Phase 1 dose-escalation study of the PARP inhibitor CEP-9722 as monotherapy or in combination with temozolomide in patients with solid tumors

Talazoparib for the Treatment of Metastatic Castration-resistant Prostate Cancer: A Narrative Review

Breast and prostate cancer are among the most commonly diagnosed cancers worldwide. Recent advances in tumor sequencing and gene studies have led to a paradigm shift from treatment centered on the type of tumor to therapy more focused on specific immune phenotype markers and molecular alterations. In this review, we discuss the utility and function of talazoparib concerning prostate cancer treatment and summarize recent and planned clinical trials on talazoparib. We searched medical databases for articles relating to the use of talazoparib in prostate cancer from inception. Poly ADP ribose polymerase (PARP) is a family of 17 necessary DNA repair enzymes responsible for base excision repair, single-strand break repair, and double-strand break repair. PARP inhibitors are a class of oral targeted therapies that compete for the NAD + binding site on PARP molecules. Talazoparib, a potent PARP inhibitor, has emerged as a significant therapeutic option in the treatment of metastatic castration-resistant prostate cancer (mCRPC), particularly for patients with specific genetic alterations. Its role as a PARP inhibitor makes it a targeted therapy, focusing on cancer cells with DNA repair deficiencies. Talazoparib's role as a biomarker-directed therapy in advanced prostate cancer has been increasingly recognized. The TALAPRO-1 demonstrated durable antitumor activity in mCRPC patients. TALAPRO-2 is a notable clinical trial, specifically examining the effectiveness of Talazoparib when used in combination therapies. Current investigations demonstrate a significant improvement in survival outcomes for the patients of mCRPC, making Talazoparib a promising intervention.

The combination therapy of isomucronulatol 7-O-beta-glucoside (IMG) and CEP-9722 targeting ferroptosis-related biomarkers in non-small cell lung cancer (NSCLC)

BACKGROUND

NSCLC is a malignant tumor with a high incidence. Ferroptosis presents an essential function in regulating carcinogenesis and tumor progression. However, the ferroptosis-associated prognostic model based on single-cell sequencing of NSCLC remains unexplored. Our study aims to establish a potential predictive model for NSCLC patients and provide available targeted drugs for clinical treatment.

METHODS

The data on NSCLC patients were collected from TCGA and GEO databases to analyze their gene expression profiles. ConsensusCluster was adopted to divide the patients into different groups based on ferroptosis-related genes. Then, the univariable Cox and LASSO analyses were applied to data analysis and model establishment. Single-cell analysis was used to explore the risk score genes in different cell populations and states. The protein levels of these genes were also investigated through the HPA database. Drug sensitivity was evaluated in CellMiner database. CCK8 and colony formation assays were performed to validate potential drugs' effects on lung cancer cell lines.

RESULTS

A ferroptosis-related prognostic model involving 14 genes in NSCLC patients was established. The risk score model was developed in training set GSE31210 and validated in the test set TCGA. The low-risk score group showed a better prognosis than the high-risk score group. The single-cell analysis revealed that the risk score genes were mainly derived from lung tumor cells. Most risk score genes were more highly expressed in tumor tissue than in normal tissue, according to the HPA database. Besides, these genes were associated with 106 drugs in CellMiner database. Finally, the drug effects on NSCLC cell growth were evaluated by cck8 and colony formation.

CONCLUSIONS

We identified an effective ferroptosis-related prognostic model based on single-cell sequencing. The potential prediction model is devoted to exploring clinical therapeutic targets for NSCLC.

Cellular and molecular basis of therapeutic approaches to breast cancer

In recent decades, there has been a significant amount of research into breast cancer, with some important breakthroughs in the treatment of both primary and metastatic breast cancers. It's a well-known fact that treating breast cancer is still a challenging endeavour even though physicians have a fantastic toolset of the latest treatment options at their disposal. Due to limitations of current clinical treatment options, traditional chemotherapeutic drugs, and surgical options are still required to address this condition. In recent years, there have been several developments resulting in a wide range of treatment options. This review article discusses the cellular and molecular foundation of chemotherapeutic drugs, endocrine system-based treatments, biological therapies, gene therapy, and innovative techniques for treating breast cancer.

