5-HT3 receptor antagonists (ondansetron, palonosetron) are first-line antiemetics for chemotherapy-induced nausea/vomiting (CINV). Palonosetron is preferred for delayed CINV over ondansetron due to:

• A Palonosetron has a lower 5-HT3 binding affinity that prevents receptor downregulation
• B Palonosetron has 30-fold higher 5-HT3 receptor binding affinity and a much longer half-life (~40 hours vs ~5 hours), plus allosteric binding that causes receptor internalisation ✓
• C Palonosetron also blocks 5-HT4 receptors in the gut, preventing delayed emesis triggered by peripheral sensitisation
• D Palonosetron inhibits substance P release, which is the primary mediator of delayed CINV

Explanation

Palonosetron (second-generation 5-HT3 antagonist) has 30-fold higher binding affinity for the 5-HT3 receptor compared to first-generation agents (ondansetron, granisetron). Its half-life is ~40 hours vs ~5 hours for ondansetron, allowing single-dose coverage for up to 5 days. Critically, palonosetron binds in an allosteric fashion that triggers receptor internalisation (receptor trafficking off the cell surface), prolonging blockade beyond what drug plasma levels alone would predict. The delayed phase of CINV (days 2–5) involves both 5-HT3 and substance P/NK1 receptor pathways, and palonosetron combined with an NK1 antagonist (aprepitant) forms the backbone of triple antiemetic regimens for highly emetogenic chemotherapy.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.