PROTACs: New method to degrade transcription regulating proteins

Discovery of ganoderic acid A (GAA) PROTACs as MDM2 protein degraders for the treatment of breast cancer

Breast cancer is one of the most common female malignant tumors, with triple-negative breast cancer (TNBC) being the most specific, highly invasive, metastatic and associated with a poor prognosis. Our previous study showed that the natural product ganoderic acid A (GAA) has a certain affinity for MDM2. In this study, two series of novel GAA PROTACs C1-C10 and V1-V10 were designed and synthesized for the treatment of breast cancer. The antitumor activity of these compounds was evaluated against four human tumor cell lines (MCF-7, MDA-MB-231, SJSA-1, and HepG2). Among them, V9 and V10 showed stronger anti-proliferative effects against breast cancer cells, and V10 showed the best selectivity in MDA-MB-231 cells (TNBC), which was 5-fold higher than that of the lead compound GAA. Preliminary structure-activity analysis revealed that V-series GAA PROTACs had better effects than C-series, and the introduction of 2O-4O PEG linkers could significantly improve the antitumor activity. Molecular docking, surface plasmon resonance (SPR), cellular thermal shift assay (CETSA), and Western blot researches showed that both V9 and V10 could bind with MDM2, and degrade the protein through the ubiquitin-proteasome system. Molecular dynamics simulation (MD) revealed that V10 is a bifunctional molecule that can bind to von Hippel-Lindau (VHL) at one end and target MDM2 at the other. In addition, V10 promoted the upregulation of p21 in p53-mutant MDA-MB-231 cells, and induced apoptosis via down-regulation of the bcl-2/bax ratio and the expression of cyclin B1. Finally, in vivo experiments showed that, V10 also exhibited good tumor inhibitory activity in xenografted TNBC zebrafish models, with an inhibition rate of 27.2% at 50 μg/mL. In conclusion, our results suggested that V10 has anti-tumor effects on p53-mutant breast cancer in vitro and in vivo, and may be used as a novel lead compound for the future development of TNBC.

Decoding Allosteric Control in Hypoxia-Inducible Factors

The mammalian family of basic helix-loop-helix-PER-ARNT-SIM (bHLH-PAS) transcription factors possess the ability to sense and respond to diverse environmental and physiological cues. These proteins all share a common structural framework, comprising a bHLH domain, two PAS domains, and transcriptional activation or repression domain. To function effectively as transcription factors, members of the family must form dimers, bringing together bHLH segments to create a functional unit that allows for DNA response element binding. The significance of bHLH-PAS family is underscored by their involvement in many major human diseases, offering potential avenues for therapeutic intervention. Notably, the clear identification of ligand-binding cavities within their PAS domains enables the development of targeted small molecules. Two examples are Belzutifan, targeting hypoxia-inducible factor (HIF)-2α, and Tapinarof, targeting the aryl hydrocarbon receptor (AHR), both of which have gained regulatory approval recently. Here, we focus on the HIF subfamily. The crystal structures of all three HIF-α proteins have been elucidated, revealing their bHLH and tandem PAS domains are used to engage their dimerization partner aryl hydrocarbon receptor nuclear translocator (ARNT, also called HIF-1β). A broad range of recent findings point to a shared allosteric modulation mechanism among these proteins, whereby small-molecules at the PAS-B domains exert direct influence over the HIF-α transcriptional functions. As our understanding of the architectural and allosteric mechanisms of bHLH-PAS proteins continues to advance, the possibility of discovering new therapeutic drugs becomes increasingly promising.

Proteostasis and neurodegeneration: a closer look at autophagy in Alzheimer's disease

Alzheimer's disease (AD) is characterized by the accumulation of misfolded amyloid-beta and tau proteins. Autophagy acts as a proteostasis process to remove protein clumps, although it progressively weakens with aging and AD, thus facilitating the accumulation of toxic proteins and causing neurodegeneration. This review examines the impact of impaired autophagy on the progression of AD disease pathology. Under normal circumstances, autophagy removes abnormal proteins and damaged organelles, but any dysfunction in this process can lead to the exacerbation of amyloid and tau pathology, particularly in AD. There is increasing attention to therapeutic tactics to revitalize autophagy, including reduced caloric intake, autophagy-stimulating drugs, and genetic therapy. However, the translation of these strategies into clinical practice faces several hurdles. In summary, this review integrates the understanding of the intricate role of autophagy dysfunction in Alzheimer's disease progression and reinforces the promising prospects of autophagy as a beneficial target for treatments to modify the course of Alzheimer's disease.

