Challenges and Opportunities of Metal Chelation Therapy in Trace Metals Overload-Induced Alzheimer's Disease

Essential trace metals like zinc (Zn), iron (Fe), and copper (Cu) play an important physiological role in the metabolomics and healthy functioning of body organs, including the brain. However, abnormal accumulation of trace metals in the brain and dyshomeostasis in the different regions of the brain have emerged as contributing factors in neuronal degeneration, Aβ aggregation, and Tau formation. The link between these essential trace metal ions and the risk of AD has been widely studied, although the conclusions have been ambiguous. Despite the absence of evidence for any clinical benefit, therapeutic chelation is still hypothesized to be a therapeutic option for AD. Furthermore, the parameters like bioavailability, ability to cross the BBB, and chelation specificity must be taken into consideration while selecting a suitable chelation therapy. The data in this review summarizes that the primary intervention in AD is brain metal homeostasis along with brain metal scavenging. This review evaluates the impact of different trace metals (Cu, Zn, Fe) on normal brain functioning and their association with neurodegeneration in AD. Also, it investigates the therapeutic potential of metal chelators in the management of AD. An extensive literature search was carried out on the "Web of Science, PubMed, Science Direct, and Google Scholar" to investigate the effect of trace elements in neurological impairment and the role of metal chelators in AD. In addition, the current review highlights the advantages and limitations of chelation therapies and the difficulties involved in developing selective metal chelation therapy in AD patients.

Synthesis, physicochemical characterization and neuroprotective evaluation of novel 1-hydroxypyrazin-2(1H)-one iron chelators in an in vitro cell model of Parkinson's disease

Iron dysregulation, dopamine depletion, cellular oxidative stress and α-synuclein protein mis-folding are key neuronal pathological features seen in the progression of Parkinson's disease. Iron chelators endowed with one or more therapeutic modes of action have long been suggested as disease modifying therapies for its treatment. In this study, novel 1-hydroxypyrazin-2(1H)-one iron chelators were synthesized and their physicochemical properties, iron chelation abilities, antioxidant capacities and neuroprotective effects in a cell culture model of Parkinson's disease were evaluated. Physicochemical properties (log β, log D7.4, pL0.5) suggest that these ligands have a poorer ability to penetrate cell membranes and form weaker iron complexes than the closely related 1-hydroxypyridin-2(1H)-ones. Despite this, we show that levels of neuroprotection provided by these ligands against the catecholaminergic neurotoxin 6-hydroxydopamine in vitro were comparable to those seen previously with the 1-hydroxypyridin-2(1H)-ones and the clinically used iron chelator Deferiprone, with two of the ligands restoring cell viability to ≥89% compared to controls. Two of the ligands were endowed with additional phenol moieties in an attempt to derive multifunctional chelators with dual iron chelation/antioxidant activity. However, levels of neuroprotection with these ligands were no greater than ligands lacking this moiety, suggesting the neuroprotective properties of these ligands are due primarily to chelation and passivation of intracellular labile iron, preventing the generation of free radicals and reactive oxygen species that otherwise lead to the neuronal cell death seen in Parkinson's disease.

Metals and Metal-Nanoparticles in Human Pathologies: From Exposure to Therapy

An increasing number of pathologies correlates with both toxic and essential metal ions dyshomeostasis. Next to known genetic disorders (e.g., Wilson's Disease and β-Thalassemia) other pathological states such as neurodegeneration and diabetes are characterized by an imbalance of essential metal ions. Metal ions can enter the human body from the surrounding environment in the form of free metal ions or metal-nanoparticles, and successively translocate to different tissues, where they are accumulated and develop distinct pathologies. There are no characteristic symptoms of metal intoxication, and the exact diagnosis is still difficult. In this review, we present metal-related pathologies with the most common onsets, biomarkers of metal intoxication, and proper techniques of metal qualitative and quantitative analysis. We discuss the possible role of drugs with metal-chelating ability in metal dyshomeostasis, and present recent advances in therapies of metal-related diseases.

