B-973, a novel piperazine positive allosteric modulator of the α7 nicotinic acetylcholine receptor

Recent Advances in the Discovery of Nicotinic Acetylcholine Receptor Allosteric Modulators

Positive allosteric modulators (PAMs), negative allosteric modulators (NAMs), silent agonists, allosteric activating PAMs and neutral or silent allosteric modulators are compounds capable of modulating the nicotinic receptor by interacting at allosteric modulatory sites distinct from the orthosteric sites. This survey is focused on the compounds that have been shown or have been designed to interact with nicotinic receptors as allosteric modulators of different subtypes, mainly α7 and α4β2. Minimal chemical changes can cause a different pharmacological profile, which can then lead to the design of selective modulators. Experimental evidence supports the use of allosteric modulators as therapeutic tools for neurological and non-neurological conditions.

Synthesis and Biological Evaluation of Novel Triazine Derivatives as Positive Allosteric Modulators of α7 Nicotinic Acetylcholine Receptors

Enhancing neuronal α7 nicotinic acetylcholine receptor (α7 nAChR) function can alleviate cognitive deficits. Here, we report the design, synthesis, and evaluation of N-(4-(trifluoromethoxy)phenyl)-1,3,5-triazin-2-amine derivatives 8-10 as a series of novel α7 nAChR positive allosteric modulators (PAMs). The representative compound 10e functions as a type I PAM with an EC50 of 3.0 μM and approximately 38-fold enhancement of α7 current in the presence of agonist acetylcholine (100 μM). It specifically enhances α7 current with high selectivity. Compound 10e shows good pharmacokinetic property in mice. Intraperitoneal injection of 10e (3 mg/kg) exhibits sufficient blood-brain barrier penetration in mice. Furthermore, 10e can also rescue the auditory gating deficit in mice with schizophrenia-like behavior. Molecular docking of 10e with homopentameric α7 nAChR reveals a new mode of action. These results support the potential of 10e for treatment for schizophrenia and Alzheimer's disease.

Therapeutic Targeting of α7 Nicotinic Acetylcholine Receptors

The α7-type nicotinic acetylcholine receptor is one of the most unique and interesting of all the members of the cys-loop superfamily of ligand-gated ion channels. Since it was first identified initially as a binding site for α-bungarotoxin in mammalian brain and later as a functional homomeric receptor with relatively high calcium permeability, it has been pursued as a potential therapeutic target for numerous indications, from Alzheimer disease to asthma. In this review, we discuss the history and state of the art for targeting α7 receptors, beginning with subtype-selective agonists and the basic pharmacophore for the selective activation of α7 receptors. A key feature of α7 receptors is their rapid desensitization by standard "orthosteric" agonist, and we discuss insights into the conformational landscape of α7 receptors that has been gained by the development of ligands binding to allosteric sites. Some of these sites are targeted by positive allosteric modulators that have a wide range of effects on the activation profile of the receptors. Other sites are targeted by direct allosteric agonist or antagonists. We include a perspective on the potential importance of α7 receptors for metabotropic as well as ionotropic signaling. We outline the challenges that exist for future development of drugs to target this important receptor and approaches that may be considered to address those challenges. SIGNIFICANCE STATEMENT: The α7-type nicotinic acetylcholine receptor (nAChR) is acknowledged as a potentially important therapeutic target with functional properties associated with both ionotropic and metabotropic signaling. The functional properties of α7 nAChR can be regulated in diverse ways with the variety of orthosteric and allosteric ligands described in this review.

Drug Development in Channelopathies: Allosteric Modulation of Ligand-Gated and Voltage-Gated Ion Channels

Ion channels have been characterized as promising drug targets for treatment of numerous human diseases. Functions of ion channels can be fine-tuned by allosteric modulators, which interact with channels and modulate their activities by binding to sites spatially discrete from those of orthosteric ligands. Positive and negative allosteric modulators have presented a plethora of potential therapeutic advantages over traditionally orthosteric agonists and antagonists in terms of selectivity and safety. This thematic review highlights the discovery of representative allosteric modulators for ligand-gated and voltage-gated ion channels, discussing in particular their identifications, locations, and therapeutic uses in the treatment of a range of channelopathies. Additionally, structures and functions of selected ion channels are briefly described to aid in the rational design of channel modulators. Overall, allosteric modulation represents an innovative targeting approach, and the corresponding modulators provide an abundant but challenging landscape for novel therapeutics targeting ligand-gated and voltage-gated ion channels.

