Polymorphic cytochromes P450 and drugs used in psychiatry

The potential impact of CYP2D6 (*2/*4/*10) gene variants among Egyptian epileptic children: A preliminary study

BACKGROUND

The cytochrome P450 (CYP) isoenzymes have an indispensable role in the metabolic phase of different medications during the treatment of multiple neuropsychiatric disorders. The foremost goal of this study is to evaluate the correlation of the allelic variants within CYP2D6 (*2/*4/*10) gene with the susceptibility for epileptic syndrome as well as the assessment the degree of resistance towards antiepileptic drugs (AEDs).

METHODS

This work was designed based on the involvement of 200 participants [100 unrelated healthy controls, 50 AEDs responsive, and 50 AEDs resistant]. Genomic DNA for the CYP2D6 variants was genotyped utilizing the T-ARMS-PCR technique.

RESULTS

The distributions of the CYP2D6*2 (rs16947; c.886C > T) and CYP2D6*4 (rs3892097; c.506-1G > A) variants were significantly correlated with elevated risk among epileptic patients compared to healthy controls (P-value < 0.05). Furthermore, the CYP2D6*2 variant was statistically associated with disease risk among AEDs responsive patients, while the CYP2D6*4 variant was statistically correlated with disease risk among AEDs resistant patients (P-value < 0.05). Interestingly, the allelic variants of the CYP2D6*4 (A allele) and CYP2D6*10 (T allele) were associated with elevated risk among AEDs resistant compared to AEDs responsive patients (P-value = 0.008 and 0.040, respectively).

CONCLUSIONS

The CYP2D6*2 and CYP2D6*4 variants were recognized as independent risk factors among epileptic patients, but not the CYP2D6*10 variant.

Adherence to Antipsychotic Medication in Bipolar Disorder and Schizophrenic Patients: A Systematic Review

Antipsychotics are the drugs prescribed to treat psychotic disorders; however, patients often fail to adhere to their treatment, and this has a severe negative effect on prognosis in these kinds of illnesses. Among the wide range of risk factors for treatment nonadherence, this systematic review covers those that are most important from the point of view of clinicians and patients and proposes guidelines for addressing them. Analyzing 38 studies conducted in a total of 51,796 patients, including patients with schizophrenia spectrum disorders and bipolar disorder, we found that younger age, substance abuse, poor insight, cognitive impairments, low level of education, minority ethnicity, poor therapeutic alliance, experience of barriers to care, high intensity of delusional symptoms and suspiciousness, and low socioeconomic status are the main risk factors for medication nonadherence in both types of disorder. In the future, prospective studies should be conducted on the use of personalized patient-tailored treatments, taking into account risk factors that may affect each individual, to assess the ability of such approaches to improve adherence and hence prognosis in these patients.

Citalopram-Induced Long QT Syndrome and the Mammalian Dive Reflex

While SCUBA diving, a 44-year-old Caucasian patient had an abnormal cardiac rhythm, presumably Torsade de Pointes (TdP), during the initial descent to depth. Upon surfacing, she developed ventricular fibrillation and died. The patient had been treated for mild depression for nearly a year with citalopram 60 mg per day, a drug known to cause prolonged QT interval. She had also been treated with two potentially hepatotoxic drugs. Liver impairment causes selective loss of cytochrome P450 (CYP) 2C19 activity, the major pathway for metabolism of citalopram. The post mortem blood level of citalopram was 1300 ng/mL. The patient was found to be an intermediate metabolizer via CYP2D6, the major pathway for metabolism of desmethylcitalopram; the level of which was also abnormally high. It is suggested that drug-induced long QT syndrome (DILQTS), caused by citalopram, combined with the mammalian dive reflex triggered malignant ventricular rhythms resulting in the patient’s death. It is further suggested that, in general, the dive reflex increases the risk of fatal cardiac rhythms when the QT interval is prolonged by drugs.

