Structure-activity analysis of biased agonism at the human adenosine A₃ receptor

Biased agonism at G protein-coupled receptors (GPCRs) has significant implications for current drug discovery, but molecular determinants that govern ligand bias remain largely unknown. The adenosine A₃ GPCR (A₃AR) is a potential therapeutic target for various conditions, including cancer, inflammation, and ischemia, but for which biased agonism remains largely unexplored. We now report the generation of bias "fingerprints" for prototypical ribose containing A₃AR agonists and rigidified (N)-methanocarba 59-N-methyluronamide nucleoside derivatives with regard to their ability to mediate different signaling pathways. Relative to the reference prototypical agonist IB-MECA, (N)-methanocarba 59-Nmethyluronamide nucleoside derivatives with significant N⁶ or C2 modifications, including elongated aryl-ethynyl groups, exhibited biased agonism. Significant positive correlation was observed between the C2 substituent length (in Å) and bias toward cell survival. Molecular modeling suggests that extended C2 substituents on (N)-methanocarba 59-N-methyluronamide nucleosides promote a progressive outward shift of the A₃AR transmembrane domain 2, which may contribute to the subset of A₃AR conformations stabilized on biased agonist binding.