Exotoxin-Targeted Drug Modalities as Antibiotic Alternatives

Repurposing FDA-approved disulfiram for targeted inhibition of diphtheria toxin and the binary protein toxins of Clostridium botulinum and Bacillus anthracis

Many bacteria act pathogenic by the release of AB-type protein toxins that efficiently enter human or animal cells and act as enzymes in their cytosol. This leads to disturbed cell functions and the clinical symptoms characteristic for the individual toxin. Therefore, molecules that directly target and neutralize these toxins provide promising novel therapeutic options. Here, we found that the FDA-approved drug disulfiram (DSF), used for decades to treat alcohol abuse, protects cells from intoxication with diphtheria toxin (DT) from Corynebacterium diphtheria, the causative agent of diphtheria, lethal toxin (LT) from Bacillus anthracis, which contributes to anthrax, and C2 enterotoxin from Clostridium botulinum when applied in concentrations lower than those found in plasma of patients receiving standard DSF treatment for alcoholism (up to 20 µM). Moreover, this inhibitory effect is increased by copper, a known enhancer of DSF activity. LT and C2 are binary toxins, consisting of two non-linked proteins, an enzyme (A) and a separate binding/transport (B) subunit. To act cytotoxic, their proteolytically activated B subunits PA63 and C2IIa, respectively, form barrel-shaped heptamers that bind to their cellular receptors and form complexes with their respective A subunits LF and C2I. The toxin complexes are internalized via receptor-mediated endocytosis and in acidified endosomes, PA63 and C2IIa form pores in endosomal membranes, which facilitate translocation of LF and C2I into the cytosol, where they act cytotoxic. In DT, A and B subunits are located within one protein, but DT also forms pores in endosomes that facilitate translocation of the A subunit. If cell binding, membrane translocation, or substrate modification is inhibited, cells are protected from intoxication. Our results implicate that DSF neither affects cellular binding nor the catalytic activity of the investigated toxins to a relevant extend, but interferes with the toxin pore-mediated translocation of the A subunits of DT, LT and C2 toxin, as demonstrated by membrane-translocation assays and toxin pore conductivity experiments in the presence or absence of DSF. Since toxin translocation across intracellular membranes represents a central step during cellular uptake of many bacterial toxins, DSF might neutralize a broad spectrum of medically relevant toxins.

Recent advances in cell membrane camouflaged nanotherapeutics for the treatment of bacterial infection

Infectious diseases caused by bacterial infections are common in clinical practice. Cell membrane coating nanotechnology represents a pioneering approach for the delivery of therapeutic agents without being cleared by the immune system in the meantime. And the mechanism of infection treatment should be divided into two parts: suppression of pathogenic bacteria and suppression of excessive immune response. The membrane-coated nanoparticles exert anti-bacterial function by neutralizing exotoxins and endotoxins, and some other bacterial proteins. Inflammation, the second procedure of bacterial infection, can also be suppressed through targeting the inflamed site, neutralization of toxins, and the suppression of pro-inflammatory cytokines. And platelet membrane can affect the complement process to suppress inflammation. Membrane-coated nanoparticles treat bacterial infections through the combined action of membranes and nanoparticles, and diagnose by imaging, forming a theranostic system. Several strategies have been discovered to enhance the anti-bacterial/anti-inflammatory capability, such as synthesizing the material through electroporation, pretreating with the corresponding pathogen, membrane hybridization, or incorporating with genetic modification, lipid insertion, and click chemistry. Here we aim to provide a comprehensive overview of the current knowledge regarding the application of membrane-coated nanoparticles in preventing bacterial infections as well as addressing existing uncertainties and misconceptions.