Therapeutic drug monitoring: perspectives of psychiatrists in Turkey

OBJECTIVES

Although the medical and economic implications of therapeutic drug monitoring have been intensely discussed over the past years, little is known about the experiences and attitudes of psychiatrists in their clinical practice. The aim of this study was to investigate psychiatrists' daily practice with therapeutic drug monitoring in Turkey.

METHODS

A nation-wide cross-sectional survey among adult and child psychiatry specialist psychiatrists in Turkey was conducted.

RESULTS

We found that 98.4% (n = 380) of the study participants used TDM in clinical practice and 1.6% (n = 6) did not. However, TDM use is limited to mood stabilizers (lithium 96.3%, valproate 97.6%) to a great extent. Only a small number of psychiatrists perform TDM for other psychotropic drugs, e.g., clozapine 2.4%, tricyclic antidepressants 1.3%, benzodiazepines 1.1%, and selective serotonin reuptake inhibitors 0,8%.

CONCLUSIONS

Most of the psychiatrists in Turkey have a positive attitude toward use of therapeutic drug monitoring although there is also a considerable difficulty to reach services for the therapeutic drug monitoring of psychotropics other than mood stabilizers.

Pharmacogenetics in psychiatry: are we ready for widespread clinical use?

There are high expectations about the capabilities of pharmacogenetics to tailor psychotropic treatment and "personalize" treatment. While a large number of associations, with generally small effect size, have been discovered, a "test" with widespread use and adoption is still missing. A more realistic picture, recognizing the important contribution of clinical and environmental factors toward overall clinical outcome has emerged. In this emerging view, genetic findings, if considered individually, may have limited clinical applications. Thus, in recent years, combinations of information in several genes have been used for the selection of appropriate therapeutic doses and for the prediction of agranulocytosis, hyperlipidemia, and response to antipsychotic and antidepressant medications. While these tests based on multiple genes show greater predictive ability than individual allele tests, their net impact on clinical consequence and costs is limited, thus leading to limited penetration into widespread clinical use. As one looks at other branches of medicine, there are successful examples of pharmacogenetic tests guiding treatment, and thus, it is reasonable to hope that with the incorporation of clinical and environmental information and the identification of new genes drawn from genome-wide analysis, will improve the predictive utility of these tests leading to their increased use by clinicians.

Rates of In Vivo Methylation of Desipramine and Nortriptyline

Routine monitoring of an 81-year-old man receiving treatment with nortriptyline for generalized anxiety disorder and depression revealed plasma concentrations of both amitriptyline and nortriptyline. In humans, the tricyclic antidepressant (TCA) tertiary amines imipramine and amitriptyline are typically metabolized by demethylation to the secondary active metabolites desipramine and nortriptyline, respectively. However, to our knowledge, methylation of secondary amine TCAs has been reported in only one case report of nortriptyline overdose and in two studies involving desipramine. In a retrospective analysis of patients from five Veterans Affairs medical centers, the rate of methylation of desipramine and nortriptyline was 8.9 % (five of 56 patients) and 14.6% (36 of 247), respectively. Possible explanations for methylation include genetic polymorphisms in cytochrome P450 metabolizing enzymes, polymorphism of amine N-methyltransferase enzyme, drug-drug interactions, smoking, and alcohol consumption. However, the mechanism by which methylation occurs is unclear and warrants further investigation. Awareness of the phenomenon could help in discouraging repeated laboratory tests and unnecessary adjustments of drug therapies, resulting in cost savings and better patient outcomes.

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 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.

