Using a molecular modeling approach for Tau-binding sites, we modified our previously reported imaging agent, [¹²⁵I]INFT, for the potential improvement of binding properties to Tau in an Alzheimer's disease (AD) brain. Two new derivatives, namely [¹²⁵I]ISAS and [¹²⁵I]NIPZ, were designed, where binding energies at site 1 of Tau were -7.4 and -6.0 kcal/mole, respectively, compared to [¹²⁵I]INFT (-7.6 kcal/mole). The radiosynthesis of [¹²⁵I]ISAS and [¹²⁵I]NIPZ was carried out by using iodine-125 and purified chromatographically to achieve >90% purity. In vitro binding affinities (IC₅₀) for Tau were as follows: INFT = 7.3 × 10^-8 M; ISAS = 4.7 × 10^-8 M; NIPZ > 10^-6 M. The binding of [¹²⁵I]ISAS to gray matter (GM) correlated with the presence of Tau in the AD brain, confirmed by anti-Tau immunohistochemistry. [¹²⁵I]NIPZ did not bind to Tau, with similar levels of binding observed in GM and white matter (WM). Four radiotracers were compared and the rank order of binding to Tau was found to be [¹²⁵I]IPPI > [¹²⁵I]INFT > [¹²⁵I]ISAS >>> [¹²⁵I]NIPZ with GM/WM ratios of [¹²⁵I]IPPI = 7.74 > [¹²⁵I]INFT = 4.86 > [¹²⁵I]ISAS = 3.62 >> [¹²⁵I]NIPZ = 1.24. The predictive value of Chimera-AutoDock for structurally related compounds binding to the Tau binding sites (measured as binding energy) was good. A binding energy of less than -7 kcal/mole is necessary and less than -8 kcal/mole will be more suitable for developing imaging agents.

Therapeutic antibodies for reducing Aβ plaque load in Alzheimer’s disease (AD) is currently making rapid progress. The diagnostic imaging of Aβ plaque load in AD has been underway and is now used in clinical studies. Here, we report our preliminary findings on imaging a therapeutic antibody, Lecanemab, in a postmortem AD brain anterior cingulate. [125I]5-iodo-3-pyridinecarboxamido-Lecanemab ([125I]IPC-Lecanemab) was prepared by coupling N-succinimidyl-5-([125I]iodo)-3-pyridinecarboxylate with Lecanemab in modest yields. The distinct binding of [125I]IPC-Lecanemab to Aβ-rich regions in postmortem human AD brains was higher in grey matter (GM) containing Aβ plaques compared to white matter (WM) (GM/WM was 1.6). Anti-Aβ immunostaining was correlated with [125I]IPC-Lecanemab regional binding in the postmortem AD human brains. [125I]IPC-Lecanemab binding was consistent with the binding of Aβ small molecules, [18F]flotaza and [125I]IBETA, in the same subjects. [18F]Flotaza and [125I]IBETA, however, exhibited significantly higher GM/WM ratios (>20) compared to [125I]IPC-Lecanemab. Our results suggest that radiolabeled [125I]IPC-Lecanemab retains the ability to bind to Aβ in human AD and may therefore be useful as a PET imaging radiotracer when labeled as [124I]IPC-Lecanemab. The ability to directly visualize in vivo a promising therapeutic antibody for AD may be useful in treatment planning and dosing and could be complimentary to small-molecule diagnostic imaging to assess outcomes of therapeutic interventions.