<b>PI-2620</b> 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 <b>PI-2014</b> was tested in healthy controls and subjects with Alzheimer's disease (AD). As <b>PI-2014</b> 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 <i>N</i>-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. <b>PI-2620</b> (2-(2-fluoropyridin-4-yl)-9<i>H</i>-pyrrolo[2,3-<i>b</i>:4,5-<i>c</i>']dipyridine) emerged as a best candidate displaying high Tau binding, low MAO-A binding, high brain uptake, and fast and complete brain washout. Furthermore, <b>PI-2620</b> showed Tau binding on brain sections from corticobasal degeneration, progressive supranuclear palsy, and Pick's disease.