Applications of covalent chemistry in targeted protein degradation

Design, synthesis and structure-activity relationship of novel 2-pyrimidinylindole derivatives as orally available anti-obesity agents

Due to the emerging global epidemic of obesity, developing safe and effective agents for anti-obesity is urgently needed. Our previous study found that 2-pyrimidinylindole derivative Wd3d exhibited potential anti-obesity activity. Herein, to further optimize the potential moiety, structural modifications were proceeded for two rounds in this study. Firstly, we designed, synthesized, and evaluated 36 new derivatives of 2-pyrimidinylindole scaffold with different substituents on the indole ring and pyrimidine ring to investigate their structure-activity relationship (SAR). Then, analogs with potent activity had the aldehyde group replaced with the acylhydrazone group to reduce cytotoxicity and improve metabolic stability. Detailed SAR studies and animal evaluation experiments led to the discovery of the compound 9ga, which significantly reduced TG accumulation with an EC50 value of 0.07 μM and showed relatively low cytotoxicity with an IC50 value of around 24 μM. Oral administration of 9ga effectively prevented the excessive growth of body weight and lessened fat mass as well as liver mass, decreased lipid accumulation in the liver and blood, and improved the heart injury parameter in the diet-induced obesity mouse model significantly better than Wd3d. A mechanism study showed that 9ga regulated the lipid metabolism during early adipogenesis by inhibiting PPARγ pathway. In conclusion, our study further highlights the anti-obesity potential of 2-pyrimidinylindole derivatives in diet-induced obesity.

Trends in covalent drug discovery: a 2020-23 patent landscape analysis focused on select covalent reacting groups (CRGs) found in FDA-approved drugs

INTRODUCTION

Covalent drugs contain electrophilic groups that can react with nucleophilic amino acids located in the active sites of proteins, particularly enzymes. Recently, there has been considerable interest in using covalent drugs to target non-catalytic amino acids in proteins to modulate difficult targets (i.e. targeted covalent inhibitors). Covalent compounds contain a wide variety of covalent reacting groups (CRGs), but only a few of these CRGs are present in FDA-approved covalent drugs.

AREAS COVERED

This review summarizes a 2020-23 patent landscape analysis that examined trends in the field of covalent drug discovery around targets and organizations. The analysis focused on patent applications that were submitted to the World International Patent Organization and selected using a combination of keywords and structural searches based on CRGs present in FDA-approved drugs.

EXPERT OPINION

A total of 707 patent applications from >300 organizations were identified, disclosing compounds that acted at 71 targets. Patent application counts for five targets accounted for ~63% of the total counts (i.e. BTK, EGFR, FGFR, KRAS, and SARS-CoV-2 Mpro). The organization with the largest number of patent counts was an academic institution (Dana-Farber Cancer Institute). For one target, KRAS G12C, the discovery of new drugs was highly competitive (>100 organizations, 186 patent applications).

Discovery of Covalent MLKL PROTAC Degraders via Optimization of a Theophylline Derivative Ligand for Treating Necroptosis

Mixed lineage kinase domain-like pseudokinase (MLKL) initiates necroptosis and could serve as a therapeutic target related to a series of human diseases. Proteolysis-targeting chimeras (PROTACs) are useful tools for degrading pathological proteins and blocking disease processes. Using computer-aided modeling and molecular dynamics simulations, we developed a series of covalent MLKL PROTACs by linking and optimizing a theophylline derivative that covalently targets MLKL. Via structure-activity relationship studies, MP-11 was identified as a potent MLKL PROTAC degrader. Furthermore, MP-11 showed lower toxicity than the original MLKL ligand, exhibiting nanomolar-scale antinecroptotic activity on human cell lines. Xenograft model studies showed that MP-11 effectively degraded MLKL in vivo. Importantly, our study demonstrates that the covalent binding strategy is an effective approach for designing MLKL-targeting PROTACs, serving as a model for developing PROTACs to treat future necroptosis-related human diseases.

Reversible Conjugation of Polypeptides and Proteins Utilizing a [3.3.1] Scaffold under Mild Conditions

An investigation of reversible protein conjugation and deconjugation is presented. Despite numerous available protein conjugation methods, there has been limited documentation of achieving protein conjugation in a controlled and reversible manner. This report introduces a protocol that enables protein modification in a multicomponent fashion under aqueous buffer and mild conditions. A readily available mercaptobenzaldehyde derivative can modify the primary amine of peptides and proteins with a distinctive [3.3.1] scaffold. This modification can be reversed under mild conditions in a controlled fashion, restoring the original protein motif. The effectiveness of this approach has been demonstrated in the modification and quantifiable regeneration of insulin protein.

