Design, antimicrobial activity and mechanism of action of Arg-rich ultra-short cationic lipopeptides

Antibacterial efficacy and membrane mechanism of action of the Serratia-derived non-ionic lipopeptide, serrawettin W2-FL10

The study aimed to investigate the antibacterial activity, cytotoxicity, and mechanism of action of the non-ionic, cyclic lipopeptide, serrawettin W2-FL10 against Staphylococcus aureus. W2-FL10 exhibited potent activity against the Gram-positive bacteria S. aureus, Enterococcus faecalis, Enterococcus faecium, Listeria monocytogenes, and Bacillus subtilis, with minimum inhibitory concentration (MIC) values ranging from 6.3 to 31.3 μg/mL, while no activity was observed against Gram-negative bacteria. Broth microdilution assays showed that W2-FL10 interacted with key cell membrane components, such as lipid phosphatidyl glycerol and lipoteichoic acid of S. aureus. Upon membrane interaction, W2-FL10 dissipated membrane potential within 12 min and increased S. aureus membrane permeability within 28-40 min, albeit at slower rates and higher concentrations than the lytic peptide melittin. The observed membrane permeability, as detected with propidium iodide (PI), may be attributed to transmembrane pores/lesions, possibly dependent on dimer-driven lipopeptide oligomerization in the membrane. Scanning electron microscopy (SEM) imaging also visually confirmed the formation of lesions in the cell wall of one of the S. aureus strains, and cell damage within 1 h of exposure to W2-FL10, corroborating the rapid time-kill kinetics of the S. aureus strains. This bactericidal action against the S. aureus strains corresponded to membrane permeabilization by W2-FL10, indicating that self-promoted uptake into the cytosol may be part of the mode of action. Finally, this lipopeptide exhibited low to moderate cytotoxicity to the Chinese hamster ovarian (CHO) cell line in comparison to the control (emetine) with an optimal lipophilicity range (log D value of 2.5), signifying its potential as an antibiotic candidate.

IMPORTANCE

Antimicrobial resistance is a major public health concern, urgently requiring antibacterial compounds exhibiting low adverse health effects. In this study, a novel antibacterial lipopeptide analog is described, serrawettin W2-FL10 (derived from Serratia marcescens), with potent activity displayed against Staphylococcus aureus. Mechanistic studies revealed that W2-FL10 targets the cell membrane of S. aureus, causing depolarization and permeabilization because of transmembrane lesions/pores, resulting in the leakage of intracellular components, possible cytosolic uptake of W2-FL10, and ultimately cell death. This study provides the first insight into the mode of action of a non-ionic lipopeptide. The low to moderate cytotoxicity of W2-FL10 also highlights its application as a promising therapeutic agent for the treatment of bacterial infections.

Mechanism of Action of Oxazoline-Based Antimicrobial Polymers Against Staphylococcus aureus: In Vivo Antimicrobial Activity Evaluation

Antimicrobial-resistant pathogens have reached alarming levels, becoming one of the most pressing global health issues. Hence, new treatments are necessary for the fight against antimicrobial resistance. Synthetic nanoengineered antimicrobial polymers (SNAPs) have emerged as a promising alternative to antimicrobial peptides, overcoming some of their limitations while keeping their key features. Herein, a library of amphiphilic oxazoline-based SNAPs using cationic ring-opening polymerization (CROP) is designed. Amphipathic compounds with 70% cationic content exhibit the highest activity against clinically relevant Staphylococcus aureus isolates, maintaining good biocompatibility in vitro and in vivo. The mechanism of action of the lead compounds against S. aureus is assessed using various microscopy techniques, indicating cell membrane disruption, while the cell wall remains unaffected. Furthermore, a potential interaction of the compounds with bacterial DNA is shown, with possible implications on bacterial division. Finally, one of the compounds exhibits high efficacy in vivo in an insect infection model.

Designing peptide amphiphiles as novel antibacterials and antibiotic adjuvants against gram-negative bacteria

Gram-negative strains are intrinsically resistant to most antibiotics due to the robust and impermeable characteristic of their outer membrane. Self-assembling cationic peptide amphiphiles (PAs) have the ability to disrupt bacteria membranes, constituting an excellent antibacterial alternative to small molecule drugs that can be used alone or as antibiotic adjuvants to overcome bacteria resistance. PA1 (C16KHKHK), self-assembled into micelles, which exhibited low antibacterial activity against all strains tested, and showed strong synergistic antibacterial activity in combination with Vancomycin with a Fractional Inhibitory Concentration index (FICi) of 0.15 against E. coli. The molecules, PA2 (C16KRKR) and PA3 (C16AAAKRKR), also self-assembled into micelles, displayed a broad-spectrum antibacterial activity against all strains tested, and low susceptibility to resistance development over 21 days. Finally, PA1, PA 2 and PA3 displayed low cytotoxicity against mammalian cells, and PA2 showed a potent antibacterial activity and low toxicity in preliminary in vivo models using G. mellonella. The results show that PAs are a great platform for the future development of effective antibiotics to slow down the antibiotic resistance and can act as antibiotic adjuvants with synergistic mechanism of action, which can be repurposed for use with existing antibiotics commonly used to treat gram-positive bacteria to treat infections caused by gram-negative bacteria.

