Discovery of ERD-308 as a Highly Potent Proteolysis Targeting Chimera (PROTAC) Degrader of Estrogen Receptor (ER)

Discovery of an orally active VHL-recruiting PROTAC that achieves robust HMGCR degradation and potent hypolipidemic activity in vivo

HMG-CoA reductase (HMGCR) protein is usually upregulated after statin (HMGCR inhibitor) treatment, which inevitably diminishes its therapeutic efficacy, provoking the need for higher doses associated with adverse effects. The proteolysis targeting chimera (PROTAC) technology has recently emerged as a powerful approach for inducing protein degradation. Nonetheless, due to their bifunctional nature, developing orally bioavailable PROTACs remains a great challenge. Herein, we identified a powerful HMGCR-targeted PROTAC (21c) comprising a VHL ligand conjugated to lovastatin acid that potently degrades HMGCR in Insig-silenced HepG2 cells (DC₅₀ = 120 nmol/L) and forms a stable ternary complex, as predicated by a holistic modeling protocol. Most importantly, oral administration of the corresponding lactone 21b reveled favorable plasma exposures referring to both the parent 21b and the conversed acid 21c. Further in vivo studies of 21b demonstrated robust HMGCR degradation and potent cholesterol reduction in mice with diet-induced hypercholesterolemia, highlighting a promising strategy for treating hyperlipidemia and associated diseases.

From Pure Antagonists to Pure Degraders of the Estrogen Receptor: Evolving Strategies for the Same Target

Pure antiestrogens, or selective estrogen receptor degraders (SERDs), have proven to be effective in treating breast cancer that has progressed on tamoxifen and/or aromatase inhibitors. However, the only FDA-approved pure antiestrogen, fulvestrant, is limited in efficacy by its low bioavailability. The search for orally bioavailable SERDs has continued for nearly as long as the clinical history of the injection-only fulvestrant. Oral SERDs that have been developed and tested in patients ranged from nonsteroidal ER binders containing an acrylic acid or amino side chain to bifunctional proteolysis-targeting chimera (PROTAC) pure ER degraders. Structural evolution in the development of oral SERD molecules has been closely associated with quantifiable ER-degrading potency, as seen in the structural comparison analysis of acrylic acid and basic amino side-chain-bearing SERDs. Failure to improve on fulvestrant in the clinical trials by numerous acidic SERDs and early basic SERDs is blamed on tolerability and/or insufficient efficacy, which will likely be overcome by the new-generation basic SERD molecules and PROTAC ER degraders with improved oral bioavailability, low toxicity, and superior efficacy of receptor degradation.

Bispecific Estrogen Receptor α Degraders Incorporating Novel Binders Identified Using DNA-Encoded Chemical Library Screening

Bispecific degraders (PROTACs) of ERα are expected to be advantageous over current inhibitors of ERα signaling (aromatase inhibitors/SERMs/SERDs) used to treat ER+ breast cancer. Information from DNA-encoded chemical library (DECL) screening provides a method to identify novel PROTAC binding features as the linker positioning, and binding elements are determined directly from the screen. After screening ∼120 billion DNA-encoded molecules with ERα WT and 3 gain-of-function (GOF) mutants, with and without estradiol to identify features that enrich ERα competitively, the off-DNA synthesized small molecule exemplar 7 exhibited nanomolar ERα binding, antagonism, and degradation. Click chemistry synthesis on an alkyne E3 ligase engagers panel and an azide variant of 7 rapidly generated bispecific nanomolar degraders of ERα, with PROTACs 18 and 21 inhibiting ER+ MCF7 tumor growth in a mouse xenograft model of breast cancer. This study validates this approach toward identifying novel bispecific degrader leads from DECL screening with minimal optimization.

Targeted degradation of transcription factors by TRAFTACs: TRAnscription Factor TArgeting Chimeras

Many diseases, including cancer, stem from aberrant activation or overexpression of oncoproteins that are associated with multiple signaling pathways. Although proteins with catalytic activity can be successfully drugged, the majority of other protein families, such as transcription factors, remain intractable due to their lack of ligandable sites. In this study, we report the development of TRAnscription Factor TArgeting Chimeras (TRAFTACs) as a generalizable strategy for targeted transcription factor degradation. We show that TRAFTACs, which consist of a chimeric oligonucleotide that simultaneously binds to the transcription factor of interest (TOI) and to HaloTag-fused dCas9 protein, can induce degradation of the former via the proteasomal pathway. Application of TRAFTACs to two oncogenic TOIs, NF-κB and brachyury, suggests that TRAFTACs can be successfully employed for the targeted degradation of other DNA-binding proteins. Thus, TRAFTAC technology is potentially a generalizable strategy to induce degradation of other transcription factors both in vitro and in vivo.

Identification of probe-quality degraders for Poly(ADP-ribose) polymerase-1 (PARP-1)

Poly(ADP-ribose) polymerase-1 (PARP-1), a critical DNA repair enzyme in the base excision repair pathway, has been pursued as an attractive cancer therapeutic target. Intervention with PARP-1 has been proved to be more sensitive to cancer cells carrying BRCA1/2 mutations. Several PARP-1 inhibitors have been available on market for the treatment of breast, ovarian and prostatic cancer. Promisingly, the newly developed proteolysis targeting chimaeras (PROTACs) may provide a more potential strategy based on the degradation of PARP-1. Here we report the design, synthesis, and evaluation of a proteolysis targeting chimaera (PROTAC) based on the combination of PARP-1 inhibitor olaparib and the CRBN (cereblon) ligand lenalidomide. In SW620 cells, our probe-quality degrader compound 2 effectively induced PARP-1 degradation which results in anti-proliferation, cells apoptosis, cell cycle arresting, and cancer cells migratory inhibition. Thus, our findings qualify a new chemical probe for PARP-1 knockdown.

