Paracetamol as a test drug to determine glucuronide formation in man. Effects of inducers and of smoking

Exploring the Smoking-Epilepsy Nexus: a systematic review and meta-analysis of observational studies : Smoking and epilepsy

BACKGROUND

Epilepsy, characterized by recurrent unprovoked seizures, poses significant challenges to affected individuals globally. While several established risk factors for epilepsy exist, the association with cigarette smoking remains debated. This study aims to conduct systematic review and meta-analysis to elucidate the potential association between smoking and the likelihood of epilepsy.

METHODS

The search was performed on March 31st, 2023, using the Medline, Embase, Web of Science, Scopus, and ScienceDirect. We included cohort, cross-sectional, and case-control studies in our meta-analysis, conducting subgroup analyses based on smoking history, sex, and epilepsy type to yield specific insights.

RESULTS

We identified 2550 studies, of which 17 studies were finally included in this study. The pooled odds ratio of epilepsy was 1.14 (0.96-1.36) in smokers compared to non-smokers. In current smokers compared to non-smokers, the odds ratio was 1.46 (1.13-1.89), while, in former smokers compared to non-smokers, the odds ratio was 1.14 (0.83-1.56).

CONCLUSIONS

While the overall association between smoking and epilepsy did not reach statistical significance, a notable association was found among current smokers. The study emphasizes the importance of smoking cessation as a potential preventive measure against epilepsy, especially given the proconvulsive effects of nicotine. Future research should address limitations and explore specific clinical scenarios to enhance our understanding of the complex relationship between cigarette use and epilepsy.

SYSTEMATIC REVIEW REGISTRATION

CRD42022342510.

How do smoking, vaping, and nicotine affect people with epilepsy and seizures? A scoping review protocol

BACKGROUND

Epilepsy is a prevalent disease that requires personalized care to control seizures, reduce side effects, and ameliorate the burden of comorbidities. Smoking is a major cause of preventable death and disease. There is evidence that patients with epilepsy smoke at high rates and that smoking may increase seizure frequency. However, there is a lack systematically synthesized evidence on the interactions between epilepsy and seizures and smoking, tobacco use, vaping, and smoking cessation.

METHODS AND ANALYSIS

This scoping review protocol guided by the Joanna Briggs Institute Manual for Evidence Synthesis and the PRISMA Extension for Scoping Reviews will investigate what is known about the interactions between smoking and epilepsy. This review will include the population of persons with all types of epilepsy or seizures and examine an inclusive list of concepts including tobacco use, vaping, nicotine replacement, and smoking cessation. The MEDLINE, Embase, APA Psycinfo, CINAHL, Cochrane, Scopus, and Web of Science databases will be searched. Following systematic screening of records, data will be charted, synthesized, and summarized for presentation and publication.

ETHICS AND DISSEMINATION

No ethical approval is required for this literature-based study. The results of this scoping review will be submitted for publication in a peer-reviewed journal. This synthesis will be informative to clinicians and direct further research that may improve health outcomes for people with epilepsy.

REGISTRATION

This protocol is registered with the Open Science Framework (DOI: https://doi.org/10.17605/OSF.IO/D3ZK8).

Drug-drug interactions that alter the exposure of glucuronidated drugs: Scope, UDP-glucuronosyltransferase (UGT) enzyme selectivity, mechanisms (inhibition and induction), and clinical significance

Drug-drug interactions (DDIs) arising from the perturbation of drug metabolising enzyme activities represent both a clinical problem and a potential economic loss for the pharmaceutical industry. DDIs involving glucuronidated drugs have historically attracted little attention and there is a perception that interactions are of minor clinical relevance. This review critically examines the scope and aetiology of DDIs that result in altered exposure of glucuronidated drugs. Interaction mechanisms, namely inhibition and induction of UDP-glucuronosyltransferase (UGT) enzymes and the potential interplay with drug transporters, are reviewed in detail, as is the clinical significance of known DDIs. Altered victim drug exposure arising from modulation of UGT enzyme activities is relatively common and, notably, the incidence and importance of UGT induction as a DDI mechanism is greater than generally believed. Numerous DDIs are clinically relevant, resulting in either loss of efficacy or an increased risk of adverse effects, necessitating dose individualisation. Several generalisations relating to the likelihood of DDIs can be drawn from the known substrate and inhibitor selectivities of UGT enzymes, highlighting the importance of comprehensive reaction phenotyping studies at an early stage of drug development. Further, rigorous assessment of the DDI liability of new chemical entities that undergo glucuronidation to a significant extent has been recommended recently by regulatory guidance. Although evidence-based approaches exist for the in vitro characterisation of UGT enzyme inhibition and induction, the availability of drugs considered appropriate for use as 'probe' substrates in clinical DDI studies is limited and this should be research priority.

Over-the-Counter Monocyclic Non-Steroidal Anti-Inflammatory Drugs in Environment-Sources, Risks, Biodegradation

Recently, the increased use of monocyclic non-steroidal anti-inflammatory drugs has resulted in their presence in the environment. This may have potential negative effects on living organisms. The biotransformation mechanisms of monocyclic non-steroidal anti-inflammatory drugs in the human body and in other mammals occur by hydroxylation and conjugation with glycine or glucuronic acid. Biotransformation/biodegradation of monocyclic non-steroidal anti-inflammatory drugs in the environment may be caused by fungal or bacterial microorganisms. Salicylic acid derivatives are degraded by catechol or gentisate as intermediates which are cleaved by dioxygenases. The key intermediate of the paracetamol degradation pathways is hydroquinone. Sometimes, after hydrolysis of this drug, 4-aminophenol is formed, which is a dead-end metabolite. Ibuprofen is metabolized by hydroxylation or activation with CoA, resulting in the formation of isobutylocatechol. The aim of this work is to attempt to summarize the knowledge about environmental risk connected with the presence of over-the-counter anti-inflammatory drugs, their sources and the biotransformation and/or biodegradation pathways of these drugs.

The UDP-glucuronosyltransferases of the blood-brain barrier: their role in drug metabolism and detoxication

UDP-glucuronosyltransferases (UGTs) form a multigenic family of membrane-bound enzymes expressed in various tissues, including brain. They catalyze the formation of β-D-glucuronides from structurally unrelated substances (drugs, other xenobiotics, as well as endogenous compounds) by the linkage of glucuronic acid from the high energy donor, UDP-α-D-glucuronic acid. In brain, UGTs actively participate to the overall protection of the tissue against the intrusion of potentially harmful lipophilic substances that are metabolized as hydrophilic glucuronides. These metabolites are generally inactive, except for important pharmacologically glucuronides such as morphine-6-glucuronide. UGTs are mainly expressed in endothelial cells and astrocytes of the blood brain barrier (BBB). They are also associated to brain interfaces devoid of BBB, such as circumventricular organ, pineal gland, pituitary gland and neuro-olfactory tissues. Beside their key-role as a detoxication barrier, UGTs play a role in the steady-state of endogenous compounds, like steroids or dopamine (DA) that participate to the function of the brain. UGT isoforms of family 1A, 2A, 2B and 3A are expressed in brain tissues to various levels and are known to present distinct but overlapping substrate specificity. The importance of these enzyme species with regard to the formation of toxic, pharmacologically or physiologically relevant glucuronides in the brain will be discussed.