Evaluation of the Effect of Antibacterial Peptides on Model Monolayers

The aim of the study was to assess the effect of the synthesized antibacterial peptides: P2 (WKWK)2-KWKWK-NH2, P4 (C12)2-KKKK-NH2, P5 (KWK)2-KWWW-NH2, and P6 (KK)2-KWWW-NH2 on the physicochemical properties of a model biological membrane made of azolectin or lecithin. The Langmuir Wilhelmy method was used for the experiments. Based on the compressibility factor, it was determined that the monolayers formed of azolectin and peptides in the aqueous subphase are in the condensed liquid phase. At the boundary between the condensed and expanded liquid phases, there was a monolayer made of lecithin and P4, P5 or P6 in the aqueous subphase. In turn, the film consisting of lecithin alone (37.7 mN/m) and lecithin and P2 (42.6 mN/m) in the water subphase was in the expanded liquid phase. All peptides change, to varying degrees, the organization and packing of molecules in the monolayer, both those made of azolectin and of lecithin. The test results can be used for further research to design a system with the expected properties for specific organisms.

Structural and biological characterization of shortened derivatives of the cathelicidin PMAP-36

Cathelicidins, a family of host defence peptides in vertebrates, play an important role in the innate immune response, exhibiting antimicrobial activity against many bacteria, as well as viruses and fungi. This work describes the design and synthesis of shortened analogues of porcine cathelicidin PMAP-36, which contain structural changes to improve the pharmacokinetic properties. In particular, 20-mers based on PMAP-36 (residues 12-31) and 13-mers (residues 12-24) with modification of amino acid residues at critical positions and introduction of lipid moieties of different lengths were studied to identify the physical parameters, including hydrophobicity, charge, and helical structure, required to optimise their antibacterial activity. Extensive conformational analysis, performed by CD and NMR, revealed that the substitution of Pro25-Pro26 with Ala25-Lys26 increased the α-helix content of the 20-mer peptides, resulting in broad-spectrum antibacterial activity against Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Staphylococcus epidermidis strains. Interestingly, shortening to just 13 residues resulted in only a slight decrease in antibacterial activity. Furthermore, two sequences, a 13-mer and a 20-mer, did not show cytotoxicity against HaCat cells up to 64 µM, indicating that both derivatives are not only effective but also selective antimicrobial peptides. In the short peptide, the introduction of the helicogenic α-aminoisobutyric acid forced the helix toward a prevailing 310 structure, allowing the antimicrobial activity to be maintained. Preliminary tests of resistance to Ser protease chymotrypsin indicated that this modification resulted in a peptide with an increased in vivo lifespan. Thus, some of the PMAP-36 derivatives studied in this work show a good balance between chain length, antibacterial activity, and selectivity, so they represent a good starting point for the development of even more effective and proteolysis-resistant active peptides.

Ultrashort lipo-tetrapeptide with potent antibacterial activity and local therapeutic effect against Staphylococcus aureus

OBJECTIVES

Mastitis in dairy cows is a common infectious disease on dairy farms and a major danger to the dairy industry. The harmful bacteria with the greatest clinical isolation rate are Staphylococcus aureus. As a result, bacterial mastitis in dairy cows can lead to decreased milk output, quality, and costs. Traditional antibiotics are currently used to treat mastitis in dairy cows. Nonetheless, long-term usage of high doses of antibiotics increases the risk of the establishment of drug-resistant strains, and the problem of drug residues is becoming more prevalent. We investigated the antibacterial effects of varying molecular side chain length lipopeptides on Staphylococcus aureus ATCC25923 and GS1311 using five tetrapeptide ultrashort lipopeptides developed and synthesised in this study.

METHODS

To evaluate the application value of the synthesized lipopeptides in the prevention and treatment of mastitis, the lipopeptides with the best antibacterial action were chosen for safety testing and a mouse mastitis model treatment test.

RESULTS

Three of the lipopeptides produced have strong antibacterial properties. Within the drug's safe concentration range, C16KGGK has an excellent antibacterial action and can have a therapeutic influence on mastitis induced by Staphylococcus aureus infection in mice.

