Characterization of Phase I and Phase II hepatic drug metabolism activities in a panel of human liver preparations

Evaluation of Human Hepatocyte Drug Metabolism Carrying High-Risk or Protection-Associated Liver Disease Genetic Variants

Metabolic-dysfunction-associated steatotic liver disease (MASLD), which affects 30 million people in the US and is anticipated to reach over 100 million by 2030, places a significant financial strain on the healthcare system. There is presently no FDA-approved treatment for MASLD despite its public health significance and financial burden. Understanding the connection between point mutations, liver enzymes, and MASLD is important for comprehending drug toxicity in healthy or diseased individuals. Multiple genetic variations have been linked to MASLD susceptibility through genome-wide association studies (GWAS), either increasing MASLD risk or protecting against it, such as PNPLA3 rs738409, MBOAT7 rs641738, GCKR rs780094, HSD17B13 rs72613567, and MTARC1 rs2642438. As the impact of genetic variants on the levels of drug-metabolizing cytochrome P450 (CYP) enzymes in human hepatocytes has not been thoroughly investigated, this study aims to describe the analysis of metabolic functions for selected phase I and phase II liver enzymes in human hepatocytes. For this purpose, fresh isolated primary hepatocytes were obtained from healthy liver donors (n = 126), and liquid chromatography-mass spectrometry (LC-MS) was performed. For the cohorts, participants were classified into minor homozygotes and nonminor homozygotes (major homozygotes + heterozygotes) for five gene polymorphisms. For phase I liver enzymes, we found a significant difference in the activity of CYP1A2 in human hepatocytes carrying MBOAT7 (p = 0.011) and of CYP2C8 in human hepatocytes carrying PNPLA3 (p = 0.004). It was also observed that the activity of CYP2C9 was significantly lower in human hepatocytes carrying HSD17B13 (p = 0.001) minor homozygous compared to nonminor homozygous. No significant difference in activity of CYP2E1, CYP2C8, CYP2D6, CYP2E1, CYP3A4, ECOD, FMO, MAO, AO, and CES2 and in any of the phase II liver enzymes between human hepatocytes carrying genetic variants for PNPLA3 rs738409, MBOAT7 rs641738, GCKR rs780094, HSD17B13 rs72613567, and MTARC1 rs2642438 were observed. These findings offer a preliminary assessment of the influence of genetic variations on drug-metabolizing cytochrome P450 (CYP) enzymes in healthy human hepatocytes, which may be useful for future drug discovery investigations.

Pharmacokinetic Interactions of a Licorice Dietary Supplement with Cytochrome P450 Enzymes in Female Participants

Licorice, the roots and rhizomes of Glycyrrhiza glabra L., has been used as a medicinal herb, herbal adjuvant, and flavoring agent since ancient times. Recently, licorice extracts have become popular as dietary supplements used by females to alleviate menopausal symptoms. Exposure to licorice products containing high levels of glycyrrhizic acid can cause hypokalemia, but independent from this effect, preclinical data indicate that licorice can inhibit certain cytochrome P450 (P450) enzymes. To evaluate whether clinically relevant pharmacokinetic interactions of licorice with P450 enzymes exist, a phase 1 clinical investigation was carried out using a licorice extract depleted in glycyrrhizic acid (content <1%) and a cocktail containing caffeine, tolbutamide, alprazolam, and dextromethorphan, which are probe substrates for the enzymes CYP1A2, CYP2C9, CYP3A4/5, and CYP2D6, respectively. The botanically authenticated and chemically standardized extract of roots from G. glabra was consumed by 14 healthy menopausal and postmenopausal female participants twice daily for 2 weeks. The pharmacokinetics of each probe drug were evaluated immediately before and after supplementation with the licorice extract. Comparison of the average areas under the time-concentration curves (AUCs) for each probe substrate in serum showed no significant changes from licorice consumption, whereas time to reach peak concentration for caffeine and elimination half-life for tolbutamide showed small changes. According to the US Food and Drug Administration guidance, which is based on changes in the AUC of each probe substrate drug, the investigated licorice extract should not cause any clinically relevant pharmacokinetic interactions with respect to CYP3A4/5, CYP2C9, CYP2D6, or CYP1A2. SIGNIFICANCE STATEMENT: Despite generally-recognized-as-safe status, the licorice species Glycyrrhiza glabra has been associated with some toxicity. Preclinical studies suggest that G. glabra might cause pharmacokinetic drug interactions by inhibiting several cytochrome P450 enzymes. This phase 1 clinical study addressed these concerns by evaluating clinically relevant effects with respect to CYP3A4/5, CYP2C9, CYP2D6, and CYP1A2. These results showed that a standardized G. glabra extract did not cause any clinically relevant pharmacokinetic drug interactions with four major cytochrome P450 enzymes.

Han Chinese specific cytochrome P450 polymorphisms and their impact on the metabolism of anti-hypertensive drugs with adrenoreceptor blocking properties

Introduction: Cytochrome P450 (CYP) is a monooxygenase superfamily mediating the elimination of anti-hypertensive drugs. Polymorphisms of CYP would lead to differential drug efficacy. Building relationships between genotype and phenotype will benefit individual medical treatment of hypertension.Areas covered: The review systematically summarizes the polymorphisms of four CYPs (CYP2C9, CYP2C19, CYP2D6, and CYP3A4) concentrated distributed in the Han Chinese population. Moreover, the activity of variants on metabolizing anti-hypertensive drugs are reviewed, especially drugs with adrenoceptor blocking properties, as well as their clinical relevancies.Expert opinion: The polymorphisms of CYP can cause stratification in drug exposure of antihypertensive drugs. Although the clinical relevance has been built partially, the translational medicine still lacks reliable data support. Furthermore, the studies have demonstrated that even the same CYP variant will exhibit different catalytic capability for different drugs, which is another obstacle to hinder its application. With the deepening of multiomics research and structural biology, nucleotide polymorphisms can be combined with transcriptome, proteome, metabolome and molecular structure analyses to study the susceptibility to hypertension and drug efficacy. A complete data chain would be further estabolished by combining studies of pharmacokinetics-pharmacodynamics, which can effectively promote the precise application of anti-hypertensive drugs.

A Comprehensive Overview of Antibiotic Selection and the Factors Affecting It

In order to prescribe an antibiotic, a physician must go through a series of decision-making processes that involve both the drug and the host. In this review article, we outline exactly what those decision-making processes are and some of their limitations. Before a medication can be prescribed, a physician has to determine if the antibiotic works against the host pathogen. To do this, basic science techniques are employed including phenotypic methods such as broth dilution methods, Kirby-Bauer susceptibility testing, Epsilometer test (E-test), and genotypic methods such as the new and upcoming automated tests. After determining if a drug has potential to work, the physician must consider the drug’s mechanism of action in order to determine a dosing regimen. Some groups of drugs should be administered at high concentrations infrequently, others should be given more frequently in smaller doses, and others lie somewhere between this spectrum. Finally, external factors such as the patient's age, especially for pediatrics and geriatrics patients, need to be considered, as these groups have the highest health care burden but are among the most vulnerable when it comes to the side effects of drugs.

