PARP inhibition potentiates the cytotoxic activity of C-1305, a selective inhibitor of topoisomerase II, in human BRCA1-positive breast cancer cells

Chemoresistance to additive PARP/PI3K dual inhibition in triple-negative breast cancer cell lines is associated with adaptive stem cell-like prevalence

Cancer stem-like cells (CSCs) are posited to exhibit specialized oncogenic capacity to drive malignancies. CSCs are distinguished by enhanced hallmarks of cancer, including apoptosis avoidance, phenotypic plasticity and aberrant growth pathway signaling. Standard-of-care chemotherapies targeted to rapidly cycling cells routinely fail to eliminate this resistant subpopulation, leading to disease recurrence and metastasis. Triple-negative breast cancer (TNBC), a highly aggressive subtype of breast cancer, is enriched for tumor-propagating CD44+/CD24-/low CSCs, which are poorly ablated by chemotherapeutics and are associated with poor prognosis. CD44 governs sustained PI3K signaling in breast cancer, which is essential for CSC maintenance. PI3K inhibition can elicit DNA damage and down-regulate BRCA1 expression, which in turn enhance the synthetic lethality of PARP inhibitors. Here, we examined a dual chemotherapeutic approach targeting these pathways by combining a pan-PI3K inhibitor (Buparlisib) and a PARP1 inhibitor (Olaparib) on a panel of TNBC cell lines with distinct mutational profiles and proportions of CSCs. We observed differential sensitivity to this dual inhibition strategy and varying cellular stress and resistance responses across eight TNBC lines. The dual chemotherapeutic effect is associated with a reduction in S-phase cells, an increased in apoptotic cells and elevated expression of cleaved PARP, indicating a provoked replicative stress response. We observed that PARP/PI3K inhibition efficacy was potentiated by repeated administration in some TNBC lines and identified critical treatment schedules, which further potentiated the dual chemotherapeutic effect. Dual inhibition induced small but significant increases in CSC relative abundance as marked by CD44+/CD24-/low or ALDH1+ cells and increased stress and survival signaling in multiple TNBC cell lines, suggesting this sub-population contributes to TNBC chemoresistance. These results suggest the additive effects of PARP and PI3K inhibition against CSC phenotypes may be enhanced by temporally-staged administration in TNBC cells.

Therapeutic strategies of dual-target small molecules to overcome drug resistance in cancer therapy

Despite some advances in targeted therapeutics of human cancers, curative cancer treatment still remains a tremendous challenge due to the occurrence of drug resistance. A variety of underlying resistance mechanisms to targeted cancer drugs have recently revealed that the dual-target therapeutic strategy would be an attractive avenue. Compared to drug combination strategies, one agent simultaneously modulating two druggable targets generally shows fewer adverse reactions and lower toxicity. As a consequence, the dual-target small molecule has been extensively explored to overcome drug resistance in cancer therapy. Thus, in this review, we focus on summarizing drug resistance mechanisms of cancer cells, such as enhanced drug efflux, deregulated cell death, DNA damage repair, and epigenetic alterations. Based upon the resistance mechanisms, we further discuss the current therapeutic strategies of dual-target small molecules to overcome drug resistance, which will shed new light on exploiting more intricate mechanisms and relevant dual-target drugs for future cancer therapeutics.

Combined PARP and Dual Topoisomerase Inhibition Potentiates Genome Instability and Cell Death in Ovarian Cancer

Although ovarian cancer is a rare disease, it constitutes the fifth leading cause of cancer death among women. It is of major importance to develop new therapeutic strategies to improve survival. Combining P8-D6, a novel dual topoisomerase inhibitor with exceptional anti-tumoral properties in ovarian cancer and compounds in preclinical research, and olaparib, a PARP inhibitor targeting DNA damage repair, is a promising approach. P8-D6 induces DNA damage that can be repaired by base excision repair or homologous recombination in which PARP plays a major role. This study analyzed benefits of combining P8-D6 and olaparib treatment in 2D and 3D cultures with ovarian cancer cells. Measurement of viability, cytotoxicity and caspase activity were used to assess therapy efficacy and to calculate the combination index (CI). Further DNA damage was quantified using the biomarkers RAD51 and γH2A.X. The combinational treatment led to an increased caspase activity and reduced viability. CI values partially show synergisms in combinations at 100 nM and 500 nM P8-D6. More DNA damage accumulated, and spheroids lost their membrane integrity due to the combinational treatment. While maintaining the same therapy efficacy as single-drug therapy, doses of P8-D6 and olaparib can be reduced in combinational treatments. Synergisms can be seen in some tested combinations. In summary, the combination therapy indicates benefits and acts synergistic at 100 nM and 500 nM P8-D6.