CONCLUSION

The findings of this study can be used to develop new antibacterial medications and their therapeutic application in the treatment of mastitis in dairy cows.

Biodegradable Cationic and Ionizable Cationic Lipids: A Roadmap for Safer Pharmaceutical Excipients

Cationic and ionizable cationic lipids are broadly applied as auxiliary agents, but their use is associated with adverse effects. If these excipients are rapidly degraded to endogenously occurring metabolites such as amino acids and fatty acids, their toxic potential can be minimized. So far, synthesized and evaluated biodegradable cationic and ionizable cationic lipids already showed promising results in terms of functionality and safety. Within this review, an overview about the different types of such biodegradable lipids, the available building blocks, their synthesis and cleavage by endogenous enzymes is provided. Moreover, the relationship between the structure of the lipids and their toxicity is described. Their application in drug delivery systems is critically discussed and placed in context with the lead compounds used in mRNA vaccines. Moreover, their use as preservatives is reviewed, guidance for their design is provided, and an outlook on future developments is given.

Development of novel broad-spectrum antimicrobial lipopeptides derived from plantaricin NC8 β

Bacterial resistance towards antibiotics is a major global health issue. Very few novel antimicrobial agents and therapies have been made available for clinical use during the past decades, despite an increasing need. Antimicrobial peptides have been intensely studied, many of which have shown great promise in vitro. We have previously demonstrated that the bacteriocin Plantaricin NC8 αβ (PLNC8 αβ) from Lactobacillus plantarum effectively inhibits Staphylococcus spp., and shows little to no cytotoxicity towards human keratinocytes. However, due to its limitations in inhibiting gram-negative species, the aim of the present study was to identify novel antimicrobial peptidomimetic compounds with an enhanced spectrum of activity, derived from the β peptide of PLNC8 αβ. We have rationally designed and synthesized a small library of lipopeptides with significantly improved antimicrobial activity towards both gram-positive and gram-negative bacteria, including the ESKAPE pathogens. The lipopeptides consist of 16 amino acids with a terminal fatty acid chain and assemble into micelles that effectively inhibit and kill bacteria by permeabilizing their cell membranes. They demonstrate low hemolytic activity and liposome model systems further confirm selectivity for bacterial lipid membranes. The combination of lipopeptides with different antibiotics enhanced the effects in a synergistic or additive manner. Our data suggest that the novel lipopeptides are promising as future antimicrobial agents, however additional experiments using relevant animal models are necessary to further validate their in vivo efficacy.

Structural insights on the selective interaction of the histidine-rich piscidin antimicrobial peptide Of-Pis1 with membranes

Of-Pis1 is a potent piscidin antimicrobial peptide (AMP), recently isolated from rock bream (Oplegnathus fasciatus). This rich in histidines and glycines 24-amino acid peptide displays high and broad antimicrobial activity and no significant hemolytic toxicity against human erythrocytes, suggesting low toxicity. To better understand the mechanism of action of Of-Pis1 and its potential selectivity, using NMR and CD spectroscopies, we studied the interaction with eukaryotic and procaryotic membranes and membrane models. Anionic sodium dodecyl sulfate (SDS) and lipopolysaccharide (LPS) micelles were used to mimic procaryotic membranes, while zwitterionic dodecyl phosphocholine (DPC) was used as eukaryotic membrane surrogate. In an aqueous environment, Of-Pis1 adopts a flexible random coil conformation. In DPC and SDS instead, the N-terminal region of Of-Pis1 forms an amphipathic α-helix with the non-polar face in close contact with the micelles. Slower solvent exchange and higher pK_as of the histidine residues in SDS than in DPC suggest that Of-Pis1 interacts more tightly with SDS. Of-Pis1 also binds tightly and structurally perturbs LPS micelles. Of-Pis1 interacts with both Escherichia coli and mammalian cell membranes, but only in the presence of Escherichia coli membranes it populates the helical conformation. Furthermore, ligand-based NMR experiments support a tighter and more specific interaction with bacterial than with eukaryotic membranes. Overall, these data clearly show the selective interaction of this broadly active AMP with bacterial over eukaryotic membranes. The conformational information is discussed in terms of Of-Pis1 amino acid sequence and composition to provide insights useful to design more potent and selective AMPs.

Efficacy of natural antimicrobial peptides versus peptidomimetic analogues: a systematic review

Aims: This systematic review was carried out to determine whether synthetic peptidomimetics exhibit significant advantages over antimicrobial peptides in terms of in vitro potency. Structural features - molecular weight, charge and length - were examined for correlations with activity. Methods: Original research articles reporting minimum inhibitory concentration  values against Escherichia coli, indexed until 31 December 2020, were searched in PubMed/ScienceDirect/Google Scholar and evaluated using mixed-effects models. Results: In vitro antimicrobial activity of peptidomimetics resembled that of antimicrobial peptides. Net charge significantly affected minimum inhibitory concentration values (p < 0.001) with a trend of 4.6% decrease for increments in charge by +1. Conclusion: AMPs and antibacterial peptidomimetics exhibit similar potencies, providing an opportunity to exploit the advantageous stability and bioavailability typically associated with peptidomimetics.

