Question 23

Classify anti-emetic agents by describing their mechanism of action and provide examples.

[Click here to toggle visibility of the answers]

College Answer

Successful candidates were those who discussed the multiple locations in the vomiting 
pathway where a drug acts, the receptors involved and discussed 5 or more classes.
Several approaches can be taken to this topic. Classification by drug group works well and 
then allows more detail to be provided about possible receptor activity. 
For example:
Anithistamines: Promethazine H1 ++++ M ++ D2 ++ 5-HT3
Anitcholinergics: Scopalamine H1 + M ++++ D2 + 5-HT3
Benzamines: Metoclopromide H1 + M -- D2 +++ 5-HT3 ++, 
Neuroleptics: Droperidol H1 + M - D2 +++ 5-HT3 +, 
5-HT3 Antagonists: Ondansetron H1 - M - D2 - 5-HT3 ++++, Granisetron H1 ++++ M ++ 
D2 ++ 5-HT3 ; Glucocorticoids: Dexamethasone H1 - M - D2 - 5-HT3 -,
Propofol: GABBA 
Cannabinoids: direct on vomiting center. 
An alternative approach involves a discussion of the distribution of receptor sites: nucleus 
vestibularis H1 M, area postrema (chemoreceptor trigger zone) 5-HT3 D2 M1 H1, nucleus 
tractus solitarius 5-HT3 D2 M H2 and then discuss which drugs act where. This material has 
also been previously covered in the viva examination and knowledge of both drug properties 
and receptor distribution is often required. 
Few answers detailed relative activity at various receptors types – “+” scale illustrated above 
would be sufficient to convey an understanding to the examiners.