γ-secretase targeting in Alzheimer's disease

Abstract

Alzheimer's disease (AD) is one of the most prevalent neurodegenerative disorders and is characterized by memory loss and cognitive decline. The amyloid cascade hypothesis posits that the pathogenesis of AD is initiated by the oligomerization and accumulation of toxic amyloid-β (Aβ) peptides within the brain. The aspartic protease γ-secretase, which catalyzes the final step in the cellular production of Aβ peptides, has been identified as a potential target for anti-amyloid intervention strategies. This target has attracted increasing attention in recent years, and novel small molecules have been developed as selective γ-secretase inhibitors and γ-secretase modulators. This review aims to discuss the role of γ-secretase protein hydrolysis activity in the pathogenesis of AD and to review the molecular mechanisms and prospects for the future development of strategies that target γ-secretase to intervene in AD development, which is expected to provide new ideas for the treatment of AD.

Keywords

Alzheimer's disease amyloid-β peptide Aβ peptides neurodegeneration γ-secretase γ-secretase inhibitors γ-secretase modulators

Introduction

Alzheimer's disease (AD) is a progressive neurodegenerative disease with a complex etiology, including hereditary, sporadic and insidious onset. As the most predominant form of dementia, AD presents significant and escalating global challenges.¹ According to the World Health Organization's 2022 Global Status Report on dementia research, an estimated 55.2 million individuals are affected worldwide. This number is projected to increase to 78 million by 2030.² AD patients gradually worsen with the progression of the disease, typically initiating with mild cognitive impairment (MCI), such as social withdrawal, depression, anxiety, and altered sleep patterns. Disease progression leads to severe cognitive decline, personality and mood changes, neuropsychiatric symptoms such as hallucinations and delusions, and an inability to care for oneself.^3–5 This ultimately results in a high disability rate and a significant burden on individuals, families and societies.⁶ Despite extensive research efforts, the etiology of AD is complex and multifactorial, and a cure remains elusive.⁶ In recent decades, most clinical drugs for AD treatment have been discontinued because of their limited effectiveness or adverse effects.^7,8 To date, the U.S. Food and Drug Administration (FDA) has approved seven drugs for the management of AD.⁶ The majority of these drugs primarily offer temporary symptomatic relief but are generally ineffective in halting the long-term progression of the disease and are accompanied by undesirable side effects, including headaches, nausea, and amyloid-related imaging abnormalities (ARIAs).^9,10 However, aducanumab^11,12 and lecanemab,^13,14 recently approved by the FDA, are monoclonal antibodies that target amyloid-β (Aβ) and offer potential disease-modifying effects.¹⁵ Nevertheless, the long-term efficacy and safety of these drugs require further validation. Consequently, more extensive trials are needed to ascertain or improve the efficacy and safety of these therapeutic agents.

The precise mechanisms underlying AD remain incompletely understood, and our current understanding of AD pathologies involves various hypotheses. One is the ‘amyloid cascade hypothesis’, which has attracted significant attention for its assertion that the deposition of Aβ in the brain constitutes the initiating and central event in the pathogenesis of AD.¹⁶ This hypothesis posits that the aberrant accumulation of Aβ peptides in the brains of AD patients results in the formation of neurotoxic substances, initiating a pathological cascade as the primary etiological factor in AD.^17,18 The generation of Aβ from amyloid-β protein precursor (AβPP) involves sequential cleavage by β-secretase (BACE1), and γ-secretase.^19–21 As a pivotal enzymatic component in AβPP processing, γ-secretase plays a pivotal role in the final steps of Aβ peptide generation. The inhibition of γ-secretase activity has been shown to reduce Aβ production and oxidative stress, enhance mitochondrial activity and decrease susceptibility to apoptosis.^22,23 The inhibition of γ-secretase activity is a key objective in the treatment of AD. γ-secretase is an aspartic protease with four components, namely, presenilin 1 (PS-1), anterior pharynx-defective 1 (APH-1), presenilin enhancer 2 (PEN-2), and nicastrin (NCSTN).²⁴ Recent studies have demonstrated that the ganglioside GM1 interacts with the N-terminus of PS-1, altering the conformation of γ-secretase and specifically accelerating the hydrolysis of AβPP by γ-secretase while also increasing the production of Aβ. This, in turn, increases amyloid plaque deposition and exacerbates cognitive dysfunction in AD mouse models.²⁵ Therefore, in depth exploration of the mechanisms affecting the abnormal regulation of Aβ production by γ-secretase and the search for specific intervention strategies are of great theoretical and practical value. This is particularly relevant in the context of the prevention and treatment of AD.

Structural and biological functions of the γ-secretase complex in AD

Amyloid plaques in the brains of AD patients primarily consist of Aβ, a small peptide of 38–43 amino acids derived from the transmembrane amyloid precursor protein.²⁶ β-secretase cleaves AβPP to generate a C-terminal fragment of β (CTF-β), designated C99, which is subsequently cleaved by γ-secretase at several sites, resulting in the generation of various lengths of Aβ fragments.¹⁶ Evidence suggests that Aβ has the potential to play a physiological role in the regulation of cognitive functions and synaptic functions.²⁷ The concentration, aggregation form and specific fragment length of Aβ in vivo are crucial factors in understanding its function.²⁸ The maintenance of Aβ under physiological conditions is dependent on the equilibrium between production and clearance mechanisms. Under physiological conditions, the concentration of Aβ in the human brain and cerebrospinal fluid is in the picomolar range.²⁹ Low concentrations of Aβ have been shown to increase long-term potentiation (LTP) and dendritic spine density and to promote docking vesicles, thus playing a positive role in the regulation of synaptic function.^30,31 Conversely, excessive concentrations of Aβ have been demonstrated to impair synaptic function.^25,32,33 Furthermore, Aβ exists in several monomeric and multimeric forms and can aggregate into fibers and plaques. An alternative hypothesis is that Aβ oligomers, rather than protofibrils or monomers, are the neurotoxic forms.³⁴ Aβ monomers have neuroprotective properties, while Aβ oligomers can induce dendritic spine defects and impair cognitive function in mice even at physiological concentrations.^15,35 In addition to the aggregated forms, the Aβ monomers themselves are thought to have different functions. The length of Aβ is dependent on the cleavage site, with the two most common isoforms being Aβ₄₀ and Aβ₄₂. N-terminal Aβ fragments and shorter Aβ fragments can prevent or even reverse Aβ-induced neurotoxicity and synaptic plasticity defects.^36–38 The hydrophobic C-terminal domains with oligomer formation, such as Aβ₄₂, increase rigidity, which is closely related to neurotoxicity. Consequently, Aβ₄₂ exhibits higher aggregation propensity and amyloid plaque formation compared to Aβ₄₀.^18,39

