In vitro pharmacological characterization of a novel selective alpha7 neuronal nicotinic acetylcholine receptor agonist ABT-107

Side Groups Convert the α7 Nicotinic Receptor Agonist Ether Quinuclidine into a Type I Positive Allosteric Modulator

The quinuclidine scaffold has been extensively used for the development of nicotinic acetylcholine receptor (nAChR) agonists, with hydrophobic substituents at position 3 of the quinuclidine framework providing selectivity for α7 nAChRs. In this study, six new ligands (4-9) containing a 3-(pyridin-3-yloxy)quinuclidine moiety (ether quinuclidine) were synthesized to gain a better understanding of the structural-functional properties of ether quinuclidines. To evaluate the pharmacological activity of these ligands, two-electrode voltage-clamp and single-channel recordings were performed. Only ligand 4 activated α7 nAChR. Ligands 5 and 7 had no effects on α7 nAChR, but ligands 6, 8, and 9 potentiated the currents evoked by ACh. Ligand 6 was the most potent and efficacious of the potentiating ligands, with an estimated EC50 for potentiation of 12.6 ± 3.32 μM and a maximal potentiation of EC20 ACh responses of 850 ± 120%. Ligand 6 increased the maximal ACh responses without changing the kinetics of the current responses. At the single-channel level, the potentiation exerted by ligand 6 was evidenced in the low micromolar concentration range by the appearance of prolonged bursts of channel openings. Furthermore, computational studies revealed the preference of ligand 6 for an intersubunit site in the transmembrane domain and highlighted some putative key interactions that explain the different profiles of the synthesized ligands. Notably, Met276 in the 15' position of the transmembrane domain 2 almost abolished the effects of ligand 6 when mutated to Leu. We conclude that ligand 6 is a novel type I positive allosteric modulator (PAM-I) of α7 nAChR.

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.

PNU-120596, a positive allosteric modulator of mammalian α7 nicotinic acetylcholine receptor, is a negative modulator of ligand-gated chloride-selective channels of the gastropod Lymnaea stagnalis

Excitatory α7 neuronal nicotinic receptors (nAChR) are widely expressed in the central and peripheral nervous and immune systems and are important for learning, memory, and immune response regulation. Specific α7 nAChR ligands, including positive allosteric modulators are promising to treat cognitive disorders, inflammatory processes, and pain. One of them, PNU-120596, highly increased the neuron response to α7 agonists and retarded desensitization, showing selectivity for α7 as compared to heteromeric nAChRs, but was not examined at the inhibitory ligand-gated channels. We studied PNU-120596 action on anion-conducting channels using voltage-clamp techniques: it slightly potentiated the response of human glycine receptors expressed in PC12 cells, of rat GABA_A receptors in cerebellar Purkinje cells and mouse GABA_A Rs heterologously expressed in Xenopus oocytes. On the contrary, PNU-120596 exerted an inhibitory effect on the receptors mediating anion currents in Lymnaea stagnalis neurons: two nAChR subtypes, GABA and glutamate receptors. Acceleration of the current decay, contrary to slowing down desensitization in mammalian α7 nAChR, was observed in L. stagnalis neurons predominantly expressing one of the two nAChR subtypes. Thus, PNU-120596 effect on these anion-selective nAChRs was just opposite to the action on the mammalian cation-selective α7 nAChRs. A comparison of PNU-120596 molecule docked to the models of transmembrane domains of the human α7 AChR and two subunits of L. stagnalis nAChR demonstrated some differences in contacts with the amino acid residues important for PNU-120596 action on the α7 nAChR. Thus, our results show that PNU-120596 action depends on a particular subtype of these Cys-loop receptors.

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.

Nicotine Self-Administration as Paradigm for Medication Discovery for Smoking Cessation: Recent Findings in Medications Targeting the Cholinergic System

Tobacco kills every year approximately six million people as a direct result of direct use, and it is still considered one of the most excruciating threats for human health worldwide. The low successful rates of the currently available pharmacotherapies to assist in quitting tobacco use suggest there is a need for more effective treatments.The intravenous self-administration (IVSA) paradigm is considered the gold standard to study voluntary drug intake in animal models, including nicotine. The IVSA paradigm has been used to identify key mechanisms involved in the addictive properties of nicotine in both rodents and nonhuman primates. In this chapter we describe how the IVSA paradigm has served to further investigate the role of nicotinic acetylcholine receptors (nAChRs) in the reinforcing properties of nicotine. Notably, this review will cover recent advances (i.e., research carried out during the past decade) using the IVSA paradigm, with a focus on the status of research on current smoking cessation medications (such as varenicline and bupropion) and of other nAChR ligands.The combination of the IVSA paradigm with pharmacological and genetic tools (e.g., knockout animals) has greatly contributed to our understanding of the role of specific subtype nAChRs in nicotine reinforcement processes. We also discuss some of the limitations of the IVSA paradigm so these can be taken into consideration when interpreting and designing new studies.

Pharmacological modulation of the α7 nicotinic acetylcholine receptor in a mouse model of mecamylamine-precipitated nicotine withdrawal

RATIONALE

Recent preclinical data has implicated the α7 nicotinic acetylcholine receptor (nAChR) as a target in modulating nicotine reward. However, the role of the channel properties of the α7 nAChR in nicotine withdrawal is unknown.

OBJECTIVES

This study aimed to investigate the impact of α7 nAChR pharmacological modulation on mecamylamine-precipitated nicotine withdrawal behaviors in mice by using positive allosteric modulators (PAMs).

METHODS

The effect of the orthosteric α7 nAChR full agonist PNU282987 (1, 3, 9 mg/kg, s.c.), type I α7 PAM NS1738 (1 and 10 mg/kg; i.p.) and the type II α7 PAM PNU120596 (3 and 9 mg/kg, i.p.) on anxiety-like behavior, somatic signs, and hyperalgesia was measured in mice undergoing mecamylamine-precipitated nicotine withdrawal. Mice were infused with 24 mg/kg/day nicotine or saline for 14 days using s.c. osmotic minipumps. Nicotine withdrawal signs were precipitated upon administration of the non-selective nAChR antagonist mecamylamine (3.5 mg/kg, i.p.).

