A critical appraisal of blood-based biomarkers for Alzheimer's disease.

Biomarkers that predict the clinical onset of Alzheimer's disease (AD) enable the identification of individuals in the early, preclinical stages of the disease. Detecting AD at this point may allow for more effective therapeutic interventions and optimized enrollment for clinical trials of novel drugs. The current biological diagnosis of AD is based on the AT(N) classification system with the measurement of brain deposition of amyloid-ß (Aß) ("A"), tau pathology ("T"), and neurodegeneration ("N"). Diagnostic cut-offs for Aß1-42, the Aß1-42/Aß1-40 ratio, tau and hyperphosphorylated-tau concentrations in cerebrospinal fluid have been defined and may support AD clinical diagnosis. Blood-based biomarkers of the AT(N) categories have been described in the AD continuum. Cross-sectional and longitudinal studies have shown that the combination of blood biomarkers tracking neuroaxonal injury (neurofilament light chain) and neuroinflammatory pathways (glial fibrillary acidic protein) enhance sensitivity and specificity of AD clinical diagnosis and improve the prediction of AD onset. However, no international accepted cut-offs have been identified for these blood biomarkers. A kit for blood Aß1-42/Aß1-40 is commercially available in the U.S.; however, it does not provide a diagnosis, but simply estimates the risk of developing AD. Although blood-based AD biomarkers have a great potential in the diagnostic work-up of AD, they are not ready for the routine clinical use.

Monitoring synaptic pathology in Alzheimer's disease through fluid and PET imaging biomarkers: a comprehensive review and future perspectives.

Alzheimer's disease (AD) is currently constrained by limited clinical treatment options. The initial pathophysiological event, which can be traced back to decades before the clinical symptoms become apparent, involves the excessive accumulation of amyloid-beta (Aß), a peptide comprised of 40-42 amino acids, in extraneuronal plaques within the brain. Biochemical and histological studies have shown that overaccumulation of Aß instigates an aberrant escalation in the phosphorylation and secretion of tau, a microtubule-binding axonal protein. The accumulation of hyperphosphorylated tau into intraneuronal neurofibrillary tangles is in turn correlated with microglial dysfunction and reactive astrocytosis, culminating in synaptic dysfunction and neurodegeneration. As neurodegeneration progresses, it gives rise to mild clinical symptoms of AD, which may eventually evolve into overt dementia. Synaptic loss in AD may develop even before tau alteration and in response to possible elevations in soluble oligomeric forms of Aß associated with early AD. These findings largely rely on post-mortem autopsy examinations, which typically involve a limited number of patients. Over the past decade, a range of fluid biomarkers such as neurogranin, α-synuclein, visinin-like protein 1 (VILIP-1), neuronal pentraxin 2, and ß-synuclein, along with positron emission tomography (PET) markers like synaptic vesicle glycoprotein 2A, have been developed. These advancements have facilitated the exploration of how synaptic markers in AD patients correlate with cognitive impairment. However, fluid biomarkers indicating synaptic loss have only been validated in cerebrospinal fluid (CSF), not in plasma, with the exception of VILIP-1. The most promising PET radiotracer, [11C]UCB-J, currently faces significant challenges hindering its widespread clinical use, primarily due to the necessity of a cyclotron. As such, additional research geared toward the exploration of synaptic pathology biomarkers is crucial. This will not only enable their extensive clinical application, but also refine the optimization process of AD pharmacological trials.

Integrative metabolomics science in Alzheimer's disease: Relevance and future perspectives.

