Production, Characterisation and Testing of an Ovine Antitoxin against Ricin; Efficacy, Potency and Mechanisms of Action

Refinement of an ovine-based immunoglobulin therapy against SARS-CoV-2, with comparison of whole IgG versus F(ab')2 fragments

The development of new therapies against SARS-CoV-2 is required to extend the toolkit of intervention strategies to combat the global pandemic. In this study, hyperimmune plasma from sheep immunised with whole spike SARS-CoV-2 recombinant protein has been used to generate candidate products. In addition to purified IgG, we have refined candidate therapies by removing non-specific IgG via affinity binding along with fragmentation to eliminate the Fc region to create F(ab')2 fragments. These preparations were evaluated for in vitro activity and demonstrated to be strongly neutralising against a range of SARS-CoV-2 strains, including Omicron B2.2. In addition, their protection against disease manifestations and viral loads were assessed using a hamster SARS-CoV-2 infection model. Results demonstrated protective effects of both IgG and F(ab')2, with the latter requiring sequential dosing to maintain in vivo activity due to rapid clearance from the circulation.

Comparative Aspects of Ricin Toxicity by Inhalation

The pathogenesis of ricin toxicity following inhalation has been investigated in many animal models, including the non-human primate (predominantly the rhesus macaque), pig, rabbit and rodent. The toxicity and associated pathology described in animal models are broadly similar, but variation appears to exist. This paper reviews the published literature and some of our own unpublished data and describes some of the possible reasons for this variation. Methodological variation is evident, including method of exposure, breathing parameters during exposure, aerosol characteristics, sampling protocols, ricin cultivar, purity and challenge dose and study duration. The model species and strain used represent other significant sources of variation, including differences in macro- and microscopic anatomy, cell biology and function, and immunology. Chronic pathology of ricin toxicity by inhalation, associated with sublethal challenge or lethal challenge and treatment with medical countermeasures, has received less attention in the literature. Fibrosis may follow acute lung injury in survivors. There are advantages and disadvantages to the different models of pulmonary fibrosis. To understand their potential clinical significance, these factors need to be considered when choosing a model for chronic ricin toxicity by inhalation, including species and strain susceptibility to fibrosis, time it takes for fibrosis to develop, the nature of the fibrosis (e.g., self-limiting, progressive, persistent or resolving) and ensuring that the analysis truly represents fibrosis. Understanding the variables and comparative aspects of acute and chronic ricin toxicity by inhalation is important to enable meaningful comparison of results from different studies, and for the investigation of medical countermeasures.

Drugs as Chemical Weapons: Past and Perspectives

The emergence of modern chemical weapons and chemical warfare is traditionally associated with World War I, but the use of poisons in the military has its roots deep in the past. The sources of these poisons have always been natural agents that also served as medicines. This relationship between poison and medicine, and nowadays between chemical warfare and medicine, or between 'military chemistry' and pharmacy, appears to be very important for understanding not only the history but also the possible future of both phenomena. This article looks at some historical examples of the use of drugs as chemical weapons and, conversely, the use of chemical weapons as medicines. It seeks to find answers to some questions that are particularly relevant to the implementation of the Chemical Weapons Convention, which aims to achieve a world without chemical warfare.

RIPpore: A Novel Host-Derived Method for the Identification of Ricin Intoxication through Oxford Nanopore Direct RNA Sequencing

Ricin is a toxin which enters cells and depurinates an adenine base in the sarcin-ricin loop in the large ribosomal subunit, leading to the inhibition of protein translation and cell death. We postulated that this depurination event could be detected using Oxford Nanopore Technologies (ONT) direct RNA sequencing, detecting a change in charge in the ricin loop. In this study, A549 cells were exposed to ricin for 2–24 h in order to induce depurination. In addition, a novel software tool was developed termed RIPpore that could quantify the adenine modification of ribosomal RNA induced by ricin upon respiratory epithelial cells. We provided demonstrable evidence for the first time that this base change detected is specific to RIP activity using a neutralising antibody against ricin. We believe this represents the first detection of depurination in RNA achieved using ONT sequencers. Collectively, this work highlights the potential for ONT and direct RNA sequencing to detect and quantify depurination events caused by ribosome-inactivating proteins such as ricin. RIPpore could have utility in the evaluation of new treatments and/or in the diagnosis of exposure to ricin.

