Synthesis and biological activities of 4-phenyl-5-pyridyl-1,3-thiazole derivatives as selective adenosine A3 antagonists

To investigate the potency of an adenosine A(3) receptor (A(3)AR) antagonist as an anti-asthmatic drug, a novel series of 4-phenyl-5-pyridyl-1,3-thiazole derivatives was synthesized and evaluated in human adenosine A(1), A(2A) and A(3) receptor and rat adenosine A(3) receptor binding assays. From investigation of the SAR study, compound 7af was identified as a highly potent human and rat A(3)AR antagonist. This compound inhibited IB-MECA-induced plasma protein extravasation in the skin of rats and showed good oral absorption. Also, compound 7af significantly inhibited antigen-induced hyper-responsiveness to acetylcholine in actively sensitized Brown Norway rats. These results show that 4-phenyl-5-pyridyl-1,3-thiazole derivatives are potential candidates to enable the evaluation of A(3)AR antagonists. Further evaluation of this class of compounds may afford a novel anti-inflammatory agent such as an anti-asthmatic drug.