A Cyclopropene Electrophile that Targets Glutathione S‐Transferase Omega‐1 in Cells

Proteome-wide Ligand and Target Discovery by Using Strain-Enabled Cyclopropane Electrophiles

The evolving use of covalent ligands as chemical probes and therapeutic agents could greatly benefit from an expanded array of cysteine-reactive electrophiles for efficient and versatile proteome profiling. Herein, to expand the current repertoire of cysteine-reactive electrophiles, we developed a new class of strain-enabled electrophiles based on cyclopropanes. Proteome profiling has unveiled that C163 of lactate dehydrogenase A (LDHA) and C88 of adhesion regulating molecule 1 (ADRM1) are ligandable residues to modulate the protein functions. Moreover, fragment-based ligand discovery (FBLD) has revealed that one fragment (Y-35) shows strong reactivity toward C66 of thioredoxin domain-containing protein 12 (TXD12), and its covalent binding has been demonstrated to impact its downstream signal pathways. TXD12 plays a pivotal role in enabling Y-35 to exhibit its antisurvival and antiproliferative effects. Finally, dicarbonitrile-cyclopropane has been demonstrated to be an electrophilic warhead in the development of GSTO1-involved dual covalent inhibitors, which is promising to alleviate drug resistance.

Tuneable thiol exchange linkers for traceless drug release applications in prodrugs and ADCs

We describe a versatile and tuneable thiol responsive linker system using thiovinylketones, which relies on the conjugate addition-elimination mechanism of Michael acceptors for the traceless release of therapeutics. In a proof-of-principle study, we translate our findings to exhibit potent thiol-cleavable antibiotic prodrugs and antibody-drug conjugates.

The expanding repertoire of covalent warheads for drug discovery

The reactive functionalities of drugs that engage in covalent interactions with the enzyme/receptor residue in either a reversible or an irreversible manner are called 'warheads'. Covalent warheads that were previously neglected because of safety concerns have recently gained center stage as a result of their various advantages over noncovalent drugs, including increased selectivity, increased residence time, and higher potency. With the approval of several covalent inhibitors over the past decade, research in this area has accelerated. Various strategies are being continuously developed to tune the characteristics of warheads to improve their potency and mitigate toxicity. Here, we review research progress in warhead discovery over the past 5 years to provide valuable insights for future drug discovery.

Late-Stage Aryl C-H Bond Cyclopropenylation with Cyclopropenium Cations

Herein, we disclose the first regio-, site- and chemoselective late-stage (hetero)aryl C-H bond cyclopropenylation with cyclopropenium cations (CPCs). The process is fast, operationally simple and shows an excellent functional group tolerance in densely-functionalized drug molecules, natural products, agrochemicals and fluorescent dyes. Moreover, we discovered that the installation of the cyclopropene ring in drug molecules could not only be used to shield against metabolic instability but also as a synthetic tool to reach medicinally-relevant sp3 -rich scaffolds exploiting the highly-strained nature of the cyclopropene ring with known transformations.

Chemistry and biology of enzymes in protein glutathionylation

Protein S-glutathionylation is emerging as a central oxidation that regulates redox signaling and biological processes linked to diseases. In recent years, the field of protein S-glutathionylation has expanded by developing biochemical tools for the identification and functional analyses of S-glutathionylation, investigating knockout mouse models, and developing and evaluating chemical inhibitors for enzymes involved in glutathionylation. This review will highlight recent studies of two enzymes, glutathione transferase omega 1 (GSTO1) and glutaredoxin 1 (Grx1), especially introducing their glutathionylation substrates associated with inflammation, cancer, and neurodegeneration and showcasing the advancement of their chemical inhibitors. Lastly, we will feature protein substrates and chemical inducers of LanC-like protein (LanCL), the first enzyme in protein C-glutathionylation.

α-Lactam Electrophiles for Covalent Chemical Biology

Electrophilic groups are one of the key pillars of contemporary chemical biology and medicinal chemistry. For instance, 3-membered N-heterocyclic compounds-such as aziridines, azirines, and oxaziridines-possess unique electronic and structural properties which underlie their potential and applicability as covalent tools. The α-lactams are also members of this group of compounds, however, their utility within the field remains unexplored. Here, we demonstrate an α-lactam reagent (AM2) that is tolerant to aqueous buffers while being reactive towards biologically relevant nucleophiles. Interestingly, carboxylesterases 1 and 2 (CES1/2), both serine hydrolases with key roles in endo- and xenobiotic metabolism, were found as primary covalent targets for AM2 in HepG2 liver cancer cells. All in all, this study constitutes the starting point for the further development and exploration of α-lactam-based electrophilic probes in covalent chemical biology.