Toward precision medicine in pediatric population using cytochrome P450 phenotyping approaches and physiologically based pharmacokinetic modeling

The activity of drug-metabolizing enzymes (DME) shows high inter- and intra-individual variability. Genetic polymorphisms, exposure to drugs, and environmental toxins are known to significantly alter DME expression. In addition, the activity of these enzymes is highly age-dependent due to maturation processes that occur during development. Currently, there is a vast choice of phenotyping methods in adults using exogenous probes to characterize the activity of these enzymes. However, this can hardly be applied to children since it requires the intake of non-therapeutic xenobiotics. In addition, sampling may be challenging in the pediatric population for a variety of reasons: limited volume (e.g., blood), inappropriate sampling methods for age (e.g., urine), and metric requiring invasive or multiple blood samples. This review covers the main existing methods that can be used in the pediatric population to determine DME activity, with a particular focus on cytochrome P450 enzymes. Less invasive tools are described, including phenotyping using endogenous probes. Finally, the potential of pediatric model-informed precision dosing using physiologically based pharmacokinetic modeling is discussed.

Oxidative metabolism of typical phenolic compounds of Danshen by electrochemistry coupled to quadrupole time-of-flight tandem mass spectrometry

An electrochemistry coupled to online quadrupole time-of-flight tandem mass spectrometry (EC/Q-TOF/MS) was applied to investigate the oxidative transformation and metabolic pathway of five phenolic acids in Danshen sample. Simulation of the phase I oxidative metabolism was carried out in an electrochemical reactor equipped with a glassy carbon working electrode. The phase II reactivity of the generated oxidative products towards biomolecules (such as glutathione) was investigated by ways of covalent adduct formation experiments. The results obtained by EC/MS were compared with well-known in vitro studies by conducting rat liver microsome incubations. Structures of the electrochemically produced metabolites were identified by accurate mass measurement and previously results in vivo metabolites. It was indicated that the electrochemical oxidation was in good accordance with similar products found in vivo experiments. In conclusion, this work confirmed that EC/Q-TOF/MS was a promising analytical tool in the prediction of metabolic transformations of functional foods.

Myclobutanil accumulation, transcriptional alteration, and tissue injury in lizards (Eremias argus) treated with myclobutanil enantiomers

Enantioselective toxicokinetics, accumulation, and toxicity of myclobutanil were investigated by oral exposure of myclobutanil enantiomers to lizards. After a single oral administration, the absorption half-lives ( [Formula: see text] ) and elimination half-lives (t_1/2k) were in the range of 0.133-14.828 and 3.641-17.682 h, respectively. The absorption and elimination half-lives of (+)-myclobutanil showed no significant differences from those of (-)-myclobutanil in lizard blood, whereas preferential enrichment of (-)-enantiomer was observed in the liver, fat, skin, intestine, lung and kidney. In the bioaccumulation experiments, the residue of (-)-myclobutanil was detected in most tissues at 7, 14, and 28 days, while (+)-myclobutanil was found only in lizard skin, at a concentration lower than that of (-)-myclobutanil. Thus, (-)-myclobutanil was preferentially accumulated in lizards. The transcriptional responses of metabolic enzyme genes indicated that cytochrome P450 1a1 (cyp1a1), cyp2d3, cyp2d6, cyp3a4 and cyp3a7 played a crucial role in the metabolism of (+)-myclobutanil, whereas cyp1a1, cyp2d3, cyp2d6, cyp2c8, and cyp3a4 contributed to the metabolism of (-)-myclobutanil. The difference in metabolism pathways may be a reason for the enantioselectivity of myclobutanil in lizard. Myclobutanil also affected the expression of antioxidant enzyme genes, and the (+)-myclobutanil treatment might produce higher oxidative stress in lizard liver when compared with its antipode. Hepatic histopathological changes such as hepatocellular hypertrophy, nuclear pyknosis, vacuolation, and non-zonal macrovesicular lipid accumulation were observed in the liver of lizards for both (+)-myclobutanil and (-)-myclobutanil treatments. Thus, myclobutanil could affect lizard liver upon multiple exposure. The findings of this study provide specific insights into the enantioselective metabolism and toxicity of chiral triazole fungicides in lizards.

Enantioselective Distribution, Degradation, and Metabolite Formation of Myclobutanil and Transcriptional Responses of Metabolic-Related Genes in Rats

Myclobutanil (MT), a chiral fungicide, can be metabolized enantioselectively in organisms. In this work, the associated absorption, distribution, metabolism and transcriptional responses of MT in rats were determined following a single-dose (10 mg·kg^-1 body weight) exposure to rac-, (+)- or (-)-MT. The enantiomer fractions (EFs) were less than 0.5 with time in the liver, kidney, heart, lung, and testis, suggesting preferential enrichment of (-)-MT in these tissues. Furthermore, there was conversion of (+)-form to (-)-form in the liver and kidney after 6 h exposure to enantiopure (+)-MT. Enrichment and degradation of the two enantiomers differed between rac-MT and MT-enantiomers groups, suggesting that MT bioaccumulation is enantiomer-specific. Interestingly, the degradation half-life of MT in the liver with rac-MT treatment was shorter than that with both MT-enantiomer treatments. One reason may be that the gene expression levels of cytochrome P450 1a2 ( cyp1a2) and cyp3a2 genes in livers treated with rac-MT were the highest among the three exposure groups. In addition, a positive correlation between the expression of cyp2e1 and cyp3a2 genes and rac-MT concentration was found in livers exposed to rac-MT. Simultaneously, five chiral metabolites were detected, and the enantiomers of three metabolites, RH-9090, RH-9089, and M2, were separated. The detected enantiomers of (+)-MT metabolites were in complete contrast with those of (-)-MT metabolites. According to the results, a metabolic pathway of MT in male rats was proposed, which included the following five metabolites: RH-9089, RH-9090, RH-9090 Sulfate, M1, and M2. The possible metabolic enzymes were marked in the pathway. The findings of this study provide more specific insights into the enantioselective metabolic mechanism of chiral triazole fungicides.