PROTACs, molecular glues and bifunctionals from bench to bedside: Unlocking the clinical potential of catalytic drugs

The vast majority of currently marketed drugs rely on small molecules with an 'occupancy-driven' mechanism of action (MOA). Therefore, the efficacy of these therapeutics depends on a high degree of target engagement, which often requires high dosages and enhanced drug exposure at the target site, thus increasing the risk of off-target toxicities (Churcher, 2018 [1]). Although small molecule drugs have been successfully used as treatments for decades, tackling a variety of disease-relevant targets with a defined binding site, many relevant therapeutic targets remain challenging to drug due, for example, to lack of well-defined binding pockets or large protein-protein interaction (PPI) interfaces which resist interference (Dang et al., 2017 [2]). In the quest for alternative therapeutic approaches to address different pathologies and achieve enhanced efficacy with reduced side effects, ligand-induced targeted protein degradation (TPD) has gained the attention of many research groups both in academia and in industry in the last two decades. This therapeutic modality represents a novel paradigm compared to conventional small-molecule inhibitors. To pursue this strategy, heterobifunctional small molecule degraders, termed PROteolysis TArgeting Chimeras (PROTACs) have been devised to artificially redirect a protein of interest (POI) to the cellular protein homeostasis machinery for proteasomal degradation (Chamberlain et al., 2019 [3]). In this chapter, the development of PROTACs will first be discussed providing a historical perspective in parallel to the experimental progress made to understand this novel therapeutic modality. Furthermore, common strategies for PROTAC design, including assays and troubleshooting tips will be provided for the reader, before presenting a compendium of all PROTAC targets reported in the literature to date. Due to the recent advancement of these molecules into clinical trials, consideration of pharmacokinetics and pharmacodynamic properties will be introduced, together with the biotech landscape that has developed from the success of PROTACs. Finally, an overview of subsequent strategies for targeted protein degradation will be presented, concluding with further scientific quests triggered by the invention of PROTACs.

Discovery of thalidomide-based PROTAC small molecules as the highly efficient SHP2 degraders

SHP2, a non-receptor tyrosine phosphatase, plays a pivotal role in numerous oncogenic cell-signaling cascades like RAS-ERK, PI3K-AKT and JAK-STAT. On the other hand, proteolysis targeting chimera (PROTAC) has emerged as a promising strategy for the degradation of disease-related protein of interest (POI). SHP2 degradation via the PROTAC strategy will provide an alternative startegy for SHP2-mediated cancer therapy. Herein we described the design, synthesis and evaluation of a series of thalidomide-based heterobifunctional molecules and identified 11(ZB-S-29) as the highly efficient SHP2 degrader with a DC₅₀ of 6.02 nM. Further mechanism investigation illustrated that 11 came into function through targeted SHP2 protein degradation.

PROTACs: A Hope for Breast Cancer Patients?

BACKGROUND

Breast Cancer (BC) is the most widely recognized disease in women. A massive number of women are diagnosed with breast cancer and many lost their lives every year. Cancer is the subsequent driving reason for dying, giving rise to it one of the current medication's most prominent difficulties.

OBJECTIVES

The main objective of the study is to examine and explore novel therapy (PROTAC) and its effectiveness against breast cancer.

METHODS

The literature search was done across Medline, Cochrane, ScienceDirect, Wiley Online, Google Scholar, PubMed, Bentham Sciences from 2001 to 2020. The articles were collected; screened, segregated, and selected papers were included for writing the review article.

RESULTS AND CONCLUSION

A novel innovation emerged around two decades ago that has great potential to not only overcome the limitations but also can provide future direction for the treatment of many diseases which has presently not many therapeutic options available and regarded as incurable with traditional techniques; that innovation is called PROTAC (Proteolysis Targeting Chimera) and able to efficaciously ubiquitinate and debase cancer encouraging proteins by noncovalent interaction. PROTACs are constituted of two active regions isolated by a linker and equipped for eliminating explicit undesirable protein. It is empowering greater sensitivity to "drug-resistant targets" as well as a more prominent opportunity to influence non-enzymatic function. PROTACs have been demonstrated to show better target selectivity contrasted with traditional small-molecule inhibitors. So far, the most investigation into PROTACs possesses particularly concentrated on applications to cancer treatment including breast cancer, the treatment of different ailments may profit from this blossoming innovation.

Tackling Immune Targets for Breast Cancer: Beyond PD-1/PD-L1 Axis

The burden of breast cancer is imposing a huge global problem. Drug discovery research and novel approaches to treat breast cancer have been carried out extensively over the last decades. Although immune checkpoint inhibitors are showing promising preclinical and clinical results in treating breast cancer, they are facing multiple limitations. From an immunological perspective, a recent report highlighted breast cancer as an "inflamed tumor" with an immunosuppressive microenvironment. Consequently, researchers have been focusing on identifying novel immunological targets that can tune up the tumor immune microenvironment. In this context, several novel non-classical immune targets have been targeted to determine their ability to uncouple immunoregulatory pathways at play in the tumor microenvironment. This article will highlight strategies designed to increase the immunogenicity of the breast tumor microenvironment. It also addresses the latest studies on targets which can enhance immune responses to breast cancer and discusses examples of preclinical and clinical trial landscapes that utilize these targets.

Proteolytic Targeting Chimeras with Specificity for Plasma Membrane and Intracellular Estrogen Receptors

Decisions regarding the assignment of hormonal therapy for breast cancer are based solely upon the presence of nuclear estrogen receptors (ERs) in biopsied tumor tissue. This is despite the fact that the G-protein-coupled estrogen receptor (GPER) is linked to advanced breast cancer and is required for breast cancer stem cell survival, an observation that suggests that effective endocrine therapy should also target this receptor. Here, two ER/GPER-targeting proteolytic chimeras (UI-EP001 and UI-EP002) are described that effectively degrade ERα, ERβ, and GPER. These chimeras form high-affinity interactions with GPER and ER with binding dissociation constants of ∼30 nM and 10-20 nM, respectively. Plasma membrane and intracellular GPER and nuclear ER were degraded by UI-EP001 and UI-EP002, but not by a partial proteolytic targeting chimera (PROTAC) lacking its estrogen-targeting domain. Pretreatment of cells with the proteasomal inhibitor, MG132, blocked UI-EP001 and UI-EP002 proteolysis, while the lysosomotrophic inhibitor, chloroquine, had no effect. The off-target activity was not observed against recombinant β1-adrenergic receptor or CXCR4. Target specificity was further demonstrated in human MCF-7 cells where both drugs effectively degraded ERα, ERβ, and GPER, sparing the progesterone receptor (PR). UI-EP001 and UI-EP002 induced cytotoxicity and G2/M cell cycle arrest in MCF-7 breast cancer and human SKBR3 (ERα-ERβ-GPER+) breast cancer cells but not human MDA-MB-231 breast cancer cells that do not express functional GPER/ER. These results suggest that it is possible to develop a receptor-based strategy of antiestrogen treatment for breast cancer that targets both plasma membrane and intracellular estrogen receptors.

