Validation and use of cloned, expressed human drug-metabolizing enzymes in heterologous cells for analysis of drug metabolism and drug-drug interactions

Variability in Human In Vitro Enzyme Kinetics

There are many factors which are known to cause variability in human in vitro enzyme kinetic data. Factors such as the source of enzyme and how it was prepared, the genetics and background of the donor, how the in vitro studies are designed, and how the data are analyzed contribute to variability in the resulting kinetic parameters. It is important to consider not only the factors which cause variability within an experiment, such as selection of a probe substrate, but also those that cause variability when comparing kinetic data across studies and laboratories. For example, the artificial nature of the microsomal lipid membrane and microenvironment in some recombinantly expressed enzymes, relative to those found in native tissue microsomes, has been shown to influence enzyme activity and thus can be a source of variability when comparing across the two different systems. All of these factors, and several others, are discussed in detail in the chapter below. In addition, approaches which can be used to visualize the uncertainty arising from the use of enzyme kinetic data within the context of predicting human pharmacokinetics are discussed.

Predicting Drug Extraction in the Human Gut Wall: Assessing Contributions from Drug Metabolizing Enzymes and Transporter Proteins using Preclinical Models

Intestinal metabolism can limit oral bioavailability of drugs and increase the risk of drug interactions. It is therefore important to be able to predict and quantify it in drug discovery and early development. In recent years, a plethora of models-in vivo, in situ and in vitro-have been discussed in the literature. The primary objective of this review is to summarize the current knowledge in the quantitative prediction of gut-wall metabolism. As well as discussing the successes of current models for intestinal metabolism, the challenges in the establishment of good preclinical models are highlighted, including species differences in the isoforms; regional abundances and activities of drug metabolizing enzymes; the interplay of enzyme-transporter proteins; and lack of knowledge on enzyme abundances and availability of empirical scaling factors. Due to its broad specificity and high abundance in the intestine, CYP3A is the enzyme that is frequently implicated in human gut metabolism and is therefore the major focus of this review. A strategy to assess the impact of gut wall metabolism on oral bioavailability during drug discovery and early development phases is presented. Current gaps in the mechanistic understanding and the prediction of gut metabolism are highlighted, with suggestions on how they can be overcome in the future.

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.

Variability in human in vitro enzyme kinetics

There are many factors which are known to cause variability in human in vitro enzyme kinetic data. Factors such as the source of enzyme and how it was prepared, the genetics and background of the donor, how the in vitro studies are designed, and how the data are analyzed contribute to variability in the resulting kinetic parameters. It is important to consider not only the factors which cause variability within an experiment, such as selection of a probe substrate, but also those that cause variability when comparing kinetic data across studies and laboratories. For example, the artificial nature of the microsomal lipid membrane and microenvironment in some recombinantly expressed enzymes, relative to those found in native tissue microsomes, has been shown to influence enzyme activity and thus can be a source of variability when comparing across the two different systems. All of these factors, and several others, are discussed in detail in the chapter below.

The blood-brain barrier: In vitro methods and toxicological applications

The blood-brain barrier (BBB) is reviewed with reference to in vitro cell culture models and their use and potential use in toxicological studies. The structure, function and in vitro study of brain microvessel endothelial cells (BMEC) is briefly described, as well as the effects of a number of xenobiotics, such as solvents, metals, polycations and herbicides, on the viability and barrier function of the BBB model. The biotransformation of xenobiotics is increasingly thought to be responsible for many toxic reactions seen in living systems. Few studies have addressed the effects of the products of biotransformation on the integrity of the barrier model. Many of the specific human bioactivating enzymes, such as cytochrome P-450s, can now be conveniently studied in eukaryotic in vitro gene expression systems. The combination of such systems with a well characterized porcine BMEC culture model might be useful in the study of reactive metabolites on the BBB, in terms of changes in indices of functional and structural BMEC viability. The potential applications and the value of such an experimental approach are discussed.

High Expression of Human CYP2C in Immortalized Human Liver Epithelial Cells

Cell lines stably expressing high levels of single isozymes of human CYP2C genes (CYP2C8, CYP2C9, CYP2C18 and CYP2C19) have been successfully generated by transfecting liver epithelial human cells (THLE) with an appropriate expression vector. To this aim, cDNAs encoding for each CYP2C gene were inserted by blunt-ended cloning into the unique insertion site of the singular expression vector pCMVneo. The recombinant pCMV2C8, pCMV2C9, pCMV2C18 and pCMV2C19 vectors were liposome-mediated transfected into THLE cells. The resulting transgenic cells, designated as T5-2C8, T5-2C9, T5-2C18 and T5-2C19, were cloned and the expression of the ectopic gene, mRNA and protein, was investigated by RT-PCR and Western blot analysis. The functionality of each expressed CYP2C was assessed by determining specific catalytic activities in these cells, that is, taxol-6-hydroxylation for CYP2C8; diclofenac-4'-hydroxylation for CYP2C9; S-mephenytoin-4'-hydroxylation for CYP2C18; S-mephenytoin-4'-hydroxylation for CYP2C19. As a result of the combined strategies used here, the transfected cells showed activities four to seven times higher than those of 24-hour cultured hepatocytes.

