Ribonucleotide reductase inhibition enhances chemoradiosensitivity of human cervical cancers

Dose finding, bioavailability, and PK-PD of oral triapine with concurrent chemoradiation for locally advanced cervical cancer and vaginal cancer (ETCTN 9892)

BACKGROUND

The addition of IV triapine to chemoradiation appeared active in phase I and II studies but drug delivery is cumbersome. We examined PO triapine with cisplatin chemoradiation.

METHODS

We implemented a 3 + 3 design for PO triapine dose escalation with expansion, starting at 100 mg, five days a week for five weeks while receiving radiation with weekly IV cisplatin for locally advanced cervical or vaginal cancer. Maximum tolerated dose (MTD), dose limiting toxicity (DLT), adverse events, pharmacokinetics (PK), pharmacodynamics (PD), and metabolic complete response (mCR) were assessed.

RESULTS

19/21 patients were DLT evaluable. DLTs included grade 4 neutropenia (n = 2), leukopenia (n = 2), lymphopenia (n = 2), and hypokalemia (n = 1). Grade 3 toxicities at least possibly related were as expected for cisplatin chemoradiation: lymphopenia (n = 12), anemia (n = 10), neutropenia (n = 4), leukopenia (n = 8), decreased platelets (n = 2), hypertension (n = 1), and hyponatremia (n = 1). MTD and RP2D were established at 100 mg. 8/13 evaluable patients had a mCR. Triapine had a bioavailability of 59%. Methemoglobin levels correlated with triapine exposure. Smoking almost doubled CYP1A2 mediated triapine clearance.

CONCLUSIONS

Oral triapine is safe when given with cisplatin chemoradiation, convenient, bioavailable. Exposure is negatively impacted by smoking, and methemoglobin is a biomarker of exposure.

CLINICAL TRIAL REGISTRATION

NCT02595879.

Novel Therapies in Gynecologic Cancer

During the past decade, considerable strides have been made in the understanding and treatment of gynecologic cancers. The advent of PARP inhibitors, antiangiogenic therapies, immunotherapy combinations, and targeted agents have altered the standard of care in ovarian, endometrial, and cervical cancers. However, continued advancement in the treatment of gynecologic cancers is critical. Fortunately, exciting work defining new therapeutic targets and novel treatment strategies is on the horizon. Here, we discuss emerging treatments for gynecologic cancers, including endometrial, cervical, ovarian, and rare gynecologic cancers. We highlight research that has deepened our understanding of the unique biology and molecular underpinnings of these cancers and is being translated into powerful new treatment approaches. We particularly highlight the advent of immunotherapy in endometrial cancer; radiosensitizers in cervical, vaginal, and vulvar cancers; targeted therapies in ovarian cancer; and molecularly driven approaches to treat rare gynecologic cancers. Continued basic, translational, and clinical research holds the promise to change the landscape of gynecologic cancer and improve the lives of all women impacted by these diseases.

Combined modality including novel sensitizers in gynecological cancers

Standard treatment of locally advanced gynecological cancers relies mainly on platinum-based concurrent chemoradiotherapy followed by brachytherapy. Current chemotherapeutic drugs are only transiently effective and patients with advanced disease often develop resistance and subsequently, distant metastases despite significant initial responses of the primary tumor. In addition, some patients still develop local failure or progression, suggesting that there is still a place for increasing the anti-tumor radiation effect. Several strategies are being developed to increase the probability of curing patients. Vaginal cancer and vulva cancer are rare diseases, which resemble cervical cancer in their histology and pathogenesis. These gynecological cancers are predominantly associated with human papilloma virus infection. Treatment strategies in other unresectable gynecologic cancers are usually derived from evidence in locally advanced cervical cancers. In this review, we discuss mechanisms by which novel therapies could work synergistically with conventional chemoradiotherapy, from pre-clinical and ongoing clinical data. Trimodal, even quadrimodal treatment are currently being tested in clinical trials. Novel combinations derived from a metastatic setting, and being tested in locally advanced tumors, include anti-angiogenic agents, immunotherapy, tumor-infiltrating lymphocytes therapy, adoptive T-cell therapy and apoptosis inducers to enhance chemoradiotherapy efficacy through complementary molecular pathways. In parallel, radiosensitizers, such as nanoparticles and radiosensitizers of hypoxia aim to maximize the effect of radiotherapy locally.

Exploiting somatic alterations as therapeutic targets in advanced and metastatic cervical cancer

It is estimated that 604,127 patients were diagnosed with cervical cancer worldwide in 2020. While a small percentage of patients will have metastatic disease at diagnosis, a large percentage (15-61%) later develop advanced disease. For this cohort, treatment with systemic chemotherapy remains the standard of care, with a static 5-year survival rate over the last thirty years. Data on targetable molecular alterations in cervical cancer have lagged behind other more common tumor types thus stunting the development of targeted agents. In recent years, tumor genomic testing has been increasingly incorporated into our clinical practice, opening the door for a potential new era of personalized treatment for advanced cervical cancer. The interim results from the NCI-MATCH study reported an actionability rate of 28.4% for the cervical cancer cohort, suggesting a subset of patients may harbor mutations which that are targetable. This review sets out to summarize the key targeted agents currently under exploration either alone or in combination with existing treatments for cervical cancer.

Inhibiting RRM2 to enhance the anticancer activity of chemotherapy

RRM2, the small subunit of ribonucleotide reductase, is identified as a tumor promotor and therapeutic target. It is common to see the overexpression of RRM2 in chemo-resistant cancer cells and patients. RRM2 mediates the resistance of many chemotherapeutic drugs and could become the predictor for chemosensitivity and prognosis. Therefore, inhibition of RRM2 may be an effective means to enhance the anticancer activity of chemotherapy. This review tries to discuss the mechanisms of RRM2 overexpression and the role of RRM2 in resistance to chemotherapy. Additionally, we compile the studies on small interfering RNA targets RRM2, RRM2 inhibitors, kinase inhibitors, and other ways that could overcome the resistance of chemotherapy or exert synergistic anticancer activity with chemotherapy through the expression inhibition or the enzyme inactivation of RRM2.

