N-thiolated beta-lactams: Studies on the mode of action and identification of a primary cellular target in Staphylococcus aureus

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.

Applications and Opportunities in Using Disulfides, Thiosulfinates, and Thiosulfonates as Antibacterials

Sulfur-containing molecules have a long history of bioactivity, especially as antibacterial agents in the fight against infectious pathogens. Organosulfur compounds from natural products have been used to treat infections throughout history. Many commercially available antibiotics also have sulfur-based moieties in their structural backbones. In the following review, we summarize sulfur-containing antibacterial compounds, focusing on disulfides, thiosulfinates, and thiosulfonates, and opportunities for future developments in the field.

Densely Functionalized 2-Methylideneazetidines: Evaluation as Antibacterials

Twenty-two novel, variously substituted nitroazetidines were designed as both sulfonamide and urethane vinylogs possibly endowed with antimicrobial activity. The compounds under study were obtained following a general procedure recently developed, starting from 4-nitropentadienoates deriving from a common β-nitrothiophenic precursor. While being devoid of any activity against fungi and Gram-negative bacteria, most of the title compounds performed as potent antibacterial agents on Gram-positive bacteria (E. faecalis and three strains of S. aureus), with the most potent congener being the 1-(4-chlorobenzyl)-3-nitro-4-(p-tolyl)azetidine 22, which displayed potency close to that of norfloxacin, the reference antibiotic (minimum inhibitory concentration values 4 and 1-2 μg/mL, respectively). Since 22 combines a relatively efficient activity against Gram-positive bacteria and a cytotoxicity on eucharyotic cells only at 4-times higher concentrations (inhibiting concentration on 50% of the cultured eukaryotic cells: 36 ± 10 μM, MIC: 8.6 μM), it may be considered as a promising hit compound for the development of a new series of antibacterials selectively active on Gram-positive pathogens. The relatively concise synthetic route described herein, based on widely available starting materials, could feed further structure-activity relationship studies, thus allowing for the fine investigation and optimization of the toxico-pharmacological profile.

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.

C4-Phenylthio β-lactams: Effect of the chirality of the β-lactam ring on antimicrobial activity

C4-phenylthio β-lactams are a new family of antibacterial agents that have activity against two phylogenetically distant bacteria - Mycobacterium tuberculosis (Mtb) and Moraxella catarrhalis (M. cat). These compounds are effective against β-lactamase producing Mtb and M. cat unlike the clinically relevant β-lactam antibiotics. The structure-activity relationship for the C4 phenylthio β-lactams has not yet been completely defined. Earlier efforts in our laboratories established that the C4-phenylthio substituent is essential for antimicrobial activity, while the N1 carbamyl substituent plays a more subtle role. In this present study, we investigated the role that the stereochemistry at C4 plays in these compounds' antibacterial activity. This was achieved by synthesizing and testing the antimicrobial activity of diastereomers with a chiral carbamyl group at N1. Our findings indicate that a strict stereochemistry for the C4-phenylthio β-lactams is not required to obtain optimal anti-Mtb and anti-M. cat activity. Furthermore, the structure-bioactivity profiles more closely relate to the electronic requirement of the phenylthiogroup. In addition, the MICs of Mtb are sensitive to growth medium composition. Select compounds showed activity against non-replicating and multi-drug resistant Mtb.

An Apparent Binary Choice in Biochemistry: Mutual Reactivity Implies Life Chooses Thiols or Nitrogen-Sulfur Bonds, but Not Both

A fundamental goal of biology is to understand the rules behind life's use of chemical space. Established work focuses on why life uses the chemistry that it does. Given the enormous scope of possible chemical space, we postulate that it is equally important to ask why life largely avoids certain areas of chemical space. The nitrogen-sulfur bond is a prime example, as it rarely appears in natural molecules, despite the very rich N-S bond chemistry applied in various branches of industry (e.g., industrial materials, agrochemicals, pharmaceuticals). We find that, out of more than 200,000 known, unique compounds made by life, only about 100 contain N-S bonds. Furthermore, the limited number of N-S bond-containing molecules that life produces appears to fall into a few very distinctive structural groups. One may think that industrial processes are unrelated to biochemistry because of a greater possibility of solvents, catalysts, and temperatures available to industry than to the cellular environment. However, the fact that life does rarely make N-S bonds, from the plentiful precursors available, and has evolved the ability to do so independently several times, suggests that the restriction on life's use of N-S chemistry is not in its synthesis. We present a hypothesis to explain life's extremely limited usage of the N-S bond: that the N-S bond chemistry is incompatible with essential segments of biochemistry, specifically with thiols. We support our hypothesis by (1) a quantitative analysis of the occurrence of N-S bond-containing natural products and (2) reactivity experiments between selected N-S compounds and key biological molecules. This work provides an example of a reason why life nearly excludes a distinct region of chemical space. Combined with future examples, this potentially new field of research may provide fresh insight into life's evolution through chemical space and its origin and early evolution.

