Association between cytochrome P4502D6 (CYP2D6) genotype, antipsychotic exposure, and abnormal involuntary movement scale (AIMS) score

Individualization of drug therapy in older people

Older people are high consumers of prescription drugs and are at increasing risk of polypharmacy and adverse reactions. Pharmacokinetic and pharmacodynamic modifications due to age and co-morbidities are an important consideration, but pharmacological background evidence to guide safe and effective therapeutic approaches are often inadequate since the older population is under-represented in clinical trials. We review the pharmacokinetic and pharmacodynamic changes that are characteristic in old age and consider evidence regarding potentially safer prescription and monitoring of drugs commonly used in older patients. We also introduce the possible role of pharmacogenomics and therapeutic drug monitoring as tools to guide the individualization of drug therapy.

Genetic variability of drug-metabolizing enzymes: the dual impact on psychiatric therapy and regulation of brain function

Polymorphic drug-metabolizing enzymes (DMEs) are responsible for the metabolism of the majority of psychotropic drugs. By explaining a large portion of variability in individual drug metabolism, pharmacogenetics offers a diagnostic tool in the burgeoning era of personalized medicine. This review updates existing evidence on the influence of pharmacogenetic variants on drug exposure and discusses the rationale for genetic testing in the clinical context. Dose adjustments based on pharmacogenetic knowledge are the first step to translate pharmacogenetics into clinical practice. However, also clinical factors, such as the consequences on toxicity and therapeutic failure, must be considered to provide clinical recommendations and assess the cost-effectiveness of pharmacogenetic treatment strategies. DME polymorphisms are relevant not only for clinical pharmacology and practice but also for research in psychiatry and neuroscience. Several DMEs, above all the cytochrome P (CYP) enzymes, are expressed in the brain, where they may contribute to the local biochemical homeostasis. Of particular interest is the possibility of DMEs playing a physiological role through their action on endogenous substrates, which may underlie the reported associations between genetic polymorphisms and cognitive function, personality and vulnerability to mental disorders. Neuroimaging studies have recently presented evidence of an effect of the CYP2D6 polymorphism on basic brain function. This review summarizes evidence on the effect of DME polymorphisms on brain function that adds to the well-known effects of DME polymorphisms on pharmacokinetics in explaining the range of phenotypes that are relevant to psychiatric practice.

Pharmacogenetics and schizophrenia

The wide interindividual variability in clinical response and tolerability of antipsychotic medications has led investigators to postulate that these variabilities may be under genetic control. Although not always consistent, there are promising indications from emergent pharmacogenetic studies that efficacy of antipsychotic medications for the various symptom domains of psychopathology in schizophrenia may be genetically regulated. This is an encouraging approach. Moreover, there are also suggestive findings that the side-effect profiles of second-generation antipsychotic medications and their propensity to cause weight gain and glucose and lipid abnormalities as well as tardive dyskinesia may be related to pharmacogenetic factors in this patient population. Ultimately, such approaches could drive choices of antipsychotic medication based on the likelihood of clinical response and development of side effects in light of a particular patient's genetic profile. In the future, this targeted approach (personalized medicine) may become informative for clinicians choosing an antipsychotic medication for an individual patient with schizophrenia.

Cytochrome P450 testing for prescribing antipsychotics in adults with schizophrenia: systematic review and meta-analyses

There is wide variability in the response of individuals to standard doses of antipsychotic drugs. It has been suggested that this may be partly explained by differences in the cytochrome P450 (CYP450) enzyme system responsible for metabolizing the drugs. We conducted a systematic review and meta-analyses to consider whether testing for CYP450 single nucleotide polymorphisms in adults starting antipsychotic treatment for schizophrenia predicts and leads to improvements in clinical outcomes. High analytic validity in terms of sensitivity and specificity was seen in studies reporting P450 testing. However, there was limited evidence of the role of CYP2D6 polymorphisms in antipsychotic efficacy, although there was an association between CYP2D6 genotype and extrapyramidal adverse effects. No studies reported on the prospective use of CYP2D6 genotyping tests in clinical practice. In conclusion, evidence of clinical validity and utility of CYP2D6 testing in patients being prescribed antipsychotics is lacking, and thus, routine pharmacogenetic testing prior to antipsychotic prescription cannot be supported at present. Further research is required to improve the evidence base and to generate data on clinical validity and clinical utility.

