Identification of a new variant CYP2D6 allele lacking the codon encoding Lys-281: possible association with the poor metabolizer phenotype

A CYPome-wide study reveals new potential players in the pathogenesis of Parkinson's disease

Genetic and environmental factors lead to the manifestation of Parkinson's disease (PD) but related mechanisms are only rudimentarily understood. Cytochromes P450 (P450s) are involved in the biotransformation of toxic compounds and in many physiological processes and thus predestinated to be involved in PD. However, so far only SNPs (single nucleotide polymorphisms) in CYP2D6 and CYP2E1 have been associated with the susceptibility of PD. Our aim was to evaluate the role of all 57 human P450s and their redox partners for the etiology and pathophysiology of PD and to identify novel potential players which may lead to the identification of new biomarkers and to a causative treatment of PD. The PPMI (Parkinson's Progression Markers Initiative) database was used to extract the gene sequences of all 57 P450s and their three redox partners to analyze the association of SNPs with the occurrence of PD. Applying statistical analyses of the data, corresponding odds ratios (OR) and confidence intervals (CI) were calculated. We identified SNPs significantly over-represented in patients with a genetic predisposition for PD (GPD patients) or in idiopathic PD (IPD patients) compared to HC (healthy controls). Xenobiotic-metabolizing P450s show a significant accumulation of SNPs in PD patients compared with HC supporting the role of toxic compounds in the pathogenesis of PD. Moreover, SNPs with high OR values (>5) in P450s catalyzing the degradation of cholesterol (CYP46A1, CY7B1, CYP39A1) indicate a prominent role of cholesterol metabolism in the brain for PD risk. Finally, P450s participating in the metabolism of eicosanoids show a strong over-representation of SNPs in PD patients underlining the effect of inflammation on the pathogenesis of PD. Also, the redox partners of P450 show SNPs with OR > 5 in PD patients. Taken together, we demonstrate that SNPs in 26 out of 57 P450s are at least 5-fold over-represented in PD patients suggesting these P450s as new potential players in the pathogenesis of PD. For the first time exceptionally high OR values (up to 12.9) were found. This will lead to deeper insight into the origin and development of PD and may be applied to develop novel strategies for a causative treatment of this disease.

Recommendations for Clinical CYP2D6 Genotyping Allele Selection: A Joint Consensus Recommendation of the Association for Molecular Pathology, College of American Pathologists, Dutch Pharmacogenetics Working Group of the Royal Dutch Pharmacists Association, and the European Society for Pharmacogenomics and Personalized Therapy

The goals of the Association for Molecular Pathology Clinical Practice Committee's Pharmacogenomics (PGx) Working Group are to define the key attributes of pharmacogenetic alleles recommended for clinical testing, and to determine a minimal set of variants that should be included in clinical PGx genotyping assays. This document series provides recommendations on a minimal panel of variant alleles (Tier 1) and an extended panel of variant alleles (Tier 2) that will aid clinical laboratories in designing assays for PGx testing. When developing these recommendations, the Association for Molecular Pathology PGx Working Group considered the functional impact of the variant alleles, allele frequencies in multiethnic populations, the availability of reference materials, as well as other technical considerations with regard to PGx testing. The ultimate goal of this Working Group is to promote standardization of PGx gene/allele testing across clinical laboratories. This document is focused on clinical CYP2D6 PGx testing that may be applied to all cytochrome P450 2D6-metabolized medications. These recommendations are not meant to be interpreted as prescriptive but to provide a reference guide for clinical laboratories that may be either implementing PGx testing or reviewing and updating their existing platform.

CYP2D6 haplotypes with enhancer single-nucleotide polymorphism rs5758550 and rs16947 (*2 allele): implications for CYP2D6 genotyping panels

INTRODUCTION

CYP2D6 metabolizes ∼25% of all clinically used drugs, with numerous genetic polymorphisms affecting enzyme activity and drug response. Clinical utility of current CYP2D6 genotyping is partially compromised the unresolved complex haplotype structure of the CYP2D6 locus. We have identified a distal enhancer single-nucleotide polymorphism rs5758550 that robustly increases CYP2D6 expression, whereas rs16947 (CYP2D6*2), previously considered inert, reduces correct mRNA splicing and expression, thereby affecting presumed activity of other alleles on the *2 haplotype.

OBJECTIVE

This study aims to determine the structure and frequency of haplotypes containing either rs5758550 or rs16947, or both, together with other relevant CYP2D6 alleles, assigning predictive enzyme activity scores to each, and addressing ambiguities in estimating diplotypes in different populations.

METHODS

The structure and frequency of haplotypes containing rs5758550 and/or rs16947 in different populations were determined by using phased genotype data from 'The 1000 Genomes Project'. The assigned haplotype-phenotype relationship was tested by associating assigned CYP2D6 activity score with CYP2D6 enzyme activity in a cohort of 122 human liver microsomes.

RESULTS

Addition of enhancer single-nucleotide polymorphism rs5758550 and *2 to a CYP2D6 panel improves prediction of CYP2D6 activity. Moreover, the haplotype containing rs5758550 and rs16947 predict extensive CYP2D6 activity more accurately than CYP2D6*2A, a surrogate marker for extensive activity.

CONCLUSION

With further studies, the results support possible incorporation of rs5758550 and rs16947 into CYP2D6 biomarker panels for more accurate prediction of CYP2D6 metabolizer status.

CYP2D6 genotype can help to predict effectiveness and safety during opioid treatment for chronic low back pain: results from a retrospective study in an Italian cohort

Background

Opioids are widely used for chronic low back pain (CLBP); however, it is still unclear how to predict their effectiveness and safety. Codeine, tramadol and oxycodone are metabolized by CYP/CYP450 2D6 (CYP2D6), a highly polymorphic enzyme linked to allele-specific related differences in metabolic activity.

Purpose

CYP2D6 genetic polymorphisms could potentially help to predict the effectiveness and safety of opioid-based drugs in clinical practice, especially in the treatment of CLBP.

Patients and methods

A cohort of 224 Italian patients with CLBP treated with codeine or oxycodone was retrospectively evaluated to determine whether adverse reactions and effectiveness were related to CYP2D6 single-nucleotide polymorphisms. CYP2D6 genotyping was performed using the xTAG^® CYP2D6 Kit v3 (Luminex) to determine CYP2D6 metabolizer phenotype (poor, intermediate, rapid and ultrarapid). Subjects from the cohort were categorized into two groups according to the occurrence of side effects (Case) or benefit (Control) after chronic analgesic treatment. The impact of CYP2D6 polymorphism on treatment outcome was tested at the metabolizer phenotype, diplotype and haplotype levels.

Results

CYP2D6 polymorphism was significantly associated with opioid treatment outcome (Omnibus P=0.018, for both global haplotype and diplotype distribution test). CYP2D6*6 and *9 carriers, alleles characterized by a reduced (*9) or absent (*6) enzymatic activity, were significantly (P<0.05) associated with therapeutic failure. CYP2D6 ultrarapid metabolizers (CYP2D6*2N patients) showed an increased risk of side effects, as would be predicted. Despite their low frequency, CYP2D6 *1/*11, *4/*6 and *41/* 2N diplotypes showed significant (P<0.05) associations of efficacy and side effects with chronic opioid treatment.

Conclusion

Our results showed that reduced CYP2D6 activity is correlated with lack of therapeutic effect. We found that the pharmacogenetic analysis of CYP2D6 could be helpful in foreseeing the safety and effectiveness of codeine or oxycodone treatment in CLBP.

Ten Years' Experience with the CYP2D6 Activity Score: A Perspective on Future Investigations to Improve Clinical Predictions for Precision Therapeutics

The seminal paper on the CYP2D6 Activity Score (AS) was first published ten years ago and, since its introduction in 2008, it has been widely accepted in the field of pharmacogenetics. This scoring system facilitates the translation of highly complex CYP2D6 diplotype data into a patient’s phenotype to guide drug therapy and is at the core of all CYP2D6 gene/drug pair guidelines issued by the Clinical Pharmacogenetics Implementation Consortium (CPIC). The AS, however, only explains a portion of the variability observed among individuals and ethnicities. In this review, we provide an overview of sources in addition to CYP2D6 genotype that contribute to the variability in CYP2D6-mediated drug metabolism and discuss other factors, genetic and non-genetic, that likely contribute to the observed variability in CYP2D6 enzymatic activity.

