the compounds of the following formula : wherein r , r 2 , r 3 and a have the meanings given in the specification , are endowed with selective a 3 adenosine receptor antagonist activity . these compounds can be used in a pharmaceutical composition to treat disorders caused by excessive activation of the a 3 receptor , or can be used in a diagnostic application to determine the relative binding of other compounds to the a 3 receptor . the compounds can be labeled , for example with fluorescent or radiolabels , and the labels used in vivo or in vitro to determine the presence of tumor cells which possess a high concentration of adenosine a 3 receptors .