Chemical, biochemical and microbiological properties of a brominated nitrovinylfuran with broad-spectrum antibacterial activity

Halogenated Antimicrobial Agents to Combat Drug-Resistant Pathogens

Antimicrobial resistance presents us with a potential global crisis as it undermines the abilities of conventional antibiotics to combat pathogenic microbes. The history of antimicrobial agents is replete with examples of scaffolds containing halogens. In this review, we discuss the impacts of halogen atoms in various antibiotic types and antimicrobial scaffolds and their modes of action, structure-activity relationships, and the contributions of halogen atoms in antimicrobial activity and drug resistance. Other halogenated molecules, including carbohydrates, peptides, lipids, and polymeric complexes, are also reviewed, and the effects of halogenated scaffolds on pharmacokinetics, pharmacodynamics, and factors affecting antimicrobial and antivirulence activities are presented. Furthermore, the potential of halogenation to circumvent antimicrobial resistance and rejuvenate impotent antibiotics is addressed. This review provides an overview of the significance of halogenation, the abilities of halogens to interact in biomolecular settings and enhance pharmacological properties, and their potential therapeutic usages in preventing a postantibiotic era. SIGNIFICANCE STATEMENT: Antimicrobial resistance and the increasing impotence of antibiotics are critical threats to global health. The roles and importance of halogen atoms in antimicrobial drug scaffolds have been established, but comparatively little is known of their pharmacological impacts on drug resistance and antivirulence activities. This review is the first to extensively evaluate the roles of halogen atoms in various antibiotic classes and pharmacological scaffolds and to provide an overview of their ability to overcome antimicrobial resistance.

Targeting the binding pocket of the fluorophore 8-anilinonaphthalene-1-sulfonic acid in the bacterial enzyme MurA

8-Anilinonaphthalene-1-sulfonic acid (ANS) has been extensively used as a fluorescent probe to detect conformational changes of proteins. It has been cocrystallized with several of the proteins it is used to monitor, including the bacterial cell wall synthesis enzyme MurA. MurA catalyzes the first committed step of peptidoglycan biosynthesis, converting UDP-N-acetylglucosamine (UDP-GlcNAc) into enolpyruvyl UDP-GlcNAc. It has been reported before that ANS binds to MurA from Enterobacter cloacae without inhibiting the enzyme's activity up to a concentration of 1 mM ANS. In this study, we present evidence that ANS inhibits the activity of several isoforms of MurA with IC50 values of 18, 22, and 31 µM against wild-type Escherichia coli, C115D E. coli, and E. cloacae MurA, respectively. This prompted us to test a larger series of structural analogs of ANS for the inhibition of these MurA enzymes, which led to the discovery of compound 26. This ANS analog showed enhanced inhibition of MurA (WT and C115D MurA from E. coli, and E. cloacae MurA) with IC50 values of 2.7, 10, and 14 µM, respectively. Based on our results, the ANS binding pocket was identified as a novel target site for the development of potential antibiotics.

Potential Inhibitors Targeting Escherichia coli UDP-N-Acetylglucosamine Enolpyruvyl Transferase (MurA): An Overview

Antibiotic resistance is one of the biggest challenges that is escalating and affecting humanity across the globe. To overcome this increasing burden of resistance, discovering novel hits by targeting the enzymes involved in peptidoglycan (murein) biosynthesis has always been considered better in antimicrobial drug discovery. UDP-N-acetylglucosamine enolpyruvyl transferase (MurA) enzyme has been identified as essential for Escherichia coli survival and catalyzes the early-stage step in bacterial cell wall synthesis. The present article gives a brief overview of the role of enzymes in peptidoglycan synthesis and MurA enzyme (previously known as MurZ in E. coli), in particular, including its structural and active site features. This review also provides an insight into the current knowledge of the reported MurA inhibitors, their mechanism of action and drawbacks of these hits that hinder their clinical trials, which would be helpful for synthesis and discovering potent molecules.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12088-021-00988-6.

Screening of compound library identifies novel inhibitors against the MurA enzyme of Escherichia coli

Bacterial cell has always been an attractive target for anti-infective drug discovery. MurA (UDP-N-acetylglucosamine enolpyruvyl transferase) enzyme of Escherichia coli (E.coli) is crucial for peptidoglycan biosynthetic pathway, as it is involved in the early stages of bacterial cell wall biosynthesis. In the present study we aim to identify novel chemical structures targeting the MurA enzyme. For screening purpose, we used in silico approach (pharmacophore based strategy) for 52,026 library compounds (Chembridge, Chemdiv and in house synthetics) which resulted in identification of 50 compounds. These compounds were screened in vitro against MurA enzyme and release of inorganic phosphate (Pi) was estimated. Two compounds (IN00152 and IN00156) were found to inhibit MurA enzyme > 70% in primary screening and IC₅₀ of 14.03 to 32.30 μM respectively. These two hits were further evaluated for their mode of inhibition studies and whole-cell activity where we observed 2-4 folds increase in activity in presence of Permeabilizer EDTA (Ethylenediaminetetraacetic acid). Combination studies were also performed with known antibiotics in presence of EDTA. Hits are reported for the first time against this target and our report also support the use of OM permeabilizer in combination with antibacterial compounds to address the permeability and efficacy issue. These lead hits can be further optimized for drug discovery. KEY POINTS: • Emerging Gram negative resistant strains is a matter of concern. • Need for new screening strategies to cope with drying up antibiotics pipeline. • Outer membrane permeabilizers could be useful to improve potency of molecules to reach its target.

