The 5-HT2A/2C inverse agonist nelotanserin alleviates L-DOPA-induced dyskinesia in the MPTP-lesioned marmoset.

Nelotanserin is a serotonin 2A and 2C (5-HT2A/2C) inverse agonist that was previously tested in the clinic for rapid-eye movement sleep behaviour disorder and psychosis in patients with Parkinson's disease (PD) dementia. Its effect on L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia has however not been investigated. As 5-HT2A antagonism/inverse agonism is a validated approach to alleviate dyskinesia, we undertook the current study to evaluate the anti-dyskinetic potential of nelotanserin in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset. Parkinsonism was induced in six common marmosets (Callithrix jacchus, three females and three males) that were then chronically treated with L-DOPA to induce dyskinesia. On experimental days, they were administered L-DOPA in combination with vehicle or nelotanserin (0.1, 0.3 and 1 mg/kg) subcutaneously, in a randomised fashion. Dyskinesia and parkinsonism were rated post hoc by a blinded observer. In comparison to vehicle, the addition of nelotanserin 0.3 and 1 mg/kg to L-DOPA diminished peak dose dyskinesia by 47% (P < 0.05) and 69% (P < 0.001). Nelotanserin 0.3 and 1 mg/kg also reduced the severity of global dyskinesia, by 40% (P < 0.01) and 55% (P < 0.001), when compared to vehicle. Nelotanserin 0.1 mg/kg did not alleviate peak dose or global dyskinesia severity. Nelotanserin had no impact on the anti-parkinsonian action of L-DOPA. Our results highlight that nelotanserin may represent an efficacious anti-dyskinetic drug and provide incremental evidence of the potential benefit of 5-HT2A/2C antagonism/inverse agonism for drug-induced dyskinesia in PD.

Effect of the mGlu2 positive allosteric modulator biphenyl-indanone A as a monotherapy and as adjunct to a low dose of L-DOPA in the MPTP-lesioned marmoset.

Activation of metabotropic glutamate 2 (mGlu2) receptors is a potential novel therapeutic approach for the treatment of parkinsonism. Thus, when administered as monotherapy or as adjunct to a low dose of L-3,4-dihydroxyphenylalanine (L-DOPA), the mGlu2 positive allosteric modulator (PAM) LY-487,379 alleviated parkinsonism in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned primates. Here, we sought to investigate the effect of biphenyl-indanone A (BINA), a highly selective mGlu2 PAM whose chemical scaffold is unrelated to LY-487,379, to determine if a structurally different mGlu2 PAM would also confer anti-parkinsonian benefit. In monotherapy experiments, MPTP-lesioned marmosets were injected with either vehicle, L-DOPA/benserazide (15/3.75 mg/kg, positive control) or BINA (0.1, 1, 10 mg/kg). In adjunct to a low L-DOPA dose experiments, MPTP-lesioned marmosets were injected with L-DOPA/benserazide (7.5/1.875 mg/kg) in combination with vehicle or BINA (0.1, 1, 10 mg/kg). Parkinsonism, dyskinesia and psychosis-like behaviours (PLBs) were then quantified. When administered alone, BINA 1 and 10 mg/kg decreased parkinsonism severity by ~22% (p < 0.01) and ~47% (p < 0.001), when compared with vehicle, which was comparable with the global effect of a high L-DOPA dose. When administered in combination with a low L-DOPA dose, BINA 1 and 10 mg/kg decreased global parkinsonism by ~38% (p < 0.001) and ~53% (p < 0.001). BINA 10 mg/kg decreased global dyskinesia by ~94% (p < 0.01) and global PLBs by ~92% (p < 0.01). Our results provide additional evidence that mGlu2 positive allosteric modulation elicits anti-parkinsonian effects. That this benefit is not related to a particular chemical scaffold suggests that it may be a class effect rather than the effect of a specific molecule.

Activation of mGlu 2/3 receptors with the orthosteric agonist LY-404,039 alleviates dyskinesia in experimental parkinsonism.

