Transporters and xenobiotic disposition

Stoichiometric ratios for biotics and xenobiotics capture effective metabolic coupling to re(de)fine biodegradation

Preserving human and environmental health requires anthropogenic pollutants to be biologically degradable. Depending on concentration, both nutrients and pollutants induce and activate metabolic capacity in the endemic bacterial consortium, which in turn aids their degradation. Knowledge on such 'acclimation' is rarely implemented in risk assessment cost-effectively. As a result, an accurate description of the mechanisms and kinetics of biodegradation remains problematic. In this study, we defined a yield 'effectivity', comprising the effectiveness at which a pollutant (substrate) enhances its own degradation by inducing (biomass) cofactors involved therein. Our architecture for calculation represents the interplay between concentration and metabolism via both stoichiometric and thermodynamic concepts. The calculus for yield 'effectivity' is biochemically intuitive, implicitly embeds co-metabolism and distinguishes 'endogenic' from 'exogenic' substances' reflecting various phenomena in biodegradation and bio-transformation studies. We combined data on half-lives of pollutants/nutrients in wastewater and surface water with transition-state rate theory to obtain also experimental values for effective yields. These quantify the state of acclimation: the portion of biodegradation kinetics attributable to (contributed by) 'natural metabolism', in view of similarity to natural substances. Calculated and experimental values showed statistically significant correspondence. Particularly, carbohydrate metabolism and nucleic acid metabolism appeared relevant for acclimation (R² = 0.11-0.42), affecting rates up to 10^4.9(±0.7) times: under steady-state acclimation, a compound stoichiometrically identical to carbohydrates or nucleic acids, is 10^3.2 to 10^4.9 times faster aerobically degraded than a compound marginally similar. Our new method, simulating (contribution by) the state of acclimation, supplements existing structure-biodegradation and kinetic models for predicting biodegradation in wastewater and surface water. The accuracy of prediction may increase when characterizing nutrients/co-metabolites in terms of, e.g., elemental analysis. We discuss strengths and limitations of our approach by comparison to empirical and mechanism-based methods.

From Magic Bullet to Magic Bomb: Reductive Bioactivation of Antiparasitic Agents

Paul Ehrlich coined the term "magic bullet" to describe how a drug kills the parasite inside its human host without harming the host itself. Ehrlich concluded that the drug must have a greater affinity to the parasite than to human cells. Today, the specificity of drug action is understood in terms of the drug target. An ideal target is a protein that is essential for the proliferation of the pathogen but absent in human cells. Examples are the enzymes of folate synthesis or of the nonmevalonate pathway in the malaria parasites. However, there are other ways how a drug can kill selectively. Of particular relevance is the specific activation of a prodrug inside the pathogen but not in the host, as this is how the current frontrunners of parasite chemotherapy work. Artemisinins for malaria, fexinidazole for human African trypanosomiasis, benznidazole for Chagas' disease, metronidazole for intestinal protozoa: these molecules are "magic bombs" that are triggered selectively. They are prodrugs that need to be activated by chemical reduction, i.e., the acquisition of an electron, which occurs in the parasite. Such a mode of action is shared by the novel antimalarial peroxides arterolane and artefenomel, which are activated by reduction of the endoperoxide bond with ferrous heme as the likely electron donor, a metabolic end-product of Plasmodium falciparum. Here we provide an overview on the molecular basis of selectivity of antiparasitic drug action with particular reference to the ozonides, the new generation of antimalarial peroxides designed by Jonathan Vennerstrom.

A high-throughput cell-based gaussia luciferase reporter assay for measurement of CYP1A1, CYP2B6, and CYP3A4 induction

The induction of cytochrome P450s can result in reduced drug efficacy and lead to potential drug-drug interactions. The xenoreceptors-aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR)-play key roles in CYP induction by xenobiotics. In order to be able to rapidly screen for the induction of three enzymes (CYP1A1, CYP2B6, and CYP3A4), we generated a stable AhR-responsive HepG2 cell line, a stable CAR-responsive HepG2 cell line, and a stable PXR-responsive HepG2 cell line.To validate these stable xenoreceptor-responsive HepG2 cell lines, we evaluated the induction of the different Gaussia reporter activities, as well as the mRNA and protein expression levels of endogenous CYPs in response to different inducers.The induction of luciferase activity in the stable xenoreceptor-responsive HepG2 cell lines by specific inducers occurred in a concentration dependent manner. There was a positive correlation between the induction of luciferase activities and the induction endogenous CYP mRNA expression levels. These xenoreceptor-responsive HepG2 cell lines were further validated with known CYP1A1, CYP2B6, and CYP3A4 inducers.These stable xenoreceptor-responsive HepG2 cell lines may be used in preclinical research for the rapid and sensitive detection of AhR, CAR, and PXR ligands that induce CYP450 isoforms.

Applicability of free drug hypothesis to drugs with good membrane permeability that are not efflux transporter substrates: A microdialysis study in rats

In clinical pharmacology, the free drug hypothesis has been widely applied in the interpretation of the relationship between pharmacokinetics and pharmacodynamics (PK/PD). The free drug hypothesis assumes that the unbound drug concentration in blood is the same as that in the site of action at steady state. The objective of this study is to demonstrate whether the free drug hypothesis is universally applicable for all drugs. The unbound concentrations of the 18 compounds in blood and in brain interstitial fluids (ISF) at steady state following constant intravenous infusion were simultaneously monitored up to 6 hours via in vivo microdialysis technique. Based on the permeability and efflux ratio (ER), the test compounds can be divided into two classes. Class I includes the compounds with good membrane permeability that are not substrates of efflux transporters (eg, P-gp, BCRP, and MRPs), whereas Class II includes the compounds that are substrates of efflux transporters. The steady-state unbound drug concentrations in blood, brain, and CSF are quantitatively very similar for Class I compounds, whereas the steady-state unbound concentrations in the brain and CSF are significantly lower than those in blood for Class II compounds. These results strongly suggest that the free drug hypothesis is not universal for all drugs but is only applicable for drugs with good permeability that are not substrates of efflux transporters.

