A missense mutation in exon 6 of the CYP2D6 gene leading to a histidine 324 to proline exchange is associated with the poor metabolizer phenotype of sparteine

Molecular Mechanisms Underlying Single Nucleotide Polymorphism-Induced Reactivity Decrease in CYP2D6

Cytochrome P450 2D6 (CYP2D6) is one of the most important enzymes involved in drug metabolism. Genetic polymorphism can influence drug metabolism by CYP2D6 such that a therapy is seriously affected by under- or overdosing of drugs. However, a general explanation at the atomistic level for poor activity is missing so far. Here we show for the 20 most common single nucleotide polymorphisms (SNPs) of CYP2D6 that poor metabolism is driven by four mechanisms. We found in extensive all-atom molecular dynamics simulations that the rigidity of the I-helix (central helix), distance between central phenylalanines (stabilizing bound substrate), availability of basic residues on the surface of CYP2D6 (binding of cytochrome P450 reductase), and position of arginine 132 (electron transfer to heme) are essential for an extensive function of the enzyme. These results were applied to SNPs with unknown effects, and potential SNPs that may lead to poor drug metabolism were identified. The revealed molecular mechanisms might be important for other drug-metabolizing cytochrome P450 enzymes.

The genomic landscape of CYP2D6 variation in the Indian population

Aim: The CYP2D6 gene is highly polymorphic, causing large interindividual variability in the metabolism of several clinically important drugs. Materials & methods: The authors investigated the diversity and distribution of CYP2D6 alleles in Indians using whole genome sequences (N = 1518). Functional consequences were assessed using pathogenicity scores and molecular dynamics simulations. Results: The analysis revealed population-specific CYP2D6 alleles (*86, *7, *111, *112, *113, *99) and remarkable differences in variant and phenotype frequencies with global populations. The authors observed that one in three Indians could benefit from a dose alteration for psychiatric drugs with accurate CYP2D6 phenotyping. Molecular dynamics simulations revealed large conformational fluctuations, confirming the predicted reduced function of *86 and *113 alleles. Conclusion: The findings emphasize the utility of comprehensive CYP2D6 profiling for aiding precision public health.

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 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.

Evaluating the impact of missenses mutations in CYP2D6*7 and CYP2D6*14A: does it compromise tamoxifen metabolism?

CYP2D6 is a high polymorphic enzyme from P450, responsible for metabolizing almost 25% of drugs. The distribution of different mutations among CYP2D6 alleles has been associated with poor, intermediate, extensive and ultra-metabolizers.

AIM

To evaluate how missenses mutations in CYP2D6*7 and CYP2D6*14A poor metabolizer alleles affect CYP2D6 stability and function.

MATERIALS & METHODS

CYPalleles database was used to collect polymorphisms data present in 105 alleles. We selected only poor metabolizers alleles that presented exclusively missenses mutations. They were analyzed through seven algorithms to predict the impact on CYP2D6 structure and function.

RESULTS

H324P, the unique mutation in CYP2D6*7, has high impact in enzyme function due to its occurrence between two alpha-helixes involved in active site dynamics. G169R, a mutation that occurs only in CYP2D6*14A, leads to the gain of solvent accessibility and severe protein destabilization.

CONCLUSION

Our in silico analysis showed that missenses mutations in CYP2D6*7 and CYP2D6*14A cause CYP2D6 dysfunction.

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.

Evolution of detoxifying systems: the role of environment and population history in shaping genetic diversity at human CYP2D6 locus

OBJECTIVE

The transition from food collection to food production (FP) modified the nature of selective pressures, and several studies illustrate that genetic adaptation to new lifestyle has occurred in humans since the agricultural revolution. Here we test the hypothesis that high levels of genetic variation at CYP2D6, a locus coding for a detoxifying enzyme of the cytochrome P450 complex, reflect this change.

METHODS

We compared DNA sequences and predicted the levels of enzyme activity across 10 African, Asian and European populations, six of which currently rely on hunting and gathering (HG) while four on food production (FP).

RESULTS AND CONCLUSION

HG and FP showed similar levels of CYP2D6 diversity, but displayed different substitution patterns at coding DNA sites possibly related to selective differences. Comparison with variation at presumably neutral independent loci confirmed this finding, despite the confounding effects of population history, resulting in higher overall variation in Africans than in Eurasians. The differences between HG and FP populations suggest that new lifestyle and dietary habits acquired in the transition to agriculture affected the variation pattern at CYP2D6, leading to an increase in FP populations of the frequency of alleles that are associated with a slower rate of metabolism. These alleles reached a balanced co-existence with other important and previously selected variants. We suggest that the pronounced substrate-dependent activity of most of these enzymes expanded the spectrum of the metabolic response.

