Ribonucleotide Reductase Inhibitor Research Articles

Triapine Derivatives Act as Copper Delivery Vehicles to Induce Deadly Metal Overload in Cancer Cells

Thiosemicarbazones continue to attract the interest of researchers as potential anticancer drugs. For example, 3-aminopyridine-2-carboxaldehyde thiosemicarbazone, or triapine, is the most well-known representative of this class of compounds that has entered multiple phase I and II clinical trials. Two new triapine derivatives HL1 and HL2 were prepared by condensation reactions of 2-pyridinamidrazone and S-methylisothiosemicarbazidium chloride with 3-N-(tert-butyloxycarbonyl) amino-pyridine-2-carboxaldehyde, followed by a Boc-deprotection procedure. Subsequent reaction of HL1 and HL2 with CuCl2·2H2O in 1:1 molar ratio in methanol produced the complexes [CuII(HL1)Cl2]·H2O (1·H2O) and [CuII(HL2)Cl2] (2). The reaction of HL2 with Fe(NO3)3∙9H2O in 2:1 molar ratio in the presence of triethylamine afforded the complex [FeIII(L2)2]NO3∙0.75H2O (3∙0.75H2O), in which the isothiosemicarbazone acts as a tridentate monoanionic ligand. The crystal structures of HL1, HL2 and metal complexes 1 and 2 were determined by single crystal X-ray diffraction. The UV-Vis and EPR spectroelectrochemical measurements revealed that complexes 1 and 2 underwent irreversible reduction of Cu(II) with subsequent ligand release, while 3 showed an almost reversible electrochemical reduction in dimethyl sulfoxide (DMSO). Aqueous solution behaviour of HL1 and 1, as well as of HL2 and its complex 2, was monitored as well. Complexes 1−3 were tested against ovarian carcinoma cells, as well as noncancerous embryonic kidney cells, in comparison to respective free ligands, triapine and cisplatin. While the free ligands HL1 and HL2 were devoid of antiproliferative activity, their respective metal complexes showed remarkable antiproliferative activity in a micromolar concentration range. The activity was not related to the inhibition of ribonucleotide reductase (RNR) R2 protein, but rather to cancer cell homeostasis disturbance—leading to the disruption of cancer cell signalling.

Design, synthesis, molecular docking and cytotoxic activity of novel urea derivatives of 2-amino-3-carbomethoxythiophene

An efficient feasible route for the one-pot synthesis of novel series of urea derivatives (2a–2j) from 2-amino-3-carbomethoxythiophene (1) via in situ isocyanate has been developed, and their corresponding anticancer activities were accomplished. The series of urea derivatives were characterized by using 1H, 13C nuclear magnetic resonance and mass spectroscopic analysis. The cytotoxic activities were evaluated against human cervical (HeLa) and human lung (NCI-H23) cancer cell lines. These studies revealed satisfactory activity for some of the compounds, which could potentially serve as lead compounds for drug discovery and development. Furthermore, molecular docking studies supported in identifying the potential binding sites between the urea derivatives and eukaryotic ribonucleotidereductase (RR). High ambiguity driven docking (HADDOCK) modelling was specifically employed to determine the model complex of RR and urea derivatives. The proposed model has provided a deep insight into the molecular level interactions of RR-urea model complexes in understanding the exact pharmacophore for designing highly potent RR inhibitors. Overall, the present work has shed light in developing a feasible and robust approach for the synthesis of novel urea derivatives of 2-amino-3-carbomethoxythiophene and identified a part of molecular structure that is responsible for a specific biological interaction leading to potential anticancer activities. We report herein, the experimental design, synthesis and characterization of a novel series of urea derivatives of 2-amino-3carbomethoxythiophene with pyrimidine amine and benzyl amine analogues as both derivatives which exhibited potential antitumor activity via one pot synthesis and subsequently studied the structure activity relationships (SAR), and anticancer activities. The docking studies identified a part of molecularstructure that is responsible for a specific biological interaction leading to the destruction of cancer cells.

