Synthetic mimics of antimicrobial peptides from triaryl scaffolds

Design, synthesis, biological evaluation and molecular docking studies of novel pleuromutilin derivatives containing nitrogen heterocycle and alkylamine groups

A series of pleuromutilin derivatives containing alkylamine and nitrogen heterocycle groups were designed and synthesised under mild conditions. The in vitro antibacterial activity of these semisynthetic derivatives against four strains of Staphylococcus aureus (MRSA ATCC 43300, S.aureus ATCC 29213, S.aureus AD3, and S.aureus 144) were evaluated by the broth dilution method. Compound 13 was found to have excellent antibacterial activity against MRSA (MIC = 0.0625 μg/mL). Furthermore, compound 13 was further studied by the time-killing kinetics and the post-antibiotic effect approach. In the mouse thigh infection model, compound 13 exhibited superior antibacterial efficacy than that of tiamulin. Meanwhile, compound 13 showed a lower inhibitory effect than that of tiamulin on RAW264.7 and 16HBE cells at the concentration of 10 μg/mL. Molecular docking study revealed that compound 13 can effectively bind to the active site of the 50S ribosome (the binding free energy = −9.66 kcal/mol).

Targeting Multidrug Resistance With Antimicrobial Peptide-Decorated Nanoparticles and Polymers

As a category of small peptides frequently found in nature, antimicrobial peptides (AMPs) constitute a major part of the innate immune system of various organisms. Antimicrobial peptides feature various inhibitory effects against fungi, bacteria, viruses, and parasites. Due to the increasing concerns of antibiotic resistance among microorganisms, development of antimicrobial peptides is an emerging tool as a favorable applicability prospect in food, medicine, aquaculture, animal husbandry, and agriculture. This review presents the latest research progress made in the field of antimicrobial peptides, such as their mechanism of action, classification, application status, design techniques, and a review on decoration of nanoparticles and polymers with AMPs that are used in treating multidrug resistance. Lastly, we will highlight recent progress in antiviral peptides to treat emerging viral diseases (e.g., anti-coronavirus peptides) and discuss the outlook of AMP applications.

Detection of bacteria using antimicrobial polymer derived via ring-opening metathesis (romp) pathway

There is growing interest in the detection of bacteria in consumables, for example, in the food and water sectors. In this study, the aim was to produce a polymer-based bacteria biosensor via ROMP (ring opening metathesis polymerization). In the first part of the study, block and random copolymers were synthesized, and their biocidal activities were tested on the glass surface. Interdigitated electrode arrays coated with the polymers possessing the highest activity were used to screen the affinity towards different bacterial strains by monitoring impedance variations in real-time. The polymer-coated electrode could detect gram-positive and gram-negative bacteria strains at a concentration of 107 cfu/mL. The results show that ROMP-based polymer offers bacterial detection and can be used in developing biosensor devices for efficiently detecting pathogenic bacteria.

Effect of Non-natural Hydrophobic Amino Acids on the Efficacy and Properties of the Antimicrobial Peptide C18G

Antimicrobial peptides (AMPs) have been an area of great interest, due to the high selectivity of these molecules toward bacterial targets over host cells and the limited development of bacterial resistance to these molecules through evolution. The peptides are known to selectively bind to bacterial cell surfaces through electrostatic interactions, and subsequently, the peptides insert into the cell membrane and cause local disruptions of membrane integrity leading to cell death. Previous experiments showed that replacing the Leu residues in the AMP C18G with other naturally occurring hydrophobic residues resulted in side-chain-dependent activities. This work extends the investigation to non-natural hydrophobic amino acids and the effect on peptide activity. Minimal inhibitory concentration (MIC) results demonstrated that amino acid substitutions containing long flexible carbon chains maintained or increased antimicrobial activity compared to natural analogues. In solution, the peptide showed aggregation only with the most hydrophobic non-natural amino acid substitutions. Binding assays using Trp fluorescence confirm a binding preference for anionic lipids while quenching experiments demonstrated that the more hydrophobic peptides are more deeply buried in the anionic lipid bilayers compared to the zwitterionic bilayers. The most effective peptides at killing bacteria were also those which showed some level of disruption of bacterial membranes; however, one peptide sequence exhibited very strong activity and very low levels of red blood cell hemolysis, yielding a promising target for future development.

