TSPO and Alzheimer disease: In the case of [11C]PK11195 - the prototypical TSPO ligand - the number of available binding sites (Bmax) was found to be significantly increased in the frontal cortex of AD post-mortem tissue, as compared to controls, while [3H](R)-PK11195 binding correlated significantly with immunohistochemically labeled activated microglia [67].