Alzheimer's disease (AD) is determined by various pathophysiological mechanisms starting 10-25 years before the onset of clinical symptoms. As multiple functionally interconnected molecular/cellular pathways appear disrupted in AD, the exploitation of high-throughput unbiased omics sciences is critical to elucidating the precise pathogenesis of AD. Among different omics, metabolomics is a fast-growing discipline allowing for the simultaneous detection and quantification of hundreds/thousands of perturbed metabolites in tissues or biofluids, reproducing the fluctuations of multiple networks affected by a disease. Here, we seek to critically depict the main metabolomics methodologies with the aim of identifying new potential AD biomarkers and further elucidating AD pathophysiological mechanisms. From a systems biology perspective, as metabolic alterations can occur before the development of clinical signs, metabolomics - coupled with existing accessible biomarkers used for AD screening and diagnosis - can support early disease diagnosis and help develop individualized treatment plans. Presently, the majority of metabolomic analyses emphasized that lipid metabolism is the most consistently altered pathway in AD pathogenesis. The possibility that metabolomics may reveal crucial steps in AD pathogenesis is undermined by the difficulty in discriminating between the causal or epiphenomenal or compensatory nature of metabolic findings.

Plasma ATN(I) classification and precision pharmacology in Alzheimer's disease.

Evaluating potential therapies for Alzheimer's disease (AD) depends on use of biomarkers for appropriate subject selection and monitoring disease progression. Biomarkers that predict onset of clinical symptoms are particularly important for AD because they enable intervention before irreversible neurodegeneration occurs. The amyloid-ß-tau-neurodegeneration (ATN) classification system is currently used as a biological staging model for AD and is based on three classes of biomarkers evaluating amyloid-ß (Aß), tau pathology and neurodegeneration or neuronal injury. Promising blood-based biomarkers for each of these categories have been identified (Aß42/Aß40 ratio, phosphorylated tau, neurofilament light chain), and this matrix is now being expanded toward an ATN(I) system, where "I" represents a neuroinflammatory biomarker. The plasma ATN(I) system, together with APOE genotyping, offers a basis for individualized evaluation and a move away from the classic "one size fits all" approach toward a biomarker-driven individualisation of therapy for patients with AD.

Investigational treatments for neurodegenerative diseases caused by inheritance of gene mutations: lessons from recent clinical trials.

We reviewed recent major clinical trials with investigational drugs for the treatment of subjects with neurodegenerative diseases caused by inheritance of gene mutations or associated with genetic risk factors. Specifically, we discussed randomized clinical trials in subjects with Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis bearing pathogenic gene mutations, and glucocerebrosidase-associated Parkinson's disease. Learning potential lessons to improve future therapeutic approaches is the aim of this review. Two long-term, controlled trials on three anti-ß-amyloid monoclonal antibodies (solanezumab, gantenerumab and crenezumab) in subjects carrying Alzheimer's disease-linked mutated genes encoding for amyloid precursor protein or presenilin 1 or presenilin 2 failed to show cognitive or functional benefits. A major trial on tominersen, an antisense oligonucleotide designed to reduce the production of the huntingtin protein in subjects with Huntington's disease, was prematurely interrupted because the drug failed to show higher efficacy than placebo and, at highest doses, led to worsened outcomes. A 28-week trial of tofersen, an antisense oligonucleotide for superoxide dismutase 1 in patients with amyotrophic lateral sclerosis with superoxide dismutase 1 gene mutations failed to show significant beneficial effects but the 1-year open label extension of this study indicated better clinical and functional outcomes in the group with early tofersen therapy. A trial of venglustat, a potent and brain-penetrant glucosylceramide synthase inhibitor, in Parkinson's disease subjects with heterozygous glucocerebrosidase gene mutations revealed worsened clinical and cognitive performance of patients on the enzyme inhibitor compared to placebo. We concluded that clinical trials in neurodegenerative diseases with a genetic basis should test monoclonal antibodies, antisense oligonucleotides or gene editing directed against the mutated enzyme or the mutated substrate without dramatically affecting physiological wild-type variants.

Role of monomeric amyloid-ß in cognitive performance in Alzheimer's disease: Insights from clinical trials with secretase inhibitors and monoclonal antibodies.

