Alzheimer’s disease (AD) is a progressive neurodegenerative disorder characterized by the building up of aberrant proteins, inflammation, and progressive loss of neurons and synapses in the brain. This progressive neurodegenerative disorder is marked by the slow degeneration of brain neuron cells, causing memory loss, cognitive decline, and changes in behavior [1–11].

The pathology of AD is characterized by the deposition of two main proteins: amyloid beta (Aβ) peptides and tau protein. Aβ peptides are produced when a larger protein known as amyloid precursor protein (APP) is cleaved by β-secretase and γ-secretase enzymes, giving rise to the formation of insoluble fibrils that aggregate into senile plaques, a hallmark of the pathology of AD [12–14]. Tau protein, on the other hand, is microtubule-associated protein that get hyperphosphorylated and forms neurofibrillary tangles (NFTs), which are another characteristic feature of AD [15]. The progression of AD pathology also begins with the deposition of Aβ peptides in the brain, followed by the formation of NFTs and synapse loss [16]. As the disease advances, the building up of these abnormal proteins leads to the activation of immune cells, such as microglia and astrocytes, which release pro-inflammatory cytokines and reactive oxygen species (ROS), causing further tissue damage and neuronal death [17, 18]. The loss of neurons and synapses leads to cognitive decline, memory impairment, and eventually dementia [19–21]. Furthermore, AD pathology also involves disrupting various cellular processes, including synaptic plasticity, mitochondrial function, and energy metabolism [20]. Mitochondrial dysfunction has been linked to AD by building up Aβ peptides, which can impair adenosine triphosphate (ATP) production and increase ROS production [22–24]. Additionally, alterations in gene expression and epigenetic modifications have been observed in AD brains, giving rise to changes in the regulation of gene transcription and protein synthesis [25, 26].

Aβ is a protein fragment that plays an essential role in AD development. It consists of 39–43 amino acid-long peptide that is generated through the proteolytic processing or sequential pathological cleavage of the APP by enzymes known as β-secretase and γ-secretase [27–29]. In healthy individuals, APP is also cleaved into a soluble form of Aβ40, which is less neurotoxic and aggregates more slowly. However, in AD, Aβ peptides, particularly Aβ42, aggregate into oligomers, fibrils, and plaques in the brain, disrupting neuronal function. These aggregates induce neurotoxicity by triggering oxidative stress, inflammation, and synaptic dysfunction, ultimately contributing to neurodegeneration and cognitive decline [30]. While Aβ40 is less prone to aggregation, it still contributes to the overall amyloid burden and may modulate the aggregation of Aβ42 [30]. The ratio of Aβ42 to Aβ40 in cerebrospinal fluid (CSF) has emerged as a potential biomarker for AD diagnosis and progression, with a lower ratio indicating a higher likelihood of AD pathology [31]. The amyloid cascade hypothesis suggests that Aβ accumulation is an early and central event in AD pathology, leading to tau protein hyperphosphorylation, NFTs, and neuronal death [32]. Aβ can bind to various receptors, including the receptor for advanced glycation end-products (RAGE), which can activate downstream signaling pathways that promote neuroinflammation and oxidative stress [33]. The aggregation of Aβ into insoluble fibrils is thought to be a significant step in the pathogenesis of AD [34]. The amyloid fibrils can deposit in the brain, forming characteristic amyloid plaques that are associated with neuronal degeneration and death [34–37]. The exact mechanisms by which Aβ aggregation leads to neurodegeneration are still not fully understood. Still, it is thought to involve the activation of various cellular stress pathways, including the unfolded protein response and the autophagy-lysosome pathway [14, 38].

In AD, Aβ has been identified as one of the primary pathological hallmarks of the disease. Mutations in genes encoding APP or the enzymes involved in its processing, such as β-secretase and γ-secretase, have been linked to early-onset familial AD (EOFAD), highlighting the critical role of Aβ in disease pathogenesis [39, 40]. Furthermore, Aβ has been shown to be elevated in the CSF of individuals with AD compared to healthy controls, providing a potential biomarker for diagnosing and monitoring disease progression [31, 41].

