Haven't got a glue: Protein surface variation for the design of molecular glue degraders

Dual-site molecular glues for enhancing protein-protein interactions of the CDK12-DDB1 complex

Protein-protein interactions (PPIs) stabilization with molecular glues plays a crucial role in drug discovery, albeit with significant challenges. In this study, we propose a dual-site approach, targeting the PPI region and its dynamic surroundings. We conduct molecular dynamics simulations to identify critical sites on the PPI that stabilize the cyclin-dependent kinase 12 - DNA damage-binding protein 1 (CDK12-DDB1) complex, resulting in further cyclin K degradation. This exploration leads to the creation of LL-K12-18, a dual-site molecular glue, which enhances the glue properties to augment degradation kinetics and efficiency. Notably, LL-K12-18 demonstrates strong inhibition of gene transcription and anti-proliferative effects in tumor cells, showing significant potency improvements in MDA-MB-231 (88-fold) and MDA-MB-468 cells (307-fold) when compared to its precursor compound SR-4835. These findings underscore the potential of dual-site approaches in disrupting CDK12 function and offer a structural insight-based framework for the design of cyclin K molecular glues.

A molecular glue degrader of the WIZ transcription factor for fetal hemoglobin induction

Sickle cell disease (SCD) is a prevalent, life-threatening condition attributable to a heritable mutation in β-hemoglobin. Therapeutic induction of fetal hemoglobin (HbF) can ameliorate disease complications and has been intently pursued. However, safe and effective small-molecule inducers of HbF remain elusive. We report the discovery of dWIZ-1 and dWIZ-2, molecular glue degraders of the WIZ transcription factor that robustly induce HbF in erythroblasts. Phenotypic screening of a cereblon (CRBN)-biased chemical library revealed WIZ as a previously unknown repressor of HbF. WIZ degradation is mediated by recruitment of WIZ(ZF7) to CRBN by dWIZ-1, as resolved by crystallography of the ternary complex. Pharmacological degradation of WIZ was well tolerated and induced HbF in humanized mice and cynomolgus monkeys. These findings establish WIZ degradation as a globally accessible therapeutic strategy for SCD.

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.

Molecular glues for protein-protein interactions: Progressing toward a new dream

The modulation of protein-protein interactions with small molecules is one of the most rapidly developing areas in drug discovery. In this review, we discuss advances over the past decade (2014-2023) focusing on molecular glues (MGs)-monovalent small molecules that induce proximity, either by stabilizing native interactions or by inducing neomorphic interactions. We include both serendipitous and rational discoveries and describe the different approaches that were used to identify them. We classify the compounds in three main categories: degradative MGs, non-degradative MGs or PPI stabilizers, and MGs that induce self-association. Diverse, illustrative examples with structural data are described in detail, emphasizing the elements of molecular recognition and cooperative binding at the interface that are fundamental for a MG mechanism of action.

Artemisinins ameliorate polycystic ovarian syndrome by mediating LONP1-CYP11A1 interaction

Polycystic ovary syndrome (PCOS), a prevalent reproductive disorder in women of reproductive age, features androgen excess, ovulatory dysfunction, and polycystic ovaries. Despite its high prevalence, specific pharmacologic intervention for PCOS is challenging. In this study, we identified artemisinins as anti-PCOS agents. Our finding demonstrated the efficacy of artemisinin derivatives in alleviating PCOS symptoms in both rodent models and human patients, curbing hyperandrogenemia through suppression of ovarian androgen synthesis. Artemisinins promoted cytochrome P450 family 11 subfamily A member 1 (CYP11A1) protein degradation to block androgen overproduction. Mechanistically, artemisinins directly targeted lon peptidase 1 (LONP1), enhanced LONP1-CYP11A1 interaction, and facilitated LONP1-catalyzed CYP11A1 degradation. Overexpression of LONP1 replicated the androgen-lowering effect of artemisinins. Our data suggest that artemisinin application is a promising approach for treating PCOS and highlight the crucial role of the LONP1-CYP11A1 interaction in controlling hyperandrogenism and PCOS occurrence.

Small molecule induced STING degradation facilitated by the HECT ligase HERC4

Stimulator of interferon genes (STING) is a central component of the cytosolic nucleic acids sensing pathway and as such master regulator of the type I interferon response. Due to its critical role in physiology and its' involvement in a variety of diseases, STING has been a focus for drug discovery. Targeted protein degradation (TPD) has emerged as a promising pharmacology for targeting previously considered undruggable proteins by hijacking the cellular ubiquitin proteasome system (UPS) with small molecules. Here, we identify AK59 as a STING degrader leveraging HERC4, a HECT-domain E3 ligase. Additionally, our data reveals that AK59 is effective on the common pathological STING mutations, suggesting a potential clinical application of this mechanism. Thus, these findings introduce HERC4 to the fields of TPD and of compound-induced degradation of STING, suggesting potential therapeutic applications.

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.

Protein degraders - from thalidomide to new PROTACs

Recently, the development of protein degraders (protein-degrading compounds) has prominently progressed. There are two remarkable classes of protein degraders: proteolysis-targeting chimeras (PROTACs) and molecular glue degraders (MGDs). Almost 70 years have passed since thalidomide was initially developed as a sedative-hypnotic drug, which is currently recognized as one of the most well-known MGDs. During the last two decades, a myriad of PROTACs and MGDs have been developed, and the molecular mechanism of action (MOA) of thalidomide was basically elucidated, including identifying its molecular target cereblon (CRBN). CRBN forms a Cullin Ring Ligase 4 with Cul4 and DDB1, whose substrate specificity is controlled by its binding ligands. Thalidomide, lenalidomide and pomalidomide, three CRBN-binding MGDs, were clinically approved to treat several intractable diseases (including multiple myeloma). Several other MGDs and CRBN-based PROTACs (ARV-110 and AVR-471) are undergoing clinical trials. In addition, several new related technologies regarding PROTACs and MGDs have also been developed, and achievements of protein degraders impact not only therapeutic fields but also basic biological science. In this article, I introduce the history of protein degraders, from the development of thalidomide to the latest PROTACs and related technologies.