Unveiling the reaction mechanism of novel copper N-alkylated tetra-azacyclophanes with outstanding superoxide dismutase activity

Quantum chemical and multiscale calculations reveal the mechanistic pathway of two superoxide dismutase mimetic N-alkylated tetra-azacyclophane copper complexes with remarkable activity. The arrangement of the binding site afforded by the bulky alkyl substituents and the coordinated water molecule as a proton source play key roles in the reaction mechanism.

Recent Developments in Metal-Based Drugs and Chelating Agents for Neurodegenerative Diseases Treatments

The brain has a unique biological complexity and is responsible for important functions in the human body, such as the command of cognitive and motor functions. Disruptive disorders that affect this organ, e.g. neurodegenerative diseases (NDDs), can lead to permanent damage, impairing the patients' quality of life and even causing death. In spite of their clinical diversity, these NDDs share common characteristics, such as the accumulation of specific proteins in the cells, the compromise of the metal ion homeostasis in the brain, among others. Despite considerable advances in understanding the mechanisms of these diseases and advances in the development of treatments, these disorders remain uncured. Considering the diversity of mechanisms that act in NDDs, a wide range of compounds have been developed to act by different means. Thus, promising compounds with contrasting properties, such as chelating agents and metal-based drugs have been proposed to act on different molecular targets as well as to contribute to the same goal, which is the treatment of NDDs. This review seeks to discuss the different roles and recent developments of metal-based drugs, such as metal complexes and metal chelating agents as a proposal for the treatment of NDDs.

Advances in Multi-Functional Ligands and the Need for Metal-Related Pharmacology for the Management of Alzheimer Disease

Alzheimer’s disease (AD) is the age linked neurodegenerative disorder with no disease modifying therapy currently available. The available therapy only offers short term symptomatic relief. Several hypotheses have been suggested for the pathogenesis of the disease while the molecules developed as possible therapeutic agent in the last decade, largely failed in the clinical trials. Several factors like tau protein hyperphosphorylation, amyloid-β (Aβ) peptide aggregation, decline in acetyl cholinesterase and oxidative stress might be contributing toward the pathogenesis of AD. Additionally, biometals dyshomeostasis (Iron, Copper, and Zinc) in the brain are also reported to be involved in the pathogenesis of AD. Thus, targeting these metal ions may be an effective strategy for the development of a drug to treat AD. Chelation therapy is currently employed for the metal intoxication but we lack a safe and effective chelating agents with additional biological properties for their possible use as multi target directed ligands for a complex disease like AD. Chelating agents possess the ability to disaggregate Aβ aggregation, dissolve amyloid plaques, and delay the cognitive impairment. Thus there is an urgent need to develop disease modifying therapeutic molecules with multiple beneficial features like targeting more than one factor responsible of the disease. These molecules, as disease modifying therapeutic agents for AD, should possess the potential to inhibit Aβ-metal interactions, the formation of toxic Aβ aggregates; and the capacity to reinstate metal homeostasis.

How Zn can impede Cu detoxification by chelating agents in Alzheimer's disease: a proof-of-concept study

The role of Cu and Zn ions in Alzheimer's disease is linked to the consequences of their coordination to the amyloid-β (Aβ) peptide, i.e. to the modulation of Aβ aggregation and to the production of Reactive Oxygen Species (ROS), two central events of the so-called amyloid cascade. The role of both ions in Aβ aggregation is still controversial. Conversely the higher toxicity of the redox competent Cu ions (compared to the redox inert Zn ions) in ROS production is acknowledged. Thus the Cu ions can be considered as the main therapeutic target. Because Zn ions are present in higher quantity than Cu ions in the synaptic cleft, they can prevent detoxification of Cu by chelators unless they have an unusually high Cu over Zn selectivity. We describe a proof-of-concept study where the role of Zn on the metal swap reaction between two prototypical ligands and the Cu(Aβ) species has been investigated by several complementary spectroscopic techniques (UV-Vis, EPR and XANES). The first ligand has a higher Cu over Zn selectivity relative to the one of Aβ peptide while the second one exhibits a classical Cu over Zn selectivity. How Zn impacts the effect of the ligands on Cu-induced ROS production and Aβ aggregation is also reported.

