Stereochemical aspects of parasympathomimetics and their antagonists: recent developments

Substances isolated from Mandragora species

The present state of knowledge in the chemistry of mandragora plant is reviewed. Isolations and identifications of the compounds were done from all parts of this plant. Up-to-date more than 80 substances were identified in different species of the genus Mandragora.

Conformation of ligands bound to the muscarinic acetylcholine receptor

Many biogenic amines evoke a variety of physiological responses by acting on G protein-coupled receptors. We have determined the conformation of two acetylcholine analogs, (S)-methacholine and (2S,4R,5S)-muscarine, bound to the M(2) muscarinic acetylcholine receptor (M(2) mAChR) by NMR spectroscopy. The analysis of the transferred nuclear Overhauser effect indicated that the receptor selectively recognized the conformers of (S)-methacholine and (2S,4R,5S)-muscarine with the gauche O-C2-C1-N dihedral angle at +60 degrees. This is distinct from the predominant conformations of these ligands in solution with O-C2-C1-N dihedral angle (+80 to approximately 85 degrees ) in the absence of the M(2) mAChR, as assessed by analyses of the coupling constants and nuclear Overhauser effect spectroscopy. We have also built a molecular model of the M(2) mAChR-(S)-methacholine complex, based on the X-ray crystallographic structure of rhodopsin. This model indicated that the conformation with the gauche O-C2-C1-N dihedral angle at +55.5 degrees, which is similar to the one determined by NMR measurement, is energetically favored in the binding of (S)-methacholine to the receptor. We suggest that this conformation represents the binding of the agonist to the M(2) mAChR in the absence of G protein.

Conformational studies on the four stereoisomers of the novel anticholinergic 4-(dimethylamino)-2-phenyl-2-(2-pyridyl)pentanamide

To interpret differences in the anticholinergic activity among the four stereoisomers of 4-(dimethylamino)-2-phenyl-2-(2-pyridyl)pentanamide (1-4), we performed conformational studies using the semiempirical molecular orbital method. The structures of the global minimum-energy conformations obtained for 1-4, however, could not explain the different activities, particularly in terms of distances between the essential pharmacophores. We thus implemented superimposition studies, using the energetically stable conformations of the most active stereoisomer, 1(2S,4R), as a template. The energy penalties for a conformation change of the less active stereoisomers 2-4 from their global minimum-energy structure to a new conformation, fitting onto the global minimum-energy conformation of 1, appear to account for the differences in the pharmacological potency better than using the other conformations of 1 as a template. We thus presume that the global minimum-energy conformation of 1 is closely related to the bioactive conformation for these anticholinergics, and also that the pharmacological potency is linked to how readily these substances can change their conformations to fit the muscarinic receptor.

Dissociation constants and relative efficacies of acetylcholine, (+)- and (-)-methacholine at muscarinic receptors in the guinea-pig ileum

Dissociation constants (KA) and relative efficacies of acetylcholine, (+)-methacholine and (-)-methacholine at muscarinic receptors in the guinea-pig isolated ileum were determined in the absence and presence of the cholinesterase inhibitor diisopropylfluorophosphate. The method used involved analysis of dose-response data before and after fractional inactivation of receptors with propylbenzilylcholine mustard. The KA values, estimated after cholinesterase inhibition, of acetylcholine, (+)- and (-)-methacholine were 1.7, 2.0 and 620 microM, respectively. The large (730 fold) difference in spasmogenic activity between (+)- and (-)-methacholine is due primarily to a difference in affinity for ileal muscarinic receptors although differences in efficacy (2 to 4 fold) also contribute. It is suggested that the methyl group at the chiral centre of (+)-methacholine has no apparent effect on the binding to muscarinic receptors, whereas the corresponding methyl group of (-)-methacholine interferes with binding, presumably by stabilizing a conformation of the drug which does not fit the receptor very well.

Affinity and efficacy correlate with chemical structure more than potency does in a series of pentatomic cyclic muscarinic agonists

The efficacy and affinity of nine pentatomic cyclic muscarinic agonists were determined on the guinea-pig ileum, according to the method of Furchgott & Bursztyn (1967). The efficacy and affinity of these agonists are affected differently by structural modifications. Our results suggest that a strong dipole oriented in the same direction as that of the hydroxy group of muscarine, or the presence of a polarizable atom in the same position, are important for efficacy.

Stereochemical requirements for central and peripheral muscarinic and antimuscarinic activity of some acetylenic compounds related to oxotremorine

1 The enantiomers of some analogues of the central muscarinic agent, oxotremorine, were prepared and investigated for tremorogenic and tremorolytic activity in intact mice and for muscarinic and antimuscarinic activity on the isolated ileum of the guinea-pig. 2 The R-isomers were more potent than the S-isomers both in vivo and in vitro regardless of whether the compounds are agonists, partial agonists or competitive antagonists. 3 It is suggested that in the oxotremorine series, agonists and antagonists interact with a common receptor site, in contrast to classical muscarinic antagonists which are believed to bind also to accessory receptor areas, located close to the agonist binding site.