Γ-secretase Modulators Research Articles

Condensed Pyridopyrimidines and Pyridopyrazines Containing a Bridgehead Nitrogen Atom: Synthesis, Chemical Properties and Biological Activity

The investigation of effective and green synthetic routes to approach to novel fused heterocycles with pyridopyrimidine and pyridopyrazine scaffolds stirs up broad interest from scientists as they are capable of providing valuable properties such as anticancer and antimicrobial activities and they are proved γ-secretase modulators, polymers, and corrosion inhibitors. This causes a steady increase in the number of publications on titled condensed systems. The present review article summarizes recent literature data from 2010 to 2020 on the methods of synthesis, chemical transformations and biological properties of condensed bicyclic systems of pyridopyrimidine and pyridopyrazine with a bridgehead nitrogen atom to establish the current state of the art in this area.

Role of γ-Secretase Inhibitors for the Treatment of Diverse Disease Conditions through Inhibition of Notch Signaling Pathway.

γ-secretase is an intramembrane protease sub-assembly that sunders transmembrane proteins. It is involved in intramembrane proteolysis and also contributes to the regeneration of transmembrane protein. The amyloid precursor proteins (APPs) are typical γ-secretase substrates. These proteins are cleaved to produce 36-43 amyloid-beta (Aβ) amino acid peptides. Abnormal folding of these proteins fragments leads to amyloid plaques, which are frequently encountered in Alzheimer's disease. Some Type I class of integral membrane proteins is processed under the influence of γ-secretase, such as receptor tyrosine-protein kinase erbB4 and CD44 glycoprotein. γ-Secretase is being explored in several diseases as a clinical goal. Both γ-secretase inhibitors (GSIs) and γ-secretase modulators (GSMs) are being evaluated for this purpose. A large amount of γ-secretase inhibitors (GSIs) from peptide to non-peptide have been disclosed, offering several lead compounds for the design and optimization of γ-secretase targets, but most GSIs lack sufficient potency, exhibit low penetration in the brain, and manifest low selectiveness. γ-secretase inhibitors are obliquely a regulator of a γ-secretase substrate Notch, and valuable in the development of β-amyloid peptide (Aβ). These γ-secretase inhibitors block the Notch signaling pathway in autoimmune and lymphoproliferative disorders, like autoimmune lymphoproliferative syndrome (ALPS) and systemic lupus erythematosus (SLE), and perhaps even in cancerous cell proliferation, angiogenesis, and cellular differentiation of human-induced pluripotent stem cells (hiPSC). The current review portrays the mechanism, regulation, and inhibition of γ-secretase in the management of a wide assortment of diseases.

Modulation of γ-Secretase Activity by a Carborane-Based Flurbiprofen Analogue.

All over the world, societies are facing rapidly aging populations combined with a growing number of patients suffering from Alzheimer’s disease (AD). One focus in pharmaceutical research to address this issue is on the reduction of the longer amyloid-β (Aβ) fragments in the brain by modulation of γ-secretase, a membrane-bound protease. R-Flurbiprofen (tarenflurbil) was studied in this regard but failed to show significant improvement in AD patients in a phase 3 clinical trial. This was mainly attributed to its low ability to cross the blood–brain barrier (BBB). Here, we present the synthesis and in vitro evaluation of a racemic meta-carborane analogue of flurbiprofen. By introducing the carborane moiety, the hydrophobicity could be shifted into a more favourable range for the penetration of the blood–brain barrier, evident by a logD7.4 value of 2.0. Furthermore, our analogue retained γ-secretase modulator activity in comparison to racemic flurbiprofen in a cell-based assay. These findings demonstrate the potential of carboranes as phenyl mimetics also in AD research.

Selective Secretase Targeting for Alzheimer's Disease Therapy.

