Levodopa-induced dyskinesias

Once-daily transdermal rivastigmine in the treatment of Alzheimer's disease

During the past decade, transdermal delivery systems (TDS) have become increasingly important for treating neurologic and psychiatric disorders. The rivastigmine patch was the first patch to be approved to treat Alzheimer’s disease (AD). The 9.5 mg/24 h patch has equal efficacy to the capsules and reduces gastrointestinal adverse events, such as nausea and vomiting, by two-thirds. This treatment is well tolerated by patients because drug delivery is even and continuous, reducing fluctuation in drug plasma level, and attenuating the development of centrally mediated cholinergic side effects. Furthermore, once-a-day application of the patch enables an easy treatment schedule, ease of handling, infrequent skin irritations, and a patient- and caregiver-friendly mode of administration. Improved compliance with a subsequent drug administration may contribute to better clinical efficacy, reduce caregiver burden, result in a slower rate of institutionalization, and lead to a decrease in healthcare and medical costs. Because of these advantages, the rivastigmine patch has enabled great progress in the treatment of AD, and represents an excellent alternative to the orally administered cholinesterase inhibitors.

Evaluation of D2 and D3 dopamine receptor selective compounds on L-dopa-dependent abnormal involuntary movements in rats

A panel of novel D2 and D3 dopamine receptor selective antagonists, partial agonists and full agonists have been evaluated for the ability to attenuate L-dopa-associated abnormal involuntary movements (AIMs) in 6-hydroxydopamine (6-OHDA) unilaterally lesioned male Sprague Dawley rats, which is an animal model of L-dopa-induced dyskinesia (LID). LID is often observed in patients with Parkinson's Disease following chronic treatment with L-dopa. The intrinsic activity of these dopaminergic compounds was determined using a forskolin-dependent adenylyl cyclase inhibition assay with transfected HEK 293 cells expressing either the human D2Long or D3 dopamine receptor subtype. For the initial experiments the 5-HT1A receptor selective partial agonist buspirone was used to verify our ability to quantitate changes in total AIMs and AIMs minus locomotor scores. Two D2 dopamine receptor selective antagonists, SV 156 and SV 293, were evaluated and found to minimally attenuate AIM scores in these animals. Four members of our WC series of D3 dopamine receptor selective compounds of varying intrinsic activity at the D3 dopamine receptor subtype, WC 10, WC 21, WC 26 and WC 44, were also evaluated and found to attenuate AIM scores in a dose dependent manner. The in vivo efficacy of the compounds increased when they were administered simultaneously with L-dopa, as compared to when the compounds were administered 60 min prior to the L-dopa/benserazide. It was also found that the D3 receptor antagonist WC 10 could inhibit the involuntary movements after they had achieved maximum intensity. Unlike the D1-like dopamine receptor selective agonist SKF 81297 and the D2-like dopamine receptor agonist bromocriptine which can precipitate abnormal involuntary movements in these unilaterally lesioned animals, abnormal involuntary movements were not observed after administration of our D3 receptor selective agonist WC 44. In addition, we evaluated the effect of these four D3 dopamine receptor selective compounds for their effect on a) spontaneous locomotion and b) coordination and agility using a rotarod apparatus. We also used a cylinder test to assess the effect of L-dopa on spontaneous and independent use of each of the rat's forelimbs in the presence or absence of test compound. The results of these studies suggest that substituted phenylpiperazine D3 dopamine receptor selective compounds are potential pharmacotherapeutic agents for the treatment of L-dopa-associated dyskinesia in patients with Parkinson's Disease.

Enhancing aromatic L-amino acid decarboxylase activity: implications for L-DOPA treatment in Parkinson's disease

Aromatic L-amino acid decarboxylase (AAAD) is an essential enzyme for the formation of catecholamines, indolamines, and trace amines. Moreover, it is a required enzyme for converting L-DOPA to dopamine when treating patients with Parkinson's disease (PD). There is now substantial evidence that the activity of AAAD in striatum is regulated by activation and induction, and second messengers play a role. Enzyme activity can be modulated by drugs acting on a number of neurotransmitter receptors including dopamine (D1-4), glutamate (NMDA), serotonin (5-HT(1A), 5-HT(2A)) and nicotinic acetylcholine receptors. Generally, antagonists enhance AAAD activity; while, agonists may diminish it. Enhancement of AAAD activity is functional, as the formation of dopamine from exogenous L-DOPA mirrors activity. Following a lesion of nigrostriatal dopaminergic neurons, AAAD in striatum responds more robustly to pharmacological manipulations, and this is true for the decarboxylation of exogenous L-DOPA as well. We review the evidence for parallel modulation of AAAD activity and L-DOPA decarboxylation and propose that this knowledge can be exploited to optimize the formation of dopamine from exogenous L-DOPA. This information can be used as a blue print for the design of novel L-DOPA treatment adjuvants to benefit patients with PD.

