Design and characterization of ebolavirus GP prehairpin intermediate mimics as drug targets.

Ebolaviruses are highly lethal filoviruses that cause hemorrhagic fever in humans and nonhuman primates. With no approved treatments or preventatives, the development of an anti-ebolavirus therapy to protect against natural infections and potential weaponization is an urgent global health need. Here, we describe the design, biophysical characterization, and validation of peptide mimics of the ebolavirus N-trimer, a highly conserved region of the GP2 fusion protein, to be used as targets to develop broad-spectrum inhibitors of ebolavirus entry. The N-trimer region of GP2 is 90% identical across all ebolavirus species and forms a critical part of the prehairpin intermediate that is exposed during viral entry. Specifically, we fused designed coiled coils to the N-trimer to present it as a soluble trimeric coiled coil as it appears during membrane fusion. Circular dichroism, sedimentation equilibrium, and X-ray crystallography analyses reveal the helical, trimeric structure of the designed N-trimer mimic targets. Surface plasmon resonance studies validate that the N-trimer mimic binds its native ligand, the C-peptide region of GP2. The longest N-trimer mimic also inhibits virus entry, thereby confirming binding of the C-peptide region during viral entry and the presence of a vulnerable prehairpin intermediate. Using phage display as a model system, we validate the suitability of the N-trimer mimics as drug screening targets. Finally, we describe the foundational work to use the N-trimer mimics as targets in mirror-image phage display, which will be used to identify D-peptide inhibitors of ebolavirus entry.

Discovery of (2S)-1-[4-(2-{6-amino-8-[(6-bromo-1,3-benzodioxol-5-yl)sulfanyl]-9H-purin-9-yl}ethyl)piperidin-1-yl]-2-hydroxypropan-1-one (MPC-3100), a purine-based Hsp90 inhibitor.

Modulation of Hsp90 (heat shock protein 90) function has been recognized as an attractive approach for cancer treatment, since many cancer cells depend on Hsp90 to maintain cellular homeostasis. This has spurred the search for small-molecule Hsp90 inhibitors. Here we describe our lead optimization studies centered on the purine-based Hsp90 inhibitor 28a containing a piperidine moiety at the purine N9 position. In this study, key SAR was established for the piperidine N-substituent and for the congeners of the 1,3-benzodioxole at C8. These efforts led to the identification of orally bioavailable 28g that exhibits good in vitro profiles and a characteristic molecular biomarker signature of Hsp90 inhibition both in vitro and in vivo. Favorable pharmacokinetic properties along with significant antitumor effects in multiple human cancer xenograft models led to the selection of 28g (MPC-3100) as a clinical candidate.

Characterization of the cellular and antitumor effects of MPI-0479605, a small-molecule inhibitor of the mitotic kinase Mps1.

Mps1 is a dual specificity protein kinase that is essential for the bipolar attachment of chromosomes to the mitotic spindle and for maintaining the spindle assembly checkpoint until all chromosomes are properly attached. Mps1 is expressed at high levels during mitosis and is abundantly expressed in cancer cells. Disruption of Mps1 function induces aneuploidy and cell death. We report the identification of MPI-0479605, a potent and selective ATP competitive inhibitor of Mps1. Cells treated with MPI-0479605 undergo aberrant mitosis, resulting in aneuploidy and formation of micronuclei. In cells with wild-type p53, this promotes the induction of a postmitotic checkpoint characterized by the ATM- and RAD3-related-dependent activation of the p53-p21 pathway. In both wild-type and p53 mutant cells lines, there is a growth arrest and inhibition of DNA synthesis. Subsequently, cells undergo mitotic catastrophe and/or an apoptotic response. In xenograft models, MPI-0479605 inhibits tumor growth, suggesting that drugs targeting Mps1 may have utility as novel cancer therapeutics.

