Genetic polymorphism of catechol-O-methyltransferase and levodopa pharmacokinetic-pharmacodynamic pattern in patients with Parkinson's disease

An update on the pharmacogenetic considerations when prescribing dopamine receptor agonists for Parkinson's disease

INTRODUCTION

Parkinson's disease is a chronic neurodegenerative multisystemic disorder that affects approximately 2% of the population over 65 years old. This disorder is characterized by motor symptoms which are frequently accompanied by non-motor symptoms such as cognitive disorders. Current drug therapies aim to reduce the symptoms and increase the patient's life expectancy. Nevertheless, there is heterogeneity in therapy response in terms of efficacy and adverse effects. This wide range in response may be linked to genetic variability. Thus, it has been suggested that pharmacogenomics may help to tailor and personalize drug therapy for Parkinson's disease.

AREAS COVERED

This review describes and updates the clinical impact of genetic factors associated with the efficacy and adverse drug reactions related to common medications used to treat Parkinson's disease. Additionally, we highlight current informative recommendations for the drug treatment of Parkinson's disease.

EXPERT OPINION

The pharmacokinetic, pharmacodynamic, and safety profiles of Parkinson's disease drugs do not favor the development of pharmacogenetic tests with a high probability of success. The chances of obtaining ground-breaking pharmacogenetics biomarkers for Parkinson's disease therapy are limited. Nevertheless, additional information on the metabolism of certain drugs, and an analysis of the potential of pharmacogenetics in novel drugs could be of interest.

Association of Catechol-O-Methyltransferase Gene Polymorphisms and Haplotypes in the Levodopa-Induced Adverse Events in Subjects with Parkinson's Disease

The presence of dyskinesia is the most common side effect of chronic administration of levodopa in Parkinson's disease (PD) subjects. Genetic polymorphisms in levodopa metabolizing gene, catechol-O-methyl transferase (COMT), is shown to influence the inter-individual variability in drug response and adverse events. In the present study, the association of COMT rs6269, rs4633, rs4818, and rs4680 polymorphisms and haplotypes on pharmacokinetics and adverse events with levodopa was investigated in 150 PD patients. The age of onset of PD was 58.00 ± 10 yrs. The most common side effect faced by 78% of the subjects was dyskinesia. The AUC of levodopa was found to be significantly higher in subjects with dyskinesia (1695 ± 113 ng/ml/hr, p < 0.0001) than those without dyskinesia (1550 ± 122 ng/ml/hr). We found that the frequency of subjects presenting dyskinesia was significantly higher in subjects carrying variant genotype of COMT rs6269, rs4633, and rs4680 than that with wild genotype and these subjects presented higher AUC of levodopa. In addition, in subjects with dyskinesia, the AUC of levodopa was found to be significantly higher with low COMT (ACCG) haplotype. The association of COMT rs6269, COMT rs4633, COMT rs4818, and COMT rs4680 variant genotypes with the risk of dyskinesia due to levodopa therapy showed an ROC AUC of 0.67 indicating the moderate prediction of dyskinesia (p = 0.0021) with these COMT variants. In conclusion, PD subjects carrying the variant genotypes of COMT strongly influence high levodopa-induced dyskinesia. Hence the genotyping of COMT before the levodopa therapy will be useful to reduce the adverse events associated with the chronic levodopa treatment.

Association of Catechol-O-Methyltransferase Gene rs4680 Polymorphism and Levodopa Induced Dyskinesia in Parkinson's Disease: A Meta-Analysis and Systematic Review

INTRODUCTION

Long-term levodopa therapy for Parkinson's disease (PD) can cause levodopa induced dyskinesia (LID). Genetic predisposition has a significant role to play in inter-individual heterogeneity in the clinical manifestation of LID. Despite accumulating evidence for the role of COMT gene polymorphism (rs4680) as a genetic basis for LID, to date results have been inconsistent. Early assessment of the Catechol-O-Methyltransferase (COMT) genotype might be helpful to stratify PD patients concerning their individual risk for LID.

METHOD

In this meta-analysis, we have used 9 studies, which were selected through online databases. Statistical analysis was performed using R (v-3.6) software. 5 genetic models have been used in the present study: Allele model (A vs. G), Dominant model (AA+AG vs. GG), Homozygote model (AA vs. GG), Co-dominant/heterozygote model (AG vs. GG), and Recessive model (AA vs. AG + GG).

RESULTS

The results indicated a significant association between COMT rs4680 (Val158Met) polymorphism and LID risk. The genotype AA of COMT rs4680 is a risk factor for LID in PD patients under the recessive model (AA vs GG+AG) in the random-effect model. Analysis based on ethnicity showed that COMT rs4680 SNP allele A is a risk factor for LID development in Asian PD patients, while GG genotype is a risk factor for LID development in non-Asian PD patients using different genetic models.

CONCLUSION

The results of the present meta-analysis support that the COMT Val158Met polymorphism is a risk factor for the development of LID in PD patients having ethnic variations.

Pharmacogenomics-a New Frontier for Individualized Treatment of Parkinson's Disease

BACKGROUND

Parkinson's disease (PD) is the second most common neurodegenerative disease with a significant public health burden. It is characterized by the gradual degeneration of dopamine neurons in the central nervous system. Although symptomatic pharmacological management remains the primary therapeutic method for PD, clinical experience reveals significant inter-individual heterogeneity in treatment effectiveness and adverse medication responses. The mechanisms behind the observed interindividual variability may be elucidated by investigating the role of genetic variation in human-to-human variances in medication responses and adverse effects.

OBJECTIVE

This review aims to explore the impact of gene polymorphism on the efficacy of antiparkinsonian drugs. The identification of factors associated with treatment effectiveness variability might assist the creation of a more tailored pharmacological therapy with higher efficacy, fewer side outcomes, and cheaper costs.

METHODS

In this review, we conducted a thorough search in databases such as PubMed, Web of Science, and Google Scholar, and critically examined current discoveries on Parkinson's disease pharmacogenetics. The ethnicity of the individuals, research methodologies, and potential bias of these studies were thoroughly compared, with the primary focus on consistent conclusions.

RESULTS

This review provides a summary of the existing data on PD pharmacogenetics, identifies its limitations, and offers insights that may be beneficial for future research. Previous studies have investigated the impact of gene polymorphism on the effectiveness and adverse effects of levodopa. The trendiest genes are the COMT gene, DAT gene, and DRD2 gene. However, limited study on other anti-Parkinson's drugs has been conducted.

CONCLUSION

Therefore, In order to develop an individualized precision treatment for PD, it is an inevitable trend to carry out multi-center, prospective, randomized controlled clinical trials of PD pharmacogenomics covering common clinical anti-PD drugs in large, homogeneous cohorts.

Association of COMT rs4680 and MAO-B rs1799836 polymorphisms with levodopa-induced dyskinesia in Parkinson's disease-a meta-analysis

BACKGROUND AND PURPOSE

Polymorphisms of the catechol-O-methyl transferase (COMT) or monoamine oxidase B (MAO-B) genes may affect the occurrence of dyskinesia in Parkinson's disease (PD) patients. However, the findings are inconsistent. Thus, we performed a meta-analysis to assess whether COMT and MAO-B genetic variants are associated with an increased incidence of levodopa-induced dyskinesia (LID) in PD patients.

