Merging PROTAC and molecular glue for degrading BTK and GSPT1 proteins concurrently

Targeted protein degradation: current molecular targets, localization, and strategies

Targeted protein degradation (TPD) has revolutionized drug discovery by selectively eliminating specific proteins within and outside the cellular context. Over the past two decades, TPD has expanded its focus beyond well-established targets, exploring diverse proteins beyond cancer-related ones. This evolution extends the potential of TPD to various diseases. Notably, TPD can target proteins at demanding locations, such as the extracellular matrix (ECM) and cellular membranes, presenting both opportunities and challenges for future research. In this review, we comprehensively examine the exciting opportunities in the burgeoning field of TPD, highlighting different targets, their cellular environment, and innovative strategies for modern drug discovery.

Advances in molecular glues: exploring chemical space and design principles for targeted protein degradation

The discovery of the E3 ligase cereblon (CRBN) as the target of thalidomide and its analogs revolutionized the field of targeted protein degradation (TPD). This ubiquitin-mediated degradation pathway was first harnessed by bivalent degraders. Recently, the emergence of low-molecular-weight molecular glue degraders (MGDs) has expanded the TPD landscape, because MGDs operate via the same mechanism while offering attractive physicochemical properties that are consistent with small-molecule therapeutics. This review delves into the discovery and advancement of MGDs, with case studies on cyclin K and the zinc finger protein IKZF2, highlighting the design principles, biological assays and therapeutic applications. Additionally, it examines the chemical space of molecular glues and outlines the collaborative efforts that are fueling innovation in this field.

Design of bifunctional molecules to accelerate post-translational modifications: achievements and challenges

Post-translational modifications (PTMs) of proteins are crucial for regulating biological processes and their dysregulation is linked to various diseases, highlighting PTM regulation as a significant target for drug development. Traditional drug targets often interact with multiple proteins, resulting in lower selectivity and inevitable adverse effects, which limits their clinical applicability. Recent advancements in bifunctional molecules, such as proteolysis-targeting chimeras (PROTACs), have shown promise in targeting PTMs precisely. However, regulatory mechanisms for many of the >600 known PTMs remain underexplored. This review examines current progress and challenges in designing bifunctional molecules for PTM regulation, focusing on effector selection and ligand design strategies, aiming to propel the utilization and advancement of bifunctional molecules to the forefront of PTM research.

Targeted protein degradation: from mechanisms to clinic

Targeted protein degradation refers to the use of small molecules to induce the selective degradation of proteins. In its most common form, this degradation is achieved through ligand-mediated neo-interactions between ubiquitin E3 ligases - the principal waste disposal machines of a cell - and the protein targets of interest, resulting in ubiquitylation and subsequent proteasomal degradation. Notable advances have been made in biological and mechanistic understanding of serendipitously discovered degraders. This improved understanding and novel chemistry has not only provided clinical proof of concept for targeted protein degradation but has also led to rapid growth of the field, with dozens of investigational drugs in active clinical trials. Two distinct classes of protein degradation therapeutics are being widely explored: bifunctional PROTACs and molecular glue degraders, both of which have their unique advantages and challenges. Here, we review the current landscape of targeted protein degradation approaches and how they have parallels in biological processes. We also outline the ongoing clinical exploration of novel degraders and provide some perspectives on the directions the field might take.

Targeted protein degradation in hematologic malignancies: clinical progression towards novel therapeutics

Targeted therapies, such as small molecule kinase inhibitors, have made significant progress in the treatment of hematologic malignancies by directly modulating protein activity. However, issues such as drug toxicity, drug resistance due to target mutations, and the absence of key active sites limit the therapeutic efficacy of these drugs. Targeted protein degradation (TPD) presents an emergent and rapidly evolving therapeutic approach that selectively targets proteins of interest (POI) based on endogenous degradation processes. With an event-driven pharmacology of action, TPD achieves efficacy with catalytic amounts, avoiding drug-related toxicity. Furthermore, TPD has the unique mode of degrading the entire POI, such that resistance derived from mutations in the targeted protein has less impact on its degradation function. Proteolysis-targeting chimeras (PROTACs) and molecular glue degraders (MGDs) are the most maturely developed TPD techniques. In this review, we focus on both preclinical experiments and clinical trials to provide a comprehensive summary of the safety and clinical effectiveness of PROTACs and MGDs in hematologic malignancies over the past two decades. In addition, we also delineate the challenges and opportunities associated with these burgeoning degradation techniques. TPD, as an approach to the precise degradation of specific proteins, provides an important impetus for its future application in the treatment of patients with hematologic malignancies.