Adenosine A2A receptors and Parkinson's disease

The drug treatment of Parkinson's disease (PD) is accompanied by a loss of drug efficacy, the onset of motor complications, lack of effect on non-motor symptoms, and a failure to modify disease progression. As a consequence, novel approaches to therapy are sought, and adenosine A(2A) receptors (A(2A)ARs) provide a viable target. A(2A)ARs are highly localized to the basal ganglia and specifically to the indirect output pathway, which is highly important in the control of voluntary movement. A(2A)AR antagonists can modulate gamma-aminobutyric acid (GABA) and glutamate release in basal ganglia and other key neurotransmitters that modulate motor activity. In both rodent and primate models of PD, A(2A)AR antagonists produce alterations in motor behavior, either alone or in combination with dopaminergic drugs, which suggest that they will be effective in the symptomatic treatment of PD. In clinical trials, the A(2A)AR antagonist istradefylline reduces "off" time in patients with PD receiving optimal dopaminergic therapy. However, these effects have proven difficult to demonstrate on a consistent basis, and further clinical trials are required to establish the clinical utility of this drug class. Based on preclinical studies, A(2A)AR antagonists may also be neuroprotective and have utility in the treatment of neuropsychiatric disorders. We are only now starting to explore the range of potential uses of A(2A)AR antagonists in central nervous system disorders, and their full utility is still to be uncovered.

Levodopa-induced dyskinesias and their management

This paper reviews the epidemiology, pathophysiology, clinical features and rationale for managing dyskinesias associated with Parkinson's disease. These are a common clinical problem occurring in up to 90% of patients and more frequently affect those with early-onset. Dyskinesias have a negative impact on quality of life and are an important cause of disability. Their precise etiology is still poorly understood, although it is recognized that dopaminergic pre-synaptic and post-synaptic mechanisms are involved together with extra-dopaminergic factors. The phenomenology of dyskinesias encompasses a variable mixture of two prevalent features: dystonia and chorea. We have studied their time course following a single acute levodopa challenge and have found that dystonia occurs throughout the duration of the on period, whereas choreiform movements occur only at the peak of therapeutic dopaminergic motor responses. This allows a schematic relationship to be drawn between a short duration motor response and the occurrence of dystonia and chorea. There is currently no satisfactory treatment for dyskinesias. Managing the therapeutic window does not provide an adequate solution due to the appearance of a dyskinesia threshold dose that narrows the therapeutic margin. High frequency stimulation of the subthalamic nucleus probably has some specific anti-dyskinetic action, but is limited by the small number of patients who are candidates for this treatment. Research efforts are currently focused on the development of specific anti-dyskinetic medications. Their availability will certainly change the current clinical practice and will widen again the therapeutic window of dopaminergic medications that has now become too narrow.

The nociceptin/orphanin FQ (NOP) receptor antagonist J-113397 enhances the effects of levodopa in the MPTP-lesioned nonhuman primate model of Parkinson's disease

The anti-parkinsonian and levodopa-sparing potential of the nociceptin/orphanin FQ receptor (NOP) antagonist J-113397 has been demonstrated in rodent models of Parkinson's disease. Here, we describe the levodopa-sparing potential of J-113397 in MPTP-lesioned marmosets. Coadministration of J-113397 (30 mg/kg) with a sub-therapeutic dose of levodopa (12.5 mg/kg) produced an anti-parkinsonian action equivalent to that of a therapeutic dose of levodopa. However, these effects were accompanied by an equivalent level of dyskinesia. The actions of NOP antagonists seen in rodents translate to nonhuman primates. However, the present study raises the possibility that these levodopa-sparing benefits may be offset by a propensity to exacerbate dyskinesia.

NMDA receptors in clinical neurology: excitatory times ahead

Since the N-methyl-D-aspartate receptor (NMDAR) subunits were cloned less than two decades ago, a substantial amount of research has been invested into understanding their physiological function in the healthy CNS. Research has also been directed at their pathological roles in various neurological diseases, including disorders resulting from acute excitotoxic insults (eg, ischaemic stroke, traumatic brain injury), diseases due to chronic neurodegeneration (eg, Alzheimer's, Parkinson's, and Huntington's diseases and amyotrophic lateral sclerosis), disorders arising from sensitisation of neurons (eg, epilepsy, neuropathic pain), and neurodevelopmental disorders associated with NMDAR hypofunction (eg, schizophrenia). Selective NMDAR antagonists have not produced positive results in clinical trials. However, there are other NMDAR-targeted therapies used in current practice that are effective for treating some neurological disorders. In this Review, we describe the evidence for the use of these therapies and provide an overview of drugs being investigated in clinical trials. We also discuss new NMDAR-targeted strategies in clinical neurology.

