Selectively guanidinylated aminoglycosides as antibiotics

The emergence of virulent, drug-resistant bacterial strains coupled with a minimal output of new pharmaceutical agents to combat them makes this a critical time for antibacterial research. Aminoglycosides are a well-studied, highly potent class of naturally occurring antibiotics with scaffolds amenable to modification, and therefore, they provide an excellent starting point for the development of semisynthetic, next-generation compounds. To explore the potential of this approach, we synthesized a small library of aminoglycoside derivatives selectively and minimally modified at one or two positions with a guanidine group replacing the corresponding amine or hydroxy functionality. Most guanidino-aminoglycosides showed increased affinity for the ribosomal decoding rRNA site, the cognate biological target of the natural products, when compared with their parent antibiotics, as measured by an in vitro fluorescence resonance energy transfer (FRET) A-site binding assay. Additionally, certain analogues showed improved minimum inhibitory concentration (MIC) values against resistant bacterial strains, including methicillin-resistant Staphylococcus aureus (MRSA). An amikacin derivative holds particular promise with activity greater than or equal to the parent antibiotic in the majority of bacterial strains tested.     

Identification of a novel protein synthesis inhibitor active against gram-positive bacteria

In an effort to identify novel antibacterial chemotypes, we performed a whole‐cell screen for inhibitors of Staphylococcus aureus growth and pursued those compounds with previously uncharacterized antibacterial activity. This process resulted in the identification of a benzothiazolium salt, ABTZ‐1, that displayed potent antibacterial activity against Gram‐positive pathogens. Several clinically desirable qualities were demonstrated for ABTZ‐1 including potent activity against multidrug‐resistant clinical isolates of methicillin‐resistant S. aureus (MRSA) and vancomycin‐resistant enterococci (VRE), retention of this activity in human serum, and low hemolytic activity. The antibacterial activity of ABTZ‐1 was attributed to its inhibition of bacterial translation, as this compound prevented the incorporation of [³⁵S]methionine into S. aureus proteins, and ABTZ‐1‐resistant strains were cross‐resistant to known inhibitors of bacterial translation. ABTZ‐1 represents a promising new class of antibacterial agents.     

Phytol Derivatives as Drug Resistance Reversal Agents

Phytol was chemically transformed into fifteen semi‐synthetic derivatives, which were evaluated for their antibacterial and drug resistance reversal potential in combination with nalidixic acid against E. coli strains CA8000 and DH5α. The pivaloyl ( 4 ), 3,4,5‐trimethoxybenzoyl ( 9 ), 2,3‐dichlorobenzoyl ( 10 ), cinnamoyl ( 11 ), and aldehyde ( 14 ) derivatives of phytol ((2E,7R,11R)‐3,7,11,15‐tetramethyl‐2‐hexadecen‐1‐ol) were evaluated by using another antibiotic, tetracycline, against the MDREC‐KG4 clinical isolate of E. coli. Derivative 4 decreased the maximal inhibitory concentration (MIC) of the antibiotics by 16‐fold, while derivatives 9 , 10 , 11 , and 14 reduced MIC values of the antibiotics up to eightfold against the E. coli strains. Derivatives 4 , 9 , 10 , 11 , and 14 inhibited the ATP‐dependent efflux pump; this was also supported by their in silico binding affinity and down‐regulation of the efflux pump gene yojI, which encodes the multidrug ATP‐binding cassette transporter protein. This study supports the possible use of phytol derivatives in the development of cost‐effective antibacterial combinations.     

