The effects of the COMT inhibitor entacapone on haemodynamics and peripheral catecholamine metabolism during exercise

The Catechol O-Methyltransferase Inhibitor Entacapone in the Treatment of Parkinson's Disease: Personal Reflections on a First-in-Class Drug Development Programme 40 Years On

In the 1980s, Orion Pharma, then a mid-ranking Nordic area pharmaceutical company, established a drug development programme on the inhibition of catechol O-methyltransferase (COMT). This enzyme, which plays an important role in the inactivation of catecholamine neurotransmitters and drugs with a catechol structure, thus came under consideration as a target in the innovative translational and clinical programme we describe in this historical review. The starting point was the conjecture that a peripherally acting COMT inhibitor might improve entry of levodopa into the brain. This had potentially significant implications for the medical treatment of Parkinson's disease (PD). The rationale was that more efficient delivery of levodopa to the brain might allow the high therapeutic doses of levodopa to be reduced and the dose interval to be extended. Elucidation of structure-activity relations paved the way for the discovery and development of entacapone, a 5-nitrocatechol that was a potent and highly specific inhibitor of COMT. Experience in phase III clinical trials established that entacapone, used as an adjunct to regular or controlled-release levodopa preparations (also including a peripherally acting dopa-decarboxylase inhibitor), increased ON-time and reduced OFF-time and improved clinical condition in patients with PD experiencing wearing-off, often with a reduced daily levodopa dose. Several of these studies also identified that entacapone improved patients' quality of life and was cost-effective. Subsequently, entacapone has been amalgamated into a triple-combination preparation (Stalevo®) with levodopa and carbidopa to create a flexible and convenient drug therapy for patients with PD who have end-of-dose motor fluctuations not stabilised on levodopa/dopa-decarboxylase inhibitor treatment. This review offers a historical perspective on a successful programme of drug development by researchers who played central roles in the progress from exploratory hypothesis to registered pharmaceutical product.

Exercise-induced changes in gene expression do not mediate post exertional malaise in Gulf War illness

Background

Post-exertional malaise (PEM) is considered a characteristic feature of chronic multi-symptom illnesses (CMI) like Gulf War illness (GWI); however, its pathophysiology remains understudied. Previous investigations in other CMI populations (i.e., Myalgic Encephalomyelitis/Chronic Fatigue Syndrome) have reported associations between PEM and expression of genes coding for adrenergic, metabolic, and immune function.

Objectives

To investigate whether PEM is meditated by gene expression in Veterans with GWI.

Methods

Veterans with GWI (n = 37) and healthy control Gulf War Veterans (n = 25) provided blood samples before and after 30-min of cycling at 70% of age-predicted heart rate reserve. Relative quantification of gene expression, symptom measurements, and select cardiopulmonary parameters were compared between groups at pre-, 30 minpost-, and 24 hpost-exercise using a doubly multivariate repeated measures analysis of variance (RM-MANOVA). Mediation analyses were used to test indirect effects of changes in gene expression on symptom responses (i.e., PEM) to the standardized exercise challenge.

Results

Veterans with GWI experienced large symptom exacerbations following exercise compared to controls (Cohen's d: 1.65; p < 0.05). Expression of β -actin (ACTB), catechol-O-methyltransferase (COMT), and toll-like receptor 4 (TLR4) decreased in Veterans with GWI at 30 min (p < 0.05) and 24 h post-exercise (p < 0.05). Changes in gene expression did not mediate post-exercise symptom exacerbation in GWI (Indirect Effect Slope Coefficient: 0.06 - 0.02; 95% CI: 0.19, 0.12).

Conclusion

An acute bout of moderate intensity cycling reduced the expression of select structural, adrenergic, and immune genes in Veterans with GWI, but the pathophysiological relevance to PEM is unclear.

Neurocardiology: therapeutic implications for cardiovascular disease

The term "neurocardiology" refers to physiologic and pathophysiological interplays of the nervous and cardiovascular systems. This selective review provides an update about cardiovascular therapeutic implications of neurocardiology, with emphasis on disorders involving primary or secondary abnormalities of catecholamine systems. Concepts of scientific integrative medicine help understand these disorders. Scientific integrative medicine is not a treatment method or discipline but a way of thinking that applies systems concepts to acute and chronic disorders of regulation. Some of these concepts include stability by negative feedback regulation, multiple effectors, effector sharing, instability by positive feedback loops, allostasis, and allostatic load. Scientific integrative medicine builds on systems biology but is also distinct in several ways. A large variety of drugs and non-drug treatments are now available or under study for neurocardiologic disorders in which catecholamine systems are hyperfunctional or hypofunctional. The future of therapeutics in neurocardiology is not so much in new curative drugs as in applying scientific integrative medical ideas that take into account concurrent chronic degenerative disorders and interactions of multiple drug and non-drug treatments with each other and with those disorders.

