MDR1 gene polymorphisms and response to acute risperidone treatment

Pharmacogenetics and Schizophrenia-Can Genomics Improve the Treatment with Second-Generation Antipsychotics?

Schizophrenia (SCZ) is a complex psychiatric disorder of multifactorial origin, in which both genetic and environmental factors have an impact on its onset, course, and outcome. Large variability in response and tolerability of medication among individuals makes it difficult to predict the efficacy of a chosen therapeutic method and create universal and precise guidelines for treatment. Pharmacogenetic research allows for the identification of genetic polymorphisms associated with response to a chosen antipsychotic, thus allowing for a more effective and personal approach to treatment. This review focuses on three frequently prescribed second-generation antipsychotics (SGAs), risperidone, olanzapine, and aripiprazole, and aims to analyze the current state and future perspectives in research dedicated to identifying genetic factors associated with antipsychotic response. Multiple alleles of genes involved in pharmacokinetics (particularly isoenzymes of cytochrome P450), as well as variants of genes involved in dopamine, serotonin, and glutamate neurotransmission, have already been identified as ones of significant impact on antipsychotic response. It must, however, be noted that although currently obtained results are promising, trials with bigger study groups and unified protocols are crucial for standardizing methods and determining objective antipsychotic response status.

Genetic Testing for Antipsychotic Pharmacotherapy: Bench to Bedside

There is growing research interest in learning the genetic basis of response and adverse effects with psychotropic medications, including antipsychotic drugs. However, the clinical utility of information from genetic studies is compromised by their controversial results, primarily due to relatively small effect and sample sizes. Clinical, demographic, and environmental differences in patient cohorts further explain the lack of consistent results from these genetic studies. Furthermore, the availability of psychopharmacological expertise in interpreting clinically meaningful results from genetic assays has been a challenge, one that often results in suboptimal use of genetic testing in clinical practice. These limitations explain the difficulties in the translation of psychopharmacological research in pharmacogenetics and pharmacogenomics from bench to bedside to manage increasingly treatment-refractory psychiatric disorders, especially schizophrenia. Although these shortcomings question the utility of genetic testing in the general population, the commercially available genetic assays are being increasingly utilized to optimize the effectiveness of psychotropic medications in the treatment-refractory patient population, including schizophrenia. In this context, patients with treatment-refractory schizophrenia are among of the most vulnerable patients to be exposed to the debilitating adverse effects from often irrational and high-dose antipsychotic polypharmacy without clinically meaningful benefits. The primary objective of this comprehensive review is to analyze and interpret replicated findings from the genetic studies to identify specific genetic biomarkers that could be utilized to enhance antipsychotic efficacy and tolerability in the treatment-refractory schizophrenia population.

ABCB1, ABCG2 and CYP2D6 polymorphism effects on disposition and response to long-acting risperidone

The relevance of the multidrug resistance (ABCB1) and breast cancer resistance (ABCG2) protein transporter polymorphisms for treatment with long-acting intramuscular (LAI) risperidone is largely unknown. We explored the relationship between these polymorphisms and cytochrome P450 (CYP) 2D6 genotype-predicted phenotype in their effects on drug disposition and clinical outcomes in adults with schizophrenia. In a 24-week observational study, patients initiated on LAI-risperidone (n=101) were genotyped [enzymes (CYP2D6 dupl,*3,*4,*5,*6,*41; CYP3A4*22, CYP3A5*3), transporters (ABCG2 421C>A; ABCB1 1236C>T, 2677G>T/A, 3435C>T)] and evaluated for steady-state (weeks 6-8) serum levels of dose-corrected risperidone, 9-OH-risperidone, risperidone+9-OH-risperidone (active moiety), and for response to treatment (PANSS, reduction vs. baseline ≥30% at week 12 and ≥45% at week 24). CYP2D6 normal/ultrarapid metabolizers (NM/UM) (vs. other) had lower risperidone (29%) and active moiety levels (24%) (9-OH-risperidone not affected). The effect on the three analytes was mild (0 to 23% reduction) in ABCG2 wild-type homozygotes and pronounced (44-55% reduction) in ABCG2 variant allele carriers. ABCG2 variant had no effect on disposition in CYP2D6 "other" phenotypes, while the effect was pronounced in CYP2D6 NM/UM subjects (31-37% reduction). ABCB1 polymorphisms had no effect on exposure to risperidone. CYP2D6 NM/UM phenotype tended to lower odds of PANSS response, ABCG2 variant was associated with 4-fold higher odds and ABCB1 (1236C>T, 2677G>T/A, 3435C>T) overall mainly wild-type genotype was associated with around 4--fold lower odds of response. In patients treated with LAI-risperidone, CYP2D6 phenotype effect on systemic exposure is conditional on the ABCG2 421C>A polymorphism. ABCG2 and ABCB1 polymorphisms affect clinical response independently of systemic risperidone disposition.

