Triazol: a privileged scaffold for proteolysis targeting chimeras

FLT3-PROTACs for combating AML resistance: Analytical overview on chimeric agents developed, challenges, and future perspectives

The urgent and unmet medical demand of acute myeloid leukemia (AML) patients has driven the drug discovery process for expansion of the landscape of AML treatment. Despite the several agents developed for treatment of AML, more than 60 % of treated patients undergo relapse again after re-emission, thus, no complete cure for this complex disease has been reached yet. Targeted oncoprotein degradation is a new paradigm that can be employed to solve drug resistance, disease relapse, and treatment failure in complex diseases as AML, the most lethal hematological malignancy. AML is an aggressive blood cancer form and the most common type of acute leukemia, with bad outcomes and a very poor 5-year survival rate. FLT3 mutations occur in about 30 % of AML cases and FLT3-ITD is associated with poor prognosis of this disease. Prevalent FLT3 mutations include internal tandem duplication and point mutations (e.g., D835) in the tyrosine kinase domain, which induce FLT3 kinase activation and result in survival and proliferation of AML cells again. Currently approved FLT3 inhibitors suffer from limited clinical efficacy due to FLT3 reactivation by mutations, therefore, alternative new treatments are highly needed. Proteolysis-targeting chimera (PROTAC) is a bi-functional molecule that consists of a ligand of the protein of interest, FLT3 inhibitor in our case, that is covalently linked to an E3 ubiquitin ligase ligand. Upon FLT3-specific PROTAC binding to FLT3, the PROTAC can recruit E3 for FLT3 ubiquitination, which is subsequently subjected to proteasome-mediated degradation. In this review we tried to address the question if PROTAC technology has succeeded in tackling the disease relapse and treatment failure of AML. Next, we explored the latest FLT3-targeting PROTACs developed in the past few years such as quizartinib-based PROTACs, dovitinib-based PROTACs, gilteritinib-based PROTACs, and others. Then, we followed with a deep analysis of their advantages regarding potency improvement and overcoming AML drug resistance. Finally, we discussed the challenges facing these chimeric molecules with proposed future solutions to circumvent them.

Characteristic roadmap of linker governs the rational design of PROTACs

Proteolysis targeting chimera (PROTAC) technology represents a groundbreaking development in drug discovery, leveraging the ubiquitin‒proteasome system to specifically degrade proteins responsible for the disease. PROTAC is characterized by its unique heterobifunctional structure, which comprises two functional domains connected by a linker. The linker plays a pivotal role in determining PROTAC's biodegradative efficacy. Advanced and rationally designed functional linkers for PROTAC are under development. Nonetheless, the correlation between linker characteristics and PROTAC efficacy remains under-investigated. Consequently, this study will present a multidisciplinary analysis of PROTAC linkers and their impact on efficacy, thereby guiding the rational design of linkers. We will primarily discuss the structural types and characteristics of PROTAC linkers, and the optimization strategies used for their rational design. Furthermore, we will discuss how factors like linker length, group type, flexibility, and linkage site affect the biodegradation efficiency of PROTACs. We believe that this work will contribute towards the advancement of rational linker design in the PROTAC research area.

