Unconventional Antibacterials and Adjuvants

Synthesis of 2-Amino-quinazolin-4(3H)-ones Using 2-Bromo-N-phenylbenzamide and Cyanamide Ullmann Cross-Coupling

Herein, an approach for synthesizing 2-amino-3-substituted quinoline-4(3H)-ones and N-phenylbenzamide derivatives was developed. A series of quinoline-4(3H)-ones were synthesized through Ullmann cross-coupling under air conditions using inexpensive, readily available cyanamide with 2-bromo-N-phenylbenzamide as the starting material, copper iodide as the catalyst, and potassium tert-butoxide as the base in dimethyl sulfoxide. Noteworthy aspects of this method include its cost-effectiveness, the accessibility of raw materials, wide substrate applicability, ligand-free, open-air conditions, and simple operating procedures. Furthermore, investigations under similar reaction conditions further show that substituting water, alcohols, phenols, amines, or mercaptans for cyanamide as the nucleophilic reagent can be used to prepare 2-functionalized N-phenylbenzamides.

Retention of methicillin susceptibility in Staphylococcus aureus using natural adjuvant as an allosteric modifier of penicillin-binding protein 2a

The emergence of methicillin-resistant Staphylococcus aureus (MRSA) poses a significant global public health challenge due to its resistance to conventional antibiotics, primarily mediated by the mutated penicillin-binding protein, PBP2a. This study aims to investigate the potential of phytochemicals derived from medicinal plants in the Indian subcontinent to serve as adjuvants, enhancing the efficacy of methicillin against MRSA through allosteric modification of PBP2a using molecular docking and molecular dynamics (MD) simulation. After comprehensive Absorption, Distribution, Metabolism, and Excretion (ADME) profiling, along with AMES and hepatotoxicity tests, 9 compounds were shortlisted as suitable adjuvant candidates. Among them, nimbolide, quercetin, emodin, daidzein, eriodictyol, luteolin, and apigenin exhibited strong binding affinity to the allosteric site of PBP2a, with docking scores ranging from -8.7 to -7.3 kcal/mol. These phytochemicals facilitated enhanced methicillin binding, as evidenced by improved docking scores ranging from -6.1 to -6.8 kcal/mol, compared to -5.6 kcal/mol for methicillin alone. Molecular dynamics simulations confirmed the stability and favorable conformations of phytochemical-PBP2a complexes. Quercetin and daidzein were identified as the most promising adjuvant candidates, forming stable and energetically favorable complexes with PBP2a. Experimental validation showed that quercetin, at 30 mg/mL, effectively retained methicillin's antibacterial efficacy against MRSA. This study underscores the potential of natural compounds in overcoming antibiotic resistance and suggests that phytochemical-antibiotic synergism could be a viable strategy to combat multidrug-resistant bacterial infections.

Lytic transglycosylase Slt of Pseudomonas aeruginosa as a periplasmic hub protein

Peptidoglycan is a major constituent of the bacterial cell wall. Its integrity as a polymeric edifice is critical for bacterial survival and, as such, it is a preeminent target for antibiotics. The peptidoglycan is a dynamic crosslinked polymer that undergoes constant biosynthesis and turnover. The soluble lytic transglycosylase (Slt) of Pseudomonas aeruginosa is a periplasmic enzyme involved in this dynamic turnover. Using amber-codon-suppression methodology in live bacteria, we incorporated a fluorescent chromophore into the structure of Slt. Fluorescent microscopy shows that Slt populates the length of the periplasmic space and concentrates at the sites of septation in daughter cells. This concentration persists after separation of the cells. Amber-codon-suppression methodology was also used to incorporate a photoaffinity amino acid for the capture of partner proteins. Mass-spectrometry-based proteomics identified 12 partners for Slt in vivo. These proteomics experiments were complemented with in vitro pulldown analyses. Twenty additional partners were identified. We cloned the genes and purified to homogeneity 22 identified partners. Biophysical characterization confirmed all as bona fide Slt binders. The identities of the protein partners of Slt span disparate periplasmic protein families, inclusive of several proteins known to be present in the divisome. Notable periplasmic partners (KD < 0.5 μM) include PBPs (PBP1a, KD = 0.07 μM; PBP5 = 0.4 μM); other lytic transglycosylases (SltB2, KD = 0.09 μM; RlpA, KD = 0.4 μM); a type VI secretion system effector (Tse5, KD = 0.3 μM); and a regulatory protease for alginate biosynthesis (AlgO, KD < 0.4 μM). In light of the functional breadth of its interactome, Slt is conceptualized as a hub protein within the periplasm.

