What are the therapeutic candidates targeting M1?

Overview of M1 Receptor

Biological Role and Importance
The M1 muscarinic acetylcholine receptor (mAChR) is one of the five subtypes of muscarinic receptors expressed widely throughout the central nervous system (CNS) and peripheral tissues. Biologically, the M1 receptor predominantly couples to Gq proteins, leading to the activation of phospholipase C, increased intracellular calcium, and activation of protein kinase C. In the brain, the M1 receptor is primarily expressed in cortical and hippocampal regions, where it plays a critical role in modulating cognitive functions such as learning, memory, attention, and sensory processing. Research has also indicated its involvement in neuronal excitability and synaptic plasticity, supporting its importance in normal brain functions and in the regulation of cortical arousal.

Clinical Significance
The significance of the M1 receptor in clinical contexts is underscored by its potential contribution to the pathophysiology of various neuropsychiatric and neurodegenerative disorders. Decreased or dysregulated M1 receptor activity has been associated with cognitive deficits, schizophrenia, Alzheimer’s disease, and even pain processing circuitry. Moreover, preclinical and clinical studies suggest that modulating M1 receptor activity can improve cognitive function and ameliorate psychotic symptoms, making it a promising target for therapeutic intervention. As our understanding of central cholinergic signaling has deepened, M1 receptor modulation has evolved into one of the primary targets for novel drug discovery programs aimed at addressing complex disorders with limited treatment options.

Therapeutic Candidates Targeting M1

Current Drugs and Compounds
A number of therapeutic candidates have emerged that specifically target the M1 receptor. These candidates can be broadly categorized into orthosteric agonists, positive allosteric modulators (PAMs), and bitopic ligands:

• Orthosteric Agonists: Early efforts in targeting the M1 receptor used compounds that directly bind to the orthosteric site. Xanomeline represents one of the best-known examples. Although xanomeline was originally recognized for its overall muscarinic agonist activity, its preferential stimulation of M1 in the CNS has been exploited in clinical trials for schizophrenia, even when combined with a peripherally restricted antagonist (trospium) to mitigate cholinergic side effects. This combination, known as KarXT, has demonstrated promising clinical outcomes in phase II studies.

• M1 Selective Agonists and Partial Agonists: Several compounds have been designed to achieve greater selectivity for M1 over other subtypes. For example, compounds such as AF102B (cevimeline), AF150(S), and AF267B have been studied for their ability to engage M1 receptors selectively while offering additional neurotrophic effects and protecting against amyloid pathology. Cevimeline is approved for indications like Sjögren’s syndrome, and its excellent pharmacokinetic profile has spurred interest in its potential repurposing for CNS indications related to cognitive deficits.

• Positive Allosteric Modulators (PAMs): An emerging class of compounds that target the M1 receptor involves PAMs, which enhance the receptor’s response to endogenous acetylcholine without directly activating the receptor in the absence of agonist. Examples include ACETYLCHOLINE M1 MUSCARIN RECEPTOR POSITIVE ALLOSTERIC modulators discussed in multiple patent references and additional PAMs that have been characterized in preclinical studies. These modulators are designed to impart a degree of physiological specificity, as they modulate receptor function only when acetylcholine is present, thereby reducing the risk of overstimulation and associated side effects.

• Bitopic Ligands: Certain ligands have been engineered to interact with both the orthosteric and allosteric sites of the M1 receptor. Bitopic ligands such as 77-LH-28-1 and AC-42 exemplify this approach, enabling high selectivity and the potential for improved efficacy by simultaneously engaging both binding domains. These compounds are promising in that their dual interaction can lead to enhanced receptor activation with fewer off-target effects.

• Other Compounds: Additional candidates described in the literature include TBPB and LuAE51090, which are M1-selective allosteric agonists showing robust in vivo efficacy and good blood–brain barrier (BBB) penetration. Moreover, compound “30,” identified via high-throughput screening (HTS) and virtual screening techniques, has been reported to possess improved pharmacokinetic and selective properties for M1, further expanding the repertoire of candidate molecules.

Developmental Pipeline
Recent developments in the M1 therapeutic space include a diverse pipeline encompassing early discovery candidates, preclinical molecules, and compounds in various stages of clinical trials:

• Preclinical Discovery: Several research groups have reported novel selective M1 agonists identified through structure-based drug design and high-throughput screening techniques. The utilization of molecular neuroimaging and pharmacophore modeling has led to the discovery of new scaffolds and candidate compounds that exhibit high receptor selectivity and favorable kinetic profiles.

