KF-26777 is a drug which acts as a potent and selective antagonist for the adenosine A₃ receptor, with sub-nanomolar affinity (A₃ 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 A₃ subtype and must be extended by expanding the ring system and adding an aromatic group to give high A₃ affinity and selectivity.