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

Elucidating the function of hypothetical PE_PGRS45 protein of Mycobacterium tuberculosis as an oxido-reductase: a potential target for drug repurposing for the treatment of tuberculosis

Mycobacterium tuberculosis (Mtb) encodes a total of 67 PE_PGRS proteins and definite functions of many of them are still unknown. This study reports PE_PGRS45 (Rv2615c) protein from Mtb as NADPH dependent oxido-reductase having substrate specificity for fatty acyl Coenzyme A. Computational studies predicted PE_PGRS45 to be an integral membrane protein of Mtb. Expression of PE_PGRS45 in non-pathogenic Mycobacterium smegmatis, which does not possess PE_PGRS genes, confirmed its membrane localization. This protein was observed to have NADPH binding motif. Experimental validation confirmed its NADPH dependent oxido-reductase activity (Km value = 34.85 ± 9.478 μM, Vmax = 96.77 ± 7.184 nmol/min/mg of protein). Therefore, its potential to be targeted by first line anti-tubercular drug Isoniazid (INH) was investigated. INH was predicted to bind within the active site of PE_PGRS45 protein and experiments validated its inhibitory effect on the oxido-reductase activity of PE_PGRS45 with IC50/Ki values of 5.66 μM. Mtb is resistant to first line drugs including INH. Therefore, to address the problem of drug resistant TB, docking and Molecular Dynamics (MD) simulation studies between PE_PGRS45 and three drugs (Entacapone, Tolcapone and Verapamil) which are being used in Parkinson's and hypertension treatment were performed. PE_PGRS45 bound the three drugs with similar or better affinity in comparison to INH. Additionally, INH and these drugs bound within the same active site of PE_PGRS45. This study discovered Mtb's PE_PGRS45 protein to have an oxido-reductase activity and could be targeted by drugs that can be repurposed for TB treatment. Furthermore, in-vitro and in-vivo validation will aid in drug-resistant TB treatment. HIGHLIGHTSIn-silico and in-vitro studies of hypothetical protein PE_PGRS45 (Rv2615c) of Mycobacterium tuberculosis (Mtb) reveals it to be an integral membrane proteinPE_PGRS45 protein has substrate specificity for fatty acyl Coenzyme A (fatty acyl CoA) and possess NADPH dependent oxido-reductase activityDocking and simulation studies revealed that first line anti-tubercular drug Isoniazid (INH) and other drugs with anti-TB property have strong affinity for PE_PGRS45 proteinOxido-reductase activity of PE_PGRS45 protein is inhibited by INHPE_PGRS45 protein could be targeted by drugs that can be repurposed for TB treatmentCommunicated by Ramaswamy H. Sarma.

Antiparkinsonian Agents in Investigational Polymeric Micro- and Nano-Systems

Parkinson's disease (PD) is a devastating neurodegenerative disease characterized by progressive destruction of dopaminergic tissue in the central nervous system (CNS). To date, there is no cure for the disease, with current pharmacological treatments aimed at controlling the symptoms. Therefore, there is an unmet need for new treatments for PD. In addition to new therapeutic options, there exists the need for improved efficiency of the existing ones, as many agents have difficulties in crossing the blood-brain barrier (BBB) to achieve therapeutic levels in the CNS or exhibit inappropriate pharmacokinetic profiles, thereby limiting their clinical benefits. To overcome these limitations, an interesting approach is the use of drug delivery systems, such as polymeric microparticles (MPs) and nanoparticles (NPs) that allow for the controlled release of the active ingredients targeting to the desired site of action, increasing the bioavailability and efficacy of treatments, as well as reducing the number of administrations and adverse effects. Here we review the polymeric micro- and nano-systems under investigation as potential new therapies for PD.

