HBT1, a Novel AMPA Receptor Potentiator with Lower Agonistic Effect, Avoided Bell-Shaped Response in In Vitro BDNF Production

Novel α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR) potentiator LT-102: A promising therapeutic agent for treating cognitive impairment associated with schizophrenia

AIMS

We aimed to evaluate the potential of a novel selective α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR) potentiator, LT-102, in treating cognitive impairments associated with schizophrenia (CIAS) and elucidating its mechanism of action.

METHODS

The activity of LT-102 was examined by Ca2+ influx assays and patch-clamp in rat primary hippocampal neurons. The structure of the complex was determined by X-ray crystallography. The selectivity of LT-102 was evaluated by hERG tail current recording and kinase-inhibition assays. The electrophysiological characterization of LT-102 was characterized by patch-clamp recording in mouse hippocampal slices. The expression and phosphorylation levels of proteins were examined by Western blotting. Cognitive function was assessed using the Morris water maze and novel object recognition tests.

RESULTS

LT-102 is a novel and selective AMPAR potentiator with little agonistic effect, which binds to the allosteric site formed by the intradimer interface of AMPAR's GluA2 subunit. Treatment with LT-102 facilitated long-term potentiation in mouse hippocampal slices and reversed cognitive deficits in a phencyclidine-induced mouse model. Additionally, LT-102 treatment increased the protein level of brain-derived neurotrophic factor and the phosphorylation of GluA1 in primary neurons and hippocampal tissues.

CONCLUSION

We conclude that LT-102 ameliorates cognitive impairments in a phencyclidine-induced model of schizophrenia by enhancing synaptic function, which could make it a potential therapeutic candidate for CIAS.

AMPA receptor neurotransmission and therapeutic applications: A comprehensive review of their multifaceted modulation

The neuropharmacological community has shown a strong interest in AMPA receptors as critical components of excitatory synaptic transmission during the last fifteen years. AMPA receptors, members of the ionotropic glutamate receptor family, allow rapid excitatory neurotransmission in the brain. AMPA receptors, which are permeable to sodium and potassium ions, manage the bulk of the brain's rapid synaptic communications. This study thoroughly examines the recent developments in AMPA receptor regulation, focusing on a shift from single chemical illustrations to a more extensive investigation of underlying processes. The complex interplay of these modulators in modifying the function and structure of AMPA receptors is the main focus, providing insight into their influence on the speed of excitatory neurotransmission. This research emphasizes the potential of AMPA receptor modulation as a therapy for various neurological disorders such as epilepsy and Alzheimer's disease. Analyzing these regulators' sophisticated molecular details enhances our comprehension of neuropharmacology, representing a significant advancement in using AMPA receptors for treating intricate neurological conditions.

5-Nitroisoxazoles in SNAr Reactions: A Novel Chemo- and Regioselective Approach to Isoxazole-Based Bivalent Ligands of AMPA Receptors

An efficient regioselective approach to novel functionalized bis(isoxazoles) with a variety of aromatic and aliphatic linkers was elaborated, based on the heterocyclization reaction of electrophilic alkenes under the treatment with tetranitromethane-triethylamine complex affording 3-EWG-5-nitroisoxazoles. The subsequent SNAr reactions of 5-nitroisoxazoles with various O,O-, N,N- and S,S-bis(nucleophiles) provide a wide range of bis(isoxazole) derivatives in good isolated yields. Employing an elaborated method, a series of novel bis(3-EWG-isoxazoles) as the promising allosteric modulators of AMPA receptors were designed and synthesized. The effect of the compounds on the kainate-induced currents was studied in the patch clamp experiments, revealing modulator properties for several of them. The best positive modulator potency was found for dimethyl 5,5'-(ethane-1,2-diylbis(sulfanediyl))bis(isoxazole-3-carboxylate), which potentiated the kainate-induced currents in a wide concentration range (10-12-10-6 M) with maximum potentiation of 77% at 10-10 M. The results were rationalized using molecular docking and molecular dynamics simulations of modulator complexes with the dimeric ligand-binding domain of the GluA2 AMPA receptor. The predicted physicochemical, ADMET, and PAINS properties confirmed that the AMPA receptor modulators based on the bis(isoxazole) scaffold may serve as potential lead compounds for the development of neuroprotective drugs.