Inactivation of CYP2D6 by methylenedioxymethamphetamine in different recombinant expression systems

Recombinantly expressed CYP450 systems (rCYPs) are often used to screen for irreversible/quasi-irreversible enzyme inhibitors during drug development. The concentration- and time-dependent inactivation of CYP2D6 by methylenedioxymethamphetamine (MDMA) was compared in three different rCYP2D6 systems (yeast microsomes, Supersomestrade mark and Bactosomestrade mark) under the conditions of the most commonly used protocols in assessing mechanism-based inactivation (MBI). MDMA (2-20microM) was pre-incubated with enzyme for 0, 2.5 and 5min followed by a five-fold dilution and further incubation with dextromethorpan (DEX) (50microM). The formation of dextrorphan (DOR) from DEX was used as a specific marker of CYP2D6 activity. Concentration- and time-dependent inactivation of CYP2D6 by MDMA was observed with each rCYP system. However, the apparent kinetic parameters for MBI (k(inact), the maximum inactivation rate constant and K(I), the inhibitor concentration associated with half maximal rate of inactivation) were significantly greater (p<0.05) for Bactosomestrade mark (0.95+/-0.33min(-1), 42.9+/-20.1microM) than those found using yeast microsomes (0.28+/-0.04min(-1), 2.86+/-1.18microM) and Supersomestrade mark (0.38+/-0.05min(-1), 3.66+/-0.10microM). After correction for depletion of MDMA during pre-incubation, k(inact) and K(I) values determined using Bactosomestrade mark decreased significantly but remained higher than for the other rCYP systems (p<0.05). Substantial metabolism of DOR after its formation from DEX was also observed using Supersomestrade mark and Bactosomestrade mark. Sub-optimal study design when investigating MBI may compromise the quantitative characterization of inhibitory characteristics using some rCYP systems.

Metabolism of MK-0524, a prostaglandin D2 receptor 1 antagonist, in microsomes and hepatocytes from preclinical species and humans

(3R)-4-(4-Chlorobenzyl)-7-fluoro-5-(methylsulfonyl)-1,2,3,4-tetrahydrocyclopenta[b]indol-3-yl acetic acid (MK-0524) is a potent orally active human prostaglandin D(2) receptor 1 antagonist that is currently under development for the prevention of niacin-induced flushing. The major in vitro and in vivo metabolite of MK-0524 is the acyl glucuronic acid conjugate of the parent compound, M2. To compare metabolism of MK-0524 across preclinical species and humans, studies were undertaken to determine the in vitro kinetic parameters (K(m) and V(max)) for the glucuronidation of MK-0524 in Sprague-Dawley rat, beagle dog, cynomolgus monkey, and human liver microsomes, human intestinal microsomes, and in recombinant human UDP glucuronosyltransferases (UGT). A comparison of K(m) values indicated that UGT1A9 has the potential to catalyze the glucuronidation of MK-0524 in the liver, whereas UGT1A3 and UGT2B7 have the potential to catalyze the glucuronidation in the intestine. MK-0524 also was subject to phase I oxidative metabolism; however, the rate was significantly lower than that of glucuronidation. The rate of phase I metabolism was ranked as follows: rat approximately monkey > human intestine > dog > human liver with qualitatively similar metabolite profiles across species. In all the cases, the major metabolites were the monohydroxylated epimers (M1 and M4) and the keto-metabolite, M3. Use of inhibitory monoclonal antibodies and recombinant human cytochromes P450 suggested that CYP3A4 was the major isozyme involved in the oxidative metabolism of MK-0524, with a minor contribution from CYP2C9. The major metabolite in hepatocyte preparations was the acyl glucuronide, M2, with minor amounts of M1, M3, M4, and their corresponding glucuronides. Overall, the in vivo metabolism of MK-0524 is expected to proceed via glucuronidation, with minor contributions from oxidative pathways.

