Pharmacogenetics of the clozapine response

H2 antihistamines: May be useful for combination therapies in cancer?

Cancer morbimortality is still a great concern despite advances in research and therapies. Histamine and its receptors' ligands can modulate different biological responses according to the cell type and the receptor subtype involved. Besides the wide variety of histamine functions in normal tissues, diverse roles in the acquisition of hallmarks of cancer such as sustained proliferative signaling, resistance to cell death, angiogenesis, metastasis, altered immunity and modified microenvironment have been described. This review summarizes the present knowledge of the various roles of histamine H2 receptor (H2R) ligands in neoplasias. A bioinformatic analysis of human tumors showed dissimilar results in the expression of the H2R gene according to tumor type when comparing malignant versus normal tissues. As well, the relationship between patients' survival parameters and H2R gene expression levels also varied, signaling important divergences in the role of H2R in neoplastic progression in different cancer types. Revised experimental evidence showed multiple effects of H2R antihistamines on several of the cited hallmarks of cancer. Interventional and retrospective clinical studies evaluated different H2R antihistamines in cancer patients with two main adjuvant uses: improving antitumor efficacy (which includes regulation of immune response) and preventing toxic adverse effects produced by chemo or radiotherapy. While there is a long path to go, research on H2R antihistamines may provide new opportunities for developing more refined combination therapeutic strategies for certain cancer types to improve patients' survival and health-related quality of life.

Clozapine's multiple cellular mechanisms: What do we know after more than fifty years? A systematic review and critical assessment of translational mechanisms relevant for innovative strategies in treatment-resistant schizophrenia

Almost fifty years after its first introduction into clinical care, clozapine remains the only evidence-based pharmacological option for treatment-resistant schizophrenia (TRS), which affects approximately 30% of patients with schizophrenia. Despite the long-time experience with clozapine, the specific mechanism of action (MOA) responsible for its superior efficacy among antipsychotics is still elusive, both at the receptor and intracellular signaling level. This systematic review is aimed at critically assessing the role and specific relevance of clozapine's multimodal actions, dissecting those mechanisms that under a translational perspective could shed light on molecular targets worth to be considered for further innovative antipsychotic development. In vivo and in vitro preclinical findings, supported by innovative techniques and methods, together with pharmacogenomic and in vivo functional studies, point to multiple and possibly overlapping MOAs. To better explore this crucial issue, the specific affinity for 5-HT₂R, D1R, α_2c, and muscarinic receptors, the relatively low occupancy at dopamine D2R, the interaction with receptor dimers, as well as the potential confounder effects resulting in biased ligand action, and lastly, the role of the moiety responsible for lipophilic and alkaline features of clozapine are highlighted. Finally, the role of transcription and protein changes at the synaptic level, and the possibility that clozapine can directly impact synaptic architecture are addressed. Although clozapine's exact MOAs that contribute to its unique efficacy and some of its severe adverse effects have not been fully understood, relevant information can be gleaned from recent mechanistic understandings that may help design much needed additional therapeutic strategies for TRS.

Clozapine Pharmacogenetic Studies in Schizophrenia: Efficacy and Agranulocytosis

Clozapine is an efficacious atypical antipsychotic for treatment-refractory schizophrenia. Clinical response and appearance of adverse events vary among individual patients receiving clozapine, with genetic and non-genetic factors potentially contributing to individual variabilities. Pharmacogenetic studies investigate associations between genetic variants and drug efficacy and toxicity. To date, most pharmacogenetic studies of clozapine have been conducted through candidate gene approaches. A recent advance in technology made it possible to perform comprehensive genetic mapping underlying clinical phenotypes and outcomes, which allow novel findings beyond biological hypotheses based on current knowledge. In this paper, we will summarize the studies on clozapine pharmacogenetics that have extensively examined clinical response and agranulocytosis. While there is still limited evidence on clozapine efficacy, recent genome-wide studies provide further evidence of the involvement of the human leukocyte antigen (HLA) region in clozapine-induced agranulocytosis.

Underuse of clozapine in treatment-resistant schizophrenia

Up to a third of patients with schizophrenia develop treatment resistance. Clozapine has been established as the most effective antipsychotic medication for treatment-resistant schizophrenia. However, it is significantly underutilised in this population. Possible reasons include clinicians' lack of experience with the drug and negative attitudes towards it. African-Caribbean patients are less likely to be treated with clozapine compared with their White counterparts and more likely to have treatment discontinued due to perceived risk of agranulocytosis. This paper discusses the current evidence relating to the underuse of clozapine and factors responsible for this.

