PROTAC-Mediated Degradation of Estrogen Receptor in the Treatment of Cancer

Development of PROTACs targeting estrogen receptor: an emerging technique for combating endocrine resistance

Despite the success of endocrine therapies in treating ER-positive breast cancer, the development of resistance remains a significant challenge. Estrogen receptor targeting proteolysis-targeting chimeras (ER PROTACs) offer a unique approach by harnessing the ubiquitin-proteasome system to degrade ER, potentially bypassing resistance mechanisms. In this review, we present the drug design, efficacy and early clinical trials of these ER PROTACs. This review underscores the academic and industrial opportunities presented by this emerging technology, as well as the challenges that must be addressed to translate these findings into effective clinical therapies.

Mechanisms of resistance to tyrosine kinase inhibitor-targeted therapy and overcoming strategies

Tyrosine kinase inhibitor (TKI)-targeted therapy has revolutionized cancer treatment by selectively blocking specific signaling pathways crucial for tumor growth, offering improved outcomes with fewer side effects compared with conventional chemotherapy. However, despite their initial effectiveness, resistance to TKIs remains a significant challenge in clinical practice. Understanding the mechanisms underlying TKI resistance is paramount for improving patient outcomes and developing more effective treatment strategies. In this review, we explored various mechanisms contributing to TKI resistance, including on-target mechanisms and off-target mechanisms, as well as changes in the tumor histology and tumor microenvironment (intrinsic mechanisms). Additionally, we summarized current therapeutic approaches aiming at circumventing TKI resistance, including the development of next-generation TKIs and combination therapies. We also discussed emerging strategies such as the use of dual-targeted antibodies and PROteolysis Targeting Chimeras. Furthermore, we explored future directions in TKI-targeted therapy, including the methods for detecting and monitoring drug resistance during treatment, identification of novel targets, exploration of dual-acting kinase inhibitors, application of nanotechnologies in targeted therapy, and so on. Overall, this review provides a comprehensive overview of the challenges and opportunities in TKI-targeted therapy, aiming to advance our understanding of resistance mechanisms and guide the development of more effective therapeutic approaches in cancer treatment.

Dysregulation of deubiquitination in breast cancer

Breast cancer (BC) is a highly frequent malignant tumor that poses a serious threat to women's health and has different molecular subtypes, histological subtypes, and biological features, which act by activating oncogenic factors and suppressing cancer inhibitors. The ubiquitin-proteasome system (UPS) is the main process contributing to protein degradation, and deubiquitinases (DUBs) are reverse enzymes that counteract this process. There is growing evidence that dysregulation of DUBs is involved in the occurrence of BC. Herein, we review recent research findings in BC-associated DUBs, describe their nature, classification, and functions, and discuss the potential mechanisms of DUB-related dysregulation in BC. Furthermore, we present the successful treatment of malignant cancer with DUB inhibitors, as well as analyzing the status of targeting aberrant DUBs in BC.

Ubiquitination of Immune System and Cancer Therapy

Ubiquitination is a post-translational modification mechanism which regulates a variety of signaling pathways and crucial biological processes. It has long been known that ubiquitination regulates the fundamental cellular processes through the induction of proteasomal degradation of target proteins. Meanwhile, the nondegradative types of polyubiquitination modification have been appreciated as important regulatory machinery by modulating the activity or subcellular localization of key signaling proteins. The function of ubiquitination plays an important role in immune responses, which helps to maintain the stability of the internal environment and to control over protein stability and function and are thus critical for the regulation of both innate and adaptive immunity. Furthermore, ubiquitination also regulates both tumor-suppressing and tumor-promoting pathways in cancer. In this review, we will discuss recent progress regarding how ubiquitination regulates immune responses, focusing on Toll-like receptors signaling in innate immunity, T cell activation, TCR signaling, and tumor immunotherapy.

Complete elimination of estrogen receptor α by PROTAC estrogen receptor α degrader ERD-148 in breast cancer cells

PURPOSE

Estrogen Receptor α (ERα) is a well-established therapeutic target for Estrogen Receptor (ER)-positive breast cancers. Both Selective Estrogen Receptor Degraders (SERD) and PROTAC ER degraders are synthetic compounds suppressing the ER activity through the degradation of ER. However, the differences between SERD and PROTAC ER degraders are far from clear.

METHODS

The effect of PROTAC ER degrader ERD-148 and SERD fulvestrant on protein degradation was evaluated by western blot analysis. The cell proliferation was tested by WST-8 assays and the gene expressions were assessed by gene microarray and real-time RT-PCR analysis after the compound treatment.

RESULTS

ERD-148 is a potent and selective PROTAC ERα degrader. It degrades not only unphosphorylated ERα but also the phosphorylated ERα in the cells. In contrast, the SERD fulvestrant showed much-reduced degradation potency on the phosphorylated ERα. The more complete degradation of ERα by ERD-148 translates into a greater maximum cell growth inhibition. However, ERD-148 and fulvestrant share a similar gene regulation profile except for the variation of regulation potency. Further studies indicate that ERD-148 degrades the ERα in fulvestrant-resistant cells.