Hydrophobicity-Modulated Small Antibacterial Molecule Eradicates Biofilm with Potent Efficacy against Skin Infections

The role of molecular arrangement of hydrophobic and hydrophilic groups for designing membrane-active molecules remains largely ambiguous. To explore this aspect, herein we report a series of membrane-active small molecules by varying the spatial distribution of hydrophobic groups. The two terminal amino groups of linear triamines such as diethylene triamine, bis(trimethylene)triamine, and bis(hexamethylene)triamine were conjugated with cationic amino acids bearing variable side chain hydrophobicity (such as diaminobutyric acid, ornithine, and lysine). The hydrophobicity was also modulated through conjugation of different long chain fatty acids with the central secondary amino group of the triamine. Molecules with constant backbone hydrophobicity displayed an enhanced antibacterial activity and decreased hemolytic activity upon increasing the side chain hydrophobicity of amino acids. On the other hand, increased hydrophobicity in the backbone introduced a slight hemolytic activity but a higher increment in antibacterial activity, resulting in better selective antibacterial compounds. The optimized lead compound derived from structure-activity-relationship (SAR) studies was the dodecanoyl analogue of a lysine series of compounds consisting of bis(hexamethylene)triamine as the backbone. This compound was active against various Gram-positive and Gram-negative bacteria at a low concentration (MIC ranged between 3.1 and 6.3 μg/mL) and displayed low toxicity toward mammalian cells (HC₅₀ = 890 μg/mL and EC₅₀ against HEK = 85 μg/mL). Additionally, it was able to kill metabolically inactive bacterial cells and eradicate preformed biofilms of MRSA. This compound showed excellent activity in a mouse model of skin infection with reduction of ∼4 log MRSA burden at 40 mg/kg dose without any sign of skin toxicity even at 200 mg/kg. More importantly, it revealed potent efficacy in an ex vivo model of human skin infection (with reduction of 85% MRSA burden at 50 μg/mL), which indicates great potential of the compound as an antibacterial agent to treat skin infections.

Guanidine functionalized anthranilamides as effective antibacterials with biofilm disruption activity

We describe a library of amphiphilic anthranilamide compounds as antimicrobial peptide (AMP) mimics. These contain a hydrophobic naphthoyl side chain and different hydrophilic cationic groups such as amino, quaternary ammonium and guanidino groups. These are prepared via the ring-opening of different isatoic anhydrides. The antibacterial activity against S. aureus and E. coli of compounds containing guanidino cationic groups was greater than that for amino and quaternary ammonium cationic groups. The fluoro-substituted guanidinium compound 9b showed a minimum inhibitory concentration (MIC) of 2.0 μM against S. aureus, and reduced established biofilms of S. aureus by 92% at 64 μM concentration. The bromo-substituted guanidinium compound 9d exhibited good MIC against S. aureus (3.9 μM) and E. coli (15.6 μM) and disrupted established biofilms of S. aureus by 83% at 62.4 μM concentration. Cytoplasmic membrane permeability studies suggested that depolarization and disruption of the bacterial cell membrane could be a possible mechanism for antibacterial activity and the in vitro toxicity studies against MRC-5 human lung fibroblast cells showed that the potent compounds are non-toxic against mammalian cells.

