Development of protein degradation inducers of oncogenic BCR-ABL protein by conjugation of ABL kinase inhibitors and IAP ligands

Exploiting the potential of the ubiquitin-proteasome system in overcoming tyrosine kinase inhibitor resistance in chronic myeloid leukemia

The advent of tyrosine kinase inhibitors (TKI) targeting BCR-ABL has drastically changed the treatment approach of chronic myeloid leukemia (CML), greatly prolonged the life of CML patients, and improved their prognosis. However, TKI resistance is still a major problem with CML patients, reducing the efficacy of treatment and their quality of life. TKI resistance is mainly divided into BCR-ABL-dependent and BCR-ABL-independent resistance. Now, the main clinical strategy addressing TKI resistance is to switch to newly developed TKIs. However, data have shown that these new drugs may cause serious adverse reactions and intolerance and cannot address all resistance mutations. Therefore, finding new therapeutic targets to overcome TKI resistance is crucial and the ubiquitin-proteasome system (UPS) has emerged as a focus. The UPS mediates the degradation of most proteins in organisms and controls a wide range of physiological processes. In recent years, the study of UPS in hematological malignant tumors has resulted in effective treatments, such as bortezomib in the treatment of multiple myeloma and mantle cell lymphoma. In CML, the components of UPS cooperate or antagonize the efficacy of TKI by directly or indirectly affecting the ubiquitination of BCR-ABL, interfering with CML-related signaling pathways, and negatively or positively affecting leukemia stem cells. Some of these molecules may help overcome TKI resistance and treat CML. In this review, the mechanism of TKI resistance is briefly described, the components of UPS are introduced, existing studies on UPS participating in TKI resistance are listed, and UPS as the therapeutic target and strategies are discussed.

BCR::ABL1 Proteolysis-targeting chimeras (PROTACs): The new frontier in the treatment of Ph+ leukemias?

BCR::ABL1 tyrosine kinase inhibitors (TKIs) have turned chronic myeloid leukemia (CML) from a lethal condition into a chronic ailment. With optimal management, the survival of CML patients diagnosed in the chronic phase is approaching that of age-matched controls. However, only one-third of patients can discontinue TKIs and enter a state of functional cure termed treatment-free remission (TFR), while the remainder require life-long TKI therapy to avoid the recurrence of active leukemia. Approximately 10% of patients exhibit primary or acquired TKI resistance and eventually progress to the blast phase. It is thought that recurrence after attempted TFR originates from CML stem cells (LSCs) surviving despite continued suppression of BCR::ABL1 kinase. Although kinase activity is indispensable for induction of overt CML, kinase-independent scaffold functions of BCR::ABL1 are known to contribute to leukemogenesis, raising the intriguing but as yet hypothetical possibility, that degradation of BCR::ABL1 protein may accomplish what TKIs fail to achieve - eliminate residual LSCs to turn functional into real cures. The advent of BCR::ABL1 proteolysis targeting chimeras (PROTACs), heterobifunctional molecules linking a TKI-based warhead to an E3 ligase recruiter, has moved clinical protein degradation into the realm of the possible. Here we examine the molecular rationale as well as pros and cons of degrading BCR::ABL1 protein. We review reported BCR::ABL1 PROTACs, point out limitations of available data and compounds and suggest directions for future research. Ultimately, clinical testing of a potent and specific BCR::ABL1 degrader will be required to determine the efficacy and tolerability of this approach.

Reversible control of kinase signaling through chemical-induced dephosphorylation

The coordination between kinases and phosphatases is crucial for regulating the phosphorylation levels of essential signaling molecules. Methods enabling precise control of kinase activities are valuable for understanding the kinase functions and for developing targeted therapies. Here, we use the abscisic acid (ABA)-induced proximity system to reversibly control kinase signaling by recruiting phosphatases. Using this method, we found that the oncogenic tyrosine kinase BCR::ABL1 can be inhibited by recruiting various cytoplasmic phosphatases. We also discovered that the oncogenic serine/threonine kinase BRAF(V600E), which has been reported to bypass phosphorylation regulation, can be positively regulated by protein phosphatase 1 (PP1) and negatively regulated by PP5. Additionally, we observed that the dual-specificity kinase MEK1 can be inhibited by recruiting PP5. This suggests that bifunctional molecules capable of recruiting PP5 to MEK or RAF kinases could be promising anticancer drug candidates. Thus, the ABA-induced dephosphorylation method enables rapid screening of phosphatases to precisely control kinase signaling.

Discovery of Potent Degraders of the Dengue Virus Envelope Protein

Targeted protein degradation has been widely adopted as a new approach to eliminate both established and previously recalcitrant therapeutic targets. Here, it is reported that the development of small molecule degraders of the envelope (E) protein of dengue virus. Two classes of bivalent E-degraders are developed by linking two previously reported E-binding small molecules, GNF-2, and CVM-2-12-2, to a glutarimide-based recruiter of the CRL4CRBN ligase to effect proteosome-mediated degradation of the E protein. ZXH-2-107 (based on GNF-2) is an E-degrader with ABL inhibitory activity while ZXH-8-004 (based on CVM-2-12-2) is a selective and potent E-degrader. These two compounds provide proof of concept that difficult-to-drug targets such as a viral envelope protein can be effectively eliminated using a bivalent degrader and provide starting points for the future development of a new class of direct-acting antiviral drugs.

