Preparation of a First 18F-Labeled Agonist for M1 Muscarinic Acetylcholine Receptors.

M1 muscarinic acetylcholine receptors (mAChRs) are abundant in postsynaptic nerve terminals of all forebrain regions and have been implicated in the cognitive decline associated with Alzheimer's disease and other CNS pathologies. Consequently, major efforts have been spent in the development of subtype-selective positron emission tomography (PET) tracers for mAChRs resulting in the development of several 11C-labeled probes. However, protocols for the preparation of 18F-labeled mAChR-ligands have not been published so far. Here, we describe a straightforward procedure for the preparation of an 18F-labeled M1 mAChR agonist and its corresponding pinacol boronate radiolabeling precursor and the non-radioactive reference compound. The target compounds were prepared from commercially available aryl fluorides and Boc protected 4-aminopiperidine using a convergent reaction protocol. The radiolabeling precursor was prepared by a modification of the Miyaura reaction and labeled via the alcohol-enhanced Cu-mediated radiofluorination. The developed procedure afforded the radiotracer in a non-decay-corrected radiochemical yield of 17 ± 3% (n = 3) and in excellent radiochemical purity (>99%) on a preparative scale. Taken together, we developed a straightforward protocol for the preparation of an 18F-labeled M1 mAChR agonist that is amenable for automation and thus provides an important step towards the routine production of a 18F-labeled M1 selective PET tracer for experimental and diagnostic applications.

Discovery and structure-activity relationships study of positive allosteric modulators of the M3 muscarinic acetylcholine receptor.

The M3 muscarinic acetylcholine receptor (mAChR) is a member of the family of mAChRs, which are associated with a variety of physiological functions including the contraction of various smooth muscle tissues, stimulation of glandular secretion, and regulation of a range of cholinergic processes in the central nerve system. We report here the discovery and a comprehensive structure--activity relationships (SARs) study of novel positive allosteric modulators (PAMs) of the M3 mAChR through a high throughput screening (HTS) campaign. Compound 9 exhibited potent in vitro PAM activity towards the M3 mAChR and significant enhancement of muscle contraction in a concentration-dependent manner when applied to isolated smooth muscle strips of rat bladder. Compound 9 also showed excellent subtype selectivity over other subtypes of mAChRs including M1, M2, and M4 mAChRs, and moderate selectivity over the M5 mAChR, indicating that compound 9 is an M3-preferring M3/M5 dual PAM. Moreover, compound 9 displayed acceptable pharmacokinetics profiles after oral dosing to rats. These results suggest that compound 9 may be a promising chemical probe for the M3 mAChR for further investigation of its pharmacological function both in vitro and in vivo.

Novel BQCA- and TBPB-Derived M1 Receptor Hybrid Ligands: Orthosteric Carbachol Differentially Regulates Partial Agonism.

Recently, investigations of the complex mechanisms of allostery have led to a deeper understanding of G protein-coupled receptor (GPCR) activation and signaling processes. In this context, muscarinic acetylcholine receptors (mAChRs) are highly relevant due to their exemplary role in the study of allosteric modulation. In this work, we compare and discuss two sets of putatively dualsteric ligands, which were designed to connect carbachol to different types of allosteric ligands. We chose derivatives of TBPB [1-(1'-(2-tolyl)-1,4'-bipiperidin-4-yl)-1H-benzo[d]imidazol-2(3H)-one] as M1 -selective putative bitopic ligands, and derivatives of benzyl quinolone carboxylic acid (BQCA) as an M1 positive allosteric modulator, varying the distance between the allosteric and orthosteric building blocks. Luciferase protein complementation assays demonstrated that linker length must be carefully chosen to yield either agonist or antagonist behavior. These findings may help to design biased signaling and/or different extents of efficacy.

Optical Control of Cardiac Function with a Photoswitchable Muscarinic Agonist.

