Probing the structural requirements of non-electrophilic naphthalene-based Nrf2 activators

Non-electrophilic NRF2 activators promote wound healing in human keratinocytes and diabetic mice and demonstrate selective downstream gene targeting

The transcription factor NRF2 plays an important role in many biological processes and is a promising therapeutic target for many disease states. NRF2 is highly expressed in the skin and is known to play a critical role in diabetic wound healing, a serious disease process for which treatment options are limited. However, many existing NRF2 activators display off-target effects due to their electrophilic mechanism, underscoring the need for alternative approaches. In this work, we investigated two recently described non-electrophilic NRF2 activators, ADJ-310 and PRL-295, and demonstrated their efficacy in vitro and in vivo in human keratinocytes and Leprdb/db diabetic mice. We also compared the downstream targets of PRL-295 to those of the widely used electrophilic NRF2 activator CDDO-Me by RNA sequencing. Both ADJ-310 and PRL-295 maintained human keratinocyte cell viability at increasing concentrations and maintained or improved cell proliferation over time. Both compounds also increased cell migration, improving in vitro wound closure. ADJ-310 and PRL-295 enhanced the oxidative stress response in vitro, and RNA-sequencing data showed that PRL-295 activated NRF2 with a narrower transcriptomic effect than CDDO-Me. In vivo, both ADJ-310 and PRL-295 improved wound healing in Leprdb/db diabetic mice and upregulated known downstream NRF2 target genes in treated tissue. These results highlight the non-electrophilic compounds ADJ-310 and PRL-295 as effective, innovative tools for investigating the function of NRF2. These compounds directly address the need for alternative NRF2 activators and offer a new approach to studying the role of NRF2 in human disease and its potential as a therapeutic across multiple disease states.

The Bioactive Gamma-Oryzanol from Oryza sativa L. Promotes Neuronal Differentiation in Different In Vitro and In Vivo Models

Gamma-oryzanol (ORY), found in rice (Oryza sativa L.), is a mixture of ferulic acid esters with triterpene alcohols, well-known for its antioxidant and anti-inflammatory properties. Our past research demonstrated its positive impact on cognitive function in adult mice, influencing synaptic plasticity and neuroprotection. In this study, we explored whether ORY can exert neuro-differentiating effects by using different experimental models. For this purpose, chemical characterization identified four components that are most abundant in ORY. In human neuroblastoma cells, we showed ORY's ability to stimulate neurite outgrowth, upregulating the expression of GAP43, BDNF, and TrkB genes. In addition, ORY was found to guide adult mouse hippocampal neural progenitor cells (NPCs) toward a neuronal commitment. Microinjection of ORY in zebrafish Tg (-3.1 neurog1:GFP) amplified neurog1-GFP signal, islet1, and bdnf mRNA levels. Zebrafish nrf2a and nrf2b morphants (MOs) were utilized to assess ORY effects in the presence or absence of Nrf2. Notably, ORY's ability to activate bdnf was nullified in nrf2a-MO and nrf2b-MO. Furthermore, computational analysis suggested ORY's single components have different affinities for the Keap1-Kelch domain. In conclusion, although more in-depth studies are needed, our findings position ORY as a potential source of bioactive molecules with neuro-differentiating potential involving the Nrf2 pathway.

Bivalent inhibitors of the BTB E3 ligase KEAP1 enable instant NRF2 activation to suppress acute inflammatory response

Most BTB-containing E3 ligases homodimerize to recognize a single substrate by engaging multiple degrons, represented by E3 ligase KEAP1 dimer and its substrate NRF2. Inactivating KEAP1 to hinder ubiquitination-dependent NRF2 degradation activates NRF2. While various KEAP1 inhibitors have been reported, all reported inhibitors bind to KEAP1 in a monovalent fashion and activate NRF2 in a lagging manner. Herein, we report a unique bivalent KEAP1 inhibitor, biKEAP1 (3), that engages cellular KEAP1 dimer to directly release sequestered NRF2 protein, leading to an instant NRF2 activation. 3 promotes the nuclear translocation of NRF2, directly suppressing proinflammatory cytokine transcription. Data from in vivo experiments showed that 3, with unprecedented potency, reduced acute inflammatory burden in several acute inflammation models in a timely manner. Our findings demonstrate that the bivalent KEAP1 inhibitor can directly enable sequestered substrate NRF2 to suppress inflammatory transcription response and dampen various acute inflammation injuries.

Targeting kelch-like (KLHL) proteins: achievements, challenges and perspectives

Kelch-like proteins (KLHLs) are a large family of BTB-containing proteins. KLHLs function as the substrate adaptor of Cullin 3-RING ligases (CRL3) to recognize substrates. KLHLs play pivotal roles in regulating various physiological and pathological processes by modulating the ubiquitination of their respective substrates. Mounting evidence indicates that mutations or abnormal expression of KLHLs are associated with various human diseases. Targeting KLHLs is a viable strategy for deciphering the KLHLs-related pathways and devising therapies for associated diseases. Here, we comprehensively review the known KLHLs inhibitors to date and the brilliant ideas underlying their development.

Strategies for converting turn-motif and cyclic peptides to small molecules for targeting protein-protein interactions

The development of small molecules that interact with protein-protein interactions is an ongoing challenge. Peptides offer a starting point in the drug discovery process for targeting protein-interactions due to their larger, more flexible structure and the structurally diverse properties that allow for a greater interaction with the protein. The techniques for rapidly identifying potent cyclic peptides and turn-motif peptides are highly effective, but this potential has not yet transferred to approved drug candidates. By applying the properties of the peptide-protein interaction the development of small molecules for drug discovery has the potential to be more efficient. In this review, we discuss the methods that allow for the unique binding properties of peptides to proteins, and the methods deployed to transfer these qualities to potent small molecules.

Structural modification of C2-substituents on 1,4-bis(arylsulfonamido)benzene or naphthalene-N,N'-diacetic acid derivatives as potent inhibitors of the Keap1-Nrf2 protein-protein interaction

The Keap1-Nrf2-ARE signaling pathway is an attractive therapeutic target for the prevention and treatment of oxidative stress-associated diseases by activating the cellular expression of cytoprotective enzymes and proteins. Small molecule inhibitors can directly disrupt the Keap1-Nrf2 protein-protein interaction (PPI), resulting in elevated levels of Nrf2 protein and subsequent stimulation of related antioxidant responses. Previously, we found that 1,4-bis(arylsulfonamido)benzene or naphthalene-N,N'-diacetic acid derivatives with an ether type C2-substituent on the benzene or naphthalene core exhibited potent inhibitory activities with IC50's in the submicromolar or nanomolar range. We here describe a more detailed structure-activity relationship study around the C2 substituents containing various polar linkers shedding new insight on their binding interactions with the Keap1 Kelch domain. The key observation from our findings is that the substituents at the C2-position of the benzene or naphthalene scaffold impact their inhibitory potencies in biochemical assays as well as activities in cell culture. The biochemical FP and TR-FRET assays revealed that the naphthalene derivatives 17b and 18 with an additional carboxylate at the C2 were the most active inhibitors against Keap1-Nrf2 PPI. In the cell-based assay, the two compounds were shown to be potent Nrf2 activators of the transcription of the Nrf2-dependent genes, such as HMOX2, GSTM3, and NQO1.

