Phase II study of darinaparsin in patients with advanced hepatocellular carcinoma

Targeting Drug Chemo-Resistance in Cancer Using Natural Products

Cancer is one of the leading causes of death globally. The development of drug resistance is the main contributor to cancer-related mortality. Cancer cells exploit multiple mechanisms to reduce the therapeutic effects of anticancer drugs, thereby causing chemotherapy failure. Natural products are accessible, inexpensive, and less toxic sources of chemotherapeutic agents. Additionally, they have multiple mechanisms of action to inhibit various targets involved in the development of drug resistance. In this review, we have summarized the basic research and clinical applications of natural products as possible inhibitors for drug resistance in cancer. The molecular targets and the mechanisms of action of each natural product are also explained. Diverse drug resistance biomarkers were sensitive to natural products. P-glycoprotein and breast cancer resistance protein can be targeted by a large number of natural products. On the other hand, protein kinase C and topoisomerases were less sensitive to most of the studied natural products. The studies discussed in this review will provide a solid ground for scientists to explore the possible use of natural products in combination anticancer therapies to overcome drug resistance by targeting multiple drug resistance mechanisms.

Effects of Selenium on Arsenic-Induced Liver Lesions in Broilers

The aim of the present study was to investigate the abilities of selenium to counteract the toxic damage of arsenic (As). Two hundred 1-day-old healthy male broilers were randomly divided into five groups and fed the following diets: control group (0.1 mg/kg As + 0.2 mg/kg Se), As group (3 mg/kg As + 0.2 mg/kg Se), As + Se group I (3 mg/kg As + 5 mg/kg Se), As + Se group II (3 mg/kg As + 10 mg/kg Se), and As + Se group III (3 mg/kg As + 15 mg/kg Se), respectively. The relative weight of the liver, hepatic protein content, GSH-Px levels, SOD activities, NO contents, iNOS and tNOS activities, and increased malondialdehyde contents, ALT and AST activities, and the apoptotic hepatocytes were analyzed. Adding 3 mg/kg arsenic to the diet caused the growth and development of chicken liver to be blocked, resulting in decrease of protein contents in liver tissue, decrease of SOD and GSH-Px activities, increase of MDA contents, decrease of NO contents, decrease of iNOS and TNOs activities, increase of ALT and AST activities, increase of apoptosis rates of liver cells. Compared to the 3-mg/kg arsenic group, adding 5 mg/kg and 10 mg/kg selenium, respectively, could repair the liver growth retardation and steatosis caused by arsenic, increase the protein contents in liver tissue, increase the activities of SOD and GSH-Px, reduce the contents of MDA, increase the contents of NO, enhance the activities of iNOS and TNOs, reduce the activities of ALT and AST, and reduce the rates of apoptosis of liver cells, in which the best effects are to add 10 mg/kg selenium. While 15 mg/kg of sodium selenite may induce progression of As-induced hepatic lesions, the results indicated that 5 and 10 mg/kg of sodium selenite supplied in the diet, through mechanisms of oxidative stress and apoptosis regulation, may ameliorate As-induced hepatic lesions in a dose-dependent manner.

Phase I studies of darinaparsin in patients with relapsed or refractory peripheral T-cell lymphoma: a pooled analysis of two phase I studies conducted in Japan and Korea

OBJECTIVE

Two phase I studies of darinaparsin including Japanese and Korean patients with relapsed/refractory peripheral T-cell lymphoma were performed to evaluate its safety (primary purpose), efficacy and pharmacokinetic profile (ClinicalTrials.gov: NCT01435863 and NCT01689220).

METHODS

Patients received intravenous darinaparsin for 5 consecutive days at 200 mg/m2/day in 4-week cycles, 300 mg/m2/day in 4-week cycles or 300 mg/m2/day in 3-week cycles.

RESULTS

Seventeen Japanese and 6 Korean patients were enrolled and treated. Drug-related adverse events developed in 18 patients (78%). Dose-limiting toxicity, grade 3 hepatic dysfunction, was reported on Day 15 of cycle 1 in 1 Japanese patient who received 300 mg/m2/day. The most common drug-related, grade ≥ 3 adverse events were lymphopenia (9%), neutropenia (9%) and thrombocytopenia (9%). No deaths occurred. In 14 evaluable patients, 1 and 3 patients had complete response and partial response, respectively. The plasma concentration-time profiles of arsenic, a surrogate marker for darinaparsin, were similar between Japanese and Korean patients. No significant difference was found in its pharmacokinetic profile.