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 Bioactivity of Polymer-Based Synthetic Mimics of Antimicrobial Peptides (SMAMPs) Made from Asymmetrically Disubstituted Itaconates

A series of asymmetrically disubstituted diitaconate monomers is presented. Starting from itaconic anhydride, functional groups could be placed selectively at the two nonequivalent carbonyl groups. By using 2D NMR spectroscopy, it was shown that the first functionalization step occurred at the carbonyl group in the β position to the double bond. These monomers were copolymerized with N,N-dimethylacrylamide (DMAA) to yield polymer-based synthetic mimics of antimicrobial peptides (SMAMPs). They were obtained by free radical polymerization, a metal-free process, and still maintained facial amphiphilicity at the repeat unit level. This eliminates the need for laborious metal removal and is advantageous from a regulatory and product safety perspective. The poly(diitaconate-co-DMAA) copolymers obtained were statistical to alternating, and the monomer feed ratio roughly matched that of the repeat unit content of the copolymers. Investigations of varied R group hydrophobicity, repeat unit ratio, and molecular mass on antimicrobial activity against Escherichia coli and on compatibility with human keratinocytes showed that the polymers with the longest R groups and lowest DMAA content were the most antimicrobial and hemolytic. This is in agreement with the biological activity of previously reported SMAMPs. Thus, the design concept of facial amphiphilicity has successfully been transferred, but the selectivity of these polymers for bacteria over mammalian cells still needs to be optimized.

Rational Design of Dimeric Lysine N-Alkylamides as Potent and Broad-Spectrum Antibacterial Agents

Antibiotic resistance is one of the biggest threats to public health, and new antibacterial agents hence are in an urgent need to combat infectious diseases caused by multidrug-resistant (MDR) pathogens. Utilizing dimerization strategy, we rationally designed and efficiently synthesized a new series of small molecule dimeric lysine alkylamides as mimics of AMPs. Evaluation of these mimics against a panel of Gram-positive and Gram-negative bacteria including MDR strains was performed, and a broad-spectrum and potent compound 3d was identified. This compound displayed high specificity toward bacteria over mammalian cell. Time-kill kinetics and mechanistic studies suggest that compound 3d quickly eliminated bacteria in a bactericidal mode by disrupting bacterial cell membrane. In addition, lead compound 3d could inhibit biofilm formation and did not develop drug resistance in S. aureus and E. coli over 14 passages. These results suggested that dimeric lysine nonylamide has immense potential as a new type of novel small molecular agent to combat antibiotic resistance.

Advances in Development of Antimicrobial Peptidomimetics as Potential Drugs

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), β³-peptides, α/β³-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.

Broad Spectrum Macromolecular Antimicrobials with Biofilm Disruption Capability and In Vivo Efficacy

In this study, antimicrobial polymers are synthesized by the organocatalytic ring-opening polymerization of an eight-membered heterocyclic carbonate monomer that is subsequently quaternized with methyl iodide. These polymers demonstrate activity against clinically relevant Gram-positive Staphylococcus epidermidis and Staphylococcus aureus, Gram-negative Escherichia coli and Pseudomonas aeruginosa, and fungus Candida albicans with fast killing kinetics. Importantly, the polymer efficiently inhibits biofilm growth and lyses existing biofilm, leading to a reduction in biomass and cell viability. In addition, the macromolecular antimicrobial is less likely to induce resistance as it acts via a membrane-lytic mechanism. The polymer is not cytotoxic toward mammalian cells with LD₅₀ of 99.0 ± 11.6 mg kg^-1 in mice through i.v. injection. In an S. aureus blood stream infection mouse model, the polymer removes bacteria from the blood more rapidly than the antibiotic Augmentin. At the effective dose, the polymer treatment does not damage liver and kidney tissues or functions. In addition, blood electrolyte balance remains unchanged after the treatment. The low cost of starting materials, ease of synthesis, nontoxicity, broad spectrum activity with fast killing kinetics, and in vivo antimicrobial activity make these macromolecular antimicrobials ideal candidates for prevention of sepsis and treatment of infections.

