-Organic-inorganic hybrid molecules, which are composed of organic-ligand(s) and metal(s), are indispensable as synthetic reagents in chemistry, but they have made very little in the way of contributions to biological research. Previously, we reported that the cytotoxicity of organic-inorganic hybrid molecules in vascular endothelial cells depends on interactions between the intramolecular metal and ligand, but remains independent of the hydrophobicity of the intramolecular metal(s). Herein, we show a synergistic cytotoxicity produced by forming a complex of copper and 2,9-dimethyl-1,10-phenanthroline in vascular endothelial cells that depends on the intracellular accumulation of copper.