Mangiferin attenuates osteoporosis by inhibiting osteoblastic ferroptosis through Keap1/Nrf2/SLC7A11/GPX4 pathway

BACKGROUND

Ferroptosis is a crucial contributor to impaired osteoblast function in osteoporosis. Mangiferin, a xanthonoid glucoside isolated from mangoes, exhibits anti-osteoporosis effects. However, its potential mechanism is not fully understood.

PURPOSE

This study explores the potencies of mangiferin on osteoblastic ferroptosis and deciphers its direct target in the context of solute carrier family 7-member 11 (SLC7A11)/glutathione peroxidases 4 (GPX4) pathway.

METHODS

In vivo models include bilateral ovariectomy induced osteoporosis mice, iron-dextran induced iron-overloaded mice, and nuclear factor-erythroid 2-related factor 2 (Nrf2)-knockout mice. Mice are orally administrated mangiferin (10, 50 or 100 mg.kg-1.d-1) for 12 weeks. In vitro osteoblast models include iron-dextran induced iron-overloaded cells, erastin induced ferroptosis cells, and gene knockout cells. RNA sequencing is applied for investigating the underlying mechanisms. The direct target of mangiferin is studied using a cellular thermal shift assay, silico docking, and surface plasmon resonance.

RESULTS

Mangiferin promotes bone formation and inhibits ferroptosis in vivo models (osteoporosis mice, iron-overloaded mice) and in vitro models (ferroptosis osteoblast, iron-overloaded osteoblasts). Mechanismly, mangiferin directly binds to the kelch-like ECH-associated protein 1 (Keap1) and activates the downstream Nrf2/SLC7A11/GPX4 pathway in both the in vivo and in vitro models. Mangiferin failed to restore the osteoporosis and ferroptosis in Nrf2-knockout mice. Silencing Nrf2, SLC7A11 or GPX4 abolished the anti-ferroptosis effect of mangiferin in erastin-induced cells. Addition of the ferroptosis agonist RSL-3 also blocked the protective effects of mangiferin on iron-overloaded cells. Furthermore, mangiferin had better effects on osteogenesis than the ferroptosis inhibitor (ferrostatin-1) and the Nrf2 agonists (sulforaphane, dimethyl fumarate, and bardoxolone).

CONCLUSIONS

We identify for the first time mangiferin as a ferroptosis inhibitor and a direct Keap1 conjugator that promotes bone formation and alleviates osteoporosis. This work also provides a potentially practical pharmacological approach for treating ferroptosis-driven diseases.

A Multimodal Deep Learning Framework for Predicting PPI-Modulator Interactions

Protein-protein interactions (PPIs) are essential for various biological processes and diseases. However, most existing computational methods for identifying PPI modulators require either target structure or reference modulators, which restricts their applicability to novel PPI targets. To address this challenge, we propose MultiPPIMI, a sequence-based deep learning framework that predicts the interaction between any given PPI target and modulator. MultiPPIMI integrates multimodal representations of PPI targets and modulators and uses a bilinear attention network to capture intermolecular interactions. Experimental results on our curated benchmark data set show that MultiPPIMI achieves an average AUROC of 0.837 in three cold-start scenarios and an AUROC of 0.994 in the random-split scenario. Furthermore, the case study shows that MultiPPIMI can assist molecular docking simulations in screening inhibitors of Keap1/Nrf2 PPI interactions. We believe that the proposed method provides a promising way to screen PPI-targeted modulators.

Advances in developing noncovalent small molecules targeting Keap1

Kelch-like ECH-associated protein 1 (Keap1) is a drug target for diseases involving oxidative stress and inflammation. There are three covalent Keap1-binding drugs on the market, but noncovalent compounds that inhibit the interaction between Keap1 and nuclear factor erythroid 2-related factor 2 (Nrf2) represent an attractive alternative. Both compound types prevent degradation of Nrf2, leading to the expression of antioxidant and antiinflammatory proteins. However, their off-target profiles differ as do their exact pharmacodynamic effects. Here, we discuss the opportunities and challenges of targeting Keap1 with covalent versus noncovalent inhibitors. We then provide a comprehensive overview of current noncovalent Keap1-Nrf2 inhibitors, with a focus on their pharmacological effects, to examine the therapeutic potential for this compound class.

Revealing how phenytoin triggers liver damage and the potential protective effects of Balanites Aegyptiaca fruit extracts: Exploring Nrf2/MAPK/ Beclin-1 signaling pathways

Phenytoin-induced liver injury (PHT ILII) is a serious condition that may necessitate discontinuation of the drug. This study investigates the mechanisms of PHT ILII and evaluates the protective effects of Balanites Aegyptiaca (BA) fruit extracts on the liver. We focus on the Nrf2/MAPK/NF-κB/Beclin-1 signaling pathways involved in oxidative stress and inflammation from drug-induced liver injury. Phytochemical analyses of BA fruit extracts (Bu-F and EA-F) are conducted. Molecular docking techniques explore the interaction between phenytoin (PHT) and the Nrf2/MAPK/NF-κB/Beclin-1 pathways. Thirty-six male rats are divided into Control, Bu-F, EA-F, PHT, Bu-F/PHT, and EA-F/PHT groups, and they are observed for 45 days. EA-F extract is rich in phenolics/flavonoids, while Bu-F extract mainly contains saponins.PHT ILII causes histological damage in liver tissues and affects Nrf-2, MAPK, TNF-α, IL-1β, Mcp-1, Beclin-1, iNOS expression, and liver function markers (ALT, AST, ALP). However, EA-F/Bu-F extracts effectively improve the histological structure and significantly reduce biochemical/immunohistochemical parameters, restoring them to near-normal levels. EA-F extract is particularly effective.In conclusion, the Nrf2/MAPK /Beclin-1 pathways play a critical role in the development of PHT ILII. BA fruit extracts show promise as hepato-protective agents, with the EA-F extract demonstrating superior efficacy. These results lay the groundwork for new treatments for PHT ILII and drug-induced liver injuries.

Medicinal Chemistry Insights into the Development of Small-Molecule Kelch-Like ECH-Associated Protein 1-Nuclear Factor Erythroid 2-Related Factor 2 (Keap1-Nrf2) Protein-Protein Interaction Inhibitors

Oxidative stress has been implicated in a wide range of pathological conditions. The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) exerts a central role in regulating the cellular defense system against oxidative and electrophilic insults. Nonelectrophilic inhibition of the protein-protein interaction (PPI) between Kelch-like ECH-associated protein 1 (Keap1) and Nrf2 has become a promising approach to activate Nrf2. Recently, multiple drug discovery strategies have facilitated the development of small-molecule Keap1-Nrf2 PPI inhibitors with potent activity and favorable drug-like properties. In this Perspective, we summarize the latest progress of small-molecule Keap1-Nrf2 PPI inhibitors from medicinal chemistry insights and discuss future prospects and challenges in this field.

