Pro-moieties of antimicrobial peptide prodrugs

A Review on Bacteriocin Extraction Techniques from Lactic Acid Bacteria

Lactic acid bacteria (LAB) are widely known for the production of secondary metabolites such as organic acids and other bioactive compounds such as bacteriocins. Finding a broad application in food and healthcare, bacteriocins have received increased attention due to their inherent antimicrobial properties. However, the extraction of bacteriocins is often plagued with low yields due to the complexity of the extraction processes and the diversity of bacteriocins themselves. Here, we review the current knowledge related to bacteriocin extraction on the different extraction techniques for isolating bacteriocins from LAB. The advantages and disadvantages of each technique will also be critically appraised, taking into account factors such as extraction efficiency, scalability and cost-effectiveness. This review aims to guide researchers and professionals in selecting the most suitable approach for bacteriocin extraction from LAB by illuminating the respective advantages and limitations of various extraction techniques.

Current research status of anti-cancer peptides: Mechanism of action, production, and clinical applications

The escalating rate of cancer cases, together with treatment deficiencies and long-term side effects of currently used cancer drugs, has made this disease a global burden of the 21st century. The number of breast and lung cancer patients has sharply increased worldwide in the last few years. Presently, surgical treatment, radiotherapy, chemotherapy, and immunotherapy strategies are used to cure cancer, which cause severe side effects, toxicities, and drug resistance. In recent years, anti-cancer peptides have become an eminent therapeutic strategy for cancer treatment due to their high specificity and fewer side effects and toxicity. This review presents an updated overview of different anti-cancer peptides, their mechanisms of action and current production strategies employed for their manufacture. In addition, approved and under clinical trials anti-cancer peptides and their applications have been discussed. This review provides updated information on therapeutic anti-cancer peptides that hold great promise for cancer treatment in the near future.

Structure-activity relationships of antibacterial peptides

Antimicrobial peptides play a crucial role in innate immunity, whose components are mainly peptide-based molecules with antibacterial properties. Indeed, the exploration of the immune system over the past 40 years has revealed a number of natural peptides playing a pivotal role in the defence mechanisms of vertebrates and invertebrates, including amphibians, insects, and mammalians. This review provides a discussion regarding the antibacterial mechanisms of peptide-based agents and their structure-activity relationships (SARs) with the aim of describing a topic that is not yet fully explored. Some growing evidence suggests that innate immunity should be strongly considered for the development of novel antibiotic peptide-based libraries. Also, due to the constantly rising concern of antibiotic resistance, the development of new antibiotic drugs is becoming a priority of global importance. Hence, the study and the understanding of defence phenomena occurring in the immune system may inspire the development of novel antibiotic compound libraries and set the stage to overcome drug-resistant pathogens. Here, we provide an overview of the importance of peptide-based antibacterial sources, focusing on accurately selected molecular structures, their SARs including recently introduced modifications, their latest biotechnology applications, and their potential against multi-drug resistant pathogens. Last, we provide cues to describe how antibacterial peptides show a better scope of action selectivity than several anti-infective agents, which are characterized by non-selective activities and non-targeted actions toward pathogens.

Rational Design of RN15m4 Cathelin Domain-Based Peptides from Siamese Crocodile Cathelicidin Improves Antimicrobial Activity

Antimicrobial peptides are becoming a new generation of antibiotics due to their therapeutic potential and ability to decrease drug-resistant bacteria development. Cathelicidins are known as effective peptides of vertebrate immunity that play crucial roles in the defensive strategy against pathogens. To improve its potency, the RN15 antibacterial peptide derived from the cathelin domain of Crocodylus siamensis cathelicidin has been modified and its antimicrobial properties investigated. Peptides were derived by template-based and physicochemical designation. The RN15 derivative peptides were predicted through their structure modeling, antimicrobial potency, and peptide-membrane calculation. The antimicrobial and cytotoxic activities of candidate peptides were investigated. Simultaneous consideration of physicochemical characteristics, secondary structure modeling, and the result of antimicrobial peptide tools prediction indicated that RN15m4 peptide was a candidate derivative antimicrobial peptide. The RN15m4 peptide expresses antimicrobial activity against most Gram-positive and Gram-negative bacteria and fungi with a lower minimum inhibition concentration (MIC) than the parent peptide. Besides, the time-killing assay shows that the designed peptide performed its ability to quickly kill bacteria better than the original peptide. Scanning electron microscopy (SEM) displayed the destruction of the bacterial cell membrane caused by the RN15m4 peptide. In addition, the RN15m4 peptide exhibits low hemolytic activity and low cytotoxic activity as good as the template peptide. The RN15m4 peptide performs a range of antimicrobial activities with low cell toxicity. Our study has illustrated the combination approach to peptide design for potent antibiotic peptide discovery.

