Fluorescence and autoradiographic evaluation of tau PET ligand PBB3 to α-synuclein pathology

Background: The tau PET ligand 2-((1E,3E)-4-(6-([¹¹C]methylamino)pyridin-3-yl)buta-1,3-dienyl)benzo[d]thiazol-6-ol ([¹¹C]PBB3) binds to a wide range of tau pathology; however, binding property of PBB3 to non-tau inclusions remains unknown. To clarify whether [¹¹C]PBB3 binds to α-synuclein pathology, reactivity of PBB3 was assessed by in vitro fluorescence and autoradiographic labeling of brain sections from α-synucleinopathies patients. Method: Of 10 pure Lewy body disease and 120 multiple system atrophy (MSA) cases in the Mayo Clinic brain bank, we selected 3 Lewy body disease and 4 MSA cases with a range of α-synuclein severity based on the quantitative analysis of α-synuclein burden. PBB3 fluorescence labeling, double or single immunostaining for α-synuclein and phospho-tau, Prussian blue staining, and in vitro autoradiography with [¹¹C]PBB3 were performed for these selected samples. Results: PBB3 fluorescence labeled various α-synuclein lesions including Lewy bodies, Lewy neurites, spheroids, glial cytoplasmic inclusions, and neuronal cytoplasmic inclusions. Meanwhile, autoradiographic labeling with [¹¹C]PBB3 at 10 nM demonstrated no significant binding in Lewy body disease cases. In contrast, significant autoradiographic binding of [¹¹C]PBB3 to the striatopallidal fibers was found in 2 MSA cases, which had high densities of glial cytoplasmic inclusions without tau or iron deposits in this region. Conclusions: Given that the maximum concentration of [¹¹C]PBB3 in human PET scans is approximately 10 nM, the present data imply that α-synuclein pathology in Lewy body disease is undetectable by [¹¹C]PBB3-PET, whereas those in a subset of MSA cases with high densities of glial cytoplasmic inclusions could be captured by this radioligand.