Influence of omeprazole on multidrug resistance protein 3 expression in human liver

In silico and biological analyses of missense variants of the human biliary efflux transporter ABCC2: effects of novel rare missense variants

BACKGROUND AND PURPOSE

The ATP-dependent biliary efflux transporter ABCC2, also known as multidrug resistance protein 2 (MRP2), is essential for the cellular disposition and detoxification of various xenobiotics including drugs as well as endogenous metabolites. Common functionally relevant ABCC2 genetic variants significantly alter drug responses and contribute to side effects. The aim of this study was to determine functional consequences of rare variants identified in subjects with European ancestry using in silico tools and in vitro analyses.

EXPERIMENTAL APPROACH

Targeted next-generation sequencing of the ABCC2 gene was used to identify novel variants in European subjects (n = 143). Twenty-six in silico tools were used to predict functional consequences. For biological validation, transport assays were carried out with membrane vesicles prepared from cell lines overexpressing the newly identified ABCC2 variants and estradiol β-glucuronide and carboxydichlorofluorescein as the substrates.

KEY RESULTS

Three novel rare ABCC2 missense variants were identified (W227R, K402T, V489F). Twenty-five in silico tools predicted W227R as damaging and one as potentially damaging. Prediction of functional consequences was not possible for K402T and V489F and for the common linked variants V1188E/C1515Y. Characterisation in vitro showed increased function of W227R, V489F and V1188E/C1515Y for both substrates, whereas K402T function was only increased for carboxydichlorofluorescein.

CONCLUSION AND IMPLICATIONS

In silico tools were unable to accurately predict the substrate-dependent increase in function of ABCC2 missense variants. In vitro biological studies are required to accurately determine functional activity to avoid misleading consequences for drug therapy.

A Systematic Review of Gastric Acid-Reducing Agent-Mediated Drug-Drug Interactions with Orally Administered Medications

BACKGROUND AND OBJECTIVE

Several review articles have been published discussing gastric acid-related drug-drug interactions (DDIs) mediated by coadministration of antacids, histamine H₂ receptor antagonists, or proton pump inhibitors, but are not sufficiently comprehensive in capturing all documented DDIs with acid-reducing agents (ARAs) and tend to focus on gastric pH-dependent DDIs and/or basic drugs. Subsequently, several new drugs have been approved, and new information is available in the literature. The objective of this systematic review is to comprehensively identify oral medications that have clinically meaningful DDIs, including loss of efficacy or adverse effects, with gastric ARAs, and categorize these medications according to mechanism of interaction.

METHODS

An indepth search of clinical data in the PDR3D: Reed Tech Navigator™ for Drug Labels, University of Washington Drug-Drug Interaction Database, DailyMed, Drugs@FDA.gov, and UpToDate^®/Lexicomp^® Drug and Drug Interaction screening tool was conducted from 1 June to 1 August 2018. The PDR3D, University of Washington Drug-Drug Interaction Database, and DailyMed were searched with terms associated with gastric acid and ARAs. Conflicting findings were further investigated using the UpToDate^®/Lexicomp^® screening tool. Clinical relevance was assessed on whether an intervention was needed, and prescribing information and/or literature supporting the DDI.

RESULTS

Through the search strategy, 121 medications were found to clinically meaningfully interact with ARAs. For 38 medications the mechanism of interaction with ARAs was identified as gastric pH dependent, and for 83 medications the interaction was found to be not gastric pH mediated, with mechanisms involving metabolic enzymes, transporters, chelation, and urine alkalization. Additionally, 109 medications were studied and did not have a clinically meaningful interaction with ARAs.

CONCLUSION

This review may provide a resource to healthcare professionals in aiding the care of patients by increasing awareness of interactions with ARAs and may also identify and potentially aid in avoiding clinically relevant DDIs and preventing risk of treatment failure and/or adverse effects. Advances in non-clinical predictions of gastric pH-mediated DDIs may guide the need for a future clinical evaluation.

Prevalence of ABCC3-1767G/A polymorphism among patients with antiretroviral-associated hepatotoxicity

Background

Plasma concentrations of antiretrovirals (ARVs) regimens have considerably varied in individuals of human immunodeficiency virus (HIV) because of variations in the expression of drug‐metabolizing and transporter genes. Transporter genes play an important role in the disposition of drugs. Polymorphism in transporter gene (ABCC3) affects the MRP3 expression and varies the treatment outcome.

Method

We examined the polymorphism of ABCC3‐1767G/A gene in a total of 165 HIV patients (out of 165 HIV patients, 34 were with and 131 were without hepatotoxicity) and 156 healthy individuals using the polymerase chain reaction–restriction fragment length polymorphism method.

Results

In univariate analysis, we found a decreased prevalence of ABCC3 1767GA, 1767GA+AA genotypes, and 1767A allele in patients with hepatotoxicity as compared to patients without hepatotoxicity (23.5% vs. 28.2% and 23.5% vs. 30.53%; 11.76% vs. 16.41%), while a higher prevalence of 1767AA genotype was observed in HIV patients in comparison with healthy controls (2.3% vs. 1.3%, odds ratio [OR] = 1.71, 95% confidence interval [CI]: 0.23–15.03, p = .89). The frequency of ABCC3‐1767AA genotype was dispersed higher in individuals with early and advanced HIV disease stage in comparison with healthy controls (5.3% vs. 1.3%, OR = 4.73, p = .70; 8.9% vs. 1.3%, OR = 1.89, p = .91). A higher occurrence of ABCC3‐1767AA genotype was found in tobacco using HIV patients without hepatotoxicity compared with nonusers (4.7% vs. 1.1%, OR = 4.28, p = .52). The distribution of ABCC3‐1767GA genotype was higher in nevirapine receiving HIV patients irrespective of their hepatotoxicity status as compared to nonusers (30.4% vs. 9.1%, OR = 3.34, p = .22; 29.4% vs. 16.7%, OR = 1.69, p = .77). In multivariate analysis, HIV patients receiving nevirapine and with hepatotoxicity was found to have a significant risk for severity of hepatotoxicity (OR = 4.56, 95% CI: 1.60–12.99, p = .004).