Overcoming Temozolomide Resistance in Glioblastoma via Enhanced NAD+ Bioavailability and Inhibition of Poly-ADP-Ribose Glycohydrolase

Glioblastoma multiforme (GBM) is an incurable brain cancer with an average survival of approximately 15 months. Temozolomide (TMZ) is a DNA alkylating agent for the treatment of GBM. However, at least 50% of the patients treated with TMZ show poor response, primarily due to elevated expression of the repair protein O6-methylguanine-DNA methyltransferase (MGMT) or due to defects in the mismatch repair (MMR) pathway. These resistance mechanisms are either somatic or arise in response to treatment, highlighting the need to uncover treatments to overcome resistance. We found that administration of the NAD+ precursor dihydronicotinamide riboside (NRH) to raise cellular NAD+ levels combined with PARG inhibition (PARGi) triggers hyperaccumulation of poly(ADP-ribose) (PAR), resulting from both DNA damage-induced and replication-stress-induced PARP1 activation. Here, we show that the NRH/PARGi combination enhances the cytotoxicity of TMZ. Specifically, NRH rapidly increases NAD+ levels in both TMZ-sensitive and TMZ-resistant GBM-derived cells and enhances the accumulation of PAR following TMZ treatment. Furthermore, NRH promotes hyperaccumulation of PAR in the presence of TMZ and PARGi. This combination strongly suppresses the cell growth of GBM cells depleted of MSH6 or cells expressing MGMT, suggesting that this regimen may improve the efficacy of TMZ to overcome treatment resistance in GBM.

Perspective on the Use of DNA Repair Inhibitors as a Tool for Imaging and Radionuclide Therapy of Glioblastoma

Simple Summary

The current routine treatment for glioblastoma (GB), the most lethal high-grade brain tumor in adults, aims to induce DNA damage in the tumor. However, the tumor cells might be able to repair that damage, which leads to therapy resistance. Fortunately, DNA repair defects are common in GB cells, and their survival is often based on a sole backup repair pathway. Hence, targeted drugs inhibiting essential proteins of the DNA damage response have gained momentum and are being introduced in the clinic. This review gives a perspective on the use of radiopharmaceuticals targeting DDR kinases for imaging in order to determine the DNA repair phenotype of GB, as well as for effective radionuclide therapy. Finally, four new promising radiopharmaceuticals are suggested with the potential to lead to a more personalized GB therapy.

Abstract

Despite numerous innovative treatment strategies, the treatment of glioblastoma (GB) remains challenging. With the current state-of-the-art therapy, most GB patients succumb after about a year. In the evolution of personalized medicine, targeted radionuclide therapy (TRT) is gaining momentum, for example, to stratify patients based on specific biomarkers. One of these biomarkers is deficiencies in DNA damage repair (DDR), which give rise to genomic instability and cancer initiation. However, these deficiencies also provide targets to specifically kill cancer cells following the synthetic lethality principle. This led to the increased interest in targeted drugs that inhibit essential DDR kinases (DDRi), of which multiple are undergoing clinical validation. In this review, the current status of DDRi for the treatment of GB is given for selected targets: ATM/ATR, CHK1/2, DNA-PK, and PARP. Furthermore, this review provides a perspective on the use of radiopharmaceuticals targeting these DDR kinases to (1) evaluate the DNA repair phenotype of GB before treatment decisions are made and (2) induce DNA damage via TRT. Finally, by applying in-house selection criteria and analyzing the structural characteristics of the DDRi, four drugs with the potential to become new therapeutic GB radiopharmaceuticals are suggested.

Studies Towards Hypoxia-Activated Prodrugs of PARP Inhibitors

Poly(ADP-ribose)polymerase (PARP) inhibitors (PARPi) have recently been approved for the treatment of breast and ovarian tumors with defects in homologous recombination repair (HRR). Although it has been demonstrated that PARPi also sensitize HRR competent tumors to cytotoxic chemotherapies or radiotherapy, normal cell toxicity has remained an obstacle to their use in this context. Hypoxia-activated prodrugs (HAPs) provide a means to limit exposure of normal cells to active drug, thus adding a layer of tumor selectivity. We have investigated potential HAPs of model PARPi in which we attach a bioreducible “trigger” to the amide nitrogen, thereby blocking key binding interactions. A representative example showed promise in abrogating PARPi enzymatic activity in a biochemical assay, with a ca. 160-fold higher potency of benzyl phthalazinone 4 than the corresponding model HAP 5, but these N-alkylated compounds did not release the PARPi upon one-electron reduction by radiolysis. Therefore, we extended our investigation to include NU1025, a PARPi that contains a phenol distal to the core binding motif. The resulting 2-nitroimidazolyl ether provided modest abrogation of PARPi activity with a ca. seven-fold decrease in potency, but released the PARPi efficiently upon reduction. This investigation of potential prodrug approaches for PARPi has identified a useful prodrug strategy for future exploration.

