Combination treatment using DDX3 and PARP inhibitors induces synthetic lethality in BRCA1-proficient breast cancer

Triple-negative breast cancers have unfavorable outcomes due to their inherent aggressive behavior and lack of targeted therapies. Breast cancers occurring in BRCA1 mutation carriers are mostly triple-negative and harbor homologous recombination deficiency, sensitizing them to inhibition of a second DNA damage repair pathway by, e.g., PARP inhibitors. Unfortunately, resistance against PARP inhibitors in BRCA1-deficient cancers is common and sensitivity is limited in BRCA1-proficient breast cancers. RK-33, an inhibitor of the RNA helicase DDX3, was previously demonstrated to impede non-homologous end-joining repair of DNA breaks. Consequently, we evaluated DDX3 as a therapeutic target in BRCA pro- and deficient breast cancers and assessed whether DDX3 inhibition could sensitize cells to PARP inhibition. High DDX3 expression was identified by immunohistochemistry in breast cancer samples of 24% of BRCA1 (p = 0.337) and 21% of BRCA2 mutation carriers (p = 0.624), as compared to 30% of sporadic breast cancer samples. The sensitivity to the DDX3 inhibitor RK-33 was similar in BRCA1 pro- and deficient breast cancer cell lines, with IC50 values in the low micromolar range (2.8–6.6 μM). A synergistic interaction was observed for combination treatment with RK-33 and the PARP inhibitor olaparib in BRCA1-proficient breast cancer, with the mean combination index ranging from 0.59 to 0.62. Overall, we conclude that BRCA pro- and deficient breast cancers have a similar dependency upon DDX3. DDX3 inhibition by RK-33 synergizes with PARP inhibitor treatment, especially in breast cancers with a BRCA1-proficient background.     

BRCA1/2‐negative hereditary triple‐negative breast cancers exhibit BRCAness

BRCA1/2‐associated breast cancers are sensitive to poly(ADPribose) polymerase (PARP) inhibitors and platinum compounds mainly due to their deficiency in DNA repair via homologous recombination (HR). However, approximately only 15% of triple‐negative breast cancers (TNBCs) are BRCA1/2‐associated. TNBCs that exhibit BRCAness (a phenotype reflecting impaired HR in BRCA1/2‐negative tumors) are also regarded sensitive to PARP inhibitors and platinum compounds. Thus, we hypothesized that hereditary BRCA1/2‐negative TNBCs may exhibit BRCAness. To find a subset of hereditary BRCA1/2‐negative TNBCs among 360 TNBCs, we first identified a group of 41 hereditary TNBCs by analyzing the family histories of the patients. Next, we tested this group for the presence of germline BRCA1/2 mutations, and finally, we compared the expression levels of 120 genes involved in HR and five other major mechanisms of DNA damage repair between BRCA1/2‐associated and BRCA1/2‐negative subgroups of hereditary TNBCs using real‐time PCR arrays. Approximately 73% of the hereditary TNBCs were BRCA1/2‐associated and 27% were BRCA1/2‐negative. The expression levels of the analyzed genes showed no significant differences between these two subgroups indicating the BRCAness of the BRCA1/2‐negative hereditary TNBCs and thereby distinguishing a novel subset of TNBCs as a potential target for PARP inhibitors or platinum‐based therapy. The results show the significance of family history in selecting patients with TNBC for therapies directed at incompetent DNA repair (e.g., PARP inhibitors and/or platinum‐based therapies) and indicate that a relatively simple strategy for broadening the target group for these modes of treatment is to identify patients with hereditary TNBCs.     

