Review on anti-alzheimer drug development: approaches, challenges and perspectives

Abstract

Alzheimer is an irreversible progressive neurodegenerative disease that causes failure of cerebral neurons and disability of the affected person to practice normal daily life activities. There is no concrete evidence to identify the exact reason behind the disease, so several relevant hypotheses emerged, highlighting many possible therapeutic targets, such as acetylcholinesterase, cholinergic receptors, N-methyl D-aspartate receptors, phosphodiesterase, amyloid β protein, protein phosphatase 2A, glycogen synthase kinase-3 beta, β-secretase, γ-secretase, α-secretase, serotonergic receptors, glutaminyl cyclase, tumor necrosis factor-α, γ-aminobutyric acid receptors, and mitochondria. All of these targets have been involved in the design of new potential drugs. An extensive number of these drugs have been studied in clinical trials. However, only galantamine, donepezil, and rivastigmine (ChEIs), memantine (NMDA antagonist), and aducanumab and lecanemab (selective anti-Aβ monoclonal antibodies) have been approved for AD treatment. Many drugs failed in the clinical trials to such an extent that questions have been posed about the significance of some of the aforementioned targets. On the contrary, the data of other drugs were promising and shed light on the significance of their targets for the development of new potent anti-alzheimer drugs.

---

Abdallah E. Abdallah

Abdallah E. Abdallah received his Bachelor's degree in pharmaceutical sciences in 2004, Master's degree in pharmaceutical chemistry in 2015, and PhD degree in pharmaceutical chemistry in 2018. Now, he is an associate professor at Faculty of Pharmacy, Al-Azhar university, Egypt. He is interested in the discovery of new therapeutic candidates, in particular, anticancer, immunomodulatory, and anti-SARS-CoV agents, especially in fields that are of high significance to society. Furthermore, he is interested in complex diseases that are difficult to treat e.g., Alzheimer disease.

---

1. Introduction

1.1. Background

Alzheimer disease (AD) is an irreversible and progressive neurodegenerative disorder leading to ultimate damage and death of brain neurons.^1,2 It causes cognitive impairment, behavioral defects, psychological disorders, memory loss, and disturbances in daily life activities such as eating, drinking, reading, writing, walking, communication, etc.³ It significantly affects not only the life of patients, but also that of their family.⁴ It mainly affects old people at or above 65 years old, but nowadays it seems to touch younger adults as a consequence of modern lifestyles.³ AD is the primary cause of dementia and affects approximately 10% of people over the age of 65 and 50% over the age of 85.⁵ It was reported that 6.7 million American people lived with clinical AD in 2023, and this figure is expected to reach 8.5 million by the year 2030.⁶ In a separate report, the estimate of worldwide people with AD reaches 74.7 million, with a care cost of about US$2 trillion by 2030.⁵

1.2. Etiology

Unfortunately, the etiology of AD is still incompletely understood. There is no definite reason evident to be behind the disease. Alternatively, AD is considered to be related to different reasons, among which are genetic factors and environmental effects such as mental stress, food habits, and lifestyle.⁷ However, histopathological studies revealed relevant multifactorial disorders from which different hypotheses originated to shed light on the likely mechanisms and effective targets of the disease. The emerged hypotheses include the following: (a) cholinergic hypothesis, (b) amyloid cascade hypothesis, (c) tau hypothesis, (d) mitochondrial cascade hypothesis, (e) oxidative stress hypothesis (f) excitotoxicity hypothesis, (g) neuroinflammatory hypothesis, and (h) others (like genetic factors, environmental factors, etc.).

1.2.1. Cholinergic hypothesis. It is evident that the whole central neurotransmitter system is affected in AD; however, the cholinergic system remains comparatively the most deteriorated one.^8–10 Furthermore, a correlation between the central cholinergic deficit and the degree of cognitive disorder was detected.^11–13 Because ACh was found to be involved in cognitive processes, work on increasing ACh levels gained a lot of attention in order to restore cognitive normality (the so-called cholinergic hypothesis). Consequently, many drugs have been developed as acetylcholinesterase inhibitors (AChEIs) in order to increase the ACh levels in the brain.¹⁴ It is obvious that such drugs do not repair the damaged neurons; instead, they alleviate the symptoms by increasing the levels and duration of action of the central ACh. Accordingly, they can be described as symptomatic therapy without curing or even preventing the progression of the disease.^13,15 In such a case, new perspectives on disease control appear to be highly reasonable. As a consequence, some other mechanisms and targets were proposed, as we can see below.

1.2.2. Amyloid cascade hypothesis. One of the most characteristic pathological hallmarks in AD brains is extracellular senile plaques, which lead to neuronal damage, pathogenesis, and disease progression.¹⁶ It was discovered that the amyloid β protein (Aβ) is the primary component of the amyloid plaques in AD.¹⁷ In general, there are three forms of Aβ: soluble monomer, soluble oligomer, and insoluble fibril, which are normally degraded and released away from the neurons.¹⁸ Furthermore, amyloid precursor protein (APP) plays a crucial role in neurite development and neuronal membrane trafficking.¹⁸ However, in AD, two enzymes (β- and γ-secretase) were identified to be involved in the overproduction of insoluble Aβ by cleaving APP. This process comprises two successive steps. β-Site amyloid precursor protein cleaving enzyme-1 (BACE1), the main form of β-secretase in the CNS, mediates the first step by cleaving APP to sAPPb and membrane bound C-terminal APP fragment (C99).¹⁹ The latter is then cleaved by γ-secretase to give Aβ peptides, which include Aβ40 and Aβ42. Due to a genetic mutation, Aβ42 is overproduced in AD. It was found that Aβ42 is hydrophobic and tends to accumulate, forming amyloid plaques rather than Aβ40.^20,21 Relatively high levels of BACE1 were detected in sporadic AD brains, accelerating the first step (the rate-limiting one) in the generation of Aβ from APP.^22,23

These facts draw attention to amyloid cascade hypothesis as a considerable theory of AD pathogenesis.²⁴ Over decades, researchers considered Aβ aggregation as the main cause of all AD pathological changes such as neurotoxicity, neuronal inflammation, and neuron loss.^16,18 Accordingly, several attempts at developing anti-Alzheimer drugs targeting Aβ have been made.^25,26 But repeated failures of Aβ-targeted clinical trials pose a question concerning the significance of the amyloid hypothesis.¹⁶ On the basis of such deficiencies in the amyloid hypothesis as well as some recent findings, researchers suggested that the major factor underlying the development and progression of AD is tau rather than Aβ.²⁷

1.2.3. Tau hypothesis. Tau is one of the microtubule-associated proteins that regulate the stability of tubulin assemblies.²⁷ Pathological hyperphosphorylation of tau is observed in AD brains, causing the accumulation of phosphorylated tau inside neuronal cells in the form of neurofibrillary tangles that eventually cause neuronal death.^27–29 Hyperphosphorylated tau is cytotoxic. It inhibits the assembly and functions of tubulin, disrupting proper intracellular transportation. In addition, it negatively affects the integrity of the mitochondrial membrane, leading to mitochondrial swelling and functional defects.^18,29,30 Tau hyperphosphorylation is correlated to dysfunction of glycogen synthase kinase-3 beta (GSK-3β) and/or protein phosphatase 2A (PP2A).^31,32 GSK-3β and PP2A are enzymes involved in the regulation of tau phosphorylation.^32,33

In contrast to amyloid plaques that aggregate extracellularly, tau-based neurofibrillary tangles are accumulated intracellularly. It was proposed that tau pathology is the crucial factor and occurs earlier than Aβ aggregation in sporadic AD.²⁹ So, the tau hypothesis is likely to reveal more predominant effects on neurons than the amyloid hypothesis.²⁷

1.2.3.1. Mitochondrial hypothesis. Mitochondrial function changes over time under the effects of genetic and/or environmental factors. Oxidative stress is a fundamental element that disrupts mitochondrial function and induces mitochondrial fragmentation during the pathogenesis of AD.³⁴ The mitochondrial hypothesis states that in sporadic, late-onset AD, changes in mitochondrial function affect APP expression and amyloid accumulation in a manner that triggers the pathogenesis of amyloid cascade.³⁵ Correlations have been increasingly recognized between mitochondrial function, Aβ amyloidosis, and tau phosphorylation.³⁶

1.2.4. Oxidative stress hypothesis. However, sporadic AD is not linked to genetic mutation; it was suggested to be likely associated with the pathological role of oxidative stress raised by abnormal metabolic reactions in the CNS.³⁷ The pathological hallmarks of the disease, such as amyloid plaques and neurofibrillary tangles, are a result of abnormalities in protein metabolism. This contributes to the oxidative stress in the sense that Aβ was found to generate free radicals.³⁷ Furthermore, there is some evidence that supports increased oxidative stress in AD brain, such as increased levels of Fe, Al, and Hg that can generate free radicals. In addition to increased lipid peroxidation and decreased polyunsaturated fatty acids, there is also increased protein and DNA oxidation in the AD brain. Moreover, diminished energy metabolism and decreased cytochrome c oxidase were observed in AD brain.³⁷ Accordingly, AD brains exhibit constant evidence of reactive oxygen species (ROS) and reactive nitrogen species (RNS)-mediated injury.³⁸

1.2.5. Excitotoxicity hypothesis. Overstimulation of N-methyl D-aspartate (NMDA) receptors by endogenous glutamate (Glu) causes excitotoxic neuronal degeneration in acute central nervous system injury syndromes such as stroke and trauma.^39,40 Similarly, continuous mild activation of NMDA leads, under chronic conditions, to neuronal damage.⁴¹ Furthermore, neural plasticity is likely to be impaired not only from neuronal damage but also from continuous activation of NMDA receptors.⁴¹ In AD, this disorder originates primarily from the toxic effects of accumulated Aβ on certain synapses, targeting the glutamate receptors NMDA and 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propanoic acid (AMPA). The consequences of this effect are the dysregulation and reduction of expression of NMDA and AMPA, and hence the accumulation of the excitatory amino acids glutamate and D-serine. Eventually, this results in synapse failure in AD.⁴² To date, synapse failure is considered one of the primary pathological markers linked to cognitive decline and AD pathogenesis.^43,44 Some researchers refer to this mechanism as synaptic failure hypothesis.⁴⁵

1.2.6. Neuroinflammation hypothesis. It is evident that AD is linked to neuroinflammation, which is triggered by several factors during the disease progression. High levels of proinflammatory cytokines such as tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) are detected in AD brain.⁴⁶ Furthermore, activated microglial cells and astrocytes were observed in AD brain.⁴⁷ It was suggested that activated microglial cells stimulate the release of proinflammatory mediators, leading to neurotoxicity, neuronal damage, and impairment of Aβ clearance.^47,48 Activation of microglial cells occurs as a response to some chronic disorders in the brain, among which are amyloid plaques.^48,49 So, accumulation of microglial cells was detected at the precipitated Aβ.⁵⁰ Additionally, neuronal overexpression of cyclooxygenase-2 was identified at different stages of AD.⁵¹ Some studies have revealed that neuroinflammation is a relatively early pathological feature of AD.⁵¹

1.2.7. Genetic factor of AD. In familial AD, genetic mutations in APP, presenilin 1, and presenilin 2 have been recognized. γ-Secretase, an enzyme linked to overproduction of insoluble Aβ, is encoded by presenilin 1 and presenilin 2.⁵² In sporadic AD, polymorphism in multiple genes has been identified.⁵³ Among them is polymorphism in the ε4 and ε2 variants of the apolipoprotein E (APOE) gene.⁵⁴ So, APOE is considered one of the most fundamental risk factors of sporadic AD.⁵⁵ Some studies revealed that the genetic factor accounts for about 80% of AD.⁵⁴

1.3. Challenges

There are some challenges in the treatment of AD, such as a lack of evidence of the exact mechanism and the primary target on which researchers can work.⁵⁶ Furthermore, no neuronal protective or regenerative drug is available nowadays.⁵⁷ There is no cure for damaged neurons, but there are attempts to stop or delay the worsening of AD. The development of AD drugs was based on different hypotheses that shed light on the histopathological disorders in AD. However, the results of clinical trials questioned almost all of these hypotheses and revealed that none of them can be considered the sole approach to treatment. The data from clinical trials also reflect the complexity of the disease, which has different essential factors contributing to its pathogenesis. Furthermore, AD takes a relatively longer time to complete the clinical study in comparison to most other therapeutic fields.⁵⁸ One more challenge is that AD is a progressive disease. Each stage may require its own specific study because early-stage drugs are likely to be ineffective in later stages. AD is also chronic, which reflects the requirement of drug safety for long-term use.

However, there are advances in anti-alzheimer drugs and their mechanisms of action. The following section concerns the drugs approved for the treatment of AD, and the drugs passed to clinical trials. The results of these clinical studies and their significance are shown below, considering the mechanisms of action extended to a wide range of targets identified in this review.

2. Discussion

In terms of their mechanisms of action, anti-alzheimer drugs can be classified as the following.

2.1. Cholinergic drugs

2.1.1. ChEI. On the basis of the cholinergic hypothesis, choline esterase inhibitors have been developed in order to improve the symptoms of AD. Two types of enzymes have been employed: acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). BChE was found to distribute in the brain and have the ability to degrade ACh. There is some evidence that BChE plays a major role in the hydrolysis of ACh and compensates for the function of AChE when its level is decreased or its production is inhibited in advanced AD.^59,60 In fact, in advanced AD, AChE level declines to 90% in comparison to the levels in a healthy brain. On the other hand, the level of BChE was found to continuously increase in advanced AD.⁶¹

Three drugs are currently FDA-approved for the treatment of AD as ChEIs: galantamine, donepezil, and rivastigmine. Donepezil and galantamine are selective AChEIs,⁶² while rivastigmine has a dual inhibitory effect on AChE and BChE.⁶³

2.1.1.1. AChEIs. Galantamine is a natural alkaloid based on benzofuro[4,3-cd]azepine (Fig. 1). It is a reversible AChE inhibitor with no effect on BChE. Furthermore, its cholinergic activity is enhanced by binding to allosteric nicotinic sites. It is exposed to minor biotransformation that includes demethylation of about 5–6% of the dose. It is excreted mainly in the urine.⁶⁴

Fig. 1 The chemical structures of approved cholinesterase inhibitors.

Donepezil is another reversible AChE inhibitor that is based on an inden-1-one nucleus (Fig. 1). It is indicated for symptomatic treatment of patients suffering from mild-to-moderate AD. Donepezil is extensively bound to plasma proteins (about 96%), so its elimination half-life is prolonged to 70 h.⁶⁴

2.1.1.2. Dual AChE and BChE inhibitors. Rivastigmine is a pseudoirreversible noncompetitive carbamate inhibitor of AChE and BChE (Fig. 1). Although its half-life is limited to only 2 h, its inhibition of ChEs is extended to approximately 10 h. This prolonged effect is attributed to the slow dissociation of the drug enzyme complex. In April 2000, rivastigmine was approved by the Food and Drug Administration (FDA) for the treatment of mild-to-moderate AD.⁶⁴

Tacrine is another dual AChE and BChE inhibitor, based on an acridine nucleus (Fig. 1). Due to its hepatotoxicity, it has been withdrawn from the market.⁶⁵ This class of drugs represents just a symptomatic treatment without preventing the progression of the disease.^66–69

Metrifonate, dimethyl (2,2,2-trichloro-1-hydroxyethyl)phosphonate, is an irreversible inhibitor for both ChEs, with higher selectivity for BChE than AChE. It is a prodrug converted nonenzymatically to the active metabolite dichlorvos, which is responsible for the sustained cholinesterase inhibition (see Fig. 2). Its clinical evaluation in mild-to-moderate AD revealed its toxicity, so its use was suspended for AD patients.⁶⁴

Fig. 2 Chemical structures of organophosphate molecules as cholinesterase inhibitors.

2.1.2. Nicotinic receptor agonist. A different approach to developing new selective anti-alzheimer drugs was targeting the α7 nicotinic receptor instead of acting on all cholinergic receptors, in the sense that this receptor plays a significant role in memory, learning, and executive functions. In contrast to currently marketed AChE inhibitors, which are known to have considerable side effects, the new approach is more likely to eventually develop drugs of relatively high safety.^70,71

ABT-126 (Fig. 3) was identified as a selective α7 nicotinic receptor agonist and suggested as a monotherapy in mild to moderate AD. In phase 2a clinical trials, ABT-126 improved cognition to some extent in patients with mild to moderate AD, while it failed in phase 2b study to reveal any significant improvement⁷² and showed no therapeutic effect.⁷³ With respect to safety profile, ABT-126 was generally well tolerated.⁷³

Fig. 3 The chemical structures of some nicotinic receptor agonists.

