PHOTACs enable optical control of protein degradation

Deciphering the shape selective conformational equilibrium of E- and Z-locked azobenzene-tetraethylammonium ion in regulating photo-switchable K+-ion channel blocking

The search for photo-switchable optopharmacological agents that can block ion channels has been a prevalent area owing to its prime advantages of reversibility and specificity over the traditional blockers. However, the quest for a higher blocking ability shown by a less stable photo-isomer to perfectly suit the requirement of the optopharmacological agents is still ongoing. To date, only a marginal improvement in terms of blocking ability is observed by the less stable E-isomer of para-substituted locked azobenzene with TEA (LAB-TEA) for the K+-ion channel. Thus, rationalization of the limitation for achieving high activity by the E-isomer is rather essential to aid the improvement of the efficiency of photoswitchable blocker drugs. Herein, we report a molecular-level analysis on the mechanism of blocking by E- and Z-LAB-TEA with the bacterial KcsA K+-ion channel using Molecular Dynamics (MD) simulation and Quantum Mechanical (QM) calculations. The positively charged TEA fragment engages in stronger electrostatic interactions, while the neutral LAB fragment engages in weaker dispersive interactions. The binding free energy calculated by Molecular Mechanics Poisson-Boltzmann Surface Area (MMPBSA) for E-LAB-TEA (-22.3 kcal mol-1) shows less thermodynamic preference for binding with K+-ion channels than Z-LAB-TEA (-21.6 kcal mol-1) corroborating the experimental observation. The correlation between the structure and the binding ability of E- and Z-isomers of LAB-TEA indicates that the channel gate is narrow and acts as a bottleneck for the entry of the binder molecule inside the large cavity. Upon irradiation, the Z-isomer converts into a less stable but long and planar E-isomer (ΔE of photoisomerism = 7.0 kcal mol-1, at SA2-CASPT2(6,4)/6-31+G(d)//CASSCF(6,4)/6-31+G(d)), which is structurally more suitable to fit into the narrow channel gate rather than the curved and non-planar Z-LAB-TEA. Thus, a reduction in the ionic current is observed owing to the preferential entry and subsequent blocking by E-LAB-TEA. Discontinuing the irradiation leads to conversion to the Z-isomer, the curved nature of which hinders its spontaneous release outside the cavity, thereby contributing only a small increase in the ionic current.

Ultrasound-Activated PROTAC Prodrugs Overcome Immunosuppression to Actuate Efficient Deep-Tissue Sono-Immunotherapy in Orthotopic Pancreatic Tumor Mouse Models

The degradation of oncoproteins mediated by proteolysis-targeting chimera (PROTAC) has emerged as a potent strategy in cancer therapy. However, the clinical application of PROTACs is hampered by challenges such as poor water solubility and off-target adverse effects. Herein, we present an ultrasound (US)-activatable PROTAC prodrug termed NPCe6+PRO for actuating efficient sono-immunotherapy in a spatiotemporally controllable manner. Specifically, US irradiation, which exhibits deep-tissue penetration capability, results in Ce6-mediated generation of ROS, facilitating sonodynamic therapy (SDT) and inducing immunogenic cell death (ICD). Simultaneously, the generated ROS cleaves the thioketal (TK) linker through a ROS-responsive mechanism, realizing the on-demand activation of the PROTAC prodrug in deep tissues. This prodrug activation results in the degradation of the target protein BRD4, while simultaneously reversing the upregulation of PD-L1 expression associated with the SDT process. In the orthotopic mouse model of pancreatic tumors, NPCe6+PRO effectively suppressed tumor growth in conjunction with US stimulation.

PROTAC: Novel degradable approach for different targets to treat breast cancer

The revolutionary Proteolysis Targeting Chimera (PROTACs) have the exciting potential to reshape the pharmaceutical industry landscape by leveraging the ubiquitin-proteasome system for targeted protein degradation. Breast cancer, the most prevalent cancer in women, could be treated using PROTAC therapy. Although substantial work has been conducted, there is not yet a comprehensive overview or progress update on PROTAC therapy for breast cancer. Hence, in this article, we've compiled recent research progress focusing on different breast cancer target proteins, such as estrogen receptor (ER), BET, CDK, HER2, PARP, EZH2, etc. This resource aims to serve as a guide for future PROTAC-based breast cancer treatment design.

New-generation advanced PROTACs as potential therapeutic agents in cancer therapy

Proteolysis-targeting chimeras (PROTACs) technology has garnered significant attention over the last 10 years, representing a burgeoning therapeutic approach with the potential to address pathogenic proteins that have historically posed challenges for traditional small-molecule inhibitors. PROTACs exploit the endogenous E3 ubiquitin ligases to facilitate degradation of the proteins of interest (POIs) through the ubiquitin-proteasome system (UPS) in a cyclic catalytic manner. Despite recent endeavors to advance the utilization of PROTACs in clinical settings, the majority of PROTACs fail to progress beyond the preclinical phase of drug development. There are multiple factors impeding the market entry of PROTACs, with the insufficiently precise degradation of favorable POIs standing out as one of the most formidable obstacles. Recently, there has been exploration of new-generation advanced PROTACs, including small-molecule PROTAC prodrugs, biomacromolecule-PROTAC conjugates, and nano-PROTACs, to improve the in vivo efficacy of PROTACs. These improved PROTACs possess the capability to mitigate undesirable physicochemical characteristics inherent in traditional PROTACs, thereby enhancing their targetability and reducing off-target side effects. The new-generation of advanced PROTACs will mark a pivotal turning point in the realm of targeted protein degradation. In this comprehensive review, we have meticulously summarized the state-of-the-art advancements achieved by these cutting-edge PROTACs, elucidated their underlying design principles, deliberated upon the prevailing challenges encountered, and provided an insightful outlook on future prospects within this burgeoning field.

PROTACs in Ovarian Cancer: Current Advancements and Future Perspectives

Ovarian cancer is the deadliest gynecologic malignancy. The majority of patients diagnosed with advanced ovarian cancer will relapse, at which point additional therapies can be administered but, for the most part, these are not curative. As such, a need exists for the development of novel therapeutic options for ovarian cancer patients. Research in the field of targeted protein degradation (TPD) through the use of proteolysis-targeting chimeras (PROTACs) has significantly increased in recent years. The ability of PROTACs to target proteins of interest (POI) for degradation, overcoming limitations such as the incomplete inhibition of POI function and the development of resistance seen with other inhibitors, is of particular interest in cancer research, including ovarian cancer research. This review provides a synopsis of PROTACs tested in ovarian cancer models and highlights PROTACs characterized in other types of cancers with potential high utility in ovarian cancer. Finally, we discuss methods that will help to enable the selective delivery of PROTACs to ovarian cancer and improve the pharmacodynamic properties of these agents.

