Infections with enterococci are challenging to treat due to intrinsic resistance to several antibiotics. Especially vancomycin-resistant Enterococcus faecium and Enterococcus faecalis are of considerable concern with a limited number of efficacious therapeutics available. From an initial screening of 20 peptidomimetics, 11 stable peptide/β-peptoid hybrids were found to have antibacterial activity against eight E. faecium and E. faecalis isolates. Microbiological characterization comprised determination of minimal inhibitory concentrations (MICs), probing of synergy with antibiotics in a checkerboard assay, time–kill studies, as well as assessment of membrane integrity. E. faecium isolates proved more susceptible than E. faecalis isolates, and no differences in susceptibility between the vancomycin-resistant (VRE) and -susceptible E. faecium isolates were observed. A test of three peptidomimetics (Ac-[hArg-βNsce]6-NH2, Ac-[hArg-βNsce-Lys-βNspe]3-NH2 and Oct-[Lys-βNspe]6-NH2) in combination with conventional antibiotics (vancomycin, gentamicin, ciprofloxacin, linezolid, rifampicin or azithromycin) revealed no synergy. The same three potent analogues were found to have a bactericidal effect with a membrane-disruptive mode of action. Peptidomimetics Ac-[hArg-βNsce-Lys-βNspe]3-NH2 and Oct-[Lys-βNspe]6-NH2 with low MIC values (in the ranges 2–8 µg/mL and 4–16 µg/mL against E. faecium and E. faecalis, respectively) and displaying weak cytotoxic properties (i.e., <10% hemolysis at a ∼100-fold higher concentration than their MICs; IC50 values of 73 and 41 µg/mL, respectively, against HepG2 cells) were identified as promising starting points for further optimization studies.
Killing them softly. The in vitro elimination of malaria parasites with biomimetic oligomers is described. Antiplasmodial activity and varying hemolytic and prehemolytic effects were observed with a α-peptide-β-peptoid chimera that selectively perturbed Plasmodium falciparum-infected red blood cells (IC50 21 μM; see figure). The fact that structural variations can result in different profiles of antiplasmodial versus hemolytic/prehemolytic effects opens new possibilities in peptidomimetic research. (Chemical Equation Presented)
[structure: see text] We describe the synthesis and characterization of the first generation of oligomers consisting of alternating repeats of alpha-amino acids and chiral N-alkyl-beta-alanine (beta-peptoid) residues. These chimeras are stable toward proteolysis, non-hemolytic, and possess antibacterial activity comparable to well-known antimicrobial agents. Moreover, the chimeras exhibit length-dependent, concentration-dependent, solvent-dependent, and ion-strength-dependent ellipticity, indicating the presence of a secondary structure in solution. Thus, alpha-peptide/beta-peptoid oligomers represent a promising novel peptidomimetic backbone construct for biologically active ligands.
Recently, a novel type of antimicrobial and proteolytically stable peptidomimetic oligomers having an alpha-peptide-beta-peptoid chimeric backbone was reported. The present paper describes efficient protocols for the preparation of a wide range of dimeric building blocks, displaying different types of side-chains, for use in solid-phase synthesis (SPS) of libraries of this type of oligomers. The beta-peptoid monomers were obtained by microwave-assisted aza-Michael additions to acrylic esters. Subsequent solution-phase peptide coupling with suitably protected alpha-amino acids afforded dimeric intermediates. Even sluggish peptide couplings, involving sterically hindered N-alkyl-beta-alanines or amino acids with bulky side-chains, gave high yields on multigram-scale when using microwave (MW) irradiation. Protecting group and side-chain manipulations were performed as one-pot solution-phase procedures to afford ten different building blocks in good to excellent yields. Finally, the efficiency of SPS oligomerization of a representative dimer was demonstrated by preparing 10- to 16-residue homomers and by the assembly of four different building blocks to give a diversely functionalized octamer.