Application and challenges of nitrogen heterocycles in PROTAC linker

The absence of effective active pockets makes traditional molecularly targeted drug strategies ineffective against 80 % of human disease-related proteins. The PROTAC technology effectively makes up for the deficiency of traditional molecular targeted drugs, which produces drug activity by degrading rather than inhibiting the target protein. The degradation of PROTAC is not only affected by POI ligand and E3 ligand, but by the selection of suitable linker which can play an important role in the efficiency and selectivity of the degradation. In the early exploring stage of the PROTAC, flexible chains were priorly applied as the linker of PROTAC. Although PROTAC with flexible chains as linkers sometimes perform well in vitro bioactivity evaluations, the introduction of lipophilic flexible chains reduces the hydrophilicity of these molecules, resulting in generally poor pharmacokinetic characteristics and pharmacological activities in vivo. In addition, recent reports have also shown that some PROTAC with flexible chains have some risks to causing hemolysis in vivo. Therefore, PROTAC with flexible chains show less druggability and large difficulty to entering the clinical trial stage. On the other hand, the application of nitrogen heterocycles in the design of PROTAC linkers has been widely reported in recent years. More and more reports have shown that the introduction of nitrogen heterocycles in the linker not only can effectively improves the metabolism of PROTAC in vivo, but also can enhance the degradation efficiency and selectivity of PROTAC. These PROTAC with nitrogen heterocycle linkers have attracted much attention of pharmaceutical chemists. The introduction of nitrogen heterocycles in the linker deserves priority consideration in the primary design of the PROTAC based on various druggabilities including pharmacokinetic characteristics and pharmacological activity. In this work, we summarized the optimization process and progress of nitrogen heterocyclic rings as the PROTAC linker in recent years. However, there were still limited understanding of how to discover, design and optimize PROTAC. For example, the selection of the types of nitrogen heterocycles and the optimization sites of this linker are challenges for researchers, choosing between four to six-membered nitrogen heterocycles, selecting from saturated to unsaturated ones, and even optimizing the length and extension angle of the linker. There is a truly need for theoretical explanation and elucidation of the PROTAC to guide the developing of more effective and valuable PROTAC.

Accelerating PROTACs Discovery Through a Direct-to-Biology Platform Enabled by Modular Photoclick Chemistry

Proteolysis targeting chimeras (PROTACs) have emerged as a promising strategy for drug discovery and exploring protein functions, offering a revolutionary therapeutic modality. Currently, the predominant approach to PROTACs discovery mainly relies on an empirical design-synthesis-evaluation process involving numerous cycles of labor-intensive synthesis-purification and bioassay data collection. Therefore, the development of innovative methods to expedite PROTAC synthesis and exploration of chemical space remains highly desired. Here, a direct-to-biology strategy is reported to streamline the synthesis of PROTAC libraries on plates, enabling the seamless transfer of reaction products to cell-based bioassays without the need for additional purification. By integrating amide coupling and light-induced primary amines and o-nitrobenzyl alcohols cyclization (PANAC) photoclick chemistry into a plate-based synthetic process, this strategy produces PROTAC libraries with high efficiency and structural diversity. Moreover, by employing this platform for PROTACs screening, we smoothly found potent PROTACs effectively inhibit triple-negative breast cancer (TNBC) cell growth and induce rapid, selective targeted degradation of cyclin-dependent kinase 9 (CDK9). The study introduces a versatile platform for assembling PROTACs on plates, followed by direct biological evaluation. This approach provides a promising opportunity for high-throughput synthesis of PROTAC libraries, thereby enhancing the efficiency of exploring chemical space and accelerating the discovery of PROTACs.

PROTAC Beyond Cancer- Exploring the New Therapeutic Potential of Proteolysis Targeting Chimeras

In the realm of oncology, the transformative impact of PROTAC (PROteolysis TAgeting Chimeras) technology has been particularly pronounced since its introduction in the 21st century. Initially conceived for cancer treatment, PROTACs have evolved beyond their primary scope, attracting increasing interest in addressing a diverse array of medical conditions. This expanded focus includes not only oncological disorders but also viral infections, bacterial ailments, immune dysregulation, neurodegenerative conditions, and metabolic disorders. This comprehensive review explores the broadening landscape of PROTAC application, highlighting ongoing developments and innovations aimed at deploying these molecules across a spectrum of diseases. Careful consideration of the design challenges associated with PROTACs reveals that, when appropriately addressed, these compounds present significant advantages over traditional therapeutic approaches, positioning them as promising alternatives. To evaluate the efficacy of PROTAC molecules, a diverse array of assays is employed, ranging from High-Throughput Imaging (HTI) assays to Cell Painting assays, CRBN engagement assays, Fluorescence Polarization assays, amplified luminescent proximity homogeneous assays, Timeresolved fluorescence energy transfer assays, and Isothermal Titration Calorimetry assays. These assessments collectively contribute to a nuanced understanding of PROTAC performance. Looking ahead, the trajectory of PROTAC technology suggests its potential recognition as a versatile therapeutic strategy for an expansive range of medical conditions. Ongoing progress in this field sets the stage for PROTACs to emerge as valuable tools in the multifaceted landscape of medical treatments.

