Glutathione S-transferase mu genotype (GSTM1*0) in Alzheimer's patients with tacrine transaminitis

Evolving Role of Pharmacogenetic Biomarkers to Predict Drug-Induced Hematological Disorders

ABSTRACT

Drug-induced hematological disorders constitute up to 30% of all blood dyscrasias seen in the clinic. Hematologic toxicity from drugs may range from life-threatening marrow aplasia, agranulocytosis, hemolysis, thrombosis to mild leukopenia, and thrombocytopenia. Pathophysiologic mechanisms underlying these disorders vary from an extension of the pharmacological effect of the drug to idiosyncratic and immune-mediated reactions. Predicting these reactions is often difficult, and this makes clinical decision-making challenging. Evidence supporting the role of pharmacogenomics in the management of these disorders in clinical practice is rapidly evolving. Despite the Clinical Pharmacology Implementation Consortium and Pharmacogenomics Knowledge Base recommendations, few tests have been incorporated into routine practice. This review aims to provide a comprehensive summary of the various drugs which are implicated for the hematological adverse events, their underlying mechanisms, and the current evidence and practical recommendations to incorporate pharmacogenomic testing in clinical care for predicting these disorders.

GSTM1 Null Genotype and GSTP1 Ile105Val Polymorphism Are Associated with Alzheimer's Disease: a Meta-Analysis

Published studies on the associations between glutathione S-transferase (GST) polymorphisms and Alzheimer's disease reported controversial findings. A meta-analysis of published studies was performed to assess the associations between polymorphisms of GSTM1, GSTT1 and GSTP1, and Alzheimer's disease. PubMed, Embase, and other databases were searched for case-control on the associations between polymorphisms of GSTM1, GSTT1 and GSTP1, and Alzheimer's disease. The odds ratio (OR) and 95% confidence interval (95% CI) were used to assess the associations. Eleven articles were finally included into the meta-analysis, including eight studies on GSTM1 null genotype, six studies on GSTT1 null genotype, and six studies on GSTP1 Ile105Val polymorphism. Overall, GSTM1 null genotype was associated with increased risk of Alzheimer's disease (fixed effect OR = 1.34, 95% CI 1.10-1.64, P = 0.004). GSTT1 null genotype was not associated with risk of Alzheimer's disease (random effect OR = 1.15, 95% CI 0.68-1.92, P = 0.60). Besides, GSTP1 Ile105Val polymorphism was significantly associated with increased risk of Alzheimer's disease (Val vs Ile: OR = 1.45, 95% CI 1.05-1.99, P = 0.023; ValVal vs IleIle: OR = 1.87, 95% CI 1.30-2.69, P = 0.001; ValVal vs IleIle + IleVal: OR = 1.76, 95% CI 1.24-2.51, P = 0.002). No obvious risk of publication bias was observed in the meta-analysis. GSTM1 null genotype and GSTP1 Ile105Val polymorphism are associated with increased risk of Alzheimer's disease. More studies with large sample size are needed to validate the findings in the meta-analysis.

Role of genes GSTM1, GSTT1, and MnSOD in the development of late-onset Alzheimer disease and their relationship with APOE*4

INTRODUCTION

Several studies have reported increased oxidation of lipids, proteins and DNA in the brains of patients with Alzheimer disease (AD). Moreover, these patients display differences in the activity and polymorphisms of the genes encoding the enzymes GST (T1, M1) and MnSOD. For these reasons, we designed a study of the variability in GSTT1, GSTM1, and MnSOD genes in healthy and AD groups from a Venezuelan population.

METHODS

We included 179 unrelated Venezuelan subjects classified as either AD patients (n=79) or healthy individuals (n=100). Presence or absence of the GSTT1/GSTM1 genes was determined using PCR-SSP, and polymorphisms of MnSOD and APOE genes were identified with PCR-RFLP.