Applications of protein ubiquitylation and deubiquitylation in drug discovery

The ubiquitin (Ub)-proteasome system (UPS) is the major machinery mediating specific protein turnover in eukaryotic cells. By ubiquitylating unwanted, damaged, or harmful proteins and driving their degradation, UPS is involved in many important cellular processes. Several new UPS-based technologies, including molecular glue degraders and PROTACs (proteolysis-targeting chimeras) to promote protein degradation, and DUBTACs (deubiquitinase-targeting chimeras) to increase protein stability, have been developed. By specifically inducing the interactions between different Ub ligases and targeted proteins that are not otherwise related, molecular glue degraders and PROTACs degrade targeted proteins via the UPS; in contrast, by inducing the proximity of targeted proteins to deubiquitinases, DUBTACs are created to clear degradable poly-Ub chains to stabilize targeted proteins. In this review, we summarize the recent research progress in molecular glue degraders, PROTACs, and DUBTACs and their applications. We discuss immunomodulatory drugs, sulfonamides, cyclin-dependent kinase-targeting molecular glue degraders, and new development of PROTACs. We also introduce the principle of DUBTAC and its applications. Finally, we propose a few future directions of these three technologies related to targeted protein homeostasis.

Protein degraders - from thalidomide to new PROTACs

Recently, the development of protein degraders (protein-degrading compounds) has prominently progressed. There are two remarkable classes of protein degraders: proteolysis-targeting chimeras (PROTACs) and molecular glue degraders (MGDs). Almost 70 years have passed since thalidomide was initially developed as a sedative-hypnotic drug, which is currently recognized as one of the most well-known MGDs. During the last two decades, a myriad of PROTACs and MGDs have been developed, and the molecular mechanism of action (MOA) of thalidomide was basically elucidated, including identifying its molecular target cereblon (CRBN). CRBN forms a Cullin Ring Ligase 4 with Cul4 and DDB1, whose substrate specificity is controlled by its binding ligands. Thalidomide, lenalidomide and pomalidomide, three CRBN-binding MGDs, were clinically approved to treat several intractable diseases (including multiple myeloma). Several other MGDs and CRBN-based PROTACs (ARV-110 and AVR-471) are undergoing clinical trials. In addition, several new related technologies regarding PROTACs and MGDs have also been developed, and achievements of protein degraders impact not only therapeutic fields but also basic biological science. In this article, I introduce the history of protein degraders, from the development of thalidomide to the latest PROTACs and related technologies.

131I Induced In Vivo Proteolysis by Photoswitchable azoPROTAC Reinforces Internal Radiotherapy

Photopharmacology, incorporating photoswitches such as azobenezes into drugs, is an emerging therapeutic method to realize spatiotemporal control of pharmacological activity by light. However, most photoswitchable molecules are triggered by UV light with limited tissue penetration, which greatly restricts the in vivo application. Here, this study proves that 131I can trigger the trans-cis photoisomerization of a reported azobenezen incorporating PROTACs (azoPROTAC). With the presence of 50 µCi mL-1 131I, the azoPROTAC can effectively down-regulate BRD4 and c-Myc levels in 4T1 cells at a similar level as it does under light irradiation (405 nm, 60 mW cm-2). What's more, the degradation of BRD4 can further benefit the 131I-based radiotherapy. The in vivo experiment proves that intratumoral co-adminstration of 131I (300 µCi) and azoPROTC (25 mg kg-1) via hydrogel not only successfully induce protein degradation in 4T1 tumor bearing-mice but also efficiently inhibit tumor growth with enhanced radiotherapeutic effect and anti-tumor immunological effect. This is the first time that a radioisotope is successfully used as a trigger in photopharmacology in a mouse model. It believes that this study will benefit photopharmacology in deep tissue.

Enhanced cellular therapy: revolutionizing adoptive cellular therapy

Enhanced cellular therapy has emerged as a novel concept following the basis of cellular therapy. This treatment modality applied drugs or biotechnology to directly enhance or genetically modify cells to enhance the efficacy of adoptive cellular therapy (ACT). Drugs or biotechnology that enhance the killing ability of immune cells include immune checkpoint inhibitors (ICIs) / antibody drugs, small molecule inhibitors, immunomodulatory factors, proteolysis targeting chimera (PROTAC), oncolytic virus (OV), etc. Firstly, overcoming the inhibitory tumor microenvironment (TME) can enhance the efficacy of ACT, which can be achieved by blocking the immune checkpoint. Secondly, cytokines or cytokine receptors can be expressed by genetic engineering or added directly to adoptive cells to enhance the migration and infiltration of adoptive cells to tumor cells. Moreover, multi-antigen chimeric antigen receptors (CARs) can be designed to enhance the specific recognition of tumor cell-related antigens, and OVs can also stimulate antigen release. In addition to inserting suicide genes into adoptive cells, PROTAC technology can be used as a safety switch or degradation agent of immunosuppressive factors to enhance the safety and efficacy of adoptive cells. This article comprehensively summarizes the mechanism, current situation, and clinical application of enhanced cellular therapy, describing potential improvements to adoptive cellular therapy.

Tumor-targeted PROTAC prodrug nanoplatform enables precise protein degradation and combination cancer therapy

Proteolysis targeting chimeras (PROTACs) have emerged as revolutionary anticancer therapeutics that degrade disease-causing proteins. However, the anticancer performance of PROTACs is often impaired by their insufficient bioavailability, unsatisfactory tumor specificity and ability to induce acquired drug resistance. Herein, we propose a polymer-conjugated PROTAC prodrug platform for the tumor-targeted delivery of the most prevalent von Hippel-Lindau (VHL)- and cereblon (CRBN)-based PROTACs, as well as for the precise codelivery of a degrader and conventional small-molecule drugs. The self-assembling PROTAC prodrug nanoparticles (NPs) can specifically target and be activated inside tumor cells to release the free PROTAC for precise protein degradation. The PROTAC prodrug NPs caused more efficient regression of MDA-MB-231 breast tumors in a mouse model by degrading bromodomain-containing protein 4 (BRD4) or cyclin-dependent kinase 9 (CDK9) with decreased systemic toxicity. In addition, we demonstrated that the PROTAC prodrug NPs can serve as a versatile platform for the codelivery of a PROTAC and chemotherapeutics for enhanced anticancer efficiency and combination benefits. This study paves the way for utilizing tumor-targeted protein degradation for precise anticancer therapy and the effective combination treatment of complex diseases.

Aryl Azocyclopropeniums: Minimalist, Visible-Light Photoswitches

We report convenient syntheses of aryl azocyclopropeniums and a study of their photochemical properties. Incorporation of the smallest arene leads to pronounced redshift of the π-π* absorbance band, compared to azobenzenes. Photoisomerization under purple or green light irradiation affords Z- or E-isomers in ratios up to 94% Z or 90% E, and the switches proved stable over multiple irradiation cycles. Thermal half-lives of metastable Z-isomers range from minutes to hours in acetonitrile and water. These properties together with the concise, versatile syntheses render aryl azocyclopropeniums exciting additions to the tool kit of readily available molecular photoswitches for wide ranging applications.