Development of non-β-Lactam covalent allosteric inhibitors targeting PBP2a in Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA), a Gram-positive bacterial pathogen, continues to pose a serious threat to the current public health system in our society. The high level of resistance to β-lactam antibiotics in MRSA is attributed to the expression of penicillin-binding protein 2a (PBP2a), which catalyzes cell wall cross-linking. According to numerous research reports, the activity of the PBP2a protein is known to be regulated by an allosteric site distinct from the active site where cell wall cross-linking occurs. Here, we conducted a screening of 113 compounds containing a 1,3,4-oxadiazole core to design new covalent inhibitors targeting the allosteric site of PBP2a and establish their structural-activity relationship. The stereochemically selective synthesis of sulfonyl oxadiazole compounds identified in the initial screening resulted in a maximum eightfold enhancement in cell inhibition activity. The sulfonyl oxadiazole-based compounds formulated as PEG-based ointments, with low toxicity test results on human cells (CC 50 : >78μM), demonstrated potent antimicrobial effects not only in a mouse skin wound infection model but also against oxacillin-resistant clinical isolate MRSA (IC 50 ≈ 1μM), as evidenced by the results. Furthermore, additional studies utilizing LC-MS/MS and in-silico approaches clearly support the allosteric site covalent binding mechanism through the nucleophilic aromatic substitution (S N Ar) reaction, as well as its association with the closure of the major active site of PBP2a.

Novel aminothiazoximone-corbelled ethoxycarbonylpyrimidones with antibiofilm activity to conquer Gram-negative bacteria through potential multitargeting effects

The emergence of drug-resistant microorganisms threatens human health, and it is usually exacerbated by the formation of biofilm, which forces the development of new antibacterial agents with antibiofilm activity. In this work, a novel category of aminothiazoximone-corbelled ethoxycarbonylpyrimidones (ACEs) was designed and synthesized, and some of the prepared ACEs showed potent bioactivity against the tested bacteria. In particular, imidazolyl ACE 6c showed better inhibitory activity towards Acinetobacter baumannii and Escherichia coli with MIC values both of 0.0066 mmol/L than norfloxacin. It was also revealed that imidazolyl ACE 6c not only possessed inconspicuous hemolytic rate and cytotoxicity, low drug resistance and no risk of penetrating the blood-brain barrier, but also exhibited obvious biofilm inhibition and eradication activities. The preliminary mechanism research suggested that imidazolyl ACE 6c could induce metabolic dysfunction by deactivating lactate dehydrogenase and promote the accumulation of reactive oxygen species to decrease the reduced glutathione and ultimately cause oxidative damage in bacteria. Furthermore, ACE 6c was also found that could insert into DNA to form the supramolecular complex of 6c-DNA and trigger cell death. The multidimensional effect might promote bacterial cell rupture, leading to the leakage of intracellular content. These findings manifested that novel imidazolyl ACE 6c as a potential multitargeting antibacterial agent with potent antibiofilm activity could provide new possibility for the treatment of refractory biofilm-intensified bacterial infections.

Robust integration of light-driven carbon quantum dots with bacterial cellulose enables excellent mechanical and antibacterial biodegradable yarn

Due to the overuse of antimicrobial drugs, bacterial resistance became an urgent problem to be solved. In this study, carbon quantum dots (CQDs) with high photodynamic antibacterial activity were synthesized by a one-pot hydrothermal method and introduced into bacterial cellulose (BC) dispersion solution. Through a wet-spinning and wet-twisting processing strategy, bionic ordering nanocomposite macrofiber (BC/CQDs-based yarn) based on BC were obtained. The results showed that BC/CQDs-based yarn had excellent tensile strength (226.8 MPa) and elongation (22.2 %). Utilizing the light-driven generation of singlet oxygen (1O2) and hydroxyl radical (·OH), BC/CQDs-based yarn demonstrated remarkable antibacterial efficacy, with 99.9999 % (6 log, P < 0.0001) and 96.54 % (1.46 log, P < 0.0004) effectiveness against E. coli and S. aureus, respectively. At the same time, BC/CQDs-based yarn also displayed the characteristics of photothermal, fluorescent, and biodegradability. In summary, the application potential of BC/CQDs-based yarn is significant, opening up a new strategy for the development of sustainable green weaving and bio-based multi-function yarn.

