Susceptibility of the antioxidant selenoenyzmes thioredoxin reductase and glutathione peroxidase to alkylation-mediated inhibition by anticancer acylfulvenes

Lipid peroxidation-derived modification and its effect on the activity of glutathione peroxidase 1

Oxidative stress and the resulting lipid peroxidation are associated with various pathological states, including neurodegenerative diseases and cancer. The end products of lipid peroxidation, such as 4-oxo-2(E)-nonenal (ONE), 4-hydroxy-2(E)-nonenal (HNE), and methylglyoxal (MG), exert several biological effects through modification of various cellular components, including DNA and proteins. Glutathione peroxidase 1 (GPx1) is an intracellular antioxidant enzyme that uses glutathione (GSH) to reduce a variety of peroxides, thereby modulating cellular oxidative stress and redox-mediated responses. GPx1 contains nucleophilic amino acids at its active (one Sec) and GSH-binding (four Arg and one Lys) sites. We found that lipid peroxidation-derived reactive aldehydes (ONE, HNE, and MG) modified the GSH-binding site, resulting in the inhibition of GPx1 activity. Mass spectrometry-based proteomic analysis identified the sites modified by each aldehyde (ONE, 14 sites; HNE, 7 sites; MG, 9 sites). The GSH-binding sites modified were as follows: ONE, Arg57, 103, 184, and 185; HNE, Lys91; MG, Arg103. Upon incubation of GPx1 with each aldehyde, ONE reduced GPx1 activity more significantly than did HNE or MG in a dose- and time-dependent manner. The addition of GSH to GPx1 3 h after incubation with ONE prevented further inhibition by trapping ONE as a ONE-GSH adduct. However, the activity of GPx1 was not restored to the initial level, indicating that ONE modified GPx1 irreversibly. This study suggests that oxidative damage to lipids, resulting in the formation of reactive aldehydes, can amplify cellular oxidative stress via direct inactivation of GPx1, which increases the production of intracellular peroxides.

Half-sandwich Ru(II) N-heterocyclic carbene complexes in anticancer drug design

The ruthenium arene fragment is a rich source for the design of anticancer drugs; in this design, the co-ligand is a critical factor for obtaining effective anticancer complexes. In comparison with other types of ligands, N-heterocyclic carbenes (NHCs) have been less explored, despite the versatility in structural modifications and the marked stabilization of metal ions, being these characteristics important for the design of metal drugs. However, notable advances have been made in the development of NHC Ruthenium arene as anticancer agents. These advances include high antitumor activities, proven both in in vitro and in in vivo models and, in some cases, with marked selectivity against tumorigenic cells. The versatility of the structure has played a fundamental role, since they have allowed a selective interaction with their molecular targets through, for example, bio-conjugation with known anticancer molecules. For this reason, the structure-activity relationship of the imidazole, benzimidazole, and abnormal NHC ruthenium (II) η6-arene complexes have been studied. Taking into account this study, several synthetic aspects are provided to contribute to the next generations of this kind of complexes. Moreover, in recent years nanotechnology has provided innovative nanomedicines, where half-sandwich Ruthenium(II) complexes are paving their way. In this review, the recent developments in nanomaterials functionalized with Ruthenium complexes for targeted drug delivery to tumors will also be highlighted.

