Demystifying the Role of Neuroinflammatory Mediators as Biomarkers for Diagnosis, Prognosis, and Treatment of Alzheimer’s Disease: A Review

Abstract

Neuroinflammatory mediators play a pivotal role in the pathogenesis of Alzheimer’s Disease (AD), influencing its onset, progression, and severity. The precise mechanisms behind AD are still not fully understood, leading current treatments to focus mainly on managing symptoms rather than preventing or curing the condition. The amyloid and tau hypotheses are the most widely accepted explanations for AD pathology; however, they do not completely account for the neuronal degeneration observed in AD. Growing evidence underscores the crucial role of neuroinflammation in the pathology of AD. The neuroinflammatory hypothesis presents a promising new approach to understanding the mechanisms driving AD. This review examines the importance of neuroinflammatory biomarkers in the diagnosis, prognosis, and treatment of AD. It delves into the mechanisms underlying neuroinflammation in AD, highlighting the involvement of various mediators such as cytokines, chemokines, and ROS. Additionally, this review discusses the potential of neuroinflammatory biomarkers as diagnostic tools, prognostic indicators, and therapeutic targets for AD management. By understanding the intricate interplay between neuroinflammation and AD pathology, this review aims to help in the development of efficient diagnostic and treatment plans to fight this debilitating neurological condition. Furthermore, it elaborates recent advancements in neuroimaging techniques and biofluid analysis for the identification and monitoring of neuroinflammatory biomarkers in AD patients.

Alzheimer’s disease is a complex neurological condition marked by diminishing cognitive functions, memory deterioration, and diminished functional skills. It stands as the primary contributor to dementia, encompassing roughly 60–80% of dementia occurrences. Generally affecting the elderly population, particularly those aged 65 and above (10%), and even more prominently in individuals aged 85 and older (30%), the likelihood of developing AD escalates with advancing age.¹ Two characteristic neuropathological traits are common in AD pathology: Extracellular amyloid-beta (Aβ) plaques and hyperphosphorylated tau protein characteristically present in intracellular neurofibrillary tangles (NFTs).² Cognitive impairment is the result of neuronal loss, synaptic dysfunction, and other pathological alterations. These pathogenic alterations cause neurodegeneration, synaptic malfunction, and subsequently cognitive decline.

The clinical presentation of AD typically follows a progressive course, beginning with subtle memory impairment and eventually progressing to severe cognitive deficits and functional dependence. The disease has a huge impact not just on the individuals affected, but also on caretakers and healthcare systems throughout the world. Although the actual etiology of Alzheimer’s is still unknown, a range of variables are likely to be involved in both the onset and progression of the illness. The variables include environmental factors, different pathophysiological processes, such as neuroinflammation, protein misfolding, oxidation dysfunction and genetic susceptibility.³

An increasingly important role for neuroinflammation, which is characterized by stimulation of microglia and astrocytes and release of inflammatory chemicals in the pathogenesis of AD has been identified. Persistent neuroinflammation worsens neuronal harm and aids in the advancement of the disease by fostering the buildup of Aβ, hyperphosphorylation of tau proteins, and malfunctioning of synapses. Neuroinflammation in AD is generally linked to glial alterations in the brain. This intricate process involves microglial cells and astrocytes, which are associated with a cascade of inflammatory mediators and modulators. Increasing evidence indicates that the inflammatory process may begin in the early stages of AD, promoting the accumulation of insoluble Aβ and tau. While inflammatory changes are often viewed as a response to Aβ and tau pathologies, it is also possible that they trigger the deposition of these proteins in the brain. Additionally, early inflammation might offer protection against protein accumulation, which could explain the conflicting results seen in some studies. Consequently, biomarkers of neuroinflammation could be valuable for early diagnosis, prognosis, and identifying potential drug targets for the secondary prevention of AD.⁴

Comprehending the intricate interaction between neuroinflammation and the pathology of AD is vital for developing prognostic indicators, treatment approaches, and diagnostic instruments for the management of AD. Neuroinflammatory mediators play a crucial role in AD, but there are gaps in our understanding. This review aims to fill knowledge gaps in understanding neuroinflammatory mediators in AD by analyzing their roles as biomarkers, their contributions to disease pathology, and evaluating current and future therapeutic strategies. Additionally, the current review supports the development of more precise, individualized, and effective methods for diagnosing and treating AD, contributing to better patient outcomes and advancing neurodegenerative research. The review aims to clarify the mechanisms by which these mediators impact disease progression and explore how they may be used in therapeutic contexts to improve patient compliance. This comprehensive review will provide a better understanding of the problems and potential of using neuroinflammatory biomarkers in the management of AD.

Neuroinflammation and Alzheimer’s Disease

Alzheimer’s disease is marked by persistent neuroinflammation, a key player in disease onset and progression. Neuroinflammation leads to stimulation of immune cells in the Central Nervous System (CNS), notably microglia and astrocytes, alongside the production of inflammatory agents. This complex relationship between neuroinflammation and AD pathology worsens neuronal impairment and fuels cognitive deterioration.⁵ In AD, astrocytes, become activated and produce inflammatory mediators and cytokines in response to inflammation and neuronal damage, thereby intensifying the brain’s inflammatory response.⁶ Chronic neuroinflammation creates a persistent inflammatory state within the brain, contributing to neuronal damage and degeneration, as inflammatory mediators can directly harm neurons and disrupt synaptic function, leading to cognitive decline.⁷ Neuroinflammation interacts with the degenerative mechanisms of AD, such as the formation of Aβ plaques and hyperphosphorylated tau protein. Inflammatory mediators can stimulate the production and accumulation of Aβ and tau proteins, while these pathological proteins can activate microglia and astrocytes, sustaining the inflammatory response (Figure 1).

Figure 1.

(A) The Aβ Hypothesis in AD. (B) The tau hypotheses in AD. The Aβ is generated through cleavage of APP by β-secretase and γ-secretase, leading to the formation of Aβ plaques. Concurrently, hyperphosphorylated tau proteins assemble and form neurofibrillary tangles. β-secretase (a), γ-secretase (b). AD: Alzheimer’s disease; APP: amyloid precursor protein; Aβ: amyloid-β.

In severe AD, microglial dysfunction impairs the ability to effectively clear Aβ and other debris, resulting in the continued accumulation of harmful proteins and sustained neuroinflammation.⁸ Considering the importance of neuroinflammation in AD progression, there is ongoing exploration of targeting inflammatory pathways as potential therapeutic approaches, including modulating microglial activation, reducing pro-inflammatory cytokine levels, and repairing the integrity of the blood-brain barrier (BBB) to mitigate the detrimental effects of neuroinflammation on neuronal survival and function.⁹ In AD, the depolarisation of astrocytes and microglia stands out as key aspects of neuroinflammation. Initially, these cells react to pathological triggers in an effort to restore balance and minimize neuronal harm.¹⁰ However, prolonged and uncontrolled activation can lead to neurotoxic effects and further disease advancement. Thus, comprehending the mechanisms driving microglial and astrocytic activation, as well as their interplay, is essential for crafting therapeutic approaches to regulate neuroinflammation in AD and other neurological conditions.