Crbn-based molecular Glues: Breakthroughs and perspectives

CRBN is a substrate receptor for the Cullin Ring E3 ubiquitin ligase 4 (CRL4) complex. It has been observed that CRBN can be exploited by small molecules to facilitate the recruitment and ubiquitination of non-natural CRL4 substrates, resulting in the degradation of neosubstrate through the ubiquitin-proteasome system. This phenomenon, known as molecular glue-induced protein degradation, has emerged as an innovative therapeutic approach in contrast to traditional small-molecule drugs. One key advantage of molecular glues, in comparison to conventional small-molecule drugs adhering to Lipinski's Rule of Five, is their ability to operate without the necessity for specific binding pockets on target proteins. This unique characteristic empowers molecular glues to interact with conventionally intractable protein targets, such as transcription factors and scaffold proteins. The ability to induce the degradation of these previously elusive targets by hijacking the ubiquitin-proteasome system presents a promising avenue for the treatment of recalcitrant diseases. Nevertheless, the rational design of molecular glues remains a formidable challenge due to the limited understanding of their mechanisms and actions. This review offers an overview of recent advances and breakthroughs in the field of CRBN-based molecular glues, while also exploring the prospects for a systematic approach to designing these compounds.

SuFEx-based chemical diversification for the systematic discovery of CRBN molecular glues

Molecular glues are small molecules that stabilize protein-protein interactions, enabling new molecular pharmacologies, such as targeted protein degradation. They offer advantages over proteolysis targeting chimeras (PROTACs), which present challenges associated with the size and properties of heterobifunctional constructions, but glues lack the rational design principles analogous to PROTACs. One notable exception is the ability to alter the structure of Cereblon (CRBN)-based molecular glues and redirect their activity toward new neo-substrate proteins. We took a focused approach toward modifying the CRBN ligand, 5'-amino lenalidomide, to alter its neo-substrate specificity using high-throughput chemical diversification by parallelized sulfur(VI)-fluoride exchange (SuFEx) transformations. We synthesized over 3,000 analogs of 5'-amino lenalidomide using this approach and screened the crude products using a phenotypic screen for cell viability, identifying dozens of analogs with differentiated activity. We characterized four compounds that degrade G-to-S phase transition 1 (GSPT1) protein, providing a proof-of-concept model for SuFEx-based discovery of CRBN molecular glues.

Molecular hybridization: a powerful tool for multitarget drug discovery

INTRODUCTION

The current drug discovery paradigm of 'one drug, multiple targets' has gained attention from both the academic medicinal chemistry community and the pharmaceutical industry. This is in response to the urgent need for effective agents to treat multifactorial chronic diseases. The molecular hybridization strategy is a useful tool that has been widely explored, particularly in the last two decades, for the design of multi-target drugs.

AREAS COVERED

This review examines the current state of molecular hybridization in guiding the discovery of multitarget small molecules. The article discusses the design strategies and target selection for a multitarget polypharmacology approach to treat various diseases, including cancer, Alzheimer's disease, cardiac arrhythmia, endometriosis, and inflammatory diseases.

EXPERT OPINION

Although the examples discussed highlight the importance of molecular hybridization for the discovery of multitarget bioactive compounds, it is notorious that the literature has focused on specific classes of targets. This may be due to a deep understanding of the pharmacophore features required for target binding, making targets such as histone deacetylases and cholinesterases frequent starting points. However, it is important to encourage the scientific community to explore diverse combinations of targets using the molecular hybridization strategy.

C2H2 Zinc Finger Transcription Factors Associated with Hemoglobinopathies

In humans, specific aberrations in β-globin results in sickle cell disease and β-thalassemia, symptoms of which can be ameliorated by increased expression of fetal globin (HbF). Two recent CRISPR-Cas9 screens, centered on ∼1500 annotated sequence-specific DNA binding proteins and performed in a human erythroid cell line that expresses adult hemoglobin, uncovered four groups of candidate regulators of HbF gene expression. They are (1) members of the nucleosome remodeling and deacetylase (NuRD) complex proteins that are already known for HbF control; (2) seven C2H2 zinc finger (ZF) proteins, including some (ZBTB7A and BCL11A) already known for directly silencing the fetal γ-globin genes in adult human erythroid cells; (3) a few other transcription factors of different structural classes that might indirectly influence HbF gene expression; and (4) DNA methyltransferase 1 (DNMT1) that maintains the DNA methylation marks that attract the MBD2-associated NuRD complex to DNA as well as associated histone H3 lysine 9 methylation. Here we briefly discuss the effects of these regulators, particularly C2H2 ZFs, in inducing HbF expression for treating β-hemoglobin disorders, together with recent advances in developing safe and effective small-molecule therapeutics for the regulation of this well-conserved hemoglobin switch.

Molecular glue degraders: exciting opportunities for novel drug discovery

INTRODUCTION

Molecular Glue Degraders (MGDs) is a concept that refers to a class of compounds that facilitate the interaction between two proteins or molecules within a cell. These compounds act as bridge that enhances specific Protein-Protein Interactions (PPIs). Over the past decade, this technology has gained attention as a potential strategy to target proteins that were traditionally considered undruggable using small molecules.

AREAS COVERED

This review presents the concept of cellular homeostasis and the balance between protein synthesis and protein degradation. The concept of protein degradation is concerned with molecular glues, which form part of the broader field of Targeted Protein Degradation (TPD). Next, pharmacochemical strategies for the rational design of MGDs are detailed and illustrated by examples of Ligand-Based (LBDD), Structure-Based (SBDD) and Fragment-Based Drug Design (FBDD).

