Inhibition Of poly(ADP-ribose) Glycohydrolase Research Articles

Abstract 3366: FoRx-06-428 is a novel PARG inhibitor with potent anti-tumor efficacy in preclinical cancer models

Abstract The formation of poly(ADP-ribose) (PAR) chains from NAD+ precursors by PARP enzymes is a characteristic posttranslational modification during DNA damage repair. PAR chains function as docking platforms for DNA repair proteins that are required to resolve DNA lesions. Equally important is the subsequent removal of PAR chains from protein substrates to conclude the repair process and restore genomic integrity. Poly(ADP-ribose) glycohydrolase (PARG) is the key enzyme in this process and has recently garnered significant attention as a novel cancer target. FoRx-06-428 is a novel, orally bioavailable and highly potent, low molecular weight PARG inhibitor. It binds to the catalytic domain of human PARG with high affinity, as evidenced by surface plasmon resonance (SPR), and displays potent inhibition of enzymatic activity. FoRx-06-428 shows selective reduction of viability in a panel of cancer cell lines highlighting therapeutic potential in multiple cancer indications. Pharmacological inhibition of PARG leads to accumulation of PAR chains and is indicative of target engagement, accompanied by modulation of DNA damage response markers, such as phosphorylation of replication protein A (pRPA) and phosphorylation of H2AX (γH2AX). Compelling anti-tumor efficacy as single agent was observed in several xenograft models (breast and gastric cancer) with excellent tolerability. Dose-dependent accumulation of PAR chains was demonstrated in vivo and constitutes a tractable target engagement biomarker for clinical exploration. Sensitivity to PARG inhibition is partly driven by distinct genomic instabilities, such as defects in homologous recombination (HR) repair, however the activity of PARG inhibitors is also influenced by aberrations in other cancer-relevant pathways. This report represents the first disclosure of the pharmacological characterization of FoRx-06-428 and highlights the value of PARG inhibition as a novel targeted approach to treat DNA repair-deficient cancers. Citation Format: Olivier Querolle, Ulrich Lücking, Luca Iacovino, Alena Freudenmann, Giacomo Rossetti, Marina Gysin, Serena Bologna, Vasilis Dionellis, Marta Malattia, Alessandro Potenza, Nicolas Bocquet, Irena Konstantinova, Andreas Goutopoulos, Sotirios Sotiriou, Thanos Halazonetis, Tarig Bashir, Frank Zenke. FoRx-06-428 is a novel PARG inhibitor with potent anti-tumor efficacy in preclinical cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3366.

Abstract 7129: DNA polymerase beta expression in head & neck cancer modulates the poly-ADP-ribose-mediated replication checkpoint

Abstract Head and Neck Squamous Cell Carcinoma (HNSCC) impose a significant health burden, necessitating innovative therapeutic strategies to enhance treatment efficacy. Current standard treatments, including surgery, radiation therapy, and chemotherapy, often exhibit limited effectiveness and cause substantial side effects. This underscores the critical need for novel therapies that selectively target cancer cells, sparing normal tissue. Survival from HNSCC is poor, and importantly, a racial disparity between African Americans (AfAm) and European Americans (EuAm) has been known for some time. Ancestry Informative Markers (AIMs) are associated with altered mRNA expression of Polβ where higher expression of Polβ was observed among AfAm patients in comparison to EuAm patients. Our research focuses on DNA polymerase beta (Polβ) and its role in replication stress and response to chemotherapy and DNA damage response modifiers. Elevated expression of Polβ is linked to increased DNA damage and genomic instability, contributing to treatment resistance. In this regard, our investigations reveal a regulatory role for base excision repair (BER) proteins, including Polβ, in the cellular response to inhibitors of poly(ADP-ribose) glycohydrolase (PARG), an enzyme involved in poly(ADP-ribose) (PAR) degradation. Inhibition of PARG triggers a robust intra-S phase response, activating ATR and CHK1 signaling. Exploiting the synthetic lethality between PARG inhibition and ATR/CHK1 inhibition, we have demonstrated the ability to overcome Polβ overexpression mediated treatment resistance in HNSCC cells. Specifically, our in vitro studies demonstrate that the response to a combination of an ATR inhibitor and PARG inhibitor is impacted by Polβ expression in HNSCC cells, with synergistic effects surpassing the efficacy of the individual inhibitors. Similarly, the combination of a CHK1 inhibitor and PARG inhibitor proves effective in cells with elevated Polβ expression, offering a potential dual therapeutic strategy. These combinations exhibit promising potential for HNSCC treatment when Polβ expression is elevated. In conclusion, our findings propose a novel therapeutic paradigm for HNSCC, employing combination therapy with ATR or CHK1 inhibitors and PARG inhibitors. This approach offers a targeted strategy, presenting a promising avenue for the development of more effective and less toxic treatment modalities in the management of HNSCC and may help overcome resistance from elevated expression of Polβ. Citation Format: Md Maruf Khan, Anusha Angajala, Denise Y. Gibbs, Jeffrey C. Liu, Camille Ragin, Robert W. Sobol. DNA polymerase beta expression in head & neck cancer modulates the poly-ADP-ribose-mediated replication checkpoint [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 7129.

