Self-Entrapment of Antimicrobial Peptides in Silica Particles for Stable and Effective Antimicrobial Peptide Delivery System

Antimicrobial peptides (AMPs) have emerged as a promising solution to tackle bacterial infections and combat antibiotic resistance. However, their vulnerability to protease degradation and toxicity towards mammalian cells has hindered their clinical application. To overcome these challenges, our study aims to develop a method to enhance the stability and safety of AMPs applicable to effective drug-device combination products. The KR12 antimicrobial peptide was chosen, and in order to further enhance its delivery and efficacy the human immunodeficiency virus TAT protein-derived cell-penetrating peptide (CPP) was fused to form CPP-KR12. A new product, CPP-KR12@Si, was developed by forming silica particles with self-entrapped CPP-KR12 peptide using biomimetic silica precipitability because of its cationic nature. Peptide delivery from CPP-KR12@Si to bacteria and cells was observed at a slightly delivered rate, with improved stability against trypsin treatment and a reduction in cytotoxicity compared to CPP-KR12. Finally, the antimicrobial potential of the CPP-KR12@Si/bone graft substitute (BGS) combination product was demonstrated. CPP-KR12 is coated in the form of submicron-sized particles on the surface of the BGS. Self-entrapped AMP in silica nanoparticles is a safe and effective AMP delivery method that will be useful for developing a drug-device combination product for tissue regeneration.

The Antimicrobial, Hemostatic, and Anti-Adhesion Effects of a Peptide Hydrogel Constructed by the All-d-Enantiomer of Antimicrobial Peptide Jelleine-1

Peptide hydrogels are believed to be potential biomaterials with wide application in the biomedical field because of their good biocompatibility, injectability, and 3D printability. Most of the previously reported polypeptide hydrogels are composed of l-peptides, while the hydrogels formed by self-assembly of d-peptides are rarely reported. Herein, a peptide hydrogel constructed by D-J-1, which is the all-d-enantiomer of antimicrobial peptide Jelleine-1 (J-1) is reported. Field emission scanning electron microscope (FE-SEM) and rheologic study are performed to characterize the hydrogel. Antimicrobial, hemostatic, and anti-adhesion studies are carried out to evaluate its biofunction. The results show that D-J-1 hydrogel is formed by self-assembly and cross-linking driven by hydrogen bonding, hydrophobic interaction, and π-π stacking force of aromatic ring in the structure of D-J-1. It exhibits promising antimicrobial activity, hemostatic activity, and anti-adhesion efficiency in a rat sidewall defect-cecum abrasion model. In addition, it also exhibits good biocompatibility. Notably, D-J-1 hydrogel shows improved in vitro and in vivo stability when compared with its l-enantiomer J-1 hydrogel. Therefore, the present study will provide new insight into the application of d-peptide hydrogel, and provides a new peptide hydrogel with antibacterial, hemostatic, and anti-adhesion efficacy for clinical use.

Molecular hybridization strategy for tuning bioactive peptide function

The physicochemical and structural properties of antimicrobial peptides (AMPs) determine their mechanism of action and biological function. However, the development of AMPs as therapeutic drugs has been traditionally limited by their toxicity for human cells. Tuning the physicochemical properties of such molecules may abolish toxicity and yield synthetic molecules displaying optimal safety profiles and enhanced antimicrobial activity. Here, natural peptides were modified to improve their activity by the hybridization of sequences from two different active peptide sequences. Hybrid AMPs (hAMPs) were generated by combining the amphipathic faces of the highly toxic peptide VmCT1, derived from scorpion venom, with parts of four other naturally occurring peptides having high antimicrobial activity and low toxicity against human cells. This strategy led to the design of seven synthetic bioactive variants, all of which preserved their structure and presented increased antimicrobial activity (3.1-128 μmol L-1). Five of the peptides (three being hAMPs) presented high antiplasmodial at 0.8 μmol L-1, and virtually no undesired toxic effects against red blood cells. In sum, we demonstrate that peptide hybridization is an effective strategy for redirecting biological activity to generate novel bioactive molecules with desired properties.