METHODS

A literature search of PubMed, Embase, and Cochrane Library was conducted to identify relevant studies published up to January 2021. The strength of the association between the polymorphisms and LID susceptibility was estimated by odds ratio (OR) and associated 95% confidence interval (CI). The pooled ORs were assessed in different genetic models.

RESULTS

Ten studies involving 2385 PD patients were included in the meta-analysis. Analysis of pooled ORs and 95% CIs suggested that the AA genotype of COMT(rs4680) was associated with LID (OR = 1.39, 95%CI: 1.02-1.89, P = 0.039) in the recessive model, and this correlation was more obvious in Brazilian samples in the analysis stratified by ethnicity. For the AG genotype of MAO-B(rs1799836), the pooled OR was 1.66 (95% CI: 1.04-2.65, P = 0.03) in patients with LID versus those without LID in the heterozygote model.

CONCLUSIONS

Our meta-analysis implicates the AA genotype of the COMT rs4680 polymorphism as potentially increasing the risk of LID in a recessive genetic model for PD patients. Furthermore, the AG genotype of the MAO-B rs1799836 polymorphism may influence the prevalence of LID in PD patients in the heterozygote model. However, further well-designed studies with larger PD patient cohorts are required to validate these results after adjusting for confounding factors.

A Systematic Review of Parkinson's Disease Pharmacogenomics: Is There Time for Translation into the Clinics?

BACKGROUND

Parkinson's disease (PD) is the second most frequent neurodegenerative disease, which creates a significant public health burden. There is a challenge for the optimization of therapies since patients not only respond differently to current treatment options but also develop different side effects to the treatment. Genetic variability in the human genome can serve as a biomarker for the metabolism, availability of drugs and stratification of patients for suitable therapies. The goal of this systematic review is to assess the current evidence for the clinical translation of pharmacogenomics in the personalization of treatment for Parkinson's disease.

METHODS

We performed a systematic search of Medline database for publications covering the topic of pharmacogenomics and genotype specific mutations in Parkinson's disease treatment, along with a manual search, and finally included a total of 116 publications in the review.

RESULTS

We analyzed 75 studies and 41 reviews published up to December of 2020. Most research is focused on levodopa pharmacogenomic properties and catechol-O-methyltransferase (COMT) enzymatic pathway polymorphisms, which have potential for clinical implementation due to changes in treatment response and side-effects. Likewise, there is some consistent evidence in the heritability of impulse control disorder via Opioid Receptor Kappa 1 (OPRK1), 5-Hydroxytryptamine Receptor 2A (HTR2a) and Dopa decarboxylase (DDC) genotypes, and hyperhomocysteinemia via the Methylenetetrahydrofolate reductase (MTHFR) gene. On the other hand, many available studies vary in design and methodology and lack in sample size, leading to inconsistent findings.

CONCLUSIONS

This systematic review demonstrated that the evidence for implementation of pharmacogenomics in clinical practice is still lacking and that further research needs to be done to enable a more personalized approach to therapy for each patient.

The Genetics of Parkinson's Disease and Implications for Clinical Practice

The genetic landscape of Parkinson’s disease (PD) is characterised by rare high penetrance pathogenic variants causing familial disease, genetic risk factor variants driving PD risk in a significant minority in PD cases and high frequency, low penetrance variants, which contribute a small increase of the risk of developing sporadic PD. This knowledge has the potential to have a major impact in the clinical care of people with PD. We summarise these genetic influences and discuss the implications for therapeutics and clinical trial design.

Population pharmacokinetics of levodopa gel infusion in Parkinson's disease: effects of entacapone infusion and genetic polymorphism

Levodopa-entacapone-carbidopa intestinal gel (LECIG) provides continuous drug delivery through intrajejunal infusion. The aim of this study was to characterize the population pharmacokinetics of levodopa following LECIG and levodopa-carbidopa intestinal gel (LCIG) infusion to investigate suitable translation of dose from LCIG to LECIG treatment, and the impact of common variations in the dopa-decarboxylase (DDC) and catechol-O-methyltransferase (COMT) genes on levodopa pharmacokinetics. A non-linear mixed-effects model of levodopa pharmacokinetics was developed using plasma concentration data from a double-blind, cross-over study of LCIG compared with LECIG in patients with advanced Parkinson’s disease (n = 11). All patients were genotyped for rs4680 (polymorphism of the COMT gene), rs921451 and rs3837091 (polymorphisms of the DDC gene). The final model was a one compartment model with a high fixed absorption rate constant, and a first order elimination, with estimated apparent clearances (CL/F), of 27.9 L/h/70 kg for LCIG versus 17.5 L/h/70 kg for LECIG, and apparent volume of distribution of 74.4 L/70 kg. Our results thus suggest that the continuous maintenance dose of LECIG, on a population level, should be decreased by approximately 35%, to achieve similar drug exposure as with LCIG. An effect from entacapone was identified on all individuals, regardless of COMT rs4680 genotype. The individuals with higher DDC and COMT enzyme activity showed tendencies towards higher levodopa CL/F. The simultaneous administration of entacapone to LCIG administration results in a 36.5% lower apparent levodopa clearance, and there is a need for lower continuous maintenance doses, regardless of patients’ COMT genotype.

Accurate and sensitive detection of Catechol-O-methyltransferase activity by liquid chromatography with fluorescence detection

Catechol-O-methyltransferase (COMT) is a major drug metabolizing enzyme in humans. COMT expression is directedly associated with various mental diseases and cancers due to its essential role in catalyzing metabolic inactivation of endogenous catecholamines and catechol estrogens. However, a practical method to precisely measure COMT activities in biological samples is lacking. In the current study, we established a liquid chromatography-fluorescence detection (LC-FD) method based on fluorometric detection of the methylated product of 3-BTD, a fluorescent probe for COMT, to sensitively quantify COMT activities in human erythrocytes and cell homogenates. Assay validation of the established LC-FD based method was conducted for selectivity and sensitivity, range of linearity, precision and accuracy, recovery, biological matrices effect and stability. The limit of quantification for 3-BTMD (the methylated product of 3-BTD by COMT) of this method was 0.0083 nM, which is nearly 10-fold lower than that for previously published methods. The method was precise with intra- and inter-day relative standard deviation (RSD) lower than 5%. In addition, this method showed an excellent anti-interference ability with no effects of the endogenous substances on the fluorometric detection of 3-BTMD. The practical use of this method was established by its successful application for the measurement of COMT activities in individual human erythrocytes (n = 13), and in cell homogenates generated from four different human cell lines. Our results suggest that this method will be of great value in accurately determining the native activity of COMT in biological samples, which is beneficial for a complete understand of the role of COMT both in physiological and pathological conditions.