Novel Radiosensitization Strategies in Uterine Cervix Cancer

The clinical, molecular, and genetic heterogeneity of uterine cervix cancers makes the discovery of effective therapies a challenge. Optimal evaluation of effective radiotherapy-agent combinations requires sophisticated trial strategies from the United States National Cancer Institute and its pharmaceutical collaborators. One strategy involves the phase 0 trial, which falls under the United States Food and Drug Administration Exploratory Investigational New Drug Guidance, or xIND. As currently envisioned for radiotherapy-based trials, the phase 0 trial provides a platform for study of pharmacodynamic effects linked to pharmacokinetic exposures, designed to screen a new experimental agent's dose or schedule, in combination with standard radiotherapy regimens, in a very small number (10-15) of subjects. In the phase 0 trial, radiotherapy-agent combinations are intended to be biologically active, but a new experimental agent's low dose or infrequent schedule is considered nontoxic and nonbeneficial. The phase 0 trial primary endpoint is an individual subject's pharmacodynamic response. Regimens move on from phase 0 trial development if and when a predetermined all-subject pharmacodynamic response rate is crossed. An initial safety experience during and after the radiotherapy-agent combination determines future feasibility. For this article, the clinical example of women with abdominopelvic lymph node-positive uterine cervix cancer is used to elaborate the phase 0 trial approach to the discovery of novel radiosensitizing oncological agents. It is expected that phase 0 radiotherapy-agent trials will become more prevalent in near-term clinical development.

Randomized Phase II Trial of Triapine-Cisplatin-Radiotherapy for Locally Advanced Stage Uterine Cervix or Vaginal Cancers

Uterine cervix or vaginal cancers have inherent overactivity of ribonucleotide reductase (RNR), making these cancers rational targets for therapy based on interruption of cisplatin-radiotherapy-induced DNA damage repair. We conducted a pilot, open-label randomized phase II trial to evaluate the efficacy and safety of cisplatin-radiotherapy with or without triapine, a small molecule with RNR-inhibitory activity, in patients with advanced-stage uterine cervix or vaginal cancers (NCT01835171), as a lead in to a randomized phase III study (NCT02466971). A total of 26 women were randomly assigned to receive 6 weeks of daily radiotherapy followed by brachytherapy (80 Gy) and once-weekly cisplatin (40 mg m⁻²)—with or without three-times weekly intravenous triapine (25 mg m⁻²)—in one 56-days cycle. Primary end points were metabolic complete response by positron emission tomography and safety. Additional end points included the rate of clinical response, rate of methemoglobinemia, and progression-free survival. The addition of triapine to cisplatin-radiotherapy improved the rate of metabolic complete response from 69 to 92% (P = 0.32) and raised the 3-year progression-free survival estimate from 77 to 92% (hazard ratio for progression, 0.30; P = 0.27). The most frequent grade 3 or 4 adverse events in either treatment group included reversible leukopenia, neutropenia, fatigue, or electrolyte abnormalities. No significant differences were seen between the two groups in the rate of adverse events. Symptomatic methemoglobinemia was not encountered after triapine infusion. In conclusion, the addition of triapine to cisplatin-radiotherapy improved the rate of metabolic complete response in patients with advanced-stage uterine cervix or vaginal cancers without significant toxicity. A phase III trial adequately powered to evaluate progression-free and overall survival is underway (NCT02466971).

Radiopharmaceuticals for Persistent or Recurrent Uterine Cervix Cancer

Uterine cervix cancers pose therapeutic challenges because of an overactive ribonucleotide reductase, which provides on-demand deoxyribonucleotides for DNA replication or for a DNA damage repair response. Ribonucleotide reductase overactivity bestows cancer cell resistance to the effects of radiotherapy and chemotherapy used to treat disease; but nevertheless, this same biologic overexpression provides opportune vulnerabilities relatively specific to uterine cervix cancers for new therapeutic strategies to take advantage. The discovery of human epidermal growth factor receptor 2 (ErbB2 or HER2) overexpression on metastatic uterine cervix cancer cells provides an opportunity for clinical trials of targeted radiopharmaceuticals in combination with DNA damage response modifying drugs. The National Cancer Institute's clinical trial infrastructure and its experimental therapeutics portfolio can now offer clinical trial evaluation of molecularly-targeted and tolerated radiopharmaceutical-drug combinations for women with persistent or recurrent metastatic uterine cervix cancer. This article discusses the current thinking of the National Cancer Institute in regard to attractive radiopharmaceutical strategies for this disease and others.

Anticancer Thiosemicarbazones: Chemical Properties, Interaction with Iron Metabolism, and Resistance Development

SIGNIFICANCE

During the past decades, thiosemicarbazones were clinically developed for a variety of diseases, including tuberculosis, viral infections, malaria, and cancer. With regard to malignant diseases, the class of α-N-heterocyclic thiosemicarbazones, and here especially 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (Triapine), was intensively developed in multiple clinical phase I/II trials. Recent Advances: Very recently, two new derivatives, namely COTI-2 and di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC) have entered phase I evaluation. Based on the strong metal-chelating/metal-interacting properties of thiosemicarbazones, interference with the cellular iron (and copper) homeostasis is assumed to play an important role in their biological activity.

CRITICAL ISSUES

In this review, we summarize and analyze the data on the interaction of (α-N-heterocyclic) thiosemicarbazones with iron, with the special aim of bridging the current knowledge on their mode of action from chemistry to (cell) biology. In addition, we highlight the difference to classical iron(III) chelators such as desferrioxamine (DFO), which are used for the treatment of iron overload.

FUTURE DIRECTIONS

We want to emphasize that thiosemicarbazones are not solely removing iron from the cells/organism. In contrast, they should be considered as iron-interacting drugs influencing diverse biological pathways in a complex and multi-faceted mode of action. Consequently, in addition to the discussion of physicochemical properties (e.g., complex stability, redox activity), this review contains an overview on the diversity of cellular thiosemicarbazone targets and drug resistance mechanisms.

