new research into alzheimer's disease

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Alzheimer’s Breakthrough: Researchers Discover Novel Way To Potentially Halt Disease Progression

By The Mount Sinai Hospital / Mount Sinai School of Medicine June 1, 2024

Mount Sinai researchers have discovered a potential new method to treat Alzheimer’s by targeting the plexin-B1 protein to improve plaque clearance in the brain, opening avenues for future therapeutic strategies. Credit: SciTechDaily.com

Innovative research from Mount Sinai has also identified new pathways for research.

Researchers at the Icahn School of Medicine at Mount Sinai have achieved a major breakthrough in Alzheimer’s disease research. Their study identifies a promising method that could potentially slow or even stop the progression of the disease. Focusing on the role of reactive astrocytes and the plexin-B1 protein in Alzheimer’s disease, the research offers vital insights into how brain cells communicate. This opens up new avenues for innovative treatment approaches. The findings were published on May 27 in the journal Nature Neuroscience .

This groundbreaking work is centered on the manipulation of the plexin-B1 protein to enhance the brain’s ability to clear amyloid plaques, a hallmark of Alzheimer’s disease. Reactive astrocytes, a type of brain cell that becomes activated in response to injury or disease, were found to play a crucial role in this process. They help control the spacing around amyloid plaques, affecting how other brain cells can access and clear these harmful deposits.

“Our findings offer a promising path for developing new treatments by improving how cells interact with these harmful plaques,” said Roland Friedel, PhD, Associate Professor of Neuroscience, and Neurosurgery, at Icahn Mount Sinai and a senior author of the study. The research was driven by the analysis of complex data comparing healthy individuals to those with Alzheimer’s, aiming to understand the disease’s molecular and cellular foundations.

Icahn Mount Sinai researchers find PLXNB1, a hub gene predicted to drive a gene subnetwork causally linked to human AD, is upregulated in reactive astrocytes surrounding amyloid plaques. Credit: Bin Zhang, PhD, Icahn Mount Sinai

Broad Implications and Validation of Gene Network Models

Hongyan Zou, PhD, Professor of Neurosurgery, and Neuroscience, at Icahn Mount Sinai and one of the study’s lead authors, highlighted the broader implications of their findings: “Our study opens new pathways for Alzheimer’s research, emphasizing the importance of cellular interactions in developing neurodegenerative disease treatments.”

One of the study’s most significant achievements is its validation of multiscale gene network models of Alzheimer’s disease. “This study not only confirms one of the most important predictions from our gene network models but also significantly advances our understanding of Alzheimer’s. It lays a solid foundation for developing novel therapeutics targeting such highly predictive network models,” said Bin Zhang, PhD, Willard T.C. Johnson Research Professor of Neurogenetics at Icahn Mount Sinai and one of the study’s lead authors. By demonstrating the critical role of plexin-B1 in Alzheimer’s disease, the research underscores the potential of targeted therapies to disrupt the disease’s progression.

The research team emphasizes that while their findings mark a significant advance in the fight against Alzheimer’s, more research is needed to translate these discoveries into treatments for human patients.

“Our ultimate goal is to develop treatments that can prevent or slow down Alzheimer’s progression,” Dr. Zhang added, outlining the team’s commitment to further exploring the therapeutic potential of plexin-B1.

Reference: “Regulation of cell distancing in peri-plaque glial nets by Plexin-B1 affects glial activation and amyloid compaction in Alzheimer’s disease” by Yong Huang, Minghui Wang, Haofei Ni, Jinglong Zhang, Aiqun Li, Bin Hu, Chrystian Junqueira Alves, Shalaka Wahane, Mitzy Rios de Anda, Lap Ho, Yuhuan Li, Sangjo Kang, Ryan Neff, Ana Kostic, Joseph D. Buxbaum, John F. Crary, Kristen J. Brennand, Bin Zhang, Hongyan Zou and Roland H. Friedel, 27 May 2024, Nature Neuroscience . DOI: 10.1038/s41593-024-01664-w

This study is supported by the NIH National Institute on Aging (NIA) grants U01AG046170 and RF1AG057440 and is part of the NIA-led Accelerating Medicines Partnership – Alzheimer’s Disease (AMP-AD) Target Discovery and Preclinical Validation program. This public-private partnership aims to shorten the time between the discovery of potential drug targets and the development of new drugs for Alzheimer’s disease treatment and prevention.

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Sanders-Brown study: Long-read RNA sequencing reveals key gene expressions in Alzheimer’s disease

LEXINGTON, Ky. (June 6, 2024) — Researchers at the University of Kentucky Sanders-Brown Center on Aging are working to develop a pre-symptomatic disease diagnostic tool for Alzheimer’s disease.

“While the need for better treatments is clear, such treatments will not be very meaningful if they are administered after symptoms have onset. By then, Alzheimer’s disease has been ravaging the brain for decades to the point the brain can no longer compensate for the extreme cellular death,” said Mark T. W. Ebbert, Ph.D., Sanders-Brown faculty and an associate professor in the Department of Internal Medicine in the College of Medicine with a joint appointment in the Department of Neuroscience.

Ebbert is leading the research team behind the study titled “Mapping medically relevant RNA isoform diversity in the aged human frontal cortex with deep long-read RNA-seq” that was recently published in Nature Biotechnology .

He is also the principal investigator on multiple awards totaling nearly $5 million from the National Institutes of Health along with grants from the Alzheimer’s Association, the BrightFocus Foundation and PhRMA Foundation that funded that project. 

Ebbert’s team includes lead authors Bernardo Aguzzoli Heberle, a Ph.D. candidate at Sanders-Brown, and Jason A. Brandon, Ph.D., a scientist at Sanders-Brown and the Department of Internal Medicine.   

The team is using a cutting-edge technique known as long-read sequencing. These technologies allow for a new level of analysis of DNA and RNA, which is the molecule that helps translate genetic code from DNA into proteins.

A single human gene can produce multiple different RNA and protein molecules, called isoforms, allowing it to perform multiple functions. Long-read sequencing helps Sanders-Brown researchers identify and measure these RNA isoforms from a single gene across the entire genome. This also overcomes technical limitations from standard short-read sequencing.

“As a proof of principle, we sequenced aged frontal cortex brain tissue — both healthy brains and brains with Alzheimer’s disease. Our team identified 99 RNA isoforms that were either increased or decreased in Alzheimer’s brains, even when the overall gene activity didn’t change,” said Ebbert. “This shows the importance of understanding isoforms and their unique functions in a gene along with their roles in human health and disease. In fact, we found more than 1,900 genes expressing multiple RNA isoforms related to human disease.”

The team at Sanders-Brown found some of those genes are medically relevant in brain-related diseases, including Alzheimer’s disease, Parkinson’s disease, autism spectrum disorder and substance use disorder.

“This step in understanding the human genome is exciting, but it is simply not enough. We have so much more to do to understand how individual RNA isoforms are involved in diseases, including Alzheimer’s disease, and how to target them therapeutically. There is so much work to do if we are going to defeat Alzheimer’s disease,” said Ebbert.

The team also discovered five new, complex RNA variants from mitochondrial DNA. Researchers believe this is the first study to identify this genetic material in human tissue.

“Although their expression is low, these genes could serve as biomarkers for mitochondrial function, which play an important role in many age-related diseases. It’s crucial to understand the role these new isoforms play in human health and disease,” said Ebbert.

Sanders-Brown scientists hope these findings can lead to new and more precise targets for disease treatment and diagnosis across a broad range of complex human diseases.

“With this method, we’ve shown there’s potential to specifically target isoforms that are either promoting cellular health or dysfunction rather than treating a gene as a single entity,” said Ebbert. “The analysis can also help us reveal unique signatures in Alzheimer’s disease not detectable at the gene level.”

Researchers say larger studies are needed to better understand the RNA patterns in complex diseases and deep long-read RNA sequencing will be a necessary tool for that work.

“We are also incredibly grateful to the patients who’ve donated to the UK Alzheimer’s Disease Center Tissue Bank,” said Ebbert. “Without their participation, this level of scientific study would not be possible.”

This study brought together a team of researchers from the UK College of Medicine’s Department of Pharmacology and Nutritional Sciences; Department of Pathology and Laboratory Medicine; and Department of Microbiology, Immunology, and Molecular Genetics; Emory University School of Medicine in Georgia; University College of London in the United Kingdom; Cold Spring Harbor Laboratory in New York; and the Mayo Clinic in Arizona.

The full study can be found online .

Research reported in this publication was supported by the National Institute of General Medical Sciences of the National Institutes of Health under Award Number R35GM138636 and the National Institute on Aging of the National Institutes of Health under Award Numbers R01AG068331 and P30AG072946. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

Acknowledgment is made to the donors of Alzheimer's Disease Research, a program of BrightFocus Foundation, for support of this research.

This work was supported by a grant from the Alzheimer's Association 2019-AARG-644082.

As the state’s flagship, land-grant institution, the University of Kentucky exists to advance the Commonwealth. We do that by preparing the next generation of leaders — placing students at the heart of everything we do — and transforming the lives of Kentuckians through education, research and creative work, service and health care. We pride ourselves on being a catalyst for breakthroughs and a force for healing, a place where ingenuity unfolds. It's all made possible by our people — visionaries, disruptors and pioneers — who make up 200 academic programs, a $476.5 million research and development enterprise and a world-class medical center, all on one campus.   

In 2022, UK was ranked by Forbes as one of the “Best Employers for New Grads” and named a “Diversity Champion” by INSIGHT into Diversity, a testament to our commitment to advance Kentucky and create a community of belonging for everyone. While our mission looks different in many ways than it did in 1865, the vision of service to our Commonwealth and the world remains the same. We are the University for Kentucky.   

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A new wave of treatment for Alzheimer’s disease

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Alzheimer's disease, the appalling and baffling degenerative brain illness that plagues many elderly people, may be caused by several distinct mechanisms driven by various genetic and lifestyle factors, says Li-Huei Tsai, Picower Professor of Neuroscience at MIT. To fully understand such conditions, she says, we must study the aging brain as a system rather than focusing on one or two types of ailing cells.

Neurodegenerative conditions take years to develop, partly because the brain is a highly plastic organ with many ways to adapt. “If there's one thing wrong, usually our nerve cells can figure out a way to continue to maintain the function of the brain,” Tsai says. “By the time someone shows any symptoms, the brain has already run out of any compensatory mechanism to mask the disruption. As you can imagine, this is a very systemic problem, with many things going wrong.”

Her work has led to a surprising approach to treat Alzheimer's, by increasing the strength of a certain frequency of our brainwaves. This noninvasive method has done well in early clinical trials carried out both by MIT and a startup firm co-founded by Tsai.

Director of The Picower Institute for Learning and Memory, Tsai also spearheads the “miBrain” project to create integrated multicellular models of the human brain, with all the major types of brain cells within a network of blood vessels. The miBrain models seek to offer more realistic representations of brain tissue that will allow improved testing of drug candidates — and eventually support treatments that are personalized to each patient, identified in miBrains built from their own cells. (Patients can donate skin cells that can be re-engineered to become brain cells.) Tsai is welcoming potential commercial partners to join this ambitious effort.

Catching the gamma wave

The brain bundles together nerve cells, supporting cells such as astrocytes and microglia, and blood vessels. “In Alzheimer's disease, all of these cells are disrupted,” Tsai says. “So how do you simultaneously take care of all these different systems and different cells?” Her lab has long explored stimulation methods that can engage multiple regions and cell types across the brain.

Decades ago, researchers discovered that light presented at certain frequencies in mammals can elicit nerve cells in the brain to follow along in synch, creating or strengthening brainwaves.

Tsai and MIT neuroscientist Christopher Moore examined this phenomenon in mice with a cutting-edge lab technology called optogenetics (which was originally co-developed by MIT researchers Ed Boyden and Feng Zhang when they were at Stanford University). The collaborators successfully used optogenetics to increase the power of gamma waves in rodent brains.

Tsai's former graduate student Hunter Iaccarino followed up to see if boosting gamma waves could produce meaningful effects in mice models of Alzheimer's disease. Working with Boyden and MIT Professor Emery Brown, Iaccarino discovered that enhancing 40-cycle-per-second gamma waves via flickering light stimulation could significantly reduce levels of the amyloid protein that is a lead indicator of Alzheimer's. The partners published these striking results in the journal Nature in 2016.

“We subsequently identified that using gamma sound stimulation also can engage nerve cells in the brain and force them to fire at the gamma frequency,” Tsai says.

The waves generated a surprising range of beneficial effects in animal models. Experiments also showed that the effect reaches key parts of the brain, such as the prefrontal cortex, where we do planning and reasoning, and the hippocampus, where we make memories.

“Today, we go cell type by cell type, system by system, to comb through all of the possible mechanisms for this effect,” she says. “If we know how it works, people will be more willing to really embrace it.”

Promise in the clinic

Early clinical trials of gamma-wave treatments have shown dramatic results.

In 2016, Tsai and Boyden were scientific co-founders of Cognito Therapeutics, a startup that has raised $93 million to commercialize gamma-wave technology. In July 2023, Cognito reported positive preliminary results for a phase 2 trial of its proprietary goggle-like device among early-stage Alzheimer's patients. Participants displayed decreased loss of brain volume and a significant slowing in functional and cognitive decline. Cognito is going forward with a phase 3 study designed to enroll 500 patients.

At MIT, Tsai and her collaborators also conducted a small-scale clinical trial on early-stage Alzheimer's subjects. Rather than giving the participants goggles, the researchers installed an LED light panel and stereo in their homes. “We reduced brain volume loss and increased connectivity,” Tsai says. This study was shut down by the pandemic, but she and her collaborators are now resuming clinical tests.

Despite employing very different devices, the Cognito and MIT trials produced similar benefits. Gamma-wave devices should be far more accessible, and safer, than the drugs available to date, Tsai suggests. Unlike the Alzheimer's drugs recently approved by the Food and Drug Administration (FDA), the therapy doesn't require highly expensive infusions or pose the risks of brain swelling and bleeding.

Modeling your whole brain with miBrain

The gamma-wave research is one thread among many in Tsai's lab aimed at understanding the entire aging brain, with its genetic complexities, and to use that understanding to personalize treatments for neurodegenerative illnesses.

“You can think of Alzheimer’s disease as like breast cancer: Depending on what genes are disrupted, people will get different therapies,” Tsai says. “I think this is probably true for other degenerative diseases, like Parkinson's.”

Her team plans to take a huge step up in modeling human brain structures with the miBrain platform, basically multicellular brain chips built with stem cell technology. (Scientists can take human skin cells and induce them to become “pluripotent” stem cells, which means they can be reprogrammed to become various brain and blood vessel cells. Those cells can then be cultured together to form a complex approximation of brain tissue.)

“This miBrain system contains all the different cell types that normally you'll see in the brain,” says Tsai. “This is essential, because the cells in the brain don't exist in isolation. They're all together and they communicate with each other through secreted factors or cell-to-cell contact, and that's a very important part of how they maintain health and functionality.”

The integrated miBrain system will empower both basic research and drug screening. “For example, the blood-brain barrier prevents many molecules from entering the brain.” she says. “We can use this in-vitro-assembled blood-brain barrier to test whether a chemical targeting a brain disease can even get into the brain.”

She points out that the FDA recently decided that it would not always require animal testing data before approving drug candidates for trial. This regulatory move should accelerate the use of in-vitro models such as the miBrain for drug testing.

Over time, Tsai hopes the miBrain will evolve into a translational platform to deliver precision medicine, enabling individualized treatments for brain illnesses. “We can reprogram your skin cells into stem cells, and then we can make a miBrain out of your cells,” she says. “If you have Parkinson's disease, we can test certain therapeutic agents and see how your miBrain responds, and then further optimize how to treat you.”

She and her MIT collaborators are now launching a miBrain center, aimed to boost both basic and translational medical research.

Scaling up this center, however, will be no easy task. “The biggest challenge is manpower, because producing all the cell types and then assembling them into a miBrain is extremely labor-intensive,” she says. “It would be very helpful if we can team up with a company and develop this together.”

MIT offers tremendous opportunities for such collaborations. “I think MIT is in the best position to lead brain disease research, because we have people who are leaders in their disciplines here, doing not just brain research but engineering and artificial intelligence,” Tsai remarks. “We really hope that we can gather people together — scientists, engineers, policymakers and economists — to figure this out. This is the future of brain research; we need people using very different approaches to work together.”

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Researchers examine evidence for a novel neuronal mechanism driving Alzheimer's disease

by Neuro-Bio Ltd

An international team of clinicians and neuroscientists have published a new perspective on the process of neurodegeneration. Their findings review evidence for a mechanism upstream of amyloid including the key neurochemical driving this process.

The paper, published in Alzheimer's & Dementia , focuses on a selective group of neurons ("the isodendritic core"). These cells have a different provenance from neurons in the rest of the brain and have previously been identified as primarily vulnerable in Alzheimer's disease (AD).

The authors acknowledge that amyloid is a significant factor in AD once it has progressed, but note that it is not present in these neurons in the early stages. If damage occurs to these vulnerable neurons in adulthood, these neurons will respond by mobilizing a retaliatory mechanism. This mechanism normally promotes growth of neurons in embryos and early life but in adulthood is detrimental.

The review describes how the pivotal molecule that drives this process, is a bioactive 14-mer peptide, T14, that selectively activates a single target receptor. In the mature brain instead of restoring normal function, T14 leads to the death of neurons and initiates an adverse snowball effect that gains momentum over time.

The isodendritic core neurons, found deep in the brain, are responsible for arousal and sleep/wake cycles and are not directly related to higher functions such as memory. Hence, the process of degeneration can continue without obvious symptoms until the damage spreads to areas that underlie cognition.

The explanation offered in this article may account for the long-time lag of 10 to 20 years from onset of neuronal loss to appearance of cognitive impairment.

The review reports how T14 can be detected at a very early stage of AD and therefore could be a pre-symptomatic indication that neurodegeneration had started and, as such, could be developed as a biomarker.

In addition, the authors describe how a cyclated version, NBP14, can act as a blocker of T14. NBP14 has been shown to prevent memory dysfunction in a mouse model of AD and its mechanism of action has been demonstrated in a variety of studies including in post mortem human brain tissue. As such, NBP14 could eventually inspire a new therapeutic strategy.

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New breakthroughs on Alzheimer’s

MIT scientists have pinpointed the first brain cells to show signs of neurodegeneration in the disorder and identified a peptide that holds potential as a treatment.

• Anne Trafton archive page

Neuronal hyperactivity and the gradual loss of neuron function are key features of Alzheimer’s disease. Now researchers led by Li-Huei Tsai, director of MIT’s Picower Institute for Learning and Memory, have identified the cells most susceptible to this damage, suggesting a good target for treatment. Even more exciting, Tsai and her colleagues have found a way to reverse neurodegeneration and other symptoms by interfering with an enzyme that is typically overactive in the brains of Alzheimer’s patients. 

In one study , the researchers used single-­cell RNA sequencing to distinguish two populations of neurons in the mammillary bodies, a pair of structures in the hypothalamus that play a role in memory and are affected early in the disease. Previous work by Tsai’s lab found that they had the highest density of amyloid beta plaques, abnormal clumps of protein that are thought to cause many Alzheimer’s symptoms. 

The researchers found that neurons in the lateral mammillary body showed much more hyperactivity and degeneration than those in the larger medial mamillary body. They also found that this damage led to memory impairments in mice and that they could reverse those impairments with a drug used to treat epilepsy.

In the other study , the researchers treated mice with a peptide that blocks a hyperactive version of an enzyme called CDK5, which plays an important role in development of the central nervous system. They found dramatic reductions in neurodegeneration and DNA damage in the brain, and the mice got better at tasks such as learning to navigate a water maze.

CDK5 is activated by a smaller protein known as P35, allowing it to add a phosphate molecule to its targets. However, in Alzheimer’s and other neurodegenerative diseases, P35 breaks down into a smaller protein called P25, which allows CDK5 to phosphorylate other molecules—including the Tau protein, leading to the Tau tangles that are another characteristic of Alzheimer’s.

Pharmaceutical companies have tried to target P25 with small-molecule drugs, but these drugs also interfere with other essential enzymes. The MIT team instead used a peptide—a string of amino acids, in this case a sequence matching that of a CDK5 segment that is critical to binding P25.

In tests on neurons in a lab dish, the researchers found that treatment with the peptide moderately reduced CDK5 activity. But in a mouse model that has hyperactive CDK5, they saw myriad beneficial effects, including reductions in DNA damage, neural inflammation, and neuron loss. 

The treatment also produced dramatic improvements in a different mouse model of Alzheimer’s, which has a mutant form of the Tau protein. Tsai hypothesizes that the peptide might confer resilience to cognitive impairment in the brains of people with Tau tangles.

“We found that the effect of this peptide is just remarkable,” she says. “We saw wonderful effects in terms of reducing neurodegeneration and neuroinflammatory responses, and even rescuing behavior deficits.”

The researchers hope the peptide could eventually be used as a treatment not only for Alzheimer’s but for frontotemporal dementia, HIV-induced dementia, diabetes-­linked cognitive impairment, and other conditions. 

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Alzheimer's disease: From immunotherapy to immunoprevention

Affiliations.

• 1 Department of Cellular Neurology, Hertie Institute for Clinical Brain Research, University of Tübingen, Tübingen, Germany; German Center for Neurodegenerative Diseases (DZNE), 72076 Tübingen, Germany. Electronic address: [email protected].
• 2 Department of Neurology and Emory National Primate Research Center, Emory University, Atlanta, GA 30322, USA. Electronic address: [email protected].
• PMID: 37729908
• PMCID: PMC10578497 (available on 2024-09-28 )
• DOI: 10.1016/j.cell.2023.08.021

Recent Aβ-immunotherapy trials have yielded the first clear evidence that removing aggregated Aβ from the brains of symptomatic patients can slow the progression of Alzheimer's disease. The clinical benefit achieved in these trials has been modest, however, highlighting the need for both a deeper understanding of disease mechanisms and the importance of intervening early in the pathogenic cascade. An immunoprevention strategy for Alzheimer's disease is required that will integrate the findings from clinical trials with mechanistic insights from preclinical disease models to select promising antibodies, optimize the timing of intervention, identify early biomarkers, and mitigate potential side effects.

