Similar biological activities of two isostructural ruthenium and osmium complexes

Structure-activity relationships for organometallic osmium arene phenylazopyridine complexes with potent anticancer activity

We report the synthesis and characterisation of 32 half sandwich phenylazopyridine Os(II) arene complexes [Os(η(6)-arene)(phenylazopyridine)X](+) in which X is chloride or iodide, the arene is p-cymene or biphenyl and the pyridine and phenyl rings contain a variety of substituents (F, Cl, Br, I, CF(3), OH or NO(2)). Ten X-ray crystal structures have been determined. Cytotoxicity towards A2780 human ovarian cancer cells ranges from high potency at nanomolar concentrations to inactivity. In general the introduction of an electron-withdrawing group (e.g. F, Cl, Br or I) at specific positions on the pyridine ring significantly increases cytotoxic activity and aqueous solubility. Changing the arene from p-cymene to biphenyl and the monodentate ligand X from chloride to iodide also increases the activity significantly. Activation by hydrolysis and DNA binding appears not to be the major mechanism of action since both the highly active complex [Os(η(6)-bip)(2-F-azpy)I]PF(6) (9) and the moderately active complex [Os(η(6)-bip)(3-Cl-azpy)I]PF(6) (23) are very stable and inert towards aquation. Studies of octanol-water partition coefficients (log P) and subcellular distributions of osmium in A2780 human ovarian cancer cells suggested that cell uptake and targeting to cellular organelles play important roles in determining activity. Although complex 9 induced the production of reactive oxygen species (ROS) in A2780 cells, the ROS level did not appear to play a role in the mechanism of anticancer activity. This class of organometallic osmium complexes has new and unusual features worthy of further exploration for the design of novel anticancer drugs.

Organometallic Pyridylnaphthalimide Complexes as Protein Kinase Inhibitors

A new metal-containing scaffold for the design of protein kinase inhibitors is introduced. Key feature is a 3-(2-pyridyl)-1,8-naphthalimide "pharmacophore chelate ligand" which is designed to form two hydrogen bonds with the hinge region of the ATP-binding site and is at the same time capable of serving as a stable bidentate ligand through C-H-activation at the 4-position of the electron-deficient naphthalene moiety. This C-H-activation leads to a reduced demand for coordinating heteroatoms and thus sets the basis for a very efficient three-step synthesis starting from 1,8-naphthalic anhydride. The versatility of this ligand is demonstrated with the discovery of a ruthenium complex that functions as a nanomolar inhibitor for myosin light-chain kinase (MYLK or MLCK).

Organometallic cis-Dichlorido Ruthenium(II) Ammine Complexes

Bifunctional neutral half-sandwich RuII complexes of the type [(η6-arene)Ru(NH3)Cl2] where arene is p-cym (1) or bip (2) were synthesised by the reaction of N,N-dimethylbenzylamine (dmba), NH4PF6 and the corresponding RuII arene dimer, and were fully characterised. X-ray crystallographic studies of [(η6-p-cym)Ru(NH3)Cl2]·{(dmba-H)(PF6)} (1a) and [(η6-bip)Ru(NH3)Cl2] (2) show extensive H-bond interactions in the solid state, mainly involving the NH3 and the Cl ligands, as well as weak aromatic stacking interactions. The half-lives for the sequential hydrolysis of 1 and 2 determined by UV/Vis spectroscopy at 310 K ranged from a few minutes for the first aquation to ca. 45 min for the second aquation; the diaqua adducts were the predominant species at equilibrium. Arene loss during the aquation of complex 2 was observed. Upon hydrolysis, both complexes readily formed mono- and di-9-ethylguanine (9-EtG) adducts in aqueous solution at 310 K. The reaction reached equilibrium after ca. 1.8 h in the case of complex 1 and was slower but more complete for complex 2 (before the onset of arene loss at ca. 2.7 h). Complexes 1 and 2 were not cytotoxic towards A2780 human ovarian cancer cells up to the maximum concentration tested (100 μM).

