Radiosynthesis and Evaluation of Acetamidobenzoxazolone Based Radioligand [11C]N’-MPB for Visualization of 18 kDa TSPO in Brain

The 18 kDa translocator protein (TSPO) is a viable target for imaging of inflammation in brain. In recent past we have explored a pharmacophore skeleton acetamidobenzoxazolone for positron emission tomography (PET) imaging of TSPO expression in brain. Here we eva-luated a new radioligand for visualization of TSPO namely [11C]N-(2-methoxyoxyphenyl)-N-methyl-2-(5-nitro-2-oxobenzo[d]oxazol-3(2H)-yl)acetamide ([11C]N’-MPB). This PET ligand exhibited high binding affinity towards TSPO (Ki = 4.9 nM) and a suitable lipophilicity (Log D) of 2.65 for the brain imaging. Biodistribution study on mice showed high accumulation of radioactivity in the TSPO-rich organs, such as the lungs, heart, kidneys, and adrenal glands. Metabolite analysis for rat brain homogenate showed 98% intact [11C]N’-MPB at 30 min after injection. To determine the specific binding of radioligand with TSPO on neuroinflammation of the brain, in vitro autoradiography and PET studies were performed in an ischemic rat model. In vitro autoradiography indicated significantly increased binding on the ipsilateral side compared with that on the contralateral side of ischemic rat brains. This result was sup-ported firmly by the contrast of radioactivity in PET images. Displacement experiments with PK11195 minimized the difference in radioactivity uptake between the two sides. In sum-mary, [11C]N’-MPB is a potential PET imaging radioligand for TSPO and, consequently, for understanding more insight for up-regulation of microglia during neuroinflammation.