Clinical and Preclinical Evidence for M1 Muscarinic Acetylcholine Receptor Potentiation as a Therapeutic Approach for Rett Syndrome

Rett syndrome (RTT) is a neurodevelopmental disorder that is characterized by developmental regression, loss of communicative ability, stereotyped hand wringing, cognitive impairment, and central apneas, among many other symptoms. RTT is caused by loss-of-function mutations in a methyl-reader known as methyl-CpG-binding protein 2 (MeCP2), a protein that links epigenetic changes on DNA to larger chromatin structure. Historically, target identification for RTT has relied heavily on Mecp2 knockout mice; however, we recently adopted the alternative approach of performing transcriptional profiling in autopsy samples from RTT patients. Through this mechanism, we identified muscarinic acetylcholine receptors (mAChRs) as potential therapeutic targets. Here, we characterized a cohort of 40 temporal cortex samples from individuals with RTT and quantified significantly decreased levels of the M1, M2, M3, and M5 mAChRs subtypes relative to neurotypical controls. Of these four subtypes, M1 expression demonstrated a linear relationship with MeCP2 expression, such that M1 levels were only diminished in contexts where MeCP2 was also significantly decreased. Further, we show that M1 potentiation with the positive allosteric modulator (PAM) VU0453595 (VU595) rescued social preference, spatial memory, and associative memory deficits, as well as decreased apneas in Mecp2+/- mice. VU595's efficacy on apneas in Mecp2+/- mice was mediated by the facilitation of the transition from inspiration to expiration. Molecular analysis correlated rescue with normalized global gene expression patterns in the brainstem and hippocampus, as well as increased Gsk3β inhibition and NMDA receptor trafficking. Together, these data suggest that M1 PAMs could represent a new class of RTT therapeutics.

Optimized Administration of the M4 PAM VU0467154 Demonstrates Broad Efficacy, but Limited Effective Concentrations in Mecp2+/- Mice

Hypofunction of cholinergic circuits and diminished cholinergic tone have been associated with the neurodevelopmental disorder Rett syndrome (RTT). Specifically, deletion of Mecp2 in cholinergic neurons evokes the same social and cognitive phenotypes in mice seen with global Mecp2 knockout, and decreased choline acetyltransferase activity and vesamicol binding have been reported in RTT autopsy samples. Further, we recently identified significant decreases in muscarinic acetylcholine receptor subtype 4 (M₄) expression in both the motor cortex and cerebellum of RTT patient autopsies and established proof of concept that an acute dose of the positive allosteric modulator (PAM) VU0467154 (VU154) rescued phenotypes in Mecp2^+/- mice. Here, we expand the assessment of M₄ PAMs in RTT to address clinically relevant questions of tolerance, scope of benefit, dose response, chronic treatment, and mechanism. We show that VU154 has efficacy on anxiety, social preference, cognitive, and respiratory phenotypes in Mecp2^+/- mice; however, the therapeutic range is narrow, with benefits seen at 3 mg/kg concentrations, but not 1 or 10 mg/kg. Further, sociability was diminished in VU154-treated Mecp2^+/- mice, suggestive of a potential adverse effect. Compound efficacy on social, cognitive, and respiratory phenotypes was conserved with a 44-day treatment paradigm, with the caveat that breath rate was moderately decreased with chronic treatment in Mecp2^+/+ and Mecp2^+/- mice. VU154 effects on respiratory function correlated with an increase in Gsk3β inhibition in the brainstem. These results identify the core symptom domains where efficacy and adverse effects may present with M₄ administration in RTT model mice and advocate for the continued evaluation as potential RTT therapeutics.

VU6036720: The First Potent and Selective In Vitro Inhibitor of Heteromeric Kir4.1/5.1 Inward Rectifier Potassium Channels

