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Original Contributions |

Mechanism of Amyloid Removal in Patients With Alzheimer Disease Treated With Gantenerumab

Susanne Ostrowitzki, MD; Dennis Deptula, PhD; Lennart Thurfjell, PhD; Frederik Barkhof, MD; Bernd Bohrmann, PhD; David J. Brooks, MD, DSc; William E. Klunk, MD; Elizabeth Ashford, BSc; Kisook Yoo, PhD; Zhi-Xin Xu, MD; Hansruedi Loetscher, PhD; Luca Santarelli, MD
Arch Neurol. 2012;69(2):198-207. doi:10.1001/archneurol.2011.1538.
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Background Gantenerumab is a fully human anti-Aβ monoclonal antibody in clinical development for the treatment of Alzheimer disease (AD).

Objectives To investigate whether treatment with gantenerumab leads to a measurable reduction in the level of Aβ amyloid in the brain and to elucidate the mechanism of amyloid reduction.

Design A multicenter, randomized, double-blind, placebo-controlled, ascending-dose positron emission tomographic study. Additionally, ex vivo studies of human brain slices from an independent sample of patients who had AD were performed.

Setting Three university medical centers.

Patients Patients with mild-to-moderate AD.

Intervention Two consecutive cohorts of patients received 2 to 7 infusions of intravenous gantenerumab (60 or 200 mg) or placebo every 4 weeks. Brain slices from patients who had AD were coincubated with gantenerumab at increasing concentrations and with human microglial cells.

Main Outcome Measures Percent change in the ratio of regional carbon 11–labeled Pittsburgh Compound B retention in vivo and semiquantitative assessment of gantenerumab-induced phagocytosis ex vivo.

Results Sixteen patients with end-of-treatment positron emission tomographic scans were included in the analysis. The mean (95% CI) percent change from baseline difference relative to placebo (n = 4) in cortical brain amyloid level was −15.6% (95% CI, −42.7 to 11.6) for the 60-mg group (n = 6) and −35.7% (95% CI, −63.5 to −7.9) for the 200-mg group (n = 6). Two patients in the 200-mg group showed transient and focal areas of inflammation or vasogenic edema on magnetic resonance imaging scans at sites with the highest level of amyloid reduction. Gantenerumab induced phagocytosis of human amyloid in a dose-dependent manner ex vivo.

Conclusion Gantenerumab treatment resulted in a dose-dependent reduction in brain amyloid level, possibly through an effector cell–mediated mechanism of action.

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Figures

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Figure 1. Flowchart of 2 consecutive cohorts of patients who received 2 to 7 infusions of intravenous gantenerumab (60 or 200 mg) or placebo every 4 weeks. All patients in the 200-mg cohort discontinued treatment as per decision by the sponsor. End-of-treatment scans were performed 2 to 4 weeks after the seventh dose in the 60-mg cohort and 4 to 14 weeks after the last dose in the 200-mg cohort. Late follow-up scans were performed 11 to 14 months after the seventh dose in the 60-mg cohort and 7 to 9 months after the last dose in the 200-mg cohort. As per request by a health authority, an additional 3-month scan was performed for the 60-mg cohort at sites where the respective protocol amendment was approved in time. Six patients who were to be dosed at the next highest dose had baseline scans; however, no postbaseline scans were performed owing to the early discontinuation of the study. PET indicate positron emission tomography; PiB, carbon 11–labeled Pittsburgh Compound Pittsburgh Compound B.

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Figure 2. Effect of gantenerumab on amyloid load as indexed by standard uptake value ratios (SUVRs) using carbon 11–labeled Pittsburgh Compound B positron emission tomography. Actual cortical composite SUVRs at baseline (A) and at the end of treatment (B), and actual change from baseline at the end of treatment (C), are shown for patients who received infusions of intravenous gantenerumab (60 or 200 mg) or placebo every 4 weeks.