Sequence-Defined Backbone Modifications Regulate Antibacterial Activity of OligoTEAs

In response to the urgent need for new antibiotic development strategies, antimicrobial peptides (AMPs) and other synthetic polymers are being actively investigated as promising alternatives to traditional antibiotics. Although most AMPs display lytic activity against several types of bacteria, they have poor toxicology profiles and are susceptible to proteolysis in vivo. While many synthetic variants have been created to mimic AMPs by tuning the hydrophobic to cationic ratio of the side-chain groups, few have decoupled the effects of charge from hydrophobicity in discrete systems, and none have investigated the effect of backbone hydrophobicity. We recently developed a rapid and efficient approach for the assembly of synthetic sequence-defined oligothioetheramides (oligoTEAs) that are resistant to protease activity. Our oligoTEA assembly scheme allows direct access to the oligomer backbone, which enables precise tuning of oligoTEA hydrophobicity while keeping charge constant. In this study, we synthesized a new class of antibacterial oligoTEAs (AOTs) with precise control over backbone hydrophobicity and composition. Our studies suggest that AOTs lyse cells via membrane permeabilization and that hydrophobicity and macromolecular conformation are key properties that regulate AOT activity. Some of our AOTs show highly promising antibacterial activity (MIC ∼ 0.5-5 μM) against clinically relevant pathogens in the presence of serum, with little to no toxicity against RBCs and HEK293 cells. Taken together, our data identify design parameters and criteria that may be useful for assembling the next generation of potent and selective AOTs.

Activity of Scorpion Venom-Derived Antifungal Peptides against Planktonic Cells of Candida spp. and Cryptococcus neoformans and Candida albicans Biofilms

The incidence of fungal infections has been increasing in the last decades, while the number of available antifungal classes remains the same. The natural and acquired resistance of some fungal species to available therapies, associated with the high toxicity of these drugs on the present scenario and makes an imperative of the search for new, more efficient and less toxic therapeutic choices. Antimicrobial peptides (AMPs) are a potential class of antimicrobial drugs consisting of evolutionarily conserved multifunctional molecules with both microbicidal and immunomodulatory properties being part of the innate immune response of diverse organisms. In this study, we evaluated 11 scorpion-venom derived non-disulfide-bridged peptides against Cryptococcus neoformans and Candida spp., which are important human pathogens. Seven of them, including two novel molecules, showed activity against both genera with minimum inhibitory concentration values ranging from 3.12 to 200 μM and an analogous activity against Candida albicans biofilms. Most of the peptides presented low hemolytic and cytotoxic activity against mammalian cells. Modifications in the primary peptide sequence, as revealed by in silico and circular dichroism analyses of the most promising peptides, underscored the importance of cationicity for their antimicrobial activity as well as the amphipathicity of these molecules and their tendency to form alpha helices. This is the first report of scorpion-derived AMPs against C. neoformans and our results underline the potential of scorpion venom as a source of antimicrobials. Further characterization of their mechanism of action, followed by molecular optimization to decrease their cytotoxicity and increase antimicrobial activity, is needed to fully clarify their real potential as antifungals.

Optimal Hydrophobicity in Ring-Opening Metathesis Polymerization-Based Protein Mimics Required for siRNA Internalization

Exploring the role of polymer structure for the internalization of biologically relevant cargo, specifically siRNA, is of critical importance to the development of improved delivery reagents. Herein, we report guanidinium-rich protein transduction domain mimics (PTDMs) based on a ring-opening metathesis polymerization scaffold containing tunable hydrophobic moieties that promote siRNA internalization. Structure-activity relationships using Jurkat T cells and HeLa cells were explored to determine how the length of the hydrophobic block and the hydrophobic side chain compositions of these PTDMs impacted siRNA internalization. To explore the hydrophobic block length, two different series of diblock copolymers were synthesized: one series with symmetric block lengths and one with asymmetric block lengths. At similar cationic block lengths, asymmetric and symmetric PTDMs promoted siRNA internalization in the same percentages of the cell population regardless of the hydrophobic block length; however, with 20 repeat units of cationic charge, the asymmetric block length had greater siRNA internalization, highlighting the nontrivial relationships between hydrophobicity and overall cationic charge. To further probe how the hydrophobic side chains impacted siRNA internalization, an additional series of asymmetric PTDMs was synthesized that featured a fixed hydrophobic block length of five repeat units that contained either dimethyl (dMe), methyl phenyl (MePh), or diphenyl (dPh) side chains and varied cationic block lengths. This series was further expanded to incorporate hydrophobic blocks consisting of diethyl (dEt), diisobutyl (diBu), and dicyclohexyl (dCy) based repeat units to better define the hydrophobic window for which our PTDMs had optimal activity. High-performance liquid chromatography retention times quantified the relative hydrophobicities of the noncationic building blocks. PTDMs containing the MePh, diBu, and dPh hydrophobic blocks were shown to have superior siRNA internalization capabilities compared to their more and less hydrophobic counterparts, demonstrating a critical window of relative hydrophobicity for optimal internalization. This better understanding of how hydrophobicity impacts PTDM-induced internalization efficiencies will help guide the development of future delivery reagents.

