Association of cytochrome P450 induction with oxidative stress in vivo as evidenced by 3-hydroxylation of salicylate

Toxicological and pharmacokinetic properties of sucralose-6-acetate and its parent sucralose: in vitro screening assays

The purpose of this study was to determine the toxicological and pharmacokinetic properties of sucralose-6-acetate, a structural analog of the artificial sweetener sucralose. Sucralose-6-acetate is an intermediate and impurity in the manufacture of sucralose, and recent commercial sucralose samples were found to contain up to 0.67% sucralose-6-acetate. Studies in a rodent model found that sucralose-6-acetate is also present in fecal samples with levels up to 10% relative to sucralose which suggest that sucralose is also acetylated in the intestines. A MultiFlow® assay, a high-throughput genotoxicity screening tool, and a micronucleus (MN) test that detects cytogenetic damage both indicated that sucralose-6-acetate is genotoxic. The mechanism of action was classified as clastogenic (produces DNA strand breaks) using the MultiFlow® assay. The amount of sucralose-6-acetate in a single daily sucralose-sweetened drink might far exceed the threshold of toxicological concern for genotoxicity (TTCgenotox) of 0.15 µg/person/day. The RepliGut® System was employed to expose human intestinal epithelium to sucralose-6-acetate and sucralose, and an RNA-seq analysis was performed to determine gene expression induced by these exposures. Sucralose-6-acetate significantly increased the expression of genes associated with inflammation, oxidative stress, and cancer with greatest expression for the metallothionein 1 G gene (MT1G). Measurements of transepithelial electrical resistance (TEER) and permeability in human transverse colon epithelium indicated that sucralose-6-acetate and sucralose both impaired intestinal barrier integrity. Sucralose-6-acetate also inhibited two members of the cytochrome P450 family (CYP1A2 and CYP2C19). Overall, the toxicological and pharmacokinetic findings for sucralose-6-acetate raise significant health concerns regarding the safety and regulatory status of sucralose itself.

Xenobiotica-metabolizing enzyme induction potential of chemicals in animal studies: NanoString nCounter gene expression and peptide group-specific immunoaffinity as accelerated and economical substitutions for enzyme activity determinations?

Xenobiotica-metabolizing enzyme (XME) induction is a relevant biological/biochemical process vital to understanding the toxicological profile of xenobiotics. Early recognition of XME induction potential of compounds under development is therefore important, yet its determination by traditional XME activity measurements is time consuming and cost intensive. A proof-of-principle study was therefore designed due to the advent of faster and less cost-intensive methods for determination of enzyme protein and transcript levels to determine whether two such methods may substitute for traditional measurement of XME activity determinations. The results of the study show that determination of enzyme protein levels by peptide group-specific immunoaffinity enrichment/MS and/or determination of gene expression by NanoString nCounter may serve as substitutes for traditional evaluation methodology and/or as an early predictor of potential changes in liver enzymes. In this study, changes of XME activity by the known standard XME inducers phenobarbital, beta-naphthoflavone and Aroclor 1254 were demonstrated by these two methods. To investigate the applicability of these methods to demonstrate XME-inducing activity of an unknown, TS was also examined and found to be an XME inducer. More specifically, TS was found to be a phenobarbital-type inducer (likely mediated by CAR rather than PXR as nuclear receptor), but not due to Ah receptor-mediated or antioxidant response element-mediated beta-naphthoflavone-type induction. The results for TS were confirmed via enzymatic activity measurements. The results of the present study demonstrate the potential applicability of NanoString nCounter mRNA quantitation and peptide group-specific immunoaffinity enrichment/MS protein quantitation for predicting compounds under development to be inducers of liver XME activity.

Variation in Mitochondria-Derived Transcript Levels Associated With DDT Resistance in the 91-R Strain of Drosophila melanogaster (Diptera: Drosophilidae)

