Human botulism immune globulin for the treatment of infant botulism

Cell entry strategy of clostridial neurotoxins

Tetanus neurotoxin and botulinum neurotoxins are the causative agents of tetanus and botulism. They block the release of neurotransmitters from synaptic vesicles in susceptible animals and man and act in nanogram quantities because of their ability to specifically attack motoneurons. They developed an ingenious strategy to enter neurons. This involves a concentration step via complex polysialo gangliosides at the plasma membrane and the uptake and ride in recycling synaptic vesicles initiated by binding to a specific protein receptor. Finally, the neurotoxins shut down the synaptic vesicle cycle, which they had misused before to enter their target cells, via specific cleavage of protein core components of the cellular membrane fusion machinery. The uptake of four out of seven known botulinum neurotoxins into synaptic vesicles has been demonstrated to rely on binding to intravesicular segments of the synaptic vesicle proteins synaptotagmin or synaptic vesicle protein 2. This review summarizes the present knowledge about the cell receptor molecules and the mode of toxin-receptor interaction that enables the toxins' sophisticated access to their site of action.

Selection and characterization of a human monoclonal neutralizing antibody for Clostridium Botulinum neurotoxin serotype B

Botulinum neurotoxins (BoNTs) are causative agents for botulism and are identified as a category A bioterror agents by the Centers for Disease Control and Prevention (CDC). Current antitoxins against BoNTs intoxication have some limitations including side effects or limited supply. As an alternative, neutralizing monoclonal antibodies will play an increasing role as BoNTs therapeutics. To date, no human anti-BoNT/B neutralizing monoclonal antibodies have yet to be reported. Herein, we describe an improved selection approach and characterization of a human monoclonal antibody, F2, which is capable of binding BoNT/B with high specificity and displays neutralizing activity in an in vitro cell-based assay. Through surface plasmon resonance studies, we have determined its association and dissociation rate constants. In sum, our data demonstrate that monoclonal antibody F2 is a promising BoNT/B therapeutic lead for further development.

The strange case of the botulinum neurotoxin: using chemistry and biology to modulate the most deadly poison

In the classic novella "The Strange Case of Dr. Jekyll and Mr. Hyde", Robert Louis Stevenson paints a stark picture of the duality of good and evil within a single man. Botulinum neurotoxin (BoNT), the most potent known toxin, possesses an analogous dichotomous nature: It shows a pronounced morbidity and mortality, but it is used with great effect in much lower doses in a wide range of clinical scenarios. Recently, tremendous strides have been made in the basic understanding of the structure and function of BoNT, which have translated into widespread efforts towards the discovery of biomacromolecules and small molecules that specifically modulate BoNT activity. Particular emphasis has been placed on the identification of inhibitors that can counteract BoNT exposure in the event of a bioterrorist attack. This Review summarizes the current advances in the development of therapeutics, including vaccines, peptides, and small-molecule inhibitors, for the prevention and treatment of botulism.

Infant botulism: two recent cases and literature review

Infant botulism is a cause for significant pediatric morbidity in the United States, though early recognition and supportive care can greatly improve clinical outcomes. Since the approval of human botulism immune globulin by the United States Food and Drug Administration (FDA) for the treatment of infant botulism in 2003, the importance of prompt initiation of therapy has been emphasized, with clinical suspicion remaining the mainstay of diagnosis. In this report, 2 cases of infant botulism are described. Each presented to the Pediatric Neurology service at our institution in Upstate New York in the spring and summer months of 2007 and were felt to be related to markedly dusty environmental conditions and the probable ingestion of C. botulinum organisms present in soil. Following this, a comprehensive review of the literature regarding infant botulism in the United States is presented, wherein the pathophysiology, clinical features, epidemiology, and treatment are discussed.

Infant botulism type Ba: first culture-confirmed case in the United Arab Emirates

We report on a 3-month-old girl with culture-confirmed infant botulism caused by a rare double toxin-producing Clostridium botulinum type Ba. This case was not related to honey-feeding. The clinical course was prolonged, with minimal spontaneous improvement at onset, and a period of fluctuating motor weakness and nasogastric feeding dependence afterward. Neurophysiologic studies produced normal results. Human botulism immune globulin was administered empirically on day 23 of presentation, with rapid full recovery. This case highlights the importance of pursuing diagnoses of infant botulism despite normal results of neurophysiologic testing and no history of honey-feeding. Our case also demonstrates a favorable response to human botulism immune globulin, despite the relatively late treatment.