Catecholamines 101

This review of clinical catecholamine neurochemistry is based on the Streeten Memorial Lecture at the 19th annual meeting of the American Autonomic Society and lectures at a satellite of the 6th Congress of the International Society of Autonomic Neuroscience. Here I provide historical perspective, describe sources and meanings of plasma levels of catecholamines and their metabolites, present a model of a sympathetic noradrenergic neuron that conveys how particular aspects of sympathetic nervous function affect plasma levels of catecholamines and their metabolites, and apply the model to understand plasma neurochemical patterns associated with some drugs and disease states.

Decreased catecholamine degradation associates with shock and kidney injury after cardiac surgery

Enzymatic pathways involving catechol-O-methyltransferase (COMT) catabolize circulating catecholamines. A G-to-A polymorphism in the fourth exon of the COMT gene results in a valine-to-methionine amino acid substitution at codon 158, which leads to thermolability and low ("L"), as opposed to high ("H"), enzymatic activity. We enrolled 260 patients postbypass surgery to test the hypothesis that COMT gene variants impair circulating catecholamine metabolism, predisposing to shock and acute kidney injury (AKI) after cardiac surgery. In accordance with the Hardy-Weinberg equilibrium, we identified 64 (24.6%) homozygous (LL), 123 (47.3%) heterozygous (HL), and 73 (28.1%) homozygous (HH) patients. Postoperative catecholamines were higher in homozygous LL patients compared with heterozygous HL and homozygous HH patients (P < 0.01). During their intensive care stay, LL patients had both a significantly greater frequency of vasodilatory shock (LL: 69%, HL: 57%, HH: 47%; P = 0.033) and a significantly longer median duration of shock (LL: 18.5 h, HL: 14.0 h, HH: 11.0 h; P = 0.013). LL patients also had a greater frequency of AKI (LL: 31%, HL: 19.5%, HH: 13.7%; P = 0.038) and their AKI was more severe as defined by a need for renal replacement therapy (LL: 7.8%, HL: 2.4%, HH: 0%; P = 0.026). The LL genotype associated with intensive care and hospital length of stay (P < 0.001 and P = 0.002, respectively), and we observed a trend for higher mortality. Cross-validation analysis revealed a similar graded relationship of adverse outcomes by genotype. In summary, this study identifies COMT LL homozygosity as an independent risk factor for shock, AKI, and hospital stay after cardiac surgery. (ClinicalTrials.gov number, NCT00334009).

Balancing short-term symptom control and long-term functional outcomes in patients with Parkinson's disease

Levodopa has played a central role in the treatment of Parkinson's disease for nearly 40 years and remains the single most effective symptomatic treatment for the disease. However, the response to levodopa therapy changes over time, and its long-term use is commonly associated with disabling motor complications. For this reason, the appropriate role of levodopa in the treatment of Parkinson's disease-in particular, the question of when to initiate therapy with the drug-has been a matter of controversy. Because levodopa is the most effective treatment for Parkinson's disease, the management of this disease becomes a matter of balancing short-term symptom control with long-term functional outcomes. This article provides an overview of the basis for levodopa-associated motor complications and their impact on patients' clinical function and quality of life, followed by a discussion of strategies for managing these complications to achieve optimum symptom control while minimizing the adverse effects of long-term therapy.

Catechol-o-methyltransferase and blood pressure in humans

BACKGROUND

Whether catechol-O-methyltransferase (COMT), the enzyme that metabolizes extraneuronal norepinephrine, contributes to blood pressure regulation in humans is unknown.

METHODS AND RESULTS

We studied incremental doses of the COMT inhibitor entacapone, the sympathetic stimulant yohimbine, and placebo in 7 patients with multiple system atrophy (Shy Drager syndrome). We selected these unique subjects because norepinephrine exerts an exaggerated increase in blood pressure in these patients. Autonomic regulation was characterized with intravenous phenylephrine, nitroprusside, and trimethaphan. Patients were extremely hypersensitive to phenylephrine and nitroprusside. Trimethaphan elicited a profound depressor response. Phenylephrine sensitivity increased only slightly during ganglionic blockade. Entacapone increased systolic blood pressure dose-dependently; however, the pressor response to yohimbine was approximately 3.5 times greater than the maximal response to entacapone.

CONCLUSIONS

COMT inhibition elicits a moderate, dose-dependent pressor response in the setting of severely impaired baroreflex buffering. Patients with multiple system atrophy allow for the characterization of subtle manipulations of norepinephrine turnover and blood pressure regulation in small numbers of subjects.