The tau protein is one of the key players in the development of AD [42]. Tau is a microtubule-associated protein that is primarily found in the brain and plays an important role in maintaining the stability and organization of microtubules, which are essential for neuronal function and survival [43]. In healthy individuals, tau is normally phosphorylated at specific serine and threonine residues, which allows it to bind to microtubules and regulate their dynamics [44]. However, in AD, tau protein becomes abnormally hyperphosphorylated at multiple sites, leading to its aggregation into insoluble fibrils that can build up in the brain [45]. The abnormal tau protein aggregates, known as NFTs, are a hallmark pathological feature of AD and are thought to contribute to the disruption of normal brain function and neuronal degeneration [46]. Over and above that, NFTs are highly correlated with cognitive decline and dementia severity in AD patients [47, 48]. The aggregation of tau protein is thought to occur through a process called seeding, where misfolded tau protein interact with each other and recruit more native tau protein to form insoluble fibers [49]. Research has shown that mutations in the MAPT gene, which encodes the tau protein, can increase the risk of developing frontotemporal dementia (FTD), including frontotemporal lobar degeneration (FTLD) and progressive supranuclear palsy (PSP) [50]. Moreover, studies have identified specific genetic variants of the MAPT gene that are associated with an increased risk of AD and FTD [51].

Neuroinflammation plays a vital role in the progression of AD, a chronic and neurodegenerative disorder characterized by the accumulation of Aβ plaques, NFTs, and synaptic loss. As proposed by Heneka et al. [52], “neuroinflammation is an integral component of AD pathophysiology, and its resolution is critical for effective therapy”. In AD, neuroinflammation is triggered by the deposition of Aβ peptides, which are generated from the proteolytic cleavage of APP. Aβ building up in the brain gives rise to the activation of microglia, the resident immune cells of the central nervous system (CNS), which then release pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha), interleukin-1 beta (IL-1β), and IL-6 [53–55]. These pro-inflammatory mediators activate astrocytes, another type of glial cell, leading to the production of ROS, nitric oxide (NO), and excitotoxic neurotransmitters, which further exacerbate neuroinflammation [56]. Moreover, activated microglia also produce reactive astrocytes, which can lead to an imbalance in glutamate homeostasis, resulting in excitotoxicity and neuronal death. Over and above that, neuroinflammation can lead to blood-brain barrier disruption, allowing toxic substances to enter the CNS and exacerbating neurodegeneration [57, 58]. Furthermore, neuroinflammation can also contribute to the formation of NFTs by promoting tau protein hyperphosphorylation and aggregation. Tau protein is a microtubule-associated protein that becomes abnormally phosphorylated and forms insoluble aggregates in the brains of individuals with AD. Activated microglia can release factors that promote tau protein phosphorylation and aggregation, leading to the building up of NFTs [13]. The aggregation of Aβ fibrils in (Figure 1) triggers a devastating chain reaction of neurodegeneration and, ultimately, cell death.

Diagnostic methods for AD typically involve a combination of several approaches, including 1) clinical evaluation, including medical history and physical examination [59]; 2) cognitive and neuropsychological assessments [59–61]; 3) neuroimaging studies using computed tomography (CT) or magnetic resonance imaging (MRI): used to obtain images of the brain to rule out other causes of dementia, such as tumors, strokes, or significant brain atrophy indicative of AD [59, 60, 62]; 4) biomarker analysis, such as Aβ and tau protein [63, 64]; 5) genetic testing [65]; 6) laboratory tests: blood tests to rule out other potential causes of memory loss, such as vitamin deficiencies, thyroid dysfunction, or infections [66].

