Regarding the mechanism of action and clinical use of antiemetic drugs, all of the following statements are correct EXCEPT:

Question

Accepted Answer

B. Domperidone is preferred over metoclopramide in Parkinson's disease because it crosses the blood-brain barrier and blocks central dopamine receptors more effectively. ## Mechanism of Antiemetic Drugs

### Dopamine Antagonists

**Key Point:** Metoclopramide and domperidone are both dopamine antagonists, but they differ critically in their blood-brain barrier (BBB) penetration.

| Drug | BBB Penetration | Mechanism | Clinical Use |
|------|-----------------|-----------|---------------|
| Metoclopramide | **High** (crosses BBB) | Blocks D2 receptors centrally + increases gastric motility | CINV, gastroesophageal reflux, gastroparesis |
| Domperidone | **Low** (polar, doesn't cross BBB) | Peripheral D2 antagonism only | CINV, gastroparesis, safe in Parkinson's disease |

**Warning:** Domperidone does NOT cross the BBB because it is a quaternary ammonium compound (highly polar). This is why it is **preferred in Parkinson's disease** — it avoids worsening parkinsonian symptoms by not blocking central dopamine.

### 5-HT3 Receptor Antagonists

**High-Yield:** Ondansetron and other 5-HT3 antagonists block serotonin receptors at:
- The chemoreceptor trigger zone (CTZ)
- The gastrointestinal tract (peripheral)
- The nucleus tractus solitarius

They are the gold standard for **chemotherapy-induced nausea and vomiting (CINV)** and postoperative nausea and vomiting (PONV).

### Corticosteroids in CINV

**Clinical Pearl:** Dexamethasone is used as an **adjunctive antiemetic** in highly emetogenic chemotherapy. It potentiates 5-HT3 antagonists and NK1 antagonists through mechanisms not fully understood but likely involving:
- Inhibition of prostaglandin synthesis
- Reduction of inflammatory mediators
- Synergy with serotonin antagonism

## Why the Correct Answer Is Wrong

Option 3 (domperidone) contains a **false claim**: domperidone does NOT cross the BBB and therefore does NOT block central dopamine receptors. This is precisely why it is safe in Parkinson's disease — it avoids central dopaminergic blockade. The statement reverses the actual pharmacological advantage.

[cite:KD Tripathi 8e Ch 47]
[cite:Harrison 21e Ch 297]

Answer

A. Dexamethasone potentiates the antiemetic effect of 5-HT3 antagonists and is used as an adjunct in highly emetogenic chemotherapy regimens

Answer

C. Metoclopramide acts as a dopamine antagonist and increases gastric motility by blocking D2 receptors in the chemoreceptor trigger zone

Answer

D. Ondansetron is a 5-HT3 receptor antagonist that is particularly effective for chemotherapy-induced nausea and vomiting by blocking serotonin at the chemoreceptor trigger zone

Regarding the mechanism of action and clinical use of antiemetic drugs, all of the following statements are correct EXCEPT:

Explanation

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