Therapeutic potential of PARP inhibitors for metastatic breast cancer

Increasing understanding of the cellular aberrations inherent to cancer cells has allowed the development of therapies to target biological pathways, an important step towards individualization of breast cancer therapy. The clinical development of poly(ADP-ribose) polymerase (PARP) inhibitors, with their novel and selective mechanism of action, are an example of this strategy. PARP plays a key role in DNA repair mechanisms, particularly the base excision repair pathway. Initially developed as inhibitors able to enhance the cytotoxicity of radiation and certain DNA-damaging agents, they have more recently been shown to have single-agent activity in certain tumors. Inhibition of PARP in a DNA repair-defective tumor can lead to gross genomic instability and cell death by exploiting the paradigm of synthetic lethality. Several studies have evaluated the role of PARP inhibitors for treatment of breast cancer, particularly in the context of BRCA-mutated and triple-negative breast cancers. In addition, inhibition of PARPs repair functions for chemotherapy-induced DNA lesions has been shown to potentiate the effect of some chemotherapy regimens. This article discusses the current understanding of PARP inhibition as a treatment for metastatic breast cancer, evidence from clinical trials and addresses its future implications.     

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.     

Novel splice‐switching oligonucleotide promotes BRCA1 aberrant splicing and susceptibility to PARP inhibitor action

Tumors carrying hereditary mutations in BRCA1, which attenuate the BRCA1 DNA damage repair pathway, are more susceptible to dual treatment with PARP inhibitors and DNA damaging therapeutics. Conversely, breast cancer tumors with nonmutated functional BRCA1 are less sensitive to PARP inhibition. We describe a method that triggers susceptibility to PARP inhibition in BRCA1‐functional tumor cells. BRCA1 exon 11 is a key for the function of BRCA1 in DNA damage repair. Analysis of the BRCA1 exon 11 splicing mechanism identified a key region within this exon which, when deleted, induced exon 11 skipping. An RNA splice‐switching oligonucleotide (SSO) developed to target this region was shown to artificially stimulate skipping of exon 11 in endogenous BRCA1 pre‐mRNA. SSO transfection rendered wild‐type BRCA1 expressing cell lines more susceptible to PARP inhibitor treatment, as demonstrated by a reduction in cell survival at all SSO concentrations tested. Combined SSO and PARP inhibitor treatment increased γH2AX expression indicating that SSO‐dependent skipping of BRCA1 exon 11 was able to promote DSBs and therefore synthetic lethality. In conclusion, this SSO provides a new potential therapeutic strategy for targeting BRCA1‐functional breast cancer by enhancing the effect of PARP inhibitors.     

The role of poly adenosine diphosphate ribose polymerase inhibitors in breast and ovarian cancer: current status and future directions

Poly adenosine diphosphate ribose polymerase (PARP) inhibitors have demonstrated single agent activity in the treatment of patients with recurrent BRCA1-mutated and BRCA2-mutated breast and ovarian cancers. They also appear to have a potential role as maintenance therapy following chemotherapy in patients with platinum sensitive recurrent sporadic and BRCA1/2 related high-grade serous ovarian cancers. The concept of BRCAness raises the possibility that PARP inhibitors may be active in selected patients with homologous recombination (HR) DNA repair-deficient tumors, even if they do not harbor a BRCA1/2 germline mutation. Further research will be required to identify the subset of patients with sporadic cancers who may benefit from PARP inhibitor therapy. Precise details on the mechanisms of action, relative potency and anti-cancer effects of different PARP inhibitors remain to be clarified and are being investigated. PARP inhibitors are known to inhibit the base excision repair (BER) pathway but in addition, recent reports indicate that aberrant activation of the error-prone non-homologous end-joining (NHEJ) pathway occurs in HR-deficient cells and that cell death provoked by PARP inhibition is dependent on NHEJ-induced genomic instability. Characterization of the precise molecular mechanisms responsible for PARP inhibitor activity should lead to the identification of predictive biomarkers of response and help identify which patients should be treated with PARP inhibitors. This is a very active field of research and the current status and future directions are reviewed.     

PARP inhibitors and breast cancer: update and perspectives

Poly(ADP-ribose) polymerase (PARP) family has become a promising therapeutic target in various malignancies including breast cancer. When homologous recombination repair is deficient, as it is observed in BRCA1/2-mutated tumor models, inhibition of PARP was shown to induce massive and selective tumor cell death (the so-called "synthetic lethality"). In breast cancer, PARP inhibitors have been developed as single-agent in BRCA1/2-mutated tumors or in combination with chemotherapy. Recently, a randomized phase III clinical trial failed to demonstrate any survival improvement by combining the iPARP iniparib to chemotherapy in triple-negative metastatic breast cancer patients. This emphasizes the need for future development of this class of compounds to resolve critical issues such as optimal schedule of administration and association to other anticancer treatments, as well as identification of pertinent biomarkers predictive for efficacy.     

