Targeting of the Translocator Protein 18 kDa (TSPO): A Valuable Approach for Nuclear and Optical Imaging of Activated Microglia

The aim of the present review is to give a concise and updated analysis of the imaging tools for the visualization of activated microglia. After an overview on the important pathologies where activated microglia are involved, we first describe the role played by the translocator protein-18 kDa (TSPO) as an important target for the visualization of activated microglia. Second, imaging tools based on TSPO ligands radiolabeled for positron emission tomography (PET) are summarized with particular emphasis to the TSPO ligands alternative to the standard radioligand [(11)C]PK11195 or (R)-[(11)C]PK11195. In this regard, an updated list of (11)C- and (18)F-labeled TSPO radioligands is shown. Moreover, a detailed analysis based on TSPO ligands bearing fluorescent probes for fluorescence microscopy is also provided. This last optical imaging technique represents an area of large and increasing interest due to the advantages offered by the use of simple instrumentation and safer experimental conditions. The scope and limitations of the nuclear and optical imaging techniques are discussed. Finally, a perspective on the plausible advances in this area is also presented.