Effects of tolcapone, a catechol-O-methyltransferase inhibitor, on motor symptoms and pharmacokinetics of levodopa in patients with Parkinson's disease

Empirical and semi-mechanistic modelling of double-peaked pharmacokinetic profile phenomenon due to gastric emptying

Models have been developed to explain double-peaked plasma concentration-time profiles using mechanisms such as variable absorption and enterohepatic recirculation. Interruption of gastric emptying has also been shown to produce double-peaks, and this work proposes models for analysis of such data. In the presence of levodopa, gastric emptying is interrupted at times associated with double-peaks in pharmacokinetic profiles. Data from a simultaneous scintigraphy and paracetamol absorption study with levodopa was obtained, and models with compartments for stomach, intestine, central and peripheral tissue were developed to describe levodopa and paracetamol pharmacokinetics, including the double-peak phenomenon. The empirical model uses two gastric emptying parameter rates which are applied over separate time periods to describe the varying gastric emptying rate. The semi-mechanistic model uses a feedback mechanism acting via an effect compartment to link the plasma concentration of levodopa to the rate of gastric emptying, allowing levodopa pharmacokinetics to vary the rate of gastric emptying and give rise to a multiple-peaked plasma pharmacokinetic profile. The models were applied to plasma levodopa and paracetamol pharmacokinetic data with and without simultaneous analysis of scintigraphy data, in both cases giving a good fit and in the absence of scintigraphy data adequately predicting the stomach profile. For the semi-mechanistic model, the first-order constant governing gastric emptying was shown to switch between fast and slow values at a critical levodopa effect compartment concentration. New models have thus been proposed for analysis of plasma concentration profiles that exhibit double-peak phenomenon and applied successfully to levodopa data.

A systematic review of catechol-0-methyltransferase inhibitors: efficacy and safety in clinical practice

Catechol-O-methyltransferase (COMT) inhibitors are drugs commonly used in the management of patients with Parkinson disease complicated by motor fluctuations. Among them, entacapone is the most commonly used. Tolcapone has been reintroduced in patients where entacapone has proved to be ineffective after being withdrawn from the market because of sporadic cases of hepatotoxicity. The last COMT inhibitor is nebicapone, which use in clinical practice is still under study.

OBJECTIVES

The objectives of this study were to analyze the clinical efficacy in reducing motor complications and to evaluate their use in clinical practice and the adverse events reported in the literature.

METHODS

Scientific articles of the main previously mentioned drugs have been reviewed.

RESULTS

All these 3 drugs have proved to be effective in improving wearing-off and significantly reduce the daily dose of levodopa at the number of daily intakes. Tolcapone is undoubtedly the most effective drug, although in clinical practice sporadic cases of hepatotoxicity have limited its use in patients unresponsive to entacapone. Nebicapone is effective, and its safety is still under evaluation. Entacapone is generally well tolerated, and no significant adverse events are reported.

CONCLUSIONS

To manage motor fluctuations, the use of COMT inhibitors is now consolidated in the common clinical practice. Tolcapone is used as a second choice in patients with severe motor fluctuations not responsive to entacapone.

Utility of tolcapone in fluctuating Parkinson's disease

Fluctuating Parkinson's disease (PD) represents a clinical management challenge. The primary utility of levodopa in patients with PD is moderated by the "wearing off" phenomena seen with long-term use. COMT inhibitors slow down the rapid metabolism of levodopa, resulting in a more-sustained response to dopaminergic therapy. Tolcapone is a selective, reversible catechol-O-methyltransferase (COMT) inhibitor, shown to have both peripheral and central effects. In clinical trials, tolcapone has been shown to reduce "off" time, increase "on" time, improve patient and clinician assessments of disease severity, and improve patient quality of life. In a SWITCH study, tolcapone was associated with greater duration of "on" time than remaining on entacapone. Adverse effects of tolcapone are related to the class, with the exception of rare cases of hepatotoxicity. Tolcapone has been recently reintroduced on the European market and recent guidance from the US Food and Drug Administration has reduced the hepatic monitoring requirements for patients initiating tolcapone therapy. With proper monitoring, tolcapone is an effective, well-tolerated drug useful in the management of patients with fluctuating PD.

