Development of Highly Potent Carbazole Amphiphiles as Membrane-Targeting Antimicrobials for Treating Gram-Positive Bacterial Infections

Discovery of new quaternized norharmane dimers as potential anti-MRSA agents

INTRODUCTION

Methicillin-resistant Staphylococcus aureus (MRSA)-caused infections greatly threaten public health. The discovery of natural-product-based anti-MRSA agents for treating infectious diseases has become one of the current research focuses.

OBJECTIVES

This study aims to identify promising anti-MRSA agents with a clear mechanism based on natural norharmane modified by quaternization or dimerization.

METHODS

A total of 32 norharmane analogues were prepared and characterized. Their antibacterial activities and resistance development propensity were tested by the broth double-dilution method. Cell counting kit-8 and hemolysis experiments were used to assess their biosafety. The plasma stability, bactericidal mode, and biofilm disruption effects were examined by colony counting and crystal violet staining assays. Fluorescence microscopy, metabolomic analysis, docking simulation and spectra titration revealed its anti-MRSA mechanisms. The mouse skin infection model was used to investigate the in vivo efficacy.

RESULTS

Compound 5a was selected as a potential anti-MRSA agent, which exhibited potent anti-MRSA activity in vitro and in vivo, low cytotoxicity and hemolysis under an effective dose. Moreover, compound 5a showed good stability in 50% plasma, a low tendency of resistance development and capabilities to disrupt bacterial biofilms. The mechanism studies revealed that compound 5a could inhibit the biosynthesis of bacteria cell walls, damage the membrane, disturb energy metabolism and amino acid metabolism pathways, and interfere with protein synthesis and nucleic acid function.

CONCLUSIONS

These results suggested that compound 5a is a promising candidate for combating MRSA infections, providing valuable information for further exploiting a new generation of therapeutic antibiotics.

Development of Xanthoangelol-Derived Compounds with Membrane-Disrupting Effects against Gram-Positive Bacteria

Infections caused by multidrug-resistant pathogens have emerged as a serious threat to public health. To develop new antibacterial agents to combat such drug-resistant bacteria, a class of novel amphiphilic xanthoangelol-derived compounds were designed and synthesized by mimicking the structure and function of antimicrobial peptides (AMPs). Among them, compound 9h displayed excellent antimicrobial activity against the Gram-positive strains tested (MICs = 0.5-2 μg/mL), comparable to vancomycin, and with low hemolytic toxicity and good membrane selectivity. Additionally, compound 9h demonstrated rapid bactericidal effects, low resistance frequency, low cytotoxicity, and good plasma stability. Mechanistic studies further revealed that compound 9h had good membrane-targeting ability and was able to destroy the integrity of bacterial cell membranes, causing an increase in intracellular ROS and the leakage of DNA and proteins, thus accelerating bacterial death. These results make 9h a promising antimicrobial candidate to combat bacterial infection.

Development of novel bisphenol derivatives with a membrane-targeting mechanism as potent gram-positive antibacterial agents

Antibiotic resistance is becoming increasingly severe. The development of small molecular antimicrobial peptides is regarded as a promising design strategy for antibiotics. Here, a series of bisphenol derivatives with amphiphilic structures were designed and synthesized as antibacterial agents by imitating the design strategy of antimicrobial peptides. After a series of structural optimizations, lead compound 43 was identified, which exhibited excellent antibacterial activity against Gram-positive bacterial strains (MICs = 0.78-1.56 μg/mL), poor hemolytic activity (HC50 > 200 μg/mL), and low cytotoxicity (CC50 > 100 μg/mL). Further biological evaluation results indicated that 43 exerted antibacterial effects by directly destroying bacterial cell membranes and displayed rapid bactericidal properties (within 0.5-1 h), leading to a very low probability of drug resistance. Moreover, in a murine model of corneal infection, 43 exhibited a strong in vivo antibacterial efficacy. These findings indicate that 43 is a promising candidate compound for the treatment of bacterial infections.

Pyridine-2,6-Dicarboxamide Proligands and their Cu(II)/Zn(II) Complexes Targeting Staphylococcus Aureus for the Attenuation of In Vivo Dental Biofilm

In the pursuit to combat stubborn bacterial infections, particularly those stemming from gram-positive bacteria, this study is an attempt to craft a precision-driven platform characterized by unparalleled selectivity, specificity, and synergistic antimicrobial mechanisms. Leveraging remarkable potential of metalloantibiotics in antimicrobial applications, herein, this work rationally designs, synthesizes, and characterizes a new library of Pyridine-2,6-dicarboxamide ligands and their corresponding transition metal Cu(II)/Zn(II) complexes. The lead compound L11 demonstrates robust antibacterial properties against Staphylococcus aureus (Minimum Inhibitory Concentration (MIC) = 2-16 µg mL-1), methicillin and vancomycin-resistant S. aureus (MIC = 2-4 µg mL-1) and exhibit superior antibacterial activity when compared to FDA-approved vancomycin, the drug of last resort. Additionally, the compound exhibits notable antimicrobial efficacy against resistant enterococcus strains (MIC = 2-8 µg mL-1). To unravel mechanistic profile, advanced imaging techniques including SEM and AFM are harnessed, collectively suggesting a mechanistic pathway involving cell wall disruption. Live/dead fluorescence studies further confirm efficacy of L11 and its complexes against S. aureus membranes. This translational exploration extends to a rat model, indicating promising in vivo therapeutic potential. Thus, this comprehensive research initiative has capabilities to transcends the confines of this laboratory, heralding a pivotal step toward combatting antibiotic-resistant pathogens and advancing the frontiers of metalloantibiotics-based therapy with a profound clinical implication.