Interactions of Indoleamine 2,3-dioxygenase-expressing LAMP3+ dendritic cells with CD4+ regulatory T cells and CD8+ exhausted T cells: synergistically remodeling of the immunosuppressive microenvironment in cervical cancer and therapeutic implications

BACKGROUND

Cervical cancer (CC) is the fourth most common cancer in women worldwide. Although immunotherapy has been applied in clinical practice, its therapeutic efficacy remains far from satisfactory, necessitating further investigation of the mechanism of CC immune remodeling and exploration of novel treatment targets. This study aimed to investigate the mechanism of CC immune remodeling and explore potential therapeutic targets.

METHODS

We conducted single-cell RNA sequencing on a total of 17 clinical specimens, including normal cervical tissues, high-grade squamous intraepithelial lesions, and CC tissues. To validate our findings, we conducted multicolor immunohistochemical staining of CC tissues and constructed a subcutaneous tumorigenesis model in C57BL/6 mice using murine CC cell lines (TC1) to evaluate the effectiveness of combination therapy involving indoleamine 2,3-dioxygenase 1 (IDO1) inhibition and immune checkpoint blockade (ICB). We used the unpaired two-tailed Student's t-test, Mann-Whitney test, or Kruskal-Wallis test to compare continuous data between two groups and one-way ANOVA with Tukey's post hoc test to compare data between multiple groups.

RESULTS

Malignant cervical epithelial cells did not manifest noticeable signs of tumor escape, whereas lysosomal-associated membrane protein 3-positive (LAMP3+ ) dendritic cells (DCs) in a mature state with immunoregulatory roles were found to express IDO1 and affect tryptophan metabolism. These cells interacted with both tumor-reactive exhausted CD8+ T cells and CD4+ regulatory T cells, synergistically forming a vicious immunosuppressive cycle and mediating CC immune escape. Further validation through multicolor immunohistochemical staining showed co-localization of neoantigen-reactive T cells (CD3+ , CD4+ /CD8+ , and PD-1+ ) and LAMP3+ DCs (CD80+ and PD-L1+ ). Additionally, a combination of the IDO1 inhibitor with an ICB agent significantly reduced tumor volume in the mouse model of CC compared with an ICB agent alone.

CONCLUSIONS

Our study suggested that a combination treatment consisting of targeting IDO1 and ICB agent could improve the therapeutic efficacy of current CC immunotherapies. Additionally, our results provided crucial insights for designing drugs and conducting future clinical trials for CC.

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.

Decoding Oncofusions: Unveiling Mechanisms, Clinical Impact, and Prospects for Personalized Cancer Therapies

Cancer-associated gene fusions, also known as oncofusions, have emerged as influential drivers of oncogenesis across a diverse range of cancer types. These genetic events occur via chromosomal translocations, deletions, and inversions, leading to the fusion of previously separate genes. Due to the drastic nature of these mutations, they often result in profound alterations of cellular behavior. The identification of oncofusions has revolutionized cancer research, with advancements in sequencing technologies facilitating the discovery of novel fusion events at an accelerated pace. Oncofusions exert their effects through the manipulation of critical cellular signaling pathways that regulate processes such as proliferation, differentiation, and survival. Extensive investigations have been conducted to understand the roles of oncofusions in solid tumors, leukemias, and lymphomas. Large-scale initiatives, including the Cancer Genome Atlas, have played a pivotal role in unraveling the landscape of oncofusions by characterizing a vast number of cancer samples across different tumor types. While validating the functional relevance of oncofusions remains a challenge, even non-driver mutations can hold significance in cancer treatment. Oncofusions have demonstrated potential value in the context of immunotherapy through the production of neoantigens. Their clinical importance has been observed in both treatment and diagnostic settings, with specific fusion events serving as therapeutic targets or diagnostic markers. However, despite the progress made, there is still considerable untapped potential within the field of oncofusions. Further research and validation efforts are necessary to understand their effects on a functional basis and to exploit the new targeted treatment avenues offered by oncofusions. Through further functional and clinical studies, oncofusions will enable the advancement of precision medicine and the drive towards more effective and specific treatments for cancer patients.

