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Caspases are a family of cysteine-dependent proteases with important cellular functions in inflammation and apoptosis, while also implicated in human diseases. Classical chemical tools to study caspase functions lack selectivity for specific caspase family members due to highly conserved active sites and catalytic machinery. To overcome this limitation, we targeted a non-catalytic cysteine residue (C264) unique to caspase-6 (C6), an enigmatic and understudied caspase isoform. Starting from disulfide ligands identified in a cysteine trapping screen, we used a structure-informed covalent ligand design to produce potent, irreversible inhibitors (3a) and chemoproteomic probes (13-t) of C6 that exhibit unprecedented selectivity over other caspase family members and high proteome selectivity. This approach and the new tools described will enable rigorous interrogation of the role of caspase-6 in developmental biology and in inflammatory and neurodegenerative diseases.

The nonstructural protein 3 (NSP3) of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) contains a conserved macrodomain enzyme (Mac1) that is critical for pathogenesis and lethality. While small molecule inhibitors of Mac1 have great therapeutic potential, at the outset of the COVID-19 pandemic there were no well-validated inhibitors for this protein nor, indeed, the macrodomain enzyme family, making this target a pharmacological orphan. Here, we report the structure-based discovery and development of several different chemical scaffolds exhibiting low- to sub-micromolar affinity for Mac1 through iterations of computer-aided design, structural characterization by ultra-high resolution protein crystallography, and binding evaluation. Potent scaffolds were designed with in silico fragment linkage and by ultra-large library docking of over 450 million molecules. Both techniques leverage the computational exploration of tangible chemical space and are applicable to other pharmacological orphans. Overall, 160 ligands in 119 different scaffolds were discovered, and 152 Mac1-ligand complex crystal structures were determined, typically to 1 Å resolution or better. Our analyses discovered selective and cell-permeable molecules, unexpected ligand-mediated protein dynamics within the active site, and key inhibitor motifs that will template future drug development against Mac1.Significance StatementSARS-CoV-2 encodes a viral macrodomain protein (Mac1) that hydrolyzes ribo-adenylate marks on viral proteins, disrupting the innate immune response to the virus. Catalytic mutations in the enzyme make the related SARS-1 virus less pathogenic and non-lethal in animals, suggesting that Mac1 will be a good antiviral target. However, no potent inhibitors of this protein class have been described, and pharmacologically the enzyme remains an orphan. Here, we computationally designed potent inhibitors of Mac1, determining 150 inhibitor-enzyme structures to ultra-high resolution by crystallography. In silico fragment linking and molecular docking of > 450 million virtual compounds led to inhibitors with submicromolar activity. These molecules may template future drug discovery efforts against this crucial but understudied viral target.

The Asp233-Asp246 pair is highly conserved in Class A β-lactamases, which hydrolyze β-lactam antibiotics. Here, we characterize its function using CTX-M-14 β-lactamase. The D233N mutant displayed decreased activity that is substrate-dependent, with reductions in kcat /Km ranging from 20% for nitrocefin to 6-fold for cefotaxime. In comparison, the mutation reduced the binding of a known reversible inhibitor by 10-fold. The mutant structures showed movement of the 213-219 loop and the loss of the Thr216-Thr235 hydrogen bond, which was restored by inhibitor binding. Mutagenesis of Thr216 further highlighted its contribution to CTX-M activity. These results demonstrate the importance of the aspartate pair to CTX-M hydrolysis of substrates with bulky side chains, while suggesting increased protein flexibility as a means to evolve drug resistance.

Chemical tools capable of detecting ferrous iron with oxidation-state specificity have only recently become available. Coincident with this development in chemical biology has been increased study and appreciation for the importance of ferrous iron during infection and more generally in host-pathogen interaction. Some of the recent findings are surprising and challenge long- standing assumptions about bacterial iron homeostasis and the innate immune response to infection. Here we review these recent developments and their implications for antibacterial therapy.

Ferroptosis is an iron-dependent form of cell death resulting from loss or inhibition of cellular machinery that protects from the accumulation of lipid hydroperoxides. Ferroptosis likely serves a tumor suppressing function in normal cellular homeostasis but certain cancers exploit, and become highly dependent on specific nodes of the pathway, presumably to survive under conditions of increased oxidative stress and elevated labile ferrous iron levels. Here we introduce Ferroptosis Inducing Peroxide for Chemoproteomics-1 (FIPC-1), a reactivity-based probe that couples Fenton-type reaction with ferrous iron to subsequent protein labeling via concomitant carbon-centered radical generation. We show that FIPC-1 induces ferroptosis in susceptible cell types and labels cellular proteins in an iron-dependent fashion. Use of FIPC-1 in a quantitative chemoproteomics workflow reproducibly enriched protein targets in the thioredoxin, oxidoreductase, and protein disulfide isomerase (PDI) families, among others. In further interrogating the saturable targets of FIPC-1, we identified the PDI family member P4HB and the functionally uncharacterized protein NT5DC2, a member of the haloacid dehalogenase (HAD) superfamily, as previously unrecognized modulators of ferroptosis. Knockdown of these target genes sensitized cells to known ferroptosis inducers, while PACMA31, a previously reported inhibitor of P4HB, directly induced ferroptosis, and was highly synergistic with erastin. Overall, this study introduces a new reactivity-based probe of the ferrous iron-dependent interactome, and uncovers new targets for the therapeutic modulation of ferroptosis.