Small molecular adjuvants repurpose antibiotics towards Gram-negative bacterial infections and multispecies bacterial biofilms

Gram-negative bacterial infections pose a significant challenge due to two major resistance elements, including the impermeability of the outer membrane and the overexpression of efflux pumps, which contribute to antibiotic resistance. Additionally, the coexistence of multispecies superbugs in mixed species biofilms further complicates treatment, as these infections are refractory to most antibiotics. To address this issue, combining obsolete antibiotics with non-antibiotic adjuvants that target bacterial membranes has shown promise in combating antibacterial resistance. However, the clinical translation of this cocktail therapy has been hindered by the toxicity associated with these membrane active adjuvants, mainly due to a limited understanding of their structure and mechanism of action. Towards this goal, herein, we have designed a small molecular adjuvant by tuning different structural parameters, such as the balance between hydrophilic and hydrophobic groups, spatial positioning of hydrophobicity and hydrogen bonding interactions, causing moderate membrane perturbation in bacterial cells without any toxicity to mammalian cells. Moderate membrane perturbation not only enhances the internalization of antibiotics, but also increases the intracellular concentration of drugs by hampering the efflux machinery. This revitalises the efficacy of various classes of antibiotics by 32-512 fold, without inducing toxicity. The leading combination not only exhibits potent bactericidal activity against A. baumannii biofilms but also effectively disrupts mature multispecies biofilms composed of A. baumannii and methicillin-resistant Staphylococcus aureus (MRSA), which is typically resistant to most antibiotics. Importantly, the combination therapy demonstrates good biocompatibility and excellent in vivo antibacterial efficacy (>99% reduction) in a skin infection model of A. baumannii. Interestingly, A. baumannii shows reduced susceptibility to develop resistance against the leading combination, underscoring its potential for treating multi-drug resistant infections.

Unraveling the mechanism of ceftaroline-induced allosteric regulation in penicillin-binding protein 2a: insights for novel antibiotic development against methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) acquires high-level resistance against β-lactam antibiotics by expressing penicillin-binding protein 2a (PBP2a). PBP2a is a cell wall-synthesizing protein whose closed active site exhibits a reduced binding affinity toward β-lactam antibiotics. Ceftaroline (CFT), a fifth-generation cephalosporin, can effectively inhibit the PBP2a activity by binding to an allosteric site to trigger the active site opening, allowing a second CFT to access the active site. However, the essential mechanism behind the allosteric behavior of PBP2a remains unclear. Herein, computational simulations are employed to elucidate how CFT allosterically regulates the conformation and dynamics of the active site of PBP2a. While CFT stabilizes the allosteric domain surrounding it, it simultaneously enhances the dynamics of the catalytic domain. Specifically, the study successfully captured the opening process of the active pocket in the allosteric CFT-bound systems and discovered that CFT alters the potential signal-propagating pathways from the allosteric site to the active site. These findings reveal the implied mechanism of the CFT-mediated allostery in PBP2a and provide new insights into dual-site drug design or combination therapy against MRSA targeting PBP2a.

A Review on Five and Six-Membered Heterocyclic Compounds Targeting the Penicillin-Binding Protein 2 (PBP2A) of Methicillin-Resistant Staphylococcus aureus (MRSA)

