Tuning Structures and Properties for Developing Novel Chemical Tools toward Distinct Pathogenic Elements in Alzheimer's Disease

Current and future therapeutic strategies for Alzheimer's disease: an overview of drug development bottlenecks

Alzheimer's disease (AD) is the most common chronic neurodegenerative disease worldwide. It causes cognitive dysfunction, such as aphasia and agnosia, and mental symptoms, such as behavioral abnormalities; all of which place a significant psychological and economic burden on the patients' families. No specific drugs are currently available for the treatment of AD, and the current drugs for AD only delay disease onset and progression. The pathophysiological basis of AD involves abnormal deposition of beta-amyloid protein (Aβ), abnormal tau protein phosphorylation, decreased activity of acetylcholine content, glutamate toxicity, autophagy, inflammatory reactions, mitochondria-targeting, and multi-targets. The US Food and Drug Administration (FDA) has approved five drugs for clinical use: tacrine, donepezil, carbalatine, galantamine, memantine, and lecanemab. We have focused on the newer drugs that have undergone clinical trials, most of which have not been successful as a result of excessive clinical side effects or poor efficacy. Although aducanumab received rapid approval from the FDA on 7 June 2021, its long-term safety and tolerability require further monitoring and confirmation. In this literature review, we aimed to explore the possible pathophysiological mechanisms underlying the occurrence and development of AD. We focused on anti-Aβ and anti-tau drugs, mitochondria-targeting and multi-targets, commercially available drugs, bottlenecks encountered in drug development, and the possible targets and therapeutic strategies for future drug development. We hope to present new concepts and methods for future drug therapies for AD.

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.

Multipronged diagnostic and therapeutic strategies for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and a major contributor to dementia cases worldwide. AD is clinically characterized by learning, memory, and cognitive deficits. The accumulation of extracellular amyloid β (Aβ) plaques and neurofibrillary tangles (NFTs) of tau are the pathological hallmarks of AD and are explored as targets for clinical diagnosis and therapy. AD pathology is poorly understood and there are no fully approved diagnosis and treatments. Notwithstanding the gap, decades of research in understanding disease mechanisms have revealed the multifactorial nature of AD. As a result, multipronged and holistic approaches are pertinent to targeting multiple biomarkers and targets for developing effective diagnosis and therapeutics. In this perspective, recent developments in Aβ and tau targeted diagnostic and therapeutic tools are discussed. Novel indirect, combination, and circulating biomarkers as potential diagnostic targets are highlighted. We underline the importance of multiplexing and multimodal detection of multiple biomarkers to generate biomarker fingerprints as a reliable diagnostic strategy. The classical therapeutics targeting Aβ and tau aggregation pathways are described with bottlenecks in the strategy. Drug discovery efforts targeting multifaceted toxicity involving protein aggregation, metal toxicity, oxidative stress, mitochondrial damage, and neuroinflammation are highlighted. Recent efforts focused on multipronged strategies to rationally design multifunctional modulators targeting multiple pathological factors are presented as future drug development strategies to discover potential therapeutics for AD.

Excited-State Intramolecular Hydrogen Transfer of Compact Molecules Controls Amyloid Aggregation Profiles

Developing chemical methodologies to directly modify harmful biomolecules affords the mitigation of their toxicity by persistent changes in their properties and structures. Here we report compact photosensitizers composed of the anthraquinone (AQ) backbone that undergo excited-state intramolecular hydrogen transfer, effectively oxidize amyloidogenic peptides, and, subsequently, alter their aggregation pathways. Density functional theory calculations showed that the appropriate position of the hydroxyl groups in the AQ backbone and the consequent intramolecular hydrogen transfer can facilitate the energy transfer to triplet oxygen. Biochemical and biophysical investigations confirmed that these photoactive chemical reagents can oxidatively vary both metal-free amyloid-β (Aβ) and metal-bound Aβ, thereby redirecting their on-pathway aggregation into off-pathway as well as disassembling their preformed aggregates. Moreover, the in vivo histochemical analysis of Aβ species produced upon photoactivation of the most promising candidate demonstrated that they do not aggregate into oligomeric or fibrillar aggregates in the brain. Overall, our combined computational and experimental studies validate a light-based approach for designing small molecules, with minimum structural complexity, as chemical reagents targeting and controlling amyloidogenic peptides associated with neurodegenerative disorders.