Influence of ABCB1 polymorphisms and serum concentrations on venlafaxine response in patients with major depressive disorder

BACKGROUND

The pharmacokinetics and the pharmacodynamics of antidepressants show large inter-individual variations which result in unpredictable clinical responses.

AIM

The aim of the study was to examine the effect of ABCB1 polymorphisms and the serum concentrations on the efficacy and tolerability of venlafaxine in patients with major depressive disorder (MDD).

METHODS

Fifty-two outpatients who met the Diagnostic and Statistical Manual of Mental Disorders Fourth Edition (DSM-IV) criteria for MDD were recruited for the study. The severity of depression was assessed using the 17-item Hamilton Rating Scale for Depression scale (HDRS₁₇) and tolerability was assessed based on a query regarding side-effects for 6 weeks. The ABCB1 C3435T/A and G2677T/A polymorphisms were genotyped by PCR/RFLP and steady-state serum venlafaxine concentrations were measured by high-performance liquid chromatography.

RESULTS

Patients with the TT genotype for the C3435T and the TT/TA genotype for the G2677T/A polymorphism showed significantly higher frequencies in venlafaxine-induced akathisia. This relationship was not observed for efficacy. As regards serum venlafaxine concentrations, patient groups showed no significant differences in efficacy and tolerability.

CONCLUSION

The results suggest that individuals with the TT-TT/TA genotypes for the C3435T-G2677T/A polymorphisms of ABCB1 may be pre-disposed to a risk of akathisia.

Pharmacokinetics of lamotrigine and its metabolite N-2-glucuronide: Influence of polymorphism of UDP-glucuronosyltransferases and drug transporters

AIMS

This study aimed to develop a population pharmacokinetic model for quantitative evaluation of the influence of genetic variants in metabolic enzymes and transporters on lamotrigine pharmacokinetics while taking into account the influence of various clinical, biochemical and demographic factors.

METHODS

We included 100 patients with epilepsy on stable dosing with lamotrigine as mono or adjunctive therapy. Lamotrigine and lamotrigine N-2-glucuronide concentrations were determined in up to two plasma samples per patient. Patients were genotyped for UGT1A4, UGT2B7, ABCB1 and SLC22A1. Population pharmacokinetic analysis was performed by non-linear mixed effects modelling. Prior knowledge from previous pharmacokinetic studies was incorporated to stabilize the modelling process. A parent-metabolite model was developed to get a more detailed view on the covariate effects on lamotrigine metabolism.

RESULTS

With a base model absorption rate (interindividual variability) was estimated at 1.96 h(-1) (72.8%), oral clearance at 2.32 l h(-1) (41.4%) and distribution volume at 77.6 l (30.2%). Lamotrigine clearance was associated with genetic factors, patient's weight, renal function, smoking and co-treatment with enzyme inducing or inhibiting drugs. In patients with UGT2B7-161TT genotype clearance was lower compared with GT and GG genotypes. Clearance was particularly high in patients with UGT2B7 372 GG genotype (compared with AA genotype it was 117%; 95% CI 44.8, 247% higher).

