Bis-picolinamide Ruthenium(III) Dihalide Complexes: Dichloride-to-Diiodide Exchange Generates Single trans Isomers with High Potency and Cancer Cell Selectivity

A library of new bis-picolinamide ruthenium(III) dihalide complexes of the type [RuX₂L₂] (X=Cl or I, L=picolinamide) have been synthesised and characterised. The complexes exhibit different picolinamide ligand binding modes, whereby one ligand is bound (N,N) and the other bound (N,O). Structural studies revealed a mixture of cis and trans isomers for the [RuCl₂L₂] complexes but upon a halide exchange reaction to yield [RuI₂L₂], only single trans isomers were detected. High cytotoxic activity against human cancer cell lines was observed, with the potencies of some complexes similar to or better than cisplatin. The conversion to [RuI₂L₂] substantially increased the activity towards cancer cell lines by more than twelvefold. The [RuI₂L₂] complexes displayed potent activity against the A2780cis (cisplatin-resistant human ovarian cancer) cell line, with a more than fourfold higher potency than cisplatin. Equitoxic activity was observed against normoxic and hypoxic cancer cells, which indicates the potential to eradicate both the hypoxic and aerobic fractions of solid tumours with similar efficiency. The activity of selected complexes against non-cancer ARPE-19 cells was also tested. The [RuI₂L₂] complexes were found to be more potent than the [RuCl₂L₂] analogues and also more selective towards cancer cells with a selectivity factor in excess of sevenfold.