Abstract 4176: Involvement of the lysosome in the cytotoxicity of Gallium Maltolate in human pediatric brain tumor cell lines

Abstract Introduction: Gallium maltolate (GaM) is a novel small molecule metallocompound that acts as an iron mimetic Trojan Horse to disrupt iron-dependent proteins essential in tumor biology. We recently reported that GaM retards the growth of orthotopically implanted adult human glioblastoma (GBM) xenografts in a rodent brain tumor model and is cytotoxic to glioblastoma stem cells (Mol Cancer Ther 17:1240, 2018). GaM's mechanisms of action include inhibition of both mitochondrial function and iron-dependent ribonucleotide reductase. In the present study, we further explored whether GaM has antitumor activity in pediatric brain tumor cells. Experimental procedures: The cytotoxicity of GaM in pediatric GBM and Atypical Teratoid Rhabdoid Tumor (ATRT) cell lines was examined by MTT and clonogenic assays. Apoptotic cell death was assessed by flow cytometry with annexin V (AV) and propidium iodide (PI) staining. Cellular bioenergetics was measured using a Seahorse XF96 Extracellular Flux Analyzer. GaM interaction with lysosomes was analyzed by immunofluorescence and electron microscopy (EM). Results: In a panel of pediatric brain tumor cell lines, CHLA-266 ATRT and GBM cells displayed the greatest sensitivity to GaM. Complete inhibition of colony growth was achieved with 6 - 7 µM GaM over a 96-hour incubation. With a shorter incubation of 48-h, 25 µM GaM induced apoptosis in > 60% of CHLA-266 ATRT cells. In contrast, a longer incubation of 96-h with 25 µM GaM was required to induce apoptosis in > 50% of CHLA-200 GBM, 69% of SF188 GBM and 60% of SJ-GBM2 cell lines. After a 24-h of incubation with GaM, prior to detectable cell death, CHLA-266 and GBM cells displayed a marked reduction in mitochondrial reserve capacity, glycolysis, and glycolytic capacity. Immunofluorescence staining for the lysosomal marker LAMP1 showed that GaM produced an increase in lysosome size and number after a 24-h or 48-h incubation relative to control cells. EM studies demonstrated a gallium signal within the lysosomal compartment as early as 6-h of incubation. EM findings consistent with GaM-induced apoptosis were noted after a 24-h incubation, thus confirming the results of the AV/PI assay. After 24-h however, gallium was no longer detectable in the lysosomes, suggesting that lysosomal disruption led to the release of gallium into the cytoplasm. Conclusions: Our results identify GaM as a novel compound with significant activity against a panel of cell lines representative of pediatric brain tumors. Most of the cell lines used in our study were derived from tumors taken from heavily pre-treated patients and were resistant to conventional chemotherapeutic drugs. Their sensitivity of GaM suggests that GaM does not share cross-resistance with other antineoplastic drugs. Our study also provides new insight into the lysosomes as a target for gallium. The prognosis of adult and pediatric brain tumors is dismal and there is an urgent need to develop newer drugs for this malignancy. Further investigation of GaM as a therapeutic agent for brain tumors is warranted. Citation Format: Salvatore Molino, Christopher R. Chitambar. Involvement of the lysosome in the cytotoxicity of Gallium Maltolate in human pediatric brain tumor cell lines [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4176.

Abstract 6269: Effect of 3AP on neuroendocrine tumor cell proliferation, apoptosis and activation of DNA repair pathway

Abstract Background: Neuroendocrine tumors (NETs) are a heterogeneous group of neoplasms arising from multiple organs. Peptide Receptor Radionuclide Therapy (PRRT) is a novel therapy approved for treating NETs with a treatment response rate of 20-30%. To improve therapeutic response of PRRT, 3-AP (Triapine), which is a ribonucleotide reductase (RR) inhibitor with antineoplastic and radiation sensitizing properties, can be used in combination with PRRT. The purpose of our study is to examine effects of Triapine treatment on cellular proliferation, apoptosis and activation of DNA repair pathway in NET cells in vitro. Methods: For these studies, we utilized two human NET cell lines: BON, a pancreatic carcinoid tumor, and QGP-1, a somatostatinoma. (i) To determine effective dose range of Triapine (500 - 2500nM), proliferation was measured at 48, 72, and 96 h with sulforhodamine B (SRB) assay. (ii) To evaluate the effects of radiosensitivity, cultures were treated with increasing doses (250 - 5,000nM), received 2 Gy radiation (XRT) 1h later, exposed to Triapine for 24 hours, rinsed, and fed Triapine-free media; colonies were counted and quantified with SRB assays 14 days later and survival curves were generated. (iii) To further characterize cellular responses, cells were treated with Triapine (250 - 5,000 nM), irradiated 1h later, and harvested 24 h later to examine expression levels of Cyclin D1, cleaved PARP, p-DNA-PK (S2056), p-ATM (S1981) and p-ATR (S428) by Western blot. Results: (i) SRB proliferation assay demonstrated the effective dose range, determined by half maximal inhibitory concentration (IC-50), was 1,3 µM in BON cells and 1.73 µM in QGP-1 cells at 96 h. (ii) Exposure to Triapine before irradiation resulted in a significant increase in the radiosensitivity of each of the tumor cell lines with dose enhancement factors at a surviving fraction of 0.10 of 1.8 and 1.6 for BON and QGP-1, respectively. (iii) Triapine treatment alone activated DNA repair pathway in a dose-dependent fashion as determined by pDNA-PK (S2056) expression. Increasing p-ATM (S1981) expression was observed in a dose-dependent fashion in QGP-1 cells, but not in BON cells. In addition, p-ATR (S428) expression was not significantly different in either cell line. (iv) Triapine treatment alone or in combination with 2 and 4 Gy XRT resulted in dose-dependent decrease in cyclin D1 expression in both cell lines, however, no significant increase in cleaved PARP expression at 24 and 48h. Conclusion: Our results demonstrated rapid and dose dependent decrease in cyclin D1 expression after Triapine treatment, dose-dependent activation of DNA-PK, the key component of the non-homologous end joining pathway and significant increase in the radiosensitivity of each of the tumor cell lines. In conclusion, our findings provide a rationale for inclusion of Triapine as a radiosensitizer in combination with PRRT in treating NETs. Citation Format: Zeta Chow, Aman Chauhan, Lowell Anthony, Courtney Townsend, B. Mark Evers, Piotr Rychahou. Effect of 3AP on neuroendocrine tumor cell proliferation, apoptosis and activation of DNA repair pathway [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6269.