A model-informed method to retrieve intrinsic from apparent cooperativity and project cellular target occupancy for ternary complex-forming compounds

There is an increasing interest to develop therapeutics that modulate challenging or undruggable target proteins via a mechanism that involves ternary complexes. In general, such compounds can be characterized by their direct affinities to a chaperone and a target protein and by their degree of cooperativity in the formation of the ternary complex. As a trend, smaller compounds have a greater dependency on intrinsic cooperativity to their thermodynamic stability relative to direct target (or chaperone) binding. This highlights the need to consider intrinsic cooperativity of ternary complex-forming compounds early in lead optimization, especially as they provide more control over target selectivity (especially for isoforms) and more insight into the relationship between target occupancy and target response via estimation of ternary complex concentrations. This motivates the need to quantify the natural constant of intrinsic cooperativity (α) which is generally defined as the gain (or loss) in affinity of a compound to its target in pre-bound vs. unbound state. Intrinsic cooperativities can be retrieved via a mathematical binding model from EC₅₀ shifts of binary binding curves of the ternary complex-forming compound with either a target or chaperone relative to the same experiment but in the presence of the counter protein. In this manuscript, we present a mathematical modeling methodology that estimates the intrinsic cooperativity value from experimentally observed apparent cooperativities. This method requires only the two binary binding affinities and the protein concentrations of target and chaperone and is therefore suitable for use in early discovery therapeutic programs. This approach is then extended from biochemical assays to cellular assays (i.e., from a closed system to an open system) by accounting for differences in total ligand vs. free ligand concentrations in the calculations of ternary complex concentrations. Finally, this model is used to translate biochemical potency of ternary complex-forming compounds into expected cellular target occupancy, which could ultimately serve as a way for validation or de-validation of hypothesized biological mechanisms of action.

The future of cancer immunotherapy: DNA vaccines leading the way

Immuno-oncology has revolutionized cancer treatment and has opened up new opportunities for developing vaccination methods. DNA-based cancer vaccines have emerged as a promising approach to activating the bodily immune system against cancer. Plasmid DNA immunizations have shown a favorable safety profile and there occurs induction of generalized as well as tailored immune responses in preclinical and early-phase clinical experiments. However, these vaccines have notable limitations in immunogenicity and heterogeneity and these require refinements. DNA vaccine technology has been focusing on improving vaccine efficacy and delivery, with parallel developments in nanoparticle-based delivery systems and gene-editing technologies such as CRISPR/Cas9. This approach has showcased great promise in enhancing and tailoring the immune response to vaccination. Strategies to enhance the efficacy of DNA vaccines include the selection of appropriate antigens, optimizing insertion in a plasmid, and studying combinations of vaccines with conventional strategies and targeted therapies. Combination therapies have attenuated immunosuppressive activities in the tumor microenvironment and enhanced the capability of immune cells. This review provides an overview of the current framework of DNA vaccines in oncology and focuses on novel strategies, including established combination therapies and those still under development.The challenges that oncologists, scientists, and researchers need to overcome to establish DNA vaccines as an avant-garde approach to defeating cancer, are also emphasized. The clinical implications of the immunotherapeutic approaches and the need for predictive biomarkers have also been reviewed upon. We have also tried to extend the role of Neutrophil extracellular traps (NETs) to the DNA vaccines. The clinical implications of the immunotherapeutic approaches have also been reviewed upon. Ultimately, refining and optimizing DNA vaccines will enable harnessing the immune system's natural ability to recognize and eliminate cancer cells, leading the world towards a revolution in cancer cure.

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.

New insights into KLFs and SOXs in cancer pathogenesis, stemness, and therapy

Despite the development of cancer therapies, the success of most treatments has been impeded by drug resistance. The crucial role of tumor cell plasticity has emerged recently in cancer progression, cancer stemness and eventually drug resistance. Cell plasticity drives tumor cells to reversibly convert their cell identity, analogous to differentiation and dedifferentiation, to adapt to drug treatment. This phenotypical switch is driven by alteration of the transcriptome. Several pluripotent factors from the KLF and SOX families are closely associated with cancer pathogenesis and have been revealed to regulate tumor cell plasticity. In this review, we particularly summarize recent studies about KLF4, KLF5 and SOX factors in cancer development and evolution, focusing on their roles in cancer initiation, invasion, tumor hierarchy and heterogeneity, and lineage plasticity. In addition, we discuss the various regulation of these transcription factors and related cutting-edge drug development approaches that could be used to drug "undruggable" transcription factors, such as PROTAC and PPI targeting, for targeted cancer therapy. Advanced knowledge could pave the way for the development of novel drugs that target transcriptional regulation and could improve the outcome of cancer therapy.