Demethylzeylasteral exhibits strong inhibition towards UDP-glucuronosyltransferase (UGT) 1A6 and 2B7

Inhibition of UDP-glucuronosyltransferase (UGT) isoforms can result in severe clinical results, including clinical drug-drug interactions (DDI) and metabolic disorders of endogenous substances. The present study aims to investigate the inhibition of demethylzeylasteral (an important active component isolated from Tripterygium wilfordii Hook F.) towards three important UGT isoforms UGT1A6, UGT1A9 and UGT2B7. The results showed that 100 μM of demethylzeylasteral exhibited strong inhibition towards UGT1A6 and UGT2B7, with negligible influence towards UGT1A9. Furthermore, Dixon and Lineweaver-Burk plots showed the inhibition of UGT1A6 and UGT2B7 by demethylzeylasteral was best fit to competitive inhibition, and the inhibition kinetic parameters (Ki) were calculated to be 0.6 μM and 17.3 μM for UGT1A6 and UGT2B7, respectively. This kind of inhibitory effect need much attention when demethylzeylasteral and demethylzeyasteral-containing herbs (e.g., Tripterygium wilfordii Hook F.) were co-administered with the drugs mainly undergoing UGT1A6, UGT2B7-catalyzed metabolism. However, when extrapolating the in vivo clinical results using our present in vitro data, many complex factors might affect final results, including the contribution of UGT1A6 and UGT2B7 to the metabolism of compounds, and the herbal or patients’ factors affecting the in vivo concentration of demethylzeylasteral.

Strong inhibition of celastrol towards UDP-glucuronosyl transferase (UGT) 1A6 and 2B7 indicating potential risk of UGT-based herb-drug interaction

Celastrol, a quinone methide triterpene isolated from Tripterygium wilfordii Hook F., has various biochemical and pharmacological activities, and is now being developed as a promising anti-tumor agent. Inhibitory activity of compounds towards UDP-glucuronosyltransferase (UGT) is an important cause of clinical drug-drug interactions and herb-drug interactions. The aim of the present study is to investigate the inhibition of celastrol towards two important UDP-glucuronosyltransferase (UGT) isoforms UGT1A6 and UGT2B7. Recombinant UGT isoforms and non-specific substrate 4-methylumbelliferone (4-MU) were used. The results showed that celastrol strongly inhibited the UGT1A6 and 2B7-mediated 4-MU glucuronidation reaction, with 0.9 ± 0.1% and 1.8 ± 0.2% residual 4-MU glucuronidation activity at 100 μM of celastrol, respectively. Furthermore, inhibition kinetic study (Dixon plot and Lineweaver-Burk plot) demonstrated that celastrol noncompetitively inhibited the UGT1A1-mediated 4-MU glucuronidation, and competitively inhibited UGT2B7-catalyzed 4-MU glucuronidation. The inhibition kinetic parameters (Ki) were calculated to be 0.49 μM and 0.045 μM for UGT1A6 and UGT2B7, respectively. At the therapeutic concentration of celastrol for anti-tumor utilization, the possibility of celastrol-drug interaction and celastrol-containing herbs-drug interaction were strongly indicated. However, given the complicated nature of herbs, these results should be viewed with more caution.

Interindividual variability in hepatic drug glucuronidation: studies into the role of age, sex, enzyme inducers, and genetic polymorphism using the human liver bank as a model system

The human liver bank has provided an invaluable model system for the study of interindividual variability in expression and activity of the major hepatic UGTs, including UGT1A1, 1A4, 1A6, 1A9, 2B7, and 2B15. Based on studies using UGT-isoform-selective probes, the rank order of activity variability is UGT 1A1>1A6>2B15>1A4 = 1A9>2B7, with coefficient of variation values ranging from 92 to 45%. Liver donor age, sex, enzyme inducers, and genetic polymorphism are factors that have been implicated as sources of this variability in UGT activity. The expression of UGTs prior to, and immediately following, birth is quite limited, explaining the susceptibility of neonates to certain drug toxicities. Old age appears to have minimal effect on UGT function. Sex differences in UGT activity are relatively small and are confined to several UGTs, including UGT2B15, which shows higher activity in males, compared with females. Enzyme inducers, including coadministered drugs, smoking, and alcohol, may increase hepatic UGT levels. Human liver bank phenotype-genotype studies, using UGT-isoform-selective probes have identified common genetic polymorphisms that are predictive of glucuronidation activity in vitro and that were subsequently verified as predictors of probe-drug clearance by glucuronidation in vivo.

Evaluation of effects of polymorphism for metabolic enzymes on pharmacokinetics of carvedilol by population pharmacokinetic analysis

In our previous study it was observed that the frequencies of UGT1A1*6, UGT2B7*3 and CYP2D6*10 in patients who have a low level ability of glucuronidation were significantly higher than those in patients with a high level of ability of glucuronidation. The same tendency was found in the frequency of CYP2D6*5, though there was no significant difference. The purpose of this study was to evaluate the effects of the polymorphism on pharmacokinetics of carvedilol by population pharmacokinetic analysis. Population pharmacokinetic analysis was performed using 373 plasma concentrations from 41 patients with chronic heart failure or angina pectoris. A one compartment pharmacokinetic model with first-order absorption (for oral dosing) was used to describe the concentration-versus-time data for carvedilol. We examined the effects of various clinical and genetic covariables in the regression models for clearance and volume of distribution. The results suggested that the factors of interindividual variation for carvedilol clearance were creatinine clearance and polymorphisms of UGT2B7 and CYP2D6 in the Japanese population with heart disease. It was estimated that UGT2B7*3 decreased the clearance of carvedilol by 37%, but UGT2B7*2 did not show any effect. Clearance in the patients who have intermediate activity of CYP2D6 was decreased by 39%.

Inducibility of UDP-glucuronosyltransferase 1As by β-naphthoflavone in HepG2 cells

UDP-glucuronosyltransferases (UGTs) are conjugation enzymes, which are regulated in a tissue-specific manner by endogenous and environmental factors. In this study, we focused on UGT1A isoforms (UGT1A1, UGT1A6 and UGT1A9), mainly expressed in the human liver, and examined the inducibility of UGT1As by beta-naphthoflavone (BNF) in human hepatoma HepG2 cells. The cells were pretreated for 72 h with BNF at concentrations of 25, 50 and 100 microM. 7-Ethyl-10-hydroxycamptothecin (SN-38) glucuronidation, used as a probe for UGT1A1, showed sigmoidal kinetics with a Hill coefficient (n) of 1.2-1.3 in control and BNF-pretreated HepG2 cells. The Vmax values were significantly increased 3.6- to 4.3-fold by BNF, whereas there was no significant change in the S50 values by BNF at any concentration examined. On the other hand, 4-methylumbelliferone (4-MU) glucuronidation as a probe for UGT1A6 and UGT1A9 in the control and BNF-pretreated HepG2 cells exhibited a biphasic kinetic pattern. Although Km1 values for the low-Km phase were similar between the control and BNF-pretreated HepG2 cells, Km2 values for the high-Km phase of BNF-pretreated HepG2 cells were reduced to 54-69% of control HepG2 cells. The values of Vmax1 and Vmax2 for the low- and high-Km phases, respectively, were significantly increased 1.9- to 2.6-fold by BNF at 25 and/or 50 microM but not 100 microM. With respect to Vmax (Vmax1 and Vmax2) and Vmax/Km (Vmax1/Km1 and Vmax2/Km2), the values were significantly increased 2.0- to 3.2-fold by BNF at all concentrations examined. Furthermore, real-time reverse transcription polymerase chain reaction using TaqMan probes demonstrated that BNF concentration-dependently induced mRNA levels of UGT1A1 but not UGT1A6 or UGT1A9 in HepG2 cells (1.3- to 6.0-fold). These results suggest that the inducibility of UGT1A isoforms in HepG2 cells by BNF is different from other aryl hydrocarbon receptor agonists previously reported, and should provide useful information for the prediction of drug-drug interactions and toxicological assessment of environmental chemicals.