Going Beyond Host Defence Peptides: Horizons of Chemically Engineered Peptides for Multidrug-Resistant Bacteria

Multidrug-resistant (MDR) bacteria are considered a health threat worldwide, and this problem is set to increase over the decades. The ESKAPE, a group of six pathogens including Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter spp. is the major source of concern due to their high death incidence and nosocomial acquired infection. Host defence peptides (HDPs) are a class of ribosomally synthesised peptides that have shown promising results in combating MDR, including the ESKAPE group, in- and outside bacterial biofilms. However, their poor pharmacokinetics in physiological mediums may impede HDPs from becoming viable clinical candidates. To circumvent this problem, chemical engineering of HDPs has been seen as an emergent approach to not only improve their pharmacokinetics but also their efficacy against pathogens. In this review, we explore several chemical modifications of HDPs that have shown promising results, especially against ESKAPE pathogens, and provide an overview of the current findings with respect to each modification.

Challenges and advances in antimicrobial peptide development

Microbial resistance is a major concern for public health worldwide, mainly because of the inappropriate use of antimicrobials. In this scenario, antimicrobial peptides (AMPs) have emerged as a potential therapeutic alternative means by which to control infectious diseases, because of their broad spectrum of action. However, some challenges can make their clinical application problematic, including metabolic instability and toxicity. Here, we provide a clear description of AMPs as promising molecules for the development of unusual antimicrobial drugs. We also describe current strategies used to overcome the main difficulties related to AMP clinical application, including different peptide designs and nanoformulation.

Boosting stability and therapeutic potential of proteolysis-resistant antimicrobial peptides by end-tagging β-naphthylalanine

Recently, much emphasis has been placed on solving the intrinsic defects of antimicrobial peptides (AMPs), especially their susceptibility to protease digestion for the systemic application of antibacterial biomaterials. Although many strategies have increased the protease stability of AMPs, antimicrobial activity was severely compromised, thereby substantially weakening their therapeutic effect. To address this issue, we introduced hydrophobic group modifications at the N-terminus of proteolysis-resistant AMPs D1 (AArIIlrWrFR) through end-tagging with stretches of natural amino acids (W and I), unnatural amino acid (Nal) and fatty acids. Of these peptides, N1 tagged with a Nal at N-terminus showed the highest selectivity index (GM_SI = 19.59), with a 6.73-fold improvement over D1. In addition to potent broad-spectrum antimicrobial activity, N1 also exhibited high antimicrobial stability toward salts, serum and proteases in vitro and ideal biocompatibility and therapeutic efficacy in vivo. Furthermore, N1 killed bacteria through multiple mechanisms, involving disruption of bacterial membranes and inhibition of bacterial energy metabolism. Indeed, appropriate terminal hydrophobicity modification opens up new avenues for developing and applying high-stability peptide-based antibacterial biomaterials. STATEMENT OF SIGNIFICANCE: To improve the potency and stability of proteolysis-resistant antimicrobial peptides (AMPs) without increasing toxicity, we constructed a convenient and tunable platform based on different compositions and lengths of hydrophobic end modifications. By tagging an Nal at the N-terminal, the obtained target compound N1 exhibited strong antimicrobial activity and desirable stability under multifarious environments in vitro (protease, salts and serum), and also showed favorable biocompatibility and therapeutic efficacy in vivo. Notably, N1exerted its bactericidal effect by damaging bacterial cell membranes and inhibiting bacterial energy metabolism in a dual mode. The findings provide a potential method for designing or optimizing proteolysis-resistant AMPs thus promoting the development and application of peptide-based antibacterial biomaterial.

