New neurogenic lipoic-based hybrids as innovative Alzheimer's drugs with σ-1 agonism and β-secretase inhibition

Lipoic acid scaffold applications in the design of multitarget-directed ligands against Alzheimer's disease

Alzheimer's disease (AD) is becoming a fast-growing public health problem which can result in psychological problems as well as loss of speech, language, short-term memory, and motor coordination. Many medications were developed and produced to treat AD, however due to the complexity of the pathology involved in the illness, many of these medications often failed in clinical or preclinical studies. The main issue with the current anti-AD medications is their low efficacy since they use a single target. Multi-target-directed ligands (MTDLs) based on "one molecule; multiple targets" have been introduced to address these two fundamental issues. MTDLs have demonstrated improved efficacy and safety since they regulate many biological targets simultaneously. Alpha-lipoic acid (LA), a natural molecule with distinct properties, is a viable scaffold for developing new MTDLs in treating many neurodegenerative diseases, particularly AD. It is a key mitochondrial enzymes' cofactor and an organic molecule with disulfide functionality. It also has potent antioxidant characteristics that enhance mitochondrial activity. Considering the neuroprotective and anti-inflammatory effects of LA, various hybrids of LA with tacrine, rivastigmine, coumarin and chromone, ibuprofen, melatonin, niacin have been synthesized and biologically evaluated as the MTDLs. In this article, we review the design of LA-based hybrids or conjugates, their biological activities, and structure-activity relationship studies, to develop new MTDLs in the field of AD pharmacotherapy.

Discovery of a potent melatonin-based inhibitor of quinone reductase-2 with neuroprotective and neurogenic properties

5-Methoxy-3-(5-methoxyindolin-2-yl)-1H-indole (3), whose structure was unambiguously elucidated by X-ray analysis, was identified as a multi-target compound with potential application in neurodegenerative diseases. It is a low nanomolar inhibitor of QR2 (IC50 = 7.7 nM), with greater potency than melatonin and comparable efficacy to the most potent QR2 inhibitors described to date. Molecular docking studies revealed the potential binding mode of 3 to QR2, which explains its superior potency compared to melatonin. Furthermore, compound 3 inhibits hMAO-A, hMAO-B and hLOX-5 in the low micromolar range and is an excellent ROS scavenger. In phenotypic assays, compound 3 showed neuroprotective activity in a cellular model of oxidative stress damage, it was non-toxic, and was able to activate neurogenesis from neural stem-cell niches of adult mice. These excellent biological properties, together with its both good in silico and in vitro drug-like profile, highlight compound 3 as a promising drug candidate for neurodegenerative diseases.

Synthesis and evaluation of lipoic acid - donepezil hybrids for Alzheimer's disease using a straightforward strategy

Alzheimer's disease is associated with a progressive loss of neurons and synaptic connections in the cholinergic system. Oxidative stress contributes to neuronal damages and to the development of amyloid plaques and neurofibrillary tangles. Therefore, antioxidants have been widely studied to mitigate the progression of Alzheimer's disease, and among these, lipoic acid has demonstrated a neuroprotective effect. Here, we present the synthesis, the molecular modelling, and the evaluation of lipoic acid-donepezil hybrids based on O-desmethyldonepezil. As compounds 5 and 6 display a high inhibition of acetylcholinesterase (IC50 = 7.6 nM and 9.1 nM, respectively), selective against butyrylcholinesterase, and a notable neuroprotective effect, slightly better than that of lipoic acid, the present study suggests that O-desmethyldonepezil could serve as a platform for the straightforward design of donepezil hybrids.

