Reactive chemistry for covalent probe and therapeutic development

Modulation of iron metabolism by new chemicals interacting with the iron regulatory system

Despite the vital role of iron and vulnerability of iron metabolism in disease states, it remains largely unknown whether chemicals interacting with cellular proteins are responsible for perturbation of iron metabolism. We previously demonstrated that cisplatin was an inhibitor of the iron regulatory system by blocking IRP2 (iron regulatory protein 2) binding to an iron-responsive element (IRE) located in the 3'- or 5'-UTR (untranslated region) of key iron metabolism genes such as transferrin receptor 1 (TfR1) and ferritin mRNAs. To guide the development of new chemical probes to modulate the IRP-IRE regulatory system, we used an artificial intelligence (AI)-based ligand design and screened a chemical library composed of cysteine-reactive warheads. Using wild type and mutant IRE-luciferase reporter cells, we identified new IRP-IRE inhibitors such as V004-0872 harboring chloroacetamide, while its analog V011-6261 with chloropropanamide completely lost the inhibitory activity. V004-0872 inhibited the human IRP2 via Cys512 and caused decreased iron levels through reciprocal TfR1 downregulation and ferritin upregulation. V004-0872 increased production of mitochondrial reactive oxygen species (ROS) and exhibited cytotoxicity that was inhibited by N-acetyl cysteine but not the ferroptosis inhibitor ferrostatin-1. Furthermore, we found that widely used haloketone protease inhibitors and acetamide herbicides inhibit the IRP-IRE system. Since IRP2 overexpression is responsible for iron excess conditions to promote growth of several cancers and exacerbation of iron-overload diseases, these results and new compounds lay the groundwork for new reagents and strategies to limit the availability of iron and oxidative stress in iron-overloaded disease conditions.

Molecular Bidents with Two Electrophilic Warheads as a New Pharmacological Modality

A systematic strategy to develop dual-warhead inhibitors is introduced to circumvent the limitations of conventional covalent inhibitors such as vulnerability to mutations of the corresponding nucleophilic residue. Currently, all FDA-approved covalent small molecules feature one electrophile, leaving open a facile route to acquired resistance. We conducted a systematic analysis of human proteins in the protein data bank to reveal ∼400 unique targets amendable to dual covalent inhibitors, which we term "molecular bidents". We demonstrated this strategy by targeting two kinases: MKK7 and EGFR. The designed compounds, ZNL-8162 and ZNL-0056, are ATP-competitive inhibitors that form two covalent bonds with cysteines and retain potency against single cysteine mutants. Therefore, molecular bidents represent a new pharmacological modality with the potential for improved selectivity, potency, and drug resistance profile.

A covalent compound selectively inhibits RNA demethylase ALKBH5 rather than FTO

N 6-Methyladenosine (m6A) is the most prevalent mRNA modification and is required for gene regulation in eukaryotes. ALKBH5, an m6A demethylase, is a promising target, particularly for anticancer drug discovery. However, the development of selective and potent inhibitors of ALKBH5 rather than FTO remains challenging. Herein, we used a targeted covalent inhibition strategy and identified a covalent inhibitor, TD19, which selectively inhibits ALKBH5 compared with FTO demethylase in protein-based and tumor cell-based assays. TD19 irreversibly modifies the residues C100 and C267, preventing ALKBH5 from binding to m6A-containing RNA. Moreover, TD19 displays good anticancer efficacy in acute myeloid leukemia and glioblastoma multiforme cell lines. Thus, the ALKBH5 inhibitor developed in this study, which selectively targets ALKBH5 compared with FTO, can potentially be used as a probe for investigating the biological functions of RNA demethylase and as a lead compound in anticancer research.