The emerging role of targeted protein degradation to treat and study cancer

The evolution of cancer treatment has provided increasingly targeted strategies both in the upfront and relapsed disease settings. Small-molecule inhibitors and immunotherapy have risen to prominence with chimeric antigen receptor T-cells, checkpoint inhibitors, kinase inhibitors, and monoclonal antibody therapies being deployed across a range of solid organ and haematological malignancies. However, novel approaches are required to target transcription factors and oncogenic fusion proteins that are central to cancer biology and have generally eluded successful drug development. Thalidomide analogues causing protein degradation have been a cornerstone of treatment in multiple myeloma, but a lack of in-depth mechanistic understanding initially limited progress in the field. When the protein cereblon (CRBN) was found to mediate thalidomide analogues' action and CRBN's neo-targets were identified, existing and novel drug development accelerated, with applications outside multiple myeloma, including non-Hodgkin's lymphoma, myelodysplastic syndrome, and acute leukaemias. Critically, transcription factors were the first canonical targets described. In addition to broadening the application of protein-degrading drugs, resistance mechanisms are being overcome and targeted protein degradation is widening the scope of druggable proteins against which existing approaches have been ineffective. Examples of targeted protein degraders include molecular glues and proteolysis targeting chimeras (PROTACs): heterobifunctional molecules that bind to proteins of interest and cause proximity-induced ubiquitination and proteasomal degradation via a linked E3 ligase. Twenty years since their inception, PROTACs have begun progressing through clinical trials, with early success in targeting the oestrogen receptor and androgen receptor in breast and prostate cancer respectively. This review explores important developments in targeted protein degradation to both treat and study cancer. It also considers the potential advantages and challenges in the translational aspects of developing new treatments. © 2024 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Discovery of Potent Degraders of the Dengue Virus Envelope Protein

Targeted protein degradation has been widely adopted as a new approach to eliminate both established and previously recalcitrant therapeutic targets. Here we report the development of small molecule degraders of the envelope (E) protein of dengue virus. We developed two classes of bivalent E-degraders, linking two previously reported E-binding small molecules, GNF-2 and CVM-2-12-2, to a glutarimide-based recruiter of the CRL4CRBN ligase to effect proteosome-mediated degradation of the E protein. ZXH-2-107 (based on GNF-2) is an E degrader with ABL inhibition while ZXH-8-004 (based on CVM-2-12-2) is a selective and potent E-degrader. These two compounds provide proof-of-concept that difficult-to-drug targets such as a viral envelope protein can be effectively eliminated using a bivalent degrader and provide starting points for the future development of a new class antiviral drugs.

PROteolysis-Targeting Chimeras (PROTACs) in leukemia: overview and future perspectives

Leukemia is a heterogeneous group of life-threatening malignant disorders of the hematopoietic system. Immunotherapy, radiotherapy, stem cell transplantation, targeted therapy, and chemotherapy are among the approved leukemia treatments. Unfortunately, therapeutic resistance, side effects, relapses, and long-term sequelae occur in a significant proportion of patients and severely compromise the treatment efficacy. The development of novel approaches to improve outcomes is therefore an unmet need. Recently, novel leukemia drug discovery strategies, including targeted protein degradation, have shown potential to advance the field of personalized medicine for leukemia patients. Specifically, PROteolysis-TArgeting Chimeras (PROTACs) are revolutionary compounds that allow the selective degradation of a protein by the ubiquitin-proteasome system. Developed against a wide range of cancer targets, they show promising potential in overcoming many of the drawbacks associated with conventional therapies. Following the exponential growth of antileukemic PROTACs, this article reviews PROTAC-mediated degradation of leukemia-associated targets. Chemical structures, in vitro and in vivo activities, pharmacokinetics, pharmacodynamics, and clinical trials of PROTACs are critically discussed. Furthermore, advantages, challenges, and future perspectives of PROTACs in leukemia are covered, in order to understand the potential that these novel compounds may have as future drugs for leukemia treatment.

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

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

The Progress of Small Molecule Targeting BCR-ABL in the Treatment of Chronic Myeloid Leukemia

Chronic myelogenous leukemia (CML) is a malignant myeloproliferative disease. According to the American Cancer Society's 2021 cancer data report, new cases of CML account for about 15% of all leukemias. CML is generally divided into three stages: chronic phase, accelerated phase, and blast phase. Nearly 90% of patients are diagnosed as a chronic phase. Allogeneic stem cell transplantation and chemotherapeutic drugs, such as interferon IFN-α were used as the earliest treatments for CML. However, they could generate obvious side effects, and scientists had to seek new treatments for CML. A new era of targeted therapy for CML began with the introduction of imatinib, the first-generation BCR-ABL kinase inhibitor. However, the ensuing drug resistance and mutant strains led by T315I limited the further use of imatinib. With the continuous advancement of research, tyrosine kinase inhibitors (TKI) and BCR-ABL protein degraders with novel structures and therapeutic mechanisms have been discovered. From biological macromolecules to classical target protein inhibitors, a growing number of compounds are being developed to treat chronic myelogenous leukemia. In this review, we focus on summarizing the current situation of a series of candidate small-molecule drugs in CML therapy, including TKIs and BCR-ABL protein degrader. The examples provided herein describe the pharmacology activity of small-molecule drugs. These drugs will provide new enlightenment for future treatment directions.

Recent advancements in targeted protein knockdown technologies-emerging paradigms for targeted therapy

A generalized therapeutic strategy for various disease conditions, including cancer, is to deplete or inactivate harmful protein targets. Various forms of protein or gene silencing molecules, e.g., small molecule inhibitors, RNA interference (RNAi), and microRNAs (miRNAs) have been used against druggable targets. Over the past few years, targeted protein degradation (TPD) approaches have been developed for direct degradation of candidate proteins. Among the TPD approaches, proteolysis targeting chimeras (PROTACs) have emerged as one of the most promising approaches for the selective elimination of proteins via the ubiquitin-proteasome system. Other than PROTACs, TPD methods with potential therapeutic use include intrabody-mediated protein knockdown and tripartite motif-21 (TRIM-21) mediated TRIM-Away. In this review, protein knockdown approaches, their modes of action, and their advantages over conventional gene knockdown approaches are summarized. In cancers, disease-associated protein functions are often executed by specific post-translational modifications (PTMs). The role of TRIM-Away is highlighted in the direct knockdown of PTM forms of target proteins. Moreover, the application challenges and the prospective clinical use of TPD approaches in various diseases are also discussed.