A definitive diagnosis of AD during life remains challenging due to the need for specific biomarkers and clinical evaluations. Current methods combine neuroimaging techniques, CSF analysis, and neuropsychological assessments. Positron emission tomography (PET) scans using radiotracers can identify Aβ plaques in the brain, a hallmark of AD. CSF analysis for Aβ, tau protein, and phosphorylated tau protein also helps distinguish AD from other neurodegenerative disorders. Structural MRI can detect brain atrophy patterns typical of AD, particularly in the hippocampus and entorhinal cortex. Advanced neuropsychological testing assesses cognitive function and can identify the presence and progression of dementia. These methods significantly improve diagnostic accuracy [67–69].

Molecular diagnostic methods have revolutionized the field of AD diagnosis, enabling the detection of specific biomarkers in body fluids or tissues to accurately diagnose and monitor the progression of the disease. One such method is CSF analysis, which has been shown to be a reliable and non-invasive approach for detecting AD biomarkers [98]. The Aβ42 peptide, a hallmark of AD, is a common biomarker measured in CSF using enzyme-linked immunosorbent assay (ELISA) or mass spectrometry-based methods [99]. Elevated levels of Aβ42 in CSF have been consistently associated with AD pathology and cognitive decline, making it a valuable diagnostic tool in AD [100]. Another molecular diagnostic method is the measurement of tau protein, another key component of NFTs, a hallmark of AD pathology. Tau protein can be measured in CSF using ELISA or western blotting, and elevated levels have been linked to AD severity and progression [101]. Furthermore, the phosphorylated form of tau protein at specific residues (e.g., tau-181P) has been shown to be a more specific biomarker for AD than total tau protein levels [102]. This specificity is vital for an accurate diagnosis, as other neurodegenerative disorders, such as FTD, can also exhibit elevated tau protein levels.

Imaging-based molecular diagnostic methods have also gained popularity in recent years. Neuroimaging modalities shown in Table 1 are useful in diagnosis and disease progression monitoring. PET imaging with amyloid-binding tracers, such as [¹⁸F]florbetapir or [¹⁸F]flutemetamol, can visualize amyloid plaques in the brain with high accuracy, providing a non-invasive means of diagnosing AD [103–105]. These tracers target specific epitopes on the Aβ peptide, allowing for the detection of early-stage amyloid deposition before symptoms emerge. In addition to these established methods, novel approaches are being explored to improve the accuracy and sensitivity of molecular diagnostics for AD. For example, liquid biopsy analysis has been proposed as a promising technique for detecting circulating biomarkers in blood samples [106]. This approach involves analyzing circulating cell-free DNA or RNA fragments for AD-related biomarkers, which could potentially replace traditional tissue-based diagnostics.

Molecular diagnostic methods have become essential tools for diagnosing and monitoring AD. The combination of CSF analysis for Aβ42 and tau protein, PET imaging with amyloid-binding tracers, and emerging technologies like liquid biopsy analysis offers a multifaceted approach to diagnosing AD with high accuracy and specificity. As our understanding of disease pathophysiology continues to evolve, these methods will likely play a very important role in early diagnosis and treatment strategies [107–110].

The Aβ imaging radiotracers approved by the Food and Drug Administration (FDA) for AD, along with their merits, include: [¹⁸F]florbetapir (Amyvid) is a PET imaging agent that targets the aggregation of Aβ in the brain. It was approved by the FDA in 2012 for the detection of Aβ plaques in patients with AD and other cerebral amyloidosis. [¹⁸F]Florbetapir binds to Aβ in the brain, allowing for visualization of these deposits on PET imaging. Merits include: 1) high sensitivity of about 93% and specificity of about 92% for detecting Aβ plaques in AD; 2) can be used to diagnose AD at an early stage; 3) may help identify individuals at risk of developing AD; 4) can be used to monitor disease progression and treatment response [132].

[¹⁸F]Florbetaben (Neuraceq) is another PET imaging agent that targets Aβ plaques in the brain. It was approved by the FDA in 2014 as a diagnostic aid to estimate the presence or absence of Aβ plaque deposition in the brain. Its merits are similar to those of [¹⁸F]florbetapir, and these include: 1) [¹⁸F]florbetaben has a high sensitivity of about 85% and a specificity of about 91% for detecting Aβ plaques; 2) it can be used to diagnose AD at an early stage; 3) it may help identify individuals at risk of developing AD; 4) it has a longer half-life than [¹⁸F]florbetapir, which may make it more convenient for patients [131, 132].