Triapine Radiochemotherapy in Advanced Stage Cervical Cancer

Clinical ribonucleotide reductase (RNR) inhibitors have reinvigorated enthusiasm for radiochemotherapy treatment of patients with regionally advanced stage cervical cancers. About two-thirds of patients outlive their cervical cancer (1), even though up to half of their tumors retain residual microscopic disease (2). The National Cancer Institute Cancer Therapy Evaluation Program conducted two prospective trials of triapine–cisplatin–radiation to improve upon this finding by precisely targeting cervical cancer’s overactive RNR. Triapine’s potent inactivation of RNR arrests cells at the G1/S cell cycle restriction checkpoint and enhances cisplatin–radiation cytotoxicity. In this article, we provide perspective on challenges encountered in and future potential of clinical development of a triapine–cisplatin–radiation combination for patients with regionally advanced cervical cancer. New trial results and review presented here suggest that a triapine–cisplatin–radiation combination may offer molecular cell cycle target control to maximize damage in cancers and to minimize injury to normal cells. A randomized trial now accrues patients with regionally advanced stage cervical cancer to evaluate triapine’s contribution to clinical benefit after cisplatin–radiation (clinicaltrials.gov, NCT02466971).

Thymidylate synthase prompts metastatic progression through the dTMP associated EMT process in pancreatic ductal adenocarcinoma

As a fundamental metabolic enzyme, anti-Thymidylate synthase (TS) strategy has been shown to be an effective therapy for human cancers. However, the genuine effects of TS in pancreatic ductal adenocarcinoma (PDA) are still conflicting. We systemically assessed the prognostic value and whether TS associated with malignant progression in PDA. Protein and mRNA expression level of TS were evaluated in en bloc PDA samples, the prognostic effect of TS expressed in cytoplasm or cytonuclear was determined separately in the first time. The impact of TS on tumor cell behaviors was assessed in in vitro assays, and the TS associated metastatic potential was further determined in two different PDA metastatic models. The retrospective clinical analysis firstly demonstrated that tumor cytonuclear TS expression was positively correlated with lymphatic metastasis and negatively correlated with the overall survival (OS) in PDA patients. The subsequent experiments further confirmed that TS depletion can effectively abate EMT (epithelial to mesenchymal) process in in vitro and decline most of the metastatic lesions in two different PDA mice models, and the deoxythymidine monophosphate (dTMP) biosynthesis malfunction resulted imbalanced dNTP pools may be the fundamental causation. Collectively, the present study suggested the prospective strategy of combined anti-TS scheme for metastatic PDA, and we strongly suggest further clinical standardization research with a large cohort to verify the prognostic value and the therapeutic potential of TS in PDA.

Phase I Trial of Triapine-Cisplatin-Paclitaxel Chemotherapy for Advanced Stage or Metastatic Solid Tumor Cancers

Ribonucleotide reductase (RNR) is an enzyme involved in the de novo synthesis of deoxyribonucleotides, which are critical for DNA replication and DNA repair. Triapine is a small-molecule RNR inhibitor. A phase I trial studied the safety of triapine in combination with cisplatin–paclitaxel in patients with advanced stage or metastatic solid tumor cancers in an effort to capitalize on disrupted DNA damage repair. A total of 13 patients with various previously treated cancers were given a 96-h continuous intravenous (i.v.) infusion of triapine (40–120 mg/m²) on day 1, and then 3-h i.v. paclitaxel (80 mg/m²) followed by 1-h i.v. cisplatin (50–75 mg/m²) on day 3. This combination regimen was repeated every 21 days. The maximum tolerated dose (MTD) for each agent was identified to be triapine (80 mg/m²), cisplatin (50 mg/m²), and paclitaxel (80 mg/m²). Common grade 3 or 4 toxicities included reversible anemia, leukopenia, thrombocytopenia, or electrolyte abnormalities. The combination regimen of triapine–cisplatin–paclitaxel resulted in no objective responses; however, five (83%) of six patients treated at the MTD had stable disease between 1 and 8 months duration. This phase I study showed that the combination regimen of triapine–cisplatin–paclitaxel was safe and provides a rational basis for a follow-up phase II trial to evaluate efficacy and progression-free survival in women with metastatic or recurrent uterine cervix cancer.

Modulating the DNA Damage Response to Improve Treatment Response in Cervical Cancer

Cervical cancer is the fourth most common cause of cancer-related death in women worldwide and new therapeutic approaches are needed to improve clinical outcomes for this group of patients. Current treatment protocols for locally advanced and metastatic disease consist of ionising radiation and chemotherapy. Chemoradiation induces cytotoxic levels of DNA double-strand breaks, which activates programmed cell death via the DNA damage response (DDR). Cervical cancers are unique given an almost exclusive association with human papillomavirus (HPV) infection; a potent manipulator of the DDR, with the potential to alter tumour sensitivity to DNA-damaging agents and influence treatment response. This review highlights the wide range of therapeutic strategies in development that have the potential to modulate DDR and sensitise cervical tumours to DNA-damaging agents in the context of HPV oncogenesis.

Three Pt(ii) complexes based on thiosemicarbazone: synthesis, HSA interaction, cytotoxicity, apoptosis and cell cycle arrest

Three new Pt-based complexes with better IC₅₀values than cisplatin displayed different cytotoxicity, cycle arrest and cell uptake manners.

Phase I trial of daily triapine in combination with cisplatin chemotherapy for advanced-stage malignancies

PURPOSE

Advanced-stage malignancies have increased deoxyribonucleotide demands in DNA replication and repair, making deoxyribonucleotide supply a potential exploitable target for therapy based on ribonucleotide reductase (RNR) inhibition.

METHODS

A dose-finding phase I trial was conducted of intravenous (i.v.) triapine, a small-molecule RNR inhibitor, and cisplatin chemotherapy in patients with advanced-stage solid tumor malignancies. Patients received dose-finding levels of i.v. triapine (48-96 mg/m²) and i.v. cisplatin (20-75 mg/m²) on 1 of 3 different schedules. The primary endpoint was to identify the maximum tolerated dose of a triapine-cisplatin combination. Secondary endpoints included the rate of triapine-cisplatin objective response and the pharmacokinetics and bioavailability of a single oral triapine dose. (Clinicaltrials.gov number, NCT00024323).

