Peptidomimetic antibiotics disrupt the lipopolysaccharide transport bridge of drug-resistant Enterobacteriaceae.

The rise of antimicrobial resistance poses a substantial threat to our health system, and, hence, development of drugs against novel targets is urgently needed. The natural peptide thanatin kills Gram-negative bacteria by targeting proteins of the lipopolysaccharide transport (Lpt) machinery. Using the thanatin scaffold together with phenotypic medicinal chemistry, structural data, and a target-focused approach, we developed antimicrobial peptides with drug-like properties. They exhibit potent activity against Enterobacteriaceae both in vitro and in vivo while eliciting low frequencies of resistance. We show that the peptides bind LptA of both wild-type and thanatin-resistant Escherichia coli and Klebsiella pneumoniae strains with low-nanomolar affinities. Mode of action studies revealed that the antimicrobial activity involves the specific disruption of the Lpt periplasmic protein bridge.

Antibiotic polymyxin arranges lipopolysaccharide into crystalline structures to solidify the bacterial membrane.

Polymyxins are last-resort antibiotics with potent activity against multi-drug resistant pathogens. They interact with lipopolysaccharide (LPS) in bacterial membranes, but mechanistic details at the molecular level remain unclear. Here, we characterize the interaction of polymyxins with native, LPS-containing outer membrane patches of Escherichia coli by high-resolution atomic force microscopy imaging, along with structural and biochemical assays. We find that polymyxins arrange LPS into hexagonal assemblies to form crystalline structures. Formation of the crystalline structures is correlated with the antibiotic activity, and absent in polymyxin-resistant strains. Crystal lattice parameters alter with variations of the LPS and polymyxin molecules. Quantitative measurements show that the crystalline structures decrease membrane thickness and increase membrane area as well as stiffness. Together, these findings suggest the formation of rigid LPS-polymyxin crystals and subsequent membrane disruption as the mechanism of polymyxin action and provide a benchmark for optimization and de novo design of LPS-targeting antimicrobials.

Macrocycle Therapeutics to Treat Life-threatening Diseases.

Polyphor's macrocycle platform led to the discovery of novel antibiotics addressing specifically Gramnegative bacteria by targeting outer membrane proteins. Furthermore, POL6014, an inhibitor of neutrophile elastase and balixafortide, a CXCR4 inhibitor have been discovered and developed from the platform. Currently a combination of balixafortide and eribulin is in Phase III clinical trial for the treatment of patients with advanced metastatic HER2-negative breast cancer.

Anti-biofilm activity of murepavadin against cystic fibrosis Pseudomonas aeruginosa isolates.

OBJECTIVES: To determine the activity of murepavadin in comparison with tobramycin, colistin and aztreonam, against cystic fibrosis (CF) Pseudomonas aeruginosa isolates growing in biofilms. The biofilm-epidemiological cut-off (ECOFF) values that include intrinsic resistance mechanisms present in biofilms were estimated. METHODS: Fifty-three CF P. aeruginosa isolates from respiratory samples were tested using the Calgary (closed system) device, while 4 [2 clinical (one smooth, one mucoid) and 2 reference strains] were tested using the BioFlux, a microfluidic open model of biofilm testing. Biofilm was stained with SYTO9® and propidium iodide. The minimal biofilm inhibitory concentration (MBIC) and the minimal biofilm eradication concentration (MBEC) were determined. The MBIC-ECOFF and the MBEC-ECOFF were calculated. RESULTS: Colistin, tobramycin and murepavadin presented similar MBIC50/MBIC90 values (4/32, 8/64 and 2/32, respectively). Murepavadin exhibited the lowest MBEC90 (64 mg/L). Aztreonam MBIC and MBEC values were higher than those of the other antibiotics tested. Tobramycin and murepavadin had the lowest MBEC-ECOFF (64 and 128 mg/L, respectively), while those of aztreonam and colistin exceeded 512 mg/L. Using the BioFlux, for the PAO1, PAO mutS and the smooth clinical strain, a significant difference (P < 0.0125) was observed when comparing the fluorescence of treated and untreated biofilms. For the mucoid strain, only the biofilm treated with aztreonam (MBIC and MBEC) and tobramycin (MBEC) showed differences with respect to the untreated biofilm. CONCLUSIONS: Murepavadin demonstrated good activity against P. aeruginosa biofilms both in open and closed systems. The MBIC-ECOFF and the MBEC-ECOFF are proposed as new parameters to estimate the activity of antibiotics on biofilms.