New Allosteric Modulators of AMPA Receptors: Synthesis and Study of Their Functional Activity by Radioligand-Receptor Binding Analysis

The synthetic approaches to three new AMPA receptor modulators-derivatives of 1,11-dimethyl-3,6,9-triazatricyclo[7.3.1.1^3,11]tetradecane-4,8,12-trione-had been developed and all steps of synthesis were optimized. The structures of the compounds contain tricyclic cage and indane fragments necessary for binding with the target receptor. Their physiological activity was studied by radioligand-receptor binding analysis using [³H]PAM-43 as a reference ligand, which is a highly potent positive allosteric modulator of AMPA receptors. The results of radioligand-binding studies indicated the high potency of two synthesized compounds to bind with the same targets as positive allosteric modulator PAM-43 (at least on AMPA receptors). We suggest that the Glu-dependent specific binding site of [³H]PAM-43 or the receptor containing this site may be one of the targets of the new compounds. We also suggest that enhanced radioligand binding may indicate the existence of synergistic effects of compounds 11b and 11c with respect to PAM-43 binding to the targets. At the same time, these compounds may not compete directly with PAM-43 for its specific binding sites but bind to other specific sites of this biotarget, changing its conformation and thereby causing a synergistic effect of cooperative interaction. It can be expected that the newly synthesized compounds will also have pronounced effects on the glutamatergic system of the mammalian brain.

Exposure to Tetrabutylammonium Bromide Impairs Cranial Neural Crest Specification, Neurogenic Program, and Brain Morphogenesis

Tetrabutylammonium bromide (TBAB) is a widely used industrial reagent and is commonly found in our aquatic ecosystem as an industrial byproduct. In humans, the ingestion of TBAB causes severe neurological impairments and disorders such as vertigo, hallucinations, and delirium. Yet, the extent of environmental risk and TBAB toxicity to human health is poorly understood. In this study, we aim to determine the developmental toxicity of TBAB using zebrafish embryos as a model and provide novel insights into the mechanism of action of such chemicals on neurodevelopment and the overall embryonic program. Our results show that exposure to TBAB results in impaired development of the brain, inner ear, and pharyngeal skeletal elements in the zebrafish embryo. TBAB treatment resulted in aberrations in the specification of the neural crest precursors, hindbrain segmentation, and otic neurogenesis. TBAB treatment also induced a surge in apoptosis in the head, tail, and trunk regions of the developing embryo. Long-term TBAB exposure resulted in cardiac edema and craniofacial defects. Further, in silico molecular docking analysis indicated that TBAB binds to AMPA receptors and modulates neural developmental genes such as olfactomedin and acetylcholinesterase in the embryonic brain. To summarize, our study highlights the novel effects of TBAB on embryonic brain formation and segmentation, ear morphogenesis, and craniofacial skeletal development.

3,4-Dihydrobenzo[e][1,2,3]oxathiazine 2,2-dioxide analogs act as potential AMPA receptor potentiators with antidepressant activity

Major depressive disorder is a common psychiatric disorder, with ∼30% of patients suffering from treatment-resistant depression. Based on preclinical studies on ketamine, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) activation may be a promising therapeutic approach. In this study, we synthesized a series of novel 3,4-dihydrobenzo[e][1,2,3]oxathiazine 2,2-dioxide analogs and analyzed their potential as AMPAR potentiators. Compounds 5aa and 7k exhibited high potentiation with little agonist activity in a high-throughput screen using a calcium influx assay in cultured hippocampal primary neurons. In rats, compound 7k had better pharmacokinetic properties and oral bioavailability (F = 67.19%); it also exhibited an acceptable safety profile in vital internal organs based on hematoxylin and eosin staining. We found that 7k produced a rapid antidepressant-like effect in chronic restraint stress-induced mice 1 h after intraperitoneal administration. Our study presented a series of novel AMPAR potentiators and identified 7k as a promising drug-like candidate against major depressive disorders.