Lowered cAMP and cGMP signalling in the brain during levodopa-induced dyskinesias in hemiparkinsonian rats: new aspects in the pathogenetic mechanisms

Dysregulation of dopamine receptors is thought to underlie levodopa-induced dyskinesias in experimental models of Parkinson's disease. It is unknown whether an imbalance of the second messengers, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), is involved in the alterations of levodopa/dopamine signal transduction. We examined cAMP and cGMP signalling in the interconnected cortico-striatal-pallidal loop at the peak of levodopa-induced dyskinesias in rats with 6-hydroxydopamine lesions in the substantia nigra. In addition, we examined the role of phosphodiesterase (PDE) and the rate of cAMP and cGMP degradation on the severity of levodopa-induced dyskinesias in animals pretreated with PDE inhibitor, zaprinast. Unilateral lesion of substantia nigra led to an increase in cAMP but a decrease in cGMP levels in the ipsilateral basal ganglia. After chronic levodopa treatment, cAMP and cGMP were differentially regulated in eukinetic animals: the cAMP level increased in the cortex and striatum but decreased in the globus pallidus of both hemispheres, whereas the cGMP decreased below baseline levels in the contralateral cortico-striatal-pallidal regions. In dyskinetic animals chronic levodopa treatment led to an absolute decrease in cAMP and cGMP levels in cortico-striatal-pallidal regions of both hemispheres. Pretreatment with zaprinast reduced the severity of levodopa-induced dyskinesias, and partly prevented the decrease in cyclic nucleotides compared with pretreatment with saline-levodopa. In conclusion, using a rat model of hemiparkinsonism, we observed a significant reduction in the levels of cyclic nucleotides in both hemispheres at the peak of levodopa-induced dyskinesias. We propose that such a decrease in cyclic nucleotides may partly result from increased catabolism through PDE overactivity.

Continuous and intermittent nicotine treatment reduces L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesias in a rat model of Parkinson's disease

The development of abnormal involuntary movements (AIMs) or dyskinesias is a serious complication of L-DOPA [L-3,4-dihydroxyphenylalanine] therapy for Parkinson's disease. Our previous work had shown that intermittent nicotine dosing reduced L-DOPA-induced dyskinetic-like movements in nonhuman primates. A readily available nicotine formulation is the nicotine patch, which provides a constant source of nicotine. However, constant nicotine administration more readily desensitizes nicotinic receptors, to possibly yield alternate behavioral outcomes. Therefore, we investigated whether constant nicotine administration reduced L-DOPA-induced AIMs in a rat parkinsonian model, with results compared with those with intermittent nicotine dosing. Rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion were exposed to either intermittent (drinking water) or constant (minipump) nicotine for > or = 2 weeks at doses that yielded plasma levels of the nicotine metabolite cotinine similar to those in smokers. The rats were next treated with L-DOPA/benserazide (8 or 12 mg/kg/15 mg/kg) for > or = 3 weeks to allow for the development of AIMs, with nicotine treatment continued. Both modes of nicotine administration resulted in > or = 50% decline in L-DOPA-induced AIMs. Nicotine treatment also significantly reduced AIMs in L-DOPA-primed rats using either dosing regimen, whereas nicotine removal led to an increase in AIMs. There was no effect of nicotine on various measures of motor performance in 6-OHDA-lesioned rats. In summary, nicotine provided either via the drinking water or minipump reduced L-DOPA-induced AIMs in a rat model of Parkinson's disease. These results suggest that either intermittent or constant nicotine treatment may be useful in the treatment of L-DOPA-induced dyskinesias in patients with Parkinson's disease.

