Exploring Covalent Allosteric Inhibition of Antigen 85C from Mycobacterium tuberculosis by Ebselen Derivatives

Trifluoromethylcinnamanilides - Effective dual inhibitors of Mycobacterium smegmatis and Plasmodium falciparum

A series of eighteen new 2-trifluoromethylcinnamanilides (1a-r) were synthesized by microwave synthesis and investigated for their antimycobacterial and antimalarial activities, along with the complementary (2E)-3-[3-(trifluoromethyl)phenyl]-N-arylprop-2-enanilides (2a-r) and (2E)-3-[4-(trifluoromethyl)phenyl]-N-arylprop-2-enanilides (3a-r) prepared earlier. All the compounds were evaluated in vitro against Mycobacterium smegmatis ATCC 700084 and a chloroquine-sensitive strain of Plasmodium falciparum 3D7/MRA-102. The most active compounds against M. smegmatis (MIC values in the range of 1.17-11.1 µM, more effective than rifampicin) were anilides substituted by 3,5-CF3 (1q, 2q, 3q), 4-OCF3 (1k), and 4-CF3 (1j, 2j). The most effective agents against P. falciparum (IC50 values in the range of 0.32-4.5 µM, comparable to chloroquine) were anilides substituted by 3,5-CF3 (1q, 2q, 3q), 2-Br-4-OCF3 (1r), 4-CF3 (1j, 3j), 4-F (2d), 4-Cl (2g), 2-Cl (1e, 2e). A preliminary in vitro cytotoxicity screening was assessed using human leukemic cell lines and human dermal fibroblasts, revealing the toxic effect of 3,5-CF3 substituted anilides. On the other hand, the other investigated agents showed insignificant cytotoxic effects. Stability assays using rat liver microsomes demonstrated that compounds 1r (R = 2-Br-4-OCF3) and 1q (R = 3,5-CF3) are neither metabolized nor affect cytochrome P450 metabolizing capacity in vitro. Furthermore, complex in silico studies were performed - a combined approach (docking/MD simulations/QTAIM calculations) helped to define the molecular interactions that were applied during the binding of active agents and the subsequent inhibition of their molecular targets - InhA (activity against M. smegmatis) and arginase (activity against P. falciparum). In conclusion, promising active agents with dual antimycobacterial and antimalarial effects were identified.

Enhance the Antimycobacterial Activity of Streptomycin with Ebselen as an Antibiotic Adjuvant Through Disrupting Redox Homeostasis

Purpose

Tuberculosis (TB) remains a major health threat worldwide, and the spread of drug-resistant (DR) TB impedes the reduction of the global disease burden. Ebselen (EbSe) targets bacterial thioredoxin reductase (bTrxR) and causes an imbalance in the redox status of bacteria. Previous work has shown that the synergistic action of bTrxR and sensitization to common antibiotics by EbSe is a promising strategy for the treatment of DR pathogens. Thus, we aimed to evaluate whether EbSe could enhance anti-TB drugs against Mycobacterium marinum (M. marinum) which is genetically related to Mycobacterium tuberculosis (Mtb) and resistant to many antituberculosis drugs.

Methods

Minimum inhibitory concentrations (MIC) of isoniazid (INH), rifampicin (RFP), and streptomycin (SM) against M. marinum were determined by microdilution. The Bliss Independence Model was used to determine the adjuvant effects of EbSe over the anti-TB drugs. Thioredoxin reductase activity was measured using the DTNB assay, and its effects on bacterial redox homeostasis were verified by the elevation of intracellular ROS levels and intracellular GSH levels. The adjuvant efficacy of EbSe as an anti-TB drug was further evaluated in a mouse model of M. marinum infection. Cytotoxicity was observed in the macrophage cells Raw264.7 and mice model.

Results

The results reveal that EbSe acts as an antibiotic adjuvant over SM on M. marinum. EbSe + SM disrupted the intracellular redox microenvironment of M. marinum by inhibiting bTrxR activity, which could rescue mice from the high bacterial load, and accelerated recovery from tail injury with low mammalian toxicity.

Conclusion

The above studies suggest that EbSe significantly enhanced the anti-Mtb effect of SM, and its synergistic combination showed low mammalian toxicity in vitro and in vivo. Further efforts are required to study the underlying mechanisms of EbSe as an antibiotic adjuvant in combination with anti-TB drug MS.

