N,N-dialkylated leucine enkephalins as potential delta opioid receptor antagonists.

A series of N,N-dialkylated leucine enkephalins were prepared in order to study the effect of substitution on antagonist activity at the delta opioid receptor. The target peptides 1-7 were evaluated in the mouse vas deferens (MVD) and guinea pig ileum (GPI) at 1 microM. All of the compounds except [N,N-di-2-phenethyl,Leu5]enkephalin (7) showed antagonist activity in the MVD against the delta receptor agonist [D-Ala2,D-Leu5]enkephalin. The most potent congener, [N,N-dibenzyl,Leu5]enkephalin (3), was 2.5-fold more potent than [N,N-diallyl,Leu5]enkephalin (1). None of the compounds at 1 microM showed any antagonist activity against agonists for other receptor types. The N,N-di-2-phenethyl (7) and N,N-dioctyl (6) analogues showed significant agonist activity at 1 microM in the MVD.