Phase II study of Triapine in patients with metastatic renal cell carcinoma: a trial of the National Cancer Institute of Canada Clinical Trials Group (NCIC IND.161)

Ironing out the role of the cyclin-dependent kinase inhibitor, p21 in cancer: Novel iron chelating agents to target p21 expression and activity

Iron (Fe) has become an important target for the development of anti-cancer therapeutics with a number of Fe chelators entering human clinical trials for advanced and resistant cancer. An important aspect of the activity of these compounds is their multiple molecular targets, including those that play roles in arresting the cell cycle, such as the cyclin-dependent kinase inhibitor, p21. At present, the exact mechanism by which Fe chelators regulate p21 expression remains unclear. However, recent studies indicate the ability of chelators to up-regulate p21 at the mRNA level was dependent on the chelator and cell-type investigated. Analysis of the p21 promoter identified that the Sp1-3-binding site played a significant role in the activation of p21 transcription by Fe chelators. Furthermore, there was increased Sp1/ER-α and Sp1/c-Jun complex formation in melanoma cells, suggesting these complexes were involved in p21 promoter activation. Elucidating the mechanisms involved in the regulation of p21 expression in response to Fe chelator treatment in neoplastic cells will further clarify how these agents achieve their anti-tumor activity. It will also enhance our understanding of the complex roles p21 may play in neoplastic cells and lead to the development of more effective and specific anti-cancer therapies.

Synthesis and biological evaluation of biotin-conjugated anticancer thiosemicarbazones and their iron(III) and copper(II) complexes

Triapine, the most prominent anticancer drug candidate from the substance class of thiosemicarbazones, was investigated in >30 clinical phase I and II studies. However, the results were rather disappointing against solid tumors, which can be explained (at least partially) due to inefficient delivery to the tumor site. Hence, we synthesized the first biotin-functionalized thiosemicarbazone derivatives in order to increase tumor specificity and accumulation. Additionally, for Triapine and one biotin conjugate the iron(III) and copper(II) complexes were prepared. Subsequently, the novel compounds were biologically evaluated on a cell line panel with different biotin uptake. The metal-free biotin-conjugated ligands showed comparable activity to the reference compound Triapine. However, astonishingly, the metal complexes of the biotinylated derivative showed strikingly decreased anticancer activity. To further analyze possible differences between the metal complexes, detailed physico- and electrochemical experiments were performed. However, neither lipophilicity or complex solution stability, nor the reduction potential or behavior in the presence of biologically relevant reducing agents showed strong variations between the biotinylated and non-biotinylated derivatives (only some differences in the reduction kinetics were observed). Nonetheless, the metal-free biotin-conjugate of Triapine revealed distinct activity in a colon cancer mouse model upon oral application comparable to Triapine. Therefore, this type of biotin-conjugated thiosemicarbazone is of interest for further synthetic strategies and biological studies.

New Water-Soluble Copper(II) Complexes with Morpholine-Thiosemicarbazone Hybrids: Insights into the Anticancer and Antibacterial Mode of Action

Six morpholine-(iso)thiosemicarbazone hybrids HL¹–HL⁶ and their Cu(II) complexes with good-to-moderate solubility and stability in water were synthesized and characterized. Cu(II) complexes [Cu(L^1–6)Cl] (1–6) formed weak dimeric associates in the solid state, which did not remain intact in solution as evidenced by ESI-MS. The lead proligands and Cu(II) complexes displayed higher antiproliferative activity in cancer cells than triapine. In addition, complexes 2–5 were found to specifically inhibit the growth of Gram-positive bacteria Staphylococcus aureus with MIC₅₀ values at 2–5 μg/mL. Insights into the processes controlling intracellular accumulation and mechanism of action were investigated for 2 and 5, including the role of ribonucleotide reductase (RNR) inhibition, endoplasmic reticulum stress induction, and regulation of other cancer signaling pathways. Their ability to moderately inhibit R2 RNR protein in the presence of dithiothreitol is likely related to Fe chelating properties of the proligands liberated upon reduction.

Understanding the metabolism of the anticancer drug Triapine: electrochemical oxidation, microsomal incubation and in vivo analysis using LC-HRMS

α-N-Heterocyclic thiosemicarbazones are among the most promising ribonucleotide reductase inhibitors identified so far. Triapine, the most prominent representative of this class of substances, has been investigated in multiple phase I and II clinical trials. With regard to clinical practice, Triapine showed activity against hematological diseases, but ineffectiveness against a variety of solid tumors. However, the reasons are still vague and the amount of ADME (absorption, distribution, metabolism and excretion) data for Triapine available in the literature is very limited. Therefore, different analytical tools were used to investigate the metabolism of Triapine including electrochemical oxidations, liver microsomes and in vivo samples from mice. The main metabolic reactions, observed by all three methods, were dehydrogenation and hydroxylations, confirming that electrochemistry, as a purely instrumental approach, can be applied for the simulation of metabolic pathways. The dehydrogenated metabolite M1 was identified as a thiadiazole ring-closed oxidation product of Triapine. From a biological point of view, M1, as a key metabolite, is of interest since the crucial chemical property of α-N-heterocyclic thiosemicarbazones to bind metal ions is lost and cytotoxicity studies showed no anticancer activity of M1. The in vivo data of the urine samples revealed very high levels of the metabolites and Triapine itself already 15 min after treatment. This clearly indicates that Triapine is rapidly metabolised and excreted, which represents an important step forward to understand the possible reason for the inefficiency of Triapine against solid tumors.

