Edith C. Glazer

Strained ruthenium complexes are potent light-activated anticancer agents.

Strained ruthenium (Ru) complexes have been synthesized and characterized as novel agents for photodynamic therapy (PDT). The complexes are inert until triggered by visible light, which induces ligand loss and covalent modification of DNA. An increase in cytotoxicity of 2 orders of magnitude is observed with light activation in cancer cells, and the compounds display potencies superior to cisplatin against 3D tumor spheroids. The use of intramolecular strain may be applied as a general paradigm to develop light-activated ruthenium complexes for PDT applications.

Eilatin Ru(II) Complexes Display Anti‐HIV Activity and Enantiomeric Diversity in the Binding of RNA

Eilatin‐containing octahedral ruthenium complexes inhibit HIV‐1 replication in CD4+ HeLa cells and in human peripheral blood monocytes with IC50 values of approximately 1 μM. Similar metal complexes that lack eilatin display 15–100‐fold lower anti‐HIV activities. [Ru(bpy)2“pre‐eilatin”]2+, a complex that contains a nonplanar analogue of eilatin, shows significantly lower nucleic acid binding and lower anti‐HIV activity than eilatin complexes. This result indicates that the extended planar surface presented by eilatin is important for both activities. Rev peptide and ethidium bromide displacement assays are used to probe the nucleic acid affinity and specificity of Λ‐ and Δ‐[Ru(bpy)2eilatin]2+. Two HIV‐1 RNA sites are compared and a significant binding preference for the Rev response element over the transactivation response region is found. Simple DNA duplexes show a consistent selectivity for Λ‐[Ru(bpy)2eilatin]2+ compared to Δ‐[Ru(bpy)2eilatin]2+, while RNAs show more diverse enantiomeric selectivities.

Modifying Charge and Hydrophilicity of Simple Ru(II) Polypyridyl Complexes Radically Alters Biological Activities: Old Complexes, Surprising New Tricks

Compounds capable of light-triggered cytotoxicity are appealing potential therapeutics, because they can provide spatial and temporal control over cell killing to reduce side effects in cancer therapy. Two simple homoleptic Ru(II) polypyridyl complexes with almost-identical photophysical properties but radically different physiochemical properties were investigated as agents for photodynamic therapy (PDT). The two complexes were identical, except for the incorporation of six sulfonic acids into the ligands of one complex, resulting in a compound carrying an overall −4 charge. The negatively charged compound exhibited significant light-mediated cytotoxicity, and, importantly, the negative charges resulted in radical alterations of the biological activity, compared to the positively charged analogue, including complete abrogation of toxicity in the dark. The charges also altered the subcellular localization properties, mechanism of action, and even the mechanism of cell death. The incorporation of negative charged ligands provides a simple chemical approach to modify the biological properties of light-activated Ru(II) cytotoxic agents.

Symmetrical dinuclear complexes with high DNA affinity based on [Ru(dpq)2(phen)]2+Electronic supplementary information (ESI) available: method for determining equilibrium binding constants and representative spectroscopic data for each experiment. See http://www.rsc.org/suppdata/cc/b3/b316917k/

Symmetrical homometallic dinuclear complexes of the type [(Ru(dpq)2)2(phen-SOS-phen)]4+, with a flexible 2-mercaptoethyl ether linker joining the two [Ru(dpq)2(phen)]2+-based sub-units, have DNA dissociation constants (Kd) in the nM range.

Photoactive Ru(II) complexes with dioxinophenanthroline ligands are potent cytotoxic agents.

Two novel strained ruthenium(II) polypyridyl complexes containing a 2,3-dihydro-1,4-dioxino[2,3-f]-1,10-phenanthroline (dop) ligand selectively ejected a methylated ligand when irradiated with >400 nm light. The best compound exhibited a 1880-fold increase in cytotoxicity in human cancer cells upon light-activation and was 19-fold more potent than the well-known chemotherapeutic, cisplatin.