Analogues of 4-[(7-Bromo-2-methyl-4-oxo-3H-quinazolin-6-yl)methylprop-2-ynylamino]-N-(3-pyridylmethyl)benzamide (CB-30865) as potent inhibitors of nicotinamide phosphoribosyltransferase (Nampt).

We have shown previously that the target of the potent cytotoxic agent 4-[(7-bromo-2-methyl-4-oxo-3H-quinazolin-6-yl)methyl-prop-2-ynylamino]-N-(3-pyridylmethyl)benzamide (CB38065, 1) is nicotinamide phosphoribosyltransferase (Nampt). With its cellular target known we sought to optimize the biochemical and cellular Nampt activity of 1 as well as its cytotoxicity. It was found that a 3-pyridylmethylamide substituent in the A region was critical to cellular Nampt activity and cytotoxicity, although other aromatic substitution did yield compounds with submicromolar enzymatic inhibition. Small unsaturated groups worked best in the D-region of the molecule, with 3,3-dimethylallyl providing optimal potency. The E region required a quinazolin-4-one or 1,2,3-benzotriazin-4-one group for activity, and many substituents were tolerated at C² of the quinazolin-4-one. The best compounds showed subnanomolar inhibition of Nampt and low nanomolar cytotoxicity in cellular assays.

Chemical proteomics identifies Nampt as the target of CB30865, an orphan cytotoxic compound.

Drug discovery based on cellular phenotypes is impeded by the challenge of identifying the molecular target. To alleviate this problem, we developed a chemical proteomic process to identify cellular proteins that bind to small molecules. CB30865 is a potent (subnanomolar) and selective cytotoxic compound of previously unknown mechanism of action. By combining chemical proteomics with biochemical and cellular pharmacology we have determined that CB30865 cytotoxicity is due to subnanomolar inhibition of nicotinamide phosphoribosyltransferase (Nampt), an enzyme present in the NAD biosynthetic pathway. Cancer cells develop dependence on Nampt due to increased energy requirements and the elevated activity of NAD consuming enzymes such as sirtuins and mono and poly(ADP-ribose) polymerases (PARPs). These findings suggest new chemical starting points for Nampt inhibitors and further implicate this enzyme as a target in cancer.

The human endosomal sorting complex required for transport (ESCRT-I) and its role in HIV-1 budding.

Efficient human immunodeficiency virus type 1 (HIV-1) budding requires an interaction between the PTAP late domain in the viral p6(Gag) protein and the cellular protein TSG101. In yeast, Vps23p/TSG101 binds both Vps28p and Vps37p to form the soluble ESCRT-I complex, which functions in sorting ubiquitylated protein cargoes into multivesicular bodies. Human cells also contain ESCRT-I, but the VPS37 component(s) have not been identified. Bioinformatics and yeast two-hybrid screening methods were therefore used to identify four novel human proteins (VPS37A-D) that share weak but significant sequence similarity with yeast Vps37p and to demonstrate that VPS37A and VPS37B bind TSG101. Detailed studies produced four lines of evidence that human VPS37B is a Vps37p ortholog. 1) TSG101 bound to several different sites on VPS37B, including a putative coiled-coil region and a PTAP motif. 2) TSG101 and VPS28 co-immunoprecipitated with VPS37B-FLAG, and the three proteins comigrated together in soluble complexes of the correct size for human ESCRT-I ( approximately 350 kDa). 3) Like TGS101, VPS37B became trapped on aberrant endosomal compartments in the presence of VPS4A proteins lacking ATPase activity. 4) Finally, VPS37B could recruit TSG101/ESCRT-I activity and thereby rescue the budding of both mutant Gag particles and HIV-1 viruses lacking native late domains. Further studies of ESCRT-I revealed that TSG101 mutations that inhibited PTAP or VPS28 binding blocked HIV-1 budding. Taken together, these experiments define new components of the human ESCRT-I complex and characterize several TSG101 protein/protein interactions required for HIV-1 budding and infectivity.