Ebselen: A Review on its Synthesis, Derivatives, Anticancer Efficacy and Utility in Combating SARS-COV-2

Ebselen is a selenoorganic chiral compound with antioxidant properties comparable to glutathione peroxidase. It is also known as 2-phenyl-1,2-benzisoselenazol-3(2H)-one. In studies examining its numerous pharmacological activities, including antioxidant, anticancer, antiviral, and anti- Alzheimer's, ebselen has demonstrated promising results. This review's primary objective was to emphasize the numerous synthesis pathways of ebselen and their efficacy in fighting cancer. The data were collected from multiple sources, including Scopus, PubMed, Google Scholar, Web of Science, and Publons. The starting reagents for the synthesis of ebselen are 2-aminobenzoic acid and N-phenyl benzamide. It was discovered that ebselen has the ability to initiate apoptosis in malignant cells and prevent the formation of new cancer cells by scavenging free radicals. In addition, ebselen increases tumor cell susceptibility to apoptosis by inhibiting TNF-α mediated NF-kB activation. Ebselen can inhibit both doxorubicin and daunorubicin-induced cardiotoxicity. Allopurinol and ebselen administered orally can be used to suppress renal ototoxicity and nephrotoxicity. Due to excessive administration, diclofenac can induce malignancy of the gastrointestinal tract, which ebselen can effectively suppress. Recent research has demonstrated ebselen to inhibit viral function by binding to cysteinecontaining catalytic domains of various viral proteases. It was discovered that ebselen could inhibit the catalytic dyad function of Mpro by forming an irreversible covalent bond between Se and Cys145, thereby altering protease function and inhibiting SARS-CoV-2. Ebselen may also inhibit the activation of endosomal NADPH oxidase of vascular endothelial cells, which is believed to be required for thrombotic complications in COVID-19. In this review, we have included various studies conducted on the anticancer effect of ebselen as well as its inhibition of SARS-CoV-2.

Glutathione Peroxidase-like Antioxidant Activity of 1,3-Benzoselenazoles: Synthesis and In Silico Molecular Docking Studies as Pancreatic Lipase Inhibitors

The synthesis of 1,3-benzoselenazoles was achieved by the reaction of corresponding bis[3-amino-N-(p-tolyl)benzamide-2-yl] diselenide, bis[3-amino-N-(4-methoxyphenyl)benzamide-2-yl] diselenide, and bis[3-amino-N-(4-(dimethylamino)phenyl) benzamide-2-yl] diselenide with aryl aldehydes. The 1,3-benzoselenazoles continued to exist as planar molecules due to the presence of secondary Se···O interactions as revealed by the single-crystal X-ray analysis. The presence of secondary Se···O interactions in 1,3-benzoselenazoles was confirmed using natural bond orbital (NBO) and atoms in molecules (AIM) calculations. Nucleus-independent chemical shift (NICS) values suggested the presence of aromatic character in a five-membered benzoselenazole heterocyclic ring. The glutathione peroxidase (GPx)-like antioxidant activity of all 1,3-benzoselenazoles was assessed using a thiophenol assay, exhibiting greater antioxidant activity than Ph2Se2 used as a reference. The most active catalyst carrying a strong electron-donating group (-NMe2) at the ortho-position to the benzoselenazole ring was further investigated at different concentrations of thiophenol, H2O2, and 1,3-benzoselenazoles as catalyst for determining their catalytic parameters. Moreover, the potential applications of all 1,3-benzoselenazoles against pancreatic lipase (PL) have been identified using in silico interactions between the active sites of the 1LPB protein as evaluated using a molecular docking study.

Ellagic Acid and Its Metabolites as Potent and Selective Allosteric Inhibitors of Liver Pyruvate Kinase

Liver pyruvate kinase (PKL) has recently emerged as a new target for non-alcoholic fatty liver disease (NAFLD), and inhibitors of this enzyme could represent a new therapeutic option. However, this breakthrough is complicated by selectivity issues since pyruvate kinase exists in four different isoforms. In this work, we report that ellagic acid (EA) and its derivatives, present in numerous fruits and vegetables, can inhibit PKL potently and selectively. Several polyphenolic analogues of EA were synthesized and tested to identify the chemical features responsible for the desired activity. Molecular modelling studies suggested that this inhibition is related to the stabilization of the PKL inactive state. This unique inhibition mechanism could potentially herald the development of new therapeutics for NAFLD.

