Synthetic lethal targeting of PTEN mutant cells with PARP inhibitors

Genome-wide transcriptome profiling of homologous recombination DNA repair

Homologous recombination (HR) repair deficiency predisposes to cancer development, but also sensitizes cancer cells to DNA damage-inducing therapeutics. Here we identify an HR defect (HRD) gene signature that can be used to functionally assess HR repair status without interrogating individual genetic alterations in cells. By using this HRD gene signature as a functional network analysis tool, we discover that simultaneous loss of two major tumour suppressors BRCA1 and PTEN extensively rewire the HR repair-deficient phenotype, which is found in cells with defects in either BRCA1 or PTEN alone. Moreover, the HRD gene signature serves as an effective drug discovery platform to identify agents targeting HR repair as potential chemo/radio sensitizers. More importantly, this HRD gene signature is able to predict clinical outcomes across multiple cancer lineages. Our findings, therefore, provide a molecular profile of HR repair to assess its status at a functional network level, which can provide both biological insights and have clinical implications in cancer.

PTEN and PI3K/AKT in non-small-cell lung cancer

Non-small-cell lung cancer (NSCLC) is the leading cause of cancer deaths worldwide. In the last years, the identification of activating EGFR mutations, conferring increased sensitivity and disease response to tyrosine kinase inhibitors, has changed the prospect of NSCLC patients. The PTEN/PI3K/AKT pathway regulates multiple cellular functions, including cell growth, differentiation, proliferation, survival, motility, invasion and intracellular trafficking. Alterations in this pathway, mainly PTEN inactivation, have been associated with resistance to EGFR-tyrosine kinase inhibitor therapy and lower survival in NSCLC patients. In this review, we will briefly discuss the main PTEN/PI3K/AKT pathway alterations found in NSCLC, as well as the cell processes regulated by PTEN/PI3K/AKT leading to tumorigenesis.

Targeted Therapies for Triple-Negative Breast Cancer: Combating a Stubborn Disease

Triple-negative breast cancers (TNBCs) constitute a heterogeneous subtype of breast cancers that have a poor clinical outcome. Although no approved targeted therapy is available for TNBCs, molecular-profiling efforts have revealed promising molecular targets, with several candidate compounds having now entered clinical trials for TNBC patients. However, initial results remain modest, thereby highlighting challenges potentially involving intra- and intertumoral heterogeneity and acquisition of therapy resistance. We present a comprehensive review on emerging targeted therapies for treating TNBCs, including the promising approach of immunotherapy and the prognostic value of tumor-infiltrating lymphocytes. We discuss the impact of pathway rewiring in the acquisition of drug resistance, and the prospect of employing combination therapy strategies to overcome challenges towards identifying clinically-viable targeted treatment options for TNBC.

Combinations of PARP Inhibitors with Temozolomide Drive PARP1 Trapping and Apoptosis in Ewing's Sarcoma

Ewing's sarcoma is a malignant pediatric bone tumor with a poor prognosis for patients with metastatic or recurrent disease. Ewing's sarcoma cells are acutely hypersensitive to poly (ADP-ribose) polymerase (PARP) inhibition and this is being evaluated in clinical trials, although the mechanism of hypersensitivity has not been directly addressed. PARP inhibitors have efficacy in tumors with BRCA1/2 mutations, which confer deficiency in DNA double-strand break (DSB) repair by homologous recombination (HR). This drives dependence on PARP1/2 due to their function in DNA single-strand break (SSB) repair. PARP inhibitors are also cytotoxic through inhibiting PARP1/2 auto-PARylation, blocking PARP1/2 release from substrate DNA. Here, we show that PARP inhibitor sensitivity in Ewing's sarcoma cells is not through an apparent defect in DNA repair by HR, but through hypersensitivity to trapped PARP1-DNA complexes. This drives accumulation of DNA damage during replication, ultimately leading to apoptosis. We also show that the activity of PARP inhibitors is potentiated by temozolomide in Ewing's sarcoma cells and is associated with enhanced trapping of PARP1-DNA complexes. Furthermore, through mining of large-scale drug sensitivity datasets, we identify a subset of glioma, neuroblastoma and melanoma cell lines as hypersensitive to the combination of temozolomide and PARP inhibition, potentially identifying new avenues for therapeutic intervention. These data provide insights into the anti-cancer activity of PARP inhibitors with implications for the design of treatment for Ewing's sarcoma patients with PARP inhibitors.

Small Molecule Inhibitors of 8-Oxoguanine DNA Glycosylase-1 (OGG1)

The DNA base excision repair (BER) pathway, which utilizes DNA glycosylases to initiate repair of specific DNA lesions, is the major pathway for the repair of DNA damage induced by oxidation, alkylation, and deamination. Early results from clinical trials suggest that inhibiting certain enzymes in the BER pathway can be a useful anticancer strategy when combined with certain DNA-damaging agents or tumor-specific genetic deficiencies. Despite this general validation of BER enzymes as drug targets, there are many enzymes that function in the BER pathway that have few, if any, specific inhibitors. There is a growing body of evidence that suggests inhibition of 8-oxoguanine DNA glycosylase-1 (OGG1) could be useful as a monotherapy or in combination therapy to treat certain types of cancer. To identify inhibitors of OGG1, a fluorescence-based screen was developed to analyze OGG1 activity in a high-throughput manner. From a primary screen of ∼50,000 molecules, 13 inhibitors were identified, 12 of which were hydrazides or acyl hydrazones. Five inhibitors with an IC50 value of less than 1 μM were chosen for further experimentation and verified using two additional biochemical assays. None of the five OGG1 inhibitors reduced DNA binding of OGG1 to a 7,8-dihydro-8-oxoguanine (8-oxo-Gua)-containing substrate, but all five inhibited Schiff base formation during OGG1-mediated catalysis. All of these inhibitors displayed a >100-fold selectivity for OGG1 relative to several other DNA glycosylases involved in repair of oxidatively damaged bases. These inhibitors represent the most potent and selective OGG1 inhibitors identified to date.

Homologous Recombination Deficiency: Exploiting the Fundamental Vulnerability of Ovarian Cancer

Approximately 50% of epithelial ovarian cancers (EOC) exhibit defective DNA repair via homologous recombination (HR) due to genetic and epigenetic alterations of HR pathway genes. Defective HR is an important therapeutic target in EOC as exemplified by the efficacy of platinum analogues in this disease, as well as the advent of PARP inhibitors, which exhibit synthetic lethality when applied to HR-deficient cells. Here, we describe the genotypic and phenotypic characteristics of HR-deficient EOCs, discuss current and emerging approaches for targeting these tumors, and present challenges associated with these approaches, focusing on development and overcoming resistance.

SIGNIFICANCE

Defective DNA repair via HR is a pivotal vulnerability of EOC, particularly of the high-grade serous histologic subtype. Targeting defective HR offers the unique opportunity of exploiting molecular differences between tumor and normal cells, thereby inducing cancer-specific synthetic lethality; the promise and challenges of these approaches in ovarian cancer are discussed in this review.

