Phase I and pharmacokinetic study of the ribonucleotide reductase inhibitor, 3-aminopyridine-2-carboxaldehyde thiosemicarbazone, administered by 96-hour intravenous continuous infusion

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.

A sensitive liquid chromatography-mass spectrometry bioanalytical assay for a novel anticancer candidate--ZMC1

ZMC1 {azetidinecarbothioic acid, [1-(2-pyridinyl) ethylidene] hydrazide} is a lead compound being developed as one of the first mutant p53 targeted anti-cancer drugs. Establishing a precise quantitative method is an integral component of this development. The aim of this study was to develop a sensitive LC/MS/MS assay suitable for assessing purity, stability and preclinical pharmacokinetic studies of ZMC1. Acetonitrile protein precipitation extraction was chosen for plasma sample preparation with satisfactory recovery (84.2-92.8%) for ZMC1. Chromatographic separation was achieved on an Xterra C18 column (50 × 4.6 mm, 3.5 µm) using a gradient elution with mobile phase of 0.1% formic acid in water and acetonitrile. ZMC1 and internal standard 2-amino-6-bromobenzothiazole were identified using selected-ion monitoring mode at m/z 235.2/178.2 and m/z 231.0/150.0 at retention times of 5.2 and 6.3 min, respectively. The method was validated with a linearity range of 3.9-500.0 ng/mL in human plasma and showed acceptable reproducibility with intra- and interday precisions <5.9 and 10.5%, and accuracy within ±5.4% of nominal values. This analytical method together with basic stability data in plasma and plasma binding experiments provides a reliable protocol for the study of ZMC1 pharmacokinetics. This will greatly facilitate the pre-clinical development of this novel anti-cancer drug.

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.

Novel thiosemicarbazones regulate the signal transducer and activator of transcription 3 (STAT3) pathway: inhibition of constitutive and interleukin 6-induced activation by iron depletion

Pharmacologic manipulation of metal pools in tumor cells is a promising strategy for cancer treatment. Here, we reveal how the iron-binding ligands desferrioxamine (DFO), di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT), and di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC) inhibit constitutive and interleukin 6-induced activation of signal transducer and activator of transcription 3 (STAT3) signaling, which promotes proliferation, survival, and metastasis of cancer cells. We demonstrate that DFO, Dp44mT, and DpC significantly decrease constitutive phosphorylation of the STAT3 transcription factor at Tyr705 in the pancreatic cancer cell lines PANC-1 and MIAPaCa-2 as well as the prostate cancer cell line DU145. These compounds also significantly decrease the dimerized STAT3 levels, the binding of nuclear STAT3 to its target DNA, and the expression of downstream targets of STAT3, including cyclin D1, c-myc, and Bcl-2. Examination of upstream mediators of STAT3 in response to these ligands has revealed that Dp44mT and DpC could significantly decrease activation of the nonreceptor tyrosine kinase Src and activation of cAbl in DU145 and MIAPaCa-2 cells. In contrast to the effects of Dp44mT, DpC, or DFO on inhibiting STAT3 activation, the negative control compound di-2-pyridylketone 2-methyl-3-thiosemicarbazone, or the DFO:Fe complex, which cannot bind cellular iron, had no effect. This demonstrates the role of iron-binding in the activity observed. Immunohistochemical staining of PANC-1 tumor xenografts showed a marked decrease in STAT3 in the tumors of mice treated with Dp44mT or DpC compared with the vehicle. Collectively, these studies demonstrate suppression of STAT3 activity by iron depletion in vitro and in vivo, and reveal insights into regulation of the critical oncogenic STAT3 pathway.

Effects of terminal dimethylation and metal coordination of proline-2-formylpyridine thiosemicarbazone hybrids on lipophilicity, antiproliferative activity, and hR2 RNR inhibition

The nickel(II), copper(II), and zinc(II) complexes of the proline-thiosemicarbazone hybrids 3-methyl-(S)-pyrrolidine-2-carboxylate-2-formylpyridine thiosemicarbazone (L-Pro-FTSC or (S)-H2L(1)) and 3-methyl-(R)-pyrrolidine-2-carboxylate-2-formylpyridine thiosemicarbazone (D-Pro-FTSC or (R)-H2L(1)), as well as 3-methyl-(S)-pyrrolidine-2-carboxylate-2-formylpyridine 4,4-dimethyl-thiosemicarbazone (dm-L-Pro-FTSC or (S)-H2L(2)), namely, [Ni(L-Pro-FTSC-2H)]2 (1), [Ni(D-Pro-FTSC-2H)]2 (2), [Ni(dm-L-Pro-FTSC-2H)]2 (3), [Cu(dm-L-Pro-FTSC-2H)] (6), [Zn(L-Pro-FTSC-2H)] (7), and [Zn(D-Pro-FTSC-2H)] (8), in addition to two previously reported, [Cu(L-Pro-FTSC-2H)] (4), [Cu(D-Pro-FTSC-2H)] (5), were synthesized and characterized by elemental analysis, one- and two-dimensional (1)H and (13)C NMR spectroscopy, circular dichroism, UV-vis, and electrospray ionization mass spectrometry. Compounds 1-3, 6, and 7 were also studied by single-crystal X-ray diffraction. Magnetic properties and solid-state high-field electron paramagnetic resonance spectra of 2 over the range of 50-420 GHz were investigated. The complex formation processes of L-Pro-FTSC with nickel(II) and zinc(II) were studied in aqueous solution due to the excellent water solubility of the complexes via pH potentiometry, UV-vis, and (1)H NMR spectroscopy. The results of the antiproliferative activity in vitro showed that dimethylation improves the cytotoxicity and hR2 RNR inhibition. Therefore, introduction of more lipophilic groups into thiosemicarbazone-proline backbone becomes an option for the synthesis of more efficient cytotoxic agents of this family of compounds.

Exploring the anti-cancer activity of novel thiosemicarbazones generated through the combination of retro-fragments: dissection of critical structure-activity relationships

Thiosemicarbazones (TSCs) are an interesting class of ligands that show a diverse range of biological activity, including anti-fungal, anti-viral and anti-cancer effects. Our previous studies have demonstrated the potent in vivo anti-tumor activity of novel TSCs and their ability to overcome resistance to clinically used chemotherapeutics. In the current study, 35 novel TSCs of 6 different classes were designed using a combination of retro-fragments that appear in other TSCs. Additionally, di-substitution at the terminal N4 atom, which was previously identified to be critical for potent anti-cancer activity, was preserved through the incorporation of an N4-based piperazine or morpholine ring. The anti-proliferative activity of the novel TSCs were examined in a variety of cancer and normal cell-types. In particular, compounds 1d and 3c demonstrated the greatest promise as anti-cancer agents with potent and selective anti-proliferative activity. Structure-activity relationship studies revealed that the chelators that utilized “soft” donor atoms, such as nitrogen and sulfur, resulted in potent anti-cancer activity. Indeed, the N,N,S donor atom set was crucial for the formation of redox active iron complexes that were able to mediate the oxidation of ascorbate. This further highlights the important role of reactive oxygen species generation in mediating potent anti-cancer activity. Significantly, this study identified the potent and selective anti-cancer activity of 1d and 3c that warrants further examination.