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.

Functional analysis of CYP2D6.31 variant: Homology modeling suggests possible disruption of redox partner interaction by Arg440His substitution

Cytochrome P450 2D6 (CYP2D6) is an important human drug-metabolizing enzyme that exhibits a marked genetic polymorphism. Numerous CYP2D6 alleles have been characterized at a functional level, although the consequences for expression and/or catalytic activity of a substantial number of rare variants remain to be investigated. One such allele, CYP2D6*31, is characterized by mutations encoding three amino acid substitutions: Arg296Cys, Arg440His and Ser486Thr. The identification of this allele in an individual with an apparent in vivo poor metabolizer phenotype prompted us to analyze the functional consequence of these substitutions on enzyme activity using yeast as a heterologous expression system. We demonstrated that the Arg440His substitution, alone or in combination with Arg296Cys and/or Ser486Thr, altered the respective kinetic parameters [Km (microM) and kcat (min(-1))] of debrisoquine 4-hydroxylation (wild-type, 25; 0.92; variants, 43-68; 0.05-0.11) and dextromethorphan O-demethylation (wild-type, 1; 4.72; variants, 12-23; 0.64-1.43), such that their specificity constants (kcat/Km) were decreased by more than 95% compared to those observed with the wild-type enzyme. The rates of oxidation of rac-metoprolol at single substrate concentrations of 40 and 400 microM were also markedly decreased by approximately 90% with each CYP2D6 variant containing the Arg440His substitution. These in vitro data confirm that the CYP2D6*31 allele encodes an enzyme with a severely impaired but residual catalytic activity and, furthermore, that the Arg440His exchange alone is the inactivating mutation. A homology model of CYP2D6 based on the crystal structure of rabbit CYP2C5 locates Arg440 on the proximal surface of the protein. Docking the structure of the FMN domain of human cytochrome P450 reductase to the CYP2D6 model suggests that Arg440 is a key member of a cluster of basic amino acid residues important for reductase binding.

Effects of Ciprofloxacin on the Stereoselective Disposition of Mexiletine in Man

Mexiletine is extensively metabolized in man, with less than 10% of the dose being excreted unchanged in urine. Clinical drug-drug interaction studies as well as in vitro drug metabolism studies suggest that CYP1A2, in addition to CYP2D6, is involved in the metabolism of mexiletine in man. Therefore, the objective of the study was to determine whether potential inhibition of CYP1A2 by the quinolone antibiotic agent ciprofloxacin would alter the stereoselective disposition of mexiletine. Nineteen healthy men (10 smokers and 9 nonsmokers) received a single 200-mg oral dose of racemic mexiletine hydrochloride on 2 occasions: once alone and once during concomitant administration of ciprofloxacin 750 mg BID (starting 3 days before and up to 2 days after the administration of mexiletine). Serial blood and urine samples were collected for 48 hours, and pharmacokinetic parameters were derived. Total clearances of R-(-)- and S-(+)-mexiletine were 42% and 63% higher in smokers compared with nonsmokers (P < 0.05). This observation is in agreement with increased clearance of mexiletine under conditions of increased CYP1A2 activity. On the other hand, ciprofloxacin administration only marginally decreased R-(-)- and S-(+)-mexiletine clearances (2 to 5 L/h; P < 0.05) secondary to a decrease in mexiletine nonrenal clearance. In conclusion, the increase in mexiletine nonrenal clearance in smokers and its decrease during the combined administration of ciprofloxacin confirm the role of CYP1A2 in the overall clearance of the drug. Nevertheless, results obtained in this study suggest that no major drug interaction is to be expected during the concomitant administration of ciprofloxacin and mexiletine in patients.

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.

Diphenhydramine alters the disposition of venlafaxine through inhibition of CYP2D6 activity in humans

CYP2D6 is the major enzyme involved in the metabolism of venlafaxine. Subjects with a low CYP2D6 activity have increased plasma concentrations of venlafaxine that may predispose them to cardiovascular side effects. In vitro and in vivo studies showed that diphenhydramine, a nonprescription antihistamine, can inhibit CYP2D6 activity. Therefore, the authors investigated in this study a potential drug interaction between diphenhydramine and venlafaxine. Fifteen male volunteers, nine with the extensive metabolizer (EM) and six with the poor metabolizer (PM) phenotype of CYP2D6, received venlafaxine hydrochloride 18.75 mg orally every 12 hours for 48 hours on two occasions (1 week apart): once alone and once during the concomitant administration of diphenhydramine hydrochloride (50 mg every 12 hours). Blood and urine samples were collected for 12 hours under steady-state conditions. In EMs, diphenhydramine decreased venlafaxine oral clearance from 104+/-60 L/hr to 43+/-23 L/hr (mean +/- SD; p < 0.05) without any effect on renal clearance (4+/-1 L/hr during venlafaxine alone and 4+/-2 L/hr during venlafaxine plus diphenhydramine). In PMs, coadministration of diphenhydramine did not cause significant changes in oral clearance and partial metabolic clearances of venlafaxine to its various metabolites. Diphenhydramine disposition was only slightly affected by genetically determined low CYP2D6 activity or concomitant administration of venlafaxine. In conclusion, diphenhydramine, at therapeutic doses, inhibits CYP2D6-mediated metabolism of venlafaxine in humans. Clinically significant interactions could be encountered during the concomitant administration of diphenhydramine and other antidepressant or antipsychotic drugs that are substrates of CYP2D6.

