Pharmacokinetics, pharmacodynamics, and tolerability of tolcapone: a review of early studies in volunteers

Levodopa Dose Equivalency in Parkinson's Disease: Updated Systematic Review and Proposals

BACKGROUND

To compare drug regimens across clinical trials in Parkinson's disease (PD) conversion formulae between antiparkinsonian drugs have been developed. These are reported in relation to levodopa as the benchmark drug in PD pharmacotherapy as 'levodopa equivalent dose' (LED). Currently, the LED conversion formulae proposed in 2010 by Tomlinson et al. based on a systematic review are predominantly used. However, new drugs with established and novel mechanisms of action and novel formulations of longstanding drugs have been developed since 2010. Therefore, consensus proposals for updated LED conversion formulae are needed.

OBJECTIVES

To update LED conversion formulae based on a systematic review.

METHODS

The MEDLINE, CENTRAL, and Embase databases were searched from January 2010 to July 2021. Additionally, in a standardized process according to the GRADE grid method, consensus proposals were issued for drugs with scarce data on levodopa dose equivalency.

RESULTS

The systematic database search yielded 3076 articles of which 682 were eligible for inclusion in the systematic review. Based on these data and the standardized consensus process, we present proposals for LED conversion formulae for a wide range of drugs that are currently available for the pharmacotherapy of PD or are expected to be introduced soon.

CONCLUSIONS

The LED conversion formulae issued in this Position Paper will serve as a research tool to compare the equivalence of antiparkinsonian medication across PD study cohorts and facilitate research on the clinical efficacy of pharmacological and surgical treatments as well as other non-pharmacological interventions in PD. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Parkinson's disease in women: Mechanisms underlying sex differences

Parkinson's disease is a neurodegenerative disease which is associated with different motor, cognitive and mood-related problems. Though it has been established that Parkinson's disease is less prevalent in women in comparison to men, the differences tend to diminish with the advancing age. Different genetic, hormonal, neuroendocrinal and molecular players contribute towards the differences in the Parkinson's disease pathogenesis. Furthermore, data available with respect to the therapeutic management of Parkinson's disease in females is limited; women often tend to suffer more from the side effects of the currently available drugs. The present review highlights the sex-specific differences which play a role in the manifestation of these symptoms and side effects of the currently available therapeutic strategies. We have also discussed the current and upcoming therapeutic strategies which are in the clinical trials such as adenosine 2A (A2A) receptor antagonists, estrogen replacement therapy, α-synuclein targeting vaccines and antibodies, Botulinum toxin A, Fas-associated factor-1 (FAF-1) inhibitors, thiazolidinediones, 5-HT1_A receptor agonists, dopamine D1/D5 receptor agonists, Glucagon-like peptide 1 (GLP-1) analogues and certain plant based principles for the treatment of Parkinson's disease in women.

The catechol-O-methyltransferase inhibitor tolcapone modulates alcohol consumption and impulsive choice in alcohol use disorder

RATIONALE

Individuals suffering from alcohol use disorder (AUD) demonstrate difficulty with decision-making and impulsivity that may be associated with impaired frontal cortical function. Therapeutics that enhance frontal dopamine tone could decrease impulsivity and in turn reduce alcohol consumption in individuals with AUD.

OBJECTIVES

To determine if the catechol-O-methyltransferase (COMT) inhibitor tolcapone can attenuate alcohol consumption in individuals with AUD and whether this attenuation correlates with tolcapone-induced changes in laboratory-based decision-making tasks.

METHODS

We used daily self-report and a novel group laboratory bar task to assess the effects of randomized double-blind crossover administration of tolcapone (100 mg TID for 5 days) on alcohol consumption and laboratory tasks assessing impulsivity in 55 non-treatment-seeking subjects with AUD.

RESULTS

Tolcapone significantly reduced self-reported alcohol consumption (t (54) = 2.05, p = 0.045). The effects of tolcapone on drinking significantly correlated with changes in impulsive decision-making, such that subjects with the greatest decrease in impulsive choice on tolcapone also reported the greatest decrease in alcohol consumption (r (45) = 0.40, p = 0.0053). We did not see effects of tolcapone on laboratory bar consumption. Adverse event (AE) reporting was low, with no significant difference in frequency or severity of AEs on tolcapone versus placebo.