Ubiquitin E3 ligases assisted technologies in protein degradation: Sharing pathways in neurodegenerative disorders and cancer

E3 ligases, essential components of the ubiquitin-proteasome-mediated protein degradation system, play a critical role in cellular regulation. By covalently attaching ubiquitin (Ub) molecules to target proteins, these ligases mark them for degradation, influencing various bioprocesses. With over 600 E3 ligases identified, there is a growing realization of their potential as therapeutic candidates for addressing proteinopathies in cancer and neurodegenerative disorders (NDDs). Recent research has highlighted the need to delve deeper into the intricate roles of E3 ligases as nexus points in the pathogenesis of both cancer and NDDs. Their dysregulation is emerging as a common thread linking these seemingly disparate diseases, necessitating a comprehensive understanding of their molecular intricacies. Herein, we have discussed (i) the fundamental mechanisms through which different types of E3 ligases actively participate in selective protein degradation in cancer and NDDs, followed by an examination of common E3 ligases playing pivotal roles in both situations, emphasising common players. Moving to, (ii) the functional domains and motifs of E3 ligases involved in ubiquitination, we have explored their interactions with specific substrates in NDDs and cancer. Additionally, (iii) we have explored techniques like PROTAC, molecular glues, and other state-of-the-art methods for hijacking neurotoxic and oncoproteins. Lastly, (iv) we have provided insights into ongoing clinical trials, offering a glimpse into the evolving landscape of E3-based therapeutics for cancer and NDDs. Unravelling the intricate network of E3 ligase-mediated regulation holds the key to unlocking targeted therapies that address the specific molecular signatures of individual patients, heralding a new era in personalized medicines.

Molecular glue degraders: exciting opportunities for novel drug discovery

INTRODUCTION

Molecular Glue Degraders (MGDs) is a concept that refers to a class of compounds that facilitate the interaction between two proteins or molecules within a cell. These compounds act as bridge that enhances specific Protein-Protein Interactions (PPIs). Over the past decade, this technology has gained attention as a potential strategy to target proteins that were traditionally considered undruggable using small molecules.

AREAS COVERED

This review presents the concept of cellular homeostasis and the balance between protein synthesis and protein degradation. The concept of protein degradation is concerned with molecular glues, which form part of the broader field of Targeted Protein Degradation (TPD). Next, pharmacochemical strategies for the rational design of MGDs are detailed and illustrated by examples of Ligand-Based (LBDD), Structure-Based (SBDD) and Fragment-Based Drug Design (FBDD).

EXPERT OPINION

Expanding the scope of what can be effectively targeted in the development of treatments for diseases that are incurable or resistant to conventional therapies offers new therapeutic options. The treatment of microbial infections and neurodegenerative diseases is a major societal challenge, and the discovery of MGDs appears to be a promising avenue. Combining different approaches to discover and exploit a variety of innovative therapeutic agents will create opportunities to treat diseases that are still incurable.

Photoswitchable PROTACs for Reversible and Spatiotemporal Regulation of NAMPT and NAD. Angew Chem Int Ed Engl 2024;63:e202315997. [PMID: 38282119 DOI: 10.1002/anie.202315997] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Nicotinamide adenine dinucleotide (NAD+ ) is an essential coenzyme with diverse biological functions in DNA synthesis. Nicotinamide phosphoribosyltransferase (NAMPT) is a key rate-limiting enzyme involved in NAD+ biosynthesis in mammals. We developed the first chemical tool for optical control of NAMPT and NAD+ in biological systems using photoswitchable proteolysis-targeting chimeras (PS-PROTACs). An NAMPT activator and dimethylpyrazolazobenzene photoswitch were used to design highly efficient PS-PROTACs, enabling up- and down-reversible regulation of NAMPT and NAD+ in a light-dependent manner and reducing the toxicity associated with inhibitor-based PS-PROTACs. PS-PROTAC was activated under 620 nm irradiation, realizing in vivo optical manipulation of antitumor activity, NAMPT, and NAD+ .

Progress in the controllability technology of PROTAC

Proteolysis-targeting chimaera (PROTAC) technology functions by directly targeting proteins and catalysing their degradation through an event-driven mode of action, a novel mechanism with significant clinical application prospects for various diseases. Currently, the most advanced PROTAC drug is undergoing phase III clinical trials (NCT05654623). Although PROTACs exhibit significant advantages over traditional small-molecule inhibitors, their catalytic degradation of normal cellular proteins can potentially cause toxic side effects. Therefore, to achieve targeted release of PROTACs and minimize adverse reactions, researchers are actively exploring diverse controllable PROTACs. In this review, we comprehensively summarize the control strategies to provide a theoretical basis for the innovative application of PROTAC technology.

Development of visible-light-activatable photocaged PROTACs

Photocaged proteolysis-targeting chimeras (PROTACs), which employ light as a stimulus to control protein degradation, have recently garnered considerable attention as both powerful chemical tools and a promising therapeutic strategy. However, the poor penetration depth of traditionally used ultraviolet light and the deficiency of alternative caging positions have restricted their applications in biological systems. By installing a diverse array of photocaged groups, with excitation wavelengths ranging from 365 nm to 405 nm, onto different positions of cereblon (CRBN) and Von Hippel-Lindau (VHL)-recruiting Brd4 degraders, we conducted the first comprehensive study on visible-light-activatable photocaged PROTACs to the best of our knowledge. We found the A2, A4 and B3 positions to be most effective at regulating the activity of the degraders, and to provide the resulting molecules (9-12 and 17) as potent visible-light-controlled degraders in live cells.

Dual-Programmable Semiconducting Polymer NanoPROTACs for Deep-Tissue Sonodynamic-Ferroptosis Activatable Immunotherapy

Proteolysis-targeting chimeras (PROTACs) can provide promising opportunities for cancer treatment, while precise regulation of their activities remains challenging to achieve effective and safe therapeutic outcomes. A semiconducting polymer nanoPROTAC (SPNFeP ) is reported that can achieve ultrasound (US) and tumor microenvironment dual-programmable PROTAC activity for deep-tissue sonodynamic-ferroptosis activatable immunotherapy. SPNFeP is formed through a nano-precipitation of a sonodynamic semiconducting polymer, a ferroptosis inducer, and a newly synthesized PROTAC molecule. The semiconducting polymers work as sonosensitizers to produce singlet oxygen (1 O2 ) via sonodynamic effect under US irradiation, and ferroptosis inducers react with intratumoral hydrogen peroxide (H2 O2 ) to generate hydroxyl radical (·OH). Such a dual-programmable reactive oxygen species (ROS) generation not only triggers ferroptosis and immunogenic cell death (ICD), but also induces on-demand activatable delivery of PROTAC molecules into tumor sites. The effectively activated nanoPROTACs degrade nicotinamide phosphoribosyl transferase (NAMPT) to suppress tumor infiltration of myeloid-derived suppressive cells (MDSCs), thus promoting antitumor immunity. In such a way, SPNFeP mediates sonodynamic-ferroptosis activatable immunotherapy for entirely inhibiting tumor growths in both subcutaneous and 2-cm tissue-covered deep tumor mouse models. This study presents a dual-programmable activatable strategy based on PROTACs for effective and precise cancer combinational therapy.

