Brain distribution of a bispecific antibody targeting Aβ
Sammanfattning: Alzheimer’s disease (AD) is characterised by aberrant protein aggregation in the brain with subsequent synaptic loss, neuroinflammation, and brain atrophy that ultimately clinically manifests as cognitive impairment. Histopathological findings in AD are extracellular plaques of the protein amyloid-beta (Aβ), Aβ in blood vessels (CAA), and intracellular neurofibrillary tangles (NFT) of hyperphosphorylated tau. The FDA recently approved the antibody aducanumab for AD treatment, and several antibodies are now in clinical phase 3 trials, demonstrating that Aβ-directed immunotherapy is a viable treatment option in AD. In this thesis we evaluated the therapeutic Aβ antibodies 3D6 and RmAb158 in comparison with the bispecific RmAb158-scFv8D3, which penetrates the blood-brain barrier (BBB) by transferrin receptor mediated transcytosis. Emphasis lies in antibody brain uptake and intra brain distribution, in their use as potential treatment options in AD and how such treatment affects BBB integrity. Vascular disturbances are common side effects of anti-Aβ immunotherapy. However, we demonstrated that the BBB permeability of large molecules is unchanged following acute 3D6 treatment in an Aβ mouse model (paper I). Next, brain uptake and distribution of radioiodinated RmAb158 and its bispecific variant RmAb158-scFv8D3 were investigated with SPECT in an Aβ mouse model. Due to its active transport across BBB, RmAb158-scFv8D3 had a higher brain uptake than RmAb158, resulting in greater total brain exposure, and higher concentration at Aβ plaques (paper II). In paper III, we labelled RmAb158-scFv8D3 with the radiometal indium-111 (111In), using chelators CHX-A”-DTPA or DOTA, and SPECT was used to investigate brain retention and biodistribution. The 111In-labelled bispecific antibody entered the brain, and although brain retention was higher in Aβ mice, the wild type (wt) background was high. SPECT revealed high bone uptake of all tracers, and subsequent ex vivo measurement pinpointed retention to the bone marrow. With the knowledge gained from paper II, we addressed whether RmAb158-scFv8D3 would improve treatment efficacy in different treatment regimes. We also assessed the immunogenicity of different antibody constructs upon chronic administration (paper IV). As all tested bispecific antibody constructs elicited a humoral response, immune cell depletion was necessary before repeated antibody treatment. Overall, long-term treatment of RmAb158-scFv8D3 did lower total brain Aβ but compared with RmAb158, it did not improve treatment efficacy.In conclusion, acute anti-Aβ immunotherapy did not negatively affect BBB integrity, and bispecific antibodies displayed improved brain distribution and long-term accumulation at parenchymal Aβ. However, this did not translate into an added treatment effect in a chronic therapeutic setting.
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