Translation of nondopaminergic treatments for levodopa-induced dyskinesia from MPTP-lesioned nonhuman primates to phase IIa clinical studies: keys to success and roads to failure

Differential effects of opioid receptor antagonism on the anti-dyskinetic and anti-parkinsonian effects of sub-anesthetic ketamine treatment in a preclinical model

Sub-anesthetic ketamine treatment has been shown to be an effective therapy for treatment-resistant depression and chronic pain. Our group has previously shown that sub-anesthetic ketamine produces acute anti-parkinsonian, and acute anti-dyskinetic effects in preclinical models of Parkinson's disease (PD). Ketamine is a multifunctional drug and exerts effects through blockade of N-methyl-d-aspartate receptors but also through interaction with the opioid system. In this report, we provide detailed pharmacokinetic rodent data on ketamine and its main metabolites following an intraperitoneal injection, and second, we explore the pharmacodynamic properties of ketamine in a rodent PD model with respect to the opioid system, using naloxone, a pan-opioid receptor antagonist, in unilateral 6-hydroxydopamine-lesioned male rats, treated with 6 mg/kg levodopa (l-DOPA) to establish a model of l-DOPA-induced dyskinesia (LID). As previously reported, we showed that ketamine (20 mg/kg) is highly efficacious in reducing LID and now report that the magnitude of this effect is resistant to naloxone (3 and 5 mg/kg). The higher naloxone dose of 5 mg/kg, however, led to an extension of the time-course of the LID, indicating that opioid receptor activation, while not a prerequisite for the anti-dyskinetic effects of ketamine, still exerts an acute modulatory effect. In contrast to the mild modulatory effect on LID, we found that naloxone added to the anti-parkinsonian activity of ketamine, further reducing the akinetic phenotype. In conclusion, our data show opioid receptor blockade differentially modulates the acute anti-parkinsonian and anti-dyskinetic actions of ketamine, providing novel mechanistic information to support repurposing ketamine for individuals with LID.

Antagonism of kappa opioid receptors accelerates the development of L-DOPA-induced dyskinesia in a preclinical model of moderate dopamine depletion

Levels of the opioid peptide dynorphin, an endogenous ligand selective for kappa-opioid receptors (KORs), its mRNA and pro-peptide precursors are differentially dysregulated in Parkinson's disease (PD) and following the development of l-DOPA-induced dyskinesia (LID). It remains unclear whether these alterations contribute to the pathophysiological mechanisms underlying PD motor impairment and the subsequent development of LID, or whether they are part of compensatory mechanisms. We sought to investigate nor-BNI, a KOR antagonist, 1) in the dopamine (DA)-depleted PD state, 2) during the development phase of LID, and 3) via measuring of tonic levels of striatal DA. While nor-BNI (3 mg/kg; s.c.) did not lead to functional restoration in the DA-depleted state, it affected the dose-dependent development of abnormal voluntary movements (AIMs) in response to escalating doses of l-DOPA in a rat PD model with a moderate striatal 6-hydroxdopamine (6-OHDA) lesion. We tested five escalating doses of l-DOPA (6, 12, 24, 48, 72 mg/kg; i.p.), and nor-BNI significantly increased the development of AIMs at the 12 and 24 mg/kg l-DOPA doses. However, after reaching the 72 mg/kg l-DOPA, AIMs were not significantly different between control and nor-BNI groups. In summary, while blocking KORs significantly increased the rate of development of LID induced by chronic, escalating doses of l-DOPA in a moderate-lesioned rat PD model, it did not contribute further once the overall severity of LID was established. While we observed an increase of tonic DA levels in the moderately lesioned dorsolateral striatum, there was no tonic DA change following administration of nor-BNI.

Antagonism of kappa opioid receptors accelerates the development of L-DOPA-induced dyskinesia in a preclinical model of moderate dopamine depletion

Levels of the opioid peptide dynorphin, an endogenous ligand selective for kappa-opioid receptors (KORs), its mRNA and pro-peptide precursors are differentially dysregulated in Parkinson disease (PD) and following the development of L-DOPA-induced dyskinesia (LID). It remains unclear, whether these alterations contribute to the pathophysiological mechanisms underlying PD motor impairment and the subsequent development of LID, or whether they are part of compensatory mechanisms. We sought to investigate nor-BNI, a KOR antagonist, 1) in the dopamine (DA)-depleted PD state, 2) during the development phase of LID, and 3) with measuring tonic levels of striatal DA. Nor-BNI (3 mg/kg; s.c.) did not lead to functional restoration in the DA-depleted state, but a change in the dose-dependent development of abnormal voluntary movements (AIMs) in response to escalating doses of L-DOPA in a rat PD model with a moderate striatal 6-hydroxydopamine (6-OHDA) lesion. We tested five escalating doses of L-DOPA (6, 12, 24, 48, 72 mg/kg; i.p.), and nor-BNI significantly increased the development of AIMs at the 12 and 24 mg/kg L-DOPA doses. However, after dosing with 72 mg/kg L-DOPA, AIMs were not significantly different between control and nor-BNI groups. In summary, while blocking KORs significantly increased the rate of development of LID induced by chronic, escalating doses of L-DOPA in a moderate-lesioned rat PD model, it did not contribute further once the overall severity of LID was established. While we saw an increase of tonic DA levels in the moderately lesioned dorsolateral striatum, there was no tonic DA change following administration of nor-BNI.

7-Nitroindazole reduces L-DOPA-induced dyskinesias in non-human Parkinsonian primate

Nitric oxide (NO) plays a pivotal role in integrating dopamine transmission in the basal ganglia and has been implicated in the pathogenesis of Parkinson disease (PD). The objective of this study was to ascertain whether the NO synthase inhibitor, 7-nitroindazole (7-NI), is able to reduce L-DOPA-induced dyskinesias (LIDs) in a non-human primate model of PD chronically intoxicated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Six Parkinsonian macaques were treated daily with L-DOPA for 3-4 months until they developed LIDs. Three animals were then co-treated with a single dose of 7-NI administered 45 min before each L-DOPA treatment. Dyskinetic MPTP-treated monkeys showed a significant decrease in LIDs compared with their scores without 7-NI treatment (p < 0.05). The anti-Parkinsonian effect of L-DOPA was similar in all three monkeys with and without 7-NI co-treatment. This improvement was significant with respect to the intensity and duration of LIDs while the beneficial effect of L-DOPA treatment was maintained and could represent a promising therapy to improve the quality of life of PD patients.

A Randomized, Double-Blind, Controlled Phase II Study of Foliglurax in Parkinson's Disease

BACKGROUND

Agents targeting the metabotropic glutamate receptor 4 have emerged as a potentially attractive new class of drugs for the treatment of Parkinson's disease (PD).

OBJECTIVE

The objective of this study was to evaluate the efficacy and safety of foliglurax in reducing off time and dyskinesia in patients with PD.

METHODS

This was a 28-day, multicenter, randomized, placebo-controlled, double-blind clinical trial of foliglurax 10 and 30 mg as adjunct to levodopa in 157 randomly assigned patients with PD and motor complications.