CONCLUSIONS

These data demonstrate that COMT inhibitors such as tolcapone may be useful therapeutics for AUD.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT02740582.

General Aggregation-Induced Emission Probes for Amyloid Inhibitors with Dual Inhibition Capacity against Amyloid β-Protein and α-Synuclein

Amyloid self-assembly is pathologically linked to many neurodegenerative diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD). While many inhibitors have been developed individually for specific amyloid proteins, there are a few effective platforms to screen on a large scale general amyloid inhibitors against different amyloid proteins. Herein, we developed a new class of amyloid inhibitor probes by site-specific conjugation of aggregation-induced emission (AIE) molecules with amyloid proteins (i.e., AIE@amyloid probes) to realize a high-throughput screening of small-molecule inhibitors. Optimization of site-specific AIE conjugation with two amyloid proteins, amyloid-β protein (Aβ) and α-synuclein (αSN), enabled us to retain their high amyloidogenic properties; i.e., AIE-amyloid probes alone exhibited strong fluorescence due to amyloid-like aggregation, but they showed no fluorescence in the presence of amyloid inhibitors to prevent amyloid aggregation. From integration of AIE@amyloid probes and computational virtual screening from a large drug database, it was found that tolcapone possessed a dual inhibition against the aggregation and cytotoxicity of both Aβ and αSN. More importantly, tolcapone significantly improved the spatial cognition and recognition of Aβ-treated mice. This work represents an innovative attempt to design an AIE-based anti-amyloid drug platform for identifying new small-molecule inhibitors against amyloidogenesis in both AD and PD or other amyloid diseases.

Highs and lows of cannabinoid-dopamine interactions: effects of genetic variability and pharmacological modulation of catechol-O-methyl transferase on the acute response to delta-9-tetrahydrocannabinol in humans

RATIONALE

The catechol-O-methyl transferase (COMT) enzyme has been implicated in determining dopaminergic tone and the effects of delta-9-tetrahydrocannabinol (THC) in the human brain.

OBJECTIVE

This study was designed to evaluate the effect of (1) a functional polymorphism and (2) acute pharmacological inhibition of COMT on the acute response to THC in humans.

METHODS

Sub-study I: The effect of intravenous (IV) THC (0.05 mg/kg) was investigated in 74 healthy subjects genotyped for the COMT rs4680 (Val/Met) polymorphism in a 2-test-day double-blind, randomized, placebo-controlled study. Sub-study II: COMT rs4680 homozygous subjects (Val/Val and Met/Met) from sub-study I received the COMT enzyme inhibitor tolcapone (200 mg) followed by IV THC or placebo on two additional test days. Subjective, behavioral, and cognitive data were obtained periodically on each test day.

RESULTS

Sub-study I: Val/Val individuals were most sensitive to THC-induced attention and working memory deficits. In contrast, the psychotomimetic and subjective effects of THC were not influenced by COMT genotype. Sub-study II: Tolcapone reduced THC-induced working memory deficits, but not THC's psychotomimetic effects. Tolcapone and COMT genotype (met/met) were associated with an increased report of feeling "mellow."

CONCLUSIONS

The interaction between COMT rs4680 polymorphisms and tolcapone on the cognitive, but not on the psychotomimetic and overall subjective effects of THC, suggests that modulation of dopaminergic signaling may selectively influence specific cannabinoid effects in healthy individuals. The role of dopaminergic signaling in the cognitive effects of cannabinoids should be considered in drug development efforts targeting these effects. CLINICALTRIALS.GOV REGISTRATION: https://clinicaltrials.gov/ct2/show/NCT00678730?term=NCT00678730&rank=1 ClinicalTrials.gov Identifier: NCT00678730.

Tolcapone-Enhanced Neurocognition in Healthy Adults: Neural Basis and Predictors

BACKGROUND

Failure of procognitive drug trials in schizophrenia may reflect the clinical heterogeneity of schizophrenia, underscoring the need to identify biomarkers of treatment sensitivity. We used an experimental medicine design to test the procognitive effects of a putative procognitive agent, tolcapone, using an electroencephalogram-based cognitive control task in healthy subjects.

