Harnessing the E3 Ligase KEAP1 for Targeted Protein Degradation

Expanding the ligand spaces for E3 ligases for the design of protein degraders

Targeted protein degradation (TPD) has recently emerged as an exciting new drug modality. However, the strategy of developing small molecule-based protein degraders has evolved over the past two decades and has now established molecular tags that are already in clinical use, as well as chimeric molecules, PROteolysis TArgeting Chimeras (PROTACs), based mainly on ligand systems developed for the two E3 ligases CRBN and VHL. The large size of the human E3 ligase family suggests that PROTACs can be developed by targeting a large diversity of E3 ligases, some of which have restricted expression patterns with the potential to design disease- or tissue-specific degraders. Indeed, many new E3 ligands have been published recently, confirming the druggability of E3 ligases. This review summarises recent data on E3 ligases and highlights the challenges in developing these molecules into efficient PROTACs rivalling the established degrader systems.

Targeted protein degradation in CNS disorders: a promising route to novel therapeutics?

Targeted protein degradation (TPD) is a rapidly expanding field, with various PROTACs (proteolysis-targeting chimeras) in clinical trials and molecular glues such as immunomodulatory imide drugs (IMiDs) already well established in the treatment of certain blood cancers. Many current approaches are focused on oncology targets, leaving numerous potential applications underexplored. Targeting proteins for degradation offers a novel therapeutic route for targets whose inhibition remains challenging, such as protein aggregates in neurodegenerative diseases. This mini review focuses on the prospect of utilizing TPD for neurodegenerative disease targets, particularly PROTAC and molecular glue formats and opportunities for novel CNS E3 ligases. Some key challenges of utilizing such modalities including molecular design of degrader molecules, drug delivery and blood brain barrier penetrance will be discussed.

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.

Applications of protein ubiquitylation and deubiquitylation in drug discovery

The ubiquitin (Ub)-proteasome system (UPS) is the major machinery mediating specific protein turnover in eukaryotic cells. By ubiquitylating unwanted, damaged, or harmful proteins and driving their degradation, UPS is involved in many important cellular processes. Several new UPS-based technologies, including molecular glue degraders and PROTACs (proteolysis-targeting chimeras) to promote protein degradation, and DUBTACs (deubiquitinase-targeting chimeras) to increase protein stability, have been developed. By specifically inducing the interactions between different Ub ligases and targeted proteins that are not otherwise related, molecular glue degraders and PROTACs degrade targeted proteins via the UPS; in contrast, by inducing the proximity of targeted proteins to deubiquitinases, DUBTACs are created to clear degradable poly-Ub chains to stabilize targeted proteins. In this review, we summarize the recent research progress in molecular glue degraders, PROTACs, and DUBTACs and their applications. We discuss immunomodulatory drugs, sulfonamides, cyclin-dependent kinase-targeting molecular glue degraders, and new development of PROTACs. We also introduce the principle of DUBTAC and its applications. Finally, we propose a few future directions of these three technologies related to targeted protein homeostasis.

The new direction of drug development: Degradation of undruggable targets through targeting chimera technology

Imbalances in protein and noncoding RNA levels in vivo lead to the occurrence of many diseases. In addition to the use of small molecule inhibitors and agonists to restore these imbalances, recently emerged targeted degradation technologies provide a new direction for disease treatment. Targeted degradation technology directly degrades target proteins or RNA by utilizing the inherent degradation pathways, thereby eliminating the functions of pathogenic proteins (or RNA) to treat diseases. Compared with traditional therapies, targeted degradation technology which avoids the principle of traditional inhibitor occupation drive, has higher efficiency and selectivity, and widely expands the range of drug targets. It is one of the most promising and hottest areas for future drug development. Herein, we systematically introduced the in vivo degradation systems applied to degrader design: ubiquitin-proteasome system, lysosomal degradation system, and RNA degradation system. We summarized the development progress, structural characteristics, and limitations of novel chimeric design technologies based on different degradation systems. In addition, due to the lack of clear ligand-binding pockets, about 80% of disease-associated proteins cannot be effectively intervened with through traditional therapies. We deeply elucidated how to use targeted degradation technology to discover and design molecules for representative undruggable targets including transcription factors, small GTPases, and phosphatases. Overall, this review provides a comprehensive and systematic overview of targeted degradation technology-related research advances and a new guidance for the chimeric design of undruggable targets.

Methotrexate-based PROTACs as DHFR-specific chemical probes

Methotrexate (MTX) is a tight-binding dihydrofolate reductase (DHFR) inhibitor, used as both an antineoplastic and immunosuppressant therapeutic. MTX, like folate undergoes folylpolyglutamate synthetase-mediated γ-glutamylation, which affects cellular retention and target specificity. Mechanisms of MTX resistance in cancers include a decrease in MTX poly-γ-glutamylation and an upregulation of DHFR. Here, we report a series of potent MTX-based proteolysis targeting chimeras (PROTACs) to investigate DHFR degradation pharmacology and one-carbon biochemistry. These on-target, cell-active PROTACs show proteasome- and E3 ligase-dependent activity, and selective degradation of DHFR in multiple cancer cell lines. By comparison, treatment with MTX increases cellular DHFR protein expression. Importantly, these PROTACs produced distinct, less-lethal phenotypes compared to MTX. The chemical probe set described here should complement conventional DHFR inhibitors and serve as useful tools for studying one-carbon biochemistry and dissecting complex polypharmacology of MTX and related drugs. Such compounds may also serve as leads for potential autoimmune and antineoplastic therapeutics.

