Hybrids: a new paradigm to treat Alzheimer’s disease

Synthesis and pharmacological evaluation of novel N-aryl-cinnamoyl-hydrazone hybrids designed as neuroprotective agents for the treatment of Parkinson's disease

Molecular hybridization between structural fragments from the structures of curcumin (1) and resveratrol (2) was used as a designing tool to generate a new N-acyl-cinnamoyl-hydrazone hybrid molecular architecture. Twenty-eight new compounds were synthesized and evaluated for multifunctional activities related to Parkinson's disease (PD), including neuroprotection, antioxidant, metal chelating ability, and Keap1/Nrf2 pathway activation. Compounds 3b (PQM-161) and 3e (PQM-164) were highlighted for their significant antioxidant profile, acting directly as induced free radical stabilizers by DPPH and indirectly by modulating intracellular inhibition of t-BOOH-induced ROS formation in neuronal cells. The mechanism of action was determined as a result of Keap1/Nrf2 pathway activation by both compounds and confirmed by different experiments. Furthermore, compound 3e (PQM-164) exhibited a significant effect on the accumulation of α-synuclein and anti-inflammatory activity, leading to an expressive decrease in gene expression of iNOS, IL-1β, and TNF-α. Overall, these results highlighted compound 3e as a promising and innovative multifunctional drug prototype candidate for PD treatment.

Imidazole-thiadiazole hybrids: A multitarget de novo drug design approach, in vitro evaluation, ADME/T, and in silico studies

A library of imidazole-thiadiazole compounds (1-24) was synthesized to explore their therapeutic applications. The compounds were subjected to meticulous in vitro evaluation against α-glucosidase, α-amylase, acetylcholinesterase (AChE), and butylcholinesterase (BChE) enzymes. Compounds were also investigated for antioxidant activities using cupric reducing antioxidant capacity (CUPRAC), ferric reducing antioxidant power (FRAP), and 1,1-diphenyl-2-picrylhydrazyl (DPPH) assays. Derivatives 5-7, 9-11, 18, and 19 displayed potent inhibitory activities with IC50 values of 1.4 ± 0.01 to 13.6 ± 0.01 and 0.9 ± 0.01 to 12.8 ± 0.02 µM against α-glucosidase, and α-amylase enzymes, respectively, compared to the standard acarbose (IC50 = 14.8 ± 0.01 µM). Compounds 11-13, 16, 20, and 21 exhibited potent activity IC50 = 8.6 ± 0.02 to 34.7 ± 0.03 µM against AChE enzyme, compared to donepezil chloride (IC50 = 39.2 ± 0.05 µM). Compound 21 demonstrated comparable inhibition IC50 = 45.1 ± 0.09 µM against BChE, compared to donepezil chloride (IC50 = 44.2 ± 0.05 µM). All compounds also demonstrated excellent antioxidant activities via CUPRAC, FRAP, and DPPH methods. Complementing the experimental studies, extensive kinetics, ADME/T, and molecular docking analysis were also conducted to unravel the pharmacokinetics and safety profiles of the designed compounds. These studies supported the experimental findings and facilitated the prioritization of hit candidates for subsequent stages of drug development.

Metal-free synthesis of functionalized tacrine derivatives and their evaluation for acetyl/butyrylcholinesterase and α-glucosidase inhibition

A mild and greener protocol was developed for C-C (C(sp3)-H functionalization) and C-N bond formation to synthesize functionalized tacrine derivatives using a biodegradable and reusable deep eutectic solvent [(DES) formed from N,N'-dimethyl urea and L-(+)-tartaric acid in a 3 : 1 ratio at 80 °C]. The condensation of 9-chloro-1,2,3,4-tetrahydroacridines with a variety of aromatic aldehydes gave unsaturated compounds via C(sp3)-H functionalization (at the C-4 position) with good yields. The substituted N-aryl tacrine derivatives were obtained from the condensed products of 9-chloro-1,2,3,4-tetrahydroacridine with substituted anilines via the nucleophilic substitution reaction (SN2 type) in the DES with good yields. This is the first example of C4-functionalized tacrine derivatives, highlighting the dual capacity of the DES to serve as both a catalyst and a solvent for facilitating C-N bond formation on acridine. The generated compounds were evaluated for acetyl/butyrylcholinesterase (AChE/BChE) and α-glucosidase inhibitory activity. It was found that the majority of the compounds reported here were significantly more potent inhibitors than the standard inhibitor tacrine (AChE IC50 = 203.51 nM; BChE IC50 = 204.01 nM). Among the compounds screened, 8m was found to be more potent with IC50 = 125.06 nM and 119.68 nM towards AChE and BChE inhibition respectively. The α-glucosidase inhibitory activity of the compounds was tested using acarbose as a standard drug (IC50 = 23 100 nM) and compound 8j was found to be active with IC50 = 19 400 nM.

Theophylline-based hybrids as acetylcholinesterase inhibitors endowed with anti-inflammatory activity: synthesis, bioevaluation, in silico and preliminary kinetic studies

In this study, we investigated the conjugation of theophylline with different compounds of natural origin hoping to construct new hybrids with dual activity against cholinergic and inflammatory pathways as potential agents for the treatment of Alzheimer's disease (AD). Out of 28 tested hybrids, two hybrids, acefylline-eugenol 6d and acefylline-isatin 19, were able to inhibit acetylcholinesterase (AChE) at low micromolar concentration displaying IC50 values of 1.8 and 3.3 μM, respectively, when compared to the galantamine standard AChE inhibitor. Moreover, the prepared hybrids exhibited a significant anti-inflammatory effect against lipopolysaccharide induced inflammation in RAW 264.7 and reduced nitric oxide (NO), tumor necrosis alpha (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6) levels in a dose dependent manner. These hybrids demonstrated significant reductions in nitric oxide (NO), tumor necrosis alpha (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6) levels in RAW 264.7 cells induced by lipopolysaccharide (LPS). The findings of this study were further explained in light of network pharmacology analysis which suggested that AChE and nitric oxide synthase were the main targets of the most active compounds. Molecular docking studies revealed their ability to bind to the heme binding site of nitric oxide synthase 3 (NOS-3) and effectively occupy the active site of AChE, interacting with both the peripheral aromatic subsite and catalytic triad. Finally, the compounds demonstrated stability in simulated gastric and intestinal environments, suggesting potential absorption into the bloodstream without significant hydrolysis. These findings highlight the possible therapeutic potential of acefylline-eugenol 6d and acefylline-isatin 19 hybrids in targeting multiple pathological mechanisms involved in AD, offering promising avenues for further development as potential treatments for this devastating disease.

