Role of PARP Inhibitors in Glioblastoma and Perceiving Challenges as Well as Strategies for Successful Clinical Development

Glioblastoma multiform is the most aggressive primary type of brain tumor, representing 54% of all gliomas. The average life span for glioblastoma multiform is around 14-15 months instead of treatment. The current treatment for glioblastoma multiform includes surgical removal of the tumor followed by radiation therapy and temozolomide chemotherapy for 6.5 months, followed by another 6 months of maintenance therapy with temozolomide chemotherapy (5 days every month). However, resistance to temozolomide is frequently one of the limiting factors in effective treatment. Poly (ADP-ribose) polymerase (PARP) inhibitors have recently been investigated as sensitizing drugs to enhance temozolomide potency. However, clinical use of PARP inhibitors in glioblastoma multiform is difficult due to a number of factors such as limited blood-brain barrier penetration of PARP inhibitors, inducing resistance due to frequent use of PARP inhibitors, and overlapping hematologic toxicities of PARP inhibitors when co-administered with glioblastoma multiform standard treatment (radiation therapy and temozolomide). This review elucidates the role of PARP inhibitors in temozolomide resistance, multiple factors that make development of these PARP inhibitor drugs challenging, and the strategies such as the development of targeted drug therapies and combination therapy to combat the resistance of PARP inhibitors that can be adopted to overcome these challenges.

Promise and limits of the CellSearch platform for evaluating pharmacodynamics in circulating tumor cells

Circulating tumor cells (CTCs), which are captured from blood with anti-epithelial cell adhesion molecule (EpCAM) antibodies, have established prognostic value in specific epithelial cancers, but less is known about their utility for assessing patient response to molecularly targeted agents via measurement of pharmacodynamic (PD) endpoints. We discuss the use of CellSearch (Janssen Diagnostics, LLC, Raritan, NJ) CTC isolation technology for monitoring PD response in early phase trials. We present representative data from three clinical trials with the poly(ADP-ribose) polymerase (PARP) inhibitor veliparib (ABT-888) suggesting that CTCs can be used to measure PD effects. However, while often leading to hypothesis-generating information, our experience points to the difficulty in obtaining sufficient EpCAM-expressing CTCs from patients with advanced disease to reach statistically significant conclusions about PD effects from each trial. Overall, the level of phenotypic heterogeneity observed in specimens from patients with advanced carcinomas suggests caution in the use of cell-surface differentiation marker-based methods for isolating CTCs.

Translating pharmacodynamic biomarkers from bench to bedside: analytical validation and fit-for-purpose studies to qualify multiplex immunofluorescent assays for use on clinical core biopsy specimens

Multiplex pharmacodynamic (PD) assays have the potential to increase sensitivity of biomarker-based reporting for new targeted agents, as well as revealing significantly more information about target and pathway activation than single-biomarker PD assays. Stringent methodology is required to ensure reliable and reproducible results. Common to all PD assays is the importance of reagent validation, assay and instrument calibration, and the determination of suitable response calibrators; however, multiplex assays, particularly those performed on paraffin specimens from tissue blocks, bring format-specific challenges adding a layer of complexity to assay development. We discuss existing multiplex approaches and the development of a multiplex immunofluorescence assay measuring DNA damage and DNA repair enzymes in response to anti-cancer therapeutics and describe how our novel method addresses known issues.

Randomized phase II trial of cyclophosphamide and the oral poly (ADP-ribose) polymerase inhibitor veliparib in patients with recurrent, advanced triple-negative breast cancer

Background In tumors carrying BRCA mutations, DNA damage caused by standard cytotoxic chemotherapy can be potentiated by poly [ADP-ribose] polymerase (PARP) inhibitors, leading to increased cell death through synthetic lethality. Individuals carrying mutations in BRCA have an increased incidence of triple negative breast cancer (TNBC). In order to assess the role of PARP inhibition in the treatment of TNBC, we conducted a randomized phase II trial of the combination of veliparib, a small molecule PARP inhibitor, with the cytotoxic agent cyclophosphamide versus cyclophosphamide alone in patients with refractory TNBC. Methods Adult patients with TNBC were randomized to receive oral cyclophosphamide 50 mg once daily with or without oral veliparib at 60 mg daily in 21-day cycles. Patients on the cyclophosphamide arm could crossover to the combination arm at disease progression. Results Forty-five patients were enrolled; 18 received cyclophosphamide alone and 21 received the combination as their initial treatment regimen. Lymphopenia was the most common grade 3/4 toxicity noted in both arms. One patient in the cyclophosphamide alone arm, and 2 in the combination arm had objective responses. Response rates and median progression free survival did not significantly differ between both treatment arms. Conclusion The addition of veliparib to cyclophosphamide, at the dose and schedule evaluated, did not improve the response rate over cyclophosphamide treatment alone in patients with heavily pre-treated triple-negative breast cancer.

