Monocyclic β-lactams as antibacterial agents: Facing antioxidant activity of N-methylthio-azetidinones

N-Sulfenylation of β-Lactams: Radical Reaction of N-Bromo-azetidinones by TEMPO Catalysis

Azetidinones with a sulfenyl group on the β-lactam nitrogen atom show interesting biological activities as antimicrobial agents and enzyme inhibitors. We report in the present study a versatile synthesis of N-sulfenylated azetidinones starting from the corresponding N-bromo derivatives by means of the (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) radical as the catalyst and disulfides. Preparation of N-halo-azetidinones was studied and optimized. The reactivity of N-bromo-azetidinone 2a as a model compound in the presence of TEMPO radical was investigated by NMR and electron paramagnetic resonance (EPR) spectroscopy studies. Optimization of the reaction conditions allowed the access of N-alkylthio- or N-arylthio-azetidinones from 55 to 92% yields, and the method exhibited a good substrate scope.

N-Thio-β-lactams targeting L,D-transpeptidase-2, with activity against drug-resistant strains of Mycobacterium tuberculosis

Effective treatment of tuberculosis is frequently hindered by the emerging antimicrobial resistance of Mycobacterium tuberculosis. The present study evaluates monocyclic β-lactam compounds targeting the mycobacterial cell wall remodeling. Novel N-thio-β-lactams were designed, synthesized, and characterized on the L,D-transpeptidase-2, a validated target in M. tuberculosis. The candidates were evaluated in biochemical assays identifying five compounds presenting target-specific kinetic constants equal or superior to meropenem, a carbapenem currently considered for tuberculosis therapy. Mass spectrometry in line with the crystal structures of five target-ligand complexes revealed that the N-thio-β-lactams act via an unconventional mode of adduct formation, transferring the thio-residues from the lactam ring to the active-site cysteine of Ldt_Mt2. The resulting stable adducts lead to a long-term inactivation of the target protein. Finally, the candidates were evaluated in vitro against a drug-susceptible and multidrug-resistant clinical isolates of M. tuberculosis, confirming the antimycobacterial effect of these novel compounds.

Endeavors towards transformation of M. tuberculosis thymidylate kinase (MtbTMPK) inhibitors into potential antimycobacterial agents

As the last enzyme in nucleotide synthesis as precursors for DNA replication, thymidylate kinase of M. tuberculosis (MtbTMPK) attracts significant interest as a target in the discovery of new anti-tuberculosis agents. Earlier, we discovered potent MtbTMPK inhibitors, but these generally suffered from poor antimycobacterial activity, which we hypothesize is due to poor bacterial uptake. To address this, we herein describe our efforts to equip previously reported MtbTMPK inhibitors with targeting moieties to increase the whole cell activity of the hybrid analogues. Introduction of a simplified Fe-chelating siderophore motif gave rise to analogue 17 that combined favorable enzyme inhibitory activity with significant activity against M. tuberculosis (MIC of 12.5 μM). Conjugation of MtbTMPK inhibitors with an imidazo[1,2-a]pyridine or 3,5-dinitrobenzamide scaffold afforded analogues 26, 27 and 28, with moderate MtbTMPK enzyme inhibitory potency, but sub-micromolar activity against mycobacteria without significant cytotoxicity. These results indicate that conjugation with structural motifs known to favor mycobacterial uptake may be a valid approach for discovering new antimycobacterial agents.

Antibacterial and antioxidant activities for natural and synthetic dual-active compounds

Antimicrobial resistance is widely recognized as a grave threat to global health in the 21st century, since the past decades have seen a dramatic increase in human-pathogenic bacteria that are resistant to one or multiple antibiotics. New antimicrobial agents are urgently required, particularly in the treatment of chronic infections such as cystic fibrosis, often associated with persistent colonization by drug-resistant pathogens and epithelial damage by pulmonary oxidative stress. In such events, it would be favourable to find agents that could have antioxidant and antibacterial activities combined in one molecule. The discovery of compounds that can show a dual-target activity considerably increased in the last years, reflecting the growing confidence that this new approach could lead to better therapeutic solutions for complex multigenic diseases. The aim of this review is to report those natural and synthetic compounds displaying significant antioxidant and antibacterial activities. In recent years there has been a growing attention on plant-derived antimicrobials as an alternative to antibiotics, for their efficacy and low tendency in developing bacterial resistance. Moreover, it was found that some natural products could enhance the activity of common antibiotics displaying a synergistic effect. We then report some selected synthetic compounds with an in-built capacity to act on two targets or with the combination in a single structure of two pharmacophores with antioxidant and antibacterial activities. Recent literature instances were screened and the most promising examples of dual-active antibacterial-antioxidant molecules were highlighted.