Identification of potential pharmacological and toxicological targets differentiating structural analogs by a combination of transcriptional profiling and promoter analysis in LS-180 and Caco-2 adenocarcinoma cell lines

Detecting and understanding the potential for off-target pharmacological effects is critical in the optimization of lead compounds in drug discovery programs. Compound-mediated activation of the pregnane X receptor (PXR; NR1I2), a key regulator for drug metabolism genes, is often monitored to avoid potential drug-drug interactions. Two structural analogs, MRL-1 and MRL-2, were determined to be equivalent PXR activators in trans-activation assays. To differentiate these two PXR activators, their transcriptional effects were examined in PXR-sufficient (LS180) and PXR-deficient (Caco-2) adenocarcinoma cell lines. Both compounds regulated drug-management genes (e.g. CYP3A4, CYP2B6, UGT1A1 and ABCB1) in LS180 cells, but not in PXR-deficient Caco-2 cells. The potency of MRL-1 and MRL-2 on PXR activation was again equivalent as revealed by a set of 113 genes that were regulated by four prototypical PXR agonists (rifampicin, ritonavir, troglitazone and dexamethasone) in the LS180 cells. The specificity of the PXR signature genes was supported by the enrichment of putative PXR binding sites uncovered by sequence-based promoter analyses. Interestingly, an additional off-target activity of MRL-2 was suggested where sterol response element binding protein binding sites were found enriched in a subset of PXR signature genes. These genes, involved in cholesterol and fatty acid synthesis, were significantly regulated by ritonavir, chlorpromazine and MRL-2, which were linked to the manifestation of phospholipidosis. The present study demonstrates the utility of our approach in the differentiation and selection of lead compounds for drug development.

Modulation of UDP-glucuronosyltransferase 1A1 in primary human hepatocytes by prototypical inducers

The primary objective of this study was to evaluate the modulation of UGT1A1 expression in human hepatocytes using prototypical CYP450 inducers. A bank of 16 human livers was utilized to obtain an estimate of the range of UGT1A1 protein expression and catalytic activity. Concentration-dependent changes in UGT1A1 response were evaluated in hepatocyte cultures after treatment with 3-methylchloranthrene, beta-napthoflavone, rifampicin, or phenobarbital. Pharmacodynamic analyses of UGT1A1 expression were conducted and compared to those of CYP450 after treatment with inducers in 2-3 different hepatocyte preparations. Additionally, expression of UGT1A1 mRNA and protein was evaluated in human hepatocytes treated with 14 different compounds known to activate differentially the human pregnane-X-receptor or constitutive androstane receptor. Pharmacodynamic modeling revealed EC50 values statistically significant between UGT1A1 and CYP2B6 after treatment with PB, but not statistically distinguishable between UGT1A1 and CYP's 1A2 or 3A4 after treatment with 3-methylchloranthrene or rifampicin, respectively. UGT1A1 was most responsive to the pregnane-X-receptor-agonists rifampicin, ritonavir, and clotrimazole at the mRNA level and, to a lesser extent, the constitutive androstane receptor-activators, phenobarbital and phenytoin. Pharmacodynamic analyses support a mechanism of coordinate regulation between UGT1A1 and a number of CYP450 enzymes by multiple nuclear receptors.

Sulpho-conjugation of ethanol in humans in vivo and by individual sulphotransferase forms in vitro

We studied whether ethanol is sulphonated in humans with the perspective of using the urinary excretion of ethyl sulphate after ethanol consumption as a biomarker for SULT (sulphotransferase) activity. We developed a sensitive and selective HPLC-MS/MS method for determining ethyl sulphate in urine. Ten volunteers received a low dose of ethanol (0.1 g/kg of body mass). In general, excretion of ethyl sulphate was maximal in the first or second hour after dosage. Within 8 h, 2.5-6.8 micromol of ethyl sulphate was excreted. A 5-fold increase in the dose of ethanol led to an increase in the amount of ethyl sulphate excreted within 8 h (28-95 micromol) and the presence of this metabolite in urine for at least 24 h. Since ethyl sulphate was still being excreted for a substantial period after the elimination of ethanol, it might be used as a medium-time biomarker for preceding ethanol consumption. We have expressed previously all human SULT forms identified in Salmonella typhimurium. Ethanol sulphonation was studied in cytosolic preparations of these strains. The highest activities were observed with SULT1A2, 1B1 and 1C2, followed by 1A3. Activities were markedly lower with SULT1E1, 1A1 and 2A1, and were negligible with SULT1C1, 2B1a, 2B1b and 4A1. If the expression levels in tissues are additionally taken into account, SULT1A3 might be the predominant form for the sulphonation of ethanol in vivo, although a robust estimate requires further studies. With this limitation, urinary ethyl sulphate excretion appears very promising as a biomarker for SULT activity in vivo.

UDP glucuronosyltransferase (UGT) 1A6 pharmacogenetics: I. Identification of polymorphisms in the 5'-regulatory and exon 1 regions, and association with human liver UGT1A6 gene expression and glucuronidation

UDP glucuronosyltransferase (UGT) 1A6 is a major isoform in human liver that glucuronidates numerous drugs, toxins, and endogenous substrates with high interindividual variability. The molecular basis for this variability remains unknown, although it likely involves genetic and environmental factors. Phenotype-genotype studies were conducted using a well characterized human liver bank (n = 54) and serotonin glucuronidation as a UGT1A6-specific phenotype marker. A positive moderate-to-heavy alcohol use history (>14 drinks per week) was the only demographic factor examined that correlated with phenotype and was associated with 2-fold higher serotonin glucuronidation (p < 0.001), UGT1A6 protein content (p = 0.004), and UGT1A6 mRNA content (p = 0.025). UGT1A6 gene resequencing identified three nonsynonymous polymorphisms (S7A, T181A, and R184S) in exon 1 and eight novel polymorphisms in the 5'-regulatory region (to -2052 base pairs). S7A was in complete linkage disequilibrium with three 5'-regulatory region polymorphisms (-1710c-->g, -1310del5, and -652g-->a). Initial univariate analyses did not identify any significant phenotype-genotype associations. However, in livers without substantial alcohol exposure, 50% lower UGT1A6 mRNA levels (p = 0.026) were found in carriers of the linked S7A-enhancer polymorphisms compared with noncarriers but without significant effect on UGT1A6 protein content or glucuronidation activities. Three major haplotypes, including (*)1A (reference), (*)1B (-1535g-->a and -427g-->c), and (*)2 (-1710c-->g, -1310del5, -652g-->a, S7A, T181A, and R184S), were identified, accounting for 90% of alleles. No association of haplotype with any of the phenotype measures could be discerned. In conclusion, although the identified UGT1A6 polymorphisms did not explain the observed glucuronidation variability, there does seem to be a significant role for environmental factors associated with alcohol consumption.

