Structure–Signal Relationships of the <i>δ</i>-Opioid-Receptor (DOR)-Selective Agonist KNT-127—Part II: Quinoline Ring Modifications for Enhanced G-Protein Signaling and Reduced <i>β</i>-Arrestin Recruitment

The δ-opioid receptor (DOR) continues to attract attention as a therapeutic target for the development of safer analgesics due to its ability to mediate pain relief with a lower risk of adverse effects compared to the μ-opioid receptor (MOR). Building upon our previous findings on KNT-127, a DOR-selective agonist with a morphinan scaffold, this study further explores the structure-signal relationships between quinoline ring modifications and the signaling bias toward Gi-protein activation while minimizing β-arrestin-2 recruitment. Our findings highlight the critical role of the 5'-position in modulating signaling bias. Bulky hydrophobic substituents, such as isopropoxy and cyclohexanoxy groups, effectively reduce β-arrestin-2 recruitment without compromising DOR binding affinity or Gi-protein activation. Molecular-docking and molecular dynamics simulations provided mechanistic insights, showing that these modifications change ligand interactions with the V281^6.55-W284^6.58-L300^7.35 sub-pocket, thus selectively favoring Gi-protein signaling. These insights clarify the key interactions for the signaling bias in DOR agonists, offering a new framework for the design of DOR-targeted therapies with an improved therapeutic profile.