Analysis of epitope information related to Bacillus anthracis and Clostridium botulinum

We have reviewed the information about epitopes of immunological interest from Clostridium botulinum and Bacillus anthracis, by mining the Immune Epitope Database and Analysis Resource. For both pathogens, the vast majority of epitopes reported to date are derived from a single protein: the protective antigen of B. anthracis and the neurotoxin type A of C. botulinum. A detailed analysis of the data was performed to characterize the function, localization and conservancy of epitopes identified as neutralizing and/or protective. In order to broaden the scope of this analysis, we have also included data describing immune responses against defined fragments (over 50 amino acids long) of the relevant antigens. The scarce information on T-cell determinants and on epitopes from other antigens besides the toxins, highlights a gap in our knowledge and identifies areas for future research. Despite this, several distinct structures at the epitope and fragment level are described herein, which could be potential additions to future vaccines or targets of novel immunotherapeutics and diagnostic reagents.

Characterization of new formalin-detoxified botulinum neurotoxin toxoids

Antigenicities of several formalin-detoxified botulinum neurotoxin preparations were measured by inhibition and sandwich enzyme-linked immunosorbent assay (ELISA), and immunogenicity was studied in mice. The toxoids were derived primarily from the serotype A 150-kDa neurotoxin protein, while one toxoid was derived from the naturally occurring 900-kDa toxin-hemagglutinin complex. Antigenicity was severely compromised in two commercially available toxoids. A variety of new toxoids were synthesized in-house by optimizing formaldehyde reaction conditions. Three of the resulting toxoids were found to be antigenically identical to the native toxin, as measured by inhibition ELISA, in spite of showing a reduction of toxicity by more than 100,000-fold. Sandwich ELISAs indicated that the in-house toxoids were two- to threefold less antigenic than the neurotoxin compared to commercial toxoids, which were about 100-fold less antigenic. Mice were immunized twice, on day 0 and day 14. By day 28, relatively high toxin-specific immunoglobulin G (IgG) titers were detected in animals that had received any of the in-house toxoids, with greater than 99% being IgG1 and the remainder being IgG2. These immunized mice remained asymptomatic after being challenged with 50 to 1,000,000 50% lethal dose (LD(50)) units of the 900-kDa neurotoxin. In contrast, animals immunized with several different batches of commercially available toxoids did not develop measurable toxin-specific antibody titers. However, these mice survived neurotoxin challenges with 2 LD(50) units but died when challenged with 6 LD(50) units. Neutralizing titers measured from pools of sera generated with the in-house toxoid preparations ranged from 2.5 to 5 U/ml. In terms of predicting immunogenicity, inhibition ELISAs comparing each formalin toxoid to the parent toxin provided good insight for screening the new toxoids as well as for estimating their relative in vivo potencies. Inhibition ELISA data indicate that those toxoids that most closely resemble the native toxin are highly immunogenic and protective. The superior quality of these new toxoids makes them useful tools for continued use in ELISA development and for antitoxin production.

Global occurrence of infant botulism, 1976-2006

OBJECTIVE

To summarize the worldwide occurrence of reported infant (intestinal toxemia) botulism cases since first recognition of the disease in 1976.

PATIENTS AND METHODS

We collected information on infant botulism cases by active and passive surveillance, by provision of therapeutic Human Botulism Immune Globulin to suspected cases, and by searching the medical literature. We defined a case as laboratory-confirmed botulism that occurred in an infant <or=12 months of age that was not caused by the ingestion of botulinum toxin in food.

RESULTS

Twenty-six countries representing 5 continents reported the occurrence of at least 1 case of infant botulism among their residents. The United States, Argentina, Australia, Canada, Italy, and Japan, in this order, reported the largest number of cases. A history of honey exposure was significantly more common among case subjects hospitalized outside of the United States than among those who were recently hospitalized in California.