Screening study for genetic polymorphisms affecting pharmacokinetics of talniflumate

Talniflumate is a phthalidyl ester of niflumic acid, which has potent analgesic and anti-inflammatory effects and is widely used to treat inflammatory disorders, such as rheumatoid arthritis. To screen the possible genetic factors affecting the pharmacokinetics (PK) of talniflumate, 23 male Korean volunteers were enrolled from two separate bioequivalence studies. All subjects received 740 mg (two tablets) talniflumate in a standard 2×2 cross-over model in a randomized order. For the genetic study, PK parameters of the reference drug were used. We used Illumina Human610Quad v1.0 DNA Analysis BeadChip for whole genome single nucleotide polymorphism (SNP) analysis and whole genome genotyping data were processed by linear regression analysis for PK parameters. Whole genome analysis revealed 1498 significant SNPs (P < 0.0001) for Cmax, 65 significant SNPs (P < 0.0001) for T max, and 1491 significant SNPs (P < 0.0001) for AUC inf. For clinical pharmacological purposes, we selected SNPs from drug metabolizing enzymes and transporters, and analyzed the PK parameters of various genotypes. Two SNPs (rs11165069 from ABCA4 (p=0.00002); rs17847036 from CYP2C9 (p=0.000001)) showed significant associations with talniflumate C max. In the T max group, two SNPs (rs3787555 from CYP24A1 (p=0.00035); rs2275034 from ABCA4 (p=0.000587)) showed significant associations with talniflumate T max. In the AUC inf group, two SNPs (rs11165069 from ABCA4 (p=0.00002); rs12461006 from SLC1A6 (p=0.00008)) exhibited significant associations with talniflumate absorption. These results show that genetic factors could affect the PK parameters, and provide information that may be used in the development of personalized talniflumate therapy.

Drug Metabolism in the Liver

Metabolism is a biotransformation process, where endogenous and exogenous compounds are converted to more polar products to facilitate their elimination from the body. The process of metabolism is divided into 3 phases. Phase I metabolism involves functionalization reactions. Phase II drug metabolism is a conjugation reaction. Phase III refers to transporter-mediated elimination of drug and/or metabolites from body normally via liver, gut, kidney, or lung. This review presents basic information on drug-metabolizing enzymes and potential factors that might affect the metabolic capacities of the enzyme or alter drug response or drug-mediated toxicities.

PROXIMAL: a method for Prediction of Xenobiotic Metabolism

Background

Contamination of the environment with bioactive chemicals has emerged as a potential public health risk. These substances that may cause distress or disease in humans can be found in air, water and food supplies. An open question is whether these chemicals transform into potentially more active or toxic derivatives via xenobiotic metabolizing enzymes expressed in the body. We present a new prediction tool, which we call PROXIMAL (Prediction of Xenobiotic Metabolism) for identifying possible transformation products of xenobiotic chemicals in the liver. Using reaction data from DrugBank and KEGG, PROXIMAL builds look-up tables that catalog the sites and types of structural modifications performed by Phase I and Phase II enzymes. Given a compound of interest, PROXIMAL searches for substructures that match the sites cataloged in the look-up tables, applies the corresponding modifications to generate a panel of possible transformation products, and ranks the products based on the activity and abundance of the enzymes involved.

Results

PROXIMAL generates transformations that are specific for the chemical of interest by analyzing the chemical’s substructures. We evaluate the accuracy of PROXIMAL’s predictions through case studies on two environmental chemicals with suspected endocrine disrupting activity, bisphenol A (BPA) and 4-chlorobiphenyl (PCB3). Comparisons with published reports confirm 5 out of 7 and 17 out of 26 of the predicted derivatives for BPA and PCB3, respectively. We also compare biotransformation predictions generated by PROXIMAL with those generated by METEOR and Metaprint2D-react, two other prediction tools.

Conclusions

PROXIMAL can predict transformations of chemicals that contain substructures recognizable by human liver enzymes. It also has the ability to rank the predicted metabolites based on the activity and abundance of enzymes involved in xenobiotic transformation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-015-0241-4) contains supplementary material, which is available to authorized users.

Single nucleotide polymorphism of CYP3A4 intron 2 and its influence on CYP3A4 mRNA expression and liver enzymatic activity in human liver

In adult liver, CYP3A4 plays an important role in the metabolism of a wide range of endogenous and exogenous compounds. To investigate whether there is a single nucleotide polymorphism (SNP) of CYP3A4 intron 2 in the liver and its effects on the mRNA expression and enzymatic activity of CYP3A4, genomic DNA was extracted from 96 liver tissue samples obtained from patients who had undergone liver surgery. An SNP of CYP3A4 intron 2 was identified by polymerase chain reaction (PCR)-single-strand confirmation polymorphism and DNA sequencing. The mRNA expression of CYP3A4 was determined by the fluorescence quantitative PCR technique. The enzymatic activity of CYP3A4 was measured using erythromycin and testosterone as probe substrates. Twelve patients were found to have the SNP/T4127G CYP3A4 within intron 2. The mRNA levels of CYP3A4 in wild-type and SNP/T4127G samples were 2.62±1.09 and 2.79±1.63, respectively (P>0.05). Erythromycin N-demethylase activity in wild-type and SNP/T4127G samples were 121.2±32.8 and 124.7±61.6 nmol·mg(-1)·min(-1), respectively (P>0.05). The activity of testosterone 6β-hydroxylase was significantly different between wild-type (648±173 pmol·mg(-1)·min(-1)) and SNP/T4127G samples (540±196 pmol·mg(-1)·min(-1); P<0.05). In conclusion, the SNP/T4127G of CYP3A4 intron 2 exists in the liver. This SNP does not affect the mRNA expression of CYP3A4 but significantly decreases the hepatic microsomal testosterone 6β-hydroxylase activity of CYP3A4. Furthermore, this study indicates that the appropriate selection of probe substrates is very important in studying the relationship between the genotype and phenotype of CYP3A4.

Endogenous and exogenous mediators of quercetin bioavailability

Quercetin is a dietary flavonol that has poor and highly variable bioavailability. Epidemiological studies suggest that higher dietary intakes of quercetin decease cardiovascular disease (CVD) risk. However, experimental findings examining its cardioprotective activities are inconsistent, thereby precluding a full understanding of its health benefits. Bioavailability of dietary constituents is a critical mediator of their health benefits. Thus, a better understanding of the factors regulating quercetin bioavailability is expected to support its potential role in managing CVD risk. This review provides an update on the evidence describing endogenous and exogenous factors responsible for the limited and highly variable bioavailability of quercetin. It focuses on pharmacokinetics studies in clinical and animal models, while also describing strategies aimed at improving quercetin bioavailability to better realize its cardioprotective activities in vivo that are routinely observed in vitro. Although significant advances have been made in understanding determinants of quercetin bioavailability, additional research in controlled trials is needed to more comprehensively examine dose-response effects, whether its cardioprotective activities improve in response to its greater bioavailability, and if the putative health benefits of quercetin are mediated directly or indirectly from one or more of its metabolites generated during xenobiotic metabolism.

