A randomized, controlled trial of remacemide for motor fluctuations in Parkinson's disease

Targeting Striatal Glutamate and Phosphodiesterases to Control L-DOPA-Induced Dyskinesia

A large body of work during the past several decades has been focused on therapeutic strategies to control L-DOPA-induced dyskinesias (LIDs), common motor complications of long-term L-DOPA therapy in Parkinson's disease (PD). Yet, LIDs remain a clinical challenge for the management of patients with advanced disease. Glutamatergic dysregulation of striatal projection neurons (SPNs) appears to be a key contributor to altered motor responses to L-DOPA. Targeting striatal hyperactivity at the glutamatergic neurotransmission level led to significant preclinical and clinical trials of a variety of antiglutamatergic agents. In fact, the only FDA-approved treatment for LIDs is amantadine, a drug with NMDAR antagonistic actions. Still, novel agents with improved pharmacological profiles are needed for LID therapy. Recently other therapeutic targets to reduce dysregulated SPN activity at the signal transduction level have emerged. In particular, mechanisms regulating the levels of cyclic nucleotides play a major role in the transduction of dopamine signals in SPNs. The phosphodiesterases (PDEs), a large family of enzymes that degrade cyclic nucleotides in a specific manner, are of special interest. We will review the research for antiglutamatergic and PDE inhibition strategies in view of the future development of novel LID therapies.

What Do Randomized Controlled Trials Inform Us About Potential Disease-Modifying Strategies for Parkinson's Disease?

Research advances have shed new insight into cellular pathways contributing to PD pathogenesis and offer increasingly compelling therapeutic targets. In this review, we made a broad survey of the published literature that report possible disease-modifying effects on PD. While there are many studies that demonstrate benefits for various therapies for PD in animal and human studies, we confined our search to human "randomised controlled trials" and with the key words "neuroprotection" or "disease-modifying". It is hoped that through studying the results of these trials, we might clarify possible mechanisms that underlie idiopathic PD. This contrasts with studying the effect of pathophysiology of familial PD, which could be carried out by gene knockouts and animal models. Randomised controlled trials indicate promising effects of MAO-B inhibitors, dopamine agonists, NMDA receptor antagonists, metabotropic glutamate receptor antagonists, therapies related to improving glucose utilization and energy production, therapies related to reduction of excitotoxicity and oxidative stress, statin use, therapies related to iron chelation, therapies related to the use of phytochemicals, and therapies related to physical exercise and brain reward pathway on slowing PD progression. Cumulatively, these approaches fall into two categories: direct enhancement of dopaminergic signalling, and reduction of neurodegeneration. Overlaps between the two categories result in challenges in distinguishing between symptomatic versus disease-modifying effects with current clinical trial designs. Nevertheless, a broad-based approach allows us to consider all possible therapeutic avenues which may be neuroprotective. While the traditional standard of care focuses on symptomatic management with dopaminergic drugs, more recent approaches suggest ways to preserve dopaminergic neurons by attenuating excitotoxicity and oxidative stress.

Glutamate modulation for the treatment of levodopa induced dyskinesia: a brief review of the drugs tested in the clinic

Levodopa is the standard treatment for Parkinson's disease, but its use is marred by the emergence of dyskinesia, for which treatment options remain limited. Here, we review the glutamatergic modulators that were assessed for their antidyskinetic potential in clinical trials, including N-methyl-D-aspartate (NMDA) antagonists, agonists at the glycine-binding site on NMDA receptors, metabotropic glutamate (mGlu) 4 agonists, mGlu₅ antagonists, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) antagonists and glutamate release inhibitors. Several agents that were investigated are not selective for their targets, raising uncertainty about the extent to which glutamatergic modulation contributed to their effects. Except for amantadine, the use of glutamatergic modulators for the treatment of dyskinesia in Parkinson's disease remains largely investigational, with promising results obtained with mGlu₅ negative allosteric modulation.