Heteromeric Kir4.1/Kir5.1 (KCNJ10/KCNJ16) inward rectifier potassium (Kir) channels play key roles in the brain and kidney, but pharmacological tools for probing their physiology and therapeutic potential have not been developed. Here, we report the discovery, in a high-throughput screening of 80,475 compounds, of the moderately potent and selective inhibitor VU0493690, which we selected for characterization and chemical optimization. VU0493690 concentration-dependently inhibits Kir4.1/5.1 with an IC50 of 0.96 μM and exhibits at least 10-fold selectivity over Kir4.1 and ten other Kir channels. Multidimensional chemical optimization of VU0493690 led to the development of VU6036720, the most potent (IC50 = 0.24 μM) and selective (>40-fold over Kir4.1) Kir4.1/5.1 inhibitor reported to date. Cell-attached patch single-channel recordings revealed that VU6036720 inhibits Kir4.1/5.1 activity through a reduction of channel open-state probability and single-channel current amplitude. Elevating extracellular potassium ion by 20 mM shifted the IC50 6.8-fold, suggesting that VU6036720 is a pore blocker that binds in the ion-conduction pathway. Mutation of the "rectification controller" asparagine 161 to glutamate (N161E), which is equivalent to small-molecule binding sites in other Kir channels, led to a strong reduction of inhibition by VU6036720. Renal clearance studies in mice failed to show a diuretic response that would be consistent with inhibition of Kir4.1/5.1 in the renal tubule. Drug metabolism and pharmacokinetics profiling revealed that high VU6036720 clearance and plasma protein binding may prevent target engagement in vivo. In conclusion, VU6036720 represents the current state-of-the-art Kir4.1/5.1 inhibitor that should be useful for probing the functions of Kir4.1/5.1 in vitro and ex vivo. SIGNIFICANCE STATEMENT: Heteromeric inward rectifier potassium (Kir) channels comprising Kir4.1 and Kir5.1 subunits play important roles in renal and neural physiology and may represent inhibitory drug targets for hypertension and edema. Herein, we employ high-throughput compound library screening, patch clamp electrophysiology, and medicinal chemistry to develop and characterize the first potent and specific in vitro inhibitor of Kir4.1/5.1, VU6036720, which provides proof-of-concept that drug-like inhibitors of this channel may be developed.

Development of : A Potent, Highly Selective, and Systemically Active Orthosteric Antagonist of the M5 Muscarinic Acetylcholine Receptor for the Treatment of Opioid Use Disorder

The muscarinic acetylcholine receptor (mAChR) subtype 5 (M₅) represents a novel potential target for the treatment of multiple addictive disorders, including opioid use disorder. Through chemical optimization of several functional high-throughput screening hits, VU6019650 (27b) was identified as a novel M₅ orthosteric antagonist with high potency (human M₅ IC₅₀ = 36 nM), M₅ subtype selectivity (>100-fold selectivity against human M_1-4) and favorable physicochemical properties for systemic dosing in preclinical addiction models. In acute brain slice electrophysiology studies, 27b blocked the nonselective muscarinic agonist oxotremorine-M-induced increases in neuronal firing rates of midbrain dopamine neurons in the ventral tegmental area, a part of the mesolimbic dopaminergic reward circuitry. Moreover, 27b also inhibited oxycodone self-administration in male Sprague-Dawley rats within a dose range that did not impair general motor output.

The effects of predator odor (TMT) exposure and mGlu3 NAM pretreatment on behavioral and NMDA receptor adaptations in the brain

A stressor can trigger lasting adaptations that contribute to neuropsychiatric disorders. Predator odor (TMT) exposure is an innate stressor that may activate the metabotropic glutamate receptor 3 (mGlu3) to produce stress adaptations. To evaluate functional involvement, the mGlu3 negative allosteric modulator (NAM, VU6010572; 3 mg/kg, i.p.) was administered before TMT exposure in male, Long Evans rats. Two weeks after, rats underwent context re-exposure, elevated zero maze (ZM), and acoustic startle (ASR) behavioral tests, followed by RT-PCR gene expression in the insular cortex and bed nucleus of the stria terminalis (BNST) to evaluate lasting behavioral and molecular adaptations from the stressor. Rats displayed stress-reactive behaviors in response to TMT exposure that were not affected by VU6010572. Freezing and hyperactivity were observed during the context re-exposure, and mGlu3-NAM pretreatment during stressor prevented the context freezing response. TMT exposure did not affect ZM or ASR measures, but VU6010572 increased time spent in the open arms of the ZM and ASR habituation regardless of stressor treatment. In the insular cortex, TMT exposure increased expression of mGlu (Grm3, Grm5) and NMDA (GriN2A, GriN2B, GriN2C, GriN3A, GriN3B) receptor transcripts, and mGlu3-NAM pretreatment blocked GriN3B upregulation. In the BNST, TMT exposure increased expression of GriN2B and GriN3B in vehicle-treated rats, but decreased expression in the mGlu3-NAM group. Similar to the insular cortex, mGlu3-NAM reversed the stressor-induced upregulation of GriN3B in the BNST. mGlu3-NAM also upregulated GriN2A, GriN2B, GriN3B and Grm2 in the control group, but not the TMT group. Together, these data implicate mGlu3 receptor signaling in some lasting adaptations of predator odor stressor and anxiolytic-like effects.

