Abstract

We evaluated the pharmacological profiles of (2R)-N-[1-(6- aminopyridin-2-ylmethyl)piperidin-4-yl]-2-[(1R)-3,3-difluorocyclopentyl]-2-hydroxy-2-phenylacetamide(compound A), which is a novel muscarinic receptor antagonist with M₂-sparing antagonistic activity. Compound A inhibited [³H]NMS binding to cloned human muscarinic m1, m2, m3, m4, and m5 receptors expressed in Chinese hamster ovary cells with K _i values (nM) of 1.5, 540, 2.8, 15, and 7.7, respectively. In isolated rat tissues, compound A inhibited carbachol-induced responses with 540-fold selectivity for trachea (K _B = 1.2 nM) over atria (K _B = 650 nM). In in vivo rat assays, compound A inhibited acetylcholine-induced bronchoconstriction and bradycardia with intravenous ED₅₀ values of 0.022 mg/kg and ≥10 mg/kg, respectively. Furthermore, in dogs, compound A (0.1–1 mg/kg p.o.) dose dependently shifted the methacholine concentration-respiratory resistance curves. In mice, compound A (10 mg/kg i.v.) did not inhibit oxotremorine-induced tremor. The brain/plasma ratio (K _p) of compound A (3 mg/kg i.v.) was 0.13 in rats; this K _p was less than that of scopolamine (1.7) and darifenacin (0.24). The inhibition of compound A (3 mg/kg i.v.) on ex vivo binding in rat cerebral cortex was almost similar to that of NMS. These findings demonstrate that compound A has high selectivity for M₃receptors over M₂ receptors, displays a potent, oral M₃ antagonistic activity without inhibition of central muscarinic receptors because of low brain penetration. It is well known that central muscarinic antagonists may have diverse CNS effects, and M₂ receptors regulate cardiac pacing and act as autoreceptors in the lung and bladder. Thus, compound A may have fewer cardiac or CNS side effects than nonselective compounds.