## Quaternary vs Tertiary Ammonium Cholinergic Drugs **Key Point:** Quaternary ammonium compounds are permanently ionized, highly polar, and cannot cross lipid membranes — including the blood-brain barrier (BBB). Tertiary amines are lipophilic and readily penetrate the BBB. ### Classification of Cholinesterase Inhibitors | Drug | Chemical Class | BBB Penetration | Clinical Use | | --- | --- | --- | --- | | **Neostigmine** | Quaternary ammonium | No | Myasthenia gravis, anticholinesterase overdose reversal | | **Physostigmine** | Tertiary amine | Yes | Anticholinergic toxidrome, glaucoma | | **Donepezil** | Tertiary amine | Yes | Alzheimer's disease | | **Tacrine** | Tertiary amine | Yes | Alzheimer's disease (hepatotoxic) | **High-Yield:** Neostigmine's inability to cross the BBB makes it unsuitable for CNS cholinergic deficiency (e.g., Alzheimer's) but ideal for peripheral neuromuscular disorders like myasthenia gravis. **Mnemonic:** **QUAT = Quaternary = Cannot cross = Peripheral only** - Neostigmine, edrophonium, pyridostigmine are quaternary. - Physostigmine, donepezil, rivastigmine are tertiary (lipophilic → CNS penetration). **Clinical Pearl:** In anticholinergic poisoning (atropine overdose), physostigmine is preferred over neostigmine because it crosses the BBB and reverses both central and peripheral anticholinergic effects.
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