R Dopamine D2/D3 receptor antagonists

DA receptors are key mediators of the emetic reflex, with DA agonists, including apomorphine, evoking emesis in ferret and dog. The use of subtype-selective DA receptor agonists has implicated the D2/D3 receptors in the emetic response to dopamine agonists, a conclusion supported by the observation that apomorphine-induced emesis is blocked by domperidone and haloperidol (38). The effectiveness of DA receptor antagonists at inhibiting the emetic response to clinically relevant emetogens has also been studied. Radiation-induced emesis in the ferret was only weakly inhibited by domperidone, whereas emesis induced by cyclophosphamide or morphine was effectively inhibited by droperidol (39), which unlike domperidone, penetrates the blood-brain barrier.

DA receptors play a role in the emetic reflex in humans. Clinically experience with drugs that activate DA receptors, such as apomorphine, or increase DA (e.g., l-DOPA) induce nausea and vomiting. Clinical experience with DA antagonists in the control of nausea and vomiting has been mixed. Studies have shown that domperidone has only weak activity in controlling nausea and vomiting in response to powerful emetogenic agents like cisplatin, although more impressive results have been obtained with less emetogenic chemotherapeutic regimes. DA antagonists that cross the blood-brain barrier, e.g., droperidol, haloperidol, and prochlorperazine (40), are effective antiemetic agents, even controlling the emetic response to highly emetogenic therapy, although adverse events such as sedation are common.

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