A pharmacodynamic and pharmacokinetic comparison of intravenous quinaprilat and oral quinapril

Quinaprilat is the active metabolite of quinapril, an orally active angiotensin-converting enzyme (ACE) inhibitor. The dose-response and duration-of-effect after single intravenous doses of quinaprilat and placebo (part A) and after administration of oral quinapril solution and intravenous quinaprilat (part B) were assessed in a randomized, crossover study of two groups of 12 healthy volunteers. Pharmacodynamic effects of quinaprilat and oral quinapril were assessed by measurement of blood pressure changes after an infusion of angiotensin I IA-I) at a dose previously determined to produce an increase in diastolic blood pressure of 25 mmHg under standardized conditions (A I presser response). A clear dose-response relationship was demonstrated for quinaprilat in this pharmacodynamic model, with 0.5 mg as the lowest effective dose. Doses of 1.0 mg and higher partially suppressed A-I presser response for at least 6 hours. Onset of action was observed within 15 minutes of intravenous administration of quinaprilat and was independent of dose, whereas peak effect and duration of action appeared to be dose related. Quinaprilat doses of 2.5 mg and 10 mg achieved approximately 50% and >80% inhibition of the A-I presser response, respectively. In part B, these doses of intravenous quinaprilat were compared with oral doses of quinapril previously found to produce 50% (2.5 mg) and 90% (10 mg) inhibition of the A-I presser response. The magnitude of effect was similar after administration of 20 mg quinapril orally and 10 mg quinaprilat intravenously. Duration of action was longer, however, after administration of intravenous quinaprilat (10 mg) than after oral quinapril (20 mg), due to the higher maximum plasma concentration (C-max) of quinaprilat. Mean area under the plasma concentration-time curve extrapolated to infinity (AUC(0-infinity)) of quinaprilat was similar after the 10-mg dose ef intravenous quinaprilat and the 20-mg dose of oral quinapril. Based on the concentrations of quinaprilat observed in this study, the absolute bioavailability of quinapril was approximately 50%; intravenous quinaprilat should therefore produce a pharmacodynamic response similar to that obtained with oral quinapril at approximately half the dose.