Randomized Trial of Verubecestat for Prodromal Alzheimer's Disease

Multiple BACE1 inhibitors abnormally increase the BACE1 protein level in neurons by prolonging its half-life

INTRODUCTION

There is keen interest in elucidating the biological mechanisms underlying recent failures of β-site amyloid precursor protein-cleaving enzyme-1 (BACE1) inhibitors in Alzheimer's disease trials.

METHODS

We developed a highly sensitive and specific immunoassay for BACE1 in cell lines and iPSC-derived human neurons to systematically analyze the effects of eight clinically relevant BACE1 inhibitors.

RESULTS

Seven of 8 inhibitors elevated BACE1 protein levels. Among protease inhibitors tested, the elevation was specific to BACE1 inhibitors. The inhibitors did not increase BACE1 transcription but extended the protein's half-life. BACE1 became elevated at concentrations below the IC₅₀ for amyloid β (Aβ).

DISCUSSION

Elevation of BACE1 by 7 of 8 BACE1 inhibitors raises new concerns about advancing such β-secretase inhibitors for AD. Chronic elevation could lead to intermittently uninhibited BACE1 when orally dosed inhibitors reach trough levels, abnormally increasing substrate processing. Compounds such as roburic acid that lower Aβ by dissociating β/γ secretase complexes are better candidates because they neither inhibit β- and γ-secretase nor increase BACE1 levels.

Further analyses of the safety of verubecestat in the phase 3 EPOCH trial of mild-to-moderate Alzheimer's disease

BACKGROUND

Verubecestat, a BACE1 inhibitor that reduces Aβ levels in the cerebrospinal fluid of humans, was not effective in a phase 3 trial (EPOCH) of mild-to-moderate AD and was associated with adverse events. To assist in the development of BACE1 inhibitors, we report detailed safety findings from EPOCH.

METHODS

EPOCH was a randomized, double-blind, placebo-controlled 78-week trial evaluating verubecestat 12 mg and 40 mg in participants with mild-to-moderate AD diagnosed clinically. The trial was terminated due to futility close to its scheduled completion. Of 1957 participants who were randomized and took treatment, 652 were assigned to verubecestat 12 mg, 652 to verubecestat 40 mg, and 653 to placebo. Adverse events and relevant laboratory, vital sign, and ECG findings were assessed.

RESULTS

Verubecestat 12 mg and 40 mg were associated with an increase in the percentage of participants reporting adverse events versus placebo (89 and 92% vs. 82%), although relatively few participants discontinued treatment due to adverse events (8 and 9% vs. 6%). Adverse events that were increased versus placebo included falls and injuries, suicidal ideation, weight loss, sleep disturbance, rash, and hair color change. Most were mild to moderate in severity. Treatment differences in suicidal ideation emerged within the first 3 months but did not appear to increase after 6 months. In contrast, treatment differences in falls and injuries continued to increase over time.

CONCLUSIONS

Verubecestat was associated with increased risk for several types of adverse events. Falls and injuries were notable for progressive increases over time. While the mechanisms underlying the increased adverse events are unclear, they may be due to BACE inhibition and should be considered in future clinical development programs of BACE1 inhibitors.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01739348 , registered on 29 November 2012.

Characterization of Cerebrospinal Fluid BACE1 Species

The β-site amyloid precursor protein cleaving enzyme 1 (BACE1) is the main brain β-secretase responsible for the amyloidogenic processing of the amyloid precursor protein (APP). Previous studies have suggested that cerebrospinal fluid (CSF) β-secretase activity may be a candidate diagnostic biomarker for Alzheimer's disease (AD), but biochemical characterization of BACE1 protein in CSF is needed. CSF samples from 19 AD patients and 19 age-matched non-AD controls (n = 19) were classified according to their Aβ42, total tau, and P-tau CSF biomarker levels. We found that β-secretase activity was higher in the CSF of AD subjects than in that of the controls. We found that the majority of the β-secretase activity in the CSF, measured using a peptide substrate homologous to the BACE1 cleavage site, was not inhibited by specific BACE1 inhibitors. We defined enzymatic activity attributable specifically to BACE1 as the activity that was blocked by the specific inhibitors, which is still higher in AD subjects. BACE1 protein levels were characterized by lectin binding, immunoprecipitation, blue native-PAGE, and western blotting using antibodies against specific protein domains. BACE1 was found to be present in human CSF as a mature form of ~ 70 kDa that probably comprised truncated and full-length species, and also as an immature form of ~ 50 kDa that retains the prodomain. CSF-BACE1 was found to assemble into hetero-complexes containing distinct species. Immunoblotting with an antibody against the C-terminus of BACE1 revealed significantly higher levels of the 70-kDa full-length BACE1, while the 50 kDa immature form remained unaltered. When the 70-kDa species was probed with an antibody against the N-terminus of BACE1 (which does not discriminate between truncated and full-length forms), no increase in immunoreactivity was observed, suggesting that truncated forms of BACE1 do not increase in AD. In conclusion, the complexity of BACE1 species in CSF has to be taken into consideration when determining BACE1 activity and protein levels in CSF as biomarkers of AD.