Activation of Microglia and Astrocytes

Microglia, the immune cells native to the CNS, are instrumental in responding to neuronal injury and pathological stressors. In AD, microglia undergo chronic activation, adopting a pro-inflammatory state marked by the release of cytokines, chemokines, and ROS.¹¹ This sustained microglial activation contributes to neuroinflammation and exacerbates neuronal damage in AD. Microglial activation in AD is driven by various triggering factors such as injury, infection, neurodegeneration, and the presence of abnormal protein aggregates like beta-amyloid. Microglia can assume several activation states, traditionally categorized as M1 (pro-inflammatory) and M2 (pro-repair), with a shift toward the pro-inflammatory M1 phenotype commonly observed in AD. These cells play a crucial role in the phagocytosis of cellular debris, including the clearance of beta-amyloid plaques. However, during chronic neuroinflammation, the phagocytic capacity of microglia may become impaired, resulting in the accumulation of pathological proteins.¹² Similarly, astrocytes, another glial cell type within the CNS, undergo activation in response to AD pathology. Stimulated astrocytes release inflammatory cytokines and chemokines, exacerbating the inflammatory milieu within the brain. Furthermore, astrocyte dysfunction impairs their supportive roles, such as regulating neurotransmitter levels and maintaining synaptic integrity, further contributing to neurodegeneration in AD.

Astrocyte activation in response to CNS injury or inflammation involves reactive gliosis, characterized by morphological changes, increased glial protein activity, and functional modifications. Activated astrocytes release inflammatory mediators such as interleukin-1β (IL-1β), interleukin-6 (IL-6), chemokines, and prostaglandins, contributing to neuroinflammation.¹³ Despite this, astrocytes also perform neuroprotective functions, including maintaining homeostasis, providing metabolic support to neurons, and participating in tissue regeneration and repair. Furthermore, astrocytes interact with microglia through direct cell communication and the release of signaling molecules, influencing microglial activation and the overall inflammatory response in the CNS.¹⁴

Interplay between Aβ Accumulation and Tau Pathology

Aβ peptide accumulation and aggregation are influenced by neuroinflammation, a characteristic component of AD pathogenesis. Pro-inflammatory mediators produced by active microglia and astrocytes, which alter amyloid precursor protein processing and interfere with Aβ clearance mechanisms, promote the production of Aβ peptides. Furthermore, Aβ peptides can cause glial cells to become inflamed, which starts a vicious cycle of neuroinflammation and Aβ build-up.^15,16 Microglia and astrocytes play crucial roles in reducing Aβ build-up and tau pathology in AD, which is primarily caused by the degradation of amyloid precursor protein. However, during chronic neuroinflammation, microglia may exhibit dysfunctional phagocytosis, leading to impaired Aβ clearance and facilitating its aggregation into plaques.¹⁷ Additionally, pro-inflammatory mediators released by microglial stimulation and ROS, which can enhance Aβ synthesis and aggregation, develop a positive feedback loop between neuroinflammation and Aβ accumulation. Astrocytes also contribute to Aβ metabolism by secreting enzymes such as neprilysin and insulin-degrading enzyme involved in Aβ degradation, though their capacity may be insufficient to prevent its accumulation in AD. Activated astrocytes release inflammatory mediators that can influence Aβ production and aggregation, with chronic activation contributing to Aβ-induced neurotoxicity and synaptic dysfunction.¹⁸ Additionally, neuroinflammation exacerbates tau pathology in AD by promoting tau protein hyperphosphorylation and aggregation, which ultimately results in the formation of NFTs. Tau hyperphosphorylation can be directly induced by pro-inflammatory cytokines and ROS, which are generated during neuroinflammation. This can impair tau’s normal activity and encourage its aggregation into NFTs. The cognitive deficit associated with AD is exacerbated by tau pathology, which also exacerbates neuronal dysfunction and synaptic loss.¹⁹ Tau pathology exacerbates neuronal dysfunction, synaptic loss, and cognitive deficits associated with AD. Both astrocytes and microglia contribute to tau pathology; activated microglia can induce astrocytes to release cytokines and chemokines that further promote tau hyperphosphorylation. The interaction between Aβ and hyperphosphorylated tau contributes to neurotoxicity, synaptic functional loss, neuronal degradation, and cognitive deficits in AD.²⁰

Disruption of Neurotransmission and Synaptic Function

Chronic neuroinflammation disrupts neurotransmission and synaptic function, further exacerbating cognitive decline in AD. Cytokines produced during neuroinflammation reduce synaptic plasticity and neurotransmitter release, cause death of neurons. Additionally, neuroinflammation induces the production of excitotoxic molecules, including glutamate, which contribute to synaptic dysfunction and neurodegeneration.²¹

Microglial and astrocytic activation significantly impact synaptic function, particularly in the context of neuroinflammation. Microglia perform synaptic pruning, essential for neuronal network development, but excessive or dysregulated pruning during neuroinflammation can eliminate healthy synapses, leading to dysfunction.²² Activated microglia release inflammatory mediators like tumor necrosis factor-α, IL-1β, and ROS, directly impairing synaptic function and plasticity by affecting neurotransmitter release and receptor function. Additionally, microglia normally provide neurotrophic support through growth factor secretion, but this support is compromised during neuroinflammation, reducing neuronal survival and impairing synaptic maintenance.²³ Astrocytes also play a crucial role, particularly in regulating neurotransmitter levels like glutamate. Dysregulated glutamate uptake and clearance by astrocytes can cause excitotoxicity, neuronal damage, and synaptic dysfunction.²⁴ Activated astrocytes release inflammatory mediators, including cytokines, chemokines, and prostaglandins, directly impairing synaptic transmission and plasticity.²³ Moreover, astrocytes are involved in bidirectional communication with neurons through glio-transmission and neuroinflammation disrupts, this communication affecting synaptic function and neuronal excitability. The combined effects of chronic neuroinflammation lead to synaptic loss, a hallmark of neurodegenerative diseases like AD, significantly contributing to cognitive decline and impairment. Inflammatory mediators released by activated glial cells induce neurotoxicity, causing neuronal dysfunction and cell death, further exacerbating synaptic deficits. Additionally, prolonged neuroinflammation hampers synaptic plasticity, crucial for cognitive and motor functions, leading to further cognitive impairments.²⁵

Neuroinflammatory Biomarkers for Diagnosis of AD

The prompt intervention and effective treatment of AD depend heavily on an early and precise diagnosis. Neuroinflammatory biomarkers have become a more viable diagnostic tool for AD, offering insights into underlying pathogenic mechanisms and disease progression. In this section, we delve into the role of neuroinflammatory biomarkers, particularly biomarkers derived from blood and CSF for the diagnosis of AD.

Blood-Based Biomarkers

They offer several advantages for the diagnosis of AD, including accessibility, ease of collection, and potential for large-scale screening. They provide valuable information about systemic and CNS inflammation, reflecting underlying pathogenic processes in AD. Several blood-based biomarkers have been examined to determine their potential use in diagnosing AD, such as cytokines, chemokines, and acute-phase proteins.²⁶

In AD, patients exhibit higher levels of cytokines and chemokines, such as IL-1β, IL-6, TNF-α, and monocyte chemoattractant protein-1 in peripheral blood, compared to healthy controls. These elevated levels are linked to neuroinflammation and disease severity, with changes in the pro-inflammatory to anti-inflammatory cytokine ratio potentially serving as diagnostic markers for AD.²⁷ Acute-phase proteins, indicators of systemic inflammation like serum amyloid A and C-reactive protein, are also elevated in individuals with AD and correlate with cognitive decline and disease progression, making them useful for diagnosis and monitoring. Oxidative stress markers, such as malondialdehyde and 8-hydroxy-2′-deoxyguanosine, are found at higher concentrations in the peripheral blood of AD patients and are associated with synapse dysfunction, neuronal damage, and cognitive decline.²⁸ These markers provide insights into disease severity and progression, serving as potential diagnostic and prognostic indicators. Additionally, alterations in peripheral immune cell profiles, such as lymphocyte subsets and monocyte/macrophage populations, reflect a systemic immune response to AD pathology. Characterizing these profiles may provide valuable information about neuroinflammation and immune dysregulation in AD, aiding in disease diagnosis and monitoring.²⁹

