Targeting the Ubiquitin-Proteasome System for Cancer Therapeutics by Small-Molecule Inhibitors

The ubiquitin-proteasome system (UPS) is a critical regulator of cellular protein levels and activity. It is, therefore, not surprising that its dysregulation is implicated in numerous human diseases, including many types of cancer. Moreover, since cancer cells exhibit increased rates of protein turnover, their heightened dependence on the UPS makes it an attractive target for inhibition via targeted therapeutics. Indeed, the clinical application of proteasome inhibitors in treatment of multiple myeloma has been very successful, stimulating the development of small-molecule inhibitors targeting other UPS components. On the other hand, while the discovery of potent and selective chemical compounds can be both challenging and time consuming, the area of targeted protein degradation through utilization of the UPS machinery has seen promising developments in recent years. The repertoire of proteolysis-targeting chimeras (PROTACs), which employ E3 ligases for the degradation of cancer-related proteins via the proteasome, continues to grow. In this review, we will provide a thorough overview of small-molecule UPS inhibitors and highlight advancements in the development of targeted protein degradation strategies for cancer therapeutics.

Target protein localization and its impact on PROTAC-mediated degradation

Proteolysis-targeting chimeras (PROTACs) bring a protein of interest (POI) into spatial proximity of an E3 ubiquitin ligase, promoting POI ubiquitylation and proteasomal degradation. PROTACs rely on endogenous cellular machinery to mediate POI degradation, therefore the subcellular location of the POI and access to the E3 ligase being recruited potentially impacts PROTAC efficacy. To interrogate whether the subcellular context of the POI influences PROTAC-mediated degradation, we expressed either Halo or FKBP12^F36V (dTAG) constructs consisting of varying localization signals and tested the efficacy of their degradation by von Hippel-Lindau (VHL)- or cereblon (CRBN)-recruiting PROTACs targeting either Halo or dTAG. POIs were localized to the nucleus, cytoplasm, outer mitochondrial membrane, endoplasmic reticulum, Golgi, peroxisome or lysosome. Differentially localized Halo or FKBP12^F36V proteins displayed varying levels of degradation using the same respective PROTACs, suggesting therefore that the subcellular context of the POI can influence the efficacy of PROTAC-mediated POI degradation.

De Novo Design of an Androgen Receptor DNA Binding Domain-Targeted peptide PROTAC for Prostate Cancer Therapy

Androgen receptor splice variant-7 (AR-V7), one of the major driving factors, is the most attractive drug target in castration-resistant prostate cancer (CRPC). Currently, no available drugs efficiently target AR-V7 in clinical practice. The DNA binding domain (DBD) is indispensable for the transcriptional activity of AR full length and AR splice variants, including AR-V7. Based on the homodimerization structure of the AR DBD, a novel peptide-based proteolysis-targeting chimera (PROTAC) drug is designed to induce AR and AR-V7 degradation in a DBD and MDM2-dependent manner, without showing any activity on other hormone receptors. To overcome the short half-life and poor cell penetrability of peptide PROTAC drugs, an ultrasmall gold (Au)-peptide complex platform to deliver the AR DBD PROTAC in vivo is developed. The obtained Au-AR pep-PROTAC effectively degrades AR and AR-V7 in prostate cancer cell lines, particularly in CWR22Rv1 cells with DC50 values 48.8 and 79.2 nM, respectively. Au-AR pep-PROTAC results in suppression of AR levels and induces tumor regression in both enzalutamide sensitive and resistant prostate cancer animal models. Further optimization of the Au-AR pep-PROTAC can ultimately lead to a new therapy for AR-V7-positive CRPC.

Cancer-Specific Delivery of Proteolysis-Targeting Chimeras (PROTACs) and Their Application to Cancer Immunotherapy

Proteolysis-targeting chimeras (PROTACs) are rapidly emerging as a potential therapeutic strategy for cancer therapy by inducing the degradation of tumor-overexpressing oncogenic proteins. They can specifically catalyze the degradation of target oncogenic proteins by recruiting E3 ligases and utilizing the ubiquitin-proteasome pathway. Since their mode of action is universal, irreversible, recyclable, long-lasting, and applicable to 'undruggable' proteins, PROTACs are gradually replacing the role of conventional small molecular inhibitors. Moreover, their application areas are being expanded to cancer immunotherapy as various types of oncogenic proteins that are involved in immunosuppressive tumor microenvironments. However, poor water solubility and low cell permeability considerably restrict the pharmacokinetic (PK) property, which necessitates the use of appropriate delivery systems for cancer immunotherapy. In this review, the general characteristics, developmental status, and PK of PROTACs are first briefly covered. Next, recent studies on the application of various types of passive or active targeting delivery systems for PROTACs are introduced, and their effects on the PK and tumor-targeting ability of PROTACs are described. Finally, recent drug delivery systems of PROTACs for cancer immunotherapy are summarized. The adoption of an adequate delivery system for PROTAC is expected to accelerate the clinical translation of PROTACs, as well as improve its efficacy for cancer therapy.

