The compounds of the following formula: wherein R, R2, R3 and A have the meanings given in the specification, are endowed with selective A3 adenosine receptor antagonist activity. These compounds can be used in a pharmaceutical composition to treat disorders caused by excessive activation of the A3 receptor, or can be used in a diagnostic application to determine the relative binding of other compounds to the A3 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 A3 receptors.