The γ-secretase complex is a promiscuous aspartyl protease that is responsible for the final intramembrane cleavage of various type I transmembrane proteins. γ-secretase can mediate regulatory intramembrane protein hydrolysis (RIP) of approximately 150 known membrane proteins, and these substrates demonstrate the broad range of roles of γ-secretase in protein degradation, tissue homeostasis, embryonic development and adult signal transduction.⁴⁰ Furthermore, the enzyme is responsible for the hydrolysis of the transmembrane structural domain (TMD) of the amyloid precursor protein in AD, which is a pivotal step in the formation of hydrophobic Aβ deposits.⁴¹ Although γ-secretase is present in healthy humans, the phagocytosis and clearance of deposited Aβ by microglia maintain a dynamic equilibrium between Aβ production and clearance mechanisms.⁴² γ-secretase mutation is common in familial AD (FAD), and some studies have shown that the mutation of the enzyme may contribute to the acceleration of AD pathology by a variety of mechanisms, including affecting the activity of γ-secretase and endolysosomal homeostasis, and so on. For example, γ-secretase mutations result in a deficiency in the carboxypeptidase function of γ-secretase. This impairment impedes the trimming function of initially formed long Aβ peptides, consequently leading to an increase in Aβ_42/40 ratio and aggregation propensity.⁴³ Furthermore, Anika Perdok confirmed that mutations in γ-secretase accelerated amyloid plaque deposition and caused more severe memory deficits by impacting endolysosomal homeostasis and synaptic function in AD-associated brain circuits.⁴⁴ These mechanisms have led to the development of drugs, including γ-secretase inhibitors and modulators, aimed at reducing Aβ production and thereby delaying the disease process.

Human γ-secretase is a membrane-embedded protease complex composed of four essential subunits: presenilin (PS), nicastrin (NCSTN), anterior pharynx defective 1 (APH-1), and presenilin enhancer protein 2 (Pen-2), collectively forming a complex assembly with twenty transmembrane domains.^24,45 The structure of the intact human γ-secretase complex was determined for the first time at 4.5 Å resolution, revealing a horseshoe-shaped TMD and a large extracellular domain belonging to nicastrin. The latter is located directly above the hollow space formed by the transmembrane horseshoe and interacts with the TMD loops.⁴⁶ Presenilin, a 50–55 kDa multisite transmembrane protein with nine transmembrane structural domains, serves as the catalytic core of γ-secretase responsible for Aβ-generating proteolytic activity.^21,47 The subunit is encoded by two homologous genes, PS1 and PS2.²¹ The absence of the PS1 and PS2 genes in mouse cells has been reported to decrease Aβ production.^48,49 Additionally, dysfunction of the γ-secretase enzyme is postulated to be a causal factor in AD, in which the majority of FAD-derived mutations are associated with PS1.⁵⁰ Nicastrin contains a large N-terminal extracellular glycosylated ectodomain, a transmembrane structural domain, and a very short C-terminal tail.^45,51 The glycosylated extracellular domain is thought to be responsible for substrate recognition.^51,52 NCSTN regulates the efficiency of AβPP cleavage by γ-secretase by engaging in direct interactions with AβPP/Aβn substrates, which consequently influences the length of Aβ products.⁵³ Seven transmembrane (TM) helices have been identified in the APH-1 subunit, which form a stable structure within the membrane, making APH-1 a scaffolding protein for γ-secretase.⁴⁵ APH-1 ensures the efficient catalytic function of the γ-secretase complex by providing conformational support for nicastrin, facilitating the flexibility of PS1, and regulating the formation of Aβ and subsequent neurotoxicity.^45,54 APH-1 exists in both the APH-1A and APH-1B isoforms. A previous study revealed that APH-1B expression levels are increased in the blood of AD patients and are associated with cortical Aβ deposition.⁵⁵ Moreover, mutations in APH-1 directly increase the cleavage activity of γ-secretase and Aβ production.⁵⁶ Pen-2, the smallest γ-secretase subunit, is a 101-amino-acid-long protein with three TMs, two of which traverse the membrane only halfway from the intracellular side.^57,58 Pen-2 may contribute to the activation of the γ-secretase complex by directly binding to the TMD4 of PS1.^57,59 In addition, Pen-2 mutations are associated with AD.⁶⁰ The four γ-secretase subunits regulate enzyme activity through coexpression, and their binding occurs during the transport of proteins from the endoplasmic reticulum to the cell surface.²⁴

Presenilin is an essential component of γ-secretase with a pivotal functional role. Twenty years ago, Wolfe, Xia and Selkoe demonstrated that two aspartic acid residues in Presenilin constitute the active site of the γ-secretase complex.⁶¹ Moreover, they reported that mutation of these aspartic acid residues inhibits the production of Aβ.⁶¹ The subunit is encoded by two homologous genes, PS1 and PS2.²¹ PS1 is a frequent site of mutations in patients with early-onset familial AD, and over 200 mutations in the genes encoding the PS1 protein have been found to be associated with AD patients, highlighting its critical influence on the AD genetic landscape.^47,50,62 The current study proposes that the active site of γ-secretase is situated on the convex side of the transmembrane horseshoe-shaped structure of the catalytic subunit PS1.⁴⁵ The accessibility and flexibility of this active site permit γ-secretase and AβPP to bind effectively, thereby triggering substantial conformational changes upon binding, which in turn affects the catalytic efficiency of the enzyme.⁴⁵ AD-derived mutations affect residues at two hotspots in PS1, each located at the center of a different four-TM bundle.⁴⁵ Mutations, insertions, or deletions in PS1 that result in an incorrect amino acid sequence are predominantly located in transmembrane regions.^62,63 These mutations may lead to a number of pathological consequences, including aberrant signaling, memory deficits, synaptic dysfunction, and elevated Aβ₄₂/Aβ₄₀ ratios.^47,64 A point mutation in TMD1 has been demonstrated to significantly reduce the production of Aβ₄₂, as well as Aβ₄₅ and Aβ₄₈.⁶⁵ Hydrophilic loop 1 (HL1) is an extracellular region situated between TMD1 and TMD2. The α-helical structural region of HL1 and the C-terminal region of PS1 are distinct substrate-binding sites.⁶⁶ Recent studies have identified Hl1 of PS1 as critical for stepwise cleavage by γ-secretase.⁶⁷ TMD3 plays a pivotal role in regulating the Aβ₄₂ ratio, whereas TMD4 may influence protein hydrolysis activity by stabilizing the TMD3 position through its high flexibility.⁶⁸ Zhou et al. first reported a high-resolution atomic structure of a transmembrane segment of AβPP bound to human γ-secretase,⁶² which revealed interactions between the AβPP TM helix and PS1 TMs, forming a hybrid β sheet.⁶² This β-fold comprises β1 and β2 chains derived from the TMD 6-terminal and loop 2 structural domains in the PS1 protein, along with an induced β3 chain in the AβPP substrate, and serves to stabilize the substrate within the catalytic cleft.⁶² The residues at the interface between PS1 and AβPP are the sites of recurrent mutations observed in AD patients.⁶² Furthermore, recent findings have revealed that the phosphorylation of multiple variable sites on PS1 can result in the formation of a pathogenic ‘closed’ conformation of PS1, which in turn leads to the degradation of β-CTF and a reduction in Aβ production.⁴⁷ Thus, the majority of small-molecule compounds designed to target γ-secretase are based on this subunit.

