Effect of xenobiotics on monooxygenase activities in cultured human hepatocytes

Induction of drug metabolizing enzyme and drug transporter expression by antifungal triazole pesticides in human HepaSH hepatocytes

Triazole pesticides are widely used fungicides, to which humans are rather highly exposed. They are known to activate drug-sensing receptors regulating expression of hepatic drug metabolizing enzymes and drug transporters, thus suggesting that the hepatic drug detoxification system is modified by these agrochemicals. To investigate this hypothesis, the effects of 9 triazole fungicides towards expression of drug metabolizing enzymes and transporters were characterized in cultured human HepaSH cells, that are human hepatocytes deriving from chimeric humanized liver TK-NOG mice. Most of triazoles used at 10 μM were found to act as inducers of cytochrome P-450 (CYP) 1A1, CYP1A2, CYP2B6, CYP3A4 and UDP-glucuronosyltransferase 1A1 mRNA levels and of CYP3A4 protein; some triazoles also enhanced mRNA expression of the canalicular transporters P-glycoprotein/MDR1, multidrug resistance-associated protein 2 and breast cancer resistance protein. Triazoles however concomitantly inhibited CYP2B6 and CYP3A4 activities and thus appeared as dual regulators of these CYPs, being both inducers of their expression and inhibitors of their activity. The inducing effect however predominated, at least for bromuconazole, propiconazole and tebuconazole. Bromuconazole was moreover predicted to enhance CYP2B6 and CYP3A4 expression in humans exposed to this fungicide in a chronic, acute or occupational context. These data demonstrate that key-actors of the human hepatic detoxification system are impacted by triazole pesticides, which may have to be considered for the risk assessment of these agrochemicals. They additionally highlight that the use of human HepaSH cells as surrogates to primary human hepatocytes represents an attractive and promising way for studying hepatic effects of environmental chemicals.

Induction by Phenobarbital of Phase I and II Xenobiotic-Metabolizing Enzymes in Bovine Liver: An Overall Catalytic and Immunochemical Characterization

In cattle, phenobarbital (PB) upregulates target drug-metabolizing enzyme (DME) mRNA levels. However, few data about PB's post-transcriptional effects are actually available. This work provides the first, and an almost complete, characterization of PB-dependent changes in DME catalytic activities in bovine liver using common probe substrates and confirmatory immunoblotting investigations. As expected, PB increased the total cytochrome P450 (CYP) content and the extent of metyrapone binding; moreover, an augmentation of protein amounts and related enzyme activities was observed for known PB targets such as CYP2B, 2C, and 3A, but also CYP2E1. However, contradictory results were obtained for CYP1A, while a decreased catalytic activity was observed for flavin-containing monooxygenases 1 and 3. The barbiturate had no effect on the chosen hydrolytic and conjugative DMEs. For the first time, we also measured the 26S proteasome activity, and the increase observed in PB-treated cattle would suggest this post-translational event might contribute to cattle DME regulation. Overall, this study increased the knowledge of cattle hepatic drug metabolism, and further confirmed the presence of species differences in DME expression and activity between cattle, humans, and rodents. This reinforced the need for an extensive characterization and understanding of comparative molecular mechanisms involved in expression, regulation, and function of DMEs.

Transfection of Primary Hepatocytes with Liver-Enriched Transcription Factors Using Adenoviral Vectors

Primary cultured hepatocytes are probably the best model to study endogenous metabolic pathways, toxicity, or drug metabolism. Many of these studies require expression of ectopic genes. It would be desirable to use a method of transfection that allows dose-response studies, high efficiency of transfection, and the possibility to express several genes at the same time. Adenoviral vectors fulfill these requirements, becoming a valuable tool for primary hepatocyte transfection. Moreover, they are easy to generate and do not require a high level of biocontainment. In the present chapter, we describe the generation, cloning, amplification, and purification of an adenoviral vector capable of infecting primary cultured hepatocytes. This recombinant adenovirus induces robust expression of the protein of interest in hepatocytes within a wide range of doses.

The Use of Long-term Hepatocyte Cultures for Detecting Induction of Drug Metabolising Enzymes: The Current Status

In this report, metabolically competent in vitro systems have been reviewed, in the context of drug metabolising enzyme induction. Based on the experience of the scientists involved, a thorough survey of the literature on metabolically competent long-term culture models was performed. Following this, a prevalidation proposal for the use of the collagen gel sandwich hepatocyte culture system for drug metabolising enzyme induction was designed, focusing on the induction of the cytochrome P450 enzymes as the principal enzymes of interest. The ultimate goal of this prevalidation proposal is to provide industry and academia with a metabolically competent in vitro alternative for long-term studies. In an initial phase, the prevalidation study will be limited to the investigation of induction. However, proposals for other long-term applications of these systems should be forwarded to the European Centre for the Validation of Alternative Methods for consideration. The prevalidation proposal deals with several issues, including: a) species; b) practical prevalidation methodology; c) enzyme inducers; and d) advantages of working with independent expert laboratories. Since it is preferable to include other alternative tests for drug metabolising enzyme induction, when such tests arise, it is recommended that they meet the same level of development as for the collagen gel sandwich long-term hepatocyte system. Those tests which do so should begin the prevalidation and validation process.

