N-terminal guanidinylation of TIPP (Tyr-Tic-Phe-Phe) peptides results in major changes of the opioid activity profile

Abstract

Derivatives of peptides of the TIPP (Tyr-Tic-Phe-Phe; Tic=1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) family containing a guanidino (Guan) function in place of the N-terminal amino group were synthesized in an effort to improve their blood-brain barrier permeability. Unexpectedly, N-terminal amidination significantly altered the in vitro opioid activity profiles. Guan-analogues of TIPP-related δ opioid antagonists showed δ partial agonist or mixed δ partial agonist/μ partial agonist activity. Guanidinylation of the mixed μ agonist/δ antagonists H-Dmt-Tic-Phe-Phe-NH2 (DIPP-NH2) and H-Dmt-TicΨ[CH2NH]Phe-Phe-NH2 (DIPP-NH2[Ψ]) converted them to mixed μ agonist/δ agonists. A docking study revealed distinct positioning of DIPP-NH2 and Guan-DIPP-NH2 in the δ receptor binding site. Lys(3)-analogues of DIPP-NH2 and DIPP-NH2[Ψ] (guanidinylated or non-guanidinylated) turned out to be mixed μ/κ agonists with δ antagonist-, δ partial agonist- or δ full agonist activity. Compounds with some of the observed mixed opioid activity profiles have therapeutic potential as analgesics with reduced side effects or for treatment of cocaine addiction.

Keywords: (5α,7α,8β-(−)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]benzeneacetamide; 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid; 1,3-diisopropylcarbodiimide; 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate; 2′,6′-dimethyltyrosine; 6-chloro-1-hydroxybenzotriazole; BBB; Boc; Cl-HOBt; DAMGO; DIC; DIPP; DIPP-NH(2); DIPP-NH(2)[Ψ]; DSLET; Dmt; ES-MS; GPI; Guan; Guanidinylated opioid peptides; H-Dmt-Tic-Phe-Phe-NH(2); H-Dmt-Tic-Phe-Phe-OH; H-Dmt-TicΨ[CH(2)NH]Phe-Phe-NH(2); H-Tyr-D-Ala-Gly-Phe(NMe)-Gly-ol; H-Tyr-D-Ser-Gly-Phe-Leu-Thr-OH; H-Tyr-Tic-Phe-Phe-OH; H-Tyr-TicΨ[CH(2)NH]Phe-Phe-OH; HBTU; HPLC; MVD; Peptide synthesis; TIPP; TIPP[Ψ]; Tic; U69,593; blood–brain barrier; electrospray mass spectrometry; guanidino; guinea pig ileum; high performance liquid chromatography; mouse vas deferens; tert-butyloxycarbonyl; δ Opioid antagonists; δ Partial opioid agonists; μ Opioid agonist/δ opioid agonists; μ Opioid agonist/δ opioid antagonists.

Figure 1

δ Receptor docking studies. H-Dmt-Tic-Phe-Phe-NH₂ (4a) in green bound to the δ receptor in the inactive state (key residues depicted in white and Asp¹²⁸ in red), and Guan-Dmt-Tic-Phe-Phe-NH₂ (4) in magenta bound to the δ receptor in the activated state (key residues depicted in yellow and Asp¹²⁸ in orange).