Potential of a γ-glutamyl-transpeptidase-stable glutathione analogue against amyloid-β toxicity

Lead optimization based design, synthesis, and pharmacological evaluation of quinazoline derivatives as multi-targeting agents for Alzheimer's disease treatment

The complexity and multifaceted nature of Alzheimer's disease (AD) have driven us to further explore quinazoline scaffolds as multi-targeting agents for AD treatment. The lead optimization strategy was utilized in designing of new series of derivatives (AK-1 to AK-14) followed by synthesis, characterization, and pharmacological evaluation against human cholinesterase's (hChE) and β-secretase (hBACE-1) enzymes. Amongst them, compounds AK-1, AK-2, and AK-3 showed good and significant inhibitory activity against both hAChE and hBACE-1 enzymes with favorable permeation across the blood-brain barrier. The most active compound AK-2 revealed significant propidium iodide (PI) displacement from the AChE-PAS region and was non-neurotoxic against SH-SY5Y cell lines. The lead molecule (AK-2) also showed Aβ aggregation inhibition in a self- and AChE-induced Aβ aggregation, Thioflavin-T assay. Further, compound AK-2 significantly ameliorated Aβ-induced cognitive deficits in the Aβ-induced Morris water maze rat model and demonstrated a significant rescue in eye phenotype in the Aꞵ-phenotypic drosophila model of AD. Ex-vivo immunohistochemistry (IHC) analysis on hippocampal rat brains showed reduced Aβ and BACE-1 protein levels. Compound AK-2 suggested good oral absorption via pharmacokinetic studies and displayed a good and stable ligand-protein interaction in in-silico molecular modeling analysis. Thus, the compound AK-2 can be regarded as a lead molecule and should be investigated further for the treatment of AD.

Design, synthesis, and biological evaluation of some 2-(3-oxo-5,6-diphenyl-1,2,4-triazin-2(3H)-yl)-N-phenylacetamide hybrids as MTDLs for Alzheimer's disease therapy

Inspite of established symptomatic relief drug targets, a multi targeting approach is highly in demand to cure Alzheimer's disease (AD). Simultaneous inhibition of cholinesterase (ChE), β secretase-1 (BACE-1) and Dyrk1A could be promising in complete cure of AD. A series of 18 diaryl triazine based molecular hybrids were successfully designed, synthesized, and tested for their hChE, hBACE-1, Dyrk1A and Aβ aggregation inhibitory potentials. Compounds S-11 and S-12 were the representative molecules amongst the series with multi-targeted inhibitory effects. Compound S-12 showed hAChE inhibition (IC50 value = 0.486 ± 0.047 μM), BACE-1 inhibition (IC50 value = 0.542 ± 0.099 μM) along with good anti-Aβ aggregation effects in thioflavin-T assay. Only compound S-02 of the series has shown Dyrk1A inhibition (IC50 value = 2.000 ± 0.360 μM). Compound S-12 has also demonstrated no neurotoxic liabilities against SH-SY5Y as compared to donepezil. The in vivo behavioral studies of the compound S-12 in the scopolamine- and Aβ-induced animal models also demonstrated attanuation of learning and memory functions in rats models having AD-like characteristics. The ex vivo studies, on the rat hippocampal brain demonstrated reduction in certain biochemical markers of the AD brain with a significant increase in ACh level. The Western blot and Immunohistochemistry further revealed lower tau, APP and BACE-1 molecular levels. The drosophilla AD model also revealed improved eyephenotype after treatment with compound S-12. The molecular docking studies of the compounds suggested that compound S-12 was interacting with the ChE-PAS & CAS residues and catalytic dyad residues of the BACE-1 enzymes. The 100 ns molecular dynamics simulation studies of the ligand-protein complexed with hAChE and hBACE-1 also suggested stable ligand-protein confirmation throughout the simulation run.

