8-Hydroxyquinoline and its derivatives have a broad variety of pharmacological properties, which make them an ideal bioactive building block in the development of metalbased anticancer drugs. In this account we aimed to rationalize the antiproliferative efficacy of organoruthenium compounds featuring 8-hydroxyquinoline-derived ligands and to elucidate structural determinants by using biological assays and bioanalytical methods. By systematically varying the halide substitution pattern at the 5-and 7-positions of the 8hydroxyquinoline ligand, as well as the halido leaving group, a series of 5,7-dihalido-8-hydroxyquinoline Ru II (η 6 -p-cymene) complexes were obtained. Studies on their cytotoxic activity revealed the minor impact of the substitution pattern (with the exception of complexes of 8-hydroxyquinoline) on their activity. Notably, the cellular accumulation showed no correlation with the cytotoxic activity, while the nature of the halido leaving group only had a significant influence in the case of the 8-hydroxyquinoline organoruthenium compounds. However, the compounds were shown to be very stable under a wide variety of pH conditions, making them possible candidates for further development as orally active anticancer agents.

One of the most promising classes of iron chelators are α-N-heterocyclic thiosemicarbazones with Triapine as the most prominent representative. In several clinical trials Triapine showed anticancer activity against hematological diseases, however, studies on solid tumors failed due to widely unknown reasons. Some years ago, it was recognized that “terminal dimethylation” of thiosemicarbazones can lead to a more than 100-fold increased activity, probably due to interactions with cellular copper depots. To better understand the structural requirements for the switch to nanomolar cytotoxicity, we systematically synthesized all eight possible N-methylated derivatives of Triapine and investigated their potential against Triapine-sensitive as well as -resistant cell lines. While only the “completely” methylated compound exerted nanomolar activity, the data revealed that all compounds with at least one N-dimethylation were not affected by acquired Triapine resistance. In addition, these compounds were highly synergistic with copper treatment accompanied by induction of reactive oxygen species and massive necrotic cell death.

The promise of the metal(arene) structure as an anticancer pharmacophore has prompted intensive exploration of this chemical space. While N-heterocyclic carbene (NHC) ligands are widely used in catalysis, they have only recently been considered in metal complexes for medicinal applications. Surprisingly, a comparatively small number of studies have been reported in which the NHC ligand was coordinated to the Ru II (arene) pharmacophore and even less with an Os II (arene) pharmacophore. Here, we present a systematic study in which we compared symmetrically substituted methyl and benzyl derivatives with the nonsymmetric methyl/benzyl analogues. Through variation of the metal center and the halido ligands, an in-depth study was conducted on ligand exchange properties of these complexes and their biomolecule binding, noting in particular the stability of the M−C NHC bond. In addition, we demonstrated the ability of the complexes to inhibit the selenoenzyme thioredoxin reductase (TrxR), suggested as an important target for anticancer metal−NHC complexes, and their cytotoxicity in human tumor cells. It was found that the most potent TrxR inhibitor diiodido(1,3-dibenzylbenzimidazol-2-ylidene)(η 6 -p-cymene)ruthenium(II) 1b I was also the most cytotoxic compound of the series, with the antiproliferative effects in general in the low to middle micromolar range. However, since there was no clear correlation between TrxR inhibition and antiproliferative potency across the compounds, TrxR inhibition is unlikely to be the main mode of action for the compound type and other target interactions must be considered in future.

Anticancer active metal complexes with biologically active ligands have the potential to interact with more than one biological target, which could help to overcome acquired and/or intrinsic resistance of tumors to small molecule drugs. In this paper we present the preparation of 2-hydroxy-[1,4]-naphthoquinone-derived ligands and their coordination to a Ru(II)(η(6)-p-cymene)Cl moiety. The synthesis of oxime derivatives resulted in the surprising formation of nitroso-naphthalene complexes, as confirmed by X-ray diffraction analysis. The compounds were shown to be stable in aqueous solution but reacted with glutathione and ascorbic acid rather than undergoing reduction. One-electron reduction with pulse radiolysis revealed different behavior for the naphthoquinone and nitroso-naphthalene complexes, which was also observed in in vitro anticancer assays.