Galantamine-Curcumin Hybrids as Dual-Site Binding Acetylcholinesterase Inhibitors

Curcumin hybrid molecules for the treatment of Alzheimer's disease: Structure and pharmacological activities

Alzheimer's disease (AD) is the most common neurodegenerative disease among the elderly. Contemporary treatments can only relieve symptoms but fail to delay disease progression. Curcumin is a naturally derived compound that has demonstrated significant therapeutic effects in AD treatment. Recently, molecular hybridization has been utilized to combine the pharmacophoric groups present in curcumin with those of other AD drugs, resulting in a series of novel compounds that enhance the therapeutic efficacy through multiple mechanisms. In this review, we firstly provide a concise summary of various pathogenetic hypotheses of AD and the mechanism of action of curcumin in AD, as well as the concept of molecular hybridization. Subsequently, we focus on the recent development of hybrid molecules derived from curcumin, summarizing their structures and pharmacological activities, including cholinesterase inhibitory activity, Aβ aggregation inhibitory activity, antioxidant activity, and other activities. The structure-activity relationships were further discussed.

Synthetic and Natural Bioactive Molecules in Balancing the Crosstalk among Common Signaling Pathways in Alzheimer's Disease: Understanding the Neurotoxic Mechanisms for Therapeutic Intervention

The structure and function of the brain greatly rely on different signaling pathways. The wide variety of biological processes, including neurogenesis, axonal remodeling, the development and maintenance of pre- and postsynaptic terminals, and excitatory synaptic transmission, depends on combined actions of these molecular pathways. From that point of view, it is important to investigate signaling pathways and their crosstalk in order to better understand the formation of toxic proteins during neurodegeneration. With recent discoveries, it is established that the modulation of several pathological events in Alzheimer's disease (AD) due to the mammalian target of rapamycin (mTOR), Wnt signaling, 5'-adenosine monophosphate activated protein kinase (AMPK), peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α), and sirtuin 1 (Sirt1, silent mating-type information regulator 2 homologue 1) are central to the key findings. These include decreased amyloid formation and inflammation, mitochondrial dynamics control, and enhanced neural stability. This review intends to emphasize the importance of these signaling pathways, which collectively determine the fate of neurons in AD in several ways. This review will also focus on the role of novel synthetic and natural bioactive molecules in balancing the intricate crosstalk among different pathways in order to prolong the longevity of AD patients.

Molecular and Pharmacokinetic Aspects of the Acetylcholinesterase-Inhibitory Potential of the Oleanane-Type Triterpenes and Their Glycosides

The acetylcholinesterase-inhibitory potential of the oleanane-type triterpenes and their glycosides from thebark of Terminalia arjuna (Combreatceae), i.e.,arjunic acid, arjunolic acid, arjungenin, arjunglucoside I, sericic acid and arjunetin, is presented. The studies are based on in silico pharmacokinetic and biomimetic studies, acetylcholinesterase (AChE)-inhibitory activity tests and molecular-docking research. Based on the calculated pharmacokinetic parameters, arjunetin and arjunglucoside I are indicated as able to cross the blood-brain barrier. The compounds of interest exhibit a marked acetylcholinesterase inhibitory potential, which was tested in the TLC bioautography test. The longest time to reach brain equilibrium is observed for both the arjunic and arjunolic acids and the shortest one for arjunetin. All of the compounds exhibit a high and relatively similar magnitude of binding energies, varying from ca. -15 to -13 kcal/mol. The superposition of the most favorable positions of all ligands interacting with AChE is analyzed. The correlation between the experimentally determined IC50 values and the steric parameters of the molecules is investigated. The inhibition of the enzyme by the analyzed compounds shows their potential to be used as cognition-enhancing agents. For the most potent compound (arjunglucoside I; ARG), the kinetics of AChE inhibition were tested. The Michaelis-Menten constant (Km) for the hydrolysis of the acetylthiocholine iodide substrate was calculated to be 0.011 mM.