Genetic association of BDNF val66met and GSK-3beta-50T/C polymorphisms with tardive dyskinesia

AIMS

Neurodegenerative processes may be involved in the pathogenesis of tardive dyskinesia (TD), and a growing body of evidence suggests that brain-derived neurotrophic factor (BDNF) plays a role in both the antipsychotic effects and the pathogenesis of TD. BDNF and glycogen synthase kinase (GSK)-3beta are important in neuronal survival, and thus abnormal regulation of BDNF and GSK-3beta may contribute to TD pathophysiology. This study investigated the relationship between two polymorphisms, val66met in the BDNF coding region and -50T/C in the GSK-3beta promoter, and susceptibility to TD among a matched sample of patients having schizophrenia with TD (n = 83), patients with schizophrenia without TD (n = 78), and normal control subjects (n = 93).

METHODS

All subjects were Korean. The BDNF val66met and GSK-3beta-50T/C genotypes were determined by polymerase chain reaction and restriction fragment length polymorphism analyses.

RESULTS

Polymerase chain reaction analysis revealed no significant difference in the occurrence of the polymorphisms among the TD, non-TD, and control subjects, but a significant interaction was observed among the groups possessing BDNF val allele in compound genotypes (P = 0.001). We found that the schizophrenic subjects with the C/C GSK-3beta genotype, who carry the val allele of the BDNF gene, are expected to have a decreased risk of developing neuroleptic-induced tardive dyskinesia (P < 0.001).

CONCLUSIONS

Our results demonstrate that the GSK-3beta C/C genotype with the BDNF val allele is associated with patients having schizophrenia without TD. This study also suggests that the BDNF and GSK-3beta gene polymorphisms work in combination, but not individually, in predisposing patients with schizophrenia to TD.

Identification of CYP2D6 null variants among long-stay, chronic psychiatric inpatients: is it strictly necessary?

We identified the null variants *3,*4,*5,*6,*7 and *8 of the CYP2D6 gene [encoding for cytochrome P450 (debrisoquine hydroxylase)] in a group of 84 chronic-stay psychiatric inpatients with severe schizophrenia or related disorders and receiving treatment with one or more CYP2D6 substrates for years. We also studied a group of 100 healthy controls of similar ethnic origin (Spanish Caucasians). Three patients were poor metabolizers (PMs) for antipsychotic drugs according to their CYP2D6 genotype (i.e. homozygous for the *4 allele) but they exhibited no adverse drug reaction over the years despite chronic treatment with CYP2D6 substrates. We suggest that CYP2D6 genetic screening is more useful in other type of psychiatric patients, particularly in younger ones starting treatment protocols.

Pharmacogenetics of antipsychotic adverse effects: Case studies and a literature review for clinicians

There is a growing body of literature supporting the contribution of genetic variability to the mechanisms responsible for the adverse effects of antipsychotic medications particularly movement disorders and weight gain. Despite the current gap between research studies and the practical tools available to the clinician to identify such risks, it is hoped that in the foreseeable future, pharmacogenetics will become a critical aid to guide the development of personalized therapeutic regimes with fewer adverse effects. We provide a summary of two cases that are examples of using cytochrome P450 pharmacogenetics in an attempt to guide treatment in the context of recent literature concerning the role of pharmacogenetics in the manifestation of adverse effects of antipsychotic therapies. These examples and the review of recent literature on pharmacogenetics of antipsychotic adverse effects illustrate the potential for applying the principles of predictive, preventive, and personalized medicine to the therapy of psychotic disorders.

