Hybrid molecules from xanomeline and tacrine: enhanced tacrine actions on cholinesterases and muscarinic M1 receptors.

A set of amide- and amine-linked hybrid molecules comprising moieties of the orthosteric M(1) muscarinic receptor agonist xanomeline and the cholinesterase inhibitor and allosteric receptor modulator tacrine were prepared with varying spacer length of 10-17 atoms. The hybrids inhibited acetylcholinesterase with similar or higher potency compared to tacrine. M(1) receptor binding affinity was similar or higher relative to xanomeline and far higher relative to tacrine. Affinities hardly changed when the receptors' orthosteric site was occupied by an inverse agonist ligand. When occupied by the orthosteric activator acetylcholine, affinity for the hybrids declined to unmeasureably low levels. Hybrids did not activate M(1) receptors. In vivo studies assaying cognition impairment in rats induced by scopolamine revealed pronounced enhancement of scopolamine action. Taken together, instead of dualsteric (simultaneous allosteric/orthosteric) binding, the hybrids seem to prefer purely allosteric binding at the inactive M(1) receptor.