Mechanism insights of curcumin and its analogues in cancer: An update

Cancer is the world's second leading cause of mortality and one of the major public health problems. Cancer incidence and mortality rates remain high despite the great advancements in existing therapeutic, diagnostic, and preventive approaches. Therefore, a quest for less toxic and more efficient anti-cancer strategies is still at the forefront of the current research. Traditionally important, curcumin commonly known as a wonder molecule has received considerable attention as an anti-cancer, anti-inflammatory, and antioxidant candidate. However, limited water solubility and low bioavailability restrict its extensive utility in different pathological states. The investigators are making consistent efforts to develop newer strategies to overcome its limitations by designing different analogues with better pharmacokinetic and pharmacodynamic properties. The present review highlights the recent updates on curcumin and its analogues with special emphasis on various mechanistic pathways involved in anti-cancer activity. In addition, the structure-activity relationship of curcumin analogues has also been precisely discussed. This article will also provide key information for the design and development of newer curcumin analogues with desired pharmacokinetic and pharmacodynamic profiles and will provide in depth understanding of molecular pathways involved in the anti-cancer activities.

Targeted Protein Degradation: Principles, Strategies, and Applications

Protein degradation is a general process to maintain cell homeostasis. The intracellular protein quality control system mainly includes the ubiquitin-proteasome system and the lysosome pathway. Inspired by the physiological process, strategies to degrade specific proteins have developed, which emerge as potent and effective tools in biological research and drug discovery. This review focuses on recent advances in targeted protein degradation techniques, summarizing the principles, advantages, and challenges. Moreover, the potential applications and future direction in biological science and clinics are also discussed.

Single amino acid-based PROTACs trigger degradation of the oncogenic kinase BCR-ABL in chronic myeloid leukemia (CML)

Proteolysis-targeting chimera (PROTAC) that specifically targets harmful proteins for destruction by hijacking the ubiquitin-proteasome system is emerging as a potent anticancer strategy. How to efficiently modulate the target degradation remains a challenging issue. In this study, we employ a single amino acid-based PROTAC, which uses the shortest degradation signal sequence as the ligand of the N-end rule E3 ubiquitin ligases to degrade the fusion protein BCR (breakpoint cluster region)-ABL (Abelson proto-oncogene), an oncogenic kinase that drives the progression of chronic myeloid leukemia. We find that the reduction level of BCR-ABL can be easily adjusted by substituting different amino acids. Furthermore, a single PEG linker is found to achieve the best proteolytic effect. Our efforts have resulted in effective degradation of BCR-ABL protein by the N-end rule pathway and efficient growth inhibition of K562 cells expressing BCR-ABL in vitro and blunted tumor growth in a K562 xenograft tumor model in vivo. The PROTAC presented has unique advantages including lower effective concentration, smaller molecular size, and modular degradation rate. Demonstrating the efficacy of the N-end rule-based PROTACs in vitro and in vivo, our study further expands the limited degradation pathways currently available for PROTACs in vivo and is easily adapted for broader applications in targeted protein degradation.

PROTACs: A novel strategy for cancer drug discovery and development

Proteolysis targeting chimera (PROTAC) technology has become a powerful strategy in drug discovery, especially for undruggable targets/proteins. A typical PROTAC degrader consists of three components: a small molecule that binds to a target protein, an E3 ligase ligand (consisting of an E3 ligase and its small molecule recruiter), and a chemical linker that hooks first two components together. In the past 20 years, we have witnessed advancement of multiple PROTAC degraders into the clinical trials for anticancer therapies. However, one of the major challenges of PROTAC technology is that only very limited number of E3 ligase recruiters are currently available as E3 ligand for targeted protein degradation (TPD), although human genome encodes more than 600 E3 ligases. Thus, there is an urgent need to identify additional effective E3 ligase recruiters for TPD applications. In this review, we summarized the existing RING-type E3 ubiquitin ligase and their small molecule recruiters that act as effective E3 ligands of PROTAC degraders and their application in anticancer drug discovery. We believe that this review could serve as a reference in future development of efficient E3 ligands of PROTAC technology for cancer drug discovery and development.

Synthesis of SNIPERs against BCR-ABL with kinase inhibitors and a method to evaluate their growth inhibitory activity derived from BCR-ABL degradation

Specific and nongenetic IAP-dependent Protein Erasers (SNIPERs) are a kind of PROTACs recruiting IAP ubiquitin ligases to induce degradation of target proteins. We have developed a series of SNIPERs against BCR-ABL oncogenic kinases by employing kinase inhibitors as target ligands. Some of these SNIPERs show potent activities to degrade BCR-ABL protein and inhibit CML cell growth. However, since SNIPERs also inhibit kinase activity, it takes some ingenuity to show that degradation of BCR-ABL plays a significant role on growth inhibitory activity. Here we describe protocols for synthesizing SNIPERs against BCR-ABL oncogenic kinase that contain kinase inhibitors as target ligands, and methods for evaluating the growth inhibitory activity against cancer cells, especially focusing on a method to discriminate the significance of protein degradation from that of kinase inhibition.

Targeting androgen receptor degradation with PROTACs from bench to bedside

Inhibition of androgen receptor (AR) has been extensively investigated to treat prostate cancer. Resistance mechanisms such as increased levels of androgen production, increased AR gene, enhancer expression and AR point mutations always reduce the clinical efficacy. Design and discovery of small-molecule PROTAC AR degraders have been pursued as a new therapeutic strategy to overcome common resistance mechanisms developed during prostate cancer treatment. In the last two decades, potent and efficacious PROTAC AR degraders have been gotten rapid development and several such compounds have been advanced into preclinical phase and phase I/II trials for the treatment of human prostate cancers. Especially, the first PROTAC to enter the clinic, ARV-110, has shown good clinical effects in patients with mCRPC. This fully demonstrates the high clinical value of PROTAC strategy in treatment of human diseases. Here, we summarized the recent advances in the development of these potential clinical-stage PROTAC AR degraders.

Progress of small molecules for targeted protein degradation: PROTACs and other technologies

Recent years have witnessed the rapid development of targeted protein degradation (TPD), especially proteolysis targeting chimeras. These degraders have manifested many advantages over small molecule inhibitors. To date, a huge number of degraders have been excavated against over 70 disease-related targets. In particular, degraders against estrogen receptor and androgen receptor have crowded into phase II clinical trial. TPD technologies largely expand the scope of druggable targets, and provide powerful tools for addressing intractable problems that can not be tackled by traditional small molecule inhibitors. In this review, we mainly focus on the structures and biological activities of small molecule degraders as well as the elucidation of mechanisms of emerging TPD technologies. We also propose the challenges that exist in the TPD field at present.

