Muscarinic antagonists in development for disorders of smooth muscle function

Area of Science:

• Pharmacology
• Gastroenterology
• Urology

Background:

• Muscarinic M3 receptors regulate smooth muscle tone, making them targets for conditions like incontinence and IBS.
• Selective M3 antagonists offer potential for better toleration than non-selective compounds.

Purpose of the Study:

• To evaluate the affinity, selectivity, and functional activity of darifenacin at muscarinic M3 receptors.
• To assess darifenacin's tissue selectivity and efficacy in animal models.

Main Methods:

• In vitro receptor binding assays to determine affinity and selectivity for M3, M1, and M2 receptors.
• Ex vivo studies on smooth muscle preparations (ileum, trachea, bladder) and salivary glands.
• In vivo studies in conscious dogs to assess M3-mediated effects on intestinal motility and other responses.

Main Results:

• Darifenacin demonstrated high affinity (pKi 9.12) and selectivity (9-74 fold) for human M3 receptors over M1 and M2.
• The compound potently inhibited M3-mediated smooth muscle responses in vitro and in vivo.
• Darifenacin showed functional tissue selectivity, inhibiting intestinal motility at lower doses than those affecting M1 or M2 mediated responses.

Conclusions:

• Darifenacin exhibits high affinity and selectivity for the muscarinic M3 receptor.
• Its selective M3 antagonism translates to potent inhibition of M3-mediated smooth muscle contraction with potential for improved toleration.
• Ongoing clinical studies will further define darifenacin's efficacy and safety profile.