4-(Phenoxy) and 4-(benzyloxy)benzamides as potent and selective inhibitors of mono-ADP-ribosyltransferase PARP10/ARTD10.

Human Diphtheria toxin-like ADP-ribosyltranferases (ARTD) 10 is an enzyme carrying out mono-ADP-ribosylation of a range of cellular proteins and affecting their activities. It shuttles between cytoplasm and nucleus and influences signaling events in both compartments, such as nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling and S phase DNA repair. Furthermore, overexpression of ARTD10 induces cell death. We recently reported on the discovery of a hit compound, OUL35 (compound 1), with 330 nM potency and remarkable selectivity towards ARTD10 over other enzymes in the human protein family. Here we aimed at establishing a structure-activity relationship of the OUL35 scaffold, by evaluating an array of 4-phenoxybenzamide derivatives. By exploring modifications on the linker between the aromatic rings, we identified also a 4-(benzyloxy)benzamide derivative, compound 32, which is potent (IC50 = 230 nM) and selective, and like OUL35 was able to rescue HeLa cells from ARTD10-induced cell death. Evaluation of an enlarged series of derivatives produced detailed knowledge on the structural requirements for ARTD10 inhibition and allowed the discovery of further tool compounds with submicromolar cellular potency that will help in understanding the roles of ARTD10 in biological systems.

2-Phenylquinazolinones as dual-activity tankyrase-kinase inhibitors.

Tankyrases (TNKSs) are enzymes specialized in catalyzing poly-ADP-ribosylation of target proteins. Several studies have validated TNKSs as anti-cancer drug targets due to their regulatory role in Wnt/ß-catenin pathway. Recently a lot of effort has been put into developing more potent and selective TNKS inhibitors and optimizing them towards anti-cancer agents. We noticed that some 2-phenylquinazolinones (2-PQs) reported as CDK9 inhibitors were similar to previously published TNKS inhibitors. In this study, we profiled this series of 2-PQs against TNKS and selected kinases that are involved in the Wnt/ß-catenin pathway. We found that they were much more potent TNKS inhibitors than they were CDK9/kinase inhibitors. We evaluated the compound selectivity to tankyrases over the ARTD enzyme family and solved co-crystal structures of the compounds with TNKS2. Comparative structure-based studies of the catalytic domain of TNKS2 with selected CDK9 inhibitors and docking studies of the inhibitors with two kinases (CDK9 and Akt) revealed important structural features, which could explain the selectivity of the compounds towards either tankyrases or kinases. We also discovered a compound, which was able to inhibit tankyrases, CDK9 and Akt kinases with equal µM potency.

Development of an Inhibitor Screening Assay for Mono-ADP-Ribosyl Hydrolyzing Macrodomains Using AlphaScreen Technology.

Protein mono-ADP-ribosylation is a posttranslational modification involved in the regulation of several cellular signaling pathways. Cellular ADP-ribosylation is regulated by ADP-ribose hydrolases via a hydrolysis of the protein-linked ADP-ribose. Most of the ADP-ribose hydrolases share a macrodomain fold. Macrodomains have been linked to several diseases, such as cancer, but their cellular roles are mostly unknown. Currently, there are no inhibitors available targeting the mono-ADP-ribose hydrolyzing macrodomains. We have developed a robust AlphaScreen assay for the screening of inhibitors against macrodomains having mono-ADP-ribose hydrolysis activity. We utilized this assay for validatory screening against human MacroD1 and identified five compounds inhibiting the macrodomain. Dose-response measurements and an orthogonal assay further validated four of these compounds as MacroD1 inhibitors. The developed assay is homogenous, easy to execute, and suitable for the screening of large compound libraries. The assay principle can also be adapted for other ADP-ribose hydrolyzing macrodomains, which can utilize a biotin-mono-ADP-ribosylated protein as a substrate.

Discovery of a Novel Series of Tankyrase Inhibitors by a Hybridization Approach.

A structure-guided hybridization approach using two privileged substructures gave instant access to a new series of tankyrase inhibitors. The identified inhibitor 16 displays high target affinity on tankyrase 1 and 2 with biochemical and cellular IC50 values of 29 nM, 6.3 nM and 19 nM, respectively, and high selectivity toward other poly (ADP-ribose) polymerase enzymes. The identified inhibitor shows a favorable in vitro ADME profile as well as good oral bioavailability in mice, rats, and dogs. Critical for the approach was the utilization of an appropriate linker between 1,2,4-triazole and benzimidazolone moieties, whereby a cyclobutyl linker displayed superior affinity compared to a cyclohexane and phenyl linker.

Small-Molecule Chemical Probe Rescues Cells from Mono-ADP-Ribosyltransferase ARTD10/PARP10-Induced Apoptosis and Sensitizes Cancer Cells to DNA Damage.

Members of the human diphtheria toxin-like ADP-ribosyltransferase (ARTD or PARP) family play important roles in regulating biological activities by mediating either a mono-ADP-ribosylation (MARylation) of a substrate or a poly-ADP-ribosylation (PARylation). ARTD10/PARP10 belongs to the MARylating ARTDs (mARTDs) subfamily, and plays important roles in biological processes that range from cellular signaling, DNA repair, and cell proliferation to immune response. Despite their biological and disease relevance, no selective inhibitors for mARTDs are available. Here we describe a small-molecule ARTD10 inhibitor, OUL35, a selective and potent inhibitor for this enzyme. We characterize its selectivity profile, model its binding, and demonstrate activity in HeLa cells where OUL35 rescued cells from ARTD10 induced cell death. Using OUL35 as a cell biology tool we show that ARTD10 inhibition sensitizes the cells to the hydroxyurea-induced genotoxic stress. Our study supports the proposed role of ARTD10 in DNA-damage repair and provides a tool compound for selective inhibition of ARTD10-mediated MARylation.

