Organoselenium Compounds as Potential Neuroprotective Therapeutic Agents

Evaluation of the Neuroprotective Effect of Organic Selenium Compounds: An in Vitro Model of Alzheimer's Disease

Selenium (Se) is an essential trace element for human health and plays an important role in the development and maintenance of central nervous system functions. Se deficiency has been associated with cognitive decline and increased oxidative stress. The increase in oxidative stress is one of the hypotheses for the emergence and worsening of neurodegenerative diseases, such as Alzheimer's disease (AD). To investigate the neuroprotective effects of organic Se compounds in human neuroblastoma cells (SH-SY5Y) differentiated into cholinergic neurons-like. The SH-SY5Y cells were differentiated into cholinergic neuron-like with retinoic acid (RA) and brain-derived neurotrophic factor (BDNF). AD was mimicked exposing the cells to okadaic acid (OA) and beta-amyloid protein (Aβ). The neuroprotective effect of organic Se compounds, selenomethionine (SeMet) and Ebselen, was evaluated through cell viability tests, acetylcholinesterase and antioxidant enzyme activities, and detection of reactive oxygen species (ROS). None of the SeMet concentrations tested protected against the toxic effect of OA + Aβ. On the other hand, previous exposure to 0.1 and 1 µM Ebselen protected cells from the toxic effect of OA + Aβ. Cell differentiation induced by RA and BDNF exposure was effective, showing characteristics of neuronal cells, and pointing to a promising model of AD. Ebselen showed a protective effect, but more studies are needed to identify the mechanism of action.

Diphenyl diselenide modulate antioxidant status, inflammatory and redox-sensitive genes in diesel exhaust particle-induced neurotoxicity

This study seeks to determine the influence of diphenyl diselenide (DPDSe) on redox status, inflammatory and redox-sensitive genes in diesel exhaust particle (DEP)-induced neurotoxicity in male albino rats. Male Wistar albino rats were administered nasally with DEP (30 and 60 μg/kg) and treated with intraperitoneal administration of 10 mg/kg DPDSe. Non-enzymatic (lipid peroxidation and conjugated diene concentrations) and enzymatic (catalase, superoxide dismutase, glutathione peroxidase) antioxidant indices and activity of acetylcholinesterase enzyme were evaluated in brain tissues of the rats. Furthermore, the expression of genes linked to oxidative stress (HO-1, Nrf2), pro-inflammatory (NF-KB, IL-8, TNF-α) anti-inflammatory (IL-10) and brain-specific (GFAP, ENO-2) genes were also determined. The results indicated that DPDSe caused a notable reduction in the high levels of thiobarbituric acid reactive substances and conjugated diene observed in the brain of DEP-administered rats. DPDSe also reversed the observed reduction in catalase, superoxide dismutase and glutathione peroxidase enzyme activities in the brain of DEP-administered rats. Lastly, the downregulation of genes associated with redox homeostasis, anti-inflammatory and brain-specific genes and upregulation of pro-inflammatory genes observed in the DEP-treated groups were ameliorated by DPDSe. The immediate restoration of altered biochemical conditions and molecular expression in the brain of DEP-treated rats by DPDSe further validates its use as a promising therapeutic candidate for restoring neurotoxicity linked with DEP-induced oxidative stress.

Selanzylimidazopyridine abolishes inflammation- and stress-induced depressive-like behaviors and decreases oxidonitrosative stress in the prefrontal cortex and hippocampus of mice

The 3-[(4-methoxyphenyl)selanyl]-2-phenylimidazo[1,2-a] pyridine (MPI), a novel organic selenium compound, has been receiving increased attention due to its antioxidant effects and its ability to protect against depression-like behaviours. However, it remains elusive whether MPI is able to reverse depressive-like symptoms and biochemical alterations in mice. In the present work, we explored the ability of MPI (10 mg/kg, i.g.) to reverse inflammation- and stress-induced depression-like behaviours in mice injected with tumour necrosis factor (TNF-α) or submitted to acute restraint stress. Depression-like behaviours were evaluated by the tail suspension and splash test and the open field test was used to evaluate the locomotor activity of mice. The prefrontal cortex and hippocampus of mice were used for the evaluation of parameters of oxidonitrosative stress. Here, we showed that a single administration of MPI abolished the depressive-like behaviours induced by TNF-α and acute restraint stress without having an effect per se. The oxidative and nitrosative stress presented in mice with depression-like behaviours were also decreased by MPI in the prefrontal cortex and hippocampus. Our findings suggest that MPI presents antidepressant-like activity which is associated with the biochemical regulation of oxidative stress in the prefrontal cortex and hippocampus of mice, arising as a promising strategy for the management of depressive symptoms.

