ABSTRACT
Introduction: Alzheimer’s disease (AD) vaccination is one of the last therapeutic options after two decades of stagnation in terms of drug development. About 140 (85%) immunization procedures against Aβ deposition and 25 (15%) against Tau have been reported, but no Food and Drug Administration approval of any AD vaccine has been achieved. This might be attributed to deficient pathogenic targets, inappropriate models, defective immunotherapeutic procedures, and inadequate clinical trial design.
Areas covered: The issues covered include the following: AD pathogenic mechanisms, rationale for active and passive immunization, vaccine targets, anti-Aβ/Tau vaccines, vaccine technologies, animal models, and clinical trials.
Expert opinion: A vaccine against AD is technically feasible; however, important methodological aspects should be changed for a tentative clinical success, including (i) the development of multitarget AD immunotherapies; (ii) the optimization of antibody titers and epitopes; (iii) the pharmacogenetic/pharmacoepigenetic validation of the immunization procedure; (iv) the prophylactic treatment of genetically stratified patients at a pre-symptomatic stage; and (v) the definition of primary endpoints in prevention, based on objective/multifactorial biomarkers. Even with exquisite protocols, a successful vaccine would be potentially useful in at most 20–30% of defined cases, according to the genetic, epigenetic, and pharmacogenetic background of AD patients.
1. Introduction
After almost 20 years of therapeutic stagnation in Alzheimer’s disease (AD), despite important investment and scientific effort [Citation1], some reflections are opportune concerning strategies in drug development, especially immunotherapy and AD vaccination. It was a great achievement to demonstrate that AD is the result of a neurodegenerative process which commences decades before the onset of the disease, caused by multifactorial events (genomic defects, epigenetic dysregulation, cerebrovascular damage, and diverse environmental factors), in order to implement potential prophylactic/preventive strategies.
Since the pioneering studies of Schenk and coworkers in 1999 [Citation2], several categories of vaccines and immunotherapeutic procedures have been developed for the treatment of AD (–) based on the two most prevalent pathogenic hypotheses (amyloidosis vs. tauopathy) that give rise to the characteristic AD neuropathological phenotype: extracellularly aggregated amyloid-β (Aβ) peptides deposited in senile plaques and intracellular neurofibrillary tangle (NFT) formation (resulting from the hyperphosphorylation of protein Tau). Two decades later, with millions of dollars invested in passive and active immunotherapy in experimental AD models and in clinical trials, and over 1000 papers published on AD immunotherapy (40–50 papers/year), about 140 (85%) immunization procedures against Aβ deposition, and 25 (15%) against Tau have been reported, but no Food and Drug Administration (FDA) approval of any AD vaccine has been achieved. Meta-analytic studies conclude that no significant benefit in clinical terms has been obtained with immunotherapy in AD [Citation3].
Table 1. Anti-Aβ immunization procedures and vaccine products.
Table 2. Anti-Tau immunization procedures and vaccine products.
Taking into consideration the health and economic impact of AD in our society, further vaccine development deserves a deep reflection by the scientific community, the pharmaceutical industry, and the health authorities worldwide. The intention of this editorial article is to delve into the technical and methodological aspects of immunization procedures that are not frequently addressed in the current literature on this subject, analyzing the plausibility of using immunotherapy as a potential strategy for the prevention and/or treatment of AD in the future.
Assuming that mutations in the Amyloid Precursor Protein (APP) gene represent one of the primary causes of AD, the rationale of the initial active immunization studies was partially correct, demonstrating that pre-symptomatic immunization of PDAPP transgenic mice, which overexpress mutant human APP (Phe717Val), prevented Aβ-plaque formation, neuritic dystrophy, and astrogliosis, and immunization with Aβ42, the most abundant isoform of pathogenic Aβ peptides, in animals with manifest AD neuropathology reduced the extent and progression of AD-like neurodegeneration markers [Citation2]. Aβ immunization also improved cognition in the TgCRND8 murine model of AD without altering total Aβ brain levels [Citation4]. These studies were replicated in over 400 experimental studies with different immunization procedures (). However, the initial clinical trial with an active Aβ vaccine (AN1792) was halted due to the development of severe complications (i.e. acute meningoencephalitis, micro-hemorrhages) in a number of immunized patients. Some of these adverse reactions were associated with a T-cell-mediated pro-inflammatory response and other still unknown mechanisms, but fortunately most of these have been ameliorated or eliminated in recent times with improved immunization procedures (i.e. new adjuvants, polymers, recombinant plant virus-based nanoparticles (PVNs), liposomes enriched with trophic factors, wheat germ agglutinin (WGA) (axonal transporter carrier), nanocarriers, bee venom phospholipase A2, 6copy-Aβ1-6-PA-BLP, mesoporous silicon particles, HIV-1-derived immunogen HIVconsv, helper T-cell peptide epitopes (UBITh®), Th2-biased delivery systems, D,L-lactide co-glycolide (PLG) microparticles, oligomer-specific mimotopes as immunogens, DNA vaccines, Aβ retroparticles, Aβ-Cys peptides, CRM197 carrier protein, and CpG oligodeoxynucleotides).