EXPERT OPINION

Expanding the scope of what can be effectively targeted in the development of treatments for diseases that are incurable or resistant to conventional therapies offers new therapeutic options. The treatment of microbial infections and neurodegenerative diseases is a major societal challenge, and the discovery of MGDs appears to be a promising avenue. Combining different approaches to discover and exploit a variety of innovative therapeutic agents will create opportunities to treat diseases that are still incurable.

Screening for molecular glues - Challenges and opportunities

Molecular glues are small molecules, typically smaller than PROTACs, and usually with improved physicochemical properties that aim to stabilise the interaction between two proteins. Most often this approach is used to improve or induce an interaction between the target and an E3 ligase, but other interactions which stabilise interactions to increase activity or to inhibit binding to a natural effector have also been demonstrated. This review will describe the effects of induced proximity, discuss current methods used to identify molecular glues and introduce approaches that could be adapted for molecular glue screening.

TAC-tics for Leveraging Proximity Biology in Drug Discovery

Chemically induced proximity (CIP) refers to co-opting naturally occurring biological pathways using synthetic molecules to recruit neosubstrates that are not normally encountered or to enhance the affinity of naturally occurring interactions. Leveraging proximity biology through CIPs has become a rapidly evolving field and has garnered considerable interest in basic research and drug discovery. PROteolysis TArgeting Chimera (PROTAC) is a well-established CIP modality that induces the proximity between a target protein and an E3 ubiquitin ligase, causing target protein degradation via the ubiquitin-proteasome system. Inspired by PROTACs, several other induced proximity modalities have emerged to modulate both proteins and RNA over recent years. In this review, we summarize the critical advances and opportunities in the field, focusing on protein degraders, RNA degraders and non-degrader modalities such as post-translational modification (PTM) and protein-protein interaction (PPI) modulators. We envision that these emerging proximity-based drug modalities will be valuable resources for both biological research and therapeutic discovery in the future.

Proteolysis-targeting chimeras with reduced off-targets

Proteolysis-targeting chimeras (PROTACs) are molecules that induce proximity between target proteins and E3 ligases triggering target protein degradation. Pomalidomide, a widely used E3 ligase recruiter in PROTACs, can independently degrade other proteins, including zinc-finger (ZF) proteins, with vital roles in health and disease. This off-target degradation hampers the therapeutic applicability of pomalidomide-based PROTACs, requiring development of PROTAC design rules that minimize off-target degradation. Here we developed a high-throughput platform that interrogates off-target degradation and found that reported pomalidomide-based PROTACs induce degradation of several ZF proteins. We generated a library of pomalidomide analogues to understand how functionalizing different positions of the phthalimide ring, hydrogen bonding, and steric and hydrophobic effects impact ZF protein degradation. Modifications of appropriate size on the C5 position reduced off-target ZF degradation, which we validated through target engagement and proteomics studies. By applying these design principles, we developed anaplastic lymphoma kinase oncoprotein-targeting PROTACs with enhanced potency and minimal off-target degradation.

Degradation by Design: New Cyclin K Degraders from Old CDK Inhibitors

Small molecules that induce protein degradation hold the potential to overcome several limitations of the currently available inhibitors. Monovalent or molecular glue degraders, in particular, enable the benefits of protein degradation without the disadvantages of high molecular weight and the resulting challenge in drug development that are associated with bivalent molecules like Proteolysis Targeting Chimeras. One key challenge in designing monovalent degraders is how to build in the degrader activity─how can we convert an inhibitor into a degrader? If degradation activity requires very specific molecular features, it will be difficult to find new degraders and challenging to optimize those degraders toward drugs. Herein, we demonstrate that an unexpectedly wide range of modifications to the degradation-inducing group of the cyclin K degrader CR8 are tolerated, including both aromatic and nonaromatic groups. We used these findings to convert the pan-CDK inhibitors dinaciclib and AT-7519 to Cyclin K degraders, leading to a novel dinaciclib-based compound with improved degradation activity compared to CR8 and confirm the mechanism of degradation. These results suggest that general design principles can be generated for the development and optimization of monovalent degraders.

Ubiquiton-An inducible, linkage-specific polyubiquitylation tool

The posttranslational modifier ubiquitin regulates most cellular processes. Its ability to form polymeric chains of distinct linkages is key to its diverse functionality. Yet, we still lack the experimental tools to induce linkage-specific polyubiquitylation of a protein of interest in cells. Here, we introduce a set of engineered ubiquitin protein ligases and matching ubiquitin acceptor tags for the rapid, inducible linear (M1-), K48-, or K63-linked polyubiquitylation of proteins in yeast and mammalian cells. By applying the so-called "Ubiquiton" system to proteasomal targeting and the endocytic pathway, we validate this tool for soluble cytoplasmic and nuclear as well as chromatin-associated and integral membrane proteins and demonstrate how it can be used to control the localization and stability of its targets. We expect that the Ubiquiton system will serve as a versatile, broadly applicable research tool to explore the signaling functions of polyubiquitin chains in many biological contexts.

Selective CK1α degraders exert antiproliferative activity against a broad range of human cancer cell lines

Molecular-glue degraders are small molecules that induce a specific interaction between an E3 ligase and a target protein, resulting in the target proteolysis. The discovery of molecular glue degraders currently relies mostly on screening approaches. Here, we describe screening of a library of cereblon (CRBN) ligands against a panel of patient-derived cancer cell lines, leading to the discovery of SJ7095, a potent degrader of CK1α, IKZF1 and IKZF3 proteins. Through a structure-informed exploration of structure activity relationship (SAR) around this small molecule we develop SJ3149, a selective and potent degrader of CK1α protein in vitro and in vivo. The structure of SJ3149 co-crystalized in complex with CK1α + CRBN + DDB1 provides a rationale for the improved degradation properties of this compound. In a panel of 115 cancer cell lines SJ3149 displays a broad antiproliferative activity profile, which shows statistically significant correlation with MDM2 inhibitor Nutlin-3a. These findings suggest potential utility of selective CK1α degraders for treatment of hematological cancers and solid tumors.