Effects of cholesterol content on activity of P-glycoproteins and membrane physical state, and consequences for anthelmintic resistance in the nematode Haemonchus contortus

Eukaryote plasma membranes protect cells from chemical attack. Xenobiotics, taken up through passive diffusion, accumulate in the membranes, where they are captured by transporters, among which P-glycoproteins (Pgps). In nematodes such as Haemonchus contortus, eggshells and cuticles provide additional protective barriers against xenobiotics. Little is known about the role of these structures in the transport of chemical molecules. Pgps, members of the ABC transporter family, are present in eggshells and cuticles. Changes in the activity of these proteins have also been correlated with alterations in lipids, such as cholesterol content, in eggshells. However, the cellular mechanisms underlying these effects remain unclear. We show here that an experimental decrease in the cholesterol content of eggshells of Haemonchus contortus, with Methyl-beta-CycloDextrin (MβCD), results in an increase in membrane fluidity, favouring Pgp activity and leading to an increase in resistance to anthelmintics. This effect is modulated by the initial degree of anthelminthic resistance of the eggs. These results suggest that eggshell fluidity plays a major role in the modulation of Pgp activity. They confirm that Pgp activity is highly influenced by the local microenvironment, in particular sterols, as observed in some vertebrate models. Thus, eggshell barriers could play an active role in the transport of xenobiotics.

Cellular Models and In Vitro Assays for the Screening of modulators of P-gp, MRP1 and BCRP

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are highly expressed in tumor cells, as well as in organs involved in absorption and secretion processes, mediating the ATP-dependent efflux of compounds, both endogenous substances and xenobiotics, including drugs. Their expression and activity levels are modulated by the presence of inhibitors, inducers and/or activators. In vitro, ex vivo and in vivo studies with both known and newly synthesized P-glycoprotein (P-gp) inducers and/or activators have shown the usefulness of these transport mechanisms in reducing the systemic exposure and specific tissue access of potentially harmful compounds. This article focuses on the main ABC transporters involved in multidrug resistance [P-gp, multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP)] expressed in tissues of toxicological relevance, such as the blood-brain barrier, cardiovascular system, liver, kidney and intestine. Moreover, it provides a review of the available cellular models, in vitro and ex vivo assays for the screening and selection of safe and specific inducers and activators of these membrane transporters. The available cellular models and in vitro assays have been proposed as high throughput and low-cost alternatives to excessive animal testing, allowing the evaluation of a large number of compounds.

Acute toxicities of pharmaceuticals toward green algae. mode of action, biopharmaceutical drug disposition classification system and quantile regression models

The acute toxicities of 36 pharmaceuticals towards green algae were estimated from a set of quantile regression models representing the first global quantitative structure-activity relationships. The selection of these pharmaceuticals was based on their predicted environmental concentrations. An agreement between the estimated values and the observed acute toxicity values was found for several families of pharmaceuticals, in particular, for antidepressants. A recent classification (BDDCS) of drugs based on ADME properties (Absorption, Distribution, Metabolism and Excretion) was clearly correlated with the acute ecotoxicities towards algae. Over-estimation of toxicity from our QSAR models was observed for classes 2, 3 and 4 whereas our model results were in agreement for the class 1 pharmaceuticals. Clarithromycin, a class 3 antibiotic characterized by weak metabolism and high solubility, was the most toxic to algae (molecular stability and presence in surface water).

Functional xenobiotic metabolism and efflux transporters in trout hepatocyte spheroid cultures

Prediction of xenobiotic fate in fish is important for the regulatory assessment of chemicals under current legislation. Trout hepatocyte spheroids are a promising in vitro model for this assessment. In this investigation, the gene expression and function for xenobiotic metabolism and cellular efflux were characterised. Using fluorescence, transport and real time PCR analysis, the expression and functionality of a variety of genes related to xenobiotic metabolism and drug efflux were assessed in a range of trout hepatocyte culture preparations. Significantly greater levels of expression of genes involved in xenobiotic metabolism and efflux were measured in spheroids (which have been shown to remain viable in excess of 30 days), compared to hepatocytes cultured using conventional suspension and monolayer culture techniques. A transient decline in the expression of genes related to both xenobiotic metabolism and transport was determined during spheroid development, with a subsequent recovery in older spheroids. The most mature spheroids also exhibited an expression profile most comparable to that reported in vivo. Functionality of efflux transporters in spheroids was also demonstrated using fluorescent markers and specific inhibitors. In conclusion, the more physiologically relevant architecture in spheroid cultures provides a high functional integrity in relation to xenobiotic metabolism and efflux. Together with the enhanced gene expression and longevity of the model, hepatocytes in spheroid culture may prove to be an accurate alternative model to study the mechanisms of these processes in fish liver and provide an assay to determine the bioaccumulation potential of environmental contaminants.

Herb-drug interaction of Epimedium sagittatum (Sieb. et Zucc.) maxim extract on the pharmacokinetics of sildenafil in rats

Epimedium sagittatum (Sieb. et Zucc.) Maxim is one of the herbs used to treat erectile dysfunction in Traditional Chinese Medicine. Sildenafil is a phosphodiesterase 5 inhibitor used to treat erectile dysfunction in Western Medicine. This study evaluates the herbal-drug interaction of Epimedium sagittatum extract on the pharmacokinetics of sildenafil in rats by ultra-performance liquid chromatography. The rat plasma was sampled from each anesthetized rat after pretreatment with 3-days Epimedium sagittatum extract (1/2 g/kg/day) and intravenous injection with sildenafil (10/30 mg/kg). The pharmacokinetic data demonstrate that the area under the concentration-time curve (AUC) of sildenafil (10 mg/kg) was significantly decreased in groups that received a high dose of Epimedium sagittatum extract. In conclusion, the study demonstrates that there was significant herb-drug interaction of Epimedium sagittatum extract on the pharmacokinetics of sildenafil at low and high daily doses, suggesting co-administration use of Epimedium sagittatum extract and sildenafil in clinical practice should be prevented due to possible herb-drug interactions.

