Effect of entacapone, a COMT inhibitor, on the pharmacokinetics and metabolism of levodopa after administration of controlled-release levodopa-carbidopa in volunteers

Comparison of the pharmacokinetics of an oral extended-release capsule formulation of carbidopa-levodopa (IPX066) with immediate-release carbidopa-levodopa (Sinemet(®)), sustained-release carbidopa-levodopa (Sinemet(®) CR), and carbidopa-levodopa-entacapone (Stalevo(®))

IPX066 (extended‐release carbidopa‐levodopa [ER CD‐LD]) is an oral extended‐release capsule formulation of carbidopa and levodopa. The single‐dose pharmacokinetics of ER CD‐LD (as 2 capsules; total dose, 97.5 mg‐390 mg CD‐LD) versus immediate‐release (IR) CD‐LD (25 mg‐100 mg), sustained‐release (CR) CD‐LD (25 mg‐100 mg), and CD‐LD‐entacapone (25 mg‐100 mg‐200 mg) was evaluated in healthy subjects. Following IR dosing, LD reached peak concentrations (C_max) at 1 hour; LD concentrations then decreased rapidly and were less than 10% of peak by 5 hours. With CR CD‐LD and CD‐LD‐entacapone, LD C_max occurred at 1.5 hours, and concentrations were less than 10% of peak by 6.3 and 7.5 hours, respectively. The initial increase in LD concentration was similar between ER CD‐LD and IR CD‐LD and faster than for CR CD‐LD and CD‐LD‐entacapone. LD concentrations from ER CD­­‐LD were sustained for approximately 5 hours and did not decrease to 10% of peak until 10.1 hours. Dose‐normalized LD C_max values for ER CD‐LD were significantly lower (P< .05) than for the other CD‐LD products. Bioavailability of LD from ER CD‐LD was 83.5%, 78.3%, and 58.8% relative to IR CD‐LD, CR CD‐LD, and CD‐LD‐entacapone, respectively.

Treatment of advanced Parkinson’s disease

Parkinson's disease is a progressive neurological disorder characterized by tremor, bradykinesia, rigidity, gait and postural instability and a variety of nonmotor symptoms. While these and other motor signs typically improve with levodopa, the so-called axial signs, such as dysarthria, dysphagia, postural instability and freezing, and most nonmotor signs, such as depression, cognitive decline and dysautonomia, usually do not respond satisfactorily to levodopa. Furthermore, the use of levodopa may be limited by the development of motor fluctuations, dyskinesias and other adverse effects. This manuscript reviews the medical management of advanced Parkinson's disease, focusing on the treatment of motor fluctuations and dyskinesias and of nonmotor and nonlevodopa responsive symptoms.

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.

The effect of peripheral enzyme inhibitors on levodopa concentrations in blood and CSF

Levodopa combined with a dopa-decarboxylase inhibitor, such as carbidopa, shifts the metabolism to the COMT pathway. Adding the peripheral acting COMT inhibitor entacapone provides improvement for patients with PD suffering from motor fluctuations. We studied the effects of the enzyme inhibitors entacapone and carbidopa on the levodopa concentrations in CSF and in blood. Five PD patients with wearing-off underwent lumbar drainage and intravenous microdialysis. Samples were taken 12 h daily for 3 days. Day 1; intravenous levodopa was given, day 2; additional oral entacapone 200 mg tid, day 3; additional oral entacapone 200 mg tid and carbidopa 25 mg bid. Levodopa in CSF and in dialysates was analysed. The AUC for levodopa increased both in blood and CSF when additional entacapone was given alone and in combination with carbidopa. The C(max) of levodopa in both CSF and blood increased significantly. Additional entacapone to levodopa therapy gives an increase of C(max) in CSF and in blood. The increase is more evident when entacapone is combined with carbidopa.

