Understanding the Metabolism of Proteolysis Targeting Chimeras (PROTACs): The Next Step toward Pharmaceutical Applications

Application and challenges of nitrogen heterocycles in PROTAC linker

The absence of effective active pockets makes traditional molecularly targeted drug strategies ineffective against 80 % of human disease-related proteins. The PROTAC technology effectively makes up for the deficiency of traditional molecular targeted drugs, which produces drug activity by degrading rather than inhibiting the target protein. The degradation of PROTAC is not only affected by POI ligand and E3 ligand, but by the selection of suitable linker which can play an important role in the efficiency and selectivity of the degradation. In the early exploring stage of the PROTAC, flexible chains were priorly applied as the linker of PROTAC. Although PROTAC with flexible chains as linkers sometimes perform well in vitro bioactivity evaluations, the introduction of lipophilic flexible chains reduces the hydrophilicity of these molecules, resulting in generally poor pharmacokinetic characteristics and pharmacological activities in vivo. In addition, recent reports have also shown that some PROTAC with flexible chains have some risks to causing hemolysis in vivo. Therefore, PROTAC with flexible chains show less druggability and large difficulty to entering the clinical trial stage. On the other hand, the application of nitrogen heterocycles in the design of PROTAC linkers has been widely reported in recent years. More and more reports have shown that the introduction of nitrogen heterocycles in the linker not only can effectively improves the metabolism of PROTAC in vivo, but also can enhance the degradation efficiency and selectivity of PROTAC. These PROTAC with nitrogen heterocycle linkers have attracted much attention of pharmaceutical chemists. The introduction of nitrogen heterocycles in the linker deserves priority consideration in the primary design of the PROTAC based on various druggabilities including pharmacokinetic characteristics and pharmacological activity. In this work, we summarized the optimization process and progress of nitrogen heterocyclic rings as the PROTAC linker in recent years. However, there were still limited understanding of how to discover, design and optimize PROTAC. For example, the selection of the types of nitrogen heterocycles and the optimization sites of this linker are challenges for researchers, choosing between four to six-membered nitrogen heterocycles, selecting from saturated to unsaturated ones, and even optimizing the length and extension angle of the linker. There is a truly need for theoretical explanation and elucidation of the PROTAC to guide the developing of more effective and valuable PROTAC.

Defeating MYC with drug combinations or dual-targeting drugs

Members of the MYC family of proteins are a major target for cancer drug discovery, but the development of drugs that block MYC-driven cancers has not yet been successful. Approaches to achieve success may include the development of combination therapies or dual-acting drugs that target MYC at multiple nodes. Such treatments hold the possibility of additive or synergistic activity, potentially reducing side effect profiles and the emergence of resistance. In this review, we examine the prominent MYC-related targets and highlight those that have been targeted in combination and/or dual-target approaches. Finally, we explore the challenges of combination and dual-target approaches from a drug development perspective.

Welding PROxAb Shuttles: A Modular Approach for Generating Bispecific Antibodies via Site-Specific Protein-Protein Conjugation

Targeted protein degradation is an innovative therapeutic strategy to selectively eliminate disease-causing proteins. Exemplified by proteolysis-targeting chimeras (PROTACs), they have shown promise in overcoming drug resistance and targeting previously undruggable proteins. However, PROTACs face challenges, such as low oral bioavailability and limited selectivity. The recently published PROxAb Shuttle technology offers a solution enabling the targeted delivery of PROTACs using antibodies fused with PROTAC-binding domains derived from camelid single-domain antibodies (VHHs). Here, a modular approach to quickly generate PROxAb Shuttles by enzymatically coupling PROTAC-binding VHHs to off-the-shelf antibodies was developed. The resulting conjugates retained their target binding and internalization properties, and incubation with BRD4-targeting PROTACs resulted in formation of defined PROxAb-PROTAC complexes. These complexes selectively induced degradation of the BRD4 protein, resulting in cytotoxicity specifically to cells expressing the antibody's target. The chemoenzymatic approach described herein provides a versatile and efficient solution for generating antibody-VHH conjugates for targeted protein degradation applications, but it could also be used to combine antibodies and VHH binders to generate bispecific antibodies for further applications.

Metabolism-driven in vitro/in vivo disconnect of an oral ERɑ VHL-PROTAC

Targeting the estrogen receptor alpha (ERα) pathway is validated in the clinic as an effective means to treat ER+ breast cancers. Here we present the development of a VHL-targeting and orally bioavailable proteolysis-targeting chimera (PROTAC) degrader of ERα. In vitro studies with this PROTAC demonstrate excellent ERα degradation and ER antagonism in ER+ breast cancer cell lines. However, upon dosing the compound in vivo we observe an in vitro-in vivo disconnect. ERα degradation is lower in vivo than expected based on the in vitro data. Investigation into potential causes for the reduced maximal degradation reveals that metabolic instability of the PROTAC linker generates metabolites that compete for binding to ERα with the full PROTAC, limiting degradation. This observation highlights the requirement for metabolically stable PROTACs to ensure maximal efficacy and thus optimisation of the linker should be a key consideration when designing PROTACs.

