Antibacterial action of synthetic antilipopolysaccharide peptides (SALP) involves neutralization of both membrane-bound and free toxins

Supramolecular assembly of micellar aggregates is the basis of low endotoxin recovery (LER) in a drug formulation that can be resolved by a whole blood assay

Lipopolysaccharide (LPS, endotoxin) is ubiquitous and represents a harmful contaminant of pharmaceutical compounds, recombinant biologicals and drug products. The pyrogen can induce severe immune responses and pathology in vitro and in vivo. Health authorities require strict control of endotoxin in parenteral drugs. However, for research and pre-clinical compound analysis, endotoxin testing is not a required quality control, which may cause potential drawbacks in the translational pipeline. Endotoxin testing is usually performed by the Limulus amebocyte lysate (LAL) assay, which is hampered by the so-called low endotoxin recovery (LER) effect when certain drug formulations are tested. A comprehensive study including structural, biophysical, and biological analyses was conducted to identify LER root cause for phosphate- and polysorbate-containing parenteral drug products. LPS in water showed extended ribbon-like aggregate structures. In placebo (formulation buffer without drug) and in drug product (drug in formulation buffer), a reaggregation of LPS into a network of interlinked micelles with hidden head group charges, and a strong reduction of the negative surface potential was observed. The non-accessibility of the LPS backbone has a direct impact leading (i) to a loss of activation of the LAL-cascade, (ii) reduced activation of the TLR4/MD-2 receptor system, and (iii) increased survival in a mouse model of endotoxemia. These data provide a structure-based explanation of the LER-underlying mechanisms. A human whole blood assay is shown to resolve LER and detect the pyrogenic activity of endotoxin with high sensitivity. This may open new test options to improve quality control in drug development and drug safety.

Mechanism of Antimicrobial Peptides: Antimicrobial, Anti-Inflammatory and Antibiofilm Activities

Antimicrobial peptides (AMPs) are regarded as a new generation of antibiotics. Besides antimicrobial activity, AMPs also have antibiofilm, immune-regulatory, and other activities. Exploring the mechanism of action of AMPs may help in the modification and development of AMPs. Many studies were conducted on the mechanism of AMPs. The present review mainly summarizes the research status on the antimicrobial, anti-inflammatory, and antibiofilm properties of AMPs. This study not only describes the mechanism of cell wall action and membrane-targeting action but also includes the transmembrane mechanism of intracellular action and intracellular action targets. It also discusses the dual mechanism of action reported by a large number of investigations. Antibiofilm and anti-inflammatory mechanisms were described based on the formation of biofilms and inflammation. This study aims to provide a comprehensive review of the multiple activities and coordination of AMPs in vivo, and to fully understand AMPs to realize their therapeutic prospect.

Mass Spectrometric Quantification of the Antimicrobial Peptide Pep19-2.5 with Stable Isotope Labeling and Acidic Hydrolysis

Sepsis is the number one cause of death in intensive care units. This life-threatening condition is caused by bacterial infections and triggered by endotoxins of Gram-negative bacteria that leads to an overreaction of the immune system. The synthetic anti-lipopolysaccharide peptide Pep19-2.5 is a promising candidate for the treatment of sepsis as it binds sepsis-inducing lipopolysaccharides and thus prevents initiation of septic shock. For clinical evaluation precise quantification of the peptide in blood and tissue is required. As the peptide is not extractable from biological samples by commonly used methods there is a need for a new analysis method that does not rely on extraction of the peptide. In order to quantify the peptide by mass spectrometry, the peptide was synthesized containing ¹³C₉,¹⁵N₁-labeled phenylalanine residues. This modification offers high stability during acidic hydrolysis. Following acidic hydrolysis of the samples, the concentration of ¹³C₉,¹⁵N₁-labeled phenylalanine determined by LC-MS could be unambiguously correlated to the content of Pep19-2.5. Further experiments validated the accuracy of the data. Moreover, the quantification of Pep19-2.5 in different tissues (as studied in Wistar rats) was shown to provide comparable results to the results obtained with radioactively-labeled (¹⁴C) Pep19-2.5- Radioactive labeling is considered as the gold standard for quantification of compounds that refrain from reliable extraction methods. This novel method represents a valuable procedure for the determination of Pep19-2.5 and sticky peptides with unpredictable extraction properties in general.