Strategies for improving antimicrobial peptide production

Antimicrobial peptides (AMPs) found in a wide range of animal, insect, and plant species are host defense peptides forming an integral part of their innate immunity. Although the exact mode of action of some AMPs is yet to be deciphered, many exhibit membrane lytic activity or interact with intracellular targets. The ever-growing threat of antibiotic resistance has brought attention to research on AMPs to enhance their clinical use as a therapeutic alternative. AMPs have several advantages over antibiotics such as broad range of antimicrobial activities including anti-fungal, anti-viral and anti-bacterial, and have not reported to contribute to resistance development. Despite the numerous studies to develop efficient production methods for AMPs, limitations including low yield, degradation, and loss of activity persists in many recombinant approaches. In this review, we outline available approaches for AMP production and various expression systems used to achieve higher yield and quality. In addition, recent advances in recombinant strategies, suitable fusion protein partners, and other molecular engineering strategies for improved AMP production are surveyed.

PEGylation enhances the antibacterial and therapeutic potential of amphibian host defence peptides

Aurein 2.1, aurein 2.6 and aurein 3.1 are amphibian host defence peptides that kill bacteria via the use of lytic amphiphilic α-helical structures. The C-terminal PEGylation of these peptides led to decreased antibacterial activity (Minimum Lethal Concentration (MLCs) ↓ circa one and a half to threefold), reduced levels of amphiphilic α-helical structure in solvents (α-helicity ↓ circa 15.0%) and lower surface activity (Δπ ↓ > 1.5 mN m^-1). This PEGylation of aureins also led to decreased levels of amphiphilic α-helical structure in the presence of anionic membranes and zwitterionic membranes (α-helicity↓ > 10.0%) as well as reduced levels of penetration (Δπ ↓ > 3.0 mN m^-1) and lysis (lysis ↓ > 10.0%) of these membranes. Based on these data, it was proposed that the antibacterial action of PEGylated aureins involved the adoption of α-helical structures that promote the lysis of bacterial membranes, but with lower efficacy than their native counterparts. However, PEGylation also reduced the haemolytic activity of native aureins to negligible levels (haemolysis ↓ from circa 10% to 3% or less) and improved their relative therapeutic indices (RTIs ↑ circa three to sixfold). Based on these data, it is proposed that PEGylated aureins possess the potential for therapeutic development; for example, to combat infections due to multi-drug resistant strains of S. aureus, designated as high priority by the World Health Organization.

Gut microbiome-immune system interaction in reptiles

Reptiles are ectothermic amniotes in a world dominated by endotherms. Reptiles originated more than 300 million years ago and they often dwell in polluted environments which may expose them to pathogenic micro-organisms, radiation and/or heavy metals. Reptiles also possess greater longevity and may live much longer than similar-sized land mammals, for example, turtles, tortoises, crocodiles and tuatara are long-lived reptiles living up to 100 years or more. Many recent studies have emphasized the pivotal role of the gut microbiome on its host; thus, we postulated that reptilian gut microbiome and/or its metabolites and the interplay with their robust immune system may contribute to their longevity and overall hardiness. Herein, we discuss the composition of the reptilian gut microbiome, immune system-gut microbiome cross-talk, antimicrobial peptides, reptilian resistance to infectious diseases and cancer, ageing, as well the current knowledge of the genome and epigenome of these remarkable species. Preliminary studies have demonstrated that microbial gut flora of reptiles such as crocodiles, tortoises, water monitor lizard and python exhibit remarkable anticancer and antibacterial properties, as well as comprise novel gut bacterial metabolites and antimicrobial peptides. The underlying mechanisms between the gut microbiome and the immune system may hold clues to developing new therapies overall for health, and possible extrapolation to exploit the ancient defence systems of reptiles for Homo sapiens benefit.

PEGylated Oligothioetheramide Prodrugs Activated by Host Serum Proteases

Due to the increasing prominence of antibiotic resistance, novel drug discovery and delivery approaches targeting bacteria are essential. In this work we evaluate a prodrug design to improve the cytotoxic profile of polycationic oligothioetheramides (oligoTEAs), which are promising antimicrobials. Herein we chemically modify the oligoTEA, PDT-4G, with a polyethylene glycol (PEG) and show that 1, 2, and 5 kDa PEGs mitigate cytotoxicity. As PEGylation reduces antibacterial activity, we evaluate two peptide linkers which, unlike oligoTEAs, are susceptible to proteolytic cleavage in serum. To gain insight into the prodrug reactivation, two linkers were tested, the 5-residue peptide sequence LMPTG, and the dipeptide sequence VC-PABC. In the presence of 20 % serum, prodrugs made with the VC-PABC linker successfully inhibited bacterial growth. Overall, we observed reactivation of oligoTEAs facilitated by serum protease cleavage of the peptide linkers. This work opens the door to the future design of antimicrobial prodrugs with tunable release profiles.