Second-generation Aβ-active immunotherapies, anti-Aβ monoclonal antibodies targeting different Aβ epitopes, and anti-Tau immunotherapies have dominated the scenario of AD vaccines during the past decade (–) [Citation5–Citation7]. Among anti-Tau strategies, AADvac1 is an active immunotherapy against tau pathology (Phase-II clinical trials). AADvac1-related antibodies target conformational epitopes in the microtubule-binding region of tau, preventing the spreading of tau aggregation and promoting tau clearance, with an apparently safe profile [Citation6].
Some dual vaccines (EB101) [Citation8], Aβ3-10-KLH vaccine [Citation9], or active full-length DNA-Aβ42 trimer immunization [Citation10] demonstrated a capacity to reduce both amyloid and tau aggregation and accumulation in transgenic animals, probably via the proteasome [Citation11]; and some Tau oligomer-specific monoclonal antibodies may also reduce Aβ load.
Despite excellent preclinical studies, all clinical trials so far have failed, probably due to (i) deficient pathogenic targets, (ii) inappropriate models, (iii) defective immunotherapeutic procedures, and (iv) inadequate clinical trial design.
Multiple genomic defects play an important role in AD pathogenesis, highly influenced by epigenetic aberrations, cerebrovascular dysfunction, and environmental factors, leading to the AD phenotype (classic neuropathological hallmarks and clinical picture: cognitive, behavioral, and psychomotor decline). Over 600 genes distributed across the human genome may contribute to AD-related neurodegeneration. Some of them are identified as primary pathogenetic genes (i.e. APP, PSEN1, APOE), and many others are secondarily relevant by association with pathogenic cascades potentially responsible for premature neuronal death. Mutations in the APP and MAPT genes are present in less than 5% of AD cases, though conformational changes in Aβ and hyperphosphorylation of tau contribute to the major neuropathological hallmarks of AD (Aβ plaques and NFTs). As a primary target, it is very unlikely that active or passive immunization procedures addressing specific epitopes at Aβ and/or Tau structures will be effective in more than 20–30% of AD cases, using changes in plaques and tangles as major surrogate markers of immunotherapeutic efficacy. Epitope selection is also important in vaccination procedures (i.e. pGlu-3 Aβ immunoglobulin G1 (IgG1) monoclonal antibody 07/1 vs. general Aβ IgG1 monoclonal antibody 3A1) [Citation12]. The toxicity of Aβ oligomers depends on size and conformation. The Aβ1-15 N-terminus contains a B-cell surface antigen necessary for antibody production that prevents the adverse reactions associated with Aβ1-42. Aggregated tau and hyperphosphorylated tau activate the immune system inducing CD4+ T cell responses which escape tolerance. Neuronal uptake and efficacy of tau antibodies for preventing tau toxicity and pathological seeding depend on antibody charge. In tautologic terms, AD-related amyloidopathy and/or tauopathy are but phenotypes of an upstream pathogenic event that induces premature neuronal death. The correction of this neurodegenerative phenotype at most slows down disease progression but does not eradicate disease etiology or modify disease genotype (or the disrupted epigenomic mechanism that leads to the AD geno-phenotype). The physiological and/or pathological consequences of endogenous Aβ clearance also require further analysis in the long term, and there is no conclusive proof that chronic Aβ and NFT clearance is therapeutically effective without future repercussions in brain function and disease progression (i.e. encephalitogenic and neurotoxic potential of Aβ/Tau immunotherapy; adjuvant-related neurotoxicity; CD4+ T cell response; Aβ/Tau-related Ig response; evading innate and adaptive immune mechanisms; neuroinflammatory reactions; cytokine reactivity; immune tolerance; immunosenescence; immunization-related neoangiogenesis, vasogenic edema and cerebrovascular regulation; cerebral amyloid angiopathy; generation of autoantibodies; impact on neuronal loss and dendritic dearborization; interference with neurotrophic factors; induction of epigenetic aberrations; and effects of vaccine decay over time).