PARP Inhibitors and Prostate Cancer: To Infinity and Beyond BRCA

The U.S. Food and Drug Administration recently approved two poly‐adenosine diphosphate‐ribose polymerase (PARP) inhibitors, olaparib and rucaparib, for treatment of biomarker‐positive metastatic castrate resistant prostate cancer. The benefits of PARP inhibition have been well characterized in patients who have BRCA1 and BRCA2 mutations in several forms of cancer. BRCA1 and BRCA2 occupy key roles in DNA damage repair, which is comprised of several different pathways with numerous participants. Patients with mutations in other key genes within the DNA damage repair pathway may also respond to treatment with PARP inhibitors, and identification of these alterations could significantly increase the percentage of patients that may benefit from PARP inhibition. This review focuses on the potential for synthetically lethal interactions between PARP inhibitors and non‐BRCA DNA damage repair genes.Implications for PracticeThe treatment potential of PARP inhibition has been well characterized in patients with BRCA1 and BRCA2 mutations, but there is compelling evidence for expanding the use of PARP inhibitors to mutations of other non‐BRCA DNA damage repair (DDR) genes. This could increase the percentage of patients that may benefit from treatment with PARP inhibitors alone or in combination with other therapies. Understanding the significance of PARP inhibitor‐sensitizing alterations in other common non‐BRCA DDR genes will help guide clinical decisions to provide targeted treatment options to a wider population of patients.     

A decade of clinical development of PARP inhibitors in perspective

Genomic instability is a hallmark of cancer, and often is the result of altered DNA repair capacities in tumour cells. DNA damage repair defects are common in different cancer types; these alterations can also induce tumour-specific vulnerabilities that can be exploited therapeutically. In 2009, a first-in-man clinical trial of the poly(ADP-ribose) polymerase (PARP) inhibitor olaparib clinically validated the synthetic lethal interaction between inhibition of PARP1, a key sensor of DNA damage, and BRCA1/BRCA2 deficiency. In this review, we summarize a decade of PARP inhibitor clinical development, a work that has resulted in the registration of several PARP inhibitors in breast (olaparib and talazoparib) and ovarian cancer (olaparib, niraparib and rucaparib, either alone or following platinum chemotherapy as maintenance therapy). Over the past 10 years, our knowledge on the mechanism of action of PARP inhibitor as well as how tumours become resistant has been extended, and we summarise this work here. We also discuss opportunities for expanding the precision medicine approach with PARP inhibitors, identifying a wider population who could benefit from this drug class. This includes developing and validating better predictive biomarkers for patient stratification, mainly based on homologous recombination defects beyond BRCA1/BRCA2 mutations, identifying DNA repair deficient tumours in other cancer types such as prostate or pancreatic cancer, or by designing combination therapies with PARP inhibitors.     

PARP Inhibitors for the Treatment and Prevention of Breast Cancer

Poly (ADP-ribose) polymerase (PARP) inhibitors, a novel class of drugs that target tumors with DNA repair defects, have received tremendous enthusiasm. Early preclinical studies identified BRCA1 and BRCA2 tumors to be highly sensitive to PARP inhibitors as a result of homologous recombination defect. Based on this premise, PARP inhibitors have been tested in early phase clinical trials as a single agent in BRCA1 or BRCA2 mutation carriers and in combination with chemotherapy in triple-negative breast cancer patients. For high-risk populations, use of PARP inhibition as a prevention agent has been postulated, but no robust preclinical or clinical studies exist yet. We review the preclinical and clinical studies in treatment of breast cancer and rationale for use of PARP inhibitors as a prevention agent for high-risk populations. Of significance, PARP inhibitors vary significantly in mechanism of action, dosing intervals, and toxicities, which are highlighted in this review.     

