Structure-function relationships for a new series of pyridine-2-carboxaldehyde thiosemicarbazones on ribonucleotide reductase activity and tumor cell growth in culture and in vivo

ORAOV1, CCND1, and MIR548K Are the Driver Oncogenes of the 11q13 Amplicon in Squamous Cell Carcinoma

11q13 amplification is a frequent event in human cancer and in particular in squamous cell carcinomas (SCC). Despite almost invariably spanning 10 genes, it is unclear which genetic components of the amplicon are the key driver events in SCC. A combination of computational, in vitro, ex vivo, and in vivo models leveraging efficient primary human keratinocyte genome editing by Cas9-RNP electroporation, identified ORAOV1, CCND1, and MIR548K as the critical drivers of the amplicon in head and neck SCC. CCND1 amplification drives the cell cycle in a CDK4/6/RB1-independent fashion and may confer a novel dependency on RRM2. MIR548K contributes to epithelial-mesenchymal transition. Finally, we identify ORAOV1 as an oncogene that acts likely via its ability to modulate reactive oxygen species. Thus, the 11q13 amplicon drives SCC through at least three independent genetic elements and suggests therapeutic targets for this morbid and lethal disease.

IMPLICATIONS

This work demonstrates novel mechanisms and ways to target these mechanisms underlying the most common amplification in squamous cell carcinoma, one of the most prevalent and deadly forms of human cancer.

Inhibitors of the Cancer Target Ribonucleotide Reductase, Past and Present

Ribonucleotide reductase (RR) is an essential multi-subunit enzyme found in all living organisms; it catalyzes the rate-limiting step in dNTP synthesis, namely, the conversion of ribonucleoside diphosphates to deoxyribonucleoside diphosphates. As expression levels of human RR (hRR) are high during cell replication, hRR has long been considered an attractive drug target for a range of proliferative diseases, including cancer. While there are many excellent reviews regarding the structure, function, and clinical importance of hRR, recent years have seen an increase in novel approaches to inhibiting hRR that merit an updated discussion of the existing inhibitors and strategies to target this enzyme. In this review, we discuss the mechanisms and clinical applications of classic nucleoside analog inhibitors of hRRM1 (large catalytic subunit), including gemcitabine and clofarabine, as well as inhibitors of the hRRM2 (free radical housing small subunit), including triapine and hydroxyurea. Additionally, we discuss novel approaches to targeting RR and the discovery of new classes of hRR inhibitors.

In silico screening identifies a novel small molecule inhibitor that counteracts PARP inhibitor resistance in ovarian cancer

Poly ADP-ribose polymerase (PARP) inhibitors are promising targeted therapy for epithelial ovarian cancer (EOC) with BRCA mutations or defective homologous recombination (HR) repair. However, reversion of BRCA mutation and restoration of HR repair in EOC lead to PARP inhibitor resistance and reduced clinical efficacy of PARP inhibitors. We have previously shown that triapine, a small molecule inhibitor of ribonucleotide reductase (RNR), impaired HR repair and sensitized HR repair-proficient EOC to PARP inhibitors. In this study, we performed in silico screening of small molecule libraries to identify novel compounds that bind to the triapine-binding pocket on the R2 subunit of RNR and inhibit RNR in EOC cells. Following experimental validation of selected top-ranking in silico hits for inhibition of dNTP and DNA synthesis, we identified, DB4, a putative RNR pocket-binding inhibitor markedly abrogated HR repair and sensitized BRCA-wild-type EOC cells to the PARP inhibitor olaparib. Furthermore, we demonstrated that the combination of DB4 and olaparib deterred the progression of BRCA-wild type EOC xenografts and significantly prolonged the survival time of tumor-bearing mice. Herein we report the discovery of a putative small molecule inhibitor of RNR and HR repair for combination with PARP inhibitors to treat PARP inhibitor-resistant and HR repair-proficient EOC.