Primary hepatocyte cultures as prominent in vitro tools to study hepatic drug transporters

Before any drug can be placed on the market, drug efficacy and safety must be ensured through rigorous testing. Animal models are used for this purpose, though currently increasing attention goes to the use of alternative in vitro systems. In particular, liver-based testing platforms that allow the prediction of pharmacokinetic (PK) and pharmacotoxicological properties during the early phase of drug development are of interest. They also enable the screening of potential effects on hepatic drug transporters. The latter are known to affect drug metabolism and disposition, thereby possibly underlying drug-drug interactions, which, in turn, may result in liver toxicity. Clearly, stable in vivo-like functional expression of drug transporters in hepatic in vitro settings is a prerequisite to be applicable in routine PK and pharmacotoxicological testing. In the first part of the article, an updated overview of hepatic drug transporters is provided, followed by a state-of-the-art review of drug-transporter production and activity in primary hepatocyte cultures (PHCs), being the gold-standard in vitro system. Specific focus is hereby put on strategies to maintain long-term functional expression, in casu of drug transporters, in these systems. In the second part, the use of PHCs to assess hepatobiliary transport and transporter-mediated interactions is outlined.

Role of MDR1 C3435T and GABRG2 C588T gene polymorphisms in seizure occurrence and MDR1 effect on anti-epileptic drug (phenytoin) absorption

AIMS

To assess the role of MDR1 and gamma-aminobutyric acid receptor-gamma 2 sub unit (GABRG2) gene polymorphism in seizure susceptibility in generalized seizure (GS) and febrile seizure (FS) patients and to evaluate MDR1 C3435T gene polymorphism's role in absorption of the anti-epileptic drug, phenytoin (PHT) in a cohort of patients.

METHODS

One hundred twenty-seven cases of seizure (86 GS and 41 FS) patients were analyzed for MDR1 C3435T and GABRG2 C588T gene polymorphisms using restriction fragment length polymorphism-polymerase chain reaction. Serum PHT levels were analyzed.

RESULTS

The T allele of MDR1 C3435T and GABRG2 C588T gene polymorphism was higher in GS in the Indian population compared with controls. From the data in GS, CT and TT genotype carriers of the MDR1 gene and TT genotype carriers of the GABRG2 gene had more recurrent seizures compared with others. MDR1 T allele carriers in the seizure reoccurrence (SR) group of GS and FS were high compared with the well-controlled seizure group (with no seizures after treatment). TT genotype carriers in SR group were high in FS (with regard to MDR1 gene polymorphism) and GS (with regard to GABRG2 gene polymorphism) compared with a well-controlled seizure group. MDR1 C3435T gene polymorphism affects serum PHT levels (p<0.015). Association of dose PHT ratio and genotype groups of MDR1 C3435T gene polymorphism showed a significant association (p<0.05). MDR1*CC genotype was more common in cases with low serum PHT levels.In addition, it is evident that CT and TT genotype carriers have a high percentage of SR with elevated serum PHT levels.

CONCLUSIONS

Our results show that the MDR1 3435T and GABRG2 588T alleles play a role in seizure occurrence. Moreover, the MDR1 3435T allele also affects PHT absorption. We suggest MDR1 C3435T and GABRG2 C588T genotyping would be of value in order to lower the risk of concentration-dependent drug toxicity and for better patient management.

Interaction of fluvastatin with the liver-specific Na+ -dependent taurocholate cotransporting polypeptide (NTCP)

It has been reported that polymorphisms in the organic anion transporting polypeptide 1B1 (OATP1B1, SLCO1B1) result in decreased hepatic uptake of simvastatin carboxy acid, the active metabolite of simvastatin. This is not the case for fluvastatin and it has been hypothesized that for this drug other hepatic uptake pathways exist. Here, we studied whether Na(+)-dependent taurocholate co-transporting polypeptide (NTCP, SLC10A1) can be an alternative hepatic uptake route for fluvastatin. Chinese Hamster Ovary cells transfected with human NTCP (CHO-NTCP) were used to investigate the inhibitory effect of fluvastatin and other statins on [(3)H]-taurocholic acid uptake ([(3)H]-TCA). Statin uptake by CHO-NTCP and cryopreserved human hepatocytes was assessed via LC-MS/MS. Fluvastatin appeared to be a potent and competitive inhibitor of [(3)H]-TCA uptake (IC(50) of 40μM), pointing to an interaction at the level of the bile acid binding pocket of NTCP. The inhibitory action of other statins was also studied, which revealed that statin inhibitory potency increased with molecular descriptors of lipophilicity: calculated logP (r(2)=0.82, p=0.034), logD(7.4) (r(2)=0.77, p=0.0001). Studies in CHO-NTCP cells showed that fluvastatin was indeed an NTCP substrate (K(m) 250±30μM, V(max) 1340±50ng/mg total cell protein/min). However, subsequent studies revealed that at clinically relevant plasma concentrations, NTCP contributed minimally to overall accumulation in human hepatocytes. In conclusion, fluvastatin interacts with NTCP at the level of the bile acid binding pocket and is an NTCP substrate. However, under normal conditions, NTCP-mediated uptake of this drug seems not to be a significant hepatocellular uptake pathway.

Fruit juice inhibition of uptake transport: a new type of food-drug interaction

A new type of interaction in which fruit juices diminish oral drug bioavailability through inhibition of uptake transport is the focus of this review. The discovery was based on an opposite to anticipated finding when assessing the possibility of grapefruit juice increasing oral fexofenadine bioavailability in humans through inhibition of intestinal MDR1-mediated efflux transport. In follow-up investigations, grapefruit or orange juice at low concentrations potentially and selectively inhibited in vitro OATP1A2-mediated uptake compared with MDR1-caused efflux substrate transport. These juices at high volume dramatically depressed oral fexofenadine bioavailability. Grapefruit was the representative juice to characterize the interaction subsequently. A volume-effect relationship study using a normal juice amount halved average fexofenadine absorption. Individual variability and reproducibility data indicated the clinical interaction involved direct inhibition of intestinal OATP1A2. Naringin was a major causal component suggesting that other flavonoids in fruits and vegetables might also produce the effect. Duration of juice clinical inhibition of fexofenadine absorption lasted more than 2 h but less than 4 h indicating the interaction was avoidable with appropriate interval of time between juice and drug consumption. Grapefruit juice lowered the oral bioavailability of several medications transported by OATP1A2 (acebutolol, celiprolol, fexofenadine, talinolol, L-thyroxine) while orange juice did the same for others (atenolol, celiprolol, ciprofloxacin, fexofenadine). Juice clinical inhibition of OATP2B1 was unresolved while that of OATP1B1 seemed unlikely. The interaction between grapefruit juice and etoposide also seemed relevant. Knowledge of both affected uptake transporter and drug hydrophilicity assisted prediction of the clinical interaction with grapefruit or orange juice.