Neuropharmacological assessment in mice and molecular docking of piperazine derivative LQFM212

Piperazine derivatives are an attractive class of chemical compounds for the treatment of various mental illness. Herein, we demonstrated the synthesis of LQFM212, a piperazine derivative, behavioral evaluation in mice and computational studies. In neuropharmacological assessment, LQFM212 treatment at doses of 18, 54 or 162 μmol/kg increased the sleep duration in sodium pentobarbital-induced sleep test. LQFM212 at dose of 162 μmol/kg increased climbing time in the chimney test and decreased the number of squares crossed in the open field test, suggesting that LQFM212 in high doses reduces spontaneous movement. However, LQFM212 treatment at the doses of 18 or 54 μmol/kg increased the preference for the center of field which could be indicative of anxiolytic-like effects. In elevated plus maze and light-dark box tests, LQFM212 treatment altered all parameters observed that demonstrate anxiolytic-like activity. These effects were reversed by flumazenil, mecamylamine, WAY-100635 and PCPA, but not with ketanserin, showing that anxiolytic-like activity involve benzodiazepine site of GABA_A receptor, nicotinic and serotonergic pathways. Molecular docking of LQFM212 showed that the ligand has more interactions with GABA_A receptor than with 5-HT_1A receptor. Despite the involvement of benzodiazepine site on anxiolytic-like effect of LQFM212, treatment with this compound did not alter cognitive function in the step-down avoidance test. In this sense, this piperazine derivative is a good prototype for treating anxiety disorders with putative mechanism of action.

More than Smoke and Patches: The Quest for Pharmacotherapies to Treat Tobacco Use Disorder

Tobacco use is a persistent public health issue. It kills up to half its users and is the cause of nearly 90% of all lung cancers. The main psychoactive component of tobacco is nicotine, primarily responsible for its abuse-related effects. Accordingly, most pharmacotherapies for smoking cessation target nicotinic acetylcholine receptors (nAChRs), nicotine's major site of action in the brain. The goal of the current review is twofold: first, to provide a brief overview of the most commonly used behavioral procedures for evaluating smoking cessation pharmacotherapies and an introduction to pharmacokinetic and pharmacodynamic properties of nicotine important for consideration in the development of new pharmacotherapies; and second, to discuss current and potential future pharmacological interventions aimed at decreasing tobacco use. Attention will focus on the potential for allosteric modulators of nAChRs to offer an improvement over currently approved pharmacotherapies. Additionally, given increasing public concern for the potential health consequences of using electronic nicotine delivery systems, which allow users to inhale aerosolized solutions as an alternative to smoking tobacco, an effort will be made throughout this review to address the implications of this relatively new form of nicotine delivery, specifically as it relates to smoking cessation. SIGNIFICANCE STATEMENT: Despite decades of research that have vastly improved our understanding of nicotine and its effects on the body, only a handful of pharmacotherapies have been successfully developed for use in smoking cessation. Thus, investigation of alternative pharmacological strategies for treating tobacco use disorder remains active; allosteric modulators of nicotinic acetylcholine receptors represent one class of compounds currently under development for this purpose.

Nicotinic receptor pharmacology in silico: Insights and challenges

Nicotinic acetylcholine receptors (nAChR) are homo- or hetero-pentameric ligand-gated ion channels of the Cys-loop superfamily and play important roles in the nervous system and muscles. Studies on nAChR benefit from in silico modeling due to the lack of high-resolution structures for most receptor subtypes and challenges in experiments addressing the complex mechanism of activation involving allosteric sites. Although there is myriad of computational modeling studies on nAChR, the multitude of the methods and parameters used in these studies makes modeling nAChR a daunting task, particularly for the non-experts in the field. To address this problem, the modeling literature on Torpedo nAChR and α7 nAChR were focused on as examples of heteromeric and homomeric nAChR, and the key in silico modeling studies between the years 1995-2019 were concisely reviewed. This was followed by a critical analysis of these studies by comparing the findings with each other and with the emerging experimental and computational data on nAChR. Based on these critical analyses, suggestions were made to guide the future researchers in the field of in silico modeling of nAChR. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Selective α7 nicotinic receptor agonists and positive allosteric modulators for the treatment of schizophrenia - a review