Genotyping of CYP2D6 in Parkinson's disease

Parkinson's disease is characterized by progressive degradation of dopaminergic neurons. Cytochrome P450 CYP2D6 enzyme is one of the most investigated and highly polymorphic isoforms, which metabolizes many drugs and is also involved in the metabolism of dopamine. Using allele-specific multiplex PCR, we genotyped 186 subjects for CYP2D6 *3, *4, *6, *7, and *8 alleles in order to estimate allelic, genotype and predicted phenotype frequencies in the control and patient groups, and to investigate the possible statistical difference between Parkinson's disease patients (n=41) and healthy controls (n=145). Parkinson's disease patients were further divided into two subgroups according to Hoehn and Yahr staging of the disease (HY), i.e. groups with HY stage less than 2.5 (HY <2.5; n=27) and more than 2.5 (HY >2.5; n=14). A subgroup of Parkinson's disease patients exhibiting side effects such as "on-off" phenomenon and dyskinesia (both suggesting favorable response to therapy) were compared with a subgroup of patients showing no such response. The preliminary results of this study showed that only the prevalence of CYP2D6 *4 allele differed significantly between the PD patients and control group (20.7% vs. 11.0%; p=0.027; RR=2.1, 95%CI 1.113-3.994). In the HY >2.5 subgroup, the CYP2D6*4 allelic difference was even greater (25.0% vs. 11.0% in controls; p=0.062, RR=2.69, 95%CI 1.090-6.624). Genotype frequencies differed only in the HY >2.5 subgroup, however with a level of significance of p=0.095.

Effect of gender, sex hormones, time variables and physiological urinary pH on apparent CYP2D6 activity as assessed by metabolic ratios of marker substrates

The effects of gender, time variables, menstrual cycle phases, plasma sex hormone concentrations and physiologic urinary pH on CYP2D6 phenotyping were studied using two widely employed CYP2D6 probe drugs, namely dextromethorphan and metoprolol. Phenotyping on a single occasion of 150 young, healthy, drug-free women and men revealed that the dextromethorphan: dextrorphan metabolic ratio (MR) was significantly lower (P < 0.0001) in 56 female extensive metabolizers (0.008+/-0.021) compared to 86 male extensive metabolizers (0.020 +/-0.040). Urinary pH was a significant predictor of dextromethorphan: dextrorphan MRs in men and women (P < 0.001). Once-a-month phenotyping with dextromethorphan of 12 healthy young men (eight extensive metabolizers and four poor metabolizers) over a 1-year period, as well as every-other-day phenotyping with dextromethorphan of healthy, pre-menopausal women (10 extensive metabolizers and 2 poor metabolizers) during a complete menstrual cycle, did not follow a particular pattern and showed similar intrasubject variability ranging from 24.1% to 74.5% (mean 50.9%) in men and from 20.5% to 96.2% (mean 52.0%) in women, independent of the CYP2D6 phenotype (P = 0.342). Using metoprolol as a probe drug, considerable intrasubject variability (38.6+/- 12.0%) but no correlation between metoprolol: alpha-hydroxymetoprolol MRs and pre-ovulatory, ovulatory and luteal phases (mean +/- SD metoprolol: a-hydroxymetoprolol MRs: 1.086+/- 1.137 pre-ovulatory; 1.159+/-1.158 ovulatory and 1.002+/-1.405 luteal phase; P> 0.9) or 17beta-oestradiol, progesterone or testosterone plasma concentrations was observed. There was a significant inverse relationship between physiologic urinary pH and sequential dextromethorphan: dextrorphan MRs as well as metoprolol: alpha-hydroxymetoprolol MRs in men and women, with metabolic ratios varying up to six-fold with metoprolol and up to 20-fold with dextromethorphan (ANCOVA P < 0.001). We conclude that apparent CYP2D6 activity is highly variable, independent of menstrual cycle phases, sex hormones, time variables or phenotype. Up to 80% of the observed variability can be explained by variations of urinary pH within the physiological range. An apparent phenotype shift as a result of variations in urinary pH may be observed in individuals who have metabolic ratios close to the population antimode.