Abstract 5212: The benefits of Didox in triple negative breast cancer treatment targeting mutant p53

Abstract Triple-negative breast cancer (TNBC) is an aggressive histological subtype of malignancy with limited treatment options. Frequently, doxorubicin (DOXO)- based chemotherapy is utilized in this patient population due to the lack of molecular targets available. While DOXO is an effective chemotherapeutic agent, its efficacy is limited due to acquired drug resistance and cardiotoxicity. Therefore the identification of other treatment options for TNBC is needed. TNBC is a heterogeneous malignancy, with 70% of cases classified as a basal subtype, which further complicates the search for an effective molecular target. However, it has been identified that 80% of TNBC samples carry TP53 (tumor protein p53) mutations, which leads to a mutant p53 protein no longer able to act as a tumor suppressor and thus failing to halt cell growth. Mutant p53, when present in malignant cells, is far less susceptible to degradation than its wild-type counterpart, and thus accumulates within the cell. As such, the TP53 is an attractive therapeutic target for TNBC. We have previously identified that ribonucleotide reductase M2 (RRM2) is upregulated in TNBC cells and is a key contributor to acquired drug resistance. The ribonucleotide reductase inhibitor Didox has been shown to be a powerful free radical scavenger and iron chelator. Through these mechanisms, Didox alleviates the non-selective cytotoxic effects of DOXO on non-cancerous tissue, including cardiotoxicity and potentiates DOXO's efficacy against malignant cells. We have previously reported that Didox in combination with DOXO effectively treats TNBC by enhancing tumor inhibition and minimizing DOXO-induced heart damage. Our current study shows that Didox alone or in combination with DOXO downregulates mutant p53 protein levels in the mutant p53 TNBC cell lines MDA-MB 468, MDA-MB 231 and BT-20. This effect is notably absent in breast cancer cell lines with wild-type p53, including MCF7, A3B5 and ZR751, where p53 successfully acts as a tumor suppressor allowing for cell cycle arrest or apoptosis after DNA damage. We hypothesize that mutant p53 is a valid therapeutic target in TNBC. As shown here, the addition of Didox downregulates mutant p53 levels and thus enhances tumor inhibition while alleviating DOXO-associated cardiotoxicity. Citation Format: Nahid Sultana, Elizabeth A. Wilson, Khyati N. Shah, Howard L. Elford, Jesika S. Faridi. The benefits of Didox in triple negative breast cancer treatment targeting mutant p53 [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5212.

Genome-Wide Screens Reveal that Resveratrol Induces Replicative Stress in Human Cells

Resveratrol is a natural product associated with wide-ranging effects in animal and cellular models, including lifespan extension. To identify the genetic target of resveratrol in human cells, we conducted genome-wide CRISPR-Cas9 screens to pinpoint genes that confer sensitivity or resistance to resveratrol. An extensive network of DNA damage response and replicative stress genes exhibited genetic interactions with resveratrol and its analog pterostilbene. These genetic profiles showed similarity to the response to hydroxyurea, an inhibitor of ribonucleotide reductase that causes replicative stress. Resveratrol, pterostilbene, and hydroxyurea caused similar depletion of nucleotide pools, inhibition of replication fork progression, and induction of replicative stress. The ability of resveratrol to inhibit cell proliferation and S phase transit was independent of the histone deacetylase sirtuin 1, which has been implicated in lifespan extension by resveratrol. These results establish that a primary impact of resveratrol on human cell proliferation is the induction of low-level replicative stress.