Analysis of biliary excretion of icariin in rats

Icariin is a bioactive herbal ingredient isolated from Epimedii Herba. This study evaluates the distribution of icariin in rats by microdialysis sampling and high-performance liquid chromatography with ultraviolet detection (HPLC-UV). Microdialysis probes were simultaneously placed in the jugular vein, brain striatum, and bile duct of each anesthetized rat for sampling after the administration of icariin (dose=10 or 20 mg/kg) via the femoral vein. The role of P-glycoprotein (P-gp) on icariin distribution was assessed by pretreatment with cyclosporine (CsA, dose=20 mg/kg). This study is the first report of the biliary excretion of icarin in rats, defined as the blood-to-bile distribution (k value), calculated by dividing the area under the concentration-time curve (AUC) of icariin in bile by that in blood (k=AUCbile/AUCblood). The k values were 19.0±5.9 and 18.8±3.8 at the doses of 10 and 20 mg/kg, respectively. The decreased biliary excretion of icariin due to pretreatment with CsA was evidenced by the reduced k values (18.8±3.8 vs 9.9±1.9, p=0.005). This work demonstrates that biliary excretion is the major elimination pathway for icariin disposition and that transporters, such as P-gp, might be related to icariin's biliary excretion.

Chronopharmacological Study of Cephalexin in Dogs

Recent studies have identified a 24 h rhythm in the expression and function of PEPT1 in rats, with significantly higher levels during the nighttime than daytime. Similarly, temporal variations have been described in glomerular filtration rate and renal blood flow, both being maximal during the activity phase and minimal during the rest phase in laboratory rodents. The aim of this study was to assess the hypothesis that the absorption of the first-generation cephalosporin antibiotic cephalexin by dogs would be less and the elimination would be slower after evening (rest span) compared to morning (activity span) administration, and whether such administration-time changes could impair the medication's predicted clinical efficacy. Six (3 male, 3 female; age 4.83+/-3.12 years) healthy beagle dogs were studied. Each dog received a single dose of 25 mg/kg of cephalexin monohydrate per os at 10:00 and 22:00 h, with a two-week interval of time between the two clock-time experiments. Plasma cephalexin concentrations were determined by microbiological assay. Cephalexin peak plasma concentration was significantly reduced to almost 77% of its value after the evening compared to morning (14.52+/-2.7 vs. 18.77+/-2.8 microg/mL) administration. The elimination half-life was prolonged 1.5-fold after the 22:00 h compared to the 10:00 h administration (2.69+/-0.9 vs. 1.79+/-0.2 h). The area under the curve and time to reach peak plasma concentration did not show significant administration-time differences. The duration of time that cephalexin concentrations remained above the minimal inhibitory concentrations (MIC) for staphylococci susceptiblity (MIC=0.5 microg/mL) was>70% of each of the 12 h dosing intervals (i.e., 10:00 and 22:00 h). It can be concluded that cephalexin pharmacokinetics vary with time of day administration. The findings of this acute single-dose study require confirmation by future steady-state, multiple-dose studies. If such studies are confirmatory, no administration-time dose adjustment is required to ensure drug efficacy in dogs receiving an oral suspension of cephalexin in a dosage of 25 mg/kg at 12 h intervals.

Enantioselective disposition of fexofenadine with the P-glycoprotein inhibitor verapamil

AIMS

The aim was to compare possible effects of verapamil, as a P-glycoprotein (P-gp) inhibitor, on the pharmacokinetics of each fexofenadine enantiomer, as a P-gp substrate.

METHODS

Thirteen healthy Japanese volunteers (10 male and three female) were enrolled. In a randomized, two-phase, crossover design, verapamil was dosed 80 mg three times daily (with total daily doses of 240 mg) for 6 days, and on day 6, a single 120-mg dose of fexofenadine was administered along with an 80-mg dose of verapamil. Subsequently, fexofenadine was administered alone after a 2-week wash-out period. The plasma concentrations of fexofenadine enantiomers were measured up to 24 h after dosing.

RESULTS

During the control phase, the mean AUC(0-infinity) of S(-)- and R(+)-fexofenadine was 700 ng h(-1) ml(-1)[95% confidence interval (CI) 577, 823] and 1202 ng h(-1) ml(-1) (95% CI 1007, 1396), respectively, with a significant difference (P < 0.001). Verapamil had a greater effect on the pharmacokinetic parameters of S(-)-fexofenadine compared with those of the R(+)-enantiomer, and increased AUC(0-infinity) of S(-)-fexofenadine and R(+)-fexofenadine by 3.5-fold (95% CI of differences 1.9, 5.1; P < 0.001) and by 2.2-fold (95% CI of differences 1.7, 3.0; P < 0.001), respectively. The R/S ratio for the AUC(0-infinity) was reduced from 1.76 to 1.32 (P < 0.001) by verapamil treatments.

CONCLUSION

This study indicates that P-gp plays a key role in the stereoselectivity of fexofenadine pharmacokinetics, since the pharmacokinetics of fexofenadine enantiomers were altered by the P-gp inhibitor verapamil, and this effect was greater for S-fexofenadine compared with R-fexofenadine.

