Therapeutic potential of PARP inhibitors for metastatic breast cancer

The Combination of Icotinib Hydrochloride and Fluzoparib Enhances the Radiosensitivity of Biliary Tract Cancer Cells

Background

Radiotherapy and chemotherapy are the main clinical treatments for biliary tract cancers (BTCs). Patients with advanced disease have a very poor prognosis, yet no molecular targets have been proven effective for the adjuvant therapy of BTCs. In this study, we aimed to explore the effect of combination treatment with icotinib hydrochloride (IH) and fluzoparib (FZ) on radiosensitivity and clarify its underlying mechanism in the HCCC-9810 and GBC-SD human BTC cell lines.

Methods

Cell proliferation was measured by Cell Counting Kit-8 (CCK-8) assay. The cell cycle distribution and apoptosis were analyzed by flow cytometry. The phosphorylation of EGFR and its downstream signaling molecules and the expression of RAD51 were measured by Western blot analysis. γ-H2AX foci in the cellular nuclei were visualized using immunofluorescence staining. A colony formation assay was performed to demonstrate cell radiosensitivity with IH and FZ combination treatment.

Results

In the HCCC-9810 and GBC-SD human BTC cell lines, combined treatment with IH and FZ with synergetic radiation significantly inhibited cell proliferation, redistributed the cell cycle, enhanced apoptosis and delayed DNA damage repair by suppressing activation of the EGFR signaling pathway and attenuating expression of the homologous recombination (HR) protein RAD51.

Conclusion

This study demonstrates that combined treatment with IH and FZ may be an applicable therapy to enhance the radiosensitivity of BTCs and that RAD51 may serve as a biomarker for this combination treatment.

Poly(ADP-ribose) Glycohydrolase Inhibition Sequesters NAD+ to Potentiate the Metabolic Lethality of Alkylating Chemotherapy in IDH-Mutant Tumor Cells

NAD⁺ is an essential cofactor metabolite and is the currency of metabolic transactions critical for cell survival. Depending on tissue context and genotype, cancer cells have unique dependencies on NAD⁺ metabolic pathways. PARPs catalyze oligomerization of NAD⁺ monomers into PAR chains during cellular response to alkylating chemotherapeutics, including procarbazine or temozolomide. Here we find that, in endogenous IDH1-mutant tumor models, alkylator-induced cytotoxicity is markedly augmented by pharmacologic inhibition or genetic knockout of the PAR breakdown enzyme PAR glycohydrolase (PARG). Both in vitro and in vivo, we observe that concurrent alkylator and PARG inhibition depletes freely available NAD⁺ by preventing PAR breakdown, resulting in NAD⁺ sequestration and collapse of metabolic homeostasis. This effect reversed with NAD⁺ rescue supplementation, confirming the mechanistic basis of cytotoxicity. Thus, alkylating chemotherapy exposes a genotype-specific metabolic weakness in tumor cells that can be exploited by PARG inactivation. SIGNIFICANCE: Oncogenic mutations in the isocitrate dehydrogenase genes IDH1 or IDH2 initiate diffuse gliomas of younger adulthood. Strategies to maximize the effectiveness of chemotherapy in these tumors are needed. We discover alkylating chemotherapy and concurrent PARG inhibition exploits an intrinsic metabolic weakness within these cancer cells to provide genotype-specific benefit.See related commentary by Pirozzi and Yan, p. 1629.This article is highlighted in the In This Issue feature, p. 1611.

Systems biology approaches based discovery of a small molecule inhibitor targeting both c-Met/PARP-1 and inducing cell death in breast cancer

Breast cancer is the second most common types of cancer worldwide. Molecular strategies have developed rapidly; however, novel treatments strategies with high efficacy and lower toxicity are still urgently demanded. Notably, biological networks estimated from microarray data and functional activity network analysis could be utilized to identify and validate potential targets. In this study, two microarray data (GSE13477, GSE31192) were firstly selected, and analyzed by multi-functional activity network analysis to generate the core protein-protein-interaction (PPI) network. Several potential targets were subsequently identified and c-Met and poly (ADP-ribose) polymerase-1 (PARP-1) were manually chosen as the key targets in breast cancer. Furthermore, virtual screening and molecular dynamics (MD) simulations were utilized to recognize novel c-Met/PARP-1 inhibitors in Specs products database. Three small molecules, namely, ZINC19909930, ZINC20032678 and ZINC13562414 were selected. Additionally, these compounds were synthesized, and two breast cancer cell lines, MDA-MB-231 and MCF-7 cells were used to validate our bioinformatic findings in vitro. MTT assay and Hoechst staining showed that ZINC20032678 significantly induced breast cancer cell death, which was mediated through apoptosis by flow cytometry. Furthermore, ZINC20032678 was shown to target the active sites of the both targets and recruitment of downstream apoptotic signaling pathways, eventually inducing breast cancer cell apoptosis. Collectively, our findings not only offer systems biology approaches based drug target identification, but also provide the new clues for developing novel inhibitors for future breast cancer research.