RESULTS

Although dose-dependent decreases in daily awake off time were apparent following treatment with foliglurax, the change from baseline to day 28 in off time (primary endpoint) and dyskinesia (secondary endpoint) did not improve significantly compared with placebo for either foliglurax dose. Treatment with foliglurax was generally safe, and there were no relevant safety signals.

CONCLUSIONS

There was no evidence in this study that foliglurax has efficacy in improving levodopa-induced motor complications in PD. © 2022 International Parkinson and Movement Disorder Society.

A novel phosphodiesterase 1 inhibitor reverses L-dopa-induced dyskinesia, but not motivation deficits, in monkeys

The enzyme phosphodiesterase 1 (PDE1) is highly expressed in the striatum and cortex. However, its role in corticostriatal function has not been fully investigated. The present study was aimed at evaluating the therapeutic potential of PDE1 inhibitors in treating motivation deficits and 3,4-dihydroxy-L-phenylalanine (L-dopa)-induced dyskinesia, which are pathological conditions of the corticostriatal system. We used a novel PDE1 inhibitor 3-ethyl-2-{[trans-4-(methoxymethyl)cyclohexyl]oxy}-7-(tetrahydro-2H-pyran-4-yl)-imidazo[5,1-f][1,2,4]triazin-4(3H)-one (DSR-143136), which was identified in our drug discovery program. Motivation in monkeys was measured using a progressive ratio task. L-Dopa-induced dyskinesia and disability scores were measured in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys. DSR-143136 had a high selectivity for PDE1 over other PDE families and 67 other biologic targets. A dopamine D₁ receptor antagonist SCH-39166 at 0.01, 0.03 and 0.1 mg/kg potently decreased motivation in monkeys. However, DSR-143136 at 0.3 and 3 mg/kg did not affect motivation deficits induced by low-dose SCH-39166 (0.01 mg/kg). On the other hand, DSR-143136 at 3 mg/kg potently decreased L-dopa-induced dyskinesia in the Parkinsonian monkey model. Importantly, this antidyskinesic efficacy was NOT accompanied by detrimental effects on motor function. Further, this compound decreased on-time with marked or severe dyskinesia, without affecting on-time itself. These findings suggest that PDE1 inhibitor could be a therapeutic candidate for treating L-dopa-induced dyskinesia in Parkinson's disease, but not for motivation deficits.

δ-opioid receptor activation protects against Parkinson's disease-related mitochondrial dysfunction by enhancing PINK1/Parkin-dependent mitophagy

Our previous studies have shown that the δ-opioid receptor (DOR) is an important neuroprotector via the regulation of PTEN-induced kinase 1 (PINK1), a mitochondria-related molecule, under hypoxic and MPP⁺ insults. Since mitochondrial dysfunctions are observed in both hypoxia and MPP⁺ insults, this study further investigated whether DOR is cytoprotective against these insults by targeting mitochondria. Through comparing DOR-induced responses to hypoxia versus MPP⁺-induced parkinsonian insult in PC12 cells, we found that both hypoxia and MPP⁺ caused a collapse of mitochondrial membrane potential and severe mitochondrial dysfunction. In sharp contrast to its inappreciable effect on mitochondria in hypoxic conditions, DOR activation with UFP-512, a specific agonist, significantly attenuated the MPP⁺-induced mitochondrial injury. Mechanistically, DOR activation effectively upregulated PINK1 expression and promoted Parkin's mitochondrial translocation and modification, thus enhancing the PINK1-Parkin mediated mitophagy. Either PINK1 knockdown or DOR knockdown largely interfered with the DOR-mediated mitoprotection in MPP⁺ conditions. Moreover, there was a major difference between hypoxia versus MPP⁺ in terms of the regulation of mitophagy with hypoxia-induced mitophagy being independent from DOR-PINK1 signaling. Taken together, our novel data suggest that DOR activation is neuroprotective against parkinsonian injury by specifically promoting mitophagy in a PINK1-dependent pathway and thus attenuating mitochondrial damage.

Preclinical evidence in support of repurposing sub-anesthetic ketamine as a treatment for L-DOPA-induced dyskinesia

Parkinson's disease (PD) is the second most common neurodegenerative disease. Pharmacotherapy with L-DOPA remains the gold-standard therapy for PD, but is often limited by the development of the common side effect of L-DOPA-induced dyskinesia (LID), which can become debilitating. The only effective treatment for disabling dyskinesia is surgical therapy (neuromodulation or lesioning), therefore effective pharmacological treatment of LID is a critical unmet need. Here, we show that sub-anesthetic doses of ketamine attenuate the development of LID in a rodent model, while also having acute anti-parkinsonian activity. The long-term anti-dyskinetic effect is mediated by brain-derived neurotrophic factor-release in the striatum, followed by activation of ERK1/2 and mTOR pathway signaling. This ultimately leads to morphological changes in dendritic spines on striatal medium spiny neurons that correlate with the behavioral effects, specifically a reduction in the density of mushroom spines, a dendritic spine phenotype that shows a high correlation with LID. These molecular and cellular changes match those occurring in hippocampus and cortex after effective sub-anesthetic ketamine treatment in preclinical models of depression, and point to common mechanisms underlying the therapeutic efficacy of ketamine for these two disorders. These preclinical mechanistic studies complement current ongoing clinical testing of sub-anesthetic ketamine for the treatment of LID by our group, and provide further evidence in support of repurposing ketamine to treat individuals with PD. Given its clinically proven therapeutic benefit for both treatment-resistant depression and several pain states, very common co-morbidities in PD, sub-anesthetic ketamine could provide multiple therapeutic benefits for PD in the future.

Neuroprotection or Neurotoxicity of Illicit Drugs on Parkinson's Disease

Parkinson's Disease (PD) is currently the most rapid growing neurodegenerative disease and over the past generation, its global burden has more than doubled. The onset of PD can arise due to environmental, sporadic or genetic factors. Nevertheless, most PD cases have an unknown etiology. Chemicals, such as the anthropogenic pollutant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and amphetamine-type stimulants, have been associated with the onset of PD. Conversely, cannabinoids have been associated with the treatment of the symptoms'. PD and medical cannabis is currently under the spotlight, and research to find its benefits on PD is on-going worldwide. However, the described clinical applications and safety of pharmacotherapy with cannabis products are yet to be fully supported by scientific evidence. Furthermore, the novel psychoactive substances are currently a popular alternative to classical drugs of abuse, representing an unknown health hazard for young adults who may develop PD later in their lifetime. This review addresses the neurotoxic and neuroprotective impact of illicit substance consumption in PD, presenting clinical evidence and molecular and cellular mechanisms of this association. This research area is utterly important for contemporary society since illicit drugs' legalization is under discussion which may have consequences both for the onset of PD and for the treatment of its symptoms.

