- Ondansetron is effectively used to treat nausea and vomiting caused by cancer treatment, surgery or viral infections involving gastrointestinal symptoms. It is administered either orally, intramuscularly or intravenously.
- Ondansetron is a selective antagonist of the serotonin receptor subtype 5-HT3. The release of serotonin from the enterochromaffin cells of the small intestine initiates a vomiting reflex through stimulation of 5-HT3 receptors located on the vagal afferents. This activation may also cause a central release of serotonin from the chemoreceptor trigger zone of the area postrema located on the floor of the forth ventricle. Ondansetron blocks the initiation of this reflex and acts on the 5-HT3 receptors on neurons located in both peripheral and central nervous system.
- Adverse effects of Ondansetron can include: headache, constipation, weakness, drowsiness and chills.
- Be aware of slow administration and maximum dose amount to avoid dystonic reactions.
- If indicated, adrenaline is not withheld because of concerns regarding drug interactions.
- There isn’t a great deal of information behind overdose quantities of Ondansetron. Some patients that have taken between 48mg-72mg have reported vision problems, hypotension and neuromuscular abnormalities.
Pharmacokinetics
- Absorbed through the gastrointestinal tract and under goes first pass metabolism. Bioavailability enhanced by presence of food and has been recorded at 56%-60% in health patients.
- Volume of distribution has been recorded at approximately 160L.
- Ondansetron is a substrate for human hepatic cytochrome including CYP34A. This plays a dominant role in the overall turnover on ondansetron. Because of the multiplicity of metabolic enzymes capable of metabolising ondansetron it is likely that inhibition of loss of one enzyme will be compensated by others and result in little change in overall rates of Ondansetron clearance. Metabolites include glucuronide and sulphate conjugates and hydroxylation products. Primary pathway is hydroxylation on the indole ring followed by subsequent glucuronide or sulphate conjunction.
- Ondansetron is excreted in the urine and faeces. Less than 10% of the dose is unchanged in the urine.
Pharmacodynamics
- The half-life of Ondansetron is approximately 3-4 hours however can be 6-8 hours in the elderly.
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