Genetic variations in catechol-O-methyltransferase gene are associated with levodopa response variability in Chinese patients with Parkinson's disease

Catechol-O-methyltransferase (COMT) is one of the main enzymes in dopamine metabolism and is reported to be associated with susceptibility to Parkinson’s disease (PD) and pharmacotherapy. However, researchers mostly focus on the most common polymorphism, rs4680. In this case-control study, we investigated the association of SNPs other than rs4680 with the levodopa (L-dopa) response and other clinical features in Chinese PD patients. Eleven single nucleotide polymorphisms (SNPs) in the COMT gene were genotyped, and clinical data were collected. Patients with the TT genotype of rs165728 or rs174699 had larger daily levodopa equivalent doses (LEDs) than the patients with CC and CT genotypes under the dominant model (p = 0.01421 for rs165728 and p = 0.02302 for rs174699). Under the dominant model, the patients with GG at rs4680 G > A had a lower occurrence of dyskinesia than those with AA and AG (p = 0.0196). Patients with CC at rs4633 had a lower occurrence of dyskinesia than those with TT and TC (p = 0.0429) under the dominant model. The frequencies of the rs174675 T and rs933271 C alleles were higher in PD patients than in the controls (p < 0.05). Our primary results showed the possible association of SNPs other than the most common functional rs4680 in COMT with interindividual variance in the L-dopa daily dose and susceptibility to dyskinesia in Chinese patients, although this was an exploratory study based on a small sample size. Larger and more randomized samples are necessary for further investigation.

MAO-B and COMT Genetic Variations Associated With Levodopa Treatment Response in Patients With Parkinson's Disease

The most commonly used Parkinson's disease (PD) treatment is the replacement of dopamine by its levodopa precursor (l-dopa). Monoamine oxidase-B (MAO-B) and catechol-o-methyl transferase (COMT) are enzymes involved in the metabolism and regulation of dopamine availability. In our study we investigated the possible relation among selected single-nucleotide polymorphisms (SNPs) in the MAO-B (rs1799836) and COMT (rs4680) genes and the therapeutic response to levodopa (l-dopa). A total of 162 Brazilian patients from the Pro-Parkinson service of Clinics Hospital of Pernambuco diagnosed with sporadic PD and treated with levodopa were enrolled. PD patients were stratified into 2 groups according to the daily levodopa dose. MAO-B and COMT SNP genotyping was conducted by polymerase chain reaction-restriction fragment length polymorphism. After multivariate analysis, we observed a significant difference between PD groups for the following variables: sex (P = .02), longer duration of disease (P = .02), longer levodopa therapy duration (P = .01), younger onset of PD (P = .01), and use of COMT inhibitor (P = .02). We observed that patients carrying MAO-B (rs1799836) A and AA genotypes and COMT (rs4680) LL genotype suffered more frequently from levodopa-induced-dyskinesia. In addition, we found an increased risk of 2.84-fold for male individuals carrying the MAO-B G allele to be treated with higher doses of levodopa (P = .04). We concluded that before beginning PD pharmacological treatment, it is important to consider the genetic variants of the MAO-B and COMT genes and the sex, reinforcing the evidence that sexual dimorphism in the genes related to dopamine metabolism might affect PD treatment.

Molecular basis of dopamine replacement therapy and its side effects in Parkinson's disease

There is currently no cure for Parkinson's disease. The symptomatic therapeutic strategy essentially relies on dopamine replacement whose efficacy was demonstrated more than 50 years ago following the introduction of the dopamine precursor, levodopa. The spectacular antiparkinsonian effect of levodopa is, however, balanced by major limitations including the occurrence of motor complications related to its particular pharmacokinetic and pharmacodynamic properties. Other therapeutic strategies have thus been developed to overcome these problems such as the use of dopamine receptor agonists, dopamine metabolism inhibitors and non-dopaminergic drugs. Here we review the pharmacology and molecular mechanisms of dopamine replacement therapy in Parkinson's disease, both at the presynaptic and postsynaptic levels. The perspectives in terms of novel drug development and prediction of drug response for a more personalised medicine will be discussed.

Genetics and Treatment Response in Parkinson's Disease: An Update on Pharmacogenetic Studies

Parkinson's disease (PD) is a complex neurodegenerative disorder characterized by a progressive loss of dopamine neurons of the central nervous system. The disease determines a significant disability due to a combination of motor symptoms such as bradykinesia, rigidity and rest tremor and non-motor symptoms such as sleep disorders, hallucinations, psychosis and compulsive behaviors. The current therapies consist in combination of drugs acting to control only the symptoms of the illness by the replacement of the dopamine lost. Although patients generally receive benefits from this symptomatic pharmacological management, they also show great variability in drug response in terms of both efficacy and adverse effects. Pharmacogenetic studies highlighted that genetic factors play a relevant influence in this drug response variability. In this review, we tried to give an overview of the recent progresses in the pharmacogenetics of PD, reporting the major genetic factors identified as involved in the response to drugs and highlighting the potential use of some of these genomic variants in the clinical practice. Many genes have been investigated and several associations have been reported especially with adverse drug reactions. However, only polymorphisms in few genes, including DRD2, COMT and SLC6A3, have been confirmed as associated in different populations and in large cohorts. The identification of genomic biomarkers involved in drug response variability represents an important step in PD treatment, opening the prospective of more personalized therapies in order to identify, for each person, the better therapy in terms of efficacy and toxicity and to improve the PD patients' quality of life.

Pharmacogenetics

Pharmacogenetics is the study of how genetics influences drug treatment outcomes. Much research has been conducted to identify and characterize gene variants that impact the pharmacokinetic and pharmacodynamic aspects of medications used to treat neurologic and psychiatric disorders. This chapter reviews the current state of pharmacogenetic aspects of these treatments. Medications with supporting pharmacogenetic information in product labeling, clinical guidelines, or important mechanistic implications are discussed. At this time, clinically relevant genetic variation in drug metabolizing enzymes may inform drug dosing for a number of medications metabolized in the liver. Additionally, genetic variation in immunological genes may be tested to assess risk for severe hypersensitivity reactions to some anticonvulsant drugs. Finally, a growing body of research highlights that genetic polymorphisms in drug targets may influence symptom response or tolerability to some treatments.

A systematic review and integrative approach to decode the common molecular link between levodopa response and Parkinson's disease

Background

PD is a progressive neurodegenerative disorder commonly treated by levodopa. The findings from genetic studies on adverse effects (ADRs) and levodopa efficacy are mostly inconclusive. Here, we aim to identify predictive genetic biomarkers for levodopa response (LR) and determine common molecular link with disease susceptibility. A systematic review for LR was conducted for ADR, and drug efficacy, independently. All included articles were assessed for methodological quality on 14 parameters. GWAS of PD were also reviewed. Protein-protein interaction (PPI) analysis using STRING and functional enrichment using WebGestalt was performed to explore the common link between LR and PD.

Results

From 37 candidate studies on levodopa toxicity, 18 genes were found associated, of which, CA_n STR 13, 14 (DRD2) was most significantly associated with dyskinesia, followed by rs1801133 (MTHFR) with hyper-homocysteinemia, and rs474559 (HOMER1) with hallucination. Similarly, 8 studies on efficacy resulted in 4 genes in which rs28363170, rs3836790 (SLC6A3) and rs4680 (COMT), were significant. To establish the molecular connection between LR with PD, we identified 35 genes significantly associated with PD. With 19 proteins associated with LR and 35 with PD, two independent PPI networks were constructed. Among the 67 nodes (263 edges) in LR, and 62 nodes (190 edges) in PD pathophysiology, UBC, SNCA, FYN, SRC, CAMK2A, and SLC6A3 were identified as common potential candidates.

Conclusion

Our study revealed the genetically significant polymorphism concerning the ADRs and levodopa efficacy. The six common genes may be used as predictive markers for therapy optimization and as putative drug target candidates.

Electronic supplementary material

The online version of this article (10.1186/s12920-017-0291-0) contains supplementary material, which is available to authorized users.