Epirubicin Glucuronidation and UGT2B7 Developmental Expression

The usefulness of epirubicin in the treatment of adult and childhood malignant diseases is related in part to the potential reduction in cardiac toxicity compared with that of other anthracyclines given at equivalent doses. An important pathway for epirubicin detoxification is UGT2B7-dependent glucuronidation. This study was implemented to provide a preclinical evaluation of the metabolism of epirubicin with respect to age-related changes in epirubicin glucuronidation in pediatric liver microsomes. Rates of epirubicin glucuronidation and levels of UGT2B7 were determined for liver microsomes from four pediatric age categories (n = 32) and one adult age category (n = 8). Both sets of data showed an increase in UGT2B7 activity and content with increasing age. Epirubicin glucuronidation activity in the adult group was statistically higher compared with all pediatric age groups (p < or = 0.01). UGT2B7 expression also was statistically higher in adults compared with children below 11 years of age, with evidence of significant differences in protein levels among the pediatric age categories. A positive correlation (r = 0.68) between UGT2B7 levels and postnatal age was observed, suggesting a progressive increase in UGT2B7 protein expression with increasing age. However, allometric scaling using the (3/4) power rule suggested no difference in activity between any of the pediatric age categories and the adult, although only a single neonatal sample was included in the analysis. In summary, these in vitro data show differences in epirubicin glucuronidation and UGT2B7 content within pediatric age groups and support the use of epirubicin in pediatric patients at least 6 months of age.

Characterization of human cytochrome P450 isoenzymes involved in the metabolism of vinorelbine

Vinorelbine (VRL) (IV Navelbine) is a semi-synthetic vinca alkaloid, used in therapeutics for the treatment of non-small-cell lung cancer and advanced breast cancer. The aim of this study was to characterize the cytochrome P450 (CYP) isoenzymes involved in VRL metabolism. VRL was incubated at 1.28 x 10(-5) m for 90 min with human hepatic microsomes prepared from 14 donors (one woman and 13 men aged from 27 to 76 years old) and characterized for CYP1A2, CYP2D6, CYP2E1 and CYP3A4 activities. A four-combined-approach study was performed, including correlation between CYP activities and VRL metabolism among the 14 batches of microsomes, inhibition of VRL biotransformation by isoform-selective substrates and by specific inhibitory antibodies, and incubation with supersomes. Analysis of unchanged VRL and its metabolites was performed using an HPLC method coupled with both radioactive and UV detections. No correlation between CYP1A2 or CYP2E1 and VRL metabolism was observed in the 14 batches of microsomes used. A correlation was shown between VRL metabolism and CYP3A4 activity as determined with the dextromethorphan N-demethylase and nifedipine oxidase activities (r(2)=0.80 and 0.59, respectively). These results were strengthened by a correlation between the metabolism extent of VRL and CYP3A4 protein content determined by immunoblotting (r(2)=0.75). Furthermore, VRL biotransformation was inhibited by troleandomycine, the CYP3A4-specific inhibitor substrate (80% of inhibition) and by anti-CYP3A antibodies (36% of inhibition). On the contrary, a low correlation with CYP2D6 activity as determined by dextrometorphan O-demethylation (r(2)=0.31) was established. CYP2D6 supersomes did not metabolize the drug whereas 63.4% of VRL were metabolized by microsomes overexpressing CYP3A4 isoform. These data indicated that CYP3A4 is the main enzyme involved in the hepatic metabolism of VRL in human, whereas CYP2D6 is not involved.

Genetic variation of human UDP-glucuronosyltransferase: implications in disease and drug glucuronidation

The uridine diphosphate (UDP)-glucuronosyltransferases (UGTs) are key enzymes in human detoxication of xeno- and endobiotics. Potentially toxic endogenous compounds such as bilirubin, or exogenous compounds such as drugs, pesticides, and carcinogens, are generally transformed into water-soluble glucuronides for excretion in bile and urine. The UGTs are encoded by a multigene family in humans. A relatively small number of human enzymes catalyze the glucuronidation of thousands of compounds. Genetic variations and single nucleotide polymorphisms (SNPs) within the UGT genes are remarkably common, and lead to genetic polymorphisms. The multiplicity of transferases, some exhibiting overlapping substrate specificity, may provide functional compensation for genetic deficit in some cases. Genetic variation may cause different phenotypes by affecting expression levels or activities of individual UGTs. This inter-individual variation in UGTs has resulted in functional deficit affecting endogenous metabolism and leading to jaundice and other diseases. Disruption of the normal metabolic physiology, by the reduction of bile acid excretion or steroid glucuronidation, may lead to cholestasis and organ dysfunction. Deficient glucuronidation of drugs and xenobiotics have an important pharmacological impact, which may lead to drug-induced adverse reactions, and even cancer. Additional novel polymorphisms in this gene family are yet to be revealed and studied, but will have a profound effect on the development of new drugs and therapies.

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.

Rapid and quantitative determination of metabolites from multiple cytochrome P450 probe substrates by gradient liquid chromatography-electrospray ionization-ion trap mass spectrometry

A rapid quantitative assay method, developed by combining fast gradient liquid chromatography and electrospray ionization-ion trap mass spectrometry, is described for the simultaneous determination of CYP450 probe substrate metabolites (4-aminophenol for CYP2E1, acetaminophen for CYP1A2, dextrorphan for CYP2D6, 4'-hydroxymephenytoin for CYP2C19, 4-hydroxytolbutamide for CYP2C9 and 6beta-hydroxytestosterone for CYP3A4) in microsomal incubations. Using this method Michaelis-Menten kinetic parameters K(m) and V(max) for the probe substrates in human liver microsomes were obtained. This LC-MS-MS method, developed with the use of LC-ESI-ion trap MS instrumentation, can efficiently be used to improve the throughput and cost-effectiveness in the preclinical drug metabolism studies.