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.

Cellular uptake of the antitumor agent Dp44mT occurs via a carrier/receptor-mediated mechanism

The chelator di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT) shows potent and selective anticancer and antimetastatic activity. However, the mechanism by which it is initially transported into cells to induce cytotoxicity is unknown. Hence, the current investigation examined the cellular uptake of ¹⁴C-Dp44mT relative to two structurally related ligands, namely the aroylhydrazone ¹⁴C-pyridoxal isonicotinoyl hydrazone (¹⁴C-PIH) and the thiosemicarbazone (¹⁴C-2-benzoylpyridine 4-ethyl-3-thiosemicarbazone (¹⁴C-Bp4eT). In marked contrast to the cellular uptake of ¹⁴C-PIH and ¹⁴C-Bp4eT, which were linear as a function of concentration, ¹⁴C-Dp44mT uptake was saturable using SK-N-MC neuroepithelioma cells (Bmax, 4.28 × 10⁷ molecules of chelator/cell; and Kd, 2.45 μM). Together with the fact that ¹⁴C-Dp44mT uptake was temperature-dependent and significantly (P < 0.01) decreased by competing unlabeled Dp44mT, these observations indicated a saturable transport mechanism consistent with carrier/receptor-mediated transport. Other unlabeled ligands that shared the saturated N4 structural moiety with Dp44mT significantly (P < 0.01) inhibited ¹⁴C-Dp44mT uptake, illustrating its importance for carrier/receptor recognition. Nevertheless, unlabeled Dp44mT most markedly decreased (¹⁴C-Dp44mT uptake, demonstrating that the putative carrier/receptor shows high selectivity for Dp44mT. Interestingly, in contrast to ¹⁴C-Dp44mT, uptake of its Fe complex [Fe(¹⁴C-Dp44mT)₂] was not saturable as a function of concentration and was much greater than the ligand alone, indicating an alternate mode of transport. Studies examining the tissue distribution of ¹⁴C-Dp44mT injected intravenously into a mouse tumor model demonstrated the ¹⁴C label was primarily identified in the excretory system. Collectively, these findings examining the mechanism of Dp44mT uptake and its distribution and excretion have clinical implications for its bioavailability and uptake in vivo.

Simultaneous determination of the novel thiosemicarbazone anti-cancer agent, Bp4eT, and its main phase I metabolites in plasma: application to a pilot pharmacokinetic study in rats

Novel thiosemicarbazone metal chelators are extensively studied anti-cancer agents with marked and selective activity against a wide variety of cancer cells, as well as human tumor xenografts in mice. This study describes the first validated LC-MS/MS method for the simultaneous quantification of 2-benzoylpyridine 4-ethyl-3-thiosemicarbazone (Bp4eT) and its main metabolites (E/Z isomers of the semicarbazone structure, M1-E and M1-Z, and the amidrazone metabolite, M2) in plasma. Separation was achieved using a C18 column with ammonium formate/acetonitrile mixture as the mobile phase. Plasma samples were treated using solid-phase extraction on 96-well plates. This method was validated over the concentration range of 0.18-2.80 μM for Bp4eT, 0.02-0.37 μM for both M1-E and M1-Z, and 0.10-1.60 μM for M2. This methodology was applied to the analysis of samples from in vivo experiments, allowing for the concentration-time profile to be simultaneously assessed for the parent drug and its metabolites. The current study addresses the lack of knowledge regarding the quantitative analysis of thiosemicarbazone anti-cancer drugs and their metabolites in plasma and provides the first pharmacokinetic data on a lead compound of this class.

Identification of ribonucleotide reductase M2 as a potential target for pro-senescence therapy in epithelial ovarian cancer

Epithelial ovarian cancer (EOC) is the leading cause of gynecological-related cancer deaths in the United States. There is, therefore, an urgent need to develop novel therapeutic strategies for this devastating disease. Cellular senescence is a state of stable cell growth arrest that acts as an important tumor suppression mechanism. Ribonucleotide reductase M2 (RRM2) plays a key role in regulating the senescence-associated cell growth arrest by controlling biogenesis of 2'-deoxyribonucleoside 5'-triphosphates (dNTPs). The role of RRM2 in EOC remains poorly understood. Here we show that RRM2 is expressed at higher levels in EOCs compared with either normal ovarian surface epithelium (P<0.001) or fallopian tube epithelium (P<0.001). RRM2 expression significantly correlates with the expression of Ki67, a marker of cell proliferation (P<0.001). Moreover, RRM2 expression positively correlates with tumor grade and stage, and high RRM2 expression independently predicts a shorter overall survival in EOC patients (P<0.001). To delineate the functional role of RRM2 in EOC, we knocked down RRM2 expression in a panel of EOC cell lines. Knockdown of RRM2 expression inhibits the growth of human EOC cells. Mechanistically, RRM2 knockdown triggers cellular senescence in these cells. Notably, this correlates with the induction of the DNA damage response, a known mediator of cellular senescence. These data suggest that targeting RRM2 in EOCs by suppressing its activity is a novel pro-senescence therapeutic strategy that has the potential to improve survival of EOC patients.

Copper(II) complexes with highly water-soluble L- and D-proline-thiosemicarbazone conjugates as potential inhibitors of Topoisomerase IIα

Two proline-thiosemicarbazone bioconjugates with excellent aqueous solubility, namely, 3-methyl-(S)-pyrrolidine-2-carboxylate-2-formylpyridine thiosemicarbazone [L-Pro-FTSC or (S)-H2L] and 3-methyl-(R)-pyrrolidine-2-carboxylate-2-formylpyridine thiosemicarbazone [D-Pro-FTSC or (R)-H2L], have been synthesized and characterized by elemental analysis, one- and two-dimensional (1)H and (13)C NMR spectroscopy, and electrospray ionization mass spectrometry. The complexation behavior of L-Pro-FTSC with copper(II) in an aqueous solution and in a 30% (w/w) dimethyl sulfoxide/water mixture has been studied via pH potentiometry, UV-vis spectrophotometry, electron paramagnetic resonance, (1)H NMR spectroscopy, and spectrofluorimetry. By the reaction of copper(II) acetate with (S)-H2L and (R)-H2L in water, the complexes [Cu(S,R)-L] and [Cu(R,S)-L] have been synthesized and comprehensively characterized. An X-ray diffraction study of [Cu(S,R)-L] showed the formation of a square-pyramidal complex, with the bioconjugate acting as a pentadentate ligand. Both copper(II) complexes displayed antiproliferative activity in CH1 ovarian carcinoma cells and inhibited Topoisomerase IIα activity in a DNA plasmid relaxation assay.