Toxicology and pharmacology of synthetic organoselenium compounds: an update

Here, we addressed the pharmacology and toxicology of synthetic organoselenium compounds and some naturally occurring organoselenium amino acids. The use of selenium as a tool in organic synthesis and as a pharmacological agent goes back to the middle of the nineteenth and the beginning of the twentieth centuries. The rediscovery of ebselen and its investigation in clinical trials have motivated the search for new organoselenium molecules with pharmacological properties. Although ebselen and diselenides have some overlapping pharmacological properties, their molecular targets are not identical. However, they have similar anti-inflammatory and antioxidant activities, possibly, via activation of transcription factors, regulating the expression of antioxidant genes. In short, our knowledge about the pharmacological properties of simple organoselenium compounds is still elusive. However, contrary to our early expectations that they could imitate selenoproteins, organoselenium compounds seem to have non-specific modulatory activation of antioxidant pathways and specific inhibitory effects in some thiol-containing proteins. The thiol-oxidizing properties of organoselenium compounds are considered the molecular basis of their chronic toxicity; however, the acute use of organoselenium compounds as inhibitors of specific thiol-containing enzymes can be of therapeutic significance. In summary, the outcomes of the clinical trials of ebselen as a mimetic of lithium or as an inhibitor of SARS-CoV-2 proteases will be important to the field of organoselenium synthesis. The development of computational techniques that could predict rational modifications in the structure of organoselenium compounds to increase their specificity is required to construct a library of thiol-modifying agents with selectivity toward specific target proteins.

Diphenyl diselenide dietary supplementation alleviates behavior impairment and brain damage in grass carp (Ctenopharyngodon idella) exposed to methylmercury chloride

Mercury (Hg) induces neurobehavioral disorders through reactive oxygen species (ROS) elevation and impairment of brain key enzyme activities. Nevertheless, the therapeutic and toxic selenium concentrations for fish are very close; diphenyl diselenide (Ph₂Se₂), an organoselenium compound with neuroprotective effects, may be an alternative to elemental Se. Therefore, the aim of this study was to determine whether dietary supplementation with Ph₂Se₂ prevented or reduced the neurobehavioral alterations and oxidative damage elicited by CH₃HgCl in grass carp Ctenopharyngodon idella. Fish exposed to CH₃HgCl exhibited significantly reduced distance travelled and swimming speed compared to the control group, as well as augmented cortisol and ROS levels and xanthine oxidase (XO) activities. CH₃HgCl exposure significantly increased lipid peroxidation (LOOH) and protein carbonylation (PC) levels compared to those of the control group, while acetylcholinesterase (AChE) and sodium-potassium pump (Na⁺, K⁺-ATPase) activities were inhibited. Dietary supplementation with 3 mg/kg Ph₂Se₂ ameliorated locomotor activity impairment and prevented the augmented brain cortisol and ROS levels as well as XO activity. The supplement reduced lipid and protein damage elicited by CH₃HgCl and exerted protective effects on brain AChE and Na⁺, K⁺-ATPase activities. Exposure to an environmental concentration of CH₃HgCl elicited neurobehavioral alterations linked to reduced locomotor activity, a finding that can be explained by oxidative damage and reduced activity of AChE and Na⁺, K⁺-ATPase in telencephalon and mesencephalon structures. Dietary supplementation with Ph₂Se₂ prevented CH₃HgCl-induced locomotor impairment. This effect appeared to be mediated by antioxidant action. Ph₂Se₂ may be a viable approach to prevention or reduction CH₃HgCl-mediated neurotoxic effects.