Prochelator strategies for site-selective activation of metal chelators

Metal dyshomeostasis has been involved in the etiology of a host of pathologies such as Wilson's, Alzheimer's, Parkinson's, Huntington's, transfusion-related iron overload diseases and cancer. Although metal chelating agents represent a necessary therapeutic strategy in metal overload diseases, long-term use of strong chelators that are not selective, can be anticipated perturbing normal physiological functions of essential metal-requiring biomolecules. In this context, the last decade has seen a growing interest in the development of molecules, referred to as "prochelators", that have little affinity for metal ions until they are activated in response to specific stimuli. Here, we present the main strategies applied to develop safe prochelators and focus on chosen examples to provide an overview of this field to date.

Novel 1-Phenyl-3-hydroxy-4-pyridinone Derivatives as Multifunctional Agents for the Therapy of Alzheimer's Disease

A series of novel 1-phenyl-3-hydroxy-4-pyridinone derivatives were designed and synthesized as multifunctional agents for Alzheimer's disease (AD) therapy through incorporation of 3-hydroxy-4-pyridinone moiety from deferiprone into the scaffold of H3 receptor antagonists. Most of these new compounds displayed designed quadruple functions, H3 receptor antagonism, Aβ aggregation inhibition, metal ion chelation, and radical scavenging. Especially, the most promising compound 5c displayed nanomolar IC50 values in H3 receptor antagonism with high selectivity, efficient capability to interrupt the formation of Aβ(1-42) fibrils, good copper and iron chelating properties, and more potent 2,2'-azino-bis(3-ethyl-benzothiazoline-6-sulfonic acid) radical cation (ABTS(•+)) scavenging activity than Trolox. Further biological evaluation revealed that it did not show obvious cytotoxicity and hERG potassium channel inhibition at micromolar concentration. In addition, compound 5c demonstrated suitable pharmacokinetic properties and acceptable blood-brain barrier (BBB) permeability in vivo. All these results indicate that compound 5c is a potential multifunctional candidate for AD therapy.

Identification of 2-subsituted benzothiazole derivatives as triple-functional agents with potential for AD therapy

A series of 2-subsituted benzothiazole derivatives were designed and synthesized as MDTLs for potential AD therapy.

Overview of Alzheimer's Disease and Some Therapeutic Approaches Targeting Aβ by Using Several Synthetic and Herbal Compounds

Alzheimer's disease (AD) is a complex age-related neurodegenerative disease. In this review, we carefully detail amyloid-β metabolism and its role in AD. We also consider the various genetic animal models used to evaluate therapeutics. Finally, we consider the role of synthetic and plant-based compounds in therapeutics.

3-Hydroxypyridinone derivatives as metal-sequestering agents for therapeutic use

Although iron is one of the most important metal ions for living organisms, it becomes toxic when in excess or misplaced. This review presents a glance at representative examples of hydroxypyridinone-based chelators, which have been recently developed as potential clinically useful drugs for metal overload diseases, mostly associated with excess of iron but also other hard metal-ions. It also includes a detailed discussion on the factors assisting chelator design strategy toward fulfillment of the most relevant biochemical properties of hydroxypyridinone chelators, highlighting structure-activity relationships and a variety of potential clinical applications, beyond chelatotherapy. This study appears as a response to the growing interest on metal chelation therapy and opens new perspectives of possible applications in future medicine.