UDP-glucuronosyltransferase 1A6: structural, functional, and regulatory aspects

Glucuronidation, catalyzed by two families of UDP-glucuronosyltransferases (UGTs), represents a major phase II reaction of endo- and xenobiotic biotransformation. UGT1A6 is the founding member of the rat and human UGT1 family. It is expressed in liver and extrahepatic tissues, such as intestine, kidney, testis, and brain, and conjugates planar phenols and arylamines. Serotonin has been identified as a selective endogenous substrate of the human enzyme. UGT1A6 is also involved in conjugation of the drug paracetamol (acetaminophen) and of phenolic metabolites of benzo[a]pyrene (together with rat UGT1A7 and human UGT1A9). High interindividual variability of human liver protein levels is due to a number of influences, including genetic, tissue-specific, and environmental factors. Evidence shows that homo- and heterozygotic expression of UGT1A6 alleles markedly affects enzyme activity. HNF1 may be responsible for tissue-specific UGT1A6 expression. Multiple environmental factors controlling UGT1A6 expression have been identified, including the pregnane X receptor, the constitutive androstane receptor, the aryl hydrocarbon receptor, and Nrf2, a bZIP transcription factor mediating stress responses. However, marked differences have been noted in the expression of rat and human UGT1A6. Regulatory factors have been studied in detail in the human Caco-2 colon adenocarcinoma cell model.

Coordinate Regulation of Drug Metabolism by Xenobiotic Nuclear Receptors: UGTs Acting Together with CYPs and Glucuronide Transporters

Xenobiotic nuclear receptors (PXR, CAR, and the Ah receptor) coordinately induce genes involved in all phases of xenobiotic metabolism including oxidative metabolism, conjugation, and transport. The comment--dedicated to honor the memory of Herbert Remmer, mentor of the author K. W. B.--discusses mechanistic, functional, and evolutionary aspects of xenobiotic nuclear receptors which induce UGTs together with CYPs and glucuronide transporters in human and rodent liver and intestine. Recent findings on regulation of CYPs, UGTs, and transporters suggest that while nuclear receptor signaling induces different CYPs, regulation may converge on single UGTs and transporters. Functional consequences of co-regulation are discussed using examples from the metabolism of xeno- and endobiotics (drugs, bilirubin, bile salts, steroid hormones, and carcinogens). Animal-plant interactions may have been a major driving force in the evolutionary divergence of CYPs and UGTs in mammals and insects as well as in their regulation by nuclear receptors. In addition, regulation by nuclear receptors was probably shaped by the need for homeostatic control of endobiotic signals in the evolution of multicellular organisms.

Pharmacogenomics of human UDP-glucuronosyltransferase enzymes

UDP-glucuronosyltransferase (UGT) enzymes comprise a superfamily of key proteins that catalyze the glucuronidation reaction on a wide range of structurally diverse endogenous and exogenous chemicals. Glucuronidation is one of the major phase II drug-metabolizing reactions that contributes to drug biotransformation. This biochemical process is also involved in the protection against environmental toxicants, carcinogens, dietary toxins and participates in the homeostasis of numerous endogenous molecules, including bilirubin, steroid hormones and biliary acids. Over the years, significant progress was made in the field of glucuronidation, especially with regard to the identification of human UGTs, study of their tissue distribution and substrate specificities. More recently, the degree of allelic diversity has also been revealed for several human UGT genes. Some polymorphic UGTs have demonstrated a significant pharmacological impact in addition to being relevant to drug-induced adverse reactions and cancer susceptibility. This review focuses on human UGTs, the description of the nature of polymorphic variations and their functional impact. The pharmacogenomic implication of polymorphic UGTs is presented, more specifically the role of UGT polymorphisms in modifying cancer risk and their impact on individual risk to drug-induced toxicities.

AN ASSESSMENT OF UDP-GLUCURONOSYLTRANSFERASE INDUCTION USING PRIMARY HUMAN HEPATOCYTES

Uridine diphosphate glucuronosyltransferases (UGTs) catalyze the glucuronidation of a wide range of xenobiotics and endogenous substrates. However, there is a lack of information concerning the response of human UGTs to inducers, and this observation prompted the current investigation. The glucuronidation of estradiol (3- and 17-positions), naphthol, propofol, and morphine (3- and 6-positions) was assessed against a battery of recombinant human UGTs to determine selective glucuronidation reactions for induction studies. The potential induction of the glucuronidation of estradiol at the 3-position, naphthol, propofol, and morphine at the 3-position was subsequently investigated in cultured primary human hepatocytes against a range of prototypic inducers including dexamethasone, 3-methylcholanthrene (3-MC), phenobarbital, rifampicin, and omeprazole. Treatment with 3-MC induced estradiol-3-glucuronidation (up to 2.5-fold) in four of five donors investigated. Statistically significant increases in naphthol glucuronidation (up to 1.7-fold) were observed following treatment with carbamazepine. UGT1A9-mediated propofol glucuronidation was induced by phenobarbital (up to 2.2-fold) and rifampicin (up to 1.7-fold). However, treatment with alpha-naphthoflavone and tangeretin resulted in a decrease in propofol glucuronidation (30% of control values). Statistically significant induction of morphine-3-glucuronidation was observed in at least three donors following treatment with phenobarbital, rifampicin, and carbamazepine. Each UGT isoform investigated displayed a distinct induction profile. Although statistically significant increases in glucuronidation were observed for each reaction studied, the level of induction was less than that observed for CYP1A2 or CYP3A4 and exhibited a large interdonor variability. The clinical relevance of the induction responses obtained in this study is unclear.

The impact of current tobacco use on the outcome of paracetamol poisoning

BACKGROUND

Tobacco smoke contains a number of substances that are capable of inducing cytochrome P450. Consequently, current tobacco use may enhance the hepatotoxicity from a paracetamol overdose by increasing the oxidative metabolism of paracetamol.

AIM

To evaluate, by multivariate analysis, the effect of current tobacco use on the morbidity and mortality from paracetamol-induced hepatotoxicity.

METHODS

A retrospective study was carried out on the basis of the hospital charts of 602 patients admitted with single-dose paracetamol poisoning for whom information on current tobacco use was available.

RESULTS

In patients admitted with paracetamol poisoning, the rate of current daily tobacco use of 70% (424 of 602 patients) was considerably higher than the rate of 31% in the background population (chi-squared test: P < 0.0001). Current tobacco use was an independent risk factor for the development of hepatic encephalopathy (odds ratio, 2.68; 95% confidence interval, 1.28-5.62) and mortality (odds ratio, 3.64; 95% confidence interval, 1.23-10.75). Current tobacco use was independently associated with high peak values of alanine transaminase and the international normalized ratio.