Evaluation of the cytotoxicity of 10 chemicals in human and rat hepatocytes and in cell lines: Correlation between in vitro data and human lethal concentration

The cytotoxicity of 10 chemicals from the Multicentre Evaluation of In vitro Cytotoxicity (MEIC) list (nos 21-30) was evaluated in human and rat cultured hepatocytes and in two established cell lines (HepG2 and 3T3) according to the MEIC programme organized by the Scandinavian Society of Cell Toxicology. The MTT test was used as the endpoint of cytotoxicity after 24hr of exposure to the chemicals. Theophylline, phenobarbital and paraquat were the least cytotoxic compounds in the cellular systems (IC(50) = 450-17,000 mum) except for the 3T3 cells. The seven remaining chemicals (dextropropoxyphene, propranolol, arsenic trioxide, cupric sulfate, mercuric chloride, thioridazine and thallium sulfate) showed a similar relative cytotoxic ranking in the four in vitro systems in the lower range of concentrations (IC(50) = 2-350 mum). The data suggest that these 10 chemicals have a basal cytotoxic effect common to the four in vitro systems, and probably none of these compounds could be considered either hepatotoxic or species specific. The correlation between in vitro data and human lethal blood concentrations showed that the predictability of the in vitro systems was similar to that of in vivo rodent tests (LD(50)) only when low cytotoxic concentrations (IC(10)) were used for correlation.

Evaluation of the cytotoxicity of ten chemicals on human cultured hepatocytes: Predictability of human toxicity and comparison with rodent cell culture systems

The cytotoxic effect of the first 10 chemicals on the MEIC list (evaluated in the Multicentre Evaluation of In Vitro Cytotoxicity organized by the Scandinavian Society of Cell Toxicology) was evaluated on human and rat cultured hepatocytes and in the non-hepatic murine 3T3 cell line. The MTT test was used as an endpoint to evaluate cytotoxicity after 24 hr of exposure to the chemicals. The predictability of human toxicity using human hepatocytes was analysed and compared with the results using rodent cell culture systems and rat and mouse LD(50) tests. Ferrous sulphate, diazepam and isopropyl alcohol produced about the same toxicity in all three cell culture models; paracetamol and acetylsalicylic acid were more toxic to human and rat hepatocytes than to mouse 3T3 cells; amitriptyline, ethylene glycol, methanol and ethanol were more toxic to human hepatocytes than to rodent cells. Digoxin was the most cytotoxic chemical to human hepatocytes (IC(50), 4.9 nm), the alcoholic compounds (isopropanol, ethylene glycol, ethanol and methanol) were the least toxic (IC(50), 125-819 mm) and paracetamol, acetylsalicylic acid, ferrous sulphate, diazepam and amitriptyline showed intermediate cytotoxicities (IC(50), 0.05-6 mm). The data suggest that for these 10 chemicals, acute toxicity in humans was more accurately predicted using human hepatocytes than using rat hepatocytes or mouse non-hepatic 3T3 cells.

Expression of Cytochromes P-450 2E1, 3A4 and 1A1/1A2 in Growing and Confluent Human HepG2 Hepatoma Cells-Effect of Ethanol

In cultured human hepatoma HepG2 cells, cytochrome (CYP) 1A-associated 7-ethoxyresorufin-O-deethylase (EROD), CYP 3A-associated benzyloxyresorufin O-debenzylase (BROD) and CYP 2E1-associated p-nitrophenol-hydroxylase (PNPH) decreased during time in culture. The enzyme activities in cells at confluence were 35-60% of the activities in cells 24 hours after seeding. Similarly, CYP 3A and CYP 2E1 proteins were present at higher concentrations in growing (G) than in confluent (C) HepG2 cells. CYP 1A1/1A2 protein was not detected, neither in G nor in C HepG2 cells but was strongly induced by 3-methylcholanthrene (3-MC) treatment. Ethanol (EtOH) was shown to increase CYP 2E1 and CYP 3A proteins and CYP 1A1/1A2-, CYP 2E1- and CYP 3A-associated mixed-function oxidase activities (MFOs) in HepG2 cells, as has been previously reported for primary cultures of human hepatocytes. These effects were observed only at the beginning of culture, in growing HepG2 cells, demonstrating the influence of the growth stage of HepG2 cells on their response to EtOH treatment. This is, to our knowledge, the first report on increases in CYP proteins and associated MFOs by EtOH in HepG2 cells. It suggests that growing HepG2 cells provide a useful in vitro model system in which to study the regulation of human CYPs by EtOH.

Permissive and suppressive effects of dexamethasone on enzyme induction in hepatocyte co-cultures

1. Steroids are known to act as permissive factors in hepatocytes. This study shows that dexamethasone (DEX) is a permissive factor for induction of CYP2B1/2, CYP3A1, CYP2A1 and probably also CYP2C11 in cultures with primary rat hepatocytes. 2. The induction factor of phenobarbital (PB)-induced formation of 16beta-hydroxytestosterone (OHT), a testosterone biotransformation product predominantly formed by CYP2B1, is increased 18-fold by the addition of 32 nM DEX to the culture medium. Interestingly, higher concentrations of DEX up to 1000 nM led to a concentration-dependent maximally 5-fold decrease (p = 0.002) of phenobarbital-induced 16beta-OHT formation compared with the effect observed with 32 nM DEX. Thus, DEX shows permissive and suppressive effects on enzyme induction depending on the concentration of the glucocorticoid. 3. Qualitatively similar but smaller permissive and suppressive effects of DEX were observed for PB-induced CYP3A1 activity as evidenced by formation of 2beta-, 6beta- and 15beta-OHT. 4. DEX is a permissive factor for induction of CYP2A1 activity by 3-methylcholanthrene (3MC), as evidenced by the formation of 7alpha-OHT. Without addition of DEX, 3MC did not induce formation of 7alpha-OHT, whereas an almost 3-fold induction occurred in the presence of DEX. In contrast to CYP2B and CYP3A, concentrations up to 1000 nM DEX were not suppressive for the induction of CYP2A1. 5. We described recently a technique that allows preparation of cultures from cryopreserved hepatocytes. An almost identical influence of dexamethasone on enzyme induction was observed here in cultures from cryopreserved compared with freshly isolated hepatocytes. 6. Cultures with primary hepatocyte cultures represent a well-established technique for the study of drug-drug interactions. However, a large interlaboratory variation is known. Our study provides evidence that differences in glucocorticoid concentration in the culture medium contribute to this variation.