Homologous Recombination Deficiency: Exploiting the Fundamental Vulnerability of Ovarian Cancer

Approximately 50% of epithelial ovarian cancers (EOC) exhibit defective DNA repair via homologous recombination (HR) due to genetic and epigenetic alterations of HR pathway genes. Defective HR is an important therapeutic target in EOC as exemplified by the efficacy of platinum analogues in this disease, as well as the advent of PARP inhibitors, which exhibit synthetic lethality when applied to HR-deficient cells. Here, we describe the genotypic and phenotypic characteristics of HR-deficient EOCs, discuss current and emerging approaches for targeting these tumors, and present challenges associated with these approaches, focusing on development and overcoming resistance.

SIGNIFICANCE

Defective DNA repair via HR is a pivotal vulnerability of EOC, particularly of the high-grade serous histologic subtype. Targeting defective HR offers the unique opportunity of exploiting molecular differences between tumor and normal cells, thereby inducing cancer-specific synthetic lethality; the promise and challenges of these approaches in ovarian cancer are discussed in this review.

Discovery of 2-[1-(4,4-Difluorocyclohexyl)piperidin-4-yl]-6-fluoro-3-oxo-2,3-dihydro-1H-isoindole-4-carboxamide (NMS-P118): A Potent, Orally Available, and Highly Selective PARP-1 Inhibitor for Cancer Therapy

The nuclear protein poly(ADP-ribose) polymerase-1 (PARP-1) has a well-established role in the signaling and repair of DNA and is a prominent target in oncology, as testified by the number of candidates in clinical testing that unselectively target both PARP-1 and its closest isoform PARP-2. The goal of our program was to find a PARP-1 selective inhibitor that would potentially mitigate toxicities arising from cross-inhibition of PARP-2. Thus, an HTS campaign on the proprietary Nerviano Medical Sciences (NMS) chemical collection, followed by SAR optimization, allowed us to discover 2-[1-(4,4-difluorocyclohexyl)piperidin-4-yl]-6-fluoro-3-oxo-2,3-dihydro-1H-isoindole-4-carboxamide (NMS-P118, 20by). NMS-P118 proved to be a potent, orally available, and highly selective PARP-1 inhibitor endowed with excellent ADME and pharmacokinetic profiles and high efficacy in vivo both as a single agent and in combination with Temozolomide in MDA-MB-436 and Capan-1 xenograft models, respectively. Cocrystal structures of 20by with both PARP-1 and PARP-2 catalytic domain proteins allowed rationalization of the observed selectivity.

PARP inhibitors in the management of breast cancer: current data and future prospects

Poly(ADP-ribose) polymerases (PARP) are enzymes involved in DNA-damage repair. Inhibition of PARPs is a promising strategy for targeting cancers with defective DNA-damage repair, including BRCA1 and BRCA2 mutation-associated breast and ovarian cancers. Several PARP inhibitors are currently in trials in the adjuvant, neoadjuvant, and metastatic settings for the treatment of ovarian, BRCA-mutated breast, and other cancers. We herein review the development of PARP inhibitors and the basis for the excitement surrounding these agents, their use as single agents and in combinations, as well as their toxicities, mechanisms of acquired resistance, and companion diagnostics.

Regulation of recombination and genomic maintenance

Recombination is a central process to stably maintain and transmit a genome through somatic cell divisions and to new generations. Hence, recombination needs to be coordinated with other events occurring on the DNA template, such as DNA replication, transcription, and the specialized chromosomal functions at centromeres and telomeres. Moreover, regulation with respect to the cell-cycle stage is required as much as spatiotemporal coordination within the nuclear volume. These regulatory mechanisms impinge on the DNA substrate through modifications of the chromatin and directly on recombination proteins through a myriad of posttranslational modifications (PTMs) and additional mechanisms. Although recombination is primarily appreciated to maintain genomic stability, the process also contributes to gross chromosomal arrangements and copy-number changes. Hence, the recombination process itself requires quality control to ensure high fidelity and avoid genomic instability. Evidently, recombination and its regulatory processes have significant impact on human disease, specifically cancer and, possibly, neurodegenerative diseases.

A phase II evaluation of selumetinib (AZD6244, ARRY-142886), a selective MEK-1/2 inhibitor in the treatment of recurrent or persistent endometrial cancer: an NRG Oncology/Gynecologic Oncology Group study

BACKGROUND

Activation of the mitogen activated protein kinase pathway plays a pivotal role in cell proliferation and is frequently activated in endometrial cancer. We sought to evaluate the efficacy/safety of selumetinib, a selective MEK-1/2 inhibitor in women with recurrent endometrial cancer.

METHODS

This was a phase II, single-arm, open-label study evaluating response and 6-month event-free survival (EFS) as primary endpoints. Eligible patients had measurable disease, 1-2 prior cytotoxic regimens, and performance status 0-2. Selumetinib 75mg PO BID was administered daily until progression or intolerance. One cycle was 28days.

RESULTS

Fifty-four patients were enrolled; 2 were excluded due to improper pre-study treatment (1) and never treated (1), leaving 52 evaluable for efficacy/safety. Median age was 62; histology included endometrioid (58%), serous (17%) and mixed (23%). Seventeen patients (33%) had 2 prior cytotoxic regimens. The median number of cycles administered was 2 (1-34). Three (6%) patients had objective response (1 CR, 2 PR); 13 had SD as best response. The proportion of patients with 6-month EFS was 12%. Median EFS, progression-free and overall survival was 2.1, 2.3, and 8.5months, respectively. Drug-attributed grade 3/4 adverse events were observed (≥5%) were fatigue (15%), anemia (10%), pain (10%), extremity edema (8%), and dyspnea (6%). There was 1 grade 4 infection (renal), 1 grade 4 anemia, and 1 death due to hemorrhage (rectum).

CONCLUSIONS

Selumetinib was tolerable in this population but did not meet pre-trial specifications for clinical efficacy.

PARP1 Inhibitors: antitumor drug design

The poly (ADP-ribose) polymerase 1 (PARP1) enzyme is one of the promising molecular targets for the discovery of antitumor drugs. PARP1 is a common nuclear protein (1-2 million molecules per cell) serving as a "sensor" for DNA strand breaks. Increased PARP1 expression is sometimes observed in melanomas, breast cancer, lung cancer, and other neoplastic diseases. The PARP1 expression level is a prognostic indicator and is associated with a poor survival prognosis. There is evidence that high PARP1 expression and treatment-resistance of tumors are correlated. PARP1 inhibitors are promising antitumor agents, since they act as chemo- and radiosensitizers in the conventional therapy of malignant tumors. Furthermore, PARP1 inhibitors can be used as independent, effective drugs against tumors with broken DNA repair mechanisms. Currently, third-generation PARP1 inhibitors are being developed, many of which are undergoing Phase II clinical trials. In this review, we focus on the properties and features of the PARP1 inhibitors identified in preclinical and clinical trials. We also describe some problems associated with the application of PARP1 inhibitors. The possibility of developing new PARP1 inhibitors aimed at DNA binding and transcriptional activity rather than the catalytic domain of the protein is discussed.

