Stereoselectivity of procyclidine binding to muscarinic receptor subtypes M 1, M 2 and M 4

Abstract

The goals of the present study were: (1) to investigate the binding properties of (R)- and (S)-procyclidine and two achiral derivatives of muscarinic M 1, M 2 and M 4 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 M 1 and rat striatum muscarinic M 4 receptors, as compared to rat cardiac M 2 receptors. (S)-Procyclidine had a 130-fold lower affinity than (R)-procyclidine for M 1 and M 4 receptors, and a 40-fold lower affinity for M 2 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 M 1 and M 4 receptors. as compared to (R)-procyclidine, and a three-fold lower affinity for M 2 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 M 1, M 2 and M 4 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 M 1 and M 4 muscarinic receptors for (R)-procyclidine reflected the better fit of the cyclohexyl group of (R)-procyclidine to the subsite of M 1 and M 4 as compared to M 2 receptors.