Discovery of novel 2H-chromene-3-carbonyl derivatives as selective estrogen receptor degraders (SERDs): Design, synthesis and biological evaluation

Selective estrogen receptor degraders (SERDs) not only block ERα activity but degrade this receptor at the same time and are effective in relapsed ERα positive breast cancer patients who have accepted other endocrine therapies. Herein, through scaffold hopping of coumarin skeleton, a series of 2H-chromene-3-carbonyl-based SERDs with phenyl acrylic acid group as the side chain were designed and synthesized. Compound XH04 containing 7-hydroxy-2H-chromene-3-carbonyl skeleton exhibited the most potent activities in 2D (IC₅₀ = 0.8 μM) and 3D cells culture models (MCF-7) and had the best ERα binding affinity as well. Furthermore, the significant antiestrogen property of compound XH04 was confirmed by inhibiting the expression of progesterone receptor (PgR) mRNA in MCF-7 cells. On the other hand, the outgoing ERα degradation property of compound XH04 was qualitatively and quantificationally verified by immunofluorescence analysis and Western blot assay in MCF-7 cells. Besides, compound XH04 repressed the expression level of Ki67 in MCF-7 cells and induced the apoptosis increase of this tumor cells in a dose-dependent manner like approved-SERD fulvestrant (2), while compound XH04 exhibited better preliminary pharmacokinetics in human and rat liver microsomes in vitro and a lower LogD_7.4 value than fulvestrant. And further molecular docking study revealed that compound XH04 possessed a proverbial and typical binding model with ERα like other reported SERD. All these results confirmed that 7-hydroxy-2H-chromene-3-carbonyl structure could be a feasible skeleton for design of ERα antagonists including SERDs and compound XH04 is a promising candidate for further development of ERα + breast cancer therapy agents.

The therapeutic potential of PROTACs

INTRODUCTION

PROTACs represent a novel class of heterobifunctional molecules that simultaneously bind to a target protein and to an E3 ligase complex, resulting in the transfer of ubiquitin and initiating a process ultimately causing the proteasomal degradation of the target protein. This mechanism of action imbues PROTACs with the ability to modulate target biology in unique ways compared to inhibitors, and the development of PROTACs as therapeutic agents is expected to result in new medicines to treat multiple diseases.

AREAS COVERED

This review includes published PCT (WO) patent applications covering January 2013 through June 2020. Only English-language patent applications with exemplified PROTACs reported to degrade a target protein(s) were deemed in scope, and the definition of 'PROTAC' was restricted to a bifunctional molecule which contains a discrete binding element for a specific degradation target(s), as well as a separate discrete E3 ligase-binding moiety.

EXPERT OPINION

Delivering on the enormous potential of PROTACs will require the development of PROTAC medicines that are differentiated from traditional small-molecule inhibitors. The modular composition of PROTACs affords both opportunities and challenges in securing robust intellectual property, and we envision that requirements for novelty are likely to evolve as this area matures.

Preclinical and Clinical Advances of Targeted Protein Degradation as a Novel Cancer Therapeutic Strategy: An Oncologist Perspective

PROteolysis Targeting Chimeras (PROTACs) are a family of heterobifunctional small molecules that specifically target cellular proteins for degradation. Given that their mode of action is distinct from that of small-molecule inhibitors widely used in clinical practice, PROTACs have the potential to improve current cancer therapies. Multiple studies have suggested that PROTACs exhibit enhanced pharmacodynamics and reduced toxicity both in vitro and in vivo compared to clinically relevant small-molecule kinase inhibitors. In addition, PROTACs have been reported to be less prone to mutation-mediated drug resistance in specific disease settings. Since its development in 2001, the field of targeted protein degradation, in which PROTACs are used, has expanded rapidly. However, earlier studies focused on the advancement of the technology itself, while preclinical and clinical data on the disease-modifying effect of PROTACs have only recently been reported. As preclinical and clinical evidence accumulates, the efficacy of PROTACs as targeted therapeutics-distinct from that of small-molecule kinase inhibitors-suggests potential translational benefit in the clinical setting. In this short review, we aim to describe translational potentials of PROTACs. We offer our perspectives as practicing oncologists on the preclinical and clinical data on PROTACs as novel therapeutics for both solid and hematological malignancies.

Selective Estrogen Receptor Degraders (SERDs): A Promising Strategy for Estrogen Receptor Positive Endocrine-Resistant Breast Cancer

Estrogen receptor (ER) plays important roles in gene transcription and the proliferation of ER positive breast cancers. Selective modulation of ER has been a therapeutic target for this specific type of breast cancer for more than 30 years. Selective estrogen receptor modulators (SERMs) and aromatase inhibitors (AIs) have been demonstrated to be effective therapeutic approaches for ER positive breast cancers. Unfortunately, 30-50% of ER positive tumors become resistant to SERM/AI treatment after 3-5 years. Fulvestrant, the only approved selective estrogen receptor degrader (SERD), is currently an important therapeutic approach for the treatment of endocrine-resistant breast cancers. The poor pharmacokinetic properties of fulvestrant have inspired the development of a new generation of oral SERDs to overcome drug resistance. In this review, we describe recent advances in ERα structure, functions, and mechanisms of endocrine resistance and summarize the development of oral SERDs in both academic and industrial areas.

Proteolysis targeting chimera (PROTAC) in drug discovery paradigm: Recent progress and future challenges

Proteolysis targeting chimera (PROTAC), hijacking protein of interest (POI) and recruiting E3 ligase for target degradation via the ubiquitin-proteasome pathway, is a novel drug discovery paradigm which has been widely used as biological tools and medicinal molecules with the potential of clinical application value. Currently, ARV-110, an orally small molecule PROTAC was designed to specifically target Androgen receptor (AR), firstly enters clinical phase I trials for the treatment of metastatic castration-resistant prostate cancer, which turns a new avenue for the development of PROTAC. We herein provide a detail summary on the latest one year progress of PROTAC target various proteins and elucidate the advantages of PROTAC technology. Finally, the potential challenges of this vibrant field are also discussed.

The Quest for Orally Available Selective Estrogen Receptor Degraders (SERDs)

Estrogen receptor-alpha (ERα) is the target of endocrine therapies for the treatment of more than 70 % of ERα-positive breast cancers. Selective estrogen receptor degraders (SERDs) antagonize estrogen binding and target the receptor for degradation, representing the last line of treatment for resistant metastatic breast cancer patients. However, the clinical efficacy of the lone clinically approved SERD (Fulvestrant) is limited by its poor oral bioavailability. Recently, several analogues of GW5638, an acrylic acid-based ERα ligand developed by Glaxo Research Institute in 1994, have been reported as promising orally bioavailable SERDs. Some of these compounds are currently in clinical trials, while various other structurally novel SERDs have also been reported by pharma as well as academic research groups. This review provides a critical analysis of the recent developments in orally available SERDs, with a focus on the structure-activity relationships, binding interactions and pharmacokinetic properties of these compounds.