RESULTS

The genotype GSTT1+/GSTM1- seems to favour development of AD (OR=2.06, P=.01). The risk level is higher when it is combined with the ɛ4 allele of the APOE gene: GSTT1+/GSTM1-/ɛ3ɛ4 (OR=3.07, P=.05), GSTT1+/GSTM1-/ɛ4ɛ4 (OR=5.52, P=.02). The Ala-9Val polymorphism does not appear to be related to AD. However, the presence of the Ala/Ala genotype increases the risk provided by the ɛ4 allele of the APOE gene: AlaAla/ɛ3ɛ4 (OR=3.47, P=.03), AlaAla/ɛ4ɛ4 (OR=6.3, P=.01).

CONCLUSIONS

The results support the hypothesis that impaired mitochondrial function and increased oxidative damage are involved in the pathogenesis of AD. It is important to study other genes related to oxidative stress and antioxidant pathways which could be involved in susceptibility to AD.

Pharmacogenomics in neurology: current state and future steps

In neurology, as in any other clinical specialty, there is a need to develop treatment strategies that allow stratification of therapies to optimize efficacy and minimize toxicity. Pharmacogenomics is one such method for therapy optimization: it aims to elucidate the relationship between human genome sequence variation and differential drug responses. Approaches have focused on candidate approaches investigating absorption-, distribution-, metabolism, and elimination (ADME)-related genes (pharmacokinetic pathways), and potential drug targets (pharmacodynamic pathways). To date, however, only few genetic variants have been incorporated into clinical algorithms. Unfortunately, a large number of studies have thrown up contradictory results due to a number of deficiencies, including small sample sizes, inadequate phenotyping, and genotyping strategies. Thus, there still exists an urgent need to establish biomarkers that could help to select for patients with an optimal benefit to risk relationship. Here we review recent advances, and limitations, in pharmacogenomics for agents used in neuroimmunology, neurodegenerative diseases, ischemic stroke, epilepsy, and primary headaches. Further work is still required in all of these areas, which really needs to progress on several fronts, including better standardized phenotyping, appropriate sample sizes through multicenter collaborations and judicious use of new technological advances such as genome-wide approaches, next generation sequencing and systems biology. In time, this is likely to lead to improvements in the benefit-harm balance of neurological therapies, cost efficiency, and identification of new drugs.

Polymorphisms in glutathione S-transferase omega-1 gene and increased risk of sporadic Alzheimer disease

Previous studies examining the association between the glutathione S-transferase omega-1 (GSTO1) single-nucleotide polymorphisms (SNPs) and Alzheimer disease (AD) have yielded conflicting results. Furthermore, an effect of GSTO1 rs4925 on the age-at-onset (AAO) of AD was found in different studies on sporadic and familial AD cases, but with contrasting findings. A total sample of 103 AD patients, and 157 age- and sex-matched unrelated caregivers from Apulia, southern Italy, were genotyped for the apolipoprotein E (APOE) polymorphism and the GSTO1 rs4925 and rs1804834 SNPs. Furthermore, we performed a haplotype analysis on these two SNPs on the GSTO1 locus and evaluated the possibility of interaction with APOE. Significant differences were observed in rs4925 genotype distribution between AD patients and age- and sex-matched healthy controls. Both the C/A (odds ratio [OR] = 3.116; 95% confidence interval [CI], 1.749-5.550) and the A/A (OR = 10.802; 95% CI, 3.605-32.128) genotypes resulted in an association with AD. A higher frequency of the allele A was observed in AD patients than in age- and sex-matched controls (OR = 3.789; 95% CI, 2.442-5.878). No significant differences were observed in the rs1804834 genotype or allele frequencies between AD patients and controls. No significant influence of the GSTO1 genotypes on the AAO was observed. No significant interaction was found among the GSTO1 SNPs and APOE. In both AD and controls, no important linkage disequilibrium (LD) was observed among the markers investigated. Whereas the C-A haplotype appeared to be protective against AD (OR = 0.303; 95% CI, 0.204-0.451), the A-A haplotype appeared to be at increased risk for AD (OR = 4.014,; 95% CI, 2.528-6.382). Our findings supported a role of the GSTO1 rs4925 SNP in the risk of sporadic AD in southern Italy, suggesting that this and other variants of the GSTO1 gene could be implicated in AD pathogenesis.