2-(2-Methyl-2-nitrovinyl)furan but Not Furvina Interfere with Staphylococcus aureus Agr Quorum-Sensing System and Potentiate the Action of Fusidic Acid against Biofilms

Quorum sensing (QS) plays an essential role in the production of virulence factors, in biofilm formation and antimicrobial resistance. Consequently, inhibiting QS is being considered a promising target for antipathogenic/anti-virulence therapies. This study aims to screen 2-nitrovinylfuran derivatives structurally related to Furvina (a broad-spectrum antibiotic already used for therapeutic purposes) for their effects on QS and in biofilm prevention/control. Furvina and four 2-nitrovinylfuran derivatives (compounds 1–4) were tested to assess the ability to interfere with QS of Staphylococcus aureus using bioreporter strains (S. aureus ALC1742 and ALC1743). The activity of Furvina and the most promising quorum-sensing inhibitor (QSI) was evaluated in biofilm prevention and in biofilm control (combined with fusidic acid). The biofilms were further characterized in terms of biofilm mass, viability and membrane integrity. Compound 2 caused the most significant QS inhibition with reductions between 60% and 80%. Molecular docking simulations indicate that this compound interacts preferentially with the protein hydrophobic cleft in the LytTR domain of AgrA pocket. Metabolic inactivations of 40% for S. aureus ALC1742 and 20% for S. aureus ALC1743 were reached. A 24 h-old biofilm formed in the presence of the QSI increased the metabolic inactivation by fusidic acid to 80%, for both strains. The overall results highlight the effects of compound 2 as well as the potential of combining QSI with in-use antibiotics for the management of skin and soft tissues infections.

Antibacterial Agents Targeting the Bacterial Cell Wall

The introduction of antibiotics to treat bacterial infections either by killing or blocking their growth has been accompanied by the studies of mechanism that allows the drugs to kill the bacteria or to stop their proliferation. In such a scenario, the emergence of antibacterial agents active on the bacterial cell wall has been of fundamental importance in the fight against bacterial agents responsible for severe diseases. As a matter of fact, the cell wall, which plays many roles during the lifecycle, is an essential constituent of most bacteria. This overview focuses on the intracellular steps of peptidoglycan biosynthesis and the research of new antibacterial agents based on the enzymes involved in these early steps of the formation of cell membrane components.

Peptidoglycan pathways: there are still more!

The discovery of 3rd and 4th generations of currently existing classes of antibiotics has not hindered bacterial resistance, which is escalating at an alarming global level. This review follows WHO recommendations through implementing new criteria for newly discovered antibiotics. These recommendations focus on abandoning old scaffolds and hitting new targets. In light of these recommendations, this review discusses seven bacterial proteins that no commercial antibiotics have targeted yet, alongside their reported chemical scaffolds.

Contributions of reactive oxygen species, oxidative DNA damage and glutathione depletion to the sensitivity of Acinetobacter baumannii to 2-(2-nitrovinyl) furan

2-(2-nitrovinyl) furan is a broad-spectrum antibacterial agent with activity against Gram-positive and Gram-negative bacteria. In this study, the contributions of reactive oxygen species, oxidative DNA damage and glutathione depletion to its activity against Acinetobacter baumannii was investigated. Inactivation of sodB, katG and recA lowered the minimum inhibitory concentration of 2-(2-nitrovinyl) furan. Furthermore, the inactivation increased the superoxide anion radical and hydrogen peroxide contents of 2-(2-nitrovinyl) furan-treated A. baumannii. Antioxidant (thiourea) reversed the elevated levels of superoxide anion radical and hydrogen peroxide. In addition, thiourea lowered the susceptibility of A. baumannii to 2-(2-nitrovinyl) furan. 2-(2-nitrovinyl) furan depleted reduced glutathione (GSH) contents of parental, sodB, katG and recA strains of A. baumannii. NAD+/NADH ratio parental, sodB, katG and recA strains of A. baumannii exposed to 2-(2-nitrovinyl) furan increased significantly. Inactivation of type-I NADH dehydrogenase lowered the reactive oxygen species generation in 2-(2-nitrovinyl) furan-treated A. baumannii. It is evident from this study that 2-(2-nitrovinyl) furan stimulates respiratory chain activity of A. baumannii leading to enhanced ROS generation, which depletes GSH and reacts with Fe²⁺ to produce hydroxyl radical that damage DNA.