Engineering Approaches for the Development of Antimicrobial Peptide-Based Antibiotics

Antimicrobial peptides (AMPs) have received increasing attention as potential alternatives for future antibiotics because of the rise of multidrug-resistant (MDR) bacteria. AMPs are small cationic peptides with broad-spectrum antibiotic activities and different action mechanisms to those of traditional antibiotics. Despite the desirable advantages of developing peptide-based antimicrobial agents, the clinical applications of AMPs are still limited because of their enzymatic degradation, toxicity, and selectivity. In this review, structural modifications, such as amino acid substitution, stapling, cyclization of peptides, and hybrid AMPs with conventional antibiotics or other peptides, will be presented. Additionally, nanodelivery systems using metals or lipids to deliver AMPs will be discussed based on the structural properties and action mechanisms of AMPs.

Discovery of a Novel Antimicrobial Peptide, Temporin-PKE, from the Skin Secretion of Pelophylax kl. esculentus, and Evaluation of Its Structure-Activity Relationships

Bacterial resistance against antibiotics has led to increasing numbers of treatment failures, and AMPs are widely accepted as becoming potential alternatives due to their advantages. Temporin-PKE is a novel peptide extracted from the skin secretion of Pelophylax kl. esculentus and it displays a strong activity against Gram-positive bacteria, with an extreme cytotoxicity. Incorporating positively charged residues and introducing D-amino acids were the two main strategies adopted for the modifications. The transformation of the chirality of Ile could reduce haemolytic activity, and an analogue with appropriate D-isoforms could maintain antimicrobial activity and stability. The substitution of hydrophobic residues could bring about more potent and broad-spectrum antimicrobial activities. The analogues with Lys were less harmful to the normal cells and their stabilities remained at similarly high levels compared to temporin-PKE. The optimal number of charges was three, and the replacement on the polar face was a better choice. Temporin-PKE-3K exerted dually efficient functions includingstrong antimicrobial and anticancer activity. This analogue showed a reduced possibility for inducing resistance in MRSA and Klebsiella pneumoniae, a rather strong antimicrobial activity in vivo, and it exhibited the highest therapeutic index such that temporin-PKE-3K has the potential to be developed as a clinical drug.

Strategies employed in the design of antimicrobial peptides with enhanced proteolytic stability

Due to the alarming developing rate of multidrug-resistant bacterial pathogens, the development and modification of antimicrobial peptides (AMPs) are unprecedentedly active. Despite the fact that considerable efforts have been expended on the discovery and design strategies of AMPs, the clinical translation of peptide antibiotics remains inadequate. A large number of articles and reviews credited the limited success of AMPs to their poor stability in the biological environment, particularly their poor proteolytic stability. In the past forty years, various design strategies have been used to improve the proteolytic stability of AMPs, such as sequence modification, cyclization, peptidomimetics, and nanotechnology. Herein, we focus our discussion on the progress made in improving the proteolytic stability of AMPs and the principle, successes, and limitations of various anti-proteolytic design strategies. It is of prospective significance to extend current insights into the degradation-related inactivation of AMPs and also alleviate/overcome the problem.

Serum Stable and Low Hemolytic Temporin-SHa Peptide Analogs Disrupt Cell Membrane of Methicillin-Resistant Staphylococcus aureus (MRSA)