Light-triggered reversible modulation of physiological functions offers the promise of enabling on-demand spatiotemporally controlled therapeutic interventions. Optogenetics has been successfully implemented in the heart, but significant barriers to its use in the clinic remain, such as the need for genetic transfection. Herein, we present a method to modulate cardiac function with light through a photoswitchable compound and without genetic manipulation. The molecule, named PAI, was designed by introduction of a photoswitch into the molecular structure of an M2 mAChR agonist. In vitro assays revealed that PAI enables light-dependent activation of M2 mAChRs. To validate the method, we show that PAI photoisomers display different cardiac effects in a mammalian animal model, and demonstrate reversible, real-time photocontrol of cardiac function in translucent wildtype tadpoles. PAI can also effectively activate M2 receptors using two-photon excitation with near-infrared light, which overcomes the scattering and low penetration of short-wavelength illumination, and offers new opportunities for intravital imaging and control of cardiac function.

Synthesis and Preliminary Evaluation of 11 C-Labeled VU0467485/AZ13713945 and Its Analogues for Imaging Muscarinic Acetylcholine Receptor Subtype†4.

Muscarinic acetylcholine receptors (mAChRs) have five distinct subunits (M1 -M5 ) and are involved in the action of the neurotransmitter acetylcholine in the central and peripheral nervous system. Attributed to the promising clinical efficacy of xanomeline, an M1 /M4 -preferring agonist, in patients of schizophrenia and Alzheimer's disease, M1 - or M4 -selective mAChR modulators have been developed that target the topographically distinct allosteric sites. Herein we report the synthesis and preliminary evaluation of 11 C-labeled positron emission tomography (PET) ligands based on a validated M4 R positive allosteric modulator VU0467485 (AZ13713945) to facilitate drug discovery. [11 C]VU0467485 and two other ligands were prepared in high radiochemical yields (>30 %, decay-corrected) with high radiochemical purity (>99 %) and high molar activity (>74 GBq µmol-1 ). In vitro autoradiography studies indicated that these three ligands possess moderate-to-high in vitro specific binding to M4 R. Nevertheless, further physiochemical property optimization is necessary to overcome the challenges associated with limited brain permeability.

Muscarinic agonist, (±)-quinuclidin-3-yl-(4-fluorophenethyl)(phenyl)carbamate: High affinity, but low subtype selectivity for human M1 - M5 muscarinic acetylcholine receptors.

Novel quinuclidinyl N-phenylcarbamate analogs were synthesized, and binding affinities at M1-M5 muscarinic acetylcholine receptor (mAChR) subtypes were determined using Chinese hamster ovary (CHO) cell membranes stably expressing one specific subtype of human mAChR. Although not subtype selective, the lead analog (±)-quinuclidin-3-yl-(4-fluorophenethyl)(phenyl)carbamate (3c) exhibited the highest affinity (Ki = 2.0, 13, 2.6, 2.2, 1.8 nM) at each of the M1-M5 mAChRs, respectively. Based on results from the [3H]dopamine release assay using rat striatal slices, 3c acted as an agonist at mAChRs. The effect of 3c was inhibited by the nonselective mAChR antagonist, scopolamine, and 3c augmented release evoked by oxotremorine. A potent analog from the same scaffold, (±)-quinuclidin-3-yl-(4-methoxyphenethyl)(phenyl)-carbamate (3b) exhibited the greatest selectivity (17-fold) at M3 over M2 mAChRs. These analogs could serve as leads for further discovery of novel subtype-selective muscarinic ligands with the goal of providing therapeutics for substance use disorders and chronic obstructive pulmonary disease.

Synthesis and Pharmacological Evaluation of Heterocyclic Carboxamides: Positive Allosteric Modulators of the M1 Muscarinic Acetylcholine Receptor with Weak Agonist Activity and Diverse Modulatory Profiles.