Discovery of first-in-class PROTACs targeting maternal embryonic leucine zipper kinase (MELK) for the treatment of Burkitt lymphoma

Maternal embryonic leucine zipper kinase (MELK) is a novel target for the treatment of various kinds of B-cell malignancies. However, the toxicity of inhibitors of MELK has led to clinical failures in cancer treatments. Moreover, inactivation of MELK catalytic domain is insufficient for achieving cancer cell apoptosis. To further confirm the role of MELK in Burkitt lymphoma treatment, we describe herein a structure-guided design of PROTACs targeting MELK. Through design, computer-assisted optimization and SAR studies, we developed the first-in-class MELK-targeting PROTAC MGP-39, which promoted a rapid and potent degradation of MELK in RAMOS cells. Additionally, the newly designed MELK degrader induced significant cell cycle arrest and apoptosis in cancer cells. Notably, compared to MELK inhibitors, MGP-39 has better anti-cancer activity and lower toxicity, indicating the practical role of PROTACs in avoiding the side effects of traditional inhibitors. More importantly, our results show that the use of a PROTAC can be adopted as a general and effective strategy for targeted cancer therapy.

Development and therapeutic potential of GSPT1 molecular glue degraders: A medicinal chemistry perspective

Unprecedented therapeutic targeting of previously undruggable proteins has now been achieved by molecular-glue-mediated proximity-induced degradation. As a small GTPase, G1 to S phase transition 1 (GSPT1) interacts with eRF1, the translation termination factor, to facilitate the process of translation termination. Studied demonstrated that GSPT1 plays a vital role in the acute myeloid leukemia (AML) and MYC-driven lung cancer. Thus, molecular glue (MG) degraders targeting GSPT1 is a novel and promising approach for treating AML and MYC-driven cancers. In this Perspective, we briefly summarize the structural and functional aspects of GSPT1, highlighting the latest advances and challenges in MG degraders, as well as some representative patents. The structure-activity relationships, mechanism of action and pharmacokinetic features of MG degraders are emphasized to provide a comprehensive compendium on the rational design of GSPT1 MG degraders. We hope to provide an updated overview, and design guide for strategies targeting GSPT1 for the treatment of cancer.

Targeting the undruggables-the power of protein degraders

Undruggable targets typically refer to a class of therapeutic targets that are difficult to target through conventional methods or have not yet been targeted, but are of great clinical significance. According to statistics, over 80% of disease-related pathogenic proteins cannot be targeted by current conventional treatment methods. In recent years, with the advancement of basic research and new technologies, the development of various new technologies and mechanisms has brought new perspectives to overcome challenging drug targets. Among them, targeted protein degradation technology is a breakthrough drug development strategy for challenging drug targets. This technology can specifically identify target proteins and directly degrade pathogenic target proteins by utilizing the inherent protein degradation pathways within cells. This new form of drug development includes various types such as proteolysis targeting chimera (PROTAC), molecular glue, lysosome-targeting Chimaera (LYTAC), autophagosome-tethering compound (ATTEC), autophagy-targeting chimera (AUTAC), autophagy-targeting chimera (AUTOTAC), degrader-antibody conjugate (DAC). This article systematically summarizes the application of targeted protein degradation technology in the development of degraders for challenging drug targets. Finally, the article looks forward to the future development direction and application prospects of targeted protein degradation technology.

Discovery of Novel ERα and Aromatase Dual-Targeting PROTAC Degraders to Overcome Endocrine-Resistant Breast Cancer

Estrogen receptor α (ERα) plays a pivotal role in the proliferation, differentiation, and migration of breast cancer (BC) cells, and aromatase (ARO) is a crucial enzyme in estrogen synthesis. Hence, it is necessary to inhibit estrogen production or the activity of ERα for the treatment of estrogen receptor-positive (ER+) BC. Herein, we present a new category of dual-targeting PROTAC degraders designed to specifically target ERα and ARO. Among them, compound 18c bifunctionally degrades and inhibits ERα/ARO, thus effectively suppressing the proliferation of MCF-7 cells while showing negligible cytotoxicity to normal cells. In vivo, 18c promotes the degradation of ERα and ARO and inhibits the growth of MCF-7 xenograft tumors. Finally, compound 18c demonstrates promising antiproliferative and ERα degradation activity against the ERαMUT cells. These findings suggest that 18c, being the inaugural dual-targeting degrader for ERα and ARO, warrants further advancement for the management of BC and the surmounting of endocrine resistance.

