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 μM against trypomastigotes and a 50% inhibitory concentration (IC50) of 1.3 ± 0.2 μM against amastigotes, while it displayed a 50% cytotoxic concentration (CC50) of 51.4 ± 0.2 μM 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 μmol/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.