Combined anticancer therapy with imidazoacridinone C-1305 and paclitaxel in human lung and colon cancer xenografts-Modulation of tumour angiogenesis

The acridanone derivative 5-dimethylaminopropylamino-8-hydroxytriazoloacridinone (C-1305) has been described as a potent inhibitor of cancer cell growth. Its mechanism of action in in vitro conditions was attributed, among others, to its ability to bind and stabilize the microtubule network and subsequently exhibit its tumour-suppressive effects in synergy with paclitaxel (PTX). Therefore, the objective of the present study was to analyse the effects of the combined treatment of C-1305 and PTX in vivo. In addition, considering the results of previous genomic analyses, particular attention was given to the effects of this treatment on tumour angiogenesis. Treatment with C-1305 revealed antitumor effect in A549 lung cancer cells, and combined treatment with PTX showed tendency to anticancer activity in HCT116 colon cancer xenografts. It also improved tumour blood perfusion in both tumour models. The plasma level of CCL2 was increased and that of PDGF was decreased after combined treatment with C-1305 and PTX. The experimental results showed that the levels of FGF1, TGF-β and Ang-4 decreased, whereas the levels of ERK1/2 and Akt phosphorylation increased in HCT116 tumour tissue following combined treatment with both drugs. The results of in vitro capillary-like structure formation assay demonstrated the inhibiting effect of C-1305 on this process. Although previous in vitro and in vivo studies suggested a positive effect of C-1305 on cancer cells, combined treatment of HCT116 human colon and A549 lung cancer cells with both PTX and C-1305 in vivo showed that the antitumor activity was restricted and associated with the modulation of tumour angiogenesis.

Dual-target inhibitors based on PARP1: new trend in the development of anticancer research

PARP1 is a hot target, and its inhibitors have been approved for cancer therapy. However, some undesirable properties restrict the application of PARP1 inhibitors, including drug resistance, side effects and low efficiency. For multifactorial diseases, dual-target drugs have exhibited excellent synergistic effects, such as reduced drug resistance, low side effects and high therapeutic efficacy, by simultaneously regulating the main pathogenic and compensatory signal pathways of diseases. In recent years, several dual-target inhibitors based on PARP1 have been reported and have demonstrated unique advantages. In this review we summarize the research progress in dual-target inhibitors based on PARP1 and discuss the related drug design strategies and structure-activity relationships. This work is expected to provide references for the development of PARP1 inhibitors.

Dual targeting, a new strategy for novel PARP inhibitor discovery

As a hallmark for cancer treatment, PARP inhibitors can effectively kill tumor cells with a mechanism termed as synthetic lethality, and are used to treat various cancers including ovarian, breast, prostate, pancreatic and others with DNA repair defects. However, along with the clinical trials progressing, the limitations of PARP-1 inhibitors became apparent such as limited activity and indications. Studies have shown that a molecule that is able to simultaneously restrict two or more targets involving in tumors is more effective in preventing and treating cancers due to the enhancing synergies. In order to make up for the shortcomings of PARP inhibitors, reduce the development cost and overcome the pharmacokinetic defects, multiple works were carried out to construct dual targeting PARP inhibitors for cancer therapy. Herein, they were summarized briefly.

