Moreover, radio-HPLC only represents the metabolite distribution in platelet-poor plasma, and the binding profile of the metabolites is currently unknown. Tracer uptake in the liver and the kidneys suggested renal excretion as the primary mechanism of tracer elimination. PET data indicated also uptake in the intestines, which suggests that hepatobiliary elimination may also be a contributing factor. In order to assess specificity of the PET tracer in vivo, blocking studies are typically carried out. This currently remains unfeasible for the P2Y12R due to the unavailability of alternative brain-permeable P2Y12R inhibitors, and intra-cisterna magna administration of P2Y12R inhibitors represents a rather invasive intervention. Instead, PLX treated mice were used as negative control, exhibiting an almost complete reduction in P2Y12R expression as a consequence of microglia depletion, which was confirmed by IHC. In addition to WT mice, Trem2 -/-mice were used based on reports of elevated expression of homeostatic genes and increased P2Y12R density. A significant difference in tracer uptake was observed in PET between microglia-depleted PLX mice and WT mice, as well as Trem2 -/-controls, which is in accordance with IHC results, showing higher microglia and P2Y12R levels in cortical sections of the WT and Trem2 -/-control group. But, PLX treated mice also exhibited substantial tracer retention in the brain, which may be indicative for microglia-and P2Y12R-independent non-specific binding in the brain parenchyma. Also, no significant difference in brain signal was detected between WT and Trem2 -/-mice. The ex vivo Iba1 or P2Y12R IHC signal was correlated with the in vivo PET signal in the cortex of WT mice and revealed a statistically significant dependency between the two output measures. This correlation analysis strongly supports the conclusion that the PET signal reflects microglial P2Y12R expression levels and that the PET signal can be attributed to tracer binding to P2Y12R on microglia. So far, it is unclear if the tracer can be applied to longitudinal imaging, and if the tracer is capable of indicating time-dependant changes in P2Y12R density, such as a down-regulation of receptor expression in the context of chronic neuroinflammation. Further modification of the structure will be required to decrease NSB, and higher molar activities could improve sensitivity in case of low target levels.