Clozapine as a Model for Antipsychotic Development

Schizophrenia is a devastating illness that affects up to 1% of the population; it is characterized by a combination of positive symptoms, negative symptoms, and cognitive impairment. Currently, treatment consists of one class of medications known as antipsychotics, which include typical (first-generation) and atypical (second-generation) agents. Unfortunately, antipsychotic medications have limited efficacy, with up to a third of patients lacking a full response. Clozapine, the first atypical antipsychotic developed, is the only medication shown to be superior to all other antipsychotics. However, owing to several life-threatening side effects and required enrollment in a registry with routine blood monitoring, clozapine is greatly underutilized in the US. Developing a medication as efficacious as clozapine with limited side effects would likely become the first-line therapy for schizophrenia and related disorders. In this review, we discuss the history of clozapine, landmark studies, and its clinical advantages and disadvantages. We further discuss the hypotheses for clozapine's superior efficacy based on neuroreceptor binding, and the limitations of a receptor-based approach to antipsychotic development. We highlight some of the advances from pharmacogenetic studies on clozapine and then focus on studies of clozapine using unbiased approaches such as pharmacogenomics and gene expression profiling. Finally, we examine how these approaches could provide insights into clozapine's mechanism of action and side-effect profile, and lead to novel and improved therapeutics.

Treatment-resistant schizophrenia: current insights on the pharmacogenomics of antipsychotics

Up to 30% of people with schizophrenia do not respond to two (or more) trials of dopaminergic antipsychotics. They are said to have treatment-resistant schizophrenia (TRS). Clozapine is still the only effective treatment for TRS, although it is underused in clinical practice. Initial use is delayed, it can be hard for patients to tolerate, and clinicians can be uncertain as to when to use it. What if, at the start of treatment, we could identify those patients likely to respond to clozapine - and those likely to suffer adverse effects? It is likely that clinicians would feel less inhibited about using it, allowing clozapine to be used earlier and more appropriately. Genetic testing holds out the tantalizing possibility of being able to do just this, and hence the vital importance of pharmacogenomic studies. These can potentially identify genetic markers for both tolerance of and vulnerability to clozapine. We aim to summarize progress so far, possible clinical applications, limitations to the evidence, and problems in applying these findings to the management of TRS. Pharmacogenomic studies of clozapine response and tolerability have produced conflicting results. These are due, at least in part, to significant differences in the patient groups studied. The use of clinical pharmacogenomic testing - to personalize clozapine treatment and identify patients at high risk of treatment failure or of adverse events - has moved closer over the last 20 years. However, to develop such testing that could be used clinically will require larger, multicenter, prospective studies.

Pharmacogenetics of clozapine response and induced weight gain: A comprehensive review and meta-analysis

Clozapine (CLZ) is the prototype atypical antipsychotic and it has many advantages over other antipsychotic drugs. Several data suggest that both CLZ response and induced weight gain are strongly determined by genetic variability. However, results remain mainly inconclusive. We aim to review the literature data about pharmacogenetics studies on CLZ efficacy, focusing on pharmacodynamic genes. Further, we performed meta-analyses on response when at least three studies for each polymorphism were available. Sensitivity analyses were conducted on Caucasian population when feasible. Electronic literature search was performed to identify pertinent studies published until May 2014 using PubMed, ISI Web of Knowledge and PsycINFO databases. For meta-analyses, data were entered and analyzed through RevMan version 5.2 using a random-effect model. Our literature search yielded 9266 articles on CLZ; among these, we identified 59 pertinent pharmacogenetic studies. Genotype data were retrieved for 14 polymorphisms in 9 genes. Among these, we had available data from at least three independent samples for 8 SNPs in 6 genes to perform meta-analyses: DRD2 rs1799732, DRD3 rs6280, HTR2A rs6313, rs6311, rs6314, HTR2C rs6318, HTR3A rs1062613, TNFa rs1800629. Although literature review provided conflicting results, in meta-analyses three genetic variants within serotonin genes resulted associated to CLZ response: rs6313 and rs6314 within HTR2A gene and rs1062613 within HT3A gene. On the other hand, no clear finding emerged for CLZ-induced weight gain. Our results suggest a possible serotonergic modulation of CLZ clinical response.