CONCLUSIONS

These data indicate the good tolerability and potential efficacy of darinaparsin in patients with relapsed/refractory peripheral T-cell lymphoma. Darinaparsin 300 mg/m2/day for 5 consecutive days in 3-week cycles is the recommended regimen for phase II study.

Biotransformation of arsenic trioxide by AS3MT favors eradication of acute promyelocytic leukemia: revealing the hidden facts

Arsenic trioxide (ATO) is one of the most effective drugs for treatment of acute promyelocytic leukemia (APL). It could specifically target the PML/RARα fusion oncoprotein stability and induces APL cell differentiation as well as apoptosis. Although many studies have been conducted to document the anticancer effects and mechanism of ATO, there is little information about the association between biotransformation of ATO to active arsenic metabolites and APL therapy. Generally, ATO can be rapidly converted into trivalent methylated metabolites by arsenic (+3 oxidation state) methyltransferase (AS3MT) mostly in liver and redistributed to bloodstream of APL patients who receiving ATO treatment, thereby leading to a balance between cytotoxicity and differentiation, which is proposed to be the key event in successful treatment of APL. In this review, we comprehensively discussed possible roles of AS3MT and methylated arsenic metabolites in APL therapy, so as to reveal the association between individual differences of AS3MT expression and activity with the therapeutic efficacy of ATO in APL patients.

Mechanisms of Multidrug Resistance in Cancer Chemotherapy

Cancer is one of the main causes of death worldwide. Despite the significant development of methods of cancer healing during the past decades, chemotherapy still remains the main method for cancer treatment. Depending on the mechanism of action, commonly used chemotherapeutic agents can be divided into several classes (antimetabolites, alkylating agents, mitotic spindle inhibitors, topoisomerase inhibitors, and others). Multidrug resistance (MDR) is responsible for over 90% of deaths in cancer patients receiving traditional chemotherapeutics or novel targeted drugs. The mechanisms of MDR include elevated metabolism of xenobiotics, enhanced efflux of drugs, growth factors, increased DNA repair capacity, and genetic factors (gene mutations, amplifications, and epigenetic alterations). Rapidly increasing numbers of biomedical studies are focused on designing chemotherapeutics that are able to evade or reverse MDR. The aim of this review is not only to demonstrate the latest data on the mechanisms of cellular resistance to anticancer agents currently used in clinical treatment but also to present the mechanisms of action of novel potential antitumor drugs which have been designed to overcome these resistance mechanisms. Better understanding of the mechanisms of MDR and targets of novel chemotherapy agents should provide guidance for future research concerning new effective strategies in cancer treatment.

Deciphering the molecular events during arsenic induced transcription signal cascade activation in cellular milieu

Anthropogenic sources of arsenic poses and creates unintentional toxico-pathological concerns to humans in many parts of the world. The understanding of toxicity of this metalloid, which shares properties of both metal and non-metal is principally structured on speciation types and holy grail of toxicity prevention. Visible symptoms of arsenic toxicity include nausea, vomiting, diarrhea and abdominal pain. In this review, we focused on the dermal cell stress caused by trivalent arsenic trioxide and pentavalent arsanilic acid. Deciphering the molecular events involved during arsenic toxicity and signaling cascade interaction is key in arsenicosis prevention. FoxO1 and FoxO2 transcription factors, members of the Forkhead/Fox family, play important roles in this aspect. Like Foxo family proteins, ATM/CHK signaling junction also plays important role in DNA nuclear factor guided cellular development. This review will summarize and discuss current knowledge about the interplay of these pathways in arsenic induced dermal pathogenesis.

Metabolism, toxicity and anticancer activities of arsenic compounds

A variety of studies indicated that inorganic arsenic and its methylated metabolites have paradoxical effects, namely, carcinogenic and anticancer effects. Epidemiological studies have shown that long term exposure to arsenic can increase the risk of cancers of lung, skin or bladder in man, which is probably associated with the arsenic metabolism. In fact, the enzymatic conversion of inorganic arsenic by Arsenic (+3 oxidation state) methyltransferase (AS3MT) to mono- and dimethylated arsenic species has long been considered as a major route for detoxification. However, several studies have also indicated that biomethylation of inorganic arsenic, particularly the production of trivalent methylated metabolites, is a process that activates arsenic as a toxin and a carcinogen. On the other hand, arsenic trioxide (As2O3) has recently been recognized as one of the most effective drugs for the treatment of APL. However, elaboration of the cytotoxic mechanisms of arsenic and its methylated metabolites in eradicating cancer is sorely lacking. To provide a deeper understanding of the toxicity and carcinogenicity along with them use of arsenic in chemotherapy, caution is required considering the poor understanding of its various mechanisms of exerting toxicity. Thereby, in this review, we have focused on arsenic metabolic pathway, the roles of the methylated arsenic metabolites in toxicity and in the therapeutic efficacy for the treatments of solid tumors, APL and/or non-APL malignancies.