Cholinesterase inhibitors in Alzheimer's disease and Lewy body spectrum disorders: the emerging pharmacogenetic story

This review provides an update on the current state of pharmacogenetic research in the treatment of Alzheimer's disease (AD) and Lewy body disease (LBD) as it pertains to the use of cholinesterase inhibitors (ChEI). AD and LBD are first reviewed from clinical and pathophysiological perspectives. This is followed by a discussion of ChEIs used in the symptomatic treatment of these conditions, focusing on their unique and overlapping pharmacokinetic and pharmacodynamic profiles, which can be used to identify candidate genes for pharmacogenetics studies. The literature published to date is then reviewed and limitations are discussed. This is followed by a discussion of potential endophenotypes which may help to refine future pharmacogenetic studies of response and adverse effects to ChEIs.

Drug-induced liver injury

Many drugs and environmental chemicals are capable of evoking some degree of liver injury. The liver represents a primary target for adverse drug reactions due to its central role in biotransformation and excretion of foreign compounds, its portal location within the circulation exposing it to a wide variety of substances, and its anatomic and physiologic structure. Drug-induced liver injury (DILI) remains the single most common adverse indication leading to drug candidate failure or withdrawal from the market. However, the absolute incidence of DILI is low, and this presents a challenge to mechanistic studies. DILI remains unpredictable making prevention very difficult. In this chapter, we focus on the current understanding of DILI. We begin with an overview regarding the significance and epidemiology of DILI and then examine the clinical presentation and susceptibility factors related to DILI. This is followed by a review of the current literature regarding the proposed pathogenesis of DILI, which involves the participation of a drug, or most often a reactive metabolite of the drug, that either directly affects cellular function or elicits an immune response. It is our hope that this chapter will shed light on the major problems associated with DILI in regards to the pharmaceutical industry, drug regulatory agencies, physicians and pharmacists, and patients.

Drug-induced liver injury: insights from genetic studies

Drug-induced liver injury (DILI) is an increasing health problem and a challenge for physicians, regulatory bodies and the pharmaceutical industry, not only because of its potential severity and elusive pathogenesis but also because it is often inaccurately diagnosed, commonly missed entirely and more often not reported. The general view is that idiosyncratic DILI, which is not predictable whether based on the pharmacology of the drug or on the dose administered, is determined by the presence in the recipient of variants in, or expression of, genes coding for key metabolic pathways and/or the immune response, and the interaction of these genetic variants with environmental variables. Furthermore, idiosyncratic DILI is an example of a complex-trait disease with two or more susceptibility loci, as reflected by the frequency of genetic variants in the population often being higher than the occurrence of significant liver injury. Polymorphisms of bioactivation/toxification pathways via the CYP450 enzymes (Phase I), detoxification reactions (Phase II) and excretion/transport (Phase III), together with immunological factors that might determine DILI are reviewed. Challenges such as gene-trait association studies and whole-genome studies, and future approaches to the study of DILI are explored. Better knowledge of the candidate genes involved could provide further insight for the prospective identification of susceptible patients at risk of developing drug-induced hepatotoxicity, development of new diagnostic tools and new treatment strategies with safer drugs.

HLA-B genotyping to detect carbamazepine-induced Stevens-Johnson syndrome: implications for personalizing medicine

Preventing severe adverse drug reactions by identifying people at risk with a simple genetic test is the goal of many pharmacogenomic studies. Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) are related, life-threatening cutaneous adverse reactions, most often caused by medication. The overall incidence and the commonly offending drugs vary among different ethnic populations. Susceptibility to such idiosyncratic reactions is thought to be genetically determined and immune mediated. Finding a strong genetic association between a particular human leukocyte antigen (HLA)-B allele and the reaction to a specific drug provides evidence that the pathogenesis of the severe cutaneous adverse drug reactions involves major histocompatibility complex-restricted presentation of a drug or its metabolites for T-cell activation. In the case of carbamazepine-induced SJS/TEN, the tight association of the HLA-B*1502 allele (sensitivity 100%, specificity 97% and odds ratio 2504) provides a plausible basis for further development of such a test to identify individuals at risk of developing this life-threatening condition.