CONCLUSIONS

Variability in lamotrigine pharmacokinetics is large and quantification of its sources may lead to more precise individual treatment. Genotyping for UGT2B7 may be useful in various clinical settings.

Impact of Genetic Polymorphisms of ABCB1 (MDR1, P-Glycoprotein) on Drug Disposition and Potential Clinical Implications: Update of the Literature

ATP-binding cassette transporter B1 (ABCB1; P-glycoprotein; multidrug resistance protein 1) is an adenosine triphosphate (ATP)-dependent efflux transporter located in the plasma membrane of many different cell types. Numerous structurally unrelated compounds, including drugs and environmental toxins, have been identified as substrates. ABCB1 limits the absorption of xenobiotics from the gut lumen, protects sensitive tissues (e.g. the brain, fetus and testes) from xenobiotics and is involved in biliary and renal secretion of its substrates. In recent years, a large number of polymorphisms of the ABCB1 [ATP-binding cassette, sub-family B (MDR/TAP), member 1] gene have been described. The variants 1236C>T (rs1128503, p.G412G), 2677G>T/A (rs2032582, p.A893S/T) and 3435C>T (rs1045642, p.I1145I) occur at high allele frequencies and create a common haplotype; therefore, they have been most widely studied. This review provides an overview of clinical studies published between 2002 and March 2015. In summary, the effect of ABCB1 variation on P-glycoprotein expression (messenger RNA and protein expression) and/or activity in various tissues (e.g. the liver, gut and heart) appears to be small. Although polymorphisms and haplotypes of ABCB1 have been associated with alterations in drug disposition and drug response, including adverse events with various ABCB1 substrates in different ethnic populations, the results have been majorly conflicting, with limited clinical relevance. Future research activities are warranted, considering a deep-sequencing approach, as well as well-designed clinical studies with appropriate sample sizes to elucidate the impact of rare ABCB1 variants and their potential consequences for effect sizes.

Applicability of gene expression and systems biology to develop pharmacogenetic predictors; antipsychotic-induced extrapyramidal symptoms as an example

Pharmacogenetics has been driven by a candidate gene approach. The disadvantage of this approach is that is limited by our current understanding of the mechanisms by which drugs act. Gene expression could help to elucidate the molecular signatures of antipsychotic treatments searching for dysregulated molecular pathways and the relationships between gene products, especially protein-protein interactions. To embrace the complexity of drug response, machine learning methods could help to identify gene-gene interactions and develop pharmacogenetic predictors of drug response. The present review summarizes the applicability of the topics presented here (gene expression, network analysis and gene-gene interactions) in pharmacogenetics. In order to achieve this, we present an example of identifying genetic predictors of extrapyramidal symptoms induced by antipsychotic.

[Pharmacogenetic-based risk assessment of antipsychotic-induced extrapyramidal symptoms]

