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.
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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
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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
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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). [
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Zr]Zr-DFO-NCS-Adu-8D3 and [
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Zr]Zr-DFO-
N
-suc-Adu-8D3 showed significantly lower uptake (< 0.65 %ID/g), being in the same range as for the
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Zr-labeled controls (B12-8D3). Autoradiography of [
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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 [
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Zr]Zr-DFO-conjugates and the control antibody groups did not show any amyloid-beta related uptake pattern, indicating that DFO is inferior for
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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 [
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Zr]Zr-DFO*-NCS-Adu-8D3. In wild type animals, such an uptake wa...