Selective degradation of cellular BRD3 and BRD4-L promoted by PROTAC molecules in six cancer cell lines

Targeted degradation of BET family proteins BRD2/3/4 or only BRD4 with PROTAC molecules has been a promising strategy for the treatment of human cancer. Meanwhile, selective degradation of cellular BRD3 and BRD4-L remains a challenging task. We report herein a novel PROTAC molecule 24 that promoted selective degradation of cellular BRD3 and BRD4-L, but not BRD2 or BRD4-S, in a panel of six cancer cell lines. The observed target selectivity was partially attributed to differences in protein degradation kinetics and in types of cell lines. In a MM.1S mouse xenograft model, an optimized lead compound 28 promoted selective degradation of BRD3 and BRD4-L in vivo and exhibited robust antitumor activity. In summary, we have demonstrated that selective degradation of BRD3 and BRD4-L over BRD2 and BRD4-S is a feasible and robust approach in multiple cancer cell lines and an animal model, which could be helpful for further investigations on BRD3 and BRD4-L that ultimately benefitting cancer research and therapeutics.

Protein-Targeted Degradation Agents Based on Natural Products

Natural products are an important source of drug lead compounds, and natural products with significant biological activity are constantly being discovered and used in clinical practice. At present, natural products play an important role in the targeted therapy of cancer, cardiovascular and cerebrovascular diseases, nervous system diseases, and autoimmune diseases. Meanwhile, in recent years, the rise of protein-targeted degradation technologies, such as proteolysis-targeting chimeras (PROTACs) and molecular glues, has provided a new solution for drug resistance caused by clinical molecular-targeting drugs. It is noteworthy that natural products and their derivatives, as important components of PROTACs and molecular glues, play an important role in the development of protein-targeting drugs. Hence, this review summarized the protein-targeted degradation agents based on natural products, such as PROTACs and molecular glues. More natural products with the potential to be used in the development of PROTACs and molecular glues as targeted protein degradation agents are still being investigated.

Clinical cancer immunotherapy: Current progress and prospects

Immune checkpoint therapy via PD-1 antibodies has shown exciting clinical value and robust therapeutic potential in clinical practice. It can significantly improve progression-free survival and overall survival. Following surgery, radiotherapy, chemotherapy, and targeted therapy, cancer treatment has now entered the age of immunotherapy. Although cancer immunotherapy has shown remarkable efficacy, it also suffers from limitations such as irAEs, cytokine storm, low response rate, etc. In this review, we discuss the basic classification, research progress, and limitations of cancer immunotherapy. Besides, by combining cancer immunotherapy resistance mechanism with analysis of combination therapy, we give our insights into the development of new anticancer immunotherapy strategies.

Current strategies and progress for targeting the "undruggable" transcription factors

Transcription factors (TFs) specifically bind to DNA, recruit cofactor proteins and modulate target gene expression, rendering them essential roles in the regulation of numerous biological processes. Meanwhile, mutated or dysregulated TFs are involved in a variety of human diseases. As multiple signaling pathways ultimately converge at TFs, targeting these TFs directly may prove to be more specific and cause fewer side effects, than targeting the upfront conventional targets in these pathways. All these features together endue TFs with great potential and high selectivity as therapeutic drug targets. However, TFs have been historically considered "undruggable", mainly due to their lack of structural information, especially about the appropriate ligand-binding sites and protein-protein interactions, leading to relatively limited choices in the TF-targeting drug design. In this review, we summarize the recent progress of TF-targeting drugs and highlight certain strategies used for targeting TFs, with a number of representative drugs that have been approved or in the clinical trials as examples. Various approaches in targeting TFs directly or indirectly have been developed. Common direct strategies include aiming at defined binding pockets, proteolysis-targeting chimaera (PROTAC), and mutant protein reactivation. In contrast, the indirect ones comprise inhibition of protein-protein interactions between TF and other proteins, blockade of TF expression, targeting the post-translational modifications, and targeting the TF-DNA interactions. With more comprehensive structural information about TFs revealed by the powerful cryo-electron microscopy technology and predicted by machine-learning algorithms, plus more efficient compound screening platforms and a deeper understanding of TF-disease relationships, the development of TF-targeting drugs will certainly be accelerated in the near future.