Clinical pharmacokinetic and pharmacodynamic properties of drugs used in the treatment of Parkinson's disease

Current research in Parkinson's disease (PD) focuses on symptomatic therapy and neuroprotective interventions. Drugs that have been used for symptomatic therapy are levodopa, usually combined with a peripheral decarboxylase inhibitor, synthetic dopamine receptor agonists, centrally-acting antimuscarinic drugs, amantadine, monoamine oxidase-B (MAO-B) inhibitors and catechol-O-methyltransferase (COMT) inhibitors. Drugs for which there is at least some evidence for neuroprotective effect are certain dopamine agonists, amantadine and MAO-B inhibitors (selegiline). Levodopa remains the most effective drug for the treatment of PD. Several factors contribute to the complex clinical pharmacokinetics of levodopa: erratic absorption, short half-life, peripheral O-methylation and facilitated transport across the blood-brain barrier. In patients with response fluctuations to levodopa, the concentration-effect curve becomes steeper and shifts to the right compared with patients with stable response. Pharmacokinetic-pharmacodynamic modelling can affect decisions regarding therapeutic strategies. The dopamine agonists include ergot derivatives (bromocriptine, pergolide, lisuride and cabergoline), non-ergoline derivatives (pramipexole, ropinirole and piribedil) and apomorphine. Most dopamine agonists have their specific pharmacological profile. They are used in monotherapy and as an adjunct to levodopa in early and advanced PD. Few pharmacokinetic and pharmacodynamic data are available regarding centrally acting antimuscarinic drugs. They are characterised by rapid absorption after oral intake, large volume of distribution and low clearance relative to hepatic blood flow, with extensive metabolism. The mechanism of action of amantadine remains elusive. It is well absorbed and widely distributed. Since elimination is primarily by renal clearance, accumulation of the drug can occur in patients with renal dysfunction and dosage reduction must be envisaged. The COMT inhibitors entacapone and tolcapone dose-dependently inhibit the formation of the major metabolite of levodopa, 3-O-methyldopa, and improve the bioavailability and reduce the clearance of levodopa without significantly affecting its absorption. They are useful adjuncts to levodopa in patients with end-of-dose fluctuations. The MAO-B inhibitor selegiline may have a dual effect: reducing the catabolism of dopamine and limiting the formation of neurotoxic free radicals. The pharmacokinetics of selegiline are highly variable; it has low bioavailability and large volume of distribution. The oral clearance is many-fold higher than the hepatic blood flow and the drug is extensively metabolised into several metabolites, some of them being active. Despite the introduction of several new drugs to the antiparkinsonian armamentarium, no single best treatment exists for an individual patient with PD. Particularly in the advanced stage of the disease, treatment should be individually tailored.

The effect of COMT inhibition with entacapone on cardiorespiratory responses to exercise in patients with Parkinson's disease

The inhibition of catechol-O-methyltransferase (COMT) may impair catecholamine clearance resulting in unwanted cardiac and hemodynamic events. We therefore studied the effects of entacapone, an inhibitor of peripheral COMT, on cardiorespiratory and plasma noradrenaline (NA) responses to exercise and on respiratory muscle strength in l-dopa treated patients with Parkinson's disease (PD). A randomized, double-blind, cross-over study with two 1week treatment periods was performed in 15 PD patients. The test battery included analysis of hemodynamics, gas exchange parameters and plasma NA during a maximal exercise test, assessment of maximal static airway pressures and pre- and post-exercise motor scores of the Unified Parkinson's Disease Rating Scale (UPDRS). The first test was done after withholding l-dopa overnight ('run-in' test, off-phase). The second and third tests were done in on-phase after 1week treatment with either entacapone 200mg or placebo given with each dose of l-dopa. No differences in maximal work load, plasma NA, or in cardiorespiratory responses to either maximal or work rate standardized submaximal exercise were observed between entacapone and placebo, except for O(2) pulse, which was slightly lower (p < 0.05) after entacapone at submaximal exercise level. Maximal airway pressures were similar between the study treatments and run-in. Exercise had no effect on motor UPDRS after either study treatment or during the run-in test. No serious adverse events were observed. The results of this study suggest that entacapone does not change the work capacity, work efficiency or respiratory muscle strength in l-dopa treated PD patients with mild to moderate disease severity, and that its use with l-dopa seems to be safe in conditions of maximal physical effort. However, data from the long-term use of COMT inhibitors are needed to confirm these findings.

Entacapone: a catechol-O-methyltransferase inhibitor for the adjunctive treatment of Parkinson's disease

BACKGROUND

When levodopa therapy is used in Parkinson's disease, degradation of the drug in the peripheral nervous system is associated with dyskinesias and motor fluctuations. Much of this degradation is produced by catechol-O-methyltransferase (COMT), an enzyme involved in the metabolism of catecholamines and catechol compounds. Inhibition of COMT activity prolongs the action of levodopa and reduces fluctuations in response. Entacapone is a selective inhibitor of COMT whose activity is primarily in the peripheral nervous system, with little effect in the central nervous system.

OBJECTIVE

This paper reviews the pharmacologic properties and clinical usefulness of entacapone in the treatment of Parkinson's disease.

METHODS

Recent studies, abstracts, and reviews published in the English-language literature were identified through searches of MEDLINE (1966-September 2000), International Pharmaceutical Abstracts (1970-September 2000), and PharmaProjects (September 2000 version), and from the Web sites of Parkinson's disease conferences held from 1996 to September 2000. Relevant human studies provided further information on the pharmacologic properties and clinical usefulness of entacapone.