RESULTS

The MTD was 96 mg/m² triapine daily days 1-4 and 75 mg/m² cisplatin split over day 2 and day 3. Frequent grade 3 or 4 adverse events included fatigue, dyspnea, leukopenia, thrombocytopenia, and electrolyte abnormalities. No objective responses were observed; 5 (50%) of 10 patients treated at the MTD had stable disease. Pharmacokinetics indicated an oral triapine bioavailability of 88%.

CONCLUSIONS

The triapine-cisplatin combination may be given safely in patients with advanced-stage solid tumor malignancies. On the basis of these results, a phase I trial adequately powered to evaluate oral triapine bioavailability in women with advanced-stage uterine cervix or vulvar cancers is underway.

The effect of human papillomavirus on DNA repair in head and neck squamous cell carcinoma

Much of the current literature regarding the molecular pathophysiology of human papillomavirus (HPV) in head and neck squamous cell carcinoma (HNSCC) has focused on the virus's effect on cell cycle modulation and cell proliferation. A second mechanism of pathogenicity employed by HPV, dysregulation of cellular DNA repair processes, has been more sparsely studied. The purpose of this review is to describe current understanding about the effect of HPV on DNA repair in HNSCC, taking cues from cervical cancer literature. HPV affects DNA-damage response pathways by interacting with many proteins, including ATM, ATR, MRN, γ-H2AX, Chk1, Chk2, p53, BRCA1, BRCA2, RAD51, Rb-related proteins 107 and 130, Tip60, and p16INK4A. Further elucidation of these pathways could lead to development of targeted therapies and improvement of current treatment protocols.

Radiation-Therapeutic Agent Clinical Trials: Leveraging Advantages of a National Cancer Institute Programmatic Collaboration

A number of oncology phase II radiochemotherapy trials with promising results have been conducted late in the overall experimental therapeutic agent development process. Accelerated development and approval of experimental therapeutic agents have stimulated further interest in much earlier radiation-agent studies to increase the likelihood of success in phase III trials. To sustain this interest, more forward-thinking preclinical radiobiology experimental designs are needed to improve discovery of promising radiochemotherapy plus agent combinations for clinical trial testing. These experimental designs should better inform next-step radiation-agent clinical trial dose, schedule, exposure, and therapeutic effect. Recognizing the need for a better strategy to develop preclinical data supporting radiation-agent phase I or II trials, the National Cancer Institute (NCI)-Cancer Therapy Evaluation Program (CTEP) and the NCI-Molecular Radiation Therapeutics Branch of the Radiation Research Program have partnered to promote earlier radiobiology studies of CTEP portfolio agents. In this Seminars in Radiation Oncology article, four key components of this effort are discussed. First, we outline steps for accessing CTEP agents for preclinical testing. Second, we propose radiobiology studies that facilitate transition from preclinical testing to early phase trial activation. Third, we navigate steps that walk through CTEP agent strategic development paths available for radiation-agent testing. Fourth, we highlight a new NCI-sponsored cooperative agreement grant supporting in vitro and in vivo radiation-CTEP agent testing that informs early phase trial designs. Throughout the article, we include contemporary examples of successful radiation-agent development initiatives.

Association of the Genetic Polymorphisms RRM1 -756T>C and -269C>A With Cervical Neoplasia

Cervical neoplasia is one of the most prevalent malignant neoplasms worldwide. Ribonucleotide reductase 1 (RRM1) is thought to play an essential role in modulating the development and progression of cervical neoplasia. Two novel genetic polymorphisms, RRM1 -756T>C and -269 C>A, are significantly correlated with RRM1 expression. Some epidemiological studies have demonstrated that genetic variants play a crucial role in susceptibility to cervical cancer. The present study aimed to identify the genetic polymorphisms RRM1 -756T>C and -269 C>A in patients with cervical neoplasia and healthy controls. In total, 493 subjects, comprising 324 healthy controls and 169 patients with cervical neoplasia, were enrolled for this study. The allelic discrimination of the RRM1 -756T>C (rs11030918) and -269C>A (rs12806698) polymorphisms was assessed using the ABI StepOne™ real-time polymerase chain reaction system and analyzed using Software Design Specification (SDS), Version 3.0, software with TaqMan assays. The risk of cervical cancer was examined, revealing adjusted odds ratios and 95% confidence intervals of 1.25 [0.51, 3.08] and 1.09 [0.43, 2.78] for individuals with CC alleles of RRM1 -756T>C and for individuals with AA alleles of RRM1 -269C>A genetic polymorphisms, respectively, compared to individuals with wild-type RRM1 genetic polymorphisms. No significant genetic interaction effect was observed in susceptibility to cervical neoplasia, and no association was found between genetic polymorphisms and clinical statuses of invasive cervical cancer. The genetic polymorphisms RRM1 -756T>C and -269C>A may not be a factor for susceptibility to cervical neoplasia.

Targeting HGF/c-MET induces cell cycle arrest, DNA damage, and apoptosis for primary effusion lymphoma

Kaposi sarcoma-associated herpesvirus (KSHV) is a principal causative agent of primary effusion lymphoma (PEL) with a poor prognosis in immunocompromised patients. However, it still lacks effective treatment which urgently requires the identification of novel therapeutic targets for PEL. Here, we report that the hepatocyte growth factor (HGF)/c-MET pathway is highly activated by KSHV in vitro and in vivo. The selective c-MET inhibitor, PF-2341066, can induce PEL apoptosis through cell cycle arrest and DNA damage, and suppress tumor progression in a xenograft murine model. By using microarray analysis, we identify many novel genes that are potentially controlled by HGF/c-MET within PEL cells. One of the downstream candidates, ribonucleoside-diphosphate reductase subunit M2 (RRM2), also displays the promising therapeutic value for PEL treatment. Our findings provide the framework for development of HGF/c-MET-focused therapy and implementation of clinical trials for PEL patients.

A phase I-II evaluation of veliparib (NSC #737664), topotecan, and filgrastim or pegfilgrastim in the treatment of persistent or recurrent carcinoma of the uterine cervix: an NRG Oncology/Gynecologic Oncology Group study

PURPOSE

The aim of this study was to evaluate the tolerability and efficacy of poly(ADP-ribose) polymerase (PARP) inhibition by veliparib during cytotoxic topotecan administration with filgrastim or pegfilgrastim neutrophil support in women with persistent or recurrent uterine cervix cancer.