Analysis of immune subtypes based on immunogenomic profiling identifies prognostic signature for cutaneous melanoma

Skin cutaneous melanoma (SKCM) is the most invasive form of skin cancer with poor prognosis. Growing evidence has demonstrated that tumor immune microenvironment plays a key contributing role in tumorigenesis and predicting clinical outcomes. The aim of this study was to recognize immune classification and a reliable prognostic signature for patients with SKCM. By using single-sample gene set enrichment (ssGSEA) and hierarchical clustering analyses, we evaluated the immune infiltration landscape of SKCM afflicted patients from The Cancer Genome Atlas (TCGA) dataset and named two SKCM subtypes: Immunity-high and Immunity-low. The Immunity-high subgroup was characterized by up-regulation of immune response and favorable survival probability. Seven candidate small molecule drugs which potentially reverse SKCM immune status were identified by using Connectivity map (CMap) database. A prognostic five-immune-associated gene (IAG) signature consisting IFITM1, TNFSF13B, APOBEC3G, CCL8 and KLRK1 with high predictive value was established using the LASSO Cox regression analysis in training set, and validated in testing set as well as Gene Expression Omnibus (GEO) external validation cohort (P < 0.05). Lower tumor purity and active immune-related signaling pathways were obviously presented in low-risk group. Furthermore, a novel composite nomogram based upon the five-IAG signature and other clinical parameters was built with excellent calibration curves. Collectively, comprehensively characterizing the SKCM subtypes based upon immune microenvironment landscape and development of a survival-related IAG signature may provide a promising avenue for improving individualized treatment design and prognosis prediction for patients with SKCM.

Inhibition of c-MET increases the antitumour activity of PARP inhibitors in gastric cancer models

Gastric cancer is the fifth most common malignancy and the third leading cause of cancer-related death worldwide. Activation of c-MET increases tumour cell survival through the initiation of the DNA damage repair pathway. PARP is an essential key in the DNA damage repair pathway. The primary role of PARP is to detect and initiate an immediate cellular response to single-strand DNA breaks. Tumours suppressor genes such as BRCA1/2 are closely associated with the DNA repair pathway. In BRCA1/2 mutations or deficiency status, cells are more likely to develop additional genetic alterations and chromosomal instability and can lead to cancer. In this study, we investigate the role of c-MET and PARP inhibition in a gastric cancer model. We exploited functional in vitro and in vivo experiments to assess the antitumour potential of co-inhibition of c-MET (SU11274) and PARP (NU1025). This leads to a reduction of gastric cancer cells viability, especially after knockdown of BRCA1/2 through apoptosis and induction of γ-Η2ΑΧ. Moreover, in AGS xenograft models, the combinatorial treatment of NU1025 plus SU11274 reduced tumour growth and triggers apoptosis. Collectively, our data may represent a new therapeutic approach for GC thought co-inhibition of c-MET and PARP, especially for patients with BRCA1/2 deficiency tumours.

The ascidian natural product eusynstyelamide B is a novel topoisomerase II poison that induces DNA damage and growth arrest in prostate and breast cancer cells

As part of an anti-cancer natural product drug discovery program, we recently identified eusynstyelamide B (EB), which displayed cytotoxicity against MDA-MB-231 breast cancer cells (IC₅₀ = 5 μM) and induced apoptosis. Here, we investigated the mechanism of action of EB in cancer cell lines of the prostate (LNCaP) and breast (MDA-MB-231). EB inhibited cell growth (IC₅₀ = 5 μM) and induced a G2 cell cycle arrest, as shown by a significant increase in the G2/M cell population in the absence of elevated levels of the mitotic marker phospho-histone H3. In contrast to MDA-MB-231 cells, EB did not induce cell death in LNCaP cells when treated for up to 10 days. Transcript profiling and Ingenuity Pathway Analysis suggested that EB activated DNA damage pathways in LNCaP cells. Consistent with this, CHK2 phosphorylation was increased, p21^CIP1/WAF1 was up-regulated and CDC2 expression strongly reduced by EB. Importantly, EB caused DNA double-strand breaks, yet did not directly interact with DNA. Analysis of topoisomerase II-mediated decatenation discovered that EB is a novel topoisomerase II poison.