Estrogen Receptor-α Targeting: PROTACs, SNIPERs, Peptide-PROTACs, Antibody Conjugated PROTACs and SNIPERs

Targeting selective estrogen subtype receptors through typical medicinal chemistry approaches is based on occupancy-driven pharmacology. In occupancy-driven pharmacology, molecules are developed in order to inhibit the protein of interest (POI), and their popularity is based on their virtue of faster kinetics. However, such approaches have intrinsic flaws, such as pico-to-nanomolar range binding affinity and continuous dosage after a time interval for sustained inhibition of POI. These shortcomings were addressed by event-driven pharmacology-based approaches, which degrade the POI rather than inhibit it. One such example is PROTACs (Proteolysis targeting chimeras), which has become one of the highly successful strategies of event-driven pharmacology (pharmacology that does the degradation of POI and diminishes its functions). The selective targeting of estrogen receptor subtypes is always challenging for chemical biologists and medicinal chemists. Specifically, estrogen receptor α (ER-α) is expressed in nearly 70% of breast cancer and commonly overexpressed in ovarian, prostate, colon, and endometrial cancer. Therefore, conventional hormonal therapies are most prescribed to patients with ER + cancers. However, on prolonged use, resistance commonly developed against these therapies, which led to selective estrogen receptor degrader (SERD) becoming the first-line drug for metastatic ER + breast cancer. The SERD success shows that removing cellular ER-α is a promising approach to overcoming endocrine resistance. Depending on the mechanism of degradation of ER-α, various types of strategies of developed.

Targeting micro-environmental pathways by PROTACs as a therapeutic strategy

Tumor microenvironment (TME) composes of multiple cell types and non-cellular components, which supports the proliferation, metastasis and immune surveillance evasion of tumor cells, as well as accounts for the resistance to therapies. Therefore, therapeutic strategies using small molecule inhibitors (SMIs) and antibodies to block potential targets in TME are practical for cancer treatment. Targeted protein degradation using PROteolysis-TArgeting Chimera (PROTAC) technic has several advantages over traditional SMIs and antibodies, including overcoming drug resistance. Thus many PROTACs are currently under development for cancer treatment. In this review, we summarize the recent progress of PROTAC development that target TME pathways and propose the potential direction of future PROTAC technique to advance as novel cancer treatment options.

Targeting the PTP1B-Bcr-Abl1 interaction for the degradation of T315I mutant Bcr-Abl1 in chronic myeloid leukemia

Small-molecule-induced degradation of mutant Bcr-Abl1 provides a potential approach to overcome Bcr-Abl1 tyrosine kinase inhibitor (TKI)-resistant chronic myeloid leukemia (CML). Our previous study reported that a synthetic steroidal glycoside SBF-1 showed remarkable anti-CML activity by inducing the degradation of native Bcr-Abl1 protein. Here, we observed the comparable growth inhibition for SBF-1 in CML cells harboring T315I mutant Bcr-Abl1 in vitro and in vivo. SBF-1 triggered its degradation through disrupting the interaction between protein-tyrosine phosphatase 1B (PTP1B) and Bcr-Abl1. Using SBF-1 as a tool, we found that Tyr46 in the PTP1B catalytic domain and Tyr852 in the Bcr-Abl1 pleckstrin-homology (PH) domain are critical for their interaction. Moreover, the phosphorylation of Tyr1086 within the Bcr-Abl1 SH2 domain recruited the E3 ubiquitin ligase c-Cbl to catalyze K27-linked ubiquitin chains, which serve as a recognition signal for p62-dependent autophagic degradation. PTP1B dephosphorylated Bcr-Abl1 at Tyr1086 and prevented the recruitment of c-Cbl, leading to the stability of Bcr-Abl1. This study unravels the action mechanism of PTP1B in stabilizing Bcr-Abl1 protein and indicates that the PTP1B-Bcr-Abl1 interaction might be one of druggable targets for TKI-resistant CML with point mutations.

Recent Advances in PROTACs for Drug Targeted Protein Research

Proteolysis-targeting chimera (PROTAC) is a heterobifunctional molecule. Typically, PROTAC consists of two terminals which are the ligand of the protein of interest (POI) and the specific ligand of E3 ubiquitin ligase, respectively, via a suitable linker. PROTAC degradation of the target protein is performed through the ubiquitin–proteasome system (UPS). The general process is that PROTAC binds to the target protein and E3 ligase to form a ternary complex and label the target protein with ubiquitination. The ubiquitinated protein is recognized and degraded by the proteasome in the cell. At present, PROTAC, as a new type of drug, has been developed to degrade a variety of cancer target proteins and other disease target proteins, and has shown good curative effects on a variety of diseases. For example, PROTACs targeting AR, BR, BTK, Tau, IRAK4, and other proteins have shown unprecedented clinical efficacy in cancers, neurodegenerative diseases, inflammations, and other fields. Recently, PROTAC has entered a phase of rapid development, opening a new field for biomedical research and development. This paper reviews the various fields of targeted protein degradation by PROTAC in recent years and summarizes and prospects the hot targets and indications of PROTAC.

PROTACs: The Future of Leukemia Therapeutics

The fight to find effective, long-lasting treatments for cancer has led many researchers to consider protein degrading entities. Recent developments in PROteolysis TArgeting Chimeras (PROTACs) have signified their potential as possible cancer therapies. PROTACs are small molecule, protein degraders that function by hijacking the built-in Ubiquitin-Proteasome pathway. This review mainly focuses on the general design and functioning of PROTACs as well as current advancements in the development of PROTACs as anticancer therapies. Particular emphasis is given to PROTACs designed against various types of Leukemia/Blood malignancies.