Activation of the Nrf2 Signaling Pathway Involving KLF9 Plays a Critical Role in Allicin Resisting Against Arsenic Trioxide-Induced Hepatotoxicity in Rats

Arsenic trioxide (As₂O₃) is both the most prevalent, naturally occurring inorganic arsenical threatening human health and an efficient therapeutic for acute promyelocytic leukemia. Regretfully, As₂O₃-treated cancer patients often suffer from hepatotoxicity. While effective antioxidant and anticarcinogenic actions of allicin have previously been demonstrated, studies indicating how allicin affects As₂O₃-induced hepatotoxicity and arsenic accumulation are lacking. Our study, for the first time, elaborates potential details of the hepatoprotective mechanisms of allicin against As₂O₃-induced liver injury. Wistar rats were administrated allicin (30 mg/kg) 1 h before As₂O₃ (3 mg/kg) by daily gavage for 2 weeks. Our results indicate that allicin ameliorated As₂O₃-induced liver dysfunction, oxidative stress, and arsenic accumulation in the liver. Meanwhile, allicin decreased NF-κB level and upregulated expression of proteins reduced by As₂O₃ including nuclear factor erythroid 2-related factor 2 (Nrf2), heme oxygenase 1, nicotinamide adenine dinucleotide phosphate:quinone oxidoreductase 1, and Krüppel-like factor 9 (KLF9). In addition, allicin promoted B cell lymphoma-extra large expression and suppressed B cell lymphoma-2-associated X protein levels regulated by As₂O₃. However, neither allicin nor As₂O₃ affected cytochrome P450 2E1 mRNA expression. In conclusion, allicin attenuated As₂O₃-induced hepatotoxicity by activating the Nrf2 signaling pathway involving KLF9 to inhibit oxidative stress and apoptosis. Our findings elucidate a detailed mechanism by which allicin provides protection against As₂O₃-induced liver injury and support its potential role as an adjunctive therapy for patients suffering from chronic arsenic exposure.

Thiolated arsenicals in arsenic metabolism: Occurrence, formation, and biological implications

Arsenic (As) is a notoriously toxic pollutant of health concern worldwide with potential risk of cancer induction, but meanwhile it is used as medicines for the treatment of different conditions including hematological cancers. Arsenic can undergo extensive metabolism in biological systems, and both toxicological and therapeutic effects of arsenic compounds are closely related to their metabolism. Recent studies have identified methylated thioarsenicals as a new class of arsenic metabolites in biological systems after exposure of inorganic and organic arsenicals, including arsenite, dimethylarsinic acid (DMA^V), dimethylarsinous glutathione (DMA^IIIGS), and arsenosugars. The increasing detection of thiolated arsenicals, including monomethylmonothioarsonic acid (MMMTA^V), dimethylmonothioarsinic acid (DMMTA^V) and its glutathione conjugate (DMMTA^VGS), and dimethyldithioarsinic acid (DMDTA^V) suggests that thioarsenicals may be important metabolites and play important roles in arsenic toxicity and therapeutic effects. Here we summarized the reported occurrence of thioarsenicals in biological systems, the possible formation pathways of thioarsenicals, and their toxicity, and discussed the biological implications of thioarsenicals on arsenic metabolism, toxicity, and therapeutic effects.

Dual oncogenic and tumor suppressor roles of the promyelocytic leukemia gene in hepatocarcinogenesis associated with hepatitis B virus surface antigen

Proteasome-mediated degradation of promyelocytic leukemia tumor suppressor (PML) is upregulated in many viral infections and cancers. We previously showed that PML knockdown promotes early-onset hepatocellular carcinoma (HCC) in hepatitis B virus surface antigen (HBsAg)-transgenic mice. Here we report the effects of PML restoration on late-onset HBsAg-induced HCC. We compared protein expression patterns, genetic mutations and the effects of pharmacologically targeting PML in wild-type, PML-/-, PML+/+HBsAgtg/o and PML-/-HBsAgtg/o mice. PML-/- mice exhibited somatic mutations in DNA repair genes and developed severe steatosis and proliferative disorders, but not HCC. PML-/-HBsAgtg/o mice exhibited early mutations in cancer driver genes and developed hyperplasia, fatty livers and indolent adipose-like HCC. In PML+/+HBsAg-transgenic mice, HBsAg expression declined over time, and HBsAg-associated PML suppression was concomitantly relieved. Nevertheless, these mice accumulated mutations in genes contributing to oxidative stress pathways and developed aggressive late-onset angiogenic trabecular HCC. PML inhibition using non-toxic doses of arsenic trioxide selectively killed long-term HBsAg-affected liver cells in PML+/+HBsAgtg/o mice with falling HBsAg and rising PML levels, but not normal liver cells or early-onset HCC cells in PML-/-HBsAgtg/0 mice. These findings suggest dual roles for PML as a tumor-suppressor lost in early-onset HBsAg-induced hepatocarcinogenesis and as an oncogenic promoter in late-onset HBsAg-related HCC progression.

