Kinetic and functional properties of [3H]ZM241385, a high affinity antagonist for adenosine A2A receptors

We have characterized the binding of [2-3H]-4-(2-[7-Amino-2-(2-furyl)-[1,2,4]-triazolo-[2,3-a]-[1,3,5]-triazin-5-ylamino]ethyl)phenol ([3H]ZM241385) to adenosine A2A receptors in membranes of rat striatum and transfected CHO cells. Saturation experiments showed that [3H]ZM241385 binds to a single class of binding sites with high affinity (Kd = 0.23 nM and 0.14 nM in CHO cell and striatal membranes, respectively). The membranes of CHO cells required pretreatment with adenosine deaminase (ADA) to achieve high-affinity binding, while ADA had no influence on the ligand binding properties in striatal membranes. The binding of [3H]ZM241385 was fast and reversible, achieving equilibrium within 20 minutes at all radioligand concentrations. The kinetic analysis of the [3H]ZM241385 interaction with A2A receptors indicated that the reaction had at least two subsequent steps. The first step corresponds to a fast equilibrium, which also determines the antagonist potency to competitively inhibit CGS21680-induced accumulation of cAMP (first equilibrium constant KA = 6.6 nM). The second step corresponds to a slow process of conformational isomerization (equilibrium constant Ki = 0.03). The combination of the two steps gives the dissociation constant Kd = 0.20 nM based on the kinetic data, which is in good agreement with the directly measured value. The data obtained shed light on the mechanism of the [3H]ZM241385 interaction with adenosine A2A receptors from different sources in vitro. The isomerization step of the A2A antagonist radioligand binding has to be taken into account for the interpretation of the binding parameters obtained from the various competition assays and explain the discrepancy between antagonist affinity in saturation experiments versus its potency in functional assays.