Regulation of copper and iron homeostasis by metal chelators: a possible chemotherapy for Alzheimer's disease

With the increase of life expectancy of humans in more than two-thirds of the countries in the World, aging diseases are becoming the frontline health problems. Alzheimer's disease (AD) is now one of the major challenges in drug discovery, since, with the exception of memantine in 2003, all clinical trials with drug candidates failed over the past decade. If we consider that the loss of neurons is due to a high level of oxidative stress produced by nonregulated redox active metal ions like copper linked to amyloids of different sizes, regulation of metal homeostasis is a key target. The difficulty for large copper-carrier proteins to directly extract copper ions from metalated amyloids might be considered as being at the origin of the rupture of the copper homeostasis regulation in AD brains. So, there is an urgent need for new specific metal chelators that should be able to regulate the homeostasis of metal ions, specially copper and iron, in AD brains. As a consequence of that concept, chelators promoting metal excretion from brain are not desired. One should favor ligands able to extract copper ions from sinks (amyloids being the major one) and to transfer these redox-active metal ions to copper-carrier proteins or copper-containing enzymes. Obviously, the affinity of these chelators for the metal ion should not be a sufficient criterion, but the metal specificity and the ability of the chelators to release the metal under specific biological conditions should be considered. Such an approach is still largely unexplored. The requirements for the chelators are very high (ability to cross the brain-blood barrier, lack of toxicity, etc.), few chemical series were proposed, and, among them, biochemical or biological data are scarce. As a matter of fact, the bioinorganic pharmacology of AD represents less than 1% of all articles dedicated to AD drug research. The major part of these articles deals with an old and rather toxic drug, clioquinol and related analogs, that do not efficiently extract copper from soluble amyloids. We have designed and developed new tetradendate ligands such as 21 and PA1637 based on bis(8-aminoquinolines) that are specific for copper chelation and are able to extract copper(II) from amyloids and then can release copper ion upon reduction with a biological reducing agent. These studies contribute to the understanding of the physicochemical properties of the tetradentate copper ligands compared with bidentate ligands like clioquinol. One of these copper ligands, PA1637, after selection with a nontransgenic mouse model that is able to efficiently monitor the loss of episodic memory, is currently under preclinical development.

Anti-mycobacterial activities of copper(II) complexes. Part II. Lipophilic hydroxypyridinones derived from maltol

Eight lipophilic 3-hydroxy-4-pyridinones have been prepared from a microwave-mediated reaction along with the corresponding copper(II) complexes. All complexes have been obtained elementally pure and X-ray diffraction studies on two of the copper complexes have confirmed the structure of these compounds. Some of these complexes showed a promising degree of anti-mycobacterial activity against Mycobacterium tuberculosis, where activity seemed to vary by substitution at the pyridinone nitrogen atom.

Thioflavin-based molecular probes for application in Alzheimer's disease: from in silico to in vitro models

Alzheimer's disease (AD) is a neurological disease of confusing causation with no cure or prevention available. The definitive diagnosis is made postmortem, in part through the presence of amyloid-beta plaques in the brain tissue, which can be done with the small molecule thioflavin-T (ThT). Plaques are also found to contain elevated amounts of metal ions Cu(ii) and Zn(ii) that contribute to the neurotoxicity of amyloid-beta (Aβ). In this paper, we report in silico, in vitro, and ex vivo studies with ThT-derived metal binders 2-(2-hydroxyphenyl)benzoxazole (HBX), 2-(2-hydroxyphenyl)benzothiazole (HBT) and their respective iodinated counterparts, HBXI and HBTI. They exhibit low cytotoxicity in a neuronal cell line, potential blood-brain barrier penetration, and interaction with Aβ fibrils from senile plaques present in human and transgenic mice AD models. Molecular modelling studies have also been undertaken to understand the prospective ligand-Aβ complexes as well as to rationalize the experimental findings. Overall, our studies demonstrate that HBX, HBT, HBXI, and HBTI are excellent agents for future use in in vivo models of AD, as they show in vitro efficacy and biological compatibility. In addition to this, we present the glycosylated form of HBX (GBX), which has been prepared to take advantage of the benefits of the prodrug approach. Overall, the in vitro and ex vivo assays presented in this work validate the use of the proposed ThT-based drug candidate series as chemical tools for further in vivo development.

