Alzheimer’s disease (AD), the most common form of dementia, was first described roughly one hundred years ago in Bavaria by Dr. Alois Alzheimer in his presentation of an early-onset (<60 years) case. AD is a progressive and fatal neurodegenerative disease that impairs memory and overall cognition. There are more than 5 million documented patients according to best estimates, with some experts suggesting that may be only 20% of the total number actually affected. The number of new cases grows by more than 10% per year. Alzheimer’s disease is the seventh leading cause of death for people of all ages, the fifth leading cause of death in people age 65 and older, and is the only one of the major diseases (heart disease, breast and prostate cancer and stroke) to be increasing in mortality; up almost 33% from 2002 to 2004. Medicare expenditures for Alzheimer’s and other dementias in 2005 were $91 billion; this total is projected to increase to $160 billion by 2010. State and federal Medicaid spending for nursing home care for people with Alzheimer’s and other dementias was estimated at $21 billion in 2005, and is projected to increase to $24 billion by 2010. Given these estimates and no significant containment or decrease in Alzheimer’s, Medicaid and Medicare expenses for Alzheimer’s and related dementias will be approximately $184 billion by 2010, or approximately 27% of the entire combined anticipated expenditure for Medicare and Medicaid in 2010.
Alzheimer’s Disease: Risk Factors
While age is the strongest risk factor, second is family history. Study of families affected by Alzheimer’s disease reveal a strong genetic component. A study of more than 14,000 identical twins in Sweden showed that of pairs of twins in which one was affected with Alzheimer’s disease, the other twin also had the disease roughly 80% of the time. Researchers now agree on four key genes, three of which (APP, PSEN1, PSEN2) can carry any of over 200 mutations that directly cause early-onset, familial AD with virtual certainty when inherited. These mutations account for two to five percent of AD cases. A particular variant (E4) of another gene called APOE increases risk for roughly half of the more common, late-onset cases of AD. Since the APOE-E4 variant does not guarantee onset of AD, other genes and envi¬ronmental factors likely work together with this genetic variant to bring on the disease. All told, the three early-onset genes and APOE account for about 30% of the genetic activity contribut¬ing to AD. Thus, 70% of the genetic activity in AD is yet to be determined, and almost certainly will hold critical clues about the root causes of the disease and how to stop it. In a highly complex genetic disease, it is imperative to know which genes are contributing to risk for the disease and how they are operating to gain a sufficient understanding to treat and prevent the disease. Every new AD gene identified provides another window into the cause of AD and a novel target for drug discovery. In fact, over 99% of all current Alzheimer’s research in academia and novel AD drug development in industry would not be possible without the identification of these genes, particularly those causing early-onset AD.
Another risk factor is gender. After adjusting for lifespan, women are more affected with AD than men, although the reason for this is unclear.
Among environmental factors, head trauma and stroke are the leading risk factors. Recent studies have shown that the mechanism for “causing” or accelerating Alzheimer’s from head trauma or Traumatic Brain Injury (TBI) is very similar to that resulting from stroke. This is particularly important as more attention is given to TBI in general as a result of the high occurrence of this kind of injury incurred by the military in Iraq, and by the growing number of head injuries in sports at all levels.
Cardiovascular risk factors, such as, high cholesterol and high homocysteine (a modified amino acid naturally occurring in the body) levels also are linked to increased risk for Alzheimer’s disease. Physical exercise has also been shown to protect against Alzheimer’s disease. At least the anecdotal conclusion from observations about cholesterol, homocysteine and exercise is that “what is good for the heart is good for the brain.”
Finally, some claim that intellectual stimula¬tion, e.g. crossword puzzles, has beneficial effects. This may be the case, but the literature so far shows more evidence from the benefits of physical exercise than from intellectual activity. Finally, caregivers and elder care workers report that those people who stay active socially take longer to develop patterns of dementia.
Alzheimer’s Disease: Lessons from genetics about the causes
While we do not know the exact cause of Alzheimer’s disease (AD), most researchers agree that excessive accumulation in the brain of a peptide (small protein) called the amyloid beta peptide (A-beta), and especially the form of it that consists of 42 amino acids (A-beta 42), is the major culprit. More specifically, the ratio between A-beta 42 and the more common form, A-beta 40, appears to be increased in the AD brain. The more out of balance they are, the more likely that A-beta 42 will accumu¬late to pathological levels in the brain, increasing the likelihood of AD pathology. A-beta 42 is the key component of beta-amyloid deposits in the brain called “senile plaques,” which accumulate outside of nerve cells, and in clumps on brain blood vessels. Beta-amyloid also drives the formation of “tangles” in¬side of nerve cells, which leading to neuronal degeneration and nerve cell death. While we still do not understand precisely how A-beta leads to tangles and nerve cell death, we do know that A-beta produced by the activities of certain enzymes (beta-secretase and gamma-secretase), which clip the amyloid precursor protein (APP), produced by one of the early-onset AD genes.
