A snapshot of chemoresistance to PARP inhibitors

Alkylation of nucleobases by 2-chloro-N,N-diethylethanamine hydrochloride (CDEAH) sensitizes PARP1-deficient tumors

Targeting BRCA1- and BRCA2-deficient tumors through synthetic lethality using poly(ADP-ribose) polymerase inhibitors (PARPi) has emerged as a successful strategy for cancer therapy. PARPi monotherapy has shown excellent efficacy and safety profiles in clinical practice but is limited by the need for tumor genome mutations in BRCA or other homologous recombination genes as well as the rapid emergence of resistance. In this study, we identified 2-chloro-N,N-diethylethanamine hydrochloride (CDEAH) as a small molecule that selectively kills PARP1- and xeroderma pigmentosum A-deficient cells. CDEAH is a monofunctional alkylating agent that preferentially alkylates guanine nucleobases, forming DNA adducts that can be removed from DNA by either a PARP1-dependent base excision repair or nucleotide excision repair. Treatment of PARP1-deficient cells leads to the formation of strand breaks, an accumulation of cells in S phase and activation of the DNA damage response. Furthermore, CDEAH selectively inhibits PARP1-deficient xenograft tumor growth compared to isogenic PARP1-proficient tumors. Collectively, we report the discovery of an alkylating agent inducing DNA damage that requires PARP1 activity for repair and acts synergistically with PARPi.

Recent Progress of the research on the benzimidazole PARP-1 inhibitors

Poly (ADP-ribose) polymerase-1 (PARP-1) is a multifunctional protein that plays an important role in DNA repair and genome integrity. PARP-1 inhibitors can be used as effective drugs not only to treat BRCA-1/2 deficient cancers because of the effect of synthetically lethal, but also to treat non-BRCA1/2 deficient tumours because of the effect of PARP capture. Therefore, the PARP inhibitors have become a focus of compelling research. Among these inhibitors, substituted benzimidazole derivatives were mainly concerned lead compounds. However, the commercial available benzimidazole PARP-1 inhibitors have some shortcomings such as serious toxicity in combination with chemotherapy drugs, in vivo cardiovascular side effects such as anemia. Therefore it's crucial for scientists to explore more structure-activity relationships of the benzimidazole PARP-1 inhibitors and access safer and more effective PARP inhibitors. As the binding region of PARP-1 and the substrates is usually characterized as NI site and AD site, the modification of benzimidazoles mainly occurs on the benzimidazole skeleton (NI site), and the side chain of benzimidazole on 2-C position (AD site). Herein, the recent progresses of the researches of benzamides PARP inhibitors were introduced. We noticed that even though many efforts were taken to the modification of NI sites, there were still lacks of optimistic and impressive results. However, the structure-activity relationships of the modification of AD sites have not thoroughly discovered yet. We hope that enlightened by the previous researches, more researches of AD site should be occurred and more effective benzimidazole PARP-1 inhibitors could be designed, synthesized, and applied to clinics.

Recent advancements in PARP inhibitors-based targeted cancer therapy

Poly(ADP-ribose) polymerase inhibitors (PARPi) are a new class of agents with unparalleled clinical achievement for driving synthetic lethality in BRCA-deficient cancers. Recent FDA approval of PARPi has motivated clinical trials centered around the optimization of PARPi-associated therapies in a variety of BRCA-deficient cancers. This review highlights recent advancements in understanding the molecular mechanisms of PARP ‘trapping’ and synthetic lethality. Particular attention is placed on the potential extension of PARPi therapies from BRCA-deficient patients to populations with other homologous recombination-deficient backgrounds, and common characteristics of PARPi and non-homologous end-joining have been elucidated. The synergistic antitumor effect of combining PARPi with various immune checkpoint blockades has been explored to evaluate the potential of combination therapy in attaining greater therapeutic outcome. This has shed light onto the differing classifications of PARPi as well as the factors that result in altered PARPi activity. Lastly, acquired chemoresistance is a crucial issue for clinical application of PARPi. The molecular mechanisms underlying PARPi resistance and potential overcoming strategies are discussed.

A Phase 1 dose-escalation study to evaluate safety, pharmacokinetics and pharmacodynamics of AsiDNA, a first-in-class DNA repair inhibitor, administered intravenously in patients with advanced solid tumours

Background

AsiDNA, a first-in-class oligonucleotide-mimicking double-stranded DNA breaks, acts as a decoy agonist to DNA damage response in tumour cells. It also activates DNA-dependent protein kinase and poly (adenosine diphosphate [ADP]-ribose) polymerase enzymes that induce phosphorylation of H2AX and protein PARylation.

