Saw palmetto extracts potently and noncompetitively inhibit human α1‐adrenoceptors in vitro

Abstract

BACKGROUND We wanted to test whether phytotherapeutic agents used in the treatment of lower urinary tract symptoms have α1-adrenoceptor antagonistic properties in vitro. METHODS Preparations of β-sitosterol and extracts of stinging nettle, medicinal pumpkin, and saw palmetto were obtained from several pharmaceutical companies. They were tested for their ability to inhibit [3H]tamsulosin binding to human prostatic α1-adrenoceptors and [3H]prazosin binding to cloned human α1A- and α1B-adrenoceptors. Inhibition of phenylephrine-stimulated [3H]inositol phosphate formation by cloned receptors was also investigated. RESULTS Up to the highest concentration which could be tested, preparations of β-sitosterol, stinging nettle, and medicinal pumpkin were without consistent inhibitory effect in all assays. In contrast, all tested saw palmetto extracts inhibited radioligand binding to human α1-adrenoceptors and agonist-induced [3H]inositol phosphate formation. Saturation binding experiments in the presence of a single saw palmetto extract concentration indicated a noncompetitive antagonism. The relationship between active concentrations in vitro and recommended therapeutic doses for the saw palmetto extracts was slightly lower than that for several chemically defined α1-adrenoceptor antagonists. CONCLUSIONS Saw palmetto extracts have α1-adrenoceptor-inhibitory properties. If bioavailability and other pharmacokinetic properties of these ingredients are similar to those of the chemically defined α1-adrenoceptor antagonists, α1-adrenoceptor antagonism might be involved in the therapeutic effects of these extracts in patients with lower urinary tract symptoms suggestive of benign prostatic obstruction. Prostate 38:208–215, 1999. © 1999 Wiley-Liss, Inc.