Review article: The prevalence and clinical relevance of cytochrome P450 polymorphisms

BACKGROUND

Most drugs currently used in clinical practice are effective in only 25% to 60% of patients, while adverse drug reactions (ADRs) as a consequence of treatment are estimated to cost billions of US dollars and tens of thousands of deaths.

AIM

To review the prevalence and clinical significance of cytochrome P450 polymorphisms.

RESULTS

The cytochrome P450 enzyme families 1-3 are responsible for 70 to 80% of all phase I dependent drug metabolisms. In 90% metabolic activity dependents on six enzymes: CYP1A2, CYP3A, CYP2C9, CYP2C19, CYP2D6 and CYP2E1. Polymorphisms in the CYP450 gene can influence metabolic activity of the subsequent enzymes. A poor metabolizer (PM) has no or very poor enzyme activity. A consequence of PM is drug toxicity if no other metabolic route is available, or when multiple drugs are metabolized by the same cytochrome. In that case dose reduction is an option to prevent toxic effects.

CONCLUSIONS

In the future genotyping should be considered to identify patients who might be at risk of severe toxic responses, in order to guide appropriate individual dosage. Medical therapy should be a close cooperation between clinicians, pharmacologists and laboratory specialists, leading to reduced therapeutic errors, ADRs and health care costs.

Pharmacogenetics and schizophrenia

Emergent pharmacogenetic studies indicate that the efficacy of antipsychotic medications in schizophrenia may be predicted through genetic analysis. There also is evidence that the side-effect profiles of second-generation antipsychotic medications and their propensity to cause weight gain, glucose and lipid abnormalities, and tardive dyskinesia may be predicted by pharmacogenetic analysis in this patient population. In the future, this targeted approach with the choice of antipsychotic medication based on the likelihood of clinical response and development of side effects in light of a particular patient's genetic status may gain hold as new treatments are developed with even fewer side effects.

Effects of CYP2D6 polymorphisms on neuroleptic malignant syndrome

OBJECTIVE

Neuroleptic malignant syndrome (NMS) is one of the most serious adverse reactions to antipsychotic medications. We accumulated data on Japanese NMS patients and, in a study designed to examine the effects of drug metabolism on the occurrence of NMS, tested the possibility of association between NMS and CYP2D6 polymorphisms.

METHODS

We studied 53 patients who had experienced NMS and 112 healthy individuals. We determined what drugs the patients with NMS had been given and retrospectively identified candidates for drugs causing NMS. We screened the prevalence of CYP2D6 genotypes using polymerase chain reaction and restriction fragment length polymorphism analyses.

RESULTS

The prevalence of *5 alleles in the group of all patients with NMS was higher than that in the controls, though this difference was not statistically significant (10.4% vs. 5.4%; P = 0.107; odds ratio (OR) 2.05; 95% confidence interval (CI) 0.87-4.80). No association was found between the frequency of *10 alleles and the occurrence of NMS. We found *4 and duplicated alleles in only one patient each among the patients with NMS. A total of 29 patients appeared to have developed NMS as a result of having taking CYP2D6 substrates. The prevalence of *5 alleles in these 29 patient was significantly higher than that in the controls (15.5% vs. 5.4%; P = 0.020; OR 3.25; 95% CI 1.30-8.13).

CONCLUSION

Our findings suggest that the CYP2D6*5 allele is likely to affect vulnerability to development of NMS.

Effects of DRD2 and CYP2D6 genotypes on delta EEG power response to aripiprazole in healthy male volunteers: a preliminary study

