Brain interstitial fluid pharmacokinetics and therapeutic effect of a BBB penetrating amyloid beta antibody measured by microdialysis.

The disease-modifying antibody lecanemab for treating Alzheimer's disease (AD) was initially designed to target amyloid-beta (Aβ) protofibrils, i.e. soluble aggregates of Aβ, but it has also been successful in clearing insoluble amyloid plaques in clinical studies. Therefore, this study aimed to investigate how a brain penetrating, bispecific murine variant of lecanemab (RmAb158-scFv8D3) distributes in the brain and interacts with different pools of aggregated Aβ in APP transgenic mice. The alpha-synuclein targeting antibody RmAbSynO2-scFv8D3 was used as control. Further, by performing in vivo high cut-off microdialysis in freely moving animals, brain interstitial fluid (ISF) was continuously collected across 24 h to assess concentrations of free antibody in the brain. Post mortem distribution of the antibodies was analyzed by sequential extraction of brain tissue. RmAb158-scFv8D3 showed rapid ISF clearance as well as a redistribution from brain extracts containing small, soluble Aβ species toward brain extracts containing insoluble, plaque-associated Aβ with time. A treatment effect was detected already at 12 h post injection, whereby the RmAb158-scFv8D3-treated animals showed lower concentrations of the smallest, most soluble Aβ aggregates. Collectively, these findings suggest that within the first 24 h after a single injection of the bispecific RmAb158-scFv8D3 antibody we can capture the antibody's initial brain distribution and interactions with both soluble Aβ aggregates and insoluble, plaque-associated Aβ. These interactions mediate a swift reduction of soluble Aβ, while clearance of insoluble Aβ requires longer treatment time.