mGlu2 Receptor Agonism, but Not Positive Allosteric Modulation, Elicits Rapid Tolerance towards Their Primary Efficacy on Sleep Measures in Rats

Dusting off old blueprints: Is it time to reconsider metabotropic glutamate receptor 2 for therapeutic drug development?

The metabotropic glutamate receptor 2 (mGlu2) is a heavily studied therapeutic target in neuropsychiatry for which we anticipate a renewed interest in the near future. We review the rationale and the outcome of clinical trials with mGlu2/3 receptor agonists in schizophrenia, a field of intense research since a seminal publication by Patel and colleagues (2007). We summarize evidence about selective, potent and safe agents with quantifiable CNS penetration that can be used to test hypotheses of mGlu2 receptors involvement in neuropsychiatric diseases. We summarize lessons learned from previous programs that should be considered to maximize the probability of success when targeting orthosteric and allosteric enhancement of mGlu2 receptor function in schizophrenia and beyond. First, we propose expanding our focus beyond presynaptic mGlu2 receptor stimulation in schizophrenia to novel hypotheses and that the choice of a therapeutic indication no longer be dictated by commercial opportunity but following science as a driver. Second, evidence on internal validity of preclinical studies supporting efficacy claims in the mGlu2 field is very limited. This gap will need to be closed when reviewing the rationale to re-initiate efforts in this field. Third, the pomaglumetad program was halted due to insufficient clinical efficacy, partly because of the inability to identify a treatment responder population. In preclinical studies, effects of mGlu2/3 receptor stimulation also seemed to vary significantly between laboratories. Definition of the responsive subject population and development of response-predicting biomarkers is therefore one of the main avenues of further research in the mGlu2 field.

Metabotropic glutamate 2,3 receptor stimulation desensitizes agonist activation of G-protein signaling and alters transcription regulators in mesocorticolimbic brain regions

Metabotropic glutamate (mGlu) receptors are regulators of glutamate release and targets for development of therapies for hyperactive glutamatergic signaling. However, the effects of long-term stimulation of mGlu receptors on cellular signaling in the brain have not been described. This study investigated the effects of 2-day and 14-day osmotic mini-pump administration of the mGlu2,3 agonist LY379268 (3.0 mg kg-1  day-1 ) to rats on receptor-mediated G-protein activation and signaling in mesocorticolimbic regions in rat brain sections. A significant reduction in LY379268-stimulated [35 S]GTPγS binding was observed in the 14-day group in some cortical regions, prefrontal cortex, nucleus accumbens, and ventral pallidum. The 14-day LY379268 treatment group exhibited mGlu2 mRNA levels significantly lower in hippocampus, nucleus accumbens, caudate, and ventral pallidum. In both 2-day and 14-day treatment groups immunodetectable phosphorylated cAMP Response Element-Binding protein (CREB) was significantly reduced across all brain regions. In the 2-day group, we observed significantly lower immunodetectable CREB protein across all brain regions, which was subsequently increased in the 14-day group but failed to achieve control values. Neither immunodetectable extracellular signal-regulated kinase (ERK) protein nor phosphorylated ERK from 2-day or 14-day treatment groups differed significantly from control across all brain regions. However, the ratio of phosphorylated ERK to total ERK protein was significantly greater in the 14-day treatment group compared with the control. These results identify compensatory changes to mGlu2,3 signal transduction in rat brains after chronic systemic administration of agonist, which could be predictive of the mechanism of action in human pharmacotherapies.

Group II metabotropic glutamate receptor activation in the basolateral amygdala mediates individual differences in stress-induced changes in rapid eye movement sleep

Group II metabotropic glutamate receptors (mGluR2/3s) have been implicated in stress and trauma related disorders including post-traumatic stress disorder (PTSD). PTSD is characterized by flashbacks, anxiety, and sleep disturbances. While many people are exposed to trauma in their lifetime, only a small percentage go on to develop PTSD, indicating individual differences in stress and emotional processing. Wistar strain rats display directionally different rapid-eye movement sleep (REM) responses to footshock stress, with resilient rats having no change or an increase in REM and vulnerable rats having a significant reduction in REM compared to baseline. The basolateral nucleus of the amygdala (BLA) is key in regulating individual differences in stress-induced alterations in sleep. Group II metabotropic glutamate receptors (mGluR2/3s) negatively modulate glutamate and are implicated in fear, fear memory, and sleep. The current study evaluated the effect of mGluR2/3 agonist LY379268 (LY37) in BLA on stress and fear memory induced changes in sleep, EEG spectra, behavioral fear expression and physiological stress. These data indicate that vulnerable rats treated with LY37 have an attenuation of the REM reductions generally seen in vulnerable rats. Furthermore, LY37 altered EEG spectra in the delta (0.5-4.5 Hz) and theta (5-9.5 Hz) frequency. LY37 did not impact behavioral fear expression or physiological stress. Therefore, mGluR2/3s within BLA are implicated in regulating individual differences in sleep responses to fear- and stress-related memories.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