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.

Discovery of Kinetin in inhibiting colorectal cancer progression via enhancing PSMB1-mediated RAB34 degradation

Colorectal cancer (CRC) is one of the most prevalent malignancies worldwide. Understanding the underlying mechanism driving CRC progression and identifying potential therapeutic drug targets are of utmost urgency. We previously utilized LC-MS-based proteomic profiling to identify proteins associated with postoperative progression in stage II/III CRC. Here, we revealed that proteasome subunit beta type-1 (PSMB1) is an independent predictor for postoperative progression in stage II/III CRC. Mechanistically, PSMB1 binds directly to onco-protein RAB34 and promotes its proteasome-dependent degradation, potentially leading to the inactivation of the MEK/ERK signaling pathway and inhibition of CRC progression. To further identify potential anticancer drugs, we screened a library of 2509 FDA-approved drugs using computer-aided drug design (CADD) and identified Kinetin as a potentiating agent for PSMB1. Functional assays confirmed that Kinetin enhanced the interaction between PSMB1 and RAB34, hence facilitated the degradation of RAB34 protein and decreased the MEK/ERK phosphorylation. Kinetin suppresses CRC progression in patient-derived xenograft (PDX) and liver metastasis models. Conclusively, our study identifies PSMB1 as a potential biomarker and therapeutic target for CRC, and Kinetin as an anticancer drug by enhancing proteasome-dependent onco-protein degradation.

Molecular glues targeting GSPT1 in cancers: A potent therapy

G1 to S phase transition 1 (GSPT1) is a key translation termination factor that significantly overexpressed in various cancer tissues and cells. Molecular glue is a kind of small molecule, which can bind to an E3 ligase such as cereblon (CRBN) and subsequently recruit neosubstrate proteins for ubiquitination-proteasomal degradation. This emerging therapeutic approach shows great potential in treating cancers and other diseases. This review aims to introduce current understanding of antitumor mechanism of molecular glues targeting GSPT1, summarize pharmacology profiles of existing molecular glues, and outline development strategies of novel molecular glues. The insights provided in this review will be valuable for future studies.

From Thalidomide to Rational Molecular Glue Design for Targeted Protein Degradation

Thalidomide and its derivatives are powerful cancer therapeutics that are among the best-understood molecular glue degraders (MGDs). These drugs selectively reprogram the E3 ubiquitin ligase cereblon (CRBN) to commit target proteins for degradation by the ubiquitin-proteasome system. MGDs create novel recognition interfaces on the surface of the E3 ligase that engage in induced protein-protein interactions with neosubstrates. Molecular insight into their mechanism of action opens exciting opportunities to engage a plethora of targets through a specific recognition motif, the G-loop. Our analysis shows that current CRBN-based MGDs can in principle recognize over 2,500 proteins in the human proteome that contain a G-loop. We review recent advances in tuning the specificity between CRBN and its MGD-induced neosubstrates and deduce a set of simple rules that govern these interactions. We conclude that rational MGD design efforts will enable selective degradation of many more proteins, expanding this therapeutic modality to more disease areas.

Facilitating the development of molecular glues: Opportunities from serendipity and rational design

Molecular glues can specifically induce interactions between two or more proteins to modulate biological functions and have been proven to be a powerful therapeutic modality in drug discovery. It plays a variety of vital roles in several biological processes, such as complex stabilization, interactome modulation and transporter inhibition, thus enabling challenging therapeutic targets to be druggable. Most known molecular glues were identified serendipitously, such as IMiDs, auxin, and rapamycin. In recent years, more rational strategies were explored with the development of chemical biology and a deep understanding of the interaction between molecular glues and proteins, which led to the rational discovery of several molecular glues. Thus, in this review, we aim to highlight the discovery strategies of molecular glues from three aspects: serendipitous discovery, screening methods and rational design principles. We expect that this review will provide a reasonable reference and insights for the discovery of molecular glues.