Staphylococcus aureus is a common human pathogen. Methicillin-resistant Staphylococcus aureus (MRSA) infections pose significant and challenging therapeutic difficulties. MRSA often acquires the non-native gene PBP2a, which results in reduced susceptibility to β-lactam antibiotics, thus conferring resistance. PBP2a has a lower affinity for methicillin, allowing bacteria to maintain peptidoglycan biosynthesis, a core component of the bacterial cell wall. Consequently, even in the presence of methicillin or other antibiotics, bacteria can develop resistance. Due to genes responsible for resistance, S. aureus becomes MRSA. The fundamental premise of this resistance mechanism is well-understood. Given the therapeutic concerns posed by resistant microorganisms, there is a legitimate demand for novel antibiotics. This review primarily focuses on PBP2a scaffolds and the various screening approaches used to identify PBP2a inhibitors. The following classes of compounds and their biological activities are discussed: Penicillin, Cephalosporins, Pyrazole-Benzimidazole-based derivatives, Oxadiazole-containing derivatives, non-β-lactam allosteric inhibitors, 4-(3H)-Quinazolinones, Pyrrolylated chalcone, Bis-2-Oxoazetidinyl macrocycles (β-lactam antibiotics with 1,3-Bridges), Macrocycle-embedded β-lactams as novel inhibitors, Pyridine-Coupled Pyrimidinones, novel Naphthalimide corbelled aminothiazoximes, non-covalent inhibitors, Investigational-β-lactam antibiotics, Carbapenem, novel Benzoxazole derivatives, Pyrazolylpyridine analogues, and other miscellaneous classes of scaffolds for PBP2a. Additionally, we discuss the penicillin-binding protein, a crucial target in the MRSA cell wall. Various aspects of PBP2a, bacterial cell walls, peptidoglycans, different crystal structures of PBP2a, synthetic routes for PBP2a inhibitors, and future perspectives on MRSA inhibitors are also explored.

Novel Thiazolylketenyl Quinazolinones as Potential Anti-MRSA Agents and Allosteric Modulator for PBP2a

Bacterial infections caused by methicillin-resistant Staphylococcus aureus have seriously threatened public health. There is an urgent need to propose an existing regimen to overcome multidrug resistance of MRSA. A unique class of novel anti-MRSA thiazolylketenyl quinazolinones (TQs) and their analogs were developed. Some synthesized compounds showed good bacteriostatic potency. Especially TQ 4 was found to exhibit excellent inhibition against MRSA with a low MIC of 0.5 μg/mL, which was 8-fold more effective than norfloxacin. The combination of TQ 4 with cefdinir showed stronger antibacterial potency. Further investigation revealed that TQ 4, with low hemolytic toxicity and low drug resistance, was not only able to inhibit biofilm formation but also could reduce MRSA metabolic activity and showed good drug-likeness. Mechanistic explorations revealed that TQ 4 could cause leakage of proteins by disrupting membrane integrity and block DNA replication by intercalated DNA. Furthermore, the synergistic antibacterial effect with cefdinir might be attributed to TQ 4 with the ability to induce PBP2a allosteric regulation of MRSA and further trigger the opening of the active site to promote the binding of cefdinir to the active site, thus inhibiting the expression of PBP2a, thereby overcoming MRSA resistance and significantly enhancing the anti-MRSA activity of cefdinir. A new strategy provided by these findings was that TQ 4, possessing both excellent anti-MRSA activity and allosteric effect of PBP2a, merited further development as a novel class of antibacterial agents to overcome increasingly severe MRSA infections.

Isopropoxy Benzene Guanidine Ameliorates Streptococcus suis Infection In Vivo and In Vitro

Streptococcus suis, an encapsulated zoonotic pathogen, has been reported to cause a variety of infectious diseases, such as meningitis and streptococcal-toxic-shock-like syndrome. Increasing antimicrobial resistance has triggered the need for new treatments. In the present study, we found that isopropoxy benzene guanidine (IBG) significantly attenuated the effects caused by S. suis infection, in vivo and in vitro, by killing S. suis and reducing S. suis pathogenicity. Further studies showed that IBG disrupted the integrity of S. suis cell membranes and increased the permeability of S. suis cell membranes, leading to an imbalance in proton motive force and the accumulation of intracellular ATP. Meanwhile, IBG antagonized the hemolysis activity of suilysin and decreased the expression of Sly gene. In vivo, IBG improved the viability of S. suis SS3-infected mice by reducing tissue bacterial load. In conclusion, IBG is a promising compound for the treatment of S. suis infections, given its antibacterial and anti-hemolysis activity.

