Treatment of Prostate Cancers and Kennedy's Disease by PROTAC-Androgen Receptor 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.

MiR-124-3p inhibits tumor progression in prostate cancer by targeting EZH2

Prostate cancer (PCa) is widespread cancer with significant morbidity and mortality rates. MicroRNAs (miRNAs) have been identified as important post-transcriptional modulators in various malignancies. This study investigated the miR-124-3p effect on PCa cell proliferation, infiltration, and apoptosis. EZH2 and miR-124-3p expression levels were measured in PCa tissues. PCa cell lines DU145 and PC3 were transfected with miR-124-3p inhibitors or analogs. EZH2 and miR-124-3p linkage was validated by conducting the luciferase enzyme reporter test. The cell viability and apoptosis were assessed by flow cytometry and MTT test. Cell movement was noted during infiltration using transwell assays. EZH2, AKT, and mTOR contents were assessed using qRT-PCR and western blotting. In clinical PCa specimens, miR-124-3p and EZH2 contents were inversely correlated. Further research has demonstrated that EZH2 is the miR-124-3p direct target. Furthermore, miR-124-3p overexpression reduced EZH2 levels and lowered cell viability, infiltration, and promoted cell death, whereas miR-124-3p silencing had the opposite effect. Overexpression of miR-124-3p decreased the phosphorylation level of AKT and mTOR, whereas miR-124-3p downregulation produced the opposite result. Our findings depict that miR-124-3p prevents PCa proliferative and invasive processes while promoting apoptosis by targeting EZH2.

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.

Therapeutic Strategies to Target the Androgen Receptor

The androgen receptor (AR) plays a key role in the maintenance of muscle and bone and the support of male sexual-related functions, as well as in the progression of prostate cancer. Accordingly, AR-targeted therapies have been developed for the treatment of related human diseases and conditions. AR agonists are an important class of drugs in the treatment of bone loss and muscle atrophy. AR antagonists have also been developed for the treatment of prostate cancer, including metastatic castration-resistant prostate cancer (mCRPC). Additionally, selective AR degraders (SARDs) have been reported. More recently, heterobifunctional degrader molecules of AR have been developed, and four such compounds are now in clinical development for the treatment of human prostate cancer. This review attempts to summarize the different types of compounds designed to target AR and the current frontiers of research on this important therapeutic target.

PROTACs: Novel approach for cancer breakdown by breaking proteins

Ubiquitination defects have been reported in various diseases, including neurodegenerative diseases, metabolic disorders and cancer. Balance between degradation and synthesis of the proteins to treat cancer can be managed by designing a chimeric molecule, known as Proteolysis Targeting Chimeric molecule (Lee, Kim et al. 2021). Proteolysis-targeting chimeras (PROTACs) acts as a tool for conducting therapeutic intervention. It eradicates or reduces the proteins that are responsible for causing diseases. Each PROTAC contains a target warhead, an E3 ligand and a linker. E3 ligases are recruited by these bifunctional molecules, and the Ubiquitin (Ub) Proteasome System (UPS) is used to target the degradation of specific proteins. As compared to inhibition, this degradation offers several advantages in the drug resistance, selectivity, and potency. Thus, numerous small molecule PROTACs are identified so far. In this review, the development of PROTACs, historical milestones, the biological mechanism, advantages and recent progress, and role of PROTAC in prostate cancer, breast cancer, non-hodgkin lymphoma, multiple myeloma, and malignant peripheral nerve sheath tumors are summarized.

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.

Chemical degrader enhances the treatment of androgen receptor-positive triple-negative breast cancer

Androgen receptor (AR) is a promising therapeutic target for AR-positive triple-negative breast cancer (TNBC). However, clinical trials of AR inhibitors only reveal modest therapeutic efficacy for AR-positive TNBC, and drug resistance is also inevitable. To address these challenges, we herein report the use of an AR-targeting proteolysis targeting chimera (AR-PROTAC) to treat AR-positive TNBC. We demonstrated that AR-PROTAC potently degraded AR protein via the ubiquitin-proteasome pathway in AR-positive TNBC BT549 cells, with a half degradation concentration of ∼46.9 nM. By evaluating the therapeutic efficacies in vitro and in vivo, we validated that AR-PROTAC was superior to enzalutamide, an AR inhibitor. Specifically, AR-PROTAC at 100 nM reduced BT549 cell viability by up to ∼80%, and AR-PRTOAC at 10 mg/kg suppressed tumor growth by ∼60% when administrated intratumorally in subcutaneous BT549 tumor mice model. Overall, these results demonstrate for the first time that PROTAC holds promise to enhance the treatment of AR-positive TNBC.

