Selective allosteric modulation of muscarinic acetylcholine receptors for the treatment of schizophrenia and substance use disorders

Evaluation of the effect of biperiden (cholinergic muscarinic receptor antagonist) on ethanol self-administration in rats

Evidence from the literature suggests that alcohol use disorder (AUD) is closely associated with alterations in neuronal plasticity and the memory system, related to modifications in muscarinic receptors. The present study aimed to investigate the effects of systemic injection of biperiden, a muscarinic cholinergic antagonist, in experimental protocols of progressive ratio, 24-hour binge alcohol consumption, and reinstatement of context-induced ethanol self-administration. The results demonstrated that biperiden at doses of 5 and 10 mg/kg (i.p.) effectively reduced the animals' motivation for alcohol consumption, as evidenced by decreased responses on the active lever and a reduction in the last ratio achieved in the progressive ratio protocol. Furthermore, these same doses significantly reduced ethanol consumption in the 24-hour binge protocol. Treatment with biperiden at doses of 1, 5, and 10 mg/kg also attenuated alcohol-seeking behavior after re-exposure to the context associated with ethanol self-administration. These findings suggest that biperiden may have therapeutic potential in the treatment of alcohol use disorder, indicating that modulation of the cholinergic system could be a promising avenue for further investigation.

Novel pharmaceutical treatment approaches for schizophrenia: a systematic literature review

PURPOSE

Schizophrenia is a chronic and debilitating neuropsychiatric disorder affecting approximately 1% of the global population. Traditional antipsychotic treatments, while effective for positive symptoms, often have significant side effects and fail to address cognitive and negative symptoms. Novel pharmacological treatments targeting muscarinic receptors, TAAR1 agonists, serotonergic pathways, and glutamate modulation have emerged as promising alternatives.

AIM

This systematic literature review aims to critically evaluate the efficacy, safety, and mechanisms of action of novel pharmacological agents in the treatment of schizophrenia.

METHODS

A comprehensive search was conducted across PubMed, Embase, Cochrane Library, Scopus, and Web of Science for randomized controlled trials (RCTs) and clinical trials published between April 2014 and March 2024. Studies evaluating novel treatments targeting muscarinic receptors, TAAR1 agonists, serotonergic agents, and glutamate modulation were included. Primary outcomes focused on symptom reduction and quality of life, while secondary outcomes included cognitive function and adverse events. The Joanna Briggs Institute (JBI) tool was used for quality assessment.

RESULTS

Eleven studies involving 4614 participants (mean age 37-43 years, predominantly male) were included. Drugs evaluated included xanomeline-trospium (KarXT), pimavanserin, ulotaront, emraclidine, and bitopertin. Significant improvements in PANSS and CGI-S scores were observed, with xanomeline-trospium showing a mean reduction of 17.4 points (p < 0.001). Adverse events were mostly mild and transient, with nausea, constipation, and somnolence being common.

CONCLUSION

Novel treatments for schizophrenia show promise in managing both positive and negative symptoms, with generally favorable safety profiles. Future studies should focus on large-scale, long-term trials to refine their efficacy, safety, and clinical applicability.

[11C]MK-6884 PET imaging reveals lower M4 muscarinic acetylcholine receptor availability following cocaine self-administration in male rats

BACKGROUND

Cocaine Use Disorder (CUD) remains a significant problem in the United States, with high rates of relapse and no present FDA-approved treatment. The acetylcholine neurotransmitter system, specifically through modulation of muscarinic acetylcholine receptor (mAChR) function, has shown promise as a therapeutic target for multiple aspects of CUD. Enhancement of the M4 mAChR subtype via positive allosteric modulation has been shown to inhibit the behavioral and neurochemical effects of cocaine across several rodent models of CUD. However, it is unclear how cocaine exposure affects M4 mAChR expression or distribution.

OBJECTIVES

To evaluate the effects of cocaine self-administration on M4 mAChR availability using [11C]MK-6884 in vivo PET imaging in rats that self-administered cocaine (cocaine SA) or sucrose pellets (control).