Crosstalk of DNA double‐strand break repair pathways in poly(ADP‐ribose) polymerase inhibitor treatment of breast cancer susceptibility gene 1/2‐mutated cancer

Germline mutations in breast cancer susceptibility gene 1 or 2 (BRCA1 or BRCA2) significantly increase cancer risk in hereditary breast and ovarian cancer syndrome (HBOC). Both genes function in the homologous recombination (HR) pathway of the DNA double‐strand break (DSB) repair process. Therefore, the DNA‐repair defect characteristic of cancer cells brings about a therapeutic advantage for poly(ADP‐ribose) polymerase (PARP) inhibitor‐induced synthetic lethality. PARP inhibitor‐based therapeutics initially cause cancer lethality but acquired resistance mechanisms have been found and need to be elucidated. In particular, it is essential to understand in detail the mechanism of DNA damage and repair to PARP inhibitor treatment. Further investigations have shown the roles of BRCA1/2 and its associations to other molecules in the DSB repair system. Notably, the repair pathway chosen in BRCA1‐deficient cells could be entirely different from that in BRCA2‐deficient cells after PARP inhibitor treatment. The present review describes synthetic lethality and acquired resistance mechanisms to PARP inhibitor through the DSB repair pathway and subsequent repair process. In addition, recent knowledge of resistance mechanisms is discussed. Our model should contribute to the development of novel therapeutic strategies.     

PARP抑制剂治疗晚期乳腺癌的作用机制及相关研究进展

多聚二磷酸腺苷核糖聚合酶（poly ADP-ribose polymerase，PARP）抑制剂可使乳腺癌细胞的单链DNA损伤修复受阻，而BRCA突变可造成乳腺癌细胞的双链DNA损伤修复功能缺失，因此PARP抑制剂治疗乳腺癌易感基因（breast cancer susceptibility gene，BRCA）突变乳腺癌是通过同时阻断单链DNA和双链DNA损伤修复，导致细胞的DNA损伤修复失败，使癌细胞死亡。目前已研发出多种敏感性和特异性较高的PARP抑制剂，该类药物主要抑制PARP1和PARP2两种亚型。本文总结PARP抑制剂用于治疗BRCA突变乳腺癌的作用机制，并对多种PARP抑制剂单用或联合化疗药物治疗晚期乳腺癌的研究进展进行综述。      

PARP inhibitors in breast cancer: Bringing synthetic lethality to the bedside

Individuals with breast and ovarian cancer susceptibility gene 1 (BRCA1) or BRCA2 germline mutations have a significantly increased lifetime risk for breast and ovarian cancers. BRCA‐mutant cancer cells have abnormal homologous recombination (HR) repair of DNA. In these tumors, the base excision repair (BER) pathway is important for cell survival. The poly(adenosine diphosphate‐ribose) polymerase (PARP) enzymes play a key role in BER, and PARP inhibitors are effective in causing cell death in BRCA‐mutant cells while sparing normal cells—a concept called synthetic lethality. PARP inhibitors are the first cancer therapeutics designed to exploit synthetic lethality. Recent clinical trials in BRCA‐mutant, metastatic breast cancer demonstrated improved outcomes with single‐agent PARP inhibitors (olaparib and talazoparib) over chemotherapy. However, resistance to PARP inhibitors remains a challenge. Primarily due to myelosuppression, the combination of PARP inhibitors with chemotherapy has been difficult. Novel combinations with chemotherapy, immunotherapy, and other targeted therapies are being pursued. In this review, the authors discuss current knowledge of PARP inhibitors in BRCA‐mutant breast cancer and potential future directions for these agents. Cancer 2018;124:2498‐506 . © 2018 American Cancer Society.     

PARP inhibitors in breast cancer: BRCA and beyond

DNA repair is essential for the survival of both normal and cancer cells. An elaborate set of signaling pathways detect single-strand and double-strand DNA breaks and mediate either DNA repair or apoptosis if the damage is too great to repair. Poly(adenosine diphosphate [ADP]-ribose) polymerases (PARPs) play a key role in the repair of base damage via the base excision repair pathway. Pharmacological inhibition of PARP induces cell death in tumors with mutations in certain DNA repair pathways--such as the BRCA pathways of double-strand break repair--and when combined with chemotherapies that cause DNA damage. PARP inhibitors are being investigated as a monotherapy for the treatment of patients with BRCA 1/2 mutations; in the treatment of triple-negative breast cancer, because of its molecular similarities to BRCA1-mutated malignancies; and as a strategy to potentiate the DNA-damaging effects of chemotherapy and radiation. The aim of this article is to review the preclinical data and rationale for PARP inhibitor use in the aforementioned settings, as well as the current status of the clinical development of these agents in the treatment of breast cancer, along with future directions for research in this field. Trials have been identified via searches of PubMed, clinicaltrials.gov, and the Proceedings of the American Society of Clinical Oncology Annual Meeting and the San Antonio Breast Cancer Symposium.     

