AD is a chronic, irreversible brain disorder that progressively impairs memory, thought, reasoning, and other cognitive abilities. Additionally, it leads to behavioral problems that may interfere with a person’s regular activities and daily life. A patient suspected of having AD should undergo several tests, including medical and family history, neurological examination, Magnetic resonance imaging (MRI) for the neurons, and vitamin B12 lab tests [42].

For a very long time, vitamin B12 insufficiency has been linked to neurologic issues, and some studies have found that it raises the chance of AD. Elevated homocysteine levels are a specific indicator of vitamin B12 deficiency and are toxic to the brain because they increase oxidative stress, calcium influx, and apoptosis. The diagnosis of vitamin B12 deficiency may be made using measurements of serum levels of vitamin B12, complete blood counts, and serum homocysteine levels [43, 44]. The pathophysiologic process of AD starts years or even decades before clinical dementia and apparent cognitive abnormalities. As a result, the phrases “AD dementia” and “AD pathophysiologic process” are distinct.

There have been no clinical indications in preclinical AD. Measurable alterations in molecular and imaging biomarkers are being studied for pre-symptomatic evaluation.

With proper assessment, mild cognitive impairment AD can show moderate periodic changes in cognitive state and memory that do not interfere with everyday activities. It is essential to rule out any other dementia-causing factors. Genetic testing is an option in cases with early-onset familial AD to look for specific mutations.

The hallmark of Alzheimer’s dementia is a growing cognitive impairment that makes day-to-day tasks challenging. It may be possible to confirm the diagnosis using abnormal biomarkers such as elevated levels of tau protein in the CSF, low levels of A$\beta$ on positron emission tomography (PET) imaging, and temporal lobe atrophy on MRI [45].

A complex neurological ailment called AD causes memory loss and progressive cognitive deterioration. For early identification, diagnosis, and the creation of efficient treatments, it is essential to comprehend the clinical stages, genetic components, and biomarkers related to AD. Here is a more thorough examination of the current state of knowledge, including any gaps and disagreements that may exist.

Diagnosing AD, tracking the course of the condition, and determining the effectiveness of treatment depends heavily on biomarkers. Three biomarkers can be distinguished: neurodegenerative markers, tau biomarkers, and A$\beta$ biomarkers. A$\beta$ biomarkers include indicators of A$\beta$ buildup in the brain, such as amyloid PET imaging and A$\beta$-42 levels in CSF. Tau protein levels in CSF or by tau PET imaging are measured as tau biomarkers. Neurodegeneration markers, such as measurements of CSF tau, CSF neurofilament light (NfL), and structural and functional imaging, evaluate neuronal damage or neurodegeneration. Although these biomarkers have potential, their applicability and standardization in various clinical contexts and patient populations still need to be improved [46].

Even though amyloid precursor protein (APP) and its derivatives have been discussed and extensively studied over the previous 20 years, it is still unclear how their normal physiological functions and metabolites work. There is proof that APP is associated with gene regulation, cell-cell, and cell-substrate interaction, brain development, and synaptogenesis. It is found in many tissues and as a membrane protein in the brain [57, 58].

APP was the first gene susceptible to AD located on chromosome 21 and encoded for a single 770 amino acid transmembrane-spanning polypeptide [59, 60].

The APP gene is duplicated due to the triplication of chromosome 21, which may improve APP expression and A$\beta$ buildup. Patients with Down syndrome reportedly had earlier AD pathology (the development of neurofibrillary tangles and senile plaques) than people without the condition [61]. These results imply that AD pathogenesis may be connected to APP overexpression. The APP gene has 18 exons used to make the APP protein. Exons 16 and 17 code for the A$\beta$ peptide. At least five isoforms of the APP protein contain the A$\beta$ peptide sequence after transcription and alternative splicing [62].

However, 21 and 3 mutations at Exons 17 and 16 suggest that APP is a very uncommon risk factor for AD since most pathogenic APP mutations were found close to the regions where the $\alpha$-secretase family of secretory and transmembrane proteins, which have an average length of 750 amino acids, are widely expressed and have roles in the proteolytic processing of the ectodomains of various cell-surface receptors and signaling molecules, and cell adhesion. In the non-amyloidogenic route, a disintegrin and metalloprotease10 (ADAM10) is the primary $\alpha$-secretase that breaks down APP, preventing the creation of A$\beta$, whose buildup and aggregation causes neuronal degeneration in AD. In addition to APP, the membrane-anchored metalloprotease ADAM10 also sheds the ectodomains of numerous other cell-surface proteins, such as cytokines, adhesion molecules, and notch. sAPP, an APP-derived fragment produced due to ADAM10 cleavage, is neuroprotective [63],

$\beta$, and $\gamma$-secretase enzymes cleave the A$\beta$ peptide; these mutations contributed to the development of AD [64, 65]. The endosomal/lysosomal and $\beta$-secretase cleavages of A$\beta$ are susceptible to N-terminal alterations in the A$\beta$ sequence. $\beta$-secretase, contain 501 amino acids with a signal peptide of 21 amino acids at the amino terminus and an amino acid range from 22 to 45 in the proprotein domain. Beta site cleaving enzyme 1 (BACE1) shares less than 30% amino acid sequence with members of the human pepsin family. The BACE1 gene is found on chromosome 11q23.3 and is unrelated to AD [66].

A transmembrane domain with 17 residues and a 24-amino-acid-long cytoplasmic tail of 24 amino acids follow the mature protein’s lumenal domain, which runs from 46 to 460. The lumenal domain of BACE1 has two active site motifs, each containing the remarkably preserved hallmark aspartic protease sequence. These are located at amino acids 93 to 96 and 289 to 292. BACE1 is anticipated to function as a type I transmembrane protein, with the location of its active site positioned on the lumenal side of the membrane. -secretase enzyme cleaves APP at the amino terminus of A$\beta$ in this location, resulting in membrane-bound C99 and the secretion of a modified form of APP known as sAPP. The BACE1-specific amino acid sequence is believed to determine this protein’s transmembrane orientation [67].

The complex of $\gamma$-secretase, which consists of a membrane-embedded protease known as presenilin 1 (PSEN1), presenilin enhance 2 (Pen-2), Aph-1, and Nicastrin, goes on to cleave C99, which is then involved in many essential cellular processes. Amyloid peptide aggregates due to abnormal amyloid precursor protein cleavage, which leads to AD building up in the brain. The transmembrane domain of $\gamma$-secretase is structured like a horseshoe and has 19 transmembrane segments (T.M.s). Nicastrin’s (NCT) large extracellular domain (ECD) just over the depression is situated space created by the T.M. horseshoe. Interesting similarities exist between Nicastrin ECD and an extensive family of peptidases, including the Prostate-specific membrane antigen (PSMA) or glutamate carboxypeptidase. Understanding the $\gamma$-secretase complex’s functioning processes is greatly aided by its structure to produce the intracellular carboxy-terminal fragment (CTF) and the amino acid [68].

