Physostigmine is a secondary metabolite produced by the African tropical vine Physostigma venosum, known as the calabar bean. This compound is used as a drug in humans to treat both glaucoma and myasthenia gravis, and was at one time considered as an Alzheimer’s disease treatment. It functions as a reversible inhibitor of cholinesterase, the protein that degrades acetylcholine. Most of the body’s movement relies on the transfer of signals from nerve cells to muscles that is mediated by acetylcholine.
Inhibitors of cholinesterase, known as anticholinesterases, inhibit the breakdown of acetylcholine. Generally, they are highly toxic to animals because they form an irreversible complex with cholinesterase. This causes the muscles to keep contracting, and paralysis and death can ensue. Many insecticides use this mode of action, as do some nerve gases. In contrast, physostigmine binds cholinesterase reversibly, which enables it to be used as a drug in humans.
Physostigmine sulfate is the form that is used as a drug in the United States, and is used is in the treatment of glaucoma. Physostigmine is effective at getting the extra fluid to drain out of the eye. It also functions as a miotic, causing the pupils to constrict. This property can also help in the treatment of glaucoma. Sometimes it is used as a miotic after the eyes have been dilated during an eye examination.
The muscle weakening disease myasthenia gravis can be treated effectively with this anticholinesterase. The action of this disorder keeps acetylcholine from serving its function of activating the muscles. Thus, treatment with physostigmine can help to relieve the symptoms of this disease.
Clinical trials were conducted with physostigmine to determine if it could improve the functioning of patients with Alzheimer’s disease because there was a hypothesis that the symptoms of the disease were due to a lack of adequate amounts of acetylcholine. Treatment with this compound caused a small but measurable increase in cognitive skills. Patients suffered from drastic side effects, however, and most of them dropped out of the tests. The conclusion was that this drug would not be useful to treat Alzheimer’s disease.
An additional property of physostigmine is that it can cross the blood-brain barrier and affect the central nervous system (CNS). This enables it to be used to treat overdoses of drugs that cause the production of too much acetylcholine. This is called a cholinergic effect. Overdoses of the plant compounds atropine and scopolamine can cause this effect. A common and potentially lethal source of such drugs is the consumption or inhalation of jimson weed, Datura stramonium, as an attempted hallucinogen.
The chemical synthesis of physostigmine is a highly difficult undertaking, given the complexity of the molecule. An additional complication is that there are two forms of the compound, known as stereoisomers, but only one form is active as a drug. Chemists discovered a way to synthesize this chemical in the laboratory in 1935, and a number of different methods have been developed. Generally this chemical is isolated from ripe, dry seeds of the calabar bean plant, rather than being synthesized from scratch.