Optimized Ebselen-Based Inhibitors of Bacterial Ureases with Nontypical Mode of Action

Screening of 25 analogs of Ebselen, diversified at the N-aromatic residue, led to the identification of the most potent inhibitors of Sporosarcina pasteurii urease reported to date. The presence of a dihalogenated phenyl ring caused exceptional activity of these 1,2-benzisoselenazol-3(2H)-ones, with K_i value in a low picomolar range (<20 pM). The affinity was attributed to the increased π-π and π-cation interactions of the dihalogenated phenyl ring with αHis323 and αArg339 during the initial step of binding. Complementary biological studies with selected compounds on the inhibition of ureolysis in whole Proteus mirabilis cells showed a very good potency (IC₅₀ < 25 nM in phosphate-buffered saline (PBS) buffer and IC₉₀ < 50 nM in a urine model) for monosubstituted N-phenyl derivatives. The crystal structure of S. pasteurii urease inhibited by one of the most active analogs revealed the recurrent selenation of the Cys322 thiolate, yielding an unprecedented Cys322-S-Se-Se chemical moiety.

Antibacterial Activity of Ebselen

Ebselen is a low-molecular-weight organoselenium compound that has been broadly studied for its antioxidant, anti-inflammatory, and cytoprotective properties. These advantageous properties were initially associated with mimicking the activity of selenoprotein glutathione peroxidase, but the biomedical impact of this compound appear to be far more complex. Ebselen serves as a substrate or inhibitor with multiple protein/enzyme targets, whereas inhibition typically originates from the covalent modification of cysteine residues by opening the benzisoselenazolone ring and S-Se bond formation. The inhibition of enzymes of various classes and origins has been associated with substantial antimicrobial potential among other activities. In this contribution, we summarize the current state of the art regarding the antibacterial activity of ebselen. This activity, alone and in combination with commercial pharmaceuticals, against pathogens, including those resistant to drugs, is presented, together with the molecular mechanism behind the reactivity. The specific inactivation of thioredoxin reductase, bacterial toxins, and other resistance factors is considered to have certain therapeutic implications. Synergistic action and sensitization to common antibiotics assisted with the use of ebselen appear to be promising directions in the treatment of persistent infections.

Discovery and mechanism studies of a novel ATG4B inhibitor Ebselen by drug repurposing and its anti-colorectal cancer effects in mice

Cysteine protease ATG4B, a key autophagy protein, is an attractive target for colorectal cancer therapy. However, ATG4B inhibitors with higher efficiency, safety, and clear mechanism are still limited. In this study, we discovered ATG4B inhibitors based on the FDA-approved drug library through FRET-based high-throughput screening and gel-based analysis. Among the nine hits, compound Ebselen showed the most potent ATG4B inhibitory activity (IC₅₀ = 189 nM) and exhibited controllable selectivity and structural optimizable possibility against ATG4A and caspases. We then performed mass spectrometry assay and cysteine mutations to confirm that Ebselen could covalently bind to ATG4B at Cys74. Moreover, Cys292 and Cys361 instead of Cys74 are responsible for the redox-oligomerization and efficient activity inhibition of ATG4B. Ultimately through cell culture and mouse xenograft tumor models, we established the impact of Ebselen on autophagy and tumor suppression via ATG4B inhibition other than apoptosis. These results suggest that old drug Ebselen as an ATG4B inhibitor through oxidative modification may be repurposed as a promising anti-colorectal cancer drug.

Small-Molecule Ebselen Binds to YTHDF Proteins Interfering with the Recognition of N 6-Methyladenosine-Modified RNAs

