Synthesis of novel 1,2-diarylpyrazolidin-3-one-based compounds and their evaluation as broad spectrum antibacterial agents

Discovery of 1,2-diaryl-3-oxopyrazolidin-4-carboxamides as a new class of MurA enzyme inhibitors and characterization of their antibacterial activity

The cytoplasmic steps of peptidoglycan synthesis represent an important targeted pathway for development of new antibiotics. Herein, we report the synthesis of novel 3-oxopyrazolidin-4-carboxamide derivatives with variable amide side chains as potential antibacterial agents targeting MurA enzyme, the first committed enzyme in these cytosolic steps. Compounds 15 (isoindoline-1,3-dione-5-yl), 16 (4-(1H-pyrazol-4-yl)phenyl), 20 (5-cyanothiazol-2-yl), 21 and 31 (5-nitrothiazol-2-yl derivatives) exhibited the most potent MurA inhibition, with IC50 values of 9.8-12.2 μM. Compounds 15, 16 and 21 showed equipotent inhibition of the C115D MurA mutant developed by fosfomycin-resistant Escherichia coli. NMR binding studies revealed that some of the MurA residues targeted by 15 also interacted with fosfomycin, but not all, indicating an overlapping but not identical binding site. The antibacterial activity of the compounds against E. coli ΔtolC suggests that inhibition of MurA accounts for the observed effect on bacterial growth, considering that a few potent MurA inhibitors could not penetrate the bacterial outer membrane and were therefore inactive as proven by the bacterial cell uptake assay. The most promising compounds were also evaluated against a panel of Gram-positive bacteria. Remarkably, compounds 21 and 31 (MurA IC50 = 9.8 and 10.2 μM respectively) exhibited a potent activity against Clostridioides difficile strains with MIC values ranging from 0.125 to 1 μg/mL, and were also shown to be bactericidal with MBC values between 0.25 and 1 μg/mL. Furthermore, both compounds were shown to have a limited activity against human normal intestinal flora and showed high safety towards human colon cells (Caco-2) in vitro. The thiolactone derivative (compound 5) exhibited an interesting broad spectrum antibacterial activity despite its weak MurA inhibition. Altogether, the presented series provides a promising class of antibiotics that merits further investigation.

Identification and Biochemical Characterization of Pyrrolidinediones as Novel Inhibitors of the Bacterial Enzyme MurA

To develop novel antibiotics, targeting the early steps of cell wall peptidoglycan biosynthesis seems to be a promising strategy that is still underutilized. MurA, the first enzyme in this pathway, is targeted by the clinically used irreversible inhibitor fosfomycin. However, mutations in its binding site can cause bacterial resistance. We herein report a series of novel reversible pyrrolidinedione-based MurA inhibitors that equally inhibit wild type (WT) MurA and the fosfomycin-resistant MurA C115D mutant, showing an additive effect with fosfomycin for the inhibition of WT MurA. For the most potent inhibitor 46 (IC50 = 4.5 μM), the mode of inhibition was analyzed using native mass spectrometry and protein NMR spectroscopy. The compound class was nontoxic against human cells and highly stable in human S9 fraction, human plasma, and bacterial cell lysate. Taken together, this novel compound class might be further developed toward antibiotic drug candidates that inhibit cell wall synthesis.

Dearomatization of Cyclic Diphenylhydrazines: Harnessing the o-Semidine Rearrangement for the Synthesis of Spirocyclic Tetrahydroquinolines

The synthesis of novel tetrahydroquinoline-containing spirocycles has been achieved through an acid-promoted dearomatization of cyclic diarylhydrazines. The reaction, proceeding through a dearomative o-semidine rearrangement, furnishes a stable, yet reactive spirocyclohexadieniminium ion, which can further be used as an electrophile or a diene in a one-pot sequence. These transformations efficiently produce novel diazaspirocycles and allow for further synthetic elaboration of the cyclohexadienamine products.

N,N-Diisopropylethylamine-Mediated Electrochemical Reduction of Azobenzenes in Dichloromethane

We report a cathodic reduction-dominated electrochemical approach for the hydrogenation of azobenzenes in dichloromethane. With cheap and readily available N,N-diisopropylethylamine as a catalytic mediator, the reaction proceeded smoothly in a simple undivided cell under constant-current electrolysis. A series of azobenzenes were successfully reduced to the corresponding hydrazobenzenes in moderate to high yields at room temperature. Preliminarily mechanistic studies indicate that solvent dichloromethane acts as a hydrogen source. The use of a common solvent as a hydrogen source, no need for stoichiometric mediators or metallic reductants, and mild conditions make this work a more straightforward and sustainable protocol for hydrogenation of azobenzenes.

Nanoparticle Fraught Liposomes: A Platform for Increased Antibiotic Selectivity in Multidrug Resistant Bacteria

Increasing antibiotic concentrations within bacterial cells while reducing them in mammalian ones would ultimately result in an enhancement of antibacterial actions, overcoming multidrug resistance, all while minimizing toxicity. Nanoparticles (NPs) have been used in numerous occasions to overcome antibiotic resistance, poor drug solubility, and stability. However, the concomitant increase in antibiotic concentration in mammalian cells and the resultant toxicity are usually overlooked. Without compromising bacterial cell fusion, large liposomes (Lip) have been reported to show reduced uptake in mammalian cells. Therefore, in this work, small NP fraught liposomes (NP-Lip) were formulated with the aim of increasing NP uptake and antibiotic delivery in bacterial cells but not in mammalian ones. Small polylactic-co-glycolic acid NPs were therefore loaded with erythromycin (Er), an antibiotic with low membrane permeability that is susceptible to drug efflux, and 3c, a 5-cyanothiazolyl urea derivative with low solubility and stability. In vitro experiments demonstrated that the incorporation of small NPs into large Lip resulted in a reduction in NP uptake by HEK293 cells while increasing it in Gram-negative bacteria (Escherichia coli DH5α, E. coli K12, and Pseudomonas aeruginosa), consequently resulting in an enhancement of antibiotic selectivity by fourfold toward E. coli (both strains) and eightfold toward P. aeruginosa. Ocular administration of NP-Lip in a P. aeruginosa keratitis mouse model demonstrated the ability of Er/3c-loaded NP-Lip to result in a complete recovery. More importantly, in comparison to NPs, the ocular administration of NP-Lip showed a reduction in TNF-alpha and IL-6 levels, implying reduced interaction with mammalian cells in vivo. This work therefore clearly demonstrated how tailoring the nano-bio interaction could result in selective drug delivery and a reduction in toxicity.

Visible-Light-Promoted Diboron-Mediated Transfer Hydrogenation of Azobenzenes to Hydrazobenzenes

A visible-light-promoted transfer hydrogenation of azobenzenes has been developed. In the presence of B₂pin₂ and upon visible-light irradiation, the reactions proceeded smoothly in methanol at ambient temperature. The azobenzenes with diverse functional groups have been reduced to the corresponding hydrazobenzenes with a yield of up to 96%. Preliminary mechanistic studies indicated that the hydrogen atom comes from the solvent and the transformation is achieved through a radical pathway.