To cleave C89 and shortened amyloid species, $\beta$-Secretase may cleave APP in the A$\beta$ region; nevertheless, most APP proceeds via a non-amyloidogenic cleavage mechanism. The A$\beta$ domain of APP is cut by $\alpha$-secretase, producing membrane-bound C83 and the secreted form of APP (sAPP). By cleaving C83, $\gamma$-secretase creates the intracellular CTF and extracellular fragment p3 (Fig. 3). Presenilin (P.S.), NCT, Pen-2, and anterior pharynx defective 1 make up the $\gamma$-secretase complex. Although other proteases may be involved, numerous members of the ADAM (a disintegrin and metalloproteinase) family, including ADAM-9, ADAM-10, and tumor necrosis factor- $\alpha$-convertase (also known as ADAM-17), are linked to the activity of $\alpha$-secretase.

In the APP gene, there have been discovered to be about 63 coding mutations associated with AD [69]. Most of them are risk factors for AD, including the A673V and the Swedish mutation (KM670/671NL). They may either encourage A$\beta$ accumulation or increase the creation of the A$\beta$-42/A$\beta$-40 ratio [70, 71]. Following the identification of this protective mutation in 2012, the Icelandic mutation A673T has drawn the interest of an increasing number of researchers [72].

Recent research has identified an uncommon new mutation, APP S198P, found in the ectodomain of the APP, acting as a helix-breaker rather than inside or next to the A$\beta$ domain. Experiments demonstrating increased A$\beta$ buildup in the APP S198P transgenic mouse brain provided evidence of pathogenicity. Accelerated endoplasmic reticulum (E.R) folding and quick transport to endosomal-lysosomal compartments caused by APP S198P in cultured cells increased A$\beta$ aggregate.

What mechanisms does the APP S198P mutation use to affect APP metabolism? In the APP 695 ectodomain, involving two unique structural domains of about 160 amino acids and about 190 amino acids, referred to as the E1 and E2, respectively, serine 198 is found. This amino acid is part of a versatile and prolonged acidic-rich domain (Ala 191-Val 290) [73]. In contrast to the creation and aggregation of the wild-type serine at position 198 of full-length APPS we, from which these metabolites were generated. Researchers discovered that soluble, extracellular APPs and cellular APP C-terminal fragments (CTF) originating from full-length APP S198P precursors comprising the Swedish mutation (APPSwe-S198P) at earlier points and accumulated to elevated amounts.

Furthermore, researchers present immunoprecipitation results implying transitory folding intermediates are generated more quickly in APPSwe-S198P-expressing cells than in APPSwe-expressing cells. This significant discovery may prove that the S198P variation encounters faster folding. The N-terminal E1 domain of APP has a structural epitope that is sulfhydryl-dependent, and pulse-chase tests using the P2-1 antibody have demonstrated that this domain does fold more quickly in cells showing the APPSwe-S198P variation than in cells expressing APPSwe. This discovery confirms the dependence of this structural epitope on sulfhydryl. According to the researchers, the proline at position 198 accelerates the folding process. It makes it easier for newly created synthetic APPSwe-S198P to migrate from the endoplasmic reticulum to the Golgi complex late compartments, where the enzymes BACE1 and $\gamma$-secretase are active to cleave APP at the $\beta$-site and $\gamma$-site respectively.

Compared to APPSwe, how can the folding pace of the APPSwe-S198P version be sped up? Proline, an amino acid that either bends or breaks a helix as it lacks amide hydrogen, is the deciding factor because of how much conformational space it takes up due to its bulky pyrrolidine ring. Interestingly, a study of the Protein Data Bank was used to determine the local conformations of each proline residue, indicating that the residue before the proline is crucial for defining the protein’s conformation. There is a high chance that the proline will adopt the conformation when it follows an aspartate residue ($\alpha$:$\beta$ = 9:1). Aspartate is included in the APP residue that comes right before residue 198.

The formation of a local helix is facilitated by an Asp-Pro pair, which in turn accelerates the folding of the neighboring E1 and E2 domains and the adjacent adaptable domain. Pulse-chase experiments were carried out to support this assertion, utilizing the sulfhydryl-dependent, structural-epitope-specific antibody mAbP2-1. These investigations revealed that APPSwe-S198P binds preferentially over APPSwe [74].

The processing of APP may change as a result of mutations (Table 2, Ref. [74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123]) around the cleavage site of $\alpha$-secretase (Glu693Lys, Glu693Gly, Glu693del, and Asp694Asn), which may increase the A$\beta$ peptide’s proteolytic resistance [124, 125]. Previously researchers studied the APP mutations near the cleavage of $\gamma$-secretase (Fig. 4). In the APP, 13 missense mutations at codons 714–715 lowered A$\beta$-40 secretions, while mutations at codons 716–717 enhanced A$\beta$-42 secretions. According to this study, the ratio of A$\beta$-42 to A$\beta$-40 may rise due to $\gamma$-secretase cleavage [126, 127].

Frontotemporal dementia, with or without supranuclear palsy, corticobasal syndrome, or parkinsonism, is associated with 40 mutations in the microtubule-associated tau (MAPT) gene, as reported by AlZFORUM [128]. The site of a mutation in a gene determines how it affects tau mRNA and protein as well as the disease that results. Exons 9 to 12 and exon 13 are where most coding mutations are found; however, Exon 1 mutations and intronic mutations in the intron after exon 10 that influence exon 10 splicing have also been described [129, 130].

Only two sequence changes in exon 7 of MAPT have been identified: Pro176 (silent point mutation) and Ala178Thr, although neither is harmful [131, 132]. Several types of tauopathies without mutations in the MAPT gene fall into one of several categories, such as Argyrophilic grain disease (AGD), corticobasal degeneration (CBD), progressive supranuclear palsy (PSP), and neurofibrillary tangle dementia, Pick’s disease, and AD. There are also other so-called unclassifiable tauopathies. These categories are based on tau immunoreactive structures’ cellular and anatomical distribution [133, 134].