After A-beta is made in the brain it is usually shuttled into the bloodstream for degradation. The balance of A-beta production in the brain versus clearance out of the brain determines how much A-beta will accumulate and potentially form toxic beta-amyloid aggregates. If production is too high and/or clearance is too low, A-beta accumulates to abnormally high levels in the brain. It can then aggregate into clusters called “oligomers”, prior to being deposited into senile plaques. While earlier, controversial hypotheses, e.g. the “amyloid cascade hypothesis” favored senile plaques as the cause of Alzheimer’s disease, it now appears that the A-beta oligomers, which precede plaques, are the actual neurotoxic entities in Alzheimer’s disease. As A-beta oligomers accumulate in the brain, communication between nerve cells is impaired eventually leading to formation of tangles and, ultimately, neuronal cell death. This is referred to at the “A-beta Hypothesis of AD”.
Novel Therapies based on knowledge gained from the early-onset AD genes
Below is a summary of promising new AD drugs in development based on the “A-beta Hypothesis of AD” described above. These drugs are aimed at retarding disease progression by curbing the accumulation of A-beta, and particularly, A-beta42, in the brain. The four established AD genes (APP, presenilins 1 and 2, and APOE) have taught us that the common pathological feature in the AD brains of patients carrying defects in any of these four genes is the excessive of accumulation of neurotoxic A-beta. There are two basic ways to achieve this: either promote the clearance of A-beta42 from brain, or turn down production of A-beta42 in brain. Below, we review some of the more promising therapeutic candidates currently in clinical trials for treating AD.
Promoting A-beta42 Clearance
These drugs are aimed at blocking excessive aggregation of A-beta into toxic forms and clearing A-beta out of the brain.
There are five major trials in this category:
1. The amyloid vaccine (Wyeth/Elan and 15 others)
In the vaccine approach, there are two basic strategies: active vaccination and passive immunization. Active vaccination involves immunizing a patient with aggregated A-beta. This stimulates the patient to make antibodies to A-beta. The first clinical trial (Wyeth/Elan) using this approach was terminated because several of those treated developed encephalitic. In the passive immunization approach, antibodies targeted against A-beta are injected intravenously into the bloodstream. The antibodies then bind to A-beta peptides that have been exported from the brain and do not let them re-enter the brain. In this manner, A-beta levels in the brain are progressively reduced. This is a promising strategy, which is being pursued by at least 15 different companies and is currently in large phase III trials at Wyeth/Elan (Bapineuzumab). While promising, this therapy can be relatively expensive.
2. IVIg (Gammagard, Baxter International)
The alternative to the amyloid vaccine approach is the intravenous “IV-IgG” approach, which attempts to achieve the same goal as the vaccine. In this case, instead of injecting purified antibodies to A-beta that have been carefully prepared and purified in the lab, they inject the whole set of antibodies from donors’ plasma. The expectation is that some of the antibodies will be naturally targeted to A-beta. Clinical studies are in the very early stages. A 24-patient study unveiled last summer by Baxter showed 16 patients on Gammagard had a better cognitive response than eight patients on a placebo.
3. Alzhemed (Neurochem)
Alzhemed is an orally available drug known as a GAG-mimetic, which is designed to bind to A-beta peptides and prevent them from aggregating. In this way, the drug is intended to block A-beta from aggregating into senile plaques. Phase II and III trials of this drug have failed, however, Neurochem may attempt a revised Phase III trial in the future.