RESULTS

Entacapone is rapidly absorbed, with a time to maximum concentration of approximately 1 hour. Its plasma elimination half-life is 0.4 to 0.7 hour in the beta phase and 2.4 hours in the gamma phase, and it has 35% absolute bioavailability after oral administration, secondary to first-pass clearance. Entacapone is 98% protein bound; thus, it is not distributed widely in tissues and is almost completely metabolized before excretion (0.1%-0.2% of dose unchanged in urine). The drug inhibits erythrocyte-soluble COMT activity in a dose-dependent fashion (48% after a 400-mg dose, 82% after an 800-mg dose). The inhibitory effect is reversible, with recovery of soluble COMT activity within 4 to 8 hours. In levodopa-treated patients with Parkinson's disease who experience motor fluctuations, clinical trials have demonstrated entacapone's effectiveness in increasing "on" time (the period during which medications relieve the symptoms of Parkinson's disease) by up to 1.2 hours, decreasing "off" time (the period during which symptoms increase) by 0.9 to 1.3 hours, and producing overall total improvement in scores on the Unified Parkinson's Disease Rating Scale. The recommended dosage of entacapone is 200 mg administered orally with each dose of levodopa/carbidopa, up to 8 doses per day. The drug is generally well tolerated, with most adverse effects attributed to levodopa-related dopaminergic effects, including dyskinesias and nausea.

CONCLUSIONS

In clinical trials, adjuvant treatment with entacapone appeared to be an effective and well-tolerated therapeutic strategy in patients with Parkinson's disease who experience fluctuations in the response to levodopa therapy. The increased elimination half-life of levodopa with concomitant entacapone results in greater and more sustained plasma levodopa levels, with more constant dopaminergic stimulation in the brain and greater amelioration of parkinsonian symptoms.

The effect of catechol-O-methyltransferase inhibition with entacapone on cardiovascular autonomic responses in L-Dopa-treated patients with Parkinson's disease

We have compared the effects of entacapone, a peripherally acting catechol-O-methyltransferase (COMT) inhibitor, and placebo on cardiovascular autonomic responses in L-Dopa/dopa decarboxylase inhibitor-treated patients with Parkinson's disease (PD). In a double-blind, randomized, crossover study with two consecutive 1-week treatment periods, a battery of cardiovascular reflex tests (orthostatic, Valsalva, deep breathing, and isometric hand grip tests) was performed in a group of 15 patients with idiopathic PD. The first set of tests was performed after withholding L-Dopa overnight (control, "off" stage). The second and third sets of tests were performed in "on" stage after 1-week treatment with either entacapone 200 mg or placebo administered with each dose of L-Dopa/dopa decarboxylase (DDC) inhibitor. Valsalva, deep breathing, and orthostatic tests demonstrated no statistically significant differences in the ratio of the longest and shortest electrocardiographic R-to-R wave (R-R) intervals between entacapone and placebo or between study treatments and control. Blood pressure responses to both orthostatic challenge and prolonged isometric work (hand grip test) were similar between treatments. Systolic orthostatic hypotension was observed in only one patient during the control test, but it occurred more frequently after L-Dopa/DDC inhibitor, regardless of concomitant administration of either entacapone (n = 3) or placebo (n = 4). Peripheral COMT inhibition with entacapone does not significantly alter cardiovascular autonomic responses in L-Dopa-treated patients with PD.

Twelve-month safety of entacapone in patients with Parkinson's disease

The safety of entacapone combined with levodopa and a dopadecarboxylase (DDC) inhibitor was tested in a 12-month double-blind study of 326 patients with idiopathic Parkinson's disease (PD). The study population represented 'typical' PD outpatients, including patients with varying disease severity and with various concomitant medications. Two-thirds of the patients were randomized to receive 200 mg of entacapone with each of 2--10 daily levodopa doses, and one-third to receive placebo. All entacapone patients were included in the safety evaluation of adverse events (AEs), vital signs, ECG, and laboratory parameters. Entacapone was well tolerated with a discontinuation rate due to AEs of 14% compared with 11% with placebo (NS). As expected, due to dopaminergic enhancement, dyskinesia was more frequent as an AE with entacapone than with placebo. Dryness of mouth, urine discoloration and diarrhoea were more frequent non-dopaminergic AEs with entacapone than with placebo. Entacapone had no adverse effects on hepatic enzyme activity, ECG or haemodynamic parameters, and there was no evidence of any toxicity. As an indication of levodopa enhancement with entacapone, patients taking 5--10 doses of levodopa, most likely representing predominantly fluctuating patients, showed a significant decrease in their mean daily levodopa dose of 94 mg in the entacapone group compared with a decrease of 39 mg in the placebo group (P < 0.01). The interval between the first two morning doses of levodopa increased by 17% with entacapone, whereas with placebo no extension was observed (P < 0.05). Despite levodopa dose reduction, efficacy of entacapone was maintained. As further evidence of efficacy, Parkinsonian symptoms markedly worsened in all patients after withdrawal of entacapone. We conclude that entacapone is safe in optimizing levodopa in long-term treatment of idiopathic Parkinson's disease. Monitoring of liver or other safety parameters during entacapone treatment is not required.