YTHDF proteins bind the N 6-methyladenosine (m6A)-modified mRNAs, influencing their processing, stability, and translation. Therefore, the members of this protein family play crucial roles in gene regulation and several physiological and pathophysiological conditions. YTHDF proteins contain a hydrophobic pocket that accommodates the m6A embedded in the RRACH consensus sequence on mRNAs. We exploited the presence of this cage to set up an m6A-competitive assay and performed a high-throughput screen aimed at identifying ligands binding in the m6A pocket. We report the organoselenium compound ebselen as the first-in-class inhibitor of the YTHDF m6A-binding domain. Ebselen, whose interaction with YTHDF proteins was validated via orthogonal assays, cannot discriminate between the binding domains of the three YTHDF paralogs but can disrupt the interaction of the YTHDF m6A domain with the m6A-decorated mRNA targets. X-ray, mass spectrometry, and NMR studies indicate that in YTHDF1 ebselen binds close to the m6A cage, covalently to the Cys412 cysteine, or interacts reversibly depending on the reducing environment. We also showed that ebselen engages YTHDF proteins within cells, interfering with their mRNA binding. Finally, we produced a series of ebselen structural analogs that can interact with the YTHDF m6A domain, proving that ebselen expansion is amenable for developing new inhibitors. Our work demonstrates the feasibility of drugging the YTH domain in YTHDF proteins and opens new avenues for the development of disruptors of m6A recognition.

Improving the biotransformation efficiency of soybean phytosterols in Mycolicibacterium neoaurum by the combined deletion of fbpC3 and embC in cell envelope synthesis

Biotransformation of soybean phytosterols into 9α-hydroxy-4-androstene-3,17-dione (9-OHAD) by mycobacteria is the core step in the synthesis of adrenocortical hormone. However, the low permeability of the dense cell envelope largely inhibits the overall conversion efficiency of phytosterols. The antigen 85 (Ag85) complex encoded by fbpA, fbpB, and fbpC was proposed as the key factor in the combined catalysis of mycoloyl for producing mycolyl-arabinogalactan (m-AG) and trehalose dimycolate (TDM) in mycobacterial cell envelope. Herein, we confirmed that fbpC3 was essential for the biotransformation of trehalose monomycolate (TMM) to TDM in Mycolicibacterium neoaurum. The deficiency of this gene raised the cell permeability, thereby enhancing the steroid uptake and utilization. The 9-OHAD yield in the fbpC3-deficient 9-OHAD-producing strain was increased by 21.3%. Moreover, the combined deletion of fbpC3 and embC further increased the 9-OHAD yield compared to the single deletion of fbpC3. Finally, after 96 h of bioconversion in industrial resting cells, the 9-OHAD yield of 11.2 g/L was achieved from 20 g/L phytosterols and the productivity reached 0.116 g/L/h. In summary, this study suggested the critical role of the fbpC3 gene in the synthesis of TDM in M. neoaurum and verified the feasibility of improving the bioconversion efficiency of phytosterols through the cell envelope engineering strategy.

The Se-S Bond Formation in the Covalent Inhibition Mechanism of SARS-CoV-2 Main Protease by Ebselen-like Inhibitors: A Computational Study

The inhibition mechanism of the main protease (Mpro) of SARS-CoV-2 by ebselen (EBS) and its analog with a hydroxyl group at position 2 of the benzisoselenazol-3(2H)-one ring (EBS-OH) was studied by using a density functional level of theory. Preliminary molecular dynamics simulations on the apo form of Mpro were performed taking into account both the hydrogen donor and acceptor natures of the Nδ and Nε of His41, a member of the catalytic dyad. The potential energy surfaces for the formation of the Se-S covalent bond mediated by EBS and EBS-OH on Mpro are discussed in detail. The EBS-OH shows a distinctive behavior with respect to EBS in the formation of the noncovalent complex. Due to the presence of canonical H-bonds and noncanonical ones involving less electronegative atoms, such as sulfur and selenium, the influence on the energy barriers and reaction energy of the Minnesota hybrid meta-GGA functionals M06, M06-2X and M08HX, and the more recent range-separated hybrid functional wB97X were also considered. The knowledge of the inhibition mechanism of Mpro by the small protease inhibitors EBS or EBS-OH can enlarge the possibilities for designing more potent and selective inhibitor-based drugs to be used in combination with other antiviral therapies.