Trial watch - inhibiting PARP enzymes for anticancer therapy

Poly(ADP-ribose) polymerases (PARPs) are a members of family of enzymes that catalyze poly(ADP-ribosyl)ation (PARylation) and/or mono(ADP-ribosyl)ation (MARylation), two post-translational protein modifications involved in crucial cellular processes including (but not limited to) the DNA damage response (DDR). PARP1, the most abundant family member, is a nuclear protein that is activated upon sensing distinct types of DNA damage and contributes to their resolution by PARylating multiple DDR players. Recent evidence suggests that, along with DDR, activated PARP1 mediates a series of prosurvival and proapoptotic processes aimed at preserving genomic stability. Despite this potential oncosuppressive role, upregulation and/or overactivation of PARP1 or other PARP enzymes has been reported in a variety of human neoplasms. Over the last few decades, several pharmacologic inhibitors of PARP1 and PARP2 have been assessed in preclinical and clinical studies showing potent antineoplastic activity, particularly against homologous recombination (HR)-deficient ovarian and breast cancers. In this Trial Watch, we describe the impact of PARP enzymes and PARylation in cancer, discuss the mechanism of cancer cell killing by PARP1 inactivation, and summarize the results of recent clinical studies aimed at evaluating the safety and therapeutic profile of PARP inhibitors in cancer patients.

Histone deacetylase inhibitor treatment induces 'BRCAness' and synergistic lethality with PARP inhibitor and cisplatin against human triple negative breast cancer cells

There is an unmet need to develop new, more effective and safe therapies for the aggressive forms of triple negative breast cancers (TNBCs). While up to 20% of women under 50 years of age with TNBC harbor germline mutations in BRCA1, and these tumors are sensitive to treatment with poly(ADP) ribose polymerase inhibitors, a majority of TNBCs lack BRCA1 mutations or loss of expression. Findings presented here demonstrate that by attenuating the levels of DNA damage response and homologous recombination proteins, pan-histone deacetylase inhibitor (HDI) treatment induces ‘BRCAness’ and sensitizes TNBC cells lacking BRCA1 to lethal effects of PARP inhibitor or cisplatin. Treatment with HDI also induced hyperacetylation of nuclear hsp90. Similar effects were observed following shRNA-mediated depletion of HDAC3, confirming its role as the deacetylase for nuclear HSP90. Furthermore, cotreatment with HDI and ABT-888 induced significantly more DNA strand breaks than either agent alone, and synergistically induced apoptosis of TNBC cells. Notably, co-treatment with HDI and ABT-888 significantly reduced in vivo tumor growth and markedly improved the survival of mice bearing TNBC cell xenografts. These findings support the rationale to interrogate the clinical activity of this novel combination against human TNBC, irrespective of its expression of mutant BRCA1.

Orthogonal targeting of EGFRvIII expressing glioblastomas through simultaneous EGFR and PLK1 inhibition

We identified a synthetic lethality between PLK1 silencing and the expression of an oncogenic Epidermal Growth Factor Receptor, EGFRvIII. PLK1 promoted homologous recombination (HR), mitigating EGFRvIII induced oncogenic stress resulting from DNA damage accumulation. Accordingly, PLK1 inhibition enhanced the cytotoxic effects of the DNA damaging agent, temozolomide (TMZ). This effect was significantly more pronounced in an Ink4a/Arf(-/-) EGFRvIII glioblastoma model relative to an Ink4a/Arf(-/-) PDGF-β model. The tumoricidal and TMZ-sensitizing effects of BI2536 were uniformly observed across Ink4a/Arf(-/-) EGFRvIII glioblastoma clones that acquired independent resistance mechanisms to EGFR inhibitors, suggesting these resistant clones retain oncogenic stress that required PLK1 compensation. Although BI2536 significantly augmented the anti-neoplastic effect of EGFR inhibitors in the Ink4a/Arf(-/-) EGFRvIII model, durable response was not achieved until TMZ was added. Our results suggest that optimal therapeutic effect against glioblastomas requires a "multi-orthogonal" combination tailored to the molecular physiology associated with the target cancer genome.

The proteomic landscape of triple-negative breast cancer

Triple-negative breast cancer is a heterogeneous disease characterized by poor clinical outcomes and a shortage of targeted treatment options. To discover molecular features of triple-negative breast cancer, we performed quantitative proteomics analysis of twenty human-derived breast cell lines and four primary breast tumors to a depth of more than 12,000 distinct proteins. We used this data to identify breast cancer subtypes at the protein level and demonstrate the precise quantification of biomarkers, signaling proteins, and biological pathways by mass spectrometry. We integrated proteomics data with exome sequence resources to identify genomic aberrations that affect protein expression. We performed a high-throughput drug screen to identify protein markers of drug sensitivity and understand the mechanisms of drug resistance. The genome and proteome provide complementary information that, when combined, yield a powerful engine for therapeutic discovery. This resource is available to the cancer research community to catalyze further analysis and investigation.

The potential of PARP inhibitors in neuro-oncology

DNA damaging agents have an integral role in the treatment of brain tumors. Recent advances in our understanding of how cancer cells repair DNA damage have made it possible to consider modification of the DNA damage response as a way in which resistance to radiotherapy and chemotherapy might be overcome. PARP inhibitors are potent but nontoxic drugs that inhibit repair of DNA single-strand breaks and increase the cytotoxic effects of radiotherapy and alkylating chemotherapy agents, including temozolomide. PARP inhibitors have potential applications in neuro-oncology because there is increasing evidence that their radio- and chemo-sensitizing effects are tumor specific. This review explores the mechanisms of action of PARP inhibitors and describes their putative mechanisms of radio- and chemo-sensitization in the context of CNS oncology. The authors go on to review their development in recent clinical trials, with a focus on glioblastoma.

Tumor heterogeneity: evolution through space and time in EGFR mutant non small cell lung cancer patients

NSCLC patients with mutations in epidermal growth factor receptor (EGFR) gene have dramatic responses with the EGFR tyrosine kinase inhibitors (TKI) in the majority of patients. However, all patients will eventually present progression of disease because of both primary and acquired resistance to EGFR TKI. In the recent years several studies have identified mechanisms involved in primary and secondary resistance to EGFR TKI treatment that can also be potential therapeutic strategies, although up to 30% of cases of acquired resistance to EGFR TKI are still unexplained. In this review we describe the mechanisms of resistance to EGFR TKIs in NSCLC patients that have been discovered and potential therapeutic strategies to overcome EGFR TKI resistance. Additionally we highlight the importance of performing additional biopsies not only at time of acquired resistance to EGFR TKI but also immediately after initiation of therapy to discover the remaining unknown mechanisms of acquired resistance to EGFR TKI as well as the underlying molecular basis of the heterogeneity in response to EGFR TKI.