Targeting allosteric sites at M1 muscarinic acetylcholine receptors is a promising strategy for the treatment of Alzheimer's disease. Positive allosteric modulators not only may potentiate binding and/or signaling of the endogenous agonist acetylcholine (ACh) but also may possess direct agonist activity (thus referred to as PAM-agonists). Recent studies suggest that PAM-agonists with robust intrinsic efficacy are more likely to produce adverse effects in vivo. Herein we present the synthesis and pharmacological evaluation of a series of pyrrole-3-carboxamides with a diverse range of allosteric profiles. We proposed structural modifications at top, core, or pendant moieties of a prototypical molecule. Although generally there was a correlation between the degree of agonist activity and the modulatory potency of the PAMs, some derivatives displayed weak intrinsic efficacy yet maintained strong allosteric modulation. We also identified molecules with the ability to potentiate mainly the affinity or both affinity and efficacy of ACh.

Structure-Based Design and Discovery of New M2 Receptor Agonists.

Muscarinic receptor agonists are characterized by apparently strict restraints on their tertiary or quaternary amine and their distance to an ester or related center. On the basis of the active state crystal structure of the muscarinic M2 receptor in complex with iperoxo, we explored potential agonists that lacked the highly conserved functionalities of previously known ligands. Using structure-guided pharmacophore design followed by docking, we found two agonists (compounds 3 and 17), out of 19 docked and synthesized compounds, that fit the receptor well and were predicted to form a hydrogen-bond conserved among known agonists. Structural optimization led to compound 28, which was 4-fold more potent than its parent 3. Fortified by the discovery of this new scaffold, we sought a broader range of chemotypes by docking 2.2 million fragments, which revealed another three micromolar agonists unrelated either to 28 or known muscarinics. Even pockets as tightly defined and as deeply studied as that of the muscarinic reveal opportunities for the structure-based design and the discovery of new chemotypes.

Diverse Effects on M1 Signaling and Adverse Effect Liability within a Series of M1 Ago-PAMs.

Both historical clinical and recent preclinical data suggest that the M1 muscarinic acetylcholine receptor is an exciting target for the treatment of Alzheimer's disease and the cognitive and negative symptom clusters in schizophrenia; however, early drug discovery efforts targeting the orthosteric binding site have failed to afford selective M1 activation. Efforts then shifted to focus on selective activation of M1 via either allosteric agonists or positive allosteric modulators (PAMs). While M1 PAMs have robust efficacy in rodent models, some chemotypes can induce cholinergic adverse effects (AEs) that could limit their clinical utility. Here, we report studies aimed at understanding the subtle structural and pharmacological nuances that differentiate efficacy from adverse effect liability within an indole-based series of M1 ago-PAMs. Our data demonstrate that closely related M1 PAMs can display striking differences in their in vivo activities, especially their propensities to induce adverse effects. We report the discovery of a novel PAM in this series that is devoid of observable adverse effect liability. Interestingly, the molecular pharmacology profile of this novel PAM is similar to that of a representative M1 PAM that induces severe AEs. For instance, both compounds are potent ago-PAMs that demonstrate significant interaction with the orthosteric site (either bitopic or negative cooperativity). However, there are subtle differences in efficacies of the compounds at potentiating M1 responses, agonist potencies, and abilities to induce receptor internalization. While these differences may contribute to the differential in vivo profiles of these compounds, the in vitro differences are relatively subtle and highlight the complexities of allosteric modulators and the need to focus on in vivo phenotypic screening to identify safe and effective M1 PAMs.

4-Phenylpyridin-2-one Derivatives: A Novel Class of Positive Allosteric Modulator of the M1 Muscarinic Acetylcholine Receptor.

