Synthesis and opioid activities of [D-Leu-8]Dynorphin(1-8) analogs containing a reduced peptide bond, psi(CH2-NH).

[D-Leu8]Dynorphin(1--8)-NH2 analogs, in which each peptide bond was systematically replaced with a psi(CH2NH) peptide bond, were synthesized by the solid-phase method. The psi(CH2NH) bond was introduced by the Boc-amino acid aldehyde/NaCNBH3 method on a solid support. In the syntheses of the analogs, undesirable double alkylation took place at the sequences of Tyr1 psi(CH2NH)Gly2 and Gly2 psi(CH2NH)Gly3, possibly due to the low steric hindrance of the glycine residue. To suppress the double alkylation, a minimum amount of aldehydes was employed. In the receptor binding assay, the psi(CH2NH) replacement of N-terminal peptide bonds which led to 1 psi 2-(2) and 2 psi 3-analogs (3) resulted in a marked reduction in binding affinities for mu-, delta-, and kappa-opioid receptors, while that of the other peptide bonds afforded analogs with a high kappa-receptor affinity. A 3 psi 4-analogs (4) showed extremely high kappa-receptor selectivity (mu/kappa Ki ratio = 339, delta/kappa Ki ratio = 24104). In the in vitro bioactivity assay (guinea pig ileum assay), 2 showed a very low IC50 ratio (2.0) in the presence and absence of peptidase inhibitors whereas those of other analogs were >27, suggesting that the introduction of the CH2NH isostere at Tyr1-Gly2 greatly enhanced the enzymatic stability of the parent peptide. Furthermore, analogs 2 and 3 showed a very low sensitivity to the inhibitory effect of NaCl plus 5'-guanylylimidodiphosphate on their binding at a kappa-receptor site as compared with the other analogs and the parent peptide. These results suggest that the two analogs (2 and 3) have partial kappa-antagonist properties.