Entacapone and selegiline with L-dopa in patients with Parkinson's disease: an interaction study

Both the catechol-O-methyltransferase (COMT) inhibitor entacapone and the monoamine oxidase B (MAO-B) inhibitor selegiline are L-dopa extenders. Both are used, often simultaneously, as adjuncts to L-dopa/dopa decarboxylase (DDC) inhibitor treatment of Parkinson's disease (PD). Their possible interactions have not been previously studied in a double-blind manner.We studied clinical response, tolerability, haemodynamics and cardiac rhythm in 16 PD patients with end-of-dose-type motor fluctuations. The patients' individual L-dopa/DDC inhibitor treatment was stabilized before the experimental treatments. This was followed by three consecutive, randomized, double-blind 2-week treatment periods with entacapone (200mg with each L-dopa dose), selegiline (10mg o.d.) or both entacapone and selegiline with the L-dopa/DDC inhibitor medication. Clinical efficacy (L-dopa test with repeated motor and dyskinesia scoring) and safety (orthostatic test, 24-h ambulatory ECG, haematological and clinical chemistry variables and adverse events) evaluations were performed before each treatment (control) and at the end of each treatment period.All three treatments, entacapone, selegiline, and entacapone+selegiline as adjunct to L-dopa/DDC inhibitor improved (p<0.05) clinical disability compared to L-dopa only but they did not differ significantly from each other. Dyskinesias increased with all the treatments, statistically significantly (p<0.01) with entacapone+selegiline. No significant differences in haemodynamics were observed between control and any of the experimental treatments, or between the experimental treatments in the orthostatic test. One patient already had symptomatic orthostatism before experimental treatments (control). In two other patients orthostatism emerged after the introduction of selegiline, and in one after every experimental treatment. Twenty-four-hour ECG did not show any differences in supraventricular or ventricular extrasystoles or heart rate between treatments. No statistically significant differences were observed in adverse events or in haematology and clinical chemistry variables. One patient treated with entacapone+selegiline discontinued the study due to dizziness and insomnia. Our results suggest that co-administration of entacapone with L-dopa/DDC inhibitor, with or without selegiline, improves clinical disability, is safe, but may also enhance dopamine-related adverse events to some extent in PD patients with end-of-dose type motor fluctuations.

Heart rate variability in Parkinson's disease patients treated with tolcapone

Following the introduction of tolcapone, a potent, reversible Catechol-O-methyltransferase (COMT) inhibitor, it has been possible to optimise the management of Parkinson's disease (PD) patients in chronic Levodopa (L-dopa) therapy. The interaction between tolcapone and the endogenous metabolism of catecholamines points to a possible influence on autonomic cardiovascular function.Cardiovascular reflexes have been analysed in a group of seven PD patients (four males, three females; mean age 69.7years, mean disease duration 14.1years) by means of the heart rate variability (HRV) method using a continuous 24-h ECG (ECGD), before and after six months of treatment with tolcapone (in addition to L-dopa).We have observed no statistically significant differences in HRV parameters, nor any changes in the incidence of hyperkinetic and hypokinetic arrhythmias, which suggest that autonomic cardiovascular function in PD patients is not influenced by six months of treatment with tolcapone.

Clinical pharmacology, therapeutic use and potential of COMT inhibitors in Parkinson's disease

