Identification of structural characteristics of some potential H2-receptor antagonists that determine the interaction with rat hepatic P-450.

Several potential H2-receptor antagonists have been tested in vitro, using liver microsomal preparations from untreated rats, in order to study their interaction with P-450. The aim of this investigation was to establish structure-activity relationships for the P-450-inhibition developed by cimetidine and related drugs. Most of the compounds tested demonstrate an inhibitory activity and a binding ability to P-450, via type II (ligand type) binding. Our results strongly indicate that the cyano-guanidine moiety is an essential structural feature for both the inhibition of a ferrocytochrome P-450-metabolic intermediate complex formation occurring during the metabolism of tofenacine, and the binding of the compounds to the heme iron of P-450. The presence of an imidazole group is not necessary for these activities. Furthermore, it is pointed out that the lipophilic character of the cyano-guanidine side chain contributes to the interaction of the test compounds with P-450, since a trend for a parabolic relationship between lipophilicity and inhibitory activity or binding ability is observed. Finally, under the experimental conditions used, no increase of the inhibitory activity of cimetidine on the metabolism of tofenacine and 7-ethylresorufin is observed after preincubation of rat liver microsomes with cimetidine, confirming earlier results in similar studies.