Neuroprotective Properties of Oleanolic Acid-Computational-Driven Molecular Research Combined with In Vitro and In Vivo Experiments

Oleanolic acid (OA), as a ubiquitous compound in the plant kingdom, is studied for both its neuroprotective and neurotoxic properties. The mechanism of acetylcholinesterase (AChE) inhibitory potential of OA is investigated using molecular dynamic simulations (MD) and docking as well as biomimetic tests. Moreover, the in vitro SH-SY5Y human neuroblastoma cells and the in vivo zebrafish model were used. The inhibitory potential towards the AChE enzyme is examined using the TLC-bioautography assay (the IC50 value is 9.22 μM). The CH-π interactions between the central fragment of the ligand molecule and the aromatic cluster created by the His440, Phe288, Phe290, Phe330, Phe331, Tyr121, Tyr334, Trp84, and Trp279 side chains are observed. The results of the in vitro tests using the SH-SY5Y cells indicate that the viability rate is reduced to 71.5%, 61%, and 43% at the concentrations of 100 µg/mL, 300 µg/mL, and 1000 µg/mL, respectively, after 48 h of incubation, whereas cytotoxicity against the tested cell line with the IC50 value is 714.32 ± 32.40 µg/mL. The in vivo tests on the zebrafish prove that there is no difference between the control and experimental groups regarding the mortality rate and morphology (p > 0.05).

Unfolding the Antibacterial Activity and Acetylcholinesterase Inhibition Potential of Benzofuran-Triazole Hybrids: Synthesis, Antibacterial, Acetylcholinesterase Inhibition, and Molecular Docking Studies

In this study, a series of novel benzofuran-based 1,2,4-triazole derivatives (10a-e) were synthesized and evaluated for their inhibitory potential against acetylcholinesterase (AChE) and bacterial strains (E. coli and B. subtilis). Preliminary results revealed that almost all assayed compounds displayed promising efficacy against AChE, while compound 10d was found to be a highly potent inhibitor of AChE. Similarly, these 5-bromobenzofuran-triazoles 10a-e were screened against B. subtilis QB-928 and E. coli AB-274 to evaluate their antibacterial potential in comparison to the standard antibacterial drug penicillin. Compound 10b was found to be the most active among all screened scaffolds, with an MIC value of 1.25 ± 0.60 µg/mL against B. subtilis, having comparable therapeutic efficacy to the standard drug penicillin (1 ± 1.50 µg/mL). Compound 10a displayed excellent antibacterial therapeutic efficacy against the E. coli strain with comparable MIC of 1.80 ± 0.25 µg/mL to that of the commercial drug penicillin (2.4 ± 1.00 µg/mL). Both the benzofuran-triazole molecules 10a and 10b showed a larger zone of inhibition. Moreover, IFD simulation highlighted compound 10d as a novel lead anticholinesterase scaffold conforming to block entrance, limiting the swinging gate, and disrupting the catalytic triad of AChE, and further supported its significant AChE inhibition with an IC50 value of 0.55 ± 1.00 µM. Therefore, compound 10d might be a promising candidate for further development in Alzheimer's disease treatment, and compounds 10a and 10b may be lead antibacterial agents.

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.

Ex Vivo Antioxidant and Cholinesterase Inhibiting Effects of a Novel Galantamine-Curcumin Hybrid on Scopolamine-Induced Neurotoxicity in Mice