PROTACs: The Future of Leukemia Therapeutics

The fight to find effective, long-lasting treatments for cancer has led many researchers to consider protein degrading entities. Recent developments in PROteolysis TArgeting Chimeras (PROTACs) have signified their potential as possible cancer therapies. PROTACs are small molecule, protein degraders that function by hijacking the built-in Ubiquitin-Proteasome pathway. This review mainly focuses on the general design and functioning of PROTACs as well as current advancements in the development of PROTACs as anticancer therapies. Particular emphasis is given to PROTACs designed against various types of Leukemia/Blood malignancies.

Beyond Proteolysis-Targeting Chimeric Molecules: Designing Heterobifunctional Molecules Based on Functional Effectors

In recent years, with the successful development of proteolysis-targeting chimeric molecules (PROTACs), the potential of heterobifunctional molecules to contribute to reenvisioning drug design, especially small-molecule drugs, has been increasingly recognized. Inspired by PROTACs, diverse heterobifunctional molecules have been reported to simultaneously bind two or more molecules and bring them into proximity to interaction, such as ribonuclease-recruiting, autophagy-recruiting, lysosome-recruiting, kinase-recruiting, phosphatase-recruiting, glycosyltransferase-recruiting, and acetyltransferase-recruiting chimeras. On the basis of the heterobifunctional principle, more opportunities for advancing drug design by linking potential effectors to a protein of interest (POI) have emerged. Herein, we introduce heterobifunctional molecules other than PROTACs, summarize the limitations of existing molecules, list the main challenges, and propose perspectives for future research directions, providing insight into alternative design strategies based on substrate-proximity-based targeting.

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.

Novel CRBN-Recruiting Proteolysis-Targeting Chimeras as Degraders of Stimulator of Interferon Genes with In Vivo Anti-Inflammatory Efficacy

The activation of the cyclic GMP-AMP synthase-stimulator of interferon gene (STING) pathway has been associated with the pathogenesis of many autoimmune and inflammatory disorders, and small molecules targeting STING have emerged as a new therapeutic strategy for the treatment of these diseases. While several STING inhibitors have been identified with potent anti-inflammatory effects, we would like to explore STING degraders based on the proteolysis-targeting chimera (PROTAC) technology as an alternative strategy to target the STING pathway. Thus, we designed and synthesized a series of STING protein degraders based on a small-molecule STING inhibitor (C-170) and pomalidomide (a CRBN ligand). These compounds demonstrated moderate STING-degrading activities. Among them, SP23 achieved the highest degradation potency with a DC₅₀ of 3.2 μM. Importantly, SP23 exerted high anti-inflammatory efficacy in a cisplatin-induced acute kidney injury mouse model by modulating the STING signaling pathway. Taken together, SP23 represents the first PROTAC degrader of STING deserving further investigation as a new anti-inflammatory agent.

Signaling Pathway and Small-Molecule Drug Discovery of FGFR: A Comprehensive Review

Targeted therapy is a groundbreaking innovation for cancer treatment. Among the receptor tyrosine kinases, the fibroblast growth factor receptors (FGFRs) garnered substantial attention as promising therapeutic targets due to their fundamental biological functions and frequently observed abnormality in tumors. In the past 2 decades, several generations of FGFR kinase inhibitors have been developed. This review starts by introducing the biological basis of FGF/FGFR signaling. It then gives a detailed description of different types of small-molecule FGFR inhibitors according to modes of action, followed by a systematic overview of small-molecule-based therapies of different modalities. It ends with our perspectives for the development of novel FGFR inhibitors.

Targeted protein degradation: mechanisms, strategies and application

Traditional drug discovery mainly focuses on direct regulation of protein activity. The development and application of protein activity modulators, particularly inhibitors, has been the mainstream in drug development. In recent years, PROteolysis TArgeting Chimeras (PROTAC) technology has emerged as one of the most promising approaches to remove specific disease-associated proteins by exploiting cells’ own destruction machinery. In addition to PROTAC, many different targeted protein degradation (TPD) strategies including, but not limited to, molecular glue, Lysosome-Targeting Chimaera (LYTAC), and Antibody-based PROTAC (AbTAC), are emerging. These technologies have not only greatly expanded the scope of TPD, but also provided fresh insights into drug discovery. Here, we summarize recent advances of major TPD technologies, discuss their potential applications, and hope to provide a prime for both biologists and chemists who are interested in this vibrant field.

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.