Cytochrome P450 2D6 and Parkinson's Disease: Polymorphism, Metabolic Role, Risk and Protection

Cytochrome P450 (CYP) 2D6 is one of the most highly active, oxidative and polymorphic enzymes known to metabolize Parkinsonian toxins and clinically established anti-Parkinson's disease (PD) drugs. Albeit CYP2D6 gene is not present in rodents, its orthologs perform almost the similar function with imprecise substrate and inhibitor specificity. CYP2D6 expression and catalytic activity are found to be regulated at every stage of the central dogma except replication as well as at the epigenetic level. CYP2D6 gene codes for a set of alternate splice variants that give rise to a range of enzymes possessing variable catalytic activity. Case-control studies, meta-analysis and systemic reviews covering CYP2D6 polymorphism and PD risk have demonstrated that poor metabolizer phenotype possesses a considerable genetic susceptibility. Besides, ultra-rapid metabolizer offers protection against the risk in some populations while lack of positive or inverse association is also reported in other inhabitants. CYP2D6 polymorphisms resulting into deviant protein products with differing catalytic activity could lead to inter-individual variations, which could be explained to certain extent on the basis of sample size, life style factors, food habits, ethnicity and tools used for statistical analysis across various studies. Current article describes the role played by polymorphic CYP2D6 in the metabolism of anti-PD drugs/Parkinsonian toxins and how polymorphisms determine PD risk or protection. Moreover, CYP2D6 orthologs and their roles in rodent models of Parkinsonism have also been mentioned. Finally, a perspective on inconsistency in the findings and futuristic relevance of CYP2D6 polymorphisms in disease diagnosis and treatment has also been highlighted.

Pharmacogenetics of CYP2D6 and tamoxifen therapy: Light at the end of the tunnel?

The clinical usefulness of assessing the enzymatic activity of CYPD6 in patients taking tamoxifen had been longly debated. In favour of preemptive evaluation of phenotypic profile of patients is the strong pharmacologic rationale, being that the formation of endoxifen, the major and clinically most important metabolite of tamoxifen, is largely dependent on the activity of CYP2D6. This enzyme is highly polymorphic for which the activity is largely depending on genetics, but that can also be inhibited by a number of drugs, i.e. antidepressants, which are frequently used in patients with cancer. Unfortunately, the clinical trials that have been published in the last years are contradicting each other on the association between CYP2D6 and significant clinical endpoints, and for this reason CYP2D6 genotyping is at present not generally recommended. Despite this, the CYP2D6 genotyping test for tamoxifen is available in many laboratories and it may still be an appropriate test to use it in specific cases.

Effect of lymphovascular space invasion on survival of stage I epithelial ovarian cancer

OBJECTIVE

To evaluate the effect of lymphovascular space invasion on survival of patients with early-stage epithelial ovarian cancer.

METHODS

A multicenter retrospective study was conducted for patients with stage IA-C epithelial ovarian cancer who underwent primary comprehensive surgery including lymphadenectomy. Histopathology slides for ovarian tumors were examined by gynecologic pathologists for the presence or absence of lymphovascular space invasion. Survival analysis was performed examining tumoral factors.

RESULTS

A total of 434 patients were included in the analysis. Lymphovascular space invasion was detected in 76 (17.5%) patients associated with histology (P=.042) and stage (P=.044). Lymphovascular space invasion was significantly associated with decreased survival outcomes (disease-free survival, 5-year rate 78.4% compared with 90.7%, P=.024 and overall survival, 84.9% compared with 93.2%, P=.031) in univariate analysis. In multivariate analysis, lymphovascular space invasion did not remain a significant variable for disease-free survival (hazard ratio [HR] 1.98, 95% confidence interval [CI] 0.97-3.97, P=.059) or overall survival (HR 2.41, 95% CI 0.99-5.85, P=.052). Lymphovascular space invasion was associated with increased risk of hematogenous and lymphatic metastasis (HR 4.79, 95% CI 1.75-13.2, P=.002) but not peritoneal metastasis (P=.33) in multivariate analysis. Among lymphovascular space invasion-expressing tumors, patients who received fewer than six cycles of postoperative chemotherapy had significantly poorer disease-free survival than those who received six or more cycles (HR 4.59, 95% CI 1.20-17.5, P=.015).

CONCLUSION

Lymphovascular space invasion is an important histologic feature to identify a subgroup of patients with increased risk of recurrence in stage I epithelial ovarian cancer.

LEVEL OF EVIDENCE

III.

Detection and characterization of the CYP2D6*9x2 gene duplication in two Spanish populations: resolution of AmpliChip CYP450 test no-calls

BACKGROUND

CYP2D6 is a major drug-metabolizing enzyme. Polymorphic variation includes copy number variants such as gene deletions, duplications and multiplications of functional and nonfunctional gene units. In this article we describe the first systematic characterization of a CYP2D6*9x2 gene duplication. CYP2D6*9 is an allelic variant conferring reduced enzymatic activity. This novel gene duplication was discovered in two unrelated Spanish psychiatric patients. Both subjects were initially tested with the AmpliChip CYP450 test, which indicated the presence of a duplication and the CYP2D6*9 allele, but did not make a genotype call. The goal of the study was to resolve this issue by characterizing the CYP2D6 gene locus in these patients.

MATERIALS & METHODS

Both individuals and one offspring were regenotyped using our own CYP2D6 genotyping strategy employing long-range PCR and TaqMan-based SNP detection. In addition, gene resequencing and genotyping of duplication-specific long-range PCR products and quantitative gene copy number analysis was applied.

RESULTS

The duplication was mapped to the CYP2D6*9 allele and copy number analysis determined a CYP2D6*9x2 gene duplication in all three individuals. Because CYP2D6*9x2 is not recognized by the AmpliChip CYP450 test, this structural arrangement was responsible for the 'no call' on the AmpliChip CYP450 test report.

CONCLUSION

The full characterization of this allele will aid in the interpretation of AmpliChip CYP450 test results for clinical and research applications. Original submitted 8 June 2011; Revision submitted 18 July 2011.

Pharmacogenomics of CYP2D6: molecular genetics, interethnic differences and clinical importance

CYP2D6 has received intense attention since the beginning of the pharmacogenetic era in the 1970s. This is because of its involvement in the metabolism of more than 25% of the marketed drugs, the large geographical and inter-ethnic differences in the genetic polymorphism and possible drug-induced toxicity. Many interesting reviews have been published on CYP2D6 and this review aims to reinstate the importance of the genetic polymorphism of CYP2D6 in different populations as well as some clinical implications and important drug interactions.

Polymorphic metabolism by functional alterations of human cytochrome P450 enzymes

The study of cytochrome P450 pharmacogenomics is of particular interest because of its promise in the development of rational means to optimize drug therapy with respect to patient's genotype to ensure maximum efficacy with minimal adverse effects. Drug metabolizing P450 enzymes are polymorphic and are the main phase I enzymes responsible for the metabolism of clinical drugs. Therefore, polymorphisms in the P450s have the most impact on the fate of clinical drugs in phase I metabolism since almost 80% of drugs in use today are metabolized by these enzymes. Predictive genotyping for P450 enzymes for a more effective therapy will be routine for specific drugs in the future. In this review, we discuss the current knowledge of polymorphic metabolism by functional alterations in nonsynonymous SNPs of P450 1A2, 2A6, 2C8, 2C9, 2C19, 2D6, and 3A4 enzymes.