Keywords: ARIA; Tau; aducanumab; aduhelm; cerebral amyloid antipathy; dementia; donanemab; lecanemab; neurofilament; β-amyloid.

Copyright © 2023 Elsevier Inc. All rights reserved.

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Study Identifies Genetic Variant as a Clear Cause of Alzheimer's—Does This Mean You Should Get Tested?

• New research suggests that having two copies of the gene variant APOE4 could be a cause of developing Alzheimer’s.
• The study also found that people with two copies of APOE4 were more likely to develop Alzheimer’s earlier in life.
• Experts don't typically advise genetic testing for Alzheimer's for most people, and a study author said the new research doesn't change current recommendations.

Recent research suggests that some Alzheimer’s cases can be traced back to a direct genetic cause, raising questions about the future of diagnosis and treatment.

Currently, most cases of Alzheimer’s don't have an identifiable underlying cause. Still, scientists have long known that inheriting a copy of a gene variant called APOE4 can elevate the chances of a diagnosis. People with two copies, who make up 2 to 5% of the general population, are at even higher risk.  

Now, researchers are proposing that having two copies of APOE4 doesn’t just raise the odds of developing the disease but can, in fact, cause it. 

“This expands our understanding to encompass 15 to 20% of cases where we can identify a genetic cause,” Juan Fortea, PhD , who led the study at the Sant Pau Research Institute in Barcelona, Spain, told Health . “This is crucial as it opens new avenues for specific research and interventions, enhancing our grasp on the disease’s underlying mechanisms.”

The new findings, published in the journal Nature Medicine , also mean more people could receive an Alzheimer’s diagnosis before they begin to show any symptoms. Medical experts say this research will hopefully spur the development of treatments and kickstart targeted clinical trials focused on this population.

“Our findings represent more than scientific progress,” Fortea said. “They are a step towards translating hope into tangible strategies for those affected by Alzheimer's.”

Here’s how the scientists made their discovery, what it could mean for Alzheimer’s treatment, and whether experts advise testing to see if you have the genetic variant.

SDI Productions / Getty Images

A New Genetic Disorder

Researchers analyzed data from 3,297 brains donated for medical research and 10,000 people participating in U.S. and European Alzheimer’s studies.

They found that 273 people had two copies of APOE4. Of those, nearly all showed signs of Alzheimer’s in their brains. The researchers concluded that having two copies of APOE4 should now be considered a genetic form of Alzheimer’s.

The team also found that patients developed Alzheimer’s pathology relatively young .

By age 55, participants with two APOE4 copies were accumulating more amyloid, a protein that forms plaques in the brain that signal Alzheimer’s, than those with just one copy of another form of APOE—the APOE3 allele. By 65, almost all had abnormal levels of amyloid. And many started developing symptoms of Alzheimer’s at age 65, younger than most people without the APOE4 variant.

“The study beautifully demonstrates that having two copies of the APOE4 gene predictably leads to pathological changes in the brain in almost every carrier,” Jim Ray, PhD , director of the Belfer Neurodegeneration Consortium at MD Anderson Cancer Center who was not involved in the study, told Health . 

Fortea acknowledged the study had notable limitations, including that almost all participants were White. Because the risk associated with APOE varies across different ethnic backgrounds, the findings might not be universally applicable.

Implications for Treatment

Ray and other experts said that the research could be a catalyst for the development of new drugs to treat people with two copies of the gene variant—both before and after they experience symptoms.

“This study argues strongly that we need therapeutics targeting the APOE4 gene for the millions of people who are at risk for AD,” he said.

No cure exists for Alzheimer’s, but some medications can temporarily improve symptoms. 

One of the more common prescriptions is Leqembi, which works to break apart some of the amyloid in the brain. Experts have been hesitant to widely prescribe the drug, however, because it carries an FDA-required black-box warning on the label that “serious and life-threatening events” like bleeding and swelling in the brain can occur, specifically in people with two copies of APOE4.  

“We are at the threshold of a new era with the advent of disease-modifying treatments, which holds significant promise for those with genetic markers linked to Alzheimer’s,” Fortea said.

Should You Get Genetic Testing?

The new study raises the question whether asymptomatic people should get tested to determine whether they have two copies of APOE4.

People whose parents were both diagnosed with Alzheimer’s relatively early—most likely in their 60s—are most likely to carry two APOE4 genes.

Currently, genetic tests aren’t routinely used in clinical settings to diagnose or predict the risk of developing Alzheimer’s. Many experts don’t advise it because of the complexities involved in interpreting the results.  

Fortea said she doesn’t think the new study should change anything regarding testing recommendations.

“At this stage, our findings do not advocate for changes in testing practices for Alzheimer’s,” Fortea said. “More research is needed, particularly in developing preventive treatments and accurately assessing risk, before we can offer concrete recommendations for genetic testing or counseling in the context of these findings.”

If you are curious about whether you have one or two copies of the APOE4 gene or concerned that you might, consult a doctor or genetic counselor about whether testing might be right for you.

Fortea J, Pegueroles J, Alcolea D, et al. APOE4 homozygozity represents a distinct genetic form of Alzheimer’s disease . Nat Med . 2024;30(5):1284-1291. doi:10.1038/s41591-024-02931-w

National Institute on Aging. Alzheimer's disease genetics fact sheet .

National Library of Medicine: DailyMed. Leqembi—lecanemab injection, solution [drug label].

Related Articles

• Open access
• Published: 02 October 2023

Clinical trials of new drugs for Alzheimer disease: a 2020–2023 update

• Li-Kai Huang 1 , 2 , 3   na1 ,
• Yi-Chun Kuan 2 , 3 , 4 , 5   na1 ,
• Ho-Wei Lin 6 &
• Chaur-Jong Hu   ORCID: orcid.org/0000-0002-4900-5967 1 , 2 , 3 , 4  

Journal of Biomedical Science volume  30 , Article number:  83 ( 2023 ) Cite this article

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Alzheimer's disease (AD) is the leading cause of dementia, presenting a significant unmet medical need worldwide. The pathogenesis of AD involves various pathophysiological events, including the accumulation of amyloid and tau, neuro-inflammation, and neuronal injury. Clinical trials focusing on new drugs for AD were documented in 2020, but subsequent developments have emerged since then. Notably, the US-FDA has approved Aducanumab and Lecanemab, both antibodies targeting amyloid, marking the end of a nearly two-decade period without new AD drugs. In this comprehensive report, we review all trials listed in clinicaltrials.gov, elucidating their underlying mechanisms and study designs. Ongoing clinical trials are investigating numerous promising new drugs for AD. The main trends in these trials involve pathophysiology-based, disease-modifying therapies and the recruitment of participants in earlier stages of the disease. These trends underscore the significance of conducting fundamental research on pathophysiology, prevention, and intervention prior to the occurrence of brain damage caused by AD.

Alzheimer disease (AD) represents a major global medical, social, and economic burden. The World Alzheimer Report 2022 revealed that more than 55 million people have AD or related conditions worldwide, and this number is projected to reach 82 million by 2030 and 138 million by 2050 [ 1 ]. Typically, AD first manifests as progressive memory decline accompanied or followed by other cognitive dysfunctions, such as visuospatial abnormalities, navigation difficulties, executive problems, and language disturbances. These cognitive impairments affect the performance of activities of daily living. During the course of AD, many behavioral and psychological symptoms of dementia (BPSD) occur [ 2 , 3 , 4 ].

Although the exact causes of AD remain unclear, the disease has two pathological hallmarks: plaques composed of amyloid-beta (Aβ) fibrils and neurofibrillary tangles (NFTs) consisting of hyperphosphorylated tau protein [ 5 , 6 , 7 ]. The key event in AD pathogenesis is believed to be Aβ accumulation. Cerebral Aβ fibril deposition may occur decades before the onset of clinical symptoms [ 8 ]. Brain atrophy, particularly in the hippocampus, is major indicator of early Aβ accumulation, particularly in the presubiculum [ 9 , 10 ]. Aβ accumulation was discovered to be crucial by three independent research groups in 1991 [ 11 , 12 , 13 ]. In familial AD, mutant autosomal-dominant genes, including the genes for amyloid precursor protein ( APP ), presenilin-1 ( PSEN1 ), and presenilin-2 ( PSEN2 ), encode the major proteins involved in amyloid metabolism [ 13 , 14 , 15 ]. Individuals with trisomy 21 (Down syndrome) have an extra copy of the APP gene, which may result in increased amyloid production and AD risk in middle age [ 16 ]. At present, the predominant theory regarding the cause of AD is the amyloid hypothesis; crucial advancements in AD therapy have been made on the basis of the proposed role of amyloid accumulation in the AD development. The United States Food and Drug Administration (US FDA) granted traditional approval for Leqembi (lecanemab-irmb) on July 6, 2023, for the treatment of AD [ 17 ]. The approval of this treatment not only affirms the pathophysiological significance of amyloid in AD but also marks a notable advance in clinical approaches to AD treatment, remedying the scarcity of new drugs in the market for nearly two decades.

Tau is a microtubule-associated protein that aids in microtubule assembly and stabilization. In AD, tau becomes hyperphosphorylated and aggregates to form paired helical filaments, a major component of NFTs within the neuronal cytoplasm. As the disease progresses, the gradual spread of tau pathology throughout brain regions has been suggested to be caused by the transfer of abnormal types of tau protein from one neuron to another [ 18 ]. The accumulation of NFTs might be initiated between the accumulation of Aβ and the development of clinical symptoms of AD [ 19 ]. NFTs and quantitative neuronal loss may be more closely correlated with disease severity and dementia progression than the amyloid plaque burden [ 20 , 21 , 22 ]. Positron emission tomography (PET) investigations have revealed a strong correlation between the binding characteristics of tau tracers and the severity of clinical manifestations in individuals with AD [ 23 ]. Molecular imaging modalities (PET) and cerebrospinal fluid (CSF) and blood–based biomarkers have extended the diagnostic scope of AD pathology to both clinical and even preclinical settings. The analysis of a combination of biomarkers such as amyloid, tau, and neurodegeneration (collectively, ATN classification) has been proposed by research on AD [ 24 , 25 ]. Furthermore, the exceptional diagnostic accuracy of plasma-based biomarkers has facilitated the clinical transition of fluid biomarkers from research settings to clinical practice. A recent presentation at the Alzheimer’s Association International Conference in 2023 highlighted the clinical and research applications of two fundamental AD biomarker categories, labeled as A and T. The A category pertains to biomarkers associated with the Aβ proteinopathy pathway, and the T category pertains to biomarkers linked to tau proteinopathy [ 26 ].

Aβ serves as a proinflammatory agent and triggers the nuclear factor κB (NF-κB) pathway in astrocytes, increasing complement C3 release. Subsequently, by binding to C3a receptors, C3 causes neuronal dysfunction and microglial activation [ 27 ]. In the early stage of AD, activated microglia may play a protective, anti-neuroinflammatory role by clearing amyloids and releasing nerve growth factors. However, activated microglia induce neurotoxic A1 astrocyte reactivity through the release of IL-1α, C1q, and TNF-α, resulting in a feedback loop of dysregulated inflammation in AD [ 28 ]. The excessive accumulation of Aβ or other toxic compounds activates proinflammatory phenotypes, resulting in neuronal damage [ 29 ]. Sustained inflammation has been observed in the brains of patients with AD [ 30 , 31 ]. The inadequate clearance of Aβ along with the aggregation of tau disrupts microglial defense mechanisms, resulting in sustained and harmful microglial activation [ 32 ]. The sequential occurrence of amyloid plaque formation, microglial activation, and the pathological phosphorylation and aggregation of tau proteins to form NFTs is the fundamental notion of the amyloid cascade–inflammation hypothesis. In the Multi-Ethnic Study of Atherosclerosis (multiple covariates were controlled for), vascular risk factor profiles and Aβ deposition significantly predicted cognitive decline [ 33 ]. Vascular risk factors can also lead to inflammation in the brain, which damages neuronal cells and further increases the likelihood of AD dementia [ 34 ].

The role of autophagy impairment is proposed in a novel hypothesis concerning plaque formation in AD. Among neurons that are compromised but still maintain some integrity, autophagic vacuoles (AVs) containing abundant Aβ are notably present. These AVs cluster within expansive membrane blebs, exhibiting a distinctive flower-like arrangement termed PANTHOS. These formations constitute the primary source of the majority of amyloid plaques found in mouse models of AD [ 35 ]. Neuroprotective therapies, including free radical scavengers, regeneration enhancers, and the suppression of excitable amino acid signaling pathways, have been proposed for preventing neuronal death or brain atrophy caused by amyloid, tau, and neuroinflammation [ 36 ]. Pathological evidence indicates that AD is also associated with degeneration in cholinergic neuron-rich regions, such as the nucleus basalis of Meynert, frontal cortex, and anterior and posterior cingulate cortex, which can lead to the symptoms of memory impairment and agitation. Acetylcholine (ACh) plays a vital role in memory function, including memory encoding, consolidation, and retrieval processes, and increasing Ach levels by using cholinesterase inhibitors (AChEIs) has become a standard therapy for the symptoms of AD [ 37 ].

Clinical trials of early or preventive interventions based on amyloid/tau theory and those targeting other pathophysiologies are ongoing or have been initiated. Many ongoing clinical trials on AD are focused on disease-modifying therapies (DMTs) that target the causes and can change the course of AD. The other trials involve symptomatic treatments—for example, enhancing cognitive function and relieving BPSD (Fig.  1 ). In this review, we summarize the new drugs being examined in ongoing trials (listed on ClinicalTrials.gov) and discuss the trends in and obstacles in AD clinical trials.

According to the amyloid hypothesis, the pathophysiology and clinical course of Alzheimer's disease progress as follows: amyloid accumulation, neuroinflammation, tau accumulation, brain metabolism dysfunction, brain atrophy, cognitive decline (from mild cognitive impairment to dementia), and the development of dementia symptoms. Novel drugs should target at least one of these events. AD Alzheimer's disease, aMCI amnestic mild cognitive impairment, BPSD behavioral psychological symptoms of dementia

Anti-amyloid therapy

Table 1 summarizes the US FDA approval status of anti-amyloid agents. Tables 2 and 3 summarize the ongoing phase 3 and phase 2 trials of anti-amyloid therapy respectively.

Aducanumab (brand name: Aduhelm) is a high-affinity, fully human immunoglobulin gamma 1 (IgG1) monoclonal antibody that binds to the N-terminus of Aβ fibrils and blocks amyloid aggregation [ 38 ]. In August 2015, two phase 3 clinical trials, namely ENGAGE and EMERGE studies, were initiated. These trials compared monthly intravenous infusions of aducanumab at one of three doses with infusions of placebo over 18 months, and the primary outcomes were cognitive and functional decline, which were assessed using the Clinical Dementia Rating (CDR) scale Sum of Boxes (CDR-SB). The secondary outcomes were other cognitive and functional measures. The trials were conducted in 150 centers across North America, Europe, Australia, and Asia. However, the findings of the EMERGE trial reached statistical significance, whereas the primary endpoint was not reached in the ENGAGE trial. An exploratory analysis revealed that a subgroup of the participants in the ENGAGE trial who received a high dose of aducanumab exhibited slow decline, which was similar to that observed among the participants in the EMERGE trial. The US FDA approved aducanumab in June 2021 on the basis of the data of the EMERGE and ENGAGE trials. Both trials presented evidence of an intermediate effect of the drug on biomarkers, indicating amyloid removal, which is likely linked to the clinical benefit of aducanumab. Further trials must be conducted to confirm the potential benefit of aducanumab [ 39 ]. The phase 3b/4 ENVISION trial (NCT05310071), which began in 2022, will enroll 1,512 patients with early AD who will receive either monthly doses of aducanumab of up to 10 mg/kg or placebo for 18 months. The aim of the trial is to determine the efficacy of aducanumab in delaying cognitive and functional decline in comparison with placebo, which would be determined on the basis of CDR-SB scores. The secondary endpoints of the trial include scores on the Alzheimer’s Disease Assessment Scale–Cognitive Subscale (ADAS-Cog) 13, Alzheimer’s Disease Cooperative Study–Activities of Daily Living Inventory (ADCS-ADL)–Mild Cognitive Impairment Version, Integrated Alzheimer’s Disease Rating Scale (iADRS), Mini-Mental State Examination, and Neuropsychiatric Inventory. The trial intends to recruit 18% of its participants from Black and Latinx populations in the United States and will have a long-term follow-up of up to 4 years, with results expected by 2026. The EMBARK trial (NCT04241068) is a phase 3b open-label study including 1,696 participants from previous aducanumab trials (from trials 221AD103, 221AD301, 221AD302, and 221AD205) that will assess aducanumab safety and tolerability over 100 weeks after a wash-out period. Participants will receive an intravenous infusion of aducanumab at 10 mg/kg monthly for 2 years, and eligible participants will continue to receive the infusion for another 52 weeks during the long-term extended treatment period. The primary outcomes are safety and tolerability, and the efficacy endpoints are the same as those in the EMERGE and ENGAGE trials, and Caregiver Global Impression of Change evaluations will be conducted every 6 months. All participants will undergo volumetric magnetic resonance imaging (MRI) scans, and a subset of the study population will undergo biomarker testing, including amyloid PET, tau PET, and CSF testing.

Lecanemab (brand name: Leqembi), a humanized IgG1 antibody derived from mAb158, selectively binds to soluble Aβ protofibrils [ 40 ]. The US FDA approved it on January 6, 2023, through an accelerated approval pathway on the basis of evidence of amyloid removal in a phase 2 trial (NCT01767311) and because it had a likelihood of having clinical benefits [ 41 ] A double-blind, placebo-controlled phase 2 trial recruited 856 patients with AD with mild cognitive impairment (MCI) or mild dementia and verified amyloid pathology through amyloid PET or CSF Aβ1-42 [ 42 ]. The results revealed a significant and dose-dependent reduction of amyloid plaques in the lecanemab group (10 mg/kg, intravenous infusion every 2 weeks) from baseline to week 79 compared with the placebo group. At the time of writing this paper, three phase 3 clinical trials on lecanemab are underway. The first trial, Clarity AD (NCT03887455), was initiated in March 2019 and was conducted at 250 sites around the world. It reported favorable outcomes for all primary and secondary measures, including ADAS-Cog14, AD Composite Score (ADCOMS), and ADCS-MCI-ADL scores [ 43 ]. The second trial is AHEAD 3–45 (NCT04468659), which was initiated in July 2020 as a 4-year trial comprising two substudies, one of which is A3, and the other one is A45. A3 is enrolling 400 people whose amyloid levels are below the brain-wide threshold for positivity; participants will receive 5 mg/kg lecanemab titrated to 10 mg/kg or placebo every month for 216 weeks. A45 is enrolling 1,000 cognitively healthy participants with positive amyloid PET scans, and they will receive lecanemab titrated to 10 mg/kg every 2 weeks for 96 weeks, followed by 10 mg/kg every month through week 216. The trial is expected to run until October 2027. The third phase 3 clinical trial is the Dominantly Inherited Alzheimer Network Trials Unit (DIAN-TU) Next Generation trial (DIAN-TU-001 (E2814), NCT05269394), in which a combination of lecanemab and the anti-tau antibody E2814 (phase 2) will be administered to 168 people with familial AD mutations. On July 6, 2023, Leqembi (lecanemab-irmb) received traditional approval from the US FDA for the treatment of AD based on Phase 3 data from the Clarity AD clinical trial [ 17 ].

The appropriate use recommendations (AURs) for lecanemab and aducanumab highlight the importance of patient selection, surveillance for adverse events, and clinician preparedness [ 44 , 45 ]. The AURs for both drugs have several similarities with respect to age criteria, biomarker requirements (positive amyloid PET or CSF findings indicative of AD), diagnosis (MCI due to AD or mild AD dementia), and MRI exclusion criteria (e.g., microhemorrhages and cortical infarction). The AURs also emphasize the importance of monitoring for amyloid-related imaging abnormalities (ARIAs), which can occur in patients receiving these drugs. APOE genotyping is recommended for informing risk discussions with candidate participants because APOE4 allele carriers, especially APOE4 homozygotes, are at a high risk of ARIAs. Patients receiving treatment must have care partners or family members who can provide necessary support and who clearly understand the nature and requirements of the therapy. Discontinuation of treatment is recommended in the following situations: when a patient is taking drugs with associated risks, such as anticoagulation agents for conditions like atrial fibrillation, deep vein thrombosis, or pulmonary embolism; or when any of the following conditions occur: a hypercoagulable state, or the development of any of the following: cerebral macrohemorrhage, multiple areas of superficial siderosis, more than 10 instances of microhemorrhages since treatment initiation, severe symptoms of ARIAs, or two or more episodes of ARIAs.

