Interaction of SSR161421, a novel specific adenosine A3 receptor antagonist with adenosine A3 receptor agonists both in vitro and in vivo

A novel adenosine A3 receptor antagonist (SSR161421) was characterized by both receptor binding assays and pharmacological tests. Binding studies on cloned human adenosine receptors showed that SSR161421 has high affinity for adenosine hA3 receptors (Ki=0.37 nM) with at least 1000-fold selectivity compared to hA1, hA2A and hA2B receptors. The receptor antagonist nature of SSR161421 was determined in a functional study on Chinese hamster ovarian cells (CHO) cells expressing human adenosine A3 receptors. SSR161421 competitively antagonized the effect of 2-chloro-N6-(3-iodobenzyl)-adenosine-5′-N-methylcarboxamide (Cl-IB-MECA) on cAMP production with a pA2 value in a luciferase reporter gene construct. In mice, intravenously administered SSR161421 inhibited the N6-(4-aminobenzyl)-adenosine-5′-N-methyl-uronamide dihydrochloride (AB-MECA) induced increase in plasma histamine levels (ED50=2.0 mg/kg) and the Cl-IB-MECA evoked plasma extravasation (ID50=2.9 mg/kg) and oedema formation (ID50=4.6 mg/kg) in mouse ear.