Cytochrome p450 phenotyping/genotyping in patients receiving antipsychotics: useful aid to prescribing?

Contribution of allelic variations to the phenotype of response to antidepressants and antipsychotics

Individualized medicine through molecular pharmacogenetics is one of the major future goals in clinical medicine. In psychopharmacology, pharmacogenetics became an expanding research component. Major research results were already attained: first, it is now feasible to predict a major proportion of the interindividual variation of plasma levels of most antidepressants and antipsychotics by using the DNA-sequence variation in genes for crucial CYP P450-enzymes as CYP2D6. Second, it is now possible to relate serious side effects (tardive dyskinesia, weight gain) of antipsychotics to specific genetic variants of genes for target proteins. Third, a long list of mainly functional variants in target protein genes was explored for their predictive power for the beneficial and adverse treatment outcome. Although specific results transferable into clinical practice were not yet obtained in this respect, the proof of principle could be demonstrated.

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.

Influence of cytochrome P450 polymorphisms on drug therapies: pharmacogenetic, pharmacoepigenetic and clinical aspects

The polymorphic nature of the cytochrome P450 (CYP) genes affects individual drug response and adverse reactions to a great extent. This variation includes copy number variants (CNV), missense mutations, insertions and deletions, and mutations affecting gene expression and activity of mainly CYP2A6, CYP2B6, CYP2C9, CYP2C19 and CYP2D6, which have been extensively studied and well characterized. CYP1A2 and CYP3A4 expression varies significantly, and the cause has been suggested to be mainly of genetic origin but the exact molecular basis remains unknown. We present a review of the major polymorphic CYP alleles and conclude that this variability is of greatest importance for treatment with several antidepressants, antipsychotics, antiulcer drugs, anti-HIV drugs, anticoagulants, antidiabetics and the anticancer drug tamoxifen. We also present tables illustrating the relative importance of specific common CYP alleles for the extent of enzyme functionality. The field of pharmacoepigenetics has just opened, and we present recent examples wherein gene methylation influences the expression of CYP. In addition microRNA (miRNA) regulation of P450 has been described. Furthermore, this review updates the field with respect to regulatory initiatives and experience of predictive pharmacogenetic investigations in the clinics. It is concluded that the pharmacogenetic knowledge regarding CYP polymorphism now developed to a stage where it can be implemented in drug development and in clinical routine for specific drug treatments, thereby improving the drug response and reducing costs for drug treatment.

Functional polymorphisms of the cytochrome P450 1A2 (CYP1A2) gene and prolonged QTc interval in schizophrenia

CYP1A2 is an important inducible enzyme involved in the metabolism of antipsychotics. This study examined two functional polymorphisms in the gene as potential markers in predicting prolongation of QTc interval in patients treated with antipsychotics. QT intervals were measured by 12-lead electrocardiography (ECG) for patients with a DSM-IV diagnosis of schizophrenia. Genomic DNA extracted from venous blood were genotyped for the two polymorphisms by PCR-RFLP. Statistically significant result for CYP1A2(*)1F was noted for all patients receiving chlorpromazine equivalent doses of above 300 mg and also for a further subgroup on antipsychotics known to be CYP1A2 substrates (p=0.007, mean QTc in ms for A/A: 395.5+/-15.1, A/C: 425.7+/-25.1, C/C: 427.3+/-20.7). For CYP1A2(*)1C, there was no statistically significant association between genotypes and mean QTc interval. Overall, there was a trend of those with the C allele of the CYP1A2(*)1F polymorphism having longer QTc intervals. The results of this study suggest that the CYP1A2(*)1F polymorphism may contribute to the risk of developing prolonged QT-interval in patients who are treated with higher doses of antipsychotics.

Depot Haloperidol Treatment in Outpatients With Schizophrenia on Monotherapy: Impact of CYP2D6 Polymorphism on Pharmacokinetics and Treatment Outcome

Haloperidol and several other antipsychotic drugs are at least partially metabolized by the polymorphic cytochrome P450 2D6 (CYP2D6). The interindividual variation in metabolic capacity of CYP2D6 might be of importance when dosing. In this study, 26 outpatients with schizophrenia and depot haloperidol as monotherapy were genotyped. The authors found 1 patient with no functional alleles, 8 with one functional allele, 16 with two functional alleles, and 1 with three functional alleles. The daily dose of haloperidol ranged from 0.45 to 14.29 mg. Steady state plasma concentrations were measured at peak (range, 1.6-67 nmol/L) and at trough (range, 1.0-49 nmol/L). The Positive and Negative Syndrome scale for Schizophrenia and the Extrapyramidal Symptom Rating Scale were used to evaluate the clinical effect. The authors found a clear correlation between haloperidol plasma concentration and number of active CYP2D6 alleles. No correlation was found between plasma concentration of haloperidol or number of CYP2D6 alleles and treatment outcome or side effects. A model to predict plasma concentration from dose and number of active CYP2D6 alleles was formed from the obtained data by means of multiple linear regression.

CYP2D6 genotype in relation to perphenazine concentration and pituitary pharmacodynamic tissue sensitivity in Asians: CYP2D6–serotonin–dopamine crosstalk revisited

OBJECTIVES

Hyperprolactinemia is a common side effect of first-generation antipsychotics mediated by antagonism of dopaminergic neurotransmission in the pituitary. Most first-generation antipsychotics are metabolized by CYP2D6 in the liver. Further, CYP2D6 is expressed in the human brain as a 5-methoxyindolethylamine O-demethylase potentially contributing to regeneration of serotonin from 5-methoxytryptamine. As dopaminergic neurotransmission is subject to regulation by serotonin, CYP2D6 may exert a nuanced (serotonergic) influence on dopaminergic tone in the pituitary. CYP2D6*10 is an allele associated with reduced enzyme function and occurs in high frequency (about 50%) in Asians. We prospectively evaluated significance of CYP2D6 genetic variation for prolactin response to perphenazine (a model first-generation antipsychotic) in Asians.

METHODS

A single oral dose of perphenazine (0.1 mg/kg) or placebo was administered to 22 medication-free nonsmoker healthy male Chinese-Canadian volunteers, following a double-blind within-subject randomized design. Blood samples were drawn at baseline and 2, 3, 4, 5 and 6 h after drug administration.

RESULTS

In volunteers with CYP2D6*10/CYP2D6*10 genotype, the mean area under curve (AUC0-6) for perphenazine concentration was 2.9-fold higher than those who carry the CYP2D6*1 allele (P<0.01). Notably, volunteers homozygous for CYP2D6*10 exhibited a significant reduction (66%) in mean pharmacodynamic tissue sensitivity as measured by the (prolactin-AUC0-6/perphenazine-AUC0-6) ratio (P=0.02).

CONCLUSIONS

CYP2D6 genotype is a significant contributor to perphenazine concentration in Chinese-Canadians. Importantly, prolactin response, when normalized per unit perphenazine concentration, appears to be blunted in volunteers homozygous for CYP2D6*10. We suggest that CYP2D6 genetic variation may potentially influence pharmacodynamic tissue sensitivity in the pituitary, presumably through disposition of an endogenous substrate (e.g. 5-methoxytryptamine).