Clinically Applicable Inhibitors Impacting Genome Stability

Advances in technology have facilitated the molecular profiling (genomic and transcriptomic) of tumours, and has led to improved stratification of patients and the individualisation of treatment regimes. To fully realize the potential of truly personalised treatment options, we need targeted therapies that precisely disrupt the compensatory pathways identified by profiling which allow tumours to survive or gain resistance to treatments. Here, we discuss recent advances in novel therapies that impact the genome (chromosomes and chromatin), pathways targeted and the stage of the pathways targeted. The current state of research will be discussed, with a focus on compounds that have advanced into trials (clinical and pre-clinical). We will discuss inhibitors of specific DNA damage responses and other genome stability pathways, including those in development, which are likely to synergistically combine with current therapeutic options. Tumour profiling data, combined with the knowledge of new treatments that affect the regulation of essential tumour signalling pathways, is revealing fundamental insights into cancer progression and resistance mechanisms. This is the forefront of the next evolution of advanced oncology medicine that will ultimately lead to improved survival and may, one day, result in many cancers becoming chronic conditions, rather than fatal diseases.

Targeting DNA Repair

Genomic instability is a characteristic of most human cancers and plays critical roles in both cancer development and progression. There are various forms of genomic instability arising from many different pathways, such as DNA damage from endogenous and exogenous sources, centrosome amplification, telomere damage, and epigenetic modifications. DNA-repair pathways can enable tumor cells to survive DNA damage. The failure to respond to DNA damage is a characteristic associated with genomic instability. Understanding of genomic instability in cancer is still very limited, but the further understanding of the molecular mechanisms through which the DNA damage response (DDR) operates, in combination with the elucidation of the genetic interactions between DDR pathways and other cell pathways, will provide therapeutic opportunities for the personalized medicine of cancer.

Management of QT prolongation induced by anti-cancer drugs: Target therapy and old agents. Different algorithms for different drugs

The side effects of anticancer drugs still play a critical role in survival and quality of life. Although the recent progresses of cancer therapies have significantly improved the prognosis of oncologic patients, side effects of antineoplastic treatments are still responsible for the increased mortality of cancer survivors. Cardiovascular toxicity is the most dangerous adverse effect induced by anticancer therapies. A survey conducted by the National Health and Nutrition Examination, showed that 1807 cancer survivors followed up for seven years: 51% died of cancer and 33% of heart disease (Vejpongsa and Yeh, 2014). Moreover, the risk of cardiotoxicity persists even with the targeted therapy, the newer type of cancer treatment, due to the presence of on-target and off-target effects related to this new class of drugs. The potential cardiovascular toxicity of anticancer agents includes: QT prolongation, arrhythmias, myocardial ischemia, stroke, hypertension (HTN), thromboembolism, left ventricular dysfunction and heart failure (HF). Compared to other cardiovascular disorders, the interest in QT prolongation and its complications is fairly recent. However, oncologists have to deal with it and to evaluate the risk-benefit ratio before starting the treatment or during the same. Electrolyte abnormalities, low levels of serum potassium and several drugs may favour the acquired QT prolongation. Treatment of marked QT prolongation includes cardiac monitoring, caution in the use or suspension of cancer drugs and correction of electrolyte abnormalities (hypokalaemia, hypomagnesaemia, hypocalcaemia). Syndrome of QT prolongation can be associated with potentially fatal cardiac arrhythmias and its treatment consists of intravenous administration of magnesium sulphate and the use of electrical cardioversion.