Intrinsic sexually dimorphic expression of the principal human CYP3A4 correlated with suboptimal activation of GH/glucocorticoid-dependent transcriptional pathways in men

Cytochrome P450 (CYP)3A4 is the principal and most abundant human isoform of CYP responsible for the metabolism of more than 50% of all consumed drugs and innumerable endogenous compounds. Expression of CYP3A4 is sexually dimorphic and regulated by the combined actions of GH and glucocorticoids. In the case of the rat, nearly all of the CYPs are "intrinsically" or "inherently" sexually dimorphic, meaning that the expressed sex differences are permanent and irreversible. Using primary hepatocyte cultures derived from men and women exposed to physiologic-like levels of continuous GH (the feminine circulating profile) alone, dexamethasone alone, and the combined regimen, we observed a dramatic inherent CYP3A4 sexual dimorphism (women more than men) with all treatments. The molecular basis for this intrinsic sexually dimorphic expression of CYP3A4 appears to be due, at least in part, to a greater level of hormone-dependent activation and nuclear translocation of both hepatocyte nuclear factor-4α (HNF-4α) and pregnane X receptor in female hepatocytes. Furthermore, these transcription factors exhibited significantly higher DNA binding levels to their specific motifs on the CYP3A4 promoter in female hepatocytes, inferring a possible explanation for the elevated expression of CYP3A4 in women. Accordingly, experiments using HepG2 cells treated with small inhibitory RNA-induced knockdown of HNF-4α and/or transfected with luciferase reporter constructs containing a CYP3A4 promoter lacking HNF-4α-binding motifs demonstrated that GH, to a greater extent dexamethasone, and to the greatest extent the combine hormone regimen, stimulated HNF-4α and pregnane X receptor promoter transactivation, signifying enhanced transcription of CYP3A4 and, thus, identifying a molecular mechanism contributing to the intrinsic sexual dimorphic expression of human CYP3A4.

Ginkgo biloba extract induces gene expression changes in xenobiotics metabolism and the Myc-centered network

The use of herbal dietary supplements in the United States is rapidly growing, and it is crucial that the quality and safety of these preparations be ensured. To date, it is still a challenge to determine the mechanisms of toxicity induced by mixtures containing many chemical components, such as herbal dietary supplements. We previously proposed that analyses of the gene expression profiles using microarrays in the livers of rodents treated with herbal dietary supplements is a potentially practical approach for understanding the mechanism of toxicity. In this study, we utilized microarrays to analyze gene expression changes in the livers of male B6C3F1 mice administered Ginkgo biloba leaf extract (GBE) by gavage for 2 years, and to determine pathways and mechanisms associated with GBE treatments. Analysis of 31,802 genes revealed that there were 129, 289, and 2,011 genes significantly changed in the 200, 600, and 2,000 mg/kg treatment groups, respectively, when compared with control animals. Drug metabolizing genes were significantly altered in response to GBE treatments. Pathway and network analyses were applied to investigate the gene relationships, functional clustering, and mechanisms involved in GBE exposure. These analyses indicate alteration in the expression of genes coding for drug metabolizing enzymes, the NRF2-mediated oxidative stress response pathway, and the Myc gene-centered network named "cell cycle, cellular movement, and cancer" were found. These results indicate that Ginkgo biloba-related drug metabolizing enzymes may cause herb-drug interactions and contribute to hepatotoxicity. In addition, the outcomes of pathway and network analysis may be used to elucidate the toxic mechanisms of Ginkgo biloba.

Synergistic antifibrotic effect of verapamil and interferon-gamma in rats: partially based on enhanced verapamil oral bioavailability

OBJECTIVE

The objective of this study was to investigate the synergistic antifibrotic effect of verapamil and interferon-gamma (IFN-gamma) on rat liver fibrosis and its potential pharmacokinetic-based mechanism.

METHODS

Rat liver fibrosis model was successfully established, and both the therapeutic effects and pharmacokinetic parameters of verapamil were evaluated after the administration of verapamil with or without IFN-gamma. The activities of cytochrome P450 3A (CYP3A) and the expression of multidrug resistance (Mdr) mRNA were measured in liver and small intestine.

RESULTS

The results showed the synergistic antifibrotic effect of verapamil and IFN-gamma in rat liver fibrosis, in terms of decreased serum L-alanine aminotransferase activity and liver hydroxyproline content and improved liver histopathology, when compared with rats treated with verapamil or IFN-gamma alone. Meanwhile, the area under the curve of verapamil increased significantly after single administration of verapamil and IFN-gamma and the concentration of verapamil in plasma increased, but the metabolite : parent ratio of verapamil decreased after consecutive administrations of verapamil and IFN-gamma. Furthermore, the activities of CYP3A in both the liver and the small intestine and the expression of Mdr in small intestine decreased in rats treated with verapamil and IFN-gamma.

CONCLUSION

All these results indicated that the combination of verapamil and IFN-gamma exerts a synergistic antifibrotic effect on rat liver fibrosis. The mechanism was partially based on the enhanced oral bioavailability of verapamil by increasing the intestinal absorption as well as reducing the first-pass metabolism, through inhibition of CYP3A activity and P-glycoprotein expression by IFN-gamma

In vitro evaluation of hepatic and extra-hepatic metabolism of coumarins using rat subcellular fractions: correlation of in vitro clearance with in vivo data

7-Ethoxycoumarin (7-EC) and 7-hydroxycoumarin (7-HC) were chosen as model compounds to study hepatic and extra-hepatic metabolism in rat tissue subcellular (microsomal and S9) fractions and to scale the observed in vitro clearance to in vivo plasma clearance. 7-EC and 7-HC showed significant metabolic degradation in liver subcellular fractions as compared to subcellular fractions obtained from intestine, kidney, lung and brain. The total in vitro metabolic clearance for 7-EC and 7-HC was determined by adding the individual in vitro organ clearance values obtained in hepatic and extra-hepatic microsomes or S9 fractions. The predicted in vivo clearance for 7-HC was 63.6 and 81.6 ml/min/kg by in vitro scaling from microsomes and S9 fractions, respectively. For 7-EC, the values were 78.5 and 76.8 ml/min/kg, respectively. The predicted clearance was found to be reasonably accurate with slight over- and underprediction. Interestingly, the relative contribution of hepatic and extra-hepatic metabolism to the total clearance of 7-EC and 7-HC was remarkably high, ranging from 62-77% and 22-38%, respectively, of the total metabolic clearance. It is concluded that the model of multi-organ subcellular fractions is a useful in vitro tool for the prediction of in vivo metabolic clearance, as it can provide information about the relative contribution of extra-hepatic and hepatic metabolism to total metabolic clearance.

Cytochrome P450 system as a toxicity biomarker of industrial wastewater in rat tissues

In the present study the effect of laboratory exposure to wastewaters from Aligarh (AWW) and Saharanpur (SWW) on the activities of cytochrome P450 (CYP450) isozymes like ethoxyresorufin-o-deethylase (EROD), pentoxyresorufin-o-deethylase (PROD) and N-nitrosodimethylamine demethylase (NDMA-d) were investigated in the liver and kidney of rats. The industrial wastewater samples from Saharanpur city of northern India resulted 12, 3.5 and 1.5-fold rise in the EROD (CYP1A1), PROD (CYP2B1) and NDMA-d (CYP2E1) activity, respectively, in the liver of treated animals. Renal EROD and PROD activities were found to be enhanced by around 5 and 7-folds, respectively, as a result of SWW treatment. On the other hand, Aligarh samples showed significant inhibition in these test CYP450 enzymes both in hepatic as well as renal tissues. Strong induction of CYP1A1 (>10-fold) suggests that EROD can serve as a potent biomarker of SWW in the liver of treated animal. However, PROD and EROD can also act as fairly good biomarkers in case of renal tissue. Marked elevation of EROD activity in SWW treated animals strongly suggests the overwhelming levels of EROD inducers in the Saharanpur sample while a meagre amount of inducers accompanied with significant levels of inhibitors in the Aligarh sample.