Blood-based biomarkers offer valuable insights into neuroinflammation and systemic immune responses in AD. Chemokines, cytokines, markers of oxidative stress, and acute-phase proteins, and peripheral immune cell profiles are among the biomarkers investigated for their utility in AD diagnosis.³⁰ Incorporating these biomarkers into diagnostic algorithms may improve the accuracy and early detection of AD, facilitating timely intervention and personalized treatment strategies. While blood-based biomarkers offer advantages such as accessibility, ease of collection, and potential for large-scale screening, their use in routine AD diagnosis is still under investigation due to challenges in specificity and sensitivity. However, further validation and standardization of blood-based biomarkers are needed to establish their clinical utility in routine practice. This requires overcoming challenges such as variability in biomarker levels across different populations and the need for large-scale, multicenter studies to establish their clinical utility in routine practice.^31,32

Cerebrospinal Fluid Biomarkers

The perfect fluid biomarker should possess qualities such as reliability, reproducibility, noninvasiveness, disease specificity, simplicity, affordability, and suitability for large-scale implementation. Blood-based biomarkers fulfill these criteria and offer potential for screening individuals at risk of AD in primary healthcare settings. While cerebrospinal fluid (CSF) collection lacks noninvasiveness, it remains valuable in elucidating brain biochemical alterations during the preclinical phases of AD. Distinct markers, including reduced Aβ-42 levels and more total Tau and phospho-Tau concentrations in cerebrospinal fluid, hold predictive value for cognitive decline progression over time.^33,34

AD is a common neurodegenerative condition characterized by the abnormal metabolism and deposition of Aβ peptides, mainly Aβ-42. According to the amyloid cascade theory, soluble Aβ peptides aggregate into insoluble plaques, causing neuronal death and synaptic malfunction that ultimately results in dementia and cognitive decline. Brain interstitial fluid, CSF, and plasma are all filled with Aβ peptides that are produced by cleaving the amyloid precursor protein, with Aβ-40 being the most common isoform.^35,36 The disparity among Aβ development and removal, particularly of Aβ-42, is implicated in Alzheimer’s pathogenesis. While Aβ-40 is abundant in vascular amyloid, Aβ-42 predominates in senile plaques, exhibiting greater fibrillogenicity and hydrophobicity.³⁷ This pathological mechanism is supported by clinical, biochemical, and genetic evidence, underlining the significance of Aβ aggregation in AD progression.

The CSF Aβ-42 peptide, which originates from neurons, emerges as a vital diagnostic marker in AD detection. Reduced levels in CSF characterize Alzheimer’s patients, exhibiting lower quantities compared to healthy individuals. Meta-analytical findings suggest the potential for distinguishing Alzheimer’s cases from controls based on diminished CSF Aβ42 levels. Research indicates elevated Aβ-42 concentrations, particularly in the early and midstages of AD, offering valuable insights for differentiation from other neurodegenerative disorders.³⁸⁻⁴⁰ CSF Aβ42 proves instrumental in discerning Alzheimer’s from alternate conditions, aiding in predictive assessments of individuals with mild cognitive impairment (MCI) who are prone to developing the disease. Studies tracking disease progression underscore the prognostic significance of diminished CSF Aβ-42 levels in Alzheimer’s development. Research findings indicate that individuals diagnosed with MCI and exhibiting decreased CSF Aβ-42 levels tend to progress more rapidly to AD.^41,42

The CSF Aβ-40 peptide, a 40-amino acid residue, is a key component of amyloid plaques in AD. It interacts with Aβ-42, a key component of AD, enhancing its diagnostic accuracy.^26,43 The concentration of CSF Aβ-40 complements Aβ-42 levels, enhancing their interpretation. CSF biomarkers, such as Aβ-42/Aβ-40 and Aβ-38/Aβ-42 ratios, have a 51% sensitivity and 82% specificity for AD diagnosis. The Aβ-40/Aβ-42 ratio significantly enhances diagnostic accuracy, distinguishing AD from non-AD cases.⁴⁴ The Aβ-40/Aβ-42 ratio is highly effective in distinguishing AD from other dementia forms, especially during the prodromal phase when clinical diagnosis presents challenges. Other promising biomarkers include Aβ-42/AH-40 and Aβ-38/Aβ-42 for early AD diagnosis.^45,46 PSEN1 and BACE1 enzyme levels are often found to be elevated in the CSF of patients with MCI, a condition that can precede AD. However, the diagnostic value of BACE1 as a biomarker for AD is limited, particularly in individuals without the APOEε4 allele, which is a known genetic risk factor for AD. This suggests that while BACE1 may have a role in the pathological processes underlying AD, its effectiveness as a stand-alone biomarker is reduced in the absence of the APOEε4 genotype. Consequently, there is a need for more refined biomarker research that takes into account genetic variations, such as APOEε4 status, to enhance the predictive accuracy and utility of these enzymes in AD diagnosis and monitoring.⁴⁷

Examining neuroinflammation and synaptic impairment is crucial for identifying early indicators of AD. Certain markers of these processes may exhibit a more direct association with cognitive decline. Patients with MCI displayed markedly elevated levels of numerous proteins associated with vesicular transport, synapse development and stabilization, and immune function in their CSF in contrast to AD patients and healthy controls.^48,49 This trend was especially pronounced in MCI individual who transitioned to AD, underscoring the potential relevance of these protein levels as predictive indicators for progression of disease.⁵⁰ YKL-40 activity has been demonstrated to be more linked in several studies to an increased chance of developing MCI from normal cognitive state. Furthermore, a quicker transition from MCI to AD and an acceleration of cognitive deterioration have also been linked to increased Visinin like protein-1 levels.³³ These biomarkers hold promise in facilitating early detection and monitoring of neurodegenerative diseases.

Triggering receptor expressed on myeloid cells-2 (TREM2), interferon-gamma inducible protein of 10 kDa, and neurogranin are key proteins in controlling brain immune responses and inflammatory cytokines. Neurogranin, a synaptic protein, may help differentiate individuals with early stage symptomatic AD due to its association with synaptic dysfunction and cognitive impairment. These proteins could be therapeutic targets for regulating immune responses and improving cognition in neurodegenerative disorders.^51,52 Alterations in CSF lipid profiles and microRNAs (miRNAs), which regulate mature miRNA abundance, could serve as potential AD biomarkers. The brain is the primary site for over 70% of known miRNAs, and their expression in exosomes could be used as clinical diagnostic biomarkers. Dysregulation of specific miRNAs may elucidate the considerable downregulation of numerous brain miRNAs over time in regions vulnerable to AD progression. The miRNAs have the potential to target pathogenic AD-related genes, which suggests their possible use in future therapeutic strategies, though this application is still under research.^53,54

Imaging Biomarkers

Neurophysiological and brain imaging methods offer pathways to study AD’s structural and functional alterations. Neurophysiological markers including transcranial magnetic stimulation and event-related potentials, as well as quantitative electroencephalography, have shown promise in predicting the transition from mild MCI to AD and differentiating between dementia types. AD imaging modalities can be broadly categorized into structural and functional approaches.^55,56

Structural Imaging Approaches

The medial temporal lobe, demonstrates atrophy in AD patients. Magnetic resonance imaging (MRI) and computed tomography scans (CT) serve as the primary modalities for assessing brain anatomy in AD.⁵⁷ Both techniques effectively detect diffuse cerebral atrophy, characterized by ventricular enlargement and cortical sulci expansion. However, CT is not typically recommended for early diagnosis due to limitations such as reduced sensitivity to early changes, especially in uncooperative individuals, limited quantitative capabilities, and lower image quality. In contrast, MRI is more successful at identifying early structural alterations in the brain and provides superior image quality.⁵⁸ Recognized as the gold standard for assessing atrophy, MRI offers precise quantitative measurements and the ability to detect subtle differences in atrophy among AD patients.