LY-404,039 is an orthosteric agonist at metabotropic glutamate 2 and 3 (mGlu 2/3 ) receptors, with a possible additional agonist effect at dopamine D 2 receptors. LY-404,039 and its pro-drug, LY-2140023, have previously been tested in clinical trials for psychiatric indications and could therefore be repurposed if they were shown to be efficacious in other conditions. We have recently demonstrated that the mGlu 2/3 orthosteric agonist LY-354,740 alleviated L-3,4-dihydroxyphenylalanine (L-DOPA)-induced abnormal involuntary movements (AIMs) in the 6-hydroxydopamine (6-OHDA)-lesioned rat without hampering the anti-parkinsonian action of L-DOPA. Here, we seek to take advantage of a possible additional D 2 -agonist effect of LY-404,039 and see if an anti-parkinsonian benefit might be achieved in addition to the antidyskinetic effect of mGlu 2/3 activation. To this end, we have administered LY-404,039 (vehicle, 0.1, 1 and 10 mg/kg) to 6-OHDA-lesioned rats, after which the severity of axial, limbs and oro-lingual (ALO) AIMs was assessed. The addition of LY-404,039 10 mg/kg to L-DOPA resulted in a significant reduction of ALO AIMs over 60-100 min (54%, P  < 0.05). In addition, LY-404,039 significantly enhanced the antiparkinsonian effect of L-DOPA, assessed through the cylinder test (76%, P  < 0.01). These results provide further evidence that mGlu 2/3 orthosteric stimulation may alleviate dyskinesia in PD and, in the specific case of LY-404,039, a possible D 2 -agonist effect might also make it attractive to address motor fluctuations. Because LY-404,039 and its pro-drug have been administered to humans, they could possibly be advanced to Phase IIa trials rapidly for the treatment of motor complications in PD.

[3H]-NFPS binding to the glycine transporter 1 in the hemi-parkinsonian rat brain.

L-3,4-dihydroxyphenylalanine (L-DOPA) is the main treatment for Parkinson's disease (PD) but with long term administration, motor complications such as dyskinesia are induced. Glycine transporter 1 (GlyT1) inhibition was shown to produce an anti-dyskinetic effect in parkinsonian rats and primates, coupled with an improvement in the anti-parkinsonian action of L-DOPA. The expression of GlyT1 in the brain in the dyskinetic state remains to be investigated. Here, we quantified the levels of GlyT1 across different brain regions using [3H]-NFPS in the presence of Org-25,935. Brain sections were chosen from sham-lesioned rats, L-DOPA-naïve 6-hydroxydopamine (6-OHDA)-lesioned rats and 6-OHDA-lesioned rats exhibiting mild or severe abnormal involuntary movements (AIMs). [3H]-NFPS binding decreased in the ipsilateral and contralateral thalamus, by 28% and 41%, in 6-OHDA-lesioned rats with severe AIMs compared to sham-lesioned animals (P < 0.01 and 0.001). [3H]-NFPS binding increased by 21% in the ipsilateral substantia nigra of 6-OHDA-lesioned rats with severe AIMs compared to 6-OHDA-lesioned rats with mild AIMs (P < 0.05). [3H]-NFPS binding was lower by 19% in the contralateral primary motor cortex and by 20% in the contralateral subthalamic nucleus of 6-OHDA-lesioned rats with mild AIMs animals compared to rats with severe AIMs (both P < 0.05). The severity of AIMs scores positively correlated with [3H]-NFPS binding in the ipsilateral substantia nigra (P < 0.05), ipsilateral entopeduncular nucleus (P < 0.05) and contralateral primary motor cortex (P < 0.05). These data provide an anatomical basis to explain the efficacy of GlyT1 inhibitors in dyskinesia in PD.

Effect of the mGlu2 positive allosteric modulator CBiPES on dyskinesia, psychosis-like behaviours and parkinsonism in the MPTP-lesioned marmoset.

Advanced Parkinson's disease (PD) is often complicated by the occurrence of dyskinesia, motor fluctuations and psychosis. To this day, few treatment options are available for each of these phenomena, and they are at times not effective or elicit adverse events, leaving some patients short of therapeutic options. We have recently shown that positive allosteric modulation of metabotropic 2 (mGlu2) receptors with the prototypical positive allosteric modulator (PAM) LY-487,379 is efficacious at alleviating both dyskinesia and psychosis-like behaviours (PLBs), while simultaneously enhancing the anti-parkinsonian action of L-3,4-dihydroxyphenylalanine (L-DOPA), in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset. Here, we assessed the effects of CBiPES, a mGlu2 PAM derived from LY-487,379, but with improved pharmacokinetic properties. Six MPTP-lesioned marmosets with reproducible dyskinesia and PLBs were administered L-DOPA in combination with vehicle or CBiPES (0.1, 1 and 10 mg/kg), after which their behaviour was rated. CBiPES 10 mg/kg reduced global dyskinesia by 60% (P < 0.0001), while peak dose dyskinesia was reduced by 66% (P < 0.001), compared to L-DOPA/vehicle. CBiPES 10 mg/kg also diminished global PLBs by 56% (P < 0.0001), while peak dose PLBs were reduced by 64% (P < 0.001), compared to L-DOPA/vehicle. Lastly, CBiPES enhanced the anti-parkinsonian action of L-DOPA, by reducing global parkinsonian disability by 43% (P < 0.01), compared to L-DOPA/vehicle. Our results provide further evidence that mGlu2 positive allosteric modulation may be an approach that could be efficacious for the treatment of dyskinesia, psychosis and motor fluctuations in PD.