Abstract 2667: PARG inhibition augments CHK1 inhibitor-induced replication stress and synergistically kills ovarian cancer cells

Abstract Ovarian cancer (OC) is one of the leading causes of cancer-related deaths in women in the United States. PARP inhibitors showed promising clinical responses; however, most of the patients eventually relapse and succumb to the resistant disease. Currently, it lacks effective therapies to prevent and treat OC, which warrants novel drugs and combination therapies to improve patients' prognoses. Epithelial OCs are heavily dependent on ATR-CHK1-mediated DNA damage checkpoint signaling for survival, which is also regulated by hedgehog/GLI1 (Hh) signaling in cancer cells. However, both Hh/GLI1 and CHK1 inhibitors failed to show promising results in clinical trials as single agents. Poly(ADP-ribose) glycohydrolase (PARG) is the critical enzyme that removes Poly-ADP ribosylated (PARylated) proteins upon replication stress. We hypothesized that inhibition of PARG could amplify CHK1 inhibitor-induced replication stress by trapping PARylated proteins on the chromatin and compromising DNA repair and causing a metabolic and mitotic catastrophe. We evaluated CHK1 inhibitor prexasertib in combination with PARG inhibitor (PARGi) to target metabolic and DNA repair crosstalk to effectively kill OC cells and to overcome resistance. Our studies demonstrate that prexasertib treatment induces PARylation by inducing replication stress. Similarly, PARG inhibition by (PDD00017273) fails to remove PARylation and activates checkpoint kinase 1 (CHK1) by phosphorylating at S296, S317, and S345 in OC cells. Based on these observations, we hypothesized that a combination of CHK1 and PARG inhibition would augment oncogenic replication stress in OC cells and cause increased DNA damage and cell death. Interestingly, our evaluation of prexasertib and PARGi combination treatment by clonogenic survival and MTT assays showed synergistic killing in a panel of OC cells, including in isogenic chemoresistant models as well as 3D organoid models of primary OC cells compared to individual drugs. As predicted, combined treatment increased DNA double-strand breaks as demonstrated by COMET assays, and increased γH2AX foci, pRPA32 foci, perturbed S/G2 phase arrest, and nuclear disintegration relative to individual-drug treatment. Furthermore, significant depletion of NAD+ levels was seen in PARGi-treated OC cells compared to untreated cells. Together, these results indicate that this combination treatment cause persistence of heavy PARylation at DNA damage sites abrogates cell cycle checkpoint mechanisms, exhausts cellular NAD+ levels, and inhibits DNA repair leading to the metabolic and mitotic collapse of cancer cells and synergistic lethality in OC cells. Currently, prexasertib is under clinical trials and our studies will provide novel mechanistic insight into the therapeutic potential of this combination and provides preclinical evidence to develop further this combination therapy in treating OC. Citation Format: Ganesh Acharya, Chinnadurai Mani, Mark B. Reedy, Komaraiah Palle. PARG inhibition augments CHK1 inhibitor-induced replication stress and synergistically kills ovarian cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2667.

PARG suppresses tumorigenesis and downregulates genes controlling angiogenesis, inflammatory response, and immune cell recruitment

Chemokines are highly expressed in tumor microenvironment and play a critical role in all aspects of tumorigenesis, including the recruitment of tumor-promoting immune cells, activation of cancer-associated fibroblasts, angiogenesis, metastasis, and growth. Poly (ADP-ribose) polymerase (PARP) is a multi-target transcription regulator with high levels of poly(ADP-ribose) (pADPr) being reported in a variety of cancers. Furthermore, poly (ADP-ribose) glycohydrolase (PARG), an enzyme that degrades pADPr, has been reported to be downregulated in tumor tissues with abnormally high levels of pADPr. In conjunction to this, we have recently reported that the reduction of pADPr, by either pharmacological inhibition of PARP or PARG’s overexpression, disrupts renal carcinoma cell malignancy in vitro. Here, we use 3 T3 mouse embryonic fibroblasts, a universal model for malignant transformation, to follow the effect of PARG upregulation on cells’ tumorigenicity in vivo. We found that the overexpression of PARG in mouse allografts produces significantly smaller tumors with a delay in tumor onset. As downregulation of PARG has also been implicated in promoting the activation of pro-inflammatory genes, we also followed the gene expression profile of PARG-overexpressing 3 T3 cells using RNA-seq approach and observed that chemokine transcripts are significantly reduced in those cells. Our data suggest that the upregulation of PARG may be potentially useful for the tumor growth inhibition in cancer treatment and as anti-inflammatory intervention.

