Dose-independent pharmacokinetics of ondansetron in rats: contribution of hepatic and intestinal first-pass effects to low bioavailability

The pharmacokinetic parameters of ondansetron were evaluated after its intravenous (at doses of 1, 4, 8 and 20 mg/kg) and oral (4, 8 and 20 mg/kg) administration to rats. The gastric, intestinal and hepatic first-pass effects of ondansetron were also evaluated after its intravenous, oral, intraportal, intragastric and intraduodenal administration at a dose of 8 mg/kg to rats. After intravenous and oral administration of ondansetron, the drug exhibits dose-independent pharmacokinetics in rats. After oral administration of ondansetron at a dose of 8 mg/kg, the unabsorbed fraction was 0.0158 of the dose, the extent of absolute oral bioavailability (F) value was 0.0407, and the hepatic and intestinal first-pass effects were 40.0% and 34.2% of the oral dose, respectively. The low F of ondansetron in rats was mainly due to considerable hepatic and intestinal first-pass effects. The lower F of ondansetron in rats (4.07%) than that in humans (62+/-15%) was mainly due to greater hepatic metabolism of the drug in rats. Ondansetron was stable in the rat gastric juices and various buffer solutions having pHs ranging from 1 to 13. The equilibrium plasma-to-blood cells partition ratio of ondansetron was 1.74-5.31. Protein binding of ondansetron to fresh rat plasma was 53.2%.