Tolcapone in the management of Parkinson's disease

Although levodopa remains the gold standard treatment for Parkinson's disease, many patients develop motor complications with chronic levodopa exposure. Tolcapone is a catechol-O-methyltransferase inhibitor that extends the action of levodopa. When used in conjunction with levodopa, tolcapone has been shown to be effective in improving motor fluctuations and reducing levodopa requirements in Parkinson's disease patients. However, rare reports of severe hepatotoxicity have limited its use. A recent review of the data on tolcapone-treated patients suggests that, with proper monitoring of liver function, the potential for hepatotoxicity with tolcapone use is negligibly small.

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

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

Catechol-O-methyltransferase inhibitors for levodopa-induced complications in Parkinson's disease

BACKGROUND

As Parkinson's disease progresses the control of motor symptoms often requires the addition of other drugs to levodopa. The principle aim of COMT inhibitor therapy is to increase the duration of effect of each levodopa dose and thus reduce the time patients spend in the relatively immobile 'off' phase.

OBJECTIVES

To compare the efficacy and safety of adjuvant COMT inhibitor therapy versus placebo in patients with Parkinson's disease, already established on levodopa and suffering from motor complications.

SEARCH STRATEGY

Electronic searches of the Cochrane Controlled Trials Register, (The Cochrane Library Issue 1, 2003), MEDLINE (1966-2003), EMBASE (1974-2003), were conducted. Grey literature was hand searched and the reference lists of identified studies and reviews examined. The manufacturers of COMT inhibitors were contacted.

SELECTION CRITERIA

Randomised controlled trials of adjuvant COMT inhibitor therapy versus a placebo in patients with a clinical diagnosis of idiopathic Parkinson's disease and long-term complications of levodopa therapy.

DATA COLLECTION AND ANALYSIS

Data were abstracted independently by the authors and differences settled by discussion. The outcome measures used included Parkinson's disease rating scales, levodopa dosage, 'off' time measurements and the frequency of withdrawals and adverse events.

MAIN RESULTS

Fourteen trials fulfilled the inclusion criteria. 2566 patients with Parkinson's disease and motor fluctuations were included in this review. Eight trials examined entacapone versus placebo in a total of 1560 patients. These trials were between two and twelve months in duration. Six trials examined tolcapone versus placebo in a total of 1006 patients. These trials were between six weeks and twelve months in duration. Both tolcapone and entacapone reduced 'off' time, reduced levodopa dose and modestly improved motor impairments and disability. This was at the expense of increased risk of dyskinesias, nausea, vomiting, and diarrhoea. A few participants taking tolcapone were found to have raised liver enzyme levels.

REVIEWERS' CONCLUSIONS

In the management of the motor complications seen in Parkinson's disease, tolcapone and entacapone can be used to reduce off time, reduce levodopa dose, and modestly improve motor impairment and disability. This is based on, at best, medium term evidence. However some participants on tolcapone had raised liver enzymes. This combined with three cases of fatal hepatic toxicity found during post-marketing surveillance has raised concerns over the safety of 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.

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.