Micro- and Nano-Systems Developed for Tolcapone in Parkinson’s Disease

To date there is no cure for Parkinson’s disease (PD), a devastating neurodegenerative disorder with levodopa being the cornerstone of its treatment. In early PD, levodopa provides a smooth clinical response, but after long-term therapy many patients develop motor complications. Tolcapone (TC) is an effective adjunct in the treatment of PD but has a short elimination half-life. In our work, two new controlled delivery systems of TC consisting of biodegradable PLGA 502 (poly (D,L-lactide-co-glycolide acid) microparticles (MPs) and nanoparticles (NPs) were developed and characterized. Formulations MP-TC4 and NP-TC3 were selected for animal testing. Formulation MP-TC4, prepared with 120 mg TC and 400 mg PLGA 502, exhibited a mean encapsulation efficiency (EE) of 85.13%, and zero-order in vitro release of TC for 30 days, with around 95% of the drug released at this time. Formulation NP-TC3, prepared with 10 mg of TC and 50 mg of PLGA 502, exhibited mean EE of 56.69%, particle size of 182 nm, and controlled the release of TC for 8 days. Daily i.p. (intraperitoneal) doses of rotenone (RT, 2 mg/kg) were given to Wistar rats to induce neurodegeneration. Once established, animals received TC in saline (3 mg/kg/day) or encapsulated within formulations MP-TC4 (amount of MPs equivalent to 3 mg/kg/day TC every 14 days) and NP-TC3 (amount of NPs equivalent to 3 mg/kg/day TC every 3 days). Brain analyses of Nissl-staining, GFAP (glial fibrillary acidic protein), and TH (tyrosine hydroxylase) immunohistochemistry as well as behavioral testing (catalepsy, akinesia, swim test) showed that the best formulation was NP-TC3, which was able to revert PD-like symptoms of neurodegeneration in the animal model assayed.

Phenols in Pharmaceuticals: Analysis of a Recurring Motif

Phenols and phenolic ethers are significant scaffolds recurring both in nature and among approved small-molecule pharmaceuticals. This compendium presents the first comprehensive compilation and analysis of the structures of U.S. FDA-approved molecules containing phenol or phenolic ether fragments. This dataset comprises 371 structures, which are strongly represented by natural products. A total of 55 of the compounds described here are on the World Health Organization's list of essential medicines. Structural analysis reveals significant differences in the physicochemical properties imparted by phenols versus phenol ethers, each having benefits and drawbacks for drug developability. Despite trends over the past decade to increase the fraction of sp³ centers in drug leads, thereby "escaping flatland", phenols and phenolic ethers are represented in 62% of small-molecule drugs approved in 2020, suggesting that this aromatic moiety holds a special place in drugs and natural products.

Redefining the strategy for the use of COMT inhibitors in Parkinson's disease: the role of opicapone

INTRODUCTION

Levodopa remains the gold-standard Parkinson's disease (PD) treatment, but the inevitable development of motor complications has led to intense activity in pursuit of its optimal delivery.

AREAS COVERED

Peripheral inhibition of dopa-decarboxylase has long been considered an essential component of levodopa treatment at every stage of illness. In contrast, only relatively recently have catechol-O-methyltransferase (COMT) inhibitors been utilized to block the other major pathway of degradation and optimize levodopa delivery to the brain. First and second-generation COMT inhibitors were deficient because of toxicity, sub-optimal pharmacokinetics or a short duration of effect. As such, they have only been employed once 'wearing-off' has developed. However, the third-generation COMT inhibitor, opicapone has overcome these difficulties and exhibits long-lasting enzyme inhibition without the toxicity observed with previous generations of COMT inhibitors. In clinical trials and real-world PD studies opicapone improves the levodopa plasma profile and results in a significant improvement in ON time in 'fluctuating' disease, but it has not yet been included in the algorithm for early treatment.

EXPERT OPINION

This review argues for a shift in the positioning of COMT inhibition with opicapone in the PD algorithm and lays out a pathway for proving its effectiveness in early disease.

Glutamic Acid Transporters: Targets for Neuroprotective Therapies in Parkinson's Disease

Parkinson's disease (PD) is a common neurodegenerative disease in middle-aged and elderly individuals. At present, no effective drug has been developed to treat PD. Although a variety of drugs exist for the symptomatic treatment of PD, they all have strong side effects. Most studies on PD mainly focus on dopaminergic neurons. This review highlights the function of glutamic acid transporters (GLTs), including excitatory amino acid transporters (EAATs) and vesicular glutamate transporters (VGLUTs), during the development of PD. In addition, using bioinformatics, we compared the expression of different types of glutamate transporter genes in the cingulate gyrus of PD patients and healthy controls. More importantly, we suggest that the functional roles of glutamate transporters may prove beneficial in the treatment of PD. In summary, VGLUTs and EAATs may be potential targets in the treatment of PD. VGLUTs and EAATs can be used as clinical drug targets to achieve better efficacy. Through this review article, we hope to enable future researchers to improve the condition of PD patients.