Reactions of Nitroxides, Part 17. Synthesis, Fungistatic and Bacteriostatic Activity of Novel Five- and Six-Membered Nitroxyl Selenoureas and Selenocarbamates

The reactions of 3-isoselenocyanato-2,2,5,5-tetramethylpyrrolidine-1-oxyl, 3-isoselenocyanatomethyl-2,2,5,5-tetramethyl-3-pyrrolidine-1-oxyl, and 4-isoselenocyanato-2,2,6,6-tetramethylpiperidine-1-oxyl with selected amines and alcohols give the corresponding novel nitroxyl selenoureas and selenocarbamates, all bearing a nitroxyl moiety. Synthesized selenoureas and selenocarbamates show significant activity against pathogenic fungi and bacteria. In contrast to piperidine nitroxides, pyrrolidine, five-membered nitroxyl selenoureas and selenocarbamates show excellent antifungal and antibacterial activity against pathogenic fungi and bacteria, respectively.

New selenosteroids as antiproliferative agents

Starting from natural steroids (diosgenin, hecogenin, smilagenin, estrone), we have prepared a wide panel of selenoderivatives, including benzoselenazolones, selenosemicarbazones, isoselenocyanates, selenoureas, selenocyanates and diselenides, with the aim of developing new families of potential chemotherapeutic agents. The modification of the organoselenium moieties, and their position on the steroid provided valuable information concerning the antiproliferative activities. Among all the families accessed herein, the best profile was achieved for selenoureas on the A ring of estrone, which exhibited GI₅₀ values in the range 2.0-4.1 μM for all the tested tumor cell lines, with increased potency compared with commonly used chemotherapeutic agents, like 5-fluorouracil and cisplatin. Cell cycle analysis revealed that selenoureas induced accumulation of cells in the G₁ phase of the cell cycle in the breast cancer cell lines HBL-100 and T-47D; therefore, a different mechanism than cisplatin, that induces cell cycle accumulation in the S phase as a result of DNA damage, must be involved. In the rest of the tumor cells, a slight increase of the S compartment was observed. Moreover, selenosteoids turned out to be excellent glutathione peroxidase (GPx) mimics for the catalytic removal of deleterious H₂O₂ (t_1/2 8.0-22.5 min) and alkyl peroxides (t_1/2 23.0-38.9 min) when used in substoichiometric amounts (1% molar ratio), thus providing a valuable tool for reducing the intrinsic oxidative stress in tumor progression.

Organoselenium compounds as mimics of selenoproteins and thiol modifier agents

Selenium is an essential trace element for animals and its role in the chemistry of life relies on a unique functional group: the selenol (-SeH) group. The selenol group participates in critical redox reactions. The antioxidant enzymes glutathione peroxidase (GPx) and thioredoxin reductase (TrxR) exemplify important selenoproteins. The selenol group shares several chemical properties with the thiol group (-SH), but it is much more reactive than the sulfur analogue. The substitution of S by Se has been exploited in organic synthesis for a long time, but in the last 4 decades the re-discovery of ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one) and the demonstration that it has antioxidant and therapeutic properties has renovated interest in the field. The ability of ebselen to mimic the reaction catalyzed by GPx has been viewed as the most important molecular mechanism of action of this class of compound. The term GPx-like or thiol peroxidase-like reaction was previously coined in the field and it is now accepted as the most important chemical attribute of organoselenium compounds. Here, we will critically review the literature on the capacity of organoselenium compounds to mimic selenoproteins (particularly GPx) and discuss some of the bottlenecks in the field. Although the GPx-like activity of organoselenium compounds contributes to their pharmacological effects, the superestimation of the GPx-like activity has to be questioned. The ability of these compounds to oxidize the thiol groups of proteins (the thiol modifier effects of organoselenium compounds) and to spare selenoproteins from inactivation by soft-electrophiles (MeHg⁺, Hg²⁺, Cd²⁺, etc.) might be more relevant for the explanation of their pharmacological effects than their GPx-like activity. In our view, the exploitation of the thiol modifier properties of organoselenium compounds can be harnessed more rationally than the use of low mass molecular structures to mimic the activity of high mass macromolecules that have been shaped by millions to billions of years of evolution.

Synthesis, antioxidant and antitumoral activities of 5'-arylchalcogeno-3-aminothymidine (ACAT) derivatives

This article presents the preparation and in vitro biological activities of new 5'-arylchalcogeno-3-aminothymidine derivatives as antioxidants (inhibition of lipid peroxidation, scavenging of the free radical 2,2-diphenylpicrylhydrazyl and demonstration of a thiol peroxidase-like activity) as well as antitumoral agents against bladder carcinoma 5637. The chalcogeno-aminothymidines presented prominent activity in the tests for both biological properties, showing a direct relation with the chalcogenium atom.