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.

The glutathione S-transferase polymorphisms in a control population and in Alzheimer's disease patients

In this study, we investigated the role of glutathione S-transferase P1 (GSTP1) polymorphisms in the pathogenesis of Alzheimer's disease (AD). We genotyped the GSTP1 polymorphisms in exon 5 (A313G) and exon 6 (C341T) by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) in 56 Croatian patients with AD and 231 controls. Distributions and frequencies of GSTP1 genetic variants were not statistically different between AD patients and healthy controls. Higher frequencies of the mutant genotypes were observed in AD patients (13% for both A313G and C341T) when compared with control subjects (7% for A313G and 8% for C341T), but association of GSTP1 GG (OR 2.057, 95% CI 0.796-5.315, p=0.094) and TT (OR 1.691, 95% CI 0.669-4.270, p=0.514) genotypes with an increased risk of AD was not confirmed by statistical analysis. The frequencies of GSTP1 alleles (A, B, C, D) did not significantly differ between AD patients and controls and they were indicated as follows: 52.7%, 15.2%, 12.5% and 19.6% for AD cases and 58.4%, 14.1%, 14.1% and 13.4% for controls. The estimation of the GSTP1 haplotype distribution showed that GSTP1*A/GSTP1*B and GSTP1*A/GSTP1*C haplotypes were less frequent, while GSTP1*B/GSTP1*B and GSTP1*C/GSTP1*D haplotypes were more frequent in AD patients than in controls. In conclusion, the involvement of GSTP1 alleles in individual susceptibility to AD was not confirmed as statistically significant in the tested Croatian Caucasian population. A possible role of GSTP1 in the complex etiopathogenesis of AD is further discussed, based on observed differences in haplotype distribution and higher frequencies of mutant genotypes in AD patients.

Polymorphisms in glutathione S-transferase omega-1 and AD, vascular dementia, and stroke

BACKGROUND

Glutathione S-transferase omega-1 (GSTO1) protects from oxidative stress, a risk factor for Alzheimer disease (AD), vascular dementia (VaD), and stroke. Polymorphisms in GSTO1 might influence the function of the protein and thus the risk of AD, VaD, and stroke.

METHODS

The GSTO1 gene was screened for variations. The effect of the detected polymorphisms on the risk of AD, VaD, and stroke was evaluated. CSF levels of cholesterol and plasma homocysteine levels were compared according to the GSTO1 genotype.

RESULTS

Two missense polymorphisms in exon 4 of GSTO1 (Ala140Asp and Glu155DeltaGlu) were detected and tested for their association with AD, VaD, and stroke. The Asp/Asp and Ala/Asp genotypes increased the risk of stroke (p = 0.003, OR = 2.1), and the Asp/Asp genotype increased the risk of VaD (p = 0.02, OR = 2.2). GSTO1 polymorphisms did not influence the risk of AD, but the Asp allele influenced the age at onset (p = 0.05). In nondemented probands CSF levels of cholesterol were increased in carriers of the Asp/Asp genotype (p = 0.004); however, in patients with manifest dementia the authors found decreased CSF levels of cholesterol in carriers of the Asp/Asp genotype (p = 0.028). Serum homocysteine levels in stroke patients were higher in carriers of at least one Asp allele (p = 0.011).

CONCLUSION

The GSTO1 Asp allele may be a genetic risk factor for cerebrovascular diseases, and might influence the course of Alzheimer disease, even though effects vary in different studies.

