Chemical modification of ring c of himbacine: Discovery of a pharmacophoric element for M2-selectivity

Removal of the carbonyl oxygen of himbacine furnishes a molecule which binds with 20-fold and 4-fold lower affinity at the M2 and M1 receptor sites, respectively. Thus, the carbonyl oxygen constitutes an important pharmacophoric element responsible for some of the M2 selectivity of himbacine. Other modifications of the C-ring of the tricyclic portion of himbacine were examined and tested for potency at M1 and M2 sites, and the data confirm that M2selectivity is optimal with a closed heterocyclic ring and with the presence of a proximal carbonyl oxygen