Similarly, α4β2 neuronal nicotinic receptors are associated with cognitive functions such as learning, memory, and attention.⁷⁴ ABT-089 (Fig. 3) was developed as a selective α4β2 partial agonist,⁷⁵ but it showed no therapeutic effects against alzheimer in clinical trials.⁷⁶

2.1.3. Muscarinic receptor agonists. Oxotremorine (Fig. 4) was developed as a stimulant for CNS muscarinic receptors. The evaluation of the drug as a potential treatment for AD revealed that oxotremorine increased the levels of ACh up to 40% in the rats' brains. Despite this promising effect that appears to help AD patients, the data from other studies were disappointing. So, the usefulness of oxotremorine in AD is highly disputed by many researchers.⁷⁷

Fig. 4 Oxotremorine chemical structure.

Xanomeline is a selective muscarinic agonist based on the pyridinylthiadiazole scaffold (Fig. 5). It showed a significant improvement in AD patients in clinical trials.⁷⁸ On the other side, it triggered a lot of unwanted side effects. However, it was considered a lead for the development of anti-alzheimer drugs.⁷⁹

Fig. 5 Development of EUK1001 as a xanomeline analog.

EUK1001 is a fluorinated derivative of xanomeline, as can be noticed from Fig. 5. EUK1001 was found to decrease AD-like neurodegenerative disorder in presenilin-deficient mice, which has no Aβ pathology. Furthermore, EUK1001 revealed a significant improvement in cognitive functions in AD mice as well as a reduction in Aβ42.⁷⁹ Meanwhile, another study indicated that EUK1001 improved memory function in aged mice.⁸⁰ According to these data, EUK1001 was suggested as a promising compound for the treatment of AD.⁸¹

2.2. Glutamatergic drugs

Memantine (Fig. 6), an adamantine derivative, was approved by the FDA, and hence it is currently used for the treatment of AD as an NMDA antagonist.⁸² It clinically enhances cognitive ability and improves behavioral disturbance with an excellent safety profile, whether it is used alone or in combination with donepezil.^83–85 Memantine blocks the NMDA receptor, reducing calcium ion influx into the neurons, so that it prevents the activation of toxic intracellular events.⁴⁰ It was found to have a low-to-moderate affinity for NMDA.⁸⁶ On the other side, antagonists with high affinity, such as phencyclidine (Fig. 6), revealed severe adverse effects that make their use for alzheimer not practical.⁸⁶

Fig. 6 The chemical structure of some glutamatergic drugs.

2.3. Phosphodiesterase inhibitors (PDEIs)

Inhibition of PDE was suggested to prevent and improve AD by increasing the levels of cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP).⁸⁷

2.3.1. PDE1 inhibitors. Vinpocetine (Fig. 7) is a PDE1 inhibitor.⁸⁸ In Europe, it has been approved for the treatment of dementia.⁸⁹ Its preclinical data indicated some significant effects, such as repairing cognitive impairment in a rodent AD model,⁹⁰ downregulating BACE1,⁹⁰ decreasing oxidative stress,^91,92 and reducing mitochondrial dysfunction.⁹³ Despite the significant preclinical findings, the results of clinical studies were disappointing with regard to the improvement of AD patients.⁹⁴

Fig. 7 The chemical structures of vinpocetine and cilostazol.

2.3.2. PDE3 inhibitors. Cilostazol, a 6-substituted quinolinone (Fig. 7) PDE3 inhibitor, showed inconclusive clinical trials in terms of efficacy in the treatment of AD.⁸⁷

2.3.3. PDE4 inhibitors. On the other hand, PDE4 inhibitors were promising to a great extent.⁹⁵ HT-0712, a 3,5-disubstituted piperidinone (Fig. 8), has completed a phase II study, showing enhancement in the long-term memory of patients with age-related memory impairment.⁹⁶ It is now under another clinical trial study designed to further evaluate its effect on AD improvement.⁹⁷

Fig. 8 The chemical structures of some PDE4 inhibitors.

Meanwhile, a phase I study showed that a combination of roflumilast, a N-pyridinylbenzamide-based molecule (Fig. 8), and donepezil improved verbal learning performance.⁹⁶ In a recent study, roflumilast reduced neuroinflammation, amyloidogenesis, oxidative stress, cholinergic impairments, and phosphorylated tau levels in the rat hippocampus exposed to streptozotocin-induced sporadic AD.⁹⁸

Zatolmilast (BPN14770), a phenylacetic acid derivative (Fig. 8), was reported as a PDE4D-negative allosteric inhibitor. It was found to show an improvement in memory and cognitive functions. Two clinical studies revealed that BPN14770 is safe and well tolerated. It was designed not to completely inhibit the enzyme in order to reduce the emetic effect.⁹⁶

Clinical trials of denbufylline, a xanthine PDE4 inhibitor (Fig. 9), were inconclusive and preliminary.⁹⁹

Fig. 9 The chemical structure of denbufylline and sildenafil.

2.3.4. PDE5 inhibitors. Sildenafil, a pyrazolopyrimidine-based PDE5 inhibitor (Fig. 9), showed improvement in cognitive functions, but these promising results are still preliminary and inconclusive.^87,96

2.3.5. PDE9 inhibitors. BI-409306 and PF-04447943 are other pyrazolopyrimidine derivatives (Fig. 10) that were developed as potent and selective PDE9 inhibitors.^100,101 However, their clinical trials revealed no significant effect on AD.^100,101

Fig. 10 The chemical structure of some PDE9 inhibitors.

2.3.6. Broad spectrum PDE inhibitors. Propentofylline, a xanthine derivative (Fig. 11), is a broad-spectrum PDE inhibitor. Five phase III studies on it revealed enhancement in cognitive functions, reduction in dementia severity, and improvement in daily life activities in mild-to-moderate AD patients. However, two books claimed, based on a MedScape article, that an 18 months phase III trial failed, so it was discontinued.⁸⁷

Fig. 11 Xanthine based broad spectrum PDEI.

2.4. Anti-Aβ drugs

Anti-Aβ drugs have been designed to degrade the amyloid plaques either chemically or immunologically by activating phagocytosis or microglia. This is likely to prevent the neuronal damage triggered by the amyloid plaques.^57,102–104

2.4.1. Immunotherapy. The most studied approaches were active and passive immunotherapies. In fact, nearly a dozen anti-Aβ monoclonal antibodies have been developed and clinically studied for AD.¹⁰⁵
2.4.1.1. Passive immunotherapy. For years, a lot of molecules have been designed and evaluated for their effects on senile amyloid plaques as a significant hallmark of AD pathogenesis. Many therapeutic targets directly or indirectly linked to Aβ protein have been involved. As a result of such a huge effort, aducanumab and lecanemab were approved by the FDA.^57,106,107

Aducanumab is a selective anti-Aβ monoclonal antibody with the ability to clear Aβ plaques. On the basis of these proven data, the FDA approved it in June 2021 as the first drug underlying the pathophysiology of Alzheimer's disease (AD). On the other side, the correlation between clearance of Aβ plaques and improvement in cognitive functions lacks evidence. Moreover, the data obtained from two phase III studies were controversial and insufficient to prove aducanumab efficacy. So, the accelerated approval of aducanumab by the FDA generated a debate among scientists. Some researchers consider this approval an obstacle to progress and pose a question concerning the cost and safety profile of aducanumab. A further argument in support of the insignificance of aducanumab is the rejection of it by the European Medicines Agency in December 2021. Now, Biogen is designing a confirmatory study, named ENVISION, required by the FDA, and it is expected to be complete in 2026.^108–112

Lecanemab is a monoclonal antibody directed against both soluble and insoluble forms of Aβ polypeptides.¹¹³ Lecanemab received FDA-accelerated approval in January 2023.^113–116 After further investigation, it received traditional approval from FDA in July 2023 for the treatment of early-stage AD.¹⁰⁶ Lecanemab showed both amyloid clearance and a slowing of clinical decline in early AD.¹¹⁷

On the contrary, solanezumab,^118,119 bapineuzumab,^120,121 ponezumab,¹²² and gantenerumab^119,123,124 failed in clinical trials as monoclonal anti-Aβ antibodies.⁵⁷ Similarly, clinical trials on the natural anti-Aβ antibodies obtained from the blood of healthy adults or Alzheimer patients revealed no clinical effects on AD.⁵⁷

2.4.1.2. Active immunotherapy. One important aspect of immunotherapy is vaccination. It takes successive stages of clinical trials. Firstly, clinical trials of vaccines composed of purified Aβ-42 polypeptide (AN1792)¹²⁵ were disappointing due to the toxicity detected in about 6% of AN1792-treated patients, such as cytotoxic T-cell-induced meningoencephalitis.^126,127 Furthermore, AN1792 failed to clear amyloid plaques; however, it activated the production of Aβ antibodies in AD patients' blood.¹²⁸ Due to the detected toxicity linked to the full-length Aβ1-42 vaccine, the concept has been modified to develop a new generation such as vanutide cridificar (ACC-001) that was designed to stimulate antibodies against N-terminal Aβ1-7. The results obtained were acceptable with regard to safety but showed no therapeutic effects.⁵⁷ Therefore, the next clinical trials involved a combination of ACC-001 and QS-21 adjuvant. The results revealed a good safety profile and high levels of anti-amyloid beta IgG in mild to moderate AD patients after long-term exposure to the combination.^129,130 This high level of anti-amyloid-beta IgG declined after some time.¹³¹ In another trial, ABvac40 vaccine was designed to initiate the production of antibodies against the C-terminal end of Aβ40. It showed good tolerability and developed Aβ40 specific antibodies.⁵⁷ ABvac40 phase II clinical studies are still being processed.¹³²

2.4.2. Small molecules. Tramiprosate, 3-aminopropane-1-sulfonic acid (Fig. 12), is an anti-amyloid oral small molecule that revealed promising clinical results. It was found to selectively and strongly inhibit Aβ42 oligomer formation and the subsequent amyloid aggregation without binding to plaques.^133–136 In a phase 2 study in AD patients, tramiprosate was found to pass the BBB and reduce Aβ42 levels in a dose-dependent manner.¹³⁷ In a phase III clinical study conducted on mild to moderate AD patients, tramiprosate showed significant efficacy in APOE4/4 homozygotes, and intermediate efficacy in APOE4 heterozygotes. But no effects have been observed on non-carrier patients.¹³⁷ The observed side effects were nausea, vomiting, and weight loss.¹³⁷ Indeed, further large and controlled trials are required to confirm the significant results of tramiprosate.¹³⁵

Fig. 12 Tramiprosate and its optimized prodrug valiltramiprosate.

Valiltramiprosate (ALZ-801) was designed as an optimized prodrug (Fig. 12) to address the extensive gastrointestinal metabolism and gastrointestinal irritation detected for the active drug tramiprosate.¹³³

Phase I clinical trials of ALZ-801 revealed good tolerability and no serious side effects.¹³³ ALZ-801 was also found to pass across the blood–brain barrier (BBB) efficiently, showing an intracranial concentration of about 40% of its plasma levels.¹³⁸ ALZ-801 phase II and phase III studies on APOE4 carries are ongoing.¹³⁹

2.5. Tau protein targeting drugs

Tau protein represents a crucial factor in AD pathogenesis. Nevertheless, the benefits of clinical trials on molecules targeting tau hyperphosphorylation were insignificant in terms of efficacy and/or safety.⁵⁷

2.5.1. PP2A activators. One of the fundamental regulators of tau phosphorylation is PP2A. It acts directly or indirectly by affecting GSK-3β.³³ So, it was a promising target for the development of potential anti-alzheimer drugs.

In addition to its action as an NMDA antagonist, memantine (Fig. 6) was found to activate PP2A, leading to inhibition of tau hyperphosphorylation.¹⁴⁰

Sodium selenate is another phosphate modifier that increases the activity of PP2A.¹⁴⁰ The data of a phase IIa clinical trial, in patients with mild to moderate AD, revealed some benefits for sodium selenate on diffusion MRI. But it failed to show any considerable improvement in other measures such as cognition and CSF levels of Aβ and tau proteins.¹⁴¹

Methylthioninium chloride (MTC) (Fig. 13), known as methylene blue, was the first molecule considered to inhibit tau aggregation without affecting the physiological function of tau as a stabilizer for neuronal microtubules.¹⁴² On the other side, the poor pharmacokinetics and intolerability of MTC were likely to be the reasons behind inefficacy in phase 2 clinical trials.⁵⁷

Fig. 13 The chemical structure of MTC.

2.5.2. GSK-3β inhibitors. The regulation of tau phosphorylation is primarily achieved by a balance between the two opposite proteins tau kinase and phosphatase activities. Tau hyperphosphorylation is suggested to be triggered when this balance is disrupted.¹⁴³ Accordingly, tau kinases emerged as interesting targets for the development of anti-alzheimer drugs.¹⁴³ In particular, the overexpression of GSK3β was found to initiate tau phosphorylation and trigger neurodegeneration.¹⁴⁴

Tideglusib, a thiadiazolinedione derivative (Fig. 14), is developed to irreversibly inhibit GSK3β without interaction with the ATP binding domain.^145,146 In animal models of AD, it caused a reduction in neurofibrillary tangles and amyloid plaques with an improvement in memory.^147,148 These data were confirmed by the preliminary data of a larger-scale phase II clinical trial in which tideglusib caused an improvement in cognitive functions along with a reduction in CSF β-secretase levels in a subgroup of mild AD patients. However, the precise analysis of the entire study revealed the insignificance of the improvement caused by tideglusib.¹⁴⁹

Fig. 14 Structure of tideglusib.

Lithium chloride (or ‘lithium’) was identified as a GSK3β inhibitor.¹⁵⁰ Preliminary results of lithium clinical trials showed some improvement with few side effects in elderly patients with AD. Further studies are required to evaluate the efficacy and safety of lithium in AD patients.¹⁴⁰

2.6. β-secretase acting drugs

2.6.1. BACE-1 inhibitors. LY2811376 [(S)-4-(2,4-difluoro-5-pyrimidin-5-yl-phenyl)-4-methyl-5,6-dihydro-4H-[1,3]thiazin-2-yl-amine] (Fig. 15) was constructed through fragment-based drug design as an oral BACE1 inhibitor. It showed good oral bioavailability and promising effects in animal models, which reflects its safety and tolerability in healthy volunteers. Meanwhile, subsequent oral doses of 30 or 90 mg of LY2811376 revealed a considerable and long-term reduction in Aβ levels in CSF. In a dose dependent manner, an increase in Aβ5-40 and a decrease in Aβ1-34 were associated with the use of LY2811376 in AD.¹⁵¹ In contrast to these promising pharmacodynamic features, its long-term use in preclinical studies showed toxic effects, which prevented any further progress in the clinical trials, and accordingly, the drug was discontinued in phase II.¹⁵²

Fig. 15 The chemical structures of some BACE1 inhibitors.

LY2886721 is a picolinamide-based (Fig. 15) BACE1 inhibitor. Its oral administration in a transgenic mouse model-based study showed a significant reduction in the levels of Aβ and soluble APPβ (sAPPβ). Similar promising results were obtained in another study that revealed a great and continuous reduction in the levels of Aβ in CSF after oral use of LY2886721 in a cannulated beagle dog model. In addition to this, LY2886721 was found to have good BBB penetration, showing high CSF concentrations.^16,153 The drug LY2886721 has been exposed to clinical trials because of its good pharmacology and safety profile. In a phase II clinical study, LY2886721 induced an abnormal elevation in liver enzymes, so the study was terminated.¹⁵⁴

Umibecestat (CNP520) is another picolinamide-based (Fig. 16) oral BACE1 inhibitor.¹⁵⁵ CNP520 was designed to meet the requirements of prevention treatment. In the preclinical studies, CNP520 revealed a considerable reduction in acute and chronic Aβ along with an acceptable safety profile.¹⁵⁶ In 2019, CNP520 passed the phase II clinical trial, showing promising efficacy and tolerability. But it failed in phase III as it caused cognitive worsening in the treatment groups.¹⁵⁷ The Alzheimer's Prevention Initiative Generation Program (Generation Study 1) announced that CNP520 worsens cognitive functions.¹⁵⁵

Fig. 16 Some picolinamide based BACE1 inhibitors.