Anti-microbial peptides (AMPs) have attracted major attention due to their potential bio-activities against some multidrug resistant pathogens. The present study evaluated the mechanism of actions of highly potent AMP temporin-SHa analogs, i.e., [G4a]-SHa, [G7a]-SHa, and [G10a]-SHa, against methicillin-resistant Staphylococcus aureus (MRSA) NCTC (13277) with minimum inhibitory concentrations (MICs) of 14.35, 7.16, and 3.58 µM, respectively. These analogs exhibited significant anti-MRSA activity at physiological salt concentration, 30% fetal bovine serum, and 30% human serum. [G4a]-SHa and [G7a]-SHa were non-hemolytic and non-cytotoxic to normal mouse fibroblast 3T3 cell and human Caco-2 cell line. Atomic force microscopy revealed that these analogs have profound effect on the morphological changes in MRSA surface with significant leakage of cell cytoplasmic content. Propidium iodide uptake kinetic assay and (bis-(1,3-dibutylbarbituric acid) trimethine oxonol) DiBAC₄(3) membrane depolarization assay demonstrated that these analogs display a membrane disrupting property, characterized by elevation of plasma membrane permeability and rapid transmembrane potential depolarization. [G10a]-SHa showed a significant anti-biofilm activity against biofilm forming S. aureus (ATCC 6538). Acute in vivo toxicity studies revealed that [G10a]-SHa possesses some toxic effect at 100-mg/kg dose. While [G4a]-SHa at 100 mg/kg, i.p. has no toxic effect even after 48 h, [G7a]-SHa also did not show any toxic effect at the dose of 100 mg/kg, i.p. during 24-h observation of animals. In conclusion, [G4a]-SHa, [G7a]-SHa, and [G10a]-SHa show improved activity against MRSA and stability compared to SHa peptide. Although highly potent, [G10a]-SHa, due to its hemolytic activity, might be more suitable for topical application, whereas [G4a]-SHa and [G7a]-SHa have potential to be used for systemic application.

Zwitterionic Peptides Reduce Accumulation of Marine and Freshwater Biofilm Formers

Zwitterionic peptides are facile low-fouling compounds for environmental applications as they are biocompatible and fully biodegradable as their degradation products are just amino acids. Here, a set of histidine (H) and glutamic acid (E), as well as lysine (K) and glutamic acid (E) based peptide sequences with zwitterionic properties were synthesized. Both oligopeptides (KE)₄K and (HE)₄H were synthesized in d and l configurations to test their ability to resist the nonspecific adsorption of the proteins lysozyme and fibrinogen. The coatings were additionally tested against the attachment of the marine organisms Navicula perminuta and Cobetia marina as well as the freshwater bacterium Pseudomonas fluorescens on the developed coatings. While the peptides containing lysine performed better in protein resistance assays and against freshwater bacteria, the sequences containing histidine were generally more resistant against marine organisms. The contribution of amino acid-intrinsic properties such as side chain pK_a values and hydrophobicity, as well as external parameters such as pH and salinity of fresh water and seawater on the resistance of the coatings is discussed. In this way, a detailed picture emerges as to which zwitterionic sequences show advantages in future generations of biocompatible, sustainable, and nontoxic fouling release coatings.

Effect of L- to D-Amino Acid Substitution on Stability and Activity of Antitumor Peptide RDP215 against Human Melanoma and Glioblastoma

The study investigates the antitumor effect of two cationic peptides, R-DIM-P-LF11-215 (RDP215) and the D-amino acid variant 9D-R-DIM-P-LF11-215 (9D-RDP215), targeting the negatively charged lipid phosphatidylserine (PS) exposed by cancer cells, such as of melanoma and glioblastoma. Model studies mimicking cancer and non-cancer membranes revealed the specificity for the cancer-mimic PS by both peptides with a slightly stronger impact by the D-peptide. Accordingly, membrane effects studied by DSC, leakage and quenching experiments were solely induced by the peptides when the cancer mimic PS was present. Circular dichroism revealed a sole increase in β-sheet conformation in the presence of the cancer mimic for both peptides; only 9D-RDP215 showed increased structure already in the buffer. Ex vitro stability studies by SDS-PAGE as well as in vitro with melanoma A375 revealed a stabilizing effect of D-amino acids in the presence of serum, which was also confirmed in 2D and 3D in vitro experiments on glioblastoma LN-229. 9D-RDP215 was additionally able to pass a BBB model, whereupon it induced significant levels of cell death in LN-229 spheroids. Summarized, the study encourages the introduction of D-amino acids in the design of antitumor peptides for the improvement of their stable antitumor activity.

