Allosteric enhancers of A1 adenosine receptors: State of the art and new horizons for drug development

Adenosine is an important autocoid, exerting its physiological effects on the human body by activation of four different G-protein-coupled-receptors (GPCRs) classified as A(1), A(2A), A(2B), and A(3). These receptors are coupled to secondary messenger systems including adenylate cyclase, inositol phosphate metabolism, and K(+), K(ATP) and Ca(2+) channels. Pharmacological agents that increase the activation of A(1) adenosine receptors in response to adenosine would be useful for treatment of cardiovascular, central nervous system, and inflammatory pathologies. Compounds that are able to enhance the activity of the A(1)-adenosine receptors by the endogenous ligand within specific tissues may have potential therapeutic advantages over non-endogenous agonists. Such an opportunity for intervention is provided by the concept of allosteric modulation of GPCRs. Therefore the use of allosteric enhancers to increase the responsiveness of the A(1) receptors to endogenous adenosine at sites of its production is an appealing alternative to activation by exogenous agonists. This approach minimizes side effects because allosteric enhancers amplify the action of the agonist by stabilizing the agonist-A(1)-receptor-G protein ternary complex. The allosteric enhancement of the GABA(A) receptor by benzodiazepines is the most famous and successful example of this strategy. The aim of this article is to give an overview of the results obtained in this field and discuss the opportunities and challenges that this class of ligands might offer for medicinal chemistry and pharmacology.