CONCLUSIONS

Current tobacco use was very frequent in patients admitted with paracetamol poisoning. It was an independent risk factor of severe hepatotoxicity, acute liver failure and death following paracetamol overdose.

The effect of food or water deprivation on paracetamol pharmacokinetics in calves

This study investigated the effect of food or water deprivation on the pharmacokinetics of paracetamol in 30 Holstein-Friesian preruminant calves (10 controls, 10 food withheld and 10 water-deprived) aged 24-25 days. Control calves were given paracetamol at 24-25 days and again at 28-29 days of age. In the food withheld and water-deprived calves paracetamol studies were performed before and after 4 days of food or water deprivation. In the control group there were no significant differences in pharmacokinetic parameters for paracetamol in 24-25 and 28-29-day-old calves. Witholding food for 4 days was associated with an increase in the mean residence time (MRT) of paracetamol (P < 0.01). When food was withheld total body clearance (ClB) of paracetamol was significantly decreased (P < 0.05). The volume of distribution (Vss) was not significantly altered. Similarly, water deprivation was associated with a significant increase in MRT and significant decrease in ClB of paracetamol (P < 0.01). The Vss was not significantly altered. Food or water deprivation also influenced the formation of major metabolites (glucuronide and sulphate) of paracetamol. It is concluded that food or water deprivation may impair the elimination drugs that undergo metabolism by UDP-glucuronyltransferase and sulphotransferase in cattle.

Drug interactions with clinafloxacin

Many fluoroquinolone antibiotics are inhibitors of cytochrome P450 enzyme systems and may produce potentially important drug interactions when administered with other drugs. Studies were conducted to determine the effect of clinafloxacin on the pharmacokinetics of theophylline, caffeine, warfarin, and phenytoin, as well as the effect of phenytoin on the pharmacokinetics of clinafloxacin. Concomitant administration of 200 or 400 mg of clinafloxacin reduces mean theophylline clearance by approximately 50 and 70%, respectively, and reduces mean caffeine clearance by 84%. (R)-Warfarin concentrations in plasma during clinafloxacin administration are 32% higher and (S)-warfarin concentrations do not change during clinafloxacin treatment. An observed late pharmacodynamic effect was most likely due to gut flora changes. Phenytoin has no effect on clinafloxacin pharmacokinetics, while phenytoin clearance is 15% lower during clinafloxacin administration.

High-performance liquid chromatographic assay for acetaminophen glucuronide in human liver microsomes

A rapid and specific high-performance liquid chromatographic assay was developed for the determination of acetaminophen glucuronide formed by human liver microsomes. In addition, incubation conditions were systematically evaluated. Conditions that yielded the optimal rate of acetaminophen glucuronide formation over various concentrations of acetaminophen (0.15-30 mM) consisted of the following: 0.1 M potassium phosphate buffer, 1 mM magnesium chloride, 30 microg/mg alamethicin, 4 mM uridine 5'-diphosphoglucuronic acid at a pH of 7.1. Alamethicin produced higher and more consistent APAPG formation rates compared to Brij-58. Adding saccharolactone to the incubation medium reduced the velocity of the reaction. Acetaminophen glucuronide, acetaminophen, and the internal standard (paraxanthine), were analyzed on a C18 column with UV detection at 250 nm. The mean correlation coefficient (r2) of the standard curves for acetaminophen glucuronide was >0.99 over the range of 0.1-25 nmol. The intra- and inter-day coefficients of variation were <4%. This method is suitable for in vitro studies using acetaminophen glucuronide formation as an index reaction for UGT activity.

Glucuronidation and sulphation of paracetamol in HIV-positive patients and patients with AIDS

AIMS

To gauge the effect of disease state and disease progression on the glucuronidation and sulphation of paracetamol (APAP) among HIV-positive patients and patients with AIDS.

METHODS

The extent of APAP glucuronidation and APAP sulphation was assessed using a spot urine sample collected 4 h after the oral administration of 500 mg of APAP to 108 patients with AIDS or HIV infection. The molar concentrations of APAP and its glucuronide and sulphate metabolites were determined using a validated h.p.l.c. method and glucuronidation and sulphation indices were constructed using APAP metabolite/APAP molar concentration ratios.

RESULTS

No effect of disease state, AIDS vs asymptomatic HIV positive vs control, on APAP glucuronidation or sulphation was observed. The patient population was studied over time and disease progression also did not significantly alter the calculated glucuronidation and sulphation indices. The effect of the concomitant administration of other therapeutic agents was assessed and in the cross sectional portion of the study dapsone appeared to significantly decrease APAP sulphation as did lamivudine. In the longitudinal portion of the study the latter effect was not observed but zidovudine was seen to increase APAP glucuronidation. The data also indicates that APAP glucuronidation may be reduced in patients who are >10% below their ideal body weight.

Functions and transcriptional regulation of PAH-inducible human UDP-glucuronosyltransferases

Functions and regulation of selected human UDP-glucuronosyltransferases (UGT1A1, UGT1A4, UGT1A6, UGT1A9, UGT2B7, UGT2B15) are summarized. Evidence for at least two PAH-inducible UGTs (UGT1A6 and UGT1A9) is presented, which, however, are also constitutively expressed in a tissue- and cell-specific manner. These isoforms have recently been characterized to conjugate planar and bulky phenols, respectively. Using a selective RT-PCR method, UGT1A6 expression was detected in a variety of tissues (liver, kidney, lung, intestine, and pharyngeal mucosa). PAH-inducible UGTs may cooperate in the metabolism of phenolic metabolites of benzo(a)pyrene. Studies with stably expressed isoforms suggest that UGT1A9 is responsible for the formation of benzo(a)pyrene-3.6-diphenol diglucuronide, the major biliary metabolite of benzo(a)pyrene.

Direct assay of nonopioid analgesics and their metabolites in human urine by capillary electrophoresis and capillary electrophoresis-mass spectrometry

Capillary electrophoresis (CE) was used for the analysis of nonopioid analgesics and their metabolites directly in urine samples. A simple, reliable screening method was developed that allows identification of the drug and/or its metabolites in urine after oral application of paracetamol, acetylsalicylic acid, antipyrine, ibuprofen, naproxen, ketoprofen and propyphenazone by their migration in CE and by their UV spectra recorded with a diode-array detector in a common CE-UV system with 50 mM borax pH 9.4 as separation buffer. For the CE-electrospray (ESI)-MS coupling a volatile 50 mM ammonium acetate buffer at pH 9.8 was used. In order to analyze the metabolic pattern in more detail different methods were developed for each drug. The separation of the metabolites of acetylsalicylic acid could be improved by injection of the urine sample at the cathodic side of the capillary. In order to identify antipyrine as neutral compound as well as its neutral metabolites-a micellar electrokinetic chromatography (MEKC) method was developed.