Antimicrobial and Anticancer Properties of Synthetic Peptides Derived from the Wasp Parachartergus fraternus

Agelaia-MPI and protonectin are antimicrobial peptides isolated from the wasp Parachartergus fraternus that show antimicrobial and neuroactive activities. Previously, two analogues of these peptides, neuroVAL and protonectin-F, were designed to reduce nonspecific toxicity and improve potency. Here, the three-dimensional structures of neuroVAL, protonectin and protonectin-F were determined by using circular dichroism and NMR spectroscopy. Antibacterial, antifungal, cytotoxic and hemolytic activities were tested for the parent peptides and analogues. All peptides showed moderate antimicrobial activity against Gram-positive bacteria, with agelaia-MPI being the most active. Protonectin and protonectin-F were found to be toxic to cancerous and noncancerous cell lines. Internalization experiments revealed that these peptides accumulate inside both cell types. By contrast, neuroVAL was nontoxic to all tested cells and was able to enter cells without accumulating. In summary, neuroVAL has potential as a nontoxic cell-penetrating peptide, while protonectin-F needs further modification to realize its potential as an antitumor peptide.

Self-Assembling, Ultrashort Peptide Gels as Antimicrobial Biomaterials

Supramolecular antimicrobial hydrogels based on peptides are attractive soft materials for the treatment of infections, considering their ease of preparation and benign fate in biological settings and in the environment. In particular, stimuli-responsive systems that can be assembled/disassembled ad hoc could offer the opportunity to switch on/off their bioactivity as needed. Besides, the shorter is the peptide, the lower its cost of production. However, a structure-to-function relationship is yet to be defined and reported activities are generally not yet competitive relative to traditional antibiotics. Inspiration for their design can be found in host defense peptides (HDPs), which can self-assemble to exert their function. This article reviews research developments in this emerging area, and it examines features, differences and similarities between antimicrobial and amyloid peptides to open the avenue towards the next generation of supramolecular antimicrobial peptides as innovative therapeutic materials.

D- and Unnatural Amino Acid Substituted Antimicrobial Peptides With Improved Proteolytic Resistance and Their Proteolytic Degradation Characteristics

The transition of antimicrobial peptides (AMPs) from the laboratory to market has been severely hindered by their instability toward proteases in biological systems. In the present study, we synthesized derivatives of the cationic AMP Pep05 (KRLFKKLLKYLRKF) by substituting L-amino acid residues with D- and unnatural amino acids, such as D-lysine, D-arginine, L-2,4-diaminobutanoic acid (Dab), L-2,3-diaminopropionic acid (Dap), L-homoarginine, 4-aminobutanoic acid (Aib), and L-thienylalanine, and evaluated their antimicrobial activities, toxicities, and stabilities toward trypsin, plasma proteases, and secreted bacterial proteases. In addition to measuring changes in the concentration of the intact peptides, LC-MS was used to identify the degradation products of the modified AMPs in the presence of trypsin and plasma proteases to determine degradation pathways and examine whether the amino acid substitutions afforded improved proteolytic resistance. The results revealed that both D- and unnatural amino acids enhanced the stabilities of the peptides toward proteases. The derivative DP06, in which all of the L-lysine and L-arginine residues were replaced by D-amino acids, displayed remarkable stability and mild toxicity in vitro but only slight activity and severe toxicity in vivo, indicating a significant difference between the in vivo and in vitro results. Unexpectedly, we found that the incorporation of a single Aib residue at the N-terminus of compound UP09 afforded remarkably enhanced plasma stability and improved activity in vivo. Hence, this derivative may represent a candidate AMP for further optimization, providing a new strategy for the design of novel AMPs with improved bioavailability.