Randomized Trial of Oral Cyclophosphamide and Veliparib in High-Grade Serous Ovarian, Primary Peritoneal, or Fallopian Tube Cancers, or BRCA-Mutant Ovarian Cancer

PURPOSE

Veliparib, a PARP inhibitor, demonstrated clinical activity in combination with oral cyclophosphamide in patients with BRCA-mutant solid tumors in a phase I trial. To define the relative contribution of PARP inhibition to the observed clinical activity, we conducted a randomized phase II trial to determine the response rate of veliparib in combination with cyclophosphamide compared with cyclophosphamide alone in patients with pretreated BRCA-mutant ovarian cancer or in patients with pretreated primary peritoneal, fallopian tube, or high-grade serous ovarian cancers (HGSOC).

EXPERIMENTAL DESIGN

Adult patients were randomized to receive cyclophosphamide alone (50 mg orally once daily) or with veliparib (60 mg orally once daily) in 21-day cycles. Crossover to the combination was allowed at disease progression.

RESULTS

Seventy-five patients were enrolled and 72 were evaluable for response; 38 received cyclophosphamide alone and 37 the combination as their initial treatment regimen. Treatment was well tolerated. One complete response was observed in each arm, with three partial responses (PR) in the combination arm and six PRs in the cyclophosphamide alone arm. Genetic sequence and expression analyses were performed for 211 genes involved in DNA repair; none of the detected genetic alterations were significantly associated with treatment benefit.

CONCLUSION

This is the first trial that evaluated single-agent, low-dose cyclophosphamide in HGSOC, peritoneal, fallopian tube, and BRCA-mutant ovarian cancers. It was well tolerated and clinical activity was observed; the addition of veliparib at 60 mg daily did not improve either the response rate or the median progression-free survival.

Preferential potentiation of topoisomerase I poison cytotoxicity by PARP inhibition in S phase

BACKGROUND

Topoisomerase I (Topo I) poisons (e.g., camptothecin (CPT)), used to treat cancer, cause DNA breaks that are most cytotoxic during S phase. PARP-1 promotes DNA repair and PARP inhibitors (PARPi) sensitise cells to Topo I poisons. We aimed to determine whether chemosensitisation is also S phase specific using rucaparib, a potent PARPi in advanced clinical evaluation.

METHODS

The impact of rucaparib, on CPT-induced cytotoxicity was measured in human colon cancer (LoVo) and leukaemic (K562) cells in asynchronous and cell cycle phase-separated cultures. Topoisomerase I and PARP levels and activity and the effect of rucaparib on DNA single-strand breaks (SSBs), double-strand breaks (DSBs) and collapsed replication fork induction and repair were determined in cell cycle phase-separated cells.

RESULTS

The cytotoxicity of CPT was greatest during S phase, partially attributable to high Topo I activity, and rucaparib preferentially sensitised S-phase cells. Rucaparib increased CPT-induced DNA SSBs in all phases of the cell cycle, and increased DSB and γH2AX foci in S and G2, with γH2AX foci being highest in S-phase cells. Repair of SSBs and DSBs was most rapid during S then G2 phases and was substantially hindered by rucaparib.

CONCLUSIONS

Rucaparib preferentially sensitises S-phase cells by increasing the frequency of collapsed replication forks.

Dynamic regulation of Rad51 by E2F1 and p53 in prostate cancer cells upon drug-induced DNA damage under hypoxia

