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, [7] and AZD-4635 [8] )