Cysteine Modification by Ebselen Reduces the Stability and Cellular Levels of 14-3-3 Proteins

The 14-3-3 proteins constitute a family of adaptor proteins with many binding partners and biological functions, and they are considered promising drug targets in cancer and neuropsychiatry. By screening 1280 small-molecule drugs using differential scanning fluorimetry (DSF), we found 15 compounds that decreased the thermal stability of 14-3-3ζ Among these compounds, ebselen was identified as a covalent, destabilizing ligand of 14-3-3 isoforms ζ, ε, γ, and η Ebselen bonding decreased 14-3-3ζ binding to its partner Ser19-phosphorylated tyrosine hydroxylase. Characterization of site-directed mutants at cysteine residues in 14-3-3ζ (C25, C94, and C189) by DSF and mass spectroscopy revealed covalent modification by ebselen of all cysteines through a selenylsulfide bond. C25 appeared to be the preferential site of ebselen interaction in vitro, whereas modification of C94 was the main determinant for protein destabilization. At therapeutically relevant concentrations, ebselen and ebselen oxide caused decreased 14-3-3 levels in SH-SY5Y cells, accompanied with an increased degradation, most probably by the ubiquitin-dependent proteasome pathway. Moreover, ebselen-treated zebrafish displayed decreased brain 14-3-3 content, a freezing phenotype, and reduced mobility, resembling the effects of lithium, consistent with its proposed action as a safer lithium-mimetic drug. Ebselen has recently emerged as a promising drug candidate in several medical areas, such as cancer, neuropsychiatric disorders, and infectious diseases, including coronavirus disease 2019. Its pleiotropic actions are attributed to antioxidant effects and formation of selenosulfides with critical cysteine residues in proteins. Our work indicates that a destabilization of 14-3-3 may affect the protein interaction networks of this protein family, contributing to the therapeutic potential of ebselen. SIGNIFICANCE STATEMENT: There is currently great interest in the repurposing of established drugs for new indications and therapeutic targets. This study shows that ebselen, which is a promising drug candidate against cancer, bipolar disorder, and the viral infection coronavirus disease 2019, covalently bonds to cysteine residues in 14-3-3 adaptor proteins, triggering destabilization and increased degradation in cells and intact brain tissue when used in therapeutic concentrations, potentially explaining the behavioral, anti-inflammatory, and antineoplastic effects of this drug.

Ebselen and Analogues: Pharmacological Properties and Synthetic Strategies for Their Preparation

Ebselen is the leader of selenorganic compounds, and starting from its identification as mimetic of the key antioxidant enzyme glutathione peroxidase, several papers have appeared in literature claiming its biological activities. It was the subject of several clinical trials and it is currently in clinical evaluation for the treatment of COVID-19 patients. Given our interest in the synthesis and pharmacological evaluation of selenorganic derivatives with this review, we aimed to collect all the papers focused on the biological evaluation of ebselen and its close analogues, covering the timeline between 2016 and most of 2021. Our analysis evidences that, even if it lacks specificity when tested in vitro, being able to bind to every reactive cysteine, it proved to be always well tolerated in vivo, exerting no sign of toxicity whatever the administered doses. Besides, looking at the literature, we realized that no review article dealing with the synthetic approaches for the construction of the benzo[d][1,2]-selenazol-3(2H)-one scaffold is available; thus, a section of the present review article is completely devoted to this specific topic.

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.

Ebselen, Disulfiram, Carmofur, PX-12, Tideglusib, and Shikonin Are Nonspecific Promiscuous SARS-CoV-2 Main Protease Inhibitors

Among the drug targets being investigated for SARS-CoV-2, the viral main protease (M^pro) is one of the most extensively studied. M^pro is a cysteine protease that hydrolyzes the viral polyprotein at more than 11 sites. It is highly conserved and has a unique substrate preference for glutamine in the P1 position. Therefore, M^pro inhibitors are expected to have broad-spectrum antiviral activity and a high selectivity index. Structurally diverse compounds have been reported as M^pro inhibitors. In this study, we investigated the mechanism of action of six previously reported M^pro inhibitors, ebselen, disulfiram, tideglusib, carmofur, shikonin, and PX-12, using a consortium of techniques including FRET-based enzymatic assay, thermal shift assay, native mass spectrometry, cellular antiviral assays, and molecular dynamics simulations. Collectively, the results showed that the inhibition of M^pro by these six compounds is nonspecific and that the inhibition is abolished or greatly reduced with the addition of reducing reagent 1,4-dithiothreitol (DTT). Without DTT, these six compounds inhibit not only M^pro but also a panel of viral cysteine proteases including SARS-CoV-2 papain-like protease and 2A^pro and 3C^pro from enterovirus A71 (EV-A71) and EV-D68. However, none of the compounds inhibits the viral replication of EV-A71 or EV-D68, suggesting that the enzymatic inhibition potency IC₅₀ values obtained in the absence of DTT cannot be used to faithfully predict their cellular antiviral activity. Overall, we provide compelling evidence suggesting that these six compounds are nonspecific SARS-CoV-2 M^pro inhibitors and urge the scientific community to be stringent with hit validation.