Thioxanthenone-based derivatives as multitarget therapeutic leads for Alzheimer's disease

A set of twenty-five thioxanthene-9-one and xanthene-9-one derivatives, that were previously shown to inhibit cholinesterases (ChEs) and amyloid β (Aβ₄₀) aggregation, were evaluated for the inhibition of tau protein aggregation. All compounds exhibited a good activity, and eight of them (5-8, 10, 14, 15 and 20) shared comparable low micromolar inhibitory potency versus Aβ₄₀ aggregation and human acetylcholinesterase (AChE), while inhibiting human butyrylcholinesterase (BChE) even at submicromolar concentration. Compound 20 showed outstanding biological data, inhibiting tau protein and Aβ₄₀ aggregation with IC₅₀ = 1.8 and 1.3 μM, respectively. Moreover, at 0.1-10 μM it also exhibited neuroprotective activity against tau toxicity induced by okadoic acid in human neuroblastoma SH-SY5Y cells, that was comparable to that of estradiol and PD38. In preliminary toxicity studies, these interesting results for compound 20 are somewhat conflicting with a narrow safety window. However, compound 10, although endowed with a little lower potency for tau and Aβ aggregation inhibition additionally demonstrated good inhibition of ChEs and rather low cytotoxicity. Compound 4 is also worth of note for its high potency as hBChE inhibitor (IC₅₀ = 7 nM) and for the three order of magnitude selectivity versus hAChE. Molecular modelling studies were performed to explain the different behavior of compounds 4 and 20 towards hBChE. The observed balance of the inhibitory potencies versus the relevant targets indicates the thioxanthene-9-one derivatives as potential MTDLs for AD therapy, provided that the safety window will be improved by further structural variations, currently under investigation.

Multi-Target Directed Ligands (MTDLs) Binding the σ1 Receptor as Promising Therapeutics: State of the Art and Perspectives

The sigma-1 (σ1) receptor is a 'pluripotent chaperone' protein mainly expressed at the mitochondria-endoplasmic reticulum membrane interfaces where it interacts with several client proteins. This feature renders the σ1 receptor an ideal target for the development of multifunctional ligands, whose benefits are now recognized because several pathologies are multifactorial. Indeed, the current therapeutic regimens are based on the administration of different classes of drugs in order to counteract the diverse unbalanced physiological pathways associated with the pathology. Thus, the multi-targeted directed ligand (MTDL) approach, with one molecule that exerts poly-pharmacological actions, may be a winning strategy that overcomes the pharmacokinetic issues linked to the administration of diverse drugs. This review aims to point out the progress in the development of MTDLs directed toward σ1 receptors for the treatment of central nervous system (CNS) and cancer diseases, with a focus on the perspectives that are proper for this strategy. The evidence that some drugs in clinical use unintentionally bind the σ1 protein (as off-target) provides a proof of concept of the potential of this strategy, and it strongly supports the promise that the σ1 receptor holds as a target to be hit in the context of MTDLs for the therapy of multifactorial pathologies.

Studies on the affinity of 6-[(n-(cyclo)aminoalkyl)oxy]-4H-chromen-4-ones for sigma 1/2 receptors

Sigma (σ) receptors represent attractive targets for the development of potential agents for the treatment of several disorders, including Alzheimer's disease and neuropathic pain. In the search for multitarget small molecules (MSMs) against such disorders, we have re-discovered chromenones as new affine σ₁/σ₂ ligands. 6-(4-(Piperidin-1-yl)butoxy)-4H-chromen-4-one (7), a previously identified MSM with potent dual-target activities against acetylcholinesterase and monoamine oxidase B, also exhibited σ₁/σ₂ affinity. 6-(3-(Azepan-1-yl)propoxy)-4H-chromen-4-one (20) showed a K _i value for σ₁ of 27.2 nM (selectivity (σ₁/σ₂) = 28), combining the desired σ₁ receptor affinity with a dual inhibitory capacity against both acetyl- and butyrylcholinesterase. 6-((5-Morpholinopentyl)oxy)-4H-chromen-4-one (12) was almost equipotent to S1RA, an established σ₁ receptor antagonist.