According to the ß-amyloid (Aß) hypothesis of Alzheimer's disease (AD), brain Aß accumulation is the primary cascade event leading to cognitive deficit and dementia. Numerous anti-Aß drugs either inhibiting production or aggregation of Aß or stimulating its clearance have failed to show clinical benefit in large scale AD trials, with ß- and γ-secretase inhibitors consistently worsening cognitive and clinical decline. In June 2021, the FDA approved aducanumab, an anti-Aß monoclonal antibody for early AD based on its ability to reduce brain amyloid plaques, while two other amyloid-clearing antibodies (lecanemab and donanemab) have recently produced encouraging cognitive and clinical results. We reviewed AD trials using PubMed, meeting abstracts and ClinicalTrials.gov and evaluated the effects of such drugs on cerebrospinal fluid (CSF) Aß levels, correlating them with cognitive effects. We found that ß-secretase and γ-secretase inhibitors produce detrimental cognitive effects by significantly reducing CSF Aß levels. We speculate that monoclonal antibodies targeting Aß protofibrils, fibrils or plaques may improve cognitive performance in early AD by increasing soluble Aß levels through Aß aggregate disassembly and/or stabilization of existing Aß monomers.These findings suggest that the real culprit in AD may be decreased levels of soluble monomeric Aß due to sequestration into brain Aß aggregates and plaques.

Plasma Amyloid-ß dynamics in late-life major depression: a longitudinal study.

Depressed individuals are twice as likely to develop Alzheimer's disease (AD) as compared to controls. Brain amyloid-ß (Aß) deposition is believed to have a major role in AD pathogenesis but studies also suggest associations of Aß dynamics and depression. The aim of this study was to test if plasma Aß levels are longitudinally associated to late-life depression. We measured plasma levels of amyloid-ß1-40 (Aß40) and amyloid-ß1-42 (Aß42) peptides longitudinally for three consecutive years in 48 cognitively intact elderly subjects with late-life major depressive disorder (LLMD) and 45 age-matched cognitively healthy controls. We found that the Aß42/Aß40 plasma ratio was significantly and steadily lower in depressed subjects compared to controls (p < 0.001). At screening, Aß42/Aß40 plasma did not correlate with depression severity (as measured with Hamilton Depression Scale) or cognitive performance (as measured with Mini-Mental State Examination) but was associated to depression severity at 3 years after adjustment for age, education, cognitive performance, and antidepressants use. This study showed that reduced plasma Aß42/Aß40 ratio is consistently associated with LLMD diagnosis and that increased severity of depression at baseline predicted low Aß42/Aß40 ratio at 3 years. Future studies are needed to confirm these findings and examine if the consistently lower plasma Aß42/Aß40 ratio in LLMD reflects increased brain amyloid deposition, as observed in AD subjects, and an increased risk for progressive cognitive decline and AD.

A critical appraisal of tau-targeting therapies for primary and secondary tauopathies.

INTRODUCTION: Primary tauopathies are neurological disorders in which tau protein deposition is the predominant pathological feature. Alzheimer's disease is a secondary tauopathy with tau forming hyperphosphorylated insoluble aggregates. Tau pathology can propagate from region to region in the brain, while alterations in tau processing may impair tau physiological functions. METHODS: We reviewed literature on tau biology and anti-tau drugs using PubMed, meeting abstracts, and ClnicalTrials.gov. RESULTS: The past 15 years have seen >30 drugs interfering with tau aggregation, processing, and accumulation reaching the clinic. Initial results with tau aggregation inhibitors and anti-tau monoclonal antibodies have not shown clinical efficacy. DISCUSSION: The reasons for these clinical failures are unclear but could be linked to the clearing of physiological forms of tau by non-specific drugs. Research is now concentrating efforts on developing reliable translational animal models and selective compounds targeting specific tau epitopes, neurotoxic tau aggregates, and post-translational tau modifications.

What have we learned from past failures of investigational drugs for Alzheimer's disease?