Granisetron, a selective 5-HT3 antagonist, reduces L-3,4-dihydroxyphenylalanine-induced abnormal involuntary movements in the 6-hydroxydopamine-lesioned rat.

Administration of L-3,4-dihydroxyphenylalanine (L-DOPA) provides Parkinson's disease patients with effective symptomatic relief. However, long-term L-DOPA therapy is often marred by complications such as dyskinesia. We have previously demonstrated that serotonin type 3 (5-HT3) receptor blockade with the clinically available and highly selective antagonist ondansetron alleviates dyskinesia in the 6-hydroxydopamine (6-OHDA)-lesioned rat. Here, we sought to explore the antidyskinetic efficacy of granisetron, another clinically available 5-HT3 receptor antagonist. Rats were rendered hemi-parkinsonian by 6-OHDA injection in the medial forebrain bundle. Following induction of stable abnormal involuntary movements (AIMs), granisetron (0.0001, 0.001, 0.01, 0.1 and 1 mg/kg) or vehicle was acutely administered in combination with L-DOPA and the severity of AIMs, both duration and amplitude, was determined. We also assessed the effect of granisetron on L-DOPA antiparkinsonian action by performing the cylinder test. Adding granisetron (0.0001, 0.001, 0.01, 0.1 and 1 mg/kg) to L-DOPA resulted in a significant reduction of AIMs duration and amplitude, with certain parameters being reduced by as much as 38 and 45% (P < 0.05 and P < 0.001, respectively). The antidyskinetic effect of granisetron was not accompanied by a reduction of L-DOPA antiparkinsonian action. These results suggest that 5-HT3 blockade may reduce L-DOPA-induced dyskinesia without impairing the therapeutic efficacy of L-DOPA. However, a U-shaped dose-response curve obtained with certain parameters may limit the therapeutic potential of this strategy and require further investigation.

The highly selective mGlu2 receptor positive allosteric modulator LY-487,379 alleviates l-DOPA-induced dyskinesia in the 6-OHDA-lesioned rat model of Parkinson's disease.

l-3,4-Dihydroxyphenylalanine (l-DOPA) is the most effective treatment for Parkinson's disease (PD), but its use over a long period is marred by motors complications such as dyskinesia. We previously demonstrated that selective metabotropic glutamate 2/3 (mGlu2/3 ) receptor activation with LY-354,740 alleviates dyskinesia in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset and the 6-hydroxydopamine (6-OHDA)-lesioned rat. Here, we sought to determine the role played by selective mGlu2 activation in the anti-dyskinetic effect of mGlu2/3 stimulation and have investigated the effect of the highly selective mGlu2 positive allosteric modulator LY-487,379 at alleviating established, and preventing the development of, l-DOPA-induced dyskinesia in the 6-OHDA-lesioned rat. First, dyskinetic 6-OHDA-lesioned rats were administered l-DOPA in combination with LY-487,379 (0.1, 1 and 10 mg/kg) or vehicle, and the severity of dyskinesia was determined. Second, 6-OHDA-lesioned rats were administered LY-487,379 (0.1 or 1 mg/kg), started concurrently with l-DOPA, once daily for 22 days, and dyskinesia severity was evaluated weekly for four consecutive weeks. We also assessed the effect of LY-487,379 on l-DOPA anti-parkinsonian effect. We found that acute challenges of LY-487,379 0.1 mg/kg in combination with l-DOPA, significantly diminished dyskinesia severity, by ≈54% (p < .01), when compared to vehicle. Moreover, animals treated with l-DOPA/LY-487,379 0.1 and 1 mg/kg during the dyskinesia induction phase exhibited milder dyskinesia, by ≈74% and ≈61%, respectively (both p < .01), when compared to l-DOPA/vehicle. LY-487,379 did not impair l-DOPA anti-parkinsonian activity. These results suggest that mGlu2 activation may be an effective and promising therapeutic strategy to alleviate the severity and prevent the development of dyskinesia.

Combined mGlu2 orthosteric stimulation and positive allosteric modulation alleviates L-DOPA-induced psychosis-like behaviours and dyskinesia in the parkinsonian marmoset.