Thiadiazole – A promising structure in design and development of anti-Alzheimer agents

The design and development of effective multitargeted agents in treating Alzheimer disease (AD) has always been a hot topic in the field of drug discovery. Since AD is a multifactorial disorder, various key hidden players such as deficit of acetylcholine (ACh), tau-protein aggregation, and oxidative stress have been associated with the incidence and progress of AD. In pursuit of improving efficacy and expanding the range of pharmacological activities of current AD drugs, the molecular hybridization method is also used intensively. Five-membered heterocyclic systems such as thiadiazole scaffolds have previously been shown to have therapeutic activity. Thiadiazole analogs as an anti-oxidant compound have been known to include a wide range of biological activity from anti-cancer to anti-Alzheimer properties. The suitable pharmacokinetic and physicochemical properties of the thiadiazole scaffold have introduced it as a therapeutic target in medicinal chemistry. The current review portrays the critical role of the thiadiazole scaffold in the design of various compounds with potential effects in the treatment of Alzheimer's disease. Furthermore, the rationale used behind hybrid-based design strategies and the outcomes achieved through the hybridization of Thiadiazole analogs with various core structures have been discussed. In addition, the data in the present review may help researchers in the design of new multidrug combinations that may provide new options for the treatment of AD.

An ongoing journey of chalcone analogues as single and multi-target ligands in the field of Alzheimer's disease: A review with structural aspects

Alzheimer's disease (AD) is a chronic and irreversible neurodegenerative disorder with progressive dementia and cognitive impairment. AD poses severe health challenge in elderly people and become one of the leading causes of death worldwide. It possesses complex pathophysiology with several hypotheses (cholinergic hypothesis, amyloid hypothesis, tau hypothesis, oxidative stress, mitochondrial dysfunction etc.). Several attempts have been made for the management of multifactorial AD. Acetylcholinesterase is the only target has been widely explored in the management of AD to the date. The current review set forth the chalcone based natural, semi-synthetic and synthetic compounds in the search of potential anti-Alzheimer's agents. The main highlights of current review emphasizes on chalcone target different enzymes and pathways like Acetylcholinesterase, β-secretase (BACE1), tau proteins, MAO, free radicals, Advanced glycation end Products (AGEs) etc. and their structure activity relationships contributing in the inhibition of above mentioned various targets of AD.

Photobiomodulation in Alzheimer's Disease-A Complementary Method to State-of-the-Art Pharmaceutical Formulations and Nanomedicine?

Alzheimer's disease (AD), as a neurodegenerative disorder, usually develops slowly but gradually worsens. It accounts for approximately 70% of dementia cases worldwide, and is recognized by WHO as a public health priority. Being a multifactorial disease, the origins of AD are not satisfactorily understood. Despite huge medical expenditures and attempts to discover new pharmaceuticals or nanomedicines in recent years, there is no cure for AD and not many successful treatments are available. The current review supports introspection on the latest scientific results from the specialized literature regarding the molecular and cellular mechanisms of brain photobiomodulation, as a complementary method with implications in AD. State-of-the-art pharmaceutical formulations, development of new nanoscale materials, bionanoformulations in current applications and perspectives in AD are highlighted. Another goal of this review was to discover and to speed transition to completely new paradigms for the multi-target management of AD, to facilitate brain remodeling through new therapeutic models and high-tech medical applications with light or lasers in the integrative nanomedicine of the future. In conclusion, new insights from this interdisciplinary approach, including the latest results from photobiomodulation (PBM) applied in human clinical trials, combined with the latest nanoscale drug delivery systems to easily overcome protective brain barriers, could open new avenues to rejuvenate our central nervous system, the most fascinating and complex organ. Picosecond transcranial laser stimulation could be successfully used to cross the blood-brain barrier together with the latest nanotechnologies, nanomedicines and drug delivery systems in AD therapy. Original, smart and targeted multifunctional solutions and new nanodrugs may soon be developed to treat AD.

N-Substituted piperidine-3-carbohydrazide-hydrazones against Alzheimer's disease: Synthesis and evaluation of cholinesterase, beta-amyloid inhibitory activity, and antioxidant capacity

A series of piperidine-3-carbohydrazide-hydrazones bearing phenylethyl, phenylpropyl, and phenylbutyl substituents on piperidine nitrogen were designed and synthesized as cholinesterase (ChE) inhibitors. The title compounds were screened for acetylcholinesterase (AChE), butyrylcholinesterase (BuChE) inhibitory activities and antioxidant capacities, and the active ones for Aβ₄₂ self-aggregation inhibition, in vitro. The chemiluminescence method was used to determine the effect of the selected compounds on the reactive oxygen species (ROS) levels in brain tissue. Physicochemical properties were calculated by the MOE program. Kinetic analysis and molecular modeling studies were also carried out for the most active compounds. Generally, the final compounds exhibited moderate to good AChE or BuChE inhibitory activity. Among them, 3g and 3j showed the most potent activity against AChE (IC₅₀  = 4.32 µM) and BuChE (IC₅₀  = 1.27 µM), respectively. The kinetic results showed that both compounds exhibited mixed-type inhibition. Among the selected compounds, nitro derivatives (3g, 4g, and 5g) provided better Aβ₄₂ inhibition. According to the chemiluminescence assay, 4i exhibited the most active superoxide free-radical scavenger activity and 3g, 3j, and 4i showed similar scavenger activity on other ROS. All results suggested that 3g, 3j, and 4i have good AChE/BuChE, Aβ₄₂ inhibitory potentials and antioxidant capacities and can therefore be suggested as promising multifunctional agents to combat Alzheimer's disease.

