Determination of alpha 1-adrenoceptor antagonists in plasma by radioreceptor assay

A simple, rapid and sensitive radioreceptor assay (RRA) for the quantification of alpha 1-adrenoceptor antagonists such as prazosin in plasma is described. The method involves the use of an RRA based on [3H]prazosin displacement in rat cerebral cortical membranes. The method is reliable, with intra-assay and inter-assay RSDs ranging from 5.9 to 9.2%. The limit of detection is 0.2 (prazosin hydrochloride), 0.05 (tamsulosin hydrochloride) and 0.3 (bunazosin hydrochloride) pmol per assay. Using this method the plasma levels of prazosin hydrochloride were determined in beagle dogs administered orally 2.39 mumol kg-1 of this drug. The plasma levels of prazosin in beagle dogs are in good agreement with those obtained using a high-performance liquid chromatography (HPLC). This RRA proved to be applicable to the monitoring of plasma prazosin levels in patients with essential hypertension and/or benign prostatic hypertrophy receiving therapy with this drug with the therapeutic dosage schedule. Thus, the concentrations of alpha 1-adrenoceptor antagonists in plasma can be adequately monitored by RRA as well as by HPLC.