Cholinesterase Inhibitors and Cholinergic Blocking Agents (P. Chemistry | Ch-6 | Unit-4): D Pharma 1st year Notes

Cholinesterase Inhibitors (Indirect Acting Cholinergic Agents)

Cholinesterase inhibitors (ChEIs) take a different approach to boosting cholinergic activity. Rather than directly activating receptors, they block the enzyme that breaks down acetylcholine — acetylcholinesterase. The result is that acetylcholine accumulates at the synapse and continues stimulating both muscarinic and nicotinic receptors for longer.

ChEIs are broadly used in three clinical areas:

• Neuromuscular disorders (myasthenia gravis)
• Post-surgical reversal of muscle relaxants
• Alzheimer’s disease and other dementias

They also divide into reversible and irreversible types. Reversible inhibitors are used therapeutically. Irreversible inhibitors (organophosphates) are used in glaucoma eye drops but are also the basis of certain insecticides and nerve agents.

---

1. Neostigmine

Neostigmine is the most tested ChEI in pharmacy exams. It is a quaternary ammonium compound — the positive charge on nitrogen prevents it from crossing the blood-brain barrier, so all its effects are peripheral (outside the CNS).

IUPAC Name: 3-{[(Dimethylamino)carbonyl]oxy}-N,N,N-trimethylbenzenaminium

Mechanism: Reversible inhibitor of acetylcholinesterase (peripherally acting)

Uses: Treatment of myasthenia gravis — a condition where the immune system attacks nicotinic receptors at the neuromuscular junction, causing muscle weakness. Also used after surgery to reverse non-depolarizing neuromuscular blockers (such as pancuronium or vecuronium). Additionally used for urinary retention and post-operative intestinal ileus.

Formulations: Tablet

Brand Names: Bloxiverz, Prostigmin, Vagostigmin

---

2. Edrophonium Chloride

Edrophonium is unique because of its extremely short duration of action — effects last only about 5 to 10 minutes. This makes it useless as a treatment drug but perfect as a diagnostic tool.

IUPAC Name: N-ethyl-3-hydroxy-N,N-dimethylbenzenaminium chloride

Mechanism: Reversible, ultra-short acting acetylcholinesterase inhibitor

Uses: The Tensilon test — a diagnostic procedure where edrophonium is injected intravenously. If a patient has myasthenia gravis, their muscle strength improves dramatically within seconds and then returns to baseline within minutes. This rapid, transient improvement confirms the diagnosis. Also used in glaucoma and ophthalmic preparations.

Formulations: Tablet, Eye Drops

Brand Names: Tensilon, Enlon

---

3. Tacrine Hydrochloride

Tacrine holds an important place in medical history as the first centrally acting cholinesterase inhibitor approved for Alzheimer’s disease. Unlike neostigmine and edrophonium, tacrine crosses the blood-brain barrier and acts in the CNS. However, it was later largely replaced by safer drugs like donepezil and rivastigmine due to hepatotoxicity concerns.

IUPAC Name: 1,2,3,4-Tetrahydroacridin-9-amine hydrochloride

Mechanism: Reversible, centrally acting acetylcholinesterase inhibitor

Uses: Management of mild to moderate Alzheimer’s disease by improving cholinergic neurotransmission in the brain.

Formulations: Tablet

Brand Names: Cognex

---

4. Echothiophate Iodide

Echothiophate is fundamentally different from the three drugs above — it is an organophosphate compound and an irreversible acetylcholinesterase inhibitor. Once it binds to the enzyme, the enzyme is permanently inactivated (a process called aging). The body must synthesize new enzyme molecules to restore function.

IUPAC Name: 2-(Diethoxyphosphorylsulfanyl)ethyl-N,N,N-trimethylazanium iodide

Mechanism: Irreversible organophosphate acetylcholinesterase inhibitor

Uses: Used as eye drops to treat elevated intraocular pressure in certain types of glaucoma (particularly aphakic glaucoma) and accommodative esotropia — a form of crossed eyes caused by uncorrected farsightedness.

Formulations: Tablet, Eye Drops

Brand Names: Phospholine Iodide

Note: Because it is irreversible, echothiophate has a much longer duration than reversible ChEIs. However, it carries a risk of systemic absorption and cholinergic toxicity with prolonged use.

---

Comparison of Cholinesterase Inhibitors

Drug	Reversibility	CNS Action	Main Use
Neostigmine	Reversible	No (peripheral only)	Myasthenia gravis, reversal of muscle relaxants
Edrophonium	Reversible (ultra-short)	No	Diagnosis of myasthenia gravis
Tacrine	Reversible	Yes (crosses BBB)	Alzheimer’s disease
Echothiophate	Irreversible	No (topical eye use)	Glaucoma

---

Cholinergic Blocking Agents (Anticholinergics)

Cholinergic blocking agents do the opposite of cholinergic drugs — they block muscarinic receptors and prevent acetylcholine from producing its effects. The classic anticholinergic effects are remembered by the mnemonic: Dry as a bone, Blind as a bat, Red as a beet, Hot as a hare, Mad as a hatter (dry mouth, blurred vision, flushed skin, fever, confusion).

These drugs are divided into natural agents (derived from plants) and synthetic agents.