[¹⁸F]Flutemetamid (Vizamyl) is a PET imaging agent that targets Aβ plaques in the brain. It was approved by the FDA in 2018 as a diagnostic aid to estimate the presence or absence of Aβ plaque deposition in the brain. Merits are: 1) has a high sensitivity of about 92.6% and specificity of about 96% for detecting Aβ plaques in AD; 2) can be used to diagnose AD at an early stage; 3) may help identify individuals at risk of developing AD; 4) it has a shorter half-life than other Aβ imaging agents, which may make it more suitable for patients with kidney or liver impairment [132]. All three FDA-approved Aβ imaging radiotracers have high sensitivity and specificity for detecting Aβ plaques in the brain, making them useful diagnostic tools for AD. While they share similar merits, each has unique characteristics that may make it more suitable for certain patients or clinical scenarios.

Imaging Aβ tracers, in Table 1, are the most commonly used radiotracers that have been approved by the FDA for PET imaging of Aβ plaques in clinical settings. Other radiotracers, such as [¹¹C]PiB and [¹⁸F]NAV4694 (AZD4694), are used for research purposes other than clinical use. Figure 2 shows some of the Aβ tracers for AD [4, 120, 147, 154, 155].

Some limitations exist when comparing [¹¹C]PiB to [¹⁸F] tracers like: 1) shorter half-life and on-site production requirement: [¹¹C]PiB has a half-life of 20.3 min, requiring on-site production at a cyclotron, whereas [¹⁸F] tracers such as [¹⁸F]florbetapir have a longer half-life (109.7 min) and can be produced at a centralized facility [156, 157]. 2) The short half-life of [¹¹C]PiB necessitates regular transportation from a nearby cyclotron or on-site production, which can be logistically challenging and expensive [158]. On the other hand, the transportation of [¹⁸F] tracers, such as [¹⁸F]florbetapir, is easier and more accessible [159]. 3) Due to the complexity of production and distribution, the cost of [¹¹C]PiB is generally higher than that of [¹⁸F] tracers [160]. 4) Some studies have shown that [¹¹C]PiB may have lower image quality in certain brain regions, such as the cerebellum, compared to [¹⁸F] tracers due to its shorter half-life and higher radiation dosing requirements, leading to lower image quality in certain regions [125]. 5) The higher radiation dose required for [¹¹C]PiB imaging may result in higher patient radiation exposure compared to [¹⁸F] tracers, which could be a concern for long-term imaging studies or repeated scans [161]. 6) Commercial software for analyzing [¹¹C]PiB images is less widespread than for [¹⁸F] tracers, which may limit the availability of accurate image analysis tools, leading to a limited availability of commercial software for image analysis [162]. 7) While [¹¹C]PiB, [¹⁸F]florbetapir, [¹⁸F]flutemetamol, and [¹⁸F]florbetaben tracers are approved by regulatory agencies for research use, only few [¹⁸F] tracers are FDA-approved for clinical use, which may impact their adoption in clinical settings for the clinical diagnosis and differential diagnosis of AD. [¹⁸F]florbetapir, [¹⁸F]florbetaben, and [¹⁸F]flutemetamol have favorable binding and imaging properties [157]. Another promising tracer, [¹⁸F]NAV4694, is currently in clinical trials [147]. The longer half-life of these ¹⁸F-labeled tracers, approximately 109.7 min, allows for wider distribution and use in clinical settings. They are expected to produce less noisy images and provide more precise quantification of Aβ accumulation in the early stages of the disease due to increased radioactivity counts during the scan. Additionally, the imaging protocols for each tracer differ in terms of scanning window, visual interpretation, and quantitative analysis of the images [4, 120, 147, 155, 163].

