Lead Optimization Highlights the Importance of Off-Target Assays to Develop a PET Tracer for the Detection of Pathological Aggregated Tau in Alzheimer's Disease and Other Tauopathies.

The first candidate was tested in healthy controls and subjects with Alzheimer's disease (AD). As displayed off-target binding to monoamine oxidase A (MAO-A), a new lead with improved binding to Tau and decreased MAO-A binding was required. For compound optimization, Tau binding assays based on both human AD brain homogenate and Tau-paired helical filaments were employed. Furthermore, two MAO-A screening assays based on (1) human-recombinant MAO-A and (2) displacement of 2-fluoro-ethyl-harmine from mouse brain homogenate were employed. Removing the -methyl group from the tricyclic core resulted in compounds displaying improved Tau binding. For the final round of optimization, the cyclic amine substituents were replaced by pyridine derivatives. (2-(2-fluoropyridin-4-yl)-9-pyrrolo[2,3-:4,5-']dipyridine) emerged as a best candidate displaying high Tau binding, low MAO-A binding, high brain uptake, and fast and complete brain washout. Furthermore, showed Tau binding on brain sections from corticobasal degeneration, progressive supranuclear palsy, and Pick's disease.