MK-4101, a Potent Inhibitor of the Hedgehog Pathway, Is Highly Active against Medulloblastoma and Basal Cell Carcinoma.

Aberrant activation of the Hedgehog (Hh) signaling pathway is implicated in the pathogenesis of many cancers, including medulloblastoma and basal cell carcinoma (BCC). In this study, using neonatally irradiated Ptch1(+/-) mice as a model of Hh-dependent tumors, we investigated the in vivo effects of MK-4101, a novel SMO antagonist, for the treatment of medulloblastoma and BCC. Results clearly demonstrated a robust antitumor activity of MK-4101, achieved through the inhibition of proliferation and induction of extensive apoptosis in tumor cells. Of note, beside antitumor activity on transplanted tumors, MK-4101 was highly efficacious against primary medulloblastoma and BCC developing in the cerebellum and skin of Ptch1(+/-) mice. By identifying the changes induced by MK-4101 in gene expression profiles in tumors, we also elucidated the mechanism of action of this novel, orally administrable compound. MK-4101 targets the Hh pathway in tumor cells, showing the maximum inhibitory effect on Gli1 MK-4101 also induced deregulation of cell cycle and block of DNA replication in tumors. Members of the IGF and Wnt signaling pathways were among the most highly deregulated genes by MK-4101, suggesting that the interplay among Hh, IGF, and Wnt is crucial in Hh-dependent tumorigenesis. Altogether, the results of this preclinical study support a therapeutic opportunity for MK-4101 in the treatment of Hh-driven cancers, also providing useful information for combination therapy with drugs targeting pathways cooperating with Hh oncogenic activity. Mol Cancer Ther; 15(6); 1177-89. ©2016 AACR.

Identification of MK-5710 ((8aS)-8a-methyl-1,3-dioxo-2-[(1S,2R)-2-phenylcyclo- propyl]-N-(1-phenyl-1H-pyrazol-5-yl)hexahydro-imidazo[1,5-a]pyrazine-7(1H)-carboxamide), a potent smoothened antagonist for use in Hedgehog pathway dependent malignancies, part 2.

The Hedgehog (Hh-) signaling pathway is a key developmental pathway which gets reactivated in many human tumors, and smoothened (Smo) antagonists are emerging as novel agents for the treatment of malignancies dependent on the Hh-pathway, with the most advanced compounds demonstrating encouraging results in initial clinical trials. A novel series of potent bicyclic hydantoin Smo antagonists was reported in the preceding article, these have been resolved, and optimized to identify potent homochiral derivatives with clean off-target profiles and good pharmacokinetic properties in preclinical species. While showing in vivo efficacy in mouse allograft models, unsubstituted bicyclic tetrahydroimidazo[1,5-a]pyrazine-1,3(2H,5H)-diones were shown to epimerize in plasma. Alkylation of the C-8 position blocks this epimerization, resulting in the identification of MK-5710 (47) which was selected for further development.

Loss of histone deacetylase 4 causes segregation defects during mitosis of p53-deficient human tumor cells.

We investigated the role of histone deacetylase 4 (HDAC4) using RNA interference (RNAi) and knockout cells to specifically address its role in cell cycle progression in tumor and normal cells. Ablation of HDAC4 led to growth inhibition in human tumor cells but not to detectable effects in normal human dermal fibroblasts (NHDF) or myelopoietic progenitors. HDAC4-/+ or HDAC4-/- murine embryonic fibroblasts showed no detectable growth defects. On the other hand, HDAC4 RNAi in HeLa cells produced mitotic arrest followed by caspase-dependent apoptosis. Mitotically arrested cells showed chromosome segregation defects. Even though the growth of both p53-wild-type and p53-null tumor cells were affected by HDAC4 ablation, segregation defects were observed only in p53-null cells. HDAC4 associates with the PP2A-B56 regulatory subunit, which is known to be involved in chromosome segregation, and RNAi of either the structural subunit A or the regulatory subunit B56 of PP2A also caused chromosome segregation defects. We conclude that HDAC4 is required for cell cycle progression of tumor cells by multiple mechanisms, one of which seems to be specific to p53-deficient cells through chromosome segregation defects. On the contrary, HDAC4 is not required for the progression of NHDF. We therefore suggest that systemic selective interference with the expression or function of HDAC4 is expected to have a significant therapeutic window, in particular, for p53-deficient tumors.

Crystal structure of a eukaryotic zinc-dependent histone deacetylase, human HDAC8, complexed with a hydroxamic acid inhibitor.