The tau protein tracers are used to visualize the accumulation of tau protein, a hallmark of AD, in the brain. Some of the tau protein tracers used include: 1) [¹⁸F]Flortaucipir, which was approved in 2018, is a radiolabeled ligand that binds to tau protein aggregates in the brain, allowing for visualization of tau pathology. It is used to help diagnose AD and monitor treatment responses in patients with MCI, or AD. The FDA approved [¹⁸F]flortaucipir based on the results of the fluorine-18 T807 (flortaucipir) ([¹⁸F]AV-1451) phase III clinical trial, which showed that the tracer was effective in detecting tau pathology in patients with AD and distinguished it from normal aging and other neurodegenerative disorders [164]. 2) [¹¹C]PiB is also a tau protein-specific PET agent that was developed by Avid Radiopharmaceuticals and received FDA orphan drug designation for AD. While not yet approved for commercial use, it has been used in clinical trials to evaluate its efficacy in detecting tau protein pathology.

[¹¹C]PiB is primarily designed to bind to Aβ plaques, a hallmark of AD pathology, rather than tau protein. However, some studies have reported that [¹¹C]PiB can also bind to tau protein, although to a lesser degree than Aβ [159, 165]. [¹¹C]PiB binds to soluble and insoluble tau protein with a higher affinity for tau tangles and NFTs [166]. The mechanism of action behind PiB’s binding to the tau protein is not fully understood, but it is believed to involve the same mechanism as its binding to Aβ, which involves the molecule’s ability to form hydrogen bonds and pi-cation interactions with the target protein [167, 168]. Despite [¹¹C]PiB’s ability to bind to tau protein, other tau-binding tracers, such as [¹⁸F]-T807 and [35S]-BTA-1, have been developed as potential diagnostic tools for AD, offering more specific binding to tau protein pathology [69, 169, 170].

To identify and visualize brain damage caused by the accumulation of tau protein in AD and other types of dementia, scientists have designed tau protein tracers for trails including MK-6240 and pyridinyl-butadienyl-benzothiazole-3 ([¹¹C]PBB3), as shown in Figure 3. These radiotracers are specifically designed to bind to tau protein tangles in the brain, allowing for accurate imaging and quantification of tau protein pathology [4, 171]. The [¹¹C]PP3 radiotracer, for example, has a high affinity for tau protein and is suitable for imaging tau protein-related disorders [147]. The newer and second-generation selective tau protein tracers are currently being developed, including [¹⁸F]MK6240, [¹⁸F]RO-948, [¹⁸F]GTP1, [¹⁸F]JNJ-311, [¹⁸F]P12620, and [¹⁸F]JNJ-067, as shown in (Figure 3). These newer tracers are being designed to improve their in vivo characteristics, enabling more effective imaging and the diagnosis of tau protein-related diseases [147].

Tau protein tracers have several merits, including 1) improved diagnostic accuracy by helping to diagnose AD with higher accuracy than conventional imaging methods, such as MRI and CT scans; 2) earlier detection: detecting AD-related changes in the brain years before symptoms appear, allowing for earlier intervention and potentially improving treatment outcomes; 3) monitoring disease progression: tracking disease progression and monitoring the response to treatment, enabling more effective management of AD patients [119, 147, 154, 163, 171–173].

The inflammation tracers approved by the FDA for use in AD are radiolabeled ligands that bind to specific biomarkers of inflammation, allowing for imaging of the brain’s inflammatory activity. Among others, the most commonly used inflammation tracers include [¹¹C]PK11195 and [¹⁸F]DPA-713. [¹¹C]PK11195 tracer binds to the translocator protein (TSPO) on activated microglia, a type of immune cell in the brain that plays a vital role in inflammation. [¹¹C]PK11195 has been approved by the FDA for use in diagnosing AD and has been shown to be sensitive to changes in brain inflammation over time [109]. [¹⁸F]DPA-713 tracer also binds to TSPO and has been shown to be more sensitive than [¹¹C]PK11195 in detecting inflammation in the brains of AD patients [174]. [¹⁸F]DPA-713 has been approved by the FDA for use in clinical trials and is currently being evaluated for its potential as a diagnostic tool for AD.