Oxidative stress is an essential factor in the development and progression of Alzheimer's disease (AD). An excessive amount of reactive oxygen species (ROS) induces the peroxidation of lipid membranes, reduces the activity of antioxidant enzymes and causes neurotoxicity. In this study, we investigated the antioxidant and cholinesterase inhibitory potential of a novel galantamine-curcumin hybrid, named 4b, administered orally in two doses (2.5 mg/kg and 5 mg/kg) in scopolamine (SC)-induced neurotoxicity in mice. To evaluate the effects of 4b, we used galantamine (GAL) (3 mg/kg) and curcumin (CCN) (25 mg/kg) as positive controls. Ex vivo experiments on mouse brains showed that the higher dose of 4b (5 mg/kg) increased reduced glutathione (GSH) levels by 46%, catalase (CAT) and superoxide dismutase (SOD) activity by 57%, and glutathione peroxidase (GPx) activity by 108%, compared with the SC-treated group. At the same time, 4b (5 mg/kg) significantly reduced the brain malondialdehyde (MDA) level by 31% and acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) activities by 40% and 30%, respectively, relative to the SC-impaired group. The results showed that 4b acted as an antioxidant agent and brain protector, making it promising for further experimental research in the field of neurodegenerative diseases.

Virtual Screening and Hit Selection of Natural Compounds as Acetylcholinesterase Inhibitors

Acetylcholinesterase (AChE) is one of the classical targets in the treatment of Alzheimer's disease (AD). Inhibition of AChE slows down the hydrolysis of acetycholine and increases choline levels, improving the cognitive function. The achieved success of plant-based natural drugs acting as AChE inhibitors, such as galantamine (GAL) from Galanthus genus and huperzine A from Huperzia serrate (approved drug in China), in the treatment of AD, and the fact that natural compounds (NCs) are considered as safer and less toxic compared to synthetic drugs, led us to screen the available NCs (almost 150,000) in the ZINC12 database for AChE inhibitory activity. The compounds were screened virtually by molecular docking, filtered for suitable ADME properties, and 32 ligands from 23 structural groups were selected. The stability of the complexes was estimated via 1 μs molecular dynamics simulation. Ten compounds formed stable complexes with the enzyme and had a vendor and a reasonable price per mg. They were tested for AChE inhibitory and antioxidant activity. Five compounds showed weak AChE inhibition and three of them exhibited high antioxidant activity.

Molecular Insights into Therapeutic Potentials of Hybrid Compounds Targeting Alzheimer's Disease

Alzheimer's disease (AD) is one of the most complex progressive neurological disorders involving degeneration of neuronal connections in brain cells leading to cell death. AD is predominantly detected among elder people (> 65 years), mostly diagnosed with the symptoms of memory loss and cognitive dysfunctions. The multifarious pathogenesis of AD comprises the accumulation of pathogenic proteins, decreased neurotransmission, oxidative stress, and neuroinflammation. The conventional therapeutic approaches are limited to symptomatic benefits and are ineffective against disease progression. In recent years, researchers have shown immense interest in the designing and fabrication of various novel therapeutics comprised of naturally isolated hybrid molecules. Hybrid therapeutic compounds are developed from the combination of pharmacophores isolated from bioactive moieties which specifically target and block various AD-associated pathogenic pathways. The method of designing hybrid molecules has numerous advantages over conventional multitarget drug development methods. In comparison to in silico high throughput screening, hybrid molecules generate quicker results and are also less expensive than fragment-based drug development. Designing hybrid-multitargeted therapeutic compounds is thus a prospective approach in developing an effective treatment for AD. Nevertheless, several issues must be addressed, and additional researches should be conducted to develop hybrid therapeutic compounds for clinical usage while keeping other off-target adverse effects in mind. In this review, we have summarized the recent progress on synthesis of hybrid compounds, their molecular mechanism, and therapeutic potential in AD. Using synoptic tables, figures, and schemes, the review presents therapeutic promise and potential for the development of many disease-modifying hybrids into next-generation medicines for AD.