An Optimized Workflow for the Discovery of New Antimicrobial Compounds Targeting Bacterial RNA Polymerase Complex Formation

Bacterial resistance represents a major health problem worldwide and there is an urgent need to develop first-in-class compounds directed against new therapeutic targets. We previously developed a drug-discovery platform to identify new antimicrobials able to disrupt the protein–protein interaction between the β’ subunit and the σ⁷⁰ initiation factor of bacterial RNA polymerase, which is essential for transcription. As a follow-up to such work, we have improved the discovery strategy to make it less time-consuming and more cost-effective. This involves three sequential assays, easily scalable to a high-throughput format, and a subsequent in-depth characterization only limited to hits that passed the three tests. This optimized workflow, applied to the screening of 5360 small molecules from three synthetic and natural compound libraries, led to the identification of six compounds interfering with the β’–σ⁷⁰ interaction, and thus was capable of inhibiting promoter-specific RNA transcription and bacterial growth. Upon supplementation with a permeability adjuvant, the two most potent transcription-inhibiting compounds displayed a strong antibacterial activity against Escherichia coli with minimum inhibitory concentration (MIC) values among the lowest (0.87–1.56 μM) thus far reported for β’–σ PPI inhibitors. The newly identified hit compounds share structural feature similarities with those of a pharmacophore model previously developed from known inhibitors.

Strategies to Improve the Activity and Biocompatibility: Modification of Peptide Antibiotics

As host defense peptides, peptide antibiotics exist in almost all organisms. Many of their activities come from their inactivation of bacteria, yeast, fungi, and even cancer cells. However, natural peptide antibiotics are relatively poor in stability and penetration, and have high hemolytic properties, which makes them difficult to directly apply. Therefore, natural peptide antibiotics can be modified to enhance their activity and biocompatibility. Based on the characteristics of amino acids, amino acid substitutions can be performed to study the effect of amino acid types on the activity of peptide antibiotics. The design of ultrashort peptides, cyclic peptides, and self-assembling peptides is also a way to improve the activity of peptide antibiotics. In addition, antibacterial peptides can also be conjugated with antibiotics, lipids, or metal ions to prepare antibacterial peptides with special activities. This review introduces several methods for modifying peptide antibiotics and their specific applications, providing a theoretical basis for the further application of peptide antibiotics.

Induction of Systemic Resistance in Maize and Antibiofilm Activity of Surfactin From Bacillus velezensis MS20

Surfactin lipopeptide is an eco-friendly microbially synthesized bioproduct that holds considerable potential in therapeutics (antibiofilm) as well as in agriculture (antifungal). In the present study, production of surfactin by a marine strain Bacillus velezensis MS20 was carried out, followed by physico-chemical characterization, anti-biofilm activity, plant growth promotion, and quantitative Reverse Transcriptase-Polymerase Chain Reaction (q RT-PCR) studies. From the results, it was inferred that MS20 was found to produce biosurfactant (3,300 mg L-1) under optimized conditions. From the physicochemical characterization [Thin layer chromatography (TLC), Fourier Transform Infrared (FTIR) Spectroscopy, Liquid Chromatography/Mass Spectroscopy (LC/MS), and Polymerase Chain Reaction (PCR) amplification] it was revealed to be surfactin. From bio-assay and scanning electron microscope (SEM) images, it was observed that surfactin (MIC 50 μg Ml-1) has appreciable bacterial aggregation against clinical pathogens Pseudomonas aeruginosa MTCC424, Escherichia coli MTCC43, Klebsiella pneumoniae MTCC9751, and Methicillin resistant Staphylococcus aureus (MRSA) and mycelial condensation property against a fungal phytopathogen Rhizoctonia solani. In addition, the q-RTPCR studies revealed 8-fold upregulation (9.34 ± 0.11-fold) of srfA-A gene compared to controls. Further, treatment of maize crop (infected with R. solani) with surfactin and MS20 led to the production of defense enzymes. In conclusion, concentration and synergy of a carbon source with inorganic/mineral salts can ameliorate surfactin yield and, application wise, it has antibiofilm and antifungal activities. In addition, it induced systemic resistance in maize crop, which makes it a good candidate to be employed in sustainable agricultural practices.