Histone deacetylase inhibitor, suberoylanilide hydroxamic acid (SAHA), enhances anti-tumor effects of the poly (ADP-ribose) polymerase (PARP) inhibitor olaparib in triple-negative breast cancer cells

Introduction

Olaparib, a poly (ADP-ribose) polymerase (PARP) inhibitor, has been found to have therapeutic potential for treating cancers associated with impaired DNA repair capabilities, particularly those with deficiencies in the homologous recombination repair (HRR) pathway. Histone deacetylases (HDACs) are important for enabling functional HRR of DNA by regulating the expression of HRR-related genes and promoting the accurate assembly of HRR-directed sub-nuclear foci. Thus, HDAC inhibitors have recently emerged as a therapeutic agent for treating cancer by inhibiting DNA repair. Based on this, HDAC inhibition could be predicted to enhance the anti-tumor effect of PARP inhibitors in cancer cells by blocking the HRR pathway.

Methods

We determined whether suberoylanilide hydroxamic acid (SAHA), a HDAC inhibitor, could enhance the anti-tumor effects of olaparib on breast cancer cell lines using a cytotoxic assay, cell cycle analysis, and Western blotting. We evaluated how exposure to SAHA affects the expression of HRR-associated genes. The accumulation of DNA double strand breaks (DSBs) induced by combination treatment was assessed. Induction of autophagy was monitored by imaging green fluorescent protein-tagged microtubule-associated protein 1A/1B-light chain 3 (LC3) expression following co-treatment with olaparib and SAHA. These in vitro data were validated in vivo using a human breast cancer xenograft model.

Results

Triple-negative breast cancer cell (TNBC) lines showed heterogeneous responses to the PARP and HDAC inhibitors. Co-administration of olaparib and SAHA synergistically inhibited the growth of TNBC cells that expressed functional Phosphatase and tensin homolog (PTEN). This effect was associated with down-regulation of the proliferative signaling pathway, increased apoptotic and autophagic cell death, and accumulation of DNA damage. The combined anti-tumor effect of olaparib and SAHA was also observed in a xenograft model. These data suggest that PTEN expression in TNBC cells can sensitize the cell response to simultaneous inhibition of PARP and HDAC both in vitro and in vivo.

Conclusion

Our findings suggest that expression of functional PTEN may serve as a biomarker for selecting TNBC patients that would favorably respond to a combination of olaparib with SAHA. This provides a strong rationale for treating TNBC patients with PTEN expression with a combination therapy consisting of olaparib and SAHA.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-015-0534-y) contains supplementary material, which is available to authorized users.

An update on PARP inhibitors for the treatment of cancer

The development of poly (adenosine diphosphate [ADP]) ribose polymerase (PARP) inhibitors (PARPi) has progressed greatly over the last few years and has shown encouraging results in the BRCA1/2 mutation–related cancers. This article attempts to summarize the rationale and theory behind PARPi, the clinical trials already reported, as well as ongoing studies designed to determine the role of PARPi in patients with and without germline mutations of BRCA genes. Future plans for PARPi both as monotherapy and in combination with standard cytotoxics, other biological agents, and as radiosensitizers are also covered. The widening scope of PARPi adds another important targeted agent to the growing list of molecular inhibitors; future and ongoing trials will identify the most effective role for PARPi, including for patients other than BRCA germline mutation carriers.

PTEN: Multiple Functions in Human Malignant Tumors

PTEN is the most important negative regulator of the PI3K signaling pathway. In addition to its canonical, PI3K inhibition-dependent functions, PTEN can also function as a tumor suppressor in a PI3K-independent manner. Indeed, the PTEN network regulates a broad spectrum of biological functions, modulating the flow of information from membrane-bound growth factor receptors to nuclear transcription factors, occurring in concert with other tumor suppressors and oncogenic signaling pathways. PTEN acts through its lipid and protein phosphatase activity and other non-enzymatic mechanisms. Studies conducted over the past 10 years have expanded our understanding of the biological role of PTEN, showing that in addition to its ability to regulate proliferation and cell survival, it also plays an intriguing role in regulating genomic stability, cell migration, stem cell self-renewal, and tumor microenvironment. Changes in PTEN protein levels, location, and enzymatic activity through various molecular mechanisms can generate a continuum of functional PTEN levels in inherited syndromes, sporadic cancers, and other diseases. PTEN activity can indeed, be modulated by mutations, epigenetic silencing, transcriptional repression, aberrant protein localization, and post-translational modifications. This review will discuss our current understanding of the biological role of PTEN, how PTEN expression and activity are regulated, and the consequences of PTEN dysregulation in human malignant tumors.

Rapamycin-resistant poly (ADP-ribose) polymerase-1 overexpression is a potential therapeutic target in lymphangioleiomyomatosis

Lymphangioleiomyomatosis (LAM) is a female-predominant cystic lung disease that can lead to respiratory failure. LAM cells typically have inactivating tuberous sclerosis complex 2 (TSC2) mutations and mammalian target of rapamycin (mTOR) complex (mTORC) 1 activation. Clinical response to the mTORC1 inhibitors has been limited, prompting a search for additional therapy for LAM. In this study, we investigated the impact of TSC2 on the expression of poly (ADP-ribose) polymerase (PARP)-1 that initiates the DNA repair pathway, and tested the efficacy of PARP1 inhibitors in the survival of TSC2-deficient (TSC2(-)) cells. We analyzed publicly available expression arrays of TSC2(-) cells and validated the findings using real-time RT-PCR, immunoblotting, and immunohistochemistry. We examined the impact of rapamycin and Torin 1 on PARP1 expression. We also tested the effect of PARP1 inhibitors, 8-hydroxy-2-methylquinazoline-4-one and 3,4-dihydro-5[4-(1-piperindinyl)butoxy]-1(2H)-isoquinoline, on the survival of TSC2(-) cells. We identified the up-regulation of PARP1 in TSC2(-) cells relative to cells in which wild-type TSC2 has been reintroduced (TSC2-addback [TSC2(+)] cells). The transcript levels of PARP1 in TSC2(-) cells were not affected by rapamycin. PARP1 levels were increased in TSC2(-) cells, xenograft tumors of rat-derived TSC2(-) cells, renal cystadenomas from Tsc2(+/-) mice, and human LAM nodules. RNA interference of mTOR failed to reduce PARP1 levels. Proliferation and survival of TSC2(-) cells was reduced in response to PARP1 inhibitor treatment, more so than TSC2(+) cells. TSC2(-) cells exhibit higher levels of PARP1 relative to TSC2(+) cells in an mTOR-insensitive manner. PARP1 inhibitors selectively suppress the growth and induce apoptosis of TSC2(-) cells from patients with LAM. Targeting PARP1 may be beneficial in the treatment of LAM and other neoplasm with mTORC1 activation.

