Synergy between L-DOPA and a novel positive allosteric modulator of metabotropic glutamate receptor 4: implications for Parkinson's disease treatment and dyskinesia.

Group III metabotropic glutamate (mGlu) receptors are localized in presynaptic terminals within basal ganglia (BG) circuitry that become hyperactive due to dopamine depletion in Parkinson's disease (PD). For this reason, group III mGlu receptors, in particular mGlu4, have been considered as key strategic targets for non-dopaminergic pharmacological treatments aimed at modulating these synapses, without producing the well known side-effects of l-DOPA, in particular the highly disabling l-DOPA-induced dyskinesia (LID). Herein we add physiological and functional support to this hypothesis using Lu AF21934, a novel selective and brain-penetrant mGlu4 receptor positive allosteric modulator (PAM) tool compound. By in vitro electrophysiological recordings we demonstrate that Lu AF21934 inhibits corticostriatal synaptic transmission and enhances the effect of the orthosteric mGlu4 receptor-preferred agonist LSP1-2111. In naïve rats, Lu AF21934 dose-dependently (10 and 30 mg/kg) alleviated haloperidol-induced catalepsy. In hemiparkinsonian rats (unilateral 6-hydroxydopamine lesion of the substantia nigra pars compacta), Lu AF21934 alone did not affect akinesia at the doses tested (10 and 30 mg/kg). However, when Lu AF21934 was combined with sub-threshold doses of l-DOPA (1 and 5 mg/kg), it acted synergistically in alleviating akinesia in a dose-dependent manner and, notably, also reduced the incidence of LID but not its severity. Interestingly, these effects occurred at Lu AF21934 brain free concentrations that showed functional activity in in vitro screens (calcium flux and electrophysiology assays). These results support the potential for antiparkinsonian clinical use of a combined treatment consisting in l-DOPA and a mGlu4 receptor PAM to reduce efficacious l-DOPA doses (generally known as l-DOPA sparing), while maintaining the same benefit on PD motor troubles, and at the same time minimizing the development of LID. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

4-(1-Phenyl-1H-pyrazol-4-yl)quinolines as novel, selective and brain penetrant metabotropic glutamate receptor 4 positive allosteric modulators.

4-(1-Phenyl-1H-pyrazol-4-yl)quinoline (1) was identified by screening the Lundbeck compound collection, and characterized as having mGlu4 receptor positive allosteric modulator properties. Compound 1 is selective over other mGlu receptors and a panel of GPCRs, ion channels and enzymes, but has suboptimal lipophilicity and high plasma and brain non-specific binding. In view of the challenges at the hit-to-lead stage previously reported in the development of mGlu4 receptor positive allosteric modulators (PAMs), a thorough structure-mGlu4 PAM activity relationship study was conducted to interrogate the chemical tractability of this chemotype. The central pyrazole ring tolerates the addition of one or two methyl groups. The C-7 position of the quinoline ring provides a site tolerant to hydrophilic substituents, enabling the design of diverse analogs with good in vitro mGlu4 PAM potency and efficacy, as well as improved microsomal turnover in vitro, compared to 1. In spite of the excellent ligand efficiency of 1 (LE=0.43), optimization of in vitro potency for this series reached a plateau around EC(50)=200 nM.

6-Aryl-3-pyrrolidinylpyridines as mGlu5 receptor negative allosteric modulators.

A series of 6-aryl-3-pyrrolidinylpyridine analogs was explored as structurally novel negative allosteric modulators of the mGlu5 receptor lacking an alkyne or oxadiazole moiety. Compounds in this series were characterized by tractable SAR, good in vitro potencies and brain penetration in rodents.

Tricyclic thiazolopyrazole derivatives as metabotropic glutamate receptor 4 positive allosteric modulators.

There is an increasing amount of evidence to support that activation of the metabotropic glutamate receptor 4 (mGlu4 receptor), either with an orthosteric agonist or a positive allosteric modulator (PAM), provides impactful interventions in diseases such as Parkinson's disease, anxiety, and pain. mGlu4 PAMs may have several advantages over mGlu4 agonists for a number of reasons. As part of our efforts in identifying therapeutics for central nervous system (CNS) diseases such as Parkinson's disease, we have been focusing on metabotropic glutamate receptors. Herein we report our studies with a series of tricyclic thiazolopyrazoles as mGlu4 PAMs.