METHODS

Sprague-Dawley rats self-administered cocaine or sucrose pellets for 15 days under 2-h or 4-h sessions followed by PET imaging with [11C]MK-6884, a radiolabeled M4 selective positive allosteric modulator to determine the effects of cocaine on [11C]MK-6884 standard uptake values with cerebellum as reference (SUVr).

RESULTS

Cumulative cocaine intake ranged between 324 and 776 mg/kg. Cocaine self-administration was associated with significantly lower [11C]MK-6884 SUVrs in the cortex, hippocampus, and striatum compared to cocaine-naive rats, with a negative correlation between radiotracer SUVrs and cocaine intake in the hippocampus.

CONCLUSIONS

These results suggest that cocaine self-administration decreases M4 mAChR availability, providing further support for pursuing activation/enhancement of M4 mAChR function as a viable pharmacotherapeutic approach for CUD.

G protein-coupled receptor modulation of striatal dopamine transmission: Implications for psychoactive drug effects

Dopamine transmission in the striatum is a critical mediator of the rewarding and reinforcing effects of commonly misused psychoactive drugs. G protein-coupled receptors (GPCRs) that bind a variety of neuromodulators including dopamine, endocannabinoids, acetylcholine and endogenous opioid peptides regulate dopamine release by acting on several components of dopaminergic circuitry. Striatal dopamine release can be driven by both somatic action potential firing and local mechanisms that depend on acetylcholine released from striatal cholinergic interneurons. GPCRs that primarily regulate somatic firing of dopamine neurons via direct effects or modulation of synaptic inputs are likely to affect distinct aspects of behaviour and psychoactive drug actions compared with those GPCRs that primarily regulate local acetylcholine-dependent dopamine release in striatal regions. This review will highlight mechanisms by which GPCRs modulate dopaminergic transmission and the relevance of these findings to psychoactive drug effects on physiology and behaviour.

The potential of muscarinic M1 and M4 receptor activators for the treatment of cognitive impairment associated with schizophrenia

Cognitive impairment is a core symptom of schizophrenia and a major determinant of poor long-term functional outcomes. Despite considerable efforts, we do not yet have any approved pharmacological treatments for cognitive impairment associated with schizophrenia (CIAS). A combination of advances in pre-clinical research and recent clinical trial findings have led to a resurgence of interest in the cognition-enhancing potential of novel muscarinic acetylcholine receptor (mAChR) agonists in schizophrenia. This article provides an overview of the scientific rationale for targeting M1 and M4 mAChRs. We describe the evolution of neuroscience research on these receptors since early drug discovery efforts focused on the mAChR agonist xanomeline. This work has revealed that M1 and M4 mAChRs are highly expressed in brain regions that are implicated in cognition. The functional significance of M1 and M4 mAChRs has been extensively characterized in animal models via use of selective receptor subtype compounds through neuronal and non-neuronal mechanisms. Recent clinical trials of a dual M1/M4 mAChR agonist show promising, replicable evidence of potential pro-cognitive effects in schizophrenia, with several other mAChR agonists in clinical development.

Co-stimulation of muscarinic M1 and M4 acetylcholine receptors prevents later cocaine reinforcement in male and female mice, but not place-conditioning

Acute stimulation of M1 or M4 muscarinic cholinergic receptors reduces cocaine abuse-related effects in mice and rats. The combined activation of these receptor subtypes produces synergistic effects on some behavioural endpoints in mice. M1 and M1 + M4 receptor stimulation in a cocaine vs. food choice assay in rats and microdialysis in rats showed delayed and lasting "anticocaine effects". Here, we tested whether these putative lasting neuroplastic changes are sufficient to occlude the reinforcing effects of cocaine at the behavioural level in mice. Mice were pre-treated with the M1 receptor partial agonist VU0364572, M4 receptor positive allosteric modulator VU0152100, or VU0364572 + VU0152100 two weeks prior to acquisition of cocaine intravenous self-administration (IVSA). Male C57BL/6JRj mice received vehicle, VU0364572, VU0152100, or VU0364572 + VU0152100. Female mice were tested with two VU0364572 + VU0152100 dose combinations or vehicle. To attribute potential effects to either reduced rewarding effects or increased aversion to cocaine, we tested VU0364572 alone and VU0364572 + VU0152100 in acquisition of cocaine-conditioned place preference (CPP) in male mice using an unbiased design. The acquisition of cocaine IVSA was drastically reduced and/or slowed in male and female mice receiving VU0364572 + VU0152100, but not either drug alone. Food-maintained operant behaviour was unaffected, indicating that the treatment effects were cocaine-specific. No treatment altered the acquisition of cocaine-CPP, neither in the post-test, nor in a challenge 14 days later. The cocaine IVSA findings confirm unusual long-lasting "anticocaine" effects of muscarinic M1 + M4 receptor stimulation. Thus, in mice, simultaneous stimulation of both receptor subtypes seems to produce potential neuroplastic changes that yield lasting effects.

