Binding affinity of adenosine receptor agonists and antagonists at human cloned A3 adenosine receptors

The A(3) adenosine receptor is one of the four adenosine receptors which have thus far been identified. Cloning of the A(3) receptor from animal species such as rat, sheep and human has shown that there are interspecies differences in its peripheral distribution, and binding affinity for various adenosine receptor ligands. The adenosine derivative, 4-aminobenzyl-5'-N-methylcarboxamidoadenosine (AB-MECA), is a potent A(3) receptor agonist which is used as a reference drug. In this report we have characterized the binding of selected adenosine receptor agonists and antagonists to HEK 293 cells transfected with the human A(3) adenosine receptor using [I-125]AB-MECA as radioligand. HE-NECA and NECA were the most potent compounds showing K-i values in the low nanomolar range, while the recently discovered non-xanthine A(2A) receptor antagonists ZM 241385, SCH 58261 and SCH 63390 showed affinity values in the micromolar range. These data further indicate the need to examine the affinity of new adenosine receptor ligands directly in human A(3) receptors. (C) 1998 Elsevier Science Inc.