Discovery of a Potent Proteolysis Targeting Chimera Enables Targeting the Scaffolding Functions of FK506-Binding Protein 51 (FKBP51)

The FK506-binding protein 51 (FKBP51) is a promising target in a variety of disorders including depression, chronic pain, and obesity. Previous FKBP51-targeting strategies were restricted to occupation of the FK506-binding site, which does not affect core functions of FKBP51. Here, we report the discovery of the first FKBP51 proteolysis targeting chimera (PROTAC) that enables degradation of FKBP51 abolishing its scaffolding function. Initial synthesis of 220 FKBP-focused PROTACs yielded a plethora of active PROTACs for FKBP12, six for FKBP51, and none for FKBP52. Structural analysis of a binary FKBP12:PROTAC complex revealed the molecular basis for negative cooperativity. Linker-based optimization of first generation FKBP51 PROTACs led to the PROTAC SelDeg51 with improved cellular activity, selectivity, and high cooperativity. The structure of the ternary FKBP51:SelDeg51:VCB complex revealed how SelDeg51 establishes cooperativity by dimerizing FKBP51 and the von Hippel-Lindau protein (VHL) in a glue-like fashion. SelDeg51 efficiently depletes FKBP51 and reactivates glucocorticoid receptor (GR)-signalling, highlighting the enhanced efficacy of full protein degradation compared to classical FKBP51 binding.

Serendipitous discovery of Class I HDAC inhibitors from rational design of molecular glue degraders targeting HDAC

Zinc-dependent histone deacetylases (HDACs) play an essential role as epigenetic regulators and are becoming increasingly important drug targets for the treatment of cancer. Although five HDAC inhibitors have been approved for treating several cancers, only one of them is a Class I HDAC inhibitor, which may have advantages over pan-HDAC inhibitors due to the various side effects associated with the latter. On the other hand, the emerging strategy of molecular glue degraders offers a unique advantage for targeting therapeutic proteins. In this study, we synthesized a series of candidate compounds based on the molecule glue, pomalidomide, using a "merger principle", initially aiming to obtain molecular glue degraders that can target HDAC degradation. However, we serendipitously discovered that compounds 2f and 3f may be potent Class I HDAC selective inhibitors. After further evaluation, we found that compounds 2f and 3f exhibit selective inhibitory effects on Class I HDAC in cancer cells. Moreover, they showed the robust antiproliferative activities against various hematological tumor cells, comparable to that of the approved Class I HDAC inhibitor, Chidamide. These results suggest that pomalidomide-derivatized compounds have promising potential as Class I HDAC inhibitors with therapeutic applications in cancer treatment.

Induced protein degradation for therapeutics: past, present, and future

The concept of induced protein degradation by small molecules has emerged as a promising therapeutic strategy that is particularly effective in targeting proteins previously considered "undruggable." Thalidomide analogs, employed in the treatment of multiple myeloma, stand as prime examples. These compounds serve as molecular glues, redirecting the CRBN E3 ubiquitin ligase to degrade myeloma-dependency factors, IKZF1 and IKZF3. The clinical success of thalidomide analogs demonstrates the therapeutic potential of induced protein degradation. Beyond molecular glue degraders, several additional modalities to trigger protein degradation have been developed and are currently under clinical evaluation. These include heterobifunctional degraders, polymerization-induced degradation, ligand-dependent degradation of nuclear hormone receptors, disruption of protein interactions, and various other strategies. In this Review, we will provide a concise overview of various degradation modalities, their clinical applications, and potential future directions in the field of protein degradation.

Design principles for cyclin K molecular glue degraders

Molecular glue degraders are an effective therapeutic modality, but their design principles are not well understood. Recently, several unexpectedly diverse compounds were reported to deplete cyclin K by linking CDK12-cyclin K to the DDB1-CUL4-RBX1 E3 ligase. Here, to investigate how chemically dissimilar small molecules trigger cyclin K degradation, we evaluated 91 candidate degraders in structural, biophysical and cellular studies and reveal all compounds acquire glue activity via simultaneous CDK12 binding and engagement of DDB1 interfacial residues, in particular Arg928. While we identify multiple published kinase inhibitors as cryptic degraders, we also show that these glues do not require pronounced inhibitory properties for activity and that the relative degree of CDK12 inhibition versus cyclin K degradation is tuneable. We further demonstrate cyclin K degraders have transcriptional signatures distinct from CDK12 inhibitors, thereby offering unique therapeutic opportunities. The systematic structure-activity relationship analysis presented herein provides a conceptual framework for rational molecular glue design.

Discovery of Molecular Glue Degraders via Isogenic Morphological Profiling

Molecular glue degraders (MGDs) are small molecules that degrade proteins of interest via the ubiquitin-proteasome system. While MGDs were historically discovered serendipitously, approaches for MGD discovery now include cell-viability-based drug screens or data mining of public transcriptomics and drug response datasets. These approaches, however, have target spaces restricted to the essential proteins. Here we develop a high-throughput workflow for MGD discovery that also reaches the nonessential proteome. This workflow begins with the rapid synthesis of a compound library by sulfur(VI) fluoride exchange chemistry coupled to a morphological profiling assay in isogenic cell lines that vary in levels of the E3 ligase CRBN. By comparing the morphological changes induced by compound treatment across the isogenic cell lines, we were able to identify FL2-14 as a CRBN-dependent MGD targeting the nonessential protein GSPT2. We envision that this workflow would contribute to the discovery and characterization of MGDs that target a wider range of proteins.