Conjugates of Tacrine and Salicylic Acid Derivatives as New Promising Multitarget Agents for Alzheimer's Disease

A series of previously synthesized conjugates of tacrine and salicylamide was extended by varying the structure of the salicylamide fragment and using salicylic aldehyde to synthesize salicylimine derivatives. The hybrids exhibited broad-spectrum biological activity. All new conjugates were potent inhibitors of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) with selectivity toward BChE. The structure of the salicylamide moiety exerted little effect on anticholinesterase activity, but AChE inhibition increased with spacer elongation. The most active conjugates were salicylimine derivatives: IC₅₀ values of the lead compound 10c were 0.0826 µM (AChE) and 0.0156 µM (BChE), with weak inhibition of the off-target carboxylesterase. The hybrids were mixed-type reversible inhibitors of both cholinesterases and displayed dual binding to the catalytic and peripheral anionic sites of AChE in molecular docking, which, along with experimental results on propidium iodide displacement, suggested their potential to block AChE-induced β-amyloid aggregation. All conjugates inhibited Aβ₄₂ self-aggregation in the thioflavin test, and inhibition increased with spacer elongation. Salicylimine 10c and salicylamide 5c with (CH₂)₈ spacers were the lead compounds for inhibiting Aβ₄₂ self-aggregation, which was corroborated by molecular docking to Aβ₄₂. ABTS^•+-scavenging activity was highest for salicylamides 5a-c, intermediate for salicylimines 10a-c, low for F-containing salicylamides 7, and non-existent for methoxybenzoylamides 6 and difluoromethoxybenzoylamides 8. In the FRAP antioxidant (AO) assay, the test compounds displayed little or no activity. Quantum chemical analysis and molecular dynamics (MD) simulations with QM/MM potentials explained the AO structure-activity relationships. All conjugates were effective chelators of Cu²⁺, Fe²⁺, and Zn²⁺, with molar compound/metal (Cu²⁺) ratios of 2:1 (5b) and ~1:1 (10b). Conjugates exerted comparable or lower cytotoxicity than tacrine on mouse hepatocytes and had favorable predicted intestinal absorption and blood-brain barrier permeability. The overall results indicate that the synthesized conjugates are promising new multifunctional agents for the potential treatment of AD.

Molecular engineering and activity improvement of acetylcholinesterase inhibitors: Insights from 3D-QSAR, docking, and molecular dynamics simulation studies

The carbamate molecule rivastigmine was found to possess promising anti-acetylcholinesterase activity, enabling to target and occupy choline binding sites, and as a result, widely used to improve the treatment of Alzheimer's disease (AD). Higher dose of rivastigmine indicates rapid onset but more adverse effects, such as the large fluctuations in plasma concentration level and frequent incidence of gastrointestinal side effect. To solve the dilemma, we developed a three-dimensional quantitative structure-activity relationship (3D-QSAR), docking and molecular dynamics (MD) simulation strategy to construct a dismountable nanoplatform of inhibitor engineering, verification and application for improving the inhibitory activity per unit concentration. With the aid of 3D-QSAR method, we constructed a model by using 25 molecules reported, and verified well the rationality of these QSAR models by non-cross validation coefficient (r² = 0.902). Docking and MD results show that rivastigmine, as a control, does target exactly the binding sites of acetylcholinesterase, those already observed experimentally, in turn, confirming the reliability of the present 3D-QSAR results. The method suggests that groups with electron-donating chemical property can improve the inhibitory activity, and screens out two novel inhibitors L-1 and L-2 with more activity from database (about 8000 compounds). Moreover, L-1 and L-2 not only target exactly the same binding sites of acetylcholinesterase as the rivastigmine does, but also hold stronger binding energy, showing a more powerful inhibitory ability. More broadly, this work showcases an approach in the engineering of carbamate inhibitors to enhance their inhibitory activity using electron-donating groups, which simplifies the design process of complex bioactive molecules.

New Multifunctional Agents for Potential Alzheimer's Disease Treatment Based on Tacrine Conjugates with 2-Arylhydrazinylidene-1,3-Diketones

Alzheimer's disease (AD) is considered a modern epidemic because of its increasing prevalence worldwide and serious medico-social consequences, including the economic burden of treatment and patient care. The development of new effective therapeutic agents for AD is one of the most urgent and challenging tasks. To address this need, we used an aminoalkylene linker to combine the well-known anticholinesterase drug tacrine with antioxidant 2-tolylhydrazinylidene-1,3-diketones to create 3 groups of hybrid compounds as new multifunctional agents with the potential for AD treatment. Lead compounds of the new conjugates effectively inhibited acetylcholinesterase (AChE, IC50 0.24-0.34 µM) and butyrylcholinesterase (BChE, IC50 0.036-0.0745 µM), with weak inhibition of off-target carboxylesterase. Anti-AChE activity increased with elongation of the alkylene spacer, in agreement with molecular docking, which showed compounds binding to both the catalytic active site and peripheral anionic site (PAS) of AChE, consistent with mixed type reversible inhibition. PAS binding along with effective propidium displacement suggest the potential of the hybrids to block AChE-induced β-amyloid aggregation, a disease-modifying effect. All of the conjugates demonstrated metal chelating ability for Cu2+, Fe2+, and Zn2+, as well as high antiradical activity in the ABTS test. Non-fluorinated hybrid compounds 6 and 7 also showed Fe3+ reducing activity in the FRAP test. Predicted ADMET and physicochemical properties of conjugates indicated good CNS bioavailability and safety parameters acceptable for potential lead compounds at the early stages of anti-AD drug development.

