Therapeutic efficacy of equine botulism antitoxin in Rhesus macaques

Abstract

BackgroundThere are currently no licensed vaccines available for prevention of botulism in humans. The vaccination is not desirable due to expanding therapeutic indications of botulinum toxins. The only available specific treatment for botulism is antitoxin to remove circulating toxin, thus, preventing further neuronal damage. BAT® (Botulism Antitoxin Heptavalent (A, B, C, D, E, F, G)—(Equine)) has been developed and its therapeutic efficacy evaluated against botulinum neurotoxin serotype A (BoNT/A) in Rhesus macaques.Methods and findingsIn a post-exposure prophylaxis (PEP) study, animals were exposed to 4x LD50/kg of BoNT/A and administered intravenously with either BAT (1x or 0.1x scaled human dose), or placebo at 4 hours post-exposure. The animals were monitored for 14 days. For the therapeutic intervention studies, animals were exposed to a 1.7x LD50/kg of BoNT/A and treated intravenously with either placebo or BAT at a 1x scaled human dose at the onset of clinical signs. Animals were monitored on an hourly basis for 14 or 21 days. In the PEP study, all animals tolerated equine based antitoxin without any adverse clinical signs. A 100% survival was observed in groups treated with the BAT compared to 0% survival in those treated with the placebo (p<0.001, Fisher’s exact test). BAT antitoxin prevented the development of signs of neurotoxicity of botulinum toxin. In a therapeutic study, treatment with the BAT at scaled 1x human dose after the onset of clinical signs significantly enhanced survival compared to the placebo (46.6% vs. 0%, p<0.0001, Fisher’s exact test). Additionally, treatment with the BAT delayed the progression of signs (muscular weakness, respiratory distress, oral/nasal discharge) of toxin intoxication and reduced the severity of the disease.ConclusionsA single dose of BAT, when administered to symptomatic monkeys, resulted in a statistically significant survival benefit compared to the placebo. Additionally, BAT completely protected monkeys from the clinical signs of intoxication and subsequent death when administered as PEP treatment. These data in part supported the licensure of BAT under the Animal Rule in the United States by the Food and Drug Administration.

A Novel Rabbit Spirometry Model of Type E Botulism and Its Use for the Evaluation of Postsymptom Antitoxin Efficacy

“…Almost all animals treated at 24 h postexposure (14 IU/kg antitoxin or higher) survived, whereas even a 30-fold-higher antitoxin dose failed to confer any protection when administered at the first signs of disease (39). However, in another recent rhesus macaque study, different results were obtained, as treatment with HBAT after the onset of clinical signs following intravenous exposure to BoNT/A (1.7 LD 50 ) conferred partial protection (46% survival) (42).…”

“…As a consequence, the use of antitoxin in animal studies has been related mostly to time postintoxication, whereas data regarding treatment after the onset of symptoms have been collected only post factum (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32). To the best of our knowledge, only a few attempts to directly measure postsymptom antitoxin efficacy were reported, mostly for BoNT/A-intoxicated nonhuman primates (14,(39)(40)(41)(42). In those studies, as in all other animal models reported thus far, symptoms were based on subjective observations, and therefore, their onset might not be witnessed precisely and may vary substantially among different studies.…”

Symptomatic treatment of botulism with a clinically approved small molecule

“…Here, we test the hypothesis that 3,4-DAP improves respiratory and skeletal muscle paralysis in vivo at clinically relevant time points after exposure to BoNT serotypes A, B, and E. These studies were conducted in mice intoxicated with up to 5 LD 50 of toxin. Based on correlation of the rate of disease progression between nonhuman primates and clinical patients, 5 LD 50 is estimated to represent an upper range for typical human foodborne exposure to serotypes A, B, or E (44,45). We found that human-equivalent doses of 3,4-DAP reversed limb and respiratory paralysis caused by each BoNT serotype.…”

“…Clinical severity scoring (CSS) was conducted by at least 3 blinded participants. A CSS rubric used in guinea pigs and nonhuman primates was modified to remove ptosis and hypersalivation, which are either absent or difficult to reliably measure in mice (44). The resulting modified CSS rubric was (scores are cumulative): mild abdominal paradox (score of 1), moderate abdominal paradox (score of 2), severe abdominal paradox and/or agonal respiratory pattern (score of 3), lethargy (score of 1), generalized body weakness (score of 2), total body paralysis (lack of righting reflex, score of 3), or death (16).…”

Studying the differential efficacy of postsymptom antitoxin treatment in type A versus type B botulism using a rabbit spirometry model

“…In most reported botulism animal studies aimed at evaluating postexposure treatment efficacy, antitoxin was administered in a time postexposure mode. Only a few attempts have been made to study PSAE ( Adler and Franz, 2016 ; Dack and Wood, 1928 ; Kodihalli et al, 2017 ; Oberst et al, 1968 ; Food and Drug Administration BAT ® Data Sheet, ). Moreover, symptoms in these studies were limited to those that can only be noticed by simple observation.…”