Deciphering the insights of poly(ADP-ribosylation) in tumor progression

Poly (ADP-ribose) polymerase (PARP) inhibitors are particularly efficient against tumors with defects in the homologous recombination repair pathway. Nonetheless poly(ADP-ribosylation) (PARylation) modulates prometastasic activities and adaptation of tumor to a hostile microenvironment. Modulation of metastasis-promoting traits is possible through the alteration of key transcription factors involved in the regulation of the hypoxic response, the recruitment of new vessels (or angiogenesis), and the stimulation of epithelial to mesenchymal transition (EMT). In this review, we summarized some of the findings that focalize on PARP-1's action on tumor aggressiveness, suggesting new therapeutic opportunities against an assembly of tumors not necessarily bearing DNA repair defects. Metastasis accounts for the vast majority of mortality derived from solid cancer. PARP-1 is an active player in tumor adaptation to metastasis and PARP inhibitors, recognized as promising therapeutic agents against homologous recombination deficient tumors, has novel properties responsible for the antimetastatic actions in different tumor settings.

PARP inhibitors

Poly (ADP-ribose) polymerases, abbreviated as PARPs, are a group of familiar proteins that play a central role in DNA repair employing the base excision repair (BER) pathway. There about 17 proteins in this family out of which the primary nuclear PARPs are PARP-1, PARP-2, PARP-3, and tankyrases 1 and 2 (PARP-5a and -5b) .The PARP family members are known to engage in a wide range of cellular activities, for example, DNA repair, transcription, cellular signaling, cell cycle regulation and mitosis amongst others. The chief functional units of PARP-1 are an amino terminal DNA binding domain (DBD), a central auto modification domain (AMD), and a carboxyl-terminal catalytic domain (CD). PARP inhibitors are currently undergoing clinical trials as targeted treatment modalities of breast, uterine, colorectal and ovarian cancer. This review summarizes current insights into the mechanism of action of PARP inhibitors, its recent clinical trials, and potential next steps in the evaluation of this promising class of anti-cancer drugs.

Discovery of 1-substituted benzyl-quinazoline-2,4(1H,3H)-dione derivatives as novel poly(ADP-ribose)polymerase-1 inhibitors

Poly(ADP-ribose)polymerase-1 (PARP-1) has emerged as a promising anticancer drug target due to its key role in the DNA repair process. In this work, a novel series of 1-benzyl-quinazoline-2,4(1H,3H)-dione derivatives were designed and synthesized as human PARP-1 inhibitors, structure-activity relationships were conducted and led to a number of potent PARP-1 inhibitors having IC50 values of single or double digit nanomolar level. Compound 7j was a potent PARP-1 and PARP-2 inhibitor and it could selectively kill the breast cancer cells MX-1 and MDA-MB-468 with mutated BRCA1/2 and PTEN, respectively, in comparison with homologous recombination proficient cell types such as breast cancer cells MDA-MB-231. In addition, compound 7j displayed the strongest potentiation effect on temozolomide in MX-1 cells (PF50=3.77) in this series of PARP-1 inhibitors.

The PI3K/AKT/mTOR interactive pathway

The phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of the rapamycin (mTOR) signalling pathway is hyperactivated or altered in many cancer types and regulates a broad range of cellular processes including survival, proliferation, growth, metabolism, angiogenesis and metastasis.

Poly (ADP-ribose) polymerase inhibitor: an evolving paradigm in the treatment of prostate cancer

Recent phase I studies have reported single-agent activities of poly (ADP-ribose) polymerase (PARP) inhibitor in sporadic and in BRCA-mutant prostate cancers. Two of the most common genetic alterations in prostate cancer, ETS gene rearrangement and loss of PTEN, have been linked to increased sensitivity to PARP inhibitor in preclinical models. Emerging evidence also suggests that PARP1 plays an important role in mediating the transcriptional activities of androgen receptor (AR) and ETS gene rearrangement. In this article, the preclinical work and early-phase clinical trials in developing PARP inhibitor-based therapy as a new treatment paradigm for metastatic prostate cancer are reviewed.

Combined image and genomic analysis of high-grade serous ovarian cancer reveals PTEN loss as a common driver event and prognostic classifier

BACKGROUND

TP53 and BRCA1/2 mutations are the main drivers in high-grade serous ovarian carcinoma (HGSOC). We hypothesise that combining tissue phenotypes from image analysis of tumour sections with genomic profiles could reveal other significant driver events.

RESULTS

Automatic estimates of stromal content combined with genomic analysis of TCGA HGSOC tumours show that stroma strongly biases estimates of PTEN expression. Tumour-specific PTEN expression was tested in two independent cohorts using tissue microarrays containing 521 cases of HGSOC. PTEN loss or downregulation occurred in 77% of the first cohort by immunofluorescence and 52% of the validation group by immunohistochemistry, and is associated with worse survival in a multivariate Cox-regression model adjusted for study site, age, stage and grade. Reanalysis of TCGA data shows that hemizygous loss of PTEN is common (36%) and expression of PTEN and expression of androgen receptor are positively associated. Low androgen receptor expression was associated with reduced survival in data from TCGA and immunohistochemical analysis of the first cohort.

CONCLUSION

PTEN loss is a common event in HGSOC and defines a subgroup with significantly worse prognosis, suggesting the rational use of drugs to target PI3K and androgen receptor pathways for HGSOC. This work shows that integrative approaches combining tissue phenotypes from images with genomic analysis can resolve confounding effects of tissue heterogeneity and should be used to identify new drivers in other cancers.

JF-305, a pancreatic cancer cell line is highly sensitive to the PARP inhibitor olaparib

Poly(ADP-ribose) polymerase-1 (PARP-1) is a DNA nick sensor involved in the base excision repair (BER) pathway. Olaparib, a PARP inhibitor, has demonstrated antitumor activity in homologous recombination (HR)-deficient cancers. To extend this specific therapy to other types of carcinomas, a panel of 11 different cancer cells were screened in the present study. JF-305, a pancreatic cancer cell line of Chinese origin, demonstrated sensitivity to the PARP inhibitor 6(5H)-phenanthridinone. In the present study, 3 μM olaparib conferred a cell survival rate of 25% following four days of treatment. The colony formation efficiency was 83% at 10 nM, and dropped to 12% at 1 μM following seven days of treatment. Furthermore, olaparib induced cell cycle arrest in the S and G₂/M phases prior to the initiation of apoptosis. Although the incidence of double-strand breaks (DSBs) was increased in the olaparib-treated JF-305 cells, the RAD51 foci were well formed at the sites of γ-H2AX recruitment, indicating an activated HR mechanism. Furthermore, tumor growth was reduced by 49.8% following 22 days of consecutive administration of 10 mg/kg olaparib in the JF-305 xenograft mouse model. In summary, the JF-305 cell line was sensitive to olaparib and provided a prospective model for the preclinical assessment of PARP inhibitors in the therapy of pancreatic cancer.

