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

Multitarget Therapeutic Strategies for Alzheimer's Disease: Review on Emerging Target Combinations

Neurodegenerative diseases represent nowadays one of the major health problems. Despite the efforts made to unveil the mechanism leading to neurodegeneration, it is still not entirely clear what triggers this phenomenon and what allows its progression. Nevertheless, it is accepted that neurodegeneration is a consequence of several detrimental processes, such as protein aggregation, oxidative stress, and neuroinflammation, finally resulting in the loss of neuronal functions. Starting from these evidences, there has been a wide search for novel agents able to address more than a single event at the same time, the so-called multitarget-directed ligands (MTDLs). These compounds originated from the combination of different pharmacophoric elements which endowed them with the ability to interfere with different enzymatic and/or receptor systems, or to exert neuroprotective effects by modulating proteins and metal homeostasis. MTDLs have been the focus of the latest strategies to discover a new treatment for Alzheimer's disease (AD), which is considered the most common form of dementia characterized by neurodegeneration and cognitive dysfunctions. This review is aimed at collecting the latest and most interesting target combinations for the treatment of AD, with a detailed discussion on new agents with favorable in vitro properties and on optimized structures that have already been assessed in vivo in animal models of dementia.

Rational design, synthesis and biological evaluation of novel multitargeting anti-AD iron chelators with potent MAO-B inhibitory and antioxidant activity

A series of (3-hydroxypyridin-4-one)-coumarin hybrids were developed and investigated as potential multitargeting candidates for the treatment of Alzheimer's disease (AD) through the incorporation of iron-chelating and monoamine oxidase B (MAO-B) inhibition. This combination endowed the hybrids with good capacity to inhibit MAO-B as well as excellent iron-chelating effects. The pFe³⁺ values of the compounds were ranging from 16.91 to 20.16, comparable to more potent than the reference drug deferiprone (DFP). Among them, compound 18d exhibited the most promising activity against MAO-B, with an IC₅₀ value of 87.9 nM. Moreover, compound 18d exerted favorable antioxidant activity, significantly reversed the amyloid-β_1-42 (Aβ_1-42) induced PC12 cell damage. More importantly, 18d remarkably ameliorated the cognitive dysfunction in a scopolamine-induced mice AD model. In brief, a series of hybrids with potential anti-AD effect were successfully obtained, indicating that the design of iron chelators with MAO-B inhibitory and antioxidant activities is an attractive strategy against AD progression.

Schisantherin A Improves the Learning and Memory by Reducing the Phosphorylation of Tau Protein of the Hippocampus in AD Mice

Memory disorders are the main symptoms of aging and Alzheimer’s disease and seriously affect the quality of life. Schisandra, as a famous traditional Chinese medicine, has been used for modulating “the internal organs” for a thousand years. The total lignans from Schisandra have been scientifically proved to improve learning and memory ability. Since it is unclear which monomer in Schisandra total lignans exerts such a function, we evaluated the potential effects of Schisantherin A (SCA), the main monomer from Schisandra, on improving learning ability and memory in amyloid β-protein (Aβ1-42)-induced Alzheimer’s disease (AD) model mice. We found that SCA (5 mg/kg) significantly prolonged the latency and reduced the number of errors in a step-through test. SCA significantly shortened the time of finding the platform and increased the number of crossing the platform and the residence time in a Morris water maze test. SCA increased superoxide dismutase activities and reduced the Malondialdehyde level of the hippocampal tissue, suggesting its role in reducing oxidative stress in the AD mice. Furthermore, we found that SCA significantly decreased the hyperphosphorylation of Tau by altering glycogen synthase kinase-3β (GSK-3β) phosphorylation on Tyr216 and Ser9. Our results revealed the mechanism underlying SCA-mediated learning and memory improvement by regulating GSK-3β activity and lowering the hyperphosphorylation of Tau protein in the hippocampus of AD mice.