When peripheral decarboxylation is blocked by carbidopa or benserazide, the main metabolic pathway of levodopa is O-methylation by catechol-O-methyltransferase (COMT). Entacapone and tolcapone are new potent, selective and reversible nitrocatechol-type COMT inhibitors. Animal studies have demonstrated that entacapone mainly has a peripheral effect whereas tolcapone also inhibits O-methylation in the brain. In human volunteers, both entacapone and tolcapone dose-dependently inhibit the COMT activity in erythrocytes, improve the bioavailability and decrease the elimination of levodopa, and inhibit the formation of 3-O-methyldopa (3-OMD). Entacapone is administered with every scheduled dose of levodopa whereas tolcapone is administered 3 times daily. The different administration regimens for these agents are based on their different pharmacokinetic and pharmacodynamic profiles. Both entacapone and tolcapone enhance and extend the therapeutic effect of levodopa in patients with advanced and fluctuating Parkinson's disease. They prolong the duration of levodopa effect. Clinical studies show that they increase the daily ON time by an average 1 to 3 hours, improve the activities of daily living and allow daily levodopa dosage to be decreased. Correspondingly, they significantly reduce the daily OFF time. No comparative studies between entacapone and tolcapone have been performed. Tolcapone also appears to have a beneficial effect in patients with nonfluctuating Parkinson's disease. The main adverse effects of the COMT inhibitors are related to their dopaminergic and gastrointestinal effects. Enhancement of dopaminergic activity may cause an initial worsening of levodopa-induced adverse effects, such as dyskinesia, nausea, vomiting, orthostatic hypotension, sleep disorders and hallucinations. Levodopa dose adjustment is recommended to avoid these events. Tolcapone is associated with diarrhoea in about 16 to 18% of patients and entacapone in less than 10% of patients. Diarrhoea has led to discontinuation in 5 to 6% of patients treated with tolcapone and in 2.5% of those treated with entacapone. Urine discoloration to dark yellow or orange is related to the colour of COMT inhibitors and their metabolites. Elevated liver transaminase levels are reported in 1 to 3% of patients treated with tolcapone but very rarely, if at all, in patients treated with entacapone. The descriptions of acute, fatal fulminant hepatitis and potentially fatal neurological reactions, such as neuroleptic malignant syndrome and rhabdomyolysis, in association with tolcapone led to the suspension of its marketing authorisation in the European Community and Canada. In many other countries, the use of tolcapone is restricted to patients who are not responding satisfactorily to other therapies. Regular monitoring of liver enzymes is required if tolcapone is used. No such adverse reactions have so far been described for entacapone and no laboratory monitoring has been proposed. COMT inhibitors added to levodopa therapy are beneficial, particularly in patients with fluctuating disease. They may be combined with other antiparkinsonian drugs, such as dopamine agonists, selegiline and anticholinergics without adverse interactions. They provide a new treatment possibility in patients with Parkinson's disease who have problems with their present levodopa therapy.

Entacapone. A review of its use in Parkinson's disease

Entacapone is a potent and specific peripheral catechol-O-methyltransferase (COMT) inhibitor. It has been shown to improve the clinical benefits of levodopa plus an aromatic L-amino acid decarboxylase inhibitor (AADC) when given to patients with Parkinson's disease and end-of-dose deterioration in the response to levodopa (the 'wearing off' phenomenon). The efficacy of entacapone is currently being assessed in patients with stable Parkinson's disease. In 2 well conducted trials of 6 months' duration and smaller short term studies, treatment with entacapone (200 mg with each dose of levodopa/AADC inhibitor) was associated with significant increases in daily 'on' time and decreases in 'off' time. Changes in Unified Parkinson's Disease Rating Scale (UPDRS) scores concurred with changes in 'on' and 'off' times: entacapone improved total, activities of daily living and motor function scores, but it had no effect on mentation scores. Entacapone also provided benefits when given with controlled release levodopa/ AADC inhibitor or with standard levodopa/AADC inhibitor and selegiline in small trials. Dopaminergic events, including dyskinesia and nausea, are among the most common events with entacapone, and are related to the drug's ability to potentiate the effects of levodopa. Diarrhoea, abdominal pain, constipation and urine discolouration are the most common nondopaminergic events, although the latter event is the only one to occur consistently more frequently with entacapone than with placebo. However, adverse events of any type infrequently led to study discontinuation.

CONCLUSIONS

The efficacy and tolerability of entacapone administered with levodopa/AADC inhibitor have not yet been compared with those of other strategies for the treatment of Parkinson's disease. However, once the decision to initiate levodopa therapy has been made, studies generally support the use of entacapone as an adjunct to levodopa in patients with Parkinson's disease and the 'wearing off' phenomenon.

New, selective catechol-O-methyltransferase inhibitors as therapeutic agents in Parkinson's disease

Levodopa remains the most effective drug for Parkinson's disease (PD). However, its benefits are limited owing to extensive metabolism by catechol-O-methyltransferase (COMT), especially if levodopa is used in combination with peripheral dopa-decarboxylase inhibitors. A new generation of potent, orally active, selective, and reversible COMT inhibitors has become available recently. Among these, tolcapone and entacapone have been best characterised. Preclinical and clinical studies have shown that COMT inhibitors markedly enhance levodopa availability and prolong its plasma half-life. In recent large clinical trials they proved to be able to ameliorate motor fluctuations, reduce disability, and decrease levodopa requirements in PD patients. The tolerability profiles of entacapone and tolcapone are good. COMT inhibition promises to become an important means of extending the benefits of levodopa therapy in PD.