Dandelion-derived vesicles-laden hydrogel dressings capable of neutralizing Staphylococcus aureus exotoxins for the care of invasive wounds

Delayed wound healing caused by bacterial infection remains a major challenge in clinical treatment. Exotoxins incorporated in bacterial extracellular vesicles play a key role as the disease-causing virulence factors. Safe and specific antivirulence agents are expected to be developed as an effective anti-bacterial infection strategy, instead of single antibiotic therapy. Plant-derived extracellular vesicle-like nanoparticles have emerged as promising therapeutic agents for skin diseases, but the elucidations of specific mechanisms of action and clinical transformation still need to be advanced. Here, dandelion-derived extracellular vesicle-like nanoparticles (TH-EVNs) are isolated and exert antivirulence activity through specifically binding to Staphylococcus aureus (S. aureus) exotoxins, thereby protecting the host cell from attack. The neutralization of TH-EVNs against exotoxins has considerable binding force and stability, showing complete detoxification effect in vivo. Then gelatin methacryloyl hydrogel is developed as TH-EVNs-loaded dressing for S. aureus exotoxin-invasive wounds. Hydrogel dressings demonstrate good physical and mechanical properties, thus achieving wound retention and controlled release of TH-EVNs, in addition to promoting cell proliferation and migration. In vivo results show accelerated re-epithelialization, promotion of collagen maturity and reduction of inflammation after treatment. Collectively, the developed TH-EVNs-laden hydrogel dressings provide a potential therapeutic approach for S. aureus exotoxin- associated trauma.

Recent advancements in the specific determination of carcinoembryonic antigens using MOF-based immunosensors

Carcinoembryonic antigens (CEAs) are prominent cancer biomarkers that enable the early detection of numerous cancers. For effective CEA screening, rapid, portable, efficient, and sensitive diagnosis approaches should be devised. Metal-organic frameworks (MOFs) are porous crystalline materials that have received major attention for application in high-efficiency signal probes owing to their advantages such as large specific surface area, superior chemical stability and tunability, high porosity, easy surface functional modification, and adjustable size and morphology. Immunoassay strategies using antigen-antibody specific interaction are one of the imperative means for rapid and accurate measurement of target molecules in biochemical fields. The emerging MOFs and their nanocomposites are synthesized with excellent features, providing promising potential for immunoassays. This article outlines the recent breakthroughs in the synthesis approaches of MOFs and overall functionalization mechanisms of MOFs with antigen/antibody and their uses in the CEA immunoassays, which operate according to electrochemical, electrochemiluminescent and colorimetric techniques. The prospects and limitations of the preparation and immunoassay applications of MOF-derived hybrid nanocomposites are also discussed at the end.

Antimicrobials: An update on new strategies to diversify treatment for bacterial infections

Ninety-five years after Fleming's discovery of penicillin, a bounty of antibiotic compounds have been discovered, modified, or synthesised. Diversification of target sites, improved stability and altered activity spectra have enabled continued antibiotic efficacy, but overwhelming reliance and misuse has fuelled the global spread of antimicrobial resistance (AMR). An estimated 1.27 million deaths were attributable to antibiotic resistant bacteria in 2019, representing a major threat to modern medicine. Although antibiotics remain at the heart of strategies for treatment and control of bacterial diseases, the threat of AMR has reached catastrophic proportions urgently calling for fresh innovation. The last decade has been peppered with ground-breaking developments in genome sequencing, high throughput screening technologies and machine learning. These advances have opened new doors for bioprospecting for novel antimicrobials. They have also enabled more thorough exploration of complex and polymicrobial infections and interactions with the healthy microbiome. Using models of infection that more closely resemble the infection state in vivo, we are now beginning to measure the impacts of antimicrobial therapy on host/microbiota/pathogen interactions. However new approaches are needed for developing and standardising appropriate methods to measure efficacy of novel antimicrobial combinations in these contexts. A battery of promising new antimicrobials is now in various stages of development including co-administered inhibitors, phages, nanoparticles, immunotherapy, anti-biofilm and anti-virulence agents. These novel therapeutics need multidisciplinary collaboration and new ways of thinking to bring them into large scale clinical use.

