New treatment strategies for Alzheimer's disease: is there a hope?

Therapeutic targets for disease modification in AD

The recognition of core and secondary pathophysiological mechanisms in AD has led to the identification of molecular targets for the development of specific drugs. In fact, more than 200 pharmaceutical compounds are currently undergoing phase 2 and 3 trials11. These compounds can be grossly divided into anti-amyloid agents and drugs that target other pathological pathways. Anti-amyloid compounds can be subdivided into drugs designed to block or inhibit the overproduction or aggregation of Aβ, or to favour its clearance from the brain (Table I; Fig. 2)20, whereas the latter group can be subdivided according to predominant mechanism of action of the drug, i.e., neurotransmitter and cell-signalling agents, glial cell modulators, neuroprotective agents, and Tau-based therapies (Table II). Studies involving stem-cell and gene therapy are also under way, but at more incipient stages of experimental validation.

Table I Compounds targeted to anti-beta-amyloid treatment

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Fig. 2

Stages of amyloid (Aβ) beta production with possible targets for treatment. Red arrows indicate possible interventional targets with respective agent.

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Table II Overall pharmacologic treatments other than anti-amyloid therapy under research for Alzheimer's disease113233

In AD, pathophysiological mechanisms change soluble Aβ peptides into fibrillary oligomers and insoluble fibrils, which accumulate extracellularly in the neural tissue and also in the intima of brain and systemic vessels34. Extracellular Aβ oligomers and fibrillary forms cause synaptic dysfunction, affect axons and dendritic spines, and eventually lead to neuronal loss in AD35. Toxic Aβ species also trigger secondary pathological mechanisms (e.g., oxidative stress and inflammation), which hasten neuronal dysfunction and death36. Therefore, pharmacological compounds that favour the clearance of Aβ from the brain, or prevent its aggregation, may represent a strategy to delay the progression of the pathological process in AD. Intracerebral amyloidosis may start in the brain of individuals with AD many years before the onset of clinical symptoms3738. Evidence of this pathological process can be depicted at prodromal or even at preclinical stages of the disease by the analysis of cerebrospinal fluid (CSF) and molecular neuroimaging biomarkers373839. Concentrations of soluble Aβ in the CSF, which are known to be reduced in patients with MCI who convert to dementia, have been inversely correlated to the rate of cognitive decline and to the severity of neurodegeneration3940. Although molecular imaging with PET provides measurement of pathological deposition of extracellular fibrillar Aβ, which brings plaques into the brain, there is no definitive neuroimaging confirmation for synaptic dysfunction with currently available tracers41.

Anti-amyloid therapy

Anti-amyloid strategies comprise pharmaceutical compounds with distinct mechanisms of action, namely drugs that (i) facilitate the clearance; (ii) inhibit the production; or (iii) prevent the aggregation of Aβ32. As shown in Table I, many pharmacological compounds have been developed to tackle the “amyloid cascade”, with the prospect of reducing the Aβ burden in the brain of mild to moderately demented AD patients2042. However, there is evidence that these interventions must be implemented at earlier stages of the disease process, i.e., at the stage of incipient dementia or prior to the conversion from mild cognitive impairment to dementia (in the MCI-AD continuum), or even earlier. It is accepted that two thirds of individuals with MCI may have intracerebral amyloid burden comparable with those with clinically manifested AD, indicating that intracerebral amyloidosis is an early event in the natural history of the disease4344.

Both active and passive immunization target the reduction of intracerebral Aβ load by eliciting humoral response against the Aβ peptide, facilitating its clearance from the brain by immune-mediated mechanisms45. Highly encouraging findings were presented by preclinical studies with transgenic mice with high Aβ load, submitted to active and passive immunization; these strategies proved effective reducing the amount of Aβ in the mouse brain, which was supposedly associated with improvements on behaviour and cognition4647. These studies have provided the rationale for the first generation of immunotherapeutic agents for AD, which was based on the active immunization of AD patients with the actual Aβ peptide48. Therefore, this strategy induces an IgM response to generate antibodies against pathogenic Aβ, which further mobilize microglia to clean plaques through phagocytosis3245. Also, the immune response prevents Aβ deposition by removing the excess of soluble Aβ forms from the circulation45. Phase 2A studies with the active anti-Aβ vaccine (AN 1792) proved efficacious to remove Aβ plaques from the brain of AD patients; however, the trial was interrupted due to the occurrence of severe adverse events in a substantial proportion of subjects (6%) who received this intervention, which was associated with the induction of a cytotoxic T-cell reaction in brain vessels leading to acute meningoencephalitis21. Interim analyses indicated that although the immunization promoted Aβ clearance, it was not associated with clinically relevant benefits, i.e., the immunological response was not associated with the successfully completed clinical trials2223.

