Altered activity and physicochemical properties of short cationic antimicrobial peptides by incorporation of arginine analogues

Positively charged residues play a significant role in enhancing the antibacterial activity of calcitermin

A systematic study on the human antimicrobial peptide calcitermin (VAIALKAAHYHTHKE) and its carefully designed derivatives was undertaken to verify the impact of divalent copper and zinc ions on the stability, coordination and antimicrobial activity of the formed complexes. In this work we investigate the calcitermin mutants where the alanine in position 7 and 8 is substituted with an arginine residue, with the aim of enhancing the antibacterial activity. Additionally, the analogue where alanine in position 7 is replaced with a histidine is considered, to obtain a chelating sequence with four histidines in alternate position; the aim of this change was to increase the cationic properties of the peptide under acidic conditions and possibly enhance its binding ability towards the metal ions. Through a comprehensive analytical approach involving potentiometric titrations, mass spectrometry, UV-Vis spectrophotometry, NMR and circular dichroism, we delved into the formation equilibria and coordination chemistry of the formed copper(II) and zinc(II) complexes. Antimicrobial assays are also performed to assess the bioactivity of the compounds against a broad spectrum of microorganisms, revealing the pivotal role of positively charged residues in enhancing the antibacterial activity of calcitermin. The obtained results serve as an important stepping stone towards the development of novel metal-based antimicrobial agents.

Non-canonical amino acid bioincorporation into antimicrobial peptides and its challenges

The rise of antimicrobial resistance and multi-drug resistant pathogens has necessitated explorations for novel antibiotic agents as the discovery of conventional antibiotics is becoming economically less viable and technically more challenging for biopharma. Antimicrobial peptides (AMPs) have emerged as a promising alternative because of their particular mode of action, broad spectrum and difficulty that microbes have in becoming resistant to them. The AMPs bacitracin, gramicidin, polymyxins and daptomycin are currently used clinically. However, their susceptibility to proteolytic degradation, toxicity profile, and complexities in large-scale manufacture have hindered their development. To improve their proteolytic stability, methods such as integrating non-canonical amino acids (ncAAs) into their peptide sequence have been adopted, which also improves their potency and spectrum of action. The benefits of ncAA incorporation have been made possible by solid-phase peptide synthesis. However, this method is not always suitable for commercial production of AMPs because of poor yield, scale-up difficulties, and its non-'green' nature. Bioincorporation of ncAA as a method of integration is an emerging field geared towards tackling the challenges of solid-phase synthesis as a green, cheaper, and scalable alternative for commercialisation of AMPs. This review focusses on the bioincorporation of ncAAs; some challenges associated with the methods are outlined, and notes are given on how to overcome these challenges. The review focusses particularly on addressing two key challenges: AMP cytotoxicity towards microbial cell factories and the uptake of ncAAs that are unfavourable to them. Overcoming these challenges will draw us closer to a greater yield and an environmentally friendly and sustainable approach to make AMPs more druggable.

Flexible and Biocompatible Antifouling Polyurethane Surfaces Incorporating Tethered Antimicrobial Peptides through Click Reactions

Efficient, simple antibacterial materials to combat implant-associated infections are much in demand. Herein, the development of polyurethanes, both cross-linked thermoset and flexible and versatile thermoplastic, suitable for "click on demand" attachment of antibacterial compounds enabled via incorporation of an alkyne-containing diol monomer in the polymer backbone, is described. By employing different polyolic polytetrahydrofurans, isocyanates, and chain extenders, a robust and flexible material comparable to commercial thermoplastic polyurethane is prepared. A series of short synthetic antimicrobial peptides are designed, synthesized, and covalently attached in a single coupling step to generate a homogenous coating. The lead material is shown to be biocompatible and does not display any toxicity against either mouse fibroblasts or reconstructed human epidermis according to ISO and OECD guidelines. The repelling performance of the peptide-coated materials is illustrated against colonization and biofilm formation by Staphylococcus aureus and Staphylococcus epidermidis on coated plastic films and finally, on coated commercial central venous catheters employing LIVE/DEAD staining, confocal laser scanning microscopy, and bacterial counts. This study presents the successful development of a versatile and scalable polyurethane with the potential for use in the medical field to reduce the impact of bacterial biofilms.

