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Antitoxins are needed that can be produced economically with improved safety and shelf life compared to conventional antisera-based therapeutics. Here we report a practical strategy for development of simple antitoxin therapeutics with substantial advantages over currently available treatments. The therapeutic strategy employs a single recombinant 'targeting agent' that binds a toxin at two unique sites and a 'clearing Ab' that binds two epitopes present on each targeting agent. Co-administration of the targeting agent and the clearing Ab results in decoration of the toxin with up to four Abs to promote accelerated clearance. The therapeutic strategy was applied to two Botulinum neurotoxin (BoNT) serotypes and protected mice from lethality in two different intoxication models with an efficacy equivalent to conventional antitoxin serum. Targeting agents were a single recombinant protein consisting of a heterodimer of two camelid anti-BoNT heavy-chain-only Ab V(H) (VHH) binding domains and two E-tag epitopes. The clearing mAb was an anti-E-tag mAb. By comparing the in vivo efficacy of treatments that employed neutralizing vs. non-neutralizing agents or the presence vs. absence of clearing Ab permitted unprecedented insight into the roles of toxin neutralization and clearance in antitoxin efficacy. Surprisingly, when a post-intoxication treatment model was used, a toxin-neutralizing heterodimer agent fully protected mice from intoxication even in the absence of clearing Ab. Thus a single, easy-to-produce recombinant protein was as efficacious as polyclonal antiserum in a clinically-relevant mouse model of botulism. This strategy should have widespread application in antitoxin development and other therapies in which neutralization and/or accelerated clearance of a serum biomolecule can offer therapeutic benefit.

Background. Clostridium botulinum strain IBCA10-7060, isolated from a patient with infant botulism, produced botulinum neurotoxin type B (BoNT/B) and another BoNT that, by use of the standard mouse bioassay, could not be neutralized by any of the Centers for Disease Control and Prevention-provided monovalent polyclonal botulinum antitoxins raised against BoNT types A-G. Methods and Results. The combining of antitoxins to neutralize the toxicity of known bivalent C. botulinum strains Ab, Ba, Af, and Bf also failed to neutralize the second BoNT. Analysis of culture filtrate by double immunodiffusion yielded a single line of immunoprecipitate with anti-A, anti-B, and anti-F botulinum antitoxins but not with anti-E antitoxin. A heptavalent F(ab')₂ botulinum antitoxin A-G obtained from the US Army also did not neutralize the second BoNT. An antitoxin raised against IBCA10-7060 toxoid protected mice against BoNT/B (Okra) and against the second BoNT but did not protect mice against BoNT/A (Hall) or BoNT/F (Langeland). Conclusion. The second BoNT thus fulfilled classic criteria for being designated BoNT/H. IBCA10-7060 is the first C. botulinum type Bh strain to be identified. BoNT/H is the first new botulinum toxin type to be recognized in >40 years, and its recognition could not have been accomplished without the availability of the mouse bioassay.

Botulinum neurotoxins (BoNTs) are highly potent, neuroparalytic protein toxins that block the release of acetylcholine from motor neurons and autonomic synapses. The unparalleled toxicity of BoNTs results from the highly specific and localized cleavage of presynaptic proteins required for nerve transmission. Currently, the only pharmacotherapy for botulism is prophylaxis with antitoxin, which becomes progressively less effective as symptoms develop. Treatment for symptomatic botulism is limited to supportive care and artificial ventilation until respiratory function spontaneously recovers, which can take weeks or longer. Mechanistic insights into intracellular toxin behavior have progressed significantly since it was shown that toxins exploit synaptic endocytosis for entry into the nerve terminal, but fundamental questions about host-toxin interactions remain unanswered. Chief among these are mechanisms by which BoNT is internalized into neurons and trafficked to sites of molecular toxicity. Elucidating how receptor-bound toxin is internalized and conditions under which the toxin light chain engages with target SNARE proteins is critical for understanding the dynamics of intoxication and identifying novel therapeutics. Here, we discuss the implications of newly discovered modes of synaptic vesicle recycling on BoNT uptake and intraneuronal trafficking.

A patient with obstruction of the terminal ileum from Crohn's disease developed complete paralysis in week 1 of hospitalization. Features initially suggested Guillain-Barre syndrome, but botulinum toxin was identified in serum and stool specimens from week 1 and type A toxin - producing Clostridium botulinum in stool specimens from weeks 3 to 19, confirming botulism due to intestinal colonization. In week 19, the inflamed small bowel was resected, and C. botulinum disappeared from the stool. In week 31, the patient was able to breath without assistance. Testing for an active immune response with neutralizing antibodies to C. botulinum at week 19 was positive; these antibodies remained at a protective level for > 1 year. Intestinal colonization botulism, rare in adults, should be considered for patients with descending paralysis, especially those with a preceding alteration in small bowel function. An active immune response to botulinum toxin with production of protective antibodies has not been demonstrated previously in a patient with botulism and may have contributed to this patient's recovery.

An aluminum hydroxide ( alum)-adsorbed, purified, botulinum F toxoid (Bot F) vaccine was manufactured to be administered as a stand-alone monovalent vaccine or to be added to the current botulinum pentavalent toxoid vaccine to make a hexavalent vaccine. We conducted a phase II trial of the Bot F vaccine over 3 years in 144 healthy adult volunteers to identify one of three vaccination schedules that was safe and maximally immunogenic for adult volunteers. We vaccinated 116 volunteers 1–3 times with Bot vaccine, and 28 volunteers 1–3 times with a licensed, alum-adsorbed hepatitis B vaccine (Engerix-B) as a reaction control group. After 1 year, 42 Bot volunteers with low, mouse anti-toxin titers (<0.10 IU/ml) received a booster injection and were followed for an additional year. The Bot vaccine inoculated three times over 28–42 days was generally well tolerated and safe, whether injected by the subcutaneous (s.c.) or intramuscular (i.m.) route, although it caused significantly more local reactions than did the HBV vaccine. Two vaccination schedules of three primary injections over 42 days (days 0, 14 and 42 or days 0, 21 and 42) provided significantly better protective immunity (anti-toxin levels >0.02 IU/ml) than did vaccinations given over 28 days (days 0, 7 and 28). Vaccine reactogenicity and immunogenicity were similar over 42 days whether administered subcutaneously or intramuscularly. However, even the most immunogenic schedule left 7–16% of volunteers unprotected at day 56 and 33–42% of vaccinees unprotected at 1 year. The booster dose administered at 1 year induced high levels of protective serum anti-toxin in all persons assayed which persisted for at least one additional year. A more potent vaccine formulation will be required to protect more individuals after primary immunization.

The need for a human-derived immune globulin to replace the equine antitoxins currently used in the treatment of botulism is well recognized. A small group of individuals who had received multiple immunizations with pentavalent botulinal toxoid were plasmapheresed for the purpose of collecting a botulism-immune plasma of human origin to be fractionated for the production of immune globulin. Human-derived immune globulin will offer the advantage over equine antitoxins of not inducing reactions to foreign protein and of having a prolonged effective half-life.