Targeting Ribonucleotide Reductase Research Articles

Inhibition of hepatitis B virus replication by targeting ribonucleotide reductase M2 protein

Chronic hepatitis B virus (HBV) infection is a key factor for hepatocellular carcinoma worldwide. Ribonucleotide reductase (RR) regulates the deoxyribonucleoside triphosphates biosynthesis and serves as a target for anti-cancer therapy. Here, we demonstrate that RR is essential for HBV replication and the viral covalently-closed-circular DNA (cccDNA) synthesis in host liver cells. By performing computer-assisted virtual screening against the crystal structure of RR small subunit M2 (RRM2), osalmid, was identified as a potential RRM2-targeting compound. Osalmid was shown to be 10-fold more active in inhibiting RR activity than hydroxyurea, and significantly inhibited HBV DNA and cccDNA synthesis in HepG2.2.15 cells. In contrast, hydroxyurea and the RR large subunit (RRM1)-inhibitory drug gemcitabine showed little selective activity against HBV replication. In addition, osalmid also was shown to possess potent activity against a 3TC-resistant HBV strain, suggesting utility in treating drug-resistant HBV infections. Interestingly, osalmid showed synergistic effects with lamivudine (3TC) in vitro and in vivo without significant toxicity, and was shown to inhibit RR activity in vivo, thus verifying its in vivo function. Furthermore, 4-cyclopropyl-2-fluoro-N-(4-hydroxyphenyl) benzamide (YZ51), a novel derivative of osalmid, showed higher efficacy than osalmid with more potent RR inhibitory activity. These results suggest that RRM2 might be targeted for HBV inhibition, and the RRM2-targeting compound osalmid and its derivative YZ51 could be a novel class of anti-HBV candidates with potential use for hepatitis B and HBV-related HCC treatment.

Targeting Ribonucleotide Reductase M2 and NF-κB Activation with Didox to Circumvent Tamoxifen Resistance in Breast Cancer.

Tamoxifen is widely used as an adjuvant therapy for patients with estrogen receptor (ERα)-positive tumors. However, the clinical benefit is often limited because of the emergence of drug resistance. In this study, overexpression of ribonucleotide reductase M2 (RRM2) in MCF-7 breast cancer cells resulted in a reduction in the effectiveness of tamoxifen, through downregulation of ERα66 and upregulation of the 36-kDa variant of ER (ERα36). We identified that NF-κB, HIF1α, and MAPK/JNK are the major pathways that are affected by RRM2 overexpression and result in increased NF-κB activity and increased protein levels of EGFR, HER2, IKKs, Bcl-2, RelB, and p50. RRM2-overexpressing cells also exhibited higher migratory and invasive properties. Through time-lapse microscopy and protein profiling studies of tamoxifen-treated MCF-7 and T-47D cells, we have identified that RRM2, along with other key proteins, is altered during the emergence of acquired tamoxifen resistance. Inhibition of RRM2 using siRRM2 or the ribonucleotide reductase (RR) inhibitor didox not only eradicated and effectively prevented the emergence of tamoxifen-resistant populations but also led to the reversal of many of the proteins altered during the process of acquired tamoxifen resistance. Because didox also appears to be a potent inhibitor of NF-κB activation, combining didox with tamoxifen treatment cooperatively reverses ER-α alterations and inhibits NF-κB activation. Finally, inhibition of RRM2 by didox reversed tamoxifen-resistant in vivo tumor growth and decreased in vitro migratory and invasive properties, revealing a beneficial effect of combination therapy that includes RRM2 inhibition to delay or abrogate tamoxifen resistance.

Potent effect of adenoviral vector expressing short hairpin RNA targeting ribonucleotide reductase large subunit M1 on cell viability and chemotherapeutic sensitivity to gemcitabine in non-small cell lung cancer cells