A review of genetic alterations in the serotonin pathway and their correlation with psychotic diseases and response to atypical antipsychotics

Serotonin is a neurotransmitter that plays a predominant role in mood regulation. The importance of the serotonin pathway in controlling behavior and mental status is well recognized. All the serotonin elements - serotonin receptors, serotonin transporter, tryptophan hydroxylase and monoamine oxidase proteins - can show alterations in terms of mRNA or protein levels and protein sequence, in schizophrenia and bipolar disorder. Additionally, when examining the genes sequences of all serotonin elements, several single nucleotide polymorphisms (SNPs) have been found to be more prevalent in schizophrenic or bipolar patients than in healthy individuals. Several of these alterations have been associated either with different phenotypes between patients and healthy individuals or with the response of psychiatric patients to the treatment with atypical antipsychotics. The complex pattern of genetic diversity within the serotonin pathway hampers efforts to identify the key variations contributing to an individual's susceptibility to the disease. In this review article, we summarize all genetic alterations found across the serotonin pathway, we provide information on whether and how they affect schizophrenia or bipolar disorder phenotypes, and, on the contribution of familial relationships on their detection frequencies. Furthermore, we provide evidence on whether and how specific gene polymorphisms affect the outcome of schizophrenic or bipolar patients of different ethnic groups, in response to treatment with atypical antipsychotics. All data are discussed thoroughly, providing prospective for future studies.

Genetics of schizophrenia: implications for treatment

Schizophrenia is a common, debilitating illness for which treatment is empirical and unsatisfactory. Intense efforts to identify etiological factors have been launched in order to facilitate rational therapy. Such efforts have included gene-mapping studies since a significant heritability has been proved. In common with other polygenic/multifactorial disorders, mapping efforts for schizophrenia pose daunting challenges. Faced with such complexities, attempts to detect genetic associations with pharmacological response have been initiated. Although intriguing associations have been reported, formal replication is required. Suggestions for the design of replicate studies are proposed. This review is restricted to studies that investigated response to clozapine (Clozaril, Novartis) and other novel antipsychotic medications.

Scientific challenges and implementation barriers to translation of pharmacogenomics in clinical practice

The mapping of the human genome and subsequent advancements in genetic technology had provided clinicians and scientists an understanding of the genetic basis of altered drug pharmacokinetics and pharmacodynamics, as well as some examples of applying genomic data in clinical practice. This has raised the public expectation that predicting patients' responses to drug therapy is now possible in every therapeutic area, and personalized drug therapy would come sooner than later. However, debate continues among most stakeholders involved in drug development and clinical decision-making on whether pharmacogenomic biomarkers should be used in patient assessment, as well as when and in whom to use the biomarker-based diagnostic tests. Currently, most would agree that achieving the goal of personalized therapy remains years, if not decades, away. Realistic application of genomic findings and technologies in clinical practice and drug development require addressing multiple logistics and challenges that go beyond discovery of gene variants and/or completion of prospective controlled clinical trials. The goal of personalized medicine can only be achieved when all stakeholders in the field work together, with willingness to accept occasional paradigm change in their current approach.

Pharmacogenetics and antipsychotics: therapeutic efficacy and side effects prediction

IMPORTANCE OF THE FIELD

Antipsychotic drug is the mainstay of treatment for schizophrenia, and there are large inter-individual differences in clinical response and side effects. Pharmacogenetics provides a valuable tool to fulfill the promise of personalized medicine by tailoring treatment based on one's genetic markers.

AREAS COVERED IN THIS REVIEW

This article reviews the pharmacogenetic literature from early 1990s to 2010, focusing on two aspects of drug action: pharmacokinetics and pharmacodynamics. Genetic variants in the neurotransmitter receptors including dopamine and 5-HT and metabolic pathways of drugs including CYP2D6 and COMT were discussed in association with clinical drug response and side effects.

WHAT THE READER WILL GAIN

Readers are expected to learn the up-to-date evidence in pharmacogenetic research and to gain familiarity to the issues and challenges facing the field.

TAKE HOME MESSAGE

Pharmacogenetic research of antipsychotic drugs is both promising and challenging. There is consistent evidence that some genetic variants can affect clinical response and side effects. However, more studies that are designed specifically to test pharmacogenetic hypotheses are clearly needed to advance the field.

Influence of serotonin transporter gene polymorphisms on clozapine response in Brazilian schizophrenics