The aim of the present study was to evaluate the effects of polymorphisms in dopamine D2 receptor (DRD2) and cytochrome P450 (CYP) 2D6 genes on delta EEG power response to aripiprazole in healthy male volunteers. Seventeen volunteers were recruited according to the DRD2 Taq1A genotype, and separated into the following groups: homozygous wild-type (A2/A2, n = 7), heterozygous (A2/A1, n = 5) and homozygous variant-type (A1/A1, n = 5) groups. After enrollment in this study, they were genotyped for CYP2D6. The volunteers received single 10 mg oral doses of aripiprazole, in accordance with an open-label parallel group study design. Plasma levels of aripiprazole and its metabolite were determined and EEGs were obtained simultaneously. The pharmacodynamic parameter was absolute delta power in the Cz channel. The changes of delta power were not different according to DRD2 Taq1A genotypes. As to the CYP2D6 allele, the subjects had the following CYP2D6 genotypes: *10/*10 (n = 4), *1/*10 (n = 5), *1/*5 (n = 2), *1/*1 (n = 3), *2/*41 (n = 1), *2/*2 (n = 1), *2N/*10 (n = 1). Subjects exhibiting the *1/*5 and *1/*10 genotypes showed a trend toward high area under the plasma aripiprazole concentration-time curve (AUC), which was linearly related to area under the EEG response-time curve (AUEC). Our results demonstrate a need for further evaluation of the CYP2D6 genotypic effect on the pharmacodynamics of aripiprazole.

The influence of long-term treatment with psychotropic drugs on cytochrome P450: the involvement of different mechanisms

This paper emphasises that besides the direct action of psychotropic drugs on cytochrome P450 (CYP) (i.e., the binding of the parent drug to the enzyme) indirect mechanisms of CYP-psychotropic interactions, namely the formation of CYP-reactive metabolite complexes and their influence on enzyme regulation, are also very important. The described interactions that are time-, drug- and CYP isoform-dependent may overlap during long-term treatment. The final result of the overlapping depends on the dosage and time interval after the last administration of a drug, which determines the concentration of the parent drug and its metabolites in the environment of the enzyme. These interactions may occur not only in the liver, but also in the brain, and may change the activity of CYP towards the metabolism of drugs, sex steroids, neurosteroids and amine neurotransmitters. The role of the CNS in the regulation of CYP by psychotropics and the significance of CYP-psychotropic interactions for pharmacological and clinical profiling of these drugs is discussed. In addition, different experimental approaches for studying CNS-acting drugs are compared.

CYP2D6 gene deletion allele in patients with neuroleptic malignant syndrome: preliminary report

Neuroleptic malignant syndrome (NMS) is a potentially fatal adverse reaction to psychopharmacologic treatment. Reported herein are two NMS patients with schizophrenia who were found to possess a CYP2D6 gene deletion allele (CYP2D6*5). The deletion results in decreased CYP2D6 activity, possibly leading to drug accumulation. Both patients with NMS had been treated with neuroleptics, including CYP2D6 substrates. Polymerase chain reaction (PCR) followed by restriction fragment length polymorphism analyses and long PCR were performed to detect CYP2D6 genotype. One patient was found to possess *5/*10; the other had a *1/*5 genotype. The present preliminary report suggests that pharmacokinetic factors cannot be excluded and the CYP2D6 polymorphism is possibly associated with the etiology of NMS.

Genetic association analysis of functional polymorphisms in the cytochrome P450 1A2 (CYP1A2) gene with tardive dyskinesia in Japanese patients with schizophrenia

OBJECTIVE

Recent studies have revealed positive associations between tardive dyskinesia (TD) and genetic polymorphisms of several cytochrome P450 (CYP) subfamilies that are involved in pharmacokinetic process of antipsychotic drugs. In the present study, we analyzed the relationship between TD and two polymorphisms of the CYP1A2 gene, 734C/A and -2964G/A, in a sample of Japanese patients with schizophrenia.

METHODS

We studied 199 Japanese patients with schizophrenia. We used the Abnormal Involuntary Movement Scale to evaluate TD. Two polymorphisms of the CYP1A2 gene, 734C/A and -2964 G/A were genotyped by means of polymerase chain reaction and restriction fragment length polymorphism analysis.

RESULTS

Neither the 734C/A nor the -2964G/A polymorphism was associated with TD [734C/A genotype: chi2=0.02, degrees of freedom (df)=2, P=1.00; allele: chi2=0.02, df=1, P=0.89; -2964G/A genotype: chi2=0.21, df=2, P=0.90; allele: chi2=0.15, df=1, P=0.70]. In addition, CYP1A2 haplotype was associated with TD (chi2=0.24, df=3, P=0.97). Furthermore, in both the subgroup of smokers and the subgroup of patients receiving high-dosage antipsychotics (chlorpromazine equivalent >1000 mg), neither the 734C/A nor the -2964G/A polymorphism was associated with TD.