Predictive Value of Parkinsonian Primates in Pharmacologic Studies: A Comparison between the Macaque, Marmoset, and Squirrel Monkey

The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned primate is the gold-standard animal model of Parkinson disease (PD) and has been used to assess the effectiveness of experimental drugs on dyskinesia, parkinsonism, and psychosis. Three species have been used in most studies-the macaque, marmoset, and squirrel monkey-the last much less so than the first two species; however, the predictive value of each species at forecasting clinical efficacy, or lack thereof, is poorly documented. Here, we have reviewed all the published literature detailing pharmacologic studies that assessed the effects of experimental drugs on dyskinesia, parkinsonism, and psychosis in each of these species and have calculated their predictive value of success and failure at the clinical level. We found that, for dyskinesia, the macaque has a positive predictive value of 87.5% and a false-positive rate of 38.1%, whereas the marmoset has a positive predictive value of 76.9% and a false-positive rate of 15.6%. For parkinsonism, the macaque has a positive predictive value of 68.2% and a false-positive rate of 44.4%, whereas the marmoset has a positive predictive value of 86.9% and a false-positive rate of 41.7%. No drug that alleviates psychosis in the clinic has shown efficacy at doing so in the macaque, whereas the marmoset has 100% positive predictive value. The small number of studies conducted in the squirrel monkey precluded us from calculating its predictive efficacy. We hope our results will help in the design of pharmacologic experiments and will facilitate the drug discovery and development process in PD.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Treatment of Parkinson's disease: what's in the non-dopaminergic pipeline?

Dopamine depletion resulting from degeneration of nigrostriatal dopaminergic neurons is the primary neurochemical basis of the motor symptoms of Parkinson's disease (PD). While dopaminergic replacement strategies are effective in ameliorating these symptoms early in the disease process, more advanced stages of PD are associated with the development of treatment-related motor complications and dopamine-resistant symptoms. Other neurotransmitter and neuromodulator systems are expressed in the basal ganglia and contribute to the extrapyramidal refinement of motor function. Furthermore, neuropathological studies suggest that they are also affected by the neurodegenerative process. These non-dopaminergic systems provide potential targets for treatment of motor fluctuations, levodopa-induced dyskinesias, and difficulty with gait and balance. This review summarizes recent advances in the clinical development of novel pharmacological approaches for treatment of PD motor symptoms. Although the non-dopaminergic pipeline has been slow to yield new drugs, further development will likely result in improved treatments for PD symptoms that are induced by or resistant to dopamine replacement.

The treatment of tremor

Tremor is a hyperkinetic movement disorder characterized by rhythmic oscillations of one or more body parts. It can be disabling and may impair quality of life. Various etiological subtypes of tremor are recognized, with essential tremor (ET) and Parkinsonian tremor being the most common. Here we review the current literature on tremor treatment regarding ET and head and voice tremor, as well as dystonic tremor, orthostatic tremor, tremor due to multiple sclerosis (MS) or lesions in the brainstem or thalamus, neuropathic tremor, and functional (psychogenic) tremor, and summarize main findings. Most studies are available for ET and only few studies specifically focused on other tremor forms. Controlled trials outside ET are rare and hence most of the recommendations are based on a low level of evidence. For ET, propranolol and primidone are considered drugs of first choice with a mean effect size of approximately 50 % tremor reduction. The efficacy of topiramate is also supported by a large double-blind placebo-controlled trial, while other drugs have less supporting evidence. With a mean effect size of about 90 % deep brain stimulation in the nucleus ventralis intermedius or the subthalamic nucleus may be the most potent treatment; however, there are no controlled trials and it is reserved for severely affected patients. Dystonic limb tremor may respond to anticholinergics. Botulinum toxin improves head and voice tremor. Gabapentin and clonazepam are often recommended for orthostatic tremor. MS tremor responds only poorly to drug treatment. For patients with severe MS tremor, thalamic deep brain stimulation has been recommended. Patients with functional tremor may benefit from antidepressants and are best be treated in a multidisciplinary setting. Several tremor syndromes can already be treated with success. But new drugs specifically designed for tremor treatment are needed. ET is most likely covering different entities and their delineation may also improve treatment. Modern study designs and long-term studies are needed.