Synthetic peptidomimetics may be designed to mimic functions of antimicrobial peptides, including potentiation of antibiotics, yet possessing improved pharmacological properties. Pairwise screening of 42 synthetic peptidomimetics combined with the antibiotics azithromycin and rifampicin in multidrug-resistant (MDR) Escherichia coli ST131 and Klebsiella pneumoniae ST258 led to identification of two subclasses of α-peptide/β-peptoid hybrids that display synergy with azithromycin and rifampicin (fractional inhibitory concentration indexes of 0.03–0.38). Further screening of the best three peptidomimetics in combination with a panel of 21 additional antibiotics led to identification of peptidomimetics that potentiated ticarcillin/clavulanate and erythromycin against E. coli, and clindamycin against K. pneumoniae. The study of six peptidomimetics was extended to Pseudomonas aeruginosa, confirming synergy with antibiotics for five of them. The most promising compound, H-(Lys-βNPhe)8-NH2, exerted only a minor effect on the viability of mammalian cells (EC50 ≥ 124–210 μM), and thus exhibited the highest selectivity toward bacteria. This compound also synergized with rifampicin and azithromycin at sub-micromolar concentrations (0.25–0.5 μM), thereby inducing susceptibility to these antibiotics at clinically relevant concentrations in clinical MDR isolates. This peptidomimetic lead and its analogs constitute promising candidates for efficient repurposing of rifampicin and azithromycin against Gram-negative pathogens.
Previous optimisation studies of peptide/peptoid hybrids typically comprise comparison of structurally related analogues displaying different oligomer length and diverse side chains. The present work concerns a systematically constructed series of 16 closely related 12-mer oligomers with an alternating cationic/hydrophobic design, representing a wide range of hydrophobicity and differences in relative side-chain lengths. The aim was to explore and rationalise the structure–activity relationships within a subclass of oligomers displaying variation of three structural features: (i) cationic side-chain length, (ii) hydrophobic side-chain length, and (iii) type of residue that is of a flexible peptoid nature. Increased side-chain length of cationic residues led to reduced hydrophobicity till the side chains became more extended than the aromatic/hydrophobic side chains, at which point hydrophobicity increased slightly. Evaluation of antibacterial activity revealed that analogues with lowest hydrophobicity exhibited reduced activity against E. coli, while oligomers with the shortest cationic side chains were most potent against P. aeruginosa. Thus, membrane-disruptive interaction with P. aeruginosa appears to be promoted by a hydrophobic surface of the oligomers (comprised of the aromatic groups shielding the cationic side chains). Peptidomimetics with short cationic side chains exhibit increased hemolytic properties as well as give rise to decreased HepG2 (hepatoblastoma G2 cell line) cell viability. An optimal hydrophobicity window could be defined by a threshold of minimal hydrophobicity conferring activity toward E. coli and a threshold for maximal hydrophobicity, beyond which cell selectivity was lost.
Pseudomonas aeruginosa infection is a predominant cause of morbidity and mortality in patients with cystic fibrosis infection and with a compromised immune system. Emergence of bacterial resistance renders existing antibiotics inefficient, and therefore discovery of new antimicrobial agents is highly warranted. In recent years, numerous studies have demonstrated that antimicrobial peptides (AMPs) constitute potent agents against a range of pathogenic bacteria. However, AMPs possess a number of drawbacks such as susceptibility to proteolytic degradation with ensuing low bioavailability. To circumvent these undesired properties of AMPs unnatural amino acids or altered backbones have been incorporated to provide stable peptidomimetics with retained antibacterial activity. Here, we report on antimicrobial α-peptide/β-peptoid lysine-based peptidomimetics that exhibit high potency against clinical drug-resistant P. aeruginosa strains obtained from cystic fibrosis patients. These clinical strains possess phoQ and/or pmrB mutations that confer high resistance to colistin, the last-resort antibiotic for treatment of infections caused by P. aeruginosa. The lead peptidomimetic LBP-2 demonstrated a 12-fold improved anti-pseudomonal activity as compared to colistin sulfate as well as favorable killing kinetics, similar antibiofilm activity, and moderate cytotoxicity.
