Novel Hexahydrospiro[piperidine-4,1‘-pyrrolo[3,4-c]pyrroles]:  Highly Selective Small-Molecule Nociceptin/Orphanin FQ Receptor Agonists

Novel hexahydrospiro[piperidine-4,1‘-pyrrolo[3,4-c]pyrroles that act as potent and selective orphanin FQ/nociceptin (N/OFQ) receptor (NOP) agonists were identified. The best compound, (+)-5a, potently inhibited 3H−N/OFQ binding to the NOP receptor (K i = 0.49 nM) but was >1000-fold less potent in binding to MOP, KOP, and DOP opiate receptors. Further, (+)-5a potently stimulated GTPγS binding to NOP membranes (EC50 = 65 nM) and inhibited forskolin-mediated cAMP accumulation in NOP-expressing cells (EC50 = 9.1 nM) with a potency comparable to that of the natural peptide agonist N/OFQ. These results indicate that (+)-5a is a highly selective and potent small-molecule full agonist of the NOP receptor.