While certain MAPT gene mutations display characteristics of sporadic tauopathies, others have notable variances, such as a broader anatomical distribution and an elevated load of tau immunoreactivity. This study describes a patient with a progressive neuropsychiatric condition and a neuropathological tauopathy phenotype incompatible with sporadic disease entities. In addition, we report on the discovery of a MAPT gene variant in exon 7 [135]. Exons 2, 3, and 10 of the human genome are used for alternative splicing; this might generate up to six distinct tau isoform variations [136, 137].

The domain that binds to microtubules of tau interacts with microtubules through C terminal repeats, which explains why 4R tau isoforms are more likely than 3R tau isoforms to stimulate microtubule assembly [138]. In the mammalian brain, tau is a crucial regulator of axonal transport that is highly expressed in neurons and normally localizes largely to axons [139].

However, tau gene deletion had little effect on axonal transport, suggesting that Microtubule-associated proteins 1 (MAP1) and Microtubule-associated proteins 2 (MAP2) may make up for tau in other proteins involved in microtubule regulation or binding [140].

Although tau is involved in oligodendrocyte process outgrowth and myelination, it is still uncertain if tau regulates the physiological functions of astrocytes. Tau also plays two other physiological roles, regulating insulin signaling and iron export [141].

PSENs were discovered for the first time in 1995 while searching for the genes causing beginning, autosomal dominant variants of familial AD (FAD) [142, 143]. A few years after their identification, it was determined that PSENs encode proteins that sustain the cleavage of APP by $\gamma$-secretase to create A$\beta$ peptides [144, 145]. PSEN genes have exhibited significant levels of evolutionary conservation, with homologs present in taxa as numerous as lower chordates, Drosophila melanogaster, and Caenorhabditis elegans [146].

In several tissues from mice and humans, including parts of the brain, PSEN1 and PSEN2 mRNA are conveyed equally and widely [147], with the cerebellum and hippocampus expressing at the highest levels. They can be seen in glial cells but are primarily expressed in neurons [148, 149]. PSEN-1 on chromosome-14 (14q24.3) and PSEN-2 on chromosome 1 (1q42.2). The membranes of the plasma membrane, the Golgi apparatus, the E.R, and the endosomes where PSEN-1 and PSEN-2 are 50-kDa polytopic transmembrane (PTM) proteins interacting with around 65 percent of their amino acid sequences [150].

PSENs have lately been concentrated at membranes connected to mitochondria (MAM) [151, 152, 153]. The placenta and skeletal muscle have PSEN-2, while the brain, heart, liver, pancreas, kidney, and placenta all include PSEN-2 isoform-2. Isoform 2 lacks amino acids 263–296 [148]. The hydrophilic, flexible N-terminus of PSENS is organized in the cytoplasm, but the C-terminus of PSENS protrudes into the extracellular space’s lumen. On PSENS, there are nine helical T.M. domains. PSENs make up the catalytic core of the $\gamma$-secretase [154].

Nicastrin, anterior pharynx deficient 1, P.S., and the heterotetrameric compound with a high molecular weight (PSEN-1 or PSEN-2) make up this enzyme (PEN-2) [155]. Voltage-dependent Na+channel subunit 2, APP, Notch, Delta1, E- and N-cadherins, CD44, Nectina-1, and ErbB4, are also included [156]. The NTF and CTF, formed by the endoproteolytic cleavage of the N- and C-terminal regions of the PSEN-1 or PSEN-2 subunit, respectively, remain connected to one another in a stable manner throughout $\gamma$-secretase assembly and development [154, 157].

The immature holoprotein 7th hydrophobic domain undergoes an autocatalytic cleavage when P.S. is integrated into the enzyme complex, yielding about 20 kDa for the CTF and 30 kDa for the NTF (which is part of a long cytosolic loop). P.S. cleavage is autocatalytic; however, P.S. maturation is a saturable process and calls for additional molecular partners. Full length (F.L.), an excessive amount of which is an immature form of PSENS results in its proteasomal breakdown. Because of this deterioration, FL-PS has a much lower half-life than the mature form, 24 hours instead of 1.5 hours. FAD is caused by about 150 autosomal dominant mutations in PSEN-1 and 14 in PSEN-2 [158].

According to recent research, FAD-linked PSEN mutants are more active and produce more A$\beta$ peptides in various cell types and transgenic mice with FAD-PSEN mutations. This aligns with the discovery that the catalytic center of the $\gamma$-secretase comprises PSENs [159]. Other investigations have cast doubt on this initial hypothesis, revealing decreased total enzymatic activity in FAD-PS expressing mice with just an elevation in the ratio of A$\beta$-42/A$\beta$-40, primarily due to a decrease in A$\beta$-40 production [160].

Consequently, although this topic is still intensely debated, the initial gain-of-function theory of loss of function has resulted from FAD-PSEN mutation. The total amount of A$\beta$ generated reduces due to the less precise cleavage of APP brought on by FAD-PSENs. As a result, there is an increase in the synthesis of A$\beta$-42 and a decrease in the creation of A$\beta$-40, changing the physiological homeostasis of the process. Another approach has recently been proposed, according to which PSEN1 mutations reduce $\gamma$-secretase’s ability to trim proteins by limiting its ability to operate as a carboxypeptidase favoring the production of A$\beta$-42 [159, 161].

PSENs are well-known for their pleiotropic $\gamma$-secretase-independent functions in addition to their well-described catalytic activity within the $\gamma$-secretase complex, particularly in maintaining Ca${}^{2+}$ homeostasis but also in protein trafficking, cell adhesion, and autophagy.

$\beta$-Amyloid (A$\beta$) synthesis, oligomerization, and deposition are all known to be crucial factors in AD. The hypothesis that excessive A$\beta$ production contributes to the onset of AD has been underlined by mutations in the APP and presenilin genes, which enhance the synthesis of A$\beta$ (1-42). Evidence shows that the apolipoprotein E (APOE) protein’s isoform APOE4 may interact with the A$\beta$ cascade and cause AD pathogenesis.

First, APOE is present in neuritic plaques, and AD patients with APOE4 have higher amounts of A$\beta$ in their brains. Similar results have been seen in transgenic mice expressing human APOE isoforms [172].

Second, research has indicated that APOE3 and APOE4 attach to A$\beta$ differently. However, it is unknown whether APOE4 actively promotes A$\beta$ aggregation and deposition or if APOE3 and E2 play a protective function by preventing A$\beta$ aggregation or encouraging A$\beta$ clearance. Compared to APOE3, APOE4 has a higher affinity for A$\beta$ in a lipid-free state. However, it has also been shown that APOE3 and E2 bind to A$\beta$ faster when bound to lipoproteins [173]. It has been discovered that APOE dose significantly impacts plaque deposition in mice models, indicating that APOE is actively involved in plaque formation. However, research has revealed that whereas APOE4 promotes greater A$\beta$ deposition, APOE3 improves A$\beta$ clearance [174].