Tacrine inhibits topoisomerases and DNA synthesis to cause mitochondrial DNA depletion and apoptosis in mouse liver

After several weeks of treatment, levels of alanine aminotransferase (ALT) increase in 50% of patients treated with tacrine for Alzheimer's disease. We looked for progressive effects on DNA to explain delayed toxicity. We first studied the in vitro effects of tacrine on DNA replication and topoisomerase-mediated DNA relaxation. We then treated mice with doses of tacrine reproducing the human daily dose on a body area basis and studied the effects of tacrine administration for up to 28 days on hepatic DNA, mitochondrial function, and cell death. In vitro, tacrine impaired DNA polymerase gamma-mediated DNA replication and also poisoned topoisomerases I and II to increase the relaxation of a supercoiled plasmid. In vivo, administration of tacrine markedly decreased incorporation of [(3)H]thymidine into mitochondrial DNA (mtDNA), progressively and severely depleted mtDNA, and partly unwound supercoiled mtDNA into circular mtDNA. Incorporation of [(3)H]thymidine into nuclear DNA (nDNA) was barely decreased, and nDNA levels were unchanged. After 12 to 28 days of treatment, administration of tacrine increased p53, Bax, mitochondrial permeability transition, cytosolic cytochrome c, and caspase-3 activity and triggered hepatocyte apoptosis and/or necrosis. In conclusion, the intercalating drug tacrine poisons topoisomerases and impairs DNA synthesis. Tacrine has been shown to accumulate within mitochondria, and it particularly targets mtDNA. After several weeks of treatment, the combination of severe mtDNA depletion and a genotoxic stress enhancing p53, Bax, and permeability transition trigger hepatocyte necrosis and/or apoptosis.

Clinical pharmacokinetics and pharmacodynamics of cholinesterase inhibitors

Cholinesterase inhibitors are the 'first-line' agents in the treatment of Alzheimer's disease. This article presents the latest information on their pharmacokinetic properties and pharmacodynamic activity. Tacrine was the first cholinesterase inhibitor approved by regulatory agencies, followed by donepezil, rivastigmine and recently galantamine. With the exception of low doses of tacrine, the cholinesterase inhibitors exhibit a linear relationship between dose and area under the plasma concentration-time curve. Cholinesterase inhibitors are rapidly absorbed through the gastrointestinal tract, with time to peak concentration usually less than 2 hours; donepezil has the longest absorption time of 3 to 5 hours. Donepezil and tacrine are highly protein bound, whereas protein binding of rivastigmine and galantamine is less than 40%. Tacrine is metabolised by hepatic cytochrome P450 (CYP) 1A2, and donepezil and galantamine are metabolised by CYP3A4 and CYP2D6. Rivastigmine is metabolised by sulfate conjugation. Two cholinesterase enzymes are present in the body, acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). Tacrine and rivastigmine inhibit both enzymes, whereas donepezil and galantamine specifically inhibit AChE. Galantamine also modulates nicotine receptors, thereby enhancing acetylcholinergic activity at the synapse. These different pharmacological profiles provide distinctions between these agents. Cholinesterase inhibitors show a nonlinear relationship between dose and cholinesterase inhibition, where a plateau effect occurs. Cholinesterase inhibitors display a different profile as each agent achieves its plateau at different doses. In clinical trials, cholinesterase inhibitors demonstrate a dose-dependent effect on cognition and functional activities. Improvement in behavioural symptoms also occurs, but without a dose-response relationship. Gastrointestinal adverse events are dose-related. Clinical improvement occurs with between 40 and 70% inhibition of cholinesterase. A conceptual model for cholinesterase inhibitors has been proposed, linking enzyme inhibition, clinical efficacy and adverse effects. Currently, measurement of enzyme inhibition is used as the biomarker for cholinesterase inhibitors. New approaches to determining the efficacy of cholinesterase inhibitors in the brain could involve the use of various imaging techniques. The knowledge base for the pharmacokinetics and pharmacodynamics of cholinesterase inhibitors continues to expand. The increased information available to clinicians can optimise the use of these agents in the management of patients with Alzheimer's disease.