The role of glutamate receptors and their interactions with dopamine and other neurotransmitters in the development of tardive dyskinesia: preclinical and clinical results

Tardive dyskinesia is a serious, disabling, movement disorder associated with the ongoing use of antipsychotic medication. Current evidence regarding the pathophysiology of tardive dyskinesia is mainly based on preclinical animal models and is still not completely understood. The leading preclinical hypothesis of tardive dyskinesia development includes dopaminergic imbalance in the direct and indirect pathways of the basal ganglia, cholinergic deficiency, serotonin receptor disturbances, neurotoxicity, oxidative stress, and changes in synaptic plasticity. Although, the role of the glutamatergic system has been confirmed in preclinical tardive dyskinesia models it seems to have been neglected in recent reviews. This review focuses on the role and interactions of glutamate receptors with dopamine, acetylcholine, and serotonin in the neuropathology of tardive dyskinesia development. Moreover, preclinical and clinical results of the differentiated effectiveness of N-methyl-D-aspartate (NMDA) receptor antagonists are discussed with a special focus on antagonists that bind with the GluN2B subunit of NMDA receptors. This review also presents new combinations of drugs that are worth considering in the treatment of tardive dyskinesia.

Pathophysiological clues to therapeutic applications of glutamate mGlu5 receptor antagonists in levodopa-induced dyskinesia

Levodopa remains to be the mainstay for treatment of Parkinson disease (PD). Long-term levodopa treatment bears a risk for developing levodopa-induced dyskinesia (LID). LID significantly overshadows patients' quality of life and therapeutic efficacy of levodopa. Pre- and post-synaptic changes in dopamine secretion and signaling, along with altered glutamate receptor expression and glutamatergic signaling in striatal neurons, and the resulting disinhibition-like changes in the corticostriatal circuitry, lead to aberrant activity of motor cortex and formation of LID. Research has highlighted the role of group I metabotropic glutamate receptors especially the metabotropic glutamate receptor 5 (mGlu5) in formation of LID through potentiating of ionotropic glutamate NMDA receptors and dopamine D₁/D₅ receptors in direct pathway. Accordingly, MTEP and MPEP were the first mGlu5 receptor antagonists which were shown to attenuate LID in animal models through suppression of downstream signaling cascades involving mitogen-activated protein kinase (MAPK) and FosB/delta FosB activation, as well as modulation of prodynorphinegic, preproenkephalinergic, and GABA-ergic neurotransmission systems. Beneficial effects of other mGlu5 receptor antagonists such as AFQ056/mavoglurant and ADX48621/dipraglurant in amelioration of LID has been shown not only in animal models but also in clinical trials. Considering the presence of mGlu receptor dysregulation in rapid eye movement (REM) sleep behavior disorder and depression, which are prodromal signs of PD, along with the neuroprotective effects of mGlu receptor antagonists, and their cognitive benefits, potential effectiveness of mGlu receptor antagonists in early prevention of PD remains to be investigated.

Pridopidine, a clinic-ready compound, reduces 3,4-dihydroxyphenylalanine-induced dyskinesia in Parkinsonian macaques

BACKGROUND

Pridopidine, in development for Huntington's disease, may modulate aberrant l-dopa-induced effects including l-dopa-induced dyskinesia (LID).

OBJECTIVE

This study investigated whether pridopidine could reduce LID in the MPTP macaque model of Parkinson's disease and characterized the observed behavioral effects in terms of receptor occupancy.

METHODS

The pharmacokinetic profile and effects of pridopidine (15-30 mg/kg) on parkinsonism, dyskinesia, and quality of on-time, in combination with l-dopa, were assessed in MPTP macaques with LID. Pridopidine receptor occupancy was estimated using known in vitro binding affinities to σ1 and dopamine D2 receptors, in vivo PET imaging, and pharmacokinetic profiling across different species.

RESULTS

Pridopidine produced a dose-dependent reduction in dyskinesia (up to 71%, 30 mg/kg) and decreased the duration of on-time with disabling dyskinesia evoked by l-dopa by 37% (20 mg/kg) and 60% (30 mg/kg). Pridopidine did not compromise the anti-parkinsonian benefit of l-dopa. Plasma exposures following the ineffective dose (15 mg/kg) were associated with full σ1 occupancy (>80%), suggesting that σ1 engagement alone is unlikely to account for the antidyskinetic benefits of pridopidine. Exposures following effective doses (20-30 mg/kg), while providing full σ1 occupancy, provide only modest dopamine D2 occupancy (<40%). However, effective pridopidine doses clearly engage a range of receptors (including adrenergic-α2C , dopamine-D3 , and serotoninergic-5-HT1A sites) to a higher degree than D2 and might contribute to the antidyskinetic actions.

CONCLUSIONS

In MPTP macaques, pridopidine produced a significant decrease in LID without compromising the antiparkinsonian benefit of l-dopa. Although the actions of pridopidine were associated with full σ1 occupancy, effective exposures are more likely associated with occupancy of additional, non-sigma receptors. This complex pharmacology may underlie the effectiveness of pridopidine against LID. © 2018 International Parkinson and Movement Disorder Society.

Viewpoint: Developing drugs for levodopa-induced dyskinesia in PD: Lessons learnt, what does the future hold?

The drive to develop drugs to treat PD starts and ends with the patient. Herein, we discuss how the experience with drug development for LID has led the field in translational studies in PD with advancing ground-breaking science via rigorous clinical trial design, to deliver clinical proof-of-concepts across multiple therapeutic targets. However, issues remain in advancing drugs efficacious preclinically to the clinic, and future studies need to learn from past successes and failures. Such lessons include implementing better early indicators of tolerability, for instance evaluating non-motor symptoms in preclinical models; improving patient-related outcome measures in clinical trials, as well as considering the unique nature of dyskinesia in an individual patient. The field of translational studies needs to become more patient focused to improve successful outcomes.

The combination of the opioid glycopeptide MMP-2200 and a NMDA receptor antagonist reduced l-DOPA-induced dyskinesia and MMP-2200 by itself reduced dopamine receptor 2-like agonist-induced dyskinesia

Dopamine (DA)-replacement therapy utilizing l-DOPA is the gold standard symptomatic treatment for Parkinson's disease (PD). A critical complication of this therapy is the development of l-DOPA-induced dyskinesia (LID). The endogenous opioid peptides, including enkephalins and dynorphin, are co-transmitters of dopaminergic, GABAergic, and glutamatergic transmission in the direct and indirect striatal output pathways disrupted in PD, and alterations in expression levels of these peptides and their precursors have been implicated in LID genesis and expression. We have previously shown that the opioid glycopeptide drug MMP-2200 (a.k.a. Lactomorphin), a glycosylated derivative of Leu-enkephalin mediates potent behavioral effects in two rodent models of striatal DA depletion. In this study, the mixed mu-delta agonist MMP-2200 was investigated in standard preclinical rodent models of PD and of LID to evaluate its effects on abnormal involuntary movements (AIMs). MMP-2200 showed antiparkinsonian activity, while increasing l-DOPA-induced limb, axial, and oral (LAO) AIMs by ∼10%, and had no effect on dopamine receptor 1 (D1R)-induced LAO AIMs. In contrast, it markedly reduced dopamine receptor 2 (D2R)-like-induced LAO AIMs. The locomotor AIMs were reduced by MMP-2200 in all three conditions. The N-methyl-d-aspartate receptor (NMDAR) antagonist MK-801 has previously been shown to be anti-dyskinetic, but only at doses that induce parkinsonism. When MMP-2200 was co-administered with MK-801, MK-801-induced pro-parkinsonian activity was suppressed, while a robust anti-dyskinetic effect remained. In summary, the opioid glycopeptide MMP-2200 reduced AIMs induced by a D2R-like agonist, and MMP-2200 modified the effect of MK-801 to result in a potent reduction of l-DOPA-induced AIMs without induction of parkinsonism.

