Indirect Acting: Physostigmine, Neostigmine, Pyridostigmine, Edrophonium chloride : Pharmaguideline -->

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Indirect Acting: Physostigmine, Neostigmine, Pyridostigmine, Edrophonium chloride

The enzyme cholinesterase is inhibited by anticholinesterases or indirectly acting cholinergic medications.
The enzyme cholinesterase is inhibited by anticholinesterases or indirectly acting cholinergic medications. Acetylcholine hydrolysis is carried out by this enzyme (ACh). So, ACh is not metabolized and accumulates at muscarinic and nicotinic sites to carry out cholinergic actions.

Mechanism of Action:
True and pseudocholinesterases rapidly hydrolyses Acetylcholine.

Steps:
  • ACh bind to anionic and esteratic sites of cholinesterase
  • Acetylated enzyme
  • Rapid hydrolysis
  • Acetate and free enzyme
  • Carbamates: binds to both anionic and esteratic sites of cholinesterase resulting in a carbamoylated enzyme. This enzyme slowly hydrolyses to release the enzyme.
  • Edrophonium: It binds only to anionic site of cholinesterase. It forms weak hydrogen bonds with ChE and has a duration of action of 8 to 10 minutes.
  • Organophosphates: It binds covalently to esteratic site cholinesterase. It inhibits ChE irreversibly as the phosphorylated enzyme hydrolyses slowly. Echothiophate binds to both anionic and esteratic sites of the enzyme.
Reversible Anticholinesterases: Physostigmine, Neostigmine, Pyridostigmine, Edrophinium chloride, Galantamine, Rivastigmine, Donepezil

Physostigmine

  • It is an alkaloid obtained from Physostigma venenosum
  • It is a tertiary amine with good penetrating ability through tissues
  • It has the following uses:-
  • Glaucoma: It reduces intraocular pressure (IOP) by producing miosis. This promotes the drainage of aqueous humour.
  • Atropine poisoning: Intravenous physostigmine is used in severe atropine poisoning.

SAR-
The difference in chain length affects the AchE block.
The most potent AChE inhibitor of the series was found to have 7 methyl groups attached to the series.
Modification of the xanthone nucleus and other aryl groups reduces AChE inhibition activity.
The presence of a white band on the part of the scent is important for the functioning of the tree.
When a flexible chain is replaced by a strong ethyne group, drug activity is lost, indicating that, a flexible chain is needed for the job.

Neostigmine

  • It is a synthetic anticholinesterase
  • Its actions are more prominent on neuromuscular junction, gastrointestinal tract and urinary bladder compared to its effects on cardiovascular system and eye
  • It does not pass across the BBB (Blood Brain Barrier) and has no central side effects. This is the reason physostigmine is preferred over neostigmine in myasthenia gravis
  • Routes of administration: oral, subcutaneous, intravenous, intramuscular
It has both direct and indirect actions on skeletal muscles:-

Direct actions: As it is structurally similar to ACh, neostigmine directly acts on Nm receptors at the neuromuscular junction. In myasthenic patients, it improves muscular power.

Indirect actions: It works by inhibiting cholinesterases and increasing ACh levels at the neuromuscular junction (NMJ).

It can be used in:
  • Curare poisoning
  • Myasthenia gravis
  • Postoperative urinary retention and paralytic ileus

SAR-
Neostigmine increases acetylcholine availability at synapse by inhibiting acetylcholinestrase enzyme.

The enzyme acetylcholinestrase is responsible for the metabolism of acetylcholine, therefore, the inhibition of acetylcholinestrase indirectly stimulates nicotinic and muscarinic receptors.

Pyridostigmine

Pyridostigmine is very similar to neostigmine
  • It has a longer duration of action.
  • It can be given two times a day in sustained release form
  • It is used in myasthenia gravis. As it is less potent that neostigmine and has a longer duration of action, it is preferred to treat myasthenia patients.
SAR-
The drug competes with acetylcholine by attaching it to anticholinestrase, thus, blocking anticholinestrase acetylcholine.

This increases the concentration of acetylcholine in the synaptic canal and as a result, increases the efficiency of cholinergic transmission.

Edrophonium

  • It is a quaternary compound
  • It is administered intravenously
  • It has a rapid onset but a short duration of action of about 8-10 minute
  • It is used in:
  • Diagnosis of myasthenia gravis
  • Differentiating myasthenic and cholinergic crisis
  • Curare poisoning (due to rapid onset of action)
Therapeutic Uses of these Drugs:
  • Glaucoma
  • Myasthenia gravis
  • Pupillary dilation reversal after refraction testing
  • Postoperative urinary retention and paralytic ileus
  • Alzheimer’s disease
  • Curare poisoning
  • Belladonna poisoning
Adverse Effects of Anticholinesterases:
Adverse effects generally occur only when there is repeated stimulation of the muscarinic and nicotinic receptors. This results in the following:
  • Sweating, nausea, vomiting
  • Abdominal cramps
  • Hypotension
  • Bradycardia
  • Tremors
  • Diarrhoea
SAR-
Edrophonium Chloride is a type of chloride salt of edrophonium, a short-acting and fast-acting cholinesterase inhibitor with parasympathomimetic activity. Echothiophate chloride enhances the effectiveness of acetylcholine by inhibiting acetylcholinesterase which hydrolyze acetylcholine. When applied topically to the eye, this agent enhances the activity of parasympathetic receptors in the neuromuscular segments of the longitudinal muscle of the ciliary body.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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