Direct degradation and stabilization of proteins: New horizons in treatment of nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) is featured with excessive hepatic lipid accumulation and its global prevalence is soaring. Nonalcoholic steatohepatitis (NASH), the severe systemic inflammatory subtype of NAFLD, is tightly associated with metabolic comorbidities, and the hepatocytes manifest severe inflammation and ballooning. Currently the therapeutic options for treating NASH are limited. Potent small molecules specifically intervene with the signaling pathways that promote pathogenesis of NASH. Nevertheless they have obvious adverse effects and show long-term ineffectiveness in clinical trials. It poses the fundamental question to efficiently and safely inhibit the pathogenic processes. Targeted protein degradation (TPD) belongs to the direct degradation strategies and is a burgeoning strategy. It utilizes the small molecules to bind to the target proteins and recruit the endogenous proteasome, lysosome and autophagosome-mediated degradation machineries. They effectively and specifically degrade the target proteins. It has exhibited promising therapeutic effects in treatment of cancer, neurodegenerative diseases and other diseases in a catalytic manner at low doses. We critically discuss the principles of multiple direct degradation strategies, especially PROTAC and ATTEC. We extensively analyze their emerging application in degradation of excessive pathogenic proteins and lipid droplets, which promote the progression of NASH. Moreover, we discuss the opposite strategy that utilizes the small molecules to recruit deubiquinases to stabilize the NASH/MASH-suppressing proteins. Their advantages, limitations, as well as the solutions to address the limitations have been analyzed. In summary, the innovative direct degradation strategies provide new insights into design of next-generation therapeutics to combat NASH with optimal safety paradigm and efficiency.

Recent advances in targeted protein degraders as potential therapeutic agents

Targeted protein degradation (TPD) technology has gradually become widespread in the past 20 years, which greatly boosts the development of disease treatment. Contrary to small inhibitors that act on protein kinases, transcription factors, ion channels, and other targets they can bind to, targeted protein degraders could target "undruggable targets" and overcome drug resistance through ubiquitin-proteasome pathway (UPP) and lysosome pathway. Nowadays, some bivalent degraders such as proteolysis-targeting chimeras (PROTACs) have aroused great interest in drug discovery, and some of them have successfully advanced into clinical trials. In this review, to better understand the mechanism of degraders, we elucidate the targeted protein degraders according to their action process, relying on the ubiquitin-proteasome system or lysosome pathway. Then, we briefly summarize the study of PROTACs employing different E3 ligases. Subsequently, the effect of protein of interest (POI) ligands, linker, and E3 ligands on PROTAC degradation activity is also discussed in detail. Other novel technologies based on UPP and lysosome pathway have been discussed in this paper such as in-cell click-formed proteolysis-targeting chimeras (CLIPTACs), molecular glues, Antibody-PROTACs (Ab-PROTACs), autophagy-targeting chimeras, and lysosome-targeting chimeras. Based on the introduction of these degradation technologies, we can clearly understand the action process and degradation mechanism of these approaches. From this perspective, it will be convenient to obtain the development status of these drugs, choose appropriate degradation methods to achieve better disease treatment and provide basis for future research and simultaneously distinguish the direction of future research efforts.

Vialinin A alleviates oxidative stress and neuronal injuries after ischaemic stroke by accelerating Keap1 degradation through inhibiting USP4-mediated deubiquitination

BACKGROUND

Oxidative stress is known as a hallmark of cerebral ischaemia‒reperfusion injury and it exacerbates the pathologic progression of ischaemic brain damage. Vialinin A, derived from a Chinese edible mushroom, possesses multiple pharmacological activities in cancer, Kawasaki disease, asthma and pathological scarring. Notably, vialinin A is an inhibitor of ubiquitin-specific peptidase 4 (USP4) that shows anti-inflammatory and antioxidative properties. However, the precise effect of vialinin A in ischaemic stroke, as well as its underlying mechanisms, remains largely unexplored.

PURPOSE

The present research focuses on the impacts of vialinin A on oxidative stress and explores the underlying mechanisms involved while also examining its potentiality as a therapeutic candidate for ischaemic stroke.

METHODS

Mouse ischaemic stroke was conducted by MCAO surgery. Vialinin A was administered via lateral ventricular injection at a dose of 2 mg/kg after reperfusion. Subsequent experiments were meticulously conducted at the appropriate time points. Stroke outcomes were evaluated by TTC staining, neurological score, Nissl staining and behavioural analysis. Co-IP assays were operated to examine the protein-protein interactions. Immunoblot analysis, qRT-PCR, and luciferase reporter assays were conducted to further investigate its underlying mechanisms.

RESULTS

In this study, we initially showed that administration of vialinin A alleviated cerebral ischaemia‒reperfusion injury-induced neurological deficits and neuronal apoptosis. Furthermore, vialinin A, which is an antioxidant, reduced oxidative stress injury, promoted the activation of the Keap1-Nrf2-ARE signaling pathway and increased the protein degradation of Keap1. The substantial neuroprotective effects of vialinin A against ischaemic stroke were compromised by the overexpression of USP4. Mechanistically, vialinin A inhibited the deubiquitinating enzymatic activity of USP4, leading to enhanced ubiquitination of Keap1 and subsequently promoting its degradation. This cascade caused the activation of Nrf2-dependent antioxidant response, culminating in a reduction of neuronal apoptosis and the amelioration of neurological dysfunction following ischaemic stroke.

CONCLUSIONS

This study demonstrates that inhibition of USP4 to activate Keap1-Nrf2-ARE signaling pathway may represent a mechanism by which vialinin A conferred protection against cerebral ischaemia‒reperfusion injury and sheds light on its promising prospects as a therapeutic intervention for ischaemic stroke.

DCAF1-based PROTACs with activity against clinically validated targets overcoming intrinsic- and acquired-degrader resistance

Targeted protein degradation (TPD) mediates protein level through small molecule induced redirection of E3 ligases to ubiquitinate neo-substrates and mark them for proteasomal degradation. TPD has recently emerged as a key modality in drug discovery. So far only a few ligases have been utilized for TPD. Interestingly, the workhorse ligase CRBN has been observed to be downregulated in settings of resistance to immunomodulatory inhibitory drugs (IMiDs). Here we show that the essential E3 ligase receptor DCAF1 can be harnessed for TPD utilizing a selective, non-covalent DCAF1 binder. We confirm that this binder can be functionalized into an efficient DCAF1-BRD9 PROTAC. Chemical and genetic rescue experiments validate specific degradation via the CRL4DCAF1 E3 ligase. Additionally, a dasatinib-based DCAF1 PROTAC successfully degrades cytosolic and membrane-bound tyrosine kinases. A potent and selective DCAF1-BTK-PROTAC (DBt-10) degrades BTK in cells with acquired resistance to CRBN-BTK-PROTACs while the DCAF1-BRD9 PROTAC (DBr-1) provides an alternative strategy to tackle intrinsic resistance to VHL-degrader, highlighting DCAF1-PROTACS as a promising strategy to overcome ligase mediated resistance in clinical settings.

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.

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.