Histone deacetylases (HDACs) are a family of enzymes involved in the regulation of gene expression, DNA repair, and stress response. These processes often are altered in tumors, and HDAC inhibitors have had pronounced antitumor activity with promising results in clinical trials. Here, we report the crystal structure of human HDAC8 in complex with a hydroxamic acid inhibitor. Such a structure of a eukaryotic zinc-dependent HDAC has not be described previously. Similar to bacterial HDAC-like protein, HDAC8 folds in a single alpha/beta domain. The inhibitor and the zinc-binding sites are similar in both proteins. However, significant differences are observed in the length and structure of the loops surrounding the active site, including the presence of two potassium ions in HDAC8 structure, one of which interacts with key catalytic residues. CD data suggest a direct role of potassium in the fold stabilization of HDAC8. Knockdown of HDAC8 by RNA interference inhibits growth of human lung, colon, and cervical cancer cell lines, highlighting the importance of this HDAC subtype for tumor cell proliferation. Our findings open the way for the design and development of selective inhibitors of HDAC8 as possible antitumor agents.

Mechanism of activation of human heparanase investigated by protein engineering.

The aim of this study was to investigate the mechanism of activation of human heparanase, a key player in heparan sulfate degradation, thought to be involved in normal and pathologic cell migration processes. Active heparanase arises as a product of a series of proteolytic processing events. Upon removal of the signal peptide, the resulting, poorly active 65 kDa species undergoes the excision of an intervening 6 kDa fragment generating an 8 kDa polypeptide and a 50 kDa polypeptide, forming the fully active heterodimer. By engineering of tobacco etch virus protease cleavage sites at the N- and C-terminal junctions of the 6 kDa fragment, we were able to reproduce the proteolytic activation of heparanase in vitro using purified components, showing that cleavage at both sites leads to activation in the absence of additional factors. On the basis of multiple-sequence alignment of the N-terminal fragment, we conclude that the first beta/alpha/beta element of the postulated TIM barrel fold is contributed by the 8 kDa subunit and that the excised 6 kDa fragment connects the second beta-strand and the second alpha-helix of the barrel. Substituting the 6 kDa fragment with the topologically equivalent loop from Hirudinaria manillensis hyaluronidase or connecting the 8 and 50 kDa fragments with a spacer of three glycine-serine pairs resulted in constitutively active, single-chain heparanases which were comparable to the processed, heterodimeric enzyme with regard to specific activity, chromatographic profile of hydrolysis products, complete inhibition at NaCl concentrations above 600 mM, a pH optimum of pH approximately 5, and inhibition by heparin with IC(50)s of 0.9-1.5 ng/microL. We conclude that (1) the heparanase heterodimer (alpha/beta)(8)-TIM barrel fold is contributed by both 8 and 50 kDa subunits with the 6 kDa connecting fragment leading to inhibition of heparanase by possibly obstructing access to the active site, (2) proteolytic excision of the 6 kDa fragment is necessary and sufficient for heparanase activation, and (3) our findings open the way to the production of recombinant, constitutively active single-chain heparanase for structural studies and for the identification of inhibitors.

A scintillation proximity active site binding assay for the hepatitis C virus serine protease.

A binding assay suitable for the identification of active site-directed inhibitors of the hepatitis C virus serine protease NS3 was developed. A C-terminal extension of 13 residues that is specifically recognized by the Escherichia coli biotin holoenzyme synthetase (Bir A) was fused to a truncated NS3 protease domain, allowing the efficient production of in vivo biotinylated protease. This enzyme was purified and shown to have the same properties as its wild-type counterpart concerning substrate binding and turnover, interaction with a cofactor peptide, and inhibition by three different classes of inhibitors. Immobilization of the biotinylated protease, using streptavidin-coated scintillation proximity beads, allowed detection, by scintillation counting, of its interaction with a tritiated active site ligand spanning the whole substrate binding site of the protease from P6 to P4('). Immobilization did not measurably affect accessibility to either the active site or the cofactor binding site of the protease as judged by the unchanged affinities for a cofactor peptide and for two active site binders. Using the displacement of the radioligand as readout, we were able to set up a rapid, robust, and fully automated assay, suitable for the selective identification of novel active site ligands of the NS3 protease.

A designed P1 cysteine mimetic for covalent and non-covalent inhibitors of HCV NS3 protease.

The difluoromethyl group was designed by computational chemistry methods as a mimetic of the canonical P1 cysteine thiol for inhibitors of the hepatitis C virus NS3 protease. This modification led to the development of competitive, non-covalent inhibitor 4 (K(i) 30 nM) and reversible covalent inhibitors (6, K(i) 0.5 nM; and 8 K*(i) 10 pM).