Deletion of Glyoxalase 1 exacerbates acetaminophen-induced hepatotoxicity in mice

Acetaminophen (APAP) overdose triggers a cascade of intracellular oxidative stress events culminating in acute liver injury. The clinically used antidote, N-acetylcysteine (NAC) has a narrow therapeutic window and early treatment is essential for satisfactory therapeutic outcome. For more versatile therapies that can be effective even at late-presentation, the intricacies of APAP-induced hepatotoxicity must be better understood. Accumulation of advanced glycation end-products (AGEs) and consequent activation of the receptor for AGEs (RAGE) are considered one of the key mechanistic features of APAP toxicity. Glyoxalase-1 (Glo-1) regulates AGE formation by limiting the levels of methylglyoxal (MEG). In this study, we studied the relevance of Glo-1 in APAP mediated activation of RAGE and downstream cell-death cascades. Constitutive Glo-1 knockout mice (GKO) and a cofactor of Glo-1, ψ-GSH, were employed as tools. Our findings show elevated oxidative stress, activation of RAGE and hepatocyte necrosis through steatosis in GKO mice treated with high-dose APAP compared to wild type controls. A unique feature of the hepatic necrosis in GKO mice is the appearance of microvesicular steatosis as a result of centrilobular necrosis, rather than inflammation seen in wild type. The GSH surrogate and general antioxidant, ψ-GSH alleviated APAP toxicity irrespective of Glo-1 status, suggesting that oxidative stress being the primary driver of APAP toxicity. Overall, exacerbation of APAP hepatotoxicity in GKO mice suggests the importance of this enzyme system in antioxidant defense against initial stages of APAP overdose.

Iron homeostasis imbalance and ferroptosis in brain diseases

Brain iron homeostasis is maintained through the normal function of blood-brain barrier and iron regulation at the systemic and cellular levels, which is fundamental to normal brain function. Excess iron can catalyze the generation of free radicals through Fenton reactions due to its dual redox state, thus causing oxidative stress. Numerous evidence has indicated brain diseases, especially stroke and neurodegenerative diseases, are closely related to the mechanism of iron homeostasis imbalance in the brain. For one thing, brain diseases promote brain iron accumulation. For another, iron accumulation amplifies damage to the nervous system and exacerbates patients' outcomes. In addition, iron accumulation triggers ferroptosis, a newly discovered iron-dependent type of programmed cell death, which is closely related to neurodegeneration and has received wide attention in recent years. In this context, we outline the mechanism of a normal brain iron metabolism and focus on the current mechanism of the iron homeostasis imbalance in stroke, Alzheimer's disease, and Parkinson's disease. Meanwhile, we also discuss the mechanism of ferroptosis and simultaneously enumerate the newly discovered drugs for iron chelators and ferroptosis inhibitors.

Design, Synthesis, and Biological Evaluation of Piperazine and N-Benzylpiperidine Hybrids of 5-Phenyl-1,3,4-oxadiazol-2-thiol as Potential Multitargeted Ligands for Alzheimer's Disease Therapy

Our present work demonstrates the successful design and synthesis of a new class of compounds based upon a multitargeted directed ligand design approach to discover new agents for use in Alzheimer's disease (AD). All the compounds were tested for their in vitro inhibitory potential against human acetylcholinesterase (hAChE), human butylcholinesterase (hBChE), β-secretase-1 (hBACE-1), and amyloid β (Aβ) aggregation. Compounds 5d and 5f have shown hAChE and hBACE-1 inhibition comparable to donepezil, while hBChE inhibition was comparable to rivastigmine. Compounds 5d and 5f also demonstrated a significant reduction in the formation of Aβ aggregates through the thioflavin T assay and confocal, atomic force, and scanning electron microscopy studies and significantly displaced the total propidium iodide, that is, 54 and 51% at 50 μM concentrations, respectively. Compounds 5d and 5f were devoid of neurotoxic liabilities against RA/BDNF (RA = retinoic acid; BDNF = brain-derived neurotrophic factor)-differentiated SH-SY5Y neuroblastoma cell lines at 10-80 μM concentrations. In both the scopolamine- and Aβ-induced mouse models for AD, compounds 5d and 5f demonstrated significant restoration of learning and memory behaviors. A series of ex vivo studies of hippocampal and cortex brain homogenates showed that 5d and 5f elicit decreases in AChE, malondialdehyde, and nitric oxide levels, an increase in glutathione level, and reduced levels of pro-inflammatory cytokines, tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) mRNA. The histopathological examination of mice revealed normal neuronal appearance in the hippocampal and cortex regions of the brain. Western blot analysis of the same tissue indicated a reduction in Aβ, amyloid precursor protein (APP)/Aβ, BACE-1, and tau protein levels, which were non-significant compared to the sham group. The immunohistochemical analysis also showed significantly lower expression of BACE-1 and Aβ levels, which was comparable to donepezil-treated group. Compounds 5d and 5f represent new lead candidates for developing AD therapeutics.

