CHF5074, a novel gamma-secretase modulator, attenuates brain beta-amyloid pathology and learning deficit in a mouse model of Alzheimer's disease

Amyloid precursor protein selective gamma-secretase inhibitors for treatment of Alzheimer's disease

INTRODUCTION

Inhibition of gamma-secretase presents a direct target for lowering Aβ production in the brain as a therapy for Alzheimer's disease (AD). However, gamma-secretase is known to process multiple substrates in addition to amyloid precursor protein (APP), most notably Notch, which has limited clinical development of inhibitors targeting this enzyme. It has been postulated that APP substrate selective inhibitors of gamma-secretase would be preferable to non-selective inhibitors from a safety perspective for AD therapy.

METHODS

In vitro assays monitoring inhibitor potencies at APP γ-site cleavage (equivalent to Aβ40), and Notch ε-site cleavage, in conjunction with a single cell assay to simultaneously monitor selectivity for inhibition of Aβ production vs. Notch signaling were developed to discover APP selective gamma-secretase inhibitors. In vivo efficacy for acute reduction of brain Aβ was determined in the PDAPP transgene model of AD, as well as in wild-type FVB strain mice. In vivo selectivity was determined following seven days x twice per day (b.i.d.) treatment with 15 mg/kg/dose to 1,000 mg/kg/dose ELN475516, and monitoring brain Aβ reduction vs. Notch signaling endpoints in periphery.

RESULTS

The APP selective gamma-secretase inhibitors ELN318463 and ELN475516 reported here behave as classic gamma-secretase inhibitors, demonstrate 75- to 120-fold selectivity for inhibiting Aβ production compared with Notch signaling in cells, and displace an active site directed inhibitor at very high concentrations only in the presence of substrate. ELN318463 demonstrated discordant efficacy for reduction of brain Aβ in the PDAPP compared with wild-type FVB, not observed with ELN475516. Improved in vivo safety of ELN475516 was demonstrated in the 7d repeat dose study in wild-type mice, where a 33% reduction of brain Aβ was observed in mice terminated three hours post last dose at the lowest dose of inhibitor tested. No overt in-life or post-mortem indications of systemic toxicity, nor RNA and histological end-points indicative of toxicity attributable to inhibition of Notch signaling were observed at any dose tested.

CONCLUSIONS

The discordant in vivo activity of ELN318463 suggests that the potency of gamma-secretase inhibitors in AD transgenic mice should be corroborated in wild-type mice. The discovery of ELN475516 demonstrates that it is possible to develop APP selective gamma-secretase inhibitors with potential for treatment for AD.

Presenilin-1 but not amyloid precursor protein mutations present in mouse models of Alzheimer's disease attenuate the response of cultured cells to γ-secretase modulators regardless of their potency and structure

γ-Secretase modulators (GSMs) inhibit the generation of amyloidogenic Aβ42 peptides and are promising agents for treatment or prevention of Alzheimer's disease (AD). Recently, a second generation of GSMs with favorable pharmacological properties has emerged, but preclinical studies to assess their efficacy in vivo are lacking. Such studies rely on transgenic mouse models that express amyloid precursor protein (APP) and presenilin (PSEN) mutations associated with early-onset familial AD. Previously, we have shown that certain PSEN1 mutations attenuated the response of cultured cells to GSMs and potentially confound in vivo studies in AD mouse models. However, different combinations of familial AD mutations might have synergistic or opposing effects, and we have now systematically determined the response of APP and PSEN1 mutations present in current AD models. Using a potent acidic GSM, we found that APP mutations, either single mutations or in combination, did not affect the potency of GSMs. In contrast, all PSEN1 mutations that have been used to accelerate pathological changes in AD models strongly attenuated the Aβ42-lowering activity of GSMs with two exceptions (M146L, A246E). Similar results were obtained with potent non-acidic GSMs indicating that the attenuating effect of PSEN1 mutations cannot simply be overcome by increased potency or structural changes. Notably, two non-acidic compounds fully compensated the attenuating effect of the PSEN1-G384A mutation. Taken together, our findings indicate that most AD models with rapid pathology and advanced phenotypes are unsuitable for preclinical GSM studies. However, we also provide evidence that additional compound screens could discover GSMs that are able to break the attenuating effects of PSEN mutations.

