PSB-10 is a drug which acts as a selective antagonist for the adenosine A<sub>3</sub> receptor (k<sub>i</sub> value at human A<sub>3</sub> receptor is 0.44 nM), with high selectivity over the other three adenosine receptor subtypes (k<sub>i</sub> values at human A<sub>1</sub>, A<sub>2A</sub> and A<sub>2B</sub> receptors are 4.1, 3.3 and 30 üM). Further pharmacological experiments in a [<sup>35</sup>S]GTPóS binding assay using hA<sub>3</sub>-CHO-cells indicated that PSB-10 acts as an inverse agonist (IC<sub>50</sub> = 4 nM). It has been shown to produce antiinflammatory effects in animal studies. 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. The affinity towards adenosine A<sub>3</sub> subtype was measured against the radioligand PSB-11.