Complications, Adverse Reactions, and Insights With the Use of Botulinum Toxin

EXOTOXINS OF the anaerobic spore-forming bacterium Clostridium Botulinum, botulinum toxins (BTX) are the causative agents of the severe food-borne illness botulism. The lethal dose of these agents is approximating 10 9 g/kg body weight, and these neurotoxins are the most toxic naturally occurring substances. The bacterium C. Botulinum has eight serotypes (A, B, C a, C b, D, E, F, and G) that produce seven serologically distinct neurotoxins. Of these, types A, B, and E are most commonly associated with botulism in humans. The different serotypes are structurally and functionally similar; however, specific differences in neuronal acceptor binding sites, intracellular enzymatic sites, and species sensitivities suggest that each serotype is its own unique pharmacologic entity. Neutralizing antibodies developed against one serotype have not been reported to block the biologic activity of another serotype. BTXs exist in their native states as large protein complexes consisting of the neurotoxin moiety (approximately 150 kDa) and one or more nontoxic proteins that function to protect the toxin molecule and may or may not have hemagglutinin activity. Stabilized through noncovalent bonds, nontoxic proteins account for up to 70% of the total mass. BTX-B associates with the nontoxic proteins to form a complex with a total molecular weight of approximately 700 kDa, whereas the BTX-A complex is estimated to be approximately 900 kDa. These large BTX complexes are most stable in the pH range of 5 to 7. At pH values above 7, the protein subunits may dissociate. Although the intracellular targets of the toxins are variable, they all ultimately prevent release of acetylcholine at the neuromuscular junction of striated muscles and thus produce chemical denervation and paralysis of the muscles. This chemical denervation, which is effective for both striated muscle and eccrine glands, is the most studied mode of action of these agents. Pharmacologic and morphologic studies suggest that the toxin enters the nerve endings via endocytosis and inhibits the release of the vesiclebound acetylcholine within the nerve terminal itself. The toxin is rapidly (within 90 minutes) and irreversibly bound to the presynaptic neuron at the neuromuscular junction. The BTX is internalized and then acts on a zinc-dependent endoprotease to disrupt some of the peptides necessary for acetylcholine release. This action may not be complete for 2 weeks and effectively disrupts the affected neuromuscular junction, causing muscular paralysis. There is an ongoing turnover of neuromuscular junctions, however, that is enhanced by toxin exposure. Muscular function begins to return at approximately 3 months and is usually complete by 6 months. The return of muscular function is due to sprout formation, but the original neuromuscular endplate is eventually totally restored. Paralytic activity in the mouse is the mode of measuring the potency of this toxin. One unit is that amount of the toxin that kills 50% (LD50) of a standardized mouse model when injected intraperitoneally. This is affected by many factors such as the strain of mouse and housing conditions. Specific potency can be reduced by the presence of contaminating proteins or inactive, aggregated toxins (toxoid) that may have been carried through in the purification procedures. These molecules may increase the risk for antibody formation. Indeed, an excess of toxoid was considered very early on to be one of the great worries in preparing the agent. It was the goal to produce toxin with as little toxoid as possible. The therapeutic activity of BTX most understood at this time is the peripheral blockade of neuromuscular activity through inhibition of acetylcholine release that causes both muscular weakness and inhibition of eccrine function. Nevertheless, numerous other modes of action are being studied, such as its ability to affect afferent pain signals as well as the communication receptors on peripheral muscle spindles. Some evidence exists that suggests that central reorganization occurs after large amounts of BTX are used peripherally. Nevertheless, there has been no evidence to suggest any permanent degeneration or atrophy of muscles in those patients treated for dystonic or spastic disorders who have been injected