Intratumoral hypoxia has been proposed to create a "mutator" phenotype through downregulation of DNA repair, leading to increased genomic instability and drug resistance. Such downregulation of DNA repair has been proposed to sensitize hypoxic cancer cells to DNA-damaging agents and inhibitors of DNA repair. Here, we showed that prostate cancer cells with mutant p53 were resistant to the poly(ADP-ribose) polymerase inhibitor, veliparib (2-[(2R)-2-methylpyrrolidin-2-yl]-1H-benzimidazole-4-carboxamide, dihydrochloride; Abbott Laboratories, Abbott Park, IL), and the DNA-damaging topoisomerase I inhibitor camptothecin-11 (CPT-11) or SN38 (7-ethyl-10-hydroxycamptothecin) under hypoxia. Upregulation of Rad51 by E2F1 upon DNA damage under hypoxia contributed to such resistance, which was reversed by either inhibiting RAD51 transcription with small interfering RNA or by expressing wild-type p53 in the p53 null prostate cancer line. Accumulation of endogenous p53 but not E2F1 and suppressed RAD51 transcription was observed in prostate cancer line with wild-type p53 after DNA damage under hypoxia. Combining veliparib with CPT-11 significantly enhanced DNA damage and apoptosis under both hypoxic and normoxic culture conditions. Such enhanced DNA damage and antitumor activities were seen in the presence of Rad51 upregulation and confirmed in vivo with PC3 mouse xenografts. These data illustrate a dynamic regulation of Rad51 by E2F1 and p53 in prostate cancer cells' response to hypoxia and DNA damage. The veliparib and CPT-11 combination can be further explored as a treatment of metastatic castration-resistant prostate cancers that have frequent p53 mutations and enriched genomic instability.

PARP inhibitors in ovarian cancer: current status and future promise

Clinical investigation of poly(ADP-ribose) polymerase (PARP) inhibitors for ovarian cancer treatment has rapidly evolved from observations of single-agent in vitro activity of these agents in BRCA-deficient cancer cells in 2005 to the initiation of multiple phase III studies in 2013. With clinical trial design and treatment of ovarian cancer increasingly based on histological and molecular characteristics, PARP inhibitors are on the horizon of becoming the first biologic agents to be used to treat ovarian cancer based upon pre-selection characteristics of the patient's cancer. PARP inhibitors are most active in ovarian cancers that have defects or aberrations in DNA repair; use of these agents has been of particular interest in high grade serous cancers (HGSC), where studies have shown that ~50% of HGSC have abnormalities of DNA repair through BRCA germline and somatic mutation, post-translational changes of BRCA, and abnormalities of other DNA repair molecules. In addition, as aberrant DNA pathways in other histological subtypes of ovarian cancer are identified, and through the combination of PARP inhibitors with other biologic agents, the pool of eligible patients who may benefit from PARP inhibitors will likely expand. Pending review by the Food and Drug Administration (FDA) and the outcome of confirmatory phase III studies, PARP inhibitors could become the first FDA-approved biologic agent for ovarian cancer and also the first new FDA-approval in ovarian cancer since carboplatin and gemcitabine were approved for platinum sensitive ovarian cancer in 2006. This review discusses the PARP inhibitors that are currently in testing for ovarian cancer treatment and the future of this class of anti-cancer agents.

Inhibiting the DNA damage response as a therapeutic manoeuvre in cancer

The DNA damage response (DDR), consisting of an orchestrated network of proteins effecting repair and signalling to cell cycle arrest, to allow time to repair, is essential for cell viability and to prevent DNA damage being passed on to daughter cells. The DDR is dysregulated in cancer with some pathways up-regulated and others down-regulated or lost. Up-regulated pathways can confer resistance to anti-cancer DNA damaging agents. Therefore, inhibitors of key components of these pathways have the potential to prevent this therapeutic resistance. Conversely, defects in a particular DDR pathway may lead to dependence on a complementary pathway. Inhibition of this complementary pathway may result in tumour-specific cell killing. Thus, inhibitors of the DDR have the potential to increase the efficacy of DNA damaging chemotherapy and radiotherapy and have single-agent activity against tumours with a specific DDR defect. This review describes the compounds that have been designed to inhibit specific DDR targets and summarizes the pre-clinical and clinical evaluation of these inhibitors of DNA damage signalling and repair.

LINKED ARTICLES

This article is part of a themed section on Emerging Therapeutic Aspects in Oncology. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.169.issue-8.

PARP inhibitors for anticancer therapy

PARP-1 [poly(ADP-ribose) polymerase-1], which plays a key role in DNA repair, was discovered 50 years ago. PARPi (PARP inhibitors), originally made to probe the function of the enzyme, inhibit DNA repair and increase the potency of anticancer cytotoxic agents. PARPi of increasing potency were developed as chemo- and radio-sensitizers and first entered clinical trial in cancer patients in 2003. However, it was the revelation in 2005 that they were synthetically lethal in cells with DNA repair defects, found almost exclusively in some tumours, that led to a major interest in this class of drug. Several PARPi have entered clinical trials and show promising activity in breast, ovarian and other cancers associated with BRCA (breast cancer early-onset) mutations or other defects in homologous recombination DNA repair. It is likely that at least one of these will be licensed soon. The present review describes key events from the discovery to clinical application of PARPi.

