In vitro high affinity α‐synuclein binding sites for the amyloid imaging agent PIB are not matched by binding to Lewy bodies in postmortem human brain1

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Abstract
Amyloid containing deposits are a defining neuropathological feature of a wide range of dementias and movement disorders. The positron emission tomography tracer PIB (Pittsburgh Compound‐B, 2‐[4′‐(methylamino)phenyl]‐6‐hydroxybenzothiazole) was developed to target senile plaques, an amyloid containing pathological hallmark of Alzheimer’s disease, formed from the amyloid‐β peptide. Despite the fact that PIB was developed from the pan‐amyloid staining dye thioflavin T, no detailed characterisation of its interaction with other amyloid structures has been reported. In this study, we demonstrate the presence of a high affinity binding site (Kd∼4 nM) for benzothiazole derivatives, including [3H]‐PIB, on α‐synuclein (AS) filaments generated in vitro, and further characterise this binding site through the use of radioligand displacement assays employing 4‐N‐methylamino‐4′‐hydroxystilbene (SB13) (Ki = 87 nM) and 2‐(1‐{6‐[(2‐fluoroethyl(methyl)amino]‐2‐naphthyl}ethylidene)malononitrile (FDDNP) (Ki = 210 nM). Despite the presence of a high‐affinity binding site on AS filaments, no discernible interaction of [3H]‐PIB was detected with amygdala sections from Parkinson’s disease cases containing frequent AS‐immunoreactive Lewy bodies and related neurities. These findings suggest that the density and/or accessibility of AS binding sites in vivo are significantly less than those associated with amyloid‐β peptide lesions. Lewy bodies pathology is therefore unlikely to contribute significantly to the retention of PIB in positron emission tomography imaging studies.