4. PBT2 (Prana Biotechnology)
PBT2 is based on the “Metal Hypothesis of AD”, which contends that copper and zinc drive the formation of toxic forms of A-beta, e.g. oligomers. This hypothesis has been championed by Drs. Rudy Tanzi and Ashley Bush who co-founded Prana Biotechnology. PBT2 is referred to as a “metal protein attenuation compound” (MPAC) that strips zinc and copper from A-beta and thereby prevents A-beta from aggregating and from forming neurotoxic A-beta oligomers, which can impair cognition. PBT2 has been shown to dramatically reduce A-beta aggregation and accumulation in transgenic AD mouse models. It is also able to block the detrimental effects of A-beta on neuronal synapses, neurotransmission, and cognition. Phase IIa clinical trial results, which were announced in March (2008), were highly encouraging. After 12 weeks of oral administration in 78 mild-moderate AD patients, PBT2 significantly lowered A-beta42 levels in the cerebrospinal fluid, which is representative of brain A-beta42 levels in mild to moderate patients. Moreover, the drug significantly improved cognition in the treated AD patients (versus placebo treated) based on their performance on two neuropsychiatric tests for “executive memory”. Importantly, the drug had no side effects or adverse events and was well tolerated. Prana is now seeking a big pharma partner to proceed to a larger phase IIb (or phase III depending on the partner) clinical trial of PBT2.
5. AZD-103 (Transition Therapeutics)
AZD-103 is a sugar-like compound known as an “inositol” that is aimed at breaking down A-beta aggregates. In a phase I clinical study, AZD-103 was well tolerated. Transition as now partnered with Elan to carry out a phase II clinical trial.
Regulating A-beta Production
These drugs are aimed at regulating the generation of A-beta in the brain.
There are four major trials here:
1. LY450139, gamma-secretase inhibitor (Lilly)
LY450139 is a gamma secretase inhibitor (GSI) targeted at blocking the activity of gamma-secretase, an enzyme necessary for the production of A-beta. While this particular drug has done quite well recently proceeding all the way to phase III clinical trials, this class of drug has generally been shrouded with potential safety concerns. This is because the enzyme, gamma-secretase, is normally needed to process many other proteins beyond the APP. For example, gamma-secretase is required to processes the essential protein called Notch. When this event is blocked, the result can be skin cancer. Yet, the fact that Lilly’s candidate has made it all the way through to phase III trials would attest to this inhibitor’s safety.
2. Flurizan (Myriad)
Alternatives to GSI’s are “gamma-secretase modulators” (GSM). This class of drugs also targets gamma-secretase, but instead of inhibiting overall gamma secretase enzyme activity, GSM’s “modulate” gamma secretase enzyme activity. More specifically, this class of drugs allows gamma-secretase to carry out its normal functions, including the production of A-beta40, but selectively block gamma secretase’s ability to produce the neurotoxic Abeta-42. In the brain, ~90% of the A-beta made is A-beta40 and ~10% is A-beta42. A-beta42 is considered the more dangerous form of A-beta because it is able to oligomerize more readily into neurotoxic forms. All but a handful of the early-onset, familial AD mutations in APP and the presenilins (which are a part of the gamma secretase enzyme complex), have the same pathogenic effect: they increase the ratio of A-beta42:A-beta40 in the brain.
Ibuprofen (and other non-steroidal anti-inflammatories; NSAIDs) was first reported to have GSM properties, i.e. they lower the A-beta42:A-beta40 ratio. Clinical trials were run for NSAIDs and mostly failed. Myriad then developed a NSAID called “flurizan”. While the Phase II trial did not show statistically significant effects on AD, phase III clinical trials on high dose flurizan are underway with reports expected this year. Given the large size of the phase III flurizan trial, it is possible that this time, statistically significant endpoints might be reached.
3. E2012 (Eisai)
E2012 is a novel, more potent gamma secretase modulator than Flurizan. It was developed by Eisai in a partnership TorreyPines Therapeutics (TPTX). Eisai carried out a screen for novel GSM’s in parallel with TPTX while also licensing first rights to TPTX’s GSMs. E2012, which has the same core structure as one of TPTX’s lead GSM’s, was advanced into a Phase I clinical trial by Eisai. In February 2007, the E2012 trial was put on hold because some side effects were observed in the eyes of rats after 13 weeks of treatment. After further testing, the side effect was not observed and the hold was lifted in April 2008. The drug is now in a Phase I clinical trial. Meanwhile, at TPTX, screening for novel GSM’s has led to the development of additional potent GSMs.
4. CTS-21166 (CoMentis)
CTS21166 is an inhibitor of the second enzyme required to generate Abeta, beta-secretase (BACE). BACE and gamma-secretase serially cleave the amyloid precursor protein to produce A-beta. The Phase I study indicated that CTS-21166 was safe and well tolerated. A Phase II study is expected later this year. Like gamma-secretase, BACE does not only process APP, but processes several other important proteins as well. Thus, similar safety concerns as those mentioned for GSI’s apply here for this BACE inhibitor.