Ebselen, disulfiram, carmofur, PX-12, tideglusib, and shikonin are non-specific promiscuous SARS-CoV-2 main protease inhibitors

There is an urgent need for vaccines and antiviral drugs to combat the COVID-19 pandemic. Encouraging progress has been made in developing antivirals targeting SARS-CoV-2, the etiological agent of COVID-19. Among the drug targets being investigated, the viral main protease (M pro ) is one of the most extensively studied drug targets. M pro is a cysteine protease that hydrolyzes the viral polyprotein at more than 11 sites and it is highly conserved among coronaviruses. In addition, M pro has a unique substrate preference for glutamine in the P1 position. Taken together, it appears that M pro inhibitors can achieve both broad-spectrum antiviral activity and a high selectivity index. Structurally diverse compounds have been reported as M pro inhibitors, with several of which also showed antiviral activity in cell culture. In this study, we investigated the mechanism of action of six previously reported M pro inhibitors, ebselen, disulfiram, tideglusib, carmofur, shikonin, and PX-12 using a consortium of techniques including FRET-based enzymatic assay, thermal shift assay, native mass spectrometry, cellular antiviral assays, and molecular dynamics simulations. Collectively, the results showed that the inhibition of M pro by these six compounds is non-specific and the inhibition is abolished or greatly reduced with the addition of reducing reagent DTT. In the absence of DTT, these six compounds not only inhibit M pro , but also a panel of viral cysteine proteases including SARS-CoV-2 papain-like protease, the 2A pro and 3C pro from enterovirus A71 (EV-A71) and EV-D68. However, none of the compounds inhibits the viral replication of EV-A71 or EV-D68, suggesting that the enzymatic inhibition potency IC 50 values obtained in the absence of DTT cannot be used to faithfully predict their cellular antiviral activity. Overall, we provide compelling evidence suggesting that ebselen, disulfiram, tideglusib, carmofur, shikonin, and PX-12 are non-specific SARS-CoV-2 M pro inhibitors, and urge the scientific community to be stringent with hit validation.

Review of the Syntheses and Activities of Some Sulfur-Containing Drugs

Background:

Sulfur-containing compounds represent an important class of chemical compounds due to their wide range of biological and pharmaceutical properties. Moreover, sulfur-containing compounds may be applied in other fields, such as biological, organic, and materials chemistry. Several studies on the activities of sulfur compounds have already proven their anti-inflammatory properties and use to treat diseases, such as Alzheimer’s, Parkinson’s, and HIV. Moreover, examples of sulfur-containing compounds include dapsone, quetiapine, penicillin, probucol, and nelfinavir, which are important drugs with known activities.

Objective:

This review will focus on the synthesis and application of some sulfur-containing compounds used to treat several diseases, as well as promising new drug candidates.

Results:

Due to the variety of compounds containing C-S bonds, we have reviewed the different synthetic routes used toward the synthesis of sulfur-containing drugs and other compounds.

Fragment-Based Design of Mycobacterium tuberculosis InhA Inhibitors

Tuberculosis (TB) remains a leading cause of mortality among infectious diseases worldwide. InhA has been the focus of numerous drug discovery efforts as this is the target of the first line pro-drug isoniazid. However, with resistance to this drug becoming more common, the aim has been to find new clinical candidates that directly inhibit this enzyme and that do not require activation by the catalase peroxidase KatG, thus circumventing the majority of the resistance mechanisms. In this work, the screening and validation of a fragment library are described, and the development of the fragment hits using a fragment growing strategy was employed, which led to the development of InhA inhibitors with affinities of up to 250 nM.

