Establishing the selective phospholipid membrane coordination, permeation and lysis properties for a series of 'druggable' supramolecular self-associating antimicrobial amphiphiles

The rise of antimicrobial resistance remains one of the greatest global health threats facing humanity. Furthermore, the development of novel antibiotics has all but ground to a halt due to a collision of intersectional pressures. Herein we determine the antimicrobial efficacy for 14 structurally related supramolecular self-associating amphiphiles against clinically relevant Gram-positive methicillin resistant Staphylococcus aureus and Gram-negative Escherichia coli. We establish the ability of these agents to selectively target phospholipid membranes of differing compositions, through a combination of computational host:guest complex formation simulations, synthetic vesicle lysis, adhesion and membrane fluidity experiments, alongside our novel ¹H NMR CPMG nanodisc coordination assays, to verify a potential mode of action for this class of compounds and enable the production of evermore effective next-generation antimicrobial agents. Finally, we select a 7-compound subset, showing two lead compounds to exhibit 'druggable' profiles through completion of a variety of in vivo and in vitro DMPK studies.

Antimicrobial Peptide Mimics for Clinical Use: Does Size Matter?

The search for efficient antimicrobial therapies that can alleviate suffering caused by infections from resistant bacteria is more urgent than ever before. Infections caused by multi-resistant pathogens represent a significant and increasing burden to healthcare and society and researcher are investigating new classes of bioactive compounds to slow down this development. Antimicrobial peptides from the innate immune system represent one promising class that offers a potential solution to the antibiotic resistance problem due to their mode of action on the microbial membranes. However, challenges associated with pharmacokinetics, bioavailability and off-target toxicity are slowing down the advancement and use of innate defensive peptides. Improving the therapeutic properties of these peptides is a strategy for reducing the clinical limitations and synthetic mimics of antimicrobial peptides are emerging as a promising class of molecules for a variety of antimicrobial applications. These compounds can be made significantly shorter while maintaining, or even improving antimicrobial properties, and several downsized synthetic mimics are now in clinical development for a range of infectious diseases. A variety of strategies can be employed to prepare these small compounds and this review describes the different compounds developed to date by adhering to a minimum pharmacophore based on an amphiphilic balance between cationic charge and hydrophobicity. These compounds can be made as small as dipeptides, circumventing the need for large compounds with elaborate three-dimensional structures to generate simplified and potent antimicrobial mimics for a range of medical applications. This review highlight key and recent development in the field of small antimicrobial peptide mimics as a promising class of antimicrobials, illustrating just how small you can go.

Short Cationic Peptidomimetic Antimicrobials

The rapid growth of antimicrobial resistance against several frontline antibiotics has encouraged scientists worldwide to develop new alternatives with unique mechanisms of action. Antimicrobial peptides (AMPs) have attracted considerable interest due to their rapid killing and broad-spectrum activity. Peptidomimetics overcome some of the obstacles of AMPs such as high cost of synthesis, short half-life in vivo due to their susceptibility to proteolytic degradation, and issues with toxicity. This review will examine the development of short cationic peptidomimetics as antimicrobials.

Norbornane-based cationic antimicrobial peptidomimetics targeting the bacterial membrane

The design, synthesis and evaluation of a small series of potent amphiphilic norbornane antibacterial agents has been performed (compound 10 MIC = 0.25 μg/mL against MRSA). Molecular modelling indicates rapid aggregation of this class of antibacterial agent prior to membrane association and insertion. Two fluorescent analogues (compound 29 with 4-amino-naphthalimide and 34 with 4-nitrobenz-2-oxa-1,3-diazole fluorophores) with good activity (MIC = 0.5 μg/mL against MRSA) were also constructed and confocal microscopy studies indicate that the primary site of interaction for this family of compounds is the bacterial membrane.

Synthesis and Properties of Carbaporphyrin and Carbachlorin Dimethyl Esters Derived from Cyclopentanedialdehydes

Norbornenes with two ester substituents were prepared by Diels-Alder cycloadditions of cyclopentadiene with dimethyl fumarate and dimethyl 1,1-ethylenedicarboxylate. Oxidation with potassium permanganate gave good yields of related diols that were oxidatively ring-opened to afford cyclopentane dialdehydes. MacDonald-type "3 + 1" condensations with a tripyrrane, followed by oxidation with DDQ in refluxing toluene, gave carbaporphyrin or carbachlorin products in good yields. The macrocyclic products were highly diatropic and produced porphyrin-like UV-vis spectra. The carbaporphyrin was converted into silver(III) and gold(III) organometallic derivatives. Reaction with methyl iodide in the presence of potassium carbonate gave mono- and dialkylation products, and treatment of the former with Ni(OAc)₂ or Pd(OAc)₂ afforded nickel(II) and palladium(II) complexes. The free base carbaporphyrin and carbachlorin, and the nickel and palladium complexes, were characterized by X-ray crystallography. The carbachlorin also reacted with silver(I) acetate to give a silver(III) derivative. Carbaporphyrins and carbachlorins underwent deuterium exchange at the meso-positions with deuteriated TFA, and this observation indicates that protonation is occurring at the bridging carbons. The new route to carbaporphyrins and carbachlorins has enabled detailed studies on the properties of these systems and provides the foundations for future investigations.

