Abstract 2681A: Structure-guided design of potent and selective inhibitors of B-Raf kinase displaying on-mechanism in vivo activity

The Ras-Raf-MAPK signaling pathway plays a central role in diverse cellular processes including differentiation, proliferation and survival. This pathway is activated in a large percentage of cancers as a result of mutations in Ras or in B-Raf Ser/Thr kinase. The activating mutation V600E in B-Raf is present in approximately 60% of melanomas and occurs with lower, yet still significant, frequency in human colon and thyroid cancers. Hence, V600EB-Raf has received considerable interest as a small-molecule drug discovery target within the pharmaceutical industry. A high-throughput screen of Amgen's internal kinase preferred library against a recombinant V600EB-Raf kinase domain enzyme identified a class of biarylamide compounds as potent inhibitors of this kinase, and potential starting points for medicinal chemistry efforts. However, these compounds suffered from poor kinase selectivity, particularly against tyrosine kinases from the Src-, VEGFR and PDGFR-families. Crystal structures of representative examples of this scaffold in B-Raf and other tyrosine kinases identified a unique pocket in B-Raf that could be accessed to achieve selective inhibitors of this enzyme. Consequently a novel series of isoquinoline-based compounds was derived which demonstrated potent inhibition against V600EB-Raf enzyme and inhibition of MAPK pathway signaling in cell lines harboring V600EB-Raf. Further optimization within this inhibitor class yielded compounds with favorable in vivo properties as demonstrated by robust inhibition of ERK phosphorylation in V600EB-Raf driven pharmacodynamic models. Crystallographic work supporting the optimization of this series of selective B-Raf inhibitors will be presented.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2681A.

14-3-3 proteins act as negative regulators of the mitotic inducer Cdc25 in Xenopus egg extracts

Cdc25, the dual-specificity phosphatase that dephosphorylates the Cdc2-cyclin B complex at mitosis, is highly regulated during the cell cycle. In Xenopus egg extracts, Cdc25 is associated with two isoforms of the 14-3-3 protein. Cdc25 is complexed primarily with 14-3-3epsilon and to a lesser extent with 14-3-3zeta. The association of these 14-3-3 proteins with Cdc25 varies dramatically during the cell cycle: binding is high during interphase but virtually absent at mitosis. Interaction with 14-3-3 is mediated by phosphorylation of Xenopus Cdc25 at Ser-287, which resides in a consensus 14-3-3 binding site. Recombinant Cdc25 with a point mutation at this residue (Cdc25-S287A) is incapable of binding to 14-3-3. Addition of the Cdc25-S287A mutant to Xenopus egg extracts accelerates mitosis and overrides checkpoint-mediated arrests of mitotic entry due to the presence of unreplicated and damaged DNA. These findings indicate that 14-3-3 proteins act as negative regulators of Cdc25 in controlling the G2-M transition.

The effect of amino acid substitutions in the conserved aromatic region of subunit II of cytochrome c oxidase in Saccharomyces cerevisiae

Mitochondrial encoded subunit II of cytochrome c oxidase carries the metal center, which acts as the initial acceptor of electrons from cytochrome c. Among the conserved features of this protein is a region in which five aromatic and three non-aromatic amino acids are conserved in a wide variety of organisms. This aromatic region has been postulated to be involved in transfer of electrons from the copper center in subunit II to the remaining metal centers of cytochrome oxidase in subunit I. To test the functional importance of two conserved, aromatic tryptophan residues and one conserved, non-aromatic glycine residue, yeast strains with alterations at these positions were characterized. The strains with altered codons were tested for their ability to carry out cellular respiration, for their growth rates on non-fermentable carbon sources, and for their cytochrome c oxidase activity. The results demonstrate that the aromatic character of the tryptophan residues appears necessary for subunit II function, while the conserved glycine can be replaced with other, small, uncharged residues.