Candidate PET Radioligand Development for Neurofibrillary Tangles: Two Distinct Radioligand Binding Sites Identified in Postmortem Alzheimer’s Disease Brain

[(18)F]THK-523 and [(18)F]807 are promising radioligands for imaging neurofibrillary tangles (NFTs) with positron emission tomography (PET) in neurodegenerative diseases, such as Alzheimer's disease (AD) and traumatic brain injury. Although [(18)F]THK-523 and [(18)F]T807 are considered high-affinity selective radioligands for NFTs, uncertainty has existed as to whether PET radioligands for imaging NFTs bind to the same molecular site because in vitro assays for ligands binding to NFTs have been lacking. We labeled THK-523 and T807 with tritium to serve as reference radioligands for in vitro binding assays with AD brain homogenates for newly synthesized ligands. With these radioligands, we identified two distinct binding sites for small molecules, one site with high affinity for THK-523 and the other with high affinity for T807. Moreover, binding assays with [(3)H]PIB confirmed that the two newly identified binding sites are also distinct from the thioflavin-T binding site where all current clinically useful PET radioligands for imaging β-amyloid plaque bind with high affinity. The two newly identified binding sites are considered to reside on NFTs rather than on β-amyloid plaques. Furthermore, we applied all three binding assays to a set of newly prepared compounds, based on chain modifications to THK-523. Some compounds with high affinity and selectivity for the THK-523 binding site emerged from this set, including one with amenability to labeling with fluorine-18, namely, ligand 10b.