Activity and electrochemical properties: iron complexes of the anticancer drug triapine and its analogs

Triapine (3-AP), is an iron-binding ligand and anticancer drug that is an inhibitor of human ribonucleotide reductase (RNR). Inhibition of RNR by 3-AP results in the depletion of dNTP precursors of DNA, thereby selectively starving fast-replicating cancer cells of nucleotides for survival. The redox-active form of 3-AP directly responsible for inhibition of RNR is the Fe(II)(3-AP)₂ complex. In this work, we synthesize 12 analogs of 3-AP, test their inhibition of RNR in vitro, and study the electronic properties of their iron complexes. The reduction and oxidation events of 3-AP iron complexes that are crucial for the inhibition of RNR are modeled with solution studies. We monitor the pH necessary to induce reduction in iron complexes of 3-AP analogs in a reducing environment, as well as the kinetics of oxidation in an oxidizing environment. The oxidation state of the complex is monitored using UV-Vis spectroscopy. Isoquinoline analogs of 3-AP favor the maintenance of the biologically active reduced complex and possess oxidation kinetics that allow redox cycling, consistent with their effective inhibition of RNR seen in our in vitro experiments. In contrast, methylation on the thiosemicarbazone secondary amine moiety of 3-AP produces analogs that form iron complexes with much higher redox potentials, that do not redox cycle, and are inactive against RNR in vitro. The catalytic subunit of human Ribonucleotide Reductase (RNR), contains a tyrosyl radical in the enzyme active site. Fe(II) complexes of 3-AP and its analogs can quench the radical and, subsequently, inactivate RNR. The potency of RNR inhibitors is highly dependent on the redox properties of the iron complexes, which can be tuned by ligand modifications. Complexes are found to be active within a narrow redox window imposed by the cellular environment.

Combination of triapine, olaparib, and cediranib suppresses progression of BRCA-wild type and PARP inhibitor-resistant epithelial ovarian cancer

PARP inhibitors target BRCA mutations and defective homologous recombination repair (HRR) for the treatment of epithelial ovarian cancer (EOC). However, the treatment of HRR-proficient EOC with PARP inhibitors remains challenging. The objective of this study was to determine whether the combination of triapine (ribonucleotide reductase inhibitor), cediranib (vascular endothelial growth factor receptor tyrosine kinase inhibitor), and the PARP inhibitor olaparib synergized against BRCA wild-type and HRR-proficient EOC in xenograft mouse models. In addition, the mechanisms by which cediranib augmented the efficacy of triapine and olaparib were investigated. BRCA-wild type and PARP inhibitor-resistant EOC cell lines were implanted subcutaneously (s.c.) into nude mice or injected intraperitoneally (i.p.) into SCID-Beige mice. Mice were then treated i.p. with olaparib, cediranib, triapine, various double and triple combinations. The volume of s.c tumor in nude mice and the abdominal circumference of SCID-Beige mice were measured to evaluate the effectiveness of the treatment to delay tumor growth and prolong the survival time of mice. In both xenograft mouse models, the combination of triapine, olaparib and cediranib resulted in marked suppression of BRCA-wild type EOC growth and significant prolongation of the survival time of mice, with efficacy greater than any double combinations and single drugs. Furthermore, we identified that cediranib abrogated pro-survival and anti-apoptotic AKT signaling, thereby enhancing the efficacy of triapine and olaparib against BRCA-wild type EOC cells. Taken together, our results demonstrate a proof-of-principle approach and the combination regiment holds promise in treating BRCA-wild type and PARP inhibitor-resistant EOC.