Various pharmacogenetic aspects of antiepileptic drug therapy: a review

Pharmacogenetics concerns the influence of an individual's genetic background on the pharmacokinetics and pharmacodynamics of xenobiotics. Much of the pharmacogenetic data in the field of epilepsy deals with the pharmacokinetics of antiepileptic drugs (AEDs). In particular, two polymorphisms of cytochrome P450 2C9 are known to slow down the metabolism of phenytoin to a degree that increases the risk of the neurotoxic adverse effects of this drug among carriers of these polymorphisms. A significant number of patients with epilepsy do not respond to AEDs and such pharmacoresistance is a major, largely unsolved, problem that is likely to be multifactorial in nature. In this regard, genetic factors may influence transmembrane drug transporter proteins, thereby modifying the intracerebral penetration of AEDs. Monogenic idiopathic epilepsies are rare and frequently associated with ion channel mutations; however, to date, a consistent relationship between changes in channel properties and clinical phenotype has not been established nor has any association between genotype and response to specific treatment options. Polymorphisms of drug targets may represent another genetic facet in epilepsy: a recent study demonstrated for the first time a polymorphism of a drug target (the alpha-subunit of a voltage-gated sodium channel) associated in clinical practice with differing response to two classic AEDs. Adverse drug reactions and teratogenicity of AEDs remain a major concern. Whole-genome single nucleotide polymorphism profiling might in the future help to determine genetic predisposing factors for adverse drug reactions. Recently, in Han Chinese treated with carbamazepine and presenting with Stevens-Johnson syndrome, a strong association was found with HLA B*1502. If genetically targeted drug development becomes more affordable/cost efficient in the near future, the development of new drugs for relatively rare diseases could become economically viable for the pharmaceutical industry. The synergy of lower trial costs and efficacy-based prescribing may reduce the cost of medical treatment for a particular disease. This hypothetical advantage of the practical use of pharmacogenetics is, however, counterbalanced by several possible dangers, including illicit data mining and the development of a human 'genetic underclass' with the risk of exclusion from, for example employment or health insurance, because of an 'unfavourable' genetic profile.

The convergence of conventional therapeutic drug monitoring and pharmacogenetic testing in personalized medicine: focus on antidepressants

The development and prospects of conventional therapeutic drug monitoring (TDM) and pharmacogenetic testing as aids in personalized treatment with antidepressants and antipsychotics are described. Our own experience is discussed in relation to international guidelines for rational TDM. Emphasis is put on the usefulness of TDM combined with genotyping of cytochrome P450 2D6 (CYP2D6), the key enzyme involved in the polymorphic metabolism of the majority of antidepressants (both tricyclics and selective serotonin reuptake inhibitors) and antipsychotic drugs. This combination of methods is particularly useful in verifying concentration-dependent adverse drug reactions (ADRs) due to poor metabolism, 'and diagnosing pharmacokinetic reasons (ultrarapid metabolism (UM)) for drug failure. This is because ADRs may mimic the psychiatric illness itself and therapeutic failure due to UM may be mistaken for poor compliance with the prescription.

Serotonin and dopamine receptor gene polymorphisms and the risk of extrapyramidal side effects in perphenazine-treated schizophrenic patients

RATIONALE

Perphenazine, a classical antipsychotic drug, has the potential to induce extrapyramidal side effects (EPS). Dopaminergic and serotonergic pathways are involved in the therapeutic and adverse effects of the drug.

OBJECTIVES

To evaluate the impact of polymorphisms in the dopamine D(2) and D(3) and serotonin 2A and 2C receptor genes (DRD2, DRD3, HTR2A, and HTR2C) on short-term effects of perphenazine monotherapy in schizophrenic patients.

MATERIALS AND METHODS

Forty-seven Estonian inpatients were evaluated before and after 4-6 weeks of treatment by Simpson-Angus rating scale, Barnes scale, and Positive and Negative Symptom Scale. Genotyping was performed for common DRD2, DRD3, HTR2A, and HTR2C gene polymorphisms, previously reported to influence receptor expression and/or function.

RESULTS

Most of the patients (n = 37) responded to the treatment and no significant association was observed between the polymorphisms and antipsychotic response. The 102C allele of HTR2A and the -697C and 23Ser alleles of HTR2C were more frequent among patients with EPS (n = 25) compared to patients without EPS (n = 22) (p = 0.02, 0.01, and 0.02, respectively). The difference between patients with and without EPS in variant allele frequencies remained significant after multiple model analyses including age, gender, and duration of antipsychotic treatment as covariants. There was no significant association between EPS occurrence and polymorphisms in the DRD2 and DRD3 genes.

CONCLUSIONS

An association was observed between polymorphisms in HTR2A and HTR2C genes and occurrence of acute EPS in schizophrenic patients treated with perphenazine monotherapy. Larger study populations are needed to confirm our findings.

Low frequency of CYP2D6 poor metabolizers among schizophrenia patients

CYP2D6 has been suggested to be functionally similar to the dopamine transporter. The present study was aimed at analysing the frequency of CYP2D6 alleles and genotype among schizophrenic patients compared to healthy volunteers. CYP2D6 *3, *4, *5, *6, *10 and duplicated alleles were analysed in 128 unselected schizophrenia inpatients (SP) and 142 unrelated white European Spanish healthy volunteers (HV). SP and HV with >2, 2, 1 or 0 CYP2D6 active genes were 4.7, 64.8, 28.1 and 2.3%, and 6.3, 52.1, 33.1 and 8.5%, respectively. The frequency of homozygous for CYP2D6 inactive alleles or poor metabolizers (PMs) was lower (P<0.05) in SP than in HV. Furthermore, the frequency of CYP2D6 inactive alleles was also lower in SP than in HV (16.8 vs 25.7; P<0.05), specifically the CYP2D6*6 allele was not found among patients. The present study shows a lower frequency of PMs in schizophrenic patients than in healthy volunteers supporting the hypothesis of a potential role of CYP2D6 in the vulnerability to schizophrenia.

Identifying drugs needing pharmacogenetic monitoring in a Korean hospital

PURPOSE

A decision matrix for identifying drugs for which pharmacogenetic drug monitoring (PDM) provides the greatest benefit in a Korean setting is described.

SUMMARY

We developed a decision matrix including the ethnic frequency of clinically relevant polymorphic cytochrome P-450 (CYP) enzymes, and the metabolic profiles and adverse drug reactions of drugs. Using the developed decision matrix based on the population allele frequencies of CYP enzymes, we identified potential candidates for PDM among the most commonly used drugs at Seoul National University Hospital (SNUH). Collectively, 17 of these drugs were largely metabolized by at least one polymorphic CYP enzyme. Pharmacogenetic information was used to identify CYP2C9, CYP2C19, and CYP2D6 as the major CYP enzymes of clinical importance for pharmacologic effect and safety in Koreans. The frequencies of poor and intermediate metabolizers among Koreans were 0% and 2.3-12% for CYP2C9, 12% and 42% for CYP2C19, and 0.44% and 28% for CYP2D6, respectively. The frequency of ultrarapid metabolizers of CYP2D6 was 2.28%. The decision matrix and pharmacogenetic information were used to identify seven drugs for PDM: warfarin, glimepiride, diazepam, amitriptyline, nortriptyline, codeine, and oxycodone. This approach can be applied to other institutional hospitals or other ethnic populations and would be helpful for advancing pharmacy practice. Further work is required to assess the practical and potential clinical relevance of pharmacogenetic variations on drugs of interest before the implementation of PDM.

CONCLUSION

A decision matrix helped identify drugs for which PDM provides the greatest potential benefit at one Korean hospital.

Could endogenous substrates of drug-metabolizing enzymes influence constitutive physiology and drug target responsiveness?