Neuroprotection in Oxidative Stress-Related Neurodegenerative Diseases: Role of Endocannabinoid System Modulation

SIGNIFICANCE

Redox imbalance may lead to overproduction of reactive oxygen and nitrogen species (ROS/RNS) and subsequent oxidative tissue damage, which is a critical event in the course of neurodegenerative diseases. It is still not fully elucidated, however, whether oxidative stress is the primary trigger or a consequence in the process of neurodegeneration. Recent Advances: Increasing evidence suggests that oxidative stress is involved in the propagation of neuronal injury and consequent inflammatory response, which in concert promote development of pathological alterations characteristic of most common neurodegenerative diseases.

CRITICAL ISSUES

Accumulating recent evidence also suggests that there is an important interplay between the lipid endocannabinoid system [ECS; comprising the main cannabinoid 1 and 2 receptors (CB1 and CB2), endocannabinoids, and their synthetic and metabolizing enzymes] and various key inflammatory and redox-dependent processes.

FUTURE DIRECTIONS

Targeting the ECS to modulate redox state-dependent cell death and to decrease consequent or preceding inflammatory response holds therapeutic potential in a multitude of oxidative stress-related acute or chronic neurodegenerative disorders from stroke and traumatic brain injury to Alzheimer's and Parkinson's diseases and multiple sclerosis, just to name a few, which will be discussed in this overview. Antioxid. Redox Signal. 29, 75-108.

A Phase Ib Randomized Controlled Study to Evaluate the Effectiveness of a Single-Dose of the NR2B Selective N-Methyl-D-Aspartate Antagonist MK-0657 on Levodopa-Induced Dyskinesias and Motor Symptoms in Patients With Parkinson Disease

OBJECTIVES

Blockade of N-methyl-D-aspartate receptors containing the NR2B subunit has been shown to be therapeutic in animal models of Parkinson disease (PD). However, findings with investigational NR2B receptor antagonists in PD patients have been mixed. The objective of this study was to evaluate the effects of the NR2B selective N-methyl-D-aspartate receptor antagonist MK-0657 on levodopa-induced dyskinesias and motor symptoms in PD patients.

METHODS

A randomized, double-blind, single-dose, 2-period crossover study was conducted in 22 patients with PD and levodopa-induced peak-dose dyskinesias. Patients received oral MK-0657 (7 mg) or placebo, in randomized order, on each of 2 test days. On both days, levodopa was administered as a 2-hour intravenous infusion at greater than or equal to 1 mg/kg per hour with frequent assessments of dyskinesia, motor function, and pharmacokinetics.

RESULTS

MK-0657 7 mg had no significant effect on dyskinesias (difference versus placebo in modified Abnormal Involuntary Movement Scale mean change from baseline area under the curve over 5 hours, -2.3; 95% confidence interval, -5.1 to 0.4) or motor function (difference versus placebo in Unified Parkinson's Disease Rating Scale Part III mean change from baseline area under the curve over 5 hours, 13.9; 95% confidence interval, -1.7 to 29.5). MK-0657 7 mg achieved the target mean maximum plasma concentration of 400 nM.

CONCLUSIONS

These data suggest that a single dose of MK-0657 7 mg is not effective in improving levodopa-induced dyskinesias and motor symptoms in PD patients.

CLINICAL TRIAL REGISTRATION

clinicaltrials.gov NCT00505843.

Repurposing drugs to treat l-DOPA-induced dyskinesia in Parkinson's disease

In this review, we discuss the opportunity for repurposing drugs for use in l-DOPA-induced dyskinesia (LID) in Parkinson's disease. LID is a particularly suitable indication for drug repurposing given its pharmacological diversity, translatability of animal-models, availability of Phase II proof-of-concept (PoC) methodologies and the indication-specific regulatory environment. A compound fit for repurposing is defined as one with appropriate human safety-data as well as animal safety, toxicology and pharmacokinetic data as found in an Investigational New Drug (IND) package for another indication. We first focus on how such repurposing candidates can be identified and then discuss development strategies that might progress such a candidate towards a Phase II clinical PoC. We discuss traditional means for identifying repurposing candidates and contrast these with newer approaches, especially focussing on the use of computational and artificial intelligence (AI) platforms. We discuss strategies that can be categorised broadly as: in vivo phenotypic screening in a hypothesis-free manner; in vivo phenotypic screening based on analogy to a related disorder; hypothesis-driven evaluation of candidates in vivo and in silico screening with a hypothesis-agnostic component to the selection. To highlight the power of AI approaches, we describe a case study using IBM Watson where a training set of compounds, with demonstrated ability to reduce LID, were employed to identify novel repurposing candidates. Using the approaches discussed, many diverse candidates for repurposing in LID, originally envisaged for other indications, will be described that have already been evaluated for efficacy in non-human primate models of LID and/or clinically. This article is part of the Special Issue entitled 'Drug Repurposing: old molecules, new ways to fast track drug discovery and development for CNS disorders'.

Current and investigational non-dopaminergic agents for management of motor symptoms (including motor complications) in Parkinson's disease

INTRODUCTION

Parkinson's disease is characterized by a heterogeneous combination of motor and non motor symptoms. The nigrostriatal dopamine deficit is one of its essential pathophysiologic features. Areas covered: This invited narrative review provides an overlook over current available and future promising non dopaminergic therapeutics to modulate altered dopaminergic neurotransmission in Parkinson's disease. Current research strategies aim to proof clinical efficacy by amelioration of motor symptoms and preponderant levodopa related movement fluctuations. These so-called motor complications are characterized by involuntary movements as a result of an overstimulation of the nigrostriatal dopaminergic system or by temporary recurrence of motor symptoms, when beneficial effects of dopamine substituting drugs vane. Expert opinion: Non dopaminergic modulation of dopamine replacement is currently mostly investigated in well defined and selected patients with motor complications to get approval. However, the world of daily maintenance of patients with its individually adapted, so-called personalised, therapy will determine the real value of these therapeutics. Here the clinical experience of the treating neurologists and the courage to use unconventional drug combinations are essential preconditions for successful treatments of motor and associated non motor complications in cooperation with the patients and their care giving surroundings.

