Original articleCoordination of nitro-thiosemicarbazones to ruthenium(II) as a strategy for anti-trypanosomal activity improvement
Graphical abstract
The association of a nitro-thiosemicarbazone with ruthenium(II), dppb = 1,4-bis(diphenylphospine)butane and bipyridine (bipy) or phenanthroline (phen) in one same complex proved to be an excellent strategy of anti-T. cruzi activity improvement.
Introduction
Trypanosoma cruzi (T. cruzi) is the etiologic agent of Chagas disease or American trypanosomiasis [1], [2]. Cruzain was shown to be one of the most relevant proteases in T. cruzi [2]. Thiosemicarbazones represent an interesting class of compounds with a wide range of pharmacological applications [3] and have been identified as lead scaffolds of cruzain inhibitors [1], [2]. Considering that many anti-trypanosomal drugs contain a nitro group, which produces the nitro anion radical, highly toxic to the parasite [4], [5], [6], we recently started an investigation of the pharmacological profile of 4-nitroacetophenone-derived thiosemicarbazones. We demonstrated that these thiosemicarbazones and their copper(II) complexes present significant in vitro anti-trypanosomal activity, the complexes resulting to be at least 5 times more active than the free ligands. [Cu(4NO2Ac4M)2], with N4-methyl-4-nitroacetophenone thiosemicarbazones (H4NO2Ac4M) proved to be at least 12.5 times more active than the thiosemicarbazone counterpart [7].
It has been shown that ruthenium complexes with inhibitors of sterol biosyntheses such as clotrimazole and ketoconazole are more active against T. cruzi than the corresponding free ligands [8]. [Ru(Cl2(CTZ)2] (CTZ = clotrimazole) was able to inhibit 90% of the proliferation of epimastigote form of T. cruzi at a concentration where the parent compound presented a modest effect. The complex was able to eradicate experimental infection by the highly infective intracellular amastigotes of T. cruzi grown on mammalian cells at concentrations as low as10−8 mol L−1, which represented a 10-fold enhancement of CTZ’s activity [8].
A series of ruthenium(II) complexes derived from 5-nitrofurylsemicarbazones (HL), [RuIICl2(DMSO)2HL], were developed by some of us, which were able to produce free radicals and redox cycling into the parasite [9]. However, their high protein binding capacity and hydrophilicity did not allow identifying in vitro active compounds against T. cruzi.
The strategy of linking a nitro-thiosemicarbazone to ruthenium could in principle lead to new anti-trypanosomal drug candidates. In the present work ruthenium(II) complexes containing N4-methyl-4-nitrobenzaldehyde thiosemicarbazone (H4NO2Fo4M), or N4-methyl-4-nitroacetophenone thiosemicarbazone (H4NO2Ac4M) and 1,4-bis(diphenylphospine)butane (dppb), 2,2′-bipyridine (bipy), 4,4′-dimethyl-2,2′-bipyridine (Mebipy) or 1,10-phenanthroline (phen) as co-ligands were prepared (Fig. 1).
The nitro-thiosemicarbazones, the new complexes and the ruthenium(II) precursors [RuCl2(bipy)(dppb)], [RuCl2(Mebipy)(dppb)], and [RuCl2(phen)(dppb)], were evaluated for their ability to in vitro suppress the growth of T. cruzi Tulahuen 2 strain [10].
Section snippets
Microanalyses and molar conductivity studies
Microanalyses suggest the formation of [RuCl(H4NO2Fo4M)(bipy)(dppb)]PF6 (1), [RuCl(H4NO2Fo4M)(Mebipy)(dppb)]PF6 (2), [RuCl(H4NO2Fo4M)(phen)(dppb)]PF6 (3), [RuCl(H4NO2Ac4M)(bipy)(dppb)]PF6 (4), [RuCl(H4NO2Ac4M)(Mebipy)(dppb)]PF6 (5) and [RuCl(H4NO2Ac4M)(phen)(dppb)]PF6 (6). The molar conductivity data indicate that all complexes are 1:1 electrolytes in accordance with the proposed formulations.
Infrared spectral studies
In the infrared spectra, the ν(C
N) stretching vibrations of H4NO2Fo4M and H4NO2Ac4M at 1599 cm−1 and
Conclusions
The developed ruthenium complexes (1–6) present a monodentate thiosemicarbazone, which is rarely found in the literature. The foregoing results indicate that the studied compounds might be investigated in vivo as anti-trypanosomal agents, since they are able to in vitro inhibit the growth of the parasite. Especially complex 2, with the lowest ID50 against T. cruzi, resulted non-toxic in the red blood cells assay, presenting a selectivity index (SI), defined as SI = ID50, red blood cells/ID50,
Physical measurements
Elemental analyses were performed on a Fison equipment, model EA 1108. A Radiometer Copenhagen Meter Lab., model CDM 230 was employed for molar conductivity measurements. Infrared spectra (KBr pellets) were obtained using a BOMEM MICHELSON instrument, model 102. NMR spectra were obtained at room temperature with a Bruker DRX-400 Avance (400 MHz) spectrometer. For 31P{1H} NMR (161 MHz) measurements CH2Cl2 was used as solvent and H3PO4 85% as external reference.
The electrochemical experiments
Supplementary material available
CCDC 702060 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments
Financial support from FAPEMIG, CNPq Instituto do Milênio-Inovação e Desenvolvimento de Novos Fármacos e Medicamentos (IM-INOFAR, Proc. CNPq 420015/05-1), CNPq-PROSUL and Fapesp from Brazil, RIDIMEDCHAG-CYTED is acknowledged.
References (30)
- et al.
J. Inorg. Biochem.
(2008) - et al.
J. Biol. Chem.
(1984) - et al.
Arch. Biochem. Biophys.
(1982) - et al.
Biochem. Pharmacol.
(1981) - et al.
Eur. J. Med. Chem.
(2008) - et al.
Eur. J. Med. Chem.
(2006) - et al.
Polyhedron
(2008) - et al.
Polyhedron
(2003) - et al.
Polyhedron
(2001) - et al.
J. Electroanal. Chem.
(2001)