RESULTS

Anxiety-like behavior in nicotine withdrawn mice was only attenuated by PNU282987 in a dose-related fashion. Somatic signs were reduced by PNU282987 and NS1738. PNU120596 was the only compound that reversed precipitated nicotine withdrawal-induced hyperalgesia.

CONCLUSIONS

Taken together, our results suggest that modulation of the α7 nAChR can play important roles in mecamylamine-precipitated nicotine withdrawal behaviors in mice. In addition, the effects of PAMs in this study suggest that endogenous acetylcholine/choline tone is sufficient to attenuate some aspects of precipitated nicotine withdrawal. These findings highlight a beneficial effect of using α7 nAChR PAMs in some aspects of precipitated nicotine withdrawal.

Design and synthesis of a novel series of (1'S,2R,4'S)-3H-4'-azaspiro[benzo[4,5]imidazo[2,1-b]oxazole-2,2'-bicyclo[2.2.2]octanes] with high affinity for the α7 neuronal nicotinic receptor

We describe an efficient and convergent synthesis of a series of (1'S,2R,4'S)-3H-4'-azaspiro[benzo[4,5]imidazo[2,1-b]oxazole-2,2'-bicyclo[2.2.2]octanes] displaying potency for the α7 nicotinic acetylcholine receptor (nAChR) and good selectivity vs. the related 5-HT_3A receptor.

BMS-933043, a Selective α7 nAChR Partial Agonist for the Treatment of Cognitive Deficits Associated with Schizophrenia

The therapeutic treatment of negative symptoms and cognitive dysfunction associated with schizophrenia is a significant unmet medical need. Preclinical literature indicates that α7 neuronal nicotinic acetylcholine (nACh) receptor agonists may provide an effective approach to treating cognitive dysfunction in schizophrenia. We report herein the discovery and evaluation of 1c (BMS-933043), a novel and potent α7 nACh receptor partial agonist with high selectivity against other nicotinic acetylcholine receptor subtypes (>100-fold) and the 5-HT_3A receptor (>300-fold). In vivo activity was demonstrated in a preclinical model of cognitive impairment, mouse novel object recognition. BMS-933043 has completed Phase I clinical trials.

Design and synthesis of a novel series of 4-heteroarylamino-1'-azaspiro[oxazole-5,3'-bicyclo[2.2.2]octanes as α7 nicotinic receptor agonists 2. Development of 4-heteroaryl SAR

Quinuclidine-containing spirooxazolines, as described in the previous report in this series, were demonstrated to have utility as α7 nicotinic acetylcholine receptor (α7 nAChR) partial agonists. In this work, the SAR of this chemotype was expanded to include an array of diazine heterocyclic substitutions. Many of the heterocyclic analogs were potent partial agonists of the α7 receptor, selective against other nicotinic receptors and the serotinergic 5HT_3A receptor. (1'S,3'R,4'S)-N-(6-phenylpyrimidin-4-yl)-4H-1'-azaspiro[oxazole-5,3'-bicyclo[2.2.2]octan]-2-amine, a potent and selective α7 nAChR partial agonist, was demonstrated to improve cognition in the mouse novel object recognition (NOR) model of episodic memory.

Design and Synthesis of a New Series of 4-Heteroarylamino-1'-azaspiro[oxazole-5,3'-bicyclo[2.2.2]octanes as α7 Nicotinic Receptor Agonists. 1. Development of Pharmacophore and Early Structure-Activity Relationship

The design and synthesis of a series of quinuclidine-containing spirooxazolidines ("spiroimidates") and their utility as α7 nicotinic acetylcholine receptor partial agonists are described. Selected members of the series demonstrated excellent selectivity for α7 over the highly homologous 5-HT_3A receptor. Modification of the N-spiroimidate heterocycle substituent led to (1S,2R,4S)-N-isoquinolin-3-yl)-4'H-4-azaspiro[bicyclo[2.2.2]octane-2,5'oxazol]-2'-amine (BMS-902483), a potent α7 partial agonist, which improved cognition in preclinical rodent models.

Varying Chirality Across Nicotinic Acetylcholine Receptor Subtypes: Selective Binding of Quinuclidine Triazole Compounds

The novel quinuclidine anti-1,2,3-triazole derivatives T1-T6 were designed based on the structure of QND8. The binding studies revealed that the stereochemistry at the C3 position of the quinuclidine scaffold plays an important role in the nAChR subtype selectivity. Whereas the (R)-enantiomers are selective to α7 over α4β2 (by factors of 44-225) and to a smaller degree over α3β4 (3-33), their (S)-counterparts prefer α3β4 over α4β2 (62-237) as well as over α7 (5-294). The (R)-derivatives were highly selective to α7 over α3β4 subtypes compared to (RS)- and (R)-QND8. The (S)-enantiomers are 5-10 times more selective to α4β2 than their (R) forms. The overall strongest affinity is observed for the (S)-enantiomer binding to α3β4 (K_i, 2.25-19.5 nM) followed by their (R)-counterpart binding to α7 (K_i, 22.5-117 nM), with a significantly weaker (S)-enantiomer binding to α4β2 (K_i, 414-1980 nM) still above the very weak respective (R)-analogue affinity (K_i, 5059-10436 nM).

1,3-diphenylpropan-1-ones as allosteric modulators of α7 nACh receptors with analgesic and antioxidant properties

Nicotine acethylcholine receptors (nAChRs) play critical roles in cognitive processes, neuroprotection and inflammation.

