Urinary Excretion Contributes to Long-Lasting Blockade of Bladder Muscarinic Receptors by Imidafenacin: Effect of Bilateral Ureteral Ligation

Imidafenacin is a potent and selective antagonist of M-1 and M-3 muscarinic receptors that is safe, efficacious, and well tolerated for controlling the symptoms of overactive bladder (OAB). However, the precise mechanisms responsible for the bladder-selective pharmacological effects of this agent remain unclear. The in vivo pharmacologic effects of imidafenacin result from receptor occupancy. Therefore, the present study was performed to characterize in vivo muscarinic receptor binding by tritium-labeled imidafenacin with high specific activity ([H-3]imidafenacin) in the bladder and other tissues of mice, and to clarify the mechanisms underlying selective binding of imidafenacin to bladder muscarinic receptors. After intravenous injection of [H-3] imidafenacin, its binding to muscarinic receptors in the bladder and other tissues of mice was assessed by a radioligand binding assay. [H-3] Imidafenacin showed a significantly longer duration of binding to muscarinic receptors in the bladder than in other tissues, and muscarinic receptor binding of [H-3] imidafenacin was markedly suppressed in the bladder alone after bilateral ligation of the ureters. After intravenous injection, the [H-3] imidafenacin concentration was markedly higher in the urine than in the plasma, suggesting that urinary excretion may contribute significantly to the selective and long-lasting binding of imidafenacin to bladder muscarinic receptors. These findings suggest that the intra-vesicular concentration of an antimuscarinic agent and its active metabolites may have a substantial influence on its pharmacological effect and duration of action in patients with OAB. In addition, factors that modulate urine production may influence the efficacy and safety of antimuscarinic agents.