Beyond Antioxidant Effects: Nature-Based Templates Unveil New Strategies for Neurodegenerative Diseases

The complex network of malfunctioning pathways occurring in the pathogenesis of neurodegenerative diseases (NDDs) represents a huge hurdle in the development of new effective drugs to be used in therapy. In this context, redox reactions act as crucial regulators in the maintenance of neuronal microenvironment homeostasis. Particularly, their imbalance results in the severe compromising of organism’s natural defense systems and subsequently, in the instauration of deleterious OS, that plays a fundamental role in the insurgence and progress of NDDs. Despite the huge efforts in drug discovery programs, the identification process of new therapeutic agents able to counteract the relentless progress of neurodegenerative processes has produced low or no effective therapies. Consequently, a paradigm-shift in the drug discovery approach for these diseases is gradually occurring, paving the way for innovative therapeutical approaches, such as polypharmacology. The aim of this review is to provide an overview of the main pharmacological features of most promising nature-based scaffolds for a possible application in drug discovery, especially for NDDs, highlighting their multifaceted effects against OS and neuronal disorders.

The recent development of donepezil structure-based hybrids as potential multifunctional anti-Alzheimer's agents: highlights from 2010 to 2020

Dementia is a term used to define different brain disorders that affect memory, thinking, behavior, and emotion. Alzheimer's disease (AD) is the second cause of dementia that is generated by the death of cholinergic neurons (especially acetylcholine (ACh)), which have a vital role in cognition. Acetylcholinesterase inhibitors (AChEI) affect acetylcholine levels in the brain and are broadly used to treat Alzheimer's. Donepezil, rivastigmine, and galantamine, which are FDA-approved drugs for AD, are cholinesterase inhibitors. In addition, scientists are attempting to develop hybrid molecules and multi-target-directed ligands (MTDLs) that can simultaneously modulate multiple biological targets. This review highlights recent examples of MTDLs and fragment-based strategy in the rational design of new potential AD medications from 2010 onwards.

This review highlights recent examples of multi-target-directed ligands (MTDLs) based on donepezil structure modification from 2010 onwards.

Dual-targeting Approach on Histamine H3 and Sigma-1 Receptor Ligands as Promising Pharmacological Tools in the Treatment of CNS-linked Disorders

With the recent market approval of Pitolisant (Wakix®), the interest in clinical application for novel multifunctional histamine H3 receptor antagonists has clearly increased. Several combinations of different H3R pharmacophores with pharmacophoric elements of other G-protein coupled receptors, transporters, or enzymes have been synthesized by numerous pharmaceutical companies and academic institutions. Since central nervous system disorders are characterized by diverse physiological dysfunctions and deregulations of a complex network of signaling pathways, optimal multipotent drugs should simultaneously and peculiarly modulate selected groups of biological targets. Interestingly, very recent studies have shown that some clinically evaluated histamine H3 receptor antagonists possess a nanomolar affinity for sigma-1 receptor binding sites, suggesting that this property might play a role in their overall efficacy. The sigma-1 receptor, unusual and yet obscure protein, is supposed to be involved in numerous CNS pathologies through neuroprotection and neuroplasticity. These two different biological structures, histamine H3 and sigma-1 receptors, combined, can represent a potential fruitful target for therapeutic developments in tackling numerous human diseases.

Small Molecules Selectively Targeting Sigma-1 Receptor for the Treatment of Neurological Diseases

The sigma-1 (σ1) receptor, an enigmatic protein originally classified as an opioid receptor subtype, is now understood to possess unique structural and functional features of its own and play critical roles to widely impact signaling transduction by interacting with receptors, ion channels, lipids, and kinases. The σ1 receptor is implicated in modulating learning, memory, emotion, sensory systems, neuronal development, and cognition and accordingly is now an actively pursued drug target for various neurological and neuropsychiatric disorders. Evaluation of the five selective σ1 receptor drug candidates (pridopidine, ANAVEX2-73, SA4503, S1RA, and T-817MA) that have entered clinical trials has shown that reaching clinical approval remains an evasive and important goal. This review provides up-to-date information on the selective targeting of σ1 receptors, including their history, function, reported crystal structures, and roles in neurological diseases, as well as a useful collation of new chemical entities as σ1 selective orthosteric ligands or allosteric modulators.