Evaluation of antioxidant and antiproliferative activities of 1,2-bis (p-amino-phenoxy) ethane derivative Schiff bases and metal complexes

In this study, the effects of the two Schiff base derivatives and their metal complexes were tested for MDA concentration, which is an indicator of lipid peroxidation, antioxidant vitamin A, vitamin E, and vitamin C levels in cell culture. A comparison was performed among the groups and it was observed that MDA, vitamin A, vitamin E, and vitamin C concentrations were statistically changed. According to the results, all compounds caused a significant oxidative stress without Zn complexes. Moreover, Mn(II), Cu(II), Zn(II), and Ni(II) complexes of Schiff bases derived from a condensation of 1,2-bis (p-aminophenoxy) ethane with naphthaldehydes and 4-methoxy benzaldehyde were examined in terms of antitumor activity against MCF-7 human breast cancer and L1210 murine leukemia cells. Furthermore, the derivatives were tested for antioxidative and prooxidative effects on MCF-7 breast cancer cells. The compounds which were tested revealed that there was an antitumor activity for MCF-7 and L 1210 cancer cells. Also, some of the compounds induced oxidative harmful.

Targeting Cyclin-Dependent Kinases for Treatment of Gynecologic Cancers

Ovarian, uterine/endometrial, and cervical cancers are major gynecologic malignancies estimated to cause nearly 30,000 deaths in 2018 in US. Defective cell cycle regulation is the hallmark of cancers underpinning the development and progression of the disease. Normal cell cycle is driven by the coordinated and sequential rise and fall of cyclin-dependent kinases (CDK) activity. The transition of cell cycle phases is governed by the respective checkpoints that prevent the entry into the next phase until cellular or genetic defects are repaired. Checkpoint activation is achieved by p53- and ATM/ATR-mediated inactivation of CDKs in response to DNA damage. Therefore, an aberrant increase in CDK activity and/or defects in checkpoint activation lead to unrestricted cell cycle phase transition and uncontrolled proliferation that give rise to cancers and perpetuate malignant progression. Given that CDK activity is also required for homologous recombination (HR) repair, pharmacological inhibition of CDKs can be exploited as a synthetic lethal approach to augment the therapeutic efficacy of PARP inhibitors and other DNA damaging modalities for the treatment of gynecologic cancers. Here, we overview the basic of cell cycle and discuss the mechanistic studies that establish the intimate link between CDKs and HR repair. In addition, we present the perspective of preclinical and clinical development in small molecule inhibitors of CDKs and CDK-associated protein targets, as well as their potential use in combination with hormonal therapy, PARP inhibitors, chemotherapy, and radiation to improve treatment outcomes.

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.

Triapine potentiates platinum-based combination therapy by disruption of homologous recombination repair

Background:

Platinum resistance may be attributable to inherent or acquired proficiency in homologous recombination repair (HRR) in epithelial ovarian cancer (EOC). The objective of this study was to evaluate the efficacy of the small molecule inhibitor triapine to disrupt HRR and sensitise BRCA wild-type EOC cells to platinum-based combination therapy in vitro and in vivo.

Methods:

The sensitivity of BRCA wild-type cancer cells to olaparib, cisplatin, carboplatin, doxorubicin, or etoposide in combination with triapine was evaluated by clonogenic survival assays. The effects of triapine on HRR activity in cells were measured with a DR-GFP reporter assay. The ability of triapine to enhance the effects of the carboplatin-doxil combination on EOC tumour growth delay was determined using a xenograft tumour mouse model.

Results:

Platinum resistance is associated with wild-type BRCA status. Triapine inhibits HRR activity and enhances the sensitivity of BRCA wild-type cancer cells to cisplatin, olaparib, and doxorubicin. However, sequential combination of triapine and cisplatin is necessary to achieve synergism. Moreover, triapine potentiates platinum-based combination therapy against BRCA wild-type EOC cells and produces significant delay of EOC tumour growth.

Conclusions:

Triapine promises to augment the clinical efficacy of platinum-based combination regimens for treatment of platinum-resistant EOC with wild-type BRCA and proficient HRR activity.