In recent studies performed in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset model of Parkinson's disease (PD), we have demonstrated that activation of the metabotropic glutamate 2 (mGlu2) receptor with the orthosteric agonist (OA) LY-354,740 and the positive allosteric modulator (PAM) LY-487,379 is effective at alleviating both dyskinesia and psychosis-like behaviours (PLBs) triggered by the administration of L-3,4-dihydroxyphenylalanine (L-DOPA). Because mGlu2 OAs and PAMs bind to different sites on the receptor, we hypothesised that greater reductions of dyskinesia and PLBs would be obtained upon concurrent administration of LY-354,740 and LY-487,379. In experiments performed in six MPTP-lesioned marmosets, we administered LY-354,740 (0.1 mg/kg), LY-487,379 (1 mg/kg), LY-354,740 (0.1 mg/kg) + LY-487,379 (1 mg/kg), or vehicle, in combination with L-DOPA and determined the effect of each treatment on dyskinesia, PLBs, and parkinsonism. When compared to vehicle, LY-354,740 and LY-487,379, administered alone or concurrently, significantly reduced dyskinesia. The combination LY-354,740 + LY-487,379 provided mild additional benefit when compared to LY-487,379 alone, but not compared to LY-354,740. For PLBs, when compared to vehicle treatment, LY-354,740, LY-487,379, and combination thereof all alleviated the abnormal behaviours, but the combination LY-354,740 + LY-487,379 did not provide greater relief than either drug alone. The anti-parkinsonian effect of L-DOPA was not altered by any of the treatments. Our results provide further evidence that mGlu2 activation might be a novel approach to treat L-DOPA-induced dyskinesia and dopaminergic psychosis in PD. However, they do not suggest that greater therapeutic effect would be achieved upon combining an mGlu2 OA and an mGlu2 PAM.

Pharmacokinetic profile of the selective 5-HT3 receptor antagonist ondansetron in the rat: an original study and a minireview of the behavioural pharmacological literature in the rat.

The availability of agonists and antagonists to modulate the activity of the 5-hydroxytryptamine (5-HT) type 3 (5-HT3) receptor has renewed interest in its role as a therapeutic target. Ondansetron is a highly selective 5-HT3 receptor antagonist that is well tolerated as an anti-emetic for patients undergoing chemotherapy. Preclinical studies in rat have shown the effects of small doses of ondansetron on cognition, behavioural sensitisation, and epilepsy. However, the pharmacokinetic (PK) profile of ondansetron in rat has not been described, which limits the translational relevance of these findings. Here, we aim to determine, in the rat, the PK profile of ondansetron in the plasma and to determine associated brain levels. The plasma PK profile was determined following acute subcutaneous administration of ondansetron (0.1, 1, and 10 µg/kg). Brain levels were measured following subcutaneous administration of ondansetron at 1 µg/kg. Plasma and brain levels of ondansetron were determined using high-performance liquid chromatography - tandem mass spectrometry. Following administration of all three doses, measured ondansetron plasma levels (≈30-3000 pg/mL) were below levels achieved with doses usually administered in the clinic, with a rapid absorption phase and a short half-life (≈30-40 min). We also found that brain levels of ondansetron at 1 µg/kg were significantly lower than plasma levels, with brain to plasma ratios of 0.45 and 0.46 in the motor and pre-frontal cortices. We discuss our findings in the context of a minireview of the literature. We hope that our study will be helpful to the design of preclinical studies with therapeutic end-points.

Classic animal models of Parkinson's disease: a historical perspective.

The quest to better understand the pathophysiology of Parkinson's disease (PD) and to find new therapies to provide greater relief to affected patients continues. The use of animal models of PD has been invaluable in the process. Here, we review, through a historical lens, some of the contribution of the 6-hydroxydopamine-lesioned rat and of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned nonhuman primate, in refining our understanding of PD and its treatment-related complications. We examine the mechanisms underlying the toxicity of 6-hydroxydopamine and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, and then explore some of the advances at the molecular, pharmacological, electrophysiological and surgical levels made while experimenting on these animal models. We also discuss behavioural testing that can be performed with these animal models and highlight some of their limitations.

5-HT2A blockade for dyskinesia and psychosis in Parkinson's disease: is there a limit to the efficacy of this approach? A study in the MPTP-lesioned marmoset and a literature mini-review.