Another picolinamide-based (Fig. 16) oral BACE-1 inhibitor is verubecestat (MK8931), which has been reported to reduce Aβ levels in AD patients.^158,159 Nevertheless, in clinical trials, it caused no improvement in cognitive function in mild to moderate AD patients.¹⁵⁸ Furthermore, some studies reported a greater decline in cognitive functions among patients receiving verubecestat than those receiving placebo.¹⁶⁰

Atabecestat (JNJ-54861911), a picolinamide derivative (Fig. 17), was designed by Pharmaceutical Janssen as an oral BACE-1 inhibitor. In 2013, some subsequent clinical phase I studies of atabecestat have been conducted. It was launched in Belgium with a single increasing dose model, followed by a second study in addition to a similar trial performed in Japan. These studies were conducted on a limited number of healthy elderly volunteers and concluded promising results for atabecestat, which reduced Aβ aggregation after single or multiple doses.^161–163 A subsequent phase IIb/III clinical trial (NCT02569398) was conducted to investigate the efficiency and safety of the drug, but it was terminated in 2018 because of the observed adverse effects, which include hepatotoxicity.^164,165 In addition, other adverse effects on cognition, sleep, depression, and anxiety were reported for atabecestat in some other studies.^164,166

Fig. 17 The chemical structure of some BACE1 inhibitors.

Lanabecestat (Fig. 17) was developed as an anti-alzheimer drug that has the ability to pass the BBB and improve the clinical features, along with preventing the progression of the disease through inhibition of BACE1.¹⁶⁷ The data obtained from some studies reflected adverse effects that include psychiatric disorders, weight loss, and change in hair color, while there was no considerable improvement in primary or secondary efficacy measures.^167,168

Elenbecestat (E2609) is a pyrazinecarboxamide-based (Fig. 18) BACE1 inhibitor. It showed a dose-dependent reduction in the levels of Aβ in CSF. However, it showed no difference from placebo with regard to some other AD-related measures, such as the Alzheimer's Disease Composite Score (ADCOMS, p = 0.38) and CDR-SB (p = 0.55). Two 24 months studies of elenbecestat in a large number of mild AD patients with positive amyloid pathology biomarkers were discontinued in the sense that the toxicity induced outweighed the benefits obtained.¹⁶⁹

Fig. 18 The chemical structure of elenbecestat.

In general, and on the basis of the published data relating to the effects of the oral BACE1 inhibitors in transgenic mouse models of AD, it can be concluded that BACE1 inhibitors are able to reduce the levels of Aβ in brain and CSF in a dose-dependent manner, but there is no solid evidence that they can improve cognitive functions effectively. In addition to the unfavorable toxicities observed in such studies and linked to BACE1 inhibitors, seventeen BACE1 inhibitors have failed in clinical trials to show considerable improvements in AD patients. Many of these trials were discontinued due to toxicity and/or cognitive worsening.¹⁷⁰

2.6.2. Dual BACE-1 and BACE-2 inhibitors. NB-360 was developed by Novartis Pharmaceuticals Corporation as a picolinamide derivative (Fig. 19) with potent BACE-1 and BACE-2 inhibition properties. It demonstrated an IC₅₀ of 5.0 nM and 6.0 nM, against the two enzymes, respectively.¹⁷¹ In preclinical studies, NB-360 significantly inhibited the accumulation of Aβ in APP transgenic mouse brains. Similar data were obtained from clinical studies in rats and dogs. It also showed good BBB permeability.^172,173 However, the safety profile of NB-360 was found disappointing. In some studies, it caused a hypopigmentation phenotype, which was attributed to the inhibition of BACE-2, which plays a crucial role in melanogenesis. Due to the effects of NB-360 on melanosome maturation and triggering hair depigmentation in a mouse model,¹⁷³ studies on NB-360 were discontinued prior to clinical trials.¹⁷¹

Fig. 19 Picolinamide based inhibitors for both BACE1 and BACE2.

RG712 (Fig. 19) was generated by Roche as a substituted phenylpicolinamide derivative carrying an oxazoline moiety. It was developed as an oral BACE inhibitor, and it showed an IC₅₀ of 30 nM in the preclinical studies with minor selectivity over BACE2.¹⁷⁴ It was evaluated in combination with an anti-Aβ antibody (gantenerumab) in some studies that involved AD transgenic mouse models, and it was found to reduce the amyloid plaques with slowing down the disease progression.¹⁷⁴ However, the hepatotoxicity induced by RG712 prevented the completion of any further related clinical trials conducted by Roch.¹⁷⁵

2.7. γ-Secretase acting drugs

In addition to the role of γ-secretase in the generation of Aβ from APP, it also proteolyzes many other type I integral membrane proteins, in particular the Notch receptor, which plays a role in many essential steps during cell differentiation.¹⁷⁶ So targeting γ-secretase as a significant approach to the treatment of AD should avoid affecting Notch signaling in order to show a good safety profile.¹⁷⁷

2.7.1. γ-Secretase inhibitors (GSIs). Semagacestat (LY450139) (Fig. 20) is a benzoazepine-based γ-secretase inhibitor developed by the Eli Lilly pharmaceutical company.¹⁷⁸ Semagacestat decreases in a dose-dependent manner brain, CSF, and plasma Aβ in animals as well as CSF and plasma Aβ in humans in comparison to placebo-treated patients.¹⁷⁹ However, in a phase III clinical study, semagacestat did not improve cognitive functions and triggered, in high doses, adverse effects such as worsening functional abilities, infections, and skin cancers.¹⁸⁰

Fig. 20 The chemical structure of some γ-secretase inhibitors.

MK-0752 (Fig. 20) is another GSI that does not differentiate between γ-secretase and Notch receptor.¹⁵⁴ In a phase I clinical trial, evaluation of its safety, pharmacokinetics, and pharmacodynamics on the basis of a single oral dose has been conducted in 27 healthy volunteers.¹⁸¹ The data obtained revealed that MK-0752 is safe and reaches its maximum plasma concentration in approximately 3–4 h with a t_1/2 of 20 h as well as greatly reducing Aβ1-40 levels in CSF for 12 h.¹⁵⁴ However, the safety profile was disappointing in a phase II study because it did not differentiate between γ-secretase and Notch.¹⁵⁴

Avagacestat (BMS-708163) is a benzenesulfonamide-based (Fig. 21) potent and selective GSI developed by Bristol-Myers Squibb.¹⁸² It revealed a 193-fold selectivity for APP over Notch blockade. It inhibited Aβ40 production with an IC₅₀ of 0.30 nM, resulting in lowering the levels of Aβ40 in CSF, brain, and plasma as studied in dogs and rats.¹⁸³ Phase I study demonstrated that the safety, tolerability, pharmacokinetics, and pharmacodynamics properties of BMS-708163 were promising after oral administration in healthy, young volunteers (NCT01454115). Accordingly, it was decided to be evaluated by further clinical trials. In phase II trials, BMS-708163 was evaluated in 209 outpatients with a median age of 75 years, diagnosed with mild-to-moderate AD. Different doses were examined in this study, and the results revealed that it cannot be tolerated at a dose of 100 mg or above. Additionally, a worsening of cognition was observed at such doses. Meanwhile, the dose up to 50 mg day⁻¹ showed results similar to those of placebo.¹⁸⁴ The obtained results have been confirmed by a subsequent study conducted on CSF biomarker-negative volunteers, which demonstrated deterioration in the health conditions of patients with the occurrence of nausea, vomiting, diarrhea, rash, and nonmelanoma skin cancers. Meanwhile, there were no therapeutic benefits to the lower doses.¹⁸⁵

Fig. 21 Benzenesulfonamide based GSI.

Begacestat (GSI-953), 2,5-disubstituted thiophene sulfonamide (Fig. 22), is considered a second generation GSI. It was generated by Wyeth (now Pfizer).¹⁸⁶ Researchers at Wyeth applied the high throughput screening technique to get the molecule LI (Fig. 22).¹⁸⁷ L1 was then modified to afford the more optimized L2, which in turn led to the discovery of L3, which showed much potent inhibition for Aβ production,¹⁸³ as can be seen in Fig. 22. The most important character of L2 and L3 is that they revealed much better selectivity for APP over Notch. But unfortunately, they were metabolically unstable, and this was attributed to the easy oxidation of the methylene group. So, the alkyl side chain was contracted to afford a series of new molecules, from which GSI-953 was discovered (Fig. 22). GSI-953 was found to be relatively more resistant to rapid metabolism, showing a half-life of more than 90 min. Meanwhile, GSI-953 displayed comparatively better properties with respect to inhibition of Aβ production and selectivity for APP over Notch, as presented in Fig. 22.^183,186 GSI-953 revealed promising data in phase I clinical trials as it inhibited Aβ production at nanomolar concentrations, and showed 16-fold selectivity for APP over Notch. In a study conducted on a human APP-overexpressing Tg2576 transgenic mouse model, oral use of GSI-953 led to a significant reduction in Aβ levels in CNS, brain, and plasma. Similar results have been obtained when the effects of oral use of GSI-953 were evaluated in healthy human volunteers.¹⁸⁸

Fig. 22 Lead optimization in the development of GSI-953.

PF-3084014 is an imidazolylpentamide-based Notch-sparing GSI (Fig. 23). It was reported as a non-competitive reversible human GSI with an IC₅₀ of 1.3 nM.^189,190 With respect to selectivity, PF-3084014 displayed minor inhibition of Notch signaling (IC₅₀ = 19.15 nM). The selectivity index of PF-3084014 was found to be 1473 for APP over Notch. So it was considered a Notch sparing GSI. It showed good penetration to the BBB with a reduction in Aβ levels in animals. However, there is a lack of data about its effect on amyloid plaque deposition in transgenic mice, as well as no data available about its behavioral effects in AD animal models.¹⁵⁴

Fig. 23 Notch sparing GSI.

In clinical trials, GSIs such as semagacestat and avagacestat reduced Aβ production in AD patients.^184,191 However, the multiple effects of such inhibitors prevented further progress in the clinical trials due to inhibition of Notch signaling,¹⁹² which induces adverse effects such as gastrointestinal disturbance, infection, worsening cognition, and the risk of skin cancer.¹⁹³ To address this problem, a new class of γ-secretase acting molecules that specifically regulate or modulate this enzyme are required.¹⁹³

2.7.2. γ-Secretase modulators (GSMs). This class of drugs have the ability to modulate the activity of γ-secretase by regulating specific activities of the enzyme rather than the whole inhibition of the enzyme. GSMs are far more interesting disease-modifying agents than GSIs because GSMs (1) selectively inhibit Aβ42 production and aggregation; (2) increase the production of shorter Aβ37 or Aβ38 rather than Aβ42; (3) do not affect the total Aβ production and the accumulation of APP-CTF; and (4), most importantly, spare Notch signaling.¹⁹⁴
2.7.2.1. First-generation GSMs. Nonsteroidal anti-inflammatory drugs (NSAIDs) represented the first generation of GSI as they were found to modulate the activity of γ-secretase in a manner that reduces Aβ42 and increases the soluble Aβ38 without inhibiting Notch. Ibuprofen, indomethacin, and sulindac sulfide (Fig. 24) are examples of this class of NSAIDs.^194,195

Fig. 24 First generation of NSAIDs as GSMs.

Unfortunately, ibuprofen, indomethacin, and sulindac sulfide lack good brain penetration, so they showed insignificant data in clinical trials associated with AD models.¹⁹⁶

2.7.2.2. Second-generation GSMs. The poor brain penetration of NSAIDs was required to be resolved in the second-generation GSMs, which can be classified into different categories, including NSAID-derived carboxylic acid GSMs, non-NSAID-derived imidazole GSMs, and natural product-derived GSMs.¹⁹⁴
2.7.2.3. NSAID-derived carboxylic acid GSMs. Tarenflurbil ((R)-flurbiprofen), an aryl propionic acid derivative (Fig. 25), was strongly suggested as a clinical candidate for AD treatment.¹⁹⁷ It showed no Notch inhibition-related adverse effects;¹⁹⁸ however, it still showed insufficient brain penetration and minor efficacy, leading to its failure in phase III clinical study.^199–201

Fig. 25 NSAID derived carboxylic acid GSMs.

JNJ-40418677 was generated by Janssen as an analog of flurbiprofen by addition of substituents on the core aryl ring (Fig. 25). JNJ-40418677 has sufficient lipophilicity to penetrate the brain and reveal its therapeutic activities. It was found to reduce Aβ42 levels and elevate the levels of Aβ38 in the brain without affecting the levels of total Aβ in the brain. The safety profile of JNJ-40418677 still needs to be evaluated.²⁰²

EVP-0015962 is a cyclobutyl group containing analog of (R)-flurbiprofen (Fig. 25), it was generated by Forum Pharmaceuticals. It displayed good data, such as a considerable reduction in Aβ42 levels when administered orally with good efficacy in animals.²⁰³ Nevertheless, the high lipophilicity was considered an unfavorable property of this drug.¹⁹⁶ So its clinical trials were discontinued after phase II.²⁰⁴

Itanapraced (CHF5074) is another carboxylic acid derivative (Fig. 26) designed by Chiesi Pharmaceuticals as a GSM. It was found to enhance memory and reduce microglial activation in Tg2576 mice.²⁰⁵ It did not show a reduction in Aβ42 levels either in plasma or CSF; instead, it decreased the levels of CD40, which is considered a marker for microglia activation. So CHF5074 is now classified as a microglia modulator.^196,206

Fig. 26 The chemical structure of itanapraced.

Biogen's research was focused on the discovery of NSAID derived carboxylic acid GSMs. In 2011, Biogen reported²⁰³ the discovery of a potential preclinical GSM candidate (BIIB042). Its development was based on the optimization of the previously examined L4, as shown in Fig. 27. This lead compound showed considerable potency, but it had poor brain penetration. So, the researchers worked on improving this criterion. More than one modification has been done on the basis of deduced SAR, which revealed that the activity was improved by the creation of a chiral center by adding a methyl group at the α position of the carboxyl group in addition to a rigid piperidinyl moiety, as can be noticed in Fig. 27. This optimization afforded the preclinical candidate BIIB042, which showed a reduction in Aβ42 levels with a concomitant elevation in Aβ38 levels and no effect on Aβ40 levels. Several clinical studies have been conducted by Biogen, which has published about 66 in vivo efficacy studies of BIIB042. The published data indicated that BIIB042 decreased Aβ42 levels and increased Aβ38 levels, but showed no effect on Aβ40 levels in the brains and plasma of mice and rats. These pharmacodynamic properties have been confirmed by similar results concluded from a study conducted on monkeys administered orally a single dose of BIIB042.²⁰³

Fig. 27 Biogen's NSAID-derived carboxylic acid GSM 1 and an optimized modulator BIIB042.

In addition to these promising data, BIIB042 did not inhibit Notch signaling in an in vitro model.²⁰⁷ In a more advanced study conducted on human APP-overexpressing Tg2576 mice, BIIB042 reduced the levels of Aβ42 and decreased the amyloid plaque burden.²⁰⁸ BIIB042 is a candidate drug for AD.²⁰⁹

2.7.2.4. Non NSAID imidazole GSMs. This class of compounds attracted the attention of researchers of Eisai group as they focused on the development of non-carboxylic acid GSMs. At first, nearly all non-carboxylic acid GSMs had a general structural feature of an arylimidazole moiety linked via an olefin to a lactam, as in E2012, or a heterocycle, as in E2212 (Fig. 28). They developed E2012 as a substantial GSM with an IC₅₀ of 83 nM and it was the first non-carboxylic acid molecule to be studied clinically as a GSM.¹⁵⁴ The clinical trials revealed a question concerning the safety of E2012, which induced cataracts in rats.²¹⁰ As a consequence, its clinical trials have been terminated and exposed to a series of modifications by Eisai, which finally developed E2212 (Fig. 28).²¹¹

Fig. 28 Non NSAID imidazole GSMs: E2012 and E2212.

It was reported that E2012 was modified to E2212 for improving the drug-likeness.¹⁵⁴ E2212, in its phase I clinical trials, showed comparatively better safety in comparison to E2012.¹⁹⁸ Diarrhea was the most observed adverse effect for E2212.¹⁹⁸ It also revealed promising efficacy by reducing the levels of Aβ42 by 54%,²¹² showing an IC50 of 17 nM.²¹³ Although the structure of E2212 has never been revealed, it is predicted by several Eisai process chemistry patents. The further development of this compound has not been reported.¹⁵⁴

Compound BMS-869780 (Fig. 29) was developed by Bristol-Myers Squibb as a non-NSAID imidazole GSM. It showed promising activity in mouse and rat brains by a significant reduction of Aβ42 levels after oral administration.²¹⁴ Although there was no evidence that BMS-869780 triggers side effects related to Notch inhibition, further clinical studies have been terminated because of a potential toxicity issue, as the daily dose required to provide therapeutic effects in human AD was very high (700 mg). BMS-869780 was then modified to BMS-932481 (IC₅₀ of 7 nM) in order to improve the pharmacodynamics, as shown in Fig. 29. It can be noticed that the chloroimidazole nucleus was replaced by the methyltriazole moiety.

