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Ondansetron used mainly as an antiemetic to treat nausea and vomiting following chemotherapy. Its effects are thought to be on both peripheral and central nerves. Ondansetron reduces the activity of the vagus nerve, which activates the vomiting center in the medulla oblongata, and also blocks serotonin receptors in the chemoreceptor trigger zone. It has little effect on vomiting caused by motion sickness, and does not have any effect on dopamine receptors or muscarinic receptors.
Ondansetron is a selective 5-HT3 receptor antagonist. While its mechanism of action has not been fully characterized, Ondansetron is not a dopamine-receptor antagonist. Serotonin receptors of the 5-HT3 type are present both peripherally on vagal nerve terminals and centrally in the chemoreceptor trigger zone of the area postrema. It is not certain whether Ondansetron's antiemetic action is mediated centrally, peripherally, or in both sites. However, cytotoxic chemotherapy appears to be associated with release of serotonin from the enterochromaffin cells of the small intestine. In humans, urinary 5-HIAA (5-hydroxyindoleacetic acid) excretion increases after cisplatin administration in parallel with the onset of emesis. The released serotonin may stimulate the vagal afferents through the 5-HT3 receptors and initiate the vomiting reflex.
Ondansetron antagonises 5-HT3 receptor, blocking serotonin, both peripherally on vagal nerve terminalsand centrally in the chemoreceptor trigger zone.
Onset: Approx 30 minutes.
Absorption: Peak plasma concentrations: Oral: 1.5 hours; rectal: 6 hours.
Distribution: Extensively distributed. Protein-binding: 70-75%.
Metabolism: Hepatic via multiple enzymatic pathways.
Excretion: Via urine (44-60% as metabolites, 5-10% as unchanged), faeces (approx 25%). Terminal elimination half-life: Oral/parenteral: 3 hours; rectal: 6 hours; elderly/renal impairment: prolonged, approx 5 hours; severe hepatic impairment: 15-32 hours.
May cause QT prolongation; caution when used in cardiac diseases, patients who are on medications that can prolong QT or patients with electrolyte abnormalities. Severe hepatic impairment. May mask progressive ileus and/or gastric distension. Pregnancy, lactation.