Design and Application of Antimicrobial Peptide Conjugates

Antimicrobial peptides (AMPs) are an interesting class of antibiotics characterized by their unique antibiotic activity and lower propensity for developing resistance compared to common antibiotics. They belong to the class of membrane-active peptides and usually act selectively against bacteria, fungi and protozoans. AMPs, but also peptide conjugates containing AMPs, have come more and more into the focus of research during the last few years. Within this article, recent work on AMP conjugates is reviewed. Different aspects will be highlighted as a combination of AMPs with antibiotics or organometallic compounds aiming to increase antibacterial activity or target selectivity, conjugation with photosensitizers for improving photodynamic therapy (PDT) or the attachment to particles, to name only a few. Owing to the enormous resonance of antimicrobial conjugates in the literature so far, this research topic seems to be very attractive to different scientific fields, like medicine, biology, biochemistry or chemistry.

Membrane-Active Small Molecules: Designs Inspired by Antimicrobial Peptides

Infectious diseases continue to be one of the major contributors to human morbidity. The rapid rate at which pathogenic microorganisms have developed resistance against frontline antimicrobials has compelled scientists to look for new alternatives. Given their vast antimicrobial repertoire, substantial research effort has been dedicated toward the development of antimicrobial peptides (AMPs) as alternative drugs. However, inherent limitations of AMPs have driven substantial efforts worldwide to develop synthetic mimics of AMPs. This review focuses on the progress that has been made toward the development of small molecules that emulate the properties of AMPs, both in terms of design and biological activity. Herein we provide an extensive discussion of the structural features of various designs and we examine biological properties that have been exploited. Furthermore, we raise a number of questions for which the field has yet to provide solutions and discuss possible future research directions that remain either unexploited or underexploited.

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).

Development of protein mimics for intracellular delivery

Designing delivery agents for therapeutics is an ongoing challenge. As treatments and desired cargoes become more complex, the need for improved delivery vehicles becomes critical. Excellent delivery vehicles must ensure the stability of the cargo, maintain the cargo's solubility, and promote efficient delivery and release. In order to address these issues, many research groups have looked to nature for design inspiration. Proteins, such as HIV-1 trans-activator of transcription (TAT) and Antennapedia homeodomain protein, are capable of crossing cellular membranes. However, due to the complexities of their structures, they are synthetically challenging to reproduce in the laboratory setting. Being able to incorporate the key features of these proteins that enable cell entry into simpler scaffolds opens up a wide range of opportunities for the development of new delivery reagents with improved performance. This review charts the development of protein mimics based on cell-penetrating peptides (CPPs) and how structure-activity relationships (SARs) with these molecules and their protein counterparts ultimately led to the use of polymeric scaffolds. These scaffolds deviate from the normal peptide backbone, allowing for simpler, synthetic procedures to make carriers and tune chemical compositions for application specific needs. Successful design of polymeric protein mimics would allow researchers to further understand the key features in proteins and peptides necessary for efficient delivery and to design the next generation of more efficient delivery reagents.

