A novel arsenical has antitumor activity toward As2O3-resistant and MRP1/ABCC1-overexpressing cell lines

AcGlcAs: A Novel P53-Targeting Arsenical with Potent Cellular Uptake and Cancer Cell Selectivity

Arsenic trioxide (ATO) targets PML/RARα and leads to miraculous success in treating acute promyelocytic leukemia. Notably, ATO also targets p53, the most frequently mutated protein in cancers, through a similar binding mechanism. However, p53-targeting ATO trials are challenging due to the poor cellular uptake and cancer selectivity of ATO. Here, we analyzed the structure-activity relationship of arsenicals and rationally developed a novel arsenical (designated AcGlcAs) by conjugating arsenic to sulfur atoms and tetraacetyl-β-d-thioglucose. AcGlcAs exhibited remarkable cellular uptake through a thiol-mediated pathway (maximally 127-fold higher than ATO), thereby potently targeting PML/RARα and mutant p53. Among the 55 tested cell lines, AcGlcAs preferentially killed cancer lines rather than normal lines. In preclinical studies, AcGlcAs significantly extended the survival of mice bearing a xenograft tumor with p53 mutation while showing high plasma stability and oral bioavailability. Thus, AcGlcAs is a potential clinical candidate for precisely treating numerous p53-mutated cancers.

The organic arsenical-derived thioredoxin and glutathione system inhibitor ACZ2 induces apoptosis and autophagy in gastric cancer via ROS-dependent ER stress

Developing novel drugs for gastric cancer (GC) is greatly needed, and a reactive oxygen species (ROS)-modulating strategy has been demonstrated to be useful for cancer treatment. However, no organic arsenical-derived ROS-modulating drug has been developed in GC. Here, we constructed ACZ2 and investigated its efficacy and potential mechanism for GC in vitro and in vivo. Our data showed that ACZ2 could inhibit GC proliferation and cause G2/M phase arrest. Moreover, ACZ2 induced ROS accumulation by depleting glutathione (GSH) and TrxR1, triggering a subsequent ER stress response by activating the PERK/EIF2/ATF4/CHOP signalling pathways, which is a crucial step for ACZ2-mediated apoptosis and autophagy. Vitally, ROS scavenger (NAC) and ER stress inhibitor (4PBA) reversed ACZ2/ROS/ER stress-mediated apoptosis and autophagy. Our in vivo results clearly demonstrated that ACZ2 suppressed tumour growth in a GC xenograft model. Collectively, our data indicated that ACZ2 is a potential agent against GC.

Cytotoxic Effects of Darinaparsin, a Novel Organic Arsenical, against Human Leukemia Cells

To explore the molecular mechanisms of action underlying the antileukemia activities of darinaparsin, an organic arsenical approved for the treatment of peripheral T-cell lymphoma in Japan, cytotoxicity of darinaparsin was evaluated in leukemia cell lines NB4, U-937, MOLT-4 and HL-60. Darinaparsin was a more potent cytotoxic than sodium arsenite, and induced apoptosis/necrosis in NB4 and HL-60 cells. In NB4 cells exhibiting the highest susceptibility to darinaparsin, apoptosis induction was accompanied by the activation of caspase-8/-9/-3, a substantial decrease in Bid expression, and was suppressed by Boc-D-FMK, a pancaspase inhibitor, suggesting that darinaparsin triggered a convergence of the extrinsic and intrinsic pathways of apoptosis via Bid truncation. A dramatic increase in the expression level of γH2AX, a DNA damage marker, occurred in parallel with G2/M arrest. Activation of p53 and the inhibition of cdc25C/cyclin B1/cdc2 were concomitantly observed in treated cells. Downregulation of c-Myc, along with inactivation of E2F1 associated with the activation of Rb, was observed, suggesting the critical roles of p53 and c-Myc in darinaparsin-mediated G2/M arrest. Trolox, an antioxidative reagent, suppressed the apoptosis induction but failed to correct G2/M arrest, suggesting that oxidative stress primarily contributed to apoptosis induction. Suppression of Notch1 signaling was also confirmed. Our findings provide novel insights into molecular mechanisms underlying the cytotoxicity of darinaparsin and strong rationale for its new clinical application for patients with different types of cancer.

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.