CONCLUSION

PROTAC ER degrader has a different mechanism of action compared to SERD which may be used in treating fulvestrant-resistant cancers.

Development of heterodimeric estrogen receptor alpha antagonists to target simultaneously the ligand and coactivator binding site

One-third of breast cancer patients will develop recurrent cancer within 15 years of endocrine treatment. Notably, tumor growth in a hormone-refractory state still relies on the interaction between estrogen receptor alpha (ERα) and upregulated coactivators. Herein, we suggest that simultaneous targeting of the primary ligand binding site (LBS) and the coactivator binding site (CABS) at ERα represents a promising alternative therapeutic strategy to overcome mutation-driven resistance in breast cancer. We synthesized two series of compounds that connect the LBS-binder (E)-3-{4-[8-fluoro-4-(4-hydroxyphenyl)-2,3-dihydrobenzo[b]oxepin-5-yl]phenyl}acrylic acid 8 with the coactivator binding site inhibitors (CBIs) 4,6-bis(isobutyl(methyl)amino)pyrimidine or 3-(5-methoxy-1H-benzo[d]imidazol-2-yl)propanoic acid via covalent linkage. The most active benzoxepine-pyrimidine conjugate 31 showed strong inhibition of estradiol-induced transactivation (IC₅₀  =  18.2 nM (ERα) and 61.7 nM (ERβ)) in a luciferase reporter gene assay as well as high antiproliferative effects in MCF-7 (IC₅₀  = 65.9 nM) and tamoxifen-resistant MCF-7/TamR (IC₅₀  = 88.9 nM) breast cancer cells. All heterodimers exhibited two- to sevenfold higher antagonism at ERα (compared with ERβ) and were superior to the acrylic acid precursor 8 in terms of ER antagonism and antiproliferative activity. It was demonstrated on the example of 31 that the compounds did not influence the ERα content in MCF-7 cells and therefore act as pure antiestrogens without downregulating potency. Possible interactions of the CBI at the receptor surface, which enhanced the biological activities, were evaluated using molecular docking studies.

Discovery of highly potent proapoptotic antiestrogens in a series of androst-5,16-dienes D-modified with imidazole-annulated pendants

Heterocyclic derivatives of steroid hormones are potent anticancer agents, which are used in the chemotherapy of breast and prostate cancers. Here, we describe a novel series of androstenes, D-modified with imidazole-annulated pendants, with significant anticancer activity. Novel C17-linked imidazole-annulated heterocyclic derivatives of dehydropregnenolone acetate were synthesized by the cyclocondensation with amidines using 3β-acetoxy-21-bromopregna-5,16-dien-20-one as the substrate. The antiproliferative potency of all the synthesized compounds was evaluated against human prostate (22Rv1) and human breast (MCF7) cancer cell lines and cytochromes P450. The lead compound, imidazo[1,2-a]pyridine derivative 3h, was revealed to be a promising candidate for future anticancer drug design, particularly against ERα-positive breast cancer. Lead compound 3h was found to be selective against MCF7 cells with IC₅₀ of 0.1μM and to act as both a potent selective agent blocking estrogen receptor α, which is involved in the stimulation of breast cancer growth, and an effective apoptosis inducer. The potential ability of compound 3h to bind to ERα was studded using molecular docking and molecular dynamics simulation. The selectivity analysis showed that lead steroid 3h produces no effects on cytochromes P450 CYP17A1, CYP7A1, and CYP21A2.

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.

An overview of PROTACs: a promising drug discovery paradigm

Proteolysis targeting chimeras (PROTACs) technology has emerged as a novel therapeutic paradigm in recent years. PROTACs are heterobifunctional molecules that degrade target proteins by hijacking the ubiquitin-proteasome system. Currently, about 20-25% of all protein targets are being studied, and most works focus on their enzymatic functions. Unlike small molecules, PROTACs inhibit the whole biological function of the target protein by binding to the target protein and inducing subsequent proteasomal degradation. PROTACs compensate for limitations that transcription factors, nuclear proteins, and other scaffolding proteins are difficult to handle with traditional small-molecule inhibitors. Currently, PROTACs have successfully degraded diverse proteins, such as BTK, BRD4, AR, ER, STAT3, IRAK4, tau, etc. And ARV-110 and ARV-471 exhibited excellent efficacy in clinical II trials. However, what targets are appropriate for PROTAC technology to achieve better benefits than small-molecule inhibitors are not fully understood. And how to rationally design an efficient PROTACs and optimize it to be orally effective poses big challenges for researchers. In this review, we summarize the features of PROTAC technology, analyze the detail of general principles for designing efficient PROTACs, and discuss the typical application of PROTACs targeting different protein categories. In addition, we also introduce the progress of relevant clinical trial results of representative PROTACs and assess the challenges and limitations that PROTACs may face. Collectively, our studies provide references for further application of PROTACs.