Fig. 29 Non NSAID derived GSMs.

BMS-932481 was evaluated for pharmacokinetics, which appeared to be promising. However, further studies have been terminated due to safety issues.¹⁵⁴

PF-06648671 (Fig. 30) was developed by Pfizer, and it was found to be of good tolerability at single doses given to healthy persons. It decreased the plasma levels of Aβ40 and Aβ42. Meanwhile, it increased the levels of Aβ37 and Aβ38.²¹⁵ However, it was discontinued due to Pfizer's discontinuation of R&D in neurology in 2018.

Fig. 30 The chemical structure of the non NSAID imidazole GSM developed by Pfizer.

In 2017, researchers at Merck reported the discovery of a new series of 17 tetrahydroisoxazole molecules as GSMs developed on the basis of Eisai's E2012 (ref. 216) and their own preceding GSM molecules, among which compounds is L5 (ref. 196) (Fig. 31). The double bond and the lactam carbonyl in Eisai's E2012 are sites of potential metabolism, so they were replaced by a tetrahydrobenzisoxazole moiety, as shown in Fig. 31. Another benefit of the tetrahydrobenzisoxazole group is avoiding the stereochemistry of the double bond. The synthetic compounds showed variable activities, from which SAR has been deduced to develop some compounds with good selectivity and potency. The most promising candidates were compounds 1 and 2, showing an IC₅₀ of 39 nM and 29 nM against Aβ42, respectively (Fig. 31). It can be noticed that 1 and 2 were far more potent than E2012 and L5. The promising criteria of 1 and 2 encouraged their evaluation in a preclinical rat model. Compound 1 reduced CSF Aβ42 levels by 58%, compared to only a 20% reduction in Aβ levels reported for compound 2 after 3 h of a single oral dose in rats. Interestingly, compound 2 was more potent than 1 in vitro. To find an explanation for this controversy, the levels of both drugs in the plasma and brain of rats have been assessed. The concentration of compound 1 in the brain was found to be sixfold that of compound 2, which gave an account of the better results of compound 1 in animal model study. Meanwhile, the efflux ratio of 2 was four-fold higher than that of 1. Despite these promising data, we did not find any further data concerning the safety and efficacy of compound 1 or its future evaluation in clinical trials.¹⁹⁶

Fig. 31 Merck Research Laboratory's oxadiazine L5 and tetrahydrobenzisoxazole analogs 1 and 2.

2.7.2.5. Natural products as gamma secretase modulators. Compound SPI-014 (Fig. 32), isolated from the extract of Actaea racemosa (black cohosh), was found to exhibit activities like GSMs. Several modifications have been done on SPI-014, and a lot of semisynthetic analogs have been developed. The modifications included the substitution of both sugar and acetate moieties with more stable groups to enhance drug-like properties, as can be seen in Fig. 32. These modifications led to the discovery of a new drug molecule, SPI-1865 (Fig. 32). Despite the improvement in the pharmacodynamics of the modified molecule, it did not pass the clinical trials due to the unanticipated off-target adrenal toxicity reported.²¹⁷

Fig. 32 Natural and semisynthetic analogs as GSMs.

2,3-Bis((Z)-4-methoxybenzylidene)succinonitrile (N1) (Fig. 33) was extracted from the marine sponge-derived fungus Dichotomomyces cejpii. It was found to decline the excessive production of Aβ42 in an AD cellular assay.²¹⁸

Fig. 33 Natural GSM.

Dihydroergocristine (DHEC) (Fig. 34) is an FDA-approved natural drug that revealed significant data with respect to selective inhibition of APP. It has a Leu-Phe-Pro motif, which plays a crucial role in binding to the allosteric site of the enzyme. This allosteric site represents the region to which APP-C99 interacts to be cleaved. So DHEC competes with APP for binding to γ-secretase, reducing the production of Aβ42 in the brain.²¹⁹ DHEC was found to reduce the levels of Aβ at micromolar concentrations when examined in vivo.²¹⁹

Fig. 34 The chemical structure of DHEC.

2.8. α-Secretase activators

Activation of α-secretase was suggested as a potential therapeutic strategy in order to inhibit the aggregation of amyloid plaques.²²⁰ This is because of the role of α-secretase in enhancing the proteolysis of APP in the non-amyloidogenic pathway and, hence, decreasing the formation of amyloid plaques. A series of membrane-bound proteases (a disintegrin and metalloprotease family) regulate the α-secretase.²²¹ ADAM10, ADAM17, and ADAM9 have been suggested as α-secretases.²²²

Synthetic retinoids were proven to improve the nonamyloidogenic proteolysis of APP. One of the important synthetic retinoids in this regard is acitretin, a vitamin A analog (Fig. 35). Acitretin was evaluated in a phase II clinical study and was found to increase ADAM10 expression as well as reduce the levels of Aβ in APP/PS-1 transgenic mice.²²³ Furthermore, it enhances the stimulation of the mature ADAM10, resulting in higher activity of α-secretase in neuroblastoma cells.²²³ One of the encouraging features of acitretin is its ability to cross the BBB easily, and its level is not affected by glycoprotein (P-gp).²²⁴ On the other side, it was linked to some severe toxicity, such as alopecia, peeling, cheilitis, and hepatotoxicity.¹⁵⁴

Fig. 35 The chemical structure of acitretin.

Etazolate (EHT-0202), a pyrazolopyridine derivative (Fig. 36), is a gamma-aminobutyric acid-A (GABA_A) receptor modulator. It was found to stimulate α-secretase and enhance sAPPα production.²²⁵ EHT-0202 has been assessed in a phase II clinical study in mild-to-moderate AD subjects. It showed cognitive improvement along with a good safety profile and tolerability.^225,226

Fig. 36 The chemical structure of etazolate.

2.9. 5HT-receptors

One of the receptors that emerged lately as potential targets for cognitive disorders and AD is serotonin.^227,228 In this regard, 5-HT6R and 5-HT7R are the most extensively studied serotonin receptors because of their high distribution in the brain and association with cognitive properties in vivo.²²⁹ Furthermore, 5-HT6R signaling was found to be associated with changes in cholinergic and glutamatergic functions in the brain, with little peripheral effect.²³⁰ However, the clinical trials against AD revealed no evidence for the therapeutic activity of any of the selective 5-HT6R or 5-HT7R drugs.⁵⁷

PF-05212377 (SAM-760), a benzimidazole derivative, and idalopirdine, an indole-based molecule, are selective 5-HT6R antagonists (Fig. 37).⁵⁷ Clinical studies demonstrated that SAM-760 had no therapeutic benefits with regard to cognitive disorders; however, it showed good safety and tolerability.^57,231

Fig. 37 The chemical structures of selective 5-HT6R antagonists.

Similarly, idalopirdine was ineffective for AD patients, with a risk of elevated liver enzymes and vomiting.²³² These data reveal the insignificance of idalopirdine for the treatment of AD.²³³

2.10. Glutaminyl cyclase (QC) inhibitors

The importance of QC in the development of anti-alzheimer drugs originated from the information that QC catalyzes the formation of cerebral pyroglutamate-Aβ 3 (AβpE3), which is considered one of the most neurotoxic forms of Aβ.^234,235 So, inhibition of QC is suggested as a potential therapeutic approach to AD treatment.²³⁴ The work on the development of QC inhibitors has drawn attention in the last decade, where the design developed a zinc-binding group in order to coordinate the Zn²⁺ ion incorporated in the active site in addition to other common features.²³⁵ One of the outstanding works in this regard was done by the Probiodrug company, which rationally developed some promising QC inhibitors and identified the imidazole nucleus as a zinc chelating weak QC inhibitor.²³⁵

PQ-912, a benzimidazole-based molecule (Fig. 38), showed competitive inhibition for QC with a Ki value of 25 nM. It was found to interact through coordination of the zinc ion in the QC's active site.²³⁵ PQ912 was evaluated for pharmacokinetic properties, which were found to be acceptable with good safety and tolerability in doses up to 200 mg.²³⁶ The studies have identified the maximum tolerated dose to support further studies at lower doses.²³⁷ Despite the promising cognitive improvement reported, many studies on PQ912 have been discontinued due to high dose toxicity.²³⁸

Fig. 38 Benzimidazole based QC inhibitor.

The results obtained from clinical evaluation of PQ912 have proven that QC is a significant AD druggable target. Furthermore, QC inhibition was found to improve synaptic functions by decreasing the toxic effects of AβpE3. In addition to this, long-term use is likely to modify the disease and reduce the neuroinflammations associated with AD.^234,235

2.11. Anti-inflammatory agents

Chronic inflammation of the cerebral neurons is one of the pathological hallmarks linked to AD.²³⁹ It was suggested that the origin of neuroinflammation is the activation of glial cells by triggering factors such as neural environment or neuronal injury. One of the crucial factors in this scenario is TNF-α, which plays a pivotal role in neuronal excitotoxicity, neuroinflammation, and synapse loss. Another role for TNF-α associated directly with AD pathogenesis is enhancing amyloidogenesis and upregulating BACE-1 expression.^240–242

Etanercept is a competitive TNF-α inhibitor. It is a fully human dimeric fusion protein consisting of the extracellular ligand-binding domain of the human 75 kilodalton TNF-α receptor linked to the Fc portion of human immunoglobulin G1 (IgG1).²⁴³ Clinical studies revealed that it had no therapeutic benefits in clinical studies against alzheimer.²⁴³ Etanercept limitation is related to its pharmacokinetics; it is unable to cross the brain–blood barrier.²⁴³

Thalidomide, another TNF-α inhibitor, is a small molecule composed of a phthalimide nucleus attached to a glutarimide moiety (Fig. 39). It also showed no therapeutic benefits in the clinical trial against alzheimer due to poor tolerability.²⁴⁴

Fig. 39 The chemical structure of thalidomide.

2.12. GABA

2.12.1. GABA_A agonist. Hyperexcitation of neuronal activity is one of the toxic factors that eventually lead to neuronal death and AD progression. It was found in AD mouse models that GABAergic neurotransmission is upregulated in the hippocampus before neuronal death.^245,246 Accordingly, it is more likely to be a significant target in order to neutralize the abnormal hyperexcitation.

Etazolate, a pyrazolopyridine derivative (Fig. 36), is a GABA_A receptor modulator that was found to show neuroprotective properties against the toxic effects of Aβ. Moreover, it revealed cognitive improvement and anti-inflammatory activity in traumatic brain injury.^247,248 Further investigation into the mechanisms of its neuroprotective effect revealed GABA_A receptor activation as well as stimulating α-secretase cleavage of APP. The importance of its GABA_A role is highlighted by the full block of its neuroprotective effect by GABA_A antagonists.²²⁶

Other examples of GABA_A agonists that showed promising results are muscimol (5-(aminomethyl)isoxazol-3(2H)-one) and propofol (2,6-diisopropylphenol) (Fig. 40).^249,250 Muscimol pretreatment was found to effectively inhibit the Aβ25–35-induced neuronal death in cultured rat cortical neurons.²⁵⁰

Fig. 40 The chemical structures of some GABA_A agonists.

Long-term use of propofol for the treatment of aged mice was found to reduce Aβ40 and Aβ42 levels in the brain and decrease the expression level of BACE1, thereby decreasing the aggregation of amyloid plaques.²⁵⁰ These data reflected the importance of chronic GABA_A receptor activation by propofol in neuroprotection and decreasing Aβ levels in brain. Furthermore, propofol was proven to improve cognitive function in both WT and APP/PS1 mice.²⁴⁹

2.12.2. GABA_B antagonist. In AD mouse models as well as in human AD patients, the levels of released γ-aminobutyric acid (GABA) were found to be significantly increased. The high levels of GABA could in turn bind to GABA_B receptors, inhibiting synaptic release in APP/PS1 mice.²⁴⁵ GABA_B antagonists were proposed to decrease the inhibition of synaptic function and enhance cognition in AD.²⁴⁵

SGS742 (CGP36742), a phosphinic acid derivative (Fig. 41), was the first GABA_B antagonist evaluated for AD in clinical trials. Its effects on rodents and monkeys were outstanding, as evidenced by significant improvements in cognition and learning tasks.²⁵¹ In addition, SGS742 was found to be well tolerated not only in experimental animals but also in human volunteers. In a phase II clinical trial, 8 weeks of oral administration of SGS742 revealed considerable attention improvement and memory enhancement in patients suffering mild cognitive impairment.^251–253

Fig. 41 GABA_B antagonists.

CGP55845 is another phosphinic acid-based (Fig. 41) GABA_B receptor antagonist. In an aged rat model with cognitive impairment, CGP55845 was found to completely improve the olfactory discrimination learning deficits and retrieve performance.²⁵⁴ These data indicated the significance of GABA_B receptors as a potential target for improving cognitive disorders.²⁵⁵

2.13. Antioxidants

2.13.1. Vitamins. Vitamin E (α-tocopherol, Fig. 42) is a powerful lipid-soluble chain-breaking antioxidant, which plays a pivotal role in preventing the toxic effects of free radicals on neuronal cells and, hence, reducing the rate of progression of dementia in mammalian cells.²⁵⁶ In experimental studies, vitamin E was found to improve cognition as well as prevent the toxic effect of Aβ in rodents.^257,258 Similar results were reported in studies conducted on AD patients.²⁵⁹ These data suggest that vitamin E, as a powerful lipid-soluble antioxidant, has the ability to significantly delay the clinical deterioration of cognitive functions in AD patients. This suggestion was supported by the finding that vitamin E considerably inhibited tau-induced neurotoxicity in Drosophila.²⁶⁰

Fig. 42 The chemical structure of vitamin E.

2.13.2. Natural compounds. Caffeine (Fig. 43) is an antioxidant that was reported to inhibit amyloidosis and amyloid plaque production, reducing Aβ levels in the brain of transgenic mouse models for early-onset familial AD.²⁶¹

Fig. 43 Natural antioxidants.

Another herbal antioxidant that showed promising data with regard to improvement of AD is silibinin (silybin) (Fig. 43), a flavonoid derived from the herb milk thistle (Silybum marianum). Silibinin was reported to prevent memory impairment and to eliminate the oxidative stress induced by Aβ in mice, so it is likely to be a potential candidate for AD treatment.²⁶²

Curcumin is a di-phenolic molecule (Fig. 44) extracted from turmeric and is known for diversity in its biological effects, such as antioxidant, anticarcinogenic, and anti-inflammatory. In the last decade, its pharmacological effects on AD have been discovered. It exhibited neuroprotection, and inhibition of Aβ aggregation, and Aβ-induced inflammation, so it is likely to be helpful in treating AD as one of the neurodegenerative diseases.²⁶³ Curcumin has also been found to inhibit AChE in in vitro studies.²⁶³

Fig. 44 The chemical structure of curcumin.

Similarly, the polyphenolic quercetin (Fig. 45) exhibited neuroprotective properties that encourage researchers to utilize it as a lead compound for developing drugs against neurodegenerative disorders such as AD. However, the oral bioavailability of quercetin is low, so the clinical trials were impeded. It was reported to reduce β-amyloidosis, astrogliosis, tauopathy, and microgliosis in the hippocampus and amygdala.²⁶⁴

Fig. 45 The poly phenolic quercetin.

Luteolin (Fig. 46) is a flavonoid compound that was found to act as a neuroprotective agent in a streptozotocin-induced alzheimer's rat model. This reported effect was suggested to be due to luteolin's antioxidant properties, which are mediated by blocking free radicals and dispersing amyloid plaques. Accordingly, luteolin is proposed as a potential therapeutic candidate for neuronal disorders, e.g., AD; however, further investigation is still required.²⁶⁵

Fig. 46 The chemical structure of luteolin.

2.13.3. Mitochondria-targeted antioxidants (MTAs). Antioxidants such as α-lipoic acid (LA), coenzyme Q10 (CoQ10) (Fig. 47), and glutathione are likely to have a potential therapeutic effect in the treatment of some neurodegenerative diseases. Mitochondrial dysfunction was considered a proposed mechanism involved in the neuronal pathogenesis associated with some neurodegenerative diseases, such as AD. So, antioxidants that protect the mitochondria and prevent their malfunction emerged as potential therapeutic agents in many neurodegenerative diseases, including AD. Because overproduction of ROS by mitochondria is a major element in the progression of AD, many metabolic antioxidants such as LA and CoQ10 that easily penetrate the cell membrane to reach the mitochondria are more likely to provide considerable protection in AD.²⁶¹

Fig. 47 The chemical structure of lipoic acid and CoQ10.