Mechanical Forces between Mycobacterial Antigen 85 Complex and Fibronectin

Adhesion to extracellular matrix proteins is an important first step in host invasion, employed by many bacterial pathogens. In mycobacteria, the secreted Ag85 complex proteins, involved in the synthesis of the cell envelope, are known to bind to fibronectin (Fn) through molecular forces that are currently unknown. In this study, single-molecule force spectroscopy is used to study the strength, kinetics and thermodynamics of the Ag85-Fn interaction, focusing on the multidrug-resistant Mycobacterium abscessus species. Single Ag85 proteins bind Fn with a strength of ~75 pN under moderate tensile loading, which compares well with the forces reported for other Fn-binding proteins. The binding specificity is demonstrated by using free Ag85 and Fn peptides with active binding sequences. The Ag85-Fn rupture force increases with mechanical stress (i.e., loading rate) according to the Friddle–Noy–de Yoreo theory. From this model, we extract thermodynamic parameters that are in good agreement with previous affinity determinations by surface plasmon resonance. Strong bonds (up to ~500 pN) are observed under high tensile loading, which may favor strong mycobacterial attachment in the lung where cells are exposed to high shear stress or during hematogenous spread which leads to a disseminated infection. Our results provide new insight into the pleiotropic functions of an important mycobacterial virulence factor that acts as a stress-sensitive adhesin.

Development and Therapeutic Potential of Selenazo Compounds

Incorporation of selenium (Se) atom into small molecules can substantially enhance their antioxidant, anti-inflammatory, antimutagenic, antitumoral or chemopreventive, antiviral, antibacterial, antifungal, antiparasitic, and neuroprotective effects. Specifically, selenazo compounds have received great attention owing to their chemical properties, pharmaceutical applications, and low toxicity. In this Perspective, we compile extensive literature evidence with the description and discussion of the most recent advances in different selenazo and selenadiazo motifs as potential pharmacological candidates. We also provide some perspectives on the challenges and future directions in the advancement of these selenazo compounds, each of which could generate drug candidates for various diseases.

Identification of ebselen as a potent inhibitor of insulin degrading enzyme by a drug repurposing screening

Insulin-degrading enzyme, IDE, is a metalloprotease implicated in the metabolism of key peptides such as insulin, glucagon, β-amyloid peptide. Recent studies have pointed out its broader role in the cell physiology. In order to identify new drug-like inhibitors of IDE with optimal pharmacokinetic properties to probe its multiple roles, we ran a high-throughput drug repurposing screening. Ebselen, cefmetazole and rabeprazole were identified as reversible inhibitors of IDE. Ebselen is the most potent inhibitor (IC_50(insulin) = 14 nM). The molecular mode of action of ebselen was investigated by biophysical methods. We show that ebselen induces the disorder of the IDE catalytic cleft, which significantly differs from the previously reported IDE inhibitors. IDE inhibition by ebselen can explain some of its reported activities in metabolism as well as in neuroprotection.

Trehalose Conjugation Enhances Toxicity of Photosensitizers against Mycobacteria

Trehalose is a natural glucose-derived disaccharide found in the cell wall of mycobacteria. It enters the mycobacterial cell through a highly specific trehalose transporter system. Subsequently, trehalose is equipped with mycolic acid species and is incorporated into the cell wall as trehalose monomycolate or dimycolate. Here, we investigate the phototoxicity of several photosensitizer trehalose conjugates and take advantage of the promiscuity of the extracellular Ag85 complex, which catalyzes the attachment of mycolic acids to trehalose and its analogues. We find that processing by Ag85 enriches and tethers photosensitizer trehalose conjugates directly into the mycomembrane. Irradiation of the conjugates triggers singlet oxygen formation, killing mycobacterial cells more efficiently, as compared to photosensitizers without trehalose conjugation. The conjugates are potent antimycobacterial agents that are, per se, affected neither by permeability issues nor by detoxification mechanisms via drug efflux. They could serve as interesting scaffolds for photodynamic therapy of mycobacterial infections.

Mycolyltransferase from Mycobacterium tuberculosis in covalent complex with tetrahydrolipstatin provides insights into antigen 85 catalysis