CONCLUSIONS

We did not find significant associations between the 734C/A and -2964G/A polymorphisms of CYP1A2 gene and TD in Japanese patients with schizophrenia. Our results suggest that these CYP1A2 gene polymorphisms may not contribute to TD susceptibility.

Inhibition of rat liver CYP2D in vitro and after 1-day and long-term exposure to neuroleptics in vivo-possible involvement of different mechanisms

The aim of the present study was to investigate the influence of classic and atypical neuroleptics on the activity of rat CYP2D measured as a rate of ethylmorphine O-deethylation. The reaction was studied in control liver microsomes in the presence of neuroleptics, as well as in microsomes of rats treated intraperitoneally (i.p.) for 1-day or 2-weeks (twice a day) with pharmacological doses of the drugs (promazine, levomepromazine, thioridazine, perazine 10 mg kg(-1); chlorpromazine 3 mg kg(-1); haloperidol 0.3 mg kg(-1); risperidone 0.1 mg kg(-1); sertindole 0.05 mg kg(-1)), in the absence of the neuroleptics in vitro. Neuroleptics added in vitro to control liver microsomes decreased the activity of the rat CYP2D by competitive or mixed inhibition of the enzyme. Thioridazine (Ki=15 microM) was the most potent inhibitor of the rat CYP2D among the drugs studied, whose effect was more pronounced than that of the other neuroleptics tested: phenothiazines (Ki=18-23 microM), haloperidol (Ki=32 microM), sertindole (Ki=51 microM) or risperidone (Ki=165 microM). The investigated neuroleptics-when given to rats in vivo-also seemed to exert an inhibitory effect on CYP2D via other mechanisms. One-day exposure of rats to the classic neuroleptics decreased the activity of CYP2D in rat liver microsomes. After chronic treatment with the investigated neuroleptics, the decreased CYP2D activity produced by the phenothiazines was still maintained, while that caused by haloperidol diminished. Moreover, risperidone decreased the activity of that enzyme. The obtained results indicate drug- and time-dependent interactions between the investigated neuroleptics and the CYP2D subfamily of rat cytochrome P-450, which may proceed via different mechanisms: (1) competitive or mixed inhibition of CYP2D shown in vitro, the inhibitory effects of phenothiazines being stronger than those of haloperidol or atypical neuroleptics, but weaker than the effects of the respective drugs on human CYP2D6; (2) in vivo inhibition of CYP2D, produced by both 1-day and chronic treatment with phenothiazines, which suggests inactivation of enzyme by intermediate metabolites; (3) in vivo inhibition of CYP2D by risperidone, produced only by chronic treatment with the drug, which suggests its influence on the enzyme regulation.

Neuropsychiatric pharmacogenetics: moving toward a comprehensive understanding of predicting risks and response

Pharmacogenetic research in the area of neuropsychiatric illnesses is rapidly evolving. Due to the complexity of the human brain, it is not surprising that our knowledge about the interaction between genetics and the treatment of these illnesses is very small. The Human Genome Project (HGP) has identified > 30,000 genes; several thousand of which have been found to occur in the brain or serve a role that enhances the brain’s function. Much of the research in the post-HGP era is being driven by a desire to use genetics to predict which patients deviate from the norm in terms of drug response or side effects. By identifying these people, we will be able to direct clinical practice such that therapies for these disorders can be individualized. With this in mind, the following review is intended to cover a broad understanding of CNS pharmacogenetics with the goal of summarizing available literature on promising candidate gene targets, which may eventually help us predict clinical outcomes in patients taking medications commonly used to treat neuropsychiatric disorders.