Magnetic Resonance Imaging

MRI stands as the predominant technique in diagnosing AD, prized for its exceptional spatial resolution, capacity to discern subtle tissue variations, and noninvasive nature. It effectively identifies microstructural and biochemical brain alterations, notably hippocampal volume changes, recognized as a key AD biomarker for diagnosis and monitoring.⁵⁹ Nevertheless, the precise pathological mechanisms underlying these alterations remain elusive, possibly stemming from synaptic and dendritic losses.

Despite its utility, hippocampal volumetry exhibits limitations, notably in the prodromal AD stage, necessitating more sensitive MR-derived biomarkers for early disease detection. While individual case assessment poses challenges in clinical practice, large-scale research studies readily detect characteristic AD-related atrophy patterns.⁶⁰ Whole hippocampal volume assessment proves inadequate in preclinical AD due to conflicting findings from different analysis methods. Recent studies have adopted segmentation techniques targeting functionally distinct hippocampal subfields, revealing heightened sensitivity in detecting subtle atrophy. Surface deflections across these subfields emerge as promising biomarkers for distinguishing early AD from nondemented individuals.⁶¹

Various tools and methodologies, including manual tracing, automated Voxel-based morphometry, and segmentation based on brain atlases, support volumetric assessment and image analysis in AD research. These approaches leverage histological data and high-resolution MRI scans for accurate segmentation. In vivo AD diagnosis hinges on discerning an individual’s brain atrophy pattern, often reflected in a continuous AD score. Furthermore, structural MRI enables the identification of cortical thinning in the entorhinal cortex, which serves as an extremely responsive indicator of anatomical alterations in individuals with MCI and AD. Methodological challenges, such as susceptibility to motion artifacts and differences in spatial resolution, underscore the need for high-resolution scanning to accurately delineate neural volume or thickness loss.¹⁴

Diffusion Tensor Imaging (DTI)

DTI is used to quantify nonrandom diffusion of water molecules and map brain tract fibers. It is frequently used to look at the microstructural characteristics of white matter in the brain.⁶² Studies focusing on AD reveal alterations in fiber tracts, particularly in the corpus callosum, posterior temporal, and frontal lobes. These changes demonstrate a gradient pattern, with more pronounced alterations observed in posterior regions compared to anterior regions. Meta-analyses suggest DTI’s moderate capability in distinguishing AD from controls by analyzing limbic regions. DTI has also shown promise in predicting cognitive decline and differentiating between different stages of AD progression. Additionally, it could function as a possible sensor for monitoring treatment response in clinical trials targeting white matter integrity.⁶³

DTI holds promise in clinical trials for tracking responses to disease-modifying drugs and detecting early functional changes preceding structural abnormalities in disease progression.⁶⁴ Yet, its role in AD diagnosis remains nascent and necessitates further investigation. Technical constraints, such as interscanner variability and the struggle to handle complications within voxels, including fiber bending and crossing, hinder the clinical utility of DTI. Future prospects lie in employing high angular resolution diffusion imaging, facilitating accurate modeling of diffusion with an orientation distribution function, thus capturing multiple orientations within a voxel. This cutting-edge imaging method may offer more precise details on microstructural alterations in the brain, enabling an earlier and more precise diagnosis of AD. By overcoming current technical limitations, high angular resolution diffusion imaging may significantly enhance our understanding of the development of the disorder and eventually result in more potent therapeutic approaches.^65,66

Proton Magnetic Resonance Spectroscopy

Proton magnetic resonance spectroscopy (MRS), an imaging technique, holds promise as a biochemical indicator in AD. It assesses chemical concentrations in the brain, with notable compounds including N-acetyl aspartate, creatine and glutamate. Since 1992, studies have demonstrated a loss in N-acetyl aspartate, a neuronal metabolite, in autopsy brain samples from AD patients, correlating with brain tangle and plaque levels, indicative of neuroaxonal density and viability.⁵⁵ Additionally, MRS has revealed increased myo-inositol levels, a glial marker, in various brain regions of AD patients. Although choline has been investigated as a potential MRS biomarker, data remain inconsistent. MRS, being more accessible, cost-effective, and radiation-free, proves valuable for diagnosis and treatment. However, additional studies is necessary to standardize methods, contrast results with other biomarkers, and clarify the pathophysiological underpinnings of metabolite abnormalities.⁶⁷

Functional Imaging Approaches

Functional imaging approaches include recently discovered methods not yet widely adopted in standard clinical settings globally. Below are detailed explanations of several functional imaging approaches.

SPECT and Perfusion Imaging

Single-photon emission computed tomography (SPECT), employing highly focused radiotracers, is a molecular technique used to detect cellular and chemical alterations linked to a disease. It serves as a valuable tool in differentiating individuals with AD. When assessing cerebral perfusion, SPECT shows a strong association with modifications in metabolism. Perfusion hexamethyl propylene amine oxime SPECT is considered an alternative to fluorodeoxyglucose F-18 positron emission tomography (FDG-PET) for diagnosing AD, despite its lower sensitivity and specificity compared to FDG. Hypoperfusion patterns, typically affecting temporoparietal areas bilaterally, show anomalies on perfusion SPECT in AD, with the cerebellum and sensory motor cortices remaining unaffected. These abnormalities are mostly seen in the posterior cingulate and medial temporal areas. Studies on SPECT’s sensitivity and specificity in diagnosing AD show rates of 65–85% for AD and 72–87% for other dementias. Although SPECT is more cost-effective, accessible, and preferred by patients compared to FDG PET, it exhibits lower diagnostic accuracy. Consequently, it may be suitable for distinguishing between dementia and nondementia conditions but not for differentiating AD from other forms like DLB.⁶⁸⁻⁷¹

Positron Emission Tomography

Positron emission tomography imaging is an advanced molecular imaging technique that offers three-dimensional visualization of the brain, rendering it a precise diagnostic tool for AD. PET scanning can confirm or exclude the suspected etiology in patients with suspected AD who do not meet the National Institute on Aging–Alzheimer’s Association criteria for dementia but have a distinct clinical and cognitive profile. This facilitates a timely and accurate diagnosis while preventing the unnecessary use of overpriced and sometimes risky therapies.⁷²

A variety of tracers, including tau fibrillary tangle deposition, Aβ deposition, and synaptic dysfunctions. The most widely used PET ligands in AD have been shown to have significant diagnostic and predictive value. For example, 18F-FDG PET assesses brain glucose metabolism, which helps in preclinical dementia detection and early diagnosis. Across several brain areas, hypometabolism, which signals neuronal malfunction and synaptic illness, is consistently observed in typical AD patients.^73,74 In the meantime, amyloid imaging uses PET to take pictures of the brain that show aberrant amyloid buildup when a radio-labeled ligand that targets amyloid aggregates is injected. Despite the initial ligand, PiB, demonstrating potential, it has several drawbacks, including a brief half-life, reliance on onsite cyclotron and 11C radiochemistry knowledge, retention in the brains of nondemented individuals, strong affinity for vascular deposits, and restricted capacity to identify highly pathogenic soluble oligomeric Aβ conformations.⁷⁵