Genetic susceptibility to adverse drug reactions

Adverse drug reactions (ADRs) are a major clinical problem. Genetic factors can determine individual susceptibility to both dose-dependent and dose-independent ADRs. Determinants of susceptibility include kinetic factors, such as gene polymorphisms in cytochrome P450 enzymes, and dynamic factors, such as polymorphisms in drug targets. The relative importance of these factors will depend on the nature of the ADR; however, it is likely that more than one gene will be involved in most instances. In the future, whole genome single nucleotide polymorphism (SNP) profiling might allow an unbiased method of determining genetic predisposing factors for ADRs, but might be limited by the lack of adequate numbers of patient samples. The overall clinical utility of genotyping in preventing ADRs needs to be proven by the use of prospective randomized controlled clinical trials.

Mechanisms of idiosyncratic hypersensitivity reactions to antiepileptic drugs

Hypersensitivity reactions to the aromatic antiepileptic drugs (AEDs) phenytoin (PHT) and carbamazepine (CBZ) appear to have an immune etiology. Current models of drug hypersensitivity center around the concept of drug bioactivation to reactive metabolites that irreversibly modify cellular proteins. These modified proteins are believed to initiate (or serve as targets of) an autoimmune-like attack on specific drug-modified proteins in target organs (e.g., liver, skin) of susceptible individuals. Consistent with this model, antibodies to drug-modified and native proteins have been identified in the sera of patients experiencing several drug hypersensitivity reactions. New models must incorporate an understanding of the mechanisms by which drug-modified proteins are processed and presented to the immune system in the appropriate context to culminate in the clinical manifestations of "hypersensitivity." Idiosyncratic toxicities associated with new AEDs, such as lamotrigine and felbamate, appear mechanistically distinct from PHT and CBZ hypersensitivity but may involve similar processes: bioactivation, detoxification, covalent adduct formation, processing and presentation of antigen to the immune system, and consequent formation of antibody and T-cell immune effectors. The goal of research is to develop a "susceptibility profile" for identifying individuals at risk for these forms of drug toxicity.

Tissue-specific expression and alternative splicing of human microsomal epoxide hydrolase

Human microsomal epoxide hydrolase (HYL1) plays an important role in the detoxification of environmental compounds and drugs, such as the aromatic anticonvulsants phenytoin, carbamazepine, and phenobarbital, by converting their P450-generated epoxide metabolites into nontoxic diols. Recently, we have shown that a genetic defect altering the structure and function of the HYL1 protein is unlikely to be responsible for predisposing individuals to idiosyncratic hypersensitivity reactions from anticonvulsants. To evaluate the possible involvement of regulatory mechanisms, we used 5' rapid amplification of cDNA ends (RACE) and reverse transcription polymerase chain reaction (RT-PCR) to identify and characterize HYL1 5' cDNA ends. In addition to exon 1 (E1) previously isolated from a liver cDNA library, we isolated four new exons (E1-a, E1-c, E1-d, and E1-e) from various tissues. E1 was always directly connected to exon 2 (E2) where the translation start codon is located. E1-a, E1-c E1-d, and E1-e are alternatively spliced to E2, having either E1-a or E1-a' (a truncated form of E1-a) at the 5' end of their respective transcript. Genomic data indicate that exons E1-a and E1-c are located at least 7 kb upstream from E1. Furthermore, we demonstrated a tissue-specific expression pattern for E1-containing mRNA species, whereas E1-a-containing transcripts appear to be expressed ubiquitously. Our results provide evidence that microsomal epoxide hydrolase is regulated by multiple untranslated exons flanked by tissue-specific promoters.

Bioactivation and inactivation of aflatoxin B1 by human, mouse and rat liver preparations: effect on SCE in human mononuclear leucocytes