Association between CYP2D6 genotype and tardive dyskinesia in Korean schizophrenics

The CYP2D6 gene codes for human cytochrome P450 2D6 enzyme, which is responsible for the metabolism of many psychiatric drugs. In schizophrenic patients treated with neuroleptics, decreased or loss of function CYP2D6 alleles may contribute to the development of tardive dyskinesia (TD), a movement disorder that frequently occurs with chronic neuroleptic treatment. The goal of this study was to determine whether the occurrence of TD is associated with CYP2D6 genotype in a cohort of Korean schizophrenics by employing a CYP450 GeneChip((R)) oligonucleotide microarray and PCR assays to screen for 19 CYP2D6 alleles. Our results revealed that males with at least one decreased or loss of function allele have a moderately greater chance of developing TD than males with only wild-type alleles. Female schizophrenics did not have a significantly greater chance of developing TD. Our results demonstrate the utility of CYP2D6 microarrays to assess genotype status in this Korean cohort.

A perspective on molecular genetic studies of tardive dyskinesia: one clue for individualized antipsychotic drug therapy

Interindividual genetic profile differences related to antipsychotic drug therapy may be determined based on molecular genetic studies of the pathogenesis of schizophrenia and studies of antipsychotic drug responses (therapeutic as well as adverse responses). In the present article, we review molecular genetic studies of tardive dyskinesia (TD), which is a representative adverse response to antipsychotic drugs. Such studies have been performed to explore the gene-associated pharmacokinetic and pharmacodynamic processes of antipsychotic drugs. Positive associations between several genes and TD have been reported. The accumulation of results from such studies will hopefully lead to individualized antipsychotic drug therapies that involve the application of new genomic techniques, including DNA microarrays. Subsequently, antipsychotic drugs may in the future be prescribed for smaller subgroups of patients who have been classified as having a particular genetic profile.

Tardive dyskinesia is not associated with the polymorphisms of 5-HT2A receptor gene, serotonin transporter gene and catechol-o-methyltransferase gene

BACKGROUND

The pathophysiology of tardive dyskinesia (TD) is not completely understood.Aim. - To assess the relationship of TD with 5-HT2A receptor gene, serotonin transporter gene (5 HTT), and catechol-o-methyltransferase (COMT) gene polymorphisms.

METHOD

Our study comprised 111 unrelated subjects who strictly met DSM-IV criteria for schizophrenia and 32 TD, and 79 healthy unrelated controls; all the subjects were of Turkish origin. The analyses of 5-HT2A receptor gene, 5 HTT gene, and COMT gene polymorphisms were performed using polymerase chain reaction (PCR) technique.

RESULTS

The polymorphisms of these genes were not significantly different between the schizophrenic patients, TD and control subjects.

CONCLUSION

Our findings indicated that 5-HT2A receptor gene, 5 HTT gene, and COMT gene polymorphisms were similar in schizophrenia with non-TD, schizophrenia with TD, and healthy controls. These polymorphisms, though, do not help to evaluate the susceptibility to TD.

Abnormal movements and tardive dyskinesia in smokers and nonsmokers with schizophrenia genotyped for cytochrome P450 2D6

STUDY OBJECTIVE

To investigate the relationships between cytochrome P450 (CYP) 2D6 genotype, antipsychotic drug exposure, abnormal movements and tardive dyskinesia, and cigarette smoking.

DESIGN

Prospective, longitudinal study.

SETTING

University mental health research center.

PATIENTS

Thirty-seven patients with schizophrenia.

INTERVENTION

Patients were genotyped for CYP2D6*1, *3, and *4 alleles, and data were collected on their psychiatric symptoms, cigarette smoking status, and antipsychotic drug exposure. Abnormal movements were measured using the Abnormal Involuntary Movement Scale (AIMS). Presence of tardive dyskinesia was also evaluated.

MEASUREMENTS AND MAIN RESULTS

A linear regression model used the AIMS scores as the dependent variable, and genotype, sex, smoking status, and antipsychotic drug exposure as independent variables. Antipsychotic drug exposure, genotype, and cigarette smoking interaction was significant (p<0.0212) for patients with the CYP2D6*1/*3, *4 genotype. Seventy-eight percent of smokers with the CYP2D6*1/*3, *4 genotype had tardive dyskinesia compared with 20-33% of patients in other groups.