Development of Clickable Photoaffinity Ligands for Metabotropic Glutamate Receptor 2 Based on Two Positive Allosteric Modulator Chemotypes

The metabotropic glutamate receptor 2 (mGlu₂) is a transmembrane-spanning class C G protein-coupled receptor that is an attractive therapeutic target for multiple psychiatric and neurological disorders. A key challenge has been deciphering the contribution of mGlu₂ relative to other closely related mGlu receptors in mediating different physiological responses, which could be achieved through the utilization of subtype selective pharmacological tools. In this respect, allosteric modulators that recognize ligand-binding sites distinct from the endogenous neurotransmitter glutamate offer the promise of higher receptor-subtype selectivity. We hypothesized that mGlu₂-selective positive allosteric modulators could be derivatized to generate bifunctional pharmacological tools. Here we developed clickable photoaffinity probes for mGlu₂ based on two different positive allosteric modulator scaffolds that retained similar pharmacological activity to parent compounds. We demonstrate successful probe-dependent incorporation of a commercially available clickable fluorophore using bioorthogonal conjugation. Importantly, we also show the limitations of using these probes to assess in situ fluorescence of mGlu₂ in intact cells where significant nonspecific membrane binding is evident.

Allosteric modulation of metabotropic glutamate receptors in alcohol use disorder: Insights from preclinical investigations

Metabotropic glutamate (mGlu) receptors are family C G protein-coupled receptors (GPCRs) that modulate neuronal excitability and synaptic transmission throughout the nervous system. Owing to recent advances in development of subtype-selective allosteric modulators of mGlu receptors, individual members of the mGlu receptor family have been proposed as targets for treating a variety of neurological and psychiatric disorders, including substance use disorders. In this chapter, we highlight preclinical evidence that allosteric modulators of mGlu receptors could be useful for reducing alcohol consumption and preventing relapse in alcohol use disorder (AUD). We begin with an overview of the preclinical models that are used to study mGlu receptor involvement in alcohol-related behaviors. Alcohol exposure causes adaptations in both expression and function of various mGlu receptor subtypes, and pharmacotherapies aimed at reversing these adaptations have the potential to reduce alcohol consumption and seeking. Positive allosteric modulators (PAMs) of mGlu₂ and negative allosteric modulators of mGlu₅ show particular promise for reducing alcohol intake and/or preventing relapse. Finally, this chapter discusses important considerations for translating preclinical findings toward the development of clinically useful drugs, including the potential for PAMs to avoid tolerance issues that are frequently observed with repeated administration of GPCR agonists.

Spike-Wave Discharges and Sleep-Wake States during Circadian Desynchronization: No Effects of Agomelatine upon Re-Entrainment

Rapid changes in the light-dark cycle cause circadian desynchronization between rhythms of spike-wave discharges (SWDs) and motor activity in genetic epileptic rats, and this is accompanied by an increase in epileptic activity. Given the close relationship between absence seizures and sleep-wake states, the present study assessed firstly a putative relationship between vigilance rhythms and SWDs during re-synchronization, and secondly sleep-wake patterns responsible for increased epileptic activity. Lastly, in a view of existing evidence that melatonin and its agonists accelerate re-synchronization, the effects of different doses of agomelatine upon the speed of re-synchronization of different sleep-wake states and SWDs were investigated. Simultaneous electroencephalographic and electromyographic recordings were made in symptomatic WAG/Rij rats, before, during and 10 days following an 8 h light phase delay. Agomelatine was orally administered acutely and sub-chronically, during 10 post-shift days. The magnitude of the advance after the shift and the speed of re-synchronization were specific for various rhythms. Most prominent change was the increase in REM sleep duration during the dark phase. A post-shift increase in passive wakefulness and a reduction in deep slow-wave sleep coincided with an aggravation of SWDs during the light phase. Agomelatine showed neither an effect on sleep-wake parameters and SWDs, nor affected re-synchronization. The same speed of re-synchronization of SWDs and light slow-wave sleep suggests that both are controlled by a common circadian mechanism. The redistribution of SWDs and their increase in the light phase after the shift may be of importance for patients with absence epilepsy planning long trans-meridian flight across time zones.