Importance of Nanoparticles for the Delivery of Antiparkinsonian Drugs

Parkinson's disease (PD) affects around ten million people worldwide and is considered the second most prevalent neurodegenerative disease after Alzheimer's disease. In addition, there is a higher risk incidence in the elderly population. The main PD hallmarks include the loss of dopaminergic neurons and the development of Lewy bodies. Unfortunately, motor symptoms only start to appear when around 50-70% of dopaminergic neurons have already been lost. This particularly poses a huge challenge for early diagnosis and therapeutic effectiveness. Actually, pharmaceutical therapy is able to relief motor symptoms, but as the disease progresses motor complications and severe side-effects start to appear. In this review, we explore the research conducted so far in order to repurpose drugs for PD with the use of nanodelivery systems, alternative administration routes, and nanotheranostics. Overall, studies have demonstrated great potential for these nanosystems to target the brain, improve drug pharmacokinetic profile, and decrease side-effects.

Prospecting for new catechol-O-methyltransferase (COMT) inhibitors as a potential treatment for Parkinson's disease: a study by molecular dynamics and structure-based virtual screening

Parkinson's disease (PD) is a neurodegenerative, chronic, and progressive disease, common in the elderly. The catechol-O-methyltransferase (COMT) is a monomeric enzyme involved in dopamine (DA) degradation, the neurotransmitter in deficit in patients with PD. The reference treatment of PD consists of levodopa (L-dopa) administration, which is the precursor of DA. The inhibition of COMT is an adjuvant treatment in PD since it keeps DA levels constant. The goal of this study was to identify drug candidates capable of inhibiting COMT for the treatment of PD and identify important fragments of these molecules. Initially, we analyzed the flexibility of COMT and defined its main conformations in solution regarding the absence (system I) and presence of the S-adenosyl-L-methionine (SAM) cofactor (system II) through molecular dynamics (MD) simulations. Two regions in these structures were selected for molecular docking, firstly the entire cavity where the cofactor and substrates are bound and secondly the specific biding region of the enzyme substrates. Based on the conformations of the MD, the virtual screening (VS) was performed against FDA Approved and Zinc Natural Products databases aiming at the selection of the best compounds. Subsequently, the absorption, distribution, metabolization, excretion, and toxicity (ADMET) properties, as well as drug-score and drug-likeness indexes of the most promising compounds were analyzed. After a detailed analysis of the compounds selected by structure-based VS, it was possible to highlight the fragments most frequently involved in their stability: 2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole, 9H-Benz(c)indole(3,2,1-ij)(1,5)naphthyridin-9-one and (10R,13S)-10,13-dimethyl-1,2,6,7,8,9,11,12,14,15,16,17dodecahydrocyclopenta[a]phenanthren-3-one. The identification of these potential fragments is essential for the prospection of more specific inhibitors against COMT using the technique of Fragment-based lead discovery (FBLD). Besides, this study allowed us to identify the potential COMT inhibitors through a complete understanding of molecular-level interactions based on the flexibility of this protein.Communicated by Ramaswamy H. Sarma.

Pharmacokinetic Enhancers (Boosters)-Escort for Drugs against Degrading Enzymes and Beyond

Pharmacokinetic enhancers (boosters) are compounds used in combination with a primary therapeutic agent (drug) and are not used for their direct effects on the disease but because they enhance or restore the activity of the primary agent. Hence, in certain cases, they represent an indispensable escort for enzyme-labile drugs. Pharmacokinetic enhancers can exert their activity on different ways. In the most common case, they inhibit enzymes such as human cytochrome P450 enzymes in the liver or other organs and, thereby, block or reduce undesired metabolism and inactivation of the primary drug. In this review, an overview will be given on the therapeutically most important classes of pharmacokinetic enhancers like β-lactamase inhibitors, inhibitors of CYP (cytochrome P450) enzymes in HIV therapy and hepatitis C, boosters for fluoropyrimidine-type anticancer agents, compounds utilized for enabling therapy of Parkinson's disease with levodopa, and others. Inhibitors of efflux pumps in both pathogenic bacteria and tumor cells will be addresses shortly.