Orally Bioavailable Prodrugs of ψ-GSH: A Potential Treatment for Alzheimer's Disease

Addressing glycation-induced oxidative stress in Alzheimer's disease (AD) is an emerging pharmacotherapeutic strategy. Restoration of the brain glyoxalase enzyme system that neutralizes reactive dicarbonyls is one such approach. Toward this end, we designed, synthesized, and evaluated a γ-glutamyl transpeptidase-resistant glyoxalase substrate, ψ-GSH. Although mechanistically successful, the oral efficacy of ψ-GSH appeared as an area in need of improvement. Herein, we describe our rationale for the creation of prodrugs that mask the labile sulfhydryl group. In vitro and in vivo stability studies identified promising prodrugs that could deliver pharmacologically relevant brain levels of ψ-GSH. When administered orally to a mouse model generated by the intracerebroventricular injection of Aβ1-42, the compounds conferred cognitive benefits. Biochemical and histological examination confirmed their effects on neuroinflammation and oxidative stress. Collectively, we have identified orally efficacious prodrugs of ψ-GSH that are able to restore brain glyoxalase activity and mitigate inflammatory and oxidative pathology associated with AD.

Sulfanegen stimulates 3-mercaptopyruvate sulfurtransferase activity and ameliorates Alzheimer's disease pathology and oxidative stress in vivo

Increased oxidative stress and inflammation are implicated in the pathogenesis of Alzheimer's disease. Treatment with hydrogen sulfide (H2S) and H2S donors such as sodium hydrosulfide (NaSH) can reduce oxidative stress in preclinical studies, however clinical benefits of such treatments are rather ambiguous. This is partly due to poor stability and bioavailability of the H2S donors, requiring impractically large doses that are associated with dose-limiting toxicity. Herein, we identified a bioavailable 3-mercaptopyruvate prodrug, sulfanegen, which is able to pose as a sacrificial redox substrate for 3-mercaptopyruvate sulfurtransferase (3MST), one of the H2S biosynthetic enzymes in the brain. Sulfanegen is able to mitigate toxicity emanating from oxidative insults and the Aβ1-42 peptide by releasing H2S through the 3MST pathway. When administered to symptomatic transgenic mouse model of AD (APP/PS1; 7 and 12 months) and mice that were intracerebroventricularly administered with the Aβ1-42 peptide, sulfanegen was able to reverse oxidative and neuroinflammatory consequences of AD pathology by restoring 3MST function. Quantitative neuropathological analyses confirmed significant disease modifying effect of the compound on amyloid plaque burden and brain inflammatory markers. More importantly, sulfanegen treatment attenuated progressive neurodegeneration in these mice, as evident from the restoration of TH+ neurons in the locus coeruleus. This study demonstrates a previously unknown concept that supplementation of 3MST function in the brain may be a viable approach for the management of AD. Finally, brought into the spotlight is the potential of sulfanegen as a promising AD therapeutic for future drug development efforts.

Dipeptide of ψ-GSH Inhibits Oxidative Stress and Neuroinflammation in an Alzheimer's Disease Mouse Model

Supplementation of glutathione (GSH) levels through varying formulations or precursors has thus far appeared to be a tenable strategy to ameliorate disease-associated oxidative stress. Metabolic liability of GSH and its precursors, i.e., hydrolysis by the ubiquitous γ-glutamyl transpeptidase (γ-GT), has limited successful clinical translation due to poor bioavailability. We addressed this problem through the design of γ-GT-resistant GSH analogue, ψ-GSH, which successfully substituted in GSH-dependent enzymatic systems and also offered promise as a therapeutic for Alzheimer's disease (AD). With the aim to improve its bioavailability, we studied the utility of a ψ-GSH precursor, dipeptide 2, as a potential AD therapeutic. Compound 2 retains the γ-GT stable ureide linkage and the thiol group for antioxidant property. By engaging glutathione synthetase, compound 2 was able to generate ψ-GSH in vivo. It was found to be a modest cofactor of glutathione peroxidase and prevented cytotoxicity of Aβ^1-42-aggregates in vitro. Studies of compound 2 in an acute AD model generated by intracerebroventricular injection of Aβ^1-42 showed cognitive benefits, which were augmented by its combination with glycine along with mitigation of oxidative stress and inflammatory pathology. Collectively, these results support further optimization and evaluation of ψ-GSH dipeptide as a potential therapeutic in transgenic AD models.