Clinical Advances of Hypoxia-Activated Prodrugs in Combination With Radiation Therapy

With the increasing incidence of cancer worldwide, the need for specific, effective therapies is ever more urgent. One example of targeted cancer therapeutics is hypoxia-activated prodrugs (HAPs), also known as bioreductive prodrugs. These prodrugs are inactive in cells with normal oxygen levels but in hypoxic cells (with low oxygen levels) undergo chemical reduction to the active compound. Hypoxia is a common feature of solid tumors and is associated with a more aggressive phenotype and resistance to all modes of therapy. Therefore, the combination of radiation therapy and bioreductive drugs presents an attractive opportunity for synergistic effects, because the HAP targets the radiation-resistant hypoxic cells. Hypoxia-activated prodrugs have typically been precursors of DNA-damaging agents, but a new generation of molecularly targeted HAPs is emerging. By targeting proteins associated with tumorigenesis and survival, these compounds may result in greater selectivity over healthy tissue. We review the clinical progress of HAPs as adjuncts to radiation therapy and conclude that the use of HAPs alongside radiation is vastly underexplored at the clinical level.

PARP inhibitors as precision medicine for cancer treatment

Personalized or precision medicine is an emerging treatment approach tailored to individuals or certain groups of patients based on their unique characteristics. These types of therapies guided by biomarkers tend to be more effective than traditional approaches, especially in cancer. The inhibitor against poly (ADP-ribose) polymerase (PARP), olaparib (Lynparza, AstraZeneca), which was approved by the US Food and Drug Administration (FDA) in 2014, demonstrated efficacy specifically for ovarian cancer patients harboring mutations in BRCA genes, which encode proteins in DNA double-strand break repairs. However, the response to PARP inhibitors has been less encouraging in other cancer types that also carry defects in the BRCA genes. Thus, furthering our understanding of the underlying mechanism of PARP inhibitors and resistance is critical to improve their efficacy. In this review, we summarize the results of preclinical studies and the clinical application of PARP inhibitors, and discuss the future direction of PARP inhibitors as a potential marker-guided personalized medicine for cancer treatment.

A phase I study of intravenous and oral rucaparib in combination with chemotherapy in patients with advanced solid tumours

BACKGROUND

This study evaluated safety, pharmacokinetics, and clinical activity of intravenous and oral rucaparib, a poly(ADP-ribose) polymerase inhibitor, combined with chemotherapy in patients with advanced solid tumours.

METHODS

Initially, patients received escalating doses of intravenous rucaparib combined with carboplatin, carboplatin/paclitaxel, cisplatin/pemetrexed, or epirubicin/cyclophosphamide. Subsequently, the study was amended to focus on oral rucaparib (once daily, days 1-14) combined with carboplatin (day 1) in 21-day cycles. Dose-limiting toxicities (DLTs) were assessed in cycle 1 and safety in all cycles.

RESULTS

Eighty-five patients were enrolled (22 breast, 15 ovarian/peritoneal, and 48 other primary cancers), with a median of three prior therapies (range, 1-7). Neutropenia (27.1%) and thrombocytopenia (18.8%) were the most common grade ⩾3 toxicities across combinations and were DLTs with the oral rucaparib/carboplatin combination. Maximum tolerated dose for the combination was 240 mg per day oral rucaparib and carboplatin area under the curve 5 mg ml-1 min-1. Oral rucaparib demonstrated dose-proportional kinetics, a long half-life (≈17 h), and good bioavailability (36%). Pharmacokinetics were unchanged by carboplatin coadministration. The rucaparib/carboplatin combination had radiologic antitumour activity, primarily in BRCA1- or BRCA2-mutated breast and ovarian/peritoneal cancers.

CONCLUSIONS

Oral rucaparib can be safely combined with a clinically relevant dose of carboplatin in patients with advanced solid tumours (Trial registration ID: NCT01009190).

Effectiveness and safety of poly (ADP-ribose) polymerase inhibitors in cancer therapy: A systematic review and meta-analysis

Poly (ADP-ribose) polymerase (PARP) inhibitors are a class of small-molecule drugs suppressing PARP enzymes activity, inducing the death of cells deficient in homologous recombination repair (HRR). HRR deficiency is common in tumor cells with BRCA gene mutation. Since their first clinical trial in 2003, PARP inhibitors have shown benefit in the treatment of HRR-deficient tumors. Recently, several randomized clinical trials (RCTs) have been conducted to investigate the potential benefit of administration of PARP inhibitors in cancer patients. However, the results remain controversial. To evaluate the efficiency and safety of PARP inhibitors in patients with cancer, we performed a comprehensive meta-analysis of RCTs. According to our study, PARP inhibitors could clearly improve progression-free survival (PFS), especially in patients with BRCA mutation. However, our study showed no significant difference in overall survival (OS) between the PARP inhibitors and controls, even in the BRCA mutation group. Little toxicity was reported in the rate of treatment correlated adverse events (AEs) in PARP inhibitor group compared with controls. In conclusion, PARP inhibitors do well in improving PFS with little toxicity, especially in patients with BRCA deficiency.