METHODS

Healthy men and women (n=27; ages 18-35 years), homozygous for either the Met/Met or Val/Val rs4680 genotype, received placebo and tolcapone 200 mg orally across 2 test days separated by 1 week in a double-blind, randomized, counterbalanced, within-subject design. On each test day, neurocognitive performance was assessed using the MATRICS Consensus Cognitive Battery and an electroencephalogram-based 5 Choice-Continuous Performance Test.

RESULTS

Tolcapone enhanced visual learning in low-baseline MATRICS Consensus Cognitive Battery performers (d=0.35) and had an opposite effect in high performers (d=0.5), and enhanced verbal fluency across all subjects (P=.03) but had no effect on overall MATRICS Consensus Cognitive Battery performance. Tolcapone reduced false alarm rate (d=0.8) and enhanced frontal P200 amplitude during correctly identified nontarget trials (d=0.6) in low-baseline 5 Choice-Continuous Performance Test performers and had opposite effects in high performers (d=0.5 and d=0.25, respectively). Tolcapone's effect on frontal P200 amplitude and false alarm rate was correlated (rs=-0.4, P=.05). All neurocognitive effects of tolcapone were independent of rs4680 genotype.

CONCLUSION

Tolcapone enhanced neurocognition and engaged electroencephalogram measures relevant to cognitive processes in specific subgroups of healthy individuals. These findings support an experimental medicine model for identifying procognitive treatments and provide a strong basis for future biomarker-informed procognitive studies in schizophrenia patients.

The "gender factor" in wearing-off among patients with Parkinson's disease: a post hoc analysis of DEEP study

Background. The early detection of wearing-off in Parkinson disease (DEEP) observational study demonstrated that women with Parkinson's disease (PD) carry an increased risk (80.1%) for wearing-off (WO). This post hoc analysis of DEEP study evaluates gender differences on WO and associated phenomena. Methods. Patients on dopaminergic treatment for ≥1 year were included in this multicenter observational cross-sectional study. In a single visit, WO was diagnosed based on neurologist assessment as well as the use of the 19-item wearing-off questionnaire (WOQ-19); WO was defined for scores ≥2. Post hoc analyses were conducted to investigate gender difference for demographic and clinical features with respect to WO. Results. Of 617 patients enrolled, 236 were women and 381 were men. Prevalence of WO was higher among women, according to both neurologists' judgment (61.9% versus 53.8%, P = 0.045) and the WOQ-19 analysis (72.5% versus 64.0%, P = 0.034). In patients with WO (WOQ-19), women experienced ≥1 motor symptom in 72.5% versus 64.0% in men and ≥1 nonmotor symptom in 44.5% versus 36.7%, in men. Conclusions. Our results suggest WO as more common among women, for both motor and nonmotor symptoms. Prospective studies are warranted to investigate this potential gender-effect.

Effect of tolcapone on brain activity during a variable attentional control task: a double-blind, placebo-controlled, counter-balanced trial in healthy volunteers

BACKGROUND

Attention is the capacity to flexibly orient behaviors and thoughts towards a goal by selecting and integrating relevant contextual information. The dorsal cingulate (dCC) and prefrontal (PFC) cortices play critical roles in attention. Evidence indicates that catechol-O-methyltransferase (COMT) modulates dopaminergic tone in the PFC and dCC.

OBJECTIVE

In this study, we explored the effect of tolcapone, a CNS penetrant COMT inhibitor that increases cortical dopamine levels, on brain activity during a Variable Attentional Control (VAC) task.

STUDY DESIGN

We performed a double-blinded, placebo-controlled, counter-balanced trial with tolcapone (Tasmar, tablets, 100 mg three times a day for 1 day and then 200 mg three times a day for 6 days; ClinicalTrials.gov identifier: NCT00044083).

SETTING

The study was conducted in the Clinical Center of the National Institute of Mental Health from 2005 to 2009.