Synthesis and biological evaluation of acridine-based histone deacetylase inhibitors as multitarget agents against Alzheimer's disease

Multitarget agents simultaneously trigger molecules in functionally complementary pathways, and are therefore considered to have potential in effectively treating Alzheimer's disease (AD), which has a complex pathogenetic mechanism. In this study, the HDAC inhibitor core is incorporated into the acetylcholine esterase (ACE) inhibitor acridine-derived moiety and resulted in compounds that exhibited higher class IIa HDAC (4, 5, 7, and 9)- and class IIb HDAC6-inhibiting activity when compared to the pan-HDAC inhibitor SAHA in clinical practice. One of these compounds, 11b, displayed greater selectivity toward HDAC6 than other isoform enzymes. In contrast, the activity of compound 6a was selective toward class IIa HDAC and HDAC6. These two compounds exhibited strong activity against Aβ-aggregation as well as significantly disrupted Aβ-oligomer. Additionally, 11b and 6a strongly inhibited AChE. These experimental findings demonstrate that compounds 11b and 6a are HDAC-Aβ-aggregation-AChE inhibitors. Notably, they can enhance neurite outgrowth, but with no significant neurotoxicity. Further biological evaluation revealed the various cellular effects of multitarget compounds 11b and 6a, which have the potential to treat AD.

Current and emerging therapeutic targets of alzheimer's disease for the design of multi-target directed ligands

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease, and a major cause of death worldwide. The number of people suffering from this debilitating disorder is rising at an unprecedented rate, with a subsequent surge in healthcare costs. Only four drugs are clinically available for the treatment of AD symptoms, but they are not disease-modifying. Consequently, there is an urgent need for a cure. Although the cause of this debilitating condition remains poorly understood, it is believed that several factors may be involved in combination - including, health and lifestyle, environmental, and genetic factors. In recent years, a number of hallmarks of the disease have also been discovered, and it is believed that these factors may play an important role in the development of AD. Amyloid aggregation is one such factor which has been highly investigated, in addition to cholinesterase enzymes and tau aggregation. In the last decade, multi-target drugs have been increasingly investigated for their application to AD treatment. By combining two or more pharmacophores in a single compound, it is possible to synthesise a drug which can target several factors that are involved in AD development. This is a particularly attractive approach as it would avoid the use of combination therapies. As a result, it could reduce the burden on carers and families, and decrease healthcare and social care costs. Many active pharmacophores have been employed for the development of hybrid drugs, due to their abilities to inhibit the factors currently widely recognised to be involved in AD. These compounds have demonstrated promising results; however, research is still required to optimise the pharmacological profiles of the drugs, in addition to their potencies. Meanwhile, extensive research is continuously being performed into other potential targets for the treatment of AD. Based on the results obtained thus far, it is likely that multi-target compounds will continue to be increasingly studied in the future as potential treatments for AD.

Reasonably activating Nrf2: A long-term, effective and controllable strategy for neurodegenerative diseases

Neurodegenerative diseases are a variety of debilitating and fatal disorder in central nervous system (CNS). Besides targeting neuronal activity by influencing neurotransmitters or their corresponding receptors, modulating the underlying processes that lead to cell death, such as oxidative stress and mitochondrial dysfunction, should also be emphasized as an assistant strategy for neurodegeneration therapy. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) has been closely verified to be related to anti-inflammation and oxidative stress, rationally regulating its belonging pathway and activating Nrf2 is emphasized to be a potential treatment approach. There have existed multiple Nrf2 activators with different mechanisms and diverse structures, but those applied for neuro-disorders are still limited. On the basis of research arrangement and compound summary, we put forward the limitations of existing Nrf2 activators for neurodegenerative diseases and their future developing directions in enhancing the blood-brain barrier permeability to make Nrf2 activators function in CNS and designing Nrf2-based multi-target-directed ligands to affect multiple nodes in pathology of neurodegenerative diseases.

Discovery of novel series of 2-substituted benzo[d]oxazol-5-amine derivatives as multi-target directed ligands for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is associated with multifactorial neuropathological conditions, which include cholinergic deficit, amyloid-beta plaques formation, loss of neuronal plasticity and neuronal death. Treating such multifactorial conditions with a single target directed approach is considered to be inadequate. Accordingly, multi-target directed ligand (MTDL) strategy has been evolved as an auspicious approach for the treatment of AD. In light of that, a library of 2-substituted benzo[d]oxazol-5-amine derivatives (29-39; 86-107) was designed using the scaffold hopping guided MTDLs strategy, synthesized and evaluated through various in-vitro and in-vivo biological studies. The optimal compound 92 exhibited potent inhibitory activities against AChE (IC₅₀ = 0.052 ± 0.010 μM), BuChE (IC₅₀ = 1.085 ± 0.035 μM), and significant amyloid-beta aggregation (20 μM) inhibition. The compound possessed better blood-brain barrier permeability (Pe = 10.80 ± 0.055 × 10^-6 cm s^-1) in PAMPA assay and neuro protective properties (40 μM) on SH-SY5Y neuroblastoma cell lines. Furthermore, in-vivo behavioural studies were performed on Y-maze test (scopolamine-induced amnesia model) and Morris water maze test (Aβ_1-42 induced ICV rat model). The compound 92, at a dose of 10 mg/kg oral administration, demonstrated a substantial improvement of the cognitive and special memory impairment. In summary, both in-vitro and in-vivo investigations evidenced that compound 92 was a potential lead for the discovery of safe and effective disease-modifying agents for AD.