Gene expression profiling as an initial approach for mechanistic studies of toxicity and tumorigenicity of herbal plants and herbal dietary supplements

Dietary supplements are consumed by more than 300 million people worldwide, and herbal dietary supplements represent the most rapidly growing portion of this industry. Even though adverse health effects of many herbal dietary supplements have been reported, safety assurances are not being addressed adequately. Toxicological data on the identification of genotoxic and tumorigenic ingredients in many raw herbs are also lacking. Currently, more than 30 herbal dietary supplements and active ingredients have been selected by the National Toxicology Program (NTP) for toxicity and tumorigenicity studies. Due to the complexity of the chemical components present in plant extracts, there are no established methodologies for determining the mechanisms of toxicity (particularly tumorigenicity) induced by herbs, such as Gingko biloba leaf extract (GBE) and other herbal plant extracts. Consequently, the understanding of toxicity of herbal dietary supplements remains limited. We have proposed that application of DNA microarrays could be a highly practical initial approach for revealing biological pathways and networks associated with toxicity induced by herbal dietary supplements and the generation of hypotheses to address likely mechanisms. The changes in expression of subsets of genes of interest, such as the modulation of drug metabolizing genes, can be analyzed after treatment with an herbal dietary supplement. Although levels of gene expression do not represent fully the levels of protein activities, we propose that subsequent biochemical and genomic experiments based on these initial observations will enable elucidation of the mechanisms leading to toxicity, including tumorigenicity. This review summarizes the current practices of microarray analysis of gene expressions in animals treated with herbal dietary supplements and discusses perspectives for the proposed strategy.

Gene expression profiling in male B6C3F1 mouse livers exposed to kava identifies--changes in drug metabolizing genes and potential mechanisms linked to kava toxicity

The association of kava products with liver-related health risks has prompted regulatory action in many countries. We used a genome-wide gene expression approach to generate global gene expression profiles from the livers of male B6C3F1 mice administered kava extract by gavage for 14 weeks, and identified the differentially expressed drug metabolizing genes in response to kava treatments. Analyses of gene functions and pathways reveal that the levels of significant numbers of genes involving drug metabolism were changed and that the pathways involving xenobiotics metabolism, Nrf2-mediated oxidative stress response, mitochondrial functions and others, were altered. Our results indicate that kava extract can significantly modulate drug metabolizing enzymes, potentially leading to herb-drug interactions and hepatotoxicity.

Systematic and simultaneous gene profiling of 84 drug-metabolizing genes in primary human hepatocytes

Drug-metabolizing enzymes are an important battery of proteins that are involved in drug metabolism, xenobiotic detoxification, and drug-induced toxicity. Systematic, efficient, and simultaneous evaluation of drug-metabolizing gene expression in response to chemicals has a wide variety of implications in drug development, disease prevention, and personalized medicine and nutrition. In the current study, the authors have systematically and simultaneously evaluated the hepatic expression profile of drug-metabolizing enzymes in cultured human hepatocytes exposed to the xenobiotics rifampicin, omeprazole, and 3-methylcholanthrene (3-MC) using the Drug Metabolism RT(2)Profiler PCR Arrays. This new high-throughput tool allowed the authors to evaluate the expression of genes coding for 84 drug-metabolizing enzymes (including phase 1 and phase 2 drug-metabolizing enzymes and transporters) simultaneously, in a 96-well format using a small amount of experimental materials. To validate the quality of the Drug Metabolism RT(2)Profiler PCR Arrays, the PCR Array was compared with the well-documented platform TaqMan assay, and a high concordance was shown between these 2 methods, indicating the high reliability of the Drug Metabolism RT(2)Profiler PCR Arrays. In addition, increasing or decreasing the expression of drug-metabolizing enzymes by these 3 compounds was observed, and underlying mechanisms are discussed.

Determination of a Human Hepatic Microsomal Scaling Factor for Predicting in Vivo Drug Clearance

PURPOSE

To determine a microsomal scaling factor for human liver suitable for prediction of in vivo drug clearance from in vitro data and to explore the role of inter-liver variability in this factor on the reported underprediction from microsomal parameters.

METHODS

Cytochrome P450 (henceforth P450) content in whole homogenates and microsomes from 38 donor livers was used to determine a microsomal scaling factor. In a subset (n = 20) of these preparations, individual P450 enzymes were examined by Western blotting and selective probe activities were determined.

RESULTS

The scaling factor from 38 livers averaged 40 mg microsomal protein per gram liver with a coefficient of variation of 31%. Western blotting experiments indicated that there was no P450 enzyme-specific trend in the distribution of individual P450 enzymes in liver microsomes relative to whole homogenate. Predictions based on an average scaling factor resulted in a satisfactory prediction of intrinsic clearance of three benzodiazepines similar to that obtained using individual factors for the same livers.

CONCLUSION

A value for human liver microsomal scaling of 40 mg microsomal protein per gram liver has been established. The reason for underprediction previously reported for 52 different drug substrates was not the use of an incorrect value for the scaling factor.

Early metabolism evaluation making traditional Chinese medicine effective and safe therapeutics

Increasing attention is being paid to the scientific evaluation of traditional Chinese medicine (TCM). As many TCMs are capable of biotransformation in the gastrointestinal tract, attention to biotransformation of TCM in the gastrointestinal tract may lead to discovery of the active components and active mechanisms. In this article, we review reports that host metabolic enzymes and intestinal bacteria may be responsible for the metabolism of TCM. Good understanding of the in vivo course of TCM will help us to know how to conduct metabolism evaluation of TCM by using in vitro human-derived system. This evaluation system will create new views on TCM as effective and safe therapeutic agents.