Collectively, γ-secretase plays a pivotal role in the processing of AβPP and the production of Aβ peptides. The structure and function of the enzyme are inextricably linked. Each subunit is essential for regulating γ-secretase activity and γ-secretase cleavage during AβPP metabolism. The structural characteristics of γ-secretase, including the flexibility of the transmembrane helix, the accessibility of the active site, and the support of the component proteins, directly impact its catalytic function. However, mutations may result in the dysfunction of γ-secretase, potentially disrupting the conformation of the active site or substrate binding. This dysfunction, in turn, may be associated with the onset of AD. Consequently, the development of compounds targeting γ-secretase represents a promising strategy for the treatment of AD. The recent revelation of the atomic structure of γ-secretase and its binding to AβPP substrates has important scientific implications and potential applications in understanding the substrate recognition and cleavage mechanism of γ-secretase, as well as in the design of specific drugs targeting γ-secretase.

Advances in the development of small-molecule compounds that target γ-secretase to ameliorate AD

The finesse of γ-secretase inhibitors

The primary mechanism of action of γ-secretase inhibitors (GSIs) is the complete inhibition of enzyme activity, which results in a reduction in Aβ production.⁶⁹ GSIs are usually present as small molecules, and the specific mechanism of action is directly binding to the active or regulatory sites of the γ-secretase complex. This binding may cause conformational changes and interference in the substrate recruitment of the enzyme, thereby interfering with the cleavage of the substrate AβPP. For example, the structure of MRK-560 in complex with PS1 and PS2 was recently determined via cryo-electron microscopy, which revealed that MRK-560 can specifically occupy the substrate binding site of PS1. MRK-560 binding induces the formation of two β-strands and the movement of the PAL loop, thereby impairing the enzyme's catalytic function and consequently decreasing the cleavage of AβPP.⁷⁰ Shelton et al.⁷¹ created AGSIs, which bind specifically to allosteric sites within γ-secretase to cause conformational changes at the S2 and S1 subsites, thereby inhibiting γ-secretase cleavage and reducing Aβ production. GSI L685,458 is a competitive transition state analog inhibitor with a structural design that allows it to mimic the transition state of the substrate in a catalytic reaction.⁷² By directly binding to the catalytic aspartic acid residue of PS1, it exerts its inhibitory effect by preventing the normal conversion of the substrate during enzyme catalysis, which in turn inhibits the cleavage of the substrate.⁷³ The GSIs DAPT, LY450,139 (Semagacestat), BMS708,163 (Avagacestat), and LY-411,575 occupy positions in dynamic enzyme structures near the substrate binding site at high concentrations. This leads in turn to structural changes in the active site and consequently, affects the catalytic process of the enzyme, including the substrate recognition and processive cleavage of Aβ.^73,74

Over the past two decades, a substantial number of potent GSIs have been developed experimentally. Preclinical studies have consistently demonstrated the efficacy of GSIs in reducing Aβ production. DAPT treatment reduced Aβ₄₂ or Aβ₄₀ levels in the cerebrospinal fluid, plasma or brain of Tg2576 mice.⁷⁵ Furthermore, both semagacestat (LY-450139) and avagacestat (BMS-708163) ameliorated cognitive deficits in AD model animals to varying degrees.⁷⁶ However, these GSIs were found to impair normal cognition in wild-type mice.⁷⁶ Several GSIs have been subjected to clinical trials as potential treatments for AD.⁷⁷ Semagacestat (LY-450139) was the first GSI to enter phase III clinical trials. Phase I and II clinical trials of LY450,139 indicated a dose-dependent reduction in plasma Aβ levels in both healthy volunteers and patients.⁷⁸ However, in the phase III trial, all trial groups exhibited effects on cognitive performance, along with several adverse reactions, such as gastrointestinal symptoms, skin cancer, and infections.^79,80 Avagacestat (BMS-708163) exhibited greater selectivity for APP-C99 over Notch 1 than semagacestat did.^73,81 The Aβ-lowering efficacy of avagacestat was demonstrated in phase I study. Nevertheless, adverse effects, including gastrointestinal issues and skin cancer, were observed in the high-dose treatment group in the phase II study.⁸² Recently cryo-EM structural analysis of γ-secretase complexed with avagacestat revealed that it occupy the same general location as the β strand of APP-C99 or Notch-C100.⁷³ These GSIs have not demonstrated the anticipated efficacy in clinical trials. The inefficacy of the treatment (AD patients treated showed cognitive worsening) and the side effects (nausea, vomiting, diarrhea, skin cancer, infections, etc.) experienced by the subjects in the clinical trials ultimately resulted in the failure of the clinical trials of the aforementioned GSIs (LY450139,⁷⁹ BMS708163,^82,83 MK0752,⁸⁴ etc.). The reason for this was primarily the lack of selectivity of these fully inhibitory GSIs, which resulted in effects on the proteolytic processing of hundreds of substrates.^73,85 This was particularly the case with the inhibition of intramembrane protein hydrolysis of the substrate Notch, which is deeply involved in the development and homeostasis of several tissues and organs.⁸⁶ Consequently, GSIs may disrupt these essential physiological processes, leading to adverse effects in AD patients.⁸⁷ However, several γ-secretase substrates, such as Notch, ErbB4, CD44, Cadherins, VEGFR1, IGF1R, MUC1, etc., are closely related to the occurrence and development of cancer. This understanding has prompted the development of GSIs as potential anticancer therapeutics.⁸⁸

To address this issue, researchers have developed GSIs with enhanced selectively for AβPP processing over Notch signaling. MRK-560 demonstrates preferential inhibition PS1 over PS2, the latter of which plays an important role in Notch-mediated physiological functions in the gut and skin.^70,89 Therefore, MRK-560 has been shown to effectively reduce cerebral Aβ production while avoiding Notch-related toxicity in Tg2576 mouse models.⁹⁰ Using MRK-560 as the lead structure, the novel spirocyclic thione series of GSIs enhances activity and substrate selectivity by increasing the stability of the additional spirocyclic ring system, particularly the selective inhibition of PS1 isoforms (33-fold versus PS2), which has the potential to mitigate the adverse effects of clinical GSIs.⁹¹ AGSIs exert their therapeutic effects through selective inhibition of Aβ₄₂ production by specifically binding to various sites within γ-secretase complex.⁷¹ Utilizing high-throughput screening and subsequent optimization by Wyeth's researchers, GSI953 (Begacestat) was identified as a potent GSI, showing 16.8-fold selectivity for AβPP over Notch.⁹² Oral administration of GSI953 (100 mg/kg) in Tg2576 transgenic mice demonstrated significant reductions in Aβ₄₀ and Aβ₄₂ levels in the plasma, brain, and cerebrospinal fluid. These preclinical findings were subsequently validated in a phase I clinical trial involving healthy volunteers and AD patients.⁹³

The development of γ-secretase-targeting compounds for Aβ reduction is significantly challenged by the enzyme's essential biological functions, particularly its role in mediating the proteolytic cleavage of Notch and numerous other substrates.⁹⁴ The adverse effects of GSIs primarily stem from their differential inhibition of protein hydrolysis within the membranes of AβPP and Notch.⁹⁵ Consequently, direct inhibition of γ-secretase activity constitutes a clinically risky strategy for AD treatment. Despite the extensive use of these drugs, extensive research efforts are currently focused on optimization of doses, dosage forms, routes of administration and combination therapies to mitigate adverse effects and enhance treatment adherence.^96,97 Alternatively, developing γ-secretase modulators with enhanced substrate specificity represents a promising approach to achieve targeted reduction of amyloid plaque deposition while preserving physiological Notch signaling.

