[3H]8-Ethyl-4-methyl-2-phenyl-(8R)-4,5,7,8-tetrahydro-1H-imidazo[2,1-i]-purin-5-one ([3H]PSB-11), a Novel High-Affinity Antagonist Radioligand for Human A3 Adenosine Receptors

This study describes the preparation and binding properties of [3H]PSB-11, a novel, potent, and selective antagonist radioligand for human A3 adenosine receptors (ARs). [3H]PSB-11 binding to membranes of Chinese hamster ovary (CHO) cells expressing the human A3 AR was saturable and reversible. Saturation experiments showed that [3H]PSB-11 labeled a single class of binding sites with high affinity (KD=4.9 nM) and limited capacity (Bmax=3500 fmol/mg of protein). PSB-11 is highly selective versus the other adenosine receptor subtypes. The new radioligand shows an extraordinarily low degree of non-specific binding rendering it a very useful tool for studying the (patho)physiological roles of A3 ARs.