Aptamer-PROTAC Conjugates (APCs) for Tumor-Specific Targeting in Breast Cancer

An insight into therapeutic efficacy of heterocycles as histone modifying enzyme inhibitors that targets cancer epigenetic pathways

Histone modifying enzymes are the key regulators involved in the post-translational modification of histone and non-histone. These enzymes are responsible for the epigenetic control of cellular functions. However, deregulation of the activity of these enzymes results in uncontrolled disorders such as cancer and inflammatory and neurological diseases. The study includes histone acetyltransferases, deacetylases, methyl transferases, demethylases, DNA methyl transferases, and their potent inhibitors which are in a clinical trial and used as medicinal drugs. The present review covers the heterocycles as target-specific inhibitors of histone-modifying enzyme, more specifically histone acetyltransferases. This review also confers more recent reports on heterocycles as potential HAT inhibitors covered from 2016-2022 and future perspectives of these heterocycles in epigenetic therapy.

Recent advances in aptamer-based targeted drug delivery systems for cancer therapy

The past decade has become an important strategy in precision medicine for the targeted therapy of many diseases, expecially various types of cancer. As a promising targeted element, nucleic acid aptamers are single-stranded functional oligonucleotides which have specific abilities to bind with various target molecules ranging from small molecules to entire organisms. They are often named ‘chemical antibody’ and have aroused extensive interest in diverse clinical studies on account of their advantages, such as considerable biostability, versatile chemical modification, low immunogenicity and quick tissue penetration. Thus, aptamer-embedded drug delivery systems offer an unprecedented opportunity in bioanalysis and biomedicine. In this short review, we endeavor to discuss the recent advances in aptamer-based targeted drug delivery platforms for cancer therapy. Some perspectives on the advantages, challenges and opportunities are also presented.

Nucleic-Acid-Based Targeted Degradation in Drug Discovery

Targeted protein degradation (TPD), represented by proteolysis-targeting chimera (PROTAC), has emerged as a novel therapeutic modality in drug discovery. However, the application of conventional PROTACs is limited to protein targets containing cytosolic domains with ligandable sites. Recently, nucleic-acid-based modalities, such as modified oligonucleotide mimics and aptamers, opened new avenues to degrade protein targets and greatly expanded the scope of TPD. Beyond constructing protein-degrading chimeras, nucleic acid motifs can also serve as substrates for targeted degradation. Particularly, the new type of chimeric RNA degrader termed ribonuclease-targeting chimera (RIBOTAC) has shown promising features in drug discovery. Here, we provide an overview of the newly emerging TPD strategies based on nucleic acids as well as new strategies for targeted degradation of nucleic acid (RNA) targets. The design strategies, case studies, potential applications, and challenges are focused on.

Engineered bioorthogonal POLY-PROTAC nanoparticles for tumour-specific protein degradation and precise cancer therapy

PROteolysis TArgeting Chimeras (PROTACs) has been exploited to degrade putative protein targets. However, the antitumor performance of PROTACs is impaired by their insufficient tumour distribution. Herein, we present de novo designed polymeric PROTAC (POLY-PROTAC) nanotherapeutics for tumour-specific protein degradation. The POLY-PROTACs are engineered by covalently grafting small molecular PROTACs onto the backbone of an amphiphilic diblock copolymer via the disulfide bonds. The POLY-PROTACs self-assemble into micellar nanoparticles and sequentially respond to extracellular matrix metalloproteinase-2, intracellular acidic and reductive tumour microenvironment. The POLY-PROTAC NPs are further functionalized with azide groups for bioorthogonal click reaction-amplified PROTAC delivery to the tumour tissue. For proof-of-concept, we demonstrate that tumour-specific BRD4 degradation with the bioorthogonal POLY-PROTAC nanoplatform combine with photodynamic therapy efficiently regress tumour xenografts in a mouse model of MDA-MB-231 breast cancer. This study suggests the potential of the POLY-PROTACs for precise protein degradation and PROTAC-based cancer therapy.

Proteolysis targeting chimeras (PROTACs) have emerged as promising cancer therapy agents but have suffered from systemic toxicity issues. Here, the authors report on the creation of polymeric PROTAC nanoparticles for tumour targeting delivery and demonstrate protein degradation in vivo, in combination with photodynamic therapy.

Enriching Proteolysis Targeting Chimeras with a Second Modality: When Two Are Better Than One

Proteolysis targeting chimera (PROTAC)-mediated protein degradation has prompted a radical rethink and is at a crucial stage in driving a drug discovery transition. To fully harness the potential of this technology, a growing paradigm toward enriching PROTACs with other therapeutic modalities has been proposed. Could researchers successfully combine two modalities to yield multifunctional PROTACs with an expanded profile? In this Perspective, we try to answer this question. We discuss how this possibility encompasses different approaches, leading to multitarget PROTACs, light-controllable PROTACs, PROTAC conjugates, and macrocycle- and oligonucleotide-based PROTACs. This possibility promises to further enhance PROTAC efficacy and selectivity, minimize side effects, and hit undruggable targets. While PROTACs have reached the clinical investigation stage, additional steps must be taken toward the translational development of multifunctional PROTACs. A deeper and detailed understanding of the most critical challenges is required to fully exploit these opportunities and decisively enrich the PROTAC toolbox.

