Choosing an antiemetic

Antiemetic drugs work by binding to specific receptor sites in the chemoreceptor trigger zone (CTZ) or vomiting centre (VC) in the brain stem. At each site there are several receptors; the more strongly the drug binds to its receptor, the more potent its antiemetic activity.

The most potent antiemetic at the CTZ is the dopamine antagonist haloperidol. At the VC, the non-sedating antihistamine cyclizine and the antimuscarinic hyoscine hydrobromide have similar efficacy, but the side effects of centrally acting antimuscarinics reduce their usefulness and makes cyclizine the drug of choice.

Levomepromazine is a phenothiazine with affinity for receptors at both the CTZ and the VC. Although its binding affinity is lower than cyclizine or haloperidol for the same receptors, it may be a useful broad spectrum antiemetic. It must

DA receptors • Haloperidol 5HT3 receptors "-setrons"

His, receptors

• Cyclizine Achm receptors

• Hyoscine hydrobromide 5HT2 receptors

• Levomepromazine

Autonomic afferents

Brainstem centres

Autonomic afferents

Brainstem centres

Viscera:

• Peritoneum

• Meninges, etc

Vomiting reflex

T

Upper Gl motility changes

DA receptors 5HT4 receptors

• Metoclopramide

• Domperidone 5HT3 receptors

• "-setrons"

Key:

CTZ = Chemoreceptor trigger zone VC = Vomiting centre

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