The second-generation of active anti-Aβ immunotherapeutic agents was designed to minimize the risk of eliciting such secondary inflammatory responses or vasogenic oedema, by stimulating soluble Aβ derivative immunogens. These vaccines elicit the immune response to raise antibodies against Aβ monomers and oligomers. Studies with the vaccine CAD106 at phase 1 indicated that it was able to reduce Aβ accumulation in cortical and subcortical brain regions by binding to Aβ aggregates and blocking cellular toxicity, with no evidence of microhaemorrhage, vasogenic oedema, or inflammatory reactions subsequent to activation of T-cells24.

Conversely, passive immunotherapy is based on the intravenous administration of full monoclonal antibodies or antibody fragments from specific exogenous origins, which directly target Aβ34. This strategy is supposedly not associated with a significant risk of eliciting T-cell inflammatory reactions45. In passive immunization, the binding of antibodies to specific Aβ epitopes induces plaque clearance through microglial activation48. In transgenic mice, peripheral infusion of antibodies raised specifically against the Aβ peptide was shown to reduce brain amyloid load27. As occurred with active immunotherapy, successful preclinical studies with passive immunotherapy were followed by phase 1 and phase 2 trials for AD. Afterward, advantageous results with transgenic animal models encouraged several centers to design clinical trials in order to establish effective treatments for AD patients and to change the disease course. Currently, some passive immunotherapies are being developed. A challengeable aspect concerns the efficacy against pathophysiology of the disease with avoidance of undesired side effects such as microhaemorrhage, vasogenic oedema, encephalitis, or neuroimaging abnormalities27. Across the screening cohorts enrolling placebo-treated patients, rarely occurrences of brain areas of microhaemorrhage have been described4950. Conversely, another group reported vasogenic oedema with this drug28.

Several passive immunotherapeutic agents have been evaluated by RCTs over the past years, namely bapineuzumab, solanezumab, gantenerumab, ponezumab, and crenezumab. These monoclonal antibodies have high affinity to antigenic determinant epitopes of Aβ, binding either to soluble forms or in plaques, being further recognized by B- and T-cells to promote its clearance from the brain2748. In addition, monoclonal antibodies may delay amyloid-β burden or stop its accumulation in the brain2748. Bapineuzumab represents the humanized Aβ monoclonal antibody that has been most tested as a candidate drug for the treatment of AD, and is currently undergoing phase 3 studies. In phase 2 studies, this compound was examined in a randomized placebo controlled trial to determine dose, safety, and efficacy in 234 patients with mild to moderate AD. Patients were given six infusions, in 13 distinct weeks, with last evaluation at week 7825. Ascending doses of bapineuzumab were shown to reduce intracerebral Aβ load through increased clearance from the brain; however, this effect was not associated with a significant benefit in clinical outcomes. Reversible and asymptomatic or transient vasogenic oedema was detected in 9.7 per cent of immunized patients, being more frequent in those receiving higher doses and in APOE*4 carriers25. Recently, during The European Federation of Neurological Societies annual meeting, in Stockholm, Sweden (2012), researchers reported that bapineuzumab failed to protect against cognitive and functional decline of AD patients undergoing a phase 3 trial23.

Solanezumab is another humanized monoclonal antibody that binds to the mid domain of soluble forms of Aβ peptide. Preliminary results indicated that it was effective promoting Aβ clearance from the brain; however, two placebo-controlled phase 3 trials failed to demonstrate clinical benefits23. Rare occurrences of micro-haemorrhage and vasogenic oedema have also been described with this drug2849. In a study conducted in Japan with AD patients with mild to moderate dementia, treatment with solanezumab was associated with a significant increase in plasma concentrations of the Aβ peptide, reflecting the increased clearance of Aβ from the brain, in the absence of relevant side effects50.