Distinctive EPR signals provide an understanding of the affinity of bis-(3-hydroxy-4-pyridinonato) copper(ii) complexes for hydrophobic environments

We put forward the use of EPR spectroscopy to assess the affinity of bis-(3-hydroxy-4-pyridinonato) copper(ii) complexes for a hydrophobic environment.

Neuroprotective Role of a Novel Copper Chelator against Aβ 42 Induced Neurotoxicity

Alzheimer's disease (AD) is a progressive neurodegenerative disease and associated with the extracellular deposits of amyloid- β peptide in hippocampus region. Metal ions like Cu, Fe and Zn are known to associate with the amyloid beta (A β ) at high concentration and interaction of these ions with soluble and aggregated forms of A β peptide help in development of AD. Here we showed Cu mediated neurotoxicity in the eye tissues of transgenic Drosophila expressing human amyloid β and its rescue through a novel Cu chelator. In this context, we have synthesised and characterized the compound L 2,6-Pyridinedicarboxylic acid, 2,6-bis[2-[(4-carboxyphenyl) methylene] hydrazide] by Mass spectra (MS) and Elemental analysis (EA). The Cu chelation potential of the compound L is tested in vivo in Drosophila. Oral administration of Copper to the transgenic larvae resulted in severe degeneration in eye tissues, which was rescued by the supplementation of compound L. The levels of anti-oxidant markers like SOD and MDA were measured in compound L treated flies and found a significant rescue (P < 0.001). Further rescue of the eye degeneration phenotypes as revealed by SEM affirm the role of copper in A β toxicity. Hence, use of compound L, an amidoamine derivative, could be a possible therapeutic measure for A β induced neurotoxicity.

A potent antioxidant small molecule aimed at targeting metal-based oxidative stress in neurodegenerative disorders

Metal-ion misregulation and oxidative stress have been linked to the progressive neurological decline associated with multiple neurodegenerative disorders. Transition metal-mediated oxidation of biomolecules via Fenton chemical reactions plays a role in disease progression. Herein we report the synthesis, characterization and antioxidant activity of 2; a pyclen derivative with enhanced antioxidant character.

A new water-soluble Cu(II) chelator that retrieves Cu from Cu(amyloid-β) species, stops associated ROS production and prevents Cu(II)-induced Aβ aggregation

The synthesis of the H(2)L(2-) ligand (N,N'-Bis[(5-sulfonato-2-hydroxy)benzyl]-N,N'-dimethyl-ethane-1,2-diamine) and characterizations of the corresponding Cu(II) complex [Cu(L)(H(2)O)](2-) (1) by X-ray diffraction, EPR, UV-Visible and potentiometry are described. At pH 7.4, the affinity of Cu(II) for this ligand is approximately 4 × 10(14)M(-1). Coordination of redox active metal ions such as copper or iron to the amyloid-β (Aβ) peptide has been linked to deleterious processes encountered in the etiology of Alzheimer disease (AD), such as Aβ aggregation and reactive oxygen species (ROS) production. In this context, the ability of the H(2)L(2-) to extract Cu(II) from Cu(Aβ) species where Aβ is the peptide involved in AD, is reported as well as its capacity to redox silence the Cu(Aβ) induced ROS formation and to prevent Cu(II)-induced Aβ aggregation. Such water soluble sulfonato-derivatives of Cu(II) chelators are very interesting counterparts for in vitro study of chelators' properties required to attend further biological applications as therapeutic tools against AD.

