Physostigmine (also known as eserine from éséré, the West African name for the Calabar bean) is a highly toxic parasympathomimetic alkaloid, specifically, a reversible cholinesterase inhibitor. It occurs naturally in the Calabar bean and the fruit of the Manchineel tree. The chemical was synthesized for the first time in 1935 by Percy Lavon Julian and Josef Pikl. It is available in the U.S. under the trade names Antilirium and Isopto Eserine, and as eserine salicylate and eserine sulfate. Today, physostigmine is most commonly used for its medicinal value. However, before its discovery by Sir Robert Christison in 1846, it was much more prevalent as an ordeal poison. The positive medical applications of the drug were first suggested in the gold medal-winning final thesis of Thomas Richard Fraser at the University of Edinburgh in 1862.
Medical uses Physostigmine, an acetylcholinesterase inhibitor, can be used to treat glaucoma and delayed gastric emptying. Because it enhances the transmission of acetylcholine signals in the brain and can cross the blood–brain barrier, physostigmine salicylate is used to treat anticholinergic poisoning (that is, poisoning by substances that interfere with the transmission of acetylcholine signaling, such as atropine, scopolamine, and other anticholinergic drug overdoses). It is also used to reverse neuromuscular blocking. Physostigmine is the antidote of choice for Datura stramonium poisoning. It is also an antidote for Atropa belladonna poisoning, the same as for atropine. It has also been used as an antidote for poisoning with GHB, but is poorly effective and often causes additional toxicity, so is not a recommended treatment. It can also be used as an antidote for dimenhydrinate or diphenhydramine poisoning. It has been shown to improve long-term memory, and was once explored as a therapy for Alzheimer's disease, but in clinical trials it was not shown to confer convincing benefits, and it led to very common moderate to severe side-effects such as nausea, vomiting, diarrhea, loss of appetite, abdominal pain, and tremors, resulting in a high rate of withdrawal. Physostigmine's poor tolerability led to it being abandoned in favor of later acetylcholinesterase inhibitors, three of which are currently in use: donepezil, galantamine, and rivastigmine. Recently, it has begun to be used in the treatment of orthostatic hypotension. Recently, physostigmine has been proposed as an antidote for intoxication with gamma hydroxybutyrate (GHB, a potent sedative-hypnotic agent that can cause loss of consciousness, loss of muscle control, and death). Physostigmine may counteract GHB by producing a nonspecific state of arousal. However, not enough scientific evidence shows physostigmine properly treats GHB toxicity. Furthermore, lower doses of GHB produce a stronger action at the GHB receptor than at the GABAB receptor, resulting in a stimulating effect which would act synergistically with physostigmine and produce hyperstimulation when the GHB blood levels begin to drop. Physostigmine also has other proposed uses: it could reverse undesired side effects of benzodiazepines such as diazepam, alleviating anxiety and tension. Another proposed use of physostigmine is to reverse the effects of barbiturates (any of a group of barbituric acids derived for use as sedatives or hypnotics).
Pharmacology Physostigmine acts by interfering with the metabolism of acetylcholine. It is a reversible inhibitor of acetylcholinesterase and butyrylcholinesterase, the enzymes responsible for the breakdown of acetylcholine in the synaptic cleft of the neuromuscular junction and other organs. It indirectly stimulates both nicotinic and muscarinic acetylcholine receptors. Physostigmine has an LD50 of 3 mg/kg in mice.
Bioactivity Physostigmine functions as an acetylcholinesterase inhibitor. Its mechanism is to prevent the hydrolysis of acetylcholine by acetylcholinesterase at the transmitted sites of acetylcholine. This inhibition enhances the effect of acetylcholine, making it useful for the treatment of cholinergic disorders and myasthenia gravis. More recently, physostigmine has been used to improve the memory of Alzheimer's patients due to its potent anticholinesterase activity. However, its drug form, physostigmine salicylate, has poor bioavailability. Physostigmine also has a miotic function, causing pupillary constriction. It is useful in treating mydriasis. Physostigmine also increases outflow of the aqueous humor in the eye, making it useful in the treatment of glaucoma.
Side effects An overdose can cause cholinergic syndrome. Other side effects may include nausea, vomiting, diarrhea, anorexia, dizziness, headache, stomach pain, sweating, dyspepsia, and seizures. The carbamate functional group readily hydrolyses in water, and in bodily conditions. The metabolite thus formed from physostigmine and some other alkaloids (e.g. cymserine) is eseroline, which research has suggested may be neurotoxic to humans. Death can occur rapidly following overdose as a result of respiratory arrest and paralysis of the heart.
Synthesis
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