Functional fluorescent nanomaterials for the detection, diagnosis and control of bacterial infection and biofilm formation: Insight towards mechanistic aspects and advanced applications

Infectious diseases resulting from the high pathogenic potential of several bacteria possesses a major threat to human health and safety. Traditional methods used for screening of these microorganisms face major issues with respect to detection time, selectivity and specificity which may delay treatment for critically ill patients past the optimal time. Thus, a convincing and essential need exists to upgrade the existing methodologies for the fast detection of bacteria. In this context, increasing number of newly emerging nanomaterials (NMs) have been discovered for their effective use and applications in the area of diagnosis in bacterial infections. Recently, functional fluorescent nanomaterials (FNMs) are extensively explored in the field of biomedical research, particularly in developing new diagnostic tools, nanosensors, specific imaging modalities and targeted drug delivery systems for bacterial infection. It is interesting to note that organic fluorophores and fluorescent proteins have played vital role for imaging and sensing technologies for long, however, off lately fluorescent nanomaterials are increasingly replacing these due to the latter's unprecedented fluorescence brightness, stability in the biological environment, high quantum yield along with high sensitivity due to enhanced surface property etc. Again, taking advantage of their photo-excitation property, these can also be used for either photothermal and photodynamic therapy to eradicate bacterial infection and biofilm formation. Here, in this review, we have paid particular attention on summarizing literature reports on FNMs which includes studies detailing fluorescence-based bacterial detection methodologies, antibacterial and antibiofilm applications of the same. It is expected that the present review will attract the attention of the researchers working in this field to develop new engineered FNMs for the comprehensive diagnosis and treatment of bacterial infection and biofilm formation.

Domperidone Inhibits Clostridium botulinum C2 Toxin and Bordetella pertussis Toxin

Bordetella pertussis toxin (PT) and Clostridium botulinum C2 toxin are ADP-ribosylating toxins causing severe diseases in humans and animals. They share a common translocation mechanism requiring the cellular chaperones Hsp90 and Hsp70, cyclophilins, and FK506-binding proteins to transport the toxins' enzyme subunits into the cytosol. Inhibitors of chaperone activities have been shown to reduce the amount of transported enzyme subunits into the cytosol of cells, thus protecting cells from intoxication by these toxins. Recently, domperidone, an approved dopamine receptor antagonist drug, was found to inhibit Hsp70 activity. Since Hsp70 is required for cellular toxin uptake, we hypothesized that domperidone also protects cells from intoxication with PT and C2. The inhibition of intoxication by domperidone was demonstrated by analyzing the ADP-ribosylation status of the toxins' specific substrates. Domperidone had no inhibitory effect on the receptor-binding or enzyme activity of the toxins, but it inhibited the pH-driven membrane translocation of the enzyme subunit of the C2 toxin and reduced the amount of PTS1 in cells. Taken together, our results indicate that domperidone is a potent inhibitor of PT and C2 toxins in cells and therefore might have therapeutic potential by repurposing domperidone to treat diseases caused by bacterial toxins that require Hsp70 for their cellular uptake.

Recent Advances in Monoclonal Antibody-Based Approaches in the Management of Bacterial Sepsis

Sepsis is a life-threatening condition characterized by an uncontrolled inflammatory response to an infectious agent and its antigens. Immune cell activation against the antigens causes severe distress that mediates a strong inflammatory response in vital organs. Sepsis is responsible for a high rate of morbidity and mortality in immunosuppressed patients. Monoclonal antibody (mAb)-based therapeutic strategies are now being explored as a viable therapy option for severe sepsis and septic shock. Monoclonal antibodies may provide benefits through two major strategies: (a) monoclonal antibodies targeting the pathogen and its components, and (b) mAbs targeting inflammatory signaling may directly suppress the production of inflammatory mediators. The major focus of mAb therapies has been bacterial endotoxin (lipopolysaccharide), although other surface antigens are also being investigated for mAb therapy. Several promising candidates for mAbs are undergoing clinical trials at present. Despite several failures and the investigation of novel targets, mAb therapy provides a glimmer of hope for the treatment of severe bacterial sepsis and septic shock. In this review, mAb candidates, their efficacy against controlling infection, with special emphasis on potential roadblocks, and prospects are discussed.