MiR-363-3p promotes prostate cancer tumor progression by targeting Dickkopf 3

BACKGROUND

Prostate cancer (PCa) is a frequent malignant tumor worldwide with high morbidity along with mortality. MicroRNAs (miRNAs) have been identified as key posttranscriptional modulators in diverse cancers. Herein, we purposed to explore the impacts of miR-363-3p on PCa growth, migration, infiltration along with apoptosis.

METHODS

The expressions of miR-363-3p along with Dickkopf 3 (DKK3) were assessed in clinical PCa specimens. We adopted the PCa cell line PC3 and transfected it using miR-363-3p repressors or mimic. The relationship of miR-363-3p with DKK3 was verified by a luciferase enzyme reporter assay. Cell viability along with apoptosis were determined by MTT assay coupled with flow cytometry analysis. Cell migration along infiltration were detected via wound healing, as well as Transwell assays. The contents of DKK3, E-cadherin, vimentin along with N-cadherin were analyzed via Western blotting accompanied with qRT-PCR.

RESULTS

MiR-363-3p was found to be inversely associated with the content of DKK3 in clinical PCa specimens. Further investigations revealed that DKK3 was miR-363-3p's direct target. Besides, overexpression of miR-363-3p decreased the contents of DKK3, promoted cell viability, migration coupled with infiltration, and reduced cell apoptosis, while silencing of miR-363-3p resulted in opposite influence. Upregulation of miR-363-3p diminished E-cadherin contents but increased vimentin along with N-cadherin protein contents in PC3 cells; in contrast, miR-363-3p downregulation produced the opposite result.

CONCLUSION

Our study indicates that miR-363-3p promotes PCa growth, migration coupled with invasion while dampening apoptosis by targeting DKK3.

Strategies toward Discovery of Potent and Orally Bioavailable Proteolysis Targeting Chimera Degraders of Androgen Receptor for the Treatment of Prostate Cancer

Proteolysis targeting chimera (PROTAC) small-molecule degraders have emerged as a promising new type of therapeutic agents, but the design of PROTAC degraders with excellent oral pharmacokinetics is a major challenge. In this study, we present our strategies toward the discovery of highly potent PROTAC degraders of androgen receptor (AR) with excellent oral pharmacokinetics. Employing thalidomide to recruit cereblon/cullin 4A E3 ligase and through the rigidification of the linker, we discovered highly potent AR degraders with good oral pharmacokinetic properties in mice with ARD-2128 being the best compound. ARD-2128 achieves 67% oral bioavailability in mice, effectively reduces AR protein and suppresses AR-regulated genes in tumor tissues with oral administration, leading to the effective inhibition of tumor growth in mice without signs of toxicity. This study supports the development of an orally active PROTAC AR degrader for the treatment of prostate cancer and provides insights and guidance into the design of orally active PROTAC degraders.

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.

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 Present and Future of Novel Protein Degradation Technology

Proteolysis targeting chimeras (PROTACs), as a novel therapeutic modality, play a vital role in drug discovery. Each PROTAC contains three key parts; a protein-of-interest (POI) ligand, a E3 ligase ligand, and a linker. These bifunctional molecules could mediate the degradation of POIs by hijacking the activity of E3 ubiquitin ligases for POI ubiquitination and subsequent degradation via the ubiquitin proteasome system (UPS). With several advantages over other therapeutic strategies, PROTACs have set off a new upsurge of drug discovery in recent years. ENDTAC, as the development of PROTACs technology, is now receiving more attention. In this review, we aim to summarize the rapid progress from 2018 to 2019 in protein degradation and analyze the challenges and future direction that need to be addressed in order to efficiently develop potent protein degradation technology.

Triazol: a privileged scaffold for proteolysis targeting chimeras

Current traditional drugs such as enzyme inhibitors and receptor agonists/antagonists present inherent limitations due to occupancy-driven pharmacology as the mode of action. Proteolysis targeting chimeras (PROTACs) are composed of an E3 ligand, a connecting linker and a target protein ligand, and are an attractive approach to specifically knockdown-targeted proteins utilizing an event-driven mode of action. The length, hydrophilicity and rigidity of connecting linkers play important role in creating a successful PROTAC. Some PROTACs with a triazole linker have displayed promising anticancer activity. This review provides an overview of PROTACs with a triazole scaffold and discusses its structure-activity relationship. Important milestones in the development of PROTACs are addressed and a critical analysis of this drug discovery strategy is also presented.