Conclusion

ABCC3 1767G/A polymorphism was not significantly associated with susceptibility to ARV‐associated hepatotoxicity, although ABCC3 1767AA genotype designated a risk for acquisition of hepatotoxicity and advancement of the disease. Nevirapine usage emerged as an independent risk factor for hepatotoxicity severity.

Modulation of Hepatic MRP3/ABCC3 by Xenobiotics and Pathophysiological Conditions: Role in Drug Pharmacokinetics

Liver transporters play an important role in the pharmacokinetics and disposition of pharmaceuticals, environmental contaminants, and endogenous compounds. Among them, the family of ATP-Binding Cassette (ABC) transporters is the most important due to its role in the transport of endo- and xenobiotics. The ABCC sub-family is the largest one, consisting of 13 members that include the cystic fibrosis conductance regulator (CFTR/ABCC7); the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) and the multidrug resistanceassociated proteins (MRPs). The MRP-related proteins can collectively confer resistance to natural, synthetic drugs and their conjugated metabolites, including platinum-containing compounds, folate anti-metabolites, nucleoside and nucleotide analogs, among others. MRPs can be also catalogued into "long" (MRP1/ABCC1, -2/C2, -3/C3, -6/C6, and -7/C10) and "short" (MRP4/C4, -5/C5, -8/C11, -9/C12, and -10/C13) categories. While MRP2/ABCC2 is expressed in the canalicular pole of hepatocytes, all others are located in the basolateral membrane. In this review, we summarize information from studies examining the changes in expression and regulation of the basolateral hepatic transporter MPR3/ABCC3 by xenobiotics and during various pathophysiological conditions. We also focus, primarily, on the consequences of such changes in the pharmacokinetic, pharmacodynamic and/or toxicity of different drugs of clinical use transported by MRP3.

Population Scale Retrospective Analysis Reveals Potential Risk of Cholestasis in Pregnant Women Taking Omeprazole, Lansoprazole, and Amoxicillin

In nearly 50% of patients with drug-induced liver injury, the bile flow is impaired known as cholestasis. Intrahepatic cholestasis of pregnancy (ICP) is the most common liver disease that happens in pregnancy. Some of the clinical symptoms include pruritus, dark urine, and abnormal liver function tests. A rise of serum bile acids is the most accurate diagnostic evidence. ICP may lead to premature birth, fetal distress, and even postpartum hemorrhage or stillbirth in some severe cases. Higher bile acid levels (> 40 μmol/L) are associated with higher rates of adverse fetal outcomes. Due to the multifactorial nature of ICP, its etiology is still not fully understood. Therefore, the current treatments of ICP are limited to control symptoms and protect fetuses. Among various causing factors, drug exposure during pregnancy is one common factor, and it can be prevented if we know drugs with increasing risk of cholestasis. Here we analyzed over 9.5 million FDA adverse effect reports to identify drugs with increasing risks of cholestasis as an adverse effect. Patients treated for cholestasis or liver diseases were removed. The odds ratio analysis reveals that lansoprazole (LSPZ), omeprazole (OMPZ) and amoxicillin (AMXC) are associated with an increased risk of cholestasis. LSPZ is associated with increased reported cholestasis by a factor of 2.32 (OR with 95% confidence interval [2.21, 2.43]). OMPZ is associated with increased reported cholestasis by a factor of 2.61 [2.54, 2.69]. AMXC is associated with increased reported cholestasis adverse effect by a factor of 6.79 [6.49, 7.11]. The risk of cholestasis associated with these three drugs is further increased in pregnant women. These findings justify careful reassessment of the safety of the three identified drugs.

Pharmacokinetic Interaction Study Between Saxagliptin and Omeprazole, Famotidine, or Magnesium and Aluminum Hydroxides Plus Simethicone in Healthy Subjects: An Open-Label Randomized Crossover Study

Saxagliptin is an orally administered, highly potent, and selective dipeptidyl peptidase-4 inhibitor for the management of type 2 diabetes mellitus. This study was conducted to determine the effect of magnesium and aluminum hydroxides plus simethicone, famotidine, and omeprazole on the pharmacokinetics of saxagliptin and its active metabolite, 5-hydroxy saxagliptin. This was an open-label, randomized, 5-treatment, 5-period, 3-way crossover study in 15 healthy subjects. Mean C_max of saxagliptin was 26% lower, but AUC was almost unchanged when saxagliptin was coadministered with Maalox Max. Mean C_max was 14% higher, but AUC was almost unchanged when saxagliptin was coadministered with famotidine. Changes in pharmacokinetics of 5-hydroxy saxagliptin generally paralleled the changes in saxagliptin. These pharmacokinetic changes were unlikely to be clinically meaningful. Coadministration of omeprazole did not affect saxagliptin C_max or AUC. Saxagliptin in combination with these medicines resulted in no unexpected safety or tolerability findings in these healthy subjects. No dose adjustment of saxagliptin or separation in the time of saxagliptin dosing is necessary with medicines that raise gastric pH when coadministered with saxagliptin.