Donanemab is a humanized monoclonal antibody developed from mouse mE8-IgG2a. It recognizes Aβ (3–42), an aggregated form of Aβ found in amyloid plaques [ 46 ]. It was discovered to be bound to approximately one-third of amyloid plaques in postmortem brain samples of patients with AD or Down syndrome, and it strongly reacted with the plaque core [ 47 ]. In the phase 2 TRAILBLAZER-ALZ study, the safety, tolerability, and efficacy of donanemab alone and in combination with the Beta-Secretase 1 (BACE1) inhibitor LY3202626 (developed by Eli Lilly and Company) were evaluated over 18 months. The trial met its primary endpoint of delaying decline—which was determined on the basis of iADRS scores—by 32% compared with placebo. Amyloid burden reduction was correlated with improvement in iADRS scores only in ApoE4 carriers [ 48 ]. Donanemab reduced the tau burden in the temporal, parietal, and frontal lobes and significantly decreased plasma pTau217 by 24% in the treatment group, whereas the placebo group exhibited a 6% increase in plasma pTau217 at the end of the trial [ 49 ]. At the time of writing this paper, five phase 3 trials of donanemab are underway: TRAILBLAZER-ALZ 2, TRAILBLAZER-ALZ 3, TRAILBLAZER-ALZ 4, TRAILBLAZER-ALZ 5 and TRAILBLAZER-ALZ 6. The TRAILBLAZER-ALZ 2 (NCT04437511) trial was initially started in June 2020 as a phase 2 safety and efficacy trial, and 500 patients with early AD were recruited. Inclusion criteria of TRAILBLAZER-ALZ 2 are similar to those of TRAILBLAZER-ALZ: a ≥ 6-month history of worsening memory and positive amyloid (flortaucipir) PET. The trial was subsequently extended to a phase 3 trial with 1,800 participants. The primary outcome is iADRS, and the effectiveness of treatment is being measured using a disease-progression model rather than solely on the basis of changes at the final time point. Trial results for 1,736 participants were published to report donanemab’s impact on early symptomatic AD. Using PET imaging to categorize individuals into groups with low/medium or high tau pathology load, the study spanned 18 months and assessed cognitive and functional scales. Donanemab achieved significant cognitive improvement in the low/medium tau group (iADRS change: − 6.02 vs. − 9.27 placebo) and combined population (change: − 10.2 vs. − 13.1 placebo). The drug notably reduced decline by 60% in patients with early-stage AD, supporting the efficacy of short-term dosing. Twenty-four outcomes were evaluated, with significant findings for 23 outcomes. Adverse effects included amyloid-related imaging problems (24% donanemab vs. 2.1% placebo) and infusion-related reactions (8.7% donanemab vs. 0.5% placebo). The study findings indicated the potential of donanemab to slow AD progression, particularly in the early stage [ 50 ]. In the TRAILBLAZER-ALZ study, donanemab slowed disease progression by 32% at 18 months ( p  = 0.04 vs. placebo), thus demonstrating clinical efficacy [ 51 ]. TRAILBLAZER-ALZ 3 (NCT05026866) is a placebo-controlled phase 3 prevention trial that was started in August 2021. The trial plans to enroll 3,300 cognitively healthy people aged 50–55 years who are at high risk of AD, as determined by elevated plasma pTau217 levels and Telephone Interview for Cognitive Status-modified scores. The primary outcome is the time to clinical progression, which is measured using global CDR scores. Participants are to be monitored every 6 months until cognitive impairment is noted (i.e., a score above 0 on the CDR for two consecutive evaluations) in 434 participants. The trial has a decentralized design and is being conducted at more than 200 sites in the United States, Japan, and Puerto Rico until November 2027. TRAILBLAZER-ALZ 4 (NCT05108922) is a phase 3, open-label, head-to-head comparison of amyloid clearance by either donanemab or aducanumab that began in November 2021 after the US FDA approval of aducanumab. The trial enrolled 200 people with early symptomatic AD, as indicated by a global CDR score of 0.5 or 1, at 31 sites in the United States. The primary outcome is the percentage of participants who achieve complete amyloid plaque clearance after 6 months for each treatment group, with clearance determined using amyloid (florbetapir) PET. The trial has 17 secondary outcomes, which are all related to amyloid PET measurements at up to 18 months. The preliminary results were presented at the 2022 Clinical Trial of AD (CTAD) conference: 38% of the patients on donanemab exhibited amyloid levels below the amyloid positivity threshold after 6 months, whereas only 2% of the patients on aducanumab has such findings. Plasma pTau217 levels decreased by 25% for the participants receiving donanemab, but not at all for those receiving aducanumab. The side effect of ARIA-edema occurred in 22% of the participants in both groups. TRAILBLAZER-ALZ 5 (NCT05508789) is being conducted to assess the safety and efficacy of donanemab in individuals with early symptomatic AD. The trial started in October 2022; 1,500 participants will be recruited by using the same criteria as those of TRAILBLAZER-ALZ 2 from 148 sites across China, Korea, Taiwan, and Europe; and the trial is expected to run until mid-2025. Participants will be administered monthly infusions of either donanemab or placebo, and the primary outcome will be measured on the basis of iADRS score changes after 18 months. TRAILBLAZER-ALZ 6 (NCT05738486) is a phase 3b study that will assess the impact of various dosing regimens of donanemab on the occurrence and severity of ARIA-E (ARIA with edema or effusion) in 800 adults with early symptomatic AD. The study also seeks to identify participant characteristics that predict the risk of ARIA-E. The trial is divided into four arms, each with a distinct donanemab dose.

Remternetug is a monoclonal antibody that recognizes a pyroglutamated form of Aβ that aggregates into amyloid plaques. In August 2022, Eli Lilly and Company initiated a phase 3 trial called TRAILRUNNER-ALZ1 (NCT05463731) that will randomize 600 patients with early symptomatic AD across 75 sites in the United States and 2 sites in Japan into groups receiving the antibody or placebo through intravenous infusion or subcutaneous injection for 1 year. The primary outcome is the percentage of patients whose amyloid plaques are cleared by the end of the treatment period. The secondary outcomes include the measurement of amyloid clearance, pharmacokinetics, and treatment-emergent anti-drug antibodies. The study also plans to conduct a year-long, blinded crossover extension. An additional safety cohort of 640 patients will receive open-label remternetug for 1 year.

Solanezumab is a humanized monoclonal antibody that targets the mid-domain of the Aβ peptide for increasing Aβ clearance [ 52 ]. Phase 3 trials of solanezumab, including EXPEDITION-1 and EXPEDITION-2, which enrolled 2,052 patients with mild-to-moderate AD, did not reveal improvements in ADAS-Cog11 and ADCS-ADL scores, which were the primary outcome measures. Similarly, the phase 3 trial EXPEDITION-3 demonstrated that 400 mg solanezumab administered every 4 weeks did not have significant effects on cognitive decline in patients with mild AD [ 52 ]. A4 (NCT02008357) is a phase 3 prevention trial focused on slowing memory and cognitive decline in elderly individuals without cognitive impairment or dementia. A4 is using a sensitive cognitive battery—the Alzheimer Disease Cooperative Study Preclinical Alzheimer Cognitive Composite—and was initiated in February 28, 2014. On March 8, 2023, Eli Lilly and Company reported that solanezumab did not slow cognitive decline or clear amyloid plaques in individuals with preclinical AD in the A4 study. DIAN-TU-001 (NCT01760005) is another ongoing phase 3 clinical trial that is testing the combination of solanezumab and gantenerumab in 210 asymptomatic and mildly symptomatic carriers of autosomal-dominant mutations in AD genes. However, on February 10, 2020, the study investigators announced that the primary endpoint was not achieved in the trial, namely treatment-related changes on the DIAN-Multivariate Cognitive Endpoint. The results indicated that the solanezumab-treated group had greater cognitive decline on some measures relative to placebo, and that solanezumab treatment did not exert any beneficial effects on downstream biomarkers, whereas gantenerumab significantly reduced amyloid plaques, CSF total tau, and phospho-tau181 and attenuated increases in neurofilament light chain [ 53 ]. The participants were offered an open-label extension with high-dose gantenerumab because of its positive effects on imaging and other biomarkers, such as normalized CSF Aβ42, and because it reduced CSF total tau and pTau181 levels.

ALZ-801 is a prodrug of tramiprosate, a small molecule of anti-Aβ oligomers and an aggregation inhibitor [ 54 ]. The phase 3 trial APOLLOE4 (NCT04770220) is evaluating the safety and efficacy of ALZ-801 for patients with early AD and carrying the homozygous ε4 allele on the apolipoprotein E gene ( APOE4/4 ). The recruited patients are receiving 265 mg ALZ-801 or placebo twice daily for 18 months. The trial started in May 2021. The primary endpoint is ADAS-Cog scores, and the secondary endpoints are scores of the Disability Assessment for Dementia, CDR-SB, and Amsterdam-iADL. The biomarkers of interest include the hippocampal volume, as determined through MRI and based on CSF and plasma pTau181 levels. Another phase 2 trial (NCT04693520) is investigating the effects of oral ALZ-801 administered to participants with early AD who have the APOE4/4 or APOE3/4 genotype with biomarkers of core AD pathology. The study is also assessing the efficacy, safety, and tolerability of ALZ-801.

Simufilam (PTI-125) is a drug that binds to filamin, a scaffolding protein that stabilizes the interaction between soluble Aβ42 and the α7 nicotinic acetylcholine receptor [ 55 ]. Two phase 3 trials, namely RETHINK-ALZ (NCT04994483) and REFOCUS-ALZ (NCT05026177), were commenced in November 2021. Both are safety and efficacy studies of simufilam and have enrolled participants with mild-to-moderate AD. RETHINK-ALZ will randomize 750 participants with AD and CDR scores of 0.5, 1, or 2 into groups receiving either placebo or 100 mg of simufilam twice a day for 1 year (52 weeks). The coprimary outcomes of this trial are ADAS-Cog12 and ADCS-ADL scores, and the trial is set to run through October 2023. REFOCUS-ALZ will randomize 1,083 participants into groups receiving placebo or 50 or 100 mg of simufilam (1:1:1) for 76 weeks. The primary outcome measures are similar to those of the RETHINK-ALZ trial. A phase 3 trial of simufilam (NCT05575076) was started in November 2022 to assess the long-term safety and tolerability of simufilam in participants with mild-to-moderate AD. That open-label extension study is intended to assess the long-term safety and tolerability of simufilam 100 mg twice daily in patients who have completed the RETHINK-ALZ or REFOCUS-ALZ Phase 3 clinical trials. The primary outcome measure is adverse event monitoring from baseline to week 52.

Varoglutamstat (PQ912) is a glutaminyl cyclase inhibitor that reduces pGlu-Aβ generation [ 56 ]. Glutaminyl cyclase catalyzes the cyclization of an exposed glutamate at the N-terminus of Aβ, resulting in the formation of toxic pGlu-Aβ, a major component of amyloid plaques. Two ongoing phase 2 clinical trials, namely VIVA-MIND and VIVIAD, are evaluating the safety, tolerability, and efficacy of varoglutamstat in participants with MCI and mild dementia due to AD. VIVA-MIND (NCT03919162) is a phase 2A multicenter, randomized, double-blind, placebo-controlled, parallel-group study of varoglutamstat, with a stage gate to phase 2B. Phase 2A involves an adaptive dosing evaluation of three doses of varoglutamstat or placebo for ≥ 24 weeks. VIVIAD (NCT04498650) is a phase 2B, multicenter, randomized, double-blind, placebo-controlled, parallel-group, dose-finding study being conducted to evaluate the safety, tolerability, and efficacy of varoglutamstat in 259 participants with MCI and mild dementia due to AD.

ABBV-916 is a monoclonal antibody to Aβ. It recognizes N-terminal truncated Aβ modified with pyroglutamate at position 3 (N3), a form of Aβ that is aggregated into amyloid plaques. A two-stage phase 2 trial of ABBV-916 is ongoing (NCT05291234). Stage A is a multiple ascending dose study, and participants have a 25% chance of receiving placebo. Stage B is a proof-of-concept study, and participants have a 20% chance of receiving placebo. The first 6 months of the study are a double-blinded period, which is to be followed by a 2-year extension period in which all participants receive ABBV-916. Approximately 195 participants aged 50–90 years are to be enrolled at approximately 90 sites across the world. The participants are to receive intravenous doses of ABBV-916 or placebo once every 4 weeks for 24 weeks and are to be followed up for an additional 16 weeks.

CT1812 is a ligand that targets the component 1 subunit of the sigma2/progesterone membrane receptor. It functions as a negative allosteric regulator, reducing the affinity of oligomeric Aβ and interfering with Aβ-induced synaptic toxicity [ 57 ]. START(COG0203) study (NCT05531656) is a phase 2, multicenter, randomized, double-blind, placebo-controlled trial that was initiated in September 2022 for evaluating the efficacy and safety of CT1812. START is comparing the effects of CT1812 (100 or 300 mg) with those of placebo over 18 months in 540 people with MCI or mild dementia due to AD. The SHINE (COG0201) study (NCT03507790) is a multicenter, randomized, double-blind, placebo-controlled, parallel-group, 36-week phase 2 study of two doses of CT1812 in adults with mild-to-moderate AD. The study is evaluating the safety, tolerability, pharmacokinetics, and efficacy of CT1812.

Anti-tau therapy

Table 4 summarizes the ongoing phase 2 trials of anti-tau therapy.

Bepranemab (UCB0107) is a monoclonal IgG4 antibody that targets a central tau epitope. An ongoing phase 2 trial (NCT04867616) enrolling 421 participants with prodromal or mild AD is investigating the safety, tolerability, and efficacy of bepranemab. After an 80-week double-blinded treatment period, the participants are eligible to enter a 48-week open-label extension period, in which they are to receive bepranemab treatment for 44 weeks. Subsequently, they are to participate in a safety evaluation visit 20 weeks after the last infusion. The primary outcome measure is the CDR-SB score.

JNJ-63733657 is a humanized IgG1 monoclonal antibody that targets the microtubule-binding region of tau and prevents the cell-to-cell propagation of pathogenic tau aggregates. The AUTONOMY trial (NCT04619420) is an ongoing phase 2, randomized, double-blind, placebo-controlled, parallel-group multicenter study. Participants with early AD symptoms and a positive tau PET scan are randomized to groups receiving JNJ-63733657 or placebo. This trial is enrolling 420 participants and is expected to be completed by November 2025. The primary outcome measure is clinical decline, as determined using the iADRS.

ACI-35 is a liposome-based vaccine that targets pathological conformations of phosphorylated tau. A phase 1b/2a multicenter, double-blind, randomized, placebo-controlled trial (NCT04445831) was conducted to evaluate the safety, tolerability, and immunogenicity of various doses, regimens, and combinations of tau-targeting vaccines in individuals with early AD. The vaccines tested were JACI-35.054 and ACI-35.030 at various dose levels. The findings were presented at the 2022 CTAD conference. The results indicated that participants who received ACI-35.030 exhibited a strong and sustained immune response against pathological tau proteins (pTau) and nonphosphorylated tau (ePHF), particularly in the mid- and low-dose groups. Recipients of JACI-35.054 also displayed a robust immune response against ePHF and pTau, but without a clear dose–effect relationship. The trial has been conducted across nine centers in Finland, Sweden, the Netherlands, and the United Kingdom and is expected to be completed by October 2023.

E2814 is a monoclonal IgG1 antibody that targets an HVPGG epitope in the microtubule-binding domain of tau, prevents cell-to-cell propagation, and mediates the clearance of pathogenic tau proteins. The DIAN-TU-001 (E2814) trial (NCT05269394) is a phase 2/3 multicenter, randomized, double-blind, placebo-controlled platform trial of potential disease-modifying therapies with biomarker, cognitive, and clinical endpoints. The trial is enrolling patients with dominantly inherited AD. The study design involves the use of the anti-amyloid antibody lecanemab. Some participants are receiving a matching placebo plus lecanemab, whereas others are receiving concurrent therapy with E2814 plus lecanemab.

LY3372689 is a small-molecule inhibitor of O-GlcNAcase, which promotes tau glycosylation and prevents tau aggregation [ 58 ]. A phase 2 trial (NCT05063539) was initiated in September 2021 for assessing the safety, tolerability, and efficacy of LY3372689 in 330 patients with early symptomatic AD with progressive memory changes for ≥ 6 months and who met the criterion of having a positive flortaucipir-PET scan.

BIIB080 is a tau DNA/RNA-based antisense oligonucleotide that inhibits the translation of tau mRNA into protein, thus suppressing tau expression. CELIA (NCT05399888) is an ongoing phase 2 trial that is aiming to determine whether BIIB080 can delay AD progression in comparison with placebo and to identify the most effective dose of BIIB080. In March 2019, Biogen/Ionis performed a 4-year open-label extension trial of quarterly injections for individuals who completed the randomized portion of the trial. The initial data of this trial were reported at the Alzheimer’s Association International Conference (2021), revealing no serious adverse events from the intrathecal injection of BIIB080 at either of three doses every month for 3 months or two high-dose injections 3 months apart. BIIB080 led to a dose-dependent reductions of 30%–50% in total tau and pTau181 levels in CSF.

Neuroprotectors and cognitive enhancers

Table 5 summarizes the ongoing phase 3 trials for therapies other than anti-amyloid/tau treatment.

The active metabolite of fosgonimeton (ATH-1017) is a positive modulator of hepatocyte growth factor (HGF)/MET signaling [ 59 ]. A phase 3 trial of fosgonimeton (NCT04488419) was initiated in September 2020 and is expected to be completed in February 2024. This study is evaluating the safety and efficacy of fosgonimeton in participants with mild-to-moderate AD, with double-blind, parallel-arm treatment implemented for 26 weeks. The primary outcome measure is the overall treatment effect of fosgonimeton, as measured using the Global Statistical Test, which combines cognition (ADAS-Cog) and function (ADCS-ADL) scores.

AR-1001 selectively inhibits phosphodiesterase 5 and suppresses cGMP hydrolysis, resulting in the activation of protein kinase G and the increased phosphorylation of the cAMP-responsive element-binding protein at Ser133. It can rescue long-term potentiation impairment and cognitive dysfunction in animal models of AD [ 60 ]. A phase 3 trial of AR-1001 (NCT05531526) was started in December 2022 and is estimated to be completed in December 2027. The study aims to evaluate the efficacy and safety of AR1001 in participants with early AD. The primary outcome measure is the change in the CDR-SB from baseline to week 52.

BPDO-1603 is a potential cognitive-enhancing drug for AD, but its mechanism of action remains unknown [ 61 ]. A phase 3 trial of BPDO-1603 (NCT04229927) was started in February 2020 and is estimated to be completed in March 2023. The study has been undertaken to evaluate the efficacy and safety of BPDO-1603 in patients with moderate-to-severe AD. The primary outcome measures are the change in Severe Impairment Battery total scores from baseline to week 24, and CIBIC-plus total scores at week 24.

Buntanetap is a novel translational inhibitor of multiple neurotoxic proteins, including APP, tau, and α-synuclein, by enhancing the binding of the atypical iron response element in the 5′UTR regions of the mRNA of the neurotoxic proteins to iron regulatory protein 1 [ 62 ]. In February 2023, phase 2 and 3 trials (NCT05686044) were initiated to measure the efficacy and safety of three doses of buntanetap in comparison with placebo in participants with mild-to-moderate AD. The primary outcome measures are ADAS-Cog and ADCS Clinical Global Impression of Change (ADCS-CGIC) scores.

Caffeine is an adenosine receptor antagonist that has been reported to be associated with slower cognitive decline and lower cerebral amyloid accumulation [ 63 ]. A phase 3 trial of caffeine (NCT04570085) was started in March 2021 to evaluate the efficacy of 30 weeks of caffeine intake in comparison with placebo on cognitive decline in patients with mild-to-moderate AD dementia (Mini-Mental State Examination scores: 16–24). The primary outcome measure is changes in neuropsychological test battery scores between the randomized value and the value after 30 weeks of treatment.

Hydralazine may have anti-neurodegenerative effects because it activates the Nrf2 pathway, which involves more than 200 antioxidant proteins; improves mitochondrial function; and increases respiration capacity and the production of adenosine triphosphate; hydralazine also activates autophagy, which aids in the clearance of intracellular aggregates [ 64 , 65 , 66 ]. A phase 3 trial of hydralazine (NCT04842552) was started in August 2021 and is anticipated to be completed in December 2023. The study is comparing the effects of 75 mg hydralazine versus placebo in patients with mild-to-moderate AD. Various cognitive and function tests, including olfactory tests, biochemical analyses, and adverse effect monitoring, are being conducted regularly during follow-up.

KarXT (xanomeline-trospium), comprised of muscarinic agonist xanomeline and muscarinic antagonist trospium, is designed to preferentially activate muscarinic receptor in the CNS and ameliorate the peripheral muscarinic side effects. It is reported that KarXT improves cognition in patients with AD and schizophrenia [ 67 ]. A 38-week phase 3 trial comparing the effects of KarXT (NCT05511363) and placebo in participants with psychosis associated with AD dementia was started in August 2022. The trial is analyzing the time from randomization to relapse (primary outcome) as well as the time from randomization to discontinuation for any reason and the safety and tolerability of KarXT (secondary outcomes).

Metformin, a commonly prescribed antidiabetic medication, has been reported to improve cognition or mood in many neurological disorders [ 68 , 69 ]. A phase 3 trial of metformin (NCT04098666) was started in March 2021 and is anticipated to be completed in April 2026. The primary outcome measure is the total recall of the Free and Cued Selective Reminding Test at 24 months.

Nilotinib is a tyrosine kinase inhibitor that preferentially targets discoidin domain receptors and can effectively reduce the occurrence of misfolded proteins in animal models of neurodegeneration by crossing the blood–brain barrier and promoting Aβ and tau degradation [ 70 ]. A phase 3 trial (NCT05143528) was initiated in February 2022 to investigate the safety and efficacy of nilotinib BE (bioequivalent) in individuals with early AD. The primary outcome measure is changes in CDR-SB scores between baseline and week 72.

Piromelatine is a melatonin MT1/2/3 and serotonin 5-HT-1A/1D receptor agonist and was developed as a treatment for mild AD [ 71 ]. In May 2022, a randomized trial (NCT05267535) was initiated in 225 noncarriers of a specific polymorphism, and these participants with mild dementia due to AD are allocated at a ratio of 1:1 to receive piromelatine or placebo for 26 weeks. A 12-month extension involves treating the placebo group with piromelatine to assess the drug’s disease-modifying effects. The primary analysis will be conducted after the initial 26 weeks. If efficacy is not confirmed, the study is to end without the extension phase.

Semaglutide is a peptidic GLP-1 receptor agonist that may regulate the aggregation of Aβ in AD. GLP-1 receptors are involved in cognition, synaptic transmission in hippocampal neurons, and cell apoptosis; thus, they may serve as targets for exploring candidate drugs with neuroprotective and cognition-enhancing effects [ 72 ]. A phase 3 trial of semaglutide (NCT04777396) was started in May 2021 to investigate the efficacy of semaglutide in individuals with early AD. The primary outcome measure is changes in the CDR-SB score from baseline to week 104.

Tricaprilin, a semisynthetic medium-chain triglyceride, is hydrolyzed to octanoic acid after administration and is further metabolized to ketones, which serve as an alternative energy substrate for the brain [ 73 ]. Therefore, tricaprilin can be used as a ketogenic source for the management of mild-to-moderate AD. A phase 3 trial (NCT04187547) was started in June 2022 to evaluate the efficacy and safety of tricaprilin in participants with mild-to-moderate AD. The primary outcome measure is changes in ADAS-Cog scores from baseline to week 20.