Amphiphilic small molecule antimicrobials: From cationic antimicrobial peptides (CAMPs) to mechanism-related, structurally-diverse antimicrobials

Bacterial infections are characterized by their rapid and widespread proliferation, leading to significant morbidity. Despite the availability of a variety of antimicrobial drugs, the resistance exhibited by pathogenic microorganisms towards these drugs demonstrates a consistent upward trajectory year after year. This trend can be attributed to the abuse or misuse of antibiotics. Although antimicrobial peptides can avoid the emergence of drug resistance to a certain extent, their clinical application has been hindered by factors such as their high production cost, poor in vivo stability, and potential cytotoxicity. Consequently, there arises an urgent need for the development of novel antimicrobial drugs. Small-molecule amphiphatic antimicrobials have a good prospect for research and development. These peptides hold the potential to address several issues, including the high cost of antimicrobial peptide production, poor in vivo stability, and cytotoxicity. Moreover, they exhibit the capability to overcome bacterial resistance, thereby considerably satisfying market demands and clinical needs. This paper reviews recent research pertaining to small molecule host-defending amphiphatic antimicrobials with cationic amphiphilic structures. It focuses on the design concepts, inherent relationships, drug-like properties, antimicrobial activities, application prospects, and emerging screening methods for novel antimicrobial. This review assumes paramount importance in mitigating the current shortcomings of antimicrobial agents. It also provides potential new ideas and methodologies for the research and development of antimicrobial agents.

Antimicrobial Peptides Incorporating Halogenated Marine-Derived Amino Acid Substituents

Small synthetic mimics of cationic antimicrobial peptides represent a promising class of compounds with leads in clinical development for the treatment of persistent microbial infections. The activity and selectivity of these compounds rely on a balance between hydrophobic and cationic components, and here, we explore the activity of 19 linear cationic tripeptides against five different pathogenic bacteria and fungi, including clinical isolates. The compounds incorporated modified hydrophobic amino acids inspired by motifs often found in bioactive marine secondary metabolites in combination with different cationic residues to probe the possibility of generating active compounds with improved safety profiles. Several of the compounds displayed high activity (low μM concentrations), comparable with the positive controls AMC-109, amoxicillin, and amphotericin B. A higher activity was observed against the fungal strains, and a low in vitro off-target toxicity was observed against erythrocytes and HeLa cells, thereby illustrating effective means for tuning the activity and selectivity of short antimicrobial peptides.

Thinking on the Construction of Antimicrobial Peptide Databases: Powerful Tools for the Molecular Design and Screening

With the accelerating growth of antimicrobial resistance (AMR), there is an urgent need for new antimicrobial agents with low or no AMR. Antimicrobial peptides (AMPs) have been extensively studied as alternatives to antibiotics (ATAs). Coupled with the new generation of high-throughput technology for AMP mining, the number of derivatives has increased dramatically, but manual running is time-consuming and laborious. Therefore, it is necessary to establish databases that combine computer algorithms to summarize, analyze, and design new AMPs. A number of AMP databases have already been established, such as the Antimicrobial Peptides Database (APD), the Collection of Antimicrobial Peptides (CAMP), the Database of Antimicrobial Activity and Structure of Peptides (DBAASP), and the Database of Antimicrobial Peptides (dbAMPs). These four AMP databases are comprehensive and are widely used. This review aims to cover the construction, evolution, characteristic function, prediction, and design of these four AMP databases. It also offers ideas for the improvement and application of these databases based on merging the various advantages of these four peptide libraries. This review promotes research and development into new AMPs and lays their foundation in the fields of druggability and clinical precision treatment.

Effect of regio- and stereoisomerism on antifouling 2,5-diketopiperazines

Marine biofouling is a problem that plagues all maritime industries at vast economic and environmental cost. Previous and current methods to prevent biofouling have employed the use of heavy metals and other toxic or highly persistent chemicals, and these methods are now coming under immense regulatory pressure. Recent studies have illustrated the potential of nature-inspired tetrasubstituted 2,5-diketopiperazines (2,5-DKPs) as eco-friendly marine biocides for biofouling control. These highly active symmetrically substituted 2,5-DKPs can be generated by combining structural motifs from cationic innate defence peptides and natural marine antifoulants. A balance between a threshold hydrophobic contribution and sufficient cationic charge has been established as key for bioactivity, and our current study further increases understanding of the antifouling mechanism by investigating the effect of both regio- and stereochemistry. Novel synthetic routes for the generation of unsymmetrical 2,5-DKPs were developed and a library of nine compounds was prepared. The compounds were screened against a series of four model macrofouling organisms (Ciona savignyi, Mytilus galloprovincialis, Spirobranchus cariniferus, and Undaria pinnatifida). Several of the evaluated compounds displayed inhibitory activity at sub-micromolar concentrations. The structural contributions to antifouling bioactivity were studied using NMR spectroscopy and molecular modelling, revealing a strong dependence on a stable amphiphilic solution structure regardless of substitution pattern.

