Poly(ADP-ribose) polymerase (PARP) inhibitors: Exploiting a synthetic lethal strategy in the clinic

Abstract IA9: Cross talk of the androgen receptor and DNA damage pathways: Molecular and translational prostate cancer relevance

Prostate cancers (PCa) are exquisitely dependent on the action of the androgen receptor (AR) for cell survival and proliferation, and there is a significant need to develop new means for targeting recurrent AR activity in both locally advanced and castration-resistant PCa(1, 2). PARP1 (Poly ADP-ribose polymerase 1) is an enzyme that modifies a subset of nuclear proteins by poly (ADP-ribose)-ylation, and is known to play a critical role in base excision repair(3). This function of PARP1 has been cultivated as a therapeutic target for tumors that harbor alterations of specific DNA repair pathways(4, 5). Multiple enzymatic inhibitors of PARP1 function are in clinical trial; while little dose limiting toxicity has been observed, suppressing PARP1-mediated DNA damage repair in BRCA1/2 deficient tumors leads to synthetic lethality and heightened clinical response to chemotherapy. Recently, it has been revealed that PARP1 has a second major cellular function on chromatin as a transcriptional coregulator, capable of modulating chromatin structure and selected transcription factor activity(6–8).

New observations in our laboratory point toward PARP1 inhibitors as a means to simultaneously dampen AR activity and sensitize PCa cells to genotoxic insult. This premise is based on three major arms of investigation. First, abrogation of PARP1 activity results in sensitization of both androgen deprivation-therapy (ADT) naïve and castration-resistant PCa cells to ionizing radiation, thus indicating that PARP1 activity plays a significant role in the cellular response to radiotherapy. Second, PARP1 activity was found to be increased as a function of tumor progression in model systems of human disease, suggesting that gain of PARP1 activity may promote resistance to combined ADT and radiotherapy. Third, robust molecular analyses indicate that PARP1 is recruited to sites of AR activity on chromatin, and therein serves as a requisite cofactor for AR activity. The dependence of AR on PARP1 activity is conserved in cells that failed hormone therapy, thus indicating that the requirement for PARP1 is maintained or enhanced during the process of tumor progression. Together, these data strongly support a model wherein the dual functions of PARP1 in controlling AR activity and the response to radiotherapy can be leveraged to improve treatment of locally advanced prostate cancer.

Citation Format: Matthew J. Schiewer, Adam P. Dicker, Jonathan R. Brody, John M. Pascal, Karen E. Knudsen, Jonathan F. Goodwin, J. Chad Brenner, Michael A. Augello, Fengzhi Liu, Jamie L. Planck, Randy S. Schrecengost, Felix Y. Feng, Arul M. Chinnaiyan. Cross talk of the androgen receptor and DNA damage pathways: Molecular and translational prostate cancer relevance [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr IA9.

Optimizing molecular-targeted therapies in ovarian cancer: the renewed surge of interest in ovarian cancer biomarkers and cell signaling pathways

The hallmarks of ovarian cancer encompass the development of resistance, disease recurrence and poor prognosis. Ovarian cancer cells express gene signatures which pose significant challenges for cancer drug development, therapeutics, prevention and management. Despite enhancements in contemporary tumor debulking surgery, tentative combination regimens and abdominal radiation which can achieve beneficial response rates, the majority of ovarian cancer patients not only experience adverse effects, but also eventually relapse. Therefore, additional therapeutic possibilities need to be explored to minimize adverse events and prolong progression-free and overall response rates in ovarian cancer patients. Currently, a revival in cancer drug discovery is devoted to identifying diagnostic and prognostic ovarian cancer biomarkers. However, the sensitivity and reliability of such biomarkers may be complicated by mutations in the BRCA1 or BRCA2 genes, diverse genetic risk factors, unidentified initiation and progression elements, molecular tumor heterogeneity and disease staging. There is thus a dire need to expand existing ovarian cancer therapies with broad-spectrum and individualized molecular targeted approaches. The aim of this review is to profile recent developments in our understanding of the interrelationships among selected ovarian tumor biomarkers, heterogeneous expression signatures and related molecular signal transduction pathways, and their translation into more efficacious targeted treatment rationales.

A force field with discrete displaceable waters and desolvation entropy for hydrated ligand docking

In modeling ligand-protein interactions, the representation and role of water are of great importance. We introduce a force field and hydration docking method that enables the automated prediction of waters mediating the binding of ligands with target proteins. The method presumes no prior knowledge of the apo or holo protein hydration state and is potentially useful in the process of structure-based drug discovery. The hydration force field accounts for the entropic and enthalpic contributions of discrete waters to ligand binding, improving energy estimation accuracy and docking performance. The force field has been calibrated and validated on a total of 417 complexes (197 training set; 220 test set), then tested in cross-docking experiments, for a total of 1649 ligand-protein complexes evaluated. The method is computationally efficient and was used to model up to 35 waters during docking. The method was implemented and tested using unaltered AutoDock4 with new force field tables.