The therapeutic potential of indole hybrids, dimers, and trimers against drug-resistant ESKAPE pathogens

Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter (ESKAPE) species as causative agents are characterized by increased levels of resistance toward multiple classes of first-line as well as last-resort antibiotics and represent serious global health concerns, creating a critical need for the development of novel antibacterials with therapeutic potential against drug-resistant ESKAPE species. Indole derivatives with structural and mechanistic diversity demonstrated broad-spectrum antibacterial activity against various clinically important pathogens including drug-resistant ESKAPE. Moreover, several indole-based agents that are exemplified by creatmycin have already been used in clinics or under clinical trials for the treatment of bacterial infections, demonstrating that indole derivatives hold great promise for the development of novel antibacterials. This review is an endeavor to highlight the current scenario of indole hybrids, dimers, and trimers with therapeutic potential against drug-resistant ESKAPE pathogens, covering articles published from 2020 to the present, to open new avenues for the exploration of novel antidrug-resistant ESKAPE candidates.

Bacterial cell death to overcome drug resistance with multitargeting bis-naphthalimides as potent antibacterial agents against Enterococcus faecalis

The increasing frequency of drug-resistant pathogens poses serious health issues to humans around the globe, leading to the development of new antibacterial agents to conquer drug resistance and bacterial infections. In view of this, we have synthesized a series of bis-naphthalimides to respond to awful drug resistance. Bioactivity assay and structure-activity relationship disclosed that compounds 5d and 5o exhibit potent antibacterial activity against E. faecalis, outperforming the marketed antibiotics. These drug candidates not only inhibit the biofilm formation of E. faecalis but also display rapid bactericidal properties, thus delaying the development of drug resistance within 20 passages. To explore the mechanism of antibacterial activity against E. faecalis, biofunctional examination was carried out which unveiled that 5d and 5o effectively disrupt bacterial cell membranes, causing the leakage of cytoplasmic contents and metabolic activity loss. Concurrently, 5d and 5o effectively intercalate with DNA to block DNA replication, causing the build-up of excessive reactive oxygen species and inhibiting the glutathione activity, ultimately leading to oxidative damage of E. faecalis and cell death. In addition, these compounds readily bind with HSA with a high binding constant, indicating that these drug candidates could be easily delivered to the target site. The above finding manifested that these newly synthesized bis-naphthalimides with multitargeting antibacterial properties offer a new prospect to overcome drug resistance.

Discovery of antibiotics that selectively kill metabolically dormant bacteria

There is a need to discover and develop non-toxic antibiotics that are effective against metabolically dormant bacteria, which underlie chronic infections and promote antibiotic resistance. Traditional antibiotic discovery has historically favored compounds effective against actively metabolizing cells, a property that is not predictive of efficacy in metabolically inactive contexts. Here, we combine a stationary-phase screening method with deep learning-powered virtual screens and toxicity filtering to discover compounds with lethality against metabolically dormant bacteria and favorable toxicity profiles. The most potent and structurally distinct compound without any obvious mechanistic liability was semapimod, an anti-inflammatory drug effective against stationary-phase E. coli and A. baumannii. Integrating microbiological assays, biochemical measurements, and single-cell microscopy, we show that semapimod selectively disrupts and permeabilizes the bacterial outer membrane by binding lipopolysaccharide. This work illustrates the value of harnessing non-traditional screening methods and deep learning models to identify non-toxic antibacterial compounds that are effective in infection-relevant contexts.

Discovery of novel Thymol-TPP antibiotics that eradicate MRSA persisters

The high prevalence of methicillin-resistant Staphylococcus aureus (MRSA) strains and the formation of non-growing, dormant "persisters" subsets help bacteria evade antibiotic treatment and enhance bacterial resistance, which poses a serious threat to human life and health. It is urgent to discover novel antibacterial therapies effective against MRSA persisters. Thymol is a common nutraceutical with weak antibacterial and antitumor activities. A series of Thymol triphenylphosphine (TPP) conjugates (TPP-Thy3) was designed and synthesized. These compounds showed significantly improved inhibitory activity against Gram-positive bacteria compared with Thymol. Among them, Thy3d displayed a low probability of resistance selection and showed excellent biocompatibility. Interestingly, Thy3d elicited a rapid killing effect of MRSA persisters (99.999%) at high concentration. Fluorescence experiments, electron microscopy, molecular dynamics simulation and bilayer experiment confirmed that Thy3d conjugates exerted potent antimicrobial activity by disrupting the integrity of the membrane of bacterial even the persister. Furthermore, Thy3d exhibited considerable efficacy in a mouse model of subcutaneous murine MRSA infection. In summary, TPP-Thy3 conjugates are a series of novel antibacterial agents and could serve as a new therapeutic strategy for combating antibiotic resistance.

Development of cannabidiol derivatives as potent broad-spectrum antibacterial agents with membrane-disruptive mechanism

The emergence of antibiotic resistance has brought a significant burden to public health. Here, we designed and synthesized a series of cannabidiol derivatives by biomimicking the structure and function of cationic antibacterial peptides. This is the first report on the design of cannabidiol derivatives as broad-spectrum antibacterial agents. Through the structure-activity relationship (SAR) study, we found a lead compound 23 that killed both Gram-negative and Gram-positive bacteria via a membrane-targeting mechanism of action with low resistance frequencies. Compound 23 also exhibited very weak hemolytic activity, low toxicity toward mammalian cells, and rapid bactericidal properties. To further validate the membrane action mechanism of compound 23, we performed transcriptomic analysis using RNA-seq, which revealed that treatment with compound 23 altered many cell wall/membrane/envelope biogenesis-related genes in Gram-positive and Gram-negative bacteria. More importantly, compound 23 showed potent in vivo antibacterial efficacy in murine corneal infection models caused by Staphylococcus aureus or Pseudomonas aeruginosa. These findings would provide a new design idea for the discovery of novel broad-spectrum antibacterial agents to overcome the antibiotic resistance crisis.

