Palonosetron is a selective 5-HT3 receptor antagonist with high binding affinity for the receptor and minimal or no affinity for other receptors. Chemotherapy drugs are believed to cause nausea and vomiting by releasing serotonin from enterochromaffin cells in the small intestine. This serotonin activates 5-HT3 receptors located on vagal nerve terminals in the periphery and in the chemoreceptor trigger zone of the area postrema in the central nervous system, initiating the vomiting reflex. Postoperative nausea and vomiting is caused by multiple patient-related, surgical, and anesthetic factors and involves serotonin release and a cascade of neuronal events in both the central nervous system and gastrointestinal tract. The 5-HT3 receptor plays a key role in the emetic response. Palonosetron works by blocking serotonin action at 5-HT3 receptors, mainly in the gastrointestinal tract and possibly also in the brain.
Pharmacokinetics: Palonosetron shows linear dose-proportional pharmacokinetics over a dose range of 1–90 mcg/kg in both healthy subjects and cancer patients. After intravenous administration in cancer patients, the mean peak plasma concentration (Cmax) ranges from 0.89 to 336 ng/mL, and the area under the curve (AUC0–∞) ranges from 13.8 to 957 ng·h/mL. It has a volume of distribution of approximately 6.9–7.9 L/kg, with about 62% plasma protein binding. Around 50% is metabolized into two inactive metabolites with less than 1% 5-HT3 antagonist activity. Metabolism occurs via the liver mainly through CYP2D6, CYP3A4, and CYP1A2 enzymes, while about 40% is excreted unchanged in urine within 144 hours. Total body clearance is approximately 160 ± 35 mL/h/kg, and renal clearance is 66.5 ± 18.2 mL/h/kg in healthy individuals. Palonosetron has a long elimination half-life of about 40 hours and a strong receptor binding affinity.