Immunohistochemical evaluation of a trial of gantenerumab or solanezumab in dominantly inherited Alzheimer disease

Abstract

Clinical trials of anti-amyloid-β (Aβ) monoclonal antibodies in Alzheimer disease (AD) infer target engagement from Aβ positron emission tomography (PET) and/or fluid biomarkers such as cerebrospinal fluid (CSF) Aβ42/40. However, these biomarkers measure brain Aβ deposits indirectly and/or incompletely. In contrast, neuropathologic assessments allow direct investigation of treatment effects on brain Aβ deposits-and on potentially myriad 'downstream' pathologic features. From a clinical trial of anti-Aβ monoclonal antibodies in dominantly inherited AD (DIAD), in the largest study of its kind, we measured immunohistochemistry area fractions (AFs) for Aβ deposits (10D5), tauopathy (PHF1), microgliosis (IBA1), and astrocytosis (GFAP) in 10 brain regions from 10 trial cases-gantenerumab (n = 4), solanezumab (n = 4), placebo/no treatment (n = 2)-and 10 DIAD observational study cases. Strikingly, in proportion to total drug received, Aβ deposit AFs were significantly lower in the gantenerumab arm versus controls in almost all areas examined, including frontal, temporal, parietal, and occipital cortices, anterior cingulate, hippocampus, caudate, putamen, thalamus, and cerebellar gray matter; only posterior cingulate and cerebellar white matter comparisons were non-significant. In contrast, AFs of tauopathy, microgliosis, and astrocytosis showed no differences across groups. Our results demonstrate with direct histologic evidence that gantenerumab treatment in DIAD can reduce parenchymal Aβ deposits throughout the brain in a dose-dependent manner, suggesting that more complete removal may be possible with earlier and more aggressive treatment regimens. Although AFs of tauopathy, microgliosis, and astrocytosis showed no clear response to partial Aβ removal in this limited autopsy cohort, future examination of these cases with more sensitive techniques (e.g., mass spectrometry) may reveal more subtle 'downstream' effects.

Keywords: Alzheimer disease; Anti-amyloid-β monoclonal antibodies; CSF; Clinical trial; Digital pathology; PiB PET.

Fig. 1

An exemplar imaging-neuropathology comparison. (a) Aβ PET imaging from a participant in the gantenerumab arm (top row, DIAN-TU-001 Case #5 in Table 1) and a participant in the placebo arm (bottom row, DIAN-TU-001 Case #6 in Table 1) showing annualized increases (red) and decreases (blue) in SUVR over the course of the trial in the caudate and putamen (here shown near the coronal plane of the anterior commissure). (b) Corresponding coronal slices from the participants’ brain donations after formalin-fixation. Differences in ventricularvolume between antemortem imaging and postmortem photography are due to ex vivo fixation. (c) Digitized sixmicron histology sections representing the caudate and putamen from both participants, batch-stained with standard DAB immunohistochemistry for Aβ (10D5 antibody). The red square indicates the location of the detail shown in the next panel. (d) Detail of the 10D5 staining. Scale bars are 250 μm

Fig. 2

Longitudinal change in Aβ PET and CSF biomarkers. (a) Longitudinal change in regional Aβ PET SUVRs in the gantenerumab arm (blue, n=4), control group (black, n=7), and solanezumab arm (red, n=3). The transparency of each dot corresponds to the total drug received by the time of the final Aβ PET (darker colors represent higher doses; exact dose values can be found in Table 1). (b) Longitudinal change in CSF Aβ42/40, ptau181, and t-tau in the gantenerumab arm (blue, n=4), control group (black, n=7), and solanezumab arm (red, n=4). The transparency of each dot corresponds to the total drug received by the time of the final CSF (darker colors represent higher doses; exact dose values can be found in Table 1). Note that there are no participants from the solanezumab group (n=4) with CSF Aβ42/40 measurements available for this study. Also note that five participants from the control group did not have longitudinal CSF measurements, so, for all comparisons, there is an n=7 for the control group. Asterisks denote p-values < 0.05 (*) associated with the t-value of the Gant:Time interaction term in the linear mixed-effects model Outcome~Drug*Time+(1|Case); no Sola:Time interaction was significant. Abbreviations: Gant=Gantenerumab, Sola=Solanezumab

Fig. 3

Aβ deposits and Gantenerumab dose-response. (a) Regional Aβ area fractions in the gantenerumab arm (blue, n=4), control group (black, n=12), and solanezumab arm (red, n=4). The transparency of each datapoint corresponds to the total drug dose received at that point in time (darker colors represent higher doses; exact dose values can be found in Table 1). For thalamus, data were available from only three participants from the solanezumab arm. †One participant from the gantenerumab arm with comparatively very high levels of Aβ deposition in the caudate and putamen (over 1.5 times the interquartile range above the third quartile) was treated as an outlier for the statistical comparison versus controls in the caudate and putamen. (b) Detail of digitized 10D5 slides of the caudate (location approximates the red square in Fig. 1) from the four gantenerumab-treated participants sorted from lowest total drug received (top) to highest (bottom). (c) Detail of digitized 10D5 slides of the caudate from four participants from the control group, approximately matched to the gantenerumab-treated participants on the basis of family mutation [49] and age at death (Supplementary Table 1). Scale bars are 250 μm. Whole-mount images of slides from which photomicrographs in b and c were taken are shown in Supplementary Fig. 1. Asterisks denote p-values < 0.05 (*) associated with Welch two-sample t-tests comparing Aβ area fractions between the control group and the gantenerumab arm or the solanezumab arm. Abbreviations: Gant=gantenerumab, Sola=solanezumab. Gantenerumab dose response at (d) final Aβ PET visit and (e) postmortem Aβ (10D5) neuropathologic assessment (inset shows results with outliers removed). The transparency of each datapoint corresponds to the total drug dose received at that point in time (darker colors represent higher doses). Asterisks denote p-values < 0.05 (*) associated with the t-value of the Dose term in the linear mixed-effects model Outcome~Dose+(1|Case), where Outcome is Aβ PET SUVR or Aβ area fraction, Dose is a fixed effect term indicating the cumulative dose of gantenerumab received for each case, and Case is a random intercept term