Covalent ligands of nuclear receptors

Nuclear receptors (NRs) are ligand-induced transcriptional factors implicated in several physiological pathways. Naïve ligands bind to their cognate receptors and modulate gene expression as agonists or antagonists. It has been observed that some ligands bind via covalent bonding with the NR Ligand Binding Domain (LBD) residues. While many such instances have been known since the 1980s, a consolidated account of these ligands and their interactions with NR-LBD is yet to be documented. To negate this, we have culled out the human NR-LBDs that form a covalent attachment with ligands. According to the study, 16 of the 48 human NRs have been targeted by covalent ligands. It was found that conserved cysteines prone to covalent attachment are predominantly located in NR-LBD helices 3 and 11. These conserved cysteines are also observed in many of the remaining NRs, which can be probed for their reactivity. Thus, the structural insights into NR-LBD interactions with covalent ligands presented here would aid drug discovery efforts targeting NRs.

Covalent fragment approaches targeting non-cysteine residues

Covalent fragment approaches combine advantages of covalent binders and fragment-based drug discovery (FBDD) for target identification and validation. Although early applications focused mostly on cysteine labeling, the chemistries of available warheads that target other orthosteric and allosteric protein nucleophiles has recently been extended. The range of different warheads and labeling chemistries provide unique opportunities for screening and optimizing warheads necessary for targeting non-cysteine residues. In this review, we discuss these recently developed amino-acid-specific and promiscuous warheads, as well as emerging labeling chemistries, which includes novel transition metal catalyzed, photoactive, electroactive, and noncatalytic methodologies. We also highlight recent applications of covalent fragments for the development of molecular glues and proteolysis-targeting chimeras (PROTACs), and their utility in chemical proteomics-based target identification and validation.

Chemoproteomic capture of RNA binding activity in living cells

Proteomic methods for RNA interactome capture (RIC) rely principally on crosslinking native or labeled cellular RNA to enrich and investigate RNA-binding protein (RBP) composition and function in cells. The ability to measure RBP activity at individual binding sites by RIC, however, has been more challenging due to the heterogenous nature of peptide adducts derived from the RNA-protein crosslinked site. Here, we present an orthogonal strategy that utilizes clickable electrophilic purines to directly quantify protein-RNA interactions on proteins through photoaffinity competition with 4-thiouridine (4SU)-labeled RNA in cells. Our photo-activatable-competition and chemoproteomic enrichment (PACCE) method facilitated detection of >5500 cysteine sites across ~3000 proteins displaying RNA-sensitive alterations in probe binding. Importantly, PACCE enabled functional profiling of canonical RNA-binding domains as well as discovery of moonlighting RNA binding activity in the human proteome. Collectively, we present a chemoproteomic platform for global quantification of protein-RNA binding activity in living cells.

Protein-protein interactions: developing small-molecule inhibitors/stabilizers through covalent strategies

The development of small-molecule inhibitors or stabilizers of selected protein-protein interactions (PPIs) of interest holds considerable promise for the development of research tools as well as candidate therapeutics. In this context, the covalent modification of selected residues within the target protein has emerged as a promising mechanism of action to obtain small-molecule modulators of PPIs with appropriate selectivity and duration of action. Different covalent labeling strategies are now available that can potentially allow for a rational, ground-up discovery and optimization of ligands as PPI inhibitors or stabilizers. This review article provides a synopsis of recent developments and applications of such tactics, with a particular focus on site-directed fragment tethering and proximity-enabled approaches.

Global Discovery of Covalent Modulators of Ribonucleoprotein Granules

Stress granules (SGs) and processing-bodies (PBs, P-bodies) are ubiquitous and widely studied ribonucleoprotein (RNP) granules involved in cellular stress response, viral infection, and the tumor microenvironment. While proteomic and transcriptomic investigations of SGs and PBs have provided insights into molecular composition, chemical tools to probe and modulate RNP granules remain lacking. Herein, we combine an immunofluorescence (IF)-based phenotypic screen with chemoproteomics to identify sulfonyl-triazoles (SuTEx) capable of preventing or inducing SG and PB formation through liganding of tyrosine (Tyr) and lysine (Lys) sites in stressed cells. Liganded sites were enriched for RNA-binding and protein-protein interaction (PPI) domains, including several sites found in RNP granule-forming proteins. Among these, we functionally validate G3BP1 Y40, located in the NTF2 dimerization domain, as a ligandable site that can disrupt arsenite-induced SG formation in cells. In summary, we present a chemical strategy for the systematic discovery of condensate-modulating covalent small molecules.

