Binding thermodynamics at the human cannabinoid CB1 and CB2 receptors

The thermodynamic parameters ΔG°, ΔH° and ΔS° of the binding equilibrium of agonists and antagonists at cannabinoid CB1 and CB2 receptors were determined by means of affinity measurements at different temperatures and van't Hoff plots were constructed. Affinity constants were measured on CHO cells transfected with the human CB1 and CB2 receptors by inhibition assays of the binding of the cannabinoid receptor agonist [3H]-CP-55,940. van't Hoff plots were linear for agonists and antagonists in the temperature range 0-30 °C. The thermodynamic parameters for CB1 receptors fall in the ranges 17 ≤ ΔH° ≤ 59 kJ/mol and 213 ≤ ΔS° ≤ 361 kJ/mol for agonists and -52 ≤ ΔH° ≤ -26 kJ/mol and -12 ≤ ΔS° ≤ 38 kJ/mol for antagonists. The thermodynamic parameters for CB2 receptors fall in the ranges 27 ≤ ΔH° ≤ 48 kJ/mol and 234 ≤ ΔS° ≤ 300 kJ/mol for agonists and -19 ≤ ΔH° ≤ -17 kJ/mol and 43 ≤ ΔS° ≤ 74 kJ/mol for antagonists. Collectively, these data show that agonist binding is always totally entropy-driven while antagonist binding is enthalpy and entropy-driven, indicating that CB1 and CB2 receptors are thermodynamically discriminated. These data could give new details on the nature of the forces driving the CB1 and CB2 binding at a molecular level. Enthalpy, entropy, free energy and binding affinity for each ligand to its receptor can all be assessed and therefore the optimal binding profile discovered. Carrying out these binding investigations as early as possible in the discovery process increases the probability that a lead compound will become a successful pharmaceutical compound.