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.

CYP2D6 mutations and therapeutic outcome in schizophrenic patients

STUDY OBJECTIVE

To investigate whether a relationship exists between the most common known cytochrome P450 (CYP) isozyme 2D6 mutations and schizophrenia. Because most antipsychotic and antidepressant agents interact with CYP2D6, we also investigated clinical outcomes in schizophrenic poor metabolizers (PMs) and extensive metabolizers (EMs).

DESIGN

Prospective, observational study.

SETTING

Two psychiatric hospitals and a university-affiliated nonpsychiatric hospital.

SUBJECTS

Thirty-nine consecutive schizophrenic patients (POP 1), 89 schizophrenics of French Canadian origin (POP 2), and 384 healthy French Canadians (POP 3).

INTERVENTION

All study subjects were genotyped for CYP2D6 mutant alleles. POP 1 patients were evaluated before and after 21 or more days of treatment with antipsychotic drugs metabolized at least in part by CYP2D6.

MEASUREMENTS AND MAIN RESULTS

Whole blood was collected to determine CYP2D6 alleles *1, *3, *4, *5, *6, and *7 using standard restriction fragment length polymorphisms and polymerase chain reaction techniques. In comparison, CYP2D6 genotypes were determined in POP 2 and POP 3. Twenty-three (59.0%) of 39 patients in POP 1 were genotypically EM homozygotes, 15 (38.4%) were EM heterozygotes, and 1 (2.6%) was a PM. Similar genotype distributions were determined in POP 2 and in POP 3. Genotype distributions for all three populations were in Hardy-Weinberg equilibrium (p>0.05), and there was no significant difference among them (p=0.857). In POP 1, no differences were seen among genotypes in disease symptom severity, number and severity of adverse drug effects, or attitudes toward drug treatment at baseline and at the end of the study. In fact, all patients improved significantly during their hospital stay (all p<0.05), although independent of the CYP2D6 genotype.

CONCLUSION

Common CYP2D6 mutant alleles were not associated with schizophrenia or with disease symptoms, antipsychotic-related adverse effects, or attitudes toward treatment.

Polymorphic cytochromes P450 and drugs used in psychiatry

1. The cytochrome P450 monooxygenases, CYP2D6, CYP2C19, and CYP2C9, display polymorphism. CYP2D6 and CYP2C19 have been studied extensively, and despite their low abundance in the liver, they catalyze the metabolism of many drugs. 2. CYP2D6 has numerous allelic variants, whereas CYP2C19 has only two. Most variants are translated into inactive, truncated protein or fail to express protein. 3. CYP2C9 is expressed as the wild-type enzyme and has two variants, in each of which one amino acid residue has been replaced. 4. The nucleotide base sequences of the cDNAs of the three polymorphic genes and their variants have been determined, and the proteins derived from these genes have been characterized. 5. An absence of CYP2D6 and/or CYP2C19 in an individual produces a poor metabolizer (PM) of drugs that are substrates of these enzymes. 6. When two drugs that are substrates for a polymorphic CYP enzyme are administered concomitantly, each will compete for that enzyme and competitively inhibit the metabolism of the other substrate. This can result in toxicity. 7. Patients can be readily phenotyped or genotyped to determine their CYP2D6 or CYP2C19 enzymatic status. Poor metabolizers (PMs), extensive metabolizers (EMs), and ultrarapid metabolizers (URMs) can be identified. 8. Numerous substrates and inhibitors of CYP2D6, CYP2C19, and CYP2C9 are identified. 9. An individual's diet and age can influence CYP enzyme activity. 10. CYP2D6 polymorphism has been associated with the risk of onset of various illnesses, including cancer, schizophrenia, Parkinson's disease, Alzheimer's disease, and epilepsy.

Study of the association between cytochromes P450 2D6 and 2E1 genotypes and the risk of drug and chemical induced idiosyncratic aplastic anaemia