Anti-neuroinflammation therapy

Masitinib, an oral tyrosine kinase inhibitor, exerts effects by inhibiting mast cell and microglia/macrophage activity, with significant CNS penetration [ 74 ]. It is currently undergoing a phase 3 trial (NCT05564169) with 600 participants, employing a randomized, double-blind, placebo-controlled, parallel-group design over 24 weeks, followed by a 24-week extension phase. Quadruple masking ensures blinding. The study aims to evaluate Masitinib as an adjunct therapy for mild to moderate AD. Estimated to conclude on December 15, 2025, the trial assesses primary outcomes through changes from baseline in ADAS-Cog-11 and ADCS-ADL scores, measuring cognitive and functional abilities, respectively.

NE3107 is an anti-inflammatory insulin sensitizer that can cross the blood–brain barrier and bind to ERK. NE3107 can selectively inhibit inflammation-driven ERK- and NF-κB-stimulated inflammatory mediators, including TNF-α, without disturbing their homeostatic functions [ 75 ]. A multicenter phase 3 trial (NCT04669028) was started in August 2021 to investigate the safety and efficacy of NE3107 at 20 mg that was orally administered twice daily versus placebo in adult participants with mild-to-moderate AD. The primary outcome measures are changes in ADAS-Cog12 and ADCS-CGIC scores from baseline to week 30 [ 76 ].

BPSD-relieving therapy

Masupirdine, a selective 5‐HT6 receptor antagonist with favorable physicochemical properties and absorption, distribution, metabolism, and excretion properties, may have beneficial effects on agitation, aggression, and psychosis in patients with moderate AD [ 77 ]. A phase 3 trial (NCT05397639) was started in November 2022 to evaluate the efficacy, safety, tolerability, and pharmacokinetics of masupirdine in comparison with placebo for treating agitation in participants with AD dementia. The primary outcome measure is the change in the score of the Cohen–Mansfield Agitation Inventory from baseline to week 12.

Nabilone is a partial agonist of cannabinoid receptor 1 (CB1) and CB2 in the brain and in peripheral tissues, and it has been reported to provide effective treatment for agitation in patients with AD [ 78 ]. A phase 3 trial (NCT04516057) was started in February 2021 to investigate whether nabilone is an effective treatment for agitation in AD patients. The primary outcome measure is agitation (Cohen–Mansfield Agitation Inventory) between baseline and week 8.

Phase 4 and repurposing trials

Table 6 summarizes ongoing phase 4 trials.

Escitalopram, a selective-serotonin reuptake inhibitor, is a commonly used antidepressant. It ameliorates cognitive impairment and could selectively attenuate phosphorylated tau accumulation in stressed rats by regulating hypothalamic–pituitary–adrenal axis activity and the insulin receptor substrate/glycogen synthase kinase-3β signaling pathway [ 79 ]. A phase 4 trial (NCT05004987) was started in February 2022 to investigate whether a reduction in depressive symptoms owing to the administration of escitalopram oxalate is associated with the normalization of AD biomarkers in CSF and inflammatory markers in the peripheral blood. The primary outcome measures are changes in CSF Aβ40 and Aβ42 levels, vascular dysfunction biomarker levels, and scores of the Montgomery–Asberg Depression Ratio Scale at week 8.

Sodium oligomannate (GV-971), a marine-derived oligosaccharide, can reconstitute the gut microbiota, reduce bacterial metabolite–driven peripheral infiltration of immune cells into the brain, inhibit amyloid-β fibril formation, and inhibit neuroinflammation in the brain, as demonstrated in animal studies [ 80 , 81 ]. A phase 4 trial (NCT05181475) was initiated in December 2021 to examine the long-term efficacy and safety of GV-971 as well as changes in blood and gut microbiota biomarkers and thereby validate its mechanism of action and establish guidance for the more rational use of drugs in clinical practice. The primary outcome measure is changes in ADAS-Cog11 scores from baseline to week 48. Another phase 4 trial was started in July 2022 and is comparing the efficacy and safety of memantine and GV-971 monotherapy and combination therapy in patients with moderate-to-severe AD. The primary outcome measure is changes in cognitive function at weeks 12, 24, 36, and 48.

Spironolactone, an aldosterone mineralocorticoid receptor antagonist, has been commonly used to treat cardiovascular diseases, including hypertension. It has anti-inflammatory effects on the peripheral tissues and central nervous system and therefore may have beneficial effects on neurological disorders [ 82 ]. A phase 4 trial (NCT04522739) was started in September 2022 to investigate whether spironolactone can be tolerated by older Black American adults with MCI and to determine its effect on memory and thinking abilities, as measured by participant performance on cognitive tests. The primary outcome measures are the number of adverse events and the attrition rate.

Published results

Among the clinical trials newly registered in the last 4 years, four articles pertaining to two trials have been published in peer-reviewed scientific journals. The characteristics of the published randomized controlled trials are summarized in Table 7 [ 43 , 53 , 83 , 84 ]. Two articles reported the results of NCT03887455 [ 43 , 84 ], and the other two reported the results of NCT01760005 [ 53 , 83 ]. The articles were published between 2018 and 2023. The results of both NCT03887455 (Clarity AD) and NCT01760005 have been discussed in the anti-amyloid section. The methodological quality of these studies is summarized in Table 8 . Both trials (NCT03887455 and NCT01760005) had a overall low risk of bias [ 43 , 53 , 83 , 84 ].

Our understanding of AD originated from clinical research, and how pathological findings are associated with clinical presentation of AD has continued to intrigue the neuroscience research community over the past century. DMTs have become the core of new drug development, and the accumulation of knowledge is leading to the evolution of diagnostic criteria and clinical outcome measurements. The view of clinical outcomes has shifted from considering them as solely determinative to considering them to be just one of the determinants. In accordance with the 2018 NIA-AA Research Framework criteria [ 25 ] or the new 2023 NIA-AA revised criteria for AD [ 26 ], the incorporation of biomarkers is necessary in clinical practice.

This review documented that in terms of the number of AD drug trials and the number of recruited participants, the majority of trials continue to focus on mechanisms involving amyloid and tau. Our 2020 report highlighted that due to the failure of early anti-amyloid trials to achieve their intended outcomes, particularly studies involving BACE inhibitors and monoclonal antibodies, some have questioned whether amyloid remains clinically relevant in AD. This shift in perspective has led to a change in the focus of research toward populations in the prodromal or preclinical stage with positive results for diagnostic biomarkers. Additionally, the validity of the amyloid hypothesis has been contested, resulting in a significant reduction in the number of anti-amyloid phase 3 trials since 2019. However, the targets of both phase 1 and phase 2 trials are diverse, with a noticeable increase in the number of phase 1 trials focusing on neuroprotection and phase 2 trials focusing on anti-neuroinflammation [ 85 ]. Since the positive outcomes in terms of slow decline in cognitive abilities in the lecanemab Clarity AD trial [ 43 ] and the donanemab trial TRAILBLAZER-ALZ [ 86 ], the impact of amyloid and consequent pathological alterations is likely to become the main focus of clinical trials. The incorporation of amyloid-related therapy either as an add-on or as a link to specific aspects of AD pathophysiology might become an important trend in clinical trials of new drugs in the future. However, despite this expansion of research areas, the scope of indications for novel anti-amyloid monoclonal antibody therapy remains limited. The mode of treatment administration and the high monitoring costs along with the need for specialized facilities and imaging scans remain challenges. Other unmet needs, such as addressing BPSD and enhancing cognitive function, necessitate pharmaceutical research. Examining drugs with diverse mechanisms necessitates thorough evaluation that extends beyond mere clinical measurements to encompass their intermediate impact on biomarkers. It is essential to investigate the potential synergy between a new drug and existing medications approved by the US FDA. This approach could even be extended to situations where adjuvant treatment, such as tau-related treatments, is provided after amyloid clearance has been achieved. Clinical trials related to AD have also exhibited a shift in focus toward the earlier stages of AD, such as MCI, or even cognitively healthy participants for developing prevention interventions.

Successful phase 3 trials such as Clarity AD (lecanemab) and EMERGE (aducanumab) have evaluated anti-amyloid treatment in mild AD (Fig.  2 ). Trials that do not target specific pathophysiologies are becoming fewer in all phases (Figs.  2 and 3 ). However, an increasing number of early-phase trials of therapies for symptoms, including cognitive enhancers and agents for relieving BPSD, are being conducted. This reflects the unmet clinical need for such therapies (Figs.  2 and 3 ). Similarly, an increasing number of phase 1 trials involving DMTs, particularly those targeting both anti-amyloid and anti-tau mechanisms, has been noted, indicating the importance of basic research (Fig.  3 ). Outcome measurement tools have also become more diverse, which has enabled meaningful improvements in AD and the efficacy of treatments to be clearly determined in clinical trials. Overall, the field of AD clinical trials is evolving, and additional promising treatments for AD are likely to be developed in the near future.

Trends in Phase 3 trials, 2020–2023, categorized according to event-related themes in ClinicalTrials.gov. Left: Number of Phase 3 trials. Right: Percentage of Phase 3 trials. A anti-amyloid therapy, B anti-tau therapy, C neuroprotection, D anti-neuroinflammation, E cognitive enhancer, F relief of behavioral psychological symptoms of dementia, G others, U undisclosed

Trends in Phase 1 and 2 trials, 2020–2023, categorized according to event-related themes in ClinicalTrials.gov. Left: Number of Phase 2 trials. Right: Number of Phase 1 trials; A anti-amyloid therapy, B anti-tau therapy, C neuroprotection, D anti-neuroinflammation, E cognitive enhancer, F relief of behavioral psychological symptoms of dementia, G others, U undisclosed

Availability of data and materials

Not applicable.

Abbreviations

Amyloid-beta

Acetylcholine

Cholinesterase inhibitors

• Alzheimer disease

Alzheimer’s Disease Assessment Scale–Cognitive Subscale

Alzheimer’s Disease Cooperative Study–Activities of Daily Living Inventory–Mild Cognitive Impairment Version

Apolipoprotein gene

Amyloid precursor protein

Amyloid-related imaging abnormalities

Amyloid, tau, and neurodegeneration biomarkers

Appropriate use recommendations

Autophagic vacuoles

Beta-secretase 1

Behavioral psychological symptoms of dementia

Clinical Dementia Rating scale

Clinical Dementia Rating scale Sum of Box

Caregiver Global Impression of Change

Cerebrospinal fluid

Clinical Trial of AD

Disease-modifyung therapies

Integrated Alzheimer’s Disease Rating Scale

Immunoglobulin gamma 1

Mild cognitive impairment

Magnetic resonance imaging

Nuclear factor κB

Neurofibrillary tangles

Neuropsychiatric Inventory

Positron emission tomography

Presenilin-1

Presenilin-2

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Li-Kai Huang and Yi-Chun Kuan contributed equally to this work.

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PhD Program in Medical Neuroscience, College of Medical Science and Technology, Taipei Medical University, No. 291, Zhong Zheng Road, Zhonghe District, New Taipei City, Taiwan

Li-Kai Huang & Chaur-Jong Hu

Taipei Neuroscience Institute, Taipei Medical University, New Taipei City, Taiwan

Li-Kai Huang, Yi-Chun Kuan & Chaur-Jong Hu

Dementia Center and Department of Neurology, Shuang-Ho Hospital, Taipei Medical University, New Taipei City, Taiwan

Department of Neurology, School of Medicine, College of Medicine, Taipei Medical University, Taipei, Taiwan

Yi-Chun Kuan & Chaur-Jong Hu

Department of Biomedical Engineering, National Taiwan University, Taipei, Taiwan

Yi-Chun Kuan

School of Medicine, College of Medicine, Taipei Medical University, Taipei, Taiwan

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LKH and YCK: Conducted literature search, developed the study concept and design, extracted information from trials and studies, and contributed to manuscript drafting and revision. HWL: Extracted information from trials and studies and contributed to manuscript drafting and revision. CJH: Contributed to the study concept and design, interpreted the data and information, finalized and revised the manuscript, and provided overall supervision of the entire project.

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Huang, LK., Kuan, YC., Lin, HW. et al. Clinical trials of new drugs for Alzheimer disease: a 2020–2023 update. J Biomed Sci 30 , 83 (2023). https://doi.org/10.1186/s12929-023-00976-6

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New class of Alzheimer's drugs showing promise in patients in early stage of disease

For decades scientists and families have been frustrated by the intractable nature of Alzheimer's disease.

Key points:

• A study showed donanemab could slow Alzheimer's disease progression by 35pc in patients in the earliest stages of the disease
• Geriatrician Michael Woodward says the medical community is excited by the results
• The donanemab study findings were similar to those of its predecessor lecanemab

As the population ages and more people develop the devastating condition, there have been no new treatments coming onto the market and for many, no hope in sight.

That was until two years ago.

In a short time, decades of research has started to come to fruition, with at least three new drugs demonstrating the first glimmers of promise.

The latest is called donanemab, with the findings of a global trial involving 1,700 patients presented at a major Alzheimer's conference in The Netherlands.

Sixteen Australians took part in the trial at eight sites in Victoria and New South Wales.

The drug, from pharmaceutical giant Eli Lilly, was able to slow Alzheimer's disease progression by 35 per cent in patients in the earliest stages of the disease.

Across the whole study, there was a 22 per cent slowdown in the disease's progress at the 18-month mark.

Michael Woodward, a geriatrician who has been involved in Alzheimer's research for decades, was at the Alzheimer's conference and said the medical community was excited by the results.

"I would regard this as the end of the beginning in Alzheimer's therapies," he said.

"The word breakthrough is used perhaps a little too often, but this is a major breakthrough.

"We now have three drugs that have been shown that can critically slow down the decline."

How does the new drug work?

Donanemab is a monoclonal antibody designed to clear the brain of amyloid plaque, which experts believe plays a role in Alzheimer's disease.

Researchers have long been trying to work out whether a protein called beta-amyloid plaque (BAP) or another protein called tau is responsible for Alzheimer's, or a combination of the two.

Those in the study were all in the early stages of Alzheimer's and aged between 60 and 85.

At the 12-month mark, the researchers said 47 per cent had no evidence of amyloid plaques, compared with 29 per cent in the placebo group.

Patients also did not need indefinite treatment, with injections being able to reduce amyloid to non-existent levels where they would not re-accumulate for many years.

Stephen Macfarlane had three patients in the study through his work with The Dementia Centre at HammondCare in Victoria.

He said the medication was the equivalent of slowing the rate of the disease by seven and a half months compared to someone who was not taking it.

"These drugs slow the progression of the disease, they don't cause people to improve," Dr Macfarlane said.

He said it was the most promising drug in two decades for Alzheimer's research.

"It's the most effective, and the safety data seems to be on a par with similar drugs," he said.

The findings show there was a risk of brain bleeding and swelling in a subset of patients, including 1.6 per cent of participants who experienced serious forms, and three who died.

"Bearing in mind that Alzheimer's disease is a fatal and otherwise untreatable illness, some degree of risk is inherent in the process," Dr Macfarlane said.

Drug follows on heels of another, lecanemab

The donanemab study findings were similar to those of its predecessor lecanemab, sold under the brand name Leqembi.

It reduced cognitive decline by 27 per cent in patients with early Alzheimer's in a study published last year.

Lou Coenen is among the Australian patients in a lecanemab trial.

This drug from Japanese drug maker Eisa is being tested in four trials that include Australian sites across 18 locations.

The 72-year-old was diagnosed with Alzheimer's about five years ago and had a family history of the disease.

"You just start feeling your thinking doesn't work quite as fast," Mr Coenen said.

"You start to wonder why."

He decided to take part in a clinical trial of lecanemab through the KaRa Institute of Neurological Diseases to help give back to the health community.

He says he does notice a difference on the medication.

It is allowing him to spend more time with his wife and family and still participate in community activities such as The Men's Shed.

"I know compared to other people this is working," he said. "But I don't have a comparative of another me that says otherwise."

On June 30 Australia's Therapeutic Goods Administration (TGA) started work to consider approving lecanemab in Australia.

This drug has shown similar results to donanemab in patients with early Alzheimer's but also comes with risks of brain swelling and bleeding in a small subset of patients.

How much will it cost?

New Alzheimer's drugs to the market are predicted to be hugely expensive for governments because of the significant time and cost they took to develop.

Leqembi is priced at about $US26,500 ($39,974) for a year's supply of infusions every two weeks but there is no potential price for donanemab yet, which will involve monthly injections.

"That's going to be a big challenge," Dr Woodward said.

"But we've got to look also at the savings. The total cost of care for Alzheimer's disease is probably closer to about $6-7 billion per year in Australia."

Dr Macfarlane said the drug would also mean Australia would need to revamp its Alzheimer's infrastructure so PET scans were more available for early diagnoses, regular hospital infusions were easier to access, and patients were diagnosed much sooner.

"We know in Australia that on average there's about a three-year delay between people first experiencing symptoms of memory loss and actually receiving a diagnosis," he said.

Biogen drug caused controversy

The drugs follow the groundbreaking but controversial release of Biogen's Aducanumab in 2021.

It is another monoclonal antibody that also works by removing the build-up of amyloid plaque proteins.

It was controversial because of the way the research was structured and the pharmaceutical company's relationship with US regulators.

In June this year the Therapeutic Goods Administration found the drug did not meet its safety and efficacy requirements for approval in Australia and Biogen withdrew its application.

Latest findings bring hope for patients

For Melbourne grandmother Jan Cody, the first sign she knew her memory was failing was when her three children met to discuss her declining mental state.

The 75-year-old had to give up her work as a psychologist, as well as cooking and driving.

"My world just shrank. There's really no medication to take," she said.

She has been involved in some Alzheimer's trials but was not eligible for donanemab.

"The slowing it down takes a long time," she said. "So one really doesn't know whether you're going to last."

"But now I do have a glimmer of hope."

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Alzheimer's researchers are looking beyond plaques and tangles for new treatments.

Jon Hamilton

Scientists say research into Alzheimer's needs to take a broader view of how the disease affects the brain — whether that's changes in the cortex or the role of inflammation. Matt York/AP hide caption

Scientists say research into Alzheimer's needs to take a broader view of how the disease affects the brain — whether that's changes in the cortex or the role of inflammation.

The field of Alzheimer's research is branching out.

After decades of focusing on the sticky amyloid plaques and tangled tau fibers associated with the disease, brain researchers are searching for other potential causes of impaired memory and thinking.

That search is on full display this week at the Alzheimer's Association International Conference in San Diego, where sessions are exploring factors including genes, brain injury, clogged arteries and inflammation.

A group of researchers from Seattle even unveiled a highly detailed atlas showing how different types of brain cells change in Alzheimer's. The goal is to help scientists identify new approaches to treatment.

"Certainly, plaques and tangles are a hallmark," says Maria Carrillo , chief science officer of the Alzheimer's Association. "It doesn't mean plaques are the cause of cell death."

Plaques are clumps of a protein called beta-amyloid that appear in the spaces between neurons. Tangles are made up of a protein called tau that appears inside a neuron.

Both proteins tend to accumulate in the brains of people with Alzheimer's. But their role in killing brain cells is still unclear.

Carrillo says the Alzheimer's field needs to look to cancer research where a deeper understanding of the disease has led to better treatments.

The shift comes after a series of experimental drugs have succeeded in removing amyloid plaques and tau tangles from the brain, but failed to halt the disease.

The Food and Drug Administration has approved one amyloid drug, Aduhelm, but is still evaluating whether it actually helps patients.

An Alzheimer's Atlas

The study that produced the atlas is emblematic of how researchers are recalibrating.

"What we're trying to do with this study is to look at cell vulnerability early on in disease, before [people] have plaques and tangles, before they have cognitive impairment," says Dr. C. Dirk Keene , a neuropathologist at the University of Washington.

To create the atlas, Keene and a team of researches analyzed more than a million cells from 84 brains donated by people who'd signed up for Alzheimer's research projects run by the University of Washington and Kaiser Permanente Washington Research Institute.

The brains came from donors "at all different stages of disease" Keene says, "so we can pinpoint what's happening from the earliest levels all the way through to people with advanced disease."

The effort is funded by the National Institute on Aging and grew out of the federal BRAIN initiative launched by President Obama in 2013.

The atlas came from the realization that "If we want to treat diseases of an extremely complex cellular organ, you need to understand that organ much better than we do," says Ed Lein , a senior investigator at the Allen Institute for Brain Science, which played a key role in analyzing the brain tissue.

So the team spent years studying cells in healthy brains before looking at brains affected by Alzheimer's.

"We've defined what a normal adult brain looks like," Lein says, "and now we can use that knowledge and look for changes that are happening in specific kinds of cells."

Future Alzheimer's Treatments Aim To Do More Than Clear Plaques From The Brain

Finding vulnerable brain cells.

At the Alzheimer's meeting, the team described changes they saw in more than 100 types of cells taken from the cortex — an area of the brain which is important to memory and thinking.

One finding was that neurons that make connections within the cortex itself were much more likely to die than those that connect to distant areas of the brain.

"What we're seeing is a profound effect on cortical circuitry that very plausibly is the reason we have cognitive decline," Lein says.

If so, a treatment designed to protect those vulnerable neurons might prevent declines in memory and thinking linked to Alzheimer's.

The team also found a proliferation of brain cells that contribute to inflammation. These included certain immune cells and a type of cell that responds to injury.

"So while the neurons are lost, the non-neuronal cells are actually increasing and changing" Lein says.

The finding supports the idea that inflammation plays an important role in Alzheimer's, and that anti-inflammatory drugs might help protect the brain.

The Seattle team hopes other scientists will use the brain cell atlas to come up with new treatments for Alzheimer's.

"We've created an open-access resource where the whole community can come and look at this data," Lein says. "They can mine it to speed up progress in the field as a whole."

Speeding up progress is one reason Kyle Travaglini , a researcher at the Allen Institute, jumped at the chance to work on the Alzheimer's project.

"My grandmother started developing Alzheimer's disease when I was just going off to college," says Travaglini, who received his PhD in 2021.

Travaglini says the atlas project is appealing because it isn't based on a preconceived idea about what causes Alzheimer's.

"It's like looking at the same disease that everyone has been looking at but in an entirely different way," he says.