Use of ionic liquids in amidation reactions for proteolysis targeting chimera synthesis

Selective degradation of disease-causing proteins using proteolysis targeting chimeras (PROTACs) has gained great attention, thanks to its several advantages over traditional therapeutic modalities. Despite the advances made so far, the structural chemical complexity of PROTACs poses challenges in their synthetic approaches. PROTACs are typically prepared through a convergent approach, first synthesizing two fragments separately (target protein and E3 ligase ligands) and then coupling them to produce a fully assembled PROTAC. The amidation reaction represents the most common coupling exploited in PROTACs synthesis. Unfortunately, the overall isolated yields of such synthetic procedures are usually low due to one or more purification steps to obtain the final PROTAC with acceptable purity. In this work, we focused our attention on the optimization of the final amidation step for the synthesis of an anti-SARS-CoV-2 PROTAC by investigating different amidation coupling reagents and a range of alternative solvents, including ionic liquids (ILs). Among the ILs screened, [OMIM][ClO4] emerged as a successful replacement for the commonly used DMF within the HATU-mediated amidation reaction, thus allowing the synthesis of the target PROTAC under mild and sustainable conditions in very high isolated yields. With the optimised conditions in hand, we explored the scalability of the synthetic approach and the substrate scope of the reaction by employing different E3 ligase ligand (VHL and CRBN)-based intermediates containing linkers of different lengths and compositions or by using different target protein ligands. Interestingly, in all cases, we obtained high isolated yields and complete conversion in short reaction times.

Generation of tau dephosphorylation-targeting chimeras for the treatment of Alzheimer's disease and related tauopathies

Abnormal hyperphosphorylation and accumulation of tau protein play a pivotal role in neurodegeneration in Alzheimer's disease (AD) and many other tauopathies. Selective elimination of hyperphosphorylated tau is promising for the therapy of these diseases. We have conceptualized a strategy, named dephosphorylation-targeting chimeras (DEPTACs), for specifically hijacking phosphatases to tau to debilitate its hyperphosphorylation. Here, we conducted the step-by-step optimization of each constituent motif to generate DEPTACs with reasonable effectiveness in facilitating the dephosphorylation and subsequent clearance of pathological tau. Specifically, for one of the selected chimeras, D16, we demonstrated its significant efficiency in rescuing the neurodegeneration caused by neurotoxic K18-tau seeds in vitro. Moreover, intravenous administration of D16 also alleviated tau pathologies in the brain and improved memory deficits in AD mice. These results suggested DEPTACs as targeted modulators of tau phosphorylation, which hold therapeutic potential for AD and other tauopathies.

Structure-Activity Relationship (SAR) Studies of Novel Monovalent AR/AR-V7 Dual Degraders with Potent Efficacy against Advanced Prostate Cancer

Androgen receptor (AR) has been extensively established as a potential therapeutic target for nearly all stages of prostate cancer (PCa). However, acquired resistance to AR-targeted drugs inevitably develops and severely limits their clinical efficacy. Particularly, there currently exists no efficient treatment for patients expressing the constitutively active AR splice variants, such as AR-V7. Herein, we report the structure-activity relationship studies of 55 N-heterocycle-substituted hydantoins, which identified the structural motifs required for AR/AR-V7 degradation. Among them, the most potent compound 27c exhibited selective AR/AR-V7 degradation over other hormone receptors and excellent antiproliferative activities in LNCaP and 22RV1 cells. RNA sequence analysis confirmed that 27c effectively suppressed transcriptional activity of the AR signaling pathway. Importantly, 27c demonstrated potent antitumor efficacy in an enzalutamide-resistant 22RV1 xenograft model. These results highlight the potential of 27c as a promising dual AR/AR-V7 degrader for overcoming drug resistance in advanced PCa expressing AR splice variants.

Design, synthesis, and biological evaluation of first-in-class indomethacin-based PROTACs degrading SARS-CoV-2 main protease and with broad-spectrum antiviral activity

To date, Proteolysis Targeting Chimera (PROTAC) technology has been successfully applied to mediate proteasomal-induced degradation of several pharmaceutical targets mainly related to oncology, immune disorders, and neurodegenerative diseases. On the other hand, its exploitation in the field of antiviral drug discovery is still in its infancy. Recently, we described two indomethacin (INM)-based PROTACs displaying broad-spectrum antiviral activity against coronaviruses. Here, we report the design, synthesis, and characterization of a novel series of INM-based PROTACs that recruit either Von-Hippel Lindau (VHL) or cereblon (CRBN) E3 ligases. The panel of INM-based PROTACs was also enlarged by varying the linker moiety. The antiviral activity resulted very susceptible to this modification, particularly for PROTACs hijacking VHL as E3 ligase, with one piperazine-based compound (PROTAC 6) showing potent anti-SARS-CoV-2 activity in infected human lung cells. Interestingly, degradation assays in both uninfected and virus-infected cells with the most promising PROTACs emerged so far (PROTACs 5 and 6) demonstrated that INM-PROTACs do not degrade human PGES-2 protein, as initially hypothesized, but induce the concentration-dependent degradation of SARS-CoV-2 main protease (Mpro) both in Mpro-transfected and in SARS-CoV-2-infected cells. Importantly, thanks to the target degradation, INM-PROTACs exhibited a considerable enhancement in antiviral activity with respect to indomethacin, with EC50 values in the low-micromolar/nanomolar range. Finally, kinetic solubility as well as metabolic and chemical stability were measured for PROTACs 5 and 6. Altogether, the identification of INM-based PROTACs as the first class of SARS-CoV-2 Mpro degraders demonstrating activity also in SARS-CoV-2-infected cells represents a significant advance in the development of effective, broad-spectrum anti-coronavirus strategies.

Discovery of the first ataxia telangiectasia and Rad3-related (ATR) degraders for cancer treatment

The first examples of ataxia telangiectasia and Rad3-related (ATR) PROTACs were designed and synthesized. Among them, the most potent degrader, ZS-7, demonstrated selective and effective ATR degradation in ATM-deficient LoVo cells, with a DC50 value of 0.53 μM. Proteasome-mediated ATR degradation by ZS-7 lasted approximately 12 h after washout in the LoVo cell lines. Notably, ZS-7 demonstrated reasonable PK profiles and, as a single agent or in combination with cisplatin, showed improved antitumor activity and safety profiles compared with the parent inhibitor AZD6738 in a xenograft mouse model of LoVo human colorectal cancer cells upon intraperitoneal (i.p.) administration.