Small molecules regulating reactive oxygen species homeostasis for cancer therapy

Elevated intracellular reactive oxygen species (ROS) and antioxidant defense systems have been recognized as one of the hallmarks of cancer cells. Compared with normal cells, cancer cells exhibit increased ROS to maintain their malignant phenotypes and are more dependent on the "redox adaptation" mechanism. Thus, there are two apparently contradictory but virtually complementary therapeutic strategies for the regulation of ROS to prevent or treat cancer. The first strategy, that is, chemoprevention, is to prevent or reduce intracellular ROS either by suppressing ROS production pathways or by employing antioxidants to enhance ROS clearance, which protects normal cells from malignant transformation and inhibits the early stage of tumorigenesis. The second strategy is the ROS-mediated anticancer therapy, which stimulates intracellular ROS to a toxicity threshold to activate ROS-induced cell death pathways. Therefore, targeting the regulation of intracellular ROS-related pathways by small-molecule candidates is considered to be a promising treatment for tumors. We herein first briefly introduce the source and regulation of ROS, and then focus on small molecules that regulate ROS-related pathways and show efficacy in cancer therapy from the perspective of pharmacophores. Finally, we discuss several challenges in developing cancer therapeutic agents based on ROS regulation and propose the direction of future development.

Possible differences in the mechanism of malignant transformation of HaCaT cells by arsenite and its dimethyl metabolites, particularly dimethylthioarsenics

BACKGROUND

As a confirmed human carcinogen, arsenic can cause skin cancer, lung cancer, etc. However, its carcinogenic mechanism is still unclear. In recent years, the oxidative stress hypothesis has become widely accepted. In mammals it has been found that arsenic can be converted to dimethylarsinous acid (DMAIII) and dimethylmonothioarsinic acid (DMMTAV) through a series of methylation and redox reactions. DMAIII and DMMTAV are highly toxic.

METHODS

Human keratinocytes (HaCaT) were exposed to different concentrations of NaAsO2 (IAsIII), DMMTAV and DMAIII for 24 h. Reactive oxygen species (hydrogen peroxide and superoxide), oxidative damage markers (8-hydroxydeoxyguanosine and malondialdehyde), and antioxidant markers (glutathione and superoxide dismutase) were measured. In addition, sulfane sulfurs were measured in HaCaT cells and a cell-free system.

RESULTS

In the DMMTAV and DMAIII treatment groups, the levels of hydrogen peroxide and superoxide in HaCaT cells were higher than in the IAsIII treatment groups at the same dose. Levels of 8-OHdG and MDA in the DMMTAV and DMAIII treatment groups were also higher than those in the IAsIII treatment groups at the same dose. However, in the DMMTAV and DMAIII treatment groups, the levels of GSH and SOD activity were lower than that in the IAsIII treatment groups. In DMMTAV-treated HaCaT cells, sulfane sulfurs were produced. Further, it was found that DMMTAV could react with DMDTAV to form persulfide in the cell-free system, which may explain the mechanism of the formation of sulfane sulfurs in DMMTAV-treated HaCaT cells.

CONCLUSIONS

DMMTAV and DMAIII more readily induce reactive oxygen species (ROS) and cause oxidative damage in HaCaT cells than inorganic arsenic. Further, the persulfide formed by the reaction of DMMTAV and DMDTAV produced from the metabolism of DMMTAV may induce a stronger reductive defense mechanism than GSH against the intracellular oxidative stress of DMMTAV. However, the cells exposed to arsenite are transformed by the continuous nuclear translocation of Nrf2 due to oxidative stress, and the persulfide from dimethylthioarsenics may promote Nrf2 by the combination with thiol groups, especially redox control key protein, Keap1, eventually cause nuclear translocation of sustained Nrf2.

Dual Inhibition of Pyruvate Dehydrogenase Complex and Respiratory Chain Complex Induces Apoptosis by a Mitochondria-Targeted Fluorescent Organic Arsenical in vitro and in vivo

Based on the potential therapeutic value in targeting mitochondria and the fluorophore tracing ability, a fluorescent mitochondria-targeted organic arsenical PDT-PAO-F16 was fabricated, which not only visualized the cellular distribution, but also exerted anti-cancer activity in vitro and in vivo via targeting pyruvate dehydrogenase complex (PDHC) and respiratory chain complexes in mitochondria. In details, PDT-PAO-F16 mainly accumulated into mitochondria within hours and suppressed the activity of PDHC resulting in the inhibition of ATP synthesis and thermogenesis disorder. Moreover, the suppression of respiratory chain complex I and IV accelerated the mitochondrial dysfunction leading to caspase family-dependent apoptosis. In vivo, the acute promyelocytic leukemia was greatly alleviated in the PDT-PAO-F16 treated group in APL mice model. Our results demonstrated the organic arsenical precursor with fluorescence imaging and target-anticancer efficacy is a promising anticancer drug.