Bio-inspired melanin nanoparticles induce cancer cell death by iron adsorption

Dysregulation of iron metabolism is a common characteristic of cancer cells. The rapid proliferation of the tumour cells means that there is an increased dependence upon iron compared to healthy cells. Chelation of iron can be undertaken with a number of different compounds, however, simply lowering systemic iron levels to control tumour growth is not possible since iron is essential for cellular metabolism in the rest of the body. Nanoparticulate iron chelators could overcome this difficulty by targeting to the tumour either by the passive enhanced permeation and retention effect, or by targeting ligands on the surface. Nanoparticles were prepared from melanin, which is a naturally occurring pigment that is widely distributed within the body, but that can chelate iron. The prepared nanoparticles were shown to be ~220 nm, and could adsorb 16.45 mmoles iron/g melanin. The nanoparticles showed no affect on control fibroblast cells at a concentration of 200 μM, whereas the immortalised cancer cell lines showed at least 56% reduction in cell growth. At a concentration of 1 mM melanin nanoparticles the cell growth could be reduced by 99% compared to the control. The nanoparticles also show no significant haemotoxicity, even at concentration of 500 μM. Melanin nanoparticles are therefore a viable prospect for destroying cancer cells via iron starvation.

The thiosemicarbazone Me2NNMe2 induces paraptosis by disrupting the ER thiol redox homeostasis based on protein disulfide isomerase inhibition

Due to their high biological activity, thiosemicarbazones have been developed for treatment of diverse diseases, including cancer, resulting in multiple clinical trials especially of the lead compound Triapine. During the last years, a novel subclass of anticancer thiosemicarbazones has attracted substantial interest based on their enhanced cytotoxic activity. Increasing evidence suggests that the double-dimethylated Triapine derivative Me₂NNMe₂ differs from Triapine not only in its efficacy but also in its mode of action. Here we show that Me₂NNMe₂- (but not Triapine)-treated cancer cells exhibit all hallmarks of paraptotic cell death including, besides the appearance of endoplasmic reticulum (ER)-derived vesicles, also mitochondrial swelling and caspase-independent cell death via the MAPK signaling pathway. Subsequently, we uncover that the copper complex of Me₂NNMe₂ (a supposed intracellular metabolite) inhibits the ER-resident protein disulfide isomerase, resulting in a specific form of ER stress based on disruption of the Ca²⁺ and ER thiol redox homeostasis. Our findings indicate that compounds like Me₂NNMe₂ are of interest especially for the treatment of apoptosis-resistant cancer and provide new insights into mechanisms underlying drug-induced paraptosis.

Nanoparticle-based delivery of an anti-proliferative metal chelator to tumor cells

This paper describes the preparation and characterization of polymeric nanoparticles loaded with a potent anti-tumor metal chelator, Di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT) for delivery to cancer cells. Metal chelators have been increasingly studied for their anti-cancer properties that rely on the high demand of neoplastic cells for iron. Dp44mT has previously shown great antiproliferative characteristics in several cancers including breast cancer and melanoma. To further expand the application of this highly cytotoxic agent for cancer treatment and to enable its specific delivery to malignant cells, here we apply nano-scale particles (NPs) of biodegradable poly(lactic-co-glycolide) (PLGA) for encapsulation of Dp44mT and evaluate its effectiveness in vitro. The results demonstrated that Dp44mT was efficiently encapsulated in PLGA particles. Resulting NPs were uniform in size and shape and had good colloidal stability. Moreover, Dp44mT encapsulation in PLGA enhanced the water solubility of this agent. Lastly, the present formulation showed high level of cytotoxicity in glioma cells. Together, these results show the potential of PLGA NPs as a nano-carrier for Dp44mT with no apparent impact on the anti-tumor activity of this compound.

Four Cu(ii) complexes based on antitumor chelators: synthesis, structure, DNA binding/damage, HSA interaction and enhanced cytotoxicity

Four novel copper(ii) complexes [Cu(II)(Bp4mT)(μ-Cl)]2 (), [Cu(II)(μ-Bp4mT)Br]2 (), [Cu(II)(HBpT)Cl] (), and [Cu(II)(HBpT)Br] () (Bp4mT = 2-benzoylpyridine-4-methylthiosemicarbazone, HBpT = 2-benzoylpyridine thiosemicarbazone), were synthesized and characterized using single-crystal X-ray diffraction, elemental analysis, infrared, and ultraviolet-visible spectroscopy. X-ray analysis revealed that complexes and based on the Bp4mT ligand presented dimeric structures in which the Cu(ii) ions were located in a five-coordinated distorted square-pyramidal geometry, whereas both and complexes were mononuclear with the Cu(ii) ions exhibiting a tetracoordinated square planar configuration. Their interactions with calf thymus DNA (CT-DNA) were investigated using viscosity measurements and fluorescence spectroscopy. Multispectroscopic evidence has shown interactions between these complexes and human serum albumin (HSA). All these complexes have exhibited efficient oxidative cleavage of supercoiled DNA in the presence of hydrogen peroxide, presumably via an oxidative mechanism. Furthermore, in vitro cytotoxicity studies of against human liver hepatocellular carcinoma cells (HepG-2), human large cell lung carcinoma cells (NCI-H460), and human cervical carcinoma cells (HeLa) indicated their promising antitumor activity with quite low IC50 values in the range of 0.08-1.98 μM, which are 83 times lower than those of cisplatin. The mechanistic studies revealed that four complexes, which induced early apoptosis, were involved in reactive oxygen species generation and DNA cleavage for their antitumor activities.

Piperazinyl fragment improves anticancer activity of Triapine

A new class of TSCs containing piperazine (piperazinylogs) of Triapine, was designed to fulfill the di-substitution pattern at the TSCs N4 position, which is a crucial prerequisite for the high activity of the previously obtained TSC compounds–DpC and Dp44mT. We tested the important physicochemical characteristics of the novel compounds L¹-L¹². The studied ligands are neutral at physiological pH, which allows them to permeate cell membranes and bind cellular Fe pools more readily than less lipid-soluble ligands, e.g. DFO. The selectivity and anti-cancer activity of the novel TSCs were examined in a variety of cancer cell types. In general, the novel compounds demonstrated the greatest promise as anti-cancer agents with both a potent and selective anti-proliferative activity. We investigated the mechanism of action more deeply, and revealed that studied compounds inhibit the cell cycle (G1/S phase). Additionally we detected apoptosis, which is dependent on cell line’s specific genetic profile. Accordingly, structure-activity relationship studies suggest that the combination of the piperazine ring with Triapine allows potent and selective anticancer chelators that warrant further in vivo examination to be identified. Significantly, this study proved the importance of the di-substitution pattern of the amine N4 function.