Metal-based drugs that break the rules

Cisplatin and other platinum compounds have had a huge impact in the treatment of cancers and are applied in the majority of anticancer chemotherapeutic regimens. The success of these compounds has biased the approaches used to discover new metal-based anticancer drugs. In this perspective we highlight compounds that are apparently incompatible with the more classical (platinum-derived) concepts employed in the development of metal-based anticancer drugs, with respect to both compound design and the approaches used to validate their utility. Possible design approaches for the future are also suggested.

Determination of multiple human arsenic metabolites employing high performance liquid chromatography inductively coupled plasma mass spectrometry

During the metabolism of different arsenic-containing compounds in human, a variety of metabolites are produced with significantly varying toxicities. Currently available analytical methods can only detect a limited number of human metabolites in biological samples during one run due to their diverse characteristics. In addition, co-elution of species is often unnoticeable with most detection techniques leading to inaccurate metabolic profiles and assessment of toxicity. A high performance liquid chromatography inductively coupled mass spectrometry (HPLC-ICP-MS) method was developed that can identify thirteen common arsenic metabolites possibly present in human with special attention dedicated to thiolated or thiol conjugated arsenicals. The thirteen species included in this study are arsenite (As(III)), arsino-glutathione (As(GS)3), arsenate (As(V)), monomethylarsonous acid (MMA(III)), monomethylarsino-glutathione (MMA(III)(GS) 2), monomethylarsonic acid (MMA(V)), dimethylarsinous acid (DMA(III) (from DMA(III)I)), S-(dimethylarsinic)cysteine (DMA(III) (Cys)), dimethylarsino-glutathione (DMA(III)(GS)), dimethylarsinic acid (DMA(V)), dimethylmonothioarsinic acid (DMMTA(V)), dimethyldithioarsinic acid (DMDTA(V)), dimethylarsinothioyl glutathione (DMMTA(V)(GS)). The developed method was applied for the analysis of cancer cells that were incubated with darinaparsin (DMA(III)(GS)), a novel chemotherapeutic agent for refractory malignancies, and the arsenic metabolic profile obtained was compared to results using a previously developed method. This method provides a useful analytical tool which is much needed in unequivocally identifying the arsenicals formed during the metabolism of environmental arsenic exposure or therapeutic arsenic administration.

NMFBFS: A NMF-Based Feature Selection Method in Identifying Pivotal Clinical Symptoms of Hepatocellular Carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) is a highly aggressive malignancy. Traditional Chinese Medicine (TCM), with the characteristics of syndrome differentiation, plays an important role in the comprehensive treatment of HCC. This study aims to develop a nonnegative matrix factorization- (NMF-) based feature selection approach (NMFBFS) to identify potential clinical symptoms for HCC patient stratification.

METHODS

The NMFBFS approach consisted of three major steps. Firstly, statistics-based preliminary feature screening was designed to detect and remove irrelevant symptoms. Secondly, NMF was employed to infer redundant symptoms. Based on NMF-derived basis matrix, we defined a novel similarity measurement of intersymptoms. Finally, we converted each group of redundant symptoms to a new single feature so that the dimension was further reduced.

RESULTS

Based on a clinical dataset consisting of 407 patient samples of HCC with 57 symptoms, NMFBFS approach detected 8 irrelevant symptoms and then identified 16 redundant symptoms within 6 groups. Finally, an optimal feature subset with 39 clinical features was generated after compressing the redundant symptoms by groups. The validation of classification performance shows that these 39 features obviously improve the prediction accuracy of HCC patients.

CONCLUSIONS

Compared with other methods, NMFBFS has obvious advantages in identifying important clinical features of HCC.