Nucleoside-5'-phosphorothioate analogues are biocompatible antioxidants dissolving efficiently amyloid beta-metal ion aggregates

Amyloid beta (Aβ) peptide is known to precipitate and form aggregates with zinc and copper ions in vitro and, in vivo in Alzheimer's disease (AD) patients. Metal-ion-chelation was suggested as therapy for the metal-ion-induced Aβ aggregation, metal-ion overload, and oxidative stress. In a quest for biocompatible metal-ion chelators potentially useful for AD therapy, we tested a series of nucleoside 5'-phosphorothioate derivatives as re-solubilization agents of Cu(+)/Cu(2+)/Zn(2+)-induced Aβ-aggregates, and inhibitors of Fenton reaction in Cu(+) or Fe(2+)/H(2)O(2) system. The most promising chelator in this series was found to be APCPP-γ-S. This nucleotide was found to be more efficient than EDTA in re-solubilization of Aβ(40)-Cu(2+) aggregates as observed by the lower diameter, d(H), (86 vs. 64 nm, respectively) obtained in dynamic light scattering measurements. Likewise, APCPP-γ-S dissolved Aβ(40)-Cu(+) and Aβ(42)-Cu(2+)/Zn(2+) aggregates, as monitored by (1)H-NMR and turbidity assays, respectively. Furthermore, addition of APCPP-γ-S to nine-day old Aβ(40)-Cu(2+)/Zn(2+) aggregates, resulted in size reduction as observed by transition electron microscopy (diameter reduction from 2.5 to 0.1 μm for Aβ(40)-Cu(2+) aggregates). APCPP-γ-S proved to be more efficient than ascorbic acid and GSH in reducing OH radical production in Fe(2+)/H(2)O(2) system (IC(50) values 85, 216 and, 92 μM, respectively). Therefore, we propose APCPP-γ-S as a potential AD therapy capable of both reducing OH radical production and re-solubilization of Aβ(40/42)-M(n+) aggregates.

Misfolded proteins in Alzheimer's disease and type II diabetes

This tutorial review presents descriptions of two amyloidogenic proteins, amyloid-β (Aβ) peptides and islet amyloid polypeptide (IAPP), whose misfolding propensities are implicated in Alzheimer's disease (AD) and type II diabetes, respectively. Protein misfolding diseases share similarities, as well as some unique protein-specific traits, that could contribute to the initiation and/or development of their associated conditions. Aβ and IAPP are representative amyloidoses and are used to highlight some of the primary considerations for studying misfolded proteins associated with human diseases in this review. Among these factors, their physiological formation, aggregation, interactions with metal ions and other protein partners, and toxicity are presented. Small molecules that target and modulate the metal-Aβ interaction and neurotoxicity are included to illustrate one of the current approaches for uncovering the complexities of protein misfolding at the molecular level.

Novel drug targets based on metallobiology of Alzheimer's disease

IMPORTANCE OF THE FIELD

Increased localization of Zn, Fe, Cu and Al within the senile plaques (SP) exacerbates amyloid beta (Aβ)-mediated oxidative damage, and acts as catalyst for Aβ aggregation in Alzheimer's disease (AD). Thus, disruption of aberrant metal-peptide interactions via chelation therapy holds considerable promise as a rational therapeutic strategy against Alzheimer's amyloid pathogenesis.

AREAS COVERED IN THIS REVIEW

The complexities of metal-induced genesis of SP are reviewed. The recent advances in the molecular mechanism of action of metal chelating agents are discussed with critical assessment of their potential to become drugs.

WHAT THE READER WILL GAIN

Taking into consideration the interaction of metals with the metal-responsive elements on the Alzheimer's amyloid precursor protein (APP), readers will gain understanding of several points to bear in mind when developing a screening campaign for AD-therapeutics.

TAKE HOME MESSAGE

A functional iron-responsive element (IRE) RNA stem loop in the 5' untranslated region (UTR) of the APP transcript regulates neural APP translation. Desferrioxamine, clioquinol, tetrathiolmolybdate, dimercaptopropanol, VK-28, and natural antioxidants, such as curcumin and ginko biloba need critical evaluation as AD therapeutics. There is a necessity for novel screens (related to metallobiology) to identify therapeutics effective in AD.