New Fluorescent Probes Targeting the Mitochondrial-Located Translocator Protein 18 kDa (TSPO) as Activated Microglia Imaging Agents

To evaluate the utility of new Translocator protein 18 kDa (TSPO)-targeted fluorescent probes for in vivo molecular imaging of activated microglia. Compounds 2-4 were synthesized; their stability and affinity for TSPO were determined. Compounds 2-4 were incubated both with Ra2 cells in the presence of LPS, a potent activator of microglia, and with tissue sections of normal and chemically injured brains. Compounds 2-4 were injected into carotid artery or directly in striatum of mice. Cells and tissue sections from these in vitro and in vivo studies were observed by fluorescence microscopy after histochemical treatments. Compounds 2-4 are stable in both buffer and physiological medium and showed high affinity for TSPO and were found to stain live Ra2 microglial cells effectively. Double staining with Mito Tracker Red suggested that binding sites of compounds 2 and 3 may exist on mitochondria. In vivo studies showed that compounds 2-4 may penetrate in part into brain; moreover, cells in mouse striatum were stained with compounds 2-4 and microglial marker CD11b. Compounds 2-4 can fluorescently label activated microglia in vitro and in vivo.