Direct determination of paracetamol and its metabolites in urine and serum by capillary electrophoresis with ultraviolet and mass spectrometric detection

The use of capillary electrophoresis (CE) for the determination of paracetamol and its main metabolites in urine and serum is described. Due to its high efficacy, CE enables the analysis of drugs directly in complex matrices. Thus, simple, rapid and reliable assays could be developed that made use of some of the main advantages of this analytical technique. In order to prevent the peaks from tailing, a water zone was injected behind the sample. Occasionally occurring peak splittings of paracetamol were investigated and methods to suppress these splittings were developed. Paracetamol, its main metabolites, paracetamol glucuronide, paracetamol sulfate as well as paracetamol cysteinate and paracetamol mercapturate, as metabolites of the oxidative pathway were identified in urine using diode-array detection and coupling of the CE instruments to electrospray-mass spectrometry. The assays were validated. Their usefulness was demonstrated by applying them to the analysis of urine and serum samples of healthy volunteers as well as to urine samples from children under anticancer therapy.

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.

Effect of cigarette smoke on UDP-glucuronosyltransferase activity and cytochrome P450 content in liver, lung and kidney microsomes in mice

The effect of cigarette smoke on the expression of several cytochromes P450 (CYP) and UDP-glucuronosyl-transferases (UGT) was studied in mice. The animals were exposed to cigarette smoke for 4 to 30 days. Enzymatic activities supported by CYP1A1, 1A2, 2B, 2E1 and the glucuronidation activity toward phenols were measured in lung, liver and kidney microsomes. Cigarette smoke induced several CYPs, especially in lung. CYP2E1 was more induced than CYP1A1 in this organ. The expression of CYP2E1 was also increased in kidney (5.6 times after 30 days). The glucuronidation in kidney was non-sensitive to the treatment whatever substrate used. In contrast, this activity was enhanced in liver and particularly in lung, in which the glucuronidation of 1-naphthol and 2-hydroxybiphenyl was increased by 122 and 180%, respectively. Interestingly, the times of induction differed according to the substrate used, thus suggesting the presence of different UGTs active toward phenols that were differentially affected by cigarette smoke. The UGT activities toward phenols were low in lung, when compared with those measured in liver or kidney. In conclusion, cigarette smoke greatly affected both glucuronidation activity and the hydroxylation reactions supported by CYPs in mouse liver and lung.

Pattern of self-administered paracetamol and codeine analgesic consumption after mandibular third-molar surgery

Pattern of analgesic consumption after unilateral mandibular third-molar surgery was investigated in an open study in 201 patients. All patients were supplied with six analgesic tablets containing 500 mg paracetamol and 30 mg codeine. Instructions for use were given. A mean consumption of 4.9 tablets over the 1st week and 3.6 tablets the day of operation was found. Eight (4%) patients indicated inadequate or no effect of the medication. The remaining patients were able to control pain, to a level of one-third of maximum pain, by using from one to five tablets. One hundred and thirty-two (68%) patients followed instructions with regard to start of medication. No difference in mean tablet consumption was found between compliant patients and those who delayed the intake of the first analgesic dose by more than 1 h. Predictor analysis showed the most powerful predictors to be preoperative depth of the third molar and moderate or heavy smoking. Thirteen per cent explanatory power of all predictors together was found.

Paracetamol metabolism in two ethnically different Spanish populations

The 24 h urinary excretion of paracetamol and its metabolites following a single oral dose of 1.5 g was compared in two ethnically different Spanish populations: 39 volunteers from the Basque country and 32 from Alicante. The urinary concentrations of unchanged paracetamol and its glucuronide, sulphate, cysteine, and mercapturic acid conjugates were determined by high-performance liquid chromatography. Statistically significant differences in the urinary excretion of unchanged paracetamol and the fractional urinary recovery of each conjugate between subjects from Alicante and subjects from the Basque country were not found. In both populations, an inverse relationship between glucuronide and sulphate conjugation following a bimodal frequency distribution pattern was found. In contrast to paracetamol oxidation, intersubject variation in paracetamol conjugation was negligible. The urinary excretion of unchanged paracetamol was higher in smokers than in nonsmokers. As compared with other studies, the urinary excretion of oxidation-derived paracetamol metabolites in both Spanish populations was intermediate and significantly different than that found in Caucasians from Scotland and West Africans (Ghana). This may determine a susceptibility to paracetamol hepatotoxicity following overdosage in the Spanish population.

Metabolism of quinine in man: identification of a major metabolite, and effects of smoking and rifampicin pretreatment

Our previous studies have shown that cigarette smoking and rifampicin pretreatment enhance the elimination of quinine, metabolism assumed, by analogy with quinidine, to be carried out by CYP3A (P450IIIA). This finding is unexpected since it has been shown that smoking induces the CYP1A rather than the CYP3A enzyme family, suggesting that the metabolism of quinine may be catalysed by CYP1A. Therefore, we conducted this study to identify possible quinine metabolites in human urine and to determine which metabolic pathway is induced by cigarette smoking and rifampicin pretreatment. A specific HPLC procedure was employed to identify metabolites of quinine in urine samples collected from healthy volunteers after an oral dose of 600 mg quinine sulphate. The results showed that there were at least seven possible metabolites of quinine detected in human urine. Three of these were identified as 2'-oxoquininone, quinine glucuronide and 3-hydroxyquinine. The 3-hydroxyquinine appeared to be a major metabolite of quinine in urine samples from every subject who took an oral dose of quinine. Although cigarette smoking and rifampicin pretreatment were shown to cause a marked increase in the elimination of quinine there were no significant changes in the formation of 3-hydroxyquinine as measured in the urine samples. This suggests that the effects of smoking and rifampicin are more complicated than we expected and require further investigation.

Induction and autoinduction properties of rifamycin derivatives: a review of animal and human studies

Animal studies have demonstrated that the mouse and rabbit are far more responsive to the inductive properties of rifamycin derivatives than the rat and guinea pig. The rat hepatic cytochrome P450 system seems to be resistant to the action of rifampicin unless very high doses are used. Mouse hepatic microsomal mixed-function oxidase activity is markedly increased by repeated dosing with rifampicin, whereas administration of rifabutin may be ineffective. In humans, both rifampicin and rifabutin are extensively metabolized and induce their own metabolism. The induced metabolic pathways remain essentially unknown. Under autoinduction conditions, the elimination half-life of rifampicin decreases, whereas that of rifabutin is not altered. Although the effects of repeated administration of rifampicin and rifabutin on the various forms of cytochrome P450 in humans have not been extensively examined, there is convincing evidence that the P4503A subfamily is induced by either drug, whereas the P4501A subfamily and P4502D6 do not appear to be affected by rifampicin. Limited reliable information is available concerning the induction of human glucuronyltransferase activities by rifampicin and rifabutin which, however, do not seem to influence zidovudine glucuronide formation in healthy subjects.