Some of the merits of these tracers include: 1) high sensitivity and specificity for detecting inflammation in the brain [109, 174]; 2) the ability to quantify changes in brain inflammation over time, allowing for monitoring of disease progression [109]; 3) the potential to aid in the diagnosis and monitoring of treatment responses in AD [174]; 4) they are non-invasive and relatively safe, as they are administered as radiotracers and do not require tissue sampling [109].

These radiotracers allow for the precise targeting and visualization of specific molecular pathways involved in AD, providing valuable diagnostic information for both clinicians and patients [163]. [¹¹C]PK11195 was the first successfully developed TSPO radiotracer for PET imaging in humans [147]. Several second-generation TSPO radioligands were developed and tested, including [¹¹C]PBR28, [¹⁸F]DPA714, [¹⁸F]FEPPA, [¹¹C]DAA1106, [¹⁸F]FEMPA, and [¹⁸F]FEDAA1106, as shown in Figure 4 [147]. While these radioligands are generally more sensitive than [¹¹C]PK11195, research has revealed that their brain uptake is influenced by the TSPO gene rs6971 polymorphism. This means that individual subjects must be phenotyped to ensure accurate imaging results based on their TSPO affinity status. The third generation and most recent is [¹¹C]ER176, which is being developed and evaluated is also shown in (Figure 4) [4, 109, 147, 163, 173, 175].

For over a decade, researchers have been trying hard to find out the diagnostic tools for AD. Currently, AD diagnosis is primarily based on neuropsychological testing. Clinically diagnosing AD requires neuroimaging [78] and monitoring accepted biomarkers, such as the accumulation of Aβ peptides as well as total and hyperphosphorylated tau protein in CSF [118, 132, 176–178].

Neuroimaging plays a vital role in the diagnosis of AD, a progressive neurological disorder characterized by cognitive decline, memory impairment, and brain atrophy. The primary neuroimaging modalities used in the diagnosis of AD include MRI and PET scans [179]. MRI provides structural information about the brain, allowing for the detection of atrophy, particularly in the hippocampus and entorhinal cortex, which are regions commonly affected in AD. On the other hand, PET scans utilize radiotracers to visualize regional brain glucose metabolism, Aβ deposition, and tau protein, which are hallmark features of AD [125]. Among others, [¹¹C]PiB, [¹⁸F]florbetapir, [¹⁸F]florbetaben, [¹⁸F]flortaucipir, and 18F-fluorodeoxyglucose ([¹⁸F]FDG), are the most commonly used PET tracers, which bind to Aβ plaques, fibrillary tau protein aggregates, and also utilization of glucose metabolism in the brain by [¹⁸F]FDG, to visualize and identify the biomarkers [126]. These imaging biomarkers can help differentiate AD from other dementias, such as FTD, and provide an objective measure of disease progression [180]. Furthermore, neuroimaging can aid in monitoring treatment responses and identifying potential therapeutic targets. For instance, amyloid-targeted therapies such as monoclonal antibodies may reduce Aβ load as measured by PET scans, indicating treatment efficacy [181]. Research findings indicate that damage to the neurons in the hippocampus is observed as a reduction in the volume of the hippocampus. Volumetric MRI images of the hippocampus atrophy are a commonly used approach to evaluate AD pathology. MRI has proven to be a highly effective method for evaluating the size of the hippocampus. It can accurately estimate the volume of the hippocampus, which is strongly associated with the number of neurons present. This indicates that volumetric MRI measures are anatomically reliable. Furthermore, a separate study discovered that decreases in the size of the hippocampus are initial signs of AD pathology, which can be measured using MRI [182]. MRI can be used to study the pathogenesis of the disease as well as the progress of the disease [182, 183].