BackgroundRibonucleotide reductase large subunit (RRM1) is the main enzyme responsible for synthesis of the deoxyribonucleotides used during DNA synthesis. It is also a cellular target for gemcitabine (GEM). Overexpression of RRM1 is reportedly associated with resistance to GEM and the poor prognosis for many types of malignant tumours. Aim of the present study is to establish gene therapy against RRM1-overexpressing tumours. MethodAn adenoviral vector that encoded a short hairpin siRNA targeting the RRM1 gene (Ad-shRRM1) was constructed. Two RRM1-overexpressing non-small cell lung cancer (NSCLC) lines, MAC10 and RERF-LC-MA, were used. Finally, a human tumour xenograft model in nude mice was prepared by subcutaneously implanting tumours derived from RERF-LC-MA cells. ResultsAd-shRRM1 effectively downregulated RRM1 mRNA and protein in both types of NSCLC cells and significantly reduced the percentage of viable cells as detected by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay (p<0.005). Caspase 3/7 analysis revealed that transfection with Ad-RRM1 increased the percentage of apoptotic cells in culture containing either type of RRM1-overexpressing cell (p<0.001). Treatment with Ad-shRRM1 exerted a potent antitumour effect against the RRM1-overexpressing RERF-LC-MA xenografts (p<0.05). Furthermore, Ad-shRRM1-mediated inhibition of RRM1 specifically increased sensitivity to gemcitabine of each type of RRM1-overexpressing tumour cell. Combination treatment with Ad-shRRM1 and GEM exerted significantly greater inhibition on cell proliferation than Ad-shRRM1 or GEM treatment alone. ConclusionRRM1 appeared to be a promising target for gene therapy, and Ad-shRRM1 had strong antitumour effects, specifically anti-proliferative and pro-apoptotic effects, against NSCLC cells that overexpressed RRM1. Combination therapy with Ad-shRRM1 and GEM may become a new treatment option for patients with NSCLC.

Abstract 1961: Anti-endocrine therapy in breast cancerresistance can be circumvented by targeting ribonucleotide reductase M2 usingthe pharmaceutical agent Didox

Abstract Breast Cancers (BC) that are sensitive to anti-endocrine therapies such as tamoxifen develop resistance despite their initial favorable response. To gain mechanistic insight on endocrine resistance, gene expression profiling was performed to determine the transcripts that are differentially expressed post-tamoxifen therapy under conditions of prolonged drug treatment and development of resistance. This resistance outcome is the result of the cancer cells invoking survival functions that are up-regulated leading to enhanced cell proliferation and cell motility networks. These networks were validated in a larger panel of breast cancer cells. Here, we report the activation of Growth factor signaling and IKK-βsignaling, NFκB signaling, in BC cells upon acquired resistance to ER blockade by anti-endocrine therapy. Moreover, ribonucleotide reductase (RRM2) was revealed as a key contributor to acquired tamoxifen resistance. An RNA interference screen demonstrated that suppression of RRM2 by the RR inhibitor Didox results in down-regulation of Her2 and EGFR, modulates ER-α signaling, alters NFκB activity and expression, and decreases anti-apoptotic pathways. Indeed, concomitant treatment of tamoxifen and Didox causes blockade of ER and RRM2 induced prolonged ER inhibition and severely impairs the growth of resistant tumor cells. Collectively, these results identify distinct mechanisms that mediate secondary resistance to anti-endocrine therapies, all of which reactivate RR signaling. These observations provide a rational strategy to overcome the multifaceted clonal heterogeneity that emerges when tumors are treated with targeted agents. We propose that Didox and an anti-hormonal therapy should be tested in BC patients who become refractory to anti-endocrine therapies. Citation Format: Khyati N. Shah, Howard L. Elford, Jesika S. Faridi. Anti-endocrine therapy in breast cancerresistance can be circumvented by targeting ribonucleotide reductase M2 usingthe pharmaceutical agent Didox. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1961. doi:10.1158/1538-7445.AM2015-1961

A novel reduction-sensitive modified polyethylenimine oligonucleotide vector

A reduction-sensitive cross-linked polyethylenimine derivative PEI-SS-OA was synthesized and evaluated for oligonucleotide delivery. PEI-SS-OA was shown to condense LOR-2501, an oligonucleotide targeting ribonucleotide reductase R1 subunit (RRM1), into positively charged complexes. The reductive degradation of the PEI-SS-OA induced by dithiothreitol was confirmed by a gel retardation assay. In vitro experiments revealed that the reduction-sensitive PEI-SS-OA was less cytotoxic and more effective in oligonucleotide delivery than the control 25kDa PEI. This study demonstrates that a reducibly degradable cationic polymer PEI-SS-OA possesses both higher oligonucleotide delivery efficiency and lower cytotoxicity than PEI (25kDa), therefore is an attractive candidate for further in vivo evaluation.

Enhanced antisense oligonucleotide delivery using cationic liposomes incorporating fatty acid-modified polyethylenimine.