Effect of Ester to Amide or N‐Methylamide Substitution on Bacterial Membrane Depolarization and Antibacterial Activity of Novel Cyclic Lipopeptides

Cyclic lipopeptides derived from the fusaricidin/LI‐F family of naturally occurring antibiotics represent particularly attractive candidates for the development of new antibacterial agents. In comparison with natural products, these derivatives may offer better stability under physiologically relevant conditions and lower nonspecific toxicity, while preserving their antibacterial activity. In this study we assessed the ability of cyclic lipodepsipeptide 1 and its analogues—amide 2 , N‐methylamide 3 , and linear peptide 4 —to interact with the cytoplasmic membranes of selected Gram‐positive bacteria. We also investigated their bacteriostatic/bactericidal modes of action and in vivo potency by using a Galleria mellonella model of MRSA infection. Cyclic lipopeptides 1 and 2 depolarize the cytoplasmic membranes of Gram‐positive bacteria in a concentration‐dependent manner. The degree of membrane depolarization was influenced by the structural and physical properties of 1 and 2 , with the more flexible and hydrophobic peptide 1 being most efficient. However, membrane depolarization does not correlate with bacterial cell lethality, suggesting that membrane‐targeting activity is not the main mode of action for this class of antibacterial peptides. Conversely, substitution of the depsipeptide bond in 1 with an N‐methylamide bond in 3 , or its hydrolysis to peptide 4 , lead to a complete loss of antibacterial activity and indicate that the conformation of cyclic lipopeptides plays a role in their antibacterial activities. Cyclic lipopeptides 1 and 2 are also capable of improving the survival of G. mellonella larvae infected with MRSA at varying efficiencies, reflecting their in vitro activities. Gaining more insight into the structure–activity relationship and mode of action of these cyclic lipopeptides may enable the development of new antibiotics of this class with improved antibacterial activity.     

Simplified Novel Muraymycin Analogues; using a Serine Template Strategy for Linking Key Pharmacophores

The present status of antibiotic research requires the urgent invention of novel agents that act on multidrug-resistant bacteria. The World Health Organization has classified antibiotic-resistant bacteria into critical, high and medium priority according to the urgency of need for new antibiotics. Naturally occurring uridine-derived “nucleoside antibiotics” have shown promising activity against numerous priority resistant organisms by inhibiting the transmembrane protein MraY (translocase I), which is yet to be explored in a clinical context. The catalytic activity of MraY is an essential process for bacterial cell viability and growth including that of priority organisms. Muraymycins are one subclass of naturally occurring MraY inhibitors. Despite having potent antibiotic properties, the structural complexity of muraymycins advocates for simplified analogues as potential lead structures. Herein, we report a systematic structure-activity relationship (SAR) study of serine template-linked, simplified muraymycin-type analogues. This preliminary SAR lead study of serine template analogues successfully revealed that the complex structure of naturally occurring muraymycins could be easily simplified to afford bioactive scaffolds against resistant priority organisms. This study will pave the way for the development of novel antibacterial lead compounds based on a simplified serine template.     

Design,Synthesis, and Antibacterial Activity of Demethylvancomycin Analogues against Drug‐Resistant Bacteria

Five novel N‐substituted demethylvancomycin derivatives were rationally designed and synthesized by using a structure‐based approach. The in vitro antibacterial activities against methicillin‐resistant Staphylococcus aureus (MRSA), gentamicin‐resistant Enterococcus faecalis (GRE), methicillin‐resistant Streptococcus pneumoniae (MRS), and vancomycin‐resistant Enterococcus faecalis (VRE) were evaluated. One of the compounds, N‐(6‐phenylheptyl)demethylvancomycin ( 12 a ), was found to exhibit more potent antibacterial activity than vancomycin and demethylvancomycin. Compound 12 a was also found to be ～18‐fold more efficacious than vancomycin against MRSA; however, the two compounds were found to have similar efficacy against MRS. Furthermore, compound 12 a exhibited a favorable pharmacokinetic profile with a half‐life of 5.11±0.52 h, which is longer than that of vancomycin (4.3±1.9 h). These results suggest that 12 a is a promising antibacterial drug candidate for further preclinical evaluation.     