Distinct mechanisms of cell-kill by triapine and its terminally dimethylated derivative Dp44mT due to a loss or gain of activity of their copper(II) complexes

Triapine, currently being evaluated as an antitumor agent in phase II clinical trials, and its terminally dimethylated derivative Dp44mT share the α-pyridyl thiosemicarbazone backbone that functions as ligands for transition metal ions. Yet, Dp44mT is approximately 100-fold more potent than triapine in cytotoxicity assays. The aims of this study were to elucidate the mechanisms underlying their potency disparity and to determine their kinetics of cell-kill in culture to aid in the formulation of their clinical dosing schedules. The addition of Cu(2+) inactivated triapine in a 1:1 stoichiometric fashion, while it potentiated the cytotoxicity of Dp44mT. Clonogenic assays after finite-time drug-exposure revealed that triapine produced cell-kill in two phases, one completed within 20 min that caused limited cell-kill, and the other occurring after 16 h of exposure that produced extensive cell-kill. The ribonucleotide reductase inhibitor triapine at 0.4 μM caused immediate complete arrest of DNA synthesis, whereas Dp44mT at this concentration did not appreciably inhibit DNA synthesis. The inhibition of DNA synthesis by triapine was reversible upon its removal from the medium. Cell death after 16 h exposure to triapine paralleled the appearance of phospho-(γ)H2AX, a marker of DNA double-strand breaks induced by collapse of DNA replication forks after prolonged replication arrest. In contrast to triapine, Dp44mT produced robust cell-kill within 1h in a concentration-dependent manner. The short-term action of both agents was prevented by thiols, indicative of the involvement of reactive oxygen species. The time dependency in the production of cell-kill by triapine should be considered in treatment regimens.

Triapine disrupts CtIP-mediated homologous recombination repair and sensitizes ovarian cancer cells to PARP and topoisomerase inhibitors

PARP inhibitors exploit synthetic lethality to target epithelial ovarian cancer (EOC) with hereditary BRCA mutations and defects in homologous recombination repair (HRR). However, such an approach is limited to a small subset of EOC patients and compromised by restored HRR due to secondary mutations in BRCA genes. Here, it was demonstrated that triapine, a small-molecule inhibitor of ribonucleotide reductase, enhances the sensitivity of BRCA wild-type EOC cells to the PARP inhibitor olaparib and the topoisomerase II inhibitor etoposide. Triapine abolishes olaparib-induced BRCA1 and Rad51 foci, and disrupts the BRCA1 interaction with the Mre11-Rad50-Nbs1 (MRN) complex in BRCA1 wild-type EOC cells. It has been shown that phosphorylation of CtIP (RBBP8) is required for the interaction with BRCA1 and with MRN to promote DNA double-strand break (DSB) resection during S and G(2) phases of the cell cycle. Mechanistic studies within reveal that triapine inhibits cyclin-dependent kinase (CDK) activity and blocks olaparib-induced CtIP phosphorylation through Chk1 activation. Furthermore, triapine abrogates etoposide-induced CtIP phosphorylation and DSB resection as evidenced by marked attenuation of RPA32 phosphorylation. Concurrently, triapine obliterates etoposide-induced BRCA1 foci and sensitizes BRCA1 wild-type EOC cells to etoposide. Using a GFP-based HRR assay, it was determined that triapine suppresses HRR activity induced by an I-SceI-generated DSB. These results suggest that triapine augments the sensitivity of BRCA wild-type EOC cells to drug-induced DSBs by disrupting CtIP-mediated HRR.

IMPLICATIONS

These findings provide a strong rationale for combining triapine with PARP or topoisomerase inhibitors to target HRR-proficient EOC cells.

Inhibitory mechanisms of heterocyclic carboxaldehyde thiosemicabazones for two forms of human ribonucleotide reductase