INTRODUCTION: In the last 15 years, huge efforts against Alzheimer's disease (AD) with drugs targeting ß-amyloid (Aß) and tau have produced poor clinical results. Aducanumab, a recently FDA-approved anti-Aß monoclonal antibody has been greeted with distrust by most experts, hospitals and insurance companies for its level of efficacy and poor tolerability. AREA COVERED: We reviewed literature on Alzheimer trials using PubMed, meeting abstracts and ClnicalTrials.gov and discuss what we can learn from past failures of investigational drugs for Alzheimer's disease, especially anti-Aß and anti-tau drugs. EXPERT OPINION: It is our opinion that previous failures of anti-AD drugs suggest that soluble Aß and tau are not appropriate drug targets. In addition, pivotal clinical trials of future clinical candidates should avoid major protocol amendments and futility analyses. Study protocols should adopt better measures to protect study blinding and minimize the potential introduction of major biases in the evaluation of clinical results. Finally, alternative biological targets should be pursued as well as more multimodal approaches to addressing neurodegeneration in AD.

Evidence of upregulation of the cholinergic anti-inflammatory pathway in late-life depression.

BACKGROUND: Decreased cholinergic tone associated with increased proinflammatory cytokines has been observed in several human diseases associated with low-grade inflammation. We examined if this attenuated cholinergic anti-inflammatory pathway (CAP) mechanism contributed to increased neuroinflammation observed in depression. METHODS: We measured cerebrospinal fluid (CSF) cholinergic markers (AChE and BChE activities) in 28 individuals with longstanding late-life major depression (LLMD) and 19 controls and their relationship to central and peripheral levels of pro-inflammatory cytokines (IL-6 and IL-8). Additionally, we examined if these cholinergic indices were related to CSF markers of microglial activation and neuroinflammation (sTREM2 and complement C3). RESULTS: Compared with controls, LLMD patients had a significant reduction in CSF BChE levels. Lower CSF BChE and AChE activities were associated with lower CSF markers of microglial and neuroinflammation (sTREM2 and C3). In addition, in LLMD patients we found an inverse relationship between peripheral marker of inflammation (plasma IL-6) and CSF BChE and AChE levels. CONCLUSIONS: Our results suggest an upregulation of the CAP mechanism in LLMD with an elevation in peripheral markers of inflammation and concomitant reduction in markers of glial activation associated with a higher cholinergic tone. Future studies should confirm these findings in a larger sample including individuals with acute and more severe depressive episodes and across all ages.

Accelerating Alzheimer's disease drug discovery and development: what's the way forward?

Introduction: As the global burden of Alzheimer's disease (AD) grows, an effective disease-modifying therapy remains a distant prospect following the repeated failure of multiple therapeutics targeting ß-amyloid and (it seems) tau over many years of costly effort. The repeated failure of single-target therapies to meaningfully modify disease progression raises major questions about the validity of many aspects of drug development in this area, especially target selection.Area covered: The authors explore the critical questions raised by a review of the collective experience to date, relating to why findings with non-clinical models and clinical biomarkers so frequently fail to translate to positive outcomes in clinical trials, which alternatives should be considered, and how we can design and conduct clinical trials that can successfully identify and quantify meaningful benefits in the future.Expert opinion: It is our opinion that we must recognize and accept the need to consider less specific, more multimodal approaches to addressing neurodegeneration in AD if we are to make progress - and we must avoid repeating the well intentioned, but ultimately erroneous, assumptions of the past.

Can Anti-ß-amyloid Monoclonal Antibodies Work in Autosomal Dominant Alzheimer Disease?