Ruthenium anticancer compounds: myths and realities of the emerging metal-based drugs

Ruthenium anticancer drugs have attracted an increasing interest in the last 20 years and two of them have entered clinical trials. Compared to platinum drugs, the complexes based on ruthenium are often identified as less toxic and capable of overcoming the resistance induced by platinum drugs in cancer cells. These activities were attributed to the transportation to tumour cells by transferrin and to the selective activation to more reactive species by the reducing environment of solid tumours as compared to healthy tissues. Ruthenium anticancer drugs have been almost always designed to mimic platinum drugs, particularly for targeting DNA. Indeed, none of the above properties has never been clearly demonstrated even for the ruthenium drugs that entered clinical trials. The suggestion for the future is to change the perspective when designing new chemical entities, abandoning the philosophy that guided the actual panel of ruthenium drugs and to look further into the fine mechanism by which the most relevant ruthenium complexes available kill the target tumour cells, then focusing on targets selective of tumour cells and responsible for cell growth and malignancy.

Synthesis and Biological Evaluation of JAHAs: Ferrocene-Based Histone Deacetylase Inhibitors

N(1)-Hydroxy-N(8)-ferrocenyloctanediamide, JAHA (7), an organometallic analogue of SAHA containing a ferrocenyl group as a phenyl bioisostere, displays nanomolar inhibition of class I HDACs, excellent selectivity over class IIa HDACs, and anticancer action in intact cells (IC(50) = 2.4 μM, MCF7 cell line). Molecular docking studies of 7 in HDAC8 (a,b) suggested that the ferrocenyl moiety in 7 can overlap with the aryl cap of SAHA and should display similar HDAC inhibition, which was borne out in an in vitro assay (IC(50) values against HDAC8 (μM, SD in parentheses): SAHA, 1.41 (0.15); 7, 1.36 (0.16). Thereafter, a small library of related JAHA analogues has been synthesized, and preliminary SAR studies are presented. IC(50) values as low as 90 pM toward HDAC6 (class IIb) have been determined, highlighting the excellent potential of JAHAs as bioinorganic probes.

Structure of anticancer ruthenium half-sandwich complex bound to glycogen synthase kinase 3β

The 3.15-Å-resolution crystal structure of the R enantiomer of the highly bioactive and antiproliferative half-sandwich ruthenium complex DW12 bound to the ATP binding site of glycogen synthase kinase 3β (GSK-3β) is reported and the binding is compared with the GSK-3β binding of staurosporine and other organic inhibitors. The structure reveals a close packing of the organometallic inhibitor in the ATP binding site of GSK-3β via an induced-fit mechanism. The molecular structure of (R)-DW12 with the CO ligand oriented perpendicular to the pyridocarbazole heterocycle allows the complex to stretch the whole distance sandwiched between the faces of the N- and C-terminal lobes and to interact tightly with the flexible glycine-rich loop, which is uncommon for the interaction of GSK-3β with organic inhibitors.

From imide to lactam metallo-pyridocarbazoles: distinct scaffolds for the design of selective protein kinase inhibitors

Organometallic pyridocarbazole scaffolds are investigated as protein kinase inhibitors. Whereas our previous designs employed solely a maleimide pharmacophore for achieving the two crucial canonical hydrogen bonds to the hinge region of the ATP binding site, we have now extended our investigations to include the related lactam metallo-pyridocarbazoles. The synthetic access of the two regioisomeric lactam pyridocarbazoles is described, and the distinct biological properties of the two lactam scaffolds are revealed by employing a ruthenium half sandwich complex as a model system, resulting in organometallic lead structures for the inhibition of the protein kinases TrkA and CLK2. These new lactam metallo-pyridocarbazoles expand our existing molecular toolbox and assist toward the generation of metal complex scaffolds as lead structures for the design of selective inhibitors for numerous kinases of the human kinome.