Integration of genomic data from pharmacokinetic pathways and drug targets is an emerging trend in bioinformatics, but is there a clear separation of pharmacokinetic pathways and drug targets? Should we also consider the potential interactions of endogenous substrates of drug-metabolizing enzymes with receptors and other molecular drug targets as we combine pharmacogenomic datasets? We discuss these overarching questions through a specific pharmacogenomic case study of the cytochrome P450 (CYP)2D6, serotonin and dopamine triad. Importantly, CYP2D6 may contribute to the regeneration of serotonin from 5-methoxytryptamine by virtue of its catalytic function as a 5-methoxyindolethylamine O-demethylase. Furthermore, serotonergic neurons provide a regulatory feedback on dopaminergic neurotransmission. Hence, we hypothesize that independent of its role as a pharmacokinetic gene, CYP2D6 may nuance the regulation of serotonergic and dopaminergic neurophysiology. Additionally, we reflect upon the contribution of hyperspecialization in biomedicine to the present disconnect between research on pharmacokinetics and drug targets, and the potential for remedying this important gap through informed dialogue among clinical pharmacologists, human geneticists, bioethicists and applied social scientists.

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.

Pharmacokinetic Variability of Aripiprazole and the Active Metabolite Dehydroaripiprazole in Psychiatric Patients

Aripiprazole is a new atypical antipsychotic drug with a partial agonist activity at dopamine 2 and serotonin 1A receptors. The metabolism of aripiprazole involves both cytochrome P450 2D6 (CYP2D6) and CYP3A4. This study investigated the pharmacokinetic variability of aripiprazole and the active metabolite dehydroaripiprazole on the basis of 155 drug monitoring samples from psychiatric patients treated with therapeutic doses of aripiprazole (10-30 mg/day). Serum concentrations of drug and metabolite were determined by liquid chromatographic and tandem mass spectrometric detection. Pharmacokinetic variability was expressed as the range in concentration/dose (C/D) ratios, and the effect of sex and occasionally coprescribed CYP2D6 or CYP3A4 inhibitors/inducers was studied. In addition, the dose-concentration relationship and combined interquartile range of concentrations obtained at low dose (10-15 mg/day) and high dose (20-30 mg/day) were described. Individual C/D ratios ranged 37-fold for aripiprazole, 78-fold for dehydroaripiprazole, and 27-fold for the active sum of aripiprazole + dehydroaripiprazole. Median C/D ratios in male and female patients differed by less than 15%, and none of the differences were significant (P > 0.14). Cases of concurrent CYP3A4 inducers/inhibitors were not found, but three patients were coprescribed the potent CYP2D6 inhibitors paroxetine or fluoxetine. No consistent difference in C/D ratio was observed in these three patients compared with the rest of the patients. There was a proportional dose-concentration relationship in the population, and the combined interquartile ranges were 230 to 960 nmol/L for aripiprazole and 330 to 1210 nmol/L for aripiprazole + dehydroaripiprazole. In conclusion, pharmacokinetic variability of aripiprazole is extensive in psychiatric patients but apparently not dependent on dose or sex. The variability of the pharmacologic active sum of aripiprazole + dehydroaripiprazole is 25% to 30% less than aripiprazole, suggesting that variability of aripiprazole is partly determined by metabolism to dehydroaripiprazole.

Can pharmacogenetics help rescue drugs withdrawn from the market?

Observations over the later half of the last century have suggested that genetic factors may be the prime determinant of drug response, at least for some drugs. Retrospectively gathered data have provided further support to the notion that genotype-based prescribing will improve the overall efficacy rates and minimize adverse drug reactions (ADRs), making personalized medicine a reality. During the last 16 years, 38 drugs have been withdrawn from major markets due to safety concerns. Inevitably, a question arises as to whether it might be possible to 'rescue' some of these drugs by promoting genotype-based prescribing. However, ironically pharmacogenetics has not perceptibly improved the risk/benefit of a large number of genetically susceptible drugs that are already in wide clinical use and are associated with serious ADRs. Drug-induced hepatotoxicity and QT interval prolongation (with or without torsade de pointes) account for 24 (63%) of these 38 drug withdrawals. In terms of the number of drugs implicated, both these toxicities are on the increase. Many others have had to be withdrawn due to their inappropriate use. This paper discusses the criteria that a drug would need to fulfill, and summarizes the likely regulatory requirements, before its pharmacogenetic rescue can be considered to be realistic. One drug that fulfils these criteria is perhexiline (withdrawn worldwide in 1988) and is discussed in some detail. For the majority of these 38 drugs there are, at present, no candidates for genetic traits to which the toxicity that led to their withdrawal may be linked. For a few other drugs where a potential candidate for a genetic trait might explain the toxicity of concern, the majority of patients who experienced the index toxicity had easily managed nongenetic risk factors. It may be possible to rescue these drugs simply by careful attention to their dose, interaction potential and prescribing patterns, but without the need for any pharmacogenetic test. In addition, the pharmacogenetic rescue of drugs might not be as effective as anticipated as hardly any pharmacogenetic test is known to have the required test efficiency to promote individualized therapy. Multiple pathways of drug elimination, contribution to toxicity by metabolites as well as the parent drug, gene-gene interactions, multiple mechanisms of toxicity and inadequate characterization of phenotype account for this lack of highly predictive tests. The clinical use of tests that lack the required efficiency carries the risks of over- or under-dosing some patients, denying the drug to others and decreasing physician vigilance of patients. Above all, at present, prescribing physicians lack an adequate understanding of pharmacogenetics and its limitations. It is also questionable whether their prescribing will comply with the requirements for pretreatment pharmacogenetic tests to make pharmacogenetic rescue a realistic goal.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

Assay for the simultaneous detection of the *1C and *1F alleles of the CYP1A2 gene by real-time polymerase chain reaction and melting curve analysis

Pharmacogenetic variation is an important factor in the therapeutic outcome of many drug treatments. The cytochrome P450 isoform CYP1A2 is involved in the metabolism of a number of antipsychotic drugs. Variable expression of this enzyme may result in idiosyncratic drug responses, including adverse reactions. A number of DNA sequence polymorphisms have been identified in the CYP1A2 gene. Of these, two alleles, CYP1A2*1C and CYP1A2*1F, have been linked to changes in gene expression among smokers. In addition, these polymorphisms have been linked to susceptibility to tardive dyskinesia in some patient populations receiving antipsychotic drug therapy. Here, we present a rapid and robust method for simultaneously genotyping the CYP1A2*1C and *1F alleles using fluorescent hybridization probes and a widely available real-time polymerase chain reaction platform. Such an assay would offer the opportunity to routinely establish the CYP1A2 genotype of a patient prior to commencing drug therapy.

Update on pharmacogenetics in epilepsy: a brief review

Recent developments in the pharmacogenetics of antiepileptic drugs provide new prospects for predicting the efficacy of treatment and potential side-effects. Epilepsy is a common, serious, and treatable neurological disorder, yet current treatment is limited by high rates of adverse drug reactions and lack of complete seizure control in a significant proportion of patients. The disorder is especially suitable for pharmacogenetic investigation because treatment response can be quantified and side-effects can be assessed with validated measures. Additionally, there is substantial knowledge of the pharmacodynamics and kinetics of antiepileptic drugs, and some candidate genes implicated in the disorder have been identified. However, recent studies of the association of particular genes and their genetic variants with seizure control and adverse drug reactions have not provided unifying conclusions. This article reviews the published work and summarises the state of research in this area. Future directions for research and the application of this technology to the clinical practice of individualising treatment for epilepsy are discussed.