Non-human primate models of PD to test novel therapies

Non-human primate (NHP) models of Parkinson disease show many similarities with the human disease. They are very useful to test novel pharmacotherapies as reviewed here. The various NHP models of this disease are described with their characteristics including the macaque, the marmoset, and the squirrel monkey models. Lesion-induced and genetic models are described. There is no drug to slow, delay, stop, or cure Parkinson disease; available treatments are symptomatic. The dopamine precursor, L-3,4-dihydroxyphenylalanine (L-Dopa) still remains the gold standard symptomatic treatment of Parkinson. However, involuntary movements termed L-Dopa-induced dyskinesias appear in most patients after chronic treatment and may become disabling. Dyskinesias are very difficult to manage and there is only amantadine approved providing only a modest benefit. In this respect, NHP models have been useful to seek new drug targets, since they reproduce motor complications observed in parkinsonian patients. Therapies to treat motor symptoms in NHP models are reviewed with a discussion of their translational value to humans. Disease-modifying treatments tested in NHP are reviewed as well as surgical treatments. Many biochemical changes in the brain of post-mortem Parkinson disease patients with dyskinesias are reviewed and compare well with those observed in NHP models. Non-motor symptoms can be categorized into psychiatric, autonomic, and sensory symptoms. These symptoms are present in most parkinsonian patients and are already installed many years before the pre-motor phase of the disease. The translational usefulness of NHP models of Parkinson is discussed for non-motor symptoms.

Trial of dextromethorphan/quinidine to treat levodopa-induced dyskinesia in Parkinson's disease

BACKGROUND

Nondopaminergic pathways represent potential targets to treat levodopa-induced dyskinesia in Parkinson's disease (PD). This pilot-study (NCT01767129) examined the safety/efficacy of the sigma-1 receptor-agonist and glutamatergic/monoaminergic modulator, dextromethorphan plus quinidine (to inhibit rapid dextromethorphan metabolism), for treating levodopa-induced dyskinesia.

METHODS

PD patients were randomized to dextromethorphan/quinidine (45 mg/10 mg twice daily)/placebo in two 2-week double-blind, crossover treatment periods, with intervening 2-week washout. After 14 days, a 2-hour intravenous levodopa-infusion was administered. Patient examinations were videotaped before infusion ("off" state) and every 30 minutes during and afterwards until patients returned to "off." The primary endpoint was dyskinesia-severity during infusion measured by Unified Dyskinesia Rating Scale part 3 area-under-curve scores (blinded expert rated). Additional endpoints included other dyskinesia/motor assessments, global measures of clinical-change, and adverse-events.

RESULTS

A total of 13 patients were randomized and completed the study (efficacy-evaluable population). Dyskinesia-severity was nonsignificantly lower with dextromethorphan/quinidine than placebo during infusion (area-under-curve 966.5 vs 1048.8; P = .191 [efficacy-evaluable patients]), and significantly lower in a post-hoc sensitivity analysis of the per-protocol-population (efficacy-evaluable patients with ≥ 80% study-drug-compliance, n = 12) when measured from infusion start to 4-hours post-infusion completion (area-under-curve 1585.0 vs 1911.3; P = .024). Mean peak dyskinesia decreased significantly from infusion-start to return to "off" (13.3 vs 14.9; P = .018 [efficacy-evaluable patients]). A total of 9 patients rated dyskinesia "much/very much improved" on dextromethorphan/quinidine versus 1-patient on placebo. Dextromethorphan/quinidine did not worsen PD-motor scores, was generally well tolerated, and was associated with more frequent adverse events.

CONCLUSION

This study provides preliminary evidence of clinical benefit with dextromethorphan/quinidine for treating levodopa-induced dyskinesia in PD. Larger studies with a longer treatment duration need to corroborate these early findings. © 2017 International Parkinson and Movement Disorder Society.

Memantine and reduced time with dyskinesia in Parkinson's Disease

OBJECTIVES

The partial glutamate antagonist amantadine is currently used in clinical practice, to reduce dyskinesia developing as a side-effect of levodopa treatment in patients suffering from Parkinson's disease (PD). This study was aimed at evaluating the antidyskinetic effect of another glutamate antagonist, memantine.

METHODS AND MATERIALS

We performed a randomized, double-blind and placebo-controlled crossover clinical trial of memantine (20 mg), with a 3-week treatment period, and 15 patients completed the study.

RESULTS

The primary outcome measure, a change in observed dyskinesia ratings, did not reach significance. Seven of the 15 patients reduced the L-dopa-induced dyskinesias by 32%, whereas for three patients, they increased by 33%, and for five patients, they did not change. Data from the self-administered diaries, as a secondary outcome measure, did show a significant 35% reduction in the percentage of time of the day spent with dyskinesia, from 25% (placebo) to 16% (memantine). Memantine was well tolerated, without any serious adverse events, or worsening in the parkinsonian motor score.

CONCLUSION

The results suggest that memantine may be a useful antidyskinetic drug, and a larger clinical study is warranted.

Striatal Plasticity in L-DOPA- and Graft-Induced Dyskinesia; The Common Link?

One of the major symptoms of the neurodegenerative condition Parkinson's disease (PD) is a slowness or loss of voluntary movement, yet frustratingly therapeutic strategies designed to restore movement can result in the development of excessive abnormal movements known as dyskinesia. These dyskinesias commonly develop as a result of pharmacotherapy in the form of L-DOPA administration, but have also been identified following deep brain stimulation (DBS) and intrastriatal cell transplantation. In the case of L-DOPA these movements can be treatment limiting, and whilst they are not long lasting or troubling following DBS, recognition of their development had a near devastating effect on the field of cell transplantation for PD.Understanding the relationship between these therapeutic approaches and the development of dyskinesia may improve our ability to restore function without disabling side effects. Interestingly, despite the fact that dopaminergic cell transplantation repairs many of the changes induced by the disease process and through L-DOPA treatment, there appears to be a relationship between the two. In rodent models of the disease, the severity of dyskinesia induced by L-DOPA prior to the transplantation procedure correlated with post-transplantation, graft-induced dyskinesia. A review of clinical data also suggested that the worse preoperational dyskinesia causes worsened graft-induced dyskinesia (GID). Understanding how these aberrant behaviors come about has been of keen interest to open up these therapeutic options more widely and one major underlying theory is the effects of these approaches on the plasticity of synapses within the basal ganglia. This review uniquely brings together developments in understanding the role of striatal synaptic plasticity in both L-DOPA and GID to guide and stimulate further investigations on the important striatal plasticity.

Drugs of abuse and Parkinson's disease

The term "drug of abuse" is highly contextual. What constitutes a drug of abuse for one population of patients does not for another. It is therefore important to examine the needs of the patient population to properly assess the status of drugs of abuse. The focus of this article is on the bidirectional relationship between patients and drug abuse. In this paper we will introduce the dopaminergic systems of the brain in Parkinson's and the influence of antiparkinsonian drugs upon them before discussing this synergy of condition and medication as fertile ground for drug abuse. We will then examine the relationship between drugs of abuse and Parkinson's, both beneficial and deleterious. In summary we will draw the different strands together and speculate on the future merit of current drugs of abuse as treatments for Parkinson's disease.