Targeting Protein Degradation Pathways in Tumors: Focusing on their Role in Hematological Malignancies

Cells must maintain their proteome homeostasis by balancing protein synthesis and degradation. This is facilitated by evolutionarily-conserved processes, including the unfolded protein response and the proteasome-based system of protein clearance, autophagy, and chaperone-mediated autophagy. In some hematological malignancies, including acute myeloid leukemia, misfolding or aggregation of the wild-type p53 tumor-suppressor renders cells unable to undergo apoptosis, even with an intact p53 DNA sequence. Moreover, blocking the proteasome pathway triggers lymphoma cell apoptosis. Extensive studies have led to the development of proteasome inhibitors, which have advanced into drugs (such as bortezomib) used in the treatment of certain hematological tumors, including multiple myeloma. New therapeutic options have been studied making use of the so-called proteolysis-targeting chimeras (PROTACs), that bind desired proteins with a linker that connects them to an E3 ubiquitin ligase, resulting in proteasomal-targeted degradation. This review examines the mechanisms of protein degradation in the cells of the hematopoietic system, explains the role of dysfunctional protein degradation in the pathogenesis of hematological malignancies, and discusses the current and future advances of therapies targeting these pathways, based on an extensive search of the articles and conference proceedings from 2005 to April 2022.

Key Considerations in Targeted Protein Degradation Drug Discovery and Development

Targeting proteins' enzymatic functions with small molecule inhibitors, as well as functions of receptor proteins with small-molecule agonists and antagonists, were the major forms of small-molecule drug development. These small-molecule modulators are based on a conventional occupancy-driven pharmacological approach. For proteome space traditionally considered undruggable by small-molecule modulators, such as enzymes with scaffolding functions, transcription factors, and proteins that lack well-defined binding pockets for small molecules, targeted protein degraders offer the opportunity to drug the proteome with an event-driven pharmacological approach. A degrader molecule, either PROTAC or molecular glue, brings the protein of interest (POI) and E3 ubiquitin ligase in close proximity and engages the ubiquitin-proteasome system (UPS), the cellular waste disposal system for the degradation of the POI. For the development of targeted protein degraders to meet therapeutic needs, several aspects will be considered, namely, the selective degradation of disease-causing proteins, the oral bioavailability of degraders beyond Lipinski's rule of five (bRo5) scope, demands of new E3 ubiquitin ligases and molecular glue degraders, and drug resistance of the new drug modality. This review will illustrate several under-discussed key considerations in targeted protein degradation drug discovery and development: 1) the contributing factors for the selectivity of PROTAC molecules and the design of PROTACs to selectively degrade synergistic pathological proteins; 2) assay development in combination with a multi-omics approach for the identification of new E3 ligases and their corresponding ligands, as well as molecular glue degraders; 3) a molecular design to improve the oral bioavailability of bRo5 PROTACs, and 4) drug resistance of degraders.

Recent advances in IAP-based PROTACs (SNIPERs) as potential therapeutic agents

Proteolytic targeting chimaeras (PROTACs) have been developed as an effective technology for targeted protein degradation. PROTACs are heterobifunctional molecules that can trigger the polyubiquitination of proteins of interest (POIs) by recruiting the ubiquitin-proteasome system, thereby inhibiting the intracellular level of POIs. To date, a variety of small-molecule PROTACs (CRBN, VHL, IAP, and MDM2-based PROTACs) have been developed. IAP-based PROTACs, also known as specific and nongenetic IAP-dependent protein erasers (SNIPERs), are used to degrade the target proteins closely related to diseases. Their structures consist of three parts, including target protein ligand, E3 ligase ligand, and the linker between them. So far, many SNIPERs have been extensively studied worldwide and have performed well in multiple diseases, especially cancer. In this review, we will present the most relevant advances in the field of SNIPERs and provide our perspective on the opportunities and challenges for SNIPERs to become therapeutic agents.

The progress of small-molecules and degraders against BCR-ABL for the treatment of CML

Chronic myeloid leukemia (CML) is a malignant disease of the hematopoietic system with crucial pathogenic protein named BCR-ABL, which endangers the life of patients severely. As a milestone of targeted drug, Imatinib has achieved great success in the treatment of CML. Nevertheless, inevitable drug resistance of Imatinib has occurred frequently in clinical due to the several mutations in the BCR-ABL kinase. Subsequently, the second-generation of tyrosine kinase inhibitors (TKIs) against BCR-ABL was developed to address the mutants of Imatinib resistance, except T315I. To date, the third-generation of TKIs targeting T315I has been developed for improving the selectivity and safety. Notably, the first allosteric inhibitor has been in market which could overcome the mutations in ATP binding site effectively. Meanwhile, some advanced technology, such as proteolysis-targeting chimeras (PROTAC) based on different E3 ligand, are highly expected to overcome the drug resistance by selectively degrading the targeted proteins. In this review, we summarized the current research progress of inhibitors and degraders targeting BCR-ABL for the treatment of CML.

Targeted Protein Degradation and Drug Discovery

Targeted protein degradation attracts attention as a novel modality for drug discovery as well as for basic research. Various types of degrader molecules have been developed so far, which include PROTACs and SNIPERs, E3 modulators, hydrophobic tagging molecules, IAP antagonists, and deubiquitylase inhibitors. PROTACs and SNIPERs are chimeric degrader molecules consisting of a target ligand linked to another ligand that binds to an E3 ubiquitin ligase. In the cells, they recruit an E3 ligase to the target protein, thereby inducing ubiquitylation and proteasomal degradation of the target protein. Because of their modular structure, novel PROTACs and SNIPERs targeting proteins of your interest can be rationally developed by substituting target ligands. In this article, various compounds capable of inducing protein degradation were overviewed, including SNIPER compounds developed in our laboratory.

Overcoming Cancer Drug Resistance Utilizing PROTAC Technology

Cancer drug resistance presents a major barrier to continued successful treatment of malignancies. Current therapies inhibiting proteins indicated in cancer progression are consistently found to lose efficacy as a result of acquired drug resistance, often caused by mutated or overexpressed protein targets. By hijacking the cellular ubiquitin-proteasome protein degradation machinery, proteolysis-targeting chimeras (PROTACs) offer an alternative therapeutic modality to cancer treatments with various potential advantages. PROTACs specific for a number of known cancer targets have been developed in the last 5 years, which present new options for remission in patients with previously untreatable malignancies and provide a foundation for future-generation compounds. One notable advantage of PROTACs, supported by evidence from a number of recent studies, is that they can overcome some of the resistance mechanisms to traditional targeted therapies. More recently, some groups have begun researching the use of PROTACs to successfully degrade mutated targets conferring cancer resistance against first-line treatments. In this review, we focus on analyzing the developments in PROTACs geared towards cancer resistance and targets that confer it in the search for new and successful therapies.

