Interindividual variability in P450-dependent generation of neoantigens in halothane hepatitis

Pharmacogenomics, concepts for the future of perioperative medicine and pain management: A review

Pharmacogenomics is the study of how genetic differences between individuals affect pharmacokinetics and pharmacodynamics. These differences are apparent to clinicians when taking into account the wide range of responses to medications given in clinical practice. A review of literature involving pharmacogenomics and pain management was performed. The implementation of preoperative pharmacogenomics will allow us to better care for our patients by delivering personalized, safer medicine. This review describes the current state of pharmacogenomics as it relates to many aspects of clinical practice and how clinicians can use these tools to improve patient outcomes.

Examining pathogenic concepts of autoimmune hepatitis for cues to future investigations and interventions

BACKGROUND

Multiple pathogenic mechanisms have been implicated in autoimmune hepatitis, but they have not fully explained susceptibility, triggering events, and maintenance or escalation of the disease. Furthermore, they have not identified a critical defect that can be targeted. The goals of this review are to examine the diverse pathogenic mechanisms that have been considered in autoimmune hepatitis, indicate investigational opportunities to validate their contribution, and suggest interventions that might evolve to modify their impact. English abstracts were identified in PubMed by multiple search terms. Full length articles were selected for review, and secondary and tertiary bibliographies were developed. Genetic and epigenetic factors can affect susceptibility by influencing the expression of immune regulatory genes. Thymic dysfunction, possibly related to deficient production of programmed cell death protein-1, can allow autoreactive T cells to escape deletion, and alterations in the intestinal microbiome may help overcome immune tolerance and affect gender bias. Environmental factors may trigger the disease or induce epigenetic changes in gene function. Molecular mimicry, epitope spread, bystander activation, neo-antigen production, lymphocytic polyspecificity, and disturbances in immune inhibitory mechanisms may maintain or escalate the disease. Interventions that modify epigenetic effects on gene expression, alter intestinal dysbiosis, eliminate deleterious environmental factors, and target critical pathogenic mechanisms are therapeutic possibilities that might reduce risk, individualize management, and improve outcome. In conclusion, diverse pathogenic mechanisms have been implicated in autoimmune hepatitis, and they may identify a critical factor or sequence that can be validated and used to direct future management and preventive strategies.

In Vitro Drug-Drug Interaction Potential of Sulfoxide and/or Sulfone Metabolites of Albendazole, Triclabendazole , Aldicarb, Methiocarb, Montelukast and Ziprasidone

Background:

The use of polypharmacy in the present day clinical therapy has made the identification of clinical drug-drug interaction risk an important aspect of drug development process. Although many drugs can be metabolized to sulfoxide and/or sulfone metabolites, seldom is known on the CYP in-hibition potential and/or the metabolic fate for such metabolites.

Objective:

The key objectives were: a) to evaluate the in vitro CYP inhibition potential of selected parent drugs with sulfoxide/sulfone metabolites; b) to assess the in vitro metabolic fate of the same panel of par-ent drugs and metabolites.

Methods:

In vitro drug-drug interaction potential of test compounds was investigated in two stages; 1) assessment of CYP450 inhibition potential of test compounds using human liver microsomes (HLM); and 2) assessment of test compounds as substrate of Phase I enzymes; including CYP450, FMO, AO and MAO using HLM, recombinant human CYP enzymes (rhCYP), Human Liver Cytosol (HLC) and Human Liver Mitochondrial (HLMit). All samples were analysed by LC-MS-MS method.

