Binding thermodynamics of adenosine A2a receptor ligands

The thermodynamic parameters delta G degree, delta H degree, and delta S degree of the binding equilibrium of seven adenosine agonists and five xanthine antagonists binding specifically to adenosine A2a receptors were determined by means of affinity measurements at six different temperatures (0, 10, 20, 25, 30 and 35 degrees) and van't Hoff plots. Affinity constants were measured on rat striatum membranes by saturation experiments for the selective A2a agonist 2-[p-(carboxy-ethyl)-phenethylamino-]5'-(N-ethyl)carboxamidoadenos ine ([3H]CGS 21680) and by inhibition assays of [3H]CGS 21680 binding for all other compounds. Scatchard plots were monophasic in the full range of temperatures, indicating a single class of high affinity binding sites whose receptor density, BMAX, is essentially temperature independent. Van't Hoff plots were linear in the temperature range 0-30 degrees for agonists and 0-35 degrees for antagonists; their thermodynamic parameters fall, respectively, in the ranges 7 < or = delta H degree < or = 50 kJ/mol and 177 < or = delta S degree < or = 278 J K-1 mol-1 and -36 < or = delta H degree < or = -7 kJ/mol and -33 < or = delta S degree < or = 94 J K-1 mol-1, showing that agonist binding is entropy-driven while antagonist binding is enthalpy-driven. The results are compared with those already reported for the binding of the same compounds to rat brain minus striatum adenosine A1 receptors obtained by displacing [3H]CHA as A1 selective radioligand (Borea PA et al., Mol Neuropharmacol 2: 273-281, 1992). The comparison suggests that the two receptors are very similar as far as their binding sites are concerned and possibly philogenetically related. The analysis of thermodynamical data makes it possible to propose an analogical model of drug-receptor interaction which may account for both affinity and intrinsic activity properties.