11.5 Study Notes on Acetylcholinesterase Inhibitors and their Pharmacology

Acetylcholinesterase Inhibitors

Introduction to Acetylcholinesterase and Inhibitors

• Acetylcholinesterase (AChE) is an enzyme that catalyzes the breakdown of acetylcholine (ACh), thereby regulating neurotransmitter levels in the cholinergic system.

• This section covers the function, classification, pharmacological effects, therapeutic uses, safety aspects regarding insecticides, and toxicity related to AChE inhibitors.

Cholinesterases Overview

• Cholinesterases are enzymes that catalyze the destruction of acetylcholine and other choline esters.

• Types of cholinesterases:

  • Acetylcholinesterase (AChE):

  • Found in neuronal tissue; primarily responsible for metabolic breakdown of acetylcholine.

  • Membrane-bound enzyme.

  • Half-life (T-half) of acetylcholine is approximately 100 microseconds due to rapid hydrolysis by AChE.

  • Butyrylcholinesterase (BuChE):

  • Found in blood, liver, and other organ systems (including lungs); also known as plasma or pseudocholinesterase.

  • Deals with dietary cholinesters entered into circulation.

Anticholinesterases

• Anticholinesterases are classified as acetylcholinesterase inhibitors, which block the action of AChE and BuChE.

• This results in an increased concentration of acetylcholine in the body.

Classification of Acetylcholinesterase Inhibitors

• 1. Duration of Action:

  • Short-acting: Drugs that do not form covalent bonds with the enzyme.

  • Example: Adrophonium (moderate affinity, quaternary nitrogen, rapidly excreted).

  • Medium-duration: Reversible carbamates that form covalent bonds and inhibit AChE for a period.

  • Examples: Neostigmine, Pyridostigmine, Physostigmine. Neostigmine and Pyridostigmine have quaternary nitrogen (charged), while Physostigmine is a tertiary amine (uncharged, crosses blood-brain barrier).

  • Long-acting: Irreversible organophosphate inhibitors that form permanent covalent bonds with AChE.

  • Examples: Echothiophate, Parathione, Malathione, Sarin (chemical weapons).

• 2. Mechanism of Action:

  • Medium-duration inhibitors allow enzyme regeneration after a few minutes, whereas long-acting inhibitors require new enzyme synthesis, taking hours or days to regenerate.

Pharmacological Effects of Acetylcholinesterase Inhibitors

• ACh levels increase, leading to stimulation of both muscarinic and nicotinic receptors throughout the body.

• Effects on Organ Systems:

  • GI tract: Increases motility, causing cramps, nausea, and diarrhea.

  • Urinary system: Urinary urgency (increased need to urinate).

  • Cardiovascular system: Bradycardia (slowed heart rate) and hypotension (lower blood pressure).

  • Respiratory system: Bronchoconstriction and increased secretions, making breathing difficult.

  • Ocular effects: Miosis (pinpoint pupils) and increased lens accommodation for near vision.

  • Sweat glands: Increased secretion (diaphoresis).

Therapeutic Uses of Acetylcholinesterase Inhibitors

• Glaucoma Treatment:

  • Increases drainage of aqueous humor through activation of muscarinic receptors, reducing intraocular pressure.

  • Medications: Pilocarpine (muscarinic agonist), Phystigmine, Ecothiophate.

• Myasthenia Gravis Treatment:

  • Condition characterized by autoimmune destruction of nicotinic ACh receptors at the neuromuscular junction.

  • Symptoms include muscle weakness and fatigue.

  • Medications: Pyridostigmine, Neostigmine, Amionium.

• Antidote for Anticholinergic Toxicity:

  • Anticholinergic drugs (e.g., Atropine) block muscarinic activity.

  • Acetylcholinesterase inhibitors can increase ACh levels to compete with anticholinergics.

• Prophylaxis Against Nerve Agent Exposure:

  • Example: Low doses of Pyridostigmine were used by troops to prevent toxicity from chemical warfare agents such as sarin.

Insecticides and Safety

• Many insecticides are irreversible AChE inhibitors, leading to increased ACh levels in insects, causing their death.

• Primary insecticide examples: Malathione; affected by metabolic pathways in humans that detoxify the compounds more effectively than in insects.

Toxicity of Acetylcholinesterase Inhibitors

• Symptoms of toxicity stem from excessive ACh:

  • Muscarinic Symptoms: Salivation, Lacrimation, Urination, Diarrhea, Gastrointestinal distress, Emesis (SLUDGE).

  • Nicotinic Symptoms: Muscle weakness, cramps, tachycardia, twitching, and fast circulation (days of the week mnemonic).