A genomic instability score in discriminating nonequivalent outcomes of BRCA1/2 mutations and in predicting outcomes of ovarian cancer treated with platinum-based chemotherapy

Detecting mutation in BRCA1/2 is a generally accepted strategy for screening ovarian cancers that have impaired homologous recombination (HR) ability and improved sensitivity to PARP inhibitor. However, a substantial subset of BRCA-mutant ovarian cancer patients shows less impaired or unimpaired HR ability, resulting in nonequivalent outcome after ovarian cancer development. We hypothesize that genomic instability provides a lifetime record of DNA repair deficiency and predicts ovarian cancer outcome. Based on the multi-dimensional TCGA ovarian cancer data, we developed a biological rationale-driven genomic instability score integrating somatic mutation and copy number change in a tumor genome. The score successfully divided BRCA-mutant ovarian tumors into cases of significantly improved outcome and cases of unimproved outcome. The score was also capable of discriminating HR-deficiency indicated by BRCA1 epigenetically silencing, EMSY amplification and homozygous deletion of core HR genes. We further found that the score was positively correlated with the complete response rate of chemotherapy and the rate of platinum-sensitivity, and predicted improved outcome of ovarian cancer, regardless of BRCA-mutation status. The score may have important value in outcome prediction and clinical trial design.

Suppression of homologous recombination sensitizes human tumor cells to IGF-1R inhibition

Inhibition of type 1 IGF receptor (IGF-1R) sensitizes to DNA-damaging cancer treatments, and delays repair of DNA double strand breaks (DSBs) by non-homologous end-joining and homologous recombination (HR). In a recent screen for mediators of resistance to IGF-1R inhibitor AZ12253801, we identified RAD51, required for the strand invasion step of HR. These findings prompted us to test the hypothesis that IGF-1R-inhibited cells accumulate DSBs formed at endogenous DNA lesions, and depend on residual HR for their repair. Indeed, initial experiments showed time-dependent accumulation of γH2AX foci in IGF-1R -inhibited or -depleted prostate cancer cells. We then tested effects of suppressing HR, and found that RAD51 depletion enhanced AZ12253801 sensitivity in PTEN wild-type prostate cancer cells but not in cells lacking functional PTEN. Similar sensitization was induced in prostate cancer cells by depletion of BRCA2, required for RAD51 loading onto DNA, and in BRCA2(-/-) colorectal cancer cells, compared with isogenic BRCA2(+/-) cells. We also assessed chemical HR inhibitors, finding that RAD51 inhibitor BO2 blocked RAD51 focus formation and sensitized to AZ12253801. Finally, we tested CDK1 inhibitor RO-3306, which impairs HR by inhibiting CDK1-mediated BRCA1 phosphorylation. R0-3306 suppressed RAD51 focus formation consistent with HR attenuation, and sensitized prostate cancer cells to IGF-1R inhibition, with 2.4-fold reduction in AZ12253801 GI50 and 13-fold reduction in GI80. These data suggest that responses to IGF-1R inhibition are enhanced by genetic and chemical approaches to suppress HR, defining a population of cancers (PTEN wild-type, BRCA mutant) that may be intrinsically sensitive to IGF-1R inhibitory drugs.

Novel neoadjuvant therapy paradigms for bladder cancer: results from the National Cancer Center Institute Forum

OBJECTIVE

To bridge gaps in translational science and develop the concepts for 2 novel biomarker-driven clinical trials: one in the presurgical setting and the other in the setting of bladder preservation with chemoradiation.

METHODS AND MATERIALS

The National Cancer Institute sponsored a forum, "Novel Neoadjuvant Therapy for Bladder Cancer," which brought leading clinical and laboratory-based scientists together with the advocacy community.

RESULTS

The group designed a neoadjuvant clinical trial to compare the clinical efficacy of the two frontline chemotherapy regimens (gemcitabine plus cisplatin versus MVAC) and the ability of a gene expression profiling-based algorithm (CoXEN) to predict complete pathological response. The trial was recently opened under the leadership of the Southwest Oncology Group (SWOG, S1314), receiving support for the biomarker studies from the NCI's BISQFP resource. A second clinical trial was planned that will examine the relationship between expression of the DNA repair protein MRE11 and complete response in patients treated with concurrent 5-fluorouracil/mitomycin C plus radiation.

CONCLUSION

The meeting provided a unique opportunity to launch a collective effort to establish molecular-based therapies for muscle-invasive urothelial cancer. The goal is to use this framework to develop comparable trials with immunotherapy in non-muscle invasive cancers and to exploit the neoadjuvant platform to develop targeted therapy in muscle-invasive disease.

[PARP inhibitors and radiotherapy: rational and prospects for a clinical use]

Poly(ADP-ribosyl)ation is a ubiquitous protein modification involved in the regulation of many cellular processes that is carried out by the poly(ADP-ribose) polymerase (PARP) family. The PARP-1, PARP-2 and PARP-3 are the only PARPs known to be activated by DNA damage. The absence of PARP-1 and PARP-2, that are both activated by DNA damage and participate in DNA damage repair processes, results in hypersensitivity to ionizing radiation and alkylating agents. PARP inhibitors that compete with NAD(+) at the enzyme's activity site can be used in BRCA-deficient cells as single agent therapies acting through the principle of synthetic lethality exploiting these cells deficient DNA double-strand break repair. Preclinical data showing an enhancement of the response of tumors to radiation has been documented for several PARP inhibitors. However, whether this is due exclusively to impaired DNA damage responses or whether tumor re-oxygenation contributes to this radio-sensitization via the vasoactive effects of the PARP inhibitors remains to be fully determined. These promising results have paved the way for the evaluation of PARP inhibitors in combination with radiotherapy in phase I and phase II clinical trials for malignant glioma, head and neck, and breast cancers. A number of challenges remain that are also reviewed in this article, including the optimization of treatment schedules for combined therapies and the validation of biomarkers that will identify which patients will most benefit from either PARP inhibitors in combination with radiotherapy.

Synthetic lethality and cancer therapy: lessons learned from the development of PARP inhibitors

The genetic concept of synthetic lethality, in which the combination or synthesis of mutations in multiple genes results in cell death, provides a framework to design novel therapeutic approaches to cancer. Already there are promising indications, from clinical trials exploiting this concept by using poly(ADP-ribose) polymerase (PARP) inhibitors in patients with germline BRCA1 or BRCA2 gene mutations, that this approach could be beneficial. We discuss the biological rationale for BRCA-PARP synthetic lethality, how the synthetic lethal approach is being assessed in the clinic, and how mechanisms of resistance are starting to be dissected. Applying the synthetic lethal concept to target non-BRCA-mutant cancers also has clear potential, and we discuss how some of the principles learned in developing PARP inhibitors might also drive the development of additional genetic approaches.

