Onsia Mechanism of Action

Pharmacology: Pharmacodynamics: Ondansetron is a competitive, highly selective 5-HT₃ receptor antagonist, acting on both central and peripheral nervous systems.
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. Chemotherapeutic agents and radiotherapy may cause release of 5-HT₃ in the small intestine initiating a vomiting reflex by activating vagal afferents via 5-HT₃ receptors. Thus, Ondansetron is used for the management of the nausea and vomiting induced by cytotoxic chemotherapy and radiotherapy.
Pharmacokinetics: After oral administration, Ondansetron is well absorbed via GI tract. The absolute bioavailability is only about 60%, mainly because of hepatic first-pass metabolism. Ondansetron is extensively distributed into the body and about 70-75% plasma protein bound. It is metabolized in the liver through cytochrome P450 isoenzymes, primarily CYP3A4, partially CYP1A2 and CYP2D6. Less than 5% of a dose is excreted 95% unchanged in the urine.