Cellular responses to replication stress: Implications in cancer biology and therapy

DNA replication is essential for cell proliferation. Any obstacles during replication cause replication stress, which may lead to genomic instability and cancer formation. In this review, we summarize the physiological DNA replication process and the normal cellular response to replication stress. We also outline specialized therapies in clinical trials based on current knowledge and future perspectives in the field.

PARP inhibitors as antitumor agents: a patent update (2013-2015)

INTRODUCTION

PARP inhibitors have been extensively explored as antitumor agents and have shown potent efficacy both in vitro and in vivo. They can be used in monotherapy under the synthetic lethality concept or in combination with radiotherapy or chemotherapy, inducing a synergistic effect. Areas covered: This review covers relevant efforts in the development of PARP inhibitors with a particular focus on recently patented PARP inhibitors, combination therapy involving PARP inhibitors, tumor responsiveness to PARP inhibitors as detailed in reports made from 2013 - 2015, and PARP drugs in clinical trials and other novel inhibitors that emerged in 2013 - 2015. Expert opinion: Clinical studies and applications of PARP inhibitors as antitumor agents have gained considerable recognition in the last few years. In addition to FDA-approved olaparib, an increasing number of new inhibitors have been designed and synthesized, some of which are under preclinical or clinical evaluation. Novel inhibitors are still required, especially new scaffold compounds or drugs with improved properties, such as higher selectivity, higher potency and lower toxicity. The development of combination therapies involving PARP inhibitors and the exploration of biomarkers to predict outcomes with PARP inhibitors would expand the applications of these inhibitors, allowing more patients to benefit from this promising class of drugs in the future.

An open-label, dose-escalation study to evaluate the safety and pharmacokinetics of CEP-9722 (a PARP-1 and PARP-2 inhibitor) in combination with gemcitabine and cisplatin in patients with advanced solid tumors

Poly (ADP-ribose) polymerase-1 (PARP-1) inhibitors may potentiate chemotherapy by hindering DNA damage repair pathways. CEP-9722 is the prodrug of CEP-8983, a selective inhibitor of PARP-1 and PARP-2. Preclinical studies and a prior phase 1 study suggested that CEP-9722 may cause less myelosuppression than has been observed with other oral PARP inhibitors. The primary objective of this study was to determine the maximum-tolerated dose of CEP-9722 in combination with gemcitabine and cisplatin in patients with advanced solid tumors. All patients received cisplatin 75 mg/m(2) on day 1 and gemcitabine 1250 mg/m(2) on days 1 and 8 of a 21-day cycle. Patients who completed one cycle of chemotherapy alone continued chemotherapy in combination with CEP-9722 150, 200, 300, or 400 mg orally twice daily on days 2-7, with dose-limiting toxicity assessed in cycle 2. Patients experiencing clinical benefit could continue treatment until disease progression or unacceptable toxicity. Thirty-two patients enrolled; 18 patients completed cycle 1 and received chemotherapy plus CEP-9722. The median (range) treatment administration with CEP-9722 was five (1-12) cycles. No patient experienced dose-limiting toxicity with CEP-9722 treatment. Grade 3/4 hematologic adverse events included neutropenia (28%) and leukopenia (11%); adverse events led to discontinuation in 33% of patients. One patient achieved complete response, three had partial responses, and 11 had stable disease; however, the relative contribution of CEP-9722 and/or the chemotherapeutic agents cannot be determined from this single-arm design. This study was discontinued before determination of the maximum-tolerated dose because of highly variable CEP-8983 exposure in all cohorts and toxicity, particularly chemotherapy-induced myelosuppression.

