Antiemetic Drugs

Question 1

Where is the vomiting centre and chemoreceptor trigger zone (CTZ) located?

Answer 1

In the medulla oblongata

VC coordinates muscles involved in vomiting

CTZ leads to the vomiting reflex once it is stimulated

Question 2

What neurotransmitters are involved in the vomiting reflex?

Answer 2

Histamine (H1 receptors)
ACh (muscarinic receptors)
5-HT (5-HT3 receptors)
Dopamine (DA2 receptors)
Substance P (via neurokinin 1 receptors)

Question 3

How to antiemetic drugs function?

Answer 3

Most drugs are receptor antagonists that inhibit neurotransmitter action in the vomiting reflex

Question 4

What is an example of a 5-HT (serotonin) antagonist?

Answer 4

*Ondansetron (Zofran)

Used in cancer chemotherapy and postoperative N/V

Question 5

What drugs are Dopamine (DA2) receptor antagonists?

Answer 5

Antipsychotic (Neuroleptic) agents

• Block DA2 receptors in the CTZ
• *Chlorpromazine (Largactil)
• Used for more severe vomiting (cancer, radiation therapy)

Prokinetic agents (increase GI motility)

• Block DA receptors in the CTZ (antiemetic action) and GI
• *Metoclopramide (Maxeran)

Question 6

What is an example of an anticholinergic agent and when is it used for antiemetic purposes?

Answer 6

• Scopolamine
• Used for motion sickness

Anticholinergic agents block ACh receptors in the inner ear labyrinth

Blocks transmission of nauseating stimuli to VC

Question 7

What is an example of an antihistamine drug with antiemetic properties?

Answer 7

Dimenhydrinate (Gravol)

- Blocks H1 receptors

Question 8

What are some adverse effects of antiemetics?

Answer 8

Ondansetron - dysrhythmia
DA antagonists - movement disorders
Scopolamine - typical anticholinergic effects
Antihistamines - sedation and typical anticholinergic effects

Question 9

What is an example of a Substance P receptor antagonists?

Answer 9

Aprepitant

• Blocks neurokinin 1-type receptors in CTZ
• Prevents postoperative and cancer chemotherapy N/V
• Prolonged duration of action

Question 10

When should you used antiemitics prophylacticlly?

Answer 10

In chemotherapy, given 30 mins to 3 hours before

Motion sickness

• Dimenhydrinate 1 hour before travel
• Scopolamine patch applied 4h before travel