A Synthetic Peptide Designed to Neutralize Lipopolysaccharides Attenuates Metaflammation and Diet-Induced Metabolic Derangements in Mice

Metabolic endotoxemia has been suggested to play a role in the pathophysiology of metaflammation, insulin-resistance and ultimately type-2 diabetes mellitus (T2DM). The role of endogenous antimicrobial peptides (AMPs), such as the cathelicidin LL-37, in T2DM is unknown. We report here for the first time that patients with T2DM compared to healthy volunteers have elevated plasma levels of LL-37. In a reverse-translational approach, we have investigated the effects of the AMP, peptide 19-2.5, in a murine model of high-fat diet (HFD)-induced insulin-resistance, steatohepatitis and T2DM. HFD-fed mice for 12 weeks caused obesity, an impairment in glycemic regulations, hypercholesterolemia, microalbuminuria and steatohepatitis, all of which were attenuated by Peptide 19-2.5. The liver steatosis caused by feeding mice a HFD resulted in the activation of nuclear factor kappa light chain enhancer of activated B cells (NF-ĸB) (phosphorylation of inhibitor of kappa beta kinase (IKK)α/β, IκBα, translocation of p65 to the nucleus), expression of NF-ĸB-dependent protein inducible nitric oxide synthase (iNOS) and activation of the NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome, all of which were reduced by Peptide 19-2.5. Feeding mice, a HFD also resulted in an enhanced expression of the lipid scavenger receptor cluster of differentiation 36 (CD36) secondary to activation of extracellular signal-regulated kinases (ERK)1/2, both of which were abolished by Peptide 19-2.5. Taken together, these results demonstrate that the AMP, Peptide 19-2.5 reduces insulin-resistance, steatohepatitis and proteinuria. These effects are, at least in part, due to prevention of the expression of CD36 and may provide further evidence for a role of metabolic endotoxemia in the pathogenesis of metaflammation and ultimately T2DM. The observed increase in the levels of the endogenous AMP LL-37 in patients with T2DM may serve to limit the severity of the disease.

A Real-Time Thermal Sensor System for Quantifying the Inhibitory Effect of Antimicrobial Peptides on Bacterial Adhesion and Biofilm Formation

The increasing rate of antimicrobial resistance (AMR) in pathogenic bacteria is a global threat to human and veterinary medicine. Beyond antibiotics, antimicrobial peptides (AMPs) might be an alternative to inhibit the growth of bacteria, including AMR pathogens, on different surfaces. Biofilm formation, which starts out as bacterial adhesion, poses additional challenges for antibiotics targeting bacterial cells. The objective of this study was to establish a real-time method for the monitoring of the inhibition of (a) bacterial adhesion to a defined substrate and (b) biofilm formation by AMPs using an innovative thermal sensor. We provide evidence that the thermal sensor enables continuous monitoring of the effect of two potent AMPs, protamine and OH-CATH-30, on surface colonization of bovine mastitis-associated Escherichia (E.) coli and Staphylococcus (S.) aureus. The bacteria were grown under static conditions on the surface of the sensor membrane, on which temperature oscillations generated by a heater structure were detected by an amorphous germanium thermistor. Bacterial adhesion, which was confirmed by white light interferometry, caused a detectable amplitude change and phase shift. To our knowledge, the thermal measurement system has never been used to assess the effect of AMPs on bacterial adhesion in real time before. The system could be used to screen and evaluate bacterial adhesion inhibition of both known and novel AMPs.

Anti-Infective and Anti-Inflammatory Mode of Action of Peptide 19-2.5

The polypeptide Pep19-2.5 (Aspidasept^®) has been described to act efficiently against infection-inducing bacteria by binding and neutralizing their most potent toxins, i.e., lipopolysaccharides (LPS) and lipoproteins/peptides (LP), independent of the resistance status of the bacteria. The mode of action was described to consist of a primary Coulomb/polar interaction of the N-terminal region of Pep19-2.5 with the polar region of the toxins followed by a hydrophobic interaction of the C-terminal region of the peptide with the apolar moiety of the toxins. However, clinical development of Aspidasept as an anti-sepsis drug requires an in-depth characterization of the interaction of the peptide with the constituents of the human immune system and with other therapeutically relevant compounds such as antibiotics and non-steroidal anti-inflammatory drugs (NSAIDs). In this contribution, relevant details of primary and secondary pharmacodynamics, off-site targets, and immunogenicity are presented, proving that Pep19-2.5 may be readily applied therapeutically against the deleterious effects of a severe bacterial infection.

Application of Biophysical Techniques to Investigate the Interaction of Antimicrobial Peptides With Bacterial Cells

Gaining new understanding on the mechanism of action of antimicrobial peptides is the basis for the design of new and more efficient antibiotics. To this aim, it is important to detect modifications occurring to both the peptide and the bacterial cell upon interaction; this will help to understand the peptide structural requirement, if any, at the base of the interaction as well as the pathways triggered by peptides ending in cell death. A limited number of papers have described the interaction of peptides with bacterial cells, although most of the studies published so far have been focused on model membrane-peptides interactions. Investigations carried out with bacterial cells highlighted the limitations connected to the use of oversimplified model membranes and, more importantly, helped to identify molecular targets of antimicrobial peptides and changes occurring to the bacterial membrane. In this review, details on the mechanism of action of antimicrobial peptides, as determined by the application of spectroscopic techniques, as well as scattering, microscopy, and calorimetry techniques, to complex systems such as peptide/bacteria mixtures are discussed.