Biosynthesis of the Peptide Antibiotic Feglymycin by a Linear Nonribosomal Peptide Synthetase Mechanism

Feglymycin, a peptide antibiotic produced by Streptomyces sp. DSM 11171, consists mostly of nonproteinogenic phenylglycine‐type amino acids. It possesses antibacterial activity against methicillin‐resistant Staphylococcus aureus strains and antiviral activity against HIV. Inhibition of the early steps of bacterial peptidoglycan synthesis indicated a mode of action different from those of other peptide antibiotics. Here we describe the identification and assignment of the feglymycin (feg) biosynthesis gene cluster, which codes for a 13‐module nonribosomal peptide synthetase (NRPS) system. Inactivation of an NRPS gene and supplementation of a hydroxymandelate oxidase mutant with the amino acid l ‐Hpg proved the identity of the feg cluster. Feeding of Hpg‐related unnatural amino acids was not successful. This characterization of the feg cluster is an important step to understanding the biosynthesis of this potent antibacterial peptide.     

Activation of the glmS Ribozyme Confers Bacterial Growth Inhibition

The ever‐growing number of pathogenic bacteria resistant to treatment with antibiotics call for the development of novel compounds with as‐yet unexplored modes of action. Here, we demonstrate the in vivo antibacterial activity of carba‐α‐d ‐glucosamine (CGlcN). In this mode of action study, we provide evidence that CGlcN‐mediated growth inhibition is due to glmS ribozyme activation, and we demonstrate that CGlcN hijacks an endogenous activation pathway, hence utilizing a prodrug mechanism. This is the first report describing antibacterial activity mediated by activating the self‐cleaving properties of a ribozyme. Our results open the path towards a compound class with an entirely novel and distinct molecular mechanism.     

Search for Shorter Portions of the Proline-Rich Antimicrobial Peptide Fragment Bac5(1–25) That Retain Antimicrobial Activity by Blocking Protein Synthesis

The spread of antibiotic-resistant pathogens has boosted the search for new antimicrobial drugs. Proline-rich antimicrobial peptides are promising lead compounds for the development of next-generation antibiotics, given their very low cytotoxicity and their good antimicrobial activity targeting the bacterial ribosome. Bac5(1–25) is an N-terminal fragment of the bovine proline-rich antimicrobial peptide Bac5, whose mode of action has been recently described. In this work we tested a number of Bac5(1–25) fragments, and we characterized their antimicrobial activity against Escherichia coli, Acinetobacter baumannii, Klebsiella pneumoniae, Staphylococcus aureus, Salmonella enterica, and Pseudomonas aeruginosa. We evaluated their cytotoxicity toward human cells and their efficacy in inhibiting bacterial protein synthesis. This allowed us to identify some shorter fragments of Bac5(1–25) with a good balance between antibacterial efficacy, protein synthesis inhibition, and ease/cost-effectiveness of synthesis, suitable as lead compounds to develop new antibacterials.     

Investigations into Viomycin Biosynthesis by Using Heterologous Production in Streptomyces lividans

Viomycin and capreomycin are members of the tuberactinomycin family of antituberculosis drugs. As with many antibacterial drugs, resistance to the tuberactinomycins is problematic in treating tuberculosis; this makes the development of new derivatives of these antibiotics to combat this resistance of utmost importance. To take steps towards developing new derivatives of this family of antibiotics, we have focused our efforts on understanding how these antibiotics are biosynthesized by the producing bacteria so that metabolic engineering of these pathways can be used to generate desired derivatives. Here we present the heterologous production of viomycin in Streptomyces lividans 1326 and the use of targeted‐gene deletion as a mechanism for investigating viomycin biosynthesis as well as the generation of viomycin derivatives. Deletion of vioQ resulted in nonhydroxylated derivatives of viomycin, while strains lacking vioP failed to acylate the cyclic pentapeptide core of viomycin with β‐lysine. Surprisingly, strains lacking vioL produced derivatives that had the carbamoyl group of viomycin replaced by an acetyl group. Additionally, the acetylated viomycin derivatives were produced at very low levels. These two observations suggested that the carbamoyl group of the cyclic pentapeptide core of viomycin was introduced at an earlier step in the biosynthetic pathway than previously proposed. We present biochemical evidence that the carbamoyl group is added to the β‐amino group of L ‐2,3‐diaminopropionate prior to incorporation of this amino acid by the nonribosomal peptide synthetases that form the cyclic pentapeptide cores of both viomycin and capreomycin.     

