## Classification of Cholinergic Drugs by Ionization **Key Point:** Quaternary ammonium compounds are permanently ionized at physiological pH and are highly polar, making them unable to cross lipid membranes including the blood-brain barrier (BBB). ### Quaternary vs. Tertiary Amines | Property | Quaternary Amines | Tertiary Amines | |----------|-------------------|------------------| | **Ionization** | Permanently charged (always ionized) | Partially ionized; can exist in non-ionized form | | **BBB Penetration** | No | Yes | | **CNS Effects** | Absent | Present | | **Examples** | Neostigmine, Edrophonium, Pyridostigmine | Physostigmine, Donepezil, Tacrine | ### Why Neostigmine is Quaternary Neostigmine contains a quaternary nitrogen atom (permanently positively charged), making it: - Highly hydrophilic - Excluded from the CNS - Useful for **peripheral** cholinergic effects only (myasthenia gravis, postoperative ileus, urinary retention) **High-Yield:** Physostigmine, despite being a carbamate inhibitor like neostigmine, is a **tertiary amine** and DOES cross the BBB — it can be used for anticholinergic toxicity (atropine poisoning) because it reaches central muscarinic receptors. **Clinical Pearl:** The inability of neostigmine to cross the BBB is why it cannot reverse anticholinergic effects in the brain, whereas physostigmine can. [cite:KD Tripathi 8e Ch 6]
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