Potential of the nanoplatform and PROTAC interface to achieve targeted protein degradation through the Ubiquitin-Proteasome system

In eukaryotic cells, the ubiquitin-proteasome system (UPS) plays a crucial role in selectively breaking down specific proteins. The ability of the UPS to target proteins effectively and expedite their removal has significantly contributed to the evolution of UPS-based targeted protein degradation (TPD) strategies. In particular, proteolysis targeting chimeras (PROTACs) are an immensely promising tool due to their high efficiency, extensive target range, and negligible drug resistance. This breakthrough has overcome the limitations posed by traditionally "non-druggable" proteins. However, their high molecular weight and constrained solubility impede the delivery of PROTACs. Fortunately, the field of nanomedicine has experienced significant growth, enabling the delivery of PROTACs through nanoscale drug-delivery systems, which effectively improves the stability, solubility, drug distribution, tissue-specific accumulation, and stimulus-responsive release of PROTACs. This article reviews the mechanism of action attributed to PROTACs and their potential implications for clinical applications. Moreover, we present strategies involving nanoplatforms for the effective delivery of PROTACs and evaluate recent advances in targeting nanoplatforms to the UPS. Ultimately, an assessment is conducted to determine the feasibility of utilizing PROTACs and nanoplatforms for UPS-based TPD. The primary aim of this review is to provide innovative, reliable solutions to overcome the current challenges obstructing the effective use of PROTACs in the management of cancer, neurodegenerative diseases, and metabolic syndrome. Therefore, this is a promising technology for improving the treatment status of major diseases.

Design and Evaluation of PROTACs Targeting Acyl Protein Thioesterase 1

PROTAC linker design remains mostly an empirical task. We employed the PRosettaC computational software in the design of sulfonyl-fluoride-based PROTACs targeting acyl protein thioesterase 1 (APT1). The software efficiently generated ternary complex models from empirically-designed PROTACs and suggested alkyl linkers to be the preferred type of linker to target APT1. Western blotting analysis revealed efficient degradation of APT1 and activity-based protein profiling showed remarkable selectivity of an alkyl linker-based PROTAC amongst serine hydrolases. Collectively, our data suggests that combining PRosettaC and chemoproteomics can effectively assist in triaging PROTACs for synthesis and providing early data on their potency and selectivity.

Recommendations on the Use of Multiple Labels in Human Mass Balance Studies

The administration of radiolabeled drug candidates is considered the gold standard in absorption, distribution, metabolism, and excretion studies for small-molecule drugs since it allows facile and accurate quantification of parent drug, metabolites, and total drug-related material independent of the compound structure. The choice of the position of the radiolabel, typically 14C or 3H, is critical to obtain relevant information. Sometimes, a biotransformation reaction may lead to cleavage of a part of the molecule. As a result, only the radiolabeled portion can be followed, and information on the fate of the nonlabeled metabolite may be lost. Synthesis and administration of two or more radiolabeled versions of the parent drug as a mixture or in separate studies may resolve this issue but comes with additional challenges. In this paper, we address the questions that may be considered to help make the right choice whether to use a single or multiple radiolabel approach and discuss the pros and cons of different multiple-labeling strategies that can be taken as well as alternative methods that allow the nonlabeled part of the molecule to be followed. SIGNIFICANCE STATEMENT: Radiolabeled studies are the gold standard in drug metabolism research, but molecules can undergo cleavage with loss of the label. This often results in discussions around potential use of multiple labels, which seem to be occurring with increased frequency since an increasing proportion of the small-molecule drugs are tending towards larger molecular weights. This review provides insight and decision criteria in considering a multiple-label approach as well as pros and cons of different strategies that can be followed.

Discovery of the cereblon-recruiting tubulin PROTACs effective in overcoming Taxol resistance in vitro and in vivo

Overexpression of β3-tubulin is a common occurrence in human tumors and is associated with resistance to microtubule-targeting agents. PROTAC strategy has demonstrated significant potential in overcoming drug resistance. Herein, we report the discovery of W13 as the first PROTAC against tubulin, which was created by connecting a CRBN ligand to the widely recognized microtubule-destabilizing agent CA-4. Notably, it retains the inhibitory activity of the parental CA-4 and further exhibits substantial degradation of α/β/β3-tubulin in both A549 and A549/Taxol cell lines. The degradation of tubulin was subsequently verified to be mediated by the ubiquitin-proteasome system. Importantly, tumor xenograft research clearly showed W13's promising antitumor activity against human lung cancer. Taken together, the discovery of W13 demonstrated the practicality and feasibility of PROTAC targeting tubulin, hence establishing a potential therapeutic approach for treating NSCLC caused by the overexpression of β3-tubulin.

Development of de-novo coronavirus 3-chymotrypsin-like protease (3CLpro) inhibitors since COVID-19 outbreak: A strategy to tackle challenges of persistent virus infection

Although no longer a public health emergency of international concern, COVID-19 remains a persistent and critical health concern. The development of effective antiviral drugs could serve as the ultimate piece of the puzzle to curbing this global crisis. 3-chymotrypsin-like protease (3CLpro), with its substrate specificity mirroring that of the main picornavirus 3C protease and conserved across various coronaviruses, emerges as an ideal candidate for broad-spectrum antiviral drug development. Moreover, it holds the potential as a reliable contingency option to combat emerging SARS-CoV-2 variants. In this light, the approved drugs, promising candidates, and de-novo small molecule therapeutics targeting 3CLpro since the COVID-19 outbreak in 2020 are discussed. Emphasizing the significance of diverse structural characteristics in inhibitors, be they peptidomimetic or nonpeptidic, with a shared mission to minimize the risk of cross-resistance. Moreover, the authors propose an innovative optimization strategy for 3CLpro reversible covalent PROTACs, optimizing pharmacodynamics and pharmacokinetics to better prepare for potential future viral outbreaks.