Optimization of the C2 substituents on the 1,4-bis(arylsulfonamido)naphthalene-N,N'-diacetic acid scaffold for better inhibition of Keap1-Nrf2 protein-protein interaction

Direct inhibition of the protein-protein interaction (PPI) between Kelch-like ECH-associated protein 1 (Keap1) and nuclear factor erythroid 2-related factor 2 (Nrf2) reduces the ubiquitination and subsequent degradation of Nrf2, leading to Nrf2 accumulation in the cytosol and the nuclear translocation of Nrf2. Once inside the nucleus, Nrf2 binds to and activates the expression of antioxidant response element (ARE) genes involved in redox homeostasis and detoxification. Herein, we report a series of 1,4-bis(arylsulfonamido)naphthalene-N,N'-diacetic acid analogs with varying C2 substituents to explore the structure-activity relationships at this position of the central naphthalene core. The Keap1-binding activities were first screened with a fluorescence polarization (FP) assay followed by further evaluation of the more potent compounds using a more sensitive time-resolved fluorescence energy transfer (TR-FRET) assay. It was found that compound 24a with C2-phthalimidopropyl group was the most potent in this series showing an IC50 of 2.5 nM in the TR-FRET assay with a Ki value in the subnanomolar range. Our docking study indicated that the C2-phthalimidopropyl group in compound 24a provided an extra hydrogen bonding interaction with the key residue Arg415 that may be responsible for the observed boost in binding affinity. In addition, compounds 12b, 15, and 24a were shown to activate the Nrf2 signaling pathway in NCM460D cells resulting in elevated mRNA levels of GSTM3, HMOX1 and NQO1 by 2.4-11.7 fold at 100 μM as compared to the vehicle control.

KEAP1-NRF2 protein-protein interaction inhibitors: Design, pharmacological properties and therapeutic potential

The transcription factor nuclear factor erythroid 2-related factor 2 (NRF2) is considered the master regulator of the phase II antioxidant response. It controls a plethora of cytoprotective genes related to oxidative stress, inflammation, and protein homeostasis, among other processes. Activation of these pathways has been described in numerous pathologies including cancer, cardiovascular, respiratory, renal, digestive, metabolic, autoimmune, and neurodegenerative diseases. Considering the increasing interest of discovering novel NRF2 activators due to its clinical application, initial efforts were devoted to the development of electrophilic drugs able to induce NRF2 nuclear accumulation by targeting its natural repressor protein Kelch-like ECH-associated protein 1 (KEAP1) through covalent modifications on cysteine residues. However, off-target effects of these drugs prompted the development of an innovative strategy, the search of KEAP1-NRF2 protein-protein interaction (PPI) inhibitors. These innovative activators are proposed to target NRF2 in a more selective way, leading to potentially improved drugs with the application for a variety of diseases that are currently under investigation. In this review, we summarize known KEAP1-NRF2 PPI inhibitors to date and the bases of their design highlighting the most important features of their respective interactions. We also discuss the preclinical pharmacological properties described for the most promising compounds.

Development of Noncovalent Small-Molecule Keap1-Nrf2 Inhibitors by Fragment-Based Drug Discovery

Targeting the protein-protein interaction (PPI) between the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) and its repressor, Kelch-like ECH-associated protein 1 (Keap1), constitutes a promising strategy for treating diseases involving oxidative stress and inflammation. Here, a fragment-based drug discovery (FBDD) campaign resulted in novel, high-affinity (K_i = 280 nM), and cell-active noncovalent small-molecule Keap1-Nrf2 PPI inhibitors. We screened 2500 fragments using orthogonal assays─fluorescence polarization (FP), thermal shift assay (TSA), and surface plasmon resonance (SPR)─and validated the hits by saturation transfer difference (STD) NMR, leading to 28 high-priority hits. Thirteen co-structures showed fragments binding mainly in the P4 and P5 subpockets of Keap1's Kelch domain, and three fluorenone-based fragments featuring a novel binding mode were optimized by structure-based drug discovery. We thereby disclose several fragment hits, including their binding modes, and show how FBDD can be performed to find new small-molecule Keap1-Nrf2 PPI inhibitors.

Sanguinarine protects against indomethacin-induced small intestine injury in rats by regulating the Nrf2/NF-κB pathways

In recent years, small intestine as a key target in the treatment of Inflammatory bowel disease caused by NSAIDs has become a hot topic. Sanguinarine (SA) is one of the main alkaloids in the Macleaya cordata extracts with strong pharmacological activity of anti-tumor, anti-inflammation and anti-oxidant. SA is reported to inhibit acetic acid-induced colitis, but it is unknown whether SA can relieve NSAIDs-induced small intestinal inflammation. Herein, we report that SA effectively reversed the inflammatory lesions induced by indomethacin (Indo) in rat small intestine and IEC-6 cells in culture. Our results showed that SA significantly relieved the symptoms and reversed the inflammatory lesions of Indo as shown in alleviation of inflammation and improvement of colon macroscopic damage index (CMDI) and tissue damage index (TDI) scores. SA decreased the levels of TNF-α, IL-6, IL-1β, MDA and LDH in small intestinal tissues and IEC-6 cells, but increased SOD activity and ZO-1 expression. Mechanistically, SA dose-dependently promoted the expression of Nrf2 and HO-1 by decreasing Keap-1 level, but inhibited p65 phosphorylation and nuclear translocation in Indo-treated rat small intestine and IEC-6 cells. Furthermore, in SA treated cells, the colocalization between p-p65 and CBP in the nucleus was decreased, while the colocalization between Nrf2 and CBP was increased, leading to the movement of gene expression in the nucleus to the direction of anti-inflammation and anti-oxidation. Nrf2 silencing blocked the effects of SA. Together our results suggest that SA can significantly prevent intestinal inflammatory lesions induced by Indo in rats and IEC-6 cells through regulation of the Nrf2 pathway and NF-κBp65 pathway.

Structure-activity relationships of 1,4-bis(arylsulfonamido)-benzene or naphthalene-N,N'-diacetic acids with varying C2-substituents as inhibitors of Keap1-Nrf2 protein-protein interaction

The Keap1-Nrf2-ARE pathway plays an important role in responding to oxidative stress and maintaining the redox homeostasis. Small molecule inhibitors targeting directly the Keap1-Nrf2 protein-protein interaction (PPI) can potentially be developed into effective preventive and therapeutic agents for numerous chronic inflammatory diseases. To improve the drug-like properties and inhibitory potency of these inhibitors, a series of 1,4-bis(arylsulfonamido)benzene or naphthalene-N,N'-diacetic acids with varying substituents at C-2 position of the benzene or naphthalene core were designed and synthesized. Among them, compound 12d with 2-(4-fluorobenzyloxy) group was the most potent direct inhibitor of Keap1-Nrf2 PPI with an IC₅₀ of 64.5 nM in the fluorescent polarization (FP) assay and 14.2 nM in a time-resolved fluorescence resonance energy transfer (TR-FRET) assay. Moreover, cell-based biological assay showed that 12d significantly increased the mRNA levels of Nrf2 downstream genes, GSTM3, HMOX2 and NQO1, through Nrf2 activation. The discovery of the new scaffolds possessing diverse O-linked fragments at the C2 position offers opportunities to further modify the chemical structures of Keap1-Nrf2 PPI inhibitors to improve their pharmacokinetic, efficacy and safety profiles.

Crystallography-Guided Optimizations of the Keap1-Nrf2 Inhibitors on the Solvent Exposed Region: From Symmetric to Asymmetric Naphthalenesulfonamides

Directly inhibiting the Keap1-Nrf2 protein-protein interaction has been investigated as a promising strategy to activate Nrf2 for anti-inflammation. We previously reported a naphthalensulfonamide Keap1-Nrf2 inhibitor NXPZ-2, but have not determined the exact binding mode with Keap1. This symmetric naphthalenesulfonamide compound has relatively low solubility. Herein, we first determined a crystal complex (resolution: 2.3 Å) of human Keap1 Kelch domain with NXPZ-2. Further optimizations on the solvent exposed region obtained asymmetric naphthalenesulfonamides and three crystal structures of Keap1 in complex with designed compounds. Among them, the asymmetric piperazinyl-naphthalenesulfonamide 6k with better aqueous solubility showed the best K_D2 value of 0.21 μM to block the interaction. The productions of ROS and NO and the expression of TNF-α were inhibited by 6k in the in vitro model. This compound could relieve inflammations by significantly increasing the Nrf2 nuclear translocation in the LPS-induced ALI model with promising pharmacokinetic properties.