The mutational impact of Illudin S on human cells

Illudin S (ILS) is a fungal sesquiterpene secondary metabolite with potent genotoxic and cytotoxic properties. Early genetic studies and more recent genome-wide CRISPR screens showed that Illudin-induced lesions are preferentially repaired by transcription-coupled nucleotide excision repair (TC-NER) with some contribution from post-replication repair pathways. In line with these results, Irofulven, a semi-synthetic ILS analog was recently shown to be particularly effective on cell lines and patient-derived xenografts with impaired NER (e.g. ERCC2/3 mutations), raising hope that ILS-derived molecules may soon enter the clinic. Despite the therapeutic potential of ILS and its analogs, we still lack a global understanding of their mutagenic potential. Here, we characterize the mutational signatures associated with chronic exposure to ILS in human cells. ILS treatment rapidly stalls DNA replication and transcription, leading to the activation of the replication stress response and the accumulation of DNA damage. Novel single and double base substitution signatures as well as a characteristic indel signature indicate that ILS treatment preferentially alkylates purine residues and induces oxidative stress, confirming prior in vitro data. Many mutation contexts exhibit a strong transcriptional strand bias, highlighting the contribution of TC-NER to the repair of ILS lesions. Finally, collateral mutations are also observed in response to ILS, suggesting a contribution of translesion synthesis pathways to ILS tolerance. Accordingly, ILS treatment led to the rapid recruitment of the Y-family DNA polymerase kappa onto chromatin, supporting its preferential use for ILS lesion bypass. Altogether, our work provides the first global assessment of the genomic impact of ILS, demonstrating the contribution of multiple DNA repair pathways to ILS resistance and mutagenicity.

Thioredoxin reductase is a major regulator of metabolism in leukemia cells

Despite the fact that AML is the most common acute leukemia in adults, patient outcomes are poor necessitating the development of novel therapies. We identified that inhibition of Thioredoxin Reductase (TrxR) is a promising strategy for AML and report a highly potent and specific inhibitor of TrxR, S-250. Both pharmacologic and genetic inhibition of TrxR impairs the growth of human AML in mouse models. We found that TrxR inhibition leads to a rapid and marked impairment of metabolism in leukemic cells subsequently leading to cell death. TrxR was found to be a major and direct regulator of metabolism in AML cells through impacts on both glycolysis and the TCA cycle. Studies revealed that TrxR directly regulates GAPDH leading to a disruption of glycolysis and an increase in flux through the pentose phosphate pathway (PPP). The combined inhibition of TrxR and the PPP led to enhanced leukemia growth inhibition. Overall, TrxR abrogation, particularly with S-250, was identified as a promising strategy to disrupt AML metabolism.

The acylfulvene alkylating agent, LP-184, retains nanomolar potency in non-small cell lung cancer carrying otherwise therapy-refractory mutations

More than 40% of non-small cell lung cancer (NSCLC) patients lack actionable targets and require non-targeted chemotherapeutics. Many become refractory to drugs due to underlying resistance-associated mutations. KEAP1 mutant NSCLCs further activate NRF2 and upregulate its client PTGR1. LP-184, a novel alkylating agent belonging to the acylfulvene class is a prodrug dependent upon PTGR1. We hypothesized that NSCLC with KEAP1 mutations would continue to remain sensitive to LP-184. LP-184 demonstrated highly potent anticancer activity both in primary NSCLC cell lines and in those originating from brain metastases of primary lung cancers. LP-184 activity correlated with PTGR1 transcript levels but was independent of mutations in key oncogenes (KRAS and KEAP1) and tumor suppressors (TP53 and STK11). LP-184 was orders of magnitude more potent in vitro than cisplatin and pemetrexed. Correlative analyses of sensitivity with cell line gene expression patterns indicated that alterations in NRF2, MET, EGFR and BRAF consistently modulated LP-184 sensitivity. These correlations were then extended to TCGA analysis of 517 lung adenocarcinoma patients, out of which 35% showed elevated PTGR1, and 40% of those further displayed statistically significant co-occurrence of KEAP1 mutations. The gene correlates of LP-184 sensitivity allow additional personalization of therapeutic options for future treatment of NSCLC.

A machine learning-based gene signature of response to the novel alkylating agent LP-184 distinguishes its potential tumor indications

BACKGROUND

Non-targeted cytotoxics with anticancer activity are often developed through preclinical stages using response criteria observed in cell lines and xenografts. A panel of the NCI-60 cell lines is frequently the first line to define tumor types that are optimally responsive. Open data on the gene expression of the NCI-60 cell lines, provides a unique opportunity to add another dimension to the preclinical development of such drugs by interrogating correlations with gene expression patterns. Machine learning can be used to reduce the complexity of whole genome gene expression patterns to derive manageable signatures of response. Application of machine learning in early phases of preclinical development is likely to allow a better positioning and ultimate clinical success of molecules. LP-184 is a highly potent novel alkylating agent where the preclinical development is being guided by a dedicated machine learning-derived response signature. We show the feasibility and the accuracy of such a signature of response by accurately predicting the response to LP-184 validated using wet lab derived IC50s on a panel of cell lines.