Schizophrenia is a severe mental illness affecting around 1% of adults with a high degree of morbidity and mortality. Affected individuals are at increased risk for unemployment, handicap, obesity, diabetes mellitus, hearth attack and suicide. Antipsychotic drugs are the best treatment for this disease, but about 20% of patients display drug resistance, or refractoriness, and may receive a special neuroleptic named Clozapine. Despite its superiority from other neuroleptics, only 30-60% of drug-resistant patients are responsive to clozapine. Clozapine's action results from interactions between dopaminergic and serotonergic neurotransmitter systems and since clozapine appears to exert its effect strongly through the serotonergic systems, alterations in serotonin synaptic levels may influence antipsychotic response. The serotonin transporter (5-HTT) is responsible for pre-synaptic re-uptake of serotonin, making this transporter a logical candidate gene for prediction of clozapine response and to increase understanding about mechanisms of refractoriness. Therefore, we investigated the influence of two polymorphisms in the 5-HTT gene (HTTLPR/rs25531 and VNTR Stin2) in clozapine response in a sample of 116 schizophrenic individuals of European descent from South-Brazil. Significant differences between responders and non-responders to clozapine were observed for the HTTLPR/rs25531 polymorphism. Nonresponders to clozapine showed a higher frequency of S'-allele (P = 0.01) and also were more likely to be S'/S' homozygous or S'/L' heterozygous than those who did respond (P = 0.04). After controlling for confounding variables, logistic regression analyses confirmed this association (OR = 3.15; 95% CI: 1.13-8.80). The observed association suggests that increased availability of extracellular serotonin concentrations at all synapses may reduce clozapine effect.

Review: The biological basis of antipsychotic response in schizophrenia

Schizophrenia is a severe mental illness affecting approximately 1% of the population worldwide. Antipsychotic drugs are effective in symptom control in up to two-thirds of patients, but in at least one-third of patients the response is poor. The reason for this is not clear, but one possibility is that good and poor responders have different neurochemical pathologies, and may therefore benefit from different treatment approaches. In this selective review we summarise research findings investigating the biological differences between patients with schizophrenia who show a good or a poor response to treatment with antipsychotic drugs.

Pharmacogenetics and psychoactive drug therapy: ready for the patient?

Psychiatry is one of the most promising areas for bringing pharmacogenomics to the patient. Psychiatric disorders such as depression and schizophrenia contribute significantly to worldwide morbidity and mortality. Forecasts rank depression second only to ischemic heart disease by 2020. In depression and schizophrenia, 30% to 50% of all patients do not respond sufficiently to the initial treatment regime. Genetic variability has been demonstrated to play an important role in the response to pharmacotherapy. Most data are available with regard to polymorphisms in the genes coding for drug-metabolizing enzymes and recommendations for the choice of personalized dosages based on genotyping results are available. Clinical outcome, in particular adverse effects, has been shown to correlate with the results from genotyping. Incorporating pharmacogenomics into clinical practice has, however, been slow and it is still not clear in which clinical situations genotyping should be performed and what the benefit of such procedures could be beyond therapeutic drug monitoring. Additionally, many studies in psychiatry focus on genetic variation in candidate genes of drug targets. However, despite promising reports, no clear recommendation can be given at present to perform such testing in clinical use.

Pharmacogenetics of antipsychotic treatment response and side effects

Antipsychotic drugs are particularly interesting in pharmacogenetic studies as they are associated with a large interindividual variability in terms of response and side effects and, therefore, frequently need to be discontinued, requiring switches to other antipsychotics. Any information that allows the prediction of outcome to a given antipsychotic in a particular patient will, therefore, be of great help for the clinician to minimize time and find the right drug for the right patient, thus optimizing response and minimizing side effects. This will also have a substantial impact on compliance and doctor-patient relationships. Moreover, antipsychotic drug treatments are often required for life-long treatment and are also frequently prescribed to the more 'vulnerable' populations: children, adolescents and the elderly. This article focuses on some important studies performed with candidate gene variants associated with antipsychotic response. In addition, important findings in pharmacogenetic studies of antipsychotic-induced side effects will be briefly summarized, such as antipsychotic treatment induced tardive dyskinesia and weight gain.

Pharmacogenetic of response efficacy to antipsychotics in schizophrenia: pharmacodynamic aspects. Review and implications for clinical research

Pharmacogenetics constitutes a new and growing therapeutic approach in the identification of the predictive factors of the response to antipsychotic treatment. This review aims to summarize recent finding into pharmacodynamic approach of pharmacogenetics of antipsychotics and particularly second generation. Studies were identified in the MEDLINE database from 1993 to July 2008 by combining the following Medical Subject Heading search terms: genetic, polymorphism, single nucleotide polymorphism, pharmacogenetics, antipsychotics, and response to treatment as well as individual antipsychotics names. Only pharmacodynamics studies were analyzed and we focused on efficacy studies. We also reviewed the references of ll identified articles. Most studies follow a polymorphism-by-polymorphism approach, and concern polymorphisms of genes coding for dopamine and serotonin receptors. Haplotypic approach has been considered in some studies. Few have studied the combinations of polymorphisms of several genes as a predictive factor of the response to antipsychotics. We present this gene-by-gene approach while detailing the features of the polymorphisms being studied (functionality, linkage disequilibrium) and the features of the studies (studied treatment(s), prospective/retrospective study, pharmacological dosage). We discuss the heterogeneity of the results and their potential clinical implications and extract methodological suggestions for the future concerning phenotype characterization, genotypes variants studied and methodological and statistical approach.

