Prazosin kinetics in hypertension

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Abstract
Prazosin kinetics were studied after single doses (i.v. and oral, 0.5 mg) and after increasing multiple doses (0.5-5 mg 3 times/d) in 8 patients with hypertension. After i.v. administration the kinetics could be described by a linear 2-compartment open model. Terminal half-life (t1/2.beta.) was .apprx. 3 h and apparent volume of distribution (Vd.beta.) .apprx. 0.6 l/kg. After oral doses bioavailability ranged from 55-82%. Since total plasma clearance was low (0.14 l/kg per h) incomplete bioavailability was the result of incomplete absorption rather than of 1st-pass liver metabolism. The estimated extraction ratio was .apprx. 14%. Renal clearance was negligible; only 1-2% of the dose was recovered unchanged in urine. Binding to plasma proteins to both albumin and .alpha.1-acid glycoprotein was substantial (97%), with albumin being most important. Increasing multiple doses showed that prazosin followed 1st-order kinetics with a linear correlation between dose and steady-state plasma concentration (p < 0.001). There were substantial variations in plasma concentrations between patients and there were also day-to-day variations in concentration within the same patient.

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