Pneumolysin as a target for new therapies against pneumococcal infections: A systematic review

BACKGROUND

This systematic review evaluates pneumolysin (PLY) as a target for new treatments against pneumococcal infections. Pneumolysin is one of the main virulence factors produced by all types of pneumococci. This toxin (53 kDa) is a highly conserved protein that binds to cholesterol in eukaryotic cells, forming pores that lead to cell destruction.

METHODS

The databases consulted were MEDLINE, Web of Science, and Scopus. Articles were independently screened by title, abstract, and full text by two researchers, and using consensus to resolve any disagreements that occurred. Articles in other languages different from English, patents, cases report, notes, chapter books and reviews were excluded. Searches were restricted to the years 2000 to 2021. Methodological quality was evaluated using OHAT framework.

RESULTS

Forty-one articles describing the effects of different molecules that inhibit PLY were reviewed. Briefly, the inhibitory molecules found were classified into three main groups: those exerting a direct effect by binding and/or blocking PLY, those acting indirectly by preventing its effects on host cells, and those whose mechanisms are unknown. Although many molecules are proposed as toxin blockers, only some of them, such as antibiotics, peptides, sterols, and statins, have the probability of being implemented as clinical treatment. In contrast, for other molecules, there are limited studies that demonstrate efficacy in animal models with sufficient reliability.

DISCUSSION

Most of the studies reviewed has a good level of confidence. However, one of the limitations of this systematic review is the lack of homogeneity of the studies, what prevented to carry out a statistical comparison of the results or meta-analysis.

CONCLUSION

A panel of molecules blocking PLY activity are associated with the improvement of the inflammatory process triggered by the pneumococcal infection. Some molecules have already been used in humans for other purposes, so they could be safe for use in patients with pneumococcal infections. These patients might benefit from a second line treatment during the initial stages of the infection preventing acute respiratory distress syndrome and invasive pneumococcal diseases. Additional research using the presented set of compounds might further improve the clinical management of these patients.

Interplay between biomaterials and the immune system: Challenges and opportunities in regenerative medicine

The use of biomaterials for tissue engineering and regenerative medicine applications has increased dramatically over recent years. However, the clinical uptake of a wide variety of biomaterials remains limited due to adverse effects commonly exhibited by patients, which are caused by the host immune response. Despite this, current in vitro evaluation standards (ISO-10993) for assessing the host response to biomaterials have limitations in predicting the likelihood of in vivo biomaterial acceptance. Furthermore, endotoxin contamination of biomaterials is rarely considered, despite its ability to significantly affect the performance of biomaterials and engineered tissues. This review highlights the importance of the immune response to biomaterials and discusses existing challenges and opportunities in the development and standardised assessment of the immune response to biomaterials, including the importance of endotoxin levels. In addition, the properties of biomaterials that impact the host immune response and the exploitation of immunomodulatory biomaterials in regenerative medicine are explored. Finally, a standardised in vitro pathway of evaluating the immune response to biomaterials (hydrogels) and their regenerative potential is proposed, aiming to ensure safety and consistency, while reducing costs and the use of animals in the biomaterials research for tissue engineering and regenerative medicine. STATEMENT OF SIGNIFICANCE: This review presents a critical analysis of the role of the interactions between the immune system and biomaterials in determining the therapeutic success of biomaterial-based approaches. No such review addressing the lack of understanding of biomaterial-immune system interactions during the developmental and pre-clinical stages of biomaterials, including the impact of the endotoxin levels of biomaterials on the immune response, is published. As there is a lack of in vitro regulations to evaluate the immune response to biomaterials, a standardised in vitro pathway to evaluate the immune response to biomaterials (hydrogels) and their immunomodulatory and regenerative potential for use in tissue engineering/regenerative medicine applications is presented. The aim of the proposed pathway of biomaterial evaluation is to ensure safety and consistency in the biomaterials research community, while reducing costs and animal use (through the concept of the 3R's - reduction, refinement, and replacement of animals).