Ion channels as drug targets in central nervous system disorders

Ion channel targeted drugs have always been related with either the central nervous system (CNS), the peripheral nervous system, or the cardiovascular system. Within the CNS, basic indications of drugs are: sleep disorders, anxiety, epilepsy, pain, etc. However, traditional channel blockers have multiple adverse events, mainly due to low specificity of mechanism of action. Lately, novel ion channel subtypes have been discovered, which gives premises to drug discovery process led towards specific channel subtypes. An example is Na(+) channels, whose subtypes 1.3 and 1.7-1.9 are responsible for pain, and 1.1 and 1.2 - for epilepsy. Moreover, new drug candidates have been recognized. This review is focusing on ion channels subtypes, which play a significant role in current drug discovery and development process. The knowledge on channel subtypes has developed rapidly, giving new nomenclatures of ion channels. For example, Ca(2+)s channels are not any more divided to T, L, N, P/Q, and R, but they are described as Ca(v)1.1-Ca(v)3.3, with even newer nomenclature α1A-α1I and α1S. Moreover, new channels such as P2X1-P2X7, as well as TRPA1-TRPV1 have been discovered, giving premises for new types of analgesic drugs.

Metabotropic glutamate receptors for Parkinson's disease therapy

Excessive glutamatergic signalling within the basal ganglia is implicated in the progression of Parkinson's disease (PD) and inthe emergence of dyskinesia associated with long-term treatment with L-DOPA. There is considerable research focus on the discovery and development of compounds that modulate glutamatergic signalling via glutamate receptors, as treatments for PD and L-DOPA-induced dyskinesia (LID). Although initial preclinical studies with ionotropic glutamate receptor antagonists showed antiparkinsonian and antidyskinetic activity, their clinical use was limited due to psychiatric adverse effects, with the exception of amantadine, a weak N-methyl-d-aspartate (NMDA) antagonist, currently used to reduce dyskinesia in PD patients. Metabotropic receptor (mGlu receptor) modulators were considered to have a more favourable side-effect profile, and several agents have been studied in preclinical models of PD. The most promising results have been seen clinically with selective antagonists of mGlu5 receptor and preclinically with selective positive allosteric modulators of mGlu4 receptor. The growing understanding of glutamate receptor crosstalk also raises the possibility of more precise modulation of glutamatergic transmission, which may lead to the development of more effective agents for PD.

Target- and mechanism-based therapeutics for neurodegenerative diseases: strength in numbers

The development of new therapeutics for the treatment of neurodegenerative pathophysiologies currently stands at a crossroads. This presents an opportunity to transition future drug discovery efforts to target disease modification, an area in which much still remains unknown. In this Perspective we examine recent progress in the areas of neurodegenerative drug discovery, focusing on some of the most common targets and mechanisms: N-methyl-d-aspartic acid (NMDA) receptors, voltage gated calcium channels (VGCCs), neuronal nitric oxide synthase (nNOS), oxidative stress from reactive oxygen species, and protein aggregation. These represent the key players identified in neurodegeneration and are part of a complex, intertwined signaling cascade. The synergistic delivery of two or more compounds directed against these targets, along with the design of small molecules with multiple modes of action, should be explored in pursuit of more effective clinical treatments for neurodegenerative diseases.

Treatment of motor symptoms in advanced Parkinson's disease: a practical approach

Patients with advanced Parkinson's disease (PD) are known to develop motor complications after a few years of levodopa (L-dopa) therapy. Motor fluctuations develop with increasing severity of the disease, owing to loss of dopaminergic neurons and loss of the buffering capacity of the neurons to fluctuating dopamine levels. Dyskinesias develop as a result of pulsatile stimulation of the receptors and alterations in neuronal firing patterns. L-dopa remains the gold standard medication for the treatment of patients with advanced PD. However, once motor complications on L-dopa therapy emerge, clinicians may add on other classes of antiparkinsonian drugs such as dopamine agonists, catechol-O-methyl transferase inhibitors (COMTIs) or monoamine oxidase type B inhibitors (MAOBIs). The individualisation of the treatment seems to be the key for the best approach of advanced PD patients. The present review provides the most important current clinical data in the pharmacological treatment of motor symptoms in advanced PD and provides the clinician a simple algorithm in order to determine the best suitable treatment to advanced parkinsonian patients.