Proteolysis Targeting Chimeras (PROTACs) based on celastrol induce multiple protein degradation for triple-negative breast cancer treatment

The pursuit of single drugs targeting multiple targets has become a prominent trend in modern cancer therapeutics. Natural products, known for their multi-targeting capabilities, accessibility, and cost-effectiveness, hold great potential for the development of multi-target drugs. However, their therapeutic efficacy is often hindered by complex structural modifications and limited anti-tumor activity. In this study, we present a novel approach using celastrol (CST)-based Proteolysis Targeting Chimeras (PROTACs) for breast cancer therapy. Through rational design, we have successfully developed compound 6a, a potent multiple protein degrader capable of selectively degrading GRP94 and CDK1/4 in tumor cells via the endogenous ubiquitin-proteasome system. Furthermore, compound 6a has demonstrated remarkable inhibitory effects on cell proliferation and migration, and induction of apoptosis in 4T1 cells through cell cycle arrest and activation of the Bcl-2/Bax/cleaved Caspase-3 apoptotic pathway. In vivo administration of compound 6a has effectively suppressed tumor growth with an acceptable safety profile. Our findings suggest that the CST-based PROTACs described herein can be readily extended to other natural products, offering a potential avenue for the development of natural product-based PROTACs for cancer treatment.

Peptide-based PROTACs: Current Challenges and Future Perspectives

Proteolysis-targeting chimeras (PROTACs) are an attractive means to target previously undruggable or drug-resistant mutant proteins. While small molecule-based PROTACs are stable and can cross cell membranes, there is limited availability of suitable small molecule warheads capable of recruiting proteins to an E3 ubiquitin ligase for degradation. With advances in structural biology and in silico protein structure prediction, it is now becoming easier to define highly selective peptides suitable for PROTAC design. As a result, peptide-based PROTACs are becoming a feasible proposition for targeting previously "undruggable" proteins not amenable to small molecule inhibition. In this review, we summarize recent progress in the design and application of peptide-based PROTACs as well as several practical approaches for obtaining candidate peptides for PROTACs. We also discuss the major hurdles preventing the translation of peptide-based PROTACs from bench to bedside, such as their delivery and bioavailability, with the aim of stimulating discussion about how best to accelerate the clinical development of peptide- based PROTACs in the near future.

A Chemical Strategy for the Degradation of the Main Protease of SARS-CoV-2 in Cells

SARS-CoV-2 is the virus that causes the global pandemic of COVID-19. The main protease (Mpro) of SARS-CoV-2 is essential for viral infection and is one of the major therapeutic targets for COVID-19. Here, we report the design, synthesis, and biological characterization of a novel heterobifunctional small molecule that could effectively induce the degradation of SARS-CoV-2 Mpro and its drug-resistant mutants in HEK 293T cells, thus demonstrating a new alternative strategy for intervening with proteins important for this novel coronavirus.

Targeted protein degradation in drug development: Recent advances and future challenges

Targeted protein degradation (TPD) has emerged as a promising therapeutic approach with potential advantages over traditional occupancy-based inhibitors in terms of dosing, side effects and targeting "undruggable" proteins. Targeted degraders can theoretically bind any nook or cranny of targeted proteins to drive degradation. This offers convenience versus the small-molecule inhibitors that must function in a well-defined pocket. The degradation process depends mainly on two cell self-destruction mechanisms, namely the ubiquitin-proteasome system and the lysosomal degradation pathway. Various TPD strategies (e.g., proteolytic-targeting chimeras, molecular glues, lysosome-targeting chimeras, and autophagy-targeting chimeras) have been developed. These approaches hold great potential for targeting dysregulated proteins, potentially offering therapeutic benefits. In this article, we systematically review the mechanisms of various TPD strategies, potential applications to drug discovery, and recent advances. We also discuss the benefits and challenges associated with these TPD strategies, aiming to provide insight into the targeting of dysregulated proteins and facilitate their clinical applications.