Pharmacogenetics of cytochrome P450 (CYP) in the elderly

The genetics of cytochrome P450 (CYP) is a very active area of multidisciplinary research, overlapping the interest of medicine, biology and pharmacology, being the CYP enzyme system responsible for the metabolism of more than 80% of the commercially available drugs. Variations in CYP encoding genes are responsible for inter-individual differences in CYP production or function, with severe clinical consequences as therapeutic failures (TFs) and adverse drug reactions (ADRs), being ADRs worldwide primary causes of morbidity and mortality in elderly people. In fact, the prevalence of both TFs and ADRs strongly increased in the presence of multiple pharmacological treatments, a common status in subjects aging 65 years and over. The present article explored some basic concepts of human genetics that have important implications in the genetics of CYP. An attempted to transfer these basic concepts to the genetic data reported by the Home Page of The Human Cytochrome P450 (CYP) Allele Nomenclature Committee was also made, focusing on the current knowledge of CYP genetics. The status of what we know and what we need to know is the base for the clinical applications of pharmacogenetics, in which personalized drug treatments constituted the main aim, in particular in patients attending a geriatric ward.

CYP2D6 gene variants: association with breast cancer specific survival in a cohort of breast cancer patients from the United Kingdom treated with adjuvant tamoxifen

Introduction

Tamoxifen is one of the most effective adjuvant breast cancer therapies available. Its metabolism involves the phase I enzyme, cytochrome P4502D6 (CYP2D6), encoded by the highly polymorphic CYP2D6 gene. CYP2D6 variants resulting in poor metabolism of tamoxifen are hypothesised to reduce its efficacy. An FDA-approved pre-treatment CYP2D6 gene testing assay is available. However, evidence from published studies evaluating CYP2D6 variants as predictive factors of tamoxifen efficacy and clinical outcome are conflicting, querying the clinical utility of CYP2D6 testing. We investigated the association of CYP2D6 variants with breast cancer specific survival (BCSS) in breast cancer patients receiving tamoxifen.

Methods

This was a population based case-cohort study. We genotyped known functional variants (n = 7; minor allele frequency (MAF) > 0.01) and single nucleotide polymorphisms (SNPs) (n = 5; MAF > 0.05) tagging all known common variants (tagSNPs), in CYP2D6 in 6640 DNA samples from patients with invasive breast cancer from SEARCH (Studies of Epidemiology and Risk factors in Cancer Heredity); 3155 cases had received tamoxifen therapy. There were 312 deaths from breast cancer, in the tamoxifen treated patients, with over 18000 years of cumulative follow-up. The association between genotype and BCSS was evaluated using Cox proportional hazards regression analysis.

Results

In tamoxifen treated patients, there was weak evidence that the poor-metaboliser variant, CYP2D6*6 (MAF = 0.01), was associated with decreased BCSS (P = 0.02; HR = 1.95; 95% CI = 1.12-3.40). No other variants, including CYP2D6*4 (MAF = 0.20), previously reported to be associated with poorer clinical outcomes, were associated with differences in BCSS, in either the tamoxifen or non-tamoxifen groups.

Conclusions

CYP2D6*6 may affect BCSS in tamoxifen-treated patients. However, the absence of an association with survival in more frequent variants, including CYP2D6*4, questions the validity of the reported association between CYP2D6 genotype and treatment response in breast cancer. Until larger, prospective studies confirming any associations are available, routine CYP2D6 genetic testing should not be used in the clinical setting.

New insights into the structural characteristics and functional relevance of the human cytochrome P450 2D6 enzyme

To date, the crystal structures of at least 12 human CYPs (1A2, 2A6, 2A13, 2C8, 2C9, 2D6, 2E1, 2R1, 3A4, 7A1, 8A1, and 46A1) have been determined. CYP2D6 accounts for only a small percentage of all hepatic CYPs (< 2%), but it metabolizes approximately 25% of clinically used drugs with significant polymorphisms. CYP2D6 also metabolizes procarcinogens and neurotoxins, such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 1,2,3,4-tetrahydroquinoline, and indolealkylamines. Moreover, the enzyme utilizes hydroxytryptamines and neurosteroids as endogenous substrates. Typical CYP2D6 substrates are usually lipophilic bases with an aromatic ring and a nitrogen atom, which can be protonated at physiological pH. Substrate binding is generally followed by oxidation (5-7 A) from the proposed nitrogen-Asp301 interaction. A number of homology models have been constructed to explore the structural features of CYP2D6, while antibody studies also provide useful structural information. Site-directed mutagenesis studies have demonstrated that Glu216, Asp301, Phe120, Phe481, and Phe483 play important roles in determining the binding of ligands to CYP2D6. The structure of human CYP2D6 has been recently determined and shows the characteristic CYP fold observed for other members of the CYP superfamily. The lengths and orientations of the individual secondary structural elements in the CYP2D6 structure are similar to those seen in other human CYP2 members, such as CYP2C9 and 2C8. The 2D6 structure has a well-defined active-site cavity located above the heme group with a volume of approximately 540 A(3), which is larger than equivalent cavities in CYP2A6 (260 A(3)), 1A2 (375 A(3)), and 2E1 (190 A(3)), but smaller than those in CYP3A4 (1385 A(3)) and 2C8 (1438 A(3)). Further studies are required to delineate the molecular mechanisms involved in CYP2D6 ligand interactions and their implications for drug development and clinical practice.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Association of graded allele-specific changes in CYP2D6 function with imipramine dose requirement in a large group of depressed patients

The inactivation and clearance of the tricyclic antidepressant imipramine is dependent on CYP2D6 activity. First, CYP2C19 converts imipramine into the active metabolite desipramine, which is then inactivated by CYP2D6. This retrospective single center study aimed to prove whether CYP2C19 and ample CYP2D6 genotyping (taking into consideration four null alleles and three decreased-activity alleles) could be used to predict imipramine and desipramine plasma concentrations in depressed patients, and whether genotype-based drug dose recommendations might assist in the early management of imipramine pharmacotherapy. In 181 subjects with major depressive disorder, drug doses were recorded, imipramine and desipramine plasma concentrations were monitored and CYP2C19 (*2) and CYP2D6 genotype (*3, *4, *5, *6, *9, *10, *41 and gene duplication) were obtained, yielding graded allele-specific CYP2D6 patient groups. Desipramine and imipramine+desipramine plasma concentration per drug dose unit, imipramine dose at steady state, and imipramine dose requirement significantly depended on CYP2D6 genotype (Kruskal-Wallis test, P<0.0001). Mean (+/-s.d.) drug dose requirements were 131 (+/-109), 155 (+/-70), 217 (+/-95), 245 (+/-125), 326 (+/-213), and 509 (+/-292) mg imipramine/day in carriers of 0, 0.5, 1, 1.5, 2, and >2 active CYP2D6 genes, respectively. Our protocol for CYP2D6 genotyping will thus importantly aid in the prediction of imipramine metabolism, allowing for the use of an adjusted starting dose and faster achievement of predefined imipramine+desipramine plasma levels in all genetic patient subgroups. Therefore, therapeutic efficacy and efficiency may be improved, the number of adverse drug reactions decreased, and hospital stay reduced.

Analysis of CYP2D6 substrate interactions by computational methods

Cytochrome P450 CYP2D6 is involved in the oxidation of well over 150 drugs and, in general, those which contain a basic nitrogen atom in the molecule. To clarify how the residues of CYP2D6 are utilized for orientating a wide range of its specific substrates and distinguishing them from a variety of other organic compounds, docking studies by AutoDock and molecular dynamics (MD) simulations were conducted. Specific ligands were docked to both the homology model and crystal structures optimally to estimate the site of reaction on the ligand molecule and the binding energy for the complex, which were generally in good agreement with the experimental data. MD simulation for the CYP2D6-propranolol complex was then carried out to reveal the amino acid residues interacting with the substrate at the active site. Phe-120, Glu-216, Asp-301, and Phe-483 are identified as the substrate-binding residues in agreement with previously reported site-directed mutagenesis data and the crystal structure reported recently (PDB code: 2F9Q). As well as these residues, our theoretical prediction suggests that Phe-219 and Glu-222 are also important residues for mediating oxidation of substrates, especially propranolol.