Shapeshifting bullvalene-linked vancomycin dimers as effective antibiotics against multidrug-resistant gram-positive bacteria

The alarming rise in superbugs that are resistant to drugs of last resort, including vancomycin-resistant enterococci and staphylococci, has become a significant global health hazard. Here, we report the click chemistry synthesis of an unprecedented class of shapeshifting vancomycin dimers (SVDs) that display potent activity against bacteria that are resistant to the parent drug, including the ESKAPE pathogens, vancomycin-resistant Enterococcus (VRE), methicillin-resistant Staphylococcus aureus (MRSA), as well as vancomycin-resistant S. aureus (VRSA). The shapeshifting modality of the dimers is powered by a triazole-linked bullvalene core, exploiting the dynamic covalent rearrangements of the fluxional carbon cage and creating ligands with the capacity to inhibit bacterial cell wall biosynthesis. The new shapeshifting antibiotics are not disadvantaged by the common mechanism of vancomycin resistance resulting from the alteration of the C-terminal dipeptide with the corresponding d-Ala-d-Lac depsipeptide. Further, evidence suggests that the shapeshifting ligands destabilize the complex formed between the flippase MurJ and lipid II, implying the potential for a new mode of action for polyvalent glycopeptides. The SVDs show little propensity for acquired resistance by enterococci, suggesting that this new class of shapeshifting antibiotic will display durable antimicrobial activity not prone to rapidly acquired clinical resistance.

Chemical Basis of Combination Therapy to Combat Antibiotic Resistance

The antimicrobial resistance crisis is a global health issue requiring discovery and development of novel therapeutics. However, conventional screening of natural products or synthetic chemical libraries is uncertain. Combination therapy using approved antibiotics with inhibitors targeting innate resistance mechanisms provides an alternative strategy to develop potent therapeutics. This review discusses the chemical structures of effective β-lactamase inhibitors, outer membrane permeabilizers, and efflux pump inhibitors that act as adjuvant molecules of classical antibiotics. Rational design of the chemical structures of adjuvants will provide methods to impart or restore efficacy to classical antibiotics for inherently antibiotic-resistant bacteria. As many bacteria have multiple resistance pathways, adjuvant molecules simultaneously targeting multiple pathways are promising approaches to combat multidrug-resistant bacterial infections.

Repurposing Inhibitors of Phosphoinositide 3-kinase as Adjuvant Therapeutics for Bacterial Infections

The rise in antimicrobial resistance and the decline in new antibiotics has created a great need for novel approaches to treat drug resistant bacterial infections. Increasing the burden of antimicrobial resistance, bacterial virulence factors allow for survival within the host, where they can evade host killing and antimicrobial therapy within their intracellular niches. Repurposing host directed therapeutics has great potential for adjuvants to allow for more effective bacterial killing by the host and antimicrobials. To this end, phosphoinositide 3-kinase inhibitors are FDA approved for cancer therapy, but also have potential to eliminate intracellular survival of pathogens. This review describes the PI3K pathway and its potential as an adjuvant target to treat bacterial infections more effectively.

Enzymatic Synthesis of Vancomycin-Modified DNA

Many potent antibiotics fail to treat bacterial infections due to emergence of drug-resistant strains. This surge of antimicrobial resistance (AMR) calls in for the development of alternative strategies and methods for the development of drugs with restored bactericidal activities. In this context, we surmised that identifying aptamers using nucleotides connected to antibiotics will lead to chemically modified aptameric species capable of restoring the original binding activity of the drugs and hence produce active antibiotic species that could be used to combat AMR. Here, we report the synthesis of a modified nucleoside triphosphate equipped with a vancomycin moiety on the nucleobase. We demonstrate that this nucleotide analogue is suitable for polymerase-mediated synthesis of modified DNA and, importantly, highlight its compatibility with the SELEX methodology. These results pave the way for bacterial-SELEX for the identification of vancomycin-modified aptamers.