Spermine-Conjugated Short Proline-Rich Lipopeptides as Broad-Spectrum Intracellular Targeting Antibacterial Agents

Toward the design of new proline-rich peptidomimetics, a short peptide segment, present in several proline-rich antimicrobial peptides (AMPs), was selected. Fatty acids of varying lengths and spermine were conjugated at the N- and C-terminals of the peptide, respectively. Spermine-conjugated lipopeptides, C₁₀-PR-Spn and C₁₂-PR-Spn, exhibited minimum inhibitory concentrations within 1.5-6.2 μM against the tested pathogens including resistant bacteria and insignificant hemolytic activity against human red blood cells up to 100 μM concentrations and demonstrated resistance against trypsin digestion. C₁₀-PR-Spn and C₁₂-PR-Spn showed synergistic antimicrobial activity against multidrug-resistant methicillin-resistant Staphylococcus aureus with several tested antibiotics. These lipopeptides did not permeabilize bacterial membrane-mimetic lipid vesicles or damage the Escherichia coli membrane like the nonmembrane-lytic AMP, buforin-II. The results suggested that C₁₀-PR-Spn and C₁₂-PR-Spn could interact with the 70S ribosome of E. coli and inhibit its protein synthesis. C₁₀-PR-Spn and C₁₂-PR-Spn demonstrated superior clearance of bacteria from the spleen, liver, and kidneys of mice, infected with S. aureus ATCC 25923 compared to levofloxacin.

Short Peptides and Their Mimetics as Potent Antibacterial Agents and Antibiotic Adjuvants

Antimicrobial resistance (AMR) has been increasing unrelentingly worldwide, thus negatively impacting human health. The discovery and development of novel antibiotics is an urgent unmet need of the hour. However, it has become more challenging, requiring increasingly time-consuming efforts with increased commercial risks. Hence, alternative strategies are urgently needed to potentiate the existing antibiotics. In this context, short cationic peptides or peptide-based antimicrobials that mimic the activity of naturally occurring antimicrobial peptides (AMPs) could overcome the disadvantages of AMPs having evolved as potent antibacterial agents. Besides their potent antibacterial efficacy, short peptide conjugates have also gained attention as potent adjuvants to conventional antibiotics. Such peptide antibiotic combinations have become an increasingly cost-effective therapeutic option to tackle AMR. This Review summarizes the recent progress for peptide-based small molecules as promising antimicrobials and as adjuvants for conventional antibiotics to counter multidrug resistant (MDR) pathogens.

Gemini Lipopeptide Bearing an Ultrashort Peptide for Enhanced Transfection Efficiency and Cancer-Cell-Specific Cytotoxicity

Cationic gemini lipopeptides are a relatively new class of amphiphilic compounds to be used for gene delivery. Through the possibility of incorporating short peptides with cell-penetrating functionalities, these lipopeptides may be advantageous over traditional cationic lipids. Herein, we report the design, synthesis, and application of a novel cationic gemini lipopeptide for gene delivery. An ultrashort peptide, containing four amino acids, arginine-cysteine-cysteine-arginine, serves as a cationic head group, and two α-tocopherol moieties act as hydrophobic anchoring groups. The new lipopeptide (ATTA) is incorporated into the conventional liposomes, containing 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and 1,2-dioleoyl-sn-glycerol-3-phosphoethanolamine (DOPE), at different molar ratios. The formulated liposomes are characterized and screened for better transfection efficiency. Transfection activity in multiple human cell lines from cancerous and noncancerous origins indicates that the inclusion of an optimal ratio of ATTA in the liposomes substantially enhances the transfection efficiency, superior to that of a traditional liposome, DOTAP-DOPE. Cytotoxicity of ATTA-containing formulations against multiple cell lines indicates potentially distinct activity between cancer and noncancer cell lines. Furthermore, lipoplexes of the ATTA-containing formulations with anticancer therapeutic gene, plasmid encoding tumor necrosis factor-related apoptosis-inducing ligand (pTRAIL), induce obviously more cytotoxicity than conventional formulations. The results indicate that arginine-rich cationic lipopeptide appears to be a promising ingredient in gene delivery vector formulations to enhance transfection efficiency and cell-selective cytotoxicity.

Antimicrobial peptides towards clinical application: Delivery and formulation

Antimicrobial peptides hold promise to supplement small molecules antibiotics and combat the multidrug resistant microbes. There are however technical hurdles towards the clinical applications, largely due to the inherent limitations of peptides including stability, cytotoxicity and bioavailability. Here we review recent studies concerning the delivery and formulation of antimicrobial peptides, by categorizing the different strategies as driven by physical interactions or chemical conjugation reactions, and carriers ranging from inorganic based ones (including gold, silver and silica based solid nanoparticles) to organic ones (including micelle, liposome and hydrogel) are covered. Besides, targeted delivery of antimicrobial peptides or using antimicrobial peptides as the targeting moiety, and responsive release of the peptides after delivery are also reviewed. Lastly, strategies towards the increase of oral bioavailability, from both physical or chemical methods, are highlighted. Altogether, this article provides a comprehensive review of the recent progress of the delivery and formulation of antimicrobial peptides towards clinical application.