Chlorhexidine as a Keap1-Nrf2 inhibitor: a new target for an old drug for Parkinson's disease therapy

Oxidative stress plays a vital role in the pathophysiology of most neurodegenerative diseases such as Parkinson's disease (PD). The Keap1-Nrf2-ARE pathway, one of the internal defense mechanisms, curbs the reactive oxygen species (ROS) generated in the cellular environment. The pathway leads to the expression of antioxidant genes such as HO-1, GCLC, and NQO1, which act as cellular redox switches and protect the cellular environment. Keap1, the negative regulator of Nrf2, is a potential therapeutic target for treating age-related neurodegenerative diseases. Tecfidera (Dimethyl fumarate), used in the intervention for relapsing multiple sclerosis, is the only commercial drug known to regulate the Nrf2 function. Here, we have identified a repurposing drug, chlorhexidine (LBP125), through ligand-based pharmacophore development and screening against the DrugBank, as a potential inhibitor of the β-propeller domain of Keap1 (Keap1-DC). Chlorhexidine, an antimicrobial agent, is widely used as a mouthwash, skin cleanser, and intervening bacterial infection during childbirth. The biochemical assay confirmed a significant binding affinity of 30 µM and competitively inhibited the Nrf2 peptide interaction. Moreover, chlorhexidine also exerts cytoprotection in a neurotoxic cell model of PD through Keap1-Nrf2 disruption leading to nuclear translocation of Nrf2 and expression of downstream genes, HO-1, and NQO1. Hence, the chemical scaffold of chlorhexidine is a potential lead to develop new chemical libraries with drug-like properties for treating PD. Communicated by Ramaswamy H. Sarma.

Synthesis and biological evaluation of 1,2,4-triazole derivatives as potential Nrf2 activators for the treatment of cerebral ischemic injury

Acute ischemic stroke is a leading cause of disability and death. The development of neuroprotectants is an emerging strategy for the treatment of ischemic stroke. In this work, we designed and synthesized a series of 1,3,5-triaryl substituent triazole derivatives by introducing a phenolic group and phenyl ring to 3,5-diaryl substituents oxadiazole. Structure-activity relationship (SAR) analysis showed that compounds with alkyl groups or with substituents at the 3-position possessed better protective effects. Among the derivatives, 3,5-dimethyl substituted compound 24 exhibited the best neuroprotective effect with weak cytotoxicity. Compound 24 possessed a high plasma protein binding rate, moderate hERG inhibition, low acute toxicity, and suitable pharmacokinetic properties. In vivo experiments demonstrated that compound 24 exerted a protective effect by reducing cerebral infarction size, improving neurological behavior, and restoring redox balance in middle cerebral artery occlusion rats. Further investigation indicated that compound 24 exerted a protective effect against sodium nitroprusside (SNP) induced cell damage by scavenging intracellular reactive oxygen species and restoring mitochondrial membrane potential. Moreover, compound 24 induced the nuclear translocation of Nuclear factor erythroid 2-related factor (Nrf2) and promoted the generation of antioxidative proteins, including Heme oxygenase-1 (HO-1), NAD(P)H quinone dehydrogenase (NQO1), and glutamate-cysteine ligase catalytic (GCLC). Surface plasmon resonance (SPR) experiments indicated that compound 24 might activate the Nrf2 signaling pathway by interacting with the Keap1 Kelch domain. Taken together, these facts indicate that compound 24 might have potential in the treatment of ischemic stroke.

Phenyl Bis-Sulfonamide Keap1-Nrf2 Protein-Protein Interaction Inhibitors with an Alternative Binding Mode

Inhibitors of Kelch-like ECH-associated protein 1 (Keap1) increase the activity of the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) by stalling its ubiquitination and degradation. This enhances the expression of genes encoding proteins involved in drug detoxification, redox homeostasis, and mitochondrial function. Nrf2 activation offers a potential therapeutic approach for conditions including Alzheimer's and Parkinson's diseases, vascular inflammation, and chronic obstructive airway disease. Non-electrophilic Keap1-Nrf2 protein-protein interaction (PPI) inhibitors may have improved toxicity profiles and different pharmacological properties to cysteine-reactive electrophilic inhibitors. Here, we describe and characterize a series of phenyl bis-sulfonamide PPI inhibitors that bind to Keap1 at submicromolar concentrations. Structural studies reveal that the compounds bind to Keap1 in a distinct "peptidomimetic" conformation that resembles the Keap1-Nrf2 ETGE peptide complex. This is different to other small molecule Keap1-Nrf2 PPI inhibitors, including bicyclic aryl bis-sulfonamides, offering a starting point for new design approaches to Keap1 inhibitors.

Importance of Binding Site Hydration and Flexibility Revealed When Optimizing a Macrocyclic Inhibitor of the Keap1-Nrf2 Protein-Protein Interaction

Upregulation of the transcription factor Nrf2 by inhibition of the interaction with its negative regulator Keap1 constitutes an opportunity for the treatment of disease caused by oxidative stress. We report a structurally unique series of nanomolar Keap1 inhibitors obtained from a natural product-derived macrocyclic lead. Initial exploration of the structure-activity relationship of the lead, followed by structure-guided optimization, resulted in a 100-fold improvement in inhibitory potency. The macrocyclic core of the nanomolar inhibitors positions three pharmacophore units for productive interactions with key residues of Keap1, including R415, R483, and Y572. Ligand optimization resulted in the displacement of a coordinated water molecule from the Keap1 binding site and a significantly altered thermodynamic profile. In addition, minor reorganizations of R415 and R483 were accompanied by major differences in affinity between ligands. This study therefore indicates the importance of accounting both for the hydration and flexibility of the Keap1 binding site when designing high-affinity ligands.

The isoquinoline PRL-295 increases the thermostability of Keap1 and disrupts its interaction with Nrf2

Transcription factor Nrf2 and its negative regulator Keap1 orchestrate a cytoprotective response against oxidative, metabolic, and inflammatory stress. Keap1 is a drug target, with several small molecules in drug development. Here, we show that the isoquinoline PRL-295 increased Keap1 thermostability in lysates from cells expressing fluorescently tagged Keap1. The thermostability of endogenous Keap1 also increased in intact cells and murine liver following PRL-295 treatment. Fluorescence Lifetime Imaging-Förster Resonance Energy Transfer (FLIM-FRET) experiments in cells co-expressing sfGFP-Nrf2 and Keap1-mCherry further showed that PRL-295 prolonged the donor fluorescence lifetime, indicating disruption of the Keap1-Nrf2 protein complex. Orally administered PRL-295 to mice activated the Nrf2transcriptional target NAD(P)H:quinone oxidoreductase 1 (NQO1) in liver and decreased the levels of plasma alanine aminotransferase and aspartate aminotransferase upon acetaminophen-induced hepatic injury. Thus, PRL-295 engages the Keap1 protein target in cells and in vivo, disrupting its interaction with Nrf2, leading to activation of Nrf2-dependent transcription and hepatocellular protection.