The organochloride insecticide dichlorodiphenyltrichloroethane (DDT) and its metabolites can increase cellular levels of reactive oxygen species (ROS), cause mitochondrial dysfunction, and induce apoptosis. The highly DDT-resistant Drosophila melanogaster Meigen 1830 (Drosophila) strain, 91-R, and its susceptible control, 91-C, were used to investigate functional and structural changes among mitochondrial-derived pathways. Resequencing of mitochondrial genomes (mitogenomes) detected no structural differences between 91-R and 91-C, whereas RNA-seq suggested the differential expression of 221 mitochondrial-associated genes. Reverse transcriptase-quantitative PCR validation of 33 candidates confirmed that transcripts for six genes (Cyp12d1-p, Cyp12a4, cyt-c-d, COX5BL, COX7AL, CG17140) were significantly upregulated and two genes (Dif, Rel) were significantly downregulated in 91-R. Among the upregulated genes, four genes are duplicated within the reference genome (cyt-c-d, CG17140, COX5BL, and COX7AL). The predicted functions of the differentially expressed genes, or known functions of closely related genes, suggest that 91-R utilizes existing ROS regulation pathways of the mitochondria to combat increased ROS levels from exposure to DDT. This study represents, to our knowledge, the initial investigation of mitochondrial genome sequence variants and functional adaptations in responses to intense DDT selection and provides insights into potential adaptations of ROS management associated with DDT selection in Drosophila.

Cytochromes p450 and skin cancer: role of local endocrine pathways

Skin is the largest body organ forming a metabolically active barrier between external and internal environments. The metabolic barrier is composed of cytochromes P450 (CYPs) that regulate its homeostasis through activation or inactivation of biologically relevant molecules. In this review we focus our attention on local steroidogenic and secosteroidogenic systems in relation to skin cancer, e.g., prevention, attenuation of tumor progression and therapy. The local steroidogenic system is composed of locally expressed CYPs involved in local production of androgens, estrogens, gluco- and mineralo-corticosteroids from cholesterol (initiated by CYP11A1) or from steroid precursors delivered to the skin, and of their metabolism and/or inactivation. Cutaneous 7-hydroxylases (CYP7A1, CYP7B1 and CYP39) potentially can produce 7-hydroxy/oxy-steroids/sterols with modifying effects on local tumorigenesis. CYP11A1 also transforms 7-dehydrocholesterol (7DHC)→22(OH)7DHC→20,22(OH)2-7DHC→7-dehydropregnenolone, which can be further metabolized to other 5,7- steroidal dienes. These 5,7-dienal intermediates are converted by ultraviolet radiation B (UVB) into secosteroids which show pro-differentiation and anti-cancer properties. Finally, the skin is the site of activation of vitamin D3 through two alternative pathways. The classical one involves sequential hydroxylation at positions 25 and 1 to produce active 1,25(OH)2D3, which is further inactivated through hydroxylation at C24. The novel pathway is initiated by CYP11A1 with predominant production of 20(OH)D3 which is further metabolized to biologically active but non-calcemic D3-hydroxyderivatives. Classical and non-classical (novel) vitamin D analogs show pro-differentiation, anti-proliferative and anticancer properties. In addition, melatonin is metabolized by local CYPs. In conclusion cutaneously expressed CYPs have significant effects on skin physiology and pathology trough regulation of its chemical milieu.

Characterization of the environmental quality of sediments from two estuarine systems based on different in-vitro bioassays

This study characterizes the environmental quality of sediments from the Arade and Guadiana estuaries using different in-vitro bioassays: a) fish hepatoma cell line (PLHC-1) to determine cytotoxicity and presence of CYP1A and oxidative stress inducing agents; b) gonad subcellular fractions from sea bass (Dicentrarchus labrax) to detect compounds that are likely to act as endocrine disrupters by interfering with the synthesis of androgens (CYP17, CYP11β) and estrogens (CYP19). Approximately 60% of extracts from the Arade estuary were cytotoxic when tested at 60 mg eQsed/mL, while only one sample from Guadiana showed cytotoxicity. Sediments from Arade collected close to harbours and waste water effluents were enriched with CYP1A inducing agents, while those from the upper Guadiana induced oxidative stress in PLHC-1 cells. On the other hand, several extracts from both estuaries were able to significantly inhibit CYP17, CYP11β and CYP19 activities in gonad subcellular fractions of sea bass, which indicates the presence of endocrine disrupters, particularly in several sites from the Arade estuary. Overall, the study highlights the usefulness of in-vitro bioassays to identify those sediments that could pose risk to aquatic organisms and that require further action to improve their environmental quality.