Pro-Oxidant Activity of Amine-Pyridine-Based Iron Complexes Efficiently Kills Cancer and Cancer Stem-Like Cells

Differential redox homeostasis in normal and malignant cells suggests that pro-oxidant-induced upregulation of cellular reactive oxygen species (ROS) should selectively target cancer cells without compromising the viability of untransformed cells. Consequently, a pro-oxidant deviation well-tolerated by nonmalignant cells might rapidly reach a cell-death threshold in malignant cells already at a high setpoint of constitutive oxidative stress. To test this hypothesis, we took advantage of a selected number of amine-pyridine-based Fe(II) complexes that operate as efficient and robust oxidation catalysts of organic substrates upon reaction with peroxides. Five of these Fe(II)-complexes and the corresponding aminopyridine ligands were selected to evaluate their anticancer properties. We found that the iron complexes failed to display any relevant activity, while the corresponding ligands exhibited significant antiproliferative activity. Among the ligands, none of which were hemolytic, compounds 1, 2 and 5 were cytotoxic in the low micromolar range against a panel of molecularly diverse human cancer cell lines. Importantly, the cytotoxic activity profile of some compounds remained unaltered in epithelial-to-mesenchymal (EMT)-induced stable populations of cancer stem-like cells, which acquired resistance to the well-known ROS inducer doxorubicin. Compounds 1, 2 and 5 inhibited the clonogenicity of cancer cells and induced apoptotic cell death accompanied by caspase 3/7 activation. Flow cytometry analyses indicated that ligands were strong inducers of oxidative stress, leading to a 7-fold increase in intracellular ROS levels. ROS induction was associated with their ability to bind intracellular iron and generate active coordination complexes inside of cells. In contrast, extracellular complexation of iron inhibited the activity of the ligands. Iron complexes showed a high proficiency to cleave DNA through oxidative-dependent mechanisms, suggesting a likely mechanism of cytotoxicity. In summary, we report that, upon chelation of intracellular iron, the pro-oxidant activity of amine-pyrimidine-based iron complexes efficiently kills cancer and cancer stem-like cells, thus providing functional evidence for an efficient family of redox-directed anti-cancer metallodrugs.

The elements of life and medicines

Which elements are essential for human life? Here we make an element-by-element journey through the periodic table and attempt to assess whether elements are essential or not, and if they are, whether there is a relevant code for them in the human genome. There are many difficulties such as the human biochemistry of several so-called essential elements is not well understood, and it is not clear how we should classify elements that are involved in the destruction of invading microorganisms, or elements which are essential for microorganisms with which we live in symbiosis. In general, genes do not code for the elements themselves, but for specific chemical species, i.e. for the element, its oxidation state, type and number of coordinated ligands, and the coordination geometry. Today, the biological periodic table is in a position somewhat similar to Mendeleev's chemical periodic table of 1869: there are gaps and we need to do more research to fill them. The periodic table also offers potential for novel therapeutic and diagnostic agents, based on not only essential elements, but also non-essential elements, and on radionuclides. Although the potential for inorganic chemistry in medicine was realized more than 2000 years ago, this area of research is still in its infancy. Future advances in the design of inorganic drugs require more knowledge of their mechanism of action, including target sites and metabolism. Temporal speciation of elements in their biological environments at the atomic level is a major challenge, for which new methods are urgently needed.

Multivalency in the inhibition of oxidative protein folding by arsenic(III) species

The renewed use of arsenicals as chemotherapeutics has rekindled interest in the biochemistry of As(III) species. In this work, simple bis- and tris-arsenical derivatives were synthesized with the aim of exploiting the chelate effect in the inhibition of thiol-disulfide oxidoreductases (here, Quiescin sulfhydryl oxidase, QSOX, and protein disulfide isomerase, PDI) that utilize two or more CxxC motifs in the catalysis of oxidative protein folding. Coupling 4-aminophenylarsenoxide (APAO) to acid chloride or anhydride derivatives yielded two bis-arsenical prototypes, BA-1 and BA-2, and a tris-arsenical, TA-1. Unlike the monoarsenical, APAO, these new reagents proved to be strong inhibitors of oxidative protein folding in the presence of a realistic intracellular concentration of competing monothiol (here, 5 mM reduced glutathione, GSH). However, this inhibition does not reflect direct inactivation of QSOX or PDI, but avid binding of MVAs to the reduced unfolded protein substrates themselves. Titrations of reduced riboflavin-binding protein with MVAs show that all 18 protein −SH groups can be captured by these arsenicals. With reduced RNase, addition of substoichiometric levels of MVAs is accompanied by the formation of Congo Red- and Thioflavin T-positive fibrillar aggregates. Even with K_d values of ∼50 nM, MVAs are ineffective inhibitors of PDI in the presence of millimolar levels of competing GSH. These results underscore the difficulties of designing effective and specific arsenical inhibitors for folded enzymes and proteins. Some of the cellular effects of arsenicals likely reflect their propensity to associate very tightly and nonspecifically to conformationally mobile cysteine-rich regions of proteins, thereby interfering with folding and/or function.