Mechanistic studies on metabolic interactions between gemfibrozil and statins

A series of studies were conducted to explore the mechanism of the pharmacokinetic interaction between simvastatin (SV) and gemfibrozil (GFZ) reported recently in human subjects. After administration of a single dose of SV (4 mg/kg p.o.) to dogs pretreated with GFZ (75 mg/kg p.o., twice daily for 5 days), there was an increase (approximately 4-fold) in systemic exposure to simvastatin hydroxy acid (SVA), but not to SV, similar to the observation in humans. GFZ pretreatment did not increase the ex vivo hydrolysis of SV to SVA in dog plasma. In dog and human liver microsomes, GFZ exerted a minimal inhibitory effect on CYP3A-mediated SVA oxidation, but did inhibit SVA glucuronidation. After i.v. administration of [(14)C]SVA to dogs, GFZ treatment significantly reduced (2-3-fold) the plasma clearance of SVA and the biliary excretion of SVA glucuronide (together with its cyclization product SV), but not the excretion of a major oxidative metabolite of SVA, consistent with the in vitro findings in dogs. Among six human UGT isozymes tested, UGT1A1 and 1A3 were capable of catalyzing the glucuronidation of both GFZ and SVA. Further studies conducted in human liver microsomes with atorvastatin (AVA) showed that, as with SVA, GFZ was a less potent inhibitor of the CYP3A4-mediated oxidation of this drug than its glucuronidation. However, with cerivastatin (CVA), the glucuronidation as well as the CYP2C8- and CYP3A4-mediated oxidation pathways were much more susceptible to inhibition by GFZ than was observed with SVA or AVA. Collectively, the results of these studies provide metabolic insight into the nature of drug-drug interaction between GFZ and statins, and a possible explanation for the enhanced susceptibility of CVA to interactions with GFZ.

Evaluation of approach to predict the contribution of multiple cytochrome P450s in drug metabolism using relative activity factor: effects of the differences in expression levels of NADPH-cytochrome P450 reductase and cytochrome b(5) in the expression system and the differences in the marker activities

The concept of relative activity factor (RAF) to extrapolate data obtained with recombinant cytochrome P450(CYP)s to human liver microsomes has been proposed. To evaluate the approach to predict the contribution of multiple CYPs using RAF, we investigated the effects of the differences in the expression levels of NADPH-cytochrome P450 reductase (OR) and cytochrome b(5) (b(5)) in recombinant CYPs from baculovirus-infected insect cells and the differences in the marker activities. Because we previously clarified that azelastine, an antiallergy and antiasthmatic drug, is N-demethylated by CYP1A2, CYP2D6, and CYP3A4 in humans, the reaction was used as a model. For calculation of RAF, three lots of recombinant CYP1A2, CYP2D6, and CYP3A4 from baculovirus-infected insect cells with different expression levels of OR and b(5) were used. The OR/CYP ratios for recombinant CYP1A2, CYP2D6, and CYP3A4 were 3.9-4.8, 5.1-8.7, and 8.0-11.3, respectively. The b(5)/CYP ratio for recombinant CYP3A4 was 2.1-18.7. As marker activities, ethoxyresorufin O-deethylation and phenacetin O-deethylation for CYP1A2, bufuralol 1'-hydroxylation and debrisoquin 4-hydroxylation for CYP2D6, testosterone 6beta-hydroxylation and midazolam 1'-hydroxylation for CYP3A4 were compared. Our results indicated that the differences in the expression levels of OR and b(5) coexpressed in baculovirus-infected insect cells would not be a critical factor for the quantitative prediction using RAF. In addition, we confirmed that differences in the marker activities did not significantly affect the calculation of RAF values, when the marker activities are specific for a certain CYP isoform. It was suggested that the RAF approach using recombinant CYPs from baculovirus-infected insect cells coexpressing OR (and b(5) if required) could be valuable for the prediction of the contribution of each CYP in drug metabolism.

