Structure-Bias Relationship of μ-Opioid Receptor Agonists

The μ-opioid receptor (μOR) is the primary drug target of opioid analgesics such as morphine and fentanyl. Activation of μORs in the central nervous system inhibits ascending pain signaling to the cortex, thereby producing analgesic effects. However, the clinical use of opioid analgesics is severely limited by adverse side effects, including respiratory depression, constipation, addiction, and the development of tolerance. μOR-mediated signaling involves both the G_i/o/z protein pathway and the β-arrestin1/2 pathway. Recent research has indicated that G protein-biased agonists, which preferentially activate the G_i/o/z pathway over the β-arrestin1/2 pathway, may provide effective analgesia with reduced side effects, thus offering improved therapeutic potential. In this chapter, we review the molecular basis of μOR-biased agonism. By integrating findings from structural and dynamic studies, we summarize the structure-bias relationships of various μOR agonists, aiming to provide valuable insights for the development of next-generation μOR-biased agonists.