In vivo Imaging of brain lesions with [11C]CLINME, a new PET radioligand of peripheral benzodiazepine receptors

The peripheral benzodiazepine receptor (PBR) is expressed by microglial cells in many neuropathologies involving neuroinflammation. PK11195, the reference compound for PBR, is used for positron emission tomography (PET) imaging but has a limited capacity to quantify PBR expression. Here we describe the new PBR ligand CLINME as an alternative to PK11195. In vitro and in vivo imaging properties of [C-11]CLINME were studied in a rat model of local acute neuroinflammation, and compared with the reference compound [C-11]PK11195, using autoradiography and PET imaging. Immunohistochemistry study was performed to validate the imaging data. [C-11]CLINME exhibited a higher contrast between the PBR-expressing lesion site and the intact side of the same rat brain than [C-11]PK11195 (2.14 +/- 0.09 vs. 1.62 +/- 0.05 fold increase respectively). The difference was due to a lower uptake for [C-11]CLINME than for [C-11]PK11195 in the non-inflammatory part of the brain in which PBR was not expressed, while uptake levels in the lesion were similar for both tracers. Tracer localization correlated well with that of activated microglial cells, demonstrated by immunohistochemistry and PBR expression detected by autoradiography. Modeling using the simplified tissue reference model showed that R-1 was similar for both ligands (R-1 similar to 1), with [C-11]CLINME exhibiting a higher binding potential than [C-11]PK11195 (1.07 +/- 0.30 vs. 0.66 +/- 0.15). The results show that [C-11]CLINME performs better than [C-11]PK11195 in this model. Further studies of this new compound should be carried out to better define its capacity to overcome the limitations of [C-11]PK11195 for PBR PET imaging. (C) 2007 Wiley-Liss, Inc.