• Clinical Candidates: One of the most advanced candidates in the pipeline is xanomeline when given in combination with trospium (KarXT). This combination aims to maximize central efficacy while reducing the peripheral cholinergic adverse effects reported with xanomeline monotherapy. Early phase clinical studies suggest that this candidate improves cognitive and behavioral symptoms in schizophrenia and perhaps could offer benefits in Alzheimer’s disease as well.

• Allosteric Modulators in Clinical Development: Beyond the direct agonists, positive allosteric modulators targeting the M1 receptor are currently being advanced through the preclinical and early clinical stages. The promise of PAMs lies in their ability to fine-tune receptor activity with a reduced propensity for receptor desensitization. Their progression is being closely monitored with respect to their safety and tolerability profiles in early-phase studies.

• Combination Approaches and Adjunctive Therapies: Some therapeutic approaches are exploring the combination of M1 receptor activation with other pharmacological agents, either for synergistic effects or for counteracting potential side effects. The combination of xanomeline with a peripheral antagonist is an excellent example of such a strategy, aiming to enhance central receptor activation while protecting against systemic adverse events.

Mechanisms of Action

Pharmacological Mechanisms
The M1 receptor activation engages several intracellular signaling cascades:

• Gq-mediated Signaling: Upon binding of agonists, the M1 receptor typically activates the phospholipase C pathway, leading to the hydrolysis of phosphatidylinositol bisphosphate, generation of inositol trisphosphate, and subsequent release of calcium from intracellular stores. This signaling cascade is critical for modulating neuronal excitability and synaptic plasticity, thereby playing an essential role in learning and memory processes.

• Modulation of Neurotransmission and Cognitive Effects: M1 receptor activation can also result in the inhibition of amyloid precursor protein (APP) processing, shifting the pathway to the non-amyloidogenic route, which may have implications for Alzheimer’s disease. Moreover, by modulating acetylcholine release at central synapses, M1 receptor activation can enhance cognitive performance and ameliorate psychotic symptoms observed in clinical populations.

• Allosteric Regulation and Bitopic Effects: Positive allosteric modulators operate by binding to regions distinct from the orthosteric site, stabilizing the receptor in an active conformation while preserving the temporal and spatial aspects of endogenous acetylcholine release. Bitopic ligands that simultaneously engage both orthosteric and allosteric sites may lead to a more robust receptor activation, ensuring both efficacy and selectivity.

• Improved Selectivity via Structural Engineering: Recent developments in structure-based design have allowed medicinal chemists to target the non-conserved allosteric sites on the M1 receptor. This strategy improves receptor subtype selectivity and reduces undesirable side effects attributable to off-target activation of other muscarinic receptor subtypes, such as M2 and M3, which are predominantly associated with peripheral cholinergic responses.

Targeting Strategies
There are several strategies to selectively modulate the M1 receptor in the therapeutic context:

• Orthosteric Agonism: Direct agonists bind to the orthosteric site with the inherent risk of activating other muscarinic subtypes. Xanomeline, while effective, exemplifies these challenges; peripheral side effects necessitated a re-formulation strategy involving trospium to block unwanted peripheral activity.

• Allosteric Modulation: Given the high structural conservation of the orthosteric pocket among muscarinic receptors, one promising strategy is to employ PAMs that bind to allosteric sites, which are less conserved. Such compounds offer the advantage of modulating receptor activity in the presence of acetylcholine without inducing excessive receptor activation in its absence. This approach is being actively pursued through numerous preclinical studies and early clinical investigations.

• Bitopic Ligand Design: This innovative strategy involves designing molecules that possess dual binding modes through engagement of both orthosteric and allosteric domains. Bitopic ligands, for instance 77-LH-28-1 and AC-42, have been shown to display improved selectivity and efficacy, capitalizing on the synergistic effects of simultaneous binding events.

• Combination Therapies: Another strategy that has attracted attention is the combination of an M1-targeted agonist with a peripherally restricted antagonist. This method helps to focus the therapeutic action on the CNS, thereby minimizing peripheral cholinergic stimulation and associated side effects. The KarXT combination (xanomeline with trospium) is a prime example of this approach, demonstrating how molecular pairing can be employed to optimize clinical outcomes.

Clinical Trials and Research

Ongoing and Completed Trials
Clinical research on M1 receptor-targeting compounds has progressed significantly over the past decade. Several candidates are at various stages of development:

• Xanomeline-Trospium (KarXT): Among the most advanced therapeutic candidates is the combination of xanomeline, a potent M1 receptor agonist, with trospium, a peripherally acting muscarinic antagonist. Phase I studies established the tolerability of xanomeline in healthy volunteers, while subsequent Phase II trials—particularly designed for schizophrenia—have demonstrated significant improvements in cognitive and behavioral symptoms. This approach leverages the central efficacy of xanomeline while mitigating peripheral cholinergic adverse effects, marking a novel and clinically promising treatment paradigm.