Correction for ‘Expanded scope of Griesbaum co-ozonolysis for the preparation of structurally diverse sensors of ferrous iron’ by Jun Chen et al., RSC Adv., 2021, 11, 34338–34342, DOI: 10.1039/d1ra05932g.

Parkinson's disease (PD) is a common neurodegenerative disorder without effective disease-modifying therapeutics. Here, we establish a chemogenetic dopamine (DA) neuron ablation model in larval zebrafish with mitochondrial dysfunction and robustness suitable for high-content screening. We use this system to conduct an in vivo DA neuron imaging-based chemical screen and identify the Renin-Angiotensin-Aldosterone System (RAAS) inhibitors as significantly neuroprotective. Knockdown of the angiotensin receptor 1 (Parkinson’s disease is caused by the slow death and deterioration of brain cells, in particular of the neurons that produce a chemical messenger known as dopamine. Certain drugs can mitigate the resulting drop in dopamine levels and help to manage symptoms, but they cause dangerous side-effects. There is no treatment that can slow down or halt the progress of the condition, which affects 0.3% of the population globally. Many factors, both genetic and environmental, contribute to the emergence of Parkinson’s disease. For example, dysfunction of the mitochondria, the internal structures that power up cells, is a known mechanism associated with the death of dopamine-producing neurons. Zebrafish are tiny fish which can be used to study Parkinson’s disease, as they are easy to manipulate in the lab and share many characteristics with humans. In particular, they can be helpful to test the effects of various potential drugs on the condition. Here, Kim et al. established a new zebrafish model in which dopamine-producing brain cells die due to their mitochondria not working properly; they then used this assay to assess the impact of 1,403 different chemicals on the integrity of these cells. A group of molecules called renin-angiotensin-aldosterone (RAAS) inhibitors was shown to protect dopamine-producing neurons and stopped them from dying as often. These are already used to treat high blood pressure as they help to dilate blood vessels. In the brain, however, RAAS worked by restoring certain mitochondrial processes. Kim et al. then investigated whether these results are relevant in other, broader contexts. They were able to show that RAAS inhibitors have the same effect in other animals, and that Parkinson’s disease often progresses more slowly in patients that already take these drugs for high blood pressure. Taken together, these findings therefore suggest that RAAS inhibitors may be useful to treat Parkinson’s disease, as well as other brain illnesses that emerge because of mitochondria not working properly. Clinical studies and new ways to improve these drugs are needed to further investigate and capitalize on these potential benefits.

Sterically shielded 1,2,4-trioxolanes prepared by Griesbaum co-ozonolysis have been utilized as chemical sensors of ferrous iron in several recently described chemical probes of labile iron. Here we report optimized conditions for co-ozonolysis that proceed efficiently and with high diastereoselectivity across an expanded range of substrates, and should enable a new generation of labile iron probes with altered reaction kinetics and physicochemical properties.

Viruses are responsible for some of the most deadly human diseases, yet available vaccines and antivirals address only a fraction of the potential viral human pathogens. Here, we provide a methodology for managing human herpesvirus (HHV) infection by covalently inactivating the HHV maturational protease via a conserved, non-catalytic cysteine (C161). Using human cytomegalovirus protease (HCMV Pr) as a model, we screened a library of disulfides to identify molecules that tether to C161 and inhibit proteolysis, then elaborated hits into irreversible HCMV Pr inhibitors that exhibit broad-spectrum inhibition of other HHV Pr homologs. We further developed an optimized tool compound targeted toward HCMV Pr and used an integrative structural biology and biochemical approach to demonstrate inhibitor stabilization of HCMV Pr homodimerization, exploiting a conformational equilibrium to block proteolysis. Irreversible HCMV Pr inhibition disrupts HCMV infectivity in cells, providing proof of principle for targeting proteolysis via a non-catalytic cysteine to manage viral infection.