Potential of flavonoids as anti-Alzheimer's agents: bench to bedside

Developing therapies for neurodegenerative diseases are challenging because of the presence of blood-brain barrier and Alzheimer being one of the commonest and uprising neurodegenerative disorders possess the need for developing novel therapies. Alzheimer's is attributed to be the sixth leading cause of death in the USA and the number of cases is estimated to be increased from 58 million in 2021 to 88 million by 2050. Natural drugs have benefits of being cost-effective, widely available, fewer side effects, and immuno-booster can be useful in managing Alzheimer. Flavonoids can slow the neuronal degeneration as they have shown activity in central nervous system and are able to cross the blood-brain barrier. These can be easily extracted from fruits, vegetable, and plants. In Alzheimer disease, flavonoids scavenges the reactive oxygen species and reduces the production of amyloid beta protein. Agents from sub-classes of flavonoids such as flavanones, flavanols, flavones, flavonols, anthocyanins, and isoflavones having pharmacological action in treating Alzheimer disease are discussed in this review.

Conjugates of Tacrine with Salicylamide as Promising Multitarget Agents for Alzheimer's Disease

New conjugates of tacrine and salicylamide with alkylene spacers were synthesized and evaluated as potential multifunctional agents for Alzheimer's disease (AD). The compounds exhibited high acetylcholinesterase (AChE, IC₅₀ to 0.224 μM) and butyrylcholinesterase (BChE, IC₅₀ to 0.0104 μM) inhibitory activities. They were also rather poor inhibitors of carboxylesterase, suggesting a low tendency to exert potential unwanted drug-drug interactions in clinical use. The conjugates were mixed-type reversible inhibitors of both cholinesterases and demonstrated dual binding to the catalytic and peripheral anionic sites of AChE in molecular docking that, along with experimental results on propidium iodide displacement, suggest their potential to block AChE-induced β-amyloid aggregation. The new conjugates exhibited high ABTS^.+ -scavenging activity. N-(6-(1,2,3,4-Tetrahydroacridin-9-ylamino)hexyl)salicylamide is a lead compound that also demonstrates metal chelating ability toward Cu²⁺ , Fe²⁺ and Zn²⁺ . Thus, the new conjugates have displayed the potential to be multifunctional anti-AD agents for further development.

Fluoxetine and sertraline based multitarget inhibitors of cholinesterases and monoamine oxidase-A/B for the treatment of Alzheimer's disease: Synthesis, pharmacology and molecular modeling studies

For the potential therapy of Alzheimer's disease (AD), cholinesterases (ChE) and monoamine oxidase (MAO) are key enzymes that regulate the level of acetylcholinesterase (AChE)/butyrylcholinesterase (BChE) and monoamines. The aim of current research is the synthesis of multi-target compounds that can concomitantly inhibit ChEs and MAO. A series of fluoxetine and sertraline hybrids was designed and evaluated as multi-target inhibitors of ChEs and hMAO. In-vitro enzyme inhibition studies demonstrated that a number of compounds displayed excellent inhibition in submicromolar to nanomolar range. However, compounds 17, 22, 38-40 possess excellent concomitant inhibitory activity against ChEs and hMAO-A/B enzymes and thus emerged as optimal multi-target hybrids. In-vivo acute toxicity study showed the safety of synthesized compounds up to 2000 mg/kg dose. The examinations of brain tissue in Swiss albino mice suggested that selected most active MAO-B inhibitors 17 and 22 have a propensity to block the MAO-B activity that could be responsible for their neurodegenerative effect in mice. The in-vitro inhibitory manner of interaction of these multipotent compounds on all four targets were confirmed by molecular docking investigations.

The hybrid compounds as multi-target ligands for the treatment of Alzheimer's Disease: Considerations on Donepezil

The strategies to combat Alzheimer's Disease (AD) have been changing with respect to the failures of many drug candidates assessed in clinical studies, the complex pathophysiology of AD, and the limitations of the current drugs employed. So far, none of the targets, either validated or nonvalidated, have been shown to be purely causative in the generation and development of AD. Considering the progressive and the neurodegenerative characteristics of the disease, the main strategy has been based on the design of molecules capable of showing activity on more than one receptor, and it is defined as multi-target ligand design strategy. The hybrid molecule concept is an outcome of this approach. Donepezil, as one of the currently employed drugs for AD therapy, has also been utilized in hybrid drug design studies. This review has aimed to present the promising donepezil-like hybrid molecules introduced in the recent period. Particularly, multi-target ligands with additional activities concomitant to cholinesterase inhibition are preferred.

Interest of novel N-alkylpyridinium-indolizine hybrids in the field of Alzheimer's disease: Synthesis, characterization and evaluation of antioxidant activity, cholinesterase inhibition, and amyloid fibrillation interference

A small library of molecules combining indolizine and N-alkyl pyridinium was synthesized and evaluated in a multi-target-directed-ligand strategy for Alzheimer's disease (AD) treatment. The new compounds were classified in three series depending on the number of methylene residues linking the two heterocycles (Ind-PyCx with x = 0, 2 or 3). The molecules were synthesized from the corresponding bis-pyridines by two-step formation of the indolizine core including mono-alkylation of pyridine and 1,3-dipolar cycloaddition with an alkylpropiolate. Their activities against AD's key-targets were evaluated in vitro: acetyl- and butyrylcholinesterase (AChE and BChE) inhibition, antioxidant properties and inhibition of amyloid fibril formation. None of the three series showed significant activities against all the targets. The Ind-PyC2 and Ind-PyC3 series are active on eeAChE and hAChE (µM IC₅₀ values). Most of the positively charged molecules from these two series also appeared active against eqBChE, however they lost their activity on hBChE. Comparative molecular modeling of 13 and 15 docked in hAChE and hBChE highlighted the importance of the substituent (p-methoxybenzoyl or methyloxycarbonyl, respectively) located on the indolizine C-3 for the binding. The larger molecule 13 fits more tightly at the active site of the two enzymes than 15 that shows a larger degree of freedom. The Ind-PyC2 and Ind-PyC3 hybrids displayed some antioxidant activity when tested at 750 µg/mL (up to 95% inhibition of DPPH radical scavenging for 10). In both series, most hybrids were also able to interact with amyloid fibers, even if the inhibitory effect was observed at a high 100 µM concentration. The Ind-PyC0 molecules stand out completely due to their spectroscopic properties which prevent their evaluation by Ellman's and ThT assays. However, these molecules showed interesting features in the presence of preformed fibers. In particular, the strong increase in fluorescence of 3 in the presence of amyloid fibers is very promising for its use as a fibrillation fluorescent reporter dye.

