Aza-stilbenes as potent and selective c-RAF inhibitors Octerloney McDonald, Karen Lackey, Ronda Davis-Ward, Edgar Wood, Vicente Samano, Patrick Maloney, Felix Deanda and Robert Hunter * GlaxoSmithKline, Research Triangle Park, NC 27709, USA Received 23 May 2006; revised 19 July 2006; accepted 21 July 2006 Available online 4 August 2006 Abstract—The synthesis of several novel aza-stilbene derivatives was carried out. The compounds were tested for their c-RAF enzyme inhibition. Compound 27 possesses significant potency against c-RAF and demonstrates selectivity over other protein kinases. A hypothesis for the binding mode, activity, and selectivity is proposed. Ó 2006 Elsevier Ltd. All rights reserved. The kinome has become a widely targeted class of sig- naling proteins for drug intervention points due to the multitude of cellular regulatory roles protein kinases play. 1 At the basic level, all protein kinase enzymes bind a molecule of ATP and peptide substrate in order to propagate the signal cascade via transfer of the terminal phosphate group to a tyrosine or serine/threonine. Early research in the area targeted the protein substrate bind- ing domain under the assumption that there was a great- er probability for achieving selectivity. 2,3 Pioneering work on piceatannol derivatives and diversely substitut- ed tyrphostins showed a number of very important con- cepts in the field of kinase inhibition. 4,5 Peptide mimics of the protein substrate proved difficult to make signifi- cantly potent as compared with ATP binding kinase activity inhibitors. 6 However, small changes in substitu- tion patterns of non-peptide inhibitors of the protein substrate phosphorylation, especially compound varia- tions that would normally emerge from a medicinal chemistry analog program, demonstrated interesting mechanism of inhibition changes (Fig. 1). 7 For example, polyhydroxylated styrenes 1 and stilbenes (piceatannol) were inhibitors by virtue of their ability to compete with peptide substrate in a kinase enzyme assay. 5 When the stilbene scaffold was substituted with halogens, as in 2, the inhibition of kinase activity was competitive with ATP, suggesting a different binding interaction. 8 In our research, we sought to use these types of small molecules as potential starting points in our effort to find diverse series to inhibit the MAPK signaling path- way since this may offer an ideal point to block cellular proliferation in cancer cells. 9 Examples of c-RAF inhib- itors currently in the literature include Sorfenib (BAY- 439006) and GW5074. 11,12 We began by performing a substructure search of the GlaxoSmithKline compound collection to select a few thousand compounds for screening in a kinase enzyme cascade assay that included c-RAF/MEK-1/ERK-2. 10 0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2006.07.063 Keywords: Kinase; c-RAF; Aza-stilbene. * Corresponding author. Tel.: +1 919 483 5463; e-mail: bob.n.hunter@gsk.com O H O H OH OH O H O H CN CN N Cl Cl CN Piceatannol Tyrphostin (1) competative. vs ATP (2) Figure 1. Historical development of modulating kinase activity via peptide substrate inhibition shifting to ATP competitive inhibition. Bioorganic & Medicinal Chemistry Letters 16 (2006) 5378–5383