The purpose of this study was to investigate the use of human and animal subcellular liver fractions in an in vitro evaluation of carcinogenic risk. The bioactivation and bioinactivation of the known genotoxic carcinogen aflatoxin B1 by human, mouse and rat liver preparations was investigated using the SCE assay in human lymphocytes as a genotoxic endpoint. There was a 10-fold variation in SCE response (1.1-11.6 SCE/Cell) in human mononuclear leucocytes (MNLs) after aflatoxin B1 was activated by human liver microsomes (n = 6). Activation correlated with the CYP1A2 phenotype of livers (r = 0.8; p < 0.05), but there was no correlation with either GST M1 genotype or epoxide hydrolase phenotype. Mouse liver microsomes activated aflatoxin B1 to a greater extent [(1 micro M) 12.8 +/- 2.51 SCE/Cell] than either rat [(10 micro M) 12.0 +/- 3.84 SCE/Cell or human (L25) [(10 micro M) 8.8 +/- 2.00 SCE/Cell liver microsomes. The addition of mouse liver cytosol and reduced glutathione (GSH) significantly (p < 0.001) reduced aflatoxin B1-dependent genotoxicity, whereas the addition of either human or rat cytosol (+GSH) was without effect. These data indicate that species variation in both bioactivation and bioinactivation can exist. Therefore there is a necessity for careful selection of activation systems from species whose biochemical profile reflects that of man.

Genetic analysis of microsomal epoxide hydrolase in patients with carbamazepine hypersensitivity

Carbamazepine therapy is occasionally complicated by hypersensitivity reactions, the mechanism of which is poorly understood. It has been suggested that affected individuals may have a genetically-determined defect of microsomal epoxide hydrolase. The aim of this study was to determine whether a single genetic mutation or pattern of mutations could be used to predict individual susceptibility to carbamazepine-hypersensitivity. DNA was isolated from 10 carbamazepine-hypersensitive patients and 10 healthy volunteers. The patients had developed various forms of toxicity with carbamazepine, including toxic epidermal necrolysis, Stevens-Johnson syndrome, hepatitis and pneumonitis. The technique of polymerase chain reaction single-strand conformation polymorphism analysis (PCR-SSCP) was used to screen for mutations in all nine exons of the microsomal epoxide hydrolase gene. Any new mutations detected by this method were characterised by direct sequencing of the DNA. In addition, in the most severely affected patient, we sequenced all nine exons of the gene. There was a higher frequency of mutations in the hypersensitive group when compared with the controls, but there was no consistent mutation (or pattern of mutations) in the microsomal epoxide hydrolase gene which was common to the hypersensitive group. DNA sequencing of all nine exons of the microsomal epoxide hydrolase gene from the most severely affected patient showed the sequence to be "wild-type," when compared to the previously published sequences. The results of this study suggest that a single mutation within the coding region of the microsomal epoxide hydrolase gene cannot be the sole determinant of the predisposition to carbamazepine hypersensitivity.

Kinetic parameters of lymphocyte microsomal epoxide hydrolase in carbamazepine hypersensitive patients. Assessment by radiometric HPLC

Idiosyncratic hypersensitivity reactions with carbamazepine have been postulated to be due to a deficiency of microsomal epoxide hydrolase (HYL1), although this is based on indirect evidence. Using 3H-cis stilbene oxide (0.5 Ci/mmol) as a substrate, we have developed a radiometric HPLC assay sensitive enough to measure the kinetic parameters of HYL1 in lymphocytes. The intra-assay coefficient of variation was 8%. Enzyme activity has been measured in lymphocytes from six carbamazepine hypersensitive patients, six patients on carbamazepine without any adverse effects, and twelve drug-naive healthy volunteers. No significant difference was observed in three kinetic parameters of the enzyme among these three groups. The values for Km, Vmax, and intrinsic clearance ranged from 6.1-89.9 microM, 3.0-23.2 pmoles diol formed/min/mg protein, and 0.147-0.493 microliter/min/mg protein. There was no difference in enzyme activity between patients currently on carbamazepine and healthy volunteers, indicating a lack of induction of lymphocyte HYL1 by carbamazepine. Co-incubation of lymphocytes with 1,1,1-trichloropropene oxide, an inhibitor of hepatic HYL1, resulted in an 82% inhibition of activity, similar to that observed with the hepatic enzyme. The healthy volunteers were genotyped as being either GSTM1 positive (n = 6) or GSTM1 negative (n = 6). This did not affect the kinetic parameters of lymphocyte microsomal epoxide hydrolase. Our results suggest that there is normal HYL1 activity in lymphocytes of hypersensitive patients using cis-stilbene oxide as a substrate.