In vitro characterization of the human biotransformation and CYP reaction phenotype of ET-743 (Yondelis®, Trabectidin®), a novel marine anti-cancer drug

ET-743 is a potent marine anti-cancer drug and is currently being investigated in phase I and II clinical trials, e.g. in combination with other anti-cancer agents. To assess the biotransformation and CYP reaction phenotype and their potential implications for human pharmacology and toxicology, the in vitro metabolism of ET-743 was characterized using incubations with human liver preparations, cytochrome P450 (CYP) and uridine diphosphoglucuronosyl transferase (UGT) supersomes.CYP supersomes and liver microsomes showed that ET-743 was metabolized mainly by CYP3A4, but also by CYP2C9, 2C19, 2D6, and 2E1. ET-743 showed the highest affinity for CYP3A4 and the highest maximal metabolic rate for CYP2D6 among the CYPs shown to metabolize ET-743. In addition, the Km value of ET-743 in female microsomes was significantly lower compared to male microsomes, while the Vmax values did not differ. ET-743 glucuronidation, catalyzed by UGT2B15, was observed in microsomes and S9 fraction. In addition, conjugation by glutathione-S-transferase and no sulphation was observed for ET-743 in cytosol and S9 fraction. ET-743 was more extensively metabolized when CYP activity was combined with phase II enzymes UGT and glutathione-S-transferase (GST), indicating that CYP, UGT, and GST simultaneously metabolize ET-743 in the S9 fraction. These results provide evidence that CYP3A4 has a major role in the metabolism of ET-743 in vitro with additional involvement of CYP2C9, 2C19, 2D6, and 2E1. Furthermore, ET-743 is conjugated by UGT and GST. This information could be important for interpretation of the pharmacokinetic data of clinical trials and prediction of drug-drug interactions.

Pharmacogenetics of drug-metabolizing enzymes: implications for a safer and more effective drug therapy

The majority of phase I- and phase II-dependent drug metabolism is carried out by polymorphic enzymes which can cause abolished, quantitatively or qualitatively decreased or enhanced drug metabolism. Several examples exist where subjects carrying certain alleles do not benefit from drug therapy due to ultrarapid metabolism caused by multiple genes or by induction of gene expression or, alternatively, suffer from adverse effects of the drug treatment due to the presence of defective alleles. It is likely that future predictive genotyping for such enzymes might benefit 15-25% of drug treatments, and thereby allow prevention of adverse drug reactions and causalities, and thus improve the health of a significant fraction of the patients. However, it will take time before this will be a reality within the clinic. We describe some important aspects in the field with emphasis on cytochrome P450 and discuss also polymorphic aspects of foetal expression of CYP3A5 and CYP3A7.

Therapeutic Drug Monitoring and Pharmacogenetic Tests as Tools in Pharmacovigilance

Therapeutic drug monitoring (TDM) and pharmacogenetic tests play a major role in minimising adverse drug reactions and enhancing optimal therapeutic response. The response to medication varies greatly between individuals, according to genetic constitution, age, sex, co-morbidities, environmental factors including diet and lifestyle (e.g. smoking and alcohol intake), and drug-related factors such as pharmacokinetic or pharmacodynamic drug-drug interactions. Most adverse drug reactions are type A reactions, i.e. plasma-level dependent, and represent one of the major causes of hospitalisation, in some cases leading to death. However, they may be avoidable to some extent if pharmacokinetic and pharmacogenetic factors are taken into consideration. This article provides a review of the literature and describes how to apply and interpret TDM and certain pharmacogenetic tests and is illustrated by case reports. An algorithm on the use of TDM and pharmacogenetic tests to help characterise adverse drug reactions is also presented. Although, in the scientific community, differences in drug response are increasingly recognised, there is an urgent need to translate this knowledge into clinical recommendations. Databases on drug-drug interactions and the impact of pharmacogenetic polymorphisms and adverse drug reaction information systems will be helpful to guide clinicians in individualised treatment choices.

The impact of Cytochrome P450-2D6 genotype on the use and interpretation of therapeutic drug monitoring in long-stay patients treated with antidepressant and antipsychotic drugs in daily psychiatric practice

PURPOSE

This retrospective follow-up study investigates whether cytochrome P450-2D6 (CYP2D6) genotype explains variability in plasma concentrations of psychotropic drugs in daily psychiatric practice.

METHODS

The study population consisted of 62 hospitalised psychiatric patients genotyped for CYP2D6. Primary endpoint was the normalised plasma concentration ratio which was defined as the [measured concentration]/[mean therapeutic concentration] allowing comparison of plasma concentrations of different substrates. Secondary endpoint was a plasma concentration above the therapeutic range. The determinant was CYP2D6 genotype classified as ultrarapid metaboliser (UM), extensive metaboliser (EM), intermediate metaboliser (IM), or poor metaboliser (PM). The relation between CYP2D6 genotype and the normalised plasma concentration ratio was assessed with a linear mixed-effects model after adjustment for the Prescribed Daily Dose (PDD). The risk of having a plasma concentration above the therapeutic range was assessed with a logistic mixed-effects model.

RESULTS

For antidepressants, CYP2D6 genotype PM (1.68 (95%CI: 1.01-2.28)) and IM (1.09 (95%CI: 0.77-1.29)) were associated with higher normalised plasma concentration ratios of antidepressants compared to EMs (0.56 (95%CI: 0.26-0.74)). In addition, the risk of a plasma concentration above the therapeutic range was increased for PMs (OR 33.1 (95%CI: 2.0-544.6)) and IMs (OR 8.2 (95%CI: 1.1-60.3)) relative to EMs using antidepressants. CYP2D6 genotype could not clearly explain variability in plasma concentrations of antipsychotics possibly due to a low frequency of therapeutic drug monitoring (TDM) in antipsychotics primarily metabolised by CYP2D6 in daily psychiatric practice.

CONCLUSIONS

CYP2D6 genotype contributes to clinically relevant variability in plasma concentrations of antidepressants but probably not antipsychotics in daily clinical practice.

Pharmacogenomics in depression and antidepressants

Genetic factors are believe y a major role in the variation of treatment response and the incidence of adverse effects to medication. The aim of pharmacogenetics is to elucidate this variability according to hereditary differences. Considering current hypotheses for the mechanisms of action of antidepressants, most investigations to date have concentrated on mutations in genes coding either for the pathways in the serotonergic and noradrenergic systems or for drug-metabolizing enzymes. Recent studies shifted the emphasis on the mains mechanism of drug action from changes in neurotransmitter concentration or receptor function toward long-lasting adaptive processes within the neurons. Although the results are controversial, many studies support the hypothesis that psychopharmacogenetics will help predict an individual's drug response, while minimizing the side effects. The inclusion of functional genomics, investigate the complex gene and/or protein expression in response to a given drug, may lead to the development of novel and safer drugs.

The AGNP-TDM Expert Group Consensus Guidelines: focus on therapeutic monitoring of antidepressants

Therapeutic drug monitoring (TDM) of psychotropic drugs such as antidepressants has been widely introduced for optimization of pharmacotherapy in psychiatric patients. The interdisciplinary TDM group of the Arbeitsgemeinschaft für Neuropsychopharmakologie und Pharmakopsychiatrie (AGNP) has worked out consensus guidelines with the aim of providing psychiatrists and TDM laboratories with a tool to optimize the use of TDM. Five research-based levels of recommendation were defined with regard to routine monitoring of drug plasma concentrations: (i) strongly recommended; (ii) recommended; (iii) useful; (iv) probably useful; and (v) not recommended. In addition, a list of indications that justify the use of TDM is presented, eg, control of compliance, lack of clinical response or adverse effects at recommended doses, drug interactions, pharmacovigilance programs, presence of a genetic particularity concerning drug metabolism, and children, adolescents, and elderly patients. For some drugs, studies on therapeutic ranges are lacking, but target ranges for clinically relevant plasma concentrations are presented for most drugs, based on pharmacokinetic studies reported in the literature. For many antidepressants, a thorough analysis of the literature on studies dealing with the plasma concentration-clinical effectiveness relationship allowed inclusion of therapeutic ranges of plasma concentrations. In addition, recommendations are made with regard to the combination of pharmacogenetic (phenotyping or genotyping) tests with TDM. Finally, practical instructions are given for the laboratory practitioners and the treating physicians how to use TDM: preparation of TDM, drug analysis, reporting and interpretation of results, and adequate use of information for patient treatment TDM is a complex process that needs optimal interdisciplinary coordination of a procedure implicating patients, treating physicians, clinical pharmacologists, and clinical laboratory specialists. These consensus guidelines should be helpful for optimizing TDM of antidepressants.