S-Dimethylarsino-glutathione (darinaparsin®) targets histone H3.3, leading to TRAIL-induced apoptosis in leukemia cells

Histone H3.3 was identified as an arsenic-binding protein of S-dimethylarsino-glutathione (ZIO-101, darinaparsin®) in leukemia cells by GE-ICP-MS, leading to TRAIL-induced apoptosis.

Differences in apoptotic signaling and toxicity between dimethylmonothioarsinic acid (DMMTAV) and its active metabolite, dimethylarsinous acid (DMAIII), in HepaRG cells: Possibility of apoptosis cascade based on diversity of active metabolites of DMMTAV

Dimethylmonothioarsinical acid (DMMTA^V), a metabolite of arsenosugars (AsSug) and arsenolipids (AsLP), which are major organoarsenicals contained in seafoods, has been a focus of our attention due to its toxicity. It has been reported that the toxicity of DMMTA^V differs according to the host cell type and that dimethylarsinous acid (DMA^III), which is a higher active metabolite of inorganic and organo arsenic compounds, may be the ultimate substance. To further elucidate the details of the mechanisms of DMMTA^V, we carried out toxicological characterization by comparing DMMTA^V and DMA^III using HepaRG cells, which are terminally differentiated hepatic cells derived from a human hepatic progenitor cell line that retains many characteristics, e.g, primary human hepatocytes including the morphology and expression of key metabolic enzymes (P450 s and GSTs, etc.) and complete expression of all nuclear receptors. HepaRG cells were induced to undergo differentiation by DMSO, which result red in increased levels of metabolic enzymes such as P450 and GST, in non-differentiated cells the cellular toxicities of DMMTA^V and DMA^III were reduced and the induction of toxicity by DMMTA^V was increased by GSH but not by DMA^III. Both DMA^III and DMMTA^V induce apoptosis and increase caspase 3/7 activity. DMA^III exposure increased the activity of caspase-9. On the contrary, DMMTA^V exposure resulted in markedly elevated activity of caspase-8 as well as caspase-9. These results suggest there are differences between the signaling pathways of apoptosis in DMA^III and DMMTA^V and that between their active metabolites. Consequently, the ultimate metabolic substance of toxicity induction of DMMTA^V may not only be DMA^III, but may also be partly due to other metabolic substances produced through the activation mechanism by GSH.

Selenium uptake through cystine transporter mediated by glutathione conjugation

Selenium (Se) is an essential trace element and is regarded as a protective agent against cancer. In particular, antioxidant effects of selenoenzymes contribute to cancer prevention. Se can also produce reactive oxygen species and, thereby, exert cancer-selective cytotoxicity. Selenodiglutathione (SDG) is a primary Se metabolite conjugated to two glutathione (GSH) moieties. SDG increases intracellular Se accumulation and is more toxic than selenous acid (H₂SeO₃), but the mechanisms for importing Se compounds into cells are not fully understood. Here, we propose a novel mechanism for importing Se, in the form of SDG. Cellular intake of Se compounds was assessed based on Se accumulation, as detected by ICP-MS. SDG incorporation was decreased in the presence of thiols (GSH, cysteine or their oxidized forms, GSSG and cystine), whereas H₂SeO₃ uptake was increased by addition of GSH or cysteine. Cellular SDG uptake was decreased by pretreatment with specific inhibitors against gamma-glutamyl transpeptidase (GGT) or the cystine/glutamate antiporter (system x_c^-). Furthermore, siRNA against xCT, which is the light chain component of system x_c^-, significantly decreased SDG incorporation. These data suggest an involvement of SDG in Se incorporation, with SDG processed at the cell surface by GGT, leading to formation of selenodicysteine which, in turn, is likely to be imported via xCT. Because GGT and xCT are highly expressed in cancer cells, these mechanisms mediated by the cystine transporter might underlie the cancer-selective toxicity of Se. In addition, the system described in our study appears to represent a physiological transport mechanism for the essential element Se.