An Activity-Based Oxaziridine Platform for Identifying and Developing Covalent Ligands for Functional Allosteric Methionine Sites: Redox-Dependent Inhibition of Cyclin-Dependent Kinase 4

Activity-based protein profiling (ABPP) is a versatile strategy for identifying and characterizing functional protein sites and compounds for therapeutic development. However, the vast majority of ABPP methods for covalent drug discovery target highly nucleophilic amino acids such as cysteine or lysine. Here, we report a methionine-directed ABPP platform using Redox-Activated Chemical Tagging (ReACT), which leverages a biomimetic oxidative ligation strategy for selective methionine modification. Application of ReACT to oncoprotein cyclin-dependent kinase 4 (CDK4) as a representative high-value drug target identified three new ligandable methionine sites. We then synthesized a methionine-targeting covalent ligand library bearing a diverse array of heterocyclic, heteroatom, and stereochemically rich substituents. ABPP screening of this focused library identified 1oxF11 as a covalent modifier of CDK4 at an allosteric M169 site. This compound inhibited kinase activity in a dose-dependent manner on purified protein and in breast cancer cells. Further investigation of 1oxF11 found prominent cation-π and H-bonding interactions stabilizing the binding of this fragment at the M169 site. Quantitative mass-spectrometry studies validated 1oxF11 ligation of CDK4 in breast cancer cell lysates. Further biochemical analyses revealed cross-talk between M169 oxidation and T172 phosphorylation, where M169 oxidation prevented phosphorylation of the activating T172 site on CDK4 and blocked cell cycle progression. By identifying a new mechanism for allosteric methionine redox regulation on CDK4 and developing a unique modality for its therapeutic intervention, this work showcases a generalizable platform that provides a starting point for engaging in broader chemoproteomics and protein ligand discovery efforts to find and target previously undruggable methionine sites.

A modification-centric assessment tool for the performance of chemoproteomic probes

Chemoproteomics has emerged as a key technology to expand the functional space in complex proteomes for probing fundamental biology and for discovering new small-molecule-based therapies. Here we report a modification-centric computational tool termed pChem to provide a streamlined pipeline for unbiased performance assessment of chemoproteomic probes. The pipeline starts with an experimental setting for isotopically coding probe-derived modifications that can be automatically recognized by pChem, with masses accurately calculated and sites precisely localized. pChem exports on-demand reports by scoring the profiling efficiency, modification homogeneity and proteome-wide residue selectivity of a tested probe. The performance and robustness of pChem were benchmarked by applying it to eighteen bioorthogonal probes. These analyses reveal that the formation of unexpected probe-derived modifications can be driven by endogenous reactive metabolites (for example, bioactive aldehydes and glutathione). pChem is a powerful and user-friendly tool that aims to facilitate the development of probes for the ever-growing field of chemoproteomics.

Covalent fragment-based ligand screening approaches for identification of novel ubiquitin proteasome system modulators

Ubiquitination is a key regulatory mechanism vital for maintenance of cellular homeostasis. Protein degradation is induced by E3 ligases via attachment of ubiquitin chains to substrates. Pharmacological exploitation of this phenomenon via targeted protein degradation (TPD) can be achieved with molecular glues or bifunctional molecules facilitating the formation of ternary complexes between an E3 ligase and a given protein of interest (POI), resulting in ubiquitination of the substrate and subsequent proteolysis by the proteasome. Recently, the development of novel covalent fragment screening approaches has enabled the identification of first-in-class ligands for E3 ligases and deubiquitinases revealing so far unexplored binding sites which highlights the potential of these methods to uncover and expand druggable space for new target classes.