Positive allosteric modulators (PAMs) of the M1 muscarinic acetylcholine receptor (M1 mAChR) are a promising strategy for the treatment of the cognitive deficits associated with diseases including Alzheimer's and schizophrenia. Herein, we report the design, synthesis, and characterization of a novel family of M1 mAChR PAMs. The most active compounds of the 4-phenylpyridin-2-one series exhibited comparable binding affinity to the reference compound, 1-(4-methoxybenzyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (BQCA) (1), but markedly improved positive cooperativity with acetylcholine, and retained exquisite selectivity for the M1 mAChR. Furthermore, our pharmacological characterization revealed ligands with a diverse range of activities, including modulators that displayed both high intrinsic efficacy and PAM activity, those that showed no detectable agonism but robust PAM activity and ligands that displayed robust allosteric agonism but little modulatory activity. Thus, the 4-phenylpyridin-2-one scaffold offers an attractive starting point for further lead optimization.

Discovery of dihydroquinazolinone derivatives as potent, selective, and CNS-penetrant M(1) and M(4) muscarinic acetylcholine receptors agonists.

We designed and synthesized a series of dihydroquinazolinone derivatives as selective M1 and M4 muscarinic acetylcholine receptors agonists. Introduction of the N-carbethoxy piperidine unit into a HTS hit compound followed by optimization of the amine linker and the carbamoyl moiety led to the identification of compound 1 as a potential candidate. The identified compound 1 showed high selectivity for M1 and M4 muscarinic acetylcholine receptors with M4 partial agonistic activity. In addition, compound 1 showed good brain penetration and reversed methamphetamine-induced hyperlocomotion in rats (ED50=3.0 mg/kg, sc).

Rational design of partial agonists for the muscarinic m1 acetylcholine receptor.

Aiming to design partial agonists for a G-protein-coupled receptor based on dynamic ligand binding, we synthesized three different series of bipharmacophoric ligands composed of the orthosteric building blocks iperoxo and 1 linked to allosteric modulators (BQCA-derived compounds, BQCAd; TBPB-derived compound, TBPBd). Their interactions were studied with the human muscarinic acetylcholine M1-receptor (hM1) with respect to receptor binding and Gq-protein signaling. Results demonstrate that iperoxo/BQCAd (2, 3) and 1/BQCAd hybrids (4) act as M1 partial agonists, whereas 1/TBPBd hybrids (5) did not activate M1-receptors. Among the iperoxo/BQCAd-hybrids, spacer length in conjunction with the pattern of substitution tuned efficacy. Most interestingly, a model of dynamic ligand binding revealed that the spacer length of 2a and 3a controlled the probability of switch between the inactive purely allosteric and the active bitopic orthosteric/allosteric binding pose. In summary, dynamic ligand binding can be exploited in M1 receptors to design partial agonists with graded efficacy.

Synthesis and biological evaluation of a novel series of heterobivalent muscarinic ligands based on xanomeline and 1-[3-(4-butylpiperidin-1-yl)propyl]-1,2,3,4-tetrahydroquinolin-2-one (77-LH-28-1).

Novel bitopic hybrids, based on the M1/M4 muscarinic acetylcholine receptor (mAChR) orthosteric agonist xanomeline (1) and the putative M1 mAChR allosteric agonist 1-[3-(4-butylpiperidin-1-yl)propyl]-1,2,3,4-tetrahydroquinolin-2-one (77-LH-28-1, 3) connected by an aliphatic linker of variable length, were prepared. The novel heterobivalent hybrids 4a-f along with the intermediate alcohols 5a-f were pharmacologically evaluated in radioligand binding assays and some of them for their functional efficacies in bioluminescence resonance energy transfer (BRET)-based assays to give an insight into the structure-activity relationships of bivalent and linker-attached compounds in mAChRs. The hybrid 4d exhibited high efficacy for ß-arrestin2 engagement in M1 mAChR and alcohol 5c behaved much like 3 at M1 mAChR and showed full antagonism in both Gi activation and ß-arrestin2 engagement at M4 mAChR. Moreover, docking simulations on the M1 mAChR model were performed to elucidate how the binding mode of the proposed compounds is influenced by the linker length.

Dualsteric muscarinic antagonists--orthosteric binding pose controls allosteric subtype selectivity.