Methylated and thiolated arsenic species for environmental and health research - A review on synthesis and characterization

Hundreds of millions of people around the world are exposed to elevated concentrations of inorganic and organic arsenic compounds, increasing the risk of a wide range of health effects. Studies of the environmental fate and human health effects of arsenic require authentic arsenic compounds. We summarize here the synthesis and characterization of more than a dozen methylated and thiolated arsenic compounds that are not commercially available. We discuss the methods of synthesis for the following 14 trivalent (III) and pentavalent (V) arsenic compounds: monomethylarsonous acid (MMA^III), dicysteinylmethyldithioarsenite (MMA^III(Cys)₂), monomethylarsonic acid (MMA^V), monomethylmonothioarsonic acid (MMMTA^V) or monothio-MMA^V, monomethyldithioarsonic acid (MMDTA^V) or dithio-MMA^V, monomethyltrithioarsonate (MMTTA^V) or trithio-MMA^V, dimethylarsinous acid (DMA^III), dimethylarsino-glutathione (DMA^III(SG)), dimethylarsinic acid (DMA^V), dimethylmonothioarsinic acid (DMMTA^V) or monothio-DMA^V, dimethyldithioarsinic acid (DMDTA^V) or dithio-DMA^V, trimethylarsine oxide (TMAO^V), arsenobetaine (AsB), and an arsenicin-A model compound. We have reviewed and compared the available methods, synthesized the arsenic compounds in our laboratories, and provided characterization information. On the basis of reaction yield, ease of synthesis and purification of product, safety considerations, and our experience, we recommend a method for the synthesis of each of these arsenic compounds.

Pharmacological GLI2 inhibition prevents myofibroblast cell-cycle progression and reduces kidney fibrosis

Chronic kidney disease is characterized by interstitial fibrosis and proliferation of scar-secreting myofibroblasts, ultimately leading to end-stage renal disease. The hedgehog (Hh) pathway transcriptional effectors GLI1 and GLI2 are expressed in myofibroblast progenitors; however, the role of these effectors during fibrogenesis is poorly understood. Here, we demonstrated that GLI2, but not GLI1, drives myofibroblast cell-cycle progression in cultured mesenchymal stem cell-like progenitors. In animals exposed to unilateral ureteral obstruction, Hh pathway suppression by expression of the GLI3 repressor in GLI1+ myofibroblast progenitors limited kidney fibrosis. Myofibroblast-specific deletion of Gli2, but not Gli1, also limited kidney fibrosis, and induction of myofibroblast-specific cell-cycle arrest mediated this inhibition. Pharmacologic targeting of this pathway with darinaparsin, an arsenical in clinical trials, reduced fibrosis through reduction of GLI2 protein levels and subsequent cell-cycle arrest in myofibroblasts. GLI2 overexpression rescued the cell-cycle effect of darinaparsin in vitro. While darinaparsin ameliorated fibrosis in WT and Gli1-KO mice, it was not effective in conditional Gli2-KO mice, supporting GLI2 as a direct darinaparsin target. The GLI inhibitor GANT61 also reduced fibrosis in mice. Finally, GLI1 and GLI2 were upregulated in the kidneys of patients with high-grade fibrosis. Together, these data indicate that GLI inhibition has potential as a therapeutic strategy to limit myofibroblast proliferation in kidney fibrosis.

The novel organic arsenical darinaparsin induces MAPK-mediated and SHP1-dependent cell death in T-cell lymphoma and Hodgkin lymphoma cells and human xenograft models

PURPOSE

Darinaparsin (Zio-101) is a novel organic arsenical compound with encouraging clinical activity in relapsed/refractory T-cell lymphoma (TCL) and Hodgkin lymphoma (HL); however, little is known about its mechanism of action.

EXPERIMENTAL DESIGN

TCL cell lines (Jurkat, Hut78, and HH) and HL cell lines (L428, L540, and L1236) were examined for in vitro cell death by MTT assay and Annexin V-based flow cytometry. Jurkat and L540-derived xenografts in SCID mice were examined for in vivo tumor inhibition and survival. Biologic effects of darinaparsin on the MAPK pathway were investigated using pharmacologic inhibitors, RNAi and transient transfection for overexpression for SHP1 and MEK.

RESULTS

Darinaparsin treatment resulted in time- and dose-dependent cytotoxicity and apoptosis in all TCL and HL cell lines. In addition, darinaparsin had more rapid, higher, and sustained intracellular arsenic levels compared with arsenic trioxide via mass spectrometry. In vivo experiments with Jurkat (TCL) and L540 (HL)-derived lymphoma xenografts showed significant inhibition of tumor growth and improved survival in darinaparsin-treated SCID mice. Biologically, darinaparsin caused phosphorylation of ERK (and relevant downstream substrates) primarily by decreasing the inhibitory SHP1 phosphatase and coimmunoprecipitation showed significant ERK/SHP1 interaction. Furthermore, ERK shRNA knockdown or constitutive overexpression of SHP1 resulted in increased apoptosis, whereas cotreatment with pharmacologic MEK inhibitors resulted in synergistic cell death. Conversely, SHP1 blockade (via pharmacologic inhibition or RNAi) and MEK constitutive activation decreased darinaparsin-related cell death.

