Abstract
Amyloid beta (Aβ) plaque is a defining pathologic feature of Alzheimer disease (AD). Aducanumab, a monoclonal IgG1 that selectively binds aggregated species of Aβ, has been shown by amyloid positron emission tomography (Amyloid PET) to reduce Aβ plaques in patients with prodromal and mild AD. This is the first autopsy report of the AD neuropathology in a patient previously treated with aducanumab. The patient was an 84-year-old woman who was randomized to the placebo arm of the PRIME Phase 1b study (221AD103). The patient progressed to moderate dementia (MMSE = 14/30), beyond the targeted early AD treatment stage, before receiving aducanumab in the long-term extension (LTE). The patient then received 32 monthly doses of aducanumab, titrated up to 6 mg/kg, for a cumulative dose of 186 mg/kg. In the LTE, Amyloid PET scans demonstrated robust Aβ plaque reduction, from a composite standard uptake value ratio (SUVR) of 1.5 at screening to < 1.1 at 56 weeks post-aducanumab dosing. MRI examinations were negative for amyloid-related imaging abnormalities (ARIA). She passed away in hospice care 4 months after her last dose of aducanumab. The postmortem neuropathologic examination confirmed AD neuropathologic changes. Aβ and IBA1 immunohistochemistry assays demonstrated sparse residual Aβ plaque engaged by amoeboid reactive microglia. Phospho-Tau (pTau) immunohistochemistry demonstrated neocortical neurofibrillary degeneration (Braak stage V, NIA/AA Stage B3). However, the density of pTau neuropathology, including neuritic plaque pTau (NP-Tau), appeared lower in the PRIME LTE Patient compared to a reference cohort of untreated Braak stage V-VI, NIA/AA Stage B3 AD cases. Taken together, this case report is the first to provide Amyloid PET and neuropathologic evidence substantiating the impact of aducanumab to reduce Aβ plaque neuropathology in a patient with AD. Furthermore, this report underscores the critical importance of autopsy neuropathology studies to augment our understanding of aducanumab's mechanism of action and impact on AD biomarkers.