Discovery of Unique Bis-Substituted Aromatic Amide Derivatives as Novel Highly Potent Antibiotics for Combating Methicillin-Resistant Staphylococcus aureus (MRSA)

Due to the increasing antibiotic resistance, developing novel antimicrobials to fight infections caused by resistant bacteria is imperative. Herein, a series of novel bis-substituted aromatic amides were designed and synthesized through modifying the hit compound 1, and their antimicrobial activities were evaluated. Among them, compound 4t, as the most potent lead, exhibited excellent antimicrobial activities against Gram-positive bacteria, including clinical methicillin-resistant Staphylococcus aureus (MRSA) isolates, while keeping weak hemolytic and mammalian cytotoxic activities. Furthermore, compound 4t displayed rapid bactericidal capabilities, low tendency to produce resistance, and favorable capacities to destroy bacterial biofilms. Further explorations indicated that compound 4t induces bacterial death by binding to cardiolipin (CL) on the bacterial membrane, disrupting the cell membrane, and facilitating the accumulation of reactive oxygen species (ROS). Additionally, compound 4t showed remarkable anti-MRSA activity in vivo, demonstrating compound 4t could be developed as a potential candidate to combat MRSA infections.

Trilogy of drug repurposing for developing cancer and chemotherapy-induced heart failure co-therapy agent

Chemotherapy-induced complications, particularly lethal cardiovascular diseases, pose significant challenges for cancer survivors. The intertwined adverse effects, brought by cancer and its complication, further complicate anticancer therapy and lead to diminished clinical outcomes. Simple supplementation of cardioprotective agents falls short in addressing these challenges. Developing bi-functional co-therapy agents provided another potential solution to consolidate the chemotherapy and reduce cardiac events simultaneously. Drug repurposing was naturally endowed with co-therapeutic potential of two indications, implying a unique chance in the development of bi-functional agents. Herein, we further proposed a novel "trilogy of drug repurposing" strategy that comprises function-based, target-focused, and scaffold-driven repurposing approaches, aiming to systematically elucidate the advantages of repurposed drugs in rationally developing bi-functional agent. Through function-based repurposing, a cardioprotective agent, carvedilol (CAR), was identified as a potential neddylation inhibitor to suppress lung cancer growth. Employing target-focused SAR studies and scaffold-driven drug design, we synthesized 44 CAR derivatives to achieve a balance between anticancer and cardioprotection. Remarkably, optimal derivative 43 displayed promising bi-functional effects, especially in various self-established heart failure mice models with and without tumor-bearing. Collectively, the present study validated the practicability of the "trilogy of drug repurposing" strategy in the development of bi-functional co-therapy agents.

Uncovering the anti-biofilm activity of Ilicicolin B against Staphylococcus aureus

The formation of bacterial biofilms reduces the entry of antibiotics into bacteria and helps bacteria tolerate otherwise lethal concentrations of antimicrobials, leading to antibiotic resistance. Therefore, clearing bacterial biofilm is an effective strategy to tackle drug resistance. Currently, there are no approved antibiotics for inhibiting bacterial biofilm formation. We found that Ilicicolin B had excellent antibacterial activity against MRSA without obvious hemolytic activity. More importantly, Ilicicolin B effectively inhibited the biofilm formation in a concentration-dependent manner by crystal violet colorimetric assay and fluorescence microscopy analysis. Exposure of Staphylococcus aureus to Ilicicolin B for 24 h reduced the protein and polysaccharide components in EPS, suggesting that Ilicicolin B disintegrated the biofilms by dissociating the EPS in a matrix. In addition, Ilicicolin B demonstrated strong antibacterial effects in a murine abscess model of S. aureus. Our findings suggest that Ilicicolin B has the potential to treat S. aureus infection by inhibiting biofilm formation.

Discovery of novel cannabidiol derivatives with augmented antibacterial agents against methicillin-resistant Staphylococcus aureus

Drug-resistant bacterium infections are a severe threat to public health and novel antimicrobial agents combating drug-resistant bacteria are an unmet medical need. Although cannabidiol (CBD) has been reported to show antibacterial effects, whether its antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA) can be improved remains unclear. Herein, a series of novel CBD derivatives were designed and synthesized using various chemical approaches including amidation, Friedel-Crafts alkylation, and Negishi cross-coupling reaction for the modifications at the C-7, C-2', C-4', and C-6' positions of CBD skeleton. Derivative 21f showed augmented antibacterial activity against MRSA with a minimum inhibitory concentration of 4 μM without cytotoxic effect in microglia BV2 cells. Further mechanistic studies suggested that 21f inhibited the formation of biofilms, induced excess reactive oxygen species, and reduced bacterial metabolism, which collectively led to the acceleration of bacterial death. Findings from this study expand the understanding of CBD derivatives as promising antibacterial agents, which provides useful information for the development of cannabinoid-based antibacterial agents.

Confronting the Threat: Designing Highly Effective bis-Benzimidazolium Agents to Overcome Biofilm Persistence and Antimicrobial Resistance

The objective of this study is to take the initial steps toward developing novel antibiotics to counteract the escalating problem of antimicrobial and bacterial persistence, particularly in relation to biofilms. Our approach involves emulating the structural characteristics of cationic antimicrobial peptides. To circumvent resistance development, we have designed a library of bis-benzimidazolium salts that selectively target the microbial membranes in a nonspecific manner. To explore their structure-activity relationship, we conducted experiments using these compounds on various pathogens known for their resistance to conventional antibiotics, including Gram-positive methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus faecium (VRE), and Gram-negative Escherichia coli (E. coli). Notably, two bis-benzimidazolium salts exhibited robust antimicrobial activity while maintaining a high level of selectivity compared with mammalian cells. Our investigations revealed significant antibiofilm activity, as these compounds rapidly acted against established biofilms. In addition, bis-benzimidazolium compounds exhibited consistent results in resistance development and cross-resistance studies. Consequently, amphiphilic bis-benzimidazolium salts hold promise as potential candidates to combat resistance-associated infections.