RRM2 regulates Bcl-2 in head and neck and lung cancers: a potential target for cancer therapy

PURPOSE

Ribonucleotide reductase subunit M2 (RRM2) plays an active role in tumor progression. Recently, we reported that depletion of RRM2 by systemic delivery of a nanoparticle carrying RRM2-specific siRNA suppresses head and neck tumor growth. The aim of this study is to clarify the underlying mechanism by which RRM2 depletion inhibits tumor growth.

EXPERIMENTAL DESIGN

siRNA-mediated gene silencing was carried out to downregulate RRM2. Immunoblotting, reverse-transcriptase PCR, confocal microscopy, tissue fractionation, gene overexpression and knockdown were employed to analyze critical apoptosis signaling. Conventional immunohistochemistry and quantum dot-based immunofluorescence were applied to detect RRM2 and Bcl2 expression and localization in tissue samples from patients and mice.

RESULTS

Knockdown of RRM2 led to apoptosis through the intrinsic pathway in head and neck squamous cell carcinoma (HNSCC) and non-small cell lung cancer (NSCLC) cell lines. We showed that Bcl-2 is a key determinant controlling apoptosis, both in vitro and in vivo, and that RRM2 depletion significantly reduces Bcl-2 protein expression. We observed that RRM2 regulates Bcl-2 protein stability, with RRM2 suppression leading to increased Bcl-2 degradation, and identified their colocalization in HNSCC and NSCLC cells. In a total of 50 specimens each from patients with HNSCC and NSCLC, we identified the colocalization of Bcl-2 and RRM2 and found a significant positive correlation between their expression in HNSCC (R = 0.98; P < 0.0001) and NSCLC (R = 0.92; P < 0.0001) tumor tissues.

CONCLUSIONS

Our novel findings add to the knowledge of RRM2 in regulating expression of the antiapoptotic protein Bcl-2 and reveal a critical link between RRM2 and Bcl-2 in apoptosis signaling.

Redox activation of Fe(III)-thiosemicarbazones and Fe(III)-bleomycin by thioredoxin reductase: specificity of enzymatic redox centers and analysis of reactive species formation by ESR spin trapping

Thiosemicarbazones such as Triapine (Tp) and Dp44mT are tridentate iron (Fe) chelators that have well-documented antineoplastic activity. Although Fe-thiosemicarbazones can undergo redox cycling to generate reactive species that may have important roles in their cytotoxicity, there is only limited insight into specific cellular agents that can rapidly reduce Fe(III)-thiosemicarbazones and thereby promote their redox activity. Here we report that thioredoxin reductase-1 (TrxR1) and glutathione reductase (GR) have this activity and that there is considerable specificity to the interactions between specific redox centers in these enzymes and various Fe(III) complexes. Site-directed variants of TrxR1 demonstrate that the selenocysteine (Sec) of the enzyme is not required, whereas the C59 residue and the flavin have important roles. Although TrxR1 and GR have analogous C59/flavin motifs, TrxR is considerably faster than GR. For both enzymes, Fe(III)(Tp)2 is reduced faster than Fe(III)(Dp44mT)2. This reduction promotes redox cycling and the generation of hydroxyl radical (HO) in a peroxide-dependent manner, even with low-micromolar levels of Fe(Tp)2. TrxR also reduces Fe(III)-bleomycin and this activity is Sec-dependent. TrxR cannot reduce Fe(III)-EDTA at significant rates. Our findings are the first to demonstrate pro-oxidant reductive activation of Fe(III)-based antitumor thiosemicarbazones by interactions with specific enzyme species. The marked elevation of TrxR1 in many tumors could contribute to the selective tumor toxicity of these drugs by enhancing the redox activation of Fe(III)-thiosemicarbazones and the generation of reactive oxygen species such as HO.

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.

A phase I study of prolonged infusion of triapine in combination with fixed dose rate gemcitabine in patients with advanced solid tumors

PURPOSE

Prolonged exposure of cancer cells to triapine, an inhibitor of ribonucleotide reductase, followed by gemcitabine enhances gemcitabine activity in vitro. Fixed-dose-rate gemcitabine (FDR-G) has improved efficacy compared to standard-dose. We conducted a phase I trial to determine the maximum tolerated dose (MTD), safety, pharmacokinetics (PK), pharmacodynamics (PD), and preliminary efficacy of prolonged triapine infusion followed by FDR-G.

EXPERIMENTAL DESIGN

Triapine was given as a 24-hour infusion, immediately followed by FDR-G (1000 mg/m(2) over 100-minute). Initially, this combination was administered days 1 and 8 of a 21-day cycle (Arm A, triapine starting dose 120 mg); but because of myelosuppression, it was changed to days 1 and 15 of a 28-day cycle (Arm B, starting dose of triapine 75 mg). Triapine steady-state concentrations (Css) and circulating ribonucleotide reductase M2-subunit (RRM2) were measured.

RESULTS

Thirty-six patients were enrolled. The MTD was determined to be triapine 90 mg (24-hour infusion) immediately followed by gemcitabine 1000 mg/m(2) (100-minute infusion), every 2 weeks of a 4-week cycle. DLTs included grade 4 thrombocytopenia, leukopenia and neutropenia. The treatment was well tolerated with fatigue, nausea/vomiting, fever, transaminitis, and cytopenias being the most common toxicities. Among 30 evaluable patients, 1 had a partial response and 15 had stable disease. Triapine PK was similar, although more variable, compared to previous studies using doses normalized to body-surface-area. Steady decline in circulating levels of RRM2 may correlate with outcome.

CONCLUSIONS

This combination was well tolerated and showed evidence of preliminary activity in this heavily pretreated patient population, including prior gemcitabine failure.

A dose escalation and pharmacodynamic study of triapine and radiation in patients with locally advanced pancreas cancer

PURPOSE

Triapine, a novel inhibitor of the M2 subunit of ribonucleotide reductase (RR), is a potent radiosensitizer. This phase 1 study, sponsored by the National Cancer Institute Cancer Therapy Evaluation Program, assessed the safety and tolerability of triapine in combination with radiation (RT) in patients with locally advanced pancreas cancer (LAPCA).

METHODS AND MATERIALS

We evaluated 3 dosage levels of triapine (24 mg/m2, 48 mg/m2, 72 mg/m2) administered with 50.4 Gy of RT in 28 fractions. Patients with LAPCA received triapine thrice weekly, every other week during the course of RT. Dose-limiting toxicity (DLT) was assessed during RT and for 4 weeks after its completion. Dynamic contrast-enhanced magnetic resonance imaging and serum RR levels were evaluated as potential predictors for early response.

RESULTS

Twelve patients were treated. Four patients (1 nonevaluable) were enrolled at dosage level 1 (DL1), 3 patients at DL2, and 5 patients (2 nonevaluable) at DL3. No DLTs were observed, and the maximum tolerated dose was not reached. Two patients (17%) achieved partial response, and 6 patients (50%) had stable disease. One patient underwent R0 resection after therapy. Ninety-two percent of patients (100% at DL3) experienced freedom from local tumor progression. In 75% of patients who eventually experienced progression, metastases developed without local progression. RR levels did not seem to predict outcome. In 4 patients with available data, dynamic contrast-enhanced magnetic resonance imaging may predict early response or resistance to therapy.

