ONDANSETRON - A NOVEL ANTIEMETIC AGENT

Ondansetron hydrochloride is a new serotonin receptor antagonist that is effective in preventing emesis associated with cancer chemotherapy. The antiemetic effect appears to be exerted through a peripheral vaga l blocking within the gastrointestinal tract, as well as an inhibitory effect within the chemoreceptor trigger zone (CTZ). Plasma concentrat ions of ondansetron peak 1 hour after an oral dose, and the tablet has an absolute bioavailability of 59%. Ondansetron undergoes extensive h epatic oxidative metabolism in the liver. The half-life of ondansetron is 3.5 hours in healthy volunteers; elderly patients have a slightly reduced clearance, and pediatric patients have increased clearance. Al though less than 10% of ondansetron is recovered unchanged in the urin e, most metabolites are eliminated by this route. The recommended dose of ondansetron is 0.15 mg/kg for three doses on the day of chemothera py (30 minutes before chemotherapy and 4 and 8 hours afterward). An al ternative regimen includes a single-day dose of 32 mg IV in adult pati ents before chemotherapy. The efficacy of ondansetron therapy for dela yed emesis has not been determined. Ondansetron has proven to be appro priate as a single agent or as an addition to standard antiemetic ther apy (ie, corticosteroids, benzodiazepines, neurotransmitter blockers) in preventing and treating acute chemotherapy-induced emesis (CIE). In itial results of clinical trials in prevention of radiotherapy-induced emesis and anesthesia-induced emesis appear positive. Ondansetron is well tolerated, with few adverse events (eg, headache, sedation).