Human UDP-glucuronosyltransferases: metabolism, expression, and disease

In vertebrates, the glucuronidation of small lipophilic agents is catalyzed by the endoplasmic reticulum UDP-glucuronosyltransferases (UGTs). This metabolic pathway leads to the formation of water-soluble metabolites originating from normal dietary processes, cellular catabolism, or exposure to drugs and xenobiotics. This classic detoxification process, which led to the discovery nearly 50 years ago of the cosubstrate UDP-glucuronic acid (19), is now known to be carried out by 15 human UGTs. Characterization of the individual gene products using cDNA expression experiments has led to the identification of over 350 individual compounds that serve as substrates for this superfamily of proteins. This data, coupled with the introduction of sophisticated RNA detection techniques designed to elucidate patterns of gene expression of the UGT superfamily in human liver and extrahepatic tissues of the gastrointestinal tract, has aided in understanding the contribution of glucuronidation toward epithelial first-pass metabolism. In addition, characterization of the UGT1A locus and genetic studies directed at understanding the role of bilirubin glucuronidation and the biochemical basis of the clinical symptoms found in unconjugated hyperbilirubinemia have uncovered the structural gene polymorphisms associated with Crigler-Najjar's and Gilbert's syndrome. The role of the UGTs in metabolism and different disease states in humans is the topic of this review.

Usefulness of statistic experimental designs in enzymology: example with recombinant hCYP3A4 and 1A2

First, the effects of 10 incubation factors were screened altogether on nifedipine dehydrogenase (NIF) and methoxyresorufin O-deethylase (MROD) activities catalyzed by recombinant human CYP3A4 and 1A2, respectively. Using a statistic experimental design, only 36 assays were needed to be exhaustive. Eight factors influenced CYP3A4-mediated NIF activity: buffer type, pH, temperature, Mg/EDTA, cytochrome b5, NADPH-P450 reductase, NADH, and solvent. Two factors had no significant effect: total ionic concentration and catalase/reduced glutathione. Six factors influenced CYP1A2-mediated MROD rates: buffer type, pH, temperature, Mg/EDTA, NADH, and glycerol. Four factors had no significant effect: total ionic concentration, cytochrome b5, reductase, and NAD+. Secondly, the combined effects of ionic strength and Mg concentration on NIF/CYP3A4 were studied and easily modeled using another statistic experimental design. The effect of Mg was strong at a constant ionic strength of 100 mM and negligible at a constant ionic strength of 500 mM. Thirdly, the effects of influencing factors and their interactions on MROD/CYP1A2 were modeled after 40 assays using a third statistic experimental design. Later experiments confirmed the predictivity of the models and the efficiency of optimized conditions. This approach can be applied to other biochemistry areas.

Structural and functional studies of UDP-glucuronosyltransferases

UDP-Glucuronosyltransferases (UGTs) are glycoproteins localized in the endoplasmic reticulum (ER) which catalyze the conjugation of a broad variety of lipophilic aglycon substrates with glucuronic acid using UDP-glucuronic acid (UDP-GIcUA) as the sugar donor. Glucuronidation is a major factor in the elimination of lipophilic compounds from the body. In this review, current information on the substrate specificities of UGT1A and 2B family isoforms is discussed. Recent findings with regard to UGT structure and topology are presented, including a dynamic topological model of UGTs in the ER. Evidence from experiments on UGT interactions with inhibitors directed at specific amino acids, photoaffinity labeling, and analysis of amino acid alignments suggest that UDP-GIcUA interacts with residues in both the N- and C-terminal domains, whereas aglycon binding sites are localized in the N-terminal domain. The amino acids identified so far as crucial for substrate binding and catalysis are arginine, lysine, histidine, proline, and residues containing carboxylic acid. Site-directed mutagenesis experiments are critical for unambiguous identification of the active-site architecture.

Hepatic metabolism of diclofenac: role of human CYP in the minor oxidative pathways

The aim of this study was to re-examine the human hepatic metabolism of diclofenac, with special focus on the generation of minor hydroxylated metabolites implicated in the idiosyncratic hepatotoxicity of the drug. Different experimental approaches were used: human hepatocytes, human microsomes, and engineered cells expressing single human CYP (cytochromes P450). Human hepatocytes formed 3'-hydroxy-, 4'-hydroxy-, 5-hydroxy- 4',5-dihydroxy-, and N,5-dihydroxydiclofenac, as well as several lactams. Formation of 4'- and 5-hydroxydiclofenac by human liver microsomes followed a Michaelis-Menten kinetics (Km 9 +/- 1 microM; Vmax 432 +/- 15 pmol/min/mg and Km 43 +/- 5 microM; and Vmax 15.4 +/- 0.6 pmol/min/mg, respectively). Secondary metabolites were detected after incubation of 5-hydroxydiclofenac with human liver microsomes, yielding 4',5-dihydroxydiclofenac (Km 15 +/- 1 microM; Vmax 96 +/- 3 pmol/min/mg) and small amounts of N,5-dihydroxydiclofenac (non-Michaelis-Menten kinetics). Based on microsome studies and the incubations with human hepatocytes and engineered cells, we estimated that in vivo CYP2C9 would be exclusively responsible for the 4' hydroxylation of diclofenac (>99.5%) as well as 5-hydroxydiclofenac (>97%). CYP2C9 was exclusively responsible for the formation of 3'-hydroxydiclofenac. Multiple regression analysis evidenced that the rate of production of 5-hydroxydiclofenac in human microsomes followed the algorithm: 0.040 x S-mephenytoin 4'-hydroxylation + 0.083 x tolbutamide methylhydroxylation, (multiple correlation coefficient = 0.969). However, the incubation of diclofenac with cell lines expressing different human CYP suggested that 7 isoforms could be involved. Comparison of data obtained with CYP-expressing cells and human hepatocytes suggests that CYP2C8 > CYP2C19 approximately CYP2C18 >> CYP2B6 are the isoforms implicated in the 5-hydroxylation of diclofenac in vivo.