A substance found in young spinal fluid helps old mice remember

Scientists look to people with Down syndrome to test Alzheimer's drugs

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NIH funds next step of cutting-edge research into Alzheimer’s disease genome

Scientists will analyze genome sequence data to identify gene risk, protective factors.

Teams of scientists will use support from the National Institutes of Health to conduct research into the genetic underpinnings of Alzheimer’s disease, analyzing how genome sequences — the order of chemical letters in a cell’s DNA — may contribute to increased risk or protect against the disease. The NIH awarded grants for using innovative new technologies and computational methods for the analysis. The scientists also will seek insights into why some people with known risks do not develop the disease.

The awards, expected to total $24 million over four years, go to eight academic medical centers that have been at the forefront of research in Alzheimer’s genetics: University of Pennsylvania, Philadelphia; Case Western Reserve University, Cleveland; University of Miami; Columbia University, New York City; Boston University; University of Washington, Seattle; Washington University in St. Louis, and University of Texas, Houston.

The investigators will analyze the genome sequencing data generated during the first phase of the Alzheimer’s Disease Sequencing Project (ADSP), an innovative collaboration that began in 2012 between the National Institute on Aging (NIA) and the National Human Genome Research Institute (NHGRI), both part of NIH. The first phase of the project determined the order of all 3 billion letters in the individual genomes of 580 participants. It also generated whole exome sequencing data (focused on the proteins influencing the disorder) of an additional 11,000 volunteers — 6,000 with Alzheimer’s compared to 5,000 controls. Funds supporting the new analysis come from fiscal 2014 additions to the NIA budget directed at intensifying Alzheimer’s research.

“We are delighted to support the important research being accomplished under this broad-based, collaborative effort. This team effort is vital to advancing a deeper understanding of the genetic variants involved in this complex and devastating disease and to the shared goal of finding targets for effective interventions,” said NIH Director Francis S. Collins, M.D., Ph.D.

The effort is critical to accomplishing the genetic research goals outlined in the National Plan to Address Alzheimer’s Disease , first announced by the U.S. Department of Health and Human Services in May 2012 and updated annually. Developed under the National Alzheimer’s Project Act (PDF - 125KB) , the plan provides a framework for a coordinated and concentrated effort in research, care and services for Alzheimer’s and related dementias. Its primary research goal is to prevent and effectively treat Alzheimer’s disease by 2025. The recommendation to conduct these specific types of studies came from the NIH-supported Alzheimer's Disease Research Summit 2012: Path to Treatment and Prevention , and applications were submitted in response to an NIH call for applications issued in May 2012.

“Working closely with our NHGRI colleagues to build, store and make freely accessible to researchers the ADSP datasets, we have opened up new avenues for research. Building on that cache of data, we have moved quickly to this next stage of analyzing the data in new and innovative ways,” said NIA Director Richard J. Hodes, M.D.

With these awards, multiple research teams will use the ADSP data to identify rare genetic variants that protect against, or contribute to, Alzheimer’s disease, explore differences in data from different racial/ethnic groups, and examine how brain images and other biomarkers are associated with genome sequences.

The projects made possible by the new funding are:

• The Consortium for Alzheimer’s Sequence Analysis (CASA). This five-university collaboration received a $12.6 million grant to analyze ADSP whole exome and whole genome sequence data generated from 6,000 volunteers with Alzheimer’s disease and 5,000 older participants free of the disorder. They also will study genomic data from 111 large families, a portion of which are of Caribbean Hispanic descent, that include multiple members with Alzheimer’s disease. The goal is to identify rare genetic variants that protect against or cause Alzheimer’s disease. CASA principal investigators are: Lindsay Farrer, Ph.D., Boston University; Jonathan Haines, Ph.D., Case Western Reserve University, Cleveland; Richard Mayeux, M.D., Columbia University, New York City; Margaret A. Pericak-Vance, Ph.D., University of Miami; and Gerard D. Schellenberg, Ph.D., University of Pennsylvania, Philadelphia. (NIA grant UF1AG047133)
• Genome mapping in families affected with Alzheimer’s disease. Ellen Wijsman, Ph.D., University of Washington, Seattle, will receive grants expected to total $2.8 million over four years to work toward a more refined mapping of the Alzheimer’s genome in families. By identifying genomic regions that are likely to contain rare high-risk or protective Alzheimer’s variants in individual families or groups of families, this analysis of ADSP data may lead to the identification of gene variants that affect not only specific families, but that may be common to specific ethnic groups. (NIA grant U01AG049507)
• Protective gene variants. Alison Goate, D.Phil., Washington University in St. Louis, will receive grants expected to total $1.7 million over four years to identify gene variants that protect against Alzheimer’s in people who are at greater risk for developing the disorder because they carry the APOE4 allele. She will examine ADSP and additional datasets to determine whether certain gene variants protect all who carry them, or only those who also carry specific genetic risk factors, and investigate whether these protective factors reduce risk in both Europeans and African-Americans. Her study will also explore how gene variants influence the age of onset of the disease. (NIA grant U01AG09508)
• Risk and protective genes and the Alzheimer’s phenotype. Sudha Seshadri, M.D., Boston University, will receive grants expected to total $3 million over four years to detect genetic variants associated with increased risk of, or protection from, Alzheimer’s in ADSP data from 5,000 people who developed the disease despite being at relatively low risk due to age or APOE genotype, and 5,000 cognitively normal older participants who likely lack these risk gene variants. The study also involves over 10,000 additional persons from the Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) Consortium. (NIA grant U01AG049505)
• Identifying risk-raising and protective copy number variations. Eric Boerwinkle, Ph.D., University of Texas Health Sciences Center, Houston, will receive grants expected to total $3.8 million over four years to identify novel copy number variations (CNV) — or the number of copies of a particular gene or region of the genome that varies from one individual to the next — that are associated with risk for, or protection, from Alzheimer’s in ADSP and CHARGE consortium datasets. The investigators will use sophisticated bioinformatics and computational tools to explore the function of genes disrupted or overlapped by CNVs and their impact on disease risk in multiple ethnic and racial groups. To gain further insight, they will examine whether CNVs influence memory performance, brain images and other biomarkers of Alzheimer’s disease. (NIA grant U01AG049506)

To further the ADSP goals, the grantees will collaborate with the NHGRI Large-Scale Sequencing and Analysis Centers program, an NIH-supported consortium that generates and analyzes large genome sequence datasets for biomedical research projects.

“The ADSP data generated over the last two years are now paving the way for cutting-edge investigations that may lead to new targets for drug development. The upcoming data analyses will be pivotal for realizing that vision,” said NHGRI Director Eric D. Green, M.D., Ph.D.

The National Human Genome Research Institute is one of the 27 Institutes and Centers at the National Institutes of Health. The NHGRI Extramural Research Program supports grants for research and training and career development at sites nationwide. Additional information about NHGRI can be found at http://www.genome.gov .

The National Institute on Aging leads the federal government effort conducting and supporting research on aging and the health and well-being of older people. It provides information on age-related cognitive change and neurodegenerative disease specifically at its Alzheimer’s Disease Education and Referral (ADEAR) Center at http://www.nia.nih.gov/Alzheimers .

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit www.nih.gov .

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• Alois Alzheimer's theory that plaques – insoluble clumps of amyloid-beta protein found in the brain – are the cause of Alzheimer’s disease still stands more than 100 years after his discovery.
• However, researchers in the field believe this fails to explain why the presence of plaques doesn't always cause neurological symptoms.
• Or why clinical trials using drugs that reduce these plaques have been largely unsuccessful.
• They recently investigated whether the amount of plaques in the brain or the amount of soluble amyloid-beta 42 remaining is more important for disease progression in people genetically predisposed to Alzheimer's.
• Those with high levels of amyloid-beta 42 in their cerebrospinal fluid (the liquid around the brain and spinal cord) were protected and their cognition was preserved over the study period, their findings show.
• This could be key in treating Alzheimer’s and other protein aggregation diseases, such as Parkinson’s and motor neuron disease, the researchers say.

In 1906, Alois Alzheimer, a psychiatrist and neuroanatomist, reported “a peculiar severe disease process of the cerebral cortex” to a gathering of psychiatrists in Tübingen, Germany. The case was a 50-year-old woman who suffered from memory loss, delusions, hallucinations, aggression and confusion – all of which worsened until her untimely death five years later.

In the autopsy, Alzheimer noticed distinctive plaques on her brain. These plaques – clumps of amyloid-beta protein – are still considered to be the cause of Alzheimer’s disease.

However, this theory has two major problems. First, it does not explain why many subjects (even old people) have plaques in their brains in the absence of any neurological symptoms, such as memory loss. Second, clinical trials for drugs that reduce these plaques have been unsuccessful – with one recent exception , but more of that later.

When amyloid-beta protein accumulates in the form of plaques (insoluble clumps), the original soluble form of the protein, which performs important functions in the brain, is consumed and lost. Some studies have shown that reduced levels of soluble amyloid-beta – called amyloid-beta 42 – have led to patients having worse clinical outcomes.

Have you read?

Driving a global effort against alzheimer’s.

In a recent study , published in the Journal of Alzheimer’s Disease , we investigated whether it’s the amount of plaques in the brain or the amount of amyloid-beta 42 remaining that is more important for Alzheimer’s disease progression.

To answer this question, we studied data on a group of people who have a rare inherited gene mutation that puts them at high risk of developing Alzheimer’s disease. The participants were from the Dominantly Inherited Alzheimer Network cohort study.

We found that the depletion of amyloid-beta 42 (the functional version of amyloid-beta) is more harmful than the amount of plaques (the insoluble clumps of amyloid beta).

Participants had an average of three years follow-up and we found that those with high levels of amyloid-beta 42 in their cerebrospinal fluid (the liquid around the brain and spinal cord) were protected and their cognition was preserved over the study period. This chimes with many studies that showed important functions of amyloid-beta 42 in memory and cognition .

It is also relevant because we studied people with the genetic mutation who develop Alzheimer’s disease, a group that is considered to provide the strongest evidence supporting the idea that amyloid-beta plaques are harmful. However, even in this group, those with higher cerebronspinal fluid (CSF) levels of amyloid-beta 42 remained cognitively normal regardless of the amount of plaques in their brains.

It is also worth mentioning that in some rare, inherited forms of Alzheimer’s disease – for example, in carriers of the so-called Osaka gene mutation or Arctic mutation – people can develop dementia having low levels of amyloid-beta 42 and no detectable plaques. This suggests that plaques aren’t the cause of their dementia, but low levels of amyloid-beta 42 might be.

Alzheimer’s Diesease, a result of rapid ageing that causes dementia, is a growing concern. Dementia, the seventh leading cause of death worldwide, cost the world $1.25 trillion in 2018, and affected about 50 million people in 2019. Without major breakthroughs, the number of people affected will triple by 2050, to 152 million.

To catalyse the fight against Alzheimer's, the World Economic Forum is partnering with the Global CEO Initiative (CEOi) to form a coalition of public and private stakeholders – including pharmaceutical manufacturers, biotech companies, governments, international organizations, foundations and research agencies.

The initiative aims to advance pre-clinical research to advance the understanding of the disease, attract more capital by lowering the risks to investment in biomarkers, develop standing clinical trial platforms, and advance healthcare system readiness in the fields of detection, diagnosis, infrastructure and access.

Lecanemab – the one recent exception

How will our findings affect drug development and clinical trials for Alzheimer’s disease? Until the recent trial with lecanemab , an antibody drug that reduces plaques, all the drug trials in Alzheimer’s disease have failed.

Some drugs were designed to reduce the levels of amyloid-beta 42, based on the rationale that if levels of the normal protein are reduced, patients will accumulate fewer plaques. Unfortunately, these drugs often made the patient’s condition worse .

Lecanemab was recently reported to have a small but significant effect in reducing cognitive decline. According to previous studies , this drug increases the levels of amyloid-beta 42 in the CSF. This is, again, in line with our hypothesis, namely that the increase of the normal amyloid protein can be beneficial.

We will know more when the results of the lecanemab trial are published. At the moment, all we have is a press release from the makers of the drug.

We think that it will be important for future trials to focus on the levels of amyloid-beta 42, and whether it is beneficial to increase and restore its levels to normal values instead of targeting it for removal. This could be achieved using proteins similar to amyloid-beta 42 – so-called “protein analogues” – but that clump together less than the natural ones.

This active protein replacement approach might become a promising new avenue of treatment for Alzheimer’s and other protein aggregation diseases, such as Parkinson’s and motor neuron disease.

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EDITORIAL article

Editorial: new insights into atypical alzheimer's disease: from clinical phenotype to biomarkers.

• 1 Department of Neurology, Mayo Clinic, Rochester, MN, United States
• 2 Department of Radiology, Mayo Clinic, Rochester, MN, United States

Editorial on the Research Topic New insights into atypical Alzheimer's disease: from clinical phenotype to biomarkers

Alzheimer's disease (AD) is a neurodegenerative disease commonly associated with memory loss and is thought as a disease affecting the elderly ( Frisoni et al., 2022 ). However, atypical clinical presentations can also occur, and they have been gaining interest in the past few decades, with research now focusing on investigating the underlying mechanisms and improving diagnosis of these atypical presentations of AD ( Gorno-Tempini et al., 2008 ; Crutch et al., 2012 ; Townley et al., 2020 ; Ossenkoppele et al., 2022 ; Shir et al., 2023 ). Atypical AD presentations are said to account for at least 25% of all AD cases ( Murray et al., 2011 ) and are characterized by the presence of initial and predominant deficits in non-memory domains, such as visual ( Crutch et al., 2012 ), language ( Gorno-Tempini et al., 2008 ), behavioral ( Ossenkoppele et al., 2015b ), executive ( Townley et al., 2020 ), or motor ( Shir et al., 2023 ) difficulties, and younger age at onset ( Graff-Radford et al., 2021 ). Despite significant clinical and neuroimaging advancements, about half of these patients are misdiagnosed or receive a delayed diagnosis ( Balasa et al., 2011 ) and they do not meet eligibility for AD clinical trials as being amnestic is a requirement in these trials. Hence, there is a need to raise awareness of these atypical AD phenotypes.

Through this Research Topic we intend to advocate that atypical AD phenotypes are valuable to the AD field and should be the focus of future research. We also propose treating these heterogenous patients as a part of the AD continuum, instead of considering them atypical. We present six articles that touch upon the concepts of the “heterogeneity in AD,” “atypical AD is a clinical spectrum” and “are atypical AD variants really atypical.”

Clinical presentations of AD are heterogenous and are associated with different neurodegenerative and neuropathological patterns, despite similar topographic distribution of amyloid. However, recent research has shown significant overlap across domains of clinical presentations, gray-matter atrophy, and tau deposition patterns in atypical AD phenotypes ( Migliaccio et al., 2009 ; Owens et al., 2018 ; Singh et al., 2024 ). A study by Phillips et al. in this Research Topic also highlighted the heterogeneity and overlap in neuroimaging abnormalities across the AD continuum by investigating white-matter integrity and microstructure differences. In this study, they found both distinct and overlapping white-matter degeneration profiles across the AD continuum, involving key regions and fiber tracts, which mirrored the findings of previous studies on atypical AD employing various imaging modalities ( Caso et al., 2015 ; Madhavan et al., 2016 ; Sintini et al., 2019 ; Singh et al., 2023 ). Phillips et al. effectively extended their study by assessing differences relative to typical AD patients and reporting a good correspondence between phenotypic white-matter differences and previously reported gray-matter atrophy. Therefore, gray-matter progression may partially be related to the white-matter degeneration seen in atypical AD phenotypes. Likewise, Mohanty et al. explored heterogeneity in atrophy across the full cognitive spectrum of amyloid positive individuals. They found atrophy-based severity to be strongly associated with tau burden and performance in executive and language domains, emphasizing the importance of language as a relevant cognitive sphere for testing across the AD continuum.

In complementary studies, Putcha et al. , Rezaii et al. , and Ahmed et al. highlighted the overlap in clinical features across atypical AD phenotypes. Putcha et al. investigated the immediate auditory naming performance in atypical AD patients, particularly the visual variant of AD. They assessed naming in the absence of a visual perceptual demand, which is inherent in most tests used to study anomia. More than half of the visual variant patients in their study demonstrated anomia and had a naming difficulty, which makes a valuable contribution to the emerging literature describing language deficits in the visual variant of AD ( Putcha et al., 2018 ; Tetzloff et al., 2021 ; Singh et al., 2024 ). Rezaii et al. on a similar note explored the distinction in language performance between visually-dependent and visually-independent contexts. They proposed language as a sensitive diagnostic domain and suggested that the language abnormalities recently being identified in the visual variant of AD may be byproducts of visuospatial processing deficits. The current literature reports the spread of neurodegeneration beyond visual areas into regions involving language processing as a potential reason for language abnormalities in the visual variant ( Migliaccio et al., 2009 ; Ossenkoppele et al., 2015a ; Singh et al., 2024 ). However, Rezaii et al. offers an alternate theory by proposing that visuospatial deficits may hinder transfer of sensory information required for a modal language processing. Ahmed et al. furthered our understanding of language deficits in the visual variant of AD by investigating the language measures temporally. They reported longer preparation, utterance duration, silent pause duration, speech duration and notably longer time to initiate response in the visual AD variant. They effectively implied that there may be impaired interplay between linguistic and cognitive abilities, by presenting evidence of vulnerability in cognitive systems overseeing planning, execution, and attentional control mechanisms. Together these studies highlight how quickly the disease progresses into secondary domains and the importance of comprehensive clinical and neuropsychological assessments in atypical AD phenotypes.

To truly appreciate the significance of this Research Topic, one must understand how challenging and necessary it is to conduct research in atypical AD phenotypes. Whitwell's perspective article beautifully emphasizes these challenges, such as how most atypical AD patients are only diagnosed at specialized centers that employ a biomarker-based approach for evidence of AD. Despite detailed clinical characterization of these phenotypes, there is a lack of consideration when compared to the typical-amnestic presentation of AD. Although atypical AD patients meet eligibility for treatment with current AD therapeutics based on abnormal amyloid biomarkers, they are not commonly considered for AD clinical trials. Moreover, the phenotypic overlap across the variants of atypical AD creates further diagnostic challenges for categorical phenotyping. For these reasons, labeling these patients as “atypical” may have diminished their importance and value in the field and it may be better to think of them as AD with cognitive impairment that affects multiple domains.

In conclusion, this Research Topic contains a treasure-trove of information. These studies have collectively advanced our understanding and hold immense potential for informing future work in atypical AD phenotypes.

Author contributions

NS: Conceptualization, Writing – original draft, Writing – review & editing. IS: Writing – original draft, Writing – review & editing.

The author(s) declare that no financial support was received for the research, authorship, and/or publication of this article.

Acknowledgments

We thank Dr. Keir Yong and Dr. Marianne Chapleau for their time, effort, and contribution to this Research Topic.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher's note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Balasa, M., Gelpi, E., Antonell, A., Rey, M. J., Sanchez-Valle, R., Molinuevo, J. L., et al. (2011). Clinical features and APOE genotype of pathologically proven early-onset Alzheimer disease. Neurology 76, 1720–1725. doi: 10.1212/WNL.0b013e31821a44dd

PubMed Abstract | Crossref Full Text | Google Scholar

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Crutch, S. J., Lehmann, M., Schott, J. M., Rabinovici, G. D., and Rossor, M. N. N. C. (2012). Posterior cortical atrophy. Lancet Neurol. 11, 170–178. doi: 10.1016/S1474-4422(11)70289-7

Frisoni, G. B., Altomare, D., Thal, D. R., Ribaldi, F., Van Der Kant, R., Ossenkoppele, R., et al. (2022). The probabilistic model of Alzheimer disease: the amyloid hypothesis revised. Nat. Rev. Neurosci. 23, 53–66. doi: 10.1038/s41583-021-00533-w

Gorno-Tempini, M. L., Brambati, S. M., Ginex, V., Ogar, J., Dronkers, N. F., Marcone, A., et al. (2008). The logopenic/phonological variant of primary progressive aphasia. Neurology 71, 1227–1234. doi: 10.1212/01.wnl.0000320506.79811.da

Graff-Radford, J., Yong, K. X. X., Apostolova, L. G., Bouwman, F. H., Carrillo, M., Dickerson, B. C., et al. (2021). New insights into atypical Alzheimer's disease in the era of biomarkers. Lancet Neurol. 20, 222–234. doi: 10.1016/S1474-4422(20)30440-3

Madhavan, A., Schwarz, C. G., Duffy, J. R., Strand, E. A., Machulda, M. M., Drubach, D. A., et al. (2016). Characterizing white matter tract degeneration in syndromic variants of Alzheimer's disease: a diffusion tensor imaging study. J. Alzheimers Dis. 49, 633–643. doi: 10.3233/JAD-150502

Migliaccio, R., Agosta, F., Rascovsky, K., Karydas, A., Bonasera, S., Rabinovici, G. D., et al. (2009). Clinical syndromes associated with posterior atrophy: early age at onset AD spectrum. Neurology 73, 1571–1578. doi: 10.1212/WNL.0b013e3181c0d427

Murray, M. E., Graff-Radford, N. R., Ross, O. A., Petersen, R. C., and Duara, R. D. W. (2011). Neuropathologically defined subtypes of Alzheimer's disease with distinct clinical characteristics: a retrospective study. Lancet Neurol. 10, 785–796. doi: 10.1016/S1474-4422(11)70156-9

Ossenkoppele, R., Cohn-Sheehy, B. I., La Joie, R., Vogel, J. W., Moller, C., Lehmann, M., et al. (2015a). Atrophy patterns in early clinical stages across distinct phenotypes of Alzheimer's disease. Hum. Brain Mapp. 36, 4421–4437. doi: 10.1002/hbm.22927

Ossenkoppele, R., Pijnenburg, Y. A., Perry, D. C., Cohn-Sheehy, B. I., Scheltens, N. M., Vogel, J. W., et al. (2015b). The behavioural/dysexecutive variant of Alzheimer's disease: clinical, neuroimaging and pathological features. Brain 138, 2732–2749. doi: 10.1093/brain/awv191

Ossenkoppele, R., Singleton, E. H., Groot, C., Dijkstra, A. A., Eikelboom, W. S., Seeley, W. W., et al. (2022). Research criteria for the behavioral variant of Alzheimer disease: a systematic review and meta-analysis. JAMA Neurol. 79, 48–60. doi: 10.1001/jamaneurol.2021.4417

Owens, T. E., Machulda, M. M., Duffy, J. R., Strand, E. A., Clark, H. M., Boland, S., et al. (2018). Patterns of neuropsychological dysfunction and cortical volume changes in logopenic aphasia. J. Alzheimers Dis. 66, 1015–1025. doi: 10.3233/JAD-171175

Putcha, D., Mcginnis, S. M., Brickhouse, M., Wong, B., and Sherman, J. C. B. C. (2018). Executive dysfunction contributes to verbal encoding and retrieval deficits in posterior cortical atrophy. Cortex 106, 36–46. doi: 10.1016/j.cortex.2018.04.010

Shir, D., Pham, N. T. T., Botha, H., Koga, S., Kouri, N., Ali, F., et al. (2023). Clinicoradiologic and neuropathologic evaluation of corticobasal syndrome. Neurology 101, e289–e299. doi: 10.1212/WNL.0000000000207397

Singh, N. A., Graff-Radford, J., Machulda, M. M., Carlos, A. F., Schwarz, C. G., Senjem, M. L., et al. (2024). Atypical Alzheimer's disease: new insights into an overlapping spectrum between the language and visual variants. J. Neurol . 12, 1–16. doi: 10.1007/s00415-024-12297-1

Singh, N. A., Graff-Radford, J., Machulda, M. M., Pham, N. T. T., Schwarz, C. G., Reid, R. I., et al. (2023). Diffusivity changes in posterior cortical atrophy and logopenic progressive aphasia: a longitudinal diffusion tensor imaging study. J. Alzheimers Dis. 94, 709–725. doi: 10.3233/JAD-221217

Sintini, I., Schwarz, C. G., Martin, P. R., Graff-Radford, J., Machulda, M. M., Senjem, M. L., et al. (2019). Regional multimodal relationships between tau, hypometabolism, atrophy, and fractional anisotropy in atypical Alzheimer's disease. Hum. Brain Mapp. 40, 1618–1631. doi: 10.1002/hbm.24473

Tetzloff, K. A., Duffy, J. R., Strand, E. A., Machulda, M. M., Schwarz, C. G., Senjem, M. L., et al. (2021). Phonological errors in posterior cortical atrophy. Dement. Geriatr. Cogn. Disord. 50, 195–203. doi: 10.1159/000516481

Townley, R. A., Graff-Radford, J., Mantyh, W. G., Botha, H., Polsinelli, A. J., Przybelski, S. A., et al. (2020). Progressive dysexecutive syndrome due to Alzheimer's disease: a description of 55 cases and comparison to other phenotypes. Brain Commun. 2:fcaa068. doi: 10.1093/braincomms/fcaa068

Keywords: atypical Alzheimer's disease, posterior cortical atrophy, logopenic progressive aphasia, dysexecutive Alzheimer's disease, corticobasal syndrome

Citation: Singh NA and Sintini I (2024) Editorial: New insights into atypical Alzheimer's disease: from clinical phenotype to biomarkers. Front. Neurosci. 18:1414443. doi: 10.3389/fnins.2024.1414443

Received: 08 April 2024; Accepted: 16 April 2024; Published: 30 April 2024.

