Carcinogenicity of methapyrilene hydrochloride, mepyramine hydrochloride, thenyldiamine hydrochloride, and pyribenzamine hydrochloride in Sprague-Dawley rats.

Four histamine antagonists, methapyrilene, thenyldiamine, mepyramine and pyribenzamine were tested for carcinogenicity in rats by continuous application in drinking water. Only methapyrilene displayed significant carcinogenic effects, inducing liver tumors in a dose-related pattern. Analogues not containing a thiophene ring (mepyramine, pyribenzamine) did not exhibit neoplastic effects under the experimental conditions.