Bioinspired PROTAC-induced macrophage fate determination alleviates atherosclerosis

Atherosclerosis is a major cause of death and disability in cardiovascular disease. Atherosclerosis associated with lipid accumulation and chronic inflammation leads to plaques formation in arterial walls and luminal stenosis in carotid arteries. Current approaches such as surgery or treatment with statins encounter big challenges in curing atherosclerosis plaque. The infiltration of proinflammatory M1 macrophages plays an essential role in the occurrence and development of atherosclerosis plaque. A recent study shows that TRIM24, an E3 ubiquitin ligase of a Trim family protein, acts as a valve to inhibit the polarization of anti-inflammatory M2 macrophages, and elimination of TRIM24 opens an avenue to achieve the M2 polarization. Proteolysis-targeting chimera (PROTAC) technology has emerged as a novel tool for the selective degradation of targeting proteins. But the low bioavailability and cell specificity of PROTAC reagents hinder their applications in treating atherosclerosis plaque. In this study we constructed a type of bioinspired PROTAC by coating the PROTAC degrader (dTRIM24)-loaded PLGA nanoparticles with M2 macrophage membrane (MELT) for atherosclerosis treatment. MELT was characterized by morphology, size, and stability. MELT displayed enhanced specificity to M1 macrophages as well as acidic-responsive release of dTRIM24. After intravenous administration, MELT showed significantly improved accumulation in atherosclerotic plaque of high fat and high cholesterol diet-fed atherosclerotic (ApoE-/-) mice through binding to M1 macrophages and inducing effective and precise TRIM24 degradation, thus resulting in the polarization of M2 macrophages, which led to great reduction of plaque formation. These results suggest that MELT can be considered a potential therapeutic agent for targeting atherosclerotic plaque and alleviating atherosclerosis progression, providing an effective strategy for targeted atherosclerosis therapy.

S, Abbineni C, Vaishampayan U, Lyssiotis CA, Parolia A, Xiao L, Chinnaiyan AM. Development of an orally bioavailable mSWI/SNF ATPase degrader and acquired mechanisms of resistance in prostate cancer

Mammalian switch/sucrose nonfermentable (mSWI/SNF) ATPase degraders have been shown to be effective in enhancer-driven cancers by functioning to impede oncogenic transcription factor chromatin accessibility. Here, we developed AU-24118, an orally bioavailable proteolysis-targeting chimera (PROTAC) degrader of mSWI/SNF ATPases (SMARCA2 and SMARCA4) and PBRM1. AU-24118 demonstrated tumor regression in a model of castration-resistant prostate cancer (CRPC) which was further enhanced with combination enzalutamide treatment, a standard of care androgen receptor (AR) antagonist used in CRPC patients. Importantly, AU-24118 exhibited favorable pharmacokinetic profiles in preclinical analyses in mice and rats, and further toxicity testing in mice showed a favorable safety profile. As acquired resistance is common with targeted cancer therapeutics, experiments were designed to explore potential mechanisms of resistance that may arise with long-term mSWI/SNF ATPase PROTAC treatment. Prostate cancer cell lines exposed to long-term treatment with high doses of a mSWI/SNF ATPase degrader developed SMARCA4 bromodomain mutations and ABCB1 (ATP binding cassette subfamily B member 1) overexpression as acquired mechanisms of resistance. Intriguingly, while SMARCA4 mutations provided specific resistance to mSWI/SNF degraders, ABCB1 overexpression provided broader resistance to other potent PROTAC degraders targeting bromodomain-containing protein 4 and AR. The ABCB1 inhibitor, zosuquidar, reversed resistance to all three PROTAC degraders tested. Combined, these findings position mSWI/SNF degraders for clinical translation for patients with enhancer-driven cancers and define strategies to overcome resistance mechanisms that may arise.

An updated patent review of BRD4 degraders

INTRODUCTION

Bromodomain-containing protein 4 (BRD4), an important epigenetic reader, is closely associated with the pathogenesis and development of many diseases, including various cancers, inflammation, and infectious diseases. Targeting BRD4 inhibition or protein elimination with small molecules represents a promising therapeutic strategy, particularly for cancer therapy.

AREAS COVERED

The recent advances of patented BRD4 degraders were summarized. The challenges, opportunities, and future directions for developing novel potent and selective BRD4 degraders are also discussed. The patents of BRD4 degraders were searched using the SciFinder and Cortellis Drug Discovery Intelligence database.

EXPERT OPINION

BRD4 degraders exhibit superior efficacy and selectivity to BRD4 inhibitors, given their unique mechanism of protein degradation instead of protein inhibition. Excitingly, RNK05047 is now in phase I/II clinical trials, indicating that selective BRD4 protein degradation may offer a viable therapeutic strategy, particularly for cancer. Targeting BRD4 with small-molecule degraders provides a promising approach with the potential to overcome therapeutic resistance for treating various BRD4-associated diseases.

Synthetic lethality: targeting the SMARCA2 bromodomain for degradation in SMARCA4-deficient tumors - a review of patent literature from 2019-June 2023

INTRODUCTION

SMARCA2 and SMARCA4 are subunits of the SWI/SNF complex which is a chromatin remodeling complex and a key epigenetic regulator that facilitates gene expression. Tumors with loss of function mutations in SMARCA4 rely on SMARCA2 for cell survival and this synthetic lethality is a potential therapeutic strategy to treat cancer.

AREAS COVERED

The current review focuses on patent applications that claim proteolysis-targeting chimeras (PROTAC) degraders that bind the bromodomain site of SMARCA2 and are published between January 2019-June 2023. A total of 29 applications from 9 different applicants were evaluated.

EXPERT OPINION

SMARCA2/4 bromodomain inhibitors do not lead to desired effects on cancer proliferation; however, companies have converted bromodomain binders into PROTACs to degrade the protein, with a preference for SMARCA2 over SMARCA4. Selective degradation of SMARCA2 is most likely required to be efficacious in the SMARCA4-deficient setting, while allowing for sufficient safety margin in normal tissues. With several patent applications disclosed recently, interest in targeting SMARCA2 should continue, especially with a selective SMARCA2 PROTAC now in the clinic from Prelude Therapeutics. The outcome of the clinical trials will influence the evolution of selective SMARCA2 PROTACs development.