A state-of-the-art review on the NRF2 in Hepatitis virus-associated liver cancer

According to a paper released and submitted to WHO by IARC scientists, there would be 905,700 new cases of liver cancer diagnosed globally in 2020, with 830,200 deaths expected as a direct result. Hepatitis B virus (HBV) hepatitis C virus (HCV), and hepatitis D virus (HDV) all play critical roles in the pathogenesis of hepatocellular carcinoma (HCC), despite the rising prevalence of HCC due to non-infectious causes. Liver cirrhosis and HCC are devastating consequences of HBV and HCV infections, which are widespread worldwide. Associated with a high mortality rate, these infections cause about 1.3 million deaths annually and are the primary cause of HCC globally. In addition to causing insertional mutations due to viral gene integration, epigenetic alterations and inducing chronic immunological dysfunction are all methods by which these viruses turn hepatocytes into cancerous ones. While expanding our knowledge of the illness, identifying these pathways also give possibilities for novel diagnostic and treatment methods. Nuclear factor erythroid 2-related factor 2 (NRF2) activation is gaining popularity as a treatment option for oxidative stress (OS), inflammation, and metabolic abnormalities. Numerous studies have shown that elevated Nrf2 expression is linked to HCC, providing more evidence that Nrf2 is a critical factor in HCC. This aberrant Nrf2 signaling drives cell proliferation, initiates angiogenesis and invasion, and imparts drug resistance. As a result, this master regulator may be a promising treatment target for HCC. In addition, the activation of Nrf2 is a common viral effect that contributes to the pathogenesis, development, and chronicity of virus infection. However, certain viruses suppress Nrf2 activity, which is helpful to the virus in maintaining cellular homeostasis. In this paper, we discussed the influence of Nrf2 deregulation on the viral life cycle and the pathogenesis associated with HBV and HCV. We summed up the mechanisms for the modulation of Nrf2 that are deregulated by these viruses. Moreover, we describe the molecular mechanism by which Nrf2 is modulated in liver cancer, liver cancer stem cells (LCSCs), and liver cancer caused by HBV and HCV. Video Abstract.

Efficient, multi-hundred-gram scale access to E3 ubiquitin ligase ligands for degrader development

Small molecule heterobifunctional degraders (commonly also known as PROTACs) offer tremendous potential to deliver new therapeutics in areas of unmet medical need. To deliver on this promise, a new discipline directed at degrader design and optimization has emerged within medicinal chemistry to address a central challenge, namely how to optimize relatively large, heterobifunctional molecules for activity, whilst maintaining drug-like properties. This process involves simultaneous optimization of the three principle degrader components: E3 ubiquitin ligase ligand, linker, and protein of interest (POI) ligand. A substantial degree of commonality exists with the E3 ligase ligands typically used at the early stages of degrader development, resulting in demand for these compounds as chemical building blocks in degrader research programs. We describe herein a collation of large scale, high-yielding syntheses to access the most utilized E3 ligase ligands to support early-stage degrader development.

Expanding PROTACtable genome universe of E3 ligases

Proteolysis-targeting chimera (PROTAC) and other targeted protein degradation (TPD) molecules that induce degradation by the ubiquitin-proteasome system (UPS) offer new opportunities to engage targets that remain challenging to be inhibited by conventional small molecules. One fundamental element in the degradation process is the E3 ligase. However, less than 2% amongst hundreds of E3 ligases in the human genome have been engaged in current studies in the TPD field, calling for the recruiting of additional ones to further enhance the therapeutic potential of TPD. To accelerate the development of PROTACs utilizing under-explored E3 ligases, we systematically characterize E3 ligases from seven different aspects, including chemical ligandability, expression patterns, protein-protein interactions (PPI), structure availability, functional essentiality, cellular location, and PPI interface by analyzing 30 large-scale data sets. Our analysis uncovers several E3 ligases as promising extant PROTACs. In total, combining confidence score, ligandability, expression pattern, and PPI, we identified 76 E3 ligases as PROTAC-interacting candidates. We develop a user-friendly and flexible web portal ( https://hanlaboratory.com/E3Atlas/ ) aimed at assisting researchers to rapidly identify E3 ligases with promising TPD activities against specifically desired targets, facilitating the development of these therapies in cancer and beyond.

Unlocking DCAFs To Catalyze Degrader Development: An Arena for Innovative Approaches

Chemically induced proximity-based targeted protein degradation (TPD) has become a prominent paradigm in drug discovery. With the clinical benefit demonstrated by certain small-molecule protein degraders that target the cullin-RING E3 ubiquitin ligases (CRLs), the field has proactively strategized to tackle anticipated drug resistance by harnessing additional E3 ubiquitin ligases to enrich the arsenal of this therapeutic approach. Here, we endeavor to explore the collaborative efforts involved in unlocking a broad range of CRL4DCAF for degrader drug development. Throughout the discussion, we also highlight how both conventional and innovative approaches in drug discovery can be taken to realize this objective. Moving ahead, we expect a greater allocation of resources in TPD to pursue these high-hanging fruits.

Bromodomain inhibitors and therapeutic applications

The bromodomain acts to recognize acetylated lysine in histones and transcription proteins and plays a fundamental role in chromatin-based cellular processes including gene transcription and chromatin remodeling. Many bromodomain proteins, particularly the bromodomain and extra terminal domain (BET) protein BRD4 have been implicated in cancers and inflammatory disorders and recognized as attractive drug targets. Although clinical studies of many BET bromodomain inhibitors have made substantial progress toward harnessing the therapeutic potential of targeting the bromodomain proteins, the development of this new class of epigenetic drugs is met with challenges, especially on-target dose-limiting toxicity. In this review, we highlight the current development of new-generation small molecule inhibitors for the BET and non-BET bromodomain proteins and discuss the research strategies used to target different bromodomain proteins for a wide array of human diseases including cancers and inflammatory disorders.