Iron is essential to all life, and competition for this vital nutrient is central to host-pathogen interactions during infection. The opportunistic Gram-negative pathogen Pseudomonas aeruginosa utilizes a diverse array of iron-acquisition strategies, including those enabling import of extracellular ferrous iron. We hypothesize that soluble and redox-active ferrous iron can be employed to activate caged antibiotics at sites of infection in vivo. Here we describe new chemistry that expands the application of our laboratory's Fe2+-activated-prodrug chemistry to cage hydroxamic acids, a class of drugs that present manifold development challenges. We synthesize the caged form of a known LpxC inhibitor and show that it is efficacious in an acute P. aeruginosa mouse-lung infection model, despite showing little activity in cell-culture experiments. Overall, our results are consistent with the Fe2+-promoted uncaging of an antibacterial payload at sites of infection in an animal and lend support to recent reports indicating that extracellular pools of ferrous iron can be utilized by bacterial pathogens like P. aeruginosa during infection.

KRAS mutations drive a quarter of cancer mortality, and most are undruggable. Several inhibitors of the MAPK pathway are FDA approved but poorly tolerated at the doses needed to adequately extinguish RAS/RAF/MAPK signaling in the tumor cell. We found that oncogenic KRAS signaling induced ferrous iron (Fe2+) accumulation early in and throughout mutant KRAS-mediated transformation. We converted an FDA-approved MEK inhibitor into a ferrous iron–activatable drug conjugate (FeADC) and achieved potent MAPK blockade in tumor cells while sparing normal tissues. This innovation allowed sustainable, effective treatment of tumor-bearing animals, with tumor-selective drug activation, producing superior systemic tolerability. Ferrous iron accumulation is an exploitable feature of KRAS transformation, and FeADCs hold promise for improving the treatment of KRAS-driven solid tumors.

The recent emergence of oxidation state selective probes of cellular iron has produced a more nuanced understanding of how cells utilize this crucial nutrient to empower enzyme function, and also how ‘labile’ ferrous iron contributes to iron-dependent cell death (ferroptosis) and other disease pathologies including cancer, bacterial infections, and neurodegeneration. These findings, viewed in light of the Fenton chemistry promoted by ferrous iron, suggest a new category of therapeutics exhibiting ferrous iron–dependent pharmacology. While still in its infancy, this nascent field draws inspiration from the remarkable activity and tremendous clinical impact of the antimalarial artemisinin. Here we review recent insights into the role of labile ferrous iron in biology and disease, and describe new therapeutic approaches designed to exploit this divalent transition metal.

Schistosomiasis, a parasitic disease caused by blood flukes of the genus Schistosoma, is a global health problem with over 200 million people infected. Treatment relies on just one drug, and new chemotherapies are needed. Schistosoma mansoni cathepsin B1 (SmCB1) is a critical peptidase for the digestion of host blood proteins and a validated drug target. We screened a library of peptidomimetic vinyl sulfones against SmCB1 and identified the most potent SmCB1 inhibitors reported to date that are active in the subnanomolar range with second order rate constants (k2nd) of ∼2 × 105 M-1 s-1. High resolution crystal structures of the two best inhibitors in complex with SmCB1 were determined. Quantum chemical calculations of their respective binding modes identified critical hot spot interactions in the S1' and S2 subsites. The most potent inhibitor targets the S1' subsite with an N-hydroxysulfonic amide moiety and displays favorable functional properties, including bioactivity against the pathogen, selectivity for SmCB1 over human cathepsin B, and reasonable metabolic stability. Our results provide structural insights for the rational design of next-generation SmCB1 inhibitors as potential drugs to treat schistosomiasis.

Compared to aryl–aryl π-stacking interactions, the analogous stacking of heteroarenes on amide π systems is less well understood and vastly underutilized in structure-based drug design. Recent theoretical studies have delineated the important geometric coordinates of the interaction, some of which have been confirmed with synthetic model systems based on Rebek imides. Unfortunately, a broadly useful and tractable protein–ligand model system of this interaction has remained elusive. Here we employed a known inhibitor scaffold to study π-stacking of diverse heteroarene substituents on the amide face of Gly238 in the cephalosporinases CTX-M-14 and CTX-M-27. Biochemical inhibition constants (K(i)) and biophysical binding constants (K(d)) were determined for nineteen new analogues against both enzymes, while multiple high-resolution co-crystal structures revealed remarkably consistent placement of the probe heteroarene on Gly238. The data presented support the predicted importance of opposing dipoles in amide-heteroarene interactions and should be useful for evaluating other theoretical predictions concerning these interactions.