Effects of Linkers and Substitutions on Multitarget Directed Ligands for Alzheimer’s Diseases: Emerging Paradigms and Strategies

Alzheimer’s disease (AD) is multifactorial, progressive and the most predominant cause of cognitive impairment and dementia worldwide. The current “one-drug, one-target” approach provides only symptomatic relief to the condition but is unable to cure the disease completely. The conventional single-target therapeutic approach might not always induce the desired effect due to the multifactorial nature of AD. Hence, multitarget strategies have been proposed to simultaneously knock out multiple targets involved in the development of AD. Herein, we provide an overview of the various strategies, followed by the multitarget-directed ligand (MTDL) development, rationale designs and efficient examples. Furthermore, the effects of the linkers and substitutional functional groups on MTDLs against various targets of AD and their modes of action are also discussed.

Mechanistic Insight into the Design of Chemical Tools to Control Multiple Pathogenic Features in Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia and is characterized by memory loss and cognitive decline. Approximately 50 million people worldwide are suffering from AD and related dementias. Very recently, the first new drug targeting amyloid-β (Aβ) aggregates has been approved by the United States Food and Drug Administration, but its efficacy against AD is still debatable. Other available treatments temporarily relieve the symptoms of AD. The difficulty in discovering effective therapeutics for AD originates from its complicated nature, which results from the interrelated pathogenic pathways led by multiple factors. Therefore, to develop potent disease-modifying drugs, multiple pathological features found in AD should be fully elucidated.Our laboratory has been designing small molecules as chemical tools to investigate the individual and interrelated pathologies triggered by four pathogenic elements found in the AD-affected brain: metal-free Aβ, metal-bound Aβ, reactive oxygen species (ROS), and acetylcholinesterase (AChE). Aβ peptides are partially folded and aggregate into oligomers, protofibrils, and fibrils. Aβ aggregates are considered to be neurotoxic, causing membrane disruption, aberrant cellular signaling, and organelle dysfunction. In addition, highly concentrated metal ions accumulate in senile plaques mainly composed of Aβ aggregates, which indicates that metal ions can directly interact with Aβ. Metal binding to Aβ affects the aggregation and conformation of the peptide. Moreover, the impaired homeostasis of redox-active Fe(II/III) and Cu(I/II) induces the overproduction of ROS through Fenton chemistry and Fenton-like reactions, respectively. Dysregulated ROS prompt oxidative-stress-damaging biological components such as lipids, proteins, and nucleic acids and, consequently, lead to neuronal death. Finally, the loss of cholinergic transmission mediated by the neurotransmitter acetylcholine (ACh) contributes to cognitive deficits observed in AD.In this Account, we illustrate the design principles for small-molecule-based chemical tools with reactivities against metal-free Aβ, metal-bound Aβ, ROS, and AChE. More importantly, mechanistic details at the molecular level are highlighted with some examples of chemical tools that were developed by our group. The aggregation of metal-free Aβ can be modulated by modifying amino acid residues responsible for self-assembling Aβ or disassembling preformed fibrils. To alter the aggregation and cytotoxicity profiles of metal-bound Aβ, ternary complexation, metal chelation, and modifications onto metal-binding residues can be effective tactics. The presence and production of ROS are able to be controlled by small molecules with antioxidant and metal-binding properties. Finally, inhibiting substrate access or substrate binding at the active site of AChE can diminish its activity, which restores the levels of ACh. Overall, our rational approaches demonstrate the feasibility of developing small molecules as chemical tools that can target and modulate multiple pathological factors associated with AD and can be useful for gaining a greater understanding of the multifaceted pathology of the disease.

Drug repurposing: small molecules against Cu(II)-amyloid-β and free radicals

Alzheimer's disease (AD) presents a complex pathology entangling numerous pathological factors, including amyloid-β (Aβ), metal ions, and reactive oxygen species (ROS). Increasing evidence reveals pathological connections among these distinct components in AD. For instance, the association between the amyloid cascade and metal ion hypotheses has introduced a novel pathogenic target: metal-bound Aβ. Investigation of such interconnections requires substantial research and can be expedited by chemical reagents that are able to modify multiple pathogenic factors in AD. Drug repurposing is an efficient approach for rediscovering previously utilized molecules with desirable biological and pharmaceutical properties as chemical reagents. Herein, we report the evaluation of three pre-approved drug molecules, selected based on their chemical structure and properties, as chemical reagents that can be used for elucidating the complicated pathology of AD.