A tacrine-tetrahydroquinoline heterodimer potently inhibits acetylcholinesterase activity and enhances neurotransmission in mice

Cholinergic neurons are ubiquitous and involved in various higher brain functions including learning and memory. Patients with Alzheimer's disease exhibit significant dysfunction and loss of cholinergic neurons. Meanwhile, such cholinergic deficits can be potentially relieved pharmacologically by increasing acetylcholine. Acetylcholinesterase (AChE) inhibitors have been used to improve cholinergic transmission in the brain for two decades and have proven effective for alleviating symptoms in the early stages of Alzheimer's disease. Therefore, the search for AChE inhibitors for drug development is ongoing. The enzymatic pocket of AChE has long been the target of several drug designs over the last two decades. The peripheral and catalytic sites of AChE are simultaneously bound by several dimeric molecules, enabling more-efficient inhibition. Here, we used 6-chlorotacrine and the tetrahydroquinolone moiety of huperzine A to design and synthesize a series of heterodimers that inhibit AChE at nanomolar potency. Specifically, compound 7b inhibits AChE with an IC₅₀ < 1 nM and spares butyrylcholinesterase. Administration of 7b to mouse brain slices restores synaptic activity impaired by pirenzepine, a muscarinic M1-selective antagonist. Moreover, oral administration of 7b to C57BL/6 mice enhances hippocampal long-term potentiation in a dose-dependent manner and is detectable in the brain tissue. All these data supported that 7b is a potential cognitive enhancer and is worth for further exploration.

Structural Characterisation and Assessment of the Novel Bacillus amyloliquefaciens RK3 Exopolysaccharide on the Improvement of Cognitive Function in Alzheimer's Disease Mice

In this study Bacillus amyloliquefaciens RK3 was isolated from a sugar mill effluent-contaminated soil and utilised to generate a potential polysaccharide with anti-Alzheimer's activity. Traditional and molecular methods were used to validate the strain. The polysaccharide produced by B. amyloliquefaciens RK3 was purified, and the yield was estimated to be 10.35 gL^-1. Following purification, the polysaccharide was structurally and chemically analysed. The structural analysis revealed the polysaccharide consists of α-d-mannopyranose (α-d-Manp) and β-d-galactopyranose (β-d-Galp) monosaccharide units connected through glycosidic linkages (i.e., β-d-Galp(1→6)β-d-Galp (1→6)β-d-Galp(1→2)β-d-Galp(1→2)[β-d-Galp(1→6)]β-d-Galp(1→2)α-d-Manp(1→6)α-d-Manp (1→6)α-d-Manp(1→6)α-d-Manp(1→6)α-d-Manp). The scanning electron microscopy and energy-dispersive X-ray spectroscopy imaging of polysaccharides emphasise their compactness and branching in the usual tubular heteropolysaccharide structure. The purified exopolysaccharide significantly impacted the plaques formed by the amyloid proteins during Alzheimer's disease. Further, the results also highlighted the potential applicability of exopolysaccharide in various industrial and pharmaceutical applications.

1H-1,2,3-triazole grafted tacrine-chalcone conjugates as potential cholinesterase inhibitors with the evaluation of their behavioral tests and oxidative stress in mice brain cells

The present paper explicates the synthesis of 1H-1,2,3-triazole tethered tacrine-chalcone conjugates and evaluation of their AChE and BuChE inhibitory activity. In-vitroAChE inhibition assay revealed three compounds, 9h, 9i, and 11f, being more potent than the standard drug tacrine and further evaluated against butyrylcholinesterase. The present study was extended to investigate the anti-amnestic effect of promising compoundson scopolamine-induced behavioral and neurochemical changes in mice. Inclined plane model and Elevated plus-maze model were performed to assess general limb motor activity and anxiety-like behavior, respectively, in mice pre-treated with scopolamine. Oxidative stress parameters reduced glutathione contents (GSH) and lipid peroxidation products (TBARS) in the brain homogenates as estimated using ex-vivo studies. Furthermore, molecular docking studies were performed for the potent compounds to decipher the mechanism of observed activities.

Multi-target Natural and Nature-Inspired Compounds against Neurodegeneration: A Focus on Dual Cholinesterase and Phosphodiesterase Inhibitors

Alzheimer’s disease is a memory-related neurodegenerative condition leading to cognitive impairment. Cholinergic deficit, together with other underlying mechanisms, leads the to onset and progression of the disease. Consequently, acetylcholinesterase inhibitors are used for the symptomatic treatment of dementia, even if limited efficacy is observed. More recently, some specific phosphodiesterase isoforms emerged as promising, alternative targets for developing inhibitors to contrast neurodegeneration. Phosphodiesterase isoforms 4, 5 and 9 were found to be expressed in brain regions that are relevant for cognition. Given the complex nature of Alzheimer’s disease and the combination of involved biochemical mechanisms, the development of polypharmacological agents acting on more than one pathway is desirable. This review provides an overview of recent reports focused on natural and Nature-inspired small molecules, or plant extracts, acting as dual cholinesterase and phosphodiesterase inhibitors. In the context of the multi-target directed ligand approach, such molecules would pave the way for the development of novel agents against neurodegeneration. More precisely, according to the literature data, xanthines, other alkaloids, flavonoids, coumarins and polyphenolic acids represent promising scaffolds for future optimization.