Thirdly, APP processing, A$\beta$ clearance, and A$\beta$ production may all include neuronal APOE receptors in one way or another. APOE4 has been reported to improve the synthesis of A$\beta$ by encouraging the endocytic recycling of APP. Last but not least, in vitro research has shown that APOE amplifies the neurotoxicity of A$\beta$, with APOE4 having more neurotoxic effects than APOE3. Furthermore, the buildup of intraneuronal A$\beta$ has been linked to the defects in neuroplasticity brought on by APOE4 in a transgenic mouse model. These results indicate a synergistic pathogenic interaction between APOE4 and A$\beta$ [175].

Liposomes, specifically their phospholipid bilayer, are a potential solution for transporting medication across the BBB, but the BBB is not easily penetrable. Therefore, numerous surface modifications have been made to enhance the transport of liposomal carriers across the BBB [221]. The BBB surface contains proteins, peptides, antibodies, and other ligand receptors, which can aid in transcytosis. Ligands present in these compounds facilitate the movement of cationic liposomes through the BBB. To enhance their movement throughout the body, glucose and other nutrients are incorporated into the liposomes [222]. The passive diffusion mechanism is started by the brain’s passive efflux when the liposomes reach the brain [222]. The substance releases at the same rate as before. New tactics have been developed that adapt to changes in the patient’s physiological parameters and control the drug’s release. pH changes, changes in enzyme activity, or changes in glutathione levels can all cause liposome drug release [223]. Acetylcholine (ACh) is bound by amyloid peptides, causing the dissolution of amyloid plaque and reduction of inflammation in the brain. This supports the preservation of healthy neurons and serves as a treatment option for AD once it has developed. To facilitate active internalization, liposomes can be modified with ligands due to the negative charge of the BBB and the occurrence of electrostatic forces [224].

To improve the pharmacokinetic profile of liposomes, PEG11 or polysaccharides can be added to the particles’ surface, extending their circulation time and improving their distribution into the brain by inhibiting rapid clearance via the reticuloendothelial system (RES). However, there is no guarantee that liposomes will cross the BBB, even with “stealth” liposomes that can significantly reduce circulation time. Several emerging strategies to enhance the stealth liposomes include incorporating peptides, antibodies, or small molecules that specifically bind to receptors or transporters overexpressed on brain endothelial cells. Among the various methods of delivering liposomes to the brain, receptor-mediated transcytosis is the most efficient and commonly used approach due to the precise interactions between receptors and ligands [225].

The transmembrane glycoprotein TfR13 is a frequently targeted receptor due to its high concentration in brain endothelial cells. However, the binding of endogenous Tf inhibits the binding of TfR-targeting ligands to the receptor. To overcome this, experiments involving Tf14-functionalized liposomes have utilized antibodies that bind to unique sites on the TfR receptors to reduce ligand antagonism and enhance the effectiveness of the liposomes in crossing the BBB [226, 227]. A different approach, mammalian iron-binding glycoprotein lactoferrin, has also functionalized liposomes. Lactoferrin interacts with Lf15 receptors and is widely dispersed throughout the BBB. Using receptor-mediated transcytosis, lactoferrin-modified liposomes were developed as modified nanocarriers for BBB crossing [228].

To determine whether combining the two BBB-piercing peptides may have a synergistic effect and whether curcumin interfered with the activity of the BBB-targeting ligands, the previous research was coupled with curcumin and two BBB-invading peptides [229].

The surface of multifunctional liposomes is modified by introducing two BBB-binding ligands and a curcumin derivative, which targets transferrin and LDL receptors. Researchers studied the inhibitory effects of these surface-modified liposomes on A$\beta$ in hCMEC/D3 cells. The ability of these liposomes to target the BBB and bind amyloid was investigated while examining the effects of one or more modifications [229, 230]. To develop a nanomedicine for Alzheimer’s disease, PEGylated immunoliposomes were created by combining two monoclonal antibodies (mAbs) targeted at the transferrin receptor and A$\beta$ peptide. The surface of the liposomes was bound with transferrin receptors antibody (OX26), an anti-transferrin receptor mAb, and 19B8, which were conjugated to maleimide and streptavidin-biotin. When administered intravenously to male Wistar rats, these immunoliposomes were absorbed by porcine brain capillary endothelial cells and crossed the BBB effectively in vivo. The persistent BBB breach caused by the disease allowed 19B8 to cross the BBB, and it was found that using immunoliposomes with two ligand-targeting antibodies was an effective treatment for Alzheimer’s disease [231].

Gold nanoparticles (GNPs) have several advantages over other metal NPs, including chemical inertness, biocompatibility, and lack of cytotoxicity. Humans have been using gold internally for 50 years [232]. The nano size, surface area, and photothermic nature directly impact the antibacterial activity of GNPs [233]. Another accepted theory is that GNPs connect intracellularly to the Sulphur base found in cells as the thiol group in enzymes, causing an abrupt disruption of the respiratory chain by forming many free radicals in death. GNPs, on the other hand, lower Adenosine tri-phosphate (ATP) activities by reducing t-RNA and ribosome interaction [234]. GNPs also decrease bacterial growth by blocking transmembrane hydrogen efflux; yet, at lower concentrations, GNPs can reduce bacterial growth by 250-fold. Because GNPs are smaller than bacterial cells, they attach to pathogen cell walls and cause cell death by delaying cell processes [234].

GNPs apply to several industries, including electronics, energy, optical, and health care. GNPs are studied for multiple purposes, like ultra-sensitive biomarkers [235], targeted heat therapy, and drug delivery vectors [236]. Gold has a higher radio density compared to contrast agents already in use. GNPs have exhibited more potent contrast qualities than those now used as a contrast agent for CT scanning. GNPs can also be used as a drug delivery vector for various reasons. For example, the same GNPs can be linked to several functional groups, enabling customized drug delivery [234]. Nanoscale particles are particularly well suited to this purpose due to their high surface area to volume ratio; additionally, light absorption by the plasmon of GNPs can be exploited to induce the heat-sensitive release of medicinal chemicals. The plasmonics of GNPs also enables localized heat therapy through the absorption of specific wavelengths of light [237]. The absorbed energy is then diffused onto the surrounding region. Combining this method of cancer tumor attack with the production of heat-sensitive anti-carcinogens is wildly successful [238].