Edited and reviewed by: Einar M. Sigurdsson , New York University, United States

Copyright © 2024 Singh and Sintini. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Neha Atulkumar Singh, singh.nehaatulkumar@mayo.edu

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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Comprehensive Review on Alzheimer’s Disease: Causes and Treatment

Alzheimer’s disease (AD) is a disorder that causes degeneration of the cells in the brain and it is the main cause of dementia, which is characterized by a decline in thinking and independence in personal daily activities. AD is considered a multifactorial disease: two main hypotheses were proposed as a cause for AD, cholinergic and amyloid hypotheses. Additionally, several risk factors such as increasing age, genetic factors, head injuries, vascular diseases, infections, and environmental factors play a role in the disease. Currently, there are only two classes of approved drugs to treat AD, including inhibitors to cholinesterase enzyme and antagonists to N -methyl d -aspartate (NMDA), which are effective only in treating the symptoms of AD, but do not cure or prevent the disease. Nowadays, the research is focusing on understanding AD pathology by targeting several mechanisms, such as abnormal tau protein metabolism, β-amyloid, inflammatory response, and cholinergic and free radical damage, aiming to develop successful treatments that are capable of stopping or modifying the course of AD. This review discusses currently available drugs and future theories for the development of new therapies for AD, such as disease-modifying therapeutics (DMT), chaperones, and natural compounds.

1. Introduction

Alzheimer’s disease (AD) (named after the German psychiatric Alois Alzheimer) is the most common type of dementia and can be defined as a slowly progressive neurodegenerative disease characterized by neuritic plaques and neurofibrillary tangles ( Figure 1 ) as a result of amyloid-beta peptide’s (Aβ) accumulation in the most affected area of the brain, the medial temporal lobe and neocortical structures [ 1 ]. Alois Alzheimer noticed a presence of amyloid plaques and a massive loss of neurons while examining the brain of his first patient that suffered from memory loss and change of personality before dying and described the condition as a serious disease of the cerebral cortex. Emil Kraepelin named this medical condition Alzheimer’s disease for the first time in his 8th edition psychiatry handbook [ 2 , 3 ]. Progressive loss of cognitive functions can be caused by cerebral disorder like Alzheimer’s disease (AD) or other factors such as intoxications, infections, abnormality in the pulmonary and circulatory systems, which causes a reduction in the oxygen supply to the brain, nutritional deficiency, vitamin B12 deficiency, tumors, and others [ 4 , 5 ].

The physiological structure of the brain and neurons in ( a ) healthy brain and ( b ) Alzheimer’s disease (AD) brain.

At present, there are around 50 million AD patients worldwide and this number is projected to double every 5 years and will increase to reach 152 million by 2050. AD burden affects individuals, their families, and the economy, with estimated global costs of US$1 trillion annually. At present, there is no cure for Alzheimer’s disease, although there are available treatments that just improve the symptoms [ 6 , 7 ]. The purpose of this review is to give a brief description about AD diagnosis, pathology, causes, and current treatments, and to highlight the recent development of compounds that could prevent or treat AD by targeting several pathogenic mechanisms, such as Aβ and tau aggregation, and misfolding, inflammation, oxidative damage, and others.

2. Alzheimer’s Disease Diagnostic Criteria

A patient suspected to have AD should undergo several tests, including neurological examination, magnetic resonance imaging (MRI) for neurons, laboratory examinations such as vitamin B12, and other tests besides the medical and family history of the patients [ 8 ]. Vitamin (vit.) B12 deficiency has been long known for its association with neurologic problems and increasing risks of AD, according to some studies. A special marker of vit. B12 deficiency is elevated homocysteine levels, which can cause brain damage by oxidative stress, increasing calcium influx and apoptosis. Diagnoses of vit. B12 deficiency can be done by measuring serum vit. B12 level alongside complete blood count and serum homocysteine levels tests [ 9 , 10 ].

In 1984, The National Institute of Neurological and Communicative Disorders and Stroke (NINCDS) and the Alzheimer’s Disease and Related Disorders Association (ADRDA) formed a work group (NINCDS-ADRDA) to establish a clinical diagnostic’s criteria for Alzheimer’s disease. This criteria includes: (1) probable Alzheimer’s disease, which can be diagnosed by dementia that is confirmed by neuropsychological tests, progressive memory loss, impaired daily-life activity, and other symptoms like aphasia (impairment of a language), apraxia (a motor skills disorder), and agnosia (a loss of perception). All of these symptoms can start from age 40–90, with the absence of any systemic or brain diseases, (2) possible Alzheimer’s disease can be applied in the absence of neurologic, psychiatric disorders, and the presence of another illness like systemic or brain disorder, but they are not the primary cause of dementia, and (3) definite Alzheimer’s disease, that is confirmed by histopathologic confirmation obtained from a biopsy or autopsy [ 11 , 12 ].

In 2011, The National Institute on Aging—Alzheimer’s Association made several changes and updated the 1984 NINCDS-ADRDA criteria for higher specificity and sensitivity in the diagnosis of Alzheimer’s disease. The newly proposed criteria include probable and possible AD dementia for the use in clinical settings and probable or possible AD dementia with pathophysiological evidence for research purposes, in addition to clinical biomarkers. There are two categories of Alzheimer’s disease biomarkers: (a) markers of brain amyloid such as positron emission tomography (PET) and cerebrospinal fluid (CSF), and (b) markers of neuronal injury like cerebrospinal fluid tau, fluorodeoxyglucose (FDG) for metabolic activity, and magnetic resonance imaging (MRI) for atrophy measurement [ 13 , 14 , 15 ].

3. Alzheimer’s Disease’s Neuropathology

There are two types of neuropathological changes in AD which provide evidence about disease progress and symptoms and include: (1) positive lesions (due to accumulation), which are characterized by the accumulation of neurofibrillary tangles, amyloid plaques, dystrophic neurites, neuropil threads, and other deposits found in the brains of AD patients. In addition to (2) negative lesions (due to losses), that are characterized by large atrophy due to a neural, neuropil, and synaptic loss. Besides, other factors can cause neurodegeneration such as neuroinflammation, oxidative stress, and injury of cholinergic neurons [ 16 , 17 , 18 ].

3.1. Senile Plaques (SP)

The senile plaques are extracellular deposits of beta-amyloid protein (Aβ) with different morphological forms, including neuritic, diffuse, dense-cored, or classic and compact type plaques. Proteolytic cleavage enzymes such as β-secretase and γ-secretase are responsible for the biosynthesis of Aβ deposits from the transmembrane amyloid precursor protein (APP) [ 19 , 20 , 21 ]. These enzymes cleave APP into several amino acid fragments: 43, 45, 46, 48, 49, and 51 amino acids, which reach the final forms Aβ40 and Aβ42. There are several types of Aβ monomers, including large and insoluble amyloid fibrils which can accumulate to form amyloid plaques and soluble oligomers that can spread throughout the brain. Aβ plays a major role in neurotoxicity and neural function, therefore, accumulation of denser plaques in the hippocampus, amygdala, and cerebral cortex can cause stimulation of astrocytes and microglia, damage to axons, dendrites, and loss of synapses, in addition to cognitive impairments [ 21 , 22 , 23 ].

3.2. Neurofibrillary Tangles (NFTs)

NFT are abnormal filaments of the hyperphosphorylated tau protein that in some stages can be twisted around each other to form paired helical filament (PHF) and accumulate in neuralperikaryal cytoplasm, axons, and dendrites, which cause a loss of cytoskeletal microtubules and tubulin-associated proteins. The hyperphosphorylated tau protein is the major constituent of NFTs in the brains of AD patients, and its evolution can reflect NFTs morphological stages, which include: (1) pre-tangle phase, one type of NFT, where phosphorylated tau proteins are accumulated in the somatodendritic compartment without the formation of PHF, (2) mature NFTs, which are characterized by filament aggregation of tau protein with the displacement of the nucleus to the periphery part of the soma, and (3) the extracellular tangles, or the ghost NFTs stage, that results from a neuronal loss due to large amounts of filamentous tau protein with partial resistance to proteolysis [ 24 , 25 ].

3.3. Synaptic Loss

A synaptic damage in the neocortex and limbic system causes memory impairment and generally is observed at the early stages of AD. Synaptic loss mechanisms involve defects in axonal transport, mitochondrial damage, oxidative stress, and other processes that can contribute to small fractions, like the accumulation of Aβ and tau at the synaptic sites. These processes eventually lead to a loss of dendritic spines, pre-synaptic terminals, and axonal dystrophy [ 26 ]. Synaptic proteins serve as biomarkers for the detection of synapses loss, and severity, such as neurogranin, a postsynaptic neuronal protein, visinin-like protein-1 (VILIP-1), and synaptotagmin-1 [ 27 , 28 ].

4. The Stages of Alzheimer’s Disease

The clinical phases of Alzheimer’s disease can be classified into (1) pre-clinical or the pre-symptomatic stage, which can last for several years or more. This stage is characterized by mild memory loss and early pathological changes in cortex and hippocampus, with no functional impairment in the daily activities and absence of clinical signs and symptoms of AD [ 1 , 29 , 30 ]. (2) The mild or early stage of AD, where several symptoms start to appear in patients, such as a trouble in the daily life of the patient with a loss of concentration and memory, disorientation of place and time, a change in the mood, and a development of depression [ 30 , 31 ]. (3) Moderate AD stage, in which the disease spreads to cerebral cortex areas that results in an increased memory loss with trouble recognizing family and friends, a loss of impulse control, and difficulty in reading, writing, and speaking [ 30 ]. (4) Severe AD or late-stage, which involves the spread of the disease to the entire cortex area with a severe accumulation of neuritic plaques and neurofibrillary tangles, resulting in a progressive functional and cognitive impairment where the patients cannot recognize their family at all and may become bedridden with difficulties in swallowing and urination, and eventually leading to the patient’s death due to these complications [ 1 , 32 ].

5. Causes and Risk Factors of Alzheimer’s Disease

AD has been considered a multifactorial disease associated with several risk factors ( Figure 2 ) such as increasing age, genetic factors, head injuries, vascular diseases, infections, and environmental factors (heavy metals, trace metals, and others). The underlying cause of pathological changes in Alzheimer’s disease (Aβ, NFTs, and synaptic loss) is still unknown. Several hypotheses were proposed as a cause for AD but two of them are believed to be the main cause: some believe that an impairment in the cholinergic function is a critical risk factor for AD, while others suggest that alteration in amyloid β-protein production and processing is the main initiating factor. However, at present, there is no accepted theory for explaining the AD pathogenesis [ 33 , 34 ].

The risk factors for Alzheimer’s disease.

5.1. Alzheimer’s Disease Hypotheses

5.1.1. cholinergic hypothesis.

In the 1970s, neocortical and presynaptic cholinergic deficits were reported to be related to the enzyme choline acetyltransferase (ChAT), which is responsible for the synthesis of acetylcholine (ACh). Due to the essential role of ACh in cognitive function, a cholinergic hypothesis of AD was proposed. ACh is synthesized in the cytoplasm of cholinergic neurons from choline and acetyl-coenzyme A by the ChAT enzyme and transported to the synaptic vesicles by vesicular acetylcholine transporter (VAChT) ( Figure 3 ). In the brain, ACh is involved in several physiological processes such as memory, attention, sensory information, learning, and other critical functions. Degeneration of the cholinergic neurons was found to take place in AD and to cause alternation in cognitive function and memory loss. Β -amyloid is believed to affect cholinergic neurotransmission and to cause a reduction in the choline uptake and a release of ACh. Studies demonstrated that cholinergic synaptic loss and amyloid fibril formation are related to Aβ oligomers’ neurotoxicity and to interactions between AChE and Aβ peptide. Additional factors also contribute to the progression of AD, such as a reduction in nicotinic and muscarinic (M2) Ach receptors, located on presynaptic cholinergic terminals, and the deficit in excitatory amino acid (EAA) neurotransmission, where glutamate concentration and D-aspartate uptake are significantly reduced in many cortical areas in AD brains. This is in addition to the use of cholinergic receptor antagonists such as scopolamine, which was found to induce amnesia. This effect can be reversed by using compounds that activate acetylcholine formation [ 35 , 36 , 37 ].

The pathway for the synthesis and transportation of acetylcholine between presynaptic and postsynaptic nerve terminals.

As a result, the cholinergic hypothesis is based on three concepts: reduced presynaptic cholinergic markers in the cerebral cortex, severe neurodegeneration of nucleus basalis of Meynert (NBM) in the basal forebrain, which is the source of cortical cholinergic innervation, and the role of cholinergic antagonists in memory decline compared to the agonists, which have the opposite effect [ 38 ].

5.1.2. Amyloid Hypothesis

For decades, it was recognized that abnormal deposition of β-sheets in the central nervous system has a strong correlation with dementia, which led to the concept of the amyloid hypothesis. However, it was found that the amyloid plaques (AP) also deposit in normal healthy brains with aging, which raised the question of whether AP deposition is responsible for AD onset or not? Therefore, in the recent years, alternative hypotheses were proposed for the non-inherited form of AD (NIAD), but at present, the amyloid hypothesis remains the most accepted pathological mechanism for inherited AD (IAD). The amyloid hypothesis suggests that the degradation of Aβ, derived from APP by β- and γ-secretase, is decreased by age or pathological conditions, which leads to the accumulation of Aβ peptides (Aβ40 and Aβ42). Increasing the ratio of Aβ42/Aβ40 induces Aβ amyloid fibril formation, resulting in neurotoxicity and tau pathology induction, and consequently, leading to neuronal cell death and neurodegeneration. AD risk factors and mutations of several genes like APP, PSEN1, and PSEN2 were found to affect Aβ catabolism and anabolism, which rapidly cause an accumulation of Aβ and fast progression of neurodegeneration [ 39 , 40 , 41 ].

5.2. Alzheimer’s Disease Risk Factors

5.2.1. aging.

The most important risk factor in AD is aging. Younger individuals rarely have this disease, and most AD cases have a late onset that starts after 65 years of age [ 42 ]. Aging is a complex and irreversible process that occurs through multiple organs and cell systems with a reduction in the brain volume and weight, a loss of synapses, and ventricles’ enlargement in specific areas accompanied by SP deposition and NFT. Moreover, several conditions might emerge during aging such as glucose hypometabolism, cholesterol dyshomeostasis, mitochondria dysfunction, depression, and cognitive decline. These changes also appear in normal aging, which makes it difficult to distinguish the cases in early AD [ 43 , 44 ]. AD can be divided based on age of onset into early-onset AD (EOAD), the rare form with around 1–6% of cases, in which most of them are familial AD characterized by having more than one member in more than one generation with AD, and ranges from 30–60 or 65 years. The second type is the late-onset AD (LOAD), which is more common with age of onset above 65 years. Both types may occur in people who have a family with a positive history of AD and families with a late-onset disease [ 45 ].

5.2.2. Genetics

Genetic factors were discovered over the years and were found to play a major role in the development of AD. 70% of the AD cases were related to genetic factors: most cases of EOAD are inherited in an autosomal dominant pattern and mutations in the dominant genes such as Amyloid precursor protein (APP) , Presenilin-1 (PSEN-1), Presenilin-2 (PSEN-2) , and apolipoprotein E (ApoE) are associated with AD [ 46 , 47 ].

Herein, we discuss the strong genetic risk factors in AD.

• Amyloid Precursor Protein (APP)

APP is a type I transmembrane protein cleaved by α-, β-, and γ-secretase to release Aβ and other proteins and is encoded by the APP gene on chromosome 21. Thirty mutations have been found in the APP gene in which twenty-five of them are related to AD and cause an accumulation of Aβ with elevated amounts. Meanwhile, there is one protective mutation, A673T, which protects against AD by decreasing Aβ, Aβ40, and Aβ42 secretion [ 48 , 49 ]. All mutations surround the secretase cleavage site, for example, the KM670/671NL mutation in mouse models has shown an increasing level of amyloid plaques in the hippocampus and cortex with no NFTs. A673V, D678H, D678N, E682K, and K687N mutations have shown cortical atrophy, whereas E682K has shown hippocampal atrophy. Neuropathological reports for the A673V mutation demonstrated a presence of NFTs and Aβ, activation of microglia and astrocytes, and neuronal loss, compared to the rest of the mentioned mutations, which show no change in the intracellular Aβ according to neuropathological reports [ 48 , 50 ]. Other mutations such as T714I, V715A, V715M, V717I, V717L, L723P, K724N, and I716V affect the γ-secretase cleavage site and cause an increase in the Aβ42/Aβ40 ratio, while E693G, E693K, D694N, and A692G mutations affect the α-secretase cleavage site and cause polymorphic aggregates with the ability to disrupt bilayer integrity. Also, the E693delta is a deletion mutation that enhances the formation of synaptotoxic Aβ [ 51 , 52 ].

• Presenilin-1 (PSEN-1) and Presenilin-2 (PSEN-2)

PSEN1 and PSEN2 genes are also the autosomal dominant form of EOAD located on chromosomes 14 and 1, respectively. PSEN-2 and PSEN-1 are homologous, with 67% similarity, with a difference in the N -terminus and the hydrophilic region. Mutation in PSEN1 gene is more common, with more than 200 mutations, while a rare form with less than 40 mutations was identified in the PSEN2 gene [ 53 , 54 ].

PSEN1 is a core protein that activates the γ-secretase complex and plays an important role in the production of Aβ from APP. Knockout studies of PSEN1 showed synaptic dysfunction and memory impairment in mice, which indicate its essential role in maintaining memory and neurons [ 51 ]. PSEN1 mutations are simple ones which include single amino acid substitution, and severe mutation can result from the substitutions of two amino acids [ 55 ]. Mutations in the PSEN1 gene increase the ratio of Aβ42/Aβ40 by decreasing Aβ40 levels. The results obtained by Sun et al. study demonstrated that C410Y or L435F mutations in PSEN1 knock-in mice increased the Aβ42/Aβ40 ratio due to a greater reduction in Aβ40 [ 56 ].

In contrast, PSEN-2 mutations are rare and play a minor role in Aβ production. Any mutation in PSEN-2 might have a severe effect on the Aβ 42/40 ratio, causing familial AD in the presence of normal PSEN-1 alleles. Some of the PSEN-2 mutations cause a significant increase in γ-secretase activity with an elevation in the Aβ-42 and Aβ 42/40 ratio level, such as N141I, T122P, M239V, and M239I, while others are rare polymorphisms and have no effect on Aβ-42, -40, and Aβ 42/40 ratio levels and are not considered as pathogenic mutations [ 53 , 57 ].

• Apolipoprotein E (ApoE)

ApoE protein is a glycoprotein expressed highly in the liver and brain astrocytes and some microglia and serves as a receptor-mediated endocytosis ligand for lipoprotein particles like cholesterol, which is essential for myelin production and normal brain function. The ApoE gene located on chromosome 19 has three isoforms, ApoE2, ApoE3, and ApoE4, due to single-nucleotide polymorphisms (SNPs) which cause changes in the coding sequence. The ApoEε4 allele is a strong risk factor for both EOAD and LOAD compared to ApoEε2 and ApoEε3 alleles that are associated with a lower risk and protective effect, respectively [ 58 ]. ApoEε4 plays an important role in Aβ deposition as a senile plaque and causes cerebral amyloid angiopathy (CAA), which is known as a marker for AD [ 59 ]. ApoEε4 was also shown to be associated with vascular damage in the brain, which leads to AD pathogenesis [ 60 ].