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.     

BRCA1/2突变和同源重组修复缺陷(HRD)检测在乳腺癌中的临床研究进展

乳腺癌已成为发病率最高的癌症。DNA修复缺陷是乳腺癌最重要的特征之一。先前的研究表明,乳腺癌易感基因1/2(breast cancer susceptibility gene 1/2,BRCA1/2)突变是预测乳腺癌同源重组修复缺陷(homologous recombination deficiency, HRD)最主要的生物标志物,能识别铂类药物和多腺苷二磷酸核糖聚合酶(poly ADP ribose polymerase, PARP)抑制剂治疗的获益人群。美国食品药品监督管理局(FDA)已批准Olaparib和Talazoparib两种PARP抑制剂,用于BRCA1/2突变的早期和晚期乳腺癌的辅助治疗。但中国尚未获批。现有研究表明,一部分非BRCA1/2突变的乳腺癌患者也具有HRD特征,可以从铂类药物或PARP抑制剂中获益。本综述总结了涉及到BRCA1/2突变、同源重组修复(homologous recombination repair, HRR)基因突变和HRD状态检测的临床研究。阐明了各种检测方法在识别乳腺癌患者HRD状态和预测疗效方面的价值,并提出应尽快开发用于中国乳腺癌HR...     

Homologous recombination deficiency and ovarian cancer

The discovery that PARP inhibitors block an essential pathway of DNA repair in cells harbouring a BRCA mutation has opened up a new therapeutic avenue for high-grade ovarian cancers. BRCA1 and BRCA2 proteins are essential for high-fidelity repair of double-strand breaks of DNA through the homologous recombination repair (HRR) pathway. Deficiency in HRR (HRD) is a target for PARP inhibitors. The first PARP inhibitor, olaparib, has now been licensed for BRCA-mutated ovarian cancers. While mutated BRCA genes are individually most commonly associated with HRD other essential HRR proteins may be mutated or functionally deficient potentially widening the therapeutic opportunities for PARP inhibitors. HRD is the first phenotypically defined predictive marker for therapy with PARP inhibitors in ovarian cancer. Several different PARP inhibitors are being trialled in ovarian cancer and this class of drugs has been shown to be a new selective therapy for high-grade ovarian cancer. Around 20% of high-grade serous ovarian cancers harbour germline or somatic BRCA mutations and testing for BRCA mutations should be incorporated into routine clinical practice. The expanded use of PARP inhibitors in HRD deficient (non-BRCA mutant) tumours using a signature of HRD in clinical practice requires validation.     

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 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.     

胚系BRCA基因突变乳腺癌的临床诊治进展

BRCA基因是最常见的乳腺癌易感基因,胚系BRCA突变患者罹患乳腺癌的风险显著增加.随着对BRCA基因的深入研究以及聚ADP-核糖聚合酶(poly ADP-ribose polymerase,PARP)抑制剂的出现,BRCA突变已成为乳腺癌治疗的新靶点.在BRCA突变乳腺癌的治疗中,PARP抑制剂和铂类为两大主要药物选...     

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.     

BRCA1基因相关的DNA修复通路异常与三阴乳腺癌化疗药物选择的关系

三阴乳腺痛以雌激素受体(estrogen receptor,ER)、孕激素受体(progestogen receptor,PR)以及Her-2阴性为特征,临床表现为组织分化差,术后易出现局部复发和远处转移,而且脏器比骨骼更容易发生转移.因对内分泌治疗和以Her-2为靶向分子的治疗无效,化疗成为治疗方案中的主体.与其他类型乳腺癌比较,三阴乳腺癌患者与乳腺基底细胞样癌和乳癌基因1(BRCA1)相关性乳腺癌在表型和分子生物学水平方面具有很多共性.不同个体DNA损伤修复能力的差异和乳腺癌的发病密切相关.BRCA1以及相关的DNA修复通路中的不同基因,如BRCA2、ATM、RAD51和CHEK2对维护基因组稳定性方面有重要作用.因此,三阴乳腺痛患者BRCA1相关的DNA修复通路异常而导致DNA修复能力异常,影响了其对化疗药物的敏感性,进而可能促进了高转移特性的形成.导致三阴乳腺癌重要脏器转移高发的原因,是肿瘤本身的转移相关基因异常的内因所致,还是对所接受的治疗耐药而使肿瘤细胞处于逃逸状态而引起,目前尚不明确.因此本文对BRCA1相关的DNA修复通路异常与三阴乳腺癌高转移之间的关系进行综述.     

