Nociceptin /Orphanin FQ Peptide (NOP) Receptor Modulators: An Update in Structure-Activity Relationships

Nociceptin /Orphanin FQ Peptide" receptor (NOPr) is a G-protein-coupled receptor with the nociceptin/orphanin FQ peptide (N/OFQ) as endogenous agonist. It is expressed in the nervous system as well as in some non-neural tissues. Its activation has pronociceptive effect at the supraspinal level, whereas at the spinal level it produces nociceptive effects at low doses and antinociceptive effects at higher doses. NOPr is also involved in mood and blood pressure regulation, immunoregulation, airway constriction, feeding, urination, bowel motility, learning and memory. Selective NOPr agonists have been tested clinically as anxiolytics and antitussives, and the antagonists as analgesics, antidepressants and in the treatment of alcohol addiction. Two NOPr radioligands have also been tested in humans as neuroimaging agents. Furthermore, the partial agonist peptide SER100 and N/OFQ have been used in clinical trials, respectively for congestive heart failure and overactive bladder. The evidence of interactions between NOP and μ-opioid receptor (MOPr) receptors has been exploited in the use of mixed NOPr/MOPr modulators as analgesics and in the treatment of drug addiction. These drugs are devoid of typical opioid liabilities. In this review, we outline the latest advances in the structure-activity relationships (SAR) of NOPr agonists and antagonists, with emphasis on affinity, activity, selectivity and pharmacokinetic features.