Pharmacogenetics of schizophrenia

Patients display significant differences in response to therapeutic agents which may be caused by a variety of factors. Among them, genetic components presumably play a major role. Pharmacogenetics is the field of research that attempts to unravel the relationship between genetic variation affecting drug metabolism (pharmacokinetic level) or drug targets (pharmacodynamic level) and interindividual differences in pharmacoresponse. In schizophrenia, pharmacokinetic studies have shown the role of genetic variants of the cytochrome P450 enzymes CYP2D6, CYP2C19, and CYP2C9 in the metabolism of neuroleptic drugs. At the level of the drug target, variants of the dopamine D3 and D4, and 5-HT2A and 5-HT2C receptors have been examined. A general problem of pharmacogenetic studies in schizophrenia is the high number of controversial findings which may be related to the lack of standardized phenotype definition. Recently, guidelines for an exact and comparable phenotype characterization have been proposed and will aid in designing and evaluating pharmacogenetic studies in the future. The final goal of pharmacogenetic studies-making a prediction of drug response at the level of the individual patient-will require a simultaneous look at a large number of response-determining genetic variants by applying the tools of pharmacogenomics, e.g. large-scale Single Nucleotide Polymorphism (SNP) detection and genotyping.

Toxicological interactions involving psychiatric drugs and alcohol: an update

This review focuses on the toxicological interactions between alcohol (ethanol) and psychiatric drugs (antidepressants and antipsychotics), including those leading to fatal poisoning. Acute or chronic ingestion of alcohol when combined with psychiatric drugs may lead to several clinically significant toxicological interactions. The metabolism of these drugs is generally but not always delayed by acute alcohol ingestion. Drugs undergoing metabolism may also show increased metabolic clearance with chronic alcohol ingestion. Therefore, the net effect may be influenced by internal (e.g. disease, age, gender), external (e.g. environment, diet) and pharmacokinetic (e.g. dose, timing of ingestion, gastrointestinal absorption, distribution and elimination) factors. Cases of fatal poisoning involving coadministration of psychiatric drugs, alcohol and other drugs prompted this review.

Antidepressant drugs in the elderly--role of the cytochrome P450 2D6

Depression, the most common mental health problem of the elderly, is often under-diagnosed and under-treated. As patients age, antidepressant pharmacologic treatment becomes more complicated due to an increased risk of adverse drug events. These risks are associated with age-related physiological changes and individual variability in drug metabolism related to several factors, the most frequent of which is polymedication as a result of coexisting chronic illnesses. Comedications induce drug interactions that depend on the patient's metabolic capacity linked to the genetically determined cytochrome P450 enzyme (CYP450) function. The effect of some isoenzyme polymorphism on the pharmacokinetics of many antidepressants and other psychotropic drugs is well characterized. The author approaches successively the notions of the cytochrome P450 (2D6), its role in the drug biotransformation, and the importance of knowing its substrates, inhibitors and inducers in order to predict drug interactions. The clinical significance of this notion, and the help that could be given by genotyping and phenotyping, are also explained. The author's experience on the relation between drug side effects and patient metabolic status, and on the antidepressant interactions with fluoxetine, fluvoxamine and citalopram, is given in order to rationalize and individualize antidepressant choice in elderly.

Cytochrome P450 2J2 Polymorphism in Healthy Caucasians and those with Diabetes Mellitus

OBJECTIVE

Cytochrome P450 (CYP) 2J2 plays an important role in the biosynthesis of the biologically active cis-epoxyeicosatrienoic acids. An allelic variant named CYP2J2*6, which encodes an enzyme that is almost inactive in the metabolism of arachidonic acid, has recently been described. We investigated the frequency of the CYP2J2*6 variant in a Caucasian population and the relationship between this polymorphism and the development of micro- and macrovascular complications and hypertension in patients with type 1 or type 2 diabetes mellitus.

METHODS

Genomic DNA was extracted from peripheral blood cells and the fragment containing the A/T single nucleotide polymorphism at position 25 661 in exon 8 of the CYP2J2 gene was amplified. The 532 bp amplified product was subsequently digested with Tsp509I and analyzed on 12% polyacrylamide gel electrophoresis.