Differential Potential of Pharmacological PARP Inhibitors for Inhibiting Cell Proliferation and Inducing Apoptosis in Human Breast Cancer Cells

BRCA1/2-mutant cells are hypersensitive to inactivation of poly(ADP-ribose) polymerase 1 (PARP-1). We recently showed that inhibition of PARP-1 by NU1025 is strongly cytotoxic for BRCA1-positive BT-20 cells, but not BRCA1-deficient SKBr-3 cells. These results raised the possibility that other PARP-1 inhibitors, particularly those tested in clinical trials, may be more efficacious against BRCA1-deficient SKBr-3 breast cancer cells than NU1025. Thus, in the presented study the cytotoxicity of four PARP inhibitors under clinical evaluation (olaparib, rucaparib, iniparib and AZD2461) was examined and compared to that of NU1025. The sensitivity of breast cancer cells to the PARP-1 inhibition strongly varied. Remarkably, BRCA-1-deficient SKBr-3 cells were almost completely insensitive to NU1025, olaparib and rucaparib, whereas BRCA1-expressing BT-20 cells were strongly affected by NU1025 even at low doses. In contrast, iniparib and AZD2461 were cytotoxic for both BT-20 and SKBr-3 cells. Of the four tested PARP-1 inhibitors only AZD2461 strongly affected cell cycle progression. Interestingly, the anti-proliferative and pro-apoptotic potential of the tested PARP-1 inhibitors clearly correlated with their capacity to damage DNA. Further analyses revealed that proteomic signatures of the two studied breast cancer cell lines strongly differ, and a set of 197 proteins was differentially expressed in NU1025-treated BT-20 cancer cells. These results indicate that BT-20 cells may harbor an unknown defect in DNA repair pathway(s) rendering them sensitive to PARP-1 inhibition. They also imply that therapeutic applicability of PARP-1 inhibitors is not limited to BRCA mutation carriers but can be extended to patients harboring deficiencies in other components of the pathway(s).

Triazolopyridopyrimidines: an emerging family of effective DNA photocleavers. DNA binding. Antileishmanial activity

Triazolopyridopyrimidines 3-phenyl-6,8-di(2-pyridyl)-[1,2,3]triazolo[5',1':6,1]pyrido[2,3-d]pyrimidine (1a), 6,8-di(pyridin-2-yl)-[1,2,3]triazolo[1',5':1,6]pyrido[2,3-d]pyrimidine (1b) and 3-methyl-6,8-di(2-pyridyl)-[1,2,3]triazolo[5',1':6,1]pyrido[2,3-d]pyrimidine (1c) were prepared and their electrochemical and luminescence properties were studied in depth. The DNA binding ability of this series of compounds has been investigated by means of UV-vis absorption and fluorescence titrations, steady-state emission quenching with ferrocyanide as well as viscosity measurements. Results have shown that triazolopyridopyrimidine 1a interacts strongly at DNA grooves. This compound also displays preferential binding to GC-rich sequences and the ability to photooxidize guanine. Moreover, these studies have revealed the key role of the phenyl substituent at the triazole ring in the binding affinity of 1a-c. Compounds 1b and 1c did not show appreciable propensity for DNA binding, however these triazolopyridopyrimidines demonstrated to present photoinduced DNA cleavage activity, 1b being more active than 1c. DNA photocleavage mediated by these compounds takes place mainly through single strand scission events and, in a minor extent, through double strand cuts. Mechanistic investigations using radical scavengers showed that both 1b and 1c generate reactive oxygen species (singlet oxygen, superoxide and hydroxyl radicals) upon irradiation. Both type I and type II mechanisms are involved in the photocleavage process. Furthermore, compounds 1a-c were tested for their antiprotozoal activity against four different Leishmania spp. (L. infantum, L. braziliensis, L. guyanensis and L. amazonensis). Triazolopyridopyrimidines 1a and 1c resulted to be more active and selective than the reference drug (miltefosine) in vitro against L. infantum amastigotes. Compound 1a exhibited high leishmanicidal activity against L. infantum spleen forms in the in vivo test.

Acridine and its derivatives: a patent review (2009 - 2013)

INTRODUCTION

Acridine derivatives have been extensively explored as potential therapeutic agents for the treatment of a number of diseases, such as cancer, Alzheimer's, and bacterial and protozoan infections. Their mode of action is mainly attributed to DNA intercalation and the subsequent effects on the biological processes linked to DNA and its related enzymes.