Designing, Synthesis, and Anti-Breast Cancer Activity of a Series of New Quinazolin-4(1H)-one Derivatives

The inhibition of phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) protein could be a promising treatment for breast cancer. In this regard, docking studies were accomplished on various functionalized organic molecules. Among them, several derivatives of quinazolin-4(1H)-one exhibited anti-breast cancer activity and satisfied the drug likeliness properties. Further, the in vitro inhibitory studies by a series of 2-(2-phenoxyquinolin-3-yl)-2,3-dihydroquinazolin-4(1H)-one molecules showed strong anti-cancer activity than the currently available drug, wortmannin. The MTT cytotoxicity assay was used to predict the anti-proliferative activity of these drugs against MCF-7 cancer cells by inhibiting the PIK3CA protein. The dose-dependent analysis showed a striking decrease in cancer cell viability at 24 h with inhibitory concentrations (IC₅₀ ) of 3b, 3c, 3d, 3f and 3m are 15±1, 17±1, 8±1, 10±1 and 60±1 (nanomoles), respectively. This is the first report in the literature on the inhibition of PIK3CA protein by quinazolinone derivatives that can be used in the treatment of cancer. Quinazolinone analogs have the potential to be safe and economically feasible scaffolds if they are produced using a chemical technique that is both straightforward and amenable to modification. From the cancer research perspective, this study can eventually offer better care for cancer patients.

In vitro studies of the protein-interaction network of cell-wall lytic transglycosylase RlpA of Pseudomonas aeruginosa

The protein networks of cell-wall-biosynthesis assemblies are largely unknown. A key class of enzymes in these assemblies is the lytic transglycosylases (LTs), of which eleven exist in P. aeruginosa. We have undertaken a pulldown strategy in conjunction with mass-spectrometry-based proteomics to identify the putative binding partners for the eleven LTs of P. aeruginosa. A total of 71 putative binding partners were identified for the eleven LTs. A systematic assessment of the binding partners of the rare lipoprotein A (RlpA), one of the pseudomonal LTs, was made. This 37-kDa lipoprotein is involved in bacterial daughter-cell separation by an unknown process. RlpA participates in both the multi-protein and multi-enzyme divisome and elongasome assemblies. We reveal an extensive protein-interaction network for RlpA involving at least 19 proteins. Their kinetic parameters for interaction with RlpA were assessed by microscale thermophoresis, surface-plasmon resonance, and isothermal-titration calorimetry. Notable RlpA binding partners include PBP1b, PBP4, and SltB1. Elucidation of the protein-interaction networks for each of the LTs, and specifically for RlpA, opens opportunities for the study of their roles in the complex protein assemblies intimately involved with the cell wall as a structural edifice critical for bacterial survival.

Multifunctional Carbon Dots-Based Nanoplatform for Bioimaging and Quaternary Ammonium Salt/Photothermal Synergistic Antibacterial

The emergence of drug resistance and superbugs poses a devastating threat to public health, even lead to death. Thus, it is significant to develop a novel antibacterial agent to combat...

An unanticipated discovery towards novel naphthalimide corbelled aminothiazoximes as potential anti-MRSA agents and allosteric modulators for PBP2a

Available therapeutic strategies are urgently needed to conquer multidrug resistance of MRSA. A visible effort was guided towards the advancement of novel antibacterial framework of naphthalimide corbelled aminothiazoximes, and desired to assert some insight on the conjunction of individual pharmacophore with distinct biological activities and unique action mechanism. Preliminary assessment displayed that dimethylenediamine derivative 13d presented a wonderful inhibition on MRSA (MIC = 0.5 μg/mL), and showed excellent membrane selectivity (HC₅₀ > 200 μg/mL) from an electrostatic distinction of the electronegative bacterial membranes and the electroneutral mammalian membranes. Moreover, 13d could effectually relieve the development of MRSA resistance. Investigations into explaining the mechanism of anti-MRSA disclosed that 13d displayed strong lipase affinity, which facilitated its permeation into cell membrane, causing membrane depolarization, leakage of cytoplasmic contents and lactate dehydrogenase (LDH) inhibition. Meanwhile, 13d could exert interaction with DNA to hinder biological function of DNA, and disrupt the antioxidant defense system of MRSA through up-regulation of ROS subjected the strain to oxidative stress. In particular, the unanticipated mechanism for naphthalimide corbelled aminothiazoximes that 13d could suppress the expression of PBP2a by inducing allosteric modulation of PBP2a and triggering the open of the active site, was discovered for the first time. These findings of naphthalimide corbelled aminothiazoximes as a small-molecule class of anti-MRSA agents held promise in strategies for treatment of MRSA infections.