CONCLUSIONS

Altogether, these data show that darinaparsin is highly active in HL and TCL and its activity is dependent primarily on MAPK mechanisms.

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.

Triple A therapy: the molecular underpinnings of the unique sensitivity of leukemic promyelocytes to anthracyclines, all-trans-retinoic acid and arsenic trioxide

If looking for a mnemonic to remember the relevant facts about acute promyelocytic leukemia (APL), one just has to remember that APL is a disease of A's. It is acute and it is highly sensitive to treatment with anthracyclines, all-trans-retinoic acid (RA) and arsenic trioxide (ATO). The presence of fusions involving the retinoic acid receptor alpha (RARA) is without question the central player driving APL and dictating the response of this disease to these therapeutic agents. However, beyond this knowledge, the molecular mechanisms that contribute to the complicated pathogenesis and the response to treatment of APL are not completely defined. As more is understood about this hematological malignancy, there are more opportunities to refine and improve treatment based on this knowledge. In this review article, we discuss the response of APL to these "A" therapies.

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).

Role of arsenic trioxide in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is a unique subtype of acute myeloid leukemia that is characterized by distinct clinical, morphological, cytogenetic, and molecular abnormalities. It is associated with a striking risk of early hemorrhagic death due to disseminated intravascular coagulation and hyperfibrinolysis. The prognosis of APL has improved dramatically following the introduction of all-trans retinoic acid (ATRA) and its combination with anthracycline-based chemotherapy during induction and consolidation. Patients with high-risk APL, defined by a white cell count >10 × 10(9)/L at diagnosis, also appear to benefit from the addition of intermediate- or high-dose cytarabine during consolidation. Arsenic trioxide (ATO) has proved to be even more effective than ATRA as a single agent, and is now routinely used for the treatment of the 20%-30% of patients who manifest disease relapse after initial treatment with ATRA and chemotherapy. ATO has a toxicity profile that differs considerably from that of both ATRA and cytotoxic chemotherapy, and accordingly presents its own specific challenges during treatment. Optimizing a strategy for the incorporation of ATO into initial therapy is currently the focus of several cooperative group trials, with an emphasis on minimizing or even eradicating the use of chemotherapy. ATRA plus ATO without chemotherapy appears to be adequate during induction and consolidation for patients with standard-risk APL, but triple therapy that includes limited anthracycline or gemtuzumab ozogamicin (GO) during induction is required for high-risk APL. Uncertainty still exists regarding the minimum amount of chemotherapy and number of consolidation cycles necessary, the optimal scheduling of ATO, and the potential utility of oral ATO administration. Although prolonged oral maintenance therapy is usually included in most current APL treatment protocols, its value remains controversial, and the superior anti-leukemic efficacy of ATO-based therapy may facilitate its elimination in the future.

The novel arsenical darinaparsin is transported by cystine importing systems

Darinaparsin (Dar; ZIO-101; S-dimethylarsino-glutathione) is a promising novel organic arsenical currently undergoing clinical studies in various malignancies. Dar consists of dimethylarsenic conjugated to glutathione (GSH). Dar induces more intracellular arsenic accumulation and more cell death than the FDA-approved arsenic trioxide (ATO) in vitro, but exhibits less systemic toxicity. Here, we propose a mechanism for Dar import that might explain these characteristics. Structural analysis of Dar suggests a putative breakdown product: dimethylarsino-cysteine (DMAC). We show that DMAC is very similar to Dar in terms of intracellular accumulation of arsenic, cell cycle arrest, and cell death. We found that inhibition of γ-glutamyl-transpeptidase (γ-GT) protects human acute promyelocytic leukemia cells (NB4) from Dar, but not from DMAC, suggesting a role for γ-GT in the processing of Dar. Overall, our data support a model where Dar, a GSH S-conjugate, is processed at the cell surface by γ-GT, leading to formation of DMAC, which is imported via xCT, xAG, or potentially other cystine/cysteine importing systems. Further, we propose that Dar induces its own import via increased xCT expression. These mechanisms may explain the enhanced toxicity of Dar toward cancer cells compared with ATO.