Design and synthesis of 6-methylpyridin-2-one derivatives as novel and potent GluN2A positive allosteric modulators for the treatment of cognitive impairment

N-Methyl-D-aspartate receptors (NMDARs) are key regulators of synaptic plasticity in the central nervous system. Potentiation of NMDARs containing GluN2A subunit has been recently recognized as a promising therapeutic approach for neurological disorders. We identified a novel series of GluN2A positive allosteric modulator (PAM) with a pyridin-2-one scaffold. Initial lead compound 1 was discovered through in silico-based screening of virtual ligands with various monocyclic scaffolds. GluN2A PAM activity was increased by introduction of a methyl group at the 6-position of the pyridin-2-one ring and a cyano group in the side chain. Modification of the aromatic ring led to the identification of potent and brain-penetrant 6-methylpyridin-2-one 17 with a negligible binding activity for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). Oral administration of 17 significantly enhanced rat hippocampal long-term potentiation (LTP). Thus, 17 would be a useful in vivo pharmacological tool to investigate complex NMDAR functions for the discovery of therapeutics toward diseases associated with NMDAR dysfunction.

Role of the AMPA receptor in antidepressant effects of ketamine and potential of AMPA receptor potentiators as a novel antidepressant

Ketamine exerts rapid and long-lasting antidepressant effects in patients with treatment-resistant depression. However, its clinical use is limited by its undesirable psychotomimetic side effects. Accumulating evidence from preclinical studies has shown that the antidepressant effects of ketamine are dependent on α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R) activation, which triggers activation of the mechanistic target of rapamycin pathway and brain-derived neurotrophic factor release. Thus, AMPA-R has emerged as a promising new target for novel antidepressants with a rapid onset of action. However, almost all known AMPA-R potentiators carry the risk of a narrow bell-shaped dose-response curve and a poor safety margin against seizures. Our data suggest that agonistic activity is not only related to the risks of bell-shaped dose-response curves and seizures but also to the reduced synaptic transmission and procognitive effects of AMPA-R potentiators. In this review, we describe our original screening approach that led to the discovery of an investigational AMPA-R potentiator with low agonistic activity, TAK-653. We further review the in vitro and in vivo profiles of TAK-653, including its procognitive and antidepressant-like effects, as well as its safety profile, in comparison with known AMPA-R potentiators with agonistic activity and AMPA, an AMPA-R agonist. The low agnostic activity of TAK-653 may overcome limitations of known AMPA-R potentiators. This article is part of the Special Issue on 'Ketamine and its Metabolites'.

Diversity of AMPA Receptor Ligands: Chemotypes, Binding Modes, Mechanisms of Action, and Therapeutic Effects

L-Glutamic acid is the main excitatory neurotransmitter in the central nervous system (CNS). Its associated receptors localized on neuronal and non-neuronal cells mediate rapid excitatory synaptic transmission in the CNS and regulate a wide range of processes in the brain, spinal cord, retina, and peripheral nervous system. In particular, the glutamate receptors selective to α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) also play an important role in numerous neurological disorders and attract close attention as targets for the creation of new classes of drugs for the treatment or substantial correction of a number of serious neurodegenerative and neuropsychiatric diseases. For this reason, the search for various types of AMPA receptor ligands and studies of their properties are attracting considerable attention both in academic institutions and in pharmaceutical companies around the world. This review focuses mainly on the advances in this area published since 2017. Particular attention is paid to the structural diversity of new chemotypes of agonists, competitive AMPA receptor antagonists, positive and negative allosteric modulators, transmembrane AMPA regulatory protein (TARP) dependent allosteric modulators, ion channel blockers as well as their binding sites. This review also presents the studies of the mechanisms of action of AMPA receptor ligands that mediate their therapeutic effects.