The Influence of Photodynamic Antimicrobial Chemotherapy on the Microbiome, Neuroendocrine and Immune System of Crustacean Post Larvae

Photodynamic antimicrobial chemotherapy (PACT), employing a combination of light and natural photosensitizer molecules such as curcumin, has been accepted as a safe modality for removing aquatic pathogens which cause diseases such as cholera in humans and vibriosis in aquatic animals. Curcumin and its photodegradation products are generally considered as safe to animals, but the impact of reactive oxygen species (ROS) generated by these products on the growth and survival of organisms at a cellular level has not been studied in detail. The ROS generated by curcumin on photoexcitation using blue light (λ_max 405 nm, 10 mW cm^-2) disinfects more than 80% of free-living Vibrio spp. in the rearing water of Penaeus monodon. However, it is less effective against Vibrio spp. colonized inside P. monodon because the carapace of the animal prevents the transmission of more than 70% of light at the 400-450 nm range and thus reduces the formation of ROS. The influence of curcumin and photoexcited curcumin on the microbiome of P. monodon were revealed by nanopore sequencing. The photoexcited curcumin induced irregular expression of genes coding the moult-inhibiting hormone (MIH), Crustacean hyperglycaemic hormone (CHH)), prophenoloxidase (ProPO), and crustin, which indicates toxic effects of ROS generated by photoexcited curcumin on the neuroendocrine and immune systems of crustaceans, which could alter their growth and survival in aquaculture settings. The study proposed the cautious use of photodynamic therapy in aquaculture systems, and care must be taken to avoid photoexcitation when animals are experiencing moulting or environmental stress.

Crystal structures of pertussis toxin with NAD+ and analogs provide structural insights into the mechanism of its cytosolic ADP-ribosylation activity

Bordetella pertussis is the causative agent of whooping cough, a highly contagious respiratory disease. Pertussis toxin (PT), a major virulence factor secreted by B. pertussis, is an AB5-type protein complex topologically related to cholera toxin. The PT protein complex is internalized by host cells and follows a retrograde trafficking route to the endoplasmic reticulum, where it subsequently dissociates. The released enzymatic S1 subunit is then translocated from the endoplasmic reticulum into the cytosol and subsequently ADP-ribosylates the inhibitory alpha-subunits (Gαi) of heterotrimeric G proteins, thus promoting dysregulation of G protein–coupled receptor signaling. However, the mechanistic details of the ADP-ribosylation activity of PT are not well understood. Here, we describe crystal structures of the S1 subunit in complex with nicotinamide adenine dinucleotide (NAD+), with NAD+ hydrolysis products ADP-ribose and nicotinamide, with NAD+ analog PJ34, and with a novel NAD+ analog formed upon S1 subunit crystallization with 3-amino benzamide and NAD+, which we name benzamide amino adenine dinucleotide. These crystal structures provide unprecedented insights into pre- and post-NAD+ hydrolysis steps of the ADP-ribosyltransferase activity of PT. We propose that these data may aid in rational drug design approaches and further development of PT-specific small-molecule inhibitors.

Novel Strategies to Inhibit Pertussis Toxin

Pertussis, also known as whooping cough, is a respiratory disease caused by infection with Bordetella pertussis, which releases several virulence factors, including the AB-type pertussis toxin (PT). The characteristic symptom is severe, long-lasting paroxysmal coughing. Especially in newborns and infants, pertussis symptoms, such as leukocytosis, can become life-threatening. Despite an available vaccination, increasing case numbers have been reported worldwide, including Western countries such as Germany and the USA. Antibiotic treatment is available and important to prevent further transmission. However, antibiotics only reduce symptoms if administered in early stages, which rarely occurs due to a late diagnosis. Thus, no causative treatments against symptoms of whooping cough are currently available. The AB-type protein toxin PT is a main virulence factor and consists of a binding subunit that facilitates transport of an enzyme subunit into the cytosol of target cells. There, the enzyme subunit ADP-ribosylates inhibitory α-subunits of G-protein coupled receptors resulting in disturbed cAMP signaling. As an important virulence factor associated with severe symptoms, such as leukocytosis, and poor outcomes, PT represents an attractive drug target to develop novel therapeutic strategies. In this review, chaperone inhibitors, human peptides, small molecule inhibitors, and humanized antibodies are discussed as novel strategies to inhibit PT.