Glutathione-responsive PROTAC for targeted degradation of ERα in breast cancer cells

ERα (estrogen receptor-α)-targeting PROTACs (PROteolysis TArgeting Chimeras) have emerged as a novel and promising modality for breast cancer therapeutics. However, ERα PROTACs-induced degradation in normal tissues raises concerns about potential off-tissue toxicity. Tumor microenvironment-responsive strategy provides potential for specific control of the PROTAC's on-target degradation activity. The glutathione (GSH) level has been reported to be significantly increased in tumor cells. Here, we designed a GSH-responsive ERα PROTAC, which is generated by conjugating an o-nitrobenzenesulfonyl group to the hydroxyl group of VHL-based ERα PROTAC through a nucleophilic substitution reaction. The o-nitrobenzenesulfonyl group as a protecting group blocks the bioactivity of ERα PROTAC (ER-P1), and that can be specifically recognized and removed by highly abundant GSH in cancer cells. Consequently, the GSH-responsive ERα PROTAC (GSH-ER-P1) exhibits significantly enhanced degradation of ERα in cancer cells compared to that in normal cells, leading to a remarkable inhibition of breast cancer cell proliferation and less toxic effects on normal cells. This study provides a potentially valuable strategy for breast cancer treatment using tumor microenvironment-responsive PROTACs.

Current advances and development strategies of orally bioavailable PROTACs

Proteolysis-targeting chimeras (PROTACs) have been an area of intensive research with the potential to extend drug space not target to traditional molecules. In the last half decade, we have witnessed several PROTACs initiated phase I/II/III clinical trials, which inspired us a lot. However, the structure of PROTACs beyond "rule of 5" resulted in developing PROTACs with acceptable oral pharmacokinetic (PK) properties remain one of the biggest bottleneck tasks. Many reports have demonstrated that it is possible to access orally bioavailable PROTACs through rational ligand and linker modifications. In this review, we systematically reviewed and highlighted the most recent advances in orally bioavailable PROTACs development, especially focused on the medicinal chemistry campaign of discovery process and in vivo oral PK properties. Moreover, the constructive strategies for developing oral PROTACs were proposed comprehensively. Collectively, we believe that the strategies summarized here may provide references for further development of oral PROTACs.

Integrated Direct-to-Biology Platform for the Nanoscale Synthesis and Biological Evaluation of PROTACs

Proteolysis targeting chimeras (PROTACs) are heterobifunctional molecules that co-opt the cell's natural proteasomal degradation mechanisms to degrade undesired proteins. A challenge associated with PROTACs is the time and resource-intensive optimization; thus, the development of high-throughput platforms for their synthesis and biological evaluation is required. In this study, we establish an ultra-high-throughput experimentation (ultraHTE) platform for PROTAC synthesis, followed by direct addition of the crude reaction mixtures to cellular degradation assays without any purification. This 'direct-to-biology' (D2B) approach was validated and then exemplified in a medicinal chemistry campaign to identify novel BRD4 PROTACs. Using the D2B platform, the synthesis of 650 PROTACs was carried out in a 1536-well plate, and subsequent biological evaluation was performed by a single scientist in less than 1 month. Due to its ability to hugely accelerate the optimization of new degraders, we anticipate our platform will transform the synthesis and testing of PROTACs.

Targeting bacterial degradation machinery as an antibacterial strategy

The exploitation of a cell's natural degradation machinery for therapeutic purposes is an exciting research area in its infancy with respect to bacteria. Here, we review current strategies targeting the ClpCP system, which is a proteolytic degradation complex essential in the biology of many bacterial species of scientific interest. Strategies include using natural product antibiotics or acyldepsipeptides to initiate the up- or down-regulation of ClpCP activity. We also examine exciting recent forays into BacPROTACs to trigger the degradation of specific proteins of interest through the hijacking of the ClpCP machinery. These strategies represent an important emerging avenue for combatting antimicrobial resistance.

Leveraging Ligand Affinity and Properties: Discovery of Novel Benzamide-Type Cereblon Binders for the Design of PROTACs

Immunomodulatory imide drugs (IMiDs) such as thalidomide, pomalidomide, and lenalidomide are the most common cereblon (CRBN) recruiters in proteolysis-targeting chimera (PROTAC) design. However, these CRBN ligands induce the degradation of IMiD neosubstrates and are inherently unstable, degrading hydrolytically under moderate conditions. In this work, we simultaneously optimized physiochemical properties, stability, on-target affinity, and off-target neosubstrate modulation features to develop novel nonphthalimide CRBN binders. These efforts led to the discovery of conformationally locked benzamide-type derivatives that replicate the interactions of the natural CRBN degron, exhibit enhanced chemical stability, and display a favorable selectivity profile in terms of neosubstrate recruitment. The utility of the most potent ligands was demonstrated by their transformation into potent degraders of BRD4 and HDAC6 that outperform previously described reference PROTACs. Together with their significantly decreased neomorphic ligase activity on IKZF1/3 and SALL4, these ligands provide opportunities for the design of highly selective and potent chemically inert proximity-inducing compounds.

