Muscarinic receptors in the bladder: from basic research to therapeutics

• Published in: British journal of pharmacology Vol. 147; no. S2; pp. S80 - S87
• Main Author: Hegde, Sharath S.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.02.2006; Nature Publishing; Nature Publishing Group
• Subjects: Animals; antimuscarinic; Biological and medical sciences; darifenacin; Humans; Medical sciences; Mice; Mice, Knockout; micturition; Muscarinic; Muscarinic Antagonists - pharmacokinetics; Muscarinic Antagonists - pharmacology; Muscarinic Antagonists - therapeutic use; Nephrology. Urinary tract diseases; overactive bladder; oxybutynin; Patient Compliance; Pharmacology. Drug treatments; Receptor, Muscarinic M2 - metabolism; Receptor, Muscarinic M3 - metabolism; Receptors, Muscarinic - drug effects; Receptors, Muscarinic - genetics; Receptors, Muscarinic - metabolism; Review; solifenacin; tolterodine; Treatment Outcome; trospium; Urinary Bladder - drug effects; Urinary Bladder - metabolism; Urinary Bladder, Overactive - drug therapy; Urinary Bladder, Overactive - metabolism; Urinary system involvement in other diseases. Miscellaneous; Urinary tract. Prostate gland; M3 Muscarinic receptor; Urinary system disease; Muscarinic; Tolterodine; Urinary tract disease; Cholinergic receptor; trospium; Treatment; Urinary system; Micturition; Solifenacin; Antispasmodic agent; Urinary bladder; antimuscarinic; Overactive bladder; Bladder disease; Darifenacin; Oxybutynin; Muscarinic receptor; Antagonist; Parasympatholytic
• ISSN: 0007-1188; 1476-5381
• DOI: 10.1038/sj.bjp.0706560

Muscarinic receptor antagonists (antimuscarinics) serve as the cornerstone in the pharmacological management of overactive bladder (OAB) by relieving the symptoms of urgency, frequency and incontinence. These drugs operate primarily by antagonizing post‐junctional excitatory muscarinic receptors (M2/M3) in the detrusor. The combination of pharmacological and gene knockout studies has greatly advanced our understanding of the functional role of muscarinic receptors in the bladder. M3 receptors produce direct smooth muscle contraction by a mechanism that relies on entry of extracellular calcium through L‐type channels and activation of a rho kinase. M2 receptors, which predominate in number, appear to facilitate M3‐mediated contractions. M2 receptors can also produce bladder contractions indirectly by reversing cAMP‐dependent β‐adrenoceptor‐mediated relaxation, although the physiological role of β‐adrenoceptors in detrusor relaxation is controversial. Emerging evidence suggests that muscarinic receptors in the urothelium/suburothelium can modulate the release of certain factors, which in turn may affect bladder function at the efferent or afferent axis. Currently, oxybutynin, tolterodine, darifenacin, solifenacin and trospium are the five major antimuscarinics approved for the treatment of OAB. Comparative clinical studies have shown that oxybutynin and solifenacin may be marginally more effective than tolterodine, although the latter seems to be better tolerated. Pharmacokinetic–pharmacodynamic analyses using plasma concentrations of ‘total drug’ indicate that, at therapeutic doses, the clinical efficacy of darifenacin and solifenacin may be driven primarily by selective M3 receptor occupation, whereas the pharmacodynamic effects of pan‐selective molecules (such as tolterodine, trospium) may potentially involve multiple receptors, including M2 and M3. Furthermore, high M3 receptor occupation is the likely explanation for the greater propensity of darifenacin and oxybutynin to cause dry mouth and/or constipation. Although the recently introduced drugs represent a significant improvement over older drugs, especially with respect to the convenience of dosing schedule, their overall efficacy and tolerability profile is still less than optimal and patient persistence with therapy is low. Recent advances in basic research have not yet offered a clear discovery path for improving the therapeutic index of antimuscarinic molecules. There is still an unmet need for an antimuscarinic medicine with superior clinical effectiveness that can translate into better persistence on therapy. British Journal of Pharmacology (2006) 147, S80–S87. doi:10.1038/sj.bjp.0706560