Gantenerumab reduces brain amyloid-β proteoforms: Biochemical evaluation in dominantly inherited Alzheimer disease

Therapeutic strategies targeting amyloid-β (Aβ) in Alzheimer disease (AD) include antibodies directed against fibrillar amyloid plaques or soluble amyloid-β (Aβ) species, yet how these approaches reshape brain pathology remains poorly understood. Postmortem neuropathological assessment is crucial for evaluating the degree of Aβ removal, providing information and context that fluid biomarkers and positron emission tomography (PET) alone cannot, and guiding development of more effective disease modifying therapies for AD. In this study, using high resolution mass spectrometry we quantitatively measured Aβ and tau proteoforms in postmortem brain tissue homogenates to evaluate the impact of long-term treatment with anti-Aβ monoclonal antibodies gantenerumab or solanezumab in a clinical trial of dominantly inherited Alzheimer disease (DIAD). Symptomatic DIAD participants who had been treated for ∼ 4 years with anti-Aβ monoclonal antibodies (gantenerumab, n=4 or solanezumab, n=4) were compared with untreated, symptomatic DIAD mutation carriers (n=40) and with age-relevant non-DIAD controls with modest or no plaque pathology (n=18). Consistent with antemortem Aβ-PET findings in the clinical trial, gantenerumab-treated participant samples showed significantly lower levels of specific Aβ proteoforms (Aβ ₄₂ , Aβ ₄₀ ) in the insoluble plaque associated pool (range: 60-80 % reduction); they also showed lower levels of post-translationally modified Aβ proteoform Aβ _3pGlu in both the insoluble and soluble pool, compared to untreated DIAD participants, but without reaching the levels found in non-DIAD controls. Samples from gantenerumab-treated participants also showed regionally lower amounts of insoluble phosphorylated tau species (pT181, pT205, pT208) and MTBR tau species (354-369, 306-317, 299-317) without apparent differences in insoluble total tau levels. In contrast, brain tissue samples from solanezumab-treated participants did not show significant differences overall in soluble or insoluble levels of Aβ and tau proteoforms in comparison to those from non-treated DIAD individuals. These findings support that amyloid targeting treatment (ATT) with gantenerumab, an antibody that targets fibrillar Aβ, can remove insoluble Aβ from the DIAD brain and also mitigate "downstream" biochemical and pathophysiological changes associated with AD.

Question: How do amyloid targeting treatments (ATT) affect the Alzheimer disease (AD)-related brain pathologies of soluble and aggregated amyloid-β and tau proteoforms in postmortem tissue?Background: Extracellular amyloid plaques and neurofibrillary tangles (NFTs) are the primary pathological hallmarks of AD. For the past three decades, active and passive immunization against cerebral amyloid-β (Aβ) has constituted one of the primary avenues of disease-modifying intervention in AD. Recently, passive immunizations using anti-Aβ targeting antibodies have demonstrated slowing of cognitive decline in sporadic AD and possibly in dominantly inherited AD (DIAD). In these clinical trials, target engagement has been inferred from Aβ-positron emission tomography (PET) and/or biofluid biomarkers during life. However, to confirm evidence of target engagement more directly, identify which pathological proteoforms are affected, quantify the dose dependent treatment effects of these treatments, and begin to evaluate their putative downstream effects on AD pathophysiology, quantitative biochemical comparisons of amyloid plaques and tau pathology (soluble and insoluble Aβ and tau species) in postmortem brain tissue of treated and untreated AD are needed.Findings: In this study, using high resolution mass spectrometry we measured the soluble and insoluble Aβ and tau proteoforms in the postmortem brain tissue homogenates from DIAN-TU-001 trial participants who were symptomatic and had been treated for ∼ 4 years with gantenerumab (n=4) or solanezumab (n=4), two antibodies that bind to insoluble and soluble Aβ, respectively. Intervals between treatment discontinuation and death ranged from 5 months to 3 years. Comparisons were made with postmortem brain tissue samples from untreated, symptomatic DIAD participants of the DIAN-Obs study (n=40) and from age-relevant, cognitively normal, non-DIAD controls with or without only modest burdens of diffuse Aβ plaques (n=18). Consistent with antemortem Aβ-PET findings in the clinical trial, gantenerumab significantly decreased specific Aβ (Aβ ₄₂ , Aβ ₄₀ ) proteoforms in the insoluble plaque associated pool (range: 60-80% reduction) compared to the untreated DIAD participants, but without reaching the levels found in the non-DIAD controls. Samples from gantenerumab treated participants also showed significantly lower amount of post-translationally modified Aβ (Aβ _3pGlu ) and regionally lower insoluble phosphorylated tau species (pT181, pT205 and pT208), without apparent differences in insoluble total tau levels. In contrast, solanezumab treatment did not show significant effect on the soluble or insoluble levels of Aβ and tau proteoforms in comparison to samples from non-treated DIAD individuals. Significance: These findings support that ATT with gantenerumab, an antibody that targets fibrillar Aβ, can remove insoluble Aβ from the DIAD brain and also mitigate "downstream" biochemical and pathophysiological changes associated with AD.