Toxicity and elemental composition of particulate matter from outdoor and indoor air of elementary schools in Munich, Germany

Outdoor particulate matter (PM(10)) is associated with detrimental health effects. However, individual PM(10) exposure occurs mostly indoors. We therefore compared the toxic effects of classroom, outdoor, and residential PM(10). Indoor and outdoor PM(10) was collected from six schools in Munich during teaching hours and in six homes. Particles were analyzed by scanning electron microscopy and X-ray spectroscopy (EDX). Toxicity was evaluated in human primary keratinocytes, lung epithelial cells and after metabolic activation by several human cytochromes P450. We found that PM(10) concentrations during teaching hours were 5.6-times higher than outdoors (117 ± 48 μg/m(3) vs. 21 ± 15 μg/m(3), P < 0.001). Compared to outdoors, indoor PM contained more silicate (36% of particle number), organic (29%, probably originating from human skin), and Ca-carbonate particles (12%, probably originating from paper). Outdoor PM contained more Ca-sulfate particles (38%). Indoor PM at 6 μg/cm(2) (10 μg/ml) caused toxicity in keratinocytes and in cells expressing CYP2B6 and CYP3A4. Toxicity by CYP2B6 was abolished with the reactive oxygen species scavenger N-acetylcysteine. We concluded that outdoor PM(10) and indoor PM(10) from homes were devoid of toxicity. Indoor PM(10) was elevated, chemically different and toxicologically more active than outdoor PM(10). Whether the effects translate into a significant health risk needs to be determined. Until then, we suggest better ventilation as a sensible option.

PRACTICAL IMPLICATIONS

Indoor air PM(10) on an equal weight base is toxicologically more active than outdoor PM(10). In addition, indoor PM(10) concentrations are about six times higher than outdoor air. Thus, ventilation of classrooms with outdoor air will improve air quality and is likely to provide a health benefit. It is also easier than cleaning PM(10) from indoor air, which has proven to be tedious.

Dietary supplementation with vitamin E and C attenuates dexamethasone-induced glucose intolerance in rats

Glucocorticoid excess induces marked insulin resistance and glucose intolerance. A recent study has shown that antioxidants prevent dexamethasone (DEX)-induced insulin resistance in cultured adipocytes. The purpose of this investigation was to examine the effects of dietary vitamin E and C (Vit E/C) supplementation on DEX-induced glucose intolerance in rats. We hypothesized that feeding rats a diet supplemented with Vit E/C would improve glucose tolerance and restore insulin signaling in skeletal muscle, adipose, and liver and prevent alterations in AMPK signaling in these tissues. Male Wistar rats received either a control or Vit E/C-supplemented diet (0.5 g/kg diet each of L-ascorbate and DL-all rac-alpha-tocopherol) for 9 days prior to, and during, 5 days of daily DEX treatment (subcutaneous injections 0.8 mg/g body wt). DEX treatment resulted in increases in the glucose and insulin area under the curve (AUC) during an intraperitoneal glucose tolerance test. The glucose, but not insulin, AUC was lowered with Vit E/C supplementation. Improvements in glucose tolerance occurred independent of a restoration of PKB phosphorylation in tissues of rats stimulated with an intraperitoneal injection of insulin but were associated with increases in AMPK signaling in muscle and reductions in AMPK signaling and the expression of fatty acid oxidation enzymes in liver. There were no differences in mitochondrial enzymes in triceps muscles between groups. This study is the first to report that dietary Vit E/C supplementation can partially prevent DEX-induced glucose intolerance in rats.

Systematic genetic and genomic analysis of cytochrome P450 enzyme activities in human liver

Liver cytochrome P450s (P450s) play critical roles in drug metabolism, toxicology, and metabolic processes. Despite rapid progress in the understanding of these enzymes, a systematic investigation of the full spectrum of functionality of individual P450s, the interrelationship or networks connecting them, and the genetic control of each gene/enzyme is lacking. To this end, we genotyped, expression-profiled, and measured P450 activities of 466 human liver samples and applied a systems biology approach via the integration of genetics, gene expression, and enzyme activity measurements. We found that most P450s were positively correlated among themselves and were highly correlated with known regulators as well as thousands of other genes enriched for pathways relevant to the metabolism of drugs, fatty acids, amino acids, and steroids. Genome-wide association analyses between genetic polymorphisms and P450 expression or enzyme activities revealed sets of SNPs associated with P450 traits, and suggested the existence of both cis-regulation of P450 expression (especially for CYP2D6) and more complex trans-regulation of P450 activity. Several novel SNPs associated with CYP2D6 expression and enzyme activity were validated in an independent human cohort. By constructing a weighted coexpression network and a Bayesian regulatory network, we defined the human liver transcriptional network structure, uncovered subnetworks representative of the P450 regulatory system, and identified novel candidate regulatory genes, namely, EHHADH, SLC10A1, and AKR1D1. The P450 subnetworks were then validated using gene signatures responsive to ligands of known P450 regulators in mouse and rat. This systematic survey provides a comprehensive view of the functionality, genetic control, and interactions of P450s.