Emerging peptide antibiotics with therapeutic potential.

This review covers some of the recent progress in the field of peptide antibiotics with a focus on compounds with novel or established mode of action and with demonstrated efficacy in animal infection models. Novel drug discovery approaches, linear and macrocyclic peptide antibiotics, lipopeptides like the polymyxins as well as peptides addressing targets located in the plasma membrane or in the outer membrane of bacterial cells are discussed.

Murepavadin antimicrobial activity against and resistance development in cystic fibrosis Pseudomonas aeruginosa isolates.

BACKGROUND: Murepavadin, a novel peptidomimetic antibiotic, is being developed as an inhalation therapy for treatment of Pseudomonas aeruginosa respiratory infection in people with cystic fibrosis (CF). It blocks the activity of the LptD protein in P. aeruginosa causing outer membrane alterations. OBJECTIVES: To determine the in vitro activity of murepavadin against CF P. aeruginosa isolates and to investigate potential mechanisms of resistance. METHODS: MIC values were determined by both broth microdilution and agar dilution and results compared. The effect of artificial sputum and lung surfactant on in vitro activity was also measured. Spontaneous mutation frequency was estimated. Bactericidal activity was investigated using time-kill assays. Resistant mutants were studied by WGS. RESULTS: The murepavadin MIC50 was 0.125 versus 4 mg/L and the MIC90 was 2 versus 32 mg/L by broth microdilution and agar dilution, respectively. Essential agreement was >90% when determining in vitro activity with artificial sputum or lung surfactant. It was bactericidal at a concentration of 32 mg/L against 95.4% of the strains within 1-5 h. Murepavadin MICs were 2-9 two-fold dilutions higher for the mutant derivatives (0.5 to >16 mg/L) than for the parental strains. Second-step mutants were obtained for the PAO mutS reference strain with an 8×MIC increase. WGS showed mutations in genes involved in LPS biosynthesis (lpxL1, lpxL2, bamA2, lptD, lpxT and msbA). CONCLUSIONS: Murepavadin characteristics, such as its specific activity against P. aeruginosa, its unique mechanism of action and its strong antimicrobial activity, encourage the further clinical evaluation of this drug.

Identification of Genes Required for Resistance to Peptidomimetic Antibiotics by Transposon Sequencing.

Pseudomonas aeruginosa is an opportunistic human pathogen and a leading cause of nosocomial infections. Due to its high intrinsic and adaptive resistance to antibiotics, infections caused by this organism are difficult to treat and new therapeutic options are urgently needed. Novel peptidomimetic antibiotics that target outer membrane (OM) proteins have shown great promise for the treatment of P. aeruginosa infections. Here, we have performed genome-wide mutant fitness profiling using transposon sequencing (Tn-Seq) to identify resistance determinants against the recently described peptidomimetics L27-11, compounds 3 and 4, as well as polymyxin B2 (PMB) and colistin (COL). We identified a set of 13 core genes that affected resistance to all tested antibiotics, many of which encode enzymes involved in the modification of the lipopolysaccharide (LPS) or control their expression. We also identified fitness determinants that are specific for antibiotics with similar structures that may indicate differences in their modes of action. These results provide new insights into resistance mechanisms against these peptide antibiotics, which will be important for future clinical development and efforts to further improve their potency.

Author Correction: Chimeric peptidomimetic antibiotics against Gram-negative bacteria.