Therapeutic applications of PARP inhibitors: anticancer therapy and beyond

The aim of this article is to describe the current and potential clinical translation of pharmacological inhibitors of poly(ADP-ribose) polymerase (PARP) for the therapy of various diseases. The first section of the present review summarizes the available preclinical and clinical data with PARP inhibitors in various forms of cancer. In this context, the role of PARP in single-strand DNA break repair is relevant, leading to replication-associated lesions that cannot be repaired if homologous recombination repair (HRR) is defective, and the synthetic lethality of PARP inhibitors in HRR-defective cancer. HRR defects are classically associated with BRCA1 and 2 mutations associated with familial breast and ovarian cancer, but there may be many other causes of HRR defects. Thus, PARP inhibitors may be the drugs of choice for BRCA mutant breast and ovarian cancers, and extend beyond these tumors if appropriate biomarkers can be developed to identify HRR defects. Multiple lines of preclinical data demonstrate that PARP inhibition increases cytotoxicity and tumor growth delay in combination with temozolomide, topoisomerase inhibitors and ionizing radiation. Both single agent and combination clinical trials are underway. The final part of the first section of the present review summarizes the current status of the various PARP inhibitors that are in various stages of clinical development. The second section of the present review summarizes the role of PARP in selected non-oncologic indications. In a number of severe, acute diseases (such as stroke, neurotrauma, circulatory shock and acute myocardial infarction) the clinical translatability of PARP inhibition is supported by multiple lines of preclinical data, as well as observational data demonstrating PARP activation in human tissue samples. In these disease indications, PARP overactivation due to oxidative and nitrative stress drives cell necrosis and pro-inflammatory gene expression, which contributes to disease pathology. Accordingly, multiple lines of preclinical data indicate the efficacy of PARP inhibitors to preserve viable tissue and to down-regulate inflammatory responses. As the clinical trials with PARP inhibitors in various forms of cancer progress, it is hoped that a second line of clinical investigations, aimed at testing of PARP inhibitors for various non-oncologic indications, will be initiated, as well.

Poly (ADP-ribose) polymerase inhibitor LT-626: Sensitivity correlates with MRE11 mutations and synergizes with platinums and irinotecan in colorectal cancer cells

Some colorectal cancers (CRC) display microsatellite instability (MSI) leading to mutations in genes such as MRE11. The aim of this study was to determine whether MSI or MRE11 mutational status correlates with sensitivity to the PARP inhibitor LT-626 and whether LT-626 synergizes with DNA-damaging chemotherapeutic agents. CRC cells harboring biallelic MRE11 mutations were more sensitive to LT-626 and stable overexpression or knock-down of MRE11 in cell lines correlated with sensitivity. Synergism was evident between LT-626 and cisplatin, oxaliplatin and SN-38 suggesting that PARP inhibitors in combination with DNA damaging agents may be a successful strategy for treatment of CRC.

Molecular pathways: targeting PARP in cancer treatment

Poly (ADP-ribose) polymerases (PARP) are a family of nuclear protein enzymes involved in the DNA damage response. The role of PARP-1 in base excisional repair has been extensively characterized. More recent in vitro studies additionally implicate a role for PARP-1 in facilitating homologous recombination and nonhomologous end-joining. The more faithful process of homologous recombination repair of double-stranded DNA breaks involves localization of BRCA-1 and BRCA-2 to sites of DNA damage, resection of the double-stranded break, and gap-filling DNA synthesis using the homologous sister chromatid as a template. Simultaneous dysfunction of both DNA repair pathways decreases the ability of cells to compensate, and forms the basis for the concept of synthetic lethality. Treatment strategies, thus far, have focused on two main principles: (i) exploitation of the concept of synthetic lethality in homologous recombination-deficient tumors, primarily in breast and ovarian cancer patients with BRCA mutation, and (ii) as radiosensitizers and chemosensitizers. BRCA deficiency accounts for only a fraction of dysfunction in homologous recombination repair. Epigenetic alterations of BRCA function and defects within the Fanconi anemia pathway also result in defective DNA repair. Rational therapeutic combinations exploiting alternate mechanisms of defective DNA repair, abrogation of cell-cycle checkpoints, and additional functions of PARP-1 present novel opportunities for further clinical development of PARP inhibitors. On the basis of the results of clinical studies of PARP inhibitors thus far, it is imperative that future development of PARP inhibitors take a more refined approach, identifying the unique subset of patients that would most benefit from these agents, determining the optimal time for use, and identifying the optimal combination partner in any particular setting.