PATIENTS

Twenty healthy volunteers (11 males; mean age = 32.7 years) with good imaging and performance data on both arms of the study were investigated.

INTERVENTION

Participants underwent 3T blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) while performing the event-related VAC task, which varies attention over three levels of load: LOW, INT (intermediate), and HIGH.

MAIN OUTCOME MEASURE

Changes in behavioral data and individual contrast images were analyzed using ANOVA with drug and task load as co-factors.

RESULTS

There was a significant main effect of increasing task load, with resulting decreased accuracy and increased reaction time. While there was no significant effect of tolcapone on these behavioral measures, the neuroimaging data showed a significant effect on load-related changes in dCC, with significantly lower dCC activation on tolcapone compared with placebo. Further, neural activity in dCC correlated positively with COMT enzyme activity (i.e., lower COMT activity and presumably more dopamine was associated with lower activation in dCC, i.e., more efficient information processing).

CONCLUSION

Our results show that pharmacological reduction of COMT activity modulates the engagement of attentional mechanisms, selectively enhancing the efficiency of dCC processing in healthy volunteers, reflected as decreased activity for the same level of performance.

COMT Val(158)Met genotype determines the direction of cognitive effects produced by catechol-O-methyltransferase inhibition

BACKGROUND

Catechol-O-methyltransferase (COMT) metabolizes dopamine. The COMT Val(158)Met polymorphism influences its activity, and multiple neural correlates of this genotype on dopaminergic phenotypes, especially working memory, have been reported. COMT activity can also be regulated pharmacologically by COMT inhibitors. The inverted-U relationship between cortical dopamine signaling and working memory predicts that the effects of COMT inhibition will differ according to COMT genotype.

METHODS

Thirty-four COMT Met(158)Met (Met-COMT) and 33 COMT Val(158)Val (Val-COMT) men were given a single 200-mg dose of the brain-penetrant COMT inhibitor tolcapone or placebo in a randomized, double-blind, between-subjects design. They completed the N-back task of working memory and a gambling task.

RESULTS

In the placebo group, Met-COMT subjects outperformed Val-COMT subjects on the 2- back, and they were more risk averse. Tolcapone had opposite effects in the two genotype groups: it worsened N-back performance in Met-COMT subjects but enhanced it in Val-COMT subjects. Tolcapone made Met-COMT subjects less risk averse but Val-COMT subjects more so. In both tasks, tolcapone reversed the baseline genotype differences.

CONCLUSIONS

Depending on genotype, COMT inhibition can enhance or impair working memory and increase or decrease risky decision making. To our knowledge, the data are the clearest demonstration to date that the direction of effect of a drug can be influenced by a polymorphism in its target gene. The results support the inverted-U model of dopamine function. The findings are of translational relevance, because COMT inhibitors are used in the adjunctive treatment of Parkinson's disease and are under evaluation in schizophrenia and other disorders.

Tolcapone, COMT polymorphisms and pharmacogenomic treatment of schizophrenia

It is widely accepted that abnormal prefrontal cortex biology resulting in deficient cognition is a primary problem in schizophrenia and that all currently available antipsychotics fail to improve cognitive and negative symptoms originating from this deficit. Evidence from basic science has revealed the importance of prefrontal dopamine signaling for optimal prefrontal function. This article describes succinctly the progress made so far, taking into account the mechanisms involved in catechol-O-methyltransferase (COMT)-induced modulation of prefrontal dopamine signaling, the impact of COMT on cognitive function and the role of COMT gene polymorphisms. The potential role of the COMT inhibitor tolcapone to improve cognitive function in health and disease is also presented here. It will soon be understood if tolcapone represents one of the first hypothesis-driven, biology-based, genotype-specific, targeted treatments of cognitive and negative symptoms of schizophrenia.

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.