Staphylococcus pseudintermedius is the predominant opportunistic pathogen in dogs causing primarily integumentary infections such as pyoderma and otitis. The worldwide emergence of methicillin‐resistant S. pseudintermedius (MRSP) constitutes a significant health problem for companion animals in veterinary medicine. Thus, discovery of novel agents for treatment of MRSP‐associated infections is highly warranted. In the present work, structure‐activity relationships (based on testing of 37 peptidomimetics) have been explored with the aim of determining the influence of oligomer length as well as effect of fluorination, end‐group modification, and length of hydrophobic side chains. Incorporation of fluorine atoms and elongation of hydrophobic side chains both conferred overall increased potency without significantly enhancing the hemolytic properties of the compounds. Importantly, it was found that when targeting MRSP, the peptidomimetic length could be reduced from 12 to 8 residues without substantial loss of antibacterial activity. By contrast, introduction of end‐group modifications did not improve the activity against MRSP (10 strains tested), but conferred increased activity against Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa, albeit the concomitantly increased hemolytic properties resulted in a slightly lowered cell selectivity.
Infections caused by Pseudomonas aeruginosa are associated with high morbidity and mortality, especially in immunocompromised patients. These bacteria frequently grow within a biofilm matrix, rendering therapy with conventional antibiotics inefficient; a fact that emphasizes the need for new treatment strategies. Antimicrobial peptidomimetics constitute potential alternatives to traditional antimicrobial agents, however, their application remains limited due to the lack of efficient delivery to their target site in vivo and the risk of high systemic toxicity. Nanogels composed of cross-linked networks of amphiphilic polymers with a therapeutic drug molecule embedded constitute attractive drug delivery systems, as they have been shown to display unique properties such as biocompatibility and biodegrability, as well as confer improved drug stability and reduced drug-mediated cytotoxicity. Here, we report on the first formulation of biopolymer nanogels incorporating a potent antibacterial peptidomimetic. A lysine-based α-peptide/β-peptoid hybrid with potent activity against P. aeruginosa was designed and formulated into a nanogel together with octenyl succinic anhydride-modified hyaluronic acid in order to improve its cell selectivity. Twelve nanogel formulations were prepared by using a design of experiments setup in order to identify the parameters yielding the highest drug loading and the smallest particle size. Encapsulation of the peptidomimetic into nanogels significantly decreased the cytotoxicity of the peptidomimetic to eukaryotes. The most promising formulation with high encapsulation efficiency (88%) of the peptidomimetic demonstrated a three-fold reduction in cytotoxicity towards hepatocytes along with improved bacterial killing kinetics.
The rapid emergence of multidrug-resistant pathogens has evolved into a global health problem as current treatment options are failing for infections caused by pan-resistant bacteria. Hence, novel antibiotics are in high demand, and for this reason antimicrobial peptides (AMPs) have attracted considerable interest, since they often show broad-spectrum activity, fast killing and high cell selectivity. However, the therapeutic potential of natural AMPs is limited by their short plasma half-life. Antimicrobial peptidomimetics mimic the structure and biological activity of AMPs, but display extended stability in the presence of biological matrices. In the present review, focus is on the developments reported in the last decade with respect to their design, synthesis, antimicrobial activity, cytotoxic side effects as well as their potential applications as anti-infective agents. Specifically, only peptidomimetics with a modular structure of residues connected via amide linkages will be discussed. These comprise the classes of α-peptoids (N-alkylated glycine oligomers), β-peptoids (N-alkylated β-alanine oligomers), β 3-peptides, α/β 3-peptides, α-peptide/β-peptoid hybrids, α/γ N-acylated N-aminoethylpeptides (AApeptides), and oligoacyllysines (OAKs). Such peptidomimetics are of particular interest due to their potent antimicrobial activity, versatile design, and convenient optimization via assembly by standard solid-phase procedures.