An Amendment to this paper has been published and can be accessed via a link at the top of the paper.

Chimeric peptidomimetic antibiotics against Gram-negative bacteria.

There is an urgent need for new antibiotics against Gram-negative pathogens that are resistant to carbapenem and third-generation cephalosporins, against which antibiotics of last resort have lost most of their efficacy. Here we describe a class of synthetic antibiotics inspired by scaffolds derived from natural products. These chimeric antibiotics contain a ß-hairpin peptide macrocycle linked to the macrocycle found in the polymyxin and colistin family of natural products. They are bactericidal and have a mechanism of action that involves binding to both lipopolysaccharide and the main component (BamA) of the ß-barrel folding complex (BAM) that is required for the folding and insertion of ß-barrel proteins into the outer membrane of Gram-negative bacteria. Extensively optimized derivatives show potent activity against multidrug-resistant pathogens, including all of the Gram-negative members of the ESKAPE pathogens1. These derivatives also show favourable drug properties and overcome colistin resistance, both in vitro and in vivo. The lead candidate is currently in preclinical toxicology studies that-if successful-will allow progress into clinical studies that have the potential to address life-threatening infections by the Gram-negative pathogens, and thus to resolve a considerable unmet medical need.

Frontier Between Cyclic Peptides and Macrocycles.

This review describes a selection of macrocyclic natural products and structurally modified analogs containing peptidic and non-peptidic elements as structural features that potentially modulate cellular permeability. Examples range from exclusively peptidic structures like cyclosporin A or phepropeptins to compounds with mostly non-peptidic character, such as telomestatin or largazole. Furthermore, semisynthetic approaches and synthesis platforms to generate general and focused libraries of compounds at the interface of cyclic peptides and non-peptidic macrocycles are discussed.

Advances in macrocyclic peptide-based antibiotics.

Macrocyclic peptide-based natural products have provided powerful new antibiotic drugs, drug candidates, and scaffolds for medicinal chemists as a source of inspiration to design novel antibiotics. While most of those natural products are active mainly against Gram-positive pathogens, novel macrocyclic peptide-based compounds have recently been described, which exhibit potent and specific activity against some of the most problematic Gram-negative ESKAPE pathogens. This mini-review gives an up-date on recent developments.

Protein epitope mimetic macrocycles as biopharmaceuticals.

Fully synthetic medium-sized macrocyclic peptides mimicking the key ß-hairpin and α-helical protein epitopes relevant in many protein-protein interactions have emerged as a novel class of drugs with the potential to fill an important gap between small molecules and proteins. Conformationally stabilized macrocyclic scaffolds represent ideal templates for medicinal chemists to incorporate bioactive peptide and protein pharmacophores in order to generate novel drugs to treat diseases with high unmet medical need. This review describes recent approaches to design and generate large libraries of such macrocycles, for hit identification, and for their efficient optimization. Finally, this review describes some of the most advanced protein epitope mimetic (PEM) macrocycles in clinical development.

Efficacy of the Novel Antibiotic POL7001 in Preclinical Models of Pseudomonas aeruginosa Pneumonia.

The clinical development of antibiotics with a new mode of action combined with efficient pulmonary drug delivery is a priority against untreatable Pseudomonas aeruginosa lung infections. POL7001 is a macrocycle antibiotic belonging to the novel class of protein epitope mimetic (PEM) molecules with selective and potent activity against P. aeruginosa We investigated ventilator-associated pneumonia (VAP) and cystic fibrosis (CF) as indications of the clinical potential of POL7001 to treat P. aeruginosa pulmonary infections. MICs of POL7001 and comparators were measured for reference and clinical P. aeruginosa strains. The therapeutic efficacy of POL7001 given by pulmonary administration was evaluated in murine models of P. aeruginosa acute and chronic pneumonia. POL7001 showed potent in vitro activity against a large panel of P. aeruginosa isolates from CF patients, including multidrug-resistant (MDR) isolates with adaptive phenotypes such as mucoid or hypermutable phenotypes. The efficacy of POL7001 was demonstrated in both wild-type and CF mice. In addition to a reduced bacterial burden in the lung, POL7001-treated mice showed progressive body weight recovery and reduced levels of inflammatory markers, indicating an improvement in general condition. Pharmacokinetic studies indicated that POL7001 reached significant concentrations in the lung after pulmonary administration, with low systemic exposure. These results support the further evaluation of POL7001 as a novel therapeutic agent for the treatment of P. aeruginosa pulmonary infections.