Computational Drug Design Applied to the Study of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors are a family of eight GPCRs that are attractive drug discovery targets to modulate glutamate action and response. Here we review the application of computational methods to the study of this family of receptors. X-ray structures of the extracellular and 7-transmembrane domains have played an important role to enable structure-based modeling approaches, whilst we also discuss the successful application of ligand-based methods. We summarize the literature and highlight the areas where modeling and experiment have delivered important understanding for mGlu receptor drug discovery. Finally, we offer suggestions of future areas of opportunity for computational work.

In Silico Studies Targeting G-protein Coupled Receptors for Drug Research Against Parkinson's Disease

Parkinson's Disease (PD) is a long-term neurodegenerative brain disorder that mainly affects the motor system. The causes are still unknown, and even though currently there is no cure, several therapeutic options are available to manage its symptoms. The development of novel antiparkinsonian agents and an understanding of their proper and optimal use are, indeed, highly demanding. For the last decades, L-3,4-DihydrOxyPhenylAlanine or levodopa (L-DOPA) has been the gold-standard therapy for the symptomatic treatment of motor dysfunctions associated to PD. However, the development of dyskinesias and motor fluctuations (wearing-off and on-off phenomena) associated with long-term L-DOPA replacement therapy have limited its antiparkinsonian efficacy. The investigation for non-dopaminergic therapies has been largely explored as an attempt to counteract the motor side effects associated with dopamine replacement therapy. Being one of the largest cell membrane protein families, G-Protein-Coupled Receptors (GPCRs) have become a relevant target for drug discovery focused on a wide range of therapeutic areas, including Central Nervous System (CNS) diseases. The modulation of specific GPCRs potentially implicated in PD, excluding dopamine receptors, may provide promising non-dopaminergic therapeutic alternatives for symptomatic treatment of PD. In this review, we focused on the impact of specific GPCR subclasses, including dopamine receptors, adenosine receptors, muscarinic acetylcholine receptors, metabotropic glutamate receptors, and 5-hydroxytryptamine receptors, on the pathophysiology of PD and the importance of structure- and ligand-based in silico approaches for the development of small molecules to target these receptors.

Broad spectrum efficacy with LY2969822, an oral prodrug of metabotropic glutamate 2/3 receptor agonist LY2934747, in rodent pain models

BACKGROUND AND PURPOSE

A body of evidence suggests activation of metabotropic glutamate 2/3 (mGlu_2/3 ) receptors would be an effective analgesic in chronic pain conditions. Thus, the analgesic properties of a novel mGlu_2/3 receptor agonist prodrug were investigated.

EXPERIMENTAL APPROACH

After oral absorption, the prodrug LY2969822 rapidly converts to the brain penetrant, potent and subtype-selective mGlu_2/3 receptor agonist LY2934747. Behavioural assessments of allodynia, hyperalgesia and nocifensive behaviours were determined in preclinical pain models after administration of LY2969822 0.3-10 mg·kg^-1 . In addition, the ability of i.v. LY2934747 to modulate dorsal horn spinal cord wide dynamic range (WDR) neurons in spinal nerve ligated (SNL) rats was assessed.

KEY RESULTS

Following treatment with LY2934747, the spontaneous activity and electrically-evoked wind-up of WDR neurons in rats that had undergone spinal nerve ligation and developed mechanical allodynia were suppressed. In a model of sensitization, orally administered LY2969822 prevented the nociceptive behaviours induced by an intraplantar injection of formalin. The on-target nature of this effect was confirmed by blockade with an mGlu_2/3 receptor antagonist. LY2969822 prevented capsaicin-induced tactile hypersensitivity, reversed the SNL-induced tactile hypersensitivity and reversed complete Freund's adjuvant - induced mechanical hyperalgesia. The mGlu_2/3 receptor agonist prodrug demonstrated efficacy in visceral pain models, including a colorectal distension model and partially prevented the nocifensive behaviours in the mouse acetic acid writhing model.

CONCLUSIONS AND IMPLICATIONS

Following oral administration of the prodrug LY2969822, the mGlu_2/3 receptor agonist LY2934747 was formed and this attenuated pain behaviours across a broad range of preclinical pain models.

Efficacy of mGlu2 -positive allosteric modulators alone and in combination with levetiracetam in the mouse 6 Hz model of psychomotor seizures

OBJECTIVE

The metabotropic glutamate receptor subtype 2 (mGlu₂ ) possesses both orthosteric and allosteric modulatory sites, are expressed in the frontal cortex and limbic structures, and can affect excitatory synaptic transmission. Therefore, mGlu₂ is a potential therapeutic target in the treatment of epilepsy. The present study seeks to evaluate the anticonvulsant potential of mGlu₂ -acting compounds.