AREA COVERED

This review covers the relevant efforts in developing acridine derivatives with enhanced therapeutic potency and selectivity and as fluorescent materials, with particular focus on the newly patented acridine derivatives in 2009 - 2013, acridine drugs in clinical trials and preclinical studies, and other new derivatives that emerged in 2009 - 2013.

EXPERT OPINION

Thousands of acridines with therapeutic and biological activities or with photochemical properties have been developed. In addition, to modify the position and the nature of the substituent on the acridine core, more attention may be paid to the development of azaacridine or other heteroatom-substituted acridine derivatives and their synthesis methods to broaden the application of acridine derivatives. In cancer chemotherapy, the mode of action of acridine derivatives needs to be further studied. Efficient methods for identification and optimization of acridine derivatives to localize at the sites of disease need to be further developed. Moreover, acridine drugs may be combined with such bioactive agents as DNA repair proteins inhibitors to overcome tumor resistance and improve outcomes.

Bufalin induces cell death in human lung cancer cells through disruption of DNA damage response pathways

Bufalin is a key component of a Chinese medicine (Chan Su) and has been proved effective in killing various cancer cells. Its role in inducing DNA damage and the inhibition of the DNA damage response (DDR) has been reported, but none have studied such action in lung cancer in detail. In this study, we demonstrated bufalin-induced DNA damage and condensation in NCI-H460 cells through a comet assay and DAPI staining, respectively. Western blotting indicated that bufalin suppressed the protein levels associated with DNA damage and repair, such as a DNA dependent serine/threonine protein kinase (DNA-PK), DNA repair proteins breast cancer 1, early onset (BRCA1), 14-3-3 σ (an important checkpoint keeper of DDR), mediator of DNA damage checkpoint 1 (MDC1), O6-methylguanine-DNA methyltransferase (MGMT) and p53 (tumor suppressor protein). Bufalin could activate phosphorylated p53 in NCI-H460 cells. DNA damage in NCI-H460 cells after treatment with bufalin up-regulated its ATM and ATR genes, which encode proteins functioning as sensors in DDR, and also up-regulated the gene expression (mRNA) of BRCA1 and DNA-PK. But bufalin suppressed the gene expression (mRNA) of p53 and 14-3-3 σ, however, bufalin did not significantly affect the mRNA of MGMT. In conclusion, bufalin induced DNA damage in NCI-H460 cells and also inhibited its DNA repair and checkpoint function.

PJ34, a poly(ADP-ribose) polymerase (PARP) inhibitor, reverses melphalan-resistance and inhibits repair of DNA double-strand breaks by targeting the FA/BRCA pathway in multidrug resistant multiple myeloma cell line RPMI8226/R

There is still no ideal treatment for multidrug resistant multiple myeloma, looking for drugs which can reverse chemotherapy resistance and enhance curative effects of chemotherapy drugs becomes a problem that needs to be solved urgently. Poly(ADP-ribose) polymerase inhibitors appear to be an important tool for medical therapy of several malignancies. In the present study, we investigated the potential of the PARP-1 inhibitor PJ34, in vitro, to further enhance the efficacy of the traditional chemotherapy drug melphalan in the multidrug-resistant multiple myeloma cell line RPMI8226/R. The effects of different concentrations of PJ34 and melphalan on cell proliferation were determined by the CCK-8 assay. The expressions of FA/BRCA pathway-related factors were detected by western blotting and RT-PCR. The percentage of cell apoptosis was measured with flow cytometry. DNA double-strand break (DSB) repair was quantified by γH2AX immunofluorescence. In addition, DNA damage repair at the level of the individual cell was determined by comet assay. Co-administration of PJ34 and melphalan had synergistic inhibitory effects on the proliferation of RPMI8226/R cells, suggesting more powerful antitumor activities. The apoptosis percentage also was increased more obviously by the treatment of melphalan plus PJ34. The activation of FA/BRCA pathway was inhibited by downregulation of related factors including FANCD2, BRCA2 and Rad51. PJ34 significantly increased the ratio of γH2AX-positive cells and the number of foci/cells. The comet tail rate of cells, tail length, tail moment and Olive tail moment all increased after PJ34 treatment in RPMI8226/R cells. These results indicate that PJ34 combined treatment with melphalan produces synergistic effects and reverses multidrug resistance of RPMI8226/R cells effectively. PJ34 cannot induce DNA damage directly, but it may increase the DNA damage induced by melphalan through inhibiting DNA damage repair. The suppression of FA/BRCA pathway may be the mechanism. Therefore, we suggest that PARP inhibitors may deserve future investigations as tools for medical treatment of multidrug resistant multiple myeloma.