Antimicrobial benzodiazepine-based short cationic peptidomimetics

Antimicrobial peptides (AMPs) appear to be good candidates for the development of new antibiotic drugs. We describe here the synthesis of peptidomimetic compounds that are based on a benzodiazepine scaffold flanked with positively charged and hydrophobic amino acids. These compounds mimic the essential properties of cationic AMPs. The new design possesses the benzodiazepine scaffold that is comprised of two glycine amino acids and which confers flexibility and aromatic hydrophobic 'back', and two arms used for further synthesis on solid phase for incorporation of charged and hydrophobic amino acids. This approach allowed us a better understanding of the influence of these features on the antimicrobial activity and selectivity. A novel compound was discovered which has MICs of 12.5 µg/ml against Staphylococcus aureus and 25 µg/ml against Escherichia coli, similar to the well-known antimicrobial peptide MSI-78. In contrast to MSI-78, the above mentioned compound has lower lytic effect against mammalian red blood cells. These peptidomimetic compounds will pave the way for future design of potent synthetic mimics of AMPs for therapeutic and biomedical applications.

Design of aromatic-containing cell-penetrating peptide mimics with structurally modified π electronics

Cell-penetrating peptides (CPPs) and their synthetic mimics (CPPMs) represent a class of molecules that facilitate the intracellular delivery of various cargo. Previous studies indicated that the presence of aromatic functionalities improved CPPM activity. Given that aromatic functionalities play prominent roles in membrane biology and participate in various π interactions, we explored whether these interactions could be optimized for improved CPPM activity. CPPMs were synthesized by ring-opening metathesis polymerization by using monomers that contained aromatic rings substituted with electron-donating and electron-withdrawing groups and covered an electrostatic potential range from -29.69 to +15.57 kcal mol(-1) . These groups altered the quadrupole moments of the aromatic systems and were used to test if such structural modifications changed CPPM activity. CPPMs were added to dye-loaded vesicles and the release of carboxyfluorescein was monitored as a function of polymer concentration. Changes in the effective polymer concentration to release 50% of the dye (effective concentration, EC50 ) were monitored. Results from this assay showed that the strength of the electron-donating and electron-withdrawing groups incorporated in the CPPMs did not alter polymer EC50 values or activity. This suggests that other design parameters may have a stronger impact on CPPM activity. In addition, these results indicate that a wide range of aromatic groups can be incorporated without negatively impacting polymer activity.

Synthetic and structural routes for the rational conversion of peptides into small molecules

The demand for modified peptides with improved stability profiles and pharmacokinetic properties is driving extensive research effort in this field. The conversion of peptides into organic molecules, as traditional drugs, is a long and puzzled way. Many and versatile approaches have been described for designing peptide mimetics: the substitution of natural residues with modified amino acids and the rigidification and modification of the backbone are the main structural and chemical routes walked in medicinal chemistry. All of these strategies have been successfully applied to obtain active new compounds in molecular biology, drug discovery and design. Here we propose a panoramic review of the most common methods for the preparation of modified peptides and the most interesting findings of the last decade.

Therapeutic strategies to combat antibiotic resistance

With multidrug resistant bacteria on the rise, new antibiotic approaches are required. Although a number of new small molecule antibiotics are currently in the development pipeline with many more in preclinical development, the clinical options and practices for infection control must be expanded. Biologics and non-antibiotic adjuvants offer this opportunity for expansion. Nevertheless, to avoid known mechanisms of resistance, intelligent combination approaches for multiple simultaneous and complimentary therapies must be designed. Combination approaches should extend beyond biologically active molecules to include smart controlled delivery strategies. Infection control must integrate antimicrobial stewardship, new antibiotic molecules, biologics, and delivery strategies into effective combination therapies designed to 1) fight the infection, 2) avoid resistance, and 3) protect the natural microbiome. This review explores these developing strategies in the context of circumventing current mechanisms of resistance.

