Ryo-ich Takei (1), Ichiro Ikegaki (1,*), Katsushi Shibata (2), Gozoh
Tsujimoto (2) and Toshio Asano (1)
(1) Laboratory for Pharmacology, Institute for Life Science Research, Asahi
Chemical Industry, 632-1 Mifuku, Ohito-cho, Tagata-gun, Shizuoka 410-2321,
Japan
(2) Department of Molecular, Cell Pharmacology, National Children's Medical
Research Center, 3-35-31 Taishido, Setagaya-ku, Tokyo 154-0004, Japan
(*) To whom correspondence should be addressed.
Abstract: The pharmacological profiles of the alpha1-adrenoceptor
antagonists naftopidil, tamsulosin and prazosin were studied in an anesthetized
dog model that allowed the simultaneous assessment of their antagonist potency
against phenylephrine-mediated increases in prostatic pressure and mean
blood pressure. The intravenous administration of each of these compounds
dose-dependently inhibited phenylephrine-induced increases in prostatic
pressure and mean blood pressure. To further assess the ability of the three
compounds to inhibit phenylephrine-induced responses, the doses required
to produce a 50% inhibition of the phenylephrine-induced increases in prostatic
and mean blood pressure and the selectivity index obtained from the ratio
of those two doses were determined for each test compound. Forty minutes
after the intravenous administration of naftopidil, the selectivity index
was 3.76, and those of tamsulosin and prazosin were 1.23 and 0.61, respectively.
These findings demonstrated that naftopidil selectively inhibited the phenylephrine-induced
increase in prostatic pressure compared with mean blood pressure in the
anesthetized dog model. The selectivity of naftopidil for prostatic pressure
was the most potent among the test compounds. In addition, using cloned
human alpha1-adrenoceptor subtypes, naftopidil was selective
for the alpha1d-adrenoceptor with approximately 3- and 17-fold
higher affinity than for the alpha1a- and alpha1b-adrenoceptor
subtypes, respectively. The selectivity of naftopidil for prostatic pressure
may be attributable to its high binding affinity for alpha1a-
and alpha1d-adrenoceptor subtypes.
Keywords: Naftopidil, alpha1-Adrenoceptor antagonist, Prostate
pressure, Benign prostatic hyperplasia, Anesthetized dog