A genetic susceptibility to drug or chemical toxicity may provide a basis for an increased risk of idiosyncratic aplastic anaemia (AA). The cytochrome P450 enzymes are responsible for the metabolism of many drugs, some of which have been linked to AA. Mutations in the cytochrome P450 CYP2D6 gene result in absent or impaired enzyme activity in about 7% of Caucasians, whereas a specific mutation in the 5'-regulatory region of the CYP2E1 gene causes overexpression of the gene. We evaluated the frequency of allelic variants of CYP2D6 and CYP2E1 using allele-specific PCR amplification and restriction enzyme analysis of blood mononuclear cell DNA among 54 Caucasian AA patients. CYP2D6 and CYP2E1 were chosen because of the link between AA and the antipsychotic drug remoxipride (CYP2D6 substrate) and benzene (CYP2E1 substrate), respectively. Results were compared with 53 controls matched for age, sex and ethnicity. The percentage of AA patients homozygous for the CYP2D6*3, CYP2D6*4 alleles (poor metabolizer phenotype) and the CYP2E1 mutant allele (overexpression) was 0%, 4% and 0%, respectively, and the percentage of heterozygotes was 2%, 28% and 15%, respectively. For normal controls the corresponding results for homozygous mutants were 0%, 4% and 0% and for heterozygotes 4%, 25% and 6%, respectively. We concluded that there were no major differences in the frequencies of the genetic polymorphisms between this series of AA patients and controls, but due to the low number of cases with the poor metabolizer phenotype and those with a history of drug exposure, the power of the study was too low to disprove an interaction.

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.

Genetic polymorphism and Parkinson's disease in Taiwan: study of debrisoquine 4-hydroxylase (CYP2D6)

Debrisoquine 4-hydroxylase (CYP2D6) is one of the cytochrome P450 enzyme families that catalyze the breakdown of a variety of exogenous and endogenous compounds. Previous reports have suggested that genetic polymorphisms of debrisoquine 4-hydroxylase are associated with susceptibility to Parkinson's disease (PD) in Caucasians. To determine if CYP2D6 also confers susceptibility to PD in Chinese patients, we carried out a study of genetic association using three polymorphic markers of the CYP2D6 gene, 188C/T, 1934G/A (mutant B), and 4268G/C. No differences of allele or genotype frequencies of these three polymorphisms were detected upon comparison of primary PD patients (n=53) with normal controls (n=94). The 1934A allele (mutant B), which accounts for the majority of poor metabolizers in Caucasians, is extremely rare in Chinese. Our data do not support the suggestion that the CYP2D6 gene is related to PD susceptibility in Chinese.

P450 enzymes and Parkinson's disease: the story so far

Environmental or endogenous toxins may cause nigral cell death in Parkinson's disease (PD) as a result of genetic susceptibility conferred by altered expression of P450 enzymes. Attention over the last 10 years has focused on CYP2D6 polymorphisms and susceptibility to PD. This review summarizes reports arising from both phenotypic and genotypic studies involving CYP2D6 and PD. Phenotypic studies have failed to support a link between CYP2D6 and PD. The more powerful genetic studies initially indicated a link between CYP2D6B mutations and PD, but critical analysis of the literature and recent studies emerging from independent laboratories fail to confirm this. Mutations in CYP2D6B are also not implicated in familial PD. As yet, there is no conclusive evidence to suggest that CYP2D6 polymorphisms confer susceptibility to PD. Whether polymorphisms in other P450s (for example, CYP1A1 and CYP2E1) are implicated in PD remains to be established.

Effect of codeine on gastrointestinal motility in relation to CYP2D6 phenotype

BACKGROUND

Codeine is widely used as an analgesic and antitussive drug. The analgesic effect of codeine is mediated by its metabolite morphine, which is formed by the polymorphically expressed enzyme CYP2D6; therefore poor metabolizers have no analgesia after administration of codeine. Like other opiates, codeine causes a delay of gastric emptying and spastic constipation. It is not yet known whether the effect on gastrointestinal motility is mediated by codeine or its metabolite morphine.

METHODS

To test the hypothesis that the metabolite morphine is responsible for the effects of codeine on gastrointestinal motility, a randomized, double-blind, two-way crossover study was performed. The orocecal transit time was studied in five extensive and five poor metabolizers of sparteine with the sulfasalazine-sulfapyridine method, assuming that no effects are observed in poor metabolizers because negligible amounts of morphine are formed.

RESULTS

No differences of orocecal transit times were observed between extensive metabolizers and poor metabolizers after oral placebo administration. However, after oral codeine administration orocecal transit time was significantly prolonged in extensive metabolizer but not poor metabolizer subjects. All pharmacokinetic parameters of codeine showed no differences between extensive metabolizers and poor metabolizers. The pharmacokinetic parameters (mean +/- SD) of the metabolite morphine were significantly different between extensive metabolizer and poor metabolizer subjects (peak serum concentration, 13.9 +/- 10.5 versus 0.68 +/- 0.15 pmol/ml; area under the serum concentration-time curve, 27.8 +/- 16.0 versus 1.9 +/- 0.7 hr.pmol/ml; total amount of morphine excreted in urine, 0.160 +/- 0.036 versus 0.015 +/- 0.007 mumol).

CONCLUSIONS

Because the orocecal transit time prolongation after codeine administration was observed only in extensive metabolizers, the effect of codeine on gastrointestinal motility, like the analgesia, is mediated by its metabolite morphine.

