Toward PET imaging of A2B adenosine receptors: A carbon-11 labeled triazinobenzimidazole tracer. Synthesis and imaging of a new A2B PET tracer

Introduction: A2B adenosine receptors (ARs) are commonly defined as "danger" sensors because they are triggered during cell injury when the endogenous molecule, adenosine, increases rapidly. These receptors, together with the other receptor subtypes (A1, A2A and A3), exert a wide variety of immunomodulating and (cyto)protective effects, thus representing a pivotal therapeutic target for different pathologies including diabetes, tumors, cardiovascular diseases, pulmonary fibrosis and others. The limited availability of potent and selective ligands for A2B ARs has prevented this receptor to emerge both as therapeutic and diagnostic target. Methods: Recently, a new class of potent A2B ARs antagonists was developed featuring the triazinobenzimidazole scaffold. Starting from this chemotype, we synthesized a new radiotracer, [11C]-4 (1-[11C]methyl-3-phenyl triazino[4,3-a]benzimidazol-4(1H)-one), and investigated the pharmacokinetics of this compound in vivo to define its potential use in the imaging of A2B AR with positron emission tomography. Results: [11C]-4 showed a very high chemical and blood stability. Results of in vivo and ex vivo experiments underlined the ability of this molecule to bind the A2B AR and correlated with the A2B AR protein and gene expression data. Conclusions: Although further studies are necessary, these data suggest that [11C]-4 may represent a good lead compound for the development of novel selective and potent A2B AR radiotracers, and a new option for the clinical investigation of several pathophysiological processes and chronic diseases.