Genome-wide gene-environment study identifies glutamate receptor gene GRIN2A as a Parkinson's disease modifier gene via interaction with coffee

Our aim was to identify genes that influence the inverse association of coffee with the risk of developing Parkinson's disease (PD). We used genome-wide genotype data and lifetime caffeinated-coffee-consumption data on 1,458 persons with PD and 931 without PD from the NeuroGenetics Research Consortium (NGRC), and we performed a genome-wide association and interaction study (GWAIS), testing each SNP's main-effect plus its interaction with coffee, adjusting for sex, age, and two principal components. We then stratified subjects as heavy or light coffee-drinkers and performed genome-wide association study (GWAS) in each group. We replicated the most significant SNP. Finally, we imputed the NGRC dataset, increasing genomic coverage to examine the region of interest in detail. The primary analyses (GWAIS, GWAS, Replication) were performed using genotyped data. In GWAIS, the most significant signal came from rs4998386 and the neighboring SNPs in GRIN2A. GRIN2A encodes an NMDA-glutamate-receptor subunit and regulates excitatory neurotransmission in the brain. Achieving P(2df) = 10(-6), GRIN2A surpassed all known PD susceptibility genes in significance in the GWAIS. In stratified GWAS, the GRIN2A signal was present in heavy coffee-drinkers (OR = 0.43; P = 6×10(-7)) but not in light coffee-drinkers. The a priori Replication hypothesis that "Among heavy coffee-drinkers, rs4998386_T carriers have lower PD risk than rs4998386_CC carriers" was confirmed: OR(Replication) = 0.59, P(Replication) = 10(-3); OR(Pooled) = 0.51, P(Pooled) = 7×10(-8). Compared to light coffee-drinkers with rs4998386_CC genotype, heavy coffee-drinkers with rs4998386_CC genotype had 18% lower risk (P = 3×10(-3)), whereas heavy coffee-drinkers with rs4998386_TC genotype had 59% lower risk (P = 6×10(-13)). Imputation revealed a block of SNPs that achieved P(2df)<5×10(-8) in GWAIS, and OR = 0.41, P = 3×10(-8) in heavy coffee-drinkers. This study is proof of concept that inclusion of environmental factors can help identify genes that are missed in GWAS. Both adenosine antagonists (caffeine-like) and glutamate antagonists (GRIN2A-related) are being tested in clinical trials for treatment of PD. GRIN2A may be a useful pharmacogenetic marker for subdividing individuals in clinical trials to determine which medications might work best for which patients.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Therapy for dyskinesias in Parkinson’s disease patients

Dyskinesia hampers the quality of life for most Parkinson’s disease patients following several years of therapy. However, the severity of L-Dopa-induced dyskinesia (LID) varies between patients, being quite tolerable in late-onset patients. Understanding the pathogenesis of LID has contributed to the development of a set of therapeutic strategies, including the choice, in early stages, of the least pulsatile regimen of dopamine-receptor activation. In cases where LIDs are already disabling, there is only a limited number of options: the optimization of ongoing DOPA-centered treatment, the utilization of glutamate antagonists and the exploration of the benefits of antipsychotic agents. More radical solutions are provided by deep brain stimulation in the subthalamic nucleus (or internal pallidus). This approach has proved efficacious in reducing LID, largely because it allows a reduction in dopaminergic daily doses. Stereotactic neurosurgery has fuelled several lines of investigation regarding the crosstalk between the basal ganglia and motor cortex. Here, we will present interesting evidence highlighting the potential for repetitive transcranial stimulation in reducing the occurrence of LID. The future may disclose important new avenues for the treatment of LIDs, given the current development of promising agents that might target different facets of dyskinesia, such as the impairment of striatal plasticity and non-Dopaminergic contributors such as adenosine, nitric oxide and the nucleotide cascade.