Anticancer effect of rationally designed α-helical amphiphilic peptides

Anticancer peptides (ACPs) have attracted increasing attention in cancer therapy due to their unique mechanism of action on cancer cells. The main challenge is to establish the correlation between their physicochemical properties and their selectivity and anticancer effect, leading to a clear design strategy. In this study, a series of new α-helical short peptides (coded At1-At12) with different anticancer activities were systematically designed with different amphiphilicity based on a natural α-helical antimicrobial peptide (AMP) derived from ant. Three of the designed peptides, At7, At10 and At11, showed considerable anticancer activity with low toxicity to normal skin fibroblasts. The high selectivity of the peptides is attributed to their balanced amphiphilicity and cationic nature which favours binding to the outer membrane of negatively charged cancer cells over the neutral membrane of normal mammalian cells. In addition to rapid membrane penetration, the designed peptides also damaged the mitochondria and induced mitochondrial membrane depolarization. Moreover, these peptides were found to induce apoptosis in cancer cells by up-regulating the expression of apoptotic proteins Bax and Caspase-3, down-regulating the apoptotic protein Bcl-2, and activating the Caspase enzyme-linked reaction. The results of this study reveal the potential of these peptides for clinical applications, and provide a guidance for further development of highly selective anticancer medications.

Development of potent cholinesterase inhibitors based on a marine pharmacophore

The management of neurological disorders such as dementia associated with Alzheimer's or Parkinson's disease includes the use of cholinesterase inhibitors. These compounds can slow down the progression of these diseases and can also be used in the treatment of glaucoma and myasthenia gravis. The majority of the cholinesterase inhibitors used in the clinic are derived from natural products and our current paper describes the use of a small marine pharmacophore to develop potent and selective cholinesterase inhibitors. Fourteen small inhibitors were designed based on recent discoveries about the inhibitory potential of a range of related marine secondary metabolites. The compounds were evaluated, in kinetic enzymatic assays, for their ability to inhibit three different cholinesterase enzymes and it was shown that compounds with a high inhibitory activity towards electric eel and human recombinant acetylcholinesterase (IC₅₀ between 20-70 μM) could be prepared. It was also shown that this compound class was particularly active against horse serum butyrylcholinesterase, with IC₅₀ values between 0.8-16 μM, which is an order of magnitude more potent than the clinically used positive control neostigmine. The compounds were further tested for off-target toxicity against both human umbilical vein endothelial cells and bovine and human erythrocytes and were shown to display a low mammalian cellular toxicity. Overall, the study illustrates how the brominated dipeptide marine pharmacophore can be used as a versatile natural scaffold for the design of potent, and selective cholinesterase inhibitors.

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.

An overview of in vitro, ex vivo and in vivo models for studying the transport of drugs across intestinal barriers

Oral administration is the most commonly used route for drug delivery owing to its cost-effectiveness, ease of administration, and high patient compliance. However, the absorption of orally delivered compounds is a complex process that greatly depends on the interplay between the characteristics of the drug/formulation and the gastrointestinal tract. In this contribution, we review the different preclinical models (in vitro, ex vivo and in vivo) from their development to application for studying the transport of drugs across intestinal barriers. This review also discusses the advantages and disadvantages of each model. Furthermore, the authors have reviewed the selection and validation of these models and how the limitations of the models can be addressed in future investigations. The correlation and predictability of the intestinal transport data from the preclinical models and human data are also explored. With the increasing popularity and prevalence of orally delivered drugs/formulations, sophisticated preclinical models with higher predictive capacity for absorption of oral formulations used in clinical studies will be needed.

Antimicrobial Peptides and Proteins: From Nature's Reservoir to the Laboratory and Beyond

Rapid rise of antimicrobial resistance against conventional antimicrobials, resurgence of multidrug resistant microbes and the slowdown in the development of new classes of antimicrobials, necessitates the urgent development of alternate classes of therapeutic molecules. Antimicrobial peptides (AMPs) are small proteins present in different lifeforms in nature that provide defense against microbial infections. They have been effective components of the host defense system for a very long time. The fact that the development of resistance by the microbes against the AMPs is relatively slower or delayed compared to that against the conventional antibiotics, makes them prospective alternative therapeutics of the future. Several thousands of AMPs have been isolated from various natural sources like microorganisms, plants, insects, crustaceans, animals, humans, etc. to date. However, only a few of them have been translated commercially to the market so far. This is because of some inherent drawbacks of the naturally obtained AMPs like 1) short half-life owing to the susceptibility to protease degradation, 2) inactivity at physiological salt concentrations, 3) cytotoxicity to host cells, 4) lack of appropriate strategies for sustained and targeted delivery of the AMPs. This has led to a surge of interest in the development of synthetic AMPs which would retain or improve the antimicrobial potency along with circumventing the disadvantages of the natural analogs. The development of synthetic AMPs is inspired by natural designs and sequences and strengthened by the fusion with various synthetic elements. Generation of the synthetic designs are based on various strategies like sequence truncation, mutation, cyclization and introduction of unnatural amino acids and synthons. In this review, we have described some of the AMPs isolated from the vast repertoire of natural sources, and subsequently described the various synthetic designs that have been developed based on the templates of natural AMPs or from de novo design to make commercially viable therapeutics of the future. This review entails the journey of the AMPs from their natural sources to the laboratory.