Other more relevant genomic defects (Mendelian mutations, susceptibility SNPs, and CNV) and epigenetic aberrations (DNA methylation, chromatin/histone modifications, and microRNA dysregulation), responsible for the abnormal expression of specific genes and anomalies in protein conformation, are pathogenic events that cannot be neglected in AD therapeutics (primary and secondary prevention) [Citation13,Citation14]. A typical paradigm is apolipoprotein E (ApoE). The presence of the APOE-4 allele is a major risk factor for AD, and over 40% of AD patients are carriers of this allele. However, immunization against ApoE-4 epitopes and/or APOE-4-related phenotypes (i.e. atherosclerosis and cardiovascular and cerebrovascular disorders) has not attracted much attention over the years, even considering that APOE genotypes are fundamental determinants of amyloidotic and/or tauopathic phenotypes. In fact, APOE-4/4 carriers are the worst responders to conventional AD treatments and vaccines (especially with passive immunization [Citation15,Citation16], clearly indicating that the pathogenic influence of ApoE cannot be ignored in any therapeutic intervention in AD to be holistically effective [Citation17]).
Another important issue to be revised, in agreement with previous comments, lies in the animal models in which AD immunization is experimentally tested. APPswe/PS1dE9 mice co-expressing human APP with the Swedish mutation (APPswe) and exon-9-deleted presenilin (PS1dE9) are used as transgenic models in preclinical studies; however, many other models (BALB/c, B6D2F1, C57BL/6J, NORBA, 3×Tg-AD, TgCRND8, Tg2576, PDAPP, APPswe X PS1.M146V (TASTPM), APPSwDI/NOS2−/-, Ts65Dn, hAPP751, Tau-P301S, Tau-P301L, THY-Tau22, R3m/4 (3R tau, aa151-391), NZW Rabbits, aged beagles, Caribbean vervets, Cynomolgus monkeys, Rhesus monkeys (Macaca mulatta), Microcebus murinus primates and Caenorhabditis elegans CL4176), expressing differential AD-related phenotypes, are currently used in many studies. Not all animal models reproduce with high-fidelity AD neuropathology. Transcriptomic studies of Aβ plaque pathology clearly show that some mouse models (e.g. TgCRND8 Swedish + Indiana APP; KM670/671NL + V717F) more closely recapitulate specific AD-related transcriptional responses than others (e.g. Tg2576 Swedish APP; KM670/671NL) [Citation18].
Some other factors (i.e. age, sex, immune status, vaccine organoleptic features, vaccine immunogenetics, pharmacogenetics, pharmacoepigenetics) may affect the success or failure of a specific vaccine [Citation19]. For instance, there are age-related changes in antibody production in response to vaccination associated with expansion of atypical memory B cells and B cell phenotypes which may influence vaccine efficacy and safety. DNA vaccines show advantages over peptide-protein vaccines, but immune responses might be lower in humans as compared with small animals. Genomic and epigenetic factors may also influence vaccine immune properties.
Our understanding on how vaccines activate the immune system and elicit protective immunity has substantially improved in parallel with advances in molecular biology, immunology, microbiology, genetics, epigenetics, pharmacogenetics, and vaccinomics. Modern vaccine development relies on knowledge of the interaction of immunogens with the immune system and their capacity to evade and/or counteract innate and adaptive immune mechanisms. An effective and safe anti-Aβ immunotherapy would require optimal levels of anti-Aβ antibodies, avoiding proinflammatory adjuvants and autoreactive T cells. From a technical perspective, novel AD vaccine formulations should offer (i) highly purified antigens with lower reactogenicity and a higher safety profile (sometimes vaccine antigen purity deteriorates vaccine immunogenicity, and some adjuvants (i.e. monophosphoryl lipid A (MPL)/trehalose dicorynomycolate (TDM); Quillaja saponaria (QS-21); Alum (IMM-AD04); aluminum hydroxide; AddaVax; BCG-DNA; AS01B; Escherichia coli heat-labile enterotoxin LT (R192G) and LT-IS; cholera toxin B subunit (CTB); MF59; Freund) can differentially maximize immunogenicity with no apparent effects on tolerability or safety), (ii) genetically engineered vaccines and innovative technologies (DNA, RNA, vector vaccines, vector-mediated antibody expression, non-viral naked DNA plasmids, ex vivo gene therapy with retrievable encapsulated cellular implants), (iii) tailored vaccine design (dual vaccines, vaccines enriched with neurotrophic factors), (iv) chimeric vaccines, (v) multitarget models, (vi) novel delivery systems for neuroavailability, and (vii) personalized vaccines [Citation19,Citation20].