Methods

The aim of this Phase 1 study was to determine dose-limiting toxicities (DLTs), maximum tolerated dose (MTD), safety and pharmacokinetics/pharmacodynamics of AsiDNA administered daily for 3 days in the first week then weekly thereafter. Twenty-two patients with advanced solid tumours were enrolled in 5 dose levels: 200, 400, 600, 900, and 1300 mg, using a 3 + 3 design.

Results

The MTD was not reached. IV AsiDNA was safe. Two DLTs (grade 4 and grade 3 hepatic enzymes increased at 900 and 1300 mg), and two related SAE at 900 mg (grade 3 hypotension and grade 4 hepatic enzymes increased) were reported. AsiDNA PK increased proportionally with dose. A robust activation of DNA-PK by a significant posttreatment increase of γH2AX was evidenced in tumour biopsies.

Conclusion

The dose of 600 mg was identified as the optimal dose for further clinical development.

Clinical trial registration

Clinical trial registration (NCT number): NCT03579628.

DNMT1: A key drug target in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer. Altered epigenetics regulation including DNA hypermethylation by DNA methyltransferase 1 (DNMT1) has been implicated as one of the causes of TNBC tumorigenesis. In this review, the oncogenic functions rendered by DNMT1 in TNBCs, and DNMT1 inhibitors targeting TNBC cells are presented and discussed. In summary, DNMT1 expression is associated with poor breast cancer survival, and it is overexpressed in TNBC subtype. The oncogenic roles of DNMT1 in TNBCs include: (1) Repression of estrogen receptor (ER) expression; (2) Promotion of epithelial-mesenchymal transition (EMT) required for metastasis; (3) Induces cellular autophagy and; (4) Promotes the growth of cancer stem cells in TNBCs. DNMT1 confers these phenotypes by hypermethylating the promoter regions of ER, multiple tumor suppressor genes, microRNAs and epithelial markers involved in suppressing EMT. DNMT1 inhibitors exert anti-tumorigenic effects against TNBC cells. This includes the hypomethylating agents azacitidine, decitabine and guadecitabine that might sensitize TNBC patients to immune checkpoint blockade therapy. DNMT1 represents an epigenetic target for TNBC cells destruction as well as to derail their metastatic and aggressive phenotypes.

BRCA Mutations in Pancreas Cancer: Spectrum, Current Management, Challenges and Future Prospects

Pancreatic ductal adenocarcinoma (PDAC) remains a challenging disease to treat. Despite advances in surgical techniques, radiation, and medical therapies, the 5-year survival rate remains below 9%. Over the past decade, the genomic landscape of PDAC has been well studied and BRCA mutations have emerged as a target for the development of more effective therapies. Alterations in germline BRCA and PALB2 are detected in approximately 5-9% of patients with PDAC and can lead to homologous repair deficiency (HRD). PDAC with HRD is more susceptible to cytotoxic agents, such as platinum salts and topoisomerase inhibitors, that cause DNA damage. Furthermore, PARP inhibitors have emerged as an effective non-cytotoxic approach to treating HRD-PDAC. In addition to BRCA and PALB2, germline mutations in other genes involved in the homologous DNA repair pathway - such as ATM and RAD51 - are potential targets, as are patients with the "BRCAness" phenotype and somatic mutations in the DNA repair pathway. Given the clinical implications of germline mutation related HRD in PDAC, universal germline testing is now recommended. In this review, we will discuss current and emerging biomarkers for HRD in PDAC, treatments, and the challenges associated with them.

Poly (ADP-ribose) polymerase inhibitors combined with other small-molecular compounds for the treatment of ovarian cancer

Ovarian cancer is a heterogeneous disease with complex molecular and genetic hallmarks. Benefitting from profound understanding of molecular mechanisms in ovarian cancer pathogenesis, novel targeted drugs have been actively explored in preclinical studies and clinical trials. Considered as one of the most potent and effective targeted therapies for the treatment of ovarian cancer, poly (ADP-ribose) polymerase (PARP) inhibitors (PARPis) take advantages of synthetic lethality mechanisms to prevent DNA damage repair in cancer cells and cause their death, especially in cancers with BRCA mutations. Mounting evidence has indicated that the combination of PARPis with cytotoxic drugs or other targeted drugs has shown favorable synergistic effects. Excitingly, the antitumor activity of combination therapy of PARPis has been actively tested in multiple clinical trials and in-vitro or in-vivo experiments. In this review, we will briefly discuss the molecular mechanisms of PARPis combined with other therapeutic small-molecular compounds for the treatment of ovarian cancer.