Novel small molecule therapeutic agents for Alzheimer disease: Focusing on BACE1 and multi-target directed ligands

Alzheimer's Disease (AD) is a progressive neurodegenerative disorder that effects 50 million people worldwide. In this review, AD pathology and the development of novel therapeutic agents targeting AD were fully discussed. In particular, common approaches to prevent Aβ production and/or accumulation in the brain including α-secretase activators, specific γ-secretase modulators and small molecules BACE1 inhibitors were reviewed. Additionally, natural-origin bioactive compounds that provide AD therapeutic advances have been introduced. Considering AD is a multifactorial disease, the therapeutic potential of diverse multi target-directed ligands (MTDLs) that combine the efficacy of cholinesterase (ChE) inhibitors, MAO (monoamine oxidase) inhibitors, BACE1 inhibitors, phosphodiesterase 4D (PDE4D) inhibitors, for the treatment of AD are also reviewed. This article also highlights descriptions on the regulator of serotonin receptor (5-HT), metal chelators, anti-aggregants, antioxidants and neuroprotective agents targeting AD. Finally, current computational methods for evaluating the structure-activity relationships (SAR) and virtual screening (VS) of AD drugs are discussed and evaluated.

New flavonoid - N,N-dibenzyl(N-methyl)amine hybrids: Multi-target-directed agents for Alzheimer´s disease endowed with neurogenic properties

The design of multi-target directed ligands (MTDLs) is a valid approach for obtaining effective drugs for complex pathologies. MTDLs that combine neuro-repair properties and block the first steps of neurotoxic cascades could be the so long wanted remedies to treat neurodegenerative diseases (NDs). By linking two privileged scaffolds with well-known activities in ND-targets, the flavonoid and the N,N-dibenzyl(N-methyl)amine (DBMA) fragments, new CNS-permeable flavonoid - DBMA hybrids (1-13) were obtained. They were subjected to biological evaluation in a battery of targets involved in Alzheimer's disease (AD) and other NDs, namely human cholinesterases (hAChE/hBuChE), β-secretase (hBACE-1), monoamine oxidases (hMAO-A/B), lipoxygenase-5 (hLOX-5) and sigma receptors (σ₁R/σ₂R). After a funnel-type screening, 6,7-dimethoxychromone - DBMA (6) was highlighted due to its neurogenic properties and an interesting MTD-profile in hAChE, hLOX-5, hBACE-1 and σ₁R. Molecular dynamic simulations showed the most relevant drug-protein interactions of hybrid 6, which could synergistically contribute to neuronal regeneration and block neurodegeneration.

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.

Neuroprotection in non-transgenic and transgenic mouse models of Alzheimer's disease by positive modulation of σ1 receptors