Development of a PCR-based strategy for CYP2D6 genotyping including gene multiplication of worldwide potential use

There is growing consensus on the potential use of pharmacogenetics in clinical practice, and hopes have been expressed for application to the improvement of global health. However, two major challenges may lead to widening the “biotechnological gap” between the developing and the industrial world; first the unaffordability of some current technologies for poorer countries, and second the necessity of analyzing all described alleles for every clinical case due to the inability to predict the ethnic group of a given patient. Because of its role in the metabolism of a number of drugs, cytochrome P450 2D6 (CYP2D6) is an excellent candidate for use in the optimization of drug therapy. CYP2D6 is a highly polymorphic gene locus with more than 50 variant alleles, and subjects can be classified as poor metabolizers (PM), extensive metabolizers (EM), or ultrarapid metabolizers (UM) of a given CYP2D6 substrate. Several strategies and methods for CYP2D6 genotyping exist. Some, however, are expensive and laborious. The aim of this study was to design a PCR-based genotyping methodology to allow rapid, straightforward, and inexpensive identification of 90%–95% of CYP2D6 PM or UM genotypes for routine clinical use, independent of the individual's ethnic group. CYP2D6 is amplified in initial extra longPCRs (XL-PCRs), which subsequently undergo fragment-length polymorphism analysis for the determination of carriers of CYP2D6 allelic variants. The same XL-PCRs are also used for the determination of CYP2D6 multiplication and 2D6*5 allele (abolished activity). The application of this new strategy for the detection of CYP2D6 mutated alleles and multiplications to routine clinical analysis will enable the PM and UM phenotypes to be predicted and identified at a reasonable cost in a large number of individuals at most locations.

Pharmacogenetics in drug regulation: promise, potential and pitfalls

Pharmacogenetic factors operate at pharmacokinetic as well as pharmacodynamic levels-the two components of the dose-response curve of a drug. Polymorphisms in drug metabolizing enzymes, transporters and/or pharmacological targets of drugs may profoundly influence the dose-response relationship between individuals. For some drugs, although retrospective data from case studies suggests that these polymorphisms are frequently associated with adverse drug reactions or failure of efficacy, the clinical utility of such data remains unproven. There is, therefore, an urgent need for prospective data to determine whether pre-treatment genotyping can improve therapy. Various regulatory guidelines already recommend exploration of the role of genetic factors when investigating a drug for its pharmacokinetics, pharmacodynamics, dose-response relationship and drug interaction potential. Arising from the global heterogeneity in the frequency of variant alleles, regulatory guidelines also require the sponsors to provide additional information, usually pharmacogenetic bridging data, to determine whether data from one ethnic population can be extrapolated to another. At present, sponsors explore pharmacogenetic influences in early clinical pharmacokinetic studies but rarely do they carry the findings forward when designing dose-response studies or pivotal studies. When appropriate, regulatory authorities include genotype-specific recommendations in the prescribing information. Sometimes, this may include the need to adjust a dose in some genotypes under specific circumstances. Detailed references to pharmacogenetics in prescribing information and pharmacogenetically based prescribing in routine therapeutics will require robust prospective data from well-designed studies. With greater integration of pharmacogenetics in drug development, regulatory authorities expect to receive more detailed genetic data. This is likely to complicate the drug evaluation process as well as result in complex prescribing information. Genotype-specific dosing regimens will have to be more precise and marketing strategies more prudent. However, not all variations in drug responses are related to pharmacogenetic polymorphisms. Drug response can be modulated by a number of non-genetic factors, especially co-medications and presence of concurrent diseases. Inappropriate prescribing frequently compounds the complexity introduced by these two important non-genetic factors. Unless prescribers adhere to the prescribing information, much of the benefits of pharmacogenetics will be squandered. Discovering highly predictive genotype-phenotype associations during drug development and demonstrating their clinical validity and utility in well-designed prospective clinical trials will no doubt better define the role of pharmacogenetics in future clinical practice. In the meantime, prescribing should comply with the information provided while pharmacogenetic research is deservedly supported by all concerned but without unrealistic expectations.

The dosing of atypical antipsychotics

Drug-drug interactions or genetic variability may require using doses different from those recommended for atypical antipsychotics. Dosage alterations of olanzapine and clozapine, dependent on cytochrome P450 1A2 (CYP1A2) for clearance, and quetiapine, dependent on cytochrome P450 3A (CYP3A), may be necessary when used with other drugs that inhibit or induce their metabolic enzymes. Smoking cessation can significantly increase clozapine, and perhaps olanzapine, levels. Ziprasidone pharmacokinetic drug-drug interactions are not likely to be important. Genetic variations of cytochrome P450 2D6 (CYP2D6) and drug-drug interactions causing inhibition (CYP2D6 and/or CYP3A) or induction (CYP3A) may be important for risperidone, and perhaps for aripiprazole, dosing. Adding inhibitors may cause side effects more easily in drugs with a narrow therapeutic window, such as clozapine or risperidone, than in those with a wide therapeutic window, such as olanzapine or aripiprazole. Adding inducers may be associated with a gradual development of lost efficacy.

How the monoamine transporter garden grows

In this issue of Molecular Pharmacology, Hahn et al. (p. 457) present a study of previously uncharacterized single amino acid variants of the human norepinephrine transporter. Intracellular trafficking, surface expression, transport properties, interaction with antagonists, and regulation by a protein kinase C-linked regulatory pathway were studied by heterologous expression in COS-7 cells. In recent years, there has been increasing focus on the natural variations and roles of nonsynonymous single-nucleotide polymorphisms in human membrane transporter genes (and their protein products) in human disease. As this information is assimilated and understood at the molecular and genetic level, the relationship between transporter pharmacogenomics and therapeutics in the age of individualized medicine will be greatly impacted.

Genetic polymorphisms of cytochrome P450 2D6 (CYP2D6): clinical consequences, evolutionary aspects and functional diversity

CYP2D6 is of great importance for the metabolism of clinically used drugs and about 20-25% of those are metabolised by this enzyme. In addition, the enzyme utilises hydroxytryptamines as endogenous substrates. The polymorphism of the enzyme results in poor, intermediate, efficient or ultrarapid metabolisers (UMs) of CYP2D6 drugs. It is plausible that the UM genotype, where more than one active gene on one allele occurs, is the outcome of selective dietary selection in certain populations in North East Africa. The UM phenotype affects 5.5% of the population in Western Europe. A hypothesis for the evolutionary basis behind selection for CYP2D6 gene duplications is presented in relation to selection for Cyp6 variants in insecticide resistant Drosophila strains. The polymorphism of CYP2D6 significantly affects the pharmacokinetics of about 50% of the drugs in clinical use, which are CYP2D6 substrates. The consequences of the polymorphism at ordinary drug doses can be either adverse drug reactions or no drug response. Examples are presented where CYP2D6 polymorphism affects the efficacy and costs of drug treatment. Predictive CYP2D6 genotyping is estimated by the author to be beneficial for treatment of about 30-40% of CYP2D6 drug substrates, that is, for about 7-10% of all drugs clinically used, although prospective clinical studies are necessary to evaluate the exact benefit of drug selection and dosage based on the CYP2D6 genotype.