Structural exploration of multifunctional monoamine oxidase B inhibitors as potential drug candidates against Alzheimer's disease

AD is one of the most typical neurodegenerative disorders that suffer many seniors worldwide. Recently, MAO inhibitors have received increasing attention not only for their roles involved in monoamine neurotransmitters metabolism and oxidative stress but also for their additional neuroprotective and neurorescue effects against AD. The curiosity in MAO inhibitors is reviving, and novel MAO-B inhibitors recently developed with ancillary activities (e.g., Aβ aggregation and AChE inhibition, anti-ROS and chelating activities) have been proposed as multitarget drugs foreshadowing a positive outlook for the treatment of AD. The current review describes the recent development of the design, synthesis, and screening of multifunctional ligands based on MAO-B inhibition for AD therapy. Structure-activity relationships and rational design strategies of the synthetic or natural product derivatives (chalcones, coumarins, chromones, and homoisoflavonoids) are discussed.

Cell Death via Lipid Peroxidation and Protein Aggregation Diseases

Lipid peroxidation of cellular membranes is a complicated cellular event, and it is both the cause and result of various diseases, such as ischemia-reperfusion injury, neurodegenerative diseases, and atherosclerosis. Lipid peroxidation causes non-apoptotic cell death, which is associated with cell fate determination: survival or cell death. During the radical chain reaction of lipid peroxidation, various oxidized lipid products accumulate in cells, followed by organelle dysfunction and the induction of non-apoptotic cell death. Highly reactive oxidized products from unsaturated fatty acids are detected under pathological conditions. Pathological protein aggregation is the general cause of these diseases. The cellular response to misfolded proteins is well-known as the unfolded protein response (UPR) and it is partially concomitant with the response to lipid peroxidation. Moreover, the association between protein aggregation and non-apoptotic cell death by lipid peroxidation is attracting attention. The link between lipid peroxidation and protein aggregation is a matter of concern in biomedical fields. Here, we focus on lethal protein aggregation in non-apoptotic cell death via lipid peroxidation. We reviewed the roles of protein aggregation in the initiation and execution of non-apoptotic cell death. We also considered the relationship between protein aggregation and oxidized lipid production. We provide an overview of non-apoptotic cell death with a focus on lipid peroxidation for therapeutic targeting during protein aggregation diseases.

Copper Toxicity Links to Pathogenesis of Alzheimer's Disease and Therapeutics Approaches

Alzheimer's disease (AD) is an irreversible, age-related progressive neurological disorder, and the most common type of dementia in aged people. Neuropathological lesions of AD are neurofibrillary tangles (NFTs), and senile plaques comprise the accumulated amyloid-beta (Aβ), loaded with metal ions including Cu, Fe, or Zn. Some reports have identified metal dyshomeostasis as a neurotoxic factor of AD, among which Cu ions seem to be a central cationic metal in the formation of plaque and soluble oligomers, and have an essential role in the AD pathology. Cu-Aβ complex catalyzes the generation of reactive oxygen species (ROS) and results in oxidative damage. Several studies have indicated that oxidative stress plays a crucial role in the pathogenesis of AD. The connection of copper levels in AD is still ambiguous, as some researches indicate a Cu deficiency, while others show its higher content in AD, and therefore there is a need to increase and decrease its levels in animal models, respectively, to study which one is the cause. For more than twenty years, many in vitro studies have been devoted to identifying metals' roles in Aβ accumulation, oxidative damage, and neurotoxicity. Towards the end, a short review of the modern therapeutic approach in chelation therapy, with the main focus on Cu ions, is discussed. Despite the lack of strong proofs of clinical advantage so far, the conjecture that using a therapeutic metal chelator is an effective strategy for AD remains popular. However, some recent reports of genetic-regulating copper transporters in AD models have shed light on treating this refractory disease. This review aims to succinctly present a better understanding of Cu ions' current status in several AD features, and some conflicting reports are present herein.

