PROJECT SUMMARY Alzheimer's disease (AD) is characterized by misfolding and aggregation of two major proteins, amyloid-beta (Aβ) and tau. While Aβ aggregation occurs years before clinical symptoms, tau accumulation in the brain is more closely correlated to the neuronal death and eventually the loss of cognitive function. In addition, tau inclusions are also found in other dementias, such as frontotemporal dementias and corticobasal degeneration. Thus, the ability to in vivo image tau would be important for clinical diagnosis and to evaluate effects of tau- targeted treatments. Earlier candidate radioligands for imaging tauopathies have been based on small- molecule drug compounds; however, these radioligands have displayed large degrees of off-target binding and an inability to bind to tau in dementias other than AD. On the other hand, single-chain variable antibody fragments (scFv) are attractive as therapy or diagnostic positron emission tomography (PET) markers for their greater specificity and high-affinity binding. Aβ plaque imaging probes derived from β-sheet binding dyes are already in clinical use, and a few such tau-binding dyes are being evaluated for cancer detection. Antibody- derived probes are likely to provide greater specificity for detecting tau lesions; however, their poor brain penetrance has restricted their use as PET ligands for imaging of targets within the CNS. Previously, we have successfully demonstrated that the transport of antibody scFv across the blood-brain barrier (BBB) can be facilitated through interaction with the transferrin receptor (TfR), and that the bispecific antibody-based PET ligands generated by fusion of fragments of TfR and Aβ antibodies were capable of detecting Aβ aggregates in vivo with distinctive differences, both quantitatively and visually, in brain uptake between wild type and transgenic mice, and with a good correlation with Aβ pathology. In fact, a humanized form (BAN2401) of our previously developed monoclonal antibody mAb158 showed promising results, for the first time, in a Phase IIb clinical trial as an anti-Aβ therapy against AD due to its distinctive selectivity for soluble Aβ protofibrils. Thus, in this proposal we will apply the same strategy to create, for the first time, bispecific antibody-based PET ligands for selective in vivo imaging of tauopathies. In this application, we have assembled a team of experts from the US and Sweden and we propose to build upon our experience in the development, evaluation and translation of PET tau probes to prioritize the initial tau antibody ligand collection based on their PET imaging performance profiles and select two optimal tau antibody ligands for the subsequent discriminative evaluation in tau animal models. That is, we will determine their capability as biomarkers to provide meaningful assessments of target engagement in animal models. The research team has extensive, multi-disciplinary experience in chemistry, biochemistry, pharmacology...