Anticholinesterase drugs

Posted by Piscean on
Anticholinesterase drugs

Anticholinesterase drugs block the action of the enzyme acetylcholinesterase (which breaks down the neurotransmitter acetylcholine) at cholinergic receptor sites, preventing the breakdown of acetylcholine. As acetylcholine builds up, it continues to stimulate the cholinergic receptors.
Anticholinesterase drugs are divided into two categories’reversible and irreversible.

These you can reverse…
Reversible anticholinesterase drugs have a short duration of action and include:
  • ambenonium
  • demecarium
  • donepezil
  • edrophonium
  • galantamine
  • guanidine
  • neostigmine
  • physostigmine
  • pyridostigmine
  • rivastigmine
  • tacrine.
…these you can’t
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 therapeutic usefulness: echothiophate.


Pharmacokinetics
Here’s a brief rundown of how anticholinesterase drugs move through the body.

Generally GI
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 patient 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.
Distribution
Physostigmine can cross the blood-brain barrier (a protective barrier 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.
Metabolism and excretion
Most anticholinesterase drugs are metabolized by enzymes in the plasma and excreted in urine. Donepezil, galantamine, rivastigmine, and tacrine are metabolized in the liver.

Pharmacodynamics
Anticholinesterase drugs promote the action of acetylcholine at receptor sites. Depending on the site and the drug’s dose and duration of action, they can produce a stimulant or depressant effect on cholinergic receptors.

From minutes to weeks
Reversible anticholinesterase drugs block the breakdown of acetylcholine for minutes to hours; irreversible anticholinesterase drugs do so for days or weeks

Pharmacotherapeutics
Anticholinesterase drugs are used for a variety of therapeutic purposes, 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 anticholinergic drugs), tricyclic antidepressants, belladonna alkaloids, and narcotics
  • to treat mild to moderate dementia and enhance cognition in patients with Alzheimer’s disease (primarily donepezil, galantamine, rivastigmine, and tacrine).
Drug interactions
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 phenobarbital may increase donepezil’s rate of elimination.
  • Aminoglycoside antibiotics, anesthetics, cholinergic blocking drugs (such as atropine, belladonna, propantheline, and scopolamine), 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.
  • 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 increased 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.