RESULTS

In the whole population, the frequency of the CYP2J2*6 allele was 0.0064 and the frequency of the CYP2J2*1 allele was 0.9936. Genotype distribution did not show significant differences between controls and patients with type 1 or type 2 diabetes. No homozygotes for CYP2J2*6 allele were found. No association was found between this allele and complications or hypertension in either type of diabetes.

CONCLUSION

The CYP2J2*6 allele is rare in the Caucasian population, and no association is inferred between this allelic variant and diabetic complications.

Genetic and environmental factors in cancer and neurodegenerative diseases

The aim of this review is to summarise the recent findings in the fields of carcinogenesis and neurodegenerative diseases, the both disorders are characterised by the contribution of different factors including the inheritance of mutated genes, and the exposure to endogenous or exogenous agents during the life. We first analysed the causative genes until now discovered in both processes, then we focused our attention on the role of environmental exposure, susceptibility factors, oxidative stress, apoptosis and aging to the development of such disorders. The genotype at a particular locus may account for an inter-individual susceptibility that can both increase or decrease the risk to develop the pathology especially after the exposure to environmental agents. The mechanism of apoptosis, that is an excellent strategy in order to eliminate damaged cells, seems to be lost during carcinogenesis, while it seems to be involved in the neuronal death in a lot of neurodegenerative disorders. Oxidative stress can both lead to DNA mutations or to the formation of damaged proteins, so being an important risk factor for the initiation and the progression of a disease: in fact it may be one of the causes or can arise as a consequence of a damage caused by other factors increasing then the first damage. It is well established that carcinogenesis is a multi-step process caused by series of successive mutations occurring into a cell and conferring to this cell a growth advantage, so that age is the largest risk factor for cancer in humans. Pathophysiology of neurodegenerative diseases is complex and likely involves multiple overlapping and perhaps redundant pathways of neuronal damage, characterised by the generation of anomalous proteins, often due to mutations in the corresponding gene, and by their subsequent accumulation into or outside specific areas of the brain.

Pharmacogenetics for the individualization of psychiatric treatment

Drug treatment of psychiatric disorders is troubled by severe adverse effects, low compliance and lack of efficacy in about 30% of patients. Pharmacogenetic research in psychiatry aims to elucidate the reasons for treatment failure and adverse reactions. Genetic variations in cytochrome P450 (CYP) enzymes have the potential to directly influence the efficacy and tolerability of commonly used antipsychotic and antidepressant drugs. The activity of psychiatric drugs can also be influenced by genetic alterations affecting the drug target molecule. These include the dopaminergic and serotonergic receptors, neurotransmitter transporters and other receptors and enzymes involved in psychiatric disorders. Association studies investigating the relation between genetic polymorphisms in metabolic enzymes and neurotransmitter receptors on psychiatric treatment outcome provide a step towards the individualization of psychiatric treatment through enabling the selection of the most beneficial drug according to the individual's genetic background.

Transplacental transfer of citalopram, fluoxetine and their primary demethylated metabolites in isolated perfused human placenta

OBJECTIVE

To investigate the transplacental transfer and the effects of protein binding on the transfer of citalopram, desmethylcitalopram, fluoxetine and desmethylfluoxetine in the isolated perfused human placenta model.

DESIGN

Prospective observational study.

METHODS

Fifteen term human placentas were obtained immediately after delivery with maternal consent and a 2-hour non-recirculating perfusion cycle of a single placental cotyledon was set up. Citalopram (1230 nmol/L) and desmethylcitalopram (600 nmol/L) or fluoxetine (1455 nmol/L) and desmethylfluoxetine (1525 nmol/L) were added to the maternal reservoir and their appearance to the fetal circulation was followed by repeated measurements. To investigate the effect of protein binding on the transfer of citalopram and fluoxetine, nine additional perfusions were performed without albumin in the perfusion medium. Citalopram and desmethylcitalopram concentrations were measured by reversed-phase high performance liquid chromatography. Fluoxetine and desmethylfluoxetine concentrations was measured by gas chromatography and antipyrine (used as a reference compound) concentrations spectrophotometrically.