Potential future neuroprotective therapies for neurodegenerative disorders and stroke

The cellular mechanisms underlying neuronal loss and neurodegeneration have been an area of interest in the last decade. Although neurodegenerative diseases such as Alzheimer disease, Parkinson disease, and Huntington disease each have distinct clinical symptoms and pathologies, they all share common mechanisms such as protein aggregation, oxidative injury, inflammation, apoptosis, and mitochondrial injury that contribute to neuronal loss. Although cerebrovascular disease has different causes from the neurodegenerative disorders, many of the same common disease mechanisms come into play following a stroke. Novel therapies that target each of these mechanisms may be effective in decreasing the risk of disease, abating symptoms, or slowing down their progression. Although most of these therapies are experimental, and require further investigation, a few seem to offer promise.

Glutamate receptors as therapeutic targets for Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor symptoms including tremor and bradykinesia. The primary pathophysiology underlying PD is the degeneration of dopaminergic neurons of the substantia nigra pars compacta. Loss of these neurons causes pathological changes in neurotransmission in the basal ganglia motor circuit. The ability of ionotropic and metabotropic glutamate receptors to modulate neurotransmission throughout the basal ganglia suggests that these receptors may be targets for reversing the effects of altered neurotransmission in PD. Studies in animal models suggest that modulating the activity of these receptors may alleviate the primary motor symptoms of PD as well as side effects induced by dopamine replacement therapy. Moreover, glutamate receptor ligands may slow disease progression by delaying progressive dopamine neuron degeneration. Antagonists of NMDA receptors have shown promise in reversing motor symptoms, levodopa-induced dyskinesias, and neurodegeneration in preclinical PD models. The effects of drugs targeting AMPA receptors are more complex; while antagonists of these receptors exhibit utility in the treatment of levodopa-induced dyskinesias, AMPA receptor potentiators show promise for neuroprotection. Pharmacological modulation of metabotropic glutamate receptors (mGluRs) may hold even more promise for PD treatment due to the ability of mGluRs to fine-tune neurotransmission. Antagonists of mGluR5, as well as activators of group II mGluRs and mGluR4, have shown promise in several animal models of PD. These drugs reverse motor deficits in addition to providing protection against neurodegeneration. Glutamate receptors therefore represent exciting targets for the development of novel pharmacological therapies for PD.

Minority enrollment in Parkinson's disease clinical trials

Under-representation of minorities in clinical trials limits access to information relevant to all segments of the population. We assessed the enrollment of minority subjects with Parkinson's disease (PD) into clinical trials. We searched PubMed for published studies of PD trials conducted in the US over the past 20 years and found that only 41 reported racial/ethnic participation (17%). In those trials reporting race/ethnicity, 8% of subjects were non-white, compared to 20% of the non-white US population over age 60. Results of this study identified the need for better reporting of racial composition in clinical trials and for the enrollment of more minority participants in research studies.

Pharmacological treatment of Parkinson's disease: life beyond dopamine D2/D3 receptors?

Parkinson's disease (PD) is a multisystemic disorder in which several neurotransmitters other than dopamine are affected. Drugs acting on non-dopaminergic systems are envisaged as promising agents to treat PD and levodopa-induced dyskinesias (LID). However, compounds targeting glutamate, adenosine, noradrenaline, 5-hydroxytryptamine, cannabinoid, and opioid transmitter systems have been assessed in human studies showing negative, inconsistent or unsatisfactory results. Most of these drugs had been tested previously in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned monkeys, as well as in the classic 6-hydroxydopamine-lesioned rat model. These failures raise several questions and concerns about the true reliability of animal studies, the adequacy of the working hypotheses and design of clinical trials, the validity of tools in current use to evaluate a particular effect, and the selectivity of the drugs used. More importantly, observed discrepancies between the results in models and patients, could challenge the validity of current ideas about the pathophysiology of parkinsonism and LID.

Pramipexole in levodopa-treated Parkinson disease patients of African, Asian, and Hispanic heritage

BACKGROUND

Little is known regarding the effects of antiparkinsonian drugs in US racial or ethnic minorities.

OBJECTIVE

: To evaluate the safety, tolerability, and efficacy of adjunctive pramipexole in Parkinson disease (PD) patients of African, Asian, or Hispanic heritage stably treated with levodopa.