EXPERIMENTAL DESIGN

This phase I-II trial examined twice-daily oral veliparib (10 mg) given during once-daily intravenous topotecan (0.6 mg/m²) on days 1 to 5 of each treatment cycle. Cycles were repeated every 21 days until disease progression or until toxicity prohibited further therapy. Toxicity and objective response rate were primary endpoints.

RESULTS

Twenty-seven women were enrolled. Frequently reported grade 3 or higher treatment-related toxicities were anemia (59%), thrombocytopenia (44%), leukopenia (22%), and neutropenia (19%). There were 2 partial responses (7% [90% confidence interval, 1%-22%]). Four patients had a disease progression date more than 6 months after the start of veliparib-topotecan therapy. Patients with low immunohistochemical expression (0-1+) of PARP-1 in their primary uterine cervix cancer were more likely to have a longer progression-free interval (hazard ratio, 0.25; P = 0.02) and survival (hazard ratio, 0.12; P = 0.005) after veliparib-topotecan therapy.

CONCLUSIONS

Clinical activity of a veliparib-topotecan combination was minimal in women with persistent or recurrent uterine cervix cancer. Women whose uterine cervix cancers express PARP-1 at low levels may benefit preferentially from PARP inhibitors combined with cytotoxic therapies, suggesting further study of PARP expression as an integral triage biomarker.

Emerging drugs for the treatment of cervical cancer

INTRODUCTION

Worldwide, most cervical cancer (CC) patients require the use of drug therapy either adjuvant, concurrent with radiation or palliative.

AREAS COVERED

This review briefly discusses the current achievements in treating CC with an emphasis in emerging agents.

EXPERT OPINION

Concurrent cisplatin with radiation and lately, gemcitabine-cisplatin chemoradiation has resulted in small but significant improvements in the treatment of locally advanced and high-risk early-stage patients. So far, only antiangiogenic therapy with bevacizumab added to cisplatin chemoradiation has demonstrated safety and encouraging results in a Phase II study. In advanced disease, cisplatin doublets yield median survival rates not exceeding 14 months. The first Phase III study of bevacizumab, added to standard chemotherapy cisplatin- or non-cisplatin-containing doublet, has shown significant increase in both overall survival and progression-free survival. Further studies are needed before bevacizumab plus chemotherapy can be considered the standard of care for advanced disease. The characterization of the mutational landscape of CC and developments of novel targeted therapies may result in more effective and individualized treatments for CC. The potential efficacy of knocking down the key alterations in CC, E6 and E7 human papilloma virus oncoproteins must not be overlooked.

Deoxycytidine kinase augments ATM-Mediated DNA repair and contributes to radiation resistance

Efficient and adequate generation of deoxyribonucleotides is critical to successful DNA repair. We show that ataxia telangiectasia mutated (ATM) integrates the DNA damage response with DNA metabolism by regulating the salvage of deoxyribonucleosides. Specifically, ATM phosphorylates and activates deoxycytidine kinase (dCK) at serine 74 in response to ionizing radiation (IR). Activation of dCK shifts its substrate specificity toward deoxycytidine, increases intracellular dCTP pools post IR, and enhances the rate of DNA repair. Mutation of a single serine 74 residue has profound effects on murine T and B lymphocyte development, suggesting that post-translational regulation of dCK may be important in maintaining genomic stability during hematopoiesis. Using [(18)F]-FAC, a dCK-specific positron emission tomography (PET) probe, we visualized and quantified dCK activation in tumor xenografts after IR, indicating that dCK activation could serve as a biomarker for ATM function and DNA damage response in vivo. In addition, dCK-deficient leukemia cell lines and murine embryonic fibroblasts exhibited increased sensitivity to IR, indicating that pharmacologic inhibition of dCK may be an effective radiosensitization strategy.

Long-Term Disease Control with Triapine-Based Radiochemotherapy for Patients with Stage IB2-IIIB Cervical Cancer

Background: National Cancer Institute phase I #7336 and phase II #8327 clinical trials explored the safety and efficacy of triapine (NSC #663249) added to cisplatin radiochemotherapy in untreated patients with advanced-stage cervical cancer. Triapine inhibits ribonucleotide reductase, the rate-limiting enzyme responsible for DNA-building deoxyribonucleotides, and thereby, enhances radiochemosensitivity by prolonging DNA repair time. Here, we report 3-year efficacy endpoints of pelvic locoregional relapse rate, disease-free, and overall survivals.

Methods: Eligible patients with bulky IB–IIIB cervical cancer underwent three-times weekly triapine (25 or 50 mg/m²), once-weekly cisplatin (40 mg/m²), and conventional daily pelvic radiation followed by brachytherapy. A cumulative incidence method estimated pelvic locoregional relapse rates. Disease-free survival was measured from radiochemotherapy start date to the date of first relapse or cancer-related death. Overall survival was measured from radiochemotherapy start date to the date of any-cause death. The Kaplan–Meier method estimated survivals.

Findings: Between 2006 and 2011, 24 untreated patients with cervical cancer met criteria for reporting in this study. A median 3.4 years of follow-up time (range, 0.3–7.6 years) has been observed. All had squamous cancers and the majority had either node-positive stage IB–IIA (33%) or stage IIIB (42%) disease. The 3-year pelvic locoregional relapse rate, disease-free survival, and overall survival were 4% [95% confidence interval (CI), 0–20%], 80% (95% CI: 71–89%), and 82% (95% CI: 74–90%), respectively.

Interpretation: Triapine radiochemotherapy was safe, active, and effective in patients with untreated advanced-stage cervical cancer, worthy of randomized clinical trial study.

Ribonucleotide reductase expression in cervical cancer: a radiation therapy oncology group translational science analysis

OBJECTIVE

To evaluate pretherapy ribonucleotide reductase (RNR) expression and its effect on radiochemotherapeutic outcome in women with cervical cancer.