Establishment of a Novel Oral Murine Model of Ricin Intoxication and Efficacy Assessment of Ovine Ricin Antitoxins

Ricin, produced from the castor beans of Ricinus communis, is a cytotoxin that exerts its action by inactivating ribosomes and causing cell death. Accidental (e.g., ingestion of castor beans) and/or intentional (e.g., suicide) exposure to ricin through the oral route is an area of concern from a public health perspective and no current licensed medical interventions exist to protect from the action of the toxin. Therefore, we examined the oral toxicity of ricin in Balb/C mice and developed a robust food deprivation model of ricin oral intoxication that has enabled the assessment of potential antitoxin treatments. A lethal oral dose was identified and mice were found to succumb to the toxin within 48 h of exposure. We then examined whether a despeciated ovine F(ab')2 antibody fragment, that had previously been demonstrated to protect mice from exposure to aerosolised ricin, could also protect against oral intoxication. Mice were challenged orally with an LD99 of ricin, and 89 and 44% of mice exposed to this otherwise lethal exposure survived after receiving either the parent anti-ricin IgG or F(ab')2, respectively. Combined with our previous work, these results further highlight the benefit of ovine-derived polyclonal antibody antitoxin in providing post-exposure protection against ricin intoxication.

Post-Exposure Anti-Ricin Treatment Protects Swine Against Lethal Systemic and Pulmonary Exposures

Ricin, a plant-derived toxin originating from the seeds of Ricinus communis (castor bean plant), is one of the most lethal toxins known. To date, there is no approved post-exposure therapy for ricin exposures. This work demonstrates for the first time the therapeutic efficacy of equine-derived anti-ricin F(ab’)₂ antibodies against lethal pulmonary and systemic ricin exposures in swine. While administration of the antitoxin at 18 h post-exposure protected more than 80% of both intratracheally and intramuscularly ricin-intoxicated swine, treatment at 24 h post-exposure protected 58% of the intramuscular-exposed swine, as opposed to 26% of the intratracheally exposed animals. Quantitation of the anti-ricin neutralizing units in the anti-toxin preparations confirmed that the disparate protection conferred to swine subjected to the two routes of exposure stems from variance between the two models. Furthermore, dose response experiments showed that approximately 3 times lesser amounts of antibody are needed for high-level protection of the intramuscularly compared to the intratracheally intoxicated swine. This study, which demonstrates the high-level post-exposure efficacy of anti-ricin antitoxin at clinically relevant time-points in a large animal model, can serve as the basis for the formulation of post-exposure countermeasures against ricin poisoning in humans.

Multiple metabolic pathways are predictive of ricin intoxication in a rat model

INTRODUCTION

Exposure to ricin can be lethal and treatments that are under development have short windows of opportunity for administration after exposure. It is therefore essential to achieve early detection of ricin exposure to provide the best prognosis for exposed individuals. Ricin toxin can be detected in clinical samples via several antibody-based techniques, but the efficacy of these can be limited due to the rapid processing and cellular uptake of toxin in the body and subsequent low blood ricin concentrations. Other diagnostic tools that perform, in an orthogonal manner, are therefore desirable.

OBJECTIVES

To determine time-dependent metabolic changes in Sprague-Dawley rats following intravenous exposure to ricin.

METHODS

Sprague-Dawley rats were intravenously exposed to ricin and multiple blood samples were collected from each animal for up to 48 h following exposure in two independent studies. Plasma samples were analysed applying HILIC and C₁₈ reversed phase UHPLC-MS assays followed by univariate and multivariate analysis.

RESULTS

In Sprague-Dawley rats we have demonstrated that metabolic changes measured in blood can distinguish between rats exposed intravenously to ricin and controls prior to the onset of behavioral signs of intoxication after 24 h. A total of 37 metabolites were significantly altered following exposure to ricin when compared to controls. The arginine/proline, bile acid and triacylglyceride metabolic pathways were highlighted as being important with two triacylglycerides at 8 h post exposure giving an AUROC score of 0.94. At 16 h and 24 h the AUROC score increased to 0.98 and 1.0 with the number of metabolites in the panel increasing to 5 and 7, respectively.