VHL-Modified PROteolysis TArgeting Chimeras (PROTACs) as a Strategy to Evade Metabolic Degradation in In Vitro Applications

PROteolysis TArgeting Chimeras (PROTACs) are tripartite molecules consisting of a linker connecting a ligand for a protein of interest to an E3 ligase recruiter, whose rationale relies on proteasome-based protein degradation. PROTACs have expanded as a therapeutic strategy to open new avenues for unmet medical needs. Leveraging our expertise, we undertook a series of in vitro experiments aimed at elucidating PROTAC metabolism. In particular, we focused on PROTACs recruiting the von Hippel-Lindau (VHL) E3 ligase. After high-resolution mass spectrometry measurements, a characteristic metabolite with mass reduction of 200 units was detected and successively confirmed as a product deriving from the cleavage of the VHL ligand moiety. Subsequently, we identified hepatic and extrahepatic prolyl endopeptidases as the main putative metabolic enzymes involved. Finally, we designed and synthesized analogs of the VHL ligands that we further exploited for the synthesis of novel VHL-directed PROTACs with an improved metabolic stability in in vitro applications.

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.

Small-Molecule Hydrophobic Tagging: A Promising Strategy of Druglike Technology for Targeted Protein Degradation

Targeted protein degradation (TPD) technologies have catalyzed a paradigm shift in therapeutic strategies and offer innovative avenues for drug design. Hydrophobic tags (HyTs) are bifunctional TPD molecules consisting of a ″lipophilic small-molecule tags″ group and a small-molecule ligand for the target protein. Despite the vast potential of HyTs, they have received relatively limited attention as a promising frontier. Leveraging their lower molecular weight and reduced numbers of hydrogen bond donors/acceptors (HBDs/HBAs) in comparison with proteolysis-targeting chimeras (PROTACs), HyTs present a compelling approach for enhancing druglike properties. In this Perspective, we explore the diverse range of HyT structures and their corresponding degradation mechanisms, thereby illuminating their broad applicability in targeting a diverse array of proteins, including previously elusive targets. Moreover, we scrutinize the challenges and opportunities entailed in developing this technology as a viable and fruitful strategy for drug discovery.

Lenalidomide derivatives and proteolysis-targeting chimeras for controlling neosubstrate degradation

Lenalidomide, an immunomodulatory drug (IMiD), is commonly used as a first-line therapy in many haematological cancers, such as multiple myeloma (MM) and 5q myelodysplastic syndromes (5q MDS), and it functions as a molecular glue for the protein degradation of neosubstrates by CRL4CRBN. Proteolysis-targeting chimeras (PROTACs) using IMiDs with a target protein binder also induce the degradation of target proteins. The targeted protein degradation (TPD) of neosubstrates is crucial for IMiD therapy. However, current IMiDs and IMiD-based PROTACs also break down neosubstrates involved in embryonic development and disease progression. Here, we show that 6-position modifications of lenalidomide are essential for controlling neosubstrate selectivity; 6-fluoro lenalidomide induced the selective degradation of IKZF1, IKZF3, and CK1α, which are involved in anti-haematological cancer activity, and showed stronger anti-proliferative effects on MM and 5q MDS cell lines than lenalidomide. PROTACs using these lenalidomide derivatives for BET proteins induce the selective degradation of BET proteins with the same neosubstrate selectivity. PROTACs also exert anti-proliferative effects in all examined cell lines. Thus, 6-position-modified lenalidomide is a key molecule for selective TPD using thalidomide derivatives and PROTACs.

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.

A Chemoenzymatic Approach To Produce a Cyclic Analogue of the Analgesic Drug MVIIA (Ziconotide)

Ziconotide (ω-conotoxin MVIIA) is an approved analgesic for the treatment of chronic pain. However, the need for intrathecal administration and adverse effects have limited its widespread application. Backbone cyclization is one way to improve the pharmaceutical properties of conopeptides, but so far chemical synthesis alone has been unable to produce correctly folded and backbone cyclic analogues of MVIIA. In this study, an asparaginyl endopeptidase (AEP)-mediated cyclization was used to generate backbone cyclic analogues of MVIIA for the first time. Cyclization using six- to nine-residue linkers did not perturb the overall structure of MVIIA, and the cyclic analogues of MVIIA showed inhibition of voltage-gated calcium channels (CaV 2.2) and substantially improved stability in human serum and stimulated intestinal fluid. Our study reveals that AEP transpeptidases are capable of cyclizing structurally complex peptides that chemical synthesis cannot achieve and paves the way for further improving the therapeutic value of conotoxins.

A beginner's guide to current synthetic linker strategies towards VHL-recruiting PROTACs

Over the last two decades, proteolysis targeting chimeras (PROTACs) have been revolutionary in drug development rendering targeted protein degradation (TPD) as an emerging therapeutic modality. These heterobifunctional molecules are comprised of three units: a ligand for the protein of interest (POI), a ligand for an E3 ubiquitin ligase, and a linker that tethers the two motifs together. Von Hippel-Lindau (VHL) is one of the most widely employed E3 ligases in PROTACs development due to its prevalent expression across tissue types and well-characterised ligands. Linker composition and length has proven to play an important role in determining the physicochemical properties and spatial orientation of the POI-PROTAC-E3 ternary complex, thus influencing the bioactivity of degraders. Numerous articles and reports have been published showcasing the medicinal chemistry aspects of the linker design, but few have focused on the chemistry around tethering linkers to E3 ligase ligands. In this review, we focus on the current synthetic linker strategies employed in the assembly of VHL-recruiting PROTACs. We aim to cover a range of fundamental chemistries used to incorporate linkers of varying length, composition and functionality.