Two forms of ribonucleotide reductase (RR), consisting of M1 with M2 subunits and M1 with p53R2 subunits, are involved in DNA replication and damage repair, respectively. 3-Aminopyridine-2-carboxaldehyde thiosemicarbazone (3AP), one of the heterocyclic carboxaldehyde thiosemicabazones (HCTs), is a potent RR inhibitor in clinical trial for cancer treatment. In this study, 3AP and its 7 derivatives showed 100-1000-fold higher inhibitory potency on KB nasopharyngeal carcinoma cells than hydroxyurea and were fully active against hydroxyurea- and gemcitabine-resistant KB cells. In vitro RR assays using two recombinant RRs showed that all 8 HCTs decreased the activity of both RRs in a dose-dependent manner and the efficiency was compatible with that on cell proliferation inhibition. Iron has different impact on the behavior of the compounds toward RRs. In the absence of iron, the HCTs showed more selective inhibition for p53R2-M1 than M2-M1, while addition of iron increased their activity but reduced their selectivity for two RRs. Radioligand binding assays showed that [(3)H]3AP directly bounded to the small subunits. Electron paramagnetic resonance measurements demonstrated that these HCTs generated reactive oxygen species with ferrous iron, which quenched the diiron-tyrosyl radical co-factor of the small subunits and hence the enzyme activity. While the ROS may be a common mediator responsible for the potent activity of the HCTs, the different characteristics of the small subunit proteins are probably associated with the subunit-selectivity of inhibition. Better understanding of the mechanism of action of RR inhibition may improve design of new potent and subunit-selective RR inhibitors for cancer therapy.

Phase I and pharmacokinetic study of Triapine, a potent ribonucleotide reductase inhibitor, in adults with advanced hematologic malignancies

Triapine, a potent inhibitor of ribonucleotide reductase, has demonstrated anti-leukemia activity in pre-clinical models. We conducted a Phase I study of Triapine administered as a 2 h infusion for 5 days in 25 adults with advanced leukemias. We established that Triapine at 96 mg/m2 once a day can be given safely on days 1-5 and 15-19 or 1-5 and 8-12 of a 4-week cycle. When administered twice a day on days 1-5 and 8-12, the maximum tolerated dose of Triapine appears to be 64 mg/m2, although the true criteria for DLT were not met by protocol definition. No CR or PR were observed, but 76% of patients had a >50% reduction in white blood cell counts. At all dose levels, the peak plasma concentration of Triapine (2.2-5.5 microM) was above levels required to achieve in vitro/in vivo leukemia growth inhibition. Based on these data, we conclude that Triapine warrants further investigation in hematologic malignancies.

Potent antitumor activity of novel iron chelators derived from di-2-pyridylketone isonicotinoyl hydrazone involves fenton-derived free radical generation

PURPOSE

The development of novel and potent iron chelators as clinically useful antitumor agents is an area of active interest. Antiproliferative activity of chelators often relates to iron deprivation or stimulation of iron-dependent free radical damage. Recently, we showed that novel iron chelators of the di-2-pyridylketone isonicotinoyl hydrazone (PKIH) class have potent and selective antineoplastic activity (E. Becker, et al., Br. J. Pharmacol., 138: 819-30, 2003). In this study, we assessed the effects of the PKIH analogues on the redox activity of iron in terms of understanding their antitumor activity.

EXPERIMENTAL DESIGN

We tested the PKIH analogues for their ability to promote iron-mediated ascorbate oxidation, benzoate hydroxylation, and plasmid degradation. Subsequent experiments assessed their ability to bind DNA, inhibit topoisomerase I, and cause DNA damage. To measure intracellular reactive oxygen species, we used the redox-sensitive probe, 2',7'-dichloro-fluorescein-diacetate, to measure intracellular PKIH-dependent redox activity.

RESULTS

The PKIH analogues had relatively little effect on ascorbate oxidation in the presence of Fe(III) but stimulated benzoate hydroxylation and plasmid DNA degradation in the presence of Fe(II) and H2O2. These ligands could not inhibit DNA topoisomerase I or cause DNA damage in intact cells. PKIH markedly increased the intracellular generation of reactive oxygen species, and this was inhibited by catalase. This enzyme also decreased the antiproliferative effect of PKIH, indicating H2O2 played a role in its cytotoxic activity.

CONCLUSIONS

Our results suggest that the antiproliferative effects of these chelators relates to intracellular iron chelation, followed by the stimulation of iron-mediated free radical generation via the so-formed iron complex.

