In vivo imaging of alpha-synuclein with antibody-based PET

The protein alpha-synuclein (alpha SYN) plays a central role in synucleinopathies such as Parkinsons's disease (PD) and multiple system atrophy (MSA). Presently, there are no selective alpha SYN positron emission tomography (PET) radioligands that do not also show affinity to amyloid-beta (A beta). We have previously shown that radiolabeled antibodies, engineered to enter the brain via the transferrin receptor (TfR), is a promising approach for PET imaging of intrabrain targets. In this study, we used this strategy to visualize alpha SYN in the living mouse brain. Five bispecific antibodies, binding to both the murine TfR and alpha SYN were generated and radiolabeled with iodine-125 or iodine-124. All bispecific antibodies bound to alpha SYN and mTfR before and after radiolabelling in an ELISA assay, and bound to brain sections prepared from alpha SYN overexpressing mice as well as human PD- and MSA subjects, but not control tissues in autoradiography. Brain concentrations of the bispecific antibodies were be-tween 26 and 63 times higher than the unmodified IgG format 2 h post-injection, corresponding to about 1.5% of the injected dose per gram brain tissue. Additionally, intrastriatal alpha SYN fibrils were visualized with PET in an alpha SYN deposition mouse model with one of the bispecific antibodies, [I-124]RmAbSynO2-scFv8D3. However, PET images acquired in alpha SYN transgenic mice with verified brain pathology injected with [I-124]RmAbSynO2-scFv8D3 and [I-124]RmAb48-scFv8D3 showed no increase in antibody retention compared to WT mice. Despite successful imaging of deposited extracellular alpha SYN using a brain-penetrating antibody-based radioligand with no cross-specificity towards A beta, this proof-of-concept study demonstrates challenges in imaging intracellular alpha SYN inclusions present in synucleinopathies