Design and synthesis of proteolysis-targeting chimeras (PROTACs) as degraders of glutathione peroxidase 4

Ferroptosis is a new type of regulated, non-apoptotic cell death driven by iron-dependent phospholipid peroxidation. Inducing cell ferroptosis by inactivating glutathione peroxidase 4 (GPX4) has been considered as an effective cancer treatment strategy, but only few GPX4 inhibitors have been reported to date. Targeted protein degradation is receiving increasing attention in the discovery and development of therapeutic modality, particularly proteolysis targeting chimeras (PROTACs). Herein, we reported the design, synthesis, and evaluation of different types of GPX4-targeting PROTACs using ML162 derivatives and ligands for CRBN/VHL E3 ligases. Among them, CRBN-based PROTAC GDC-11 showed a relatively balanced biological profile in GPX4 degradation (degradation rate of 33% at 10 μM), cytotoxicity (IC₅₀ = 11.69 μM), and lipid peroxides accumulation (2-foldincreaserelatedtoDMSO), suggesting a typical characteristic of ferroptosis. In silico docking and quantum chemistry theoretical calculations provided a plausible explanation for the moderate degrading effect of these synthesized PROTACs. Overall, this work lays the foundation for subsequent studies of GPX4-targeting PROTACs, and further design and synthesis of GPX4-targeting degrader are currently in progress in our group, which will be reported in due course.

Advances of bioorthogonal coupling reactions in drug development

Currently, bioorthogonal coupling reactions have garnered considerable interest due to their high substrate selectivity and less restrictive reaction conditions. During recent decades, bioorthogonal coupling reactions have emerged as powerful tools in drug development. This review describes the current applications of bioorthogonal coupling reactions in compound library building mediated by the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction and in situ click chemistry or conjunction with other techniques; druggability optimization with 1,2,3-triazole groups; and intracellular self-assembly platforms with ring tension reactions, which are presented from the viewpoint of drug development. There is a reasonable prospect that bioorthogonal coupling reactions will accelerate the screening of lead compounds, the designing strategies of small molecules and expand the variety of designed compounds, which will be a new trend in drug development in the future.

Bifunctional robots inducing targeted protein degradation

The gaining importance of Targeted Protein Degradation (TPD) and PROTACs (PROteolysis-TArgeting Chimeras) have drawn the scientific community's attention. PROTACs are considered bifunctional robots owing to their avidity for the protein of interest (POI) and E3-ligase, which induce the ubiquitination of POI. These molecules are based on event-driven pharmacology and are applicable in different conditions such as oncology, antiviral, neurodegenerative disease, acne etc., offering tremendous scope to researchers. In this review, primarily, we attempted to compile the recent works available in the literature on PROTACs for various targeted proteins. We summarized the design and development strategies with a focus on molecular information of protein residues and linker design. Rationalization of the ternary complex formation using Artificial Intelligence including machine & deep learning models and traditionally followed computational tools are also included in this study. Moreover, details describing the optimization of PROTACs chemistry and pharmacokinetic properties are added. Advanced PROTAC designs and targeting complex proteins, is summed up to cover the wide spectrum.

A practical "preTACs-cytoblot" platform accelerates the streamlined development of PROTAC-based protein degraders

With the growing importance of PROTAC-mediated protein degradation in drug discovery, robust synthetic methodologies and rapid screening assays are urgently needed. By harnessing the improved alkene hydroazidation reaction, we developed a novel strategy to introduce azido groups into the linker-E3 ligand conjugates and effectively created a range of prepacked terminal azide-labeled "preTACs" as PROTAC toolkit building blocks. Moreover, we demonstrated that preTACs are ready to conjugate to ligands targeting a protein of interest to generate libraries of chimeric degraders, which are subsequently screened for effective protein degradation directly from cultured cells with a cytoblot assay. Our study exemplifies that this practical "preTACs-cytoblot" platform allows efficient PROTAC assembly and rapid activity assessments. It may help industrial and academic investigators to accelerate their streamlined development of PROTAC-based protein degraders.