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

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

Bifunctional robots inducing targeted protein degradation

The gaining importance of Targeted Protein Degradation (TPD) and PROTACs (PROteolysis-TArgeting Chimeras) have drawn the scientific community's attention. PROTACs are considered bifunctional robots owing to their avidity for the protein of interest (POI) and E3-ligase, which induce the ubiquitination of POI. These molecules are based on event-driven pharmacology and are applicable in different conditions such as oncology, antiviral, neurodegenerative disease, acne etc., offering tremendous scope to researchers. In this review, primarily, we attempted to compile the recent works available in the literature on PROTACs for various targeted proteins. We summarized the design and development strategies with a focus on molecular information of protein residues and linker design. Rationalization of the ternary complex formation using Artificial Intelligence including machine & deep learning models and traditionally followed computational tools are also included in this study. Moreover, details describing the optimization of PROTACs chemistry and pharmacokinetic properties are added. Advanced PROTAC designs and targeting complex proteins, is summed up to cover the wide spectrum.

Protein degraders enter the clinic - a new approach to cancer therapy

Heterobifunctional protein degraders, such as PROteolysis TArgeting Chimera (PROTAC) protein degraders, constitute a novel therapeutic modality that harnesses the cell's natural protein-degradation machinery - that is, the ubiquitin-proteasome system - to selectively target proteins involved in disease pathogenesis for elimination. Protein degraders have several potential advantages over small-molecule inhibitors that have traditionally been used for cancer treatment, including their event-driven (rather than occupancy-driven) pharmacology, which permits sub-stoichiometric drug concentrations for activity, their capacity to act iteratively and target multiple copies of a protein of interest, and their potential to target nonenzymatic proteins that were previously considered 'undruggable'. Following numerous innovations in protein degrader design and rigorous evaluation in preclinical models, protein degraders entered clinical testing in 2019. Currently, 18 protein degraders are in phase I or phase I/II clinical trials that involve patients with various tumour types, with a phase III trial of one initiated in 2022. The first safety, efficacy and pharmacokinetic data from these studies are now materializing and, although considerably more evidence is needed, protein degraders are showing promising activity as cancer therapies. Herein, we review advances in protein degrader development, the preclinical research that supported their entry into clinical studies, the available data for protein degraders in patients and future directions for this new class of drugs.

Targeting the Epidermal Growth Factor Receptor with Molecular Degraders: State-of-the-Art and Future Opportunities

Epidermal growth factor receptor (EGFR) is an oncogenic drug target and plays a critical role in several cellular functions including cancer cell growth, survival, proliferation, differentiation, and motility. Several small-molecule tyrosine kinase inhibitors (TKIs) and monoclonal antibodies (mAbs) have been approved for targeting intracellular and extracellular domains of EGFR, respectively. However, cancer heterogeneity, mutations in the catalytic domain of EGFR, and persistent drug resistance limited their use. Different novel modalities are gaining a position in the limelight of anti-EGFR therapeutics to overcome such limitations. The current perspective reflects upon newer modalities, importantly the molecular degraders such as PROTACs, LYTACs, AUTECs, and ATTECs, etc., beginning with a snapshot of traditional and existing anti-EGFR therapies including small molecule inhibitors, mAbs, and antibody drug conjugates (ADCs). Further, a special emphasis has been made on the design, synthesis, successful applications, state-of-the-art, and emerging future opportunities of each discussed modality.

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.

Synthesis and Biological Activity of a VHL-Based PROTAC Specific for p38α

We report a series of small molecule proteolysis-targeting chimeras (PROTACs) that target the protein kinase p38α for degradation. These PROTACs are based on a ligand of the VHL E3 ubiquitin ligase, which is linked to an ATP competitive inhibitor of p38α. We provide evidence that these compounds can induce the specific degradation of p38α, but not p38β and other related kinases, at nanomolar concentrations in several mammalian cell lines. We also show that the p38α-specific PROTACs are soluble in aqueous solutions and therefore suitable for their administration to mice. Systemic administration of the PROTACs induces p38α degradation only in the liver, probably due to the PROTAC becoming inactivated in that organ, but upon local administration the PROTACs induce p38α degradation in mammary tumors. Our compounds provide an alternative to traditional chemical inhibitors for targeting p38α signaling in cultured cells and in vivo.

Design, Synthesis and In Vitro Investigation of Cabozantinib-Based PROTACs to Target c-Met Kinase

(1) Background: This investigation aimed at developing a series of c-Met-targeting cabozantinib-based PROTACs. (2) Methods: Purification of intermediate and target compounds was performed using column chromatography, in vitro antiproliferation activity was measured using a standard MTT assay and a c-Met degradation assay was performed via the immunoblotting technique. (3) Results: Several compounds exhibited antiproliferative activity towards different cell lines of breast cancer (T47D, MDA-MB-231, SKBR3, HCC1954 and MCF7) at the same level as parent cabozantinib and 7-demethyl cabozantinib. Two target conjugates, bearing a VHL-ligand as an E3-ligase binding moiety and glycol-based linkers, exhibited the effective inhibition of c-Met phosphorylation and an ability to decrease the level of c-Met in HCC1954 cells at micromolar concentrations. (4) Conclusions: Two compounds exhibit c-Met inhibition activity in the nanomolar range and can be considered as PROTAC molecules due to their ability to decrease the total level of c-Met in HCC1954 cells. The structures of the offered compounds can be used as starting points for further evaluation of cabozantinib-based PROTACs.

