Anticholinesterase drugs block the action of the enzyme acetyl-cholinesterase (which breaks down the neurotransmitter acetyl-choline) at cholinergic receptor sites, preventing the breakdown of acetylcholine. As acetylcholine builds up, it continues to stimu-late the cholinergic receptors. (See One day at a time: Recogniz-ing a toxic response.)
Anticholinesterase drugs are divided into two categories—re-versible and irreversible.
Reversible anticholinesterase drugs have a short duration of ac-tion and include:
Irreversible anticholinesterase drugs have long-lasting effects and are used primarily as toxic insecticides and pesticides or as nerve gas in chemical warfare. (Pyridostigmine enhances the effects of antidotes used to counteract nerve agents.) Only one has thera-peutic usefulness: echothiophate.
Here’s a brief rundown of how anticholinesterase drugs move through the body.
Many of the anticholinesterase drugs are readily absorbed from the GI tract, subcutaneous tissue, and mucous membranes.
Because neostigmine is poorly absorbed from the GI tract, the patient needs a higher dose when taking this drug orally. Because the duration of action for an oral dose is longer, however, the pa-tient doesn’t need to take it as frequently. When a rapid effect is needed, neostigmine should be given by the I.M. or I.V. route.
Physostigmine can cross the blood-brain barrier (a protective bar-rier between the capillaries and brain tissue that prevents harmful substances from entering the brain). Donepezil is highly bound to plasma proteins, tacrine is about 55% bound, rivastigmine is 40% bound, and galantamine is 18% bound.
Most anticholinesterase drugs are metabolized by enzymes in the plasma and excreted in urine. Donepezil, galantamine, rivastig-mine, and tacrine are metabolized in the liver.
Anticholinesterase drugs promote the action of acetylcholine at receptor sites. Depending on the site and the drug’s dose and du-ration of action, they can produce a stimulant or depressant effect on cholinergic receptors.
Reversible anticholinesterase drugs block the breakdown of acetylcholine for minutes to hours; irreversible anti-cholinesterase drugs do so for days or weeks.
Anticholinesterase drugs are used for a variety of therapeutic pur-poses, including:
§ to reduce eye pressure in patients with glaucoma and during eye surgery
§ to increase bladder tone
§ to improve tone and peristalsis (movement) through the GI tract in patients with reduced motility or paralytic ileus (paralysis of the small intestine)
§ to promote muscle contractions in patients with myasthenia gravis
§ to diagnose myasthenia gravis (neostigmine and edrophonium)
§ as an antidote to cholinergic blocking drugs (also called anti-cholinergic drugs), tricyclic antidepressants, belladonna alka-loids, and narcotics
§ to treat mild to moderate dementia and enhance cognition in pa-tients with Alzheimer’s disease (primarily donepezil, galantamine, rivastigmine, and tacrine).
These interactions can occur with anticholinesterase drugs:
· Other cholinergic drugs, particularly cholinergic agonists (such as bethanechol, carbachol, and pilocarpine), increase the risk of a toxic reaction when taken with anticholinesterase drugs.
· Carbamazepine, dexamethasone, rifampin, phenytoin, and phe-nobarbital may increase donepezil’s rate of elimination.
· Aminoglycoside antibiotics, anesthetics, cholinergic blocking drugs (such as atropine, belladonna, propantheline, and scopol-amine), magnesium, corticosteroids, and antiarrhythmic drugs(such as procainamide and quinidine) can reduce the effects of anticholinesterase drugs and can mask early signs of a cholinergic crisis. (See Adverse reactions to anticholinesterase drugs.)
· Other medications with cholinergic-blocking properties, such as tricyclic antidepressants, bladder relaxants, and antipsychotics, can also counteract the effects of anticholinesterase drugs.
· The effects of tacrine, donepezil, and galantamine may be in-creased when these drugs are combined with known inhibitors of cytochrome P-450 enzymes, such as cimetidine and erythromycin.
Cigarette use increases the clearance of rivastigmine.