The KEAP1-NRF2 System in Healthy Aging and Longevity

Aging is inevitable, but the inherently and genetically programmed aging process is markedly influenced by environmental factors. All organisms are constantly exposed to various stresses, either exogenous or endogenous, throughout their lives, and the quality and quantity of the stresses generate diverse impacts on the organismal aging process. In the current oxygenic atmosphere on earth, oxidative stress caused by reactive oxygen species is one of the most common and critical environmental factors for life. The Kelch-like ECH-associated protein 1-NFE2-related factor 2 (KEAP1-NRF2) system is a critical defense mechanism of cells and organisms in response to redox perturbations. In the presence of oxidative and electrophilic insults, the thiol moieties of cysteine in KEAP1 are modified, and consequently NRF2 activates its target genes for detoxification and cytoprotection. A number of studies have clarified the contributions of the KEAP1-NRF2 system to the prevention and attenuation of physiological aging and aging-related diseases. Accumulating knowledge to control stress-induced damage may provide a clue for extending healthspan and treating aging-related diseases. In this review, we focus on the relationships between oxidative stress and aging-related alterations in the sensory, glandular, muscular, and central nervous systems and the roles of the KEAP1-NRF2 system in aging processes.

A bioactive ligand-conjugated iridium(III) metal-based complex as a Keap1-Nrf2 protein-protein interaction inhibitor against acetaminophen-induced acute liver injury

Hepatotoxicity caused by an overdose of acetaminophen (APAP) is the leading reason for acute drug-related liver failure. Nuclear factor erythroid-2-related factor 2 (Nrf2) is a protein that helps to regulate redox homeostasis and coordinate stress responses via binding to the Kelch-like ECH-associated protein 1 (Keap1). Targeting the Keap1-Nrf2 interaction has recently emerged as a potential strategy to alleviate liver injury caused by APAP. Here, we designed and synthesized a number of iridium (III) and rhodium (III) complexes bearing ligands with reported activity against oxidative stress, which is associated with Nrf2 transcriptional activation. The iridium (III) complex 1 bearing a bioactive ligand 2,9-dimethyl-1,10-phenanthroline and 4-chloro-2-phenylquinoline, a derivative of the bioactive ligand 2-phenylquinoline, was identified as a direct small-molecule inhibitor of the Keap1–Nrf2 protein-protein interaction. 1 could stabilize Keap1 protein, upregulate HO-1 and NQO1, and promote Nrf2 nuclear translocation in normal liver cells. Moreover, 1 reversed APAP-induced liver damage by disrupting Keap1–Nrf2 interaction and without inducing organ damage and immunotoxicity in mice. Our study demonstrates the identification of a selective and efficacious antagonist of Keap1–Nrf2 interaction possessed good cellular permeability in cellulo and ideal pharmacokinetic parameters in vivo, and, more importantly, validates the feasibility of conjugating metal complexes with bioactive ligands to generate metal-based drug leads as non-toxic Keap1–Nrf2 interaction inhibitors for treating APAP-induced acute liver injury.

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1 reversed APAP-induced liver damage by disrupting Keap1–Nrf2 interaction without inducing organ damage or immunotoxicity.

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Complex 1 possessed good cellular permeability in cellulo and ideal pharmacokinetic parameters in vivo.

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Conjugating metal complexes with bioactive ligands opens a novel avenue for the treatment of APAP-induced liver damage.

Identification of a potent Nrf2 displacement activator among aspirin-containing prodrugs

Aspirin is a desired leaving group in prodrugs aimed at treatment of neurodegeneration and other conditions. A library of aspirin derivatives of various scaffolds potentially activating Nrf2 has been tested in Neh2-luc reporter assay which screens for direct Nrf2 protein stabilizers working via disruption of Nrf2-Keap1 interaction. Most aspirin prodrugs had a pro-alkylating or pro-oxidant motif in the structure and, therefore, were toxic at high concentrations. However, among the active compounds, we identified a molecule resembling a well-known Nrf2 displacement activator, bis-1,4-(4-methoxybenzenesulfonamidyl) naphthalene (NMBSA). The direct comparison of the newly identified compound with NMBSA and its improved analog in the reporter assay showed no quenching with N-acetyl cysteine, thus pointing to Nrf2 stabilization mechanism without cysteine alkylation. The potency of the newly identified compound in the reporter assay was much stronger than NMBSA, despite its inhibitory action in the commercial fluorescence polarization assay was observed only in the millimolar range. Molecular docking predicted that mono-deacetylation of the novel prodrug should generate a potent displacement activator. The time-course of reporter activation with the novel prodrug had a pronounced lag-period pointing to a plausible intracellular transformation leading to an active product. Treatment of the novel prodrug with blood plasma or cell lysate demonstrated stepwise deacetylation as judge by liquid chromatography-mass spectrometry (LC-MS). Hence, the esterase-catalyzed hydrolysis of the prodrug liberates only acetyl groups from aspirin moiety and generates a potent Nrf2 activator. The discovered mechanism of prodrug activation makes the newly identified compound a promising lead for future optimization studies.

Conjugation of Diclofenac with Novel Oleanolic Acid Derivatives Modulate Nrf2 and NF-κB Activity in Hepatic Cancer Cells and Normal Hepatocytes Leading to Enhancement of Its Therapeutic and Chemopreventive Potential

Combining NSAIDs with conventional therapeutics was recently explored as a new strategy in cancer therapy. Our earlier studies showed that novel oleanolic acid oximes (OAO) conjugated with aspirin or indomethacin may enhance their anti-cancer potential through modulation of the Nrf2 and NF-κB signaling pathways. This study focused on the synthesis and biological evaluation of four diclofenac (DCL)–OAO derivative conjugates in the context of these pathways’ modification and hepatic cells survival. Treatment with the conjugates 4d, 3-diclofenacoxyiminoolean-12-en-28-oic acid morpholide, and 4c, 3-diclofenacoxyiminoolean-12-en-28-oic acid benzyl ester significantly reduced cell viability in comparison to the DCL alone. In THLE-2, immortalized normal hepatocytes treated with these conjugates resulted in the activation of Nrf2 and increased expression in SOD-1 and NQO1, while the opposite effect was observed in the HepG2 hepatoma cells. In both cell lines, reduced activation of the NF-κB and COX-2 expression was observed. In HepG2 cells, conjugates increased ROS production resulting from a reduced antioxidant defense, induced apoptosis, and inhibited cell proliferation. In addition, the OAO morpholide derivative and its DCL hybrid reduced the tumor volume in mice bearing xenografts. In conclusion, our study demonstrated that conjugating diclofenac with the OAO morpholide and a benzyl ester might enhance its anti-cancer activity in HCC.

Optimization of 1,4-bis(arylsulfonamido)naphthalene-N,N'-diacetic acids as inhibitors of Keap1-Nrf2 protein-protein interaction to suppress neuroinflammation

The protein-protein interaction (PPI) between kelch-like ECH-associated protein 1 (Keap1) and nuclear factor erythroid 2-related factor 2 (Nrf2) is recognized as a promising target for the prevention and treatment of oxidative stress-related inflammatory diseases. Herein, a series of novel 1,4-bis(arylsulfonamido)naphthalene-N,N'-diacetic acid analogs (7p-t and 8c) were designed to further explore the structure-activity relationships of the series. Their activities were measured first with a fluorescence polarization (FP) assay and more potent compounds were further evaluated using a more sensitive time-resolved fluorescence energy transfer (TR-FRET) assay, demonstrating IC₅₀ values between 7.2 and 31.3 nM. In cytotoxicity studies, the naphthalene derivatives did not show noticeable toxicity to human HepG2-C8 and mouse brain BV-2 microglia cells. Among them, compound 7q bearing oxygen-containing fused rings was shown to significantly stimulate the cellular Nrf2 signaling pathway, including activation of antioxidant response element (ARE)-controlled expression of Nrf2 target genes and proteins. More importantly, 7q suppressed up-regulation of several pro-inflammatory cytokines in lipopolysaccharide (LPS)-challenged BV-2 microglial cells, representing a potential therapeutic application for controlling neuroinflammatory disorders.