Administration of LA (Fig. 47) for long periods has been reported to decrease the expression of lipid peroxidation markers without reducing the Aβ levels in the brains of AD mice. Additionally, LA was found ineffective in improving cognition.²⁶⁶

CoQ10 (Fig. 47) plays a protective role, preventing mitochondrial damage by oxidative stress as well as protecting the whole neuronal cell through reducing Aβ overproduction and intracellular neurofibrillary tangles. Furthermore, CoQ10 is an essential factor for the production of ATP by mitochondria, so it is recommended as a significant antioxidant for AD prevention.²⁶¹

2.13.4. Other antioxidants. Melatonin (Fig. 48) investigation suggested that the antioxidant properties of melatonin could play a role in inhibiting Aβ-induced toxicity²⁶⁷ and reducing tau hyperphosphorylation.^268–271 Furthermore, melatonin was examined in an APP695 transgenic mouse model and found to improve memory and learning deficits. Additionally, melatonin reduced Aβ-induced neuronal death in AD cell models.²⁷² The above data indicate that melatonin as an antioxidant is a potential therapeutic candidate for AD; however, further clinical studies remain necessary to evaluate the efficacy and safety of melatonin for AD treatment.²⁶¹

Fig. 48 The chemical structure of melatonin and selegiline.

Selegiline (Fig. 48), a selective monoamine oxidase-B inhibitor, was suggested for neurodegenerative disease treatment due to its possible antioxidant properties in addition to rapid generation of the potent vasodilator nitric oxide in cerebral blood vessels.²⁷³ In 1997, it was reported to protect neurons and decrease the progression rate of AD in patients with moderately severe impairment from AD.²⁵⁸ In 2000, the analysis of 15 clinical trials revealed that there was no solid evidence to identify selegiline as a potential treatment for AD.²⁶¹

The discussed drugs are outlined in the following table (Table 1) along with their mechanisms of action and references.

Table 1 The name, mechanism of action, and reference of the drugs discussed above

Serial	Drug	Mechanism of action	Reference
1	Galantamine	AChEI	62
2	Donepezil	AChEI	62
3	Rivastigmine	AChEI and BChEI	63
4	Tacrine	AChEI and BChEI	65
5	Metrifonate	Irreversible AChEI and BChEI	64
6	ABT-126	Selective α7 nicotinic receptor agonist	72
7	ABT-089	Selective α4β2 nicotinic partial agonist	75
8	Oxotremorine	CNS muscarinic agonist	77
9	Xanomeline	CNS muscarinic agonist	79
10	EUK1001	CNS muscarinic agonist	79
11	Memantine	NMDA antagonist	82
12	Phencyclidine	NMDA antagonist	86
13	Vinpocetine	PDE1 inhibitor	89
14	Cilostazol	PDE3 inhibitor	87
15	HT-0712	PDE4 inhibitor	96
16	Roflumilast	PDE4 inhibitor	96
17	Zatolmilast (BPN14770)	PDE4 inhibitor	96
18	Denbufylline	PDE4 inhibitor	99
19	Sildenafil	PDE5 inhibitor	87 and 96
20	BI-409306	PDE9 inhibitor	100 and 101
21	PF-04447943	PDE9 inhibitor	100 and 101
22	Propentofylline	Broad-spectrum PDE inhibitor	87
23	Aducanumab	Selective anti-Aβ monoclonal antibody	108–112
24	Lecanemab	Selective anti-Aβ monoclonal antibody	113–116
25	Solanezumab	Monoclonal anti-Aβ antibody	118 and 119
26	Bapineuzumab	Monoclonal anti-Aβ antibody	120 and 121
27	Ponezumab	Monoclonal anti-Aβ antibody	122
28	Gantenerumab	Monoclonal anti-Aβ antibody	119
29	AN1792	Purified Aβ-42 polypeptide vaccine	125
30	ACC-001	N-terminal Aβ1-7 vaccine	57
31	QS-21	Adjuvant combined with ACC-001	129 and 130
32	ABvac40	C-terminal end of Aβ40 vaccine	57
33	Tramiprosate	Inhibitor for Aβ42 oligomer formation	133–136
34	Valitramiprosate (ALZ-801)	Inhibitor for Aβ42 oligomer formation (prodrug)	133
35	Sodium selenate	PP2A activator	140
36	Methylthioninium chloride	PP2A activator	142
37	Tideglusib	GSK3β inhibitor	145 and 146
38	Lithium chloride	GSK3β inhibitor	150
39	LY2811376	BACE-1 inhibitor	151
40	LY2886721	BACE-1 inhibitor	16 and 153
41	Umibecestat (CNP520)	BACE-1 inhibitor	155
42	Verubecestat (MK8931)	BACE-1 inhibitor	158 and 159
43	Atabecestat (JNJ-54861911)	BACE-1 inhibitor	161–163
44	Lanabecestat	BACE-1 inhibitor	167
45	Elenbecestat (E2609)	BACE-1 inhibitor	169
46	NB-360	Dual BACE-1 and BACE-2 inhibitor	171
47	RG712	Dual BACE-1 and BACE-2 inhibitor	174
48	Semagacestat (LY450139)	γ-Secretase inhibitor	178
49	MK-0752	γ-Secretase inhibitor	154
50	Avagacestat (BMS-708163)	Selective γ-secretase inhibitor	182
51	Begacestat (GSI-953)	Selective γ-secretase inhibitor	186
52	PF-3084014	Notch sparing γ-secretase inhibitor	189 and 190
53	Ibuprofen	First generation GSM	194 and 195
54	Indomethacin	First generation GSM	194 and 195
55	Sulindac	First generation GSM	194 and 195
56	Tarenflurbil	Second generation GSM	197
57	JNJ-40418677	Second generation GSM	202
58	EVP-0015962	Second generation GSM	203
59	Itanapraced (CHF5074)	Second generation GSM	205
60	BIIB042	Second generation GSM	203 and 208
61	E2012	Second generation GSM	154 and 210
62	E2212	Second generation GSM	211
63	BMS-869780	Second generation GSM	214
64	BMS-932481	Second generation GSM	154
65	PF-06648671	Second generation GSM	215
66	SPI-014	Second generation GSM	217
67	SPI-1865	Second generation GSM	217
68	Dihydroergocristine	Second generation GSM	219
69	Acitretin	α-Secretase activator	223
70	Etazolate (EHT-0202)	GABAA modulator & α-secretase activator	225
71	PF-05212377 (SAM-760)	Selective 5-HT6R antagonist	57 and 231
72	Iadalopirdine	Selective 5-HT6R antagonist	233
73	Varoglutamstat (PQ-912)	QC inhibitor	235
74	Etanercept	Competitive TNF-α inhibitor	243
75	Thalidomide	Immunomodulator & TNF-α inhibitor	244
76	Muscimol	GABAA agonist	250
77	Propofol	GABAA agonist	250
78	SGS742 (CGP36742)	GABAB antagonist	251
79	CGP55845	GABAB antagonist	254
80	Vitamin E	Antioxidant	257 and 258
81	Caffeine	Antioxidant	261
82	Silibinin	Antioxidant	262
83	Curcumin	Antioxidant	263
84	Quercetin	Antioxidant	264
85	Luteolin	Antioxidant	265
86	α-Lipoic acid	Mitochondrial-targeted antioxidant	261 and 266
87	Coenzyme Q10	Mitochondrial-targeted antioxidant	261
88	Melatonin	Antioxidant	267–271
89	Selegiline	Antioxidant	258 and 261

---

3. Conclusion and perspectives

Herein, AD potential therapeutic targets have been discussed, along with the clinically studied relevant drugs. As can be seen above, only a few drugs have been approved for the treatment of AD. Galantamine, donepezil, and rivastigmine (ChEIs), memantine (NMDA antagonist), and aducanumab and lecanemab (selective anti-Aβ monoclonal antibodies) are the currently approved drugs. This limited number of clinically used drugs against AD does not reflect the large number of proteins and enzymes identified as significant therapeutic targets of AD or the extensive number of drugs studied in the clinical trials of AD. But this image reflects the complexity of the disease and the lack of concrete evidence for the exact, definite cause of the disease.

Accordingly, AD can be considered a multifactorial disease that requires a deeper understanding of its etiology to give priority to the most crucial targets. At the same time, early diagnosis of the disease is of great importance to maximize the benefits of the treatment; especially, it was found that amyloid plaques and neurofibrillary tangles can be detected decades before the appearance of symptoms.²⁷⁴ As we saw earlier, lecanemab has been approved for the early stages of AD. So, the identification of early detectable biomarkers of AD is highly significant. Furthermore, there is some evidence that the approach to prevent further formation of amyloid plaques is not enough for the treatment of AD in the sense that the neurodegeneration is triggered by the neurotoxic effects of the already aggregated Aβ. This may provide an explanation for the failure of almost all clinical trials based on the prevention of Aβ aggregation. So, all aspects should be taken into account in any further clinical study. The clinical data given in this work reveals GSM and QC inhibitors as the most promising classes involving inhibition of the formation of toxic Aβ. Additionally, α-secretase activators enhance proteolysis of APP in the non-amyloidogenic pathway. At the same time, the current study presented many therapeutic classes that have been proven to protect neurons from the toxic effects of Aβ. They include the neuroprotective GABA_A agonists, antioxidants, anti-inflammatory, and immunomodulatory. Furthermore, the significant effects of PP2A activators and GSK-3β inhibitors in preventing neurofibrillary tangles should also be considered. For future work, it may be beneficial in such a case to apply molecular hybridization in order to develop potential drugs that work on more than one target linked firmly to AD. Otherwise, a combination of AD drugs is highly recommended. Clinical trials of drugs acting on Aβ and tau protein in combination with neuroprotective agents may change the current situation and reveal a significant protocol for AD treatment.

For the design of new anti-alzheimer small molecules, two isosteric nuclei showed very significant therapeutic properties, including reduction of the production of further toxic Aβ, activation of the cleavage of APP to soluble Aβ rather than the insoluble one, neuroprotection against the toxic effects of accumulated Aβ, and symptomatic improvement in cognitive functions. These two promising isosteric nuclei are benzimidazole and pyrazolopyridine. We saw above that their derivatives revealed QC inhibition, α-secretase activation, GABA_A modulation, and 5HT antagonist activity, showing very promising clinical results. The importance of these nuclei was highlighted by the therapeutic effects of their isostere, xanthine nucleus. We found that xanthine derivatives exhibited PDE inhibition as well as antioxidant activity. The most significant of them is benzimidazole in the sense that imidazole is considered a zinc binding group in QC inhibition, and on it a class of anti-alzheimer drugs were built. This class is called non-NSAID-derived imidazole GSMs, as explained earlier. Accordingly, these nuclei can be used as a scaffold for building new candidates for potential multi-target anti-alzheimer activity, taking together the clinical results and molecular structure of their derivatives discussed in the current study.

Conflicts of interest

There is no any conflict of interest.

Acknowledgements

The author thanks the permanent scientific committee for the promotion of professors and associate professors specializing in pharmaceutical chemistry, organic chemistry, and analytical chemistry at Al-Azhar University for suggesting writing this article.

