Neuroprotective derivatives of tacrine that target NMDA receptor and acetyl cholinesterase - Design, synthesis and biological evaluation

Novel tacrine-based multi-target directed Ligands: Enhancing cholinesterase inhibition, NMDA receptor antagonism, and CNS bioavailability for Alzheimer's disease treatment

Alzheimer's disease (AD) is a multifaceted neurodegenerative disorder for which current treatments provide only symptomatic relief, primarily through cholinesterase (ChE) inhibition and N-methyl-d-aspartate receptor (NMDAR) antagonism. To improve therapeutic efficacy and safety, we designed and synthesized 16 novel tacrine derivatives modified at position 7 with various (hetero)aryl groups or deuterium substitution. Initially, in silico screening predicted favorable CNS permeability and oral bioavailability. Subsequent in vitro evaluations demonstrated significant inhibitory potency against acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), with derivatives 5i and 5m displaying particularly promising profiles. Metabolic stability assessed using human liver microsomes revealed enhanced stability for compound 5e, whereas 5i and 5m underwent rapid metabolism. Notably, compound 7 showed improved metabolic stability attributed to deuterium incorporation. The newly synthesized compounds were further tested for antagonistic activity on the GluN1/GluN2B subtype of NMDAR, with compound 5m exhibiting the most potent and voltage-independent inhibition. The ability of these compounds to permeate the blood-brain barrier (BBB) was confirmed through in vitro PAMPA assays. In preliminary hepatotoxicity screening (HepG2 cells), most derivatives exhibited higher cytotoxicity than tacrine, emphasizing the ongoing challenge in hepatotoxicity management. Based on its overall favorable profile, compound 5m advanced to in vivo pharmacokinetic studies in mice, demonstrating efficient CNS penetration, with brain concentrations exceeding plasma levels (brain-to-plasma ratio 2.36), indicating active transport across the BBB. These findings highlight compound 5m as a promising tacrine-based multi-target-directed ligand, supporting further preclinical development as a potential therapeutic candidate for AD.

Significance of NMDA receptor-targeting compounds in neuropsychological disorders: An in-depth review

N-methyl-D-aspartate receptors (NMDARs), a subclass of glutamate-gated ion channels, play an integral role in the maintenance of synaptic plasticity and excitation-inhibition balance within the central nervous system (CNS). Any irregularities in NMDAR functions, whether hypo-activation or over-activation, can destabilize neural networks and impair CNS function. Several decades of experimental and clinical investigations have demonstrated that NMDAR dysfunction is implicated in the pathophysiology of various neurological disorders. Despite designing a long list of compounds that differentially modulate NMDARs, success in developing drugs that can selectively and effectively regulate various NMDAR subtypes while showing encouraging efficacy in clinical settings remains limited. A better understanding of the basic mechanism of NMDAR function, particularly its selective regulation in pathological conditions, could aid in designing effective drugs for the treatment of neurological conditions. Here, we reviewed the experimental and clinical investigations that studied the effects of available NMDAR modulators in various neurological disorders and weighed up the pros and cons of the use of these substances on the improvement of functional outcomes of these disorders. Despite numerous efforts to develop NMDAR modulatory drugs that did not produce the desired outcomes, NMDARs remain a significant target for advancing novel drugs to treat neurological disorders. This article reviews the complexity of NMDAR signaling dysfunction in different neurological diseases, the efforts taken to examine designed compounds targeting specific subtypes of NMDARs, including challenges associated with using these substances, and the potential enhancements in drug discovery for NMDAR modulatory compounds by innovative technologies.

Intracortical Injection of Okadaic Acid Increases Locomotor Activity and Decreases Anxiety-like Behaviour in Adult Male Rats: Potential Involvement of NMDA Receptor

Background

Neuronal transmission through the N-methyl-d-aspartate receptor (NMDAR) is a key event in synaptic plasticity and excitotoxicity. The channel properties and biochemical signalling activities of the receptor are regulated by phosphatases such as protein phosphatase 1. While the immediate consequences of NMDAR activation have been reported previously, the long-term behavioural changes remain unclear.