PET ligands enable SV2A-based visualization of synapse damage in AD.⁷⁶ UCB-J PET detects 40% reduced SV2A signal in AD hippocampus, suggesting potential combined biomarker utility. This method has been underutilized in longitudinally assessing synapse loss in AD, and is promising when combined with other biomarkers like CSF, MRI, or FDG PET imaging. Measuring synapse loss in AD involves direct quantification of synapse density, which can be complemented with other biomarkers found in CSF, MRI, or FDG PET imaging. In AD, tau imaging acts as a stand-in marker for cognitive impairment or disease progression. The creation of selective tau PET tracers has advanced since 2002, although initial iterations had drawbacks such as increased retention in the striatum.^61,77

Based on the recent investigation, various novel PET tracers have been identified for human studies. However, challenges persist due to the characteristics of tau intracellular aggregates and their high amyloid deposition, complicating clinical application.⁷⁸ Future PET tracers need to exhibit greater selectivity and affinity for tau over Aβ and must traverse the BBB without undergoing metabolism. Novel imaging methodologies for investigating AD include translocator protein (TSPO) PET, which is upregulated in neuroinflammation and implicated in neurodegeneration. Increased TSPO expression has been observed in AD animal models, correlating with brain pathological regions and areas of heightened TSPO immunohistochemical staining. Furthermore, imaging epigenetics, such as [11C] Martinostat PET, focuses on mechanisms regulating gene expression, particularly histone deacetylase.^79,80

Functional MRI

Functional MRI (fMRI) is a technique used in imaging that quantifies the amount of oxygen in specific brain regions in response to certain stimuli or cognitive tasks. Research has demonstrated that individuals with AD had less coordinated activity in cortical and hippocampal regions compared to those without the disease. fMRI may be used to investigate decreased inhibition and increased hippocampus activity while the subject is in a task-activated state, or it can be used to examine synaptic integrity and circuit connection when the subject is at rest.^56,81 Functional connectivity, as discerned through rsfMRI, serves as an early indicator of synaptic pathology, particularly evident in the hippocampus’s separation from cortical input. Task-activated fMRI studies have been beneficial in observing mild AD cases, revealing diminished hippocampal activity and predicting subsequent cognitive decline.⁸²

Neuroinflammatory Biomarkers as Therapeutic Targets for AD

NF-κB as Therapeutic Target for AD

Inflammation is promoted by the overexpression of adhesion molecules, chemokines, and pro-inflammatory cytokines, which are all brought on by the transcription factor nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Its connection to amyloid plaques emphasizes its part in neuroinflammation associated with AD. The two distinct phenotypes of macrophages, which include microglia, are M1 and M2. When activated, M1 phenotypes contribute to increased neuroinflammation. In AD, toll-like receptors (TLRs) on the cell surface can activate activated microglia, which are persistently active in an M1 phenotype.^83,84

Damage-associated molecular patterns like lipopolysaccharide (LPS) bind to TLR4, causing glial cells to adopt an M1 phenotype, making them more susceptible to inflammation. This leads to interferon-β production and activation of interleukin-1 receptor-associated kinase 1 (IRAK1).^85,86 MyD88-dependent TLR signaling activates Interleukin-1 receptor-associated kinase 1, TAK1, IKK, and IκB, allowing NF-κB to translocate into the nucleus and act as a transcription factor, inducing NLR family, pyrin domain containing (NLRP3) inflammasome and cytokine synthesis.^87,88Figure 2 illustrates the link between NF-κB and NLRP3, indicating that targeting NF-κB may enable the downregulation of inflammasome recruitment, thereby reducing neuroinflammation in AD patients and enhancing their cognitive performance.

Figure 2.

Illustration of how NLRP3, TREM2, cGAS-STING, and microglial NF-κB contribute to the development of AD. Neuronal death is triggered by Aβ by activating the NLRP3 inflammasome and NF-κB pathway. DAP12 becomes phosphorylated as a result of TREM2’s interaction with it, initiating a signaling cascade. In order to control neuroinflammation, cGAS-STING activates transcription factors for type I interferons and proinflammatory cytokines. Moreover, TLRs are involved in controlling the polarization of macrophages. Extended TLR4 activation increases pro-inflammatory cytokines and chemokines, which in turn exacerbate chronic inflammation.

One particularly interesting target for delaying the onset of AD is NF-κB, given its association with neurodegeneration in AD patients when activated. Strategies aiming to disrupt various steps of NF-κB signaling, including nuclear translocation, DNA interaction, phosphorylation, and IκB degradation, could offer therapeutic benefits. However, conventional NSAIDs have not yielded positive outcomes in AD treatment.^89,90 In the realm of other neuroinflammatory conditions, there is a burgeoning interest in natural compounds that inhibit inflammation via the NF-κB pathway. This avenue of research holds promise for identifying novel therapeutic targets with preventive or curative effects. Raising IL-10 and other anti-inflammatory cytokine levels has shown potential to enhance cognitive function in microglia.⁹¹ Phytochemicals such as curcumin and omega-3 polyunsaturated fatty acids show the ability to mitigate Aβ-induced neuroinflammation through NF-κB modulation. Curcumin inhibits NF-κB activity, lowering nitric oxide and prostaglandin E2 synthesis and thus resulting in improved cognitive function via a lower level of Aβ in transgenic animal models of AD.^92,93 Omega-3 fatty acids function as a ligand for PPAR-γ, indirectly influencing NF-κB expression.⁹⁴

Resveratrol, a compound found in Polygonum cuspidatum, exhibits the ability to downregulate NF-κB by thwarting Aβ-induced IκB phosphorylation, lowering the amount of pro-inflammatory cytokine released and upregulated IL-10 levels in microglia stimulated by LPS. Yet, its practical use in clinical settings might be restricted because of its inadequate oral bioavailability, which arises from substantial hepatic and intestinal clearance.^95,96

Despite setbacks observed in clinical trials involving phytochemicals, they remain a promising approach for the treatment of AD. These compounds could act as valuable scaffolds for the development of active analogues targeting NF-κB to mitigate neuroinflammation and forestall neurodegeneration. Other NF-κB inhibitors like MW01–2–069A-SRM have shown efficacy in suppressing proinflammatory cytokines and ameliorating synaptic dysfunction and behavioral deficits in AD mouse models.^97,98

NLRP3 as Therapeutic Target for AD

Pattern recognition receptors generate vast cytoplasmic polyprotein complexes called inflammasomes, which in turn cause inflammatory responses such as the production of IL-1β and IL-18 as well as pyroptosis.⁹⁹ NLRP3, the most extensively investigated, is linked to autoinflammatory diseases like gout, diabetes, obesity, and AD.¹⁰⁰ NLRP3 inflammasomes can be triggered by canonical and noncanonical signaling pathways, with caspases 4 and 5 in humans and caspase-11 in mice involved.¹⁰¹

The buildup of Aβ in neurotoxic plaques and insoluble fibrils results in neurodegeneration. Aβ oligomers and fibrils can trigger neuroinflammation in the brain by activating microglia. Persistent NLRP3 inflammasome signaling in microglia causes malfunction, limiting their ability to remove Aβ and NFTs. This leads to a vicious cycle of Aβ and tau buildup and microglial activation. Severe AD is associated with higher levels of microglial dysfunction than healthy control brains. In AD, tau pathology is closely linked to the NLRP3 inflammasome being activated.¹⁰²

Many studies have recently been conducted on the functional significance of the NLRP3 inflammasome in AD pathogenesis, with data indicating that direct inhibition of its activation or targeting critical components could be significant implications for AD prevention and therapy.¹⁰³ Memantine and AChE inhibitors are the only two drug classes that have been approved for AD treatment. Donepezil is a cheap AChE inhibitor that inhibits AKT/MAPK, NLRP3 inflammasome, and NF-κB/STAT3 signaling to diminish neuroinflammation brought on by LPS and Aβ MCC950, often referred to as CRID3, enhances cognitive performance and Aβ elimination.¹⁰⁴