Chemical Speciation of Selenium and Mercury as Determinant of Their Neurotoxicity

The antagonism of mercury toxicity by selenium has been well documented. Mercury is a toxic metal, widespread in the environment. The main target organs (kidneys, lungs, or brain) of mercury vary depending on its chemical forms (inorganic or organic). Selenium is a semimetal essential to mammalian life as part of the amino acid selenocysteine, which is required to the synthesis of the selenoproteins. This chapter has the aim of disclosing the role of selenide or hydrogen selenide (Se^-2 or HSe^-) as central metabolite of selenium and as an important antidote of the electrophilic mercury forms (particularly, Hg²⁺ and MeHg). Emphasis will be centered on the neurotoxicity of electrophile forms of mercury and selenium. The controversial participation of electrophile mercury and selenium forms in the development of some neurodegenerative disease will be briefly presented. The potential pharmacological use of organoseleno compounds (Ebselen and diphenyl diselenide) in the treatment of mercury poisoning will be considered. The central role of thiol (-SH) and selenol (-SeH) groups as the generic targets of electrophile mercury forms and the need of new in silico tools to guide the future biological researches will be commented.

The potential toxicological insights about the anti-HIV drug azidothymidine-derived monoselenides in human leukocytes: Toxicological insights of new selenium-azidothymidine analogs

Acquired immunodeficiency syndrome (AIDS) is a worldwide disease characterized by impairments of immune function. AIDS can be associated with oxidative stress (OS) that can be linked to selenium (Se) deficiency. Se is fundamental for the synthesis of selenoproteins, such as glutathione peroxidase and thioredoxin reductase. These enzymes catalyze the decomposition of reactive oxygen species and contribute to maintain equilibrium in cell redox status. Literature data indicate that organoselenium compounds, such as ebselen and diphenyl diselenide, have antioxidant properties in vitro and in vivo models associated with OS. Nevertheless, selenocompounds can also react and oxidize thiols groups, inducing toxicity in mammals. Here, we tested the potential cytotoxic and genotoxic properties of six analogs of the prototypal anti-HIV drug azidothymidine (AZT) containing Se (5′-Se-(phenyl)zidovudine; 5′-Se-(1,3,5-trimethylphenyl)zidovudine; 5′-Se-(1-naphtyl)zidovudine; 5′-Se-(4-chlorophenyl)zidovudine) (C4); 5′-Se-(4-methylphenyl)zidovudine (C5); and 5′-(4-methylbenzoselenoate)zidovudine). C5 increased the rate of dithiothreitol oxidation (thiol oxidase activity) and C2-C4 and C6 (at 100 µM) increased DNA damage index (DI) in human leukocytes. Moreover, C5 (200 µM) decreased human leukocyte viability to about 50%. Taken together, these results indicated the low in vitro toxicity in human leukocytes of some Se-containing analogs of AZT.

Polyphenolic Composition and Evaluation of Antioxidant Activity, Osmotic Fragility and Cytotoxic Effects of Raphiodon echinus (Nees & Mart.) Schauer

Raphiodon echinus (R. echinus) is used in Brazilian folk medicine for the treatment of inflammation, coughs, and infectious diseases. However, no information is available on the potential antioxidant, cytotoxicity and genotoxicity of this plant. In this study, the polyphenolic constituents, antioxidant capacity and potential toxic effects of aqueous and ethanolic extracts of R. echinus on human erythrocytes and leukocytes were investigated for the first time. R. echinus extracts showed the presence of Gallic, chlorogenic, caffeic and ellagic acids, rutin, quercitrin and quercetin. Aqueous and ethanolic extracts of R. echinus exhibited antioxidant activity in DPPH radical scavenging with IC50 = 111.9 μg/mL (EtOH extract) and IC50 = 227.9 μg/mL (aqueous extract). The extracts inhibited Fe(2+) (10 μM) induced thiobarbituric acid reactive substances (TBARS) formation in rat brain and liver homogenates. The extracts (30-480 μg/mL) did not induce genotoxicity, cytotoxicity or osmotic fragility in human blood cells. The findings of this present study therefore suggest that the therapeutic effect of R. echinus may be, in part, related to its antioxidant potential. Nevertheless, further in vitro and in vivo studies are required to ascertain the safety margin of its use in folk medicine.