Mechanisms of chemotherapy/radiotherapy-induced emesis in animal models.

Animal models of chemotherapy/radiotherapy-induced emesis successfully predicted the clinical efficacy of the 5-HT3 receptor antagonists for the control of acute emesis. Further studies in animals have provided valuable information relating to the pathophysiology of emesis and the mechanism of action of 5-HT3 receptor antagonists. These agents inhibit emesis by blocking the action of 5-HT at 5-HT3 receptors on the vagus nerve in the gastrointestinal tract and in the hindbrain vomiting system. 5-HT is hypothesized to be released from enterochromaffin cells following cytotoxic therapy or radiation. The mechanism by which 5-HT is released from enterochromaffin cells is unknown and, although various mechanisms have been proposed, none of these have provided convincing supportive evidence. In collaboration with scientists at Glaxo we have pioneered two models of cisplatin-induced acute and delayed emesis [Rudd et al., 1994]. In the first model, ferrets are given a low dose of cisplatin (5 mg/kg i.p.) and observed for 3 days. A pattern of emesis similar to that seen in the clinic has been observed with two distinct phases of emesis. Ondansetron, and particularly ondansetron plus high-dose dexamethasone, are effective in reducing the emetic response over days 1-3. The second model uses a higher dose of cisplatin (10 mg/kg i.p.) and an observation period of 24 h. Part of the emetic response over this time is resistant to 5-HT3 receptor antagonism. Studies into the mechanism of the emesis induced in both models may give an insight into cisplatin-induced emesis in man that is not controlled with 5-HT3 receptor antagonists.