Evaluation of partial volume correction and analysis of longitudinal [18F]GTP1 tau PET imaging in Alzheimer's disease using linear mixed-effects models.

Purpose: We evaluated the impact of partial volume correction (PVC) methods on the quantification of longitudinal [18F]GTP1 tau positron-emission tomography (PET) in Alzheimer's disease and the suitability of describing the tau pathology burden temporal trajectories using linear mixed-effects models (LMEM). Methods: We applied van Cittert iterative deconvolution (VC), 2-compartment, and 3-compartment, and the geometric transfer matrix plus region-based voxelwise methods to data acquired in an Alzheimer's disease natural history study over 18 months at a single imaging site. We determined the optimal PVC method by comparing the standardized uptake value ratio change (%ΔSUVR) between diagnostic and tau burden-level groups and the longitudinal repeatability derived from the LMEM. The performance of LMEM analysis for calculating %ΔSUVR was evaluated in a natural history study and in a multisite clinical trial of semorinemab in prodromal to mild Alzheimer's disease by comparing results to traditional per-visit estimates. Results: The VC, 2-compartment, and 3-compartment PVC methods had similar performance, whereas region-based voxelwise overcorrected regions with a higher tau burden. The lowest within-subject variability and acceptable group separation scores were observed without PVC. The LMEM-derived %ΔSUVR values were similar to the per-visit estimates with lower variability. Conclusion: The results indicate that the tested PVC methods do not offer a clear advantage or improvement over non-PVC images for the quantification of longitudinal [18F]GTP1 PET data. LMEM offers a robust framework for the longitudinal tau PET quantification with low longitudinal test-retest variability. Clinical trial registration: NCT02640092 and NCT03289143.

Cross-sectional and longitudinal assessments of function in prodromal-to-mild Alzheimer's disease: A comparison of the ADCS-ADL and A-IADL-Q scales.

Introduction: Prior observational work in a heterogeneous cohort of participants with mild cognitive impairment suggests the Amsterdam Instrumental Activities of Daily Living Questionnaire (A-IADL-Q) may have greater sensitivity for functional decline than the more established Alzheimer's Disease Cooperative Study-Activities of Daily Living (ADCS-ADL) scale. However, the relative utility of the A-IADL-Q versus the ADCS-ADL for clinical trials in early Alzheimer's disease (AD) remains uncertain. Methods: We compared baseline and longitudinal performance of the A-IADL-Q and ADCS-ADL in participants with biomarker-confirmed prodromal (pAD; n = 158) or mild (mAD; n = 283) AD enrolled in the 18-month Tauriel study of semorinemab (NCT03289143). Results: The A-IADL-Q exhibited numerically stronger discrimination between pAD and mAD participants at baseline per Cohen's d analyses and similar sensitivity to longitudinal decline across cohorts over 18 months relative to the ADCS-ADL. Discussion: The comparable performance of the ADCS-ADL and A-IADL-Q supports the utility of the A-IADL-Q in early AD clinical trials. Highlights: The Amsterdam Instrumental Activities of Daily Living Questionnaire (A-IADL-Q) may be more sensitive than the Alzheimer's Disease Cooperative Study-Activities of Daily Living Scale (ADCS-ADL) for distinguishing prodromal and mild Alzheimer's disease (AD).A-IADL-Q and ADCS-ADL are similarly sensitive to decline in early AD over 18 months.Comparable performance of these indices supports A-IADL-Q use in future AD trials.Additional AD clinical trial data could extend findings across more diverse cohorts.

Safety and Efficacy of Semorinemab in Individuals With Prodromal to Mild Alzheimer Disease: A Randomized Clinical Trial.