Recent advances of thiazole hybrids in biological applications

Thiazoles have attracted much synthetic interest due to their wide variety of biological properties and are important members of heterocyclic compounds. In recent years, studies on the synthesis of thiazole compounds have been increasing because of the properties of this core. In particular, the hybrid structures in which the thiazole ring and the other nuclei are linked have gained popularity. Hybrid structures are formed by the combination of different groups of chemical reactivity and biological activity characteristics. In this review, we highlight recent developments related to hybrid structures containing a thiazole core, recently developed as anticancer, antibacterial, anti-inflammatory, analgesic, anti-tubercular, antialzheimer and antidiabetic compounds.

BACE1 inhibitors: Current status and future directions in treating Alzheimer's disease

Alzheimer's disease (AD) is an irreversible, progressive neurodegenerative brain disorder with no current cure. One of the important therapeutic approaches of AD is the inhibition of β-site APP cleaving enzyme-1 (BACE1), which is involved in the rate-limiting step of the cleavage process of the amyloid precursor protein (APP) leading to the generation of the neurotoxic amyloid β (Aβ) protein after the γ-secretase completes its function. The produced insoluble Aβ aggregates lead to plaques deposition and neurodegeneration. BACE1 is, therefore, one of the attractive targets for the treatment of AD. This approach led to the development of potent BACE1 inhibitors, many of which were advanced to late stages in clinical trials. Nonetheless, the high failure rate of lead drug candidates targeting BACE1 brought to the forefront the need for finding new targets to uncover the mystery behind AD. In this review, we aim to discuss the most promising classes of BACE1 inhibitors with a description and analysis of their pharmacodynamic and pharmacokinetic parameters, with more focus on the lead drug candidates that reached late stages of clinical trials, such as MK8931, AZD-3293, JNJ-54861911, E2609, and CNP520. In addition, the manuscript discusses the safety concerns and insignificant physiological effects, which were highlighted for the most successful BACE1 inhibitors. Furthermore, the review demonstrates with increasing evidence that despite tremendous efforts and promising results conceived with BACE1 inhibitors, the latest studies suggest that their clinical use for treating Alzheimer's disease should be reconsidered. Finally, the review sheds light on alternative therapeutic options for targeting AD.

A novel class of multitarget anti-Alzheimer benzohomoadamantane‒chlorotacrine hybrids modulating cholinesterases and glutamate NMDA receptors

The development of multitarget compounds against multifactorial diseases, such as Alzheimer's disease, is an area of very intensive research, due to the expected superior therapeutic efficacy that should arise from the simultaneous modulation of several key targets of the complex pathological network. Here we describe the synthesis and multitarget biological profiling of a new class of compounds designed by molecular hybridization of an NMDA receptor antagonist fluorobenzohomoadamantanamine with the potent acetylcholinesterase (AChE) inhibitor 6-chlorotacrine, using two different linker lengths and linkage positions, to preserve or not the memantine-like polycyclic unsubstituted primary amine. The best hybrids exhibit greater potencies than parent compounds against AChE (IC₅₀ 0.33 nM in the best case, 44-fold increased potency over 6-chlorotacrine), butyrylcholinesterase (IC₅₀ 21 nM in the best case, 24-fold increased potency over 6-chlorotacrine), and NMDA receptors (IC₅₀ 0.89 μM in the best case, 2-fold increased potency over the parent benzohomoadamantanamine and memantine), which suggests an additive effect of both pharmacophoric moieties in the interaction with the primary targets. Moreover, most of these compounds have been predicted to be brain permeable. This set of biological properties makes them promising leads for further anti-Alzheimer drug development.