The finesse of γ-secretase modulators

In addition to GSIs, γ-secretase modulators (GSMs) have emerged as promising therapeutic candidates for alleviating AD symptoms. These pharmacological agents exert their therapeutic effects by selectively modulating γ-secretase activity, thereby promoting the further cleavage of Aβ₄₂ into shorter, less toxic Aβ peptides without completely inhibiting the activity of the enzyme. This mechanism effectively reduces the accumulation of aggregation-prone Aβ peptides in the brain.⁹⁸

GSMs are categorized into two generations. The first generation of GSMs is derived from nonsteroidal anti-inflammatory drugs (NSAIDs), including ibuprofen, indomethacin, and sulforaphane sulfide, which were the earliest compounds to be discovered or developed to modulate γ-secretase activity.⁹⁹ The mechanism of action of these GSMs primarily involves their direct interaction with the γ-secretase complex, leading to conformational changes that specifically affects its activity. This modulation does not inhibit the ε-cleavage activity necessary for the physiological function of the γ-secretase substrates.¹⁰⁰ For example, ibuprofen and flurbiprofen have been demonstrated to modulate γ-secretase activity by increasing the distance between the PS1 loop region and the AβPP C-terminus.^100,101

Second-generation GSMs include NSAID-derived carboxylic acid GSMs, non-NSAID-derived imidazole GSMs, and natural product-derived GSMs.¹⁰² As the presenilin subunit constitutes the catalytic center of γ-secretase, the development of many GSMs has focused on this protein.^103,104 Second-generation GSMs regulate γ-secretase activity primarily by binding to specific sites within presenilin, thereby influencing Aβ production. Piperidine acetate-type GSM-1 is among the most representative regulators. Crump et al.^105,106 demonstrated through photoaffinity labeling experiments that GSM-1 regulates γ-secretase activity by specifically binding to the N-terminal fragment of PS and altering the conformation of the active site, thereby lowering Aβ₄₂ levels. E2012 is one of the most representative imidazole-type GSMs. E2012 was shown to specifically interact with γ-secretase at the interface of PS and NCT on the extracellular side of the cell, as demonstrated by photoaffinity labeling of E2012-Bpyne.¹⁰⁴ The target of E2012 was hydrophilic loop 1 of presenilin at a specific variant site.^67,73 E2012 also modulates the piston movement of transmembrane domain 1, thereby influencing γ-secretase activity.¹⁰⁷ These findings provide valuable insights for the development of novel therapeutic agents for AD.

First-generation GSMs have been demonstrated to effectively reduce amyloid plaque Aβ₄₂ accumulation and increase the production of soluble Aβ₃₈ in vitro and in vivo without inhibiting the Notch signaling pathway.^108,109 However, the efficacy of NSAIDs to reduce Aβ₄₂ levels in animal models is inconsistent and complex.¹¹⁰ Different doses failed to achieve the expected reduction in Aβ₄₂ levels or may nonspecifically lowered Aβ₄₀ and Aβ₄₂ levels.^111,112 These findings highlight the limitations of NSAIDs in reducing Aβ₄₂ under specific conditions. Furthermore, these drugs exhibit low in vivo potency and limited brain penetration (approximately 1%), with only modest success in clinical trials.^113,114 Consequently, second-generation GSMs are designed to enhance in vivo potency and brain penetration while addressing the challenge of high lipophilicity. Data from novel GSMs in cellular and in vitro studies have been published. BIIB042 is one of the most potent second-generation GSMs. This compound reduces Aβ₄₂ levels and increases Aβ₃₈ levels in cells and in plasma of Tg2576 mice, rats, and cynomolgus monkeys. BIIB042 exerts minimal effect on the levels of Aβ₄₀. Additionally, this treatment demonstrated clear efficacy in reducing plaque pathology.¹¹⁵ The most potent R-flurbiprofen analog, EVP-001596266, effectively reduced Aβ₄₂ production, attenuated memory deficits, and decreased Aβ plaque formation and inflammation in the brains of Tg2576 mice at oral doses of 20 or 60 mg/kg/day.¹¹⁶ The piperidine analogs GSM-1, GSM-2, and GSM-10 h have been shown to reduce Aβ₄₂ levels without affecting total Aβ production or Notch processing in both cellular and animal studies.^112,117,118 GSM-2 is the most potent compound within this subclass. Acute and subchronic administration of GSM-2 significantly ameliorated memory deficits in Tg2576 mice without impairing normal cognition in wild-type mice.⁷⁶ Soluble GSM-36 demonstrated a preferential reduction in Aβ₄₂ levels in the brain and plasma of Tg2576 mice,¹¹⁹ along with favorable brain permeability, clearance, half-life, and volume distribution.¹²⁰ Compared to GSIs, second-generation GSMs have also demonstrated efficacy in reducing Aβ₄₂ in clinical settings, with a significantly improved safety profile. Oral administration of BMS-932481 reduced Aβ₃₉, Aβ₄₀, and Aβ₄₂ levels while increasing Aβ₃₇ and Aβ₃₈ levels in the cerebrospinal fluid of both young and elderly healthy volunteers.¹²¹ Moreover, compared to first-generation GSMs, most second-generation GSMs demonstrated superior activity, pharmacokinetics, and pharmacodynamics in vivo. NGP555, a representative thiazole compound, demonstrated robust Aβ₄₂ level-lowering efficacy and favorable in vivo brain penetration, distribution, and metabolic profiles in a phase I clinical trial.¹²² Furthermore, individuals treated with 200 mg or 400 mg of NGP555 showed a positive shift in Aβ_37/42 or Aβ_38/42 compared to the group receiving placebo within 14 days, suggesting that NGP555 acts by engaging brain targets and influencing amyloid biomarkers in cerebrospinal fluid.¹²³ PF-06648671 has recently completed multiple phase I trials, demonstrating its efficacy in reducing Aβ₄₂ in the hindbrain and cerebrospinal fluid of healthy and elderly subjects through single and multiple incremental oral doses. Daily doses of up to 360 mg were administered for up to 14 days. No significant drug-related safety concerns were observed.¹²⁴

In conclusion, both GSIs and GSMs, which target γ-secretase activity, play pivotal roles in regulating γ-secretase activity and consequently influencing accumulation of Aβ peptides. GSIs are primarily designed to inhibit γ-secretase activity, thereby preventing the γ-secretase-mediated cleavage of AβPP. In contrast, GSMs regulate γ-secretase activity by targeting the variant site within the catalytic subunit presenilin, which ultimately influencing Aβ deposition. The efficacy of pharmaceutical agents may be compromised by several factors, including drug target selection, dose-dependent adverse effects, challenges associated with the permeability of the blood‒brain barrier, and the patient heterogeneity. Through ongoing investigations of the atomic structure and function of γ-secretase, the substrate selectivity of GSIs has been significantly enhanced. In contrast, GSMs selectively reduce the accumulation of the more aggregation-prone Aβ₄₂ peptides, and novel GSMs have demonstrated improved biocompatibility. Despite the incomplete understanding of the mechanisms regulating the cleavage specificity of γ-secretase and the limitations of certain small molecules in terms of stability and adverse reactions, the potential of targeting γ-secretase for AD treatment remains a subject of ongoing investigation.