Results:

CYP1A2 was inhibited by methiocarb, triclabendazole, triclabendazole sulfoxide, and ziprasi-done sulfone with IC50 of 0.71 µM, 1.07 µM, 4.19 µM, and 17.14 µM, respectively. CYP2C8 was in-hibited by montelukast, montelukast sulfoxide, montelukast sulfone, tribendazole, triclabendazole sulfox-ide, and triclabendazole sulfone with IC50 of 0.08 µM, 0.05 µM, 0.02 µM, 3.31 µM, 8.95 µM, and 1.05 µM, respectively. CYP2C9 was inhibited by triclabendazole, triclabendazole sulfoxide, triclabendazole sulfone, montelukast, montelukast sulfoxide and montelukast sulfone with IC50 of 1.17 µM, 1.95 µM, 0.69 µM, 1.34 µM, 3.61 µM and 2.15 µM, respectively. CYP2C19 was inhibited by triclabendazole and triclabendazole sulfoxide with IC50 of 0.25 and 0.22, respectively. CYP3A4 was inhibited by monte-lukast sulfoxide and triclabendazole with IC50 of 9.33 and 15.11, respectively. Amongst the studied sul-foxide/sulfone substrates, the propensity of involvement of CY2C9 and CYP3A4 enzyme was high (ap-proximately 56% of total) in the metabolic fate experiments.

Conclusion:

Based on the findings, a proper risk assessment strategy needs to be factored (i.e., perpetra-tor and/or victim drug) to overcome any imminent risk of potential clinical drug-drug interaction when sulfoxide/sulfone metabolite(s) generating drugs are coadministered in therapy.

Human cytochrome P450 2E1 mutations that alter mitochondrial targeting efficiency and susceptibility to ethanol-induced toxicity in cellular models

Human polymorphisms in the 5'-upstream regulatory regions and also protein coding regions of cytochrome P450 2E1 (CYP2E1) are known to be associated with several diseases, including cancer and alcohol liver toxicity. In this study, we report novel mutations in the N-terminal protein targeting regions of CYP2E1 that markedly affect subcellular localization of the protein. Variant W23R/W30R protein (termed W23/30R) is preferentially targeted to mitochondria but very poorly to the endoplasmic reticulum, whereas the L32N protein is preferentially targeted to the endoplasmic reticulum and poorly to mitochondria. These results explain the physiological significance of bimodal CYP targeting to the endoplasmic reticulum and mitochondria previously described. COS-7 cells and HepG2 cells stably expressing W23/30R mutations showed markedly increased alcohol toxicity in terms of increased production of reactive oxygen species, respiratory dysfunction, and loss of cytochrome c oxidase subunits and activity. Stable cells expressing the L32N variant, on the other hand, were relatively less responsive to alcohol-induced toxicity and mitochondrial dysfunction. These results further support our previous data, based on mutational studies involving altered targeting, indicating that mitochondria-targeted CYP2E1 plays an important role in alcohol liver toxicity. The results also provide an interesting new link to genetic variations affecting subcellular distribution of CYP2E1 with alcohol-induced toxicity.

Drug-induced autoimmune-like hepatitis

The clinical phenotype of classical autoimmune hepatitis can be mimicked by idiosyncratic drug-induced liver injury, and differentiation can be difficult. The goals of this review are to enumerate the major agents of drug-induced autoimmune-like hepatitis, describe the clinical findings and risk factors associated with it, detail the clinical tools by which to assess causality, discuss putative pathogenic mechanisms, and describe treatment and outcome. The frequency of drug-induced autoimmune-like hepatitis among patients with classical features of autoimmune hepatitis is 9%. Minocycline and nitrofurantoin are implicated in 90% of cases. Female predominance, acute onset, and absence of cirrhosis at presentation are important clinical manifestations. Genetic factors affecting phase I and phase II transformations of the drug, polymorphisms that protect against cellular oxidative stress, and human leukocyte antigens that modulate the immune response may be important pathogenic components. Clinical judgment is the mainstay of diagnosis as structured diagnostic methods for drug-induced liver injury are imperfect. The covalent binding of a reactive drug metabolite to a hepatocyte surface protein (commonly a phase I or phase II enzyme), formation of a neoantigen, activation of CD8 T lymphocytes with nonselective antigen receptors, and deficient immune regulatory mechanisms are the main bases for a transient loss of self-tolerance. Discontinuation of the offending drug is the essential treatment. Spontaneous improvement usually ensues within 1 month. Corticosteroid therapy is warranted for symptomatic severe disease, and it is almost invariably effective. Relapse after corticosteroid withdrawal probably does not occur, and its absence distinguishes drug-induced disease from classical autoimmune hepatitis.