Acute restraint stress redirects prefrontal cortex circuit function through mGlu5 receptor plasticity on somatostatin-expressing interneurons

Inhibitory interneurons orchestrate prefrontal cortex (PFC) activity, but we have a limited understanding of the molecular and experience-dependent mechanisms that regulate synaptic plasticity across PFC microcircuits. We discovered that mGlu5 receptor activation facilitates long-term potentiation at synapses from the basolateral amygdala (BLA) onto somatostatin-expressing interneurons (SST-INs) in mice. This plasticity appeared to be recruited during acute restraint stress, which induced intracellular calcium mobilization within SST-INs and rapidly potentiated postsynaptic strength onto SST-INs. Restraint stress and mGlu5 receptor activation each augmented BLA recruitment of SST-IN phasic feedforward inhibition, shunting information from other excitatory inputs, including the mediodorsal thalamus. Finally, studies using cell-type-specific mGlu5 receptor knockout mice revealed that mGlu5 receptor function in SST-expressing cells is necessary for restraint stress-induced changes to PFC physiology and related behaviors. These findings provide new insights into interneuron-specific synaptic plasticity mechanisms and suggest that SST-IN microcircuits may be promising targets for treating stress-induced psychiatric diseases.

Effects of acute and repeated administration of the selective M4 PAM VU0152099 on cocaine versus food choice in male rats

Ligands that stimulate muscarinic acetylcholine receptors 1 and 4 (M₁ , M₄ ) have shown promising effects as putative pharmacotherapy for cocaine use disorder in rodent assays. We have previously shown reductions in cocaine effects with acute M₄ stimulation, as well as long-lasting, delayed reductions in cocaine taking and cocaine seeking with combined M₁ /M₄ receptor stimulation or with M₁ stimulation alone. M₄ stimulation opposes dopaminergic signalling acutely, but direct dopamine receptor antagonists have proved unhelpful in managing cocaine use disorder because they lose efficacy with long-term administration. It is therefore critical to determine whether M₄ approaches themselves can remain effective with repeated or chronic dosing. We assessed the effects of repeated administration of the M₄ positive allosteric modulator (PAM) VU0152099 in rats trained to choose between intravenous cocaine and a liquid food reinforcer to obtain quantitative measurement of whether M₄ stimulation could produce delayed and lasting reduction in cocaine taking. VU0152099 produced progressively augmenting suppression of cocaine choice and cocaine intake, but produced neither rebound nor lasting effects after treatment ended. To compare and contrast effects of M₁ versus M₄ stimulation, we tested whether the M₄ PAM VU0152100 suppressed cocaine self-administration in mice lacking CalDAG-GEFI signalling factor, required for M₁ -mediated suppression of cocaine self-administration. CalDAG-GEFI ablation had no effect on M₄ -mediated suppression of cocaine self-administration. These findings support the potential usefulness of M₄ PAMs as pharmacotherapy to manage cocaine use disorder, alone or in combination with M₁ -selective ligands, and show that M₁ and M₄ stimulation modulate cocaine-taking behaviour by distinct mechanisms.

Synthesis and characterization of chiral 6-azaspiro[2.5]octanes as potent and selective antagonists of the M4 muscarinic acetylcholine receptor

In this manuscript, we report a series of chiral 6-azaspiro[2.5]octanes and related spirocycles as highly potent and selective antagonists of the muscarinic acetylcholine receptor subtype 4 (mAChR₄). Chiral separation and subsequent X-ray crystallographic analysis of early generation analogs revealed the R enantiomer to possess excellent human and rat M₄ potency, and further structure-activity relationship (SAR) studies on this chiral scaffold led to the discovery of VU6015241 (compound 19). Compound 19 is characterized by high M₄ potency and selectivity across multiple species, excellent aqueous solubility, and moderate brain exposure in rodents after intraperitoneal administration.