Antisense oligonucleotides (ASOs) have promising therapeutic potential in oncotherapy. However, low stability and efficacy limit their application in the clinic. Cationic liposomes have been investigated as delivery vehicles for ASOs. Here, we report the synthesis and evaluation of an ASO delivery vehicle comprising cationic liposomes incorporating fatty acid-modified polyethylenimine. An oleic acid derivative of branched polyethylenimine (PEI-OA) and a linoleic acid derivative of branched polyethylenimine (PEI-LA) were synthesized and incorporated into liposomes. The PEI-modified liposomes were synthesized by an ethanol injection method with composition of PEI-modified lipid/Chol/TPGS. The properties of these liposomes, including cytotoxicity, cellular uptake, ASO target silencing activity, based on mRNA and protein downregulation, were investigated. LOR-2501, an ASOs targeting ribonucleotide reductase R1 subunit (R1) was used as the therapeutic cargo. The PEI-modified liposomes showed relatively compact particle size and excellent colloidal stability for at least 25 days. PEI-modified liposomes effectively delivered LOR-2501 into KB cells and efficiently induced down-regulation of R1 mRNA and protein. Compared with regular cationic liposomes, PEI-modified liposomes was more effective, reducing R1 mRNA and protein by ~10%.

Quantitative proteomics of the yeast Hsp70/Hsp90 interactomes during DNA damage reveal chaperone-dependent regulation of ribonucleotide reductase.

This study provides the dynamic interactome of the yeast Hsp70 and Hsp90 under DNA damage which suggest key roles for the chaperones in a variety of signaling cascades. Importantly, the cancer drug target ribonucleotide reductase was shown to be a client of Hsp70 and Hsp90 in both yeast and breast cancer cells. As such, this study highlights the potential of a novel cancer therapeutic strategy that exploits the synergy of chaperone and ribonucleotide reductase inhibitors.

Abstract 3675: PAN-811 blocks anticancer drug-induced neurotoxicity

Abstract The side effects of anticancer drugs, such as chemotherapy-induced cognitive impairment or chemobrain, severely limit their dose usage and therapeutic course, and seriously affect quality of life in cancer patients. The mechanisms underlying chemobrain are still unexplained, and there are currently no treatments. The present study aimed to understand neurotoxicological mechanism of anticancer drugs and to develop therapeutic agents for chemobrain. Antimetabolites methotrexate (MTX) and 5-fluorouracil (5-FU) are widely used chemotherapeutic agents, readily crossing the blood-brain barrier (BBB). An alkylating agent cisplatin (CDDP) has also been reported to induce chemobrain although controversy exists on its capability to pass the BBB. In this study, primary cerebral neurons from embryonic rats were seeded in serum-free culture medium containing 100% antioxidants (AO). Half of the medium was replaced twice with medium without AO at day 7 (50% AO) and 9 (25% AO). By day 13, the neurons were insulted with 100μM MTX, 25μM 5-FU, or 3.5μM CDDP, and treated with 1.25, 2.5, 5 or 10 μM PAN-811·Cl·H2O (PAN-811) for 3 additional days using vehicle as the control. By day 16, media were harvested for LDH assay and cells were photographed at 300x. Five hours following a third medium replacement (12.5% AO), the cells were photographed again and evaluated by MTS analysis. No neurotoxicity was observed under 50-100% AO conditions for MTX, 5-FU or CDDP treatment. When the AO content was reduced to 25%, MTX and 5-FU all caused 70% and 75% increases in LDH release, and CDDP elicited a 112% increase, without notable morphological changes. When AO content was reduced to 12.5%, morphological cell death (reductions in cell and neurite densities) occurred in each of MTX, 5-FU and CDDP insulted groups. Additionally, 5-FU and CDDP insults resulted in 33% and 66% reductions in MTS readings, respectively. All these indicate a key role of oxidative stress in mediation of the in vitro neurotoxicity. PAN-811, an anticancer drug candidate targeting ribonucleotide reductase, inhibited MTX-, 5-FU- or CDDP- elicited LDH leakage in the 25% AO condition in a dose-dependently manner; PAN-811 at 3μM suppressed LDH released from MTX- or 5-FU- insulted neurons to control levels, and at 10μM fully blocked CDDP-induced LDH leakage. Furthermore, PAN-811 at 10μM fully inhibited 5-FU-induced MTS reduction, and elevated MTS reading by 48% for CDDP-insulted neurons under 12.5% AO condition. In addition, PAN-811 at 5μM well blocked MTX-, 5-FU- or CDDP- elicited morphological cell death. Inhibition of neurotoxicity via suppression of oxidative stress by PAN-811 should not interfere with anticancer efficacy of tested drugs since their anticancer activities do not rely on oxidative stress. Besides, PAN-811 has a distinct intracellular target, which may introduce a synergistic effect with a tested drug in suppression of cancer growth, while protecting against chemobrain. Citation Format: Zhi-Gang Jiang, Steven Fuller, Hossein A. Ghanbari. PAN-811 blocks anticancer drug-induced neurotoxicity. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3675. doi:10.1158/1538-7445.AM2014-3675