Boosted coupling of ATP hydrolysis to substrate transport upon cooperative estradiol-17-β-D-glucuronide binding in a Drosophila ATP binding cassette type-C transporter

ATP binding cassette type-C (ABCC) transporters move molecules across cell membranes upon hydrolysis of ATP; however, their coupling of ATP hydrolysis to substrate transport remains elusive. Drosophila multidrug resistance-associated protein (DMRP) is the functional ortholog of human long ABCC transporters, with similar substrate and inhibitor specificity, but higher activity. Exploiting its high activity, we kinetically dissected the catalytic mechanism of DMRP by using E₂-d-glucuronide (E₂G), the physiologic substrate of human ABCC. We examined the DMRP-mediated interdependence of ATP and E₂G in biochemical assays. We observed E₂G-dependent ATPase activity to be biphasic at subsaturating ATP concentrations, which implies at least 2 E₂G binding sites on DMRP. Furthermore, transport measurements indicated strong nonreciprocal cooperativity between ATP and E₂G. In addition to confirming these findings, our kinetic modeling with the Complex Pathway Simulator indicated a 10-fold decrease in the E₂G-mediated activation of ATP hydrolysis upon saturation of the second E₂G binding site. Surprisingly, the binding of the second E₂G allowed for substrate transport with a constant rate, which tightly coupled ATP hydrolysis to transport. In summary, we show that the second E₂G binding-similar to human ABCC2-allosterically stimulates transport activity of DMRP. Our data suggest that this is achieved by a significant increase in the coupling of ATP hydrolysis to transport.-Karasik, A., Ledwitch, K. V., Arányi, T., Váradi, A., Roberts, A., Szeri, F. Boosted coupling of ATP hydrolysis to substrate transport upon cooperative estradiol-17-β-D-glucuronide binding in a Drosophila ATP binding cassette type-C transporter.

Activation of p38 Mitogen-Activated Protein Kinase by Clotrimazole Induces Multidrug Resistance-Associated Protein 3 Activation through a Novel Transcriptional Element

Multidrug resistance-associated protein 3 (MRP3) is a basolaterally localized transporter in the liver and contributes to the transport of various metabolites such as conjugates of endogenous compounds and drugs from hepatocytes. MRP3 expression in the human liver is low under normal physiologic conditions but is induced by drug treatment. Although several studies have identified a region necessary for the basal transcription of MRP3, no region that responds to drugs has been reported. To identify the xenobiotic-responsive elements of MRP3, we constructed a luciferase reporter plasmid containing the MRP3 5'-flanking region up to -10 kb upstream from the transcription start site. Among typical nuclear receptor ligands, clotrimazole dramatically enhanced MRP3 reporter activity in HepG2 cells, whereas rifampicin had no effect. We then conducted MRP3 reporter assays with deletion or mutation constructs to identify a clotrimazole-responsive element. The element was located approximately -6.8 kb upstream from the MRP3 transcription start site. Overexpression of the pregnane X receptor did not enhance clotrimazole-mediated transcription. We found that clotrimazole was toxic to HepG2 cells and we therefore investigated whether mitogen-activated protein kinase (MAPK) activation is involved in the transactivation of MRP3 by clotrimazole. p38 MAPK inhibitor SB203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole] suppressed MRP3 mRNA expression induced by clotrimazole, whereas c-Jun N-terminal kinase inhibitor SP600125 (1,9-pyrazoloanthrone) and extracellular signal-regulated kinase inhibitor PD98059 [2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one] did not. Phosphorylated p38 MAPK was detected in HepG2 cells treated with clotrimazole. These results suggest that activation of the p38 MAPK pathway induces the transcriptional activation of MRP3.

The molecular mechanism underlying the induction of hepatic MRP3 expression and function by omeprazole

Previous work has indicated that there is increased protein expression of multidrug resistance-associated protein 3 (MRP3) in the liver samples of patients treated with omeprazole compared with those who were not. However, evidence is still lacking to show the mechanisms underlying that induction. This study aimed to assess changes in the fold-induction of MRP3 mRNA and protein expression over controls in omeprazole-treated HepG2 cells after transient transfection of human MRP3 siRNA, or after pretreatment with actinomycin D (Act-D). Furthermore, MRP3 siRNA knock-down or MRP-specific inhibition (indomethacin) was used to determine whether the MRP3 protein induced by omeprazole possessed an enhanced efflux transport. The results demonstrated that omeprazole induced MRP3 mRNA and protein expression in a concentration- and time-dependent manner. Moreover, that induction was almost completely abolished by the addition of human MRP3 siRNA and also by pretreatment with Act-D, respectively. In addition, the decay rate of MRP3 mRNA in vehicle- and omeprazole-treated cells was similar in the presence of Act-D, suggesting transcriptional up-regulation of MRP3 mRNA expression by omeprazole. Most importantly, omeprazole induced MRP3 efflux transport activity, as measured by the 5-carboxyfluorescein assay in the absence and presence of human MRP3 siRNA or indomethacin. It is concluded that omeprazole can induce MRP3 mRNA and protein expression and enhance MRP3 efflux transport activity through transcriptional up-regulation, and that omeprazole can also induce other MRP transporters.

