Electrophilic derivatives of purines as irreversible inhibitors of A1 adenosine receptors

Adenosine modulates a variety of physiological functions.1 It acts as an inhibitor of neuronal firing and the release of neurotransmitters,1b an inhibitor of platelet aggregation,1c a cardiac depressant and a vasodilator,1d a vasoconstrictor1e (e.g., in the renal afferent arterioles and in the skin), and an immunosuppressant.1f Most of the physiological effects of adenosine result from binding to discrete membrane-bound adenosine receptors of the A1 or A2 subtypes.1g The xanthine drugs caffeine and theophylline, and many synthetic analogues,2 act as competitive antagonists at adenosine receptors. Alkylating or acylating ligands that form a stable covalent bond with a receptor have been synthesized for a number of receptors, including opiate,3,4 phencyclidine,5 adrenergic,6 and benzodiazepine receptors.7 The utility of such chemically irreversibly bound ligands for characterizing receptors in membranes and in physiological systems,4,5 and in receptor identification,8 has been demonstrated. For adenosine receptors, only photoaffinity labels1g,9 have hitherto been described. We now report a set of reactive adenosine receptor ligands, derived from agonist10 and antagonist11 functionalized congeners. These ligands contain electrophilic acylating and alkylating groups capable of reaction with nucleophilic residues in proximity to the adenosine receptor binding site.