Antihistaminic/antiallergic activity of 2-dialkylaminoalkylthio(oxy)-1-substituted benzimidazoles: evaluation "in vitro" and "in vivo".

A new series of 2-dialkylamino-alkylthio(oxy)-1-substituted benzimidazoles synthesized in our laboratories was found to have promising antihistaminic activity. The results of pharmacological screening ("in vitro": radioreceptor binding and isolated organs; "in vivo": protection against mortality induced by histamine or by compound 48/80, passive cutaneous anaphylaxis, and prolongation of barbiturate-induced sleeping-time) gave clear-cut structure-activity relationships. This series of products has a general selectivity towards H1 receptors, weak antiallergic properties and negligible central effects. DF 10967 (1-ethoxyethyl-2-dimethyl-aminoethylthiobenzimidazole) was the most interesting compound, being very potent both "in vitro" (Ki = 3.2 +/- 0.8 nM) and "in vivo" (ID50 11 micrograms/kg, i.p. and 8 micrograms/kg, i.p. against histamine- and 48/80-induced mortality), with no central effects. The last finding is probably due to poor penetration into the brain (as confirmed by "in vivo" binding test with [3H]-mepyramine) and to lack of interaction with other central receptors.