Effects of Lipidation on a Proline-Rich Antibacterial Peptide

The emergence of multidrug-resistant bacteria is a worldwide health problem. Antimicrobial peptides have been recognized as potential alternatives to conventional antibiotics, but still require optimization. The proline-rich antimicrobial peptide Bac7(1-16) is active against only a limited number of Gram-negative bacteria. It kills bacteria by inhibiting protein synthesis after its internalization, which is mainly supported by the bacterial transporter SbmA. In this study, we tested two different lipidated forms of Bac7(1-16) with the aim of extending its activity against those bacterial species that lack SbmA. We linked a C12-alkyl chain or an ultrashort cationic lipopeptide Lp-I to the C-terminus of Bac7(1-16). Both the lipidated Bac-C12 and Bac-Lp-I forms acquired activity at low micromolar MIC values against several Gram-positive and Gram-negative bacteria. Moreover, unlike Bac7(1-16), Bac-C12, and Bac-Lp-I did not select resistant mutants in E. coli after 14 times of exposure to sub-MIC concentrations of the respective peptide. We demonstrated that the extended spectrum of activity and absence of de novo resistance are likely related to the acquired capability of the peptides to permeabilize cell membranes. These results indicate that C-terminal lipidation of a short proline-rich peptide profoundly alters its function and mode of action and provides useful insights into the design of novel broad-spectrum antibacterial agents.

Trends in liposomal nanocarrier strategies for the oral delivery of biologics

The number of approved macromolecular drugs such as peptides, proteins and antibodies steadily increases. Since drugs with high molecular weight are commonly not suitable for oral delivery, research on carrier strategies enabling oral administration is of vital interest. In past decades, nanocarriers, in particular liposomes, have been exhaustively investigated as oral drug-delivery platform. Despite their successful application as parenteral delivery vehicles, liposomes have up to date not succeeded for oral administration. However, a plenitude of approaches aiming to increase the oral bioavailability of macromolecular drugs administered by liposomal formulations has been published. Here, we summarize the strategies published in the last 10 years (vaccine strategies excluded) with a main focus on strategies proven efficient in animal models.

Biological and Physico-Chemical Characteristics of Arginine-Rich Peptide Gemini Surfactants with Lysine and Cystine Spacers

Ultrashort cationic lipopeptides (USCLs) and gemini cationic surfactants are classes of potent antimicrobials. Our recent study has shown that the branching and shortening of the fatty acids chains with the simultaneous addition of a hydrophobic N-terminal amino acid in USCLs result in compounds with enhanced selectivity. Here, this approach was introduced into arginine-rich gemini cationic surfactants. l-cystine diamide and l-lysine amide linkers were used as spacers. Antimicrobial activity against planktonic and biofilm cultures of ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) strains and Candida sp. as well as hemolytic and cytotoxic activities were examined. Moreover, antimicrobial activity in the presence of human serum and the ability to form micelles were evaluated. Membrane permeabilization study, serum stability assay, and molecular dynamics were performed. Generally, critical aggregation concentration was linearly correlated with hydrophobicity. Gemini surfactants were more active than the parent USCLs, and they turned out to be selective antimicrobial agents with relatively low hemolytic and cytotoxic activities. Geminis with the l-cystine diamide spacer seem to be less cytotoxic than their l-lysine amide counterparts, but they exhibited lower antibiofilm and antimicrobial activities in serum. In some cases, geminis with branched fatty acid chains and N-terminal hydrophobic amino acid resides exhibited enhanced selectivity to pathogens over human cells.

DBAASP v3: database of antimicrobial/cytotoxic activity and structure of peptides as a resource for development of new therapeutics