Template-assisted covalent modification underlies activity of covalent molecular glues

Molecular glues are proximity-inducing small molecules that have emerged as an attractive therapeutic approach. However, developing molecular glues remains challenging, requiring innovative mechanistic strategies to stabilize neoprotein interfaces and expedite discovery. Here we unveil a trans-labeling covalent molecular glue mechanism, termed 'template-assisted covalent modification'. We identified a new series of BRD4 molecular glue degraders that recruit CUL4DCAF16 ligase to the second bromodomain of BRD4 (BRD4BD2). Through comprehensive biochemical, structural and mutagenesis analyses, we elucidated how pre-existing structural complementarity between DCAF16 and BRD4BD2 serves as a template to optimally orient the degrader for covalent modification of DCAF16Cys58. This process stabilizes the formation of BRD4-degrader-DCAF16 ternary complex and facilitates BRD4 degradation. Supporting generalizability, we found that a subset of degraders also induces GAK-BRD4BD2 interaction through trans-labeling of GAK. Together, our work establishes 'template-assisted covalent modification' as a mechanism for covalent molecular glues, which opens a new path to proximity-driven pharmacology.

Expanding the horizons of targeted protein degradation: A non-small molecule perspective

Targeted protein degradation (TPD) represented by proteolysis targeting chimeras (PROTACs) marks a significant stride in drug discovery. A plethora of innovative technologies inspired by PROTAC have not only revolutionized the landscape of TPD but have the potential to unlock functionalities beyond degradation. Non-small-molecule-based approaches play an irreplaceable role in this field. A wide variety of agents spanning a broad chemical spectrum, including peptides, nucleic acids, antibodies, and even vaccines, which not only prove instrumental in overcoming the constraints of conventional small molecule entities but also provided rapidly renewing paradigms. Herein we summarize the burgeoning non-small molecule technological platforms inspired by PROTACs, including three major trajectories, to provide insights for the design strategies based on novel paradigms.

High-Throughput Miniaturized Synthesis of PROTAC-Like Molecules

The development of miniaturized high-throughput in situ screening platforms capable of handling the entire process of drug synthesis to final screening is essential for advancing drug discovery in the future. In this study, an approach based on combinatorial solid-phase synthesis, enabling the efficient synthesis of libraries of proteolysis targeting chimeras (PROTACs) in an array format is presented. This on-chip platform allows direct biological screening without the need for transfer steps.  UV-induced release of target molecules into individual droplets facilitates further on-chip experimentation. Utilizing a mitogen-activated protein kinase kinases (MEK1/2) degrader as a template, a series of 132 novel PROTAC-like molecules is synthesized using solid-phase Ugi reaction. These compounds are further characterized using various methods, including matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) imaging, while consuming only a few milligrams of starting materials in total. Furthermore, the feasibility of culturing cancer cells on the modified spots and quantifying the effect of MEK suppression is demonstrated. The miniaturized synthesis platform lays a foundation for high-throughput in situ biological screening of potent PROTACs for potential anticancer activity and offers the potential for accelerating the drug discovery process by integrating miniaturized synthesis and biological steps on the same array.

Discovery of First-in-Class PROTAC Degraders of SARS-CoV-2 Main Protease

We have witnessed three coronavirus (CoV) outbreaks in the past two decades, including the COVID-19 pandemic caused by SARS-CoV-2. Main protease (MPro), a highly conserved protease among various CoVs, is essential for viral replication and pathogenesis, making it a prime target for antiviral drug development. Here, we leverage proteolysis targeting chimera (PROTAC) technology to develop a new class of small-molecule antivirals that induce the degradation of SARS-CoV-2 MPro. Among them, MPD2 was demonstrated to effectively reduce MPro protein levels in 293T cells, relying on a time-dependent, CRBN-mediated, and proteasome-driven mechanism. Furthermore, MPD2 exhibited remarkable efficacy in diminishing MPro protein levels in SARS-CoV-2-infected A549-ACE2 cells. MPD2 also displayed potent antiviral activity against various SARS-CoV-2 strains and exhibited enhanced potency against nirmatrelvir-resistant viruses. Overall, this proof-of-concept study highlights the potential of targeted protein degradation of MPro as an innovative approach for developing antivirals that could fight against drug-resistant viral variants.