Phosphorylation of ribosomal protein S6 confers PARP inhibitor resistance in BRCA1-deficient cancers

Inhibition of poly(ADP-ribose) polymerase (PARP) is a promising therapeutic strategy for BRCA1 deficient cancers, however, the development of drug resistance limits clinical efficacy. Previously we found that the BRCA1-AKT1 pathway contributes to tumorigenesis and that the AKT1/mTOR is a novel therapeutic target for BRCA1-deficient cancers. Here, we report that phosphorylation of ribosomal protein S6, a mTOR downstream effector, is greatly increased in BRCA1 deficient cells resistant to PARP inhibition. Phosphorylation of S6 is associated with DNA damage and repair signaling during PARP inhibitor treatment. In BRCA1 deficient cells, expression of S6 lacking all five phosphorylatable sites renders the cells sensitive to PARP inhibitor and increases DNA damage signals. In addition, the S6 mutations reduce tumor formation induced by Brca1-deficiency in mice. Inhibition of S6 phosphorylation by rapamycin restores PARP sensitivity to resistant cells. Combined treatment with rapamycin and PARP inhibitor effectively suppresses BRCA1-deficient tumor growth in mice. These results provide evidence for a novel mechanism by which BRCA1 deficient cancers acquire drug resistance and suggest a new therapeutic strategy to circumvent resistance.     

Development of PARP inhibitors in gynecological malignancies

PARP inhibitors demonstrate synthetic lethality in tumors with BRCA1/2 mutations and other homologous recombination repair deficiencies by interfering with DNA repair and causing direct toxicity to DNA through PARP trapping. PARP inhibitors have been shown to be beneficial in the treatment of BRCA1/2-mutated ovarian cancers, which has led to a shift in the treatment paradigm of this disease. Further studies to establish the role of PARP inhibitors during earlier stages of treatment are ongoing. The use of PARP inhibitors in other cancers with homologous recombination repair deficiencies, such as breast cancer and prostate cancer, is gradually evolving as well, including their use in the neoadjuvant and adjuvant settings. PARP inhibitor combination strategies with chemotherapy, targeted agents, radiotherapy, and immunotherapy are also being explored. The role of predictive biomarkers, including molecular signatures and homologous recombination deficiency scores based on loss of heterozygosity and other structural genomic aberrations, will be crucial to improved patient stratification to enhance the clinical utility of PARP inhibitors. This may also allow the use of PARP inhibitors to be extended beyond tumors with specific homologous recombination DNA repair gene mutations in the future. An improved understanding of the mechanisms underlying PARP inhibitor resistance will also be important to enable the development of new approaches to increase efficacy. This is a field rich in opportunity, and the coming years should see a better understanding of which patients we should be treating with PARP inhibitors and where these agents should come in over the course of treatment.     

PARP Inhibitors in BRCA Gene-Mutated Ovarian Cancer and Beyond

Poly(ADP-ribose)polymerase (PARP) inhibitors are showing considerable promise for the treatment of BRCA mutation–associated ovarian and breast cancer. This approach exploits a synthetic lethal strategy to target the specific DNA repair pathway in cancers that harbor mutations in the BRCA1 or BRCA2 genes. Accumulating evidence suggests that PARP inhibitors may have a wider application in the treatment of sporadic, high-grade serous ovarian cancers and other cancers including endometrial cancer. In this review, we discuss the clinical development of PARP inhibitors in ovarian cancer and explore challenges that need to be addressed if the full potential of these agents is to be realized.     

