The opiate-like action of tilidine is mediated by metabolites

The analgesic effect of tilidine in rats is completely antagonized by the narcotic antagonist naloxone. Radioreceptor assays revealed, however, that the main metabolites of tilidine, nortilidine and bisnortilidine, rather than tilidine exhibit affinity to opiate receptors. These findings were confirmed in studies using the electrically stimulated guinea pig ileum and the mouse vas deferens. Chronic tilidine administration to rats caused a considerable degree of physical dependence, which was expected from the ability of the intact animal to metabolize tilidine. In the isolated ileum from chronically morphinized guinea pigs, both nortilidine and bisnortilidine fully substituted for morphine in preventing induction of withdrawal, indicating dependence liability of these metabolites.