The combination of arsenic and cryptotanshinone induces apoptosis through induction of endoplasmic reticulum stress-reactive oxygen species in breast cancer cells

Arsenic trioxide has been successfully used for the treatment of patients with acute promyelocytic leukemia (APL) worldwide. Recently, it has also been further developed to treat solid tumors in clinical trials. However, the therapeutic effects on malignant tumors appeared to be unsatisfactory, as these cells exhibited resistance towards arsenic. In this study, we explored new therapeutic strategies for treatment of human breast cancer MCF-7 cells based on arsenic metabolites. The MCF-7 cells were exposed to three arsenic species, namely, inorganic arsenite (iAs(III)) and its intermediate metabolites monomethylarsonous acid (MMA(III)) and dimethylarsinous acid (DMA(III)) either alone or in combination with cryptotanshinone (CPT) to establish their anticancer effects against MCF-7 cells. Surprisingly, MCF-7 cells were shown to be resistant to both iAs(III) and CPT when used alone; however, they were shown to be relatively sensitive to treatment when exposed to MMA(III) and DMA(III) alone. Conversely, the combination of MMA(III) with CPT showed significantly enhanced anticancer effects on MCF-7 cells at low doses, but no appreciable effect was observed upon exposure to the other two arsenic species with CPT. In addition, remarkable redistribution of pro-apoptosis related proteins Bax and Bak was observed in the mitochondria, together with activation of poly(ADP-ribose) polymerase (PARP) and caspase-9 after exposure to the combination of MMA(III) with CPT. Furthermore, we clearly found that induction of apoptosis in MCF-7 cells was predominantly triggered by endoplasmic reticulum (ER) stress after exposure to the combination of MMA(III) with CPT.

Darinaparsin inhibits prostate tumor-initiating cells and Du145 xenografts and is an inhibitor of hedgehog signaling

Prostate cancer is the leading cause of cancer-related death in men in the United States. A major cause of drug resistance in prostate and other epithelial tumors may be due to the presence of a fraction of tumor cells that retain the ability to initiate tumors and hence are termed tumor-initiating cells (TIC) or cancer stem cells. Here, we report that darinaparsin, an organic derivative of arsenic trioxide, is cytotoxic to prostate cancer cell lines as well as fresh prostate cancer cells from patients at low micromolar concentrations, and importantly inhibits the TIC subpopulations. It also inhibits growth of the castrate-resistant Du145 prostate tumor propagated as xenograft in mice and inhibits the tumor-initiating potential of prostate cancer cells. Although the mechanism by which darinaparsin acts is not completely known, we show that it kills prostate cancer cells by blocking cells in the G2-M phase of the cell cycle and inhibits Hedgehog signaling by downregulating Gli-2 transcriptional activity. These data provide a rationale for evaluating darinaparsin in patients with castrate-resistant prostate cancer.

Glutathione S-conjugates as prodrugs to target drug-resistant tumors

Living organisms are continuously exposed to xenobiotics. The major phase of enzymatic detoxification in many species is the conjugation of activated xenobiotics to reduced glutathione (GSH) catalyzed by the glutathione-S-transferase (GST). It has been reported that some compounds, once transformed into glutathione S-conjugates, enter the mercapturic acid pathway whose end products are highly reactive and toxic for the cell responsible for their production. The cytotoxicity of these GSH conjugates depends essentially on GST and gamma-glutamyl transferases (γGT), the enzymes which initiate the mercapturic acid synthesis pathway. Numerous studies support the view that the expression of GST and γGT in cancer cells represents an important factor in the appearance of a more aggressive and resistant phenotype. High levels of tumor GST and γGT expression were employed to selectively target tumor with GST- or γGT-activated drugs. This strategy, explored over the last two decades, has recently been successful using GST-activated nitrogen mustard (TLK286) and γGT-activated arsenic-based (GSAO and Darinaparsin) prodrugs confirming the potential of GSH-conjugates as anticancer drugs.