Modification of narrow‐spectrum peptidomimetic polyurethanes with fatty acid chains confers broad‐spectrum antibacterial activity

Each year, thousands of patients die from antimicrobial‐resistant bacterial infections that fail to respond to conventional antibiotic treatment. Antimicrobial polymers are a promising new method of combating antibiotic‐resistant bacterial infections. We have previously reported the synthesis of a series of narrow‐spectrum peptidomimetic antimicrobial polyurethanes that are effective against Gram‐negative bacteria, such as Escherichia coli; however, these polymers are not effective against Gram‐positive bacteria, such as Staphylococcus aureus. With the aim of understanding the correlation between chemical structure and antibacterial activity, we have subsequently developed three structural variants of these antimicrobial polyurethanes using post‐polymerization modification with decanoic acid and oleic acid. Our results show that such modifications converted the narrow‐spectrum antibacterial activity of these polymers into broad‐spectrum activity against Gram‐positive species such as S. aureus, however, also increasing their toxicity to mammalian cells. Mechanistic studies of bacterial membrane disruption illustrate the differences in antibacterial action between the various polymers. The results demonstrate the challenge of balancing antimicrobial activity and mammalian cell compatibility in the design of antimicrobial polymer compositions. © 2019 Society of Chemical Industry     

1‐(1H‐Indol‐3‐yl)ethanamine Derivatives as Potent Staphylococcus aureus NorA Efflux Pump Inhibitors

The synthesis of 37 1‐(1H‐indol‐3‐yl)ethanamine derivatives, including 12 new compounds, was achieved through a series of simple and efficient chemical modifications. These indole derivatives displayed modest or no intrinsic anti‐staphylococcal activity. By contrast, several of the compounds restored, in a concentration‐dependent manner, the antibacterial activity of ciprofloxacin against Staphylococcus aureus strains that were resistant to fluoroquinolones due to overexpression of the NorA efflux pump. Structure–activity relationships studies revealed that the indolic aldonitrones halogenated at position 5 of the indole core were the most efficient inhibitors of the S. aureus NorA efflux pump. Among the compounds, (Z)‐N‐benzylidene‐2‐(tert‐butoxycarbonylamino)‐1‐(5‐iodo‐1H‐indol‐3‐yl)ethanamine oxide led to a fourfold decrease of the ciprofloxacin minimum inhibitory concentration against the SA‐1199B strain when used at a concentration of 0.5 mg L ^?1. To the best of our knowledge, this activity is the highest reported to date for an indolic NorA inhibitor. In addition, a new antibacterial compound, tert‐butyl (2‐(3‐hydroxyureido)‐2‐(1H‐indol‐3‐yl)ethyl)carbamate, which is not toxic for human cells, was also found.     

Synthesis and Antibacterial Activity of New N-Alkylammonium and Carbonate-Triazole Derivatives within Desosamine of 14- and 15-Membered Lactone Macrolides

Desosamines of azithromycin (AZM) and clarithromycin (CLA) were modified by N-alkylation or nucleophilic substitution at the carbonyl/CuAAC sequence. Biological studies revealed a higher antibacterial potency of quaternary N-alkylammonium bromides of CLA as compared to AZM. SAR studies of CLA salts, including biological, conformation and molecular-docking analysis, enriched by physicochemical parameters, showed the importance of less bulky and unsaturated substituent for an efficient docking mode at the ribosomal tunnel and good antibacterial potency against clinical and standard Streptococcus pneumoniae and Streptococcus pyogenes strains (MICs 0.25 or 0.5 μg/mL). These CLA salts also have an at least threefold lower cytotoxicity than reference antibiotics at comparable antibacterial activity against the S. pneumoniae clinical strain. Differences in antibacterial effects noted for AZM and CLA salts bearing less bulky N-substituents can be better understood when their binding modes in the ribosomal tunnel are considered rather than their common low lipophilicity and excellent water solubility.     