Background: Although cancer has been known for decades to harbor an insatiable appetite for iron, only recently has the chemistry emerged to exploit this altered state therapeutically, by targeting the expanded cytosolic ‘labile’ iron pool (LIP), of the cancer cell. The state of the art include therapies that react with the LIP to produce cytotoxic radical species (in some cases also releasing drug payloads), and molecules that exacerbate LIP-induced oxidative stress to trigger “ferroptosis”. Effectively implementing LIP targeted therapies in patients will require biomarkers to identify those tumors with the most elevated LIP, and thus most likely to succumb to LIP targeted interventions. Toward this goal, we tested herein whether tumor uptake of the novel LIP sensing radiotracer 18F-TRX aligns with tumor sensitivity to LIP targeted therapies. Methods:18F-TRX uptake was assessed in vivo among ten subcutaneous and orthotopic human xenograft models. Glioma and renal cell carcinoma were prioritized as these tumors have the highest relative expression levels of STEAP3, the oxidoreductase that reduces ferric iron to the ferrous oxidation state, in the Cancer Cell Line Encyclopedia. The antitumor effects of the LIP activated prodrug TRX-CBI, which releases the DNA alkylator cyclopropylbenzindoline (CBI), were compared in mice bearing U251 or PC3 xenografts, tumors with high and intermediate levels of 18F-TRX uptake, respectively. Results:18F-TRX showed a wide range of tumor accumulation. An antitumor assessment study showed that the growth of U251 xenografts, the model with the highest 18F-TRX uptake, was potently inhibited by TRX-CBI. Moreover, the antitumor effects against U251 were significantly greater than those observed for PC3 tumors, consistent with the relative 18F-TRX determined LIP levels in tumors prior to therapy. Lastly, a dosimetry study showed that the estimated effective human doses for adult males and females were comparable to those of other 18F-based imaging probes. Conclusion: We report the first evidence that tumor sensitivity to a LIP targeted therapy can be predicted with a molecular imaging tool. More generally, these data bring a new dimension to the nuclear theranostic model by showing a requirement for imaging to quantify in situ the concentration of a metastable bioanalyte toward predicting tumor drug sensitivity.

The emergence of artemisinin resistance, combined with certain suboptimal properties of ozonide agents arterolane and artefenomel, has necessitated the search for new drug candidates in the endoperoxide class. Our group has focused on trioxolane analogues with substitution patterns not previously explored. Here, we describe the enantioselective synthesis of analogues bearing a trans-3″ carbamate side chain and find these to be superior, both in vitro and in vivo, to the previously reported amides. We identified multiple analogues that surpass the oral efficacy of arterolane in the Plasmodium berghei model while exhibiting drug-like properties (logD, solubility, metabolic stability) similar or superior to next-generation clinical candidates like E209 and OZ609. While the preclinical assessment of new analogues is still underway, current data suggest the potential of this chemotype as a likely source of future drug candidates from the endoperoxide class.

KRAS mutations cause a quarter of cancer mortality and currently are not sensitive to any targeted, FDA-approved agents. Several inhibitors of the MAPK pathway are FDA approved but exhibit low clinical tolerability at doses needed to adequately extinguish KRAS signaling. We discovered an avidity for ferrous iron (Fe2+) induced by and dependent on oncogenic KRAS signaling. We leveraged anFDA-approved MEK inhibitor to produce a prototypical Ferrous Iron–Activatable Drug Conjugate (FeADC) and with this novel agent achieved MAPK blockade in tumor cells while sparing normal tissues. These improvements allowed sustainable, effective treatment of tumor bearing animals with superior tolerability. Iron-activated therapeutics are unknown outside of antiparasitic therapy, but may hold significant promise for the treatment of KRAS-driven solid tumors.

The development of selective ubiquitin-specific protease-7 (USP7) inhibitors GNE-6640 and GNE-6776, which induce tumour cell death and reveal differential kinetics of Lys-48 and Lys-63-linked ubiquitin chain depolymerization by USP7. Deubiquitinating enzymes remove the small modifier protein ubiquitin from target substrates regulating their stability. One such enzyme, USP7, is a potential target for anti-cancer therapy, as its inhibition would result in the degradation of the ubiquitinated oncoprotein MDM2, leading to reactivation of the tumour suppressor protein p53. However, selective inhibitors of USP7 have remained elusive. Here, Ingrid Wertz and team develop two USP7 inhibitors, providing structural insights into the mode of action of these compounds and demonstrating their toxicity towards tumour cells. Elsewhere in this issue, David Komander and colleagues independently report the identification of two small molecules that inhibit USP7 with high affinity and specificity both in vitro and within cells, also demonstrating their ability to inhibit tumour growth. The ubiquitin system regulates essential cellular processes in eukaryotes. Ubiquitin is ligated to substrate proteins as monomers or chains and the topology of ubiquitin modifications regulates substrate interactions with specific proteins. Thus ubiquitination directs a variety of substrate fates including proteasomal degradation1. Deubiquitinase enzymes cleave ubiquitin from substrates and are implicated in disease2; for example, ubiquitin-specific protease-7 (USP7) regulates stability of the p53 tumour suppressor and other proteins critical for tumour cell survival3. However, developing selective deubiquitinase inhibitors has been challenging4 and no co-crystal structures have been solved with small-molecule inhibitors. Here, using nuclear magnetic resonance-based screening and structure-based design, we describe the development of selective USP7 inhibitors GNE-6640 and GNE-6776. These compounds induce tumour cell death and enhance cytotoxicity with chemotherapeutic agents and targeted compounds, including PIM kinase inhibitors. Structural studies reveal that GNE-6640 and GNE-6776 non-covalently target USP7 12 A distant from the catalytic cysteine. The compounds attenuate ubiquitin binding and thus inhibit USP7 deubiquitinase activity. GNE-6640 and GNE-6776 interact with acidic residues that mediate hydrogen-bond interactions with the ubiquitin Lys48 side chain5, suggesting that USP7 preferentially interacts with and cleaves ubiquitin moieties that have free Lys48 side chains. We investigated this idea by engineering di-ubiquitin chains containing differential proximal and distal isotopic labels and measuring USP7 binding by nuclear magnetic resonance. This preferential binding protracted the depolymerization kinetics of Lys48-linked ubiquitin chains relative to Lys63-linked chains. In summary, engineering compounds that inhibit USP7 activity by attenuating ubiquitin binding suggests opportunities for developing other deubiquitinase inhibitors and may be a strategy more broadly applicable to inhibiting proteins that require ubiquitin binding for full functional activity.