"Typical" antipsychotics remain the wide-prescribed drugs in modern psychiatry. But these drugs are associated with development of extrapyramidal symptoms (EPS). Preventive methods of EPS are actively developed and they concentrate on personalized approach. The method of taking into account genetic characteristics of patient for prescribing of treatment was proven as effective in cardiology, oncology, HIV-medicine. In this review the modern state of pharmacogenetic research of antipsychotic-induced EPS are considered. There are pharmacokinetic and pharmacodynamic factors which impact on adverse effects. Pharmacokinetic factors are the most well-studied to date, these include genetic polymorphisms of genes of cytochrome P450. However, evidence base while does not allow to do the significant prognosis of development of EPS based on genetic testing of CYP2D6 and CYP7A2 polymorphisms. Genes of pharmacodynamics factors, which realize the EPS during antipsychotic treatment, are the wide field for research. In separate part of review research of such systems as dopaminergic, serotonergic, adrenergic, glutamatergic, GABAergic, BDNF were analyzed. The role of oxidative stress factors in the pathogenesis of antipsychotic-induced EPS was enough detailed considered. The system of those factors may be used for personalized risk assessment of antipsychotics' safety in the future. Although there were numerous studies, the pharmacogenetic-based prevention of EPS before prescribing of antipsychotics was not introduced. However, it is possible to distinguish the most perspectives markers for further research. Furthermore, brief review of new candidate genes provides here, but only preliminary results were published. The main problem of the field is the lack of high- quality studies. Moreover, the several results were not replicated in repeat studies. The pharmacogenetic-based research must be standardized by ethnicity of patients. But there is the ethnical misbalance in world literature. These facts explain why the introduction of pharmacogenetic testing for risk assessment of antipsychotic-induced EPS is so difficult to achieve.

Pharmacogenetics of second-generation antipsychotics

This review considers pharmacogenetics of the so called 'second-generation' antipsychotics. Findings for polymorphisms replicating in more than one study are emphasized and compared and contrasted with larger-scale candidate gene studies and genome-wide association study analyses. Variants in three types of genes are discussed: pharmacokinetic genes associated with drug metabolism and disposition, pharmacodynamic genes encoding drug targets, and pharmacotypic genes impacting disease presentation and subtype. Among pharmacokinetic markers, CYP2D6 metabolizer phenotype has clear clinical significance, as it impacts dosing considerations for aripiprazole, iloperidone and risperidone, and variants of the ABCB1 gene hold promise as biomarkers for dosing for olanzapine and clozapine. Among pharmacodynamic variants, the TaqIA1 allele of the DRD2 gene, the DRD3 (Ser9Gly) polymorphism, and the HTR2C -759C/T polymorphism have emerged as potential biomarkers for response and/or side effects. However, large-scale candidate gene studies and genome-wide association studies indicate that pharmacotypic genes may ultimately prove to be the richest source of biomarkers for response and side effect profiles for second-generation antipsychotics.

Polymorphisms in folate pathway and pemetrexed treatment outcome in patients with malignant pleural mesothelioma

INTRODUCTION

A combination of pemetrexed and cisplatin has been shown to improve the outcome in patients with malignant pleural mesothelioma (MPM), however, there is a great heterogeneity in treatment response among patients. The aim of our study was to evaluate the influence of polymorphisms in folate pathway and transporter genes on pemetrexed treatment outcome in Slovenian patients with MPM.

METHODS

MPM patients treated with pemetrexed in the course of a prospective randomized clinical trial were genotyped for nineteen polymorphisms in five genes of folate pathway and six transporter genes. Logistic regression was used to assess the influence of polymorphisms on treatment efficacy and toxicity, while Cox regression was used to determine their influence on progression-free and overall survival.

RESULTS

Patients with at least one polymorphic MTHFD1 rs2236225 allele had a significantly lower response rate (p = 0.005; odds ratio [OR] = 0.12; 95% confidence interval [CI] = 0.03-0.54) and shorter progression-free survival (p = 0.032; hazard ratio [HR] = 3.10; 95% CI = 1.10-8.74) than non-carriers. Polymorphisms in transporter genes did not influence survival; however, several were associated with toxicity. Liver toxicity was significantly lower in carriers of polymorphic ABCC2 rs2273697 (p = 0.028; OR = 0.23; 95% CI = 0.06-0.85), SLCO1B1 rs4149056 (p = 0.028; OR = 0.23; 95% CI = 0.06-0.85) and rs11045879 (p = 0.014; OR = 0.18; 95% CI = 0.05-0.71) alleles compared to non-carriers, as well as in patients with SLCO1B1 GCAC haplotype (p = 0.048; OR = 0.17; 95% CI = 0.03-0.98). Gastrointestinal toxicity was much more common in patients with polymorphic ABCC2 rs717620 allele (p = 0.004; OR = 10.67; 95% CI = 2.15-52.85) and ABCC2 CAG haplotype (p = 0.006; OR = 5.67; 95% CI = 1.64-19.66).