Virtually every patient affected by Parkinson's disease (PD) eventually requires treatment with L-3,4-dihydroxyphenylalanine (L-DOPA), which leads to complications such as dyskinesia and psychosis. Whereas blockade of serotonin 2A (5-HT2A) receptors appears to be an effective way to reduce both dyskinesia and psychosis, whether it has the potential to eliminate the two phenomena remains to be determined. In a previous study, we showed that highly selective 5-HT2A receptor blockade with EMD-281,014, at plasma levels comparable to those achieved in the clinic, reduced dyskinesia and psychosis-like behaviours (PLBs), in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset. Here, we sought to determine whether further increasing the dose would result in greater therapeutic benefit or if maximal effectiveness was achieved at lower doses. Six MPTP-lesioned marmosets with stable dyskinesia and PLBs were administered EMD-281,014 (0.1, 1 and 10 mg/kg) or vehicle in combination with L-DOPA and the effect on dyskinesia, PLBs and parkinsonism was assessed. Administration of EMD-281,014 (0.1, 1 and 10 mg/kg) in combination with L-DOPA resulted in a significant reduction in the severity of dyskinesia, by up to 63%, 64% and 61% (each P < 0.001), when compared to L-DOPA/vehicle. Similarly, the addition of EMD-281,014 (0.1, 1 and 10 mg/kg) to L-DOPA also significantly decreased the severity of PLBs, by up to 54%, 55% and 53% (each P < 0.001), when compared to L-DOPA/vehicle. Our results suggest that there might be a ceiling to the reduction of dyskinesia and psychosis that can be achieved through antagonism of 5-HT2A receptors.

Effect of the selective 5-HT2A receptor antagonist EMD-281,014 on L-DOPA-induced abnormal involuntary movements in the 6-OHDA-lesioned rat.

L-3,4-Dihydroxyphenylalanine (L-DOPA) is the most effective therapy for motor symptoms of Parkinson's disease (PD); however, with repeated administration, as many as 94% of PD patients develop complications such as L-DOPA-induced dyskinesia. We previously demonstrated that EMD-281,014, a highly selective serotonin 2A (5-HT2A) receptor antagonist, reduces the severity of dyskinesia in the parkinsonian marmoset, without interfering with L-DOPA anti-parkinsonian benefit. Here, we assessed the effects of EMD-281,014 on L-DOPA-induced abnormal involuntary movements (AIMs) in the 6-hydroxydopamine (6-OHDA)-lesioned rat. We first determined the pharmacokinetic profile of EMD-281,014, to administer doses leading to clinically relevant plasma levels in the behavioural experiments. Dyskinetic 6-OHDA-lesioned rats were then administered EMD-281,014 (0.01, 0.03 and 0.1 mg/kg) or vehicle in combination with L-DOPA and AIMs severity was evaluated. We also assessed the effect of EMD-281,014 on L-DOPA anti-parkinsonian action with the cylinder test. We found that the addition of EMD-281,014 (0.01, 0.03 and 0.1 mg/kg) to L-DOPA did not reduce AIMs severity (P > 0.05), when compared to vehicle. EMD-281,014 did not compromise L-DOPA anti-parkinsonian action. Our results suggest that the highly selective 5-HT2A receptor antagonist EMD-281,014 is well-tolerated by parkinsonian rats, but does not attenuate L-DOPA-induced AIMs. Our results highlight differences between rodent and primate models of PD when it comes to determining the anti-dyskinetic action of 5-HT2A receptor antagonists.

Autoradiographic labelling of metabotropic glutamate type 2/3 receptors in the hemi-parkinsonian rat brain.