Pharmacogenetic testing in psychiatry: a review of features and clinical realities

This article focuses on the first generation of pharmacogenetic tests that are potentially useful in psychiatry. All pharmacogenetic tests currently on the market, or soon to be marketed in psychiatry, for which some information has been published in peer-reviewed journal articles (or abstracts), were selected. Five pharmacogenetic tests are reviewed in detail: the Roche AmpliChip CYP450 Test, the Luminex Tag-It Mutation Detection Kit, the LGC clozapine response test, the PGxPredict: Clozapine test, and the Genomas PhyzioType system. After reviewing these tests, three practical aspects of implementing pharmacogenetic testing in psychiatric clinical practice are briefly reviewed: (1) the evaluation of these tests in clinical practice, (2) cost-effectiveness, and (3) regulatory oversight. Finally, the future of these and other pharmacogenetic tests in psychiatry is discussed.

Pharmacogenomics: the promise of personalized medicine for CNS disorders

This review focuses first on the concept of pharmacogenomics and its related concepts (biomarkers and personalized prescription). Next, the first generation of five DNA pharmacogenomic tests used in the clinical practice of psychiatry is briefly reviewed. Then the possible involvement of these pharmacogenomic tests in the exploration of early clinical proof of mechanism is described by using two of the tests and one example from the pharmaceutical industry (iloperidone clinical trials). The initial attempts to use other microarray tests (measuring RNA expression) as peripheral biomarkers for CNS disorders are briefly described. Then the challenge of taking pharmacogenomic tests (compared to drugs) into clinical practice is explained by focusing on regulatory oversight, the methodological/scientific issues concerning diagnostic tests, and cost-effectiveness issues. Current information on medicine-based evidence and cost-effectiveness usually focuses on average patients and not the outliers who are most likely to benefit from personalized prescription. Finally, future research directions are suggested. The future of 'personalized prescription' in psychiatry requires consideration of pharmacogenomic testing and environmental and personal variables that influence pharmacokinetic and pharmacodynamic drug response for each individual drug used by each patient.

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.

Excess of transmission of the G allele of the -1438A/G polymorphism of the 5-HT2A receptor gene in patients with schizophrenia responsive to antipsychotics

BACKGROUND

The -1438A/G polymorphism of the 5-HT2A gene has been found to be associated with clinical response to clozapine and other second generation antipsychotics. Testing the impact of this marker on response to first generation antipsychotics (which have a lower affinity for the 5-HT2A receptor) provides the opportunity to help disentangling the two different roles that this polymorphism might have. A psychopharmacogenetic role should be detected only for antipsychotics with high affinity to the 5-HT2A receptor (therefore to second generation antipsychotics). An alternative role would imply tagging a subgroup of patients responsive to any antipsychotic, whatever their affinity, meaning that the association is more depending on non pharmacological charaterictics, such as clinical specificities.

METHODS

A family-based sample of 100 Algerian patients with schizophrenia (according to DSM-IV criteria) and their 200 biological parents was recruited, in order to avoid stratification biases. Patients were all treated, or have been treated, by conventional antipsychotics (mainly haloperidol) for at least four weeks, at appropriate dosage. May and Dencker scale was used to distinguish responders and non responders.

RESULTS

No allele of the -1438A/G polymorphism of the 5-HT2A gene was transmitted in excess (50 transmitted for 38 untransmitted) in the whole sample of patients with schizophrenia (p = .90). In contrast, a significant excess of transmission of the G allele was observed (p = .02) in the subgroup of patients with good treatment response (17 transmitted for 6 untransmitted).

CONCLUSION

Using a TDT approach, we showed that the G allele of the -1438A/G polymorphism of the gene coding for the 5-HT2A receptor was associated to schizophrenia with good response to conventional antipsychotics, although this conclusion is based on 88 informative patients only. Because previous data showed the same result with atypical antipsychotics, it can be concluded that the G allele tags a subgroup of schizophrenic patients with greater chance of improvement with antipsychotics of either type.

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.

