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The brain is a challenging target for antibody-based positron emission tomography (immunoPET) to image amyloid-beta (Aβ). Antibodies detect pathology with high sensitivity, but due to their size and biological half-life, they cause a high background radiation, if radiolabelled. Antibodies fused to transferrin-receptor (TfR) binders can penetrate the BBB via receptor-mediated transcytosis. 

In this thesis, I evaluated several methods to reduce the biological half-life for bispecific antibodies, which bind to Aβ and TfR, to reduce the time between injection and imaging.

In paper I, we studied two different clearing approaches – direct clearance and induced clearance – to reduce blood concentrations of a monospecific and a bispecific, brain penetrating antibody, for enhanced contrast. The direct clearing approach was too efficient to show a benefit for brain imaging. The induced clearing strategy, based on the inverse-electron demand Diels−Alder (IEDDA) reaction of a TCO and a tetrazine, proved the concept of induced clearance for the monospecific antibody, but not for the bispecific antibody. For paper II, we changed the antibody design and compared a bispecific antibody with its corresponding monospecific variant, both with and without a mutation that attenuated binding to the neonatal Fc receptor (FcRn), to decrease antibody circulation time in vivo. The mutation reduced the blood half-life and we suggested an imaging time 12 to 24 h after injection. In paper III, we radiolabelled both FcRn mutated antibody constructs with fluorine-18, to compare their pharmacokinetic profiles in WT mice with PET imaging over 9 h. The bispecific antibody, that showed higher brain uptake, was then injected into WT and AD mice (App^NL-G-F). PET scanning 12 h after injection revealed higher antibody retention in App^NL-G-F compared to WT mice. In paper IV, we tested two novel tetrazines for their potential to be used as pre-targeting agents. Pre-targeting describes a two-step approach with the aim to achieve a high contrast PET image. First a TCO-modified antibody is injected and after a while a second substance, a radiolabelled tetrazine is injected. Successful pre-targeting requires a tetrazine which can penetrate the brain and then be efficiently cleared. We could show that both fluorine-18 labelled tetrazines entered the brain, where one of them was more efficient than the other. 

In conclusion we have shown that it is possible to increase the peripheral clearance of radiolabelled antibodies and get one step closer to same-day immunoPET imaging.