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.

The role of E3 ubiquitin ligase in multiple myeloma: potential for cereblon E3 ligase modulators in the treatment of relapsed/refractory disease

INTRODUCTION

Insights into the mechanisms of protein homeostasis and proteasomal degradation have led to new strategies of redirecting the ubiquitin-proteasome system (UPS) to reduce or eliminate proteins or survival factors key to malignant pathobiology, multiple myeloma (MM) in particular. These strategies have enabled researchers to target proteins that were previously considered difficult to modulate by pharmacological means.

AREAS COVERED

This review provides a brief overview of UPS biology, particularly the role of the CRL4^CRBN E3 ubiquitin ligase complex, and summarizes current strategies for co-opting the UPS, including CELMoD compounds, SNIPERs, PROTACs, and degronimids. A detailed discussion is provided on lead CELMoD compounds iberdomide and mezigdomide, which are currently being evaluated in clinical trials in patients with MM.

EXPERT OPINION

Since a high proportion of patients develop drug resistance, it is vital to have novel therapeutic agents for treating relapsed patients with MM more effectively. It is encouraging that the expanding pathophysiological insight into cellular signaling pathways in MM increasingly translates into the development of novel therapeutic agents such as targeted protein degraders. This holds promise for improving outcomes in MM and beyond.

Development of a degrader against oncogenic fusion protein FGFR3-TACC3

Fibroblast growth factor receptor 3-transforming acidic coiled-coil containing protein 3 (FGFR3-TACC3), which has been identified in many cancers such as glioblastoma and bladder cancer, is a potent oncogenic fusion protein that induces constitutive activation of FGFR signaling, resulting in uncontrolled cell proliferation. Although several tyrosine kinase inhibitors against FGFR are currently under development, resistance to such types of inhibitors in patients has become a concern. In this study, a chimeric molecule SNIPER(TACC3)-11 (5a) was developed and found to reduce FGFR3-TACC3 levels effectively. Compound 5a conjugated KHS108 (a TACC3 ligand) to an LCL161 derivative (11) (an inhibitor of apoptosis protein [IAP] ligand) with a PEG linker (n = 2). Mechanistical analysis showed that cellular IAP1 was required for the reduction of FGFR3-TACC3 levels. Consistent with the decrease in FGFR3-TACC3 levels, compound 5a suppressed the growth of FGFR3-TACC3 positive cells. Thus, compound 5a is a candidate therapeutic with a novel drug modality against cancers that exhibit FGFR3-TACC3-dependent proliferation and exerts pharmacological effects distinct from FGFR3 kinase inhibitors because it lacks substructures crucial for kinase inhibition.

Combined GSTT1 Null, GSTM1 Null and XPD Lys/Lys Genetic Polymorphisms and Their Association with Increased Risk of Chronic Myeloid Leukemia

Purpose

Glutathione S-transferases (GSTT1 and GSTM1) are instrumental in detoxification process of activated carcinogens. Nucleotide excision repair is carried out by DNA helicase encoded by xeroderma pigmentosum group D (XPD) genes and aberrations in the XPD gene predisposes to increased risk of cancer. The present study aimed to investigate GSTT1, GSTM1 and XPD polymorphisms in newly diagnosed chronic myeloid leukemia (CML) patients and to examine the association of these polymorphisms with the risk of developing CML.

Patients and Methods

This case–control study was carried out from June 2019 to August 2021 involving 150 newly diagnosed patients with CML and an equal number of randomly selected age- and sex-matched healthy individuals. A multiplex-PCR assay was used to genotype GSTT1 null and GSTM1 null polymorphisms. XPD gene polymorphism was detected by PCR-RFLP using predesigned gene-specific primers.

Results

GSTT1 and GSTM1 null polymorphisms were detected in 42.7% and 61.3% of cases, respectively, compared to 18% and 35.3% for controls. The combination of both GST null polymorphisms revealed a significant association with CML. Frequencies of XPD Lys751Gln genotypes in cases were 62.7% heterozygous Lys/Gln, 24% homozygous Lys/Lys and 13.3% homozygous Gln/Gln, while in the controls were 74.7%, 20%, and 5.3%, respectively. Significant differences were also noted regarding the combination of GSTT1/GSTM1 null with XPD Lys/Lys, and GSTM1 null with XPD Lys/Lys.

Conclusion

In conclusion, GSTT1 null, GSTM1 null and XPD polymorphisms showed positive association with the risk of development of CML. Furthermore, age and gender did not exhibit any association with the studied polymorphisms, while CML phases were associated with GSTT1 null polymorphism.

Advancing targeted protein degradation for cancer therapy

The human proteome contains approximately 20,000 proteins, and it is estimated that more than 600 of them are functionally important for various types of cancers, including nearly 400 non-enzyme proteins that are challenging to target by traditional occupancy-driven pharmacology. Recent advances in the development of small-molecule degraders, including molecular glues and heterobifunctional degraders such as proteolysis-targeting chimeras (PROTACs), have made it possible to target many proteins that were previously considered undruggable. In particular, PROTACs form a ternary complex with a hijacked E3 ubiquitin ligase and a target protein, leading to polyubiquitination and degradation of the target protein. The broad applicability of this approach is facilitated by the flexibility of individual E3 ligases to recognize different substrates. The vast majority of the approximately 600 human E3 ligases have not been explored, thus presenting enormous opportunities to develop degraders that target oncoproteins with tissue, tumour and subcellular selectivity. In this Review, we first discuss the molecular basis of targeted protein degradation. We then offer a comprehensive account of the most promising degraders in development as cancer therapies to date. Lastly, we provide an overview of opportunities and challenges in this exciting field.

Light-Controllable PROTACs for Temporospatial Control of Protein Degradation

PROteolysis-TArgeting Chimeras (PROTACs) is an emerging and promising approach to target intracellular proteins for ubiquitination-mediated degradation, including those so-called undruggable protein targets, such as transcriptional factors and scaffold proteins. To date, plenty of PROTACs have been developed to degrade various disease-relevant proteins, such as estrogen receptor (ER), androgen receptor (AR), RTK, and CDKs. However, the on-target off-tissue and off-target effect is one of the major limitation that prevents the usage of PROTACs in clinic. To this end, we and several other groups have recently developed light-controllable PROTACs, as the representative for the third generation controllable PROTACs, by using either photo-caging or photo-switch approaches. In this review, we summarize the emerging light-controllable PROTACs and the prospective for other potential ways to achieve temporospatial control of PROTACs.

