Novel muscarinic acetylcholine receptor hybrid ligands embedding quinuclidine and 1,4-dioxane fragments

To obtain novel muscarinic acetylcholine receptor (mAChR) antagonists, the enantiomers of the hybrid compounds 3–5, in which the quinuclidin-3-yloxy fragment of solifenacin and the 6,6-diphenyl-1,4-dioxane-2-yl moiety of 2 linked by an ester or ether spacer were embedded in the same chemical entity, were prepared and evaluated for their affinity at the five mAChR subtypes (M₁-M₅). Stereochemistry and the nature of the linker between the quinuclidine moiety and the 1,4-dioxane nucleus play an important role on the affinities of the compounds. The presence of an ether bridge confers higher affinities for all mAChR subtypes to the ligand. Interestingly, the ether enantiomer (R,S)-5 shows the highest affinity at all mAChR subtypes with pK_i values similar to that of solifenacin at M₃ and higher at the other subtypes. Unlike solifenacin, it shows a preference for M₁ mAChR subtype with respect to the other subtypes. This compound, lacking a permanent positive charge on the nitrogen atom, can be a useful tool for the pharmacological study of mAChRs in the central nervous system.