Nitro/Nitrosyl-Ruthenium Complexes Are Potent and Selective Anti-Trypanosoma cruzi Agents Causing Autophagy and Necrotic Parasite Death

cis-[RuCl(NO2)(dppb)(5,5'=-mebipy)] (complex 1), cis-[Ru(NO2)(2)(dppb)(5,5'=-mebipy)] (complex 2), ct-[RuCl(NO)(dppb)(5,5'=-mebipy)](PF6)(2) (complex 3), and cc-[RuCl(NO)(dppb)(5,5'=-mebipy)](PF6)(2) (complex 4), where 5,5'=-mebipy is 5,5'=-dimethyl-2,2'=-bipyridine and dppb is 1,4-bis(diphenylphosphino) butane, were synthesized and characterized. The structure of complex 2 was determined by X-ray crystallography. These complexes exhibited a higher anti-Trypanosoma cruzi activity than benznidazole, the current antiparasitic drug. Complex 3 was the most potent, displaying a 50% effective concentration (EC50) of 2.1 +/- 0.6 mu M against trypomastigotes and a 50% inhibitory concentration (IC50) of 1.3 +/- 0.2 mu M against amastigotes, while it displayed a 50% cytotoxic concentration (CC50) of 51.4 +/- 0.2 mu M in macrophages. It was observed that the nitrosyl complex 3, but not its analog lacking the nitrosyl group, releases nitric oxide into parasite cells. This release has a diminished effect on the trypanosomal protease cruzain but induces substantial parasite autophagy, which is followed by a series of irreversible morphological impairments to the parasites and finally results in cell death by necrosis. In infected mice, orally administered complex 3 (five times at a dose of 75 mu mol/kg of body weight) reduced blood parasitemia and increased the survival rate of the mice. Combination index analysis of complex 3 indicated that its in vitro activity against trypomastigotes is synergic with benznidazole. In addition, drug combination enhanced efficacy in infected mice, suggesting that ruthenium-nitrosyl complexes are potential constituents for drug combinations.