CONCLUSION

The combination of triapine at 72 mg/m2 3 times weekly every other week and standard RT is tolerable with interesting activity in patients with LAPCA.

Methemoglobin formation by triapine, di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT), and other anticancer thiosemicarbazones: identification of novel thiosemicarbazones and therapeutics that prevent this effect

Thiosemicarbazones are a group of compounds that have received comprehensive investigation as anticancer agents. The antitumor activity of the thiosemicarbazone, 3-amino-2-pyridinecarboxaldehyde thiosemicarbazone (3-AP; triapine), has been extensively assessed in more than 20 phase I and II clinical trials. These studies have demonstrated that 3-AP induces methemoglobin (metHb) formation and hypoxia in patients, limiting its usefulness. Considering this problem, we assessed the mechanism of metHb formation by 3-AP compared with that of more recently developed thiosemicarbazones, including di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT). This was investigated using intact red blood cells (RBCs), RBC lysates, purified oxyhemoglobin, and a mouse model. The chelation of cellular labile iron with the formation of a redox-active thiosemicarbazone-iron complex was found to be crucial for oxyhemoglobin oxidation. This observation was substantiated using a thiosemicarbazone that cannot ligate iron and also by using the chelator, desferrioxamine, that forms a redox-inactive iron complex. Of significance, cellular copper chelation was not important for metHb generation in contrast to its role in preventing tumor cell proliferation. Administration of Dp44mT to mice catalyzed metHb and cardiac metmyoglobin formation. However, ascorbic acid administered together with the drug in vivo significantly decreased metHb levels, providing a potential therapeutic intervention. Moreover, we demonstrated that the structure of the thiosemicarbazone is of importance in terms of metHb generation, because the DpT analog, di-2-pyridylketone-4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC), does not induce metHb generation in vivo. Hence, DpC represents a next-generation thiosemicarbazone that possesses markedly superior properties. This investigation is important for developing more effective thiosemicarbazone treatment regimens.

Management of 3-aminopyridine-2-carboxaldehyde thiosemicarbazone-induced methemoglobinemia

The anticancer agent 3-aminopyridine-2-carboxaldehyde thiosemicarbazone is a ribonucleotide reductase inhibitor. It inactivates ribonucleotide reductase by disrupting an iron-stabilized radical in ribonucleotide reductase's small subunits, M2 and M2b (p53R2). Unfortunately, 3-aminopyridine-2-carboxaldehyde thiosemicarbazone also alters iron II (Fe(2+)) in hemoglobin. This creates Fe(3+) methemoglobin that does not deliver oxygen. Fe(2+) in hemoglobin normally auto-oxidizes to inactive Fe(3+) methemoglobin at a rate of nearly 3% per day and this is counterbalanced by a reductase system that normally limits methemoglobin concentrations to less than 1% of hemoglobin. This balance may be perturbed by symptomatic toxicity levels during 3-aminopyridine-2-carboxaldehyde thiosemicarbazone therapy. Indications of 3-aminopyridine-2-carboxaldehyde thiosemicarbazone sequelae attributable to methemoglobinemia include resting dyspnea, headaches and altered cognition. Management of methemoglobinemia includes supplemental oxygen, ascorbate and, most importantly, intravenously administered methylene blue as a therapeutic antidote.

Ribonucleotide reductase inhibition restores platinum-sensitivity in platinum-resistant ovarian cancer: a Gynecologic Oncology Group Study

Background

The potent ribonucleotide reductase (RNR) inhibitor 3-aminopyridine-2-carboxyaldehyde-thiosemicarbazone (3-AP) was tested as a chemosensitizer for restored cisplatin-mediated cytotoxicity in platinum-resistant ovarian cancer.

Methods

Preclinical in vitro platinum-resistant ovarian cancer cell survival, RNR activity, and DNA damage assays were done after cisplatin or cisplatin plus 3-AP treatments. Six women with platinum-resistant ovarian cancer underwent four-day 3-AP (96 mg/m², day one to four) and cisplatin (25 mg/m², day two and three) infusions every 21 days until disease progression or adverse effects prohibited further therapy. Pre-therapy ovarian cancer tissues were analyzed by immunohistochemistry for RNR subunit expression as an indicator of cisplatin plus 3-AP treatment response.

Results

3-AP preceding cisplatin exposure in platinum-resistant ovarian cancer cells was not as effective as sequencing cisplatin plus 3-AP together in cell survival assays. Platinum-mediated DNA damage (i.e., γH2AX foci) resolved quickly after cisplatin-alone or 3-AP preceding cisplatin exposure, but persisted after a cisplatin plus 3-AP sequence. On trial, 25 four-day overlapping 3-AP and cisplatin cycles were administered to six women (median 4.2 cycles per patient). 3-AP-related methemoglobinemia (range seven to 10%) occurred in two (33%) of six women, halting trial accrual.

Conclusions

When sequenced cisplatin plus 3-AP, RNR inhibition restored platinum-sensitivity in platinum-resistant ovarian cancers. 3-AP (96 mg/m²) infusions produced modest methemoglobinemia, the expected consequence of ribonucleotide reductase inhibitors disrupting collateral proteins containing iron.

Trial registry

ClinicalTrials.gov NCT00081276

Iron deprivation suppresses hepatocellular carcinoma growth in experimental studies

PURPOSE

Hepatocellular carcinoma (HCC) is the third most common cause of cancer-related death, and iron overload is a significant risk factor in the development of HCC. In this study, we investigated the potential application of depriving iron by a novel iron chelator, thiosemicarbazone-24 (TSC24), in HCC treatment.

EXPERIMENTAL DESIGN

Two HCC cell lines and HFE knockout (HFE(-/-)) mice were used to determine iron chelation efficiency of TSC24. The anticancer effects of TSC24 on HCC were analyzed in vitro and in athymic xenograft mouse models.

RESULTS

Treatment with TSC24 significantly decreased the cellular iron concentration in hepatoma cells and the serum iron concentration in HFE(-/-) mice by blocking iron uptake and interfering with normal regulation of iron levels. Moreover, the viability of HCC cell lines was reduced by TSC24. Confirming the mechanism of the agent, this decrease in viability could be partially rescued by addition of exogenous iron. TSC24 also suppressed tumor growth in athymic mice bearing human HCC xenografts in a concentration-dependent manner, without apparent toxicity in parallel with a decrease in the serum iron level. Further studies revealed that TSC24 efficiently triggered cell-cycle arrest and apoptosis in Hep3B and HepG2 cell lines.

CONCLUSIONS

TSC24 is a potent iron chelator that suppresses human HCC tumor growth by disrupting iron homeostasis, reducing available iron, and triggering cell-cycle arrest and apoptosis, without apparent host toxicity at effective doses. Thus, TSC24 shows great potential for the treatment of HCC.