Overview of epigenetic degraders based on PROTAC, molecular glue, and hydrophobic tagging technologies

Epigenetic pathways play a critical role in the initiation, progression, and metastasis of cancer. Over the past few decades, significant progress has been made in the development of targeted epigenetic modulators (e.g., inhibitors). However, epigenetic inhibitors have faced multiple challenges, including limited clinical efficacy, toxicities, lack of subtype selectivity, and drug resistance. As a result, the design of new epigenetic modulators (e.g., degraders) such as PROTACs, molecular glue, and hydrophobic tagging (HyT) degraders has garnered significant attention from both academia and pharmaceutical industry, and numerous epigenetic degraders have been discovered in the past decade. In this review, we aim to provide an in-depth illustration of new degrading strategies (2017-2023) targeting epigenetic proteins for cancer therapy, focusing on the rational design, pharmacodynamics, pharmacokinetics, clinical status, and crystal structure information of these degraders. Importantly, we also provide deep insights into the potential challenges and corresponding remedies of this approach to drug design and development. Overall, we hope this review will offer a better mechanistic understanding and serve as a useful guide for the development of emerging epigenetic-targeting degraders.

Degraders in epigenetic therapy: PROTACs and beyond

Epigenetics refers to the reversible process through which changes in gene expression occur without changing the nucleotide sequence of DNA. The process is currently gaining prominence as a pivotal objective in the treatment of cancers and other ailments. Numerous drugs that target epigenetic mechanisms have obtained approval from the Food and Drug Administration (FDA) for the therapeutic intervention of diverse diseases; many have drawbacks, such as limited applicability, toxicity, and resistance. Since the discovery of the first proteolysis-targeting chimeras (PROTACs) in 2001, studies on targeted protein degradation (TPD)-encompassing PROTACs, molecular glue (MG), hydrophobic tagging (HyT), degradation TAG (dTAG), Trim-Away, a specific and non-genetic inhibitor of apoptosis protein (IAP)-dependent protein eraser (SNIPER), antibody-PROTACs (Ab-PROTACs), and other lysosome-based strategies-have achieved remarkable progress. In this review, we comprehensively highlight the small-molecule degraders beyond PROTACs that could achieve the degradation of epigenetic proteins (including bromodomain-containing protein-related targets, histone acetylation/deacetylation-related targets, histone methylation/demethylation related targets, and other epigenetic targets) via proteasomal or lysosomal pathways. The present difficulties and forthcoming prospects in this domain are also deliberated upon, which may be valuable for medicinal chemists when developing more potent, selective, and drug-like epigenetic drugs for clinical applications.

Recent progress in degradation of membrane proteins by PROTACs and alternative targeted protein degradation techniques

Targeted protein degradation (TPD) is one of the key strategies of current targeted cancer therapy, and it can eliminate some of the root causes of cancer, and effectively avoid drug resistance caused by traditional drugs. Proteolysis targeting chimera (PROTAC) is a hot branch of the TPD strategy, and it has been shown to induce the degradation of target proteins by activating the inherent ubiquitin-proteasome system (UPS) in tumor cells. PROTACs have been developed for more than two decades, and some of them have been clinically evaluated. Although most of the proteins degraded by PROTACs are intracellular, degradation of some typical membrane proteins has also been reported, such as epidermal growth factor receptor (EGFR), anaplastic lymphoma kinase (ALK), programmed death ligand 1 (PD-L1), and G-protein-coupled receptor (GPCR). In addition, some other effective membrane protein-degrading strategies have also emerged, such as antibody-based PROTAC (AbTAC), lysosome targeting chimera (LYTAC), molecular glue, and nanoparticle-based PROTAC (Nano-PROTAC). Herein, we discussed the advantages, disadvantages and potential applications of several important membrane protein degradation techniques. These techniques that we have summarized are insightful in paving the way for future development of more general strategies for membrane protein degradation.

Recent Advances in Optically Controlled PROTAC

Proteolysis-targeting chimera (PROTAC) technology is a groundbreaking therapeutic approach with significant clinical potential for degrading disease-inducing proteins within targeted cells. However, challenges related to insufficient target selectivity raise concerns about PROTAC toxicity toward normal cells. To address this issue, researchers are modifying PROTACs using various approaches to enhance their target specificity. This review highlights innovative optically controlled PROTACs as anti-cancer therapies currently used in clinical practice and explores the challenges associated with their efficacy and safety. The development of optically controlled PROTACs holds the potential to significantly expand the clinical applicability of PROTAC-based technology within the realm of drug discovery.

Pervasive aggregation and depletion of host and viral proteins in response to cysteine-reactive electrophilic compounds

Protein homeostasis is tightly regulated, with damaged or misfolded proteins quickly eliminated by the proteasome and autophagosome pathways. By co-opting these processes, targeted protein degradation technologies enable pharmacological manipulation of protein abundance. Recently, cysteine-reactive molecules have been added to the degrader toolbox, which offer the benefit of unlocking the therapeutic potential of 'undruggable' protein targets. The proteome-wide impact of these molecules remains to be fully understood and given the general reactivity of many classes of cysteine-reactive electrophiles, on- and off-target effects are likely. Using chemical proteomics, we identified a cysteine-reactive small molecule degrader of the SARS-CoV-2 non- structural protein 14 (nsp14), which effects degradation through direct modification of cysteines in both nsp14 and in host chaperones together with activation of global cell stress response pathways. We find that cysteine-reactive electrophiles increase global protein ubiquitylation, trigger proteasome activation, and result in widespread aggregation and depletion of host proteins, including components of the nuclear pore complex. Formation of stress granules was also found to be a remarkably ubiquitous cellular response to nearly all cysteine-reactive compounds and degraders. Collectively, our study sheds light on complexities of covalent target protein degradation and highlights untapped opportunities in manipulating and characterizing proteostasis processes via deciphering the cysteine-centric regulation of stress response pathways.

The application of PROTAC in HDAC

Inducing protein degradation by proteolysis targeting chimera (PROTAC) has provided great opportunities for scientific research and industrial applications. Histone deacetylase (HDAC)-PROTAC has been widely developed since the first report of its ability to induce the degradation of SIRT2 in 2017. To date, ten of the eighteen HDACs (HDACs 1-8, HDAC10, and SIRT2) have been successfully targeted and degraded by HDAC-PROTACs. HDAC-PROTACs surpass traditional HDAC inhibitors in many aspects, such as higher selectivity, more potent antiproliferative activity, and the ability to disrupt the enzyme-independent functions of a multifunctional protein and overcome drug resistance. Rationally designing HDAC-PROTACs is a main challenge in development because slight variations in chemical structure can lead to drastic effects on the efficiency and selectivity of the degradation. In the future, HDAC-PROTACs can potentially be involved in clinical research with the support of the increased amount of in vivo data, pharmacokinetic evaluation, and pharmacological studies.

Cell-Specific Degradation of Histone Deacetylase Using Warhead-Caged Proteolysis Targeting Chimeras

Proteolysis targeting chimeras (PROTACs) have shifted the paradigm for drug development via target protein degradation. However, PROTACs may exhibit systemic toxicity to normal cells due to indiscriminate degradation and the utility of inhibitors as a warhead for protein targeting. Here, we propose a new strategy for developing activatable PROTACs for cell-specific degradation of histone deacetylase (HDAC) with minimal side effects via caging of the warhead. Molecular docking reveals that the hydroxyl group of the HDAC inhibitor is crucial for targeting. An enzyme-activatable PROTAC is designed by caging the hydroxyl group with the substrate for NAD(P)H: quinone oxidoreductase 1 (NQO1) overexpressed in cancer cells. We demonstrate that the caged PROTAC can be converted to its active form in response to NQO1. The enzyme-activatable PROTAC allows the efficient and specific degradation of HDAC6 and exerts antiproliferative activity in NQO1-positive cells. The generalizability of the design is further demonstrated by engineering a H2O2-responsive PROTAC for specific degradation of HDAC6 in cells with elevated H2O2. The strategy of caging the ligand for target proteins would afford a new dimension for developing activatable PROTACs with high specificity and minimal side effects.