Anticancer metal compounds in NCI's tumor-screening database: putative mode of action

Clustering analysis of tumor cell cytotoxicity profiles for the National Cancer Institute (NCI)'s open compound repository has been used to catalog over 1100 metal or metalloid containing compounds with potential anticancer activity. The molecular features and corresponding reactivity of these compounds have been analyzed in terms of properties of their metals, their associated organic components (ligands) and their capacity to inhibit tumor cell growth. Cytotoxic responses are influenced by both the identity of the metal and the properties of its coordination ligand, with clear associations between structural similarities and cytotoxicity. Assignments of mechanisms of action (MOAs) for these compounds could be segregated into four broad response classes according to preference for binding to biological sulfhydryl groups, chelation, generation of reactive oxygen species (ROS), and production of lipophilic ions. Correlations between specific cytotoxic responses and differential gene expression profiles within the NCI's tumor cell panel serve as a validation for candidate biological targets and putative MOA classes. In addition, specific sensitivity toward subsets of metal containing agents has been found for certain tumor cell panels. Taken together, our results expand the knowledge base available for evaluating, designing and developing new metal-based anticancer drugs that may provide the basis for target-specific therapeutics.

Iron chelators as therapeutic agents for the treatment of cancer

A wide variety of studies in vitro, in vivo, and in clinical trials have demonstrated that the chelator currently used to treat iron overload disease, desferrioxamine, has anti-proliferative effects against both leukemia and neuroblastoma. However, the efficacy of desferrioxamine is severely limited due to its poor ability to permeate cell membranes and chelate intracellular iron pools. These studies have led to the development of other iron chelators that are far more effective than desferrioxamine. Some of these chelators such as 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (Triapine) have entered phase I clinical trials, while other chelators such as 2-hydroxy-1-naphthylaldehyde isonicotinoyl hydrazone or tachpyridine require evaluation in animal models. The high anti-tumor activity observed with these ligands certainly suggests further development of chelators as anti-cancer agents is warranted.

Desferrithiocin is a more potent antineoplastic agent than desferrioxamine

Desferrithiocin (DFT) is an orally effective Fe chelator, with a similar high affinity and selectivity for Fe to desferrioxamine (DFO), which has been shown clinically to possess antineoplastic activity. In this study, DFT was assessed for antineoplastic potential in hepatocellular carcinoma cell lines (HCC). This was done as there are few treatments for this aggressive neoplasm. The effects of DFT on cell proliferation, cell cycle progression, Fe uptake and toxicity were examined. To establish whether DFT was selective for cancer cells a comparison was made with normal (non-proliferating) hepatocytes and non-tumorigenic (proliferating) fibroblasts (SWISS-3T3). DFT was a potent inhibitor of HCC proliferation (IC(50) approximately 40 microM). DFO also inhibited HCC proliferation under the same conditions, but was much less active (IC(50)=110 - 210 microM). When saturated with Fe, the activity of DFT, like DFO, was greatly diminished, suggesting it may act by depriving the cells of Fe or inactivating essential Fe pool(s). Indeed DFT rapidly decreased Fe uptake from Tf-(59)Fe by hepatoma cells and also by normal hepatocytes. However, DFT (and DFO) had much less effect on cell survival in hepatocytes and fibroblasts than in hepatoma cells. DFT may, like DFO, inhibit the cell cycle in the S phase of DNA synthesis. Both chelators showed low toxicity. These results indicate that DFT has potent antineoplastic activity in HCC. Further investigation into the DFT class of Fe chelators seems warranted, particularly in view of its high activity in relation to DFO, a chelator which is already in clinical trial for neuroblastoma.