Inhibition of Poly ADP-Ribose Glycohydrolase Sensitizes Ovarian Cancer Cells to Poly ADP-Ribose Polymerase Inhibitors and Platinum Agents.

BackgroundPoly ADP-ribose glycohydrolase (PARG) is responsible for the catabolism of PARP-synthesized PAR to free ADP-ribose. Inhibition of PARG leads to DNA repair interruption and consequently induces cell death. This study aims to evaluate the effect of a PARG inhibitor (PARGi) on epithelial ovarian cancer (OC) cell lines, alone and in combination with a PARP inhibitor (PARPi) and/or Cisplatin.MethodsPARG mRNA levels were studied in three different OC datasets: TCGA, Hendrix, and Meyniel. PARG protein levels were assessed in 100 OC specimens from our bio-bank. The therapeutic efficacy of PARGi was assessed using cell migration and clonogenic formation assays. Flow cytometry was used to evaluate the cell apoptosis rate and the changes in the cell cycle.ResultsPARG protein was highly expressed in 34% of the OC tumors and low expression was found in another 9%. Similarly, Hendrix, Meyneil and TCGA databases showed a significant up-regulation in PARG mRNA expression in OC samples as compared to normal tissue (P=0.001, P=0.005, P=0.005, respectively). The use of PARGi leads to decreased cell migration. PARGi in combination with PARPi or Cisplatin induced decreased survival of cells as compared to each drug alone. In the presence of PARPi and Cisplatin, PARG knockdown cell lines showed significant G2/M cell cycle arrest and cell death induction.ConclusionsPARG inhibition appears as a complementary strategy to PARP inhibition in the treatment of ovarian cancer, especially in the presence of homologous recombination defects.

Regulation of ALT-associated homology-directed repair by polyADP-ribosylation.

The synthesis of poly(ADP-ribose) (PAR) reconfigures the local chromatin environment and recruits DNA-repair complexes to damaged chromatin. PAR degradation by poly(ADP-ribose) glycohydrolase (PARG) is essential for progression and completion of DNA repair. Here, we show that inhibition of PARG disrupts homology-directed repair (HDR) mechanisms that underpin alternative lengthening of telomeres (ALT). Proteomic analyses uncover a new role for poly(ADP-ribosyl)ation (PARylation) in regulating the chromatin-assembly factor HIRA in ALT cancer cells. We show that HIRA is enriched at telomeres during the G2 phase and is required for histone H3.3 deposition and telomere DNA synthesis. Depletion of HIRA elicits systemic death of ALT cancer cells that is mitigated by re-expression of ATRX, a protein that is frequently inactivated in ALT tumors. We propose that PARylation enables HIRA to fulfill its essential role in the adaptive response to ATRX deficiency that pervades ALT cancers.

Inhibition of PARG sensitizes ovarian cancer cells to PARP inhibitors and DNA damaging agents

Objective: Poly(ADP-ribose) glycohydrolase (PARG) is responsible for PAR catabolism. The aim of this study was to evaluate the effect of a PARG inhibitor on epithelial ovarian cancer cell lines, alone and in combination with a poly(ADP-ribose) polymerase inhibitor (PARPi) and/or cisplatin.

Abstract 546: PARG and WEE1 inhibition in GI cancers: A novel synergistic, targeted therapeutic approach