GNPs have been investigated to boost the efficiency of solar panels in energy technology, and their utility is also derived from their plasmonic characteristics. Specific wavelengths of light are required for solar panels to function. Nevertheless, as the plasmon of NPs can be altered through regulated synthesis, Developing NPs that can both absorb light at frequencies that the solar panel cannot use and release light at those frequencies is possible [238]. Other metallic NPs have been studied using similar methods. Another notable application of GNPs is wiring in nano-circuitry [238]. Since the formulation of Moore’s law in 1965, there has been a lot of interest in making circuits smaller and smaller, eventually leading to courses on the nano-scale [239].

The administration of immunotherapy doses to target amyloid plaques for Alzheimer’s disease treatment results in significant adverse outcomes, including meningoencephalitis [240]. Nanoparticles coated with antibodies that target specific proteins can detect and dissolve protein clumps in brain cells, potentially reducing the harmful side effects of immunotherapy for Alzheimer’s disease. One technique to accomplish this is using metal oxide nanoparticles coated with antibodies that bind to AD-associated proteins, which can be detected using secondary ion mass spectrometry [241]. Polyethylene glycol (PEG) nanoparticles coated with specific antibodies can degrade A$\beta$-42. In Alzheimer’s disease, A$\beta$ fibrils cause neurotoxicity, but this can be significantly reduced by covering the surface of poly (lactic-co-glycolic acid) (PLGA) nanoparticles with monoclonal A$\beta$ 83-14 [242].

Iron oxide nanoparticles (NPs) are commonly used in biomedical research, and there is ongoing research to explore the possibility of creating new theranostic drugs for Alzheimer’s disease using ultra-small superparamagnetic iron oxide nanoparticles and a phenothiazine-based near-infrared (NIR) fluorescent dye. These particles have the potential to offer NIR fluorescence and magnetic resonance imaging of A$\beta$ plaques in the brains of mouse models with Alzheimer’s disease and also prevent the plaques from aggregating [242]. Protein-capped (PC) cadmium nanoparticles and PC-Fe${}_3$O${}_4$ coated with proteins may be a promising strategy for inhibiting tau aggregation in AD cells and developing anti-tau aggregation drugs. These iron oxide nanoparticles may also have applications in diagnosing and treating neurological diseases such as AD [243].

A$\beta$ peptides consisting of 39–42 amino acids are mainly present in humans, with A$\beta$-1-40 and A$\beta$-1-42 being the most abundant. These peptides can form A$\beta$ plaques due to their ability to form fibrils. The accumulation of A$\beta$ peptides, including soluble oligomers and mature fibrils, is associated with the development of AD [244]. However, only a few authorized medications may be used to treat AD due to the related toxicity and emergence of resistance [245]. Peptides, organic compounds, and synthetic peptides have shown promising results in preclinical studies in AD by either preventing the formation of aggregates or removing them. However, their effectiveness is limited due to poor in vivo stability, limited BBB penetration, and inferior efficacy, making them inefficient in treating AD [246]. The effectiveness of modern AD therapies is attributed to the use of quantum dots (QDs), which potently inhibit the formation of A$\beta$ plaques and protect cells against the harmful effects of A$\beta$ oligomers. This is due to their small size (2–10 nm) and low cytotoxicity. Additionally, A$\beta$-1-42 peptides and carbon materials can interact hydrophobically to prevent A$\beta$ plaque formation, reducing negative surface potential and enhancing the inhibitory properties of QDs [247] (Table 4, Ref. [221, 229, 248, 249, 250, 251, 252]).

Tramiprosate covalently coupled with graphene quantum dots (QDs) decreased A$\beta$ accumulation in AD synergistically [253]. The coupling of glycine-proline-glutamate (Gly-Pro-Glu, GPE) and GQDs resulted in the creation of glycine-proline-glutamate (GQDG) nanomaterial, which showed a reduction in A$\beta$-1-42 fibril aggregation when administered intravenously in APP/PS1 transgenic mice (Fig. 4, Ref. [50]). The small size and large surface area of GQDG allowed it to pass through the Madin-Darby Canine Kidney (MDCK) cell monolayer and selectively interact with the hydrophobic group of A$\beta$1-42 protein, leading to increased inhibition and improved memory and learning in mice treated for AD [254].

The distinct characteristics of QDs make them an attractive alternative to traditional imaging techniques and dyes. QDs can detect A$\beta$ aggregation states in vivo, enabling early identification of AD. To achieve this, fluorescent QD probes and an anti-A$\beta$ antibody are injected into transgenic mice carrying human APP695swe and APP717 mutations via the intracerebroventricular route [255].

In gene therapy, which was initially proposed in the 1960s, the barrier of gene delivery is overcome due to NPs. These challenges include cell-specific targeting, protecting genetic cargo from deterioration, preventing reticuloendothelial system (RES) clearance, and lysosomal and endosomal escape [256].

Inorganic NPs can be engineered to enhance gene delivery effectiveness. An ideal gene delivery method should possess the following characteristics: the capability to enter the cell’s plasma membrane, disrupt the endosomal membrane, bind and condense genetic material, safeguard the nucleic acid cargo, ensure specific delivery to the intended target, remain stable in the bloodstream and evade immune system detection [256, 257]

Extensive research into the underlying disease mechanisms of neurodegenerative disorders has identified specific genetic abnormalities that play a crucial role in disease onset. Gene therapy offers a means to deliver various types of genetic material, including messenger RNA (mRNA), small interfering RNA (siRNA), guide RNA (gRNA), and microRNA (miRNA). Scientific investigations have shown that RNA interference (RNAi), a technique that reduces the production of specific mRNA molecules using siRNA and miRNA, is highly effective for silencing genes involved in disease progression [258].

Synthetic double-stranded siRNAs, approximately 21–25 nucleotides long, are highly precise for targeting specific mRNA sequences and silencing genes within human cells [259]. The advent of RNAi has revolutionized the field of therapeutics, providing a new pathway for treating a wide range of diseases, including cancer and neurological disorders [260]. However, to practically implement siRNA-mediated gene silencing in medicine, it is crucial to have an efficient and safe delivery method, preferably a nanocarrier, to transport the siRNA to the desired site effectively. Recent advancements in gene therapy, such as genome editing, hold significant promise in specifically targeting abnormal genetic modifications in diseased areas [261].