Discovery of "Molecular Switches" within a Series of mGlu5 Allosteric Ligands Driven by a "Magic Methyl" Effect Affording Both PAMs and NAMs with In Vivo Activity, Derived from an M1 PAM Chemotype

In the course of optimizing an M₁ PAM chemotype, introduction of an ether moiety unexpectedly abolished M₁ PAM activity while engendering a "molecular switch" to afford a weak, pure mGlu₅ PAM. Further optimization was able to deliver a potent (mGlu₅ EC₅₀ = 520 nM, 63% Glu Max), centrally penetrant (K_p = 0.83), MPEP-site binding mGlu₅ PAM 17a (VU6036486) that reversed amphetamine-induced hyperlocomotion. A pronounced "magic methyl" effect was noted with a regioisomeric methyl congener, leading to a change in pharmacology to afford a potent (mGlu₅ IC₅₀ = 110 nM, 3% Glu Min), centrally penetrant (K_p = 0.94), MPEP-site binding NAM 28d (VU6044766) that displayed anxiolytic activity in a mouse marble burying assay. These data further support the growing body of literature concerning the existence of G protein-coupled receptor (GPCR) allosteric privileged structures, and the value and impact of subtle methyl group walks, as well as the highly productive fluorine walk, around allosteric ligand cores to stabilize unique GPCR conformations.

Discovery and Optimization of a Novel Series of Competitive and Central Nervous System-Penetrant Protease-Activated Receptor 4 (PAR4) Inhibitors

The detailed pharmacology and therapeutic potential of the central PAR4 receptors are poorly understood due to a lack of potent, selective, and brain-penetrant tool compounds. Despite this, robust data with biochemical and genetic tools show the therapeutic potential of PAR4 antagonists in traumatic brain injury, Alzheimer's disease, Parkinson's disease, and other neurodegenerative disorders with a neuroinflammatory component. Thus, we performed a functional HTS campaign, identified a fundamentally new PAR4 competitive inhibitor chemotype, optimized this new series (increased potency >45-fold), discovered enantiospecific activity (though opposing preference for human versus mouse PAR4), and engendered high central nervous system penetration (rat K_p's of 0.52 to 4.2 and K_p,uu's of 0.52 to 1.2).

Discovery of structurally distinct tricyclic M4 positive allosteric modulator (PAM) chemotypes - Part 2

This Letter details our efforts to develop novel tricyclic M₄ PAM scaffolds with improved pharmacological properties. This endeavor involved a "tie-back" strategy to replace the 3-amino-4,6-dimethylthieno[2,3-b]pyridine-2-carboxamide core which lead to the discovery of two novel tricyclic cores: a 7,9-dimethylpyrido[3',2':4,5]thieno[3,2-d]pyrimidine core and 2,4-dimethylthieno[2,3-b:5,4-c']dipyridine core. Both tricyclic cores displayed low nanomolar potency against the human M₄ receptor.

mGlu1 potentiation enhances prelimbic somatostatin interneuron activity to rescue schizophrenia-like physiological and cognitive deficits

Evidence for prefrontal cortical (PFC) GABAergic dysfunction is one of the most consistent findings in schizophrenia and may contribute to cognitive deficits. Recent studies suggest that the mGlu1 subtype of metabotropic glutamate receptor regulates cortical inhibition; however, understanding the mechanisms through which mGlu1 positive allosteric modulators (PAMs) regulate PFC microcircuit function and cognition is essential for advancing these potential therapeutics toward the clinic. We report a series of electrophysiology, optogenetic, pharmacological magnetic resonance imaging, and animal behavior studies demonstrating that activation of mGlu1 receptors increases inhibitory transmission in the prelimbic PFC by selective excitation of somatostatin-expressing interneurons (SST-INs). An mGlu1 PAM reverses cortical hyperactivity and concomitant cognitive deficits induced by N-methyl-d-aspartate (NMDA) receptor antagonists. Using in vivo optogenetics, we show that prelimbic SST-INs are necessary for mGlu1 PAM efficacy. Collectively, these findings suggest that mGlu1 PAMs could reverse cortical GABAergic deficits and exhibit efficacy in treating cognitive dysfunction in schizophrenia.