Acetylation Targeting Chimera Enables Acetylation of the Tumor Suppressor p53

With advances in chemically induced proximity technologies, heterobifunctional modalities such as proteolysis targeting chimeras (PROTACs) have been successfully advanced to clinics for treating cancer. However, pharmacologic activation of tumor-suppressor proteins for cancer treatment remains a major challenge. Here, we present a novel Acetylation Targeting Chimera (AceTAC) strategy to acetylate the p53 tumor suppressor protein. We discovered and characterized the first p53Y220C AceTAC, MS78, which recruits histone acetyltransferase p300/CBP to acetylate the p53Y220C mutant. MS78 effectively acetylated p53Y220C lysine 382 (K382) in a concentration-, time-, and p300-dependent manner and suppressed proliferation and clonogenicity of cancer cells harboring the p53Y220C mutation with little toxicity in cancer cells with wild-type p53. RNA-seq studies revealed novel p53Y220C-dependent upregulation of TRAIL apoptotic genes and downregulation of DNA damage response pathways upon acetylation induced by MS78. Altogether, the AceTAC strategy could provide a generalizable platform for targeting proteins, such as tumor suppressors, via acetylation.

Emerging Pharmacotherapeutic Strategies to Overcome Undruggable Proteins in Cancer

Targeted therapies in cancer treatment can improve in vivo efficacy and reduce adverse effects by altering the tissue exposure of specific biomolecules. However, there are still large number of target proteins in cancer are still undruggable, owing to the following factors including (1) lack of ligand-binding pockets, (2) function based on protein-protein interactions (PPIs), (3) the highly specific conserved active sites among protein family members, and (4) the variability of tertiary docking structures. The current status of undruggable targets proteins such as KRAS, TP53, C-MYC, PTP, are carefully introduced in this review. Some novel techniques and drug designing strategies have been applicated for overcoming these undruggable proteins, and the most classic and well-known technology is proteolysis targeting chimeras (PROTACs). In this review, the novel drug development strategies including targeting protein degradation, targeting PPI, targeting intrinsically disordered regions, as well as targeting protein-DNA binding are described, and we also discuss the potential of these strategies for overcoming the undruggable targets. Besides, intelligence-assisted technologies like Alpha-Fold help us a lot to predict the protein structure, which is beneficial for drug development. The discovery of new targets and the development of drugs targeting them, especially those undruggable targets, remain a huge challenge. New drug development strategies, better extraction processes that do not disrupt protein-protein interactions, and more precise artificial intelligence technologies may provide significant assistance in overcoming these undruggable targets.

Proteolysis-targeting chimeras in biotherapeutics: Current trends and future applications

The success of inhibitor-based therapeutics is largely constrained by the acquisition of therapeutic resistance, which is partially driven by the undruggable proteome. The emergence of proteolysis targeting chimera (PROTAC) technology, designed for degrading proteins involved in specific biological processes, might provide a novel framework for solving the above constraint. A heterobifunctional PROTAC molecule could structurally connect an E3 ubiquitin ligase ligand with a protein of interest (POI)-binding ligand by chemical linkers. Such technology would result in the degradation of the targeted protein via the ubiquitin-proteasome system (UPS), opening up a novel way of selectively inhibiting undruggable proteins. Herein, we will highlight the advantages of PROTAC technology and summarize the current understanding of the potential mechanisms involved in biotherapeutics, with a particular focus on its application and development where therapeutic benefits over classical small-molecule inhibitors have been achieved. Finally, we discuss how this technology can contribute to developing biotherapeutic drugs, such as antivirals against infectious diseases, for use in clinical practices.

Current advances of small molecule E3 ligands for proteolysis-targeting chimeras design

Proteolysis-targeting chimeras (PROTACs) as an emerging drug discovery modality has been extensively concerned in recent years. Over 20 years development, accumulated studies have demonstrated that PROTACs show unique advantages over traditional therapy in operable target scope, efficacy, and overcoming drug resistance. However, only limited E3 ligases, the essential elements of PROTACs, have been harnessed for PROTACs design. The optimization of novel ligands for well-established E3 ligases and the employment of additional E3 ligases remain urgent challenges for investigators. Here, we systematically summarize the current status of E3 ligases and corresponding ligands for PROTACs design with a focus on their discovery history, design principles, application benefits, and potential defects. Meanwhile, the prospects and future directions for this field are briefly discussed.

Discovery of Norbornene as a Novel Hydrophobic Tag Applied in Protein Degradation

Hydrophobic tagging (HyT) is a potential therapeutic strategy for targeted protein degradation (TPD). Norbornene was discovered as an unprecedented hydrophobic tag in this study and was used to degrade the anaplastic lymphoma kinase (ALK) fusion protein by linking it to ALK inhibitors. The most promising degrader, Hyt-9, potently reduced ALK levels through Hsp70 and the ubiquitin-proteasome system (UPS) in vitro without compensatory upregulation of ALK. Furthermore, Hyt-9 exhibited a significant tumor-inhibiting effect in vivo with moderate oral bioavailability. More importantly, norbornene can also be used to degrade the intractable enhancer of zeste homolog 2 (EZH2) when tagged with the EZH2 inhibitor tazemetostat. Thus, the discovery of novel hydrophobic norbornene tags shows promise for the future development of TPD technology.

PROTACs: Novel tools for improving immunotherapy in cancer

Posttranslational modifications (PTMs), such as phosphorylation, methylation, ubiquitination, and acetylation, are important in governing protein expression levels. Proteolysis targeting chimeras (PROTACs) are novel structures designed to target a protein of interest (POI) for ubiquitination and degradation, leading to the selective reduction in the expression levels of the POI. PROTACs have exhibited great promise due to their ability to target undruggable proteins, including several transcription factors. Recently, PROTACs have been characterized to improve anticancer immunotherapy via the regulation of specific proteins. In this review, we describe how the PROTACs target several molecules, including HDAC6, IDO1, EGFR, FoxM1, PD-L1, SHP2, HPK1, BCL-xL, BET proteins, NAMPT, and COX-1/2, to regulate immunotherapy in human cancers. PROTACs may provide potential treatment benefits by enhancing immunotherapy in cancer patients.

E3 Ligases Meet Their Match: Fragment-Based Approaches to Discover New E3 Ligands and to Unravel E3 Biology

Ubiquitination is a key post-translational modification of proteins, affecting the regulation of multiple cellular processes. Cells are equipped with over 600 ubiquitin orchestrators, called E3 ubiquitin ligases, responsible for directing the covalent attachment of ubiquitin to substrate proteins. Due to their regulatory role in cells, significant efforts have been made to discover ligands for E3 ligases. The recent emergence of the proteolysis targeting chimera (PROTAC) and molecular glue degrader (MGD) modalities has further increased interest in E3 ligases as drug targets. This perspective focuses on how fragment based lead discovery (FBLD) methods have been used to discover new ligands for this important target class. In some cases these efforts have led to clinical candidates; in others, they have provided tools for deepening our understanding of E3 ligase biology. Recently, FBLD-derived ligands have inspired the design of PROTACs that are able to artificially modulate protein levels in cells.