Bivalent ligands of G protein-coupled receptors have been shown to simultaneously either bind to two adjacent receptors or to bridge different parts of one receptor protein. Recently, we found that bivalent agonists of muscarinic receptors can simultaneously occupy both the orthosteric transmitter binding site and the allosteric vestibule of the receptor protein. Such dualsteric agonists display a certain extent of subtype selectivity, generate pathway-specific signaling, and in addition may allow for designed partial agonism. Here, we want to extend the concept to bivalent antagonism. Using the phthal- and naphthalimide moieties, which bind to the allosteric, extracellular site, and atropine or scopolamine as orthosteric building blocks, both connected by a hexamethonium linker, we were able to prove a bitopic binding mode of antagonist hybrids for the first time. This is demonstrated by structure-activity relationships, site-directed mutagenesis, molecular docking studies, and molecular dynamics simulations. Findings revealed that a difference in spatial orientation of the orthosteric tropane moiety translates into a divergent M2/M5 subtype selectivity of the corresponding bitopic hybrids.

Bis(ammonio)alkane-type agonists of muscarinic acetylcholine receptors: synthesis, in vitro functional characterization, and in vivo evaluation of their analgesic activity.

In this study, we synthesized and tested in vitro and in vivo two groups of bis(ammonio)alkane-type compounds, 6a-9a and 6b-9b, which incorporate the orthosteric muscarinic agonist iperoxo into a molecular fragment of the M2-selective allosteric modulators W84 and naphmethonium. The agonist potency and efficacy of these hybrid derivatives at M1, M2 and M3 muscarinic receptor subtypes and their anticholinesterase activity were evaluated on isolated tissue preparations. Their analgesic action was then assayed in vivo in the acetic acid writhing test and the occurrence of peripheral and central cholinergic side effects was also determined. The investigated hybrids behaved as potent muscarinic agonists and weak cholinesterase inhibitors. These effects were more pronounced for bisquaternary salts bearing the naphmethonium moiety than for the W84-containing analogs, and resulted in a significant analgesic activity in vivo. A promising profile was displayed by the naphmethonium-related compound 8b, which combined the most potent antinociception among the test compounds with the absence of relevant cholinergic side effects.

Targeting selective activation of M(1) for the treatment of Alzheimer's disease: further chemical optimization and pharmacological characterization of the M(1) positive allosteric modulator ML169.

The M(1) muscarinic acetylcholine receptor is thought to play an important role in memory and cognition, making it a potential target for the treatment of Alzheimer's disease (AD) and schizophrenia. Moreover, M(1) interacts with BACE1 and regulates its proteosomal degradation, suggesting selective M(1) activation could afford both palliative cognitive benefit as well as disease modification in AD. A key challenge in targeting the muscarinic acetylcholine receptors is achieving mAChR subtype selectivity. Our lab has previously reported the M(1) selective positive allosteric modulator ML169. Herein we describe our efforts to further optimize this lead compound by preparing analogue libraries and probing novel scaffolds. We were able to identify several analogues that possessed submicromolar potency, with our best example displaying an EC(50) of 310 nM. The new compounds maintained complete selectivity for the M(1) receptor over the other subtypes (M(2)-M(5)), displayed improved DMPK profiles, and potentiated the carbachol (CCh)-induced excitation in striatal MSNs. Selected analogues were able to potentiate CCh-mediated nonamyloidogenic APPsα release, further strengthening the concept that M(1) PAMs may afford a disease-modifying role in the treatment of AD.

Muscarinic and nicotinic acetylcholine receptor agonists and allosteric modulators for the treatment of schizophrenia.

