2 nM or lower. In this range of concentration, [11C]PBB3 could preferentially interact with high-affinity binding components formed by tau assemblies. An excessive amount of radioligand in the brain would result in saturation of radioligand binding to tau lesions and increased binding to low-affinity, high-capacity binding components in Aβ plaques, and such overload of free

radioligand is more likely in regions with less abundant tau pathologies. This could be even more critical in capturing early tau pathologies that originate in the hippocampal formation and may require technical improvements and Romidepsin datasheet methodological refinements, including high-resolution imaging, correction for motions of subjects during scans, and

robust definition of VOIs on the atrophic hippocampus. Although nonspecific [11C]PBB3-PET signals in control human subjects were generally low, radioligand retention in dural venous sinuses was noticeable in all scanned individuals. Possible mechanisms that underlie this property are discussed in the Supplemental Discussion. The present work has also implied the potential utility of multimodal imaging systems for translational development of therapeutic agents that counteract tau fibrillogenesis. Optical imaging with a near-infrared fluorescent probe, such as PBB5, could provide the least invasive technique to assess tau accumulation in living mouse models. As demonstrated by our in vitro NVP-BGJ398 and ex vivo fluorescence labeling, all PBBs share a similarity in terms of their reactivity with tau aggregates. Hence, PBB5 optics may be applicable to early screening of therapeutic agents that suppress tau deposition,

and the data on abundance of tau lesions obtained by this approach may be translatable to advanced stages of assessments using [11C]PBB3-PET in animal models and humans. By contrast, pharmacokinetic properties of PBB5 (Figure S5) were found to be distinct from those of electrically neutral PBBs, including PBB2 and PBB3. These considerations would be of importance in developing and using fluorescent ligands applicable to optical and PET imaging. To conclude, our class very of multimodal imaging agents offers the possibility of visual investigations of fibrillary tau pathologies at subcellular, cellular, and regional levels. These assay systems are potentially powerful tools for the longitudinal evaluation of anti-tau treatments (Marx, 2007), as a single probe may facilitate a seamless, bidirectional translation between preclinical and clinical insights. PET tracers would also serve a more immediate therapeutic purpose by enabling the assessment of the effects of anti-Aβ and anti-tau therapies on tau pathologies in living AD patients.