Problems with the present inhibitors and a relevance of new and improved COMT inhibitors in Parkinson's disease

Entacapone and tolcapone are reversible COMT inhibitors which have been approved for clinical use in patients with Parkinson disease (PD). Nebicapone is a third COMT inhibitor which has been studied in humans. COMT inhibitors are used in combination with levodopa and a dopa decarboxylase (DDC) inhibitor. Each of them has problems either in pharmacokinetics, pharmacodynamics, clinical efficacy, or in safety. All three inhibitors have short elimination half-lives, about 2-3h. Tolcapone is longer acting and more potent COMT inhibitor than entacapone; nebicapone lies in between. However, none of the present inhibitors cause a complete peripheral COMT inhibition. Tolcapone and nebicapone have increased more levodopa AUC than entacapone which is reflected also in their clinical efficacy. The most common adverse event with COMT inhibitors is dyskinesia which is usually managed by decreasing levodopa dose. The greatest problem with tolcapone and probably also with nebicapone is their liver toxicity which is not seen with entacapone. Tolcapone causes severe diarrhea more often than entacapone. Though the present COMT inhibitors have improved significantly the treatment of advanced PD patients, they still have several problems and weaknesses leaving room for developing better COMT inhibitors.

Pharmacokinetic-pharmacodynamic interaction between nebicapone, a novel catechol-o-methyltransferase inhibitor, and controlled-release levodopa/carbidopa 200 mg/50 mg : randomized, double-blind, placebo-controlled, crossover study in healthy subjects

BACKGROUND AND OBJECTIVES

Levodopa is the most effective symptomatic treatment for Parkinson's disease (PD), but its use is often associated with development of motor complications. These adverse responses to fluctuations in dopaminergic stimulation can be reduced by concomitant administration of a catechol-O-methyltransferase (COMT) inhibitor. Nebicapone is a new COMT inhibitor currently being developed for use as an adjunct to levodopa/dopa decarboxylase inhibitor in the treatment of PD. This article aimed to investigate the effect of single oral doses (50 mg, 100 mg and 200 mg) of nebicapone on levodopa pharmacokinetics and erythrocyte-soluble COMT (S-COMT) activity when coadministered with a single dose of controlled-release (CR) levodopa/carbidopa 200 mg/50 mg (Sinemet((R)) CR 200/50) in healthy subjects (n = 16).

METHODS

This was a randomized, double-blind, placebo-controlled, four-way crossover study in healthy subjects, with at least 5 days of washout between treatment periods.

RESULTS

There was a dose-dependent and significant increase in levodopa extent of exposure (area under the plasma concentration-time curve from time zero to infinity [AUC(infinity)]) without a significant change in peak exposure (maximum plasma concentration; [C(max)]). Using placebo as a reference, levodopa geometric mean ratios (GMRs) and 90% CIs following nebicapone 50 mg, 100 mg and 200 mg were, respectively, 1.13 (0.98, 1.30), 1.04 (0.90, 1.19) and 1.10 (0.96, 1.27) for C(max) and 1.26 (1.16, 1.34), 1.37 (1.27, 1.75) and 1.47 (1.42, 1.65) for AUC(infinity). For 3-O-methyldopa (3-OMD), the GMRs and 90% CIs were, respectively, 0.61 (0.55, 0.67), 0.45 (0.41, 0.50) and 0.33 (0.30, 0.36) for C(max) and 0.69 (0.61, 0.78), 0.53 (0.41, 0.61) and 0.41 (0.37, 0.47) for AUC(infinity). Nebicapone dose dependently and significantly decreased COMT activity. Maximum COMT inhibition occurred at 1.5-2.4 hours post-dose and ranged from 56% to 73% with nebicapone 50 mg and 200 mg, respectively. There was a good correlation between plasma concentrations of nebicapone and inhibition of S-COMT activity. Treatments were well tolerated.

CONCLUSION

Following concomitant administration with levodopa/carbidopa CR 200 mg/50 mg, single doses of nebicapone 50 mg, 100 mg and 200 mg significantly and dose-dependently inhibited S-COMT activity, increased systemic exposure to levodopa, and reduced 3-OMD formation.

Impact of interacting functional variants in COMT on regional gray matter volume in human brain

BACKGROUND

Functional variants in the catechol-O-methyltransferase (COMT) gene have been shown to impact cognitive function, cortical physiology and risk for schizophrenia. A recent study showed that previously reported effects of the functional val158met SNP (rs4680) on brain function are modified by other functional SNPs and haplotypes in the gene, though it was unknown if these effects are also seen in brain structure.

METHODS

We used voxel-based morphometry to investigate the impact of multiple functional variants in COMT on gray matter volume in a large group of 151 healthy volunteers from the CBDB/NIMH Genetic Study of Schizophrenia.

RESULTS

We found that the previously described rs4680 val risk variant affects hippocampal and dorsolateral prefrontal (DLPFC) gray matter volume. In addition, we found that this SNP interacts with a variant in the P2 promoter region (rs2097603) in predicting changes in hippocampal gray matter volume consistent with a nonlinear effect of extracellular dopamine.