Proteomic approaches advancing targeted protein degradation

Targeted protein degradation (TPD) is an emerging modality for research and therapeutics. Most TPD approaches harness cellular ubiquitin-dependent proteolytic pathways. Proteolysis-targeting chimeras (PROTACs) and molecular glue (MG) degraders (MGDs) represent the most advanced TPD approaches, with some already used in clinical settings. Despite these advances, TPD still faces many challenges, pertaining to both the development of effective, selective, and tissue-penetrant degraders and understanding their mode of action. In this review, we focus on progress made in addressing these challenges. In particular, we discuss the utility and application of recent proteomic approaches as indispensable tools to enable insights into degrader development, including target engagement, degradation selectivity, efficacy, safety, and mode of action.

Function, mechanism and drug discovery of ubiquitin and ubiquitin-like modification with multiomics profiling for cancer therapy

Ubiquitin (Ub) and ubiquitin-like (Ubl) pathways are critical post-translational modifications that determine whether functional proteins are degraded or activated/inactivated. To date, >600 associated enzymes have been reported that comprise a hierarchical task network (e.g., E1-E2-E3 cascade enzymatic reaction and deubiquitination) to modulate substrates, including enormous oncoproteins and tumor-suppressive proteins. Several strategies, such as classical biochemical approaches, multiomics, and clinical sample analysis, were combined to elucidate the functional relations between these enzymes and tumors. In this regard, the fundamental advances and follow-on drug discoveries have been crucial in providing vital information concerning contemporary translational efforts to tailor individualized treatment by targeting Ub and Ubl pathways. Correspondingly, emphasizing the current progress of Ub-related pathways as therapeutic targets in cancer is deemed essential. In the present review, we summarize and discuss the functions, clinical significance, and regulatory mechanisms of Ub and Ubl pathways in tumorigenesis as well as the current progress of small-molecular drug discovery. In particular, multiomics analyses were integrated to delineate the complexity of Ub and Ubl modifications for cancer therapy. The present review will provide a focused and up-to-date overview for the researchers to pursue further studies regarding the Ub and Ubl pathways targeted anticancer strategies.

Targeted Protein Degradation: Advances, Challenges, and Prospects for Computational Methods

The therapeutic approach of targeted protein degradation (TPD) is gaining momentum due to its potentially superior effects compared with protein inhibition. Recent advancements in the biotech and pharmaceutical sectors have led to the development of compounds that are currently in human trials, with some showing promising clinical results. However, the use of computational tools in TPD is still limited, as it has distinct characteristics compared with traditional computational drug design methods. TPD involves creating a ternary structure (protein-degrader-ligase) responsible for the biological function, such as ubiquitination and subsequent proteasomal degradation, which depends on the spatial orientation of the protein of interest (POI) relative to E2-loaded ubiquitin. Modeling this structure necessitates a unique blend of tools initially developed for small molecules (e.g., docking) and biologics (e.g., protein-protein interaction modeling). Additionally, degrader molecules, particularly heterobifunctional degraders, are generally larger than conventional small molecule drugs, leading to challenges in determining drug-like properties like solubility and permeability. Furthermore, the catalytic nature of TPD makes occupancy-based modeling insufficient. TPD consists of multiple interconnected yet distinct steps, such as POI binding, E3 ligase binding, ternary structure interactions, ubiquitination, and degradation, along with traditional small molecule properties. A comprehensive set of tools is needed to address the dynamic nature of the induced proximity ternary complex and its implications for ubiquitination. In this Perspective, we discuss the current state of computational tools for TPD. We start by describing the series of steps involved in the degradation process and the experimental methods used to characterize them. Then, we delve into a detailed analysis of the computational tools employed in TPD. We also present an integrative approach that has proven successful for degrader design and its impact on project decisions. Finally, we examine the future prospects of computational methods in TPD and the areas with the greatest potential for impact.

Discovery of Novel Bruton's Tyrosine Kinase PROTACs with Enhanced Selectivity and Cellular Efficacy

Bruton's tyrosine kinase (BTK) is a target for treating B-cell malignancies and autoimmune diseases, and several BTK inhibitors are already approved for use in humans. Heterobivalent BTK protein degraders are also in development, based on the premise that proteolysis targeting chimeras (PROTACs) may provide additional therapeutic benefits. However, most BTK PROTACs are based on the BTK inhibitor ibrutinib raising concerns about their selectivity profiles, given the known off-target effects of ibrutinib. Here, we disclose the discovery and in vitro characterization of BTK PROTACs based on the selective BTK inhibitor GDC-0853 and the cereblon recruitment ligand pomalidomide. PTD10 is a highly potent BTK degrader (DC50 0.5 nM) that inhibited cell growth and induced apoptosis at lower concentrations than the two parent molecules, as well as three previously reported BTK PROTACs, and had improved selectivity compared to ibrutinib-based BTK PROTACs.

