Discovery of 2-[1-(4,4-Difluorocyclohexyl)piperidin-4-yl]-6-fluoro-3-oxo-2,3-dihydro-1H-isoindole-4-carboxamide (NMS-P118): A Potent, Orally Available, and Highly Selective PARP-1 Inhibitor for Cancer Therapy.

The nuclear protein poly(ADP-ribose) polymerase-1 (PARP-1) has a well-established role in the signaling and repair of DNA and is a prominent target in oncology, as testified by the number of candidates in clinical testing that unselectively target both PARP-1 and its closest isoform PARP-2. The goal of our program was to find a PARP-1 selective inhibitor that would potentially mitigate toxicities arising from cross-inhibition of PARP-2. Thus, an HTS campaign on the proprietary Nerviano Medical Sciences (NMS) chemical collection, followed by SAR optimization, allowed us to discover 2-[1-(4,4-difluorocyclohexyl)piperidin-4-yl]-6-fluoro-3-oxo-2,3-dihydro-1H-isoindole-4-carboxamide (NMS-P118, 20by). NMS-P118 proved to be a potent, orally available, and highly selective PARP-1 inhibitor endowed with excellent ADME and pharmacokinetic profiles and high efficacy in vivo both as a single agent and in combination with Temozolomide in MDA-MB-436 and Capan-1 xenograft models, respectively. Cocrystal structures of 20by with both PARP-1 and PARP-2 catalytic domain proteins allowed rationalization of the observed selectivity.

Insights into PARP Inhibitors' Selectivity Using Fluorescence Polarization and Surface Plasmon Resonance Binding Assays.

PARP inhibitors are an exciting new class of antineoplastic drugs that have been proven to be efficacious as single agents in cancer settings with inherent DNA repair defects, as well as in combination with DNA-damaging chemotherapeutics. Currently, they are designed to target the catalytic domain of PARP-1, the most studied member of the family, with a key role in the DNA-damage repair process. Because PARP inhibitors are substrate (NAD(+)) competitors, there is a need for a deeper understanding of their cross-reactivity. This is particularly relevant for PARP-2, the PARP-1 closest homologue, for which an embryonic lethal phenotype has been observed in double knockout mice. In this study, we describe the development and validation of binding assays based on fluorescence polarization (FP) and surface plasmon resonance (SPR) techniques. PARP-1, PARP-2, PARP-3, and TNKS-1 FP displacement assays are set up by employing ad hoc synthesized probes. These assays are suitable for high-throughput screening (HTS) and selectivity profiling, thus allowing the identification of NAD(+)binding site selective inhibitors. The PARP-1 and PARP-2 complementary SPR binding assays confirm displacement data and the in-depth inhibitor characterization. Moreover, these formats have the potential to be broadly applicable to other members of the PARP family.

Identification of candidate substrates for poly(ADP-ribose) polymerase-2 (PARP2) in the absence of DNA damage using high-density protein microarrays.

Poly(ADP-ribose) polymerase-2 (PARP2) belongs to the ADP-ribosyltransferase family of enzymes that catalyze the addition of ADP-ribose units to acceptor proteins, thus affecting many diverse cellular processes. In particular, PARP2 shares with PARP1 and, as recently highlighted, PARP3 the sole property of being catalytically activated by DNA-strand breaks, implying key downstream functions in the cellular response to DNA damage for both enzymes. However, evidence from several studies suggests unique functions for PARP2 in additional processes, possibly mediated through its basal, DNA-damage unstimulated ADP-ribosylating activity. Here, we describe the development and application of a protein microarray-based approach tailored to identify proteins that are ADP-ribosylated by PARP2 in the absence of DNA damage mimetics and might thus represent useful entry points to the exploration of novel PARP2 functions. Several candidate substrates for PARP2 were identified and global hit enrichment analysis showed a clear enrichment in translation initiation and RNA helicase molecular functions. In addition, the top scoring candidates FK506-binding protein 3 and SH3 and cysteine-rich domain-containing protein 1 were selected and confirmed in a complementary assay format as substrates for unstimulated PARP2.

Identification of 4,5-dihydro-1H-pyrazolo[4,3-h]quinazoline derivatives as a new class of orally and selective Polo-like kinase 1 inhibitors.