Fast and Slow Metabolizers ofHoasca

Harmine, a major alkaloid in ayahuasca (hoasca), is a selective and reversible inhibitor of the enzyme monoamine oxidase-A (MAO-A). It is also a selective inhibitor of the human cytochrome P450 isozyme 2D6 (CYP 2D6), which metabolizes harmine to a more hydrophilic derivative for eventual excretion. CYP 2D6 exhibits a wide range of polymorphisms in human populations, and variations in this enzymatic activity could account for differences in effects between individuals who use hoasca. This report broadly describes two subgroups of CYP 2D6 phenotypes--i.e., fast and slow metabolizers of harmine-in 14 experienced male members of the União do Vegetal (UDV) who received a standardized dosage of hoasca. To compensate for metabolic variations in their normal religious practice, the administered dose of hoasca is always determined by the presiding mestre, who is responsible for deciding the actual amount for each individual. This age-old method compensates for metabolic variations between individuals and variations in both the alkaloid profile and strength of the hoasca.

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.

Pharmacogenetic studies of response to risperidone and other newer atypical antipsychotics

Risperidone and other newer atypical antipsychotics are becoming the mainstay for schizophrenia treatment. Recent studies suggest that the 5-hydroxytryptamine receptor 2A (5-HT2A) gene (HTR2A) T102C and G-1438A polymorphisms may influence treatment response of risperidone or olanzapine for schizophrenia's negative symptoms (e.g., blunted affect and social withdrawal). In addition, the HTR6 T267C polymorphism has been linked to risperidone response for positive symptoms (delusions and hallucinations). The dopamine D2 receptor (DRD2) Ser311Cys polymorphism may also play a role in determining risperidone efficacy for positive, negative and cognitive symptoms, the DRD2 Ins-A2/Del-A1 diplotype may predict better risperidone response, and the DRD3 Ser311Cys variant may affect general treatment response of several atypical agents. Although investigators have started to explore genetic effects on cognitions of schizophrenia patients receiving antipsychotics, future larger sized pharmacogenetic studies on both psychotic symptoms and cognitive functions are warranted.

Pharmacogenetics as a tool in the therapy of schizophrenia

AIM

This review summarises the present knowledge of associations between pharmacogenetics and therapeutic efficacy and side effects of antipsychotics to enable pharmacists to judge the applicability for a more tailor made therapy in patients with schizophrenia. Polymorphisms of Cytochrome P450 isoenzymes and neurotransmitter receptors involved in the efficacy and side effects of antipsychotics are highlighted in this review.

METHOD

A search was performed in Medline and EMBASE for the period 1995-August 2002. Also relevant references from the selected papers were incorporated.

RESULTS

Poor metabolism with respect to CYP2D6 seems to be related with more pronounced extrapyramidal symptoms and more specifically with a higher incidence of tardive dyskinesia. The C/C-genotype for CYP1A2 results in smokers in a reduction of enzyme activity, but an effect on the incidence of tardive dyskinesia is controversial. For dopamine D2 receptors the effect of the -141C Ins/Del polymorphism on efficacy is not clear yet, although the Taq I polymorphism is associated with greater improvement of positive, but not negative symptoms in acute psychosis. The Gly9-allele of the dopamine D3 receptor is associated with the response to clozapine, but in studies in which the choice of antipsychotics is not restricted, the role of this polymorphism is unclear. The reverse is applicable to the dopamine D(4.2/4.7) polymorphism. For the 5-HT2A receptor the His452Tyr polymorphism is associated with response to clozapine, the 102 T/C polymorphism leads to equivocal results. The polymorphism studied for 5-HT5A, 5-HT6, alpha1A- and alpha2A-receptors give no clear associations with the response to clozapine. The polymorphism studied of the dopamine D2 and D4 receptor are not related to extrapyramidal adverse effects and side effects, respectively. The 9Gly-variant of the dopamine D3 receptor, the 102C-variant, but not the His452Tyr polymorphism of the 5-HT2A-receptor and the 23Ser-variant (for females only) of the 5-HT2C receptor seem to increase the susceptibility to tardive dyskinesia. Weight gain induced by antipsychotics seems to be associated with the -759C-allele of the 5-HT2C receptor.

CONCLUSION

The results show the first careful steps toward application of pharmacogenetics in a more individualised, tailor-made, pharmacotherapy. A pre-condition seems to be a multifactorial approach, as can be expected for multifactorial processes.

No association between polymorphisms in three genes of cytochrome p450 family and paranoid schizophrenia in northern Chinese Han population

We genotyped six SNPs in the genes of p450 family among paranoid schizophrenics and normal controls. All subjects are unrelated Han Chinese. Three showed polymorphic, and no significant differences in allele or genotype frequencies were detected between patients and controls. Thus we obtained no evidence for the involvement of the polymorphisms in paranoid schizophrenia in the population investigated.

P-glycoprotein is a factor in the uptake of dextromethorphan, but not of melperone, into the mouse brain: evidence for an overlap in substrate specificity between P-gp and CYP2D6

In this study, the role of P-glycoprotein (P-gp) for the pharmacokinetics of dextromethorphan, a CYP2D6 substrate, and of melperone, a CYP2D6 inhibitor, was investigated. The substances were administered subcutaneously near the nape of the neck of wild-type mice and of abcb1ab (-/-) mice. One hour after injection, concentrations of the two drugs in cerebrum, plasma and in different organs were measured by high-performance liquid chromatography. No significant differences between wild-type mice and abcb1ab (-/-) mice were observed for melperone, suggesting that P-gp is not involved in the uptake of melperone into the brain or other organs of mice. The concentration of dextromethorphan in the brain was more than twice as high in abcb1ab (-/-) mice compared to wild-type mice. Therefore, P-gp appears to be a factor in the uptake of dextromethorphan into the mouse brain, and abcb1-polymorphisms need to be considered for CYP2D6 phenotyping experiments with this drug. There is an overlap in substrate specificity between P-gp and CYP2D6. P-gp is a factor in the uptake of dextromethorphan, but not of melperone.

Guidelines for prevention and treatment of adverse effects of antipsychotic drugs on glucose-insulin homeostasis and lipid metabolism

RATIONALE

With the antipsychotic drugs available today, especially with some of the newer, atypical antipsychotics, metabolic side effects, such as weight gain, diabetes mellitus and lipid abnormalities, have become a complication to the drug therapy that have to be recognized and treated.

OBJECTIVE

The aim of this article is to suggest guidelines for prevention and treatment of adverse effects of antipsychotics on glucose-insulin homeostasis and lipid metabolism, whereas strategies for management of antipsychotic-induced weight gain are summarized elsewhere.

METHOD

The guidelines are based on results of experimental and clinical studies presented in the article, as well as on a recently published review of 180 articles in the field.

RESULTS

Both conventional and atypical antipsychotics can indirectly, by causing obesity, promote development of insulin resistance and type-2 diabetes. In addition, some atypical agents probably directly induce hyperinsulinemia, followed by weight gain, insulin resistance and drug-induced, sometimes insulin-dependent, diabetes.