Enhancing radiosensitisation of BRCA2-proficient and BRCA2-deficient cell lines with hyperthermia and PARP1-i

Poly(ADP-ribose)polymerase1 (PARP1) is an important enzyme in regulating DNA replication. Inhibition of PARP1 can lead to collapsed DNA forks which subsequently causes genomic instability, making DNA more susceptible in developing fatal DNA double strand breaks. PARP1-induced DNA damage is generally repaired by homologous recombination (HR), in which BRCA2 proteins are essential. Therefore, BRCA2-deficient tumour cells are susceptible to treatment with PARP1-inhibitors (PARP1-i). Recently, BRCA2 was shown to be down-regulated by hyperthermia (HT) temporarily, and this consequently inactivated HR for several hours. In this study, we investigated whether HT exclusively interferes with HR by analysing thermal radiosensitisation of BRCA2-proficient and deficient cells. After elucidating the equitoxicity of PARP1-i on BRCA2-proficient and deficient cells, we studied the cell survival, apoptosis, DNA damage (γ-H2AX foci and comet assay) and cell cycle distribution after different treatments. PARP1-i sensitivity strongly depends on the BRCA2 status. BRCA2-proficient and deficient cells are radiosensitised by HT, indicating that HT does not exclusively act by inhibition of HR. In all cell lines, the addition of HT to radiotherapy and PARP1-i resulted in the lowest cell survival, the highest levels of DNA damage and apoptotic levels compared to duo-modality treatments. Concluding, HT not only inhibits HR, but also has the capability of radiosensitising BRCA2-deficient cells. Thus, in case of BRCA2-mutation carriers, combining HT with PARP1-i may boost the treatment efficacy. This combination therapy would be effective for all patients with PARP1-i regardless of their BRCA status.     

Differential Chemotherapeutic Sensitivity for Breast Tumors With “BRCAness”: A Review

BRCA1 or BRCA2 mutations predispose to cancer development, primarily through their loss of role in the repair of DNA double‐strand breaks. They play a key role in homologous recombination repair, which is a conservative, error‐free DNA repair mechanism. When mutated, other alternative, error‐prone mechanisms for DNA repair take over, leading to genomic instability. Somatic mutations are rare in sporadic breast tumors, but expression of BRCA1 and BRCA2 genes can be downregulated in other mechanistic ways. These tumors have similar features in terms of their phenotypic and genotypic profiles, which are normally regulated by these genes, and mutations lead to defective DNA repair capacity, called “BRCAness.” Attempts have been made to exploit this differentially expressed feature between tumors and normal tissues by treatment with DNA‐damaging chemotherapy agents. Cells with this functional BRCA deficiency should be selectively susceptible to DNA‐damaging drugs. Preclinical and early clinical (primarily retrospective) evidence supports this approach. In contrast, there is emerging evidence of relative resistance of tumors containing BRCA1 or BRCA2 mutations (or BRCAness) to taxanes. In this review, we summarize the data supporting differential chemotherapeutic sensitivity on the basis of defective DNA repair. If confirmed with available, clinically applicable techniques, this differential chemosensitivity could lead to treatment choices in breast cancer that have a more individualized biologic basis.     

PARP inhibitors synergize with gemcitabine by potentiating DNA damage in non-small-cell lung cancer

Poly (ADP-ribose) polymerase (PARP) inhibitors have demonstrated great promise in the treatment of patients with deficiencies in homologous recombination (HR) DNA repair, such as those with loss of BRCA1 or BRCA2 function. However, emerging studies suggest that PARP inhibition can also target HR-competent cancers, such as non-small-cell lung cancer (NSCLC), and that the therapeutic effect of PARP inhibition may be improved by combination with chemotherapy agents. In our study, it was found that PARP inhibitors talazoparib (BMN-673) and olaparib (AZD-2281) both had synergistic activity with the common first-line chemotherapeutic gemcitabine in a panel of lung cancer cell lines. Furthermore, the combination demonstrated significant in vivo antitumor activity in an H23 xenograft model of NSCLC compared to either agent as monotherapy. This synergism occurred without loss of HR repair efficiency. Instead, the combination induced synergistic single-strand DNA breaks, leading to accumulation of toxic double-strand DNA lesions in vitro and in vivo. Our study elucidates the underlying mechanisms of synergistic activity of PARP inhibitors and gemcitabine, providing a strong motivation to pursue this combination as an improved therapeutic regimen.     

