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DNA damage repair enzymes are promising targets in the development of new therapeutic agents for a wide range of cancers and potentially other diseases. The enzyme poly(ADP-ribose) glycohydrolase (PARG) plays a pivotal role in the regulation of DNA repair mechanisms; however, the lack of potent drug-like inhibitors for use in cellular and in vivo models has limited the investigation of its potential as a novel therapeutic target. Using the crystal structure of human PARG in complex with the weakly active and cytotoxic anthraquinone 8a, novel quinazolinedione sulfonamides PARG inhibitors have been identified by means of structure-based virtual screening and library design. 1-Oxetan-3-ylmethyl derivatives 33d and 35d were selected for preliminary investigations in vivo. X-ray crystal structures help rationalize the observed structure–activity relationships of these novel inhibitors.

A series of reversible inhibitors of lysine specific demethylase 1 (LSD1) with a 5-hydroxypyrazole scaffold have been developed from compound 7, which was identified from the patent literature. Surface plasmon resonance (SPR) and biochemical analysis showed it to be a reversible LSD1 inhibitor with an IC50 value of 0.23 µM. Optimisation of this compound by rational design afforded compounds with Kd values of

The recently discovered enzyme tyrosyl-DNA phosphodiesterase 2 (TDP2) has been implicated in the topoisomerase-mediated repair of DNA damage. In the clinical setting, it has been hypothesized that TDP2 may mediate drug resistance to topoisomerase II (topo II) inhibition by etoposide. Therefore, selective pharmacological inhibition of TDP2 is proposed as a novel approach to overcome intrinsic or acquired resistance to topo II-targeted drug therapy. Following a high-throughput screening (HTS) campaign, toxoflavins and deazaflavins were identified as the first reported sub-micromolar and selective inhibitors of this enzyme. Toxoflavin derivatives appeared to exhibit a clear structure-activity relationship (SAR) for TDP2 enzymatic inhibition. However, we observed a key redox liability of this series, and this, alongside early in vitro drug metabolism and pharmacokinetics (DMPK) issues, precluded further exploration. The deazaflavins were developed from a singleton HTS hit. This series showed distinct SAR and did not display redox activity; however low cell permeability proved to be a challenge.

Novel derivatives of the steroid DHEA 1, a known uncompetitive inhibitor of G6PD, were designed, synthesized, and tested for their ability to inhibit this dehydrogenase enzyme. Several compounds with approximately 10-fold improved potency in an enzyme assay were identified, and this improved activity translated to efficacy in a cellular assay. The SAR for steroid inhibition of G6PD has been substantially developed; the 3β-alcohol can be replaced with 3β-H-bond donors such as sulfamide, sulfonamide, urea, and carbamate. Improved potency was achieved by replacing the androstane nucleus with a pregnane nucleus, provided a ketone at C-20 is present. For pregnan-20-ones incorporation of a 21-hydroxyl group is often beneficial. The novel compounds generally have good physicochemical properties and satisfactory in vitro DMPK parameters. These derivatives may be useful for examining the role of G6PD inhibition in cells and will assist the future design of more potent steroid inhibitors with potential therapeutic utility.

In recent years, many proteins involved in DNA repair, such as ATR, ATM and PARP, have received considerable attention as potential points of therapeutic intervention in cancer. Indeed, these efforts have recently delivered several agents into clinical evaluation or FDA regulatory approval. However, the DNA repair protein poly(ADP ribose) glycohydrolase (PARG), which plays an equally critical role in DNA single stand break repair, to successful drug discovery efforts. Through our innovative collaboration with AstraZeneca, we have discovered a novel PARG-binding pharmacophore and have employed this information to discover drug-like chemotypes, facilitating the development of potent and selective inhibitors. This poster will describe our emerging results in this area, where a novel benzimidazolone sulphonamide scaffold has been shown potently to inhibit PARG in both biochemical and cellular assays with potencies of 40 nM and 60 nM respectively. Moreover, these agents display pharmacology consistent with the anticipated mode of action, appropriate drug-like properties and are selective against PARP1 and the close glycohydrolase homologue ARH3. The medicinal chemistry optimisation of this scaffold will be described, alongside the recent biological results obtained. Ultimately, this work has helped deliver tool compounds which may help to elucidate the true pharmacology and roles of PARG in cancer and other disease settings. Citation Format: Allan Jordan, Ben Acton, Nicola Hamilton, James Hitchin, Colin Hutton, Dominic James, Cliff Jones, Stuart Jones, Alison McGonagle, Helen Small, Kate Smith, Alex Stowell, Julie Tucker, Ian Waddell, Bohdan Waszkowycz, Donald Ogilvie. Benzimidazolone sulphonamides - potent, selective and drug-like 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 3715.

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.

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.