Piperlongumine conjugates induce targeted protein degradation

Proteolysis targeting chimeras (PROTACs) are bifunctional molecules that degrade target proteins through recruiting E3 ligases. However, their application is limited in part because few E3 ligases can be recruited by known E3 ligase ligands. In this study, we identified piperlongumine (PL), a natural product, as a covalent E3 ligase recruiter, which induces CDK9 degradation when it is conjugated with SNS-032, a CDK9 inhibitor. The lead conjugate 955 can potently degrade CDK9 in a ubiquitin-proteasome-dependent manner and is much more potent than SNS-032 against various tumor cells in vitro. Mechanistically, we identified KEAP1 as the E3 ligase recruited by 955 to degrade CDK9 through a TurboID-based proteomics study, which was further confirmed by KEAP1 knockout and the nanoBRET ternary complex formation assay. In addition, PL-ceritinib conjugate can degrade EML4-ALK fusion oncoprotein, suggesting that PL may have a broader application as a covalent E3 ligase ligand in targeted protein degradation.

Advancing targeted protein degradation for metabolic diseases therapy

The development and application of traditional drugs represented by small molecule chemical drugs and biological agents, especially inhibitors, have become the mainstream drug development. In recent years, targeted protein degradation (TPD) technology has become one of the most promising methods to remove specific disease-related proteins using cell self-destruction mechanisms. Many different TPD strategies are emerging based on the ubiquitin-proteasome system (UPS) and the autophagy-lysosomal pathway (ALP), including but not limited to proteolysis-targeting chimeras (PROTAC), molecular glues (MG), lysosome targeting chimeras (LYTAC), chaperone-mediated autophagy (CMA)-targeting chimeras, autophagy-targeting chimera (AUTAC), autophagosome-tethering compound (ATTEC), and autophagy-targeting chimera (AUTOTAC). The advent of targeted degradation technology can change most protein targets in human cells from undruggable to druggable, greatly expanding the therapeutic prospect of refractory diseases such as metabolic syndrome. Here, we summarize the latest progress of major TPD technologies, especially in metabolic syndrome and look forward to providing new insights for drug discovery.

Protein degradation: expanding the toolbox to restrain cancer drug resistance

Despite significant progress in clinical management, drug resistance remains a major obstacle. Recent research based on protein degradation to restrain drug resistance has attracted wide attention, and several therapeutic strategies such as inhibition of proteasome with bortezomib and proteolysis-targeting chimeric have been developed. Compared with intervention at the transcriptional level, targeting the degradation process seems to be a more rapid and direct strategy. Proteasomal proteolysis and lysosomal proteolysis are the most critical quality control systems responsible for the degradation of proteins or organelles. Although proteasomal and lysosomal inhibitors (e.g., bortezomib and chloroquine) have achieved certain improvements in some clinical application scenarios, their routine application in practice is still a long way off, which is due to the lack of precise targeting capabilities and inevitable side effects. In-depth studies on the regulatory mechanism of critical protein degradation regulators, including E3 ubiquitin ligases, deubiquitylating enzymes (DUBs), and chaperones, are expected to provide precise clues for developing targeting strategies and reducing side effects. Here, we discuss the underlying mechanisms of protein degradation in regulating drug efflux, drug metabolism, DNA repair, drug target alteration, downstream bypass signaling, sustaining of stemness, and tumor microenvironment remodeling to delineate the functional roles of protein degradation in drug resistance. We also highlight specific E3 ligases, DUBs, and chaperones, discussing possible strategies modulating protein degradation to target cancer drug resistance. A systematic summary of the molecular basis by which protein degradation regulates tumor drug resistance will help facilitate the development of appropriate clinical strategies.

E3 ligase ligand optimization of Clinical PROTACs

Proteolysis targeting chimeras (PROTACs) technology can realize the development of drugs for non-druggable targets that are difficult to achieve with traditional small molecules, and therefore has attracted extensive attention from both academia and industry. Up to now, there are more than 600 known E3 ubiquitin ligases with different structures and functions, but only a few have developed corresponding E3 ubiquitin ligase ligands, and the ligands used to design PROTAC molecules are limited to a few types such as VHL (Von-Hippel-Lindau), CRBN (Cereblon), MDM2 (Mouse Doubleminute 2 homolog), IAP (Inhibitor of apoptosis proteins), etc. Most of the PROTAC molecules that have entered clinical trials were developed based on CRBN ligands, and only DT2216 was based on VHL ligand. Obviously, the structural optimization of E3 ubiquitin ligase ligands plays an instrumental role in PROTAC technology from bench to bedside. In this review, we review the structure optimization process of E3 ubiquitin ligase ligands currently entering clinical trials on PROTAC molecules, summarize some characteristics of these ligands in terms of druggability, and provide some preliminary insights into their structural optimization. We hope that this review will help medicinal chemists to develop more druggable molecules into clinical studies and to realize the greater therapeutic potential of PROTAC technology.

Hypoxia deactivates epigenetic feedbacks via enzyme-derived clicking proteolysis-targeting chimeras

Epigenetic mediation through bromodomain and extraterminal (BET) proteins have progressively translated protein imbalance into effective cancer treatment. Perturbation of druggable BET proteins through proteolysis-targeting chimeras (PROTACs) has recently contributed to the discovery of effective therapeutics. Unfortunately, precise and microenvironment-activatable BET protein degradation content with promising tumor selectivity and pharmacological suitability remains elusive. Here, we present an enzyme-derived clicking PROTACs (ENCTACs) capable of orthogonally cross-linking two disparate small-molecule warhead ligands that recognize BET bromodomain-containing protein 4 (BRD4) protein and E3 ligase within tumors only upon hypoxia-induced activation of nitroreductase enzyme. This localized formation of heterobifunctional degraders promotes specific down-regulation of BRD4, which subsequently alters expression of epigenetic targets and, therefore, allows precise modulation of hypoxic signaling in live cells, zebrafish, and living mice with solid tumors. Our activation-feedback system demonstrates compelling superiorities and may enable the PROTAC technology with more flexible practicality and druggable potency for precision medicine in the near future.