Lecithin:Cholesterol Acyltransferase Activation by Sulfhydryl-Reactive Small Molecules: Role of Cysteine-31

Lecithin:cholesterol acyltransferase (LCAT) catalyzes plasma cholesteryl ester formation and is defective in familial lecithin:cholesterol acyltransferase deficiency (FLD), an autosomal recessive disorder characterized by low high-density lipoprotein, anemia, and renal disease. This study aimed to investigate the mechanism by which compound A [3-(5-(ethylthio)-1,3,4-thiadiazol-2-ylthio)pyrazine-2-carbonitrile], a small heterocyclic amine, activates LCAT. The effect of compound A on LCAT was tested in human plasma and with recombinant LCAT. Mass spectrometry and nuclear magnetic resonance were used to determine compound A adduct formation with LCAT. Molecular modeling was performed to gain insight into the effects of compound A on LCAT structure and activity. Compound A increased LCAT activity in a subset (three of nine) of LCAT mutations to levels comparable to FLD heterozygotes. The site-directed mutation LCAT-Cys31Gly prevented activation by compound A. Substitution of Cys31 with charged residues (Glu, Arg, and Lys) decreased LCAT activity, whereas bulky hydrophobic groups (Trp, Leu, Phe, and Met) increased activity up to 3-fold (P < 0.005). Mass spectrometry of a tryptic digestion of LCAT incubated with compound A revealed a +103.017 m/z adduct on Cys31, consistent with the addition of a single hydrophobic cyanopyrazine ring. Molecular modeling identified potential interactions of compound A near Cys31 and structural changes correlating with enhanced activity. Functional groups important for LCAT activation by compound A were identified by testing compound A derivatives. Finally, sulfhydryl-reactive β-lactams were developed as a new class of LCAT activators. In conclusion, compound A activates LCAT, including some FLD mutations, by forming a hydrophobic adduct with Cys31, thus providing a mechanistic rationale for the design of future LCAT activators.

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.

Allicin-inspired thiolated fluoroquinolones as antibacterials against ESKAPE pathogens

Thiolated fluoroquinolones were synthesized from ciprofloxacin and evaluated for antimicrobial activity against a panel of pathogenic bacteria. Gram-positive species including methicillin-resistant Staphylococcus aureus (MRSA) exhibited the highest level of increased sensitivity toward ciprofloxacin bound with a N-propylthio substituent. Evidence was found that the antibiotics form disulfides with low molecular weight thiols in bacteria and potentiate generation of cytosolic reactive oxygen species (ROS). In final analysis, the enhanced anti-MRSA activity of thiolated fluoroquinolones was attributed to increased cell permeability and reaction with cytosolic thiols that yields an inactive disulfide metabolite and the parent drug ciprofloxacin as an inhibitor of DNA synthesis.

Proline-based phosphoramidite reagents for the reductive ligation of S-nitrosothiols

S-Nitrosothiols have many biological implications but are rarely used in organic synthesis. In this work we report the development of proline-based phosphoramidite substrates that can effectively convert S-nitrosothiols to proline-based sulfenamides through a reductive ligation process. A unique property of this method is that the phosphine oxide moiety on the ligation products can be readily removed under acidic conditions. In conjugation with the facile preparation of S-nitrosothiols (RSNO) from the corresponding thiols (RSH), this method provides a new way to prepare proline-based sulfenamides from simple thiol starting materials.

Syntheses and Biological Evaluations of Highly Functionalized Hydroxamate Containing and N-Methylthio Monobactams as Anti-Tuberculosis and β-Lactamase Inhibitory Agents

Both the resurgence of tuberculosis (TB) and antibiotic resistance continue to threaten modern healthcare and new means of combating pathogenic bacterial infections are needed. The syntheses of monobactams possessing hydroxamate and N-methylthio functionality are described, as well as their anti-TB, in vitro β-lactamase inhibitory, and general antimicrobial evaluations. A number of compounds exhibited significant anti-TB and β-lactamase inhibitory activity, with MIC values in the range of 25 to < 0.19 μM against Mycobacteria tuberculosis (M.tb), and K_i values in the range of 25-0.03 μM against purified NDM-1 and VIM-1 lystate metallo β-lactamases. This work suggests that these scaffolds may serve as promising leads in developing new antibiotics and/or β-lactamase inhibitors.

Non-transpeptidase binding arylthioether β-lactams active against Mycobacterium tuberculosis and Moraxella catarrhalis

The prevalence of drug resistance in both clinical and community settings as a consequence of alterations of biosynthetic pathways, enzymes or cell wall architecture is a persistent threat to human health. We have designed, synthesized, and tested a novel class of non-transpeptidase, β-lactamase resistant monocyclic β-lactams that carry an arylthio group at C4. These thioethers exhibit inhibitory and cidal activity against serine β-lactamase producing Mycobacterium tuberculosis wild type strain (Mtb) and multiple (n=8) β-lactamase producing Moraxella catarrhalis clinical isolates.

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.