Treatment of cognitive deficits associated with schizophrenia: potential role of catechol-O-methyltransferase inhibitors

In the last two decades, understanding of the dynamics of dopamine function in the prefrontal cortex and its role in prefrontal cortex physiology has opened up new avenues for therapeutic interventions in conditions in which prefrontal cortex function is compromised. Neuropsychological and imaging studies of prefrontal information processing have confirmed specific cognitive and neurophysiological abnormalities in individuals with schizophrenia. Because such findings are also observed in the healthy siblings of patients with schizophrenia, they may represent intermediate phenotypes related to schizophrenia susceptibility genes.Catechol-O-methyltransferase (COMT) represents an important candidate as a susceptibility gene for cognitive dysfunction in schizophrenia because of the unique role this enzyme plays in regulating prefrontal dopaminergic function. A functional COMT polymorphism (Val158Met) predicts performance in tasks of prefrontal executive function and the neurophysiological response measured with electroencephalography and functional magnetic resonance imaging in tasks assessing working memory. In fact, individuals with the Val/Val genotype, which encodes for the high-activity enzyme resulting in lower dopamine concentrations in the prefrontal cortex, perform less well and are less efficient physiologically than Met/Met individuals. These findings raise the possibility of new pharmacological interventions for the treatment of prefrontal cortex dysfunction and of predicting outcome based on COMT genotype. One strategy consists of the use of CNS-penetrant COMT inhibitors such as tolcapone. A second strategy is to increase extracellular dopamine concentrations in the frontal cortex by blocking the noradrenaline (norepinephrine) reuptake system, a secondary mechanism responsible for the disposal of dopamine from synaptic clefts in the prefrontal cortex. A third possibility involves the use of modafinil, a drug with an unclear mechanism of action but with positive effects on working memory in rodents. The potential of these drugs to improve executive cognitive function by selectively increasing dopamine load in the frontal cortex but not in subcortical territories, and the possibility that response to them may be modified by a COMT polymorphism, provides a novel genotype-based targeted pharmacological approach without abuse potential for the treatment of cognitive disorder in schizophrenia and in other conditions involving prefrontal cortex dysfunction.

Tolcapone improves cognition and cortical information processing in normal human subjects

Prefrontal cortical dopamine (DA) regulates various executive cognitive functions, including attention and working memory. Efforts to enhance prefrontal-related cognition, which have focused on catecholaminergic stimulant drugs, have been unsatisfactory. Recently, the demonstration that a functional polymorphism in the catecholamine-O-methyltransferase (COMT) gene impacts prefrontal cognition raises the possibility of a novel pharmacological approach for the treatment of prefrontal lobe executive dysfunction. To explore in a proof of concept study the effects of tolcapone, a CNS penetrant specific COMT inhibitor, we performed a randomized, double blind, placebo controlled, and crossover design of this drug in normal subjects stratified by COMT (val158met) genotype. COMT enzyme activity was determined in peripheral blood. Forty-seven normal volunteers with no family history of psychiatric disorders underwent neuropsychological testing and 34 of those subjects underwent physiological measurement of prefrontal information processing assessed by blood oxygen level-dependent functional magnetic resonance imaging (fMRI). We found significant drug effects on measures of executive function and verbal episodic memory and a significant drug by genotype interaction on the latter, such that individuals with val/val genotypes improved, whereas individuals with met/met genotypes worsened on tolcapone. fMRI revealed a significant tolcapone-induced improvement in the efficiency of information processing in prefrontal cortex during a working memory test. This study demonstrates enhancement of prefrontal cortical function in normal human subjects with a nonstimulant drug having COMT inhibitory activity. Our results are consistent with data from animal studies and from computational models of the effects of selective enhancement of DA signaling in the prefrontal cortex.

Gender disparities in Parkinson's disease

Parkinson's disease is a chronic neurodegenerative disorder of unknown etiology. There are sparse data on gender differences in this disorder, but it is clear that there are gender discrepancies in incidence, symptoms, medication effects and treatments. There also appear to be lifecycle fluctuations in the disease course of female Parkinson's disease patients. The effect of estrogen in this disorder is multifold and its role in the development and treatment of PD will be discussed.