Spotlight on opicapone as an adjunct to levodopa in Parkinson's disease: design, development and potential place in therapy

Parkinson's disease (PD) is a progressive, chronic, neurodegenerative disease characterized by rigidity, tremor, bradykinesia and postural instability secondary to dopaminergic deficit in the nigrostriatal system. Currently, disease-modifying therapies are not available, and levodopa (LD) treatment remains the gold standard for controlling motor and nonmotor symptoms of the disease. LD is extensively and rapidly metabolized by peripheral enzymes, namely, aromatic amino acid decarboxylase and catechol-O-methyltransferase (COMT). To increase the bioavailability of LD, COMT inhibitors are frequently used in clinical settings. Opicapone is a novel COMT inhibitor that has been recently approved by the European Medicines Agency as an adjunctive therapy to combinations of LD and aromatic amino acid decarboxylase inhibitor in adult PD patients with end-of-dose motor fluctuations. We aimed to review the biochemical properties of opicapone, summarize its preclinical and clinical trials and discuss its future potential role in the treatment of PD.

Molecular recognition in chemical and biological systems

Structure-based ligand design in medicinal chemistry and crop protection relies on the identification and quantification of weak noncovalent interactions and understanding the role of water. Small-molecule and protein structural database searches are important tools to retrieve existing knowledge. Thermodynamic profiling, combined with X-ray structural and computational studies, is the key to elucidate the energetics of the replacement of water by ligands. Biological receptor sites vary greatly in shape, conformational dynamics, and polarity, and require different ligand-design strategies, as shown for various case studies. Interactions between dipoles have become a central theme of molecular recognition. Orthogonal interactions, halogen bonding, and amide⋅⋅⋅π stacking provide new tools for innovative lead optimization. The combination of synthetic models and biological complexation studies is required to gather reliable information on weak noncovalent interactions and the role of water.

Pharmacological profile of opicapone, a third-generation nitrocatechol catechol-O-methyl transferase inhibitor, in the rat

BACKGROUND AND PURPOSE

Catechol-O-methyltransferase (COMT) is an important target in the levodopa treatment of Parkinson's disease; however, the inhibitors available have problems, and not all patients benefit from their efficacy. Opicapone was developed to overcome those limitations. In this study, opicapone's pharmacological properties were evaluated as well as its potential cytotoxic effects.

EXPERIMENTAL APPROACH

The pharmacodynamic effects of opicapone were explored by evaluating rat COMT activity and levodopa pharmacokinetics, in the periphery through microdialysis and in whole brain. The potential cytotoxicity risk of opicapone was explored in human hepatocytes by assessing cellular ATP content and mitochondrial membrane potential.

KEY RESULTS

Opicapone inhibited rat peripheral COMT with ED50 values below 1.4 mg⋅kg(-1) up to 6 h post-administration. The effect was sustained over the first 8 h and by 24 h COMT had not returned to control values. A single administration of opicapone resulted in increased and sustained plasma levodopa levels with a concomitant reduction in 3-O-methyldopa from 2 h up to 24 h post-administration, while tolcapone produced significant effects only at 2 h post-administration. The effects of opicapone on brain catecholamines after levodopa administration were sustained up to 24 h post-administration. Opicapone was also the least potent compound in decreasing both the mitochondrial membrane potential and the ATP content in human primary hepatocytes after a 24 h incubation period.

CONCLUSIONS AND IMPLICATIONS

Opicapone has a prolonged inhibitory effect on peripheral COMT, which extends the bioavailability of levodopa, without inducing toxicity. Thus, it exhibits some improved properties compared to the currently available COMT inhibitors.

A dual-tracer study of extrastriatal 6-[18F]fluoro-m-tyrosine and 6-[18F]-fluoro-L-dopa uptake in Parkinson's disease

6-[(18)F]-Fluoro-L-dopa (FDOPA) has been widely used as a biomarker for catecholamine synthesis, storage, and metabolism--its intense uptake in the striatum, and fainter uptake in other brain regions, is correlated with the symptoms and pathophysiology of Parkinson's disease (PD). 6-[(18)F]fluoro-m-tyrosine (FMT), which also targets L-amino acid decarboxylase, has potential advantages over FDOPA as a radiotracer because it does not form catechol-O-methyltransferase (COMT) metabolites. The purpose of the present study was to compare the regional distribution of these radiotracers in the brains of PD patients. Fifteen Parkinson's patients were studied with FMT and FDOPA positron emission tomography (PET) as well as high-resolution structural magnetic resonance imaging (MRI). MRI's were automatically parcellated into neuroanatomical regions of interest (ROIs) in Freesurfer (http://surfer.nmr.mgh.harvard.edu); region-specific uptake rate constants (Kocc) were generated from coregistered PET using a Patlak graphical approach. The essential findings were as follows: (1) regional Kocc were highly correlated between the radiotracers and in agreement with a previous FDOPA studies that used different ROI selection techniques; (2) FMT Kocc were higher in extrastriatal regions of relatively large uptake such as amygdala, pallidum, brainstem, hippocampus, entorhinal cortex, and thalamus, whereas cortical Kocc were similar between radiotracers; (3) while subcortical uptake of both radiotracers was related to disease duration and severity, cortical uptake was not. These results suggest that FMT may have advantages for examining pathologic changes within allocortical loop structures, which may contribute to cognitive and emotional symptoms of PD.