Multiple reactivities of flavonoids towards pathological elements in Alzheimer's disease: structure-activity relationship

Amyloid-β (Aβ) accumulation, metal ion dyshomeostasis, oxidative stress, and cholinergic deficit are four major characteristics of Alzheimer's disease (AD). Herein, we report the reactivities of 12 flavonoids against four pathogenic elements of AD: metal-free and metal-bound Aβ, free radicals, and acetylcholinesterase. A series of 12 flavonoids was selected based on the molecular structures that are responsible for multiple reactivities including hydroxyl substitution and transfer of the B ring from C2 to C3. Our experimental and computational studies reveal that the catechol moiety, the hydroxyl groups at C3 and C7, and the position of the B ring are important for instilling multiple functions in flavonoids. We establish a structure-activity relationship of flavonoids that should be useful for designing chemical reagents with multiple reactivities against the pathological factors of AD.

Minimalistic Principles for Designing Small Molecules with Multiple Reactivities against Pathological Factors in Dementia

Multiple pathogenic elements, including reactive oxygen species, amyloidogenic proteins, and metal ions, are associated with the development of neurodegenerative disorders. We report minimalistic redox-based principles for preparing compact aromatic compounds by derivatizing the phenylene moiety with various functional groups. These molecular agents display enhanced reactivities against multiple targets such as free radicals, metal-free amyloid-β (Aβ), and metal-bound Aβ that are implicated in the most common form of dementia, Alzheimer's disease (AD). Mechanistic studies reveal that the redox properties of these reagents are essential for their function. Specifically, they engage in oxidative reactions with metal-free and metal-bound Aβ, leading to chemical modifications of the Aβ peptides to form covalent adducts that alter the aggregation of Aβ. Moreover, the administration of the most promising candidate significantly attenuates the amyloid pathology in the brains of AD transgenic mice and improves their cognitive defects. Our studies demonstrate an efficient and effective redox-based strategy for incorporating multiple functions into simple molecular reagents.

Mechanistic approaches for chemically modifying the coordination sphere of copper-amyloid-β complexes

Neurotoxic implications of the interactions between Cu(I/II) and amyloid-β (Aβ) indicate a connection between amyloid cascade hypothesis and metal ion hypothesis with respect to the neurodegeneration associated with Alzheimer's disease (AD). Herein, we report a mechanistic strategy for modifying the first coordination sphere of Cu(II) bound to Aβ utilizing a rationally designed peptide modifier, L1. Upon reacting with L1, a metal-binding histidine (His) residue, His14, in Cu(II)-Aβ was modified through either covalent adduct formation, oxidation, or both. Consequently, the reactivity of L1 with Cu(II)-Aβ was able to disrupt binding of Cu(II) to Aβ and result in chemically modified Aβ with altered aggregation and toxicity profiles. Our molecular-level mechanistic studies revealed that such L1-mediated modifications toward Cu(II)-Aβ could stem from the molecule's ability to 1) interact with Cu(II)-Aβ and 2) foster copper-O₂ chemistry. Collectively, our work demonstrates the development of an effective approach to modify Cu(II)-Aβ at a metal-binding amino acid residue and consequently alter Aβ's coordination to copper, aggregation, and toxicity, supplemented with an in-depth mechanistic perspective regarding such reactivity.

Tuning Properties for Blood-Brain Barrier Permeation: A Statistics-Based Analysis

In the effort to define a set of rules useful in tuning the properties for a successful blood-brain barrier (BBB) permeation, we statistically analyzed a set of 328 compounds and correlated their experimental in vivo logBB with a series of computed descriptors. Contingency tables were constructed, observed and expected distributions were calculated, and chi-square (χ²) distributions were evaluated. This allowed to point out a significant dependence of certain physicochemical properties in influencing the BBB permeation. Of over 15 computed descriptors, 9 resulted to be particularly important showing highly significant χ² distribution: polar surface area (χ² = 66.79; p = 1.08 × 10^-13), nitrogen and oxygen count (χ² = 51.17; p = 2.06 × 10^-10), logP (χ² = 47.38; p = 1.27 × 10^-9), nitrogen count (χ² = 38.29; p = 9.77 × 10^-8), logD (χ² = 36.80; p = 36.80), oxygen count (χ² = 35.83; p = 3.13 × 10^-7), ionization state (χ² = 33.02, p = 3.19 × 10^-7), hydrogen bond acceptors (χ² = 30.80; p = 3.36 × 10^-6), and hydrogen bond donors (χ² = 29.29; p = 6.81 × 10^-6). Other parameters describing the mass and size of the molecules (molecular weight: 11.18; p = 2.46 × 10^-2) resulted in being not significant since the population within the observed and expected distribution was similar. Depending on the combination of the significant descriptors, we set a three cases probabilistic scenario (BBB⁺, BBB^-, BBB⁺/BBB^-) that would prospectively be used to tune properties for BBB permeation.

