THE USE OF α-ADRENOCEPTOR ANTAGONISTS IN THE PHARMACOLOGICAL MANAGEMENT OF BENIGN PROSTATIC HYPERTROPHY: AN OVERVIEW

Abstract

Benign prostatic hypertrophy (BPH) produces symptomatic urethral obstruction in a significant percentage of older men. Since the incidence of BPH is age related, the clinical and economic impact of this disease will continue to progress as average lifespan increases. BPH is associated with growth of both glandular and stromal elements of the prostate gland. Glandular hyperplasia can be partially reversed by withdrawal of androgenic tone with androgen receptor antagonists or steroid-5- α-reductase inhibitors. However, the reduction in prostatic size produced by these agents has little effect on the dynamic tone induced by nerve mediated contraction of stromal smooth muscle. This tone is mediated by activation of α-adrenoceptors. Therefore the α-adrenoceptor antagonists represent a useful pharmacological approach to the treatment of BPH. Studies in isolated strips of human prostate show that either exogenous α-adrenoceptor agonists or electrical field stimulation will induce contraction. Studies with selective antagonists such as prazosin show that this response is mediated by the α 1-adrenoceptor, even though radioligand binding studies show the presence of α 1and α 2adrenoceptor subtypes in approximately equal density. Following the cloning of multiple α 1-adrenoceptors, the contractile response in human prostate has been assigned to the α 1Aadrenoceptor. However, recent data would suggest a functional role for another subtype, which has not yet been cloned, and designated as α 1Lbased on a relatively low affinity for prazosin. Clinical trials have shown efficacy of a variety of α-adrenoceptor antagonists in BPH, including non-selective agents such as phenoxybenzamine, as well as a variety of selective α 1-adrenoceptor antagonists, most structurally related to prazosin. The agents most commonly employed at the present time include the prazosin analogs terazosin, doxazosin and alfuzosin, as well as the structurally unrelated indoramin and tamsulosin. The design of new α 1-antagonists for BPH has concentrated on agents producing preferential blockage of urogenital vis-á-vis vascular α 1-adrenoceptors, based either on selectivity for the α 1A-adrenoceptor subtype or on functional uroselectivity in animal models. While these newer agents offer the prospect of reducing the incidence of the cardiovascular side effects associated with current therapy their superiority over nonselective α 1-adrenoceptor antagonists remains to be demonstrated in the clinical setting.