Abstract Colorectal carcinomas (CRC) and pancreatic ductal adenocarcinomas (PDAC) are the 2nd and 3rd leading causes of cancer-related deaths in the U.S. respectively. Recent work from our lab highlighted Poly ADP-Ribose Glycohydrolase (PARG) as a target in PDAC, and PARG inhibition (PARGi) sensitized PDAC cells to the DNA damaging agents, 5-FU and oxaliplatin. Other labs have shown that PARG inhibition causes replication stress in cancer cells. CRC and PDAC cells, while cellularly different, share a high frequency of KRAS and P53 inactivating mutations. High levels of replication stress (RS) has also been well documented in these tumor types. Targeting DNA Damage Response Kinases (DDRKs) in combination with chemotherapy (mostly DNA damage inducers like 5-FU, oxaliplatin, etc.) has gained recent interest in solid tumors but overlapping toxicities and tolerability have been issues clinically. Accordingly, we hypothesized that combined inhibition of PARG and DDRKs could yield synergistic effects in tumors with high replication stress while alleviating adverse events seen in other combination strategies. We determined in drug sensitivity assays that PARGi did not synergize with either the CHK1 inhibitor prexasertib or the ATR inhibitor VE-821 in our cell lines but did synergize with Wee1i (AZD1775) in both PDAC and CRC cell lines (Comb. Analysis, p<.05). Additionally, we assessed long-term cell survival using colony formation assays, which yielded a dramatic reduction in cell survival for Panc1, MiaPaCa-2, CFPAC1 (PDAC cells), HCT116 (CRC cells) and SW480 (CRC cells) cell lines with the combination of PARGi and Wee1i as compared to single-agent controls (p<.001). To validate the targeted synergy between AZD1775 and PARGi, we utilized genetic models of PARG and Wee1 silencing, including CRISPR PARG knockout cell lines, siRNA, and shRNA knockdown models. Genetic knockdown of Wee1 and PARG in PDAC and CRC cell lines yielded strikingly similar results in colony formation assays, with 2 and 12-fold changes respectively compared to Wee1i alone (P-value < .0001). HCT116 CRISPR PARG knockouts yielded a 5-fold difference in Wee1i sensitivity in colony formation assays. Mechanistically, western blot analysis demonstrated PARGi and Wee1i led to >2-fold increases in PARylation, yH2AX, and cleaved-caspase 3 as compared to single-agent controls. In conclusion, combined PARG - Wee1 inhibition is a novel targeting strategy in PDAC and CRC tumors that results in decreased cell viability. We are currently testing the efficacy of AZD1775 in PARG CRISPR-knockout tumors in in vivo xenograft models. Ongoing studies are focused on assessing the effect of Wee1i + PARGi on chromosomal aberrations, disruption in replication fork dynamics via DNA fiber labeling, and cell cycle dependencies. Citation Format: Lebaron C. Agostini, Aditi Jain, Michael Pishvaian, Charles Yeo, Jonathan Brody. PARG and WEE1 inhibition in GI cancers: A novel synergistic, targeted therapeutic approach [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 546.

Abstract NT-117: INHIBITION OF PARG, SENSITIZES OVARIAN CANCER CELLS TO PARP INHIBITORS AND DNA DAMAGING AGENTS

Abstract BACKGROUND: Poly(ADP-ribose) glycohydrolase (PARG) is responsible poly(ADP-ribose) (PAR) catabolism, which is synthesized by poly(ADP-ribose) polymerases (PARPs) at the site of DNA single-strand breaks (SSB). Faulty PAR formation or disintegration inhibits SSB repair. PARP inhibitors (PARPi) exploit the synthetic lethality between SSB repair and homologous recombination (HR) double strand break (DSB) repair. The most effective PARPi traps PARP at the SSB site, causing a double strand break during DNA synthesis that the cell cannot repair. The trapping of PARP can also be achieved through PARG inhibition. Due to the high rate of HR defects in epithelial ovarian cancer, the aim of this study is to evaluate the effect of a PARG inhibitor (PARGi) on epithelial ovarian cancer cell lines, alone and in combination with PARPi or cisplatin. METHODS: PARG protein levels were assessed in 74, unselected, snap-frozen, tumors kept in our bio-bank by western blotting and immuno-histochemistry (IHC), as well as in the TCGA database. PARG was knocked down in two cell lines resistant to PARP inhibition, OVCAR3 and SKOV3. The knockdowns were subjected to cell migration and cell proliferation assays, as well as single cell analysis. BRCA1 mutated and BRCA1 wild-type cell lines were exposed to clinically relevant doses of PARGi, PARPi, or cisplatin, separately or in various combination. Therapeutic efficacy was assessed using colony formation assay. Western Blotting was used to detect the levels of PARG and PARP proteins in the cell lines. RESULTS: PARG mRNA was expressed in 30% of tumors in TCGA. PARG protein was also detected in 30% of the biobank tumors analyzed. PARG knockdowns exhibited reduced rates of cellular proliferation and significant G2/M cell cycle arrest. In PARG over expressed ovarian cancer cell lines, PARG inhibitor similarly reduced cell migration (OVCAR3 and SKOV3: 75% reduction, SNU-251: 48% reduction). CONCLUSION: PARG inhibition appears to be a viable, complementary strategy to PARP inhibition in HR-deficient cancers. The effect of BRCA1 on the efficacy of PARG inhibition, as well as various treatment combinations, are currently under investigation. Citation Format: Emad Matanes, Tahira Baloch , David Octeau, Roy Kessous, Liron Kogan, Ido Laskov, Walter Gotlieb, Amber Yasmeen. INHIBITION OF PARG, SENSITIZES OVARIAN CANCER CELLS TO PARP INHIBITORS AND DNA DAMAGING AGENTS [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr NT-117.