Conclusion

The treatment and prevention of AD present a significant challenge because of the complex nature of the disease. γ-secretase is the key enzyme responsible for Aβ production, generating Aβ peptides by the cleavage of AβPP. These peptides are considered critical in the pathological process of AD. Therefore, γ-secretase is regarded as a promising therapeutic target for AD. In recent years, significant advances have been made in γ-secretase research, particularly in the development of novel therapeutic agents for the treatment of AD. By studying the structure and function of γ-secretase, scientists have identified a variety of potential therapeutic strategies, including GSIs to reduce Aβ production or γ-secretase modulators to control Aβ deposition (Figure 1). However, since γ-secretase is involved in numerous of important biological processes, the design of therapeutic strategies targeting γ-secretase must address highly specific selectivity. Recently, our understanding of this topic has been advanced by integrating conventional biochemical techniques with cutting-edge cryo-electron microscopy analyses to elucidate the mechanisms of these drugs in greater detail. This article reviews the recently published atomic-resolution structure of γ-secretase and highlights recent advances in small-molecule compounds targeting this enzyme. Nevertheless, further research is required to elucidate the precise cleavage mechanism of γ-secretase and the exact molecular mechanism of drugs targeting it. This information will facilitate the identification of the key factors of γ-secretase substrate specificity. Although immunotherapy is the most advanced therapeutic strategy, with a focus on traditional targets such as Aβ and tau, there is a discernible shift toward small-molecule therapeutic modalities. These modalities are characterized by their simplicity, maturity and adaptability, offering a promising route to emerging targets. As a result, the prospect of developing a new generation of small-molecule drugs for AD, with the potential to halt or reverse AD progression is exciting. GSM may be a candidate for future AD prevention trials. Further research is needed to understand drug mechanisms, assess long-term efficacy and ensure safety.

Figure 1.

The mechanism by which small molecule drugs, specifically γ-secretase inhibitors (GSIs) and γ-secretase modulators (GSMs), targeting γ-secretase for the treatment of Alzheimer's disease (AD). The administration of GSIs or GSMs to AD patients has been shown associated with to a reduction in the deposition of amyloid plaques deposition and subsequent neurodegeneration within the brain. The underlying mechanism of action of GSIs and GSMs is to inhibit or modulate the activity of γ-secretase, thereby regulating the cleavage of the substrate APP-C99, which in turn leads to a decrease in Aβ generation. Created in BioRender. g\, t3. (2025) https://BioRender.com/t83w632.

Footnotes

ORCID iDs

Lin Du

Yinxiang Wei

Gencheng Han

Author contributions

Lin Du (Conceptualization; Writing – original draft; Writing – review & editing); Ge Li (Conceptualization; Writing – review & editing); Yinxiang Wei (Supervision); Gencheng Han (Supervision; Writing – review & editing).

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Sciences Foundation of China (Grants No. 82101913 to Y.W. and 82371776 to G.H.), Beijing Natural Science Foundation (Grants No. 7244377 to G.L.), Medical Science and Technique Foundation of Henan Province (Grants No. SBGJ202102195 to Y.W.).

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data availability statement

Data sharing is not applicable to this article as no datasets were generated or analyzed during this study.

References

Romero-Márquez

Forbes-Hernández

Navarro-Hortal

, et al. Molecular mechanisms of the protective effects of olive leaf polyphenols against Alzheimer’s disease. Int J Mol Sci 2023; 24: 4353.

World Health Organization. A Blueprint for Dementia Research. 1st ed. Geneva: World Health Organization, 2022.

Knopman

Amieva

Petersen

, et al. Alzheimer disease. Nat Rev Dis Primers 2021; 7: 33.

Falgàs

Walsh

Neylan

, et al. Deepen into sleep and wake patterns across Alzheimer’s disease phenotypes. Alzheimers Dement 2021; 17: 1403–1406.

Atri

. The Alzheimer’s disease clinical spectrum: diagnosis and management. Med Clin North Am 2019; 103: 263–293.

Alzheimer's Association. 2023 Alzheimer’s disease facts and figures. Alzheimers Dement 2023; 19: 1598–1695.

de los Ríos

Marco-Contelles

. Tacrines for Alzheimer’s disease therapy. III. The PyridoTacrines. Eur J Med Chem 2019; 166: 381–389.

Jarrott

. Tacrine: in vivo veritas. Pharmacol Res 2017; 116: 29–31.

Cui

Guo

Y-E

Fang

J-H

, et al. Donepezil, a drug for Alzheimer’s disease, promotes oligodendrocyte generation and remyelination. Acta Pharmacol Sin 2019; 40: 1386–1393.

10.

Benek

Korabecny

Soukup

. A perspective on multi-target drugs for Alzheimer’s disease. Trends Pharmacol Sci 2020; 41: 434–445.

11.

Cummings

Salloway

. Aducanumab: appropriate use recommendations. Alzheimers Dement 2022; 18: 531–533.

12.

Dhillon

. Aducanumab: first approval. Drugs 2021; 81: 1437–1443.

13.

van Dyck

Swanson

Aisen

, et al. Lecanemab in early Alzheimer’s disease. N Engl J Med 2023; 388: 9–21.

14.

Larkin

. Lecanemab gains FDA approval for early Alzheimer disease. JAMA 2023; 329: 363.

15.

Zhang

Chen

, et al. Amyloid β-based therapy for Alzheimer’s disease: challenges, successes and future. Signal Transduct Target Ther 2023; 8: 248.

16.

Kepp

Robakis

Høilund-Carlsen

, et al. The amyloid cascade hypothesis: an updated critical review. Brain 2023; 146: 3969–3990.

17.

Haass

Selkoe

. If amyloid drives Alzheimer disease, why have anti-amyloid therapies not yet slowed cognitive decline? PLoS Biol 2022; 20: e3001694.

18.

Chen

Yan

, et al. Amyloid beta: structure, biology and structure-based therapeutic development. Acta Pharmacol Sin 2017; 38: 1205–1235.

19.

Vassar

Bennett

Babu-Khan

, et al. Beta-secretase cleavage of Alzheimer’s amyloid precursor protein by the transmembrane aspartic protease BACE. Science 1999; 286: 735–741.

20.

Sinha

Anderson

Barbour

, et al. Purification and cloning of amyloid precursor protein beta-secretase from human brain. Nature 1999; 402: 537–540.

21.

Wolfe

. Structure and function of the γ-secretase complex. Biochemistry 2019; 58: 2953–2966.

22.