The dominant theory of Alzheimer disease (AD) has been that amyloid-ß (Aß) accumulation in the brain is the initial cause of the degeneration leading to cognitive and functional deficits. Autosomal dominant Alzheimer disease (ADAD), in which pathologic mutations of the amyloid precursor protein (APP) or presenilins (PSENs) genes are known to cause abnormalities of Aß metabolism, should thus offer perhaps the best opportunity to test anti-Aß drugs. Two long-term preventive studies (Dominantly Inherited Alzheimer Network Trials Unit Adaptive Prevention Trial [DIAN-TU-APT] and Alzheimer Preventive Initiative-ADAD) were set up to evaluate the efficacy of monoclonal anti-Aß antibodies (solanezumab, gantenerumab, and crenezumab) in carriers of ADAD, but the results of the DIAN-TU-APT study have shown that neither solanezumab nor gantenerumab slowed cognitive decline in 144 subjects with ADAD followed for 4 years, despite one of the drugs (gantenerumab) significantly affected biomarkers relevant to their intended mechanism of action. Surprisingly, solanezumab significantly accelerated cognitive decline of both asymptomatic and symptomatic subjects. These failures further undermine the Aß hypothesis and could support the suggestion that ADAD is triggered by accumulation of other APP metabolites, rather than Aß.

Pharmacological management of dementia with Lewy bodies with a focus on zonisamide for treating parkinsonism.

INTRODUCTION: Dementia with Lewy bodies (DLB) has no approved symptomatic or disease-modifying treatments in the US and Europe, despite being the second most common cause of neurodegenerative dementia. AREAS COVERED: Herein, the authors briefly review the DLB drug development pipeline, providing a summary of the current pharmacological intervention studies. They then focus on the anticonvulsant zonisamide, a benzisoxazole derivative with a sulfonamide group and look at its value for treating parkinsonism in DLB. EXPERT OPINION: Several new compounds are being tested in DLB, the most innovative being those aimed at decreasing brain accumulation of α-synuclein. Unfortunately, new drug testing is challenging in terms of consistent diagnostic criteria and lack of reliable biomarkers. Few randomized controlled trials (RCTs) are well-designed, with enough power to detect significant drug effects. Levodopa monotherapy can treat the parkinsonism in DLB, but it can cause agitation or visual hallucination worsening. Two Phase II/III RCTs of DLB patients recently reported a statistically significant improvement in motor function in those receiving zonisamide as an adjunctive treatment to levodopa. New biomarker strategies and validated outcome measures for DLB or prodromal DLB may enhance clinical trial design for the development of specific disease-modifying treatments.

Discontinued disease-modifying therapies for Alzheimer's disease: status and future perspectives.

INTRODUCTION: Alzheimer's disease (AD) is the main cause of dementia and represents a huge burden for patients, carers, and healthcare systems. Extensive efforts for over 20 years have failed to find effective disease-modifying drugs. Although amyloid-ß (Aß) accumulation in the brain predicts cognitive decline, effective reduction of plaque load by numerous drug candidates has not yielded significant clinical benefits. A similar pattern is now emerging for drugs which target hyperphosphorylated tau, and trials with anti-inflammatory drugs have been negative despite neuroinflammation appearing to have a crucial role in AD pathogenesis. AREAS COVERED: This article reviews key drugs that have been discontinued while in development for AD and delineates the future landscape for present and alternative approaches. EXPERT OPINION: Anti-Aß drugs have failed to validate the Aß cascade hypothesis of AD. Early findings suggest that the same is happening with therapeutics targeting tau and focussing future research solely on anti-tau drugs is inappropriate. Alternative targets should be pursued, including apolipoprotein E, immunomodulation, plasma exchange, protein autophagy and clearance, mitochondrial dysfunction, abnormal glucose metabolism, neurovascular unit support, epigenetic dysregulation, synaptic loss and dysfunction, microbiota dysbiosis, and combination therapies. Meanwhile, repurposing of drugs approved for other indications is justified where scientific rationale and robust preclinical evidence exist.

A Path Toward Precision Medicine for Neuroinflammatory Mechanisms in Alzheimer's Disease.