Hepatocytes—the choice to investigate drug metabolism and toxicity in man: In vitro variability as a reflection of in vivo

The pharmaceutical industry is committed to marketing safer drugs with fewer side effects, predictable pharmacokinetic properties and quantifiable drug-drug interactions. Drug metabolism is a major determinant of drug clearance and interindividual pharmacokinetic differences, and an indirect determinant of the clinical efficacy and toxicity of drugs. Progressive advances in the knowledge of metabolic routes and enzymes responsible for drug biotransformation have contributed to understanding the great metabolic variations existing in human beings. Phenotypic as well genotypic differences in the expression of the enzymes involved in drug metabolism are the main causes of this variability. However, only a minor part of phenotypic variability in man is attributable to gene polymorphisms, thus making the definition of a normal liver complex. At present, the use of human in vitro hepatic models at early preclinical stages means that the process of selecting drug candidates is becoming much more rational. Cultured human hepatocytes are considered to be the closest model to human liver. However, the fact that hepatocytes are located in a microenvironment that differs from that of the cell in the liver raises the question: to what extent does drug metabolism variability observed in vitro actually reflect that of the liver in vivo? By comparing the metabolism of a model compound both in vitro and in vivo in the same individual, a good correlation between the in vitro and in vivo relative abundance of oxidized metabolites and the hydrolysis of the compound was observed. Thus, it is reasonable to consider that the variability observed in human hepatocytes reflects the existing phenotypic heterogeneity of the P450 expression in human liver.

Hepatocyte cell lines: their use, scope and limitations in drug metabolism studies

Gaining knowledge on the metabolism of a drug, the enzymes involved and its inhibition or induction potential is a necessary step in pharmaceutical development of new compounds. Primary human hepatocytes are considered a cellular model of reference, as they express the majority of drug-metabolising enzymes, respond to enzyme inducers and are capable of generating in vitro a metabolic profile similar to what is found in vivo. However, hepatocytes show phenotypic instability and have a restricted accessibility. Different alternatives have been explored in the past recent years to overcome the limitations of primary hepatocytes. These include immortalisation of adult or fetal human hepatic cells by means of transforming tumour virus genes, oncogenes, conditionally immortalised hepatocytes, and cell fusion. New strategies are currently being used to upregulate the expression of drug-metabolising enzymes in cell lines or to derive hepatocytes from progenitor cells. This paper reviews the features of liver-derived cell lines, their suitability for drug metabolism studies as well as the state-of-the-art of the strategies pursued in order to generate metabolically competent hepatic cell lines.

In vitro methods in human drug biotransformation research: implications for cancer chemotherapy

Anticancer drugs have a complex pharmacological and toxicological profile with a narrow therapeutic index. It is therefore critical to understand the factors that contribute to the marked intersubject variability in the pharmacokinetics and pharmacodynamics often observed with anticancer compounds. Since hepatic and extra-hepatic drug metabolism represents a major drug disposition pathway, extensive efforts are made to thoroughly investigate metabolism of anticancer compounds during the pre-clinical and clinical development phases as well as to address issues encountered during the clinical use of an approved drug. In recent years there has been a significant paradigm shift in pre-clinical/non-clinical drug metabolism studies. Most importantly, this has included a reduced reliance on animal models and increased use of human tissues (i.e. human liver microsomes and other cellular fractions, primary culture of human hepatocytes, cDNA expressed human-specific enzymes and cell-based reporter assays). Typically, experiments are performed using these tools to identify the phase I and/or phase II enzymes involved in metabolism of the drug/investigational agent and for metabolic fingerprinting. Additionally, issues pertaining to the rate, extent and mechanism(s) of the inhibition or induction of the metabolic pathways are also investigated. These studies provide important clues about various aspects of the disposition of a therapeutic agent including first-pass metabolism, elimination half-life, overall bioavailability and the potential for drug-drug interactions. The methodologies used for in vitro assessment of drug metabolism and their applications to drug development and clinical therapeutics with special emphasis on anticancer drugs are reviewed in this manuscript.

Drug metabolism by cultured human hepatocytes: how far are we from the in vivo reality?

The investigation of metabolism is an important milestone in the course of drug development. Drug metabolism is a determinant of drug pharmacokinetics variability in human beings. Fundamental to this are phenotypic differences, as well as genotypic differences, in the expression of the enzymes involved in drug metabolism. Genotypic variability is easy to identify by means of polymerase chain reaction-based or DNA chip-based methods, whereas phenotypic variability requires direct measurement of enzyme activities in liver, or, indirectly, measurement of the rate of metabolism of a given compound in vivo. There is a great deal of phenotypic variability in human beings, only a minor part being attributable to gene polymorphisms. Thus, enzyme activity measurements in a series of human livers, as well as in vivo studies with human volunteers, show that phenotypic variability is, by far, much greater than genotypic variability. In vitro models are currently used to investigate the hepatic metabolism of new compounds. Cultured human hepatocytes are considered to be the closest model to the human liver. However, the fact that hepatocytes are placed in a microenvironment that differs from that of the cells in the liver raises the question of to what extent drug metabolism variability observed in vitro actually reflects that in the liver in vivo. This issue has been examined by investigating the metabolism of the model compound, aceclofenac (an approved analgesic/anti-inflammatory drug), both in vitro and in vivo. Hepatocytes isolated from programmed liver biopsies were incubated with aceclofenac, and the metabolites formed were investigated by HPLC. The patients were given the drug during the course of clinical recovery, and the metabolites, largely present in urine, were analysed. In vitro and in vivo data from the same individual were compared. There was a good correlation between the in vitro and in vivo relative abundance of oxidised metabolites (4'-OH-aceclofenac + 4'-OH-diclofenac; Spearman's rho = 0.855), and the hydrolysis of aceclofenac (diclofenac + 4'-OH-aceclofenac + 4'-OH-diclofenac; rho = 0.691), while the conjugation of the drug in vitro was somewhat lower than in vivo. Globally, the metabolism of aceclofenac in vitro correlated with the amount of metabolites excreted in urine after 16 hours (rho = 0.95). Overall, although differing among assays, the in vitro/in vivo metabolism data for each patient were surprisingly similar. Thus, the variability observed in vitro appears to reflect genuine phenotypic variability among the donors.