CONCLUSIONS

Most countries have not yet reported cases of infant botulism. This limited reporting of the disease to date contrasts with the known global occurrence of Clostridium botulinum spores in soils and dust and suggests that infant botulism may be under-recognized, underreported, or both. When bulbar palsies, hypotonia, and weakness are present, physicians should consider the possibility of infant botulism even if the patient has not been fed honey. Publication of additional case reports and surveillance summaries will enhance understanding of the occurrence and extent of this under-recognized disease.

Overconfidence as a cause of diagnostic error in medicine

The great majority of medical diagnoses are made using automatic, efficient cognitive processes, and these diagnoses are correct most of the time. This analytic review concerns the exceptions: the times when these cognitive processes fail and the final diagnosis is missed or wrong. We argue that physicians in general underappreciate the likelihood that their diagnoses are wrong and that this tendency to overconfidence is related to both intrinsic and systemically reinforced factors. We present a comprehensive review of the available literature and current thinking related to these issues. The review covers the incidence and impact of diagnostic error, data on physician overconfidence as a contributing cause of errors, strategies to improve the accuracy of diagnostic decision making, and recommendations for future research.

Attomolar detection of botulinum toxin type A in complex biological matrices

BACKGROUND

A highly sensitive, rapid and cost efficient method that can detect active botulinum neurotoxin (BoNT) in complex biological samples such as foods or serum is desired in order to 1) counter the potential bioterrorist threat 2) enhance food safety 3) enable future pharmacokinetic studies in medical applications that utilize BoNTs.

METHODOLOGY/PRINCIPAL FINDINGS

Here we describe a botulinum neurotoxin serotype A assay with a large immuno-sorbent surface area (BoNT/A ALISSA) that captures a low number of toxin molecules and measures their intrinsic metalloprotease activity with a fluorogenic substrate. In direct comparison with the "gold standard" mouse bioassay, the ALISSA is four to five orders of magnitudes more sensitive and considerably faster. Our method reaches attomolar sensitivities in serum, milk, carrot juice, and in the diluent fluid used in the mouse assay. ALISSA has high specificity for the targeted type A toxin when tested against alternative proteases including other BoNT serotypes and trypsin, and it detects the holotoxin as well as the multi-protein complex form of BoNT/A. The assay was optimized for temperature, substrate concentration, size and volume proportions of the immuno-sorbent matrix, enrichment and reaction times. Finally, a kinetic model is presented that is consistent with the observed improvement in sensitivity.

CONCLUSIONS/SIGNIFICANCE

The sensitivity, specificity, speed and simplicity of the BoNT ALISSA should make this method attractive for diagnostic, biodefense and pharmacological applications.

Biological terrorism

A biological terrorism event could have a large impact on the general population and health care system. The impact of an infectious disaster will most likely be great to emergency departments, and the collaboration between emergency and infectious disease specialists will be critical in developing an effective response. A bioterrorism event is a disaster that requires specific preparations beyond the usual medical disaster planning. An effective response would include attention to infection control issues and plans for large-scale vaccination or antimicrobial prophylaxis. This article addresses some general issues related to preparing an effective response to a biological terrorism event. It will also review organisms and toxins that could be used in biological terrorism, including clinical features, management, diagnostic testing, and infection control.

Investigating novel therapeutic targets and molecular mechanisms to treat botulinum neurotoxin A intoxication

Botulinum neurotoxin (BoNT) has recently catapulted into the public consciousness in two seemingly disparate roles: potential use as a biological weapon and treatment of neuromuscular disorders. This bacterially produced protein is the most potent toxin known to humans, with a lethal dose estimated at approximately 1 ng/kg of body weight. BoNT intoxication occurs via a multistep process that includes recognition of peripheral neuronal cell membrane receptors, endocytosis and translocation of the light chain metalloprotease into the cytosol, and catalytic cleavage of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins critical for vesicle fusion and neurotransmitter release. Each of these distinct steps has been studied at the molecular level and is a valid target for the development of pharmacological interventions to prevent the considerable mortality associated with botulism. Alternatively, clinical uses of BoNT continue to expand at an ever-increasing rate; however, further optimization of BoNT therapy is critical to maximize efficacy.