CONCLUSIONS

These data demonstrate that metabolites may be a useful tool to diagnose and detect ricin exposure, thus increasing the effectiveness of supportive therapy and future ricin-specific medical treatments.

Ricin: An Ancient Story for a Timeless Plant Toxin

The castor plant (Ricinus communis L.) has been known since time immemorial in traditional medicine in the pharmacopeia of Mediterranean and eastern ancient cultures. Moreover, it is still used in folk medicine worldwide. Castor bean has been mainly recommended as anti-inflammatory, anthelmintic, anti-bacterial, laxative, abortifacient, for wounds, ulcers, and many other indications. Many cases of human intoxication occurred accidentally or voluntarily with the ingestion of castor seeds or derivatives. Ricinus toxicity depends on several molecules, among them the most important is ricin, a protein belonging to the family of ribosome-inactivating proteins. Ricin is the most studied of this category of proteins and it is also known to the general public, having been used for several biocrimes. This manuscript intends to give the reader an overview of ricin, focusing on the historical path to the current knowledge on this protein. The main steps of ricin research are here reported, with particular regard to its enzymatic activity, structure, and cytotoxicity. Moreover, we discuss ricin toxicity for animals and humans, as well as the relation between bioterrorism and ricin and its impact on environmental toxicity. Ricin has also been used to develop immunotoxins for the elimination of unwanted cells, mainly cancer cells; some of these immunoconjugates gave promising results in clinical trials but also showed critical limitation.

Generation of Highly Efficient Equine-Derived Antibodies for Post-Exposure Treatment of Ricin Intoxications by Vaccination with Monomerized Ricin

Ricin, a highly lethal toxin derived from the seeds of Ricinus communis (castor beans) is considered a potential biological threat agent due to its high availability, ease of production, and to the lack of any approved medical countermeasure against ricin exposures. To date, the use of neutralizing antibodies is the most promising post-exposure treatment for ricin intoxication. The aim of this work was to generate anti-ricin antitoxin that confers high level post-exposure protection against ricin challenge. Due to safety issues regarding the usage of ricin holotoxin as an antigen, we generated an inactivated toxin that would reduce health risks for both the immunizer and the immunized animal. To this end, a monomerized ricin antigen was constructed by reducing highly purified ricin to its monomeric constituents. Preliminary immunizing experiments in rabbits indicated that this monomerized antigen is as effective as the native toxin in terms of neutralizing antibody elicitation and protection of mice against lethal ricin challenges. Characterization of the monomerized antigen demonstrated that the irreversibly detached A and B subunits retain catalytic and lectin activity, respectively, implying that the monomerization process did not significantly affect their overall structure. Toxicity studies revealed that the monomerized ricin displayed a 250-fold decreased activity in a cell culture-based functionality test, while clinical signs were undetectable in mice injected with this antigen. Immunization of a horse with the monomerized toxin was highly effective in elicitation of high titers of neutralizing antibodies. Due to the increased potential of IgG-derived adverse events, anti-ricin F(ab')₂ antitoxin was produced. The F(ab')₂-based antitoxin conferred high protection to intranasally ricin-intoxicated mice; ~60% and ~34% survival, when administered 24 and 48 h post exposure to a lethal dose, respectively. In line with the enhanced protection, anti-inflammatory and anti-edematous effects were measured in the antitoxin treated mice, in comparison to mice that were intoxicated but not treated. Accordingly, this anti-ricin preparation is an excellent candidate for post exposure treatment of ricin intoxications.

Ribosome Inactivating Proteins: From Plant Defense to Treatments against Human Misuse or Diseases

Ribosome inactivating proteins (RIPs) form a vast family of hundreds of toxins from plants, fungi, algae, and bacteria. RIP activities have also been detected in animal tissues. They exert an N-glycosydase catalytic activity that is targeted to a single adenine of a ribosomal RNA, thereby blocking protein synthesis and leading intoxicated cells to apoptosis. In many cases, they have additional depurinating activities that act against other nucleic acids, such as viral RNA and DNA, or genomic DNA. Although their role remains only partially understood, their functions may be related to plant defense against predators and viruses, plant senescence, or bacterial pathogenesis.