References

1. J. C. de la Torre, Alzheimer's disease is incurable but preventable, J. Alzheimer's Dis., 2010, 20(3), 861–870.
2. F. Di Domenico, et al., The triangle of death in Alzheimer's disease brain: the aberrant cross-talk among energy metabolism, mammalian target of rapamycin signaling, and protein homeostasis revealed by redox proteomics, Antioxid. Redox Signaling, 2017, 26(8), 364–387.
3. J. Folch, et al., Current research therapeutic strategies for Alzheimer’s disease treatment, Neural Plast., 2016, 2016, 8501693.
4. M. Agrawal, et al., Nose-to-brain drug delivery: An update on clinical challenges and progress towards approval of anti-Alzheimer drugs, J. Controlled Release, 2018, 281, 139–177.
5. I. Bhushan, et al., Alzheimer's disease: Causes & treatment–A review, Ann. Biotechnol., 2018, 1(1), 1002.
6. K. B. Rajan, et al., Population estimate of people with clinical Alzheimer's disease and mild cognitive impairment in the United States (2020–2060), Alzheimer's Dementia, 2021, 17(12), 1966–1975.
7. A. Di Stefano, et al., Drug delivery strategies for Alzheimer's disease treatment, Expert Opin. Drug Delivery, 2011, 8(5), 581–603.
8. G. Benzi and A. Moretti, Is there a rationale for the use of acetylcholinesterase inhibitors in the therapy of Alzheimer's disease?, Eur. J. Pharmacol., 1998, 346(1), 1–13.
9. J. J. Buccafusco and A. V. Terry, Multiple central nervous system targets for eliciting beneficial effects on memory and cognition, J. Pharmacol. Exp. Ther., 2000, 295(2), 438–446.
10. H. C. Dringenberg, Alzheimer's disease: more than a ‘cholinergic disorder'—evidence that cholinergic–monoaminergic interactions contribute to EEG slowing and dementia, Behav. Brain Res., 2000, 115(2), 235–249.
11. E. K. Perry, et al., Correlation of cholinergic abnormalities with senile plaques and mental test scores in senile dementia, Br. Med. J., 1978, 2(6150), 1457–1459.
12. P. T. Francis, et al., Neurochemical studies of early-onset Alzheimer's disease: possible influence on treatment, N. Engl. J. Med., 1985, 313(1), 7–11.
13. M. Carreiras and J. Marco, Recent approaches to novel anti-Alzheimer therapy, Curr. Pharm. Des., 2004, 10(25), 3167.
14. A. Martorana, Z. Esposito and G. Koch, Beyond the cholinergic hypothesis: do current drugs work in Alzheimer's disease?, CNS Neurosci. Ther., 2010, 16(4), 235–245.
15. F. Hosseini, et al., Design, synthesis, and biological evaluation of novel 4-oxobenzo [d] 1, 2, 3-triazin-benzylpyridinum derivatives as potent anti-Alzheimer agents, Bioorg. Med. Chem., 2019, 27(13), 2914–2922.
16. Y. Zhang, et al., Amyloid β-based therapy for Alzheimer's disease: Challenges, successes and future, Signal Transduction Targeted Ther., 2023, 8(1), 248.
17. G. G. Glenner and C. W. Wong, Alzheimer's disease: initial report of the purification and characterization of a novel cerebrovascular amyloid protein, Biochem. Biophys. Res. Commun., 1984, 120(3), 885–890.
18. L. Chen, Hypotheses of Alzheimer's Disease Pathogenesis, 2022.
19. K. L. Chagas Monteiro, et al., BACE-1 Inhibitors Targeting Alzheimer's Disease, Curr. Alzheimer Res., 2023, 20(3), 131–148.
20. A. S. Gurjar, et al., Design, synthesis, in silico and in vitro screening of 1, 2, 4-thiadiazole analogues as non-peptide inhibitors of beta-secretase, Bioorg. Chem., 2014, 57, 90–98.
21. S. Salloway, et al., Disease-modifying therapies in Alzheimer’s disease, Alzheimer’s Dementia, 2008, 4(2), 65–79.
22. S. Azimi, et al., Discovery of imidazopyridines containing isoindoline-1, 3-dione framework as a new class of BACE1 inhibitors: design, synthesis and SAR analysis, Eur. J. Med. Chem., 2017, 138, 729–737.
23. K. R. Sadleir, et al., Presynaptic dystrophic neurites surrounding amyloid plaques are sites of microtubule disruption, BACE1 elevation, and increased Aβ generation in Alzheimer's disease, Acta Neuropathol., 2016, 132, 235–256.
24. J. A. Hardy and G. A. Higgins, Alzheimer's disease: the amyloid cascade hypothesis, Science, 1992, 256(5054), 184–185.
25. X. Liu, et al., Clusterin transduces Alzheimer-risk signals to amyloidogenesis, Signal Transduction Targeted Ther., 2022, 7(1), 325.
26. B.-L. Sun, et al., Critical thinking on amyloid-beta-targeted therapy: challenges and perspectives, Sci. China: Life Sci., 2021, 64, 926–937.
27. F. Kametani and M. Hasegawa, Reconsideration of amyloid hypothesis and tau hypothesis in Alzheimer's disease, Front. Neurosci., 2018, 12, 25.
28. R. B. Maccioni, et al., The revitalized tau hypothesis on Alzheimer's disease, Arch. Med. Res., 2010, 41(3), 226–231.
29. A. F. Arnsten, et al., Hypothesis: Tau pathology is an initiating factor in sporadic Alzheimer's disease, Alzheimer's Dementia, 2021, 17(1), 115–124.
30. K. Iqbal, Tau and Alzheimer's disease: Past, Present and Future, Cytoskeleton, 2023.
31. E. Lauretti, O. Dincer and D. Praticò, Glycogen synthase kinase-3 signaling in Alzheimer's disease, Biochim. Biophys. Acta, Mol. Cell Res., 2020, 1867(5), 118664.
32. J.-M. Sontag and E. Sontag, Protein phosphatase 2A dysfunction in Alzheimer's disease, Front. Mol. Neurosci., 2014, 7, 16.
33. W. Qian, et al., PP2A regulates tau phosphorylation directly and also indirectly via activating GSK-3β, J. Alzheimer’s Dis., 2010, 19(4), 1221–1229.
34. X. Wang, et al., Dynamin-like protein 1 reduction underlies mitochondrial morphology and distribution abnormalities in fibroblasts from sporadic Alzheimer's disease patients, Am. J. Pathol., 2008, 173(2), 470–482.
35. R. H. Swerdlow, J. M. Burns and S. M. Khan, The Alzheimer's disease mitochondrial cascade hypothesis: progress and perspectives, Biochim. Biophys. Acta, Mol. Basis Dis., 2014, 1842(8), 1219–1231.
36. R. H. Swerdlow, J. M. Burns and S. M. Khan, The Alzheimer's disease mitochondrial cascade hypothesis, J. Alzheimer’s Dis., 2010, 20(s2), S265–S279.
37. W. R. Markesbery, Oxidative stress hypothesis in Alzheimer's disease, Free Radicals Biol. Med., 1997, 23(1), 134–147.
38. D. Praticò, Oxidative stress hypothesis in Alzheimer's disease: a reappraisal, Trends Pharmacol. Sci., 2008, 29(12), 609–615.
39. J. W. Olney, D. F. Wozniak and N. B. Farber, Excitotoxic neurodegeneration in Alzheimer disease: new hypothesis and new therapeutic strategies, Arch. Neurol., 1997, 54(10), 1234–1240.
40. M. T. Kabir, et al., NMDA receptor antagonists: repositioning of memantine as a multitargeting agent for Alzheimer's therapy, Curr. Pharm. Des., 2019, 25(33), 3506–3518.
41. W. Danysz, et al., Neuroprotective and symptomatological action of memantine relevant for Alzheimer's disease—a unified glutamatergic hypothesis on the mechanism of action, Neurotoxic. Res., 2000, 2(2–3), 85–97.
42. A. C. Paula-Lima, J. Brito-Moreira and S. T. Ferreira, Deregulation of excitatory neurotransmission underlying synapse failure in Alzheimer's disease, J. Neurochem., 2013, 126(2), 191–202.
43. S. Meftah and J. Gan, Alzheimer's disease as a synaptopathy: Evidence for dysfunction of synapses during disease progression, Front. Synaptic Neurosci., 2023, 15, 1129036.
44. F. J. Rodriguez-Jimenez, et al., Alzheimer's disease and synapse Loss: What can we learn from induced pluripotent stem Cells?, J. Adv. Res., 2023, 54, 105–118.
45. P. Dourlen, et al., The new genetic landscape of Alzheimer's disease: from amyloid cascade to genetically driven synaptic failure hypothesis?, Acta Neuropathol., 2019, 138, 221–236.
46. S. W. Pimplikar, Neuroinflammation in Alzheimer's disease: from pathogenesis to a therapeutic target, J. Clin. Immunol., 2014, 34, 64–69.
47. V. Calsolaro and P. Edison, Neuroinflammation in Alzheimer’s disease: current evidence and future directions, Alzheimer’s Dementia, 2016, 12(6), 719–732.
48. B. Tamburini, et al., Emerging Roles of Cells and Molecules of Innate Immunity in Alzheimer's Disease, Int. J. Mol. Sci., 2023, 24(15), 11922.
49. C. K. Jung, et al., Fibrillar amyloid plaque formation precedes microglial activation, PLoS One, 2015, 10(3), e0119768.
50. C. A. Colton, Heterogeneity of microglial activation in the innate immune response in the brain, J. Neuroimmune Pharmacol., 2009, 4, 399–418.
51. P. Eikelenboom, et al., Neuroinflammation–an early event in both the history and pathogenesis of Alzheimer's disease, Neurodegener. Dis., 2010, 7(1–3), 38–41.
52. L. Bertram, C. M. Lill and R. E. Tanzi, The genetics of Alzheimer disease: back to the future, Neuron, 2010, 68(2), 270–281.
53. J. Andrade-Guerrero, et al., Alzheimer's Disease: An Updated Overview of Its Genetics, Int. J. Mol. Sci., 2023, 24(4), 3754.
54. R. E. Tanzi, The genetics of Alzheimer disease, Cold Spring Harbor Perspect. Med., 2012, 2(10), a006296.
55. M. Lozupone, et al., The Impact of Apolipoprotein E (APOE) Epigenetics on Aging and Sporadic Alzheimer's Disease, Biology, 2023, 12(12), 1529.
56. M. Wong-Guerra, et al., Revisiting the neuroinflammation hypothesis in Alzheimer's disease: a focus on the druggability of current targets, Front. Pharmacol., 2023, 14, 1161850.
57. C. E. Conti Filho, et al., Advances in Alzheimer's disease's pharmacological treatment, Front. Pharmacol., 2023, 14, 1101452.
58. J. B. Langbaum, et al., Recommendations to address key recruitment challenges of Alzheimer's disease clinical trials, Alzheimer's Dementia, 2023, 19(2), 696–707.
59. A. Basiri, et al., Design and synthesis of new piperidone grafted acetylcholinesterase inhibitors, Bioorg. Med. Chem. Lett., 2017, 27(2), 228–231.
60. L. Wang, et al., Design, synthesis, biological evaluation, and molecular modeling studies of chalcone-rivastigmine hybrids as cholinesterase inhibitors, Bioorg. Med. Chem., 2017, 25(1), 360–371.
61. Y. Furukawa-Hibi, et al., Butyrylcholinesterase inhibitors ameliorate cognitive dysfunction induced by amyloid-β peptide in mice, Behav. Brain Res., 2011, 225(1), 222–229.
62. D. Dimitrova, D. Getova and K. Saracheva, Effects of 3R, 16S-2-hydroxyethyl apovincaminate (HEAPO), donepezil and galantamine on learning and memory retention in naïve Wistar rats, Acta Pharm., 2023, 73(1), 91–105.
63. N. A.-E. El-Sayed, et al., Design, synthesis, in vitro and in vivo evaluation of novel pyrrolizine-based compounds with potential activity as cholinesterase inhibitors and anti-Alzheimer's agents, Bioorg. Chem., 2019, 93, 103312.
64. C. O. Wilson, et al., Wilson and Gisvold's Textbook of Organic Medicinal and Pharmaceutical Chemistry, 2004.
65. A. Kumar and A. Singh, A review on Alzheimer's disease pathophysiology and its management: an update, Pharmacol. Rep., 2015, 67(2), 195–203.
66. L. Huang, et al., Discovery of indanone derivatives as multi-target-directed ligands against Alzheimer's disease, Eur. J. Med. Chem., 2014, 87, 429–439.
67. J. Wang, et al., Synthesis and evaluation of multi-target-directed ligands for the treatment of Alzheimer's disease based on the fusion of donepezil and melatonin, Bioorg. Med. Chem., 2016, 24(18), 4324–4338.
68. Z. Gazova, et al., Multi-target-directed therapeutic potential of 7-methoxytacrine-adamantylamine heterodimers in the Alzheimer's disease treatment, Biochim. Biophys. Acta, Mol. Basis Dis., 2017, 1863(2), 607–619.
69. A. S. M. Arshad, et al., Synthesis, characterization and crystal structure of new tetrahydro-β-carboline as acetylcholinesterase inhibitor, J. Mol. Struct., 2020, 1200, 127070.
70. M. B. Colovic, et al., Acetylcholinesterase inhibitors: pharmacology and toxicology, Curr. Neuropharmacol., 2013, 11(3), 315–335.
71. J. L. Yakel, Cholinergic receptors: functional role of nicotinic ACh receptors in brain circuits and disease, Pflug. Arch. Eur. J. Physiol., 2013, 465(4), 441–450.
72. L. M. Gault, et al., ABT-126 monotherapy in mild-to-moderate Alzheimer's dementia: randomized double-blind, placebo and active controlled adaptive trial and open-label extension, Alzheimer’s Res. Ther., 2016, 8(1), 1–13.
73. H. Florian, et al., Efficacy and safety of ABT-126 in subjects with mild-to-moderate Alzheimer's disease on stable doses of acetylcholinesterase inhibitors: a randomized, double-blind, placebo-controlled study, J. Alzheimer’s Dis., 2016, 51(4), 1237–1247.
74. O. Sabri, et al., Cognitive correlates of α4β2 nicotinic acetylcholine receptors in mild Alzheimer's dementia, Brain, 2018, 141(6), 1840–1854.
75. L. E. Rueter, et al., ABT-089: pharmacological properties of a neuronal nicotinic acetylcholine receptor agonist for the potential treatment of cognitive disorders, CNS Drug Rev., 2004, 10(2), 167–182.
76. R. A. Lenz, et al., Adaptive, dose-finding phase 2 trial evaluating the safety and efficacy of ABT-089 in mild to moderate Alzheimer disease, Alzheimer Dis. Assoc. Disord., 2015, 29(3), 192–199.
77. M. Hernandez, et al., Different muscarinic receptor subtypes mediating the phasic activity and basal tone of pig isolated intravesical ureter, Br. J. Pharmacol., 1993, 110(4), 1413–1420.
78. N. C. Bodick, et al., Effects of xanomeline, a selective muscarinic receptor agonist, on cognitive function and behavioral symptoms in Alzheimer disease, Arch. Neurol., 1997, 54(4), 465–473.
79. Z. Li, et al., Xanomeline derivative EUK1001 attenuates Alzheimer's disease pathology in a triple transgenic mouse model, Mol. Med. Rep., 2017, 16(5), 7835–7840.
80. Y. Cui, et al., Enhancement of memory function in aged mice by a novel derivative of xanomeline, Cell Res., 2008, 18(11), 1151–1153.
81. W. Si, et al., A novel derivative of xanomeline improves fear cognition in aged mice, Neurosci. Lett., 2010, 473(2), 115–119.
82. B. C. Tang, Y. T. Wang and J. Ren, Basic information about memantine and its treatment of Alzheimer’s disease and other clinical applications, Ibrain, 2023, 9(3), 340–348.
83. S. Matsunaga, et al., The efficacy and safety of memantine for the treatment of Alzheimer's disease, Expert Opin. Drug Saf., 2018, 17(10), 1053–1061.
84. T. Kishi, et al., Memantine for Alzheimer's disease: an updated systematic review and meta-analysis, J. Alzheimer’s Dis., 2017, 60(2), 401–425.
85. P. Xia, et al., Memantine preferentially blocks extrasynaptic over synaptic NMDA receptor currents in hippocampal autapses, J. Neurosci., 2010, 30(33), 11246–11250.
86. M. R. Farlow and N. M. D. A. receptor antagonists, A new therapeutic approach for Alzheimer's disease, Geriatrics, 2004, 59(6), 22–27.
87. O. Sanders and L. Rajagopal, Phosphodiesterase inhibitors for Alzheimer's disease: a systematic review of clinical trials and epidemiology with a mechanistic rationale, J. Alzheimer’s Dis. Reports, 2020, 4(1), 185–215.
88. M. Shekarian, et al., Neuroprotective effects of vinpocetine, as a phosphodiesterase 1 inhibitor, on long-term potentiation in a rat model of Alzheimer's disease, BMC Neurosci., 2023, 24(1), 1–10.
89. A. E. Medina, Therapeutic utility of phosphodiesterase type I inhibitors in neurological conditions, Front. Neurosci., 2011, 5, 21.
90. A. A. Ali, et al., Vinpocetine mitigates aluminum-induced cognitive impairment in socially isolated rats, Physiol. Behav., 2019, 208, 112571.
91. H. I. Ahmed, S. A. Abdel-Sattar and H. S. Zaky, Vinpocetine halts ketamine-induced schizophrenia-like deficits in rats: impact on BDNF and GSK-3β/β-catenin pathway, Naunyn-Schmiedeberg’s Arch. Pharmacol., 2018, 391, 1327–1338.
92. V. Fattori, et al., Vinpocetine reduces diclofenac-induced acute kidney injury through inhibition of oxidative stress, apoptosis, cytokine production, and NF-κB activation in mice, Pharmacol. Res., 2017, 120, 10–22.
93. G. Svab, et al., The mitochondrial targets of neuroprotective drug vinpocetine on primary neuron cultures, brain capillary endothelial cells, synaptosomes, and brain mitochondria, Neurochem. Res., 2019, 44, 2435–2447.
94. L. J. Thal, et al., The safety and lack of efficacy of vinpocetine in Alzheimer's disease, J. Am. Geriatr. Soc., 1989, 37(6), 515–520.