The biology and future prospects of antivirulence therapies

The emergence and increasing prevalence of bacterial strains that are resistant to available antibiotics demand the discovery of new therapeutic approaches. Targeting bacterial virulence is an alternative approach to antimicrobial therapy that offers promising opportunities to inhibit pathogenesis and its consequences without placing immediate life-or-death pressure on the target bacterium. Certain virulence factors have been shown to be potential targets for drug design and therapeutic intervention, whereas new insights are crucial for exploiting others. Targeting virulence represents a new paradigm to empower the clinician to prevent and treat infectious diseases.

Subunit vaccine against the seven serotypes of botulism

Botulinum neurotoxins (BoNTs) are the most toxic proteins for humans and are classified as category A toxins. There are seven serotypes of BoNTs defined by the lack of cross-serotype toxin neutralization. Thus, an effective vaccine must neutralize each BoNT serotype. BoNTs are organized as dichain A-B toxins, where the N-terminal domain (light chain) is a zinc metalloprotease targeting soluble NSF attachment receptor proteins that is linked to the C-terminal domain (heavy chain [HC]) by a disulfide bond. The HC comprises a translocation domain and a C-terminal receptor binding domain (HCR). HCRs of the seven serotypes of BoNTs (hepta-HCR) were engineered for expression in Escherichia coli, and each HCR was purified from E. coli lysates. Immunization of mice with the E. coli-derived hepta-serotype HCR vaccine elicited an antibody response to each of the seven BoNT HCRs and neutralized challenge by 10,000 50% lethal doses of each of the seven BoNT serotypes. A solid-phase assay showed that the anti-hepta-serotype HCR sera inhibited the binding of HCR serotypes A and B to the ganglioside GT1b, the first step in BoNT intoxication of neurons. This is the first E. coli-derived vaccine that effectively neutralizes each of the seven BoNT serotypes.

Infant botulism: 20 years' experience at a single institution

Admissions from January 1, 1985, to December 31, 2005, with the diagnosis of infant botulism were reviewed to describe the clinical presentation, course, outcome, and changes related to the availability of botulism immune globulin treatment. Botulism diagnoses were confirmed by the finding of toxin or Clostridium botulinum organisms in stool samples (type A, 14; type B, 25; type not noted, 5). Twenty-four patients were admitted from 1985-1994 and 20 from 1995-2004. Infants in the two decades were similar in age, demographics, and presenting features. Ventilator support was needed in 13 of 24 (54%) in 1985-1994 and in 15 of 20 (75%) in 1995-2005; 43 required nasogastric feeding. Seventeen patients were treated with botulism immune globulin. Length of stay was shorter in infants treated with botulism immune globulin (13.5 vs 23 days, P = .009), with a trend toward reduced need for nasogastric feeding and in shorter duration of tracheal intubation. All patients recovered fully. Even with the availability of botulism immune globulin, meticulous supportive care remained essential for recovery.

Infant botulism: a 30-year experience spanning the introduction of botulism immune globulin intravenous in the intensive care unit at Childrens Hospital Los Angeles

OBJECTIVE

To report a tertiary care hospital's 30-year experience with the diagnosis, treatment, and outcome of infant botulism in the PICU before and after the availability of Botulism Immune Globulin Intravenous.

METHODS

This was a retrospective medical chart review of the 67 patients who had received a diagnosis of infant botulism and were admitted to the ICU from 1976 to 2005. The ages on presentation, length of hospital stay, length of ICU stay, length of mechanical ventilation, and type of botulism toxin were recorded and compared for patients who had received Botulism Immune Globulin Intravenous and those who had not. On the basis of our results, conclusions were drawn regarding the effect of Botulism Immune Globulin Intravenous on the morbidity of infant botulism.