In silico Exploration of Bioactive Phytochemicals Against Neurodegenerative Diseases Via Inhibition of Cholinesterases

Neurodegenerative disorders are estimated to become the second leading cause of death worldwide by 2040. Despite the widespread use of diverse allopathic drugs, these brain-associated disorders can only be partially addressed and long term treatment is often linked with dependency and other unwanted side effects. Nature, believed to be an arsenal of remedies for any illness, presents an interesting avenue for the development of novel neuroprotective agents. Interestingly, inhibition of cholinesterases, involved in the breakdown of acetylcholine in the synaptic cleft, has been proposed to be neuroprotective. This review therefore aims to provide additional insight via docking studies of previously studied compounds that have shown potent activity against acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in vitro. Indeed, the determination of potent plant-based ligands for this purpose through in silico methods enables the elimination of lengthy and costly traditional methods of drug discovery. Herein, a literature search was conducted to identify active phytochemicals which are cholinesterase inhibitors. Following which in silico docking methods were applied to obtain docking scores. Compound structures were extracted from online ZINC database and optimized using AM1 implemented in gaussian09 software. Noteworthy ligands against AChE highlighted in this study include: 19,20-dihydroervahanine A and 19, 20-dihydrotabernamine. Regarding BChE inhibition, the best ligands were found to be 8-Clavandurylkaempferol, Na-methylepipachysamine D; ebeiedinone; and dictyophlebine. Thus, ligand optimization between such phytochemicals and cholinesterases coupled with in vitro, in vivo studies and randomized clinical trials can lead to the development of novel drugs against neurodegenerative disorders.

Design, Synthesis and Biological Evaluation of New Cycloalkyl Fused Quinolines Tethered to Isatin Schiff Bases as Cholinesterase Inhibitors

AIMS AND OBJECTIVE

Alzheimer's disease is now a most prevalent neuro degenerative disease of central nervous system leading to dementia in elderly aged population. Numerous pathological changes have been associated in the progression of Alzheimer's disease. One of such pathological hypothesis is declined cholinergic activity which eventually affects cognitive and memory deficits. Inhibition cholinesterases will apparently elevate acetyl choline levels which is benefactor on cognitive symptoms of the disease. This manuscript describes the new tacrine derivatives tethered to isatin Schiff bases through alkanoyl linker and screened for cholinesterase inhibitory activity.

MATERIALS AND METHODS

Tacrine and two more cycloalkyl ring fused quinolones were synthesized and converted to Ncycloalkyl fused quinoline chloroamides. Isatin Schiff bases were also synthesized by the reaction between isatin and substituted aromatic anilines and in subsequent reaction, isatin Schiff bases were reacted with cycloalkyl fused quinolones to afford anticipated compounds 10a-i, 11a-i and 12a-i. All the compounds have been screened for acetyl and butyryl cholinesterase inhibitory activity and in vivo behavioral studies. Binding interactions of the desired compounds have also been studied by docking them in active site of both cholinesterases.

RESULTS

Three compounds 12d, 12e and 12h with propionyl and butyroyl linker between amine and isatin Schiff base scaffold have shown potent acetyl and butyryl cholinesterase inhibitory activity. However most potent cholinesterase inhibitor was 13d with IC50 value of 0.71±0.004 and 1.08±0.02 μM against acetyl and butyryl cholinesterases respectively. The hepatotoxicity of potent compounds revealed that the tested compounds were less hepatotoxic than tacrine and also exhibited encouraging in vivo behavioral studies in test animals. Docking studies of all the molecules disclosed close hydrogen bond interactions within the binding site of both cholinesterases.

CONCLUSION

New cycloalkyl fused quinolones tethered with alkoyl linker to isatin Schiff bases endowed significant and potent cholinesterase inhibitory activities. Few of the compounds have also exhibited lesser hepatotoxicity and all the synthesized compounds were good in behavioral studies. Molecular docking studies also indicated close interactions in active site of cholinesterases.

Design, synthesis and biological evaluation of edaravone derivatives bearing the N-benzyl pyridinium moiety as multifunctional anti-Alzheimer's agents

A series of multi-target directed edaravone derivatives bearing N-benzyl pyridinium moieties were designed and synthesised. Edaravone is a potent antioxidant with significant neuroprotective effects and N-benzyl pyridinium has previously exhibited positive results as part of a dual-site binding, peripheral anionic site (PAS) and catalytic anionic site (CAS), acetylcholinesterase (AChE) inhibitor. The designed edaravone-N-benzyl pyridinium hybrid compounds were docked within the AChE active site. The results indicated interactions with conserved amino acids (Trp279 in PAS and Trp84 in CAS), suggesting good dual-site inhibitory activity. Significant in vitro AChE inhibitory activities were observed for selected compounds (IC50: 1.2-4.6 µM) with limited butyrylcholinesterase inhibitory activity (IC50's >160 µM), indicating excellent selectivity towards AChE (SI: 46 - >278). The compounds also showed considerable antioxidant ability, similar to edaravone. In silico studies indicated that these compounds should cross the blood-brain barrier, making them promising lead molecules in the development of anti-Alzheimer's agents.