Recent advances in clinical trials targeting the kynurenine pathway

The kynurenine pathway (KP) is the major catabolic pathway for the essential amino acid tryptophan leading to the production of nicotinamide adenine dinucleotide. In inflammatory conditions, the activation of the KP leads to the production of several bioactive metabolites including kynurenine, 3-hydroxykynurenine, 3-hydroxyanthranilic acid, kynurenic acid and quinolinic acid. These metabolites can have redox and immune suppressive activity, be neurotoxic or neuroprotective. While the activity of the pathway is tightly regulated under normal physiological condition, it can be upregulated by immunological activation and inflammation. The dysregulation of the KP has been implicated in wide range of neurological diseases and psychiatric disorders. In this review, we discuss the mechanisms involved in KP-mediated neurotoxicity and immune suppression, and its role in diseases of our expertise including cancer, chronic pain and multiple sclerosis. We also provide updates on the clinical trials evaluating the efficacy of KP inhibitors and/or analogues in each respective disease.

Targeting of the FOXM1 Oncoprotein by E3 Ligase-Assisted Degradation

The transcription factor FOXM1 that regulates multiple proliferation-related genes through selective protein-DNA and protein-protein interactions is now considered an attractive oncotarget. There are several small-molecule inhibitors that indirectly suppress the expression of FOXM1 or block its DNA binding domain (FOXM1-DBD). However, insufficient specificity or/and efficacy are two potential drawbacks. Here, we employed in silico modeling of FOXM1-DBD with inhibitors to enable the design of an effective CRBN-recruiting molecule that induced significant FOXM1 protein degradation and exerted promising in vivo antitumor activity against TNBC xenograft models. This study is the first of its kind showcasing the use of an approach described in the literature as protein-targeting chimeras to degrade the elusive FOXM1, providing an alternative strategy to counter the pathological effects resulting from the increased transcriptional activity of FOXM1 observed in cancer cells.

Indoleamine 2,3-dioxygenase 1 (IDO1) inhibitors and PROTAC-based degraders for cancer therapy

Indoleamine 2,3-dioxygenase 1 (IDO1), a known immunosuppressive enzyme that catalyzes the rate-limiting step in the oxidation of tryptophan (Trp) to kynurenine (Kyn), has received increasing attention as an attractive immunotherapeutic target for cancer therapy. Up to now, eleven small-molecule IDO1 inhibitors have entered clinical trials for the treatment of cancers. In addition, proteolysis targeting chimera (PROTAC) based degraders also provide prospects for cancer therapy. Herein we present a comprehensive overview of the medicinal chemistry strategies and potential therapeutic applications of IDO1 inhibitors in nonclinical trials and IDO1-PROTAC degraders.

The Role of AP-1 Transcription Factors in Plasma Cell Biology and Multiple Myeloma Pathophysiology

Multiple myeloma (MM) is an incurable hematologic malignancy characterized by the clonal expansion of malignant plasma cells within the bone marrow. Activator Protein-1 (AP-1) transcription factors (TFs), comprised of the JUN, FOS, ATF and MAF multigene families, are implicated in a plethora of physiologic processes and tumorigenesis including plasma cell differentiation and MM pathogenesis. Depending on the genetic background, the tumor stage, and cues of the tumor microenvironment, specific dimeric AP-1 complexes are formed. For example, AP-1 complexes containing Fra-1, Fra-2 and B-ATF play central roles in the transcriptional control of B cell development and plasma cell differentiation, while dysregulation of AP-1 family members c-Maf, c-Jun, and JunB is associated with MM cell proliferation, survival, drug resistance, bone marrow angiogenesis, and bone disease. The present review article summarizes our up-to-date knowledge on the role of AP-1 family members in plasma cell differentiation and MM pathophysiology. Moreover, it discusses novel, rationally derived approaches to therapeutically target AP-1 TFs, including protein-protein and protein-DNA binding inhibitors, epigenetic modifiers and natural products.