Bridged Proteolysis Targeting Chimera (PROTAC) Enables Degradation of Undruggable Targets

Proteolysis Targeting Chimeras (PROTACs) are attractive therapeutic modalities for degrading disease-causing proteins. While many PROTACs have been developed for numerous protein targets, current small-molecule PROTAC approaches cannot target undruggable proteins that do not have small-molecule binders. Here, we present a novel PROTAC approach, termed bridged PROTAC, which utilizes a small-molecule binder of the target protein's binding partner to recruit the protein complex into close proximity with an E3 ubiquitin ligase to target undruggable proteins. Applying this bridged PROTAC strategy, we discovered MS28, the first-in-class degrader of cyclin D1, which lacks a small-molecule binder. MS28 effectively degrades cyclin D1, with faster degradation kinetics and superior degradation efficiency than CDK4/6, through recruiting the CDK4/6-cyclin D1 complex to the von Hippel-Lindau E3 ligase. MS28 also suppressed the proliferation of cancer cells more effectively than CDK4/6 inhibitors and degraders. Altogether, the bridged PROTAC strategy could provide a generalizable platform for targeting undruggable proteins.

Design and characterization of a heterobifunctional degrader of KEAP1

The Kelch-like ECH-associated protein 1 (KEAP1) - nuclear factor erythroid 2-related factor 2 (NRF2) signaling pathway senses reactive oxygen species and regulates cellular oxidative stress. Inhibiting KEAP1 to activate the NRF2 antioxidant response has been proposed as a promising strategy to treat chronic diseases caused by oxidative stress. Here, we developed a proteolysis targeting chimera (PROTAC) that depletes KEAP1 from cells through the ubiquitin-proteasome pathway. A previously developed KEAP1 inhibitor and thalidomide were incorporated in the heterobifunctional design of the PROTAC as ligands for KEAP1 and CRBN recruitment, respectively. Optimization of the chemical composition and linker length resulted in PROTAC 14 which exhibited potent KEAP1 degradation with low nanomolar DC₅₀ in HEK293T (11 nM) and BEAS-2B (<1 nM) cell lines. Furthermore, PROTAC 14 increased the expression of NRF2 regulated antioxidant proteins and prevented cell death induced by reactive oxygen species. Together, these results established a blueprint for further development of KEAP1-targeted heterobifunctional degraders and will facilitate the study of the biological consequences of KEAP1 removal from cells. This approach represents an alternative therapeutic strategy to existing treatments for diseases caused by oxidative stress.

Comparative structural analyses of the NHL domains from the human E3 ligase TRIM-NHL family

Tripartite motif (TRIM) proteins constitute one of the largest subfamilies of the RING-type E3 ubiquitin ligases that play a role in diverse processes from homeostasis and immune response to viral restriction. While TRIM proteins typically harbor an N-terminal RING finger, a B-box and a coiled-coil domain, a high degree of diversity lies in their C termini that contain diverse protein interaction modules, most of which, both structures and their roles in intermolecular interactions, remain unknown. Here, high-resolution crystal structures of the NHL domains of three of the four human TRIM-NHL proteins, namely TRIM2, TRIM3 and TRIM71, are presented. Comparative structural analyses revealed that, despite sharing an evolutionarily conserved six-bladed β-propeller architecture, the low sequence identities resulted in distinct properties of these interaction domains at their putative binding sites for macromolecules. Interestingly, residues lining the binding cavities represent a hotspot for genetic mutations linked to several diseases. Thus, high sequence diversity within the conserved NHL domains might be essential for differentiating binding partners among TRIM-NHL proteins.

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.

Exploring the target scope of KEAP1 E3 ligase-based PROTACs

Targeted protein degradation (TPD) uses small molecules to recruit E3 ubiquitin ligases into the proximity of proteins of interest, inducing ubiquitination-dependent degradation. A major bottleneck in the TPD field is the lack of accessible E3 ligase ligands for developing degraders. To expand the E3 ligase toolbox, we sought to convert the Kelch-like ECH-associated protein 1 (KEAP1) inhibitor KI696 into a recruitment handle for several targets. While we were able to generate KEAP1-recruiting degraders of BET family and murine focal adhesion kinase (FAK), we discovered that the target scope of KEAP1 was narrow, as targets easily degraded using a cereblon (CRBN)-recruiting degrader were refractory to KEAP1-mediated degradation. Linking the KEAP1-binding ligand to a CRBN-binding ligand resulted in a molecule that induced degradation of KEAP1 but not CRBN. In sum, we characterize tool compounds to explore KEAP1-mediated ubiquitination and delineate the challenges of exploiting new E3 ligases for generating bivalent degraders.

Natural Product-Inspired Targeted Protein Degraders: Advances and Perspectives

Targeted protein degradation (TPD), a promising therapeutic strategy in drug discovery, has great potential to regulate the endogenous degradation of undruggable targets with small molecules. As vital resources that provide diverse structural templates for drug discovery, natural products (NPs) are a rising and robust arsenal for the development of therapeutic TPD. The first proof-of-concept study of proteolysis-targeting chimeras (PROTACs) was a natural polyketide ovalicin-derived degrader; since then, NPs have shown great potential to promote TPD technology. The use of NP-inspired targeted protein degraders has been confirmed to be a promising strategy to treat many human conditions, including cancer, inflammation, and nonalcoholic fatty liver disease. Nevertheless, the development of NP-inspired degraders is challenging, and the field is currently in its infancy. In this review, we summarize the bioactivities and mechanisms of NP-inspired degraders and discuss the associated challenges and future opportunities in this field.

Predicting the structural basis of targeted protein degradation by integrating molecular dynamics simulations with structural mass spectrometry

Targeted protein degradation (TPD) is a promising approach in drug discovery for degrading proteins implicated in diseases. A key step in this process is the formation of a ternary complex where a heterobifunctional molecule induces proximity of an E3 ligase to a protein of interest (POI), thus facilitating ubiquitin transfer to the POI. In this work, we characterize 3 steps in the TPD process. (1) We simulate the ternary complex formation of SMARCA2 bromodomain and VHL E3 ligase by combining hydrogen-deuterium exchange mass spectrometry with weighted ensemble molecular dynamics (MD). (2) We characterize the conformational heterogeneity of the ternary complex using Hamiltonian replica exchange simulations and small-angle X-ray scattering. (3) We assess the ubiquitination of the POI in the context of the full Cullin-RING Ligase, confirming experimental ubiquitinomics results. Differences in degradation efficiency can be explained by the proximity of lysine residues on the POI relative to ubiquitin.