DESIGN

Multicenter, parallel-group, double-blind, randomized, placebo-controlled trial.

SETTING

: Seventeen Parkinson Study Group sites in the United States and Puerto Rico.

PATIENTS

One hundred forty-four PD patients of African, Asian, or Hispanic heritage enrolled from January 1997 to August 1998 and observed until October 1998.

INTERVENTION

Subjects received pramipexole or placebo (3:1 ratio), 0.375 mg/d to a maximum tolerated dose (<or=4.5 mg/d) over a 6-week period, achieving optimum levels (0.375, 1.5, 3.0, or 4.5 mg/d) in the 4-week maintenance period.

MAIN OUTCOME MEASURE

Change in the sum of the Unified Parkinson's Disease Rating Scale parts 2 (activities of daily living) and 3 (motor) from baseline to week 10.

RESULTS

Parkinsonism improved (mean [SD] reduction in Unified Parkinson's Disease Rating Scale activities of daily living + motor score at 10 weeks, 10.27 [11.96] pramipexole vs 6.54 [13.58] placebo, P = 0.012) and was similar in each group. Adverse events occurred in 85.3% on pramipexole and 68.6% on placebo. Hallucinations and insomnia were more common on pramipexole than placebo (18 vs 0, P = 0.023, and 15 vs 0, P = 0.045, respectively).

CONCLUSIONS

Pramipexole is an effective adjunctive antiparkinsonian therapy in PD patients of African, Asian, and Hispanic heritage. Tolerability and safety overall were similar among the groups, but differences in profiles of adverse effects and tolerability were suggested.

Self-reported adherence versus pill count in Parkinson's disease: The NET-PD experience

PURPOSE

To compare the Morisky medication adherence questionnaire to pill counts as measures of adherence in the NET-PD futility clinical trials.

BACKGROUND

Like in other chronic diseases, non-adherence with medications occurs in Parkinson's disease (PD), although nonadherence has not been of significant concern in most PD clinical trials. The most common approach to assessment is to do a pill count at each visit. The simple, 4-question Morisky medication adherence questionnaire may provide an alternative approach to monitoring treatment adherence in PD.

METHODS

Adherence data from two NET-PD Phase II clinical trials enrolling a total of 413 participants were analyzed. The association between demographic and clinical characteristics and adherence was explored.

RESULTS

Ninety-percent of participants took 80% or more of the study drug. However, the Morisky medication adherence questionnaires showed 56% report high and 44% report medium adherence. Agreement between the two measures is fair (ICC = 0.40).

CONCLUSIONS

Overall adherence as assessed by pill count appears high. The Morisky medication adherence questionnaire may be useful in PD clinical trials, since it is moderately correlated to pill count and may be more sensitive to nonadherence.

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.

Neuroprotective therapy in Parkinson's disease: current status and new directions from experimental and genetic clues

Despite successful treatment of Parkinson's disease (PD) with a wide variety of symptomatic therapy, the disease continues to progress and drug-resistance symptoms become the predominant factors producing the disability of PD patients. Neuroprotective therapies have been tested, but clinically effective drugs have not been found yet. New insights gained from studies of genetic forms of PD point to the common pathogenic mechanisms that have been suspected in sporadic forms of the disease and may provide new approaches for the future neuroprotective therapies.

Novel pharmacological strategies for motor complications in Parkinson’s disease

In advanced Parkinson's disease, the combination of disease progression and levodopa therapy leads to the development of motor problems complicating the therapeutic response, known as motor response complications. The nonphysiological, pulsatile stimulation produced by most currently available dopaminergic therapies triggers a complicated series of responses resulting in the dysregulation of glutamate receptors and many other neurotransmitter systems on striatal neurons. Although a number of novel compounds that provide a more continuous dopaminergic stimulation are becoming available, no practical way to accomplish this in a truly physiological manner currently exists. Novel strategies for pharmacological intervention with the use of nondopaminergic treatments, with drugs targeting selected transmitter receptors expressed on striatal neurons appear more promising. These include NMDA or AMPA antagonists, or drugs acting on 5-hydroxytryptamine subtype 2A, alpha2-adrenergic, adenosine A2A and cannabinoid CB1 receptors. Future strategies may also target pre- and postsynaptic components that regulate firing pattern, like synaptic vesicle proteins, or nonsynaptic gap junction communication mechanisms, or drugs with actions at the signal transduction systems that modulate the phosphorylation state of NMDA receptors. These new therapeutic strategies, alone or in combination, hold the promise of providing effective control or reversal of motor response complications.