Infrared Absorption Study of Schiff Bases of Aminohydroxyguanidine

Schiff bases of aminohydroxyguanidine were designed to contain the essential pharmacophore of hydroxyguanidine and have structural similarity to hydroxyurea, thiosemicarbazone and N-carbamyloxyurea. Most of these compounds have been shown to have antitumor and/ or antiviral activities more potent than hydroxyurea and hydroxyguanidine. Their mode of action is inhibition of ribonucleotide reductase which is the key enzyme in de novo DNA synthesis. Their infrared spectra (IR) are determined and discussed in this paper. Possible applications of the IR data in pharmaceutical analysis are presented.

Viral ribonucleotide reductase attenuates the anti-herpes activity of acyclovir in contrast to amenamevir

Amenamevir is a helicase-primase inhibitor of herpes simplex virus (HSV) and varicella-zoster virus (VZV) and is used for the treatment of herpes zoster in Japan. The half maximal effective concentrations (EC50s) of acyclovir and sorivudine for VZV increased as the time of treatment was delayed from 6 to 18 h after infection, while those of amenamevir and foscarnet were not affected. Susceptibility of infected cells at 0 and 18 h after infection was examined with four anti-herpes drugs, and the fold increases in EC50 for acyclovir, sorivudine, amenamevir, and foscarnet were 13.1, 6.3, 1.3, and 1.0, respectively. The increase in the EC50s for acyclovir in the late phase of infection in VZV and HSV was abolished by hydroxyurea, a ribonucleotide reductase (RR) inhibitor. The common mechanism affecting antiviral activities of acyclovir to HSV and VZV was examined in HSV-infected cells. The amount of HSV DNA in cells treated with amenamevir at 10 x EC50 was similar at 0 and 12 h but less than that in cells treated with acyclovir at 10 x EC50. dGTP, produced through viral RR, peaked at 4 h and decreased thereafter as it was consumed by viral DNA synthesis. Because acyclovir and amenamevir inhibited viral DNA synthesis, thus making dGTP unnecessary, dGTP was significantly more abundant in the presence of acyclovir and amenamevir than in untreated, infected cells. Abundant dGTP supplied by RR may compete with acyclovir triphosphate and attenuate its antiviral activity. In contrast, abundant dGTP did not influence the inhibitory action of amenamevir on viral helicase-primase activity. ATP was significantly decreased at 12 h after infection and significantly more abundant in untreated infected cells compared to cells treated with acyclovir and amenamevir. The anti-herpetic activity of amenamevir was not affected by the replication cycle of VZV and HSV, indicating the suitability of amenamevir for the treatment of herpes zoster and suppressive therapy for genital herpes.

Etctn 10388: A phase I trial of triapine and lutetium Lu 177 dotatate in well-differentiated somatostatin receptor-positive gastroenteropancreatic neuroendocrine tumors (GEP-NETs).

TPS4660 Background: Radiolabeled somatostatin analogues provide a means of delivering targeted radiation with a high therapeutic index to NETs that express somatostatin receptors (SSTRs). Radiolabeled somatostatin analogue Lutetium Lu 177 Dotatate (Lutathera) is a beta-emitting radionuclide, recently FDA approved for use in SSTR positive gastroenteropancreatic neuroendocrine tumors (GEPNETS) in the US based on the NETTER-1 Phase III trial. Despite favorable PFS and safety profile, the drug has limited cytoreductive capability with a 17% ORR. We hypothesize that addition of an effective radiation sensitizer could help improve antitumor activity of Lutathera. Ribonucleotide reductase (RNR) is the only enzyme responsible for conversion of ribonucleoside diphosphate to deoxyribonucleotide diphosphate (dNDP), the key building blocks for DNA synthesis. Radiation is a potent inducer of DNA double-strand breaks (DSBs), and RNR is the rate-limiting enzyme in the repair of DNA in this setting. Triapine is an inhibitor of RNR. This study will test the hypothesis that radiation sensitizer triapine can be safely combined with peptide receptor radionuclide therapy and ultimately may improve antitumor activity of Lutetium Lu 177 Dotatate. Methods: This study is an investigator initiated, NCI sponsored, multicenter phase 1 trial of triapine and Lutetium Lu 177 Dotatate in well-differentiated somatostatin receptor-positive gastroenteropancreatic neuroendocrine tumor (GEP-NETs) after the failure of at least one line of prior systemic cancer treatment. A total of 29 patients will be enrolled in the dose escalation with help of Bayesian optimal interval design (BOIN) and dose expansion cohorts. The study will be open through the NCI ETCTN (National Cancer Institute Experimental Therapeutics Clinical Trials Network) program. Patients will be treated with 177 lutetium dotatate in combination with triapine. Triapine will be administered orally (100 mg once a day starting dose) from D1-14 with each dose of PRRT [200 mCi]. Primary endpoint is to evaluate recommended phase II dose (RP2D). Secondary endpoints are to evaluate safety, pharmacokinetics, and clinical activity (ORR and PFS). We are also evaluating NETEST, a novel blood based test that evaluates levels of 51 neuroendocrine tumor gene transcripts. In addition, the study will correlate clinical outcome with baseline somatostatin receptor density, somatic tumor mutations and germline mutations. Clinical trial information: 04234568 .