Development of a Selective and High Affinity Radioligand, [3H]VU6013720, for the M4 Muscarinic Receptor

M4 muscarinic receptors are highly expressed in the striatum and cortex, brain regions that are involved in diseases such as Parkinson's disease, schizophrenia, and dystonia. Despite potential therapeutic advantages of specifically targeting the M4 receptor, it has been historically challenging to develop highly selective ligands, resulting in undesired off-target activity at other members of the muscarinic receptor family. Recently, we have reported first-in-class, potent, and selective M4 receptor antagonists. As an extension of that work, we now report the development and characterization of a radiolabeled M4 receptor antagonist, [3H]VU6013720, with high affinity (pKd of 9.5 ± 0.2 at rat M4, 9.7 at mouse M4, and 10 ± 0.1 at human M4 with atropine to define nonspecific binding) and no significant binding at the other muscarinic subtypes. Binding assays using this radioligand in rodent brain tissues demonstrate loss of specific binding in Chrm4 knockout animals. Dissociation kinetics experiments with various muscarinic ligands show differential effects on the dissociation of [3H]VU6013720 from M4 receptors, suggesting a binding site that is overlapping but may be distinct from the orthosteric site. Overall, these results demonstrate that [3H]VU6013720 is the first highly selective antagonist radioligand for the M4 receptor, representing a useful tool for studying the basic biology of M4 as well for the support of M4 receptor-based drug discovery. SIGNIFICANCE STATEMENT: This manuscript describes the development and characterization of a novel muscarinic (M) acetylcholine subtype 4 receptor antagonist radioligand, [3H]VU6013720. This ligand binds to or overlaps with the acetylcholine binding site, providing a highly selective radioligand for the M4 receptor that can be used to quantify M4 protein expression in vivo and probe the selective interactions of acetylcholine with M4 versus the other members of the muscarinic receptor family.

Discovery of a potent M5 antagonist with improved clearance profile. Part 2: Pyrrolidine amide-based antagonists

This Letter describes our ongoing effort to improve the clearance of selective M5 antagonists. Herein, we report the replacement of the previously disclosed piperidine amide (4, disclosed in Part 1) with a pyrrolidine amide core. Several compounds within this series provided good potency, subtype selectivity, and low to moderate clearance profiles. Interestingly, the left-hand side SAR for this series diverged from our earlier efforts.

Muscarinic acetylcholine receptors for psychotic disorders: bench-side to clinic

Modern interest in muscarinic acetylcholine receptor (mAChR) activators for schizophrenia began in the 1990s when xanomeline, an M₁/M₄-preferring mAChR agonist developed for cognitive symptoms of Alzheimer's disease (AD), had unexpected antipsychotic activity. However, strategies to address tolerability concerns associated with activation of peripheral mAChRs were not available at that time. The discovery of specific targeted ligands and combination treatments to reduce peripheral mAChR engagement have advanced the potential of mAChR activators as effective treatments for psychotic disorders. This review provides perspectives on the background of the identification of mAChRs as potential antipsychotics, advances in the preclinical understanding of mAChRs as targets, and the current state of mAChR activators under active clinical development for schizophrenia.