The impact of fatty acids on the antibacterial properties of N-thiolated β-lactams

Bacterial fatty acid synthesis (FAS) is a potentially important, albeit controversial, target for antimicrobial therapy. Recent studies have suggested that the addition of exogenous fatty acids (FAs) to growth media can circumvent the effects of FAS-targeting compounds on bacterial growth. Consequently, such agents may have limited in vivo applicability for the treatment of human disease, as free FAs are abundant within the body. Our group has previously developed N-thiolated β-lactams and found they function by interfering with FAS in select pathogenic bacteria, including MRSA. To determine if the FAS targeting activity of N-thiolated β-lactams can be abrogated by exogenous fatty acids, we performed MIC determinations for MRSA strains cultured with the fatty acids oleic acid and Tween 80. We find that, whilst the activity of the known FAS inhibitor triclosan is severely compromised by the addition of both oleic acid and Tween 80, exogenous FAs do not mitigate the antibacterial activity of N-thiolated β-lactams towards MRSA. Consequently, we propose that N-thiolated β-lactams are unique amongst FAS-inhibiting antimicrobials, as their effects are unimpeded by exogenous FAs.

Studies on the antifungal properties of N-thiolated beta-lactams

N-thiolated beta-lactams had previously been shown to have antibacterial activity against a narrow selection of pathogenic bacteria including Staphylococcus aureus and Bacillus anthracis, as well as apoptotic-inducing activity in a variety of human cancer cell lines. We now have found that these lactams also possess antifungal activity against Candida and other fungi by exerting powerful cytostatic effects that disrupt the structural integrity of cytoplasmic membranes. The mode of action and structure-activity trends of these lactams as antifungals parallel that previously seen in our antibacterial studies.

Unsymmetric aryl-alkyl disulfide growth inhibitors of methicillin-resistant Staphylococcus aureus and Bacillus anthracis

This study describes the antibacterial properties of synthetically produced mixed aryl-alkyl disulfide compounds as a means to control the growth of Staphylococcus aureus and Bacillus anthracis. Some of these compounds exerted strong in vitro bioactivity. Our results indicate that among the 12 different aryl substituents examined, nitrophenyl derivatives provide the strongest antibiotic activities. This may be the result of electronic activation of the arylthio moiety as a leaving group for nucleophilic attack on the disulfide bond. Small alkyl residues on the other sulfur provide the best activity as well, which for different bacteria appears to be somewhat dependent on the nature of the alkyl moiety. The mechanism of action of these lipophilic disulfides is likely similar to that of previously reported N-thiolated beta-lactams, which have been shown to produce alkyl-CoA disulfides through a thiol-disulfide exchange within the cytoplasm, ultimately inhibiting type II fatty acid synthesis. However, the mixed alkyl-CoA disulfides themselves show no antibacterial activity, presumably due to the inability of the highly polar compounds to cross the bacterial cell membrane. These structurally simple disulfides have been found to inhibit beta-ketoacyl-acyl carrier protein synthase III, or FabH, a key enzyme in type II fatty acid biosynthesis, and thus may serve as new leads to the development of effective antibacterials for MRSA and anthrax infections.

Induction of tumor cell apoptosis by a novel class of N-thiolated beta-lactam antibiotics with structural modifications at N1 and C3 of the lactam ring

The investigation of novel anti-tumor agents that preferentially select for malignant cells with a tolerable toxicity level has been the focus of anti-cancer drug discovery. Our laboratories have previously reported that certain N-alkylthiolated beta-lactams had DNA-damaging and apoptosis-inducing activity in various tumor lines but not in nontransformed cells. In the current study, we further delineated the effects of substitutions at C3 or N1 of the lactam ring for cell death-inducing capability with close attention paid to a discernible structure-activity relationship (SAR). We found that two beta-lactam analogs (JG-5 and JG-19), both containing a branched-chain moiety at C3 of the lactam ring, exhibit potent apoptosis-inducing activity. Additionally, JG-5 exhibited superior in vitro biological activity over JG-19 owing to structural modifications made to substituents at the N1 and C3 positions of the lactam ring. Furthermore, the branched beta-lactams were able to inhibit growth of mice bearing breast cancer xenografts, associated with induction of DNA damage and apoptosis in tumor tissues. Our results strongly warrant further investigation into these novel beta-lactams as potential anti-cancer therapeutics.

Anti-tumor activity of N-thiolated beta-lactam antibiotics

An ongoing strategy for cancer treatment is selective induction of apoptosis in cancer over normal cells. N-thiolated beta-lactams were found to induce DNA damage, growth arrest and apoptosis in cultured human cancer cells. However, whether these compounds have a similar effect in vivo has not been studied. We report here that treatment with the beta-lactam L-1 caused a significant inhibition of tumor growth in a breast cancer xenograft mouse model, associated with induction of DNA damage and apoptosis in vivo. These results suggest that the synthetic antibiotic N-thiolated beta-lactams hold great potential to be developed as novel anti-cancer drugs.