Synthetic and natural iron chelators: therapeutic potential and clinical use

Iron-chelation therapy has its origins in the treatment of iron-overload syndromes. For many years, the standard for this purpose has been deferoxamine. Recently, considerable progress has been made in identifying synthetic chelators with improved pharmacologic properties relative to deferoxamine. Most notable are deferasirox (Exjade(®)) and deferiprone (Ferriprox(®)), which are now available clinically. In addition to treatment of iron overload, there is an emerging role for iron chelators in the treatment of diseases characterized by oxidative stress, including cardiovascular disease, atherosclerosis, neurodegenerative diseases and cancer. While iron is not regarded as the underlying cause of these diseases, it does play an important role in disease progression, either through promotion of cellular growth and proliferation or through participation in redox reactions that catalyze the formation of reactive oxygen species and increase oxidative stress. Thus, iron chelators may be of therapeutic benefit in many of these conditions. Phytochemicals, many of which bind iron, may also owe some of their beneficial properties to iron chelation. This review will focus on the advances in iron-chelation therapy for the treatment of iron-overload disease and cancer, as well as neurodegenerative and chronic inflammatory diseases. Established and novel iron chelators will be discussed, as well as the emerging role of dietary plant polyphenols that effectively modulate iron biochemistry.

Advances in iron chelation: an update

INTRODUCTION

Oxidative stress (caused by excess iron) can result in tissue damage, organ failure and finally death, unless treated by iron chelators. The causative factor in the etiology of a variety of disease states is the presence of iron-generated reactive oxygen species (ROS), which can result in cell damage or which can affect the signaling pathways involved in cell necrosis-apoptosis or organ fibrosis, cancer, neurodegeneration and cardiovascular, hepatic or renal dysfunctions. Iron chelators can reduce oxidative stress by the removal of iron from target tissues. Equally as important, removal of iron from the active site of enzymes that play key roles in various diseases can be of considerable benefit to the patients.

AREAS COVERED

This review focuses on iron chelators used as therapeutic agents. The importance of iron in oxidative damage is discussed, along with the three clinically approved iron chelators.

EXPERT OPINION

A number of iron chelators are used as approved therapeutic agents in the treatment of thalassemia major, asthma, fungal infections and cancer. However, as our knowledge about the biochemistry of iron and its role in etiologies of seemingly unrelated diseases increases, new applications of the approved iron chelators, as well as the development of new iron chelators, present challenging opportunities in the areas of drug discovery and development.

A copper chelate of thiosemicarbazone NSC 689534 induces oxidative/ER stress and inhibits tumor growth in vitro and in vivo

In this study, a Cu(2+) chelate of the novel thiosemicarbazone NSC 689534 was evaluated for in vitro and in vivo anti-cancer activity. Results demonstrated that NSC 689534 activity (low micromolar range) was enhanced four- to fivefold by copper chelation and completely attenuated by iron. Importantly, once formed, the NSC 689534/Cu(2+) complex retained activity in the presence of additional iron or iron-containing biomolecules. NSC 689534/Cu(2+) mediated its effects primarily through the induction of ROS, with depletion of cellular glutathione and protein thiols. Pretreatment of cells with the antioxidant N-acetyl-l-cysteine impaired activity, whereas NSC 689534/Cu(2+) effectively synergized with the glutathione biosynthesis inhibitor buthionine sulfoximine. Microarray analysis of NSC 689534/Cu(2+)-treated cells highlighted activation of pathways involved in oxidative and ER stress/UPR, autophagy, and metal metabolism. Further scrutiny of the role of ER stress and autophagy indicated that NSC 689534/Cu(2+)-induced cell death was ER-stress dependent and autophagy independent. Last, NSC 689534/Cu(2+) was shown to have activity in an HL60 xenograft model. These data suggest that NSC 689534/Cu(2+) is a potent oxidative stress inducer worthy of further preclinical investigation.

The iron-chelating drug triapine causes pronounced mitochondrial thiol redox stress

Triapine (Tp) is an iron chelator with activity against several types of cancer. Iron-Tp [Fe(III)(Tp)(2)] can be redox-cycled to generate reactive oxygen species that may contribute to its cytotoxicity. However, evidence for this mechanism in cells is limited. The cytosolic and mitochondrial thioredoxins (Trx1 and Trx2, respectively) are essential for cell survival. They are normally maintained in the reduced state, and support the function of many intracellular proteins including the peroxiredoxins (Prxs). Their redox status can indicate oxidant stress in their respective subcellular compartments. Tp treatment of human lung A549 cells caused almost complete oxidation of Trx2 and its dependent peroxiredoxin (Prx3), but there was no effect on Trx1 redox status. Significant inhibition of total TrxR activity did not occur until Tp levels were 4-fold above those needed to cause Trx2 oxidation. While Tp caused a 36-45% decline in reduced glutathione (GSH) levels, GSH accounted for >99% of the total glutathione in the absence and presence of Tp. In vitro studies demonstrated that cysteine reduces Fe(III)(Tp)(2) to Fe(II)(Tp)(2), and cysteine was faster and more efficient than reduced glutathione (GSH) in this regard. Fe(III)(Tp)(2) also mediated the oxidation of purified Trx2 in vitro. Thus, Fe(III)(Tp)(2) itself, and/or various reactive species that may result from its redox cycling, could account for Trx2 and Prx3 oxidation in Tp-treated cells. The striking difference between the effects on Trx2 and Trx1 implies a pronounced thiol redox stress that is largely directed at the mitochondria. These previously unrecognized effects of Tp could contribute to its overall cytotoxicity.

2-Acetylpyridine thiosemicarbazones are potent iron chelators and antiproliferative agents: redox activity, iron complexation and characterization of their antitumor activity

Through systematic structure-activity studies of the 2-benzoylpyridine thiosemicarbazone (HBpT), 2-(3-nitrobenzoyl)pyridine thiosemicarbazone (HNBpT) and dipyridylketone thiosemicarbazone (HDpT) series of iron (Fe) chelators, we identified structural features necessary to form Fe complexes with potent anticancer activity (J. Med. Chem. 2007, 50, 3716-3729). In this investigation, we generated the related 2-acetylpyridine thiosemicarbazone (HApT) analogues to examine the influence of the methyl group at the imine carbon. Four of the six HApT chelators had potent antitumor activity (IC(50): 0.001-0.002 microM) and Fe chelation efficacy that was similar to the most effective HBpT and HDpT ligands. The HApT Fe complexes had the lowest Fe(III/II) redox potentials of any thiosemicarbazone series we have generated. This property, in combination with their ability to effectively chelate cellular Fe, make the HApT series one of the most potent antiproliferative agents developed by our group.

Phase I trial of pelvic radiation, weekly cisplatin, and 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, NSC #663249) for locally advanced cervical cancer

PURPOSE

This study assessed the safety/tolerability, pharmacokinetics, and clinical activity of three times weekly i.v. 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, NSC #663249) in combination with once-weekly i.v. cisplatin and daily pelvic radiation in patients with gynecologic malignancies. 3-AP is a novel small-molecule inhibitor of ribonucleotide reductase (RNR) and is being tested as a potential radiosensitizer and chemosensitizer.

