Clinical pharmacokinetics of prazosin.

Prazosin, a quinazoline derivative, is a peripheral vasodilator used in the treatment of arterial hypertension and more recently, congestive heart failure (CHF). Prazosin is extensively metabolised by the liver and has high first-pass metabolism and low oral bioavailability. In normal healthy volunteers, the time of peak concentration occurs between 1 and 3 hours after oral administration, with wide interindividual variations. The extent of oral absorption seems to be similar for different pharmaceutical forms and is not influenced by the presence of food in the digestive tract. Oral bioavailability of prazosin ranges from 43.5 to 69.3% (mean 56.9%). Prazosin is highly (92 to 97%) bound to human plasma proteins (albumin and alpha 1-acid glycoprotein) and the extent of binding is independent of the plasma concentration of the drug in the range of 20 to 150 ng/ml. Preliminary studies in humans indicate that pathways for biotransformation of prazosin are similar to those observed in the rat and dog. Only 6% of prazosin is excreted unchanged, mainly in the urine. The two main metabolites (0-demethylated) are almost completely excreted in bile. The time course of disappearance of prazosin from plasma after intravenous injecton indicates that prazosin disposition should be described by a model containing at least 2 kinetically distinct compartments. The mean elimination half-life is about 2.5 hours. After intravenous administration, the steady-state volume of distribution has been calculated to be 42.2 +/- 8.9L and the total body clearance 12.7 +/- 1.3L/h. In hypertensive patients with normal renal function, prazosin kinetics do not differ significantly from normals. However, prazosin disposition is modified in chronic renal failure and in congestive heart failure. In both cases, the plasma free fraction of prazosin is increased and plasma elimination half-life is longer. Prazosin kinetics may be expected to be altered in patients with liver diseases. Pharmacokinetic data do not suggest a mechanism to explain the disappearance of the first-dose effect during continued administration of prazosin. Although more investigation is needed to define prazosin kinetics in congestive heart failure and chronic renal failure, the available information about prolongation of elimination half-life, decreased protein binding and increased peak plasma concentrations suggest that prazosin dosage should be titrated cautiously in such patients.