Mu Opioid Receptor Positive Allosteric Modulator BMS-986122 Confers Agonist-Dependent G Protein Subtype Signaling Bias

The mu Opioid Receptor (MOR) is a G protein-coupled receptor (GPCR) and is responsible for the effects of all medically used opioids. Most opioids activate all inhibitory Gαi/o/z proteins through MOR, initiating signaling events that culminate in a variety of physiological effects such as analgesia, euphoria, and respiratory depression. Gaining a better understanding of how the chemical structure of opioids influences the functional activation profiles of G protein subtypes by MOR is critical for disentangling the multitude of opioid effects and the development of safer analgesics. A recent development in opioid pharmacology has been the discovery of positive allosteric modulators (PAMs) for opioid receptors, such as BMS-986122, which act at the MOR to increase the potency of full agonists and the efficacy of partial agonists. Here, we utilized a nanoBRET-based functional assay system in live HEK 293T cells to study how the pharmacological properties of opioids were uniquely affected by BMS-986122 when the MOR signaled through specific inhibitory Gα subunits. We report that BMS-986122 differentially enhanced opioid activity when the MOR signaled through different Gα subunits with the greatest difference observed with partial agonists. Additionally, the binding affinity of BMS-986122 to the MOR was significantly altered by the co-binding Gα subunit. Site-directed mutagenesis experiments revealed key amino acid residue differences on Gαi/o subunits involved in the differential effects observed. This study sheds light on the molecular features of biased signaling for both opioid ligands and G proteins, which may prove useful for the further development of biased agonists or allosteric modulators at the MOR.