Impact of sickle cell disease on presentation and progression of paediatric HIV: a retrospective cohort study.

HIV and sickle cell disease (SCD) are significant causes of morbidity and mortality in sub-Saharan Africa. Given their separate roles in immune dysregulation, our objective was to characterise the impact that SCD has on the presentation and progression of paediatric HIV. The study was a retrospective cohort study (study period 2004-2018). Cases of HIV+and SCD-afflicted patients (HIV+/SCD+) were obtained via electronic chart review from a paediatric HIV clinic in Kampala, Uganda and matched 1:3 with HIV+controls without SCD (HIV+/SCD-). Thirty-five HIV+/SCD+subjects and 95 HIV+/SCD- controls were analysed (39% female (51/130), age 3.6years (SD3.9)). At baseline, WHO clinical stage (64% total cohort Stage III/IV) and nutritional status (9.4% severe acute malnutrition) were similar for both groups, whereas HIV+/SCD+had higher though non-significant baseline CD4 count (1036 (SD713) vs 849 (SD638) cells/microlitre, P=0.20, two-tailed t-test). There were 19 deaths, 6 (17%) HIV+/SCD+and 13 (14%) HIV+/SCD-, with unadjusted/adjusted models showing no significant difference. Nutritional progression and clinical stage progression showed no significant differences between groups. Kaplan-Meier analysis showed a slower rate of treatment failures in the HIV+/SCD+cohort (P=0.11, log-rank survival test). Trajectory analysis showed that in the time period analysed, the HIV+/SCD+cohort showed a more rapid rise and higher total CD4 count (P=0.012, regression analysis). The study suggests that SCD does not adversely affect the progression of HIV in patients on ART. Further, HIV+/SCD+achieved higher CD4 counts and fewer HIV treatment failures, suggesting physiological effects due to SCD might mitigate HIV progression.

The cytotoxicity of gallium maltolate in glioblastoma cells is enhanced by metformin through combined action on mitochondrial complex 1.

New drugs are needed for glioblastoma, an aggressive brain tumor with a dismal prognosis. We recently reported that gallium maltolate (GaM) retards the growth of glioblastoma in a rat orthotopic brain tumor model by inhibiting mitochondrial function and iron-dependent ribonucleotide reductase (RR). However, GaM’s mechanism of action at the mitochondrial level is not known. Given the interaction between gallium and iron metabolism, we hypothesized that gallium might target iron-sulfur (Fe-S) cluster-containing mitochondrial proteins. Using Extracellular Flux Analyzer technology, we confirmed that after a 24-h incubation, GaM 50 μmol/L inhibited glioblastoma cell growth by <10% but inhibited cellular oxygen consumption rate by 44% and abrogated mitochondrial reserve capacity. GaM blocked mitochondrial complex I activity and produced a 2.9-fold increase in cellular ROS. NMR spectroscopy revealed that gallium binds to IscU, the bacterial scaffold protein for Fe-S cluster assembly and stabilizes its folded state. Gallium inhibited the rate of in vitro cluster assembly catalyzed by bacterial cysteine desulfurase in a reaction mixture containing IscU, Fe (II), DTT, and L-cysteine. Metformin, a complex I inhibitor, enhanced GaM’s inhibition of complex I, further increased cellular ROS levels, and synergistically enhanced GaM’s cytotoxicity in glioblastoma cells in 2-D and 3-D cultures. Metformin did not affect GaM action on cellular iron uptake or transferrin receptor1 expression nor did it enhance the cytotoxicity of the RR inhibitor Didox. Our results show that GaM inhibits complex I by disrupting iron-sulfur cluster assembly and that its cytotoxicity can be synergistically enhanced by metformin through combined action on complex I.