The CNR1 gene as a pharmacogenetic factor for antipsychotics rather than a susceptibility gene for schizophrenia

Neurobiological research suggests a significant role of the endocannabinoid system in schizophrenia vulnerability and also in the quality of response to antipsychotics. Genetics offer an opportunity to disentangle its involvement in the disease vulnerability vs an influence on antipsychotics' effects. The possible role of a tag SNP (the 1359G/A polymorphism) of the gene encoding the cannabinoid receptor type 1 (CNR1) in schizophrenia and/or therapeutic response to atypical antipsychotics was assessed in a cohort of 133 French schizophrenic patients compared to 141 normal control subjects. No difference in 1359G/A polymorphism was observed between patients and control subjects, and no relationships were noted between this polymorphism and any clinical parameter considered as potential intermediate factor. However, the G allele frequency was significantly higher among non-responsive vs responsive patients, with a dose effect of the G allele. In contrast, no association was found for three other genetic polymorphisms of the CNR1 gene. The G allele of the CNR1 gene polymorphisms could be a psychopharmacogenetic rather than a vulnerability factor regarding schizophrenia and its treatment.

Pharmacogenetics and pharmacogenomics of schizophrenia: a review of last decade of research

The last decade of research into the pharmacogenetics of antipsychotics has seen the development of genetic tests to determine the patients' metabolic status and the first attempts at personalization of antipsychotic treatment. The most significant results are the association between drug metabolic polymorphisms, mainly in cytochrome P450 genes, with variations in drug metabolic rates and side effects. Patients with genetically determined CYP2D6 poor metabolizer (PMs) status may require lower doses of antipsychotic. Alternatively, CYP2D6 ultrarapid matabolizers (UMs) will need increased drug dosage to obtain therapeutic response. Additionally, polymorphisms in dopamine and serotonin receptor genes are repeatedly found associated with response phenotypes, probably reflecting the strong affinities that most antipsychotics display for these receptors. In particular, there is important evidence suggesting association between dopamine 2 receptor (D2) polymorphisms (Taq I and -141-C Ins/Del) and a dopamine 3 receptor (D3) polymorphism (Ser9Gly) with antipsychotic response and drug-induced tardive dyskinesia. Additionally, there is accumulating evidence indicating the influence of a 5-HT2C polymorphism (-759-T/C) in antipsychotic-induced weight gain. Application of this knowledge to clinical practice is slowly gathering pace, with pretreatment determination of individual's drug metabolic rates, via CYP genotyping, leading the field. Genetic determination of patients' metabolic status is expected to bring clinical benefits by helping to adjust therapeutic doses and reduce adverse reactions. Genetic tests for the pretreatment prediction of antipsychotic response, although still in its infancy, have obvious implications for the selection and improvement of antipsychotic treatment. These developments can be considered as successes, but the objectives of bringing pharmacogenetic and pharmacogenomic research in psychiatric clinical practice are far from being realized. Further development of genetic tests is required before the concept of tailored treatment can be applied to psychopharmatherapy. This review aims to summarize the key findings from the last decade of research in the field. Current knowledge on genetic prediction of drug metabolic status, general response and drug-induced side effects will be reviewed and future pharmacogenomic and epigenetic research will be discussed.

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.

Negative symptoms of schizophrenia could explain discrepant data on the association between the 5-HT2A receptor gene and response to antipsychotics

Pharmacogenetic studies assessing the role of 5-HT(2A) receptor gene in antipsychotic efficacy yielded conflicting data. Phenotypical heterogeneity of schizophrenia might explain such discrepancies. For example, negative symptoms are known to reflect severity of illness and to restrain therapeutic response. On this basis, we re-assessed the possible influence of the -1438A/G polymorphism of the 5-HT(2A) receptor gene on the clinical efficacy of atypical antipsychotics with focus on several relevant dimensions. One hundred and sixteen French schizophrenic subjects treated for at least 1 month by atypical antipsychotics were screened for treatment response according to the May and Dencker scale. Gender, age at onset, duration and severity of illness, intensity of negative and positive symptoms at discharge were investigated. The intensity of negative symptoms at discharge was the only variable explaining May and Dencker score (p < 0.001), and was significantly associated with the AA genotype of the -1438A/G polymorphism of the 5-HT(2A) receptor gene (p = 0.03). However, the A allele was not independently associated with refractoriness to atypical antipsychotics. Accordingly, the score reached in the Scale for the Assessment of Negative Symptoms (SANS) appeared as a confounding factor between therapeutic response and the -1438A/G polymorphism of the 5-HT(2A) receptor gene, at least in our sample. This data indicate that negative symptoms are worth being systematically assessed in pharmacogenetic studies aimed at analysing candidate genes in schizophrenia.

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.