RESULTS

According to their substituents, 1,3-diphenylpropan-1-one derivatives act as α7 nAChRs negative allosteric modulators (NAM, OMe) or Type I positive allosteric modulators (PAMs, OH). Compounds 7 and 31 were the most effective (989 and 666% enhancement of ACh-induced currents) and potent (EC50: 12.9 and 6.85 μM) PAMs. They exhibited strong radical scavenging values. Compound 31, selective over other neuronal nAChR subtypes and with acceptable pharmacokinetic profile, showed antinociceptive effects in a model of inflammatory pain.

CONCLUSION

Compound 31 is a novel, potent and selective α7 nAChR PAM, displaying antioxidant and analgesic activities. The 1,3-diphenylpropan-1-one scaffold could be the base toward more advanced type I PAMs for the treatment of nAChR-mediated diseases.

ABT-089, but not ABT-107, ameliorates nicotine withdrawal-induced cognitive deficits in C57BL6/J mice

Nicotine withdrawal produces cognitive deficits that can predict relapse. Amelioration of these cognitive deficits emerges as a target in current smoking cessation therapies. In rodents, withdrawal from chronic nicotine disrupts contextual fear conditioning (CFC), whereas acute nicotine enhances this hippocampus-specific learning and memory. These modifications are mediated by β2-subunit-containing (β2*) nicotinic acetylcholine receptors in the hippocampus. We aimed to test ABT-089, a partial agonist of α4β2*, and ABT-107, an α7 nicotinic acetylcholine receptor agonist, for amelioration of cognitive deficits induced by withdrawal from chronic nicotine in mice. Mice underwent chronic nicotine administration (12.6 mg/kg/day or saline for 12 days), followed by 24 h of withdrawal. At the end of withdrawal, mice received 0.3 or 0.6 mg/kg ABT-089 or 0.3 mg/kg ABT-107 (doses were determined through initial dose-response experiments and prior studies) and were trained and tested for CFC. Nicotine withdrawal produced deficits in CFC that were reversed by acute ABT-089, but not ABT-107. Cued conditioning was not affected. Taken together, our results suggest that modulation of hippocampal learning and memory using ABT-089 may be an effective component of novel therapeutic strategies for nicotine addiction.

Reducing inflammation and rescuing FTD-related behavioral deficits in progranulin-deficient mice with α7 nicotinic acetylcholine receptor agonists

Mutations in the progranulin gene cause frontotemporal dementia (FTD), a debilitating neurodegenerative disease that involves atrophy of the frontal and temporal lobes and affects personality, behavior, and language. Progranulin-deficient mouse models of FTD exhibit deficits in compulsive and social behaviors reminiscent of patients with FTD, and develop excessive microgliosis and increased release of inflammatory cytokines. Activation of nicotinic acetylcholine receptors (nAChRs) by nicotine or specific α7 nAChR agonists reduces neuroinflammation. Here, we investigated whether activation of nAChRs by nicotine or α7 agonists improved the excessive inflammatory and behavioral phenotypes of a progranulin-deficient FTD mouse model. We found that treatment with selective α7 agonists, PHA-568487 or ABT-107, strongly suppressed the activation of NF-κB in progranulin-deficient cells. Treatment with ABT-107 also reduced microgliosis, decreased TNFα levels, and reduced compulsive behavior in progranulin-deficient mice. Collectively, these data suggest that targeting activation of the α7 nAChR pathway may be beneficial in decreasing neuroinflammation and reversing some of the behavioral deficits observed in progranulin-deficient FTD.

Structure-activity relationship studies of SEN12333 analogues: determination of the optimal requirements for binding affinities at α7 nAChRs through incorporation of known structural motifs

Alpha7 nicotinic acetylcholine receptors (nAChRs) have implications in the regulation of cognitive processes such as memory and attention and have been identified as a promising therapeutic target for the treatment of the cognitive deficits associated with schizophrenia and Alzheimer's disease (AD). Structure affinity relationship studies of the previously described α7 agonist SEN12333 (8), have resulted in the identification of compound 45, a potent and selective agonist of the α7 nAChR with enhanced affinity and improved physicochemical properties over the parent compound (SEN12333, 8).

Targeting of α7 Nicotinic Acetylcholine Receptors in the Treatment of Schizophrenia and the Use of Auditory Sensory Gating as a Translational Biomarker

Accumulating evidence suggests that the α7 subtype of nicotinic acetylcholine receptors (nAChRs) plays a key role in inflammatory processes, thought to be involved in the pathophysiology of neuropsychiatric diseases, such as schizophrenia and Alzheimer's disease. Preclinical and clinical studies showed that the diminished suppression of P50 auditory evoked potentials in patients with schizophrenia may be associated with a decreased density of α7 nAChRs in the brain. This points to a role for auditory sensory gating (P50) as a translational biomarker. A number of agonists and positive allosteric modulators (PAMs) for α7 nAChR promoted beneficial effects in animal models with sensory gating and cognitive deficits. Additionally, several clinical studies showed that α7 nAChR agonists could improve suppression in auditory P50 evoked potentials, as well as cognitive deficits, and negative symptoms in patients with schizophrenia. Taken together, α7 nAChR presents as an extremely attractive therapeutic target for schizophrenia. In this article, the author discusses recent findings on α7 nAChR agonists such as DMXB-A, RG3487, TC-5619, tropisetron, EVP-6124 (encenicline), ABT-126, AQW051 and α7 nAChR PAMs such as JNJ-39393406, PNU- 120596 and AVL-3288 (also known as UCI-4083), and their potential as therapeutic drugs for neuropsychiatric diseases, such as schizophrenia.