• Treatment:

  • Antidote: Pralidoxime (2-PAM), which reactivates inhibited AChE (not effective against carbamate poisoning).

  • Atropine for muscarinic effects, supportive care for respiratory distress and other symptoms.

Key Points Regarding Overdosing and Underdosing

• Underdosing leads to insufficient ACh for neuromuscular stimulation.

• Overdosing can cause paralysis similar to succinylcholine effects.

• Adrophonium is used diagnostically due to its fast action; improvements indicate underdosing while worsening symptoms indicate overdosing.

Conclusion

• The discussion covers AChE inhibitors' classification, pharmacological effects, risks, therapeutic applications, and toxicological implications, highlighting critical mechanisms and treatment approaches for various clinical conditions.

• Students should remember to distinguish between different types of AChE inhibitors and their unique properties regarding both therapeutic uses and potential toxicities. The next topics will expand to treatments for Alzheimer's disease in future sessions.

DETAILED

Therapeutic Uses of Acetylcholinesterase Inhibitors

• Glaucoma Treatment:

  • Glaucoma is a group of eye conditions that damage the optic nerve, often caused by abnormally high intraocular pressure (IOP). AChE inhibitors, or direct-acting muscarinic agonists like pilocarpine, are used to treat open-angle glaucoma.

  • These drugs increase the outflow of aqueous humor, the fluid within the eye, by causing miosis (pupil constriction) and contraction of the ciliary muscle. This contraction pulls on the trabecular meshwork, opening up its pores and facilitating fluid drainage, thereby reducing intraocular pressure.

  • Medications: Pilocarpine (a direct muscarinic agonist, not an AChE inhibitor but often used for the same therapeutic effect), Phystigmine (a tertiary amine that can penetrate the eye), and Ecothiophate (a long-acting irreversible inhibitor, now less commonly used due to its pronounced side effects and long duration of action).

• Myasthenia Gravis Treatment:

  • Myasthenia Gravis is a chronic autoimmune neuromuscular disease characterized by fluctuating muscle weakness and fatigue. It results from the body's immune system producing antibodies that attack and destroy or block the nicotinic acetylcholine receptors at the postsynaptic membrane of the neuromuscular junction (NMJ).

  • AChE inhibitors work by preventing the breakdown of acetylcholine in the synaptic cleft, leading to an increased concentration and prolonged presence of ACh. This allows more ACh molecules to bind to the fewer available functional nicotinic receptors, enhancing cholinergic neurotransmission and improving muscle contraction strength.

  • Medications: Pyridostigmine and Neostigmine are the primary agents used. Both contain quaternary nitrogen, restricting their action mainly to the periphery (neuromuscular junction) and limiting CNS side effects. Ambenonium (not Amionium) is another less commonly used agent with similar properties.

• Antidote for Anticholinergic Toxicity:

  • Anticholinergic toxicity occurs when there is an excessive blockade of muscarinic acetylcholine receptors, often caused by drugs like Atropine, scopolamine, tricyclic antidepressants, or certain antihistamines. Symptoms can include dry mouth, blurred vision, urinary retention, constipation, tachycardia, and central nervous system effects such as delirium and hallucinations.

  • Acetylcholinesterase inhibitors can effectively reverse these effects by increasing the levels of endogenous acetylcholine in the synaptic cleft. This elevated ACh then competes with the anticholinergic drug for binding sites on the muscarinic receptors, thereby overcoming the blockade.

  • Physostigmine, a tertiary amine, is particularly useful in this context because its lipophilic nature allows it to readily cross the blood-brain barrier, making it effective in treating both peripheral and central anticholinergic syndromes. Neostigmine, being a quaternary amine, would only address peripheral symptoms.

• Prophylaxis Against Nerve Agent Exposure:

  • Nerve agents (e.g., Sarin, Soman, VX) are highly potent organophosphate compounds that cause irreversible inhibition of acetylcholinesterase, leading to massive accumulation of ACh and severe cholinergic crisis.

  • Low doses of reversible AChE inhibitors, such as Pyridostigmine, can be administered prophylactically to military personnel or individuals at high risk of exposure. The mechanism involves reversible binding of pyridostigmine to a portion of the AChE enzymes, temporarily protecting them from irreversible phosphorylation by subsequently encountered organophosphate nerve agents.

  • When the nerve agent is encountered, it will primarily bind to the unprotected AChE molecules. The pyridostigmine-bound enzymes will eventually reactivate, allowing some level of cholinergic function to be maintained, thus mitigating the severity of the nerve agent poisoning and potentially increasing survival rates. This strategy must be followed by atropine and pralidoxime if actual exposure occurs.