title: Prediction of alpha1-adrenoceptor occupancy in the human prostate from plasma concentrations of silodosin, tamsulosin and terazosin to treat urinary obstruction in benign prostatic hyperplasia.
abstract: Alpha1-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 alpha(1)-adrenoceptors. The present study was undertaken to predict the magnitude and duration of alpha(1)-adrenoceptor occupancy in the human prostate after oral alpha(1)-adrenoceptor antagonists. Prostatic alpha(1)-adrenoceptor-binding parameters of silodosin were estimated by measuring specific [(3)H]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 prostate, alpha(1)-adrenoceptor occupancy in the human prostate was estimated to be around 60-70% at 1-6 h after oral administration of silodosin at doses of 3.0, 8.1, and 16.1 micromol. Thereafter, the receptor occupancy was periodically decreased, to 24% (8.1 micromol) and 54% (16.1 micromol) 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 alpha(1)-adrenoceptor antagonists may provide the rationale for the optimum dosage regimen of these drugs in the therapy of BPH.
authors.affiliation: Department of Pharmacokinetics and Pharmacodynamics and Center of Excellence Program in the 21st Century, University of Shizuoka School of Pharmaceutical Sciences, Sizuoka, Japan. yamada@ys7-u-shizuoka-ken.ac.jp
authors.forename: Shizuo
authors.initials: S
authors.lastname: Yamada
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Kato
authors.forename: Takashi
authors.initials: T
authors.lastname: Okura
authors.forename: Yoshiyuki
authors.initials: Y
authors.lastname: Kagawa
authors.forename: Kazuki
authors.initials: K
authors.lastname: Kawabe
mesh_terms: D000317:Adrenergic alpha-Antagonists / Q000097:blood*; D000328:Adult; D000818:Animals; D006801:Humans; D007211:Indoles / Q000097:blood*; D008297:Male; D011224:Prazosin / Q000031:analogs & derivatives* / Q000097:blood; D011467:Prostate / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D018340:Receptors, Adrenergic, alpha-1 / Q000378:metabolism*; D013449:Sulfonamides / Q000097:blood*; D000077409:Tamsulosin
chemical_list: D000317:Adrenergic alpha-Antagonists; D007211:Indoles; D018340:Receptors, Adrenergic, alpha-1; D013449:Sulfonamides; C041226:Terazosin; C095285:silodosin; D000077409:Tamsulosin; D011224:Prazosin