Neuroimaging techniques, such as MRI, PET, and MRS, have been extensively used to assess the progression of AD by providing valuable information on brain structure, function, and biochemistry [207]. MRI is particularly useful for monitoring structural changes in the brain, including cortical thinning and hippocampal atrophy, which are characteristic features of AD [208]. Studies have shown that MRI can detect changes in brain structure up to 10 years before clinical symptoms appear, making it a valuable tool for early diagnosis and monitoring disease progression [209]. PET imaging with amyloid ligands, such as [¹¹C]PiB, can quantify Aβ deposition in the brain, a hallmark of AD pathology [209]. This technique has been used to track disease progression by monitoring the accumulation of Aβ plaques in the brain over time [210]. Additionally, PET imaging with [¹⁸F]-FDG can assess glucose metabolism in the brain, which is decreased in AD patients. This technique has been used to monitor changes in brain glucose metabolism during disease progression [95]. MRS is another neuroimaging modality that has been used to monitor disease progression in AD. MRS can measure changes in brain metabolites, such as NAA, which is a marker of neuronal integrity. As proposed by Graff-Radford and Kantarci [88], it was shown that NAA levels decrease in AD patients, and this decrease can be used as a biomarker of disease progression. The combination of these neuroimaging techniques has been shown to provide a comprehensive understanding of disease progression in AD. For example, a study used MRI to monitor cortical thinning, PET with [¹¹C]PiB to track Aβ deposition, and MRS to measure changes in brain metabolites. The researcher found that these techniques were able to accurately predict disease progression over a period of 3 years [182]. Neuroimaging techniques such as MRI, PET, and MRS provide valuable information on brain structure, function, and biochemistry, allowing researchers and clinicians to track changes in the brain over time. These changes can be used to diagnose and monitor disease progression, making it possible to develop effective treatment strategies and improve patient outcomes [211].

Effective evaluation of drug efficacy provides important information for drug development and treatment plan selection. Efficacy indicators should reflect drug effectiveness, with high repeatability and feasibility. The key steps involved in evaluating drug efficacy for AD using imaging techniques [212] include 1) define the target; 2) select a radiotracer [84, 212]; 3) baseline imaging; 4) assess drug intervention; 5) quantify imaging data; 6) correlate imaging findings with clinical outcomes; 7) conduct multi-center trials. Multi-center trials can enhance the robustness and generalizability of the findings, supporting the potential use of neuroimaging as a technique for drug efficacy in AD [182, 213].

Clinical trials involving anti-amyloid monoclonal antibodies such as aducanumab, lecanemab, and donanemab showed that anti-amyloid monoclonal antibodies remove or prevent the formation of Aβ plaques to slow or halt disease progression [214]. Each of these anti-amyloid monoclonal antibodies has undergone clinical trials with neuroimaging assessments. As stated by Budd Haeberlein et al. [215], aducanumab was tested in patients with early AD in the EMERGE study, where MRI and FDG-PET were used to assess changes in brain volume and glucose metabolism. The study showed significant reductions in amyloid burden on amyloid PET scans, as well as improvements in cognition and function [215]. Similarly, lecanemab was evaluated in patients with MCI or mild AD in the CLARITY AD study, where MRI and amyloid PET were used to assess changes in brain volume and amyloid burden. The study demonstrated significant reductions in amyloid burden on amyloid PET scans and a slowing of cognitive decline [216]. Donanemab is a humanized monoclonal antibody that targets soluble Aβ oligomers, a hallmark of AD. In clinical trials, donanemab has been shown to reduce the levels of Aβ in the CSF and plasma of patients with mild MCI or early-stage AD [217, 218]. The phase 2b study of donanemab, known as the A4 trial, involved 540 patients with MCI or early-stage AD and demonstrated that treatment with donanemab significantly reduced CSF Aβ levels by up to 80% compared to placebo [219]. Additionally, the study found that patients treated with donanemab showed slower cognitive decline and improved cognitive function compared to placebo-treated patients [217, 219]. The results of the A4 trial suggest that targeting Aβ oligomers with donanemab may be a promising therapeutic approach for early-stage AD [220, 221].