Targeting ribonucleotide reductase M2 subunit by small interfering RNA exerts anti-oncogenic effects in gastric adenocarcinoma.

Ribonucleotide reductase M2 subunit(RRM2) is one of the two subunits of human ribonucleotide reductase which plays a critical role in tumor progression. The aim of the present study was to analyze its expression, clinical significance and biological functions in gastric adenocarcinoma. We observed the upregulation of RRM2 mRNA and protein in all nine gastric cancer cell lines examined. In paired primary gastric cancers, both mRNA and protein levels of RRM2 were significantly upregulated in tumors compared with the corresponding non-tumorous gastric tissues. RRM2 protein expression correlated with higher tumor grade, advanced T stage and poor disease-specific survival. RRM2 knockdown in gastric cancer cell lines AGS, MKN1 and MKN28 significantly suppressed cell proliferation, inhibited monolayer colony formation, reduced cell invasion and induced apoptosis. Downregulation of RRM2 suppressed xenograft formation invivo. Collectively, these findings suggest that RRM2 plays a crucial role in gastric tumorigenesis and may serve as a potential prognostic marker and therapeutic target in gastric cancer.

A Polyethylenimine-Linoleic Acid Conjugate for Antisense Oligonucleotide Delivery

A novel antisense oligonucleotide (ASO) carrier, polyethylenimine conjugated to linoleic acid (PEI-LA), was synthesized and evaluated for delivery of LOR-2501 to tumor cells. LOR-2501 is an ASO targeting ribonucleotide reductase R1 subunit (RRM1). In this study, PEI-LA was synthesized by reacting PEI (Mw ~ 800) with linoleoyl chloride. Gel retardation assay showed complete complexation between PEI-LA and LOR-2501 at N/P ratio above 8. No significant cytotoxicity was observed with these complexes at the tested dosage levels. Interestingly, at N/P ratio of >6, levels of cellular uptake of PEI-LA/LOR-2501 were double that of PEI/LOR-2501 complexes of the same N/P ratio. PEI-LA/LOR-2501 induced downregulation of 64% and 70% of RRM1 at mRNA and protein levels, respectively. The highest transfection activity was shown by PEI-LA/LOR-2501 complexes at N/P ratio of 10. Finally, using pathway specific inhibitors, clathrin-mediated endocytosis was shown to be the principle mechanism of cellular internalization of these complexes. In conclusion, PEI-LA is a promising agent for the delivery of ASOs and warrants further investigation.

Systemic delivery of siRNA nanoparticles targeting RRM2 suppresses head and neck tumor growth

Systemic delivery of siRNA to solid tumors remains challenging. In this study, we investigated the systemic delivery of a siRNA nanoparticle targeting ribonucleotide reductase subunit M2 (RRM2), and evaluated its intratumoral kinetics, efficacy and mechanism of action. Knockdown of RRM2 by an RNAi mechanism strongly inhibited cell growth in head and neck squamous cell carcinoma (HNSCC) and non-small cell lung cancer (NSCLC) cell lines. In a mouse xenograft model of HNSCC, a single intravenous injection led to the accumulation of intact nanoparticles in the tumor that disassembled over a period of at least 3days, leading to target gene knockdown lasting at least 10days. A four-dose schedule of siRNA nanoparticle delivering RRM2 siRNA targeted to HNSCC tumors significantly reduced tumor progression by suppressing cell proliferation and inducing apoptosis. These results show promise for the use of RRM2 siRNA-based therapy for HNSCC and possibly NSCLC.

