Distance geometry of alpha-substituted 2,2-diphenylpropionate antimuscarinics.
Quantitative structure-activity relationships between pharmacological activities and physical properties of a series of 2,2-diphenylpropionate compounds were used to define the topography of the antagonist binding site of muscarinic receptors. XICAMM, a computer molecular modeling program, was used to calculate geometrical and topological values of the compounds. The compounds were tested for their antimuscarinic activities by: (a) inhibition of [N-methyl-3H]scopolamine binding to the muscarinic receptors of N4TG1 neuroblastoma cells, (b) inhibition of carbachol-induced alpha-amylase release from rat pancreas acini, and (c) blocking of acetylcholine-induced contraction of guinea pig ileum. To evaluate as clearly as possible only the effect of the bond distance on the potency of the synthesized antimuscarinics, the compounds contained as many constant features as possible. Neither the hydrophobic nor the ester moieties of the compounds were changed, and the rings containing the protonated nitrogen were saturated and restricted. The antimuscarinic activities obtained from the three assays were significantly correlated with each other, with the exception of two compounds, 9 and 13. The latter two compounds demonstrated specificity for the m3 muscarinic receptor subtype expressed in the pancreas. Furthermore, it was demonstrated that the antimuscarinic activities were significantly related to the bond distances between the carbonyl oxygen (constant electronegative locus) and the protonated nitrogen (center of cationic charge) of the 2,2-diphenylpropionate compounds. Parabolic relationships between the pharmacological activities and bond distances were empirically established. The shortest calculated bond distance of these compounds was approximately 4.4 A, whereas the longest was about 5.9 A. The maximum antimuscarinic potency was observed with a calculated bond distance of about 5.2 A in all three assays.