A translational pharmacology approach to understanding the predictive value of abuse potential assessments

Within the drug development industry the assessment of abuse potential for novel molecules involves the generation and review of data from multiple sources, ranging from in-vitro binding and functional assays through to in-vivo nonclinical models in mammals, as well as collection of information from studies in humans. This breadth of data aligns with current expectations from regulatory agencies in both the USA and Europe. To date, there have been a limited number of reviews on the predictive value of individual models within this sequence, but there has been no systematic review on how each of these models contributes to our overall understanding of abuse potential risk. To address this, we analyzed data from 100 small molecules to compare the predictive validity for drug scheduling status of a number of models that typically contribute to the abuse potential assessment package. These models range from the assessment of in-vitro binding and functional profiles at receptors or transporters typically associated with abuse through in-vivo models including locomotor activity, drug discrimination, and self-administration in rodents. Data from subjective report assessments in humans following acute dosing of compounds were also included. The predictive value of each model was then evaluated relative to the scheduling status of each drug in the USA. In recognition of the fact that drug scheduling can be influenced by factors other than the pharmacology of the drug, we also evaluated the predictive value of each assay for the outcome of the human subjective effects assessment. This approach provides an objective and statistical assessment of the predictive value of many of the models typically applied within the pharmaceutical industry to evaluate abuse potential risk. In addition, the impact of combining information from multiple models was examined. This analysis adds to our understanding of the predictive value of each model, allows us to critically evaluate the benefits and limitations of each model, and provides a method for identifying opportunities for improving our assessment and prediction of abuse liability risk in the future.

A within-subject comparison of 6-[18F]fluoro-m-tyrosine and 6-[18F]fluoro-L-dopa in Parkinson's disease

Progression of Parkinson's disease symptoms is imperfectly correlated with positron emission tomography biomarkers for dopamine biosynthetic pathways. The radiopharmaceutical 6-[(18) F]fluoro-m-tyrosine is not a substrate for catechol-O-methyltransferase and therefore has a more favorable uptake-to-background ratio than 6-[(18) F]fluoro-L-dopa. The objective of this study was to evaluate 6-[(18) F]fluoro-m-tyrosine relative to 6-[(18) F]fluoro-L-dopa with partial catechol-O-methyltransferase inhibition as a biomarker for clinical status in Parkinson's disease. Twelve patients with early-stage Parkinson's disease, off medication, underwent Unified Parkinson Disease Rating Scale scoring, brain magnetic resonance imaging, and 3-dimensional dynamic positron emission tomography using equivalent doses of 6-[(18) F]fluoro-m-tyrosine and 6-[(18) F]fluoro-L-dopa with tolcapone, a catechol-O-methyltransferase inhibitor. Images were realigned within subject, after which the tissue-derived uptake rate constant was generated for volumes of interest encompassing the caudate nucleus, putamen, and subregions of the putamen. We computed both bivariate (Pearson) and partial (covariate of age) correlations between clinical subscores and tissue-derived uptake rate constant. Tissue-derived uptake rate constant values were correlated between the radiopharmaceuticals (r = 0.8). Motor subscores were inversely correlated with the contralateral putamen 6-[(18) F]fluoro-m-tyrosine tissue-derived uptake rate constant (|r| > 0.72, P < .005) but not significantly with the 6-[(18) F]fluoro-L-dopa tissue-derived uptake rate constant. The uptake rate constants for both radiopharmaceuticals were also inversely correlated with activities of daily living subscores, but the magnitude of correlation coefficients was greater for 6-[(18) F]fluoro-m-tyrosine. In this design, 6-[(18) F]fluoro-m-tyrosine uptake better reflected clinical status than did 6-[(18) F]fluoro-L-dopa uptake. We attribute this finding to 6-[(18) F]fluoro-m-tyrosine's higher affinity for the target, L-aromatic amino acid decarboxylase, and the absence of other major determinants of the uptake rate constant. These results also imply that L-aromatic amino acid decarboxylase activity is a major determinant of clinical status.