Importance: Neurofibrillary tangles composed of aggregated tau protein are one of the neuropathological hallmarks of Alzheimer disease (AD) and correlate with clinical disease severity. Monoclonal antibodies targeting tau may have the potential to ameliorate AD progression by slowing or stopping the spread and/or accumulation of pathological tau. Objective: To evaluate the safety and efficacy of the monoclonal anti-tau antibody semorinemab in prodromal to mild AD. Design, Setting, and Participants: This phase 2 randomized, double-blind, placebo-controlled, parallel-group clinical trial was conducted between October 18, 2017, and July 16, 2020, at 97 sites in North America, Europe, and Australia. Individuals aged 50 to 80 years (inclusive) with prodromal to mild AD, Mini-Mental State Examination scores between 20 and 30 (inclusive), and confirmed ß-amyloid pathology (by positron emission tomography or cerebrospinal fluid) were included. Interventions: During the 73-week blinded study period, participants received intravenous infusions of placebo or semorinemab (1500 mg, 4500 mg, or 8100 mg) every 2 weeks for the first 3 infusions and every 4 weeks thereafter. Main Outcomes and Measures: The primary outcomes were change from baseline on the Clinical Dementia Rating-Sum of Boxes score from baseline to week 73 and assessments of the safety and tolerability for semorinemab compared with placebo. Results: In the modified intent-to-treat cohort (n = 422; mean [SD] age, 69.6 [7.0] years; 235 women [55.7%]), similar increases were seen on the Clinical Dementia Rating-Sum of Boxes score in the placebo (n = 126; Δ = 2.19 [95% CI, 1.74-2.63]) and semorinemab (1500 mg: n = 86; Δ = 2.36 [95% CI, 1.83-2.89]; 4500 mg: n = 126; Δ = 2.36 [95% CI, 1.92-2.79]; 8100 mg: n = 84; Δ = 2.41 [95% CI, 1.88-2.94]) arms. In the safety-evaluable cohort (n = 441), similar proportions of participants experienced adverse events in the placebo (130 [93.1%]) and semorinemab (1500 mg: 89 [88.8%]; 4500 mg: 132 [94.7%]; 8100 mg: 90 [92.2%]) arms. Conclusions and Relevance: In participants with prodromal to mild AD in this randomized clinical trial, semorinemab did not slow clinical AD progression compared with placebo throughout the 73-week study period but did demonstrate an acceptable and well-tolerated safety profile. Additional studies of anti-tau antibodies may be needed to determine the clinical utility of this therapeutic approach. Trial Registration: ClinicalTrials.gov Identifier: NCT03289143.

Site-Specific Cerebrospinal Fluid Tau Hyperphosphorylation in Response to Alzheimer's Disease Brain Pathology: Not All Tau Phospho-Sites are Hyperphosphorylated.

BACKGROUND: Understanding patterns of association between CSF phosphorylated tau (p-tau) species and clinical disease severity will aid Alzheimer's disease (AD) diagnosis and treatment. OBJECTIVE: To evaluate changes in tau phosphorylation ratios to brain imaging (amyloid PET, [18F]GTP1 PET, and MRI) and cognition across clinical stages of AD in two different cohorts. METHODS: A mass spectrometry (MS)-based method was used to evaluate the relationship between p-tau/tau phosphorylation ratios on 11 sites in CSF and AD pathology measured by tau PET ([18F]GTP1) and amyloid PET ([18F]florbetapir or [18F]florbetaben). Cohort A included cognitively normal amyloid negative (n = 6) and positive (n = 5) individuals, and amyloid positive prodromal (n = 13), mild (n = 12), and moderate AD patients (n = 10); and Cohort B included amyloid positive prodromal (n = 24) and mild (n = 40) AD patients. RESULTS: In this cross-sectional analysis, we identified clusters of phosphosites with different profiles of phosphorylation ratios across stages of disease. Eight of 11 investigated sites were hyperphosphorylated and associated with SUVR measures from [18F]GTP1 and amyloid PET. Novel sites 111, 153, and 208 may be relevant biomarkers for AD diagnosis to complement tau hyperphosphorylation measures on previously established sites 181, 205, 217, and 231. Hypophosphorylation was detected on residues 175, 199, and 202, and was inversely associated with [18F]GTP1 and amyloid PET. CONCLUSION: Hyperphosphorylated and hypophosphorylated forms of tau are associated with AD pathologies, and due to their different site-specific profiles, they may be used in combination to assist with staging of disease.

Baseline [18F]GTP1 tau PET imaging is associated with subsequent cognitive decline in Alzheimer's disease.