Mathematical modeling of the in vitro hepatic disposition of mycophenolic acid and its glucuronide in sandwich-cultured human hepatocytes

In recent years, it has become increasingly important to test the safety of circulating metabolites of novel drugs as part of drug discovery and development programs. Accordingly, it is essential to develop suitable methods for identifying the major metabolites and their disposition in animal species and in humans. Mycophenolic acid (MPA), a selective inosine-5'-monophosphate dehydrogenase (IMPDH) inhibitor, is metabolized by glucuronidation and enterohepatic circulation of MPA-glucuronides is an important factor in the continuous systemic exposure of MPA. In humans, about 90% of the administered MPA dose is finally excreted as MPA phenyl-glucuronide (MPAG) in urine. Notably, the plasma concentration of MPAG is much higher than that of MPA. These factors suggest that, after its formation in hepatocytes, MPAG is excreted into bile and is also transported across the basolateral membrane to enter the circulation. In the present study, we performed metabolic/hepatobiliary transport studies of MPA and MPAG using sandwich-cultured human hepatocytes (SCHH) and constructed mathematical models of their hepatic disposition. We also performed vesicular transport studies to identify which human multidrug resistance-associated proteins (MRPs) are involved in the transport of MPAG from hepatocytes. MPAG was a preferred substrate for the biliary excretion transporter MRP2 and the hepatic basolateral transporters MRP3 and MRP4 in conventional and metabolic/hepatobiliary transport studies using SCHH and vesicular transport studies using human MRP-expressing membrane vesicles. The resulting mathematical model suggested that the basolateral transport plays an important role in the hepatic disposition of MPAG formed in hepatocytes. Our findings suggest that mathematical modeling of metabolic/hepatobiliary transport studies using SCH will provide useful information for determining the fate of metabolites formed in hepatocytes.

A perspective on efflux transport proteins in the liver

Detailed knowledge regarding the influence of hepatic transport proteins on drug disposition has advanced at a rapid pace over the past decade. Efflux transport proteins located in the basolateral and apical (canalicular) membranes of hepatocytes play an important role in the hepatic elimination of many endogenous and exogenous compounds, including drugs and metabolites. This review focuses on the role of these efflux transporters in hepatic drug excretion. The impact of these proteins as underlying factors for disease is highlighted, and the importance of hepatic efflux proteins in the efficacy and toxicity of drugs is discussed. In addition, a brief overview of methodology to evaluate the function of hepatic efflux transport proteins is provided. Current challenges in predicting the impact of altered efflux protein function on systemic, intestinal, and hepatocyte exposure to drugs and metabolites are highlighted.

Suspected methotrexate toxicity from omeprazole: a case review of carboxypeptidase G2 use in a methotrexate-experienced patient with methotrexate toxicity and a review of the literature

We report a case of methotrexate toxicity potentially induced by a drug interaction between methotrexate and omeprazole in a 25-year-old man with osteosarcoma. The patient was placed on omeprazole after his first cycle of high-dose methotrexate for stress ulcer prophylaxis, and it was discontinued before the start of the first day of the patient's second round of high-dose methotrexate. The 24-hour methotrexate level was elevated and he continued to have sustained levels for 18 days. Side effects due to elevated serum methotrexate included seizures, mucositis, acute renal failure, and thrombocytopenia. Aggressive hydration, urinary alkalinization, and leucovorin were continued during the period of elevated methotrexate levels, with the patient receiving a course of hemodialysis and a dose of carboxypeptidase G2. The patient's symptoms resolved, and his renal function returned to baseline within 2 months. The patient was able to receive future courses of chemotherapy without methotrexate. Although use of the Naranjo adverse reaction probability scale indicated a probable relationship (score of 6) between the patient's development of methotrexate toxicity and omeprazole use, we believe this was a drug-drug interaction case consistent with previous reports in the literature.

Multidrug resistance associated proteins in multidrug resistance

Multidrug resistance proteins (MRPs) are members of the C family of a group of proteins named ATP-binding cassette (ABC) transporters. These ABC transporters together form the largest branch of proteins within the human body. The MRP family comprises of 13 members, of which MRP1 to MRP9 are the major transporters indicated to cause multidrug resistance in tumor cells by extruding anticancer drugs out of the cell. They are mainly lipophilic anionic transporters and are reported to transport free or conjugates of glutathione (GSH), glucuronate, or sulphate. In addition, MRP1 to MRP3 can transport neutral organic drugs in free form in the presence of free GSH. Collectively, MRPs can transport drugs that differ structurally and mechanistically, including natural anticancer drugs, nucleoside analogs, antimetabolites, and tyrosine kinase inhibitors. Many of these MRPs transport physiologically important anions such as leukotriene C4, bilirubin glucuronide, and cyclic nucleotides. This review focuses mainly on the physiological functions, cellular resistance characteristics, and probable in vivo role of MRP1 to MRP9.

Suspicion of drug-drug interaction between high-dose methotrexate and proton pump inhibitors: a case report - should the practice be changed?

We report a case of a potential drug-drug interaction in a woman treated by a first injection of high-dose methotrexate for a T-lymphoblastic lymphoma. Valaciclovir, fluoxetine and pantoprazole were given concomitantly. A methotrexate overdosage was shown at 36 h after infusion associated with a severe renal failure. Alkaline hyperhydration, folinic acid and carboxypeptidase G2 were given. Prescription analyses by pharmacists and literature research have permitted us to suggest that a drug-drug interaction between methotrexate and proton pump inhibitors (PPI) was responsible for this renal failure. Several mechanisms of interaction were suggested and might be related to the inhibition of renal methotrexate transporters by PPI, an increase in the methotrexate efflux to the blood by an upregulation of multidrug resistance protein 3 by PPI or genetic polymorphisms. This case shows that pharmacists can help physicians to optimize patient treatment: they consensually decided on the systematic discontinuation of PPI or a switch to ranitidine when patients were treated by high-dose methotrexate.

Regulation of hepatic ABCC transporters by xenobiotics and in disease states

The subfamily of ABCC transporters consists of 13 members in mammals, including the multidrug resistance-associated proteins (MRPs), sulfonylurea receptors (SURs), and the cystic fibrosis transmembrane conductance regulator (CFTR). These proteins play roles in chemical detoxification, disposition, and normal cell physiology. ABCC transporters are expressed differentially in the liver and are regulated at the transcription and translation level. Their expression and function are also controlled by post-translational modification and membrane-trafficking events. These processes are tightly regulated. Information about alterations in the expression of hepatobiliary ABCC transporters could provide important insights into the pathogenesis of diseases and disposition of xenobiotics. In this review, we describe the regulation of hepatic ABCC transporters in humans and rodents by a variety of xenobiotics, under disease states and in genetically modified animal models deficient in transcription factors, transporters, and cell-signaling molecules.

