Applications of adenosine receptor ligands in medical imaging by positron emission tomography

In the last decade the field of purinergic pharmacology has continued to grow as the complexity of the receptor families and the various enzymes involved in purine metabolism have been defined in molecular terms. Adenosine receptors (ARs) are currently divided into the four subclasses A(1)-, A(2A)-, A(2B)- and A(3)AR. The most intensively studied subtypes are the high-affinity A(1) and A(2A) receptors, which are activated by adenosine in nano- to submicromolar concentrations. The clinical importance of the A(1) adenosine receptor (A(1)AR) and the A(2A)adenosine receptor (A(2A)AR) makes them attractive targets for radionuclide in vivo imaging. Positron Emission Tomography (PET) is an imaging modality which can determine biochemical and physiological processes in vivo in a quantitative way by using radiopharmaceuticals labeled with positron emitting radionuclides as (11)C, (13)N, (15)O and (18)F and by measuring the annihilation radiation using a coincidence technique. This includes also measurement of the pharmacokinetics of labeled drugs and the assessment of the effects of drugs on metabolism. In the present article we review the radioligands which are currently available for visualisation and quantification of ARs using PET with a special focus on the A(1)AR and A(2A)AR.