Update on Apomorphine for the Rapid Treatment of Hypomobility (“Off”) Episodes in Parkinson's Disease

As Parkinson's disease progresses, fluctuations between akinesia, or hypomobility ("off" times), and mobility ("on" times) increase in frequency despite optimized pharmacotherapy. Motor fluctuations include predictable shortening of therapeutic effects, nocturnal or early morning akinesia, random hypomobility, and delayed mobility (variable responses to individual doses of drugs). Current oral antiparkinson drugs are inadequate for rapid and consistent relief of symptoms during hypomobility. Apomorphine, an injectable dopamine agonist recently introduced in the United States, is indicated for the management of hypomobility associated with advanced Parkinson's disease. Subcutaneous apomorphine is effective for rapid and consistent rescue from hypomobility, with a magnitude of motor improvement similar to that of levodopa. The effect begins within 20 minutes after dosing and lasts approximately 100 minutes. Therapeutic rescue doses are 2-6 mg, and patients typically require approximately three rescue doses/day. Apomorphine is associated with a clinically significant potential to cause nausea and orthostatic hypotension. These potential effects can be managed with antiemetic prophylaxis and appropriate determination of the therapeutic rescue dose.

Tolcapone: a review of its use in the management of Parkinson's disease

Tolcapone (Tasmar) is a selective, reversible inhibitor of peripheral and central catechol-O-methyltransferase (COMT). Results of well designed studies indicate that oral tolcapone is an effective adjunct to levodopa plus a peripheral dopa-decarboxylase inhibitor (DDCI) in patients with fluctuating Parkinson's disease. Tolcapone significantly improves levodopa-induced motor fluctuations and significantly reduces levodopa requirements. The drug is generally well tolerated, with the most commonly occurring adverse events being dopaminergic related. Thus, tolcapone is a useful option in patients with fluctuating Parkinson's disease who are receiving levodopa/DDCI and are not responding to, or are not candidates for, other adjunctive treatments.

Cerebrospinal fluid 3,4-dihydroxyphenylacetic acid level after tolcapone administration as an indicator of nigrostriatal degeneration

The development of reliable biological markers of nigrostriatal degeneration has important implications from both experimental and clinical viewpoints, since such biomarkers could be used for diagnostic and monitoring purposes in models of parkinsonism as well as in Parkinson's disease patients. In this study, levels of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were measured in the cerebrospinal fluid (CSF) of normal and parkinsonian squirrel monkeys in order to assess their reliability as indicators of nigrostriatal injury. In particular, we tested the hypothesis that these measurements may become more accurate by inhibiting catecholamine-O-methyltransferase (COMT) activity and therefore blocking the conversion of DOPAC to homovanillic acid. Oral administration of the COMT inhibitor tolcapone (2 doses of 15 mg/kg each with a 4-h interval) significantly reduced enzyme activity in the monkey brain. Tolcapone treatment enhanced CSF DOPAC concentrations in unlesioned animals (by approximately four times) as well as monkeys rendered parkinsonian after severe nigrostriatal dopaminergic injury caused by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Importantly, however, COMT inhibition greatly magnified the differences in CSF DOPAC levels between control and parkinsonian monkeys, since MPTP-induced DOPAC depletion was 35% in the absence vs >60% in the presence of tolcapone. Thus, tolcapone administration enhances the detection of DOPAC in the CSF and, by doing so, improves the reliability of CSF DOPAC as a marker of nigrostriatal degeneration.

Entacapone in the management of Parkinson's disease

Entacapone is one of a new class of drugs, the catechol-O-methyltransferase (COMT) inhibitors, which expand the therapeutic options for Parkinson's disease by extending the action of levodopa. Entacapone is used in conjunction with levodopa and provides benefit to patients who suffer from motor fluctuations. Side effects include worsening of the dyskinesias associated with peak doses of levodopa, hypotension, constipation and urine discoloration. Unlike tolcapone, an earlier COMT inhibitor, entacapone does not require liver function monitoring.