It's a TRIM-endous view from the top: the varied roles of TRIpartite Motif proteins in brain development and disease

The tripartite motif (TRIM) protein family members have been implicated in a multitude of physiologies and pathologies in different tissues. With diverse functions in cellular processes including regulation of signaling pathways, protein degradation, and transcriptional control, the impact of TRIM dysregulation can be multifaceted and complex. Here, we focus on the cellular and molecular roles of TRIMs identified in the brain in the context of a selection of pathologies including cancer and neurodegeneration. By examining each disease in parallel with described roles in brain development, we aim to highlight fundamental common mechanisms employed by TRIM proteins and identify opportunities for therapeutic intervention.

Gluing the pieces together

Illuminating the path to degrading troublesome proteins.

Inducible mismatch repair streamlines forward genetic approaches to target identification of cytotoxic small molecules

Orphan cytotoxins are small molecules for which the mechanism of action (MoA) is either unknown or ambiguous. Unveiling the mechanism of these compounds may lead to useful tools for biological investigation and new therapeutic leads. In selected cases, the DNA mismatch repair-deficient colorectal cancer cell line, HCT116, has been used as a tool in forward genetic screens to identify compound-resistant mutations, which have ultimately led to target identification. To expand the utility of this approach, we engineered cancer cell lines with inducible mismatch repair deficits, thus providing temporal control over mutagenesis. By screening for compound resistance phenotypes in cells with low or high rates of mutagenesis, we increased both the specificity and sensitivity of identifying resistance mutations. Using this inducible mutagenesis system, we implicate targets for multiple orphan cytotoxins, including a natural product and compounds emerging from a high-throughput screen, thus providing a robust tool for future MoA studies.

Fine-tuning chemical genetics to identify physiologic drug targets

Genetics-based approaches can enable drug target identification in human cells. In this issue of Cell Chemical Biology, Nguyen et al.1 use inducible degradation of a mismatch repair protein to tune the mutation rate in HCT116 cells, thereby increasing sensitivity and selectivity in identifying resistance-conferring mutations for several cytotoxic small molecules.

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.

A platform for the rapid synthesis of molecular glues (Rapid-Glue) under miniaturized conditions for direct biological screening

Molecular glues, functioning via inducing degradation of the target protein while having similar molecular weight as traditional small molecule drugs, are emerging as a promising modality for the development of therapeutic agents. However, the development of molecular glues is limited by the lack of general principles and systematic methods. Not surprisingly, most molecular glues have been identified serendipitously or through phenotypic screening of large libraries. However, the preparation of large and diverse molecular glue libraries is not an easy task and requires extensive resources. We previously developed platforms for rapid synthesis of proteolysis targeting chimeras (PROTACs) that can be used directly for biological screening with minimal resources. Herein, we report a platform of rapid synthesis of molecular glues (Rapid-Glue) via a micromolar scale coupling reaction between hydrazide motif on the E3 ligase ligands and commercially available aldehydes with diverse structures. A pilot library of 1520 compounds is generated under miniaturized conditions in a high throughput manner without any further manipulation including purification after the synthesis. Through this platform, we identified two highly selective GSPT1 molecular glues through direct screening in cell-based assays. Three additional analogues were prepared from readily available starting materials by replacing the hydrolytic labile acylhydrazone linker with a more stable amide linker based on the two hits. All three analogues showed significant GSPT1 degradation activity and two of them possess comparable activity to the corresponding hit. The feasibility of our strategy is thus verified. Further studies by increasing the diversity and size of the library followed by appropriate assays will likely yield distinct molecular glues targeting novel neo-substrates.

Salinization Dramatically Enhance the Anti-Prostate Cancer Efficacies of AR/AR-V7 and Mnk1/2 Molecular Glue Degraders, Galeterone and VNPP433-3β Which Outperform Docetaxel and Enzalutamide in CRPC CWR22Rv1 Xenograft Mouse Model

Galeterone, 3β-(hydroxy)-17-(1H-benzimidazole-1-yl)androsta-5,16-diene (Gal, 1) and VNPP433-3β, 3β-(1H-imidazole-1-yl-17-(1H-benzimidazole-1-yl)androsta-5,16-diene (2) are potent molecular glue degrader modulators of AR/AR-V7 and Mnk1/2-eIF4E signaling pathways, and are promising Phase 3 and Phase 1 drug candidates, respectively. Because appropriate salts can be utilized to create new chemical entities with enhanced aqueous solubility, in vivo pharmacokinetics, and enhanced in vitro and in vivo efficacies, the monohydrochloride salt of Gal (3) and the mono- and di-hydrochlorides salts of compound 2, compounds 4 and 5, respectively, were synthesized. The salts were characterized using 1H NMR, 13C NMR and HRMS analyses. Compound 3 displayed enhanced in vitro antiproliferative activity (7.4-fold) against three prostate cancer cell lines but surprisingly decreased plasma exposure in the pharmacokinetics study. The antiproliferative activities of the compound 2 salts (4 and 5) were equivalent to that of compound 2, but their oral pharmacokinetic profiles were significantly enhanced. Finally, and most importantly, oral administration of the parent compounds (1 and 2) and their corresponding salts (3, 4 and 5) caused dose-dependent potent inhibition/regression of aggressive and difficult-to-treat CWR22Rv1 tumor xenografts growth, with no apparent host toxicities and were highly more efficacious than the blockbuster FDA-approved prostate cancer drugs, Enzalutamide (Xtandi) and Docetaxel (Taxotere). Thus, the HCl salts of Gal (3) and VNPP433-3β (4 and 5) are excellent orally bioavailable candidates for clinical development.

