INTRODUCTION:Positron emission tomography (PET) is a valuable tool for assessing tau pathology in Alzheimer's disease (AD), but it is not widely accessible. Clarifying the relationship between fluid tau species and tau PET in AD may allow for the identification of fluid biomarkers that could serve as more accessible surrogates for tau PET. METHODS:Cerebrospinal fluid (CSF) and plasma tau species levels were assessed across prodromal to moderate AD subjects (CSF: n = 53, plasma: n = 181) via immunoassays and liquid chromatography-mass spectrometry. Fluid measures were correlated with [18F]GTP1 tau PET at baseline for head-to-head comparisons across analytes. RESULTS:CSF C2N-eMTBR-tau243, tau phosphorylated at threonine 205 (p-tau205), p-tau217, and a peptide from the tau-441 microtubule-binding region (MTBR) (MTBR/243-254) exhibited the highest correlations with [18F]GTP1 standardized uptake value ratio. Plasma p-tau181 and p-tau217 demonstrated correlation rankings with tau PET similar to those observed with corresponding CSF analytes. DISCUSSION:CSF C2N-eMTBR-tau243, MTBR/243-254, p-tau205, and p-tau217 demonstrated comparable strong correlations with tau PET. These findings may guide future development of plasma biomarker surrogates for tau PET.

Neurofibrillary tangles (NFTs) are pathological hallmarks of Alzheimer's Disease (AD), consisting of aggregated tau protein. Tau is frequently used as a biomarker in AD clinical trials, as it provides diagnostic and prognostic information and serves as a pharmacodynamic marker to assess effects of tau-targeting treatments. Tau PET tracers specifically bind to NFTs, allowing for tau pathology imaging. However, PET imaging requires specialized infrastructure and is costly, limiting its widespread use in trials. Our study assessed the relationship between cerebrospinal fluid (CSF) soluble tau species and tau PET imaging. The aim was to identify fluid biomarkers that could serve as more accessible alternatives for assessing tau pathology in clinical trials. CSF tau species were measured at baseline in a subset of 53 prodromal-to-moderate AD participants enrolled in two anti-Tau Ph II trials ( n  = 26, n  = 27). Tau peptides were measured by data-independent acquisition mass spectrometry (DIA-MS). Total tau, pTau181, and pTau205 were measured using Elecsys immunoassays. N-term and Mid-domain tau peptides were measured by targeted LC-MS. pTau217 was measured using a Simoa assay and later with an Elecsys assay. Trial participants also completed [18F]GTP1 imaging. Standardized uptake value ratios (SUVR) were reported from the whole cortical gray matter and meta temporal regions using inferior cerebellar gray matter as the reference area. 46 pathology confirmed AD brains from the Arizona Study of Aging and Neurodegenerative Disorders at Banner Sun Health Research Institute were used to determine correlations between tau peptides and NFT burden in the fusiform gyrus. Tau peptides were measured by DIA-MS; NFT burden was measured by quantifying AT8-positive gray matter areas of adjacent brain sections. CSF and brain Tau species were correlated to [18F]GTP1 SUVR and %AT8-positive area, respectively. The analysis demonstrated CSF pTau205, pTau217, and a proteomic peptide from the 2N4R isoform MTBR region had highest correlations with [18F]GTP1 SUVR across both target regions. The same MTBR peptide was highly correlated with the %AT8 positive area in the fusiform gyrus. A proteomic peptide from the MTBR region was identified to be the highest correlated tau peptide in AD CSF and in the brain with tau pathology.

Tau tangle neuropathology can be visualized in living individuals using radioligands that bind specifically to aggregated tau. MTBR243 is a biomarker that has been found to be strongly associated with the presence of tau tangles in the brain. In this analysis we compared MTBR243 measured in CSF to Tau PET and clinical variables. MTBR243 was analyzed in 200 baseline CSF samples from study participants from two clinical trials in early and mild-to-moderate Alzheimer’s Disease (AD) (Tauriel, NCT03289143 and Lauriet, NCT03828747), including 182 participants who also had Tau PET data using [ 18 F]GTP1 tau PET tracer. All subjects were amyloid positive per study inclusion criteria. Tau PET results were analyzed as continuous variable using the Temporal Meta ROI SUVR and dichotomized as low/high based on Temporal Meta ROI SUVR of 1.33. Clinical measures at baseline included CDR-SB, ADAS-Cog11, ADCS-ADL, and MMSE. The CSF MTBR243 concentration had high correlation with Tau PET SUVR (Spearman’s ρ = 0.69, 95% CI 0.60-0.76). 69.8% were classified as high Tau PET in the participants who were analyzed for MTBR243 in CSF. The AUC-ROC of CSF MTBR243 for identifying Tau PET positivity was 0.89 (95% CI: 0.84-0.94). We found that both Tau PET and CSF MTBR243 correlated with all 4 clinical measures at baseline as well as change from baseline. The correlations for the two biomarkers with clinical measures were very similar. CSF MTBR243 correlates well with Tau PET in this amyloid positive population and discriminates well between low and high Tau PET patients. Tau PET and CSF MTBR243 have similar correlations to clinical variables, suggesting that CSF MTBR243 could be a suitable candidate as a fluid surrogate biomarker for Tau PET.