Human CYP2D6 and metabolism of m-chlorophenylpiperazine

BACKGROUND

Metabolic drug-drug interactions can occur between drugs that are substrates or inhibitors of the same cytochrome P450 (CYP) isoenzymes, but can be prevented by knowing which isoenzymes are primarily responsible for a drug's metabolism. m-Chlorophenylpiperazine (mCPP) is a psychopharmacologically active metabolite of four different psychiatric drugs. The present experiments were designed to identify the CYP isoenzymes involved in the metabolism of mCPP to its main metabolite p-hydroxy-mCPP (OH-mCPP).

METHODS

The rate of production of OH-mCPP from mCPP was correlated with isoform activities in a panel of human liver microsomes, was assessed using a panel of individual complementary DNA-expressed human CYP isoenzymes, and was investigated in the presence of a specific inhibitor of CYP2D6.

RESULTS

OH-mCPP production correlated significantly with CYP2D6 activity in human liver microsomes. Furthermore, incubations with microsomes from cells expressing CYP2D6 resulted in OH-mCPP formation, whereas no mCPP was formed from incubations with microsomes from cells expressing other individual isoforms. Finally, when the specific CYP2D6 inhibitor quinidine was preincubated with either human liver microsomes or cells expressing human CYP2D6, there was a concentration-dependent decrease in the production of OH-mCPP.

CONCLUSIONS

These results confirm that CYP2D6 is the isoform responsible for the p-hydroxylation of mCPP, and indicate that caution should be exercised in coprescribing inhibitors or substrates of CYP2D6 with drugs that have mCPP as a metabolite.

AH receptor-controlled transcriptional regulation and function of rat and human UDP-glucuronosyltransferase isoforms

Transcriptional regulation and function of rat and human PAH-inducible UDP-glucuronosyltransferase (UGT) isoforms have been studied. 1. At least two PAH-inducible UGT isoforms are expressed in a variety of tissues, the rat isoforms UGT1A6 and UGT1A7, and the human isoforms UGT1A6 and UGT1A9. 2. For the rat and human UGT1A6 isoforms two modes of tissue- and cell-specific regulation were found, PAH-inducible and constitutive expression. 3. Transient transfection studies, using human UGT1A6/CAT fusion constructs and colon carcinoma Caco-2 cells, revealed that PAH induction of human UGT1A6 is mediated by the Ah receptor. 4. Cell-expressed UGT isoforms were used to study their function in PAH metabolism. Rat UGT1A7 and human UGT1A9 appear to be more efficient than the corresponding UGT1A6 isoforms in catalyzing glucuronide formation of PAH phenols and diphenols. Several isoforms may act together in the formation of benzo(a)pyrene-3.6-diol diglucuronide, the major glucuronide found in rat bile. The results suggest complex modes of transcriptional regulation of PAH-inducible UGTs. They also suggest a major role of these UGT isoforms in detoxication of PAHs.

Relative quantities of catalytically active CYP 2C9 and 2C19 in human liver microsomes: application of the relative activity factor approach

The relative catalytic activities of CYP2C9 and CYP2C19 in human liver microsomes has been determined using the approach of relative activity factors (RAFs). Tolbutamide methylhydroxylation and S-mephenytoin 4'-hydroxylation were used as measures of CYP2C9 and CYP2C19 activity, respectively. The kinetics of these reactions were studied in human liver microsomes, in microsomes from human lymphoblastoid cells, and in insect cells expressing CYP2C9 and CYP2C19. RAFs were calculated as the ratio of Vmax (reaction velocity at saturating substrate concentrations) in human liver microsomes of the isoform-specific index reaction divided by the Vmax of the reaction catalyzed by the cDNA expressed isoform. RAFs were also determined for SUPERMIX, a commercially available mixture of cDNA expressed human drug metabolizing CYPs formulated to achieve a balance of enzyme activities similar to that found in human liver microsomes. Lymphoblast RAF2C9 in human liver microsomes ranged from 54 to 145 pmol CYP/mg protein (mean value: 87), while a value of 251 pmol CYP/mg protein was obtained for SUPERMIX. Insect cell RAF2C9 in human liver microsomes ranged from 1.6 to 143 pmol CYP/mg protein (mean value: 49), while a value of 201 pmol CYP/mg protein was obtained for SUPERMIX. Both lymphoblast and insect cell RAF2C19 in human liver microsomes ranged from 4 to 45 pmol CYP/mg protein (mean values: 29 and 28, respectively), while a value of 29 pmol CYP/mg protein was obtained for SUPERMIX. The nature of the cDNA expression system used had no effect on the kinetic parameters of CYP2C9 as a tolbutamide methylhydroxylase, or of CYP2C19 as a S-mephenytoin hydroxylase. However insect cell expressed CYP2C19 (which includes oxidoreductase) had substantially greater activity as a tolbutamide methylhydroxylase when compared to lymphoblast expressed CYP2C19. The ratio of mean lymphoblast-determined RAF2C9 to RAF2C19 in human livers was 3.0 (range 1.6-17.9; n = 10), while this ratio for SUPERMIX was 8.6. The ratio of mean insect cell-determined RAF2C9 to RAF2C19 in human livers was 1.7 (range 0.04-16.2; n = 10), while this ratio for SUPERMIX was 7.0. Neither ratio is in agreement with the 20:1 ratio of immunoquantified levels of CYP2C9 and 2C19 in human liver microsomes reported in previous studies. SUPERMIX may contain catalytically active CYP2C9 in levels higher than those in human liver microsomes.