Indoleamine 2,3-dioxygenase 1 (IDO1) inhibitors in clinical trials for cancer immunotherapy

Indoleamine 2,3-dioxygenase 1 (IDO1) is a heme enzyme that catalyzes the oxidation of L-tryptophan. Functionally, IDO1 has played a pivotal role in cancer immune escape via catalyzing the initial step of the kynurenine pathway, and overexpression of IDO1 is also associated with poor prognosis in various cancers. Currently, several small-molecule candidates and peptide vaccines are currently being assessed in clinical trials. Furthermore, the "proteolysis targeting chimera" (PROTAC) technology has also been successfully used in the development of IDO1 degraders, providing novel therapeutics for cancers. Herein, we review the biological functions of IDO1, structural biology and also extensively summarize medicinal chemistry strategies for the development of IDO1 inhibitors in clinical trials. The emerging PROTAC-based IDO1 degraders are also highlighted. This review may provide a comprehensive and updated overview on IDO1 inhibitors and their therapeutic potentials.

Is PROTAC technology really a game changer for central nervous system drug discovery?

Introduction: Central nervous system (CNS) diseases affect a large portion of the population, however, few therapeutic options are available. Furthermore, to date, clinical trials have been largely unsuccessful due to difficulty in targeting the undruggable, toxic proteins that underly many CNS disorders. PROteolysis Targeting Chimeras (PROTACs) are a rapidly emerging technology that has been proposed as a potential treatment option for various CNS diseases by hijacking the endogenous protein degradation process.Areas Covered: Herein, the authors discuss how the application of PROTACs may be beneficial in the treatment of major CNS diseases. They further discuss the main advantages and disadvantages of using PROTACs in the CNS, focusing on potential limitations such as their transient nature, localization, blood-brain barrier permeability and proteasome dysfunction.Expert opinion: It is evident that PROTACs have significant potential as a therapeutic tool for the treatment of CNS diseases and there is preliminary evidence suggesting that PROTACs could be successful in a clinical setting. Nevertheless, numerous limitations exist that must be overcome before this technology can be applied as a successful therapeutic for CNS disorders. Importantly, more in vivo studies are needed to determine the feasibility and effectiveness of using PROTACs in the brain.

Tackling Drug Resistance in EGFR Exon 20 Insertion Mutant Lung Cancer

Insertion mutations in exon 20 (Ex20ins) of the epidermal growth factor receptor (EGFR) gene are the largest class of EGFR mutations in non-small cell lung cancer (NSCLC) for which there are currently no approved targeted therapies. NSCLC patients with these mutations do not respond to clinically approved EGFR tyrosine kinase inhibitors (TKIs) and have poor outcomes. A number of early phase clinical trials are currently underway to evaluate the efficacy of a new generation of TKIs that are capable of binding to and blocking Ex20ins. Although these agents have shown some clinical activity, patient responses have been restricted by dose-limiting toxicity or rapid acquisition of resistance after a short response. Here we review the current understanding of the mechanisms of resistance to these compounds, which include on-target EGFR secondary mutations, compensatory bypass pathway activation and acquisition of an EMT phenotype. Taking lessons from conventional EGFR inhibitor therapy in NSCLC, we also consider other potential sources of resistance including the presence of drug-tolerant persister cells. We will discuss therapeutic strategies which have the potential to overcome different forms of drug resistance. We conclude by evaluating recent technological developments in drug discovery such as PROTACs as a means to better tackle TKI resistance in NSCLC harbouring Ex20ins mutations.

PROTACs to address the challenges facing small molecule inhibitors

Small molecule inhibitors of proteins represent important medicines and critical chemical tools to investigate the biology of the target proteins. Advances in various -omics technologies have fueled the pace of discovery of disease-relevant proteins. Translating these discoveries into human benefits requires us to develop specific chemicals to inhibit the proteins. However, traditional small molecule inhibitors binding to orthosteric or allosteric sites face significant challenges. These challenges include drug selectivity, therapy resistance as well as drugging undruggable proteins and multi-domain proteins. To address these challenges, PROteolysis TArgeting Chimera (PROTAC) has been proposed. PROTACs are heterobifunctional molecules containing a binding ligand for a protein of interest and E3 ligase-recruiting ligand that are connected through a chemical linker. Binding of a PROTAC to its target protein will bring a E3 ligase in close proximity to initiate polyubiquitination of the target protein ensuing its proteasome-mediated degradation. Unlike small molecule inhibitors, PROTACs achieve target protein degradation in its entirety in a catalytical fashion. In this review, we analyze recent advances in PROTAC design to discuss how PROTACs can address the challenges facing small molecule inhibitors to potentially deliver next-generation medicines and chemical tools with high selectivity and efficacy. We also offer our perspectives on the future promise and potential limitations facing PROTACs. Investigations to overcome these limitations of PROTACs will further help realize the promise of PROTACs for human benefits.