6-mercaptopurine and 9-(2-phosphonyl-methoxyethyl) adenine (PMEA) transport altered by two missense mutations in the drug transporter gene ABCC4

Multiple drug resistance protein 4 (MRP4, ABCC4) belongs to the C subfamily of the ATP-binding cassette (ABC) transporter superfamily and participates in the transport of diverse antiviral and chemotherapeutic agents such as 6-mercaptopurine (6-MP) and 9-(2-phosphonyl methoxyethyl) adenine (PMEA). We have undertaken a comprehensive functional characterization of protein variants of MRP4 found in Caucasians and other ethnicities. A total of 11 MRP4 missense genetic variants (nonsynonymous SNPs), fused to green fluorescent protein (GFP), were examined in Xenopus laevis oocytes for their effect on expression, localization, and function of the transporter. Radiolabeled 6-MP and PMEA were chosen as transport substrates. All MRP4 protein variants were found to be expressed predominantly in the oocyte membrane. A total of four variants (Y556C, E757 K, V776I, and T1142 M) exhibited a 20% to 40% reduced expression level compared to the wild type. Efflux studies showed that 6-MP is transported by MRP4 in unmodified form. Compared to wild-type MRP4, the transmembrane variant V776I, revealed a significant lower activity in 6-MP transport, while the amino acid exchange Y556C in the Walker(B) motif displayed significantly higher transport of PMEA. The transport properties of the other variants were comparable to wild-type MRP4. Our study shows that Xenopus oocytes are well suited to characterize MRP4 and its protein variants. Carriers of the rare MRP4 variants Y556C and V776I may have altered disposition of MRP4 substrates.

The tangle of nuclear receptors that controls xenobiotic metabolism and transport: crosstalk and consequences

The expression of many genes involved in xenobiotic/drug metabolism and transport is regulated by at least three nuclear receptors or xenosensors: aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR). These receptors establish crosstalk with other nuclear receptors or transcription factors controlling signaling pathways that regulate the homeostasis of bile acids, lipids, glucose, inflammation, vitamins, hormones, and others. These crosstalks are expected to modify profoundly our vision of xenobiotic/drug disposition and toxicity. They provide molecular mechanisms to explain how physiopathological stimuli affect xenobiotic/drug disposition, and how xenobiotics/drugs may affect physiological functions and generate toxic responses. In addition, the possibility that xenosensors may control other signaling pathways opens the way to new pharmacological opportunities.

Nuclear receptors RXRalpha:RARalpha are repressors for human MRP3 expression

Multidrug resistance-associated protein MRP3/Mrp3 (ABCC3) is upregulated in cholestasis, an adaptive response that may protect the liver from accumulation of toxic compounds, such as bile salts and bilirubin conjugates. However, the mechanism of this upregulation is poorly understood. We and others have previously reported that fetoprotein transcription factor/liver receptor homolog-1 is an activator of MRP3/Mrp3 expression. In searching for additional regulatory elements in the human MRP3 promoter, we have now identified nuclear receptor retinoic X receptor-alpha:retinoic acid receptor-alpha (RXRalpha:RARalpha) as a repressor of MRP3 activation by transcription factor Sp1. A luciferase reporter assay demonstrated that cotransfection of transcription factor Sp1 stimulates the MRP3 promoter activity and that additions of RXRalpha:RARalpha abrogated this activation in a dose-dependent manner. Site mutations and gel shift assays have identified a Sp1 binding GC box motif at -113 to -108 nts upstream from the MRP3 translation start site, where RXRalpha:RARalpha specifically reduced Sp1 binding to this site. Mutation of the GC box also reduced MRP3 promoter activity. The functional role of RXRalpha:RARalpha as a repressor of MRP3 expression was further confirmed by RARalpha small-interfering RNA knockdown in HepG2 cells, which upregulated endogenous MRP3 expression. In summary, our results indicate that activator Sp1 and repressor RXRalpha:RARalpha act in concert to regulate MRP3 expression. Since RXRalpha:RARalpha expression is diminished by cholestatic liver injury, loss of RXRalpha:RARalpha may lead to upregulation of MRP3/Mrp3 expression in these disorders.

Variability in human hepatic MRP4 expression: influence of cholestasis and genotype

The multidrug resistance protein 4 (MRP4) is an efflux transporter involved in the transport of endogenous substrates and xenobiotics. We measured MRP4 mRNA and protein expression in human livers and found a 38- and 45-fold variability, respectively. We sequenced 2 kb of the 5'-flanking region, all exons and intron/exon boundaries of the MRP4 gene in 95 patients and identified 74 genetic variants including 10 non-synonymous variations, seven of them being located in highly conserved regions. None of the detected polymorphisms was significantly associated with changes in the MRP4 mRNA or protein expression. Immunofluorescence microscopy indicated that none of the non-synonymous variations affected the cellular localization of MRP4. However, in cholestatic patients the MRP4 mRNA and protein expression both were significantly upregulated compared to non-cholestatic livers (protein: 299+/-138 vs 100+/-60a.u., P<0.001). Taken together, human hepatic MRP4 expression is highly variable. Genetic variations were not sufficient to explain this variability. In contrast, cholestasis is one major determinant of human hepatic MRP4 expression.