Polo-like kinase 1 (Plk1) is a fundamental regulator of mitotic progression whose overexpression is often associated with oncogenesis and therefore is recognized as an attractive therapeutic target in the treatment of proliferative diseases. Here we discuss the structure-activity relationship of the 4,5-dihydro-1H-pyrazolo[4,3-h]quinazoline class of compounds that emerged from a high throughput screening (HTS) campaign as potent inhibitors of Plk1 kinase. Furthermore, we describe the discovery of 49, 8-{[2-methoxy-5-(4-methylpiperazin-1-yl)phenyl]amino}-1-methyl-4,5-dihydro-1H-pyrazolo[4,3-h]quinazoline-3-carboxamide, as a highly potent and specific ATP mimetic inhibitor of Plk1 (IC(50) = 0.007 microM) as well as its crystal structure in complex with the methylated Plk1(36-345) construct. Compound 49 was active in cell proliferation against different tumor cell lines with IC(50) values in the submicromolar range and active in vivo in the HCT116 xenograft model where it showed 82% tumor growth inhibition after repeated oral administration.

Regulation of the wild-type and Y1235D mutant Met kinase activation.

Met receptor tyrosine kinase plays a crucial role in the regulation of a large number of cellular processes and, when deregulated by overexpression or mutations, leads to tumor growth and invasion. The Y1235D mutation identified in metastases was shown to induce constitutive activation and a motile-invasive phenotype on transduced carcinoma cells. Wild-type Met activation requires phosphorylation of both Y1234 and Y1235 in the activation loop. We mapped the major phosphorylation sites in the kinase domain of a recombinant Met protein and identified the known residues Y1234 and Y1235 as well as a new phosphorylation site at Y1194 in the hinge region. Combining activating and silencing mutations at these sites, we characterized in depth the mechanism of activation of wild-type and mutant Met proteins. We found that the phosphotyrosine mimetic mutation Y1235D is sufficient to confer constitutive kinase activity, which is not influenced by phosphorylation at Y1234. However, the specific activity of this mutant was lower than that observed for fully activated wild-type Met and induced less phosphorylation of Y1349 in the signaling site, indicating that this mutation cannot entirely compensate for a phosphorylated tyrosine at this position. The Y1194F silencing mutation yielded an enzyme that could be activated to a similar extent as the wild type but with significantly slower activation kinetics, underlying the importance of this residue, which is conserved among different tyrosine kinase receptors. Finally, we observed different interactions of wild-type and mutant Met with the inhibitor K252a that may have therapeutic implications for the selective inhibition of this kinase.

High-throughput NMR-based screening with competition binding experiments.

The Achilles heel of ligand-based NMR screening methods is their failure to detect high-affinity ligands and molecules that bind covalently to the receptor. We have developed a novel approach for performing high-throughput screening with NMR spectroscopy that overcomes this limitation. The method also permits detection of potential high-affinity molecules that are only marginally soluble, thus significantly enlarging the diversity of compounds amenable to NMR screening. The techniques developed utilize transverse and/or selective longitudinal relaxation parameters in combination with competition binding experiments. Mathematical expressions are derived for proper setup of the NMR experiments and for extracting an approximate value of the binding constant for the identified ligand from a single-point measurement. With this approach it is possible to screen thousands of compounds in a short period of time against protein or DNA and RNA fragments. The methodology can also be applied for screening plant and fungi extracts.

Crystal structure of the tyrosine kinase domain of the hepatocyte growth factor receptor c-Met and its complex with the microbial alkaloid K-252a.

The protooncogene c-met codes for the hepatocyte growth factor receptor tyrosine kinase. Binding of its ligand, hepatocyte growth factor/scatter factor, stimulates receptor autophosphorylation, which leads to pleiotropic downstream signaling events in epithelial cells, including cell growth, motility, and invasion. These events are mediated by interaction of cytoplasmic effectors, generally through Src homology 2 (SH2) domains, with two phosphotyrosine-containing sequence motifs in the unique C-terminal tail of c-Met (supersite). There is a strong link between aberrant c-Met activity and oncogenesis, which makes this kinase an important cancer drug target. The furanosylated indolocarbazole K-252a belongs to a family of microbial alkaloids that also includes staurosporine. It was recently shown to be a potent inhibitor of c-Met. Here we report the crystal structures of an unphosphorylated c-Met kinase domain harboring a human cancer mutation and its complex with K-252a at 1.8-A resolution. The structure follows the well established architecture of protein kinases. It adopts a unique, inhibitory conformation of the activation loop, a catalytically noncompetent orientation of helix alphaC, and reveals the complete C-terminal docking site. The first SH2-binding motif (1349YVHV) adopts an extended conformation, whereas the second motif (1356YVNV), a binding site for Grb2-SH2, folds as a type II Beta-turn. The intermediate portion of the supersite (1353NATY) assumes a type I Beta-turn conformation as in an Shc-phosphotyrosine binding domain peptide complex. K-252a is bound in the adenosine pocket with an analogous binding mode to those observed in previously reported structures of protein kinases in complex with staurosporine.