• M1 Selective Agonists: Compounds such as AF102B (cevimeline), AF150(S), and AF267B have been evaluated in early-phase clinical settings. These compounds have demonstrated not only potent M1 receptor activation but also exert neurotrophic effects and reduce neurodegeneration markers. Although not all these agents have advanced to late-phase clinical trials, their preclinical efficacy and favorable pharmacokinetic profiles are driving further development in cognitive disorders.

• Positive Allosteric Modulators (PAMs): Early-phase clinical trials have begun investigating M1 PAMs. Given their mechanism of potentiating the endogenous cholinergic system, they are considered promising candidates for addressing cognitive deficits with potentially fewer side effects than direct agonists. Several proprietary candidates, as described in patent literature and preclinical reports, are currently under evaluation.

• Bitopic Ligands: Although many bitopic compounds are still in the preclinical stage, promising candidates, such as those based on the AC-42 and 77-LH-28-1 scaffolds, are undergoing rigorous testing. Their ability to finely modulate M1 receptor activity positions them as potential candidates for future clinical trials focusing on neurodegenerative and psychiatric indications.

Efficacy and Safety Results
The clinical outcomes of several M1 receptor-targeting candidates highlight both promise and challenges:

• Efficacy Outcomes:
 – In trials involving xanomeline-trospium, patients with schizophrenia have shown meaningful improvements not only in positive and negative symptoms but also in cognitive performance measures. This indicates that M1 receptor modulation can influence cortical circuits effectively, suggesting a broader role for these agents beyond traditional antipsychotic effects.
 – Preclinical data for selective M1 agonists like AF267B and cevimeline indicate substantial cognitive enhancement and neuroprotective effects, which are especially relevant to Alzheimer’s disease and other forms of dementia.
 – Studies of M1 PAMs demonstrate that, when administered in relevant preclinical models, these compounds can potentiate acetylcholine-mediated responses and lead to enhanced learning and memory outcomes.

• Safety Profiles:
 – A critical challenge with orthosteric agonists such as xanomeline is the risk of peripheral cholinergic side effects, including gastrointestinal disturbances, which necessitated the innovative combination with trospium. Clinical studies indicate that this combination can reduce peripheral adverse effects by approximately 50% compared to xanomeline monotherapy, underscoring how combination therapies can improve the safety profile of M1-targeted interventions.
 – In the case of compounds like EVP-6124 (encenicline), while initial promise was seen in improving cognitive functions, severe gastrointestinal adverse effects led to its withdrawal from later-phase trials, highlighting the delicate balance required in modulating the M1 receptor.
 – Early data from allosteric modulators and bitopic ligands suggest an improved side effect profile, likely due to their dependence on endogenous acetylcholine levels, thereby reducing the chance of receptor overstimulation.

Challenges and Future Directions

Current Challenges in Targeting M1
Despite the promising advances in identifying M1 receptor-targeting therapeutics, several challenges remain:

• Selectivity and Off-Target Effects: The high degree of conservation in the orthosteric binding sites across muscarinic receptor subtypes complicates the development of truly selective M1 agonists. Agents that bind to the orthosteric site risk stimulating receptors such as M2 and M3, leading to unwanted peripheral side effects such as bradycardia, gastrointestinal disturbances, and excessive salivation.

• Receptor Desensitization: Continuous stimulation of the M1 receptor can lead to receptor desensitization and downregulation, thereby attenuating the therapeutic effects over time. This is a particular concern for direct agonists and necessitates the exploration of allosteric modulation strategies that maintain receptor responsiveness.

• Side Effects and Tolerability: Many candidates, even highly selective ones, have been associated with cholinergic side effects. While combination strategies like xanomeline-trospium have ameliorated some of these effects, optimizing the balance between efficacy and safety remains a significant hurdle.

• Pharmacokinetics and CNS Penetration: Effective therapy targeting central disorders requires compounds to have adequate blood–brain barrier permeability. Several of the promising candidates have demonstrated good BBB penetration in preclinical studies, yet achieving a consistent and reproducible pharmacokinetic profile in humans is challenging and continues to be the focus of ongoing research.