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.

Targeted protein degrader development for cancer: advances, challenges, and opportunities

Anticancer-targeted therapies inhibit various kinases implicated in cancer and have been used in clinical settings for decades. However, many cancer-related targets are proteins without catalytic activity and are difficult to target using traditional occupancy-driven inhibitors. Targeted protein degradation (TPD) is an emerging therapeutic modality that has expanded the druggable proteome for cancer treatment. With the entry of new-generation immunomodulatory drugs (IMiDs), selective estrogen receptor degraders (SERDs), and proteolysis-targeting chimera (PROTAC) drugs into clinical trials, the field of TPD has seen explosive growth in the past 10 years. Several challenges remain that need to be tackled to increase successful clinical translation of TPD drugs. We present an overview of the global landscape of clinical trials of TPD drugs over the past decade and summarize the clinical profiles of new-generation TPD drugs. In addition, we highlight the challenges and opportunities for the development of effective TPD drugs for future successful clinical translation.

PROTACs in the Management of Prostate Cancer

Cancer treatments with targeted therapy have gained immense interest due to their low levels of toxicity and high selectivity. Proteolysis-Targeting Chimeras (PROTACs) have drawn special attention in the development of cancer therapeutics owing to their unique mechanism of action, their ability to target undruggable proteins, and their focused target engagement. PROTACs selectively degrade the target protein through the ubiquitin-proteasome system, which describes a different mode of action compared to conventional small-molecule inhibitors or even antibodies. Among different cancer types, prostate cancer (PC) is the most prevalent non-cutaneous cancer in men. Genetic alterations and the overexpression of several genes, such as FOXA1, AR, PTEN, RB1, TP53, etc., suppress the immune response, resulting in drug resistance to conventional drugs in prostate cancer. Since the progression of ARV-110 (PROTAC for PC) into clinical phases, the focus of research has quickly shifted to protein degraders targeting prostate cancer. The present review highlights an overview of PROTACs in prostate cancer and their superiority over conventional inhibitors. We also delve into the underlying pathophysiology of the disease and explain the structural design and linkerology strategies for PROTAC molecules. Additionally, we touch on the various targets for PROTAC in prostate cancer, including the androgen receptor (AR) and other critical oncoproteins, and discuss the future prospects and challenges in this field.

Discovery of novel BTK PROTACs with improved metabolic stability via linker rigidification strategy

Bruton's Tyrosine Kinase (BTK) functions as a key regulator of B-cell receptor (BCR) signaling pathway, which is frequently hyperactivated in a variety of lymphoma cancers. Using Proteolysis Targeting Chimera (PROTAC) technology, we have recently discovered a highly potent ARQ-531-derived BTK PROTAC 6e, inducing effective degradation of both wild type (WT) and C481S mutant BTK proteins. However, the poor metabolic stability of PROTAC 6e have limited its further in vivo studies. Herein, we present our structure-activity relationship (SAR) studies on modifying PROTAC 6e using linker rigidification strategy to identify a novel cereblon (CRBN)-recruiting compound 3e that induced BTK degradation in a concentration-dependent manner but had no effect on reducing the level of CRBN neo-substrates. Moreover, compound 3e suppressed the cell growth more potently than the small molecule inhibitors ibrutinib and ARQ-531 in several cells. Furthermore, compound 3e with the rigid linker displayed a significantly improved metabolic stability profile with the T_1/2 increased to more than 145 min. Overall, we discovered a highly potent and selective BTK PROTAC lead compound 3e, which could be further optimized as potential BTK degradation therapy for BTK-associated human cancers and diseases.

Novel approaches to targeted protein degradation technologies in drug discovery

INTRODUCTION

Target protein degradation (TPD) provides a novel therapeutic modality, other than inhibition, through the direct depletion of target proteins. Two primary human protein homeostasis mechanisms are exploited: the ubiquitin-proteasome system (UPS) and the lysosomal system. TPD technologies based on these two systems are progressing at an impressive pace.