First-in-Class Chemical Probes against Poly(ADP-ribose) Glycohydrolase (PARG) Inhibit DNA Repair with Differential Pharmacology to Olaparib.

The enzyme poly(ADP-ribose) glycohydrolase (PARG) performs a critical role in the repair of DNA single strand breaks (SSBs). However, a detailed understanding of its mechanism of action has been hampered by a lack of credible, cell-active chemical probes. Herein, we demonstrate inhibition of PARG with a small molecule, leading to poly(ADP-ribose) (PAR) chain persistence in intact cells. Moreover, we describe two advanced, and chemically distinct, cell-active tool compounds with convincing on-target pharmacology and selectivity. Using one of these tool compounds, we demonstrate pharmacology consistent with PARG inhibition. Further, while the roles of PARG and poly(ADP-ribose) polymerase (PARP) are closely intertwined, we demonstrate that the pharmacology of a PARG inhibitor differs from that observed with the more thoroughly studied PARP inhibitor olaparib. We believe that these tools will facilitate a wider understanding of this important component of DNA repair and may enable the development of novel therapeutic agents exploiting the critical dependence of tumors on the DNA damage response (DDR).

Abstract 1995: HuR dependent inhibition of PARG enhances PARP inhibitor therapy for DNA repair proficient and deficient pancreatic cancer cells

Abstract Introduction: Despite our deep understanding of genetic drivers of the disease, pancreatic ductal adenocarcinoma (PDA) continues to be associated with dismal survival rates. Targeting the DNA repair machinery has emerged as a promising therapeutic strategy to treat pancreatic cancer patients carrying DNA damage repair (DDR) mutations. Such mutations promote tumorigenesis, but also paradoxically render tumor cells particularly susceptible to platinum-based agents and PARP inhibitors (PARPi). However, despite promising preclinical and clinical results, early data demonstrate that eventually most tumors, regardless of DDR status, become resistant to PARPi-therapies. The mRNA-binding protein HuR, predominantly expressed in the nucleus, translocates to the cytoplasm upon tumor-associated stress where it post-transcriptionally regulates select mRNA cargo, resulting in resistance to DNA damaging agents in a harsh tumor microenvironment. Here, we sought to evaluate the role of HuR in regulating PARPi efficacy. Results: In response to cellular stress induced by IC50 dosages of a panel of PARPis such as Veliparib, Olaparib, Rucaparib, Talazoparib and Niraparib, nuclear localized HuR undergoes cytoplasmic translocation. Silencing of HuR via siRNA, CRISPR and a DOX-inducible system resulted in significant decrease in long and short- term PDA cell survival, irrespective of DDR status. To complement and validate in vitro findings, we employed a heterotropic mouse xenograft model using Mia.sh290 cells wherein DOX induction significantly reduced HuR expression. Olaparib mediated PARP inhibition (50mg/kg, 5 days a week) combined with DOX-induced HuR silencing resulted in significant reduction in tumor volumes, compared to Olaparib alone or DOX alone. Mechanistically, we demonstrate that the pro-survival protein HuR facilitates PDA cells to recover from PARPi insult by, in part, regulating poly ADP ribose glycohydrolase (PARG), the major enzyme responsible for hydrolyzing poly-ADP ribose (PAR) polymers, on chromatin and associated proteins. HuR binds to two 41- 43bp long sites in the 3’ untranslated region (3’UTR) of PARG, increasing its mRNA stability and protein expression. Increased PARG activity, further validated via exogenous overexpression, promotes DNA repair efficiency and increases PDA cell survival. Functional analysis indicate that such inhibition of HuR and/or PARG significantly enhances PARPi sensitivity in PDA cells, via increased accumulation of DNA damage γH2AX foci, preventing efficient removal of PAR polymers, and enhancing detrimental trapping of PARP1 on chromatin. Conclusions: Taken together, our results indicate that HuR- mediated upregulation of PARG acts as a universal pro-survival mechanism and HuR inhibition could significantly potentiate PARPi therapy in PDA, irrespective of DNA repair status. Citation Format: Saswati N. Chand, Mahsa Zarei, Akshay R. Kamath, Matthew J. Schiewer, Carmella Romeo, Joseph A. Cozzitorto, Nicole Meisner- Kober, Eric Londin, Isidore Rigoutsos, Karen Knudsen, John M. Pascal, Charles J. Yeo, Jordan M. Winter, Jonathan R. Brody. HuR dependent inhibition of PARG enhances PARP inhibitor therapy for DNA repair proficient and deficient pancreatic cancer cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1995.