Bcl2-induced DNA replication stress promotes lung carcinogenesis in response to space radiation.

Space radiation is characterized by high-linear energy transfer (LET) ionizing radiation. The relationships between the early biological effects of space radiation and the probability of cancer in humans are poorly understood. Bcl2 not only functions as a potent antiapoptotic molecule but also as an oncogenic protein that induces DNA replication stress. To test the role and mechanism of Bcl2 in high-LET space radiation-induced lung carcinogenesis, we created lung-targeting Bcl2 transgenic C57BL/6 mice using the CC10 promoter to drive Bcl2 expression selectively in lung tissues. Intriguingly, lung-targeting transgenic Bcl2 inhibits ribonucleotide reductase activity, reduces dNTP pool size and retards DNA replication fork progression in mouse bronchial epithelial cells. After exposure of mice to space radiation derived from 56iron, 28silicon or protons, the incidence of lung cancer was significantly higher in lung-targeting Bcl2 transgenic mice than in wild-type mice, indicating that Bcl2-induced DNA replication stress promotes lung carcinogenesis in response to space radiation. The findings provide some evidence for the relative effectiveness of space radiation and Bcl-2 at inducing lung cancer in mice.

Functional Genomic Screen in Mesothelioma Reveals that Loss of Function of BRCA1-Associated Protein 1 Induces Chemoresistance to Ribonucleotide Reductase Inhibition.

Loss of function of BRCA1-associated protein 1 (BAP1) is observed in about 50% of malignant pleural mesothelioma (MPM) cases. The aim of this study was to investigate whether this aspect could be exploited for targeted therapy. A genetically engineered model was established expressing either functional or nonfunctional BAP1, and whole-genome siRNA synthetic lethality screens were performed assessing differentially impaired survival between the two cell lines. The whole-genome siRNA screen unexpectedly revealed 11 hits (FDR < 0.05) that were more cytotoxic to BAP1-proficient cells. Two actionable targets, ribonucleotide reductase (RNR) catalytic subunit M1 (RRM1) and RNR regulatory subunit M2 (RRM2), were validated. In line with the screen results, primary mesothelioma (BAP1 +/-) overexpressing BAP1 C91A (catalytically dead mutant) was more resistant to RNR inhibition, while BAP1 knockdown in the BAP1-proficient cell lines rescued the cells from their vulnerability to RNR depletion. Gemcitabine and hydroxyurea were more cytotoxic in BAP1-proficient cell line-derived spheroids compared with BAP1 deficient. Upregulation of RRM2 upon gemcitabine and hydroxyurea treatment was more profound in BAP1 mut/del cell lines. Increased lethality mediated by RNR inhibition was observed in NCI-H2452 cells reconstituted with BAP1-WT but not with BAP1 C91A. Upregulation of RRM2 in NCI-H2452-BAP1 WT spheroids was modest compared with control or C91A mutant. Together, we found that BAP1 is involved in the regulation of RNR levels during replication stress. Our observations reveal a potential clinical application where BAP1 status could serve as predictive or stratification biomarker for RNR inhibition-based therapy in MPM.

Ribonucleotide reductase inhibitors suppress SAMHD1 ara‐CTPase activity enhancing cytarabine efficacy

The deoxycytidine analogue cytarabine (ara‐C) remains the backbone treatment of acute myeloid leukaemia (AML) as well as other haematological and lymphoid malignancies, but must be combined with other chemotherapeutics to achieve cure. Yet, the underlying mechanism dictating synergistic efficacy of combination chemotherapy remains largely unknown. The dNTPase SAMHD1, which regulates dNTP homoeostasis antagonistically to ribonucleotide reductase (RNR), limits ara‐C efficacy by hydrolysing the active triphosphate metabolite ara‐CTP. Here, we report that clinically used inhibitors of RNR, such as gemcitabine and hydroxyurea, overcome the SAMHD1‐mediated barrier to ara‐C efficacy in primary blasts and mouse models of AML, displaying SAMHD1‐dependent synergy with ara‐C. We present evidence that this is mediated by dNTP pool imbalances leading to allosteric reduction of SAMHD1 ara‐CTPase activity. Thus, SAMHD1 constitutes a novel biomarker for combination therapies of ara‐C and RNR inhibitors with immediate consequences for clinical practice to improve treatment of AML.

Inhibition of the ATR-CHK1 Pathway in Ewing Sarcoma Cells Causes DNA Damage and Apoptosis via the CDK2-Mediated Degradation of RRM2.

