Synthesis, biology, NMR and conformation studies of the topographically constrained δ‐opioid selective peptide analogs of [β‐iPrPhe³]deltorphin I

Abstract

Replacement of Phe3 in the endogenous delta-opioid selective peptide deltorphin I with four optically pure stereoisomers of the topographically constrained, highly hydrophobic novel amino acid beta-isopropylphenylalanine (beta-iPrPhe) produced four pharmacologically different deltorphin I peptidomimetics. Radiolabeled ligand-binding assays and in vitro biological evaluation indicate that the stereoconfiguration of the iPrPhe residue plays a crucial role in determining the binding affinity, bioactivity and selectivity of [beta-iPrPhe3]deltorphin I analogs: a (2S,3R) configuration of the iPrPhe3 residue in [beta-iPrPhe3]deltorphin I provided the most desirable biological properties with binding affinity (IC50 = 2 nM), bioassay potency (IC50 = 1.23 nM in MVD assay) and exceptional selectivity for the delta-opioid receptor over the mu-opioid receptor (30 000). Further conformational studies based on two-dimensional NMR and computer-assisted molecular modeling suggested a model for the possible bioactive conformation in which the Tyr1 and (2S,3R)-beta-iPrPhe3 residues adopt trans side-chain conformations, and the linear peptide backbone favors a distorted beta-turn conformation.

Novel sst4-Selective Somatostatin (SRIF) Agonists. 2. Analogues with β-Methyl-3-(2-naphthyl)alanine Substitutions at Position 8

“…In the past 15 years, several conformationally constrained β-methyl-substituted amino acids, such as β-methylphenylalanine, − β-methyltyrosine, β-methyl-2‘,6‘-dimethyltyrosine, β-methyltryptophan, ,− β-methyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid, and β-methylnaphthylalanine, were synthesized and introduced into bioactive peptides, causing large differences in receptor binding and selectivity. In some cases, such substitutions also prolonged the biological activity of the given peptides by locking a favorable “bioactive conformation”. ,− We have discussed in the preceding paper and others the effect of β-methylation of betidamino acids on their Ramachandran plots (structural studies) and of betidamino acid-containing bioactive peptides on their biological activities . These findings suggest that β-alkylation is a powerful approach in peptide ligand design and has great potential for understanding ligand−receptor interactions.…”

“…In some cases, such substitutions also prolonged the biological activity of the given peptides by locking a favorable "bioactive conformation". 32,[45][46][47][48][49][50][51][52][53] We have discussed in the preceding paper 54 and others the effect of β-methylation of betidamino acids on their Ramachandran plots (structural studies) 55 and of betidamino acid-containing bioactive peptides on their biological activities. 56 These findings suggest that β-alkylation is a powerful approach in peptide ligand design and has great potential for understanding ligandreceptor interactions.…”

Peptidomimetics and Their Applications for Opioid Peptide Drug Discovery

“…Moreover, these peptides are enzymatically stable and permeable to the BBB due to their structural features [ 34 ]. Their message region, Tyr- D Ala-Phe-, was shown to form a β-turn structure stabilized by the D Ala, and the substitution of a Phe residue with a more constrained β- i PrPhe residue enhanced the conformation resulting in the exceptional selectivity (30,000 fold) over MOR [ 170 ]. The C -terminal address region was attributed to the DOR selectivity through the hydrophobic Val 5 -Val 6 residues, which stabilize an optimal conformation for the binding rather than interact with the receptor [ 31 , 171 ].…”

“…β-MePhe, β- i PrPhe, β-methyl-2,6-dimethylphenylalanine, and Tmt are the most widely used in opioid peptidomimetics [ 292 , 293 ]. The favorable side chain rotamer for DOR was the trans form (2 S , 3 R ), and [(2 S , 3 R )-β-Tmt 1 ]DPDPE and [(2 S , 3 R )-β- i PrPhe 3 ]DLT increased DOR selectivity with retained affinity [ 170 , 293 ]. D , L -amino-β-mercapto-β,β-pentamethylenepropionic acid (Apmp) has a cyclohexyl ring in place of the geminal dimethyl group of Pen, and its substitution at position 2 in DPDPE resulted in the loss of DOR affinity and selectivity due to the steric hindrance and hydrophobic property while the substitution at position 5 was tolerated [ 294 ].…”