E3 Ligase Ligands in Successful PROTACs: An Overview of Syntheses and Linker Attachment Points

Proteolysis-targeting chimeras (PROTACs) have received tremendous attention as a new and exciting class of therapeutic agents that promise to significantly impact drug discovery. These bifunctional molecules consist of a target binding unit, a linker, and an E3 ligase binding moiety. The chemically-induced formation of ternary complexes leads to ubiquitination and proteasomal degradation of target proteins. Among the plethora of E3 ligases, only a few have been utilized for the novel PROTAC technology. However, extensive knowledge on the preparation of E3 ligands and their utilization for PROTACs has already been acquired. This review provides an in-depth analysis of synthetic entries to functionalized ligands for the most relevant E3 ligase ligands, i.e. CRBN, VHL, IAP, and MDM2. Less commonly used E3 ligase and their ligands are also presented. We compare different preparative routes to E3 ligands with respect to feasibility and productivity. A particular focus was set on the chemistry of the linker attachment by discussing the synthetic opportunities to connect the E3 ligand at an appropriate exit vector with a linker to assemble the final PROTAC. This comprehensive review includes many facets involved in the synthesis of such complex molecules and is expected to serve as a compendium to support future synthetic attempts towards PROTACs.

In vivo synthetic chemistry of proteolysis targeting chimeras (PROTACs)

Chemical knockdown of therapeutic targets using proteolysis targeting chimeras (PROTACs) is a rapidly developing field in drug discovery, but PROTACs are bifunctional molecules that generally show poor bioavailability due to their relatively high molecular weight. Recent developments aimed at the development of next-generation PROTACs include the in vivo synthesis of PROTAC molecules, and the exploitation of PROTACs as chemical tools for in vivo synthesis of ubiquitinated proteins. This short review covers recent advances in these areas and discusses the prospects for in vivo synthetic PROTAC technology.

Discovery of novel BCR-ABL PROTACs based on the cereblon E3 ligase design, synthesis, and biological evaluation

Protein degradation is a promising strategy for drug development. Proteolysis-targeting chimeras (PROTACs) hijacking the E3 ligase cereblon (CRBN) exhibit enormous potential and universal degradation performance due to the small molecular weight of CRBN ligands. In this study, the CRBN-recruiting PROTACs were explored on the degradation of oncogenic fusion protein BCR-ABL, which drives the pathogenesis of chronic myeloid leukemia (CML). A series of novel PROTACs were synthesized by conjugating BCR-ABL inhibitor dasatinib to the CRBN ligand including pomalidomide and lenalidomide, and the extensive structure-activity relationship (SAR) studies were performed focusing on optimization of linker parameters. Therein, we uncovered that pomalidomide-based degrader 17 (SIAIS056), possessing sulfur-substituted carbon chain linker, exhibits the most potent degradative activity in vitro and favorable pharmacokinetics in vivo. Besides, degrader 17 also degrades a variety of clinically relevant resistance-conferring mutations of BCR-ABL. Furthermore, degrader 17 induces significant tumor regression against K562 xenograft tumors. Our study indicates that 17 as an efficacious BCR-ABL degrader warrants intensive investigation for the future treatment of BCR-ABL⁺ leukemia.

Design, synthesis, and biological evaluation of Bcr-Abl PROTACs to overcome T315I mutation

Bcr-Abl threonine 315 to isoleucine 315 (T315I) gatekeeper mutation induced drug resistance remains an unmet clinical challenge for the treatment of chronic myeloid leukemia (CML). Chemical degradation of Bcr-Abl^T315I protein has become a potential strategy to overcome drug resistance. Herein, we first described the design, synthesis, and evaluation of a new class of selective Bcr-Abl^T315I proteolysis-targeting chimeric (PROTAC) degraders based on GZD824 (reported as Bcr-Abl^T315I inhibitor by our group). One of the degrader 7o with 6-member carbon chain linkage with pomalidomide exhibits the most potent degradation efficacy with DR of 69.89% and 94.23% at 100 and 300 nmol/L, respectively, and has an IC₅₀ value of 26.8 ± 9.7 nmol/L against Ba/F3^T315I cells. Further, 7o also displays substantial tumor regression against Ba/F3-Bcr-Abl^T315I xenograft model in vivo.

Major advances in targeted protein degradation: PROTACs, LYTACs, and MADTACs

Of late, targeted protein degradation (TPD) has surfaced as a novel and innovative chemical tool and therapeutic modality. By co-opting protein degradation pathways, TPD facilitates complete removal of the protein molecules from within or outside the cell. While the pioneering Proteolysis-Targeting Chimera (PROTAC) technology and molecular glues hijack the ubiquitin-proteasome system, newer modalities co-opt autophagy or the endo-lysosomal pathway. Using this mechanism, TPD is posited to largely expand the druggable space far beyond small-molecule inhibitors. In this review, we discuss the major advances in TPD, highlight our current understanding, and explore outstanding questions in the field.

Specific non-genetic IAP-based protein erasers (SNIPERs) as a potential therapeutic strategy

As a newly emerged technology, PROTAC (proteolysis targeting chimera) is a promising therapeutic strategy for varieties of diseases. Unlike small molecule inhibitors, PROTACs catalytically induce target proteins degradation, including currently "undruggable" target proteins. In addition, PROTACs can be a potentially successful strategy to overcome drug resistance. IAPs can inhibit apoptosis by inhibiting caspase, and also exhibits the activity of E3 ubiquitin ligase. Specific and nongenetic IAP-based protein erasers (SNIPERs) are hybrid molecules that designed based on IAPs, and used to degrade the target proteins closely associated with diseases. Their structures consist of three parts, including target protein ligand, E3 ligase ligand and the linker between them. SNIPERs (PROTACs) degrade diseases-associated proteins through human inherent ubiquitin-proteasome system. So far, many SNIPERs have been developed to treat diseases that difficult to handle by traditional methods, such as radiotherapy, chemotherapy and small molecule inhibitors, and showed promising prospects in application. In this paper, the recent advances of SNIPERs were summarized, and the chances and challenges associated with this area were also highlighted.

