An adenosine receptor antagonist is a drug which acts as an antagonist of one or more of the adenosine receptors.[1] The best known are xanthines and their derivatives (natural: caffeine,[2] theophylline,[3] and theobromine; and synthetic: PSB-1901[4]), but there are also non-xanthine representatives (e.g. ISAM-140,[5] ISAM-R316,[6] Etrumadenant, and AZD-4635[7])