Physiologically based toxicokinetic models and their application in human exposure and internal dose assessment

Human populations may exhibit large interindividual variation in toxicokinetic response to chemical exposures. Rapid developments in dosimetry research have brought medicine and public health closer to understanding the biological basis of this heterogeneity. The toxicokinetic behavior of chemicals is, in part, controlled by the properties of the epithelium surrounding organs, some of which are effective barriers to penetration into the systemic circulation. Physiologically based toxicokinetic (PBTK) models have been developed and used to simulate the mechanism of uptake into the systemic circulation, to extrapolate between doses and exposure routes, and to estimate internal dosimetry and sources of heterogeneity in animals and humans. Recent improvements to PBTK models include descriptions of active transport across biological membranes, carrier-mediated clearance, and fractal kinetics. The expanding area of toxicogenetics has provided valuable insight for delineating toxicokinetic differences between individuals; genetic differences include inherited single nucleotide polymorphisms, copy number variants, and dynamic changes in the methylation pattern of imprinted genes. This chapter discusses the structure of PBTK models and how toxicogenetic information and newer biological descriptions have improved our understanding of variability in response to toxicant exposures.

Effects of endocrine disruptors on genes associated with 17beta-estradiol metabolism and excretion

In order to provide a global analysis of the effects of endocrine disruptors on the hormone cellular bioavailability, we combined 17beta-estradiol (E2) cellular flow studies with real-time PCR and Western blot expression measurements of genes involved in the hormone metabolism and excretion. Three endocrine disruptors commonly found in food were chosen for this study, which was conducted in the estrogen receptor (ER) negative hepatoblastoma HepG2 cell line: bisphenol A (BPA), genistein (GEN) and resveratrol (RES). We showed that 24 h after a single dose treatment with genistein, resveratrol or bisphenol A, the expression of ATP-binding cassette transporters (the multidrug resistance or MDR, and the multidrug resistance associated proteins or MRP) uridine diphosphate-glucuronosyltransferases (UGT) and/or sulfotransferases (ST) involved in 17beta-estradiol elimination process were significantly modulated and that 17beta-estradiol cellular flow was modified. Resveratrol induced MDR1 and MRP3 expressions, bisphenol A induced MRP2 and MRP3 expressions, and both enhanced 17beta-estradiol efflux. Genistein, on the other hand, inhibited ST1E1 and UGT1A1 expressions, and led to 17beta-estradiol cellular retention. Thus, we demonstrate that bisphenol A, genistein and resveratrol modulate 17beta-estradiol cellular bioavailability in HepG2 and that these modulations most probably involve regulations of 17beta-estradiol phase II and III metabolism proteins. Up to now, the estrogenicity of environmental estrogenic pollutants has been based on the property of these compounds to bind to ERs. Our results obtained with ER negative cells provide strong evidence for the existence of ER-independent pathways leading to endocrine disruption.

The different effects of itraconazole on the pharmacokinetics of fexofenadine enantiomers

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Recently, we have shown that the plasma concentration of R-fexofenadine is greater than that of the S-enantiomer. Although itraconazole co-administration is known to increase the bioavailability of a racemic mixture of fexofenadine, little is known about the stereoselective inhibition of P-gp activity by itraconazole.

WHAT THIS STUDY ADDS

This study indicates that the stereoselective pharmacokinetics of fexofenadine are due to P-gp-mediated transport and its stereoselectivity is altered by itraconazole, a an inhibitor of P-gp.

AIMS

The aim of this study was to determine the inhibitory effect of itraconazole, a P-glycoprotein (P-gp) inhibitor, on the stereoselective pharmacokinetics of fexofenadine.

METHODS

A two-way double-blind, placebo-controlled crossover study was performed with a 2-week washout period. Twelve healthy volunteers received either itraconazole 200 mg or matched placebo in a randomized fashion with a single oral dose of fexofenadine 60 mg simultaneously. The plasma concentrations and the amount of urinary excretion (Ae) of fexofenadine enantiomers were measured up to 24 h after dosing.

RESULTS

After placebo administration, mean AUC(0,24 h) of S- and R-fexofenadine was 474 ng ml(-1) h (95% CI 311, 638) and 798 ng ml(-1) h (95% CI 497, 1101), respectively. Itraconazole affected the pharmacokinetic parameters of S-fexofenadine more, and increased AUC(0,24 h) of S-fexofenadine and R-fexofenadine by 4.0-fold (95% CI of differences 2.8, 5.3; P < 0.001) and by 3.1-fold (95% CI of differences 2.2, 4.0; P = 0.014), respectively, and Ae(0,24 h) of S-fexofenadine and R-fexofenadine by 3.6-fold (95% CI of differences 2.6, 4.5; P < 0.001) and by 2.9-fold (95% CI of differences 2.1, 3.8; P < 0.001), respectively. Additionally, the R : S ratio for AUC(0,24 h) and Ae(0,24 h) were significantly reduced in the itraconazole phase, while t(max), t(1/2) and renal clearance were constant during the study.

CONCLUSIONS

This study indicates that the stereoselective pharmacokinetics of fexofenadine are due to P-gp-mediated transport and its stereoselectivity is altered by itraconazole, a P-gp inhibitor. However, further study will be needed because the different affinities of the two enantiomers for P-gp have not been supported by in vitro studies.

Transporter-mediated drug interactions: clinical implications and in vitro assessment

Although they are less frequently compared with the reported cases of CYP-mediated drug interactions, clinically significant transporter-mediated drug interactions, which are mainly based on efflux transporter or P-glycoprotein data, have been reported. Unlike the CYP-mediated drug interactions that can be readily defined by inhibition or induction of CYP enzymes, the evidence for the so-called transporter-mediated drug interactions is often less conclusive. The difficulty in defining transporter-mediated drug interactions is due mainly to the interplay between transporters and drug-metabolizing enzymes in drug disposition, and the lack of specific and potent inhibitors for each transporter and enzyme. An important lesson learned from animal studies is that transporter inhibition has a much greater impact on the tissue distribution of drugs than on the systemic exposure of drugs measured in plasma. The potential risk of transporter-mediated drug interactions might be underestimated if only plasma concentrations are monitored.

Primary analysis on conjugational transfer multidrug resistance-related proteins of Shigella flexneri strain

AIM: To search for the new proteins related to multidrug resistance by comparing the proteomics of whole cellular proteins between sensitive strain and conjugational transfer anti-multidrug strain of Shigella flexneri.

METHODS: Clinical sensitive Shigella flexneri strain was transduced into anti-multidrug strain by conjugational transfer trials. Immobilized pH gradient (IPG) two-dimensional electrophoresis was adopted and the gels were analyzed by Image Master 2D Platinum software. Matrix assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) were used to analyzed differential expression proteins.