Controlled Delivery of BET-PROTACs: In Vitro Evaluation of MZ1-Loaded Polymeric Antibody Conjugated Nanoparticles in Breast Cancer

Bromo and extraterminal domain (BET) inhibitors-PROteolysis TArgeting Chimera (BETi-PROTAC) is a new family of compounds that induce proteasomal degradation through the ubiquitination of the tagged to BET inhibitors Bromodomain proteins, BRD2 and BRD. The encapsulation and controlled release of BET-PROTACs through their vectorization with antibodies, like trastuzumab, could facilitate their pharmacokinetic and efficacy profile. Antibody conjugated nanoparticles (ACNPs) using PROTACs have not been designed and evaluated. In this pioneer approach, the commercial MZ1 PROTAC was encapsulated into the FDA-approved polymeric nanoparticles. The nanoparticles were conjugated with trastuzumab to guide the delivery of MZ1 to breast tumoral cells that overexpress HER2. These ACNPs were characterized by means of size, polydispersity index, and Z-potential. Morphology of the nanoparticles, along with stability and release studies, completed the characterization. MZ1-loaded ACNPs showed a significant cytotoxic effect maintaining its mechanism of action and improving its therapeutic properties.

Discovery of SK-575 as a Highly Potent and Efficacious Proteolysis-Targeting Chimera Degrader of PARP1 for Treating Cancers

The nuclear protein poly(ADP-ribose) polymerase-1 (PARP1) has a well-established role in the signaling and repair of DNA and is a validated therapeutic target for cancers and other human diseases. Here, we have designed, synthesized, and evaluated a series of small-molecule PARP1 degraders based on the proteolysis-targeting chimera (PROTAC) concept. Our efforts have led to the discovery of highly potent PARP1 degraders, as exemplified by compound 18 (SK-575). SK-575 potently inhibits the growth of cancer cells bearing BRCA1/2 mutations and induces potent and specific degradation of PARP1 in various human cancer cells even at low picomolar concentrations. SK-575 achieves durable tumor growth inhibition in mice when used as a single agent or in combination with cytotoxic agents, such as temozolomide and cisplatin. These data demonstrate that SK-575 is a highly potent and efficacious PARP1 degrader.

Design and characterization of cereblon-mediated androgen receptor proteolysis-targeting chimeras

Proteolysis-targeting chimera (PROTAC)-mediated protein degradation is a rapidly emerging therapeutic intervention that induces the degradation of targeted proteins. Herein, we report the design and biological evaluation of a series of androgen receptor (AR) PROTAC degraders for the treatment of metastatic castration-resistant prostate cancer. Predominantly, instead of thalidomide, we utilized the TD-106 scaffold, a novel cereblon (CRBN) binder that was identified in our previous study. Our results suggest that the linker position in the TD-106 CRBN binder is critical for the efficiency of AR degradation. The compounds attached to the 6-position of TD-106 promoted better degradation of AR than those at the 5- and 7-positions. Among the synthesized AR PROTACs, the representative degrader 33c (TD-802) effectively induced AR protein degradation, with a degradation concentration 50% of 12.5 nM and a maximum degradation of 93% in LNCaP prostate cancer cells. Additionally, most AR PROTAC degraders, including TD-802, displayed good liver microsomal stability and in vivo pharmacokinetic properties. Finally, we showed that TD-802 effectively inhibited tumor growth in an in vivo xenograft study.

Targeting Cullin-RING E3 Ligases for Radiosensitization: From NEDDylation Inhibition to PROTACs

As a dynamic regulator for short-lived protein degradation and turnover, the ubiquitin-proteasome system (UPS) plays important roles in various biological processes, including response to cellular stress, regulation of cell cycle progression, and carcinogenesis. Over the past decade, research on targeting the cullin-RING (really interesting new gene) E3 ligases (CRLs) in the UPS has gained great momentum with the entry of late-phase clinical trials of its novel inhibitors MLN4924 (pevonedistat) and TAS4464. Several preclinical studies have demonstrated the efficacy of MLN4924 as a radiosensitizer, mainly due to its unique cytotoxic properties, including induction of DNA damage response, cell cycle checkpoints dysregulation, and inhibition of NF-κB and mTOR pathways. Recently, the PROteolysis TArgeting Chimeras (PROTACs) technology was developed to recruit the target proteins for CRL-mediated polyubiquitination, overcoming the resistance that develops inevitably with traditional targeted therapies. First-in-class cell-permeable PROTACs against critical radioresistance conferring proteins, including the epidermal growth factor receptor (EGFR), androgen receptor (AR) and estrogen receptor (ER), cyclin-dependent kinases (CDKs), MAP kinase kinase 1 (MEK1), and MEK2, have emerged in the past 5 years. In this review article, we will summarize the most important research findings of targeting CRLs for radiosensitization.

Discovery of First-in-Class Protein Arginine Methyltransferase 5 (PRMT5) Degraders

The aberrant expression of protein arginine methyltransferase 5 (PRMT5) has been associated with multiple cancers. Using the proteolysis targeting chimera technology, we discovered a first-in-class PRMT5 degrader 15 (MS4322). Here, we report the design, synthesis, and characterization of compound 15 and two structurally similar controls 17 (MS4370) and 21 (MS4369), with impaired binding to the von Hippel-Lindau E3 ligase and PRMT5, respectively. Compound 15, but not 17 and 21, effectively reduced the PRMT5 protein level in MCF-7 cells. Our mechanism studies indicate that compound 15 degraded PRMT5 in an E3 ligase- and proteasome-dependent manner. Compound 15 also effectively reduced the PRMT5 protein level and inhibited growth in multiple cancer cell lines. Moreover, compound 15 was highly selective for PRMT5 in a global proteomic study and exhibited good plasma exposure in mice. Collectively, compound 15 and its two controls 17 and 21 are valuable chemical tools for exploring the PRMT5 functions in health and disease.