The accumulation of misfolded proteins, such as amyloid-beta oligomers and alpha-synuclein, is a key factor in various diseases. These proteins lead to endoplasmic reticulum (ER) stress and trigger ER-associated degradation, making them potential targets for gene therapy [262]. When these proteins aggregate within the ER lumen, they disrupt ER calcium homeostasis and disturb the unfolded protein response (UPR) signaling. As a result, neurons undergo apoptotic cell death through pro-apoptotic reactions [263, 264]. In the context of diseases like Parkinson’s disease (PD), it has been possible to improve protein folding and preserve dopaminergic neurons’ survival and motor function through gene therapy. Specifically, gene therapy has been employed to suppress the overexpression of the BiP gene, also known as glucose-regulated protein 78, which is associated with a reduction in the unfolded protein response. In such cases, gene silencing techniques can effectively address the pathology [265].

In AD, PD, and Huntington’s disease (HD), aberrant signaling of the rapamycin (mTOR) pathway has been noted in addition to the deregulation of epigenetics, autophagy, and dysfunctional microglia and astrocytes. As the situation worsens, specific types of dysfunction are displayed by each disease mechanism. Therefore, to provide the best care, it is essential to pinpoint the precise mechanism underlying how a patient present with these disorders. Gene therapy is a viable strategy for treating neurodegenerative diseases because it has effectively treated various problems. This is especially noteworthy given the findings on genetic anomalies in PD and AD patients [266].

Between 2017 and 2020, numerous nanoparticles were used in central nervous system (CNS) gene therapy, as shown in Table 5 (Ref. [267, 268, 269, 270, 271]). Our knowledge of targeting particular genes that are defective or aggregated proteins linked to neurodegenerative illnesses has greatly benefited from these investigations. Approaches for safe and efficient delivery using nanoparticle-based gene therapy are particularly promising in overcoming biological barriers like the BBB. The natural synthesis of NPs offers several advantages besides gene therapy, mainly when using certain extracts that can work together with therapeutic genes [267].

Applications for diagnosing and treating AD using nanotechnology are promising. Targeting particular AD-related biomarkers with functionalized nanoparticles enables non-invasive imaging approaches for early detection and diagnosis [272]. By encapsulating therapeutic chemicals and boosting their stability, bioavailability, and targeted distribution to the brain, nanoparticles can help improve drug delivery and targeted therapy [273, 274]. Additionally, functionalized nanoparticles can support neuroprotection and regeneration by providing neurotrophic factors and reducing neuroinflammation and oxidative stress [275].

However, there are challenges to overcome in translating nanotechnology into AD.

Biocompatibility and safety: To assure the long-term safety of nanomaterials, thorough studies are needed to evaluate their possible toxicity and clearance pathways [276].

Blood Brain Barrier: Effective distribution of nanoparticles across the BBB is still difficult. To increase BBB penetration, strategies like surface alterations, functionalization with targeted ligands, and focused ultrasonic techniques are being investigated [277].

Scalability and manufacture: For clinical translation, scalable and repeatable synthesis and manufacturing processes for nanoparticles are required [278].

Curcumin, found in the rhizome of turmeric (Curcuma longa), is used in Indian cuisine to add flavor and preserve food [280] (Fig. 6). An intriguing observation is that the incidence of AD among individuals aged 70–79 in India is four times lower than that in the United States, implying that a diet high in curcumin among older Indians could be accountable for the reduced risk of AD [281]. The anti-amyloidogenic, anti-inflammatory, and anti-oxidative properties of curcumin have been backed by robust in vitro and in vivo studies, which suggest that it can potentially prevent Alzheimer’s disease [35]. Curcumin and its derivative rosmarinic acid exhibit anti-amyloidogenic properties in vitro, inhibiting the formation of neurotoxic A$\beta$ fibrils and preventing their elongation from fresh A$\beta$. Additionally, they can break down preformed A$\beta$ fibrils and regenerate A$\beta$ monomers [36]. Curcumin has been identified as a potential therapeutic agent for treating and preventing AD because of its ability to target A$\beta$ fibrilization primarily. The inhibition of A$\beta$ fibril formation has been proposed as a practical therapeutic approach for AD treatment. The unique structure of curcumin, composed of two 3,4-methoxyhydroxyphenyl rings connected by a short carbohydrate chain, enables it to bind specifically to free A$\beta$, preventing the formation of A$\beta$ fibrils. However, the exact mechanism underlying curcumin’s anti-amyloidogenic effect is unclear. Additionally, curcumin may bind specifically to A$\beta$ fibrils, disrupting their $\beta$-sheet-rich structure [36].

Curcumin, a substance that can target amyloid pathology, has drawn much attention for its potential as a treatment for AD. Its use in diagnostics using imaging techniques is one area of focus. When human neuroblastoma SK-N-SH cells were subjected to H${}_2$O${}_2$, using biodegradable PLGA-curcumin nanoformulations demonstrated both safety and protective effects against oxidative damage. This formulation protects neurons against oxidative stress, a typical occurrence in AD, by inhibiting the activation of the redox-sensitive transcription factor nuclear factor erythroid 2–related factor (Nrf2) in the presence of H${}_2$O${}_2$ [282]. It has been shown that curcumin formulations with different modifications can prevent amyloid-beta (A$\beta$) from aggregating and protect against its harmful effects. These formulations include nanoliposomes, lipid-conjugate liposomes, biodegradable poly (alkyl cyanoacrylate), biotin-coupled poly (ethylene glycol) (PEGylated), PEG liposomes with anti-transferrin, and click chemistry. These alterations have demonstrated the potential of curcumin-based formulations to stop the aggregation of A$\beta$ and protect cells from its damaging effects [283, 284].

Athymic mice fed with the NanoCurcumin formulation showed decreased levels of H${}_2$O${}_2$ and increased levels of glutathione in their brains. Furthermore, the activity of the cell death-related enzymes caspase 3 and 7 dropped. These results demonstrate the promise of this therapy for AD, indicating a favorable cellular environment with enhanced redox balance. Another unique method uses curcumin-loaded polymer nanoparticles coupled to APOE3 and constructed of PBCA (poly butyl cyanoacrylate). This method has proven effective in reducing the cytotoxicity caused by A$\beta$ in AD [285, 286]. Curcumin-functionalized gold nanoparticles have shown successful interactions with amyloid protein/peptide. These nanoparticles can prevent the development of amyloid fibrils and even break up already-formed fibrils. They help to stop the aggregation of amyloid proteins and encourage their disaggregation by acting as synthetic molecular chaperones [287].