Amelioration of L-Dopa-Associated Dyskinesias with Triterpenoic Acid in a Parkinsonian Rat Model

Levo-Dopa (L-Dopa) is widely used for the oral treatment of Parkinson's disease. However, chronic treatment with L-Dopa produces abnormal involuntary movements (AIMs) known as dyskinesias. In this study, commercially available oleanolic acid (OA) that has been previously shown to ameliorate the toxic effects of 6-hydroxydopamine (6-OHDA) in preconditioning studies was used to treat AIMs in a rat model for Parkinson's disease. The forelimb-use asymmetry test was used to measure Parkinson's disease-associated motor impairment. AIMs were measured after 21 days of L-Dopa administration. Glutathione levels were measured in blood, and catalase levels were measured in the substantia nigra and striatum of both the left and right hemispheres. We found that L-Dopa alone as well as L-Dopa and OA combination treatment attenuated the limb-use asymmetry caused by the unilateral injection of 6-OHDA. Chronic L-Dopa administration produced AIMs which were attenuated by treatment with OA. Catalase concentration decreased significantly in the striatum but not in the substantia nigra of the lesioned hemisphere. L-Dopa alone as well as the combined L-Dopa and OA treatment ameliorated the effects of 6-OHDA on catalase concentration. However, intervention with L-Dopa alone as well as with L-Dopa and OA did not affect plasma glutathione concentration. These results suggest that OA administration enhances the effect of catalase on reactive oxygen species following 6-OHDA injection. OA may provide possibilities as an adjunct treatment to prevent or attenuate the development of AIMs following chronic L-Dopa treatment in Parkinson's disease.

Cannabinoids in Neurodegenerative Disorders and Stroke/Brain Trauma: From Preclinical Models to Clinical Applications

Cannabinoids form a singular family of plant-derived compounds (phytocannabinoids), endogenous signaling lipids (endocannabinoids), and synthetic derivatives with multiple biological effects and therapeutic applications in the central and peripheral nervous systems. One of these properties is the regulation of neuronal homeostasis and survival, which is the result of the combination of a myriad of effects addressed to preserve, rescue, repair, and/or replace neurons, and also glial cells against multiple insults that may potentially damage these cells. These effects are facilitated by the location of specific targets for the action of these compounds (e.g., cannabinoid type 1 and 2 receptors, endocannabinoid inactivating enzymes, and nonendocannabinoid targets) in key cellular substrates (e.g., neurons, glial cells, and neural progenitor cells). This potential is promising for acute and chronic neurodegenerative pathological conditions. In this review, we will collect all experimental evidence, mainly obtained at the preclinical level, supporting that different cannabinoid compounds may be neuroprotective in adult and neonatal ischemia, brain trauma, Alzheimer's disease, Parkinson's disease, Huntington's chorea, and amyotrophic lateral sclerosis. This increasing experimental evidence demands a prompt clinical validation of cannabinoid-based medicines for the treatment of all these disorders, which, at present, lack efficacious treatments for delaying/arresting disease progression, despite the fact that the few clinical trials conducted so far with these medicines have failed to demonstrate beneficial effects.

Symptomatic Models of Parkinson's Disease and L-DOPA-Induced Dyskinesia in Non-human Primates

Models of Parkinson's disease (PD) can be produced in several non-human primate (NHP) species by applying neurotoxic lesions to the nigrostriatal dopamine pathway. The most commonly used neurotoxin is MPTP, a compound accidentally discovered as a contaminant of street drugs. Compared to other neurotoxins, MPTP has the advantage of crossing the blood-brain barrier and can thus be administered systemically. MPTP-lesioned NHPs exhibit the main core clinical features of PD. When treated with L-DOPA, these NHP models develop involuntary movements resembling the phenomenology of human dyskinesias. In old-world NHP species (macaques, baboons), choreic and dystonic dyskinesias can be readily distinguished and quantified with specific rating scales. More recently, certain non-motor symptoms relevant to human PD have been described in L-DOPA-treated MPTP-NHPs, including a range of neuropsychiatric abnormalities and sleep disturbances. The main shortcomings of MPTP-NHP models consist in a lack of progression of the underlying neurodegenerative lesion, along with an inability to model the intracellular protein-inclusion pathology typical of PD. The strength of MPTP-NHP models lies in their face and predictive validity for symptomatic treatments of parkinsonian motor features. Indeed, these models have been instrumental to the development of several medical and surgical approaches that are currently applied to treat PD.

Dyskinesias and impulse control disorders in Parkinson's disease: From pathogenesis to potential therapeutic approaches

Dopaminergic treatment in Parkinson's disease (PD) reduces the severity of motor symptoms of the disease. However, its chronic use is associated with disabling motor and behavioral side effects, among which levodopa-induced dyskinesias (LID) and impulse control disorders (ICD) are the most common. The underlying mechanisms and pathological substrate of these dopaminergic complications are not fully understood. Recently, the refinement of imaging techniques and the study of the genetics and molecular bases of LID and ICD indicate that, although different, they could share some features. In addition, animal models of parkinsonism with LID have provided important knowledge about mechanisms underlying such complications. In contrast, animal models of parkinsonism and abnormal impulsivity, although useful regarding some aspects of human ICD, do not fully resemble the clinical phenotype of ICD in patients with PD, and until now have provided limited information. Studies on animal models of addiction could complement the previous models and provide some insights into the background of these behavioral complications given that ICD are regarded as behavioral addictions. Here we review the most relevant advances in relation to imaging, genetics, biochemistry and pharmacological interventions to treat LID and ICD in patients with PD and in animal models with a view to better understand the overlapping and unique maladaptations to dopaminergic therapy that are associated with LID and ICD.

Famotidine, a Histamine H2 Receptor Antagonist, Does Not Reduce Levodopa-Induced Dyskinesia in Parkinson's Disease: A Proof-of-Concept Study

The neural mechanisms underlying levodopa-induced dyskinesia (LID) in Parkinson's disease (PD) may involve histamine (H₂) receptors on striatopallidal pathways. We recently demonstrated that the clinically available oral histamine H₂ receptor antagonist (H₂ RA), famotidine, can reduce l-dopa-induced chorea in MPTP-lesioned macaques. We hypothesized that famotidine may be useful in the treatment of LID in PD patients. We performed a proof-of-concept, double-blind, randomized, multiple cross-over (4×) trial. Seven PD subjects with bothersome dyskinesia were randomized to oral famotidine 80, 120, and 160 mg/day and placebo. Each subject was randomized to receive each of the four treatment phases for 14 days followed by a 7-day wash-out period between each treatment phase. The primary outcome measure was change in the Unified Dyskinesia Rating Scale (UDysRS; part III) between placebo and famotidine. Secondary outcomes were UDysRS (parts I and II), Global Impression of Change, Lang-Fahn Activities of Daily Living Dyskinesia Scale, Unified Parkinson's Disease Rating part III, and adverse events (AEs). Outcomes were evaluated pre- and post-treatment per dose and analyzed using a mixed-effects linear model. There was no significant effect of famotidine treatment on any of the primary or secondary outcome measures compared to placebo (each dose and all doses combined). There were no significant AEs. Even though the sample size of the current study is limited, famotidine seems to be safe in patients with PD and LID, but showed no potential as an antidyskinetic agent.