Cycloprop-2-ene-1-carboxylates: Potential chemical biology tools in the early growth stage of Arabidopsis thaliana

Cyclopropene derivatives have been used as extremely reactive units in organic chemistry owing to their high ring-strain energy. They have become popular reagents both for bioorthogonal chemistry and for chemical biology because of their small size and ability to be genetically encoded. In this context, we conducted an exploratory study to identify the biologically active cyclopropenes that affect normal plant growth. We synthesized several cycloprop-2-ene-1-carboxylic acid derivatives and evaluated their effects on the early growth stage of Arabidopsis thaliana. Eventually, we identified the chemicals that affect apical hook development in Arabidopsis thaliana. Their mode of action is different from those of ethylene receptor inhibition and gibberellin biosynthesis inhibition. We expect that some of the chemicals reported here can be new tools in chemical biology to determine useful molecular targets for herbicides or plant growth regulators.

Ynamide Electrophile for the Profiling of Ligandable Carboxyl Residues in Live Cells and the Development of New Covalent Inhibitors

Covalent inhibitors with an electrophilic warhead have received considerable attention due to their remarkable pharmacological properties. However, the electrophilic warhead in covalent drugs is often an α, β-unsaturated amide, and the targets are mainly cysteine or lysine residues. Thus, the development of novel electrophiles that can target other amino acids is highly desirable. Ynamide, a useful and versatile building block, is commonly employed in the construction of various compounds in organic synthesis. The performance of this functional group in a proteome-wide environment has been studied here for the first time, and it has been shown that it can efficiently modify carboxyl residues in situ and in vitro. Upon incorporation of this ynamide warhead into the pharmacophores of kinase inhibitors, the resulting compound showed moderate inhibition against the EGFR L858R mutant but not against EGFR WT. This novel electrophilic group can be used in the development of new types of covalent inhibitors.

The covalent reactivity of functionalized 5-hydroxy-butyrolactams is the basis for targeting of fatty acid binding protein 5 (FABP5) by the neurotrophic agent MT-21

Covalently acting compounds experience a strong interest within chemical biology both as molecular probes in studies of fundamental biological mechanisms and/or as novel drug candidates. In this context, the identification of new classes of reactive groups is particularly important as these can expose novel reactivity modes and, consequently, expand the ligandable proteome. Here, we investigated the electrophilic reactivity of the 3-acyl-5-hydroxy-1,5-dihydro-2H-pyrrole-2-one (AHPO) scaffold, a heterocyclic motif that is e.g. present in various bioactive natural products. Our investigations were focused on the compound MT-21 – a simplified structural analogue of the natural product epolactaene – which is known to have both neurotrophic activity and ability to trigger apoptotic cell death. We found that the central N-acyl hemiaminal group of MT-21 can function as an electrophilic centre enabling divergent reactivity with both amine- and thiol-based nucleophiles, which furthermore translated to reactivity with proteins in both cell lysates and live cells. We found that in live cells MT-21 strongly engaged the lipid transport protein fatty acid-binding protein 5 (FABP5) by direct binding to a cysteine residue in the bottom of the ligand binding pocket. Through preparation of a series of MT-21 derivatives, we probed the specificity of this interaction which was found to be strongly dependent on subtle structural changes. Our study suggests that MT-21 may be employed as a tool compound in future studies of the biology of FABP5, which remains incompletely understood. Furthermore, our study has also made clear that other natural products containing the AHPO-motif may likewise possess covalent reactivity and that this property may underlie their biological activity.

In this work, it is shown that an N-acyl hemiaminal motif present in many natural products can function as an electrophilic centre, mediating covalent reactivity in biological systems, reacting with both thiols and amines.

Novel Electrophilic Warhead Targeting a Triple-Negative Breast Cancer Driver in Live Cells Revealed by "Inverse Drug Discovery"

The "inverse drug discovery" strategy is a potent means of exploring the cellular targets of latent electrophiles not typically used in medicinal chemistry. Cyclopropenone, a powerful electrophile, is generally used in bio-orthogonal reactions mediated by triarylphosphine or in photo-triggered cycloaddition reactions. Here, we have studied, for the first time, the proteome reactivity of cyclopropenones in live cells and discovered that the cyclopropenone warhead can specifically and efficiently modify a triple-negative breast cancer driver, glutathione S-transferase pi-1 (GSTP1), by covalently binding at the catalytic active site. Further structure optimization and signaling pathway validation have led to the discovery of potent inhibitors of GSTP1.