Influence of rifampicin on the toxicity and the analgesic effect of acetaminophen

The influence of rifampicin on the toxicity, analgesic effect and pharmacokinetics of acetaminophen was studied in male albino mice. Repeated administration of rifampicin (50 mg/kg i.p. daily for 6 days) shortened hexobarbital sleeping time and increased liver weight, microsomal cytochrome P-450 and heme contents, NADPH-cytochrome c reductase and ethylmorphine-N-demethylase activities. Aniline hydroxylase activity was decreased and glucuronidation of p-nitrophenol was unaffected. Rifampicin pretreatment changed neither the LD50 of acetaminophen nor the hepatic glutathione level nor the glutathione depletion provoked by the toxic dose of acetaminophen (737 mg/kg p.o.). This suggests that rifampicin has no influence on the amount of acetaminophen toxic metabolites formed in the liver. Rifampicin decreased the acetaminophen analgesic effect in mice. Rifampicin decreased the Cmax, the half-time, the MRT and the AUC of acetaminophen and accelerated its clearance. The plasma concentration of acetaminophen glucuronide and acetaminophen sulfate was increased. It is assumed that the most probable mechanism by which rifampicin decreases acetaminophen analgesia is the accelerated acetaminophen elimination.

Assessment of liver metabolic function. Clinical implications

Inter- and intraindividual variability in pharmacokinetics of most drugs is largely determined by variable liver function as described by parameters of hepatic blood flow and metabolic capacity. These parameters may be altered as a result of disease affecting the liver, genetic differences in metabolising enzymes, and various types of drug interactions, including enzyme induction, enzyme inhibition or down-regulation. With the now known large number of drug metabolising enzymes, their differential substrate specificity, and their differential induction or inhibition, each test substance of liver function should be used as a probe for its specific metabolising enzyme. Thus, the concept of model test-substances providing general information about liver function has severe limitations. To test the metabolic activity of several enzymes, either several test substances may be given (cocktail approach) or several metabolites of a single test substance may be analysed (metabolic fingerprint approach). The enzyme-specific analysis of liver function results in a preference for analysis of the metabolites rather than analysis of the clearance of the parent test substance. There are specific methods to quantify the activity of cytochrome P450 enzymes such as CYP1A2, CYP2C9, CYP2C19MEPH, CYP2D6, CYP2E1, and CYP3A, and phase II enzymes, such as glutathione S-transferases, glucuronyl-transferases or N-acetyltransferases, in vivo. Interactions based on competitive or noncompetitive inhibition should be analysed specifically for the cytochrome P450 enzyme involved. At least 5 different types of cytochrome P450 enzyme induction may result in major variability of hepatic function; this may be quantified by biochemical parameters, clearance methods, or highly enzyme-specific methods such as Western blot analysis or molecular biological techniques such as mRNA quantification in blood and tissues. Therapeutic drug monitoring is already implicitly used for quantification of the enzyme activities relevant for a specific drug. Selective impairment of hepatic enzymes due to gene mutations may have an effect on the pharmacokinetics of certain drugs similar to that caused by cirrhosis. Assessment of this heritable source of variability in liver function is possible by in vivo or ex vivo enzymological methods. For genetically polymorphic enzymes and carrier proteins involved in drug disposition, molecular genetic methods using a patient's blood sample may be used for classification of the individual into: (i) the impaired or poor metaboliser (homozygous deficient); (ii) the extensive (homozygous active) metaboliser group; and (iii) the moderately extensive metaboliser (heterozygous) group. For hepatic blood flow determinations, galactose or sorbitol given at relatively low doses may be much better indicators than the indocyanine green.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Factors and conditions affecting the glucuronidation of oxazepam

The aim of the present work was to investigate the impact of disease states and environmental and host factors on the glucuronidation of oxazepam. Glucuronidation represents quantitatively one of the most important metabolic conjugation pathways (phase II) in man for the inactivation and detoxication of xenobiotics and endogenous compounds and the liver is the major site for it to take place. Far less attention has been paid to the conjugation reactions in previous clinical research in this field compared to the immense interest in the oxidative biotransformation pathways (phase I). This fact is mainly due to the latter giving rise to active or reactive metabolites with a toxicological potential. The metabolism of oxazepam expresses exclusively the capacity for glucuronide formation. It was a prerequisite to establish the bioavailability of oxazepam prior to succeeding studies on the oral disposition of the drug. A preparation for intravenous administration was created. Clearance was chosen as measurement of the capacity to glucuronidate oxazepam. Severe decompensated liver disease was associated with a significant decrease in oxazepam clearance, that became even more obvious when corrected for by a diminished binding to plasma proteins. This increase in free fraction of oxazepam was substantial and could mainly be accounted for by low plasma albumin values. The results are in part a settlement with earlier studies on glucuronidation in liver disease and they may undoubtedly be ascribed to the severe degree of liver disease. For the first time it was shown that hypothyroidism led to a decline in the clearance and metabolism of oxazepam and paracetamol that is mainly biotransformed by glucuronidation. It was concluded that the enzymes responsible for glucuronidation in hypothyroidism are under the influence of thyroid hormones as is the case with oxidative enzymes. Further studies focused on the effect of host and environmental factors on glucuronidation. A commercially available very low calorie product for the treatment of obesity resulted in a decrease in oxazepam clearance and a lack of co-factors as a consequence of the low calorie intake was explanatorily proposed. Beta-adrenoceptor antagonists are often prescribed together with other drugs and close knowledge on interactions is mandatory but insufficient in regard of drugs being glucuronidated. Despite the mutual metabolic pathway labetalol exerted no dispositional alterations concerning oxazepam. It was moreover suggested that very elderly subjects between the age of 80 to 94 years had a reduced clearance of oxazepam.(ABSTRACT TRUNCATED AT 400 WORDS)

Paracetamol glucuronidation by recombinant rat and human phenol UDP-glucuronosyltransferases

Stably expressed human and rat phenol UDP-glucuronosyltransferases (UGTs) of the UGT1 complex (HlugP1, HlugP4 and 3-methylcholanthrene-inducible rat UGT1A1, the latter considered to be an orthologous enzyme to HlugP1) have been used to investigate the role of UGTs in paracetamol glucuronidation. Kinetic analysis of recombinant UGTs was compared to that of total UGT activities in liver microsomes. Paracetamol was found to be an overlapping substrate of several UGTs. It shows higher affinity for HlugP1 and rat UGT1A1 (apparent Km values of 2 and 3 mM, respectively) than for HlugP4 (Km = 50 mM) and other UGTs present in liver microsomes (Km values of > 12 mM). Glucuronidation of paracetamol with HlugP1 contrasts with that of 6-hydroxychrysene and of 4-methylumbelliferone, which are conjugated with higher affinity by HlugP4 than by HlugP1. Due to the wide tissue distribution of rat UGT1A1, paracetamol glucuronidation was also investigated in extrahepatic rat and human tissues. Paracetamol UGT activity was present and inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin in rat kidney, lung and spleen. It was also detected in human kidney. A selective cDNA probe for exon 1 of HlugP1 cross-reacted with mRNA from both human liver and kidney. The results demonstrate that paracetamol is conjugated by HlugP1 and its rat orthologue UGT1A1 with higher affinity than by HlugP4 and other UGTs.