Kim

Jang

, et al. Inhibition of cholesterol biosynthesis reduces γ-secretase activity and amyloid-β generation. J Alzheimers Dis 2016; 51: 1057–1068.

23.

Sheng

Gong

Niu

, et al. Inhibition of gamma-secretase activity reduces abeta production, reduces oxidative stress, increases mitochondrial activity and leads to reduced vulnerability to apoptosis: implications for the treatment of Alzheimer’s disease. Free Radic Biol Med 2009; 46: 1362–1375.

24.

Hur

J-Y

. γ-secretase in Alzheimer’s disease. Exp Mol Med 2022; 54: 433–446.

25.

Wang

Zhou

Sun

, et al. Preferential regulation of γ-secretase-mediated cleavage of APP by ganglioside GM1 reveals a potential therapeutic target for Alzheimer’s disease. Adv Sci (Weinh) 2023; 10: e2303411.

26.

Yang

, et al. Based on molecular structures: amyloid-β generation, clearance, toxicity and therapeutic strategies. Front Mol Neurosci 2022; 15: 927530.

27.

Kent

Spires-Jones

Durrant

. The physiological roles of tau and aβ: implications for Alzheimer’s disease pathology and therapeutics. Acta Neuropathol 2020; 140: 417–447.

28.

Raskatov

. What is the ‘relevant’ amyloid β42 concentration? Chembiochem 2019; 20: 1725–1726.

29.

Roher

Esh

Kokjohn

, et al. Aβ peptides in human plasma and tissues and their significance for Alzheimer’s disease. Alzheimers Dement 2009; 5: 18–29.

30.

Ortiz-Sanz

Gaminde-Blasco

Valero

, et al. Early effects of Aβ oligomers on dendritic spine dynamics and arborization in hippocampal neurons. Front Synaptic Neurosci 2020; 12: 2.

31.

Gulisano

Melone

Ripoli

, et al. Neuromodulatory action of picomolar extracellular Aβ42 oligomers on presynaptic and postsynaptic mechanisms underlying synaptic function and memory. J Neurosci 2019; 39: 5986–6000.

32.

Arbel-Ornath

Hudry

Boivin

, et al. Soluble oligomeric amyloid-β induces calcium dyshomeostasis that precedes synapse loss in the living mouse brain. Mol Neurodegener 2017; 12: 27.

33.

Wei

, et al. Amyloid β oligomers suppress excitatory transmitter release via presynaptic depletion of phosphatidylinositol-4,5-bisphosphate. Nat Commun 2019; 10: 1193.

34.

. Amyloid-β: a double agent in Alzheimer’s disease? Biomed Pharmacother 2021; 139: 111575.

35.

Wilcox

Lacor

Pitt

, et al. Aβ oligomer-induced synapse degeneration in Alzheimer’s disease. Cell Mol Neurobiol 2011; 31: 939–948.

36.

Forest

Alfulaij

Arora

, et al. Protection against β-amyloid neurotoxicity by a non-toxic endogenous N-terminal β-amyloid fragment and its active hexapeptide core sequence. J Neurochem 2018; 144: 201–217.

37.

Anni

Weiss

E-M

Guhathakurta

, et al. Aβ1-16 controls synaptic vesicle pools at excitatory synapses via cholinergic modulation of synapsin phosphorylation. Cell Mol Life Sci 2021; 78: 4973–4992.

38.

Forest

Taketa

Arora

, et al. The neuroprotective beta amyloid hexapeptide core reverses deficits in synaptic plasticity in the 5xFAD APP/PS1 mouse model. Front Mol Neurosci 2021; 14: 576038.

39.

Hefter

Ludewig

Draguhn

, et al. Amyloid, APP, and electrical activity of the brain. Neuroscientist 2020; 26: 231–251.

40.

Güner

Lichtenthaler

. The substrate repertoire of γ-secretase/presenilin. Semin Cell Dev Biol 2020; 105: 27–42.

41.

Cai

Morishima

Takagi-Niidome

, et al. Conformational dynamics of transmembrane domain 3 of presenilin 1 is associated with the trimming activity of γ-secretase. J Neurosci 2019; 39: 8600–8610.

42.

Cai

Chen

. The amyloid-beta clearance: from molecular targets to glial and neural cells. Biomolecules 2023; 13: 313.

43.

Wolfe

. Dysfunctional γ-secretase in familial Alzheimer’s disease. Neurochem Res 2019; 44: 5–11.

44.

Perdok

Van Acker

Vrancx

, et al. Altered expression of Presenilin2 impacts endolysosomal homeostasis and synapse function in Alzheimer’s disease-relevant brain circuits. Nat Commun 2024; 15: 10412.

45.

Bai

Yan

Yang

, et al. An atomic structure of human γ-secretase. Nature 2015; 525: 212–217.

46.

Bai

, et al. Three-dimensional structure of human γ-secretase. Nature 2014; 512: 166–170.

47.

Savoy

Cummins

Henrichs

. An examination of the structural association of PSEN1 with Alzheimer’s disease. FASEB J 2021; 35(S1): https://doi.org/10.1096/fasebj.2021.35.S1.03472.

48.

Herreman

Serneels

Annaert

, et al. Total inactivation of gamma-secretase activity in presenilin-deficient embryonic stem cells. Nat Cell Biol 2000; 2: 461–462.

49.

De Strooper

Saftig

Craessaerts

, et al. Deficiency of presenilin-1 inhibits the normal cleavage of amyloid precursor protein. Nature 1998; 391: 387–390.

50.

Yan

Wang

Guo

. Progress on loss-of-function hypothesis of presenilin-1 mutations in Alzheimer diseases. Zhejiang Da Xue Xue Bao Yi Xue Ban 2021; 49: 487–499.

51.

Xie

Yan

Zhou

, et al. Crystal structure of the γ-secretase component nicastrin. Proc Natl Acad Sci U S A 2014; 111: 13349–13354.

52.

Escamilla-Ayala

Wouters

Sannerud

, et al. Contribution of the presenilins in the cell biology, structure and function of γ-secretase. Semin Cell Dev Biol 2020; 105: 12–26.

53.

Petit

Hitzenberger

Lismont

, et al. Extracellular interface between APP and Nicastrin regulates Aβ length and response to γ-secretase modulators. EMBO J 2019; 38: e101494.

54.

Sarkar

Bhunia

. Delineating the role of GxxxG motif in amyloidogenesis: a new perspective in targeting amyloid-beta mediated AD pathogenesis. ACS Bio Med Chem Au 2024; 4: 4.

55.

Park

Pyun

J-M

Hodges

, et al. Dysregulated expression levels of APH1B in peripheral blood are associated with brain atrophy and amyloid-β deposition in Alzheimer’s disease. Alzheimers Res Ther 2021; 13: 183.

56.

Watanabe

Yoshida

Hidaka

, et al. Specific mutations in Aph1 cause γ-secretase activation. Int J Mol Sci 2022; 23: 507.

57.

Sun

Zhao

Yang

, et al. Structural basis of human γ-secretase assembly. Proc Natl Acad Sci U S A 2015; 112: 6003–6008.

58.