INTRODUCTION:Semorinemab, an anti-tau monoclonal antibody, was assessed in two Phase II trials for Alzheimer's disease (AD). Plasma and cerebrospinal fluid (CSF) biomarkers provided insights into the drug's potential mechanism of action. METHODS:Qualified assays were used to measure biomarkers of tau, amyloidosis, glial activity, neuroinflammation, synaptic function, and neurodegeneration from participant samples in Tauriel (NCT03289143) and Lauriet (NCT03828747) Phase II trials. RESULTS:Plasma phosphorylated Tau 181 (pTau181) and CSF chitinase-3-like protein 1 (YKL-40) increased following semorinemab treatment in both studies. In Lauriet, increasing plasma glial fibrillary protein (GFAP) concentrations stabilized with semorinemab, while this was not observed in Tauriel. Other AD pathophysiology biomarkers showed no consistent response to semorinemab. DISCUSSION:Increases in CSF YKL-40 suggest that semorinemab may stimulate microglia activation in the presence of AD-associated Tau pathology, but not in healthy controls. Stabilization of plasma GFAP in Lauriet indicates a possible impact on reactive gliosis in mild-to-moderate AD. TRIAL REGISTRATION:Tauriel ClinicalTrials.gov Identifier: NCT03289143. Lauriet ClinicalTrials.gov Identifier: NCT03828747. Phase 1 ClinicalTrials.gov Identifier: NCT02820896. HIGHLIGHTS:AD pathophysiology biomarkers were measured to assess the mechanism of action. Semorinemab increased CSF YKL-40 in participants with AD but not in healthy controls. Semorinemab possibly stabilized plasma GFAP in the Lauriet trial. Semorinemab treatment may activate microglia and moderate reactive gliosis.

INTRODUCTION:Growing evidence suggests a role for neuroinflammation in Alzheimer's disease (AD). We investigated complement pathway activity in AD patient cerebrospinal fluid (CSF) and evaluated its modulation by the anti-tau antibody semorinemab. METHODS:Immunoassays were applied to measure CSF complement proteins C4, factor B (FB), C3 and their cleavage fragments C4a, C3a, and factor Bb (Bb) in AD patients and a separate cognitively unimpaired (CU) cohort. RESULTS:All measured CSF complement proteins were increased in AD versus CU subjects, with C4a displaying the most robust increase. Finally, semorinemab did not have a significant pharmacodynamic effect on CSF complement proteins. DISCUSSION:Elevated levels of CSF C4a, C4, C3a, C3, Bb, and FB are consistent with complement activation in AD brains. Despite showing a reduction in CSF soluble tau species, semorinemab did not impact complement protein levels or activity. Further studies are needed to determine the value of complement proteins as neuroinflammation biomarkers in AD. HIGHLIGHTS:Cerebrospinal fluid (CSF) complement proteins C4a, C3a, Bb, C4, C3, and factor B levels were increased in Alzheimer's disease (AD) patients compared to a separate cognitively unimpaired (CU) cohort. Baseline CSF complement protein levels were correlated with neuro-axonal degeneration and glial activation biomarkers in AD patients. The investigational anti-tau antibody semorinemab did not impact CSF complement protein levels or activity relative to the placebo arm.

INTRODUCTION:Triggering receptor expressed on myeloid cells 2 (TREM2) agonists are being clinically evaluated as disease-modifying therapeutics for Alzheimer's disease. Clinically translatable pharmacodynamic (PD) biomarkers are needed to confirm drug activity and select the appropriate therapeutic dose in clinical trials. METHODS:We conducted multi-omic analyses on paired non-human primate brain and cerebrospinal fluid (CSF), and stimulation of human induced pluripotent stem cell-derived microglia cultures after TREM2 agonist treatment, followed by validation of candidate fluid PD biomarkers using immunoassays. We immunostained microglia to characterize proliferation and clustering. RESULTS:We report CSF soluble TREM2 (sTREM2) and CSF chitinase-3-like protein 1 (CHI3L1/YKL-40) as PD biomarkers for the TREM2 agonist hPara.09. The respective reduction of sTREM2 and elevation of CHI3L1 in brain and CSF after TREM2 agonist treatment correlated with transient microglia proliferation and clustering. DISCUSSION:CSF CHI3L1 and sTREM2 reflect microglial TREM2 agonism and can be used as clinical PD biomarkers to monitor TREM2 activity in the brain. HIGHLIGHTS:CSF soluble triggering receptor expressed on myeloid cells 2 (sTREM2) reflects brain target engagement for a novel TREM2 agonist, hPara.09. CSF chitinase-3-like protein 1 reflects microglial TREM2 agonism. Both can be used as clinical fluid biomarkers to monitor TREM2 activity in brain.