Alzheimer's disease (AD) is associated with marked atrophy of the cerebral cortex and accumulation of amyloid plaques and neurofibrillary tangles. Amyloid plaques are formed by oligomers of amyloid-β (Aβ) in the brain, with a length of 42 and 40 amino acids. α-secretase cleaves amyloid-β protein precursor (AβPP) producing the membrane-bound fragment CTFα and the soluble fragment sAβPPα with neuroprotective activity; β-secretase produces membrane-bound fragment CTFβ and a soluble fragment sAβPPβ. After α-secretase cleavage of AβPP, γ-secretase cleaves CTFα to produce the cytoplasmic fragment AICD and P3 in the non-amyloidogenic pathway. CTFβ is cleaved by γ-secretase producing AICD as well as Aβ in amyloidogenic pathways. In the last years, the study of natural products and synthetic compounds, such as α-secretase activity enhancers, β-secretase inhibitors (BACE-1), and γ-secretase activity modulators, have been the focus of pharmaceuticals and researchers. Drugs were improved regarding solubility, blood-brain barrier penetration, selectivity, and potency decreasing Aβ42. In this regard, BACE-1 inhibitors, such as Atabecestat, NB-360, Umibecestat, PF-06751979 Verubecestat, LY2886721, Lanabecestat, LY2811376 and Elenbecestat, were submitted to phase I-III clinical trials. However, inhibition of Aβ production did not recover cognitive functions or reverse disease progress. Novel strategies are being developed, aiming at a partial reduction of Aβ production, such as the development of γ-secretase modulators or α-secretase activity enhancers. Such therapeutic tools shall focus on slowing down or minimizing the progression of neuronal damage. Here, we summarize structures and activities of the latest compounds designed for AD treatment, with remarkable in vitro, in vivo, and clinical phase activities.

A promising new γ-secretase modulator for Alzheimer's disease.

Effective and safe treatments for Alzheimer's disease (AD) have been an elusive target for scientists who have been working tirelessly to gain control over a disease that is affecting millions of people, with continually rising case numbers as the population ages. However, in this issue of JEM, Rynearson et al. (2021. J. Exp. Med.https://doi.org/10.1084/jem.20202560) present a beacon of hope for this field with a preclinical evaluation of a potent and robust γ-secretase modulator (GSM).

Preclinical validation of a potent γ-secretase modulator for Alzheimer’s disease prevention

A potent γ-secretase modulator (GSM) has been developed to circumvent problems associated with γ-secretase inhibitors (GSIs) and to potentially enable use in primary prevention of early-onset familial Alzheimer's disease (EOFAD). Unlike GSIs, GSMs do not inhibit γ-secretase activity but rather allosterically modulate γ-secretase, reducing the net production of Aβ42 and to a lesser extent Aβ40, while concomitantly augmenting production of Aβ38 and Aβ37. This GSM demonstrated robust time- and dose-dependent efficacy in acute, subchronic, and chronic studies across multiple species, including primary and secondary prevention studies in a transgenic mouse model. The GSM displayed a >40-fold safety margin in rats based on a comparison of the systemic exposure (AUC) at the no observed adverse effect level (NOAEL) to the 50% effective AUC or AUCeffective, the systemic exposure required for reducing levels of Aβ42 in rat brain by 50%.

Phenyl bioisosteres in medicinal chemistry: discovery of novel γ-secretase modulators as a potential treatment for Alzheimer's disease.

Phenyl rings are one of the most prevalent structural moieties in active pharmaceutical ingredients, even if they often contribute to poor physico-chemical properties. Herein, we propose the use of a bridged piperidine (BP) moiety as a phenyl bioisostere, which could also be seen as a superior phenyl alternative as it led to strongly improved drug like properties, in terms of solubility and lipophilicity. Additionally, this BP moiety compares favorably to the recently reported saturated phenyl bioisosteres. We applied this concept to our γ-secretase modulator (GSM) project for the potential treatment of Alzheimer's disease delivering clinical candidates.

Hydrophilic loop 1 of Presenilin-1 and the APP GxxxG transmembrane motif regulate γ-secretase function in generating Alzheimer-causing Aβ peptides

γ-Secretase is responsible for the proteolysis of amyloid precursor protein (APP) into amyloid-beta (Aβ) peptides, which are centrally implicated in the pathogenesis of Alzheimer’s disease (AD). The biochemical mechanism of how processing by γ-secretase is regulated, especially as regards the interaction between enzyme and substrate, remains largely unknown. Here, mutagenesis reveals that the hydrophilic loop-1 (HL-1) of presenilin-1 (PS1) is critical for both γ-secretase step-wise cleavages (processivity) and its allosteric modulation by heterocyclic γ-modulatory compounds. Systematic mutagenesis of HL-1, including all of its familial AD mutations and additional engineered variants, and quantification of the resultant Aβ products show that HL-1 is necessary for proper sequential γ-secretase processivity. We identify Y106, L113, and Y115 in HL-1 as key targets for heterocyclic γ-secretase modulators (GSMs) to stimulate processing of pathogenic Aβ peptides. Further, we confirm that the GxxxG domain in the APP transmembrane region functions as a critical substrate motif for γ-secretase processivity: a G29A substitution in APP-C99 mimics the beneficial effects of GSMs. Together, these findings provide a molecular basis for the structural regulation of γ-processivity by enzyme and substrate, facilitating the rational design of new GSMs that lower AD-initiating amyloidogenic Aβ peptides.