CONCLUSIONS

MTHFD1 polymorphism affected treatment response and survival, while polymorphisms in ABCC2 and SLCO1B1 transporter genes influenced the risk for toxicity. These polymorphisms could serve as potential markers of pemetrexed treatment outcome in patients with MPM.

Pharmacogenetics of antipsychotics

OBJECTIVE

During the past decades, increasing efforts have been invested in studies to unravel the influence of genetic factors on antipsychotic (AP) dosage, treatment response, and occurrence of adverse effects. These studies aimed to improve clinical care by predicting outcome of treatment with APs and thus allowing for individualized treatment strategies. We highlight most important findings obtained through both candidate gene and genome-wide association studies, including pharmacokinetic and pharmacodynamic factors.

METHODS

We reviewed studies on pharmacogenetics of AP response and adverse effects published on PubMed until early 2012. Owing to the high number of published studies, we focused our review on findings that have been replicated in independent studies or are supported by meta-analyses.

RESULTS

Most robust findings were reported for associations between polymorphisms of the cytochrome P450 system, the dopamine and the serotonin transmitter systems, and dosage, treatment response, and adverse effects, such as AP-induced weight gain or tardive dyskinesia. These associations were either detected for specific medications or for classes of APs.

CONCLUSION

First promising and robust results show that pharmacogenetics bear promise for a widespread use in future clinical practice. This will likely be achieved by developing algorithms that will include many genetic variants. However, further investigation is warranted to replicate and validate previous findings, as well as to identify new genetic variants involved in AP response and for replication of existing findings.

Individualizing clozapine and risperidone treatment for schizophrenia patients

Schizophrenia is a severe disorder that significantly affects the quality of life and total functioning of patients and their caregivers. Clozapine is the first atypical antipsychotic with fewer adverse effects and established efficacy. As a rule of thumb, risperidone is one of the most reliable and effective antipsychotics for newly diagnosed and chronic schizophrenics. Pharmacogenetic studies have identified genomic variants of candidate genes that seem to be important in the way a patient responds to treatment. The recent progress made in pharmacogenomics will improve the quality of treatment, since drug doses will be tailored to the special needs of each patient. In this article, we review the available literature attempting to delineate the role of genomic variations in clozapine and risperidone response in schizophrenic patients of various ethnicities. We conclude that pharmacogenomics for these two drugs is still not ready for implementation in the clinic.

Strengths and weaknesses of pharmacogenetic studies of antipsychotic drugs: the potential value of the PEPs study

The successful application of pharmacogenetics in routine clinical practice is still a long way from becoming a reality. In order to favor the transfer of pharmacogenetic results to clinical practice, especially in psychiatry, these studies must be optimized. This article reviews the strengths and weaknesses that characterize pharmacogenetic studies in psychiatry and condition their implementation in clinical practice. We also include recommendations for improving the design of pharmacogenetic studies, which may convert their limitations into strengths and facilitate the implementation of their results into clinical practice. Finally, we discuss the potential value of naturalistic, prospective, multicenter and coordinated projects such as the 'Phenotype-genotype and environmental interaction. Application of a predictive model in first psychotic episodes' (known as the PEPs study, from the Spanish abbreviation) in pharmacogenetic studies.