CONCLUSION

In this article, guidelines for the management of adverse metabolic effects of antipsychotics are described.

Pharmacogenetics of antipsychotic-induced weight gain

RATIONALE

Antipsychotic medications have been associated with considerable weight gain. The degree of inter-individual variability and known genetic contributions to obesity suggest a combination of genetic and environmental factors. In the absence of established mechanisms and valid predictors for this relevant adverse effect, pharmacogenetic studies may provide the basis for the development of individualized treatment and preventive interventions.

OBJECTIVE

The aim of the present review is to analyze the theoretical and empirical knowledge base for the selection of the most promising target genes that may contribute to antipsychotic-induced weight gain.

METHODS

Examination of the preclinical and clinical literature that can inform the rational choice of target genes that may play a role in the development of adverse changes in body composition associated with antipsychotic treatment.

RESULTS

Theoretically, candidate gene selection can be guided by knowledge about molecular pathways associated with obesity, receptors modulated by antipsychotic drugs, and enzymes implicated in their metabolism and bioavailability. While most available data relate to the general mechanisms of obesity and few studies have directly examined the genetic contributions to antipsychotic-induced weight gain, several genes warrant further investigation. These include the 5-HT(2C), pro-opiomelanocortin, leptin, ghrelin, tumor necrosis factor alpha, adiponectin, dopamine D(2) receptor, histamine-H(1) receptor, and alpha(1), beta(2) and beta(3) adrenergic receptor genes.

CONCLUSIONS

Pharmacogenetic studies can provide powerful tools for the pre-treatment identification of individuals at high risk for antipsychotic-induced weight gain, to uncover biological mechanisms that may even generalize to non-drug-induced weight gain, and to isolate novel targets for treatments of weight gain and obesity. To enhance power, future studies should pay close attention to population selection and avoidance/control of confounds, particularly past treatment exposure.

Pharmacogenetic aspects of drug-induced torsade de pointes: potential tool for improving clinical drug development and prescribing

Drug-induced torsade de pointes (TdP) has proved to be a significant iatro-genic cause of morbidity and mortality and a major reason for the withdrawal of a number of drugs from the market in recent times. Enzymes that metabolise many of these drugs and the potassium channels that are responsible for cardiac repolarisation display genetic polymorphisms. Anecdotal reports have suggested that in many cases of drug-induced TdP, there may be a concealed genetic defect of either these enzymes or the potassium channels, giving rise to either high plasma drug concentrations or diminished cardiac repolarisation reserve, respectively. The presence of either of these genetic defects may predispose a patient to TdP, a potentially fatal adverse reaction, even at therapeutic dosages of QT-prolonging drugs and in the absence of other risk factors. Advances in pharmacogenetics of drug metabolising enzymes and pharmacological targets, together with the prospects of rapid and inexpensive genotyping procedures, promise to individualise and improve the benefit/risk ratio of therapy with drugs that have the potential to cause TdP. The qualitative and the quantitative contributions of these genetic defects in clinical cases of TdP are unclear because not all of the patients with TdP are routinely genotyped and some relevant genetic mutations still remain to be discovered. There are regulatory guidelines that recommend strategies aimed at uncovering the risk of TdP associated with new chemical entities during their development. There are also a number of guidelines that recommend integrating pharmacogenetics in this process. This paper proposes a strategy for integrating pharmacogenetics into drug development programmes to optimise association studies correlating genetic traits and endpoints of clinical interest, namely failure of efficacy or development of repolarisation abnormalities. Until pharmacogenetics is carefully integrated into all phases of development of QT-prolonging drugs and large-scale studies are undertaken during their post-marketing use to determine the genetic components involved in induction of TdP, routine genotyping of patients remains unrealistic. Even without this pharmacogenetic data, the clinical risk of TdP can already be greatly minimised. Clinically, a substantial proportion of cases of TdP are due to the use of either high or usual dosages of drugs with potential to cause TdP in the presence of factors that inhibit drug metabolism. Therefore, choosing the lowest effective dose and identifying patients with these non-genetic risk factors are important means of minimising the risk of TdP. In view of the common secondary pharmacology shared by these drugs, a standard set of contraindications and warnings have evolved over the last decade. These include factors responsible for pharmacokinetic or pharmacodynamic drug interactions. Among the latter, the more important ones are bradycardia, electrolyte imbalance, cardiac disease and co-administration of two or more QT-prolonging drugs. In principle, if large scale prospective studies can demonstrate a substantial genetic component, pharmacogenetically driven prescribing ought to reduce the risk further. However, any potential benefits of pharmacogenetics will be squandered without any reduction in the clinical risk of TdP if physicians do not follow prescribing and monitoring recommendations.

Validation of (-)-N-3-benzyl-phenobarbital as a selective inhibitor of CYP2C19 in human liver microsomes

(-)-N-3-Benzyl-phenobarbital (NBPB) was reported to be a potent and selective inhibitor of CYP2C19. To validate the selectivity of NBPB toward CYP2C19 in human liver microsomes, the inhibitory effects on major cytochrome P450 isoform-specific reactions were evaluated in the present study. In human liver microsomes, NBPB showed potent competitive inhibition on CYP2C19-mediated S-mephenytoin 4'-hydroxylation with an IC(50) value of 0.25 microM and K(i) value of 0.12 microM, whereas weak inhibition was observed for CYP1A2-, CYP2A6-, CYP2B6-, CYP2C8-, CYP2C9-, CYP2D6-, and CYP3A4-mediated reactions with IC(50) values >100, >100, 62, 34, 19, >100, and 89 microM, respectively. Importantly, its selectivity toward CYP2C19 among the CYP2C subfamily was demonstrated. Therefore, NBPB can be used as a potent and selective inhibitor to establish the relative contribution of CYP2C19 for in vitro reaction phenotyping studies. This compound can also serve as a positive control inhibitor of CYP2C19 for routine screening of P450 reversible inhibition when human liver microsomes are used as the enzyme source.

Pharmacogenetics of cytochrome P450 and its applications in drug therapy: the past, present and future

The field of cytochrome P450 pharmacogenetics has progressed rapidly during the past 25 years. All the major human drug-metabolizing P450 enzymes have been identified and cloned, and the major gene variants that cause inter-individual variability in drug response and are related to adverse drug reactions have been identified. This information now provides the basis for the use of predictive pharmacogenetics to yield drug therapies that are more efficient and safer. Today, we understand which drugs warrant dosing based on pharmacogenetics to improve drug treatment. It is anticipated that, in the future, genotyping could be used to personalize drug treatment for vast numbers of subjects, decreasing the cost of drug treatment and increasing the efficacy of drugs and health in general. I estimate that such personalized P450 gene-based treatment would be relevant for 10-20% of all drug therapy.

Pharmacogenetics of antidepressants and antipsychotics: the contribution of allelic variations to the phenotype of drug response

Genetic factors contribute to the phenotype of drug response. We systematically analyzed all available pharmacogenetic data from Medline databases (1970-2003) on the impact that genetic polymorphisms have on positive and adverse reactions to antidepressants and antipsychotics. Additionally, dose adjustments that would compensate for genetically caused differences in blood concentrations were calculated. To study pharmacokinetic effects, data for 36 antidepressants were screened. We found that for 20 of those, data on polymorphic CYP2D6 or CYP2C19 were found and that in 14 drugs such genetic variation would require at least doubling of the dose in extensive metabolizers in comparison to poor metabolizers. Data for 38 antipsychotics were examined: for 13 of those CYP2D6 and CYP2C19 genotype was of relevance. To study the effects of genetic variability on pharmacodynamic pathways, we reviewed 80 clinical studies on polymorphisms in candidate genes, but those did not for the most part reveal significant associations between neurotransmitter receptor and transporter genotypes and therapy response or adverse drug reactions. In addition associations found in one study could not be replicated in other studies. For this reason, it is not yet possible to translate pharmacogenetic parameters fully into therapeutic recommendations. At present, antidepressant and antipsychotic drug responses can best be explained as the combinatorial outcome of complex systems that interact at multiple levels. In spite of these limitations, combinations of polymorphisms in pharmacokinetic and pharmacodynamic pathways of relevance might contribute to identify genotypes associated with best and worst responders and they may also identify susceptibility to adverse drug reactions.

