Do BACE inhibitor failures in Alzheimer patients challenge the amyloid hypothesis of the disease?

KEYWORDS:

• BACE inhibitors
• dementia
• cognitive disorders
• Alzheimer’s disease
• MCI
• lifestyle

1. The amyloid-β hypothesis of Alzheimer’s disease

The amyloid-β (Aβ) cascade hypothesis of the Alzheimer’s disease (AD) assumes that the brain accumulation of this 40–42 amino acid peptide represents the initial event of the pathological process and starts 15–20 years before clinical symptoms become apparent. Point mutations of the amyloid precursor protein (APP) and the enzymes involved in its metabolism (PSEN1 and PSEN2) alter the production of Aβ and cause the familial form of AD. This observation represents the foundation of the amyloid hypothesis of AD, and during the last 20 years, intensive efforts have been made to identify compounds which antagonize the accumulation of Aβ in the AD brain. Unfortunately, all drugs interfering with Aβ production, clearance and aggregation have failed clinically. Interestingly, some of these drugs – especially inhibitors of the two enzymes responsible for the Aβ production from APP (γ-secretase and β-secretase) – were found to worsen the cognitive, psychiatric and clinical conditions of patients with established or prodromal AD [1].

2. Recent clinical trials with BACE inhibitors

In the last year, four β-site amyloid precursor protein cleaving enzyme (BACE) inhibitors have failed to show benefit in large placebo-controlled studies in patients with either early or advanced disease, and even in healthy subjects at risk of developing AD. In February 2017, a Phase III, 18-month trial of verubecestat (12 or 40 mg/day) in 1,958 mild-to-moderate AD patients (EPOCH) was stopped for lack of efficacy [2]. From a pharmacodynamic point of view, verubecestat worked efficiently to lower Aβ levels in cerebrospinal fluid (CSF) in a dose-dependent manner. Nevertheless, the drug did not improve either cognition or activity of daily living in the patients. Patients abandoning treatment for adverse events were distributed in a dose-dependent manner across the three treatment groups: 5.8%, 8.3%, and 9.4% in placebo, low and high dose groups, respectively. Apparently, the numbers of deaths were also dose-dependent: 5 (0.8%) in the placebo group, 9 (1.4%) in the 12-mg group and 12 (1.8%) in the 40-mg group. The death rate in the placebo recipients rose to 8/360 (2.2%) in the open-label extension during which they were switched to 40 mg verubecestat (p = 0.049). In February 2018, a Phase III, 2-year trial of verubecestat (12 or 40 mg/day) in 1,454 people with subjective memory decline and positive amyloid positron emission tomography (PET) (APECS) appeared futile and was halted [3]. Patients in the 40-mg verubecestat group performed significantly worse than those on placebo on clinical global performance, as measured with the Clinical Dementia Rating – Sum of Boxes (CDR-SB). The drug’s detrimental effects were seen from 3 to 24 months of treatment. Similarly, patients on verubecestat showed worse functionality than to those on placebo as measured with the Alzheimer’s Disease Cooperative Study Group-Activities of Daily Living (ADCS-ADL) scale. The detrimental effects on functionality became significant after 9 months of treatment. Compared to placebo, the drug accelerated (at both doses) the rate of conversion from prodromal AD to overt dementia (25% vs 20% per year). Subjects treated with verubecestat complained more frequently than those on placebo of anxiety, depression and disturbed sleep [3]. In terms of brain atrophy, subjects on verubecestat showed a faster reduction in brain and hippocampal volumes compared to placebo-treated subjects after 3 months of treatment [4], i.e. at the same time-point of cognitive worsening.

In May 2018, a Phase II/III, 54-month trial of atabecestat (5 or 25 mg/day) initially planned in 1,650 asymptomatic amyloid-positive subjects at risk of developing AD (EARLY), was halted during its recruitment phase due to liver toxicity and unfavorable benefit–risk ratio [5]. The EARLY study targeted a prodromal population, with participants having a CDR staging of 0 and confirmed amyloid positivity by PET or CSF. At the time of study interruption, 557 participants took atabecestat for no more than 18 months and about half of them for only three. Equal numbers of participants received 5 mg, 25 mg or placebo. The high-dose group had a statistically significant cognitive decrement compared to placebo at both 6 and 12 months of treatment on a composite cognitive scale (ADCS-PACC) and at 3 months on a neuropsychological test battery (RBANS). This group started with 183 participants at baseline, but only 110 completed the three-month assessment, 66 the six-month, and 28 the 12-month. For later time points, the number of participants was even smaller. The researchers saw no differences in ADCS-ADL in this study, but they identified more depression, anxiety, and sleep or dream-related problems [6].