Direct Modulators of K-Ras-Membrane Interactions

Protein-membrane interactions (PMIs) are ubiquitous in cellular signaling. Initial steps of signal transduction cascades often rely on transient and dynamic interactions with the inner plasma membrane leaflet to populate and regulate signaling hotspots. Methods to target and modulate these interactions could yield attractive tool compounds and drug candidates. Here, we demonstrate that the conjugation of a medium-chain lipid tail to the covalent K-Ras(G12C) binder MRTX849 at a solvent-exposed site enables such direct modulation of PMIs. The conjugated lipid tail interacts with the tethered membrane and changes the relative membrane orientation and conformation of K-Ras(G12C), as shown by molecular dynamics (MD) simulation-supported NMR studies. In cells, this PMI modulation restricts the lateral mobility of K-Ras(G12C) and disrupts nanoclusters. The described strategy could be broadly applicable to selectively modulate transient PMIs.

Clinical Translation of Targeted Protein Degraders

Targeted protein degradation (TPD) has emerged as a potentially transformational therapeutic modality with considerable promise. Molecular glue degraders remodel the surface of E3 ligases inducing interactions with neosubstrates resulting in their polyubiquitination and proteasomal degradation. Molecular glues are clinically precedented and have demonstrated the ability to degrade proteins-of-interest (POIs) previously deemed undruggable due to the absence of a traditional small molecule binding pocket. Heterobifunctional proteolysis targeting chimeras (PROTACs) possess ligands for an E3 complex and the POIs, which are chemically linked together, and similarly hijack the ubiquitin machinery to deplete the target. There has been a recent surge in the number of degraders entering clinical trials, particularly directed toward cancer. Nearly all utilize CRL4CRBN as the E3 ligase, and a relatively limited diversity of POIs are currently targeted. In this review, we provide an overview of the degraders in clinical trials and provide a perspective on the lessons learned from their development and emerging human data that will be broadly useful to those working in the TPD field.

The rise of degrader drugs

The cancer genomics revolution has served up a plethora of promising and challenging targets for the drug discovery community. The field of targeted protein degradation (TPD) uses small molecules to reprogram the protein homeostasis system to destroy desired target proteins. In the last decade, remarkable progress has enabled the rational development of degraders for a large number of target proteins, with over 20 molecules targeting more than 12 proteins entering clinical development. While TPD has been fully credentialed by the prior development of immunomodulatory drug (IMiD) class for the treatment of multiple myeloma, the field is poised for a "Gleevec moment" in which robust clinical efficacy of a rationally developed novel degrader against a preselected target is firmly established. Here, we endeavor to provide a high-level evaluation of exciting developments in the field and comment on steps that may realize the full potential of this new therapeutic modality.

Molecular Glue Discovery: Current and Future Approaches

The intracellular interactions of biomolecules can be maneuvered to redirect signaling, reprogram the cell cycle, or decrease infectivity using only a few dozen atoms. Such "molecular glues," which can drive both novel and known interactions between protein partners, represent an enticing therapeutic strategy. Here, we review the methods and approaches that have led to the identification of small-molecule molecular glues. We first classify current FDA-approved molecular glues to facilitate the selection of discovery methods. We then survey two broad discovery method strategies, where we highlight the importance of factors such as experimental conditions, software packages, and genetic tools for success. We hope that this curation of methodologies for directed discovery will inspire diverse research efforts targeting a multitude of human diseases.

Expanding Chemical Probe Space: Quality Criteria for Covalent and Degrader Probes

Within druggable target space, new small-molecule modalities, particularly covalent inhibitors and targeted degraders, have expanded the repertoire of medicinal chemists. Molecules with such modes of action have a large potential not only as drugs but also as chemical probes. Criteria have previously been established to describe the potency, selectivity, and properties of small-molecule probes that are qualified to enable the interrogation and validation of drug targets. These definitions have been tailored to reversibly acting modulators but fall short in their applicability to other modalities. While initial guidelines have been proposed, we delineate here a full set of criteria for the characterization of covalent, irreversible inhibitors as well as heterobifunctional degraders ("proteolysis-targeting chimeras", or PROTACs) and molecular glue degraders. We propose modified potency and selectivity criteria compared to those for reversible inhibitors. We discuss their relevance and highlight examples of suitable probe and pathfinder compounds.

Affinity and cooperativity modulate ternary complex formation to drive targeted protein degradation

Targeted protein degradation via "hijacking" of the ubiquitin-proteasome system using proteolysis targeting chimeras (PROTACs) has evolved into a novel therapeutic modality. The design of PROTACs is challenging; multiple steps involved in PROTAC-induced degradation make it difficult to establish coherent structure-activity relationships. Herein, we characterize PROTAC-mediated ternary complex formation and degradation by employing von Hippel-Lindau protein (VHL) recruiting PROTACs for two different target proteins, SMARCA2 and BRD4. Ternary-complex attributes and degradation activity parameters are evaluated by varying components of the PROTAC's architecture. Ternary complex binding affinity and cooperativity correlates well with degradation potency and initial rates of degradation. Additionally, we develop a ternary-complex structure modeling workflow to calculate the total buried surface area at the interface, which is in agreement with the measured ternary complex binding affinity. Our findings establish a predictive framework to guide the design of potent degraders.

Targeted protein degradation might present a novel therapeutic approach in the fight against African trypanosomiasis

African trypanosomiasis (AT) is a hemoparasitic disease caused by infection with African trypanosomes and it is prevalent in many sub-Saharan African countries, affecting both humans and domestic animals. The disease is transmitted mostly by haematophagous insects of the genus Glossina while taking blood meal, in the process spreading the parasites from an infected animal to an uninfected animal. The disease is fatal if untreated, and the available drugs are generally ineffective and resulting in toxicities. Therefore, it is still pertinent to explore novel methods and targets for drug discovery. Proteolysis-targeting chimeras (PROTACs) present a new strategy for development of therapeutic molecules that mimic cellular proteasomal-mediated protein degradation to target proteins involved in different disease types. PROTACs have been used to degrade proteins involved in various cancers, neurodegenerative diseases, and immune disorders with remarkable success. Here, we highlight the problems associated with the current treatments for AT, discuss the concept of PROTACs and associated targeted protein degradation (TPD) approaches, and provide some insights on the future potential for the use of these emerging technologies (PROTACs and TPD) for the development of new generation of anti-Trypanosoma drugs and the first "TrypPROTACs".