Selectivity of a thiosemicarbazonatocopper(ii) complex towards duplex RNA. Relevant noncovalent interactions both in solid state and solution

Thiosemicarbazones and their metal derivatives have long been screened as antitumor agents, and their interactions with DNA have been analysed. Herein, we describe the synthesis and characterization of compounds containing [CuL]⁺ entities (HL = pyridine-2-carbaldehyde thiosemicarbazone) and adenine, cytosine or 9-methylguanine, and some of their corresponding nucleotides. For the first time, crystal structures of adenine- and 9-methylguanine-containing thiosemicarbazone complexes are reported. To the best of our knowledge, the first study on the affinity thiosemicarbazone-RNA is also provided here. Experimental and computational studies have shown that [CuL(OH₂)]⁺ entities at low concentration intercalate into dsRNA poly(rA)·poly(rU) through strong hydrogen bonds involving uracil residues and π-π stacking interactions. In fact, noncovalent interactions are present both in the solid state and in solution. This behaviour diverges from that observed with DNA duplexes and creates an optimistic outlook in achieving selective binding to RNA for subsequent possible medical applications.

Mammalian Cell Division in 3D Matrices via Quantitative Confocal Reflection Microscopy

The study of how mammalian cell division is regulated in a 3D environment remains largely unexplored despite its physiological relevance and therapeutic significance. Possible reasons for the lack of exploration are the experimental limitations and technical challenges that render the study of cell division in 3D culture inefficient. Here, we describe an imaging-based method to efficiently study mammalian cell division and cell-matrix interactions in 3D collagen matrices. Cells labeled with fluorescent H2B are synchronized using the combination of thymidine blocking and nocodazole treatment, followed by a mechanical shake-off technique. Synchronized cells are then embedded into a 3D collagen matrix. Cell division is monitored using live-cell microscopy. The deformation of collagen fibers during and after cell division, which is an indicator of cell-matrix interaction, can be monitored and quantified using quantitative confocal reflection microscopy. The method provides an efficient and general approach to study mammalian cell division and cell-matrix interactions in a physiologically relevant 3D environment. This approach not only provides novel insights into the molecular basis of the development of normal tissue and diseases, but also allows for the design of novel diagnostic and therapeutic approaches.

Thiosemicarbazone-based selective proliferation inactivators inhibit gastric cancer cell growth, invasion, and migration

A series of novel thiosemicarbazone derivatives were synthesized and evaluated for their antiproliferative activity against several selected tumor cell lines of different origins using the MTT assay. The preliminary results indicated that the MGC-803 cell line was remarkably sensitive to all the synthesized compounds. Among this series, compound 5n showed the best inhibitory activity with an IC₅₀ value of 0.93 μM (about 10-fold more potent than 3-AP) against MGC-803. Further mechanism studies revealed that compound 5n could obviously inhibit the proliferation of MGC-803 cells by inducing apoptosis and arresting the cell cycle at the S phase. Compound 5n also showed marked inhibition of cell migration and invasion, without significant cytotoxicity against gastric epithelial immortalized GES-1 cells.

Clinical pharmacology and clinical trials of ribonucleotide reductase inhibitors: is it a viable cancer therapy?

PURPOSE

Ribonucleotide reductase (RR) enzymes (RR1 and RR2) play an important role in the reduction of ribonucleotides to deoxyribonucleotides which is involved in DNA replication and repair. Augmented RR activity has been ascribed to uncontrolled cell growth and tumorigenic transformation.

METHODS

This review mainly focuses on several biological and chemical RR inhibitors (e.g., siRNA, GTI-2040, GTI-2501, triapine, gemcitabine, and clofarabine) that have been evaluated in clinical trials with promising anticancer activity from 1960's till 2016. A summary on whether their monotherapy or combination is still effective for further use is discussed.

RESULTS

Among the RR2 inhibitors evaluated, GTI-2040, siRNA, gallium nitrate and didox were more efficacious as a monotherapy, whereas triapine was found to be more efficacious as combination agent. Hydroxyurea is currently used more in combination therapy, even though it is efficacious as a monotherapy. Gallium nitrate showed mixed results in combination therapy, while the combination activity of didox is yet to be evaluated. RR1 inhibitors that have long been used in chemotherapy such as gemcitabine, cladribine, fludarabine and clofarabine are currently used mostly as a combination therapy, but are equally efficacious as a monotherapy, except tezacitabine which did not progress beyond phase I trials.

CONCLUSIONS

Based on the results of clinical trials, we conclude that RR inhibitors are viable treatment options, either as a monotherapy or as a combination in cancer chemotherapy. With the recent advances made in cancer biology, further development of RR inhibitors with improved efficacy and reduced toxicity is possible for treatment of variety of cancers.

New Iminodiacetate-Thiosemicarbazone Hybrids and Their Copper(II) Complexes Are Potential Ribonucleotide Reductase R2 Inhibitors with High Antiproliferative Activity

As ribonucleotide reductase (RNR) plays a crucial role in nucleic acid metabolism, it is an important target for anticancer therapy. The thiosemicarbazone Triapine is an efficient R2 inhibitor, which has entered ∼20 clinical trials. Thiosemicarbazones are supposed to exert their biological effects through effectively binding transition-metal ions. In this study, six iminodiacetate-thiosemicarbazones able to form transition-metal complexes, as well as six dicopper(II) complexes, were synthesized and fully characterized by analytical, spectroscopic techniques (IR, UV-vis; ¹H and ¹³C NMR), electrospray ionization mass spectrometry, and X-ray diffraction. The antiproliferative effects were examined in several human cancer and one noncancerous cell lines. Several of the compounds showed high cytotoxicity and marked selectivity for cancer cells. On the basis of this, and on molecular docking calculations one lead dicopper(II) complex and one thiosemicarbazone were chosen for in vitro analysis as potential R2 inhibitors. Their interaction with R2 and effect on the Fe(III)₂-Y· cofactor were characterized by microscale thermophoresis, and two spectroscopic techniques, namely, electron paramagnetic resonance and UV-vis spectroscopy. Our findings suggest that several of the synthesized proligands and copper(II) complexes are effective antiproliferative agents in several cancer cell lines, targeting RNR, which deserve further investigation as potential anticancer drugs.