Aryl hydrocarbon or dioxin receptor: biologic and toxic responses

1. The AhR represents a ligand-activated transcription factor. Receptor agonists include planar aromatic compounds, a variety of heterocyclic plant constituents, and PCDD/PCDF. The latter lead to persistent activation of the receptor due to their strong binding affinity and long biologic half-life of over 10 years in human blood and fat. Practically every person on earth is exposed to these compounds via the diet (> 90%) and by high concentrations in mother's milk. PCDD/PCDF produced toxic responses in exposed people (primarily chloracne and immunosuppression) in the past. However, the present PCDD/PCDF levels (basal levels) in the general population are below those warranting toxicologic concern. 2. The AhR has been characterized as a helix-loop-helix transcription factor related to the Drosophila developmental genes sim and per. The cytosolic form of the receptor is present as an inactive complex with two subunits of HSP90. After ligand binding HSP90 is released and the receptor enters the nucleus as a heterodimer together with a related protein ARNT. It binds with high affinity to certain enhancer elements in the upstream region of several genes such as cytochrome P4501A1 (CYP1A1). The AhR transcriptionally activates several drug-metabolizing enzymes and proteins involved in growth/differentiation, such as the plasminogen activator inhibitor PAI-2 and IL-1 beta. In addition, it modulates the action of a number of other nuclear transcription factors such as receptors of the steroid hormone receptor superfamily and of cell surface receptors such as EGF. With the exception of CYP1A1 induction, little is known about the mechanism of transcriptional activation of the AhR-controlled genes. Many AhR-modulated biologic responses (such as modulation of the estrogen and EGF receptor) appear to be indirect. 3. Persistent activation of the AhR is probably responsible for toxic responses in experimental animals and humans. They are markedly tissue and species specific. In rodents a wasting syndrome, immunosuppression, teratogenicity, chloracne, and carcinogenicity/tumor promotion have been well studied. There is good evidence for an involvement for the AhR in these responses. However, the chain of events from receptor activation to the diverse toxic endpoints is largely unknown. Alteration of growth and differentiation of epithelial tissues may underlie most of the toxic responses. A lot has already been achieved, mostly by characterizing the AhR and transcriptional activation of CYP1A1. Still more work lies ahead of us, for example, elucidation of the physiologic roles of the AhR and of the chains of events from receptor activation to the various biologic and toxic endpoints.

Perturbation of paracetamol urinary metabolic ratios by urine flow rate

The effects of high and low urine flow rates on the urinary metabolic ratios for paracetamol glucuronidation, sulphation and oxidation were determined at steady-state in seven healthy young adult volunteers. Metabolic partial clearances were unaffected by urine flow rate, but individual paracetamol metabolic ratios varied 2.5- to 3.2-fold over a 7.4-fold range of urine flow rates (0.81-6.00 ml min-1). The change in metabolic ratios was due entirely to a 2.5-fold change in renal clearance of unchanged paracetamol. These data emphasise the limitations of the metabolic ratio as a measure of intrinsic clearance for compounds which undergo some degree of tubular reabsorption.

Urinary caffeine metabolites in man. Age-dependent changes and pattern in various clinical situations

In an exploratory study the 24-h urinary excretion pattern of caffeine and 14 of its major metabolites was studied in 32 volunteers (adults, adolescents and children), 14 patients either with end stage renal disease or liver cirrhosis, 7 heavy smokers and 27 patients on therapy with cimetidine, allopurinol, theophylline or phenytoin. Caffeine and its metabolites were quantified by UV-absorption after liquid/liquid-extraction and HPLC-separation, which ensured proper analysis of 1-methyluric acid. In adults the renal excretion of caffeine derivatives corresponded to an intake of 509 mg caffeine/day, with 1-methyluric acid as the predominant metabolite. About 69% of caffeine was degraded by the paraxanthine pathway, and theobromine- (19%) and the theophylline pathway (14%) were less important. The ratio of paraxanthine formation to urinary caffeine concentration (= clearance equivalent) was about 2.2 ml.min-1.kg-1 in adults, and the corresponding ratios for theophylline and theobromine were 0.43 ml.min-1.kg-1 and 0.59 ml.min-1.kg-1, respectively. As expected, caffeine degradation was impaired in patients with cirrhosis and was increased in persons who smoked heavily or who were on phenytoin therapy. The results document the possibility of noninvasively investigating gross differences in caffeine disposition by analysis of the urinary pattern of its metabolites.

Decreased glucuronidation and increased bioactivation of acetaminophen in Gilbert's syndrome

Gilbert's syndrome occurs in 5%-7% of the human population and is caused by an inherited deficiency in the glucuronidation of endogenous bilirubin, resulting in its accumulation and jaundice. The authors of the present study have previously shown that rats with a similar deficiency in bilirubin glucuronidation (Gunn rats) had reduced glucuronidation and enhanced susceptibility to the toxicity of the widely used analgesic, acetaminophen. Acetaminophen is eliminated primarily by glucuronidation, which prevents its cytochrome P-450-catalysed bioactivation to a hepatotoxic reactive intermediate. The purpose of this study was to determine whether people with Gilbert's syndrome had reduced glucuronidation and enhanced bioactivation of acetaminophen. Therefore, the biotransformation of acetaminophen, 20 mg/kg IV, was investigated in six subjects with Gilbert's syndrome (total bilirubin, 41 +/- 6 mumol/L; mean +/- SE) and six normal controls (total bilirubin, 11 +/- 2 mumol/L; P less than 0.01). Formation of the acetaminophen glucuronide conjugate measured by high-performance liquid chromatography was quantified by the ratio of the area under the plasma concentration-time curve (AUC) from 0 to 2 hours for the acetaminophen glucuronide divided by the AUC for acetaminophen. Acetaminophen bioactivation was quantified by the molar percentage of acetaminophen excreted in the urine during 24 hours as glutathione-derived conjugates (cysteine and mercapturic acid). Acetaminophen glucuronide formation in subjects with Gilbert's syndrome was 31% lower than that in normal controls (0.27 +/- 0.05 vs. 0.39 +/- 0.03; P less than 0.05), and bioactivation was 1.7-fold higher (3.5% +/- 0.4% vs. 2.1% +/- 0.3%; P less than 0.05). One control subject with normal bilirubin glucuronidation had substantially decreased acetaminophen glucuronide formation (0.20) and enhanced bioactivation (4.8%). Among all subjects, glucuronidation correlated inversely with bioactivation (r = -0.84; P less than 0.001), indicating that a decrease in a major pathway of elimination can shunt more drug through the toxifying route. Thus, a deficiency in bilirubin UDP-glucuronosyltransferase, evidenced by jaundice, can be paralleled by a deficiency in glucuronidation of other compounds. In these cases, jaundice can be a phenotypic determinant of enhanced acetaminophen bioactivation. On the other hand, some people with normal bilirubin glucuronidation may have a deficiency in the glucuronidation of acetaminophen; these people are not easily recognized.(ABSTRACT TRUNCATED AT 400 WORDS)

Drug glucuronidation in humans

Glucuronidation is a major metabolic pathway for a large number of drugs in humans. Conjugation of drugs and other chemicals with glucuronic acid is catalyzed by the multigene UDP-glucuronosyltransferase family. It is believed that a number (unspecified at present) of glucuronosyltransferase isozymes, which probably differ in terms of substrate specificity and regulation, contribute to drug glucuronidation. Factors known to influence the pharmacokinetics of glucuronidated drugs in man, presumably via an effect on specific glucuronosyltransferases, include age (especially the neonatal period), cigarette smoking, diet, certain disease states, coadministered drugs, ethnicity, genetics and hormonal effects.