Human Hepatic Cell Cultures: In Vitro and In Vivo Drug Metabolism

Drug metabolism is the major determinant of drug clearance, and the factor most frequently responsible for inter-individual differences in drug pharmacokinetics. The expression of drug metabolising enzymes shows significant interspecies differences, and variability among human individuals (polymorphic or inducible enzymes) makes the accurate prediction of the metabolism of a new compound in humans difficult. Several key issues need to be addressed at the early stages of drug development to improve drug candidate selection: a) how fast the compound will be metabolised; b) what metabolites will be formed (metabolic profile); c) which enzymes are involved and to what extent; and d) whether drug metabolism will be affected directly (drug-drug interactions) or indirectly (enzyme induction) by the administered compound. Drug metabolism studies are routinely performed in laboratory animals, but they are not sufficiently accurate to predict the metabolic profiles of drugs in humans. Many of these issues can now be addressed by the use of relevant human in vitro models, which speed up the selection of new candidate drugs. Human hepatocytes are the closest in vitro model to the human liver, and they are the only model which can produce a metabolic profile of a drug which is very similar to that found in vivo. However, the use of human hepatocytes is restricted, because limited access to suitable tissue samples prevents their use in high throughput screening systems. The pharmaceutical industry has made great efforts to develop fast and reliable in vitro models to overcome these drawbacks. Comparative studies on liver microsomes and cells from animal species, including humans, are very useful for demonstrating species differences in the metabolic profile of given drug candidates, and are of great value in the judicious and justifiable selection of animal species for later pharmacokinetic and toxicological studies. Cytochrome P450 (CYP)-engineered cells (or microsomes from CYP-engineered cells, for example, Supersomes™) have made the identification of the CYPs involved in the metabolism of a drug candidate more straightforward and much easier. However, the screening of compounds acting as potential CYP inducers can only be conducted in cellular systems fully capable of transcribing and translating CYP genes.

Strategies and molecular probes to investigate the role of cytochrome P450 in drug metabolism: focus on in vitro studies

Drug metabolism is the major determinant of drug clearance and, because of polymorphic or inducible expression of drug-metabolising cytochrome P450s (CYPs), is the factor most frequently responsible for interindividual differences in pharmacokinetics. A number of well characterised CYP substrates and inhibitors have been identified that allow precise measurements of individual CYP isoforms. Their use, alone or in combination, facilitates the phenotype characterisation of hepatocytes in vitro and in vivo. Two procedures are used for in vitro investigation of the metabolic profile of a drug: incubation with microsomes and incubation with metabolically competent cells. The major limitation of microsomes is that they express phase I activities, but only part of phase II activities, and can only be used for short incubation times. When intact cells are used, gene expression, metabolic pathways, cofactors/enzymes and plasma membrane are largely preserved, but fully differentiated cells such as primary cultured hepatocytes need to be used, since hepatoma cell lines have only very low and partial CYP expression. CYP-engineered cells or their microsomes ('supersomes') have made the identification of the CYPs involved in the metabolism of a drug candidate straightforward and easier. Inhibition of CYP is an undesirable feature for a drug candidate, and needs to be addressed by examining whether the drug candidate inhibits the metabolism of other compounds or whether other compounds inhibit the metabolism of the drug candidate. Such experiments can be conducted both with microsomes and in cells. The major limitation of microsomes is that inhibition parameters may not accurately reflect the situation in vivo, since the contribution of drug transport is not considered. The best picture of a potential drug-drug interaction can be obtained in metabolically competent hepatocytes. Screening of CYP inducers cannot be done in microsomes. It requires the use of a cellular system fully capable of transcribing and translating CYP genes, and can be monitored in vitro as an increase in enzyme mRNA or activity. Human hepatocytes in primary culture respond well to enzyme inducers during the first few days; this ability is lost thereafter. Rat hepatocytes are much less stable and soon become unresponsive to inducers. Hepatoma cell lines respond poorly to inducers, although the induction of a few isoenzymes has been reported. Primary cultured hepatocytes are still the unique in vitro model that allows global examination of the inductive potential of a drug.

Species comparison in P450 induction: effects of dexamethasone, omeprazole, and rifampin on P450 isoforms 1A and 3A in primary cultured hepatocytes from man, Sprague-Dawley rat, minipig, and beagle dog

Induction of P450 isoforms 1A (CYP1A) and 3A (CYP3A) by model inducers dexamethasone, omeprazole and rifampin was evaluated in primary cultured hepatocytes from man and laboratory animals. Inducer-specific species-differences were observed. Results with human hepatocytes from six human donors consistently show that both rifampin and dexamethasone were inducers of CYP3A activity (measured as testosterone 6beta-hydroxylase activity), with rifampin being more potent. Conversely, in rat hepatocytes, dexamethasone was a potent CYP3A inducer while rifampin was not an inducer. Rifampin but not dexamethasone induced CYP3A in minipig and beagle dog hepatocytes. Omeprazole was a potent inducer of CYP1A activity (measured as ethoxyresorufin-O-deethylase activity) in human, beagle dog and minipig hepatocytes, and not an inducer in rat hepatocytes. The species-differences observed suggest that human hepatocytes represent the most appropriate preclinical experimental system for the evaluation of P450 induction in human.