CONCLUSIONS

We report evidence that interacting functional variants in COMT affect gray matter regional volume in hippocampus and DLPFC, providing further in vivo validation of the biological impact of complex genetic variation in COMT on neural systems relevant for the pathophysiology of schizophrenia and extending observations of nonlinear dependence of prefrontal neurons on extracellular dopamine to the domain of human brain structure.

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.

Entacapone increases levodopa exposure and reduces plasma levodopa variability when used with Sinemet CR

Entacapone is a catechol-O-methyltransferase (COMT) inhibitor used as an adjunct to levodopa/dopa decarboxylase inhibitors in the treatment of Parkinson's disease. Entacapone increases the bioavailability and reduces the daily variation of plasma levodopa when administered with standard levodopa preparations. These parameters were studied when entacapone was administered with a controlled-release levodopa preparation after repeated administrations throughout the day in 16 healthy male volunteers. On 2 test days, 200 mg entacapone or placebo was administered 4 times during the day at 4-hour intervals concomitantly with a single dose of controlled-release levodopa/carbidopa 100 mg/25 mg (Sinemet CR). Plasma levodopa, 3-O-methyldopa (3-OMD), and carbidopa concentrations were measured before intake of the medication and then every 30 minutes for 16 hours (until midnight), and less frequently up to 24 hours after the first levodopa dose. The minimum, maximum, and average concentration of levodopa; the daily variation of levodopa concentration; and the area under the time concentration curve (AUC) were calculated. The mean (+/-SD) plasma levodopa AUC was 39% (P = 0.0001) higher with entacapone (11,802 +/- 1454 ng/hour/mL) compared with placebo (8465 +/- 927 ng/hour/mL). The daily variation of plasma levodopa was reduced by about 25% with entacapone (P < 0.01). Entacapone significantly reduced plasma 3-OMD concentration by about 50% (P = 0.0001), indicating marked COMT inhibiting activity. There were no differences in plasma carbidopa concentrations. Entacapone significantly increased the bioavailability of levodopa and reduced its daily variation when administered concomitantly with a controlled-release levodopa preparation.

High-performance liquid chromatography method for the quantification of entacapone in human plasma

A simple, sensitive and selective HPLC method with UV detection (315 nm) was developed and validated for quantitation of entacapone in human plasma, the newest addition to the group of antiparkinsonian agents. Following a single-step liquid-liquid extraction (LLE) with ethyl acetate/n-hexane (30/70, v/v), the analyte and internal standard (rofecoxib) were separated using an isocratic mobile phase of 30 mM phosphate buffer (pH 2.75)/acetonitrile (62/38, v/v) on a reverse phase C18 column. The lower limit of quantitation was 25 ng/mL, with a relative standard deviation of less than 8%. A linear range of 25-2500 ng/mL was established. This HPLC method was validated with between-batch and within-batch precision of 2.2-4.2% and 1.7-7.8%, respectively. The between-batch and within-batch accuracy was 98.7-107.5% and 97.5-106.0%, respectively. Frequently coadministered drugs did not interfere with the described methodology. Stability of entacapone in plasma was excellent, with no evidence of degradation during sample processing (autosampler) and 30 days storage in a freezer. This validated method is sensitive, simple and repeatable enough to be used in pharmacokinetic studies.

Entacapone, a catechol-O-methyltransferase inhibitor for treating Parkinson's disease: review and current status

Levodopa, a dopamine precursor administered with a decarboxylase inhibitor, is the principal therapy for treating the symptoms of Parkinson's disease. Unfortunately, after approximately 2-5 years, it frequently loses its beneficial effects as evidenced by motor fluctuations. Entacapone (Comtan) is a selective, reversible catechol-O-methyltransferase inhibitor that dose-dependently increases the peripheral bioavailability of levodopa and prolongs its duration of action. Early studies confirmed that treatment with entacapone resulted in increased striatal uptake of levodopa after iv. administration of [18F] levodopa. Preclinical studies confirmed decreased formation of COMT-dependent metabolites, including 3-O methyldopa and homovanillic acid. Clinical studies performed in patients with motor fluctuations have shown that entacapone prolonged the duration of motor response by an average of 1-1.3 h. Parkinsonian patients receiving therapeutic doses of dopamine agonists and selegiline also experienced an incremental improvement in 'on' time when entacapone was added to their drug regimen. At present, there are no published safety studies beyond six to twelve months in duration, or studies in nonfluctuating patients. Based on the clinical trial data available, entacapone is well-tolerated in the majority of patients. Dopaminergic-related adverse effects include dyskinesias, nausea and dizziness. Non-dopaminergic adverse effects include diarrhoea, abdominal discomfort and discoloration of urine. Diarrhoea is occasionally severe and may require discontinuation of therapy. Of 406 entacapone-treated subjects, there was one incidence of elevated liver transaminases, although this was attributed to an underlying disorder. In the US, Phase III trials have been completed and a New Drug Application (NDA) has been filed. In Europe, the drug received a favourable review and is currently available.