Evaluation of mRNA expression of human drug-metabolizing enzymes and transporters in chimeric mouse with humanized liver

The hepatic mRNA expression of human drug-metabolizing enzymes and transporters in chimeric mise with almost-completely humanized liver (replacement index: 71-89%) was investigated. The mRNAs of 58 human phase I enzymes, 26 human phase II enzymes, 23 human transporters, and five mouse Cyps were measured in the chimeric mice with humanized liver generated using hepatocytes from a Japanese donor. The mRNA expression of 52 human phase I enzymes, which includes 20 human CYPs, 26 human phase II enzymes and 21 human transporters was ascertained in the chimeric mouse liver. Among them, the expression of the target mRNAs vital for liver function such as the metabolism and secretion of endogenous compounds appeared to be maintained. The central value for the expression ratio in all target genes in chimeric mouse liver to the donor liver was 0.46, which was lower than the substitution rate of chimeric mouse liver by donor liver. The ratio of mouse Cyp mRNA expression of chimeric mouse liver to that of control mouse liver was 0.19 or less, except for that of Cyp2b10. There were good correlations between the mRNA expression levels of human hepatic albumin gene, the values of the rate of replacement of mouse liver by human liver, and the human blood albumin concentration in the chimeric mice. The chimeric mice with humanized liver may be a useful tool for the evaluation of drug-drug interactions such as the inhibition and induction of drug-metabolizing enzymes and transporters.

Sexually dimorphic regulation of hepatic isoforms of human cytochrome p450 by growth hormone

Sex differences in drug metabolism have been reported in numerous species, including humans. In rats and mice, sex-dependent differences in circulating growth hormone profiles are responsible for the differential expression of multiple sex-dependent isoforms of cytochrome P450, which is the basis for the sexual dimorphism in drug metabolism. In contrast, very little is known about sex differences in human isoforms of cytochrome P450 and their regulation by growth hormone. In this study, we have examined the effects of physiologic-like exposure doses to dexamethasone and/or pulsatile (masculine) or constant (feminine) human growth hormone on expression levels of CYP3A4, 1A2, 2D6, and 2E1 and the glucocorticoid and growth hormone receptors in hepatocyte cultures obtained from men and women donors. We report that growth hormone can regulate expression of CYP3A4, 1A2, and 2D6. The masculine-like pulsatile growth hormone profile suppresses dexamethasone-induced CYP3A4, 1A2, and 2D6, whereas the feminine-like constant profile is permissive allowing isoform expression to be equal to or greater than glucocorticoid induction alone. There are intrinsic sexual differences in hepatocytes of men and women resulting in different levels of responsiveness of CYP3A4, 1A2, and hormone receptor expression to the same sexually dimorphic growth hormone profiles. Last, although real, the sexually dimorphic effects of growth hormone on human cytochrome P450 expression are not as dramatic as those observed in rats and could easily be overlooked by the heterogeneous backgrounds of human populations.

GLUCURONIDATION AND SULFATION OF 7-HYDROXYCOUMARIN IN LIVER MATRICES FROM HUMAN, DOG, MONKEY, RAT, AND MOUSE

Uridine 5'-diphospho-N-acetylgalactosamine glycosyltransferases (UGTs) and sulfotransferases (SULTs) are 2 phase II enzymes that are actively involved in detoxification processes as well as in drug metabolism. Compared with cytochrome P450 enzymes, the role of UGTs and SULTs in drug metabolism has received little attention. Liver microsomes, S9 fractions, and cryopreserved hepatocytes from human, dog, cynomolgus monkey, mouse, and rat were used as matrices in the study. Single compound, 7-hydroxycoumarin (7-HC), along with necessary cofactors was dosed into the matrices and incubated at 37 degrees C; formation of two metabolites, 7-HC-glucuronide and 7-HC-sulfate, was determined with liquid chromatography with tandem mass spectrometry. Within the same species, the UGTs activities in microsomes and S9 fractions were comparable. In addition, UGTs activities in cryopreserved hepatocytes were lower than in the other matrices. Also, the SULTs activities were much higher in S9 fractions than in cryopreserved hepatocytes and microsomes. Species differences on UGTs and SULTs activities were also observed. The results indicated that S9 fractions, microsomes, and cryopreserved hepatocytes might be useful for UGTs metabolism study, whereas S9 fractions appear to be the most appropriate matrix for both UGTs and SULTs metabolism. Species differences with respect to phase II metabolism also need to be taken into consideration when selecting an in vitro system to evaluate various aspects of drug metabolism.

Flavin-containing monooxygenase activity in camel tissues: comparison with rat and human liver enzymes

We previously reported the occurrence of multiple forms of drug metabolizing enzymes in camel tissues. In this study, we demonstrated for the first time, flavin-containing monooxygenase (FMO)-dependent metabolism of two model substrates methimazole (MEM) and N,N'-dimethylaniline (DMA) by camel liver, kidney, brain and intestine. FMO-catalyzed metabolism in the microsomes of camel tissues was independent of cytochrome P450 (CYP) activity and exhibited a pH and temperature dependence characteristic of FMO enzymes. Use of inhibitors of CYP activities, SKF525A, octylamine or antibody against NADPH-P450 reductase, did not significantly alter the FMO-dependent substrate metabolism. Using MEM as a model substrate for FMO activity, we show that camel liver has an activity similar to that in rat and human livers. MEM metabolism in extrahepatic tissues in camels was significantly lower (60%-80%) than that in liver. Our results suggest occurrence of FMO in camel tissues, with catalytic properties similar to those in rat and human livers. These results may help in better understanding the effects of pharmacologically and toxicologically active compounds administered to camels.

Comparison of basal gene expression profiles and effects of hepatocarcinogens on gene expression in cultured primary human hepatocytes and HepG2 cells

Toxicogenomics is a relatively new discipline of toxicology. Microarrays and bioinformatics tools are being used successfully to understand the effects of toxicants on in vivo and in vitro model systems, and to gain a better understanding of the relevance of in vitro models commonly used in toxicological studies. In this study, cDNA filter arrays were used to determine the basal expression patterns of human cultured primary hepatocytes from different male donors; compare the gene expression profile of HepG2 to that of primary hepatocytes; and analyze the effects of three genotoxic hepatocarcinogens; aflatoxin B(1) (AFB(1)), 2-acetylaminofluorene (2AAF), and dimethylnitrosamine (DMN), as well as one non-gentoxic hepatotoxin, acetaminophen (APAP) on gene expression in both in vitro systems. Real-time PCR was used to verify differential gene expression for selected genes. Of the approximately 31,000 genes screened, 3-6% were expressed in primary hepatocytes cultured on matrigel for 16 h. Of these genes, 867 were expressed in cultured hepatocytes from all donors. HepG2 cells expressed about 98% of the genes detectable in cultured primary hepatocytes, however, 31% of the HepG2 transcriptome was unique to the cell line. A number of these genes are expressed in human liver but expression is apparently lost during culture. There was considerable variability in the response to chemical carcinogen exposure in primary hepatocytes from different donors. The transcription factors, E2F1 and ID1 mRNA were increased three-fold and six-fold (P < 0.05, P < 0.01), respectively, in AFB(1) treated primary human hepatocytes but were not altered in HepG2. ID1 expression was also increased by dimethylnitrosamine, acetylaminofluorene and acetaminophen in both primary hepatocytes and HepG2. Identification of genes that are expressed in primary hepatocytes from most donors, as well as those genes with variable expression, will aid in understanding the variability in human reactions to drugs and chemicals. This study suggests that identification of biomarkers of exposure to some chemicals may be possible in the human through microarray analysis, despite the variability in responses.