Rational Design of Multitarget-Directed Ligands: Strategies and Emerging Paradigms

Due to the complexity of multifactorial diseases, single-target drugs do not always exhibit satisfactory efficacy. Recently, increasing evidence indicates that simultaneous modulation of multiple targets may improve both therapeutic safety and efficacy, compared with single-target drugs. However, few multitarget drugs are on market or in clinical trials, despite the best efforts of medicinal chemists. This article discusses the systematic establishment of target combination, lead generation, and optimization of multitarget-directed ligands (MTDLs). Moreover, we analyze some MTDLs research cases for several complex diseases in recent years and the physicochemical properties of 117 clinical multitarget drugs, with the aim to reveal the trends and insights of the potential use of MTDLs.

Synthesis of 2-Amino-5-Carboxamide Thiazole Derivatives via Dehydrative Cyclization of Thiourea Intermediate Resin on Solid Phase

In this study, we synthesized 2-amino-5-carboxamide thiazole derivatives on solid phase. The synthesis of the library starts with the reductive amination of the 4-formyl-3-methoxy phenoxy resin to prevent isomer formation. The dehydrative cyclization of thiourea intermediate resin, which is the key step in the synthetic process, was successfully synthesized using α-bromoketone in the presence of the DMF so as to afford 2-amino-5-carboxylate thiazole resin. The resulting resin is coupled with various amines. Finally, the 2-amino-5-carboxamide thiazole resin was cleaved from the polymer support using a TFA and DCM cocktail. The physicochemical properties of the proposed 2-amino-5-carboxamide thiazole derivatives were calculated and showed potential to be an reasonable oral bioavailability drug properties as determined by Lipinski's Rule.

Design, Synthesis, and In Vitro Biological Activities of a Bio-Oxidizable Prodrug to Deliver Both ChEs and DYRK1A Inhibitors for AD Therapy

Despite their side effects, cholinesterase (ChE) inhibitors remain the only approved drugs to treat Alzheimer's disease patients, along with the N-methyl-d-aspartate (NMDA) receptor antagonist memantine. In the last few years, the dual-specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A) has also been studied as a promising target for the development of new drugs for this pathology. In this context, and based on our previous characterization of bio-oxidizable prodrugs of potent acetylcholinesterase (AChE) inhibitors, we envisioned a strategy involving the synthesis of a bio-oxidizable prodrug of both ChE and DYRK1A inhibitors. To this end, we fixed our interest on a known potent inhibitor of DYRK1A, namely INDY. The designed prodrug of both ChE and DYRK1A inhibitors was successfully synthesized, connecting both inhibitors by a carbonate link. This prodrug and its corresponding drug were then evaluated as ChEs and DYRK1A inhibitors. Remarkably, in vitro results were in accordance with the starting hypothesis, showing a relative inactivity of the prodrug against DYRK1A and ChEs and a potent inhibition of ChEs by the oxidized form. Molecular docking and kinetic studies of ChE inhibition by the active compound are also discussed in this report.

Cholinesterase inhibitory isoquinoline alkaloids from Corydalis mucronifera

Eight previously undescribed isoquinoline alkaloids, mucroniferanines H-M, together with 16 known isoquinoline alkaloids, were isolated from Corydalis mucronifera Maxim.. The structures of the previously undescribed compounds were elucidated by interpretation of 1D and 2D NMR spectroscopic and HRMS data, and their absolute configurations were established by computational electronic circular dichroism (ECD) calculations and X-ray diffraction data. Mucroniferanine L is reported as the first natural amide bond linked isoquinoline alkaloid dimer. The isolated compounds were evaluated for AChE and BuChE inhibitory activities and mucroniferanine H showed significant activities with IC₅₀ values of 2.31 μM and 36.71 μM, respectively.

Reconsideration of Anticholinesterase Therapeutic Strategies against Alzheimer's Disease

Alzheimer's disease (AD) is well-known as a severe neurodegeneration disease involving complicated etiologies, and cholinesterase inhibition remain the prevailing mode of clinical intervention in AD management. Although most clinically applied cholinesterase inhibitors (ChEIs) achieve limited clinical outcomes, research on the central cholinergic system is still thriving. Recently, an impressive amount of knowledge regarding novel acetylcholinesterase functions, as well as the close association between the central cholinergic system and other key elements for AD pathogenesis, has accumulated, highlighting that this field still has great potential for future drug development. In contrast to the overwhelmingly disappointing clinical therapeutic effects of various disease-modifying drug candidates, interesting evidence has continued to emerge over the past 20 years from the wealth of preclinical and clinical data on the usage of ChEIs, indicating underestimated clinical benefits due to physician ambivalence, a lack of persistent treatment, and inappropriate medication times or doses. Here we pinpoint several topics fit for future attention, focusing on the updated cholinergic hypothesis, especially the pleiotropic relationships with key pathogenetic signaling pathways and functions in AD, as well as possible novel therapeutic strategies, including novel ChEIs and cholinesterase inhibition-based innovative multifunctional therapeutic candidates. We intend to strengthen the future value of the precise application of cholinergic drugs, especially novel ChEIs, as a cornerstone pharmacological approach to AD treatment, either alone or in combination with other targets, to relieve symptoms and to modify disease progression.