Response of subtype-specific human breast cancer-derived cells to poly(ADP-ribose) polymerase and checkpoint kinase 1 inhibition

When DNA damage is detected, checkpoint signal networks are activated to stop the cell cycle, and DNA repair processes begin. Inhibitory compounds targeting components of DNA damage response pathways have been identified and are being used in clinical trials, in combination with chemotherapeutic agents, to enhance cancer therapy. Inhibitors of checkpoint kinases, Chk1 and Chk2, have been shown to sensitize tumor cells to DNA damaging agents, and treatment of BRCA1/2-deficient tumor cells, as well as triple negative breast cancers, with poly(ADP-ribose) polymerase (PARP) inhibitors has shown promise. But systematic studies to determine which tumor subtypes are likely to respond to these specific inhibitors have not been reported. The current study was designed to test sensitivity of specific breast cancer subtype-derived cells to two classes of these new inhibitory drugs, PARP and Chk1 inhibitors. Luminal, HER2 overexpressing, and triple negative breast cancer-derived cells were tested for sensitivity to killing by PARP inhibitors, ABT-888 and BSI-201, and Chk1 inhibitor, PF-00477736, alone or in combination with gemcitabine or carboplatin. Each of the triple negative breast cancer cell lines showed strong sensitivity to the Chk1 inhibitor, but only the BRCA1-deficient breast cancer cell lines showed sensitivity to the PARP inhibitors, suggesting that in vitro testing of cancer cell lines of specific subtypes, with panels of the different PARP and Chk1 inhibitors, will contribute to stratification of patients for clinical trials using these classes of inhibitors.     

Contemplating chemosensitivity of basal-like breast cancer based on BRCA1 dysfunction

Gene-expression profiling classified breast cancer to intrinsic subtypes, including luminal A and B, HER2 positive, normal-breast-like, and basal-like tumors. Of these, basal-like tumors that express basal cytokeratins and that are negative for estrogen receptor α, progesterone receptor, and HER2 show the most aggressive phenotype with a poor prognosis. Analyses of clinical samples and basic research indicate that basal-like breast cancer is caused by deficiencies in the breast cancer susceptibility protein, BRCA1. Indeed, conditionally deleting BRCA1 from the mammary gland causes mice to develop basal-like cancers at high rates. One of the major functions of BRCA1 is DNA double-strand break (DSB) repair, and its failure to perform causes increased sensitivity of cells to DNA damage-inducing agents, such as PARP inhibitors, DNA cross-linkers, or topoisomerase inhibitors. Therefore, BRCA1 dysfunction could be a principal target for therapeutic application of basal-like breast cancer. Recently, significant progress has been made in understanding the BRCA1 cascade in response to DSBs, where ubiquitin polymer formation plays critical roles. Ubiquitination was indeed found to be an apparent early response of breast cancer to neoadjuvant treatment with epirubicin and cyclophosphamide. Deducing the role of BRCA1 ubiquitin E3 ligase activity in this pathway is a critical challenge to further clarify its functional mechanism. In individualized treatment of breast cancer, evaluation of the DNA repair capacity by the BRCA1 pathway may be an important issue when determining proper treatment of basal-like breast cancer.     