Drug biotransformation by human hepatocytes. In vitro/in vivo metabolism by cells from the same donor

BACKGROUND/AIMS

Cultured human hepatocytes are considered a close model to human liver. However, the fact that hepatocytes are placed in a microenvironment that differs from that of the cell in the liver raises the question: to what extent does drug metabolism in vitro reflect that of the liver in vivo? This issue was examined by investigating the in vitro and in vivo metabolism of aceclofenac, an analgesic/anti-inflammatory drug.

METHODS

Hepatocytes isolated from programmed liver biopsies were incubated with aceclofenac, and the metabolites formed were investigated by HPLC. During the course of clinical recovery, patients were given the drug, and the metabolites, largely present in the urine, were analyzed. In vitro and in vivo data of the same individual were compared.

RESULTS

The relative abundance of oxidized metabolites in vitro (i.e. 4'OH-aceclofenac + 4'OH-diclofenac vs. total hydroxylated metabolites; Spearman's p = 0.855), as well the hydrolysis of aceclofenac (4'OH-diclofenac vs. 4'OH-aceclofenac + 4'OH-diclofenac; p = 0.691) correlated well with in vivo data. The conjugation of the drug in vitro (24.6 +/- 7.6%) was lower than that in vivo (44.9 +/- 5.3%). The rate of 4'OH-aceclofenac formation in vitro correlated with the amount of metabolites excreted in urine after 16 h (p = 0.95).

CONCLUSIONS

The in vitro/in vivo metabolism of the drug was surprisingly similar in each patient. The variability observed in vitro reflected an existing phenotypic variability among donors.

Comparison of the stability of some major cytochrome P450 and conjugation reactions in rat, dog and human hepatocyte monolayers

The stability of four major cytochrome P450 isoenzymes (CYPIA, CYP2B, CYP2E1 and CYP3A) and of two phase II conjugation enzymes (glucuronyl- and sulfotransferases) was investigated in primary cultures of rat, dog and human hepatocytes in the same conditions. 7-ethoxyresorufin deethylation (EROD), 7-methoxycoumarin demethylation (MCOD), chlorzoxazone (CLOX) 6-hydroxylation, 1'- and 4-hydroxylation of midazolam (MDZ), and p-nitrophenol glucuronidation and sulfation, were used respectively. The EROD activity was stable over 72 hours in rat and dog and only 48 hours in human hepatocytes. The MCOD activity was also stable in rat but decreased in dog by 30% within 72 hours The CLOX hydroxylase activity was most stable in human whereas in rat and dog it fell down to 30% within 72 and 24 hours, respectively. The MDZ hydroxylase activity showed the same unstability profile in the three species investigated. Both conjugation reactions were either stable or showed an increase by up to 60-70% in all three species over 72 hours. The enzymes tested showed different stabilities in rat, dog and human hepatocytes over 72 hours, thus demonstrating the limitations of hepatocyte monolayers as models for metabolic investigations and emphasising the need for validation/characterization studies before routine use.

Comparison of the effects of various peroxisome proliferators on peroxisomal enzyme activities, DNA synthesis, and apoptosis in rat and human hepatocyte cultures

Peroxisome proliferators (PPs) are a class of rodent nongenotoxic hepatocarcinogens that cause hepatocyte peroxisome proliferation, increased DNA synthesis, and decreased spontaneous apoptosis. We examined the effects of various PPs such as the hypolipidemic agents clofibric acid (CLO), bezafibrate (BEZA), ciprofibrate (CIPRO), and nafenopin (NAFE) and the plasticizer di-(2-ethylhexyl)phthalate (DEHP) on the various parameters in vitro in rat and human hepatocyte cultures. In rat hepatocyte cultures, after 72 h of treatment with the various PPs at 100-500 microM, a compound-dependent increase in acyl CoA oxidase (ACO) and carnitine acetyl transferase (CAT) activities, markers of peroxisome proliferation, was observed with the following potencies: CIPRO = NAFE > BEZA > CLO > DEHP. A minor (120-150%), but significant, no concentration-dependent increase in DNA synthesis and a marked, no compound-dependent and, with the exception of NAFE, no concentration-dependent 60-80% decrease in spontaneous apoptosis was observed with all tested compounds (50-250 microM) after 48 h of treatment. Inhibition of spontaneous apoptosis in PP-treated versus control rat hepatocyte cultures was also observed morphologically. Furthermore, PPs inhibited transforming growth factor beta (TGFbeta)-induced apoptosis but not tumor necrosis factor alpha (TNFalpha)/alpha Amanitine (alphaAma)-induced apoptosis in rat hepatocyte cultures. In human hepatocyte cultures, the various PPs at 50-500 microM did not affect peroxisomal enzyme activities, DNA synthesis, or spontaneous and induced (TGFbeta or TNFalpha/alphaAma) apoptosis. The compound-dependent peroxisome proliferation but no compound-dependent disruption of the mitogenic/apoptotic balance elicited by PPs in primary rat hepatocyte cultures supports the hypothesis that oxidative stress is directly linked to the hepatocarcinogenic potential of a given PP in rodents and that disruption of the mitogenic/apoptotic balance contributes to the development of PP-induced hepatocarcinogenesis. In addition, the absence of effects of all PPs on both peroxisome proliferation-associated parameters and mitogenic/apoptotic balance supports the hypothesis that human liver cells are refractory to PP-induced hepatocarcinogenesis.