Pharmacogenetics and psychiatry

Genetic factors play a significant role in predicting an individual's response to a drug. The response may be the desired therapeutic effect of the drug and also may be the undesirable development of adverse effects. This relationship between genes and drug response interests the pharmacogeneticist. This article aims to give an overview of the exciting discoveries made so far in the field of psychiatry, particularly concerning the response to antidepressants and antipsychotics, as well as to mention some of the more recent findings. The ultimate goal of pharmacogenetics is to provide medication "tailored" to the individual based on their genetic profile, and although this may currently seem a distant target, it has already begun to raise ethical questions, which also are discussed.

Candidate gene studies of antipsychotic drug efficacy and drug-induced weight gain

Converging data suggest that the identification of the molecular variants that influence antipsychotic drug response may soon be feasible. For the most part, these studies have focused on the new atypical antipsychotic agents, particularly clozapine. Although initial data in this regard has been inconclusive, recent studies have suggested that variation in the gene that codes for the dopamine D2 receptor (DRD2) may significantly influence the clinical efficacy of a number of typical and atypical antipsychotic drugs, perhaps via a variant that influences messenger RNA (mRNA) stability and translation. Studies of antipsychotic-induced weight gain have been more consistent than studies of antipsychotic drug efficacy, perhaps because weight dysregulation represents a more powerful phenotype for genetic studies, with a specific single nucleotide polymorphism (SNP) in the 5- hydroxytryptamine 2C (5-HT2c) receptor being associated with weight gain across diverse samples. Larger, more comprehensive studies are needed to confirm these results, but taken together, they suggest that pharmacogenetic strategies may be critical towards gaining a more precise understanding of the mechanism of action of antipsychotic drugs in the treatment of schizophrenia.

Role of pharmacogenomics in individualising treatment with SSRIs

The introduction of the SSRIs has significantly transformed the pharmacological treatment of a range of psychiatric disorders. In particular, individuals affected by depression, panic disorder, obsessive-compulsive disorder and social phobia have benefited substantially from their use. Compared with the previous generation of psychotropic drugs, SSRIs offer an improved tolerability to therapy while maintaining a high level of efficacy. Nevertheless, despite these advantages, not all patients benefit from treatment; an appreciable proportion do not respond adequately, while others may react adversely. This necessitates a review of the initial treatment choice, often involving extended periods of illness while a more suitable therapy is sought. Such a scenario could be avoided were it possible to determine the most suitable drug prior to treatment. Several factors are postulated to influence outcome of drug therapy; most recently, pharmacogenetic studies have demonstrated a significant influence of genetic mechanisms on the efficacy of clinically prescribed drugs. This contribution, which is primarily a reflection of alterations in genes that encode drug-metabolising enzymes, drug receptors, transporters and second messengers, may be pertinent to the success of SSRI therapy. Attesting to this potential, studies to elucidate the influence of genetic processes on SSRI efficacy now represent a major focus of pharmacogenetics research. Current evidence emerging from the field suggests that gene variants within the serotonin transporter and cytochrome P450 drug-metabolising enzymes may bear a particular importance, though further corroboration of these findings is still warranted. At the same time, it appears likely that further key participating genes remain to be identified. By comprehensively delineating these genetic components, it is envisaged that this will eventually facilitate the development of highly sensitive protocols for individualising SSRI treatment.

Investigation of promoter variants of the histamine 1 and 2 receptors in schizophrenia and clozapine response

We report the identification of four novel histamine 1 (H1-17-C/T, -974-C/A, -1023-A/G and -1536-G/C) and four novel histamine 2 promoter polymorphisms (H2-294-A/G, -592-A/G, -1018-G/A and -1077-G/A) which we have investigated for involvement in susceptibility to schizophrenia, and in clinical response to clozapine treatment. We identified a weak independent association between variants at the H1-1536-G/C locus and schizophrenia, where an excess of the H1-1536-C allele was observed amongst such patients (P = 0.036). However upon correction for multiple testing this relationship was no longer statistically significant. Similar investigation of the H2 receptor polymorphisms revealed association between genotype at the H2-1018-G/A locus and clinical response to clozapine treatment (P = 0.027), though upon correction for multiple testing this difference was no longer significant. We have concluded that the participation of these variants in the disorder is unlikely, particularly in view of their apparent lack of function and unlikely influence on receptor expression. However their nature alludes to the potential presence of other more important alterations further along these regions, where sequences encoding alternate promoters have recently been identified for each receptor that may yet be found to harbour such polymorphisms.

Receptor pharmacogenetics: relevance to CNS syndromes

Pharmacogenetic research dedicated to the investigation of inherited factors that influence drug response has produced exciting results over the past decade. Adding to the knowledge that genetic variation in metabolic enzymes may cause drug-related adverse reactions, recent studies indicate that variation in neurotransmitter receptors can also be the cause of treatment failure. In addition, recent studies have attempted to use genetic information for the prediction of treatment outcome. The aim of this review is to summarize the most significant findings in pharmacogenetic research in relation to CNS drugs and to outline how these studies could lead to the individualization of drug treatment.