In male Lacca mice, an oral curcumin lipid nanoformulation has demonstrated encouraging results in correcting the adverse effects of aluminum (AlCl${}_3$) exposure. The effectiveness of this therapy was shown by the 97% restoration of membrane lipids and a 73% recovery of acetylcholinesterase function. Additionally, PLGA-based curcumin nanoformulations have shown promise in reversing the neurotoxicity brought on by acrolein. The restoration of -glutamylcysteine synthetase levels and a decline in reactive oxygen species (ROS) and reactive nitrogen species were credited with this reversal. The drop in glutathione levels, known for their neuroprotective role, was unaffected by this treatment, which is crucial to highlight [288].

The compound N-[2-(3,4-methoxyphenyl)ethyl]-N-[2-(3,4-methoxyphenyl)ethyl]-3-phenylacrylamide, also known as GX-50, has been found in Sichuan pepper (Zanthoxylum bungeanum) and has been recognized as a potential therapeutic agent for AD [289]. Furthermore, in vitro studies showed that GX-50 can disintegrate A$\beta$ oligomers, prevent A$\beta$-induced neuronal apoptosis and apoptotic gene expression, and minimize neuronal calcium toxicity [290, 291]. Furthermore, GX-50 can inhibit microglial migration toward A$\beta$ aggregates by activating the TGF-1-Smad2 signaling pathway and decreasing CCL5 chemokine production [292]. However, GX-50 is linked to neuroinflammation, and its anti-inflammatory properties have not been fully explored. Toll-like receptor 4 (TLR4) expression is increased in activated microglia and the brains of AD patients, resulting in the activation of the NF-$\kappa$B and MAPK pathways and the production of proinflammatory cytokines.

The possibility of treatment approaches that target this receptor for treating the condition is highlighted by the function of TLR4 in the onset of AD. Numerous studies have shown that blocking TLR4 can halt the course of AD. The clearance of A$\beta$ and the induction of neurotoxic cytokines during neuroinflammation are two distinct functions of TLR4 signaling in AD. As a result, individuals with AD may experience both good and bad outcomes from TLR4 activation. TLR4 activation can be harmful because it is frequently used in trials to produce a state that resembles AD and is characterized by neuroinflammation and memory deficits [293, 294]. TLR4 inhibition and the subsequent decrease in pro-inflammatory cytokines may be the underlying molecular mechanisms for these effects. It has been demonstrated in studies employing the APP/PS1 animal model, which has cerebral amyloid deposition, that lowering TLR4 levels enhances cognitive function. The injection of TAK-242, a particular inhibitor of TLR4, improved cognitive performance, reduced A$\beta$ formation, and prevented neuronal death in these animal models. Similar to what was seen with TLR4 inhibition, baicalin has also shown neuroprotective effects in this mouse when acting through the TLR4/NF-$\kappa$B signaling pathway [295, 296]. A Sichuan pepper extract known as GX-50 as shown in (Fig. 7) has anti-inflammatory properties, particularly in the brain regions afflicted by AD. TLR4 is suppressed as part of its mechanism of action, which then lessens the recruitment of MyD88 and TRAF6. As a result, the critical signaling pathways involved in inflammation—NF-$\kappa$B nuclear translocation and MAPK phosphorylation are suppressed. In the setting of AD, GX-50 demonstrates its anti-inflammatory capabilities via modulating various molecular pathways [297, 298].

The possibility of GX-50 inhibiting A$\beta$-42, a protein linked to AD, is being studied. When GX-50 and gold nanoparticles (AuNPs) are used together, A$\beta$-42 is dramatically inhibited compared to GX-50 alone. The potential use of the GX-50-AuNPs complex in treating AD has been further verified by molecular docking studies, systems biology, and time course simulation [299].

Resveratrol, a polyphenolic phytoalexin found in berries, grapes, and red wine, has been associated with several biological and pharmacological effects [280]. Numerous epidemiological studies have reported an inverse association between wine consumption and the development of AD, indicating that resveratrol may contribute to wine’s beneficial effects in treating Alzheimer’s patients. Resveratrol as shown in (Fig. 8), like curcumin, can readily penetrate the intact BBB and enter the brain tissue [300]. Recent research has demonstrated that resveratrol possesses considerable neuroprotective properties in vivo and in vitro experiments. It has been found to hinder neuronal cell death and reduce brain damage caused by ischemia/hypoxia, trauma, and excitotoxicity [301, 302]. In various cell lines expressing the Swedish mutant APP695, resveratrol has been demonstrated to have anti-amyloidogenic properties by decreasing the secretion or intracellular accumulation of A$\beta$ peptides; however, it does not affect the enzymes involved in A$\beta$ production, such as $\beta$- or $\gamma$-secretase. The reduction of A$\beta$ levels by resveratrol was inhibited by selective proteasome inhibitors and siRNA-mediated knockdown of the proteasome subunit $\beta$5, suggesting that resveratrol modulates proteasome activity to lower A$\beta$ levels [303].

Resveratrol-Selenium Nanoparticles (RSV-SeNPs) demonstrated a potent anti-inflammatory and antioxidant impact against neurotoxicity. Additionally, because RSV-SeNPs have an anti-amyloidogenic capability, they improve the clearance of misfolded proteins. RSV-SeNPs affect several signaling pathways implicated in AD development, alleviate cholinergic deficiencies, and enhance neuropathology and neurocognitive functions. RSV-SeNPs may be utilized to treat AD in general. However, more research is needed to pinpoint the precise underlying pathways [304].

Acetylcholine (ACh) is well known for playing a critical part in memory and learning processes. A negative feedback loop that controls A$\beta$ production has been suggested due to the interplay between the cholinergic and A$\beta$ systems [305]. This feedback loop is broken by abnormal A$\beta$ buildup, affecting cholinergic transmission, especially through alpha-7 nicotinic acetylcholine receptors [306].

Based on this knowledge, cholinesterase inhibitors are efficient AD treatments, supporting Davies and Maloney’s (1976) original concept on the role of cholinergic deficiencies in AD pathology. Tacrine, Donepezil, Rivastigmine, Galantamine, Xanthostigmine, Para-aminobenzoic acid, Coumarin, Flavonoid, and Pyrroloisoxazole analogs are only a few of the drugs that have been created and explored for the treatment of AD [307, 308]. Drugs like Rivastigmine, Donepezil, and Galantamine that have received Food and Drug Administration (FDA) approval improve cholinergic function by preventing the activity of acetylcholinesterase, which breaks down acetylcholine. The brain’s ACh levels rise as a result of these drugs. Acetylcholinesterase inhibitors are often well tolerated except for tacrine; side effects are typically dose-dependent. A monoamine oxidase A and B inhibitor called Ladostigil (TV3326), currently in phase II clinical studies, also exhibit antidepressant benefits [309, 310].