Impact of CYP2D6 genetic polymorphism on tramadol pharmacokinetics and pharmacodynamics

BACKGROUND AND OBJECTIVE

Tramadol is metabolized by the highly polymorphic enzyme cytochrome P450 (CYP)2D6. Patients with different CYP2D6 genotypes may respond differently to tramadol in terms of pain relief and adverse events. In this study, we compare the pharmacokinetics and effects of tramadol in Malaysian patients with different genotypes to establish the pharmacokinetic-pharmacodynamic relationship of tramadol.

STUDY DESIGN AND SETTING

All patients received an intravenous dose of tramadol 100mg as their first postoperative analgesic. Blood was sampled at 0 minutes and subsequently at 15 and 30 minutes, 1, 2, 4, 8, 16, 20, and 24 hours for serum tramadol and analyzed by high-performance liquid chromatography (HPLC). Patients were genotyped for CYP2D6*1, *3, *4, *5, *9, *10, and *17 alleles and duplication of the gene by means of an allele-specific PCR. Pain was measured using the Visual Analog Scales, and adverse effects were recorded.

RESULTS

About half of the patients had the wild-type allele (CYP2D6*1), with the 'Asian'CYP2D6*10 allele accounting for most of the rest (40%). None of the genotypes predicted poor metabolism. Twenty-seven percent of the patients were intermediate metabolizers (IM) and 2.9% were ultra-rapid (UM) metabolizers; the remaining 70% were extensive metabolizers (EM). The mean total clearance (CL) predicted by the model was lower (19 L/h) and the half-life longer (5.9 hours) than those reported in Western populations. This may due to the high frequency of the CYP2D6*10 allele amongst Malaysian patients. The UM and EM groups had 2.6- and 1.3-times faster CL, respectively, than the IM. CL was 16, 18, 23, and 42 L/h while mean half-lives were 7.1, 6.8, 5.6, and 3.8 hours among the IM, EM1, EM2, and UM groups, respectively. However, the analgesic effects of tramadol were not measured adequately among the postoperative patients to establish its full therapeutic effects. There were significant differences in the adverse-effect profiles amongst the various genotype groups, with the IM group experiencing more adverse effects than the EM, and the EM having more adverse effects than the UM.

CONCLUSION

CYP2D6 activity may play an important role in determining the pharmacokinetics of tramadol and in predicting its adverse effects. If these results can be confirmed in a larger population, genotyping may be an important tool in determining the dose of tramadol.

No association between CYP2D6 polymorphisms and personality trait in Japanese

AIMS

The polymorphic enzyme CYP2D6 is expressed not only in liver but also in brain at low concentrations. CYP2D6 mediates, to some extent, the synthesis of the neurotransmitters, serotonin and dopamine. We investigated a possible association between the genetic polymorphism of CYP2D6 and individual personality trait.

METHODS

Mentally and physically healthy volunteers were recruited (n = 342). Temperament and Character Inventory (TCI) and CYP2D6 genotyping were performed in all subjects. We detected mutated alleles which were identified using the Amplichip CYP450 DNA chip.

RESULTS

The number of phenotypes, assumed by genotype for ultrarapid metabolizers (UM), extensive metabolizers (EM), intermediate metabolizers (IM) and poor metabolizers (PM) were 4 (1.1%), 262 (76.6%), 75 (21.9%) and 1 (0.3%), respectively. There were no differences in scores for novelty seeking, harm avoidance, reward dependence or persistence among the CYP2D6 phenotypes. The number of mutated alleles for CYP2D6 did not differ for scores of novelty seeking, harm avoidance, reward dependence or persistence. In subitem analyses, only RD3 (attachment) had a significant difference both in the CYP2D6 phenotype (P < 0.05) and genotype (P < 0.05).

CONCLUSIONS

This study did not demonstrate a significant association between CYP2D6 activity and personality trait because of the small interindividual variability in CYP2D6 activity within the Japanese population.

Enantioselective pharmacokinetics of tramadol in CYP2D6 extensive and poor metabolizers

OBJECTIVE

To describe in detail the intravenous, single oral and multiple oral dose enantioselective pharmacokinetics of tramadol in CYP2D6 extensive metabolizers (EMs) and poor metabolizers (PMs).

METHODS

Eight EMs and eight PMs conducted three phases as an open-label cross-over trial with different formulations; 150 mg single oral tramadol hydrochloride, 50 mg single oral tramadol hydrochloride every 8 h for 48 h (steady state), 100 mg intravenous tramadol hydrochloride. Urine and plasma concentrations of (+/-)-tramadol and (+/-)-M1 were determined for 48 h after administration.

RESULTS

In all three phases, there were significant differences between EMs and PMs in AUC and t(1/2) of (+)-tramadol (P< or =0.0015), (-)-tramadol (P< or =0.0062), (+)-M1 (P< or =0.0198) and (-)-M1 (P< or =0.0370), and significant differences in C(max) of (+)-M1 (P<0.0001) and (-)-M1 (P< or =0.0010). In Phase A and C, significant differences in t(max) were seen for (+)-M1 (P< or =0.0200). There were no statistical differences between the absolute bioavailability of tramadol in EMs and PMs. The urinary recoveries of (+)-tramadol, (-)-tramadol, (+)-M1 and (-)-M1 were statistically significantly different in EMs and PMs (P<0.05). Median antimodes of the urinary metabolic ratios of (+)-tramadol / (+)-M1 and (-)-M1 were 5.0 and 1.5, respectively, hereby clearly separating EMs and PMs in all three phases.

CONCLUSION

The impact of CYP2D6 phenotype on tramadol pharmacokinetics was similar after single oral, multiple oral and intravenous administration displaying significant pharmacokinetic differences between EMs and PMs of (+)-tramadol, (-)-tramadol, -(+)-M1 and (-)-M1. The O-demethylation of tramadol was catalysed stereospecific by CYP2D6 in the way that very little (+)-M1 was produced in PMs.

Identification of CYP2D6 impaired functional alleles in Mexican Americans

OBJECTIVES

To extend the genotyping analysis of the CYP2D6 gene and further explain variability of CYP2D6 activity in Mexican Americans by genetic factors.

METHODS

CYP2D6 gene sequence variations associated with *6, *7, *8, *9, *11, *14, *29, *41, *45, and *46 alleles as well as the 2988G>A SNP were examined in 264 Mexican Americans; 236 had previously been phenotyped with dextromethorphan. All subjects were previously genotyped for CYP2D6*2, *3, *4, *5, *10, *17, and the presence of a gene duplication. Associations between genotype and CYP2D6 activity were determined.

RESULTS

Mexican Americans revealed a high frequency of functional alleles (CYP2D6*1 and *2; 73.1%), followed by CYP2D6*4 (non-functional, 10.0%) and the reduced-function allele *41 (9.5%). The frequencies of CYP2D6*5, *6, *9, *10, duplication, and 2988A were 1.7%, 0.4%, 1.1%, 2.8%, 0.8%, and 5.7%, respectively. CYP2D6*3, *17, and *29 were found only in one individual (CYP2D6*2/*3, *1/*17, and *4/*29), while CYP2D6*7, *8, *11, *14, *45, and *46 were absent in this study population. Decreased CYP2D6 activity was more accurately predicted by the presence *41[-1584C] compared to *41[2988A]. One genotype/phenotype discordant subject was resolved by the presence of a CYP2D6*6 allele (*4/*6), while two other cases remained discordant (*41/*41 and *1/*1).

CONCLUSIONS

The CYP2D6*4, *5, and *6 null alleles along the reduced function alleles *9, *10, and *41 are the major cause for diminished dextromethorphan oxidative capacity in Mexican Americans. These findings may have implications for the safety and efficacy of CYP2D6 substrates taken by Mexican Americans.

Identification of Subtypes of CYP2D Gene Rearrangements among Carriers of CYP2D6 Gene Deletion and Duplication

Background: Cytochrome P450 2D6 (CYP2D6) is one of the best-known polymorphic drug-metabolizing enzymes. Rapidly evolving genotyping techniques permit the identification of single-nucleotide polymorphisms (SNPs) and thereby a prediction of individual metabolic capacities for CYP2D6 substrates. A considerable part of interindividual variability in CYP2D6 enzyme activity, however, is not related to SNPs but to gene deletions and duplications. Currently used genotyping methods assume that these gene rearrangements are homogeneous.