The α7 nAChR selective agonists as drug candidates for Alzheimer's disease

The nicotinic acetylcholine receptors (nAChRs) are ion channels distribute in the central or peripheral nervous system. They are receptors of the neurotransmitter acetylcholine and activation of them by agonists mediates synaptic transmission in the neuron and muscle contraction in the neuromuscular junction. Current studies reveal relationship between the nAChRs and the learning and memory as well as cognation deficit in various neurological disorders such as Alzheimer's disease, Parkinson's disease, schizophrenia and drug addiction. There are various subtypes in the nAChR family and the α7 nAChR is one of the most abundant subtypes in the brain. The α7 nAChR is significantly reduced in the patients of Alzheimer's disease and is believed to interact with the Aβ amyloid. Aβ amyloid is co-localized with α7 nAChR in the senile plaque and interaction between them induces neuron apoptosis and reduction of the α7 nAChR expression. Treatment with α7 agonist in vivo shows its neuron protective and procognation properties and significantly improves the learning and memory ability of the animal models. Therefore, the α7 nAChR agonists are excellent drug candidates for Alzheimer's disease and we summarized here the current agonists that have selectivity of the α7 nAChR over the other nAChR, introduced recent molecular modeling works trying to explain the molecular mechanism of their selectivity and described the design of novel allosteric modulators in our lab.

Safety and clinical effects of EVP-6124 in subjects with Alzheimer's disease currently or previously receiving an acetylcholinesterase inhibitor medication

Alzheimer's disease (AD) is a prevalent and currently incurable brain disease whose impact will continue to rise as the population ages. With limited treatment options, a variety of experimental therapies are currently in clinical trials. EVP-6124 (encenicline) is an α7 nicotinic acetylcholine receptor partial agonist under investigation for the symptomatic treatment of AD. EVP-6124 activates the α7 nicotinic acetylcholine receptor at low nanomolar brain concentrations and improves memory performance in rats. Treatment with EVP-6124 in Phase I and II trials involving patients with mild-to-moderate AD was well tolerated and showed statistically significant improvements compared with placebo on cognitive and functional measures. Two Phase III trials under the title COGNITIV AD will assess the efficacy and tolerability of EVP-6124 in patients with mild-to-moderate AD. Based on the completed clinical trials and proposed mechanism of action, EVP-6124 would appear to be a good candidate for therapy in combination with cholinesterase inhibitors.

The α7 nicotinic receptor agonist ABT-107 protects against nigrostriatal damage in rats with unilateral 6-hydroxydopamine lesions

The finding that smoking is inversely correlated with Parkinson's disease and that nicotine attenuates nigrostriatal damage in Parkinsonian animals supports the idea that nicotine may be neuroprotective. Nicotine is thought to exert this effect by acting at nicotinic receptors (nAChRs), including the α7 subtype. The objective of this study was twofold: first, to test the protective potential of ABT-107, an agonist with high selectivity for α7 nAChRs; and second, to investigate its cellular mechanism of action. Rats were implanted with minipumps containing ABT-107 (0.25mg/kg/d). In addition, we tested the effect of nicotine (1mg/kg/d) as a positive control, and also DMXB (2mg/kg/d) which acts primarily with α7 but also α4β2* nAChRs. Two weeks after minipump placement, the rats were lesioned by unilateral administration of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle. Lesioning alone decreased contralateral forelimb use and adjusted stepping, two measures of Parkinsonism. ABT-107 and nicotine treatment significantly improved these behaviors at all weeks tested, with variable improvement with DMXB. We next investigated the cellular mechanism involved. The striatal dopamine transporter (DAT), a marker of dopaminergic integrity, was reduced ~70% with lesioning. ABT-107 or nicotine treatment significantly increased DAT levels in lesioned striatum; these drugs did not alter DAT levels in intact striatum. ABT-107 and nicotine also significantly improved basal dopamine release from lesioned striatum, as well as nicotine-stimulated dopamine release mediated via α4β2* and α6β2* nAChRs. These data suggest that α7 nAChR agonists may improve motor behaviors associated with nigrostriatal damage by enhancing striatal dopaminergic function.

Administration of the nicotinic acetylcholine receptor agonists ABT-089 and ABT-107 attenuates the reinstatement of nicotine-seeking behavior in rats

Current smoking cessation pharmacotherapies have modest efficacy, and most smokers relapse within the first few days after a quit attempt. Nicotine withdrawal-induced craving and cognitive impairments predict smoking relapse during abstinence and suggest that cognitive-enhancing drugs may prevent relapse. ABT-089 and ABT-107 are subtype-selective nAChR agonists that improve cognitive performance in laboratory animals. However, there are no studies examining the effects of ABT-089 and ABT-107 on nicotine self-administration and the reinstatement of nicotine-seeking behavior, an animal model of relapse in human smokers. The goal of the present study was to determine the effects of the α4β2*/α6β2* nAChR agonist ABT-089 and the α7 nAChR agonist ABT-107 on nicotine taking and seeking in rats. The effects of acute ABT-089 and ABT-107 pretreatment on nicotine self-administration and reinstatement were tested in male Sprague Dawley rats. Parallel studies of ABT-089 and ABT-107 on sucrose self-administration and reinstatement were tested in separate groups of rats to determine if the effects of these drug treatments generalized to other reinforced behaviors. Nicotine and sucrose self-administration behaviors were not altered following acute administration of ABT-089 (0, 0.12, 1.2 and 12.0mg/kg) or ABT-107 (0, 0.03 and 0.3mg/kg). In contrast, both ABT-089 and ABT-107 pretreatment dose-dependently attenuated nicotine reinstatement. These effects were reinforcer-specific as no effects of ABT-089 or ABT-107 pretreatment on sucrose seeking were noted. Taken together, these findings suggest that ABT-089 and ABT-107 do not affect nicotine consumption, but may reduce the likelihood that a smoking lapse will lead to relapse.