RESULTS: Conjugational transfer anti-multidrug strain of Shigella flexneri was obtained successfully. It was found that there were 946±37 protein spots in the whole cellular protein 2-DE gels of sensitive strain and 1013±157 protein spots in that of conjugational transfer anti-multidrug strain. A total of 43 differential expression protein spots were found and 5 proteins related to multidrug resistance, including two new proteins (CRISPR-associated protein and heat shock protein chaperone Groel-Groes-Adp7), were identified based on peptide mass fingerprinting. ATP binding cassette transporter protein, cysteine synthase, predicted periplasmic or secreted lipoprotein protein were highly expressed in conjugational transfer anti-multidrug strain.

CONCLUSION: Some genes related to multidrug resistance of the donor can be transduced into sensitive strains and expressed highly. Meanwhile, the expression of some important cellular metabolic enzymes is up-regulated, and ATP binding cassette transporter protein plays an significant role in the mechanism of Shigella flexneri multidrug resistance.

Potential impact of ABCB1 (p-glycoprotein) polymorphisms on avermectin toxicity in humans

Several members of the ATP binding cassette (ABC) transporter protein superfamily perform xenobiotic efflux functions in mammals, limiting gut absorption, mediating excretion, and controlling entry of a wide range of chemicals to sensitive compartments such as brain, testes and foetus. Perhaps the best characterised of these is p-glycoprotein (gene name ABCB1/MDR1), a barrier epithelia expressed protein with structurally diverse substrates, including the avermectin pesticides. In specific mouse and dog strains, ABCB1 mutations have been identified that result in loss of p-glycoprotein function in the blood brain barrier (BBB) and increased susceptibility to avermectin neurotoxicity. As yet no large rearrangements of the human ABCB1 gene analogous to those in the mouse and dog have been identified. However, numerous human ABCB1 single nucleotide polymorphisms (SNPs) have been identified, the allelic frequencies of which vary with ethnicity. There is no clear consensus on whether or not SNPs, or combinations of SNPs, reduce human p-glycoprotein functionality. However, recent in vivo human data indicate that the two commonest ABCB1 haplotypes both exhibit full BBB functionality. We discuss here the role of p-glycoprotein in limiting brain absorption of avermectin pesticides, as well as the potential impact of the reported functional effects and population frequencies of known ABCB1 polymorphisms on avermectin pesticide risk assessments.

Naringin is a major and selective clinical inhibitor of organic anion-transporting polypeptide 1A2 (OATP1A2) in grapefruit juice

We showed previously that grapefruit and orange juices inhibited human enteric organic anion-transporting polypeptide (OATP)1A2 in vitro and lowered oral fexofenadine bioavailability clinically. Inhibition of OATP1A2 transport by flavonoids in grapefruit (naringin) and orange (hesperidin) was conducted in vitro. Two randomized, crossover, pharmacokinetic studies were performed clinically. In one study, 120 mg of fexofenadine was ingested with 300 ml grapefruit juice, an aqueous solution of naringin at the same juice concentration (1,200 microM), or water. In the other study, fexofenadine was administered with grapefruit juice, with or 2 h before aqueous suspension of the particulate fraction of juice containing known clinical inhibitors of enteric CYP3A4, but relatively low naringin concentration (34 microM), or with water. Naringin and hesperidin's half-maximal inhibitions were 3.6 and 2.7 microM, respectively. Fexofenadine area under the plasma drug concentration-time curves (AUCs) with grapefruit juice and naringin solution were 55% (P<0.001) and 75% (P<0.05) of that with water, respectively. Fexofenadine AUCs with grapefruit juice and particulate fractions were 57% (P<0.001), 96% (not significant (NS)), and 97% (NS) of that with water, respectively. Individuals tested in both studies (n=9 of 12) had highly reproducible fexofenadine AUC with water (r(2)=0.85, P<0.001) and extent of reduction of it with grapefruit juice (r(2)=0.72, P<0.01). Naringin most probably directly inhibited enteric OATP1A2 to decrease oral fexofenadine bioavailability. Inactivation of enteric CYP3A4 was probably not involved. Naringin appears to have sufficient safety, specificity, and sensitivity to be a clinical OATP1A2 inhibitor probe. Inherent OATP1A2 activity may be influenced by genetic factors. This appears to be the first report of a single dietary constituent clinically modulating drug transport.

Safety and efficacy of statins in Asians

Asian patients frequently have heightened responses to therapeutic drugs. As a consequence, the recommended drug doses are often lower in Asian countries than in Western countries. This practice extends to the use of cardiovascular drugs, including statins for the treatment of dyslipidemia. Pharmacokinetic investigations have noted higher plasma levels of statins in Asians compared with Caucasians, although postmarketing data for all statins have not identified any particular safety issues, even when statins are given at equivalent doses. The potential mechanisms of heightened response to statins in Asians are related to genetically based differences in the metabolism of statins at the level of hepatic enzymes and drug transporters. Studies indicate that lower statin doses achieve lipid improvements in Asian patients comparable with those observed with higher doses in Caucasians. In conclusion, prescribing lower starting doses of statins in Asians appears warranted while research on this subject continues.

