Reply To: Maropitant or metoclop after apomorphine
Apomorphine is an a potent Dopamine 1 and (to a greater degree) Dopamine 2-‘like’ receptor agonist and also has an antagonistic effect on serotonergic receptors 5-HT2 and antagonist effects on alpha adrenergic receptors.
Metoclopramide is a D2 receptor antagonist (main mechanism of anti-nausea) and also an antagonist of 5-HT3
Maropitant is an NK-1 antagonist
So I think it’s the other way round I think! Metoclopramide and Apomorphine act on the same receptors and maropitant is the different one.
The physiology of vomiting is fairly complicated with loads of receptors (Dopaminergic, serotonergic, adrenergic, histaminergic, neurokinergic) and some other ones I also can’t spell! all in the CRTZ chemoreceptor trigger zone. So when we give apomorphine this is where the action is happening on the D2 receptors. I guess the question pharmacologically is, does metoclopramide have greater affinity for the D2 receptor than apomorphine? Metoclopramide has 28.8 nM affinity at the D2 receptor site and apomorphine 52nM (higher number = lower affinity) so theoretically metoclopramide may diminish the effects of apomorphine but I couldn’t find any studies relating to this. The other question… is antagonism of the NK-1 receptor enough to counter the agonists effect on D2. I’m not sure we have an answer to this one!
I couldn’t find any literature on the specific question.
A little counter question why give an anti-emetic at all?
The duration of action of apomorphine is around 60-90minutes, the induction of vomiting and completion of vomiting will take anywhere between 5-20minutes. The info on the data sheet for maropitant suggests that it takes 45-60minutes to have reliable action (some studies administered at time point zero and then gave apomorphine at 60min, or an opioid at 45min). So when considering the timing – is it really worth it? Are we making a difference or just giving the anti-emetic because it feels like the right thing to do?
Hope this adds to your discussion!