RESULTS

The mean (SD) steady-state transplacental transfer (TPT(SS)%) for citalopram, desmethylcitalopram, fluoxetine and desmethylfluoxetine was 9.1%, 5.6% (P = 0.017 compared with citalopram), 8.7% and 9.1%, respectively, calculated as the ratio between the steady-state concentrations in fetal venous and maternal arterial sides. The TPT(SS)%s of citalopram, desmethylcitalopram, fluoxetine and desmethylfluoxetine were 86%, 50%, 88% and 91% of that of freely diffusable antipyrine. The absence of albumin significantly reduced the transfer of citalopram and fluoxetine (TPT(SS)% 1.1% and 4.8%, respectively) but not the transfer of antipyrine.

CONCLUSION

Citalopram, fluoxetine and desmethylfluoxetine all cross the human placenta, and may, therefore, affect the perinatal outcome of infants exposed to these drugs during pregnancy. The transfer of desmethylcitalopram was significantly lower, which in the clinical setting may suggest lower fetal exposure of serotonin re-uptake inhibition by citalopram compared with fluoxetine. The presence of albumin was necessary for the transplacental transfer of both citalopram and fluoxetine.

Single-Step Assays to Analyze CYP2D6 Gene Polymorphisms in Asians: Allele Frequencies and a Novel *14B Allele in Mainland Chinese

Background: Cytochrome P450-dependent monooxygenase 2D6 (CYP2D6) activity can be estimated by investigating the metabolism of model drugs or by genotyping the most common CYP2D6 alleles. For Caucasians, the CYP2D6 allele frequencies are well investigated, and single-step assays are available for genotyping, whereas allele analysis in mainland Chinese is limited.

Methods: Two tetra-primer assays and one allele-specific amplification assay were developed to easily genotype the CYP2D6 alleles *8, *10, and *14 previously detected in Asians. Applying these assays in combination with established single-tube assays, we analyzed 223 DNA samples from Chinese volunteers for the CYP2D6 alleles *3, *4, *5, *6, *8, *10, and *14 and for duplication of CYP2D6.

Results: Six different alleles were detected in mainland Chinese. The most frequent mutant allele was the intermediate metabolizer allele, CYP2D6*10, with a prevalence of 51.3%, followed by the poor metabolizer alleles CYP2D6*5 (7.2%) and a novel variant of CYP2D6*14. This novel *14B allele (2.0%) differs from the *14 allele by the absence of the C188T substitution and by the additional G1749C substitution. Furthermore, six duplication alleles of CYP2D6 were detected, including one duplication of the *10 allele (*10X2).

Conclusions: The CYP2D6 allele frequencies in mainland Chinese shows some genetic diversity compared with Chinese from other regions: a novel *14B allele, a slightly higher frequency of the *5 allele, and a slightly lower frequency of the *10 allele than in most other Chinese populations.

Inhibition kinetics of monoclonal antibodies against cytochromes P450

Monoclonal antibodies (MAbs) inhibitory to individual cytochromes P450 (P450s) are of tremendous utility in identification of P450s responsible for the metabolism of a given drug or drug candidate in pharmaceuticals. In the present study, two inhibitory MAbs against CYP2D6 (MAb(2D6-50,) IgG(2b) and MAb(2D6-184), IgG(2a)) were developed by hybridoma technology to exhibit their high specificity and potency. The MAbs were further employed to assess the quantitative role (47-93%) of CYP2D6 to the metabolism of bufuralol in human liver microsomes from seven donors. Together with the MAb inhibitory to CYP3A4 as previously reported (Mei et al., 1999), the MAbs were used to study the inhibition kinetics of dextromethorphan O-demethylation (CYP2D6), testosterone 6beta-hydroxylation (CYP3A4) and aflatoxin B1 3-hydroxylation (CYP3A4), respectively, with an adequate size of sample measurement. A kinetic model was proposed to fit the experimental observations with three-dimensional nonlinear regression, thereby resulting in a solution of kinetic parameters, i.e., K(I), K(S), V(max), alpha, and beta (changes in K(I) or K(S) and V(max) in the presence of the MAb). As a result, dissociation constants (K(I)) of the MAbs for the enzymes and the maximal inhibition (beta) values for the P450-catalyzed reactions were predicted to have 0.04 to 0.25 microM and > or =94%, respectively. The results have demonstrated that the model can accurately predict the kinetic parameters and provide some insights into the understanding of the mechanism of MAb interaction with P450 enzyme in nature and the applications of the MAbs in qualitative and quantitative identification of P450s involved in drug metabolism.