Human COMT over-expression confers a heightened susceptibility to dyskinesia in mice

Catechol-O-methyltransferase (COMT) degrades dopamine and its precursor l-DOPA and plays a critical role in regulating synaptic dopamine actions. We investigated the effects of heightened levels of COMT on dopamine-regulated motor behaviors and molecular alterations in a mouse model of dyskinesia. Transgenic mice overexpressing human COMT (TG) and their wildtype (WT) littermates received unilateral 6-OHDA lesions in the dorsal striatum and were treated chronically with l-DOPA for two weeks. l-DOPA-induced dyskinesia was exacerbated in TG mice without altering l-DOPA motor efficacy as determined by contralateral rotations or motor coordination. Inductions of FosB and phospho-acetylated histone 3 (molecular correlates of dyskinesia) were potentiated in the lesioned striatum of TG mice compared with their WT littermates. The TG mice had lower basal levels of dopamine in the striatum. In mice with lesions, l-DOPA induces a greater increase in the dopamine metabolite 3-methoxytyramine in the lesioned striatum of dyskinetic TG mice than in WT mice. The levels of serotonin and its metabolite were similar in TG and WT mice. Our results demonstrate that human COMT overexpression confers a heightened susceptibility to l-DOPA-induced dyskinesia and alters molecular and neurochemical responses in the lesioned striatum of mice.

Pharmacogenetics of Parkinson's Disease in Clinical Practice

Background

Pharmacogenetics aims to identify the genetic factors participating in the heterogeneity of drug response. The ultimate goal is to provide personalized treatment by identifying responders and non-responders, individuals at risk of developing drug adverse effects, and by adjusting dosage. Several studies have been performed in Parkinson's disease (PD), to investigate drug response variability according to genetic factors for dopamine replacement therapies.

Methods

We performed a systematic literature search of articles related to pharmacogenetic studies in PD, and found 47 studies.

Findings

Motor response and adverse reactions to dopaminergic drugs were associated with genes encoding enzymes of their metabolism as well as their receptors or targets. Despite some interesting results, considerable work remains to be done to replicate and validate their clinical relevance before translation into clinical practice.

Conclusions

There are currently no guidelines published for pharmacogenetic factors related to PD drugs. More research is need in this field in order to improve our knowledge in drug response variability in PD. Algorithms taking into account clinical, pharmacological, and genetic factors are probably the most promising way to help for a personalized medicine in PD.

How close are we to individualized medicine for Parkinson's disease?

INTRODUCTION

There is a considerable inter-individual heterogeneity in clinical features, disease course, and treatment response in Parkinson's disease (PD), which can be explained not only by disease process and clinical variables, but also by an impact from genetic factors. Evidence-based medicine relies on large randomized control trials and meta-analysis-average medicine, which ignores individual differences. However, we are now in the early phases of a paradigm shift in medicine relating to individuality and variability. The purpose of individualized medicine is to predict patients' responses to targeted therapy using diagnostic tests based on genetics or other molecular mechanisms, thus providing the right drug at the right dose at the right time.

AREAS COVERED

In this article, we outline current state of individualized medicine for PD. Expert Commentary: Pharmacogenomics, an important element of individualized medicine, is just beginning to be considered in PD. To advance the clinical use of pharmacogenomics, big data cohort for genomic research and multidisciplinary team approaches are necessary.

Pharmacogenetic considerations in the treatment of Parkinson's disease

Recently, a lot of progress has been made in the identification of genetic biomarkers of drug response. Efforts to define the role of genetic polymorphisms in optimizing pharmacotherapy of Parkinson's disease were also undertaken. This report presents the current state of knowledge on pharmacogenetics of PD, including genes encoding enzymes involved in drug metabolism, drug transporters and direct targets of antiparkinsonian drugs. In most of cases, available data on pharmacogenetic factors that could turn out to be significant modifiers of therapy with anti-PD drugs is still very incomplete and makes it impossible to reach final conclusion about their usefulness in the clinic. More extensive studies, in more uniform, large patient groups, including genome-wide association studies, should be undertaken to finally confirm or deny the value of genetic tests in PD therapy individualization.

Dopaminergic Modulation of Medial Prefrontal Cortex Deactivation in Parkinson Depression

Parkinson's disease (PD) is associated with emotional abnormalities. Dopaminergic medications ameliorate Parkinsonian motor symptoms, but less is known regarding the impact of dopaminergic agents on affective processing, particularly in depressed PD (dPD) patients. The aim of this study was to examine the effects of dopaminergic pharmacotherapy on brain activation to emotional stimuli in depressed versus nondepressed Parkinson disease (ndPD) patients. Participants included 18 ndPD patients (11 men, 7 women) and 10 dPD patients (7 men, 3 women). Patients viewed photographs of emotional faces during functional MRI. Scans were performed while the patient was taking anti-Parkinson medication and the day after medication had been temporarily discontinued. Results indicate that dopaminergic medications have opposite effects in the prefrontal cortex depending upon depression status. DPD patients show greater deactivation in the ventromedial prefrontal cortex (VMPFC) on dopaminergic medications than off, while ndPD patients show greater deactivation in this region off drugs. The VMPFC is in the default-mode network (DMN). DMN activity is negatively correlated with activity in brain systems used for external visual attention. Thus dopaminergic medications may promote increased attention to external visual stimuli among dPD patients but impede normal suppression of DMN activity during external stimulation among ndPD patients.

Clinical implications of neuropharmacogenetics

INTRODUCTION

Pharmacogenetics aims to identify the underlying genetic factors participating in the variability of drug response. Indeed, genetic variability at the DNA or RNA levels can directly or indirectly modify the pharmacokinetic or the pharmacodynamic parameters of a drug. The ultimate aim of pharmacogenetics is to move towards a personalised medicine by predicting responders and non-responders, adjusting the dose of the treatment, and identifying individuals at risk of adverse drug effects.

METHODS

A literature research was performed in which we reviewed all pharmacogenetic studies in neurological disorders including neurodegenerative diseases, multiple sclerosis, stroke and epilepsy.

RESULTS

Several pharmacogenetic studies have been performed in neurology, bringing insights into the inter-individual drug response variability and in the pathophysiology of neurological diseases. The principal implications of these studies for the management of patients in clinical practice are discussed.

CONCLUSION/DISCUSSION

Although several genetic factors have been identified in the modification of drug response in neurological disorders, most of them have a marginal predictive effect at the single gene level, suggesting mutagenic interactions as well as other factors related to drug interaction and disease subtypes. Most pharmacogenetic studies deserve further replication in independent populations and, ideally, in pharmacogenetic clinical trials to demonstrate their relevance in clinical practice.

Parkinson's disease pharmacogenomics: new findings and perspectives

Parkinson's disease (PD) is unique among neurodegenerative disorders because a highly effective pharmacological symptomatic treatment is available. The marked variability in drug response and in adverse profiles associated with this treatment led to the search of genetic markers associated with these features. We present a review of the literature on PD pharmacogenetics to provide a critical discussion of the current findings, new approaches, limitations and recommendations for future research. Pharmacogenetics studies in this field have assessed several outcomes and genes, with special focus on dopaminergic genes, mainly DRD2, which is the most important receptor in nigrostriatal pathway. The heterogeneity in methodological strategies employed by different studies is impressive. The question of whether PD pharmacogenetics studies will improve clinical management by causing a shift from a trial-and-error approach to a pharmacological regimen that takes into account the individual variability remains an open question. Collaborative longitudinal studies with larger sample sizes, better outcome definitions and replication studies are required.