Discovery of E3 Ligase Ligands for Target Protein Degradation

Target protein degradation has emerged as a promising strategy for the discovery of novel therapeutics during the last decade. Proteolysis-targeting chimera (PROTAC) harnesses a cellular ubiquitin-dependent proteolysis system for the efficient degradation of a protein of interest. PROTAC consists of a target protein ligand and an E3 ligase ligand so that it enables the target protein degradation owing to the induced proximity with ubiquitin ligases. Although a great number of PROTACs has been developed so far using previously reported ligands of proteins for their degradation, E3 ligase ligands have been mostly limited to either CRBN or VHL ligands. Those PROTACs showed their limitation due to the cell type specific expression of E3 ligases and recently reported resistance toward PROTACs with CRBN ligands or VHL ligands. To overcome these hurdles, the discovery of various E3 ligase ligands has been spotlighted to improve the current PROTAC technology. This review focuses on currently reported E3 ligase ligands and their application in the development of PROTACs.

PROTACs: Current Trends in Protein Degradation by Proteolysis-Targeting Chimeras

In the recent past, proteolysis-targeting chimera (PROTAC) technology has received enormous attention for its ability to overcome the limitations of protein inhibitors and its capability to target undruggable proteins. The PROTAC molecule consists of three components, a ubiquitin E3 ligase ligand, a linker, and a target protein ligand. The application of this technology is rapidly gaining momentum, especially in cancer therapy. In this review, we first look at the history of degraders, followed by a section on the ubiquitin proteasome system (UPS) and E3 ligases used in PROTAC development. PROTACs are dependent on the UPS for degradation of target proteins. We further discuss the scope and design of degraders and mitigation strategies for overcoming the hook effect seen with degraders. As PROTACs do not follow Lipinski's 'Rule of 5', these molecules face drug metabolism and pharmacokinetic challenges. A detailed section on absorption, distribution, metabolism, and excretion of degraders is provided wherein we discuss methodologies and strategies to surmount the challenges faced by these molecules. For understanding PROTAC-mediated degradation, the characterization and measurement of protein levels in cells is important. Currently used techniques and recent advancements in assessment tools for degraders are discussed. Furthermore, we examine the challenges and emerging technologies that need to be focused on in order to competently develop potent degraders. Many companies are working in this area of emerging new modality and a few PROTACs have already entered clinical trials; the details of the trials are included in this review.

Novel Design Strategies to Enhance the Efficiency of Proteolysis Targeting Chimeras

Despite the success of drug discovery over the past decades, many potential drug targets still remain intractable for small molecule modulation. The development of proteolysis targeting chimeras (PROTACs) that trigger degradation of the target proteins provides a conceptually novel approach to address drug targets that remained previously elusive. Currently, the main challenge of PROTAC development is the identification of efficient, tissue- and cell-selective PROTAC molecules with good drug-likeness and favorable safety profiles. This review focuses on strategies to enhance the effectiveness and selectivity of PROTACs. We provide a comprehensive summary of recently reported PROTAC design strategies and discuss the advantages and disadvantages of these strategies. Future perspectives for PROTAC design will also be discussed.

Targeted Protein Degradation to Overcome Resistance in Cancer Therapies: PROTAC and N-Degron Pathway

Extensive progress in understanding the molecular mechanisms of cancer growth and proliferation has led to the remarkable development of drugs that target cancer-driving molecules. Most target molecules are proteins such as kinases and kinase-associated receptors, which have enzymatic activities needed for the signaling cascades of cells. The small molecule inhibitors for these target molecules greatly improved therapeutic efficacy and lowered the systemic toxicity in cancer therapies. However, long-term and high-dosage treatment of small inhibitors for cancer has produced other obstacles, such as resistance to inhibitors. Among recent approaches to overcoming drug resistance to cancers, targeted protein degradation (TPD) such as proteolysis-targeting chimera (PROTAC) technology adopts a distinct mechanism of action by which a target protein is destroyed through the cellular proteolytic system, such as the ubiquitin–proteasome system or autophagy. Here, we review the currently developed PROTACs as the representative TPD molecules for cancer therapy and the N-degrons of the N-degron pathways as the potential TPD ligands.

Chemistries of bifunctional PROTAC degraders

Proteolysis targeting chimeras (PROTACs) technology is a novel and promising therapeutic strategy using small molecules to induce ubiquitin-dependent degradation of proteins. It has received extensive attention from both academia and industry as it can potentially access previously inaccessible targets. However, the design and optimization of PROTACs present big challenges for researchers, and the general strategy for its development and optimization is a lot of trial and error based on experience. This review highlights the important advances in this rapidly growing field and critical limitations of the traditional trial-and-error approach to developing PROTACs by analyzing numerous representative examples of PROTACs development. We summarize and analyze the general principles and strategies for PROTACs design and optimization from the perspective of chemical structure design, and propose potential future pathways to facilitate the development of PROTACs.

Proteolysis-targeting chimera (PROTAC) delivery system: advancing protein degraders towards clinical translation

Proteolysis Targeting Chimeras (PROTACs), an emerging therapeutic entity designed to degrade target proteins by hijacking the ubiquitin-proteasome system, have the potential to revolutionize the healthcare industry. The broad applicability of this protein degradation strategy has been verified with a few E3 ligases and a variety of distinct targets through the construction of modular chimeric structures. Despite recent efforts to promote the use of PROTACs for clinical applications, most PROTACs do not make it beyond the preclinical stage of drug development. There are several reasons that prevent PROTACs from reaching the market, and the inadequate delivery to the target site is one of the most challenging hurdles. With the increasing need for accelerating the translational process, combining the concepts of PROTACs and delivery systems has been explored to enhance the in vivo performance of PROTACs. These improved delivery strategies can eliminate unfavorable physicochemical properties of PROTACs, improve their targetability, and decrease their off-target side effects. The integration of powerful PROTACs and versatile delivery systems will inaugurate a burgeoning orientation for the field of targeted protein degradation. In this review, we will survey the latest progress in improving the in vivo degradation efficacy of PROTACs through delivery strategies, outline design principles for PROTAC-based delivery systems, discuss the current challenges with PROTACs, and outlook future opportunities in this field.