RESULTS

Sixty-seven patients' charts were reviewed; 23 male and 29 female patients did not receive Botulism Immune Globulin Intravenous. Of patients who did not receive Botulism Immune Globulin Intravenous, the median age at presentation was 71 days, median length of hospital stay was 35 days, ICU stay was 24 days, and duration of mechanical ventilation was 17 days. A total of 40% had type A toxin, and 60% had type B toxin. There was a significant difference between patients with toxin types A and B in length of hospital stay but not length of ICU stay or mechanical ventilation. Patients with type A toxin were significantly older than patients with type B toxin. Fifteen children received Botulism Immune Globulin Intravenous. There were statistically significant differences in length of hospital stay, length of ICU stay, and length of mechanical ventilation between patients who received Botulism Immune Globulin Intravenous and those who did not.

CONCLUSIONS

The use of Botulism Immune Globulin Intravenous significantly decreased the length of ICU stay, length of mechanical ventilation, and overall hospital stay in children with infant botulism.

Nurse staffing, mortality, and length of stay in for-profit and not-for-profit hospitals

The issue of differential quality in for-profit (FP) and not-for-profit (NFP) hospitals remains a critical health policy question. With research demonstrating a relationship between nurse staffing and quality, the question arises whether the relationship differs in these hospital types. Using Healthcare Cost and Utilization Project data from the period 1990-1995, we found that case mix-adjusted registered nurse (RN) staffing was significantly lower in FPs than in NFPs, and we found a superior distribution of outcomes (mortality and length of stay) obtained with a lower level of RN staffing. The differences in mortality and length of stay disappeared, however, after controlling for population and market characteristics.

Botulism diagnostics: from clinical symptoms to in vitro assays

Botulinum neurotoxin (BoNT), which cause the deadly neuroparalytic disease, botulism, is the most toxic substance known to man. BoNT can be used as potential bioterrorism agents, and therefore, pose great threat to national security and public health. Rapid and sensitive detection of BoNTs using molecular and biochemical techniques is an essential component in the diagnosis of botulism, and is yet to be achieved. The most sensitive and widely accepted assay method for BoNTs is mouse bioassay, which takes 4 days to complete. This clearly can not meet the need for clinical diagnosis of botulism, botulinum detection in field conditions, and screening of large scale samples. Consequently, the clinical diagnosis of botulism relies on the clinical symptom development, thus limiting the effectiveness of antitoxin treatment. In response to this critical need, many in vitro methods for BoNT detection are under development. This review is focused on recently developed in vitro detection methods for BoNTs, and emerging new technologies with potential for sensitive and rapid in vitro diagnostics for botulism.

Neuromuscular junction in health and disease

A number of illnesses and other factors can affect the function of the neuromuscular junction (NMJ). These may have an affect at pre- or post-junctional sites. This review outlines the anatomy and the physiology of the NMJ. It also describes the mechanisms and physiological basis of many of the disorders of the NMJ. Finally, the importance of these disorders in anaesthetic practice is discussed.

Infant botulism presenting with poor feeding and lethargy: a review of 4 cases

Infant botulism is a rare cause of hypotonia in young infants. It may present with vague symptoms such as poor feeding and lethargy. We present 4 cases of infant botulism presenting to 2 community hospitals in central Maryland. In each case, poor feeding and lethargy were the chief complaints. One patient was referred to the emergency department with suspected sepsis and one with suspected intussusception. Three patients required endotracheal intubation. All were treated with botulism immune globulin, and all eventually made full recoveries. We discuss the differential diagnosis and provide an overview of infant botulism.

Intestinal toxemia botulism in Italy, 1984–2005

Botulism in humans is caused by botulinum neurotoxins, produced in most cases by Clostridium botulinum, although other Clostridia species are implicated as well. Of the five forms of botulism in humans, three are referred to as "infective": wound botulism, infant botulism, and adult intestinal botulism; the latter two forms are also referred to as "intestinal toxemia botulism" because the organism colonizes the lumen of the intestinal tract and produces botulinum neurotoxin in vivo. Twenty-three cases of infant botulism and three cases of adult intestinal botulism occurred in Italy between 1984 and 2005. Microbiological analyses of clinical, environmental, and food samples and analysis of clinical and epidemiological data revealed two main characteristics of intestinal toxemia botulism in Italy that are not common in cases in other countries: the isolation of a strain of C. butyricum that produced botulinum neurotoxin type E in 6 of 26 cases, including two cases of adult intestinal toxemia botulism, and the onset of botulism in these cases with concomitant severe gastrointestinal symptomatology. This report summarizes the microbiological, clinical, and epidemiological data of all cases of intestinal toxemia botulism that have occurred in Italy in the period 1984-2005.