The α7 nicotinic receptor agonist ABT-107 decreases L-Dopa-induced dyskinesias in parkinsonian monkeys

Previous studies in Parkinsonian rats and monkeys have shown that β2-selective nicotinic acetylcholine receptor (nAChR) agonists reduce l-Dopa-induced dyskinesias (LIDs), a serious complication of l-Dopa therapy for Parkinson's disease. Since rodent studies also suggested an involvement of α7 nAChRs in LIDs, we tested the effect of the potent, selective α7 agonist ABT-107 [5-(6-[(3R)-1-azabicyclo[2.2.2]oct-3-yloxy] pyridazin-3-yl)-1H-indole]. MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-lesioned monkeys were gavaged with l-Dopa/carbidopa (10 and 2.5 mg/kg, respectively) twice daily, which resulted in stable LIDs. A dose-response study (0.03-1.0 mg/kg) showed that oral ABT-107 decreased LIDs by 40-60%. LIDs returned to control levels only after a 6-week ABT-107 washout, suggesting that long-term molecular changes were involved. Subsequent readministration of ABT-107 decreased LIDs by 50-60%, indicating that tolerance did not develop. ABT-107 had no effect on Parkinsonism or cognitive performance. We next tested ABT-107 together with the β2 agonist ABT-894 [(3-(5,6-dichloro-pyridin-3-yl)-1(S),5 (S)-3,6-diazabicyclo[3.2.0]heptane], previously shown to reduce LIDs in Parkinsonian monkeys. In one study, the monkeys were first given oral ABT-894 (0.01 mg/kg), which maximally decreased LIDs by 50-60%; they were then also treated with 0.1 mg/kg ABT-107, a dose that maximally reduced LIDs. The effect of combined treatment on LIDs was similar to that with either drug alone. Comparable results were observed in a group of monkeys first treated with ABT-107 and then also given ABT-894. Thus, α7 and β2 nAChR-selective drugs may function via a final common mechanism to reduce LIDs. The present results suggest that drugs targeting either α7 or β2 nAChRs may be useful as antidyskinetic agents in Parkinson's disease.

Effects of systemically applied nAChRα7 agonists and antagonists on light-induced phase shifts of hamster circadian activity rhythms

Many physiological systems in mammals are linked to the body's master circadian rhythm in the sleep/wake cycle and dysfunctions in this rhythm has been associated with neurological diseases such as major depression, Alzheimer's Disease and schizophrenia. There is some evidence that nicotinic cholinergic input to the master circadian pacemaker, the suprachiasmatic nucleus, may modulate circadian activity rhythms, but data employing in vivo preparations is sparse. Therefore we examined the ability of intraperitoneally applied nicotinic agonists and antagonists relatively selective for the α7 nicotinic receptor to modulate light-induced phase shifts of hamster circadian wheel running rhythms. Hamsters were maintained in constant darkness and exposed to light pulses early and late in their active period, mimicking dusk and dawn respectively, which elicited phase delays and advances of their circadian wheel running rhythms. The α7 receptor antagonists bPiDDB (0.03-3mg/kg) and methyllacaconitine (0.1-1mg/kg) inhibited both light- induced phase advances and delays of circadian wheel running rhythms by as much as 75% versus vehicle injections. In contrast, systemic injections of the α7 agonists PHA 543613 and ABT107, both at 0.156-2.5mg/kg, had no effect on light induced phase advances or delays. Further, α7 nicotinic receptors were identified in the hamster suprachiasmatic nucleus using an antibody that recognizes α7 nicotinic receptors. These results clearly identify the ability of α7 nicotinic receptor antagonists to inhibit light-entrainment of the hamster circadian pacemaker. Therefore, nicotinic compounds may be useful for the treatment of circadian dysfunction associated with neurological diseases.

Sazetidine-A Activates and Desensitizes Native α7 Nicotinic Acetylcholine Receptors

The aim of this study was to investigate the ability of sazetidine-A, a novel partial agonist at α4β2 nicotinic acetylcholine receptors (nAChRs), to affect the function of native α7 nAChRs in SH-SY5Y cells and primary cortical cultures. The α7-selective positive allosteric modulator PNU-120596 was used to reveal receptor activation, measured as an increase in intracellular calcium using fluorescent indicators. In the absence of PNU-120596, sazetidine-A elicited mecamylamine-sensitive increases in fluorescence in SH-SY5Y cells (EC₅₀ 4.2 µM) but no responses from primary cortical neurons. In the presence on PNU-120596, an additional response to sazetidine-A was observed in SH-SY5Y cells (EC₅₀ 0.4 µM) and robust responses were recorded in 14 % of cortical neurons. These PNU-120596-dependent responses were blocked by methyllycaconitine, consistent with the activation of α7 nAChRs. Preincubtion with sazetidine-A concentration-dependently attenuated subsequent responses to the α7-selective agonist PNU-282987 in SH-SY5Y cells (IC₅₀ 476 nM) and cortical cultures. These findings support the ability of sazetidine-A to interact with α7 nAChRs, which may contribute to sazetidine-A’s actions in complex physiological systems.

Design and synthesis of a hybrid series of potent and selective agonists of α7 nicotinic acetylcholine receptor

α7 nicotinic acetylcholine receptor agonists are promising therapeutic candidates for the treatment of cognitive impairment. As a follow up of our internal medicinal chemistry program we investigated a novel series of α7 nAChR agonists. Starting from molecular docking studies on two series of molecules recently developed in our laboratories, an alternative scaffold was designed attempting to combine the optimal features of these previously identified urea and pyrazole compounds. Based on our previous SAR knowledge and on predicted drug-like properties, a small library was synthesized in parallel manner, affording compounds with excellent α7 nAChR activity, selectivity and preliminary ADME profile.

