The accelerated approval of the monoclonal antibody (mAb) aducanumab as a treatment option for Alzheimer's Disease and the continued discussions about its efficacy have shown that a better understanding of immunotherapy for the treatment of neurodegenerative diseases is needed. 89 Zr-immuno-PET could be a suitable tool to open new avenues for the diagnosis of CNS disorders, monitoring disease progression, and assessment of novel therapeutics. Herein, three different 89 Zr-labeling strategies and direct radioiodination with 125 I of a bispecific anti-amyloid-beta aducanumab derivate, consisting of aducanumab with a C-terminal fused anti-transferrin receptor binding single chain Fab fragment derived from 8D3 (Adu-8D3), were compared ex vivo and in vivo with regard to brain uptake and target engagement in an APP/PS1 Alzheimer's disease mouse model and wild type animals. Methods: Adu-8D3 and a negative control antibody, based on the HIV specific B12 antibody also carrying C-terminal fused 8D3 scFab (B12-8D3), were each conjugated with NCS-DFO, NCS-DFO*, or TFP- N -suc-DFO-Fe-ester, followed by radiolabeling with 89 Zr. 125 I was used as a substitute for 124 I for labeling of both antibodies. 30 µg of radiolabeled mAb, corresponding to approximately 6 MBq 89 Zr or 2.5 MBq 125 I, were injected per mouse. PET imaging was performed 1, 3 and 7 days post injection (p.i.). All mice were sacrificed on day 7 p.i. and subjected to ex vivo biodistribution and brain autoradiography. Immunostaining on brain tissue was performed after autoradiography for further validation. Results: Ex vivo biodistribution revealed that the brain uptake of [ 89 Zr]Zr-DFO*-NCS-Adu-8D3 (2.19 ±0.12 %ID/g) was as high as for its 125 I-analog (2.21 ±0.15 %ID/g). [ 89 Zr]Zr-DFO-NCS-Adu-8D3 and [ 89 Zr]Zr-DFO- N -suc-Adu-8D3 showed significantly lower uptake (< 0.65 %ID/g), being in the same range as for the 89 Zr-labeled controls (B12-8D3). Autoradiography of [ 89 Zr]Zr-DFO*-NCS-Adu-8D3 and [ 125 I]I-Adu-8D3 showed an amyloid-beta related granular uptake pattern of radioactivity. In contrast, the [ 89 Zr]Zr-DFO-conjugates and the control antibody groups did not show any amyloid-beta related uptake pattern, indicating that DFO is inferior for 89 Zr-immuno-PET imaging of the brain in comparison to DFO* for Adu-8D3. This was confirmed by day 7 PET images showing only amyloid-beta related brain uptake for [ 89 Zr]Zr-DFO*-NCS-Adu-8D3. In wild type animals, such an uptake wa...

Purpose The recent conditional FDA approval of Aducanumab (Adu) for treating Alzheimer’s disease (AD) and the continued discussions around that decision have increased interest in immunotherapy for AD and other brain diseases. Reliable techniques for brain imaging of antibodies may guide decision-making in the future but needs further development. In this study, we used 89Zr-immuno-PET to evaluate the targeting and distribution of a bispecific brain-shuttle IgG based on Adu with transferrin receptor protein-1 (TfR1) shuttling mechanism, mAbAdu-scFab8D3, designated Adu-8D3, as a candidate theranostic for AD. We also validated the 89Zr-immuno-PET platform as an enabling technology for developing new antibody-based theranostics for brain disorders. Methods Adu, Adu-8D3, and the non-binding control construct B12-8D3 were modified with DFO*-NCS and radiolabeled with 89Zr. APP/PS1 mice were injected with 89Zr-labeled mAbs and imaged on days 3 and 7 by positron emission tomography (PET). Ex vivo biodistribution was performed on day 7, and ex vivo autoradiography and immunofluorescence staining were done on brain tissue to validate the PET imaging results and target engagement with amyloid-β plaques. Additionally, [89Zr]Zr-DFO*-Adu-8D3 was evaluated in 3, 7, and 10-month-old APP/PS1 mice to test its potential in early stage disease. Results A 7-fold higher brain uptake was observed for [89Zr]Zr-DFO*-Adu-8D3 compared to [89Zr]Zr-DFO*-Adu and a 2.7-fold higher uptake compared to [89Zr]Zr-DFO*-B12-8D3 on day 7. Autoradiography and immunofluorescence of [89Zr]Zr-DFO*-Adu-8D3 showed co-localization with amyloid plaques, which was not the case with the Adu and B12-8D3 conjugates. [89Zr]Zr-DFO*-Adu-8D3 was able to detect low plaque load in 3-month-old APP/PS1 mice. Conclusion 89Zr-DFO*-immuno-PET revealed high and specific uptake of the bispecific Adu-8D3 in the brain and can be used for the early detection of Aβ plaque pathology. Here, we demonstrate that 89Zr-DFO*-immuno-PET can be used to visualize and quantify brain uptake of mAbs and contribute to the evaluation of biological therapeutics for brain diseases.

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