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 older patients with ovarian and breast cancer: Young International Society of Geriatric Oncology review paper

Breast and ovarian cancer are common malignancies among older adults, causing significant morbidity and mortality. Although most cases of breast and ovarian cancer are sporadic, a significant proportion is caused by mutations in cancer susceptibility genes, most often breast cancer susceptibility genes (BRCA) 1 and 2. Furthermore, some breast and ovarian tumors are phenotypically similar to those with BRCA mutations, a phenomenon known as “BRCAness”. BRCA mutations and “BRCAness” lead to defects in DNA repair, which may be a target for therapeutic agents such as Poly ADP-Ribose Polymerase (PARP) inhibitors. PARP inhibitors are novel medications which lead to double-strand breaks resulting in cell death due to synthetic lethality, and which have been shown to be effective in patients with advanced breast and ovarian cancers with or without BRCA mutations. Three different PARP inhibitors (olaparib, niraparib, and rucaparib) have been approved for the treatment of ovarian cancer and one (olaparib) for breast cancer harboring BRCA mutations. Here, we review the currently available evidence regarding the use of PARP inhibitors for the treatment of patients with breast and ovarian cancer, with a particular focus on the inclusion of older adults in clinical trials of these therapies. Additionally, we provide an overview of currently ongoing studies of PARP inhibitors in breast and ovarian cancer, and include recommendations for increasing the evidence-base for using these medications among older patients.     

The emerging role of DNA repair proteins as predictive, prognostic and therapeutic targets in cancer

Advanced cancer is the second leading cause of death in the western world. Chemotherapy and radiation are the two main treatment modalities currently available to improve patient outcomes, but treatment related toxicity and the emergence of resistance limit their effectiveness. Hence there is an urgent need to develop novel treatment strategies. Rapid advances in cancer biology have identified key pathways involved in the repair of DNA damage induced by chemotherapeutic agents and irradiation. Efficient DNA repair in the cancer cell is an important mechanism for therapeutic resistance. Up to 130 genes have been identified that are associated with human DNA repair. Several of these proteins are emerging as important predictive and prognostic factors in solid tumours. Inhibition of DNA repair has the potential to enhance the efficacy of currently available DNA damaging agents. In recent years, several promising drug targets have been identified and novel drugs synthesised that target specific DNA repair proteins. These agents have shown impressive anti-cancer effects in preclinical studies in combination with chemotherapy or irradiation. Their role in human cancer is now being investigated in early phase clinical trials in combination with chemotherapy. MGMT inhibitors, PARP inhibitors and methoxyamine are currently in early stages of clinical development. Innovative clinical trial designs are essential to evaluate the potential of DNA repair inhibitor in cancer therapy.     

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.     

Poly(ADP-ribose) polymerase inhibitors in cancer treatment: A clinical perspective

Inbuilt mechanisms of DNA surveillance and repair are integral to the maintenance of genomic stability. Poly(ADP-ribose) polymerase (PARP) is a nuclear enzyme that plays a critical role in DNA damage response processes. PARP inhibition has been successfully employed as a novel therapeutic strategy to enhance the cytotoxic effects of DNA-damaging agents. We have shown that PARP inhibition has substantial single agent antitumour activity with a wide therapeutic index in homologous DNA repair-defective tumours such as those arising in BRCA1 and BRCA2 mutation carriers. This is the first successful clinical application of a synthetic lethal approach to targeting cancer. Exploitation of defects in DNA repair pathways through targeted inhibition of salvage repair pathways is an exciting anticancer approach, with potentially broad clinical applicability. Several PARP inhibitors are now in clinical development. This review outlines the biological function and rationale of targeting PARP, details pre-clinical and clinical data and discusses the promises and challenges involved in developing these antitumour agents.     

DNA damage and breast cancer

Cancer is intimately related to the accumulation of DNA damage, and repair failures (including mutation prone repair and hyperactive repair systems). This article relates current clinical categories for breast cancer and their common DNA damage repair defects. Information is included on the potential for accumulation of DNA damage in the breast tissue of a woman during her lifetime and the role of DNA damage in breast cancer development. We then cover endogenous and exogenous sources of DNA damage, types of DNA damage repair and basic signal transduction pathways for three gene products involved in the DNA damage response system; namely BRCA1, BRIT1 and PARP-1. These genes are often considered tumor suppressors because of their roles in DNA damage response and some are under clinical investigation as likely sources for effective new drugs to treat breast cancers. Finally we discuss some of the problems of DNA damage repair systems in cancer and the conundrum of hyper-active repair systems which can introduce mutations and confer a survival advantage to certain types of cancer cells.