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<P>Adenosine regulates a wide range of physiological functions through specific cell membrane receptors. On the basis of pharmacological studies and molecular cloning, four distinct adenosine receptors have been identified and classified as A1, A2a. A2b and A3. These adenosine receptors are members of the G-protein-coupled receptor family. <P> An intense medicinal chemistry effort made over the last 20 years has led to a variety of selective adenosine receptor agonists and antagonists. While all the agonists thus far identified are related to the adenosine structure, the antagonists available belong to different chemical classes. The prototypic antagonists are xanthine derivatives evolved from the natural compounds, caffeine and theophylline. Typically, they are 8-substituted-1,2,3-trialkylxanthine and are A1 selective antagonists. More recently, 8-styrylxanthines have been found to be selective for A2a receptors. Other non-xanthine heterocycles are potent A2a antagonists and possess different degree of selectivity. Selective antagonists are not available yet for A2b and A3 receptors. <P> Given the recent developments of A2a selective antagonists, we have reviewed their chemical structures and biological properties in the attempts to get insight into this emerging class of new interesting compounds. The development of some of the A2a antagonists will provide better understanding of the role of A2a receptors in physiological and pathological states. The compounds appear also to have the potential to be useful for the treatment of cerebral ischemia or neurodegenerative disorders, such as Parkinson's disease.</P>

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