Targeting the Nrf2/Amyloid-Beta Liaison in Alzheimer's Disease: A Rational Approach

Amyloid is a prominent feature of Alzheimer's disease (AD). Yet, a linear linkage between amyloid-β peptide (Aβ) and the disease onset and progression has recently been questioned. In this context, the crucial partnership between Aβ and Nrf2 pathways is acquiring paramount importance, offering prospects for deciphering the Aβ-centered disease network. Here, we report on a new class of antiaggregating agents rationally designed to simultaneously activate transcription-based antioxidant responses, whose lead 1 showed interesting properties in a preliminary investigation. Relying on the requirements of Aβ recognition, we identified the catechol derivative 12. In SH-SY5Y neuroblastoma cells, 12 combined remarkable free radical scavenger properties to the ability to trigger the Nrf2 pathway and induce the Nrf2-dependent defensive gene NQO1 by means of electrophilic activation of the transcriptional response. Moreover, 12 prevented the formation of cytotoxic stable oligomeric intermediates, being significantly more effective, and per se less toxic, than prototype 1. More importantly, as different chemical features were exploited to regulate Nrf2 and Aβ activities, the two pathways could be tuned independently. These findings point to compound 12 and its derivatives as promising tools for investigating the therapeutic potential of the Nrf2/Aβ cellular network, laying foundation for generating new drug leads to confront AD.

Monocyclic β-lactams loaded on hydroxyapatite: new biomaterials with enhanced antibacterial activity against resistant strains

The development of biomaterials able to act against a wide range of bacteria, including antibiotic resistant bacteria, is of great importance since bacterial colonization is one of the main causes of implant failure. In this work, we explored the possibility to functionalize hydroxyapatite (HA) nanocrystals with some monocyclic N-thio-substituted β-lactams. To this aim, a series of non-polar azetidinones have been synthesized and characterized. The amount of azetidinones loaded on HA could be properly controlled on changing the polarity of the loading solution and it can reach values up to 17 wt%. Data on cumulative release in aqueous solution show different trends which can be related to the lipophilicity of the molecules and can be modulated by suitable groups on the azetidinone. The examined β-lactams-HA composites display good antibacterial activity against reference Gram-positive and Gram-negative bacteria. However, the results of citotoxicity and antibacterial tests indicate that HA loaded with 4-acetoxy-1-(methylthio)-azetidin-2-one displays the best performance. In fact, this material strongly inhibited the bacterial growth of both methicillin resistant and methicillin susceptible clinical isolates of S. aureus from surgical bone biopsies, showing to be a very good candidate as a new functional biomaterial with enhanced antibacterial activity.

Synthesis and biological evaluation of new 1,3-thiazolidine-4-one derivatives of 2-(4-isobutylphenyl)propionic acid

New thiazolidine-4-one derivatives of 2-(4-isobutylphenyl)propionic acid (ibuprofen) have been synthesized as potential anti-inflammatory drugs. The structure of the new compounds was proved using spectral methods (FR-IR, ¹H-NMR, ¹³C-NMR, MS). The in vitro antioxidant potential of the synthesized compounds was evaluated according to the total antioxidant activity, the DPPH and ABTS radical scavenging assays. Reactive oxygen species (ROS) and free radicals are considered to be involved in many pathological events like diabetes mellitus, neurodegenerative diseases, cancer, infections and more recently, in inflammation. It is known that overproduction of free radicals may initiate and amplify the inflammatory process via upregulation of genes involved in the production of proinflammatory cytokines and adhesion molecules. The chemical modulation of acyl hydrazones of ibuprofen 3a–l through cyclization to the corresponding thiazolidine-4-ones 4a–n led to increased antioxidant potential, as all thiazolidine-4-ones were more active than their parent acyl hydrazones and also ibuprofen. The most active compounds are the thiazolidine-4-ones 4e, m, which showed the highest DPPH radical scavenging ability, their activity being comparable with vitamin E.