INTRODUCTION

Alpha 7 nicotinic acetylcholine receptor (α7 nAChR) partial agonists, agonists, and positive allosteric modulators (PAMs) have been in development for over a decade. The initial candidates were in clinical trials for a wide variety of diseases including schizophrenia, but there has yet to be a successful compound to make it to the market for any disorder. Although difficult to assess the cause of all the clinical failures, the lack of efficacy played a major role. The development of more selective compounds, may bring a successful compound to long-suffering schizophrenia patients.

AREAS COVERED

This article examines investigational agonists and positive allosteric modulators of the α7 nicotinic receptor in preclinical studies as well as clinical trials. Our search included the use of SciFinder, Google, and clinicaltrials.gov with search dates of 2015 to the present.

EXPERT OPINION

Researchers must rethink their approach should look more closely at the selectivity of new compounds and how to tackle the translational gap. Perhaps new positive allosteric modulators that can help minimize receptor desensitization and selectivity profiles can be a path forward for α7 nAChRs in schizophrenia.

Pharmacological Characterization of the Novel and Selective α7 Nicotinic Acetylcholine Receptor-Positive Allosteric Modulator BNC375

Treatments for cognitive deficits associated with central nervous system (CNS) disorders such as Alzheimer disease and schizophrenia remain significant unmet medical needs that incur substantial pressure on the health care system. The α7 nicotinic acetylcholine receptor (nAChR) has garnered substantial attention as a target for cognitive deficits based on receptor localization, robust preclinical effects, genetics implicating its involvement in cognitive disorders, and encouraging, albeit mixed, clinical data with α7 nAChR orthosteric agonists. Importantly, previous orthosteric agonists at this receptor suffered from off-target activity, receptor desensitization, and an inverted U-shaped dose-effect curve in preclinical assays that limit their clinical utility. To overcome the challenges with orthosteric agonists, we have identified a novel selective α7 positive allosteric modulator (PAM), BNC375. This compound is selective over related receptors and potentiates acetylcholine-evoked α7 currents with only marginal effect on the receptor desensitization kinetics. In addition, BNC375 enhances long-term potentiation of electrically evoked synaptic responses in rat hippocampal slices and in vivo. Systemic administration of BNC375 reverses scopolamine-induced cognitive deficits in rat novel object recognition and rhesus monkey object retrieval detour (ORD) task over a wide range of exposures, showing no evidence of an inverted U-shaped dose-effect curve. The compound also improves performance in the ORD task in aged African green monkeys. Moreover, ex vivo ¹³C-NMR analysis indicates that BNC375 treatment can enhance neurotransmitter release in rat medial prefrontal cortex. These findings suggest that α7 nAChR PAMs have multiple advantages over orthosteric α7 nAChR agonists for the treatment of cognitive dysfunction associated with CNS diseases. SIGNIFICANCE STATEMENT: BNC375 is a novel and selective α7 nicotinic acetylcholine receptor (nAChR) positive allosteric modulator (PAM) that potentiates acetylcholine-evoked α7 currents in in vitro assays with little to no effect on the desensitization kinetics. In vivo, BNC375 demonstrated robust procognitive effects in multiple preclinical models across a wide exposure range. These results suggest that α7 nAChR PAMs have therapeutic potential in central nervous system diseases with cognitive impairments.

Activators of α7 nAChR as Potential Therapeutics for Cognitive Impairment

The α7 nicotinic acetylcholine receptor (nAChR) is a promising target for the treatment of cognitive deficits associated with psychiatric and neurological disorders, including schizophrenia and Alzheimer's disease (AD). Several α7 nAChR agonists and positive allosteric modulators (PAMs) have demonstrated procognitive effects in preclinical models and early clinical trials. However, despite intense research efforts in the pharmaceutical industry and academia, none of the α7 nAChR ligands has been approved for clinical use. This chapter will focus on the α7 nAChR ligands that have advanced to clinical studies and explore the reasons why these agents have not met with unequivocal clinical success.