[Intermolecular interaction-based ubiquitin-proteasome system-targeting drug discovery]

We review recent advances of Ubiquitin-Proteasome System (UPS)-based research and development with increased focus as drug discovery approaches and introduce applications of chimera-type small molecule compounds (SNIPER/PROTAC) that selectively promote degradation of a drug target protein. UPS makes the point (polyubiquitin chain) of targeting protein as a substrate and has a property that degrade the target protein with proteasome. Protein knockout technologies degrade the drug target protein by apply this protein degrading system. In current technologies, polyubiquitin chains are artificially added to the drug target proteins through small molecules and introduce degradation of the target proteins. The approaches are divided into 2 types, one of which is E3 modulator-based technology represented by thalidomide, the other one is chimera compound-based technology represented by SNIPER/PROTAC. Furthermore, novel technologies are practically used to identify small molecule E3 binders as well as E3-targeting protein binders. These new approaches are expected to contribute to the efficient UPS-based drug discovery.

Protocols for Synthesis of SNIPERs and the Methods to Evaluate the Anticancer Effects

Inducing degradation of undruggable target proteins by the use of chimeric small molecules, represented by proteolysis-targeting chimeras, is a promising strategy for drug development. We developed a series of chimeric molecules, termed "specific and nongenetic inhibitor of apoptosis protein (IAP)-dependent protein erasers" (SNIPERs) that recruit IAP ubiquitin ligases to induce degradation of target proteins. SNIPERs also induce degradation of some IAPs, including cIAP1 and XIAP, which are antiapoptotic proteins that are overexpressed in many cancers. Such protein degraders have unique properties that could be especially useful in cancer therapy. This chapter describes (1) the design and synthesis of SNIPER compounds, (2) the methods used for the detection of target protein degradation and ubiquitylation, and (3) the protocol to evaluate the antitumor activity of SNIPER.

Snapshots and ensembles of BTK and cIAP1 protein degrader ternary complexes

Heterobifunctional chimeric degraders are a class of ligands that recruit target proteins to E3 ubiquitin ligases to drive compound-dependent protein degradation. Advancing from initial chemical tools, protein degraders represent a mechanism of growing interest in drug discovery. Critical to the mechanism of action is the formation of a ternary complex between the target, degrader and E3 ligase to promote ubiquitination and subsequent degradation. However, limited insights into ternary complex structures exist, including a near absence of studies on one of the most widely co-opted E3s, cellular inhibitor of apoptosis 1 (cIAP1). In this work, we use a combination of biochemical, biophysical and structural studies to characterize degrader-mediated ternary complexes of Bruton's tyrosine kinase and cIAP1. Our results reveal new insights from unique ternary complex structures and show that increased ternary complex stability or rigidity need not always correlate with increased degradation efficiency.

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.

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.

Design and synthesis of peptide-based chimeric molecules to induce degradation of the estrogen and androgen receptors

Peptide-based inducers of estrogen receptor (ER) α and androgen receptor (AR) degradations via the ubiquitin-proteasome system (UPS) were developed. The designated inducers were composed of two biologically active scaffolds: the helical peptide PERM3, which is an LXXLL-like mimic of the coactivator SRC-1, and various small molecules (MV1, LCL161, VH032, and POM) that bind to E3 ligases (IAPs, VHL, and cereblon, respectively), to induce ubiquitylation of nuclear receptors that bind to SRC-1. All of the synthesized chimeric E3 ligand-containing molecules induced the UPS-mediated degradation of ERα and AR. The PERM3 peptide was applicable for the development of the ERα and AR degraders using these E3 ligands.

Proteolysis targeting chimeras (PROTACs) are emerging therapeutics for hematologic malignancies

Proteolysis targeting chimeras (PROTACs) are heterobifunctional small molecules that utilize the ubiquitin proteasome system (UPS) to degrade proteins of interest (POI). PROTACs are potentially superior to conventional small molecule inhibitors (SMIs) because of their unique mechanism of action (MOA, i.e., degrading POI in a sub-stoichiometric manner), ability to target “undruggable” and mutant proteins, and improved target selectivity. Therefore, PROTACs have become an emerging technology for the development of novel targeted anticancer therapeutics. In fact, some of these reported PROTACs exhibit unprecedented efficacy and specificity in degrading various oncogenic proteins and have advanced to various stages of preclinical and clinical development for the treatment of cancer and hematologic malignancy. In this review, we systematically summarize the known PROTACs that have the potential to be used to treat various hematologic malignancies and discuss strategies to improve the safety of PROTACs for clinical application. Particularly, we propose to use the latest human pan-tissue single-cell RNA sequencing data to identify hematopoietic cell type-specific/selective E3 ligases to generate tumor-specific/selective PROTACs. These PROTACs have the potential to become safer therapeutics for hematologic malignancies because they can overcome some of the on-target toxicities of SMIs and PROTACs.

Challenges and Opportunities in Cancer Drug Resistance

There has been huge progress in the discovery of targeted cancer therapies in recent years. However, even for the most successful and impactful cancer drugs which have been approved, both innate and acquired mechanisms of resistance are commonplace. These emerging mechanisms of resistance have been studied intensively, which has enabled drug discovery scientists to learn how it may be possible to overcome such resistance in subsequent generations of treatments. In some cases, novel drug candidates have been able to supersede previously approved agents; in other cases they have been used sequentially or in combinations with existing treatments. This review summarizes the current field in terms of the challenges and opportunities that cancer resistance presents to drug discovery scientists, with a focus on small molecule therapeutics. As part of this review, common themes and approaches have been identified which have been utilized to successfully target emerging mechanisms of resistance. This includes the increase in target potency and selectivity, alternative chemical scaffolds, change of mechanism of action (covalents, PROTACs), increases in blood-brain barrier permeability (BBBP), and the targeting of allosteric pockets. Finally, wider approaches are covered such as monoclonal antibodies (mAbs), bispecific antibodies, antibody drug conjugates (ADCs), and combination therapies.