Evidence of Oatp and Mdr1 in cryopreserved rat hepatocytes

Transport proteins control uptake of drugs into the liver (e.g., organic anion transporting polypeptide (Oatp)) and excretion of drugs from the liver (e.g., multidrug resistance protein 1 (Mdr1)). In this study, cryopreserved rat hepatocytes were used to investigate the effect of different culture conditions (suspension, conventional culture and sandwich culture) on the uptake of [(3)H]-taurocholate+/-probenecid and the efflux of [(3)H]-vinblastine+/-ketoconazole; mRNA levels of Oatp1a1, Oatp1a4, Mdr1a and Mdr1b were determined using real-time reverse transcription polymerase chain reaction (RT-PCR) and protein expression of Mdr was assessed by immunocytochemistry. The uptake of [(3)H]-taurocholate was higher in cryopreserved rat hepatocytes maintained in suspension as compared to hepatocytes in culture. A significant time dependent decline in the uptake of [(3)H]-taurocholate was noticed from day 2 to day 4 in conventional and sandwich cultures. [(3)H]-taurocholate uptake was significantly reduced using the inhibitor probenecid. Oatp mRNA expression in hepatocytes in suspension was similar to that of liver, whereas much lower levels were detected in the cultures; this was in accordance with [(3)H]-taurocholate uptake results. Mdr1 activity was assessed by accumulation of the Mdr1 selective substrate, [(3)H]-vinblastine, in hepatocytes using ketoconazole as an inhibitor. The results showed Mdr1 activity in cryopreserved rat hepatocytes in conventional and sandwich cultures. A time dependent increase in Mdr1 activity was noticed from day 2 to day 4. Mdr1 activity was not found using hepatocytes in suspension. Mdr1 mRNA expression was high in cryopreserved hepatocytes from both culture systems. Immunocytochemistry showed the Mdr protein in membranes of hepatocytes in culture as well as in that of hepatocytes in liver sections. In conclusion, the present study showed that cryopreserved rat hepatocytes maintained canalicular transport activity (Mdr1) and basolateral transport activity. Hepatocytes in suspension had a higher uptake of taurocholate with a high Oatp (1a1 and 1a4) mRNA expression as compared to hepatocytes in culture. The presence of Mdr1 in both conventional and sandwich culture was confirmed at mRNA level, by protein expression as well as transport activity.

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.

Pharmacogenomics: challenges and opportunities

The outcome of drug therapy is often unpredictable, ranging from beneficial effects to lack of efficacy to serious adverse effects. Variations in single genes are 1 well-recognized cause of such unpredictability, defining the field of pharmacogenetics (see Glossary). Such variations may involve genes controlling drug metabolism, drug transport, disease susceptibility, or drug targets. The sequencing of the human genome and the cataloguing of variants across human genomes are the enabling resources for the nascent field of pharmacogenomics (see Glossary), which tests the idea that genomic variability underlies variability in drug responses. However, there are many challenges that must be overcome to apply rapidly accumulating genomic information to understand variable drug responses, including defining candidate genes and pathways; relating disease genes to drug response genes; precisely defining drug response phenotypes; and addressing analytic, ethical, and technological issues involved in generation and management of large drug response data sets. Overcoming these challenges holds the promise of improving new drug development and ultimately individualizing the selection of appropriate drugs and dosages for individual patients.

An integrated approach to model hepatic drug clearance

It has been well accepted that hepatic drug extraction depends on the blood flow, vascular binding, transmembrane barriers, transporters, enzymes and cosubstrate and their zonal heterogeneity. Models of hepatic drug clearances have been appraised with respect to their utility in predicting drug removal by the liver. Among these models, the "well-stirred" model is the simplest since it assumes venous equilibration, with drug emerging from the outflow being in equilibrium with drug within the liver, and the concentration is the same throughout. The "parallel tube" and dispersion models, and distributed model of Goresky and co-workers have been used to account for the observed sinusoidal concentration gradient from the inlet and outlet. Departure from these models exists to include heterogeneity in flow, enzymes, and transporters. This article utilized the physiologically based pharmacokinetic (PBPK) liver model and its extension that include heterogeneity in enzymes and transporters to illustrate how in vitro uptake and metabolic data from zonal hepatocytes on transport and enzymes may be used to predict the kinetics of removal in the intact liver; binding data were also necessary. In doing so, an integrative platform was provided to examine determinants of hepatic drug clearance. We used enalapril and digoxin as examples, and described a simple liver PBPK model that included transmembrane transport and metabolism occurring behind the membrane, and a zonal model in which the PBPK model was expanded three sets of sub-compartments that are arranged sequentially to represent zones 1, 2, and 3 along the flow path. The latter model readily accommodated the heterogeneous distribution of hepatic enzymes and transporters. Transport and metabolic data, piecewise information that served as initial estimates, allowed for the unknown efflux and other intrinsic clearances to be estimated. The simple or zonal PBPK model provides predictive views on the hepatic removal of drugs and metabolites.

Lack of dose-dependent effects of itraconazole on the pharmacokinetic interaction with fexofenadine

The aim of this study was to determine the inhibitory effect of itraconazole at different coadministered doses on fexofenadine pharmacokinetics. In a randomized four-phase crossover study, 11 healthy volunteers were administered a 60-mg fexofenadine hydrochloride tablet alone on one occasion (control phase) and with three different doses of 50, 100, and 200 mg of itraconazole simultaneously on the other three occasions (itraconazole phase). Although the elimination half-life and the renal clearance of fexofenadine remained relatively constant, a single administration of itraconazole with fexofenadine significantly increased mean area under the plasma concentration-time curve (AUC(0-infinity)) of fexofenadine (1701/3554, 4308, and 4107 ng h/ml for control; 50 mg, 100 mg, and 200 mg of itraconazole, respectively). Although mean itraconazole AUC(0-48) from 50 mg to 200 mg increased dose dependently from 214 to 772 ng h/ml (p = 0.003), no significant difference was noted in the three parameters, AUC (p = 0.423), C(max) (p = 0.636), and renal clearance (p = 0.495), of fexofenadine among the three doses of itraconazole. Itraconazole exposure at a lower dose (50 mg) compared with the clinical dose (200 mg once or twice daily) had the maximal effect on fexofenadine pharmacokinetics, even though itraconazole plasma concentrations gradually increased after higher doses. These findings suggest that the interaction may occur at the gut wall before reaching the portal vein circulation, and the inhibitory effect must be saturated by substantial local concentrations of itraconazole in the gut lumen after 50-mg dosing.