Methods: We analyzed the interindividual variability in CYP2D6 gene arrangements in genomic DNA from 740 Caucasian individuals by allele-specific PCR to identify common SNPs of the CYP2D6 gene that correspond to the variant alleles CYP2D6*3, *4, and *9. We investigated the presence and variability of CYP2D6*5 (gene deletion), CYP2D6x2 (gene duplication), and CYP2D6xn (gene amplification) by EcoRI and XbaI restriction fragment length polymorphism analyses and by long PCR plus KpnI and BamHI digestion. The presence of new mutations at the CYP2D locus was analyzed by sequencing.

Results: CYP2D6 gene rearrangements were present in &gt;12% of individuals. Variability in the rearrangements regarding both gene deletion and gene duplication existed, and one of the unusual arrangements led to incorrect phenotype prediction. The frequency for carriers of unusual gene rearrangements was &lt;0.3% (95% confidence interval, 0%–0.6%) in the population studied.

Conclusions: Heterogeneity in CYP2D6 gene rearrangement exists, but the allele frequency indicates that the risk for an erroneous phenotype prediction related to such variability is extremely low and that this risk can be neglected in routine analyses.

Polymorphic cytochrome P450 2D6: humanized mouse model and endogenous substrates

Cytochrome P450 2D6 (CYP2D6) is the first well-characterized polymorphic phase I drug-metabolizing enzyme, and more than 80 allelic variants have been identified for the CYP2D6 gene, located on human chromosome 22q13.1. Human debrisoquine and sparteine metabolism is subdivided into two principal phenotypes--extensive metabolizer and poor metabolizer--that arise from variant CYP2D6 genotypes. It has been estimated that CYP2D6 is involved in the metabolism and disposition of more than 20% of prescribed drugs, and most of them act in the central nervous system or on the heart. These drug substrates are characterized as organic bases containing one nitrogen atom with a distance about 5, 7, or 10 A from the oxidation site. Aspartic acid 301 and glutamic acid 216 were determined as the key acidic residues for substrate-enzyme binding through electrostatic interactions. CYP2D6 transgenic mice, generated using a lambda phage clone containing the complete wild-type CYP2D6 gene, exhibits enhanced metabolism and disposition of debrisoquine. This transgenic mouse line and its wild-type control are models for human extensive metabolizers and poor metabolizers, respectively, and would have broad application in the study of CYP2D6 polymorphism in drug discovery and development, and in clinical practice toward individualized drug therapy. Endogenous 5-methoxyindole- thylamines derived from 5-hydroxytryptamine were identified as high-affinity substrates of CYP2D6 that catalyzes their O-demethylations with high enzymatic capacity and specificity. Thus, polymorphic CYP2D6 may play an important role in the interconversions of these psychoactive tryptamines, including a crucial step in a serotonin-melatonin cycle.

Relationship between Type A and B personality and debrisoquine hydroxylation capacity

AIM

A person with Type A personality is an 'aggressor' compared with the rarely harried Type B. Although debrisoquine hydroxylase (CYP2D6) capacity has been associated with personality, no study has specifically investigated its association with personality Type A and B. Therefore the aim of this research was to study the impact of CYP2D6 on Type A and B personality.

METHODS

Type A and B personality questionnaires were administered to 48 healthy patients undergoing elective orthopaedic surgery. After obtaining informed consent, patients were genotyped for the various CYP2D6 alleles by allele-specific polymerase chain reaction. Based on the genotypes, patients were grouped as extensive metabolizer (EM)1 (normal) (CYP2D6*1/*1), EM2 (intermediate) (CYP2D6*1/*4, CYP2D6*1/*5, CYP2D6*1/*9 and CYP2D6*1/*10) and EM3 (slow) (CYP2D6*4/*10, CYP2D6*5/*10, CYP2D6*10/*10 and CYP2D6*10/*17). Chi(2) was used to determine the relationship between the groups and personality types.

RESULTS

The percentages of patients who were of the EM1, EM2 and EM3 groups were 20.8%, 52.1% and 27.1%, respectively. There was a significant difference (P = 0.032) between the three groups in terms of personality type, in which EM1 showed a tendency to be of personality Type A while EM2 and EM3 tended to be of personality Type B.

CONCLUSION

The study suggests that there is a relationship between CYP2D6 activity and Type A and B personality.

Cytochrome P450 2D6: overview and update on pharmacology, genetics, biochemistry

Of about one dozen human P450 s that catalyze biotransformations of xenobiotics, CYP2D6 is one of the more important ones based on the number of its drug substrates. It shows a very high degree of interindividual variability, which is primarily due to the extensive genetic polymorphism that influences expression and function. This so-called debrisoquine/sparteine oxidation polymorphism has been extensively studied in many different populations and over 80 alleles and allele variants have been described. CYP2D6 protein and enzymatic activity is completely absent in less than 1% of Asian people and in up to 10% of Caucasians with two null alleles, which do not encode a functional P450 protein product. The resulting "poor metabolizer" (PM) phenotype is characterized by the inability to use CYP2D6-dependent metabolic pathways for drug elimination, which affect up to 20% of all clinically used drugs. The consequences are increased risk of adverse drug reactions or lack of therapeutic response. Today, genetic testing predicts the PM phenotype with over 99% certainty. At the other extreme, the "Ultrarapid Metabolizer" (UM) phenotype can be caused by alleles carrying multiple gene copies. "Intermediate Metabolizers" (IM) are severely deficient in their metabolism capacity compared to normal "Extensive Metabolizers" (EM), but in contrast to PMs they express a low amount of residual activity due to the presence of at least one partially deficient allele. Whereas the intricate genetics of the CYP2D6 polymorphism is becoming apparent at ever greater detail, applications in clinical practice are still rare. More clinical studies are needed to show where patients benefit from drug dose adjustment based on their genotype. Computational approaches are used to predict and rationalize substrate specificity and enzymatic properties of CYP2D6. Pharmacophore modeling of ligands and protein homology modeling are two complementary approaches that have been applied with some success. CYP2D6 is not only expressed in liver but also in the gut and in brain neurons, where endogenous substrates with high-turnover have been found. Whether and how brain functions may be influenced by polymorphic expression are interesting questions for the future.

Genetic polymorphism of CYP2D6 in Chinese subjects in Malaysia

BACKGROUND

Although Malaysian Chinese share an origin with the mainland Chinese, their evolution has been influenced by intermarriages. With a gene such as CYP2D6, which is highly polymorphic, it is expected that the Malaysian Chinese would exhibit a polymorphism profile different from those of the Chinese populations in other geographical locations.

OBJECTIVE

To study the genotype distribution of CYP2D6 among the Chinese people in Malaysia.

METHOD

We obtained DNA from 236 Chinese individuals in Malaysia and used PCR-based methods to identify any common CYP2D6 alleles.

RESULTS

A total of 236 subjects were enrolled and were successfully genotyped. Malaysian Chinese were relatively heterogeneous in terms of their CYP2D6 genotypes with nine genotypes recorded. CYP2D6*4, *5, *9, *10 and *17 were detected with the most common genotype being *1/*10. No subject had genotypes that predicted poor metabolic activity. However, 40% showed genotypes (e.g. CYP2D6*10/*10, *17, *4 and *9 and *9/*9) that predicted an intermediate metabolizer phenotype. Another subject carried the defective CYP2D6*17 allele and six carried the defective CYP2D6*9 allele. Both these alleles have not been reported in other earlier Chinese studies.

CONCLUSION

This study revealed that, in terms of CYP2D6 polymorphism, Malaysian Chinese were a heterogeneous group of people. Although sharing some similarities with other Orientals, they also seemed to have some notable differences. The alleles CYP2D6*4, *5, *9, *10 and *17 were all detected. CYP2D6*3 was however absent.

Standard CYP2D6 genotyping procedures fail for the CYP2D6*5 and duplication alleles when hair roots are used as a source of DNA

BACKGROUND

Hair roots provide a useful alternative to blood as a source of DNA for genotyping. Besides simple and non-invasive collections, the DNA extraction step is also easy to perform and is fast. The aim of our study is to determine if hair roots can be used to genotype all of the common CYP2D6 alleles for routine screening purposes.

