Nuclear imaging of neuroinflammation: a comprehensive review of [11C]PK11195 challengers

Neurodegenerative, inflammatory and neoplastic brain disorders involve neuroinflammatory reactions, and a biomarker of neuroinflammation would be useful for diagnostic, drug development and therapy control of these frequent diseases. In vivo imaging can document the expression of the peripheral benzodiazepine receptor (PBR)/translocator protein 18 kDa (TSPO) that is linked to microglial activation and considered a hallmark of neuroinflammation. The prototype positron emission tomography tracer for PBR, [11C]PK11195, has shown limitations that until now have slowed the clinical applications of PBR imaging. In recent years, dozens of new PET and SPECT radioligands for the PBR have been radiolabelled, and several have been evaluated in imaging protocols. Here we review the new PBR ligands proposed as challengers of [11C]PK11195, critically analyze preclinical imaging studies and discuss their potential as neuroinflammation imaging agents.