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Figure 3. Effect of gantenerumab on amyloid load as indexed by standard uptake value ratios (SUVRs) using carbon 11–labeled Pittsburgh Compound B ([11C]PiB) positron emission tomography. Scatterplot shows percent change from baseline (specific [11C]PiB signal) in cortical composite SUVR over gantenerumab doses for all patients with an end-of-treatment scan who received gantenerumab (60 or 200 mg) or placebo every 4 weeks. The dose-response relationship is indicated by the linear regression line (% change in amyloid = 12.81 − 0.13 × dose) of the baseline-adjusted percent change residual value (vertical axis) vs actual dose of gantenerumab (horizontal axis).

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Figure 4. Effect of gantenerumab on amyloid load as indexed by standard uptake value ratios (SUVRs) using carbon 11–labeled Pittsburgh Compound B ([11C]PiB) positron emission tomography. The median SUVR percent changes from baseline (specific [11C]PiB signal) by brain region are shown for patients who received infusions of intravenous gantenerumab (60 or 200 mg) or placebo every 4 weeks.

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Figure 5. Magnetic resonance imaging (MRI) scans from an APOE ε4 homozygous patient. Images shown represent scans at baseline (A), during treatment (B), and after treatment (C) that were acquired using a fluid-attenuated inversion recovery sequence. The new area of hyperintensity on the scan performed during treatment (B) is most prominent in the right temporal lobe (arrow) and is consistent with inflammation or vasogenic edema. It first appeared on the scheduled MRI scan 2 weeks after the second drug infusion, was progressive for 6 weeks, and subsequently spontaneously completely resolved by week 17 (C).

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Figure 6. Integrated analysis of amyloid positron emission tomography (PET) and magnetic resonance imaging (MRI). The PET and MRI scans from the 2 patients (ie, patients A and B) are shown. The baseline standard uptake value ratio (SUVR) images are superimposed on the baseline MRI scans (A and C), and the binary masks of the MRI (fluid-attenuated inversion recovery) findings as outlined by the expert reader are superimposed on the baseline structural MRI scans (B and D). The end-of-treatment SUVR maps are superimposed on the baseline MRI scans (E and G), and the difference maps of SUVRs at the end of treatment minus baseline are superimposed on baseline MRI scans (F and H). The late follow-up SUVR maps are superimposed on the baseline MRI scans (I and K), and the difference maps of SUVRs at late follow-up minus baseline are superimposed on the baseline MRI scans (J and L). Crosshairs indicate positioning of the MRI finding.

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Figure 7. Percent reduction (specific carbon 11–labeled Pittsburgh Compound B signal) from baseline in cortical composite region vs fluid-attenuated inversion recovery (FLAIR) area. This figure summarizes in quantitative terms the results shown in Figure 6. In both patients A and B, reduction in the standard uptake value ratio is larger in the volume representing the magnetic resonance imaging finding (FLAIR) than in the cortical composite volume of interest (VOI). This is true for both time points: the end of treatment and posttreatment.

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Figure 8. Microglial phagocytosis of human amyloid plaques. Amyloid plaque staining of human Alzheimer disease (AD) brain sections in the absence of (A; scale bar, 100 μm) and after preincubation with gantenerumab followed by incubation with primary human microglia as effector cells (B-D; scale bars, 100 μm). Aβ amyloid plaques were decreased in the presence of human microglia after preincubation with gantenerumab in a concentration-dependent manner, with slight clearance of small plaques seen at 50 ng/mL (B) and substantial decrease of plaques at 500 ng/mL (C) and 5000 ng/mL (D). Live-cell imaging showed the removal of Alexa Fluor 555–conjugated gantenerumab (E and H; scale bars, 20 μm) by a migrating microglia cell adjacent to amyloid deposits depicted at start (E-G; scale bars, 20 μm) and 12 hours (H-J; scale bars, 20 μm). An example of a removed part of gantenerumab-stained amyloid is indicated by an arrowhead, and an example of newly phagocytosed gantenerumab within the migrating microglia cell is indicated by an arrow. Differential interference contrast (F and I) and merged (G and J) images are shown to follow the movement of the microglia cell (small arrowheads) and the intracellular uptake of gantenerumab at amyloid deposits over the incubation period.

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