Structure-activity, theoretical, and X-ray studies on the intramolecular interactions in a series of novel histamine H2 receptor antagonists.

: The furan ring of the histamine H2 receptor antagonist 3-amino-4-[[2-[[[5-[(dimethylamino)methyl]-2-furanyl]-methyl] thio]ethyl]amino]-1,2,5-thiadiazole 1-oxide (1a) was replaced by thiophene, pyridine, benzene, and pyrrole. The relative receptor affinities of these analogues were estimated by in vitro and in vivo techniques. A theoretical model for the stacking interaction, observed by single crystal X-ray analysis of 1a, was developed, and the ability to enter into this type of interaction was estimated. The X-ray analysis of the pyridine analogue of 1a revealed no intramolecular stacking interaction. The theoretical studies were evaluated in light of the observed receptor affinities, and the relevance of the solid-state geometry of 1a to the receptor-bound geometry was assessed. It is suggested that the stacked geometry found in the X-ray structure of 1a does not represent a conformation that is relevant to that bound at the histamine H2 receptor.