Abstract 3714: Optimisation of quinazolinedione sulphonamides as novel inhibitors of poly(ADP Ribose) glycohydrolase (PARG)

Abstract The macrodomain protein poly(ADP ribose) glycohydrolase (PARG) has been shown to be a critical component in the repair of single stand DNA breaks and counteracts the function of the ARTD family of poly(ADP ribose) polymerases, commonly known as the PARPs. As PARG exists as a single protein, it presents an attractive target for therapeutic intervention in cancer cells with enhanced dependence upon DNA repair. Inhibitors of this enzyme have proved difficult to discover and develop. Moreover, intact cell-active tool compounds which have the propensity to be used as robust chemical probes to understand PARG pharmacology, are absent from the literature. This poster will describe our work in this emerging area, optimising a series of drug-like quinazolinedione derivatives to deliver molecules with the correct physicochemical and biochemical properties to function as in vitro cell probe compounds. These unprecedented agents display potent on-target biochemical (5 nM) and cell (10 nM) activity with a significant window to acute 3-day cytotoxicity. Moreover, these agents are selective against PARP family members and the close glycohydrolase homologue ARH3. The medicinal chemistry optimisation of the scaffold will be described, alongside the outline pharmacology demonstrating on-target, selective inhibition of PARG in cells. Such tool compounds will be of value in revealing the detailed mechanisms of action of PARG in DNA repair and other PAR chain-mediated cellular processes, with the ultimate goal of delivering novel and clinically relevant therapeutic agents. Citation Format: Kate Smith, Ben Acton, Dominic James, Cliff Jones, Stuart Jones, Allan Jordan, Nicola Hamilton, Alison McGonagle, Daniel Mould, Helen Small, Alex Stowell, Julie Tucker, Ian Waddell, Bohdan Waszkowycz, Donald Ogilvie. Optimisation of quinazolinedione sulphonamides as novel inhibitors of poly(ADP Ribose) glycohydrolase (PARG). [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3714.

Abstract C39: First-in-class inhibitors of the putatively undruggable DNA repair target Poly(ADP-ribose) glycohydrolase (PARG)

Abstract Poly(ADP-ribose) glycohydrolase (PARG) is the only enzyme known to catalyse hydrolysis of the O-glycosidic linkages of ADP-ribose polymers, thereby reversing the effects of poly(ADP-ribose) polymerases (PARPs). PARG depletion, using RNAi, results in several effects such as PAR chain persistence, progression of single- to double-strand DNA lesions and NAD+ depletion. Given these findings, inhibition of PARG with a small molecule agent offers a potential opportunity to interfere with DNA repair mechanisms and induce cell death in those cells with increased susceptibility to DNA damage, such as tumour cells. Previous efforts to develop small molecule inhibitors of PARG activity have generally been hampered by poor physicochemical properties, off-target pharmacology and a lack of cell permeability, leading some to suggest that PARG may be undruggable. In contrast, we have now developed a series of first-in-class PARG inhibitors which display drug-like properties and attractive pharmacokinetic parameters. These compounds have proved to be useful biological tool compounds. Moreover, displaying selective activity in both biochemical and, more importantly, cellular assays of PARG function, these derivatives have allowed an exploration of the phenotypes resulting from reversible, pharmacological PARG inhibition in both in vitro cell panels and in vivo models. Furthermore, our initial bioinformatic analysis suggests that deficiency of a known tumour suppressor confers sensitivity to PARG inhibition, suggesting patient populations that will potentially benefit from PARGi therapies. Citation Format: Bohdan Waszkowycz, Dominic James, Ben Acton, Emma Fairweather, Sam Fritzl, Niall Hamilton, Nicola Hamilton, Sarah Holt, James Hitchen, Colin Hutton, Stuart Jones, Allan Jordan, Alison McGonagle, Daniel Mould, Helen Small, Kate Smith, Alexandra Stowell, Ian D. Waddell, Donald Ogilvie. First-in-class inhibitors of the putatively undruggable DNA repair target Poly(ADP-ribose) glycohydrolase (PARG). [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C39.