Triapine (3-aminopyridine-2-carboxaldehyde- thiosemicarbazone): A potent inhibitor of ribonucleotide reductase activity with broad spectrum antitumor activity

Previous studies from our laboratories have shown that (a) Triapine() is a potent inhibitor of ribonucleotide reductase activity and (b) hydroxyurea-resistant L1210 leukemia cells are fully sensitive to Triapine. In an analogous manner, Triapine was similarly active against the wild-type and a hydroxyurea-resistant subline of the human KB nasopharyngeal carcinoma. Triapine was active in vivo against the L1210 leukemia over a broad range of dosages and was curative for some mice. This agent also caused pronounced inhibition of the growth of the murine M109 lung carcinoma and human A2780 ovarian carcinoma xenografts in mice. Optimum anticancer activity required twice daily dosing due to the duration of inhibition of DNA synthesis which lasted about 10 hr in L1210 cells treated with Triapine in vivo. DNA synthesis in normal mouse tissues (i.e. duodenum and bone marrow) uniformly recovered faster than that in L1210 leukemia cells, demonstrating a pharmacological basis for the therapeutic index of this agent. Triapine was more potent than hydroxyurea in inhibiting DNA synthesis in L1210 cells in vivo, and the effects of Triapine were more pronounced. In addition, the duration of the inhibition of DNA synthesis in leukemia cells from mice treated with Triapine was considerably longer than in those from animals treated with hydroxyurea. Combination of Triapine with various classes of agents that damage DNA (e.g. etoposide, cisplatin, doxorubicin, and 1-acetyl-1,2-bis(methylsulfonyl)-2-(2-chloroethyl)hydrazine) resulted in synergistic inhibition of the L1210 leukemia, producing long-term survivors of tumor-bearing mice treated with several dosage levels of the combinations, whereas no enhancement of survival was found when Triapine was combined with gemcitabine or cytosine arabinoside. The findings demonstrate the superiority of Triapine over hydroxyurea as an anticancer agent and further suggest that prevention by Triapine of repair of DNA lesions created by agents that damage DNA may result in efficacious drug combinations for the treatment of cancer.

Development of novel aroylhydrazone ligands for iron chelation therapy: 2-pyridylcarboxaldehyde isonicotinoyl hydrazone analogs

Previous studies have demonstrated that aroylhydrazone iron (Fe) chelators of the pyridoxal isonicotinoyl hydrazone (PIH) class have high Fe chelation efficacy both in vitro and in vivo. Depending on their design, these drugs may have potential as agents for the treatment of Fe overload disease or cancer. Considering the high potential of this class of ligands, we have synthesized seven novel aroylhydrazones in an attempt to identify Fe chelators more efficient than desferrioxamine (DFO) and more soluble than those of the PIH class. These compounds belong to a new series of tridentate chelators known as the 2-pyridylcarboxaldehyde isonicotinoyl hydrazones (PCIH). In this study we have examined the Fe chelation efficacy and antiproliferative activity of these chelators including their effects on the expression of genes (WAF1 and GADD45) known to be important in mediating cell cycle arrest at G1/S. From seven chelators synthesized, three ligands, namely 2-pyridylcarbox-aldehyde benzoyl hydrazone (PCBH), 2-pyridylcarboxaldehyde m-bromobenzoyl hydrazone (PCBBH), and 2-pyridylcarboxaldehyde 2-thiophenecarboxyl hydrazone (PCTH), showed greater Fe chelation activity than DFO and comparable or greater efficiency than PIH. These ligands were highly effective at both mobilizing 59Fe from cells and preventing 59Fe uptake from 59Fe-transferrin and caused a marked increase in the RNA-binding activity of the iron-regulatory proteins (IRP). Our studies have also demonstrated that compared with the cytotoxic Fe chelator, 2-hydroxy-1-naphthylaldehyde isonicotinoyl hydrazone (311), these ligands have far less effect on cellular growth and 3H-thymidine, 3H-leucine, or 3H-uridine incorporation. In addition, in contrast to 311, which markedly increased WAF1 and GADD45 mRNA expression, PCBH and PCTH did not have any effect, whereas PCBBH increased the expression of GADD45 mRNA. Collectively, these results demonstrate the potential of several of these ligands as agents for the management of Fe overload disease.