In Silico Modeling of the α7 Nicotinic Acetylcholine Receptor: New Pharmacological Challenges Associated with Multiple Modes of Signaling

The α7 nicotinic acetylcholine receptor is a homopentameric ion-channel of the Cys-loop superfamily characterized by its low probability of opening, high calcium permeability, and rapid desensitization. The α7 receptor has been targeted for the treatment of the cognitive symptoms of schizophrenia, depression, and Alzheimer's disease, but it is also involved in inflammatory modulation as a part of the cholinergic anti-inflammatory pathway. Despite its functional importance, in silico studies of the α7 receptor cannot produce a general model explaining the structural features of receptor activation, nor predict the mode of action for various ligand classes. Two particular problems in modeling the α7 nAChR are the absence of a high-resolution structure and the presence of five potentially nonequivalent orthosteric ligand binding sites. There is wide variability regarding the templates used for homology modeling, types of ligands investigated, simulation methods, and simulation times. However, a systematic survey focusing on the methodological similarities and differences in modeling α7 has not been done. In this work, we make a critical analysis of the modeling literature of α7 nAChR by comparing the findings of computational studies with each other and with experimental studies under the main topics of structural studies, ligand binding studies, and comparisons with other nAChR. In light of our findings, we also summarize current problems in the field and make suggestions for future studies concerning modeling of the α7 receptor.

Heteromeric Neuronal Nicotinic Acetylcholine Receptors with Mutant β Subunits Acquire Sensitivity to α7-Selective Positive Allosteric Modulators

Homomeric α7 nicotinic acetylcholine receptors (nAChR) have an intrinsically low probability of opening that can be overcome by α7-selective positive allosteric modulators (PAMs), which bind at a site involving the second transmembrane domain (TM2). Mutation of a methionine that is unique to α7 at the 15' position of TM2 to leucine, the residue in most other nAChR subunits, largely eliminates the activity of such PAMs. We tested the effect of the reverse mutation (L15'M) in heteromeric nAChR receptors containing α4 and β2, which are the nAChR subunits that are most abundant in the brain. Receptors containing these mutations were found to be strongly potentiated by the α7 PAM 3a,4,5,9b-tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[c]quinoline-8-sulfonamide (TQS) but insensitive to the alternative PAM 1-(5-chloro-2,4-dimethoxyphenyl)-3-(5-methylisoxazol-3-yl)-urea. The presence of the mutation in the β2 subunit was necessary and sufficient for TQS sensitivity. The primary effect of the mutation in the α4 subunit was to reduce responses to acetylcholine applied alone. Sensitivity to TQS required only a single mutant β subunit, regardless of the position of the mutant β subunit within the pentameric complex. Similar results were obtained when β2L15'M was coexpressed with α2 or α3 and when the L15'M mutation was placed in β4 and coexpressed with α2, α3, or α4. Functional receptors were not observed when β1L15'M subunits were coexpressed with other muscle nAChR subunits. The unique structure-activity relationship of PAMs and the α4β2L15'M receptor compared with α7 and the availability of high-resolution α4β2 structures may provide new insights into the fundamental mechanisms of nAChR allosteric potentiation. SIGNIFICANCE STATEMENT: Heteromeric neuronal nAChRs have a relatively high initial probability of channel activation compared to receptors that are homomers of α7 subunits but are insensitive to PAMs, which greatly increase the open probability of α7 receptors. These features of heteromeric nAChR can be reversed by mutation of a single residue present in all neuronal heteromeric nAChR subunits to the sequence found in α7. Specifically, the mutation of the TM2 15' leucine to methionine in α subunits reduces heteromeric receptor channel activation, while the same mutation in neuronal β subunits allows heteromeric receptors to respond to select α7 PAMs. The results indicate a key role for this residue in the functional differences in the two main classes of neuronal nAChRs.

