Design of New Potent, Selective, and Long-Acting NOP Receptor Agonists through Multiple Sequential d-Amino Acid Substitutions of [Arg14 Lys15]N/OFQ(1-15)-NH2

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 [Arg14 Lys15]N/OFQ(1-15)-NH2 to improve metabolic stability while preserving receptor activity. Progressive substitutions revealed marked positional tolerance within the C-terminal address domain and identified [d-Lys13,15d-Arg14]N/OFQ(1–15)-NH2 (compound 1c) as the most promising compound. This led to the Cha1-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.