Binding thermodynamics and intrinsic activity of adenosine A1 receptor ligands

A thermodynamic analysis of the binding to rat cortex adenosine A1 receptors of 5'-deoxyribose-N6-cyclopentyladenosine (full agonist) and several 8-alkylamino homologues of N6-cyclopentyladenosine (partial agonists) was performed. The intrinsic activity of the compounds was also evaluated by measuring the inhibition of forskolin-stimulated 3'-5'-cyclic adenosine monophosphate (c-AMP) levels in isolated epididymal rat adipocytes. Standard free energy (deltaG), enthalpy (deltaH ) and entropy (deltaS ) of the binding equilibrium were determined by affinity measurements carried out at different temperatures (0, 10, 20, 25, 30 degrees C). Affinity constants of drug-receptor interactions were obtained by displacement experiments in the presence of 1nM [3H]N6-cyclohexyladenosine. Levels of c-AMP were evaluated by performing competitive binding assays. As the affinity of the ligands was found to increase with temperature enhancement, the binding of full and partial agonists is therefore totally entropy-driven. Standard entropy values of a wide series of adenosine derivatives, including the compounds under examination, are strictly correlated to those of intrinsic activity. Similarly, deltaS values appear correlated to the in vivo ability of the adenosine derivatives to inhibit rat heart rate. Thermodymanic data of adenosine A1 receptor ligands are proposed as an indicator of their pharmacodynamics.