Chemical Terrorism Attacks: Update on Antidotes

There is well-founded concern that a chemical or radioactive agent will at some point be used as a weapon of terror. There are several antidotes that, if used correctly in a timely fashion, can help lessen the harm caused by these agents. This article is meant to introduce the clinician to several such agents, along with the antidotes useful in the management of exposure to these. It covers the indications, administration, and precautions for using these antidotes.

Food Poisoning

Food poisoning is encountered throughout the world. Many of the toxins responsible for specific food poisoning syndromes are no longer limited to isolated geographic locations. With increased travel and the ease of transporting food products, it is likely that a patient may present to any emergency department with the clinical effects of food poisoning. Recognizing specific food poisoning syndromes allows emergency health care providers not only to initiate appropriate treatment rapidly but also to notify health departments early and thereby prevent further poisoning cases. This article reviews several potential food-borne poisons and describes each agent's mechanism of toxicity, expected clinical presentation, and currently accepted treatment.

Creation and development of the public service orphan drug Human Botulism Immune Globulin

The public service orphan drug Human Botulism Immune Globulin for the treatment of infant botulism would not have come into existence without the federal Orphan Drug Act and the funding mechanism that it provided to conduct pivotal clinical trials. Nonetheless, creating, developing, and achieving licensure of Human Botulism Immune Globulin took approximately 15 years and approximately $10.6 million (2005 dollars) to accomplish. Use of Human Botulism Immune Globulin to treat patients with infant botulism has resulted thus far in more than 30 years of avoided hospital stay and more than $50 million (2005 dollars) of avoided hospital costs. To provide a possible paradigm for others, the circumstances that enabled a state public health department to create, test, license, and distribute an orphan drug are described here.

Clinical mimics of infant botulism

Since 1992, Human Botulism Immune Globulin has been provided by the California Department of Health Services to infants with probable infant botulism, the intestinal toxemia form of human botulism. Human Botulism Immune Globulin became available in California in 1992-1997 within a randomized, controlled, double-blinded, pivotal clinical trial and subsequently became available nationwide in 1998-2003 in an open-label study until its licensure in October 2003 as BabyBIG. Thereafter, Human Botulism Immune Globulin remained available nationwide as an approved orphan-drug product. To achieve prompt neutralization of circulating botulinum toxin, the decision to treat with Human Botulism Immune Globulin has been based on clinical criteria that include a consistent history and physical findings of bulbar palsies, hypotonia, and weakness. After licensure, the charts of patients who did not have laboratory-confirmed infant botulism were reviewed to identify their actual diagnoses. The approximately 5% of 681 patients treated with Human Botulism Immune Globulin who did not have infant botulism fell into 5 categories: spinal muscular atrophy, metabolic disorders, other infectious diseases, miscellaneous, and probable infant botulism lacking laboratory confirmation.

An in vitro and in vivo disconnect uncovered through high-throughput identification of botulinum neurotoxin A antagonists

Among the agents classified as "Category A" by the U.S. Centers for Disease Control and Prevention, botulinum neurotoxin (BoNT) is the most toxic protein known, with microgram quantities of the protein causing severe morbidity and mortality by oral or i.v. routes. Given that this toxin easily could be used in a potential bioterrorist attack, countermeasures urgently are needed to counteract the pathophysiology of BoNT. At a molecular level, BoNT exerts its paralytic effects through intracellular cleavage of vesicle docking proteins and subsequent organism-wide autonomic dysfunction. In an effort to identify small molecules that would disrupt the interaction between the light-chain metalloprotease of BoNT serotype A and its cognate substrate, a multifaceted screening effort was undertaken. Through the combination of in vitro screening against an optimized variant of the light chain involving kinetic analysis, cellular protection assays, and in vivo mouse toxicity assays, molecules that prevent BoNT/A-induced intracellular substrate cleavage and extend the time to death of animals challenged with lethal toxin doses were identified. Significantly, the two most efficacious compounds in vivo showed less effective activity in cellular assays intended to mimic BoNT exposure; indeed, one of these compounds was cytotoxic at concentrations three orders of magnitude below its effective dose in animals. These two lead compounds have surprisingly simple molecular structures and are readily amenable to optimization efforts for improvements in their biological activity. The findings validate the use of high-throughput screening protocols to define previously unrecognized chemical scaffolds for the development of therapeutic agents to treat BoNT exposure.