Chemistries of bifunctional PROTAC degraders

Proteolysis targeting chimeras (PROTACs) technology is a novel and promising therapeutic strategy using small molecules to induce ubiquitin-dependent degradation of proteins. It has received extensive attention from both academia and industry as it can potentially access previously inaccessible targets. However, the design and optimization of PROTACs present big challenges for researchers, and the general strategy for its development and optimization is a lot of trial and error based on experience. This review highlights the important advances in this rapidly growing field and critical limitations of the traditional trial-and-error approach to developing PROTACs by analyzing numerous representative examples of PROTACs development. We summarize and analyze the general principles and strategies for PROTACs design and optimization from the perspective of chemical structure design, and propose potential future pathways to facilitate the development of PROTACs.

PROTACs: great opportunities for academia and industry (an update from 2020 to 2021)

PROteolysis TArgeting Chimeras (PROTACs) technology is a new protein-degradation strategy that has emerged in recent years. It uses bifunctional small molecules to induce the ubiquitination and degradation of target proteins through the ubiquitin-proteasome system. PROTACs can not only be used as potential clinical treatments for diseases such as cancer, immune disorders, viral infections, and neurodegenerative diseases, but also provide unique chemical knockdown tools for biological research in a catalytic, reversible, and rapid manner. In 2019, our group published a review article "PROTACs: great opportunities for academia and industry" in the journal, summarizing the representative compounds of PROTACs reported before the end of 2019. In the past 2 years, the entire field of protein degradation has experienced rapid development, including not only a large increase in the number of research papers on protein-degradation technology but also a rapid increase in the number of small-molecule degraders that have entered the clinical and will enter the clinical stage. In addition to PROTAC and molecular glue technology, other new degradation technologies are also developing rapidly. In this article, we mainly summarize and review the representative PROTACs of related targets published in 2020-2021 to present to researchers the exciting developments in the field of protein degradation. The problems that need to be solved in this field will also be briefly introduced.

Targeting key proteins involved in transcriptional regulation for cancer therapy: Current strategies and future prospective

The key proteins involved in transcriptional regulation play convergent roles in cellular homeostasis, and their dysfunction mediates aberrant gene expressions that underline the hallmarks of tumorigenesis. As tumor progression is dependent on such abnormal regulation of transcription, it is important to discover novel chemical entities as antitumor drugs that target key tumor-associated proteins involved in transcriptional regulation. Despite most key proteins (especially transcription factors) involved in transcriptional regulation are historically recognized as undruggable targets, multiple targeting approaches at diverse levels of transcriptional regulation, such as epigenetic intervention, inhibition of DNA-binding of transcriptional factors, and inhibition of the protein-protein interactions (PPIs), have been established in preclinically or clinically studies. In addition, several new approaches have recently been described, such as targeting proteasomal degradation and eliciting synthetic lethality. This review will emphasize on accentuating these developing therapeutic approaches and provide a thorough conspectus of the drug development to target key proteins involved in transcriptional regulation and their impact on future oncotherapy.

Proteolysis-targeting chimeras and their implications in breast cancer

Breast cancer (BC) is a highly heterogeneous neoplasm of the mammary tissue, causing the deaths of a large number of women worldwide. Nearly 70% and 20% of BC cases are estrogen receptor alpha positive (ERα+) and human epidermal growth factor receptor 2-positive (HER2+), respectively; therefore, ER and HER2 targeted therapies have been employed in BC treatment. However, resistance to these therapies has been reported, indicating a need for developing novel therapeutic strategies. Proteolysis-targeting chimeras (PROTACs) are new, promising therapeutic tools designed with a bimodular structure: one module allows specific binding to target proteins, and the other module allows efficient degradation of these target proteins. In this paper, PROTACs and their potential in controlling the progression of ERα and HER2+ BC are discussed.

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.

Advances in targeting 'undruggable' transcription factors with small molecules

Transcription factors (TFs) represent key biological players in diseases including cancer, autoimmunity, diabetes and cardiovascular disease. However, outside nuclear receptors, TFs have traditionally been considered 'undruggable' by small-molecule ligands due to significant structural disorder and lack of defined small-molecule binding pockets. Renewed interest in the field has been ignited by significant progress in chemical biology approaches to ligand discovery and optimization, especially the advent of targeted protein degradation approaches, along with increasing appreciation of the critical role a limited number of collaborators play in the regulation of key TF effector genes. Here, we review current understanding of TF-mediated gene regulation, discuss successful targeting strategies and highlight ongoing challenges and emerging approaches to address them.

Selective degradation of the estrogen receptor in the treatment of cancers

Estrogen receptor subtype α (ERα) plays key roles in breast cancers, and has been a target for endocrine therapy for a long time. Unfortunately, long-term treatment by Aromatase Inhibitors (AIs) or Selective Estrogen Receptor Modulators (SERMs) could cause drug resistance and also would increase the risk for uterine cancer. Therefore, novel anti-breast cancer drugs based on different mechanisms of action have received significant attention, especially through the strategies of selective degradation of ER. In this article, the latest research progress of selective targeting ER for degradation, including Selective ER Downregulators (SERDs), Proteolysis Targeting Chimaeras (PROTACs) and other techniques, was reviewed, and the applications and problems to be solved were prospected.

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.

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.

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.

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 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.

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.