Dimethylarsinothioyl glutathione as a metabolite in human multiple myeloma cell lines upon exposure to Darinaparsin

Here, we report the identification of dimethylarsinothioyl glutathione (DMMTA^V(GS)) as a metabolite in cellular extracts of dimethyarsinous glutathione (Darinaparsin, DMA^III(GS)) treated human multiple myeloma (MM) cell lines. Co-elution of sulfur and arsenic on the inductively coupled plasma mass spectrometer (ICP-MS) indicated the presence of sulfur along with arsenic in the newly observed unidentified molecule on the speciation chromatograms of cell lines treated with DMA^III(GS). Liquid chromatography–electrospray ionization–mass spectrometry of the unknown peak in the MS and tandem MS modes revealed molecular ion peaks at m/z = 443.9 and 466.0, corresponding to [DMMTA^V(GS) + H]⁺ and [DMMTA^V(GS) + Na]⁺, as well as peaks at 314.8 for the loss of glutamic acid and 231.1 for the loss of glycine. In addition, peaks were observed at 176.9 corresponding to cysteine and glycine adducts and at 137.1 for the [C₂H₆AsS]⁺ ion. An increase in the peak area of the unidentified peak was observed upon spiking the cell extracts with a standard of DMMTA^V(GS). Heat deactivation of MM cells prevented the formation of DMMTA^V(GS) raising the possibility of its formation via an enzymatic reaction. Formation studies in DMA^III(GS) treated MM cells revealed the dependence of DMMTA^V(GS) formation on the depletion of DMA^III(GS). The presence of 5 mM glutathione prevented its formation, indicating that DMA^III, a dissociation product of DMA^III(GS), is likely a precursor for the formation of DMMTA^V(GS). DMMTA^V(GS) was observed to form under acidic and neutral pH conditions (pH 3.0–7.4). In addition, DMMTA^V(GS) was found to be stable in cell extracts at both acidic and neutral pH conditions. When assessing the toxicity by exposing multiple myeloma cells to arsenicals externally, DMMTA^V(GS) was found to be much less toxic than DMA^III(GS) and DMMTA^V, potentially due to its limited uptake in the cells (10 and 16% of the uptakes of DMA^III(GS) and DMMTA^V, respectively).

Darinaparsin: solid tumor hypoxic cytotoxin and radiosensitizer

PURPOSE

Hypoxia is an important characteristic of the solid tumor microenvironment and constitutes a barrier for effective radiotherapy. Here, we studied the effects of darinaparsin (an arsenic cytotoxin) on survival and radiosensitivity of tumor cells in vitro under normoxia and hypoxia and in vivo using xenograft models, compared to effects on normal tissues.

EXPERIMENTAL DESIGN

The cytotoxicity and radiosensitization of darinaparsin were first tested in vitro in a variety of solid tumor cell lines under both normoxia and hypoxia and compared with arsenic trioxide (ATO, an arsenical with reported cytotoxic and radiosensitizing activities on tumor cells). The effects were then tested in mouse models of xenograft tumors derived from tumor cell lines and clinical tumor specimens. The potential mechanisms of darinaparsin effects, including reactive oxygen species (ROS) generation, cellular damage, and changes in global gene expression, were also investigated.

RESULTS

In comparison with ATO, darinaparsin had significantly higher in vitro cytotoxic and radiosensitizing activities against solid tumor cells under both normoxia and hypoxia. In vivo experiments confirmed these activities at doses that had no systemic toxicities. Importantly, darinaparsin did not radiosensitize normal bone marrow and actually radioprotected normal intestinal crypts. The darinaparsin-mediated antitumor effects under hypoxia were not dependent on ROS generation and oxidative damage, but were associated with inhibition of oncogene (RAS and MYC)-dependent gene expression.

CONCLUSION

Darinaparsin has significant and preferential cytotoxic and radiosensitizing effects on solid tumors as compared with normal cells. Darinaparsin may therefore increase the therapeutic index of radiation therapy and has near term translational potential.

Arsenic compounds: revived ancient remedies in the fight against human malignancies

Arsenic, the 20th most abundant element in the earth crust, is one of the oldest drugs in the world. It was used in the 18th century in treating hematopoietic malignancies, discarded in 1950s in favor of chemotherapeutic agents (busulphan and others), and was revived in the 1970s due to its dramatic efficacy on acute promyelocytic leukemia (APL) driven by the t(15;17) translocation-generated PML-RARα fusion. Arsenic represents the most potent single agent for APL, and achieves a five-year overall survival of 90% in APL patients when combined with all-trans retinoic acid (ATRA) and chemotherapy (daunorubicin and cytarabine), turning this disease from highly fatal to highly curable. Arsenic triggers sumoylation/ubiquitination and proteasomal degradation of PML-RARα via directly binding to the C3HC4 zinc finger motif in the RBCC domain of the PML moiety and induction of its homodimerization/multimerization and interaction with the SUMO E2 conjugase Ubc9. Because of its multiplicity of targets and complex mechanisms of action, arsenic is widely tested in combination with other agents in a variety of malignancies. Other arsenic containing recipes including oral formulations and organic arsenicals are being developed and tested, and progress in these areas will definitely expand the use of arsenicals in other malignant diseases.