Randomized clinical trial of topiramate for levodopa-induced dyskinesia in Parkinson's disease

BACKGROUND

The antiepileptic drug topiramate reduces levodopa-induced dyskinesia without exacerbating parkinsonism in animal models. We report a randomized, double-blind, placebo-controlled crossover trial in patients with Parkinson's disease and levodopa-induced dyskinesia.

METHODS

Fifteen patients with Parkinson's disease and stable levodopa-induced dyskinesia were enrolled into the study, of whom 13 were randomized to topiramate or placebo. The study medication was titrated to 100 mg/day over four weeks, and assessments were carried out after a further two weeks. Dyskinesia severity assessed by a blinded rater from video recordings was the primary outcome measure.

RESULTS

Seven patients (mean age 58.9 ± 12.8 years) completed the study. Patients taking topiramate vs. placebo showed a significant increase in dyskinesia severity compared to baseline (Wilcoxon signed rank test, P = 0.043). Five patients withdrew from the study whilst taking topiramate due to adverse effects.

CONCLUSIONS

Topiramate tended to worsen dyskinesia in patients with Parkinson's disease, and was poorly tolerated.

L-DOPA-treatment in primates disrupts the expression of A(2A) adenosine-CB(1) cannabinoid-D(2) dopamine receptor heteromers in the caudate nucleus

The molecular basis of priming for L-DOPA-induced dyskinesias in Parkinson's disease (PD), which depends on the indirect pathway of motor control, is not known. In rodents, the indirect pathway contains striatopallidal GABAergic neurons that express heterotrimers composed of A(2A) adenosine, CB(1) cannabinoid and D(2) dopamine receptors that regulate dopaminergic neurotransmission. The present study was designed to investigate the expression of these heteromers in the striatum of a primate model of Parkinson's disease and to determine whether their expression and pharmacological properties are altered upon L-DOPA treatment. By using the recently developed in situ proximity ligation assay and by identification of a biochemical fingerprint, we discovered a regional distribution of A(2A)/CB(1) /D(2) receptor heteromers that predicts differential D(2)-mediated neurotransmission in the caudate-putamen of Macaca fascicularis. Whereas heteromers were abundant in the caudate nucleus of both naïve and MPTP-treated monkeys, L-DOPA treatment blunted the biochemical fingerprint and led to weak heteromer expression. These findings constitute the first evidence of altered receptor heteromer expression in pathological conditions and suggest that drugs targeting A(2A)-CB(1) -D(2) receptor heteromers may be successful to either normalize basal ganglia output or prevent L-DOPA-induced side effects.

Assessing clinical prospects of silicon quantum dots: studies in mice and monkeys

Silicon nanocrystals can provide the outstanding imaging capabilities of toxic heavy-metal-based quantum dots without employing heavy metals and have potential for rapid progression to the clinic. Understanding the toxicity of silicon quantum dots (SiQDs) is essential to realizing this potential. However, existing studies of SiQD biocompatibility are limited, with no systematic progression from small-animal to large-animal studies that are more clinically relevant. Here, we test the response of both mice and monkeys to high intravenous doses of a nanoconstruct created using only SiQDs and FDA-approved materials. We show that (1) neither mice nor monkeys show overt signs of toxicity reflected in their behavior, body mass, or blood chemistry, even at a dose of 200 mg/kg. (2) This formulation did not biodegrade as expected. Elevated levels of silicon were present in the liver and spleen of mice three months post-treatment. (3) Histopathology three months after treatment showed adverse effects of the nanoformulation in the livers of mice, but showed no such effects in monkeys. This investigation reveals that the systemic reactions of the two animal models may have some differences and there are no signs of toxicity clearly attributable to silicon quantum dots.

Functional neuroimaging in Parkinson's disease

The use of functional imaging in neurodegenerative diseases has increased in recent years, with applications in research into the underlying pathophysiology, aiding in diagnosis, or evaluating new treatments. In Parkinson's disease (PD), these imaging methods have expanded our understanding of the disease beyond dopaminergic deficits. Moreover, functional imaging methods have described alterations in functional networks relating not only to the motor symptoms, but also to many nonmotor features of PD, such as cognitive dysfunction. From a clinical viewpoint, functional imaging methods can assist in monitoring disease progression, such as in the context of clinical trials, and holds the potential to aid in early diagnosis of PD and differentiation from other parkinsonian disorders.

The effects of fast-off-D2 receptor antagonism on L-DOPA-induced dyskinesia and psychosis in parkinsonian macaques

3,4-Dihydroxyphenylalanine (L-DOPA) treatment of Parkinson's disease (PD) is compromised by motor side effects, such as dyskinesia and non-motor problems, including psychosis. Because of the marked reduction in brain dopamine in PD and the resultant dopamine D2 receptor supersensitivity, it is impossible to use standard potent dopamine D2 receptor antagonists such as haloperidol to alleviate side effects without compromising the anti-parkinsonian benefits of L-DOPA. Haloperidol antagonizes D2 receptors with high affinity and slowly dissociates from D2 receptors (50% dissociation at 38min). We hypothesized that a rapidly dissociating D2 antagonist might allow some functional dopaminergic transmission and thus have a profile, with respect to reduction of dyskinesia and anti-parkinsonian effects, that was more useful therapeutically. The present study tested the principle of using a fast-off-D2 drug, CLR151 (50% dissociation at 23s) to modify L-DOPA actions in cynomolgus macaques with MPTP-parkinsonism. CLR151 (100mg/kg p.o.) reduced L-DOPA-induced dyskinesia and activity in the parkinsonian macaque by 86% and 52% respectively during peak action. CLR151 (100mg/kg) also reduced psychosis-like behaviour (i.e. reduced apparent visual hallucinations by 78%). Nevertheless, this dose of CLR151 significantly reduced the duration of anti-parkinsonian action of L-DOPA, ON-time (by 90%), and increased parkinsonian disability (by 57%). These data suggest that fast-off-D2 dopamine receptor antagonists, with D2-off-rate values close to those for CLR151, are unlikely to be useful in the treatment of dyskinesia and psychosis in PD. However, fast-off-D2 drugs could provide benefit if new congeners would have an even faster dissociation rate. Such drugs are now becoming available.

CNS penetration of the opioid glycopeptide MMP-2200: a microdialysis study

Endogenous opioid peptides enkephalin and dynorphin are major co-transmitters of striatofugal pathways of the basal ganglia. They are involved in the genesis of levodopa-induced dyskinesia and in the modulation of direct and indirect striatal output pathways that are disrupted in Parkinson's disease. One pharmacologic approach is to develop synthetic glycopeptides closely resembling endogenous peptides to restore their normal functions. Glycosylation promotes penetration of the blood-brain barrier. We investigated CNS penetration of the opioid glycopeptide MMP-2200, a mixed δ/μ-agonist based on leu-enkephalin, as measured by in vivo microdialysis and subsequent mass spectrometric analysis in awake, freely moving rats. The glycopeptide (10 mg/kg) reaches the dorsolateral striatum (DLS) rapidly after systemic (i.p.) administration and is stably detectable for the duration of the experiment (80 min). The detected level at the end of the experiment (around 250 pM) is about 10-fold higher than the level of the endogenous leu-enkephalin, measured simultaneously. This is one of the first studies to directly prove that glycosylation of an endogenous opioid peptide leads to excellent blood-brain barrier penetration after systemic injection, and explains robust behavioral effects seen in previous studies by measuring how much glycopeptide reaches the target structure, in this case the DLS.