Molecular mechanisms of genetic polymorphisms of drug metabolism

One of the major causes of interindividual variation of drug effects is genetic variation of drug metabolism. Genetic polymorphisms of drug-metabolizing enzymes give rise to distinct subgroups in the population that differ in their ability to perform certain drug biotransformation reactions. Polymorphisms are generated by mutations in the genes for these enzymes, which cause decreased, increased, or absent enzyme expression or activity by multiple molecular mechanisms. Moreover, the variant alleles exist in the population at relatively high frequency. Genetic polymorphisms have been described for most drug metabolizing enzymes. The molecular mechanisms of three polymorphisms are reviewed here. The acetylation polymorphism concerns the metabolism of a variety of arylamine and hydrazine drugs, as well as carcinogens by the cytosolic N-acetyltransferase NAT2. Seven mutations of the NAT2 gene that occur singly or in combination define numerous alleles associated with decreased function. The debrisoquine-sparteine polymorphism of drug oxidation affects the metabolism of more than 40 drugs. The poor metabolizer phenotype is caused by several "loss of function" alleles of the cytochrome P450 CYP2D6 gene. On the other hand, "ultrarapid" metabolizers are caused by duplication or amplification of an active CYP2D6 gene. Intermediate metabolizers are often heterozygotes or carry alleles with mutations that decrease enzyme activity only moderately. The mephenytoin polymorphism affects the metabolism of mephenytoin and several other drugs. Two mutant alleles of CYP2C19 have so far been identified to cause this polymorphism. These polymorphisms show recessive transmission of the poor or slow metabolizer phenotype, i.e. two mutant alleles define the genotype in these individuals. Simple DNA tests based on the primary mutations have been developed to predict the phenotype. Analysis of allele frequencies in different populations revealed major differences, thereby tracing the molecular history and evolution of these polymorphisms.

The cytochrome P450 2D6 (CYP2D6) enzyme polymorphism: screening costs and influence on clinical outcomes in psychiatry

OBJECTIVES

This study examined factors that affect cost, reliability, and the value of determining the cytochrome P450 2D6 (CYP2D6) polymorphism in clinical practice.

STUDY DESIGN

The method of deoxyribonucleic acid isolation, sample preparation, oligonucleotide primers, and polymerase chain reaction procedures were scrutinized for their effect on CYP2D6 genotyping efforts. The determination of the CYP2D6 A, B, D, E, and T alleles was used to identify the deficiency in CYP2D6 expression in 161 individuals phenotyped for CYP2D6 activity with dextromethorphan. The CYP2D6 genotype was assessed in 74 outpatients who had received diagnoses of depression. Eighteen of these patients were screened because of an adverse response to a tricyclic or antidepressant known or suspected to be a CYP2D6 substrate.

RESULTS

The CYP2D6 A, B, C, D, E, and T alleles could be detected in 13 hours at a cost of $84 per sample by judicious selection of conditions and procedures. The genotype provided an accurate predictor of CYP2D6 expression in all 134 subjects who expressed the enzyme and in all 27 unrelated individuals phenotyped as deficient in CYP2D6 activity. In the patient group that experienced adverse effects, 44% of all CYP2D6 gene copies contained the A, B, D, E, or T allele(s) associated with inactive CYP2D6 expression. This was more than twice the rate for the occurrence of mutant alleles in the other 56 psychiatric patients (21%) and in 80 random subjects from the general population (20%; p < 0.05).

CONCLUSIONS

Screening psychiatric patients for CYP2D6 expression may distinguish metabolic-based therapeutic problems from drug sensitivity caused by other mechanisms.

Influence of amino acid residue 374 of cytochrome P-450 2D6 (CYP2D6) on the regio- and enantio-selective metabolism of metoprolol