Genomic instability in human cancer: Molecular insights and opportunities for therapeutic attack and prevention through diet and nutrition

Genomic instability can initiate cancer, augment progression, and influence the overall prognosis of the affected patient. Genomic instability arises from many different pathways, such as telomere damage, centrosome amplification, epigenetic modifications, and DNA damage from endogenous and exogenous sources, and can be perpetuating, or limiting, through the induction of mutations or aneuploidy, both enabling and catastrophic. Many cancer treatments induce DNA damage to impair cell division on a global scale but it is accepted that personalized treatments, those that are tailored to the particular patient and type of cancer, must also be developed. In this review, we detail the mechanisms from which genomic instability arises and can lead to cancer, as well as treatments and measures that prevent genomic instability or take advantage of the cellular defects caused by genomic instability. In particular, we identify and discuss five priority targets against genomic instability: (1) prevention of DNA damage; (2) enhancement of DNA repair; (3) targeting deficient DNA repair; (4) impairing centrosome clustering; and, (5) inhibition of telomerase activity. Moreover, we highlight vitamin D and B, selenium, carotenoids, PARP inhibitors, resveratrol, and isothiocyanates as priority approaches against genomic instability. The prioritized target sites and approaches were cross validated to identify potential synergistic effects on a number of important areas of cancer biology.

Inhibitory effect of Embelin on human acute T cell lymphoma Jurkat cells through activation of the apoptotic pathway

It has previously been shown that Embelin inhibits proliferation, promotes apoptosis, and increases sensitivity and reduces resistance to chemotherapy drugs, in various types of tumor cells. The present study examined the effects of Embelin on the proliferation of human acute T cell lymphoma Jurkat cells. Jurkat cells were treated with various concentrations of Embelin and the effects of Embelin on the inhibition of growth of Jurkat cells were evaluated. Expression of X-linked inhibitor of apoptosis protein (XIAP); poly ADP ribose polymerase; caspase-3; caspase-8; caspase-9; the proapoptotic protein, Bax; and the antiapoptotic proteins, Bcl-xl and Bcl-2, were assessed. The results showed that Embelin significantly inhibited the growth of human acute T cell lymphoma Jurkat cells. Following treatment with 5, 10 or 20 mM Embelin for 48 h, cell viability was 82.31, 58.65 and 37.62%, respectively, which was significantly reduced compared with that of the control group and the 0.1% DMSO control group (P<0.01). Furthermore, the caspase-3 inhibitor, z-DEVD-fmk, and the caspase-9 inhibitor, Ac-LEHD-CHO, reversed this inhibitory effect. It was also shown that the apoptotic rate of cells treated with Embelin was significantly elevated. Subsequently, it was demonstrated that Embelin downregulated the expression of XIAP and the proapoptotic Bcl2 family members, Bcl-2 and Bcl-xl, while it concomitantly upregulated that of the antiapoptotic protein, Bax. These results showed that Embelin inhibited growth and induced apoptosis of Jurkat cells in vitro, by activating the endogenous caspase-dependent apoptotic pathway through inhibition of XIAP and proapoptotic Bcl-2 family members.