Protein kinase inhibition of clinically important staurosporine analogues

The isolation in 1977 of the microbial alkaloid staurosporine inaugurated research into several distinct series of related natural and synthetic compounds. This has especially included research into applications as anticancer drugs, beginning with the observation of low nanomolar inhibition of protein kinases. At present, several staurosporine cognates are in advanced clinical trials as anticancer agents, with the potential to join the 10 other protein kinase inhibitors now approved for clinical use. Staurosporine is a broadly selective and potent protein kinase inhibitor, with submicromolar binding to the vast majority of the protein kinases tested, and binding most of them more tightly than 100 nM. Crystal structures have shown the extended buried surface area interactions between the protein kinase adenine binding site and the extended aromatic plane of the inhibitor, together with protein-saccharide interactions in the ribose binding site. Together with structures of closely related analogues, there are now some 70 X-ray crystal structures in the Protein Data Bank that enable analysis of target binding properties of the clinical compounds. In this manuscript we review the discovery of these compounds, revisit crystal structures and review the observed interactions. These support the interpretation of kinase selectivity profiles of staurosporine and its analogues, including midostaurin (PKC412), for which a co-crystal structure is not yet available. Further, the mix of purely natural, biosynthetically and chemically modified compounds described here offer insights into prospects and strategies for drug discovery via bioprospecting.

Inert ruthenium half-sandwich complexes with anticancer activity

In this study, we investigate the anticancer properties of an inert half-sandwich metal complex scaffold. UV melting experiments with duplex DNA and (1)H-NMR experiments with 9-ethylguanine reveal that the apoptotic ruthenium complex DW12 does not interact with DNA. On the other hand, diminishing the kinase inhibition properties of DW12 by methylating the maleimide nitrogen (DW12-Me) abolishes the anticancer activity. Furthermore, the incorporation of a fluorine into the pyridine moiety (NP309) improves the IC(50) value for glycogen synthase kinase 3 (GSK-3) and at the same time the cytotoxicity, implying that the anticancer activity correlates with the inhibition of GSK-3 and maybe other not yet identified kinases. We demonstrate in Burkitt-like lymphoma (BJAB) cells that NP309 is not necrotic but induces apoptosis and that this apoptosis is mediated by a loss of the mitochondrial membrane potential, caspase-9 processing, and is partly dependent on Bcl-2 expression. In addition, NP309 efficiently induces apoptosis in vincristine- and cytarabine-resistant human B-cell precursor cell lines.

Large-Scale Structural Biology of the Human Proteome

The large-scale structural biology projects that target human proteins focus predominantly on the catalytic domains of potential therapeutic targets and the domains of human proteins that mediate protein-protein and protein-small-molecule interactions. Their main scientific objective is to elucidate the molecular basis for specificity and selectivity of function within large protein families of therapeutic interest, such as kinases, phosphatases, and proteins involved in epigenetic regulation. Half of the unique human protein structures determined in the past three years derive from these initiatives.

Targeting proteins with metal complexes

Unique properties of metal complexes, such as structural diversity, adjustable ligand exchange kinetics, fine-tuned redox activities, and distinct spectroscopic signatures, make them exciting scaffolds not only for binding to nucleic acids but increasingly also to proteins as non-traditional targets. This feature article discusses recent trends in this field. These include the use of chemically inert metal complexes as structural scaffolds for the design of enzyme inhibitors, new strategies for inducing selective coordination chemistry at the protein binding site, recent advances in the development of catalytic enzyme inhibitors, and the design of metal complexes that can inject electrons or holes into redox enzymes. A common theme in many of the discussed examples is that binding selectivity is at least in part achieved through weak interactions between the ligand sphere and the protein binding site. These examples hint to an exciting future in which "organic-like" molecular recognition principles are combined with properties that are unique to metals and thus promise to yield compounds with novel and unprecedented properties.