In vivo oxidative damage in rats is associated with barbiturate response but not other cytochrome P450 inducers

Previously published studies have shown that cytochrome P450 (P450) enzyme systems can produce reactive oxygen species and suggest roles of P450s in oxidative stress. However, most of the studies have been done in vitro, and the potential link between P450 induction and in vivo oxidative damage has not been rigorously explored with validated biomarkers. Male Sprague-Dawley rats were pretreated with typical P450 inducers (beta-naphthoflavone, phenobarbital (PB), Aroclor 1254, isoniazid, pregnenolone 16alpha-carbonitrile, and clofibrate) or the general P450 inhibitor 1-aminobenztriazole; induction of P4501A, -2B, -2E, -3A, and -4A subfamily enzymes was confirmed by immunoblotting and the suppression of P450 by 1-aminobenztriazole using spectral analysis. PB and Aroclor 1254 significantly enhanced malondialdehyde and H2O2 generation and NADPH oxidation in vitro and significantly enhanced formation in vivo, in both liver and plasma. Some of the other treatments changed in vitro parameters but none did in vivo. The PB-mediated increases in liver and plasma F2-isoprostanes could be ablated by 1-aminobenztriazole, implicating the PB-induced P450(s) in the F2-isoprostane elevation. The markers of in vivo oxidative stress were influenced mainly by PB and Aroclor 1254, indicative of an oxidative damage response only to barbiturate-type induction and probably related to 2B subfamily enzymes. These studies define the contribution of P450s to oxidative stress in vivo, in that the phenomenon is relatively restricted and most P450s do not contribute substantially.

An alternatively spliced cytochrome P4501A1 in human brain fails to bioactivate polycyclic aromatic hydrocarbons to DNA-reactive metabolites

CYP1A1, a cytochrome P450 enzyme, metabolizes polycyclic aromatic hydrocarbons to genotoxic metabolite(s) that bind to DNA and initiate carcinogenesis. RT-PCR amplification of the complete open reading frame of CYP1A1 generated an amplicon of 1593 bp having deletion of 87 bp of exon-6 that translated into functional P450 enzyme. Unlike wild type CYP1A1, exon 6 del CYP1A1 did not metabolize polycyclic aromatic hydrocarbons such as, benzo(a)pyrene to genotoxic, ultimate carcinogens that form DNA adducts. Exon 6 del CYP1A1 metabolized ethoxyresorufin (the classical substrate for CYP1A1) less efficiently compared with wild type CYP1A1 while pentoxy and benzyloxyresorufin (classical substrates for CYP2B) were dealkylated more efficiently. In silico docking showed alteration of the substrate access channel in exon 6 del CYP1A1 such that benzo(a)pyrene does not bind in any orientation that would permit the formation of carcinogenic metabolites. Genotyping revealed that the splice variant was not generated due to differences in genomic DNA sequence and the variant was present only in brain but not in liver, kidney, lung, or heart from the same individual. We provide evidence that unique P450 enzymes, generated by alternate splicing in a histiospecific manner can modify genotoxic potential of carcinogens such as benzo(a)pyrene by altering their biotransformation pathway.

Design of a method for generation of gas-phase hydroxyl radicals, and use of HPLC with fluorescence detection to assess the antioxidant capacity of natural essential oils

The use of natural antioxidants is of increasing importance in the human diet, because they are recognised as compounds essential to health which minimize or delay the aging process. Despite apparent simplicity, however, it is very difficult to measure and quantify such properties, for which a robust analytical method is required. Because oxidation usually is caused by the presence of OH* radicals, a new method involving the in-situ, vapour-phase generation of these radicals and their quantification in the presence and absence of potential antioxidant extracts has been developed. The oxidant atmosphere generated from hydrogen peroxide is carried by an air stream through an empty quartz chamber in which UV radiation promotes the formation of radicals by a photochemical reaction. The products then pass through a cartridge containing the essential oil, finally bubbling into an impinger containing an aqueous solution of salicylic acid, at pH 4.5, which reacts with the OH* radicals forming 2,5-dihydroxybenzoic acid. This solution is quantified by RP-HPLC using UV and fluorescence detectors connected in series. Detection and quantification limits for OH* radicals were approximately 0.01 pg g(-1) air. Description and optimization of the method are discussed, as also is the antioxidant performance of an extract of ginger (Zingiber officinale R.), which reduced the oxidation process by up to 92%.