Loss of phosphodiesterase 4D mediates acquired triapine resistance via Epac-Rap1-Integrin signaling

Triapine, an anticancer thiosemicarbazone, is currently under clinical investigation. Whereas promising results were obtained in hematological diseases, trials in solid tumors widely failed. To understand mechanisms causing triapine insensitivity, we have analysed genomic alterations in a triapine-resistant SW480 subline (SW480/tria). Only one distinct genomic loss was observed specifically in SW480/tria cells affecting the phosphodiesterase 4D (PDE4D) gene locus. Accordingly, pharmacological inhibition of PDE4D resulted in significant triapine resistance in SW480 cells. Hence, we concluded that enhanced cyclic AMP levels might confer protection against triapine. Indeed, hyperactivation of both major downstream pathways, namely the protein kinase A (PKA)-cAMP response element-binding protein (Creb) and the exchange protein activated by cAMP (Epac)-Ras-related protein 1 (Rap1) signaling axes, was observed in SW480/tria cells. Unexpectedly, inhibition of PKA did not re-sensitize SW480/tria cells against triapine. In contrast, Epac activation resulted in distinct triapine resistance in SW480 cells. Conversely, knock-down of Epac expression and pharmacological inhibition of Rap1 re-sensitized SW480/tria cells against triapine. Rap1 is a well-known regulator of integrins. Accordingly, SW480/tria cells displayed enhanced plasma membrane expression of several integrin subunits, enhanced adhesion especially to RGD-containing matrix components, and bolstered activation/expression of the integrin downstream effectors Src and RhoA/Rac. Accordingly, integrin and Src inhibition resulted in potent triapine re-sensitization especially of SW480/tria cells. In summary, we describe for the first time integrin activation based on cAMP-Epac-Rap1 signaling as acquired drug resistance mechanism. combinations of triapine with inhibitors of several steps in this resistance cascade might be feasible strategies to overcome triapine insensitivity of solid tumors.

Structure-Activity Relationships of Di-2-pyridylketone, 2-Benzoylpyridine, and 2-Acetylpyridine Thiosemicarbazones for Overcoming Pgp-Mediated Drug Resistance

Multidrug resistance (MDR) mediated by P-glycoprotein (Pgp) represents a significant impediment to successful cancer treatment. The compound, di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT), has been shown to induce greater cytotoxicity against resistant cells than their nonresistant counterparts. Herein, the structure-activity relationships of selected thiosemicarbazones are explored and the novel mechanism underlying their ability to overcome resistance is further elucidated. Only thiosemicarbazones with electron-withdrawing substituents at the imine carbon mediated Pgp-dependent potentiated cytotoxicity, which was reversed by Pgp inhibition. Treatment of resistant cells with these thiosemicarbazones resulted in Pgp-dependent lysosomal membrane permeabilization (LMP) that relied on copper (Cu) chelation, reactive oxygen species generation, and increased relative lipophilicity. Hence, this study is the first to demonstrate the structural requirements of these thiosemicarbazones necessary to overcome MDR. We also demonstrate the mechanism that enables the targeting of resistant tumors, whereby thiosemicarbazones "hijack" lysosomal Pgp and form redox-active Cu complexes that mediate LMP and potentiate cytotoxicity.

Impact of Stepwise NH2-Methylation of Triapine on the Physicochemical Properties, Anticancer Activity, and Resistance Circumvention

One of the most promising classes of iron chelators are α-N-heterocyclic thiosemicarbazones with Triapine as the most prominent representative. In several clinical trials Triapine showed anticancer activity against hematological diseases, however, studies on solid tumors failed due to widely unknown reasons. Some years ago, it was recognized that “terminal dimethylation” of thiosemicarbazones can lead to a more than 100-fold increased activity, probably due to interactions with cellular copper depots. To better understand the structural requirements for the switch to nanomolar cytotoxicity, we systematically synthesized all eight possible N-methylated derivatives of Triapine and investigated their potential against Triapine-sensitive as well as -resistant cell lines. While only the “completely” methylated compound exerted nanomolar activity, the data revealed that all compounds with at least one N-dimethylation were not affected by acquired Triapine resistance. In addition, these compounds were highly synergistic with copper treatment accompanied by induction of reactive oxygen species and massive necrotic cell death.

Radiation-Therapeutic Agent Clinical Trials: Leveraging Advantages of a National Cancer Institute Programmatic Collaboration

A number of oncology phase II radiochemotherapy trials with promising results have been conducted late in the overall experimental therapeutic agent development process. Accelerated development and approval of experimental therapeutic agents have stimulated further interest in much earlier radiation-agent studies to increase the likelihood of success in phase III trials. To sustain this interest, more forward-thinking preclinical radiobiology experimental designs are needed to improve discovery of promising radiochemotherapy plus agent combinations for clinical trial testing. These experimental designs should better inform next-step radiation-agent clinical trial dose, schedule, exposure, and therapeutic effect. Recognizing the need for a better strategy to develop preclinical data supporting radiation-agent phase I or II trials, the National Cancer Institute (NCI)-Cancer Therapy Evaluation Program (CTEP) and the NCI-Molecular Radiation Therapeutics Branch of the Radiation Research Program have partnered to promote earlier radiobiology studies of CTEP portfolio agents. In this Seminars in Radiation Oncology article, four key components of this effort are discussed. First, we outline steps for accessing CTEP agents for preclinical testing. Second, we propose radiobiology studies that facilitate transition from preclinical testing to early phase trial activation. Third, we navigate steps that walk through CTEP agent strategic development paths available for radiation-agent testing. Fourth, we highlight a new NCI-sponsored cooperative agreement grant supporting in vitro and in vivo radiation-CTEP agent testing that informs early phase trial designs. Throughout the article, we include contemporary examples of successful radiation-agent development initiatives.