CONCLUSION

Patients with a CYP2D6*3 or *4 allele may shunt antipsychotic metabolism through other pathways that are induced by cigarette smoke. This induction may result in formation of neurotoxic metabolites, leading to increased AIMS scores and a higher frequency of tardive dyskinesia compared with patients without these alleles.

Antipsychotic-induced extrapyramidal syndromes and cytochrome P450 2D6 genotype: a case-control study

To study the association between polymorphism of the cytochrome P450 2D6 gene (CYP2D6) and the risk of antipsychotic-induced extrapyramidal syndromes, as measured by the use of antiparkinsonian medication. Data for this case-control study were obtained from a psychiatric hospital where newly admitted patients are routinely screened for several CYP2D6 mutant alleles. Cases were patients prescribed antiparkinsonian medication during oral antipsychotic drug treatment in the period September 1994 to August 2000. They were divided into those using an antipsychotic drug the metabolic elimination of which depends on the activity of the CYP2D6 enzyme ('CYP2D6-dependent') and those using other antipsychotic drugs. We formed a control group of antipsychotic drug users for both case groups using a matching ratio of 3 : 1 (controls : cases). Control patients were matched on whether or not their prescribed antipsychotic drug was CYP2D6-dependent. Odds ratios for patients who were slow metabolizers versus patients who were extensive metabolizers were calculated using conditional logistic regression and were adjusted for age, gender, dose and other potential confounding factors. We identified 77 case patients who were prescribed a CYP2D6-dependent antipsychotic drug and 54 case patients who were prescribed non CYP2D6-dependent antipsychotic drugs. Among the case- and control-patients using a CYP2D6-dependent antipsychotic drug, the poor metabolizers were more than four times more likely to start with antiparkinsonian medication than the extensive metabolizers (odds ratio 4.44; 95% confidence interval 1.11-17.68). An increased risk was not observed for patients using non CYP2D6-dependent antipsychotic drugs (odds ratio 1.20; 95% confidence interval 0.21-6.79). Genetically impaired CYP2D6 activity can increase the risk of antipsychotic-induced extrapyrimidal syndromes. Poor metabolizers should have their antipsychotic drug dosage reduced when the metabolism of the prescribed drug depends on CYP2D6 activity or should receive an antipsychotic drug that is not CYP2D6-dependent.

Cytochrome P450 polymorphisms and response to antipsychotic therapy

Antipsychotic drugs are used for the treatment of schizophrenia and other related psychotic disorders. The antipsychotics currently available include older or classical compounds and newer or atypical agents. Most antipsychotic drugs are highly lipophilic compounds and undergo extensive metabolism by cytochrome P450 (CYP) enzymes in order to be excreted. There is a wide interindividual variability in the biotransformation of antipsychotic drugs, resulting in pronounced differences in steady-state plasma concentrations and, possibly, in therapeutic and toxic effects, during treatment with fixed doses. Many classical and some newer antipsychotics are metabolized to a significant extent by the polymorphic CYP2D6, which shows large interindividual variation in activity. Other CYPs, especially CYP1A2 and CYP3A4, also contribute to the interindividual variability in the kinetics of antipsychotics and occurrence of drug interactions. No relationship between CYP2D6 genotype or activity and therapeutic effects of classical antipsychotic drugs has been found in the few studies performed. On the other hand, some investigations suggest that poor metabolizers (PMs) of CYP2D6 would be more prone to over-sedation and, possibly, Parkinsonism during treatment with classical antipsychotics, while other studies, mostly retrospective, have been negative or inconclusive. For the newer antipsychotics, such data are lacking. To date, CYP2D6 phenotyping and genotyping appear, therefore, to be clinically useful for dose predicting only in special cases and for a limited number of antipsychotics, while their usefulness in predicting clinical effects must be further explored.