Structure-based design of bitopic ligands for the µ-opioid receptor.

Mu opioid receptor (µOR) agonists like fentanyl have long been used for pain management, but are considered a major public health concern due to their adverse side effects, including lethal overdose. 1 To design safer therapeutics, we report a conceptually novel approach targeting conserved sodium (Na + ) binding site 2 , observed in µOR 3 and many other class A GPCRs, by bitopic fentanyl derivatives functionalized via a linker with a positively charged guanidino group. Cryo-EM structures of the most potent bitopic ligands in complex with µOR highlight the key interactions between the ligand's guanidine and the key Asp 2.50 residue in the Na + site. While the lead bitopics maintain nanomolar potency and high efficacy at Gi subtypes, they show strongly reduced arrestin recruitment, one also shows the lowest Gz-efficacy among the panel of µOR agonists, including partial and biased, morphinan and fentanyl analogs. In mice, the best bitopic ligand displayed µOR dependent antinociception with attenuated adverse effects supporting the µOR Na + site as a potential target for the design of safer analgesics. In general, our study suggests that bitopic ligands engaging the Na + pocket in class A GPCRs can be designed to control their efficacy and functional selectivity profiles for G i/o/z subtypes and arrestins, thus modulating their in vivo pharmacology.