8p deletion is strongly linked to poor prognosis in breast cancer

Deletions of chromosome 8p occur frequently in breast cancers, but analyses of its clinical relevance have been limited to small patient cohorts and provided controversial results. A tissue microarray with 2,197 breast cancers was thus analyzed by fluorescence in-situ hybridization using an 8p21 probe in combination with a centromere 8 reference probe. 8p deletions were found in 50% of carcinomas with no special type, 67% of papillary, 28% of tubular, 37% of lobular cancers and 56% of cancers with medullary features. Deletions were always heterozygous. 8p deletion was significantly linked to advanced tumor stage (P < 0.0001), high-grade (P < 0.0001), high tumor cell proliferation (Ki67 Labeling Index; P < 0.0001), and shortened overall survival (P < 0.0001). For example, 8p deletion was seen in 32% of 290 grade 1, 43% of 438 grade 2, and 65% of 427 grade 3 cancers. In addition, 8p deletions were strongly linked to amplification of MYC (P < 0.0001), HER2 (P < 0.0001), and CCND1 (p = 0.001), but inversely associated with ER receptor expression (p = 0.0001). Remarkably, 46.5% of 8p-deleted cancers harbored amplification of at least one of the analyzed genes as compared to 27.5% amplifications in 8p-non-deleted cancers (P < 0.0001). In conclusion, 8p deletion characterizes a subset of particularly aggressive breast cancers. As 8p deletions are easy to analyze, this feature appears to be highly suited for future DNA based prognostic breast cancer panels. The strong link of 8p deletion with various gene amplifications raises the possibility of a role for regulating genomic stability.

Combined deletion of Pten and p53 in mammary epithelium accelerates triple-negative breast cancer with dependency on eEF2K

The tumor suppressors Pten and p53 are frequently lost in breast cancer, yet the consequences of their combined inactivation are poorly understood. Here, we show that mammary-specific deletion of Pten via WAP-Cre, which targets alveolar progenitors, induced tumors with shortened latency compared to those induced by MMTV-Cre, which targets basal/luminal progenitors. Combined Pten-p53 mutations accelerated formation of claudin-low, triple-negative-like breast cancer (TNBC) that exhibited hyper-activated AKT signaling and more mesenchymal features relative to Pten or p53 single-mutant tumors. Twenty-four genes that were significantly and differentially expressed between WAP-Cre:Pten/p53 and MMTV-Cre:Pten/p53 tumors predicted poor survival for claudin-low patients. Kinome screens identified eukaryotic elongation factor-2 kinase (eEF2K) inhibitors as more potent than PI3K/AKT/mTOR inhibitors on both mouse and human Pten/p53-deficient TNBC cells. Sensitivity to eEF2K inhibition correlated with AKT pathway activity. eEF2K monotherapy suppressed growth of Pten/p53-deficient TNBC xenografts in vivo and cooperated with doxorubicin to efficiently kill tumor cells in vitro. Our results identify a prognostic signature for claudin-low patients and provide a rationale for using eEF2K inhibitors for treatment of TNBC with elevated AKT signaling.

Personalized sequencing and the future of medicine: discovery, diagnosis and defeat of disease

The potential for personalized sequencing to individually optimize medical treatment in diseases such as cancer and for pharmacogenomic application is just beginning to be realized, and the utility of sequencing healthy individuals for managing health is also being explored. The data produced requires additional advancements in interpretation of variants of unknown significance to maximize clinical benefit. Nevertheless, personalized sequencing, only recently applied to clinical medicine, has already been broadly applied to the discovery and study of disease. It is poised to enable the earlier and more accurate diagnosis of disease risk and occurrence, guide prevention and individualized intervention as well as facilitate monitoring of healthy and treated patients, and play a role in the prevention and recurrence of future disease. This article documents the advancing capacity of personalized sequencing, reviews its impact on disease-oriented scientific discovery and anticipates its role in the future of medicine.

Clinicopathological significance of human apurinic/apyrimidinic endonuclease 1 (APE1) expression in oestrogen-receptor-positive breast cancer

Oestrogen metabolites can induce oxidative DNA base damage and generate potentially mutagenic apurinic sites (AP sites) in the genomic DNA. If unrepaired, mutagenic AP sites could drive breast cancer pathogenesis and aggressive phenotypes. Human apurinic/apyrimidinic endonuclease 1 (APE1) is a key DNA base excision repair (BER) protein and essential for processing AP sites generated either directly by oestrogen metabolites or during BER of oxidative base damage. Our hypothesis is that altered APE1 expression may be associated with aggressive tumour biology and impact upon clinical outcomes in breast cancer. In the current study, we have investigated APE1 protein expression in a large cohort of breast cancers (n = 1285) and correlated to clinicopathological features and survival outcomes. Low APE1 protein expression was associated with high histological grade (p < 0.000001), high mitotic index (p < 0.000001), glandular de-differentiation (p < 0.000001), pleomorphism (p = 0.003), absence of hormonal receptors (ER-/PgR-/AR-) (p < 0.0001) and presence of triple negative phenotype (p = 0.001). Low APE1 protein expression was associated with loss of BRCA1, low XRCC1, low FEN1, low SMUG1 and low pol β (ps < 0.0001). High MIB1 (p = 0.048), bcl-2 negativity (p < 0.0001) and low TOP2A (p < 0.0001) were likely in low APE1 tumours. In the ER-positive sub-group, specifically, low APE1 remains significantly associated with high histological grade, high mitotic index, glandular de-differentiation (ps < 0.00001) and poor breast cancer specific survival (p = 0.007). In the ER-positive cohort that received adjuvant endocrine therapy, low APE1 protein expression is associated with poor survival (p = 0.006). In multivariate analysis, low APE1 remains independently associated with poor survival in ER-positive tumours (p = 0.048). We conclude that low APE1 expression may have prognostic and predictive significance in ER-positive breast cancers.