In vitro and in vivo antibacterial properties of peptide AMC-109 impregnated wound dressings and gels

Synthetic mimics of antimicrobial peptides (AMPs) is a promising class of molecules for a variety of antimicrobial applications. Several hurdles must be passed before effective systemic infection therapies with AMPs can be achieved, but the path to effective topical treatment of skin, nail, and soft tissue infections appears less challenging to navigate. Skin and soft tissue infection is closely coupled to the emergence of antibiotic resistance and represents a major burden to the healthcare system. The present study evaluates the promising synthetic cationic AMP mimic, AMC-109, for treatment of skin infections in vivo. The compound is evaluated both in impregnated cotton wound dressings and in a gel formulation against skin infections caused by Staphylococcus aureus and methicillin resistant S. aureus. Both the ability to prevent colonization and formation of an infection, as well as eradicate an ongoing infection in vivo with a high bacterial load, were evaluated. The present work demonstrates that AMC-109 displays a significantly higher antibacterial activity with up to a seven-log reduction in bacterial loads compared to current clinical standard therapy; Altargo cream (1% retapamulin) and Fucidin cream (2% fusidic acid) in the in vivo wound models. It is thus concluded that AMC-109 represents a promising entry in the development of new and effective remedies for various skin infections.

Advances of peptides for antibacterial applications

In the past few decades, peptide antibacterial products with unique antibacterial mechanisms have attracted widespread interest. They can effectively reduce the probability of drug resistance of bacteria and are biocompatible, so they possess tremendous development prospects. This review provides recent research and analysis on the basic types of antimicrobial peptides (including poly (amino acid)s, short AMPs, and lipopeptides) and factors to optimize antimicrobial effects. It also summarizes the two most important modes of action of antimicrobial peptides and the latest developments in the application of AMPs, including antimicrobial agent, wound healing, preservative, antibacterial coating and others. Finally, we discuss the remaining challenges to improve the antibacterial peptides and propose prospects in the field.

Phidianidine A and Synthetic Analogues as Naturally Inspired Marine Antifoulants

Stationary and slow-moving marine organisms regularly employ a natural product chemical defense to prevent being colonized by marine micro- and macroorganisms. While these natural antifoulants can be structurally diverse, they often display highly conserved chemistries and physicochemical properties, suggesting a natural marine antifouling pharmacophore. In our current report, we investigate the marine natural product phidianidine A, which displays several chemical properties found in highly potent marine antifoulants. Phidianidine A and synthetic analogues were screened against the settlement and metamorphosis of Amphibalanus improvisus cyprids, and several of the compounds displayed inhibitory activities at low micromolar concentrations with IC₅₀ values down to 0.7 μg/mL observed. The settlement study highlights that phidianidine A is a potent natural antifoulant and that the scaffold can be tuned to generate simpler and improved synthetic analogues. The bioactivity is closely linked to the size of the compound and to its basicity. The study also illustrates that active analogues can be prepared in the absence of the natural constrained 1,2,4-oxadiazole ring. A synthetic lead analogue of phidianidine A was incorporated in a coating and included in antifouling field trials, where it was shown that the coating induced potent inhibition of marine bacteria and microalgae settlement.

Impact of non-proteinogenic amino acids in the discovery and development of peptide therapeutics

With the development of modern chemistry and biology, non-proteinogenic amino acids (NPAAs) have become a powerful tool for developing peptide-based drug candidates. Drug-like properties of peptidic medicines, due to the smaller size and simpler structure compared to large proteins, can be changed fundamentally by introducing NPAAs in its sequence. While peptides composed of natural amino acids can be used as drug candidates, the majority have shown to be less stable in biological conditions. The impact of NPAA incorporation can be extremely beneficial in improving the stability, potency, permeability, and bioavailability of peptide-based therapies. Conversely, undesired effects such as toxicity or immunogenicity should also be considered. The impact of NPAAs in the development of peptide-based therapeutics is reviewed in this article. Further, numerous examples of peptides containing NPAAs are presented to highlight the ongoing development in peptide-based therapeutics.