Advancing Strategies for Proteolysis-Targeting Chimera Design

Proteolysis-targeting chimeras (PROTACs) have shown great therapeutic potential by degrading various disease-causing proteins, particularly those related to tumors. Therefore, the introduction of PROTACs has ushered in a new chapter of antitumor drug development, marked by significant advances over recent years. Herein, we describe recent developments in PROTAC technology, focusing on design strategy, development workflow, and future outlooks. We also discuss potential opportunities and challenges for PROTAC research.

The proteolysis targeting chimera GMB-475 combined with dasatinib for the treatment of chronic myeloid leukemia with BCR::ABL1 mutants

Patients with chronic myeloid leukemia (CML) show resistance to tyrosine kinase inhibitors (TKIs) targeting ABL1 due to the emergence of BCR::ABL1 mutants, especially compound mutants during the treatment, which brings great challenges to clinical practice. Combination therapy is an effective strategy for drug resistance. GMB-475, a proteolysis targeting chimera (PROTAC) targeting the myristoyl pocket of ABL1 in an allosteric manner, degrades the BCR::ABL1 through the ubiquitin–proteasome pathway. In this study, we combined GMB-475 with orthosteric TKIs targeting ABL1 to overcome resistance. We constructed Ba/F3 cells carrying BCR::ABL1 mutants by gene cloning technology and compared the effects of combination therapy with those of monotherapy on the biological characteristics and signaling pathways in CML cells. We found that the effects of ABL1 inhibitors, including imatinib, dasatinib, ponatinib, and ABL001, on growth inhibition and promoting apoptosis of Ba/F3 cells with BCR::ABL1 mutants, especially compound mutants, were weakened. GMB-475 combined with TKIs, especially dasatinib, synergistically inhibited growth, promoted apoptosis, and blocked the cell cycle of Ba/F3 cells carrying BCR::ABL1 mutants and synergistically blocked multiple molecules in the JAK-STAT pathway. In conclusion, dasatinib enhanced the antitumor effect of GMB-475; that is, the combination of PROTAC targeting ABL1 in an allosteric manner and orthosteric TKIs, especially dasatinib, provides a novel idea for the treatment of CML patients with BCR::ABL1 mutants in clinical practice.

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

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

An overview of kinase downregulators and recent advances in discovery approaches

Since the clinical approval of imatinib, the discovery of protein kinase downregulators entered a prosperous age. However, challenges still exist in the discovery of kinase downregulator drugs, such as the high failure rate during development, side effects, and drug-resistance problems. With the progress made through multidisciplinary efforts, an increasing number of new approaches have been applied to solve the above problems during the discovery process of kinase downregulators. In terms of in vitro and in vivo drug evaluation, progress was also made in cellular and animal model platforms for better and more clinically relevant drug assessment. Here, we review the advances in drug design strategies, drug property evaluation technologies, and efficacy evaluation models and technologies. Finally, we discuss the challenges and perspectives in the development of kinase downregulator drugs.

AZD5438-PROTAC: A selective CDK2 degrader that protects against cisplatin- and noise-induced hearing loss

Cyclin-dependent kinase 2 (CDK2) is a potential therapeutic target for the treatment of hearing loss and cancer. Previously, we identified AZD5438 and AT7519-7 as potent inhibitors of CDK2, however, they also targeted additional kinases, leading to unwanted toxicities. Proteolysis Targeting Chimeras (PROTACs) are a new promising class of small molecules that can effectively direct specific proteins to proteasomal degradation. Herein we report the design, synthesis, and characterization of PROTACs of AT7519-7 and AZD5438 and the identification of PROTAC-8, an AZD5438-PROTAC, that exhibits selective, partial CDK2 degradation. Furthermore, PROTAC-8 protects against cisplatin ototoxicity and kainic acid excitotoxicity in zebrafish. Molecular dynamics simulations reveal the structural requirements for CDK2 degradation. Together, PROTAC-8 is among the first-in-class PROTACs with in vivo therapeutic activities and represents a new lead compound that can be further developed for better efficacy and selectivity for CDK2 degradation against hearing loss and cancer.

