Alzheimer’s Disease (AD) is a prime example of the inefficacy of research based on animal experiments. Despite decades of animal experiments, we are still very far away from having this age-related disease under control. A number of drugs developed on the basis of animal experiments fail when they have to prove themselves in the clinical phase in humans or are withdrawn after market launch. Because of its chronic lack of success, the "animal model" is now being questioned even by AD researchers. Furthermore, mini-brains generated from human cells offer promising research approaches.

A widespread disease

Every year, 200,000 people develop AD in Germany, and 1.2 million are currently affected by this serious disease. AD is a form of dementia. It usually occurs in elderly people; patients are rarely younger than 60 years. There is an inherited form, which only affects a minimal proportion, while age-related Alzheimer's dementia accounts for 99% of the cases (1). The progressive death of nerve cells is accompanied by memory loss, speech disorders, depression, and personality changes. The brain shrinks by up to 20% (2). This disease is an enormous burden for the patients and their entire social environment.

Drugs for AD are unsatisfactory

A number of drugs to treat AD have been developed on the basis of animal experiments in recent decades. Although there have been "successes" in countless animal studies based on mice and other animals, the effectiveness of these drugs in humans is not satisfactory, while the side effects are a major problem for patients. One study found that of over 400 human clinical trials of AD drugs, only 0.4% ended up improving clinical symptoms in patients (3).

Many of the Alzheimer's drugs that have been rated as effective and safe in animal experiments fail at the latest in clinical phases 2 and 3 in humans because they do not improve the patient’s symptoms, or because the side effects are intolerable. In the worst cases, the administration of the active ingredient even deteriorates the patient's condition. A phase 3 study was recently terminated because the active substance tested, Verubecestat, showed no improvement in Alzheimer's symptoms, although it reduced the amyloid content in the patient's brain. In addition, it even worsened the cognitive performance of Alzheimer's patients: after 2 years, patients taking the drug showed an almost 40% higher incidence of dementia than the placebo group, the brain shrank more, and memory performance deteriorated (4). There were also serious side effects such as sleep disorders, psychosis, and depression. In animal experiments in contrast, the active ingredient was classified as extremely promising.

This example shows once again that animal experiments do not offer safety and can even pose a risk to humans due to the lack of transferability. The whole class of active ingredients, to which Verubecestat belongs, namely beta-secretase inhibitors is being questioned now, because other substances that are similar to Verubecestat have previously failed in clinical phases, i.e. in tests on humans, and seem to have a negative influence on Alzheimer's symptoms (5). This is a scientific disaster because the underlying scientific amyloid hypothesis was one of the best established ones. Based on this, uncountable “animal models” were generated, which are still used in animal studies today. Of the few drugs that eventually make it onto the market, some are withdrawn due to lack of efficacy or serious side effects. In 2016, for example, Solanezumab (Eli Lilly) was recalled, and in 2019 the similarly effective Aducanumab (Biogen/Eisai) was withdrawn (6).

The formation of plaques in the brain is a phenomenon observed in all Alzheimer's patients. These are protein deposits whose core consists of the protein amyloid. The plaques form between the neurons and the amyloid protein is often also deposited in vascular cells (7). Amyloid is a cleavage product of the amyloid precursor protein APP. Most current drugs interfere with these processes. The so-called beta-secretase inhibitors, for example, block the cleavage of the APP, while amyloid antibodies intercept freely available amyloid. In both cases, the aim is to reduce the free amyloid so that the formation of plaques is prevented as far as possible. These drugs, which also include Solanezumab and Aducanumab, were developed based on the amyloid hypothesis, which posits that plaque formation directly affects the symptoms of AD. However, this scientific theory is controversial and disputed by many neurological researchers.

The reason why animal experiments fail

Genetic changes are linked to plaque formation, but these cases are very rare. Experts agree that the development and progress of AD is influenced by a complex interaction of multiple factors in the body.