METHODS

The anticonvulsant efficacy of two selective mGlu₂ -positive allosteric modulators (PAMs) (JNJ-42153605 and JNJ-40411813/ADX71149) and one mGlu_2/3 receptor agonist (LY404039) were evaluated alone and in combination with the antiseizure drug levetiracetam (LEV) in the mouse 6 Hz model.

RESULTS

In the 6 Hz (32 mA stimulus intensity) model, median effective dose (ED₅₀ ) values were determined for JNJ-42153605 (3.8 mg/kg), JNJ-40411813 (12.2 mg/kg), and LY404039 (10.9 mg/kg). At the 44 mA stimulus intensity, ED₅₀ values were determined for JNJ-42153605 (5.9 mg/kg), JNJ-40411813 (21.0 mg/kg), LY404039 (14.1 mg/kg), and LEV (345 mg/kg). In addition, subprotective doses of each mGlu₂ -acting compound, administered in combination with various doses of LEV, were able to shift the 6 Hz 44 mA ED₅₀ for LEV by >25-fold. When JNJ-42153605 was administered at varying doses in combination with a single dose of LEV (10 mg/kg), the potency of JNJ-42153605 was increased 3.7-fold. Similarly, when a moderately effective dose of LEV (350 mg/kg) was administered in combination with varying doses of JNJ-40411813, the potency of JNJ-40411813 was increased approximately 14-fold. Plasma levels of JNJ-40411813 and LEV were not different when administered alone or in combination, suggesting that increases in potency are not due to pharmacokinetic effects.

SIGNIFICANCE

These studies suggest a potential positive pharmacodynamic effect of mGlu₂ -acting compounds in combination with LEV. If this effect is translated in a clinical setting, it can support a rational polypharmacy concept in treatment of epilepsy patients.

Selective agonism of mGlu8 receptors by (S)-3,4-dicarboxyphenylglycine does not affect sleep stages in the rat

BACKGROUND

Metabotropic glutamate receptors (mGlu) play a role in a number of physiological processes and behaviors, as well as in certain pathological conditions and diseases. New drugs targetting mGlu receptors are being developed with treatment purposes. Recent data indicates that glutamate is involved in sleep, and pharmacological manipulation of distinct subtypes of mGlu receptors affect sleep. Here the consequences of selective pharmacological agonism of mGlu8 receptor upon sleep and wakefulness are explored for the first time.

METHODS

32 male Wistar rats were stereotaxically prepared for polysomnography. (S)-3,4-dicarboxyphenylglycine (S)-3,4-DCPG (5, 10, and 20mg/kg, ip), a selective and potent mGlu8 receptor agonist, or physiological saline was administered one hour after the light period began.

RESULTS

Compared to control vehicle, (S)-3,4-DCPG, did not affect, at any of the doses given, the sleep and wakefulness parameters examined in the general analysis of the three hours of recording. Drug effects across time were studied analyzing three one-hour time blocks, control and experimental groups did not show any significant difference in the sleep and wakefulness parameters analyzed. Latency to sleep stages did not significantly vary between vehicle and treatment groups.

CONCLUSIONS

Results indicate that pharmacological activation of mGlu8 receptor by (S)-3,4-DCPG (5, 10, 20mg/kg, ip) does not affect sleep and wakefulness in the rat, suggesting that pharmacological agonism of these receptors may not influence sleep. Further research is needed to verify whether new drugs acting on these receptors lack of effect upon sleep and wakefulness.

Discovery of 8-Trifluoromethyl-3-cyclopropylmethyl-7-[(4-(2,4-difluorophenyl)-1-piperazinyl)methyl]-1,2,4-triazolo[4,3-a]pyridine (JNJ-46356479), a Selective and Orally Bioavailable mGlu2 Receptor Positive Allosteric Modulator (PAM)

Positive allosteric modulators of the metabotropic glutamate 2 receptor have generated great interest in the past decade. There is mounting evidence of their potential as therapeutic agents in the treatment of multiple central nervous system disorders. We have previously reported substantial efforts leading to potent and selective mGlu2 PAMs. However, finding compounds with the optimal combination of in vitro potency and good druglike properties has remained elusive, in part because of the hydrophobic nature of the allosteric binding site. Herein, we report on the lead optimization process to overcome the poor solubility inherent to the advanced lead 6. Initial prototypes already showed significant improvements in solubility while retaining good functional activity but displayed new liabilities associated with metabolism and hERG inhibition. Subsequent subtle modifications efficiently addressed those issues leading to the identification of compound 27 (JNJ-46356479). This new lead represents a more balanced profile that offers a significant improvement on the druglike attributes compared to previously reported leads.