Complementing organic elements with a metal center provides new opportunities for building three-dimensional structures with unique and defined shapes. Such access to unexplored chemical space may lead to the discovery of molecules with unprecedented properties. Along these lines, this account article describes our successful design of highly potent and selective rutheniumbased inhibitors for the protein kinases GSK-3 and Pim-1 by using the class of indolocarbazole alkaloids as a lead structure. The described ruthenium complexes are kinetically inert scaffolds in which the ruthenium has the function to organize the orientation of the organic ligands in the three-dimensional space.

A chiral second-generation organoruthenium half-sandwich compound is disclosed that shows a remarkable selectivity and cellular potency for the inhibition of glycogen synthase kinase 3 (GSK-3). The selectivity was evaluated against a panel of 57 protein kinases, in which no other kinase was inhibited to the same extent, with a selectivity window of at least tenfold to more than 1000-fold at 100 microM ATP. Furthermore, a comparison with organic GSK-3 inhibitors demonstrated the superior cellular activity of this ruthenium compound: wnt signaling was fully induced at concentrations down to 30 nM. For comparison, the well-established organic GSK-3 inhibitors 6-bromoindirubin-3'-oxime (BIO) and kenpaullone activate the wnt pathway at concentrations that are higher by around 30-fold and 100-fold, respectively. The treatment of zebrafish embryos with the organometallic inhibitor resulted in a phenotype that is typical for the inhibition of GSK-3. No phenotypic change was observed with the mirror-imaged ruthenium complex. The latter does not, in fact, show any of the pharmacological properties for the inhibition of GSK-3. Overall, these results demonstrate the potential usefulness of organometallic compounds as molecular probes in cultured cells and whole organisms.

A general route to ruthenium pyridocarbazole half-sandwich complexes is presented and applied to the synthesis of sixteen new compounds, many of which have modulated protein kinase inhibition properties. For example, the incorporation of a fluorine into the pyridine moiety increases the binding affinity for glycogen synthase kinase 3 by almost one order of magnitude. These data are supplemented with cyclic voltammetry experiments and a protein co-crystallographic study.

BackgroundThe serine/threonine kinase PIM2 is highly expressed in human leukemia and lymphomas and has been shown to positively regulate survival and proliferation of tumor cells. Its diverse ATP site makes PIM2 a promising target for the development of anticancer agents. To date our knowledge of catalytic domain structures of the PIM kinase family is limited to PIM1 which has been extensively studied and which shares about 50% sequence identity with PIM2.Principal FindingsHere we determined the crystal structure of PIM2 in complex with an organoruthenium complex (inhibition in sub-nanomolar level). Due to its extraordinary shape complementarity this stable organometallic compound is a highly potent inhibitor of PIM kinases.SignificanceThe structure of PIM2 revealed several differences to PIM1 which may be explored further to generate isoform selective inhibitors. It has also demonstrated how an organometallic inhibitor can be adapted to the binding site of protein kinases to generate highly potent inhibitors.Enhanced version This article can also be viewed as an enhanced version in which the text of the article is integrated with interactive 3D representations and animated transitions. Please note that a web plugin is required to access this enhanced functionality. Instructions for the installation and use of the web plugin are available in Text S1.

Substitution mutations in the BRAF serine/threonine kinase are found in a variety of human cancers. Such mutations occur in ∼70% of human malignant melanomas, and a single hyperactivating V600E mutation is found in the activation segment of the kinase domain and accounts for more than 90% of these mutations. Given this correlation, the molecular mechanism for BRAF regulation as well as oncogenic activation has attracted considerable interest, and activated forms of BRAF, such as BRAF V600E , have become attractive targets for small molecule inhibition. Here we report on the identification and subsequent optimization of a potent BRAF inhibitor, CS292, based on an organometallic kinase inhibitor scaffold. A cocrystal structure of CS292 in complex with the BRAF kinase domain reveals that CS292 binds to the ATP binding pocket of the kinase and is an ATP competitive inhibitor. The structure of the kinase-inhibitor complex also demonstrates that CS292 binds to BRAF in an active conformation and suggests a mechanism for regulation of BRAF by phosphorylation and BRAF V600E oncogene-induced activation. The structure of CS292 bound to the active form of the BRAF kinase also provides a novel scaffold for the design of BRAF V600E oncogene selective BRAF inhibitors for therapeutic application.RAF 1 kinases were originally identified as cellular homologues of v-raf oncogenes acquired by retroviruses and contain three members: CRAF (RAF-1 or c-RAF-1), BRAF, and ARAF (1-3). RAF family kinases are central players in the highly conserved mitogen-activated protein kinase (MAPK) signaling pathway (RAS-RAF-MEK-ERK) which relays signals from the extracellular space through receptor tyrosine kinases (RTKs) to the nucleus to promote the expression of genes involved in cell proliferation and survival. RAF kinases function by specifically phosphorylating MEK1/2 within the kinase activation loop leading to the subsequent activation of MEK1/2, which in turn activates ERK1/2. Activated ERK1/2 translocates into the nucleus and activates transcription factors to promote cellular outcomes, including survival, growth, proliferation, and differentiation (4). RAF family kinases are † This work was supported by Grant CA 114046 from the National Institutes of Health.* To whom correspondence should be addressed. Telephone: (215) 898-5006. Fax: (215) 898-0381. marmor@wistar.org. ‖ Current address: Department of Chemistry, Philipps-Universität Marburg, Marburg, Germany 1 Abbreviations: CI, chemical ionization; c-KIT, proto-oncogene receptor tyrosine kinase; DCC, dicyclohexylcarbodiimide; DCM, dichloromethane; ERK, extracellular signal-regulated kinase; ES, electrospray; FDA, Food and Drug Administration; GSK3β, glycogen synthase kinase 3 β isoform; HRP, horseradish peroxidase; MAPK, mitogen-activated protein kinase; MEK, dual-specificity mitogenactivated protein kinase kinase; PDB, Protein Data Bank; PDGFR, platelet-derived growth factor receptor; PI3K, phosphatidylinositol 3-kinase; RAF, RAF proto-oncogene serine/threonine-protein kinase (sub...

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Mammalian sterile 20 (MST1) kinase, a member of the sterile 20 (Ste-20) family of proteins, is a proapoptotic cytosolic kinase that plays an important role in the cellular response to oxidative stress. In this study, we report on the development of a potent and selective MST1 kinase inhibitor based on a ruthenium half-sandwich scaffold. We show that the enantiopure organoruthenium inhibitor, 9E1, has an IC50 value of 45 nM for MST1 and a greater than 25-fold inhibitor selectivity over the related Ste-20 kinases, p21 activated kinase 1 (PAK1), and p21 activated kinase 4 (PAK4) and an almost 10-fold selectivity over the related Thousand and one amino acids kinase 2 (TAO2). Compound 9E1 also displays a promising selectivity profile against unrelated protein kinases, however, the proto-oncogene serine/threonine protein kinase PIM1 (PIM-1) and glycogen synthase kinase 3 (GSK-3β) are inhibited with IC50 values in the low nanomolar range. We also show that 9E1 can inhibit MST1 function in cells. A cocrystal structure of a related compound with PIM-1 and a homology model with MST1 reveals the binding mode of this scaffold to MST1 and provides a starting point for the development of improved MST1 kinase inhibitors for possible therapeutic application.

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