Correlation between hemolytic activity, cytotoxicity and systemic in vivo toxicity of synthetic antimicrobial peptides

The use of non-standard toxicity models is a hurdle in the early development of antimicrobial peptides towards clinical applications. Herein we report an extensive in vitro and in vivo toxicity study of a library of 24 peptide-based antimicrobials with narrow spectrum activity towards veterinary pathogens. The haemolytic activity of the compounds was evaluated against four different species and the relative sensitivity against the compounds was highest for canine erythrocytes, intermediate for rat and human cells and lowest for bovine cells. Selected peptides were additionally evaluated against HeLa, HaCaT and HepG2 cells which showed increased stability towards the peptides. Therapeutic indexes of 50–500 suggest significant cellular selectivity in comparison to bacterial cells. Three peptides were administered to rats in intravenous acute dose toxicity studies up to 2–8 × MIC. None of the injected compounds induced any systemic toxic effects in vivo at the concentrations employed illustrating that the correlation between the different assays is not obvious. This work sheds light on the in vitro and in vivo toxicity of this class of promising compounds and provides insights into the relationship between the different toxicity models often employed in different manners to evaluate the toxicity of novel bioactive compounds in general.

Ubiquicidin-Derived Peptides Selectively Interact with the Anionic Phospholipid Membrane

Ubiquicidin (UBI)/ribosomal protein S30 (RS30) is an intracellular protein with antimicrobial activities against various pathogens. UBI (29-41) and UBI (31-38) are two crucial peptides derived from Ubiquicidin, which have shown potential as infection imaging probes. Here, we report the interactions of UBI-derived peptides with anionic and zwitterionic phospholipid membranes. Our isothermal titration calorimetry results show that both peptides selectively interact with the anionic phospholipid membrane (a model bacterial membrane) and reside mainly on the membrane surface. The interaction of UBI-derived peptides with the anionic phospholipid membrane is exothermic and driven by both enthalpy (ΔH) and entropy (ΔS), with the entropic term TΔS being greater than ΔH. This large entropic term can be a result of the aggregation of the anionic vesicles, which is confirmed by dynamic light scattering (DLS) measurements. DLS data show that vesicle aggregation is enhanced with increasing peptide-to-lipid molar ratios (P/L) and is found to be more pronounced in the case of UBI (29-41). DLS results are found to be consistent with independent transmission measurements. To study the effects of UBI-derived peptides on the microscopic dynamics of the model bacterial membrane, quasielastic neutron scattering (QENS) measurements have been carried out. The QENS results show that both peptides restrict the lateral motion of the lipid within the leaflet. UBI (29-41) acts as a stronger stiffening agent, hindering the lateral diffusion of lipids more efficiently than UBI (31-38). To our knowledge, this is the first report illustrating the mechanism of interaction of UBI-derived peptides with model membranes. This study also has implications for the improvement and design of antimicrobial peptide-based infection imaging probes.

Understanding interactions of Citropin 1.1 analogues with model membranes and their influence on biological activity

The rapid emergence of resistant bacterial strains has made the search for new antibacterial agents an endeavor of paramount importance. Cationic antimicrobial peptides (AMPs) have the ability to kill resistant pathogens while diminishing the development of resistance. Citropin 1.1 (Cit 1.1) is an AMP effective against a broad range of pathogens. 20 analogues of Cit 1.1 were prepared to understand how sequence variations lead to changes in structure and biological activity. Various analogues exhibited an increased antimicrobial activity relative to Cit 1.1. The two most promising, AMP-016 (W3F) and AMP-017 (W3F, D4R, K7R) presented a 2- to 8-fold increase in activity against MRSA (both = 4 μg/mL). AMP-017 was active against E. coli (4 μg/mL), K. pneumoniae (8 μg/mL), and A. baumannii (2 μg/mL). NMR studies indicated that Cit 1.1 and its analogues form a head-to-tail helical dimer in a membrane environment, which differs from a prior study by Sikorska et al. Active peptides displayed a greater tendency to form α-helices and to dimerize when in contact with a negatively-charged membrane. Antimicrobial activity was observed to correlate to the overall stability of the α-helix and to a positively charged N-terminus. Biologically active AMPs were shown by SEM and flow cytometry to disrupt membranes in both Gram-positive and Gram-negative bacteria through a proposed carpet mechanism. Notably, active peptides exhibited typical serum stabilities and a good selectivity for bacterial cells over mammalian cells, which supports the potential use of Cit 1.1 analogues as a novel broad-spectrum antibiotic for drug-resistant bacterial infections.