Activation of α4β2*/α6β2* nicotinic receptors alleviates anxiety during nicotine withdrawal without upregulating nicotinic receptors

Although nicotine mediates its effects through several nicotinic acetylcholine receptor (nAChR) subtypes, it remains to be determined which nAChR subtypes directly mediate heightened anxiety during withdrawal. Relative success in abstinence has been found with the nAChR partial agonist varenicline (Chantix; Pfizer, Groton, CT); however, treatment with this drug fails to alleviate anxiety in individuals during nicotine withdrawal. Therefore, it is hypothesized that success can be found by the repurposing of other nAChR partial agonists for cessation therapies that target anxiety. It is noteworthy that the selective partial agonists for α4β2, ABT-089 [2-methyl-3-[2(S)-pyrrolidinylmethoxy]pyridine], and α7, ABT-107 [5-(6-[(3R)-1-azabicyclo[2.2.2]oct-3-yloxy] pyridazin-3-yl)-1H-indole] (AbbVie, North Chicago, IL), have not been evaluated as possible therapeutics for nicotine cessation. Therefore, we examined the effect of ABT-089 and ABT-107 on anxiety during withdrawal from nicotine in the novelty-induced hypophagia (NIH) paradigm. We found that short-term administration of ABT-089 and ABT-107 alleviate anxiety-like behavior during withdrawal from nicotine while long-term administration of ABT-089 but not ABT-107 reduces anxiety-like behavior during withdrawal. After behavioral testing, brains were harvested and β2-containing nAChRs were measured using [(3)H]epibaditine. ABT-089 and ABT-107 do not upregulate nAChRs, which is in contrast to the upregulation of nAChRs observed after nicotine. Furthermore, ABT-089 is anxiogenic in nicotine naive animals, suggesting that the effects on anxiety are specifically related to the nicotine-dependent state. Together, these studies identify additional nAChR partial agonists that may aid in the rational development of smoking cessation aids.

Nicotinic agonist-induced improvement of vigilance in mice in the 5-choice continuous performance test

Impaired attentional processing is prevalent in numerous neuropsychiatric disorders and may negatively impact other cognitive and functional domains. Nicotine - a nonspecific nicotinic acetylcholine receptor (nAChR) agonist - improves vigilance in healthy subjects and schizophrenia patients as measured by continuous performance tests (CPTs), but the nAChR mediating this effect remains unclear. Here we examine the effects of: (a) nicotine; (b) the selective α7 nAChR agonist PNU 282987; and (c) the selective α4β2 nAChR agonist ABT-418 alone and in combination with scopolamine-induced disruption of mouse 5-choice (5C-)CPT performance. This task requires the inhibition of responses to non-target stimuli as well as active responses to target stimuli, consistent with human CPTs. C57BL/6N mice were trained to perform the 5C-CPT. Drug effects were examined in extended session and variable stimulus-duration challenges of performance. Acute drug effects on scopolamine-induced disruption in performance were also investigated. Nicotine and ABT-418 subtly but significantly improved performance of normal mice and attenuated scopolamine-induced disruptions in the 5C-CPT. PNU 282-987 had no effects on performance. The similarity of nicotine and ABT-418 effects provides support for an α4β2 nAChR mechanism of action for nicotine-induced improvement in attention/vigilance. Moreover, the data provide pharmacological predictive validation for the 5C-CPT because nicotine improved and scopolamine disrupted normal performance of the task, consistent with healthy humans in the CPT. Future studies using more selective agonists may result in more robust improvements in performance.

Effects of the novel α7 nicotinic acetylcholine receptor agonist ABT-107 on sensory gating in DBA/2 mice: pharmacodynamic characterization

Nicotinic acetylcholine receptor (nAChR) agonists improve sensory gating deficits in animal models and schizophrenic patients. The aim of this study was to determine whether the novel and selective α7 nAChR full agonist 5-(6-[(3R)-1-azabicyclo[2.2.2]oct-3-yloxy]pyridazin-3-yl)-1H-indole (ABT-107) improves sensory gating deficits in DBA/2 mice. Sensory gating was measured by recording hippocampal-evoked potential P20-N40 waves and determining gating test/conditioning (T/C) ratios in a paired auditory stimulus paradigm. ABT-107 at 0.1 μmol/kg (average plasma concentration of 1.1 ng/ml) significantly improved sensory gating by lowering T/C ratios during a 30-min period after administration in unanesthetized DBA/2 mice. ABT-107 at 1.0 μmol/kg was ineffective at 30 min after administration when average plasma levels were 13.5 ng/ml. However, the 1.0 μmol/kg dose was effective 180 min after administration when plasma concentration had fallen to 1.9 ng/ml. ABT-107 (0.1 μmol/kg) also improved sensory gating in anesthetized DBA/2 mice pretreated with α7 nAChR-desensitizing doses of nicotine (6.2 μmol/kg) or ABT-107 (0.1 μmol/kg) itself. Moreover, repeated b.i.d. dosing of ABT-107 (0.1 μmol/kg) was as efficacious as a single dose. The acute efficacy of ABT-107 (0.1 μmol/kg) was blocked by the nAChR antagonist methyllycaconitine, but not by the α4β2 nAChR antagonist dihydro-β-erythroidine. These studies demonstrate that ABT-107 improves sensory gating through the activation of nAChRs, and efficacy is sustained under conditions of repeated dosing or with prior nAChR activation with nicotine.

Ion channel pharmacology under flow: automation via well-plate microfluidics

Automated patch clamping addresses the need for high-throughput screening of chemical entities that alter ion channel function. As a result, there is considerable utility in the pharmaceutical screening arena for novel platforms that can produce relevant data both rapidly and consistently. Here we present results that were obtained with an innovative microfluidic automated patch clamp system utilizing a well-plate that eliminates the necessity of internal robotic liquid handling. Continuous recording from cell ensembles, rapid solution switching, and a bench-top footprint enable a number of assay formats previously inaccessible to automated systems. An electro-pneumatic interface was employed to drive the laminar flow of solutions in a microfluidic network that delivered cells in suspension to ensemble recording sites. Whole-cell voltage clamp was applied to linear arrays of 20 cells in parallel utilizing a 64-channel voltage clamp amplifier. A number of unique assays requiring sequential compound applications separated by a second or less, such as rapid determination of the agonist EC(50) for a ligand-gated ion channel or the kinetics of desensitization recovery, are enabled by the system. In addition, the system was validated via electrophysiological characterizations of both voltage-gated and ligand-gated ion channel targets: hK(V)2.1 and human Ether-à-go-go-related gene potassium channels, hNa(V)1.7 and 1.8 sodium channels, and (α1) hGABA(A) and (α1) human nicotinic acetylcholine receptor receptors. Our results show that the voltage dependence, kinetics, and interactions of these channels with pharmacological agents were matched to reference data. The results from these IonFlux™ experiments demonstrate that the system provides high-throughput automated electrophysiology with enhanced reliability and consistency, in a user-friendly format.