Structural studies of ß-hairpin peptidomimetic antibiotics that target LptD in Pseudomonas sp.

We report structural studies in aqueous solution on backbone cyclic peptides that possess potent antimicrobial activity specifically against Pseudomonas sp. The peptides target the ß-barrel outer membrane protein LptD, which plays an essential role in lipopolysaccharide transport to the outer membrane. The peptide L27-11 contains a 12-residue loop (T(1)W(2)L(3)K(4)K(5)R(6)R(7)W(8)K(9)K(10)A(11)K(12)) linked to a DPro-LPro template. Two related peptides were also studied, one with various Lys to ornithine or diaminobutyric acid substitutions as well as a DLys(6) (called LB-01), and another containing the same loop sequence, but linked to an LPro-DPro template (called LB-02). NMR studies and MD simulations show that L27-11 and LB-01 adopt ß-hairpin structures in solution. In contrast, LB-02 is more flexible and importantly, adopts a wide variety of different backbone conformations, but not ß-hairpin conformations. L27-11 and LB-01 show antimicrobial activity in the nanomolar range against Pseudomonas aeruginosa, whereas LB-02 is essentially inactive. Thus the ß-hairpin structure of the peptide is important for antimicrobial activity. An alanine scan of L27-11 revealed that tryptophan side chains (W(2)/W(8)) displayed on opposite faces of the ß-hairpin represent key groups contributing to antimicrobial activity.

Inhibition of lipopolysaccharide transport to the outer membrane in Pseudomonas aeruginosa by peptidomimetic antibiotics.

The asymmetric outer membrane (OM) of Gram-negative bacteria contains lipopolysaccharide (LPS) in the outer leaflet and phospholipid in the inner leaflet. During OM biogenesis, LPS is transported from the periplasm into the outer leaflet by a complex comprising the OM proteins LptD and LptE. Recently, a new family of macrocyclic peptidomimetic antibiotics that interact with LptD of the opportunistic human pathogen Pseudomonas aeruginosa was discovered. Here we provide evidence that the peptidomimetics inhibit the LPS transport function of LptD. One approach to monitor LPS transport involved studies of lipid A modifications. Some modifications occur only in the inner membrane while others occur only in the OM, and thus provide markers for LPS transport within the bacterial envelope. We prepared a conditional lptD mutant of P. aeruginosa PAO1 that allowed control of lptD expression from the rhamnose promoter. With this mutant, the effects caused by the antibiotic on the wild-type strain were compared with those caused by depleting LptD in the mutant strain. When LptD was depleted in the mutant, electron microscopy revealed accumulation of membrane-like material within cells and OM blebbing; this mirrored similar effects in the wild-type strain caused by the antibiotic. Moreover, the bacterium responded to the antibiotic, and to depletion of LptD, by introducing the same lipid A modifications, consistent with inhibition by the antibiotic of LptD-mediated LPS transport. This conclusion was further supported by monitoring the radiolabelling of LPS from [¹4C]acetate, and by fractionation of IM and OM components. Overall, the results provide support for a mechanism of action for the peptidomimetic antibiotics that involves inhibition of LPS transport to the cell surface.

Peptidomimetic antibiotics target outer-membrane biogenesis in Pseudomonas aeruginosa.