Potential Therapeutic Applications of Plant-Derived Alkaloids against Inflammatory and Neurodegenerative Diseases

Alkaloids are a type of natural compound possessing different pharmacological activities. Natural products, including alkaloids, which originate from plants, have emerged as potential protective agents against neurodegenerative disorders (NDDs) and chronic inflammations. A wide array of prescription drugs are used against these conditions, however, not free of limitations of potency, side effects, and intolerability. In the context of personalized medicine, further research on alkaloids to unravel novel therapeutic approaches in reducing complications is critical. In this review, a systematic survey was executed to collect the literature on alkaloids and their health complications, from which we found that majority of alkaloids exhibit anti-inflammatory action via nuclear factor-κB and cyclooxygenase-2 (COX-2), and neuroprotective interaction through acetylcholinesterase (AChE), COX, and β-site amyloid precursor protein activity. In silico ADMET and ProTox-II-related descriptors were calculated to predict the pharmacological properties of 280 alkaloids isolated from traditional medicinal plants towards drug development. Out of which, eight alkaloids such as tetrahydropalmatine, berberine, tetrandrine, aloperine, sinomenine, oxymatrine, harmine, and galantamine are found to be optimal within the categorical range when compared to nicotine. These alkaloids could be exploited as starting materials for novel drug synthesis or, to a lesser extent, manage inflammation and neurodegenerative-related complications.

The Potential Effect of Insulin on AChE and Its Interactions with Rivastigmine In Vitro

There is no definite cure for Alzheimer’s disease (AD) due to its multifactorial origin. Drugs that inhibit acetylcholinesterase (AChE), such as rivastigmine, are promising symptomatic treatments for AD. Emerging evidence suggests that insulin therapy can hinder several aspects of AD pathology. Insulin has been shown to modify the activity of AChE, but it is still unknown how insulin and AChE interact. Combination therapy, which targets several features of the disease based on existing medications, can provide a worthy therapy option for AD management. However, to date, no studies have examined the potential interaction of insulin with AChE and/or rivastigmine in vitro. In the present study, we employed the Response Surface Methodology (RSM) as an in vitro assessment to investigate the effect of insulin on both AChE activity and rivastigmine inhibitory action using a common spectrophotometric assay for cholinesterase activity, Ellman’s method. Our results showed that insulin, even at high concentrations, has an insignificant effect on both the activity of AChE and rivastigmine’s inhibitory action. The variance of our data is near zero, which means that the dispersion is negligible. However, to improve our understanding of the possible interaction of insulin and rivastigmine, or its target AChE, more in silico modelling and in vivo studies are needed.

Chronic Stress and Oxidative Stress as Common Factors of the Pathogenesis of Depression and Alzheimer's Disease: The Role of Antioxidants in Prevention and Treatment

There is a growing body of scientific research showing the link between depression and dementia in Alzheimer’s disease (AD). The chronic stress contributes to the formation of oxidative stress in the parts of the brain involved in the development of depression and AD. The scientific literature reports the significant role of antioxidants, which are highly effective in treating these diseases. In this review, we have summarized the relationship between chronic stress, oxidative stress, and the changes in the brain they cause occurring in the brain. Among all the compounds showing antioxidant properties, the most promising results in AD treatment were observed for Vitamin E, coenzyme Q10 (CoQ10), melatonin, polyphenols, curcumin, and selenium. In case of depression treatment, the greatest potential was observed in curcumin, zinc, selenium, vitamin E, and saffron.

A Galantamine-Curcumin Hybrid Decreases the Cytotoxicity of Amyloid-Beta Peptide on SH-SY5Y Cells

Misfolded amyloid beta (Aβ) peptides aggregate and form neurotoxic oligomers. Membrane and mitochondrial damages, calcium dysregulation, oxidative stress, and fibril deposits are among the possible mechanisms of Aβ cytotoxicity. Galantamine (GAL) prevents apoptosis induced by Aβ mainly through the ability to stimulate allosterically the α7 nAChRs and to regulate the calcium cytosolic concentration. Here, we examined the cytoprotective effects of two GAL derivatives, namely compounds 4b and 8, against Aβ cytotoxicity on the human neuroblastoma cell line SH-SY5Y. The protective effects were tested at simultaneous administration, pre-incubation and post-incubation, with Aβ. GAL and curcumin (CU) were used in the study as reference compounds. It was found that 4b protects cells in a similar mode as GAL, while compound 8 and CU potentiate the toxic effects of Aβ. Allosteric stimulation of α7 nAChRs is suggested as a possible mechanism of the cytoprotectivity of 4b. These and previous findings characterize 4b as a prospective non-toxic multi-target agent against neurodegenerative disorders with inhibitory activity on acetylcholinesterase, antioxidant, and cytoprotective properties.