PARG dysfunction enhances DNA double strand break formation in S-phase after alkylation DNA damage and augments different cell death pathways

Poly(ADP-ribose) glycohydrolase (PARG) is the primary enzyme responsible for the degradation of poly(ADP-ribose). PARG dysfunction sensitizes cells to alkylating agents and induces cell death; however, the details of this effect have not been fully elucidated. Here, we investigated the mechanism by which PARG deficiency leads to cell death in different cell types using methylmethanesulfonate (MMS), an alkylating agent, and Parg−/− mouse ES cells and human cancer cell lines. Parg−/− mouse ES cells showed increased levels of γ-H2AX, a marker of DNA double strand breaks (DSBs), accumulation of poly(ADP-ribose), p53 network activation, and S-phase arrest. Early apoptosis was enhanced in Parg−/− mouse ES cells. Parg−/− ES cells predominantly underwent caspase-dependent apoptosis. PARG was then knocked down in a p53-defective cell line, MIAPaCa2 cells, a human pancreatic cancer cell line. MIAPaCa2 cells were sensitized to MMS by PARG knockdown. Enhanced necrotic cell death was induced in MIAPaCa2 cells after augmenting γ-H2AX levels and S-phase arrest. Taken together, these data suggest that DSB repair defect causing S-phase arrest, but p53 status was not important for sensitization to alkylation DNA damage by PARG dysfunction, whereas the cell death pathway is dependent on the cell type. This study demonstrates that functional inhibition of PARG may be useful for sensitizing at least particular cancer cells to alkylating agents.

PARG ACTIVITY MEDIATES POST-TRAUMATIC INFLAMMATORY REACTION AFTER EXPERIMENTAL SPINAL CORD TRAUMA

The aim of the present study was to examine the role of poly(ADP-ribose) glycohydrolase (PARG) on the modulation of the inflammatory response and tissue injury associated with neurotrauma. Spinal cord trauma was induced in WT mice by the application of vascular clips (force of 24 g) to the dura via a four-level T5-T8 laminectomy. Spinal cord injury in WT mice resulted in severe trauma characterized by edema, neutrophil infiltration, and cytokine production followed by recruitment of other inflammatory cells, production of a range of inflammation mediators, tissue damage, apoptosis and disease. The genetic disruption of the PARG gene in mice or the pharmacological inhibition of PARG with GPI 16552 (40 mg/kg intraperitoneally bolus), a novel and potent PARG inhibitor, significantly reduced the degree of spinal cord inflammation and tissue injury (histological score), neutrophils infiltration, iNOS, nitrotyrosine, COX2, and cytokines production (TNF-α and IL-1β), and apoptosis. In a separate experiment we have clearly demonstrated that PARG inhibition significantly ameliorated the recovery of limb function. Taken together, our results indicate that PARG activity modulates the inflammatory response and tissue injury events associated with spinal cord trauma and participate in target organ damage under these conditions.

Poly(ADP-Ribose) Glycohydrolase Activity Mediates Post-Traumatic Inflammatory Reaction after Experimental Spinal Cord Trauma

The aim of the present study was to examine the role of poly-(ADP-ribose) glycohydrolase (PARG) on the modulation of the inflammatory response and tissue injury associated with neurotrauma. Spinal cord trauma was induced in wild-type (WT) mice by the application of vascular clips (force of 24 g) to the dura via a two-level T(6) to T(7) laminectomy. Spinal cord injury in WT mice resulted in severe trauma characterized by edema, neutrophil infiltration, and cytokine production followed by recruitment of other inflammatory cells, production of a range of inflammation mediators, tissue damage, apoptosis, and disease. The genetic disruption of the PARG gene in mice or the pharmacological inhibition of PARG with GPI 16552 [N-bis-(3-phenyl-propyl)9-oxo-fluorene-2,7-diamide] (40 mg/kg i.p. bolus), a novel and potent PARG inhibitor, significantly reduced the degree of spinal cord inflammation and tissue injury (histological score), neutrophil infiltration, cytokine production (tumor necrosis factor-alpha and interleukin-1beta), and apoptosis. In a separate experiment, we have clearly demonstrated that PARG inhibition significantly ameliorated the recovery of limb function. Taken together, our results indicate that PARG activity modulates the inflammatory response and tissue injury events associated with spinal cord trauma and participate in target organ damage under these conditions.

PARG activity mediates intestinal injury induced by splanchnic artery occlusion and reperfusion

Poly (ADP-ribosyl)ation, an early post-translational modification in response to DNA damage, is catalyzed by poly (ADP-ribose) polymerase (PARP-1) and catabolized by poly(ADP-ribose) glycohydrolase (PARG). The aim of this study was to investigate the role of PARG on the modulation of the inflammatory response caused by splanchnic ischemia and reperfusion. SAO shock in rats and wild-type (WT) mice was associated with a significant neutrophil infiltration in the ileum and production of tumor necrosis factor-alpha (TNF-alpha). Reperfused ileum tissue sections from SAO-shocked WT mice and rats showed positive staining for P-selectin and ICAM-1 localized mainly in the vascular endothelial cells. Genetic disruption of the PARG gene in mice or pharmacological inhibition of PARG by PARG inhibitors significantly improved the histological status of the reperfused tissues associated with reduced expression of P-selectin and ICAM-1, neutrophil infiltration into the reperfused intestine, and TNF-alpha production. These results suggest that PARG activity modulates the inflammatory response in ischemia/reperfusion and participates in end (target) organ damage under these conditions.