The horizon of precision medicine in breast cancer: fragmentation, alliance, or reunification?

Genomic studies have shown that breast cancer includes a large number of targetable genomic alterations. Most of these genomic alterations are rare and can evolve during the natural history of the disease. Three paths are being followed to develop precision medicine in metastatic breast cancer. First, the conventional path will consist of fragmenting the disease and developing drugs in each rare genomic segment. This will require screening large numbers of patients for genomic alterations to run the therapeutic trials, especially the registration trials. The second path will consist in clustering rare genomic alterations in more frequent segments defined by an altered pathway. Finally, one possible path for precision medicine will be to test genomic algorithms for the whole patient population with metastatic breast cancer. This latter scenario would reunify breast cancer into a single entity and test whether the use of genomics would improve outcomes in this population of patients. Challenges and perspective in the field of precision medicine will include the prediction of resistance, the integration of immunology, and DNA repair in the genomic algorithms and the transfer of concepts to early-stage breast cancers.

The Elephant and the Blind Men: Making Sense of PARP Inhibitors in Homologous Recombination Deficient Tumor Cells

Poly(ADP-ribose) polymerase 1 (PARP1) is an important component of the base excision repair (BER) pathway as well as a regulator of homologous recombination (HR) and non-homologous end-joining (NHEJ). Previous studies have demonstrated that treatment of HR-deficient cells with PARP inhibitors results in stalled and collapsed replication forks. Consequently, HR-deficient cells are extremely sensitive to PARP inhibitors. Several explanations have been advanced to explain this so-called synthetic lethality between HR deficiency and PARP inhibition: (i) reduction of BER activity leading to enhanced DNA double-strand breaks, which accumulate in the absence of HR; (ii) trapping of inhibited PARP1 at sites of DNA damage, which prevents access of other repair proteins; (iii) failure to initiate HR by poly(ADP-ribose) polymer-dependent BRCA1 recruitment; and (iv) activation of the NHEJ pathway, which selectively induces error-prone repair in HR-deficient cells. Here we review evidence regarding these various explanations for the ability of PARP inhibitors to selectively kill HR-deficient cancer cells and discuss their potential implications.

A nanoparticle formulation that selectively transfects metastatic tumors in mice

Nanoparticle gene therapy holds great promise for the treatment of malignant disease in light of the large number of potent, tumor-specific therapeutic payloads potentially available for delivery. To be effective, gene therapy vehicles must be able to deliver their therapeutic payloads to metastatic lesions after systemic administration. Here we describe nanoparticles comprised of a core of high molecular weight linear polyethylenimine (LPEI) complexed with DNA and surrounded by a shell of polyethyleneglycol-modified (PEGylated) low molecular weight LPEI. Compared with a state-of-the-art commercially available in vivo gene delivery formulation, i.v. delivery of the core/PEGylated shell (CPS) nanoparticles provided more than a 16,000-fold increase in the ratio of tumor to nontumor transfection. The vast majority of examined liver and lung metastases derived from a colorectal cancer cell line showed transgene expression after i.v. CPS injection in an animal model of metastasis. Histological examination of tissues from transfected mice revealed that the CPS nanoparticles selectively transfected neoplastic cells rather than stromal cells within primary and metastatic tumors. However, only a small fraction of neoplastic cells (<1%) expressed the transgene, and the extent of delivery varied with the tumor cell line, tumor site, and host mouse strain used. Our results demonstrate that these CPS nanoparticles offer substantial advantages over previously described formulations for in vivo nanoparticle gene therapeutics. At the same time, they illustrate that major increases in the effectiveness of such approaches are needed for utility in patients with metastatic cancer.