AREAS COVERED

This review focuses on the TPD strategies based on UPS and lysosomal system, mainly classified into three types: Molecular Glue (MG), PROteolysis Targeting Chimera (PROTAC), and lysosome-mediated TPD. Starting with a brief background introduction of each strategy, exciting examples and perspectives on these novel approaches are provided.

EXPERT OPINION

MGs and PROTACs are two major UPS-based TPD strategies that have been extensively investigated in the past decade. Despite some clinical trials, several critical issues remain, among which is emphasized by the limitation of targets. Recently developed lysosomal system-based approaches provide alternative solutions for TPD beyond UPS' capability. The newly emerging novel approaches may partially address issues that have long plagued researchers, such as low potency, poor cell permeability, on-/off-target toxicity, and delivery efficiency. Comprehensive considerations for the rational design of protein degraders and continuous efforts to seek effective solutions are imperative to advance these strategies into clinical medications.

Structural rationalization of GSPT1 and IKZF1 degradation by thalidomide molecular glue derivatives

Thalidomide and its derivatives are molecular glues that bind cereblon (CRBN), a component of an E3 ubiquitin ligase complex, and mediate protein interactions with neosubstrates resulting in their polyubiquitination and proteasomal degradation. The structural features of neosubstrate binding have been elucidated that highlight key interactions with a β-hairpin degron containing a glycine, which is present in a wide-range of proteins, including zinc-finger transcription factors such as IKZF1, and the translation termination factor GSPT1. Here, we profile 14 closely-related thalidomide derivatives in CRBN occupancy, and IKZF1 and GSPT1 degradation cell-based assays, and use crystal structures, computational docking and molecular dynamics to delineate subtle structure-activity relationships. Our findings will enable the rational design of CRBN modulators in the future, and help avoid the degradation of GSPT1 which is broadly cytotoxic.

PROTACS: A technology with a gold rush-like atmosphere

Abnormally expressed or malfunctioning proteins may affect or even damage cells, leading to the onset of diseases. Proteolysis targeting chimera (PROTAC) technology has been proven to be a fresh therapeutic strategy, superior to conventional small molecule inhibitors for the treatment of diseases caused by pathogenic proteins. Unlike conventional small molecule inhibitors that are occupancy-driven, PROTACs are heterobifunctional small molecules with catalytic properties. They combine with E3 ligases and target proteins to form a ternary complex, rendering the target protein ubiquitous and subsequently degraded by the proteasome. This paper focuses first on significant events in the development of PROTAC technology from 2001 to 2022, followed by a brief overview of various PROTACs categorized by target proteins. In addition, the applications of PROTACs in the treatment of diseases and fundamental biology are also under discussion.

GSPT1 Functions as a Tumor Promoter in Human Liver Cancer

OBJECTIVE

This study analyzed the role of G1 to S phase transition 1 protein (GSPT1) in promoting progression of liver cancer cells.

METHODS

A bioinformatics database was used to analyze the expression levels of GSPT1 in liver cancer tissues and the prognosis of patients. Subsequently, Western blotting and quantitative PCR were used to verify the expression levels of GSPT1 between normal hepatocytes and hepatoma cells. We used a CRISPR/Cas9 system to construct knockouts of GSPT1 in HepG2 and HCCLM9 liver cancer cells. The effect of GSPT1 on liver cancer cell migration and invasion was analyzed using flow cytometry, migration, and tumor formation assays.

RESULTS

The Cancer Genome Atlas Liver Hepatocellular Carcinoma dataset indicated that GSPT1 expression was upregulated in liver cancer cell lines, and patients with liver cancer had poor prognosis. Knockout of GSPT1 in cells significantly inhibited tumor proliferation, cell migration, and growth in vivo.

CONCLUSION

In this study, we found that GSPT1 promotes the migration and invasion of liver cancer cells.

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.

Progression of Antiviral Agents Targeting Viral Polymerases

Viral DNA and RNA polymerases are two kinds of very important enzymes that synthesize the genetic materials of the virus itself, and they have become extremely favorable targets for the development of antiviral drugs because of their relatively conserved characteristics. There are many similarities in the structure and function of different viral polymerases, so inhibitors designed for a certain viral polymerase have acted as effective universal inhibitors on other types of viruses. The present review describes the development of classical antiviral drugs targeting polymerases, summarizes a variety of viral polymerase inhibitors from the perspective of chemically synthesized drugs and natural product drugs, describes novel approaches, and proposes promising development strategies for antiviral drugs.