Galantamine-Curcumin Hybrids as Dual-Site Binding Acetylcholinesterase Inhibitors

Galantamine (GAL) and curcumin (CU) are alkaloids used to improve symptomatically neurodegenerative conditions like Alzheimer's disease (AD). GAL acts mainly as an inhibitor of the enzyme acetylcholinesterase (AChE). CU binds to amyloid-beta (Aβ) oligomers and inhibits the formation of Aβ plaques. Here, we combine GAL core with CU fragments and design a combinatorial library of GAL-CU hybrids as dual-site binding AChE inhibitors. The designed hybrids are screened for optimal ADME properties and BBB permeability and docked on AChE. The 14 best performing compounds are synthesized and tested in vitro for neurotoxicity and anti-AChE activity. Five of them are less toxic than GAL and CU and show activities between 41 and 186 times higher than GAL.

Highly Significant Scaffolds to Design and Synthesis Cholinesterase Inhibitors as Anti-Alzheimer Agents

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Alzheimer, a progressive disease, is a common term for memory loss which interferes with daily life through severe influence on cognitive abilities. Based on the cholinergic hypothesis, and Xray crystallographic determination of the structure of acetylcholinesterase (AChE) enzyme, the level of acetylcholine (ACh, an important neurotransmitter associated with memory) in the hippocampus and cortex area of the brain has a direct effect on Alzheimer. This fact encourages scientists to design and synthesize a wide range of acetylcholinesterase inhibitors (AChEIs) to control the level of ACh in the brain, keeping in view the crystallographic structure of AChE enzyme and drugs approved by the Food and Drug Administration (FDA).

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AChEIs have slightly diverse pharmacological properties, but all of them work by inhibiting the segregation of ACh by blocking AChE. We reviewed significant scaffolds introduced as AChEIs. In some studies, the activity against butyrylcholinesterase (BuChE) has been evaluated as well because BuChE is a similar enzyme to neuronal acetylcholinesterase and is capable of hydrolyzing ACh. In order to study AChEIs effectively, we divided them structurally into 12 classes and briefly explained effective AChEIs and compared their activities against AChE enzyme.

Therapeutic Potential of Multifunctional Tacrine Analogues

Abstract: Tacrine is a potent inhibitor of cholinesterases (acetylcholinesterase and butyrylcholinesterase) that shows limiting clinical application by liver toxicity. In spite of this, analogues of tacrine are considered as a model inhibitor of cholinesterases in the therapy of Alzheimer’s disease. The interest in these compounds is mainly related to a high variety of their structure and biological properties. In the present review, we have described the role of cholinergic transmission and treatment strategies in Alzheimer’s disease as well as the synthesis and biological activity of several recently developed classes of multifunctional tacrine analogues and hybrids, which consist of a new paradigm to treat Alzheimer’s disease. We have also reported potential of these analogues in the treatment of Alzheimer’s diseases in various experimental systems.

Design, synthesis and biological activity of novel tacrine-isatin Schiff base hybrid derivatives

A series of novel tacrine-isatin Schiff base hybrid derivatives (7a-p) were designed, synthesized and evaluated as multi-target candidates against Alzheimer's disease (AD). The biological assays indicated that most of these compounds displayed potent inhibitory activity toward acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) and specific selectivity for AChE over BuChE. It was also found that they act as excellent metal chelators. The compounds 7k and 7m were found to be good inhibitors of AChE-induced amyloid-beta (Aβ) aggregation. Most of the compounds inhibited AChE with the IC₅₀ values, ranging from 0.42 nM to 79.66 nM. Amongst them, 7k, 7m and 7p, all with a 6 carbon linker between tacrine and isatin Schiff base exhibited the strongest inhibitory activity against AChE with IC₅₀ values of 0.42 nM, 0.62 nM and 0.95 nM, respectively. They were 92-, 62- and 41-fold more active than tacrine (IC₅₀ = 38.72 nM) toward AChE. Most of the compounds also showed a potent BuChE inhibition among which 7d with an IC₅₀ value of 0.11 nM for BuChE is the most potent one (56-fold more potent than that of tacrine (IC₅₀ = 6.21 nM)). In addition, most compounds exhibited the highest metal chelating property. Kinetic and molecular modeling studies revealed that 7k is a mixed-type inhibitor, capable of binding to catalytic and peripheral site of AChE. Our findings make this hybrid scaffold an excellent candidate to modify current drugs in treating Alzheimer's disease (AD).

Synthesis of novel vanillin derivatives: novel multi-targeted scaffold ligands against Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia worldwide, normally affecting people aged over 65. Due to the multifactorial nature of this disease, a "multi-target-directed ligands" (MTDLs) approach for the treatment of this illness has generated intense research interest in the past few years. Vanillin is a natural antioxidant and it provides a good starting point for the synthesis of new compounds with enhanced antioxidant properties, together with many biological activities, including β-amyloid peptide aggregating and acetylcholinesterase inhibiting properties. Here we report novel vanillin derivatives, bearing a tacrine or a naphthalimido moiety. All compounds exhibited improved antioxidant properties using DPPH assay, with IC50 as low as 19.5 μM, FRAP and ORAC assays, with activities up to 1.54 and 6.4 Trolox equivalents, respectively. In addition, all compounds synthesized showed inhibitory activity toward acetylcholinesterase enzyme at μmolar concentrations using the Ellman assay. Computational docking studies of selected compounds showed interactions with both the catalytic anionic site and the peripheral anionic site of the enzyme. Furthermore, these compounds inhibited Aβ(1-42) amyloid aggregation using the fluorometric ThT assay, with compound 4 showing comparable inhibitory activity to the positive control, curcumin. At cellular level compound 4 (1 μM) showed significant protective effects in neuroblastoma SH-SY5Y cell line when treated with hydrogen peroxide (400 μM). In our opinion, vanillin derivatives could provide a viable platform for future development of multi-targeted ligands against AD.

