Radionuclide selection influences imaging outcomes in immunoPET with a brain-penetrant anti-Aβ antibody

BackgroundBispecific antibodies exploiting receptor-mediated transcytosis offer a promising strategy to overcome limited blood-brain barrier permeability in Alzheimers disease (AD) therapy and imaging. Lecanemab-Fab8D3 (Lec-Fab8D3), a bispecific anti-amyloid beta (A{beta}) antibody engineered for enhanced brain delivery, holds potential as a companion immunoPET imaging diagnostic with the novel lecanemab immunotherapy. This study aimed to compare three radionuclides--zirconium-89 (89Zr), copper-64 (64Cu), and iodine-124 (124I)--for PET imaging with Lec-Fab8D3 to study its in vivo brain distribution and evaluate its potential as an AD companion diagnostic. MethodsLec-Fab8D3 was conjugated to DFO* or NODAGA for 89Zr and 64Cu radiolabeling, respectively, or directly radioiodinated with 124I. PET imaging was performed in the Tg-ArcSwe mouse model of A{beta} pathology and wild-type (WT) littermates at multiple time points post administration of the radiolabeled antibody, followed by ex vivo biodistribution, autoradiography, and A{beta} quantification to assess brain uptake, specificity, and distribution of the radiolabeled Lec-Fab8D3. ResultsRadiolabeled Lec-Fab8D3 variants showed retained binding properties with high radiochemical purity and yields. PET imaging demonstrated cortical brain uptake of all three tradiotracers in Tg-ArcSwe mice, with [89Zr]Zr-DFO*-Lec-Fab8D3 and [124I]I-Lec-Fab8D3 showing the best discrimination between Tg-ArcSwe and WT mice at 48-72 h post-injection. The highest absolute brain retention, combined with a lower brain-to-cerebellum ratio, was observed in both Tg-ArcSwe and WT mice that received the radiometal-labeled (89Zr and 64Cu) antibody, likely due to the residualizing nature of radiometals. Ex vivo analyses confirmed PET findings, and immunostaining demonstrated co-localization of Lec-Fab8D3 with A{beta} deposits. ConclusionsImmunoPET imaging with bispecific Lec-Fab8D3 enables specific detection of brain A{beta} pathology in an AD mouse model. 89Zr was superior to 64Cu due to a more compatible half-life, while 124I displayed higher regional contrast than both radiometals, despite lower overall brain signal. The combined findings from radiometal- and iodine-based immunoPET will enhance our understanding of intra-brain distribution of bispecific antibodies. Furthermore, this highlights the importance of the choice of radiolabeling strategy and how it will impact the outcome of immunoPET with bispecific A{beta} antibodies.