Intrinsic Activity at Adenosine A1 Receptors: Partial and Inverse Agonism

Novel phenomena, such as constitutive activity and inverse agonism, have led to a ligand (re)classification along an agonists-neutral antagonist-inverse agonist continuum. This review focuses on adenosine A1 receptor ligands and their intrinsic activity (α). The intrinsic activity of a ligand depends both on the chemical characteristics of the compound itself and on the experimental conditions in which it is assayed. Consequently, due to this tissue-dependency determination of a ligand's intrinsic activity is not always easily performed. Meanwhile, this feature may also be used in a profitable manner, namely to separate desired and unwanted effects of the drug. Briefly, this review provides possible screening methods to distinguish the different classes of ligands. It also deals with the structural elements and functional groups in the adenosine A1 receptor ligands that determine their intrinsic activity.