Profiling nuclear cysteine ligandability and effects on nuclear localization using proximity labeling-coupled chemoproteomics

The nucleus controls cell growth and division through coordinated interactions between nuclear proteins and chromatin. Mutations that impair nuclear protein association with chromatin are implicated in numerous diseases. Covalent ligands are a promising strategy to pharmacologically target nuclear proteins, such as transcription factors, which lack ordered small-molecule binding pockets. To identify nuclear cysteines that are susceptible to covalent liganding, we couple proximity labeling (PL), using a histone H3.3-TurboID (His-TID) construct, with chemoproteomics. Using covalent scout fragments, KB02 and KB05, we identified ligandable cysteines on proteins involved in spindle assembly, DNA repair, and transcriptional regulation, such as Cys101 of histone acetyltransferase 1 (HAT1). Furthermore, we show that covalent fragments can affect the abundance, localization, and chromatin association of nuclear proteins. Notably, the Parkinson disease protein 7 (PARK7) showed increased nuclear localization and chromatin association upon KB02 modification at Cys106. Together, this platform provides insights into targeting nuclear cysteines with covalent ligands.

Targeted Protein Degradation: Advances, Challenges, and Prospects for Computational Methods

The therapeutic approach of targeted protein degradation (TPD) is gaining momentum due to its potentially superior effects compared with protein inhibition. Recent advancements in the biotech and pharmaceutical sectors have led to the development of compounds that are currently in human trials, with some showing promising clinical results. However, the use of computational tools in TPD is still limited, as it has distinct characteristics compared with traditional computational drug design methods. TPD involves creating a ternary structure (protein-degrader-ligase) responsible for the biological function, such as ubiquitination and subsequent proteasomal degradation, which depends on the spatial orientation of the protein of interest (POI) relative to E2-loaded ubiquitin. Modeling this structure necessitates a unique blend of tools initially developed for small molecules (e.g., docking) and biologics (e.g., protein-protein interaction modeling). Additionally, degrader molecules, particularly heterobifunctional degraders, are generally larger than conventional small molecule drugs, leading to challenges in determining drug-like properties like solubility and permeability. Furthermore, the catalytic nature of TPD makes occupancy-based modeling insufficient. TPD consists of multiple interconnected yet distinct steps, such as POI binding, E3 ligase binding, ternary structure interactions, ubiquitination, and degradation, along with traditional small molecule properties. A comprehensive set of tools is needed to address the dynamic nature of the induced proximity ternary complex and its implications for ubiquitination. In this Perspective, we discuss the current state of computational tools for TPD. We start by describing the series of steps involved in the degradation process and the experimental methods used to characterize them. Then, we delve into a detailed analysis of the computational tools employed in TPD. We also present an integrative approach that has proven successful for degrader design and its impact on project decisions. Finally, we examine the future prospects of computational methods in TPD and the areas with the greatest potential for impact.

Recent advances of AIE-active materials for orthotopic tumor phototheranostics

Cancer ranks as a leading threat to human life and health. Compared to conventional cancer treatments, phototheranostics shares the advantages of integrated diagnosis and therapy, outstanding therapeutic performance and good controllability. Amid diverse phototheranostic agents, small organic luminogens with aggregation-induced emission (AIEgen) tendency show predominant advantages in terms of superior photostability, large Stokes shifts, and boosted theranostic capacity as aggregates. In the past two decades, AIE-active materials have demonstrated formidable applications in disease theranostics, especially for tumors. This review mainly highlights the recent advances of orthotopic tumor phototheranostics mediated by AIEgens with a classification of different organs. Additionally, a brief discussion of current bottlenecks and future directions is outlined. We believe this review can deepen the understanding and spur more innovations on tumor theranostics by employing AIEgens. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Small-molecule probes from bench to bedside: advancing molecular analysis of drug-target interactions toward precision medicine

Over the past decade, remarkable advances have been witnessed in the development of small-molecule probes. These molecular tools have been widely applied for interrogating proteins, pathways and drug-target interactions in preclinical research. While novel structures and designs are commonly explored in probe development, the clinical translation of small-molecule probes remains limited, primarily due to safety and regulatory considerations. Recent synergistic developments - interfacing novel chemical probes with complementary analytical technologies - have introduced and expedited diverse biomedical opportunities to molecularly characterize targeted drug interactions directly in the human body or through accessible clinical specimens (e.g., blood and ascites fluid). These integrated developments thus offer unprecedented opportunities for drug development, disease diagnostics and treatment monitoring. In this review, we discuss recent advances in the structure and design of small-molecule probes with novel functionalities and the integrated development with imaging, proteomics and other emerging technologies. We further highlight recent applications of integrated small-molecule technologies for the molecular analysis of drug-target interactions, including translational applications and emerging opportunities for whole-body imaging, tissue-based measurement and blood-based analysis.