PROTACs: New method to degrade transcription regulating proteins

Transcription is the fundamental process in all living organisms. A variety of important proteins, such as NRs, BETs, HDACs and many others are involved in transcription process. In general, overexpression of these proteins would cause many diseases. Some approved therapeutics employed inhibitors to regulate the transcription process, however, the results are far from satisfying. Therefore, it is in high demand to develop new technology to improve the therapeutic effects. In recent years, proteolysis-targeting chimaera (PROTAC) turned out to be a novel efficient therapeutic method to treat various diseases which were caused by proteins overexpression. PROTAC molecules are bifunctional small molecules that simultaneously bind a target protein and an E3-ubiquitin ligase, thus causing ubiquitination and subsequent degradation of the target protein by the proteasome. In contrast to traditional inhibitors, PROTACs showed higher efficiency to tackle the diseases which were caused by protein overexpression due to their excellent performance for degrading target proteins in transcription regulation. In this review, 29 kinds of PROTACs targeting transcription regulator proteins are summarized, and meanwhile the advantages of PROTACs are highlighted. Furthermore, several examples of PROTACs regulating the transcription for the treatment of diseases and functioning as tools for biological research are also disscussed.

The peptide PROTAC modality: a novel strategy for targeted protein ubiquitination

Despite dramatic advances in drug discovery over the decades, effective therapeutic strategies for cancers treatment are still in urgent demands. PROteolysis TArgeting Chimera (PROTAC), a novel therapeutic modality, has been vigorously promoted in preclinical and clinical applications. Unlike small molecule PROTAC, peptide PROTAC (p-PROTAC) with advantages of high specificity and low toxicity, while avoiding the limitations of shallow binding pockets through large interacting surfaces, provides promising substitutions for E3 ubiquitin ligase complex-mediated ubiquitination of "undruggable proteins". It is worth noting that successful applications of p-PROTAC still have some obstacles, including low stability and poor membrane permeability. Hence, we highlight that p-PROTAC combined with cell-penetrating peptides, constrained conformation technique, and targeted delivery systems could be the future efforts for potential translational research.

Targeting estrogen receptor α for degradation with PROTACs: A promising approach to overcome endocrine resistance

Estrogen receptor alfa (ERα) is expressed in approximate 70% of breast cancer (BC) which is the most common malignancy in women worldwide. To date, the foremost intervention in the treatment of ER positive (ER+) BC is still the endocrine therapy. However, resistance to endocrine therapies remains a major hurdle in the long-term management of ER + BC. Although the mechanisms underlying endocrine resistance are complex, cumulative evidence revealed that ERα still plays a critical role in driving BC tumor cells to grow in resistance state. Fulvestrant, a selective estrogen receptor degrader (SERD), has moved to first line therapy for metastatic ER + BC, suggesting that removing ERα would be a useful strategy to overcome endocrine resistance. Proteolysis-Targeting Chimera (PROTAC) technology, an emerging paradigm for protein degradation, has the potential to eliminate both wild type and mutant ERα in breast cancer cells. Excitingly, ARV-471, an ERα-targeted PROTAC developed by Arvinas, has been in phase 1 clinical trials. In this review, we will summarize recent progress of ER-targeting PROTACs from publications and patents along with their therapeutic opportunities for the treatment of endocrine-resistant BC.

Chimera induced protein degradation: PROTACs and beyond

Ubiquitin-proteasome system, autophagy-lysosome pathway and N-end rule pathway are crucial protein quality control mechanisms in human body. Hijacking these endogenous protein degrading measures by chimera degraders could be a revolutionary strategy for the discovery of small-molecule drugs. As the most advanced chimera degraders, PROTACs have demonstrated the potential by delivering two drug candidates into clinical trials. The development of chimera degraders exploiting these three pathways are reviewed, a focus is given on the chemical structures and their influences on biological effects from a viewpoint of medicinal chemistry.

PROTAC: A promising technology for cancer treatment

Proteolysis-targeting chimeric molecules (PROTACs), which attract much more attention today, may be a potential way to treat cancer. PROTACs are made up of ligands of target proteins, E3 ligase recruiting elements and linkers. PROTACs can hijack the intracellular inherent ubiquitin proteasome system in cells to degrade different target proteins. PROTACs targeting different cancer-related proteins have been successfully developed and outperform small inhibitors, the traditional way of treating cancer. In this review, we focus on PROTACs targeting cancer-related proteins and their superiority over inhibitors.

Discovery of Novel PDEδ Degraders for the Treatment of KRAS Mutant Colorectal Cancer

KRAS-PDEδ protein-protein interaction represents an appealing target for cancer therapy. However, fast release of high-affinity inhibitors from PDEδ hampered drug binding affinity and antiproliferative activity. To overcome the limitations, the first proteolysis-targeting chimeric (PROTAC) small molecules targeting PDEδ were designed. By employment of PDEδ inhibitor deltazinone (2) and cereblon ligand pomalidomide (6), a series of potent PROTAC PDEδ degraders were obtained. The most promising compound 17f efficiently induced PDEδ degradation and demonstrated significantly improved antiproliferative potency in KRAS mutant SW480 cells. Compound 17f also achieved significant tumor growth inhibition in the SW480 colorectal cancer xenograft model. This proof-of-concept study provided a new strategy to validate the druggability of KRAS-PDEδ interaction and offered an effective lead compound for the treatment of KRAS mutant cancer.

Two-Stage Strategy for Development of Proteolysis Targeting Chimeras and its Application for Estrogen Receptor Degraders

Proteolysis targeting chimeras (PROTACs) have emerged as useful chemical probes and potential therapeutics by taking advantage of the ubiquitin-proteasome system to degrade intracellular disease-associated proteins. PROTACs are heterobifunctional molecules composed of a target protein ligand, E3 ubiquitin ligase ligand, and a linker between them. The generation of efficient PROTACs requires screening of many parameters, especially the lengths and types of the linkers. We report our proof-of-concept study using a two-stage strategy to facilitate the development of PROTACs against the estrogen receptor (ER). In stage one, a library of close to 100 PROTACs was synthesized by simply mixing a library of ERα ligands containing a hydrazide functional group at different positions with a preassembled library of E3 ligase ligands bearing different types and lengths of linkers with a terminal aldehyde group in a 1:1 ratio. Cell-based screening occurred without further purification, because the formation of the acylhydrazone linkage is highly efficient and produces water as the only byproduct. Compound A3 was the most potent ER degrader in two ER+ cell lines (DC₅₀= ∼ 10 nM, D_max= ≥ 95%). Stage two involved transformation to a more stable amide linker to generate a more drug-like molecule. The new compound, AM-A3, showed comparable biological activity (DC₅₀ = 1.1 nM, D_max = 98%) and induced potent antiproliferation (IC₅₀= 13.2 nM, I_max= 69%) in MCF-7. This proof-of -concept study demonstrates that the two-stage strategy can significantly facilitate the development of PROTACs against ER without the tedious process of making large numbers of PROTACs one by one. It has the potential to be expanded to many other targets.