Encoding BRAF inhibitor functions in protein degraders

Various BRAF kinase inhibitors were developed to treat cancers carrying the BRAFV600E mutation. First-generation BRAF inhibitors could lead to paradoxical activation of the MAPK pathway, limiting their clinical usefulness. Here, we show the development of two series of BRAFV600E-targeting PROTACs and demonstrate that the exchange of the inhibitor scaffold from vemurafenib to paradox-breaker ligands resulted in BRAFV600E degraders that did not cause paradoxical ERK activation.

PROTACs: great opportunities for academia and industry (an update from 2020 to 2021)

PROteolysis TArgeting Chimeras (PROTACs) technology is a new protein-degradation strategy that has emerged in recent years. It uses bifunctional small molecules to induce the ubiquitination and degradation of target proteins through the ubiquitin–proteasome system. PROTACs can not only be used as potential clinical treatments for diseases such as cancer, immune disorders, viral infections, and neurodegenerative diseases, but also provide unique chemical knockdown tools for biological research in a catalytic, reversible, and rapid manner. In 2019, our group published a review article “PROTACs: great opportunities for academia and industry” in the journal, summarizing the representative compounds of PROTACs reported before the end of 2019. In the past 2 years, the entire field of protein degradation has experienced rapid development, including not only a large increase in the number of research papers on protein-degradation technology but also a rapid increase in the number of small-molecule degraders that have entered the clinical and will enter the clinical stage. In addition to PROTAC and molecular glue technology, other new degradation technologies are also developing rapidly. In this article, we mainly summarize and review the representative PROTACs of related targets published in 2020–2021 to present to researchers the exciting developments in the field of protein degradation. The problems that need to be solved in this field will also be briefly introduced.

E3 ligase ligand chemistries: from building blocks to protein degraders

In recent years, proteolysis-targeting chimeras (PROTACs), capable of achieving targeted protein degradation, have proven their great therapeutic potential and usefulness as molecular biology tools. These heterobifunctional compounds are comprised of a protein-targeting ligand, an appropriate linker, and a ligand binding to the E3 ligase of choice. A successful PROTAC induces the formation of a ternary complex, leading to the E3 ligase-mediated ubiquitination of the targeted protein and its proteasomal degradation. In over 20 years since the concept was first demonstrated, the field has grown substantially, mainly due to the advancements in the discovery of non-peptidic E3 ligase ligands. Development of small-molecule E3 binders with favourable physicochemical profiles aided the design of PROTACs, which are known for breaking the rules of established guidelines for discovering small molecules. Synthetic accessibility of the ligands and numerous successful applications led to the prevalent use of cereblon and von Hippel-Lindau as the hijacked E3 ligase. However, the pool of over 600 human E3 ligases is full of untapped potential, which is why expanding the artillery of E3 ligands could contribute to broadening the scope of targeted protein degradation. In this comprehensive review, we focus on the chemistry aspect of the PROTAC design process by providing an overview of liganded E3 ligases, their chemistries, appropriate derivatisation, and synthetic approaches towards their incorporation into heterobifunctional degraders. By covering syntheses of both established and underexploited E3 ligases, this review can serve as a chemistry blueprint for PROTAC researchers during their future ventures into the complex field of targeted protein degradation.

PROTAC targeted protein degraders: the past is prologue

Targeted protein degradation (TPD) is an emerging therapeutic modality with the potential to tackle disease-causing proteins that have historically been highly challenging to target with conventional small molecules. In the 20 years since the concept of a proteolysis-targeting chimera (PROTAC) molecule harnessing the ubiquitin-proteasome system to degrade a target protein was reported, TPD has moved from academia to industry, where numerous companies have disclosed programmes in preclinical and early clinical development. With clinical proof-of-concept for PROTAC molecules against two well-established cancer targets provided in 2020, the field is poised to pursue targets that were previously considered 'undruggable'. In this Review, we summarize the first two decades of PROTAC discovery and assess the current landscape, with a focus on industry activity. We then discuss key areas for the future of TPD, including establishing the target classes for which TPD is most suitable, expanding the use of ubiquitin ligases to enable precision medicine and extending the modality beyond oncology.

Covalent fragment-based ligand screening approaches for identification of novel ubiquitin proteasome system modulators

Ubiquitination is a key regulatory mechanism vital for maintenance of cellular homeostasis. Protein degradation is induced by E3 ligases via attachment of ubiquitin chains to substrates. Pharmacological exploitation of this phenomenon via targeted protein degradation (TPD) can be achieved with molecular glues or bifunctional molecules facilitating the formation of ternary complexes between an E3 ligase and a given protein of interest (POI), resulting in ubiquitination of the substrate and subsequent proteolysis by the proteasome. Recently, the development of novel covalent fragment screening approaches has enabled the identification of first-in-class ligands for E3 ligases and deubiquitinases revealing so far unexplored binding sites which highlights the potential of these methods to uncover and expand druggable space for new target classes.

Smart Nano-PROTACs Reprogram Tumor Microenvironment for Activatable Photo-metabolic Cancer Immunotherapy

Protease inhibitors can modulate intratumoral metabolic processes to reprogram the immunosuppressive tumor microenvironment (TME), which however suffer from the limited efficacy and off-targeted side effects. We report smart nano-proteolysis targeting chimeras (nano-PROTACs) with phototherapeutic ablation and cancer-specific protein degradation to reprogram the TME for photo-metabolic cancer immunotherapy. This nano-PROTAC has a semiconducting polymer backbone linked with a cyclooxygenase 1/2 (COX-1/2)-targeting PROTAC peptide (CPP) via a cathepsin B (CatB)-cleavable segment. CPP can be activated by the tumor-overexpressed CatB to induce the degradation of COX-1/2 via the ubiquitin-proteasome system. The persistent degradation of COX-1/2 depletes their metabolite prostaglandin E₂ which is responsible for activation of immune suppressor cells. Such a smart PROTAC strategy synergized with phototherapy specifically reprograms the immunosuppressive TME and reinvigorates antitumor immunity.