Antibiotics with new mechanisms of action are urgently required to combat the growing health threat posed by resistant pathogenic microorganisms. We synthesized a family of peptidomimetic antibiotics based on the antimicrobial peptide protegrin I. Several rounds of optimization gave a lead compound that was active in the nanomolar range against Gram-negative Pseudomonas spp., but was largely inactive against other Gram-negative and Gram-positive bacteria. Biochemical and genetic studies showed that the peptidomimetics had a non-membrane-lytic mechanism of action and identified a homolog of the beta-barrel protein LptD (Imp/OstA), which functions in outer-membrane biogenesis, as a cellular target. The peptidomimetic showed potent antimicrobial activity in a mouse septicemia infection model. Drug-resistant strains of Pseudomonas are a serious health problem, so this family of antibiotics may have important therapeutic applications.

The design, structures and therapeutic potential of protein epitope mimetics.

Using a biologically relevant peptide or protein structure as a starting point for lead identification represents one of the most powerful approaches in modern drug discovery. Here, we focus on the protein epitope mimetic (PEM) approach, where folded 3D structures of peptides and proteins are taken as starting points for the design of synthetic molecules that mimic key epitopes involved in protein-protein and protein-nucleic acid interactions. By transferring the epitope from a recombinant to a synthetic scaffold that can be produced by parallel combinatorial methods, it is possible to optimize target affinity and specificity as well as other drug-like ADMET properties. The PEM technology is a powerful tool for target validation, and for the development of novel PEM-based drugs.

Molecular characterization of the NCoA-1-STAT 6 interaction.

Many protein-protein interactions involved in cell signalling, cell adhesion and regulation of transcription are mediated by short alpha-helical recognition motifs with the sequence Leu-Xaa-Xaa-Leu-Leu (LXXLL, where Xaa is any amino acid). Originally observed in cofactors that interact with hormone-activated nuclear receptors, LXXLL motifs are now known to occur in many transcription factors, including the STAT family, which transmit signals from activated cytokine receptors at the cell surface to target genes in the nucleus. STAT 6 becomes activated in response to IL-4 and IL-13, which regulate immune and anti-inflammatory responses. Structural studies have revealed how an LXXLL motif located in 2.5 turns of an alpha-helical peptide derived from STAT 6 provide contacts through the leucine side chains to the coactivator of transcription, NCoA-1. However, since many protein-protein interactions are mediated by LXXLL motifs, it is important to understand how specificity is achieved in this and other signalling pathways. Here, we show that energetically important contacts between STAT 6 and NCoA-1 are made in residues that flank the LXXLL motif, including the underlined residues in the sequence LLPPTEQDLTKLL. We also demonstrate how the affinity for NCoA-1 of peptides derived from this region of STAT 6 can be significantly improved by optimising knobs-into-holes contacts on the surface of the protein. The results provide important new insights into the origins of binding specificity, and might be of practical value in the design of novel small-molecule inhibitors of this important protein-protein interaction.

Biaryl amino acid templates in place of D-Pro-L-Pro in cyclic beta-hairpin cationic antimicrobial peptidomimetics.

The turn-forming D-Pro-L-Pro template has been frequently used to promote regular beta-hairpin conformations in cyclic protein epitope mimetics. Here the use of three isomeric biaryl templates has been studied as alternatives to D-Pro-L-Pro in the preparation of beta-hairpin peptidomimetics. The o,o'- o,m'- and m,m'-isomers of carboxymethyl- and aminomethyl-substituted biaryl templates have been incorporated into novel macrocyclic mimics of the naturally occurring cationic antimicrobial peptide protegrin I. The presence of the o-carboxymethyl-o'-aminomethyl-biaryl template within the macrocyclic peptide resulted in the appearance of slowly interconverting atropisomers. Although none of the resulting mimetics adopted stable beta-hairpin structures in aqueous solution, they all nevertheless retained a significant antimicrobial activity against Gram positive and Gram negative bacteria. These mimetics provide interesting starting points for an optimization program in the search for potent and novel antimicrobial compounds.