Frequency, Reasons, and Risk Factors of Entacapone Discontinuation in Parkinson Disease

OBJECTIVES

To determine the rate of and identify reasons for discontinuation of entacapone treatment in Parkinson disease patients.

METHODS

A retrospective study design was used, including medical record reviews and telephone call to the patient as needed.

RESULTS

Two hundred and twenty-two Parkinson disease patients were started on entacapone over a period of 3 years. One hundred and twenty-two patients (56%) had discontinued the medication by the end of the follow up period. Reasons for discontinuation included lack of efficacy (46%), worsening of parkinsonian symptoms (28%), worsened cognition (20%), dyskinesia (17%), nausea (11%), diarrhea (9%), cost (4%), other side effect (8%), and other, non-medication related reasons (19%). Presence of wearing off at the time of initial treatment and male sex were associated with decreased dropout rates, while the presence of orthostatic hypotension increased the likelihood of discontinuation. Patient age, disease duration, Hoehn and Yahr stage, Unified Parkinson's Disease Rating Scale section II score, presence of dementia, hallucinations, dyskinesias, or depression, number of daily L-dopa doses, and concomitant treatment with dopamine agonists did not affect dropout rate.

CONCLUSIONS

Lack of efficacy seems to be the main reason for entacapone discontinuation. Male patients with wearing off and no orthostatic hypotension seem to tolerate the medication better. Advanced disease stage, presence of cognitive dysfunction, and presence of dyskinesia should not necessarily preclude patients from being treated with entacapone, when otherwise indicated.

Pharmacokinetic and pharmacodynamic profiles of BIA 3-202, a novel catechol-O-methyltransferase (COMT) inhibitor, during multiple-dose administration to healthy subjects

The tolerability, pharmacodynamics, and pharmacokinetics of BIA 3-202 (50 mg, 100 mg, and 200 mg twice daily and 200 mg thrice daily), a novel catechol-O-methyltransferase (COMT) inhibitor, were investigated in healthy volunteers. BIA 3-202 was administered to four sequential groups of 8 healthy male subjects under a double-blind, randomized, placebo-controlled design. Within each group, 2 subjects were randomized to treatment with placebo. Treatment duration was 9 days: single dose on the first and last days and twice or thrice daily on days 3 to 8. BIA 3-202 was well tolerated at all dose regimens tested. Median maximum plasma BIA 3-202 concentrations were attained at 0.5 to 2.5 hours postdose. Thereafter, concentrations declined with a t1/2 of approximately 2 to 4 hours. The increase in the extent of systemic exposure, as measured by AUC0-tau, was approximately proportional to the administered dose. Steady state of plasma BIA 3-202 concentrations occurred by day 4 in all dose groups. Less than 1% of the total dose administered was excreted in urine up to 48 hours postdose. BIA 3-202 markedly reduced soluble COMT (S-COMT) activity in erythrocytes, with maximum inhibition occurring at 1 to 2 hours postdose; enzyme activity returned to baseline levels by approximately 8 hours. Inhibition of S-COMT activity appeared to increase with increasing doses of BIA 3-202 on both day 1 and day 9. In conclusion, BIA 3-202 was well tolerated in all the oral multiple-dose regimens tested. BIA 3-202 was shown to inhibit S-COMT activity in erythrocytes, and its pharmacokinetics appeared to be linear (i.e., dose independent and time invariant).