• ATP Binding Cassette Transporter A1 (ABCA1)

Adenosine triphosphate (ATP)-binding cassette transporter A1 (ABCA1) is part of a large ABC transporters family that regulate cholesterol efflux in the circulation, like apolipoproteins-AI (ApoAI), and into the brain, like ApoE. In addition, ABCA1 maintains the stability of ApoE lipidation and serves as a mediator for high-density lipoprotein (HDL) generation, which reflects its role in atherosclerosis and cardiovascular diseases. Studies on the AD mice model showed that ABCA1 deficiency increases amyloid plaques and eliminates the lipidation of ApoE [ 61 ]. In humans, a mutation in ABCA1 results in Tangier disease, which is characterized by low levels of high-density lipoprotein (HDL) and ApoAI in plasma, accumulation of cholesterol in tissues, and AD pathogenesis [ 62 ].

• Clusterin Gene (CLU) and Bridging Integrator 1 ( BIN1 )

In contrast to PSEN1 , PSEN2 , and APP mutations, which result in familial or EOAD, clusterin ( CLU) and Bridging Integrator 1 ( BIN1 ) genes are novel risk factors for LOAD. In 2009, Genome-Wide Association Studies (GWAS) identified the CLU gene located on chromosome 8, which is upregulated in the cortex and hippocampus of AD brains, in addition to AD cerebrospinal fluid (CSF) and plasma, which make the CLU a promising biomarker for AD. The CLU may play a protective role by interacting with Aβ and promoting its clearance, or a neurotoxic role by reducing Aβ clearance. The Aβ ratio values determine whether the CLU role is neuroprotective or neurotoxic [ 63 ].

BIN1 is a Bin-Amphiphysin-Rvs (BAR) adaptor protein that is involved in the production of membrane curvature and other endocytosis cellular functions. BIN1 has several isoforms: some are found in the brain, where they interact with different proteins such as clathrin, synaptojanin, and amphiphysin 1, and others in which they regulate synaptic vesicle endocytosis. Recently, BIN1 was recognized as the second most important risk factor for LOAD after ApoE, where it plays a role in Aβ production and as a tau and NFT pathology modulator [ 64 , 65 ].

• Evolutionarily Conserved Signaling Intermediate in Toll pathway (ECSIT)

A significant accumulation of Aβ in AD brains increases protein oxidation, which reflects the critical role of mitochondria in Aβ cytotoxicity and AD pathogenesis. Evolutionarily conserved signaling intermediate in Toll pathway (ECSIT) gene is located on chromosome 19 and is associated with increasing the risk of AD. ECSIT encodes the adapting protein that functions as a cytoplasmic and signaling protein and is responsible for stabilizing the mitochondrial respiratory complex. Moreover, the adaptor protein is involved in the activation of nuclear factor (NF)-κB, interferon regulatory factors (IRFs), and activating protein-1. Also, it is involved in coupling immune toll-like receptor (TLR), homeostatic bone morphogenetic pathway (BMP), and transforming growth factor-beta (TGF-b) pathways [ 66 , 67 ].

ECSIT interacts with mitochondrial proteins such as Lon protease homolog (LONP1) and glutaryl-CoA dehydrogenase (GCDH), which are involved in intra-mitochondrial proteolysis and redox signaling respectively, followed by interactions with AD seed nitric oxide synthase (NOS3). Moreover, studies have shown certain interactions of ECSIT with the AD genes ApoE , PSEN-1 , and PSEN-2 . These interactions support the role of ECSIT as a molecular link in oxidative stress, inflammation, and mitochondrial dysfunction in AD [ 66 , 68 ].

• Estrogen Receptor Gene (ESR)

AD affects both women and men, but nearly two-thirds of AD cases are women. Several studies have shown that women with AD experience worse mental deterioration than men. Additionally, on the genetic level, some genes’ variation, like the ApoE4 allele, significantly increases AD risk in women compared to men. Other studies documented that AD risk in women is associated with the loss of ovarian hormones during menopause due to the fact that estrogen regulates several activities in the brain, such as neurotransmission, neural development, survival, protection against oxidative stress, reduction of Aβ peptide levels, and attenuation of tau hyperphosphorylation. The estrogen activity is mediated through estrogen receptors (ERs) (intracellular, transmembrane, and membrane-bound ERs). The two major subtypes of these receptors are ERα and Erβ, which are encoded by two distinct genes and are located on chromosome 6 and 14, respectively. ERα receptor is found in the hypothalamus and amygdala, whereas ERβ receptors are in the hippocampus and cortex. Single nucleotide polymorphisms (SNPs) in ERβ and ERα genes may affect exogenous estrogen in older women and influence cognitive aging. PvuII (rs9340799) and Xbal (rs223493) are examples of SNPs found in ERα and are associated with AD and cognitive impairment. Also, several SNPs in ERβ have been proven to increase the risk of AD in women [ 69 , 70 , 71 , 72 ].

• Other Genes

Other genes’ polymorphism associated with increasing the risk of AD include vitamin D receptor (VDR) gene polymorphism, which affects the affinity of vitamin D to its receptor and may cause neurodegenerative diseases and neuronal damage [ 73 ]. Moreover, epigenetic factors like DNA methylation, histone, and chromatin modifications were demonstrated to be involved in AD [ 33 , 74 ].

5.2.3. Environmental Factors

Aging and genetic risk factors cannot explain all cases of AD. Environmental risk factors including air pollution, diet, metals, infections, and many others may induce oxidative stress and inflammation and increase the risk for developing AD. Herein, we report the most important environmental factors and their relationships with AD [ 75 , 76 ].

• Air Pollution

The air pollution is characterized by modifying the nature of the atmosphere through the introduction of chemical, physical, or biological pollutants. It is associated with respiratory and cardiovascular diseases and recently, its association with AD was documented. Six air pollutants have been defined by National Ambient Air Quality Standards (NAAQSs) in the USA as a threat to human health, including ozone (O 3 ), nitrogen oxides (NO x ), carbon monoxide (CO), particulate matter (PM), sulfur dioxide (SO 2 ), and lead. Studies on animals and cellular models have shown that an exposure to high levels of air pollution can result in a damage to the olfactory mucosa and bulb, in addition to the frontal cortex region, similar to that observed in AD. In individuals exposed to air pollutants, there is a link between oxidative stress, neuroinflammation, and neurodegeneration, with the presence of hyper-phosphorylated tau and Aβ plaques in the frontal cortex. The air pollution can cause an increase in Aβ 42 formation, accumulation, and impaired cognitive function [ 77 , 78 ].

In recent years, the number of studies on the role of nutrition in AD have been increased. Several dietary supplements such as antioxidants, vitamins, polyphenols, and fish were reported to decrease the risk of AD, whereas saturated fatty acids and high-calorie intake were associated with increasing the risk of AD [ 79 ]. The food processing causes degradation of heat-sensitive micronutrients (e.g., vitamin C and folates), loss of large amounts of water, and formation of toxic secondary products (advanced glycation end products, AGEs) from non-enzymatic glycation of free amino groups in proteins, lipids, and nucleic acids. The toxic effect of AGEs is referred to as their ability to induce oxidative stress and inflammation by modifying the structure and function of the cell surface receptors and body proteins. Different studies demonstrated that elevated AGEs serum level is associated with cognitive decline and progression of AD. The AGE receptor (RAGE) is located in different places within the body, including microglia and astrocytes, and was established to be overexpressed in the brain of AD patients and serve as a transporter and a cell surface receptor for Aβ [ 80 ]. Malnutrition is another risk factor for AD. Deficiency in nutrients such as folate, vitamin B12, and vitamin D may cause a decrease in cognitive function, in addition to the fact that patients with AD suffer from problems associated with eating and swallowing, which may increase the risk of malnutrition [ 81 ].

Metals are found in nature and biological systems and can be divided into bio-metals that have a physiological function in living organisms (e.g., copper, zinc, and iron), and toxicological metals which do not possess any biological function (e.g., aluminum and lead) [ 82 ]. Aluminum is used significantly in the industries such as processed foods, cosmetics, medical preparations, medicines, and others. In the body, aluminum is bound to plasma transferrin and to citrate molecules that can mediate the transfer of aluminum to the brain. Studies demonstrated that Al accumulates in the cortex, hippocampus, and cerebellum areas, where it interacts with proteins and causes misfolding, aggregation, and phosphorylation of highly phosphorylated proteins like tau protein, characteristic of AD [ 83 ]. Lead competes with the binding site of bio-metals like calcium and can cross the blood–brain barrier (BBB) rapidly, where it can modify neural differentiation and synaptogenesis and cause severe damage. Studies revealed that an acute exposure to lead was associated with AD and caused an increase of β-secretase expression and Aβ accumulation. Cadmium is a carcinogenic water-soluble metal that can cross the BBB and cause neurological diseases like AD. Results have demonstrated that Cadmium ions are involved in the aggregation of Aβ plaques and the self-aggregation of tau in the AD brain. The data accumulated on metals support the notion that they are among the risk factors involved in the development of AD [ 84 ].

Chronic infections to the central nervous system (CNS) can cause an accumulation of Aβ plaques and NFT, therefore, they are included among the risk factors in AD. Studies by Dr. Itzhaki showed that the DNA of herpes simplex virus (HSV-1) was found in patients with ApoE-ε4 allele carriers, which explains the high risk for developing AD. HSV-1 can replicate in the brain, which can result in the activation of the inflammatory response and an increase in Aβ deposition, resulting in damage to neurons and gradual development of AD. On the other hand, the study results by Miklossy and Balin’s have revealed the role of chronic bacterial infections in AD. For example, syphilitic dementia caused by spirochete bacteria ( Treponema pallidum ), which are accumulated in the cerebral cortex, produced lesions similar to neurofibrillary tangles, which led to devastating neurodegenerative disorders. Besides, Chlamydia pneumonia bacterium can trigger late-onset AD by activation of astrocyte and cytotoxic microglia, disrupt calcium regulation and apoptosis, resulting in deterioration of cognitive function, and increase the risk of AD [ 85 , 86 , 87 ].

5.2.4. Medical Factors

Several risk factors are related to the development of Alzheimer’s disease. Adding to this list, older people with AD usually have medical conditions such as cardiovascular disease (CVD), obesity, diabetes, and others. All of these conditions are associated with increased risk of AD [ 88 , 89 ].

• Cardiovascular Disease (CVDs)

CVDs are recognized as an important risk factor for AD, such as the stroke that is associated with increased risk of dementia due to a neural tissue loss, which enhances degenerative effect and influences amyloid and tau pathology. Atrial fibrillation also causes embolisms which leads to stroke and a decrease in memory and cognitive functions. Moreover, heart failure affects the pumping function of the heart and results in insufficient blood supply to the body and hypo-perfusion of the brain that leads to hypoxia and neural damage. The coronary heart disease’s hypothesis indicates that atherosclerosis, peripheral artery disease, hypo-perfusion, and emboli are all related to increased risk of AD. Hypertension is associated with thickening of vessel walls and narrowing of the lumen which reduce the cerebral blood flow, and in chronic cases, it may cause cerebral edema, which all participate as risk factors for AD and CVD. The CVD is a modifiable risk factor and by focusing on its relationship with AD, a pathway to prevent and delay the disease can be obtained [ 89 , 90 ].

• Obesity and Diabetes

Obesity is a term used for too much body fat in individuals due to consuming more calories than they burn and can be calculated by using the body mass index (BMI). Increasing the body fat is associated with a decreased brain blood supply which promotes brain ischemia, memory loss, and vascular dementia. The obesity, unhealthy diet, and other factors can cause impaired glucose tolerance (IGT) or diabetes, which is characterized by hyperglycemia that affects peripheral tissues and blood vessels. Chronic hyperglycemia can induce cognitive impairment as a result of increasing amyloid-beta accumulation, oxidative stress, mitochondrial dysfunction, and neuroinflammation. Obesity is characterized by increasing pro-inflammatory cytokines secretions from adipose tissue, which stimulate macrophages and lymphocytes and eventually lead to local and systemic inflammation. This inflammation promotes insulin resistance, hyperinsulinemia, and as a consequence, hyperglycemia. Obesity is a well-known risk factor for type 2 diabetes, CVDs, and cancer, which are identified as risk factors for dementia and AD. The brain inflammation causes an increase in microglia and results in reduced synaptic plasticity and impaired neurogenesis. Microglia can affect insulin receptor substrate 1 (IRS-1) and block intracellular insulin signaling, which has an important role in neural health. Therefore, alteration in insulin action can result in Aβ accumulation and reduce the tau protein degradation associated with AD [ 91 , 92 , 93 , 94 ].

6. Treatment

Currently, Alzheimer’s disease cases worldwide are reported to be around 24 million, and in 2050, the total number of people with dementia is estimated to increase 4 times. Even though AD is a public health issue, as of now, there is only two classes of drugs approved to treat AD, including inhibitors to cholinesterase enzyme (naturally derived, synthetic and hybrid analogues) and antagonists to N -methyl d -aspartate (NMDA). Several physiological processes in AD destroy Ach-producing cells which reduce cholinergic transmission through the brain. Acetylcholinesterase inhibitors (AChEIs), which are classified as reversible, irreversible, and pseudo-reversible, act by blocking cholinesterase enzymes (AChE and butyrylcholinesterase (BChE)) from breaking down ACh, which results in increasing ACh levels in the synaptic cleft [ 95 , 96 , 97 ]. On the other hand, overactivation of NMDAR leads to increasing levels of influxed Ca 2+ , which promotes cell death and synaptic dysfunction. NMDAR antagonist prevents overactivation of NMDAR glutamate receptor and hence, Ca 2+ influx, and restores its normal activity. Despite the therapeutic effect of these two classes, they are effective only in treating the symptoms of AD, but do not cure or prevent the disease [ 98 , 99 ]. Unfortunately, only a few clinical trials on AD have been launched in the last decade and their outcome was a big failure. Several mechanisms have been proposed to understand AD pathology in order to modify its pathway and develop successful treatments, which include abnormal tau protein metabolism, β-amyloid, inflammatory response, and cholinergic and free radical damage [ 30 , 100 ]. On the other hand, most AD modifiable risk factors such as cardiovascular or lifestyle habits can be prevented without medical intervention. Studies showed that physical activity can improve the brain health and reduce AD by activating the brain vascularization, plasticity, neurogenesis, and reducing inflammation by decreasing Aβ production, which all result in improving cognitive function in older people. Moreover, the Mediterranean diet (MD), intellectual activity, and higher education all may reduce the progression of AD and memory loss and increase the brain capacity and cognitive functions. Several studies revealed that multi-domain intervention which includes lifestyle (diet, exercise, and cognitive training), depression of AD symptoms, and controlling cardiovascular risk factors, can increase or maintain cognitive function and prevent new cases of AD in older people [ 101 ]. Herein, we summarize the currently available drugs and theories for the development of new therapies for AD.

6.1. Symptomatic Treatment of AD

6.1.1. cholinesterase inhibitors.

According to the cholinergic hypothesis, AD is due to the reduction in acetylcholine (ACh) biosynthesis. Increasing cholinergic levels by inhibiting acetylcholinesterase (AChE) is considered one of the therapeutic strategies that increases cognitive and neural cell function. AChEIs are used to inhibit acetylcholine degradation in the synapses, which results in continuous accumulation of ACh and activation of cholinergic receptors. Tacrine (tetrahydroaminoacridine) ( 1, Figure 4 ) was the first FDA (Food and Drug Administration)-approved cholinesterase inhibitor drug for the treatment of AD, which acts by increasing ACh in muscarinic neurons, but it exited the market immediately after its introduction due to a high incidence of side effects like hepatotoxicity and a lack of benefits, which was observed in several trials. Later on, several AChEIs were introduced, such as donepezil ( 2 , Figure 4 ), rivastigmine ( 3 , Figure 4 ), and galantamine ( 4 , Figure 4 ), and are currently in use for the symptomatic treatment of AD [ 34 , 97 , 102 , 103 ]. Another strategy that may help in the treatment of AD is increasing choline reuptake and as a result, increasing acetylcholine synthesis at the presynaptic terminals. This can be achieved by targeting choline transporter (CHT1) which is responsible for supplying choline for the synthesis of ACh. Developing drugs that are capable of increasing CHT1 at the plasma membrane may become the future therapy of AD [ 36 ].

The chemical structures of approved drugs for symptomatic treatment of AD (tacrine 1 , donepezil 2 , rivastigmine 3 , galantamine 4 , and memantine 5 ) and disease-modifying compounds that entered clinical trials (semagacestat 6 , avagacestat 7 , tarenflurbil 8 , lanabecestat 9 , verubecestat 10 , atabecestat 11 , umibecestat 12 , methylene blue 13 , tideglusib 14 , and saracatinibin 15 ).

Donepezil ( 2 , Figure 4 ) is an indanonebenzylpiperidine derivative and a second generation of AChEIs and is considered the leading drug for AD treatment. Donepezil binds to acetylcholinesterase reversibly and inhibits acetylcholine hydrolysis, which leads to a higher concentration of ACh at the synapses. The drug is well-tolerated with mild and transient cholinergic side effects which are related to the gastrointestinal and nervous systems. It should be noted that donepezil is used to treat symptoms of AD such as improving cognition and behavior without altering the AD progression [ 104 , 105 , 106 ].

• Rivastigmine

Rivastigmine ( 3 , Figure 4 ) is a pseudo irreversible inhibitor of AChE and butyrylcholinesterase (BuChE) that acts by binding to the two active sites of AChE (anionic and estearic sites), which results in preventing ACh metabolism. BuChE is found mostly in glial cells with only 10% of AChE activity in the normal brain, whereas in the AD brain, its activity is increased to 40–90%, while ACh activity is reduced simultaneously, which suggests that BuChE action may indicate a moderate to severe dementia. Rivastigmine dissociates more slowly than AChE, which is why it is called a pseudo-irreversible, and it undergoes metabolism at the synapse by AChE and BuChE. The drug is used in mild to moderate AD cases. It improves cognitive functions and daily life activities. Oral administration of the drug is associated with adverse effects such as nausea, vomiting, dyspepsia, asthenia, anorexia, and weight loss. In many cases, these side effects are the main reason behind stopping taking the medicine, however, they can be settled down in time and consequently, the drug becomes more tolerated. Rivastigmine can be delivered by transdermal patches for controlled and continuous delivery of the drug through the skin, with enhanced tolerability and caregiver satisfaction. Also, the patches can deliver a lower dosage compared to pills, which results in reduced side effects. Most AD patients suffer from memory loss and swallowing problems which affect their compliance in administering oral drugs at regular intervals. Therefore, the use of transdermal patches is the most appropriate method for delivering the drug in AD patients [ 107 , 108 , 109 , 110 ].

• Galantamine (GAL)

Galantamine ( 4 , Figure 4 ) is considered a standard first-line drug for mild to moderate AD cases. GAL is a selective tertiary isoquinoline alkaloid with a dual mechanism of action in which it acts as a competitive inhibitor of AChE and can bind allosterically to the α-subunit of nicotinic acetylcholine receptors and activate them. GAL can improve behavioral symptoms, daily life activities, and cognitive performance with good efficacy and tolerability, similar to other AChE inhibitors. Several delivery systems were developed to improve the drug delivery to the brain: Wahba et al. attached GAL to ceria-containing hydroxyapatite particles for selective delivery of the drug to the affected regions in the brain. Misra et al. and Fornaguera et al. used solid-lipid nanoparticles and nano-emulsification approaches respectively, to carry GAL hydrobromide. The results of these studies demonstrated a promising strategy for safe delivery of the drug. Hanafy et al. developed nasal GAL hydrobromide/chitosan complex nanoparticles which showed good pharmacological efficacy, while Woo et al. utilized the patch system as a carrier for a controlled release dosage form of the drug [ 111 , 112 , 113 , 114 ].

6.1.2. N -methyl d -aspartate (NMDA) Antagonists

NMDAR is believed to have a dominant role in the pathophysiology of AD. NMDAR stimulation results in Ca 2+ influx which activates signal transduction and as a consequence, it triggers gene transcription essential for the formation of a long-term potentiation (LTP), which is important for synaptic neurotransmission, plasticity, and memory formation. Over-activation of NMDARs causes an abnormal level of Ca 2+ signaling and overstimulation of glutamate, which is the primary excitatory amino acid in the CNS, which results in excitotoxicity, synaptic dysfunction, neuronal cell death, and a decline in cognitive functions. Several NMDAR uncompetitive antagonists have been developed and entered clinical trials, however, most of them failed due to low efficacy and side effects. Memantine ( 5 , Figure 4 ) is the only approved drug in this category to treat moderate to severe AD; in addition, other NMDAR uncompetitive antagonist compounds are being developed, such as RL-208 (3,4,8,9-tetramethyltetracyclo [4.4.0.0 3,9 .0 4,8 ]dec-1-yl)methylamine hydrochloride), a polycyclic amine compound that may possess a promising therapeutic effect in age-related cognitive problems and AD [ 115 , 116 , 117 ].

Memantine ( 5 , Figure 4 ) is a low-affinity uncompetitive antagonist of the NMDAR, a subtype of glutamate receptor that prevents over-activation of the glutaminergic system involved in the neurotoxicity in AD cases. Memantine is used for the treatment of moderate to severe AD alone or in combination with AChEI. The drug is safe and well-tolerated, it blocks the excitatory receptor without interfering with the normal synaptic transmission due to memantine’s low affinity, where it is displaced rapidly from NMDAR by high concentrations of glutamate, thus avoiding a prolonged blockage. The latter is associated with high side effects, especially on learning and memory [ 99 , 118 ].

6.2. Promising Future Therapies

6.2.1. disease-modifying therapeutics (dmt).

