Adenine monohydrochloride hemihydrate is a potent and selective agonist for the adenine receptor (ADORA? Ki = 18 nM). It demonstrates functional activity by inhibiting forskolin-stimulated cAMP formation (pEC50 8.54) and stimulating GTPgammaS binding (pEC50 7.21) in transfected CHO cells, making it a valuable tool for studying purinergic signaling in vitro.