Ribonucleotide reductase as a chemotherapeutic target

New ruthenium(II) complexes with cyclic thio- and semicarbazone: evaluation of cytotoxicity and effects on cell migration and apoptosis of lung cancer cells

We describe the synthesis, physicochemical characterization, and in vitro antitumor assays of four novel analogous ruthenium(II) complexes with general formula cis-[Ru^II(N-L)(P-P)₂]PF₆, where P-P = bis(diphenylphosphine)methane (dppm, in complexes 1 and 2) or bis(diphenylphosphine)ethane (dppe, in complexes 3 and 4) and N-L = 5,6-diphenyl-4,5-dihydro-2H-[1,2,4]triazine-3-thione (Btsc, in complexes 1 and 3) or 5,6-diphenyltriazine-3-one (Bsc, in complexes 2 and 4). The data were consistent with cis arrangement of the biphosphine ligands. For the Btsc and Bsc ligands, the data pointed to monoanionic bidentate coordination to ruthenium(II) through N,S and N,O, respectively. Single-crystal X-ray diffraction showed that complex 1 crystallized in the monoclinic system, space group P2₁/c. Determination of the cytotoxicity profiles of complexes 1-4 gave SI values ranging from 1.19 to 3.50 against the human lung adenocarcinoma cell line A549 and the non-tumor lung cell line MRC-5. Although the molecular docking studies suggested that the interaction between DNA and complex 4 was energetically favorable, the experimental results showed that they interacted weakly. Overall, our results demonstrated that these novel ruthenium(II) complexes have interesting in vitro antitumor potential and this study may contribute to further studies in medicinal inorganic chemistry.

The pathogenic mechanism of Mycobacterium tuberculosis: implication for new drug development

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), is a tenacious pathogen that has latently infected one third of the world's population. However, conventional TB treatment regimens are no longer sufficient to tackle the growing threat of drug resistance, stimulating the development of innovative anti-tuberculosis agents, with special emphasis on new protein targets. The Mtb genome encodes ~4000 predicted proteins, among which many enzymes participate in various cellular metabolisms. For example, more than 200 proteins are involved in fatty acid biosynthesis, which assists in the construction of the cell envelope, and is closely related to the pathogenesis and resistance of mycobacteria. Here we review several essential enzymes responsible for fatty acid and nucleotide biosynthesis, cellular metabolism of lipids or amino acids, energy utilization, and metal uptake. These include InhA, MmpL3, MmaA4, PcaA, CmaA1, CmaA2, isocitrate lyases (ICLs), pantothenate synthase (PS), Lysine-ε amino transferase (LAT), LeuD, IdeR, KatG, Rv1098c, and PyrG. In addition, we summarize the role of the transcriptional regulator PhoP which may regulate the expression of more than 110 genes, and the essential biosynthesis enzyme glutamine synthetase (GlnA1). All these enzymes are either validated drug targets or promising target candidates, with drugs targeting ICLs and LAT expected to solve the problem of persistent TB infection. To better understand how anti-tuberculosis drugs act on these proteins, their structures and the structure-based drug/inhibitor designs are discussed. Overall, this investigation should provide guidance and support for current and future pharmaceutical development efforts against mycobacterial pathogenesis.

Ribonucleotide reductase: Implications of thiol S-nitrosylation and tyrosine nitration for different subunits

Ribonucleotide reductase (RNR) is a multi-subunit enzyme responsible for catalyzing the rate-limiting step in the production of deoxyribonucleotides essential for DNA synthesis and repair. The active RNR complex is composed of multimeric R1 and R2 subunits. The RNR catalysis involves the formation of tyrosyl radicals in R2 subunits and thiyl radicals in R1 subunits. Despite the quaternary structure and cofactor diversity, all the three classes of RNR have a conserved cysteine residue at the active site which is converted into a thiyl radical that initiates the substrate turnover, suggesting that the catalytic mechanism is somewhat similar for all three classes of the RNR enzyme. Increased RNR activity has been associated with malignant transformation, cancer cell growth, and tumorigenesis. Efforts concerning the understanding of RNR inhibition in designing potent RNR inhibitors/drugs as well as developing novel approaches for antibacterial, antiviral treatments, and cancer therapeutics with improved radiosensitization have been made in clinical research. This review highlights the precise and potent roles of NO in RNR inhibition by targeting both the subunits. Under nitrosative stress, the thiols of the R1 subunits have been found to be modified by S-nitrosylation and the tyrosyl radicals of the R2 subunits have been modified by nitration. In view of the recent advances and progresses in the field of nitrosative modifications and its fundamental role in signaling with implications in health and diseases, the present article focuses on the regulations of RNR activity by S-nitrosylation of thiols (R1 subunits) and nitration of tyrosyl residues (R2 subunits) which will further help in designing new drugs and therapies.

Implications of the colonic deposition of free hemoglobin-α chain: a previously unknown tissue by-product in inflammatory bowel disease

BACKGROUND

We analyzed inflamed mucosal/submucosal layers of ulcerative colitis (UC = 63) and Crohn's colitis (CC = 50), and unexpectedly, we unveiled a pool of free hemoglobin alpha (Hb-α) chain. Patients with colitides have increased reactive oxidative stress (ROS), DNA oxidation products, free iron in mucosa, in preneoplastic, and in colitis-cancers and increased risks of developing colorectal cancer. All inflammatory bowel disease-related colorectal cancer lesions are found in segments with colitis. Linking this information, we investigated whether free Hb-α is key transformational stepping that increases colitis-related colorectal cancer vulnerability.