Design, synthesis, and evaluation of novel pleuromutilin aryl acrylate derivatives as promising broad-spectrum antibiotics especially for combatting multi-drug resistant gram-negative bacteria

The emergence of drug-resistant strains presents a grave challenge for traditional antibiotics, underscoring the exigency of exploring novel antibacterial drugs. To address this, the present study endeavors to design and synthesize a collection of pleuromutilin aromatic acrylate derivatives, guided by combination principles. The antibacterial activity and structure-activity relationship of these derivatives were evaluated, and most of the derivatives displayed moderate to excellent antibacterial activity against both Gram-positive bacteria and Gram-negative bacteria. Among these derivatives, 5g exhibited the strongest antibacterial activity, with MIC (minimum inhibitory concentration) values ranging from 1-32 μg/mL, and a MIC value against clinically isolated drug-resistant strains of 4-64 μg/mL. Additionally, 5g exhibited negligible cytotoxicity, superior anti-mycoplasma activity, and a greater propensity to perturb bacterial cell membranes. Notably, the administration of 5g resulted in an increased survival rate of MRSA (Methicillin-resistant Staphylococcus aureus)-infected mice, with an ED50 (median effective dose) value of 9.04 mg/kg. These results indicated the potential of 5g to be further developed as an antibacterial drug for the clinical treatment of drug-resistant bacterial infections.

Antimicrobial activity of natural and semi-synthetic carbazole alkaloids

Since the first natural carbazole alkaloid, murrayanine, was isolated from Mwraya Spreng, carbazole alkaloid derivatives have been widely concerned for their anti-tumor, anti-viral and anti-bacterial activities. In recent decades, a growing body of data suggest that carbazole alkaloids and their derivatives have different biological activities. This is the first comprehensive description of the antifungal and antibacterial activities of carbazole alkaloids in the past decade (2012-2022), including natural and partially synthesized carbazole alkaloids in the past decade. Finally, the challenges and problems faced by this kind of alkaloids are summarized. This paper will be helpful for further exploration of this kind of alkaloids.

Evaluation of deoxythymidine-based cationic amphiphiles as antimicrobial, antibiofilm, and anti-inflammatory agents

OBJECTIVES

We recently designed a series of cationic deoxythymidine-based amphiphiles that mimic the cationic amphipathic structure of antimicrobial peptides (AMPs). Among these amphiphiles, ADG-2e and ADL-3e displayed the highest selectivity against bacterial cells. In this study, ADG-2e and ADL-3e were evaluated for their potential as novel classes of antimicrobial, antibiofilm, and anti-inflammatory agents.

METHODS

Minimum inhibitory concentrations of ADG-2e and ADL-3e against bacteria were determined using the broth microdilution method. Proteolytic resistance against pepsin, trypsin, α-chymotrypsin, and proteinase K was determined by radial diffusion and HPLC analysis. Biofilm activity was investigated using the broth microdilution and confocal microscopy. The antimicrobial mechanism was investigated by membrane depolarization, cell membrane integrity analysis, scanning electron microscopy (SEM), genomic DNA influence and genomic DNA binding assay. Synergistic activity was evaluated using checkerboard method. Anti-inflammatory activity was investigated using ELISA and RT-PCR.

RESULTS

ADG-2e and ADL-3e showed good resistance to physiological salts and human serum, and a low incidence of drug resistance. Moreover, they exhibit proteolytic resistance against pepsin, trypsin, α-chymotrypsin, and proteinase K. ADG-2e and ADL-3e were found to kill bacteria by an intracellular target mechanism and bacterial cell membrane-disrupting mechanism, respectively. Furthermore, ADG-2e and ADL-3e showed effective synergistic effects when combined with several conventional antibiotics against methicillin-resistant Staphylococcus aureus (MRSA) and multidrug-resistant Pseudomonas aeruginosa (MDRPA). Importantly, ADG-2e and ADL-3e not only suppressed MDRPA biofilm formation but also effectively eradicated mature MDRPA biofilms. Furthermore, ADG-2e and ADL-3e drastically decreased tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) gene expression and protein secretion in lipopolysaccharide (LPS)-stimulated macrophages, implying potent anti-inflammatory activity in LPS-induced inflammation.

CONCLUSION

Our findings suggest that ADG-2e and ADL-3e could be further developed as novel antimicrobial, antibiofilm, and anti-inflammatory agents to combat bacterial infections.

Development of Aromatic-Linked Diamino Acid Antimicrobial Peptide Mimics with Low Hemolytic Toxicity and Excellent Activity against Methicillin-Resistant Staphylococcus aureus (MRSA)

Infections caused by methicillin-resistant Staphylococcus aureus (MRSA) have become one of the biggest threats to public health. To develop new antibacterial agents against MRSA, a series of diamino acid compounds with aromatic nuclei linkers were designed and synthesized. Compound 8j, which exhibited low hemolytic toxicity and the best selectivity against S. aureus (SI > 2000), showed good activity against clinical MRSA isolates (MIC = 0.5-2 μg/mL). Compound 8j was able to quickly kill bacteria without inducing bacterial resistance. A mechanistic study and transcriptome analysis revealed that compound 8j can act on phosphatidylglycerol and induce the accumulation of endogenous reactive oxygen species, which can destroy bacterial membranes. Importantly, compound 8j achieved a 2.75 log reduction of MRSA count at 10 mg/kg/d in a mouse subcutaneous infection model. These findings suggested that compound 8j had the potential to be an antibacterial agent against MRSA.