RESULTS

We applied our proprietary RADR® platform to an NCI-60 discovery dataset encompassing LP-184 IC50s and publicly available gene expression data. We used multiple feature selection layers followed by the XGBoost regression model and reduced the complexity of 20,000 gene expression values to generate a 16-gene signature leading to the identification of a set of predictive candidate biomarkers which form an LP-184 response gene signature. We further validated this signature and predicted response to an additional panel of cell lines. Considering fold change differences and correlation between actual and predicted LP-184 IC50 values as validation performance measures, we obtained 86% accuracy at four-fold cut-off, and a strong (r = 0.70) and significant (p value 1.36e-06) correlation between actual and predicted LP-184 sensitivity. In agreement with the perceived mechanism of action of LP-184, PTGR1 emerged as the top weighted gene.

CONCLUSION

Integration of a machine learning-derived signature of response with in vitro assessment of LP-184 efficacy facilitated the derivation of manageable yet robust biomarkers which can be used to predict drug sensitivity with high accuracy and clinical value.

Irreversible TrxR1 inhibitors block STAT3 activity and induce cancer cell death

Because of its key role in cancer development and progression, STAT3 has become an attractive target for developing new cancer therapeutics. While several STAT3 inhibitors have progressed to advanced stages of development, their underlying biology and mechanisms of action are often more complex than would be expected from specific binding to STAT3. Here, we have identified and optimized a series of compounds that block STAT3-dependent luciferase expression with nanomolar potency. Unexpectedly, our lead compounds did not bind to cellular STAT3 but to another prominent anticancer drug target, TrxR1. We further identified that TrxR1 inhibition induced Prx2 and STAT3 oxidation, which subsequently blocked STAT3-dependent transcription. Moreover, previously identified inhibitors of STAT3 were also found to inhibit TrxR1, and likewise, established TrxR1 inhibitors block STAT3-dependent transcriptional activity. These results provide new insights into the complexities of STAT3 redox regulation while highlighting a novel mechanism to block aberrant STAT3 signaling in cancer cells.

A Chemical Proteomic Analysis of Illudin-Interacting Proteins

The illudin natural product family are fungal secondary metabolites with a characteristic spirocyclopropyl-substituted fused 6,5-bicyclic ring system. They have been extensively studied for their cytotoxicity in various tumor cell types, and semisynthetic derivatives with improved therapeutic characteristics have progressed to clinical trials. Although it is believed that this potent alkylating compound class acts mainly through DNA modification, little is known about its binding to protein sites in a cellular context. To reveal putative protein targets of the illudin family in live cancer cells, we employed a semisynthetic strategy to access a series of illudin-based probes for activity-based protein profiling (ABPP). While the probes largely retained potent cytotoxicity, proteomic profiling studies unraveled multiple protein hits, suggesting that illudins exert their mode of action not from addressing a specific protein target but rather from DNA modification and unselective protein binding.

Significance of the mitochondrial thioredoxin reductase in cancer cells: An update on role, targets and inhibitors

