(11)C-PiB and (124)I-Antibody PET Provide Differing Estimates of Brain Amyloid-β After Therapeutic Intervention

PET imaging of amyloid-β (Aβ) has become an important component of Alzheimer disease diagnosis. (11)C-Pittsburgh compound B ((11)C-PiB) and analogs bind to fibrillar Aβ. However, levels of nonfibrillar, soluble, aggregates of Aβ appear more dynamic during disease progression and more affected by Aβ-reducing treatments. The aim of this study was to compare an antibody-based PET ligand targeting nonfibrillar Aβ with (11)C-PiB after β-secretase (BACE-1) inhibition in 2 Alzheimer disease mouse models at an advanced stage of Aβ pathology. Methods: Transgenic ArcSwe mice (16 mo old) were treated with the BACE-1 inhibitor NB-360 for 2 mo, whereas another group was kept as controls. A third group was analyzed at the age of 16 mo as a baseline. Mice were PET-scanned with (11)C-PiB to measure Aβ plaque load followed by a scan with the bispecific radioligand (124)I-RmAb158-scFv8D3 to investigate nonfibrillar aggregates of Aβ. The same study design was then applied to another mouse model, App(NL-G-F). In this case, NB-360 treatment was initiated at the age of 8 mo and animals were scanned with (11)C-PiB-PET and (125)I-RmAb158-scFv8D3 SPECT. Brain tissue was isolated after scanning, and Aβ levels were assessed. Results: (124)I-RmAb158-scFv8D3 concentrations measured with PET in hippocampus and thalamus of NB-360–treated ArcSwe mice were similar to those observed in baseline animals and significantly lower than concentrations observed in same-age untreated controls. Reduced (125)I-RmAb158-scFv8D3 retention was also observed with SPECT in hippocampus, cortex, and cerebellum of NB-360–treated App(NL-G-F) mice. Radioligand in vivo concentrations corresponded to postmortem brain tissue analysis of soluble Aβ aggregates. For both models, mice treated with NB-360 did not display a reduced (11)C-PiB signal compared with untreated controls, and further, both NB-360 and control mice tended, although not reaching significance, to show higher (11)C-PiB signal than the baseline groups. Conclusion: This study demonstrated the ability of an antibody-based radioligand to detect changes in brain Aβ levels after anti-Aβ therapy in ArcSwe and App(NL-G-F) mice with pronounced Aβ pathology. In contrast, the decreased Aβ levels could not be quantified with (11)C-PiB PET, suggesting that these ligands detect different pools of Aβ.