Vaccination against Alzheimer disease

Abstract

Alzheimer disease is a devastating chronic disease without adequate therapy. More than 10 years ago, it was demonstrated in transgenic mouse models that vaccination may be a novel, disease-modifying therapy for Alzheimer. Subsequent clinical development has been a roller-coaster with some positive and many negative news. Here, we would like to summarize evidence that next generation vaccines optimized for old people and focusing on patients with mild disease stand a good chance to proof efficacious for the treatment of Alzheimer.

Introduction

Alzheimer vaccines

Alzheimer Disease (AD) is an important progressive neurodegenerative illness, and is a major form of dementia. With increasing age, the probability to develop AD steeply increases and currently affects 26 million people worldwide.¹ As the age of the human population is globally growing, the increased life expectancy and the consequent increased frequency and number of patients afflicted with this disease is becoming a large social and economic burden.^2,3 Only symptomatic and not very effective treatments are currently available and in view of the alarming epidemiological data, novel therapeutic strategies to prevent, mitigate or delay the onset of AD are pursued extensively.⁴ The hallmark of the pathology observed in AD is plaques in the brain of the afflicted patients. Active or passive vaccination against components of these plaques is currently one of the most promising therapies with disease-modifying potential.⁵

The 2 major components of the plaques are extracellular aggregates of amyloid-β (Aβ) peptides, which are cleaved out of the amyloid precursor protein (APP) by the β- and γ-secretase, and intracellular neurofibrillary tangles (NFT) consisting of hyperphosphorylated tau proteins.⁶ Under healthy conditions Aβ peptides as well as tau proteins are constantly expressed but do not show aggregate formation. The normal function of Aβ remains controversial but seems to have e.g., an anti-microbial activity.⁷ In contrast, tau proteins stabilize microtubules and ensure neuronal integrity and stimulus transfer.

Familial AD is associated with mutations in the APP protein as well as presenilin 1 and 2 as these mutations are known to enhance production of Aβ-peptides and increase their deposition in plaques. Transgenic mice expressing APP associated with familial AD also show plaque deposition and signs of the illness. Furthermore, Aβ peptides are toxic to cells under conditions of oxidative stress, and inflammation⁸ and APO ϵ4, which is a genetic risk factor for AD, causes delayed and decreased degradation of the peptide.⁹ Intracellular neurofibriallary tangles (NFT) are also considered valuable targets for treatment of Alzheimer. Progress is, however, much less advanced compared with Aβ_1-42 and is therefore not subject of this review.

In 1999, Elan pharmaceuticals reported that active immunization against Aβ_1-42 could reduce Aβ pathology in APP transgenic mice.¹⁰ Immunization with aggregated Aβ_1-42 formulated in strong adjuvants caused reduced plaque burden¹⁰ and subsequent studies also demonstrated improved mental performance in immunized mice.¹¹ These findings were not restricted to the particular transgenic mouse strain, as similar results were obtained in additional transgenic mouse models of AD using active and passive immunization (for review, see ref. 12). As passive immunization with mAbs was able to clear the plaques, it was clear that antibodies were the relevant effector molecules induced by vaccination. The exact mechanism of action is, however, still under debate. There is the possibility that Aβ_1-42 specific antibodies may act as a sink in the blood and “extract” Aβ_1-42 peptide from the brain. Even though Aβ_1-42 deposition in plaques seems to be a rather dynamic process, it remains difficult to explain how individual Aβ_1-42 peptides are released from the plaques and reach the blood to be captured by antibodies. It may also be possible that specific antibodies, which are known to penetrate the blood brain barrier to some extent, are transporting the peptides in a reverse manner from the brain into the blood. Both hypotheses seem to be supported by the fact that Aβ_1-42 levels increase in serum upon immunization as they are bound by antibodies. On the other hand, Aβ_1-42 peptide found in blood may not be from the brain but from alternative sources, such as Aβ_1-42 depositions found in blood vessels. Yet an alternative possibility is that Aβ_1-42 specific-antibodies bind to plaques and promote their removal by microglia cells. As this hypothesis does not call for large amounts of Aβ_1-42 to migrate from the brain to the blood, and since aggregates would allow for efficient high-avidity binding of antibodies, this last hypothesis may be the most plausible one. In addition to reducing the plaque-burden, specific antibodies may also mitigate some toxic effects attributed to Aβ_1-42.

First generation vaccine

In light of these promising preclinical results a number of clinical studies were initiated to assess safety and potential efficacy of vaccines against AD. The initial vaccine developed by Elan and Wyeth was based on aggregated human Aβ_1-42 (AN1792). In first phase I trials, AD patients were immunized with AN1792 formulated in the adjuvant QS21 (AN1792). Aβ specific antibodies could be found in more than half of the immunized AD patients in phase I trials without notable adverse events.¹³ Encouraged by the positive safety data, AN1792 formulated in QS21 vaccine was tested in a relatively larger phase II trial with 372 AD patients to assess safety as well as efficacy of the therapy. However, treatment had to be discontinued prematurely in this trial since 6% of the vaccinated patients developed aseptic meningo/-encephalitis.¹⁴ Although the patients responded well to treatment with corticosteroids, the risk was deemed too high and further development of the vaccine discontinued. However, detailed analysis of safety and efficacy parameters of the study yielded a wealth of information for next generation vaccine development.

