Further studies on the Dmt-Tic pharmacophore:: Hydrophobic substituents at the C-terminus endow δ antagonists to manifest μ agonism or μ antagonism

Twenty N- and/or C-modified Dmt-Tic analogues yielded similar K-i values with either [H-3]DPDPE (delta(1) agonist) or [H-3]N,N(Me)(2)-Dmt-Tic-OH (delta antagonist). N-Methylation enhanced delta antagonism while N-piperidine-1-yl, N-pyrrolidine-1-yl, and N-pyrrole-1-yl were detrimental. Dmt-Tic-X (X = -NHNH2, -NHCH3, -NH-1-adamantyl, -NH-tBu, -NH-5-tetrazolyl) had high delta affinities (K-i = 0.16 to 1 nM) with variable mu affinities to yield nonselective or weakly mu-selective analogues, N,N-(Me)(2)Dmt-Tic-NH-1-adamantane exhibited dual delta and mu receptor affinities (K(i)delta = 0.16 nM and K(i)mu =1.12 nM) and potent delta antagonism (pA(2) = 9.06) with mu agonism (IC50 = 16 nM). H-Dmt-beta HTic-OH (methylene bridge between C-alpha of Tic and carboxylate function) yielded a biostable peptide with high delta affinity (K-i = 0.85 nM) and delta antagonism (pA(2) = 8.85) without mu bioactivity, Dmt-Tic-Ala-X (X = -NHCH3, -OCH3, -NH-1-adamantyl, -NHtBu) exhibited high delta affinities (K-i = 0.06 to 0.2 nM) and elevated mu affinities (K-i = 2.5 to 11 nM), but only -Dmt-Tic-Ala-NH-1-adamantane and H-Dmt-Tic-Ala-NHtBu yielded delta receptor antagonism (pA(2) = 9.29 and 9.16, respectively). Thus, Dmt-Tic with hydrophobic C-terminal substituents enhanced mu affinity to provide delta antagonists with dual receptor affinities and bifunctional activity.