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

Abstract

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 κ, μ and δ-opioid receptors was investigated by radioligand binding assay and bioactivity by a mouse vas deferens (MVD) assay. The addition of the amino acid residue Arg to the N-terminal enhances the opioid receptor affinity of Noc(1–13)NH 2 while retaining ORL1 receptor affinity at a moderate level. The replacement of Gly in position 3 by the basic or polar amino acids—Arg, Asn, Lys(For) or its deletion led to inactive analogues. The replacement of Ser in position 10 by its d-isomer, Pro and d-Pro resulted in a series of analogues with the following order of activity: Ser 10> d-Ser 10>Pro 10> d-Pro 10. In [ d-Ser 10]Noc(1–13)NH 2, introduction of an additional Phe(NO 2) 4 led to a >60-fold increase of ORL1 affinity, completely attenuating the loss of affinity brought about by Ser 10. In other analogues, introduction of Phe(NO 2) 4 did not change the magnitude of ORL1 binding significantly. Generally, while modifications in position 3 frequently led to a loss of most or all bioactivity, modifications in position 0 (Arg 0) or 4 (Phe(NO 2) 4) and 10 ( d-Ser 10, Pro 10) are tolerated.