METHODS

UC/CC samples were profiled using matrix-assisted laser desorption/ionization mass spectrometry; protein identification was made by liquid chromatography. Diverticulitis was used as control (Ctrl). The presence of Hb(n) (n = α, β, or hemin)/Hb was validated by Western blotting and immunohistochemistry. We tested for DNA damage (DNAD) by exposing normal colonic epithelial cell line, NCM460, to 10 μM and 100 μM of Hb(n)/Hb, individually for 2, 6, and 12 hours. Quantification of Hb-α staining was done by Nikon Elements Advance Research Analysis software. ROS was measured by the production of 8-OHdG. DNAD was assessed by Comet assay. Colonic tissue homogenate antioxidants Nrf2-, CAT-, SOD-, and GPx-expressions were analyzed densitometrically/normalized by β-actin.

RESULTS

Immunohistochemistry of CC/UC mucosal/submucosal compartments stained strongly positive for Hb-α and significantly higher versus Ctrl. NCM460 exposed to Hb(n)/Hb exhibited steadily increasing ROS and subsequent DNAD. DNAD was higher in 10 μM than 100 μM in Hb-β/hemin the first 2 hours then plateaued followed by DNAD repair. This may be likely due to apoptosis in the later concentration. Nrf2 enzyme activities among UC, CC, and ulcerative colitis-associated colon cancer (UCAC) were observed impaired in all inflammatory bowel disease subjects. Decreased levels of Nrf2 among patients with UC versus patients with CC with active disease were insignificant as well as versus Ctrls but significantly lower in UCAC versus Ctrl. SOD was decreased in UC and UCAC and GPx in CC but statistically not significant. Comparing CC versus UC, SOD was significantly lower in CC (P < 0.05). CAT was observed increased among patients with CC/UC/UCAC and GPx in UC and UCAC versus Ctrl, respectively, and significantly increased in CC versus Ctrl (P < 0.01).

CONCLUSIONS

In the colitides, mucosal/submucosal tissue microenvironments demonstrated pool of free Hb-α chain. In vitro exposure of NCM460 cells to Hb(n)/Hb induced ROS and DNAD. Toxic effect of free Hb-α, in colonic epithelial cells, is therefore through production of ROS formation modulated by impairment of antioxidant effects. Targeting reduction-oxidation-sensitive pathways and transcription factors may offer options for inflammatory bowel disease-management and colitis-related cancer prevention.

Nucleotide metabolism, oncogene-induced senescence and cancer

Senescence is defined as a stable cell growth arrest. Oncogene-induced senescence (OIS) occurs when an activated oncogene is expressed in a normal cell. OIS acts as a bona fide tumor suppressor mechanism by driving stable growth arrest of cancer progenitor cells harboring the initial oncogenic hit. OIS is often characterized by aberrant DNA replication and the associated DNA damage response. Nucleotides, in particular deoxyribonucleotide triphosphates (dNTPs), are necessary for both DNA replication and repair. Imbalanced dNTP pools play a role in a number of human diseases, including during the early stages of cancer development. This review will highlight what is currently known about the role of decreased nucleotide metabolism in OIS, how nucleotide metabolism leads to transformation and tumor progression, and how this pathway can be targeted as a cancer therapeutic by inducing senescence of cancer cells.

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

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

Targeting ribonucleotide reductase for cancer therapy

INTRODUCTION

Ribonucleotide reductase (RR) is a unique enzyme, because it is responsible for reducing ribonucleotides to their corresponding deoxyribonucleotides, which are the building blocks required for DNA replication and repair. Dysregulated RR activity is associated with genomic instability, malignant transformation and cancer development. The use of RR inhibitors, either as a single agent or combined with other therapies, has proven to be a promising approach for treating solid tumors and hematological malignancies.

AREAS COVERED

This review covers recent publications in the area of RR, which include: i) the structure, function and regulation of RR; ii) the roles of RR in cancer development; iii) the classification, mechanisms and clinical application of RR inhibitors for cancer therapy and iv) strategies for developing novel RR inhibitors in the future.

EXPERT OPINION

Exploring the possible nonenzymatic roles of RR subunit proteins in carcinogenesis may lead to new rationales for developing novel anticancer drugs. Updated information about the structure and holoenzyme models of RR will help in identifying potential sites in the protein that could be targets for novel RR inhibitors. Determining RR activity and subunit levels in clinical samples will provide a rational platform for developing personalized cancer therapies that use RR inhibitors.

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.

Effect of siRNAs on HSV-1 plaque formation and relative expression levels of RR mRNA

RNA interference (RNAi) is a process by which introduced small interfering RNA (siRNA) can cause the specific degradation of mRNA with identical sequences. The human herpes simplex virus type 1 (HSV-1) RR is composed of two distinct homodimeric subunits encoded by UL39 and UL40, respectively. In this study, we applied siRNAs targeting the UL39 and UL40 genes of HSV-1. We showed that synthetic siRNA silenced effectively and specifically UL39 and UL40 mRNA expression and inhibited HSV-1 replication. Our work offers new possibilities for RNAi as a genetic tool for inhibition of HSV-1 replication.

