KF-26777 is a drug which acts as a potent and selective antagonist for the adenosine A<sub>3</sub> receptor, with sub-nanomolar affinity (A<sub>3</sub> Ki=0.2nM) and high selectivity over the other three adenosine receptor subtypes. Simple xanthine derivatives such as caffeine and DPCPX have generally low affinity for the A<sub>3</sub> subtype and must be extended by expanding the ring system and adding an aromatic group to give high A<sub>3</sub> affinity and selectivity.