In summary, one of our best chances for effectively treating and preventing AD based on what we have learned from the four established AD genes, so far, is to target A-beta with a cocktail of therapies that, on one hand, safely and specifically regulate A-beta production, and on the other hand, enhance the clearance of A-beta while also preventing its aggregation into neurotoxic oligomers. With several active trials and other promising drugs headed toward, the hope is that at least one or more of these therapeutics will successfully slow or reverse disease progress in AD.
The Alzheimer’s Genome Project
The genetic component of Alzheimer’s disease is strong with at least 80% of cases involving inheritance, according to large twin studies. In 1987, Dr. Tanzi and colleagues at Harvard U. (along with two other groups) reported the isolation of the first AD gene (APP). Dr. Tanzi and colleagues then went on to co-discover two more early-onset genes (presenilin 1 and 2) in 1995. In addition, a late-onset genetic risk factor gene, APOE, was discovered in 1992 at Duke U. These four genes and every new AD gene identified provide valuable clues about the causes of AD and novel targets for drug discovery and development. In fact, it is safe to say that greater than 99% of all current Alzheimer’s research in academia and industry would not be possible without the identification of these genes. Yet, these genes account for only 30% of the inheritance of AD with 70% still remaining a mystery. To address this, labs all around the world are trying to identify the remaining AD genes. A national consortium has been created for this purpose as well. Another effort in this area is described as the Alzheimer’s Genome Project (AGP)™ initiative. The AGP is based at Massachusetts Gen¬eral Hospital under the direction of Dr. Rudy Tanzi, who was involved in the identifi¬cation all three early-onset Alzheimer’s genes. The AGP is a three-year, approximately $3 million effort mainly being funded by the Cure Alzheimer’s Fund. It is scheduled to have first round results of the entire Alzheimer’s genome by the summer of 2008. This will be the first family-based whole genome as¬sociation study for Alzheimer’s disease, being carried out in over 1300 AD families. This study requires the newest technology, e.g. microarray gene “Chips”, sophisticated statistical analysis, large family samples for DNA, human genome project databases, and sophisticated statistical genetic analyses.
Along with the AGP, Dr. Rudy Tanzi and his colleague, Dr. Lars Bertram have created the “AlzGene” website (www.alzgene.org; supported by the Cure Alzheimer’s Fund). AlzGene is now the largest and most trusted interactive database of weekly updated information about Alzheimer’s-related genes in the research community. It is a freely available online interactive database that provides a comprehensive and continuously updated encyclopedia of genetic risk factor data published in the field of AD. Its core constituent is an analysis engine that allows for quantita¬tive summarization of the published data for each gene and genetic variant (or: polymorphism, mutation) using a statistical method called meta-analysis. With more than 1,000 scientific papers published in AD genetics over the past two decades, it has become impracticable for AD researchers to regularly track and evaluate the multitude of—many times conflicting—findings in the field in a systematic manner, making it impossible to tease out the potentially most important risk factors for AD. AlzGene closes this gap and provides weekly updated meta-analyses of all published studies, prominently displaying the most promising (“top”) results on its homepage.
Over the past two decades, it has become clear, that most of what we now know about the etiology and pathogenesis of AD, has emerged primarily from studies of the Alzheimer’s disease genes. The four established AD genes, and particularly the three early-onset AD genes, PS1, PS2, and APP have driven and continue to guide virtually all wet lab research in AD. Studies of these genes have provided an unprecedented window into the molecular underpinnings of AD. So far, all four genes point to the excessive accumulation of the toxic peptide A-beta in brain as the primary cause of AD. Accordingly, most ongoing clinical trials aimed at modifying disease progression (as opposed to just treating the symptoms) are targeting A-beta accumulation in the brain. Meanwhile, the hunt for novel AD genes continues around the world, since every new AD gene identified provides novel targets for drug discovery and unique opportunities to design innovative therapies to treat and prevent AD. The ultimate elucidation of all AD genes should also enable more accurate disease prediction and diagnosis. Thus, the overarching and long-term goal of AD genetic research is to accelerate the path toward conquering this disease by enabling reliable genetic testing, to identify those who are at greatest risk for AD, and to develop novel therapies based upon genetic findings, to prevent AD at its earliest stages.