The relationship between COMT genotype and the clinical effectiveness of tolcapone, a COMT inhibitor, in patients with Parkinson's disease

Patients with Parkinson's Disease (PD) have a variable response to tolcapone, a catechol-O-methyltransferase (COMT) inhibitor. In addition, a subset of patients develop severe diarrhea as a side effect. Two codominant alleles for the COMT gene exist, coding for low and high activity, resulting in low-, medium-, and high-activity genotypes. This study investigates the relationship between this variation in genotype and clinical effects in patients with PD taking tolcapone. To investigate the relationship between COMT polymorphism and clinical response, 24 patients who completed tolcapone clinical trials provided blood samples for COMT genotype analysis. Change in levodopa dose and United Parkinson Disease Rating Scale (UPDRS) Part III (motor subscale) were analyzed at baseline, at 1-2 weeks, and 6 months after initiation of tolcapone. Genotype analysis was performed on seven patients who had diarrhea as a side effect. There was no significant correlation between genotype and improvement in UPDRS score (p = 0.29) according to a linear models approach that adjusted for the subject's severity of PD, tolcapone dose (either 100 or 200 mg three times daily) and initial differences in baseline scores. No significant difference was seen in change in daily levodopa intake and genotype. There was also no relation between diarrhea and COMT genotype. These results indicate that, in the treatment of Parkinson's disease, COMT genotype is not a major contributor to the clinical response to tolcapone.

Clinical, pharmacokinetic, and pharmacodynamic effects of tolcapone withdrawal in levodopa-treated patients with parkinsonism

The effect and clinical significance of tolcapone withdrawal on erythrocyte catechol-O-methyltransferase (COMT) activity, levodopa pharmacokinetics, and levodopa requirements were investigated in 59 patients with fluctuating parkinsonism who were randomized to receive placebo or tolcapone 100 or 200 mg three times daily for 6 weeks. Tolcapone withdrawal caused a transient elevation in COMT activity by 64% in patients receiving 100 mg three times daily and by 128% in those receiving 200 mg three times daily at approximately 1-2 weeks after discontinuation of drug. Thereafter, COMT activity was declining but did not reach baseline values by the 12-week study endpoint. However, this had no effect on plasma levodopa and 3-O-methyldopa (3-OMD) concentrations or on levodopa requirements. During treatment, tolcapone increased "on" time and decreased "off" time; after discontinuation of study medication and levodopa dose adjustment, on and off times were similar to baseline. Withdrawal was generally well tolerated; no patients withdrew from the trial during the posttreatment period, and no serious adverse events were observed. In conclusion, the transient increase in erythrocyte COMT activity observed after discontinuation oftolcapone is not associated with changes in peripheral levodopa metabolism and therefore has no significant clinical consequences in terms of levodopa requirements, clinical symptoms, or adverse events.

Population pharmacokinetics of tolcapone in parkinsonian patients in dose finding studies

AIMS

To use pharmacostatistical models to characterize tolcapone's pharmacokinetics in parkinsonian patients, and to identify any demographic subpopulations which may be at risk of either under- or over-exposure to this catechol-O-methyltransferase (COMT) inhibitor.

METHODS

Four hundred and twelve patients participated in three multicentre, parallel, double-blind, placebo-controlled, dose-finding studies and received either placebo or tolcapone (50, 200 or 400 mg three times daily) in addition to levodopa/decarboxylase inhibitor therapy. Sparse blood samples were obtained from 275 patients for tolcapone assay and the concentrations (1414 in total) were analysed using the NONMEM program.

RESULTS

The pharmacokinetic model which best described the data was a two-compartment open model with first-order absorption and possibly a lag-time. Tolcapone pharmacokinetics were shown to be stable, with no systematic trend between 2 and 6 weeks of treatment. The absorption of the drug was shown to be rapid and concomitant food intake had only a minor effect on the relative bioavailability (10-20% reduction compared with fasting). The overall clearance of tolcapone could be estimated with good precision (approximately 4. 5-5 l h-1 ), and none of the investigated covariates (e.g. sex, age, body weight) had any clinically significant influence on this parameter. The volume of distribution showed relatively high variability and was calculated to be approximately 30 l, leading to an estimated half-life in patients of approximately 5-8 h.

CONCLUSIONS

Using sparse concentrations and mixed effect-effects modelling analysis it is possible to describe the pharmacokinetics of tolcapone in parkinsonian populations. The parameter estimates obtained agreed with those obtained from conventional pharmacokinetic studies and no subpopulation was shown to be at risk of either under- or over-exposure to tolcapone.