Action Mechanisms of Curcumin in Alzheimer's Disease and Its Brain Targeted Delivery

AD is a chronic neurodegenerative disease. Many different signaling pathways, such as Wnt/β-catenin, Notch, ROS/JNK, and PI3K/Akt/mTOR are involved in Alzheimer’s disease and crosstalk between themselves. A promising treatment involves the uses of flavonoids, and one of the most promising is curcumin; however, because it has difficulty permeating the blood–brain barrier (BBB), it must be encapsulated by a drug carrier. Some of the most frequently studied are lipid nanocarriers, liposomes, micelles and PLGA. These carriers are further conjugated with brain-targeting agents such as lactoferrin and transferrin. In this review paper, curcumin and its therapeutic effects, which have been examined in vivo, are analyzed and then the delivery systems to the brain are addressed. Overall, the analysis of the literature revealed great potential for curcumin in treating AD and indicated the challenges that require further research.

Discovery of a Novel Acetylcholinesterase Inhibitor by Fragment-Based Design and Virtual Screening

Despite extensive and intensive research efforts in recent decades, there is still no effective treatment for neurodegenerative diseases. On this background, the use of drugs inhibiting the enzyme acetylcholinesterase (AChE) remains an eternal evergreen in the symptomatic treatment of mild to moderate cognitive impairments. Even more, the cholinergic hypothesis, somewhat forgotten in recent years due to the shift in focus on amyloid cascade, is back to life, and the search for new, more effective AChE inhibitors continues. We generated a fragment-based library containing aromatic moieties and linkers originating from a set of novel AChE inhibitors. We used this library to design 1220 galantamine (GAL) derivatives following the model GAL (binding core) - linker (L) - aromatic fragment (Ar). The newly designed compounds were screened virtually for blood–brain barrier (BBB) permeability and binding to AChE. Among the top 10 best-scored compounds, a representative lead molecule was selected and tested for anti-AChE activity and neurotoxicity. It was found that the selected compound was a powerful non-toxic AChE inhibitor, 68 times more active than GAL, and could serve as a lead molecule for further optimization and development.

A Novel Galantamine-Curcumin Hybrid as a Potential Multi-Target Agent against Neurodegenerative Disorders

The acetylcholinesterase (AChE) inhibitors are the main drugs for symptomatic treatment of neurodegenerative disorders like Alzheimer’s disease. A recently designed, synthesized and tested hybrid compound between the AChE inhibitor galantamine (GAL) and the antioxidant polyphenol curcumin (CU) showed high AChE inhibition in vitro. Here, we describe tests for acute and short-term toxicity in mice as well as antioxidant tests on brain homogenates measured the levels of malondialdehide (MDA) and glutathione (GSH) and in vitro DPPH, ABTS, FRAP and LPO inhibition assays. Hematological and serum biochemical analyses were also performed. In the acute toxicity tests, the novel AChE inhibitor given orally in mice showed LD₅₀ of 49 mg/kg. The short-term administration of 2.5 and 5 mg/kg did not show toxicity. In the ex vivo tests, the GAL-CU hybrid performed better than GAL and CU themselves; in a dose of 5 mg/kg, it demonstrates 25% reduction in AChE activity, as well as a 28% and 73% increase in the levels of MDA and GSH, respectively. No significant changes in blood biochemical data were observed. The antioxidant activity of 4b measured ex vivo was proven in the in vitro tests. In the ABTS assay, 4b showed radical scavenging activity 10 times higher than the positive control butylhydroxy toluol (BHT). The GAL-CU hybrid is a novel non-toxic AChE inhibitor with high antioxidant activity which makes it a prospective multitarget drug candidate for treatment of neurodegenerative disorders.