Furvina inhibits the 3-oxo-C12-HSL-based quorum sensing system of Pseudomonas aeruginosa and QS-dependent phenotypes

Disruption of cell-cell communication or quorum sensing (QS) is considered a stimulating approach for reducing bacterial pathogenicity and resistance. Although several QS inhibitors (QSIs) have been discovered so far their clinical use remains distant. This problem can be circumvented by searching for QSI among drugs already approved for the treatment of different diseases. In this context, antibiotics have earned special attention. Whereas at high concentrations antibiotics exert a killing effect, at lower concentrations they may act as signaling molecules and as such can modulate gene expression. In this study, the antibiotic furvina was shown to be able to cause inhibition of the 3-oxo-C12-HSL-dependent QS system of Pseudomonas aeruginosa. Furvina interacts with the LasI/LasR system. The data were validated by modeling studies. Furvina can also reduce biofilm formation and decrease the production of QS-controlled virulence factors.

Antibacterial activity of the nitrovinylfuran G1 (Furvina) and its conversion products

2-Bromo-5-(2-bromo-2-nitrovinyl)furan (G1 or Furvina) is an antimicrobial with a direct reactivity against thiol groups. It is active against Gram-positive and Gram-negative bacteria, yeasts and filamentous fungi. By reacting with thiol groups it causes direct damage to proteins but, as a result, is very short-living and interconverts into an array of reaction products. Our aim was to characterize thiol reactivity of G1 and its conversion products and establish how much of antimicrobial and cytotoxic effects are due to the primary activity of G1 and how much can be attributed to its reaction products. Stability of G1 in growth media as well as its conversion in the presence of thiols was characterized. The structures of G1 decomposition products were determined using NMR and mass-spectroscopy. Concentration- and time-dependent killing curves showed that G1 is bacteriostatic for Escherichia coli at the concentration of 16 μg/ml and bactericidal at 32 μg/ml. However, G1 is inefficient against non-growing E. coli. Addition of cysteine to medium reduces the antimicrobial potency of G1. Nevertheless, the reaction products of G1 and cysteine enabled prolonged antimicrobial action of the drug. Therefore, the activity of 2-bromo-5-(2-bromo-2-nitrovinyl)furan is a sum of its immediate reactivity and the antibacterial effects of the conversion products.

The efficacy of 2-nitrovinylfuran derivatives against Leishmania in vitro and in vivo

Despite recent advances in the treatment of some forms of leishmaniasis, the available drugs are still far from ideal due to inefficacy, parasite resistance, toxicity and cost. The wide-spectrum antimicrobial activity of 2-nitrovinylfuran compounds has been described, as has their activity against Trichomonas vaginalis and other protozoa. Thus, the aim of this study was to test the antileishmanial activities of six 2-nitrovinylfurans in vitro and in a murine model of leishmaniasis. Minimum parasiticide concentration (MPC) and 50% inhibitory concentration (IC₅₀) values for these compounds against the promastigotes of Leishmania amazonensis, Leishmania infantum and Leishmania braziliensis were determined, as were the efficacies of two selected compounds in an experimental model of cutaneous leishmaniasis (CL) caused by L. amazonensis in BALB/c mice. All of the compounds were active against the promastigotes of the three Leishmania species tested. IC₅₀ and MPC values were in the ranges of 0.8-4.7 µM and 1.7-32 µM, respectively. The compounds 2-bromo-5-(2-bromo-2-nitrovinyl)-furan (furvina) and 2-bromo-5-(2-methyl-2-nitrovinyl)-furan (UC245) also reduced lesion growth in vivo at a magnitude comparable to or higher than that achieved by amphotericin B treatment. The results demonstrate the potential of this class of compounds as antileishmanial agents and support the clinical testing of Dermofural^(r) (a furvina-containing antifungal ointment) for the treatment of CL.

Promiscuity and selectivity in covalent enzyme inhibition: a systematic study of electrophilic fragments

Covalent ligand-target interactions offer significant pharmacological advantages. However, off-target reactivity of the reactive groups, which usually have electrophilic properties, must be minimized, and the selectivity of irreversible inhibitors is a crucial requirement. We therefore performed a systematic study to determine the selectivity of several electrophilic groups that can be used as building blocks for covalently binding ligands. Six reactive groups with modulated electrophilicity were combined with 11 nonreactive moieties, resulting in a small combinatorial library of 72 fragment-like compounds. These compounds were screened against a group of 11 enzyme targets to assess their selectivity and their potential for promiscuous binding to proteins. The assay results showed a considerably lower degree of promiscuity than initially expected, even for those members of the screening collection that contain supposedly highly reactive electrophiles.

Inhibitors of the peptidoglycan biosynthesis enzymes MurA-F

The widespread emergence of resistant bacterial strains is becoming a serious threat to public health. This thus signifies the need for the development of new antibacterial agents with novel mechanisms of action. Continuous efforts in the design of novel antibacterials remain one of the biggest challenges in drug development. In this respect, the Mur enzymes, MurA-F, that are involved in the formation of UDP-N-acetylmuramyl-pentapeptide can be genuinely considered as promising antibacterial targets. This review provides an in-depth insight into the recent developments in the field of inhibitors of the MurA-F enzymes. Special attention is also given to compounds that act as multiple inhibitors of two, three or more of the Mur enzymes. Moreover, the reasons for the lack of preclinically successful inhibitors and the challenges to overcome these hurdles in the next years are also debated.