The binding of 2-(4′-methylaminophenyl)benzothiazole to postmortem brain homogenates is dominated by the amyloid component

2-(4′-Methylaminophenyl)benzothiazole (BTA-1) is an uncharged derivative of thioflavin-T that has high affinity for Aβ fibrils and shows very good brain entry and clearance. In this study, we asked whether BTA-1, at concentrations typical of those achieved during positron emission tomography (PET) studies, could specifically bind to amyloid deposits in the complex milieu of human brain or whether amyloid binding was overshadowed by nonspecific binding, found even in brains that did not contain amyloid deposits. We quantitatively assessed [³H]BTA-1 binding to crude homogenates of postmortem brain obtained from nine Alzheimer's disease (AD) subjects, eight controls, and six subjects with non-AD dementia. BTA-1 binding was >10-fold higher in AD brain, and the majority (94%) of the binding was specific (displaceable). High-affinity [³H]BTA-1 was observed only in AD brain gray matter and was not present in control brain gray matter, AD brain white matter, or cerebellum. The K_d of [³H]BTA-1 for binding to AD brain (5.8±0.90 nM) was very similar to the K_d for binding to synthetic Aβ fibrils. In addition, the K_i of various BTA analogs for inhibition of [³H]BTA-1 binding to AD brain homogenates was very similar to their K_i for inhibition of [³H]BTA-1 binding to synthetic Aβ fibrils. Nanomolar concentrations of [³H]BTA-1 did not appear to bind to neurofibrillary tangles. Finally, BTA-1 did not appear to bind significantly to common neuroreceptors or transporter sites. These data suggest that the binding of BTA-1 to AD brain is dominated by a specific interaction with Aβ amyloid deposits.