Structure and function of therapeutic antibodies can be modulated by a variety of post-translational modifications (PTM). Tyrosine (Tyr) sulfation is a type of negatively charged PTM that occurs during protein trafficking through the Golgi. In this study, we discovered that an anti-interleukin (IL)-4 human IgG1, produced by transiently transfected HEK293 cells, contained a fraction of unusual negatively charged species. Interestingly, the isolated acidic species exhibited a two-fold higher affinity to IL-4 and a nearly four-fold higher potency compared to the main species. Mass spectrometry (MS) showed the isolated acidic species possessed an +80-Dalton from the expected mass, suggesting an occurrence of Tyr sulfation. Consistent with this hypothesis, we show the ability to control the acidic species during transient expression with the addition of Tyr sulfation inhibitor sodium chlorate or, conversely, enriched the acidic species from 30% to 92% of the total antibody protein when the IL-4 IgG was co-transfected with tyrosylprotein sulfotransferase genes. Further MS and mutagenesis analysis identified a Tyr residue at the light chain complementarity-determining region-1 (CDRL-1), which was sulfated specifically. These results together have demonstrated for the first time that Tyr sulfation at CDRL-1 could modulate antibody binding affinity and potency to a human immune cytokine.

Background and ObjectivesAccumulation of tau pathology in Alzheimer disease (AD) correlates with cognitive decline. Anti-tau immunotherapies were proposed as potential interventions in AD. While antibodies targeting N-terminal tau failed to demonstrate clinical efficacy in prodromal-to-mild AD, their utility at other disease stages was not evaluated in prior studies. Lauriet is a phase 2 study of an anti-tau monoclonal antibody, semorinemab, in patients with mild-to-moderate AD.MethodsThe phase 2 Lauriet study included a randomized, placebo-controlled, double-blind period, during which participants with mild-to-moderate AD received 4,500 mg of IV semorinemab or placebo every 4 weeks for 48 or 60 weeks. Participants who chose to continue in the subsequent optional open-label extension received 4,500 mg of semorinemab every 4 weeks for up to 96 weeks. Coprimary efficacy endpoints were change from baseline to week 49 or 61 on the 11-item version of the Alzheimer's Disease Assessment Scale-Cognitive Subscale (ADAS-Cog11) and the Alzheimer's Disease Cooperative Study-Activities of Daily Living (ADCS-ADL) scale. Secondary efficacy endpoints included change from baseline on the Mini-Mental State Examination (MMSE) and Clinical Dementia Rating-Sum of Boxes (CDR-SB). Safety, pharmacokinetics, and pharmacodynamic effects were also evaluated.ResultsBetween December 3, 2018, and February 27, 2020, 624 individuals were screened, 272 participants were randomized, and 238 were included in the modified intent-to-treat population (received >= 1 dose(s) of study medication and underwent baseline and >= 1 postbaseline assessment(s)). Baseline characteristics were well balanced. At week 49, the semorinemab arm demonstrated a 42.2% reduction (-2.89 points, 95% CI -4.56 to -1.21, p = 0.0008) in decline on the ADAS-Cog11 (coprimary endpoint) relative to the placebo arm. However, no treatment effects were observed on the ADCS-ADL scale (coprimary endpoint; absolute difference between the 2 treatment arms in the ADCS-ADL score change from baseline of -0.83 points, 95% CI -3.39 to 1.72, p = 0.52) or on the MMSE or CDR-SB (secondary endpoints). Semorinemab was safe and well tolerated.DiscussionBased on the results of the prespecified coprimary endpoints, this study was negative. While semorinemab had a significant effect on cognition measured by the ADAS-Cog11, this effect did not extend to improved functional or global outcomes. These results may warrant further exploration of semorinemab or other anti-tau therapies in mild-to-moderate AD.Classification of EvidenceThis study provides Class I evidence that semorinemab does not slow functional decline in patients with mild-to-moderate AD.Trial Registration InformationThe Lauriet study is registered on ClinicalTrials.gov, NCT03828747, and EudraCT 2018-003398-87.

The Tauriel Study (NCT03289143) was a Phase 2 randomized, double‐blind, placebo‐controlled, parallel‐group clinical trial that assessed the safety and efficacy of semorinemab in patients with prodromal‐to‐mild Alzheimer’s disease. We examined the concordance of fluid tau biomarkers with Tau PET for the identification of patients with high tau burden at enrollment.