The novel arsenical Darinaparsin circumvents BRG1-dependent, HO-1-mediated cytoprotection in leukemic cells

Darinaparsin (Dar) is a more potent cytotoxic arsenical than arsenic trioxide (ATO). We hypothesized that the increased cytotoxicity of Dar may be because of a decreased cytoprotective response. We observed that, unlike ATO, Dar does not induce heme oxygenase-1 (HO-1), even though it induces expression of other nuclear factor (erythroid-derived 2)-like 2 (NRF2)-dependent detoxifying enzymes to a greater extent than ATO, in both cancer cell lines and patient-derived leukemic cells. This strengthens the emerging evidence, showing that response to reactive oxygen species (ROS) is stimuli specific. Dar treatment prevents recruitment of the transcriptional coregulator Brahma-related gene 1 (BRG1) to the HMOX1 promoter, which is required for HMOX1 expression. The inability of Dar to induce HO-1 correlates with arrest in G2/M cell cycle phase and BRG1 phosphorylation. Inhibition of HO-1 increases the toxicity of ATO, but has no effect on Dar-induced apoptosis. Accordingly, the lack of HO-1 induction is involved in Dar's enhanced antileukemic properties. Our data highlight cytoprotective responses mediated by HO-1 and BRG1 as a novel target for enhancing the therapeutic range of arsenicals.

Multidrug resistance protein 1 (ABCC1) confers resistance to arsenic compounds in human myeloid leukemic HL-60 cells

Arsenic trioxide (As(2)O(3)) is established as one of the most effective drugs for treatment of patients with acute promyelocytic leukemia, as well as other types of malignant tumors. However, HL-60 cells are resistant to As(2)O(3), and little is known about the underlying resistance mechanism for As(2)O(3) and its biomethylation products, namely, monomethylarsonous acid (MMA(III)) on the treatment of tumors. In the present study, we investigated the molecular mechanisms underlying iAs(III) and its intermediate metabolite MMA(III)-induced anticancer effects in the HL-60 cells. Here, we show that the HL-60 cells exhibit resistance to inorganic iAs(III) (IC(50) = 10 μM), but are relatively sensitive to its intermediate MMA(III) (IC(50) = 3.5 μM). Moreover, we found that the multidrug resistance protein 1 (MRP1), but not MRP2, is expressed in HL-60 cells, which reduced the intracellular arsenic accumulation, and conferred resistance to inorganic iAs(III) and MMA(III). Pretreatment of HL-60 with MK571, an inhibitor of MRP1, significantly increased iAs(III) and MMA(III)-induced cytotoxicity and arsenic accumulations, suggesting that the expression of MRP1/4 may lead to HL-60 cells resistance to trivalent arsenic compounds.

Factors determining sensitivity and resistance of tumor cells to arsenic trioxide

Previously, arsenic trioxide showed impressive regression rates of acute promyelocytic leukemia. Here, we investigated molecular determinants of sensitivity and resistance of cell lines of different tumor types towards arsenic trioxide. Arsenic trioxide was the most cytotoxic compound among 8 arsenicals investigated in the NCI cell line panel. We correlated transcriptome-wide microarray-based mRNA expression to the IC(50) values for arsenic trioxide by bioinformatic approaches (COMPARE and hierarchical cluster analyses, Ingenuity signaling pathway analysis). Among the identified pathways were signaling routes for p53, integrin-linked kinase, and actin cytoskeleton. Genes from these pathways significantly predicted cellular response to arsenic trioxide. Then, we analyzed whether classical drug resistance factors may also play a role for arsenic trioxide. Cell lines transfected with cDNAs for catalase, thioredoxin, or the anti-apoptotic bcl-2 gene were more resistant to arsenic trioxide than mock vector transfected cells. Multidrug-resistant cells overexpressing the MDR1, MRP1 or BCRP genes were not cross-resistant to arsenic trioxide. Our approach revealed that response of tumor cells towards arsenic trioxide is multi-factorial.

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.

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.

Targeting PKC delta-mediated topoisomerase II beta overexpression subverts the differentiation block in a retinoic acid-resistant APL cell line

Retinoic acid (RA) relieves the maturation block in t(15:17) acute promyelocytic leukemia (APL), leading to granulocytic differentiation. However, RA treatment alone invariably results in RA resistance, both in vivo and in vitro. RA-resistant cell lines have been shown to serve as useful models for elucidation of mechanisms of resistance. Previously, we identified topoisomerase II beta (TOP2B) as a novel mediator of RA-resistance in APL cell lines. In this study, we show that both TOP2B protein stability and activity are regulated by a member of the protein kinase C (PRKC) family, PRKC delta (PRKCD). Co-treatment with a pharmacologic inhibitor of PRKCD and RA resulted in the induction of an RA responsive reporter construct, as well as the endogenous RA target genes, CEBPE, CYP26A1 and RIG-I. Furthermore, the co-treatment overcame the differentiation block in RA-resistant cells, as assessed by morphological analysis, restoration of promyelocytic leukemia nuclear bodies, induction of CD11c cell surface expression and an increase in nitro-blue-tetrazolium reduction. Cumulatively, our data suggest a model whereby inhibition of PRKCD decreases TOP2B protein levels, leading to a loss of TOP2B-mediated repressive effects on RA-induced transcription and granulocytic differentiation.