Pharmacogenetics of clinical response to risperidone

Despite risperidone's proven safety and efficacy, existing pharmacogenetic knowledge could be applied to improve its clinical use. The present work aims to summarize the information about genetic polymorphisms affecting risperidone adverse reactions and efficacy during routine clinical practice. The most relevant genes involved in the metabolism of the drug (i.e., CYP2D6, CYP3A and ABCB1) appear to have the greatest potential to predict differences in plasma concentrations of the drug and its interactions, but also relate to side effects, such as neuroleptic syndrome, weight gain or polydipsia. Other genes that have been found in association at least twice with any adverse reactions including metabolic changes, extrapyramidal symptoms or prolactine increase are: 5HT2A; 5HT2C; 5HT6; DRD2; DRD3; and BDNF. Some of these genes (5HTR2A, DRD2 and DRD3), along with 5-HTTLPR and COMT, have also been reported to be related with negative clinical outcomes. However, there is not yet enough evidence to support their routine screening during clinical practice.

No association of a set of candidate genes on haloperidol side effects

We previously investigated a sample of patients during an active phase of psychosis in the search for genetic predictors of haloperidol induced side effects. In the present work we extend the genetic association analysis to a wider panel of genetic variations, including 508 variations located in 96 genes. The original sample included 96 patients. An independent group of 357 patients from the CATIE study served as a replication sample. Outcomes in the investigation sample were the variation through time of: 1) the ESRS and UKU total scores 2) ESRS and UKU subscales (neurologic and psychic were included) related to tremors and 3) ESRS and UKU subscales that do not relate to tremors. Outcome in the replication sample was the presence vs absence of motoric side effects from baseline to visit 1 (∼ one month of treatment) as assessed by the AIMS scale test. Rs2242480 located in the CYP3A4 was associated with a different distribution of the UKU neurologic scores through time (permutated p = 0.047) along with a trend for a different haloperidol plasma levels (lower in CC subjects). This finding was not replicated in the CATIE sample. In conclusion, we did not find conclusive evidence for a major association between the investigated variations and haloperidol induced motoric side effects

Clinically significant psychotropic drug-drug interactions in the primary care setting

In recent years, the growing numbers of patients seeking care for a wide range of psychiatric illnesses in the primary care setting has resulted in an increase in the number of psychotropic medications prescribed. Along with the increased utilization of psychotropic medications, considerable variability is noted in the prescribing patterns of primary care providers and psychiatrists. Because psychiatric patients also suffer from a number of additional medical comorbidities, the increased utilization of psychotropic medications presents an elevated risk of clinically significant drug interactions in these patients. While life-threatening drug interactions are rare, clinically significant drug interactions impacting drug response or appearance of serious adverse drug reactions have been documented and can impact long-term outcomes. Additionally, the impact of genetic variability on the psychotropic drug's pharmacodynamics and/or pharmacokinetics may further complicate drug therapy. Increased awareness of clinically relevant psychotropic drug interactions can aid clinicians to achieve optimal therapeutic outcomes in patients in the primary care setting.

Antipsychotic drug dosage and therapeutic response in schizophrenia is influenced by ABCB1 genotypes: a study from a south Indian perspective

Aim: The conventional practice of using trial and error mode to select antipsychotic drugs in treatment of schizophrenia can result in symptom exacerbations, relapse and severe side effects, resulting in higher costs of treatment. P-glycoprotein (ABCB1) is known to regulate the concentration of antipsychotic drugs in the brain. Variable expressivity based on polymorphism in the gene ABCB1 may reflect on the drug response and its relationship to dosage. Materials & methods: All antipsychotic dosages administered to patients were converted to common chlorpromazine equivalents. Response to antipsychotics was based on 50% cutoff in Brief Psychiatric Rating Scale ratings after 1-year of follow-up. Using a case–control study design, ABCB1 polymorphisms were screened in 192 individuals grouped into responders and nonresponders. Results: A strong allelic, genotypic and haplotypic association, was observed, which was predictive of good response to antipsychotics. Individuals carrying the favorable homozygous genotypes of rs1045642 and rs2032582 displayed better response with increased dosage while those carrying risk genotype manifested refractoriness on increased dosage. Conclusion: The study suggests that a priori knowledge of ABCB1 genotypes can provide a significant input into evaluating the patient’s response to medication, and minimizing redundant dosing and refractoriness.

