Structural modifications to tetrahydropyridine-3-carboxylate esters en route to the discovery of M₅-preferring muscarinic receptor orthosteric antagonists

The M₅ muscarinic acetylcholine receptor is suggested to be a potential pharmacotherapeutic target for the treatment of drug abuse. We describe herein the discovery of a series of M₅-preferring orthosteric antagonists based on the scaffold of 1,2,5,6-tetrahydropyridine-3- carboxylic acid. Compound 56, the most selective compound in this series, possesses an 11-fold selectivity for the M₅ over M₁ receptor and shows little activity at M₂-M₄. This compound, although exhibiting modest affinity (K_i = 2.24 μM) for the [³H]N-methylscopolamine binding site on the M₅ receptor, is potent (IC₅₀ = 0.45 nM) in inhibiting oxotremorine-evoked [³H]DA release from rat striatal slices. Further, a homology model of human M₅ receptor based on the crystal structure of the rat M₃ receptor was constructed, and docking studies of compounds 28 and 56 were performed in an attempt to understand the possible binding mode of these novel analogues to the receptor.