The sigma-1 (σ1) receptor is an endoplasmic reticulum (ER) chaperone protein, enriched in mitochondria-associated membranes. Its activation triggers physiological responses to ER stress and modulate Ca2+ mobilization in mitochondria. Small σ1 agonist molecules activate the protein and act behaviorally as antidepressant, anti-amnesic and neuroprotective agents. Recently, several chemically unrelated molecules were shown to be σ1 receptor positive modulators (PMs), with some of them a clear demonstration of their allostericity. We here examined whether a σ1 PM also shows neuroprotective potentials in pharmacological and genetic models of Alzheimer's disease (AD). For this aim, we describe (±)-2-(3-chlorophenyl)-3,3,5,5-tetramethyl-2-oxo-[1,4,2]-oxazaphosphinane (OZP002) as a novel σ1 PM. OZP002 does not bind σ1 sites but induces σ1 effects in vivo and boosts σ1 agonist activity. OZP002 was antidepressant in the forced swim test and its effect was blocked by the σ1 antagonist NE-100 or in σ1 receptor knockout mice. It potentiated the antidepressant effect of the σ1 agonist igmesine. In mice tested for Y-maze alternation or passive avoidance, OZP002 prevented scopolamine-induced learning deficits, in a NE-100 sensitive manner. Pre-administered IP before an ICV injection of amyloid Aβ25-35 peptide, a pharmacological model of Alzheimer's disease, OZP002 prevented the learning deficits induced by the peptide after one week in the Y-maze, passive avoidance and novel object tests. Biochemical analyses of the mouse hippocampi showed that OZP002 significantly decreased Aβ25-35-induced increases in reactive oxygen species, lipid peroxidation, and increases in Bax, TNFα and IL-6 levels. Immunohistochemically, OZP002 prevented Aβ25-35-induced reactive astrogliosis and microgliosis in the hippocampus. It also alleviated Aβ25-35-induced decreases in synaptophysin level and choline acetyltransferase activity. Moreover, chronically administered in APPswe mice during 2 months, OZP002 prevented learning deficits (in all tests plus place learning in the water-maze) and increased biochemical markers. This study shows that σ1 PM with high neuropotective potential can be identified, combining pharmacological efficacy, selectivity and therapeutic safety, and identifies a novel promising compound, OZP002.

Targeting Inflammatory Pathways in Alzheimer's Disease: A Focus on Natural Products and Phytomedicines

Studies of the brains of Alzheimer's disease (AD) patients have revealed key neuropathological features, such as the deposition of aggregates of insoluble amyloid-β (Aβ) peptides and neurofibrillary tangles (NFTs). These pathological protein deposits, including Aβ peptides (which form senile plaques) and hyperphosphorylated tau (which aggregates into NFTs), have been assumed to be 'the cause of AD'. Aβ has been extensively targeted to develop an effective disease-modifying therapy, but with limited clinical success. Emerging therapies are also now targeting further pathological processes in AD, including neuroinflammation. This review focuses on the inflammatory and oxidative stress-related changes that occur in AD, and discusses some emerging anti-inflammatory natural products and phytomedicines. Many of the promising compounds are cytokine-suppressive anti-inflammatory drugs (CSAIDs), which target the proinflammatory AP1 and nuclear factor-κB signalling pathways and inhibit the expression of many proinflammatory cytokines, such as interleukin (IL)-1, IL-6, tumour necrosis factor-α, or nitric oxide produced by inducible nitric oxide synthase. However, many of these phytomedicines have not been tested in rigorous clinical trials in AD patients. It is not yet clear if the active compounds reach an effective concentration in the brain (due to limited bioavailability) or if they can slow down AD progression in long-term trials. The authors suggest that it is crucial for both the pharmacological and complementary medicine industries to conduct and fund those studies to significantly advance the field.

Neurogenic and neuroprotective donepezil-flavonoid hybrids with sigma-1 affinity and inhibition of key enzymes in Alzheimer's disease

In this work we describe neurogenic and neuroprotective donepezil-flavonoid hybrids (DFHs), exhibiting nanomolar affinities for the sigma-1 receptor (σ1R) and inhibition of key enzymes in Alzheimer's disease (AD), such as acetylcholinesterase (AChE), 5-lipoxygenase (5-LOX), and monoamine oxidases (MAOs). In general, new compounds scavenge free radical species, are predicted to be brain-permeable, and protect neuronal cells against mitochondrial oxidative stress. N-(2-(1-Benzylpiperidin-4-yl)ethyl)-6,7-dimethoxy-4-oxo-4H-chromene-2-carboxamide (18) is highlighted due to its interesting biological profile in σ1R, AChE, 5-LOX, MAO-A and MAO-B. In phenotypic assays, it protects a neuronal cell line against mitochondrial oxidative stress and promotes maturation of neural stem cells into a neuronal phenotype, which could contribute to the reparation of neuronal tissues. Molecular modelling studies of 18 in AChE, 5-LOX and σ1R revealed the main interactions with these proteins, which will be further exploited in the optimization of new, more efficient DFHs.