Animal models of Parkinson's disease: a source of novel treatments and clues to the cause of the disease

Animal models of Parkinson's disease (PD) have proved highly effective in the discovery of novel treatments for motor symptoms of PD and in the search for clues to the underlying cause of the illness. Models based on specific pathogenic mechanisms may subsequently lead to the development of neuroprotective agents for PD that stop or slow disease progression. The array of available rodent models is large and ranges from acute pharmacological models, such as the reserpine- or haloperidol-treated rats that display one or more parkinsonian signs, to models exhibiting destruction of the dopaminergic nigro-striatal pathway, such as the classical 6-hydroxydopamine (6-OHDA) rat and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse models. All of these have provided test beds in which new molecules for treating the motor symptoms of PD can be assessed. In addition, the emergence of abnormal involuntary movements (AIMs) with repeated treatment of 6-OHDA-lesioned rats with L-DOPA has allowed for examination of the mechanisms responsible for treatment-related dyskinesia in PD, and the detection of molecules able to prevent or reverse their appearance. Other toxin-based models of nigro-striatal tract degeneration include the systemic administration of the pesticides rotenone and paraquat, but whilst providing clues to disease pathogenesis, these are not so commonly used for drug development. The MPTP-treated primate model of PD, which closely mimics the clinical features of PD and in which all currently used anti-parkinsonian medications have been shown to be effective, is undoubtedly the most clinically-relevant of all available models. The MPTP-treated primate develops clear dyskinesia when repeatedly exposed to L-DOPA, and these parkinsonian animals have shown responses to novel dopaminergic agents that are highly predictive of their effect in man. Whether non-dopaminergic drugs show the same degree of predictability of response is a matter of debate. As our understanding of the pathogenesis of PD has improved, so new rodent models produced by agents mimicking these mechanisms, including proteasome inhibitors such as PSI, lactacystin and epoximycin or inflammogens like lipopolysaccharide (LPS) have been developed. A further generation of models aimed at mimicking the genetic causes of PD has also sprung up. Whilst these newer models have provided further clues to the disease pathology, they have so far been less commonly used for drug development. There is little doubt that the availability of experimental animal models of PD has dramatically altered dopaminergic drug treatment of the illness and the prevention and reversal of drug-related side effects that emerge with disease progression and chronic medication. However, so far, we have made little progress in moving into other pharmacological areas for the treatment of PD, and we have not developed models that reflect the progressive nature of the illness and its complexity in terms of the extent of pathology and biochemical change. Only when this occurs are we likely to make progress in developing agents to stop or slow the disease progression. The overarching question that draws all of these models together in the quest for better drug treatments for PD is how well do they recapitulate the human condition and how predictive are they of successful translation of drugs into the clinic? This article aims to clarify the current position and highlight the strengths and weaknesses of available models.

Synergistic antidyskinetic effects of topiramate and amantadine in animal models of Parkinson's disease

L-Dopa-induced dyskinesia in patients with Parkinson's disease can be alleviated by amantadine, an antagonist at N-methyl-D-aspartate glutamate receptors. The antiepileptic drug topiramate, which blocks α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, has also been shown to reduce dyskinesia. The purpose of this study was to examine the behavioral pharmacology of topiramate alone and in combination with amantadine in animal models of PD and L-dopa-induced dyskinesia. The effects of topiramate (5-20 mg/kg) and amantadine (5-20 mg/kg) on abnormal involuntary movements (the rat homologue of dyskinesia) and Rotarod performance were assessed alone and in combination in the 6-hydroxydopamine-lesioned rat following chronic L-dopa treatment. Dyskinesia, parkinsonian disability, and "on-time" were assessed in the MPTP-lesioned nonhuman primate following administration of topiramate (5-20 mg/kg) and amantadine (0.1-1.0 mg/kg) alone and in combination. Topiramate and amantadine dose-dependently reduced dyskinesia in the 6-hydroxydopamine-lesioned rat, whereas topiramate reduced Rotarod performance; there was no effect on parkinsonian disability in the MPTP-lesioned nonhuman primate, in which both drugs reduced dyskinesia. Topiramate and amantadine exhibited differential antidyskinetic effects on dyskinesia elicited by the dopamine D1 receptor agonist SKF 38393 (2 mg/kg). Subthreshold doses of both drugs in combination had a synergistic effect on dyskinesia in the 6-hydroxydopamine-lesioned rat, with no worsening of motor performance; this effect was confirmed in the MPTP-lesioned nonhuman primate, with a selective reduction in "bad on-time." These data confirm the antidyskinetic potential of topiramate and suggest that combination with low-dose amantadine may allow better reduction of dyskinesia with no adverse motor effects.

Effects of the novel glycopeptide opioid agonist MMP-2200 in preclinical models of Parkinson's disease

In Parkinson's disease (PD), the consequence of dopaminergic denervation is an imbalance in the activity of the direct and indirect striatofugal pathways, which include potentially important changes in opioid peptide expression and/or activity. The systemic administration of a novel glycosylated opioid peptide MMP-2200 (a.k.a. lactomorphin) was shown to have potent effects in two standard models of PD: 1) amphetamine-induced rotations in the hemi-Parkinsonian 6-hydroxydopamine (6-OHDA)-treated rat and 2) locomotion in the reserpine-treated rat. MMP-2200, an opioid mu and delta receptor agonist, reduced amphetamine-induced rotations in severely-lesioned hemi-Parkinsonian rats; this effect was fully blocked by naloxone, an opioid receptor antagonist. The selective δ-opioid receptor antagonist naltrindole only partially blocked the effect of MMP-2200. MMP-2200 alone did not induce rotations. This effect was also observed in a mild progressive rat 6-OHDA-lesion model. In animals treated with reserpine, profound akinesia was induced that was reversed with apomorphine. There was a prominent overshoot in animals that received apomorphine compared to non-reserpine treated animals, reflecting the well described phenomenon of dopamine supersensitivity indicating that apomorphine not only reversed akinesia but also induced hyper-kinesia. The opioid peptide MMP-2200 blocked the apomorphine-induced hyper-kinesia. This effect of MMP-2200 was prevented by pre-administration of naloxone. MMP-2200 had no effect in preventing the reserpine-induced akinesia, nor did it affect locomotion in control animals. Taken together, the results from these two models are consistent with the glycopeptide opioid agonist MMP-2200 having a potent effect on movements related to dopaminergic hyper-stimulation following striatal dopamine depletion that are best explained by a reduction in the downstream effects of dopamine agonists in these models.