Inhibition of the Self-Assembly of Aβ and of Tau by Polyphenols: Mechanistic Studies

The amyloid-β (Aβ) peptide and tau protein are thought to play key neuropathogenic roles in Alzheimer's disease (AD). Both Aβ and tau self-assemble to form the two major pathological hallmarks of AD: amyloid plaques and neurofibrillary tangles, respectively. In this review, we show that naturally occurring polyphenols abundant in fruits, vegetables, red wine, and tea possess the ability to target pathways associated with the formation of assemblies of Aβ and tau. Polyphenols modulate the enzymatic processing of the amyloid-β precursor protein and inhibit toxic Aβ oligomerization by enhancing the clearance of Aβ42 monomer, modulating monomer-monomer interactions and remodeling oligomers to non-toxic forms. Additionally, polyphenols modulate tau hyperphosphorylation and inhibit tau β-sheet formation. The anti-Aβ-self-assembly and anti-tau-self-assembly effects of polyphenols increase their potential as preventive or therapeutic agents against AD, a complex disease that involves many pathological mechanisms.

AD molecular: PET amyloid imaging across the Alzheimer's disease spectrum: From disease mechanisms to prevention

The advent of amyloid-beta (Aβ) positron emission tomography (PET) imaging has transformed the field of Alzheimer's disease (AD) by enabling the quantification of cortical Aβ accumulation and propagation in vivo. This revolutionary tool has made it possible to measure direct associations between Aβ and other AD biomarkers, to identify factors that influence Aβ accumulation and to redefine entry criteria into clinical trials as well as measure drug target engagement. This chapter summarizes the main findings on the associations of Aβ with other biomarkers of disease progression across the AD spectrum. It discusses investigations of the timing at which Aβ pathology starts to accumulate, demonstrates the clinical utility of Aβ PET imaging and discusses some ethical implications. Finally, it presents genetic and potentially modifiable lifestyle factors that might influence Aβ accumulation and therefore be targets for AD prevention.

Trafficking and proteolytic processing of amyloid precursor protein and secretases in Alzheimer's disease development: An up-to-date review

Alzheimer's disease (AD), which is predicted to affect 1 in 85 persons worldwide by 2050, results in progressive loss of neuronal functions, leading to impairments in memory and cognitive abilities. As being one of the major neuropathological hallmarks of AD, senile plaques mainly consist of amyloid-β (Aβ) peptides, which are derived from amyloid precursor protein (APP) via the sequential cleavage by β- and γ-secretases. Although the overproduction and accumulation of Aβ peptides are at the center of AD research, the new discoveries point out to the complexity of the disease development. In this respect, it is crucial to understand the processing and the trafficking of APP, the enzymes involved in its processing, the cleavage products and their therapeutic potentials. This review summarizes the salient features of APP processing focusing on APP, the canonical secretases as well as the novel secretases and the cleavage products with an update of the recent developments. We also discussed the intracellular trafficking of APP and secretases in addition to their potential in AD therapy.

Alzheimer's disease

Alzheimer's disease (AD) dementia refers to a particular onset and course of cognitive and functional decline associated with age together with a particular neuropathology. It was first described by Alois Alzheimer in 1906 about a patient whom he first encountered in 1901. Modern clinical diagnostic criteria have been developed, and criteria have also been proposed to recognize preclinical (or presymptomatic) stages of the disease with the use of biomarkers. The primary neuropathology was described by Alzheimer, and in the mid-1980s subsequently evolved into a more specific neuropathologic definition that recognizes the comorbid neuropathologies that frequently contribute to clinical dementia. Alzheimer's disease is now the most common form of neurodegenerative dementia in the United States with a disproportionate disease burden in minority populations. Deficits in the ability to encode and store new memories characterizes the initial stages of the disease. Subsequent progressive changes in cognition and behavior accompany the later stages. Changes in amyloid precursor protein (APP) cleavage and production of the APP fragment beta-amyloid (Aβ) along with hyperphosphorylated tau protein aggregation coalesce to cause reduction in synaptic strength, synaptic loss, and neurodegeneration. Metabolic, vascular, and inflammatory changes, as well as comorbid pathologies are key components of the disease process. Symptomatic treatment offers a modest, clinically measurable effect in cognition, but disease-modifying therapies are desperately needed.

Current and Emerging Pharmacological Targets for the Treatment of Alzheimer's Disease

No cure or disease-modifying therapy for Alzheimer's disease (AD) has yet been realized. However, a multitude of pharmacological targets have been identified for possible engagement to enable drug discovery efforts for AD. Herein, we review these targets comprised around three main therapeutic strategies. First is an approach that targets the main pathological hallmarks of AD: amyloid-β (Aβ) oligomers and hyperphosphorylated tau tangles which primarily focuses on reducing formation and aggregation, and/or inducing their clearance. Second is a strategy that modulates neurotransmitter signaling. Comprising this strategy are the cholinesterase inhibitors and N-methyl-D-aspartate receptor blockade treatments that are clinically approved for the symptomatic treatment of AD. Additional targets that aim to stabilize neuron signaling through modulation of neurotransmitters and their receptors are also discussed. Finally, the third approach comprises a collection of 'sensitive targets' that indirectly influence Aβ or tau accumulation. These targets are proteins that upon Aβ accumulation in the brain or direct Aβ-target interaction, a modification in the target's function is induced. The process occurs early in disease progression, ultimately causing neuronal dysfunction. This strategy aims to restore normal target function to alleviate Aβ-induced toxicity in neurons. Overall, we generally limit our analysis to targets that have emerged in the last decade and targets that have been validated using small molecules in in vitro and/or in vivo models. This review is not an exhaustive list of all possible targets for AD but serves to highlight the most promising and critical targets suitable for small molecule drug intervention.