Purpose

We attempted to investigate the long-term behavioural effects of N-methyl-d-aspartic acid (NMDA) injection and the role of phosphatases during NMDAR signalling.

Methods

NMDAR was activated by stereotaxic injection of NMDA into the prefrontal cortex of adult rats. To elucidate the role of phosphatases in mediating NMDAR signalling and associated animal behaviour, okadaic acid (OA), a phosphatase inhibitor, was administered before NMDA injection. The animals were tested for their general locomotion and cognitive function using behavioural assays.

Results

A single injection of NMDA impaired cognition in the long term. Interestingly, intracortical OA injection resulted in increased locomotor activity and decreased anxiety-like behaviour in animals without major cognitive effects.

Conclusion

We demonstrate that the inhibition of phosphatases during NMDAR signalling can affect locomotion and anxiety-like behaviour in adult male rats. Our study underscores the potential of modulating phosphatases as a pharmacological target for anxiety disorders.

Novel multipotent conjugate bearing tacrine and donepezil motifs with dual cholinergic inhibition and neuroprotective properties targeting Alzheimer's disease

In this work, we developed potential multifunctional agents to combat Alzheimer's disease. According to our strategy, fragments of tacrine and donepezil were merged in a unique hybrid structure. After successfully synthesizing the compounds, they were evaluated for their dual AChE/BuChE inhibitor potential and neuroprotector response using a glutamate-induced excitotoxicity model. Most of the compounds showed promising activity. Among them, the hybrid with 2,5-dimetoxysubstitution (3b) was the most potent analogue, triggering dual potent AChE/BuChE inhibition with low nanomolar affinity (IC50 ∼ 300 nM) and low toxicity to human liver cancer cells (HepG2). This analogue prevented the glutamate excitotoxic stimulus during pre/post treatment testing, maintained ATP levels, possessed an astrocytic protective response, and abolished the glutamate-induced excitotoxicity. Besides, the hit compound 3b exhibited suitable permeability in the blood-brain barrier (BBB) and low degradability in human blood-plasma. In addition, the docking studies suggested that the neuroprotectant response exhibited by 3b can be related to the direct blockage of the NMDA channel pore. Finally, an ideal neuropharmacokinetic profile was estimated for 3b. Overall, the designed conjugates provide a novel multifunctional molecular scaffold that can be used as a prototype drug in further investigations toward novel multipotent therapeutics for treating AD.

Design, synthesis, molecular modeling, in vitro evaluation of novel piperidine-containing hydrazone derivatives as cholinesterase inhibitors

In an effort to develop new and effective therapeutic agents for Alzheimer's disease, a series of hydrazone derivatives bearing piperidine rings have been designed and synthesized. The chemical structures of the compounds were characterized by various spectroscopic techniques. In vitro antioxidant and cholinesterase activities of the compounds were evaluated. Among the compounds, N12 exhibited the most antioxidant activity in all methods (CUPRAC, FRAP, DPPH, ABTS). In vitro acetylcholinesterase (AChE) activity results of the compounds showed good IC50 values between 14.124 ± 0.084 and 49.680 ± 0.110 µM were obtained (IC50 = 38.842 ± 0.053 µM for Donepezil). Among the compounds, N7 and N6 are much more effective derivatives than the standard compound donepezil with IC50 values of 14.124 ± 0.084 and 17.968 ± 0.072 µM, respectively. In vitro, butyrylcholinesterase (BChE) inhibition values of the compounds were between 13.505 ± 0.025 and 52.230 ± 0.027 μm. Among the compounds, N6 has the highest BChE inhibition with an IC50 value of 13.505 μm in the series. The cytotoxicity and AChE inhibitory activity of the compounds on SH-SY5Y cell lines were also evaluated. Kinetic studies were also performed to determine the behavior of the compounds as competitive or noncompetitive inhibitors. The binding modes of N6, which was determined to be highly effective according to in vitro analyses, with AChE and BChE were investigated using molecular docking studies, and the stability of the complexes was determined by molecular dynamics simulations. These findings indicated that AChE and BChE enzymes maintained their overall structural stability and compactness during interactions with compound N6.