Targeting of cryptic binding sites represents an attractive but underexplored approach to modulating protein function with small molecules. Using the dimeric protease (Pr) from Kaposi’s sarcoma-associated herpes-virus (KSHV) as a model system, we sought to dissect a putative allosteric network linking a cryptic site at the dimerization interface to enzyme function. Five cryogenic X-ray structures were solved of the monomeric protease with allosteric inhibitors bound to the dimer interface site. Distinct coordinated movements captured by the allosteric inhibitors were also revealed as alternative states in room-temperature X-ray data and comparative analyses of other dimeric herpesvirus proteases. A two-step mechanism was elucidated through detailed kinetic analyses and suggests an enzyme isomerization model of inhibition. Finally, a representative allosteric inhibitor from this class was shown to be efficacious in a cellular model of viral infectivity. These studies reveal a coordinated dynamic network of atomic communication linking cryptic binding site occupancy and allosteric inactivation of KHSV Pr that can be exploited to target other members of this clinically relevant family of enzymes.

We describe the first systematic study of antimalarial 1,2,4-trioxolanes bearing a substitution pattern regioisomeric to that of arterolane. Conformational analysis suggested that trans-3″-substituted trioxolanes would exhibit Fe(II) reactivity and antiparasitic activity similar to that achieved with canonical cis-4″ substitution. The chiral 3″ analogues were prepared as single stereoisomers and evaluated alongside their 4″ congeners against cultured malaria parasites and in a murine malaria model. As predicted, the trans-3″ analogues exhibited in vitro antiplasmodial activity remarkably similar to that of their cis-4″ comparators. In contrast, efficacy in the Plasmodium berghei mouse model differed dramatically for some of the congeneric pairs. The best of the novel 3″ analogues (e.g., 12i) outperformed arterolane itself, producing cures in mice after a single oral exposure. Overall, this study suggests new avenues for modulating Fe(II) reactivity and the pharmacokinetic and pharmacodynamic properties of 1,2,4-trioxolane antimalarials.

Gram-negative pathogens expressing serine β-lactamases (SBLs) and metallo-β-lactamases (MBLs), especially those with carbapenemase activity, threaten the clinical utility of almost all β-lactam antibiotics. Here we describe the discovery of a heteroaryl phosphonate scaffold that exhibits non-covalent cross-class inhibition of representative carbapenemases, specifically the SBL KPC-2 and the MBLs NDM-1 and VIM-2. The most potent lead, compound 16, exhibited low nM to low μM inhibition of KPC-2, NDM-1, and VIM-2. Compound 16 potentiated imipenem efficacy against resistant clinical and laboratory bacterial strains expressing carbapenemases, while showing some cytotoxicity toward human HEK293T cells only at concentrations above 100 μg/mL. Complex structures with KPC-2, NDM-1, and VIM-2 demonstrate how these inhibitors achieve high binding affinity to both enzyme classes. These findings provide a structurally and mechanistically new scaffold for drug discovery targeting multidrug resistant Gram-negative pathogens, and more generally highlight the active site features of carbapenemases that can be leveraged for lead discovery.

Iron is essential to all life, and competition for this vital nutrient is central to host–pathogen interactions during infection. The opportunistic Gram-negative pathogen Pseudomonas aeruginosa utilizes a diverse array of iron-acquisition strategies, including those enabling import of extracellular ferrous iron. We hypothesize that soluble and redox-active ferrous iron can be employed to activate caged antibiotics at sites of infection in vivo. Here we describe new chemistry that expands the application of our laboratory’s Fe(2+)-activated-prodrug chemistry to cage hydroxamic acids, a class of drugs that present manifold development challenges. We synthesize the caged form of a known LpxC inhibitor and show that it is efficacious in an acute P. aeruginosa mouse-lung infection model, despite showing little activity in cell-culture experiments. Overall, our results are consistent with the Fe(2+)-promoted uncaging of an antibacterial payload at sites of infection in an animal and lend support to recent reports indicating that extracellular pools of ferrous iron can be utilized by bacterial pathogens like P. aeruginosa during infection.

