Antiemetics and Prokinetic Drugs
Emesis or vomiting is a common manifestation of infections, gastrointestinal disorders, anaesthesia, motion sickness, and drug toxicity.
· D Antagonists
While D2 antagonists* such as phenothiazines and buty-rophenones, corticosteroids, cannabinoids, and antihista-mines have been discussed elsewhere (consult Index), only the 5-HT3 antagonists will be dealt with here. Benzamides and related compounds (which are also D2 antagonists) are discussed in the subsequent section on prokinetic drugs (vide infra).
Examples include alosetron, dolasetron, granisetron, ondan-setron, and tropisetron. These drugs antagonise the 5-HT3 receptors located peripherally on vagal nerve terminals and centrally in the chemoreceptor trigger zone. They are used in the management of chemotherapy-induced emesis, radiation-induced emesis and postoperative nausea.
Adverse effects include constipation, vertigo, head-ache, blurred vision, asymptomatic elevation of liver enzymes, dystonic reactions, and allergic reactions. Cases of anaphylactoid-anaphylactic reactions have been reported. Reactions consisted of urticaria, angioedema, hypotension, bronchospasm, and dyspnoea. Several cases of chest pain associated with therapeutic use of ondansetron have been reported. Bronchospasm has occurred with intravenous infusion, rarely.
Suspected cases of overdose with ondansetron have presented with the development of fever, rashes, pruritus and restlessness. Mild transient elevation of serum lactate dehydrogenase and temporary blindness of 2 to 3 minutes duration have occurred. Overdose of intravenous adminis-tration of dolasetron has resulted in hypotension, dizziness and abnormal ECG intervals. Monitoring complete blood count and liver and kidney function tests is suggested for patients with significant exposure. Activated charcoal may be considered. There is no specific antidote for ondansetron. Treatment is symptomatic and supportive. Haemodialysis/ haemoperfusion is not expected to be of benefit due to the large volume of distribution. Diphenhydramine has been used to manage adverse effects of therapeutic use including fever, rashes, pruritus and restlessness.