Presenilin mouse and zebrafish models for dementia: focus on neurogenesis

Autosomal dominant mutations in the presenilin gene PSEN cause familial Alzheimer's disease (AD), a neurological disorder pathologically characterized by intraneuronal accumulation and extracellular deposition of amyloid-β in plaques and intraneuronal, hyperphosphorylated tau aggregation in neurofibrillary tangles. Presenilins (PS/PSENs) are part of the proteolytic γ-secretase complex, which cleaves substrate proteins within the membrane. Cleavage of the amyloid precursor protein (APP) by γ-secretase releases amyloid-β peptides. Besides its role in the processing of APP and other transmembrane proteins, presenilin plays an important role in neural progenitor cell maintenance and neurogenesis. In this review, we discuss the role of presenilin in relation to neurogenesis and neurodegeneration and review the currently available presenilin animal models. In addition to established mouse models, zebrafish are emerging as an attractive vertebrate model organism to study the role of presenilin during the development of the nervous system and in neurodegenerative disorders involving presenilin. Zebrafish is a suitable model organism for large-scale drug screening, making this a valuable model to identify novel therapeutic targets for AD.

Alzheimer's disease: clinical trials and drug development

Alzheimer's disease is the most common cause of dementia in elderly people. Research into Alzheimer's disease therapy has been at least partly successful in terms of developing symptomatic treatments, but has also had several failures in terms of developing disease-modifying therapies. These successes and failures have led to debate about the potential deficiencies in our understanding of the pathogenesis of Alzheimer's disease and potential pitfalls in diagnosis, choice of therapeutic targets, development of drug candidates, and design of clinical trials. Many clinical and experimental studies are ongoing, but we need to acknowledge that a single cure for Alzheimer's disease is unlikely to be found and that the approach to drug development for this disorder needs to be reconsidered. Preclinical research is constantly providing us with new information on pieces of the complex Alzheimer's disease puzzle, and an analysis of this information might reveal patterns of pharmacological interactions instead of single potential drug targets. Several promising randomised controlled trials are ongoing, and the increased collaboration between pharmaceutical companies, basic researchers, and clinical researchers has the potential to bring us closer to developing an optimum pharmaceutical approach for the treatment of Alzheimer's disease.

gamma-secretases: from cell biology to therapeutic strategies

Presenilins form the catalytic part of the gamma-secretases, protein complexes that are responsible for the intramembranous cleavage of transmembrane proteins. The presenilins are involved in several biological functions, but are best known for their role in the generation of the beta-amyloid (Abeta) peptide in Alzheimer's disease and are therefore thought to be important drug targets for this disorder. Mutations in the presenilin genes cause early-onset familial Alzheimer's disease, but mutation carriers have substantial phenotypic heterogeneity. Recent evidence implicating presenilin mutations in non-Alzheimer's dementias, including frontotemporal dementia and Lewy body dementia, warrants further investigation. An increased understanding of the diversity of the molecular cell biology of the gamma-secretase complex and the effects of clinical mutations in the presenilin genes might help pave the way for improved development of drugs that are designed to target gamma-secretase enzymatic activity in Alzheimer's disease and potentially in other neurological diseases.

Disease-modifying approach to the treatment of Alzheimer's disease: from alpha-secretase activators to gamma-secretase inhibitors and modulators

In the last decade, advances in understanding the neurobiology of Alzheimer's disease (AD) have translated into an increase in clinical trials assessing various potential AD treatments. At present, drugs used for the treatment of AD only slightly delay the inevitable symptomatic progression of the disease and do not affect the main neuropathological hallmarks of the disease, i.e. senile plaques and neurofibrillary tangles. Brain accumulation of oligomeric species of beta-amyloid (A beta) peptides, the principal components of senile plaques, is believed to play a crucial role in the development of AD. Based on this hypothesis, huge efforts are being made to identify drugs able to interfere with proteases regulating A beta formation from amyloid precursor protein (APP). Compounds that stimulate alpha-secretase, the enzyme responsible for non-amyloidogenic metabolism of APP, are being developed and one of these, EHT-0202, has recently commenced evaluation in a phase II study. The discovery of inhibitors of beta-secretase (memapsin-2, beta-amyloid cleaving enzyme-1 [BACE-1]), the enzyme that regulates the first step of amyloidogenic APP metabolism, has proved to be particularly difficult because of inherent medicinal chemistry issues and only one compound (CTS-21166) has proceeded to clinical testing. Conversely, several compounds that inhibit gamma-secretase, the pivotal enzyme that generates A beta, have been identified, the most advanced being LY-450139 (semagacestat), presently in phase III clinical development. There has been considerable disappointment over the failure of a phase III study of tarenflurbil, a compound believed to modulate the activity of gamma-secretase, after encouraging phase II findings. Nevertheless, other promising gamma-secretase modulators are being developed and are approaching clinical testing. All these therapeutic approaches increase the hope of slowing the rate of decline in patients with AD and modifying the natural history of this devastating disease within the next 5 years.

