The cytotoxicity of several simple ruthenium(III) compounds ([Cl(NH 3 ) 5 Ru]Cl 2 , cis -[Cl 2 (NH 3 ) 4 RU]Cl, [4-picoline(NH 3 ) 5 Ru]Cl 3 , ImH trans -[(Im) 2 Cl 4 Ru] (Im = imidazole and related species) and cell nuclear nucleic acid binding were studied in cultures of HeLa and Jurkat T ag cells. Cytotoxicity studies of cis -[Cl 2 (NH 3 ) 4 Ru]Cl and ImH trans -[(IM) 2 Cl 4 Ru] revealed that both complexes were moderately toxic to HeLa cells (IC 50 ∼ 5 μM). Both complexes became more toxic to the cells at reduced P O2 and with the addition of apotransferrin to the media. Both [Cl(NH 3 ) 5 Ru]Cl 2 and cis -[Cl 2 (NH 3 ) 4 Ru]Cl 2 bind to both apo and holotransferrin, under relatively mild reaction conditions. Ruthenium bound to apotransferrin was generally more toxic to Jurkat cells than was the unbound, free ruthenium(III) complex while ruthenium bound to holotransferrin was generally as, or less toxic than the corresponding free complex.
[Cl(NH 3 ) 5 Ru]Cl 2 was used to model the interactions between ruthenium prodrugs and glutathione, a cellular heavy metal scavenger. Glutathione first reduces [Cl(NH 3 ) 5 Ru] 2+ to [H 2 O(NH 3 ) 5 Ru] 2+ before coordinating and sequestering the metal ion as [GS(NH 3 ) 5 R II ], which can be oxidized to [GS(NH 3 ) 5 Ru III ] + . The necessity of reducing the metal ion to the more labile Ru II before sequestering results in competing effects with regard to nucleic acid or other biomolecular binding. At [GSH]<[Ru], the GSH is entirely utilized in reducing the metal ion to Ru II , which generally binds more rapidly to biological ligands than the corresponding Ru III complex. At [GSH]>[Ru], there is sufficient free GSH to sequester the Ru II ion; consequently, at [GSH]/[Ru] > 1, the GSH protects DNA and other biomolecules from metal binding.
Small hexanitrogen ligand complexes of Ru III , such as [(IM) 6 Ru]Cl 3 , (Im = imidazole) are extraordinarily active immunosuppressive agents, whose mechanism appears to involve electron transfer. Such complexes are reduced by HeLa and Jurkat T ag cells and studies were undertaken to determine whether the specific cytotoxicity of these agents against antigen-activated T cells might be the result of metal-ion induced apoptosis.