Expression profiles of 50 xenobiotic transporter genes in humans and pre-clinical species: a resource for investigations into drug disposition

Carrier-mediated transporters play a critical role in xenobiotic disposition and transporter research is complicated by species differences and their selective tissue expression. The purpose of this study was to generate a comprehensive data set of xenobiotic transporter gene expression profiles in humans and the pre-clinical species mouse, rat, beagle dog and cynomolgus monkey. mRNA expression profiles of 50 genes from the ABC, SLC and SLCO transporter superfamilies were examined in 40 human tissues by microarray analyses. Transporter genes that were identified as enriched in the liver or kidney, or that were selected for their known roles in xenobiotic disposition, were then compared in 22 tissues across the five species. Finally, as clinical variability in drug response and adverse reactions may be the result of variability in transporter gene expression, variability in the expression of selected transporter genes in 75 human liver donors were examined and compared with the highly variable drug metabolizing enzyme CYP3A4.

Genetic Variations of the ABC Transporter Gene ABCC3 in a Japanese Population

An ATP-binding cassette transporter, multidrug resistance-related protein 3 (MRP3), is encoded by the ABCC3 gene. The MRP3 protein is expressed in several tissues, and functions as an efflux transporter for conjugated as well as unconjugated substrates. In this study, the 31 ABCC3 exons and their flanking introns were comprehensively screened for genetic variations in 89 Japanese subjects. Forty-six genetic variations, including 21 novel ones, were found: 8 were located in the 5'-flanking region, 14 in the coding exons (8 synonymous and 6 nonsynonymous variations), and 24 in the introns. Of these 46 variations, five novel nonsynonymous variations, 2221C>T (Gln741Stop), 2395G>A (Val799Met), 2798_2799delAG (Gln933ArgfsX64), 3657C>A (Ser1219Arg), and 4217C>T (Thr1406Met), were found as heterozygous variations. The allele frequencies were 0.011 for Ser1219Arg and 0.006 for the other four variations. Gln741Stop induces a stop codon at codon 741. Gln933ArgfsX64 causes a frame-shift at codon 933, resulting in early termination at codon 997. Both variations result in loss of 6 transmembrane helices (from the 12th to 17th helices) in the C-terminus and all regions of nucleotide binding domain 2. Thus, both variant proteins are assumed to be inactive. These data provide fundamental and useful information for pharmacogenetic studies on MRP3-transported drugs in Japanese.

Principles of hepatic organic anion transporter regulation during cholestasis, inflammation and liver regeneration

Hepatic uptake and biliary excretion of organic anions (e.g., bile acids and bilirubin) is mediated by hepatobiliary transport systems. Defects in transporter expression and function can cause or maintain cholestasis and jaundice. Recruitment of alternative export transporters in coordination with phase I and II detoxifying pathways provides alternative pathways to counteract accumulation of potentially toxic biliary constituents in cholestasis. The genes encoding for organic anion uptake (NTCP, OATPs), canalicular export (BSEP, MRP2) and alternative basolateral export (MRP3, MRP4) in liver are regulated by a complex interacting network of hepatocyte nuclear factors (HNF1, 3, 4) and nuclear (orphan) receptors (e.g., FXR, PXR, CAR, RAR, LRH-1, SHP, GR). Bile acids, proinflammatory cytokines, hormones and drugs mediate causative and adaptive transporter changes at a transcriptional level by interacting with these nuclear factors and receptors. Unraveling the underlying regulatory mechanisms may therefore not only allow a better understanding of the molecular pathophysiology of cholestatic liver diseases but should also identify potential pharmacological strategies targeting these regulatory networks. This review is focused on general principles of transcriptional basolateral and canalicular transporter regulation in inflammation-induced cholestasis, ethinylestradiol- and pregnancy-associated cholestasis, obstructive cholestasis and liver regeneration. Moreover, the potential therapeutic role of nuclear receptor agonists for the management of liver diseases is highlighted.

Multidrug resistance-associated proteins 3, 4, and 5

We summarize in this paper the recently published results on multidrug resistance-associated proteins 3, 4, and 5 (MRPs 3-5). MRP3 can transport organic compounds conjugated to glutathione, sulfate, or glucuronate, such as estradiol-17beta-glucuronide, bilirubin-glucuronides, and etoposide-glucuronide, and also bile salts and methotrexate. Studies in knockout mice have shown that Mrp3 contributes to the transport of morphine-3-glucuronide and acetaminophen-glucuronide from the liver into blood. There is no evidence for a major role of MRP3 in bile salt metabolism, at least in mice. The function of MRP3 in other tissues, notably the gut and the adrenal cortex, remains to be defined. MRP4 and MRP5 have attracted attention by their ability to transport cyclic nucleotides and many nucleotide analogs. The initial reports that MRP4 and 5 can transport cGMP with microM affinity have not been confirmed in recent work and the physiological importance of cyclic nucleotide transport by MRP4 and 5 remains to be determined. Transfected cells containing high concentrations of MRP4 and 5 are moderately resistant to base, nucleoside, and nucleotide analogs. The affinity of both transporters for nucleotide analogs is low (K (m) around 1 mM) and there is no evidence that the transport of these compounds results in resistance in vivo. The physiological function of MRP4 and 5 remains to be found.