Novel hits for acetylcholinesterase inhibition derived by docking-based screening on ZINC database

The inhibition of the enzyme acetylcholinesterase (AChE) increases the levels of the neurotransmitter acetylcholine and symptomatically improves the affected cognitive function. In the present study, we searched for novel AChE inhibitors by docking-based virtual screening of the standard lead-like set of ZINC database containing more than 6 million small molecules using GOLD software. The top 10 best-scored hits were tested in vitro for AChE affinity, neurotoxicity, GIT and BBB permeability. The main pharmacokinetic parameters like volume of distribution, free fraction in plasma, total clearance, and half-life were predicted by previously derived models. Nine of the compounds bind to the enzyme with affinities from 0.517 to 0.735 µM, eight of them are non-toxic. All hits permeate GIT and BBB and bind extensively to plasma proteins. Most of them are low-clearance compounds. In total, seven of the 10 hits are promising for further lead optimisation. These are structures with ZINC IDs: 00220177, 44455618, 66142300, 71804814, 72065926, 96007907, and 97159977.

Multi-target-directed ligands for treating Alzheimer's disease: Butyrylcholinesterase inhibitors displaying antioxidant and neuroprotective activities

The limited clinical efficacy of current symptomatic treatment and minute effect on progression of Alzheimer's disease has shifted the research focus from single targets towards multi-target-directed ligands. Here, a potent selective inhibitor of human butyrylcholinesterase was used as the starting point to develop a new series of multifunctional ligands. A focused library of derivatives was designed and synthesised that showed both butyrylcholinesterase inhibition and good antioxidant activity as determined by the DPPH assay. The crystal structure of compound 11 in complex with butyrylcholinesterase revealed the molecular basis for its low nanomolar inhibition of butyrylcholinesterase (Ki = 1.09 ± 0.12 nM). In addition, compounds 8 and 11 show metal-chelating properties, and reduce the redox activity of chelated Cu²⁺ ions in a Cu-ascorbate redox system. Compounds 8 and 11 decrease intracellular levels of reactive oxygen species, and are not substrates of the active efflux transport system, as determined in Caco2 cells. Compound 11 also protects neuroblastoma SH-SY5Y cells from toxic Aβ_1-42 species. These data indicate that compounds 8 and 11 are promising multifunctional lead ligands for treatment of Alzheimer's disease.

Molecular Consortia-Various Structural and Synthetic Concepts for More Effective Therapeutics Synthesis

The design and discovery of novel drug candidates are the initial and most probably the crucial steps in the drug development process. One of the tasks of medicinal chemistry is to produce new molecules that have a desired biological effect. However, even today the search for new pharmaceuticals is a very complicated process that is hard to rationalize. Literature provides many scientific reports on future prospects of design of potentially useful drugs. Many trends have been proposed for the design of new drugs containing different structures (dimers, heterodimers, heteromers, adducts, associates, complexes, biooligomers, dendrimers, dual-, bivalent-, multifunction drugs and codrugs, identical or non-identical twin drugs, mixed or combo drugs, supramolecular particles and various nanoindividuals. Recently much attention has been paid to different strategies of molecular hybridization. In this paper, various molecular combinations were described e.g., drug–drug or drug-non-drug combinations which are expressed in a schematic multi-factor form called a molecular matrix, consisting of four factors: association mode, connection method, and the number of elements and linkers. One of the most popular trends is to create small–small molecule combinations such as different hybrids, codrugs, drug–drug conjugates (DDCs) and small-large molecule combinations such as antibody-drug conjugates (ADCs), polymer-drug conjugates (PDCs) or different prodrugs and macromolecular therapeutics. A review of the structural possibilities of active framework combinations indicates that a wide range of potentially effective novel-type compounds can be formed. What is particularly important is that new therapeutics can be obtained in fast, efficient, and selective methods using current trends in chemical synthesis and the design of drugs such as the “Lego” concept or rational green approach.

Success stories of natural product-based hybrid molecules for multi-factorial diseases

Complex diseases comprises of highly complicated etiology resulting in limited applicability of conventional targeted therapies. Consequently, conventional medicinal compounds suffer major failure when used for such disease conditions. Additionally, development of multidrug resistance (MDR), adverse drug reactions and clinical specificity of single targeted drug therapy has increased thrust for novel drug therapy. In this rapidly evolving era, natural product-based discovery of hybrid molecules or multi-targeted drug therapies have shown promising results and are trending now a days. Historically, nature has blessed human with different sources viz. plant, animal, microbial, marine and ethnopharmaceutical sources which has given a wide variety of medicinally active compounds. These compounds from natural origin are always choice of interest of medicinal chemists because of their minimum side effects. Hybrid molecules synthesized by fusing or conjugating different active molecules obtained from these sources are reported to synergistically block different pathways which contribute in the pathogenesis of complex diseases. This review strives to encompass all natural product-derived hybrid molecules which act as multi-targeting agents striking various targets involved in different pathways of complex diseased conditions reported in literature.

Dual GSK-3β/AChE Inhibitors as a New Strategy for Multitargeting Anti-Alzheimer's Disease Drug Discovery

Designing multitarget-directed ligands (MTDLs) is considered to be a promising approach to address complex and multifactorial maladies such as Alzheimer's disease (AD). The concurrent inhibition of the two crucial AD targets, glycogen synthase kinase-3β (GSK-3β) and human acetylcholinesterase (hAChE), might represent a breakthrough in the quest for clinical efficacy. Thus, a novel family of GSK-3β/AChE dual-target inhibitors was designed and synthesized. Among these hybrids, 2f showed the most promising profile as a nanomolar inhibitor on both hAChE (IC₅₀ = 6.5 nM) and hGSK-3β kinase activity (IC₅₀ = 66 nM). It also showed good inhibitory effect on β-amyloid self-aggregation (inhibitory rate = 46%) at 20 μM. Western blot analysis revealed that compound 2f inhibited hyperphosphorylation of tau protein in mouse neuroblastoma N2a-Tau cells. In vivo studies confirmed that 2f significantly ameliorated the cognitive disorders in scopolamine-treated ICR mice and less hepatotoxicity than tacrine. This study provides new leads for assessment of GSK-3β and AChE pathway dual inhibition as a promising strategy for AD treatment.