METHODS/MATERIALS

Pretherapy RNR M1, M2, and M2b immunohistochemistry was done on cervical cancer specimens retrieved from women treated on Radiation Therapy Oncology Group (RTOG) 0116 and 0128 clinical trials. Enrollees of RTOG 0116 (node-positive stages IA-IVA) received weekly cisplatin (40 mg/m(2)) with amifostine (500 mg) and extended-field radiation then brachytherapy (85 Gy). Enrollees of RTOG 0128 (node-positive or bulky ≥5 cm, stages IB-IIA or stages IIB-IVA) received cisplatin (75 mg/m(2)) on days 1, 23, and 43 and 5-FU (1 g/m(2) for 4 days) during pelvic radiation then brachytherapy (85 Gy), plus celecoxib (400 mg twice daily, day 1 through 1 year). Disease-free survival (DFS) was estimated univariately by the Kaplan-Meier method. Cox proportional hazards models evaluated the impact of RNR immunoreactivity on DFS.

RESULTS

Fifty-one tissue samples were analyzed: 13 from RTOG 0116 and 38 from RTOG 0128. M1, M2, and M2b overexpression (3+) frequencies were 2%, 80%, and 47%, respectively. Low-level (0-1+, n = 44/51) expression of the regulatory subunit M1 did not associate with DFS (P = 0.38). High (3+) M2 expression occurred in most (n = 41/51) but without impact alone on DFS (hazard ratio, 0.54; 95% confidence interval, 0.2-1.4; P = 0.20). After adjusting for M2b status, pelvic node-positive women had increased hazard for relapse or death (hazard ratio, 5.5; 95% confidence interval, 2.2-13.8; P = 0.0003).

CONCLUSIONS

These results suggest that RNR subunit expression may discriminate cervical cancer phenotype and radiochemotherapy outcome. Future RNR biomarker studies are warranted.

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.

Gemcitabine and carboplatin demonstrate synergistic cytotoxicity in cervical cancer cells by inhibiting DNA synthesis and increasing cell apoptosis

BACKGROUND

The present study aims to investigate the subunit expression and enzyme activity of ribonucleotide reductase in cervical cancer patients, and detect the combined effect of the ribonucleotide reductase inhibitor gemcitabine and the chemotherapeutic agent carboplatin on cervical cancer cell lines.

METHODS

Using quantitative reverse transcription polymerase chain reaction, Western blotting, and cytidine 5'-diphosphate reduction assays, we tested the expression and activity of ribonucleotide reductase in cervical cancer patients. The antitumor activity of gemcitabine and/or carboplatin treatments to SiHa and CaSki human cervical cancer cell lines were assessed by Cell Counting Kit-8 viability assay, EdU incorporation assay, immunofluorescence assay, flow cytometry assay, and Western blotting methods. Additionally, synergistic efficacy was quantitatively analyzed using a combination index based on the Chou-Talalay method.

RESULTS

The mRNA levels of three ribonucleotide reductase subunits were all upregulated in the cervical cancer tissues compared with normal tissues (P<0.0001). Consistently, the protein expression and enzyme activity of ribonucleotide reductase were also increased in the cervical cancer tissues. Interestingly, gemcitabine inhibited DNA synthesis and carboplatin induced DNA damage. Further, the combined drug regime had a significant synergistic effect on inhibiting cervical cancer cell viability (log10[combination index] <0) via enhanced DNA damage and cell apoptosis.

CONCLUSION

The expression and activity of ribonucleotide reductase was increased in cervical cancer. Our study demonstrated the synergistic cytotoxicity of gemcitabine and carboplatin, through inhibiting DNA synthesis and increasing cell apoptosis in cervical cancer cell lines. This evidence might provide a rational clue of their combined application to improve cervical cancer treatment.

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.

Radiochemotherapy plus 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, NSC #663249) in advanced-stage cervical and vaginal cancers

OBJECTIVE

Cervical and vaginal cancers have virally-mediated or mutated defects in DNA damage repair responses, making these cancers sensible targets for ribonucleotide reductase inhibition during radiochemotherapy.

METHODS

We conducted a phase II study evaluating 3× weekly 2-hour intravenous 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, 25 mg/m(2)) co-administered with 1× weekly intravenous cisplatin (40 mg/m(2)) and daily pelvic radiation (45 Gy) in women with stage I(B2)-IV(B) cervical (n=22) or stage II-IV vaginal (n=3) cancers. Brachytherapy followed (40 Gy). Toxicity was monitored by common terminology criteria for adverse events (version 3.0). The primary end point of response was assessed by 3-month posttherapy 2-[(18)F] fluoro-2-deoxy-d-glucose positron emission tomography (PET/CT) and clinical examination.

RESULTS

3-AP radiochemotherapy achieved clinical responses in 24 (96% [95% confidence interval: 80-99%]) of 25 patients (median follow-up 20 months, range 2-35 months). 23 (96% [95% confidence interval: 80-99%]) of 24 patients had 3-month posttherapy PET/CT scans that recorded metabolic activity in the cervix or vagina equal or less than that of the cardiac blood pool, suggesting complete metabolic responses. The most frequent 3-AP radiochemotherapy-related adverse events included fatigue, nausea, diarrhea, and reversible hematological and electrolyte abnormalities.

CONCLUSIONS

The addition of 3-AP to cisplatin radiochemotherapy was tolerable and produced high rates of clinical and metabolic responses in women with cervical and vaginal cancers. Future randomized phase II and III clinical trials of 3-AP radiochemotherapy are warranted.

Inhibition of Poly(ADP-Ribose) Polymerase Enhances Radiochemosensitivity in Cancers Proficient in DNA Double-Strand Break Repair

Pharmacologic inhibitors of poly(ADP-ribose) polymerase (PARP) putatively enhance radiation toxicity in cancer cells. Although there is considerable information on the molecular interactions of PARP and BRCA1- and BRCA2-deficient cancers, very little is known of the PARP inhibition effect upon cancers proficient in DNA double-strand break repair after ionizing radiation or after stalled replication forks. In this work, we investigate whether PARP inhibition by ABT-888 (veliparib) augments death-provoking effects of ionizing radiation, or of the topoisomerase I poison topotecan, within uterine cervix cancers cells harboring an unfettered, overactive ribonucleotide reductase facilitating DNA double-strand break repair and contrast these findings with ovarian cancer cells whose regulation of ribonucleotide reductase is relatively intact. Cell lethality of a radiation-ABT-888 combination is radiation and drug dose dependent. Data particularly highlight an enhanced topotecan-ABT-888 cytotoxicity, and corresponds to an increased number of unrepaired DNA double-strand breaks. Overall, our findings support enhanced radiochemotherapy toxicity in cancers proficient in DNA double-strand break repair when PARP is inhibited by ABT-888.