INDUCTION OF THE MULTIDRUG RESISTANCE-ASSOCIATED PROTEIN FAMILY OF TRANSPORTERS BY CHEMICAL ACTIVATORS OF RECEPTOR-MEDIATED PATHWAYS IN MOUSE LIVER

The multidrug resistance-associated proteins (Mrp) are ATP-dependent transporters that export a variety of conjugated and unconjugated compounds out of cells. There are nine identified Mrp transporters in humans, with murine orthologs for all except Mrp8. Because nuclear receptors mediate induction of phase I enzymes, Mrp transporter expression might be similarly regulated by these receptors to coordinate metabolism and export of chemicals from liver. To test the hypothesis that Mrp expression may be coordinately regulated with phase I enzyme expression in liver, 15 different compounds were given representing known transcriptionally mediated pathways: aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR), constitutive androstane receptor (CAR), peroxisome proliferator-activated receptor alpha (PPARalpha), and nuclear factor-E2-related factor 2 (Nrf2). Each of these compounds induced expression of their respective target enzyme in liver, demonstrating that the chemical regimens were effective. The AhR ligands [2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polychlorinated biphenyl 126 (PCB126), and beta-naphthoflavone] induced Mrp2, -3, -5, and -6 mRNA expression. The CAR activator 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) induced Mrp2, -3, -4, -6, and -7 mRNA expression. Mrp3 was also induced by two other CAR activators phenobarbital and diallyl sulfide, two PXR ligands, pregnenalone-16alpha-carbonitrile and spironolactone, and the PPARalpha ligands clofibrate, ciprofibrate, and diethylhexylphthalate. The Nrf2 activators (butylated hydroxyanisole, oltipraz, and ethoxyquin) induced Mrp2-6. In conclusion, a variety of mechanisms are suggested for Mrp3 induction, including AhR, CAR, PXR, PPARalpha, and Nrf2, whereas on a whole, a predominant role for AhR and Nrf2 in hepatic induction of the Mrp family was observed. Thus, these specific transcription factors are implicated in regulation of both drug metabolism and efflux transport.

Identification and functional characterization of the natural variant MRP3-Arg1297His of human multidrug resistance protein 3 (MRP3/ABCC3)

The human multidrug resistance protein 3 (MRP3, symbol ABCC3) is an ATP-binding cassette transporter that mediates the efflux of organic anions, including lipophilic substances conjugated with glucuronate, sulphate or glutathione, across the basolateral membrane of polarized cells (e.g. hepatocytes) into blood. Genetic variants of MRP3 may affect the transport of these substances out of cells. The aims of this study were: (i) to identify MRP3 polymorphisms; (ii) to functionally characterize one relatively frequent MRP3 polymorphism; and (iii) to establish whether MRP3 transports bilirubin glucuronosides. Exonic nucleotide variants in the ABCC3 gene were identified by single-strand conformation polymorphism analysis. The 3890G>A mutation, resulting in MRP3-ArgHis, was introduced into the ABCC3 cDNA which was stably transfected into MDCKII cells. For the functional characterization of MRP3-ArgHis in comparison with MRP3, ATP-dependent transport was analysed in isolated membrane vesicles. Two non-synonymous MRP3 variants were identified with an allele frequency of 0.003 for 1643T>A (MRP3-LeuGln) and 0.08 for 3890G>A (MRP3-ArgHis). Because of the high frequency of the 3890G>A mutation, and because of the close proximity of Arg to the second nucleotide-binding domain, we pursued the functional characterization of the MRP3-ArgHis polymorphic variant. MRP3-ArgHis was correctly localized to the basolateral membrane of polarized MDCKII cells. We identified monoglucuronosyl bilirubin, bisglucuronosyl bilirubin and leukotriene C4 as substrates for both MRP3 and MRP3-ArgHis. Dehydroepiandrosterone-3-sulphate and 17beta-glucuronosyl oestradiol were transported with similar kinetics by MRP3 and MRP3-ArgHis. This experimental setup provides a useful tool to analyse the functional consequences of polymorphic variants of MRP3.

Mechanism of the pharmacokinetic interaction between methotrexate and benzimidazoles: potential role for breast cancer resistance protein in clinical drug-drug interactions

The antifolate drug methotrexate (MTX) is transported by breast cancer resistance protein (BCRP; ABCG2) and multidrug resistance-associated protein1-4 (MRP1-4; ABCC1-4). In cancer patients, coadministration of benzimidazoles and MTX can result in profound MTX-induced toxicity coinciding with an increase in the serum concentrations of MTX and its main metabolite 7-hydroxymethotrexate. We hypothesized that benzimidazoles interfere with the clearance of MTX and/or 7-hydroxymethotrexate by inhibition of the ATP-binding cassette drug transporters BCRP and/or MRP2, two transporters known to transport MTX and located in apical membranes of epithelia involved in drug disposition. First, we investigated the mechanism of interaction between benzimidazoles (pantoprazole and omeprazole) and MTX in vitro in membrane vesicles from Sf9 cells infected with a baculovirus containing human BCRP or human MRP2 cDNA. In Sf9-BCRP vesicles, pantoprazole and omeprazole inhibited MTX transport (IC50 13 microm and 36 microm, respectively). In Sf9-MRP2 vesicles, pantoprazole did not inhibit MTX transport and at high concentrations (1 mm), it even stimulated MTX transport 1.6-fold. Secondly, we studied the transport of pantoprazole in MDCKII monolayers transfected with mouse Bcrp1 or human MRP2. Pantoprazole was actively transported by Bcrp1 but not by MRP2. Finally, the mechanism of the interaction was studied in vivo using Bcrp1-/- mice and wild-type mice. Both in wild-type mice pretreated with pantoprazole to inhibit Bcrp1 and in Bcrp1-/- mice that lack Bcrp1, the clearance of i.v. MTX was decreased significantly 1.8- to 1.9-fold compared with the clearance of i.v. MTX in wild-type mice. The conclusion is as follows: benzimidazoles differentially affect transport of MTX mediated by BCRP and MRP2. Competition for BCRP may explain the clinical interaction between MTX and benzimidazoles.