Original submitted 14 February 2012; Revision submitted 14 May 2012

Functional significance of genetic polymorphisms in P-glycoprotein (MDR1, ABCB1) and breast cancer resistance protein (BCRP, ABCG2)

Recent pharmacogenomic/pharmacogenetic (PGx) studies have disclosed important roles for drug transporters in the human body. Changes in the functions of drug transporters due to drug/food interactions or genetic polymorphisms, for example, are associated with large changes in pharmacokinetic (PK) profiles of substrate drugs, leading to changes in drug response and side effects. This information is extremely useful not only for drug development but also for individualized treatment. Among drug transporters, the ATP-binding cassette (ABC) transporters are expressed in most tissues in humans, and play protective roles; reducing drug absorption from the gastrointestinal tract, enhancing drug elimination into bile and urine, and impeding the entry of drugs into the central nervous system and placenta. In addition to PK/pharmacodynamic (PD) issues, ABC transporters are reported as etiologic and prognostic factors (or biomarkers) for genetic disorders. Although a consensus has not yet been reached, clinical studies have demonstrated that the PGx of ABC transporters influences the overall outcome of pharmacotherapy and contributes to the pathogenesis and progression of certain disorders. This review explains the impact of PGx in ABC transporters in terms of PK/PD, focusing on P-glycoprotein and breast cancer resistance protein (BCRP).

Relationship between P-glycoprotein and second-generation antipsychotics

The membrane transport protein P-glycoprotein (P-gp) is an interesting candidate for individual differences in response to antipsychotics. To present an overview of the current knowledge of P-gp and its interaction with second-generation antipsychotics (SGAs), an internet search for all relevant English original research articles concerning P-gp and SGAs was conducted. Several SGAs are substrates for P-gp in therapeutic concentrations. These include amisulpride, aripiprazole, olanzapine, perospirone, risperidone and paliperidone. Clozapine and quetiapine are not likely to be substrates of P-gp. However, most antipsychotics act as inhibitors of P-gp, and can therefore influence plasma and brain concentrations of other substrates. No information was available for sertindole, ziprasidone or zotepine. Research in animal models demonstrated significant differences in antipsychotic brain concentration and behavior owing to both P-gp knockout and inhibition. Results in patients are less clear, as several external factors have to be accounted for. Patients with polymorphisms which decrease P-gp functionality tend to perform better in clinical settings. There is some variability in the findings concerning adverse effects, and no definitive conclusions can be drawn at this point.

Pharmacogenetics of response to antipsychotics in patients with schizophrenia

This review presents the findings of pharmacogenetic studies exploring the influence of gene variants on antipsychotic treatment response, in terms of both symptom improvement and adverse effects, in patients with schizophrenia. Despite numerous studies in the field, replicating findings across different cohorts that include subjects of different ethnic groups has been challenging. It is clear that non-genetic factors have an important contribution to antipsychotic treatment response. Differing clinical, demographic and environmental characteristics of the cohorts studied have added substantial complexity to the interpretation of the positive and negative findings of many studies. Pharmacogenomic genome-wide investigations are beginning to yield interesting data although they have failed to replicate the most robust findings of candidate gene studies, and are limited by the sample size, especially given the need for studying homogeneous cohorts. Most of the studies conducted on cohorts treated with single antipsychotics have investigated clozapine, olanzapine or risperidone response. These studies have provided some of the most replicated associations with treatment efficacy. Serotonergic system gene variants are significantly associated with the efficacy of clozapine and risperidone, but may have less influence on the efficacy of olanzapine. Dopamine D3 receptor polymorphisms have been more strongly associated with the efficacy of clozapine and olanzapine, and D2 genetic variants with the efficacy of risperidone. Serotonin influences the control of feeding behaviour and has been hypothesized to have a role in the development of antipsychotic-induced weight gain. Numerous studies have linked the serotonin receptor 2C (5-HT2C) -759-C/T polymorphism with weight gain. The leptin gene variant, -2548-G/A, has also been associated with weight gain in several studies. Pharmacogenetic studies support the role of cytochrome P450 enzymes and dopamine receptor variants in the development of antipsychotic-induced movement disorders, with a contribution of serotonergic receptors and other gene variants implicated in the mechanism of action of antipsychotics. Clozapine-induced agranulocytosis has been associated with polymorphisms in the major histocompatibility complex gene (HLA).