Advances in the Design and Development of PROTAC-mediated HDAC degradation

Due to developments in modern chemistry, previously undruggable targets are becoming druggable thanks to selective degradation using the ubiquitin-proteasomal degradation system. PROteolysis TArgeting Chimeras (PROTACs) are heterobifunctional molecules designed specifically to degrade target proteins (protein of interest, POI). They are of significant interest to industry and academia as they are highly specific and can target previously undruggable target proteins from transcription factors to enzymes. More than 15 degraders are expected to be evaluated in clinical trials by the end of 2021. Herein, we describe recent advances in the design and development of PROTAC-mediated degradation of histone deacetylases (HDACs). PROTAC-mediated degradation of HDACs can offer some significant advantages over direct inhibition, such as the use of substoichiometric doses and the potential to disrupt enzyme-independent HDAC function. Herein, we discuss the potential implications of the degradation of HDACs with HDAC knockout studies and the selection of HDAC inhibitors and E3 ligase ligands for the design of the PROTACs. The potential utility of HDAC PROTACs in various disease pathologies from cancer to inflammation to neurodegeneration is driving the interest in this field.

The role of MOZ/KAT6A in hematological malignancies and advances in MOZ/KAT6A inhibitors

Hematological malignancies, unlike solid tumors, are a group of malignancies caused by abnormal differentiation of hematopoietic stem cells. Monocytic leukemia zinc finger protein (MOZ), a member of the MYST (MOZ, Ybf2/Sas3, Sas2, Tip60) family, is a histone acetyltransferase. MOZ is involved in various cellular functions: generation and maintenance of hematopoietic stem cells, development of erythroid cells, B-lineage progenitors and myeloid cells, and regulation of cellular senescence. Studies have shown that MOZ is susceptible to translocation in chromosomal rearrangements to form fusion genes, leading to the fusion of MOZ with other cellular regulators to form MOZ fusion proteins. Different MOZ fusion proteins have different roles, such as in the development and progression of hematological malignancies and inhibition of cellular senescence. Thus, MOZ is an attractive target, and targeting MOZ to design small-molecule drugs can help to treat hematological malignancies. This review summarizes recent progress in biology and medicinal chemistry for the histone acetyltransferase MOZ. In the biology section, MOZ and cofactors, structures of MOZ and related HATs, MOZ and fusion proteins, and roles of MOZ in cancer are discussed. In medicinal chemistry, recent developments in MOZ inhibitors are summarized.

E3 Ligase Ligands in Successful PROTACs: An Overview of Syntheses and Linker Attachment Points

Proteolysis-targeting chimeras (PROTACs) have received tremendous attention as a new and exciting class of therapeutic agents that promise to significantly impact drug discovery. These bifunctional molecules consist of a target binding unit, a linker, and an E3 ligase binding moiety. The chemically-induced formation of ternary complexes leads to ubiquitination and proteasomal degradation of target proteins. Among the plethora of E3 ligases, only a few have been utilized for the novel PROTAC technology. However, extensive knowledge on the preparation of E3 ligands and their utilization for PROTACs has already been acquired. This review provides an in-depth analysis of synthetic entries to functionalized ligands for the most relevant E3 ligase ligands, i.e. CRBN, VHL, IAP, and MDM2. Less commonly used E3 ligase and their ligands are also presented. We compare different preparative routes to E3 ligands with respect to feasibility and productivity. A particular focus was set on the chemistry of the linker attachment by discussing the synthetic opportunities to connect the E3 ligand at an appropriate exit vector with a linker to assemble the final PROTAC. This comprehensive review includes many facets involved in the synthesis of such complex molecules and is expected to serve as a compendium to support future synthetic attempts towards PROTACs.