PROTACs for BRDs proteins in cancer therapy: a review

BRDs proteins that recognise chromatin acetylation regulate gene expression, are epigenetic readers and master transcription coactivators. BRDs proteins are now emerging as targets for new therapeutic development. Blocking the function of any of BRDs proteins can be a control agent for diseases, such as cancer. Traditional drugs like enzyme inhibitors and protein–protein inhibitors have many limitations. The therapeutic efficacy of them remains to be proven. Recently, Proteolysis-Targeting Chimaeras (PROTACs) have become an advanced tool in therapeutic intervention as they remove disease-causing proteins. Extremely potent and efficacious small-molecule PROTACs of the BRDs proteins, based on available, potent, and selective BRDs inhibitors, have been reported. This review presents a comprehensive overview of the development of PROTACs for BRDs proteins regulation in cancer, and the chances and challenges associated with this area are also highlighted.

Proteolysis-targeting chimaeras (PROTACs) as pharmacological tools and therapeutic agents: advances and future challenges

Proteolysis-targeting chimaeras (PROTACs) have been developed to be an emerging technology for targeted protein degradation and attracted the favour of academic institutions, large pharmaceutical enterprises, and biotechnology companies. The mechanism is based on the inhibition of protein function by hijacking a ubiquitin E3 ligase for protein degradation. The heterobifunctional PROTACs contain a ligand for recruiting an E3 ligase, a linker, and another ligand to bind with the protein targeted for degradation. To date, PROTACs targeting ∼70 proteins, many of which are clinically validated drug targets, have been successfully developed with several in clinical trials for diseases therapy. In this review, the recent advances in PROTACs against clinically validated drug targets are summarised and the chemical structure, cellular and in vivo activity, pharmacokinetics, and pharmacodynamics of these PROTACs are highlighted. In addition, the potential advantages, challenges, and prospects of PROTACs technology in disease treatment are discussed.

A Versatile and Sustainable Multicomponent Platform for the Synthesis of Protein Degraders: Proof-of-Concept Application to BRD4-Degrading PROTACs

The use of small molecules to induce targeted protein degradation is increasingly growing in the drug discovery landscape, and protein degraders have progressed rapidly through the pipelines. Despite the advances made so far, their synthesis still represents a significant burden and new approaches are highly demanded. Herein we report an unprecedented platform that leverages the modular nature of both multicomponent reactions and degraders to enable the preparation of highly decorated PROTACs. As a proof of principle, our platform has been applied to the preparation of potential BRD4-degrading PROTACs, resulting in the discovery of a set of degraders endowed with high degradation efficiency. Compared to the existing methods, our approach offers a versatile and cost-effective means to access libraries of protein degraders and increase the chance of identifying successful candidates.

Discovery of small molecule ligands for the von Hippel-Lindau (VHL) E3 ligase and their use as inhibitors and PROTAC degraders

The von Hippel-Lindau (VHL) Cullin RING E3 ligase is an essential enzyme in the ubiquitin-proteasome system that recruits substrates such as the hypoxia inducible factor for ubiquitination and subsequent proteasomal degradation. The ubiquitin-proteasome pathway can be hijacked toward non-native neo-substrate proteins using proteolysis targeting chimeras (PROTACs), bifunctional molecules designed to simultaneously bind to an E3 ligase and a target protein to induce target ubiquitination and degradation. The availability of high-quality small-molecule ligands with good binding affinity for E3 ligases is fundamental for PROTAC development. Lack of good E3 ligase ligands as starting points to develop PROTAC degraders was initially a stumbling block to the development of the field. Herein, the journey towards the design of small-molecule ligands binding to VHL is presented. We cover the structure-based design of VHL ligands, their application as inhibitors in their own right, and their implementation into rationally designed, potent PROTAC degraders of various target proteins. We highlight the key findings and learnings that have provided strong foundations for the remarkable development of targeted protein degradation, and that offer a blueprint for designing new ligands for E3 ligases beyond VHL.

Recent advancements in the discovery of cereblon-based protease-targeted chimeras with potential for therapeutic intervention

Protease-targeted chimeras (PROTACs) have been employed as a novel therapeutic approach, utilizing the ubiquitin-proteasome system for targeted protein degradation. PROTACs are heterobifunctional molecules consisting of an E3 ligase ligand and a small-molecule inhibitor for recruiting a protein of interest. After binding, PROTAC molecules recruit E3 ligase for ubiquitination of the protein of interest, which is followed by its proteasome-mediated degradation. PROTAC molecules have several advantages over traditional small-molecule inhibitors. A number of PROTAC molecules based on small-molecule inhibitors have been developed against various diseases, among which cereblon-based PROTAC molecules have received the greatest interest due to their promising clinical use. This article highlights the current trends in the discovery of cereblon-based PROTAC molecules along with their medicinal chemistry, clinical progression and future outlook in cancers, cardiovascular diseases and neurodegenerative disorders.

Antibody targeting of E3 ubiquitin ligases for receptor degradation

Most current therapies that target plasma membrane receptors function by antagonizing ligand binding or enzymatic activities. However, typical mammalian proteins comprise multiple domains that execute discrete but coordinated activities. Thus, inhibition of one domain often incompletely suppresses the function of a protein. Indeed, targeted protein degradation technologies, including proteolysis-targeting chimeras¹ (PROTACs), have highlighted clinically important advantages of target degradation over inhibition². However, the generation of heterobifunctional compounds binding to two targets with high affinity is complex, particularly when oral bioavailability is required³. Here we describe the development of proteolysis-targeting antibodies (PROTABs) that tether cell-surface E3 ubiquitin ligases to transmembrane proteins, resulting in target degradation both in vitro and in vivo. Focusing on zinc- and ring finger 3 (ZNRF3), a Wnt-responsive ligase, we show that this approach can enable colorectal cancer-specific degradation. Notably, by examining a matrix of additional cell-surface E3 ubiquitin ligases and transmembrane receptors, we demonstrate that this technology is amendable for ‘on-demand’ degradation. Furthermore, we offer insights on the ground rules governing target degradation by engineering optimized antibody formats. In summary, this work describes a strategy for the rapid development of potent, bioavailable and tissue-selective degraders of cell-surface proteins.