An array of β-peptoid-peptide hybrid oligomers displaying different amino acid/peptoid compositions and chain lengths was studied with respect to antimicrobial activity against Staphylococcus epidermidis both in planktonic and biofilm cultures, comparing the effects with those of the common antibiotic vancomycin. Susceptibility and time-kill assays were performed to investigate activity against planktonic cells, whilst confocal laser scanning microscopy was used to investigate the dynamics of the activity against cells within biofilms. All tested peptidomimetics were bactericidal against both exponentially growing and stationary-phase S. epidermidis cells with similar killing kinetics. At the minimum inhibitory concentration (MIC), all peptidomimetics inhibited biofilm formation, whilst peptidomimetics at concentrations above the MIC (80-160μg/mL) eradicated young (6-h-old) biofilms, whilst even higher concentrations were needed to eradicate mature (24-h-old) biofilms completely. Chiral and guanidinylated hybrids exhibited the fastest killing effects against slow-growing cells and had more favourable antibiofilm properties than analogues only containing lysine or lacking chirality in the β-peptoid residues. However, the results of the mature biofilm killing assay indicated more complex structure-activity relationships. Cytotoxicity assays showed a clear correlation between oligomer length and cell toxicity within each subclass of peptides, but all possessed a high differential toxicity favouring killing of bacterial cells. This class of peptidomimetics may constitute promising antimicrobial alternatives for the prevention and treatment of multidrug-resistant S. epidermidis infections.
The in vitro activity of a host defense peptidomimetic (HDM-4) was investigated. The compound exhibited an antimicrobial activity profile against a range of Gram-negative bacteria. HDM-4 permeabilized the outer membrane and partly depolarized the inner membrane at its minimal inhibitory concentration (MIC). Moreover, it was demonstrated that HDM-4 was distributed widely in the bacterial cell at lethal concentrations, and that it could bind to DNA. It was confirmed that the multimodal action of HDM-4 resulted in it being less likely to lead to resistance development as compared to single-target antibiotics. HDM-4 exhibited multispecies anti-biofilm activity at sub-MIC levels. Furthermore, HDM-4 modulated the immune response by inducing the release of the chemoattractants interleukin-8 (IL-8), monocyte chemotactic protein-1 (MCP-1), and MCP-3 from human peripheral blood mononuclear cells. In addition, the compound suppressed lipopolysaccharide-mediated inflammation by reducing the release of the pro-inflammatory cytokines IL-6 and tumor necrosis factor-α.
Stable peptidomimetics mimicking natural antimicrobial peptides (AMPs) have emerged as a promising class of potential novel antibiotics. In the present study, we aimed at determining whether the antibacterial activity of two α-peptide/β-peptoid peptidomimetics against a range of bacterial pathogens was affected by conditions mimicking in vivo settings. Their activity was enhanced to an unexpected degree in the presence of human blood plasma for thirteen pathogenic Gram-positive and Gram-negative bacteria. MIC values typically decreased 2- to 16-fold in the presence of a human plasma concentration that alone did not damage the cell membrane. Hence, MIC and MBC data collected in these settings appear to represent a more appropriate basis for in vivo experiments preceding clinical trials. In fact, concentrations of peptidomimetics and peptide antibiotics (e.g. polymyxin B) required for in vivo treatments might be lower than traditionally deduced from MICs determined in laboratory media. Thus, antibiotics previously considered too toxic could be developed into usable last-resort drugs, due to ensuing lowered risk of side effects. In contrast, the activity of the compounds was significantly decreased in heat-inactivated plasma. We hypothesize that synergistic interactions with complement proteins and/or clotting factors most likely are involved.
Multidrug-resistant bacteria pose a serious threat to public health worldwide. Previously, α-peptide/β-peptoid hybrid oligomers were found to display activity against Gram-negative multidrug-resistant bacteria. In the present work, the influence of hydrophobicity, fluorination and distribution of cationic/hydrophobic residues on antimicrobial, hemolytic and cytotoxic properties of α-peptide/β-peptoid hybrids were investigated. An array of 22 peptidomimetics was tested. Analogues with enhanced hydrophobicity exhibited increased activity against Gram-positive bacteria. Incorporation of fluorinated residues into the peptidomimetics conferred increased potency against Gram-positive bacteria, while hemolytic properties and activity against Gram-negative bacteria depended on the degree and type of fluorination. Generally, shorter oligomers were less potent as compared to the corresponding longer analogues. However, some short analogues exhibited equal or higher antimicrobial activity. The hydrophobic/cationic alternating design proved superior to other distribution patterns of cationic side chains and hydrophobic moieties.