Elevated ribonucleotide reductase levels associate with suppressed radiochemotherapy response in human cervical cancers

OBJECTIVE

Ribonucleotide reductase (RNR) supplies deoxyribonucleotide diphosphates demanded by cells to repair radiation-induced DNA damage. Here, we investigate the impact of pretherapy RNR M1, M2, and M2b (p53R3) subunit level upon human cervical cancer radiochemosensitivity.

MATERIALS/METHODS

Immunohistochemistry was performed on a tissue array comprised of 18 paired benign uterine cervix and stage IB2 cervical cancers to evaluate the relationship between cytosolic RNR M1, M2, and M2b staining intensity and radiochemotherapy cancer response. Patients underwent surgical hysterectomy (n = 8), or daily radiation (45 Gy), coadministered once-weekly cisplatin (40 mg/m), then low-dose rate brachytherapy (30 Gy) followed by adjuvant hysterectomy (n = 10). Radiochemotherapy response was determined by Response Evaluation Criteria In Solid Tumors version 1.0 criteria during brachytherapy. Cancer relapse rates and disease-free survival were calculated.

RESULTS

M1, M2, and M2b antibody staining intensity was low (0-1+) in benign uterine cervical tissue. M1 and M2b immunoreactivity was 2+ or 3+ in most (13/18) cervical cancers. M2 immunoreactivity was 3+ in nearly all (16/18) cervical cancers. Cervical cancers overexpressing M1 and M2b had an increased hazard for incomplete radiochemotherapy response, relapse, and shortened disease-free survival.

CONCLUSIONS

Ribonucleotide reductase subunit levels may predict human cervical cancer radiochemosensitivity and subsequent posttherapy cancer outcome. Further validation testing of RNR subunits as biomarkers for radiochemotherapy response is warranted.

Mammalian ribonucleotide reductase subunit p53R2 is required for mitochondrial DNA replication and DNA repair in quiescent cells

In postmitotic mammalian cells, protein p53R2 substitutes for protein R2 as a subunit of ribonucleotide reductase. In human patients with mutations in RRM2B, the gene for p53R2, mitochondrial (mt) DNA synthesis is defective, and skeletal muscle presents severe mtDNA depletion. Skin fibroblasts isolated from a patient with a lethal homozygous missense mutation of p53R2 grow normally in culture with an unchanged complement of mtDNA. During active growth, the four dNTP pools do not differ in size from normal controls, whereas during quiescence, the dCTP and dGTP pools decrease to 50% of the control. We investigate the ability of these mutated fibroblasts to synthesize mtDNA and repair DNA after exposure to UV irradiation. Ethidium bromide depleted both mutant and normal cells of mtDNA. On withdrawal of the drug, mtDNA recovered equally well in cycling mutant and control cells, whereas during quiescence, the mutant fibroblasts remained deficient. Addition of deoxynucleosides to the medium increased intracellular dNTP pools and normalized mtDNA synthesis. Quiescent mutant fibroblasts were also deficient in the repair of UV-induced DNA damage, as indicated by delayed recovery of dsDNA analyzed by fluorometric analysis of DNA unwinding and the more extensive and prolonged phosphorylation of histone H2AX after irradiation. Supplementation by deoxynucleosides improved DNA repair. Our results show that in nontransformed cells only during quiescence, protein p53R2 is required for maintenance of mtDNA and for optimal DNA repair after UV damage.

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.

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

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

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

Background

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

Methods

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

Results

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

Conclusions

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

Trial registry

ClinicalTrials.gov NCT00081276

Ribonucleotide reductase inhibitors: a new look at an old target for radiosensitization

Ribonucleotide reductase (RR), the rate limiting enzyme in the synthesis and repair of DNA, has been studied as a target for inhibition in the treatment of cancer for many years. While some researchers have focused on RR inhibitors as chemotherapeutic agents, particularly in hematologic malignancies, some of the most promising data has been generated in the field of radiosensitization. Early pre-clinical studies demonstrated that the addition of the first of these drugs, hydroxyurea, to ionizing radiation (IR) produced a synergistic effect in vitro, leading to a large number of clinical studies in the 1970–1980s. These studies, mainly in cervical cancer, initially produced a great deal of interest, leading to the incorporation of hydroxyurea in the treatment protocols of many institutions. However, over time, the conclusions from these studies have been called into question and hydroxyurea has been replaced in the standard of care of cervical cancer. Over the last 10 years, a number of well-done pre-clinical studies have greatly advanced our understanding of RR as a target. Those advances include the elucidation of the role of p53R2 and our understanding of the temporal relationship between the delivery of IR and the response of RR. At the same time, new inhibitors with increased potency and improved binding characteristics have been discovered, and pre-clinical and early clinical data look promising. Here we present a comprehensive review of the pre-clinical and clinical data in the field to date and provide some discussion of future areas of research.

Therapeutic Mechanisms of Treatment in Cervical and Vaginal Cancer

Cervical and vaginal cancers remain serious health problems. Worldwide, more than 530,000 women annually are diagnosed with these diseases, with most new incident cases occurring in nations with limited health resources and underdeveloped screening programs. For women whose disease is too bulky or widespread for surgery, radiochemotherapy should be looked upon as the standard of care. Randomized clinical trials have indicated that radiochemotherapy strategies that disrupt the repair of damaged DNA are key to the management of advanced stage cervical and vaginal cancers. Here, from a viewpoint of cancer cell molecular biology, treatments for advanced stage cervical and vaginal cancers are discussed.