Targeting cancer by binding iron: Dissecting cellular signaling pathways

Newer and more potent therapies are urgently needed to effectively treat advanced cancers that have developed resistance and metastasized. One such strategy is to target cancer cell iron metabolism, which is altered compared to normal cells and may facilitate their rapid proliferation. This is supported by studies reporting the anti-neoplastic activities of the clinically available iron chelators, desferrioxamine and deferasirox. More recently, ligands of the di-2-pyridylketone thiosemicarbazone (DpT) class have demonstrated potent and selective anti-proliferative activity across multiple cancer-types in vivo, fueling studies aimed at dissecting their molecular mechanisms of action. In the past five years alone, significant advances have been made in understanding how chelators not only modulate cellular iron metabolism, but also multiple signaling pathways implicated in tumor progression and metastasis. Herein, we discuss recent research on the targeting of iron in cancer cells, with a focus on the novel and potent DpT ligands. Several key studies have revealed that iron chelation can target the AKT, ERK, JNK, p38, STAT3, TGF-β, Wnt and autophagic pathways to subsequently inhibit cellular proliferation, the epithelial-mesenchymal transition (EMT) and metastasis. These developments emphasize that these novel therapies could be utilized clinically to effectively target cancer.

Interaction of Di-2-pyridylketone 2-pyridine Carboxylic Acid Hydrazone and Its Copper Complex with BSA: Effect on Antitumor Activity as Revealed by Spectroscopic Studies

The drug, di-2-pyridylketone-2-pyridine carboxylic acid hydrazone (DPPCAH) and its copper complex (DPPCAH-Cu) exhibit significant antitumor activity. However, the mechanism of their pharmacological interaction with the biological molecule bovine serum albumin (BSA) remains poorly understood. The present study elucidates the interactions between the drug and BSA through MTT assays, spectroscopic methods and molecular docking analysis. Our results indicate that BSA could attenuate effect on the cytotoxicity of DPPCAH, but not DPPCAH-Cu. Data from fluorescence quenching measurements demonstrated that both DPPCAH and DPPCAH-Cu could bind to BSA, with a reversed effect on the environment of tryptophan residues in polarity. CD spectra revealed that the DPPCAH-Cu exerted a slightly stronger effect on the secondary structure of BSA than DPPCAH. The association constant of DPPCAH with BSA was greater than that of DPPCAH-Cu. Docking studies indicated that the binding of DPPCAH to BSA involved a greater number of hydrogen bonds compared to DPPCAH-Cu. The calculated distances between bound ligands and tryptophans in BSA were in agreement with fluorescence resonance energy transfer results. Thus, the binding affinity of the drug (DPPCAH or DPPCAH-Cu) with BSA partially contributes to its antitumor activity; the greater the drug affinity is to BSA, the less is its antitumor activity.

Zinc(II)-Thiosemicarbazone Complexes Are Localized to the Lysosomal Compartment Where They Transmetallate with Copper Ions to Induce Cytotoxicity

As the di-2-pyridylketone thiosemicarbazone (DpT) and 2-acetylpyridine thiosemicarbazone (ApT) series show potent antitumor activity in vitro and in vivo, we synthesized their fluorescent zinc(II) complexes to assess their intracellular distribution. The Zn(II) complexes generally showed significantly greater cytotoxicity than the thiosemicarbazones alone in several tumor cell-types. Notably, specific structure-activity relationships demonstrated the importance of the di-2-pyridyl pharmacophore in their activity. Confocal fluorescence imaging and live cell microscopy showed that the Zn(II) complex of our lead compound, di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC), which is scheduled to enter clinical trials, was localized to lysosomes. Under lysosomal conditions, the Zn(II) complexes were shown to transmetallate with copper ions, leading to redox-active copper complexes that induced lysosomal membrane permeabilization (LMP) and cytotoxicity. This is the first study to demonstrate direct lysosomal targeting of our novel Zn(II)-thiosemicarbazone complexes that mediate their activity via transmetalation with copper ions and LMP.

Strong effect of copper(II) coordination on antiproliferative activity of thiosemicarbazone-piperazine and thiosemicarbazone-morpholine hybrids

In this study, 2-formylpyridine thiosemicarbazones and three different heterocyclic pharmacophores were combined to prepare thiosemicarbazone–piperazine mPip-FTSC (HL1) and mPip-dm-FTSC (HL2), thiosemicarbazone–morpholine Morph-FTSC (HL3) and Morph-dm-FTSC (HL4), thiosemicarbazone–methylpyrrole-2-carboxylate hybrids mPyrr-FTSC (HL5) and mPyrr-dm-FTSC (HL6) as well as their copper(II) complexes [CuCl(mPipH-FTSC-H)]Cl (1 + H)Cl, [CuCl(mPipH-dm-FTSC-H)]Cl (2 + H)Cl, [CuCl(Morph-FTSC-H)] (3), [CuCl(Morph-dm-FTSC-H)] (4), [CuCl(mPyrr-FTSC-H)(H2O)] (5) and [CuCl(mPyrr-dm-FTSC-H)(H2O)] (6). The substances were characterized by elemental analysis, one- and two-dimensional NMR spectroscopy (HL1–HL6), ESI mass spectrometry, IR and UV–vis spectroscopy and single crystal X-ray diffraction (1–5). All compounds were prepared in an effort to generate potential antitumor agents with an improved therapeutic index. In addition, the effect of structural alterations with organic hybrids on aqueous solubility and copper(II) coordination ability was investigated. Complexation of ligands HL2 and HL4 with copper(II) was studied in aqueous solution by pH-potentiometry, UV–vis spectrophotometry and EPR spectroscopy. Proton dissociation processes of HL2 and HL4 were also characterized in detail and microscopic constants for the Z/E isomers were determined. While the hybrids HL5, HL6 and their copper(II) complexes 5 and 6 proved to be insoluble in aqueous solution, precluding antiproliferative activity studies, the thiosemicarbazone–piperazine and thiosemicarbazone–morpholine hybrids HL1–HL4, as well as copper(II) complexes 1–4 were soluble in water enabling cytotoxicity assays. Interestingly, the metal-free hybrids showed very low or even a lack of cytotoxicity (IC50 values > 300 μM) in two human cancer cell lines HeLa (cervical carcinoma) and A549 (alveolar basal adenocarcinoma), whereas their copper(II) complexes were cytotoxic showing IC50 values from 25.5 to 65.1 μM and 42.8 to 208.0 μM, respectively in the same human cancer cell lines after 48 h of incubation. However, the most sensitive for HL4 and complexes 1–4 proved to be the human cancer cell line LS174 (colon carcinoma) as indicated by the calculated IC50 values varying from 13.1 to 17.5 μM.