Polymorphism of the dopamine transporter type 1 gene modifies the treatment response in Parkinson's disease

After more than 50 years of treating Parkinson's disease with l-DOPA, there are still no guidelines on setting the optimal dose for a given patient. The dopamine transporter type 1, now known as solute carrier family 6 (neurotransmitter transporter), member 3 (SLC6A3) is the most powerful determinant of dopamine neurotransmission and might therefore influence the treatment response. We recently demonstrated that methylphenidate (a dopamine transporter inhibitor) is effective in patients with Parkinson's disease with motor and gait disorders. The objective of the present study was to determine whether genetic variants of the dopamine transporter type 1-encoding gene (SLC6A3) are associated with differences in the response to treatment of motor symptoms and gait disorders with l-DOPA and methylphenidate (with respect to the demographic, the disease and the treatment parameters and the other genes involved in the dopaminergic neurotransmission). This analysis was part of a multicentre, parallel-group, double-blind, placebo-controlled, randomized clinical trial of methylphenidate in Parkinson's disease (Protocol ID:2008-005801-20; ClinicalTrials.gov:NCT00914095). We scored the motor Unified Parkinson's Disease Rating Scale and the Stand-Walk-Sit Test before and after a standardized acute l-DOPA challenge before randomization and then after 3 months of methylphenidate treatment. Patients were screened for variants of genes involved in dopamine metabolism: rs28363170 and rs3836790 polymorphisms in the SLC6A3 gene, rs921451 and rs3837091 in the DDC gene (encoding the aromatic L-amino acid decarboxylase involved in the synthesis of dopamine from l-DOPA), rs1799836 in the MAOB gene (coding for monoamine oxidase B) and rs4680 in the COMT gene (coding for catechol-O-methyltransferase). Investigators and patients were blinded to the genotyping data throughout the study. Eighty-one subjects were genotyped and 61 were analysed for their acute motor response to l-DOPA. The SLC6A3 variants were significantly associated with greater efficacy of l-DOPA for motor symptoms. The SLC6A3 variants were also associated with greater efficacy of methylphenidate for motor symptoms and gait disorders in the ON l-DOPA condition. The difference between motor Unified Parkinson's Disease Rating Scale scores for patients with different SLC6A3 genotypes was statistically significant in a multivariate analysis that took account of other disease-related, treatment-related and pharmacogenetic parameters. Our preliminary results suggest that variants of SLC6A3 are genetic modifiers of the treatment response to l-DOPA and methylphenidate in Parkinson's disease. Further studies are required to assess the possible value of these genotypes for (i) guiding l-DOPA dose adaptations over the long term; and (ii) establishing the risk/benefit balance associated with methylphenidate treatment for gait disorders.

Pharmacogenetics of drug response in Parkinson's disease

Parkinson's disease (PD) is a debilitating, demoralizing and financially devastating condition affecting 1% of population at the age of 60 years. Thus, very important issue to address is individual therapy optimization. Recent results have shown evidence that variable efficacy of treatment and risk of motor and mental complications could have genetic origin. Significant roles in that process play (pharmaco)genomic/genetic studies of PD. Variability in genes coding for drug-metabolizing enzymes, drug receptors and proteins involved in drug pathway signaling is an important factor determining inter-individual variability in drug responses. Interpersonal differences in drug responses are clearly documented although individualized treatment of PD is not widely known. Treatment with antiparkinsonian drugs is associated with the development of complications, such as L-DOPA-induced dyskinesia (LID), hallucinations and excessive daytime sleepiness. Carriers of specific genetic polymorphisms are particularly susceptible to development of some of these drug adverse effects. Pharmacogenomics aims to understand the relationship between genetic factors and inter-individual variations in drug responses, and to translate this information in therapy tailored to individual patient genetics. Relatively few efforts have been made to investigate the role of pharmacogenetics in the individual response to anti-PD drugs. Thus, many genetic variations and polymorphisms in myriad of different proteins can influence individual response to anti-PD drugs.

Association of Catechol-O-Methyltransferase and monoamine oxidase B gene polymorphisms with motor complications in parkinson's disease in a Chinese population

BACKGROUND

Catechol-O-Methyltransferase (COMT) and Monoamine oxidase B (MAO-B) are the main enzymes that metabolize dopamine in the brain. The polymorphisms of the COMT gene and MAO-B gene are associated with high, intermediate and low levels of activity. This may influence the prevalence of motor complications in Parkinson's Disease (PD).

METHODS

The study enrolled 1087 Chinese PD patients throughout the country. Sanger dideoxynucleotide chain termination methods were used for COMT and MAO-B genotyping. The researchers compared the association between presence of motor complications and COMT and MAO-B gene polymorphisms, both separately and in combination.

RESULTS

Comparison of the allele frequencies revealed that COMT (GG) was significantly more common among PD patients who exhibited wearing-off compared to PD patients without wearing-off (P < 0.05). A statistically higher frequency of the MAO-B (AG) genotype in PD patients with dyskinesias was found (P < 0.05). Although these differences were not significant after Bonferroni's correction. The combined haplotype of the MAO-B and COMT showed no increase (p < 0.05) in the risk of wearing-off and dyskinesias.

CONCLUSIONS

Our findings suggest that polymorphisms in COMT and MAO-B may increase the risk of wearing-off and dyskinesias. COMT (GG) genotype may be the risk factor of wearing-off. While MAO-B (AG) genotype may be the risk factor of dyskinesias.

Pharmacogenetics of Parkinson's disease - through mechanisms of drug actions

In the last years due to development of molecular methods a substantial progress in understanding of genetic associations with drug effects in many clinical disciplines has been observed. The efforts to define the role of genetic polymorphisms in optimizing pharmacotherapy of Parkinson’s disease (PD) were also undertaken. So far, some promising genetic loci for PD treatment were determined. In the review pharmacogenetic aspects of levodopa, dopamine agonists and COMT inhibitors are discussed.

Dopaminergic modulation of memory and affective processing in Parkinson depression

Depression is common in Parkinson's disease and is associated with cognitive impairment. Dopaminergic medications are effective in treating the motor symptoms of Parkinson's disease; however, little is known regarding the effects of dopaminergic pharmacotherapy on cognitive function in depressed Parkinson patients. This study examines the neuropsychological effects of dopaminergic pharmacotherapy in Parkinsonian depression. We compared cognitive function in depressed and non-depressed Parkinson patients at two time-points: following overnight withdrawal and after the usual morning regimen of dopaminergic medications. A total of 28 non-demented, right-handed patients with mild to moderate idiopathic Parkinson's disease participated. Ten of these patients were depressed according to DSM IV criteria. Results revealed a statistically significant interaction between depression and medication status on three measures of verbal memory and a facial affect naming task. In all cases, depressed Parkinson's patients performed significantly more poorly while on dopaminergic medication than while off. The opposite pattern emerged for the non-depressed Parkinson's group. The administration of dopaminergic medication to depressed Parkinson patients may carry unintended risks.

