Pre- and Post-synapses Contain Lecanemab-reactive Amyloid-β in Post-mortem Human Alzheimer's Disease Brain

Recently, the amyloid-beta (A{beta}) targeting antibody lecanemab has demonstrated modest therapeutic efficacy in slowing cognitive decline in people with Alzheimer's disease (AD). Lecanemab clears amyloid plaques from the brain; however, plaque load does not correlate strongly with cognitive function. The strongest neuropathological correlate of cognitive decline in AD is synapse loss, which is exacerbated in the halo surrounding neuritic amyloid plaques where A{beta} accumulates in remaining synapses. Here, we hypothesised that, through clearing plaques and the associated halo of soluble A{beta} that can directly damage synapses, lecanemab could temper plaque-associated synapse loss. High-resolution imaging of temporal cortex tissue from people who died with AD (N=20) and age-matched controls (N=19) reveals lecanemab staining within individual pre and post-synaptic excitatory terminals in addition to plaque staining. The percentage of pre-synapses containing lecanemab-positive A{beta} was over 200% higher in AD and the percentage of post-synapses was over 150% higher in AD than control tissue, with highest levels of synaptic lecanemab staining observed near plaques. These data demonstrate that lecanemab antibody recognises A{beta} within synapses, warranting future work to determine whether lecanemab treatment slows cognitive decline, at least in part, through both clearing plaques and facilitating clearance or neutralisation of synaptic A{beta}.