Polymyxin B-Triggered Assembly of Peptide Hydrogels for Localized and Sustained Release of Combined Antimicrobial Therapy

Antimicrobial peptides: An alternative strategy to combat antimicrobial resistance

Antimicrobial peptides (AMPs) are a diverse group of naturally occurring molecules produced by eukaryotes and prokaryotes. They have an important role in innate immunity via their direct microbicidal properties or immunomodulatory activities against pathogens. With the widespread occurrence of antimicrobial resistance (AMR), AMPs are considered as viable alternatives for the treatment of multidrug-resistant microbes, inflammation, and, wound healing. The broad-spectrum antibacterial activity of AMPs is predominantly attributed to membrane disruption, leading to the formation of transmembrane pores and, eventually, cell lysis. However, mechanisms related to inhibition of cell wall synthesis, nucleic acid synthesis, protein synthesis, or enzymatic activity are also associated with these peptides. In this review, we discuss our current understanding, therapeutic uses and challenges associated with the clinical applications of AMPs.

Galleria mellonella (Greater Wax Moth) as a Reliable Animal Model to Study the Efficacy of Nanomaterials in Fighting Pathogens

The spread of multidrug-resistant microbes has made it necessary and urgent to develop new strategies to deal with the infections they cause. Some of these are based on nanotechnology, which has revolutionized many fields in medicine. Evaluating the safety and efficacy of these new antimicrobial strategies requires testing in animal models before being tested in clinical trials. In this context, Galleria mellonella could represent a valid alternative to traditional mammalian and non-mammalian animal models, due to its low cost, ease of handling, and valuable biological properties to investigate host-pathogen interactions. The purpose of this review is to provide an updated overview of the literature concerning the use of G. mellonella larvae as an animal model to evaluate safety and efficacy of nanoparticles and nanomaterials, particularly, of those that are used or are under investigation to combat microbial pathogens.

Development of Nanomaterials-Based Agents for Selective Antibacterial Activity

Bacterial infections continue to threaten public health due to limitations in rapid and accurate diagnostic techniques. While broad-spectrum antibiotics offer empirical treatment, their overuse has fuelled the emergence of antimicrobial resistance (AMR) pathogens, posing a critical global public health challenge. In this critical scenario, nanomaterial-based antibacterial agents emerge as a promising solution to combat bacteria and inhibit their proliferation. However, selective elimination of pathogenic bacteria is paramount. This review highlights recent advancements in developing nanomaterials for selective antibacterial activity. We categorize these agents based on their mode of action, exploring how they selectively interact with bacteria and their potential antibacterial mechanisms. This review offers crucial insights for researchers exploring the potential of nanotechnology to address the growing threat of AMR.

Advances in the clinical treatment of multidrug-resistant pathogens using polymyxins

OBJECTIVES

Polymyxins are a vital class of antibiotics used to combat multidrug-resistant Gram-negative bacteria. However, their use is limited due to potential nephrotoxicity and the availability of alternative antibiotics. This review aims to examine the properties of polymyxins and the clinical advances in their use for treating infections caused by carbapenem-resistant Gram-negative bacteria (CR-GNB).

METHODS

This review analyses literature on polymyxin properties and various clinical approaches, including intravenous drip infusion, nebulized or dry powder inhalation, and ointment application. Treatment efficacy in terms of bacterial eradication, cure rate and mortality rate are reviewed and evaluated.

RESULTS

Polymyxins have been reintroduced to treat critical infections due to the increasing prevalence of CR-GNB. Clinical trials and studies have confirmed that polymyxins can effectively treat CR-GNB infections when the formulation and administration are appropriate, with acceptable levels of nephrotoxicity.

CONCLUSIONS

In the future, the development of polymyxin formulations will aim to improve their clinical effectiveness while reducing toxicity and side effects and preventing the emergence of polymyxin-resistant strains. Enhanced efficacy and minimized potential side effects can be achieved by developing new polymyxin-delivery systems that provide a smart and controlled release or customized patient administration.