Mycobacterium tuberculosis antigen 85 (Ag85) enzymes catalyze the transfer of mycolic acid (MA) from trehalose monomycolate to produce the mycolyl arabinogalactan (mAG) or trehalose dimycolate (TDM). These lipids define the protective mycomembrane of mycobacteria. The current model of substrate binding within the active sites of Ag85s for the production of TDM is not sterically and geometrically feasible; additionally, this model does not account for the production of mAG. Furthermore, this model does not address how Ag85s limit the hydrolysis of the acyl-enzyme intermediate while catalyzing acyl transfer. To inform an updated model, we obtained an Ag85 acyl-enzyme intermediate structure that resembles the mycolated form. Here, we present a 1.45-Å X-ray crystal structure of M. tuberculosis Ag85C covalently modified by tetrahydrolipstatin (THL), an esterase inhibitor that suppresses M. tuberculosis growth and mimics structural attributes of MAs. The mode of covalent inhibition differs from that observed in the reversible inhibition of the human fatty-acid synthase by THL. Similarities between the Ag85-THL structure and previously determined Ag85C structures suggest that the enzyme undergoes structural changes upon acylation, and positioning of the peptidyl arm of THL limits hydrolysis of the acyl-enzyme adduct. Molecular dynamics simulations of the modeled mycolated-enzyme form corroborate the structural analysis. From these findings, we propose an alternative arrangement of substrates that rectifies issues with the previous model and suggest a direct role for the β-hydroxy of MA in the second half-reaction of Ag85 catalysis. This information affords the visualization of a complete mycolyltransferase catalytic cycle.

Cyclipostins and cyclophostin analogs inhibit the antigen 85C from Mycobacterium tuberculosis both in vitro and in vivo

An increasing prevalence of cases of drug-resistant tuberculosis requires the development of more efficacious chemotherapies. We previously reported the discovery of a new class of cyclipostins and cyclophostin (CyC) analogs exhibiting potent activity against Mycobacterium tuberculosis both in vitro and in infected macrophages. Competitive labeling/enrichment assays combined with MS have identified several serine or cysteine enzymes in lipid and cell wall metabolism as putative targets of these CyC compounds. These targets included members of the antigen 85 (Ag85) complex (i.e. Ag85A, Ag85B, and Ag85C), responsible for biosynthesis of trehalose dimycolate and mycolylation of arabinogalactan. Herein, we used biochemical and structural approaches to validate the Ag85 complex as a pharmacological target of the CyC analogs. We found that CyC_7β, CyC_8β, and CyC₁₇ bind covalently to the catalytic Ser¹²⁴ residue in Ag85C; inhibit mycolyltransferase activity (i.e. the transfer of a fatty acid molecule onto trehalose); and reduce triacylglycerol synthase activity, a property previously attributed to Ag85A. Supporting these results, an X-ray structure of Ag85C in complex with CyC_8β disclosed that this inhibitor occupies Ag85C's substrate-binding pocket. Importantly, metabolic labeling of M. tuberculosis cultures revealed that the CyC compounds impair both trehalose dimycolate synthesis and mycolylation of arabinogalactan. Overall, our study provides compelling evidence that CyC analogs can inhibit the activity of the Ag85 complex in vitro and in mycobacteria, opening the door to a new strategy for inhibiting Ag85. The high-resolution crystal structure obtained will further guide the rational optimization of new CyC scaffolds with greater specificity and potency against M. tuberculosis.

Thermal and Photoinduced Copper-Promoted C-Se Bond Formation: Synthesis of 2-Alkyl-1,2-benzisoselenazol-3(2H)-ones and Evaluation against Mycobacterium tuberculosis

2-Alkyl-1,2-benzisoselenazol-3(2H)-ones, represented by ebselen (1a), are being studied intensively for a range of medicinal applications. We describe both a new thermal and photoinduced copper-mediated cross-coupling between potassium selenocyanate (KSeCN) and N-substituted ortho-halobenzamides to form 2-alkyl-1,2-benzisoselenazol-3(2H)-ones containing a C-Se-N bond. The copper ligand (1,10-phenanthroline) facilitates C-Se bond formation during heating via a mechanism that likely involves atom transfer (AT), whereas, in the absence of ligand, photoinduced activation likely proceeds through a single electron transfer (SET) mechanism. A library of 15 2-alkyl-1,2-benzisoselenazol-3(2H)-ones was prepared. One member of the library was azide-containing derivative 1j that was competent to undergo a strain-promoted azide-alkyne cycloaddition. The library was evaluated for inhibition of Mycobacterium tuberculosis (Mtb) growth and Mtb Antigen 85C (Mtb Ag85C) activity. Compound 1f was most potent with a minimal inhibitory concentration (MIC) of 12.5 μg/mL and an Mtb Ag85C apparent IC₅₀ of 8.8 μM.