Structural factors for receptor binding and activation of the Nociceptin opioid receptor by full and partial agonists

Small-molecule ligands for the nociceptin/orphanin FQ (N/OFQ) opioid peptide receptor (NOP) from the 1-(piperidin-4-yl)indolin-2-one chemical scaffold has yielded several selective NOP full or partial agonists and NOP antagonists, as well as bifunctional agonists targeting the NOP and mu opioid (MOP) receptor. We report herein further lead optimization efforts around this scaffold to design new NOP partial agonists. Our previous structure-activity relationship studies in this series showed that ligands containing lipophilic alicyclic or aromatic substituents directly attached to the piperidine nitrogen induced NOP receptor states associated with full or partial agonism, whereas linking the same lipophilic moieties via a methylene linker to the piperidine nitrogen resulted in ligands that induced inactive NOP receptor states (NOP antagonists). Polar amino acid residues in NOP receptor extracellular loop 2 (EL2) and a hydrogen-bond network between EL2, transmembrane helix 7 (TM7) and TM1 residues have been proposed to play role in ligand-driven activation of this G-protein coupled receptor (GPCR) but has not been explored for this chemical scaffold. Here we report ligand-based and structure-based discovery and structure activity relationship (SAR) of new 3-unsubstituted indolin-2-ones, indoline-2,3-diones and 3-(hydroxyimino)indolin-2-ones and identify key interactions at the NOP receptor that influence binding affinity and intrinsic activity. Our SAR and lead optimization introducing polar substituents at the 3-position of the indolinone series led to potent NOP partial agonism, in both NOP-selective and bifunctional NOP/MOP indolinone ligands such as the 3-(hydroxyimino)indolin-2-ones 1d (AT-127), 2c and 3c, which have potencies comparable to recently reported NOP partial agonists under clinical development.