DDT-induced oxidative damage in human blood mononuclear cells

Recent work indicates that DDT and its metabolites induce apoptosis in different cellular types. However, the mechanism by which DDT generates apoptosis has not been elucidated. In this study, our data demonstrate that the apoptosis induction by DDT and its metabolites in human peripheral blood mononuclear cells (PBMC) is preceded by an increase in the levels of reactive oxygen species (ROS). Cells isolated from healthy individuals were incubated for different intervals of time (0-24 h) and in the presence of increasing concentrations of p'p-DDT, p'p-DDE, or p'p-DDD (0-80 microg/ml). The induction of oxidative stress was then determined by flow cytometry, using the compound 2',7'-dichlorofluorescein diacetate. The control level of ROS was 4.46+/-0.96 IFM, for DDT- and DDD-treated cells we obtained a 19.0-fold increment, whereas for DDE, the increment was 25-fold. ROS induction by DDT and DDE was observed after 1 h of incubation, while for DDD such levels began to be detected at 3 h of incubation; a maximum effect on the ROS production for the three compounds was found at 6 h of treatment. A significant level of ROS was induced by DDT, DDE, and DDD only at 60 and 80 microg/ml. Finally, to find an association between generation of ROS and apoptosis induction, cells incubated with DDT, DDE, and DDD were evaluated for apoptosis induction and generation of oxidative stress. Our results show that an increase in ROS was accompanied by apoptosis of PBMC in vitro. Moreover, N-acetyl-L-cysteine significantly inhibits the apoptosis induction.

Constitutive expression and localization of cytochrome P‐450 1A1 in rat and human brain: presence of a splice variant form in human brain 1

Cytochrome P-450 function as mono-oxygenases and metabolize xenobiotics. CYP1A1, a cytochrome P-450 enzyme, bioactivates polycyclic aromatic hydrocarbons to reactive metabolite(s) that bind to DNA and initiate carcinogenesis. Northern and immunoblot analyses revealed constitutive expression of Cyp1a1 and CYP1A1 in rat and human brain, respectively. CYP1A1 mRNA and protein were localized predominantly in neurons of cerebral cortex, Purkinje and granule cell layers of cerebellum and pyramidal neurons of CA1, CA2, and CA3 subfields of the hippocampus. RT-PCR analyses using RNA obtained from autopsy human brain samples demonstrated the presence of a splice variant having a deletion of 87 bp of exon 6. This splice variant was present in human brain, but not in the liver from the same individual, and was absent in rat brain and liver. Structural modeling indicated broadening of the substrate access channel in the brain variant. The study demonstrates the presence of a unique cytochrome P-450 enzyme in human brain that is generated by alternate splicing. The presence of distinct cytochrome P-450 enzymes in human brain that are different from well-characterized hepatic forms indicates that metabolism of xenobiotics including drugs could occur in brain by pathways different from those known to occur in liver.

Characterization of hydroxyl radical formation by microsomal enzymes using a water-soluble trap, terephthalate

Using terephthalic acid as a water-soluble trap, we characterized hydroxyl radicals (HO?) formation by liver microsomal enzymes from isoniazid-treated rats. We found that HO? formation was entirely dependent on intact microsomal enzymes, the presence of NADPH, and iron complexed with EDTA. In contrast to the other radical traps, we found no evidence that terephthalate is a substrate for cytochrome P450. Cumene hydroperoxide, an artificial supporter of cytochrome P450-catalyzed oxidation, failed to maintain HO(.-) formation. HO(.-) formation in liver microsomes was inhibited by the HO(.-) radical scavengers: dimethyl sulfoxide (DMSO), mannitol, and citrulline. It was abolished by catalase, but not superoxide dismutase (SOD), indicating that hydrogen peroxide was the sole precursor of the HO(.-). Therefore, the generation of hydroxyl radicals by microsomal enzymes appears to be dependent on two processes: (1) the rate of hydrogen peroxide production; and (2) the availability of iron ions or other transition metals for Fenton type reactions.