Validation and use of the CALUX-bioassay for the determination of dioxins and PCBs in bovine milk

There is a strong need for the development of relatively cheap and rapid bioassays for the determination of dioxins and related compounds in food. A newly developed CALUX (Chemical-Activated LUciferase gene eXpression) bioassay was tested for its possible use to determine low levels of dioxins in bovine milk. Data show that this mammalian cell-based test is very sensitive for 2,3,7,8-substituted dioxins and related PCBs, thereby reflecting the relative potencies of these compounds in comparison to TCDD (TEF-values). The limit of detection was about 50 fg of TCDD. Furthermore, the response obtained with a mixture of dioxins was additive, in accordance with the TEF-principle. Milk fat was isolated by centrifugation followed by clean-up of the fat with n-pentane, removal of the fat on a 33% H2SO4 silica column, and determination of Ah receptor agonist activity with the CALUX-bioassay. An equivalent of 67 mg fat was tested per experimental unit, resulting in a limit of quantification around 1 pg i-TEQ/g fat. To investigate the performance of the method, butter fat was cleaned and spiked with a mixture of 17 different 2,3,7,8-substituted PCDD and PCDF congeners at 1, 3, 6, 9, 12 and 15 pg TEQ/g fat, as confirmed by GC/MS. In this concentration range, the method showed a recovery of TEQs around 67% (58-87%). The reproducibility, determined in three independent series showed a CV varying between 4% and 54%, with the exception of the sample spiked at 1 pg i-TEQ (CV 97%). The repeatability determined with the sample spiked at 6 pg i-TEQ/g showed a CV of 10%. Testing of 22 bovine milk samples, taken at different sites in The Netherlands, in the CALUX-assay showed combined dioxin and dioxin-like PCB levels equivalent to 1.6 pg TCDD/g fat (range 0.2-4.6). GC/MS analysis of these samples revealed an average level of 1.7 pg i-TEQ/g fat, varying between 0.5 and 4.7 pg i-TEQ/g fat. All five samples showing a GC/MS determined dioxin content of more than 2 pg i-TEQ/g fat gave a response in the CALUX-assay corresponding to more than 2 pg TCDD/g fat. These data clearly show that the CALUX-bioassay is a promising method for the rapid and low cost screening of dioxins in bovine milk.

Metabolic capacities in cultured human hepatocytes obtained by a new isolating procedure from non-wedge small liver biopsies

A new isolating procedure of human hepatocytes has been developed using two-step collagenase digestion by a non-perfusion procedure (NP) of non-wedge liver biopsies. 1. A yield of 2-7 x 10(6) hepatocytes/g liver, 52-95% viability and 13-75% attachment were obtained from liver biopsies weighing 6-60 g, comparable to that obtained when using the classical perfusion procedure (P) to isolate human hepatocytes from wedge liver samples of 50-150 g. 2. In culture, human hepatocytes obtained by NP remained attached to plastic for up to 5 days and displayed the usual morphological characteristics. Their metabolic capacities, assessed by liver-specific albumin and urea synthesis and by CYP-dependent and conjugation pathways, were equivalent to those of human hepatocytes obtained by P. In addition, they responded adequately to specific CYP inducers, demonstrating that they constitute a model in which human drug metabolism and toxicity studies can be performed.

Preclinical evaluation of drug-drug interaction potential: present status of the application of primary human hepatocytes in the evaluation of cytochrome P450 induction

Cytochrome P450 (CYP) inhibition and induction are the key mechanisms in drug-drug interactions. Aside from clinical studies, primary human hepatocytes may represent the most appropriate experimental system for the evaluation of CYP induction in humans. A consensus of an international panel on the present status and future research directions in the application of primary human hepatocytes in the evaluation of CYP-induction is presented here. The following observations are concluded to be generally true: (1) Human hepatocytes isolated from both biopsy samples and transplantable livers are suitable for induction studies. (2) Hormonally-defined media can be used for the evaluation of CYP induction. (3) Isozyme-selective induction of CYP1A and 3A by known inducers are observed. (4) Reproducibility of induction could be improved by using hepatocytes plated as confluent cultures. (5) Induction could be observed for hepatocytes treated at 1-3 days after culturing. (6) Treatment duration of 2 days in general leads to near maximal induction. (7) In general, there is a good qualitative correlation between human hepatocyte results in vitro and clinical observations in vivo. (8) When the same inducers were evaluated in independent laboratories, similar data were generally observed. We conclude that primary human hepatocytes represent an appropriate model for mechanistic evaluation of CYP induction and as a screening tool for CYP induction potential of xenobiotics. A set of data acceptance criteria are proposed: (1) Positive response should be observed with concurrent positive control chemicals; (2) reproducible observation should be observed with multiple human donors; (3) for negative response, the doses used should not be cytotoxic; and (4) replicate treatment and/or multiple dose treatment should be performed to allow statistical analysis. Future studies should include the further development of on: (1) The inducibility of CYP isozymes other than CYP1A and 3A, and phase II enzymes; (2) further development of culturing condition to allow optimal gene expression; (3) evaluation of the involvement of nonparenchymal cells on CYP induction of parenchymal cells; (4) the and validation of quantitative approaches to extrapolate in vitro data to in vivo data; (5) evaluation of possible individual variations and potential genetic polymorphism in inducibility; (6) further definition of species differences in CYP induction; (7) development of a 'normal' human hepatocyte cell line for CYP induction studies; (8) improvement of cryopreservation procedure of human hepatocytes; (9) definition of the molecular mechanisms of CYP induction; and (10) evaluation of the induction of phase II metabolic pathways.