Antibody-PROTAC Conjugates Enable HER2-Dependent Targeted Protein Degradation of BRD4

Targeting protein degradation with Proteolysis-Targeting Chimeras (PROTACs) is an area of great current interest in drug discovery. Nevertheless, although the high effectiveness of PROTACs against a wide variety of targets has been established, most degraders reported to date display limited intrinsic tissue selectivity and do not discriminate between cells of different types. Here, we describe a strategy for selective protein degradation in a specific cell type. We report the design and synthesis of a trastuzumab-PROTAC conjugate (Ab-PROTAC 3) in which E3 ligase-directed degrader activity is caged with an antibody linker which can be hydrolyzed following antibody-PROTAC internalization, releasing the active PROTAC and inducing catalytic protein degradation. We show that 3 selectively targets bromodomain-containing protein 4 (BRD4) for degradation only in HER2 positive breast cancer cell lines, while sparing HER2 negative cells. Using live cell confocal microscopy, we show internalization and lysosomal trafficking of the conjugate specifically in HER2 positive cells, leading to the release of active PROTAC in quantities sufficient to induce potent BRD4 degradation. These studies demonstrate proof-of-concept for tissue-specific BRD4 degradation, overcoming limitations of PROTAC selectivity, with significant potential for application to novel targets.

Computer-Aided Ligand Discovery for Estrogen Receptor Alpha

Breast cancer (BCa) is one of the most predominantly diagnosed cancers in women. Notably, 70% of BCa diagnoses are Estrogen Receptor α positive (ERα+) making it a critical therapeutic target. With that, the two subtypes of ER, ERα and ERβ, have contrasting effects on BCa cells. While ERα promotes cancerous activities, ERβ isoform exhibits inhibitory effects on the same. ER-directed small molecule drug discovery for BCa has provided the FDA approved drugs tamoxifen, toremifene, raloxifene and fulvestrant that all bind to the estrogen binding site of the receptor. These ER-directed inhibitors are non-selective in nature and may eventually induce resistance in BCa cells as well as increase the risk of endometrial cancer development. Thus, there is an urgent need to develop novel drugs with alternative ERα targeting mechanisms that can overcome the limitations of conventional anti-ERα therapies. Several functional sites on ERα, such as Activation Function-2 (AF2), DNA binding domain (DBD), and F-domain, have been recently considered as potential targets in the context of drug research and discovery. In this review, we summarize methods of computer-aided drug design (CADD) that have been employed to analyze and explore potential targetable sites on ERα, discuss recent advancement of ERα inhibitor development, and highlight the potential opportunities and challenges of future ERα-directed drug discovery.

Discovery of GNE-149 as a Full Antagonist and Efficient Degrader of Estrogen Receptor alpha for ER+ Breast Cancer

Estrogen receptor alpha (ERα) is a well-validated drug target for ER-positive (ER+) breast cancer. Fulvestrant is FDA-approved to treat ER+ breast cancer and works through two mechanisms-as a full antagonist and selective estrogen receptor degrader (SERD)-but lacks oral bioavailability. Thus, we envisioned a "best-in-class" molecule with the same dual mechanisms as fulvestrant, but with significant oral exposure. Through lead optimization, we discovered a tool molecule 12 (GNE-149) with improved degradation and antiproliferative activity in both MCF7 and T47D cells. To illustrate the binding mode and key interactions of this scaffold with ERα, we obtained a cocrystal structure of 6 that showed ionic interaction of azetidine with Asp351 residue. Importantly, 12 showed favorable metabolic stability and good oral exposure. 12 exhibited antagonist effect in the uterus and demonstrated robust dose-dependent efficacy in xenograft models.

Development of Selective Histone Deacetylase 6 (HDAC6) Degraders Recruiting Von Hippel-Lindau (VHL) E3 Ubiquitin Ligase

Histone deacetylase 6 (HDAC6) is involved in multiple cellular processes such as aggresome formation, protein stability, and cell motility. Numerous HDAC6-selective inhibitors have been developed as cellular chemical tools to elucidate the function of HDAC6. Since HDAC6 has multiple domains that cannot be studied by HDAC6-selective inhibitors, CRISPR-CAS9 and siRNA/shRNA have been employed to elucidate the nonenzymatic functions of HDAC6. However, these genetic methods have many limitations. Proteolysis targeting chimera (PROTAC) is an emerging technology for the development of small molecules that can quickly remove the entire protein in cells. We previously developed multifunctional HDAC6 degraders that can recruit cereblon (CRBN) E3 ubiquitin ligase. These HDAC6 degraders can degrade not only HDAC6 but also neo-substrates of CRBN. They are excellent candidates for the development of anticancer therapeutics, but the multifunctional nature of the CRBN-based HDAC6 degraders has limited their utility as specific chemical probes for the study of HDAC6-related cellular pathways. Herein we report the development of the first cell-permeable HDAC6-selective degraders employing Von Hippel-Lindau (VHL) E3 ubiquitin ligase, which does not have any known neo-substrates. The DC₅₀'s of the most potent compound 3j are 7.1 nM and 4.3 nM in human MM1S and mouse 4935 cell lines, respectively. The D _max's of 3j in these two cell lines are 90% and 57%, respectively.

Proteolysis-Targeting Chimeras as Therapeutics and Tools for Biological Discovery

New biological tools provide new techniques to probe fundamental biological processes. Here we describe the burgeoning field of proteolysis-targeting chimeras (PROTACs), which are capable of modulating protein concentrations at a post-translational level by co-opting the ubiquitin-proteasome system. We describe the PROTAC technology and its application to drug discovery and provide examples where PROTACs have enabled novel biological insights. Furthermore, we provide a workflow for PROTAC development and use and discuss the benefits and issues associated with PROTACs. Finally, we compare PROTAC-mediated protein-level modulation with other technologies, such as RNAi and genome editing.

Recent progress in selective estrogen receptor downregulators (SERDs) for the treatment of breast cancer

Selective estrogen receptor downregulators (SERDs) are a novel class of compounds capable of reducing the ERα protein level and blocking ER activity. Therefore, SERDs are considered as a significant therapeutic approach to treat ER+ breast cancer in both early stage and more advanced drug-resistant cases. After the FDA approval of a steroidal drug, fulvestrant, as a SERD for the treatment of breast cancer in patients who have progressed on antihormonal agents, several molecules with diverse chemical structures have been rapidly developed, studied and evaluated for selective estrogen receptor downregulation activity. Here we compile the promising SERDs reported in recent years and discuss the chemical structure and pharmacological profile of the most potent compound of the considered series. Because of the availability of only a limited number of effective drugs for the treatment of breast cancer, the quest for a potent SERD with respectable activity and bioavailability is still ongoing. The goal of this article is to make available to the reader an overview of the current progress in SERDs and provide clues for the future discovery and development of novel pharmacological potent SERDs for the treatment of breast cancer.