A deep dive into degrader-induced protein-protein interfaces

Targeted protein degradation (TPD) relies on a comprehensive understanding of interfaces between hijacked E3 ligases and their substrates. In vitro techniques often do not capture the interaction dynamics. Recently, Hanzl et al. introduced deep mutational scanning (DMS) in combination with structural and biochemical approaches to identify residues crucial for degrader activity.

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.

Protein-protein interfaces in molecular glue-induced ternary complexes: classification, characterization, and prediction

Molecular glues are a class of small molecules that stabilize the interactions between proteins. Naturally occurring molecular glues are present in many areas of biology where they serve as central regulators of signaling pathways. Importantly, several clinical compounds act as molecular glue degraders that stabilize interactions between E3 ubiquitin ligases and target proteins, leading to their degradation. Molecular glues hold promise as a new generation of therapeutic agents, including those molecular glue degraders that can redirect the protein degradation machinery in a precise way. However, rational discovery of molecular glues is difficult in part due to the lack of understanding of the protein-protein interactions they stabilize. In this review, we summarize the structures of known molecular glue-induced ternary complexes and the interface properties. Detailed analysis shows different mechanisms of ternary structure formation. Additionally, we also review computational approaches for predicting protein-protein interfaces and highlight the promises and challenges. This information will ultimately help inform future approaches for rational molecular glue discovery.

Functional E3 ligase hotspots and resistance mechanisms to small-molecule degraders

Targeted protein degradation is a novel pharmacology established by drugs that recruit target proteins to E3 ubiquitin ligases. Based on the structure of the degrader and the target, different E3 interfaces are critically involved, thus forming defined 'functional hotspots'. Understanding disruptive mutations in functional hotspots informs on the architecture of the assembly, and highlights residues susceptible to acquire resistance phenotypes. Here we employ haploid genetics to show that hotspot mutations cluster in substrate receptors of hijacked ligases, where mutation type and frequency correlate with gene essentiality. Intersection with deep mutational scanning revealed hotspots that are conserved or specific for chemically distinct degraders and targets. Biophysical and structural validation suggests that hotspot mutations frequently converge on altered ternary complex assembly. Moreover, we validated hotspots mutated in patients that relapse from degrader treatment. In sum, we present a fast and widely accessible methodology to characterize small-molecule degraders and associated resistance mechanisms.

Strategies for Conditional Regulation of Proteins

Design of the next-generation of therapeutics, biosensors, and molecular tools for basic research requires that we bring protein activity under control. Each protein has unique properties, and therefore, it is critical to tailor the current techniques to develop new regulatory methods and regulate new proteins of interest (POIs). This perspective gives an overview of the widely used stimuli and synthetic and natural methods for conditional regulation of proteins.

Inducible mismatch repair streamlines forward genetic approaches to target identification of cytotoxic small molecules

Orphan cytotoxins are small molecules for which the mechanism of action (MoA) is either unknown or ambiguous. Unveiling the mechanism of these compounds may lead to useful tools for biological investigation and in some cases, new therapeutic leads. In select cases, the DNA mismatch repair-deficient colorectal cancer cell line, HCT116, has been used as a tool in forward genetic screens to identify compound-resistant mutations, which have ultimately led to target identification. To expand the utility of this approach, we engineered cancer cell lines with inducible mismatch repair deficits, thus providing temporal control over mutagenesis. By screening for compound resistance phenotypes in cells with low or high rates of mutagenesis, we increased both the specificity and sensitivity of identifying resistance mutations. Using this inducible mutagenesis system, we implicate targets for multiple orphan cytotoxins, including a natural product and compounds emerging from a high-throughput screen, thus providing a robust tool for future MoA studies.

Targeted AURKA degradation: Towards new therapeutic agents for neuroblastoma

Aurora kinase A (AURKA) is a well-established target in neuroblastoma (NB) due to both its catalytic functions during mitosis and its kinase-independent functions, including stabilization of the key oncoprotein MYCN. We present a structure-activity relationship (SAR) study of MK-5108-derived PROTACs against AURKA by exploring different linker lengths and exit vectors on the thalidomide moiety. PROTAC SK2188 induces the most potent AURKA degradation (DC_50,24h 3.9 nM, D_max,24h 89%) and shows an excellent binding and degradation selectivity profile. Treatment of NGP neuroblastoma cells with SK2188 induced concomitant MYCN degradation, high replication stress/DNA damage levels and apoptosis. Moreover, SK2188 significantly outperforms the parent inhibitor MK-5108 in a cell proliferation screen and patient-derived organoids. Furthermore, altering the attachment point of the PEG linker to the 5-position of thalidomide allowed us to identify a potent AURKA degrader with a linker as short as 2 PEG units. With this, our SAR-study provides interesting lead structures for further optimization and validation of AURKA degradation as a potential therapeutic strategy in neuroblastoma.

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.

Advancing Targeted Protein Degradation via Multiomics Profiling and Artificial Intelligence

Only around 20% of the human proteome is considered to be druggable with small-molecule antagonists. This leaves some of the most compelling therapeutic targets outside the reach of ligand discovery. The concept of targeted protein degradation (TPD) promises to overcome some of these limitations. In brief, TPD is dependent on small molecules that induce the proximity between a protein of interest (POI) and an E3 ubiquitin ligase, causing ubiquitination and degradation of the POI. In this perspective, we want to reflect on current challenges in the field, and discuss how advances in multiomics profiling, artificial intelligence, and machine learning (AI/ML) will be vital in overcoming them. The presented roadmap is discussed in the context of small-molecule degraders but is equally applicable for other emerging proximity-inducing modalities.

