Which of the following cholinergic drugs is a quaternary ammonium compound and therefore does NOT cross the blood-brain barrier?

Question

Accepted Answer

C. Neostigmine. ## 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]

Answer

A. Tacrine

Answer

B. Physostigmine

Answer

D. Donepezil

Which of the following cholinergic drugs is a quaternary ammonium compound and therefore does NOT cross the blood-brain barrier?

Explanation

Classification of Cholinergic Drugs by Ionization

Quaternary vs. Tertiary Amines

Why Neostigmine is Quaternary

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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