Catechol-O-methyltransferase inhibition in erythrocytes and liver by BIA 3-202 (1-[3,4-dibydroxy-5-nitrophenyl]-2-phenyl-ethanone)

The present study evaluated the relationship between the degree of catechol-O-methyltransferase (COMT) inhibition in erythrocytes and liver by BIA 3-202 (1-[3,4-dihydroxy-5-nitrophenyl]-2-phenyl-ethanone) and determined its effects upon the O-methylation of L-DOPA in rats orally treated with L-DOPA plus benserazide. The soluble form of COMT (S-COMT) in erythrocytes was endowed with the same affinity as liver S-COMT for the substrate adrenaline. BIA 3-202 inhibited erythrocytes and liver S-COMT with ED50's of 1.9 (0.7, 3.1) and 1.9 (0.5, 3.2) (95% confidence limits) mg kg(-1), respectively. BIA 3-202 reduced the L-DOPA-induced rise of 3-O-methyl-L-DOPA in the peripheral circulation, striatal dialysate levels and striatum, and increased dopamine striatal levels. In BIA 3-202-treated rats the increase in L-DOPA in peripheral blood and striatal dialysates was significantly greater than in vehicle-treated rats. It is concluded that S-COMT activity in erythrocytes may provide important information on the pharmacodynamic profile of COMT inhibitors. The novel COMT inhibitor BIA 3-202 is a potent COMT inhibitor that enhances the availability of L-DOPA to the brain by reducing its O-methylation, which may prove beneficial in patients with Parkinson's disease treated with L-DOPA.

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.

Pharmacodynamic response of entacapone in rats after administration of entacapone formulations and prodrugs with varying bioavailabilities

The aim of this in vivo study was to assess the effect of improved oral bioavailability of entacapone on its actual pharmacodynamic response, COMT inhibition in erythrocytes. Rats were administered entacapone orally as a suspension, as a plain solution, an entacapone/HP-beta-CD solution, two N-alkyl-carbamate ester prodrugs and intravenously as a solution. Also the relationship between pharmacodynamic and pharmacokinetic responses of entacapone was investigated. The administration of entacapone as a solution (plain solution pH 7.4; F=34.8% or entacapone/HP-beta-CD solution pH 3.0; F = 18.5%) resulted in significantly higher degree of COMT inhibition in erythrocytes than could be achieved by administering entacapone as a suspension (pH 3.0; F=8.9%). The inhibitory Emax model did not reveal any significant differences in EC50 estimates of entacapone suspension, entacapone/HP-beta-CD solution or entacapone solution. The overall pharmacodynamic response of entacapone (AUE; area under effect-time curve) was dependent on the pharmacokinetic response (AUC; area under concentration-time curve) irrespective of the entacapone formulation and dosage form. However, this dependency did not extend to formulations producing very high peak concentrations of entacapone in plasma; high plasma concentrations reached transiently after administration of entacapone solution had only a minor effect on the overall pharmacodynamic response (AUE). The inhibitory Emax model revealed that a plateau of COMT inhibition near to Emax is attained by plasma concentrations under 2000 ng/ml, irrespective of the formulation. This supports the results concerning the dependence of AUE on AUC.

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 effects of different repeated doses of entacapone on the pharmacokinetics of L-Dopa and on the clinical response to L-Dopa in Parkinson's disease

We performed a double-blind, placebo-controlled, randomized, crossover, multiple-dose study on entacapone in 25 patients with Parkinson's disease with levodopa (L-Dopa) treatment-related fluctuations. A run-in period was followed by four 2-week treatment periods during which the patients took 4 to 6 daily doses of L-Dopa concomitantly with 100, 200, or 400 mg of entacapone or with placebo. The effects were assessed at the end of each period; the inhibition of soluble catechol-O-methyltransferase (S-COMT) activity in red blood cells and the plasma concentrations of entacapone, L-Dopa, and 3-O-methyldopa (3-OMD) were measured and clinical effects assessed on an 18-hour home diary. Twenty-one patients completed the study. Entacapone decreased the COMT activity from predose level: 100 mg by 25%, 200 mg by 33%, and 400 mg by 32% (p < 0.001 vs. placebo for each dose). Correspondingly, the 3-OMD concentrations decreased by 39%, 54%, and 66% with 100-, 200-, and 400-mg doses, respectively. The elimination half-life of L-Dopa was prolonged by 23% (p < 0.05), 26% (p < 0.001), and 48% (p < 0.001), and the area under the curve of L-Dopa increased by 17% (p < 0.05), 27% (p < 0.001), and 37% (p < 0.001) with the increasing doses. Despite a significant decrease in the daily dose of L-Dopa, entacapone decreased the proportion of daily "off" time: 100 mg by 11%, 200 mg by 18%, and 400 mg by 20% compared with placebo. However, this decrease was not statistically significant for any of the doses in this small patient population. The dyskinetic "on" time did not increase with different doses of entacapone. All doses were well tolerated, and no severe adverse events were reported. The study showed that repeated dosing of entacapone inhibits the COMT activity in a dose-dependent manner and thereby reduces the loss of L-Dopa to 3-OMD. Therefore, the area under the curve of L-Dopa is increased and the patient's clinical condition improved.

