Novel Mixed NOP/Opioid Receptor Peptide Agonists.

The nociceptin/orphanin FQ (N/OFQ)/N/OFQ receptor (NOP) system controls different biological functions including pain and cough reflex. Mixed NOP/opioid receptor agonists elicit similar effects to strong opioids but with reduced side effects. In this work, 31 peptides with the general sequence [Tyr/Dmt,Xaa]N/OFQ(1-13)-NH were synthesized and pharmacologically characterized for their action at human recombinant NOP/opioid receptors. The best results in terms of NOP versus mu opioid receptor potency were obtained by substituting both Tyr and Thr at the N-terminal portion of N/OFQ(1-13)-NH with the noncanonical amino acid Dmt. [Dmt]N/OFQ(1-13)-NH has been identified as the most potent dual NOP/mu receptor peptide agonist so far described. Experimental data have been complemented by studies to shed light on the molecular mechanisms by which the peptide binds the active form of the mu receptor. Finally, the compound exerted antitussive effects in an model of cough.