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.

Drug discovery using chemical systems biology: repositioning the safe medicine Comtan to treat multi-drug and extensively drug resistant tuberculosis

The rise of multi-drug resistant (MDR) and extensively drug resistant (XDR) tuberculosis around the world, including in industrialized nations, poses a great threat to human health and defines a need to develop new, effective and inexpensive anti-tubercular agents. Previously we developed a chemical systems biology approach to identify off-targets of major pharmaceuticals on a proteome-wide scale. In this paper we further demonstrate the value of this approach through the discovery that existing commercially available drugs, prescribed for the treatment of Parkinson's disease, have the potential to treat MDR and XDR tuberculosis. These drugs, entacapone and tolcapone, are predicted to bind to the enzyme InhA and directly inhibit substrate binding. The prediction is validated by in vitro and InhA kinetic assays using tablets of Comtan, whose active component is entacapone. The minimal inhibition concentration (MIC(99)) of entacapone for Mycobacterium tuberculosis (M.tuberculosis) is approximately 260.0 microM, well below the toxicity concentration determined by an in vitro cytotoxicity model using a human neuroblastoma cell line. Moreover, kinetic assays indicate that Comtan inhibits InhA activity by 47.0% at an entacapone concentration of approximately 80 microM. Thus the active component in Comtan represents a promising lead compound for developing a new class of anti-tubercular therapeutics with excellent safety profiles. More generally, the protocol described in this paper can be included in a drug discovery pipeline in an effort to discover novel drug leads with desired safety profiles, and therefore accelerate the development of new drugs.

Tolcapone: review of its pharmacology and use as adjunctive therapy in patients with Parkinson's disease

Levodopa has been the gold standard therapy for the motor symptoms of Parkinson’s disease for more than three decades. Although it remains the most effective treatment, its long-term use is associated with motor fluctuations and dyskinesias that can be disabling for patients and difficult for physicians to manage medically. In the last 10 years, the catechol-O-methyltransferase (COMT) inhibitor tolcapone has been studied for its efficacy as an adjunctive treatment to levodopa plus a dopa decarboxylase inhibitor. Adjunctive therapy with tolcapone can significantly reduce the dose of levodopa required. Moreover, treatment with tolcapone significantly reduces wearing off and on-off periods in fluctuating patients and improves ‘on’ time in patients with stable disease. Tolcapone has assumed a new place in the arsenal of medications for Parkinson’s disease. This paper reviews the pharmacology, safety and efficacy of tolcapone in patients with advanced Parkinson’s disease. After some initial concerns about its safety, tolcapone has been shown to be safe if used and monitored according to guidelines regarding liver function. Tolcapone produces expected dopaminergic side effects, including headache, nausea, insomnia, as well as diarrhea; however, these side effects are generally mild and as a rule do not result in discontinuation of therapy.

Norepinephrine and stimulant addiction

No pharmacotherapies are approved for stimulant use disorders, which are an important public health problem. Stimulants increase synaptic levels of the monoamines dopamine (DA), serotonin and norepinephrine (NE). Stimulant reward is attributable mostly to increased DA in the reward circuitry, although DA stimulation alone cannot explain the rewarding effects of stimulants. The noradrenergic system, which uses NE as the main chemical messenger, serves multiple brain functions including arousal, attention, mood, learning, memory and stress response. In pre-clinical models of addiction, NE is critically involved in mediating stimulant effects including sensitization, drug discrimination and reinstatement of drug seeking. In clinical studies, adrenergic blockers have shown promise as treatments for cocaine abuse and dependence, especially in patients experiencing severe withdrawal symptoms. Disulfiram, which blocks NE synthesis, increased the number of cocaine-negative urines in five randomized clinical trials. Lofexidine, an alpha(2)-adrenergic agonist, reduces the craving induced by stress and drug cues in drug users. In addition, the NE transporter (NET) inhibitor atomoxetine attenuates some of d-amphetamine's subjective and physiological effects in humans. These findings warrant further studies evaluating noradrenergic medications as treatments for stimulant addiction.