Primary biliary cirrhosis: a 2010 update

Primary biliary cirrhosis (PBC) is a chronic inflammatory autoimmune disease that mainly targets the cholangiocytes of the interlobular bile ducts in the liver. The condition primarily affects middle-aged women. Without treatment, PBC generally progresses to cirrhosis and eventually liver failure over a period of 10-20 years. PBC is a rare disease with prevalence of less than 1/2000. PBC is thought to result from a combination of multiple genetic factors and superimposed environmental triggers. The contribution of the genetic predisposition is evidenced by the familial clustering. Several risk factors, including exposure to infectious agents and chemical xenobiotics, have been suggested. Ursodeoxycholic acid (UDCA) is currently the only FDA-approved medical treatment for PBC. When administered at doses of 13-15 mg/kg/day, a majority of patients with PBC have a normal life expectancy without additional therapeutic measures. One out of three patients does not adequately respond to UDCA therapy and may need additional medical therapy and/or liver transplantation. This review summarises current knowledge on the epidemiology, ethiopathogenesis, clinical, and therapeutic aspects of PBC.

Halothane-induced liver injury is mediated by interleukin-17 in mice

Drug-induced liver injury is a major problem in drug development and clinical drug therapy. In most cases the mechanisms are still unknown, thus, it is difficult to predict or prevent these reactions. It has been known that halothane, an inhaled anesthetic, induces liver injury. To investigate the mechanisms of halothane-induced liver injury, we used a recently established mouse model of liver injury. The expression of transcription factors and cytokines specific for Th1 and Th2 (helper T cells), respectively, were compared between BALB/c and C57BL/6 mice. The mRNA expression ratios of mouse T-bet(a Th1-specific transcription factor)/GATA-binding protein (GATA-3, a Th2-specific transcription factor) and interferon gamma/interleukin (IL)-10 were lower in BALB/c mice compared with C57BL/6 mice, suggesting that a typical Th1 or Th2-dominant response could not be distinguished in halothane-induced liver injury. We observed increases of the plasma IL-17 level and hepatic macrophage inflammatory protein 2 expression in halothane-administrated BALB/c mice, as well as neutrophil infiltration. Neutralization of IL-17 suppressed the hepatotoxic effect of halothane. Administration of recombinant IL-17 (1 microg per mouse, single ip) to the halothane-treated mice resulted in a remarkable increase of alanine and aspartate aminotransferases. In conclusion, we demonstrated that IL-17 is involved in the halothane-induced liver injury.

Liver enzyme induction and inhibition: implications for anaesthesia

Recent breakthroughs in molecular biology have enabled a reclassification of drug metabolising enzymes based on their amino acid sequence. This has led to a better understanding of drug metabolism and drug interactions. The majority of these drug metabolising enzymes may be either induced or inhibited by drugs or by extraneous substances including foodstuffs, cigarette smoke and environmental pollutants. Virtually all drugs used in anaesthesia are metabolised by either hepatic phase 1 or phase II enzymes. This review considers the classification of drug metabolising enzymes, explains the mechanisms of enzyme induction and inhibition, and also considers how the action of drugs commonly used by anaesthetists, including opioids and neuromuscular blocking drugs, may be altered by this mechanism.

Role of bioactivation in drug-induced hypersensitivity reactions

Drug-induced hypersensitivity reactions are a major problem in both clinical treatment and drug development. This review covers recent developments in our understanding of the pathogenic mechanisms involved, with special focus on the potential role of metabolism and bioactivation in generating a chemical signal for activation of the immune system. The possible role of haptenation and neoantigen formation is discussed, alongside recent findings that challenge this paradigm. Additionally, the essential role of costimulation is examined, as are the potential points whereby costimulation may be driven by reactive metabolites. The relevance of local generation of metabolites in determining the location and character of a reaction is also covered.