The monoclonal antibody ponezumab targets the amino-terminal portion of Aβ_1-40. In animal models this antibody significantly reduced cerebral Aβ burden and cerebral amyloid angiopathy, along with improvements in behaviour27. In a phase 1 study, a single intravenous dose of ponezumab was shown to be safe and well tolerated, and associated with increments in plasma and CSF concentrations of the Aβ peptide, suggesting that the drug may alter central Aβ levels. However, subsequent phase 2 studies did not confirm clinical efficacy, and development of ponezumab for mild to moderate AD was interrupted26. There are other ongoing trials with humanized monoclonal antibodies raised against the Aβ peptide, such as gantenerumab and crenezumab. Gantenerumab seems to reduce the cerebral Aβ load in AD patients in a dose-dependent fashion27. However, it is yet to be determined whether treatment with this antibody can slow disease progression and improve clinical outcomes49. Data from a phase 1 clinical trial testing another humanized anti-Aβ monoclonal antibody, known as MABT5102A, to protect against Aβ_1-42 oligomer-induced cytotoxicity, showed no vasogenic oedema, even among APOE*4 carriers50.

Other anti-amyloid strategies have been addressed by clinical trials. Preliminary studies support that the production and accumulation of Aβ can be downregulated by the specific γ-secretase inhibitors avagacestat and semagacestat2628.

Recently, semagacestat, a non-selective gamma-secretase inhibitor, has been examined as a potential treatment for AD patients51. Unfortunately, preliminary results from two ongoing long-term phase III trials found no efficacy. The studies were interrupted because researchers verified it did not slow disease progression and, in addition, was associated with increasing cognitive impairment and worsening daily living activities, as well with increasing risk for skin cancer with semagacestat51. Although vasogenic oedema has been a rare vascular event, a report described exacerbation of psoriatic skin lesions with this drug2152.

Avagacestat has been considered as a potently inhibitor of Ab40 and Ab42 formation, with selectivity for effects on APP relative to Notch proteins which interfere with cell proliferation, differentiation, and apoptosis. In a study enrolling healthy subjects, this compound exerted a potent selective gamma-secretase inhibition with decreased CSF Ab levels, as well as the inhibition of the human Notch proteins53. The authors reported a good tolerability profile and no changes in histopathology of skin or in lymphocytes following 28 days of dosing, although this period was not enough to confirm such clinical alterations in humans52. A phase II study with AD outpatients reported a dose-dependent of avagacestat on CSF amyloid isoforms and Tau protein. However, the authors demonstrated no signicant reduction in these biomarkers, although they reported an acceptable safety and tolerability with this drug54. Another study was designed to compare chronic treatment of transgenic mice models of AD between the nonsteroidal anti-inflammatory CHF5074 and a prototypal gamma-secretase inhibitor (DAPT)31. The authors observed that the CHF5074 compound has effectively prevented amyloid accumulation in the brain tissue and behavioural impairment; however, no significant effects were found with DAPT treatment.

The inhibition of beta-secretase is another potential mechanism of disease modification in AD, given the major role of this enzyme in the amyloidogenic cleavage of the amyloid precursor protein (APP). BACE-1 (β-site amyloid precursor protein cleaving enzyme 1) produces two peptides (Ab40 and Ab42), and its inhibition with specific compounds precludes the excess of amyloid and its accumulation into plaques55.

An inhibitor of β-secretase (GRL-8234) was recently investigated in young transgenic mice with decreased soluble amyloid-beta in the brain tissue and with rescued behaviour performance29. These findings indicated that β-secretase inhibitors play a role in reduction of amyloid-beta plaque load at an early stage of AD pathogenesis, as well as in older mice, suggesting possible benefits for treatment of AD patients at a later stage of the disease29.