EXPERIMENTAL DESIGN

Patients with stage IB2 to IVB cervical cancer (n = 10) or recurrent uterine sarcoma (n = 1) were assigned to dose-finding cohorts of 2-hour 3-AP infusions during 5 weeks of cisplatin chemoradiation. Pharmacokinetic and methemoglobin samples and tumor biopsy for RNR activity were obtained on day 1 and day 10. Clinical response was assessed.

RESULTS

The maximum tolerated 3-AP dose was 25 mg/m(2) given three times weekly during cisplatin and pelvic radiation. Two patients experienced manageable 3-AP-related grade 3 or 4 electrolyte abnormalities. 3-AP pharmacokinetics showed a 2-hour half-life, with median peak plasma concentrations of 277 ng/mL (25 mg/m(2)) and 467 ng/mL (50 mg/m(2)). Median methemoglobin levels peaked at 1% (25 mg/m(2)) and 6% (50 mg/m(2)) at 4 hours after initiating 3-AP infusions. No change in RNR activity was found on day 1 versus day 10 in six early complete responders, whereas elevated RNR activity was seen on day 10 as compared with day 1 in four late complete responders (P = 0.02). Ten (100%) patients with stage IB2 to IVB cervical cancer achieved complete clinical response and remained without disease relapse with a median 18 months of follow-up (6-32 months).

CONCLUSIONS

3-AP was well tolerated at a three times weekly i.v. 25 mg/m(2) dose during cisplatin and pelvic radiation. Clin Cancer Res; 16(4); 1298-306.

The maximum tolerated dose and biologic effects of 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP) in combination with irinotecan for patients with refractory solid tumors

PURPOSE

3-AP is a ribonucleotide reductase inhibitor and has been postulated to act synergistically with other chemotherapeutic agents. This study was conducted to determine the toxicity and antitumor activity of 3-AP with irinotecan. Correlative studies included pharmacokinetics and the effects of ABCB1 and UGT1A1 polymorphisms.

METHODS

The treatment plan consisted of irinotecan on day 1 with 3-AP on days 1-3 of a 21-day cycle. Starting dose was irinotecan 150 mg/m(2) and 3-AP 85 mg/m(2) per day. Polymorphisms of ABCB1 were evaluated by pyrosequencing. Drug concentrations were determined by HPLC.

RESULTS

Twenty-three patients were enrolled, 10 men and 13 women. Tumor types included seven patients with pancreatic cancer, four with lung cancer, two with cholangiocarcinoma, two with mesothelioma, two with ovarian cancer, and six with other malignancies. Two patients experienced dose-limiting toxicity (DLT) at dose level 1, requiring amendment of the dose-escalation scheme. Maximal tolerated dose (MTD) was determined to be 3-AP 60 mg/m(2) per day and irinotecan 200 mg/m(2). DLTs consisted of hypoxia, leukopenia, fatigue, infection, thrombocytopenia, dehydration, and ALT elevation. One partial response in a patient with refractory non-small cell lung cancer was seen. Genotyping suggests that patients with wild-type ABCB1 have a higher rate of grade 3 or 4 toxicity than those with ABCB1 mutations.

CONCLUSIONS

The MTD for this combination was 3-AP 60 mg/m(2) per day on days 1-3 and irinotecan 200 mg/m(2) on day 1 every 21 days. Antitumor activity in a patient with refractory non-small cell lung cancer was noted at level 1.

Modulating radiation resistance by inhibiting ribonucleotide reductase in cancers with virally or mutationally silenced p53 protein

Therapeutic ionizing radiation damages DNA, increasing p53-regulated ribonucleotide reductase (RNR) activity required for de novo synthesis of the deoxyribonucleotide triphosphates used during DNA repair. This study investigated the pharmacological inhibition of RNR in cells of virally or mutationally silenced p53 cancer cell lines using 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, Triapine(R), NSC #663249), a chemotherapeutic radiosensitizer that equally inhibits RNR M2 and p53R2 small subunits. The effects of 3-AP on RNR inhibition and resulting radiosensitization were evaluated in cervical (CaSki, HeLa and C33-a) and colon (RKO, RKO-E6) cancer cells. 3-AP treatment significantly enhanced radiation-related cytotoxicity in cervical and colon cancer cells. 3-AP treatment significantly decreased RNR activity, caused prolonged radiation-induced DNA damage, and resulted in an extended G(1)/S-phase cell cycle arrest in all cell lines. Similar effects were observed in both RKO and RKO-E6 cells, suggesting a p53-independent mechanism of radiosensitization. We conclude that inhibition of ribonucleotide reductase by 3-AP enhances radiation-mediated cytotoxicity independent of p53 regulation by impairing repair processes that rely on deoxyribonucleotide production, thereby substantially increasing the radiation sensitivity of human cancers.

Synthesis and biological properties of iron chelators based on a bis-2-(2-hydroxy-phenyl)-thiazole-4-carboxamide or -thiocarboxamide (BHPTC) scaffold

Bis-2-(2-hydroxy-phenyl)-thiazole-4-carboxamides and -thiocarboxamides (BHPTCs) form a family of gemini hexacoordinated bis-tridentate chelating scaffolds. Four molecules were synthesized and shown to chelate iron(III) efficiently with a 1:1 stoichiometry. A dithioamide BHPTC displayed promising antiproliferative activity in several cancerous cell lines, making this molecule an interesting lead compound for the design of new iron-chelating anticancer drugs. Conversely, diamide BHPTCs had significant cytoprotective activity against iron overload in HepaRG cells in vitro, and were as efficient as and less toxic than deferoxamine B (DFO).

Phase II study of 3-AP Triapine in patients with recurrent or metastatic head and neck squamous cell carcinoma

BACKGROUND

Treatment options for recurrent or metastatic head and neck squamous cell carcinoma (HNSCC) are limited with response rates to cytotoxic chemotherapy of approximately 30% and median survival of 6 months.

PATIENTS AND METHODS

In a multicentre phase II study, 32 patients with recurrent or metastatic HNSCC received 3-AP Triapine (3-aminopyridine-2-carboxaldehyde thiosemicarbazone), an inhibitor of ribonucleotide reductase, 96 mg/m2, daily for 4 days every 14 days (one cycle). Eligibility criteria required Eastern Cooperative Oncology Group performance status (ECOG PS) of zero to two with a life expectancy of >3 months; one prior chemotherapy regimen was allowed.

RESULTS

Thirty patients were assessable for response and toxicity. Median age was 57 years (range 36-79) and median ECOG PS was one (range 0-2). Thirteen patients had previously been treated with chemotherapy. A total of 130 cycles were administered with a median number of cycles of 3.5 (range 1-8). Mild anaemia (40%), nausea (22%) and fatigue (22%) were commonly reported with G3 and G4 neutropenia documented in 22% and 22%, respectively. Overall response rate was 5.9% (95% confidence interval 0.2% to 28.7%). One patient achieved a partial response, eight had stable disease and 21 progressive disease. Median time to disease progression was 3.9 months.