Darinaparsin: a novel organic arsenical with promising anticancer activity

Darinaparsin is an organic arsenical composed of dimethylated arsenic linked to glutathione, and is being investigated for antitumor properties in vitro and in vivo. While other arsenicals, including arsenic trioxide, have been used clinically, none have shown significant activity in malignancies outside of acute promyelocytic leukemia. Darinaparsin has significant activity in a broad spectrum of hematologic and solid tumors in preclinical models. Here, we review the literature describing the signaling pathways and mechanisms of action of darinaparsin and compare them to mechanisms of cell death induced by arsenic trioxide. Darinaparsin has overlapping, but distinct, signaling mechanisms. We also review the current results of clinical trials with darinaparsin (both intravenous and oral formulations) that demonstrate significant antitumor activity.

Redox-directed cancer therapeutics: molecular mechanisms and opportunities

Redox dysregulation originating from metabolic alterations and dependence on mitogenic and survival signaling through reactive oxygen species represents a specific vulnerability of malignant cells that can be selectively targeted by redox chemotherapeutics. This review will present an update on drug discovery, target identification, and mechanisms of action of experimental redox chemotherapeutics with a focus on pro- and antioxidant redox modulators now in advanced phases of preclinal and clinical development. Recent research indicates that numerous oncogenes and tumor suppressor genes exert their functions in part through redox mechanisms amenable to pharmacological intervention by redox chemotherapeutics. The pleiotropic action of many redox chemotherapeutics that involves simultaneous modulation of multiple redox sensitive targets can overcome cancer cell drug resistance originating from redundancy of oncogenic signaling and rapid mutation.Moreover, some redox chemotherapeutics may function according to the concept of synthetic lethality (i.e., drug cytotoxicity is confined to cancer cells that display loss of function mutations in tumor suppressor genes or upregulation of oncogene expression). The impressive number of ongoing clinical trials that examine therapeutic performance of novel redox drugs in cancer patients demonstrates that redox chemotherapy has made the crucial transition from bench to bedside.

A phase I clinical trial of darinaparsin in patients with refractory solid tumors

PURPOSE

Darinaparsin, an organic arsenic, targets essential cell survival pathways. We determined the dose-limiting toxicity (DLT) and maximum tolerated dose of darinaparsin in patients with advanced cancer.

EXPERIMENTAL DESIGN

Patients with solid malignancies refractory to conventional therapies were treated with i.v. darinaparsin daily for 5 days every 4 weeks. The starting dose (78 mg/m(2)) escalated to 109, 153, 214, 300, 420, and 588 mg/m(2). A conventional "3 + 3" design was used.

RESULTS

Forty patients (median age, 61.5 years; median number of prior therapies, 5) received therapy; 106 cycles were given (median, 2; range, 1-12). Twenty patients reported no drug-related toxicities. No DLTs were reported at a dose of <420 mg/m(2). At 588 mg/m(2), two of four patients developed DLTs, including grade 3 altered mental status and ataxia. Of four patients treated at the de-escalated dose of 500 mg/m(2), one developed similar toxicities. De-escalating the dose to 420 mg/m(2) (n = 8) resulted in two neurologic DLTs. Further de-escalation to 300 mg/m(2) (n = 3) resulted in no drug-related toxicities. Arsenic plasma levels peaked on treatment day 3, plateaued on day 5, and returned to baseline on day 7. Plasma levels varied within cohorts but increased with increasing doses. The median plasma arsenic half-life was 16.2 hours. Seven (17.5%) patients had stable disease for > or =4 months (median, 6; range, 4-11), including 4 of 17 with colorectal and 2 of 3 with renal cancer.

CONCLUSIONS

The recommended dose for phase II trials is 300 mg/m(2) i.v. given daily for 5 days every 4 weeks.