Cytochrome P-450 2D6 (CYP2D6) is an important human drug-metabolizing enzyme responsible for the oxidation of more than 30 widely used therapeutic agents. The enzymes encoded by the published genomic [Kimura, Umeno, Skoda, Meyer and Gonzalez (1989) Am. J. Hum. Genet. 45, 889-904] and cDNA [Gonzalez, Skoda, Kimura, Umeno, Zanger, Nebert, Gelboin, Hardwick and Meyer (1988) Nature 331, 442-446] sequences of CYP2D6, and presumed to represent wild-type sequences, differ at residue 374 and encode valine (CYP2D6-Val) and methionine (CYP2D6-Met) respectively. The influence of this amino acid difference on cytochrome P-450 expression, ligand binding, catalysis and stereoselective oxidation of metoprolol was investigated by the heterologous expression of the corresponding cDNAs in the yeast Saccharomyces cerevisiae. The level of expression of apo- and holo-protein was similar with each form of CYP2D6 cDNA, and the binding affinities of a series of ligands to CYP2D6-Val and CYP2D6-Met were identical. The enantioselective O-demethylation and alpha-hydroxylation of metoprolol were also similar with each form of CYP2D6, O-demethylation being R-(+)- enantioselective (CYP2D6-Val: R/S, 1.6; CYP2D6-Met: R/S, 1.4), whereas alpha-hydroxylation showed a preference for S-(-)-metoprolol (CYP2D6-Val: R/S, 0.7; CYP2D6-Met: R/S, 0.8). However, although the favoured regiomer overall was O-demethylmetoprolol (ODM), the regioselectivity for O-demethylation of each metoprolol enantiomer was significantly greater for CYP2D6-Val [R-(+)-: ODM/alpha-hydroxymetoprolol (alpha OH), 5.9; S-(-)-: ODM/alpha OH, 2.5) than that observed for CYP2D6-Met [R-(+)-: ODM/alpha OH, 2.2; S-(-)-: ODM/alpha OH, 1.4]. The stereoselective properties of CYP2D6-Val were consistent with those observed for CYP2D6 in human liver microsomes. The difference in the stereoselective properties of CYP2D6-Val and CYP2D6-Met were rationalized with respect to a homology model of the active site of CYP2D6 based on an alignment with the crystal structure of the bacterial cytochrome P-450BM-3' CYP102.

Steady-state plasma levels of nortriptyline and its 10-hydroxy metabolite: relationship to the CYP2D6 genotype

The relationship between the CYP2D6 genotype and the steady state plasma levels of nortriptyline (NT), its main active metabolite 10-hydroxynortriptyline (10-OH-NT) and the NT/10-OH-NT ratio were studied in 21 Caucasian depressed patients treated with 100-150 mg NT daily. The patients had participated in a previously published study investigating the role of NT and 10-OH-NT for the therapeutic effect of NT, and the plasma level data were from that study. In the present follow-up study, the patients were genotyped with respect to the polymorphic CYP2D6 by allele-specific PCR amplification and EcoRI RFLP. One poor metabolizer (PM) was identified and she had the highest plasma concentration of NT. Among the 20 extensive metabolizers (EM), the genotype (homozygous versus heterozygous EM) alone was not found to explain the variance in dose-corrected NT concentrations, but contributed significantly when gender was also taken into account. Together, these factors accounted for 59% of the variability in NT levels. Female patients had higher plasma levels of NT than male patients. 10-OH-NT levels were influenced by genotype, and NT/10-OH-NT ratio by genotype and gender. The present follow-up study confirms a relationship between the CYP2D6 genotype and the plasma levels of NT and its active metabolite. Identification of PM by genotyping should be of value for the prediction of the plasma levels and, consequently, the lower than average dose of NT required for optimal therapy. Also among EM, the genotype contributes to the variability in NT and 10-OH-NT levels but alone is of limited practical value for the prediction of optimal dosage.

Propafenone in a usual dose produces severe side-effects: the impact of genetically determined metabolic status on drug therapy

We report the case of an elderly lady presenting with dizziness, a head injury resulting from a fall and bradycardia. Propafenone 150 mg t.i.d. had been prescribed for atrial fibrillation with tachyarrhythmia, induced by hyperthyroidism, 18 months earlier. A toxic concentration of parent propafenone, and no 5-hydroxy metabolite, was detected in a plasma sample. Symptoms disappeared after the discontinuation of propafenone. The poor metaboliser (PM) phenotype of sparteine/debrisoquine was assumed and subsequently confirmed by phenotyping (sparteine test) and genotyping (allele-specific polymerase chain reaction). The PM phenotype is common in European populations, with a prevalence of about 7%. If drugs with narrow therapeutic ranges undergo genetically polymorphic metabolism, toxicity may arise even with recommended doses. Individualization of doses is required to avoid adverse effects.

Dihydrocodeine: a new opioid substrate for the polymorphic CYP2D6 in humans

BACKGROUND

The opioid dihydrocodeine (DHC) is frequently used as an analgesic and antitussive agent. However, until now there have been no detailed data on dihydrocodeine metabolism in humans. We therefore investigated pathways that contribute to elimination of dihydrocodeine, and we tested the hypothesis that dihydrocodeine O-demethylation to dihydromorphine (DHM) is catalyzed by the polymorphic CYP2D6.

METHODS

A single oral dose of dihydrocodeine was administered to six extensive (metabolic ratio [MR] < or = 1), two intermediate (1 < MR < 20) and six poor metabolizers (MR > or = 20) of sparteine/debrisoquin. Serum concentrations of dihydrocodeine and dihydromorphine were measured up to 25 hours, and urinary excretion of conjugated and unconjugated dihydrocodeine, dihydromorphine, and nordihydrocodeine were determined.