CONCLUSIONS

3-AP Triapine as a single agent, at this dose and schedule, is well tolerated but has only minor activity in the treatment of advanced HNSCC.

A phase 2 consortium (P2C) trial of 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP) for advanced adenocarcinoma of the pancreas

3-Aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, Triapine) is a novel small molecule inhibitor of ribonucleotide reductase (RR) with clinical signs of activity in pancreatic cancer. Therefore, the Phase 2 Consortium (P2C) initiated a trial (two single stage studies with planned interim analysis) of 3-AP at 96 mg/m(2) intravenously days 1-4 and 15-18 of a 28-day cycle in both chemotherapy-naive and gemcitabine-refractory (GR) patients with advanced pancreatic cancer. The primary endpoint was survival at six months (chemotherapy-naive) and four months (GR). Secondary endpoints were toxicity, response, overall survival, time to progression and mechanistic studies. Fifteen patients were enrolled including one chemotherapy-naïve and 14 GR. The chemotherapy-naïve patient progressed during cycle 1 with grade 3 and 4 toxicities. Of 14 GR patients, seven received two cycles, six received one cycle and one received eight cycles. Progression precluded further treatment in 11 GR patients. Additionally, one died of an ileus in cycle 1 considered related to treatment and two stopped treatment due to toxicity. Five GR patients had grade 4 toxicities possibly related to 3-AP and six GR patients had grade 3 fatigue. Toxicities and lack of meaningful clinical benefit prompted early study closure. Four-month survival in GR patients was 21% (95% CI: 8-58%). Correlative studies confirmed that 3-AP increased the percentage of S-phase buccal mucosal cells, the presence of multidrug resistance gene polymorphisms appeared to predict leukopenia, and baseline pancreatic tumor RR M2 expression was low relative to other tumors treated with 3-AP. In conclusion, this regimen appears inactive against predominantly GR pancreatic cancer. RR M2 protein may not have a critical role in the malignant potential of pancreatic cancer.

Cancer cell iron metabolism and the development of potent iron chelators as anti-tumour agents

Cancer contributes to 50% of deaths worldwide and new anti-tumour therapeutics with novel mechanisms of actions are essential to develop. Metabolic inhibitors represent an important class of anti-tumour agents and for many years, agents targeting the nutrient folate were developed for the treatment of cancer. This is because of the critical need of this factor for DNA synthesis. Similarly to folate, Fe is an essential cellular nutrient that is critical for DNA synthesis. However, in contrast to folate, there has been limited effort applied to specifically design and develop Fe chelators for the treatment of cancer. Recently, investigations have led to the generation of novel di-2-pyridylketone thiosemicarbazone (DpT) and 2-benzoylpyridine thiosemicarbazone (BpT) group of ligands that demonstrate marked and selective anti-tumour activity in vitro and also in vivo against a wide spectrum of tumours. Indeed, administration of these compounds to mice did not induce whole body Fe-depletion or disturbances in haematological or biochemical indices due to the very low doses required. The mechanism of action of these ligands includes alterations in expression of molecules involved in cell cycle control and metastasis suppression, as well as the generation of redox-active Fe complexes. This review examines the alterations in Fe metabolism in tumour cells and the systematic development of novel aroylhydrazone and thiosemicarbazone Fe chelators for cancer treatment.

Phase I study of the ribonucleotide reductase inhibitor 3-aminopyridine-2-carboxaldehyde-thiosemicarbazone (3-AP) in combination with high dose cytarabine in patients with advanced myeloid leukemia

PURPOSE

This Phase I dose escalation study was based on the hypothesis that the addition of 3-aminopyridine-2-carboxaldehyde-thiosemicarbazone (3-AP) to cytarabine would enhance cytarabine cytotoxicity. The primary objective of the study was to establish the maximum tolerated dose of 3-AP when given in combination with a fixed dose of cytarabine.

EXPERIMENTAL DESIGN

Twenty-five patients with relapsed or refractory myeloid leukemia were enrolled to three dose levels of 3-AP. Cytarabine was administered as a 2 h infusion at a fixed dose of 1,000 mg/m2/day for 5 consecutive days. Escalating doses of 3-AP as a 2 h infusion were administered on days 2 through 5. The 3-AP infusion preceded the start of the cytarabine infusion by 4 h.

RESULTS

In general, the toxicities observed with the combination were similar to the expected toxicity profile for cytarabine when utilized as a single agent at this dose and schedule. However, two of three patients developed dose-limiting methemoglobinemia at the highest 3-AP dose studied (100 mg/m2). Transient reversible methemoglobinemia was documented in 11 of 15 patients enrolled at the 75 mg/ m2 dose level. Objective evidence of clinical activity was observed in four patients.

CONCLUSIONS

The combination of 3-AP and cytarabine given on this schedule is feasible in advanced myeloid leukemia. The recommended Phase II dose is 75 mg/m2/day of 3-AP on days 2-5 given prior to cytarabine administered at a dose of 1,000 mg/m2/day over 5 consecutive days. Methemoglobinemia is a common toxicity of this combination and requires close monitoring.

Chelators at the cancer coalface: desferrioxamine to Triapine and beyond

The importance of iron and copper in cancer biology has been well established. Iron plays a fundamental role in cellular proliferation and copper has been shown to be a significant cofactor for angiogenesis. Early observations with the chelator used for the treatment of iron overload, desferrioxamine, showed that it had promise as an anticancer agent. These results sparked great interest in the possibility of developing more effective iron chelators for cancer therapy. The recent entry into clinical trials of the iron-binding drug, Triapine, provides evidence of the potential of this antitumor strategy. Likewise, chelators originally designed to treat disorders of copper overload, such as penicillamine, trientine, and tetrathiomolybdate, have also emerged as potential anticancer drugs, as they are able to target the key angiogenic cofactor, copper. In this review, we will discuss the development of these and other chelators that show potential as anticancer agents.

Transferrin receptor 2 is frequently expressed in human cancer cell lines

Different proteins ensure the fine control of iron metabolism at the level of various tissues. Among these proteins, it was discovered a second transferrin receptor (TfR2), that seems to play a key role in the regulation of iron homeostasis. Its mutations are responsible for type 3 hemochromatosis (Type 3 HH). Although TfR2 expression in normal tissues was restricted at the level of liver and intestine, we observed that TfR2 was frequently expressed in tumor cell lines. Particularly frequent was its expression in ovarian cancer, colon cancer and glioblastoma cell lines; less frequent was its expression in leukemic and melanoma cell lines. Interestingly, in these tumor cell lines, TfR2 expression was inversely related to that of receptor 1 for transferrin (TfR1). Experiments of in vitro iron loading or iron deprivation provided evidence that TfR2 is modulated in cancer cell lines according to cellular iron levels following two different mechanisms: (i) in some cells, iron loading caused a downmodulation of total TfR2 levels; (ii) in other cell types, iron loading caused a downmodulation of membrane-bound TfR2, without affecting the levels of total cellular TfR2 content. Iron deprivation caused in both conditions an opposite effect compared to iron loading. These observations suggest that TfR2 expression may be altered in human cancers and warrant further studies in primary tumors. Furthermore, our studies indicate that, at least in tumor cells, TfR2 expression is modulated by iron through different biochemical mechanisms, whose molecular basis remains to be determined.