Designing mimics of membrane active proteins

As a semipermeable barrier that controls the flux of biomolecules in and out the cell, the plasma membrane is critical in cell function and survival. Many proteins interact with the plasma membrane and modulate its physiology. Within this large landscape of membrane-active molecules, researchers have focused significant attention on two specific classes of peptides, antimicrobial peptides (AMPs) and cell penetrating peptides (CPPs), because of their unique properties. In this Account, we describe our efforts over the last decade to build and understand synthetic mimics of antimicrobial peptides (SMAMPs). These endeavors represent one specific example of a much larger effort to understand how synthetic molecules interact with and manipulate the plasma membrane. Using both defined molecular weight oligomers and easier to produce, but heterogeneous, polymers, we have generated scaffolds with biological potency exceeding that of the natural analogues. One of these compounds has progressed through a phase II clinical trial for pan-staph infections. Modern biophysical assays have highlighted the interplay between the synthetic scaffold and lipid composition: a negative Gaussian curvature is required both for pore formation and for the initiation of endosome creation. Although work remains to better resolve the complexity of this interplay between lipids, other bilayer components, and the scaffolds, significant new insights have been discovered. These results point to the importance of considering the various aspects of permeation and how these are related to "pore formation". More recently, our efforts have expanded toward protein transduction domains, or mimics of cell penetrating peptides. Using a combination of unique molecular scaffolds and guanidinium-rich side chains, we have produced an array of polymers with robust membrane (and delivery) activity. In this new area, researchers are just beginning to understand the fundamental interactions between these new scaffolds and the plasma membrane. Negative Gaussian curvature is also important in these systems, but the detailed relationships between molecular structure, self-assembly with lipids, and translocation will require more investigation. It has become clear that the combination of molecular design, biophysical models, and biological evaluation provides a robust approach to the generation and study of novel proteinomimetics.

Polymeric systems of antimicrobial peptides--strategies and potential applications

The past decade has seen growing interest in the investigation of peptides with antimicrobial activity (AMPs). One approach utilized in infection control is incorporation of antimicrobial agents conjugated with the polymers. This review presents the recent developments on polymeric AMP carriers and their potential applications in the biomedical and pharmaceutical fields.

Combating multidrug-resistant Gram-negative bacterial infections

INTRODUCTION

Multidrug-resistant (MDR) bacterial infections, especially those caused by Gram-negative pathogens, have emerged as one of the world's greatest health threats. The development of novel antibiotics to treat MDR Gram-negative bacteria has, however, stagnated over the last half century.

AREAS COVERED

This review provides an overview of recent R&D activities in the search for novel antibiotics against MDR Gram-negatives. It provides emphasis in three key areas. First, the article looks at new analogs of existing antibiotic molecules such as β-lactams, tetracyclines, and aminoglycoside as well as agents against novel bacterial targets such as aminoacyl-tRNA synthetase and peptide deformylase. Second, it also examines alternative strategies to conventional approaches including cationic antimicrobial peptides, siderophores, efflux pump inhibitors, therapeutic antibodies, and renewed interest in abandoned treatments or those with limited indications. Third, the authors aim to provide an update on the current clinical development status for each drug candidate.

EXPERT OPINION

The traditional analog approach is insufficient to meet the formidable challenge brought forth by MDR superbugs. With the disappointing results of the genomics approach for delivering novel targets and drug candidates, alternative strategies to permeate the bacterial cell membrane, enhance influx, disrupt efflux, and target specific pathogens via therapeutic antibodies are attractive and promising. Coupled with incentivized business models, governmental policies, and a clarified regulatory pathway, it is hoped that the antibiotic pipeline will be filled with an effective armamentarium to safeguard global health.