Lighting the way to tumor destruction

Achieving tumor-specific protein loss remains a challenge in the delivery of proteolysis-targeting chimeras (PROTACs) as cancer therapeutics. As a solution, Wang et al. recently developed nanoformulated PROTACs (NAPs), a novel photoactivatable degradation approach. This innovative class of compounds harnesses near-infrared (NIR) light for precise PROTAC release and protein degradation in mouse tumors.

TACkling Cancer by Targeting Selective Protein Degradation

Targeted protein degradation has emerged as an alternative therapy against cancer, offering several advantages over traditional inhibitors. The new degrader drugs provide different therapeutic strategies: they could cross the phospholipid bilayer membrane by the addition of specific moieties to extracellular proteins. On the other hand, they could efficiently improve the degradation process by the generation of a ternary complex structure of an E3 ligase. Herein, we review the current trends in the use of TAC-based technologies (TACnologies), such as PROteolysis TArgeting Chimeras (PROTAC), PHOtochemically TArgeting Chimeras (PHOTAC), CLIck-formed Proteolysis TArgeting Chimeras (CLIPTAC), AUtophagy TArgeting Chimeras (AUTAC), AuTophagosome TEthering Compounds (ATTEC), LYsosome-TArgeting Chimeras (LYTAC), and DeUBiquitinase TArgeting Chimeras (DUBTAC), in experimental development and their progress towards clinical applications.

Novel biomolecules in targeted cancer therapy: a new approach towards precision medicine

Cancer is a major threat to human life around the globe, and the discovery of novel biomolecules continue to be an urgent therapeutic need that is still unmet. Precision medicine relies on targeted therapeutic strategies. Researchers are better equipped to develop therapies that target proteins as they understand more about the genetic alterations and molecules that cause progression of cancer. There has been a recent diversification of the sorts of targets exploited in treatment. Therapeutic antibody and biotechnology advancements enabled curative treatments to reach previously inaccessible sites. New treatment strategies have been initiated for several undruggable targets. The application of tailored therapy has been proven to have efficient results in controlling cancer progression. Novel biomolecules like SMDCs, ADCs, mABs, and PROTACS has gained vast attention in the recent years. Several studies have shown that using these novel technology helps in reducing the drug dosage as well as to overcome drug resistance in different cancer types. Therefore, it is crucial to fully untangle the mechanism and collect evidence to understand the significance of these novel drug targets and strategies. This review article will be discussing the importance and role of these novel biomolecules in targeted cancer therapies.

Photoactivated Protein Degrader for Optical Control of Synaptic Function

Hundreds of proteins determine the function of synapses, and synapses define the neuronal circuits that subserve myriad brain, cognitive, and behavioral functions. It is thus necessary to precisely manipulate specific proteins at specific sub-cellular locations and times to elucidate the roles of particular proteins and synapses in brain function. We developed PHOtochemically TArgeting Chimeras (PHOTACs) as a strategy to optically degrade specific proteins with high spatial and temporal precision. PHOTACs are small molecules that, upon wavelength-selective illumination, catalyze ubiquitylation and degradation of target proteins through endogenous proteasomes. Here, we describe the design and chemical properties of a PHOTAC that targets Ca2+/calmodulin-dependent protein kinase II alpha (CaMKIIα), which is abundant and crucial for the baseline synaptic function of excitatory neurons. We validate the PHOTAC strategy, showing that the CaMKIIα-PHOTAC is effective in mouse brain tissue. Light activation of CaMKIIα-PHOTAC removed CaMKIIα from regions of the mouse hippocampus only within 25 μm of the illuminated brain surface. The optically controlled degradation decreases synaptic function within minutes of light activation, measured by the light-initiated attenuation of evoked field excitatory postsynaptic potential (fEPSP) responses to physiological stimulation. The PHOTACs methodology should be broadly applicable to other key proteins implicated in synaptic function, especially for evaluating their precise roles in the maintenance of long-term potentiation and memory within subcellular dendritic domains.

Delivering on Cell-Selective Protein Degradation Using Chemically Tailored PROTACs

PROTACs (Proteolysis-Targeting Chimeras) have emerged as a groundbreaking class of chemical tools that facilitate the degradation of target proteins by leveraging the ubiquitin-proteasome system (UPS). However, the effective utilization of PROTACs in chemical biology studies and therapeutics encounters significant challenges when it comes to achieving cell-selective protein degradation and in vivo applications. This review article aims to shed light on recent advancements in the development of Pro-PROTACs, which exhibit controlled protein degradation capabilities in response to external stimuli or disease-related endogenous biochemical signals. The article delves into the specific chemical strategies employed to regulate the interaction between PROTACs and E3 ubiquitin ligases or target proteins. These strategies enable spatial and temporal control over the protein degradation potential of Pro-PROTACs. Furthermore, the review summarizes recent investigations regarding the delivery of PROTACs using biodegradable nanoparticles for in vivo applications and targeted protein degradation. Such delivery systems hold great promise for enabling efficient and selective protein degradation in vivo. Lastly, the article provides a perspective on the future design of multifunctional PROTACs and their intracellular delivery mechanisms, with a particular focus on achieving cell-selective protein degradation.

Targeted delivery of PROTAC-based prodrug activated by bond-cleavage bioorthogonal chemistry for microneedle-assisted cancer therapy

Proteolysis-targeting chimera (PROTAC) is emerging as a new strategy to degrade target proteins in a precise way by taking advantage of the cellular ubiquitin-proteasome system. However, the potential cytotoxicity of PROTAC should be avoided to mitigate the off-target degradation of proteins in healthy tissues or cells. To address this issue, we herein present a strategy to cage a PROTAC with 4-(vinyloxy) benzyl carbonate (MZ1-O), which can be eliminated through a 3,6-dimethyl-1,2,4,5-tetrazine (Tz)-mediated inverse electron-demand Diels-Alder (iEDDA) reaction to generate a BRD4 (bromodomain-containing protein 4) degrader, MZ1. We further propose a dissolvable microneedle-assisted strategy for site-specific activation of MZ1-O that is delivered by a targeted delivery vector through systemic route in vivo, and demonstrate such a bioorthogonal strategy is efficient and precise for tumor treatment. Our study suggests that the bioorthogonal activation of PROTAC-based prodrug offers a highly specific and precise approach for cancer therapy.