TAK-070 is a non-petpidic agent that inhibits BACE-1 in a dose-dependent and non-competitive maner. However, the inhibitory mechanism of this compound are not clear, and the reduction of amyloid-beta has been considered modest with low selectivity over other enzymes30. Using an AD transgenic mouse model, the immunogenicity and efficacy of the compound AF20513 was tested and it was found that this active vaccine induced cellular and humoral immune responses against beta-amyloid pathophysiology in the brain tissue of the animals, without inducing microglial activation56. In addition, the amyloid-β anti-aggregator curcumin could be a future therapeutic strategy22. However, large clinical trials are needed to test this and other promising drugs.

The occurrence of vasogenic oedema and multiple cortical and subcortical vascular lesions represent adverse events that must be considered in Aβ immunotherapy with monoclonal antibodies. These radiological findings are associated with cerebral amyloid angiopathy-related inflammation (CAA-ri), and can be asymptomatic or present clinically with acute or sub-acute manifestations such as headache, seizures, focal neurological deficits, and behavioural disturbances5758. This reaction was observed with bapineuzumab and other monoclonal antibodies but, interestingly, it was also reported in patients treated with the γ-secretase inhibitor avagacestat2659.

Tau-oriented strategies

Given its critical role in pathogenesis of AD, drug development may also target the production, processing (phosphorylation) and aggregation of Tau protein33. Three ‘anti-tau’ strategies have been addressed by clinical trials. Agents like methylene-blue (Rember) and NAP (AL-108) favour the stabilization of microtubules, while lithium salts may prevent Tau hyperphosphorylation through the inhibition of GSK-3β60. Two widely used mood stabilizers, lithium and valproate, are also inhibitors of this enzyme, reducing tau phosphorylation in animal models61. Proteomic studies in AD patients treated with valproate indicated ten differentially expressed proteins related to functional classes implicated in the neurobiology of the disease and to the therapeutic action of drug62. However, in a multicenter clinical trial conducted by the Alzheimer's Disease Cooperative Study (ADCS), an accelerated decrease in total brain and hippocampal volume was observed after 1 year of followup among patients treated with divalproex sodium, which was accompanied by greater cognitive impairment63. In addition, valproate treatment of moderately demented AD patients did not delay the emergence of neuropsychiatric symptoms of agitation or psychosis, and was not associated with reduction of cognitive or functional decline; rather, it was associated with adverse effects such as somnolence, gait disturbance, tremor, diarrhoea and weakness64.

Lithium, a potent GSK-3 inhibitor, has shown neuroprotective action such as reducing Tau concentration and phosphorylation, via the inhibition of GSK-3β, reducing Aβ production, axonal degeneration and releasing the transforming growing factor beta 1 (TGF-β1) in experimental studies of cultured neurons and transgenic mice656667686970. Epidemiologic studies showed a reduced risk of developing dementia among patients with bipolar disorder treated chronically with lithium7172, but there is limited evidence of the clinical benefits of lithium treatment for patients with or at risk for AD73747576. In a recent single-centre, placebo-controlled trial conducted in a sample of patients with amnestic MCI, long-term lithium treatment was associated with stabilization of cognitive and functional parameters, in addition to a reduction in CSF concentrations of phosphorylated Tau76. In a previous phase 2 study in patients with mild AD treated with lithium for a shorter (10 wk) period, no significant differences were observed in biological (GSK-3 activity and CSF biomarkers) or clinical (cognitive and functional) outcomes74. In addition to that, the safety limits of chronic lithium treatment for older adults need to be determined, given the risk of renal impairment, hypothyroidism and other metabolic adverse effects77. Other pharmaceutical compounds targeting GSK-3β, such as the specific inhibitors AR-A014418, SRN-003-55, CHIR-98014 and SB216763, have so far shown neuroprotective effects in preclinical models of AD78.

Methylthioninium chloride (known as Rember), also known as methylene-blue, is a phenothiazine widely used as a histological stain. Currently, it is a promising compound being evaluated in AD trials. The drug has anti-oxidative properties and has been shown to reduce Aβ oligomerization, given its ability to bind to the Tau domain that is required for its aggregation, therefore, preventing the formation of PHFs79. In animal models, it has been shown to decrease Aβ levels and prevent cognitive deficits in triple transgenic mice80. A phase 2b randomized trial of Rember monotherapy in mild and moderate AD patients suggested cognitive benefits in volunteers81. Results of a phase 3 trial are soon expected to assure its safety profile and efficacy as a disease-modifying drug for AD82.