L-3,4-dihydroxyphenylalanine (L-DOPA) is the treatment of choice for Parkinson's disease (PD) motor symptoms, but its chronic use is hindered by complications such as dyskinesia. Pre-clinical studies discovered that activation of metabotropic glutamate type 2 and 3 (mGlu2/3) receptors alleviates L-DOPA-induced dyskinesia. To gain mechanistic insight into the anti-dyskinetic activity of mGlu2/3 activation, we performed autoradiographic binding with [3H]-LY-341,495 in brain sections from L-DOPA-treated 6-hydroxydopamine (6-OHDA)-lesioned rats that developed mild or severe dyskinesia, as well as L-DOPA-untreated 6-OHDA-lesioned and sham-lesioned animals. In the ipsilateral hemisphere, mildly dyskinetic 6-OHDA-lesioned rats showed a decrease in [3H]-LY-341,495 binding in the entopeduncular nucleus (EPN, 30 % vs sham-lesioned rats, P<0.05), globus pallidus (GP, 28 % vs sham-lesioned rats, P<0.05; 23 % vs L-DOPA-untreated 6-OHDA-lesioned rats, P<0.001), and primary motor cortex (49 % vs sham-lesioned rats, P<0.05; 45 % vs L-DOPA-untreated 6-OHDA-lesioned rats, P<0.001). Severely dyskinetic 6-OHDA-lesioned rats exhibited an increase in binding in the primary motor cortex (43 % vs mildly dyskinetic 6-OHDA-lesioned rats, P<0.05). In the contralateral hemisphere, mildly dyskinetic 6-OHDA-lesioned rats harboured a decrease in binding in the EPN (30 % vs sham-lesioned rats; 24 % vs L-DOPA-untreated 6-OHDA-lesioned rats, both P<0.05), GP (34 % vs sham-lesioned rats, P<0.05; 23 % vs L-DOPA-untreated 6-OHDA-lesioned rats, P<0.001), and primary motor cortex (50 % vs sham-lesioned rats; 44 % vs L-DOPA-untreated 6-OHDA-lesioned rats, both P<0.05). Severely dyskinetic 6-OHDA-lesioned rats presented a decrease in binding in the GP (30 % vs sham-lesioned rats; 19 % vs L-DOPA-untreated 6-OHDA-lesioned rats, both P<0.05). Abnormal involuntary movements scores of 6-OHDA-lesioned animals were positively correlated with [3H]-LY-341,495 binding in the ipsilateral striatum, ipsilateral EPN, ipsilateral primary motor cortex and contralateral primary motor cortex (all P<0.05). These results suggest that alterations in mGlu2/3 receptor levels may be part of an endogenous compensatory mechanism to alleviate dyskinesia.

Effect of mGluR2 and mGluR2/3 activators on parkinsonism in the MPTP-lesioned non-human primate.

We have previously discovered that the selective activation of metabotropic glutamate type 2 receptors (mGluR2) and concurrent stimulation of metabotropic glutamate types 2 and 3 receptors (mGluR2/3) enhance the anti-parkinsonian action of L-3,4-dihydroxyphenylalanine (L-DOPA). Here, we sought to determine the effects of the mGluR2/3 orthosteric agonists LY-354,740 and LY-404,039, as well as the effects of the mGluR2 positive allosteric modulators LY-487,379 and CBiPES on the range of movement, bradykinesia, posture and alertness as adjuncts to L-DOPA. Ten 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmosets entered 4 experimental streams: L-DOPA + LY-354,740 (vehicle, 0.1, 0.3 and 1 mg/kg), L-DOPA + LY-404,039 (vehicle, 0.1, 1 and 10 mg/kg), L-DOPA + LY-487,379 (vehicle, 0.1, 1 and 10 mg/kg), L-DOPA + CBiPES (vehicle, 0.1, 1 and 10 mg/kg). For each molecule, treatments were randomised, and the range of movement, bradykinesia, posture and alertness were assessed by a blinded rater. None of the tested compounds significantly altered the global range of movement. LY-404,039 and CBiPES both reduced global bradykinesia, by up to 46% (both P < 0.05). LY-354,740, LY-404,039 and CBiPES each improved global posture by 35%, 44% and 39% (each P < 0.05), respectively. LY-404,039 and CBiPES both enhanced alertness by 54% (P < 0.05) and 79% (P < 0.01), respectively. LY-487,379 did not improve any of the parameters. Our results suggest that selective mGluR2 positive allosteric modulation and combined mGluR2/3 orthosteric stimulation might benefit bradykinesia, posture and alertness in PD when added to L-DOPA, which potentially represent novel therapeutic indications for molecules acting via these mechanisms.

Positive allosteric mGluR2 modulation with BINA alleviates dyskinesia and psychosis-like behaviours in the MPTP-lesioned marmoset.

There is mounting evidence that positive allosteric modulation of metabotropic glutamate type 2 receptors (mGluR2) is an efficacious approach to reduce the severity of L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia, psychosis-like behaviours (PLBs), while conferring additional anti-parkinsonian benefit. However, the mGluR2 positive allosteric modulators (PAMs) tested so far, LY-487,379 and CBiPES, share a similar chemical scaffold. Here, we sought to assess whether similar benefits would be conferred by a structurally-distinct mGluR2 PAM, biphenylindanone A (BINA). Six 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmosets exhibiting dyskinesia and PLBs were administered L-DOPA with either vehicle or BINA (0.1, 1, and 10 mg/kg) in a randomised within-subject design and recorded. Behaviour was analysed by a blinded rater who scored the severity of each of parkinsonism, dyskinesia and PLBs. When added to L-DOPA, BINA 0.1 mg/kg, 1 mg/kg, and 10 mg/kg all significantly reduced the severity of global dyskinesia, by 40%, 52% and 53%, (all P < 0.001) respectively. BINA similarly attenuated the severity of global PLBs by 35%, 48%, and 50%, (all P < 0.001) respectively. Meanwhile, BINA did not alter the effect of L-DOPA on parkinsonism exhibited by the marmosets. The results of this study provide incremental evidence of positive allosteric modulation of mGluR2 as an effective therapeutic strategy for alleviating dyskinesia and PLBs, without hindering the anti-parkinsonian action of L-DOPA. Furthermore, this therapeutic benefit does not appear to be confined to a particular chemical scaffold.