Effects of α7 nicotinic acetylcholine receptor agonists on antipsychotic efficacy in a preclinical mouse model of psychosis

RATIONALE

Antipsychotics normalize responses in the DBA/2 mouse model of prepulse inhibition (PPI), a preclinical model of sensorimotor gating deficits. The α7 nicotinic acetylcholine receptor (nAChR) as a molecular target is considered an attractive approach for improvement of cognitive deficits in schizophrenia (CDS). Assessment of clinical efficacy of novel agents in CDS involves treating patients already on antipsychotic medications.

OBJECTIVE

We evaluated the effects of the combination of α7 nAChR agonists ABT-107 (0.1-10.0 mg/kg i.p.), A-582941 (0.04-4.0 mg/kg i.p.), and PNU282987 (1.0-10.0 mg/kg i.p.) with risperidone (0.1-1.0 mg/kg i.p.) or haloperidol (0.3-3.0 mg/kg i.p.), representative atypical and typical antipsychotic agents in the DBA/2 mouse PPI model. The same α7 agonists were given alone or in combination with a dose of antipsychotic medication that induces a minimal level of catalepsy in rats, an assay with predictive validity for the induction of extrapyramidal symptoms.

RESULTS

The α7 nAChR agonists ABT-107, A-582941, and PNU282987 had no effect in DBA/2 mouse PPI when given alone yet increased the effects of haloperidol and risperidone. The α7 nAChR agonists did not cause catalepsy in rats, nor did they enhance antipsychotic-induced catalepsy.

CONCLUSIONS

When given in combination with either a typical or atypical antipsychotic, α7 nAChR agonists did not impair efficacy in the DBA/2 J mouse PPI model. The efficacy but not the motoric side effects of antipsychotics was enhanced, suggesting that adjunctive therapy of α7 nAChR agonists not only could be useful for the treatment of cognitive deficits associated with schizophrenia but also could enhance the efficacy against positive symptoms.

The role of the α7 nicotinic receptor in cognitive processing of persons with schizophrenia

PURPOSE OF REVIEW

Cognitive abilities are established as major deficits in those with schizophrenia and represent significant impediments toward successful psychosocial functioning within the community. Pharmacological treatments have historically focused on reducing the positive and negative symptoms of schizophrenia, with recent increased attention to developing agents to improve cognition as a treatment outcome. This review will highlight the recent advances in developing agents that act at neuronal α7 nicotinic acetylcholine receptors (nAChRs).

RECENT FINDINGS

A number of agents have demonstrated improvements in cognitive deficits in studies using both preclinical and clinical models. Evidence suggests that α7 nAChR activation impacts rule acquisition and initial learning in preclinical models. Preclinical studies indicate improved spatial working memory and reversal of phencyclidine-induced learning and memory deficits. Clinical studies indicate normalization of neuronal default network activity using functional magnetic resonance imaging methods. In addition, research on new agents developed to improve cognition suggests improved cognitive functioning in multiple domains (e.g., attention, memory, executive functioning, sensory gating and overall cognition) and promising safety profiles of additional agents.

SUMMARY

Improvements in cognitive functioning within preclinical and clinical studies of schizophrenia are evident through the use of α7 nAChR agonists, while positive allosteric modulators may gain increased attention in the future.

Alpha7 nicotinic acetylcholine receptor is a target in pharmacology and toxicology

Alpha7 nicotinic acetylcholine receptor (α7 nAChR) is an important part of the cholinergic nerve system in the brain. Moreover, it is associated with a cholinergic anti-inflammatory pathway in the termination of the parasympathetic nervous system. Antagonists of α7 nAChR are a wide group represented by conotoxin and bungarotoxin. Even Alzheimer's disease drug memantine acting as an antagonist in its side pathway belongs in this group. Agonists of α7 nAChR are suitable for treatment of multiple cognitive dysfunctions such as Alzheimer's disease or schizophrenia. Inflammation or even sepsis can be ameliorated by the agonistic acting compounds. Preparations RG3487, SEN34625/WYE-103914, SEN12333, ABT-107, Clozapine, GTS-21, CNI-1493, and AR-R17779 are representative examples of the novel compounds with affinity toward the α7 nAChR. Pharmacological, toxicological, and medicinal significance of α7 nAChR are discussed throughout this paper.

The nicotinic acetylcholine receptor: smoking and Alzheimer's disease revisited

Epidemiological studies regarding Alzheimer's disease (AD) in smokers currently suggest inconsistent results. The clinicopathological findings also vary as to how AD pathology is affected by smoking behavior. Even though clinicopathological, functional, and epidemiological studies in humans do not present a consistent picture, much of the in vitro data implies that nicotine has neuroprotective effects when used in neurodegenerative disorder models. Current studies of the effects of nicotine and nicotinic agonists on cognitive function in both the non-demented and those with AD are not convincing. More data is needed to determine whether repetitive activation of nAChR with intermittent or acute exposure to nicotine, acute activation of nAChR, or long-lasting inactivation of nAChR secondary to chronic nicotine exposure will have a therapeutic effect and/or explain the beneficial effects of those types of drugs. Other studies show multifaceted connections between nicotine, nicotinic agonists, smoking, and nAChRs implicated in AD etiology. Although many controversies still exist, ongoing studies are revealing how nicotinic receptor changes and functions may be significant to the neurochemical, pathological, and clinical changes that appear in AD.

