A structure-activity study of nociceptin-(1-13)-peptide amide. Synthesis of analogues substituted in positions 0, 1, 3, 4 and 10.

A series of analogues of nociceptin, Noc(1-13)NH(2) (an agonist at the ORL1 receptor) was synthesized with following modifications: (1) N-terminal extension with Arg(0); (2) replacement of Gly(3) by basic or polar amino acids-Arg, Asn, Lys(For) or deletion; (3) exchange of Phe(1) or Phe(4) by Phe(NO(2)); (4) substitution of Ser(10) with D-Ser, Pro, D-Pro. The analogs were synthesized by solid-phase methodology using Fmoc-amino acid pentafluorophenyl esters. The affinity for the ORL1 and for the kappa, micro and delta-opioid receptors was investigated by radioligand binding assay and bioactivity by a mouse vas deferens (MVD) assay.