Monitoring drug-protein interaction

A variety of therapeutic drugs can undergo biotransformation via Phase I and Phase II enzymes to reactive metabolites that have intrinsic chemical reactivity toward proteins and cause potential organ toxicity. A drug-protein adduct is a protein complex that forms when electrophilic drugs or their reactive metabolite(s) covalently bind to a protein molecule. Formation of such drug-protein adducts eliciting cellular damages and immune responses has been a major hypothesis for the mechanism of toxicity caused by numerous drugs. The monitoring of protein-drug adducts is important in the kinetic and mechanistic studies of drug-protein adducts and establishment of dose-toxicity relationships. The determination of drug-protein adducts can also provide supportive evidence for diagnosis of drug-induced diseases associated with protein-drug adduct formation in patients. The plasma is the most commonly used matrix for monitoring drug-protein adducts due to its convenience and safety. Measurement of circulating antibodies against drug-protein adducts may be used as a useful surrogate marker in the monitoring of drug-protein adducts. The determination of plasma protein adducts and/or relevant antibodies following administration of several drugs including acetaminophen, dapsone, diclofenac and halothane has been conducted in clinical settings for characterizing drug toxicity associated with drug-protein adduct formation. The monitoring of drug-protein adducts often involves multi-step laboratory procedure including sample collection and preliminary preparation, separation to isolate or extract the target compound from a mixture, identification and determination. However, the monitoring of drug-protein adducts is often difficult because of short half-lives of the protein adducts, sampling problem and lack of sensitive analytical techniques for the protein adducts. Currently, chromatographic (e.g. high performance liquid chromatography) and immunological methods (e.g. enzyme-linked immunosorbent assay) are two major techniques used to determine protein adducts of drugs in patients. The present review highlights the importance for clinical monitoring of drug-protein adducts, with an emphasis on methodology and with a further discussion of the application of these techniques to individual drugs and their target proteins.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Separation and detection methods for covalent drug–protein adducts

Covalent binding of reactive metabolites of drugs to proteins has been a predominant hypothesis for the mechanism of toxicity caused by numerous drugs. The development of efficient and sensitive analytical methods for the separation, identification, quantification of drug-protein adducts have important clinical and toxicological implications. In the last few decades, continuous progress in analytical methodology has been achieved with substantial increase in the number of new, more specific and more sensitive methods for drug-protein adducts. The methods used for drug-protein adduct studies include those for separation and for subsequent detection and identification. Various chromatographic (e.g., affinity chromatography, ion-exchange chromatography, and high-performance liquid chromatography) and electrophoretic techniques [e.g., sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), two-dimensional SDS-PAGE, and capillary electrophoresis], used alone or in combination, offer an opportunity to purify proteins adducted by reactive drug metabolites. Conventionally, mass spectrometric (MS), nuclear magnetic resonance, and immunological and radioisotope methods are used to detect and identify protein targets for reactive drug metabolites. However, these methods are labor-intensive, and have provided very limited sequence information on the target proteins adducted, and thus the identities of the protein targets are usually unknown. Moreover, the antibody-based methods are limited by the availability, quality, and specificity of antibodies to protein adducts, which greatly hindered the identification of specific protein targets of drugs and their clinical applications. Recently, the use of powerful MS technologies (e.g., matrix-assisted laser desorption/ionization time-of-flight) together with analytical proteomics have enabled one to separate, identify unknown protein adducts, and establish the sequence context of specific adducts by offering the opportunity to search for adducts in proteomes containing a large number of proteins with protein adducts and unmodified proteins. The present review highlights the separation and detection technologies for drug-protein adducts, with an emphasis on methodology, advantages and limitations to these techniques. Furthermore, a brief discussion of the application of these techniques to individual drugs and their target proteins will be outlined.