Redox cycling metals: Pedaling their roles in metabolism and their use in the development of novel therapeutics

Essential metals, such as iron and copper, play a critical role in a plethora of cellular processes including cell growth and proliferation. However, concomitantly, excess of these metal ions in the body can have deleterious effects due to their ability to generate cytotoxic reactive oxygen species (ROS). Thus, the human body has evolved a very well-orchestrated metabolic system that keeps tight control on the levels of these metal ions. Considering their very high proliferation rate, cancer cells require a high abundance of these metals compared to their normal counterparts. Interestingly, new anti-cancer agents that take advantage of the sensitivity of cancer cells to metal sequestration and their susceptibility to ROS have been developed. These ligands can avidly bind metal ions to form redox active metal complexes, which lead to generation of cytotoxic ROS. Furthermore, these agents also act as potent metastasis suppressors due to their ability to up-regulate the metastasis suppressor gene, N-myc downstream regulated gene 1. This review discusses the importance of iron and copper in the metabolism and progression of cancer, how they can be exploited to target tumors and the clinical translation of novel anti-cancer chemotherapeutics.

Novel Mechanism of Cytotoxicity for the Selective Selenosemicarbazone, 2-Acetylpyridine 4,4-Dimethyl-3-selenosemicarbazone (Ap44mSe): Lysosomal Membrane Permeabilization

Selenosemicarbazones show marked antitumor activity. However, their mechanism of action remains unknown. We examined the medicinal chemistry of the selenosemicarbazone, 2-acetylpyridine 4,4-dimethyl-3-selenosemicarbazone (Ap44mSe), and its iron and copper complexes to elucidate its mechanisms of action. Ap44mSe demonstrated a pronounced improvement in selectivity toward neoplastic relative to normal cells compared to its parent thiosemicarbazone. It also effectively depleted cellular Fe, resulting in transferrin receptor-1 up-regulation, ferritin down-regulation, and increased expression of the potent metastasis suppressor, N-myc downstream regulated gene-1. Significantly, Ap44mSe limited deleterious methemoglobin formation, highlighting its usefulness in overcoming toxicities of clinically relevant thiosemicarbazones. Furthermore, Cu-Ap44mSe mediated intracellular reactive oxygen species generation, which was attenuated by the antioxidant, N-acetyl-L-cysteine, or Cu sequestration. Notably, Ap44mSe forms redox active Cu complexes that target the lysosome to induce lysosomal membrane permeabilization. This investigation highlights novel structure-activity relationships for future chemotherapeutic design and underlines the potential of Ap44mSe as a selective anticancer/antimetastatic agent.

Nanoformulations of anticancer thiosemicarbazones to reduce methemoglobin formation and improve anticancer activity

Triapine and two derivatives were encapsulated into polymeric nanoparticles as well as liposomes. The most stable formulation showed strongly reduced methemoglobin formation and improved anticancer activity.

Novel biotin-functionalized lipidic nanocarriers for encapsulating BpT and Bp4eT iron chelators: evaluation of potential anti-tumour efficacy by in vitro, in vivo and pharmacokinetic studies in A549 mice models

This work proposes a novel strategy for delivery of iron chelators to the tumour cells which is exemplified in A549 mice models by using lipidic nanocarriers and introducing biotin based targeting.

Kinetico-mechanistic studies on methemoglobin generation by biologically active thiosemicarbazone iron(III) complexes

The oxidation of human oxyhemoglobin (HbO₂) to methemoglobin (metHb) is an undesirable side effect identified in some promising thiosemicarbazone anti-cancer drugs. This is attributable to oxidation reactions driven by Fe^III complexes of these drugs formed in vivo. In this work the Fe^III complexes of selected 2-benzoylpyridine thiosemicarbazones (HBpT), 2-acetylpyridine thiosemicarbazones (HApT), and the clinically trialled thiosemicarbazone, Triapine® (3-amino-2-pyridinecarboxaldehyde thiosemicarbazone, H3-AP), have been studied. This was achieved by time-resolved UV-Visible absorption spectroscopy and the sequential oxidation of the α- and β-chains of HbO₂ at distinctly different rates has been identified. A key structural element, namely a terminal -NH₂ group on the thiosemicarbazone moiety, was found to be an important common feature of the most active HbO₂ oxidising complexes that were investigated. Therefore, these studies indicate that an unsubstituted -NH₂ moiety at the terminus of the thiosemicarbazone group should be avoided in the design of future compounds from this class.

Differences in protein binding and excretion of Triapine and its Fe(III) complex

Triapine has been investigated as anticancer drug in multiple clinical phase I/II trials. Although promising anti-leukemic activity was observed, Triapine was ineffective against solid tumors. The reasons are currently widely unknown. The biological activity of Triapine is strongly connected to its iron complex (Fe-Triapine) which is pharmacologically not investigated. Here, novel analytical tools for Triapine and Fe-Triapine were developed and applied for cell extracts and body fluids of treated mice. Triapine and its iron complex showed a completely different behavior: for Triapine, low protein binding was observed in contrast to fast protein adduct formation of Fe-Triapine. Notably, both drugs were rapidly cleared from the body (serum half-life time <1h). Remarkably, in contrast to Triapine, where (in accordance to clinical data) basically no renal excretion was found, the iron complex was effectively excreted via urine. Moreover, no Fe-Triapine was detected in serum or cytosolic extracts after Triapine treatment. Taken together, our study will help to further understand the biological behavior of Triapine and its Fe-complex and allow the development of novel thiosemicarbazones with pronounced activity against solid tumor types.