The Database of Antimicrobial Activity and Structure of Peptides (DBAASP) is an open-access, comprehensive database containing information on amino acid sequences, chemical modifications, 3D structures, bioactivities and toxicities of peptides that possess antimicrobial properties. DBAASP is updated continuously, and at present, version 3.0 (DBAASP v3) contains >15 700 entries (8000 more than the previous version), including >14 500 monomers and nearly 400 homo- and hetero-multimers. Of the monomeric antimicrobial peptides (AMPs), >12 000 are synthetic, about 2700 are ribosomally synthesized, and about 170 are non-ribosomally synthesized. Approximately 3/4 of the entries were added after the initial release of the database in 2014 reflecting the recent sharp increase in interest in AMPs. Despite the increased interest, adoption of peptide antimicrobials in clinical practice is still limited as a consequence of several factors including side effects, problems with bioavailability and high production costs. To assist in developing and optimizing de novo peptides with desired biological activities, DBAASP offers several tools including a sophisticated multifactor analysis of relevant physicochemical properties. Furthermore, DBAASP has implemented a structure modelling pipeline that automates the setup, execution and upload of molecular dynamics (MD) simulations of database peptides. At present, >3200 peptides have been populated with MD trajectories and related analyses that are both viewable within the web browser and available for download. More than 400 DBAASP entries also have links to experimentally determined structures in the Protein Data Bank. DBAASP v3 is freely accessible at http://dbaasp.org.

Lipidation of Antimicrobial Peptides as a Design Strategy for Future Alternatives to Antibiotics

Multi-drug-resistant bacteria are becoming more prevalent, and treating these bacteria is becoming a global concern. One alternative approach to combat bacterial resistance is to use antimicrobial (AMPs) or host-defense peptides (HDPs) because they possess broad-spectrum activity, function in a variety of ways, and lead to minimal resistance. However, the therapeutic efficacy of HDPs is limited by a number of factors, including systemic toxicity, rapid degradation, and low bioavailability. One approach to circumvent these issues is to use lipidation, i.e., the attachment of one or more fatty acid chains to the amine groups of the N-terminus or a lysine residue of an HDP. In this review, we examined lipidated analogs of 66 different HDPs reported in the literature to determine: (i) whether there is a link between acyl chain length and antibacterial activity; (ii) whether the charge and (iii) the hydrophobicity of the HDP play a role; and (iv) whether acyl chain length and toxicity are related. Overall, the analysis suggests that lipidated HDPs with improved activity over the nonlipidated counterpart had acyl chain lengths of 8–12 carbons. Moreover, active lipidated peptides attached to short HDPs tended to have longer acyl chain lengths. Neither the charge of the parent HDP nor the percent hydrophobicity of the peptide had an apparent significant impact on the antibacterial activity. Finally, the relationship between acyl chain length and toxicity was difficult to determine due to the fact that toxicity is quantified in different ways. The impact of these trends, as well as combined strategies such as the incorporation of d- and non-natural amino acids or alternative approaches, will be discussed in light of how lipidation may play a role in the future development of antimicrobial peptide-based alternatives to current therapeutics.

Imaging and Targeted Antibacterial Therapy Using Chimeric Antimicrobial Peptide Micelles

Infectious diseases induced by multidrug-resistant bacteria are a challenging problem in medicine because of global rise in the drug resistance to pathogenic bacteria. Despite great efforts on the development of antibiotics and antimicrobial agents, there is still a great need to develop a strategy to early detect bacterial infections and eradicate bacteria effectively and simultaneously. The innate immune systems of various organisms produce antimicrobial peptides, which kill a broad range of bacteria with minimal cytotoxicity to mammalian cells. Therefore, antimicrobial peptides have recently attracted increasing attention as an alternative to conventional antibiotics in antibacterial medications. Here, we report a new family of antibacterial agents, which is formulated from self-assembly of a chimeric antimicrobial lipopeptide (DSPE-HnMc) and amphiphilic biodegradable polymers. HnMc micelles could effectively bind the bacterial membrane to kill a wide spectrum of bacteria and bacterial biofilms. In the studies of mouse models of drug-resistant bacterial infections, HnMc micelles could target bacterial infections with high specificity and also kill drug-resistant bacteria effectively, demonstrating the great potential of HnMc micelles as imaging and targeted antibacterial agents. These findings also provide new insight into the design of antimicrobial peptide-based nanomedicine for detection and treatment of bacterial infections.

Effect of Disulfide Cyclization of Ultrashort Cationic Lipopeptides on Antimicrobial Activity and Cytotoxicity

Ultrashort cationic lipopeptides (USCLs) are considered to be a promising class of antimicrobials with high activity against a broad-spectrum of microorganisms. However, the majority of these compounds are characterized by significant toxicity toward human cells, which hinders their potential application. To overcome those limitations, several approaches have been advanced. One of these is disulfide cyclization that has been shown to improve drug-like characteristics of peptides. In this article the effect of disulfide cyclization of the polar head of N-palmitoylated USCLs on in vitro biological activity has been studied. Lipopeptides used in this study consisted of three or four basic amino acids (lysine and arginine) and cystine in a cyclic peptide. In general, disulfide cyclization of the lipopeptides resulted in peptides with reduced cytotoxicity. Disulfide-cyclized USCLs exhibited improved selectivity between Candida sp., Gram-positive strains and normal cells in contrast to their linear counterparts. Interactions between selected USCLs and membranes were studied by molecular dynamics simulations using a coarse-grained force field. Moreover, membrane permeabilization properties and kinetics were examined. Fluorescence and transmission electron microscopy revealed damage to Candida cell membrane and organelles. Concluding, USCLs are strong membrane disruptors and disulfide cyclization of polar head can have a beneficial effect on its in vitro selectivity between Candida sp. and normal human cells.

