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.