PROTACs: A novel strategy for cancer drug discovery and development

Proteolysis targeting chimera (PROTAC) technology has become a powerful strategy in drug discovery, especially for undruggable targets/proteins. A typical PROTAC degrader consists of three components: a small molecule that binds to a target protein, an E3 ligase ligand (consisting of an E3 ligase and its small molecule recruiter), and a chemical linker that hooks first two components together. In the past 20 years, we have witnessed advancement of multiple PROTAC degraders into the clinical trials for anticancer therapies. However, one of the major challenges of PROTAC technology is that only very limited number of E3 ligase recruiters are currently available as E3 ligand for targeted protein degradation (TPD), although human genome encodes more than 600 E3 ligases. Thus, there is an urgent need to identify additional effective E3 ligase recruiters for TPD applications. In this review, we summarized the existing RING-type E3 ubiquitin ligase and their small molecule recruiters that act as effective E3 ligands of PROTAC degraders and their application in anticancer drug discovery. We believe that this review could serve as a reference in future development of efficient E3 ligands of PROTAC technology for cancer drug discovery and development.

Setting sail: Maneuvering SHP2 activity and its effects in cancer

Since the discovery of tyrosine phosphorylation being a critical modulator of cancer signaling, proteins regulating phosphotyrosine levels in cells have fast become targets of therapeutic intervention. The nonreceptor protein tyrosine phosphatase (PTP) coded by the PTPN11 gene "SHP2" integrates phosphotyrosine signaling from growth factor receptors into the RAS/RAF/ERK pathway and is centrally positioned in processes regulating cell development and oncogenic transformation. Dysregulation of SHP2 expression or activity is linked to tumorigenesis and developmental defects. Even as a compelling anti-cancer target, SHP2 was considered "undruggable" for a long time owing to its conserved catalytic PTP domain that evaded drug development. Recently, SHP2 has risen from the "undruggable curse" with the discovery of small molecules that manipulate its intrinsic allostery for effective inhibition. SHP2's unique domain arrangement and conformation(s) allow for a truly novel paradigm of inhibitor development relying on skillful targeting of noncatalytic sites on proteins. In this review we summarize the biological functions, signaling properties, structural attributes, allostery and inhibitors of SHP2.

Template-assisted covalent modification of DCAF16 underlies activity of BRD4 molecular glue degraders

Small molecules that induce protein-protein interactions to exert proximity-driven pharmacology such as targeted protein degradation are a powerful class of therapeutics1-3. Molecular glues are of particular interest given their favorable size and chemical properties and represent the only clinically approved degrader drugs4-6. The discovery and development of molecular glues for novel targets, however, remains challenging. Covalent strategies could in principle facilitate molecular glue discovery by stabilizing the neo-protein interfaces. Here, we present structural and mechanistic studies that define a trans-labeling covalent molecular glue mechanism, which we term "template-assisted covalent modification". We found that a novel series of BRD4 molecular glue degraders act by recruiting the CUL4DCAF16 ligase to the second bromodomain of BRD4 (BRD4BD2). BRD4BD2, in complex with DCAF16, serves as a structural template to facilitate covalent modification of DCAF16, which stabilizes the BRD4-degrader-DCAF16 ternary complex formation and facilitates BRD4 degradation. A 2.2 Å cryo-electron microscopy structure of the ternary complex demonstrates that DCAF16 and BRD4BD2 have pre-existing structural complementarity which optimally orients the reactive moiety of the degrader for DCAF16Cys58 covalent modification. Systematic mutagenesis of both DCAF16 and BRD4BD2 revealed that the loop conformation around BRD4His437, rather than specific side chains, is critical for stable interaction with DCAF16 and BD2 selectivity. Together our work establishes "template-assisted covalent modification" as a mechanism for covalent molecular glues, which opens a new path to proximity driven pharmacology.

Quantitative measurement of PROTAC intracellular accumulation

In recent years, Proteolysis Targeting Chimera (PROTAC) technology has emerged as one of the most promising approaches to remove disease-associated proteins by utilizing cells' own destruction machinery. To achieve successful degradation of a protein of interest (POI), the heterobifunctional PROTAC molecules must penetrate into the cells first, followed by target engagement and formation of the POI-PROTAC-E3 ligase complex. Based on this understanding, the assessment of cell permeability and in cell target engagement are of great importance to evaluate the efficacy of PROTAC candidates. PROTAC molecules can be classified as non-covalent and covalent, and covalent PROTACs can be further divided into irreversible and reversible covalent. Here, we present a high-throughput assay to prioritize different types of BTK PROTACs by measuring their intracellular accumulation quantitatively, using kinase binding assays and the NanoBRET target engagement platform.