Parkinson's disease: emerging pharmacotherapy

Parkinson's disease (PD) is the second most common neurodegenerative disease. The prevalence is increasing with age and averages approximately 0.3% in the entire population. The clinical picture is dominated by the cardinal motor symptoms such as tremor at rest, bradykinesia, muscular rigidity, stooped posture and postural instability. Psychiatric comorbidity is common, comprising dementia, depression, anxiety and psychosis. Although many drugs have been developed and introduced into the market to provide symptomatic treatment, there is still no cure for PD and not even solid evidence for disease-modifying strategies. In addition, motor complications in advanced stages of the disease, side effects of the dopaminergic therapy, and non-motor symptoms remain huge challenges during long-term therapy. Thus, new therapeutic agents are desperately needed. Here, we describe current therapies and possible future developments that we hope will contribute to sustaining quality of life in patients suffering from Parkinson's disease for many years.

Evidence-based efficacy comparison of tolcapone and entacapone as adjunctive therapy in Parkinson's disease

The relative efficacy has not been adequately established for the two catechol-O-methyltransferase (COMT) inhibitors that are currently available for adjunctive therapy in Parkinson's disease; tolcapone and entacapone. A recent Cochrane meta-analysis of 14 studies in 2566 patients, conducted to assess the efficacy and safety of tolcapone and entacapone, found both to be statistically superior to placebo in increasing ON time and decreasing OFF time. The meta-analysis also showed that the weighted mean difference from baseline to endpoint in tolcapone-treated patients was twice that in entacapone-treated patients for both placebo-corrected ON time and OFF time. Withdrawal rates were generally lower for tolcapone. Two additional studies have examined the switch between tolcapone and entacapone. In 40 Parkinson's disease patients with fluctuations who were switched from tolcapone to entacapone, improvements in ON time and reductions in OFF time were approximately twice the magnitude for tolcapone than for entacapone. In a second study examining the switch from entacapone to tolcapone, the results for several exploratory variables also suggested that tolcapone has greater efficacy than entacapone. These findings indicate that tolcapone should be considered in all patients with entacapone-refractory motor fluctuations.

Pharmacotherapy for Parkinson's disease

The available pharmacotherapies for Parkinson's disease address symptomatology because no agent has been demonstrated to provide definite neuroprotection against the disease. Choice of pharmacotherapy must include consideration of short-term benefits as well as long-term consequences. Patients with mild Parkinson's disease often function adequately without symptomatic treatment. However, recent data suggest that initiation of treatment with a well-tolerated agent (e.g., the monoamine oxidase [MAO]-B inhibitor rasagiline) in the absence of functional impairment is associated with improved long-term outcomes. Consideration should also be given to many patient-specific factors, including patient expectations, level of disability, employment status, functional as well as chronologic age, expected efficacy and tolerability of drugs, and response to previous Parkinson's disease therapies. Increasingly, initial monotherapy begins with a nondopaminergic agent or, if the patient is considered functionally young, a dopamine agonist. Since Parkinson's disease is a progressive disorder, adjustments to pharmacotherapy must be expected over time. When greater symptomatic relief is desired, or in the more frail elderly patient, levodopa therapy should be considered. If motor fluctuations develop, addition of a catechol-O-methyltransferase inhibitor or MAO-B inhibitor should be considered. For management of levodopa-induced dyskinesias, addition of amantadine is an option. Surgery may be considered when patients need additional symptomatic control or are experiencing severe motor complications despite pharmacologically optimized therapy.

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

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

Results From a 2-Year Centralized Tolcapone Liver Enzyme Monitoring Program

OBJECTIVES

Tolcapone, administered with levodopa/carbidopa to patients with Parkinson disease, is an effective and generally well-tolerated adjunctive therapy. However, 4 early cases of hepatotoxicity causing 3 deaths in patients not properly monitored gave rise to more rigorous liver function test monitoring guidelines and a liver function test monitoring program in which blood samples from tolcapone-treated patients were tested and results were collected in a central database. We analyzed these results to determine the percentages of patients with at least 1 aspartate aminotransferase (AST) or alanine aminotransferase (ALT) elevation above the upper limit of normal (ULN) and at least 1 elevation greater than 2 times the ULN.

METHODS

This retrospective, observational analysis included all AST and ALT values recorded in the database from 11,883 samples from patients who received tolcapone for up to 2 years or longer (January 1999-January 2001).

RESULTS

Of 1725 patients who had at least 1 AST or ALT measurement, 3.9% (n = 67) had elevations above the ULN. Less than 1% (15/1725) of patients had AST or ALT elevations greater than 2 times the ULN. Most values returned to normal during continued tolcapone treatment. In 472 patients monitored biweekly while receiving tolcapone for 20 to 114 consecutive weeks, only 0.6% (n = 3) had an AST or ALT value greater than 2 times the ULN; no significant elevations occurred in the remaining 469 (99.4%).

