Evolution of the Dmt-Tic pharmacophore: N-terminal methylated derivatives with extraordinary delta opioid antagonist activity

The delta opioid antagonist H-Dmt-Tic-OH (2',6'-dimethyl-L-tyrosyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) exhibits extraordinary delta receptor binding characteristics [K-i(delta) = 0.022 nM; K-i(u)/K-i(delta) = 150 000] and delta antagonism (pA(2) = 8.2; K-e = 5.7 nM). A change in chirality of Dmt at C alpha (1, 2, 6, 8, 10, 13) curtailed delta receptor parameters, while replacement of its alpha-amino function by a methyl group (3) led to inactivity; Tyr-Tic analogues 4 and 11 weakly interacted with delta receptors. N-Alkylation of H-Dmt-Tic-OH and H-Dmt-Tic-Ala-OH with methyl groups produced potent delta-opioid ligands with high delta receptor binding capabilities and enhanced delta antagonism: (i) N-Me-Dmt-Tic-OH 5 had high delta opioid binding (K-i(delta) = 0.2 nM), elevated delta antagonism on mouse vas deferens (MVD) (pA(2) = 8.5; K-e = 2.8 nM), and nondetectable mu activity with guinea pig ileum (GPI). (ii) N,N-Me-2-Dmt-Tic-OH (12) was equally efficacious in delta receptor binding (K-i(delta) = 0.12 nM; K-i(mu)/K-i(delta) = 20 000), but delta antagonism rose considerably (pA(2) = 9.4; K-e = 0.28 nM) with weak mu antagonism (pA(2) = 5.8; K-e = 1.58 mu M; GPI/MVD = 1:5640). N-Me-(9) and N,N-Me-2-Dmt-Tic-Ala-OH (15) also augmented delta opioid receptor binding, such that 15 demonstrated high affinity (K-i(delta) = 0.0755 nM) and selectivity (K-i(mu)/K-i(delta) = 20 132) with exceptional antagonist activity on MVD (pA(2) = 9.6; K-e = 0.22 nM) and weak antagonism on GPI (pA(2) = 5.8; K-e = 1.58 mu M; GPI/MVD = 1:7180). Although the amidated dimethylated dipeptide analogue 14 had high K-i(delta) (0.31 nM) and excellent antagonist activity (pA(2) = 9.9; K-e = 0.12 nM), the increased activity toward mu receptors in the absence of a free acid function at the C-terminus revealed modest delta selectivity (K-i(mu)/K-i(delta) = 1 655) and somewhat comparable bioactivity (GPI/MVD = 4500). Thus, the data demonstrate that N,N-(Me)(2)-Dmt-Tic-OH (12) and N,N-Me-2-Dmt-Tic-Ala-OH (15) retained high delta receptor affinities and delta selectivities and acquired enhanced potency in pharmacological bioassays on MVD greater than that of other peptide or non-peptide delta antagonists.