Comparative binding properties of the tau PET tracers THK5117, THK5351, PBB3, and T807 in postmortem Alzheimer brains

Background: The aim of this study was to compare the binding properties of several tau positron emission tomography tracers - THK5117, THK5351, T807 (also known as AV1451; flortaucipir), and PBB3 - head to head in the same human brain tissue. Methods: Binding assays were performed to compare the regional distribution of ³H-THK5117 and ³H-THK5351 in postmortem tissue from three Alzheimer's disease (AD) cases and three control subjects in frontal and temporal cortices as well as in the hippocampus. Competition binding assays between THK5351, THK5117, PBB3, and T807, as well as off-target binding of THK5117 and T807 toward monoamine oxidase B (MAO-B), were performed using binding assays in brain homogenates and autoradiography of three AD cases. Results: Regional binding of ³H-THK5117 and ³H-THK5351 was similar, except in the temporal cortex, which showed higher ³H-THK5117 binding. Saturation studies demonstrated two binding sites for ³H-THK5351 (K _d1 = 5.6 nM, B_max = 76 pmol/g; K _d2 = 1 nM, B_max = 40 pmol/g). Competition studies in the hippocampus between ³H-THK5351 and unlabeled THK5351, THK5117, and T807 revealed super-high-affinity sites for all three tracers (THK5351 K _i = 0.1 pM; THK5117 K _i = 0.3 pM; T807 K _i = 0.2 pM) and an additional high-affinity site (THK5351 K _i = 16 nM; THK5117 K _i = 20 nM; T807 K _i = 78nM). ¹⁸F-T807, ¹¹C-THK5351, and ¹¹C-PBB3 autoradiography of large frozen sections from three AD brains showed similar regional binding for the three tracers, with lower binding intensity for ¹¹C-PBB3. Unlabeled THK5351 and T807 displaced ¹¹C-THK5351 to a similar extent and a lower extent, respectively, compared with ¹¹C-PBB3. Competition with the MAO-B inhibitor ³H-l-deprenyl was observed for THK5117 and T807 in the hippocampus (THK5117 K _i = 286 nM; T807 K _i = 227 nM) and the putamen (THK5117 K _i = 148 nM; T807 K _i = 135 nM). ³H-THK5351 binding was displaced using autoradiography competition with unlabeled THK5351 and T807 in cortical areas by 70-80% and 60-77%, respectively, in the basal ganglia, whereas unlabeled deprenyl displaced ³H-THK5351 binding by 40% in the frontal cortex and 50% in the basal ganglia. Conclusions: THK5351, THK5117, and T807 seem to target similar binding sites, but with different affinities, whereas PBB3 seems to target its own binding site. Both THK5117 and T807 demonstrated off-target binding in the hippocampus and putamen with a ten times lower binding affinity to the MAO-B inhibitor deprenyl compared with ³H-THK5351.