Indolinone scaffold optimization reveals structural factors for potent Nociceptin opioid receptor partial agonism, including AT-127.

The Nociceptin opioid peptide receptor (NOP) is a promising target for developing novel analgesics and anxiolytics, offering potential benefits without the severe side effects associated with mu opioid receptor (MOP) activation. Current standard-of-care opioids primarily target MOP, leading to issues like addiction and respiratory depression. Understanding the precise structural factors that govern NOP receptor binding and activation is crucial for designing selective and safer compounds. Previous work on the 1-(piperidin-4-yl)indolin-2-one chemical scaffold has yielded various NOP ligands, but the specific molecular interactions driving full versus partial agonism, particularly with new indolinone derivatives, remained underexplored.

Researchers conducted lead optimization and structure-activity relationship (SAR) studies around the 1-(piperidin-4-yl)indolin-2-one scaffold. They employed both ligand-based and structure-based discovery approaches to design and synthesize new derivatives, including 3-unsubstituted indolin-2-ones, indoline-2,3-diones, and 3-(hydroxyimino)indolin-2-ones. The study focused on identifying key interactions at the NOP receptor that influence binding affinity and intrinsic activity. This involved systematically introducing polar substituents at the 3-position of the indolinone series to modulate receptor engagement and functional outcomes.

Previous SAR studies demonstrated that lipophilic alicyclic or aromatic substituents directly attached to the piperidine nitrogen induced NOP receptor states associated with full or partial agonism. Conversely, linking these same lipophilic moieties via a methylene linker resulted in ligands that induced inactive NOP receptor states, functioning as NOP antagonists. This study identified that polar amino acid residues in the NOP receptor extracellular loop 2 (EL2) and a hydrogen-bond network between EL2, transmembrane helix 7 (TM7), and TM1 residues play a critical role in ligand-driven activation of this G-protein coupled receptor (GPCR). Lead optimization, specifically introducing polar substituents at the 3-position of the indolinone series, significantly enhanced NOP partial agonism. This strategy successfully yielded potent NOP partial agonists in both NOP-selective and bifunctional NOP/MOP indolinone ligands. > Notably, compounds such as the 3-(hydroxyimino)indolin-2-ones 1d (AT-127), 2c, and 3c were identified, exhibiting potencies comparable to recently reported NOP partial agonists currently under clinical development.