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.

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.

Molecular Glues: Capable Protein-Binding Small Molecules That Can Change Protein-Protein Interactions and Interactomes for the Potential Treatment of Human Cancer and Neurodegenerative Diseases

Molecular glue (MG) compounds are a type of unique small molecule that can change the protein–protein interactions (PPIs) and interactomes by degrading, stabilizing, or activating the target protein after their binging. These small-molecule MGs are gradually being recognized for their potential application in treating human diseases, including cancer. Evidence suggests that small-molecule MG compounds could essentially target any proteins, which play critical roles in human disease etiology, where many of these protein targets were previously considered undruggable. Intriguingly, most MG compounds with high efficacy for cancer treatment can glue on and control multiple key protein targets. On the other hand, a single key protein target can also be glued by multiple MG compounds with distinct chemical structures. The high flexibility of MG–protein interaction profiles provides rich soil for the growth and development of small-molecule MG compounds that can be used as molecular tools to assist in unraveling disease mechanisms, and they can also facilitate drug development for the treatment of human disease, especially human cancer. In this review, we elucidate this concept by using various types of small-molecule MG compounds and their corresponding protein targets that have been documented in the literature.

PROTAC Degraders with Ligands Recruiting MDM2 E3 Ubiquitin Ligase: An Updated Perspective

Mouse double minute 2 (MDM2) is an E3 ubiquitin ligase which effectively degrades tumor suppressor p53. In the past two decades, many MDM2 inhibitors that disrupt the MDM2-p53 binding have been discovered and developed. Given that the MDM2-p53 forms auto-regulatory loop in which p53 is a substrate of MDM2 for targeted degradation, while MDM2 is a p53 target for transcriptional upregulation, these MDM2 inhibitors have limited efficacy due to p53 degradation by accumulated MDM2 upon rapid in vivo clearance of the MDM2 inhibitors. Fortunately, proteolysis targeting chimeras (PROTACs), a novel therapeutic strategy, overcome the limitations of MDM2 inhibitors. Some of MDM2 inhibitors developed in the past two decades have been used in PROTAC technology for two applications: 1) as component 1 to bind with endogenous MDM2 as a target for PROTAC-based degradation of MDM2; and 2) as component 2 to bind with endogenous MDM2 as a PROTAC E3 ligand for PROTAC-based degradation of other oncogenic proteins. In this review, we summarize current progress in the discovery and development of MDM2-based PROTAC drugs with future perspectives and challenges for their applications in effective treatment of human cancer.

Targeted protein degradation: mechanisms, strategies and application

Traditional drug discovery mainly focuses on direct regulation of protein activity. The development and application of protein activity modulators, particularly inhibitors, has been the mainstream in drug development. In recent years, PROteolysis TArgeting Chimeras (PROTAC) technology has emerged as one of the most promising approaches to remove specific disease-associated proteins by exploiting cells’ own destruction machinery. In addition to PROTAC, many different targeted protein degradation (TPD) strategies including, but not limited to, molecular glue, Lysosome-Targeting Chimaera (LYTAC), and Antibody-based PROTAC (AbTAC), are emerging. These technologies have not only greatly expanded the scope of TPD, but also provided fresh insights into drug discovery. Here, we summarize recent advances of major TPD technologies, discuss their potential applications, and hope to provide a prime for both biologists and chemists who are interested in this vibrant field.

An overview of kinase downregulators and recent advances in discovery approaches

Since the clinical approval of imatinib, the discovery of protein kinase downregulators entered a prosperous age. However, challenges still exist in the discovery of kinase downregulator drugs, such as the high failure rate during development, side effects, and drug-resistance problems. With the progress made through multidisciplinary efforts, an increasing number of new approaches have been applied to solve the above problems during the discovery process of kinase downregulators. In terms of in vitro and in vivo drug evaluation, progress was also made in cellular and animal model platforms for better and more clinically relevant drug assessment. Here, we review the advances in drug design strategies, drug property evaluation technologies, and efficacy evaluation models and technologies. Finally, we discuss the challenges and perspectives in the development of kinase downregulator drugs.