Phase II study of darinaparsin in patients with advanced hepatocellular carcinoma

BACKGROUND

Darinaparsin is a novel organic arsenic that reaches higher intracellular concentration with decreased toxicity compared to inorganic arsenic. We conducted a multi-center phase II study with darinaparsin in patients with advanced HCC.

METHODS

Eligibility criteria included unresectable or metastatic measurable HCC, up to two prior systemic treatments, ECOG performance status < or = 2, Child Pugh Class A or B and adequate organ functions. Darinaparsin was administered at 420 mg/m(2) intravenously, twice weekly at least 72 h apart for 3 weeks in a 4-week cycle. The primary end point was response rate. A Simon two-stage design was used.

RESULTS

Among 15 patients in the first stage, no objective responses were observed. Two patients had stable disease. The median number of cycles on study per patient was 2 (1-6). The median progression free survival and overall survival were 55 days (95% confidence interval: 50-59) and 190 days (95% confidence interval: 93-227), respectively. No treatment related hospitalizations or deaths occurred. Treatment related grade 1-2 toxicities included nausea, vomiting (26.7% each), fatigue (20%), anorexia and diarrhea (13.3% each). Grade 3 anorexia, wheezing, agitation, abdominal pain and SGPT were observed in 1 patient each (6.7%). One patient experienced grade 4 hypoglycemia (6.7%).

CONCLUSIONS

Darinaparsin could be safely administered with tolerable toxicity profiles, and no QTc prolongation in patients with advanced HCC. However, at this dose and schedule, it has shown no objective responses in HCC and this trial was terminated as planned after the first stage of efficacy analysis.

Darinaparsin induces a unique cellular response and is active in an arsenic trioxide-resistant myeloma cell line

Here, we report on the organic arsenical darinaparsin (ZIO-101, S-dimethylarsino-glutathione) and its anti-myeloma activity compared with inorganic arsenic trioxide. Darinaparsin induced apoptosis in multiple myeloma cell lines in a dose-dependent manner, and the addition of N-acetylcysteine, which increases intracellular glutathione (GSH), blocked cytotoxicity of both darinaparsin and arsenic trioxide. In contrast to arsenic trioxide, intracellular GSH does not appear to be important for darinaparsin metabolism, as an inhibitor of GSH synthesis, buthionine sulfoximine, had little effect on drug activity. This discrepancy was resolved when we determined the effects of thiols on drug uptake. The addition of exogenous GSH, L-cysteine, or D-cysteine prevented darinaparsin cellular uptake and cell death but had no effect on the uptake or activity of arsenic trioxide, suggesting a difference in the transport mechanism of these two drugs. In addition, gene expression profiling revealed differences in the signaling of protective responses between darinaparsin and arsenic trioxide. Although both arsenicals induced a transient heat shock response, only arsenic trioxide treatment induced transcription of metal response genes and anti-oxidant genes related to the Nrf2-Keap1 pathway. In contrast to the protective responses, both arsenicals induced up-regulation of BH3-only proteins. Moreover, silencing of BH3-only proteins Noxa, Bim, and Bmf protected myeloma cells from darinaparsin-induced cell death. Finally, treatment of an arsenic trioxide-resistant myeloma cell line with darinaparsin resulted in dose-dependent apoptosis, indicating that cross-resistance does not necessarily develop between these two forms of arsenic in multiple myeloma cell lines. These results suggest darinaparsin may be useful as an alternative treatment in arsenic trioxide-resistant hematologic cancers.

Effect of arsenic on regulatory T cells

INTRODUCTION

Arsenic (As) affects the function and survival of lymphocytes, and some arsenic compounds exert a relevant antineoplastic effect. We have explored the effect of As on T regulatory cells.

RESULTS AND DISCUSSION

In vitro experiments with peripheral blood mononuclear cells from healthy subjects showed that low concentrations of As tended to increase the number of natural T regulatory (nTreg) lymphocytes, whereas concentrations >5.0 muM had an opposite effect. Furthermore, rats exposed to As showed redistribution of nTreg cells, and As administration to rats with experimental allergic encephalomyelitis increased the levels of nTreg cells in spleen and diminished the severity of this condition. On the other hand, in 47 apparently healthy subjects chronically exposed to As, we found significant inverse correlation between urinary As levels and the number and function of nTreg lymphocytes. Although most of these individuals showed enhanced levels of apoptotic lymphocytes in peripheral blood, with a diminution of mitochondrial membrane potential, no significant correlation between these parameters and urinary As was detected.

CONCLUSION

Our data indicate that As seems to have a relevant and complex effect on nTreg cells.