Homologous recombination (HR) is a principal support pathway for DNA replication and for recovery from DNA breaks and interstrand crosslinks, making it a rational target for inhibition in cancer therapy. The ATPase RAD54 functions in molecular events that promote DNA sequence-preservation during HR-mediated damage repair, including homology search, DNA strand exchange, and transition to DNA repair synthesis within a displacement loop intermediate. We developed a high-throughput biochemical screen to identify small-molecule inhibitors of human RAD54, using a phosphate detection assay to monitor RAD54 ATPase activity in the presence of double-stranded DNA (dsDNA). After filtering potential DNA intercalators and ‘frequent hitters,’ we identified two chemotypes that reproducibly inhibited RAD54 ATPase in vitro. We evaluated these chemotypes for inhibition of RAD54-dsDNA binding and cancer cell survival. A halogenated carbazole/dihydroacridine scaffold inhibited a panel of SWI2/SNF2-related ATPases but not VCP/p97, an unrelated ATPase. Small molecules that interfere with key steps in HR— such as inhibitors of RAD54—may expose DNA repair-dependent vulnerabilities in cancer cells.

Serine and metallo-carbapenemases are a serious health concern due to their capability to hydrolyze nearly all β-lactam antibiotics. However, the molecular basis for their unique broad-spectrum substrate profile is poorly understood, particularly for serine carbapenemases, such as KPC-2. Using substrates and newly identified small molecules, we compared the ligand binding properties of KPC-2 with the noncarbapenemase CTX-M-14, both of which are Class A β-lactamases with highly similar active sites. Notably, compared to CTX-M-14, KPC-2 was more potently inhibited by hydrolyzed β-lactam products (product inhibition), as well as by a series of novel tetrazole-based inhibitors selected from molecular docking against CTX-M-14. Together with complex crystal structures, these data suggest that the KPC-2 active site has an enhanced ability to form favorable interactions with substrates and small molecule ligands due to its increased hydrophobicity and flexibility. Such properties are even more pronounced in metallo-carbapenemases, such as NDM-1, which was also inhibited by some of the novel tetrazole compounds, including one displaying comparable low μM affinities against both KPC-2 and NDM-1. Our results suggest that carbapenemase activity confers an evolutionary advantage on producers via a broad β-lactam substrate scope but also a mechanistic Achilles' heel that can be exploited for new inhibitor discovery. The complex structures demonstrate, for the first time, how noncovalent inhibitors can be engineered to simultaneously target both serine and metallo-carbapenemases. Despite the relatively modest activity of the current compounds, these studies also demonstrate that hydrolyzed products and tetrazole-based chemotypes can provide valuable starting points for broad-spectrum inhibitor discovery against carbapenemases.

[Image: see text] Redox cycling of iron powers various enzyme functions crucial for life, making the study of iron acquisition, storage, and disposition in the whole organism a worthy topic of inquiry. However, despite its important role in biology and disease, imaging iron in animals with oxidation-state specificity remains an outstanding problem in biology and medicine. Here we report a first-generation reactivity-based probe of labile ferrous iron suitable for positron emission tomography studies in live animals. The responses of this reagent to systemic changes in labile iron disposition were revealed using iron supplementation and sequestration treatments in mice, while the potential of this approach for in vivo imaging of cancer was demonstrated using genetically and pathologically diverse mouse models, including spontaneous tumors arising in a genetically engineered model of prostate cancer driven by loss of PTEN.

Aberrant activation of caspase-6 (C6) in the absence of other hallmarks of apoptosis has been demonstrated in cells and tissues from patients with Huntington disease (HD) and animal models. C6 activity correlates with disease progression in patients with HD and the cleavage of mutant huntingtin (mHTT) protein is thought to strongly contribute to disease pathogenesis. Here we show that the mHTT1-586 fragment generated by C6 cleavage interacts with the zymogen form of the enzyme, stabilizing a conformation that contains an active site and is prone to full activation. This shift toward enhanced activity can be prevented by a small-molecule inhibitor that blocks the interaction between C6 and mHTT1-586. Molecular docking studies suggest that the inhibitor binds an allosteric site in the C6 zymogen. The interaction of mHTT1-586 with C6 may therefore promote a self-reinforcing, feedforward cycle of C6 zymogen activation and mHTT cleavage driving HD pathogenesis.