Inhibition of poly(ADP-ribose) glycohydrolase by gallotannin selectively up-regulates expression of proinflammatory genes.

Poly(ADP-ribose)-polymerase-1 (PARP-1) and poly(ADP-ribose) (PAR) are emerging key regulators of chromatin superstructure and transcriptional activation. Accordingly, both genetic inactivation of PARP-1 and pharmacological inhibition of PAR formation impair the expression of several genes, including those of the inflammatory response. In this study, we asked whether poly(ADP-ribose) glycohydrolase (PARG), the sole depoly(ADP-ribosyl)ating enzyme identified so far, also regulates gene expression. We report the novel finding that inhibition of PARG by gallotannin triggered nuclear accumulation of PAR and concomitant PAR-dependent expression of inducible NO synthase (iNOS) and cyclooxygenase-2 (COX-2), but not of interleukin-1beta and tumor necrosis factor-alpha, in cultured RAW 264.7 macrophages. Remarkably, silencing of PARG by means of small interfering RNA selectively impaired gallotannin-induced expression of iNOS and COX-2. Consistent with a PAR-dependent transcriptional activation, increases of iNOS and COX-2 transcripts were not caused by activation of transcription factors such as nuclear factor-kappaB, activator protein-1, signal transducer and activator of transcription-1 or interferon regulatory factor-1, nor by mRNA stabilization. Overall, our data provide the first evidence that pharmacological inhibition of PARG leads to PAR-dependent alteration of gene expression profiles in macrophages.

Cytoprotective effect of gallotannin in oxidatively stressed HaCaT keratinocytes: the role of poly(ADP-ribose) metabolism.

Oxidative stress-induced cytotoxicity is mediated in part by accelerated poly-ADP ribosylation. Peroxynitrite and hydrogen peroxide cause DNA breakage triggering the activation of the DNA nick sensor enzyme poly(ADP-ribose) polymerase-1 (PARP-1). Overactivation of PARP-1 leads to cell dysfunction and cell death mainly due to depletion of NAD(+) (the substrate of PARP-1) and ATP. PARP-1 attaches most ADP-ribose residues onto itself, leading to downregulation of enzyme activity. Here, we have investigated the role of poly(ADP-ribose) glycohydrolase (PARG), the poly(ADP-ribose)-catabolyzing enzyme in oxidative stress-induced cytotoxicity in HaCaT cells. We have found that inhibition of PARG by gallotannin (GT) (50 microM) provided significant cytoprotection to peroxynitrite- or hydrogen peroxide-treated HaCaT cells, as assessed by lactate dehydrogenase release and propidium iodide uptake (parameters of necrotic cell death) as well as caspase activation (apoptotic parameter). GT pretreatment has also inhibited the depletion of cellular NAD(+) pools in hydrogen peroxide- or peroxynitrite-treated HaCaT cells. GT caused the accumulation of poly(ADP-ribose) and concomitant inhibition in cellular PARP activity in oxidatively stressed cells. Therefore, PARG is likely to contribute to maintaining the active state of PARP-1 by continuously removing inhibitory ADP-ribose residues from PARP-1.

Poly(ADP-ribose) degradation by post-nuclear extracts from human cells

The nuclear metabolism of poly(ADP-ribose) is mainly regulated by poly(ADP-ribose) polymerase-1 (PARP-1) and by poly(ADP-ribose) glycohydrolase (PARG). A PARP-like enzyme, V-PARP, and a PARG isoform are present in the extra-nuclear compartment of mammalian cells, even if poly(ADP-ribose) has never been detected therein. In this work, we demonstrate the ability of post-nuclear extracts from HeLa and HL60 cells to degrade synthetic 32P-polymers of ADP-ribose to ADP-ribose and, further, to AMP. This reaction implies the combined action of PARG and of an ADP-ribose-degrading activity, possibly corresponding to a phosphodiesterase and/or to an ADP-ribose pyrophosphatase. The inhibition of PARG or ADP-ribose-degrading enzymes allowed the demonstration that in vitro synthesized 32P-poly(ADP-ribose) is first digested to ADP-ribose monomers by a typical PARG reaction, and that ADP-ribose is further rapidly converted into AMP by an Mg 2+-dependent activity. Collectively, our results demonstrate the ability of the human cell post-nuclear fraction to convert synthetic poly(ADP-ribose) into utilizable AMP units by the concerted action of PARG and ADP-ribose-degrading activities.