The use of small molecules to investigate molecular mechanisms and therapeutic targets for treatment of botulinum neurotoxin A intoxication

Botulinum neurotoxins (BoNTs) are agents responsible for botulism, a disease characterized by peripheral neuromuscular blockade and subsequent flaccid paralysis. The potent paralytic ability of these toxins has resulted in their use as a therapeutic; however, BoNTs are also classified by the Centers for Disease Control and Prevention as one of the six highest-risk threat agents of bioterrorism. Consequently, a thorough understanding of the molecular mechanism of BoNT toxicity is crucial before effective inhibitors and, ultimately, an approved drug can be developed. In this article, we systematically detail BoNT intoxication by examining each of the discrete steps in this process. Additionally, rationally designed strategies for combating the toxicity of the most potent BoNT serotype are evaluated.

An Initial Assessment of the Systemic Pharmacokinetics of Botulinum Toxin

Botulinum toxin is an extraordinarily potent molecule that has an unusually long duration of action. Despite this, there is little information available on natural mechanisms for metabolism or elimination and virtually no information on pharmacologically induced mechanisms for metabolism and elimination. Therefore, a number of experiments were performed on laboratory animals that addressed two major issues: 1) the effect of blood on the structure, function, and biologic half-life of the toxin, and 2) the effect of neutralizing antibodies on half-life and elimination of circulating toxin. In the first series of studies, the metabolic transformation of toxin was assessed by incubating it in blood for varying lengths of time. At each time point, aliquots were examined to determine the amount of toxin, the structure of toxin, the catalytic activity of toxin, and the neuromuscular blocking activity of toxin. This work demonstrated that blood did not alter any characteristic of the toxin molecule. Experiments were also done in which toxin was administered to mice and rats at doses that produced clinical poisoning. The results demonstrated that the elimination half-life for native (nonmetabolized) toxin in blood and serum was 230 to 260 min. During the second series of studies, the rate of elimination of circulating toxin was studied in the presence of antibodies directed against the carboxyl-terminal half of the toxin molecule. This work demonstrated that neutralizing antibodies 1) enhanced clearance of toxin from the circulation and 2) enhanced tissue accumulation of toxin, particularly in liver and spleen.

Botulinum type A toxin neutralisation by specific IgG and its fragments: a comparison of mouse systemic toxicity and local flaccid paralysis assays

In this study, we have compared two in vivo assay methods to measure the type A botulinum toxin neutralising activity of specific immunoglobulin G (IgG) and its fragments (F(ab')(2), Fab', Fab) purified from pentavalent botulinum antisera raised in goats. Each assay method was repeated on three separate occasions in mice and relative potencies calculated with respect to a type A equine reference antitoxin. The conventional assay, which measures the number of mice surviving typically after 72 or 96 h following the intraperitoneal administration of a mixture of toxin and antitoxin, gave the following order of potency IgG>F(ab')(2)>Fab'>Fab (6.8>4.7>3.5>2.6 IU/mg). Differences in potency are likely to be due to differences in the pharmacokinetics of the antitoxins, which are related to their molecular weight. The alternative local flaccid paralysis assay, where toxin and antitoxin are injected subcutaneously into the left inguinocrural region, gave results with a narrower range of activities: IgG>Fab'>F(ab')(2)>Fab (6.0>5.9>5.5>4.6 IU/mg). Comparison of the two assay methods showed no significant differences for IgG, F(ab')(2) or Fab', although the Fab fragment was significantly more potent in the non-lethal assay probably because of the reduced influence of antitoxin pharmacokinetics in this localised assay. These findings show that a local flaccid paralysis assay provides a less time consuming and more humane alternative to the lethal assay for the potency testing of botulinum IgG and F(ab')(2) antitoxins.