Muscarinic and nicotinic acetylcholine (ACh) receptors (mAChRs and nAChRs) are emerging as important targets for the development of novel treatments for the symptoms associated with schizophrenia. Preclinical and early proof-of-concept clinical studies have provided strong evidence that activators of specific mAChR (M(1) and M(4)) and nAChR (α(7) and α(2)ß(4)) subtypes are effective in animal models of antipsychotic-like activity and/or cognitive enhancement, and in the treatment of positive and cognitive symptoms in patients with schizophrenia. While early attempts to develop selective mAChR and nAChR agonists provided important preliminary findings, these compounds have ultimately failed in clinical development due to a lack of true subtype selectivity and subsequent dose-limiting adverse effects. In recent years, there have been major advances in the discovery of highly selective activators for the different mAChR and nAChR subtypes with suitable properties for optimization as potential candidates for clinical trials. One novel strategy has been to identify ligands that activate a specific receptor subtype through actions at sites that are distinct from the highly conserved ACh-binding site, termed allosteric sites. These allosteric activators, both allosteric agonists and positive allosteric modulators, of mAChR and nAChR subtypes demonstrate unique mechanisms of action and high selectivity in vivo, and may provide innovative treatment strategies for schizophrenia.

Targeting glutamate synapses in schizophrenia.

Although early clinical observations implicated dopamine dysfunction in the neuropathology of schizophrenia, accumulating evidence suggests that multiple neurotransmitter pathways are dysregulated. The psychotomimetic actions of NMDA receptor antagonists point to an imbalance of glutamatergic signaling. Encouragingly, numerous preclinical and clinical studies have elucidated several potential targets for increasing NMDA receptor function and equilibrating glutamatergic tone, including the metabotropic glutamate receptors 2, 3 and 5, the muscarinic acetylcholine receptors M(1) and M(4), and the glycine transporter GlyT1. Highly specific allosteric and orthosteric ligands have been developed that modify the activity of these novel target proteins, and in this review we summarize both the glutamatergic mechanisms and the novel compounds that are increasing the promise for a multifaceted pharmacological approach to treat schizophrenia.

Hybrid molecules from xanomeline and tacrine: enhanced tacrine actions on cholinesterases and muscarinic M1 receptors.

A set of amide- and amine-linked hybrid molecules comprising moieties of the orthosteric M(1) muscarinic receptor agonist xanomeline and the cholinesterase inhibitor and allosteric receptor modulator tacrine were prepared with varying spacer length of 10-17 atoms. The hybrids inhibited acetylcholinesterase with similar or higher potency compared to tacrine. M(1) receptor binding affinity was similar or higher relative to xanomeline and far higher relative to tacrine. Affinities hardly changed when the receptors' orthosteric site was occupied by an inverse agonist ligand. When occupied by the orthosteric activator acetylcholine, affinity for the hybrids declined to unmeasureably low levels. Hybrids did not activate M(1) receptors. In vivo studies assaying cognition impairment in rats induced by scopolamine revealed pronounced enhancement of scopolamine action. Taken together, instead of dualsteric (simultaneous allosteric/orthosteric) binding, the hybrids seem to prefer purely allosteric binding at the inactive M(1) receptor.

Synthesis and pharmacological evaluation of novel N-alkyl/aryl substituted thiazolidinone arecoline analogues as muscarinic receptor 1 agonist in Alzheimer's dementia models.

Earlier we have reported the effect of arecoline thiazolidinone and morpholino arecoline analogues as muscarinic receptor 1 agonist in Alzheimer's dementia models. To elucidate further our SAR study on the chemistry and muscarinic receptor binding efficacy, a series of novel N-alkyl/aryl substituted thiazolidinone arecoline analogues 6(a-m) were designed and synthesized from 3-pyridine carboxaldehyde by reacting with different amines in the presence of gamma-ferrite as catalyst and subjected to in vitro muscarinic receptor binding studies using male Wistar rat brain membrane homogenate and extended to in vivo pharmacological evaluation of memory and learning in male Wistar rats. Derivative 6j having diphenylamine moiety attached to nitrogen of thiazolidinone showed significant affinity for the M1 receptor binding.