Phenoxytacrine derivatives: Low-toxicity neuroprotectants exerting affinity to ifenprodil-binding site and cholinesterase inhibition

Tacrine (THA), a long withdrawn drug, is still a popular scaffold used in medicinal chemistry, mainly for its good reactivity and multi-targeted effect. However, THA-associated hepatotoxicity is still an issue and must be considered in drug discovery based on the THA scaffold. Following our previously identified hit compound 7-phenoxytacrine (7-PhO-THA), we systematically explored the chemical space with 30 novel derivatives, with a focus on low hepatotoxicity, anticholinesterase action, and antagonism at the GluN1/GluN2B subtype of the NMDA receptor. Applying the down-selection process based on in vitro and in vivo pharmacokinetic data, two candidates, I-52 and II-52, selective GluN1/GluN2B inhibitors thanks to the interaction with the ifenprodil-binding site, have entered in vivo pharmacodynamic studies. Finally, compound I-52, showing only minor affinity to AChE, was identified as a lead candidate with favorable behavioral and neuroprotective effects using open-field and prepulse inhibition tests, along with scopolamine-based behavioral and NMDA-induced hippocampal lesion models. Our data show that compound I-52 exhibits low toxicity often associated with NMDA receptor ligands, and low hepatotoxicity, often related to THA-based compounds.

Lobeline: A multifunctional alkaloid modulates cholinergic and glutamatergic activities

Developing drugs for Alzheimer's disease (AD) is an extremely challenging task due to its devastating pathology. Previous studies have indicated that natural compounds play a crucial role as lead molecules in the development of drugs. Even though, there are remarkable technological advancements in the isolation and synthesis of natural compounds, the targets for many of them are still unknown. In the present study, lobeline, a piperidine alkaloid has been identified as a cholinesterase inhibitor through chemical similarity assisted target fishing method. The structural similarities between lobeline and donepezil, a known acetylcholinesterase (AChE) inhibitor encouraged us to hypothesize that lobeline may also exhibit AChE inhibitory properties. It was further confirmed by in silico, in vitro and biophysical studies that lobeline could inhibit cholinesterase. The binding profiles indicated that lobeline has a higher affinity for AChE than BChE. Since excitotoxicity is one of the major pathological events associated with AD progression, we also investigated the neuroprotective potential of lobeline against glutamate mediated excitotoxicity in rat primary cortical neurons. The cell based NMDA receptor (NMDAR) assay with lobeline suggested that neuroprotective potential of lobeline is mediated through the blockade of NMDAR activity.

Structural, harmonic force field and vibrational studies of cholinesterase inhibitor tacrine used for treatment of Alzheimer's disease

Different structures of free base (FB), two cationic forms (CA) and three hydrochloride forms (HCl) of cholinesterase inhibitor tacrine used for treatment of Alzheimer 's disease was evaluated using hybrid B3LYP calculations in order to perform their complete vibrational assignments using the scaled harmonic force fields. Structures of anhydrous form of tacrine have been optimized in gas phase and in aqueous solution. The structure of form III HCl is in agreement with the experimental determined by X-ray diffraction while the predicted IR, Raman, ¹H- ¹³C NMR and UV spectra show good correlations with the corresponding experimental ones. Energy values show that the three forms of HCl can exist in both media because these energetic values decrease from 35.15 kJ/mol in gas phase to 5.51 kJ/mol in solution. For the most stable species of tacrine, the following stability order using natural bond orbital (NBO) studies was found: form I HCl > form III HCl > form I CA > FB. CA presents the higher solvation energy value, as reported for hydrochloride species of alkaloids and antihypertensive agents. The structural parameters of form III of HCl present better concordance and corresponds to that experimental observed in the solid phase. Higher topological properties of form III together with the strong N2-H26⋯Cl31 interaction could justify the presence of this form in the solid phase and in solution and the higher stabilities in both media. The gap values support the higher reactivity of form III while FB is the less reactive species in both media. Complete vibrational assignments for FB, CA and HCl species together with the corresponding scaled force constants are reported.