CYP2D6 and CYP2C19 genotype-based dose recommendations for antidepressants: a first step towards subpopulation-specific dosages

OBJECTIVE

This review aimed to provide distinct dose recommendations for antidepressants based on the genotypes of cytochrome P450 enzymes CYP2D6 and CYP2C19. This approach may be a useful complementation to clinical monitoring and therapeutic drug monitoring.

METHOD

Our literature search covered 32 antidepressants marketed in Europe, Canada, and the United States. We evaluated studies which had compared pharmacokinetic parameters of antidepressants among poor, intermediate, extensive and ultrarapid metabolizers.

RESULTS

For 14 antidepressants, distinct dose recommendations for extensive, intermediate and poor metabolizers of either CYP2D6 or CYP2C19 were given. For the tricyclic antidepressants, dose reductions around 50% were generally recommended for poor metabolizers of substrates of CYP2D6 or CYP2C19, whereas differences were smaller for the selective serotonin reuptake inhibitors.

CONCLUSION

We have provided preliminary average dose suggestions based on the phenotype or genotype. This is a first attempt to apply the new pharmacogenetics to suggest dose-regimens that take the differences in drug metabolic capacity into account.

Review of the pharmacokinetics, pharmacogenetics, and drug interaction potential of antidepressants: focus on venlafaxine

Improving outcomes for patients with depression involves selecting the best possible drug therapy. Considerations relevant to drug product selection include: 1) pharmacokinetic issues such as half-life and time to steady-state, and protein binding; 2) pharmacodynamic drug-drug interactions; and 3) drug metabolism-related drug interactions. A comparison of selected antidepressants with an emphasis on venlafaxine's similarities and differences is presented. Based on these parameters, selecting an antidepressant medication, such as venlafaxine, that has a low potential for drug interactions at the Cytochrome P450 (CYP) enzyme system, and is easy to monitor and dose, facilitate successful treatment of patients. Venlafaxine has been evaluated in clinical studies that demonstrate low to negligible drug interaction potential at CYP2D6, CYP1A2, CYP2C19, and CYP3A4. Its short half-life and time to steady-state, when coupled with the extended release characteristics of the preferred dosage formulation allow for once daily dosing and rapid attainment of therapeutic effects. The CYP3A4 system is involved in both first-pass metabolism and systemic clearance of medications. Drug interactions at this isoenzyme have proven to be of high clinical relevance ranging from cardiovascular toxicity and death with commonly used drugs such as cisapride, to subtherapeutic levels of cyclosporine or protease inhibitors leading to transplant rejection or HIV relapse. Reasons for the under detection and reporting of drug interaction mediated adverse events include healthcare system structure, the poor return to follow up of non-adherent patients, the need for greater education and training of clinicians to recognize drug-related adverse events, and the reluctance of patients to spontaneously communicate about the unpleasant effects of their medication.