Profiling donepezil template into multipotent hybrids with antioxidant properties

Alzheimer’s disease is debilitating neurodegenerative disorder in the elderly. Current therapy relies on administration of acetylcholinesterase inhibitors (AChEIs) -donepezil, rivastigmine, galantamine, and N-methyl-d-aspartate receptor antagonist memantine. However, their therapeutic effect is only short-term and stabilizes cognitive functions for up to 2 years. Given this drawback together with other pathological hallmarks of the disease taken into consideration, novel approaches have recently emerged to better cope with AD onset or its progression. One such strategy implies broadening the biological profile of AChEIs into so-called multi-target directed ligands (MTDLs). In this review article, we made comprehensive literature survey emphasising on donepezil template which was structurally converted into plethora of MTLDs preserving anti-cholinesterase effect and, at the same time, escalating the anti-oxidant potential, which was reported as a crucial role in the pathogenesis of the Alzheimer’s disease.

Novel Sigma-1 receptor antagonists: from opioids to small molecules: what is new?

Sigma-1 (σ1) receptor has been identified as a chaperone protein that interacts with other proteins, such as N-methyl-D-aspartate (NMDA) and opioid receptors, modulating their activity. σ1 receptor antagonists have been developed to obtain useful compounds for the treatment of psychoses, pain, drug abuse and cancer. Some interesting compounds such as E-5842 (5) and MS-377 (24), haloperidol and piperazine derivatives, respectively, were endowed with high affinity for σ1 receptors (Ki σ1 = 4 and 73 nM; Ki σ2 = 220 and 6900, respectively). They were developed for the treatment of psychotic disorders and 5 also underwent Phase II clinical trials suggesting interesting potential therapeutic applications. Here, σ1 receptor antagonists have been grouped based on chemical structure and reviewed according to structure-activity relationship and potential therapeutic role.

Synthesis and evaluation of haloperidol metabolite II prodrugs as anticancer agents

The use of haloperidol metabolite II (HP-metabolite II) prodrugs is an emerging strategy in the treatment of cancer. HP-metabolite II exhibits antiproliferative properties at micromolar concentrations inducing apoptosis in different types of cancer. Thus, the application of the prodrug approach appears as a useful method leading to much more desirable pharmacokinetic and pharmacodynamic properties. Some studies have shown that the esterification of the hydroxyl group of HP-metabolite II with 4-phenylbutiric acid (4-PBA) or valproic acid enhances the anticancer therapeutic potency. The current progresses in the design, synthesis and evaluation of anticancer activity of HP metabolite II prodrugs will be discussed in this review.

Recent Advances in Neurogenic Small Molecules as Innovative Treatments for Neurodegenerative Diseases

The central nervous system of adult mammals has long been considered as a complex static structure unable to undergo any regenerative process to refurbish its dead nodes. This dogma was challenged by Altman in the 1960s and neuron self-renewal has been demonstrated ever since in many species, including humans. Aging, neurodegenerative, and some mental diseases are associated with an exponential decrease in brain neurogenesis. Therefore, the controlled pharmacological stimulation of the endogenous neural stem cells (NSCs) niches might counteract the neuronal loss in Alzheimer’s disease (AD) and other pathologies, opening an exciting new therapeutic avenue. In the last years, druggable molecular targets and signalling pathways involved in neurogenic processes have been identified, and as a consequence, different drug types have been developed and tested in neuronal plasticity. This review focuses on recent advances in neurogenic agents acting at serotonin and/or melatonin systems, Wnt/β-catenin pathway, sigma receptors, nicotinamide phosphoribosyltransferase (NAMPT) and nuclear erythroid 2-related factor (Nrf2).