Proteolysis-targeting chimera (PROTAC) delivery system: advancing protein degraders towards clinical translation

Proteolysis Targeting Chimeras (PROTACs), an emerging therapeutic entity designed to degrade target proteins by hijacking the ubiquitin-proteasome system, have the potential to revolutionize the healthcare industry. The broad applicability of this protein degradation strategy has been verified with a few E3 ligases and a variety of distinct targets through the construction of modular chimeric structures. Despite recent efforts to promote the use of PROTACs for clinical applications, most PROTACs do not make it beyond the preclinical stage of drug development. There are several reasons that prevent PROTACs from reaching the market, and the inadequate delivery to the target site is one of the most challenging hurdles. With the increasing need for accelerating the translational process, combining the concepts of PROTACs and delivery systems has been explored to enhance the in vivo performance of PROTACs. These improved delivery strategies can eliminate unfavorable physicochemical properties of PROTACs, improve their targetability, and decrease their off-target side effects. The integration of powerful PROTACs and versatile delivery systems will inaugurate a burgeoning orientation for the field of targeted protein degradation. In this review, we will survey the latest progress in improving the in vivo degradation efficacy of PROTACs through delivery strategies, outline design principles for PROTAC-based delivery systems, discuss the current challenges with PROTACs, and outlook future opportunities in this field.

Homobivalent, Trivalent, and Covalent PROTACs: Emerging Strategies for Protein Degradation

Proteolysis-targeting chimeras (PROTACs) is a fast-growing technology providing many strengths over inhibition of protein activity directly and is attracting increasing interest in new drug discovery and development. However, efficiently identifying potent and drug-like degraders is still challenging in the development of PROTACs. Complementary to traditional PROTACs, several emerging types of PROTACs, such as homobivalent PROTACs based on two E3 ligases (e.g., CRBN, VHL, MDM2, TRIM24), chemical- or biological-based trivalent/multitargeted PROTACs, and covalent PROTACs, are rising for targeted protein degradation. These new types of PROTACs have several advantages over the traditional PROTACs including high selectivity, low toxicity, better therapeutic effects, and so on. In this perspective, we will summarize the latest development of representative PROTACs focusing on research mainly in past 10 years and discuss their advantages and disadvantages. Moreover, the outlook and perspectives on the associated challenges and future directions will be provided.

Major Advances in Emerging Degrader Technologies

Recently, degrader technologies have attracted increasing interest in the academic field and the pharmaceuticals industry. As one of the degrader technologies, proteolysis-targeting chimeras (PROTACs) have emerged as an attractive pharmaceutical development approach due to their catalytic ability to degrade numerous undruggable disease-causing proteins. Despite the remarkable progress, many aspects of traditional PROTACs still remain elusive. Its expansion could lead to PROTACs with new paradigm. Currently, many reviews focused on the design and optimization strategies through summarizing classical PROTACs, application in diseases and prospect of PROTACs. In this review, we categorize various emerging PROTACs ranging from simply modified classical PROTACs to atypical PROTACs such as nucleic acid-based PROTACs, and we put more emphasis on molecular design of PROTACs with different strategies. Furthermore, we summarize alternatives of PROTACs as lysosome-targeting chimeras (LYTACs) and macroautophagy degradation targeting chimeras (MADTACs) based on different degradation mechanism despite of lysosomal pathway. Beyond these protein degraders, targeting RNA degradation with the potential for cancer and virus therapeutics has been discussed. In doing so, we provide our perspective on the potential development or concerns of each degrader technology. Overall, we hope this review will offer a better mechanistic understanding of emerging degraders and prove as useful guide for the development of the coming degrader technologies.

PROTACs: past, present and future

Proteolysis-targeting chimeras (PROTACs) are heterobifunctional molecules consisting of one ligand that binds to a protein of interest (POI) and another that can recruit an E3 ubiquitin ligase. The chemically-induced proximity between the POI and E3 ligase results in ubiquitination and subsequent degradation of the POI by the ubiquitin-proteasome system (UPS). The event-driven mechanism of action (MOA) of PROTACs offers several advantages compared to traditional occupancy-driven small molecule inhibitors, such as a catalytic nature, reduced dosing and dosing frequency, a more potent and longer-lasting effect, an added layer of selectivity to reduce potential toxicity, efficacy in the face of drug-resistance mechanisms, targeting nonenzymatic functions, and expanded target space. Here, we highlight important milestones and briefly discuss lessons learned about targeted protein degradation (TPD) in recent years and conjecture on the efforts still needed to expand the toolbox for PROTAC discovery to ultimately provide promising therapeutics.