Drug metabolism in hepatocyte sandwich cultures of rats and humans

Adult hepatocytes from rat and man were maintained for 2 weeks between two gel layers in a sandwich configuration to study the influence of this culture technique on the preservation of basal activities of xenobiotic-metabolizing phase I and phase II enzymes. The response of these enzyme activities to an enzyme inducer was investigated using rifampicin (RIF). Basal levels of cytochrome P-450 (CYP) isozymes were characterized by measuring ethoxyresorufin O-deethylation (EROD), ethoxycoumarin O-deethylation (ECOD), and the specific oxidation of testosterone (T). In hepatocytes from untreated rats, CYP isozyme levels, including the major form CYP 2C11, increased during the first 3 days in culture. After this period of recovery, the levels of CYP 2C11, CYP 2A1, and CYP 2B1 decreased, whereas CYP 3A1 increased. In contrast to these dynamic changes, CYP activities such as CYP 1A2 and the major isozyme CYP 3A4 were largely preserved until day 9 in cultures of human hepatocytes. In measuring phase II activities, a distinct increase in glucuronosyltransferase (UDP-GT) activity toward p-nitrophenol (PNP) was found for rat and human hepatocytes over 2 weeks in culture. Sulfotransferase (ST) activity toward PNP showed an initial increase, with a maximum at day 7 and day 9 in culture, respectively, and then decreased until day 14. Glutathione S-transferase (GST) activity decreased constantly during the time of culture. Effects of the enzyme-inducing drug rifampicin on phase I and phase II enzymes were investigated using cultured human hepatocytes. Rifampicin treatment (50 micromol/L) for 7 days resulted in a 3.7-fold induction of CYP 3A4 at day 9 in culture. ECOD activity was increased sixfold and phase II ST activity increased twofold compared to the initial value at day 3. No effect of rifampicin on CYP 3A was found in cultures of rat hepatocytes. These results demonstrate that rat and human hepatocytes preserve the major forms of CYP isozymes and phase II activities and respond to inducing drugs such as rifampicin. The novel hepatocyte sandwich culture is suitable for investigating drug metabolism, drug-drug interactions and enzyme induction.

Extended primary culture of human hepatocytes in a collagen gel sandwich system

To develop a strategy for extended primary culture of human hepatocytes, we placed human hepatocytes between two layers of collagen gel, called a "collagen gel sandwich." Maintenance of hepatocellular functions in this system was compared with that of identical hepatocyte preparations cultured on dry-collagen coated dishes or cocultured with rat liver epithelial cells. Human hepatocytes in a collagen gel sandwich (five separate cultures) survived for more than 4 wk, with the longest period of culture being 78 d. They maintained polygonal morphology with bile canaliculuslike structures and high levels of albumin secretion throughout the period of culture. In contrast, hepatocytes on dry-collagen became feature-less, and albumin secretion could not be detected after 14 d of culture. This loss of albumin secretion was partially recovered by overlaying one layer of collagen gel. Ethoxyresorufin O-deethylase activity, associated with cytochrome P450 1A2, was detected basally up to 29 d in collagen gel sandwich culture. These activities were induced four- to eightfold after induction with dibenz(a,h)anthracene. Cocultures also maintained basal activity up to 29 d. However, their inducibility was lower than that of hepatocytes in collagen gel sandwich. No ethoxyresorufin O-deethylase activity was detected in hepatocytes cultured on dry-collagen at 7 d. Thus, the collagen gel sandwich system preserves differentiated morphology and functions of human hepatocytes in primary culture for a prolonged period of time. This system is a promising model for studying human hepatocellular function, including protein synthesis and drug metabolism in vitro.

Acquisition and use of human in vitro liver preparations

Human in vitro liver preparations-i.e., slices, hepatocyte suspensions, primary hepatocyte cultures and microsomes-are increasingly used in the drug development process. The main applications are prediction of drug metabolite profiles, drug-drug interactions and toxicity. The use of these in vitro models is limited, however, because of their erratic availability, the absence of validated protocols and the difficulties of extrapolation of in vitro data to the in vivo situation.

Induction of cytochrome P-450 isoenzymes in cultured monkey hepatocytes

The effect of phenobarbital (PB), beta-naphthoflavone (beta-NF) and rifampicin (Rif) on the drug-metabolizing activity of cultured squirrel monkey hepatocytes was examined. The drug metabolizing activity (e.g. alkoxycoumarin dealkylase or steroid hydroxylase) gradually decreased during the culture period with 40-70% activity remaining at 72 hr. When 0.5 mM PB was added to the culture, the activities of 7-methoxycoumarin O-demethylase (MCOD) and 7-ethoxycoumarin O-deethylase (ECOD) increased to 6-7 fold higher level than those of control at 72 hr. Testosterone 6 beta-hydroxylase (6 beta-OH-T) and testosterone 16 beta-hydroxylase (16 beta-OH-T) activities were approx. 3-fold higher than those of the control. Addition of beta-NF significantly increased the activities of 7-ethoxyresorufin O-deethylase (EROD) and ECOD. Though statistically insignificant, Rif slightly increased 6 beta-OH-T activity. Western blot analysis indicated PB induced production of the CYP 2B and 3A subfamilies, while beta-NF and Rif induced that of the CYP 1A and the CYP 3A subfamily, respectively.