Computational analysis of data from a genome-wide screening identifies new PARP1 functional interactors as potential therapeutic targets

Knowledge of interaction network between different proteins can be a useful tool in cancer therapy. To develop new therapeutic treatments, understanding how these proteins contribute to dysregulated cellular pathways is an important task. PARP1 inhibitors are drugs used in cancer therapy, in particular where DNA repair is defective. It is crucial to find new candidate interactors of PARP1 as new therapeutic targets in order to increase efficacy of PARP1 inhibitors and expand their clinical utility. By a yeast-based genome wide screening, we previously discovered 90 candidate deletion genes that suppress growth-inhibition phenotype conferred by PARP1 in yeast. Here, we performed an integrated and computational analysis to deeply study these genes. First, we identified which pathways these genes are involved in and putative relations with PARP1 through g:Profiler. Then, we studied mutation pattern and their relation to cancer by interrogating COSMIC and DisGeNET database; finally, we evaluated expression and alteration in several cancers with cBioPortal, and the interaction network with GeneMANIA. We identified 12 genes belonging to PARP1-related pathways. We decided to further validate RIT1, INCENP and PSTA1 in MCF7 breast cancer cells. We found that RIT1 and INCENP affected PARylation and PARP1 protein level more significantly in PARP1 inhibited cells. Furthermore, downregulation of RIT1, INCENP and PSAT1 affected olaparib sensitivity of MCF7 cells. Our study identified candidate genes that could have an effect on PARP inhibition therapy. Moreover, we also confirm that yeast-based screenings could be very helpful to identify novel potential therapy factors.

An Effective Epigenetic-PARP Inhibitor Combination Therapy for Breast and Ovarian Cancers Independent of BRCA Mutations

Purpose: PARP inhibitors (PARPi) are primarily effective against BRCA1/2-mutated breast and ovarian cancers, but resistance due to reversion of mutated BRCA1/2 and other mechanisms is common. Based on previous reports demonstrating a functional role for DNMT1 in DNA repair and our previous studies demonstrating an ability of DNA methyltransferase inhibitor (DNMTi) to resensitize tumors to primary therapies, we hypothesized that combining a DNMTi with PARPi would sensitize PARPi-resistant breast and ovarian cancers to PARPi therapy, independent of BRCA status.Experimental Design: Breast and ovarian cancer cell lines (BRCA-wild-type/mutant) were treated with PARPi talazoparib and DNMTi guadecitabine. Effects on cell survival, ROS accumulation, and cAMP levels were examined. In vivo, mice bearing either BRCA-proficient breast or ovarian cancer cells were treated with talazoparib and guadecitabine, alone or in combination. Tumor progression, gene expression, and overall survival were analyzed.Results: Combination of guadecitabine and talazoparib synergized to enhance PARPi efficacy, irrespective of BRCA mutation status. Coadministration of guadecitabine with talazoparib increased accumulation of ROS, promoted PARP activation, and further sensitized, in a cAMP/PKA-dependent manner, breast and ovarian cancer cells to PARPi. In addition, DNMTi enhanced PARP "trapping" by talazoparib. Guadecitabine plus talazoparib decreased xenograft tumor growth and increased overall survival in BRCA-proficient high-grade serous ovarian and triple-negative breast cancer models.Conclusions: The novel combination of the next-generation DNMTi guadecitabine and the first-in-class PARPi talazoparib inhibited breast and ovarian cancers harboring either wild-type- or mutant-BRCA, supporting further clinical exploration of this drug combination in PARPi-resistant cancers. Clin Cancer Res; 24(13); 3163-75. ©2018 AACR.