Economic and health-related quality of life considerations of new therapies in Parkinson's disease

The progressive disability of Parkinson's disease results in substantial burdens for patients, their families and society in terms of increased health resource use, poorer quality of life, caregiver burden, disrupted family relationships, decreases in social and leisure activities, deteriorating emotional well-being, and direct and indirect costs of illness. Health-related quality of life (HR-QOL) measures have been used successfully in cross-sectional studies to identify and characterise these burdens; however, there is not yet substantial evidence that these instruments will be responsive to changes in patients over time and that the results will provide patients and health professionals with clinically meaningful information useful in making decisions about treatment strategies. The few studies documenting direct and indirect costs indicate increased use of ancillary health and community services, significant adaptations in home and transportation, increased use of mobility and self-care aids, and lack of access to appropriate healthcare providers. Patients with Parkinson's disease incur higher hospital expenses, have increased number of prescriptions, and experience earnings loss; the latter also applies to family caregivers. The choice, intensity and timing of therapy are determined by a variety of factors: presenting symptoms, age, employment status, comorbidity, cognitive impairment and level of functional impairment. Choices must be individually tailored to a patient's physical and personal needs. To be useful for patients with Parkinson's disease in clinical practice, clinicians should be able to use HR-QOL measures to identify appropriate medical interventions or socio-behavioural modifications to modify the HR-QOL deficits. However, while the interplay of interventions and clinical outcomes are often well understood, the effects of interventions on HR-QOL outcomes have not been studied extensively. Little research has been done that explicitly links the signs and symptoms of Parkinson's disease to the HR-QOL outcomes. The only Parkinson's disease cost-effectiveness study as yet performed indicated higher costs for patients receiving pramipexole than for those not taking the drug, but additional quality life-years were gained. Longer term effectiveness of many treatment strategies, and the usefulness of HR-QOL instruments to assess these treatments for individual patients over time, are critical areas for future research.

Entacapone

OBJECTIVE

To introduce entacapone, a new catechol-O-methyltransferase inhibitor, and discuss its pharmacology, pharmacodynamics, pharmacokinetics, clinical efficacy, drug interactions, adverse events, dosage guidelines, and therapeutic and formulary considerations.

DATA SOURCE

A MEDLINE database search (1966-December 1999) was performed to identify relevant English-language articles including recent studies, abstracts, and reviews. Search terms included entacapone, OR-611, catechol-O-methyltransferase inhibitor, and Parkinson's disease.

STUDY SELECTION

Relevant published human studies were chosen to summarize the pharmacokinetics, clinical efficacy, adverse effects, and drug interactions.

DATA EXTRACTION

All available human clinical trials were reviewed.

DATA SYNTHESIS

Entacapone is the second medication of a new class of drugs, the catechol-O-methyltransferase inhibitors, indicated for clinical use as an adjunct to levodopa/carbidopa to treat patients with idiopathic Parkinson's disease who experience the signs and symptoms of end-of-dose wearing-off. Entacapone in combination with levodopa/dopa decarboxylase inhibitor has been shown to increase the AUC of levodopa, which leads to less fluctuation of levodopa plasma concentrations. Clinically, the duration of motor response to levodopa was prolonged as reflected by an increase in the mean on time. The addition of entacapone resulted in a decrease in the mean daily dosage of levodopa. The recommended dosage of entacapone is 200 mg administered orally with each dose of levodopa/carbidopa, up to a maximum of eight doses per day. Common adverse effects include dyskinesia, nausea, diarrhea, and urine discoloration.

CONCLUSIONS

Entacapone should be considered as add-on treatment to levodopa/carbidopa in Parkinson's disease patients with end-of-dose wearing-off effect.

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.

