The Development of Drugs to Activate the Muscarinic M1 and M4 Receptors to Treat Disorders of the Human CNS

Background Accumulating evidence suggests that selective activation of the M1 and/or M4 muscarinic acetylcholine receptor (mAChR) subtypes may offer a novel therapeutic approach for the treatment of psychotic symptoms and cognitive impairments observed in psychiatric disorders, including schizophrenia. The M1 and M4 receptors are members of the family A G protein-coupled receptors; and include five different subtypes termed M1–M5, which modulate many central nervous system functions, including affective responses and cognition. Unfortunately, until the recent clinical success of CobenfyTM, M1 and/or M4 muscarinic orthosteric agonists ultimately failed in clinical development due to the lack of receptor subtype selectivity resulting in adverse side effects associated with non-selective activation of peripheral M2 and M3 mAChRs. Aims & Objectives This presentation will review recent breakthrough innovations by researchers at the Warren Center for Neuroscience Drug Discovery to identify subtype-selective M1 and M4 mAChR tool compounds and clinical candidates that activate a specific receptor subtype at sites that are less highly conserved than the orthosteric binding site of acetylcholine (ACh) and more topographically distinct, termed allosteric sites. These efforts have led to the discovery of highly, selective M1 and M4 positive allosteric modulators (PAMs), ligands that do not activate the M1 or M4 receptor directly, but markedly potentiate the response of the receptor subtype to ACh. Method HTS, medicinal chemistry and cheminformatics techniques were utilized to identify and optimize selective M1 and M4 mAChR PAMs, including VU0467319 and VU0467154 respectively; as well as selective M4 mAChR orthosteric antagonists, represented by VU6028418. In vitro cell-based assays were used to generate functional potency, efficacy, and selectivity data. Pharmacokinetic (PK) studies measured compound plasma/brain exposure in preclinical species using LC/MS detection. Compound efficacy was assessed in rodent models of antipsychotic-like activity, cognitive enhancement, and changes in sleep/wake architecture with DSI telemetry devices. Results In vitro studies revealed that VU0467319 is a moderately potent M1 PAM (EC50 = 492 nM, ACh max=71%) with minimal M1 agonism (EC50 > 30 μM), and no activity at other mAChRs up to 30μM. VU0467319 exhibited excellent multispecies PK and produced dose-dependent increases in prefrontal cortical- and hippocampal-mediated learning and memory tasks in rodents, including novel object recognition, contextual fear conditioning and eight arm radial arm maze tasks; as well as increased high frequency gamma power, a correlate of enhanced arousal. In vitro studies demonstrated that VU0467154 is highly potent (EC50=18nM at rM4; ACh max=68%) and selective M4 PAM with no activity at other mAChRs up to 30μM. VU0467154 produced reversal of psychostimulant-induced hyperlocomotion in wildtype mice and rats, but not in M4 KO mice. VU0467154 also reversed impairments in PFC-dependent cognitive tasks, including visual pairwise discrimination. VU0467154 increased non-rapid eye movement sleep, while decreasing rapid eye movement sleep and arousal; all effects were attenuated using the M4 orthosteric antagonist VU6028418. VU0467319 and VU0467154 did not induce any dose-limiting adverse side effects. Discussion & Conclusions The pharmacologic characterization of selective M1 and M4 mAChR PAMs has revealed distinct efficacy profiles that may provide innovative treatment strategies for addressing the different affective and cognitive impairments observed in schizophrenia and other neuropsychiatric disorders.