A Phase II study of 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP) and gemcitabine in advanced pancreatic carcinoma. A trial of the Princess Margaret hospital Phase II consortium

3-Aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, Triapine, Vion Pharmaceuticals, New Haven, CT) is an inhibitor of the M2 subunit of ribonucleotide reductase (RR). Preclinical testing demonstrates synergy between 3-AP and gemcitabine. Phase I studies of the combination have suggested tolerability and some initial evidence of efficacy. Therefore, a phase II study of gemcitabine plus 3-AP in advanced pancreatic carcinoma was undertaken. In this two-step phase II trial, patients with advanced pancreatic adenocarcinoma who had not received prior chemotherapy for advanced disease were treated with 3-AP 105 mg/m(2) given over 2 h. Four hours after the 3-AP infusion was completed, gemcitabine 1,000 mg/m(2) was given over 30 min. Both drugs were given on days 1, 8 and 15 of a 28-day cycle.Twenty-six patients were enrolled to the study. One patient withdrew consent prior to receiving any treatment and is excluded from all further analyses. Four patients discontinued treatment due to adverse effects. Grade 3/4 hematological adverse events included neutropenia, thrombocytopenia, lymphopenia, leukopenia and anemia and the most frequent non-hematological adverse events were fatigue and pain. No objective responses were observed. Eleven patients had stable disease (SD). In five of these eleven patients, SD lasted for more than 6 months. The median time to progression was 4.1 months and the 6 month progression-free survival rate was 29%. The median survival was 9.0 months with a 1-year survival of 28.0%. The combination of 3-AP and gemcitabine is associated with moderate toxicity in patients with advanced pancreatic cancer. This two-stage trial was stopped after stage I due to lack of antitumour activity. On the basis of this clinical trial, the combination of gemcitabine and 3-AP at this dose and schedule does not warrant further study in this patient population.

The synergistic interaction of gemcitabine and cytosine arabinoside with the ribonucleotide reductase inhibitor triapine is schedule dependent

Gemcitabine and ara-C have multiple mechanisms of action: DNA incorporation and for gemcitabine also ribonucleotide reductase (RNR) inhibition. Since dCTP competes with their incorporation into DNA, dCTP depletion can potentiate their cytotoxicity. We investigated whether additional RNR inhibition by Triapine (3-AP), a new potent RNR inhibitor, enhanced cytotoxicity of gemcitabine and ara-C in four non-small-cell-lung-cancer (NSCLC) cell lines, using the multiple-drug-effect analysis. Simultaneous and sequential exposure (preexposure to 3-AP for 24h) in a constant molar ratio of 3-AP and gemcitabine was antagonistic/additive in all cell lines. Preexposure to 3-AP at an IC(25) concentration for 24h before variable concentrations of gemcitabine was synergistic. RNR inhibition by 3-AP resulted in a more synergistic interaction in combination with ara-C, which does not inhibit RNR. Two cell lines with pronounced synergism (SW1573) or antagonism (H460) for gemcitabine/3-AP, were evaluated for accumulation of the active metabolites, dFdCTP and ara-CTP. Simultaneous exposure induced no or a small increase, but ara-CTP increased after pretreatment with 3-AP, 4-fold in SW1573 cells, but not in H460 (<1.5 fold). Ara-C and gemcitabine incorporation into DNA were more pronounced (about 2-fold increased) for sequential treatment in SW1573 compared to H460 cells (<1.5 fold). This was not related to the activity and expression of deoxycytidine kinase and the M2 subunit of RNR. In conclusion, RNR inhibition by 3-AP prior to gemcitabine or ara-C exposure stimulates accumulation of the active metabolites and incorporation into DNA. The combination 3-AP/Ara-C is more synergistic than 3-AP/gemcitabine possibly because gemcitabine already inhibits RNR, but ara-C does not.

Ribonucleotide reductase subunit M2 mRNA expression in pretreatment biopsies obtained from unresectable pancreatic carcinomas

BACKGROUND

Gemcitabine is an efficacious cytotoxic agent used in the treatment of unresectable pancreatic carcinoma (PC). Recently, gemcitabine resistance has been associated with the ribonucleotide reductase subunit M2 (RRM2). In this prospective study, we hypothesized that RRM2 expression in PC biopsy specimens would be a significant predictor of outcome.

METHODS

RRM2 mRNA expression in 35 endoscopic ultrasonography-guided fine needle aspiration biopsy (EUS-FNAB) samples was quantified using real-time quantitative reverse transcription-polymerase chain reaction.

RESULTS

Thirty-one of 35 biopsy specimens could be assessed for RRM2 expression levels. The mean RRM2 expression relative to glyceraldehyde-3-phosphate dehydrogenase was 0.248 (range, 0.00739 to 0.858). Eighteen patients (64.5%) had low RRM2 levels, and 13 patients (35.5%) had high RRM2 levels with a cutoff of 0.1. The median survival was 8.8 months for patients with low RRM2 levels and 5.0 months for patients with high levels (P < 0.05). In the low RRM2 expression group, a complete response (CR) was observed in one patient, and a partial response (PR) was observed in eight patients. In contrast, in the high RRM2 expression group, PR was observed in one patient, and CR was not observed. The overall response rate between the high and low expression groups was significantly different (50.0% vs. 7.7%, P < 0.05).

CONCLUSIONS

RRM2 mRNA expression of EUS-FNAB specimens is a key predictive marker of survival in gemcitabine-treated patients with PC.

Future of toxicology--iron chelators and differing modes of action and toxicity: the changing face of iron chelation therapy

Iron (Fe) chelation therapy was initially designed to alleviate the toxic effects of excess Fe evident in Fe-overload diseases. However, the novel toxicological properties of some Fe chelator-metal complexes have shifted appreciable focus to their application in cancer chemotherapy. Redox-inactive Fe chelator complexes are well suited for the treatment of Fe-overload diseases, whereas Fe chelator complexes with high redox activity have shown promising results as chemotherapeutics against cancer. Within this perspective, we discuss the different modes of action and toxicological profiles of Fe chelators, including analogues of 2-pyridylcarboxaldehyde isonicotinoyl hydrazone, di-2-pyridylketone isonicotinoyl hydrazone, di-2-pyridylketone thiosemicarbazone, and the clinically trialed chelator 3-aminopyridine-2-carboxaldehyde thiosemicarbazone. The potential application of these agents in the changing face of Fe chelation therapy is discussed.