Thermodynamic in vitro studies as a method to investigate the pharmacodynamic behavior of adenosine A(1) receptor ligands

Purpose. A thermodynamic analysis of the binding to rat cortex adenosine Al receptor of N-6-substituted (full agonists) and N-6-substituted-deoxyribos e (partial agonists) adenosine derivatives was performed. The intrinsic act ivity of the compounds was evaluated by measurements of the inhibition of f orskolin stimulated 3', 5'-cyclic adenosine monophosphate (c-AMP) levels in isolated epididymal rat adipocytes. Methods. The thermodynamic parameters Delta G degrees (standard free energy ), Delta H degrees (standard enthalpy), and Delta S degrees (standard entro py) of the binding equilibrium were determined by means of affinity measure ments carried out at different temperatures (0, 10, 20, 25, 30 degrees C). Levels of c-AMP were evaluated performing competitive protein binding assay s. Results. The binding of the ligands increases with temperature enhancement and, as a consequence, is totally entropy driven. Standard entropy values c orrelate significantly with intrinsic activity ones. Conclusions. It is proposed the data obtained by these in vitro experiments can be used to investigate the in vivo pharmacodynamic of A, full and part ial agonists.