In-Cell Activation of Organo-Osmium(II) Anticancer Complexes

The family of iodido Os^II arene phenylazopyridine complexes [Os(η⁶ -p-cym)(5-R¹ -pyridylazo-4-R² -phenyl))I]⁺ (where p-cym=para-cymene) exhibit potent sub-micromolar antiproliferative activity towards human cancer cells and are active in vivo. Their chemical behavior is distinct from that of cisplatin: they do not readily hydrolyze, nor bind to DNA bases. We report here a mechanism by which they are activated in cancer cells, involving release of the I^- ligand in the presence of glutathione (GSH). The X-ray crystal structures of two active complexes are reported, 1-I (R¹ =OEt, R² =H) and 2-I (R¹ =H, R² =NMe₂ ). They were labelled with the radionuclide ¹³¹ I (β^- /γ emitter, t_1/2 8.02 d), and their activity in MCF-7 human breast cancer cells was studied. 1-[¹³¹ I] and 2-[¹³¹ I] exhibit good stability in both phosphate-buffered saline and blood serum. In contrast, once taken up by MCF-7 cells, the iodide ligand is rapidly pumped out. Intriguingly, GSH catalyzes their hydrolysis. The resulting hydroxido complexes can form thiolato and sulfenato adducts with GSH, and react with H₂ O₂ generating hydroxyl radicals. These findings shed new light on the mechanism of action of these organo-osmium complexes.