Smart PROTACs Enable Controllable Protein Degradation for Precision Cancer Therapy

Proteolysis-targeting chimeras (PROTACs) are heterobifunctional chemicals that degrade proteins at the post-translational level, which represent an emerging therapeutic modality to fight cancer and other diseases. Although several PROTACs have now entered clinical trials, potential off-tissue side effects have resulted from nonspecific accumulation at non-cancerous sites after systemic administration, and this remains a major challenge. To this end, in the past 3 years, activatable PROTACs whose activity can only be launched on demand have gained tremendous momentum. In this review, we provide an overview of these new smart activatable PROTACs, which exert protein degradation action only in response to internal or external stimuli. We categorize these activatable PROTACs according to their activation mechanism contributed by different stimuli, including reduction-activatable, hypoxia-activatable, and enzyme-activatable PROTACs and photo-caged or photo-switchable PROTACs. The use of stimuli-responsive chemical blocks in these activatable PROTACs allows local activation of the antitumor effects while reducing the incidence of off-site side effects for precision cancer therapy. The design principle and category of smart PROTACs are introduced along with an overview of their therapeutic prospects and challenges.

Recent Developments in Targeting Bromodomain and Extra Terminal Domain Proteins for Cancer Therapeutics

Abstract:

Bromodomain and extra-terminal domain (BET) proteins are a well-studied family of proteins associated with a variety of diseases including malignancy and chronic inflammation. Currently, numerous pan BET inhibitors have exhibited potent efficacy in several in vivo preclinical models and entered clinical trials, but have largely stalled due to their adverse events. Therefore, the development of new selective inhibitors and PROTACs (Proteolysis Targeting Chimeras) targeting BET is urgently needed. In the present review, we summarize the BET protein structure, the recent development of BET inhibitors, focusing mainly on BRD4-selective inhibitors and PROTAC degraders.

Non-small molecule PROTACs (NSM-PROTACs): Protein degradation kaleidoscope

The proteolysis targeting chimeras (PROTACs) technology has been rapidly developed since its birth in 2001, attracting rapidly growing attention of scientific institutes and pharmaceutical companies. At present, a variety of small molecule PROTACs have entered the clinical trial. However, as small molecule PROTACs flourish, non-small molecule PROTACs (NSM-PROTACs) such as peptide PROTACs, nucleic acid PROTACs and antibody PROTACs have also advanced considerably over recent years, exhibiting the unique characters beyond the small molecule PROTACs. Here, we briefly introduce the types of NSM-PROTACs, describe the advantages of NSM-PROTACs, and summarize the development of NSM-PROTACs so far in detail. We hope this article could not only provide useful insights into NSM-PROTACs, but also expand the research interest of NSM-PROTACs.

Strategies for designing proteolysis targeting chimaeras (PROTACs)

Proteolysis targeting chimaeras (PROTACs) is a cutting edge and rapidly growing technique for new drug discovery and development. Currently, the largest challenge in the molecular design and drug development of PROTACs is efficient identification of potent and drug-like degraders. This review aims to comprehensively summarize and analyse state-of-the-art methods and strategies in the design of PROTACs. We provide a detailed illustration of the general principles and tactics for designing potent PROTACs, highlight representative case studies, and discuss the advantages and limitations of these strategies. Particularly, structure-based rational PROTAC design and emerging new types of PROTACs (e.g., homo-PROTACs, multitargeting PROTACs, photo-control PROTACs and PROTAC-based conjugates) will be focused on.

Smart Nano-PROTACs Reprogram Tumor Microenvironment for Activatable Photo-metabolic Cancer Immunotherapy

Protease inhibitors can modulate intratumoral metabolic processes to reprogram the immunosuppressive tumor microenvironment (TME), which however suffer from the limited efficacy and off-targeted side effects. We report smart nano-proteolysis targeting chimeras (nano-PROTACs) with phototherapeutic ablation and cancer-specific protein degradation to reprogram the TME for photo-metabolic cancer immunotherapy. This nano-PROTAC has a semiconducting polymer backbone linked with a cyclooxygenase 1/2 (COX-1/2)-targeting PROTAC peptide (CPP) via a cathepsin B (CatB)-cleavable segment. CPP can be activated by the tumor-overexpressed CatB to induce the degradation of COX-1/2 via the ubiquitin-proteasome system. The persistent degradation of COX-1/2 depletes their metabolite prostaglandin E₂ which is responsible for activation of immune suppressor cells. Such a smart PROTAC strategy synergized with phototherapy specifically reprograms the immunosuppressive TME and reinvigorates antitumor immunity.

G4-PROTAC: targeted degradation of a G-quadruplex binding protein

G-quadruplex (G4) binding proteins regulate important biological processes, but their interaction networks are poorly understood. We report the first use of G4 as a warhead of a proteolysis-targeting chimera (G4-PROTAC) for targeted degradation of a G4-binding protein (RHAU/DHX36). G4-PROTAC provides a new way to explore G4-protein networks and to develop potential therapeutics.