Simultaneous MAO-B and COMT inhibition in L-Dopa-treated patients with Parkinson's disease

The effect of selegiline (L-deprenyl) on plasma catecholamines, clinical response, and drug tolerability was studied in 13 patients with Parkinson's disease (PD) treated with L-Dopa/benserazide and entacapone, a peripheral catechol-O-methyltransferase (COMT) inhibitor, in a placebo-controlled double-blind study. An L-Dopa test was performed on 3 study days. The first study day was with L-Dopa/benserazide only (control), the second after 14 days of treatment with 200 mg entacapone taken concomitantly with L-Dopa/benserazide in combination with either selegiline (10 mg daily) or placebo. After a 2-week washout period, selegiline and placebo treatments were switched, and the third study day was after 14 days of treatment. During the study days, clinical response was evaluated at 30-min intervals for 6 h, by using the motor score of the Unified Parkinson's Disease Rating Scale (UPDRS). In addition, repeated blood pressure measurements were made, and plasma samples were taken for analysis of L-Dopa, 3-O-methyldopa (3-OMD), dihydroxyphenyl acetic acid (DOPAC), homovanillic acid (HVA), dopamine, noradrenaline, and 3-methoxy-4-hydroxyphenylethylene glycol (MHPG). Monoamine oxidase B (MAO-B) and COMT enzyme activities were measured from platelets and erythrocytes, respectively. Entacapone improved the clinical response to L-Dopa during both selegiline and placebo (p < 0.001) treatments. The improvement was more marked during combined selegiline and entacapone treatment than with entacapone alone (p < 0.01). Entacapone significantly increased plasma L-Dopa and DOPAC levels and decreased plasma 3-OMD and MHPG levels both with selegiline and placebo. Selegiline partially inhibited the entacapone-induced increase of plasma DOPAC. Plasma dopamine and noradrenaline levels did not change. Entacapone decreased erythrocyte COMT activity by > 35% (p < 0.001), and platelet MAO-B activity was almost completely inhibited by selegiline (p < 0.001). One patient withdrew because of diarrhea, dizziness, and loss of sleep when receiving selegiline treatment. Otherwise no differences in adverse events, mean daily blood pressures, or other safety parameters were observed between selegiline and placebo treatments. Our results suggest that entacapone can be safely administered together with L-Dopa and selegiline in patients with PD, although further studies with larger number of patients and longer treatment periods are necessary to confirm this finding.

COMT inhibition by entacapone does not affect growth hormone or prolactin secretion in healthy volunteers

We studied the effects of entacapone, a novel inhibitor of the enzyme catechol-O-methyltransferase (COMT), on spontaneous and levodopa (LD) modulated secretion of growth hormone (GH) and prolactin (PRL) in 12 healthy male volunteers. The study had a double-blind, cross-over design with two experimental settings. In the first setting the subjects received a single oral dose of 400 mg of entacapone or matching placebo in a randomized order. In the second setting, a single oral dose of 300 mg of LD and 75 mg of carbidopa was administered concomitantly with either 400 mg of entacapone or matching placebo in a randomized order. Entacapone had no effect on resting levels of GH, but PRL concentrations in plasma were slightly lower after entacapone than after placebo. As expected, LD/carbidopa increased the concentration of GH and decreased that of PRL. The effects of LD were not influenced by concomitant administration of entacapone. Compared with the administration of LD/carbidopa together with placebo, concomitant administration of entacapone increased the AUC of LD by 29% and reduced the AUC of 3-O-methyldopa (a metabolite of LD produced by COMT) by 69%. Entacapone appears not to enhance the effects of LD on hypothalamic-pituitary function, although the LD dose used may have been bigger than optimal for detection of a small modulatory influence.

Simultaneous inhibition of catechol-O-methyltransferase and monoamine oxidase A: effects on hemodynamics and catecholamine metabolism in healthy volunteers

OBJECTIVE

To evaluate the effects of simultaneous pharmacologic inhibition of catechol-O-methyltransferase (COMT) and monoamine oxidase type A (MAO-A) on hemodynamics and catecholamine metabolism in healthy volunteers at rest and during exercise.

BACKGROUND

Entacapone, a COMT inhibitor, is studied as an adjunct to levodopa treatment in patients with Parkinson's disease. Moclobemide, an MAO-A inhibitor, is already in clinical use as an antidepressant. It is likely that entacapone and moclobemide will be used concomitantly in the future in patients who have both Parkinson's disease and depression. It was therefore considered to be important to investigate the tolerability of combined COMT and MAO-A inhibition with entacapone and moclobemide.

DESIGN AND METHODS

This was a randomized, single-dose, double-blind crossover study of 12 healthy male volunteers. The treatments were either placebo, 200 mg entacapone, 150 mg moclobemide, or the combination of entacapone and moclobemide in single doses. Heart rate, blood pressure, impedance cardiography, and plasma concentrations of catecholamines and their metabolites were measured both at rest and during submaximal standardized bicycle exercise.

RESULTS

Entacapone and moclobemide (either alone or in combination) did not change heart rate, blood pressure, or any hemodynamic parameter at rest or during exercise compared with placebo. Neither were the concentrations of norepinephrine and epinephrine in plasma influenced. Both drugs had the expected effects on catecholamine metabolite concentrations in plasma. The decrease in the concentration of 3-methoxy-4-hydroxyphenylglycol (MHPG) induced by moclobemide was not potentiated by entacapone.