Positive AMPA receptor modulation in the treatment of neuropsychiatric disorders: A long and winding road

Glutamatergic transmission is widely implicated in neuropsychiatric disorders, and the discovery that ketamine elicits rapid-acting antidepressant effects by modulating α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) signaling has spurred a resurgence of interest in the field. This review explores agents in various stages of development for neuropsychiatric disorders that positively modulate AMPARs, both directly and indirectly. Despite promising preclinical research, few direct and indirect AMPAR positive modulators have progressed past early clinical development. Challenges such as low potency have created barriers to effective implementation. Nevertheless, the functional complexity of AMPARs sets them apart from other drug targets and allows for specificity in drug discovery. Additional effective treatments for neuropsychiatric disorders that work through positive AMPAR modulation may eventually be developed.

Strictly regulated agonist-dependent activation of AMPA-R is the key characteristic of TAK-653 for robust synaptic responses and cognitive improvement

Agonistic profiles of AMPA receptor (AMPA-R) potentiators may be associated with seizure risk and bell-shaped dose-response effects. Here, we report the pharmacological characteristics of a novel AMPA-R potentiator, TAK-653, which exhibits minimal agonistic properties. TAK-653 bound to the ligand binding domain of recombinant AMPA-R in a glutamate-dependent manner. TAK-653 strictly potentiated a glutamate-induced Ca2+ influx in hGluA1i-expressing CHO cells through structural interference at Ser743 in GluA1. In primary neurons, TAK-653 augmented AMPA-induced Ca2+ influx and AMPA-elicited currents via physiological AMPA-R with little agonistic effects. Interestingly, TAK-653 enhanced electrically evoked AMPA-R-mediated EPSPs more potently than AMPA (agonist) or LY451646 (AMPA-R potentiator with a prominent agonistic effect) in brain slices. Moreover, TAK-653 improved cognition for both working memory and recognition memory, while LY451646 did so only for recognition memory, and AMPA did not improve either. These data suggest that the facilitation of phasic AMPA-R activation by physiologically-released glutamate is the key to enhancing synaptic and cognitive functions, and nonselective activation of resting AMPA-Rs may negatively affect this process. Importantly, TAK-653 had a wide safety margin against convulsion; TAK-653 showed a 419-fold (plasma Cmax) and 1017-fold (AUC plasma) margin in rats. These findings provide insight into a therapeutically important aspect of AMPA-R potentiation.

[Ligands of the AMPA-subtype glutamate receptors: mechanisms of action and novel chemotypes]

Ionotropic glutamate receptors of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subtype play a key role in synaptic plasticity representing one of the mechanisms for learning and memory formation. They can also serve as targets for the development of novel classes of pharmaceuticals for the treatment or substantive correction of many serious neurodegenerative and psychoneurological disorders. The search and studies of various types of AMPA receptor ligands attract considerable attention from academic organizations and pharmaceutical companies all over the world. This review mainly focuses on recent advances in this field. The architecture and operational mechanism of the receptor as well as its major binding sites and ligand types are considered. Special attention is paid to the studies of mechanisms of action and novel chemotypes of AMPA receptor agonists and competitive antagonists, positive and negative allosteric modulators, auxiliary protein-dependent allosteric modulators, and ion channel blockers.

