GPCR Agonist-to-Antagonist Conversion: Enabling the Design of Nucleoside Functional Switches for the A 2A Adenosine Receptor.

Modulators of the G protein-coupled A _2A adenosine receptor (A _2A AR) have been considered promising agents to treat Parkinson's disease, inflammation, cancer, and central nervous system disorders. Herein, we demonstrate that a thiophene modification at the C8 position in the common adenine scaffold converted an A _2A AR agonist into an antagonist. We synthesized and characterized a novel A _2A AR antagonist, 2 (LJ-4517), with K _i = 18.3 nM. X-ray crystallographic structures of 2 in complex with two thermostabilized A _2A AR constructs were solved at 2.05 and 2.80 Å resolutions. In contrast to A _2A AR agonists, which simultaneously interact with both Ser277 ^7.42 and His278 ^7.43 , 2 only transiently contacts His278 ^7.43 , which can be direct or water-mediated. The n -hexynyl group of 2 extends into an A _2A AR exosite. Structural analysis revealed that the introduced thiophene modification restricted receptor conformational rearrangements required for subsequent activation. This approach can expand the repertoire of adenosine receptor antagonists that can be designed based on available agonist scaffolds.