Chromatin structure in double strand break repair

Cells are under constant assault by endogenous and environmental DNA damaging agents. DNA double strand breaks (DSBs) sever entire chromosomes and pose a major threat to genome integrity as a result of chromosomal fragment loss or chromosomal rearrangements. Exogenous factors such as ionizing radiation, crosslinking agents, and topoisomerase poisons, contribute to break formation. DSBs are associated with oxidative metabolism, form during the normal S phase, when replication forks collapse and are generated during physiological processes such as V(D)J recombination, yeast mating type switching and meiosis. It is estimated that in mammalian cells ∼10 DSBs per cell are formed daily. If left unrepaired DSBs can lead to cell death or deregulated growth, and cancer development. Cellular response to DSB damage includes mechanisms to halt the progression of the cell cycle and to restore the structure of the broken chromosome. Changes in chromatin adjacent to DNA break sites are instrumental to the DNA damage response (DDR) with two apparent ends: to control compaction and to bind repair and signaling molecules to the lesion. Here, we review the key findings related to each of these functions and examine their cross-talk.

Treatment of triple-negative breast cancer using anti-EGFR-directed radioimmunotherapy combined with radiosensitizing chemotherapy and PARP inhibitor

Triple-negative breast cancer (TNBC) is associated with poor survival. Chemotherapy is the only standard treatment for TNBC. The prevalence of BRCA1 inactivation in TNBC has rationalized clinical trials of poly(adenosine diphosphate ribose) polymerase (PARP) inhibitors. Similarly, the overexpression of epidermal growth factor receptor (EGFR) rationalized anti-EGFR therapies in this disease. However, clinical trials using these 2 strategies have not reached their promise. In this study, we used EGFR as a target for radioimmunotherapy and hypothesized that EGFR-directed radioimmunotherapy can deliver a continuous lethal radiation dose to residual tumors that are radiosensitized by PARP inhibitors and chemotherapy.

METHODS

We analyzed EGFR messenger RNA in published gene expression array studies and investigated EGFR protein expression by immunohistochemistry in a cohort of breast cancer patients to confirm EGFR as a target in TNBC. Preclinically, using orthotopic and metastatic xenograft models of EGFR-positive TNBC, we investigated the effect of the novel combination of (177)Lu-labeled anti-EGFR monoclonal antibody, chemotherapy, and PARP inhibitors on cell death and the survival of breast cancer stem cells.

RESULTS

In this first preclinical study of anti-EGFR radioimmunotherapy in breast cancer, we found that anti-EGFR radioimmunotherapy is safe and that TNBC orthotopic tumors and established metastases were eradicated in mice treated with anti-EGFR radioimmunotherapy combined with chemotherapy and PARP inhibitors. We showed that the superior response to this triple-agent combination therapy was associated with apoptosis and eradication of putative breast cancer stem cells.

CONCLUSION

Our data support further preclinical investigations toward the development of combination therapies using systemic anti-EGFR radioimmunotherapy for the treatment of recurrent and metastatic TNBC.

Cancer genome landscapes

Over the past decade, comprehensive sequencing efforts have revealed the genomic landscapes of common forms of human cancer. For most cancer types, this landscape consists of a small number of "mountains" (genes altered in a high percentage of tumors) and a much larger number of "hills" (genes altered infrequently). To date, these studies have revealed ~140 genes that, when altered by intragenic mutations, can promote or "drive" tumorigenesis. A typical tumor contains two to eight of these "driver gene" mutations; the remaining mutations are passengers that confer no selective growth advantage. Driver genes can be classified into 12 signaling pathways that regulate three core cellular processes: cell fate, cell survival, and genome maintenance. A better understanding of these pathways is one of the most pressing needs in basic cancer research. Even now, however, our knowledge of cancer genomes is sufficient to guide the development of more effective approaches for reducing cancer morbidity and mortality.

Prevalence and healthcare actions of women in a large health system with a family history meeting the 2005 USPSTF recommendation for BRCA genetic counseling referral

BACKGROUND

In 2005, the United States Preventive Services Task Force (USPSTF) released guidelines which outlined specific family history patterns associated with an increased risk for BRCA1/2 mutations, and recommended at-risk individuals be referred for genetic counseling and evaluation for BRCA testing. The purpose of this study was to assess the prevalence of individuals with a USPSTF increased-risk family history pattern, the frequency with which specific patterns were met, and resulting healthcare actions among women from the Henry Ford Health System.