Biology and function of the reversible sulfation pathway catalysed by human sulfotransferases and sulfatases

Sulfation and sulfate conjugate hydrolysis play an important role in metabolism, and are catalysed by members of the sulfotransferase and sulfatase enzyme super-families. In general, sulfation is a deactivating, detoxication pathway, but for some chemicals the sulfate conjugates are much more reactive than the parent compound. The range of compounds which are sulfated is enormous, yet we still understand relatively little of the function of this pathway. This review summarises current knowledge of the sulfation system and the enzymes involved, and illustrates how heterologous expression of sulfotransferases (SULTs) and sulfatases is aiding our appreciation of the properties of these important proteins. The role of sulfation in the bioactivation of procarcinogens and promutagens is discussed, and new data on the inhibition of the sulfotransferase(s) involved by common dietary components such as tea and coffee are presented. The genetic and environmental factors which are known to influence the activity and expression of human SULTs and sulfatases are also reviewed.

Specificity of human UDP-glucuronosyltransferases and xenobiotic glucuronidation

Several human liver UDP-Glucuronosyltransferases (UGTs) have been cloned and the cDNAs expressed in heterologous cell lines. This technological advance has allowed the assessment of the functional substrate specificity of these UGTs. The problems which may be encountered with the latency and assay of UGTs are briefly described. The data accumulated to date indicate that the Km, and possibly the Vmax/Km, for individual substrates are the best parameters to assess the specificity of the enzymes towards xenobiotic molecules. The substrate specificity of seven UGTs has been summarised from the currently available information. Of these, UGT1*02 and UGT2B8 appear to be key isoforms in the glucuronidation of a wide range of xenobiotic substrates. Additional UGTs have yet to be identified and characterised and their future inclusion may provide further insights. Finally, the functional role of each UGT in vivo has to be determined.

Formation of mono- and diglucuronides and other glycosides of benzo(a)pyrene-3,6-quinol by V79 cell-expressed human phenol UDP-glucuronosyltransferases of the UGT1 gene complex

Glucuronidation of quinols of polycyclic aromatic hydrocarbons (PAHs) represents an important detoxication pathway preventing toxic quinone/quinol redox cycles. Therefore, mono- and diglucuronide formation of benzo(a)pyrene-3,6-quinol was investigated and compared to that of structurally related 3,6-dihydroxychrysene and simple phenols (1-naphthol and 4-methylumbelliferone) using V79 cell-expressed human UGT1.6 (= P1) and human UGT1.7 (= P4). Properties of human UGT1.6 were compared to those of the rat ortholog. Cofactors related to UDP-glucuronic acid such as UDP-galacturonic acid and UDP-glucose were also studied. It was found that rat and human UGT1.6 and human UGT1.7 catalyse monoglucuronide formation of planar PAH quinols. Diglucuronide formation was only detectable with human UGT1.7. The UGT isozymes studied also formed galacturonides and, although only to a minor extent, glucosides. Rat UGT1.6 (but not the human ortholog) catalysed digalacturonide formation of benzo(a)pyrene-3,6-quinol; the in vivo significance of galacturonide formation remains to be established. The results suggest that planar PAH phenols and quinols are conjugated more efficiently by human UGT1.7 than by UGT1.6, which preferentially conjugates simple planar phenols.