Biochemical modulation of aracytidine (Ara-C) effects by GTI-2040, a ribonucleotide reductase inhibitor, in K562 human leukemia cells

GTI-2040 is a potent antisense to the M2 subunit of the ribonucleotide reductase (RNR), an enzyme involved in the de novo synthesis of nucleoside triphosphates. We hypothesized that combination of GTI-2040 with the cytarabine (Ara-C) could result in an enhanced cytotoxic effect with perturbed intracellular deoxynucleotide/nucleotide (dNTP/NTP) pools including Ara-C triphosphate (Ara-CTP). This study aims to provide a direct experimental support of this hypothesis by monitoring the biochemical modulation effects, intracellular levels of Ara-CTP, dNTPs/NTPs following the combination treatment of Ara-C, and GTI-2040 in K562 human leukemia cells. GTI-2040 was introduced into cells via electroporation. A hybridization-ligation ELISA was used to quantify intracellular GTI-2040 concentrations. Real-time PCR and Western blot methods were used to measure the RNR M2 mRNA and protein levels, respectively. 3-(4,5-Dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt assay was used to measure the cytotoxicity following various drug treatments. A non-radioactive HPLC-UV method was used for measuring the intracellular Ara-CTP, while a LC-MS/MS method was used to quantify intracellular dNTP/NTP pools. GTI-2040 was found to downregulate M2 mRNA and protein levels in a dose-dependent manner and showed significant decrease in dNTP but not NTP pool. When combining GTI-2040 with Ara-C, a synergistic cytotoxicity was observed with no further change in dNTP/NTP pools. Importantly, pretreatment of K562 cells with GTI-2040 was found to increase Ara-CTP level for the first time, and this effect may be due to inhibition of RNR by GTI-2040. This finding provides a laboratory justification for the current phase I/II evaluation of GTI-2040 in combination with Ara-C in patients with acute myeloid leukemia.

A phase I/II study of GTI-2040 plus docetaxel as second-line treatment in advanced non-small cell lung cancer: a study of the PMH phase II consortium

INTRODUCTION

GTI-2040, an antisense oligonucleotide, targets the ribonucleotide reductase R2 subunit, critical for DNA synthesis. This study determined the recommended phase II dose (RP2D) of docetaxel plus GTI-2040, toxicity, and response rate in advanced non-small cell lung cancer (NSCLC).

PATIENTS AND METHODS

Advanced solid tumor patients, preferably with platinum-treated NSCLC, performance status 0 to 2, no symptomatic central nervous system metastases, adequate organ and bone marrow function, and >or=1 prior chemotherapy regimen were treated with escalating doses of GTI-2040 given by 14-day continuous intravenous infusion (CVI) plus docetaxel every 21 days.

RESULTS

Twenty-nine patients were treated, (24 NSCLC, 3 hormone-refractory prostate cancer, 1 head and neck, and 1 small cell lung cancer). GTI-2040 5 mg/kg as CVI for 14 days plus docetaxel 75 mg/m(2) intravenously every 21days was determined as the RP2D. Dose-limiting toxicity was not seen. Two patients at RP2D developed grade 4/5 febrile neutropenia. One prostate specific antigen response was seen in phase I, but no objective tumor responses in the NSCLC patients. Median time to progression was 3.4 months, 3.2 months in the NSCLC patients treated at RP2D.

CONCLUSIONS

Activity of the combination at RP2D, GTI-2040 5 mg/kg/d x 14 days by CVI plus docetaxel 75 mg/m(2) does not seem superior to docetaxel alone in previously treated NSCLC.

Mechanism of inhibition of ribonucleotide reductase with motexafin gadolinium (MGd)

Motexafin gadolinium (MGd) is an expanded porphyrin anticancer agent which selectively targets tumor cells and works as a radiation enhancer, with promising results in clinical trials. Its mechanism of action is oxidation of intracellular reducing molecules and acting as a direct inhibitor of mammalian ribonucleotide reductase (RNR). This paper focuses on the mechanism of inhibition of RNR by MGd. Our experimental data present at least two pathways for inhibition of RNR; one precluding subunits oligomerization and the other direct inhibition of the large catalytic subunit of the enzyme. Co-localization of MGd and RNR in the cytoplasm particularly in the S-phase may account for its inhibitory properties. These data can elucidate an important effect of MGd on the cancer cells with overproduction of RNR and its efficacy as an anticancer agent and not only as a general radiosensitizer.

Digalloylresveratrol, a new phenolic acid derivative induces apoptosis and cell cycle arrest in human HT-29 colon cancer cells

Digalloylresveratrol (DIG) is a new synthetic ester of the naturally occurring polyhydroxyphenolic substances gallic acid and resveratrol which both exert anti-cancer activity in a number of tumor cell lines. The aim of the study was to identify the biochemical effects of DIG in HT-29 human colon cancer cells. DIG induced dose-dependently apoptosis after treatment for 72 h (40 microM DIG caused apoptosis in 45% of cells). DIG led to a substantial imbalance of deoxyribonucleoside triphosphates (dNTPs), the products of the enzyme ribonucleotide reductase (RR) and directly inhibited RR as it significantly reduced the incorporation of (14)C-labeled cytidine into the DNA of tumor cells. Furthermore, DIG affected the cell division and inhibited the transition from S to G2/M phase of the cell cycle. In contrast to resveratrol or gallic acid, DIG did not inhibit cyclooxygenases I and II. When HT-29 cells were simultaneously treated with DIG and 5-FU, the standard chemotherapeutic substance for colon cancer, additive growth inhibitory effects could be observed. With respect to the various biochemical and anti-proliferative effects of DIG in HT-29 cells, we regard DIG as a potential candidate for future treatment options of colon cancer and conclude that further preclinical and in vivo studies are warranted.