Selective Bacterial Targeting and Infection-Triggered Release of Antibiotic Colistin Conjugates

In order to render potent, but toxic antibiotics more selective, we have explored a novel conjugation strategy that includes drug accumulation followed by infection-triggered release of the drug. Bacterial targeting was achieved using a modified fragment of the human antimicrobial peptide ubiquicidin, as demonstrated by fluorophore-tagged variants. To limit the release of the effector colistin only to infection-related situations, we introduced a linker that was cleaved by neutrophil elastase (NE), an enzyme secreted by neutrophil granulocytes at infection sites. The linker carried an optimized sequence of amino acids that was required to assure sufficient cleavage efficiency. The antibacterial activity of five regioisomeric conjugates prepared by total synthesis was masked, but was released upon exposure to recombinant NE when the linker was attached to amino acids at the 1- or the 3-position of colistin. A proof-of-concept was achieved in co-cultures of primary human neutrophils and Escherichia coli that induced the secretion of NE, the release of free colistin, and an antibacterial efficacy that was equal to that of free colistin.

Exploring the possible targeting strategies of liposomes against methicillin-resistant Staphylococcus aureus (MRSA)

Multi antibiotic-resistant bacterial infections are on the rise due to the overuse of antibiotics. Methicillin-resistant Staphylococcus aureus (MRSA) is one of the pathogens listed under the category of serious threats where vancomycin remains the mainstay treatment despite the availability of various antibacterial agents. Recently, decreased susceptibility to vancomycin from clinical isolates of MRSA has been reported and has drawn worldwide attention as it is often difficult to overcome and leads to increased medical costs, mortality, and longer hospital stays. Development of antibiotic delivery systems is often necessary to improve bioavailability and biodistribution, in order to reduce antibiotic resistance and increase the lifespan of antibiotics. Liposome entrapment has been used as a method to allow higher drug dosing apart from reducing toxicity associated with drugs. The surface of the liposomes can also be designed and enhanced with drug-release properties, active targeting, and stealth effects to prevent recognition by the mononuclear phagocyte system, thus enhancing its circulation time. The present review aimed to highlight the possible targeting strategies of liposomes against MRSA bacteremia systemically while investigating the magnitude of this effect on the minimum inhibitory concentration level.

Novel phenolic antimicrobials enhanced activity of iminodiacetate prodrugs against biofilm and planktonic bacteria

Prodrugs are pharmacologically attenuated derivatives of drugs that undergo bioconversion into the active compound once reaching the targeted site, thereby maximizing their efficiency. This strategy has been implemented in pharmaceuticals to overcome obstacles related to absorption, distribution, and metabolism, as well as with intracellular dyes to ensure concentration within cells. In this study, we provide the first examples of a prodrug strategy that can be applied to simple phenolic antimicrobials to increase their potency against mature biofilms. The addition of (acetoxy)methyl iminodiacetate groups increases the otherwise modest potency of simple phenols. Biofilm-forming bacteria exhibit a heightened tolerance toward antimicrobial agents, thereby accentuating the need for new antibiotics as well as those, which incorporate novel delivery strategies to enhance activity toward biofilms.

Identification of a Novel Cathelicidin from the Deinagkistrodon acutus Genome with Antibacterial Activity by Multiple Mechanisms

The abuse of antibiotics and the consequent increase of drug-resistant bacteria constitute a serious threat to human health, and new antibiotics are urgently needed. Research shows that antimicrobial peptides produced by natural organisms are potential substitutes for antibiotics. Based on Deinagkistrodonacutus (known as five-pacer viper) genome bioinformatics analysis, we discovered a new cathelicidin antibacterial peptide which was called FP-CATH. Circular dichromatic analysis showed a typical helical structure. FP-CATH showed broad-spectrum antibacterial activity. It has antibacterial activity to Gram-negative bacteria and Gram-positive bacteria including methicillin-resistant Staphylococcus aureus (MRSA). The results of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) showed that FP-CATH could cause the change of bacterial cell integrity, having a destructive effect on Gram-negative bacteria and inducing Gram-positive bacterial surface formation of vesicular structure. FP-CATH could bind to LPS and showed strong binding ability to bacterial DNA. In vivo, FP-CATH can improve the survival rate of nematodes in bacterial invasion experiments, and has a certain protective effect on nematodes. To sum up, FP-CATH is likely to play a role in multiple mechanisms of antibacterial action by impacting bacterial cell integrity and binding to bacterial biomolecules. It is hoped that the study of FP-CATH antibacterial mechanisms will prove useful for development of novel antibiotics.