2. Expert opinion
AD immunotherapy has been the focus of scientific attention for the past two decades; however, the initial excitement on the potential effects of AD vaccines as a prophylactic strategy against AD was evanescing over time due to the failure of clinical trials with active or passive immunization procedures. The main reasons for this unfortunate drawback can be attributed to different problems: therapeutic targets, immunization processes, clinical trial design, and patient recruitment.
Concerning clinical trials, if an AD vaccine represents a prophylactic intervention for delaying disease onset (or, under optimal conditions, to avoid AD), it is nonsense to test vaccines in patients with mild–moderate dementia where brain damage is already consolidated. The purpose of a vaccine is not to resuscitate dying neurons but to prevent their premature neurodegeneration. Therefore, the clinical trial design has to be adapted to this new therapeutic modality, not reproducing obsolete procedures used for the clinical testing of symptomatic drugs (with no breakthroughs during the past 20 years) [Citation1]. In this regard, regulatory authorities (FDA, European Medicines Agency, Koseisho, and other national regulators), together with the pharmaceutical industry and the scientific community, have to design novel protocols for preventive intervention in AD.
Methodologically, a vaccine against AD is technically feasible; however, important procedural aspects should be changed for a tentative clinical success, including (i) the development of multitarget AD immunotherapies: addressing the binomial APP-MAPT target is insufficient to halt disease progression from early (asymptomatic) stages of AD; it is essential to address other targets (i.e. APOE, TOMM40, and other pathogenic gene products) which interact with APP processing to generate amyloid deposits and neurodegeneration; (ii) the optimization of antibody titers and epitopes by improving immunization procedures and vaccinomics; (iii) the pharmacogenetic/pharmacoepigenetic validation of the immunization procedure: only 20% of AD patients are extensive metabolizers for conventional treatments; immunization is not an exception and pharmaco(Epi)genetic geno-phenotyping should be a preceptive test for personalized interventions; vaccines might become, with time, a novel modality of epigenetic intervention to reversibly regulate gene expression in complex disorders such as AD; (iv) the prophylactic treatment of genetically stratified patients at a pre-symptomatic stage: the genomic background of each patient modulates disease progression and may elicit a differential immunotherapeutic response to AD vaccines (either passive or active procedures); and (v) the definition of primary endpoints in prevention, based on objective/multifactorial biomarkers (other than Aβ, Tau, and conventional psychometrics), for an efficient assessment of vaccine efficacy and safety: novel biomarkers should include genomic, epigenetic, and proteomic signatures in body fluids or functional neuroimaging; especially important are epigenetic and proteomic markers which are modifiable with disease progression and sensitive to therapeutic intervention in presymptomatic stages [Citation13,Citation21]. Even with exquisite protocols, a successful vaccine would be potentially useful in at most 20–30% of the defined cases, according to the genetic, epigenetic, and pharmacogenetic background of AD patients [Citation13]. The persistency in treating AD as a single clinical entity with a monotherapeutic vision is an unwise pathway to failure, as the past three decades of unsuccessful pharmacological development have demonstrated. Preventive intervention is an issue of paramount importance in the fight against dementia in developed countries and AD immunotherapy is the best positioned procedure for this purpose as long as technological and personalized processes are appropriately implemented.
Declaration of interest
R Cacabelos is President of the EuroEspes Group and Chair of Genomic Medicine. He has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.
Reviewer Disclosures
Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.
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References
- Cacabelos R. Have there been improvement in Alzheimer’s disease drug discovery over the past 5 years? Expert Opin Drug Discov. 2018;13(6):523–538.