Targeting NRF2 to treat cancer

NRF2 is a basic leucine zipper (bZip) transcription factor that is the master regulator of redox homeostasis. Under basal conditions, the cellular level of NRF2 is low due to a posttranslational regulation by the ubiquitin proteasome system (UPS). But, when an organism is challenged with oxidative or xenobiotic stress, the NRF2 pathway is activated by inhibition of the E3 ubiquitin ligase complex that normally marks NRF2 for destruction. For several decades, researchers have searched for molecules that can intentionally activate NRF2, as this was shown to be a means to prevent certain diseases, at least in animal models. In the present era, there are many compounds known to activate the NRF2 pathway including natural products and synthetic compounds, covalent and non-covalent compounds, and others. However, it was also revealed that like many protective pathways, the NRF2 pathway has a dark side. Just as NRF2 can protect normal cells from damage, it can protect malignant cells from damage. As cells transform, they are exposed to many stressors and aberrant upregulation of NRF2 can facilitate transformation and it can help cancer cells to grow, to spread, and to resist treatment. For this reason, researchers are also interested in the discovery and development of NRF2 inhibitors. In the present review, we will begin with a general discussion of NRF2 structure and function, we will discuss the latest in NRF2 non-covalent activators, and we will discuss the current state of NRF2 inhibitors.

Beyond direct Nrf2 activation; reinvestigating 1,2,4-oxadiazole scaffold as a master key unlocking the antioxidant cellular machinery for cancer therapy

Harnessing the antioxidant cellular machinery has sparked considerable interest as an efficient anticancer strategy. Activating Nrf2, the master switch of the cellular redox system, suppresses ROS, alleviates oxidative stress, and halts cancer progression. 1,2,4-oxadiazoles are iconic direct Nrf2 activators that disrupt Nrf2 interaction with its endogenous repressor Keap1. This study introduces rationally designed 1,2,4-oxadiazole derivatives that inhibit other Nrf2 suppressors (TrxR1, IKKα, and NF-kB) thus enhancing Nrf2 activation for preventing oxidative stress and carcinogenesis. Preliminary screening showed that the phenolic oxadiazoles 11, 15, and 19 were comparable to ascorbic acid (ROS scavenging) and EDTA (iron chelation), and superior to doxorubicin against HepG-2, MDA-MB231, and Caco-2 cells. They suppressed ROS by 3 folds and activated Nrf2 by 2 folds in HepG-2 cells. Mechanistically, they inhibited TrxR1 (IC₅₀; 13.19, 17.89, and 9.21 nM) and IKKα (IC₅₀; 11.0, 15.94, and 19.58 nM), and downregulated NF-κB (7.6, 1.4 and 1.9 folds in HepG-2), respectively. They inhibited NADPH oxidase (IC₅₀; 16.4, 21.94, and 10.71 nM, respectively) that potentiates their antioxidant activities. Docking studies predicted their important structural features. Finally, they recorded drug-like in silico physicochemical properties, ADMET, and ligand efficiency metrics.

Deconstructing Noncovalent Kelch-like ECH-Associated Protein 1 (Keap1) Inhibitors into Fragments to Reconstruct New Potent Compounds

Targeting the protein-protein interaction (PPI) between nuclear factor erythroid 2-related factor 2 (Nrf2) and Kelch-like ECH-associated protein 1 (Keap1) is a potential therapeutic strategy to control diseases involving oxidative stress. Here, six classes of known small-molecule Keap1-Nrf2 PPI inhibitors were dissected into 77 fragments in a fragment-based deconstruction reconstruction (FBDR) study and tested in four orthogonal assays. This gave 17 fragment hits of which six were shown by X-ray crystallography to bind in the Keap1 Kelch binding pocket. Two hits were merged into compound 8 with a 220-380-fold stronger affinity (K_i = 16 μM) relative to the parent fragments. Systematic optimization resulted in several novel analogues with K_i values of 0.04-0.5 μM, binding modes determined by X-ray crystallography, and enhanced microsomal stability. This demonstrates how FBDR can be used to find new fragment hits, elucidate important ligand-protein interactions, and identify new potent inhibitors of the Keap1-Nrf2 PPI.

NRF2 activation by reversible KEAP1 binding induces the antioxidant response in primary neurons and astrocytes of a Huntington's disease mouse model

Oxidative stress has been associated with pathogenesis in several diseases including Huntington's disease (HD), a neurodegenerative disorder caused by a mutation in the huntingtin gene. Oxidative stress induced reactive oxygen species (ROS) are normally controlled at the cellular level by the nuclear factor (erythroid-derived 2)-like 2 (NRF2) a transcription factor that regulates the expression of various antioxidants and detoxifying proteins. Normally NRF2 is largely inactivated in the cytoplasm by the Kelch-like ECH-associated protein 1 (KEAP1)/Cullin-3 (CUL3) mediated ubiquitination and subsequent proteosomal degradation. In the presence of ROS, KEAP1 sensor cysteines are directly or indirectly engaged resulting in NRF2 release, nuclear translocation, and activation of its target genes. Consequently the activation of NRF2 by a small-molecule drug may have the therapeutic potential to control oxidative stress by upregulation of the endogenous antioxidant responses. Here we attempted to validate the use of a reversible non-acidic KEAP1 binder (Compound 2) to activate NRF2 with better cellular activity than similar acidic compounds. When tested head to head with sulforaphane, a covalent KEAP1 binder, Compound 2 had a similar ability to induce the expression of genes known to be modulated by NRF2 in neurons and astrocytes isolated from wild-type rat, wild type mouse and zQ175 (an HD mouse model) embryos. However, while sulforaphane also negatively affected genes involved in neurotoxicity in these cells, Compound 2 showed a clean profile suggesting its mode of action has lower off-target activity. We show that Compound 2 was able to protect cells from an oxidative insult by preserving the ATP content and the mitochondrial potential of primary astrocytes, consistent with the hypothesis that neurotoxicity induced by oxidative stress can be limited by upregulation of innate antioxidant response.

Patent Review (2017-2020) of the Keap1/Nrf2 Pathway Using PatSeer Pro: Focus on Autoimmune Diseases

Research on the antioxidant pathway comprising the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) and its cytoplasmic inhibitor Kelch-like ECH-associated protein 1 (Keap1) is ever increasing. As modulators of this pathway have started to be used in clinical trials and clinical practice, Nrf2 has become the subject of several patents. To assess the patent landscape of the last three years on Nrf2 and evaluate the main fields they refer to, we used the web-based tool PatSeer Pro to identify patents mentioning the Nrf2 pathway between January 2017 and May 2020. This search resulted in 509 unique patents that focus on topics such as autoimmune, neurodegenerative, liver, kidney, and lung diseases and refer to modulators (mainly activators) of the Nrf2 pathway as potential treatments. Autoimmunity emerged as the main theme among the topics of Nrf2 patents, including a broad range of diseases, such as systemic sclerosis, systemic lupus erythematosus, multiple sclerosis, inflammatory bowel diseases, Hashimoto's thyroiditis, etc.; however, there was a dearth of experimental support for the respective patents' claims. Given that chronic inflammation is the main element of the pathophysiology of most autoimmune diseases, the majority of patents referring to activation of Nrf2 as a method to treat autoimmune diseases base their claims on the well-established anti-inflammatory role of Nrf2. In conclusion, there is strong interest in securing intellectual property rights relating to the potential use of Nrf2 pathway activators in a variety of diseases, and this trend parallels the rise in related research publications. However, in the case of autoimmunity, more research is warranted to support the potential beneficial effects of Nrf2 modulation in each disease.