Synthetic lethal vulnerabilities of cancer

The great majority of targeted anticancer drugs inhibit mutated oncogenes that display increased activity. Yet many tumors do not contain such actionable aberrations, such as those harboring loss-of-function mutations. The notion of targeting synthetic lethal vulnerabilities in cancer cells has provided an alternative approach to exploiting more of the genetic and epigenetic changes acquired during tumorigenesis. Here, we review synthetic lethality as a therapeutic concept that exploits the inherent differences between normal cells and cancer cells. Furthermore, we provide an overview of the screening approaches that can be used to identify synthetic lethal interactions in human cells and present several recently identified interactions that may be pharmacologically exploited. Finally, we indicate some of the challenges of translating synthetic lethal interactions into the clinic and how these may be overcome.

An update on PARP inhibitors--moving to the adjuvant setting

Inhibition of poly(ADP-ribose) polymerase (PARP) enzymes is a potential synthetic lethal therapeutic strategy in cancers harbouring specific DNA-repair defects, including those arising in carriers of BRCA1 or BRCA2 mutations. Since the development of first-generation PARP inhibitors more than a decade ago, numerous clinical trials have been performed to validate their safety and efficacy, bringing us to the stage at which adjuvant therapy with PARP inhibitors is now being considered as a viable treatment option for patients with breast cancer. Nevertheless, the available data do not provide clear proof that these drugs are efficacious in the setting of metastatic disease. Advancement of a therapy to the neoadjuvant and adjuvant settings without such evidence is exceptional, but seems reasonable in the case of PARP inhibitors because the target population that might benefit from this class of drugs is small and well defined. This Review describes the evolution of PARP inhibitors from bench to bedside, and provides an up-to-date description of the key published or otherwise reported clinical trials of these agents. The specific considerations and challenges that might be encountered when implementing these compounds in the adjuvant treatment of breast cancer in the clinic are also highlighted.

Novel treatment strategies in triple-negative breast cancer: specific role of poly(adenosine diphosphate-ribose) polymerase inhibition

Inhibitors of the poly(adenosine triphosphate-ribose) polymerase (PARP)-1 enzyme induce synthetic lethality in cancers with ineffective DNA (DNA) repair or homologous repair deficiency, and have shown promising clinical activity in cancers deficient in DNA repair due to germ-line mutation in BRCA1 and BRCA2. The majority of breast cancers arising in carriers of BRCA1 germ-line mutations, as well as half of those in BRCA2 carriers, are classified as triple-negative breast cancer (TNBC). TNBC is a biologically heterogeneous group of breast cancers characterized by the lack of immunohistochemical expression of the ER, PR, or HER2 proteins, and for which the current standard of care in systemic therapy is cytotoxic chemotherapy. Many "sporadic" cases of TNBC appear to have indicators of DNA repair dysfunction similar to those in BRCA-mutation carriers, suggesting the possible utility of PARP inhibitors in a subset of TNBC. Significant genetic heterogeneity has been observed within the TNBC cohort, creating challenges for interpretation of prior clinical trial data, and for the design of future clinical trials. Several PARP inhibitors are currently in clinical development in BRCA-mutated breast cancer. The use of PARP inhibitors in TNBC without BRCA mutation will require biomarkers that identify cancers with homologous repair deficiency in order to select patients likely to respond. Beyond mutations in the BRCA genes, dysfunction in other genes that interact with the homologous repair pathway may offer opportunities to induce synthetic lethality when combined with PARP inhibition.

Zinc finger oxidation of Fpg/Nei DNA glycosylases by 2-thioxanthine: biochemical and X-ray structural characterization

DNA glycosylases from the Fpg/Nei structural superfamily are base excision repair enzymes involved in the removal of a wide variety of mutagen and potentially lethal oxidized purines and pyrimidines. Although involved in genome stability, the recent discovery of synthetic lethal relationships between DNA glycosylases and other pathways highlights the potential of DNA glycosylase inhibitors for future medicinal chemistry development in cancer therapy. By combining biochemical and structural approaches, the physical target of 2-thioxanthine (2TX), an uncompetitive inhibitor of Fpg, was identified. 2TX interacts with the zinc finger (ZnF) DNA binding domain of the enzyme. This explains why the zincless hNEIL1 enzyme is resistant to 2TX. Crystal structures of the enzyme bound to DNA in the presence of 2TX demonstrate that the inhibitor chemically reacts with cysteine thiolates of ZnF and induces the loss of zinc. The molecular mechanism by which 2TX inhibits Fpg may be generalized to all prokaryote and eukaryote ZnF-containing Fpg/Nei-DNA glycosylases. Cell experiments show that 2TX can operate in cellulo on the human Fpg/Nei DNA glycosylases. The atomic elucidation of the determinants for the interaction of 2TX to Fpg provides the foundation for the future design and synthesis of new inhibitors with high efficiency and selectivity.

DNA damage response and prostate cancer: defects, regulation and therapeutic implications

DNA damage response (DDR) includes the activation of numerous cellular activities that prevent duplication of DNA lesions and maintain genomic integrity, which is critical for the survival of normal and cancer cells. Specific genes involved in the DDR such as BRCA1/2 and P53 are mutated during prostate cancer progression, while various oncogenic signaling such as Akt and c-Myc are activated, enhancing the replication stress and increasing the genomic instability of cancer cells. These events may render prostate cancer cells particularly sensitive to inhibition of specific DDR pathways, such as PARP in homologous recombination (HR) DNA repair and Chk1 in cell cycle checkpoint and DNA repair, creating opportunities for synthetic lethality or synergistic cytotoxicity. Recent reports highlight the critical role of androgen receptor (AR) as a regulator of DDR genes, providing a rationale for combining DNA-damaging agents or targeted DDR inhibitors with hormonal manipulation or AR inhibition as treatment for aggressive disease. The aims of this review are to discuss specific DDR defects in prostate cancer that occur during disease progression, to summarize recent advances in understanding the regulation of DDR in prostate cancer, and to present potential therapeutic opportunities through combinational targeting of the intact components of DDR signaling pathways.