Structural basis of γ-secretase inhibition and modulation by small molecule drugs

Development of γ-secretase inhibitors (GSIs) and modulators (GSMs) represents an attractive therapeutic opportunity for Alzheimer's disease (AD) and cancers. However, how these GSIs and GSMs target γ-secretase has remained largely unknown. Here, we report the cryoelectron microscopy (cryo-EM) structures of human γ-secretase bound individually to two GSI clinical candidates, Semagacestat and Avagacestat, a transition state analog GSI L685,458, and a classic GSM E2012, at overall resolutions of 2.6-3.1Å. Remarkably, each of the GSIs occupies the same general location on presenilin 1 (PS1) that accommodates the β strand from amyloid precursor protein or Notch, interfering with substrate recruitment. L685,458 directly coordinates the two catalytic aspartate residues of PS1. E2012 binds to an allosteric site of γ-secretase on the extracellular side, potentially explaining its modulating activity. Structural analysis reveals a set of shared themes and variations for inhibitor and modulator recognition that will guide development of the next-generation substrate-selective inhibitors.

Computational prediction and molecular mechanism of γ-secretase modulators

Selective control over Aβ production via γ-secretase modulators (GSM) is a promising strategy for treating Alzheimer's disease, yet the specific binding sites and mechanism of action of GSMs remain unknown. Using the recent cryo-electron microscopy structures of substrate-bound γ-secretase we used two distinct methods to identify four potential binding sites for pyridopyrazine-1,6-dione GSMs. We demonstrate binding to site 4 formed between PS1-TM2, PS1-TM5 and the APP-C83-TM, with experimental activity data correlating significantly (95% confidence) with our computed binding-affinities for this site. Charged protonated GSMs may display higher affinities because of π-cation interaction with the polar residue Tyr115 of PS1-NTF. Surprisingly, the pIC50 of these compounds is largely described (R2 > 0.4 for all of these) by the molecular size, hydrophobicity, and polarizability. We thus believe that we have identified the primary modulator binding site in γ-secretase for these compounds, as well as strong descriptors of GSM potency. Our results are consistent with the FIST model of γ-secretase action and suggest that GSMs work in two ways: The binding affinity itself contributes stability to the ternary enzyme-modulator-substrate complex (tight grabbing), thus preventing early release of the substrate and increasing trimming to shorter, innocent Aβ peptides. At the same time, drug size, hydrophobicity, and polarizability stabilize the more compact semi-open state over the open PS1 state, to make cleavage more precise and complete.

Molecular imaging of Alzheimer's disease-related gamma-secretase in mice and nonhuman primates.

The pathogenesis of Alzheimer's disease (AD) is primarily driven by brain accumulation of the amyloid-β-42 (Aβ42) peptide generated from the amyloid-β precursor protein (APP) via cleavages by β- and γ-secretase. γ-Secretase is a prime drug target for AD; however, its brain regional expression and distribution remain largely unknown. Here, we are aimed at developing molecular imaging tools for visualizing γ-secretase. We used our recently developed γ-secretase modulators (GSMs) and synthesized our GSM-based imaging agent, [11C]SGSM-15606. We subsequently performed molecular imaging in rodents, including AD transgenic animals, and macaques, which revealed that our probe displayed good brain uptake and selectivity, stable metabolism, and appropriate kinetics and distribution for imaging γ-secretase in the brain. Interestingly, rodents and macaques shared certain brain areas with high γ-secretase expression, suggesting a functional conservation of γ-secretase. Collectively, we have provided the first molecular brain imaging of γ-secretase, which may not only accelerate our drug discovery for AD but also advance our understanding of AD.

Generation of Leads for γ-Secretase Modulation.

Herein, we disclose three structurally differentiated γ-secretase modulators (GSMs) based on an oxadiazine scaffold. The analogues from series I potently inhibit the generation of Aβ42 in vitro when the substituents at 3 and 4 positions of the oxadiazine moiety adopt an α orientation (cf. 11). To address the concern around potential reactivity of the exocyclic double bond present in series I toward nucleophilic attack, compounds containing either an endocyclic double bond, such as 20 (series II), or devoid of an olefinic moiety, such as 27 (series III), were designed and validated as novel GSMs. Compound 11 and azepine 20 exhibit robust lowering of CSF Aβ42 in rats treated with a 30 mg/kg oral dose.