Antimicrobial Activity from Colistin-Heparin Lamellar-Phase Complexes for the Coating of Biomedical Devices

Infections arising in hospitalized patients, particularly those who have undergone surgery and are reliant on receiving treatment through biomedical devices, continue to be a rising concern. It is well-known that aqueous mixtures of oppositely charged surfactant and polymer molecules can self-assemble to form liquid crystalline structures, primarily via electrostatically driven interactions that have demonstrated great potential as tailored-release nanomaterials. Colistin is a re-emerging antibiotic used against multidrug-resistant Gram-negative bacteria. Its amphiphilic structure allows it to form micellar aggregates in solution. Thus, the aim of this study was to determine whether structured complexes form between colistin and negatively charged biopolymers, such as the highly sulfated anticoagulant, heparin. Cross-polarized light microscopy and synchrotron small-angle X-ray scattering were employed to visualize the appearance of birefringent structures and identify liquid crystalline structures, respectively, formed across the interface between solutions of colistin and heparin. A lamellar phase with a lattice parameter of ∼40 Å was formed upon contact between the oppositely charged solutions of colistin and heparin. In addition, in vitro release studies showed a slow release of colistin from the lamellar-phase gel complexes into the bulk media, and disk diffusion bioassays revealed antimicrobial activity against Pseudomonas aeruginosa. This system provides a novel, cost-effective, and simple approach to reducing the risk of infections by potentially applying the formulation as a coating for biomedical implants or tubing.

A Molecularly Complete Planar Bacterial Outer Membrane Platform

The bacterial outer membrane (OM) is a barrier containing membrane proteins and liposaccharides that fulfill crucial functions for Gram-negative bacteria. With the advent of drug-resistant bacteria, it is necessary to understand the functional role of this membrane and its constituents to enable novel drug designs. Here we report a simple method to form an OM-like supported bilayer (OM-SB), which incorporates native lipids and membrane proteins of gram-negative bacteria from outer membrane vesicles (OMVs). We characterize the formation of OM-SBs using quartz crystal microbalance with dissipation (QCM-D) and fluorescence microscopy. We show that the orientation of proteins in the OM-SB matches the native bacterial membrane, preserving the characteristic asymmetry of these membranes. As a demonstration of the utility of the OM-SB platform, we quantitatively measure antibiotic interactions between OM-SBs and polymyxin B, a cationic peptide used to treat Gram-negative infections. This data enriches understanding of the antibacterial mechanism of polymyxin B, including disruption kinetics and changes in membrane mechanical properties. Combining OM-SBs with microfluidics will enable higher throughput screening of antibiotics. With a broader view, we envision that a molecularly complete membrane-scaffold could be useful for cell-free applications employing engineered membrane proteins in bacterial membranes for myriad technological purposes.

Synthesis and evaluation of cationic norbornanes as peptidomimetic antibacterial agents

A series of structurally amphiphilic biscationic norbornanes have been synthesised as rigidified, low molecular weight peptidomimetics of cationic antimicrobial peptides. A variety of charged hydrophilic functionalities were attached to the norbornane scaffold including aminium, guanidinium, imidazolium and pyridinium moieties. Additionally, a range of hydrophobic groups of differing sizes were incorporated through an acetal linkage. The compounds were evaluated for antibacterial activity against both Gram-negative and Gram-positive bacteria. Activity was observed across the series; the most potent of which exhibited an MIC's ≤ 1 μg mL(-1) against Streptococcus pneumoniae, Enterococcus faecalis and several strains of Staphylococcus aureus, including multi-resistant methicillin resistant (mMRSA), glycopeptide-intermediate (GISA) and vancomycin-intermediate (VISA) S. aureus.

Synthesis of Norbornane Bisether Antibiotics via Silver-mediated Alkylation

A small series of norbornane bisether diguanidines have been synthesized and evaluated as antibacterial agents. The key transformation-bisalkylation of norbornane diol 6-was not successful using Williamson methodology but has been accomplished using Ag2O mediated alkylation. Further functionalization to incorporate two guanidinium groups gave rise to a series of structurally rigid cationic amphiphiles; several of which (16d, 16g and 16h) exhibited antibiotic activity. For example, compound 16d was active against a broad range of bacteria including Pseudomonas aeruginosa (MIC = 8 µg/mL), Escherichia coli (MIC = 8 µg/mL) and methicillin-resistant Staphylococcus aureus (MIC = 8 µg/mL).

