Structure-dependent in vitro cytotoxicity of the isomeric complexes [Ru(L)2Cl2] (L=o-tolylazopyridine and 4-methyl-2-phenylazopyridine) in comparison to [Ru(azpy)2Cl2]

The dichlorobis(2-phenylazopyridine)ruthenium(II) complexes, [Ru(azpy)2Cl2], are under renewed investigation due to their potential anticancer activity. The three most common isomers α-, β- and γ-[RuL2Cl2] with L=o-tolylazopyridine (tazpy) and 4-methyl-2-phenylazopyridine (mazpy) (α indicating the coordinating Cl, N(pyridine) and Nazo atoms in mutual cis, trans, cis positions, β indicating the coordinating Cl, N(pyridine) and Nazo atoms in mutual cis, cis, cis positions, and γ indicating the coordinating Cl, N(pyridine) and Nazo atoms in mutual trans, cis, cis positions) are synthesized and characterized by NMR spectroscopy. The molecular structures of γ-[Ru(tazpy)2Cl2] and α-[Ru(mazpy)2Cl2] are determined by X-ray diffraction analysis. The IC50 values of the geometrically isomeric [Ru(tazpy)2Cl2] and [Ru(mazpy)2Cl2] complexes compared with those of the parent [Ru(azpy)2Cl2] complexes are determined in a series of human tumour cell lines (MCF-7, EVSA-T, WIDR, IGROV, M19, A498 and H266). These data unambiguously show for all complexes the following trend: the α isomer shows a very high cytotoxicity, whereas the β isomer is a factor 10 less cytotoxic. The γ isomers of [Ru(tazpy)2Cl2] and [Ru(mazpy)2Cl2] display a very high cytotoxicity comparable to that of the γ isomer of the parent compound [Ru(azpy)2Cl2] and to that of the α isomer. These biological data are of the utmost importance for a better understanding of the structure–activity relationships for the isomeric [RuL2Cl2] complexes.