Allosteric Agonism of α7 Nicotinic Acetylcholine Receptors: Receptor Modulation Outside the Orthosteric Site

Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop superfamily of ligand-gated ion channels. Typically, channel activation follows the binding of agonists to the orthosteric binding sites of the receptor. α7 nAChRs have a very low probability of channel activation, which can be reversed by the binding of α7 selective positive allosteric modulators (PAMs) to putative sites within the transmembrane domains. Although typical PAMs, like PNU-120596, require coapplication of an orthosteric agonist to produce large channel activations, some, like GAT107 and B-973B [(S)-3-(3,4-difluorophenyl)-N-(1-(6-(4-(pyridin-2-yl)piperazin-1-yl)pyrazin-2-yl)ethyl)propanamide], are characterized as allosteric activating PAMs, which also bind to an allosteric activation (AA) site in the extracellular domain and activate the α7 ion channel by themselves. We had previously characterized N,N-diethyl-N'-phenylpiperazine analogs with various functions. In this work, we docked members of this family to a homology model of the α7 receptor extracellular domain. The compound 1,1-diethyl-4(naphthalene-2-yl)piperazin-1-ium (2NDEP) a weak partial agonist, showed particularly favorable docking and binding energies at the putative AA site of the receptor. We hypothesized that 2NDEP could couple with PAMs through the AA site. This hypothesis was tested with the α7 mutant C190A, which is not activated by orthosteric agonists but is effectively activated by GAT107. The results showed that 2NDEP acts as an allosteric agonist of α7C190A when coapplied with the PAM PNU-120596. Also, the allosteric activity was nearly abolished upon coapplication with the AA site-selective antagonist 2,3,5,6MP-TQS (cis-trans-4-(2,3,5,6-tetramethylphenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide), consistent with AA site involvement. Overall, our findings show a novel mode of agonism through an allosteric site in the extracellular domain of α7 nAChR.

Macroscopic and Microscopic Activation of α7 Nicotinic Acetylcholine Receptors by the Structurally Unrelated Allosteric Agonist-Positive Allosteric Modulators (ago-PAMs) B-973B and GAT107

B-973 is an efficacious type II positive allosteric modulator (PAM) of α7 nicotinic acetylcholine receptors that, like 4BP-TQS and its active isomer GAT107, can produce direct allosteric activation in addition to potentiation of orthosteric agonist activity, which identifies it as an allosteric activating (ago)-PAM. We compared the properties of B-973B, the active enantiomer of B-973, with those of GAT107 regarding the separation of allosteric potentiation and activation. Both ago-PAMs can strongly activate mutants of α7 that are insensitive to standard orthosteric agonists like acetylcholine. Likewise, the activity of both ago-PAMs is largely eliminated by the M254L mutation in the putative transmembrane PAM-binding site. Allosteric activation by B-973B appeared more protracted than that produced by GAT107, and B-973B responses were relatively insensitive to the noncompetitive antagonist mecamylamine compared with GAT107 responses. Similar differences are also seen in the single-channel currents. The two agents generate unique profiles of full-conductance and subconductance states, with B-973B producing protracted bursts, even in the presence of mecamylamine. Modeling and docking studies suggest that the molecular basis for these effects depends on specific interactions in both the extracellular and transmembrane domains of the receptor.

B-973, a Novel α7 nAChR Ago-PAM: Racemic and Asymmetric Synthesis, Electrophysiological Studies, and in Vivo Evaluation

We report here the total synthesis of B-973 (five steps), a recently identified α7 nAChR ago-PAM, its enantiomeric resolution, and its electrophysiological characterization in Xenopus oocytes to identify (-)-B-973B as the bioactive enantiomer. The asymmetric synthesis of B-973B was accomplished in 99% ee, and X-ray crystallography studies revealed its absolute "S" stereochemistry. B-973B was effective in attenuating pain behavior and decreasing paw edema (formalin test), and its analgesic effects were mediated through α7 nAChR.

New Insights on Neuronal Nicotinic Acetylcholine Receptors as Targets for Pain and Inflammation: A Focus on α7 nAChRs

BACKGROUND

Nicotine and nicotinic acetylcholine receptors (nAChRs) have been explored for the past three decades as targets for pain control. The aim of this review is to introduce readers particularly to α7 nAChRs in a perspective of pain and its modulation.

METHODS

Developments for α7 nAChR modulators and recent animal studies related to pain are reviewed.

RESULTS

Accumulating evidences suggest that selective ligands for α7 nAChRs hold promise in the treatment of chronic pain conditions as they lack many of side effects associated with other nicotinic receptor types.

CONCLUSION

This review provides the reader recent insights on α7 nAChRs from structure and function to the latest findings on the pharmacology and therapeutic targeting of these receptors for the treatment of pain and inflammation.