significant associations were found between OATP-B, MRP2 or MDR1 polymorphisms and the pharmacokinetics of pravastatin. These results suggest that haplotypes are more informative in predicting the OATP-C phenotype than single SNPs.

HMG-CoA reductase inhibitor pharmacogenomics: overview and implications for practice

HMG-CoA reductase inhibitors (statins) are widely prescribed and recommended as first-line therapy for most patients with hypercholesterolemia or established coronary heart disease. However, there is interpatient variability in lipid-lowering response to statins that is not explained by initial cholesterol levels and inadequate dosing alone. Genetic polymorphisms may contribute. This review discusses the potential contribution of polymorphisms in genes encoding proteins involved in drug metabolism and transport, cholesterol biosynthesis, lipid metabolism and others to lipid responses to statins.

Inborn errors of biliary canalicular transport systems

Cholestatic syndromes are inborn or acquired disorders of bile formation. In recent years, several inherited cholestatic syndromes were characterized at the molecular level: progressive familial intrahepatic cholestasis (PFIC) and benign recurrent intrahepatic cholestasis (BRIC). Both PFIC and BRIC were divided phenotypically in distinct subtypes; however, at the genotype level, these clinical entities overlap. PFIC starts in early childhood and progresses toward liver cirrhosis, which often requires liver transplantation within the first decade of life. The diagnosis of PFIC is usually made on the basis of clinical and laboratory findings but needs to be confirmed by genetic and histological analysis. Only recently was it recognized that BRIC, which was estimated as a milder form of PFIC-1, may be caused by more than one gene.

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.

Ethacrynic-acid-induced glutathione depletion and oxidative stress in normal and Mrp2-deficient rat liver

Oxidative stress in the liver is sometimes accompanied by cholestasis. We investigated the localization and role of multidrug-resistance-associated protein (Mrp) 2, a biliary transporter involved in bile-salt-independent bile flow, under ethacrynic acid (EA)-induced acute oxidative stress. Normal Sprague-Dawley rat (SDR) and Mrp2-deficient Eisai hyperbilirubinemic rat (EHBR) livers were perfused with 500 microM EA. The release of glutamic pyruvic transaminase (GPT) and thiobarbituric-acid-reactive substances (TBARS) from EHBR liver was markedly delayed compared with that from SDR liver. This is mainly due to the higher basal level of glutathione (GSH) in EHBR liver (59.1 +/- 0.3 nmol/mg protein) compared with SDR liver (39.7 +/- 1.5 nmol/mg protein). EA similarly induced a rapid reduction in GSH followed by mitochondrial permeability transition in the isolated mitochondria from both SDR and EHBR. Internalization of Mrp2 was detected before nonspecific disruption of the canalicular membrane and GPT release in SDR liver perfused with 100 microM EA. SDR liver preperfused with hyperosmolar buffer (405 mosmol/L) for 30 min induced internalization of Mrp2 without changing the basal GSH level, while elimination of hepatic GSH by 300 microM EA perfusion was significantly delayed thereafter. Concomitantly, hepatotoxicity assessed by the release of GPT and TBARS was also significantly attenuated under hyperosmolar conditions. In conclusion, preserved cytosolic and intramitochondrial GSH is the key factor involved in the acute hepatotoxicity induced by EA and its susceptibility could be altered by the presence of Mrp2.

FUNCTIONAL ANALYSIS OF DOG MULTIDRUG RESISTANCE-ASSOCIATED PROTEIN 2 (MRP2) IN COMPARISON WITH RAT MRP2

We investigated whether the species difference in the biliary excretion activity of some Mrp2 substrates was attributable to the intrinsic transport potential or the expression level of Mrp2, especially in rat and dog. Dog Mrp2 cDNA was isolated from beagle dog liver, and a vesicle transport study was performed using recombinant rat and dog Mrp2 expressed in insect Sf9 cells. The ATP-dependent transport of 17beta-estradiol 17-(beta-D-glucuronide) ([3H]E(2)17betaG) and leukotriene C4 ([3H]LTC4), normalized by the absolute protein expression level, was similar in both Mrp2s. The Mrp2 protein expression in dog liver was only 10% of that in rat liver and was comparable with the reported difference in the biliary excretion clearance of temocaprilat as Mrp2 substrate. In contrast to LTC4, unique transport kinetics for E(2)17betaG were evident in dog Mrp2. In addition to the high-affinity site with a K(m) value of 3.25 +/- 0.10 microM, which is similar to that in rat Mrp2 (4.81 +/- 1.21 microM), dog Mrp2 has an additional low-affinity site (>>75 microM), which makes a major contribution to the transport of E(2)17betaG (65% of the total transport capacity at tracer concentration). In summary, the difference in the biliary excretion activity of Mrp2 substrates between rat and dog depends on the Mrp2 protein expression level rather than the intrinsic transport activity of the transporter molecules. The unique transport properties of glucuronide conjugates by dog Mrp2 may lead to the species difference involving the drug-drug interaction or drug-induced hyperbilirubinemia on the bile canalicular membrane.