Metabotropic glutamate receptors in Parkinson's disease.

Parkinson's disease (PD) is a complex disorder that leads to alterations in multiple neurotransmitter systems, notably glutamate. As such, several drugs acting at glutamatergic receptors have been assessed to alleviate the manifestation of PD and treatment-related complications, culminating with the approval of the N-methyl-d-aspartate (NMDA) antagonist amantadine for l-3,4-dihydroxyphenylalanine (l-DOPA)-induced dyskinesia. Glutamate elicits its actions through several ionotropic and metabotropic (mGlu) receptors. There are 8 sub-types of mGlu receptors, with sub-types 4 (mGlu4) and 5 (mGlu5) modulators having been tested in the clinic for endpoints pertaining to PD, while sub-types 2 (mGlu2) and 3 (mGlu3) have been investigated in pre-clinical settings. In this book chapter, we provide an overview of mGlu receptors in PD, with a focus on mGlu5, mGlu4, mGlu2 and mGlu3 receptors. For each sub-type, we review, when applicable, their anatomical localization and possible mechanisms underlying their efficacy for specific disease manifestation or treatment-induced complications. We then summarize the findings of pre-clinical studies and clinical trials with pharmacological agents and discuss the potential strengths and limitations of each target. We conclude by offering some perspectives on the potential use of mGlu modulators in the treatment of PD.

Pharmacokinetic profile of bitopertin, a selective GlyT1 inhibitor, in the rat.

Bitopertin, a selective glycine transporter 1 (GlyT1) inhibitor, has been extensively studied for the treatment of schizophrenia, with known safety and tolerability profiles in the clinic. Whereas several rodent experiments have been reported, the pharmacokinetic (PK) profile of bitopertin in rodents has not been extensively reported, as only two studies disclosed limited PK parameters in male rats after oral administration. Here, we determined the PK profile of bitopertin in female Sprague-Dawley rats. Blood samples were taken serially, before and after sub-cutaneous (0.03, 0.1, 0.3, 1, and 3 mg/kg) or intra-venous (0.1 mg/kg) administration. Plasma levels were determined by high-performance liquid chromatography coupled with heat-assisted electrospray ionisation tandem mass spectrometry (HPLC-HESI MS/MS). Subsequently, PK parameters were calculated using non-compartmental analysis, including area under the curve (AUC), time (Tmax) to maximal plasma concentration (Cmax), clearance (CL), volume of distribution (Vz), as well as half-life (T1/2). Following sub-cutaneous injection, bitopertin exhibited dose-dependent AUC0-∞ (439.6-34,018.9 ng/mL) and Tmax (3.7-24.0 h), a very long terminal T1/2 (35.06-110.32 h) and low CL (0.07-0.13 L/h/kg), suggesting that bitopertin is slowly absorbed and eliminated in the rat. The observed relationship between dose and the extent of drug exposure (AUC) was linear. Following administration of all sub-cutaneous doses, measured bitopertin plasma levels were comparable to levels achieved with doses already administered in the clinic. We hope that our results will be useful in the design of pre-clinical experiments in which this drug will eventually be administered sub-cutaneously.

Anti-parkinsonian effect of the mGlu2 positive allosteric modulator LY-487,379 as monotherapy and adjunct to a low L-DOPA dose in the MPTP-lesioned marmoset.

In previous experiments, we have discovered that positive allosteric modulation of metabotropic glutamate 2 (mGlu2) receptors enhances the anti-parkinsonian action of an optimal dose of L-3,4-dihydroxyphenylalanine (L-DOPA). Whether selective mGlu2 positive allosteric modulation would also alleviate parkinsonian disability as monotherapy or as adjunct to a sub-optimal dose of L-DOPA has not been determined. Here, we assessed the anti-parkinsonian effect of mGlu2 positive allosteric modulation as monotherapy and adjunct to a sub-optimal dose of L-DOPA in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmosets. The highly selective positive allosteric modulator (PAM) LY-487,379 was utilised to activate mGlu2 receptors. When administered as monotherapy, LY-487,379 10 mg/kg diminished global parkinsonism by 48% (P < 0.001) and increased duration of on-time by 7-fold, when compared to vehicle treatment (P < 0.05). When added to a sub-optimal dose of L-DOPA, LY-487,379 10 mg/kg decreased global parkinsonism by 44% (P < 0.001) and extended duration of on-time by 2.5-fold (P < 0.01). Our results indicate that selective mGlu2 positive allosteric modulation elicits anti-parkinsonian benefits as monotherapy and as adjunct to sub-optimal dose of L-DOPA paradigms, potentially suggesting that mGlu2 PAMs may have a therapeutic niche early in the treatment of PD as DOPA-sparing agents.