95. X. Wei, et al., Targeting phosphodiesterase 4 as a therapeutic strategy for cognitive improvement, Bioorg. Chem., 2023, 130, 106278.
96. J. Prickaerts, P. R. Heckman and A. Blokland, Investigational phosphodiesterase inhibitors in phase I and phase II clinical trials for Alzheimer's disease, Expert Opin. Invest. Drugs, 2017, 26(9), 1033–1048.
97. A. Zagórska, et al., Drug Discovery and Development Targeting Dementia, Pharm., 2023, 16(2), 151.
98. N. Hasan, et al., Roflumilast reduces pathological symptoms of sporadic Alzheimer's disease in rats produced by Intracerebroventricular Streptozotocin by inhibiting NF-κB/BACE-1 mediated Aβ production in the Hippocampus and activating the cAMP/BDNF signalling pathway, Neurotoxic. Res., 2022, 40(2), 432–448.
99. T. Treves and A. Korczyn, Denbufylline in dementia: a double-blind controlled study, Dementia Geriatr. Cognit. Disord., 1999, 10(6), 505–510.
100. L. Frölich, et al., Evaluation of the efficacy, safety and tolerability of orally administered BI 409306, a novel phosphodiesterase type 9 inhibitor, in two randomised controlled phase II studies in patients with prodromal and mild Alzheimer's disease, Alzheimer’s Res. Ther., 2019, 11, 1–11.
101. E. M. Schwam, et al., A multicenter, double-blind, placebo-controlled trial of the PDE9A inhibitor, PF-04447943, in Alzheimer's disease, Curr. Alzheimer Res., 2014, 11(5), 413–421.
102. S. Da Mesquita, et al., Meningeal lymphatics affect microglia responses and anti-Aβ immunotherapy, Nature, 2021, 593(7858), 255–260.
103. L. Vaillant-Beuchot, et al., Accumulation of amyloid precursor protein C-terminal fragments triggers mitochondrial structure, function, and mitophagy defects in Alzheimer's disease models and human brains, Acta Neuropathol., 2021, 141, 39–65.
104. B. R. Troutwine, et al., Mitochondrial function and Aβ in Alzheimer's disease postmortem brain, Neurobiol. Dis., 2022, 171, 105781.
105. G. E. Vitek, B. Decourt and M. N. Sabbagh, Lecanemab (BAN2401): an anti–beta-amyloid monoclonal antibody for the treatment of Alzheimer disease, Expert Opin. Invest. Drugs, 2023, 32(2), 89–94.
106. H. D. Larkin, Lecanemab gains FDA approval for early Alzheimer disease, JAMA, J. Am. Med. Assoc., 2023, 329(5), 363.
107. J. Park, C. Simpson and K. Patel, Lecanemab: A Humanized Monoclonal Antibody for the Treatment of Early Alzheimer Disease, Ann. Pharmacother., 2023, 10600280231218253.
108. M. Vaz, et al., Role of aducanumab in the treatment of Alzheimer's disease: Challenges and opportunities, Clin. Interventions Aging, 2022, 797–810.
109. S. Walsh, et al., Aducanumab for Alzheimer’s Disease?, Br. Med. J., 2021, 374, n1682.
110. J. Sevigny, et al., The antibody aducanumab reduces Aβ plaques in Alzheimer's disease, Nature, 2016, 537(7618), 50–56.
111. S. A. Beshir, et al., Aducanumab therapy to treat Alzheimer’s disease: A narrative review, Int. J. Alzheimer’s Dis., 2022, 2022, 9343514.
112. M. Tagliapietra, Aducanumab for the treatment of Alzheimer's disease, Drugs Today, 2022, 58(10), 465–477.
113. S. M. Hoy, Lecanemab: first approval, Drugs, 2023, 83(4), 359–365.
114. J. Cummings, et al., Lecanemab: appropriate use recommendations, J. Prev. Alzheimers Dis., 2023, 1–16.
115. C. H. Van Dyck, et al., Lecanemab in early Alzheimer's disease, N. Engl. J. Med., 2023, 388(1), 9–21.
116. N. H. Parikh, et al., Current trends and updates in the treatment of Alzheimer's disease, in Alzheimer's Disease and Advanced Drug Delivery Strategies, Elsevier, 2024, pp. 373–390.
117. E. McDade, et al., Lecanemab in patients with early Alzheimer's disease: Detailed results on biomarker, cognitive, and clinical effects from the randomized and open-label extension of the phase 2 proof-of-concept study, Alzheimer’s Res. Ther., 2022, 14(1), 1–17.
118. L. S. Honig, et al., Trial of solanezumab for mild dementia due to Alzheimer's disease, N. Engl. J. Med., 2018, 378(4), 321–330.
119. S. Salloway, et al., A trial of gantenerumab or solanezumab in dominantly inherited Alzheimer's disease, Nat. Med., 2021, 27(7), 1187–1196.
120. A. I. Abushouk, et al., Bapineuzumab for mild to moderate Alzheimer's disease: a meta-analysis of randomized controlled trials, BMC Neurol., 2017, 17(1), 1–13.
121. R. Vandenberghe, et al., Bapineuzumab for mild to moderate Alzheimer's disease in two global, randomized, phase 3 trials, Alzheimer’s Res. Ther., 2016, 8(1), 1–13.
122. L. Lannfelt, et al., Perspectives on future Alzheimer therapies: amyloid-β protofibrils-a new target for immunotherapy with BAN2401 in Alzheimer's disease, Alzheimer’s Res. Ther., 2014, 6(2), 1–8.
123. S. Ostrowitzki, et al., A phase III randomized trial of gantenerumab in prodromal Alzheimer's disease, Alzheimer’s Res. Ther., 2017, 9(1), 1–15.
124. R. J. Bateman, et al., Gantenerumab: an anti-amyloid monoclonal antibody with potential disease-modifying effects in early Alzheimer's disease, Alzheimer’s Res. Ther., 2022, 14(1), 1–17.
125. R. L. Patton, et al., Amyloid-β peptide remnants in AN-1792-immunized Alzheimer's disease patients: a biochemical analysis, Am. J. Pathol., 2006, 169(3), 1048–1063.
126. E. Masliah, et al., Aβ vaccination effects on plaque pathology in the absence of encephalitis in Alzheimer disease, Neurology, 2005, 64(1), 129–131.
127. I. Ferrer, et al., Neuropathology and pathogenesis of encephalitis following amyloid β immunization in Alzheimer's disease, Brain Pathol., 2004, 14(1), 11–20.
128. S. R. Robinson, et al., Lessons from the AN 1792 Alzheimer vaccine: lest we forget, Neurobiol. Aging, 2004, 25(5), 609–615.
129. M. Hull, et al., Long-Term extensions of randomized vaccination trials of ACC-001 and QS-21 in mild to moderate alzheimer's disease, Curr. Alzheimer Res., 2017, 14(7), 696–708.
130. F. Pasquier, et al., Two phase 2 multiple ascending–dose studies of vanutide cridificar (ACC-001) and QS-21 adjuvant in mild-to-moderate Alzheimer's disease, J. Alzheimer’s Dis., 2016, 51(4), 1131–1143.
131. C. Van Dyck, et al., Vanutide Cridificar (ACC-001) and QS-21 Adjuvant in Individuals with Early Alzheimer's Disease: Amyloid Imaging Positron Emission Tomography and Safety Results from a Phase 2 Study, J. Prev. Alzheimers Dis., 2016, 3(2), 75–84.
132. E. Molina, et al., AB1601 topline results–Phase 2 study of ABvac40 in patients with amnestic mild cognitive impairment (a-MCI) or very mild Alzheimer's Disease (Vm-AD), Alzheimer's Dementia, 2022, 18, e065633.
133. J. A. Hey, et al., Clinical pharmacokinetics and safety of ALZ-801, a novel prodrug of tramiprosate in development for the treatment of Alzheimer's disease, Clin. Pharmacokinet., 2018, 57, 315–333.
134. M. Tolar, et al., Aducanumab, gantenerumab, BAN2401, and ALZ-801—the first wave of amyloid-targeting drugs for Alzheimer's disease with potential for near term approval, Alzheimer’s Res. Ther., 2020, 12, 1–10.
135. S. Manzano, et al., A review on tramiprosate (Homotaurine) in alzheimer’s disease and other neurocognitive disorders, Front. Neurol., 2020, 11, 614.
136. P. Kocis, et al., Elucidating the Aβ42 anti-aggregation mechanism of action of tramiprosate in Alzheimer’s disease: integrating molecular analytical methods, pharmacokinetic and clinical data, CNS Drugs, 2017, 31, 495–509.
137. S. Abushakra, et al., Clinical benefits of tramiprosate in alzheimer's disease are associated with higher number of APOE4 alleles: the “APOE4 gene-dose effect, J. Prev. Alzheimer's Dis., 2016, 3(4), 219–228.
138. M. Tolar, S. Abushakra and M. Sabbagh, The path forward in Alzheimer's disease therapeutics: Reevaluating the amyloid cascade hypothesis, Alzheimer's Dementia, 2019.
139. S. Abushakra, et al., Prevalence of Amyloid-Related Imaging Abnormalities in APOE4/4 Homozygotes with Early Alzheimer’s Disease: Baseline Findings from Ongoing Clinical Trials of Oral Anti-amyloid Agent ALZ-801 (Valiltramiprosate)(P5-6.003), Neurology, 2023, 100(17_supplement_2) DOI:10.1212/WNL.0000000000203550.
140. E. E. Congdon and E. M. Sigurdsson, Tau-targeting therapies for Alzheimer disease, Nat. Rev. Neurol., 2018, 14(7), 399–415.
141. C. B. Malpas, et al., A phase IIa randomized control trial of VEL015 (Sodium Selenate) in mild-moderate Alzheimer's disease, J. Alzheimer’s Dis., 2016, 54(1), 223–232.
142. V. Melis, et al., Effects of oxidized and reduced forms of methylthioninium in two transgenic mouse tauopathy models, Behav. Pharmacol., 2015, 26(4), 353.
143. L. Martin, et al., Tau protein kinases: involvement in Alzheimer's disease, Ageing Res. Rev., 2013, 12(1), 289–309.
144. J. J. Lucas, et al., Decreased nuclear β-catenin, tau hyperphosphorylation and neurodegeneration in GSK-3β conditional transgenic mice, EMBO J., 2001, 20(1–2), 27–39.
145. J. M. Domínguez, et al., Evidence for irreversible inhibition of glycogen synthase kinase-3β by tideglusib, J. Biol. Chem., 2012, 287(2), 893–904.
146. A. Martinez, et al., First non-ATP competitive glycogen synthase kinase 3 β (GSK-3β) inhibitors: thiadiazolidinones (TDZD) as potential drugs for the treatment of Alzheimer's disease, J. Med. Chem., 2002, 45(6), 1292–1299.
147. B. DaRocha-Souto, et al., Activation of glycogen synthase kinase-3 beta mediates β-amyloid induced neuritic damage in Alzheimer's disease, Neurobiol. Dis., 2012, 45(1), 425–437.
148. L. Serenó, et al., A novel GSK-3β inhibitor reduces Alzheimer's pathology and rescues neuronal loss in vivo, Neurobiol. Dis., 2009, 35(3), 359–367.
149. S. Lovestone, et al., A phase II trial of tideglusib in Alzheimer's disease, J. Alzheimer’s Dis., 2015, 45(1), 75–88.
150. V. Stambolic, L. Ruel and J. R. Woodgett, Lithium inhibits glycogen synthase kinase-3 activity and mimics wingless signalling in intact cells, Curr. Biol., 1996, 6(12), 1664–1669.
151. E. Portelius, et al., β-site amyloid precursor protein-cleaving enzyme 1 (BACE1) inhibitor treatment induces Aβ5-X peptides through alternative amyloid precursor protein cleavage, Alzheimer’s Res. Ther., 2014, 6, 1–8.
152. P. C. May, et al., Robust central reduction of amyloid-β in humans with an orally available, non-peptidic β-secretase inhibitor, J. Neurosci., 2011, 31(46), 16507–16516.
153. P. C. May, et al., The potent BACE1 inhibitor LY2886721 elicits robust central Aβ pharmacodynamic responses in mice, dogs, and humans, J. Neurosci., 2015, 35(3), 1199–1210.
154. D. Kumar, et al., Secretase inhibitors for the treatment of Alzheimer's disease: Long road ahead, Eur. J. Med. Chem., 2018, 148, 436–452.
155. L.-K. Huang, S.-P. Chao and C.-J. Hu, Clinical trials of new drugs for Alzheimer disease, J. Biomed. Sci., 2020, 27(1), 1–13.
156. U. Neumann, et al., The BACE-1 inhibitor CNP 520 for prevention trials in Alzheimer's disease, EMBO Mol. Med., 2018, 10(11), e9316.
157. A. Graf, et al., Fts3-01-01: Umibecestat (Cnp520) Is Not Associated With Changes In Hippocampal Morphology In Rats Or Changes In Csf Ad Biomarkers In Humans Treated For 3 Months, Alzheimer's Dementia, 2019, 15, P872.
158. M. F. Egan, et al., Randomized trial of verubecestat for mild-to-moderate Alzheimer's disease, N. Engl. J. Med., 2018, 378(18), 1691–1703.
159. T. Voss, et al., Progression from Prodromal Alzheimer’s Disease to Mild Alzheimer’s Disease Dementia in the Verubecestat APECS Study: Adjudicating Diagnostic Transitions, J. Alzheimer’s Dis., 2023, 92(1), 341–348.
160. M. F. Egan, et al., Randomized trial of verubecestat for prodromal Alzheimer's disease, N. Engl. J. Med., 2019, 380(15), 1408–1420.
161. M. Timmers, et al., BACE1 dynamics upon inhibition with a BACE inhibitor and correlation to downstream Alzheimer's disease markers in elderly healthy participants, J. Alzheimer’s Dis., 2017, 56(4), 1437–1449.
162. M. Timmers, et al., Profiling the dynamics of CSF and plasma Aβ reduction after treatment with JNJ-54861911, a potent oral BACE inhibitor, Alzheimer’s Dement.: Transl. Res. Clin. Interv., 2016, 2(3), 202–212.
163. M. Timmers, et al., Pharmacodynamics of atabecestat (JNJ-54861911), an oral BACE1 inhibitor in patients with early Alzheimer's disease: randomized, double-blind, placebo-controlled study, Alzheimer’s Res. Ther., 2018, 10, 1–18.
164. D. Henley, et al., Preliminary Results of a Trial of Atabecestat in Preclinical Alzheimer's Disease, The N. Engl. J. Med., 2019, 380(15), 1483–1485.
165. S. De Jonghe, et al., Biopsy pathology and immunohistochemistry of a case of immune-mediated drug-induced liver injury with Atabecestat, Hepatology, 2021, 73(1), 452–455.
166. R. Sperling, et al., Findings of efficacy, safety, and biomarker outcomes of atabecestat in preclinical Alzheimer disease: a truncated randomized phase 2b/3 clinical trial, JAMA Neurol., 2021, 78(3), 293–301.
167. A. M. Wessels, et al., Efficacy and safety of lanabecestat for treatment of early and mild Alzheimer disease: the AMARANTH and DAYBREAK-ALZ randomized clinical trials, JAMA Neurol., 2020, 77(2), 199–209.
168. J. A. Zimmer, et al., Lanabecestat: Neuroimaging results in early symptomatic Alzheimer's disease, Alzheimer’s Dement.: Transl. Res. Clin. Interv., 2021, 7(1), e12123.
169. D. Caouette, Eisai and Biogen to Discontinue Phase III Clinical Studies of BACE Inhibitor Elenbecestat in Early Alzheimer's Disease, Eisai’s press release, 2019.
170. B. P. Imbimbo and M. Watling, Investigational BACE inhibitors for the treatment of Alzheimer's disease, Expert Opin. Invest. Drugs, 2019, 28(11), 967–975.
171. U. Neumann, R. Machauer and D. R. Shimshek, The β-secretase (BACE) inhibitor NB-360 in preclinical models: From amyloid-β reduction to downstream disease-relevant effects, Br. J. Pharmacol., 2019, 176(18), 3435–3446.
172. U. Neumann, et al., A novel BACE inhibitor NB-360 shows a superior pharmacological profile and robust reduction of amyloid-β and neuroinflammation in APP transgenic mice, Mol. Neurodegener., 2015, 10(1), 1–15.
173. D. R. Shimshek, et al., Pharmacological BACE1 and BACE2 inhibition induces hair depigmentation by inhibiting PMEL17 processing in mice, Sci. Rep., 2016, 6(1), 21917.
174. H. Jacobsen, et al., Combined treatment with a BACE inhibitor and anti-Aβ antibody gantenerumab enhances amyloid reduction in APPLondon mice, J. Neurosci., 2014, 34(35), 11621–11630.
175. C. G. Parsons and G. Rammes, Preclinical to phase II amyloid beta (Aβ) peptide modulators under investigation for Alzheimer's disease, Expert Opin. Invest. Drugs, 2017, 26(5), 579–592.
176. F. Tadros Hakem, Y. Farid Fouad and R. K. Arafa, Gamma Secretase as an Important Drug Target for Management of Alzheimer's Disease: A Comprehensive Review, Curr. Top. Med. Chem., 2024, 24(2), 109–127.
177. B. De Strooper, T. Iwatsubo and M. S. Wolfe, Presenilins and γ-secretase: structure, function, and role in Alzheimer disease, Cold Spring Harbor Perspect. Med., 2012, 2(1).
178. P. C. May, et al., O3-06-07 Multi-compartmental pharmacodynamic assessment of the functional gamma-secretase inhibitor LY450139 in PDAPP transgenic mice and non-transgenic mice, Neurobiol. Aging, 2004,(25), S65.
179. D. B. Henley, et al., Development of semagacestat (LY450139), a functional γ-secretase inhibitor, for the treatment of Alzheimer's disease, Expert Opin. Pharmacother., 2009, 10(10), 1657–1664.
180. R. Doody, Alzheimer’s Disease Cooperative Study Steering Committee, E. Siemers, G. Sethuraman, R. Mohs and Semagacestat Study Group, et al.) A phase 3 trial of semagacestat for treatment of Alzheimer's disease, N. Engl. J. Med., 2013, 369, 341–350.
181. R. Rosen, et al., Efficacy and tolerability of vardenafil in men with mild depression and erectile dysfunction: the depression-related improvement with vardenafil for erectile response study, Am. J. Psychiatry, 2006, 163(1), 79–87.