Zhang

Sisodia

. The topology of pen-2, a γ-secretase subunit, revisited: evidence for a reentrant loop and a single pass transmembrane domain. Mol Neurodegener 2015; 10: 39.

59.

Watanabe

Tomita

Sato

, et al. Pen-2 is incorporated into the gamma-secretase complex through binding to transmembrane domain 4 of presenilin 1. J Biol Chem 2005; 280: 41967–41975.

60.

Andreoli

Trecroci

La Russa

, et al. Presenilin enhancer-2 gene: identification of a novel promoter mutation in a patient with early-onset familial Alzheimer’s disease. Alzheimers Dement 2011; 7: 574–578.

61.

Wolfe

Xia

Ostaszewski

, et al. Two transmembrane aspartates in presenilin-1 required for presenilin endoproteolysis and gamma-secretase activity. Nature 1999; 398: 513–517.

62.

Zhou

Yang

Guo

, et al. Recognition of the amyloid precursor protein by human γ-secretase. Science 2019; 363: eaaw0930.

63.

Wolfe

Miao

. Structure and mechanism of the γ-secretase intramembrane protease complex. Curr Opin Struct Biol 2022; 74: 102373.

64.

Sun

Zhou

Yang

, et al. Analysis of 138 pathogenic mutations in presenilin-1 on the in vitro production of Aβ42 and Aβ40 peptides by γ-secretase. Proc Natl Acad Sci U S A 2017; 114: E476–E485.

65.

Ohki

Shimada

Tominaga

, et al. Binding of longer Aβ to transmembrane domain 1 of presenilin 1 impacts on Aβ42 generation. Mol Neurodegener 2014; 9: 7.

66.

Takagi-Niidome

Sasaki

Osawa

, et al. Cooperative roles of hydrophilic loop 1 and the C-terminus of presenilin 1 in the substrate-gating mechanism of γ-secretase. J Neurosci 2015; 35: 2646–2656.

67.

Liu

Lauro

Wolfe

, et al. Hydrophilic loop 1 of Presenilin-1 and the APP GxxxG transmembrane motif regulate γ-secretase function in generating Alzheimer-causing Aβ peptides. J Biol Chem 2021; 296: 100393.

68.

Cai

Tomita

. Structure-activity relationship of presenilin in γ-secretase-mediated intramembrane cleavage. Semin Cell Dev Biol 2020; 105: 102–109.

69.

Elmaleh

Farlow

Conti

, et al. Developing effective Alzheimer’s disease therapies: clinical experience and future directions. J Alzheimers Dis 2019; 71: 715–732.

70.

Guo

Wang

Zhou

, et al. Molecular basis for isoform-selective inhibition of presenilin-1 by MRK-560. Nat Commun 2022; 13: 1–7.

71.

Shelton

Zhu

Chau

, et al. Modulation of gamma-secretase specificity using small molecule allosteric inhibitors. Proc Natl Acad Sci U S A 2009; 106: 20228–20233.

72.

Santiago

Guzmán-Ocampo

Aguayo-Ortiz

, et al. Characterizing the chemical space of γ-secretase inhibitors and modulators. ACS Chem Neurosci 2021; 12: 2765–2775.

73.

Yang

Zhou

Guo

, et al. Structural basis of γ-secretase inhibition and modulation by small molecule drugs. Cell 2021; 184: 521–533.e14.

74.

Svedružić

ŽM

Vrbnjak

Martinović

, et al. Structural analysis of the simultaneous activation and inhibition of γ-secretase activity in the development of drugs for Alzheimer’s disease. Pharmaceutics 2021; 13: 514.

75.

Lanz

Himes

Pallante

, et al. The gamma-secretase inhibitor N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester reduces A beta levels in vivo in plasma and cerebrospinal fluid in young (plaque-free) and aged (plaque-bearing) Tg2576 mice. J Pharmacol Exp Ther 2003; 305: 864–871.

76.

Mitani

Yarimizu

Saita

, et al. Differential effects between γ-secretase inhibitors and modulators on cognitive function in amyloid precursor protein-transgenic and nontransgenic mice. J Neurosci 2012; 32: 2037–2050.

77.

Xia

. γ-secretase and its modulators: twenty years and beyond. Neurosci Lett 2019; 701: 162–169.

78.

Fleisher

Raman

Siemers

, et al. Phase 2 safety trial targeting amyloid beta production with a gamma-secretase inhibitor in Alzheimer disease. Arch Neurol 2008; 65: 1031–1038.

79.

Doody

Raman

Farlow

, et al. A phase 3 trial of semagacestat for treatment of Alzheimer’s disease. N Engl J Med 2013; 369: 341–350.

80.

Gupta

Martins

. Semagacestat for treatment of Alzheimer’s disease. N Engl J Med 2013; 369: 1660–1661.

81.

Gillman

Starrett

Parker

, et al. Discovery and evaluation of BMS-708163, a potent, selective and orally bioavailable γ-secretase inhibitor. ACS Med Chem Lett 2010; 1: 120–124.

82.

Coric

Salloway

van Dyck

, et al. Targeting prodromal Alzheimer disease with avagacestat: a randomized clinical trial. JAMA Neurol 2015; 72: 1324–1333.

83.

Coric

van Dyck

Salloway

, et al. Safety and tolerability of the γ-secretase inhibitor avagacestat in a phase 2 study of mild to moderate Alzheimer disease. Arch Neurol 2012; 69: 1430–1440.

84.

Kumar

Ganeshpurkar

Kumar

, et al. Secretase inhibitors for the treatment of Alzheimer’s disease: long road ahead. Eur J Med Chem 2018; 148: 436–452.

85.

Hou

Zielonka

Serneels

, et al. The γ-secretase substrate proteome and its role in cell signaling regulation. Mol Cell 2023; 83: 4106–4122.e10.

86.

Gozlan

Sprinzak

. Notch signaling in development and homeostasis. Development 2023; 150: dev201138.

87.

Zhou

Lin

Long

, et al. Notch signaling pathway: architecture, disease, and therapeutics. Signal Transduct Target Ther 2022; 7: 95.

88.

Song

Zhang

, et al. The critical role of γ-secretase and its inhibitors in cancer and cancer therapeutics. Int J Biol Sci 2023; 19: 5089–5103.

89.

Borgegård

Gustavsson

Nilsson

, et al. Alzheimer’s disease: presenilin 2-sparing γ-secretase inhibition is a tolerable Aβ peptide-lowering strategy. J Neurosci 2012; 32: 17297–17305.

90.

Best

Smith

Reilly

, et al. The novel gamma secretase inhibitor N-[cis-4-[(4-chlorophenyl)sulfonyl]-4-(2,5-difluorophenyl)cyclohexyl]-1,1,1-trifluoromethanesulfonamide (MRK-560) reduces amyloid plaque deposition without evidence of notch-related pathology in the Tg2576 mouse. J Pharmacol Exp Ther 2007; 320: 552–558.

91.

Zhao

Pissarnitski

Josien

, et al. Discovery of a novel, potent spirocyclic series of γ-secretase inhibitors. J Med Chem 2015; 58: 8806–8817.

92.