Recapitulating the clinical scenario of BRCA-associated pancreatic cancer in pre-clinical models

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal malignancies. BRCA-associated PDAC comprises a clinically relevant subtype. A portion of these patients are highly susceptible to DNA damaging therapeutics, however, responses are heterogeneous and clinical resistance evolves. We have developed unique patient-derived xenograft (PDX) models from metastatic lesions of germline BRCA-mutated patients obtained at distinct time points; before treatment and at progression. Thus, closely mimicking clinical scenarios, to further investigate treatment naïve and resistant patients. DNA was isolated from six BRCA-mutated PDXs and classified by whole-genome sequencing to stable-genome or homologous recombination deficient (HRD)-genome. The sensitivity to DNA-damaging agents was evaluated in vivo in three BRCA-associated PDAC PDXs models: (1) HRD-genome naïve to treatments; (2) stable-genome naïve to treatment; (3) HRD-genome resistant to treatment. Correlation between disease course at tissue acquisition and response to PARP inhibitor (PARPi)/platinum was demonstrated in PDXs in vivo. Only the HRD-genome PDX, naïve to treatment, was sensitive to PARP inhibitor/cisplatin treatments. Our results demonstrate heterogeneous responses to DNA damaging agents/PARPi in BRCA-associated PDX thus reflecting the wide clinical spectrum. An HRD-genome PDX generated from a naïve to treatment biopsy was sensitive to platinum/PARPi whereas no benefit was observed in treating a HRD-genome PDXs generated from a patient that had acquired resistance nor stable-genome PDXs.

Micronuclei Frequency in Tumors Is a Predictive Biomarker for Genetic Instability and Sensitivity to the DNA Repair Inhibitor AsiDNA

Therapeutic strategies targeting DNA repair pathway defects have been widely explored, but often only benefit small numbers of patients. Here we characterized potential predictive biomarkers for treatment with AsiDNA, a novel first-in-class DNA repair inhibitor. We evaluated genetic instability and DNA repair defects by direct and indirect assays in 12 breast cancer cell lines to estimate the spontaneous occurrence of single-strand and double-strand breaks (DSB). For each cell line, we monitored constitutive PARP activation, spontaneous DNA damage by alkaline comet assay, basal micronuclei levels, the number of large-scale chromosomal rearrangements (LST), and the status of several DNA repair pathways by transcriptome and genome analysis. Sensitivity to AsiDNA was associated with a high spontaneous frequency of cells with micronuclei and LST and specific alterations in DNA repair pathways that essentially monitor DSB repair defects. A high basal level of micronuclei as a predictive biomarker for AsiDNA treatment was validated in 43 tumor cell lines from various tissues and 15 models of cell- and patient-derived xenografts. Micronuclei quantification was also possible in patient biopsies. Overall, this study identified genetic instability as a predictive biomarker for sensitivity to AsiDNA treatment. That micronuclei frequency can be measured in biopsies and does not reveal the same genetic instability as conventional genome assays opens new perspectives for refining the classification of tumors with genetic instability. Cancer Res; 77(16); 4207-16. ©2017 AACR.

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.

Expression of human poly (ADP-ribose) polymerase 1 in Saccharomyces cerevisiae: Effect on survival, homologous recombination and identification of genes involved in intracellular localization

The poly (ADP-ribose) polymerase 1 (PARP-1) actively participates in a series of functions within the cell that include: mitosis, intracellular signaling, cell cycle regulation, transcription and DNA damage repair. Therefore, inhibition of PARP1 has a great potential for use in cancer therapy. As resistance to PARP inhibitors is starting to be observed in patients, thus the function of PARP-1 needs to be studied in depth in order to find new therapeutic targets. To gain more information on the PARP-1 activity, we expressed PARP-1 in yeast and investigated its effect on cell growth and UV induced homologous recombination. To identify candidate genes affecting PARP-1 activity and cellular localization, we also developed a yeast genome wide genetic screen. We found that PARP-1 strongly inhibited yeast growth, but when yeast was exposed to the PARP-1 inhibitor 6(5-H) phenantridinone (PHE), it recovered from the growth suppression. Moreover, we showed that PARP-1 produced PAR products in yeast and we demonstrated that PARP-1 reduced UV-induced homologous recombination. By genome wide screening, we identified 99 mutants that suppressed PARP-1 growth inhibition. Orthologues of human genes were found for 41 of these yeast genes. We determined whether the PARP-1 protein level was altered in strains which are deleted for the transcription regulator GAL3, the histone H1 gene HHO1, the HUL4 gene, the deubiquitination enzyme gene OTU1, the nuclear pore protein POM152 and the SNT1 that encodes for the Set3C subunit of the histone deacetylase complex. In these strains the PARP-1 level was roughly the same as in the wild type. PARP-1 localized in the nucleus more in the snt1Δ than in the wild type strain; after UV radiation, PARP-1 localized in the nucleus more in hho1 and pom152 deletion strains than in the wild type indicating that these functions may have a role on regulating PARP-1 level and activity in the nucleus.