Factors affecting the clinical development of cytochrome p450 3A substrates

The objective of this review is to evaluate the risks associated with the discovery and development of cytochrome p450 (CYP) 3A substrates. CYP3A is the most abundant p450 enzyme in human liver and is highly expressed in the intestinal tract. The enzyme contributes substantially to metabolism of approximately 50% of currently marketed drugs that undergo oxidative metabolism. As a result, drug-drug interactions involving inhibitors of CYP3A-mediated metabolism can be of great clinical consequence. It is the position of the authors that, because of the factors responsible for the broad substrate specificity of CYP3A, discovery and development of compounds across a large and broad portfolio that are completely devoid of CYP3A metabolism is not feasible. Thus, it is important that scientifically valid approaches to the discovery and development of compounds metabolised by CYP3A be realised. The clinical relevance of CYP3A metabolism is dependent on a multitude of factors that include the degree of intestinal and hepatic CYP3A-mediated first-pass extraction, the therapeutic index of the compound and the adverse event associated with inhibition of CYP3A metabolism. Thus, a better understanding of the disposition of a CYP3A-metabolised compound relative to the projected or observed therapeutic index (or safety margin) can provide ample evidence to support the continued development of a CYP3A substrate. This document will highlight current practices as well as the benefits and risks associated with those practices.

The impact of cytochrome P450 2E1-dependent metabolic variance on a risk-relevant pharmacokinetic outcome in humans

Risk assessments include assumptions about sensitive subpopulations, such as the fraction of the general population that is sensitive and the extent that biochemical or physiological attributes influence sensitivity. Uncertainty factors (UF) account for both pharmacokinetic (PK) and pharmacodynamic (PD) components, allowing the inclusion of risk-relevant information to replace default assumptions about PK and PD variance (uncertainty). Large numbers of human organ donor samples and recent advances in methods to extrapolate in vitro enzyme expression and activity data to the intact human enable the investigation of the impact of PK variability on human susceptibility. The hepatotoxicity of trichloroethylene (TCE) is mediated by acid metabolites formed by cytochrome P450 2E1 (CYP2E1) oxidation, and differences in the CYP2E1 expression are hypothesized to affect susceptibility to TCE's liver injury. This study was designed specifically to examine the contribution of statistically quantified variance in enzyme content and activity on the risk of hepatotoxic injury among adult humans. We combined data sets describing (1) the microsomal protein content of human liver, (2) the CYP2E1 content of human liver microsomal protein, and (3) the in vitro Vmax for TCE oxidation by humans. The 5th and 95th percentiles of the resulting distribution (TCE oxidized per minute per gram liver) differed by approximately sixfold. These values were converted to mg TCE oxidized/h/kg body mass and incorporated in a human PBPK model. Simulations of 8-hour inhalation exposure to 50 ppm and oral exposure to 5 micro g TCE/L in 2 L drinking water showed that the amount of TCE oxidized in the liver differs by 2% or less under extreme values of CYP2E1 expression and activity (here, selected as the 5th and 95th percentiles of the resulting distribution). This indicates that differences in enzyme expression and TCE oxidation among the central 90% of the adult human population account for approximately 2% of the difference in production of the risk-relevant PK outcome for TCE-mediated liver injury. Integration of in vitro metabolism information into physiological models may reduce the uncertainties associated with risk contributions of differences in enzyme expression and the UF that represent PK variability.

Incorporating human interindividual biotransformation variance in health risk assessment

The protection of sensitive individuals within a population dictates that measures other than central tendencies be employed to estimate risk. The refinement of human health risk assessments for chemicals metabolized by the liver to reflect data on human variability can be accomplished through (1) the characterization of enzyme expression in large banks of human liver samples, (2) the employment of appropriate techniques for the quantification and extrapolation of metabolic rates derived in vitro, and (3) the judicious application of physiologically based pharmacokinetic (PBPK) modeling. While in vitro measurements of specific biochemical reactions from multiple human samples can yield qualitatively valuable data on human variance, such measures must be put into the perspective of the intact human to yield the most valuable predictions of metabolic differences among humans. For quantitative metabolism data to be the most valuable in risk assessment, they must be tied to human anatomy and physiology, and the impact of their variance evaluated under real exposure scenarios. For chemicals metabolized in the liver, the concentration of parent chemical in the liver represents the substrate concentration in the Michaelis Menten description of metabolism. Metabolic constants derived in vitro may be extrapolated to the intact liver, when appropriate conditions are met. Metabolic capacity Vmax; the maximal rate of the reaction) can be scaled directly to the concentration of enzyme (or enzyme fraction) contained in the liver. Several environmental, genetic and lifestyle factors can influence the concentration of cytochrome P450 forms (CYP) in the liver by affecting either (1) the extent to which the CYP forms are expressed in the endoplasmic reticulum of the cell (isolated as the microsomal fraction from tissue homogenates), or (2) the expression of microsomal protein in intact liver tissue. Biochemically sound measures of the hepatic distribution of xenobiotic metabolizing enzymes among humans, based on expression of the enzymes within microsomal protein and the distribution of microsomal protein among intact livers, can be combined with metabolic constants derived in vitro to generate values consistent with those employed in PBPK models. When completed, the distribution (and bounds) of Vmax values can be estimated and included in PBPK models. Exercising such models under plausible exposure scenarios will demonstrate the extent to which human interindividual enzyme variance can influence parameters (i.e., the detoxication of a toxic chemical through metabolism) that may influence risk. In this article, we describe a methodology and conditions which must exist for such an approach to be successful.

Variation in coumarin 7-hydroxylase activity associated with genetic polymorphism of cytochrome P450 2A6 and the body status of iron stores in adult Thai males and females

The relationships between catalytic activity of cytochrome P450 2A6 (CYP2A6), polymorphism of CYP2A6 gene, gender and levels of body iron stores were analysed in a sample group of 202 apparently healthy Thais, aged 19-47 years. Eleven individuals were found to have high activity of CYP2A6, judged by the relatively large amounts (11.2-14.6 mg) of 7-hydroyxcoumarin (7-OHC) excreted 3 h following administration of 15 mg of coumarin. Ten individuals, however, did not excrete any 7-OHC. Of these 10, four were found to have no CYP2A6 gene (whole gene deletion; CYP2A6*4 allele). The frequency of the CYP2A6 alleles; *1A, *1B and *4 in the whole sample group was 52, 40 and 8% while the frequency of the CYP2A6 gene types; *1A/*1A, *1A/*1B, *1B/*1B, *1A/*4, *1B/*4, *4/*4 was 29, 41, 16, 7, 5 and 2%. Subjects having CYP2A6*1A/*1B gene-type group were found to have higher rates of coumarin 7-hydroxylation compared with those of the CYP2A6*1B/*1B and CYP2A6*1A/*4 gene types. The inter-individual variability in CYP2A6 catalytic activity was therefore attributed in part to the CYP2A6 genetic polymorphism. Variation in CYP2A6 activity in this sample group was not associated with gender but, interestingly, it did show an inverse association with plasma ferritin; an indicator of body iron stores. Higher rates of coumarin 7-hydroxylation were found in individuals with low body iron stores (plasma ferritin < 20 microg/l) compared with subjects having normal body iron store status. Subjects (n = 16) with iron overload (plasma ferritin > 300 microg/l) also tended to have elevated rates of coumarin 7-hydroxylation. These results suggest an increased CYP2A6 expression in subjects who have excessive body iron stores. Further investigations into the underlying factors that may lead to increased expression of CYP2A6 in association with abnormal body iron stores are currently in progress in our laboratory.