Several additional medicines, including CY-09, OLT1177, and Oridonin, have demonstrated promising antineuroinflammation properties in NLRP3-related conditions. CY-09 inhibits ATP interaction alongside NLRP3 by binding directly to the NLRP3 Walker A motif, and it has been shown to protect mice against gout and type 2 diabetes.¹⁰⁵ By binding directly to and limiting the activity of NLRP3 ATPase, dipansutrile blocks the NLRP3 inflammasome from being activated.¹⁰⁶ Oridonin, produced from the herbal plant Rabdosia rubescent, reduces Aβ1–42-induced neuroinflammation by blocking the NF-κB pathway. It has been discovered that NSAIDs of the fenamate type preferentially block the NLRP3 inflammasome, hence preventing cognitive decline and lowering IL-1β levels.¹⁰⁷

It was recently shown that caspase activation and recruitment domain (CARD) and pyrin proteins function to prevent neurodegeneration and neuroinflammation.¹⁰⁸ POP1 inhibits apoptosis-associated speck-like protein (ASC) containing a CARD dependent inflammasome responses, decreasing systemic inflammation and autoinflammatory disorders by binding to ASC and preventing ASC- pattern recognition receptors interactions, while POP3 is crucial for regulating ALR inflammasomes in vivo in both human and murine macrophages. CARDs are renowned for their capacity to oligomerize and connect to one another; for example, CARD16 self-oligomerizes and interacts with ASC-CARD.¹⁰⁷ CARD18 has a contentious function in controlling inflammasome stimulation, with some research claiming it suppresses caspase-1 oligomerization and stimulation via CARD–CARD interactions.¹⁰⁹

TREM2 as Therapeutic Target for AD

Numerous genetic risk factors, such as TREM2 polymorphisms that increase AD risk 2–4 times, have been found by the genome-wide association studies (GWAS) on AD. Microglia are the primary source of expression for TREM2, which helps to maintain brain homeostasis by providing neurotrophic support and phagocytosing amyloid and cell neuronal debris. It is highly expressed in the hippocampal region, which is consistent with some clinical indications and symptoms of AD.¹¹⁰ TREM2 activates two signaling pathways: phagocytosis and anti-inflammatory actions. In vivo investigations with the amyloid mouse model demonstrate its critical role in microglial clustering around plaques, plaque compaction, and microglial proliferation.¹¹¹ Research indicates that knocking down TREM2 enhances Aβ-induced tau seeding and dissemination around plaques, while in PS19 mouse models, TREM2 loss decreases neuroinflammation despite its protective role against tau-mediated neurodegeneration.^52,112

Additional investigation is required to comprehend the diverse functions of TREM2 in microglia connection with the different clinical situations of AD. Anti-inflammatory compounds increase TREM2 activity, on the other hand, it is decreased by pro-inflammatory mediators such IL1β, LPS, tumor necrosis Factor-α, etc. TREM2 plays an anti-inflammatory role by reducing macrophage responses to TLR ligation and adversely regulating TLR-mediated dendritic cell maturation, antigen-specific T-cell proliferation and type I interferon responses.^113,114 A recent study revealed that the TREM2-mediated inflammatory response to reduce neuroinflammation and cognitive decline is associated with the PI3K/AKT/FoxO3a signaling pathway.^115,116 Various studies have focused on TREM2, a protein implicated in protecting neurons in AD. One approach involves administering recombinant sTREM2 into the brains of 5xFAD mice, leading to decreased Aβ levels and improved cognitive function. However, sTREM2 has been associated with tau pathology, suggesting that its therapeutic effects might involve the regulation of microglia.¹¹⁷

The primary goal is to reduce TREM2’s proteolytic shedding caused by disintegrin and metalloproteinases.¹¹⁸ This may have an effect on the production of soluble AβPPα that is neurotrophic and neuroprotective, and it may also promote the development of senile plaques. Researchers are looking into medications or antibodies that can specifically block TREM2 shedding while leaving other ADAM 10/17 substrates or sAPP cleavage unaffected.¹¹⁹ For this reason, several monoclonal antibodies have been prepared, AL002, a humanized monoclonal IgG1 antibody, demonstrated high tolerance and safety. NF-κB-sensitive miRNA-34a may have therapeutic applications for controlling innate immunological and phagocytic responses in neuroinflammation.¹²⁰ Subsequently it has been found that TREM2 activity is both up and downregulated in individuals with sporadic AD. It has been suggested that the mitochondrial chaperone heat shock protein 60 functions as a TREM2 agonist, promoting microglial phagocytosis and triggering TREM2 signaling. Since TREM2 modulates microglial pathogenesis in AD by acting downstream of CD33, inhibiting CD33 may potentially offer some protection. Through in vivo research with 5xFAD mice, it was shown that CD33 deletion ameliorated memory loss and decreased Aβ pathology, outcomes that TREM2 knockout reversed. Furthermore, the discovery of ligand mimetic peptides, such as COG1410, offers a brand-new AD treatment option.^121−123

cGAS-STING Pathway: A Possible Treatment Strategy for AD

The Cyclic GMP-AMP Synthase-Stimulator of Interferon Genes (cGAS-STING) pathway is a key mechanism that links pathogenic DNA detection to innate immune defense system activation. The STING not only detects pathogenic DNA and alerts the immune system to infectious pathogens, but it also works with cGAS to generate an interferon-based response that protects the host.^124,125 cGAS’s enzymatic activity is activated when it attaches to cytosolic double-stranded DNA. This process converts GTP and ATP into 2’3′-cyclic GMP-AMP (cGAMP), a cyclic dinucleotide.^126,127 The cGAMP then binds to the STING in the ER, activating it. Due to this conformational shift, the STING oligomer translocate to perinuclear areas where it interacts to TBK1. The Phosphorylation of IRF3 is then triggered by TBK1. Interferon master regulator type I. In the nucleus, IRF3 is also translocated, in which it promotes the production of Type-I interferons and other inflammatory cytokines by encouraging the transcription of genes that code for interferons and other cytokines.¹²⁸

It is believed that the cGAS-STING serves as a go-between the neuroinflammation that results in neurodegeneration when mitochondrial stress is activated.¹²⁹ Research conducted in vivo has demonstrated that the enzyme cGAS is expressed at the miRNAs level by both astrocyte and microglial cells, Moreover, microglia respond to IFN-β treatment by upregulating cGAS. Moreover, the cGAS-STING pathway triggers a natural immune response and may make a good target for the treatment of AD and tauopathy. This is initiated by the interaction of tau 3R/4R proteins with polyglutamine binding protein 1.^112,130,131

Multiple preclinical investigations have shown how does the cGAS-STING mechanism works in neurodegenerative disease-related neuroinflammation, indicating that inhibiting this approach may serve as an option for therapy. However, inhibiting cGAS-STING may also heighten the risk of acute infections and cancer may result from extended inhibition of the neuroinflammatory reaction.¹³² Therefore, the development of a safe and efficient STING modulator depends on identifying the ideal window of opportunity for STING inhibition and activation therapy. As of right now, preclinical research on cGAS-STING inhibitors particularly targets AD, but other therapeutic targets, such cGAMP and PQBP1, may require more confirmation before being considered as innovative pharmacological targets. The most recent study used raised inflammatory markers and decreased NAD+ levels. using APP/PS1 mutant transgenic mice as a model; however, these effects were reversed by administration of NAD+ precursor nicotinamide riboside, indicating that NAD+ supplementation may be a viable therapeutic strategy for treating AD through the cGAS-STING pathway.¹³³

Recent Advancements in Clinical and Preclinical Research for Managing Neuroinflammation in AD

The table. 1 outline different approach employed in preclinical and clinical studies to address neuroinflammation in neurodegenerative diseases, along with the reasons behind their use and citations to relevant clinical studies.