The effect of valproic acid on hepatic and plasma levels of 15-F2t-isoprostane in rats

The mechanism by which valproic acid (VPA) induces liver injury remains unknown, but it is hypothesized to involve the generation of toxic metabolites and/or reactive oxygen species. This study's objectives were to determine the effect of VPA on plasma and hepatic levels of the F(2)-isoprostane, 15-F(2t)-IsoP, a marker for oxidative stress, and to investigate the influence of cytochrome P450- (P450-) mediated VPA biotransformation on 15-F(2t)-IsoP levels in rats. In rats treated with VPA (500 mg/kg), plasma 15-F(2t)-IsoP was increased 2.5-fold at t(max) = 0.5 h. Phenobarbital pretreatment (80 mg/kg/d for 4 d) in VPA-treated rats increased plasma and liver levels of free 15-F(2t)-IsoP by 5-fold and 3-fold, respectively, when compared to control groups. This was accompanied by an elevation in plasma and liver levels of P450-mediated VPA metabolites. Pretreatment with SKF-525A (80 mg/kg) or 1-aminobenzotriazole (100 mg/kg), which inhibited P450-mediated VPA metabolism, did not attenuate the increased levels of plasma 15-F(2t)-IsoP in VPA-treated groups. Plasma and hepatic levels of 15-F(2t)-IsoP were further elevated after 14 d of VPA treatment compared to single-dose treatment. Our data indicate that VPA increases plasma and hepatic levels of 15-F(2t)-IsoP and this effect can be enhanced by phenobarbital by a mechanism not involving P450-catalyzed VPA biotransformation.

Effects of aspirin and/or salicylate on hydrolysis and glucuronidation of indomethacin in rat erythrocytes and hepatocytes

This study was conducted to explore the mechanism of the pharmacokinetic interaction between aspirin (ASP) and indomethacin (IND) using rat erythrocytes (RBCs) and hepatocytes. ASP was hydrolyzed to salicylic acid (SA) in both the RBCs and hepatocytes. Within RBCs, aspirin and/or salicylate (ASP/SA) increased the concentration of IND, accompanied by a constant hydrolysis of IND. In hepatocytes, a low dose of IND was subjected to glucuronidation rather than hydrolysis, and ASP/SA inhibited both the acylglucuronidation of IND and hydrolysis of IND glucuronide. A high dose of IND underwent hydrolysis with about double the glucuronidation, and ASP/SA decreased the ratio of hydrolysis to glucuronidation, accompanied by a loss of ASP, IND and their metabolites from the medium. Collectively, the results provide metabolic insight into the mechanism of drug-drug interaction between ASP/SA and IND in the hepatocytes and RBCs.

New hepatocyte in vitro systems for drug metabolism: metabolic capacity and recommendations for application in basic research and drug development, standard operation procedures

Primary hepatocytes represent a well-accepted in vitro cell culture system for studies of drug metabolism, enzyme induction, transplantation, viral hepatitis, and hepatocyte regeneration. Recently, a multicentric research program has been initiated to optimize and standardize new in vitro systems with hepatocytes. In this article, we discuss five of these in vitro systems: hepatocytes in suspension, perifusion culture systems, liver slices, co-culture systems of hepatocytes with intestinal bacteria, and 96-well plate bioreactors. From a technical point of view, freshly isolated or cryopreserved hepatocytes in suspension represent a readily available and easy-to-handle in vitro system that can be used to characterize the metabolism of test substances. Hepatocytes in suspension correctly predict interspecies differences in drug metabolism, which is demonstrated with pantoprazole and propafenone. A limitation of the hepatocyte suspensions is the length of the incubation period, which should not exceed 4hr. This incubation period is sufficiently long to determine the metabolic stability and to allow identification of the main metabolites of a test substance, but may be too short to allow generation of some minor, particularly phase II metabolites, that contribute less than 3% to total metabolism. To achieve longer incubation periods, hepatocyte culture systems or bioreactors are used. In this research program, two bioreactor systems have been optimized: the perifusion culture system and 96-well plate bioreactors. The perifusion culture system consists of collagen-coated slides allowing the continuous superfusion of a hepatocyte monolayer with culture medium as well as establishment of a constant atmosphere of 13% oxygen, 82% nitrogen, and 5% CO2. This system is stable for at least 2 weeks and guarantees a remarkable sensitivity to enzyme induction, even if weak inducers are tested. A particular advantage of this systemis that the same bioreactor can be perfused with different concentrations of a test substance in a sequential manner. The 96-well plate bioreactor runs 96 modules in parallel for pharmacokinetic testing under aerobic culture conditions. This system combines the advantages of a three-dimensional culture system in collagen gel, controlled oxygen supply, and constant culture medium conditions, with the possibility of high throughput and automatization. A newly developed co-culture system of hepatocytes with intestinal bacteria offers the possibility to study the metabolic interaction between liver and intestinal microflora. It consists of two chambers separated by a permeable polycarbonate membrane, where hepatocytes are cultured under aerobic and intestinal bacteria in anaerobic conditions. Test substances are added to the aerobic side to allow their initial metabolism by the hepatocytes, followed by the metabolism by intestinal bacteria at the anaerobic side. Precision-cut slices represent an alternative to isolated hepatocytes and have been used fo the investigation of hepatic metabolism, hepatotoxicity, and enzyme induction. A specific advantage of liver slices is the possibility to study toxic effects on hepatocytes that are mediated or modified by nonparenchymal cells (e.g., by cytokine release from Kupffer cells) because the physiological liver microarchitecture is maintained in cultured slices. For all these in vitro systems, a prevalidation has been performed using standard assays for phase I and II enzymes. Representative results with test substances and recommendations for application of these in vitro systems, as well as standard operation procedures are given.

