Pharmacological Effects of Darifenacin on Human Isolated Urinary Bladder

Abstract

Darifenacin [(S)-2--2,2-diphenylacetamide] is a novel antimuscarinic drug currently undergoing phase III trials for the treatment of overactive bladder. We investigated the functional antagonist potency of darifenacin, and the antimuscarinic drugs propiverine, oxybutynin and atropine, on human detrusor smooth muscle. Urinary bladder specimens were obtained from 20 patients who underwent total cystectomy for malignant bladder tumor. Using an organ-bath technique, the effects of the compounds on carbachol-, KCl-, CaCl₂- or electrical field stimulation (EFS)-induced contractions of the tissues were evaluated. The order of antagonist potency (pA₂values) at the muscarinic M₃ receptors was: darifenacin (9.34) > atropine (9.26) > oxybutynin (7.74) > propiverine (7.68). Darifenacin and atropine, at concentrations up to 10^–6 mol/l, did not inhibit the KCl- and CaCl₂-induced contractions (concentrations 80 and 5 mmol/l, respectively), while propiverine and oxybutynin (10^–5 mol/l) significantly inhibited these contractions. Pretreatment with darifenacin (10^–9–10^–6 mol/l), propiverine (10^–8– 10^–5 mol/l), oxybutynin (10^–8–10^–5 mol/l) and atropine (10^–9–10^–6 mol/l) significantly inhibited maximum EFS-induced contractions. Darifenacin inhibited contractions of human detrusor smooth muscle only through its antimuscarinic action, while propiverine and oxybutynin had both antimuscarinic and Ca²⁺ channel antagonist actions. These findings indicate that darifenacin is a potent antagonist at the M₃ receptor and support its use as a treatment for overactive bladder.