Cellular redox pathways as a therapeutic target in the treatment of cancer

The vulnerability of some cancer cells to oxidative signals is a therapeutic target for the rational design of new anticancer agents. In addition to their well characterized effects on cell division, many cytotoxic anticancer agents can induce oxidative stress by modulating levels of reactive oxygen species (ROS) such as the superoxide anion radical, hydrogen peroxide and hydroxyl radicals. Tumour cells are particularly sensitive to oxidative stress as they typically have persistently higher levels of ROS than normal cells due to the dysregulation of redox balance that develops in cancer cells in response to increased intracellular production of ROS or depletion of antioxidant proteins. In addition, excess ROS levels potentially contribute to oncogenesis by the mediation of oxidative DNA damage. There are several anticancer agents in development that target cellular redox regulation. The overall cellular redox state is regulated by three systems that modulate cellular redox status by counteracting free radicals and ROS, or by reversing the formation of disulfides; two of these are dependent on glutathione and the third on thioredoxin. Drugs targeting S-glutathionylation have direct anticancer effects via cell signalling pathways and inhibition of DNA repair, and have an impact on a wide range of signalling pathways. Of these agents, NOV-002 and canfosfamide have been assessed in phase III trials, while a number of others are undergoing evaluation in early phase clinical trials. Alternatively, agents including PX-12, dimesna and motexafin gadolinium are being developed to target thioredoxin, which is overexpressed in many human tumours, and this overexpression is associated with aggressive tumour growth and poorer clinical outcomes. Finally, arsenic derivatives have demonstrated antitumour activity including antiproliferative and apoptogenic effects on cancer cells by pro-oxidant mechanisms, and the induction of high levels of oxidative stress and apoptosis by an as yet undefined mechanism. In this article we review anticancer drugs currently in development that target cellular redox activity to treat cancer.

Darinaparsin is a multivalent chemotherapeutic which induces incomplete stress response with disruption of microtubules and Shh signaling

Chemotherapeutics and other pharmaceuticals are common sources of cellular stress. Darinaparsin (ZIO-101) is a novel organic arsenical under evaluation as a cancer chemotherapeutic, but the drug's precise mechanism of action is unclear. Stress granule formation is an important cellular stress response, but the mechanisms of formation, maintenance, and dispersal of RNA-containing granules are not fully understood. During stress, small, diffuse granules initially form throughout the cytoplasm. These granules then coalesce near the nucleus into larger granules that disperse once the cellular stress is removed. Complete stress granule formation is dependent upon microtubules. Human cervical cancer (HeLa) cells, pre-treated with nocodazole for microtubule depolymerization, formed only small, diffuse stress granules upon sodium arsenite treatment. Darinaparsin, as a single agent, also induced the formation of small, diffuse stress granules, an effect similar to that of the combination of nocodazole with sodium arsenite. Darinaparsin inhibited the polymerization of microtubules both in vivo and in vitro. Interestingly, upon removal of darinaparsin, the small, diffuse stress granules completed formation with coalescence in the perinuclear region prior to disassembly. These results indicate that RNA stress granules must complete formation prior to disassembly, and completion of stress granule formation is dependent upon microtubules. Finally, treatment of cells with darinaparsin led to a reduction in Sonic hedgehog (Shh) stimulated activation of Gli1 and a loss of primary cilia. Therefore, darinaparsin represents a unique multivalent chemotherapeutic acting on stress induction, microtubule polymerization, and Shh signaling.

Anticancer activity of metal complexes: involvement of redox processes

Cells require tight regulation of the intracellular redox balance and consequently of reactive oxygen species for proper redox signaling and maintenance of metal (e.g., of iron and copper) homeostasis. In several diseases, including cancer, this balance is disturbed. Therefore, anticancer drugs targeting the redox systems, for example, glutathione and thioredoxin, have entered focus of interest. Anticancer metal complexes (platinum, gold, arsenic, ruthenium, rhodium, copper, vanadium, cobalt, manganese, gadolinium, and molybdenum) have been shown to strongly interact with or even disturb cellular redox homeostasis. In this context, especially the hypothesis of "activation by reduction" as well as the "hard and soft acids and bases" theory with respect to coordination of metal ions to cellular ligands represent important concepts to understand the molecular modes of action of anticancer metal drugs. The aim of this review is to highlight specific interactions of metal-based anticancer drugs with the cellular redox homeostasis and to explain this behavior by considering chemical properties of the respective anticancer metal complexes currently either in (pre)clinical development or in daily clinical routine in oncology.