An Unbiased Oncology Compound Screen to Identify Novel Combination Strategies

Combination drug therapy is a widely used paradigm for managing numerous human malignancies. In cancer treatment, additive and/or synergistic drug combinations can convert weakly efficacious monotherapies into regimens that produce robust antitumor activity. This can be explained in part through pathway interdependencies that are critical for cancer cell proliferation and survival. However, identification of the various interdependencies is difficult due to the complex molecular circuitry that underlies tumor development and progression. Here, we present a high-throughput platform that allows for an unbiased identification of synergistic and efficacious drug combinations. In a screen of 22,737 experiments of 583 doublet combinations in 39 diverse cancer cell lines using a 4 by 4 dosing regimen, both well-known and novel synergistic and efficacious combinations were identified. Here, we present an example of one such novel combination, a Wee1 inhibitor (AZD1775) and an mTOR inhibitor (ridaforolimus), and demonstrate that the combination potently and synergistically inhibits cancer cell growth in vitro and in vivo This approach has identified novel combinations that would be difficult to reliably predict based purely on our current understanding of cancer cell biology. Mol Cancer Ther; 15(6); 1155-62. ©2016 AACR.

Role of Biomarkers in the Development of PARP Inhibitors

Defects in DNA repair lead to genomic instability and play a critical role in cancer development. Understanding the process by which DNA damage repair is altered or bypassed in cancer may identify novel therapeutic targets and lead to improved patient outcomes. Poly(adenosine diphosphate-ribose) polymerase 1 (PARP1) has an important role in DNA repair, and novel therapeutics targeting PARP1 have been developed to treat cancers with defective DNA repair pathways. Despite treatment successes with PARP inhibitors (PARPi), intrinsic and acquired resistances have been observed. Preclinical studies and clinical trials in cancer suggest that combination therapy using PARPi and platinating agents is more effective than monotherapy in circumventing drug resistance mechanisms. Additionally, identification of biomarkers in response to PARPi will lead to improved patient selection for targeted cancer treatment. Recent technological advances have provided the necessary tools to examine many potential avenues to develop such biomarkers. This review examines the mechanistic rationale of PARP inhibition and potential biomarkers in their development for personalized therapy.

PARP inhibitors in ovarian cancer: Clinical evidence for informed treatment decisions

Ovarian cancer is the fifth leading cause of female cancer deaths in the Western world. Significant progress has been made in the treatment of patients with ovarian cancer, however, the majority of patients experience disease recurrence and new therapies are being sought for such patients. Clinical investigation of poly(ADP-ribose) polymerase (PARP) inhibitors for ovarian cancer treatment has demonstrated promising activity in this disease. Here, we review the development of PARP inhibitors and their future role in the treatment of patients with ovarian cancer. Studies of olaparib, the first PARP inhibitor to be approved in Europe and the USA, in patients with recurrent ovarian cancer have demonstrated clinical efficacy with improvements in progression-free survival. In maintenance therapy of platinum-sensitive ovarian cancer there is supporting evidence of clinical benefit from exploratory endpoints that include time to first subsequent treatment and time to second subsequent treatment. Adverse events that should be monitored following treatment with PARP inhibitors include nausea, vomiting, fatigue and anaemia. Based on the evidence presented, patients who will receive the greatest benefit from PARP inhibition are those with platinum-sensitive relapsed ovarian cancer and a BRCA mutation.

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.

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.

PARP inhibitors: A new era of targeted therapy

Personalized medicine seeks to utilize targeted therapies with increased selectivity and efficacy in preselected patient cohorts. One such molecularly targeted therapy is enabled by inhibiting the enzyme poly(ADP-ribose) polymerase (PARP) by small molecule inhibitors in tumors which have a defect in the homologous DNA recombination pathway, most characteristically due to BRCA mutations. Olaparib, a highly potent PARP inhibitor, has recently been the approved for ovarian cancer therapy by the FDA and European commission in patients with platinum-sensitive, recurrent, high-grade serous ovarian cancer with BRCA1 or BRCA2 mutations. Currently, clinical trials with several PARP inhibitors are being conducted to assess the toxicities, the efficacies and the benefit of the drugs as monotherapies or combined with radiation or other chemotherapeutic agents, in ovarian, breast, prostate, rectal, lung, pancreatic, peritoneal, head and neck, brain, squamous cell carcinomas and sarcomas, to list a few. In this review, our focus is to outline the emerging molecular mechanisms, preclinical evidence and clinical applications of PARP inhibitors especially in nonBRCA cancers, and review the combination strategies compatible with PARP inhibitor therapy.