Synergies with and Resistance to Membrane-Active Peptides

Membrane-active peptides (MAPs) have long been thought of as the key to defeating antimicrobial-resistant microorganisms. Such peptides, however, may not be sufficient alone. In this review, we seek to highlight some of the common pathways for resistance, as well as some avenues for potential synergy. This discussion takes place considering resistance, and/or synergy in the extracellular space, at the membrane, and during interaction, and/or removal. Overall, this review shows that researchers require improved definitions of resistance and a more thorough understanding of MAP-resistance mechanisms. The solution to combating resistance may ultimately come from an understanding of how to harness the power of synergistic drug combinations.

Control of the Lung Residence Time of Highly Permeable Molecules after Nebulization: Example of the Fluoroquinolones

Pulmonary drug delivery is a promising strategy to treat lung infectious disease as it allows for a high local drug concentration and low systemic side effects. This is particularly true for low-permeability drugs, such as tobramycin or colistin, that penetrate the lung at a low rate after systemic administration and greatly benefit from lung administration in terms of the local drug concentration. However, for relatively high-permeable drugs, such as fluoroquinolones (FQs), the rate of absorption is so high that the pulmonary administration has no therapeutic advantage compared to systemic or oral administration. Formulation strategies have thus been developed to decrease the absorption rate and increase FQs’ residence time in the lung after inhalation. In the present review, some of these strategies, which generally consist of either decreasing the lung epithelium permeability or decreasing the release rate of FQs into the epithelial lining fluid after lung deposition, are presented in regards to their clinical aspects.

Structural design and antimicrobial properties of polypeptides and saccharide–polypeptide conjugates

The development and progress of antimicrobial polypeptides and saccharide–polypeptide conjugates in regards to their structural design, biological functions and antimicrobial mechanism.

High concentrations of middle ear antimicrobial peptides and proteins and proinflammatory cytokines are associated with detection of middle ear pathogens in children with recurrent acute otitis media

Recurrent and chronic otitis media (OM) are often refractory to antibiotics due to bacterial persistence in biofilm within the middle ear. In vitro and in vivo studies have demonstrated that antimicrobial proteins and peptides (AMPs) are bactericidal against otopathogens, indicating potential therapeutic value for recalcitrant OM. We measured concentrations of 6 AMPs and 14 cytokines in middle ear effusion (MEE) from 67 children undergoing ventilation tube insertion for recurrent acute OM. Sixty one percent of children had bacterial otopathogens detected in their MEE, 39% by PCR and 22% by PCR and culture. Groups were defined as: PCR-negative/culture-negative (absence of bacterial otopathogen), n = 26; PCR-positive/culture-negative (presence of nonculturable bacterial otopathogen), n = 26; PCR-positive/culture-positive (presence of culturable bacterial otopathogen), n = 15. Age, antibiotic usage, day-care attendance, presence of respiratory viruses in MEE and number of AOM episodes were similar between groups. AMP and cytokine concentrations were higher in children with bacterial otopathogens in their MEE compared to those with no bacterial otopathogens. Median concentrations of AMPs (except HBD2) were 3 to 56-fold higher in MEE from children with bacterial otopathogens detected in their MEE (P ≤ 0.01). Similarly, median cytokine concentrations (except TGFβ) were >16-fold higher in MEE with bacterial otopathogens detected (P ≤ 0.001). This is the first study to measure AMPs in MEE and together with the cytokine data, results suggest that elevated AMPs and cytokines in MEE are a marker of inflammation and bacterial persistence. AMPs may play an important role in OM pathogenesis.

Potential factors contributing to the poor antimicrobial efficacy of SAAP-148 in a rat wound infection model

BACKGROUND

We investigated the efficacy of a synthetic antimicrobial peptide SAAP-148, which was shown to be effective against Methicillin-resistant Staphylococcus aureus (MRSA) on tape-stripped mice skin. Unexpectedly, SAAP-148 was not effective against MRSA in our pilot study using rats with excision wounds. Therefore, we investigated factors that might have contributed to the poor efficacy of SAAP-148. Subsequently, we optimised the protocol and assessed the efficacy of SAAP-148 in an adapted rat study.

METHODS

We incubated 100 µL of SAAP-148 with 1 cm2 of a wound dressing for 1 h and determined the unabsorbed volume of peptide solution. Furthermore, 105 colony forming units (CFU)/mL MRSA were exposed to increasing dosages of SAAP-148 in 50% (v/v) human plasma, eschar- or skin extract or PBS. After 30 min incubation, the number of viable bacteria was determined. Next, ex vivo skin models were inoculated with MRSA for 1 h and exposed to SAAP-148. Finally, excision wounds on the back of rats were inoculated with 107 CFU MRSA overnight and treated with SAAP-148 for 4 h or 24 h. Subsequently, the number of viable bacteria was determined.