Isomerization of bioactive acylhydrazones triggered by light or thiols

The acylhydrazone unit is well represented in screening databases used to find ligands for biological targets, and numerous bioactive acylhydrazones have been reported. However, potential E/Z isomerization of the C=N bond in these compounds is rarely examined when bioactivity is assayed. Here we analysed two ortho-hydroxylated acylhydrazones discovered in a virtual drug screen for modulators of N-methyl-D-aspartate receptors and other bioactive hydroxylated acylhydrazones with structurally defined targets reported in the Protein Data Bank. We found that ionized forms of these compounds, which are populated under laboratory conditions, photoisomerize readily and the isomeric forms have markedly different bioactivity. Furthermore, we show that glutathione, a tripeptide involved with cellular redox balance, catalyses dynamic E⇄Z isomerization of acylhydrazones. The ratio of E to Z isomers in cells is determined by the relative stabilities of the isomers regardless of which isomer was applied. We conclude that E/Z isomerization may be a common feature of the bioactivity observed with acylhydrazones and should be routinely analysed.

Hydrogen Peroxide-Inducible PROTACs for Targeted Protein Degradation in Cancer Cells

Proteolysis-targeting chimeras (PROTACs) provide a powerful technique to degrade targeted proteins utilizing the cellular ubiquitin-proteasome system. The major concern is the host toxicity resulting from their poor selectivity. Inducible PROTACs responding to exogenous stimulus, such as light, improve their specificity, but it is difficult for photo-activation in deep tissues. Herein, we develop H2 O2 -inducible PROTAC precursors 2/5, which can be activated by endogenous H2 O2 in cancer cells to release the active PROTACs 1/4 to effectively degrade targeted proteins. This results in the intended cytotoxicity towards cancer cells while targeted protein in normal cells remains almost unaffected. The higher Bromodomain-containing protein 4 (BRD4) degradation activity and cytotoxicity of 2 towards cancer cells is mainly due to the higher endogenous concentration of H2 O2 in cancer cells (A549 and H1299), characterized by H2 O2 -responsive fluorescence probe 3. Western blot assays and cytotoxicity experiments demonstrate that 2 degrades BRD4 more effectively and is more cytotoxic in H2 O2 -rich cancer cells than in H2 O2 -deficient normal cells. This method is also extended to estrogen receptor (ER)-PROTAC precursor 5, showing H2 O2 -dependent ER degradation ability. Thus, we establish a novel strategy to induce targeted protein degradation in a H2 O2 -dependent way, which has the potential to improve the selectivity of PROTACs.

Exploitation of Proximity-Mediated Effects in Drug Discovery: An Update of Recent Research Highlights in Perturbing Pathogenic Proteins and Correlated Issues

The utilization of proximity-mediated effects to perturb pathogenic proteins of interest (POIs) has emerged as a powerful strategic alternative to conventional drug design approaches based on target occupancy. Over the past three years, the burgeoning field of targeted protein degradation (TPD) has witnessed the expansion of degradable POIs to membrane-associated, extracellular, proteasome-resistant, and even microbial proteins. Beyond TPD, researchers have achieved the proximity-mediated targeted protein stabilization, the recruitment of intracellular immunophilins to disturb undruggable targets, and the nonphysiological post-translational modifications of POIs. All of these strides provide new avenues for innovative drug discovery aimed at battling human malignancies and other major diseases. This perspective presents recent research highlights and discusses correlated issues in developing therapeutic modalities that exploit proximity-mediated effects to modulate pathogenic proteins, thereby guiding future academic and industrial efforts in this field.

Near-Infrared-Activatable PROTAC Nanocages for Controllable Target Protein Degradation and On-Demand Antitumor Therapy

As a novel protein knockdown tool, proteolysis targeting chimeras (PROTACs) can induce potent degradation of target proteins by hijacking E3 ubiquitin ligases. However, the uncontrollable protein disruption of PROTACs is prone to cause "off-target" toxicity after systemic administration. Herein, we designed a photocaged-PROTAC (phoBET1) and loaded it in UCNPs-based mesoporous silica nanoparticles (UMSNs) to construct a NIR light-activatable PROTAC nanocage (UMSNs@phoBET1) for controllable target protein degradation. Upon NIR light (980 nm) irradiation, UMSNs@phoBET1 nanocages could be activated to release active PROTAC via a controlled pattern for degrading bromodomain-containing protein 4 (BRD4) and inducing MV-4-11 cancer cell apoptosis. In vivo experiments demonstrated that UMSNs@phoBET1 nanocages were capable of responding to NIR light in tumor tissues to achieve BRD4 degradation and effectively suppress tumor growth. This NIR light-activatable PROTAC nanoplatform compensates for the current shortcomings of short-wavelength light-controlled PROTACs and presents a paradigm for the precise regulation of PROTACs in living tissues.

Self-Assembled Nano-PROTAC Enables Near-Infrared Photodynamic Proteolysis for Cancer Therapy

Confining the protein degradation activity of proteolysis-targeting chimera (PROTAC) to cancer lesions ensures precision treatment. However, it still remains challenging to precisely control PROTAC function in tumor regions in vivo. We herein describe a near-infrared (NIR) photoactivatable nano-PROTAC (NAP) for remote-controllable proteolysis in tumor-bearing mice. NAP is formed by molecular self-assembly from an amphiphilic conjugate of PROTAC linked with an NIR photosensitizer through a singlet oxygen (1O2)-cleavable linker. The activity of PROTAC is initially silenced but can be remotely switched on upon NIR photoirradiation to generate 1O2 by the photosensitizer. We demonstrated that NAP enabled tumor-specific degradation of bromodomain-containing protein 4 (BRD4) in an NIR light-instructed manner. This in combination with photodynamic therapy (PDT) elicited an effective suppression of tumor growth. This work thus presents a novel approach for spatiotemporal control over targeted protein degradation by PROTAC.

Chemically induced degradation of epigenetic targets

Proteolysis-targeting chimeras (PROTACs) are heterobifunctional small molecules that induce the ternary complex formation between a protein-of-interest (POI) and an E3 ligase, leading to targeted polyubiquitination and degradation of the POI. Particularly, PROTACs have the distinct advantage of targeting both canonical and noncanonical functions of epigenetic targets over traditional inhibitors, which typically target canonical functions only, resulting in greater therapeutic efficacy. In this review, we methodically analyze published PROTAC degraders of epigenetic writer, reader, and eraser proteins and their in vitro and in vivo effects. We highlight the mechanism of action of these degraders and their advantages in targeting both canonical and noncanonical functions of epigenetic targets in the context of cancer treatment. Furthermore, we present a future outlook for this exciting field. Overall, pharmacological degradation of epigenetic targets has emerged as an effective and attractive strategy to thwart cancer progression and growth.

Targeted protein degradation might present a novel therapeutic approach in the fight against African trypanosomiasis

African trypanosomiasis (AT) is a hemoparasitic disease caused by infection with African trypanosomes and it is prevalent in many sub-Saharan African countries, affecting both humans and domestic animals. The disease is transmitted mostly by haematophagous insects of the genus Glossina while taking blood meal, in the process spreading the parasites from an infected animal to an uninfected animal. The disease is fatal if untreated, and the available drugs are generally ineffective and resulting in toxicities. Therefore, it is still pertinent to explore novel methods and targets for drug discovery. Proteolysis-targeting chimeras (PROTACs) present a new strategy for development of therapeutic molecules that mimic cellular proteasomal-mediated protein degradation to target proteins involved in different disease types. PROTACs have been used to degrade proteins involved in various cancers, neurodegenerative diseases, and immune disorders with remarkable success. Here, we highlight the problems associated with the current treatments for AT, discuss the concept of PROTACs and associated targeted protein degradation (TPD) approaches, and provide some insights on the future potential for the use of these emerging technologies (PROTACs and TPD) for the development of new generation of anti-Trypanosoma drugs and the first "TrypPROTACs".