It is generally known that glutamate-induced excitotoxicity causes cells to become overloaded with calcium, experience a mitochondrial malfunction, produce more nitric oxide, and make a lot of oxidants, all of which contribute to neuronal apoptosis. However, NMDA receptor antagonists like memantine can lessen the harmful effects of glutamate. Memantine, which the FDA licensed in 2003 for use in patients with moderate-to-severe AD, has demonstrated some modest cognitive advantages in mild-to-moderate AD [311, 312].

Memantine protects neurons by inhibiting the activity of glycogen synthase kinase-3 (GSK-3), which lowers tau phosphorylation. Memantine is an acetylcholinesterase inhibitor that functions as a noncompetitive antagonist of the glutamatergic NMDA receptor. It can be used alone or in conjunction with other medications. It’s important to remember, though, that as compared to monotherapy, combination therapy may not result in as many positive changes [307, 313].

The APP cleavage by $\alpha$-secretase or $\beta$-secretase enzymes is followed by processing by $\gamma$-secretase. The creation of inhibitors that target this amyloidogenic pathway was prompted by the theory that aging causes “overactivation” of secretases or reduced $\alpha$-secretase processing [314].

The $\alpha$-secretase activity of specific metalloproteinases, such as ADAM10 and matrix metalloproteinase 9 (MMP-9), has been investigated. Melatonin, gemfibrozil (a PPAR $\alpha$-agonist), and serotonin 5-HT4 receptor agonists have all been suggested as ways to stimulate ADAM10 and inhibit the production of A$\beta$ [315]. Additionally, it has been demonstrated that overexpressing MMP-9 prevents cognitive abnormalities in transgenic mice models of AD [316].

Inhibitors have been developed to target the transmembrane protease BACE1, although early attempts based on molecular docking techniques to target the enzyme’s inaccessible catalytic core failed. Compared to other ADAM proteases, BACE1 inhibition has fewer adverse consequences [317, 318].

A multisubunit protease complex called $\gamma$-secretase is involved in the proteolysis of several signaling proteins. $\gamma$-secretase inhibitors have been examined; however, due to suppressing other signaling pathways, particularly the Notch system, they frequently cause serious side effects, such as gastrointestinal problems and an increased risk of skin cancer. Although notch-sparing inhibitors have been developed, the findings of their clinical trials have not been very encouraging. It has been noted that $\gamma$-secretase complex modulators have more promising outcomes than inhibitors [308].

To stop the production of A$\beta$ peptides, both $\beta$-secretase and $\gamma$-secretase must be active. Dietary modifications and antioxidants that activate the ADAM proteases may compensate for the reduction in $\gamma$-secretase activity that comes with aging. PSEN-1 and PSEN-2 genetic deficiencies in -secretase are recognized risk factors for familial AD. As a result, early trials of $\gamma$-secretase inhibitors were unsuccessful; now, modulators that target this complex show more potential [319].

The effects of $\gamma$-secretase inhibitors on other cleaved receptors, like the Notch receptor, are linked to their toxicity. Contrarily, $\gamma$-secretase modulators only affect the A cleaving site, not the complex’s other sites [320]. Furthermore, the regulation of A$\beta$ synthesis may be influenced by cholesterol and its derivatives, such as cholesterol acid, which function as $\gamma$-secretase modulators. These endogenous metabolites’ dysregulation may contribute to AD linked to metabolic syndrome, dyslipidemia, and obesity [321].

Significant research efforts have been made in recent years to create cures that aim to stop the development and aggregation of the A$\beta$ peptide. Small molecule inhibitors have been studied in clinical trials to prevent A$\beta$ aggregation, including tramiprosate (in phase III), clioquinol (in phase II), scylloinositol (in phase II), and epigallocatechin-3-gallate (in phase II/III). These medications have considerable adverse effects, even though they can stabilize A$\beta$ monomers [322].

Another strategy uses artificial peptides called “sheet breakers” generated from the iA5p sequence. Zetidine-2-carboxylic acid, 3-phenyl azetidine-2-carboxylic acid, proline, and sulfonyl proline are a few peptides that have demonstrated promise in reducing the cellular harm brought on by A$\beta$ exposure. They accomplish this by inhibiting the development of fibrils and successfully improving spatial memory [323, 324].

Additionally, stemazole has demonstrated defense mechanisms against A$\beta$-induced toxicity in SH-SY5Y cells in vitro, lowering A$\beta$-aggregation. The efficacy of substances like curcumin, T718MA, and SK-PC-B70M to protect neurons against A$\beta$-induced toxicity has also been proven. These results imply that these substances may be able to reduce the harmful effects of A$\beta$ on neuronal cells [323].

Neurofibrillary tangles, which are collections of hyperphosphorylated tau protein in the form of paired, helical-twisted filaments, are one of the therapeutic targets in treating them. Axon’s Alzheimer’s Disease vaccine (AADvac1) was the first vaccination evaluated in clinical immunotherapy trials; currently, trials for ACI-35, another vaccine based on liposomes, are being conducted [308].

It has been investigated to prevent tau proteins from becoming phosphorylated, which is a factor in the aberrant aggregation of these proteins. However, testing of tideglusib, an irreversible inhibitor of the protein kinase GSK-3 (which is implicated in tau phosphorylation), failed to produce any statistically significant advantages. Cyclin-dependent kinase 5 (CDK5), which is similarly involved in tau hyperphosphorylation, is a potential therapeutic target [309].

Several compounds are being tested in clinical studies for their potential to prevent tau aggregation. One such example is the ability of methylene blue (MB) and its metabolites azure A and azure B to enhance protein degradation and inhibit the activities of caspase-1 and caspase-3 [325]. Additionally, in mouse models genetically modified to express tau mutations associated with AD, leucomethylthioninium with a suitable counterion (LMTX, in phase III clinical trials) and methylthioninium chloride, or MTC (in phase II clinical trials) have demonstrated the capacity to decrease tau aggregation and reverse behavioral deficits [326]. Additionally, they can reduce the course of the disease in AD patients. LMTX and MTC have been shown to have neuroprotective effects in vivo, although the precise mechanisms by which they do so are still not well known. N-phenylamines, anthraquinones, phenyl thiazolyl-hydrazides, rhodanines, benzothiazoles, and phenothiazines are further interesting tau aggregation inhibitors [327, 328].