The ABC Transporters MDR1 and MRP2: Multiple Functions in Disposition of Xenobiotics and Drug Resistance

ATP-binding cassette (ABC) transporters comprise one of the largest membrane bound protein families. They are involved in transport of numerous compounds. These proteins transport substrates against a concentration gradient with ATP hydrolysis as a driving force across the membrane. Mammalian ABC proteins have important physiological, pharmacological and toxicological functions including the transport of lipids, bile salts, drugs, toxic and environmental agents. The efflux pumps serve both as natural defense mechanisms and influence the bioavailability and disposition of drugs. In general terms, the transporters remove xenobiotics from the cellular environment. For example, in cancer cells, over expression of these molecules may confer to multidrug resistance against cytostatic drugs. In addition, based on diverse structural characteristics and a broad substrate specifity, ABC transport proteins alter the intracellular concentration of a variety of therapeutically used compounds and toxicologically relevant agents. We review the function of the human multidrug resistance protein MDR1, (P-glycoprotein, ABCB1) and the multidrug resistance protein MRP2 (ABCC2). We focus on four topics namely 1) structure and physiological functions of these transporters, 2) substrates e.g., drugs, xenotoxins, and environmental toxicants including their conjugates, 3) drug-drug interactions, and the role of chemosensitizers which may be able to reverse drug resistance, and 4) pharmacologically and toxicologically relevant genetic polymorphisms in transport proteins and their clinical implications.

ATP-dependent transport of organic anions into isolated basolateral membrane vesicles from rat intestine

The mechanism for the cellular extrusion of organic anions across the intestinal basolateral membrane was examined using isolated membrane vesicles from rat jejunum, ileum, and colon. It was found that 17beta-estradiol 17beta-D-glucuronide (E217betaG) is taken up in an ATP-dependent manner into the basolateral membrane vesicles (BLMVs) but not into the brush-border or microsomal counterparts. The ATP-dependent uptake of E217betaG into BLMVs from jejunum and ileum was described by a single component with a Km value of 23.5 and 8.31 microM, respectively, whereas that into the BLMVs from colon was described by assuming the presence of high (Km=0.82 microM)- and low-affinity (Km=35.4 microM) components. Taurocholate, 6-hydroxy-5,7-dimethyl-2-methylamino-4-(3-pyridylmethyl) benzothiazole glucuronide and taurolithocholate sulfate, but not leukotriene C4, were significantly taken up by the BLMVs. In addition to such substrate specificity, the inhibitor sensitivity of the ATP-dependent transport in BLMVs was similar to that of rat multidrug resistance-associated protein 3 (Mrp3), which is located on the basolateral membrane of enterocytes. Together with the fact that the rank order of the extent of the expression of Mrp3 (jejunum < ileum << colon) is in parallel with that of the extent of the transport of ligands, these results suggest that the ATP-dependent uptake of organic anions into isolated intestinal BLMVs is at least partly mediated by Mrp3.

Interleukin-1beta represses MRP2 gene expression through inactivation of interferon regulatory factor 3 in HepG2 cells

The human multidrug resistance protein 2 (MRP2/ABCC2), expressed on the bile canalicular membrane, mediates the multispecific efflux of several organic anions, including conjugates of glucuronate, sulfate, and glutathione. Expression of MRP2 can be altered in response to environmental stimuli such as cholestasis and jaundice. We previously reported that MRP2 mRNA expression levels are decreased in the nontumorous part of hepatitis C virus-infected human liver tissues, and that inflammatory cytokines inhibit MRP2 expression in human hepatic (HepG2) cells. We investigated the molecular mechanisms by which inflammatory cytokines modulate MRP2 gene expression in hepatic cells. Treatment of human hepatic cells with interleukin-1beta (IL-1beta) or tumor necrosis factor alpha resulted in a decrease in the protein and mRNA levels of MRP2. IL-1beta inhibited the transcriptional activity of MRP2 promoter constructs by 40%, and this inhibition of MRP2 promoter activity was mediated through the interferon stimulatory response element (ISRE). Electrophoretic mobility shift assays with IL-1beta-treated nuclear extracts showed a decrease in the formation of DNA protein complexes, specifically those including interferon regulatory factor 3 (IRF3). Expression of recombinant human IRF3 increased MRP2 promoter activity. Treatment with a specific extracellular signal-regulated kinase inhibitor relieved IL-1beta-induced MRP2 mRNA downregulation and abrogated the binding of IRF3 to the ISRE element. In conclusion, IL-1beta induces downregulation of the MRP2 gene by inactivating IRF3 binding to ISRE on the MRP2 promoter in human hepatic cells; this inactivation is accomplished via interference with the extracellular signal-regulated kinase pathway.