Disassembly of self-assembling peptide hydrogels as a versatile method for cell extraction and manipulation

Self-assembling peptide hydrogels (SAPHs) are increasingly being used as two-dimensional (2D) cell culture substrates and three-dimensional (3D) matrices due to their tunable properties and biomimicry of native tissues. Despite these advantages, SAPHs often represent an end-point in cell culture, as isolating cells from them leads to low yields and disruption of cells, limiting their use and post-culture analyses. Here, we report on a protocol designed to easily and effectively disassemble peptide amphiphile (PA) SAPHs to retrieve 3D encapsulated cells with high viability and minimal disruption. Due to the pivotal role played by salt ions in SAPH gelation, tetrasodium ethylenediaminetetraacetic acid (Na4EDTA) was used as metal chelator to sequester ions participating in PA self-assembly and induce a rapid, efficient, clean, and gentle gel-to-sol transition. We characterise PA disassembly from the nano- to the macro-scale, provide mechanistic and practical insights into the PA disassembly mechanism, and assess the potential use of the process. As proof-of-concept, we isolated different cell types from cell-laden PA hydrogels and demonstrated the possibility to perform downstream biological analyses including cell re-plating, gene analysis, and flow cytometry with high reproducibility and no material interference. Our work offers new opportunities for the use of SAPHs in cell culture and the potential use of cells cultured on SAPHs, in applications such as cell expansion, analysis of in vitro models, cell therapies, and regenerative medicine.

A Paintable Small-Molecule Hydrogel with Antimicrobial and ROS Scavenging Activities for Burn Wound Healing

Delayed wound healing induced by bacterial infection and a persistent inflammatory response remains a great clinical challenge. Herein, we reported a paintable, anti-bacterial, and anti-inflammatory Nap-F3K-CA (Nap-Phe-Phe-Phe-Lys-Caffeic Acid) hydrogel for burn wound management based on caffeic acid (CA)-functionalized short peptides (Nap-Phe-Phe-Phe-Lys). Hydrogels are assembled by non-covalent interactions between gelators, and the incorporation of CA promotes the self-assembly of the hydrogel. After being applied to burn wounds, the hydrogel effectively adapted to irregular wound beds and maintained a moist protective environment at the wound. The Nap-F3K-CA hydrogel can scavenge ROS to relieve oxidative damage and downregulate proinflammatory levels. The Nap-F3K-CA hydrogel also displayed potent antibacterial activity against Gram-positive and Gram-negative bacteria, which reduced the incidence of wound infections. Moreover, the hydrogel exhibited good biocompatibility and hemostatic function. In vivo experiments demonstrated that the Nap-F3K-CA hydrogel significantly accelerated the repair of the skin structure including promoting collagen deposition, vascular regeneration, and hair follicle formation. These findings proved the clinical application potential of the Nap-F3K-CA hydrogel as a promising burn wound dressing.

Peptide-based supramolecular hydrogels and their biotherapeutic applications

In recent years, supramolecular hydrogels have made groundbreaking research progress in biomedical fields such as drug delivery, biosensing, imaging analysis, and tissue engineering. Peptides, with their unique characteristics of facile preparation, low immunogenicity and easy biodegradability, are commonly used as building blocks of supramolecular hydrogels. Peptide-based supramolecular hydrogels loaded with drugs, prepared via physical means or covalent crosslinking, exhibit unique three-dimensional network structures and strong water retention capacities. These properties enhance drug bioavailability and reduce side effects, enabling drug accumulation and responsive release at disease sites, significantly improving the therapeutic efficacy. Here, we review recent advancements in peptide-based supramolecular hydrogels and their biotherapeutic applications, including chemotherapy, photothermal therapy, photodynamic therapy, immunotherapy, gene therapy, antibacterial and anti-inflammatory treatments, and other biological applications. This review aims to provide new inspiration for the development of biomaterials in the therapeutic field and provide more personalized options for disease treatment. Additionally, challenges and limitations in this field are briefly discussed.

Elastin-Derived Peptide-Based Hydrogels as a Potential Drug Delivery System

A peptide-based hydrogel sequence was computationally predicted from the Ala-rich cross-linked domains of elastin. Three candidate peptides were subsequently synthesised and characterised as potential drug delivery vehicles. The elastin-derived peptides are Fmoc-FFAAAAKAA-NH2, Fmoc-FFAAAKAA-NH2, and Fmoc-FFAAAKAAA-NH2. All three peptide sequences were able to self-assemble into nanofibers. However, only the first two could form hydrogels, which are preferred as delivery systems compared to solutions. Both of these peptides also exhibited favourable nanofiber lengths of at least 1.86 and 4.57 µm, respectively, which are beneficial for the successful delivery and stability of drugs. The shorter fibre lengths of the third peptide (maximum 0.649 µm) could have inhibited their self-assembly into the three-dimensional networks crucial to hydrogel formation.