Serotonin drugs, including clemizole, have recently emerged as potential treatment options for Dravet syndrome. We synthesized a library of clemizole analogues and screened for anti-seizure activity using the zebrafish Dravet syndrome model. Coupled with in vitro binding, we identified 5-HT2B receptors as a mediator in the mechanism of seizure suppression., Dravet syndrome is a life-threatening early-onset epilepsy not well controlled by antiepileptic drugs. Drugs that modulate serotonin (5-HT) signalling, including clemizole, locaserin, trazodone and fenfluramine, have recently emerged as potential treatment options for Dravet syndrome. To investigate the serotonin receptors that could moderate this antiepileptic activity, we designed and synthesized 28 novel analogues of clemizole, obtained receptor binding affinity profiles, and performed in vivo screening in a scn1lab mutant zebrafish (Danio rerio) model which recapitulates critical clinical features of Dravet syndrome. We discovered three clemizole analogues with 5-HT receptor binding that exert powerful antiepileptic activity. Based on structure–activity relationships and medicinal chemistry-based analysis, we then screened an additional set of known 5-HT receptor specific drug candidates. Integrating our in vitro and in vivo data implicates 5-HT2B receptors as a critical mediator in the mechanism of seizure suppression observed in Dravet syndrome patients treated with 5-HT modulating drugs., Graphical Abstract Graphical Abstract

Here we describe a new approach for tumor targeting in which augmented concentrations of Fe(II) in cancer cells and/or the tumor microenvironment triggers drug release from an Fe(II)-reactive prodrug conjugate. The 1,2,4-trioxolane scaffold developed to enable this approach can in principle be applied to a broad range of cancer therapeutics and is illustrated here with Fe(II)-targeted forms of a microtubule toxin and a duocarmycin-class DNA-alkylating agent. We show that the intrinsic reactivity/toxicity of the duocarmycin analog is masked in the conjugated form and this greatly reduced toxicity in mice. This in turn permitted elevated dosing levels, leading to higher systemic exposure and a significantly improved response in tumor xenograft models. Overall our results suggest that Fe(II)-dependent drug delivery via trioxolane conjugates could have significant utility in expanding the therapeutic index of a range of clinical and preclinical stage cancer chemotherapeutics.

The targeting of non-catalytic cysteine residues with small molecules is drawing increased attention from drug discovery scientists and chemical biologists. From a biological perspective, genomic and proteomic studies have revealed the presence of cysteine mutations in several oncogenic proteins, suggesting both a functional role for these residues and also a strategy for targeting them in an 'allele specific' manner. For the medicinal chemist, the structure-guided design of cysteine- reactive molecules is an appealing strategy to realize improved selectivity and pharmacodynamic properties in drug leads. Finally, for chemical biologists, the modification of cysteine residues provides a unique means to probe protein structure and allosteric regulation. Here, we review three applications of cysteinemodifying small molecules: 1) the optimization of existing drug leads, 2) the discovery of new lead compounds, and 3) the use of cysteine-reactive molecules as probes of protein dynamics. In each case, structure-guided design plays a key role in determining which cysteine residue(s) to target and in designing compounds with the proper geometry to enable both covalent interaction with the targeted cysteine and productive non-covalent interactions with nearby protein residues.

Improved methods for studying intracellular reactive iron(II) are of significant interest for studies of iron metabolism and disease relevant changes in iron homeostasis. Here we describe a highly-selective reactivity-based probe in which Fenton-type reaction with intracellular labile iron(II) leads to unmasking of the aminonucleoside puromycin. Puromycin leaves a permanent and dose-dependent mark on treated cells that can be detected with high sensitivity and precision using the high-content, plate-based immunofluorescence assay described. Using this new probe and screening approach, we detected alteration of cellular labile iron(II) in response extracellular iron conditioning, overexpression of iron storage and/or export proteins, and post-translational regulation of iron export. Finally, we utilized this new tool to demonstrate the presence of augmented labile iron(II) pools in cancer cells as compared to non-tumorigenic cells.