Regulatory Genotoxicity Testing: A Critical Appraisal

This review considers current approaches to regulatory genotoxicity testing, focusing on how the use of animals can be further replaced, reduced and refined. The complementary roles of in vitro and in vivo testing, and the justification for using animals, are discussed in detail. Recommendations are made for improvements and further work, in the light of the considerable current controversy surrounding the composition and deployment of testing strategies, and the interpretation of the data generated, particularly for carcinogenicity prediction. The major problems are the oversensitivity of in vitro tests and the insensitivity of in vivo assays. On the basis of an analysis of some published databases, it is concluded that there is insufficient support for using in vivo genotoxicity assays for screening. Also, it is questionable whether the scientific benefits of using such assays always outweigh the costs to the animals involved. The considerable efforts being made to harmonise in vivo protocols and to develop improved methods for detecting genotoxicity are discussed. It is recommended that more emphasis be placed on characterising genotoxins in vitro, especially for mechanisms of activity, to optimise the benefits of any confirmatory animal tests.. Also, regulatory agencies are urged to require better-designed and more-scientifically sound protocols, in which animal numbers are minimised and data interpretation, particularly that of negative results, is facilitated. Lastly, in the development and validation of transgenic rodent systems, emphasis should be placed on developing protocols in which other acute toxicity and metabolism endpoints can be measured simultaneously with in vivo mutagenesis, while minimising animal numbers.

Identification of human CYP isoforms involved in the metabolism of propranolol enantiomers--N-desisopropylation is mediated mainly by CYP1A2

1. Studies using human liver microsomes and six recombinant human CYP isoforms (i.e. CYP1A2, 2A6, 2B6, 2D6, 2E1 and 3A4) were performed to identify the cytochrome P450 (CYP) isoform(s) involved in the ring 4-hydroxylation and side-chain N-desisopropylation of propranolol enantiomers in humans. 2. alpha-Naphthoflavone and 7-ethoxyresorufin (selective inhibitors of CYP1A1/2) inhibited the N-desisopropylation of R- and S-propranolol by human liver microsomes by 20 and 40%, respectively, while quinidine (a selective inhibitor of CYP2D6) abolished the 4-hydroxylation of both propranolol enantiomers almost completely. In contrast, sulphaphenazole (CYP2C8/9 inhibitor), S-mephenytoin (CYP2C19 inhibitor), troleandomycin (CYP3A3/4 inhibitor) and diethyldithiocarbamate (CYP2E1 inhibitor) elicited only weak inhibitory effects on propranolol metabolism via the two measured metabolic pathways. 3. Significant (P < 0.01) correlations were observed between the microsomal N-desisopropylation of both propranolol enantiomers and that for the O-deethylation of phenacetin among the 11 different human liver microsome samples (r = 0.98 and 0.77 for R- and S-propranolol, respectively). A marginally significant (r = 0.60, P congruent to 0.05) correlation was also observed between N-desisopropylation of S-, but not of R-propranolol and the 4'-hydroxylation of S-mephenytoin. No significant correlations were observed between the N-desisopropylation of propranolol enantiomers and the 2-hydroxylation of desipramine, the hydroxylation of tolbutamide or the 6 beta-hydroxylation of testosterone. 4. Significant (P < 0.01) correlations were observed between the microsomal 4-hydroxylation of R- and S-propranolol and the 2-hydroxylation of desipramine (r = 0.85 and 0.98, respectively). A weak (r = 0.66), albeit significant (P < 0.05) correlation was observed between the 4-hydroxylation of R-, but not of S-propranolol and the hydroxylation of tolbutamide. No significant correlations were observed between the 4-hydroxylation of propranolol enantiomers and the oxidation of other substrates for CYP1A2, 2C19, and 3A3/4. 5. Recombinant human CYP1A2 and CYP2D6 exhibited comparable catalytic activity with respect to the N-desisopropylation of both propranolol enantiomers; only expressed CYP2D6 exhibited a marked catalytic activity with respect to the 4-hydroxylation of both propranolol enantiomers.(ABSTRACT TRUNCATED AT 400 WORDS)

Advantages and disadvantages of using human cells for pharmacological and toxicological studies

1. Marked species differences in the distribution and affinity of drug receptors, and in the patterns of biotransformation and susceptibility to the toxicity of xenobiotics, provide the impetus for using human tissues for pharmacological and toxicological studies. 2. Studies with intact cells facilitate the correlation of xenobiotic metabolism with cellular indices of toxicity, which can provide the mechanistic basis for understanding species differences in toxicity. 3. Human cells in suspension or primary culture reflect the variability in susceptibility to toxicity in a population. 4. The current limitation to these studies is scarcity of human material, the need for improved (cryo)preservation techniques for human hepatocytes/precision-cut slices and difficulties in predicting in vivo exposure-risk relationships from in vitro dose-response relationships.

Variables affecting nicotine metabolism

Nicotine metabolism is exceedingly sensitive to perturbation by numerous host factors. To reduce the large variations and discrepancies in the literature pertaining to nicotine metabolism, investigators in future studies need to recognize and better control these host factors. Recent advances in the understanding of nicotine metabolism have suggested new approaches to elucidating underlying mechanisms of certain toxic effects associated with cigarette smoking.