Prediction of α1-adrenoceptor occupancy in the human prostate from plasma concentrations of silodosin, tamsulosin and terazosin to treat urinary obstruction in benign prostatic hyperplasia

alpha(1)-Adrenoceptor antagonists are clinically useful for the improvement of urinary obstruction due to benign prostatic hyperplasia (BPH), and their therapeutic effects are mediated through the blockade of prostatic mu(1-)adrenoceptors. The present study was undertaken to predict the magnitude and duration of alpha-adrenoceptor occupancy in the human prostate after oral mu(1)-adrenoceptor antagonists. Prostatic alpha(1)-adrenoceptor-binding parameters of silodosin were estimated by measuring specific [H-3]prazosin binding in rat prostate after oral administration of this drug. The plasma concentration of silodosin after oral administration in rats and healthy volunteers was measured using a high-performance liquid chromatographic method. The alpha(1)-adrenoceptor-binding affinities (K-i) of silodosin, tamsulosin, and terazosin in the human prostate and plasma concentrations of tamsulosin and terazosin were obtained from the literature. Using the alpha(1)-adrenoceptor binding parameters of silodosin in rat prostates alpha(1)-adrenoceptor occupancy in the human prostate was estimated to be around 60-70% at I-6 h after oral administration of silodosin at doses of 3.0, 8.1, and 16.1 mu mol. Thereafter, the receptor occupancy was periodically decreased, to 24% (8.1 mu mol) and 54% (16.1 mu mol) 24 h later. A similar magnitude and time course of alpha(1)-adrenoceptor occupancy by silodosin in the human prostate were estimated using alpha(1)-adrenoceptor-binding affinities (K-i) in the human prostate. Despite about two orders of differences in the plasma unbound concentrations after clinically effective oral dosages of silodosin, tamsulosin, and terazosin, there was a comparable magnitude of prostatic alpha(1)-adrenoceptor occupancy by these drugs. In conclusion, the prediction of alpha(1)-adrenoceptor occupancy in the human prostate by mu(1)-adrenoceptor antagonists may provide the rationale for the optimum dosage regimen of these drugs in the therapy of BPH.