Hopkins

. ACS Chemical neuroscience molecule spotlight on Begacestat (GSI-953). ACS Chem Neurosci 2012; 3: 3–4.

93.

Martone

Zhou

Atchison

, et al. Begacestat (GSI-953): a novel, selective thiophene sulfonamide inhibitor of amyloid precursor protein gamma-secretase for the treatment of Alzheimer’s disease. J Pharmacol Exp Ther 2009; 331: 598–608.

94.

Sprinzak

Blacklow

. Biophysics of notch signaling. Annu Rev Biophys 2021; 50: 157–189.

95.

De Strooper

Annaert

Cupers

, et al. A presenilin-1-dependent gamma-secretase-like protease mediates release of Notch intracellular domain. Nature 1999; 398: 518–522.

96.

Nunes

Loureiro

Pereira

. Drug delivery systems as a strategy to improve the efficacy of FDA-approved Alzheimer’s drugs. Pharmaceutics 2022; 14: 2296.

97.

Kabir

Uddin

Mamun

, et al. Combination drug therapy for the management of Alzheimer’s disease. Int J Mol Sci 2020; 21: 3272.

98.

Mandal

Buevich

Caldwell

, et al. Generation of leads for γ-secretase modulation. J Med Chem 2020; 63: 8216–8230.

99.

Weggen

Eriksen

Das

, et al. A subset of NSAIDs lower amyloidogenic Abeta42 independently of cyclooxygenase activity. Nature 2001; 414: 212–216.

100.

Kukar

Golde

. Possible mechanisms of action of NSAIDs and related compounds that modulate γ-secretase cleavage. Curr Top Med Chem 2008; 8: 47–53.

101.

Lleó

Berezovska

Herl

, et al. Nonsteroidal anti-inflammatory drugs lower Abeta42 and change presenilin 1 conformation. Nat Med 2004; 10: 1065–1066.

102.

Oehlrich

Berthelot

DJ-C

Gijsen

HJM

. γ-secretase modulators as potential disease modifying anti-Alzheimer’s drugs. J Med Chem 2011; 54: 669–698.

103.

Kounnas

Danks

Cheng

, et al. Modulation of γ-secretase reduces β-amyloid deposition in a transgenic mouse model of Alzheimer’s disease. Neuron 2010; 67: 769–780.

104.

Pozdnyakov

Murrey

Crump

, et al. γ-secretase modulator (GSM) photoaffinity probes reveal distinct allosteric binding sites on presenilin. J Biol Chem 2013; 288: 9710–9720.

105.

Jumpertz

Rennhack

Ness

, et al. Presenilin is the molecular target of acidic γ-secretase modulators in living cells. PLoS One 2012; 7: e30484.

106.

Crump

Fish

Castro

, et al. Piperidine acetic acid based γ-secretase modulators directly bind to Presenilin-1. ACS Chem Neurosci 2011; 2: 705–710.

107.

Cai

Yonaga

Tomita

. Activation of γ-secretase trimming activity by topological changes of transmembrane domain 1 of presenilin 1. J Neurosci 2017; 37: 12272–12280.

108.

Luo

Y-M

. Turning the tide on Alzheimer’s disease: modulation of γ-secretase. Cell Biosci 2022; 12: 2.

109.

Gasparini

Ongini

Wenk

. Non-steroidal anti-inflammatory drugs (NSAIDs) in Alzheimer’s disease: old and new mechanisms of action. J Neurochem 2004; 91: 521–536.

110.

Eriksen

Sagi

Smith

, et al. NSAIDs and enantiomers of flurbiprofen target gamma-secretase and lower Abeta 42 in vivo. J Clin Invest 2003; 112: 440–449.

111.

Lanz

Fici

Merchant

. Lack of specific amyloid-β(1-42) suppression by nonsteroidal anti-inflammatory drugs in young, plaque-free Tg2576 mice and in Guinea pig neuronal cultures. J Pharmacol Exp Ther 2005; 312: 399–406.

112.

Ling

I-F

Golde

Galasko

, et al. Modulation of Aβ42 in vivo by γ-secretase modulator in primates and humans. Alzheimers Res Ther 2015; 7: 55.

113.

Wilcock

Black

Hendrix

, et al. Efficacy and safety of tarenflurbil in mild to moderate Alzheimer’s disease: a randomised phase II trial. Lancet Neurol 2008; 7: 483–493.

114.

Lozupone

Solfrizzi

D’Urso

, et al. Anti-amyloid-β protein agents for the treatment of Alzheimer’s disease: an update on emerging drugs. Expert Opin Emerg Drugs 2020; 25: 319–335.

115.

Scannevin

Chollate

Brennan

, et al. BIIB042, A novel γ-secretase modulator, reduces amyloidogenic aβ isoforms in primates and rodents and plaque pathology in a mouse model of Alzheimer’s disease. Neuropharmacology 2016; 103: 57–68.

116.

Rogers

Felsenstein

Hrdlicka

, et al. Modulation of γ-secretase by EVP-0015962 reduces amyloid deposition and behavioral deficits in Tg2576 mice. Mol Neurodegener 2012; 7: 61.

117.

Mitani

Yarimizu

Akashiba

, et al. Amelioration of cognitive deficits in plaque-bearing Alzheimer’s disease model mice through selective reduction of nascent soluble Aβ42 without affecting other Aβ pools. J Neurochem 2013; 125: 465–472.

118.

Hawkins

Harrison

Ahmed

, et al. Dynamics of Aβ42 reduction in plasma, CSF and brain of rats treated with the γ-secretase modulator, GSM-10 h. Neurodegener Dis 2011; 8: 455–464.

119.

Raven

Ward

Zoltowska

, et al. Soluble gamma-secretase modulators attenuate Alzheimer’s β-amyloid pathology and induce conformational changes in presenilin 1. EBioMedicine 2017; 24: 93–101.

120.

Rynearson

Buckle

Barnes

, et al. Design and synthesis of aminothiazole modulators of the gamma-secretase enzyme. Bioorg Med Chem Lett 2016; 26: 3928–3937.

121.

Soares

Gasior

Toyn

, et al. The γ-secretase modulator, BMS-932481, modulates aβ peptides in the plasma and cerebrospinal fluid of healthy volunteers. J Pharmacol Exp Ther 2016; 358: 138–150.

122.

Kounnas

Lane-Donovan

Nowakowski

, et al. NGP 555, A γ-secretase modulator, lowers the amyloid biomarker, Aβ42, in cerebrospinal fluid while preventing Alzheimer’s disease cognitive decline in rodents. Alzheimers Dement (N Y) 2017; 3: 65–73.

123.

Kounnas

Durakoglugil

Herz

, et al. NGP 555, A γ-secretase modulator, shows a beneficial shift in the ratio of amyloid biomarkers in human cerebrospinal fluid at safe doses. Alzheimers Dement (N Y) 2019; 5: 458–467.

124.

Ahn

Carrieri

Dela Cruz

, et al. Pharmacokinetic and pharmacodynamic effects of a γ-secretase modulator, PF-06648671, on CSF amyloid-β peptides in randomized phase I studies. Clin Pharmacol Ther 2020; 107: 211–220.