Optimization of linear and cyclic peptide inhibitors of KEAP1-NRF2 protein-protein interaction

Inhibition of KEAP1-NRF2 protein-protein interaction is considered a promising strategy to selectively and effectively activate NRF2, a transcription factor which is involved in several pathologies such as Huntington's disease (HD). A library of linear peptides based on the NRF2-binding motifs was generated on the nonapeptide lead Ac-LDEETGEFL-NH₂ spanning residues 76-84 of the Neh2 domain of NRF2 with the aim to replace E78, E79 and E82 with non-acidic amino acids. A deeper understanding of the features and accessibility of the T80 subpocket was also targeted by structure-based design. Approaches to improve cell permeability were investigated using both different classes of cyclic peptides and conjugation to cell-penetrating peptides. This insight will guide future design of macrocycles, peptido-mimetics and, most importantly, small neutral brain-penetrating molecules to evaluate whether NRF2 activators have utility in HD.

Discovery of 2-oxy-2-phenylacetic acid substituted naphthalene sulfonamide derivatives as potent KEAP1-NRF2 protein-protein interaction inhibitors for inflammatory conditions

Nuclear factor erythroid 2-related factor 2 (NRF2) is a pleiotropic transcription factor which regulates the constitutive and inducible transcription of a wide array of genes and confers protection against a variety of pathologies. Directly disrupting Kelch-like ECH-associated protein 1 (KEAP1)-NRF2 protein-protein interaction (PPI) has been explored as a promising strategy to activate NRF2. We reported here the first identification of a series of 2-oxy-2-phenylacetic acid substituted naphthalene sulfonamide derivatives as potent KEAP1-NRF2 inhibitors. Our efforts led to the potent small molecule KEAP1-NRF2 inhibitor, 20c, which exhibited a K_d of 24 nM to KEAP1 and an IC₅₀ of 75 nM in disrupting KEAP1-NRF2 interaction. Subsequent biological studies provided consistent evidence across mouse macrophage cell-based and in vivo models that 20c induced NRF2 target gene expression and enhanced downstream antioxidant and anti-inflammatory activities. Our study not only demonstrated that small molecule KEAP1-NRF2 PPI inhibitors can be potential preventive and therapeutic agents for diseases and conditions involving oxidative stress and inflammation but also enriched the chemical diversity of the KEAP1-NRF2 inhibitors.

Recent progress in Keap1-Nrf2 protein-protein interaction inhibitors

Therapeutic targeting the protein-protein interaction (PPI) of Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) and its main regulator, Kelch-like ECH-Associating protein 1 (Keap1) has been emerged as a feasible way to combat oxidative stress related diseases, due to the key role of Nrf2 in oxidative stress regulation. In recent years, many efforts have been made to develop potent Keap1-Nrf2 inhibitors with new chemical structures. Various molecules with diverse chemical structures have been reported and some compounds exhibit high potency. This review summarizes peptide and small molecule Keap1-Nrf2 inhibitors reported recently. We also highlight the pharmacological effects and discuss the possible therapeutic application of Keap1-Nrf2 inhibitors.

NRF2 Regulation Processes as a Source of Potential Drug Targets against Neurodegenerative Diseases

NRF2 acts by controlling gene expression, being the master regulator of the Phase II antioxidant response, and also being key to the control of neuroinflammation. NRF2 activity is regulated at several levels, including protein degradation by the proteasome, transcription, and post-transcription. The purpose of this review is to offer a concise and critical overview of the main mechanisms of NRF2 regulation and their actual or potential use as targets for the treatment of neurodegenerative diseases.

The role of natural products in revealing NRF2 function

Covering: up to 2020The transcription factor NRF2 is one of the body's major defense mechanisms, driving transcription of >300 antioxidant response element (ARE)-regulated genes that are involved in many critical cellular processes including redox regulation, proteostasis, xenobiotic detoxification, and primary metabolism. The transcription factor NRF2 and natural products have an intimately entwined history, as the discovery of NRF2 and much of its rich biology were revealed using natural products both intentionally and unintentionally. In addition, in the last decade a more sinister aspect of NRF2 biology has been revealed. NRF2 is normally present at very low cellular levels and only activated when needed, however, it has been recently revealed that chronic, high levels of NRF2 can lead to diseases such as diabetes and cancer, and may play a role in other diseases. Again, this "dark side" of NRF2 was revealed and studied largely using a natural product, the quassinoid, brusatol. In the present review, we provide an overview of NRF2 structure and function to orient the general reader, we will discuss the history of NRF2 and NRF2-activating compounds and the biology these have revealed, and we will delve into the dark side of NRF2 and contemporary issues related to the dark side biology and the role of natural products in dissecting this biology.

Combined Peptide and Small-Molecule Approach toward Nonacidic THIQ Inhibitors of the KEAP1/NRF2 Interaction

The NRF2-ARE pathway is an intrinsic mechanism of defense against oxidative stress. Inhibition of the interaction between NRF2 and its main negative regulator KEAP1 is an attractive strategy toward neuroprotective agents. We report here the identification of nonacidic tetrahydroisoquinolines (THIQs) that inhibit the KEAP1/NRF2 protein-protein interaction. Peptide SAR at one residue is utilized as a tool to probe structural changes within a specific pocket of the KEAP1 binding site. We used structural information from peptide screening at the P2 pocket, noncovalent small-molecules inhibitors, and the outcome from an explorative SAR at position 5 of THIQs to identify a series of neutral THIQ analogs that bind to KEAP1 in the low micromolar range. These analogs establish new H-bond interactions at the P3 and P2 pockets allowing the replacement of the carboxylic acid functionality by a neutral primary carboxamide. X-ray crystallographic studies reveal the novel binding mode of these molecules to KEAP1.

A hydrogen peroxide responsive prodrug of Keap1-Nrf2 inhibitor for improving oral absorption and selective activation in inflammatory conditions

Transcription factor Nuclear factor erythroid 2-related factor 2 (Nrf2) and its negative regulator, the E3 ligase adaptor Kelch-like ECH-associated protein 1 (Keap1), control the redox and metabolic homeostasis and oxidative stress. Inhibitors of Keap1-Nrf2 interaction are promising in oxidative stress related inflammatory diseases but now hit hurdles. By utilizing thiazolidinone moiety to shield the key carboxyl pharmacophore in Keap1-Nrf2 inhibitor, a hydrogen peroxide (H₂O₂)-responsive prodrug pro2 was developed. The prodrug modification improved the physicochemical properties and cell membrane permeability of the parent drug. Pro2 was stable and stayed inactive under various physiological conditions, while became active by stimulation of H₂O₂ or inflammation derived reactive oxygen species. Moreover, pro2 exhibited proper pharmacokinetic profile suitable for oral administration and enhanced anti-inflammatory efficiency in vivo. Thus, this novel prodrug approach may not only provide an important advance in the therapy of chronic inflammatory diseases with high level of H₂O₂, but also offer a fresh solution to improve the drug-like and selectivity issues of Keap1-Nrf2 inhibitors.