An update on the efficacy of non-steroidal anti-inflammatory drugs in Alzheimer's disease

Several epidemiological studies suggest that long-term use of non-steroidal anti-inflammatory drugs (NSAIDs) may protect against Alzheimer's disease (AD), especially for patients carrying one or more epsilon4 allele of the apolipoprotein E. The biological mechanism of this protection is not completely understood and may involve inhibition of COX activity, inhibition of beta-amyloid(1-42) (Abeta42) production and aggregation, inhibition of beta-secretase activity, activation of PPAR-gamma or stimulation of neurotrophin synthesis. Unfortunately, long-term, placebo-controlled clinical trials with both non-selective and COX-2 selective NSAIDs in AD patients produced negative results. A secondary prevention study with rofecoxib in patients with mild cognitive impairment and a primary prevention study with naproxen and celecoxib in elderly subjects with a family history of AD were also negative. All these failures have diminished the hope that NSAIDs could be beneficial in the treatment of AD. It is hypothesized that the chronic use of NSAIDs may be beneficial only in the normal brain by inhibiting the production of Abeta42. Once the Abeta deposition process has started, NSAIDs are no longer effective and may even be detrimental because of their inhibiting activity on activated microglia of the AD brain, which mediates Abeta clearance and activates compensatory hippocampal neurogenesis.

Novel orally active morpholine N-arylsulfonamides gamma-secretase inhibitors with low CYP 3A4 liability

A new class of 2,6-disubstituted morpholine N-arylsulfonamide gamma-secretase inhibitors was designed based on the introduction of a morpholine core in lieu or piperidine in our lead series. This resulted in compounds with improved CYP 3A4 profiles. Several analogs that were active at lowering Abeta levels in Tg CRND8 mice upon oral administration were identified.

Selective modulation of amyloid-beta peptide degradation by flurbiprofen, fenofibrate, and related compounds regulates Abeta levels

Gamma-secretase modulators (GSMs) include selected non-steroidal anti-inflammatory drugs such as flurbiprofen that selectively lowers the neurotoxic amyloid-beta peptide Abeta(1-42). GSMs are attractive targets for Alzheimer's disease, in contrast to 'inverse GSMs,' such as fenofibrate, which selectively increase the level of Abeta(1-42). A methodology for screening of Abeta modulating drugs was developed utilizing an Abeta-producing neuroblastoma cell line stably transfected with mutant human amyloid precursor protein, immunoprecipitation of Abeta peptides, and mass spectroscopic quantitation of Abeta(1-37)/Abeta(1-38)/Abeta(1-40)/Abeta(1-42) using an Abeta internal standard. The unexpected conclusion of this work was that in this system, drug effects are independent of gamma-secretase. The methodology recapitulated reported results for modulation of Abeta by GSMs. However, control experiments in which exogenous Abeta(1-40)/Abeta(1-42) was added (i) to drug-treated wild-type cells or (ii) to conditioned media from these wild-type cells, gave comparable patterns of Abeta modulation. These results, suggesting that drugs modulate the ability of cell-derived factors to degrade Abeta, was interrogated by adding protease inhibitors and performing molecular weight cut-off fractionation. The results confirmed that modulation of Abeta(1-40)/Abeta(1-42) was mediated by selective proteolysis. Treatment of N2a cells with flurbiprofen or fenofibric acid selectively enhanced Abeta(1-42) clearance by extracellular proteolysis; treatment with HCT-1026 or fenofibrate (esters of flurbiprofen and fenobric acid) inhibited clearance of Abeta(1-40) and Abeta(1-42).