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.

Discussion of PARP inhibitors in cancer therapy

Poly (ADP-ribose) polymerases (PARP) are a family of enzymes that play a very important role in preserving the integrity of the genome. Recently, PARP inhibitors have been shown to enhance the therapeutic ratio in cancer patients due to their specific targeting of homologous recombination repair-defective tumors, through a synthetic lethal interaction. Researchers are also presently investigating novel strategies for the treatment of sporadic cancers by combining PARP inhibitors with other DNA-damaging agents. This review will focus on recently patented PARP inhibitors and literature that supports the reported claims presented in these patents. The patents reviewed were categorized into two groups: PARP inhibitors as a single-agent or in combination with other agents for the treatment of various types of cancer. These compounds are currently in clinical trials and, if successful, can greatly impact therapeutic index in cancer therapy.

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

Introduction

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

Methods

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

Results

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

Conclusions

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

The unpluggable in pursuit of the undruggable: tackling the dark matter of the cancer therapeutics universe

The notion that targeted drugs can unplug gain-of-function tumor pathways has revitalized pharmaceutical research, but the survival benefits of this strategy have so far proven modest. A weakness of oncogene-blocking approaches is that they do not address the problem of cancer progression as selected by the recessive phenotypes of genetic instability and apoptotic resistance which in turn arise from loss-of-function – i.e., undruggable – defects of caretaker (e.g., BRCA, MLH1) or gatekeeper (e.g., TP53, PTEN) suppressor genes. Genetic instability ensures that rapid cell kill is balanced by rapid selection for apoptotic resistance and hence for metastasis, casting doubt on the assumption that cytotoxicity (“response”) remains the best way to identify survival-enhancing drugs. In the absence of gene therapy, it is proposed here that caretaker-defective (high-instability) tumors may be best treated with low-lethality drugs inducing replicative (RAS-RAF-ERK) arrest or dormancy, causing “stable disease” rather than tumorilytic remission. Gatekeeper-defective (death-resistant) tumors, on the other hand, may be best managed by combining survival (PI3K-AKT-mTOR) pathway blockade with metronomic or sequential pro-apoptotic drugs.

Inhibition of DNA damage repair by artificial activation of PARP with siDNA

One of the major early steps of repair is the recruitment of repair proteins at the damage site, and this is coordinated by a cascade of modifications controlled by phosphatidylinositol 3-kinase-related kinases and/or poly (ADP-ribose) polymerase (PARP). We used short interfering DNA molecules mimicking double-strand breaks (called Dbait) or single-strand breaks (called Pbait) to promote DNA-dependent protein kinase (DNA-PK) and PARP activation. Dbait bound and induced both PARP and DNA-PK activities, whereas Pbait acts only on PARP. Therefore, comparative study of the two molecules allows analysis of the respective roles of the two signaling pathways: both recruit proteins involved in single-strand break repair (PARP, XRCC1 and PCNA) and prevent their recruitment at chromosomal damage. Dbait, but not Pbait, also inhibits recruitment of proteins involved in double-strand break repair (53BP1, NBS1, RAD51 and DNA-PK). By these ways, Pbait and Dbait disorganize DNA repair, thereby sensitizing cells to various treatments. Single-strand breaks repair inhibition depends on direct trapping of the main proteins on both molecules. Double-strand breaks repair inhibition may be indirect, resulting from the phosphorylation of double-strand breaks repair proteins and chromatin targets by activated DNA-PK. The DNA repair inhibition by both molecules is confirmed by their synthetic lethality with BRCA mutations.

Mechanisms and insights into drug resistance in cancer

Cancer drug resistance continues to be a major impediment in medical oncology. Clinically, resistance can arise prior to or as a result of cancer therapy. In this review, we discuss different mechanisms adapted by cancerous cells to resist treatment, including alteration in drug transport and metabolism, mutation and amplification of drug targets, as well as genetic rewiring which can lead to impaired apoptosis. Tumor heterogeneity may also contribute to resistance, where small subpopulations of cells may acquire or stochastically already possess some of the features enabling them to emerge under selective drug pressure. Making the problem even more challenging, some of these resistance pathways lead to multidrug resistance, generating an even more difficult clinical problem to overcome. We provide examples of these mechanisms and some insights into how understanding these processes can influence the next generation of cancer therapies.