Genomic KRAS alterations are identified in >90% of pancreatic adenocarcinomas (PDA), but there are no efficient targeted therapies for these patients. RAF-MEK-MAPK is frequently cited as a frequent potentiator in RAS-driven malignancies. Directly inhibiting the biochemical output of oncogenic Ras and Raf via MEK inhibition has been an interest for almost two decades and is now approved. However, it has not lived up to its full potential. Because MEK function is essential in many healthy tissues, on-target side effects accumulate in the eye, skin and other organs, substantially limiting the clinical tolerance of MEK inhibition (MEKi). Inspired by the remarkable pharmacology of antimalarial endoperoxides, we have extended the concept of Fe2+-dependent pharmacology to other disease states, which are characterized by elevated levels or aberrant production of free redox-active Fe2+by reimagining the 1,2,4-trioxolane ring as a selective detector of free Fe2+. We re-engineered the trioxolane pharmacophore to produce a new scaffold (denoted “TRX” herein); enabling Fe2+dependent release of a drug payload. We next found that mutant Kras alone is sufficient to initiate an elevated labile iron pool and is necessary to maintain elevated Fe2+levels in PDA. These findings encouraged us to link an FDA-approved, orally bioavailable cobimetinib (COBI) to a trioxolane pharmacophore (yielding TRX-COBI). We first assessed the stability TRX-COBI by profiling of the prodrug and parent in the plasma of normal mice after intraperitoneal (IP) administration and found the TRX-Conjugated form to be very stable, with only 3% (by AUC) of TRX-COBI being deconjugated in serum. Next, we applied orthotopically (in the pancreas) KrasG12D, Tp53 flox/floxcells grafted into syngeneic, immunocompetent FVB mice and treated the mice with MEKi or TRX-MEKi and harvested the tumors and skin after 2 weeks. We find that TRX-MEK (dosed equimolar to MEKi), inhibits pERK in PDA tumors in vivo. Additionally, these same animals demonstrate decreased skin toxicity in tail skin. To validate these findings further, we used a patient derived xenograft (PDX 232) from a KRAS mutated PDA and compared the therapeutic efficacy of COBI payloads. It shows that TRX-COBI potently inhibited pERK in human PDX cells in nude mice, and was as efficacious as the parent drug. In addition, when combined with SHP2 inhibitor, higher dose of TRX-COBI was tolerable and showing better tumor growth inhibition in another lung PDX model carrying KRASG12C. Together, these results show the in vivo activity, stability and mitigated toxicity of the TRX-prodrug approach in KRAS-driven pancreatic cancer. Citation Format: Honglin Jiang, Ryan Muir, Adam Renslo, Eric Collisson. Novel ferrous iron-dependent prodrug strategy for KRAS-driven pancreatic cancer: The preclinic efficacy and safety test [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6419.

K2P potassium channels generate leak currents that stabilize the resting membrane potential of excitable cells. Various K2P channels are implicated in pain, ischemia, depression, migraine, and anesthetic responses, making this family an attractive target for small molecule modulator development efforts. BL-1249, a compound from the fenamate class of nonsteroidal anti-inflammatory drugs is known to activate K2P2.1(TREK-1), the founding member of the thermo- and mechanosensitive TREK subfamily; however, its mechanism of action and effects on other K2P channels are not well-defined. Here, we demonstrate that BL-1249 extracellular application activates all TREK subfamily members but has no effect on other K2P subfamilies. Patch clamp experiments demonstrate that, similar to the diverse range of other chemical and physical TREK subfamily gating cues, BL-1249 stimulates the selectivity filter “C-type” gate that controls K2P function. BL-1249 displays selectivity among the TREK subfamily, activating K2P2.1(TREK-1) and K2P10.1(TREK-2) ∼10-fold more potently than K2P4.1(TRAAK). Investigation of mutants and K2P2.1(TREK-1)/K2P4.1(TRAAK) chimeras highlight the key roles of the C-terminal tail in BL-1249 action and identify the M2/M3 transmembrane helix interface as a key site of BL-1249 selectivity. Synthesis and characterization of a set of BL-1249 analogs demonstrates that both the tetrazole and opposing tetralin moieties are critical for function, whereas the conformational mobility between the two ring systems impacts selectivity. Together, our findings underscore the landscape of modes by which small molecules can affect K2P channels and provide crucial information for the development of better and more selective K2P modulators of the TREK subfamily.

Primary cilia are required for Smoothened to transduce vertebrate Hedgehog signals, but how Smoothened accumulates in cilia and is activated is incompletely understood. Here, we identify cilia-associated oxysterols that promote Smoothened accumulation in cilia and activate the Hedgehog pathway. Our data reveal that cilia-associated oxysterols bind to two distinct Smoothened domains to modulate Smoothened accumulation in cilia and tune the intensity of Hedgehog pathway activation. We find that the oxysterol synthase HSD11β2 participates in the production of Smoothened-activating oxysterols and promotes Hedgehog pathway activity. Inhibiting oxysterol biosynthesis impedes oncogenic Hedgehog pathway activation and attenuates the growth of Hedgehog pathway-associated medulloblastoma, suggesting that targeted inhibition of Smoothened-activating oxysterol production may be therapeutically useful for patients with Hedgehog-associated cancers., This work was supported by grants from the NIH (HL007731 and CA212279-01), the UCSF Physician Scientist Scholar Program, the American Society of Clinical Oncology, the Rally Foundation for Childhood Cancer Research, and the American Brain Tumor Association to D.R.R.; the NIH program Project Grant to Molecular Genetics (HL20948) to J.G.M.; Cancer Research UK (C20724 and A14414) and the European Research Council (647278) to C.S.; the NIH (AR065409 and HD092659) to S.Y.W. and L.X., respectively; the NIH (R01GM102498), the Ludwig Cancer Institute, and the Howard Hughes Medical Institute to P.A.B.; and the NIH (AR054396 and GM095941), the Burroughs Wellcome Fund, and the Packard Foundation to J.F.R.