Reversible Covalent PROTACs: Novel and Efficient Targeted Degradation Strategy

The proteolysis targeting chimeras (PROTACs), which are composed of a target protein binding moiety, a linker, and an E3 ubiquitin ligase binder, have been a promising strategy for drug design and discovery. Given the advantages of potency, selectivity, and drug resistance over inhibitors, several PROTACs have been reported in literature, which mostly focus on noncovalent or irreversible covalent binding to the target proteins. However, it must be noted that noncovalent or irreversible PROTACs have several drawbacks such as weak binding affinity and unpredictable off-target effects. Reversible covalent PROTACs, with properties of enhanced potency, selectivity, and long duration of action, have attracted an increasing amount of attention. Here, we propose a comparison between these three patterns and highlight that reversible covalent PROTACs could pave the way for a wide variety of challenging target degradations.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

From Conception to Development: Investigating PROTACs Features for Improved Cell Permeability and Successful Protein Degradation

Proteolysis Targeting Chimeras (PROTACs) are heterobifunctional degraders that specifically eliminate targeted proteins by hijacking the ubiquitin-proteasome system (UPS). This modality has emerged as an orthogonal approach to the use of small-molecule inhibitors for knocking down classic targets and disease-related proteins classified, until now, as "undruggable." In early 2019, the first targeted protein degraders reached the clinic, drawing attention to PROTACs as one of the most appealing technology in the drug discovery landscape. Despite these promising results, PROTACs are often affected by poor cellular permeability due to their high molecular weight (MW) and large exposed polar surface area (PSA). Herein, we report a comprehensive record of PROTAC design, pharmacology and thermodynamic challenges and solutions, as well as some of the available strategies to enhance cellular uptake, including suggestions of promising biological tools for the in vitro evaluation of PROTACs permeability toward successful protein degradation.

Indazole as a Privileged Scaffold: The Derivatives and their Therapeutic Applications

BACKGROUND

Heterocyclic compounds, also called heterocycles, are a major class of organic chemical compound that plays a vital role in the metabolism of all living cells. The heterocyclic compound, indazole, has attracted more attention in recent years and is widely present in numerous commercially available drugs. Indazole-containing derivatives, representing one of the most important heterocycles in drug molecules, are endowed with a broad range of biological properties.

METHODS

A literature search was conducted in PubMed, Google Scholar and Web of Science regarding articles related to indazole and its therapeutic application.

RESULTS

The mechanism and structure-activity relationship of indazole and its derivatives were described. Based on their versatile biological activities, the compounds were divided into six groups: anti-inflammatory, antibacterial, anti-HIV, antiarrhythmic, antifungal and antitumour. At least 43 indazole-based therapeutic agents were found to be used in clinical application or clinical trials.

CONCLUSION

This review is a guide for pharmacologists who are in search of valid preclinical/clinical drug compounds where the progress of approved marketed drugs containing indazole scaffold is examined from 1966 to the present day. Future direction involves more diverse bioactive moieties with indazole scaffold and greater insights into its mechanism.

Research progress of MEK1/2 inhibitors and degraders in the treatment of cancer

Mitogen-activated protein kinase kinases 1 and 2 (MEK1/2) are the crucial part of the RAS-RAF-MEK-ERK pathway (or ERK pathway), which is involved in the regulation of various cellular processes including proliferation, survival, and differentiation et al. Targeting MEK has become an important strategy for cancer therapy, and 4 MEK inhibitors (MEKis) have been approved by FDA to date. However, the application of MEKis is limited due to acquired resistance under long-term treatment. Fortunately, an emerging technology, named proteolysis targeting chimera (PROTAC), could break through this limitation by inducing MEK1/2 degradation. Compared to MEKis, MEK1/2 PROTAC is rarely studied and only three MEK1/2 PROTAC molecules, have been reported until now. This paper will outline the ERK pathway and the mechanism and research progress of MEK1/2 inhibitors, but focus on the development of MEK degraders and their optimization strategies. PAC-1 strategy which can induce MEK degradation indirectly, other PROTACs on ERK pathway, the advantages and challenges of PROTAC technology will be subsequently discussed.