Structural basis for positive allosteric modulation of AMPA and kainate receptors

This paper summarizes the present knowledge on how positive allosteric modulators (PAMs) interact with the ligand-binding domain (LBD) of AMPA and kainate receptors, based on structure determinations. AMPA and kainate receptors belong to the family of ionotropic glutamate receptors that are responsible for mediating the majority of fast excitatory neurotransmission. These receptors have been related to brain disorders, e.g. Alzheimer's disease and attention deficit hyperactivity disorder. PAMs are small molecules that potentiate AMPA and kainate receptor currents by interfering with receptor desensitization. Therefore, PAMs are considered to be of interest for the development of pharmacological tools. Whereas PAMs for AMPA receptors have been known for several years, only recently have PAMs for kainate receptors been reported. Today, >80 structures are available for AMPA receptors with PAMs. These PAMs bind at the interface between two LBD subunits in the vicinity of residue 775, which is important for functional differences between flip and flop isoforms of AMPA receptors. PAMs can be divided into five classes based on their binding mode. The most potent PAM reported to date belongs to class 3, which comprises dimerized PAMs. Three structures of the kainate receptor GluK1 were determined with PAMs belonging to class 2. One PAM enhances kainate receptor currents 5- to 59-fold but shows 100-fold lower potency compared to AMPA receptors. Selective PAMs for kainate receptors will be of great use as pharmacological tools for functional investigations in vivo and might potentially prove useful as drugs in controlling the activity of neuronal networks.

Electrophysiological characterization of a novel AMPA receptor potentiator, TAK-137, in rat hippocampal neurons

We have recently discovered an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R) potentiator TAK-137, 9-(4-phenoxyphenyl)-3,4-dihydropyrido[2,1-c][1,2,4] thiadiazine 2,2-dioxide with little agonistic effect. Under preclinical evaluation, TAK-137 demonstrated potent pro-cognitive effects with lower risks of seizure and bell-shaped dose response than LY451646, a potent AMPA-R potentiator, in rodents and monkeys. In this study, using rat primary cultured hippocampal neurons we explored the electrophysiological characterization of TAK-137 on native AMPA-Rs. TAK-137 dose-dependently enhanced AMPA-induced inward currents; its potency in the presence of AMPA was comparable to that of LY451646. The inward currents enhanced by TAK-137 were almost completely inhibited by GYKI53655, a selective AMPA-R blocker. Moreover, TAK-137 did not affect N-methyl-_D-aspartate (NMDA)-activated inward currents, which suggests the AMPA-R-selective activation by TAK-137. In the absence of AMPA-R agonist, LY451646 at 30 μM induced slowly developing large inward currents, whereas TAK-137 at 30 μM exhibited a slight impact on baseline holding currents, further supporting the lower agonistic properties of TAK-137 than LY451646. Similar to LY451646, TAK-137 also increased the potency and binding affinity of AMPA for AMPA-Rs. These results indicate that TAK-137 is a highly potent and selective potentiator with little agonistic effect against native AMPA-Rs. Much greater agonistic effects of LY451646 than of TAK-137 may contribute to the increased risks of seizure and bell-shaped dose response in vivo.

T-448, a specific inhibitor of LSD1 enzyme activity, improves learning function without causing thrombocytopenia in mice

Dysregulation of histone H3 lysine 4 (H3K4) methylation has been implicated in the pathogenesis of several neurodevelopmental disorders. Targeting lysine-specific demethylase 1 (LSD1), an H3K4 demethylase, is therefore a promising approach to treat these disorders. However, LSD1 forms complexes with cofactors including growth factor independent 1B (GFI1B), a critical regulator of hematopoietic differentiation. Known tranylcypromine-based irreversible LSD1 inhibitors bind to coenzyme flavin adenine dinucleotide (FAD) and disrupt the LSD1-GFI1B complex, which is associated with hematotoxicity such as thrombocytopenia, representing a major hurdle in the development of LSD1 inhibitors as therapeutic agents. To discover LSD1 inhibitors with potent epigenetic modulation and lower risk of hematotoxicity, we screened small molecules that enhance H3K4 methylation by the inhibition of LSD1 enzyme activity in primary cultured rat neurons but have little impact on LSD1-GFI1B complex in human TF-1a erythroblasts. Here we report the discovery of a specific inhibitor of LSD1 enzyme activity, T-448 (3-((1S,2R)-2-(cyclobutylamino)cyclopropyl)-N-(5-methyl-1,3,4-thiadiazol-2-yl)benzamide fumarate). T-448 has minimal impact on the LSD1-GFI1B complex and a superior hematological safety profile in mice via the generation of a compact formyl-FAD adduct. T-448 increased brain H3K4 methylation and partially restored learning function in mice with NMDA receptor hypofunction. T-448-type LSD1 inhibitors with improved safety profiles may provide unique therapeutic approaches for central nervous system disorders associated with epigenetic dysregulation.