Triapine and cytarabine is an active combination in patients with acute leukemia or myelodysplastic syndrome

Triapine, an iron chelator and a potent inhibitor of ribonucleotide reductase, has significant anti-leukemia activity. A phase I study of Triapine in combination with ara-C was conducted in 32 patients with refractory acute leukemia and high-risk MDS. Triapine (105 mg/m2/day 6-h infusion) was followed immediately by ara-C [100 (n=4), 200 (n=6), 400 (n=7), or 800 (n=8)mg/m2/day] as an 18-h infusion for 5 consecutive days. Dose-limiting toxicities (DLTs) were observed at the 800 mg/m2 ara-C dose level (one patient each with grade 4 mucositis; grade 4 neutropenic colitis, sepsis; grade 4 neuropathy; and grade 4 hyperbilirubinemia). Therefore, the study was amended to include an ara-C dose level of 600 mg/m2/day, no DLTs occurred in seven patients treated at this dose level. Mean Triapine C(max) and AUC were 1.13 microg/mL and 251.5 minmicrog/mL. Of 31 evaluable patients, 4 (13%) (3 AML, 1 Ph+ALL) achieved a CR (1 at a dose of 800 mg/m2; 2 at 600 mg/m2; 1 at 200mg/m2). The recommended phase II regimen is Triapine 105 mg/m2/day followed by ara-C 600 mg/m2/day for 5 consecutive days every 3-6 weeks.

Dehydration of ribonucleotides catalyzed by ribonucleotide reductase: the role of the enzyme

This article focuses on the second step of the catalytic mechanism for the reduction of ribonucleotides catalyzed by the enzyme Ribonucleotide Reductase (RNR). This step corresponds to the protonation/elimination of the substrate's C-2' hydroxyl group. Protonation is accomplished by the neighbor Cys-225, leading to the formation of one water molecule. This is a very relevant step since most of the known inhibitors of this enzyme, which are already used in the fight against certain forms of cancer, are 2'-substituted substrate analogs. Even though some theoretical studies have been performed in the past, they have modeled the enzyme with minimal gas-phase models, basically represented by a part of the side chain of the relevant amino acids, disconnected from the protein backbone. This procedure resulted in a limited accuracy in the position and/or orientation of the participating residues, which can result in erroneous energetics and even mistakes in the choice of the correct mechanism for this step. To overcome these limitations we have used a very large model, including a whole R1 model with 733 residues plus the substrate and 10 A thick shell of water molecules, instead of the minimal gas-phase models used in previous works. The ONIOM method was employed to deal with such a large system. This model can efficiently account for the restrained mobility of the reactive residues, as well as the long-range enzyme-substrate interactions. The results gave additional information about this step, which previous small models could not provide, allowing a much clearer evaluation of the role of the enzyme. The interaction energy between the enzyme and the substrate along the reaction coordinate and the substrate steric strain energy have been obtained. The conclusion was that the barrier obtained with the present model was very similar to the one previously determined with minimal gas-phase models. Therefore, the role of the enzyme in this step was concluded to be mainly entropic, rather than energetic, by placing the substrate and the two reactive residues in a position that allows for the highly favorable concerted trimolecular reaction, and to protect the enzyme radical from the solvent.

Design, synthesis, and evaluation of octahydropyranopyrrole-based inhibitors of mammalian ribonucleotide reductase

Inhibitors of mammalian ribonucleotide reductase possessing a novel octahydropyranopyrrole scaffold based on a cyclic heptapeptide inhibitor have been designed, synthesized, and evaluated. Structure-function studies reveal that the bicyclic scaffold is indeed necessary to maintain inhibitory activity.

Mechanisms of action of peptide inhibitors of mammalian ribonucleotide reductase targeting quaternary structure

Mammalian ribonucleotide reductase (mRR) is a chemotherapeutic target. The enzyme is composed of 2 subunits (mR1 and mR2) and is inhibited by Ac-FTLDADF (denoted P7), corresponding to the C-terminus of mR2, which competes with mR2 for binding to mR1. mRR has 2 physiologically important active forms, mR12mR22 and mR16(mR22)j (j = 1-3). Here we report on the mechanism of action of recently identified peptide derivatives having higher activities than P7 toward inhibition of one or both active forms. A significant feature of both P7 and these new inhibitors is that they are more potent vs. mR12mR22 than mR16(mR22)j. For some of these peptides, this is due in part to their preferential binding to the mR1 monomer. The possible application of these peptide derivatives in cancer chemotherapy is discussed.

The enantioselectivities of the active and allosteric sites of mammalian ribonucleotide reductase

Here we examine the enantioselectivity of the allosteric and substrate binding sites of murine ribonucleotide reductase (mRR). L-ADP binds to the active site and L-ATP binds to both the s- and a-allosteric sites of mR1 with affinities that are only three- to 10-fold weaker than the values for the corresponding D-enantiomers. These results demonstrate the potential of L-nucleotides for interacting with and modulating the activity of mRR, a cancer chemotherapeutic and antiviral target. On the other hand, we detect no substrate activity for L-ADP and no inhibitory activity for N3-L-dUDP, demonstrating the greater stereochemical stringency at the active site with respect to catalytic activity.