Emerging therapies for urothelial cancer

Urothelial carcinoma is one of the leading causes of death in Europe and the United States. Despite its chemosensitivity, median overall survival for advanced disease is still nearly 1 year. Most second-line chemotherapeutic agents tested have been disappointing. Thus, new treatment strategies are clearly needed. This review focuses on emerging therapies in urothelial carcinoma. Results from recent clinical trials, investigating the activity of new generation cytostatic agents, as well as results from studies assessing the toxicity and efficacy of novel targeted therapies, are discussed. In this setting, anti-epidermal growth factor receptor, angiogenesis, and phosphatidylinositol 3-kinase/mammalian target of rapamycin inhibitors account for the majority of phase I and II trials.

Targeting poly(ADP-ribose) polymerase-1 as a promising approach for immunomodulation in multiple sclerosis?

Despite significant advancement in developing therapies for multiple sclerosis (MS), drugs that cure this devastating disorder are an unmet need. Among the remedies showing efficacy in preclinical MS models, inhibitors of poly(ADP-ribose) polymerase (PARP)-1 have gained great momentum. Emerging evidence demonstrates that PARP-1 inhibitors epigenetically regulate gene expression and finely tune transcriptional activation in immune and neural cells. In this review, we present an appraisal of the effects of PARP-1 and its inhibitors on immune activation, with particular emphasis on the processes taking place during the autoimmune attack directed against the central nervous system. One explanation is that drugs inhibiting PARP-1 activity protect from neuroinflammation in MS models via immunomodulation and direct neuroprotection. PARP-1 inhibitors have already reached the clinical arena as cancer treatments, and observations made in treating these patients could help advance treatments for MS.

A snapshot of chemoresistance to PARP inhibitors

The exploitation of synthetic lethality in BRCA-deficient tumor carriers using potent inhibitors of the enzyme poly(ADP-ribose) polymerase (PARP)-1 has led to an enthusiastic response among basic scientists, oncologists and pharmaceutical companies. However, accumulating evidence demonstrates that resistance to these drugs develops in tumors in both preclinical and clinical settings. Here, I focus on literature dealing with resistance to these drugs and discuss the molecular mechanisms involved, such as restoration of BRCA function, upregulation of nonhomologous end-joining-dependent DNA repair, induction of P-glycoprotein expression and epigenetic deregulation. Clinical implications of resistance to PARP1 inhibitors are also discussed.

The changing therapeutic landscape of castration-resistant prostate cancer

Castration-resistant prostate cancer (CRPC) has a poor prognosis and remains a significant therapeutic challenge. Before 2010, only docetaxel-based chemotherapy improved survival in patients with CRPC compared with mitoxantrone. Our improved understanding of the underlying biology of CRPC has heralded a new era in molecular anticancer drug development, with a myriad of novel anticancer drugs for CRPC entering the clinic. These include the novel taxane cabazitaxel, the vaccine sipuleucel-T, the CYP17 inhibitor abiraterone, the novel androgen-receptor antagonist MDV-3100 and the radioisotope alpharadin. With these developments, the management of patients with CRPC is changing. In this Review, we discuss these promising therapies along with other novel agents that are demonstrating early signs of activity in CRPC. We propose a treatment pathway for patients with CRPC and consider strategies to optimize the use of these agents, including the incorporation of predictive and intermediate end point biomarkers, such as circulating tumor cells.

Selective killing of cancer cells by a small molecule targeting the stress response to ROS

Malignant transformation, driven by gain-of-function mutations in oncogenes and loss-of-function mutations in tumour suppressor genes, results in cell deregulation that is frequently associated with enhanced cellular stress (for example, oxidative, replicative, metabolic and proteotoxic stress, and DNA damage). Adaptation to this stress phenotype is required for cancer cells to survive, and consequently cancer cells may become dependent upon non-oncogenes that do not ordinarily perform such a vital function in normal cells. Thus, targeting these non-oncogene dependencies in the context of a transformed genotype may result in a synthetic lethal interaction and the selective death of cancer cells. Here we used a cell-based small-molecule screening and quantitative proteomics approach that resulted in the unbiased identification of a small molecule that selectively kills cancer cells but not normal cells. Piperlongumine increases the level of reactive oxygen species (ROS) and apoptotic cell death in both cancer cells and normal cells engineered to have a cancer genotype, irrespective of p53 status, but it has little effect on either rapidly or slowly dividing primary normal cells. Significant antitumour effects are observed in piperlongumine-treated mouse xenograft tumour models, with no apparent toxicity in normal mice. Moreover, piperlongumine potently inhibits the growth of spontaneously formed malignant breast tumours and their associated metastases in mice. Our results demonstrate the ability of a small molecule to induce apoptosis selectively in cells that have a cancer genotype, by targeting a non-oncogene co-dependency acquired through the expression of the cancer genotype in response to transformation-induced oxidative stress.

The ups and downs of tannins as inhibitors of poly(ADP-ribose)glycohydrolase

DNA damage to cells activates nuclear poly(ADP-ribose)polymerases (PARPs) and the poly(ADP-ribose) (PAR) synthesized is rapidly cleaved into ADP-ribose (ADPR) by PAR glycohydrolase (PARG) action. Naturally appearing tannin-like molecules have been implicated in specific inhibition of the PARG enzyme. This review deals with the in vitro and in vivo effects of tannins on PAR metabolism and their downstream actions in DNA damage signaling.