METHODS

As part of a study evaluating ovarian cancer risk perception and screening, 2,524 randomly selected participants completed a detailed interview (response rate 76%) from an initial eligible cohort of 16,720 women.

RESULTS

Approximately 6% of participants had a family history fulfilling one or more of the USPSTF patterns. Although 90% of these women had shared their family history with their provider, less than 20% had been referred for genetic counseling and only 8% had undergone genetic testing. Caucasian women with higher income and education levels were more likely to receive referrals. Among the 95 participants in the total study cohort who reported BRCA testing, 78% did not have a family history that met one of the USPSTF patterns.

CONCLUSIONS

These results suggest a higher prevalence of women with an increased-risk family history than originally predicted by the USPSTF, and lack of provider recognition and referral for genetic services.

IMPACT

Improvements in healthcare infrastructure and clinician education will be required to realize population level benefits from BRCA genetic counseling and testing.

Genetic Heterogeneity and Clonal Evolution of Tumor Cells and their Impact on Precision Cancer Medicine

The efficacy of targeted therapies in leukemias and solid tumors depends upon the accurate detection and sustained targeting of initial and evolving driver mutations and/or aberrations in cancer cells. Tumor clonal evolution of the diverse populations of cancer cells during cancer progression contributes to the longitudinal variations of clonal, morphological, anatomical, and molecular heterogeneity of tumors. Moreover, drug-resistant subclones present at initiation of therapy or emerging as a result of targeted therapies represent major challenges for achieving success of personalized therapies in providing meaningful improvement in cancer survival rates. Here, I briefly portray tumor cell clonal evolution at the cellular and molecular levels, and present the multiple types of genetic heterogeneity in tumors, with a focus on their impact on the implementation of personalized or precision cancer medicine.

Erythropoietin-driven signalling and cell migration mediated by polyADP-ribosylation

Background:

Recombinant human erythropoietin (EPO) is the leading biotechnology engineered hormone for treatment of anaemia associated with chronic conditions including kidney failure and cancer. The finding of EPO receptors on cancer cells has raised the concern that in addition to its action in erythropoiesis, EPO may promote tumour cell growth. We questioned whether EPO-induced signalling and consequent malignant cell manifestation is mediated by polyADP-ribosylation.

Methods:

Erythropoietin-mediated PARP (polyADP-ribose polymerase-1) activation, gene expression and core histone H4 acetylation were examined in UT7 cells, using western blot analysis, RT–PCR and immunofluorescence. Erythropoietin-driven migration of the human breast epithelial cell line MDA-MB-435 was determined by the scratch assay and in migration chambers.

Results:

We have found that EPO treatment induced PARP activation. Moreover, EPO-driven c-fos and Egr-1 gene expression as well as histone H4 acetylation were mediated via polyADP-ribosylation. Erythropoietin-induced cell migration was blocked by the PARP inhibitor, ABT-888, indicating an essential role for polyADP-ribosylation in this process.

Conclusions:

We have identified a novel pathway by which EPO-induced gene expression and breast cancer cell migration are regulated by polyADP-ribosylation. This study introduces new possibilities regarding EPO treatment for cancer-associated anaemia where combining systemic EPO treatment with targeted administration of PARP inhibitors to the tumour may allow safe treatment with EPO, minimising its possible undesirable proliferative effects on the tumour.

Therapeutic intervention by the simultaneous inhibition of DNA repair and type I or type II DNA topoisomerases: one strategy, many outcomes

Many anticancer drugs reduce the integrity of DNA, forming strand breaks. This can cause mutations and cancer or cell death if the lesions are not repaired. Interestingly, DNA repair-deficient cancer cells (e.g., those with BRCA1/2 mutations) have been shown to exhibit increased sensitivity to chemotherapy. Based on this observation, a new therapeutic approach termed 'synthetic lethality' has been developed, in which radiation therapy or cytotoxic anticancer agents are employed in conjunction with selective inhibitors of poly(ADP-ribose)polymerase-1 (PARP-1). Such combinations can cause severe genomic instability in transformed cells resulting in cell death. The synergistic effects of combining PARP-1 inhibition with anticancer drugs have been demonstrated. However, the outcome of this therapeutic strategy varies significantly between cancer types, suggesting that synthetic lethality may be influenced by additional cellular factors. This review focuses on the outcomes of the combined action of PARP-1 inhibitors and agents that affect the activity of DNA topoisomerases.