Inhibition of Cdk2 kinase activity selectively targets the CD44⁺/CD24⁻/Low stem-like subpopulation and restores chemosensitivity of SUM149PT triple-negative breast cancer cells

Inflammatory breast cancer (IBC) is an angioinvasive and most aggressive type of advanced breast cancer characterized by rapid proliferation, chemoresistance, early metastatic development and poor prognosis. IBC tumors display a triple-negative breast cancer (TNBC) phenotype characterized by centrosome amplification, high grade of chromosomal instability (CIN) and low levels of expression of estrogen receptor α (ERα), progesterone receptor (PR) and HER-2 tyrosine kinase receptor. Since the TNBC cells lack these receptors necessary to promote tumor growth, common treatments such as endocrine therapy and molecular targeting of HER-2 receptor are ineffective for this subtype of breast cancer. To date, not a single targeted therapy has been approved for non-inflammatory and inflammatory TNBC tumors and combination of conventional cytotoxic chemotherapeutic agents remains the standard therapy. IBC tumors generally display activation of epithelial to mesenchymal transition (EMT) that is functionally linked to a CD44+/CD24-/Low stem-like phenotype. Development of EMT and consequent activation of stemness programming is responsible for invasion, tumor self-renewal and drug resistance leading to breast cancer progression, distant metastases and poor prognosis. In this study, we employed the luminal ER+ MCF-7 and the IBC SUM149PT breast cancer cell lines to establish the extent to which high grade of CIN and chemoresistance were mechanistically linked to the enrichment of CD44+/CD24low/- CSCs. Here, we demonstrate that SUM149PT cells displayed higher CIN than MCF-7 cells characterized by higher percentage of structural and numerical chromosomal aberrations. Moreover, centrosome amplification, cyclin E overexpression and phosphorylation of retinoblastoma (Rb) were restricted to the stem-like CD44+/CD24-/Low subpopulation isolated from SUM149PT cells. Significantly, CD44+/CD24-/Low CSCs displayed resistance to conventional chemotherapy but higher sensitivity to SU9516, a specific cyclin-dependent kinase 2 (Cdk2) inhibitor, demonstrating that aberrant activation of cyclin E/Cdk2 oncogenic signaling is essential for the maintenance and expansion of CD44+/CD24-/Low CSC subpopulation in IBC. In conclusion, our findings propose a novel therapeutic approach to restore chemosensitivity and delay recurrence of IBC tumors based on the combination of conventional chemotherapy with small molecule inhibitors of the Cdk2 cell cycle kinase.

Drug repurposing screen identifies lestaurtinib amplifies the ability of the poly (ADP-ribose) polymerase 1 inhibitor AG14361 to kill breast cancer associated gene-1 mutant and wild type breast cancer cells

Introduction

Breast cancer is a devastating disease that results in approximately 40,000 deaths each year in the USA. Current drug screening and chemopreventatitive methods are suboptimal, due in part to the poor specificity of compounds for cancer cells. Poly (ADP-ribose) polymerase 1 (PARP1) inhibitor (PARPi)-mediated therapy is a promising approach for familial breast cancers caused by mutations of breast cancer-associated gene-1 and -2 (BRCA1/2), yet drug resistance frequently occurs during the treatment. Moreover, PARPis exhibit very little effect on cancers that are proficient for DNA repair and clinical efficacy for PARPis as single-agent therapies has yet to be illustrated.

Methods

Using a quantitative high-throughput screening approach, we screened a library containing 2,816 drugs, most of which are approved for human or animal use by the Food and Drug Administration (FDA) or other countries, to identify compounds that sensitize breast cancer cells to PARPi. After initial screening, we performed further cellular and molecular analysis on lestaurtinib, which is an orally bioavailable multikinase inhibitor and has been used in clinical trials for myeloproliferative disorders and acute myelogenous leukemia.