CONCLUSION

The combined use of therapeutic single doses of entacapone and moclobemide in healthy volunteers did not affect the hemodynamics or concentrations of unconjugated norepinephrine and epinephrine in plasma. Other mechanisms are capable of regulating the concentrations of norepinephrine and epinephrine in circulating blood (and apparently also at receptors in the heart and vascular tissue) when both COMT and MAO-A activity are inhibited to a significant extent. This was also the case during marked sympathetic stimulation. The changes in the catecholamine metabolite concentrations provide evidence of effective COMT and MAO inhibition. Concentrations of MHPG in plasma are determined mainly by MAO-A activity because COMT inhibition did not have an additional effect on the moclobemide-induced decrease in plasma MHPG.

Monoamine oxidase A and B activities in heavy smokers

There is a strong association between depression and smoking. Because monoamine oxidase (MAO) inhibition leads to antidepressant effect and in vitro studies have shown that cigarette smoke inhibits MAO activity, it is conceivable that smoking may have an antidepressant effect, if smokers have reduced MAO activity. Therefore, we assessed platelet MAO-B activity and plasma concentration of catecholamine metabolites reflecting MAO-A activity in heavy dependent smokers and nonsmokers matched for sociodemographic characteristics. Platelet MAO-B activity, plasma 3,4-dihydroxyphenylglycol, plasma 3,4-dihydroxyphenylacetic acid, and plasma 3,4-dihydroxyphenylalanine concentrations were significantly lower in smokers than in nonsmokers, whereas plasma norepinephrine did not differ. Significantly more smokers reported previous history of depression, manic episode, panic attack, agoraphobia, and simple phobia. Smokers had higher scores (p < 0.001) on the Montgomery-Asberg Depression Rating Scale (MADRS) and the Hamilton Anxiety Scales. It is concluded that the activities of both forms of the MAO are reduced in heavy dependent smokers.

The effect of entacapone on the disposition and hemodynamic effects of intravenous isoproterenol and epinephrine

BACKGROUND

Entacapone is a potent, selective catechol-O-methyltransferase (COMT) inhibitor. Entacapone could potentiate the hemodynamic effects of exogenously administered catecholamines, which are substrates of the COMT enzyme.

DESIGN AND METHODS

Originally, the study was to follow a placebo-controlled, randomized crossover design. Because of two cases of ventricular arrhythmia, a decision was made to terminate the study before its completion. Six subjects went through the isoproterenol and epinephrine infusions while taking placebo and five other subjects while taking entacapone. The actual design was thus one with two parallel groups with random allocation and double-blind drug administration. The subjects were given either a single dose of 400 mg entacapone or placebo 30 minutes before the start of isoproterenol or epinephrine infusions. Four dosages of epinephrine (1.5, 3, 6, or 12 micrograms/min) and isoproterenol (0.5, 1, 1.5, or 2 micrograms/min) were infused (5 minutes for each level). Heart rate and blood pressure were measured and ECG was monitored. The concentrations of isoproterenol and epinephrine in plasma were determined by HPLC.

RESULTS

The maximal increase in heart rate during isoproterenol infusion after entacapone administration (40 +/- 11 beats/min, mean +/- SD) was statistically greater (p = 0.0496) than after placebo administration (27 +/- 7 beats/min). The increase in heart rate during epinephrine infusion was 25 +/- 13 beats/min after entacapone administration and 14 +/- 9 beats/min after placebo administration (p = 0.127). There were no statistically significant differences between entacapone and placebo in blood pressure or in plasma concentrations of isoproterenol and epinephrine.

CONCLUSION

We conclude that entacapone may potentiate the chronotropic and arrhythmogenic effects of exogenously administered isoproterenol and epinephrine.

General properties and clinical possibilities of new selective inhibitors of catechol O-methyltransferase

1. The structure of catechol O-methyltransferase (COMT) has been recently characterized and a series of new and selective COMT inhibitors developed. 2. Entacapone, nitecapone and tolcapone are nitrocatechol-type potent COMT inhibitors in vitro (Ki in nanomolar range). They are also very selective for COMT and active in vivo even after oral administration. CGP 28014 is a pyridine derivative that is active only in vivo. 3. In animal studies, these compounds inhibit effectively the O-methylation of L-dopa, thus improving its bioavailability and brain penetration and potentiating its behavioural effects. 4. Entacapone and nitecapone have mainly a peripheral effect whereas tolcapone and CGP 28014 also inhibit O-methylation in the brain. 5. In man, entacapone, nitecapone and tolcapone all inhibit dose dependently the COMT activity in erythrocytes. These COMT inhibitors also decrease the amount of COMT dependent metabolites of adrenaline and noradrenaline in plasma. 6. In human volunteers, entacapone, tolcapone and CGP 28014 improve the bioavailability of L-dopa and inhibit the formation of 3-O-methyldopa. 7. In the first clinical studies in patients with Parkinson's disease, both entacapone and tolcapone potentiate and prolong the therapeutic effect of L-dopa.