Hydroxyurea Decreases Gemcitabine Resistance in Pancreatic Carcinoma Cells With Highly Expressed Ribonucleotide Reductase

This study aimed to determine whether the treatment of pancreatic carcinoma can be defined on the basis of the expression of genes involved in gemcitabine metabolism and whether combination treatment is more effective than conventional treatment. Four pancreatic carcinoma cell lines (Panc-1, MIAPaCa-2, BxPC-3, and Capan-2) were used to determine the patterns of gemcitabine-metabolizing genes and mesenchymal marker gene expressions using quantitative real-time polymerase chain reaction. Chemosensitivity and cell proliferation were measured using colorimetric assay. Gemcitabine was combined with hydroxyurea or small interfering RNA targeting ribonucleotide reductase to assess changes in chemoresistance. Panc-1 and MIAPaCa-2 cell lines were profoundly chemoresistant and expressed genes corresponding to cells with distinct mesenchymal phenotypes. In addition, Panc-1 highly expressed ribonucleotide reductase and showed a 4-fold increase in gemcitabine sensitivity after treatment with hydroxyurea. Combination treatment tailored to cells with highly expressed ribonucleotide reductase was more effective than treatment with gemcitabine alone. Moreover, phenotype and gemcitabine metabolism may independently confer chemoresistance.

Preclinical toxicity and toxicokinetics of GTI-2040, a phosphorothioate oligonucleotide targeting ribonucleotide reductase R2

GTI-2040, a 20-mer phosphorothioate oligonucleotide, was designed to hybridize to the mRNA sequence of human ribonucleotide reductase R2. GTI-2040 has been shown to inhibit human cancer cell proliferation by downregulation of R2 expression in vitro and to significantly inhibit tumor growth in xenograft models of human cancer in mice. As part of the safety evaluation for human clinical trials, the toxicity and toxicokinetics of GTI-2040 were determined in Sprague-Dawley rats and rhesus monkeys. GTI-2040 was administered to rats at 2, 10, and 50mg/kg/day by bolus intravenous injection every second day for 21days with a 21-day recovery. In monkeys, an acute study was performed with single, escalating doses of GTI-2040 ranging from 10 to 80mg/kg given as a 24-h continuous intravenous infusion. As well, a 21-day, continuous intravenous infusion study with GTI-2040 was conducted in monkeys at 2, 10, and 50mg/kg/day, with a 3-week recovery. Blood sampling was done to measure GTI-2040 plasma concentrations, metabolites, and pharmacokinetic parameters, and tissues were collected to assess the distribution of GTI-2040 and/or metabolites. The toxicities of GTI-2040 in both rats and monkeys were typical for the phosphorothioate oligonucleotide class of compounds. In monkeys, there was a dose-related increase in GTI-2040 plasma levels with concomitant increase in complement activation and prolongation of activated partial thromboplastin time. In both rats and monkeys, the tissues having the highest concentrations of GTI-2040 (kidney, liver, spleen) had the largest dose-related toxic effects. Adverse effects were diminished or absent in the recovery animals. GTI-2040 was well tolerated when infused over 24h at doses up to 80mg/kg in monkeys. In rats and monkeys, GTI-2040 was reasonably well tolerated and showed reversible toxicities when administered at doses up to 50mg/kg/day for 21days. The no observed adverse effect dose level for GTI-2040 in both animal species was 2mg/kg/day. There were no apparent sequence-specific effects related to the interaction of GTI-2040 with the R2 component of the mRNA expressing ribonucleotide reductase.

Enzyme Kinetics of GTI-2040, a Phosphorothioate Oligonucleotide Targeting Ribonucleotide Reductase