Stepwise Design of γ-Secretase Modulators with an Advanced Profile by Judicious Coordinated Structural Replacements and an Unconventional Phenyl Ring Bioisostere.

Starting from RO6800020 (1), our former γ-secretase modulator (GSM) lead compound, we utilized sequential structural replacements to improve the potency (IC50), pharmacokinetic properties including the free fraction (fraction unbound (fu)) in plasma, and in vivo efficacy. Importantly, we used novel CF3-alkoxy groups as bioisosteric replacements of a fluorinated phenyl ring and properties such as lipophilicity, solubility, metabolic stability, and free fraction could be balanced, maintaining low Pgp efflux needed for CNS penetration. In addition, by reducing aromaticity, we prevented phototoxicity. Additional substitution in the triazolopyridine core disturbed the binding to phosphatidylinositol 4-kinase, catalytic β (PIK4CB). We also introduced less lipophilic head heterocycles devoid of covalent binding (CVB) liability. After these changes, further modifications to the trifluoroethoxy bioisosteric replacement allowed rebalancing of properties, such as lipophilicity, and also potency. Our optimization strategy culminated with in vivo active RO7101556 (18B) having excellent properties and being selected as an advanced candidate.

Discovery of RO7185876, a Highly Potent γ-Secretase Modulator (GSM) as a Potential Treatment for Alzheimer's Disease.

γ-Secretase (GS) is a key target for the potential treatment of Alzheimer's disease. While inhibiting GS led to serious side effects, its modulation holds a lot of potential to deliver a safe treatment. Herein, we report the discovery of a potent and selective gamma secretase modulator (GSM) (S)-3 (RO7185876), belonging to a novel chemical class, the triazolo-azepines. This compound demonstrates an excellent in vitro and in vivo DMPK profile. Furthermore, based on its in vivo efficacy in a pharmacodynamic mouse model and the outcome of the dose range finding (DRF) toxicological studies in two species, this compound was selected to undergo entry in human enabling studies (e.g., GLP toxicology and scale up activities).

Optimization and biological evaluation of imidazopyridine derivatives as a novel scaffold for γ-secretase modulators with oral efficacy against cognitive deficits in Alzheimer’s disease model mice

Gamma-secretase modulators (GSMs) selectively lower amyloid-β42 (Aβ42) and are therefore potential disease-modifying drugs for Alzheimer’s disease (AD). Here, we report the discovery of imidazopyridine derivatives as GSMs with oral activity on not only Aβ42 levels but also cognitive function. Structural optimization of the biphenyl group and pyridine-2-amide moiety of compound 1a greatly improved GSM activity and rat microsomal stability, respectively. 5-{8-[(3,4′-Difluoro[1,1′-biphenyl]-4-yl)methoxy]-2-methylimidazo[1,2-a]pyridin-3-yl}-N-methylpyridine-2-carboxamide (1o) showed high in vitro potency and brain exposure, induced a robust reduction in brain Aβ42 levels, and exhibited undetectable inhibition of cytochrome p450 enzymes. Moreover, compound 1o showed excellent efficacy against cognitive deficits in AD model mice. These findings suggest that compound 1o is a promising candidate for AD therapeutics.

Pathogenic Aβ generation in familial Alzheimer's disease: novel mechanistic insights and therapeutic implications.

Neurotoxic amyloid-β peptide (Aβ) 42/43 species generated by β-secretase and γ-secretase from the β-amyloid precursor protein (APP) are believed to trigger Alzheimer's disease (AD). Relative increases of these species due to mutations in APP and presenilin/γ-secretase are associated with the vast majority of early onset familial AD cases. Important breakthroughs have recently been made in elucidating the mechanism(s) of these mutations, showing that altered substrate interactions and substrate-enzyme complex stabilities are underlying their pathogenic Aβ generation. Moreover, first structures of γ-secretase in complex with APP and Notch1 substrates allow insight into how substrate cleavage could be initiated and further progress has been made in the mechanistic understanding of γ-secretase modulators, advanced Aβ-lowering drugs. These insights could be exploited for future AD clinical trials.

