Substituted 4-phenylthiazoles: Development of potent and selective A1, A3 and dual A1/A3 adenosine receptor antagonists

Adenosine acts as a powerful signaling molecule via four distinct G protein-coupled receptors, designated A₁, A_2A, A_2B and A₃ adenosine receptors (ARs). A_2A and A_2B ARs are G_s-coupled, while A₁ and A₃ ARs inhibit cAMP production via G_i proteins. Antagonists for A₁ and A₃ ARs may be useful for the treatment of (neuro)inflammatory diseases including acute kidney injury and kidney failure, pulmonary diseases, and Alzheimer's disease. In the present study, we optimized the versatile 2-amino-4-phenylthiazole scaffold by introducing substituents at N2 and C5 to obtain A₁ and A₃ AR antagonists including dual-target compounds. Selective A₁ antagonists with (sub)nanomolar potency were produced, e.g. 11 and 13. These compounds showed species differences being significantly more potent at the rat as compared to the human A₁ AR, and were characterized as inverse agonists. Several potent and selective A₃ AR antagonists, e.g. 7, 8, 17 and 22 (K_i values of 5-9 nM at the human A₃ AR) were prepared, which were much less potent at the rat orthologue. Moreover, dual A₁/A₃ antagonists (10, 18) were developed showing K_i values between 8 and 42 nM. Docking and molecule dynamic simulation studies using the crystal structure of the A₁ AR and a homology model of the A₃ AR were performed to rationalize the observed structure-activity relationships.