Rapid induction of P-glycoprotein expression by high permeability compounds in colonic cells in vitro: a possible source of transporter mediated drug interactions?

P-glycoprotein (PGP) substrates with high membrane permeability, such as propranolol and verapamil, are considered to be essentially "transparent" to PGP since the transporter does not significantly limit their absorption or elimination. However, the question of whether such compounds can modulate PGP expression in epithelial cells following short-term exposure, with potential consequences for drug interactions, has not been addressed. LS180 colonic epithelial cells were exposed to propranolol or verapamil at concentrations (50-300 microM) consistent with those likely to be present in the gut lumen during oral dosing. Both compounds stimulated four to six-fold increases in MDR1 mRNA and PGP protein expression measured by quantitative real-time PCR and immunoblotting, respectively. These changes were accompanied by an induction in transporter activity measured by rhodamine 123 efflux. In contrast, metoprolol, a compound with similar permeability but no affinity for PGP had no effect on PGP expression. The induction of PGP by propranolol and verapamil was rapid with significant increases occurring within 3h with maximal stimulation after 6h exposure. Rifampicin, shown to cause clinical drug interactions via a PXR-mediated increase in PGP expression, exhibited a very similar time-course and extent of induction. In conclusion, verapamil and propranolol, whose trans-epithelial permeability are unaffected by PGP, appear to be effective inducers of PGP expression in gut epithelial cells in vitro. While the in vivo significance of these observations is unknown, this questions whether high permeability, "PGP-transparent" compounds, currently favoured in drug selection strategies, should be evaluated in terms of their potential for transporter-mediated drug interactions.

Three-dimensional pharmacology, a subject ranging from ignorance to overstatements

Stereoselectivity has been known to play a role in drug action for 100 years or more. Nevertheless, chiral drugs have been developed and used as racemates, neglecting the fact that they comprise mixtures of two or more compounds which may have quite different pharmacological properties. A very limited access to pure enantiomers in the past has been responsible for this unsatisfactory state of affairs. During the last 20 years, significant achievements have made it possible to perform stereoselective synthesis and analysis. Today, novel chiral drugs are as a rule developed as single enantiomers. Yet, studies of old racaemic drugs are still designed, performed and published without mention of the fact that two or more compounds are involved. In recent years, a number of old racaemic drugs have been re-evaluated and re-introduced into the clinical area as the pure, active enantiomer (the eutomer). While in principle correct, the clinical benefit of this shift from a well established racaemate to a pure enantiomer often seems to be limited and sometimes exaggerated. Racaemic drugs with a deleterious enantiomer that does not contribute to the therapeutic effect (the distomer), may have been sorted out in the safety evaluation process. However, in the future any pharmacological study of racaemic drugs must include the pure enantiomers. This will generate new, valuable information on stereoselectivity in drug action and interaction.

Benzimidazole drugs and modulation of biotransformation enzymes

Benzimidazole drugs (e.g., anthelmintics albendazole, fenbendazole, oxfenbendazole, thiabendazole, mebendazole; inhibitors of proton pump omeprazole, lansoprasole, pantoprasole) represent substances used in both human and veterinary medicine; however, from the point of view of induction and inhibition of biotransformation enzymes, research has been carried out mainly due to the initiative of human pharmacologists. The purpose of the present review is to inform about inductive and inhibitive effects of benzimidazole drugs in man, animals and cell cultures. Pharmacological and toxicological consequences of modulation of biotransformation enzymes are discussed and the significance of studies in the field of modulation of biotransformation enzymes in food-producing animals is explained. Since the modulating effect of benzimidazoles strongly varies depending on structure of the individual substances, the particular attention is paid to structure-modulation relationships.

Genetic polymorphisms in the multidrug resistance-associated protein 3 (ABCC3, MRP3) gene and relationship to its mRNA and protein expression in human liver

AIMS

To determine the genetic variability of multidrug resistance protein 3 (MRP3).

METHODS

Genomic DNA samples from 103 Caucasians were systematically screened for genetic variations to find a potential relationship with hepatic MRP3 expression. Sequencing comprised all 31 exons, approximately 100 bp of the flanking intronic regions and 2 kb of the 5' UTR.

RESULTS

In total, 51 mutations were identified. Fifteen SNPs were located in the coding exons of MRP3, six of which are nonsynonymous mutations. SNPs 39G>C (allele frequency: 0.5%, located in exon 1), 202C>T (1.6%, exon 2), 1037C>T (0.5%, exon 9), 1537C>A (0.5%, exon 12), 3890G>A (5.2%, exon 27) and 4267G>A (0.6%, exon 29) resulted in Lys13Asn, His68Tyr, Ser346Phe, Gln513Lys, Arg1297His and Gly1423Arg amino acid substitutions, respectively. A splice site mutation (1339-1G>T) was found at the intron 10-exon 11 boundary. To evaluate, whether mutations in the MRP3 gene correlate with human hepatic MRP3 expression, we analyzed the genetic variants in Caucasian liver samples, whose MRP3 mRNA (n = 84) and protein (n = 50) expression has been determined by real time quantitative PCR and Western Blot, respectively. We found a significant correlation of a polymorphism in the 5' promoter region (-211C>T) of MRP3 with mRNA expression. Individuals homozygous and heterozygous for the -211C>T promoter polymorphism had significantly lower MRP3 transcript levels compared to wild-type individuals (P < 0.05). Accordingly, electrophoretic mobility shift assay demonstrated that -211C>T polymorphism affected the binding of nuclear factors.

CONCLUSIONS

Multiple genetic polymorphisms of MRP3 exist in Caucasians. The -211C>T promoter polymorphism appears to be associated with altered hepatic MRP3 mRNA expression.