Although the genetic component only has an influence on the formation of plaques in a few cases and despite the uncertainty of to what extent these influence the symptoms at all, most "animal models" in Alzheimer's research are based on simple genetic mutations. 172 genetically modified "mouse and rat models" are currently used for Alzheimer's research (8). They usually have only a single gene that is altered, which is suspected of being associated with Alzheimer's in humans. In recent years, more and more "humanized animal models" have been generated, in which a human gene is integrated into the genome. This is scientific nonsense, as the expression and regulation of a human gene in a mouse organism can be very different from the one in the human body from which it originates. In addition, mice or rats would never naturally develop Alzheimer's, which further highlights the absurdity of this approach.

Although the genetic mutations cause plaques in the genetically altered animals, they do not reflect human AD with its complex symptoms such as memory loss etc. (9). Additionally, there are inadequate evaluation criteria, for example when it comes to examining the effectiveness of an Alzheimer's drug on such a transgenic mouse. Although physiological changes in the plaques in the brain or the condition of the neurons can be assessed, it remains unclear whether the drug can actually improve the symptoms of Alzheimer's patients. The mouse cannot reliably reveal whether the memory performance or even the personality is changing, and neither can it develop language problems, as seen in many patients. The researcher will also not find out whether the drug causes side effects in mice that are not physiologically apparent at first glance. The Morris water maze, a test that has been used more or less unchanged since 1979, is often used to assess the memory performance of rats and mice. A mouse is placed in a large basin of turbid water. There is a platform just below the surface of the water that the animal must find. After a few days of "training", the platform is removed. If the mouse searches in vain at the exact place where the platform was before, it is considered good memory. Comparing such a crude experimental setup with the progressive memory gaps of Alzheimer's patients is more than questionable.

There are many other obstacles, such as the human lifestyle (stress factors, social environment, etc.), which are involved significantly in the development and progress of neurological diseases, and cannot be simulated in animal experiments. Rather, the opposite is the case: the animals are kept under artificial living conditions that cannot be compared to the environment of an Alzheimer's patient. "Highly standardized conditions" are set in order to not additionally influence the research results (which vary a lot anyway) - obviously, such a setting has nothing to do with the real life of an Alzheimer's patient. Moreover, usually only male animals are used for the experiments in order to maximize the homogeneity of the research results. However, AD affects both sexes equally, so this approach is by no means target aimed.

Criticism from within

There are no effective therapies to combat Alzheimer's dementia and the roots of this complex disease have not yet been elucidated by relying on animal experiments. Criticism is now increasingly coming from the ranks of neurological researchers and physicians. Referring to the failure of the Verubecestat study, the German “Ärztezeitung” concludes that "Alzheimer's research seems to be stuck" (5). Thorsten Müller, who researches Alzheimer's at the Ruhr University in Bochum, even states that Alzheimer's research is "currently at the very beginning again" (10). In the Nature journal, one of the most prestigious scientific journals worldwide, the frustration of the researchers about the "Alzheimer animal models", lacking success for human health, is extensively reported on (9). The article discusses the creation of new "animal models", but Belgian molecular biologist and Alzheimer's researcher Bart de Strooper says: "The biggest mistake you can make is to think you can ever have a mouse with AD." It is therefore a misconception that there will ever be a mouse that develops or faithfully reflects AD. There is nothing to add to this powerful statement.

Regarding these circumstances, it is not surprising that research - despite millions of animals suffering and billions of tax money being wasted on transgenic animals and questionable research projects - is still a long way from understanding AD, let alone successfully treating or curing it. It is also not surprising that there is a lack of effective medication and that medications keep on being withdrawn from the market because all they could achieve is give the patients false hope. The animal experimental system in Alzheimer's research produces drugs for only one target group: Genetically modified, young adult male mice that live under artificial laboratory conditions without natural environmental influences and that develop a fraction of the symptoms of human Alzheimer's patients.