Neuroinflammation commences decades before Alzheimer's disease (AD) clinical onset and represents one of the earliest pathomechanistic alterations throughout the AD continuum. Large-scale genome-wide association studies point out several genetic variants-TREM2, CD33, PILRA, CR1, MS4A, CLU, ABCA7, EPHA1, and HLA-DRB5-HLA-DRB1-potentially linked to neuroinflammation. Most of these genes are involved in proinflammatory intracellular signaling, cytokines/interleukins/cell turnover, synaptic activity, lipid metabolism, and vesicle trafficking. Proteomic studies indicate that a plethora of interconnected aberrant molecular pathways, set off and perpetuated by TNF-α, TGF-ß, IL-1ß, and the receptor protein TREM2, are involved in neuroinflammation. Microglia and astrocytes are key cellular drivers and regulators of neuroinflammation. Under physiological conditions, they are important for neurotransmission and synaptic homeostasis. In AD, there is a turning point throughout its pathophysiological evolution where glial cells sustain an overexpressed inflammatory response that synergizes with amyloid-ß and tau accumulation, and drives synaptotoxicity and neurodegeneration in a self-reinforcing manner. Despite a strong therapeutic rationale, previous clinical trials investigating compounds with anti-inflammatory properties, including non-steroidal anti-inflammatory drugs (NSAIDs), did not achieve primary efficacy endpoints. It is conceivable that study design issues, including the lack of diagnostic accuracy and biomarkers for target population identification and proof of mechanism, may partially explain the negative outcomes. However, a recent meta-analysis indicates a potential biological effect of NSAIDs. In this regard, candidate fluid biomarkers of neuroinflammation are under analytical/clinical validation, i.e., TREM2, IL-1ß, MCP-1, IL-6, TNF-α receptor complexes, TGF-ß, and YKL-40. PET radio-ligands are investigated to accomplish in vivo and longitudinal regional exploration of neuroinflammation. Biomarkers tracking different molecular pathways (body fluid matrixes) along with brain neuroinflammatory endophenotypes (neuroimaging markers), can untangle temporal-spatial dynamics between neuroinflammation and other AD pathophysiological mechanisms. Robust biomarker-drug codevelopment pipelines are expected to enrich large-scale clinical trials testing new-generation compounds active, directly or indirectly, on neuroinflammatory targets and displaying putative disease-modifying effects: novel NSAIDs, AL002 (anti-TREM2 antibody), anti-Aß protofibrils (BAN2401), and AL003 (anti-CD33 antibody). As a next step, taking advantage of breakthrough and multimodal techniques coupled with a systems biology approach is the path to pursue for developing individualized therapeutic strategies targeting neuroinflammation under the framework of precision medicine.

Development of disease-modifying drugs for frontotemporal dementia spectrum disorders.

Frontotemporal dementia (FTD) encompasses a spectrum of clinical syndromes characterized by progressive executive, behavioural and language dysfunction. The various FTD spectrum disorders are associated with brain accumulation of different proteins: tau, the transactive response DNA binding protein of 43 kDa (TDP43), or fused in sarcoma (FUS) protein, Ewing sarcoma protein and TATA-binding protein-associated factor 15 (TAF15) (collectively known as FET proteins). Approximately 60% of patients with FTD have autosomal dominant mutations in C9orf72, GRN or MAPT genes. Currently available treatments are symptomatic and provide limited benefit. However, the increased understanding of FTD pathogenesis is driving the development of potential disease-modifying therapies. Most of these drugs target pathological tau - this category includes tau phosphorylation inhibitors, tau aggregation inhibitors, active and passive anti-tau immunotherapies, and MAPT-targeted antisense oligonucleotides. Some of these therapeutic approaches are being tested in phase II clinical trials. Pharmacological approaches that target the effects of GRN and C9orf72 mutations are also in development. Key results of large clinical trials will be available in a few years. However, clinical trials in FTD pose several challenges, and the development of specific brain imaging and molecular biomarkers could facilitate the recruitment of clinically homogenous groups to improve the chances of positive clinical trial results.