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Pro2 was developed by utilizing H₂O₂-responsive thiazolidinone moiety to shield carboxyl group in Keap1-Nrf2 inhibitor.

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Pro2 was stable and inactive under various physiological conditions, while became active under inflammatory conditions.

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Pro2 exhibited proper pharmacokinetic profile for oral administration and enhanced anti-inflammatory efficiency in vivo.

Nrf2 activation through the inhibition of Keap1-Nrf2 protein-protein interaction

Activation of the transcription factor Nrf2 via the Keap1-Nrf2-ARE signaling system regulates the transcription and subsequent expression of cellular cytoprotective proteins and plays a crucial role in preventing pathological conditions exacerbated by the overproduction of oxidative stress. In addition to electrophilic modulators, direct non-covalent inhibitors that interrupt the Keap1-Nrf2 protein-protein interaction (PPI) leading to Nrf2 activation have attracted a great deal of attention as potential preventive and therapeutic agents for oxidative stress-related diseases. Structural studies of Keap1-binding ligands, development of biochemical and cellular assays, and new structure-based design approaches have facilitated the discovery of small molecule PPI inhibitors. This perspective reviews the Keap1-Nrf2-ARE system, its physiological functions, and the recent progress in the discovery and the potential applications of direct inhibitors of Keap1-Nrf2 PPI.

Synthesis and Evaluation of Noncovalent Naphthalene-Based KEAP1-NRF2 Inhibitors

The oxidative stress response, gated by the protein-protein interaction of KEAP1 and NRF2, has garnered significant interest in the past decade. Misregulation in this pathway has been implicated in disease states such as multiple sclerosis, rheumatoid arthritis, and diabetic chronic wounds. Many of the known activators of NRF2 are electrophilic in nature and may operate through several biological pathways rather than solely through the activation of the oxidative stress response. Recently, our lab has reported a nonelectrophilic, monoacidic, naphthalene-based NRF2 activator which exhibited good potency in vitro. Herein, we report a detailed structure-activity relationship of naphthalene-based NRF2 activators, an X-ray crystal structure of our monoacidic KEAP1 inhibitor, and identification of an underexplored area of the NRF2 binding pocket of KEAP1.

Potential Applications of NRF2 Modulators in Cancer Therapy

The nuclear factor erythroid 2-related factor 2 (NRF2)-Kelch-like ECH-associated protein 1 (KEAP1) regulatory pathway plays an essential role in protecting cells and tissues from oxidative, electrophilic, and xenobiotic stress. By controlling the transactivation of over 500 cytoprotective genes, the NRF2 transcription factor has been implicated in the physiopathology of several human diseases, including cancer. In this respect, accumulating evidence indicates that NRF2 can act as a double-edged sword, being able to mediate tumor suppressive or pro-oncogenic functions, depending on the specific biological context of its activation. Thus, a better understanding of the mechanisms that control NRF2 functions and the most appropriate context of its activation is a prerequisite for the development of effective therapeutic strategies based on NRF2 modulation. In line of principle, the controlled activation of NRF2 might reduce the risk of cancer initiation and development in normal cells by scavenging reactive-oxygen species (ROS) and by preventing genomic instability through decreased DNA damage. In contrast however, already transformed cells with constitutive or prolonged activation of NRF2 signaling might represent a major clinical hurdle and exhibit an aggressive phenotype characterized by therapy resistance and unfavorable prognosis, requiring the use of NRF2 inhibitors. In this review, we will focus on the dual roles of the NRF2-KEAP1 pathway in cancer promotion and inhibition, describing the mechanisms of its activation and potential therapeutic strategies based on the use of context-specific modulation of NRF2.

Rutaecarpine inhibits KEAP1-NRF2 interaction to activate NRF2 and ameliorate dextran sulfate sodium-induced colitis

Inflammatory bowel disease (IBD) represents a group of chronic relapsing intestinal disorders. Rutaecarpine (RUT), isolated from the Traditional Chinese Medicine (TCM) of Evodia rutaecarpa, was reported to suppress IBD. However, the mechanism by which RUT ameliorates dextran sulfate sodium (DSS)-induced IBD is largely unknown. By use of nuclear factor-erythroid 2-related factor 2 (NRF2) knockout mice, cell-based studies, surface plasmon resonance (SPR), western blotting analysis, and molecular docking studies, the mechanism by which RUT affects DSS-induced colitis was explored. In DSS-treated wild-type mice but not in Nrf2-null mice, RUT significantly improved colitis as revealed by rescued body weight loss, improved histology and inflammation, and induced expression of NRF2 target genes in colon and ileum. Cell-based studies showed that RUT significantly increased the LD₅₀ for hydrogen peroxide (H₂O₂)-induced cell damage, activated NRF2 nuclear translocation, and suppressed the production of reactive oxygen species in H₂O₂-treated HCT116 cells, activated NRF2 luciferase reporter activities in HCT116 cells and HepG2 cells, and induced expression of NRF2 target genes in primary intestinal epithelial cells. Molecular docking in silico and SPR assays indicated that RUT interacted with kelch-like ECH-associated protein 1 (KEAP1), and extracellular incubation studies revealed that RUT bound to the KEAP1 kelch domain with a calculated equilibrium dissociation constant K_d of 19.6 μM. In conclusion, these results demonstrate that RUT ameliorates DSS-induced colitis, dependent on NRF2, and could be a potential therapeutic option for IBD patients. Mechanistically, RUT potentiates NRF2 nuclear translocation to upregulate NRF2-mediated antioxidant response by directly inhibiting KEAP1-NRF2 interaction.

Recent progress in the development of small molecule Nrf2 activators: a patent review (2017-present)

Introduction: The transcription factor Nrf2 (nuclear factor erythroid 2-related factor 2) is the first line of defense against a plethora of environmental or endogenous deviations in redox metabolism, proteostasis, inflammation, etc. Therefore, pharmacological activation of Nrf2 is a potential therapeutic approach for several diseases related to oxidative stress and inflammation, such as cancer, cardiovascular, and neurodegenerative diseases.Areas covered: The authors first describe the biological function of Nrf2 and the molecular regulatory mechanism of Keap1-Nrf2-ARE ((Kelch-like ECH-Associating protein 1)-Nrf2-(antioxidant response element)). Then, they review recent progress of covalent activators and non-covalent Keap1-Nrf2 protein-protein interaction (PPI) inhibitors from patents and publications in 2017-present, consisting of new chemical molecules, structure optimization of reported activators and progress in preclinical or clinical trials.Expert opinion: Despite significant achievements in the development of Nrf2 activators, the selectivity is the primary consideration. Due to reacting with redox-sensitive cysteines in proteins except for Keap1, electrophilic activators often exhibit off-target effects. For Keap1-Nrf2 PPI inhibitors, how to enhance in vivo efficacy and/or penetrate blood-brain barrier (BBB) to reach central nervous system (CNS) is also challenging. Fragment-based drug discovery (FBDD), carboxylic acid bioisosteric replacement and prodrug approach might be used to circumvent this challenge. Moreover, the possibility of cancer risk caused by Nrf2 activation needs to be considered carefully.