RESULTS

Contrary to Cuticell, Parafilm and Tegaderm film, < 20% of peptide solution was recovered after incubation with gauze, Mepilex border and Opsite Post-op. Furthermore, in plasma, eschar- or skin extract > 20-fold higher dosages of SAAP-148 were required to achieve a 2-log reduction (LR) of MRSA versus SAAP-148 in PBS. Exposure of ex vivo models to SAAP-148 for 24 h resulted in a 4-fold lower LR than a 1 h or 4 h exposure period. Additionally, SAAP-148 caused a 1.3-fold lower mean LR at a load of 107 CFU compared to 105 CFU MRSA. Moreover, exposure of ex vivo excision wound models to SAAP-148 resulted in a 1.5-fold lower LR than for tape-stripped skin. Finally, SAAP-148 failed to reduce the bacterial counts in an adapted rat study.

CONCLUSIONS

Several factors, such as absorption of SAAP-148 by wound dressings, components within wound exudates, re-colonisation during the exposure of SAAP-148, and a high bacterial load may contribute to the poor antimicrobial effect of SAAP-148 against MRSA in the rat model.

Snake Venom Cathelicidins as Natural Antimicrobial Peptides

Bioactive small molecules isolated from animals, plants, fungi and bacteria, including natural antimicrobial peptides, have shown great therapeutic potential worldwide. Among these peptides, snake venom cathelicidins are being widely exploited, because the variation in the composition of the venom reflects a range of biological activities that may be of biotechnological interest. Cathelicidins are short, cationic, and amphipathic molecules. They play an important role in host defense against microbial infections. We are currently facing a strong limitation on pharmacological interventions for infection control, which has become increasingly complex due to the lack of effective therapeutic options. In this review, we will focus on natural snake venom cathelicidins as promising candidates for the development of new antibacterial agents to fight antibiotic-resistant bacteria. We will highlight their antibacterial and antibiofilm activities, mechanism of action, and modulation of the innate immune response.

Evaluating the level of nitroreductase activity in clinical Klebsiella pneumoniae isolates to support strategies for nitro drug and prodrug development

To understand the potential utility of novel nitroreductase (NR)-activated prodrugs, NR enzyme activity was assessed in clinical Klebsiella pneumoniae isolates using a NR-activated fluorescent probe. NR activity was constant throughout the bacterial growth cycle, but individual K. pneumoniae isolates exhibited a wide range of NR activity levels. The genes of major NR enzymes (nfsA and nfnB) showed a number of sequence variants. Aside from a C-terminal extension of NfnB, which may be responsible for lower NR activity in specific isolates, the genetic differences did not explain the variation in activity. Analysis of important clinical strains (ST11, ST258, ST14 and ST101) showed significant variation in NR activity between isolates within the same sequence type despite conservation of nfsA/nfnB sequences. Addition of methyl viologen (MV), a known activator of soxRS, caused a significant increase in NR activity for all strains, with proportionally larger increases in activity seen for strains with low uninduced NR levels. Real-time PCR on selected strains following exposure to MV showed upregulation of soxS (15-32-fold) and nfsA (5-22-fold) in all strains tested. Expression of nfnB was upregulated 2-5-fold in 4/6 strains tested. High levels of NR activity in the absence of MV activation correlated with nitrofurantoin susceptibility. These data provide evidence that NR gene mutations and regulatory pathways influence NR activity in K. pneumoniae isolates and this is likely to impact treatment efficacy with novel nitro-containing drugs or prodrugs.

Drug delivery systems designed to overcome antimicrobial resistance

Antimicrobial resistance has emerged as a huge challenge to the effective treatment of infectious diseases. Aside from a modest number of novel anti-infective agents, very few new classes of antibiotics have been successfully developed for therapeutic use. Despite the research efforts of numerous scientists, the fight against antimicrobial (ATB) resistance has been a longstanding continued effort, as pathogens rapidly adapt and evolve through various strategies, to escape the action of ATBs. Among other mechanisms of resistance to antibiotics, the sophisticated envelopes surrounding microbes especially form a major barrier for almost all anti-infective agents. In addition, the mammalian cell membrane presents another obstacle to the ATBs that target intracellular pathogens. To negotiate these biological membranes, scientists have developed drug delivery systems to help drugs traverse the cell wall; these are called "Trojan horse" strategies. Within these delivery systems, ATB molecules can be conjugated with one of many different types of carriers. These carriers could include any of the following: siderophores, antimicrobial peptides, cell-penetrating peptides, antibodies, or even nanoparticles. In recent years, the Trojan horse-inspired delivery systems have been increasingly reported as efficient strategies to expand the arsenal of therapeutic solutions and/or reinforce the effectiveness of conventional ATBs against drug-resistant microbes, while also minimizing the side effects of these drugs. In this paper, we aim to review and report on the recent progress made in these newly prevalent ATB delivery strategies, within the current context of increasing ATB resistance.