From pharmacokinetics to pharmacogenomics: a new approach to tailor immunosuppressive therapy

One of the main tasks in the management of organ transplantation is the optimization of immunosuppressive therapy, in order to provide therapeutic efficacy limiting drug-related toxicity. In the past years major efforts have been carried out to define therapeutic windows based on blood/plasma levels of each immunosuppressant relating those concentrations to drug dosing and clinical events. Although this traditional approach is able to identify environmental and nongenetic factors that can influence drug exposure during the course of treatment, it presents limitations. Therefore, complementary strategies are advocated. The advent of the genomic era gives birth to pharmacogenomics, a science that studies how the genome as a whole, including single genes as well as gene-to-gene interactions, may affect the action of a drug. This science is of particular importance for drugs characterized by a narrow therapeutic index, such as the immunosuppressants. Preliminary studies focused on polymorphisms of genes encoding for enzymes actively involved in drug metabolism, drug transport and pharmacological target. Pharmacogenomics holds promise for improvement in the ability to individualize immunosuppressive therapy based on the patient's genetic profile, and can be viewed as a support to traditional therapeutic drug monitoring. However, the clinical applicability of this approach is still to be proven.

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.

Hepatic drug metabolism and transport in patients with kidney disease

The disposition of many drugs is altered in patients with acute (AKD) and chronic kidney disease (CKD). A decline in renal clearance of several drugs has been correlated significantly with residual renal function (ie, creatinine clearance) of subjects. Reductions in nonrenal clearance of some compounds also have been reported and associated with clearance of markers of oxidative and/or conjugative metabolism or P-glycoprotein-mediated transport. Although initial accounts of reduced hepatic microsomal cytochrome P-450 (CYP) content and activity in animal models of AKD and CKD were published almost 25 years ago, it is only in the last decade that technical advances in molecular biology and clinical pharmacology have enabled researchers to begin to characterize the phenotypic expression of individual enzymes and, importantly, distinguish the molecular and/or genetic basis for these changes. The selective modulation of hepatic CYP enzyme activity observed in kidney disease is caused, at least in part, by differentially altered expression of several CYP isoforms. This review summarizes data available through June 2003 regarding the effect of AKD and CKD on drug metabolism. Knowledge of the impact and nature of these alterations associated with kidney disease may facilitate the individualization of medication management in this patient population.

Evidence for the validity of cortisol 6 beta-hydroxylation clearance as a new index for in vivo cytochrome P450 3A phenotyping in humans

This study uses stable isotope methodology to evaluate the validity of 6beta-hydroxylation clearance of endogenous cortisol as a new index for in vivo CYP3A phenotyping in humans. Important factors contradictory to the use of a conventional index of urinary ratio of 6beta-hydroxycortisol to cortisol (6beta-OHF/F) to evaluate in vivo CYP3A activity are also discussed. Stable isotopically labeled cortisol (3-5 mg) was orally administered to three healthy adult subjects to accurately determine the fractional metabolic clearance specific for the 6beta-hydroxylation of cortisol. Plasma concentrations of labeled cortisol and urinary excreted amounts of labeled cortisol and 6beta-OHF were analyzed by gas chromatography-mass spectrometry simultaneously with their endogenous counterparts. There was a good correlation between endogenous and exogenous 6beta-hydroxylation clearances in the three subjects tested (r = 0.7733, 0.9112, and 0.9534 for 2-, 4-, and 6- to 8-h urine collection periods, respectively). This strongly suggests that the endogenous 6beta-hydroxylation clearance can be used as an appropriate index for phenotyping the in vivo CYP3A activity. Furthermore, observed intra- (2.1- to 4.6-fold) and interindividual variabilities (ca. 5-fold) in the labeled cortisol renal clearance suggest that the urinary ratio 6beta-OHF/F, a function of 6beta-hydroxylation clearance and renal clearance of cortisol, does not always reflect the in vivo CYP3A activity. When a macrolide antibiotic, clarithromycin, was administered to a healthy volunteer in a dose of 200 mg every 12 h for 6 days, the inhibitory effects of clarithromycin on the in vivo CYP3A activity were clearly seen by the 6beta-hydroxylation clearance of endogenous cortisol but not by the urinary ratio 6beta-OHF/F.

Pharmacogenetics of cytochrome p4502D6: genetic background and clinical implication

Interindividual differences in the pharmacokinetics of a number of drugs are often due to hereditary polymorphisms of drug-metabolizing enzymes. Most important is cytochrome p4502D6 (CYP2D6), also known as debrisoquine/sparteine hydroxylase. It catalyzes hydroxylation or demethylation of more than 20% of drugs metabolized in the human liver, such as neuroleptics, antidepressants, some beta-blockers and many others like codeine. About 7%-10% of Caucasians lack any CYP2D6 activity due to deletions and frame-shift or splice-site mutations of the gene. About 1%-3% of Middle-Europeans, but up to 29% of Ethiopians display gene duplications, leading to elevated so-called ultrarapid metabolization rates. Meanwhile there is now a much better understanding of the genetic background of poor, intermediate, extensive and ultrarapid metabolizers, enabling a more precise DNA genotyping-based prediction of plasma levels. Since there is evidence that deteriorated drug elimination partly accounts for drug side-effects, CYP2D6 genotyping could contribute to an individualized and therefore optimized drug therapy.

CYP2C9 gene and susceptibility to major depressive disorder

Alteration of monoaminergic neurotransmission has been implicated in the pathophysiology of mood disorders, and CYP2C9 enzyme activity has been shown to be modulated by serotonin in vitro. The present study was aimed at analysing the frequency of CYP2C9 alleles (*1, *2, *3) among patients suffering from major depressive disorder. In all, 70 such suffering psychiatric outpatients were studied. The CYP2C9 genotypes were determined by allele-specific PCR. The CYP2C9*3 allele frequency was higher (P<0.01) among the patients suffering from major depression than in a population of 89 schizophrenic patients (odds ratio=3.3) and 138 healthy volunteers (odds ratio=2.8). The results suggest that CYP2C9 genetic polymorphism may be related to a major depressive disorder due to an alteration in endogenous metabolism, although a linkage between CYP2C9 and some other gene related to depression cannot be ruled out.

CSF taurine level is influenced by plasma cholesterol and the CYP2D6 phenotype

Eight healthy male volunteers, lumbar-punctured before and during simvastatin treatment, were phenotyped for CYP2D6 analysis of the debrisoquine metabolic ratio (the ratio between the urinary recovery of debrisoquine and its 4-hydroxy metabolite) after a single oral dose of debrisoquine. The mean cerebrospinal fluid concentrations of cholesterol and taurine did not differ before and during treatment. During (but not before) treatment taurine in the CSF correlated with the debrisoquine metabolic ratio (r=-0.93; P=0.0007) Our results might indicate an influence of CYP2D6 on the level of taurine in the CSF that was secondary to the change in plasma cholesterol.