Small AntiMicrobial Peptide With in Vivo Activity Against Sepsis

Antimicrobial peptides (AMPs) are considered as potential therapeutic sources of future antibiotics because of their broad-spectrum activities and alternative mechanisms of action compared to conventional antibiotics. Although AMPs present considerable advantages over conventional antibiotics, their clinical and commercial development still have some limitations, because of their potential toxicity, susceptibility to proteases, and high cost of production. To overcome these drawbacks, the use of peptides mimics is anticipated to avoid the proteolysis, while the identification of minimalist peptide sequences retaining antimicrobial activities could bring a solution for the cost issue. We describe here new polycationic β-amino acids combining these two properties, that we used to design small dipeptides that appeared to be active against Gram-positive and Gram-negative bacteria, selective against prokaryotic versus mammalian cells, and highly stable in human plasma. Moreover, the in vivo data activity obtained in septic mice reveals that the bacterial killing effect allows the control of the infection and increases the survival rate of cecal ligature and puncture (CLP)-treated mice.

Very Short and Stable Lactoferricin-Derived Antimicrobial Peptides: Design Principles and Potential Uses

The alarming rate at which micro-organisms are developing resistance to conventional antibiotics represents one of the global challenges of our time. There is currently ample space in the antibacterial drug pipeline, and scientists are trying to find innovative and novel strategies to target the microbial enemies. Nature has remained a source of inspiration for most of the antibiotics developed and used, and the immune molecules produced by the innate defense systems, as a first line of defense, have been heralded as the next source of antibiotics. Most living organisms produce an arsenal of antimicrobial peptides (AMPs) to rapidly fend off intruding pathogens, and several different attempts have been made to transform this versatile group of compounds into the next generation of antibiotics. However, faced with the many hurdles of using peptides as drugs, the success of these defense molecules as therapeutics remains to be realized. AMPs derived from the proteolytic degradation of the innate defense protein lactoferrin have been shown to display several favorable antimicrobial properties. In an attempt to investigate the biological and pharmacological properties of these much shorter AMPs, the sequence dependence was investigated, and it was shown, through a series of truncation experiments, that these AMPs in fact can be prepared as tripeptides, with improved antimicrobial activity, via the incorporation of unnatural hydrophobic residues and terminal cappings. In this Account, we describe how this class of promising cationic tripeptides has been developed to specifically address the main challenges limiting the general use of AMPs. This has been made possible through the identification of the antibacterial pharmacophore and via the incorporation of a range of unnatural hydrophobic and cationic amino acids. Incorporation of these residues at selected positions has allowed us to extensively establish how these compounds interact with the major proteolytic enzymes trypsin and chymotrypsin and also the two major drug-binding plasma proteins serum albumin and α-1 glycoprotein. Several of the challenges associated with using AMPs relate to their size, susceptibility to rapid proteolytic degradation, and poor oral bioavailability. Our studies have addressed these issues in detail, and the results have allowed us to effectively design and prepare active and metabolically stable AMPs that have been evaluated in a range of functional settings. The optimized short AMPs display inhibitory activities against a plethora of micro-organisms at low micromolar concentrations, and they have been shown to target resistant strains of both bacteria and fungi alike with a very rapid mode of action. Our Account further describes how these compounds behave in in vivo experiments and highlights both the challenges and possibilities of the intriguing compounds. In several areas, they have been shown to exhibit comparable or superior activity to established antibacterial, antifungal, and antifouling commercial products. This illustrates their ability to effectively target and eradicate various microbes in a variety of settings ranging from the ocean to the clinic.

Investigating the nucleic acid interactions of histone-derived antimicrobial peptides

While many antimicrobial peptides (AMPs) disrupt bacterial membranes, some translocate into bacteria and interfere with intracellular processes. Buforin II and DesHDAP1 are thought to kill bacteria by interacting with nucleic acids. Here, molecular modeling and experimental measurements are used to show that neither nucleic acid binding peptide selectively binds DNA sequences. Simulations and experiments also show that changing lysines to arginines enhances DNA binding, suggesting that including additional guanidinium groups is a potential strategy to engineer more potent AMPs. Moreover, the lack of binding specificity may make it more difficult for bacteria to evolve resistance to these and other similar AMPs.