Mutation in Elongation Factor G Confers Resistance to the Antibiotic Argyrin in the Opportunistic Pathogen Pseudomonas aeruginosa

The natural myxobacterial product argyrin is a cyclic peptide exhibiting immunosuppressive activity as well as antibacterial activity directed against the highly intrinsically resistant opportunistic pathogen Pseudomonas aeruginosa. In this study, we used whole‐genome sequencing technology as a powerful tool to determine the mode of action of argyrin. Sequencing of argyrin‐resistant P. aeruginosa isolates selected in vitro uncovered six point mutations that distinguished the resistant mutants from their susceptible parental strain. All six mutations were localized within one gene: fusA1, which encodes for the elongation factor EF‐G. After the reintroduction of selected mutations into the susceptible wild type, the strain became resistant to argyrin. Surface plasmon resonance experiments confirmed the interaction of argyrin A with FusA1. Interestingly, EF‐G has been previously shown to be the target of the anti‐Staphylococcus antibiotic fusidic acid. Mapping of the mutations onto a structural model of EF‐G revealed that the mutations conveying resistance against argyrin were clustered within domain III on the side opposite to that involved in fusidic acid binding, thus indicating that argyrin exhibits a new mode of protein synthesis inhibition. Although no mutations causing argyrin resistance have been found in other genes of P. aeruginosa, analysis of the sequence identity in EF‐G and its correlation with argyrin resistance in different bacteria imply that additional factors such as uptake of argyrin play a role in the argyrin resistance of other organisms.     

Antimicrobial Peptides with Antibacterial Activity against Vancomycin-Resistant Staphylococcus aureus Strains: Classification,Structures, and Mechanisms of Action

The emergence of bacteria resistant to conventional antibiotics is of great concern in modern medicine because it renders ineffectiveness of the current empirical antibiotic therapies. Infections caused by vancomycin-resistant Staphylococcus aureus (VRSA) and vancomycin-intermediate S. aureus (VISA) strains represent a serious threat to global health due to their considerable morbidity and mortality rates. Therefore, there is an urgent need of research and development of new antimicrobial alternatives against these bacteria. In this context, the use of antimicrobial peptides (AMPs) is considered a promising alternative therapeutic strategy to control resistant strains. Therefore, a wide number of natural, artificial, and synthetic AMPs have been evaluated against VRSA and VISA strains, with great potential for clinical application. In this regard, we aimed to present a comprehensive and systematic review of research findings on AMPs that have shown antibacterial activity against vancomycin-resistant and vancomycin-intermediate resistant strains and clinical isolates of S. aureus, discussing their classification and origin, physicochemical and structural characteristics, and possible action mechanisms. This is the first review that includes all peptides that have shown antibacterial activity against VRSA and VISA strains exclusively.     

Bis‐Cyclic Guanidines as a Novel Class of Compounds Potent against Clostridium difficile

Clostridium difficile infection (CDI) symptoms range from diarrhea to severe toxic megacolon and even death. Due to its rapid acquisition of resistance, C. difficile is listed as an urgent antibiotic‐resistant threat, and has surpassed methicillin‐resistant Staphylococcus aureus (MRSA) as the most common hospital‐acquired infection in the USA. To combat this pathogen, a new structural class of pseudo‐peptides that exhibit antimicrobial activities could play an important role. Herein we report a set of bis‐cyclic guanidine compounds that show potent antibacterial activity against C. difficile with decent selectivity. Eight compounds showed high in vitro potency against C. difficile UK6 with MIC values of 1.0 μg mL^?1, and cytotoxic selectivity index (SI) values up to 37. Moreover, the most selective compound is also effective in the treatment of C. difficile‐induced disease in a mouse model of CDI, and appears to be a very promising new candidate for the treatment of CDI.     