Copper(I) Iodide-Catalyzed Synthesis of N,N′-Disubstituted Guanidines from N-Substituted Cyanamides

A facile and effective synthesis of N-alkyl-N′-arylguanidines was accomplished by the reaction of N-arylcyanamides with various primary and secondary alkylamines, under the catalysis of copper(i) iodide and Xantphos in DMF. This methodology provides a direct access to versatile N,N′-disubstituted guanidine derivatives from N-arylcyanamides that can be readily prepared from the corresponding nitriles via Tiemann rearrangement.

Supramolecular pyridyl urea gels as soft matter with antibacterial properties against MRSA and/or E. coli

The development of a family of twelve aryl pyridyl ureas, their crystallography and the ability of a number of these to form hydrogen bonding supramolecular gels with antimicrobial properties are described.

Exploring and exploiting the synergy of non-covalent interactions on the surface of gold nanoparticles for fluorescent turn-on sensing of bacterial lipopolysaccharide

The sensing of lipopolysaccharide (LPS) relies on the synergy of multiple electrostatic and hydrophobic interactions between LPS and the sensor. However, how non-covalent interactions are coordinated to impel the recognition process still remains elusive, and the exploration of which would promote the development of LPS sensors with higher specificity and sensitivity. In this work, we hypothesize that Au NPs would provide a straightforward and flexible platform for studying the synergy of non-covalent interactions. The detailed mechanism of interactions between the designed fluorescent probes and Au NPs with two distinct surface properties was systematically explored. We demonstrated that only when the electrostatic attraction and hydrophobic stacking are both present, the binding of fluorescent probes onto Au NPs can be not only highly efficient, but also positively cooperative. After that, hybrid systems that consist of Au NPs and surface-assembled fluorescent probes were exploited for fluorescent turn-on sensing of LPS. The results show that the sensitivity and selectivity to LPS relies strongly on the binding affinity between fluorescent probes and Au NPs. Fluorescent probes assembled Au NPs thus provide an attractive platform for further optimization of the sensitivity/selectivity of LPS sensing.

The chemistry and biology of organic guanidine derivatives

The chemistry and biology of organic natural guanidines are reviewed, including the isolation, structure determination, synthesis, biosynthesis and biological activities of alkaloids, non-ribosomal peptides, guanidine-bearing terpenes, polyketides and shikimic acid derivatives from natural sources.

Peptide functionalized polydiacetylene liposomes act as a fluorescent turn-on sensor for bacterial lipopolysaccharide

Mixed polydiacetylene (PDA) liposomes functionalized on their surface with a fluorescent pentalysine peptide derivative and histidine in a ratio of 1:9 can identify bacterial lipopolysaccharide (LPS). Upon photopolymerization of the self-assembled liposomes the initial fluorescence of the peptide-diacetylene amphiphiles is quenched. Interaction with LPS in aqueous solution or on the surface of E. coli DH5α restores the fluorescence. This increase in fluorescence is selective for LPS relative to other negatively charged analytes including nucleotides and ctDNA. This simple turn-on fluorescent sensor allows detecting LPS even at low micromolar concentrations.

Structure-activity studies on novel polymyxin derivatives that carry only three positive charges

Polymyxin B and colistin are pentacationic lipopeptides that possess a cyclic heptapeptide portion, a linear tripeptide portion, and a fatty acyl tail. They are used, in spite of nephrotoxicity, to treat infections caused by extremely multiresistant Gram-negative bacteria. We have recently developed novel derivatives, that carry three cationic charges only. Some of them, including NAB739, are directly antibacterial whereas others, including NAB7061, lack the direct activity but sensitize bacteria to other antibiotics. NAB739 and NAB7061 differ from the old polymyxins in their renal handling and have reduced affinity to kidney brush border membrane. To further study the structure-activity relationships, we here synthesized eight additional derivatives and tested their antibacterial activity. NAB751 carries methylheptanoyl as the fatty acyl instead of octanoyl in NAB739 and was as active as NAB739, whereas NAB750 with dodecanoyl was less active. NAB781 and NAB782 with the linear peptide portion Ser-DSer and Ser-Ser-DSer, respectively, were less active than NAB739 that carries Thr-DSer. NAB771 with Thr at position 8 in the cyclic portion (instead of Dab in NAB7061) and Thr-Dab as the linear peptide portion (instead of Thr-Abu in NAB7061), resembled NAB7061 in its activity. However, replacement of two Dab residues in the cyclic portion with Thr greatly decreased the activity, even though the loss of the cationic charges was compensated by introducing two Dab residues in the linear portion. These findings reveal that subtle structural modifications have a major effect on the antibacterial activity and that it is possible to design numerous tricationic polymyxin derivatives that are antibacterial.