BACKGROUND: The role and implementation of tau PET imaging for predicting subsequent cognitive decline in Alzheimer's disease (AD) remains uncertain. This study was designed to evaluate the relationship between baseline [18F]GTP1 tau PET and subsequent longitudinal change across multiple cognitive measures over 18 months. METHODS: Our analyses incorporated data from 67 participants, including cognitively normal controls (n = 10) and ß-amyloid (Aß)-positive individuals ([18F] florbetapir Aß PET) with prodromal (n = 26), mild (n = 16), or moderate (n = 15) AD. Baseline measurements included cortical volume (MRI), tau burden ([18F]GTP1 tau PET), and cognitive assessments [Mini-Mental State Examination (MMSE), Clinical Dementia Rating (CDR), 13-item version of the Alzheimer's Disease Assessment Scale-Cognitive Subscale (ADAS-Cog13), and Repeatable Battery for the Assessment of Neuropsychological Status (RBANS)]. Cognitive assessments were repeated at 6-month intervals over an 18-month period. Associations between baseline [18F]GTP1 tau PET indices and longitudinal cognitive performance were assessed via univariate (Spearman correlations) and multivariate (linear mixed effects models) approaches. The utility of potential prognostic tau PET cut points was assessed with ROC curves. RESULTS: Univariate analyses indicated that greater baseline [18F]GTP1 tau PET signal was associated with faster rates of subsequent decline on the MMSE, CDR, and ADAS-Cog13 across regions of interest (ROIs). In multivariate analyses adjusted for baseline age, cognitive performance, cortical volume, and Aß PET SUVR, the prognostic performance of [18F]GTP1 SUVR was most robust in the whole cortical gray ROI. When AD participants were dichotomized into low versus high tau subgroups based on baseline [18F]GTP1 PET standardized uptake value ratios (SUVR) in the temporal (cutoff = 1.325) or whole cortical gray (cutoff = 1.245) ROIs, high tau subgroups demonstrated significantly more decline on the MMSE, CDR, and ADAS-Cog13. CONCLUSIONS: Our results suggest that [18F]GTP1 tau PET represents a prognostic biomarker in AD and are consistent with data from other tau PET tracers. Tau PET imaging may have utility for identifying AD patients at risk for more rapid cognitive decline and for stratification and/or enrichment of participant selection in AD clinical trials. Trial registration ClinicalTrials.gov NCT02640092 . Registered on December 28, 2015.

Cerebrospinal fluid tau fragment correlates with tau PET: a candidate biomarker for tangle pathology.

To date, there is no validated fluid biomarker for tau pathology in Alzheimer's disease, with contradictory results from studies evaluating the correlation between phosphorylated tau in CSF with tau PET imaging. Tau protein is subjected to proteolytic processing into fragments before being secreted to the CSF. A recent study suggested that tau cleavage after amino acid 368 by asparagine endopeptidase (AEP) is upregulated in Alzheimer's disease. We used immunoprecipitation followed by mass spectrometric analyses to evaluate the presence of tau368 species in CSF. A novel Simoa® assay for quantification of tau368 in CSF was developed, while total tau (t-tau) was measured by ELISA and the presence of tau368 in tangles was evaluated using immunohistochemistry. The diagnostic utility of tau368 was first evaluated in a pilot study (Alzheimer's disease = 20, control = 20), then in a second cohort where the IWG-2 biomarker criteria were applied (Alzheimer's disease = 37, control = 45), and finally in a third cohort where the correlation with 18F-GTP1 tau PET was evaluated (Alzheimer's disease = 38, control = 11). The tau368/t-tau ratio was significantly decreased in Alzheimer's disease (P < 0.001) in all cohorts. Immunohistochemical staining demonstrated that tau fragments ending at 368 are present in tangles. There was a strong negative correlation between the CSF tau368/t-tau ratio and 18F-GTP1 retention. Our data suggest that tau368 is a tangle-enriched fragment and that the CSF ratio tau368/t-tau reflects tangle pathology. This novel tau biomarker could be used to improve diagnosis of Alzheimer's disease and to facilitate the development of drug candidates targeting tau pathology. Furthermore, future longitudinal studies will increase our understanding of tau pathophysiology in Alzheimer's disease and other tauopathies.

Cross-sectional associations between [18F]GTP1 tau PET and cognition in Alzheimer's disease.

The regional relationships between tau positron emission tomography (PET) imaging and cognitive impairment in Alzheimer's disease (AD) remain uncertain. We examined cross-sectional associations between cognitive performance, cerebral uptake of the novel tau PET tracer [18F]GTP1, and other neuroimaging indices ([18F]florbetapir amyloid PET, magnetic resonance imaging) in 71 participants with normal cognition, prodromal AD, or AD dementia. Greater [18F]GTP1 uptake was seen with increasing clinical severity and correlated with poorer cognition. [18F]GTP1 uptake and cortical volume (but not [18F]florbetapir uptake) were independently associated with cognitive performance, particularly within the temporal lobe. Delayed memory was more specifically associated with temporal [18F]GTP1 uptake; other domains correlated with a broader range of regional [18F]GTP1 uptake. These data confirm that [18F]GTP1 tau PET uptake significantly correlates with cognitive performance in AD, but regional correlations between performance in non-memory cognitive domains were less specific than reported by tau PET imaging studies that included participants with atypical focal cortical AD syndromes. Tau PET imaging may have utility as a surrogate biomarker for clinical AD progression in therapeutic trials of disease-modifying interventions.