The effect of 17-N substituents on the activity of the opioid κ receptor in nalfurafine derivatives

We have previously reported the essential structure of the opioid kappa receptor agonist nalfurafine hydrochloride (TRK-820) for binding to the kappa receptor. In the course of this study, we focused on the effect of the substituent at 17-N in nalfurafine on the binding affinity for the j receptor. The exchange of the 17-N substituent in nalfurafine from cyclopropylmethyl to fluoro-substituted alkyl groups, which are strong electron withdrawing substituents, almost completely diminished the binding affinities for the mu and delta opioid receptors, but the binding affinity for the j receptor was still maintained. As a result, nalfurafine derivatives with 17-fluoro-substituted alkyl groups showed higher selectivities for the j receptor than did nalfurafine itself. With regard to the j agonistic activities, the conversion of the 17-N substituent in nalfurafine from cyclopropylmethyl to fluoro-substituted alkyl groups led to the gradual decrease of the agonistic activities in the order corresponding to their binding affinities for the j receptor. In contrast, the derivative with the bulky 17-isobutyl group showed lower affinity and agonistic activity for the j receptor than the derivatives with the smaller functional groups. This research suggested that both the electronic property and the steric characteristics of the 17-N substituent would have a great influence on the binding property for the j receptor. (c) 2012 Elsevier Ltd. All rights reserved.