Design, Synthesis, and Biological Evaluation of HDAC Degraders with CRBN E3 Ligase Ligands

Targeted Protein Degradation (TPD) for Immunotherapy: Understanding Proteolysis Targeting Chimera-Driven Ubiquitin-Proteasome Interactions

Targeted protein degradation or TPD, is rapidly emerging as a treatment that utilizes small molecules to degrade proteins that cause diseases. TPD allows for the selective removal of disease-causing proteins, including proteasome-mediated degradation, lysosome-mediated degradation, and autophagy-mediated degradation. This approach has shown great promise in preclinical studies and is now being translated to treat numerous diseases, including neurodegenerative diseases, infectious diseases, and cancer. This review discusses the latest advances in TPD and its potential as a new chemical modality for immunotherapy, with a special focus on the innovative applications and cutting-edge research of PROTACs (Proteolysis TArgeting Chimeras) and their efficient translation from scientific discovery to technological achievements. Our review also addresses the significant obstacles and potential prospects in this domain, while also offering insights into the future of TPD for immunotherapeutic applications.

Perspectives and new aspects of histone deacetylase inhibitors in the therapy of CNS diseases

Many populations worldwide are suffering from central nervous system (CNS) diseases such as brain tumors, neurodegenerative diseases (Alzheimer's disease, Parkinson's disease and Huntington's disease) and stroke. There is a shortage of effective drugs for most CNS diseases. As one of the regulatory mechanisms of epigenetics, the particular role and therapeutic benefits of histone deacetylases (HDACs) in the CNS have been extensively studied. In recent years, HDACs have attracted increasing attention as potential drug targets for CNS diseases. In this review, we summarize the recent applications of representative histone deacetylases inhibitors (HDACis) in CNS diseases and discuss the challenges in developing HDACis with different structures and better blood-brain barrier (BBB) permeability, hoping to promote the development of more effective bioactive HDACis for the treatment of CNS diseases.

BRD4: New Hope in the Battle Against Glioblastoma

The BET family proteins, comprising BRD2, BRD3 and BRD4, represent epigenetic readers of acetylated histone marks that play pleiotropic roles in the tumorigenesis and growth of multiple human malignancies, including glioblastoma (GBM). A growing body of investigation has proven BET proteins as valuable therapeutic targets for cancer treatment. Recently, several BRD4 inhibitors and degraders have been reported to successfully suppress GBM in preclinical and clinical studies. However, the precise role and mechanism of BRD4 in the pathogenesis of GBM have not been fully elucidated or summarized. This review focuses on summarizing the roles and mechanisms of BRD4 in the context of the initiation and development of GBM. In addition, several BRD4 inhibitors have been evaluated for therapeutic purposes as monotherapy or in combination with chemotherapy, radiotherapy, and immune therapies. Here, we provide a critical appraisal of studies evaluating various BRD4 inhibitors and degraders as novel treatment strategies against GBM.

Developments of PROTACs technology in immune-related diseases

Traditional chemotherapy and immunotherapy are primary disease-treatment strategies. However, they face numerous challenges, including limited therapeutic benefits, off-target effects, serious adverse effects, drug resistance, long half-life time, poor oral bioavailability, and drugging undruggable proteins. Proteolytic targeted chimeras (PROTACs) were suggested to solve these problems. PROTACs are heterogeneous functional molecules linked by a chemical linker and contain a binding ligand for the protein of interest and a recruiting ligand for the E3 ligand. The binding of a PROTAC to a target protein brings the E3 ligand enzyme into proximity, initiating polyubiquitination of the target protein, followed by protease-mediated degradation. To date, PROTACs against dozens of immunological targets have been successfully developed, many of which have been clinically validated drug targets, and several have entered clinical trials for immune-related diseases. This article reviews the role of PROTACs-mediated degradation of critical proteins in immune disorders and cancer immunotherapy. Chemical structures, cellular and in vivo activities, and pharmacodynamics of these PROTACs are summarized. Lastly, we also discuss the prospects and potential limitations that PROTACs face.

Design, synthesis, and biological evaluation of BRD4 degraders

Epigenetic proteins are one of the important targets in the current research fields of cancer therapy. A family of bromodomain-containing (BRD) and extra terminal domain (BET) proteins act as epigenetic readers to regulate the expression of key oncogenes and anti-apoptotic proteins. Recently, although BET degraders based on PROTAC technology have achieved significant antitumor effects, the lack of selectivity for BET protein degradation has not been fully addressed. Herein, a series of small molecule BRD4 PROTACs were designed and synthesized. Most of the degraders were effective in inhibiting MM.1S and MV-4-11 cell lines, especially in MV-4-11. Among them, degrader 8b could induce the degradation of BRD4 and exhibited a time- and concentration-dependent depletion manner and there was a significant depletion of BRD4, laying a foundation for effectively treating leukemia and multiple myeloma. Moreover, 8b could also effectively prevent the activation of MRC5 cells by inducing the degradation of BRD4 protein, which preliminarily proves that the BRD4 degrader based on the PROTAC concept has great potential for the treatment of pulmonary fibrosis. Taken together, these findings laid a foundation for BRD4 degraders as an effective strategy for treating related diseases.

Solid-Phase Synthesis of Cereblon-Recruiting Selective Histone Deacetylase 6 Degraders (HDAC6 PROTACs) with Antileukemic Activity

In this work, we utilized the proteolysis targeting chimera (PROTAC) technology to achieve the chemical knock-down of histone deacetylase 6 (HDAC6). Two series of cereblon-recruiting PROTACs were synthesized via a solid-phase parallel synthesis approach, which allowed the rapid preparation of two HDAC6 degrader mini libraries. The PROTACs were either based on an unselective vorinostat-like HDAC ligand or derived from a selective HDAC6 inhibitor. Notably, both PROTAC series demonstrated selective degradation of HDAC6 in leukemia cell lines. The best degraders from each series (denoted A6 and B4) were capable of degrading HDAC6 via ternary complex formation and the ubiquitin-proteasome pathway, with DC₅₀ values of 3.5 and 19.4 nM, respectively. PROTAC A6 demonstrated promising antiproliferative activity via inducing apoptosis in myeloid leukemia cell lines. These findings highlight the potential of this series of degraders as effective pharmacological tools for the targeted degradation of HDAC6.

Development of SOS1 Inhibitor-Based Degraders to Target KRAS-Mutant Colorectal Cancer

Direct blockade of KRAS driver mutations in colorectal cancer (CRC) has been challenging. Targeting SOS1, a guanine nucleotide exchange factor, has arisen as an attractive approach for KRAS-mutant CRC. Here, we describe the development of novel SOS1 degraders and their activity in patient-derived CRC organoids (PDO). The design of these degraders as proteolysis-targeting chimera was based on the crystal structures of cereblon and SOS1. The synthesis used the 6- and 7-OH groups of a quinazoline core as anchor points to connect lenalidomide. Fifteen compounds were screened for SOS1 degradation. P7 was found to have up to 92% SOS1 degradation in both CRC cell lines and PDOs with excellent specificity. SOS1 degrader P7 demonstrated superior activity in inhibiting CRC PDO growth with an IC₅₀ 5 times lower than that of SOS1 inhibitor BI3406. In summary, we developed new SOS1 degraders and demonstrated SOS1 degradation as a feasible therapeutic strategy for KRAS-mutant CRC.

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.