RESULTS

There were no differences in the pharmacokinetics of dihydrocodeine between extensive and poor metabolizers. However, the area under the serum concentration-time curve (AUC), partial metabolic clearance, and total urinary recovery of dihydromorphine were significantly lower in poor metabolizers (10.3 +/- 6.1 nmol.hr/L; 7.0 +/- 4.1 ml/min; 1.3% +/- 0.9% of dose) compared with extensive metabolizers (75.5 +/- 42.9 nmol.hr/L; 49.7 +/- 29.9 ml/min; 8.9% +/- 6.2%; p < 0.01). There was a strong correlation between the AUCDHC/AUCDHM ratio and the urinary metabolic ratio of sparteine (rS = 0.89, p = 0.001). No significant differences between extensive and poor metabolizers were detected in urine for conjugated dihydrocodeine (extensive metabolizers, 27.7% of dose; poor metabolizers, 31.5%), unconjugated dihydrocodeine (extensive metabolizers, 31.1%; poor metabolizers, 31.1%), conjugated nordihydrocodeine (extensive metabolizers, 6.3%; poor metabolizers, 5.4%), or unconjugated nordihydrocodeine (extensive metabolizers, 15.8%; poor metabolizers, 19.5%).

CONCLUSIONS

Dihydrocodeine O-demethylation to dihydromorphine is impaired in poor metabolizers of sparteine. The main urinary metabolites after administration of dihydrocodeine are the parent compound and its conjugates in extensive and poor metabolizers.

An unequal cross-over event within the CYP2D gene cluster generates a chimeric CYP2D7/CYP2D6 gene which is associated with the poor metabolizer phenotype

1. The study of the CYP2D genotype and phenotype of a Caucasian family revealed that a XbaI-9 kb allele was associated with the poor metabolizer phenotype. 2. A Polymerase Chain Reaction (PCR)-based assay showed that the previously described mutations D6A and D6B are not associated with the XbaI-9 kb allele. 3. To explore the molecular basis of the poor metabolizer phenotype associated with the XbaI-9 kb allele, complete sequencing of the nine exons and intron-exon boundaries of the CYP2D6 gene was undertaken after amplification by PCR. 4. All the exons were successfully amplified using CYP2D6 gene-specific primers except exon 1 which required a combination of CYP2D7 gene-specific 5' primer and a CYP2D6 gene-specific 3' primer. 5. Sequence data derived from this amplified product revealed that the XbaI-9 kb allele corresponds to a novel rearrangement of the locus. This involved a deletion of an approximately 20 kilobase (kb) DNA segment generating a hybrid 5' CYP2D7/CYP2D6 3' gene. 6. The chimeric gene is non-functional presumably due to an insertion in exon 1 (characteristic of the exon 1 of the CYP2D7 gene) which causes a shift in the reading frame with premature termination of translation.

Polymorphic drug metabolism in schizophrenic patients with tardive dyskinesia

The metabolism of many neuroleptics cosegregates catalyzed by the polymorphic cytochrome P450 CYP2D6. The population can be phenotyped into extensive metabolizers (EM) and poor metabolizers (PM) with respect to this enzyme's activity. PM are likely to achieve higher than average concentrations of neuroleptic drugs in plasma, with an increased risk of extrapyramidal side effects, possibly including tardive dyskinesia. Sixteen white schizophrenic patients who had developed tardive dyskinesia during long-term neuroleptic treatment were phenotyped with debrisoquine and genotyped by CYP2D6-specific DNA amplification and EcoRI restriction fragment length polymorphism analysis. Only 1 (6%) of the 16 patients had a PM genotype, 8 (50%) were homozygous, and 7 (44%) were heterozygous EM. None had a CYP2D6 genotype indicative of ultrarapid debrisoquine hydroxylation capacity. The patients were also phenotyped with mephenytoin, a probe drug for another polymorphic cytochrome P450, CYP2C19. One patient was a PM of S-mephenytoin, which corresponds to the frequency found in healthy white volunteers. In conclusion, there was no overrepresentation of PM of debrisoquine or of S-mephenytoin among the 16 patients with neuroleptic-induced tardive dyskinesia. However, the PM of debrisoquine had the highest score on the Simpson-Angus Rating Scale and the second highest on the Abnormal Involuntary Movement Scale, despite a very low neuroleptic dose. Also, the debrisoquine MR correlated significantly with the SARS score (rs = 0.685, p < 0.05, N = 10), indicating a relationship between the degree of impaired CYP2D5 activity and the severity of extrapyramidal side effects during neuroleptic treatment.