Association Between Catechol-O-Methyltransferase (COMT) Gene Polymorphisms, Parkinson's Disease, and Levodopa Efficacy

OBJECTIVES

We investigated the association between catechol-O-methyltransferase (COMT) gene polymorphisms and Parkinson's disease (PD) susceptibility, severity of disease, and levodopa (L-Dopa) efficacy.

SUBJECTS AND METHODS

Patients (N = 97) with primary PD and healthy volunteers (N = 102) were recruited. Disease severity was assessed with the Unified Parkinson's Disease Rating Scale (UPDRS) and Hoehn & Yahr grade at 'On stage'. Genomic DNA was extracted from blood cells. Polymerase chain reaction and sequencing were used to detect COMT mutations. Data were analyzed by SPSS 18.0. False discovery rate (FDR) or Bonferroni correction was used if the result showed P < 0.05.

RESULTS

Four COMT mutations were detected in 199 subjects: rs74745580 (only in two patients with primary PD), rs4633, rs6267, and rs3838146. There were no statistical differences in frequencies of rs4633, rs6267, and rs3838146 genotypes between PD patients and the control group. The frequency of allele rs4633T was higher in PD patients than in the control group. UPDRS score was lower in rs4633 (CT/TT) carriers and rs3838146 (-C/- -) carriers than in rs4633 (CC) and rs3838146 (CC) carriers. PD patients carrying rs6267 (GT/TT) had higher UPDRS scores than patients with rs6267 (GG) (P < 0.05). The frequencies of the three polymorphisms were not statistically different between patients who did and did not receive L-Dopa; dose and duration of L-Dopa treatment did not differ between genotypes; and there was also no difference in the ratios of loss of efficacy towards levodopa.

CONCLUSIONS

The polymorphisms rs4633, rs6267, and rs3838146 were associated with severity of PD but were not associated with L-Dopa medication.

Dopa-decarboxylase gene polymorphisms affect the motor response to L-dopa in Parkinson's disease

BACKGROUND

In Parkinson's disease (PD), the response to L-dopa is highly variable and unpredictable. The major pathway for dopamine synthesis from L-dopa is decarboxylation by aromatic L-amino acid decarboxylase (AAAD, encoded by the DDC gene).

OBJECTIVE

To determine the motor response to L-dopa in PD patients as a function of the DDC gene promoter polymorphisms (rs921451 T > C polymorphism (DDC(T/C)) and rs3837091 AGAG del (DDC(AGAG/-))).

METHODS

Thirty-three Caucasian PD patients underwent an acute l-dopa challenge together with the peripheral AAAD inhibitor benserazide and were genotyped for rs921451 and rs3837091. The primary efficacy criterion was the motor response to L-dopa, as estimated by the area under the curve for the change in the Unified Parkinson's Disease Rating Scale part III (UPDRS) score relative to baseline (AUCΔUPDRS) in the 4 h following L-dopa administration. Secondary endpoints were pharmacokinetic parameters for plasma levels of L-dopa and dopamine. Investigators and patients were blinded to genotypes data throughout the study.

RESULTS

When adjusted for the L-dopa dose, the AUCΔUPDRS was significantly lower in DDC(CC/CT) patients (n = 14) than in DDC(TT) patients (n = 19) and significantly lower in DDC(-/- or AGAG/-) patients (n = 8) than in DDC(AGAG/AGAG) patients (n = 25). There were no significant intergroup differences in plasma pharmacokinetic parameters for L-dopa and dopamine.

DISCUSSION

The rs921451 and rs3837091 polymorphisms of the DDC gene promoter influence the motor response to L-dopa but do not significantly change peripheral pharmacokinetic parameters for L-dopa and dopamine. Our results suggest that DDC may be a genetic modifier of the l-dopa response in Parkinson's disease.

The Val158Met COMT polymorphism is a modifier of the age at onset in Parkinson's disease with a sexual dimorphism

The catechol-O-methyltranferase (COMT) is one of the main enzymes that metabolise dopamine in the brain. The Val158Met polymorphism in the COMT gene (rs4680) causes a trimodal distribution of high (Val/Val), intermediate (Val/Met) and low (Met/Met) enzyme activity. We tested whether the Val158Met polymorphism is a modifier of the age at onset (AAO) in Parkinson's disease (PD). The rs4680 was genotyped in a total of 16 609 subjects from five independent cohorts of European and North American origin (5886 patients with PD and 10 723 healthy controls). The multivariate analysis for comparing PD and control groups was based on a stepwise logistic regression, with gender, age and cohort origin included in the initial model. The multivariate analysis of the AAO was a mixed linear model, with COMT genotype and gender considered as fixed effects and cohort and cohort-gender interaction as random effects. COMT genotype was coded as a quantitative variable, assuming a codominant genetic effect. The distribution of the COMT polymorphism was not significantly different in patients and controls (p=0.22). The Val allele had a significant effect on the AAO with a younger AAO in patients with the Val/Val (57.1±13.9, p=0.03) than the Val/Met (57.4±13.9) and the Met/Met genotypes (58.3±13.5). The difference was greater in men (1.9 years between Val/Val and Met/Met, p=0.007) than in women (0.2 years, p=0.81). Thus, the Val158Met COMT polymorphism is not associated with PD in the Caucasian population but acts as a modifier of the AAO in PD with a sexual dimorphism: the Val allele is associated with a younger AAO in men with idiopathic PD.

Pharmacogenomics in neurology: current state and future steps

In neurology, as in any other clinical specialty, there is a need to develop treatment strategies that allow stratification of therapies to optimize efficacy and minimize toxicity. Pharmacogenomics is one such method for therapy optimization: it aims to elucidate the relationship between human genome sequence variation and differential drug responses. Approaches have focused on candidate approaches investigating absorption-, distribution-, metabolism, and elimination (ADME)-related genes (pharmacokinetic pathways), and potential drug targets (pharmacodynamic pathways). To date, however, only few genetic variants have been incorporated into clinical algorithms. Unfortunately, a large number of studies have thrown up contradictory results due to a number of deficiencies, including small sample sizes, inadequate phenotyping, and genotyping strategies. Thus, there still exists an urgent need to establish biomarkers that could help to select for patients with an optimal benefit to risk relationship. Here we review recent advances, and limitations, in pharmacogenomics for agents used in neuroimmunology, neurodegenerative diseases, ischemic stroke, epilepsy, and primary headaches. Further work is still required in all of these areas, which really needs to progress on several fronts, including better standardized phenotyping, appropriate sample sizes through multicenter collaborations and judicious use of new technological advances such as genome-wide approaches, next generation sequencing and systems biology. In time, this is likely to lead to improvements in the benefit-harm balance of neurological therapies, cost efficiency, and identification of new drugs.

Pharmacological treatment and the prospect of pharmacogenetics in Parkinson's disease

Parkinson disease (PD) is a progressive movement disorder marked by tremor, rigidity, bradykinesia and postural instability. Levodopa (l-dopa), usually combined with a peripheral dopa decarboxylase inhibitor, has been proved to provide the best symptomatic benefit for PD. However, its long-term efficacy is limited because of motor complications and drug-induced dyskinesia. Dopamine agonists, catechol-O-methyltransferase inhibitors and monoamine oxidase-B inhibitors are anti-parkinsonian (anti-PD) drugs that have been found to further improve the potency of l-dopa and prevent the onset of motor complications. However, as PD is a progressive disorder, all the drugs used for its therapy, manifest reduced efficacy and adverse effects with time. Research on the field of pharmacogenetics has pointed out that the genetic variability of each individual determines to a large extent the inter-individual variability in response to anti-PD drugs. Clinicogenetic trials show that drug efficacy or toxicity or susceptibility to side effects are features governed by genetic principles. This article is a review of the present pharmacological treatment of PD and current pharmacogenetic data for PD.

