The adenosine A1 receptor antagonist DPCPX also interacts with appreciable affinity with A2b receptors, but binding of DPCPX is virtually eliminated in animals lacking the A1 receptor (Johansson et al. 2001), suggesting that this presents a minor problem. There are several useful A2a receptor antagonists. The most selective so far is SCH58261. The structurally related ZM241385 is more readily available (Poucher et al. 1995), but shows appreciable affinity to A2b receptors (Ongini et al. 1999). The situation is somewhat more favourable in the case of A2b antagonists, where some potent and relatively selective antagonists have been found (Kim et al. 2000). The A3 receptor is—in contrast to the other adenosine receptors— notably insensitive to several xanthines. Hence, most A3 antagonists have a non-xanthine structure—including dihydropyridines, pyridines and flavonoids (Baraldi et al. 2000). One of the most selective compounds (for human, but not rat A3 receptors) is MRE-3008-F20, which is also a useful antagonist radioligand at human A3 receptors (Varani et al. 2000). MRS1523 and MRS1191 are two compounds that also show marked species difference and are selective for only the human A3 receptor (Jacobson etal. 1997).
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