Fatty Acid Conjugation Leads to Length-Dependent Antimicrobial Activity of a Synthetic Antibacterial Peptide (Pep19-4LF)

The increasing number of infections caused by multidrug-resistant bacteria requires an intensified search for new antibiotics. Pep19-4LF is a synthetic antimicrobial peptide (GKKYRRFRWKFKGKLFLFG) that was previously designed with the main focus on high antimicrobial activity. The hydrophobic motif, LFLFG, was found to be essential for antimicrobial activity. However, this motif shows several limitations such as aggregation in biological media, low solubility, and small yields in peptide synthesis. In order to obtain more appropriate peptide characteristics, the hydrophobic motif was replaced with fatty acids. For this purpose, a shortened variant of Pep19-4LF (Pep19-short; GKKYRRFRWKFKGK) was synthesized and covalently linked to saturated fatty acids of different chain lengths. The peptide conjugates were tested with respect to their antibacterial activity by microdilution experiments on different bacterial strains. The length of the fatty acid was found to be directly correlated to the antimicrobial activity up to an ideal chain length (undecanoic acid, C11:0). This conjugate showed high antimicrobial activity in absence of toxicity. Time–kill studies revealed a fast and bactericidal mode of action. Furthermore, the first in vivo experiments of the conjugate in rodents demonstrated pharmacokinetics appropriate for application as a drug. These results clearly indicate that the hydrophobic motif of the peptide can be replaced by a single fatty acid of medium length, simplifying the design of this antimicrobial peptide while retaining high antimicrobial activity in the absence of toxicity.

An update on endotoxin neutralization strategies in Gram-negative bacterial infections

INTRODUCTION

Gram-negative bacterial infections represent still a severe problem of human health care, regarding the increase in multi-resistance against classical antibiotics and the lack of newly developed antimicrobials. For the fight against these germs, anti-infective agents must overcome and/or bind to the Gram-negative outer membrane consisting of a lipopolysaccharide (LPS, endotoxin) outer leaflet and an inner leaflet from phospholipids, with additional peripheral or integral membrane proteins (OMP's).

AREAS COVERED

The current article reviews data of existing therapeutic options and summarizes newer approaches for targeting and neutralizing endotoxins, ranging from in vitro over in vivo animal data to clinical applications by using databases such as Medline.

EXPERT OPINION

Conventional antibiotic treatment of the bacteria leads to their killing, but not necessary LPS neutralization, which may be a severe problem in particular for the systemic pathway. This is the reason why there is an increasing number of therapeutic approaches, which - besides combating whole bacteria - at the same time try to neutralize endotoxin within or outside the bacterial cells mainly responsible for the high inflammation induction in Gram-negative species.

Inactivation of Bacteria by γ-Irradiation to Investigate the Interaction with Antimicrobial Peptides

The activity of antimicrobial peptides (AMPs) has been investigated extensively using model membranes composed of phospholipids or lipopolysaccharides in aqueous environments. However, from a biophysical perspective, there is a large scientific interest regarding the direct interaction of membrane-active peptides with whole bacteria. Working with living bacteria limits the usability of experimental setups and the interpretation of the resulting data because of safety risks and the overlap of active and passive effects induced by AMPs. We killed or inactivated metabolic-active bacteria using γ-irradiation or sodium azide, respectively. Microscopy, flow cytometry, and SYTOX green assays showed that the cell envelope remained intact to a high degree at the minimal bactericidal dose. Furthermore, the tumor-necrosis-factor-α-inducing activity of the lipopolysaccharides and the chemical lipid composition was unchanged. Determining the binding capacity of AMPs to the bacterial cell envelope by calorimetry is difficult because of an overlapping of the binding heat and metabolic activities of the bacteria-induced by the AMPs. The inactivation of all active processes helps to decipher the complex thermodynamic information. From the isothermal titration calorimetry (ITC) results, we propose that the bacterial membrane potential (Δψ) is possibly an underestimated modulator of the AMP activity. The negative surface charge of the outer leaflet of the outer membrane of Gram-negative bacteria is already neutralized by peptide concentrations below the minimal inhibitory concentration. This proves that peptide aggregation on the bacterial membrane surface plays a decisive role in the degree of antimicrobial activity. This will not only enable many biophysical approaches for the investigation between bacteria and membrane-active peptides in the future but will also make it possible to compare biophysical parameters of active and inactive bacteria. This opens up new possibilities to better understand the active and passive interaction processes between AMPs and bacteria.