The cause of aseptic meningo-/encephalitis

Immediately after the issues with the Elan/Wyeth trial, it was unclear whether T or B cells were responsible for the side effects. The preferred effector cells were T cells, as antibodies are thought to be the effects or molecules responsible for plaque removal. Indeed, evidence was accumulating that T cells were causing the problems. An important piece of evidence was that subsequent analysis revealed that the side effects failed to correlate with antibody titers.^14-16 In addition, infiltrating T-cells were found in the brain of diseased patients which suggested Aβ specific T-cells to be the cause of the side observed effects.^17-20 The design of the vaccine may have be a key reason for the immunopathology. As human Aβ_1-42 was used for immunization, no foreign Th cell epitope has been introduced to bypass Th cell tolerance. As a consequence, Aβ_1-42—specific tolerance had to be broken for the induction of Th cell dependent IgG responses. The combination of strong adjuvants (QS21/saponin) and the fibrillar character of the vaccine causing persistence of the antigen for extended time periods at the injection site were able to overcome Th cell tolerance and caused the generation of Th cell dependent IgG responses. Overcoming Th cell tolerance, however, apparently also caused the T cell mediated immunopathology.

Preliminary evidence for efficacy

Even though dosing of patients was stopped in the middle of the trial, there were a substantial number of patients mounting specific IgG responses. This allowed investigator-driven unblinding of a subcohort of the study. These authors described strongly decreased mental decline and delayed disease progression in antibody-responders.^13,14 Later studies assessing all patients in a pre-defined manner found some, but not as robust effects.^18,21,22 Thus, despite the serious safety issues observed in the AN1792 trial, the vaccine provided first clinical evidence that active vaccination against Aβ may be beneficial for the treatment of AD.^18,21,22 From a mechanistic point of view, it was very interesting that post-mortem analysis demonstrated strong reduction of plaque load in antibody responders. Hence, as in the transgenic mice, specific IgG was able to remove plaques consisting of Aβ_1-42 from the brain.²³ This was an important finding as many researchers doubted the ability of serum IgG antibodies to interfere with plaque burden in a secluded place such as the brain.

Next generation vaccines

The observed cases of meningo/-encephalitis represent the most significant setback for the development of a vaccine against Alzheimer disease. As discussed above, T cells were soon thought to be the pathological culprit, a hypothesis supported by the fact that mAbs specific for Aβ_1-42 have caused no signs of meningo-encephalitis. The two major strategies pursued by the industry has been introduction of Th cell epitopes to be able to bypass Th cell tolerance and shortening of the Aβ_1-42—derived peptide. Indeed, peptides of sizes smaller than 8 amino acids cannot induce T cell responses as they cannot bind to MHC molecules for presentation to T cells. Figure 1 summarizes these steps and identifies short Aβ_1-42—derived peptides coupled to a protein carrier as the ideal vaccine candidate

Figure 1. Three versions of vaccines against Aβ_1–42. The original version consisting of Aβ_1–42 only, an improved version, where a Th cell epitope has been introduced and current versions of the vaccine, where short peptides derived from Aβ_1–42 are coupled to protein carriers or virus-like particles.

Indeed, the follow up candidate for AN1792 consists of the N-terminal seven amino acids of Aβ_1-42 covalently linked to the diphtheria toxin cross reactive mutant (CRM197) protein carrier and is called ACC-001. As discussed, this peptide is below the minimum length-requirements for any T-cell epitope to bind to MHC molecules. Nevertheless, strong Aβ_1-42—specific antibody responses were induced in the transgenic mice and plaque development was efficiently blocked and cognitive functions were improved. This compound is currently subject to intense clinical testing.

Likewise Novartis, using bacteriophage based VLP technology, has embarked on the development of an Alzheimer vaccine targeting amyloid β. This vaccine, which goes by the name of CAD106 consists of the bacteriophage Qβ VLP (Qβ) displaying Aβ_1-6 on its surface by means of chemical coupling. As described for the CRM197 vaccine candidate, CAD106 was highly immunogenic for the induction of antibody in transgenic mice as well as rabbits and primates but did not induce measurable Aβ_1-42—specific T cells responses.²⁴ As expected, immunization with CAD106 inhibited amyloid cortical plaques deposition in transgenic mouse models of Novartis. Immunization of old mice exhibiting advanced pathology in the brain, resulted in reduced levels of amyloid deposition. However, as expected from immunizing old mice, responses were low compared with immunized young mice. Currently available information from first in man studies with CAD106 suggest that the vaccine is well tolerated and induces Aβ—specific immune response in the majority of the patients in the absence of meningoencephalitis.^25-27

Merck and Co., Affirisis/GSK, AC immune and United Biomedical all use a similar approach as described above with small differences in the carrier and/or peptide and adjuvant used for vaccine generation. However, none of the currently tested vaccines has clinical PoC demonstrating delayed disease progression. Another drawback of these second generation vaccines is that they are not optimized for the old and therefore are expected to induce moderate antibody responses. It will be interesting to see whether the attempt to induce anti-inflammatory T cells during vaccination as performed by Mercia Pharma Inc. will have an impact on safety and efficacy of the approach.