Development and structural analysis of adenosine site binding tankyrase inhibitors.

Tankyrases 1 and 2, the specialized members of the ARTD protein family, are druggable biotargets whose inhibition may have therapeutic potential against cancer, metabolic disease, fibrotic disease, fibrotic wound healing and HSV viral infections. We have previously identified a novel tankyrase inhibitor scaffold, JW55, and showed that it reduces mouse colon adenoma formation in vivo. Here we expanded the scaffold and profiled the selectivity of the compounds against a panel of human ARTDs. The scaffold also enables a fine modulation of selectivity towards either tankyrase 1 or tankyrase 2. In order to get insight about the binding mode of the inhibitors, we solved crystal structures of the compounds in complex with tankyrase 2. The compounds bind to the adenosine pocket of the catalytic domain and cause changes in the protein structure that are modulated by the chemical modifications of the compounds. The structural analysis allows further rational development of this compound class as a potent and selective tankyrase inhibitor.

Discovery of potent and selective nonplanar tankyrase inhibiting nicotinamide mimics.

Diphtheria toxin-like ADP-ribosyltransferases catalyse a posttranslational modification, ADP-ribosylation and form a protein family of 17 members in humans. Two of the family members, tankyrases 1 and 2, are involved in several cellular processes including mitosis and Wnt/ß-catenin signalling pathway. They are often over-expressed in cancer cells and have been linked with the survival of cancer cells making them potential therapeutic targets. In this study, we identified nine tankyrase inhibitors through virtual and in vitro screening. Crystal structures of tankyrase 2 with the compounds showed that they bind to the nicotinamide binding site of the catalytic domain. Based on the co-crystal structures we designed and synthesized a series of tetrahydroquinazolin-4-one and pyridopyrimidin-4-one analogs and were subsequently able to improve the potency of a hit compound almost 100-fold (from 11 µM to 150 nM). The most potent compounds were selective towards tankyrases over a panel of other human ARTD enzymes. They also inhibited Wnt/ß-catenin pathway in a cell-based reporter assay demonstrating the potential usefulness of the identified new scaffolds for further development.

Discovery of tankyrase inhibiting flavones with increased potency and isoenzyme selectivity.

Tankyrases are ADP-ribosyltransferases that play key roles in various cellular pathways, including the regulation of cell proliferation, and thus, they are promising drug targets for the treatment of cancer. Flavones have been shown to inhibit tankyrases and we report here the discovery of more potent and selective flavone derivatives. Commercially available flavones with single substitutions were used for structure-activity relationship studies, and cocrystal structures of the 18 hit compounds were analyzed to explain their potency and selectivity. The most potent inhibitors were also tested in a cell-based assay, which demonstrated that they effectively antagonize Wnt signaling. To assess selectivity, they were further tested against a panel of homologous human ADP-ribosyltransferases. The most effective compound, 22 (MN-64), showed 6 nM potency against tankyrase 1, isoenzyme selectivity, and Wnt signaling inhibition. This work forms a basis for rational development of flavones as tankyrase inhibitors and guides the development of other structurally related inhibitors.

para-Substituted 2-phenyl-3,4-dihydroquinazolin-4-ones as potent and selective tankyrase inhibitors.

Human tankyrases are attractive drug targets, especially for the treatment of cancer. We identified a set of highly potent tankyrase inhibitors based on a 2-phenyl-3,4-dihydroquinazolin-4-one scaffold. Substitutions at the para position of the scaffold's phenyl group were evaluated as a strategy to increase potency and improve selectivity. The best compounds displayed single-digit nanomolar potencies, and profiling against several human diphtheria-toxin-like ADP-ribosyltransferases revealed that a subset of these compounds are highly selective tankyrase inhibitors. The compounds also effectively inhibit Wnt signaling in HEK293 cells. The binding mode of all inhibitors was studied by protein X-ray crystallography. This allowed us to establish a structural basis for the development of highly potent and selective tankyrase inhibitors based on the 2-phenyl-3,4-dihydroquinazolin-4-one scaffold and outline a rational approach to the modification of other inhibitor scaffolds that bind to the nicotinamide site of the catalytic domain.

Structural basis and selectivity of tankyrase inhibition by a Wnt signaling inhibitor WIKI4.

Recently a novel inhibitor of Wnt signaling was discovered. The compound, WIKI4, was found to act through tankyrase inhibition and regulate ß-catenin levels in many cancer cell lines and human embryonic stem cells. Here we confirm that WIKI4 is a high potency tankyrase inhibitor and that it selectively inhibits tankyrases over other ARTD enzymes tested. The binding mode of the compound to tankyrase 2 was determined by protein X-ray crystallography to 2.4 Å resolution. The structure revealed a novel binding mode to the adenosine subsite of the donor NAD(+) binding groove of the catalytic domain. Our results form a structural basis for further development of potent and selective tankyrase inhibitors based on the WIKI4 scaffold.