Drug glucuronidation in clinical psychopharmacology

Glucuronidation is a phase II metabolic process and one of the most common pathways in the formation of hydrophilic drug metabolites. At least 33 families of uridine diphosphate-glucuronosyltransferases have been identified in vitro, and specific nomenclature similar to that used to classify the cytochrome (CYP) P450 system has been established. The UGT1 and UGT2 subfamilies represent the most important of these enzymes in human drug metabolism. Factors affecting glucuronidation include the following: cigarette smoking, obesity, age, and gender. In addition, several drugs have been found in vitro to be substrates, inhibitors, or inducers of UGT enzymes. Induction or inhibition of both UGT and CYP isoforms may occur simultaneously. Some important drug interactions involving glucuronidation have been documented and others can be postulated. This review summarizes the relevant literature pertaining to drug glucuronidation and its implications for clinical psychopharmacology.

Relationships between non-occupational cadmium exposure and expression of nine cytochrome P450 forms in human liver and kidney cortex samples

This study was undertaken to assess associations between age, gender, cigarette smoke and non-workplace cadmium exposure, and liver pathology and inter-individual variation in cytochrome P450 (CYP) expression in human tissues. Autopsy specimens of twenty-eight Queensland residents whose ages ranged from 3 to 89 years were analyzed for the presence of nine CYP protein isoforms by immunoblotting. All subjects were Caucasians and their liver cadmium contents ranged from 0.11 to 3.95 microg/g wet weight, while their kidney cadmium contents were in the range of 2 to 63 microg/g wet weight. CYP1A2, CYP2A6, CYP2D6, CYP3A4, and CYP3A5 were detected in liver but not in kidney, and CYP1A1 and CYP1B1 were not found in liver or kidney. Lowered liver CYP2C8/19 protein contents were found to be associated with liver pathology. Importantly, we show elevated levels of CYP2C9 protein to be associated with cadmium accumulation in liver. No mechanism that explains this association is apparent, but there are two possibilities that require further study. One is that variation in CYP2C9 protein levels may be, in part, attributed to an individual's non-workplace exposure to cadmium, or an individual's CYP2C9 genotype may be a risk factor for cadmium accumulation. A positive correlation was found between liver CYP3A4 protein and subject age. Levels of liver CYP1A2 protein, but not other CYP forms, were increased in people more exposed to cigarette smoke, but there was no association between CYP1A2 protein and cadmium. CYP2A6 protein was found in all liver samples and CYP2A6 gene typing indicated the absence of CYP2A6 null allele (CYP2A6(D)) in this sample group, confirming very low prevalence of homozygous CYP2A6(D) in Caucasians. CYP2A6 gene types W/W, W/C, and C/C were not associated with variations in liver microsomal CYP2A6 protein. CYP2D6 protein was absent in all twenty-five kidney samples tested but was detectable in liver samples of all but two subjects, indicating the prevalence of the CYP2D6 null allele (CYP2D6(D)) in this sample group to be about 7%, typical of Caucasian populations.

Identification of UDP-glucuronosyltransferases involved in the human hepatic metabolism of GV150526, a novel glycine antagonist

The major metabolic pathway for elimination of GV150526 is by glucuronidation exerted by glucuronosyl transferases (UGTs). Potential exists for the modification of GV150526 pharmacokinetics by drugs capable of inhibiting the glucuronidation of GV150526. Using human liver microsomes, 44 compounds were screened for inhibition of GV150526 glucuronidation. These compounds were selected because they are extensively glucuronidated themselves or are used as concomitant medication in the treatment of acute stroke. For 11 compounds out of the 44, full inhibition kinetics were performed to determine their Ki-value and mechanism of inhibition. To predict possible in vivo drug-drug interactions, the theoretical percentage of inhibition (i) was determined, based on in vitro determined Ki-values, and the expected Cmax plasma levels of GV150526 and the inhibitor. Of the 11 compounds examined, only propofol had an i-value of 6.6; for all other compounds i-values were lower than 2.1. These results indicate that although in vitro inhibition is observed, the likelihood of in vivo drug-drug metabolic interactions occurring is low. The inhibition results suggest that in addition to UGT1A1, also UGT1A3, UGT1A8/9, and UGT2B4 are involved in the glucuronidation of GV150526. The involvement of UGT1A1 and UGT1A8/9 was confirmed from studies using cDNA expressed human UGT cell lines.

Human hepatic metabolism of a novel 2-carboxyindole glycine antagonist for stroke: in vitro-in vivo correlations

1. The hepatic metabolism of 3-[-2(phenylcarbamoyl) ethenyl]-4,6-dichloroindole-2-carboxylic acid (GV150526), a novel glycine antagonist for stroke, was investigated. 2. After a single intravenous administration of 800 mg GV150526 to healthy volunteers, six metabolites were observed. The major metabolites detected in human plasma have been shown by mass spectrometry to be glucoronides and one sulphate conjugate. 3. After incubation of GV150526 for 6 and 24 h with human liver slices, three glucuronide metabolites were observed. After incubation of GV150526 with pooled human liver microsomes, only one metabolite was observed, with the same molecular weight and HPLC retention time as the synthetic standard GV217053 (GV150526 hydroxylated on the para-position of the phenyl ring). 4. GV150526 hydroxylase enzyme kinetics--a step before sulphation--was determined using pooled human microsomes and was shown to be catalysed by cytochrome P4502C9. Glucuronidation kinetics towards GV150526 using microsomal preparations were also determined. Glucuronidation of GV150526 was observed with UGT1A1 cDNA-expressed protein, but not with UGT1A6. 5. The above enzyme kinetic data were used to calculate intrinsic clearance after scaling-up and hepatic clearance were calculated. Since GV150526 has a high plasma protein binding capacity, the effect of GV150526 binding to microsomal protein was determined. Thus, enzyme kinetic data were corrected, plotting the free (unbound) concentration of GV150526 versus enzymatic velocities: apparent Vmax did not alter significantly but apparent Km was approximately 10-fold lower. Correlation of these corrected enzyme kinetic data to predict clearance with in vivo clearance of GV150526 was good when both fu(plasma) and fu(microsomes) were included in the clearance calculations.