Placebo influences on dyskinesia in Parkinson's disease

Clinical features that are prognostic indicators of placebo response among dyskinetic Parkinson's disease patients were determined. Placebo-associated improvements occur in Parkinsonism, but responses in dyskinesia have not been studied. Placebo data from two multicenter studies with identical design comparing sarizotan to placebo for treating dyskinesia were accessed. Sarizotan (2 mg/day) failed to improve dyskinesia compared with placebo, but both treatments improved dyskinesia compared with baseline. Stepwise regression identified baseline characteristics that influenced dyskinesia response to placebo, and these factors were entered into a logistic regression model to quantify their influence on placebo-related dyskinesia improvements and worsening. Because placebo-associated improvements in Parkinsonism have been attributed to heightened dopaminergic activity, we also examined the association between changes in Parkinsonism and dyskinesia. Four hundred eighty-four subjects received placebo treatment; 178 met criteria for placebo-associated dyskinesia improvement and 37 for dyskinesia worsening. Older age, lower baseline Parkinsonism score, and lower total daily levodopa doses were associated with placebo-associated improvement, whereas lower baseline dyskinesia score was associated with placebo-associated worsening. Placebo-associated dyskinesia changes were not correlated with Parkinsonism changes, and all effects in the sarizotan group were statistically explained by the placebo-effect regression model. Dyskinesias are affected by placebo treatment. The absence of correlation between placebo-induced changes in dyskinesia and Parkinsonism argues against a dopaminergic activation mechanism to explain placebo-associated improvements in dyskinesia. The magnitude and variance of placebo-related changes and the factors that influence them can be helpful in the design of future clinical trials of antidyskinetic agents.

Commentary: Dopaminergic dysfunction in DYT1 dystonia

A three-base-pair deletion in the torsinA gene leads to generalized torsion dystonia (DYT1) in humans, an often devastating movement disorder in which voluntary movements are disrupted by sustained muscle spasms and abnormal limb posturing. In a recent issue of Experimental Neurology, Zhao et al. (2008) have provided a thorough behavioral, anatomic, and biochemical characterization of a mouse line that over-expresses human mutant torsinA, with particular emphasis on the possible role of dopaminergic dysfunction in these animals. This commentary provides an overview of the clinical and genetic features of the human disease and of the available transgenic mouse models for DYT1 dystonia, and discusses the evidence favoring the role of dopamine in the clinical manifestations of the disease.

Parkinson's disease and resistive exercise: rationale, review, and recommendations

Individuals with Parkinson's disease (PD) are not only burdened with disease-specific symptoms (i.e., bradykinesia, rigidity, and tremor), but are also confronted with age-associated progressive loss of physical function, perhaps to a greater extent than neurologically normal adults. Suggestions for the inclusion of resistive exercise into treatment to attenuate these symptoms were made over 10 years ago, yet very few well controlled investigations are available. The objective of this review is to establish a clear rationale for the efficacy of resistance training in individuals with PD. Specifically, we highlight musculoskeletal weakness and its relationship to function as well as potential training-induced adaptive alterations in the neuromuscular system. We also review the few resistance training interventions currently available, but limit this review to those investigations that provide a quantitative exercise prescription. Finally, we recommend future lines of inquiry warranting further attention and call to question the rationale behind current exercise prescriptions. The absence of reports contraindicating resistive exercise, the potential for positive adaptation, and the noted benefits of resistance training in other populations may provide support for its inclusion into a treatment approach to PD.

PYM50028, a novel, orally active, nonpeptide neurotrophic factor inducer, prevents and reverses neuronal damage induced by MPP+ in mesencephalic neurons and by MPTP in a mouse model of Parkinson's disease

Many experimental data support the enhancement of neurotrophic factors as a means to modify neurodegeneration in Parkinson's disease. However, the translation of this to the clinic has proven problematic. This is likely due to the complex nature of the surgical gene delivery and cell-based approaches adopted to deliver proteinaceous neurotrophic factors to targets within the central nervous system. We investigated the ability of a novel, orally active, nonpeptide neurotrophic factor inducer, PYM50028 (Cogane), to restore dopaminergic function after 1-methyl-4-phenylpyridinium (MPP(+)) -induced damage to mesencephalic neurons in vitro and in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) -lesioned mice. In rat mesencephalic neurons, administration of PYM50028, either before or after MPP(+), significantly prevented and reversed both MPP(+)-induced neuronal atrophy and cell loss. These effects were potent and of a magnitude equivalent to those achieved by a combination of brain-derived neurotrophic factor (BDNF) and glial-derived neurotrophic factor (GDNF). Oral administration of PYM50028 (10 mg/kg/day for 60 days) to MPTP-lesioned mice, commencing after a striatal impairment was evident, resulted in a significant elevation of striatal GDNF (297%) and BDNF (511%), and attenuated the loss of striatal dopaminergic transporter levels and dopaminergic neurons in the substantia nigra. PYM50028 did not inhibit monoamine oxidase B in vitro, nor did it alter brain levels of MPP(+) in vivo. PYM50028 has neuroprotective and neurorestorative potential and is in clinical development for the treatment of neurodegenerative disorders, including Parkinson's disease.