Peptide inhibitors of mammalian ribonucleotide reductase

Mammalian ribonucleotide reductase (mRR) is a chemotherapeutic target. In common with other class Ia RRs, the enzyme is composed of two subunits (mR1 and mR2), with mR1 containing both the active site and allosteric effector sites and mR2 containing a stable tyrosyl radical that is essential for enzymatic activity. mRR is inhibited by Ac-FTLDADF (denoted P7), corresponding to the C-terminus of mR2, which competes with mR2 for binding to mR1. The enzyme has two physiologically important active forms, mR12mR22 and mR16(mR22)j (j=1-3), with high ATP concentrations favoring the latter. Here, we report on our progress in using structural and functional studies in conjunction with library screening to identify derivatives of tri-, tetra- and hexapeptides, and cyclic heptapeptides, having equal or significantly higher activities than P7 toward inhibition of one or both active forms. These identifications were made by screening candidate peptides both for their abilities to bind to mR1 competitively with P7 and to inhibit ribonucleotide reductase activity. A significant feature of both P7 and the newly identified derivatives is that they are stronger inhibitors of mR12mR22 than of mR16(mR22)j. For the tetrapeptides, this is due in part to their preferential binding to mR1 monomer. The possible application of these peptide derivatives in cancer chemotherapy, exploiting their preferential inhibition of mR12mR22, is considered.

More potent linear peptide inhibitors of mammalian ribonucleotide reductase

Mammalian ribonucleotide reductase (mRR) is a chemotherapeutic target. The enzyme is composed of two subunits (mR1 and mR2) and is inhibited by Ac-FTLDADF (denoted P7), corresponding to the C-terminus of mR2, which disrupts mRR quaternary structure by competing with mR2 for binding to mR1. The tripeptide FmocWFF acts similarly. Here we report on the use of small, focused libraries to identify Fmoc derivatives of tetra and hexapeptides having comparable or considerably higher activities than P7 toward inhibition of mRR.

Oligopeptide inhibition of class I ribonucleotide reductases

Class I ribonucleotide reductases (RRs), which are well-recognized targets for cancer chemotherapeutic and antiviral agents, are composed of two different subunits, R1 and R2, and are inhibited by oligopeptides corresponding to the C-terminus of R2, which compete with R2 for binding to R1. These peptides specifically inhibit the RRs from which they are derived, and closely homologous RRs, but do not inhibit less homologous RRs. Here we review results obtained for oligopeptide inhibition of RRs from several sources, including related x-ray, NMR, and modeling results. The most extensive studies have been performed on herpes simplex virus-RR (HSV-RR) and mammalian-RR (mRR). A common model fits the data obtained for both enzymes, in which the C-terminal residue of the oligopeptide (Leu for HSV-RR, Phe for mRR) binds with high specificity to a narrow and deep hydrophobic subsite, and two or more hydrophobic groups at the N-terminal portion of the peptide bind to a broad and shallow second hydrophobic subsite. The studies have led to the development of highly potent and specific inhibitors of HSV-RR and promising inhibitors of mRR, and indicate possible directions for the development of inhibitors of bacterial and fungal RRs.

Phase I and pharmacodynamic study of Triapine, a novel ribonucleotide reductase inhibitor, in patients with advanced leukemia

In a phase I study, 24 patients with refractory leukemia received Triapine, a novel ribonucleotide reductase (RR) inhibitor, as a continuous intravenous infusion over 96 h beginning on days 1 and 15 or days 1 and 8. On the days 1 and 15 regimen, the starting dose was 120 mg/m(2) per day, and the maximum tolerated dose (MTD) was 160 mg/m(2) per day. Three of eight patients receiving 160 mg/m(2) per day in the first course, and one patient escalated to this dose in a second course, developed hepatic dose-limiting toxicity (DLT). For the days 1 and 8 regimen, the first 96 h infusion was administered at a fixed dose of 140 mg/m(2) per day. The dose of the second infusion beginning on day 8 was escalated from 120 to 160 mg/m(2) per day without observing DLT. No objective responses occurred. Over 70% of patients had a >50% reduction in white blood cell counts. The steady-state levels of Triapine were between 0.6 and 1 microM. As expected from the in vitro studies, at these plasma concentrations there was a decline in dATP and dGTP pools and a decrease in DNA synthetic capacity of the circulating leukemia cells. Based on these clinical, pharmacokinetic, and pharmacodynamic data, Triapine warrants further study in patients with hematologic malignancies.

Evolving role of ribonucleoside reductase inhibitors in hematologic malignancies

Ribonucleotide reductases catalyze the de novo biosynthesis of deoxyribonucleosides for DNA synthesis. Increased ribonucleotide reductases activity has been associated with malignant transformation and tumor cell growth. The ribonucleotide reductases inhibitors may bind with the R1 subunit of the enzyme (Class 1) or the nonheme iron (Class 2). This review focuses on the therapeutic use of ribonucleotide reductases inhibitors in hematologic malignancies. Hydroxyurea, fludarabine and cladribine have established roles in the management of hematologic malignancies, while other ribonucleotide reductases inhibitors, such as gemcitabine, tezacitabine and heterocyclic carboxaldehyde thiosemicarbazones (e.g., triapine) are being evaluated in clinical trials.