Table 1. Recent Breakthrough from Innovative Clinical Trial Approaches Targeting Neuroinflammation in AD.

Method	Rationale	Ref
SIRT1 activator	• Resveratrol CSF levels of matrix metalloproteinase-9	(134)
	• modulates neuroinflammation, and promotes adaptive immunity.
	• Activation of SIRT1 by resveratrol presents a promising target for the treatment
HClO scavenger	• Anserine counteracts neuroinflammatory responses associated with AD by scavenging HClO production.	(135)
P38 MAPK down regulator	• Neflamapimod, educe neuronal damage by inhibiting P38 MAPK activity,	(136)
	• leading to cytokine release p38a MAPK in neurons worsens p-tau
Microglial modulator(CHF5074)	• Reduces brain GSK-3b	(137)
	• Lowers hyperphosphorylated tau
	• Boosts synaptophysin levels.
Lysergic acid diethylamide	• Shows anti-inflammatory effects via 5-HT2A receptor signaling	(138)
Intranasal insulin	• Prevents inflammation in CSF,	(139)
	• Boosts immunity,
	• Enhances cerebral vascular health.
Insulin sensitizer (NE3107)	• Blocks inflammation-induced ERK and NF-kB activation	(140)
Plasma fraction	• Reduce brain aging,	(141)
	• Boosts cognition, promotes neurogenesis, increases synaptic density, and decreases neuroinflammation in the host.
Ginkgo biloba	• Neuroprotective effects	(142)
	• antioxidant
	• anti-inflammatory properties.
NLRP3 inflammasome inhibitor (MCC950)	• Inhibition of the NLRP3 inflammasome reduces neuroinflammation by preventing the release of pro-inflammatory cytokines	(143)
TREM2 Agonist (AL002) (NCT04592874)	• AL002 targets TREM2, modulating microglial response and enhancing phagocytic activity, which can reduce Aβ plaque.	(144)
TNF-α Inhibitor (XPro1595)	• XPro1595 selectively neutralizes soluble TNF-α without affecting transmembrane TNF or TNF receptors, reducing neuroinflammation and white matter degeneration.	(145)
CD33 Antibody (AL003)	• AL003 targets CD33, an inhibitory receptor on microglia, potentially reducing their dysfunctional response in AD and promoting a neuroprotective state.	(146)
PDE4 Inhibitors (Roflumilast):	• PDE4 inhibitors enhance cAMP levels, leading to reduced microglial activation and decreased production of pro-inflammatory cytokines.	(147,148)
P2 × 7 receptor antagonist (JNJ-54175446)	• Inhibition of P2 × 7 receptors reduces microglial activation and neuroinflammatory cytokine release, which are involved in AD progression.	(149)
NF-κB Pathway Inhibitors (Curcumin and Others)	• Various polyphenols and alkaloids inhibit the NF-κB pathway,	(83)
	• reducing neuroinflammation and amyloid genesis in AD models.

Resveratrol as a SIRT1 Stimulant

Activation of Sirtuin 1 (SIRT1) represents a promising avenue for addressing neuroinflammation associated with AD. The activity of SIRT1’s deacetylase function is regulated by the NAD+/NADH ratio, influencing epigenetic transcriptional mechanisms. Resveratrol, a polyphenolic compound present in red grapes and peanuts, serves as a powerful stimulant of SIRT1, mimicking the actions of caloric restriction. Resveratrol supplementation has been proven to mitigate behavioral deficits and reduce Aβ accumulation within the CNS as aging progresses. A retrospective analysis revealed that 52 weeks of resveratrol administration significantly decreased cerebrospinal fluid (CSF) levels of matrix metalloproteinase-9 (MMP-9). Furthermore, it was found to elevate plasma MMP10 levels, lower interleukin-12P40 (IL-12P40) levels, interleukin-12P70 levels, and regulate normal T cell production and secretion upon stimulation. These observations indicate that resveratrol could potentially alleviate CSF MMP9 levels, regulate neuroinflammation, and prompt adaptive immune responses ^134,.¹⁵⁰

Anserine as an HClO-Scavenger

Anserine which serves as hypochlorous acid (HClO) scavenger has been previously evaluated to preserve cognitive function in individuals with by Masuoka et al. For a period of 12 weeks, participants received 500 mg of anserine or a placebo every day. The active and placebo groups’ cognitive function differed significantly, and this difference became much more pronounced when daily anserine intake was taken into account. This implies that anserine could prevent cognitive impairment in older people with MCI.¹³⁵

Neflamapimod, an Inhibitor of P38 MAPK

Neflamapimod, a pharmacological agent inhibiting P38 MAPK, is presently undergoing clinical trials to suppress the activity of P38 MAPKs, which contribute to neuronal damage. Concurrently, two experimental drugs, Neflamapimod and MW150, are under investigation. The relationship between patients’ responsiveness to Neflamapimod medication and plasma phosphorylated tau at position 181 in DLB patients.¹⁵¹ According to their research, people who were below the p-tau181 criterion showed more improvements than people who were over the cutoff for all assessed outcomes.

In another study, Jiang et al. evaluated Neflamapimod in preclinical and randomized clinical settings for basal forebrain cholinergic degeneration in a different investigation. Two secondary end points, a dementia rating scale and functional mobility, showed benefits, whereas the main goal, a cognitive test battery, did not show any meaningful impacts.¹³⁶ A 24-week phase 2 clinical trial by Prins et al. evaluated the effectiveness of Neflamapimod, a P38 alpha kinase inhibitor, in patients with moderate AD. Their findings showed significant reductions in CSF levels of T-tau and p-tau181, particularly with respect to neurogranin, following Neflamapimod administration compared to placebo. Additionally, favorable patterns were observed on the Hopkins Verbal Learning Test-Revised and Wechsler Memory Scale among individuals with elevated trough plasma Neflamapimod levels.¹⁵²

CHF5074 as a Microglial Regulator

Sharma et al. conducted a 12-week examination to evaluate CHF5074, a microglial modulator, in terms of safety, tolerability, pharmacokinetics, and pharmacodynamics among 96 individuals diagnosed with MCI. Three groups in the trial were given different dosages of CHF5074 or a placebo. The results showed that over the 12-week titrated therapy period, with dosages up to 600 mg/day, CHF5074 was well-tolerated among MCI patients. Furthermore, CHF5074 demonstrated a dose-dependent impact on biomarkers of the CNS linked to neuroinflammation. Importantly, no significant differences were observed in neuropsychological assessments.^137,153

Lysergic Acid Diethylamide

A study conducted by Family et al. identified substantial anti-inflammatory properties associated with lysergic acid diethylamide (LSD), potentially positioning it as a therapeutic agent for addressing neuroinflammation linked to neurodegenerative conditions. The investigation involved 148 healthy elderly volunteers who were administered varying doses of LSD. Results indicated that LSD plasma concentrations were undetectable in the 5 mg dosage group, with no reported adverse events throughout the study. Additionally, the assessment revealed no discernible impairment in cognitive function, balance, or proprioception among the participants.¹³⁸ The study’s findings underscored the safety and tolerability profile of LSD, thereby advocating for its continued exploration in clinical settings for the improved management and prophylaxis against AD.⁵⁵

