BOTULINUM TOXIN A

Description

Botulism, a disease of the nervous system in animals and humans, was first recorded in Germany in 1735 and was thought to be due to eating a tainted sausage. The name botulism comes from the German ‘botulus’ for sausage. Botulinum toxin (molecular weight of 100 000, consisting of 1300 amino acids) is derived from Clostridium botulinum – an anaerobic spore-forming gram-positive bacterium commonly found in soil. It can affect a wide range of animals, including mammals, fish, and birds. There are seven serological varieties of botulism denoted by the letters A–G. They are structurally similar but antigenically and serologically unique. Only the botulism types A, B, E, and F can cause toxicity in humans. Types C and D can cause toxicity in animals. From 1990 to 2000, the US Centers for Disease Control documented 263 individual cases from 160 foodborne botulism events in the United States; 4% of these resulted in fatality and 103 of these cases occurred in Alaska. They were due to traditional Alaska aboriginal foods, including the ingestion of raw whale fat or muktuk. While the numbers of cases of foodborne botulism and infant botulism are stable, the incidence of wound botulism has increased due to the use of black tar heroin. California has reported the majority of cases of wound botulism associated with tar heroin use.

Uses

The use of botulinum toxin includes medicinal (e.g., treatment of spastic conditions, hyperhidrosis), cosmetic (removal of wrinkles), or as a potential terrorism agent (e.g., via aerosolization).

Indications

Botulinum toxin purified neurotoxin complex (Botox) is a purified form of botulinum toxin type A, produced from a culture of Clostridium botulinum. Injection of botulinum toxin into muscle induces paralysis by inhibiting the release of acetylcholine from motor neurons, thereby blocking neuromuscular conduction. It is approved for the treatment of blepharospasm, strabismus, and excessive sweating. Botox is also approved for use in dermatology to induce paralysis of the muscles of facial expression to reverse deep wrinkles. The effect of an individual treatment usually becomes apparent within 3 days and lasts approximately 3 months. The effect may persist for a longer period after a series of treatments because the muscles atrophy. The major adverse effect is temporary loss of function of a muscle required for normal social functioning, as may occur after inadvertent injection of muscles required for smiling or raising the upper eyelids.

Biological Functions

Botulism is most commonly caused by ingestion of a neurotoxin produced by Clostridium botulinum in improperly canned food. Poisoning may also occur after wound contamination with the organism. Infant botulism may occur when spores of the organism germinate and manufacture the toxin in the intestinal tract of infants. Botulinum toxin works by inhibiting ACh release at all cholinergic synapses.
Botulinum toxins are classified into seven antigenically distinct types, A through G. Each consists of a polypeptide chain of about 150,000 daltons. All but one is nicked by trypsin-type enzymes to yield a light and heavy chain linked by a disulfide bridge.One end of the heavy chain mediates binding to the nerve terminal, and the other initiates internalization of the toxin. The light chain produces the intracellular inhibition of ACh release. This involves a Zn-dependent endopeptidase action to cleave synaptic target proteins that control vesicle docking and fusion with the prejunctional membrane.

Clinical Use

Botulinum toxin is used clinically in the treatment of blepharospasm, writer’s cramp, spasticities of various origins, and rigidity due to extrapyramidal disorders. It is also used to treat gustatory sweating and cosmetically to decrease facial wrinkles. Botulinum toxin A (Botox, Oculinum) injected intramuscularly produces functional denervation that lasts about 3 months. Clinical benefit is seen within 1 to 3 days.Adverse effects range from diplopia and irritation with blepharospasm to muscle weakness with dystonias.

Side effects

Botulinum toxin is the most toxic substance known. One gram of crystalline toxin adequately dispersed can kill a population of a million people, so its use in bioterrorism is a possibility. The toxin can be introduced through inhalation or ingestion but not through dermal exposure. The threat of mass inhalation poisoning is limited by the ability or inability to aerosolize the toxin for widespread dispersion. Contaminating the water or food supply is also a possibility, although the toxin is degraded by standard water treatment and by heating of foods to 85°C (185°F) for 5 minutes. Prior immunization with toxoid vaccine is advisable for personnel at risk, but prophylactic administration of trivalent equine antitoxin is not recommended.

Environmental Fate

Clostridium botulinum secretes an exotoxin (of which there are seven distinctive antigenic types labeled A–G) consisting of proteinaceous heavy and light chains linked together by a disulfide bond. Human poisonings have predominantly been due to types A (vegetables), B (meat), and E (fish). The toxin inhibits acetylcholine-mediated neurotransmitter release by binding onto SNARE proteins that facilitate exocytosis of acetylcholine synaptosomes. The heavy chain directs endocytosis of the toxin into a neuron in the peripheral nervous system. The pharmacologically active zincdependent endopeptidase light chain then binds to the SNARE proteins (synaptobrevin, SNAP-25, syntaxin), cleaving a portion of the protein and thus destroying it. The consequence is blockage of acetylcholine release. The toxin does not cross the blood–brain barrier and affects only peripheral motor nerves.

Toxicity evaluation

Inadequately sterilized processed products (e.g., canned foods) can be contaminated with neurotoxin-producing spores, leading to potential accidental exposure by ingestion (the most common route). Poor food hygiene during food preparation has been the major causes of large outbreaks of foodborne botulism. In patients with underdeveloped intrinsic gut immunity (e.g., infants), C. botulinum colonization of the gastrointestinal tract can occur. Susceptibility to colonization can also occur where the normal gastrointestinal protective mechanisms are deficient (e.g., in patients who have undergone major gastric surgery whereby acid production is reduced or where bile acid formation has ceased).
Cutaneous absorption and thus toxicity do not occur if the skin is undamaged. However, an open wound or mucous membrane exposed to the bacterium increases the risk of toxicity.