Synergistic Effect of Trabectedin and Olaparib Combination Regimen in Breast Cancer Cell Lines

Purpose

Trabectedin induces synthetic lethality in tumor cells carrying defects in homologous recombinant DNA repair. We evaluated the effect of concomitant inhibition of nucleotide-excision repair and poly (ADP-ribose) polymerase (PARP) activity with trabectedin and PARP inhibitors, respectively, and whether the synthetic lethality effect had the potential for a synergistic effect in breast cancer cell lines. Additionally, we investigated if this approach remained effective in BRCA1-positive breast tumor cells.

Methods

We have evaluated the in vitro synergistic effect of combinations of trabectedin and three different PARP inhibitors (veliparib, olaparib, and iniparib) in four breast cancer cell lines, each presenting a different BRCA1 genetic background. Antiproliferative activity, DNA damage, cell cycle perturbations and poly(ADP-ribosyl)ation were assessed by MTT assay, comet assay, flow cytometry and western blot, respectively.

Results

The combination of trabectedin and olaparib was synergistic in all the breast cancer cell lines tested. Our data indicated that the synergy persisted regardless of the BRCA1 status of the tumor cells. Combination treatment was associated with a strong accumulation of double-stranded DNA breaks, G2/M arrest, and apoptotic cell death. Synergistic effects were not observed when trabectedin was combined with veliparib or iniparib.

Conclusion

Collectively, our results indicate that the combination of trabectedin and olaparib induces an artificial synthetic lethality effect that can be used to kill breast cancer cells, independent of BRCA1 status.     

Nuclear translocation of the receptor tyrosine kinase c-MET reduces the treatment efficacies of olaparib and gemcitabine in pancreatic ductal adenocarcinoma cells

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive cancer that lack effective therapeutic strategies. The response rate of PDAC for treatment with gemcitabine, a current first-line chemotherapeutic for this tumor, is lower than 20%. Identifying key targetable molecules that mediate gemcitabine resistance and developing novel strategies for precision PDAC medicine are urgently needed. Most PDACs have either intratumoral hypoxia or high reactive oxygen species (ROS) production; cytotoxic chemotherapy can elevate ROS production in PDACs. Although excessive ROS production leads to oxidative damage of macromolecules such as DNA, pancreatic cancer cells can survive high DNA damage stress levels. Therefore, identifying molecular mechanisms of overcoming ROS-induced stress in pancreatic cancer cells is important for developing novel therapeutic strategies. ROS-induced DNA damage is predominantly repaired via poly (ADP-ribose) polymerase 1 (PARP1)-mediated DNA repair mechanisms. A recent clinical trial reported that PARP inhibitors are effective in treating pancreatic patients carrying BRCA mutations. However, only less than 10% of pancreatic cancer patients bearing BRCA mutated tumors. Activation of the receptor tyrosine kinase c-MET positively correlates with poor prognosis for PDAC, and our previous study showed that nuclear c-MET can phosphorylate PARP1 at tyrosine 907 under ROS stimulation to promote DNA repair. As described herein, we proposed to expand PARP inhibitor-targeted therapy to more pancreatic cancer patients regardless of BRCA mutation status by combining olaparib, a PARP inhibitor, with c-MET inhibitors as we demonstrated in our previous studies in breast cancer. In this prospective study, we found that ROS-inducing chemotherapeutic drugs such as gemcitabine and doxorubicin stimulated nuclear accumulation of c-MET in BxPC-3 and L3.6pl pancreatic cancer cells. We further showed that combining a c-MET inhibitor with gemcitabine or a PARP inhibitor induced more DNA damage than monotherapy did. Moreover, we demonstrated the synergistic antitumor effects of c-MET inhibitors combined with a PARP inhibitor or gemcitabine in eliminating pancreatic cancer cells. These data suggested that accumulation of ROS in pancreatic cancer cells promotes nuclear localization of c-MET, resulting in resistance to both chemotherapy and PARP inhibitors. Our findings suggest that combining c-MET inhibitors with PARP inhibitors or gemcitabine is a novel, rational therapeutic strategy for advanced pancreatic cancer.