Regional increase in P-glycoprotein function in the blood-brain barrier of patients with chronic schizophrenia: a PET study with [(11)C]verapamil as a probe for P-glycoprotein function

P-glycoprotein (P-gp), a major efflux pump in the blood-brain barrier (BBB) has a profound effect on entry of drugs, peptides and other substances into the central nervous system (CNS). The brain's permeability can be negatively influenced by modulation of the transport function of P-gp. Inflammatory mediators play a role in schizophrenia, and may be able to influence the integrity of the BBB, via P-gp modulation. We hypothesized that P-gp function in the BBB is changed in patients with schizophrenia. Positron-emission tomography was used to measure brain uptake of [(11)C]verapamil, which is normally extruded from the brain by P-gp. We found that patients with chronic schizophrenia under treatment with antipsychotic drugs compared with healthy controls showed a significant decrease in [(11)C]verapamil uptake in the temporal cortex, the basal ganglia, and the amygdala, and amygdalae, and a trend towards a significant decrease was seen throughout the brain. The decrease of [(11)C]verapamil uptake correlates with an increased activity of the P-gp pump. Increased P-gp activity may be a factor in drug resistance in schizophrenia, induced by the use of antipsychotic agents.

The role of CYP2D6 and ABCB1 pharmacogenetics in drug-naïve patients with first-episode schizophrenia treated with risperidone

PURPOSE

To evaluate the role of cytochrome 450 2D6 (CYP2D6) and ABCB1 variants on plasma risperidone concentrations and treatment response in 83 drug-naive patients experiencing a first episode of psychosis.

METHODS

All patients were treated with risperidone for 8 weeks. The CYP2D6 genotyping was performed by allele-specific PCR-restriction fragment length polymorphism analysis (for alleles *3,*4,*6) and long-distance PCR (for duplications and allele *5), while real-time PCR analysis was used for the ABCB1 G2677T/A and C3435T variants. Plasma concentrations of risperidone and 9-OH risperidone were measured by high-performance liquid chromatography.

RESULTS

The number of patients with the CYP2D6 wild type (wt)/wt, wt/mutation (mut) and mut/mut genotype was 43, 32 and 8, respectively. The number of patients with the ABCB1 2677G/G, G/T and T/T variants was 29, 42 and 12, respectively; those with the 3435CC, C/T and T/T variants was 25, 37 and 21, respectively. The CYP2D6 genotype had a strong effect on the steady-state dose-corrected plasma levels (C/D) of risperidone, its 9-OH metabolite and the active moiety, while the ABCB1 2677 T/T and 3435 T/T genotypes has similarly strong effects on the active moiety C/D. The CYP2D6 poor metabolizers had a significantly higher risperidone C/D and active moiety C/D and lower 9-OH risperidone C/D. The ABCB1 3435 T allele and the ABCB1 2667 T-3435 T haplotype carriers were more frequent among subjects without extrapyramidal syndromes. Patients showed significant improvements in positive and general symptoms, but not in negative symptoms. These changes were not related to variations in genetic and drug concentration data.

CONCLUSION

Our findings suggest that CYP2D6 and ABCB1 G2677T and C3435T may be useful determinants of risperidone plasma concentrations, but the clinical implications of these associations in relation to treatment response and side-effects remain unclear.