Cytochrome P450 induction and metabolism of alkoxyresorufins, ethylmorphine and testosterone in cultured hepatocytes from goats, sheep and cattle

Very little is known of cytochrome P450 (P450) patterns and enzyme characteristics in food-producing animal species. Oxidative metabolism of alkoxyresorufins, ethylmorphine (EtM) and testosterone (TST) was used to monitor the effects of the P450 inducers phenobarbital (PB), beta-naphthoflavone (BNF), dexamethasone (DEX) and triacetyloleandomycin (TAO) in primary cultured hepatocytes from goats, sheep and cattle. BNF effectively and specifically induced ethoxyresorufin deethylase (> 20-fold), indicating the presence of an inducible P450 1A form, and down-regulated EtM demethylation and most selected TST hydroxylations. In non-induced hepatocyte cultures, TST was metabolized to 6 beta-, 2 beta-, 12 beta-, and 11 alpha-hydroxy-TST (OHT). PB and, to a lesser extent, DEX non-specifically induced all OHT formations, and EtM demethylation. TAO almost completely inhibited OHT formation and EtM demethylation. These results indicate the involvement of principally one P450 form, or a restricted number of related P450 forms, presumably belonging to the P450 3A subfamily. In western blot analysis, cross reactivity was found with rat anti-P450 3A1 and anti-sheep P450 3A. A more specific PB effect was observed for 16 alpha-OHT, which may be formed though a ruminant P450 2B form. None of the inducers influenced pentoxyresorufin depentylase (PROD) or EtM O-deethylation. Metabolite patterns and inducibility of selected activities in ruminant hepatocytes are in accordance with previous findings in goats in vivo. Cytochrome P450 characteristics in ruminants appear to differ from those in rats whereas similarities to the situation in humans appear to exist.

Use of human hepatocyte cultures for drug metabolism studies

Among the in vitro models developed to investigate drug metabolism isolated hepatocytes have become the most powerful model. Human hepatocytes can be prepared from whole livers and surgical wedge biopsies. When placed in culture they retain their specific drug metabolizing activities including inducible cytochrome P450 enzymes for several days. Primary human hepatocyte cultures are now increasingly used for studying drug behavior during preclinical development, e.g. drug interactions, stereoselective drug metabolism and drug metabolic profiles. As a rule there is a good in vivo/in vitro correlation in drug biotransformation activity. The main metabolites found in vivo are recovered in vitro. However, quantitative differences are frequently observed.

Potentiation of heroin and methadone hepatotoxicity by ethanol: an in vitro study using cultured human hepatocytes

1. The hepatotoxic effects of heroin and methadone, and the effect of ethanol on opioid-induced hepatotoxicity, have been investigated in human cultured hepatocytes. Hepatocytes pretreated with 50 and 100 mM ethanol were exposed to increasing concentrations of heroin and methadone. 2. Cytotoxicity was evaluated by measuring leakage of intracellular lactate dehydrogenase, and by assessment of hepatocyte mitochondrial succinate dehydrogenase. The half-maximal cytotoxic concentration of heroin for human hepatocytes (TC50) was decreased by 70-55% by pre-exposure to 50 mM ethanol, and that for methadone was decreased by 60-40%. 3. Metabolic functions of human hepatocytes were significantly impaired at concentrations of opioids that had shown little cytotoxicity. Ethanol potentiated opioid-induced hepatotoxicity; concentrations of heroin and methadone that had little or no effect on hepatocyte metabolism in the absence of ethanol caused a significant decrease in urea synthesis rate, metabolism of glycogen and depletion of the intracellular GSH pool after ethanol pretreatment. 4. The increase in toxicity of heroin and methadone produced by ethanol is concomitant with a 40% increase in cytochrome P-450 levels of the pretreated hepatocytes.

Glucocorticoid-mediated potentiation of P450 induction in primary culture of rainbow trout hepatocytes

Induction of 7-ethoxyresorufin O-deethylase activity (a cytochrome P450IA-dependent activity) by beta-naphthoflavone (0.36 microM) is increased 2-3-fold by dexamethasone or cortisol (10(-9)-10(-7) M) in rainbow trout hepatocyte cultures. This potentiation does not seem to be a time-dependent process and could be a classical glucocorticoid receptor-mediated event resulting in enhanced transcriptional activation of the CYP1A, as previously shown in mammals. Since glucocorticoid levels can increase in fish exposed to pollutants, such steroids may interfere with the induction response to xenobiotics.

Potentiation of cocaine hepatotoxicity by ethanol in human hepatocytes

The hepatotoxic effects of cocaine on the human liver and the effect of ethanol on cocaine-induced hepatotoxicity have been examined in adult human hepatocytes cultured in chemically defined conditions. Cultures were exposed to concentrations of cocaine ranging from 10(-2) to 10(-5) M. Cytotoxicity was evaluated after 24 hr of continuous exposure to cocaine by measuring the leakage of intracellular LDH and the ability of cells to reduce MTT. According to these end-point parameters, half-maximal cytotoxic concentrations of cocaine for human hepatocytes (IC50) were 6.8 and 7.8 mM, respectively. Lower concentrations of cocaine, however, impaired basic metabolic functions of human hepatocytes. Exposure of cells to 2 mM cocaine for 24 hr resulted in a 50% decrease in hepatic glycogen, a 40% decrease in cellular glutathione content, and a 40% decrease in urea synthesis with respect to control values. For most of the metabolic parameters assayed, significant alterations were observed at 0.5 mM cocaine. Glycogen reloading of hepatocytes began to be inhibited in the presence of 0.60 mM cocaine (IC10). Ethanol greatly potentiated cocaine-induced hepatotoxicity. After a 48 hr pretreatment of human hepatocytes with 50 mM ethanol, low concentration of cocaine (0.25 mM) that had no effects on hepatocyte metabolism in the absence of ethanol caused a 20% inhibition of the urea synthesis rate, a 40% degradation of glycogen stores, and a 30% reduction in glutathione content. The results of our work show that ethanol increases the effects of cocaine on human hepatocytes by a factor of 10.