CONCLUSIONS

This analysis of results from the central monitoring program suggests that significant transaminase elevations are rare, typically transient, and may return to normal in the face of continued tolcapone therapy in most patients.

Efficacy of pramipexole and transdermal rotigotine in advanced Parkinson's disease: a double-blind, double-dummy, randomised controlled trial

BACKGROUND

Continuous dopaminergic drug delivery is an unmet medical need in advanced Parkinson's disease. The aim of this trial-Clinical Efficacy of Pramipexole And Transdermal Rotigotine in Advanced PD (CLEOPATRA-PD)-was to assess the efficacy of adjunct treatment with rotigotine in comparison with placebo and with pramipexole in levodopa-treated patients with advanced Parkinson's disease and wearing-off type motor fluctuations.

METHODS

In this randomised controlled trial, eligible participants were randomly assigned to receive either rotigotine (up to 16 mg/24 h as a transdermal patch), pramipexole (up to 4.5 mg/day orally), or placebo for 6 months. Primary efficacy variables were absolute change in total hours "off" (assessed by home diaries) from baseline to end of study and responder rate (defined as the proportion of patients with >or=30% reduction in absolute off time per day). Analyses were done by intention to treat. This trial is registered with the US National Institutes of Health clinical trials database (ClinicalTrials.gov), number NCT00244387.

FINDINGS

204 patients were randomly assigned to receive rotigotine, 201 to receive pramipexole, and 101 to receive placebo; 427 (84%) completed the trial. The number of discontinuations in each group was similar; most were for adverse events. The mean dose of rotigotine was 12.95 mg/24 h (SD 3.54), the mean dose of pramipexole was 3.1 mg/day (1.24). Mean absolute change in off time from baseline was -2.5 h (SE 0.20) with rotigotine, -2.8 h (0.20) with pramipexole, and -0.9 h (0.29) with placebo. The absolute change in off time from baseline compared with placebo was -1.58 h (95% CI -2.27 to -0.90; p<0.0001) for rotigotine and -1.94 h (-2.63 to -1.25; p<0.0001) for pramipexole. Responder rates were 67% (134 of 200 patients) for pramipexole, 59.7% (120 of 201 patients) for rotigotine, and 35% (35 of 100 patients) for placebo.

INTERPRETATION

In terms of change in absolute off time, rotigotine was non-inferior to pramipexole. Continuous delivery of rotigotine as transdermal patches could offer similar efficacy to oral pramipexole in patients with fluctuating Parkinson's disease over 6 months of treatment.

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.

Bisubstrate inhibitors of catechol O-methyltransferase (COMT): the crucial role of the ribose structural unit for inhibitor binding affinity

Inhibition of the enzyme catechol O-methyltransferase offers a therapeutic handle to regulate the catabolism of catecholamine neurotransmitters, providing valuable assistance in the treatment of CNS disorders such as Parkinson's disease. A series of ribose-modified bisubstrate inhibitors of COMT featuring 2'-deoxy-, 3'-deoxy-, 2'-aminodeoxy-3'-deoxy-, and 2'-deoxy-3'-aminodeoxyribose-derived central moieties and analogues containing the carbocyclic skeleton of the natural product aristeromycin were synthesized and evaluated to investigate the molecular recognition properties of the ribose binding site in the enzyme. Key synthetic intermediates in the ribose-derived series were obtained by deoxygenative [1,2]-hydride shift rearrangement of adenosine derivatives; highlights in the synthesis of carbocyclic aristeromycin analogues include a diastereoselective cyclopropanation step and nucleobase introduction with a modified Mitsunobu protocol. In vitro biological evaluation and kinetic studies revealed dramatic effects of the ribose modification on binding affinity: 3'-deoxygenation of the ribose gave potent inhibitors (IC50 values in the nanomolar range), which stands in sharp contrast to the remarkable decrease in potency observed for 2'-deoxy derivatives (IC50 values in the micromolar range). Aminodeoxy analogues were only weakly active, whereas the change of the tetrahydrofuran skeleton to a carbocycle unexpectedly led to a complete loss of biological activity. These results confirm that the ribose structural unit of the bisubstrate inhibitors of COMT is a key element of molecular recognition and that modifications thereof are delicate and may lead to surprises.

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.