Intravitreal HDAC Inhibitor Belinostat Effectively Eradicates Vitreous Seeds Without Retinal Toxicity In Vivo in a Rabbit Retinoblastoma Model

Purpose

Current melphalan-based regimens for intravitreal chemotherapy for retinoblastoma vitreous seeds are effective but toxic to the retina. Thus, alternative agents are needed. Based on the known biology of histone deacetylases (HDACs) in the retinoblastoma pathway, we systematically studied whether the HDAC inhibitor belinostat is a viable, molecularly targeted alternative agent for intravitreal delivery that might provide comparable efficacy, without toxicity.

Methods

In vivo pharmacokinetic experiments in rabbits and in vitro cytotoxicity experiments were performed to determine the 90% inhibitory concentration (IC₉₀). Functional toxicity by electroretinography and structural toxicity by optical coherence tomography (OCT), OCT angiography, and histopathology were evaluated in rabbits following three injections of belinostat 350 µg (2× IC₉₀) or 700 µg (4× IC₉₀), compared with melphalan 12.5 µg (rabbit equivalent of the human dose). The relative efficacy of intravitreal belinostat versus melphalan to treat WERI-Rb1 human cell xenografts in rabbit eyes was directly quantified. RNA sequencing was used to assess belinostat-induced changes in RB cell gene expression.

Results

The maximum nontoxic dose of belinostat was 350 µg, which caused no reductions in electroretinography parameters, retinal microvascular loss on OCT angiography, or retinal degeneration. Melphalan caused severe retinal structural and functional toxicity. Belinostat 350 µg (equivalent to 700 µg in the larger human eye) was equally effective at eradicating vitreous seeds in the rabbit xenograft model compared with melphalan (95.5% reduction for belinostat, P < 0.001; 89.4% reduction for melphalan, P < 0.001; belinostat vs. melphalan, P = 0.10). Even 700 µg belinostat (equivalent to 1400 µg in humans) caused only minimal toxicity. Widespread changes in gene expression resulted.

Conclusions

Molecularly targeted inhibition of HDACs with intravitreal belinostat was equally effective as standard-of-care melphalan but without retinal toxicity. Belinostat may therefore be an attractive agent to pursue clinically for intravitreal treatment of retinoblastoma.

Activating mGlu3 Metabotropic Glutamate Receptors Rescues Schizophrenia-like Cognitive Deficits Through Metaplastic Adaptations Within the Hippocampus

BACKGROUND

Polymorphisms in GRM3, the gene encoding the mGlu₃ metabotropic glutamate receptor, are associated with impaired cognition and neuropsychiatric disorders such as schizophrenia. Limited availability of selective genetic and molecular tools has hindered progress in developing a clear understanding of the mechanisms through which mGlu₃ receptors regulate synaptic plasticity and cognition.

METHODS

We examined associative learning in mice with trace fear conditioning, a hippocampal-dependent learning task disrupted in patients with schizophrenia. Underlying cellular mechanisms were assessed using ex vivo hippocampal slice preparations with selective pharmacological tools and selective genetic deletion of mGlu₃ receptor expression in specific neuronal subpopulations.

RESULTS

mGlu₃ receptor activation enhanced trace fear conditioning and reversed deficits induced by subchronic phencyclidine. Mechanistic studies revealed that mGlu₃ receptor activation induced metaplastic changes, biasing afferent stimulation to induce long-term potentiation through an mGlu₅ receptor-dependent, endocannabinoid-mediated, disinhibitory mechanism. Selective genetic deletion of either mGlu₃ or mGlu₅ from hippocampal pyramidal cells eliminated effects of mGlu₃ activation, revealing a novel mechanism by which mGlu₃ and mGlu₅ interact to enhance cognitive function.

CONCLUSIONS

These data demonstrate that activation of mGlu₃ receptors in hippocampal pyramidal cells enhances hippocampal-dependent cognition in control and impaired mice by inducing a novel form of metaplasticity to regulate circuit function, providing a clear mechanism through which genetic variation in GRM3 can contribute to cognitive deficits. Developing approaches to positively modulate mGlu₃ receptor function represents an encouraging new avenue for treating cognitive disruption in schizophrenia and other psychiatric diseases.