Design of New Potent, Selective, and Long-Acting NOP Receptor Agonists through Multiple Sequential <scp>d</scp> -Amino Acid Substitutions of [Arg ¹⁴ Lys ¹⁵ ]N/OFQ(1–15)-NH ₂

High Resolution Image Download MS PowerPoint Slide The nociceptin/orphanin FQ (N/OFQ) receptor (NOP) ligands are drug candidates for different diseases, but peptide ligands are often limited by rapid enzymatic degradation and short in vivo duration of action. Here, we applied a multiple D-amino acid substitution strategy to the peptide template [Arg 14 Lys 15 ]N/OFQ(1-15)-NH 2 to improve metabolic stability while preserving receptor activity. Progressive substitutions revealed marked positional tolerance within the C-terminal address domain and identified [ d -Lys 13,15 d -Arg 14 ]N/OFQ(1–15)-NH 2 (compound 1c ) as the most promising compound. This led to the Cha 1 -containing analogue 3a, which retained full agonist efficacy and high selectivity at the NOP receptor in vitro and ex vivo . In vivo, compound 3a induced a long-lasting loss of the righting reflex in mice, closely overlapping the pharmacological profile of the potent and long-acting agonist UFP-112. These findings define the stereochemical tolerance of the NOP receptor toward multiple d -amino acid substitutions within the N/OFQ peptide and demonstrate that this information can be exploited as a rational design strategy to generate NOP receptor agonists with prolonged in vivo activity.