Development of a Brigatinib degrader (SIAIS117) as a potential treatment for ALK positive cancer resistance

EML4-ALK and NPM-ALK fusion proteins possess constitutively activated ALK (anaplastic lymphoma kinase) activity, which in turn leads to the development of non-small cell lung cancer and anaplastic large-cell lymphomas (ALCLs). FDA-approved ALK inhibitor drugs cause significant cancer regression. However, drug resistance eventually occurs and it becomes a big obstacle in clinic. Novel proteolysis targeting chimera (PROTAC) technology platform provides a potential therapeutic strategy for drug resistance. Herein, we designed and synthesized a series of ALK PROTACs based on Brigatinib and VHL-1 conjunction, and screened SIAIS117 as the best degrader which not only blocked the growth of SR and H2228 cancer cell lines, but also degraded ALK protein. In addition, SIAIS117 also showed much better growth inhibition effect than Brigatinib on 293T cell line that exogenously expressed G1202R-resistant ALK proteins. Furthermore, it also degraded G1202R mutant ALK protein in vitro. At last, it has the potentially anti-proliferation ability of small cell lung cancer. Thus, we have successfully generated the degrader SIAIS117 that can potentially overcome resistance in cancer targeted therapy.

Discovery of PROTAC BCL-XL degraders as potent anticancer agents with low on-target platelet toxicity

Anti-apoptotic protein BCL-X_L plays a key role in tumorigenesis and cancer chemotherapy resistance, rendering it an attractive target for cancer treatment. However, BCL-X_L inhibitors such as ABT-263 cannot be safely used in the clinic because platelets solely depend on BCL-X_L to maintain their viability. To reduce the on-target platelet toxicity associated with the inhibition of BCL-X_L, we designed and synthesized PROTAC BCL-X_L degraders that recruit CRBN or VHL E3 ligase because both of these enzymes are poorly expressed in human platelets compared to various cancer cell lines. We confirmed that platelet-toxic BCL-X_L/2 dual inhibitor ABT-263 can be converted into platelet-sparing CRBN/VHL-based BCL-X_L specific degraders. A number of BCL-X_L degraders are more potent in killing cancer cells than their parent compound ABT-263. Specifically, XZ739, a CRBN-dependent BCL-X_L degrader, is 20-fold more potent than ABT-263 against MOLT-4 T-ALL cells and has >100-fold selectivity for MOLT-4 cells over human platelets. Our findings further demonstrated the utility of PROTAC technology to achieve tissue selectivity through recruiting differentially expressed E3 ligases.

Update on targeted cancer therapies, single or in combination, and their fine tuning for precision medicine

BACKGROUND

Until recently, patients who have the same type and stage of cancer all receive the same treatment. It has been established, however, that individuals with the same disease respond differently to the same therapy. Further, each tumor undergoes genetic changes that cause cancer to grow and metastasize. The changes that occur in one person's cancer may not occur in others with the same cancer type. These differences also lead to different responses to treatment. Precision medicine, also known as personalized medicine, is a strategy that allows the selection of a treatment based on the patient's genetic makeup. In the case of cancer, the treatment is tailored to take into account the genetic changes that may occur in an individual's tumor. Precision medicine, therefore, could be defined in terms of the targets involved in targeted therapy.

METHODS

A literature search in electronic data bases using keywords "cancer targeted therapy, personalized medicine and cancer combination therapies" was conducted to include papers from 2010 to June 2019.

RESULTS

Recent developments in strategies of targeted cancer therapy were reported. Specifically, on the two types of targeted therapy; first, immune-based therapy such as the use of immune checkpoint inhibitors (ICIs), immune cytokines, tumor-targeted superantigens (TTS) and ligand targeted therapeutics (LTTs). The second strategy deals with enzyme/small molecules-based therapies, such as the use of a proteolysis targeting chimera (PROTAC), antibody-drug conjugates (ADC) and antibody-directed enzyme prodrug therapy (ADEPT). The precise targeting of the drug to the gene or protein under attack was also investigated, in other words, how precision medicine can be used to tailor treatments.

CONCLUSION

The conventional therapeutic paradigm for cancer and other diseases has focused on a single type of intervention for all patients. However, a large literature in oncology supports the therapeutic benefits of a precision medicine approach to therapy as well as combination therapies.

Targeted degradation of activating estrogen receptor α ligand-binding domain mutations in human breast cancer

PURPOSE

Studies have identified several estrogen receptor α (ERα) ligand-binding domain (LBD) somatic mutations in endocrine therapy resistant, metastatic ER-positive breast cancers. The most common mutations, Tyr537Ser (Y537S) and Asp538Gly (D538G), are detected in ~ 30% of endocrine resistant metastatic breast cancer patients. These ESR1 mutations induce the agonist conformation of ERα, confer an estrogen-independent phenotype, and promote drug resistance to antiestrogens.

METHODS

ER-positive, estrogen-dependent MCF-7 cells were engineered to express either the Y537S or D538G mutants using CRISPR knock-in (cY537S and cD538G). These cells were used to screen several estrogen receptor degrader (ERD) compounds synthesized using the Proteolysis Targeting Chimeras (PROTAC) method to induce degradation of ERα via the ubiquitin-proteasome pathway.

RESULTS

Wild-type MCF-7 and ERα LBD mutant cells were treated with ERD-148 (10 pM-1 µM) and assayed for cellular proliferation using the PrestoBlue cell viability assay. ERD-148 attenuated ER-dependent growth with IC₅₀ values of 0.8, 10.5, and 6.1 nM in MCF-7, cY537S, and cD538G cells, respectively. Western blot analysis showed that MCF-7 cells treated with 1 nM ERD-148 for 24 h exhibited reduced ERα protein expression as compared to the mutants. The ER-regulated gene, GREB1, demonstrated significant downregulation in parental and mutant cells after 24 h of ERD-148 treatment at 10 nM. Growth of the ER-negative, estrogen-independent MDA-MB-231 breast cancer cells was not inhibited by ERD-148 at the ~ IC₉₀ observed in the ER-positive cells.