Triazole-containing hybrids with anti-Mycobacterium tuberculosis potential - Part I: 1,2,3-Triazole

Tuberculosis regimens currently applied in clinical practice require months of multidrug therapy, which imposes a major challenge of patient compliance and drug resistance development. Moreover, because of the increasing emergence of hard-to-treat tuberculosis, this disease continues to be a significant threat to the human population. 1,2,3-triazole as a privileged structure has been widely used as an effective template for drug discovery, and 1,2,3-triazole-containing hybrids that can simultaneously act on dual or multiple targets in Mycobacterium tuberculosis have the potential to circumvent drug resistance, enhance efficacy, reduce side effects and improve pharmacokinetic as well as pharmacodynamic profiles. Thus, 1,2,3-triazole-containing hybrids are useful scaffolds for the development of antitubercular agents. This review aims to highlight recent advances of 1,2,3-triazole-containing hybrids with potential activity against various forms of M. tuberculosis, covering articles published between 2015 and 2020. The structure-activity relationship and the mechanism of action are also discussed to facilitate further rational design of more effective drug candidates.

Current strategies for the design of PROTAC linkers: a critical review

PROteolysis TArgeting Chimeras (PROTACs) are heterobifunctional molecules consisting of two ligands; an “anchor” to bind to an E3 ubiquitin ligase and a “warhead” to bind to a protein of interest, connected by a chemical linker. Targeted protein degradation by PROTACs has emerged as a new modality for the knock down of a range of proteins, with the first agents now reaching clinical evaluation. It has become increasingly clear that the length and composition of the linker play critical roles on the physicochemical properties and bioactivity of PROTACs. While linker design has historically received limited attention, the PROTAC field is evolving rapidly and currently undergoing an important shift from synthetically tractable alkyl and polyethylene glycol to more sophisticated functional linkers. This promises to unlock a wealth of novel PROTAC agents with enhanced bioactivity for therapeutic intervention. Here, the authors provide a timely overview of the diverse linker classes in the published literature, along with their underlying design principles and overall influence on the properties and bioactivity of the associated PROTACs. Finally, the authors provide a critical analysis of current strategies for PROTAC assembly. The authors highlight important limitations associated with the traditional “trial and error” approach around linker design and selection, and suggest potential future avenues to further inform rational linker design and accelerate the identification of optimised PROTACs. In particular, the authors believe that advances in computational and structural methods will play an essential role to gain a better understanding of the structure and dynamics of PROTAC ternary complexes, and will be essential to address the current gaps in knowledge associated with PROTAC design.

Ce(OTf)3-Catalyzed Multicomponent Reaction of Alkynyl Carboxylic Acids, tert-Butyl Isocyanide, and Azides for the Assembly of Triazole-Oxazole Derivatives

Cerium(III) triflate-catalyzed multicomponent reactions between alkynyl carboxylic acids, tert-butyl isocyanide, and organic azides have been developed. In the presence of Ce(OTf)₃ (10 mol %), the cascade reaction of one molecule of alkynyl carboxylic acid with three molecules of tert-butyl isocyanides proceeds chemoselectively and regioselectively via a triple and ordered isocyanide insertion process at room temperature, and then the cesium-catalyzed [3 + 2] cycloaddtion reaction between the resulted alkynyl oxazole and organic azides was further initiated by the temperature elevation (100 °C), thereby leading to multisubstituted triazole-oxazole derivatives in practical, time-saving, one-pot operations. Furthermore, some of the synthesized target compounds showed potential anticancer activities against MGC803 (human gastric cancer cell) with IC₅₀ values below 20 μmol L^-1.