Blocking studies using in vitro ARG were conducted to determine specific binding for P2Y12R (Figure ). WT brain sections were co-incubated with [ 18 F]12 and a 1000-fold excess of ticagrelor, a potent and structurally non-related P2Y12R antagonist. Tracer signal of unblocked [ 18 F]12 displays a clear and uniform pattern. This aligns with the anticipated homogeneous distribution of homeostatic microglia in healthy mice observed in immunofluorescence staining with an anti-mouse P2Y12R antibody in adjacent brain sections (Figure ) and previous ARG studies with P2Y12R ligand 2. Blocking the signal with ticagrelor led to a decreased signal intensity of 50% on average, speaking for a good target specificity of the tracer. However, autoradiographs of blocked [ 18 F]12 still displayed a basal signal, potentially indicating nonspecific binding (NSB).