Membrane-bound E3 ubiquitin ligases RNF43 and ZNRF3 are overexpressed in colorectal cancer, and can be repurposed using proteolysis-targeting antibodies (PROTABs) to selectively degrade cell-surface receptors in tumours.

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

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

PROTACs bearing piperazine-containing linkers: what effect on their protonation state?

Proteolysis targeting chimeras (PROTACs) represent an emerging class of compounds for innovative therapeutic application. Their bifunctional nature induces the formation of a ternary complex (target protein/PROTAC/E3 ligase) which allows target protein ubiquitination and subsequent proteasomal-dependent degradation. To date, despite great efforts being made to improve their biological efficacy PROTACs rational design still represents a challenging task, above all for the modulation of their physicochemical and pharmacokinetics properties. Considering the pivotal role played by the linker moiety, recently the insertion of a piperazine moiety into the PROTAC linker has been widely used, as this ring can in principle improve rigidity and increase solubility upon protonation. Nevertheless, the pK _a of the piperazine ring is significantly affected by the chemical groups located nearby, and slight modifications in the linker could eliminate the desired effect. In the present study, the pK _a values of a dataset of synthesized small molecule compounds including PROTACs and their precursors have been evaluated in order to highlight how a fine modulation of piperazine-containing linkers can impact the protonation state of these molecules or similar heterobifunctional ones. Finally, the possibility of predicting the trend through in silico approaches was also evaluated.

PROTACs: past, present and future

Proteolysis-targeting chimeras (PROTACs) are heterobifunctional molecules consisting of one ligand that binds to a protein of interest (POI) and another that can recruit an E3 ubiquitin ligase. The chemically-induced proximity between the POI and E3 ligase results in ubiquitination and subsequent degradation of the POI by the ubiquitin-proteasome system (UPS). The event-driven mechanism of action (MOA) of PROTACs offers several advantages compared to traditional occupancy-driven small molecule inhibitors, such as a catalytic nature, reduced dosing and dosing frequency, a more potent and longer-lasting effect, an added layer of selectivity to reduce potential toxicity, efficacy in the face of drug-resistance mechanisms, targeting nonenzymatic functions, and expanded target space. Here, we highlight important milestones and briefly discuss lessons learned about targeted protein degradation (TPD) in recent years and conjecture on the efforts still needed to expand the toolbox for PROTAC discovery to ultimately provide promising therapeutics.

Signaling Pathway and Small-Molecule Drug Discovery of FGFR: A Comprehensive Review

Targeted therapy is a groundbreaking innovation for cancer treatment. Among the receptor tyrosine kinases, the fibroblast growth factor receptors (FGFRs) garnered substantial attention as promising therapeutic targets due to their fundamental biological functions and frequently observed abnormality in tumors. In the past 2 decades, several generations of FGFR kinase inhibitors have been developed. This review starts by introducing the biological basis of FGF/FGFR signaling. It then gives a detailed description of different types of small-molecule FGFR inhibitors according to modes of action, followed by a systematic overview of small-molecule-based therapies of different modalities. It ends with our perspectives for the development of novel FGFR inhibitors.

E3 ligase ligand chemistries: from building blocks to protein degraders

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

Selective degradation of histone deacetylase 8 mediated by a proteolysis targeting chimera (PROTAC)

We developed a first-in-class proteolysis targeting chimera (PROTAC) for selective degradation of histone deacetylase 8 (HDAC8). The PROTAC induced degradation of HDAC8 without affecting the levels of other HDACs in cellular assays, and inhibited the growth of T-cell leukemia Jurkat cells more potently than a conventional HDAC8 inhibitor.

Degraders: The Ultimate Weapon Against Amplified Driver Kinases in Cancer

Amplification of pro-oncogenic kinases is a common genetic alteration driving tumorigenic phenotypes. Cancer cells rely on the amplified kinases to sustain cell proliferation, survival, and growth, presenting an opportunity to develop therapies targeting the amplified kinases. Utilizing small molecule catalytic inhibitors as therapies to target amplified kinases is plagued by de novo resistance driven by increased expression of the target, and amplified kinases can drive tumorigenic phenotypes independent of catalytic activity. Here, we discuss the emergence of proteolysis-targeting chimeras that provide an opportunity to target these oncogenic drivers effectively. SIGNIFICANCE STATEMENT: Protein kinases contribute to tumorigenesis through catalytic and noncatalytic mechanisms, and kinase gene amplifications are well described mechanisms of resistance to small molecule catalytic inhibitors. Repurposing catalytic inhibitors for the development of protein degraders will offer improved clinical benefits by targeting noncatalytic functions of kinases that promote tumorigenesis and overcoming resistance due to amplification.

3-Aminophthalic acid, a new cereblon ligand for targeted protein degradation by O'PROTAC

In this study, we identified 3-aminophthalic acid as a new ligand of cereblon (CRBN) E3 ubiquitin ligase and developed a phthalic acid-based O'PROTAC for degradation of the ERG transcription factor. This phthalic acid-based O'PROTAC presented an efficacy in degrading ERG comparable to those displayed by pomalidomide-based ERG O'PROTACs. Moreover, phthalic acid-being more chemically stable and economical than classical immunomodulatory drugs (IMiDs)-represents, as a ligand, a new alternative for the development of PROTACs, especially O'PROTACs.