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Pharmacology: Pharmacodynamics: Ondansetron is a potent, highly selective 5HT3 receptor-antagonist. Chemotherapy and radiotherapy may cause release of 5HT in the small intestine initiating a vomiting reflex by activating vagal afferents via 5HT3 receptors. Ondansetron blocks the initiation of this reflex. Activation of vagal afferents may also cause a release of 5HT in the area postrema, located on the floor of the fourth ventricle and this may also promote emesis through a central mechanism. Thus the effect of ondansetron is probably due to antagonism of 5HT3 receptors on neurons located both in the peripheral and central nervous system.
Pharmacokinetics: Onset of effect: Within 30 minutes.
Plasma protein binding: 70% to 76%.
Metabolism: Extensively in the liver by hydroxylation, followed by glucuronide or sulfate conjugation.
Bioavailability: Oral: 56%.
Half-life: Children <15 years: 2-3 hours, adults: 3-6 hours.
Time to peak: Oral: ~2 hours.
Elimination: In urine and feces; <10% of parent drug recovered unchanged in urine.