Novel xanthone antibacterials: Semi-synthesis, biological evaluation, and the action mechanisms

α-Mangostin (α-MG) has demonstrated to display potent activities against Gram-positive bacterial. However, the contribution of phenolic hydroxyl groups of α-MG to the antibacterial activity remains obscure, severely hampering selection of structure modification to develop more potential α-MG-based anti-bacterial derivatives. Herein, twenty-one α-MG derivatives are designed, synthesized and evaluated for the antibacterial activities. The structure activity relationships (SARs) reveal that the contribution of the phenolic groups ranks as C3 > C6 > C1, and the phenolic hydroxyl group at C3 is essential to the antibacterial activity. Of note, compared to the parent compound α-MG, 10a with one acetyl at C1 exhibits the higher safety profiles due to its higher selectivity and no hemolysis, and the more potent antibacterial efficacy in an animal skin abscess model. Our evidences further present that, in comparison with α-MG, 10a has a stronger ability in depolarizing membrane potentials and leads to more leakage of bacterial proteins, consistent with the results observed by transmission electron microscopy (TEM). Transcriptomics analysis demonstrates those observations possibly relate to disturbed synthesis of proteins participating in the biological process of membrane permeability and integrity. Collectively, our findings provide a valuable insight for developing α-MG-based antibacterial agents with little hemolysis and new action mechanism via structural modifications at C1.

Small Molecular Mimetics of Antimicrobial Peptides as a Promising Therapy To Combat Bacterial Resistance

Clinically, antibiotics are widely used to treat infectious diseases; however, excessive drug abuse and overuse exacerbate the prevalence of drug-resistant bacterial pathogens, making the development of novel antibiotics extremely difficult. Antimicrobial peptide (AMP) is one of the most promising candidates for overcoming bacterial resistance owing to its unique structure and mechanism of action. This study examines the development of small molecular mimetics of AMPs over the past two decades. These mimetics can selectively disrupt membranes, which are the characteristic antibacterial mechanism of AMPs. In addition, the advantages and disadvantages of small AMP mimetics are discussed. The small molecular mimetics of AMPs are anticipated to garner interest and investment in discovering new antibiotics. This Perspective will assist in revitalizing the golden age of antibiotics in the current era of combating bacterial resistance.

Discovery of Quaternized Pyridine-Thiazole-Pleuromutilin Derivatives with Broad-Spectrum Antibacterial and Potent Anti-MRSA Activity

The quaternization of compounds has emerged as a promising molecular design strategy for the development of antibiotics. Herein, we report the design, synthesis, antibacterial activities, and structure-activity relationships of a series of novel pleuromutilin derivatives containing a quaternary amine C-14 side chain. Most of these derivatives exhibited broad-spectrum antibacterial activity against the tested bacteria. 10b was the most effective antibacterial agent that displayed excellent antibacterial activity against five clinical methicillin-resistant Staphylococcus aureus (MRSA) isolates, remarkable antimycoplasma activity, rapid bactericidal effects, and a strong ability to damage bacterial biofilms. Further mechanistic studies indicated that 10b destroyed bacterial cell membranes to exert its antibacterial effects. Moreover, 10b exhibited high survival protection and potent in vivo antibacterial efficacy (ED₅₀ = 4.94 mg/kg) in a mouse model of systemic MRSA infection. These findings suggest that 10b is a promising candidate for the treatment of multi-drug-resistant infectious diseases, especially MRSA infections.

Benzenesulfonyl thiazoloimines as unique multitargeting antibacterial agents towards Enterococcus faecalis

New efficient antimicrobial agents are urgently needed to combat invasive multidrug-resistant pathogens infections. Structurally unique benzenesulfonyl thiazoloimines (BSTIs) were exploited as novel potential antibacterial victors to confront terrific drug resistance. Some developed BSTIs exerted effectively antimicrobial efficacy against the tested strains. Notably, 2-pyridyl BSTI 14d exhibited good antibacterial activity against E. faecalis with MIC value of 1 μg/mL, which was superior to sulfathiazole and norfloxacin. The most active compound 14d not only showed rapid bactericidal properties and impeded E. faecalis biofilm formation to effectually relieve the development of drug resistance, but also performed low toxicity toward human red blood cells, human normal squamous epithelial cells and human non-neoplastic colon epithelial cells. Mechanistic investigation demonstrated that molecule 14d could exert efficient membrane destruction leading to the leakage of intracellular materials and metabolism inhibition, cause oxidative damage of E. faecalis through accumulation of excess reactive oxygen species and reduction of glutathione activity, and intercalate into DNA to hinder replication of DNA. Molecular docking indicated that the formation of 14d-dihydrofolate synthetase supramolecular complex could hinder the function of this enzyme. ADME analysis displayed that compound 14d possessed promising pharmacokinetic properties. These findings suggested that the newly developed benzenesulfonyl thiazoloimines with multitargeting antibacterial potential provided a new possibility for evading resistance.