---

Natural Cholinergic Blocking Agents

---

5. Atropine Sulphate

Atropine comes from Atropa belladonna — the deadly nightshade plant. The name belladonna (Italian for “beautiful lady”) comes from the historical use of the plant extract to dilate pupils for cosmetic effect. It is a classic and essential drug in every pharmacy curriculum.

IUPAC Name: (RS)-(8-Methyl-8-azabicyclo[3.2.1]oct-3-yl) 3-hydroxy-2-phenylpropanoate sulphate

Mechanism: Competitive muscarinic receptor antagonist (blocks M1, M2, M3)

Uses: As eye drops — dilates the pupil (mydriasis) before eye examinations and treats amblyopia (lazy eye) and cycloplegia. Systemically — used in bradycardia to increase heart rate, in organophosphate poisoning as the antidote, and before surgery to reduce secretions.

Formulations: Tablet, Eye Drops

Brand Names: Isopto Atropine

---

6. Ipratropium Bromide

Ipratropium is essentially atropine modified for inhalation use. It is a quaternary ammonium compound (like neostigmine), which means it does not cross the blood-brain barrier or the bronchial mucosa significantly — its action is almost entirely local in the lungs.

IUPAC Name: [8-methyl-8-(1-methylethyl)-8-azoniabicyclo[3.2.1]oct-3-yl] 3-hydroxy-2-phenylpropanoate

Mechanism: Competitive muscarinic antagonist — blocks bronchial smooth muscle contraction

Uses: First-line bronchodilator for COPD maintenance therapy. Prevents wheezing, breathlessness, coughing, and chest tightness. Unlike salbutamol (which works via beta-2 stimulation), ipratropium works by blocking the parasympathetic bronchoconstriction that is prominent in COPD.

Formulations: Syrup, Inhaler, Tablet

Brand Names: Atrovent

---

Synthetic Cholinergic Blocking Agents

---

7. Tropicamide

Tropicamide is the most commonly used mydriatic in modern ophthalmology, largely because its effects wear off faster than atropine — within 4 to 6 hours compared to atropine’s 7 to 14 days.

IUPAC Name: Benzeneacetamide, N-ethyl-α-(hydroxymethyl)-N-(4-pyridinylmethyl)-

Mechanism: Short-acting muscarinic antagonist (topical ophthalmic)

Uses: Dilates the pupil for fundus examination, cycloplegic refraction, and ophthalmoscopy. Also used before and after certain eye surgeries. Its short duration of action is a practical advantage in outpatient settings.

Formulations: Eye Drops (though listed with syrup/inhaler/tablet in some references)

Brand Names: Mydriacyl

---

8. Cyclopentolate Hydrochloride

Cyclopentolate is intermediate in duration between tropicamide (short) and atropine (very long). It is particularly preferred in children for cycloplegic refraction because it provides good cycloplegia without the prolonged effects of atropine.

IUPAC Name: (RS)-2-(dimethylamino)ethyl (1-hydroxycyclopentyl)(phenyl)acetate

Mechanism: Competitive muscarinic antagonist (ophthalmic)

Uses: Used to dilate the pupil before eye examination. Especially useful in detecting refractive errors in children who tend to over-accommodate, masking the true prescription.

Formulations: Eye Drops, Tablet

Brand Names: Cyclogyl, Cylate, Mydrilate, Pentolair

---

9. Clidinium Bromide

Clidinium shifts the focus from ophthalmology to gastroenterology. It is a quaternary anticholinergic used in combination with the benzodiazepine chlordiazepoxide (the combination is sold as Librax) for gastrointestinal disorders.

IUPAC Name: 3-[(2-hydroxy-2,2-diphenylacetyl)oxy]-1-methyl-1-azabicyclo[2.2.2]octan-1-ium bromide

Mechanism: Quaternary muscarinic antagonist — reduces gastrointestinal secretions and motility

Uses: Symptomatic treatment of peptic ulcer disease and relief of abdominal cramps due to irritable bowel syndrome, diverticulitis, and colicky abdominal pain.

Formulations: Syrup, Tablet

Brand Names: Librax

---

10. Dicyclomine Hydrochloride

Dicyclomine is an antispasmodic that acts by a dual mechanism — it has mild anticholinergic activity but its primary action is a direct relaxant effect on smooth muscle. This makes it relatively selective for the gut without the strong systemic anticholinergic side effects of atropine.

IUPAC Name: 2-(diethylamino)ethyl 1-cyclohexylcyclohexanecarboxylate

Mechanism: Direct smooth muscle relaxant + mild anticholinergic

Uses: Treatment of irritable bowel syndrome (IBS). Relaxes the spasms in the colon that cause cramping and pain.

Formulations: Syrup, Inhaler, Tablet

Brand Names: Bentyl

---

Anticholinergic Drugs – Quick Reference by Use

Drug	Primary Use Area	Duration of Action
Atropine sulphate	Eye exam, Emergency, Pre-op	Long (eye: 7–14 days)
Ipratropium bromide	COPD inhalation therapy	4–6 hours
Tropicamide	Routine eye examination	4–6 hours
Cyclopentolate	Paediatric refraction	6–24 hours
Clidinium bromide	Peptic ulcer, IBS	Oral dosing schedule
Dicyclomine HCl	Irritable bowel syndrome	Oral dosing schedule

FAQ – Cholinesterase Inhibitors and Cholinergic Blocking Agents