Inhibition of ribonucleotide reductase (RNR), the rate-limiting enzyme in the synthesis of deoxyribonucleotides, causes DNA replication stress and activates the ataxia telangiectasia and rad3-related protein (ATR)-checkpoint kinase 1 (CHK1) pathway. Notably, a number of different cancers, including Ewing sarcoma tumors, are sensitive to the combination of RNR and ATR-CHK1 inhibitors. However, multiple, overlapping mechanisms are reported to underlie the toxicity of ATR-CHK1 inhibitors, both as single agents and in combination with RNR inhibitors, toward cancer cells. Here, we identified a feedback loop in Ewing sarcoma cells in which inhibition of the ATR-CHK1 pathway depletes RRM2, the small subunit of RNR, and exacerbates the DNA replication stress and DNA damage caused by RNR inhibitors. Mechanistically, we identified that the inhibition of ATR-CHK1 activates CDK2, which targets RRM2 for degradation via the proteasome. Similarly, activation of CDK2 by inhibition or knockdown of the WEE1 kinase also depletes RRM2 and causes DNA damage and apoptosis. Moreover, we show that the concurrent inhibition of ATR and WEE1 has a synergistic effect in Ewing sarcoma cells. Overall, our results provide novel insight into the response to DNA replication stress, as well as a rationale for targeting the ATR, CHK1, and WEE1 pathways, in Ewing sarcoma tumors. IMPLICATIONS: Targeting the ATR, CHK1, and WEE1 kinases in Ewing sarcoma cells activates CDK2 and increases DNA replication stress by promoting the proteasome-mediated degradation of RRM2.

Abstract A063: TAS1553, a novel class of RNR inhibitor, demonstrates synergistic antitumor efficacy in combination with nucleoside analogues

Abstract Background: Ribonucleotide reductase (RNR) is an essential metabolic enzyme that catalyzes a rate-limiting step for de novo dNTP biosynthesis via converting ribonucleotides to deoxyribonucleotides. dNTPs are also supplied through nucleoside salvage pathway in which dNTP is synthesized from extracellular nucleoside. The salvage pathway is also utilized for conversion of several nucleoside analogues to their active metabolites. Given the previous report showing that the inhibition of de novo pathway activates salvage pathway at least in vitro, RNR is considered as a promising target for combination therapy with nucleoside analogues. However, in vivo efficacy of selective RNR inhibitor in combination with nucleoside analogues has not been clarified yet. Previously, we reported a novel class of RNR inhibitor TAS1553, which is a selective and orally available small molecule abrogating protein-protein interaction between RNR subunits. Here, we evaluated the synergistic activity of TAS1553 and several nucleoside analogues in vitro as well as in vivo studies. Material and methods: Antiproliferative activity was assessed by CellTiter-Glo® 2.0 assay. Intracellular and intratumoral ara-CTP amounts were measured by HPLC analysis. Antitumor efficacy was evaluated in athymic nude mice bearing MV-4-11 (human AML) and CFPAC-1 (human pancreatic cancer) cell line. Results: To examine whether TAS1553 plus nucleoside analogues combination shows synergistic efficacy, we evaluated antiproliferative activity of TAS1553 in combination with some nucleoside analogues (cytarabine, gemcitabine, decitabine, and 2F-ara-A) to calculate the combination index (CI). TAS1553 exhibited synergistic antiproliferative activity (CI &amp;lt; 1) against cancer cell lines in combination with all of the tested nucleoside analogues. Then, to analyze the mechanism underlying the synergistic effect, we asked whether treatment with TAS1553 augments the activity of salvage pathway by measuring ara-CTP, which is an active metabolite of cytarabine. The results showed that treatment with TAS1553 significantly increased ara-CTP accumulation in MV-4-11 cells implying that TAS1553 activates nucleoside salvage pathway by inhibiting de novo nucleotide synthesis. Importantly, when analyzing the timing of treatment for the combination, prior or simultaneous treatment with TAS1553 increased ara-CTP amount more than post treatment. Consistently with in vitro result, co-administration of TAS1553 and cytarabine caused significant increase of intratumoral ara-CTP compared with cytarabine administration in MV-4-11 mouse xenograft model. Following this result, the combination of TAS1553 (100 mg/kg/day, q.d.) and cytarabine (10 mg/kg/day, 5 consecutive days) showed superior antitumor efficacy to MV-4-11 tumor than each monotherapy while the combination did not cause intolerable body weight change. Furthermore, TAS1553 also demonstrated the striking anittumor efficacy in combination with gemcitabine (20 mg/kg/day, q.wk.) on CFPAC-1 mouse xenograft model. Conclusions: TAS1553, a novel class of RNR inhibitor, has synergistic antitumor efficacy in combination with nucleoside analogues. These combination therapy could be promising therapeutic options for cancer patients. Citation Format: Takuya Hoshino, Hiroyuki Ueno, Wakako Yano, Sayaka Tsukioka, Seiji Miyahara, Takamasa Suzuki, Kazutaka Miyadera, Teruhiro Utsugi, Takeshi Sagara. TAS1553, a novel class of RNR inhibitor, demonstrates synergistic antitumor efficacy in combination with nucleoside analogues [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr A063. doi:10.1158/1535-7163.TARG-19-A063