Triapine (3-aminopyridine-2-carboxaldehyde thiosemicarbazone; 3-AP): an inhibitor of ribonucleotide reductase with antineoplastic activity

The enzyme RR catalyzes the conversion of ribonucleoside diphosphates to their deoxyribonucleotide counterparts. RR is critical for the generation of the cytosine, adenine, and guanine deoxyribonucleotide 5'-triphosphate building blocks of DNA, which are present in cells as exceedingly small intracellular pools. Therefore, interference with the function of RR might well result in an agent with significant antineoplastic activity, particularly against rapidly proliferating tumor cells. HUr is the only inhibitor of RR in clinical usage; this agent, however, is a relatively poor inhibitor of the enzyme and has a short serum half-life. Consequently, HUr is a relatively weak anticancer agent. In an effort to develop a more potent inhibitor of RR with utility as an anticancer agent, we have synthesized 3-AP and demonstrated (a) potent inhibition of L1210 leukemia cells in vitro, (b) curative capacity for mice bearing the L1210 leukemia, (c) marked inhibition of RR, and (d) sensitivity of HUr-resistant cells to 3-AP. These findings collectively demonstrate the clinical potential of 3-AP as an antineoplastic agent.

The enzyme ribonucleotide reductase: target for antitumor and anti-HIV therapy

Ribonucleotide reductase is the rate-limiting enzyme of DNA synthesis, and it has been shown to be linked with malignant transformation and tumor cell proliferation. It was therefore considered as an excellent target for cancer chemotherapy. This article reviews the in vitro and in vivo effects of hydroxyurea the first inhibitor of the enzyme, which is currently used in general clinical practice. In addition, we summarize the results obtained with other inhibitors of the enzyme; for instance, polyhydroxy-substituted benzohydroxamic acid derivatives, a promising group of inhibitors of ribonucleotide reductase that was synthesized by Bart van'T Riet and investigated by our group. In vitro as well as animal data and pharmacokinetic results are reviewed and possible implications for an improvement in the management of various patient groups are outlined.

Overexpression of the multidrug resistance genes mdr1, mdr3, and mrp in L1210 leukemia cells resistant to inhibitors of ribonucleotide reductase

L1210 MQ-580 is a murine leukemia cell line resistant to the cytotoxic activity of the alpha-(N)-heterocyclic carboxaldehyde thiosemicarbazone class of inhibitors of ribonucleotide reductase. The line is cross-resistant to etoposide, daunomycin, and vinblastine. L1210 MQ-580 cells expressed 8-fold resistance to 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP), a relatively newly developed inhibitor of ribonucleotide reductase. The accumulation of [14C]3-AP by L1210 MQ-580 cells was 5- to 6-fold less than by parental L1210 cells. An increased rate of efflux of 3-AP was responsible for the lower steady-state concentration of 3-AP in resistant cells. In reverse transcription-polymerase chain reaction assays, L1210 MQ-580 cells were found to overexpress the multidrug resistance genes mdr1, mdr3, and mrp, but not the mdr2 gene, compared with parental L1210 cells. Measurement of the steady-state concentration of doxorubicin, a potential substrate for both the mdr and mrp gene products, demonstrated that L1210 MQ-580 cells accumulated 4-fold less anthracycline than parental cells. These findings indicate that drug efflux is a major determinant of the pattern of cross-resistance of L1210 MQ-580 cells. To extrapolate these observations to the human homologues of the mdr1, mdr3, and mrp murine genes, the effects of 3-AP were measured in L1210/VMDRC0.06 and NIH3T3 36-8-32 cells transfected with human MDR1 and MRP cDNAs, respectively. The transfectants were 2- to 3-fold resistant to the cytotoxic effects of 3-AP and accumulated less [14C]3-AP than their parental mock-transfected counterparts. Moreover, the cytotoxic activity of 3-AP was significantly greater in two double mrp gene knockout cell lines than in parental W 9.5 embryonic stem cells. Thus, the results suggest that 3-AP is a substrate for both the P-glycoprotein and MRP and that baseline MRP expression has the capacity to exert a protective role against the toxicity of this agent.