Design and synthesis of a tetrahydropyran-based inhibitor of mammalian ribonucleotide reductase

A tetrahydropyran-based inhibitor (2) of mammalian ribonucleotide reductase (mRR) has been designed and synthesized based on the heptapeptide, N-AcFTLDADF (1), corresponding to the C-terminus of the R2 subunit of mRR. Inhibition studies revealed that 2 is indeed a competent inhibitor, albeit less potent than 1.

Interaction of gallium nitrate with other inhibitors of ribonucleotide reductase: effects on the proliferation of human leukemic cells

Ribonucleotide reductase, a key enzyme in deoxyribonucleotide synthesis, is an important target for cancer chemotherapy. Drugs that inhibit its individual components may act synergistically to block DNA synthesis. Prior work has established that gallium inhibits the R2 subunit of ribonucleotide reductase. We show that gallium acts synergistically with the ribonucleotide reductase inhibitors gemcitabine and hydroxyurea to inhibit the proliferation of CCRF-CEM cells. In contrast, combinations of gallium with the ribonucleotide reductase inhibitors amidox, didox, or trimidox produced antagonistic effects on cell growth. Spectroscopy analysis revealed that as a result of their metal-binding properties, amidox, didox and trimidox formed complexes with gallium, thus negating potential synergistic actions. Our results have important implications in the design of clinical trials using these ribonucleotide reductase inhibitors in combination.

Iron binding capacity of didox (3,4 dihydroxybenzohydroxamic acid) and amidox (3,4 dihydroxybenzamidoxime) two inhibitors of the enzyme ribonucleotide reductase

Ribonucleotide reductase is the rate limiting enzyme of deoxynucleoside triphosphate synthesis and is considered to be an excellent target of cancer chemotherapy. Didox and amidox are newly synthesized compounds, which inhibit this enzyme and have in vitro and in vivo antitumor activity. We have now investigated the capability of didox and amidox to interfere with the iron metabolism. We show by photometric and polarographic methods, that didox and amidox are capable of forming an iron complex. However, their cytotoxic action cannot be circumvented by addition of Fe-ammoniumcitrate, indicating the iron complexing capacity not to be responsible for the mechanism of action of these compounds. When L1210 leukemia cells were incubated with the didox-iron or amidox-iron complex itself, only slight changes of the 50% growth inhibitory capacity of the complex in comparison with didox or amidox alone could be shown. We conclude, that didox and amidox are capable of forming an iron complex, but in contrast to other agents, the anticancer activity cannot be contributed to this effect alone. Further studies will have to elucidate the molecular mechanism of action of these new and promising anticancer agents.

Iron binding capacity of didox (3,4-dihydroxybenzohydroxamic acid) and amidox (3,4-dihydroxybenzamidoxime) new inhibitors of the enzyme ribonucleotide reductase

Ribonucleotide reductase is the rate limiting enzyme of deoxynucleoside triphosphate synthesis and is considered to be an excellent target of cancer chemotherapy. Didox and amidox are newly synthesized compounds, which inhibit this enzyme and have in vitro and in vivo antitumor activity. We have now investigated the capability of didox and amidox to interfere with the iron metabolism. We show by photometric and polarographic methods, that didox and amidox are capable of forming an iron complex. However, their cytotoxic action cannot be completely circumvented by addition of Fe-ammoniumcitrate, indicating that the iron complexing capacity may not be responsible for the mechanism of action of these compounds. When L1210 leukemia cells were incubated with the didox-iron or amidox-iron complex itself, changes of the 50% growth inhibitory capacity of the complex in comparison with didox or amidox alone could be shown. We conclude, that didox and amidox are capable of forming iron complexes, but in contrast to other agents, the anticancer activity cannot be contributed to this effect alone. Future studies will have to elucidate the molecular mechanism of action of these new and promising anticancer agents.

Simultaneous determination of the new anticancer agent amidox and its metabolites in rat bile and plasma by high-performance liquid chromatography

A new reversed-phase ion-pair high-performance liquid chromatography method was developed to study the first-pass hepatic metabolism of the anti cancer drug amidox in bile. Separation of the metabolites was achieved on a Spherisorb C18 column after liquid-liquid extraction using a linear gradient system of heptanesulfonic acid in potassium phosphate monobasic (pH 4.0) with increasing amounts of methanol (0-40%). The method was further applied to a pharmacokinetic study of amidox in rats after 200 mg kg-1 intraperitoneal administration. Using 50 microliters of rat bile and 300 microliters of rat plasma the limit of detection for amidox was 60 ng and 85 ng, respectively, and the assay was linear from 0.1 to 150 micrograms ml-1. This method appears to be sensitive enough to be used in further pharmacokinetic studies of amidox in human volunteers.