Membrane-Active Nonivamide Derivatives as Effective Broad-Spectrum Antimicrobials: Rational Design, Synthesis, and Biological Evaluation

Antibiotic resistance is emerging as a "global public health concern". To address the growing epidemic of multidrug-resistant pathogens, the development of novel antimicrobials is urgently needed. In this study, by biomimicking cationic antibacterial peptides, we designed and synthesized a series of new membrane-active nonivamide and capsaicin derivatives as peptidomimetic antimicrobials. Through modulating charge/hydrophobicity balance and rationalizing structure-activity relationships of these peptidomimetics, compound 51 was identified as the lead compound. Compound 51 exhibited potent antibacterial activity against both Gram-positive bacteria (MICs = 0.39-0.78 μg/mL) and Gram-negative bacteria (MICs = 1.56-6.25 μg/mL), with low hemolytic activity and low cytotoxicity. Compound 51 displayed a faster bactericidal action through a membrane-disruptive mechanism and avoided bacterial resistance development. Furthermore, compound 51 significantly reduced the microbial burden in a murine model of keratitis infected by Staphylococcus aureus or Pseudomonas aeruginosa. Hence, this design strategy can provide a promising and effective solution to overcome antibiotic resistance.

Membrane-Targeting Neolignan-Antimicrobial Peptide Mimic Conjugates to Combat Methicillin-Resistant Staphylococcus aureus (MRSA) Infections

Infections caused by methicillin-resistant Staphylococcus aureus (MRSA) continue to endanger public health. Here, we report the synthesis of neolignan isomagnolone (I) and its isomer II, and the preparation of a series of novel neolignan-antimicrobial peptide (AMP) mimic conjugates. Notably, conjugates III5 and III15 exhibit potent anti-MRSA activity in vitro and in vivo, comparable to that of vancomycin, a current effective treatment for MRSA. Moreover, III5 and III15 display not only fast-killing kinetics and low resistance frequency but also low toxicity as well as effects on bacterial biofilms. Mechanism studies reveal that III5 and III15 exhibit rapid bactericidal effects through binding to the phosphatidylglycerol (PG) and cardiolipin (CL) of the bacterial membrane, thereby disrupting the cell membranes and allowing increased reactive oxygen species (ROS) as well as protein and DNA leakage. The results indicate that these neolignan-AMP mimic conjugates could be promising antimicrobial candidates for combating MRSA infections.

Design and Synthesis of Phenyl Sulfide-Based Cationic Amphiphiles as Membrane-Targeting Antimicrobial Agents against Gram-Positive Pathogens

Due to the emergence of antimicrobial resistance and the lack of new antibacterial agents, it has become urgent to discover and develop new antibacterial agents against multidrug-resistant pathogens. Antimicrobial peptides (AMPs) serve as the first line of defense for the host. In this work, we have designed, synthesized, and biologically evaluated a series of phenyl sulfide derivatives by biomimicking the structural features and biological functions of AMPs. Among these derivatives, the most promising compound 17 exhibited potent antibacterial activity against Gram-positive bacteria (minimum inhibitory concentrations = 0.39-1.56 μg/mL), low hemolytic activity (HC₅₀ > 200 μg/mL), and high membrane selectivity. In addition, 17 can rapidly kill Gram-positive bacteria within 0.5 h through membrane-targeting action and avoid antibiotic resistance. More importantly, 17 showed high in vivo efficacy against Staphylococcus aureus in a murine corneal infection model. Therefore, 17 has great potential as a lead compound for the treatment of Gram-positive bacterial infections.

Carbazole Derivatives as Potential Antimicrobial Agents

Microbial infection is a leading cause of death worldwide, resulting in around 1.2 million deaths annually. Due to this, medicinal chemists are continuously searching for new or improved alternatives to combat microbial infections. Among many nitrogen-containing heterocycles, carbazole derivatives have shown significant biological activities, of which its antimicrobial and antifungal activities are the most studied. In this review, miscellaneous carbazole derivatives and their antimicrobial activity are discussed (articles published from 1999 to 2022).

Design, synthesis, and biological evaluation of tetrahydroquinoline amphiphiles as membrane-targeting antimicrobials against pathogenic bacteria and fungi

The rising prevalence of drug-resistant pathogens is one of the biggest threats to human health. The development of new antibiotics that can overcome drug resistance is in urgent need. Herein, we designed and synthesized a series of amphiphilic tetrahydroquinoline derivatives as small-molecule-based antimicrobial peptidomimetics. Two lead compounds 36 and 52 which contained the tetrahydroquinoline core, hydrophobic alkyl chains (n-nonyl or isoprenyl group), different spacer lengths (n = 4 or 8), and cationic guanidine moiety, showed poor hemolytic activity, low cytotoxicity, and potent broad-spectrum antimicrobial activity against Gram-positive and Gram-negative bacteria, as well as fungi. The further biological evaluation revealed that compounds 36 and 52 can kill bacteria and fungi rapidly via membrane-targeting action and avoid drug resistance development. More importantly, compounds 36 and 52 exhibited similarly potent in vivo antimicrobial activities in a murine corneal infection caused by Staphylococcus aureus ATCC29213 or Pseudomonas aeruginosa ATCC9027, as compared to vancomycin or gatifloxacin. These results suggest that compounds 36 and 52 have great potential as new broad-spectrum antimicrobial agents to combat microbial resistance.

N-Arylimidazoliums as Highly Selective Biomimetic Antimicrobial Agents

Antibiotic resistance has become one of the greatest health threats in the world. In this study, a charge-dispersed dimerization strategy is described for the antimicrobial peptide (AMP) mimics via a tunable cationic charge to improve the selectivity between prokaryotic microbes and eukaryotic cells. This strategy is demonstrated with a series of charge-dispersed AMP mimics based on N-arylimidazolium skeletons. These N-arylimidazolium AMP mimics show potent antibacterial activity against strains along with a low rate of drug resistance, good hemocompatibility, and low cytotoxicity. In addition to the elimination of planktonic bacteria, N-arylimidazolium AMP mimics can also inhibit biofilm formation and destroy the established biofilm. More importantly, methicillin-resistant Staphylococcus aureus (MRSA)-induced lung-infected mice can be effectively treated by the intravenous administration of N-arylimidazolium AMP mimic, which enable the design of N-arylimidazolium AMP mimics to offer an alternative avenue to eradicate drug-resistant bacterial infection.

