Structural Modifications to Tetrahydropyridine-3-carboxylate Esters en Route to the Discovery of M5-Preferring Muscarinic Receptor Orthosteric Antagonists

The M-5 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 M-5-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 M-5 over M-1 receptor and shows little activity at M-2-M-4. This compound, although exhibiting modest affinity (K-i = 2.24 mu M) for the [H-3]N-methylscopolamine binding site on the M-5 receptor, is potent (IC50 = 0.45 nM) in inhibiting oxotremorine-evoked [H-3]DA release from rat striatal slices. Further, a homology model of human M-5 receptor based on the crystal structure of the rat M-3 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.