CONCLUSION

ERD-148 inhibits the growth of ER-positive breast cancer cells via downregulating ERα with comparable potency to Fulvestrant with marginal non-specific toxicity.

PROTACs: A novel strategy for cancer therapy

Chemotherapeutic strategy has been widely used for treating malignance by targeting irregular expressed or mutant proteins with small molecular inhibitors (SMIs) or monoclonal antibodies (mAbs). However, most intracellular proteins lack of active sites or antigens where SMIs or mAbs bind with, and are called as non-druggable targets for a long time. From the first year of this century, PROteolysis-TArgeting Chimeras (PROTACs) has emerged to be a promising approach for proteins, including those non-druggable ones, such as transcriptional factors and scaffold proteins. The first generation of peptide-based PROTACs adopts β-TrCP and VHL as E3 ligases, but the cellular permeability and chemical stability issues restrict their clinical application. The second generation of small molecule-based PROTACs adopts MDM2, VHL, IAPs and Cereblon as E3 ligases have been tensely studied. To date, the targets of PROTACs including those overexpressed oncogenic proteins such as ER, AR and BRDs, disease-relevant fusion proteins such as NPM/EML4-ALK and BCR-ABL, cancer-driven mutant proteins such as EGFR, kinases such as CDKs and RTKs. The major disadvantage of PROTACs is the noncancer specificity and relative higher toxicity, due to its catalytic role. To overcome this, we and other have recently developed several similar light-controllable PROTACs, termed as the third generation controllable PROTACs. The degradation of targets by those PROTACs can be triggered by UVA or visible light, providing a tool box for further PROTACs design. Here in this review, we introduce the historical milestones and prospective for further PROTACs development in clinical use.

PHOTACs enable optical control of protein degradation

PROTACs (PROteolysis TArgeting Chimeras) are bifunctional molecules that target proteins for ubiquitylation by an E3 ligase complex and subsequent degradation by the proteasome. They have emerged as powerful tools to control the levels of specific cellular proteins. We now introduce photoswitchable PROTACs that can be activated with the spatiotemporal precision that light provides. These trifunctional molecules, which we named PHOTACs (PHOtochemically TArgeting Chimeras), consist of a ligand for an E3 ligase, a photoswitch, and a ligand for a protein of interest. We demonstrate this concept by using PHOTACs that target either BET family proteins (BRD2,3,4) or FKBP12. Our lead compounds display little or no activity in the dark but can be reversibly activated with different wavelengths of light. Our modular approach provides a method for the optical control of protein levels with photopharmacology and could lead to new types of precision therapeutics that avoid undesired systemic toxicity.

Protein-protein interactions: a structural view of inhibition strategies and the IL-23/IL-17 axis

Protein-protein interactions are central to biology and provide opportunities to modulate disease with small-molecule or protein therapeutics. Recent developments in the understanding of the tractability of protein-protein interactions are discussed with a focus on the ligandable nature of protein-protein interaction surfaces. General principles of inhibiting protein-protein interactions are illustrated with structural biology examples from six members of the IL-23/IL-17 signaling family (IL-1, IL-6, IL-17, IL-23 RORγT and TNFα). These examples illustrate the different approaches to discover protein-protein interaction inhibitors on a target-specific basis that has proven fruitful in terms of discovering both small molecule and biologic based protein-protein interaction inhibitors.

Proteasomal and lysosomal degradation for specific and durable suppression of immunotherapeutic targets

Cancer immunotherapy harness the body’s immune system to eliminate cancer, by using a broad panel of soluble and membrane proteins as therapeutic targets. Immunosuppression signaling mediated by ligand-receptor interaction may be blocked by monoclonal antibodies, but because of repopulation of the membrane via intracellular organelles, targets must be eliminated in whole cells. Targeted protein degradation, as exemplified in proteolysis targeting chimera (PROTAC) studies, is a promising strategy for selective inhibition of target proteins. The recently reported use of lysosomal targeting molecules to eliminate immune checkpoint proteins has paved the way for targeted degradation of membrane proteins as crucial anti-cancer targets. Further studies on these molecules’ modes of action, target-binding “warheads”, lysosomal sorting signals, and linker design should facilitate their rational design. Modifications and derivatives may improve their cell-penetrating ability and the in vivo stability of these pro-drugs. These studies suggest the promise of alternative strategies for cancer immunotherapy, with the aim of achieving more potent and durable suppression of tumor growth. Here, the successes and limitations of antibody inhibitors in cancer immunotherapy, as well as research progress on PROTAC- and lysosomal-dependent degradation of target proteins, are reviewed.

PROTACs: great opportunities for academia and industry

Although many kinds of therapies are applied in the clinic, drug-resistance is a major and unavoidable problem. Another disturbing statistic is the limited number of drug targets, which are presently only 20-25% of all protein targets that are currently being studied. Moreover, the focus of current explorations of targets are their enzymatic functions, which ignores the functions from their scaffold moiety. As a promising and appealing technology, PROteolysis TArgeting Chimeras (PROTACs) have attracted great attention both from academia and industry for finding available approaches to solve the above problems. PROTACs regulate protein function by degrading target proteins instead of inhibiting them, providing more sensitivity to drug-resistant targets and a greater chance to affect the nonenzymatic functions. PROTACs have been proven to show better selectivity compared to classic inhibitors. PROTACs can be described as a chemical knockdown approach with rapidity and reversibility, which presents new and different biology compared to other gene editing tools by avoiding misinterpretations that arise from potential genetic compensation and/or spontaneous mutations. PRTOACs have been widely explored throughout the world and have outperformed not only in cancer diseases, but also in immune disorders, viral infections and neurodegenerative diseases. Although PROTACs present a very promising and powerful approach for crossing the hurdles of present drug discovery and tool development in biology, more efforts are needed to gain to get deeper insight into the efficacy and safety of PROTACs in the clinic. More target binders and more E3 ligases applicable for developing PROTACs are waiting for exploration.

Utilizing PROTAC technology to address the on-target platelet toxicity associated with inhibition of BCL-XL

BCL-XL, an anti-apoptotic BCL-2 family protein, plays a key role in cancer cell survival. However, the potential of BCL-XL as an anti-cancer target has been hampered by the on-target platelet toxicity because platelets depend on BCL-XL to maintain their viability. Here we report the development of a PROTAC BCL-XL degrader, XZ424, which has increased selectivity for BCL-XL-dependent MOLT-4 cells over human platelets compared with conventional BCL-XL inhibitors. This proof-of-concept study demonstrates the potential of utilizing a PROTAC approach to achieve tissue selectivity.