Recent failures of new potential symptomatic treatments for Parkinson's disease: causes and solutions

One major goal of current research in Parkinson's disease (PD) is the discovery of novel agents to improve symptomatic management. The object of these new treatments should be to provide effective symptom control throughout the course of the disease without the development of side effects such as motor and psychiatric complications. Results of several clinical trials of new treatment options reported in the past 2 years have shown negative or unsatisfactory results. Most of the drugs and surgical procedures used in these studies had been tested previously in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkeys as well as in the classic 6-hydroxydopamine-lesioned rat model. They raise several questions about the true reliability of animal studies, the adequacy of the working hypotheses and design of clinical trials, the validity of tools in current use to evaluate a specific effect, and the selectivity of the drugs used. All these factors may explain failure. This review focuses on pharmacological and surgical treatments tested to improve the management of patients with motor fluctuations and dyskinesias. Some of the recent trials and possible reasons for their lack of success are critically analysed. Finally, some suggestions to avoid further failures and improve results are proposed.

Glutamate receptors and Parkinson's disease: opportunities for intervention

Parkinson's disease is a debilitating neurodegenerative movement disorder that is the result of a degeneration of dopaminergic neurons in the substantia nigra pars compacta. The resulting loss of striatal dopaminergic tone is believed to underlie a series of changes in the circuitry of the basal ganglia that ultimately lead to severe motor disturbances due to excessive basal ganglia outflow. Glutamate plays a central role in the disruption of normal basal ganglia function, and it has been hypothesised that agents acting to restore normal glutamatergic function may provide therapeutic interventions that bypass the severe motor side effects associated with current dopamine replacement strategies. Analysis of the effects of glutamate receptor ligands in the basal ganglia circuit suggests that both ionotropic and metabotropic glutamate receptors could have antiparkinsonian actions. In particular, NMDA receptor antagonists that selectively target the NR2B subunit and antagonists of the metabotropic glutamate receptor mGluR5 appear to hold promise and deserve future attention.

Levodopa-induced motor complications are associated with alterations of glutamate receptors in Parkinson's disease

Glutamate receptors were studied in the brains of controls and Parkinson's disease (PD) patients, of which 10 of 14 developed motor complications (dyskinesias and/or wearing-off) following levodopa therapy. (125)I-RTI binding to the dopamine transporter and dopamine concentrations show comparable nigrostriatal denervation between the subgroups of PD patients. (3)H-Ro 25-6981 binding to the NR1/NR2B NMDA receptor was increased in the putamen of PD patients experiencing motor complications compared to those who did not (+53%) and compared to controls (+18%) whereas binding remained unchanged in the caudate nucleus. (3)H-AMPA binding was increased in the lateral putamen (+23%) of PD patients with motor complications compared to those without whereas it was decreased in the caudate nucleus of the PD patients (-16%) compared to controls. Caudate and putamen (3)H-CGP39653 binding to NR1/NR2A NMDA receptor and NR1 subunit mRNA levels measured by in situ hybridization were unchanged in subgroups of PD patients compared to controls. These findings suggest that glutamate receptor supersensitivity in the putamen plays a role in the development of motor complications (both wearing-Off and dyskinesias) following long-term levodopa therapy in PD.

Treatment options for Parkinson's disease

PURPOSE OF REVIEW

Parkinson's disease is a common neurodegenerative disorder. Clinical trials designed to assess the safety and efficacy of novel neuroprotective as well as symptomatic medical and surgical strategies are being performed to increase and enhance treatment options. The purpose of this review is to summarize these therapeutic options, emphasizing reports published in the last year.

RECENT FINDINGS

Experimental therapeutics in Parkinson's disease has focused on prevention of levodopa complications, treatment of dyskinesias associated with levodopa therapy, surgical intervention and neuroprotection.

