Stereoselectivity of procyclidine binding to muscarinic receptor subtypes M1, M2 and M4

The goals of the present study were: (1) to investigate the binding properties of (R)- and (S)-procyclidine and two achiral derivatives of muscarinic M1, M2 and M4 receptor subtypes and (2) to identify the interactions which allow these receptors to discriminate between the two stereoisomers. (R)-Procyclidine showed a higher affinity for human neuroblastoma NB-OK 1 muscarinic M1 and rat striatum muscarinic M4 receptors, as compared to rat cardiac M2 receptors. (S)-Procyclidine had a 130-fold lower affinity than (R)-procyclidine for M1 and M4 receptors, and a 40-fold lower affinity for M2 receptors. Pyrrinol, the achiral diphenyl derivative with the cyclohexyl group of (S)-procyclidine replaced by a phenyl group, has an eight-fold lower affinity for M1 and M4 receptors, as compared to (R)-procyclidine, and a three-fold lower affinity for M2 receptors. Hexahydro-procyclidine, the corresponding achiral dicyclohexyl compound, had a 10- to 20-fold lower affinity than (R)-procyclidine for the three receptors. The increase in binding free energy, which is observed when the phenyl and cyclohexyl groups of procyclidine are separately replaced by cyclohexyl and phenyl groups, respectively, was additive in the case of M1, M2 and M4 receptors. This indicates that the muscarinic receptor stereoselectivity was based on the coexistence of two binding sites, one preferring a phenyl rather than cyclohexyl group and the second preferring a cyclohexyl rather than a phenyl group. In addition, there were also binding sites for the hydroxy moiety and the protonated amino group of the ligands. The greater affinity and stereoselectivity of M1 and M4 muscarinic receptors for (R)-procyclidine reflected the better fit of the cyclohexyl group of (R)-procyclidine to the subsite of M1 and M4 as compared to M2 receptors.