Inflammation, reactive oxygen species and cytochrome P450

Inflammation may ultimately result from damage to membrane lipids by reactive oxygen species (ROS) such as peroxide, superoxide anion, hydroxyl radical and singlet oxygen. This study compares some of the methods used to determine ROS-ethane exhalation, malondialdehyde quantified as thiobarbituric acid-reacting materials, and luminol-activated chemiluminescence (LAC)-and explores possible relationships with oedema formation in the rat foot-pad model. Iron nitrilotriacetate was the most effective of the model compounds tested in producing lipid peroxidation and ethane exhalation in mice. In the mouse and the rat, iron nitrilotriacetate caused increased ethane exhalation and concomitant increases in liver and kidney malondialdehyde. In the rat foot-pad oedema model, the challenge with Freund's complete adjuvant produced maximum malondialdehyde and maximum LAC in the inflamed paw 8 h after dosing, at which time oedema had also reached a high level. These effects were attributed mainly to hydroxyl radical and singlet oxygen. The inhibition of oedema by four anti-inflammatory drugs correlated well with LAC but less well with inhibition of malondialdehyde production. This study shows good agreement between different methods of determining ROS formation, and that inhibition of ROS formation in vivo is paralleled by a decrease in inflammation.

Enzyme induction and inhibition by new antiepileptic drugs: a review of human studies

The aim of this paper is to review a number of new antiepileptic agents (i.e. felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin and zonisamide) for their inducing and/or inhibitory properties in humans, mainly considering the interactions where they are involved as the cause rather than the object of such interactions. Two aspects have been particularly taken into account: the changes or absence of changes in plasma/serum concentrations of concomitant drugs and the direct or indirect evidence of induction, inhibition or lack of effect on the six major human hepatic CYP isozymes (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4), as well as on other CYP isozymes or enzyme systems. Felbamate clearly affects the pharmacokinetics of a number of drugs, generally increasing but also decreasing their concentrations. It induces enzymes such as CYP3A4 and inhibits enzymes such as CYP2C19 and those of the beta-oxidation pathway. Topiramate is not devoid of potential interaction properties: it decreases the plasma concentrations of ethinylestradiol, induces CYP3A4 and inhibits CYP2C19. For oxcarbazepine, no inhibitory, only inductive effects have been observed thus far. Felbamate. topiramate and oxcarbazepine may induce the metabolism of steroidal oral contraceptives. In this respect, tiagabine has been studied at a rather low dose. Pharmacodynamic or pharmacokinetic interaction seems to exist between lamotrigine and carbamazepine. Lamotrigine appears to be a weak inducer of UGTs, whereas induction of CYP3A4 seems improbable as the compound does not change the concentrations of oral contraceptives or the urinary excretion of 6beta-hydroxycortisol. Zonisamide has very peculiar pharmacokinetics and an extensive metabolism. Additional information on its enzyme inducing or inhibiting properties would be necessary, as data so far collected on its effect on the pharmacokinetics of other drugs are conflicting. Gabapentin, vigabatrin and in particular levetiracetam appear to be devoid of significant enzyme inducing or inhibiting properties.