The new inhibitors of ribonucleotide reductase--comparison of some physico-chemical properties

Amidox (AX), didox (DX) and trimidox (TX), compounds synthetized as new ribonucleotide reductase inhibitors, have been investigated by ultraviolet (UV) spectrophotometry, polarography and high performance liquid chromatography (HPLC). The experiments have been performed at various pH values. The changes in UV absorption of the compounds studied were recorded and it was demonstrated that these changes are related to the pH and to structural features of the investigated molecules. From the compounds included in our series of experiments, only amidox and trimidox are reduced during polarographic experiments in Britton-Robinson buffer. The reduction of both compounds proceeded in two one-electron steps in acidic pH. One two-electron diffuse irreversible wave was observed at basic pH. The values of the half-wave potential became more negative in accordance with the increasing pH. HPLC assay also showed changes in the retention of compounds investigated, particularly when the pH of the mobile phase was close to the dissociation constant of the particular drug. The changes of physico-chemical properties detected by the all used methods are related to different chemical structures (the most significant changes were observed in alkaline pH).

NMR structure of an inhibitory R2 C-terminal peptide bound to mouse ribonucleotide reductase R1 subunit

The structure of peptide N-AcYTLDADF when bound to the large subunit of mouse ribonucleotide reductase has been elucidated by transfer NOE. This structure suggests a general design for type 1 RR inhibitors.

Iron binding capacity of trimidox (3,4,5-trihydroxybenzamidoxime), a new inhibitor of the enzyme ribonucleotide reductase

Ribonucleotide reductase is the rate limiting enzyme of deoxynucleoside triphosphate synthesis and is considered to be an excellent target of cancer chemotherapy. Trimidox, a newly synthesized compound, inhibits this enzyme and has in vitro and in vivo antitumour activity. As trimidox was able to upregulate the expression of the transferrin receptor in HL-60 human promyelocytic leukaemia cells, we have now investigated the capability of trimidox to interfere with iron metabolism. We show by photometric and polarographic methods that trimidox is able for form an iron complex. However, its cytotoxic action cannot be circumvented by addition of iron-saturated transferrin or iron-ammonium citrate, indicating that the iron complexing capacity is not responsible for the mechanism of action of this compound. When HL-60, K562 or L1210 leukaemia cells were incubated with the trimidox-iron complex itself, we could observe increases of the 50% growth inhibitory capacity of the complex in comparison with trimidox alone. We conclude that trimidox is able to form an iron complex, but in contrast to other agents, the anticancer activity cannot be contributed to this effect alone. Further studies will have to elucidate the molecular mechanism of action of this new and promising anticancer agent.

Biochemical and antitumor activity of trimidox, a new inhibitor of ribonucleotide reductase

Trimidox (3,4,5-trihydroxybenzamidoxime), a newly synthesized analog of didox (N,3,4-trihydroxybenzamide) reduced the activity of ribonucleotide reductase (EC 1.17.4.1) in extracts of L1210 cells by 50% (50% growth-inhibitory concentration, IC50) at 5 microM, whereas hydroxyurea, the only ribonucleotide reductase inhibitor in clinical use, exhibited an IC50 of 500 microM. Ribonucleotide reductase activity was also measured in situ by incubating L1210 cells for 24 h with trimidox at 7.5 microM, a concentration that inhibits cell proliferation by 50% (IC50) or at 100 microM for 2 h; these concentrations resulted in a decrease in enzyme activity to 22% and 50% of the control value, respectively. Trimidox and hydroxyurea were cytotoxic to L1210 cells with IC50 values of 7.5 and 50 microM, respectively. Versus ribonucleotide reductase, trimidox and hydroxyurea yielded IC50 values of 12 and 87 microM, respectively. A dose-dependent increase in life span was observed in mice bearing intraperitoneally transplanted L1210 tumors. Trimidox treatment (200 mg/kg; q1dx9) significantly increased the life span of mice bearing L1210 leukemia (by 82% in male mice and 112% in female mice). The anti-tumor activity appeared more pronounced in female mice than in male mice. Viewed in concert, these findings suggest that trimidox is a new and potent inhibitor of ribonucleotide reductase and that it is a promising candidate for the chemotherapy of cancer in humans.

Cloning and characterization of subunit genes of ribonucleotide reductase, a cell-cycle-regulated enzyme, from Plasmodium falciparum

Ribonucleotide reductase (EC 1.17.4.1; RNR), a cell-cycle-regulated enzyme, catalyzes the rate-limiting step in the de novo synthesis of deoxyribonucleotides by the reduction of the corresponding ribonucleotides. The important role of the RNR in DNA synthesis and cell division makes this enzyme an excellent target for chemotherapy. However, nothing is known about this enzyme from the malaria parasite Plasmodium falciparum. We have isolated cDNA clones encoding both the large and small RNR subunits. The sequences of full-length clones of the large and small RNR subunits revealed an open reading frame encoding 806 and 349 amino acids, respectively, and showed significant identity with other RNR sequences in the data base. RNA blot analysis showed that the size of the large and small RNR subunit transcripts are 5.4 kb and 2.2 kb, respectively. Both the RNR subunit transcripts fluctuate in level during the cell cycle, reaching a peak preceding maximal DNA synthesis activity. An oligodeoxynucleotide phosphorothioate that is complementary to sequences around the translational initiation codon of the small RNR subunit showed significant inhibition of growth, as measured by the inhibition in DNA synthesis.