Cyclic peptide conjugated photosensitizer for targeted phototheranostics of gram-negative bacterial infection

Antimicrobial photodynamic therapy (PDT) is a promising alternative to antibiotics for eradicating pathogenic bacterial infections. It holds advantage of not inducing antimicrobial resistance but is limited for the treatment of gram-negative bacterial infection due to the lack of photosensitizer (PS) capable of targeted permeating the outer membrane (OM) of gram-negative bacteria. To facilitate the targeted permeability of PS, cyclic polymyxin b nonapeptide that can specifically bind to the lipopolysaccharide on OM, is conjugated to an FDA approved PS chlorin e6 via variable linkers. Based on structure to activity study, C6pCe6 with aminohexanoic linker and P2pCe6 with amino-3, 6-dioxaoctanoic linker are identified to preferentially image gram-negative bacteria. These two conjugates also exhibit improved aqueous dispersity and enhanced ROS generation, consequently enabled their selective bactericidal activities against gram-negative bacteria upon 660 nm light irradiation. The effective photobactericidal ability of P2pCe6 is further validated on P. aeruginosa infected G. mellonella. Moreover, it is demonstrated to effectively treat the P. aeruginosa infection and accelerate the healing process at the wound site of mouse. Owing to the light irradiation triggered targeted imaging and enhanced bactericidal capacities, P2pCe6 hold great potential to serve as a potent PS for mediating the phototheranostics of gram-negative bacterial infection.

Current status of development and biomedical applications of peptide-based antimicrobial hydrogels

Microbial contamination poses a serious threat to human life and health. Through the intersection of material science and modern medicine, advanced bionic hydrogels have shown great potential for biomedical applications due to their unique bioactivity and ability to mimic the extracellular matrix environment. In particular, as a promising antimicrobial material, the synthesis and practical biomedical applications of peptide-based antimicrobial hydrogels have drawn increasing research interest. The synergistic effect of peptides and hydrogels facilitate the controlled release of antimicrobial agents and mitigation of their biotoxicity while achieving antimicrobial effects and protecting the active agents from degradation. This review reports on the progress and trends of researches in the last five years and provides a brief outlook, aiming to provide theoretical background on peptide-based antimicrobial hydrogels and make suggestions for future related work.

A pH/enzyme dual responsive PMB spatiotemporal release hydrogel promoting chronic wound repair

Suppressing persistent multidrug-resistant (MDR) bacterial infections and excessive inflammation is the key for treating chronic wounds. Therefore, developing a microenvironment-responsive material with good biodegradability, drug-loading, anti-infection, and anti-inflammatory properties is desired to boost the chronic wounds healing process; however, using ordinary assembly remains a defect. Herein, we propose a pH/enzyme dual-responsive polymyxin B (PMB) spatiotemporal-release hydrogel (GelMA/OSSA/PMB), namely, the amount of OSSA and PMB released from GelMA/OSSA/PMB was closely related the wound pH and the enzyme concentration changing. The GelMA/OSSA/PMB showed better biosafety than equivalent free PMB, owing to the controlled release of PMB, which helped kill planktonic bacteria and inhibit biofilm activity in vitro. In addition, the GelMA/OSSA/PMB exhibited excellent antibacterial and anti-inflammatory properties. A MDR Pseudomonas aeruginosa caused infection was effectively resolved by the GelMA/OSSA/PMB hydrogel in vivo, thereby significantly boosting wound closure during the inflammatory phase. Furthermore, GelMA/OSSA/PMB accelerated the sequential phases of wound repair.