Design, synthesis and the biological evaluation of new 1,3-thiazolidine-4-ones based on the 4-amino-2,3-dimethyl-1-phenyl-3-pyrazolin-5-one scaffold

New thiazolidine-4-one derivatives based on the 4-aminophenazone (4-amino-2,3-dimethyl-1-phenyl-3-pyrazolin-5-one) scaffold have been synthesized as potential anti-inflammatory drugs. The pyrazoline derivatives are known especially for their antipyretic, analgesic and anti-inflammatory effects, but recently there were synthesized new compounds with important antioxidant, antiproliferative, anticancer and antidiabetic activities. The beneficial effects of these compounds are explained by nonselective inhibition of cyclooxygenase izoenzymes, but also by their potential scavenging ability for reactive oxygen and nitrogen species. The structure of the new compounds was proved using spectroscopic methods (FR-IR, ¹H-NMR, ¹³C-NMR, MS). The in vitro antioxidant potential of the synthesized compounds was evaluated according to the ferric reducing antioxidant power, phosphomolydenum reducing antioxidant power, DPPH and ABTS radical scavenging assays. The chemical modulation of 4-aminophenazone (6) through linkage to thiazolidine-propanoic acid derivatives 5a–l led to improved antioxidant potential, all derivatives 7a–l being more active than phenazone. The most active compounds are the derivatives 7e, and 7k, which showed the higher antioxidant effect depending on the antioxidant assay considered.

Molecular Targets of β-Lactam-Based Antimicrobials: Beyond the Usual Suspects

The common practice in antibacterial drug development has been to rapidly make an attempt to find ever-more stable and broad-spectrum variants for a particular antibiotic, once a drug resistance for that antibiotic is detected. We are now facing bacterial resistance toward our clinically relevant antibiotics of such a magnitude that the conversation for antimicrobial drug development ought to include effective new antibiotics with alternative mechanisms of action. The electrophilic β-lactam ring is amenable for the inhibition of different enzyme classes by a suitable decoration of the core scaffold. Monocyclic β-lactams lacking an ionizable group at the lactam nitrogen exhibit target preferences toward bacterial enzymes important for resistance and virulence. The present review intends to draw attention to the versatility of the β-lactams as antimicrobials with "unusual" molecular targets.

Synthesis and biological evaluation of new 2-azetidinones with sulfonamide structures

New series of N-(arylidene)hydrazinoacetyl sulfonamides 4a_1–6, 4b_1–6 and N-(4-aryl-3-chloro-2-oxoazetidin-1-yl)aminoacetyl sulfonamides 5a_1–6, 5b_1–6 were synthesized. The structures of the new derivatives was confirmed using spectral methods (FT-IR, ¹H-NMR, ¹³C-NMR). The antibacterial activities of these compounds against Gram positive (Staphyloccoccus aureus ATCC 6583, Staphyloccoccus epidermidis ATCC 12228, Enterococcus faecalis ATCC 25912) and Gram negative (Klebsiella pneumoniae CIP 53153, Proteus vulgaris CIP 104989, Citrobacter freundii CIP 5732, Enterobacter cloacae CIP 103475, Escherichia coli ATCC 25922, Pseudomonas aeruginosa CIP 82118) bacterial strains were evaluated using the broth micro-dilution method. Compound 4a₂ displayed the highest antibacterial activity, especially against Staphyloccoccus epidermidis, Enterococcus faecalis and Pseudomonas aeruginosa. The antioxidant potential of the synthesized compounds was also investigated according to ferric reducing power, total antioxidant activity and DPPH radical scavenging assays. All tested compounds showed excellent antioxidant activity in comparison with sulfadiazine and sulfisoxazole which were used as parent sulfonamides. Moreover, some of them showed an antioxidant activity comparable with that of ascorbic acid. In general, the compounds designed based on a sulfadiazine skeleton (compounds 4a_1–6, 5a_1–6) are more active than those obtained from sulfisoxazole (compounds 4b_1–6, 5b_1–6), and the N-(arylidene)hydrazinoacetyl sulfonamide derivatives 4a_1–6, 4b_1–6 are more active than their azetidionone analogues 5a_1–6, 5b_1–6.