N-methylthioacetylation of RYYRIK-NH₂ with enhanced specific binding affinity and high antagonist activity for nociceptin ORL1 receptor

Antagonists of the neuropeptide nociceptin are expected to be potential analgesic and antineuropathic drugs acting on ORL1 GPCR receptors. The peptide library-based antagonist Ac-RYYRIK-NH₂ inhibits the nociceptin activity mediated through ORL1, but preserves a considerably high level of agonist activity. We previously reported that the N-terminal acyl group is important for interaction with specific receptors, and developed isovarelyl-RYYRIK-NH₂, which exhibits strong antagonist activity with negligible agonist activity. In the present study, in order to obtain a more potent antagonist, we further modified the isovarelyl group by replacing its Cβ atom with an oxygen, nitrogen, or sulfur atom to give the methyl group improved interaction ability. The methyl group bound to such heteroatoms was expected to enhance the hydrophobic interaction between the peptide and the ORL1 receptor. The RYYRIK-NH₂ peptide with a methylthioacetyl group, CH₃SCH₂CO, revealed a higher receptor-binding affinity with strong antagonist activity, and the results suggested the presence of a receptor aromatic group as a complementary residue of this CH₃S group.