γ-Glutamyl-Transpeptidase-Resistant Glutathione Analog Attenuates Progression of Alzheimer's Disease-like Pathology and Neurodegeneration in a Mouse Model

Oxidative stress in Alzheimer’s disease (AD) is mediated, in part, by the loss of glutathione (GSH). Previous studies show that γ-glutamyl transpeptidase (GGT)-resistant GSH analog, Ψ-GSH, improves brain GSH levels, reduces oxidative stress markers in brains of APP/PS1 transgenic mice, a mouse model of AD, and attenuates early memory deficits in the APP/PS1 model. Herein, we examined whether Ψ-GSH can attenuate the disease progression when administered following the onset of AD-like pathology in vivo. Cohorts of APP/PS1 mice were administered Ψ-GSH for 2 months starting at 8 month or 12 months of age. We show that Ψ-GSH treatment reduces indices of oxidative stress in older mice by restoration of enzyme glyoxalase-1 (Glo-1) activity and reduces levels of insoluble Aβ. Quantitative neuropathological analyses show that Ψ-GSH treatment significantly reduces Aβ deposition and brain inflammation in APP/PS1 mice compared to vehicle-treated mice. More importantly, Ψ-GSH treatment attenuated the progressive loss of cortical TH+ afferents and the loss of TH+ neurons in the locus coeruleus (LC). Collectively, the results show that Ψ-GSH exhibits significant antioxidant activity in aged APP/PS1 mice and chronic Ψ-GSH treatment administered after the onset of AD pathology can reverse/slow further progression of AD-like pathology and neurodegeneration in vivo.

The Amyloid Aggregation Accelerator Diacetyl Prevents Cognitive Decline in Alzheimer's Mouse Models

Diacetyl (DA), a food flavorant, is linked with occupational lung disease. Our in vitro experiments described the formation of a covalent adduct by DA with Arg⁵ of the Aβ_1-42 peptide, which resulted in only a transient increase in neurotoxicity in SH-SY5Y cells. However, in vivo implications of these effects on Alzheimer's disease (AD) pathogenesis and the underlying mechanisms remain poorly understood. In the APP/PS1 transgenic AD mouse model, DA treatment did not exacerbate learning and memory deficits in the Morris water maze test. Moreover, DA increased the Aβ_1-42 plaque burden and decreased neuronal inflammation in the transgenic AD mice. Additionally, cognitive impairment induced by intracerebroventricular Aβ_1-42 was restored by the DA treatment, as assessed by the T-maze test. A corresponding mitigation of neuronal inflammation was also observed in the hippocampus of these nontransgenic mice due to the acceleration of Aβ_1-42 aggregation by DA into nontoxic plaques. The data from SDS-PAGE, dot-blot, and TEM in vitro experiments corroborated the acceleration of the Aβ_1-42 aggregation observed in vivo in AD animal models and characterized the DA-induced formation of Aβ_1-42 fibrils. Such Aβ_1-42-DA fibrils were unstable in the presence of detergent and amenable to detection by the thioflavin T reagent, thus underscoring the distinct assembly of these fibrils compared to that of the fibrils of the native Aβ_1-42. Taken together, the results of this study present for the first time the in vivo implications of the DA-induced acceleration of Aβ_1-42 and may provide a strategy for the rational design of Aβ_1-42 aggregation accelerators as AD therapeutics that promote oligomer-free Aβ_1-42 fibril formation.

Sulfur-containing therapeutics in the treatment of Alzheimer's disease

Sulfur is widely existent in natural products and synthetic organic compounds as organosulfur, which are often associated with a multitude of biological activities. OBenzothiazole, in which benzene ring is fused to the 4,5-positions of the thiazolerganosulfur compounds continue to garner increasing amounts of attention in the field of medicinal chemistry, especially in the development of therapeutic agents for Alzheimer's disease (AD). AD is a fatal neurodegenerative disease and the primary cause of age-related dementia posing severe societal and economic burdens. Unfortunately, there is no cure for AD. A lot of research has been conducted on sulfur-containing compounds in the context of AD due to their innate antioxidant potential and some are currently being evaluated in clinical trials. In this review, we have described emerging trends in the field, particularly the concept of multi-targeting and formulation of disease-modifying strategies. SAR, pharmacological targets, in vitro/vivo ADMET, efficacy in AD animal models, and applications in clinical trials of such sulfur compounds have also been discussed. This article provides a comprehensive review of organosulfur-based AD therapeutic agents and provides insights into their future development.