Pioneering technologies: Brain organoids

Even though the molecular processes concerning the amyloid protein have now been studied in great detail, very little is known about the functions of the precursor protein APP and other cleavage products. As the amyloid hypothesis begins to sway, it is important to expand research in this direction - with reliable, human-based methods, not with even more artificial "animal models”.

A research team from the Ruhr-Universität Bochum led by Thorsten Müller has uncovered a new mechanism that most likely explains the death of nerve cells in Alzheimer's patients. The scientists did not gain these important insights in animal experiments, but solely in a model that could revolutionize numerous areas of neurological research: human brain organoids (10). These are human mini-brains or organ precursors of just a few millimetres in size that are grown in the laboratory. The brain organoids are generated from induced Pluripotent Stem Cells (iPSCs) from human donors. Only a few hair root or skin cells are required for this. They are then reprogrammed into iPSCs in the laboratory. The tiny brain organoids can be used like normal cell cultures for various experiments, e.g. also for drug screening (11). A great advantage of the organoids is that they preserve the characteristics of the human donor. This means that cells from an Alzheimer's patient can be used to cultivate their own individual brain organoids which can be used for research. It is hard to imagine a better Alzheimer’s model than this one, because the organoid is naturally affected by Alzheimer's. Such a model is of great value, especially for aetiology.

The Bochum researchers speak of a "living system" in which the nerve cells behave like those in the real human brain. They have shown that the cleavage products of the amyloid precursor protein APP, which have received little attention to date, appear to play a key role in the symptoms of Alzheimer's dementia. In contrast to the amyloid protein, which forms the plaques outside the nerve cells, another cleavage product forms a protein complex that migrates into the cell nucleus, which subsequently causes the death of the nerve cell (10). This newly discovered molecular mechanism, which is not directly related to the plaques, could explain Alzheimer's symptoms such as memory loss and other cognitive impairments. The Bochum researchers even state that the procedure can also be adapted for other neurological diseases such as schizophrenia.

The industry has already recognized the enormous potential of brain organoids for biomedical research and drug development. The AxoSim group recently licensed the "Mini Brain" technology, which was developed in 2016 by the pharmacologist and toxicologist Thomas Hartung (12). On their website, the company states that "Animal testing is yesterday's gold standard." And they are right: Animal testing is a thing of the past.

11 June 2019
Dr. Tamara Zietek, PhD

References

1. Alles Wissenswerte zur Alzheimer-Krankheit. Alzheimer Forschung Initiative e.V. (AFI). Accessed 13 June 2023
2. Das Wichtigste über die Alzheimer-Krankheit. Deutsche Alzheimer Gesellschaft
3. Cummings JL et al.: AD drug-development pipeline: few candidates, frequent failures. Alzheimer’s Research & Therapy. 2014; 6(4): 37
4. Egan MF et al.: Randomized trial of Verubecestat for prodromal AD. New England Journal of Medicine. 2019; 380(15): 1408–20
5. Schlag für Demenzforschung: Anti-Alzheimer-Wirkstoff beschleunigt kognitiven Abbau. Ärzte-Zeitung, 25.4.2019. Accessed 13 June 2023
6. Große Hoffnung auf Alzheimer-Medikament zerschlagen. Ärzte gegen Tierversuche, Pressemitteilung, 26.3.2019. Accessed 13 June 2023
7. Das Wichtigste: Die neurobiologischen Grundlagen der Alzheimer-Krankheit. Deutsche Alzheimer-Gesellschaft
8. AD Research Models. ALZFORUM. Accessed 13 June 2023
9. Reardon S.: Frustrated Alzheimer’s researchers seek better lab mice. Nature. 2018; 563: 611
10. Alzheimer im Mini-Gehirn. Ruhr-Uni-Bochum, 30.4.2019. Accessed 13 June 2023
11. Reproducible Brain Organoids Could Offer New Models for Research. ALZFORUM. News, 6.6.2019. Accessed 13 June 2023
12. Axosim Accessed 13 June 2023