METHOD

The study complies with the Declaration of Helsinki. After obtaining informed consents, both blood and hair samples were collected from 92 patients for genotyping of the CYP2D6 gene. PCR was used to detect the following mutations: CYP2D6*1, *3, *4, *5, *9, *10, *17 and duplication gene. The results were compared where hair roots and blood were used as templates for DNA respectively.

RESULTS

When blood was used as a source of DNA for genotyping, all of the investigated CYP2D6 alleles were successfully amplified. However, with hair roots, the genes with the larger fragment sizes: CYP2D6*5 and the duplication gene could not be amplified and the bands of other alleles investigated were faint when visualized under UV light.

CONCLUSIONS

DNA extraction from hair roots and leucocytes yielded similar results but the DNA extracted from hair roots did not allow successful amplification of the longer genes such as the CYP2D6*5 and the duplication gene.

High-throughput analysis of informative CYP2D6 compound haplotypes

We describe a high-throughput protocol for detecting key polymorphisms in the drug-metabolizing enzyme gene CYP2D6 and a number of linked microsatellites that is both fast and relatively inexpensive to perform. This approach employs GeneScan technology to enable a researcher to determine rapidly the status of seven simple nucleotide polymorphisms in CYP2D6 and also to assay repeat number variation at five closely linked dinucleotide microsatellite loci. The method requires only three PCRs and two GeneScan runs per sample. We anticipate that this will be of value to researchers in three different ways: (1) rapid discrimination of common CYP2D6 alleles, (2) high-resolution haplotyping for association studies involving chromosome 22q13.1 using microsatellite variation, and (3) generation of compound haplotypes for investigating the evolution of CYP2D6 variation. We also report compound haplotype frequencies for an Ashkenazi Jewish and a British sample.

Correlation of tramadol pharmacokinetics and CYP2D6*10 genotype in Malaysian subjects

The aim of the present study is to investigate the influence of the CYP2D6*10 allele on the disposition of tramadol hydrochloride in Malaysian subjects. A single dose of 100 mg tramadol was given intravenously to 30 healthy orthopaedic patients undergoing various elective surgeries. After having obtained written informed consents, patients were genotyped for CYP2D6*10: the most common CYP2D6 allele among Asians by means of allele-specific polymerase chain reaction. The presence of other mutations (CYP2D6*1, *3, *4, *5, *9 and *17) was also investigated. Tramadol was extracted from 1 ml serum with an n-hexane: ethylacetate combination (4:1) after alkalinisation with ammonia (pH 10.6). Serum concentrations were measured by means of high-performance liquid chromatography. The pharmacokinetics of tramadol was studied during the 24 h after the dose. As among other Asians, the allele frequency for CYP2D6*10 among Malaysians was high (0.43). Subjects who were homozygous for CYP2D6*10 had significantly (P=0.046) longer mean serum half-life of tramadol than subjects of the normal or the heterozygous group (Kruskal-Wallis test). When patients were screened for the presence of other alleles, the pharmacokinetic parameter values were better explained. CYP2D6 activity may play a main role in determining tramadol pharmacokinetics. The CYP2D6*10 allele particularly was associated with higher serum levels of tramadol compared with the CYP2D6*1 allele. However, genotyping for CYP2D6*10 alone is not sufficient to explain tramadol disposition.

Genetic polymorphism of enzymes involved in xenobiotic metabolism and the risk of lung cancer

Chronic inhalation of cigarette smoke is a major risk factor for the development of lung cancer. It has been suggested that genetic susceptibility may contribute to the risk, because only a small portion of smokers develops the disease. Several polymorphisms that involve the metabolic activation or detoxification of carcinogens derived from cigarette smoke have been found to be associated with lung cancer risk. Many studies have focused on the relation between the distribution of polymorphic variants of different forms of the metabolic enzymes and lung cancer susceptibility. In this respect two groups of genetic polymorphisms of enzymes involved in xenobiotic metabolism, cytochrome P450 (CYP) and glutathione S-transferases (GSTs), have been discussed.CYP multigene superfamily consists of 10 subfamilies (CYP1-CYP10). A positive association between development of lung cancer and the mutant homozygous genotype ofCYP1A1 gene has been reported in several Japanese populations but such an association has not been observed in either Caucasians or African-Americans. The relation betweenCYP2D6 and lung cancer remains conflicting and inconclusive. Several polymorphisms have been identified at theCYP2E1 locus. No definitive link between the polymorphisms ofCYP2E1 and the risk of lung cancer has, however, been identified. The role of otherCYP2 isoforms in lung carcinogenesis has not been sufficiently investigated.GSTs form a superfamily of genes consisting of five distinct families, namedGSTA, GSTM, GSTP, GSTT andGSTS. The role ofGSTM, GSTT1 orGSTP1 polymorphism in modifying the lung cancer risk may be more limited than has been so far anticipated.Although some genetic polymorphisms discussed here have not shown significant increases/decreases in risk, individuals with differing genotypes may have different susceptibilities to lung cancer. Hopefully, in future studies it will be possible to screen for lung cancer using specific biomarkers.

Genetic Polymorphism of Human Cytochrome P450 Involved in Drug Metabolism

Recent advances in human gene analysis promoted by the human genome project have brought us a massive amount of information. These data can be seen and analyzed by personal computer through individual Web sites. As a result, the best use of bioinformatic is essential for recent molecular biology research. Genetic polymorphism of drug-metabolizing enzymes influences individual drug efficacy and safety through the alteration of pharmacokinetics and disposition of drugs. Considerable amounts of data have now accumulated as allelic differences of various drug metabolizing enzymes. Current understanding of genotype information on cytochrome P450 is hereby summarized, based on the Web site for their use in individual optimization of drug therapy.

Functional evaluation of cytochrome P450 2D6 with Gly42Arg substitution expressed in Saccharomyces cerevisiae

A single amino acid-substituted mutant protein, CYP2D6 (G42R) was expressed in Saccharomyces cerevisiae and its enzymatic properties were compared with those of other single (P34S, R296C and S486T) and double amino acid-substituted mutant proteins (P34S/S486T and R296C/S486T) expressed in yeast cells, all of which were known to occur in the CYP2D6 gene as single nucleotide polymorphisms. The protein levels of G42R, P34S and P34S/S486T in microsomal fractions and their oxidation capacities towards debrisoquine as a prototypic substrate and bunitrolol as a chiral substrate were different from those of wild-type CYP2D6, while the R296C, S486T and R296C/S486T behaved similarly to the wild-type in these indices. The CYP contents both in yeast microsomal and in whole cell fractions indicated that some part of G42R protein was localized in the endoplasmic reticulum membrane fraction, whereas most of G42R protein was in some subcellular fractions other than endoplasmic reticulum. In kinetic analysis, the G42R substitution increased apparent Km and decreased Vmax for debrisoquine 4-hydroxylation, while it increased both Km and Vmax for bunitrolol 4-hydroxylation. The P34S substitution did not drastically change Km but decreased Vmax for debrisoquine 4-hydroxylation, whereas Km was increased and Vmax unchanged or decreased for bunitrolol 4-hydroxylation by P34S substitution. These results suggest that the G42R substitution causes a change in the CYP2D6 conformation, which may be different from the change produced by the P34S substitution.