Molecular Strategies of Deoxynucleotide Triphosphate Supply Inhibition Used in the Treatment of Gynecologic Malignancies

Chemotherapies targeting deoxynucleotide triphosphate synthesis are of high medical interest in the treatment of gynecologic malignancies. In this article, we focus on targeted inhibitors of ribonucleotide reductase, an enzyme in charge of ribonucleotide reduction to their corresponding deoxyribonucleotide to be used as the building blocks of DNA. We also discuss human clinical trials have utilized ribonucleotide reductase subunit-specific inhibitors, particularly trials for women with cervical cancer.

A phase I and pharmacokinetic study of oral 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, NSC #663249) in the treatment of advanced-stage solid cancers: a California Cancer Consortium Study

BACKGROUND

3-Aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP) is a novel small-molecule ribonucleotide reductase inhibitor. This study was designed to estimate the maximum tolerated dose (MTD) and oral bioavailability of 3-AP in patients with advanced-stage solid tumors.

METHODS

Twenty patients received one dose of intravenous and subsequent cycles of oral 3-AP following a 3 + 3 patient dose escalation. Intravenous 3-AP was administered to every patient at a fixed dose of 100 mg over a 2-h infusion 1 week prior to the first oral cycle. Oral 3-AP was administered every 12 h for 5 consecutive doses on days 1-3, days 8-10, and days 15-17 of every 28-day cycle. 3-AP was started at 50 mg with a planned dose escalation to 100, 150, and 200 mg. Dose-limiting toxicities (DLT) and bioavailability were evaluated.

RESULTS

Twenty patients were enrolled. For dose level 1 (50 mg), the second of three treated patients had a DLT of grade 3 hypertension. In the dose level 1 expansion cohort, three patients had no DLTs. No further DLTs were encountered during escalation until the 200-mg dose was reached. At the 200 mg 3-AP dose level, two treated patients had DLTs of grade 3 hypoxia. One additional DLT of grade 4 febrile neutropenia was subsequently observed at the de-escalated 150 mg dose. One DLT in 6 evaluable patients established the MTD as 150 mg per dose on this dosing schedule. Responses in the form of stable disease occurred in 5 (25%) of 20 patients. The oral bioavailability of 3-AP was 67 ± 29% and was consistent with the finding that the MTD by the oral route was 33% higher than by the intravenous route.

CONCLUSIONS

Oral 3-AP is well tolerated and has an MTD similar to its intravenous form after accounting for the oral bioavailability. Oral 3-AP is associated with a modest clinical benefit rate of 25% in our treated patient population with advanced solid tumors.

Targeting the Large Subunit of Human Ribonucleotide Reductase for Cancer Chemotherapy

Ribonucleotide reductase (RR) is a crucial enzyme in de novo DNA synthesis, where it catalyses the rate determining step of dNTP synthesis. RRs consist of a large subunit called RR1 (α), that contains two allosteric sites and one catalytic site, and a small subunit called RR2 (β), which houses a tyrosyl free radical essential for initiating catalysis. The active form of mammalian RR is an α_nβ_m hetero oligomer. RR inhibitors are cytotoxic to proliferating cancer cells. In this brief review we will discuss the three classes of RR, the catalytic mechanism of RR, the regulation of the dNTP pool, the substrate selection, the allosteric activation, inactivation by ATP and dATP, and the nucleoside drugs that target RR. We will also discuss possible strategies for developing a new class of drugs that disrupts the RR assembly.

Deoxynucleoside salvage facilitates DNA repair during ribonucleotide reductase blockade in human cervical cancers

Cells generate 2'-deoxyribonucleoside triphosphates (dNTPs) for both replication and repair of damaged DNA predominantly through de novo reduction of intracellular ribonucleotides by ribonucleotide reductase (RNR). Cells can also salvage deoxynucleosides by deoxycytidine kinase/thymidine kinase 1 in the cytosol or by deoxyguanosine kinase/thymidine kinase 2 in mitochondria. In this study we investigated whether the salvage dNTP supply pathway facilitates DNA damage repair, promoting cell survival, when pharmacological inhibition of RNR by 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, NSC no. 663249) impairs the de novo pathway. Human cervical cancer cells were subjected to radiation with or without 3-AP under medium deoxynucleoside concentrations of 0, 0.05, 0.5 and 5.0 µM. Efficacy of DNA damage repair was assessed by γ-H2AX flow cytometry and focus counts, by single cell electrophoresis (Comet assay), and by caspase 3 cleavage assay as a marker of treatment-induced apoptosis. Cell survival was assessed by colony formation. We found that deoxyribonucleotide salvage facilitates DNA repair during RNR inhibition by 3-AP and that salvage reduces the radiochemosensitivity of human cervical cancer cells.

Radiosensitization of human cervical cancer cells by inhibiting ribonucleotide reductase: enhanced radiation response at low-dose rates

PURPOSE

To test whether pharmacologic inhibition of ribonucleotide reductase (RNR) by 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, NSC #663249) enhances radiation sensitivity during low-dose-rate ionizing radiation provided by a novel purpose-built iridium-192 cell irradiator.

METHODS AND MATERIALS

The cells were exposed to low-dose-rate radiation (11, 23, 37, 67 cGy/h) using a custom-fabricated cell irradiator or to high-dose-rate radiation (330 cGy/min) using a conventional cell irradiator. The radiation sensitivity of human cervical (CaSki, C33-a) cancer cells with or without RNR inhibition by 3-AP was evaluated using a clonogenic survival and an RNR activity assay. Alteration in the cell cycle distribution was monitored using flow cytometry.

RESULTS

Increasing radiation sensitivity of both CaSki and C33-a cells was observed with the incremental increase in radiation dose rates. 3-AP treatment led to enhanced radiation sensitivity in both cell lines, eliminating differences in cell cytotoxicity from the radiation dose rate. RNR blockade by 3-AP during low-dose-rate irradiation was associated with low RNR activity and extended G(1)-phase cell cycle arrest.

CONCLUSIONS

We conclude that RNR inhibition by 3-AP impedes DNA damage repair mechanisms that rely on deoxyribonucleotide production and thereby increases radiation sensitivity of human cervical cancers to low-dose-rate radiation.