The catechol-O-methyltransferase (COMT) inhibitor entacapone enhances the pharmacokinetic and clinical response to Sinemet CR in Parkinson's disease

OBJECTIVES

Entacapone is a specific, potent, peripherally acting catechol-O-methyltransferase (COMT) inhibitor. It has been shown to improve the bioavailability of plasma levodopa and extend its clinical effect when used as an adjunct to standard levodopa preparations, but there is little experience of the effect of entacapone on controlled release levodopa preparations.

METHODS

A double blind, placebo controlled, single dose, randomised, cross over trial was performed in 14 patients with Parkinson's disease with motor fluctuations to investigate the clinical effect of a single dose of entacapone (200 mg) when administered with either standard levodopa-carbidopa (Sinemet) or controlled release levodopa-carbidopa preparations (Sinemet CR).

RESULTS

When entacapone was administered with standard Sinemet the duration of the clinical response to standard Sinemet was longer in comparison with the response after placebo (p=0.02). Moreover, in the same patients, entacapone significantly increased the duration of the clinical response to Sinemet CR (p=0.05) without prolonging the latency of response or enhancing dyskinesias.

CONCLUSIONS

These data confirm the clinical efficacy of entacapone-standard Sinemet combination. They also indicate that adding entacapone to controlled release levodopa preparations might provide a useful treatment option in patients with Parkinson's disease with motor fluctuations. A double blind clinical trial with a chronically administered entacapone-Sinemet CR combination is, however, required to verify this viewpoint.

Interventions to achieve tonic exposure to levodopa: delaying or preventing the onset of motor complications

Variations in levodopa delivery and subsequent dopamine exposure may lead to motor fluctuations and dyskinesias in patients with Parkinson's disease, as the therapeutic window for levodopa narrows with disease progression. Long-term exposure to the oscillations in levodopa-derived dopamine also is suggested to cause postsynaptic changes within the dopaminergic system, leading to further reductions in the drug's efficacy. The need to extend the actions of levodopa led to the development of the catechol-O-methyltransferase (COMT) inhibitors, which are the newest agents introduced to manage the symptoms of Parkinson's disease. Entacapone and tolcapone are two potent, selective, and reversible COMT inhibitors that effectively augment levodopa's pharmacokinetics by increasing area under the plasma concentration versus time curve and plasma elimination half-life without significantly affecting peak levodopa concentrations. This enhanced pharmacokinetic effect results in marked improvements in patients' clinical response. In patients experiencing end-of-dose wearing-off, the addition of either entacapone or tolcapone led to improvements in "on" time, "off" time, and the Unified Parkinson's Disease Rating Scale scoring system, even though many patients already were receiving optimum dosages of various other antiparkinsonian drugs. In patients with stable responses to long-term levodopa, addition of tolcapone significantly reduced the onset of motor fluctuations. These data, combined with the potential for delaying the onset of motor fluctuations, suggest that COMT inhibition may enhance levodopa's short- and long-term efficacy.

COMT inhibition with tolcapone does not affect carbidopa pharmacokinetics in parkinsonian patients in levodopa/carbidopa (Sinemet)

AIMS

Tolcapone is a novel catechol-O-methyltransferase (COMT) inhibitor used as an adjunct to levodopa/carbidopa or levodopa/benserazide therapy to improve treatment of Parkinson's disease. The aim of the current study was to investigate the potential effect of tolcapone on the pharmacokinetics of carbidopa.

METHODS

This was an open-label study in 12 parkinsonian patients receiving optimal levodopa/carbidopa therapy and tolcapone 200 mg three times daily for 6 weeks. Blood samples were taken at baseline (i.e. before the first tolcapone intake) and after 1-2 weeks and 6 weeks so that carbidopa pharmacokinetics before and during tolcapone treatment could be assessed.

RESULTS

No changes in any pharmacokinetic parameters of carbidopa were observed. The mean AUC(0,tau) and Cmax values at baseline were 0.39 microg ml-1 h and 0. 14 microg ml-1, respectively. During tolcapone treatment these values were on average 0.35 microg ml-1 h (AUC(0,tau), week 1-2), 0. 34 microg ml-1 h (AUC(0,tau), week 6 and 0.13 microg ml-1 (Cmax, weeks 1-2 and 6). tmax remained unchanged (approx. 2 h).

CONCLUSIONS

These results indicate that tolcapone does not affect carbidopa elimination and that no interaction of any clinical relevance occurs between tolcapone and carbidopa.