Bioorthogonal PROTAC Prodrugs Enabled by On-Target Activation

Although proteolysis targeting chimeras (PROTACs) have become promising therapeutic modalities, important concerns exist about the potential toxicity of the approach owing to uncontrolled degradation of proteins and undesirable ligase-mediated off-target effects. Precision manipulation of degradation activity of PROTACs could minimize potential toxicity and side effects. As a result, extensive efforts have been devoted to developing cancer biomarker activating prodrugs of PROTACs. In this investigation, we developed a bioorthogonal on-demand prodrug strategy (termed click-release "crPROTACs") that enables on-target activation of PROTAC prodrugs and release of PROTACs in cancer cells selectively. Inactive PROTAC prodrugs TCO-ARV-771 and TCO-DT2216 are rationally designed by conjugating a bioorthogonal trans-cyclooctenes (TCO) group into the ligand of the VHL E3 ubiquitin ligase. The tetrazine (Tz)-modified RGD peptide, c(RGDyK)-Tz, which targets integrin α_vβ₃ biomarker in cancer cells, serves as the activation component for click-release of the PROTAC prodrugs to achieve targeted degradation of proteins of interest (POIs) in cancer cells versus noncancerous normal cells. The results of studies accessing the viability of this strategy show that the PROTAC prodrugs are selectively activated in an integrin α_vβ₃-dependent manner to produce PROTACs, which degrade POIs in cancer cells. The crPROTAC strategy might be a general, abiotic approach to induce selective cancer cell death through the ubiquitin-proteasome pathway.

Microneedle Patch Delivery of PROTACs for Anti-Cancer Therapy

Proteolysis-targeting chimera (PROTAC) is an emerging technique for degrading disease-related proteins. However, the current PROTACs suffer from inadequate solubility and lack of organ targeting, which has hampered their druggability. Herein, we report direct and sustained delivery of PROTACs using microneedle patches to the diseased tissues. In this study, we use an estrogen receptor alpha (ERα)-degrading PROTAC, ERD308, to treat ER-positive breast cancer. A pH-sensitive micelle, MPEG-poly(β-amino ester) (MPEG-PAE), is used to encapsulate ERD308 along with an FDA-approved CDK4/6 inhibitor, Palbociclib (Pal), before loading into biodegradable microneedle patches. These patches enable prolonged drug release into deep tumors, maintaining therapeutic levels for at least 4 days, with an excellent drug retention rate of over 87% in tumors. ERD308 released from the microneedle patches can sufficiently degrade ERα in MCF7 cells. Co-administration of ERD308 and Palbociclib exhibits excellent efficacy by over 80% tumor reduction as well as a good safety profile. Our work demonstrates the feasibility and proof-of-concept therapeutic potential of using microneedle patches to directly deliver PROTACs into tumors.

Antiviral PROTACs: Opportunity borne with challenge

Proteolysis targeting chimera (PROTAC) degradation of pathogenic proteins by hijacking of the ubiquitin-proteasome-system has become a promising strategy in drug design. The overwhelming advantages of PROTAC technology have ensured a rapid and wide usage, and multiple PROTACs have entered clinical trials. Several antiviral PROTACs have been developed with promising bioactivities against various pathogenic viruses. However, the number of reported antiviral PROTACs is far less than that of other diseases, e.g., cancers, immune disorders, and neurodegenerative diseases, possibly because of the common deficiencies of PROTAC technology (e.g., limited available ligands and poor membrane permeability) plus the complex mechanism involved and the high tendency of viral mutation during transmission and replication, which may challenge the successful development of effective antiviral PROTACs. This review highlights the important advances in this rapidly growing field and critical limitations encountered in developing antiviral PROTACs by analyzing the current status and representative examples of antiviral PROTACs and other PROTAC-like antiviral agents. We also summarize and analyze the general principles and strategies for antiviral PROTAC design and optimization with the intent of indicating the potential strategic directions for future progress.

Stimuli-activatable PROTACs for precise protein degradation and cancer therapy

The proteolysis targeting chimeras (PROTACs) approach has attracted extensive attention in the past decade, which represents an emerging therapeutic modality with the potential to tackle disease-causing proteins that are historically challengeable for conventional small molecular inhibitors. PROTAC harnesses the endogenic E3 ubiquitin ligase to degrade protein of interest (POI) via ubiquitin-proteasome system in a cycle-catalytic manner. The event-driven pharmacology of PROTAC is poised to pursue those targets that are conventionally undruggable, which enormously extends the space of drug development. Furthermore, PROTAC has the potential to address drug resistance of small molecular inhibitors by degrading the whole POI. Nevertheless, PROTACs display high-efficiency and always-on properties to degrade POI, they may cause severe side effects due to an "on-target but off-tissue" protein degradation profile at the undesirable tissues and cells. Given that, the stimuli-activatable PROTAC prodrugs have been recently exploited to confine precise protein degradation of the favorable targets, which may conquer the adverse effects of PROTAC due to uncontrollable protein degradation. Herein, we summarized the cutting-edge advances of the stimuli-activatable PROTAC prodrugs. We also overviewed the progress of PROTAC prodrug-based nanomedicine to improve PROTAC delivery to the tumors and precise POI degradation in the targeted cells.

Proteolysis-targeting chimeras in biotherapeutics: Current trends and future applications

The success of inhibitor-based therapeutics is largely constrained by the acquisition of therapeutic resistance, which is partially driven by the undruggable proteome. The emergence of proteolysis targeting chimera (PROTAC) technology, designed for degrading proteins involved in specific biological processes, might provide a novel framework for solving the above constraint. A heterobifunctional PROTAC molecule could structurally connect an E3 ubiquitin ligase ligand with a protein of interest (POI)-binding ligand by chemical linkers. Such technology would result in the degradation of the targeted protein via the ubiquitin-proteasome system (UPS), opening up a novel way of selectively inhibiting undruggable proteins. Herein, we will highlight the advantages of PROTAC technology and summarize the current understanding of the potential mechanisms involved in biotherapeutics, with a particular focus on its application and development where therapeutic benefits over classical small-molecule inhibitors have been achieved. Finally, we discuss how this technology can contribute to developing biotherapeutic drugs, such as antivirals against infectious diseases, for use in clinical practices.

Current advances of small molecule E3 ligands for proteolysis-targeting chimeras design

Proteolysis-targeting chimeras (PROTACs) as an emerging drug discovery modality has been extensively concerned in recent years. Over 20 years development, accumulated studies have demonstrated that PROTACs show unique advantages over traditional therapy in operable target scope, efficacy, and overcoming drug resistance. However, only limited E3 ligases, the essential elements of PROTACs, have been harnessed for PROTACs design. The optimization of novel ligands for well-established E3 ligases and the employment of additional E3 ligases remain urgent challenges for investigators. Here, we systematically summarize the current status of E3 ligases and corresponding ligands for PROTACs design with a focus on their discovery history, design principles, application benefits, and potential defects. Meanwhile, the prospects and future directions for this field are briefly discussed.