Comprehensive analysis of the genetic factors determining expression and function of hepatic CYP2D6

Variable expression and function of the cytochrome P4502D6 (CYP2D6) leads to distinct phenotypes termed ultrarapid (UM), extensive (EM), intermediate (IM) and poor metabolizer (PM). Whereas the PM phenotype is known to be caused by two null-alleles leading to absence of functional CYP2D6 protein, the large variability among individuals with functional alleles remained largely unexplained. In this study, we systematically investigated 76 liver biopsies from individuals with known sparteine metabolic ratios (MRS) for the relationships between CYP2D6 genotype, microsomal protein expression, bufuralol 1'-hydroxylase activity and in-vivo phenotype. Average CYP2D6 protein levels ranged from undetectable in PMs (MRS > 20) to 2.6 +/- 2.7 pmol/mg microsomal protein in IMs (1.2 < MRS< 20), 7.6 +/- 4.7 in EMs (0.2 < MRS < 1.2) and 23.8 +/- 7.7 in UMs (MRS < 0.2), respectively. Analysis with respect to genotype demonstrated gradually increased expression and function for individuals with no, one, two or three functional gene copies per genome. The recently discovered -1584 C/G promoter polymorphism was identified as another major factor for expression and function with the mutant [-1584G] promoter type being consistently associated with significantly higher expression than [-1584C]. To investigate functional differences between the detected variant protein forms CYP2D6.1, 2D6.2, 2D6.9 and 2D6.10, we expressed them recombinantly in insect cells. The most significant difference was a decrease in the relative P450 holoprotein content of all allelic forms, including the common functional variant 2D6.2, in comparison to 2D6.1, whereas only modest Km changes were observed. Taken together, these data provide further insight into the complex mechanisms that govern the highly variable expression and function of CYP2D6.

Heterogeneity of the CYP2D6 gene among Malays in Malaysia

BACKGROUND

Although Malays shared an origin with Chinese, their evolution saw substantial divergences. Phenotyping studies suggested that they differed in CYP2D6 polymorphism, with higher PM prevalence but lesser right-shift for debrisoquine MRs.

OBJECTIVE

To study the genotype distribution of CYP2D6 among the Malays in Malaysia.

METHOD

We obtained DNA from 107 Malays and used PCR to determine common CYP2D6 alleles.

RESULT

CYP2D6*1 occurred at a frequency of 36.0%, duplicated gene, 0.93%, CYP2D6*4, 2.8%, CYP2D6*5, 5.1%, CYP2D6*9, 3.3%, CYP2D6*10, 49.5% and CYP2D6*17, 0.5%. The findings of CYP2D6*17 and CYP2D6*9 were novel for Asia. The frequency for CYP2D6*10 was lower than in other Asian races. The most frequent genotypes were CYP2D6*1/*10 at 39.3%. Two subjects had genotypes that predicted PM phenotype, 35% showed genotypes that predicted intermediate metabolizers and one subject had a genotype that predicted ultra-rapid metabolism.

CONCLUSION

The genetic polymorphism of CYP2D6 in Malays is different from Chinese and Far Eastern races. They may be intermediate between East Asians and Caucasians in CYP2D6 activity. Further study in relation to the evolution of races and disease prevalence may help to identify the contributions of the polymorphism in alleged susceptibility to diseases apart from delineating its contributions to ethnic differences in the pharmacology of CYP2D6 drugs.

Characterization of (+/-)-bufuralol hydroxylation activities in liver microsomes of Japanese and Caucasian subjects genotyped for CYP2D6

Twenty-four genetic polymorphisms in the CYP2D6 gene were analysed in liver DNA samples of 39 Japanese and 44 Caucasians and compared with CYP2D6 protein levels and bufuralol 1'- and 6-hydroxylation activities in liver microsomes of these human samples. We detected 13 types of CYP2D6 genetic polymorphisms and classified these into 20 genotypes; nine types were found in Japanese and 14 types in Caucasian samples. CYP2D6*10B, but not CYP2D6*10A, was the most frequent (34.6%) in Japanese. In Caucasians, several CYP2D6 polymorphisms including CYP2D6*4, *4D, *4E, *4L, *3, *9, *5 and *2E (frequencies of 6.8, 3.4, 4.5, 9.1, 1.1, 2.3, 2.3 and 4.5%, respectively) were detected. A Caucasian having CYP2D6*3/*5 had a protein with slower gel mobility (immunoblotting with anti-CYP2D6 antibody) and very low activity for bufuralol 1'-hydroxylation. Five Caucasian samples (CYP2D6*4/*4, *4/*4L, or *4D/*4L) had no measurable CYP2D6 protein and very low bufuralol 1'-hydroxylation activities. Seven Japanese subjects with CYP2D6*10B/*10B had CYP2D6 protein at levels of approximately 20% of those present in humans with CYP2D6*1 and *2 and catalysed bufuralol 1'-hydroxylation at low rates. Kinetic analysis of bufuralol 1'- and 6-hydroxylation indicates that (i) the Km values for 1'-hydroxylation were lower in individuals with CYP2D6*1/*1, *1/*2, *1/*2X2, and *2/*2 than those with CYP2D6*4/*4, *4/*4L, *4D/*4L, or *10B/*10B and Vmax values tended to be higher in the former groups (*1, *2), and (ii) individuals with heterozygous CYP2D6*1/*4D, *1/*4L, and *1/*5 had relatively high Vmax/Km ratios, whereas individuals with heterozygous CYP2D6*1/*9, *2/4D, *2/*5, *2/*10B, *2E/*4E, *3/*5, *4L/*9, and *10B/*39 had lower Vmax/Km ratios for bufuralol 1'-hydroxylation. Quinidine inhibited bufuralol 1'-hydroxylation in liver microsomes, particularly at low substrate concentrations, in individuals with CYP2D6*1/*1, and 1/1*2, but not those with CYP2D6*4/*4 and very slightly in individuals with CYP2D6*10B/*10B. The latter two groups were found to be more sensitive to alpha-naphthoflavone than the former groups, indicative of the contribution of CYP1A2. These results support the view that CYP2D6*3, *4, *4D, and *4L are major genotypes producing poor metabolizer phenotypes in CYP2D6 in Caucasians, whereas CYP2D6*10B is a major factor in decreased CYP2D6 protein expression and catalytic activities in Japanese.

Mimicking gene defects to treat drug dependence

The genetic basis for drug dependence has focused on genes that encode receptors involved in the reinforcing properties of drugs of abuse or that determine drug-taking behavior (e.g. impulsivity, etc.). Pharmacogenetic variations in the patterns of metabolism among individuals can also importantly modulate the risk of drug dependence. Cytochrome P450 drug metabolizing enzymes (CYPs), can activate (e.g. codeine to morphine) or deactivate (e.g. nicotine to cotinine) drugs of abuse. Some CYPs are polymorphic, that is, there are gene mutations which result in individuals with no (null mutations) or decreased enzyme activity (e.g. CYP2D6*10). Individuals with two null mutations appear in the population as phenotypic poor metabolizers. Using in vitro studies, we have identified drugs of abuse that are substrates of the polymorphic enzymes CYP2D6 (codeine, amphetamines, dextromethorphan), CYP2A6 (nicotine) and CYP2C19 (flunitrazepam). In human experimental studies, we have shown that CYP phenotype and genotype affect abuse liability of CYP2D6 metabolized drugs of abuse. In addition, we inhibited CYP2D6 and decreased individuals' risk of dependence experimentally (codeine, dextromethorphan) and treated codeine dependence. In epidemiologic studies CYP2D6 and CYP2A6 null mutations protect individuals from becoming codeine and tobacco dependent, respectively. With respect to CYP2A6, individuals with mutations, smoke fewer cigarettes and can quit more easily. Inhibiting CYP2A6 (e.g. tranylcypromine, methoxsalen) decreases smoking and the activation of procarcinogens. By mimicking these gene defects the risk of dependence can be decreased in individuals and new treatments developed.

Pharmacokinetic interaction of fluvoxamine and thioridazine in schizophrenic patients

This study investigated to what extent fluvoxamine affects the pharmacokinetics of thioridazine (THD) in schizophrenic patients under steady-state conditions. Concentrations of THD, mesoridazine, and sulforidazine were measured in plasma samples obtained from 10 male inpatients, aged 36 to 78 years, at three different time points: A, during habitual monotherapy with THD at 88 +/-54 mg/day; B, after addition of a low dosage of fluvoxamine (25 mg twice a day) for 1 week; and C, 2 weeks after fluvoxamine discontinuation. After the addition of fluvoxamine, THD concentrations relative to time point A significantly increased approximately threefold from 0.40 to 1.21 micromol/L (225%) (p < 0.002), mesoridazine concentrations increased from 0.65 to 2.0 micromol/L (219%) (p < 0.004), and sulforidazine levels increased from 0.21 to 0.56 micromol/L (258%) (p < 0.004). The THD-mesoridazine and THD-sulforidazine ratios remained unchanged during the study. Mean plasma THD, mesoridazine, and sulforidazine levels decreased at time point C, but despite fluvoxamine discontinuation for 2 weeks, three patients continued to exhibit elevated concentrations of THD and its metabolites. In conclusion, fluvoxamine markedly interferes with the metabolism of THD, probably at the CYP2C19 and/or CYP1A2 enzyme level. Therefore, clinicians should be aware of the potential for a clinical drug interaction between both compounds, and careful monitoring of THD levels is valuable to prevent the accumulation of the drug and resulting toxicity.