Resistance to PARP-Inhibitors in Cancer Therapy

The pharmacological inhibitors of poly(ADP-ribose) polymerase (PARP) family of proteins have shown promising results in preclinical studies and clinical trials as a monotherapy or in combination therapy for some cancers. Thus, usage of PARP-inhibitors (PARPi) in cancer therapy is bound to increase with time, but resistance of cancer cells to PARPi is also beginning to be observed. Here we review different known and potential mechanisms by which: (i) PARPi kill cancer cells; and (ii) cancer cells develop resistance to PARPi. Understanding the lethality caused by PARPi and the countermeasures deployed by cancers cells to survive PARPi will help us rationalize the use of this new class of drugs in cancer therapy.

PARP inhibitors: mechanism of action and their potential role in the prevention and treatment of cancer

The use of poly(ADP-ribose) polymerase (PARP) inhibitors provided proof-of-concept for a synthetic lethal anti-cancer strategy as a result of their efficacy and favourable toxicity profile in BRCA1/2 mutation carriers. Efforts are underway to identify a broader group of patients with genomic susceptibility that may benefit from these agents. In an endeavour to enhance anti-tumour effects, PARP inhibitors have been combined with traditional cytotoxic therapy and radiotherapy; however, optimization of dosing schedules for these combination regimens remains key to maximizing benefit whilst mitigating the potential for increased toxicity. With ongoing clinical experience of PARP inhibition, mechanisms of resistance to these therapies are being elucidated and specific challenges to long-term administration of these drugs will need to be addressed. Development of robust predictive biomarkers of response for optimal patient selection and rational combination strategies must be pursued if the full potential of these agents is to be realized.

The NAD metabolome--a key determinant of cancer cell biology

NAD is a vital molecule in all organisms. It is a key component of both energy and signal transduction--processes that undergo crucial changes in cancer cells. NAD(+)-dependent signalling pathways are many and varied, and they regulate fundamental events such as transcription, DNA repair, cell cycle progression, apoptosis and metabolism. Many of these processes have been linked to cancer development. Given that NAD(+)-dependent signalling reactions involve the degradation of the molecule, permanent nucleotide resynthesis through different biosynthetic pathways is crucial for incessant cancer cell proliferation. This necessity supports the targeting of NAD metabolism as a new therapeutic concept for cancer treatment.

The role of PARP1 in the DNA damage response and its application in tumor therapy

Single-strand break repair protein poly(ADP-ribose) polymerase 1 (PARP1) catalyzes the poly(ADP-ribosyl)ation of many key proteins in vivo and thus plays important roles in multiple DNA damage response pathways, rendering it a promising target in cancer therapy. The tumor-suppressor effects of PARP inhibitors have attracted significant interest for development of novel cancer therapies. However, recent evidence indicated that the underlying mechanism of PARP inhibitors in tumor therapy is more complex than previously expected. The present review will focus on recent progress on the role of PARP1 in the DNA damage response and PARP inhibitors in cancer therapy. The emerging resistance of BRCA-deficient tumors to PARP inhibitors is also briefly discussed from the perspective of DNA damage and repair. These recent research advances will inform the selection of patient populations who can benefit from the PARP inhibitor treatment and development of effective drug combination strategies.

The diverse roles and clinical relevance of PARPs in DNA damage repair: current state of the art

Poly(ADP-ribose) polymerase (PARP) catalyzed poly(ADP-ribosyl)ation is one of the earliest post-translational modification of proteins detectable at sites of DNA strand interruptions. The considerable recent progress in the science of PARP in the last decade and the discovery of a PARP superfamily (17 members) has introduced this modification as a key mechanism regulating a wide variety of cellular processes including among others transcription, regulation of chromatin dynamics, telomere homeostasis, differentiation and cell death. However, the most extensive studied and probably the best characterized role is in DNA repair where it plays pivotal roles in the processing and resolution of the damaged DNA. Although much of the focus has been on PARP1 in DNA repair, recent advances highlight the emergence of other DNA-dependent PARPs (i.e. PARP2, PARP3 and possibly Tankyrase) in this process. Here we will summarize the recent insights into the molecular functions of these PARPs in different DNA repair pathways in which they emerge as specific actors. Furthermore, the DNA repair functions of PARP1 have stimulated another area of intense research in the field with the development of potent and selective PARP1 inhibitors to promote genome instability and cell death in tumor cells. Their current use in clinical trials have demonstrated potentiation of antitumoral drugs and cytotoxicity in repair deficient tumor cells.