Chemical strategies to modify amyloidogenic peptides using iridium(iii) complexes: coordination and photo-induced oxidation

Effective chemical strategies, i.e., coordination and coordination-/photo-mediated oxidation, are rationally developed towards modification of amyloidogenic peptides and subsequent control of their aggregation and toxicity.

Amyloidogenic peptides are considered central pathological contributors towards neurodegeneration as observed in neurodegenerative disorders [e.g., amyloid-β (Aβ) peptides in Alzheimer's disease (AD)]; however, their roles in the pathologies of such diseases have not been fully elucidated since they are challenging targets to be studied due to their heterogeneous nature and intrinsically disordered structure. Chemical approaches to modify amyloidogenic peptides would be valuable in advancing our molecular-level understanding of their involvement in neurodegeneration. Herein, we report effective chemical strategies for modification of Aβ peptides (i.e., coordination and coordination-/photo-mediated oxidation) implemented by a single Ir(iii) complex in a photo-dependent manner. Such peptide variations can be achieved by our rationally designed Ir(iii) complexes (Ir-Me, Ir-H, Ir-F, and Ir-F2) leading to significantly modulating the aggregation pathways of two main Aβ isoforms, Aβ₄₀ and Aβ₄₂, as well as the production of toxic Aβ species. Overall, we demonstrate chemical tactics for modification of amyloidogenic peptides in an effective and manageable manner utilizing the coordination capacities and photophysical properties of transition metal complexes.

Cu and Zn interactions with Aβ peptides: consequence of coordination on aggregation and formation of neurotoxic soluble Aβ oligomers

The coordination chemistry of transition metal ions (Fe, Cu, Zn) with the amyloid-β (Aβ) peptides has attracted a lot of attention in recent years due to its repercussions in Alzheimer's disease (AD).

Regulatory Activities of Dopamine and Its Derivatives toward Metal-Free and Metal-Induced Amyloid-β Aggregation, Oxidative Stress, and Inflammation in Alzheimer's Disease

A catecholamine neurotransmitter, dopamine (DA), is suggested to be linked to the pathology of dementia; however, the involvement of DA and its structural analogues in the pathogenesis of Alzheimer's disease (AD), the most common form of dementia, composed of multiple pathogenic factors has not been clear. Herein, we report that DA and its rationally designed structural derivatives (1-6) based on DA's oxidative transformation are able to modulate multiple pathological elements found in AD [i.e., metal ions, metal-free amyloid-β (Aβ), metal-bound Aβ (metal-Aβ), and reactive oxygen species (ROS)], with demonstration of detailed molecular-level mechanisms. Our multidisciplinary studies validate that the protective effects of DA and its derivatives on Aβ aggregation and Aβ-mediated toxicity are induced by their oxidative transformation with concomitant ROS generation under aerobic conditions. In particular, DA and the derivatives (i.e., 3 and 4) show their noticeable anti-amyloidogenic ability toward metal-free Aβ and/or metal-Aβ, verified to occur via their oxidative transformation that facilitates Aβ oxidation. Moreover, in primary pan-microglial marker (CD11b)-positive cells, the major producers of inflammatory mediators in the brain, DA and its derivatives significantly diminish inflammation and oxidative stress triggered by lipopolysaccharides and Aβ through the reduced induction of inflammatory mediators as well as upregulated expression of heme oxygenase-1, the enzyme responsible for production of antioxidants. Collectively, we illuminate how DA and its derivatives could prevent multiple pathological features found in AD. The overall studies could advance our understanding regarding distinct roles of neurotransmitters in AD and identify key interactions for alleviation of AD pathology.