Antibiotic resistance: a rundown of a global crisis

The advent of multidrug resistance among pathogenic bacteria is imperiling the worth of antibiotics, which have previously transformed medical sciences. The crisis of antimicrobial resistance has been ascribed to the misuse of these agents and due to unavailability of newer drugs attributable to exigent regulatory requirements and reduced financial inducements. Comprehensive efforts are needed to minimize the pace of resistance by studying emergent microorganisms, resistance mechanisms, and antimicrobial agents. Multidisciplinary approaches are required across health care settings as well as environment and agriculture sectors. Progressive alternate approaches including probiotics, antibodies, and vaccines have shown promising results in trials that suggest the role of these alternatives as preventive or adjunct therapies in future.

From Antimicrobial Peptides to Antimicrobial Poly(α-amino acid)s

Conventional small-molecule antibiotics are facing a significant challenge of the rapidly developed drug resistance of pathogens. In contrast, antimicrobial peptides (AMPs), an important component for innate host defenses, are now under intensive investigation as a promising antimicrobial agent for combating drug resistant pathogens. Most AMPs can effectively kill a broad spectrum of pathogens via physical disruption of microbial cellular membranes, which is identified to be difficult to develop resistance. However, the clinical applications of AMPs are still greatly limited by several inherent impediments, such as high cost of production, potential hemolysis or toxicity, and liability to proteinase degradation. Recently, cationic poly(α-amino acid)s with structures mimicking the AMPs are found to have excellent antimicrobial activity. These polymers, termed "antimicrobial poly(α-amino acid)s (APAAs)," have some advantages over AMPs, such as easy production and modification, prolonged antimicrobial activity, low cytotoxicity, and enhanced stability to protease degradation. Here, a brief introduction of mechanisms and affecting factors of microbial killing by AMPs is first presented, followed by a systematic illustration of recent advances in design and preparation of biomimetic APAAs and a perspective in this field.

An acidic model pro-peptide affects the secondary structure, membrane interactions and antimicrobial activity of a crotalicidin fragment

In order to study how acidic pro-peptides inhibit the antimicrobial activity of antimicrobial peptides, we introduce a simple model system, consisting of a 19 amino-acid long antimicrobial peptide, and an N-terminally attached, 10 amino-acid long acidic model pro-peptide. The antimicrobial peptide is a fragment of the crotalicidin peptide, a member of the cathelidin family, from rattlesnake venom. The model pro-peptide is a deca (glutamic acid). Attachment of the model pro-peptide only leads to a moderately large reduction in the binding to- and induced leakage of model liposomes, while the antimicrobial activity of the crotalicidin fragment is completely inhibited by attaching the model pro-peptide. Attaching the pro-peptide induces a conformational change to a more helical conformation, while there are no signs of intra- or intermolecular peptide complexation. We conclude that inhibition of antimicrobial activity by the model pro-peptide might be related to a conformational change induced by the pro-peptide domain, and that additional effects beyond induced changes in membrane activity must also be involved.

Emerging Nanomedicine Therapies to Counter the Rise of Methicillin-Resistant Staphylococcus aureus

In a recent report, the World Health Organisation (WHO) classified antibiotic resistance as one of the greatest threats to global health, food security, and development. Methicillin-resistant Staphylococcus aureus (MRSA) remains at the core of this threat, with persistent and resilient strains detectable in up to 90% of S. aureus infections. Unfortunately, there is a lack of novel antibiotics reaching the clinic to address the significant morbidity and mortality that MRSA is responsible for. Recently, nanomedicine strategies have emerged as a promising therapy to combat the rise of MRSA. However, these approaches have been wide-ranging in design, with few attempts to compare studies across scientific and clinical disciplines. This review seeks to reconcile this discrepancy in the literature, with specific focus on the mechanisms of MRSA infection and how they can be exploited by bioactive molecules that are delivered by nanomedicines, in addition to utilisation of the nanomaterials themselves as antibacterial agents. Finally, we discuss targeting MRSA biofilms using nano-patterning technologies and comment on future opportunities and challenges for MRSA treatment using nanomedicine.

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.