Future Research and Development
The future of M1 receptor-targeted therapeutics is likely to be shaped by several key research and development directions:

• Advances in Structure-Based Drug Design: The advent of high-resolution crystal structures of GPCRs, including muscarinic receptors, has significantly enhanced our ability to design ligands with improved selectivity profiles. Targeting allosteric sites through structure-based methodologies is expected to yield a next generation of M1 PAMs and bitopic ligands with minimal off-target interactions.

• Development of Allosteric Modulators: Given the limitations of orthosteric agonists, allosteric modulators represent a particularly promising area for future research. By selectively potentiating the effects of endogenous acetylcholine, these compounds can provide a more physiological modulation of M1 receptor activity, reducing the risks associated with receptor overstimulation and desensitization. Current preclinical findings underscore the potential of M1 PAMs to enhance cognitive performance while maintaining a favorable safety profile.

• Combination Therapy Strategies: The clinical success of the xanomeline-trospium combination illustrates the power of leveraging combination therapy to overcome single-agent limitations. Future studies may explore additional pairings or adjunctive strategies where M1 receptor activation is combined with inhibitors or modulators of peripheral muscarinic receptors to further refine therapeutic outcomes for a range of CNS disorders.

• Biomarker Development and Patient Stratification: To maximize clinical success, future trials should incorporate robust biomarkers for patient selection and response monitoring. As our molecular understanding of neuropsychiatric conditions evolves, it will be critical to define subgroups of patients who are most likely to benefit from M1-targeted interventions. This includes the development of imaging biomarkers and neurophysiological endpoints that directly reflect M1 receptor function.

• Mitigating Adverse Effects: Continuous research into the pharmacodynamics of M1 receptor activation will guide strategies to mitigate adverse effects. Efforts are underway to develop drugs with intrinsic properties—such as temporal selectivity or dose-dependent modulation—that inherently reduce side effects while preserving therapeutic efficacy. Improving the functional selectivity of candidates through innovative chemical design, such as bitopic ligand approaches, is expected to play a major role.

• Investigation in Diverse CNS Indications: While much of the current research has focused on schizophrenia and cognitive deficits in Alzheimer’s disease, future investigations will likely extend to other CNS disorders where cholinergic dysfunction is implicated. These include conditions such as Parkinson’s disease, neuropathic pain, and possibly even mood disorders. Such expansion will depend on both preclinical validations and carefully designed clinical studies to explore the full spectrum of M1 receptor involvement in CNS pathophysiology.

• Longitudinal Studies and Real-World Data Collection: As more M1-targeting candidates enter clinical trials, the accumulation of long-term efficacy and safety data will be crucial. Real-world evidence and longitudinal studies will inform dosing regimens, the durability of therapeutic responses, and determine if compensatory mechanisms emerge during prolonged modulation of the M1 receptor.

Conclusion
In summary, the therapeutic candidates targeting the M1 receptor encompass a diverse and rapidly evolving class of compounds including direct orthosteric agonists, highly selective partial agonists, positive allosteric modulators, and innovative bitopic ligands. These agents are being developed to address a broad spectrum of CNS disorders ranging from schizophrenia and Alzheimer’s disease to neuropathic pain. While conventional compounds such as xanomeline have demonstrated clinical efficacy, issues with peripheral side effects have driven the development of combination therapies like xanomeline-trospium (KarXT), which are already showing promise in clinical trials.

Through advanced structure-based drug design, medicinal chemists are now able to target less conserved allosteric sites to enhance selectivity and reduce off-target effects. In parallel, the preclinical pipeline is rich with innovative compounds like AF102B, AF267B, TBPB, and LuAE51090, as well as novel entities discovered through high-throughput screening strategies, which exhibit properties desirable for CNS therapeutics, such as excellent BBB penetration and favorable receptor pharmacodynamics. The adoption of combination therapies and the strategic targeting of allosteric sites represent promising avenues to mitigate the inherent challenges of non-selective receptor activation and receptor desensitization.

Despite these promising developments, several challenges remain—including achieving absolute receptor subtype selectivity, balancing efficacy with tolerability, and ensuring sustainable pharmacokinetic profiles in humans. Future research is expected to build on current successes by developing M1-specific biomarkers, refining combination therapy strategies, and extending the therapeutic applications of M1 modulation to additional neurological and psychiatric disorders.

Ultimately, the translation of these advanced therapeutic candidates from bench to bedside heralds an exciting era for precision neuropharmacology. Continued collaboration between academic researchers, pharmaceutical developers, and clinical investigators will be essential to overcoming the challenges inherent in M1 receptor-targeted therapy and to realizing the full potential of these compounds in improving patient outcomes. The future of M1 receptor pharmacotherapy appears promising, driven by innovative molecular design, rigorous clinical testing, and a balanced approach to efficacy and safety.