Discovery of estrogen receptor α targeting caged hypoxia-responsive PROTACs with an inherent bicyclic skeleton for breast cancer treatment

In view of the potential off-target effects of antitumor drugs, including proteolysis targeting chimera (PROTAC), certain toxic effects may be caused in normal tissues. Herein, based on the characteristics of the tumor microenvironment, we reported the first estrogen receptor α (ERα) targeting hypoxia-responsive PROTACs in order to improve their safety in breast cancer treatment by introducing two hypoxia-activated groups, nitroimidazole and nitrobenzene, into the ER ligand or E3 ligand of an active PROTAC, which has certain cytotoxicity in normal cells. Bioactivity studies showed that these hypoxia-responsive PROTACs exhibited excellent hypoxic responsiveness and ERα degradation activity under hypoxic conditions, and thus improved the toxic effects of the active PROTAC in normal cells. It is expected that our caged compounds provide a new strategy for precise functional control of PROTAC drugs for breast cancer treatment.

Photomodulation Approaches to Overcome Antimicrobial Resistance

Photopharmacology is an approach that aims to be an alternative to classical chemotherapy. Herein, the different classes of photoswitches and photocleavage compounds and their biological applications are described. Proteolysis targeting chimeras (PROTACs) containing azobenzene moieties (PHOTACs) and photocleavable protecting groups (photocaged PROTACs) are also mentioned. Furthermore, porphyrins are referenced as successful photoactive compounds in a clinical context, such as in the photodynamic therapy of tumours as well as preventing antimicrobial resistance, namely in bacteria. Porphyrins combining photoswitches and photocleavage systems are highlighted, taking advantage of both photopharmacology and photodynamic action. Finally, porphyrins with antibacterial activity are described, taking advantage of the synergistic effect of photodynamic treatment and antibiotic therapy to overcome bacterial resistance.

Photoswitchable Molecular Glues Enable Optical Control of Transcription Factor Degradation

Immunomodulatory drugs (IMiDs), which include thalidomide and its derivatives, have emerged as the standard of care against multiple myeloma. They function as molecular glues that bind to the E3 ligase cereblon (CRBN) and induce protein interactions with neosubstrates, including the transcription factors Ikaros (IKZF1) and Aiolos (IKZF3). The subsequent ubiquitylation and degradation of these transcription factors underlies the antiproliferative activity of IMiDs. Here, we introduce photoswitchable immunomodulatory drugs (PHOIMiDs) that can be used to degrade Ikaros and Aiolos in a light-dependent fashion. Our lead compound shows minimal activity in the dark and becomes an active degrader upon irradiation with violet light. It shows high selectivity over other transcription factors, regardless of its state, and could therefore be used to control the levels of Ikaros and Aiolos with high spatiotemporal precision.

The Extracellular Matrix: Its Composition, Function, Remodeling, and Role in Tumorigenesis

The extracellular matrix (ECM) is a ubiquitous member of the body and is key to the maintenance of tissue and organ integrity. Initially thought to be a bystander in many cellular processes, the extracellular matrix has been shown to have diverse components that regulate and activate many cellular processes and ultimately influence cell phenotype. Importantly, the ECM's composition, architecture, and stiffness/elasticity influence cellular phenotypes. Under normal conditions and during development, the synthesized ECM constantly undergoes degradation and remodeling processes via the action of matrix proteases that maintain tissue homeostasis. In many pathological conditions including fibrosis and cancer, ECM synthesis, remodeling, and degradation is dysregulated, causing its integrity to be altered. Both physical and chemical cues from the ECM are sensed via receptors including integrins and play key roles in driving cellular proliferation and differentiation and in the progression of various diseases such as cancers. Advances in 'omics' technologies have seen an increase in studies focusing on bidirectional cell-matrix interactions, and here, we highlight the emerging knowledge on the role played by the ECM during normal development and in pathological conditions. This review summarizes current ECM-targeted therapies that can modify ECM tumors to overcome drug resistance and better cancer treatment.

Novel approaches to targeted protein degradation technologies in drug discovery

INTRODUCTION

Target protein degradation (TPD) provides a novel therapeutic modality, other than inhibition, through the direct depletion of target proteins. Two primary human protein homeostasis mechanisms are exploited: the ubiquitin-proteasome system (UPS) and the lysosomal system. TPD technologies based on these two systems are progressing at an impressive pace.

AREAS COVERED

This review focuses on the TPD strategies based on UPS and lysosomal system, mainly classified into three types: Molecular Glue (MG), PROteolysis Targeting Chimera (PROTAC), and lysosome-mediated TPD. Starting with a brief background introduction of each strategy, exciting examples and perspectives on these novel approaches are provided.

EXPERT OPINION

MGs and PROTACs are two major UPS-based TPD strategies that have been extensively investigated in the past decade. Despite some clinical trials, several critical issues remain, among which is emphasized by the limitation of targets. Recently developed lysosomal system-based approaches provide alternative solutions for TPD beyond UPS' capability. The newly emerging novel approaches may partially address issues that have long plagued researchers, such as low potency, poor cell permeability, on-/off-target toxicity, and delivery efficiency. Comprehensive considerations for the rational design of protein degraders and continuous efforts to seek effective solutions are imperative to advance these strategies into clinical medications.

Ligation to Scavenging Strategy Enables On-Demand Termination of Targeted Protein Degradation

Event-driven bifunctional molecules, typified by proteolysis targeting chimera (PROTAC) technology, have been successfully applied in degrading many proteins of interest (POI). Due to the unique catalytic mechanism, PROTACs will induce multiple cycles of degradation until the elimination of the target protein. Here, we propose a versatile "Ligation to scavenging" approach to terminate event-driven degradation for the first time. Ligation to the scavenging system consists of a TCO-modified dendrimer (PAMAM-G5-TCO) and tetrazine-modified PROTACs (Tz-PROTACs). PAMAM-G5-TCO can rapidly scavenge intracellular free PROTACs via an inverse electron demand Diels-Alder reaction and terminate the degradation of certain proteins in living cells. Thus, this work proposes a flexible chemical knockdown approach to adjust the levels of POI on-demand in living cells, which paves the way for controlled target protein degradation.

Design, synthesis and biological evaluation of the tumor hypoxia-activated PROTACs bearing caged CRBN E3 ligase ligands

Tumor hypoxia-activated proteolysis targeting chimeras (ha-PROTACs) 9 and 10 were designed and synthesized by incorporating the hypoxia-activated leaving group (1-methyl-2-nitro-1H-imidazol-5-yl)methyl or 4‑nitrobenzyl into the structure of the cereblon (CRBN) E3 ligand of an epidermal growth factor receptor 19 deletions (EGFR^Del19-based PROTAC 8. The in vitro protein degradation assay demonstrated that 9 and 10 could effectively and selectively degrade EGFR^Del19 in tumor hypoxia. Meanwhile, these two compounds showed higher potency in inhibiting cell viability and migration, as well as in promoting cells apoptosis in tumor hypoxia. Moreover, nitroreductase reductive activation assay indicated that prodrugs 9 and 10 could successfully release the active compound 8. This study confirmed the feasibility to develop ha-PROTACs to enhance the selectivity of PROTACs by caging CRBN E3 ligase ligand.