The mGluR2/3 orthosteric agonist LY-404,039 reduces dyskinesia, psychosis-like behaviours and parkinsonism in the MPTP-lesioned marmoset.

LY-404,039 is an orthosteric agonist of metabotropic glutamate 2 and 3 receptors (mGluR2/3) that may harbour additional agonist effect at dopamine D2 receptors. LY-404,039 and its pro-drug, LY-2140023, have previously entered clinical trials as treatment options for schizophrenia. They could therefore be repurposed, if proven efficacious, for other conditions, notably Parkinson's disease (PD). We have previously shown that the mGluR2/3 orthosteric agonist LY-354,740 alleviated L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia and psychosis-like behaviours (PLBs) in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset. Unlike LY-404,039, LY-354,740 does not stimulate dopamine D2 receptors, suggesting that LY-404,039 may elicit broader therapeutic effects in PD. Here, we sought to investigate the effect of this possible additional dopamine D2-agonist action of LY-404,039 by assessing its efficacy on dyskinesia, PLBs and parkinsonism in the MPTP-lesioned marmoset. We first determined the pharmacokinetic profile of LY-404,039 in the marmoset, in order to select doses resulting in plasma concentrations known to be well tolerated in the clinic. Marmosets were then injected L-DOPA with either vehicle or LY-404,039 (0.1, 0.3, 1 and 10 mg/kg). The addition of LY-404,039 10 mg/kg to L-DOPA resulted in a significant reduction of global dyskinesia (by 55%, P < 0.01) and PLBs (by 50%, P < 0.05), as well as reduction of global parkinsonism (by 47%, P < 0.05). Our results provide additional support of the efficacy of mGluR2/3 orthosteric stimulation at alleviating dyskinesia, PLBs and parkinsonism. Because LY-404,039 has already been tested in clinical trials, it could be repurposed for indications related to PD.

Effect of the mGlu4 positive allosteric modulator ADX-88178 on parkinsonism, psychosis-like behaviours and dyskinesia in the MPTP-lesioned marmoset.

RATIONALE: Positive allosteric modulation of metabotropic glutamate type 4 (mGlu4) receptors is a promising strategy to alleviate parkinsonian disability and L-3,4-dihydroxyphenylalanine (L-DOPA) induced dyskinesia. ADX-88178 is a highly selective mGlu4 positive allosteric modulator (PAM) that previously enhanced the anti-parkinsonian action of L-DOPA in the 6-hydroxydopamine-lesioned rat model of Parkinson's disease (PD). OBJECTIVES: We sought to explore the effects of ADX-88178 on psychosis-like behaviours (PLBs) in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset. We also aimed to determine the effect of ADX-88178 on parkinsonism and dyskinesia. METHODS: Six MPTP-lesioned marmosets were administered L-DOPA chronically to induce stable PLBs and dyskinesias. They were then administered ADX-88178 (0.01, 0.1 and 1 mg/kg) or vehicle, in combination with L-DOPA/benserazide (15/3.75 mg/kg), both sub-cutaneously, in a randomised fashion. PLBs, parkinsonism and dyskinesia were then measured. RESULTS: ADX-88178 mildly worsened global PLBs at the dose of 1 mg/kg (by 13%, P = 0.020). L-DOPA alone conferred 158 min of on-time, while the duration of on-time was 212 min (34% increase, P = 0.011), after adding ADX-88178 1 mg/kg to L-DOPA. Accordingly, ADX-88178 1 mg/kg reduced global parkinsonian disability, by 38% (P = 0.0096). ADX-88178 1 mg/kg diminished peak dose dyskinesia by 34% (P = 0.015). Minimal effects were provided by lower doses. CONCLUSIONS: Whereas these results provide additional evidence of the anti-parkinsonian and anti-dyskinetic effects of mGlu4 positive allosteric modulation as an adjunct to L-DOPA, they also suggest that ADX-88178 may exacerbate dopaminergic psychosis. Further studies are needed to evaluate this possible adverse effect of mGlu4 PAMs on PD psychosis.