Investigation of the salicylaldehyde thiosemicarbazone scaffold for inhibition of influenza virus PA endonuclease

The influenza virus PA endonuclease is an attractive target for the development of novel anti-influenza virus therapeutics, which are urgently needed because of the emergence of drug-resistant viral strains. Reported PA inhibitors are assumed to chelate the divalent metal ion(s) (Mg²⁺ or Mn²⁺) in the enzyme's catalytic site, which is located in the N-terminal part of PA (PA-Nter). In the present work, a series of salicylaldehyde thiosemicarbazone derivatives have been synthesized and evaluated for their ability to inhibit the PA-Nter catalytic activity. Compounds 1-6 have been evaluated against influenza virus, both in enzymatic assays with influenza virus PA-Nter and in virus yield assays in MDCK cells. In order to establish a structure-activity relationship, the hydrazone analogue of the most active thiosemicarbazone has also been evaluated. Since chelation may represent a mode of action of such class of molecules, we studied the interaction of two of them, one with and one without biological activity versus the PA enzyme, towards Mg²⁺, the ion that is probably involved in the endonuclease activity of the heterotrimeric influenza polymerase complex. The crystal structure of the magnesium complex of the o-vanillin thiosemicarbazone ligand 1 is also described. Moreover, docking studies of PA endonuclease with compounds 1 and 2 were performed, to further analyse the possible mechanism of action of this class of inhibitors.

Triapine-mediated ABCB1 induction via PKC induces widespread therapy unresponsiveness but is not underlying acquired triapine resistance

Although triapine is promising for treatment of advanced leukemia, it failed against solid tumors due to widely unknown reasons. To address this issue, a new triapine-resistant cell line (SW480/tria) was generated by drug selection and investigated in this study. Notably, SW480/tria cells displayed broad cross-resistance against several known ABCB1 substrates due to high ABCB1 levels (induced by promoter hypomethylation). However, ABCB1 inhibition did not re-sensitize SW480/tria cells to triapine and subsequent analysis revealed that triapine is only a weak ABCB1 substrate without significant interaction with the ABCB1 transport function. Interestingly, in chemo-naive, parental SW480 cells short-time (24 h) treatment with triapine stimulated ABCB1 expression. These effects were based on activation of protein kinase C (PKC), a known response to cellular stress. In accordance, SW480/tria cells were characterized by elevated levels of PKC. Together, this led to the conclusion that increased ABCB1 expression is not the major mechanism of triapine resistance in SW480/tria cells. In contrast, increased ABCB1 expression was found to be a consequence of triapine stress-induced PKC activation. These data are especially of importance when considering the choice of chemotherapeutics for combination with triapine.

Cell death and ultrastructural alterations in Leishmania amazonensis caused by new compound 4-Nitrobenzaldehyde thiosemicarbazone derived from S-limonene

Background

The treatment of leishmaniasis with pentavalent antimonials is problematic because of their toxicity. Investigations of potentially active molecules are important to discover less toxic drugs that are viable economic alternatives for the treatment of leishmaniasis. Thiosemicarbazones are a group of molecules that are known for their wide versatility and biological activity. In the present study, we examined the antileishmania activity, mechanism of action, and biochemical alterations produced by a novel molecule, 4-nitrobenzaldehyde thiosemicarbazone (BZTS), derived from S-limonene against Leishmania amazonensis.

Results

BZTS inhibited the growth of the promastigote and axenic amastigote forms, with an IC₅₀ of 3.8 and 8.0 μM, respectively. Intracellular amastigotes were inhibited by the compound with an IC₅₀ of 7.7 μM. BZTS also had a CC₅₀ of 88.8 μM for the macrophage strain J774A1. BZTS altered the shape, size, and ultrastructure of the parasites, including damage to mitochondria, reflected by extensive swelling and disorganization of the inner mitochondrial membrane, intense cytoplasmic vacuolization, and the presence of concentric membrane structures inside the organelle. Cytoplasmic lipid bodies, vesicles inside vacuoles in the flagellar pocket, and enlargement were also observed. BZTS did not induce alterations in the plasma membrane or increase annexin-V fluorescence intensity, indicating no phosphatidylserine exposure. However, it induced the production of mitochondrial superoxide anion radicals.

Conclusions

The present results indicate that BZTS induced dramatic effects on the ultrastructure of L. amazonensis, which might be associated with mitochondrial dysfunction and oxidative damage, leading to parasite death.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-014-0236-0) contains supplementary material, which is available to authorized users.

Ni(II) and Cu(II) N(4)-ethylmorpholine citronellalthiosemicarbazonate: a comparative analysis of cytotoxic effects in malignant human cancer cell lines

In this paper we report a study conducted with two analogous complexes, bis(N(4)-ethylmorpholine citronellalthiosemicarbazonate) nickel(II) and -copper(II) on four tumour cell lines (U937, HL60, SK-N-MC and HT29). All cell lines appear to be sensitive to both metal complexes, but while in U937, HL60 and SK-N-MC, apoptosis is the main mode through which cell death occurs, HT29 cells undergo necrosis. Among the cell lines which undergo apoptosis, SK-N-MC response is characterized by the intrinsic pathway, whereas U937 and HL60 involve both the intrinsic and the extrinsic pathways. The redox activity of the two complexes provides experimental evidence that they can modulate reactive oxygen species (ROS) production as a function of both the metal and the cell line used. Among the four cell lines, HL60 does not seem to give a significant response to exposure to both compounds. In the case of the nickel derivative, ROS generation is a relatively early event, and ROS could be the mediator leading to cellular damage. HT29 shows a remarkable and rapid ROS increase and a significant induction of membrane peroxidation that could be correlated to the onset of necrosis.