Mechanistic Study of Xanthotoxin-Mediated Inactivation of CYP1A2 and Related Drug-Drug Interaction with Tacrine

Xanthotoxin (XTT) is a biologically active furanocoumarin widely present in foods and plants. The present study is designed to systematically investigate the enzymatic interaction of XTT with CYP1A2, along with pharmacokinetic alteration of tacrine resulting from the co-administration of XTT. The results showed that XTT induced a time-, concentration-, and NADPH-dependent inhibition of CYP1A2, and the inhibition was irreversible. Co-incubation of glutathione (GSH) and catalase/superoxide dismutase was unable to prevent enzyme inactivation. Nevertheless, competitive inhibitor fluvoxamine exhibited a concentration-dependent protective effect against the XTT-induced CYP1A2 inactivation. A GSH trapping experiment provided strong evidence for the production of epoxide or/and γ-ketoenal intermediates resulting from the metabolic activation of XTT. Furthermore, pretreatment of rats with XTT was found to significantly increase the C_max and area under the curve of plasma tacrine relative to those of tacrine administration alone.

Sub-pocket-focused designing of tacrine derivatives as potential acetylcholinesterase inhibitors

Human acetylcholinesterase (hAChE) has a potential role in the management of acetylcholine, one of the neurotransmitters that modulate the overall activity of cholinergic system, AChE inhibitors have a greater impact in the therapeutics. Though the atomic structure of hAChE has been extensively studied, the precise active site geometry upon binding to different ligands are yet to be explored. In the present study, an extensive structural analysis of our recently reported hAChE-tacrine complex has carried out and revealed the presence of two prominent sub-pockets located at the vicinity of the hAChE active site. Structural bioinformatics assisted studies designed 132 putative sub-pockets focused tacrine derivatives (SPFTDs), their molecular docking, free energy estimations revealed that they are stronger than tacrine in terms of binding affinity. Our in vitro studies also supported the in silico findings, all these SPFTDs are having better potencies than tacrine. Cytotoxic nature of these SPFTDs on HepG2 and Neuro-2a cell lines, diminishes the possibilities for future in vivo studies. However, the identification of these sub pockets and the SPFTDs paved a new way to the future drug discovery especially since AChE is one of the promising and approved drug targets in treatment of AD drug discovery.

Aging and aging-related diseases: from molecular mechanisms to interventions and treatments

Aging is a gradual and irreversible pathophysiological process. It presents with declines in tissue and cell functions and significant increases in the risks of various aging-related diseases, including neurodegenerative diseases, cardiovascular diseases, metabolic diseases, musculoskeletal diseases, and immune system diseases. Although the development of modern medicine has promoted human health and greatly extended life expectancy, with the aging of society, a variety of chronic diseases have gradually become the most important causes of disability and death in elderly individuals. Current research on aging focuses on elucidating how various endogenous and exogenous stresses (such as genomic instability, telomere dysfunction, epigenetic alterations, loss of proteostasis, compromise of autophagy, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, altered intercellular communication, deregulated nutrient sensing) participate in the regulation of aging. Furthermore, thorough research on the pathogenesis of aging to identify interventions that promote health and longevity (such as caloric restriction, microbiota transplantation, and nutritional intervention) and clinical treatment methods for aging-related diseases (depletion of senescent cells, stem cell therapy, antioxidative and anti-inflammatory treatments, and hormone replacement therapy) could decrease the incidence and development of aging-related diseases and in turn promote healthy aging and longevity.

Novel Chromone-Containing Allylmorpholines Induce Anxiolytic-like and Sedative Effects in Adult Zebrafish