Arsonium-containing lipophosphoramides, poly-functional nano-carriers for simultaneous antibacterial action and eukaryotic cell transfection

Gene therapy of diseases like cystic fibrosis (CF) would consist of delivering a gene medicine towards the lungs via the respiratory tract into the target epithelial cells. Accordingly, poly-functional nano-carriers are required in order to overcome the various successive barriers of such a complex environment, such as airway colonization with bacterial strains. In this work, the antibacterial effectiveness of a series of cationic lipids is investigated before evaluating its compatibility with gene transfer into human bronchial epithelial cells. Among the various compounds considered, some bearing a trimethyl-arsonium headgroup demonstrate very potent biocide effects towards clinically relevant bacterial strains. In contrast to cationic lipids exhibiting no or insufficient antibacterial potency, arsonium-containing lipophosphoramides can simultaneously inhibit bacteria while delivering DNA into eukaryotic cells, as efficiently and safely as in absence of bacteria. Moreover, such vectors can demonstrate antibacterial activity in vitro while retaining high gene transfection efficiency to the nasal epithelium as well as to the lungs in mice in vivo. Arsonium-containing amphiphiles are the first synthetic compounds shown to achieve efficient gene delivery in the presence of bacteria, a property particularly suitable for gene therapy strategies under infected conditions such as within the airways of CF patients.

Expedient Synthesis of SMAMPs via Click Chemistry

A novel series of synthetic mimics of antimicrobial peptides (SMAMPs) containing triazole linkers were assembled using click chemistry. While only moderately active in buffer alone, an increase in antimicrobial activity against Staphylococcus aureus and Escherichia coli was observed when these SMAMPs were administered in the presence of mouse serum. One compound had minimum inhibitory concentrations (MICs) of 0.39 μg/mL and 6.25 μg/mL, respectively, and an HC50 of 693 μg/mL. These values compared favorably to peptide-based antimicrobials. A correlation between the net positive charge and SMAMP antimicrobial activity was observed. The triazole linker, an amide surrogate, was found to provide better antimicrobial activity against both S. aureus and E. coli when compared to other analogues.

Synthetic mimics of antimicrobial peptides with immunomodulatory responses

A new series of aryl-based synthetic mimics of antimicrobial peptides (SMAMPs) with antimicrobial activity and selectivity have been developed via systematic tuning of the aromatic groups and charge. The addition of a pendant aromatic group improved the antimicrobial activity against Gram-negative bacteria, while the addition of charge improved the selectivity. SMAMP 4 with six charges and a naphthalene central ring demonstrated a selectivity of 200 against both Staphylococcus aureus and Escherichia coli , compared with a selectivity of 8 for the peptide MSI-78. In addition to the direct antimicrobial activity, SMAMP 4 exhibited specific immunomodulatory activities in macrophages both in the presence and in the absence of lipopolysaccharide, a TLR agonist. SMAMP 4 also induced the production of a neutrophil chemoattractant, murine KC, in mouse primary cells. This is the first nonpeptidic SMAMP demonstrating both good antimicrobial and immunomodulatory activities.

Will new generations of modified antimicrobial peptides improve their potential as pharmaceuticals?

The concept of antimicrobial peptides (AMPs) as potent pharmaceuticals is firmly established in the literature, and most research articles on this topic conclude by stating that AMPs represent promising therapeutic agents against bacterial and fungal pathogens. Indeed, early research in this field showed that AMPs were diverse in nature, had high activities with low minimal inhibitory concentrations, had broad spectrums of activity against bacterial, fungal and viral pathogens, and could easily be manipulated to alter their specificities, reduce their cytotoxicities and increase their antimicrobial activities. Unfortunately, commercial development of these peptides, for even the simplest of applications, has been very limited. With some peptides there are obstacles with their manufacture, in vivo efficacy and in vivo retention. More recently, the focus has shifted. Contemporary research now uses a more sophisticated approach to develop AMPs that surmount many of these prior obstacles. AMP mimetics, hybrid AMPs, AMP congeners, cyclotides and stabilised AMPs, AMP conjugates and immobilised AMPs have all emerged with selective or 'targeted' antimicrobial activities, improved retention, or unique abilities that allow them to bind to medical or industrial surfaces. These groups of new peptides have creative medical and industrial application potentials to treat antibiotic-resistant bacterial infections and septic shock, to preserve food or to sanitise surfaces both in vitro and in vivo.