Expanding horizons in iron chelation and the treatment of cancer: role of iron in the regulation of ER stress and the epithelial-mesenchymal transition

Cancer is a major public health issue and, despite recent advances, effective clinical management remains elusive due to intra-tumoural heterogeneity and therapeutic resistance. Iron is a trace element integral to a multitude of metabolic processes, including DNA synthesis and energy transduction. Due to their generally heightened proliferative potential, cancer cells have a greater metabolic demand for iron than normal cells. As such, iron metabolism represents an important "Achilles' heel" for cancer that can be targeted by ligands that bind and sequester intracellular iron. Indeed, novel thiosemicarbazone chelators that act by a "double punch" mechanism to both bind intracellular iron and promote redox cycling reactions demonstrate marked potency and selectivity in vitro and in vivo against a range of tumours. The general mechanisms by which iron chelators selectively target tumour cells through the sequestration of intracellular iron fall into the following categories: (1) inhibition of cellular iron uptake/promotion of iron mobilisation; (2) inhibition of ribonucleotide reductase, the rate-limiting, iron-containing enzyme for DNA synthesis; (3) induction of cell cycle arrest; (4) promotion of localised and cytotoxic reactive oxygen species production by copper and iron complexes of thiosemicarbazones (e.g., Triapine(®) and Dp44mT); and (5) induction of metastasis and tumour suppressors (e.g., NDRG1 and p53, respectively). Emerging evidence indicates that chelators can further undermine the cancer phenotype via inhibiting the epithelial-mesenchymal transition that is critical for metastasis and by modulating ER stress. This review explores the "expanding horizons" for iron chelators in selectively targeting cancer cells.

Kinetic studies on the oxidation of oxyhemoglobin by biologically active iron thiosemicarbazone complexes: relevance to iron-chelator-induced methemoglobinemia

The oxidation of oxyhemoglobin to methemoglobin has been found to be facilitated by low molecular weight iron(III) thiosemicarbazone complexes. This deleterious reaction, which produces hemoglobin protein units unable to bind dioxygen and occurs during the administration of iron chelators such as the well-known 3-aminopyridine-2-pyridinecarbaldehyde thiosemicarbazone (3-AP; Triapine), has been observed in the reaction with Fe(III) complexes of some members of the 3-AP structurally-related thiosemicarbazone ligands derived from di-2-pyridyl ketone (HDpxxT series). We have studied the kinetics of this oxidation reaction in vitro using human hemoglobin and found that the reaction proceeds with two distinct time-resolved steps. These have been associated with sequential oxidation of the two different oxyheme cofactors in the α and β protein chains. Unexpected steric and hydrogen-bonding effects on the Fe(III) complexes appear to be the responsible for the observed differences in the reaction rate across the series of HDpxxT ligand complexes used in this study.

Iron and cancer: more ore to be mined

Iron is an essential nutrient that facilitates cell proliferation and growth. However, iron also has the capacity to engage in redox cycling and free radical formation. Therefore, iron can contribute to both tumour initiation and tumour growth; recent work has also shown that iron has a role in the tumour microenvironment and in metastasis. Pathways of iron acquisition, efflux, storage and regulation are all perturbed in cancer, suggesting that reprogramming of iron metabolism is a central aspect of tumour cell survival. Signalling through hypoxia-inducible factor (HIF) and WNT pathways may contribute to altered iron metabolism in cancer. Targeting iron metabolic pathways may provide new tools for cancer prognosis and therapy.

Novel chelators for cancer treatment: where are we now?

SIGNIFICANCE

Under normal circumstances, cellular iron levels are tightly regulated due to the potential toxic effects of this metal ion. There is evidence that tumors possess altered iron homeostasis, which is mediated by the perturbed expression of iron-related proteins, for example, transferrin receptor 1, ferritin and ferroportin 1. The de-regulation of iron homeostasis in cancer cells reveals a particular vulnerability to iron-depletion using iron chelators. In this review, we examine the absorption of iron from the gut; its transport, metabolism, and homeostasis in mammals; and the molecular pathways involved. Additionally, evidence for alterations in iron processing in cancer are described along with the perturbations in other biologically important transition metal ions, for example, copper(II) and zinc(II). These changes can be therapeutically manipulated by the use of novel chelators that have recently been shown to be highly effective in terms of inhibiting tumor growth.

RECENT ADVANCES

Such chelators include those of the thiosemicarbazone class that were originally thought to target only ribonucleotide reductase, but are now known to have multiple effects, including the generation of cytotoxic radicals.

CRITICAL ISSUES

Several chelators have shown marked anti-tumor activity in vivo against a variety of solid tumors. An important aspect is the toxicology and the efficacy of these agents in clinical trials.

FUTURE DIRECTIONS

As part of the process of the clinical assessment of the new chelators, an extensive toxicological assessment in multiple animal models is essential for designing appropriate dosing protocols in humans.

Alkyl substituted 2'-benzoylpyridine thiosemicarbazone chelators with potent and selective anti-neoplastic activity: novel ligands that limit methemoglobin formation

Thiosemicarbazone chelators, including the 2'-benzoylpyridine thiosemicarbazones (BpT) class, show marked potential as anticancer agents. Importantly, 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP) has been investigated in >20 phase I and II clinical trials. However, side effects associated with 3-AP administration include methemoglobinemia. Considering this problem, novel BpT analogues were designed bearing hydrophobic, electron-donating substituents at the para position of the phenyl group (RBpT). Their Fe(III/II) redox potentials were all within the range accessible to cellular oxidants and reductants, suggesting they can redox cycle. These RBpT ligands exhibited potent and selective antiproliferative activity, which was comparable or exceeded their BpT counterparts. Major findings include that methemoglobin formation mediated by the lipophilic t-BuBpT series was significantly (p < 0.05-0.001) decreased in comparison to 3-AP in intact red blood cells and were generally comparable to the control. These data indicate the t-BuBpT ligands may minimize methemoglobinemia, which is a marked advantage over 3-AP and other potent thiosemicarbazones.