The GSM BPN-15606 as a Potential Candidate for Preventative Therapy in Alzheimer’s Disease

In the amyloid hypothesis of Alzheimer's disease (AD), the dysregulation of amyloid-β protein (Aβ) production and clearance leads to amyloid deposits, tau tangles, neuronal loss, and cognitive dysfunction. Thus far, therapies targeting the enzymes responsible for Aβ production have been found ineffective or having significant side effects. To test whether a γ-secretase modulator, BPN-15606, is an effective disease-modifying or preventative treatment in the PSAPP mouse model of AD. We treated pre-plaque (3-month-old) and post-plaque (6-month-old) PSAPP AD transgenic mice for 3 months and examined behavioral, biochemical, and pathological end points. BPN-15606 attenuated cognitive impairment and reduced amyloid plaque load, microgliosis, and astrogliosis associated with the AD phenotype of PSAPP mice when administered to pre-plaque (3-month-old) but was ineffective when administered to post-plaque (6-month-old) mice. No treatment-related toxicity was observed. BPN-15606 appears efficacious when administered prior to significant pathology.

Human-Induced Pluripotent Stem Cells and Herbal Small-Molecule Drugs for Treatment of Alzheimer's Disease.

Alzheimer’s disease (AD) is characterized by extracellular amyloid plaques composed of the β-amyloid peptides and intracellular neurofibrillary tangles and associates with progressive declines in memory and cognition. Several genes play important roles and regulate enzymes that produce a pathological accumulation of β-amyloid in the brain, such as gamma secretase (γ-secretase). Induced pluripotent stem cells from patients with Alzheimer’s disease with different underlying genetic mechanisms may help model different phenotypes of Alzheimer’s disease and facilitate personalized drug screening platforms for the identification of small molecules. We also discuss recent developments by γ-secretase inhibitors and modulators in the treatment of AD. In addition, small-molecule drugs isolated from Chinese herbal medicines have been shown effective in treating Alzheimer’s disease. We propose a mechanism of small-molecule drugs in treating Alzheimer’s disease. Combining therapy with different small-molecule drugs may increase the chance of symptomatic treatment. A customized strategy tailored to individuals and in combination with therapy may be a more suitable treatment option for Alzheimer’s disease in the future.

Novel small molecule therapeutic agents for Alzheimer disease: Focusing on BACE1 and multi-target directed ligands.

Alzheimer's Disease (AD) is a progressive neurodegenerative disorder that effects 50 million people worldwide. In this review, AD pathology and the development of novel therapeutic agents targeting AD were fully discussed. In particular, common approaches to prevent Aβ production and/or accumulation in the brain including α-secretase activators, specific γ-secretase modulators and small molecules BACE1 inhibitors were reviewed. Additionally, natural-origin bioactive compounds that provide AD therapeutic advances have been introduced. Considering AD is a multifactorial disease, the therapeutic potential of diverse multi target-directed ligands (MTDLs) that combine the efficacy of cholinesterase (ChE) inhibitors, MAO (monoamine oxidase) inhibitors, BACE1 inhibitors, phosphodiesterase 4D (PDE4D) inhibitors, for the treatment of AD are also reviewed. This article also highlights descriptions on the regulator of serotonin receptor (5-HT), metal chelators, anti-aggregants, antioxidants and neuroprotective agents targeting AD. Finally, current computational methods for evaluating the structure-activity relationships (SAR) and virtual screening (VS) of AD drugs are discussed and evaluated.

Recent developments of small molecule γ-secretase modulators for Alzheimer's disease.

Alzheimer's disease (AD) is the most common form of progressive neurodegenerative disorder, marked by memory loss and a decline in cognitive function. The major hallmarks of AD are the presence of intracellular neurofibrillary tau tangles (NFTs) composed of hyperphosphorylated tau proteins and extracellular plaques composed of amyloid beta peptides (Aβ). The amyloid (Aβ) cascade hypothesis proposes that the AD pathogenesis is initiated by the accumulation of Aβ peptides in the parenchyma of the brain. An aspartyl intramembranal protease called γ-secretase is responsible for the production of Aβ by the cleavage of the amyloid precursor protein (APP). Clinical studies of γ-secretase inhibitors (GSIs) for AD failed due to the lack of substrate specificity. Therefore, γ-secretase modulators (GSMs) have been developed as potential disease modifying agents to modulate the γ-secretase cleavage activity towards the production of toxic Aβ42 peptides. Following the first-generation 'nonsteroidal anti-inflammatory drug' (NSAID) based GSMs, second-generation GSMs (carboxylic acid based NSAID derivatives and non-NSAID derived heterocyclic analogues), as well as natural product-based GSMs, have been developed. In this review, we focus on the recent developments of small molecule-based GSMs that show potential improvements in terms of drug-like properties as well as their current status in human clinical trials and the future perspectives of GSM research.