Thioredoxin reductase 2 (TrxR2) is a key component of the mitochondrial thioredoxin system able to transfer electrons to peroxiredoxin 3 (Prx3) in a reaction mediated by thioredoxin 2 (Trx2). In this way, both the level of hydrogen peroxide and thiol redox state are modulated. TrxR2 is often overexpressed in cancer cells conferring apoptosis resistance. Due to their exposed flexible arm containing selenocysteine, both cytosolic and mitochondrial TrxRs are inhibited by a large number of molecules. The various classes of inhibitors are listed and the molecules acting specifically on TrxR2 are extensively described. Particular emphasis is given to gold(I/III) complexes with phosphine, carbene or other ligands and to tamoxifen-like metallocifens. Also chemically unrelated organic molecules, including natural compounds and their derivatives, are taken into account. An important feature of many TrxR2 inhibitors is provided by their nature of delocalized lipophilic cations that allows their accumulation in mitochondria exploiting the organelle membrane potential. The consequences of TrxR2 inhibition are presented focusing especially on the impact on mitochondrial pathophysiology. Inhibition of TrxR2, by hindering the activity of Trx2 and Prx3, increases the mitochondrial concentration of reactive oxygen species and shifts the thiol redox state toward a more oxidized condition. This is reflected by alterations of specific targets involved in the release of pro-apoptotic factors such as cyclophilin D which acts as a regulator of the mitochondrial permeability transition pore. Therefore, the selective inhibition of TrxR2 could be utilized to induce cancer cell apoptosis.

Targeting the Thioredoxin System for Cancer Therapy

Thioredoxin (Trx) and thioredoxin reductase (TrxR) are essential components of the Trx system which plays pivotal roles in regulating multiple cellular redox signaling pathways. In recent years TrxR/Trx have been increasingly recognized as an important modulator of tumor development, and hence targeting TrxR/Trx is a promising strategy for cancer treatment. In this review we first discuss the structural details of TrxR, the functions of the Trx system, and the rational of targeting TrxR/Trx for cancer treatment. We also highlight small-molecule TrxR/Trx inhibitors that have potential anticancer activity and review their mechanisms of action. Finally, we examine the challenges of developing TrxR/Trx inhibitors as anticancer agents and perspectives for selectively targeting TrxR/Trx.

Covalent Modifiers: A Chemical Perspective on the Reactivity of α,β-Unsaturated Carbonyls with Thiols via Hetero-Michael Addition Reactions

Although Michael acceptors display a potent and broad spectrum of bioactivity, they have largely been ignored in drug discovery because of their presumed indiscriminate reactivity. As such, a dearth of information exists relevant to the thiol reactivity of natural products and their analogues possessing this moiety. In the midst of recently approved acrylamide-containing drugs, it is clear that a good understanding of the hetero-Michael addition reaction and the relative reactivities of biological thiols with Michael acceptors under physiological conditions is needed for the design and use of these compounds as biological tools and potential therapeutics. This Perspective provides information that will contribute to this understanding, such as kinetics of thiol addition reactions, bioactivities, as well as steric and electronic factors that influence the electrophilicity and reversibility of Michael acceptors. This Perspective is focused on α,β-unsaturated carbonyls given their preponderance in bioactive natural products.

Cellular Protection of SNAP-25 against Botulinum Neurotoxin/A: Inhibition of Thioredoxin Reductase through a Suicide Substrate Mechanism

Botulium neurotoxins (BoNTs) are among the most lethal toxins known to man. They are comprised of seven serotypes with BoNT/A being the most deadly; yet, there is no approved therapeutic for their intoxication or one that has even advanced to clinical trials. Botulinum neurotoxicity is ultimately governed through light chain (LC) protease SNARE protein cleavage leading to a loss of neurotransmitter release. Pharmacological attempts to ablate BoNT/A intoxication have sought to either nullify cellular toxin entry or critical biochemical junctions found within its intricate mechanism of action. In these regards, reports have surfaced of nonpeptidic small molecule inhibitors, but few have demonstrated efficacy in neutralizing cellular toxicity, a key prerequisite before rodent lethality studies can be initiated. On the basis of a lead discovered in our BoNT/A cellular assay campaign, we investigated a family of N-hydroxysuccinimide inhibitors grounded upon structure activity relationship (SAR) fundamentals. Molecules stemming from this SAR exercise were theorized to be protease inhibitors. However, this proposition was overturned on the basis of extensive kinetic analysis. Unexpectedly, inhibitor data pointed to thioredoxin reductase (TrxR), an essential component required for BoNT protease translocation. Also unforeseen was the inhibitors' mechanism of action against TrxR, which was found to be brokered through a suicide-mechanism utilizing quinone methide as the inactivating element. This new series of TrxR inhibitors provides an alternative means to negate the etiological agent responsible for BoNT intoxication, the LC protease.

