Abstract
The pharmacokinetic parameters of nipradilol (NIP), a new potent antihypertensive and antianginal agent, and propranolol were determined after oral, intravenous and intraportal administration to the beagle dog implanted with cannula in portal vein at a dose of 1 mg/kg. Orally administered NIP underwent extensive first-pass metabolism leading to low bioavailability (11%), despite of complete gastrointestinal absorption. On the constant infusion for 30 min into the portal vein, hepatic extraction ratio was 0.71. The reduction in the systemic availability of orally administered NIP could partly be attributed to the fact that denitration and glucuronidation of NIP occur primarily in the intestinal tract and liver, respectively. Following oral administration of NIP, smaller amount of unchanged drug (1.9%) was excreted into the urine than that of intravenous administration (5.8%). However, in the qualitative and the quantitative aspects on urinary metabolic patterns, there was no appreciable influence of the route of administration. On the other hand, the systemic availability and the hepatic extraction ratio of propranolol were 11% and 0.86, respectively, suggesting that the first-pass metabolism through the liver actually contributes to the reduced availability.