A walk through tau therapeutic strategies

Tau neuronal and glial pathologies drive the clinical presentation of Alzheimer's disease and related human tauopathies. There is a growing body of evidence indicating that pathological tau species can travel from cell to cell and spread the pathology through the brain. Throughout the last decade, physiological and pathological tau have become attractive targets for AD therapies. Several therapeutic approaches have been proposed, including the inhibition of protein kinases or protein-3-O-(N-acetyl-beta-D-glucosaminyl)-L-serine/threonine Nacetylglucosaminyl hydrolase, the inhibition of tau aggregation, active and passive immunotherapies, and tau silencing by antisense oligonucleotides. New tau therapeutics, across the board, have demonstrated the ability to prevent or reduce tau lesions and improve either cognitive or motor impairment in a variety of animal models developing neurofibrillary pathology. The most advanced strategy for the treatment of human tauopathies remains immunotherapy, which has already reached the clinical stage of drug development. Tau vaccines or humanised antibodies target a variety of tau species either in the intracellular or extracellular spaces. Some of them recognise the amino-terminus or carboxy-terminus, while others display binding abilities to the proline-rich area or microtubule binding domains. The main therapeutic foci in existing clinical trials are on Alzheimer's disease, progressive supranuclear palsy and non-fluent primary progressive aphasia. Tau therapy offers a new hope for the treatment of many fatal brain disorders. First efficacy data from clinical trials will be available by the end of this decade.

Multitarget therapeutic strategies for Alzheimer's disease

Neurodegenerative diseases such as Alzheimer’s, Huntington’s and Parkinson’s diseases have multifaceted nature because of the different factors contributing to their progression. The complex nature of neurodegenerative diseases has developed a pressing need to design multitarget-directed ligands to address the complementary pathways involved in these diseases. The major enzyme targets for development of therapeutics for Alzheimer’s disease are cholinesterase and β-secretase enzymes. In this review, we discuss recent advances in profiling single target inhibitors based on these enzymes to multitarget-directed ligands as potential therapeutics for this devastating disease. In addition, therapeutics based on iron chelation strategy are discussed as well.

Structure-activity relationship (SAR) studies of synthetic glycogen synthase kinase-3β inhibitors: A critical review

Glycogen Synthase Kinase-3 (GSK-3) is a constitutively dynamic, omnipresent serine/threonine protein kinase regularly called as a "multitasking kinase" due to its pliable function in diverse signaling pathways. It exists in two isoforms i.e., GSK-3α and GSK-3β. Inhibition of GSK-3 may be useful in curing various diseases such as Alzheimer's disease, type II diabetes, mood disorders, cancers, chronic inflammatory agents, stroke, bipolar disorders and so on, but the approach poses significant challenges. Lithium was the first GSK-3β inhibitor to be used for therapeutic outcome and has been effectively used for many years. In recent years, a large number of structurally diverse potent GSK-3β inhibitors are reported. The present review focuses on the recent developments in the area of medicinal chemistry to explore the diverse chemical structures of potent GSK-3β inhibitors and also describes its structure-activity relationships (SAR) and molecular binding interactions of favorable applicability in various diseases.

Donepezil-based multi-functional cholinesterase inhibitors for treatment of Alzheimer's disease

Alzheimer's disease (AD) is one of the most common neurodegenerative disorders in elderly people. Considering the multifactorial nature of AD, the concept of multi-target-directed ligands (MTDLs) has recently emerged as a new strategy for designing therapeutic agents on AD. MTDLs are confirmed to simultaneously affect diverse targets which contribute to etiology of AD. As the most potent approved drug, donepezil affects various events of AD, like inhibiting cholinesterases activities, anti-Aβ aggregation, anti-oxidative stress et al. Modifications of donepezil or hybrids with pharmacophores of donepezil in recent five years are summarized in this article. On the basis of case studies, our concerns and opinions about development of donepezil derivatives, designing of MTDLs, and perspectives for AD treatments are discussed in final part.