In June 2018, a Phase III, 2-year trial of lanabecestat in 1,202 subjects early AD (AMARANTH) and a Phase III, 3-year trial in 1,899 mild AD (DAYBREAK-ALZ) were discontinued for futility [7]. In both studies, subjects were treated with placebo or lanabecestat at 20 mg/day or 50 mg/day. The primary outcome measure of efficacy in both studies was the 13-item form of the Alzheimer’s Disease Assessment Scale – Cognitive Subscale (ADAS-Cog13). In both studies, patients on lanabecestat showed faster cognitive decline than those on placebo [6].

In July 2018, a Phase II, 1-year trial of LY3202626 (3 or 12 mg/day) in 316 amyloid-positive mild AD patients (NAVIGATE-AD) was terminated after an interim futility analysis gave it little chance of success [8]. No clear effect on cognition was found, although there were hints of a worsened deficit in the 3-mg group at 24 weeks on the ADAS-Cog13 and at 52 weeks on the Mini Mental State Examination (MMSE) [6].

Another BACE inhibitor, elenbecestat, has completed an 18-month, double-blind, placebo-controlled study in 70 MCI-to-moderate AD subjects. Patients were allocated to placebo or 5, 15 or 50 mg/day with 17–19 subjects per treatment arm [9]. Twenty-seven subjects (39%) discontinued the study: 5/17 on placebo (29%) and 22/53 on elenbecestat (42%) with nightmares as a notable side effect on active drug. At 18 months, placebo and elenbecestat groups did not differ on either the Alzheimer’s Disease Composite Score (ADCOMS, p = 0.38) or CDR-SB (p = 0.55) [9].

What do these drugs do to memory? There are more than 40 known BACE1 substrates, and BACE inhibitors may block one or more of them with neurodegenerative consequences. Substrates like seizure protein 6 (SEZ6) [10], close homolog of L1 (CHL1) [11], and neuregulin-1 [12] are potential culprits. Indeed, several studies using BACE1- and BACE2-deficient mice demonstrated that these two proteases affect a wide range of physiological substrates and functions within and outside the nervous system. For BACE1 this includes axon guidance, neurogenesis, muscle spindle formation, neuronal network functions, and myelination, whereas BACE2 was been shown to be involved in pigmentation and pancreatic β-cell function [13]. Interestingly, studies have indicated that prolonged treatment with BACE1 inhibitors may negatively affect spine formation and bone density, hippocampal long-term potentiation, and cognition in wild-type mice [14].

3. Other anti-Aβ drugs accelerating AD decline

Negative effects of BACE inhibitors on cognition, functionality and clinical global performance were also observed in trials of other anti-Aβ drugs. Higher mortality rate on semagacestat (a γ-secretase inhibitor) was reported in a Phase III trial involving 1,537 mild-to-moderate AD patients [15]. This trial was discontinued prematurely because of deterioration in patient cognition. At the time of trial termination, 10.2%, 23.9% and 27.3% of patients on placebo, semagacestat 100 mg and semagacestat 140 mg daily, respectively, had withdrawn from the study because of adverse events. There were six deaths on placebo (1.2%), 11 deaths on 100 mg (2.2%), and 15 deaths on 140 mg (2.8%). A previous study has shown that the doses of semagacestat used in the Phase III trial (100 and 140 mg/day) significantly decreased the production of CNS Aβ by 47% and 52%, respectively [16]. Avagacestat, a γ-secretase inhibitor, worsened cognition in both prodromal [17] and in mild-to-moderate AD patients [18]. Tarenflurbil, a γ-secretase modulator, significantly worsened CDR-SB compared to placebo in mild AD patients, although it did not affect CSF Aβ levels [19]. CAD106, an active anti-Aβ vaccine, tended to worsen MMSE compared to placebo in patients with mild AD [20]. Another Aβ antigen, AD02, worsened cognition and functionality in patients with early AD [21]. Scyllo-inositol, an Aβ aggregation inhibitor, dose-dependently increased mortality in mild-to-moderate AD patients [22].