Using the Galleria mellonella burn wound and infection model to identify and characterize potential wound probiotics

Burn wound infection is the leading cause of mortality among burn wound patients. One of the most commonly isolated bacterial burn wound pathogens is Pseudomonas aeruginosa, a notorious nosocomial multidrug-resistant pathogen. As a consequence of its recalcitrance to frontline antibiotic therapy, there is an urgent need to develop alternative treatment avenues to tackle this pathogen. One potential alternative infection prevention measure is to seed the wound bed with probiotic bacteria. Several species of Lactobacillus, a common commensal bacterium, have been previously reported to display growth inhibition activity against wound pathogens. Various species of this genus have also been shown to augment the wound healing process, which makes it a promising potential therapeutic agent. Due to the complexity of the burn wound trauma and burn wound infection, an in vivo model is required for the development of novel therapeutics. There are multiple in vivo models that are currently available, the most common among them being the murine model. However, mammalian burn wound infection models are logistically challenging, do not lend themselves to screening approaches and come with significant concerns around ethics and animal welfare. Recently, an invertebrate burn wound and infection model using G. mellonella has been established. This model addresses several of the challenges of more advanced animal models, such as affordability, maintenance and reduced ethical concerns. This study validates the capacity of this model to screen for potential wound probiotics by demonstrating that a variety of Lactobacillus spp. can limit P. aeruginosa burn wound infection and improve survival.

Peptide Supramolecular Hydrogels with Sustained Release Ability for Combating Multidrug-Resistant Bacteria

Chronic wound infection caused by multidrug-resistant bacteria is a major threat globally, leading to high mortality rates and a considerable economic burden. To address it, an innovative supramolecular nanofiber hydrogel (Hydrogel-RL) harboring antimicrobial peptides was developed based on the novel arginine end-tagging peptide (Pep 6) from our recent study, triggering cross-linking. In vitro results demonstrated that Hydrogel-RL can sustain the release of Pep 6 up to 120 h profiles, which is biocompatible and exhibits superior activity for methicillin-resistant Staphylococcus aureus (MRSA) biofilm inhibition and elimination. A single treatment of supramolecular Hydrogel-RL on an MRSA skin infection model revealed formidable antimicrobial activity and therapeutic effects in vivo. In the chronic wound infection model, Hydrogel-RL promoted mouse skin cell proliferation, reduced inflammation, accelerated re-epithelialization, and regulated muscle and collagen fiber formation, rapidly healing full-thickness skin wounds. To show its vehicle property for wound infection combined therapy, etamsylate, an antihemorrhagic drug, was loaded into the porous network of Hydrogel-RL, which demonstrated improved hemostatic activity. Collectively, Hydrogel-RL is a promising clinical candidate agent for functional supramolecular biomaterials designed for combating multidrug-resistant bacteria and rescuing stalled healing in chronic wound infections.

Greener Cleavage of Protected Peptide Fragments from Sieber Amide Resin

Following the successful introduction of two benign solvents for cleaving protected acid peptide fragments from 2-chlorotrityl chloride (2-CTC) resin, there is a need to green the cleavage process for obtaining protected peptide amide fragments. In this work, p-xylene and toluene are introduced as greener alternates to dichloromethane (DCM) for preparing protected peptide amide fragments from a Sieber amide resin. The N-dealkylation is a demanding chemical reaction, requiring that the cleavage protocol be optimised to ensure complete cleavage from the resin. After a 30 min reaction time, only 66 % of the final peptide product was retrieved even with the conventional dichloromethane solvent. Therefore, 120 min was considered sufficient to fully cleave the peptide from the Sieber amide resin. This work reaffirms the fact that greening strategies are far from detrimental, with green alternatives often outperforming their replaced counterparts.

Strategies for Improving Peptide Stability and Delivery

Peptides play an important role in many fields, including immunology, medical diagnostics, and drug discovery, due to their high specificity and positive safety profile. However, for their delivery as active pharmaceutical ingredients, delivery vectors, or diagnostic imaging molecules, they suffer from two serious shortcomings: their poor metabolic stability and short half-life. Major research efforts are being invested to tackle those drawbacks, where structural modifications and novel delivery tactics have been developed to boost their ability to reach their targets as fully functional species. The benefit of selected technologies for enhancing the resistance of peptides against enzymatic degradation pathways and maximizing their therapeutic impact are also reviewed. Special note of cell-penetrating peptides as delivery vectors, as well as stapled modified peptides, which have demonstrated superior stability from their parent peptides, are reported.

