Recently, bioorthogonal chemistry based on the Inverse Electron-Demand Diels-Alder (IEDDA) cycloaddition between 1,2,4,5-tetrazines and trans-cyclooctene (TCO) analogues added an interesting dimension to molecular imaging. Until now, antibodies (Abs) were tagged with TCO and after pretargeting they were reacted with tetrazines substituted with reporters. However, TCO tags have the tendency to degrade under physiological conditions, and due to their hydrophobic nature are buried within the protein. This results in loss of reactivity and a low Ab functional loading. To circumvent these problems, we report for the first time an approach in which tetrazines are used as tags for antibody (Ab) modification, and TCO as the imaging agent. We developed a new Ab-tetrazine conjugate, which displays a high functional loading, good stability and reactivity. We utilized this immunoconjugate for live-cell imaging together with novel TCO probes, resulting in selective and rapid labeling of SKOV-3 cells. Our approach may be useful for in vivo pretargeted imaging.

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[(89)Zr]Df-Bz-JRF/AβN/25 binds with high affinity to Aβ1-40. The tracer displays an acceptable in vivo stability and is able to cross the blood-brain barrier. [(89)Zr]Df-Bz-JRF/AβN/25 might therefore be a potential candidate for in vivo imaging of Aβ deposition in the brain.

Introduction: The accumulation of amyloid-β is a pathological hallmark of Alzheimer’s disease and is a target for molecular imaging probes to aid in diagnosis and disease monitoring. This study evaluated the feasibility of using a radiolabeled monoclonal anti-amyloid-β antibody (JRF/AβN/25) to non-invasively assess amyloid-β burden in aged transgenic mice (APPPS1–21) with μPET imaging.Methods: We investigated the antibody JRF/AβN/25 that binds to full-length Aβ. JRF/AβN/25 was radiolabeled with a [89Zr]-desferal chelate and intravenously injected into 12–13 month aged APPPS1–21 mice and their wild-type (WT) controls. Mice underwent in vivo μPET imaging at 2, 4, and 7 days post injection and were sacrificed at the end of each time point to assess brain penetrance, plaque labeling, biodistribution, and tracer stability. To confirm imaging specificity we also evaluated brain uptake of a non-amyloid targeting [89Zr]-labeled antibody (trastuzumab) as a negative control, additionally we performed a competitive blocking study with non-radiolabeled Df-Bz-JRF/AβN/25 and finally we assessed the possible confounding effects of blood retention.Results: Voxel-wise analysis of μPET data demonstrated significant [89Zr]-Df-Bz-JRF/AβN/25 retention in APPPS1–21 mice at all time points investigated. With ex vivo measures of radioactivity, significantly higher retention of [89Zr]-Df-Bz-JRF/AβN/25 was found at 4 and 7 days pi in APPPS1–21 mice. Despite the observed genotypic differences, comparisons with immunohistochemistry revealed that in vivo plaque labeling was low. Furthermore, pre-treatment with Df-Bz-JRF/AβN/25 only partially blocked [89Zr]-Df-Bz-JRF/AβN/25 uptake indicative of a high contribution of non-specific binding.Conclusion: Amyloid plaques were detected in vivo with a radiolabeled monoclonal anti-amyloid antibody. The low brain penetrance of the antibody in addition to non-specific binding prevented an accurate estimation of plaque burden. However, it should be noted that [89Zr]-Df-Bz-JRF/AβN/25 nevertheless demonstrated in vivo binding and strategies to increase brain penetrance would likely achieve better results.