Intranasal Insulin

Initial findings from pilot clinical trials indicate that intranasal insulin may hold promise as a therapeutic intervention for AD. Phase II trial conducted in a cohort of patients with either MCI or AD which revealed a notable improvement in CSF biomarker profiles and a decelerated progression of symptoms when compared to a placebo cohort. The experiment evaluated modifications to CSF indicators linked to immunological response, vascular integrity, and inflammation. Results indicated that the group treated with insulin exhibited elevated levels of CSF interferon-γ and eotaxin, along with downregulated IL-6, for the 12-month trial period. Unexpectedly, the group treated with insulin exhibited distinct correlations between shifts in CSF markers and variations in brain volume, cognitive function, as well as levels of tau and amyloid.¹³⁹

Insulin Sensitizer NE3107

With its anti-inflammatory and insulin-sensitizing properties, NE3107 is being tested for safety and effectiveness in mild-to-moderate AD in older persons. The experiment compares the results of a 30-week treatment plan against a placebo. The research is being evaluated in this multicenterd trial using a randomized design involving 316 subjects as a cohort, the trial has demonstrated promising outcomes in cognitive function, as well as improvements in functional and behavioral characteristics. The study is registered on ClinicalTrials.gov with the number NCT04669028, and the comprehensive final results of the study are anticipated to be disseminated imminently.¹⁴⁰

GRF6019, as a Plasma Fraction

Hannestad et al. have conducted clinical research to assess the Initial assessment of the efficacy, safety, and tolerance profile of GRF6019 plasma fraction in individuals with severe AD.0 The clinical research study involving 18 subjects divided into two groups receiving either GRF6019 or a placebo via intravenous infusions administered daily over a span of 5 days. Notably, all participants completed the trial without experiencing any severe adverse events, highlighting the favorable safety, feasibility, and tolerability profile of GRF6019. Despite the absence of sufficient statistical analyses to detect significant differences in cognitive and functional evaluations between the treatment and placebo groups, the findings underscore the importance of further exploration to evaluate the potential therapeutic merits of GRF6019 and similar plasma fractions.¹⁴¹

Ginkgo Biloba Extracts: EGb 761

A research endeavor led by Morató et al. delved into the examination of how EGb 761, a standardized extract derived from Ginkgo biloba, influences individuals with mild cognitive impairment. The trial included a 12-month observation phase and a subsequent 12-month extension phase. Participants were stratified into two cohorts: the intervention group receiving daily doses of EGb 761, and the control group abstaining from the extract. After the initial 12-month period, the EGb 761-treated cohort continued their regimen, while the control cohort commenced EGb 761 supplementation. The study comprised 100 MCI patients, with 60% being female, the average age of the participants was 73.1 years, with an average duration of 2.9 years from symptom onset to diagnosis. Anticipated results are slated for release by 2023.¹⁴²

[18F]-DPA-714 and [^18F]-Ro948 for Measurement of Synaptic Density

An ongoing interventional study (NCT05911178) is investigating the connections between microglial activation, tau pathology, and synaptic density in AD using advanced neuroimaging techniques. This research utilizes tracers, including [^18F]-DPA-714 and [^18F]-Ro948, to examine tau pathology and microglial activity, as well as the novel PET radioligand [^11C]-UCB-J to measure synaptic density. The study focuses on mapping the regions of microglial activation and cortical tau accumulation and evaluating their impact on synaptic dysfunction. Employing a multimodal approach, which integrates PET/MRI imaging, cognitive assessments, and peripheral immune biomarkers, the research aims to better classify AD subgroups and enhance treatment strategies for neurodegenerative disorders.¹⁵⁴

Challenges and Future Prospects

The study of neuroinflammatory mediators as biomarkers for AD presents both challenges and promising future prospects. The complexity of the neuroinflammatory response, the heterogeneity of AD patients due to genetic background, lifestyle factors, and coexisting medical conditions, and the sensitivity and specificity of these biomarkers are significant challenges. However, advances in omics technologies and bioinformatics are enabling more comprehensive and precise profiling of the inflammatory response in AD. High-sensitivity assays and imaging techniques are improving the detection and quantification of these biomarkers. The integration of neuroinflammatory biomarkers with other biomarkers and clinical data through machine learning and artificial intelligence holds the potential to enhance diagnostic accuracy and prognostic predictions. A better understanding of neuroinflammation’s role in AD pathogenesis may reveal novel therapeutic targets, leading to more effective treatments that can modulate the inflammatory response and potentially alter the disease course. Thus, a continued exploration of neuroinflammatory mediators as biomarkers for AD not only offers a promising avenue for improving diagnosis, prognosis, and treatment but also has the potential to shape future research directions and clinical practices by identifying key therapeutic targets and enhancing patient care strategies. Ultimately, this research could contribute to better patient outcomes.

Conclusion

AD biomarker research has evolved due to advancements in analytical and visualization techniques. Modern assays and technologies enable more sensitive blood testing to identify AD pathology. Combining biomarkers, such as miRNA, can increase the specificity and accuracy of diagnosis. Extensive research examining the combinations of different markers can provide valuable information. Biomarkers have a potential application as real-time indicators for monitoring the impact of disease-modifying medications in AD clinical trials. Currently, there are roughly 182 phase II and phase III AD therapeutic studies in the ClinicalTrials.gov database, which frequently use PET imaging and CSF biomarkers to evaluate treatment results. Pro-inflammatory cytokines, Aβ40, Aβ42, P-Tau, and other plasma biomarkers have been included in some studies as additional tools for tracking AD course. There is a significant need for biomarkers that can enable widespread screening of patients in primary healthcare settings to offer dependable initial diagnoses for individuals at risk. Blood and CSF biomarkers have the potential to significantly reduce AD diagnostic costs, enabling widespread access to affordable diagnostic approaches. A better understanding of the connection between biomarker levels, lifestyle variables, and AD pathogenesis is needed. The discovery of novel, promising AD biomarkers is expected to expedite the clinical development of potent therapeutic agents, reduce total costs associated with managing the disease, and open the door to better clinical trial designs.

Glossary

Abbreviations

AD

Alzheimer’s disease

Aβ

Amyloid-beta

NFTs

neurofibrillary tangles

ROS

Reactive Oxygen species

CNS

central nervous system

BBB

Blood Brain Barrier

CSF

Cerebrospinal fluid

MCI

Mild cognitive impairments

TREM2

Triggering receptor expressed on myeloid cells 2

MRI

Magnetic resonance imaging

CT

Computed Tomography scans

DTI

Diffusion tensor imaging

MRS

Magnetic resonance spectroscopy

SPECT

Single-photon emission computed tomography

FDG

Fluorodeoxyglucose F-18

PET

Positron emission tomography

fMRI

Functional MRI

TSPO

Translocator protein

LPS

lipopolysaccharide

NF-κB

Nuclear factor kappa-light-chain-enhancer of activated B cells

TLR

Toll-like receptors

NLRP3

NLR family, pyrin domain containing

CARD

Caspase Activation and Recruitment Domain

cGAS-STING

Cyclic GMP-AMP Synthase- Stimulator of Interferon Genes

SIRT1

Sirtuin 1

LSD

lysergic acid diethylamide

Data Availability Statement

Data can be provided upon request to the corresponding authors.

Author Contributions

RRD, GJL, and KV: conceptualization, writing, reviewing, and editing completed the initial draft of the manuscript. SS: formal analysis, reviewing and editing. All authors have read and approved the final submitted manuscript.

This does not received funding from any profitable or nonprofitable organization.

The authors declare no competing financial interest.