What can we learn from mAbs

There are a number of mAbs in development. Most advanced are Bapineuzumab (PFE/JNJ/ELN) and Solanezumab. (LLY). Both antibodies have been in large phase III studies. Even though they failed to show efficacy in the overall disease population, subgroups analysis showed a strong impact of Apo ϵ4, which is involved in degradation of Aβ_1-42 and predisposes to poor responsiveness to mAb-treatment. Most remarkable, a subgroup analysis of the LLY trial with Solanezumab demonstrated efficacy in pooled data of mild but not moderate patients (a 34% reduction in cognitive decline, which was significant compared with placebo [P = 0.001]). This may be the most important insight, as such a vaccine moves toward a preventative rather than therapeutic modality. It seems rather obvious that prevention of plaque accumulation is far superior over removal of existing plaques, as the latter process is expected to simply leave “a hole in the brain,” which is not necessarily helpful for mental performance. As PET tomography now can specifically visualize Alzheimer plaques in the living patient, it will be possible to treat patients very early, in fact sometimes before they even show any symptoms.²⁴ In familial Alzheimer, plaque deposition could be visualized a decade before onset of symptoms. As most of the potential side effects will be due to interaction of antibodies with plaques, it is obvious that early immunization will also keep the side-effects low. PET may also be used as a surrogate marker for efficacy as it is now possible to demonstrate reduction in plaque load in situ in patients treated with mAbs.²⁸

There are additional biomarkers that may be used to assist diagnosis and progression of disease. Levels of Aβ peptides and tau in cerebrospinal fluid (CSF) have been shown to be altered in familial Alzheimer disease patients more than 10 y before onset of disease and may therefore represent additional ways to diagnose AD early.²⁹ In addition, levels of tau protein in CSF may also be used to follow disease development under treatment. In contrast, levels of Aβ_1-40 and Aβ_1-42 -peptides may be less useful biomarkers to monitor therapy success, as antibodies will stabilize those peptides and increase their levels at least in the blood. Indeed, antibody bound Aβ peptides in serum seen after vaccination may be viewed as auspicious sign for potential efficacy. Levels of phospho-tau, N-glycan or lipid profiles in blood may be additional promising markers; they are, however, far from routine clinical application and it is unclear how robust the signals are.³⁰

The way Aβ_1-42 deposits are recognized in treated patients may also be an important distinction between mAbs and vaccines in terms of their safety. mAbs are injected at high doses in a very short time period; interaction of antibodies and Aβ_1-42 deposits in e.g., blood vessels or the brain will therefore be rapid and strong. In contrast, onset of antibody production upon vaccination will be slow, likely reducing the potential side effects. Indeed Bapineuzumab has a number of side-effects (e.g., vasogenic edema) which do not seem to occur in vaccinated individuals.

Table 1 summarizes the various strategies to treat Alzheimer disease. It also includes classical small molecules. The marketed drugs are all not disease modifying but there is a number of new molecules as well as mAbs and vaccines in development which are expected to be disease-modifying by reducing plaque burden or preventing their build-up.

Table 1. Strategies to treat Alzheimer disease^31-35.

Molecule Type	Target	Phase
Small Molecules	Acetycholine Esterase	Market
	NMDA receptor	Market
	β-amyloid	Phase II
	Phospho-Tau	Phase II
	β/γ-secretase	Phase II
mAbs	β-amyloid	Phase III
	TNF	Phase II
Vaccine	β-amyloid	Phase II
	Phospho-Tau	Phase I

The way forward: From therapy to prophylaxis

There are two key changes that need to be implemented in new Alzheimer vaccine programs. (1) The vaccines need to be optimized for old people. This may be achieved by introducing new structural features into the vaccine backbone and/or use of optimized adjuvants as well as regimens. (2) Patients with mild disease will need to be recruited. PET-imaging will be the key to allow early diagnosis and monitoring treatment success. Thus, vaccine-based treatment of Alzheimer disease will move from late therapeutic to early almost prophylactic interventions. This will allow preventing the build-up of plaques, bringing benefit to the patient with minimal damage expected.

Side effects caused by vaccination against Aβ are expected to be minimal in the absence of Aβ_1-42 aggregates, i.e., plaques. Thus, the earlier a vaccine may be used, the smaller the expected side effects. Indeed, vaccination before plaque deposition in a prophylactic manner may be possible without causing any undue side effects. Thus, in a way similar to the childhood vaccines that we are all familiar with, a vaccine against Alzheimer disease may become the first prophylactic vaccine specifically used in senior citizens.

10.4161/hv.28183

Disclosure of Potential Conflicts of interest

All authors are involved in the development of active immunotherapies.

Acknowledgments

We would like to thank Adrian Hill, Thomas Kündig, and Pål Johansen for helpful discussions.