Studies on the mechanism of the synergistic interaction between 2'-deoxy-5-azacytidine and cisplatin

2'-Deoxy-5-azacytidine (5-aza-CdR) and cisplatin interact to produce synergistic cytotoxicity against many human tumor cell lines. Preliminary experiments designed to explore the mechanism of this synergy suggested a poor correlation between synergy and the degree of genomic hypomethylation measured following exposure to 5-aza-CdR. Subsequent studies using plasmid DNA suggested that rather than DNA hypomethylation, incorporation of 5-aza-CdR into DNA mediated increased cisplatin binding to DNA and could therefore be essential to the synergistic interaction between these two agents. In this series of experiments, we evaluated the degree of synergy with cisplatin produced against two human melanoma cell lines by two additional antimetabolites that were chosen on the basis of their biochemical properties. In addition, we investigated the synergy between 5-aza-CdR and cisplatin in parental and 5-aza-CdR-resistant murine cell lines, which differed in their sensitivity to 5-aza-CdR and DNA methylation status but incorporated similar amounts of 5-aza-CdR into DNA when exposed to this antimetabolite. In the studies testing additional antimetabolites, cytosine arabinoside, which is incorporated into DNA but does not hypomethylate it, produced synergy with cisplatin that was similar or superior to that obtained using 5-aza-CdR. With 3-deaza-adenosine, which is not incorporated into DNA but produces DNA hypomethylation through inhibition of S-adenosylhomocysteine hydrolase, a primarily antagonistic interaction was observed in the two cell lines studied. In the 5-aza-CdR-sensitive and -resistant cell lines, a very similar synergistic interaction was documented for 5-aza-CdR and cisplatin despite the significant difference observed in DNA methylation levels. Taken as a whole, these data suggest that DNA hypomethylation was not critical to the synergistic cytotoxicity produced by 5-aza-CdR and cisplatin. This finding suggests additional strategies that could further modulate this interaction.

Growth, ribonucleotide reductase and metals in murine leukemic lymphocytes

Trace metals are essential for the growth and several other properties of human lymphocytes. We studied the effects of media with variable concentrations of three metals (Fe2+, Cu2+, Zn2+), a metal chelator (deferoxamine, DFX) and a cell-growth inhibitor (hydroxyurea) on the growth, intracellular metal concentration and activity of the enzyme ribonucleotide reductase in murine leukemic lymphocytes (L1210). Intracellular concentrations of Fe and Cu fluctuated within narrow limits in normal media, but decreased to very low concentrations in metal-poor media. The intracellular Zn concentration did not vary appreciably. Growth in intact cells decreased by 50%-70% when normal media were replaced by metal-poor media, but returned to control values when media were supplemented with gradually increasing concentrations of Fe and Cu. Fe and Cu had synergistic effects, while Zn had no stimulatory action. Hydroxyurea and DFX both inhibited cell growth, but only DFX inhibition was reversed by addition of metals. The addition of the above metals and inhibitors to the cell extracts produced effects on ribonucleotide reductase activity similar to those observed on the growth of whole cell preparations (stimulation by Fe and Cu, inhibition by Zn, DFX and hydroxyurea). These findings show that (a) the intracellular metal concentration is maintained in a narrow range during cell growth; (b) ribonucleotide reductase activity varies with cell growth; (c) ribonucleotide reductase activity and cell growth increase with Fe and Cu and decrease with Zn and DFX. Our data suggest that (a) Fe, Cu and Zn may have some effect on the growth and ribonucleotide reductase activity of L1210 cells, that (b) Fe, Cu and Zn may operate in a related and interdependent way and that (c) DFX inhibits cell growth probably through inhibition of the reductase activity and chelation of the Fe of its Fe-containing subunit. We conclude that any study on one of these metals should always include the other two and that manipulation of intracellular metals should be investigated as a potential therapeutic modulator of growth in leukemic lymphocytes.

A 41 kDa transferrin related molecule acts as an autocrine growth factor for HL-60 cells

HL-60 cells produce an autostimulatory growth factor. Since the stimulatory effect of HL-60 conditioned medium is only observed in the absence of exogenous transferrin we have assayed HL-60 cells for the production of transferrin and found that they produce polypeptides which react with transferrin antibodies. 35S-methionine labelling, immunoprecipitation and subsequent separation by SDS-gel electrophoresis reveals the presence of a major transferrin related 41 +/- 2 kDa species released by HL-60 cells. Physiological levels of iron salts completely abolish the requirement of exogenous transferrin which indicates that the endogenous transferrin related polypeptides in the presence of exogenous inorganic iron salts are sufficient for the proliferation of HL-60 cells provided insulin or related growth factors are present. The addition of transferrin receptor antibodies inhibits the stimulatory action of the endogenous transferrin related activity.

Studies on the mechanisms of inhibition of L1210 cell growth by 3,4-dihydroxybenzohydroxamic acid and 3,4-dihydroxybenzamidoxime

Didox and Amidox inhibit L1210 cell growth in culture. At least one of the mechanisms in the mode(s) of action of the compounds is directed at the ribonucleotide reductase site. Partially purified preparations of ribonucleotide reductase activity are inhibited by Amidox and Didox. The formation of deoxycytidine nucleotides from [14C]cytidine in intact L1210 cells is also blocked. Didox and Amidox cause the decrease in the intracellular pools of the four dNTPs. Hydroxyurea-resistant L1210 cells are not cross-resistant to either Didox or Amidox. These data suggest that Didox and Amidox are not inhibiting ribonucleotide reductase through a mechanism similar to hydroxyurea.