4. Expert opinion

The previous multiple clinical failures of anti-Aβ drugs put doubts on the Aβ cascade hypothesis of AD. This appears particularly evident with the failure of BACE inhibitors because they block the formation of Aβ and were expected to block or attenuate the entire Aβ pathological cascade. Aβ is a widely expressed peptide with a physiological role in several brain functions including modulation of synaptic function, facilitation of neuronal growth and survival, protection against oxidative stress, and surveillance against neuroactive compounds, toxins and pathogens [23,24]. Other studies have highlighted a role for Aβ in regulating neuronal electrophysiology [25,26], synaptic plasticity and memory [27–29], long-term potentiation [30], neuronal transmission [31], learning and memory [32], hippocampal and memory consolidation [28], neurogenesis [33] and neuronal survival [34].

Aβ brain concentrations have been found to be elevated in traumatic brain injury, chronic traumatic encephalopathy, cerebral ischemia, amyotrophic lateral sclerosis, major depression and event in general anesthesia [1]. Elevations of central or peripheral Aβ concentrations were also noted during cardiac arrest and cardiac surgery, including in studies involving infants and children. Increases in CSF or brain Aβ concentrations were also observed in cognitively normal subjects after one day of sleep deprivation. The effects of sleep disruption on Aβ concentrations appear to be due to increased production rather than decreased clearance [1]. The transient increase in Aβ secretion after acute brain injury or the steady overproduction of Aβ in chronic brain diseases might represent a reparative reaction of the brain to mitigate neuronal damage or insult. In the AD brain, Aβ overproduction could represent a compensatory reaction to loss of neuronal functioning. The real causes of the initial neuronal damage in the AD brain are presently unknown but may include chronic inflammation, tau-accumulation, viral or bacterial infection, metabolic failure, abnormal microglial functioning, oxidative stress, and cholesterol metabolism derangement. Thus, Aβ overproduction could represent an adaptive response to an unknown upstream pathological event. The chronic Aβ accumulation leads to the formation of Aβ aggregates, including Aβ oligomers, with consequent neurotoxicity and fuelling of the AD pathological cascade.

5. Conclusions

Any theoretical challenge to the Aβ cascade hypothesis of AD must propose alternative explanations for the relationship between point mutations of the PSEN and APP genes and the occurrence of autosomal dominant familial AD. According to the Aβ amyloid hypothesis, PSEN and APP mutations in familial AD have a gain-of-function role. However, this view may be more complex than initially thought. A large study evaluated the effect of 138 pathogenic PSEN1 mutations on the in vitro production of Aβ_1-42 and Aβ_1-40 peptides and found that about 90% of these mutations lead to reduced production of Aβ_1-42 and Aβ_1-40 [35]. Some APP mutations lead to increases in total Aβ levels, others to decreases. Variants such as A2V, H6R, D7N (using the Aβ numbering scheme) produce Aβ_1-42/Aβ_1-40 ratios similar to wild type APP, whereas APP variants E22G, E22K, and E22Q lower this ratio [36]. A recent hypothesis on the role of iron dysregulation in the AD pathophysiology has also been proposed and may explain the existence of both familial and sporadic AD forms [37]. We cannot exclude the possibility that in AD patients the accumulation of Aβ in the brain is secondary to an unknown initial disrupting event [38,39], including viral or microbial infection [40]. The increase in brain Aβ concentrations could be a reactive compensatory response of neurons damaged by presently unknown causes.

Previous clinical failures of anti-Aβ antigens and γ-secretase inhibitors, and the recent clinical failures of BACE inhibitors and monoclonal anti-Aβ antibodies, tell us it is time to abandon anti-Aβ therapies. The detrimental cognitive effects observed in EARLY atabecestat study in asymptomatic subjects at risk of developing AD indicate that tackling the disease in the presymptomatic phase by this means is not paying. The premature discontinuation of the crenezumab and aducanumab Phase III studies in subjects with early or prodromal AD tells us that targeting Aβ oligomers is also not working [41]. This repeated clinical evidence strongly suggests that not only BACE inhibitors, but indeed all anti-Aβ approaches are dead. We should move the search of anti-AD therapeutics into other areas. Clinical studies are presently ongoing with various different approaches, including anti-tau monoclonal antibodies (RG6100, ABBV-8E12, E2814), anti-herpes simplex virus-1 (valacyclovir), and plasma replacement therapy. Pursuing modifiable AD risk factors (hypertension, dyslipidemia and obesity at midlife, diabetes mellitus, smoking, physical inactivity, depression and low levels of education) and other potential infective causes are also reasonable and cheap strategies [42].

Declaration of interest

The authors have no 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. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosures

A reviewer on this manuscript is involved in BACE inhibitor research and development. Peer reviewers on this manuscript have no other relevant financial relationships or otherwise to disclose.

Additional information

Funding

This paper was not funded.