Vancomycin-Lipopeptide Conjugates with High Antimicrobial Activity on Vancomycin-Resistant Enterococci

Multidrug-resistant bacteria represent one of the most important health care problems worldwide. While there are numerous drugs available for standard therapy, there are only a few compounds capable of serving as a last resort for severe infections. Therefore, approaches to control multidrug-resistant bacteria must be implemented. Here, a strategy of reactivating the established glycopeptide antibiotic vancomycin by structural modification with polycationic peptides and subsequent fatty acid conjugation to overcome the resistance of multidrug-resistant bacteria was followed. This study especially focuses on the structure-activity relationship, depending on the modification site and fatty acid chain length. The synthesized conjugates showed high antimicrobial potential on vancomycin-resistant enterococci. We were able to demonstrate that the antimicrobial activity of the vancomycin-lipopeptide conjugates depends on the chain length of the attached fatty acid. All conjugates showed good cytocompatibility in vitro and in vivo. Radiolabeling enabled the in vivo determination of pharmacokinetics in Wistar rats by molecular imaging and biodistribution studies. An improved biodistribution profile in comparison to unmodified vancomycin was observed. While vancomycin is rapidly excreted by the kidneys, the most potent conjugate shows a hepatobiliary excretion profile. In conclusion, these results demonstrate the potential of the structural modification of already established antibiotics to provide highly active compounds for tackling multidrug-resistant bacteria.

Polymorphic Self-Organization of Lauroyl Peptide in Response to pH and Concentration

Amphipathic peptides are attractive building blocks for the preparation of self-assembling, bio-inspired, and stimuli responsive nanomaterials with pharmaceutical interest. The bioavailability of these materials can be improved with the insertion of d amino acid residues to avoid fast proteolysis in vivo. With this knowledge, a new lauroyl peptide consisting of a sequence of glycine, glycine, d-serine, and d-lysine was designed. In spite of its simple sequence, this lipopeptide self-assembles into spherical micelles at acid pH, when the peptide moiety adopts disordered conformations. Self-aggregates reshape toward fibers at basic pH, following the conformational transition of the peptide region from random coil to β-sheet. Finally, hydrogels are achieved at basic pH and higher concentrations. The transition from random coil to β-sheet conformation of the peptide headgroup obtained by increasing pH was monitored by circular dichroism and vibrational spectroscopy. A structural analysis, performed by combining dynamic light scattering, small-angle X-ray scattering, transmission electron microscopy, and molecular dynamic simulations, demonstrated that the transition allows the self-assemblies to remodel from spherical micelles to rodlike shapes, to long fibers with rectangular cross-section and a head-tail-tail-head structure. The viscoelastic behavior of the hydrogels formed at the highest pH was investigated by rheology measurements.

Ultrashort Cationic Lipopeptides-Effect of N-Terminal Amino Acid and Fatty Acid Type on Antimicrobial Activity and Hemolysis

Ultrashort cationic lipopeptides (USCLs) are promising antimicrobial agents that hypothetically may be alternatively used to combat pathogens such as bacteria and fungi. In general, USCLs consist of fatty acid chains and a few basic amino acid residues. The main shortcoming of USCLs is their relatively high cytotoxicity and hemolytic activity. This study focuses on the impact of the hydrophobic fatty acid chain, on both antimicrobial and hemolytic activities. To learn more about this region, a series of USCLs with different straight-chain fatty acids (C8, C10, C12, C14) attached to the tripeptide with two arginine residues were synthesized. The amino acid at the N-terminal position was exchanged for proteinogenic and non-proteinogenic amino acid residues (24 in total). Moreover, the branched fatty acid residues were conjugated to N-terminus of a dipeptide with two arginine residues. All USCLs had C-terminal amides. USCLs were tested against reference bacterial strains (including ESKAPE group) and Candida albicans. The hemolytic potential was tested on human erythrocytes. Hydrophobicity of the compounds was evaluated by RP-HPLC. Shortening of the fatty acid chain and simultaneous addition of amino acid residue at N-terminus were expected to result in more selective and active compounds than those of the reference lipopeptides with similar lipophilicity. Hypothetically, this approach would also be beneficial to other antimicrobial peptides where N-lipidation strategy was used to improve their biological characteristics.