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.

Striatal [18F]fluorodopa utilization after COMT inhibition with entacapone studied with PET in advanced Parkinson's disease

The effect of peripheral catechol-O-methyltransferase (COMT) inhibition with entacapone on striatal uptake of 6-[18F]fluoro-L-dopa (FDOPA) was studied with PET both without and with entacapone in fifteen advanced parkinsonian patients and six healthy controls. Entacapone significantly enhanced the fraction of unmetabolized FDOPA in plasma from 16% to about 50% at 80 minutes after FDOPA injection in all subjects. The striatal to occipital ratios and the striatal FDOPA uptake, expressed as a modified decarboxylation coefficient (k3R0), was significantly increased in healthy controls, whereas in parkinsonian patients the increase was significant only in the caudate. On the other hand, the influx constant (Ki) decreased significantly in the caudate and putamen in parkinsonian patients; in healthy controls the Ki remained virtually unchanged. Effective peripheral COMT inhibition markedly increased the fraction of FDOPA in plasma and thus its availability in the brain for decarboxylation both in patients and control subjects. However, the change in striatal FDOPA uptake was modest in the advanced parkinsonian patients as compared to that in control subjects, because of the advanced disease, decreased storage capacity, or both.

Entacapone, a novel catechol-O-methyltransferase inhibitor for Parkinson's disease, does not impair mitochondrial energy production

Entacapone, a novel mainly peripherally acting catechol-O-methyltransferase inhibitor used in the treatment of Parkinson's disease, was evaluated for its possible uncoupling activity in cell culture, in rat liver mitochondria, and in isolated guinea-pig heart. Entacapone did not stimulate respiration in the L1210 murine T cell lymphoma cell line at the concentrations studied (5-40 microM). Furthermore, entacapone neither increased mitochondrial respiration nor impaired cardiac function at pharmacologically relevant concentrations (< 10 microM). In fact, the threshold concentration for increased mitochondrial oxygen consumption was 20 microM and half-maximal stimulation of respiration was not detected until 58 microM. Surprisingly, tolcapone, another catechol-O-methyltransferase inhibitor, which acts both peripherally and centrally, stimulated respiration in L1210 cells at the lowest concentration studied (5 microM). In addition, 1 microM tolcapone increased mitochondrial respiration, indicating that it caused uncoupling at a much lower concentration than that of 2,4-dinitrophenol, a well-known uncoupler of oxidative phosphorylation. Tolcapone also impaired the mechanical function and oxygen consumption of the isolated guinea-pig heart at 1 microM. These results show that peripherally acting entacapone, unlike the brain-penetrating tolcapone, is a safe catechol-O-methyltransferase inhibitor for the treatment of Parkinson's disease, since it does not interfere with mitochondrial energy metabolism at pharmacologically effective concentrations.

Entacapone enhances levodopa-induced reversal of motor disability in MPTP-treated common marmosets

Oral administration of levodopa (L-dopa) (2.5-25.0 mg/kg) plus carbidopa (12.5 mg/kg p.o.) to MPTP-treated common marmosets produced a dose-related increase in locomotor activity and a corresponding decrease in motor disability. Pretreatment with the peripheral COMT inhibitor entacapone (12.5 mg/kg p.o.) enhanced the intensity and duration of the increase in locomotor activity and the reversal of motor disability produced by a threshold dose of L-dopa (2.5 mg/kg p.o.) plus carbidopa. By contrast, entacapone pretreatment did not potentiate the increased locomotor activity or reversal of motor disability produced by a near-maximal dose of L-dopa (12.5 mg/kg p.o.) plus carbidopa. The effects of entacapone (5.0-25.0 mg/kg p.o.) were dose related, with doses of > 12.5 mg/kg tending to produce less potentiation of L-dopa's effects compared to lower doses. Pretreatment with entacapone (12.5 mg/kg p.o.) without carbidopa caused a short-lasting enhancement of L-dopa's (12.5 mg/kg p.o.) action, whereas pretreatment with carbidopa (12.5 mg/kg p.o.) alone had a more dramatic effect. However, pretreatment with both carbidopa and entacapone produced the greatest overall motor response. In conclusion, entacapone enhances the motor response produced by a low-threshold dose of L-dopa plus carbidopa. However, optimization of both the dose of L-dopa and entacapone appears necessary to obtain the maximal therapeutic response.