Concordance between trifluoroacetic acid and hepatic protein trifluoroacetylation after disulfiram inhibition of halothane metabolism in rats

BACKGROUND

Cytochrome P4502E1(CYP2E1)-mediated oxidation of halothane to a reactive intermediate (trifluoroacyl chloride) that covalently binds to hepatic proteins forming trifluoroacetylated neoantigens is believed to be the initiating event in a complex immunologic cascade culminating in antibody formation and severe hepatic necrosis ('halothane hepatitis') in susceptible patients. Trifluoroacyl chloride may also hydrolyze to the stable metabolite trifluoroacetic acid (TFA). CYP2E1 inactivation by disulfiram or its primary metabolite, diethyldithiocarbamate, inhibits human halothane oxidation to TFA in vitro and in vivo. Nevertheless, disulfiram effects on hepatic protein trifluoroacetylation by halothane in vivo are unknown. This investigation tested the hypotheses that disulfiram prevents halothane-dependent protein trifluoroacetylation in vivo, and that TFA represents a biomarker for hepatic protein trifluoroacetylation.

METHODS

Rats were pretreated with isoniazid (CYP2E1 induction), isoniazid followed by disulfiram (CYP2E1 inhibition), or nothing (controls), then anesthetized with halothane or nothing (controls). Plasma and urine TFA were quantified by ion HPLC; hepatic microsomal TFA-proteins were analyzed by Western blot.

RESULTS

CYP2E1 induction increased both TFA and TFA-protein formation compared with uninduced halothane-treated rats. Disulfiram, even after CYP2E1 induction, nearly abolished both TFA and TFA-protein formation. Pretreatments similarly affected both TFA and TFA-protein formation across all groups.

CONCLUSIONS

Disulfiram inhibition of CYP2E1-mediated halothane oxidation prevents hepatic protein trifluoroacetylation. Based on the concordance between TFA and TFA-protein formation, TFA appears to be a valid biomarker for TFA-protein formation. Disulfiram inhibition of human halothane oxidation in vivo, previously assessed by diminished TFA formation, probably also confers inhibition of hepatic TFA-protein formation.

Halogenated inhalational anaesthetics

The halogenated inhalational anaesthetics halothane, enflurane, isoflurane and desflurane can produce metabolic hepatocellular injury in humans to a variable extent. During metabolism of these anaesthetics, tissue acetylation occurs due to the formation of reactive intermediates. Proteins modified by acetylation may constitute neo-antigens with a potential for triggering an antibody-mediated immune response. The likelihood of suffering post-operative immune hepatitis depends on the amount of the anaesthetic metabolized and is thereby considerably less with enflurane, isoflurane or desflurane compared with halothane. Plasma inorganic fluoride concentrations are regularly increased after sevoflurane. Elevated inorganic fluoride concentrations have been associated with nephrotoxicity following methoxyflurane anaesthesia but not after sevoflurane. Another source of concern is the products of degradation from reactions with carbon dioxide absorbents. Most important is compound A, which has been shown to exhibit nephrotoxicity in rodents. However, no significant changes in renal function parameters have been reported in surgical patients.

Reactive intermediates and the dynamics of glutathione transferases

Reactive intermediates are a continuous burden in biology and several defense mechanisms have evolved. Here we focus on the functions of glutathione transferases (GSTs) with the aim to discuss the quantitative aspects of defense against reactive intermediates. Humans excrete approximately 0.1 mmol of thioether conjugates per day. As the amount of GST active sites in liver is approximately 0.5 mmol, it appears that glutathione transferase catalysts are present in tremendous excess. In fact, the known catalytic properties of GSTs reveal that the enzymes can empty the liver glutathione (GSH) pool in a matter of seconds when provided with a suitable substrate. However, based on the urinary output of conjugates (or derivatives thereof), individual GSTs turn over (i.e., catalyze a single reaction) only once every few days. Glutathione transferase overcapacity reflects the fact that there is a linear relation between GST enzyme amount and protection level (provided that GSH is not depleted). Put in a different perspective, a few reactive molecules will always escape conjugation and reach cellular targets. It is therefore not surprising that signaling systems sensing reactive intermediates have evolved resulting in the increase of GSH and GST levels. Precisely for this reason, more moderately reactive electrophiles (Michael acceptors) are receiving growing interest due to their anticarcinogenic properties. Another putative regulatory mechanism involves direct activation of microsomal GST1 by thiol-reactive electrophiles through cysteine 49. The toxicological significance of low levels of reactive intermediates are of interest also in drug development, and here we discuss the use of microsomal GST1 activation as a surrogate detection marker.