Disease-modifying treatment or therapy (DMT) alter the progression of AD by working on several pathophysiological mechanisms. This is in contrast to symptomatic therapy which works on improving the cognitive functions and decreasing symptoms such as depression or delusions without affecting or modifying the disease. DMTs, either immunotherapies or small molecules, are administrated orally and are being developed to prevent AD or decrease its progression. Several DMTs have been developed and entered the clinical trials, such as AN-1792, a synthetic Aβ peptide (human Aβ 1–42 peptide of 42-amino acids with the immune adjuvant QS-21) and the first active immunotherapy for AD which entered phase II clinical trials and discontinued due to a meningoencephalitis side effect in 6% of the patients. Other drugs were also developed and failed in the clinical trials, including the anti-Aβ antibody (solanezumab and bapineuzumab), γ-Secretase inhibitors (semagacestat 6 , avagacestat 7 , and tarenflurbil 8 ) ( Figure 4 ) and β-secretase inhibitors (BACE) (Lanabecestat 9, verubecestat 10 , and atabecestat 11 ) ( Figure 4 ). DMTs failures are due to several factors, such as starting therapy too late, giving treatment for the wrong main target, use of inappropriate drug doses, and misunderstanding of the pathophysiology of AD. Several immunotherapies described in Table 1 have been developed over decades, including: CAD106, an active Aβ immunotherapy that induces Aβ antibodies in animal models and consists of multiple copies of Aβ1–6 peptide coupled to Qβ coat protein, a virus-like particle, and is still in clinical trials, and CNP520 (umibecestat, 12 ) ( Figure 4 ), a small molecule that inhibits beta-scretase-1 (BACE-1) and therefore inhibits Aβ production. CNP520 was found to reduce Aβ plaque deposition and Aβ levels in the brain and CSF in rats, dogs, and healthy adults ≥ 60 years old, and is still under clinical trials. Furthermore, aducanumab, gantenerumab, and crenezumab are all human Aβ monoclonal antibody that bind with high affinity to aggregated Aβ, and they are still under study in the clinical phases with other DMTs described in Table 1 [ 6 , 119 , 120 , 121 , 122 , 123 , 124 ].

Disease modifying agents for the treatment of Alzheimer’s disease in clinical trials.

Another class targeting the α-secretase enzyme was developed and has been considered as therapeutic agents. α-secretase modulators or activators stimulate the cleavage of APP. There is little knowledge about the activation pathway, but research assumes that it is promoted by the phosphatidylinositol 3-kinase (PI3K)/Akt pathway or by γ-aminobutyric acid (GABA) receptor signaling. Targeting these pathways may give potential therapeutic agents for AD [ 6 ].

In addition to the anti-amyloid agents, the tau aggregation inhibitors are another promising DMT. The tau is a biomarker for neurofibrillary tangles (NFT) in AD and naturally modulates microtubule stability, signaling pathways, and axonal transport. A modification in tau conformation results in toxic aggregation. Therefore, the prevention of tau aggregation becomes an interesting approach for drug discovery to reduce AD progression. Studies in mice have shown that tau oligomers cause mitochondrial damage, disruption of neuronal signaling, synaptic loss, and memory impairment. Disease-modifying therapeutics (DMT) like small molecules can be used to inhibit the initial step in the tau aggregation and thereby reduce its accumulation. Methylene blue ( 13 , Figure 4 ) is a blue dye that inhibits the tau aggregation and entered phase II clinical trials to treat mild to moderate AD. Upon administration of the drug, the color of the urine becomes blue, which indicates a lack of binding, and because of that, the study was highly criticized. Other approaches suggest that an inhibition of specific kinases such as glycogen synthase kinase 3 (GSK3β) can inhibit tau hyperphosphorylation and block tau deposition. Examples of these entities include tideglusib ( 14 , or NP-031112 (NP-12), Figure 4 ), a thiazolidinedione-derived compound, lithium, pyrazolopyridines, pyrazolopyrazines, sodium valproate, and others. Another protein kinase inhibitor is saracatinib (AZD0530) ( 15 , Figure 4 ), which acts by inhibiting tyrosine kinase and has shown good results in improving memory in transgenic mice and is currently in phase II trials [ 125 , 126 , 127 ]. Davidowitz et al. utilized the hatu mouse model of tauopathy to study the efficacy of a lead small molecule in preventing tau accumulation. The study results demonstrated a significant reduction in tau levels and its phosphorylated form levels, which indicates the ability to inhibit the entire pathway of the tau aggregation by using an optimized lead compound [ 128 ].

6.2.2. Chaperones

Protein misfolding caused by mutations or environmental factors results in aggregations that are toxic, and their accumulation causes neurodegenerative disorders like AD. Naturally, cells develop protein quality control (PQC) systems that inhibit protein misfolding before exerting their toxic effects. With age, this balance is altered and the misfolded shapes overwhelm the PQC system, which in turn activates the unfolded protein response (UPR) that stops the protein synthesis and increases chaperone production. Generally, the cells in humans have proteins that are responsible for other proteins to function and arrive to their destination in the cell. These proteins are called “chaperones”. Chaperones are involved in protein folding and improvement of the PQC system efficiency. Therefore, it is considered a promising candidate for treating neurodegenerative diseases. It can be classified into three groups: (1) molecular chaperones, which are proteins that assist other nonnative proteins in their folding or unfolding, like overexpression of heat shock proteins (Hsps) that serve as neuroprotective agents, (2) pharmacological chaperones, which are low molecular weight compounds (enzymes or receptor-ligand or selective binding molecules) that induce refolding of proteins, stabilize their structure, and restore their function, and (3) chemical chaperones, also low molecular weight compounds, which are divided into two groups, osmolytes and hydrophobic compounds. The members in these two groups have no specific mechanism of action and need high concentrations to exert their therapeutic effects [ 129 ].

• Heat Shock Proteins (Hsps)

The causes for most neurodegenerative diseases are protein misfolding and aggregation, which lead to cell death. The molecular chaperone can be intracellular, such as in the case of heat shock proteins (e.g., Hsp40, Hsp60, Hsp70, Hsp90, Hsp100, and Hsp110), and extracellular, such as clustering and alpha-macroglobulin. HSPs play an essential role in the protein folding process and protect cells from harmful stress-related events. There are two families of Hsps: (a) classic Hsps that possess an ATP-binding site with a molecular weight of 60 kD or more. This family includes Hsp100, Hsp90, Hsp70, and Hsp60, and (b) the small Hsps such as αB-crystalline, Hsp27, Hsp20, HspB8, and HspB2/B3 that lack ATP-binding site, with a molecular weight of 40 kD or less. These proteins can assist other Hsps in their refolding function. Failure of these mechanisms can lead to oxidative stress, mitochondrial dysfunction, and many other conditions that cause damage, a loss of neurons, and a progression of neurodegenerative diseases. Different HSPs can block the aggregation process of misfolded proteins, like amyloidogenic proteins (Aβ and tau), and promote their degradation [ 130 , 131 ].

Hsp60 plays an important role in mitochondrial protein folding. Its role in AD is not clear, some believe that the protein has a protective role and others think it has a harmful effect where it can be over-expressed by activated microglia, which increases pro-inflammatory factors such as toll-like receptor 4 (TLR-4) that stimulate neuronal cell death. Therefore, inhibiting activated microglia and Hsp60 expression is a promising strategy for preventing neurodegenerative diseases. Examples of compounds that inhibit Hsp60 are mizoribine (Immunosuppressant) ( 16 , Figure 5 ) and pyrazolopyrimidine EC3016 ( 17 , Figure 5 ). Both compounds act by blocking ATPase activity of Hsp60 and inhibiting protein folding. On the other hand, avrainvillamide, a fungal metabolite ( 18 , Figure 5 ), and epolactaene, a bacterial metabolite ( 19 , Figure 5 ), act by binding to the Hsp60′s cysteine residues and inhibit its folding activity. However, Hsp60’s role in AD remains controversial and there is a need for more investigations to understand its role [ 130 ].

The chemical structures of different chaperone molecules: Mizoribine 16 , EC3016 17 , Avrainvillamide 18 , Epolaztaene 19 , MKT-077 20 , YM-01 21 , JG-98 22 , Radicicol 23 , Geldanamycin 24 , 17-AAG 25 , Pochoxime C (OS47720) 26 , R55 27 , and OT1001 28 .

Studies have shown that Hsp70 binds to Aβ42 and prevents self-aggregation. Martín-Peña et al. studied two isoforms of Hsp70, cytosolic and extracellular, in Drosophila flies AD models and evaluated their protective role against memory decline that results from Aβ42 aggregation. The animal studies showed that Hsp70 has a dual function: intracellularly and extracellularly, where it protects against Aβ42 neurotoxicity and synaptic loss. In addition to its ability to bind to tau and its hyper-phosphorylated form and prevent its formation, it decreases aggregation and promotes tau binding to microtubules. Hsp70 acts by activating microglia, insulin-degrading enzyme, and tumor growth facto r- β1, which degrades β-amyloids and prevents memory impairments [ 132 , 133 ]. Some studies in AD brain tissue demonstrated an overexpression of Hsp70 levels and a correlation with the presence of activated glia and stressed neurons. Also, it was found that Hsp70 is associated with extracellular deposits in AD. Drug therapies targeting Hsp70, mainly referring to previous anticancer drugs which target and inhibit Hsp70 ATP-binding site, are considered as candidates in AD treatment due to their ability to reduce tau levels in vitro and ex vivo. MKT-077(1-ethyl-2-(( Z )-(( E )-3-ethyl-5-(3-methylbenzo [ d ]thiazol-2(3 H )-ylidene)-4-oxothiazolidin-2-ylidene)methyl)pyridin-1-ium chloride) ( 20 , Figure 5 ), is an anticancer rhodacyanine compound that binds to mortalin, a mitochondrial Hsp70 site, and acts as an anti-proliferative agent, but the use of this compound was stopped due to toxicity side effects and low BBB penetration. On the other hand, YM-01 ( 21 , Figure 5 ), a more potent MKT-077 derivative, was developed with a single replacement of the ethyl group on the pyridinium nitrogen of MKT-077 with a methyl group. JG-98 ( 22 , Figure 5 ) is also an MKT-077 derivative with a 60-fold higher binding affinity to Hsp70 than YM-01 [ 130 , 134 , 135 , 136 ].

Hsp90 is another type of HSP that regulates the tau phosphorylation and dephosphorylation. An inhibition of Hsp90 results in a decrease in phosphorylation of tau due to a reduction in tau kinases, which is thought to be responsible for tau pathogenesis when it is hyperactivated. Hsp90 inhibitors are used for cancer therapy, but recently, they are considered as promising therapy for AD. Radicicol (RDC) ( 23 , Figure 5 ) and geldanamycin (GA) ( 24 , Figure 5 ) are Hsp90 inhibitors. GA is a natural antifungal compound and the first discovered Hsp90 inhibitor. Studies on this inhibitor were stopped due to its toxicity. On the other hand, 17-AAG (17-(Allylamino)-17-demethoxygeldanamycin) ( 25 , Figure 5 ) is a GA derivative with a lower toxicity and better pharmacokinetic profile that showed a good improvement of the cognitive function by inducing other HSPs, like Hsp70, in addition to reducing NFTs in the transgenic mouse model by blocking the tau phosphorylation pathway, indirectly [ 137 , 138 ]. Pochoxime C (OS47720) ( 26 , Figure 5 ) is also a CNS-permeable Hsp90 inhibitor that showed good safety and efficacy profiles when tested in the AD mouse model. Studies revealed that OS47720 acts by strengthening synaptic function via heat shock factor (HSF-1) activation and dependent transcriptional events [ 139 ].

The combined studies demonstrate that targeting HSPs is a promising strategy to develop drugs with a new mechanism of action for reducing pathogenic tau levels and restoring normal tau homeostasis.

• Vacuolar sorting protein 35 (VPS35)

An accumulation of proteins in neurons and glial cells leads to disturbance of cellular protein homeostasis. The endosomal-lysosomal system is responsible for transporting proteins for recycling and degradation. Any malfunction in the system can lead to several diseases, such as Alzheimer’s disease. Retromer is a complex of regulator proteins composed of sorting nexin (SNX1, 2, 5, 6) and vacuolar sorting proteins (VPS 26, 29, 35), which are responsible for transporting cargo molecules from the endosome to the trans -Golgi network. A loss of retromer’s function results in the downregulation of VPS35, which can increase Aβ formation, induce cognitive impairments, and cause synaptic dysfunction, which is reported in AD patients [ 140 , 141 ]. A study on 3xTg mice brains was conducted to evaluate the effect of VPS35 overexpression on memory function. The study showed that a significant reduction of the Aβ peptide and tau neuropathology (soluble, insoluble, and phosphorylated tau) was associated with overexpression of VPS35, in addition to a reduction in neuroinflammation and ameliorating synaptic dysfunction [ 142 ]. Therefore, VPS35 is an important promising therapeutic target for AD treatment. A small pharmacological chaperones molecule called R55 (thiophene-2,5-diylbis(methylene) dicarbamimidothioatedihydrochloride) ( 27 , Figure 5 ), a thiophenethiourea derivative, can enhance retromer stability and function by increasing retromer proteins, shifting AOO from the endosome, and reducing pathogenic processing of APP, which may serve as a promising therapeutic molecule for neurodegenerative diseases [ 143 ].

Studies demonstrated that the accumulation of gangliosides has been associated with misfolding and aggregation of proteins in neurodegenerative diseases. Abnormal levels of mono-sialoganglioside (GM1, GM2, and GM3) have been reported in AD brains. Mutant forms of Aβ, like Dutch mutant APPE693Q, showed susceptibility to pro-aggregation properties of GM2 and GM3, resulting in the formation of Aβ peptides complexes with gangliosides (ganglioside-bound Aβ (GAβ) peptide) and subsequently leading to an acceleration of aggregation and accumulation of Aβ peptides.

β-hexosaminidase (β-hex) is a lysosomal enzyme that acts by catabolizing GM2 ganglioside, and increasing its activity can lead to a reduction of GM2 levels and Aβ aggregation and accumulation. Small molecules like pharmacological chaperones (PC) can selectively bind and stabilize wild-type proteins and restore their normal folding. OT1001 ( 28 , Figure 5 ) is an iminosugar PC that targets β-hex and increases its level in the brain and reduces GAβ pathology. Studies on Dutch APPE693Q transgenic mice showed that OT1001 has good pharmacokinetics, brain penetration ability, and tolerability, with lower side effects. These make the compound a good drug candidate for increasing the β-hex activity [ 144 ].

6.2.3. Natural Extract

For a long time, natural compounds have been used as therapeutic agents for several pathological diseases, and recent studies showed that they possess a neuroprotective effect. In vitro and in vivo studies have proven that natural compounds possess a therapeutic potential for AD, which allowed some of them to enter the clinical trials stages. Nicotine was the first natural compound entered in the clinical trials for AD, then other compounds like vitamins C, E, and D gained more attention and interest due to their protective role against neuroinflammation and oxidative damage. Recently, bryostatin, a macrolide lactone extract from bryozoan Bugula neritina, has been evaluated and showed the ability to induce α-secretase activity, reduce Aβ production, and enhance the learning and memory in an AD mice model [ 145 ]. Other natural compounds used in folk medicine (traditional Chinese medicine (TCM)) demonstrated a great potential in treating AD by acting on several mechanisms, as shown in Table 2 below [ 146 ].

Natural compounds used in folk medicine and their mechanism of actions.

7. Conclusions

Alzheimer’s disease is now considered a world health concern; as a consequence, the National Institute on Aging—Alzheimer’s Association reclassified and updated the 1984 NINCDS-ADRDA criteria for higher specificity, sensitivity, and early identification of patients at risk of developing AD. Several criteria have been proposed for a more accurate diagnosis of AD, including clinical biomarkers, bodily fluids, and imaging studies. Despite that, the treatment of AD remains symptomatic, without alteration in the disease’s prognosis. Inhibitors to cholinesterase enzyme such as galantamine, donepezil, and rivastigmine, and NMDA antagonists such as memantine, improve memory and alertness but do not prevent progression. Several studies have shown that modification in lifestyle habits like diet and exercise can improve brain health and reduce AD without medical intervention and is considered as a first-line intervention for all AD patients. Recently, the research is focusing on targeting the pathological features of AD such as Aβ and p-tau. Future therapies such as disease-modifying treatment can alter the progression of AD by targeting the Aβ pathway, and many drugs have entered the clinical trials, like AN-1792, solanezumab, bapineuzumab, semagacestat, avagacestat, and tarenflurbil, but failed in demonstrating efficacy in the final clinical stages. Other DMTs are still under investigation, such as those targeting Aβ and tau pathologies, such as aducanumab, gantenerumab, crenezumab, tideglusib, lithium, and others. Other promising compounds called chaperones like heat shock proteins and vacuolar sorting protein 35 (VPS35) function by assisting other proteins to function normally and to arrive at their destination in the cell safely, and therefore can be used as a treatment for neurodegenerative diseases. Moreover, the natural extracts used in folk Chinese medicine showed great potential in treating AD by acting on several mechanisms’ pathways. In conclusion, the success of AD treatment depends on its early administration and patient monitoring for disease progression using biomarkers diagnosis. Future therapies that target tau pathology and the use of combination therapy may have a potential to slow the progression of AD pathology. Designing a potent, selective, and effective drug is urgently needed to treat patients with AD and those at risk for developing the disease.

Author Contributions

Literature survey and first draft writing were done by Z.B., and final draft, including the revisions, were accomplished by R.K. All authors have read and agreed to the published version of the manuscript.

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Alzheimer’s Takes a Financial Toll Long Before Diagnosis, Study Finds

New research shows that people who develop dementia often begin falling behind on bills years earlier.

By Ben Casselman

Long before people develop dementia, they often begin falling behind on mortgage payments, credit card bills and other financial obligations, new research shows.

A team of economists and medical experts at the Federal Reserve Bank of New York and Georgetown University combined Medicare records with data from Equifax, the credit bureau, to study how people’s borrowing behavior changed in the years before and after a diagnosis of Alzheimer’s or a similar disorder.

What they found was striking: Credit scores among people who later develop dementia begin falling sharply long before their disease is formally identified. A year before diagnosis, these people were 17.2 percent more likely to be delinquent on their mortgage payments than before the onset of the disease, and 34.3 percent more likely to be delinquent on their credit card bills. The issues start even earlier: The study finds evidence of people falling behind on their debts five years before diagnosis.

“The results are striking in both their clarity and their consistency,” said Carole Roan Gresenz, a Georgetown University economist who was one of the study’s authors. Credit scores and delinquencies, she said, “consistently worsen over time as diagnosis approaches, and so it literally mirrors the changes in cognitive decline that we’re observing.”

The research adds to a growing body of work documenting what many Alzheimer’s patients and their families already know: Decision-making, including on financial matters, can begin to deteriorate long before a diagnosis is made or even suspected. People who are starting to experience cognitive decline may miss payments, make impulsive purchases or put money into risky investments they would not have considered before the disease.

“There’s not just getting forgetful, but our risk tolerance changes,” said Lauren Hersch Nicholas, a professor at the University of Colorado School of Medicine who has studied dementia’s impact on people’s finances. “It might seem suddenly like a good move to move a diversified financial portfolio into some stock that someone recommended.”

People in the early stages of the disease are also vulnerable to scams and fraud, added Dr. Nicholas, who was not involved in the New York Fed research. In a paper published last year , she and several co-authors found that people likely to develop dementia saw their household wealth decline in the decade before diagnosis.

The problems are likely to only grow as the American population ages and more people develop dementia. The New York Fed study estimates that 600,000 delinquencies will occur over the next decade as a result of undiagnosed memory disorders.

That probably understates the impact, the researchers argue. Their data includes only issues that show up on credit reports, such as late payments, not the much broader array of financial impacts that the diseases can cause. Wilbert van der Klaauw, a New York Fed economist who is another of the study’s authors, said that after his mother was diagnosed with Alzheimer’s, his family discovered parking tickets and traffic violations that she had hidden.

“If anything, this is kind of an underestimate of the kind of financial difficulties people can experience,” he said.

Shortly before he was diagnosed with Alzheimer’s, Jay Reinstein bought a BMW he could not afford.

“I went into a showroom and I came home with a BMW,” he said. “My wife was not thrilled.”

At the time, Mr. Reinstein had recently retired as assistant city manager for Fayetteville, N.C. He had been noticing memory issues for years, but dismissed them as a result of his demanding job. Only after his diagnosis did he learn that friends and colleagues had also noticed the changes but had said nothing.

Mr. Reinstein, 63, is fortunate, he added. He has a government pension, and a wife who can keep an eye on his spending. But for those with fewer resources, financial decisions made in the years before diagnosis can have severe consequences, leaving them without money at the time when they will need it most. The authors of the New York Fed study noted that the financial effects they saw predated most of the costs associated with the disease, such as the need for long-term care.

The study expands on past research in part through its sheer scale: Researchers had access to health and financial data on nearly 2.5 million older Americans with chronic health conditions, roughly half a million of whom were diagnosed with Alzheimer’s or related disorders. (The records were anonymized, allowing researchers to combine the two sets of data without having access to identifying details on the individual patients.)

The large amount of data allowed researchers to slice the data more finely than in past studies, looking at the impact of race, sex, household size and other variables. Black people, for example, were more than twice as likely as white people to have financial problems before diagnosis, perhaps because they had fewer resources to begin with, and also because Black patients are often diagnosed later in the course of the disease.

The researchers hoped that the data could eventually allow them to develop a predictive algorithm that could flag people who might be suffering from impaired financial decision-making associated with Alzheimer’s disease — although they stressed that there were unresolved questions about who would have access to such information and how it would be used.

Until then, the researchers said, their findings should be a warning to older Americans and their families that they should prepare for the possibility of an Alzheimer’s diagnosis. That could mean taking steps such as granting a trusted person financial power of attorney, or simply paying attention to signs that someone might be behaving uncharacteristically.

Dr. Nicholas agreed.

“We should be thinking about the possibility of financial difficulties linked to a disease we don’t even know we have,” she said. “Knowing that, people should be on the lookout for these symptoms among friends and family members.”

Pam Belluck contributed reporting.

Tell us about your family's challenges with money management and Alzheimer's.

Ben Casselman writes about economics with a particular focus on stories involving data. He has covered the economy for nearly 20 years, and his recent work has focused on how trends in labor, politics, technology and demographics have shaped the way we live and work. More about Ben Casselman

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