Effectiveness of KarXT (xanomeline-trospium) for cognitive impairment in schizophrenia: post hoc analyses from a randomised, double-blind, placebo-controlled phase 2 study

The muscarinic receptor agonist xanomeline improved cognition in phase 2 trials in Alzheimer's disease and schizophrenia. We present data on the effect of KarXT (xanomeline-trospium) on cognition in schizophrenia from the 5-week, randomised, double-blind, placebo-controlled EMERGENT-1 trial (NCT03697252). Analyses included 125 patients with computerised Cogstate Brief Battery (CBB) subtest scores at baseline and endpoint. A post hoc subgroup analysis evaluated the effects of KarXT on cognitive performance in patients with or without clinically meaningful cognitive impairment at baseline, and a separate outlier analysis excluded patients with excessive intraindividual variability (IIV) across cognitive subdomains. ANCOVA models assessed treatment effects for completers and impairment subgroups, with or without removal of outliers. Sample-wide, cognitive improvement was numerically but not statistically greater with KarXT (n = 60) than placebo (n = 65), p = 0.16. However, post hoc analyses showed 65 patients did not exhibit clinically meaningful cognitive impairment at baseline, while eight patients had implausibly high IIV at one or both timepoints. Significant treatment effects were observed after removing outliers (KarXT n = 54, placebo n = 63; p = 0.04). Despite the small sample size, a robust (d = 0.50) and significant effect was observed among patients with cognitive impairment (KarXT n = 23, placebo n = 37; p = 0.03). These effects did not appear to be related to improvement in PANSS total scores (linear regression, R2 = 0.03). Collectively, these findings suggest that KarXT may have a separable and meaningful impact on cognition, particularly among patients with cognitive impairment.

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.

Examining the role of muscarinic M5 receptors in VTA cholinergic modulation of depressive-like and anxiety-related behaviors in rats

Acetylcholine is implicated in mood disorders including depression and anxiety. Increased cholinergic tone in humans and rodents produces pro-depressive and anxiogenic-like effects. Cholinergic receptors in the ventral tegmental area (VTA) are known to mediate these responses in male rats, as measured by the sucrose preference test (SPT), elevated plus maze (EPM), and the forced swim test (FST). However, these effects have not been examined in females, and the VTA muscarinic receptor subtype(s) mediating the pro-depressive and anxiogenic-like behavioral effects of increased cholinergic tone are unknown. We first examined the behavioral effects of increased VTA cholinergic tone in male and female rats, and then determined whether VTA muscarinic M5 receptors were mediating these effects. VTA infusion of the acetylcholinesterase inhibitor physostigmine (0.5 μg, 1 μg and 2 μg/side) in males and females produced anhedonic-like, anxiogenic, pro-depressive-like responses on the SPT, EPM, and FST. In females, VTA administration of the muscarinic M5 selective negative allosteric modulator VU6000181 (0.68 ng, 2.3 ng, 6.8 ng/side for a 3 μM, 10 μM, 30 μM/side infusion) did not alter SPT, EPM nor FST behavior. However, in males intra-VTA infusion of VU6000181 alone reduced time spent immobile on the FST. Furthermore, co-infusion of VU6000181 with physostigmine, in male and female rats, attenuated the pro-depressive and anxiogenic-like behavioral responses induced by VTA physostigmine alone, in the SPT, EPM, and FST. Together, these data reveal a critical role of VTA M5 receptors in mediating the anhedonic, anxiogenic, and depressive-like behavioral effects of increased cholinergic tone in the VTA.

The Microbiome and the Gut-Liver-Brain Axis for Central Nervous System Clinical Pharmacology: Challenges in Specifying and Integrating In Vitro and In Silico Models

The complexity of integrating microbiota into clinical pharmacology, environmental toxicology, and opioid studies arises from bidirectional and multiscale interactions between humans and their many microbiota, notably those of the gut. Hosts and each microbiota are governed by distinct central dogmas, with genetics influencing transcriptomics, proteomics, and metabolomics. Each microbiota's metabolome differentially modulates its own and the host's multi‐omics. Exogenous compounds (e.g., drugs and toxins), often affect host multi‐omics differently than microbiota multi‐omics, shifting the balance between drug efficacy and toxicity. The complexity of the host‐microbiota connection has been informed by current methods of in vitro bacterial cultures and in vivo mouse models, but they fail to elucidate mechanistic details. Together, in vitro organ‐on‐chip microphysiological models, multi‐omics, and in silico computational models have the potential to supplement the established methods to help clinical pharmacologists and environmental toxicologists unravel the myriad of connections between the gut microbiota and host health and disease.