The role of CYP forms in the metabolism and metabolic activation of HCFCs and other halocarbons

The use of hydrochlorofluorocarbons (HCFCs) such as HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane) and HCFC-141b (1,1-dichloro-1-fluoroethane) is becoming widespread as replacements for the ozone depleting chlorofluorocarbons. Hepatic activation of HCFC-123 or the unsaturated perchloroethylene through oxidative pathways leads to the formation of the electrophiles trifluoroacetyl chloride or trichloroacetyl chloride, respectively. These can react with epsilon-NH(2) functions of lysine in proteins and give rise to neoantigens. In the case of HCFC-123, this reaction is catalysed primarily by CYP2E1 and to a much lesser extent by the constitutive CYP2C19, CYP2B6 and CYP2C8. For perchloroethylene, the extent of activation is less and the reaction is catalysed primarily by the CYP2B family. While acute hepatotoxicity has been seen in humans exposed to HCFC-123 or halothane, little short- or long-term toxicity in rodents is observed. No immunological related toxicity of perchloroethylene has been reported in exposed humans. Long-term exposure of rats can lead to renal tubule carcinomas and in mice, hepatocellular carcinomas. These toxic reactions do not appear to be directly related to the formation of the putative trichloroacetyl chloride intermediate.

Differential effects of isoflurane and halothane on the induction of heat shock proteins

Isoflurane is considered to be a less hepatotoxic volatile anesthetic than halothane since it not only undergoes quantitatively much less metabolism to form toxic reactive intermediates, but also preserves better hepatic blood flow. However, the biochemical basis for the reduced hepatotoxicity has not been elucidated. In this study, we examined the induction of two heat shock proteins, heat shock protein 70 (HSP70) and heme oxygenase-1 (HO-1), in the livers of rats pretreated with or without phenobarbital, followed by exposure to isoflurane or halothane under hypoxic conditions. In the phenobarbital-pretreated rats, the maximal induction of HSP70 was observed by halothane-hypoxia treatment, followed by a half-maximal induction by isoflurane-hypoxia treatment, and less than 30% induction by hypoxia treatment alone. Serum alanine aminotransferase (ALT) activity, an indicator of hepatic dysfunction, which correlated well with the extent of centrilobular necrosis, showed similar changes with increases in HSP70 mRNA. In contrast, HO-1 mRNA was induced only by treatment with halothane-hypoxia. In addition, changes in the expression of HSP70 and HO-1 mRNAs were correlated with their protein expression in the liver. In non-pretreated rats, neither isoflurane-hypoxia exposure nor halothane-hypoxia exposure caused apparent hepatic injury. There was also no induction of HSP70 or HO-1 mRNA by these treatments in non-pretreated animals. These findings demonstrate that there is a significant difference in hepatic injury, and in the induction of HO-1 and HSP70 between halothane-hypoxia and isoflurane-hypoxia treatments. Isoflurane is known to be safer than halothane, which may, in part, be accounted for by the generation of less oxidative stress in the presence of isoflurane, as assessed by reduced induction of heat shock proteins compared with halothane treatment.

Cytochrome P450 2E1: its clinical and toxicological role

Cytochrome (CYP) P450 2E1 is clinically and toxicologically important and it is constitutively expressed in the liver and many other tissues. In contrast to many other CYP isoenzymes, indisputable evidence for a functionally important polymorphism of CYP2E1 in the human population is lacking. CYP2E1 metabolizes a wide variety of chemicals with different structures, in particular small and hydrophobic compounds, including potential cytotoxic and carcinogenic agents. In addition, chlorzoxazone and trimethadione metabolism are good CYP2E1 probes for liver disease in vivo and in vitro. In the future, methods for fully analysing the function of CYP2E1 using knockout mice will be established. This article reviews recent advances in our understanding of the role of human CYP2E1 in drug metabolism.