Evolution of the Dmt-Tic Pharmacophore:  N-Terminal Methylated Derivatives with Extraordinary δ Opioid Antagonist Activity

Abstract

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 [Ki delta = 0.022 nM; Ki mu/Ki delta = 150,000] and delta antagonism (pA2 = 8.2; Ke = 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 (Ki delta = 0.2 nM), elevated delta antagonism on mouse vas deferens (MVD) (pA2 = 8.5; Ke = 2.8 nM), and nondetectable mu activity with guinea pig ileum (GPI). (ii) N,N-Me2-Dmt-Tic-OH (12) was equally efficacious in delta receptor binding (Ki delta = 0.12 nM; Ki mu/Ki delta = 20000), but delta antagonism rose considerably (pA2 = 9.4; Ke = 0.28 nM) with weak mu antagonism (pA2 = 5.8; Ke = 1.58 microM; GPI/MVD = 1:5640). N-Me-(9) and N,N-Me2-Dmt-Tic-Ala-OH (15) also augmented delta opioid receptor binding, such that 15 demonstrated high affinity (Ki delta = 0.0755 nM) and selectivity (Ki mu/Ki delta = 20132) with exceptional antagonist activity on MVD (pA2 = 9.6; Ke = 0.22 nM) and weak antagonism on GPI (pA2 = 5.8; Ke = 1.58 microM; GPI/MVD = 1:7180). Although the amidated dimethylated dipeptide analogue 14 had high Ki delta (0.31 nM) and excellent antagonist activity (pA2 = 9.9; Ke = 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 (Ki mu/Ki delta = 1655) and somewhat comparable bioactivity (GPI/MVD = 4500). Thus, the data demonstrate that N,N-(Me)2-Dmt-Tic-OH (12) and N,N-Me2-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.

Ligands Raise the Constraint That Limits Constitutive Activation in G Protein-coupled Opioid Receptors

“…3c). These data are consistent with the previous findings that N-alkylation of the Dmt-Tic pharmacophore increases the binding affinity for the DOP receptor (25).…”

“…This is derived from the condensation of the two unnatural amino acids, 2Ј,6Ј-dimethyltyrosine (Dmt) and 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic). As shown previously, substitutions within the Dmt-Tic pharmacophore can generate a vast array of changes in the affinity and efficacy of ligands for the two opioid receptors (25)(26)(27)(28).…”

Role of benzimidazole (Bid) in the δ-opioid agonist pseudopeptide H-Dmt-Tic-NH-CH2-Bid (UFP-502)☆

“…Pseudopeptides (1)(2)(3)(4)(5)(6) were prepared in solution by peptide synthetic methods. Simply, BocDmt-Tic-OH 19 was coupled with H 2 N-CH 2 -hetero-/homo-cycle via WSC/HOBt.…”

“…Simply, BocDmt-Tic-OH 19 was coupled with H 2 N-CH 2 -hetero-/homo-cycle via WSC/HOBt. Final Nterminal Boc deprotection with TFA and purification by preparative HPLC, gave compounds (1)(2)(3)(4)(5)(6). The intermediates (benzo [d]oxazol-2-yl)methanamine (H 2 N-CH 2 -Boa) and (benzo [d] thiazol-2-yl)methanamine (H 2 N-CH 2 -Bta) were prepared according to the procedure of Nestor et al 13 Mixed carbonic anhydride coupling of Z-Gly-OH with o-aminophenol gave the crude intermediate amide, which was converted without purification to the desired benzoxazole (Z-NH-CH 2 -Boa) by cyclization and dehydration in refluxing propionic acid (~140°C).…”

(2S,3R)β-Methyl-2′,6′-dimethyltyrosine-l-tetrahydroisoquinoline-3-carboxylic acid [(2S,3R)TMT-l-Tic-OH] Is a Potent, Selective δ-Opioid Receptor Antagonist in Mouse Brain

“…Thus, introduction of steric constraints into the prototype ␦-opioid peptide antagonist, N,N-diallyl-Leuenkephalin (Shaw et al, 1982), yielded metabolically stable pseudopeptide antagonists, such as ICI 174,864 (Cotton et al, 1984), TIPP (Schiller et al, 1992), TIPP⌿ (Schiller et al, 1993), and Tyr-Tic (Salvadori et al, 1995), with progressively increasing ␦-selectivity. In the second generation of pseudopeptide opioid antagonists, additional steric constraints were introduced by the use of methylated Tyr analogs, leading to further improved ␦-selectivity, such as DMT-L-Tic (Salvadori et al, 1995(Salvadori et al, , 1997 and TMT-L-Tic (Liao et al, 1997).…”