Probing the Structure-Activity Relationship of the Natural Antifouling Agent Polygodial against both Micro- and Macrofoulers by Semisynthetic Modification

The current study represents the first comprehensive investigation into the general antifouling activities of the natural drimane sesquiterpene polygodial. Previous studies have highlighted a high antifouling effect toward macrofoulers, such as ascidians, tubeworms, and mussels, but no reports about the general antifouling effect of polygodial have been communicated before. To probe the structural and chemical basis for antifouling activity, a library of 11 polygodial analogues was prepared by semisynthesis. The library was designed to yield derivatives with ranging polarities and the ability to engage in both covalent and noncovalent interactions, while still remaining within the drimane sesquiterpene scaffold. The prepared compounds were screened against 14 relevant marine micro- and macrofouling species. Several of the polygodial analogues displayed inhibitory activities at sub-microgram/mL concentrations. These antifouling effects were most pronounced against the macrofouling ascidian Ciona savignyi and the barnacle Balanus improvisus, with inhibitory activities observed for selected compounds comparable or superior to several commercial antifouling products. The inhibitory activity against the microfouling bacteria and microalgae was reversible and significantly less pronounced than for the macrofoulers. This study illustrates that the macro- and microfoulers are targeted by the compounds via different mechanisms.

Chitosan Derivatives Active against Multidrug-Resistant Bacteria and Pathogenic Fungi: In Vivo Evaluation as Topical Antimicrobials

The continuous rise of antimicrobial resistance and the dearth of new antibiotics in the clinical pipeline raise an urgent call for the development of potent antimicrobial agents. Cationic chitosan derivatives, N-(2-hydroxypropyl)-3-trimethylammonium chitosan chlorides (HTCC), have been widely studied as potent antibacterial agents. However, their systemic structure-activity relationship, activity toward drug-resistant bacteria and fungi, and mode of action are very rare. Moreover, toxicity and efficacy of these polymers under in vivo conditions are yet to be established. Herein, we investigated antibacterial and antifungal efficacies of the HTCC polymers against multidrug resistant bacteria including clinical isolates and pathogenic fungi, studied their mechanism of action, and evaluated cytotoxic and antimicrobial activities in vitro and in vivo. The polymers were found to be active against both bacteria and fungi (MIC = 125-250 μg/mL) and displayed rapid microbicidal kinetics, killing pathogens within 60-120 min. Moreover, the polymers were shown to target both bacterial and fungal cell membrane leading to membrane disruption and found to be effective in hindering bacterial resistance development. Importantly, very low toxicity toward human erythrocytes (HC₅₀ = >10000 μg/mL) and embryo kidney cells were observed for the cationic polymers in vitro. Further, no inflammation toward skin tissue was observed in vivo for the most active polymer even at 200 mg/kg when applied on the mice skin. In a murine model of superficial skin infection, the polymer showed significant reduction of methicillin-resistant Staphylococcus aureus (MRSA) burden (3.2 log MRSA reduction at 100 mg/kg) with no to minimal inflammation. Taken together, these selectively active polymers show promise to be used as potent antimicrobial agents in topical and other infections.

Role of arginine and lysine in the antimicrobial mechanism of histone-derived antimicrobial peptides

Translocation of cell-penetrating peptides is often promoted by increased content of arginine or other guanidinium groups. However, relatively little research has considered the role of these functional groups on antimicrobial peptide activity. This study compared the activity of three histone-derived antimicrobial peptides-buforin II, DesHDAP1, and parasin-with variants that contain only lysine or arginine cationic residues. These peptides operate via different mechanisms as parasin causes membrane permeabilization while buforin II and DesHDAP1 translocate into bacteria. For all peptides, antibacterial activity increased with increased arginine content. Higher arginine content increased permeabilization for parasin while it improved translocation for buforin II and DesHDAP1. These observations provide insight into the relative importance of arginine and lysine in these antimicrobial peptides.

Evaluation of cationic micropeptides derived from the innate immune system as inhibitors of marine biofouling

A series of 13 short synthetic amphiphilic cationic micropeptides, derived from the antimicrobial iron-binding innate defence protein lactoferrin, have been evaluated for their capacity to inhibit the marine fouling process. The whole biofouling process was studied and microfouling organisms such as marine bacteria and microalgae were included as well as the macrofouling barnacle Balanus improvisus. In total 19 different marine fouling organisms (18 microfoulers and one macrofouler) were included and both the adhesion and growth of the microfoulers were investigated. It was shown that the majority of the peptides inhibited barnacle cyprid settlement via a reversible nontoxic mechanism, with IC50 values as low as 0.5 μg ml(-1). Six peptides inhibited adhesion and growth of microorganisms. Two of these were particularly active against the microfoulers with MIC-values ranging between 0.01 and 1 μg ml(-1), which is comparable with the commercial reference antifoulant SeaNine.