- Schenk D, Barbour R, Dunn W, et al. Immunization with amyloid-beta attenuates Alzheimer-disease-like pathology in the PDAPP mouse. Nature. 1999;400(6740):173–177.
- Foroutan N, Hopkins RB, Tarride JE, et al. Safety and efficacy of active and passive immunotherapy in mild-to-moderate Alzheimer’s disease: a systematic review and network meta-analysis. Clin Invest Med. 2019;42(1):53–65.
- Janus C, Pearson J, McLaurin J, et al. A beta peptide immunization reduces behavioural impairment and plaques in a model of Alzheimer’s disease. Nature. 2000;408(6815):979–982.
- Herline K, Drummond E, Wisniewski T. Recent advancements toward therapeutic vaccines against Alzheimer’s disease. Expert Rev Vaccines. 2018;17(8):707–721.
- Novak P, Zilka N, Zilkova M, et al. AADvac1, an active immunotherapy for Alzheimer’s disease and non Alzheimer tauopathies: an overview of preclinical and clinical development. J Prev Alzheimers Dis. 2019;6(1):63–69.
- Hoskin JL, Sabbagh MN, Al-Hasan Y, et al. Tau immunotherapies for Alzheimer’s disease. Expert Opin Investig Drugs. 2019;28(6):545–554.
- Carrera I, Fernandez-Novoa L, Aliev G, et al. Validating Immunotherapy in Alzheimer’s disease: the EB101 vaccine. Curr Pharm Des. 2016;22(7):849–858.
- Zhang HY, Zhu K, Meng Y, et al. Reduction of amyloid beta by Aβ3-10-KLH vaccine also decreases tau pathology in 3×Tg-AD mice. Brain Res Bull. 2018;142:233–240.
- Rosenberg RN, Fu M, Lambracht-Washington D. Active full-length DNA Aβ42 immunization in 3xTg-AD mice reduces not only amyloid deposition but also tau pathology. Alzheimers Res Ther. 2018;10(1):115.
- Oddo S, Billings L, Kesslak JP, et al. Abeta immunotherapy leads to clearance of early, but not late, hyperphosphorylated tau aggregates via the proteasome. Neuron. 2004;43(3):321–332.
- Frost JL, Liu B, Rahfeld JU, et al. An anti-pyroglutamate-3 Aβ vaccine reduces plaques and improves cognition in APPswe/PS1ΔE9 mice. Neurobiol Aging. 2015;36(12):3187–3199.
- Cacabelos R, Carril JC, Sanmartín A, et al. Pharmacoepigenetic processors: epigenetic drugs, drug resistance, toxicoepigenetics and nutriepigenenetics. In: Cacabelos R, editor. Pharmacoepigenetics. Amsterdam: Elsevier/Academic Press; 2019. p. 191–424.
- Cacabelos R, Cacabelos N, Carril JC. The role of pharmacogenomics in adverse drug reactions. Expert Rev Clin Pharmacol. 2019;12(5):407–442.
- Panza F, Frisardi V, Imbimbo BP, et al. Bapineuzumab: anti-β-amyloid monoclonal antibodies for the treatment of Alzheimer’s disease. Immunotherapy. 2010;2(6):767–782.
- Cribbs DH. Abeta DNA vaccination for Alzheimer’s disease: focus on disease prevention. CNS Neurol Disord Drug Targets. 2010;9(2):207–216.
- Cacabelos R. Pharmacogenomics of Alzheimer’s and Parkinson’s diseases. Neurosci Lett. [ cited 2018 Sep 17]:1733807. DOI:10.1016/j.neulet.2018.09.018.
- Rothman SM, Tanis KQ, Gandhi P, et al. Human Alzheimer’s disease gene expression signatures and immune profile in APP mouse models: a discrete transcriptomic view of Aβ plaque pathology. J Neuroinflammation. 2018;15(1):256.
- Cacabelos R, editor. World guide for drug use and pharmacogenomics. Corunna (ES): EuroEspes Publishing; 2012.
- Zepp F. Principles of vaccination. Methods Mol Biol. 2016;1403:57–84.
- Bachmann MF, Jennings GT, Vogel M. A vaccine against Alzheimer`s disease: anything left but faith? Expert Opin Biol Ther. 2019;19(1):73–78.