Induction of complementary function reductase enzymes in colon cancer cells by dithiole-3-thione versus sodium selenite

Cellular induction of reductase enzymes can alter the susceptibility of cells toward drugs and chemicals. In this study, we compared the capacity of a single dose of sodium selenite and 3H-1,2-dithiole-3-thione (D3T) to influence the drug-relevant reducing capacity of HT29 cells over time, and defined the protein-specific contribution to this activity on the basis of selected reaction monitoring mass spectrometry. Thioredoxin reductase 1 (TrxR1) protein levels and activity were inducible up to 2.2-fold by selenium. In contrast, selenium had only a minor influence on prostaglandin reductase 1 (PTGR1) and

NAD(P)H

quinone oxidoreductase 1 (NQO1) activity and protein levels. D3T, a strong Nrf2 inducer, induced all the reductases and additionally increased the cytotoxicity of hydroxymethylacylfulvene, a bioreductive DNA-alkylating drug. The data and experimental approaches allow one to define induction potency for reductase enzymes PTGR1, TrxR1, and NQO1 in HT29 cells and link these to changes in drug cytotoxicity.

Improved efficacy of acylfulvene in colon cancer cells when combined with a nuclear excision repair inhibitor

The efficacy of DNA-damaging anticancer drugs is highly influenced by cellular DNA repair capacity, and by inhibiting the relevant DNA repair pathway, efficacy of alkylating agents may be increased. Therefore, combining DNA repair inhibitors with anticancer agents that selectively target tumor tissue should improve cancer treatment. The objective of this study was to test the hypothesis that cotreatment of cancer cells with acylfulvene (AF, alkylating agent) and UCN-01 (DNA repair inhibitor) would improve drug efficacy and promote the persistence of DNA adducts. Previous data regarding the relative susceptibility of repair proficient versus deficient cells toward an AF analogue suggests that corresponding adducts are repaired by nuclear excision repair (NER), a cellular process that has been shown to be prevented with UCN-01. In this study, cells were cotreated with nontoxic levels of UCN-01 together with increasing doses of AF. The efficacy of AF was assessed by measuring cytotoxicity and DNA adducts. In addition, cells were cotreated with nontoxic levels of methoxyamine, a known base excision repair (BER) inhibitor, to determine if inhibiting BER also promotes cytotoxicity of AF. DNA-adducts were measured in a sensitive and precise manner by using stable isotope-labeled mass spectrometry analysis. The data obtained in this study demonstrate for the first time that pharmacological inhibition of the NER pathway of DNA repair leads to the persistence of AF-specific adducts and promotes AF cytotoxicity.

Selenium compounds, apoptosis and other types of cell death: an overview for cancer therapy

Selenium (Se) is an essential trace element involved in different physiological functions of the human body and plays a role in cancer prevention and treatment. Induction of apoptosis is considered an important cellular event that can account for the cancer preventive effects of Se. The mechanisms of Se-induced apoptosis are associated with the chemical forms of Se and their metabolism as well as the type of cancer studied. So, some selenocompounds, such as SeO₂ involve the activation of caspase-3 while sodium selenite induces apoptosis in the absence of the activation of caspases. Modulation of mitochondrial functions has been reported to play a key role in the regulation of apoptosis and also to be one of the targets of Se compounds. Other mechanisms for apoptosis induction are the modulation of glutathione and reactive oxygen species levels, which may function as intracellular messengers to regulate signaling pathways, or the regulation of kinase, among others. Emerging evidence indicates the overlaps between the apoptosis and other types of cell death such as autophagy. In this review we report different processes of cell death induced by Se compounds in cancer treatment and prevention.