Impact of the Halogen Substitution Pattern on the Biological Activity of Organoruthenium 8-Hydroxyquinoline Anticancer Agents

8-Hydroxyquinoline and its derivatives have a broad variety of pharmacological properties, which make them an ideal bioactive building block in the development of metal-based 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 8-hydroxyquinoline ligand, as well as the halido leaving group, a series of 5,7-dihalido-8-hydroxyquinoline Ru-II(eta(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.