Pro-apoptotic cationic host defense peptides rich in lysine or arginine to reverse drug resistance by disrupting tumor cell membrane

Host defense peptides have been demonstrated to exhibit prominent advantages in cancer therapy with selective binding ability toward tumor cells via electrostatic attractions, which can overcome the limitations of traditional chemotherapy drugs, such as toxicity on non-malignant cells and the emergence of drug resistance. In this work, we redesigned and constructed a series of cationic peptides by inserting hydrophobic residues into hydrophilic surface or replacing lysine (K) with arginine (R), based on the experience from the preliminary work of host defense peptide B1. In-depth studies demonstrated that the engineered peptides exhibited more potent anti-cancer activity against various cancer cell lines and much lower toxicity to normal cells compared with B1. Further investigation revealed that compounds I-3 and I-7 could act on cancer cell membranes and subsequently alter the permeability, which facilitated obvious pro-apoptotic activity in paclitaxel-resistant cell line (MCF-7/Taxol). The result of mitochondrial membrane potential assay (ΔΨm) demonstrated that the peptides induced ΔΨm dissipation and mitochondrial depolarization. The caspase-3 cellular activity assay showed that the anti-cancer activity of peptides functioned via caspase-3-dependent apoptosis. The study yielded compound I-7 with superior properties for antineoplastic activity in comparison to B1, which makes it a promising potential candidate for cancer therapy.

Host defense antimicrobial peptides as antibiotics: design and application strategies

This review deals with the design and application strategies of new antibiotics based on naturally occurring antimicrobial peptides (AMPs). The initial candidate can be designed based on three-dimensional structure or selected from a library of peptides from natural or laboratory sources followed by optimization via structure-activity relationship studies. There are also advanced application strategies such as induction of AMP expression from host cells by various factors (e.g., metals, amino acids, vitamin D and sunlight), the use of engineered probiotic bacteria to deliver peptides, the design of prodrug and peptide conjugates to improve specific targeting. In addition, combined uses of newly developed AMPs with existing antimicrobial agents may provide a practical avenue for effective management of antibiotic-resistant bacteria (superbugs), including biofilms. Finally, we highlight AMPs already in use or under clinical trials.

A traceless reversible polymeric colistin prodrug to combat multidrug-resistant (MDR) gram-negative bacteria

Colistin methanesulfonate (CMS) is the only prodrug of colistin available for clinical use for the treatment of infections caused by multidrug-resistant (MDR) Gram-negative bacteria. Owing to its slow and variable release, an alternative is urgently required to improve effectiveness. Herein we describe a PEGylated colistin prodrug whereby the PEG is attached via a cleavable linker (col-aaPEG) introducing an acetic acid terminated poly (ethylene glycol) methyl ether (aaPEG) onto the Thr residue of colistin. Due to the labile ester containing link, this prodrug is converted back into active colistin in vitro within 24h. Compared to CMS, it showed a similar or better antimicrobial performance against two MDR isolates of Pseudomonas aeruginosa and Acinetobacter baumannii through in vitro disk diffusion, broth dilution and time-kill studies. In a mouse infection model, col-aaPEG displayed acceptable bacterial killing against P. aeruginosa ATCC 27853 and no nephrotoxicity was found after systemic administration, suggesting it to be a potential alternative for CMS.

Prodrug Strategy in Drug Development

Prodrugs are chemically modified derivatives introduced in therapy due to their advantageous physico-chemical properties (greater stability, improved solubility, increased permeability), used in inactive form. Biological effect is exerted by the active derivatives formed in organism through chemical transformation (biotransformation). Currently, 10% of pharmaceutical products are used as prodrugs, nearly half of them being converted to active form by hydrolysis, mainly by ester hydrolysis. The use of prodrugs aims to improve the bioavailability of compounds in order to resolve some unfavorable characteristics and to reduce first-pass metabolism. Other objectives are to increase drug absorption, to extend duration of action or to achieve a better tissue/organ selective transport in case of non-oral drug delivery forms. Prodrugs can be characterized by chemical structure, activation mechanism or through the presence of certain functional groups suitable for their preparation. Currently we distinguish in therapy traditional prodrugs prepared by chemical derivatisation, bioprecursors and targeted delivery systems. The present article is a review regarding the introduction and applications of prodrug design in various areas of drug development.

