Chiral recognition of doxazosin enantiomers in 3 targets for therapy as well as adverse drug reactions in animal experiments.

Doxazosin used in benign prostatic hyperplasia has the side effects of causing hypotension and the risk of heart failure. The 3 targets of α(1A)-adrenoceptors (in the prostate), α(1D)-adrenoceptors (in the aorta), and an unknown mechanism (in the heart) are involved, respectively. We hypothesized that there is a chiral recognition of doxazosin enantiomers in the 3 targets. Using isolated rat aorta (α(1D)-adrenoceptors) and rabbit prostate (α(1A)-adrenoceptors), we examined pA(2) and pK(B) values of doxazosin enantiomers. We observed chronotropic and inotropic effects of doxazosin enantiomers in isolated rat and rabbit heart tissues. (-)Doxazosin and (+)doxazosin produced a shift to the right of concentration-contraction curves for noradrenalin (aorta) and phenylephrine (prostate smooth muscle). The pA(2) value of (-)doxazosin (8.625 ± 0.053) was smaller than (+)doxazosin (9.503 ± 0.051) in rat aorta, but their pK(B) values in rabbit prostate were the same. In rat and rabbit heart tissues, (+)doxazosin (3-30 µmol·L(-1)) significantly decreased atrial rate, and produced negative inotropic effects; however, (-)doxazosin did not affect the atrial rate, and produced positive inotropic effects in the atria. Thus, the chiral carbon atom of doxazosin does not affect its activity at the therapeutic target of α(1A)-adrenoceptors in the prostate, but significantly changes its blocking activity against α(1D)-adrenoceptors in the aorta, and produces opposite inotropic effects in the atria via an α(1)-adrenoceptor-independent mechanism.