Hepatocyte nuclear factor (HNF) 1 and HNF4 mediate hepatic multidrug resistance protein 2 up-regulation during hepatitis C virus gene expression

Hepatitis C virus (HCV) is known to induce hepatic oxidative stress that is implicated in the up-regulation of multidrug resistance proteins (MRPs). The relationship between increased prooxidant production, MRPs, and HCV has not been investigated. Here, we report that a homeodomain-containing transcription factor, hepatocyte nuclear factor (HNF) 1, plays a central role in liver gene regulation during HCV gene expression and/or subgenome replication. MRP2 protein and mRNA expression were increased and MRP2 promoter activity was increased 7-fold. Mutations within the putative HNF1 binding site of the human MRP2 promoter abrogated HCV-induced activation, implicating HNF1 in the induction of MRP2 by HCV. The mechanism by which HNF1-mediated activation occurs seems to be transcriptional, because the regulated expression of HNF4, which is known to control HNF1 expression, was also increased. Consistent with this finding, HNF1 mRNA was increased 10-fold. A promoter-luciferase construct of the human HNF1 gene was activated in an HNF4-dependent manner, and a mutant construct lacking the HNF4 binding site was not activated in HCV-positive cells. Consistent with this hypothesis, HNF4 protein and mRNA levels as well as HNF4 promoter activity and DNA binding activity were increased. The expression of HNF1 seems to play a critical role in the induction of hepatic MRP2 secondary to HCV subgenomic replication. The ability of HCV to induce HNF1 and HNF4 is attributed to 1) increased oxidative stress and 2) direct protein-protein interactions between HCV nonstructural component (NS) 5A and HNF1, leading to enhanced HNF1 DNA binding. In conclusion, we describe a novel mechanism by which HCV gene expression may induce adaptive responses involving MRP2 via HNF1 activation. This may constitute, in part, the cellular detoxification task force during HCV infection.

Functional expression of sinusoidal and canalicular hepatic drug transporters in the differentiated human hepatoma HepaRG cell line

Functional expression of both sinusoidal and canalicular hepatic drug transporters was investigated in the highly differentiated human hepatoma HepaRG cell line and also, for comparison, in primary human hepatocytes and in the hepatoma HepG2 cell line. Using RT-qPCR assays, differentiated HepaRG cells were found to display a pattern of transporter expression close to that found in primary human hepatocytes, i.e. they exhibit substantial mRNA levels of the influx transporters OCT1, OATP-B, OATP-C and NTCP, and of the secretion transporters MRP2, MRP3, BSEP and P-glycoprotein. By contrast, expression of influx transporters was not present or very weak in HepG2 cells. Drug transport assays allowed to detect functional activities of OCT1, OATPs/OAT2, NTCP, MRPs and P-glycoprotein in differentiated HepaRG cells as in primary human hepatocytes whereas HepG2 cells only showed notable MRP and P-glycoprotein activities. In addition, expression of canalicular transporters in HepaRG cells was found to be up-regulated by known inducers of transporters such as rifampicin, phenobarbital and chenodeoxycholate acting on P-glycoprotein, MRP2 and BSEP, respectively. HepaRG cells thus exhibit functional expression of both sinusoidal and canalicular drug transporters and have retained regulatory pathways controlling transporter levels. These data, associated with the known high expression of drug metabolizing enzymes in HepaRG cells, highlight the interest of such hepatoma cells for analysing hepatic drug detoxification pathways.

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.

Lignans, bacteriocides and organochlorine compounds activate the human pregnane X receptor (PXR)

The pregnane X receptor (PXR) mediates the induction of enzymes involved in steroid metabolism and xenobiotic detoxification. The receptor is expressed in liver and intestinal tissues and is activated by a wide range of compounds. The ability of a diverse range of dietary compounds to activate PXR-mediated transcription was assayed in HuH7 cells following transient transfection with human PXR (hPXR). The compounds investigated included phytochemicals such as lignans and phytoestrogens, organochlorine dietary contaminants such as polychlorinated biphenyls (PCBs) and triclosan and selected steroid, drug and herbal compounds. The hPXR activation at the top concentrations tested (10 microM) relative to the positive control 10 microM rifampicin ranged from 1.3% (trans-resveratrol) to 152% (ICI 182780). Hydroxylated compounds were marginally more potent than the parent compounds (tamoxifen activation was 74.6% whereas 4 hydroxytamoxifen activation was 84.2%) or significantly greater (vitamin D3 activation was 1.6%, while hydroxylated vitamin D3 activation was 55.6%). Enterolactone, the metabolite of common dietary lignans, was a medium activator of PXR (35.6%), compared to the lower activation of a parent lignan, secoisolariciresinol (20%). Two non-hydroxylated PCB congeners (PCB 118 and 153), which present a larger fraction of the PCB contamination of fatty foods, activated hPXR by 26.6% and 17%, respectively. The pesticide trans-nonachlor activation was 53.8%, while the widely used bacteriocide triclosan was a medium activator of hPXR at 46.2%. The responsiveness of PXR to activation by lignan metabolites suggests that dietary intake of these compounds may affect the metabolism of drugs that are CYP3A substrates. Additionally, the evidence that organochlorine chemicals, particularly the ubiquitous triclosan, activate hPXR suggests that these environmental chemicals may, in part, exhibit their endocrine disruptor activities by altering PXR-regulated steroid hormone metabolism with potential adverse health effects in exposed individuals.

Multidrug resistance in Staphylococcus aureus due to overexpression of a novel multidrug and toxin extrusion (MATE) transport protein

Efflux is an important mechanism of multidrug resistance (MDR) in bacteria. The multidrug and toxin extrusion (MATE) family is the most recently described group of MDR efflux proteins, none of which have previously been identified in Staphylococcus aureus. Two independently derived S. aureus mutants having efflux-related MDR phenotypes were studied using microarray technology and a marked overexpression of an open reading frame (ORF; mepA) encoding a protein homologous with MATE family proteins was observed in both. There was concomitant overexpression of ORFs in close proximity to mepA (approximately 100 bp) encoding a MarR-type regulator (mepR, upstream of mepA) and a protein of unknown function (mepB, downstream). Experiments in which mepA was overexpressed or disrupted revealed that the encoded protein has a broad substrate profile that includes several monovalent and divalent biocides and the fluoroquinolone antimicrobial agents norfloxacin and ciprofloxacin. The function of MepB is obscure, it does not contribute to the MDR phenotype conferred by MepA. MepR overexpression reversed the MDR phenotypes of both mutants by repressing mepA transcription. All three ORFs are preferentially transcribed as a single mepRAB unit, suggesting that the three genes form an operon.