The Best Peptidomimetic Strategies to Undercover Antibacterial Peptides

Health-care systems that develop rapidly and efficiently may increase the lifespan of humans. Nevertheless, the older population is more fragile, and is at an increased risk of disease development. A concurrently growing number of surgeries and transplantations have caused antibiotics to be used much more frequently, and for much longer periods of time, which in turn increases microbial resistance. In 1945, Fleming warned against the abuse of antibiotics in his Nobel lecture: "The time may come when penicillin can be bought by anyone in the shops. Then there is the danger that the ignorant man may easily underdose himself and by exposing his microbes to non-lethal quantities of the drug make them resistant". After 70 years, we are witnessing the fulfilment of Fleming's prophecy, as more than 700,000 people die each year due to drug-resistant diseases. Naturally occurring antimicrobial peptides protect all living matter against bacteria, and now different peptidomimetic strategies to engineer innovative antibiotics are being developed to defend humans against bacterial infections.

Low Fouling Peptides with an All (d) Amino Acid Sequence Provide Enhanced Stability against Proteolytic Degradation While Maintaining Low Antifouling Properties

Peptide-functionalized surfaces, composed of optimized l-peptides, show a high resistance toward nonspecific adsorption of proteins. As l-peptides are known to be prone to proteolytic degradation, the aim of this work is to enhance the stability against enzymatic degradation by using the all d-peptide mirror image of the optimized l-peptides and to determine if the all d-enantiomer retains the protein-resistant and antifouling properties. Two l-peptides and their d-peptide mirror images, some of them containing the nonproteinogenic amino acid α-aminoisobutyric acid (Aib), were synthesized and tested against non-specific adsorption of the proteins lysozyme and fibrinogen and the settlement of marine diatom Navicula perminuta and marine bacteria Cobetia marina. Both the d-enantiomer and the insertion of Aib protected the peptides from proteolytic degradation. Protein resistance was enhanced with the d-enantiomers while maintaining the resistance toward diatoms.

Antimicrobial Peptides with High Proteolytic Resistance for Combating Gram-Negative Bacteria

Poor proteolytic resistance is an urgent problem to be solved in the clinical application of antimicrobial peptides (AMPs), yet common solutions, such as complicated chemical modifications and utilization of d-amino acids, greatly increase the difficulty and cost of producing AMPs. In this work, a set of novel peptides was synthesized based on an antitrypsin/antichymotrypsin hydrolytic peptide structure unit (XYPX) _n (X represents I, L, and V; Y represents R and K), which was designed using a systematic natural amino acid arrangement. Of these peptides, 16 with seven repeat units had the highest average selectivity index (GM_SI = 99.07) for all of the Gram-negative bacteria tested and remained highly effective in combating Escherichia coli infection in vivo. Importantly, 16 also had dramatic resistance to a high concentration of trypsin/chymotrypsin hydrolysis and exerted bactericidal activity through a membrane-disruptive mechanism. Overall, these findings provide new approaches for the development of antiprotease hydrolytic peptides that target Gram-negative bacteria.

Synthesis of breast cancer targeting conjugate of temporin-SHa analog and its effect on pro- and anti-apoptotic protein expression in MCF-7 cells

The frog natural product temporin-SHa (FLSGIVGMLGKLF_amide) is a potent antimicrobial peptide, as is the analog [S3K]SHa. By solid-phase synthesis, we prepared temporin-SHa and several temporin-SHa analogs with one or more D-alanine residues incorporated. The natural product and the analog [G10a]SHa were found to be cytotoxic in mammalian cell lines and induce cell death. To achieve selectivity, we conjugated the analog [G10a]SHa with a breast cancer targeting peptide (BCTP). The resulting peptide temporin [G10a]SHa-BCTP conjugate was selectively active against the MCF-7 breast cancer cell line with no cytotoxicity in NIH-3T3 fibroblasts. Unlike the natural product or [G10a]SHa, the conjugated peptide induced apoptosis, downregulating the expression of Bcl-2 and survivin and upregulating Bax and caspase-3.