Ondansetron metabolism and pharmacokinetics.

Hepatic oxidative metabolism accounts for more than 95% of ondansetron clearance from the body. The major excreted metabolites are conjugates of 7-hydroxy or 8-hydroxyondansetron, which appear to contribute little to the activity of the parent drug. Ondansetron plasma clearance averages approximately 0.45 L/h/kg, is similar in young male volunteers and cancer patients undergoing cisplatin-based chemotherapy, and does not change significantly with repeated dosing. Clearance decreases with increasing age, whereas volume of distribution remains unchanged. The result is an increase in mean plasma half-life from 3.5 hours in young volunteers (18-40 years) to 5.5 hours in volunteers over 75 years of age. Clearance and volume of distribution are higher in young (7-12 years) cancer patients, resulting in a mean plasma half-life of 2.5 hours. Plasma clearance is slightly slower in females. Ondansetron clearance decreases and half-life increases in patients with severe hepatic insufficiency. Clearance may be enhanced in patients receiving known hepatic enzyme inducers. Because of large intersubject variability in clearance and the relative safety of ondansetron, adjustments in ondansetron dosing based on age or gender alone are not recommended. Ondansetron is rapidly and completely absorbed when administered as a tablet. A relationship exists between control of emesis and the area under the plasma concentration-time curve for ondansetron. All data collected to date support the thesis that ondansetron is a competitive antagonist of the 5-hydroxytryptamine (5-HT3) receptor in humans.