Pharmacokinetic and pharmacodynamic studies with two alpha-adrenoceptor antagonists, doxazosin and prazosin in the rabbit

Abstract

The cardiovascular effects of doxazosin, a quinazoline derivative related to prasozin were investigated and compared to prazosin in the rabbit. Radioligand binding studies using rabbit cerebral membranes showed that both doxazosin and prazosin were roughly equipotent at displacing [3H]-prazosin from specific binding sites. However, the lower pA2 value for doxazosin at alpha 1-adrenoceptors in isolated thoracic aorta preparations suggests a lower potency compared to prazosin. The dose-related pressor effects of intravenous phenylephrine were used to assess vascular alpha 1-adrenoceptor antagonism in vivo. There was a close agreement between alpha 1-adrenoceptor antagonist potency and maximum hypotensive effects with both doxazosin and prazosin. The alpha 1-adrenoceptor antagonist effects of doxazosin were more prolonged than those of prazosin. Studies using either radioligand binding or pressor responses to B-HT 920 showed that doxazosin did not show any significant affinity for the alpha 2-adrenoceptor. Similarly, no direct vasodilator effects were observed either in animals administered angiotensin II or in isolated thoracic aorta spiral strip preparations contracted with potassium. Doxazosin has a longer terminal elimination half-life than prazosin. The pharmacokinetics of doxazosin were linear over the dose range examined. Following pharmacological 'autonomic blockade' and treatment with prazosin, doxazosin did not cause any further fall in blood pressure. These observations suggest that doxazosin, like prazosin, appears to exert its hypotensive action through alpha 1-adrenoceptor antagonism. The prolonged fall in blood pressure and well sustained alpha 1-adrenoceptor antagonism after doxazosin raise the possibility of an active metabolite which also has alpha 1-adrenoceptor blocking properties.