Chromone-containing allylmorpholines (CCAMs) are a novel class of compounds that have demonstrated acetyl- and butyryl-cholinesterase-inhibiting and N-methyl-D-aspartate (NMDA) receptor-blocking properties in vitro, but their in vivo pharmacological activity remains underexplored. In this work, we evaluated the psychotropic activity of five different CCAMs (1 (9a), 2 (9j), 3 (9l), 4 (33a), and 5 (33b)) using the novel tank test (NTT) and light/dark box (LDB) test in adult zebrafish. The CCAMs were screened in the NTT at a range of concentrations, and they were found to induce a dose-dependent sedative effect. Compound 4 (33a) was also evaluated using the LDB test, and it was found to have anxiolytic-like properties at low concentrations. To assess the potential contribution of the glutamate and cholinergic mechanisms in the effects of the CCAMs, we conducted experiments with pre-exposure to putative antagonists, NMDA and biperiden. Neither biperiden nor NMDA were able to diminish or cancel the effects of the CCAMs, countering the in vitro data obtained in previous studies. The apparent discrepancy could be related to the specifics of CCAM metabolism or to the interspecies differences between the putative target proteins, possibly due to the relatively low identity percentage of their sequences. Although further research in mammals is required in order to establish their pharmacological properties, novel CCAMs may represent an appealing group of psychoactive drug candidates.

Tacrine Derivatives in Neurological Disorders: Focus on Molecular Mechanisms and Neurotherapeutic Potential

Tacrine is a drug used in the treatment of Alzheimer's disease as a cognitive enhancer and inhibitor of the enzyme acetylcholinesterase (AChE). However, its clinical application has been restricted due to its poor therapeutic efficacy and high prevalence of detrimental effects. An attempt was made to understand the molecular mechanisms that underlie tacrine and its analogues influence over neurotherapeutic activity by focusing on modulation of neurogenesis, neuroinflammation, endoplasmic reticulum stress, apoptosis, and regulatory role in gene and protein expression, energy metabolism, Ca2+ homeostasis modulation, and osmotic regulation. Regardless of this, analogues of tacrine are considered as a model inhibitor of cholinesterase in the therapy of Alzheimer's disease. The variety both in structural make-up and biological functions of these substances is the main appeal for researchers' interest in them. A new paradigm for treating neurological diseases is presented in this review, which includes treatment strategies for Alzheimer's disease, as well as other neurological disorders like Parkinson's disease and the synthesis and biological properties of newly identified versatile tacrine analogues and hybrids. We have also shown that these analogues may have therapeutic promise in the treatment of neurological diseases in a variety of experimental systems.

Role of Ca2+/Calmodulin-Dependent Protein Kinase Type II in Mediating Function and Dysfunction at Glutamatergic Synapses

Glutamatergic synapses harbor abundant amounts of the multifunctional Ca²⁺/calmodulin-dependent protein kinase type II (CaMKII). Both in the postsynaptic density as well as in the cytosolic compartment of postsynaptic terminals, CaMKII plays major roles. In addition to its Ca²⁺-stimulated kinase activity, it can also bind to a variety of membrane proteins at the synapse and thus exert spatially restricted activity. The abundance of CaMKII in glutamatergic synapse is akin to scaffolding proteins although its prominent function still appears to be that of a kinase. The multimeric structure of CaMKII also confers several functional capabilities on the enzyme. The versatility of the enzyme has prompted hypotheses proposing several roles for the enzyme such as Ca²⁺ signal transduction, memory molecule function and scaffolding. The article will review the multiple roles played by CaMKII in glutamatergic synapses and how they are affected in disease conditions.

Towards novel tacrine analogues: Pd(dppf)Cl2·CH2Cl2 catalyzed improved synthesis, in silico docking and hepatotoxicity studies

A plethora of 6-(hetero)aryl C–C and C–N bonded tacrine analogues has been made accessible by employing palladium mediated (Suzuki–Miyaura, Heck, Sonogashira, Stille and Buchwald) cross-coupling reactions, starting from either halogenated or borylated residues. The successful use of Pd(dppf)Cl₂·CH₂Cl₂ as a common catalytic system in realizing all these otherwise challenging transformations is the highlight of our optimized protocols. The analogues thus synthesized allow the available chemical space around the C-6 of this biologically relevant tacrine core to be explored. The in silico docking studies of the synthesized compounds were carried out against the acetylcholinesterase (AChE) enzyme. The hepatotoxicity studies of these compounds were done against complexes of CYP1A2 and CYP3A4 proteins with known inhibitors like 7,8-benzoflavone and ketoconazole, respectively.

24 synthesized compounds by various cross-coupling reactions on 6-bromo tacrine. Molecular docking and toxicity prediction studies were also performed.