Quercetin protects human brain microvascular endothelial cells from fibrillar β-amyloid1-40-induced toxicity

Amyloid beta-peptides (Aβ) are known to undergo active transport across the blood-brain barrier, and cerebral amyloid angiopathy has been shown to be a prominent feature in the majority of Alzheimer׳s disease. Quercetin is a natural flavonoid molecule and has been demonstrated to have potent neuroprotective effects, but its protective effect on endothelial cells under Aβ-damaged condition is unclear. In the present study, the protective effects of quercetin on brain microvascular endothelial cells injured by fibrillar Aβ_1–40 (fAβ_1–40) were observed. The results show that fAβ_1–40-induced cytotoxicity in human brain microvascular endothelial cells (hBMECs) can be relieved by quercetin treatment. Quercetin increases cell viability, reduces the release of lactate dehydrogenase, and relieves nuclear condensation. Quercetin also alleviates intracellular reactive oxygen species generation and increases superoxide dismutase activity. Moreover, it strengthens the barrier integrity through the preservation of the transendothelial electrical resistance value, the relief of aggravated permeability, and the increase of characteristic enzyme levels after being exposed to fAβ_1–40. In conclusion, quercetin protects hBMECs from fAβ_1–40-induced toxicity.

Neurodegeneration in Parkinson's disease: interactions of oxidative stress, tryptophan catabolites and depression with mitochondria and sirtuins

The biological underpinnings to the etiology and course of neurodegeneration in Parkinson's disease are an area of extensive research that has yet to produce an early biological marker or disease-slowing or preventative treatment. Recent conceptualizations of Parkinson's disease have integrated immuno-inflammation and oxidative and nitrosative stress occurring in depression, somatization and peripheral inflammation into the course of Parkinson's disease. We review the data showing the importance of immuno-inflammatory processes and oxidative and nitrosative stress in such classically conceived 'comorbidities', suggesting that lifetime, prodromal and concurrent depression and somatization may be intricately involved in the etiology and course of Parkinson's disease, rather than psychiatric comorbidities. This produces a longer term developmental perspective of Parkinson's disease, which incorporates tryptophan catabolites (TRYCATs), lipid peroxidation, sirtuins, cyclic adenosine monophosphate, aryl hydrocarbon receptor, and circadian genes. This integrates wider bodies of data pertaining to neuronal loss in Parkinson's disease, emphasizing how these interact with susceptibility genes to drive changes in mitochondria, blood-brain barrier permeability and intercellular signalling. We review this data here in the context of neurodegeneration in Parkinson's disease and to the future directions indicated for slowing disease progression.

Restoration of glyoxalase enzyme activity precludes cognitive dysfunction in a mouse model of Alzheimer's disease

Pathologically high brain levels of reactive dicarbonyls such as methylglyoxal or glyoxal initiate processes that lead ultimately to neurodegeneration, presented clinically as Alzheimer's disease and other cognitive or motor impairment disorders. Methylglyoxal and glyoxal result from glycolysis and normal metabolic pathways. Their reaction products with proteins (advanced glycation end products), and their primary chemical toxicities are both linked unequivocally to the primary pathologies of Alzheimer's disease, namely, amyloid plaques and neurofibrillary tangles. Generation of dicarbonyls is countered through the reduction of dicarbonyls by the glutathione-dependent glyoxalase enzyme system. Although glyoxalase-I is overexpressed in early and middle stages of Alzheimer's disease, glutathione depletion in the Alzheimer's afflicted brain cripples its efficacy. Due to the lack of a suitable pharmacological tool, the restoration of glyoxalase enzyme activity in pre-Alzheimer's or manifest Alzheimer's remains yet unvalidated as a means for anti-Alzheimer's therapy development. Disclosed herein are the results of a preclinical study into the therapeutic efficacy of ψ-GSH, a synthetic cofactor of glyoxalase, in mitigating Alzheimer's indicators in a transgenic mouse model (APP/PS1) that is predisposed to Alzheimer's disease. ψ-GSH administration completely averts the development of spatial mnemonic and long-term cognitive/cued-recall impairment. Amyloid β deposition and oxidative stress indicators are drastically reduced in the ψ-GSH-treated APP/PS1 mouse. ψ-GSH lacks discernible toxicity at strikingly high doses of 2000 mg/kg. The hypothesis that restoring brain glyoxalase activity would ameliorate neurogeneration stands validated, thus presenting a much needed new target for design of anti-Alzheimer's therapeutics. Consequently, ψ-GSH is established as a candidate for drug-development.