SUMMARY

There are at least four important implications of the recent therapeutic trials: (1) the incidence of levodopa-related dyskinesias decreases as a result of initial use of dopamine agonists; (2) surgery, primarily in the form of the bilateral, high-frequency stimulation of the subthalamic nucleus, is highly effective in patients who are responsive to levodopa but experience marked motor fluctuation or other complications; (3) while neuroprotection has not yet been demonstrated with any currently used therapeutic agent, improved understanding of mechanisms of cell death will undoubtedly result in the discovery of new drugs with potential disease-modifying effects; and (4) implantation of fetal mesencephalon tissue and other grafts may be effective in younger patients with Parkinson's disease, but is associated with significant complications and remains experimental.

Neuroprotection in Parkinson's disease; a commentary

Parkinson's disease (PD) is a worldwide neurodegenerative disorder. Although the etiology has been linked to genetic and environmental factors, curative treatment remains a challenge. Several hypotheses support different pathophysiological mechanisms related to oxidative stress, glutamate-mediated neurotoxicity, mitochondrial energetic impairment and nitric oxide (NO) over-production. Moreover, apoptotic mechanisms have been identified in PD. In this way, classical drugs such as amantadine, selegiline and dopamine agonists show only a modest neuroprotective effect. New strategies with enormous potential are now under development. These include neuroprotectants and agents that might rescue dopaminergic neurons. Glutamate receptor antagonists, neurotrophins, neuroimmunophilins, adenosine A2A receptor antagonists, iron-chelators and NO modulators, as well as caspase inhibitors have evident neuroprotective properties in experimental PD models.

A double-blind, placebo-controlled and longitudinal study of riluzole in early Parkinson's disease

BACKGROUND

To the extent that excitotoxicity may play a role in the pathogenesis of certain neurodegenerative disorders, antagonists of glutamate, an excitatory neurotransmitter, should exert neuroprotective effects in these disorders, including Parkinson's disease (PD).

METHODS

Patients in early stages of PD, not previously treated with levodopa, were randomized to receive riluzole 50mg capsules orally, taken twice daily or a matching placebo. All subjects were evaluated at baseline (pre-treatment), at 1, 3 and 6 months (post-treatment), and following a 6-week washout. After the washout, all subjects were offered an enrollment in an open label, 1-year, extension study. The principal investigator (JJ), however, remained blinded to the original assignment during the entire study. The patients were assessed by the Unified Parkinson's Disease Rating Scale (UPDRS), Activities of Daily Living (ADL), Hoehn & Yahr (HY) stage, and Schwab and England (SE) ADL scale. The quantitative assessments included Movement Time (MT) and Reaction Time (RT). Additionally, the time to initiate dopaminergic therapy was assessed. Safety was determined at each visit by clinical history and examination, a panel of blood safety laboratory tests including complete blood count, chemistry profile, and liver function studies.

RESULTS

Twenty patients with a mean age of 62+/-9.02 (range: 46-73) years and mean duration of symptoms of 18+/-9.53 (range: 6-36) months were enrolled. One patient withdrew from the study because he needed more aggressive treatment of his symptoms. Analysis of the efficacy variables showed no meaningful symptomatic effect of riluzole on UPDRS score. Likewise, there was no significant change in the median HY stage, SE ADL rating, or the MT/RT. Seventeen patients (mean age 62+/-9.26) elected to continue in the open label extension study. Although the observed deterioration in UPDRS scores seemed to be more pronounced in the placebo group than in the riluzole group, the difference did not reach statistical significance. There was no statistically significant difference in the latency between enrollment and start of symptomatic therapy when patients initially treated with riluzole were compared to those initially treated with placebo (8.3 vs 9 months).

CONCLUSIONS

This pilot and extension study showed that riluzole, 100mg/day, was well tolerated in patients with early PD. No evidence of symptomatic effect of riluzole was observed. Because of the exploratory nature of the design and small size of the study, it was not possible to determine whether riluzole affected the natural history of PD. The encouraging results from our study, however, suggest that larger, longitudinal studies are warranted.