Preclinical characterization of AMPA receptor potentiator TAK-137 as a therapeutic drug for schizophrenia

The downregulation of the glutamate system may be involved in positive, negative, and cognitive symptoms of schizophrenia. Through enhanced glutamate signaling, the activation of the α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor, an ionotropic glutamate receptor, could be a new therapeutic strategy for schizophrenia. TAK-137 is a novel AMPA receptor potentiator with minimal agonistic activity; in this study, we used rodents and nonhuman primates to assess its potential as a drug for schizophrenia. At 10 mg kg-1 p.o., TAK-137 partially inhibited methamphetamine-induced hyperlocomotion in rats, and at 3, 10, and 30 mg kg-1 p.o., TAK-137 partially inhibited MK-801-induced hyperlocomotion in mice, suggesting weak effects on the positive symptoms of schizophrenia. At 0.1 and 0.3 mg kg-1 p.o., TAK-137 significantly ameliorated MK-801-induced deficits in the social interaction of rats, demonstrating potential improvement of impaired social functioning, which is a negative symptom of schizophrenia. The effects of TAK-137 were evaluated on multiple cognitive domains-attention, working memory, and cognitive flexibility. TAK-137 enhanced attention in the five-choice serial reaction time task in rats at 0.2 mg kg-1 p.o., and improved working memory both in rats and monkeys: 0.2 and 0.6 mg kg-1 p.o. ameliorated MK-801-induced deficits in the radial arm maze test in rats, and 0.1 mg kg-1 p.o. improved the performance of ketamine-treated monkeys in the delayed matching-to-sample task. At 0.1 and 1 mg kg-1 p.o., TAK-137 improved the cognitive flexibility of subchronic phencyclidine-treated rats in the reversal learning test. Thus, TAK-137-type AMPA receptor potentiators with low intrinsic activity may offer new therapies for schizophrenia.

TAK-137, an AMPA-R potentiator with little agonistic effect, has a wide therapeutic window

Activation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPA-R) is a promising strategy to treat psychiatric and neurological diseases if issues of bell-shaped response and narrow safety margin against seizure can be overcome. Here, we show that structural interference at Ser743 in AMPA-R is a key to lower the agonistic effect of AMPA-R potentiators containing dihydropyridothiadiazine 2,2-dioxides skeleton. With this structural insight, TAK-137, 9-(4-phenoxyphenyl)-3,4-dihydropyrido[2,1-c][1,2,4]thiadiazine 2,2-dioxide, was discovered as a novel AMPA-R potentiator with a lower agonistic effect than an AMPA-R potentiator LY451646 ((R)-N-(2-(4'-cyanobiphenyl-4-yl)propyl)propane-2-sulfonamide) in rat primary neurons. TAK-137 induced brain-derived neurotrophic factor in neurons in rodents and potently improved cognition in both rats and monkeys. Compared to LY451646, TAK-137 had a wider safety margin against seizure in rats. TAK-137 enhanced neural progenitor proliferation over a broader range of doses in rodents. Thus, TAK-137 is a promising AMPA-R potentiator with potent procognitive effects and lower risks of bell-shaped response and seizure. These data may open the door for the development of AMPA-R potentiators as therapeutic drugs for psychiatric and neurological diseases.