Influence of the Multivalency of Ultrashort Arg‐Trp‐Based Antimicrobial Peptides (AMP) on Their Antibacterial Activity

Peptide dendrimers are a class of molecules of high interest in the search for new antibiotics. We used microwave‐assisted, copper(I)‐catalyzed alkyne–azide cycloaddition (CuAAC; “click” chemistry) for the simple and versatile synthesis of a new class of multivalent antimicrobial peptides (AMPs) containing solely arginine and tryptophan residues. To investigate the influence of multivalency on antibacterial activity, short solid‐phase‐ synthesized azide‐modified Arg‐Trp‐containing peptides were “clicked” to three different alkyne‐modified benzene scaffolds to access scaffolds with one, two, or three peptides. The antibacterial activity of 15 new AMPs was investigated by minimal inhibitory concentration (MIC) assays on five different bacterial strains, including a multidrug‐resistant Staphylococcus aureus (MRSA) strain. With ultrashort (2–3 residues) peptides, a clear synergistic effect of the trivalent display was observed, whereas this effect was not apparent with longer peptides. The best candidates showed activities in the low‐micromolar range against Gram‐positive MRSA. Surprisingly, the best activity against Gram‐negative Acinetobacter baumannii was observed with an ultrashort dipeptide on the trivalent scaffold (MIC: 7.5 μM ). The hemolytic activity was explored for the three most active peptides. At concentrations ten times the MIC values, <1 % hemolysis of red blood cells was observed.     

A Phytochemical–Halogenated Quinoline Combination Therapy Strategy for the Treatment of Pathogenic Bacteria

With the continued rise of drug‐resistant bacterial infections coupled with the current discouraging state of the antibiotic pipeline, the need for new antibacterial agents that operate through unique mechanisms compared with conventional antibiotics and work in synergy with other agents is at an all‐time high. We have discovered that gallic acid, a plant‐derived phytochemical, dramatically potentiates the antibacterial activities of several halogenated quinolines (up to 11 800‐fold potentiation against Staphylococcus aureus) against pathogenic bacteria, including drug‐resistant clinical isolates. S. aureus demonstrated the highest sensitivity towards gallic acid–halogenated quinoline combinations, including one halogenated quinoline that demonstrated potentiation of biofilm eradication activity against a methicillin‐resistant S. aureus (MRSA) clinical isolate. During our studies, we also demonstrated that these halogenated quionlines operate through an interesting metal(II) cation‐dependent mechanism and display promising mammalian cytotoxicity.     

Multicomponent Petasis‐borono Mannich Preparation of Alkylaminophenols and Antimicrobial Activity Studies

In this work we report the antibacterial activity of alkylaminophenols. A series of such compounds was prepared by a multicomponent Petasis‐borono Mannich reaction starting from salicylaldehyde and its derivatives. The obtained compounds were tested against a large panel of microorganisms, Gram‐positive and Gram‐negative bacteria, and a yeast. Among the several tertiary amine derivatives tested, indoline‐derived aminophenols containing a nitro group at the para‐phenol position showed considerable activity against bacteria tested with minimal inhibitory concentrations as low as 1.36 μm against Staphyloccocus aureus and Mycobacterium smegmatis. Cytotoxicity of the new para‐nitrophenol derivatives was observed only at concentrations much higher than those required for antibacterial activity.     

Benzylaminoethylureido-Tailed Benzenesulfonamides Show Potent Inhibitory Activity against Bacterial Carbonic Anhydrases

A series of benzylaminoethylureido-tailed benzenesulfonamides was analyzed for their inhibition potential against bacterial carbonic anhydrases (CAs) such as VhCA α, β, and γ from Vibrio cholerae, and BpsCA β and γ-CAs from Burkholderia pseudomallei. Growing drug resistance against antibiotics demands alternative targets and mechanisms of action. As CA is essential for the survival of bacteria, such enzymes have the potential for developing new antibiotics. Most of the compounds presented excellent inhibition potential against VhCA γ compared to α and β, with K_i values in the range of 82.5–191.4 nM. Several sulfonamides exhibited excellent inhibition against BpsCA β with K_i values in the range of 394–742.8 nM. Recently it has been demonstrated that sufonamide CA inhibitors are effective against vancomycin-resistant enterococci. These data show that CA inhibition of pathogenic bacteria may lead to a new class of antibiotics.