EVP-6124, a novel and selective α7 nicotinic acetylcholine receptor partial agonist, improves memory performance by potentiating the acetylcholine response of α7 nicotinic acetylcholine receptors

EVP-6124, (R)-7-chloro-N-quinuclidin-3-yl)benzo[b]thiophene-2-carboxamide, is a novel partial agonist of α7 neuronal nicotinic acetylcholine receptors (nAChRs) that was evaluated here in vitro and in vivo. In binding and functional experiments, EVP-6124 showed selectivity for α7 nAChRs and did not activate or inhibit heteromeric α4β2 nAChRs. EVP-6124 had good brain penetration and an adequate exposure time. EVP-6124 (0.3 mg/kg, p.o.) significantly restored memory function in scopolamine-treated rats (0.1 mg/kg, i.p.) in an object recognition task (ORT). Although donepezil at 0.1 mg/kg, p.o. or EVP-6124 at 0.03 mg/kg, p.o. did not improve memory in this task, co-administration of these sub-efficacious doses fully restored memory. In a natural forgetting test, an ORT with a 24 h retention time, EVP-6124 improved memory at 0.3 mg/kg, p.o. This improvement was blocked by the selective α7 nAChR antagonist methyllycaconitine (0.3 mg/kg, i.p. or 10 μg, i.c.v.). In co-application experiments of EVP-6124 with acetylcholine, sustained exposure to EVP-6124 in functional investigations in oocytes caused desensitization at concentrations greater than 3 nM, while lower concentrations (0.3-1 nM) caused an increase in the acetylcholine-evoked response. These actions were interpreted as representing a co-agonist activity of EVP-6124 with acetylcholine on α7 nAChRs. The concentrations of EVP-6124 that resulted in physiological potentiation were consistent with the free drug concentrations in brain that improved memory performance in the ORT. These data suggest that the selective partial agonist EVP-6124 improves memory performance by potentiating the acetylcholine response of α7 nAChRs and support new therapeutic strategies for the treatment of cognitive impairment. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Virtual screening against acetylcholine binding protein

The nicotinic acetylcholine receptors (nAChRs) are a member of the ligand-gated ion channel family and play a key role in the transfer of information across neurological networks. The X-ray crystal structure of agonist-bound α(7) acetylcholine binding protein (AChBP) has been recognized as the most appropriate template to model the ligand-binding domain of nAChR for studying the molecular mechanism of the receptor-ligand interactions. Virtual screening of the National Cancer Institute diversity set, a library of 1990 compounds with nonredundant pharmacophore profiles, using AutoDock against AChBPs revealed 51 potential candidates. In vitro radioligand competition assays using [(3)H] epibatidine against the AChBPs from the freshwater snails, Lymnaea stagnalis, and from the marine species, Aplysia californica and the mutant (AcY55W), revealed seven compounds from the list of candidates that had micromolar to nanomolar affinities for the AChBPs. Further investigation on α(7)nAChR expressing in Xenopus oocytes and on the recombinant receptors with fluorescence resonance energy transfer (FRET)-based calcium sensor expressing in HEK cells showed that seven compounds were antagonists of α(7)nAChR, only one compound (NSC34352) demonstrated partial agonistic effect at low dose (10 µM), and two compounds (NSC36369 and NSC34352) were selective antagonists on α(7)nAchR with moderate potency. These hits serve as novel templates/scaffolds for development of more potent and specific in the AChR systems.

Positive allosteric modulators as an approach to nicotinic acetylcholine receptor-targeted therapeutics: advantages and limitations

Neuronal nicotinic acetylcholine receptors (nAChR), recognized targets for drug development in cognitive and neuro-degenerative disorders, are allosteric proteins with dynamic interconversions between multiple functional states. Activation of the nAChR ion channel is primarily controlled by the binding of ligands (agonists, partial agonists, competitive antagonists) at conventional agonist binding sites, but is also regulated in either negative or positive ways by the binding of ligands to other modulatory sites. In this review, we discuss models for the activation and desensitization of nAChR, and the discovery of multiple types of ligands that influence those processes in both heteromeric nAChR, such as the high-affinity nicotine receptors of the brain, and homomeric α7-type receptors. In recent years, α7 nAChRs have been identified as a potential target for therapeutic indications leading to the development of α7-selective agonists and partial agonists. However, unique properties of α7 nAChR, including low probability of channel opening and rapid desensitization, may limit the therapeutic usefulness of ligands binding exclusively to conventional agonist binding sites. New enthusiasm for the therapeutic targeting of α7 has come from the identification of α7-selective positive allosteric modulators (PAMs) that work effectively on the intrinsic factors that limit α7 ion channel activation. While these new drugs appear promising for therapeutic development, we also consider potential caveats and possible limitations for their use, including PAM-insensitive forms of desensitization and cytotoxicity issues.

Single- and multiple-dose pharmacokinetics, safety, and tolerability of the selective alpha7 neuronal nicotinic receptor agonist, ABT-107, in healthy human volunteers

ABT-107 is a potent, selective α7 nicotinic receptor agonist under development for treatment of Alzheimer's disease and cognitive deficits associated with schizophrenia. The pharmacokinetics, safety, and tolerability of escalating single oral doses (1, 3, 10, 30, 60, 80, and 100 mg; double-blind, placebo-controlled, randomized, incomplete crossover design) and multiple oral doses (2, 6, and 15 mg once daily for 7 days; double-blind, placebo-controlled, randomized, parallel-group design) of ABT-107 were evaluated. Additionally, effect of food on ABT-107 pharmacokinetics (20-mg single dose) was evaluated using an open-label, 2-period, fasting and nonfasting, randomized, complete crossover design. ABT-107 exhibited nonlinear (more than dose-proportional) pharmacokinetics. ABT-107 half-life ranged from 7 to 10 hours, and steady state was achieved by day 6 of dosing. Food did not have a clinically meaningful effect on ABT-107 exposure. ABT-107 was safe and well tolerated over the tested dose range. The most frequently reported adverse events were nausea, headache, and tremor following single dosing and somnolence following multiple dosing. The pharmacokinetics, safety, and tolerability profiles of ABT-107 pose it as a good candidate for further development.