Mec1ATM/ATR is a multifaceted protein kinase promoting cellular response to genotoxic stress and perturbation in protein homeostasis

ATM and ATR are master regulators of the DNA damage response linked to cancer, neurodegeneration, and accelerated ageing. We find that inactivation of Mec1, an essential budding yeast ATM/ATR protein, leads to widespread protein aggregation and cell death in response to three different types of proteotoxic stresses; heat, Huntingtin (HTT), the aggregation prone Huntington’s disease protein, andazetidine 2 carboxylic acid (AZC), a proline analogue that induces protein misfolding. Conditions that activate protein catabolism (e.g. activation of autophagy) or impede protein anabolism (e.g. cycloheximide [CHX] or deletion of genes involved translation) rescues the lethality via aggregate-resolution. Inactivation of Rad53- or Dun1- kinases, the two key components of the Mec1 DNA damage checkpoint response, confers distinct sensitivity profiles: rad53K277A confers sensitivity to AZC, HTT, and CHX; in contrast, dun1Δconfers sensitive only to AZC and HTT but robust resistance to CHX. We also find that Sml1, an inhibitor of ribonucleotide reductase (RNR), which undergoes Mec1-Rad53-Dun1 dependent degradation in response to DNA damage is maintained in response to proteotoxic stress. Taken together, these results unveil a new function of Mec1 in mediating cellular response to perturbation in protein homeostasis. We propose that Mec1 is a versatile signal transduction protein that promotes resistance to both genotoxic and proteotoxic stresses via distinct mechanisms.

Potent ribonucleotide reductase inhibitors: Thiazole-containing thiosemicarbazone derivatives.

The antioxidant, antimalarial, antibacterial, and antitumor activities of thiosemicarbazones have made this class of compounds important for medicinal chemists. In addition, thiosemicarbazones are among the most potent and well-known ribonucleotide reductase inhibitors. In this study, 24 new thiosemicarbazone derivatives were synthesized, and the structures and purity of the compounds were determined by IR, 1 H NMR, 13 C NMR, mass spectroscopy, and elemental analysis. The IC50 values of these 24 compounds were determined with an assay for ribonucleotide reductase inhibition. Compounds 19, 20, and 24 inhibited ribonucleotide reductase enzyme activity at a higher level than metisazone as standard. The cytotoxic effects of these compounds were measured on the MCF7 (human breast adenocarcinoma) and HEK293 (human embryonic kidney) cell lines. Similarly, compounds 19, 20, and 24 had a selective effect on the MCF7 and HEK293 cell lines, killing more cancer cells than cisplatin as standard. The compounds (especially 19, 20, and 24 as the most active ones) were then subjected to docking experiments to identify the probable interactions between the ligands and the enzyme active site. The complex formation was shown qualitatively. The ADME (absorption, distribution, metabolism, and excretion) properties of the compounds were analyzed using in-silico techniques.

Nonmelanoma skin cancer associated with Hydroxyurea treatment: Overview of the literature and our own experience.

Nonmelanoma skin cancer is the most common malignant tumor in the fair skin population, with each year several millions of diagnosed cases. Their most common risk factors are fair skin, a history of excessive ultraviolet light exposure, chronic inflammatory skin conditions, exposure to radiation, and contact with arsenic. Certain drugs can also be associated with a higher risk of nonmelanoma skin cancer. These include hydroxyurea, which acts as a metabolic inhibitor of ribonucleotide reductase and a potent nonalkylating myelosuppressive agent. It is used for the treatment of various myeloproliferative disorders, including chronic myeloid leukemia, polycythemia vera, and essential thrombocytopenia. Several publications describe an increased occurrence of skin manifestations following hydroxyurea treatment. A growing body of evidence indicates a possible role of hydroxyurea in skin cancer progression. In this review article, we summarize some relevant observations about the association of hydroxyurea and skin cancer, and we describe our own clinical experiences to provide up to date recommendations about the care of patients on hydroxyurea therapy.

Advances of Ribonucleotide Reductase Inhibitors on Cancer

Advances of Ribonucleotide Reductase Inhibitors on Cancer