Cytochrome P450 enzyme system: genetic polymorphisms and impact on clinical pharmacology

The cytochrome P450 (CYP) enzyme system is involved in the metabolism and elimination of numerous widely used drugs. The capacity of this system varies from one person to another, leading to variable drug excretion rates and intersubject differences in the final serum drug concentrations. For this reason, therapeutic response and side-effects vary widely between patients treated with the same dose of drug. The intersubject variability in metabolic rate is largely determined by genetic factors. Some CYP enzymes, including CYP2D6 and CYP2C19, are genetically polymorphic. Several mutant alleles have been described, Environmental factors such as smoking, diet and co-administration of medications might also influence the CYP enzyme activity. By the use of genotyping or phenotyping methods every individual can be classified as either a poor, an intermediate, an extensive or an ultrarapid metabolizer. If this could be performed prior to drug therapy, the knowledge could be applied to drug selection and dose adjustment in order to reach therapeutic serum drug levels.

Update: genetic polymorphism of drug metabolizing enzymes in humans

The cytochrome P450 (P450 or CYP) monooxygenases, CYP2D6, CYP2C19, CYP2E1 and CYP2C9, and non-P450 monooxygenases, N-acetyltransferase, thioprine methyltransferases and dihydropyrimidine dehydrogenase, all display polymorphism. CYP2D6 and CYP2C19 have been studied extensively and, despite their low abundance in the liver, they have been found to catalyse the metabolism of many drugs. CYP2D6 has many allelic variants, whereas CYP2C19 has only two. Most variants are translated into inactive, truncated proteins or fail to express protein. There is, as yet, no clear information about CYP2E1 polymorphism. In addition, genetic differences in certain foreign-compound metabolizing enzymes, such as Phase II enzymes, have been shown to be associated with an increased risk of developing environmentally and occupationally related diseases such as cancer. When two drugs that are substrates of a polymorphic CYP enzyme are administered concomitantly during drug therapy, each will compete for that enzyme and competitively inhibit the metabolism of the other substrate. This can result in toxicity. Patients who are poor metabolizers (PMs), extensive metabolizers (EMs) and ultrarapid metabolizers (URMs) can be identified. Having such information will help in determining the appropriate dosage of certain drugs when treating patients with an inherited abnormality of a drug-metabolizing enzyme. In view of the remarkable progress in this particular field, it is to be expected that more genetic polymorphisms will be discovered in the near future.

Absence of effect of seven functional mutations in the CYP2D6 gene in Parkinson's disease

The reduction or loss of cytochrome P450 enzyme activity as a result of mutations in the CYP2D6 gene has been suggested as a risk factor for Parkinson's disease (PD). Conflicting results among reported studies of the prevalence of mutations among patients with PD suggested a more comprehensive genotyping and an analysis of the interactions with other suspected risk factors and family history. We determined the frequency of seven CYP2D6 mutations among 109 patients with PD and 110 control subjects. Family history of PD, age of onset, exposure to pesticides or herbicides, and well-water consumption were obtained for all cases. There was no significant difference in frequency between patients with PD and control subjects for any mutant allele and no significant association with family history, onset age, or environmental exposures. We sought to increase the power of our study by combining reports from the literature, choosing allele frequencies as the most informative measure. Although we found variability in reported allele frequencies for control subjects that made a meta-analysis problematic, summing all reports demonstrated no difference in CYP2D6 mutation frequency between patients with PD and control subjects. This comprehensive study of CYP2D6 mutations demonstrates that other genes or shared environmental exposures account for the familial risk of PD.

Genetic polymorphisms of human N-acetyltransferase, cytochrome P450, glutathione-S-transferase, and epoxide hydrolase enzymes: relevance to xenobiotic metabolism and toxicity

In this review, an overview is presented of the current knowledge of genetic polymorphisms of four of the most important enzyme families involved in the metabolism of xenobiotics, that is, the N-acetyltransferase (NAT), cytochrome P450 (P450), glutathione-S-transferase (GST), and microsomal epoxide hydrolase (mEH) enzymes. The emphasis is on two main topics, the molecular genetics of the polymorphisms and the consequences for xenobiotic metabolism and toxicity. Studies are described in which wild-type and mutant alleles of biotransformation enzymes have been expressed in heterologous systems to study the molecular genetics and the metabolism and pharmacological or toxicological effects of xenobiotics. Furthermore, studies are described that have investigated the effects of genetic polymorphisms of biotransformation enzymes on the metabolism of drugs in humans and on the metabolism of genotoxic compounds in vivo as well. The effects of the polymorphisms are highly dependent on the enzyme systems involved and the compounds being metabolized. Several polymorphisms are described that also clearly influence the metabolism and effects of drugs and toxic compounds, in vivo in humans. Future perspectives in studies on genetic polymorphisms of biotransformation enzymes are also discussed. It is concluded that genetic polymorphisms of biotransformation enzymes are in a number of cases a major factor involved in the interindividual variability in xenobiotic metabolism and toxicity. This may lead to interindividual variability in efficacy of drugs and disease susceptibility.

Expression in human prostate of drug- and carcinogen-metabolizing enzymes: association with prostate cancer risk

The role of two common polymorphisms of enzymes involved in the metabolism of drugs and carcinogens was studied in relation to prostate cancer. The gene encoding one of these enzymes (NAT2) is located in an area where frequent allelic loss occurs in prostate cancer. Mutations at the genes CYP2D6 and NAT2 were analysed by allele-specific polymerase chain reaction and restriction mapping in DNA from 94 subjects with prostate cancer and 160 male healthy control subjects. Eleven prostate specimens were analysed for genotype and enzymatic activities NAT2, CYP2D6 and CYP3A by using the enzyme-specific substrates sulphamethazine and dextromethorphan. Enzyme activities with substrate specificities corresponding to NAT2, CYP2D6 and CYP3A are present in human prostate tissue, with mean +/-s.d. activities of 4.8+/-4.4 pmol min(-1) mg(-1) protein, 156+/-91 and 112+/-72 nmol min(-1) mg(-1) protein respectively. The Km values for the prostate CYP2D6 and CYP3A enzyme activities corresponded to that of liver CYP2D6 and CYP3A activities, and the CYP2D6 enzyme activity is related to the CYP2D6 genotype. The N-acetyltransferase, in contrast, had a higher Km than NAT2 and was independent of the NAT2 genotype. The CYP2D6 and CYP3A enzymes, and an N-acetyltransferase activity that is independent of the regulation of the NAT2 gene, are expressed in human prostate tissue. The presence of carcinogen-metabolizing enzymes in human prostate with a high interindividual variability may be involved in the regulation of local levels of carcinogens and mutagens and may underlie interindividual differences in cancer susceptibility.

Rarity of debrisoquine hydroxylase gene polymorphism in Chinese patients with Parkinson's disease

Impaired debrisoquine metabolism resulting from defects in the cytochrome P450 CYP2D6-debrisoquine hydroxylase gene has been shown to be associated with the development of Parkinson's disease (PD). We studied two polymorphisms in this gene in 207 Chinese PD patients and 227 control subjects by polymerase chain reaction and restriction analysis. The G to A substitution at position 1934 in the junction of intron 3/exon 4 was detected in one sporadic PD patient and two control subjects, all of whom were heterozygous. The single base deletion at position 2637 in exon 5 was not detected in any of the study subjects. Such rarity of CYP2D6 polymorphism indicates PD in the Chinese population is associated with some other gene defects.