Analysis of the CYP2D6 gene polymorphism and enzyme activity in African-Americans in southern California

Despite its importance in drug metabolism and disease susceptibility, CYP2D6 activity and genetic polymorphism have rarely been investigated in African-American populations. In order to bridge this gap, we examined the genotype and phenotype of the enzyme in 154 African-American (AA) and 143 Caucasian (C) normal volunteers. AAs are significantly more likely to possess *17 and *5, but less likely to have *4. Overall, the two groups were similar in their CYP2D6 activity as measured with dextromethorphan as the probe (metabolic ratio 2.21 +/- 0.78 for AAs; 2.11 +/- 0.86 for Cs; t = 1.02, NS). Two of four AAs and six of seven Cs were classified as poor metabolizers and have two nonfunctioning alleles. CYP2D6 activity is determined by *17, *4, *5 and age in AAs (r2 = 0.33, f = 18.8, P < 0.001) and by *4 and *XN in Cs (r2 = 0.14, f = 10.8, P < 0.001). These results support previous findings demonstrating the importance of *17 in determining CYP2D6 activity in AAs.

Applications of pharmacogenetics in psychiatry: personalisation of treatment

In spite of the lack of epidemiological information, pharmacogenetic research has produced evidence of the relationship between genes and treatment response. Genetic variants of metabolic enzymes are related to toxic reactions; polymorphisms in genes coding for drug-targeted neurotransmitter receptors influence therapeutic efficacy. Also, recent studies have shown that response to antipsychotic drugs can be predicted by looking at the individual's pharmacogenetic profile. In addition to providing the first evidence that treatment response can be predicted by looking at a core of key genes, these studies illustrate the feasibility of individualisation of psychiatric treatment.

Pharmacogenomics and schizophrenia

Although antipsychotic drugs are effective in alleviating schizophrenic symptoms, individual differences in patient response suggest that genetic components play a major role, and pharmacogenetic studies have indicated the possibility for a more individually based pharmacotherapy. The new field of pharmacogenomics, which focuses on genetic determinants of drug response at the level of the entire human genome, is important for development and prescription of safer and more effective individually tailored drugs. DNA microarray (DNA chip) analysis enables genome-wide scanning, using the high-density single nucleotide polymorphisms map. Pharmacogenomics will aid in understanding how genetics influence disease development and drug response, and contribute to discovery of new treatments. The rate of discovery of those polymorphisms will depend on the quality of the drug response phenotype. Prospective genotyping of schizophrenic patients for the many genes at the level of the drug target, drug metabolism, and disease pathways will contribute to individualized therapy matching the patient's unique genetic make-up with an optimally effective drug.

Rapid Detection of the CYP2D6*3, CYP2D6*4, and CYP2D6*6 Alleles by Tetra-Primer PCR and of the CYP2D6*5 Allele by Multiplex Long PCR

Background: Interindividual differences in CYP2D6 activity range from total absence of metabolism of certain drugs to ultrafast metabolism and can produce adverse effects or lack of therapeutic effect under standard therapy. Several mutations have been described in the CYP2D6 gene that abolish CYP2D6 activity. However, four mutations explain the majority of the poor metabolizers. We describe four single-tube assays to detect these mutations.

Methods: Three tetra-primer PCR assays were developed to detect the mutations in the CYP2D6*3, *4, and *6 alleles. In these single-tube assays, the CYP2D6 locus is amplified directly, followed by the allele-specific amplification on this new template. In addition, a multiplex long PCR was developed to genotype the CYP2D6*5 allele. Two long PCR amplifications for detection of the deletion of CYP2D6 (*5) and for detection of the CYP2D6 gene region were combined in one tube.

Results: Analysis of 114 alleles showed no CYP2D6*3 allele, and allele frequencies of 28.1% for CYP2D6*4, 2.6% for CYP2D6*5, and 0.9% for CYP2D6*6. Re-analysis of the DNA samples by restriction fragment length polymorphism and sequencing analysis confirmed these results. Furthermore, re-analysis of sequenced genomic DNA by tetra-primer PCR analysis (7–11 times) always showed identical results.

Conclusions: Our set of single-tube assays allows rapid and reproducible genotyping of the majority of CYP2D6 poor metabolizers.

Update: genetic polymorphism of drug metabolizing enzymes in humans

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