TGFβ induces "BRCAness" and sensitivity to PARP inhibition in breast cancer by regulating DNA-repair genes

Transforming growth factor beta (TGFβ) proteins are multitasking cytokines, in which high levels at tumor sites generally correlate with poor prognosis in human patients with cancer. Previously, it was reported that TGFβ downregulates the expression of ataxia telangiectasia-mutated (ATM) and mutS homolog 2 (MSH2) in breast cancer cells through an miRNA-mediated mechanism. In this study, expression of a panel of DNA-repair genes was examined, identifying breast cancer 1, early onset (BRCA1) as a target downregulated by TGFβ through the miR181 family. Correlations between the expression levels of TGFβ1 and the miR181/BRCA1 axis were observed in primary breast tumor specimens. By downregulating BRCA1, ATM, and MSH2, TGFβ orchestrates DNA damage response in certain breast cancer cells to induce a "BRCAness" phenotype, including impaired DNA-repair efficiency and synthetic lethality to the inhibition of poly (ADP-ribose) polymerase (PARP). Xenograft tumors with active TGFβ signaling exhibited resistance to the DNA-damaging agent doxorubicin but increased sensitivity to the PARP inhibitor ABT-888. Combination of doxorubicin with ABT-888 significantly improved the treatment efficacy in TGFβ-active tumors. Thus, TGFβ can induce "BRCAness" in certain breast cancers carrying wild-type BRCA genes and enhance the responsiveness to PARP inhibition, and the molecular mechanism behind this is characterized.

IMPLICATIONS

These findings enable better selection of patients with sporadic breast cancer for PARP interventions, which have exhibited beneficial effects in patients carrying BRCA mutations.

Correlation between gene expression and mutator phenotype predicts homologous recombination deficiency and outcome in ovarian cancer

New strategies are needed to predict response to platinum-based chemotherapy and outcome of ovarian cancers. We hypothesized that the mutator phenotype in the cancer genome represents the overuse of alternative DNA repair mechanisms, which might be a sign of homologous recombination (HR) deficiency and can be captured by gene expression. Multidimensional data of ovarian cancer patients and breast cancer patients from The Cancer Genome Atlas (TCGA) database were used for the development and validation of a potential clinical information-independent score that correlates with HR deficiency and predicts outcome. Correlation of the score with platinum response, outcome, and BRCA mutations was assessed. The score correlated with increased genomic mutation rate in both ovarian cancer and breast cancer cases that harbored a substantial subset of HR-deficient samples. Significantly improved outcomes were observed in the high-scoring group versus the low-scoring group in the TCGA dataset and in three large gene expression microarray datasets. A strong correlation was found between the score and the likelihood of achieving complete response to chemotherapy. The score was also found to be highly robust to noises in genomic mutations. Sixty-four patients harboring BRCA mutations were successfully divided into two groups based on scores, with the high-scoring group showing significantly improved outcomes compared with wild-type cases and the low-scoring group showing no significance in all the same analyses. The score was significantly correlated with the response to platinum therapy and outcome. Evaluation of the score as a prognostic tool in ovarian cancer patients is warranted.

KEY MESSAGE

We develop a diagnostic signature for the HR-deficiency based on a novel hypothesis. HR-deficiency score is significantly correlated to platinum therapy and outcomes. HRDS was validated by its association with OS, PFS, DFS and CR in validation datasets. Evaluation of the score as a prognostic tool in ovarian cancer patients is warranted.

Inhibition of poly(ADP-ribosyl)ation in cancer: old and new paradigms revisited

Inhibitors of poly(ADP-ribose) polymerases actualized the biological concept of synthetic lethality in the clinical practice, yielding a paradigmatic example of translational medicine. The profound sensitivity of tumors with germline BRCA mutations to PARP1/2 blockade owes to inherent defects of the BRCA-dependent homologous recombination machinery, which are unleashed by interruption of PARP DNA repair activity and lead to DNA damage overload and cell death. Conversely, aspirant BRCA-like tumors harboring somatic DNA repair dysfunctions (a vast entity of genetic and epigenetic defects known as "BRCAness") not always align with the familial counterpart and appear not to be equally sensitive to PARP inhibition. The acquisition of secondary resistance in initially responsive patients and the lack of standardized biomarkers to identify "BRCAness" pose serious threats to the clinical advance of PARP inhibitors; a feeling is also emerging that a BRCA-centered perspective might have missed the influence of additional, not negligible and DNA repair-independent PARP contributions onto therapy outcome. While regulatory approval for PARP1/2 inhibitors is still pending, novel therapeutic opportunities are sprouting from different branches of the PARP family, although they remain immature for clinical extrapolation. This review is an endeavor to provide a comprehensive appraisal of the multifaceted biology of PARPs and their evolving impact on cancer therapeutics.

Discovery and structure-activity relationship of novel 2,3-dihydrobenzofuran-7-carboxamide and 2,3-dihydrobenzofuran-3(2H)-one-7-carboxamide derivatives as poly(ADP-ribose)polymerase-1 inhibitors

Novel substituted 2,3-dihydrobenzofuran-7-carboxamide (DHBF-7-carboxamide) and 2,3-dihydrobenzofuran-3(2H)-one-7-carboxamide (DHBF-3-one-7-carboxamide) derivatives were synthesized and evaluated as inhibitors of poly(ADP-ribose)polymerase-1 (PARP-1). A structure-based design strategy resulted in lead compound 3 (DHBF-7-carboxamide; IC₅₀ = 9.45 μM). To facilitate synthetically feasible derivatives, an alternative core was designed, DHBF-3-one-7-carboxamide (36, IC₅₀ = 16.2 μM). The electrophilic 2-position of this scaffold was accessible for extended modifications. Substituted benzylidene derivatives at the 2-position were found to be the most potent, with 3′,4′-dihydroxybenzylidene 58 (IC₅₀ = 0.531 μM) showing a 30-fold improvement in potency. Various heterocycles attached at the 4′-hydroxyl/4′-amino of the benzylidene moiety resulted in significant improvement in inhibition of PARP-1 activity (e.g., compounds 66–68, 70, 72, and 73; IC₅₀ values from 0.718 to 0.079 μM). Compound 66 showed selective cytotoxicity in BRCA2-deficient DT40 cells. Crystal structures of three inhibitors (compounds (−)-13c, 59, and 65) bound to a multidomain PARP-1 structure were obtained, providing insights into further development of these inhibitors.

Development of synthetic lethality anticancer therapeutics

The concept of synthetic lethality (the creation of a lethal phenotype from the combined effects of mutations in two or more genes) has recently been exploited in various efforts to develop new genotype-selective anticancer therapeutics. These efforts include screening for novel anticancer agents, identifying novel therapeutic targets, characterizing mechanisms of resistance to targeted therapy, and improving efficacies through the rational design of combination therapy. This review discusses recent developments in synthetic lethality anticancer therapeutics, including poly ADP-ribose polymerase inhibitors for BRCA1- and BRCA2-mutant cancers, checkpoint inhibitors for p53 mutant cancers, and small molecule agents targeting RAS gene mutant cancers. Because cancers are caused by mutations in multiple genes and abnormalities in multiple signaling pathways, synthetic lethality for a specific tumor suppressor gene or oncogene is likely cell context-dependent. Delineation of the mechanisms underlying synthetic lethality and identification of treatment response biomarkers will be critical for the success of synthetic lethality anticancer therapy.