The disease-modifying antibody lecanemab for treating Alzheimer's disease (AD) was initially designed to target amyloid-beta (Aβ) protofibrils, i.e. soluble aggregates of Aβ, but it has also been successful in clearing insoluble amyloid plaques in clinical studies. Therefore, this study aimed to investigate how a brain penetrating, bispecific murine variant of lecanemab (RmAb158-scFv8D3) distributes in the brain and interacts with different pools of aggregated Aβ in APP transgenic mice. The alpha-synuclein targeting antibody RmAbSynO2-scFv8D3 was used as control. Further, by performing in vivo high cut-off microdialysis in freely moving animals, brain interstitial fluid (ISF) was continuously collected across 24 h to assess concentrations of free antibody in the brain. Post mortem distribution of the antibodies was analyzed by sequential extraction of brain tissue. RmAb158-scFv8D3 showed rapid ISF clearance as well as a redistribution from brain extracts containing small, soluble Aβ species toward brain extracts containing insoluble, plaque-associated Aβ with time. A treatment effect was detected already at 12 h post injection, whereby the RmAb158-scFv8D3-treated animals showed lower concentrations of the smallest, most soluble Aβ aggregates. Collectively, these findings suggest that within the first 24 h after a single injection of the bispecific RmAb158-scFv8D3 antibody we can capture the antibody's initial brain distribution and interactions with both soluble Aβ aggregates and insoluble, plaque-associated Aβ. These interactions mediate a swift reduction of soluble Aβ, while clearance of insoluble Aβ requires longer treatment time.

The extent to which the cerebrovasculature is affected in various brain disorders is still not well understood. To address this, we established a transcriptomic repository of major vascular cell types and microglia to compare the global transcriptomic response in mouse models of three human brain disorders linked to neuroinflammation and associated vascular reactivity: Alzheimer's disease (AD), traumatic brain injury (TBI), and cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). Single-cell analysis of >250,000 cells at different disease stages led to identification of two previously unknown vascular cell subtypes, expanded the endothelial zonation spectrum and allowed for a detailed analysis of the cellular and molecular responses. Surprisingly, most vascular cell types lacked major transcriptomic changes across the three conditions, while microglia exhibited significant, disease-specific transcriptional changes. Notably, microglial responses converged between late-stage TBI and AD, offering insights into the predisposition for neurodegeneration following TBI.

Bispecific antibodies exploiting receptor-mediated transcytosis offer a promising strategy to overcome limited blood–brain barrier permeability in Alzheimer disease immunotherapy and imaging. Lecanemab-Fab8D3 (Lec-Fab8D3), a bispecific amyloid-β (Aβ) antibody with enhanced brain delivery, may complement lecanemab immunotherapy as an immuno-PET imaging agent. Here, we systematically assess how the choice of radionuclide affects PET detection of Lec-Fab8D3 within the brain to evaluate its potential as a companion diagnostic.

Methods: Lec-Fab8D3 was conjugated to an octadentate derivative of desferrioxamine (DFO*) or NODAGA for ⁸⁹Zr and ⁶⁴Cu radiolabeling, respectively, or directly radioiodinated with ¹²⁴I. PET imaging was performed in the Tg-ArcSwe Aβ mouse model and wild-type (WT) littermates at multiple time points after radiotracer administration, followed by biodistribution, autoradiography, and Aβ quantification to assess brain uptake, specificity, and distribution.

Results: PET imaging demonstrated high cortical brain uptake of all 3 radiotracers in Tg-ArcSwe mice. Labeling with the metals ⁸⁹Zr and ⁶⁴Cu produced the highest overall brain signal in both Tg-ArcSwe and WT mice. [⁸⁹Zr]Zr-DFO*-Lec-Fab8D3 and [¹²⁴I]I-Lec-Fab8D3 demonstrated the greatest discrimination between Tg-ArcSwe and WT mice, with [¹²⁴I]I-Lec-Fab8D3 exhibiting the most pronounced regional differences. Ex vivo analyses corroborated the PET findings, and immunostaining confirmed radiotracer colocalization with Aβ deposits.

Conclusion: Immuno-PET imaging with radiolabeled Lec-Fab8D3 enables specific detection of brain Aβ pathology. Because of its residualizing properties, ⁸⁹Zr produced the highest overall signal, whereas ¹²⁴I yielded greater regional contrast, despite lower total brain signal. These findings enhance our understanding of the intrabrain distribution of bispecific antibodies and highlight the importance of radionuclide selection and its impact on immuno-PET outcomes.