Catechol-O-methyltransferase Val158Met and the risk of dyskinesias in Parkinson's disease

BACKGROUND

The A-allele of the catechol-O-methyltransferase (COMT) Val158Met polymorphism is associated with decreased enzymatic activity and higher dopamine availability.

METHODS

We studied 219 patients with PD who were free of dyskinesias at baseline and underwent thorough annual examinations.

RESULTS

The A-allele of the COMT Val158Met polymorphism was related to an increased risk of developing dyskinesias during follow-up, in a dose-dependent manner (adjusted hazard ratios for the AG and AA genotypes [compared to GG]: 2.09 [95% confidence interval (CI), 1.07-4.06] and 2.81 [CI, 1.43-5.54], respectively).

CONCLUSIONS

This finding suggests that genetic factors may affect susceptibility to dyskinesias in PD.

The COMT Val158Met polymorphism affects the response to entacapone in Parkinson's disease: a randomized crossover clinical trial

OBJECTIVE

In Parkinson disease (PD), the selective C-O-methyltransferase (COMT) inhibitor entacapone prolongs the effect of levodopa on motor symptoms (ON time) by increasing its bioavailability. The COMT Val158Met polymorphism is equally distributed in PD patients and modulates COMT activity, which can be high (Val/Val, COMT(HH) ), intermediate (Val/Met, COMT(HL) ), or low (Met/Met, COMT(LL) ). The objective of this study was to determine the response to entacapone in COMT(HH) and COMT(LL) PD patients.

METHODS

Thirty-three PD patients, homozygous for the COMT alleles COMT(HH) (n = 17) and COMT(LL) (n = 16), were randomized in a double-blind crossover trial consisting of 2 successive acute levodopa challenges associated with 200mg entacapone or placebo. The primary endpoint was the gain in the best ON time. Secondary endpoints were levodopa pharmacokinetics and COMT activity in red blood cells.

RESULTS

The gain in the best ON time was higher in COMT(HH) than in COMT(LL) patients (39 ± 10 vs 9 ± 9 minutes, p = 0.04, interaction between treatment and genotype). Area under the concentration over time curve of levodopa increased more after entacapone in COMT(HH) than in COMT(LL) patients (+62 ± 6% vs +34 ± 8%, p = 0.01). COMT inhibition by entacapone was higher in COMT(HH) than in COMT(LL) patients (-0.54 ± 0.07 vs -0.31 ± 0.06 pmol/min/mg protein, p = 0.02).

INTERPRETATION

The COMT(HH) genotype in PD patients enhances the effect of entacapone on the pharmacodynamics and pharmacokinetics of levodopa. The response to entacapone after repeated administrations and in heterozygous patients remains to be determined.

Defining drug disposition determinants: a pharmacogenetic-pharmacokinetic strategy

In preclinical and early clinical drug development, information about the factors influencing drug disposition is used to predict drug interaction potential, estimate and understand population pharmacokinetic variability, and select doses for clinical trials. However, both in vitro drug metabolism studies and pharmacogenetic association studies on human pharmacokinetic parameters have focused on a limited subset of the proteins involved in drug disposition. Furthermore, there has been a one-way information flow, solely using results of in vitro studies to select candidate genes for pharmacogenetic studies. Here, we propose a two-way pharmacogenetic-pharmacokinetic strategy that exploits the dramatic recent expansion in knowledge of functional genetic variation in proteins that influence drug disposition, and discuss how it could improve drug development.

Effects of nebicapone on levodopa pharmacokinetics, catechol-O-methyltransferase activity, and motor fluctuations in patients with Parkinson disease

OBJECTIVE

To investigate the effects of nebicapone, a new catechol-O-methyltransferase (COMT) inhibitor, on levodopa pharmacokinetics, COMT activity, and motor fluctuations in Parkinson disease in comparison to placebo and entacapone.

METHODS

Randomized, double-blind, placebo-controlled, 4-way crossover study consisting of 4 treatment periods (6-9 days duration each) in 19 patients (65.3 +/- 8.5 years) treated with carbidopa/levodopa 3 to 7 times per day. Nebicapone/entacapone/placebo and carbidopa/levodopa doses were administered concomitantly. At the end of each period, a levodopa test was performed, and levodopa and 3-O-methyldopa levels and COMT activity were assayed.

RESULTS

After 75 mg nebicapone, 150 mg nebicapone, and 200 mg entacapone, levodopa area under the plasma concentration time curve significantly increased 28.1, 48.4, and 33.3%, and 3-O-methyldopa area under the plasma concentration time curve significantly decreased 59.2, 70.8, and 59.1%, respectively. Peak COMT inhibition was similar between active treatments, but extent of COMT inhibition was more sustained with 75 and 150 mg nebicapone than with 200 mg entacapone. After the levodopa test doses, ON time significantly increased 29 minutes with 75 mg nebicapone, 45 minutes with 150 mg nebicapone, and 16 minutes with 200 mg entacapone. Patients' diaries showed a decrease in daily OFF time of 109 minutes with 75 mg nebicapone, 103 minutes with 150 mg nebicapone, and 71 minutes with 200 mg entacapone, and an increase in daily ON time of 74, 101, and 74 minutes, respectively. Treatments were generally well tolerated and safe; no relevant changes in liver function tests were reported.

CONCLUSIONS

Nebicapone, a new COMT inhibitor, significantly decreased COMT activity, increased systemic exposure to levodopa, and improved motor response. Nebicapone deserves further evaluation in larger samples of patients.

Pharmacogenetics of antiparkinsonian drug treatment: a systematic review

Pharmacotherapy is the mainstay in the treatment of Parkinson's disease and the armamentarium of drugs available for the therapy of this disease is still expanding. Anti-Parkinson's disease drugs are effective in reducing the physical symptoms, such as hypokinesia, bradykinesia, rigidity and tremor. However, there is a large interindividual variability in response to anti-Parkinson's disease drugs with respect to both drug efficacy and toxicity. It is thought that genetic variability in genes encoding drug-metabolizing enzymes, drug receptors and proteins involved in pathway signaling is an important factor in determining interindividual variability in drug response. Pharmacogenetics aims at identifying genetic markers associated with drug response. Ideally, knowledge of these genetic markers will enable us to predict an individual's drug response in terms of both efficacy and toxicity. The role of pharmacogenetics in the treatment of Parkinson's disease is relatively unexplored. Therefore, we aim to present a systematic review of the published pharmacogenetic studies in Parkinson's disease and to describe polymorphic genes of interest for future research.