In vivo effects of new inhibitors of catechol-O-methyl transferase

1. The effects of two new synthetic compounds showing in vitro catechol-O-methyl transferase (COMT) inhibitor properties were studied in vivo and compared with the effects of nitecapone and Ro-41-0960. 2. QO IA (3-(3-hydroxy-4-methoxy-5-nitrobenzylidene)-2,4-pentanedione), QO IIR ([2-(3,4-dihydroxy-2-nitrophenyl)vinyl]phenyl ketone), nitecapone and Ro-41-0960 (30 mg kg(-1), i.p.) were given to reserpinized rats 1 h before the administration of L-DOPA/carbidopa (LD/CD, 50:50 mg kg(-1), i.p.). Locomotor activity was assessed 1 h later. All the COMT inhibitors (COMTI), with the exception of QO IA, markedly potentiated LD/CD reversal of reserpine-induced akinesia. Similar results were obtained when the COMTI were coadministered with LD/CD. The effect of compound QO IIR was dose-dependent (7.5-30 mg kg(-1), i.p.). 3. The COMTI (30 mg kg(-1), i.p.) potentiated LD/CD reversal of both catalepsy and hypothermia of reserpinized mice. 4. QO IIR, nitecapone and Ro-41-0960 (30 mg kg(-1), i.p.) reduced striatal 3-methyl-DOPA (3-OMD) levels and increased dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) levels. Compound QO IA was devoid of any effect on striatal amine levels. In contrast to the other inhibitors, Ro-41-0961 reduced HVA levels as well. The effect of QO IIR on striatal amine levels was dose-dependent (7.5-60 mg kg(-1), i.p.) 5. These results suggest that the new compound QO IIR is an effective peripherally acting COMT inhibitor in vivo.

A comparative study on neurochemistry of cerebrospinal fluid in advanced Parkinson's disease

This study addresses two issues: (1) the comparative neurochemistry of classic tremor type of Parkinson's disease or PD-A and akinetic type of Parkinson's disease or PD-B; and (2) the neurochemistry of levodopa failure syndrome (LDFS). Cerebrospinal fluid from the lateral ventricle was collected from 50 patients with idiopathic Parkinson's disease of PD-A and PD-B. Levels of monoamine neurotransmitters and metabolites were determined using high performance liquid chromatography. We have found that (1) 5-hydroxylindoleacetic acid (5-HIAA) level is significantly lower in PD-B than in PD-A; (2) 5-HIAA level is inversely associated with score of part one of United Parkinson's Disease Rating Score (UPDRS); (3) 5-HIAA level is inversely associated with score of part four of UPDRS; (4) 3-O-methyldopa (3-OMD) level is positively associated with levodopa failure syndrome (LDFS) assessed by part four of UPDRS and inversely associates with 5-HIAA. From these data, it can be inferred that serotonergic activity is decreased in PD-B to a greater extent than in PD-A and that decreased serotonergic activity plays a role in LDFS.

Tolcapone, a selective catechol-O-methyltransferase inhibitor for treatment of Parkinson's disease

Tolcapone is a selective peripheral and central catechol-O-methyltransferase (COMT) inhibitor recently approved as adjunctive therapy in patients with idiopathic Parkinson's disease who are already being treated with a levodopa-peripheral dopa decarboxylase inhibitor (DDI) combination. Tolcapone potentiates and prolongs the effect of levodopa in the central nervous system (CNS) by enhancing levodopa's delivery to the CNS and slowing dopamine's central metabolism. A short terminal disposition half-life of 2 hours mandates dosing 3 times/day. Dosage adjustment is generally unnecessary in the presence of mild to moderate renal and hepatic impairment. Coadministration of tolcapone with levodopa-DDI results in significant amelioration of the wearing-off and on-off phenomena and frequently allows significant levodopa dosage reduction. In patients with stable disease, tolcapone improves "on" time. As might be expected from its potentiation of levodopa effects, dopaminergic side effects are prominent with this agent. Although the main objective of drug treatment in Parkinson's disease remains clinical improvement with an optimum dose and frequency of levodopa administration, tolcapone may prove a useful adjunct to such therapy, especially in the presence of the wearing-off and on-off phenomena. The relative merits of this agent vis-a-vis dopamine receptor agonists are somewhat unclear at present. However, recent guidelines from the American Academy of Neurology suggest that a COMT inhibitor be added to levodopa-dopamine agonist therapy in patients with advanced disease.

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.