Synthetic cationic antimicrobial peptides bind with their hydrophobic parts to drug site II of human serum albumin

Background

Many biologically active compounds bind to plasma transport proteins, and this binding can be either advantageous or disadvantageous from a drug design perspective. Human serum albumin (HSA) is one of the most important transport proteins in the cardiovascular system due to its great binding capacity and high physiological concentration. HSA has a preference for accommodating neutral lipophilic and acidic drug-like ligands, but is also surprisingly able to bind positively charged peptides. Understanding of how short cationic antimicrobial peptides interact with human serum albumin is of importance for developing such compounds into the clinics.

Results

The binding of a selection of short synthetic cationic antimicrobial peptides (CAPs) to human albumin with binding affinities in the μM range is described. Competitive isothermal titration calorimetry (ITC) and NMR WaterLOGSY experiments mapped the binding site of the CAPs to the well-known drug site II within subdomain IIIA of HSA. Thermodynamic and structural analysis revealed that the binding is exclusively driven by interactions with the hydrophobic moieties of the peptides, and is independent of the cationic residues that are vital for antimicrobial activity. Both of the hydrophobic moieties comprising the peptides were detected to interact with drug site II by NMR saturation transfer difference (STD) group epitope mapping (GEM) and INPHARMA experiments. Molecular models of the complexes between the peptides and albumin were constructed using docking experiments, and support the binding hypothesis and confirm the overall binding affinities of the CAPs.

Conclusions

The biophysical and structural characterizations of albumin-peptide complexes reported here provide detailed insight into how albumin can bind short cationic peptides. The hydrophobic elements of the peptides studied here are responsible for the main interaction with HSA. We suggest that albumin binding should be taken into careful consideration in antimicrobial peptide studies, as the systemic distribution can be significantly affected by HSA interactions.

AVPpred: collection and prediction of highly effective antiviral peptides

In the battle against viruses, antiviral peptides (AVPs) had demonstrated the immense potential. Presently, more than 15 peptide-based drugs are in various stages of clinical trials. Emerging and re-emerging viruses further emphasize the efforts to accelerate antiviral drug discovery efforts. Despite, huge importance of the field, no dedicated AVP resource is available. In the present study, we have collected 1245 peptides which were experimentally checked for antiviral activity targeting important human viruses like influenza, HIV, HCV and SARS, etc. After removing redundant peptides, 1056 peptides were divided into 951 training and 105 validation data sets. We have exploited various peptides sequence features, i.e. motifs and alignment followed by amino acid composition and physicochemical properties during 5-fold cross validation using Support Vector Machine. Physiochemical properties-based model achieved maximum 85% accuracy and 0.70 Matthew’s Correlation Coefficient (MCC). Performance of this model on the experimental validation data set showed 86% accuracy and 0.71 MCC which is far better than the general antimicrobial peptides prediction methods. Therefore, AVPpred—the first web server for predicting the highly effective AVPs would certainly be helpful to researchers working on peptide-based antiviral development. The web server is freely available at http://crdd.osdd.net/servers/avppred.

Identification of bitterness-masking compounds from cheese

Bitterness-masking compounds were identified in a natural white mold cheese. The oily fraction of the cheese was extracted and further fractionated by using silica gel column chromatography. The four fractions obtained were characterized by thin-layer chromatography and nuclear magnetic resonance spectroscopy. The fatty acid-containing fraction was found to have the highest bitterness-masking activity against quinine hydrochloride. Bitterness-masking activity was quantitated using a method based on subjective equivalents. At 0.5 mM, the fatty acid mixture, which had a composition similar to that of cheese, suppressed the bitterness of 0.008% quinine hydrochloride to be equivalent to that of 0.0049-0.0060% and 0.5 mM oleic acid to that of 0.0032-0.0038% solution. The binding potential between oleic acid and the bitter compounds was estimated by isothermal titration calorimetry. These results suggest that oleic acid masked bitterness by forming a complex with the bitter compounds.

Synthesis of anticancer heptapeptides containing a unique lipophilic β(2,2) -amino acid building block

We report a series of synthetic anticancer heptapeptides (H-KKWβ(2,2) WKK-NH(2)) containing eight different central lipophilic β(2,2) -amino acid building blocks, which have demonstrated high efficiency when used as scaffolds in small cationic antimicrobial peptides and peptidomimetics. The most potent peptides in the present study had IC(50) values of 9-23 µm against human Burkitt's lymphoma and murine B-cell lymphoma and were all nonhaemolytic (EC(50)  > 200 µm). The most promising peptide 10e also demonstrated low toxicity against human embryonic lung fibroblast cells and peripheral blood mononuclear cells and exceptional proteolytic stability.