Optimization of PROTAC Ternary Complex Using DNA Encoded Library Approach

The proteolysis targeting chimera (PROTAC) strategy results in the down-regulation of unwanted protein(s) for disease treatment. In the PROTAC process, a heterobifunctional degrader forms a ternary complex with a target protein of interest (POI) and an E3 ligase, which results in ubiquitination and proteasomal degradation of the POI. While ternary complex formation is a key attribute of PROTAC degraders, modification of the PROTAC molecule to optimize ternary complex formation and protein degradation can be a labor-intensive and tedious process. In this study, we take advantage of DNA-encoded library (DEL) technology to efficiently synthesize a vast number of possible PROTAC molecules and describe a parallel screening approach that utilizes DNA barcodes as reporters of ternary complex formation and cooperative binding. We use a designed PROTAC DEL against BRD4 and CRBN to describe a dual protein affinity selection method and the direct discovery of novel, potent BRD4 PROTACs that importantly demonstrate clear SAR. Such an approach evaluates all the potential PROTACs simultaneously, avoids the interference of PROTAC solubility and permeability, and uses POI and E3 ligase proteins in an efficient manner.

Identification of HPCAL1 as a specific autophagy receptor involved in ferroptosis

Selective macroautophagy/autophagy maintains cellular homeostasis through the lysosomal degradation of specific cellular proteins or organelles. The pro-survival effect of selective autophagy has been well-characterized, but the mechanism by which it drives cell death is still poorly understood. Here, we use a quantitative proteomic approach to identify HPCAL1 (hippocalcin like 1) as a novel autophagy receptor for the selective degradation of CDH2 (cadherin 2) during ferroptosis. HPCAL1-dependent CDH2 depletion increases susceptibility to ferroptotic death by reducing membrane tension and favoring lipid peroxidation. Site-directed mutagenesis aided by bioinformatic analyses revealed that the autophagic degradation of CDH2 requires PRKCQ (protein kinase C theta)-mediated HPCAL1 phosphorylation on Thr149, as well as a non-classical LC3-interacting region motif located between amino acids 46-51. An unbiased drug screening campaign involving 4208 small molecule compounds led to the identification of a ferroptosis inhibitor that suppressed HPCAL1 expression. The genetic or pharmacological inhibition of HPCAL1 prevented ferroptosis-induced tumor suppression and pancreatitis in suitable mouse models. These findings provide a framework for understanding how selective autophagy promotes ferroptotic cell death.Abbreviations: ANXA7: annexin A7; ARNTL: aryl hydrocarbon receptor nuclear translocator like; CCK8: cell counting kit-8; CDH2: cadherin 2; CETSAs: cellular thermal shift assays; CPT2: carnitine palmitoyltransferase 2; DAMP, danger/damage-associated molecular pattern; DPPH: 2,2-diphenyl-1-picrylhydrazyl; DFO: deferoxamine; EBNA1BP2: EBNA1 binding protein 2; EIF4G1: eukaryotic translation initiation factor 4 gamma 1; FBL: fibrillarin; FKBP1A: FKBP prolyl isomerase 1A; FTH1: ferritin heavy chain 1; GPX4: glutathione peroxidase 4; GSDMs: gasdermins; HBSS: Hanks' buffered salt solution; HMGB1: high mobility group box 1; HNRNPUL1: heterogeneous nuclear ribonucleoprotein U like 1; HPCAL1: hippocalcin like 1; H1-3/HIST1H1D: H1.3 linker histone, cluster member; IKE: imidazole ketone erastin; KD: knockdown; LDH: lactate dehydrogenase; LIR: LC3-interacting region; MAGOH: mago homolog, exon junction complex subunit; MAP1LC3B: microtubule associated protein 1 light chain 3 beta; MDA: malondialdehyde; MLKL: mixed lineage kinase domain like pseudokinase; MPO: myeloperoxidase; MTOR: mechanistic target of rapamycin kinase; OE: overexpressing; OSTM1: osteoclastogenesis associated transmembrane protein 1; PRKC/PKC: protein kinase C; PRKAR1A: protein kinase cAMP-dependent type I regulatory subunit alpha; PRDX3: peroxiredoxin 3; PTGS2: prostaglandin-endoperoxide synthase 2; ROS: reactive oxygen species; SLC7A11: solute carrier family 7 member 11; SLC40A1: solute carrier family 40 member 1; SPTAN1: spectrin alpha, non-erythrocytic 1; STS: staurosporine; UBE2M: ubiquitin conjugating enzyme E2 M; ZYX: zyxin.

A Comprehensive Overview of Small-Molecule Androgen Receptor Degraders: Recent Progress and Future Perspectives

Prostate cancer (PC), the second most prevalent malignancy in men worldwide, has been proven to depend on the aberrant activation of androgen receptor (AR) signaling. Long-term androgen deprivation for the treatment of PC inevitably leads to castration-resistant prostate cancer (CRPC) in which AR remains a crucial oncogenic driver. Thus, there is an urgent need to develop new strategies to address this unmet medical need. Targeting AR for degradation has recently been in a vigorous development stage, and accumulating clinical studies have highlighted the benefits of AR degraders in CRPC patients. Herein, we provide a comprehensive summary of small-molecule AR degraders with diverse mechanisms of action including proteolysis-targeting chimeras (PROTACs), selective AR degraders (SARDs), hydrophobic tags (HyT), and other AR degraders with distinct mechanisms. Accordingly, their structure-activity relationships, biomedical applications, and therapeutic values are also dissected to provide insights into the future development of promising AR degradation-based therapeutics for CRPC.