Enzyme kinetics of GTI-2040 (5'-GGC TAA ATC GCT CCA CCA AG-3'), a phosphorothioate ribonucleotide reductase antisense, were investigated for the first time in 3' exonuclease solution and human liver microsomes (HLMs), using the ion-pair high-performance liquid chromatogram method for quantification of the parent drug and two major 3'N-1 and 3'N-2 metabolites. Enzyme kinetics of GTI-2040 in 3'-exonuclease solution were found to be well characterized by the Michaelis-Menten model, using the sum of formation rates of 3'N-1 and 3'N-2 (approximately total metabolism) because of sequential metabolism. In HLMs, a biphasic binding was observed for GTI-2040 with high- and low-affinity constants (K(d)s) of 0.03 and 3.8 microM, respectively. Enzyme kinetics of GTI-2040 in HLMs were found to deviate from Michaelis-Menten kinetics when the total GTI-2040 substrate was used. However, after correction for the unbound fractions, the formation rate of total metabolites could be described by Michaelis-Menten kinetics. Using the free substrate fraction, the K(m) and V(max) of GTI-2040 were determined to be 6.33 +/- 3.2 microM and 16.5 +/- 8.4 nmol/mg/h, respectively. Using these values, in vitro hepatic intrinsic clearance (CL(int)) in HLM was estimated to be 2.61 +/- 0.56 ml/h. The CL(int) was then used to predict GTI-2040's in vivo intrinsic clearance in humans by a microsomal protein scaling factor, which gave a mean value of 182.7 l/h, representing 24.1% of the observed in vivo mean scaled hepatic intrinsic clearance of 758.7 l/h in patients with acute myeloid leukemia. We concluded that the saturable nonspecific binding of GTI-2040 in HLMs complicated the interpretation of its enzyme kinetics, and scaled intrinsic clearance from HLMs only partially predicted the in vivo intrinsic clearance.

Impact of tumor‐specific targeting and dosing schedule on tumor growth inhibition after intravenous administration of siRNA‐containing nanoparticles

This study addresses issues of relevance for siRNA nanoparticle delivery by investigating the functional impact of tumor-specific targeting and dosing schedule. The investigations are performed using an experimental system involving a syngeneic mouse cancer model and a theoretical system based on our previously described mathematical model of siRNA delivery and function. A/J mice bearing subcutaneous Neuro2A tumors approximately 100 mm(3) in size were treated by intravenous injection with siRNA-containing nanoparticles formed with cyclodextrin-containing polycations (CDP). Three consecutive daily doses of transferrin (Tf)-targeted nanoparticles carrying 2.5 mg/kg of two different siRNA sequences targeting ribonucleotide reductase subunit M2 (RRM2) slowed tumor growth, whereas non-targeted nanoparticles were significantly less effective when given at the same dose. Furthermore, administration of the three doses on consecutive days or every 3 days did not lead to statistically significant differences in tumor growth delay. Mathematical model calculations of siRNA-mediated target protein knockdown and tumor growth inhibition are used to elucidate possible mechanisms to explain the observed effects and to provide guidelines for designing more effective siRNA-based treatment regimens regardless of delivery methodology and tumor type.

Gallium(III) and Iron(III) Complexes of α-N-Heterocyclic Thiosemicarbazones: Synthesis, Characterization, Cytotoxicity, and Interaction with Ribonucleotide Reductase

A series of gallium(III) and iron(III) complexes with five different 4N-substituted alpha-N-heterocyclic thiosemicarbazones, viz., 2-acetylpyridine N,N-dimethylthiosemicarbazone (1), 2-acetylpyridine N-pyrrolidinylthiosemicarbazone (2), acetylpyrazine N,N-dimethylthiosemicarbazone (3), acetylpyrazine N-pyrrolidinylthiosemicarbazone (4), and acetylpyrazine N-piperidinylthiosemicarbazone (5), with the general formula [GaLCl2] (HL = 1 and 2) and [ML2][Y] (M = Ga, HL = 1-5, Y = PF6; M = Fe, HL = 1-5, Y = FeCl4 and PF6) were synthesized and characterized by elemental analysis, a number of spectroscopic methods (NMR, IR, UV-vis), mass spectrometry, and X-ray crystallography. The in vitro antitumor potency was studied in two human cancer cell lines (41M and SK-BR-3). The central metal ions exert pronounced effects in a divergent manner: gallium(III) enhances, whereas iron(III) weakens the cytotoxicity of the ligands. The capacity of ligand 1 and its Ga(III) and Fe(III) complexes to destroy the tyrosyl radical of the presumed target ribonucleotide reductase is reported.

Deoxyribonucleoside 2'- or 3'-mixed disulfides: prodrugs to target ribonucleotide reductase and/or to inhibit HIV reverse transcription.

Herein, we report the design, synthesis, and biological effects of nucleosides bearing a disulfide function on the sugar ring as prodrugs of potentially active mercaptonucleotides that can target ribonucleotide reductase or reverse transcriptase. We show that cytidine derivatives efficiently reduce dNTP pools in human CEM/SS cells and that 3'-deoxythymidin-3'-yl methyl disulfide is able to interfere with both cellular dNTP synthesis and HIV reverse transcription.