Results

Our study indicated that lestaurtinib is highly potent against breast cancers as a mono-treatment agent. It also strongly enhanced the activity of the potent PARPi AG14361 on breast cancer cell growth both in vitro and in vivo conditions. The inhibition of cancer growth is measured by increased apoptosis and reduced cell proliferation. Consistent with this, the treatment results in activation of caspase 3/7, and accumulation of cells in the G2 phase of the cell cycle, irrespective of their BRCA1 status. Finally, we demonstrated that AG14361 inhibits NF-κB signaling, which is further enhanced by lestaurtinib treatment.

Conclusions

Lestaurtinib amplifies the ability of the PARP1 inhibitor AG14361 to kill BRCA1 mutant and wild-type breast cancer cells, at least in part, by inhibiting NF-κB signaling. Each of these drugs has been approved for clinical trials for several different cancers, thus, their combination treatment should be applicable for a breast cancer trial in the future.

Interactions Between Ataxia Telangiectasia Mutated Kinase Inhibition, Poly(ADP-ribose) Polymerase-1 Inhibition and BRCA1 Status in Breast Cancer Cells

BACKGROUND

Cells harboring BRCA1/BRCA2 mutations are hypersensitive to inhibition of poly(ADP-ribose) polymerase-1 (PARP-1). We recently showed that interference with PARP-1 activity by NU1025 is strongly cytotoxic for BRCA1-positive BT-20 cells but not BRCA1-deficient SKBr-3 cells. These unexpected observations prompted speculation that other PARP-1 inhibitor(s) may be more cytotoxic towards SKBr-3 cells. In addition, interference with the DNA damage signaling pathway via (for instance) Ataxia telangiectasia mutated (ATM) kinase inhibition may induce synthetic lethality in DNA repair-deficient breast cancer cells and pharmacological interference with ATM activity may sensitize breast cancer cells to PARP-1 inactivation.

METHODS

We determined drug cytotoxicity in human MCF-7 and SKBr-3 breast cancer cells using the CellTiterGLO Luminescent cell viability assay and a Tecan multi-label, multitask plate counter to measure generated luminescence. Changes in cell cycle progression were monitored by flow cytometric measurement of DNA content in cells stained with propidium iodide.

RESULTS

Unlike NU1025, AZD2461, a new PARP-1 inhibitor, markedly reduced the numbers of living MCF-7 and SKBr-3 cells. ATM kinase inhibition (CP466722) was also cytotoxic for both MCF-7 and SKBr-3 cells. Furthermore, AZD2461 enhanced the cytotoxicity of CP466722 in both cell lines by inducing apoptosis, and concurrent inhibition of ATM and PARP-1 reduced cell proliferation more strongly than either single treatment.

CONCLUSIONS

Our data show that inhibition of PARP-1 by AZD2461 is synthetically lethal for NU1025-resistant MCF-7 and SKBr-3 breast cancer cells. They also indicate that DNA damage signaling is essential for survival of both SKBr-3 and MCF-7 cells, especially after inactivation of PARP-1.

Predictive biomarkers for cancer therapy with PARP inhibitors

Poly(ADP-ribose) polymerase (PARP) inhibitors have raised high expectations for the treatment of multiple malignancies. PARP inhibitors, which can be used as monotherapies or in combination with DNA-damaging agents, are particularly efficient against tumors with defects in DNA repair mechanisms, in particular the homologous recombination pathway, for instance due to BRCA mutations. Thus, deficient DNA repair provides a framework for the success of PARP inhibitors in medical oncology. Here, we review encouraging results obtained in recent clinical trials investigating the safety and efficacy of PARP inhibitors as anticancer agents. We discuss emerging mechanisms of regulation of homologous recombination and how inhibition of DNA repair might be used in cancer therapy. We surmise that the identification of patients that are likely to benefit from PARP inhibition will improve the clinical use of PARP inhibitors in a defined target population. Thus, we will place special emphasis on biomarker discovery.