Functionalized congeners of 1,3-dipropyl-8-phenylxanthine: potent antagonists for adenosine receptors that modulate membrane adenylate cyclase in pheochromocytoma cells, platelets and fat cells

Six amine, amino acid and peptide derivatives derived from 1,3-dipropyl-8-(p-carboxymethylphenyl)xanthine, a functionalized congener of 1,3-dipropyl-8-phenylxanthine, have been investigated as antagonists at A2 adenosine receptors stimulatory to Adenylate Cyclase in membranes from rat pheochromocytoma PC 12 cells and human platelets and at A1 adenosine receptors inhibitory to Adenylate Cyclase from rat fat cells. The functionalized congeners and conjugates have affinity constants ranging from 80 to 310 nM at A2 receptors of PC 12 cells and from 25 to 135 nM at those of platelets. The affinity of the xanthine derivatives at A1 receptors of fat cells are in the 15 to 30 nM range. Thus, the amino acid and peptide conjugates have high potencies at both receptor subclasses and show some selectivity toward A1 adenosine receptors. Derivatives of the congeners should be useful as receptor probes and as radioiodinated ligands.