Macrodiolide Diversification Reveals Broad Immunosuppressive Activity That Impairs the cGAS-STING Pathway

The development of new immunomodulatory agents can impact various areas of medicine. In particular, compounds with the ability to modulate innate immunological pathways hold significant unexplored potential. Herein, we report a modular synthetic approach to the macrodiolide natural product (-)-vermiculine, an agent previously shown to possess diverse biological effects, including cytotoxic and immunosuppressive activity. The synthesis allows for a high degree of flexibility in modifying the macrocyclic framework, including the formation of all possible stereoisomers. In total, 18 analogues were prepared. Two analogues with minor structural modifications showed clearly enhanced cancer cell line selectivity and reduced toxicity. Moreover, these compounds possessed broad inhibitory activity against innate immunological pathways in human PBMCs, including the DNA-sensing cGAS-STING pathway. Initial mechanistic characterization suggests a surprising impairment of the STING-TBK1 interaction.

Forward Chemical Genetic Screen for Oxygen-Dependent Cytotoxins Uncovers New Covalent Fragments that Target GPX4

The identification of growth inhibitory compounds with the ability to selectively target the cellular oxygenation state may be of therapeutic interest. Here, a phenotypic screen of a covalent fragment library revealed diverse compounds containing propiolamide warheads with selective toxicity for liver cancer cells in normoxic conditions. Target identification and validation through CETSA and direct pulldown experiments demonstrated that several compounds target glutathione peroxidase 4 (GPX4) and induce ferroptotic cell death. Although being an oxidative cell death mechanism, ferroptosis can be induced also under hypoxic conditions. Prompted by the selective toxicity discovered in the screen, we mapped the oxygen-dependence of several ferroptosis-inducing compounds across three different cell lines. These studies revealed combinations with notable reductions in sensitivity under hypoxic conditions. These observations are mechanistically interesting and may be relevant for the use of ferroptosis-inducers as anti-cancer agents.

Total Synthesis of Natural Products Containing Enamine or Enol Ether Derivatives

Enamine and enol ethers are nucleophilic functional groups that are well known to most chemists. When enamine or enol ethers are present in natural products, they are nearly exclusively found as derivatives having a direct connection to electron-withdrawing groups for stabilization, and the resulting larger entities, such as enamides or enol acylates, can be further extended or modified in the framework of natural products. The restricted conformational space that is associated with even simple enamine and enol ether derivatives can be a strong determinant of the overall molecular structure, and the more polarized derivatives can endow some natural products with electrophilic properties and thus facilitate covalent interactions with biological targets.In this Account, I describe our efforts (published since 2016) to prepare natural products from several different classes that all feature enamine or enol ether derivatives as key functionalities. Our choice of targets has been guided by a desire to illuminate unknown biological mechanisms associated with the compounds or, alternatively, to improve upon known biological activities that appear to be promising from a biomedical perspective. In the present text, however, the exclusive focus will be on the syntheses.First, I will discuss the basic properties of the functional groups and briefly present a small collection of illustrative and inspirational examples from the literature for their construction in different complex settings. Next, I will provide an overview of our work on the macrocyclic APD-CLD natural products, rakicidin A and BE-43547A₁, involving the development of an efficient macrocyclization strategy and the development of methods to construct the hallmark APD group: a modified enamide. The synthesis of the meroterpenoid strongylophorine-26 is discussed next, where we developed an oxidative quinone methoxylation to build a vinylogous ester group in the final step of the synthesis and employed FeCl₃-mediated cascade reactions for the rapid assembly of the overall scaffold to enable a short semisynthesis from isocupressic acid. An efficient core scaffold assembly was also in focus in our synthesis of the alkaloid streptazone A with the signature enaminone system being assembled through a rhodium-catalyzed Pauson-Khand reaction. Sequential, site-selective redox manipulations were developed to arrive at strepatzone A and additional members of the natural product family. Finally, I discuss our work to prepare analogs of complex polyether ionophores featuring functionalized tetronic acids as cation-binding groups. A method for the construction of a suitably protected chloromethylidene-modified tetronate is presented which enabled its installation in the full structure through a C-acylation reaction. This work exemplifies how components of abundant polyether ionophores can be recycled and used to access new structures which may possess enhanced biological activities.

Cycloaddition of Strained Cyclic Alkenes and Ortho-Quinones: A Distortion/Interaction Analysis

The chemistry of strained unsaturated cyclic compounds has experienced remarkable growth in recent years via the development of metal–free click reactions. Among these reactions, the cycloaddition of cyclopropenes and their analogues to ortho-quinones has been established as a highly promising click reaction. The present work investigates the mechanism involved in the cycloaddition of strained dienes to ortho-quinones and structural factors that would influence this reaction. For this purpose, we use B97D density functional theory calculations throughout, and for relevant cases, we use spin component–scaled MP2 calculations and single–point domain-based local pair natural orbital coupled cluster (DLPNO-CCSD(T)) calculations. The outcomes are analyzed in detail using the distortion/interaction model, and suggestions for future experimental work are made.