Bryan T. Forrest

The Discovery of PLK4 Inhibitors: (E)-3-((1H-Indazol-6-yl)methylene)indolin-2-ones as Novel Antiproliferative Agents

The family of Polo-like kinases is important in the regulation of mitotic progression; this work keys on one member, namely Polo-like kinase 4 (PLK4). PLK4 has been identified as a candidate anticancer target which prompted a search for potent and selective inhibitors of PLK4. The body of the paper describes lead generation and optimization work which yielded nanomolar PLK4 inhibitors. Lead generation began with directed virtual screening, using a ligand-based focused library and a PLK4 homology model. Validated hits were used as starting points for the design and discovery of PLK4 inhibitors of novel structure, namely (E)-3-((1H-indazol-6-yl)methylene)indolin-2-ones. Computational models, based on a published X-ray structure (PLK4 kinase domain), were used to understand and optimize the in vitro activity of the series; potent antiproliferative activity was obtained. The kinase selectivity profile and cell cycle analysis of selected inhibitors are described. The results of a xenograft study with an optimized compound 50 (designated CFI-400437) support the potential of these novel PLK4 inhibitors for cancer therapy.

2,3,4,5-Tetrahydro-1H-pyrido[2,3-b and e][1,4]diazepines as inhibitors of the bacterial enoyl ACP reductase, FabI

In the search for new antibacterial agents, the enzyme FabI has been identified as an attractive target. Employing a structure guided approach, the previously reported ene-amide series of FabI inhibitors were expanded to include 2,3,4,5-tetrahydro-1H-pyrido[2,3-b and e][1,4]diazepines. These novel series incorporate additional H-bonding functions and can be more water soluble than their naphthyridinone progenitors; diazepine 16c is shown to be efficacious in a mouse infection model.

The Discovery of Orally Bioavailable Tyrosine Threonine Kinase (TTK) Inhibitors: 3-(4-(heterocyclyl)phenyl)-1H-indazole-5-carboxamides as Anticancer Agents

The acetamido and carboxamido substituted 3-(1H-indazol-3-yl)benzenesulfonamides are potent TTK inhibitors. However, they display modest ability to attenuate cancer cell growth; their physicochemical properties, and attendant pharmacokinetic parameters, are not drug-like. By eliminating the polar 3-sulfonamide group and grafting a heterocycle at the 4 position of the phenyl ring, potent inhibitors with oral exposure were obtained. An X-ray cocrystal structure and a refined binding model allowed for a structure guided approach. Systematic optimization resulted in novel TTK inhibitors, namely 3-(4-(heterocyclyl)phenyl)-1H-indazole-5-carboxamides. Compounds incorporating the 3-hydroxy-8-azabicyclo[3.2.1]octan-8-yl bicyclic system were potent (TTK IC50 < 10 nM, HCT116 GI50 < 0.1 μM), displayed low off-target activity (>500×), and microsomal stability (T(1/2) > 30 min). A subset was tested in rodent PK and mouse xenograft models of human cancer. Compound 75 (CFI-401870) recapitulated the phenotype of TTK RNAi, demonstrated in vivo tumor growth inhibition upon oral dosing, and was selected for preclinical evaluation.

Abstract LB-215: Inhibition of Polo-like kinase 4 as an anti-cancer strategy

Polo-like kinase 4 or PLK4 is a member of a conserved family of serine/threonine protein kinases that regulate multiple cellular processes, such as cell division and checkpoint regulation of mitosis. These kinases are often deregulated in cancer. PLK4 is the most structurally divergent PLK, localizes to centrosomes and is a critical regulator of centriole duplication. Over-expression of PLK4 leads to centrosome amplification and results in chromosome instability (CIN), a common characteristic observed in many types of cancers. We found that PLK4 is upregulated in breast cancer, specifically in the basal-like subtype. PLK4 expression is induced by hypoxia and suppressed by p53 in cancer cells. Consistent with this observation, PLK4 expression in cancer cells is upregulated when they are implanted and grown as xenografts in vivo. Furthermore, RNAi-mediated depletion of PLK4 inhibits the growth of cancer cells, but not normal cells (HMEC), in vitro, and tumor growth in vivo. Interestingly, siRNA knockdown of PLK4 sensitizes cancer cells to hypoxia. These findings suggest that targeting PLK4 may be a good therapeutic strategy in treating certain cancers. To this end, we initiated a discovery program that resulted in the identification of potent PLK4 inhibitors. These novel inhibitors are potent anti-prolifeartives, cause loss of mitotic checkpoint followed by apoptotic cell death, and suppress tumor growth in xenograft models. Mechanistically, inhibition of PLK4 suppresses phosphorylation of PLK4 and Histone H3, leads to failure of centrosome clustering and formation of multipolar spindles. Interestingly, breast cancer cell response to PLK4 inhibition appears to differ among subtypes of breast cancer cells and to be influenced by receptor and mutation status, such as ER and PTEN. Since multipolar division in cancer cells is not viable, due to massive missegregation of chromosomes, inhibition of PLK4 and formation of multipolar division followed by cell death may be a unique strategy for killing cancer cells. Implications of these findings in treating cancer will also be discussed. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-215. doi:10.1158/1538-7445.AM2011-LB-215