The effect of food on the pharmacokinetics of doxazosin mesylate was examined in a crossover study with twelve hypertensive subjects. Neither of these differences was statistically or clinically significant. Doxazosin mesylate is extensively metabolized in the liver, mainly by O-demethylation of the quinazoline nucleus or hydroxylation of the benzodioxan moiety. Although several active metabolites of doxazosin have been identified, the pharmacokinetics of these metabolites have not been characterized. Plasma elimination of doxazosin is biphasic, with a terminal elimination half-life of about 22 hours.
Enterohepatic recycling is suggested by secondary peaking of plasma doxazosin concentrations.
In a crossover study in 24 normotensive subjects, the pharmacokinetics and safety of doxazosin were shown to be similar with morning and evening dosing regimens. Pharmacokinetic studies in elderly patients and patients with renal impairment have shown no significant alterations compared to younger patients with normal renal function. As with any drug wholly metabolized by the liver, use of doxazosin mesylate in patients with altered liver function should be undertaken with caution. In two placebo-controlled studies, of normotensive and hypertensive BPH patients, in which doxazosin was administered in the morning and the titration interval was two weeks and one week, respectively, trough plasma concentrations of doxazosin mesylate were similar in the two populations. Linear kinetics and dose proportionality were observed.