182. G. Tong, et al., Multicenter, randomized, double-blind, placebo-controlled, single-ascending dose study of the oral γ-secretase inhibitor BMS-708163 (Avagacestat): tolerability profile, pharmacokinetic parameters, and pharmacodynamic markers, Clin. Ther., 2012, 34(3), 654–667.
183. S. C. Mayer, et al., Discovery of begacestat, a Notch-1-sparing γ-secretase inhibitor for the treatment of Alzheimer's disease, J. Med. Chem., 2008, 51(23), 7348–7351.
184. V. Coric, et al., Safety and tolerability of the γ-secretase inhibitor avagacestat in a phase 2 study of mild to moderate Alzheimer disease, Arch. Neurol., 2012, 69(11), 1430–1440.
185. V. Coric, et al., Targeting prodromal Alzheimer disease with avagacestat: a randomized clinical trial, JAMA Neurol., 2015, 72(11), 1324–1333.
186. C. R. Hopkins, ACS Chemical Neuroscience Molecule Spotlight on Begacestat (GSI-953), ACS, 2012, 3–4.
187. D. C. Cole, et al., (S)-N-(5-Chlorothiophene-2-sulfonyl)-β, β-diethylalaninol a Notch-1-sparing γ-secretase inhibitor, Bioorg. Med. Chem. letters, 2009, 19(3), 926–929.
188. R. L. Martone, et al., Begacestat (GSI-953): a novel, selective thiophene sulfonamide inhibitor of amyloid precursor protein γ-secretase for the treatment of Alzheimer's disease, J. Pharmacol. Exp. Ther., 2009, 331(2), 598–608.
189. B. P. Imbimbo, Alzheimer's disease: γ-secretase inhibitors, Drug Discovery Today: Ther. Strategies, 2008, 5(3), 169–175.
190. D. M. Barten and C. F. Albright, Therapeutic strategies for Alzheimer's disease, Mol. Neurobiol., 2008, 37, 171–186.
191. R. S. Doody, et al., A phase 3 trial of semagacestat for treatment of Alzheimer's disease, N. Engl. J. Med., 2013, 369(4), 341–350.
192. Y. Mitani, et al., Differential effects between γ-secretase inhibitors and modulators on cognitive function in amyloid precursor protein-transgenic and nontransgenic mice, J. Neurosci., 2012, 32(6), 2037–2050.
193. J.-Y. Hur, γ-Secretase in Alzheimer's disease, Exp. Mol. Med., 2022, 54(4), 433–446.
194. C. J. Crump, D. S. Johnson and Y.-M. Li, Development and mechanism of γ-secretase modulators for Alzheimer's disease, Biochemistry, 2013, 52(19), 3197–3216.
195. S. Weggen, et al., A subset of NSAIDs lower amyloidogenic Aβ42 independently of cyclooxygenase activity, Nature, 2001, 414(6860), 212–216.
196. S. Mekala, G. Nelson and Y.-M. Li, Recent developments of small molecule γ-secretase modulators for Alzheimer's disease, RSC Med. Chem., 2020, 11(9), 1003–1022.
197. T. Morihara, et al., Selective inhibition of Aβ42 production by NSAID R-enantiomers, J. Neurochem., 2002, 83(4), 1009–1012.
198. Y. Yu, et al., Safety, tolerability, pharmacokinetics, and pharmacodynamics of the novel γ-secretase modulator, E2212, in healthy human subjects, J. Clin. Pharmacol., 2014, 54(5), 528–536.
199. R. C. Green, et al., Effect of tarenflurbil on cognitive decline and activities of daily living in patients with mild Alzheimer disease: a randomized controlled trial, JAMA, J. Am. Med. Assoc., 2009, 302(23), 2557–2564.
200. J. L. Eriksen, et al., NSAIDs and enantiomers of flurbiprofen target γ-secretase and lower Aβ42 in vivo, J. Clin. Invest., 2003, 112(3), 440–449.
201. G. K. Wilcock, et al., Efficacy and safety of tarenflurbil in mild to moderate Alzheimer's disease: a randomised phase II trial, Lancet Neurol., 2008, 7(6), 483–493.
202. B. Van Broeck, et al., Chronic treatment with a novel γ-secretase modulator, JNJ-40418677, inhibits amyloid plaque formation in a mouse model of Alzheimer's disease, Br. J. Pharmacol., 2011, 163(2), 375–389.
203. K. Rogers, et al., Modulation of γ-secretase by EVP-0015962 reduces amyloid deposition and behavioral deficits in Tg2576 mice, Mol. Neurodegener., 2012, 7, 1–18.
204. J. Zhao, et al., Targeting amyloidogenic processing of APP in Alzheimer's disease, Front. Mol. Neurosci., 2020, 13, 137.
205. S. Sivilia, et al., Multi-target action of the novel anti-Alzheimer compound CHF5074: in vivo study of long term treatment in Tg2576 mice, BMC Neurosci., 2013, 14(1), 1–14.
206. B. P. Imbimbo, et al., Pharmacokinetics and pharmacodynamics of CHF5074 after short-term administration in healthy subjects, Alzheimer Dis. Assoc. Disord., 2013, 27(3), 278–286.
207. H. Peng, et al., Discovery of BIIB042, a potent, selective, and orally bioavailable γ-secretase modulator, ACS Med. Chem. Lett., 2011, 2(10), 786–791.
208. R. H. Scannevin, et al., BIIB042, a novel γ-secretase modulator, reduces amyloidogenic Aβ isoforms in primates and rodents and plaque pathology in a mouse model of Alzheimer's disease, Neuropharmacology, 2016, 103, 57–68.
209. W. Wuli, et al., Human-induced pluripotent stem cells and herbal small-molecule drugs for treatment of Alzheimer's disease, Int. J. Mol. Sci., 2020, 21(4), 1327.
210. K. Nakano-Ito, et al., E2012-induced cataract and its predictive biomarkers, Toxicol. Sci., 2014, 137(1), 249–258.
211. P. Nie, A. Vartak, and Y.-M. Li, γ-Secretase inhibitors and modulators: Mechanistic insights into the function and regulation of γ-Secretase, in Seminars in cell & developmental biology, Elsevier, 2020.
212. A. Hall and T. R. Patel, γ-Secretase modulators: current status and future directions, Prog. Med. Chem., 2014, 53, 101–145.
213. T. T. Wager, et al., Moving beyond rules: the development of a central nervous system multiparameter optimization (CNS MPO) approach to enable alignment of druglike properties, ACS Chem. Neurosci., 2010, 1(6), 435–449.
214. J. H. Toyn, et al., Identification and Preclinical Pharmacology of the-Secretase Modulator BMS-869780, Int. J. Alzheimer’s Dis., 2014, 2014, 431858.
215. J. E. Ahn, et al., Pharmacokinetic and Pharmacodynamic Effects of a γ-Secretase Modulator, PF-06648671, on CSF Amyloid-β Peptides in Randomized Phase I Studies, Clin. Pharmacol. Ther., 2020, 107(1), 211–220.
216. Z. Zhao, et al., Discovery of a tetrahydrobenzisoxazole series of γ-secretase modulators, ACS Med. Chem. Lett., 2017, 8(10), 1002–1006.
217. M. A. Findeis, et al., Natural product and natural product-derived gamma secretase modulators from Actaea Racemosa extracts, Medicines, 2015, 2(3), 127–140.
218. H. Harms, et al., Aβ-42 lowering agents from the marine-derived fungus Dichotomomyces cejpii, Steroids, 2015, 104, 182–188.
219. X. Lei, et al., The FDA-approved natural product dihydroergocristine reduces the production of the Alzheimer's disease amyloid-β peptides, Sci. Rep., 2015, 5(1), 16541.
220. S. F. Lichtenthaler and C. Haass, Amyloid at the cutting edge: activation of α-secretase prevents amyloidogenesis in an Alzheimer disease mouse model, J. Clin. Invest., 2004, 113(10), 1384–1387.
221. B. De Strooper, R. Vassar and T. Golde, The secretases: enzymes with therapeutic potential in Alzheimer disease, Nat. Rev. Neurol., 2010, 6(2), 99–107.
222. R. E. Tanzi and L. Bertram, Twenty years of the Alzheimer's disease amyloid hypothesis: a genetic perspective, Cell, 2005, 120(4), 545–555.
223. F. Tippmann, et al., Up-regulation of the α-secretase ADAM10 by retinoic acid receptors and acitretin, FASEB J., 2009, 23(6), 1643–1654.
224. D. Holthoewer, et al., Acitretin, an enhancer of alpha-secretase expression, crosses the blood–brain barrier and is not eliminated by P-glycoprotein, Neurodegener. Dis., 2012, 10(1–4), 224–228.
225. M. Marcade, et al., Etazolate, a neuroprotective drug linking GABAA receptor pharmacology to amyloid precursor protein processing, J. Neurochem., 2008, 106(1), 392–404.
226. B. Vellas, et al., EHT0202 in Alzheimer's disease: a 3-month, randomized, placebo-controlled, double-blind study, Curr. Alzheimer Res., 2011, 8(2), 203–212.
227. G. S. V. Higa, et al., 5-HT-dependent synaptic plasticity of the prefrontal cortex in postnatal development, Sci. Rep., 2022, 12(1), 21015.
228. D. V. Eremin, et al., Serotonin Receptors as a Potential Target in the Treatment of Alzheimer's Disease, Biochem., 2023, 88(12), 2023–2042.
229. K. Kucwaj-Brysz, et al., Chemical update on the potential for serotonin 5-HT6 and 5-HT7 receptor agents in the treatment of Alzheimer's disease, Bioorg. Med. Chem. Letters, 2021, 49, 128275.
230. M. Andrews, B. Tousi and M. N. Sabbagh, 5HT6 antagonists in the treatment of Alzheimer’s dementia: current progress, Neurol. Ther., 2018, 7, 51–58.
231. T. Fullerton, et al., A Phase 2 clinical trial of PF-05212377 (SAM-760) in subjects with mild to moderate Alzheimer's disease with existing neuropsychiatric symptoms on a stable daily dose of donepezil, Alzheimer’s Res. Ther., 2018, 10, 1–10.
232. S. Matsunaga, H. Fujishiro and H. Takechi, Efficacy and safety of idalopirdine for Alzheimer's disease: a systematic review and meta-analysis, Int. Psychogeriatr., 2019, 31(11), 1627–1633.
233. A. Atri, et al., Effect of idalopirdine as adjunct to cholinesterase inhibitors on change in cognition in patients with Alzheimer disease: three randomized clinical trials, JAMA, J. Am. Med. Assoc., 2018, 319(2), 130–142.
234. D. Chen, et al., Development and evolution of human glutaminyl cyclase inhibitors (QCIs): an alternative promising approach for disease-modifying treatment of Alzheimer’s disease, Front. Aging Neurosci., 2023, 15, 1209863.
235. J. R. Coimbra, et al., An overview of glutaminyl cyclase inhibitors for Alzheimer's disease, Future Med. Chem., 2019, 11(24), 3179–3194.
236. I. Lues, et al., A phase 1 study to evaluate the safety and pharmacokinetics of PQ912, a glutaminyl cyclase inhibitor, in healthy subjects, Alzheimer’s Dement.: Transl. Res. Clin. Interv., 2015, 1(3), 182–195.
237. P. Scheltens, et al., Safety, tolerability and efficacy of the glutaminyl cyclase inhibitor PQ912 in Alzheimer's disease: results of a randomized, double-blind, placebo-controlled phase 2a study, Alzheimer’s Res. Ther., 2018, 10(1), 1–14.
238. D. K. Vijayan and K. Y. Zhang, Human glutaminyl cyclase: Structure, function, inhibitors and involvement in Alzheimer's disease, Pharmacol. Res., 2019, 147, 104342.
239. Z.-Z. Si, et al., Targeting neuroinflammation in Alzheimer's disease: From mechanisms to clinical applications, Neural Regener. Res., 2023, 18(4), 708.
240. T. Song, et al., Mitochondrial dysfunction, oxidative stress, neuroinflammation, and metabolic alterations in the progression of Alzheimer's disease: A meta-analysis of in vivo magnetic resonance spectroscopy studies, Ageing Res. Rev., 2021, 72, 101503.
241. C. L. Chen, et al., Alzheimer's disease THErapy with NEuroaid (ATHENE): A randomized double-blind delayed-start trial, J. Am. Med. Dir. Assoc., 2022, 23(3), 379–386.
242. D. Plantone, et al., The Role of TNF-α in Alzheimer's Disease: A Narrative Review, Cells, 2023, 13(1), 54.
243. B. Goffe and J. C. Cather, Etanercept: an overview, J. Am. Acad. Dermatol., 2003, 49(2), 105–111.
244. B. Decourt, et al., Poor safety and tolerability hamper reaching a potentially therapeutic dose in the use of thalidomide for Alzheimer's disease: results from a double-blind, placebo-controlled trial, Curr. Alzheimer Res., 2017, 14(4), 403–411.
245. S. Jo, et al., GABA from reactive astrocytes impairs memory in mouse models of Alzheimer's disease, Nat. Med., 2014, 20(8), 886–896.
246. Z. Wu, et al., Tonic inhibition in dentate gyrus impairs long-term potentiation and memory in an Alzheimer's disease model, Nature communications, 2014, 5(1), 4159.
247. M. Bauer, et al., Delta-like 1 participates in the specification of ventral midbrain progenitor derived dopaminergic neurons, J. Neurochem., 2008, 104(4), 1101–1115.
248. J. Drott, et al., Etazolate improves performance in a foraging and homing task in aged rats, Eur. J. Pharmacol., 2010, 634(1–3), 95–100.
249. H. Shao, et al., Chronic treatment with anesthetic propofol improves cognitive function and attenuates caspase activation in both aged and Alzheimer's disease transgenic mice, J. Alzheimer’s Dis., 2014, 41(2), 499–513.
250. Y. Zhang, et al., Chronic treatment with anesthetic propofol attenuates β-amyloid protein levels in brain tissues of aged mice, Transl. Neurodegener., 2014, 3(1), 1–7.
251. W. Froestl, et al., SGS742: the first GABAB receptor antagonist in clinical trials, Biochem. Pharmacol., 2004, 68(8), 1479–1487.
252. D. Getova and N. Bowery, Effects of high-affinity GABA B receptor antagonists on active and passive avoidance responding in rodents with gamma-hydroxybutyrolactone-induced absence syndrome, Psychopharmacology, 2001, 157, 89–95.
253. K. Helm, et al., GABAB receptor antagonist SGS742 improves spatial memory and reduces protein binding to the cAMP response element (CRE) in the hippocampus, Neuropharmacology, 2005, 48(7), 956–964.
254. C. L. LaSarge, et al., Blockade of GABA (B) receptors completely reverses age-related learning impairment, Neuroscience, 2009, 164(3), 941–947.
255. Y. Li, et al., Implications of GABAergic neurotransmission in Alzheimer's disease, Front. Aging Neurosci., 2016, 8, 31.
256. C. Foy, et al., Plasma chain-breaking antioxidants in Alzheimer’s disease, vascular dementia and Parkinson’s disease, Q. J. Med., 1999, 92(1), 39–45.
257. T. Montiel, et al., Role of oxidative stress on β-amyloid neurotoxicity elicited during impairment of energy metabolism in the hippocampus: protection by antioxidants, Exp. Neurol., 2006, 200(2), 496–508.
258. E. E. Devore, et al., Dietary antioxidants and long-term risk of dementia, Arch. Neurol., 2010, 67(7), 819–825.
259. M. Sano, et al., A controlled trial of selegiline, alpha-tocopherol, or both as treatment for Alzheimer's disease, N. Engl. J. Med., 1997, 336(17), 1216–1222.
260. D. Dias-Santagata, et al., Oxidative stress mediates tau-induced neurodegeneration in Drosophila, J. Clin. Invest., 2007, 117(1), 236–245.
261. Y. Feng and X. Wang, Antioxidant therapies for Alzheimer’s disease, Oxid. Med. Cell. Longevity, 2012, 2012, 472932.
262. P. Lu, et al., Silibinin prevents amyloid β peptide-induced memory impairment and oxidative stress in mice, Br. J. Pharmacol., 2009, 157(7), 1270–1277.
263. K. Goozee, et al., Examining the potential clinical value of curcumin in the prevention and diagnosis of Alzheimer's disease, Br. J. Nutr., 2016, 115(3), 449–465.
264. L. G. Costa, et al., Mechanisms of neuroprotection by quercetin: counteracting oxidative stress and more, Oxid. Med. Cell. Longevity, 2016, 2016, 2986796.
265. H. Wang, et al., Ameliorating effect of luteolin on memory impairment in an Alzheimer's disease model, Mol. Med. Rep., 2016, 13(5), 4215–4220.
266. J. F. Quinn, et al., Chronic dietary α-lipoic acid reduces deficits in hippocampal memory of aged Tg2576 mice, Neurobiol. Aging, 2007, 28(2), 213–225.
267. J. Zhou, et al., Melatonin impairs NADPH oxidase assembly and decreases superoxide anion production in microglia exposed to amyloid-β1–42, J. Pineal Res., 2008, 45(2), 157–165.
268. Y.-q. Deng, et al., Effects of melatonin on wortmannin-induced tau hyperphosphorylation, Acta Pharmacol. Sin., 2005, 26(5), 519–526.
269. S. J. Liu and J. Z. Wang, Alzheimer-like tau phosphorylation induced by wortmannin in vivo and its attenuation by melatonin, Acta Pharmacol. Sin., 2002, 23(2), 183–187.
270. D. L. Wang, et al., Melatonin attenuates isoproterenol-induced protein kinase A overactivation and tau hyperphosphorylation in rat brain, J. Pineal Res., 2004, 37(1), 11–16.
271. X.-C. Li, et al., Effect of melatonin on calyculin A-induced tau hyperphosphorylation, Eur. J. Pharmacol., 2005, 510(1–2), 25–30.
272. Z. Feng and J.-t. Zhang, Protective effect of melatonin on β-amyloid-induced apoptosis in rat astroglioma c6 cells and its mechanism, Free Radicals Biol. Med., 2004, 37(11), 1790–1801.
273. T. Thomas, Monoamine oxidase-B inhibitors in the treatment of Alzheimers disease, Neurobiol. Aging, 2000, 21(2), 343–348.
274. A. Wuestefeld, et al., Age-related and amyloid-beta-independent tau deposition and its downstream effects, Brain, 2023, awad135.

---