Gram-selective antibacterial peptide hydrogels

The human microbiome plays fundamental roles in human health and disease. However, widely used broad-spectrum antibiotics severely disrupt human-related microbial communities, eventually leading to resistant bacteria, posing a growing threat to global medical health. Antimicrobial peptides (AMPs) are promising antimicrobial agents that barely cause bacterial resistance. Excellent broad-spectrum antimicrobial activities have been achieved using hydrogels self-assembled from AMPs, but there is still a lack of AMP hydrogels that can target Gram-positive and Gram-negative bacteria. Herein, several hydrogels self-assembled from AMPs, termed IK1, IK3, and IK4, were designed and synthesized. In vitro antibacterial results indicated that the IK1 and IK4 hydrogels specifically targeted Gram-positive and Gram-negative bacteria, respectively, while the IK3 hydrogel targeted both Gram-positive and Gram-negative bacteria. The desired broad-spectrum or Gram-selective AMP hydrogels are believed to be obtained through the rational design of the hydrophilicity, hydrophobicity, and charge properties of the peptide molecules. Good in vivo Gram-selective antibacterial properties and the ability to promote wound healing have been demonstrated via treating mouse wound models with these AMP hydrogels. We believe that these Gram-selective AMP hydrogels could potentially have important applications in treating common recurring infections.

Hybrid Nanoparticles and Composite Hydrogel Systems for Delivery of Peptide Antibiotics

The growing number of drug-resistant pathogenic bacteria poses a global threat to human health. For this reason, the search for ways to enhance the antibacterial activity of existing antibiotics is now an urgent medical task. The aim of this study was to develop novel delivery systems for polymyxins to improve their antimicrobial properties against various infections. For this, hybrid core-shell nanoparticles, consisting of silver core and a poly(glutamic acid) shell capable of polymyxin binding, were developed and carefully investigated. Characterization of the hybrid nanoparticles revealed a hydrodynamic diameter of approximately 100 nm and a negative electrokinetic potential. The nanoparticles demonstrated a lack of cytotoxicity, a low uptake by macrophages, and their own antimicrobial activity. Drug loading and loading efficacy were determined for both polymyxin B and E, and the maximal loaded value with an appropriate size of the delivery systems was 450 µg/mg of nanoparticles. Composite materials based on agarose hydrogel were prepared, containing both the loaded hybrid systems and free antibiotics. The features of polymyxin release from the hybrid nanoparticles and the composite materials were studied, and the mechanisms of release were analyzed using different theoretical models. The antibacterial activity against Pseudomonas aeruginosa was evaluated for both the polymyxin hybrid and the composite delivery systems. All tested samples inhibited bacterial growth. The minimal inhibitory concentrations of the polymyxin B hybrid delivery system demonstrated a synergistic effect when compared with either the antibiotic or the silver nanoparticles alone.

Peptide Amphiphile Hydrogels Based on Homoternary Cucurbit[8]uril Host-Guest Complexes

Supramolecular hydrogels based on peptide amphiphiles (PAs) are promising materials for tissue engineering and model extracellular matrixes for biological studies. While PA hydrogels are conventionally formed via electrostatic screening, new hydrogelation mechanisms might help to improve the design and functionality of these materials. Here, we present a host-guest-mediated PA hydrogelation method that relies on the formation of a host-guest homoternary complex with cucurbit[8]uril (CB[8]) and aromatic amino-acid-bearing PA nanofibers. As a result of the host-guest cross-linking between PA nanofibers, hierarchical morphologies and increased stiffness were found when host-guest-mediated PA hydrogels were compared to their ion-based equivalents. Additionally, both families of hydrogels exhibited similar biocompatibilities. These results demonstrate that CB[8]-mediated hydrogelation can be used as an alternative cross-linking method to upgrade the design of PA materials and extend their biomedical applications.

Molecular Engineering of Polymyxin B for Imaging and Treatment of Bacterial Infections

The emergence of multi-drug resistant bacteria and the lack of novel antibiotics to combat them have led to the revival of polymyxin B, a previously abandoned antibiotic due to its potential nephrotoxicity and neurotoxicity. To facilitate its widely clinical applications, increasing effort has been devoted to molecularly engineer polymyxin B for the targeted imaging and effective treatment of bacterial infections. Herein, the molecular engineering strategies will be summarized in this mini review, with selected recent advances for illustration. Perspective of the challenges and trends in this exciting and eagerly anticipated research area will also be provided in the end. We hope this mini review will inspire researchers from diverse fields to bring forward the next wave of exploiting molecular engineering approaches to propel the “old” polymyxin B to “new” clinical significance in combating bacterial infections.