Three Clusters of Conformational States in P450cam Reveal a Multistep Pathway for Closing of the Substrate Access Channel

Conformational changes in the substrate access channel have been observed for several forms of cytochrome P450, but the extent of conformational plasticity exhibited by a given isozyme has not been completely characterized. Here we present crystal structures of P450cam bound to a library of 12 active site probes containing a substrate analogue tethered to a variable linker. The structures provide a unique view of the range of protein conformations accessible during substrate binding. Principal component analysis of a total of 30 structures reveals three discrete clusters of conformations: closed (P450cam-C), intermediate (P450cam-I), and fully open (P450cam-O). Relative to P450cam-C, the P450cam-I state results predominantly from a retraction of helix F, while both helices F and G move in concert to reach the fully open P450cam-O state. Both P450cam-C and P450cam-I are well-defined states, while P450cam-O shows evidence of a somewhat broader distribution of conformations and includes the open form recently seen in the absence of substrate. The observed clustering of protein conformations over a wide range of ligand variants suggests a multistep closure of the enzyme around the substrate that begins by conformational selection from an ensemble of open conformations and proceeds through a well-defined intermediate, P450cam-I, before full closure to the P450cam-C state in the presence of small substrates. This multistep pathway may have significant implications for a full understanding of substrate specificity, kinetics, and coupling of substrate binding to P450 function.

Coordination of Hydroxyquinolines to a Ruthenium Bis-dimethyl-phenanthroline Scaffold Radically Improves Potency for Potential as Antineoplastic Agents

A series of ruthenium coordination complexes containing hydroxyquinoline ligands were synthesized that exhibited radically improved potencies up to 86-fold greater than clioquinol, a known cytotoxic compound. The complexes were also >100-fold more potent than clioquinol in a tumor spheroid model, with values similar to currently used chemotherapeutics for the treatment of solid tumors. Cytotoxicity occurs through rapid processes that induce apoptosis but appear to be mediated by cell-cycle independent mechanisms. The ruthenium complexes do not inhibit the proteasome at concentrations relevant for cell death, and contrary to previous reports, clioquinol and other hydroxyquinoline compounds do not act as direct proteasome inhibitors to induce cell death.

Light-sensitive ruthenium complex-loaded cross-linked polymeric nanoassemblies for the treatment of cancer

This work focuses on improving the efficacy of photoactivatable Ru complexes for photodynamic therapy by employing cross-linked nanoassemblies (CNAs) as a delivery approach. The effects of complex photoactivation, hydrophobicity, and solution ionic strength and pH on complex loading and release from CNAs were analyzed. The cell cytotoxicity of CNA formulations was similar to free Ru complexes despite reduced or eliminated DNA interactions. The release rate and the amount of each Ru complex released (%) varied inversely with complex hydrophobicity, while the effect of solution ionic strength was dependent on complex hydrophobicity. Premature release of two photoactivatable prodrugs prior to irradiation was believed to account for higher activity in cells studies compared to DNA interaction studies; however, for photostable 1O2 generator-loaded CNAs this cannot explain the high cytotoxicity and lack of DNA interactions because release was incomplete after 48 hrs. The cause remains unclear, but among other possibilities, accelerated release in a cell culture environment may be responsible.

A light-activated metal complex targets both DNA and RNA in a fluorescent in vitro transcription and translation assay.

A coupled in vitro transcription and translation (IVTT) assay that uses GFP as a fluorescent reporter allowed the potency of a light‐activated cytotoxic ruthenium agent to be quantified. The compound inhibits the function of both DNA and mRNA only upon light activation. The IVTT functional assay provides estimates of potency that are consistent with cellular cytotoxicity values, in marked contrast to the values obtained from traditional DNA‐damage assays.

Photochemical Properties and Structure‐Activity Relationships of Ru(II) Complexes with Pyridyl‐benzazole Ligands as Promising Anticancer Agents

Ruthenium complexes capable of light-triggered cytotoxicity are appealing potential prodrugs for photodynamic therapy (PDT) and photoactivated chemotherapy (PACT). Two groups of Ru(II) polypyridyl complexes with 2-(2-pyridyl)-benzazole ligands were synthesized and investigated for their photochemical properties and anticancer activity to compare strained and unstrained systems that are likely to have different biological mechanisms of action. The structure-activity relationship was focused on the benzazole core bioisosterism and replacement of coligands in Ru(II) complexes. Strained compounds rapidly ejected the 2-(2-pyridyl)-benzazole ligand after light irradiation, and possessed strong toxicity in the HL-60 cell line both under dark and light conditions. In contrast, unstrained Ru(II) complexes were non-toxic in the absence of light, induced cytotoxicity at nanomolar concentrations after light irradiation, and are capable of light-induced DNA damage. The 90-220-fold difference in light and dark IC50 values provides a large potential therapeutic window to allow for selective targeting of cells by exposure to light.