Differential In Vitro and In Vivo Toxicities of Antimicrobial Peptide Prodrugs for Potential Use in Cystic Fibrosis

There has been considerable interest in the use of antimicrobial peptides (AMPs) as antimicrobial agents for the treatment of many conditions, including cystic fibrosis (CF). The challenging conditions of the CF patient lung require robust AMPs that are active in an environment of high proteolytic activity but that also have low cytotoxicity and immunogenicity. Previously, we developed prodrugs of AMPs that limited the cytotoxic effects of AMP treatment by rendering the antimicrobial activity dependent on the host enzyme neutrophil elastase (NE). However, cytotoxicity remained an issue. Here, we describe the further optimization of the AMP prodrug (pro-AMP) model for CF to produce pro-WMR, a peptide with greatly reduced cytotoxicity (50% inhibitory concentration against CFBE41o- cells, >300 μM) compared to that of the previous group of pro-AMPs. The bactericidal activity of pro-WMR was increased in NE-rich bronchoalveolar lavage (BAL) fluid from CF patients (range, 8.4% ± 6.9% alone to 91.5% ± 5.8% with BAL fluid; P = 0.0004), an activity differential greater than that of previous pro-AMPs. In a murine model of lung delivery, the pro-AMP modification reduced host toxicity, with pro-WMR being less toxic than the active peptide. Previously, host toxicity issues have hampered the clinical application of AMPs. However, the development of application-specific AMPs with modifications that minimize toxicity similar to those described here can significantly advance their potential use in patients. The combination of this prodrug strategy with a highly active AMP has the potential to produce new therapeutics for the challenging conditions of the CF patient lung.

Cecropins from Plutella xylostella and Their Interaction with Metarhizium anisopliae

Cecropins are the most potent induced peptides to resist invading microorganisms. In the present study, two full length cDNA encoding cecropin2 (Px-cec2) and cecropin3 (Px-cec3) were obtained from P. xylostella by integrated analysis of genome and transcriptome data. qRT-PCR analysis revealed the high levels of transcripts of Px-cecs (Px-cec1, Px-cec2 and Px-cec3) in epidermis, fat body and hemocytes after 24, 30 and 36 h induction of Metarhizium anisopliae, respectively. Silencing of Spätzle and Dorsal separately caused the low expression of cecropins in the fat body, epidermis and hemocytes, and made the P.xylostella larvae more susceptible to M. anisopliae. Antimicrobial assays demonstrated that the purified recombinant cecropins, i.e., Px-cec1, Px-cec2 and Px-cec3, exerted a broad spectrum of antimicrobial activity against fungi, as well as Gram-positive and Gram-negative bacteria. Especially, Px-cecs showed higher activity against M. anisopliae than another selected fungi isolates. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that cecropins exerted the vital morphological alterations to the spores of M. anisopliae. Based on our results, cecropins played an imperative role in resisting infection of M. anisopliae, which will provide the foundation of biological control of insect pests by using cecorpins as a target in the future.

Antimicrobial peptides in 2014

This article highlights new members, novel mechanisms of action, new functions, and interesting applications of antimicrobial peptides reported in 2014. As of December 2014, over 100 new peptides were registered into the Antimicrobial Peptide Database, increasing the total number of entries to 2493. Unique antimicrobial peptides have been identified from marine bacteria, fungi, and plants. Environmental conditions clearly influence peptide activity or function. Human α-defensin HD-6 is only antimicrobial under reduced conditions. The pH-dependent oligomerization of human cathelicidin LL-37 is linked to double-stranded RNA delivery to endosomes, where the acidic pH triggers the dissociation of the peptide aggregate to release its cargo. Proline-rich peptides, previously known to bind to heat shock proteins, are shown to inhibit protein synthesis. A model antimicrobial peptide is demonstrated to have multiple hits on bacteria, including surface protein delocalization. While cell surface modification to decrease cationic peptide binding is a recognized resistance mechanism for pathogenic bacteria, it is also used as a survival strategy for commensal bacteria. The year 2014 also witnessed continued efforts in exploiting potential applications of antimicrobial peptides. We highlight 3D structure-based design of peptide antimicrobials and vaccines, surface coating, delivery systems, and microbial detection devices involving antimicrobial peptides. The 2014 results also support that combination therapy is preferred over monotherapy in treating biofilms.

Understanding bacterial resistance to antimicrobial peptides: From the surface to deep inside

Resistant bacterial infections are a major health problem in many parts of the world. The major commercial antibiotic classes often fail to combat common bacteria. Although antimicrobial peptides are able to control bacterial infections by interfering with microbial metabolism and physiological processes in several ways, a large number of cases of resistance to antibiotic peptide classes have also been reported. To gain a better understanding of the resistance process various technologies have been applied. Here we discuss multiple strategies by which bacteria could develop enhanced antimicrobial peptide resistance, focusing on sub-cellular regions from the surface to deep inside, evaluating bacterial membranes, cell walls and cytoplasmic metabolism. Moreover, some high-throughput methods for antimicrobial resistance detection and discrimination are also examined. This article is part of a Special Issue entitled: Bacterial Resistance to Antimicrobial Peptides.