Aloe emodin-conjugated sulfonyl hydrazones as novel type of antibacterial modulators against S. aureus 25923 through multifaceted synergistic effects

Aloe emodin-conjugated sulfonyl hydrazones were designed and synthesized as novel type of antibacterial modulators. Aloe emodin benzenesulfonyl hydrazone 5a (AEBH-5a) was preponderant for the treatment of S. aureus 25923 (MIC = 0.5 μg/mL) over norfloxacin and presented high selectivity between bacterial membranes and mammalian membranes. Especially, AEBH-5a could eliminate the formed biofilms and relieve the development of S. aureus 25923 resistance. The antibacterial mechanism of AEBH-5a from extracellularity to intracellularity illustrated that AEBH-5a could destroy bacterial membrane integrity, leading to the leakage of protein and nucleic acid. Besides, AEBH-5a could not only interact with DNA and induce oxidative stress but also inhibit lactate dehydrogenase (LDH) activity as well as render metabolic inactivation. In silico ADME studies prediction of AEBH-5a revealed a favorable bioavailability score and prominent drug-likeness profile. This research showed that the multifaceted synergistic effect initiated by aloe emodin-conjugated sulfonyl hydrazones is a reasonable and effective tactic to combat menacing bacterial infections.

Copper-catalyzed multicomponent reactions for the efficient synthesis of diverse spirotetrahydrocarbazoles

In the presence of copper sulfate, three- or four-component reactions of 2-methylindole, aromatic aldehydes and various cyclic dienophiles in refluxing toluene afforded diverse spirotetrahydrocarbazoles. This reaction is an important development of the Levy reaction by using 2-methylindole to replace ethyl indole-2-acetate and successfully provides facile access to important polysubstituted spiro[carbazole-3,3'-indolines], spiro[carbazole-2,3'-indolines], spiro[carbazole-3,5'-pyrimidines] and spiro[carbazole-3,1'-cycloalkanes] in satisfactory yields and with high diastereoselectivity.

Unique Carbazole-Oxadiazole Derivatives as New Potential Antibiotics for Combating Gram-Positive and -Negative Bacteria

Novel carbazole-oxadiazoles were developed as new potential antibacterial agents to combat dreadful resistance. Some target compounds displayed predominant inhibitory effects on the tested Gram-positive and -negative bacteria, and carbazole-oxadiazoles 5g, 5i-k, 16a-c, and tetrazole analogues 23b-c were found to be efficient in impeding the growth of MRSA and Pseudomonas aeruginosa ATCC 27853 (MICs = 0.25-4 μg/mL). Furthermore, compounds 5g and 23b-c not only possessed rapid bactericidal ability and low tendency to develop resistance but also exhibited low cytotoxic effects toward Hek 293T, HeLa, and red blood cells (RBCs), especially molecule 5g also showed low toxicity in vivo, which showed the therapeutic potential of these compounds. Further exploration indicated that compounds 5g, 5i, and 23b-c could disintegrate the integrity of bacterial cell membranes to leak the cytoplasmic contents, thus exerting excellent antibacterial effects. These facts mean that carbazole-based antibacterial agents might have bright prospects in confronting bacterial infections.

Natural Berberine-derived Azolyl Ethanols as New Structural Antibacterial Agents against Drug-Resistant Escherichia coli

Natural berberine-derived azolyl ethanols as new structural antibacterial agents were designed and synthesized for fighting with dreadful bacterial resistance. Partial target molecules exhibited potent activity against the tested strains, particularly, nitroimidazole derivative 4d and benzothiazole-2-thoil compound 18b, with low cytotoxicity both exerted strong antibacterial activities against multidrug-resistant Escherichia coli at low concentrations as 0.007 and 0.006 mM, respectively. Meanwhile, the active compounds 4d and 18b possessed the ability to rapidly kill bacteria and observably eradicate the E. coli biofilm by reducing exopolysaccharide content to prevent bacterial adhesion, which was conducive to alleviating the development of E. coli resistance. Preliminary mechanistic explorations suggested that the excellent antibacterial potential of molecules 4d and 18b might be attributed to their ability to disintegrate membrane, accelerate ROS accumulation, reduce bacterial metabolism, and intercalate into DNA groove. These results provided powerful information for the further exploitation of natural berberine derivatives against bacterial pathogens.

Development of Membrane-Active Honokiol/Magnolol Amphiphiles as Potent Antibacterial Agents against Methicillin-Resistant Staphylococcus aureus (MRSA)

Currently, infections caused by drug-resistant bacteria have become a new challenge in anti-infective treatment, seriously endangering public health. In our continuous effort to develop new antimicrobials, a series of novel honokiol/magnolol amphiphiles were prepared by mimicking the chemical structures and antibacterial properties of cationic antimicrobial peptides. Among them, compound 5i showed excellent antibacterial activity against Gram-positive bacteria and clinical MRSA isolates (minimum inhibitory concentrations (MICs) = 0.5-2 μg/mL) with low hemolytic and cytotoxic activities and high membrane selectivity. Moreover, 5i exhibited rapid bactericidal properties, low resistance frequency, and good capabilities of disrupting bacterial biofilms. Mechanism studies revealed that 5i destroyed bacterial cell membranes, resulting in bacterial death. Additionally, 5i displayed high biosafety and potent in vivo anti-infective potency in a murine sepsis model. Our study indicates that these honokiol/magnolol amphiphiles shed light on developing novel antibacterial agents, and 5i is a potential antibacterial candidate for combating MRSA infections.