Effect of MTHFR Polymorphisms on Hyperhomocysteinemia in Levodopa-treated Parkinsonian Patients

High plasma homocysteine levels have been observed in Parkinson's disease (PD) patients treated with levodopa. In this study, we investigated the effects of C677T and A1298C MTHFR polymorphisms, in association with L-DOPA daily dose and vitamin status, on hyperhomocysteinemia development in PD patients. Plasma homocysteine and folate/vitamin B12 levels were assayed in 49 L-DOPA-treated PD patients, and compared with those of 86 healthy subjects. Genotyping for MTHFR polymorphisms was carried out by DG-DGGE. Homocysteine levels were significantly higher in patients than in controls (16.3 +/- 5.7 vs. 11.7 +/- 2.7 micromol/l, P < 0.01). No significant differences were found between patients and controls with regard to folate/vitamin B12 levels, and MTHFR allele distribution. The TT+AA genotype was significantly more frequent in PD patients than in controls (32.5% vs. 17.4%, P < 0.05), but not associated with an increased risk for PD (OR = 2.3, CI = 1.0-5.2). Further, patients carrier of this genotype exhibited a mild hyperhomocysteinemia (22.1 +/- 4.9 micromol/l), while a protective effect was observed in patients having the CC+AA genotype (11.2 +/- 1.6 micromol/l; OR = 0.19, CI = 0.06-0.59). Interestingly, homocysteine levels were also moderately increased in patients with CT heterozygous genotype, in the context of either AA or AC (14.5 +/- 3.6 micromol/l), in comparison to subjects with the CC + AA genotype. Finally, we did not find any significant association of combined C677T and A1298C MTHFR polymorphisms with an increased risk for hyperhomocysteinemia in PD patients. A better understanding of the role of homocysteine and MTHFR genotypes in PD is needed to reveal novel approaches for disease management.

The effect of Madopar on the pharmacokinetics of ropinirole in healthy Chinese volunteers

Ropinirole is a nonergoline dopamine D(2)-receptor agonist and has been proven to be effective in both monotherapy and combination therapy for idiopathic Parkinson's disease. The purpose of the present study was to examine the effect of Madopar on the pharmacokinetics of ropinirole in healthy Chinese volunteers by using liquid chromatography tandem mass spectrometry (HPLC/MS/MS). A single dose of 1mg ropinirole was given orally after administration of the placebo or Madopar (containing 200 mg levodopa and 50 mg benserazide) to six healthy males and six healthy females in a cross-over randomized study with a minimum washout period of 8 days. Pharmacokinetic parameters were calculated for both treatments. Coadministration of ropinirole and Madopar did not result in a notable change in rate or extent of availability of ropinirole, as shown by the ratios (90% confidence intervals) of 1.045 (0.900, 1.222) for C(max) (maximum plasma concentration) and 1.167 (1.086, 1.262) for AUC(0-inf) (the area under the concentration-time curve). Likewise, no significant difference in any of the other pharmacokinetic parameters [T(max) (the time needed to reach the C(max)), MRT (mean residence time), volume of distribution (V/F), and clearance (CL/F)] was observed between the treatment groups. No clinically relevant adverse effects were detected under either conditions and there are no pharmacokinetic grounds for adjusting the dose of ropinirole when given in combination with Madopar in Chinese patients.

Catechol-O-methyltransferase and its inhibitors in Parkinson's disease

Parkinson's disease (PD) is a neurological disorder characterized by the degeneration of dopaminergic neurons, with consequent reduction in striatal dopamine levels leading to characteristic motor symptoms. The most effective treatment for this disease continues to be the dopamine replacement therapy with levodopa together with an inhibitor of aromatic amino acid decarboxylase (AADC). The efficacy of this therapy, however, decreases with time and most patients develop fluctuating responses and dyskinesias. The last decade showed that the use of catechol-O-methyltransferase inhibitors as adjuvants to the levodopa/AADC inhibitor therapy, significantly improves the clinical benefits of this therapy. The purpose of this article is to review the current knowledge on the enzyme catechol-O-methyltransferase (COMT) and the role of COMT inhibitors in PD as a new therapeutic approach to PD involving conversion of levodopa to dopamine at the target region in the brain and facilitation of the continuous action of this amine at the receptor sites. A historical overview of the discovery and development of COMT inhibitors is presented with a special emphasis on nebicapone, presently under clinical development, as well as entacapone and tolcapone, which are already approved as adjuncts in the therapy of PD. This article reviews human pharmacokinetic and pharmacodynamic properties of these drugs as well as their clinical efficacy and safety.

Polymorphisms in candidate genes: implications for the current treatment of Parkinson's disease

Pharmacological treatment remains the cornerstone of therapy in Parkinson's disease. A number of clinical and genetic factors may influence the therapeutic response and treatment-related complications. Some exploratory studies have suggested that genetic polymorphisms may influence an individual's response to dopaminergic therapy and susceptibility to drug-related complications, such as hallucinations, dyskinesias, motor fluctuations and sudden onset of sleep episodes. This article provides a concise summary and discussion of the potential utility and limitation of studies that have examined the influence of genetic polymorphisms on drug-related response and complications in Parkinson's disease.

Pharmacokinetic optimisation in the treatment of Parkinson's disease : an update

Pharmacotherapy for Parkinson's disease is focused on dopaminergic drugs, mainly the dopamine precursor levodopa and dopamine receptor agonists. The elimination half-life (t(1/2)) of levodopa from plasma (in combination with a decarboxylase inhibitor) of about 1.5 hours becomes more influential as the disease progresses. The long-duration of response to levodopa, which is evident in early Parkinson's disease, diminishes and after a few years of treatment motor performance is closely correlated to the fluctuating plasma concentrations of levodopa. Absorption of levodopa in the proximal small intestine depends on gastric emptying, which is erratic and may be slowed in Parkinson's disease. The effects of levodopa on motor function are dependent on gastric emptying in patients in the advanced stages of disease. The current treatment concept is continuous dopaminergic stimulation (CDS). Sustained-release formulations of levodopa may provide more stable plasma concentrations. Oral liquid formulations shorten the time to reach peak concentration and onset of effect but do not affect plasma levodopa variability. The t(1/2) of levodopa can be prolonged by adding a catechol-O-methyltransferase inhibitor (entacapone or tolcapone), which may reduce fluctuations in plasma concentrations, although both peak and trough concentrations are increased with frequent administration. Intravenous and enteral (duodenal/jejunal) infusions of levodopa yield stable plasma levodopa concentrations and motor performance. Enteral infusion is feasible on a long-term basis in patients with severe fluctuations. Among the dopamine receptor agonists the ergot derivatives bromocriptine, cabergoline, dihydroergocryptine and pergolide, and the non-ergot derivatives piribedil, pramipexole and ropinirole, have longer t(1/2) compared with levodopa. Thus, they stimulate dopamine receptors in a less pulsatile manner, yet pharmacokinetic studies of repeated doses of dopamine receptor agonists are few. Optimisation of these drugs is often performed with standardised titration schedules. Apomorphine and lisuride have short t(1/2) and are suitable for subcutaneous infusion, with results similar to those of levodopa infusion. Transdermal administration of dopamine receptor agonists such as rotigotine might be an alternative in the future. In general, initial dopamine receptor agonist monotherapy is associated with poorer motor performance and lower incidence of motor complications compared with levodopa. Buccal administration of the monoamine oxidase-B inhibitor selegiline (deprenyl) provides better absorption and less formation of metabolites compared with standard tablets. To conclude, several new drugs, formulations and routes of administration have been introduced in the treatment of Parkinson's disease during the last decade, mainly with CDS as the aim. CDS can be approached by optimising the use of dopaminergic drugs based on pharmacokinetic data.