Single nucleotide polymorphisms and their characterization with oligonucleotide microarrays

Small variations in human DNA sequences influence our predisposition to disease and our response to medications. Many psychiatric diseases appear to be polygenic. Because our molecular understanding of the genetic etiology of neuropsychiatric disorders is very limited, the discovery and characterization of these variations is crucial in identifying disease genes. Furthermore, this knowledge may enable the customized selection of drug treatments based on an individual's genotype so as to maximize efficacy yet avoid adverse side effects. Examples of findings using conventional methods for determining genotypes, as well as new and future technologies for the characterization of single nucleotide polymorphisms, will be discussed, with a focus on psychiatric diseases and medications.

Pharmacogenetic screening and therapeutic drugs

BACKGROUND

Pharmacogenetics is the science of the influence of heredity on pharmacological response.

ISSUES

The cost of severe adverse drug reactions in individuals has been estimated in the US alone to be in excess of US$4 billion. It has been argued that in a significant proportion of cases, the efficacy and toxicity profiles of drug therapy would be substantially improved in individuals if characteristics due to genetic variation were taken into account. Methods are now available, which make screening for susceptibility feasible.

CONCLUSIONS

There are several therapeutic areas in which screening may give rise to significant improvements in outcome with cost-benefits to both the individual and the community. However, there is currently a lack of data on which cost-benefit analysis can be based. The challenge is to provide this information for new drugs, and for drugs with established therapeutic roles.

Association analysis for NMDA receptor subunit 2B (GRIN2B) genetic variants and psychopathology and clozapine response in schizophrenia

It is known that a syndrome resembling schizophrenia is produced by the N-methyl-d-aspartate receptor antagonists. It has also been demonstrated that the level of an ionotropic N-methyl-d-aspartate 2B subunit (GRIN2B) of the glutamate receptor tends to increase after subchronic administration of clozapine, suggesting that GRIN2B may play an active role in the pathogenesis of schizophrenia and the function of clozapine medication. We studied 100 schizophrenic patients, investigating the associations for the GRIN2B genetic variants, and psychiatric symptoms and clozapine response. No significant differences were demonstrated comparing these three groups in terms of the baseline Brief Psychiatric Rating Scale (BPRS) score (P = 0.441). The percentage of patients scoring within 20% of baseline BPRS after clozapine treatment was similar for the three genotype groups (P = 0.132). A marginally higher mean clozapine dosage was revealed, however, for patients bearing the 2664C/C genotype (P = 0.013). Although replication of this research is required to confirm the results, an association for the GRIN2B C2664T polymorphism and clozapine treatment is suggested from our findings, which may assist in the prediction of optimal dosage for schizophrenic patients.

Candidate gene case-control association studies: advantages and potential pitfalls

There is increasing information on the importance of genetic polymorphisms in human genes. Polymorphisms occur on average once every 500-1000 base pairs in the human genome and are useful in the identification of genes involved in human disease. Some genetic polymorphisms have functionally significant effects on the gene product and are the most useful type of polymorphism in disease association studies while others are simply useful markers. There are two main approaches using polymorphisms in the identification of genes involved in polygenic diseases. The first involves examining inheritance patterns for genetic polymorphisms in family studies and the second case-control studies which compare genotype frequencies for candidate disease genes in unrelated individuals with the disease and healthy controls. Use of family studies is generally the preferred approach but this is only feasible if the genetic component of the disease is relatively strong, DNA samples are available from other family members and the disease is relatively easy to diagnose and is not stigmatized. Population case-control studies are useful both as an alternative and an adjunct to family studies. When performing case-control studies factors such as study design, methods for recruitment of cases and controls, functional significance of polymorphisms chosen for study and statistical analysis of data require close attention to ensure that only genuine associations are detected. To illustrate some potential problems in the design and interpretation of association studies, some specific examples of association studies on drug response and on disease susceptibility involving receptor genes, cytochrome P450 and other xenobiotic metabolizing enzyme genes and immune system genes including TNF-alpha, IL-10 and the IL-4 receptor are discussed.

Treatment-refractory schizophrenia

Treatment-refractory schizophrenia is common. Refinements in pharmacologic and psychosocial treatments of schizophrenia offer the expectation of superior outcomes for this disadvantaged patient group. This article critically reviews those articles that were published during the year 2000 that address this treatment-refractory population.