Natural Berberine-Hybridized Benzimidazoles as Novel Unique Bactericides against Staphylococcus aureus

Natural berberine-hybridized benzimidazoles as potential antibacterial agents were constructed to treat Staphylococcus aureus infection in the livestock industry. Bioassay showed that some new berberine-benzimidazole hybrids exhibited potent antibacterial efficacies, especially, the 2,4-dichlorobenzyl derivative 7d not only showed strong activity against S. aureus ATCC 29213 with the MIC value of 0.006 mM but also effectively eradicated bacterial biofilm and exhibited low toxicity toward mammalian cells. The drug combination experiments showed that compound 7d together with norfloxacin could enhance the antibacterial efficacy. Moreover, the 2,4-dichlorobenzyl derivative 7d did not show obvious propensity to develop bacterial resistance. Preliminary mechanism studies revealed that the active molecule 7d could damage the membrane integrity, stimulate ROS generation, and bind with DNA as well as S. aureus sortase A, thus exerting powerful antibacterial ability. In light of these facts, berberine-benzimidazole hybrid 7d showed a large potentiality as a new bactericide for treating S. aureus in the livestock industry.

Rational Design of Dipicolylamine-Containing Carbazole Amphiphiles Combined with Zn2+ as Potent Broad-Spectrum Antibacterial Agents with a Membrane-Disruptive Mechanism

Antibiotic resistance has become one of the most urgently important problems facing healthcare providers. A novel series of dipicolylamine-containing carbazole amphiphiles with strong Zn²⁺ chelating ability were synthesized, biomimicking cationic antimicrobial peptides. Effective broad-spectrum 16 combined with 12.5 μg/mL Zn²⁺ was identified as the most promising antimicrobial candidate. 16 combined with 12.5 μg/mL Zn²⁺ exhibited excellent antimicrobial activity against both Gram-positive and Gram-negative bacteria (MICs = 0.78-3.125 μg/mL), weak hemolytic activity, and low cytotoxicity. Time-kill kinetics and mechanism studies revealed 16 combined with 12.5 μg/mL Zn²⁺ had rapid bacterial killing properties, as evidenced by disruption of the integrity of bacterial cell membranes, effectively preventing bacterial resistance development. Importantly, 16 combined with 12.5 μg/mL Zn²⁺ showed excellent in vivo efficacy in a murine keratitis model caused by Staphylococcus aureus ATCC29213 or Pseudomonas aeruginosa ATCC9027. Therefore, 16 combined with 12.5 μg/mL Zn²⁺ could be a promising candidate for treating bacterial infections.

The Regioselective Functionalization Reaction of Unprotected Carbazoles with Donor-Acceptor Cyclopropanes

The regioselective functionalization reaction of unprotected carbazoles with donor-acceptor (D-A) cyclopropanes has been demonstrated for the first time. With Sc(OTf)₃ as Lewis acid catalyst, the N-H functionalization of carbazoles takes place through a highly selective nitrogen-initiated nucleophilic ring opening reaction. Significantly, by engaging TfOH as Brønsted acid catalyst, a straightforward C-H functionalization at the 3-position of the unprotected carbazole proceeds via Friedel-Crafts-type addition. This strategy facilitates the diversity-oriented synthesis of carbazole-containing heterocycles and expands the novel application of D-A cyclopropanes.

Multicomponent Reaction for Diastereoselective Synthesis of Spiro[carbazole-3,4'-pyrazoles] and Spiro[carbazole-3,4'-thiazoles]

In the presence of copper sulfate, the three-component reaction of aromatic aldehydes, ethylindole-3-acetate and 4-arylidene-5-methyl-2-phenylpyrazol-3-ones, in refluxing toluene afforded spiro[carbazole-3,4'-pyrazoles] in good yields with high diastereoselectivity. More importantly, the similar CuSO₄ promoted the four-component reaction of two molecular aromatic aldehydes with ethylindole-3-acetate and 5-methyl-2-phenyl-pyrazol-3-one resulted in 2,4-diarylspiro[carbazole-3,4'-pyrazoles] in satisfactory yields. Additionally, CuSO₄ promoted the four-component reaction of two molecular aromatic aldehydes, ethylindole-3-acetate and 2-phenylthiazol-4-one, in refluxing toluene gave 2,4-diarylspiro[carbazole-3,4'-thiazoles] with diastereomeric ratios in the range of 3:1 to 20:1.

Novel carbazole-oxadiazoles as potential Staphylococcus aureus germicides

Staphylococcus aureus resistance poses nonnegligible threats to the livestock industry. In light of this, carbazole-oxadiazoles were designed and synthesized for treating S. aureus infection. Bioassay discovered that 3,6-dibromocarbazole derivative 13a had effective inhibitory activities to several Gram-positive bacteria, in particular to S. aureus, S. aureus ATCC 29213, MRSA and S. aureus ATCC 25923 (MICs = 0.6-4.6 nmol/mL), which was more active than norfloxacin (MICs = 6-40 nmol/mL). Subsequent studies showed that 3,6-dibromocarbazole derivative 13a acted rapidly on S. aureus ATCC 29213 and possessed no obvious tendency to induce bacterial resistance. Further evaluations indicated that 3,6-dibromocarbazole derivative 13a showed strong abilities to disrupt bacterial biofilm and interfere with DNA, which might be the power sources of antibacterial performances. Moreover, 3,6-dibromocarbazole derivative 13a also exhibited slight cell lethality toward Hek 293 T and LO2 cells and low hemolytic toxicity to red blood cells. The above results implied that the active molecule 13a could be studied in the future development of agricultural available antibiotics.