Reatividade de compostos inorgânicos que inibem a 2-trans-enoil-ACP (COA) redutase, um alvo anti-tuberculose

Abstract

This study was performed with thionicotinamide, ethionamide, isothionicotinamide and isoniazid along with their inorganic compounds. Once these organic ligands are active against tuberculosis, this work has aimed to evaluate theirs reactivity and potentiality as metallodrug. The coordination compounds of pentacyanoferrate(II) with ethionamide and isothionicotinamide showed different coordination mode. The latter had similar spectroscopic properties of the complexes with thionicotinamide and isoniazid, which upon characterization indicated that the coordination occurred through the nitrogen of the pyridine ring. On the other hand, the characterization data for the complex with ethionamide indicated the coordination mode through thioamide group, probably due to the steric hindrance of ethyl group. As expected, we observed that the pyridine compounds with substitution on "y" position have improved back-bonding effect upon coordination to pentacyanoferrate(II). Additionally, electron transfer reactivity studies of the free thioamide ligands suggested the following general order of reactivity: isothionicotinamide > ethionamide > thionicotinamide. Hexacyanoferrate(III) was used as a functional model for ligand oxidation showing interesting results. It produced similar intermediate (S-oxide for thioamides) and final products as observed by enzymatic reactions with KatG or EtaA. Moreover the reaction with isoniazid and NAD+ triggered by this inorganic oxidizing compound provided the NAD-IZD inhibitor as KatG does. Additional reactivity studies were performed with hydrogen peroxide and these ligands showed superoxide formation. More particularly, thioamide species showed stronger superoxide formation than isoniazid following the same pattern of reactivity above noticed by spectroscopy. These results suggest that thioamide compounds could exhibit an important superoxide production role in vivo as a side effect against tuberculosis that has been suggested for isoniazid. The coordinated ligands exhibited higher reactivity compared with the free species in solution. The oxidations processes produced similar intermediates and final compounds as using the free ligands along with their native enzymes. These results suggest our initial proposal concerning the activity improvement of these compounds. Excited about this noble possibility we pursued biochemical studies. Interesting assays were performed with wild and mutant InhA enzyme, which is the main target of these free drugs. Those coordination compounds inhibited InhA enzyme faster than isoniazid on its better reaction conditions as well as the mutant enzyme. Furthermore their inhibition worked even without KatG, activating enzyme, and NADH suggesting their efficiencies against resistant strain of M. tuberculosis. Despite this inhibition takes place with a reasonable broad range of pentacyanoferrate(II) complexes it does not occur with similar complexes such as pentacyanoruthenate(Il), pentaamimuthenium(II) or hEDTA iron complexes. This result implies InhA inhibition is close related with pentacyanoferrate(II) chemistry. Biological assays performed in collaboration with the Biotechnology Group of UFRGS and UFES showed that the complex with isoniazid presents low toxicity and anti-tuberculosis activity (MIC = 0.2 µg/mL). It was interesting to notice that even the pentaamineruthenium(lI) complex, unable to inhibit InhA, have presented activity in vitro. MIC of these compounds was as low as for free isoniazid. Finally, a low visual hepatoxicity in mouse when using coordinated isoniazid was surprisingly noticed. These striking results open new avenues for metallodrug design against mycobacteria. This work is a result of a reactivity evaluation followed by a rational inorganic drug design in attempt to develop anti-tuberculosis metallodrugs. Of course, further investigations are necessary to define the real mechanism and select a suitable compound to be used as metallodrug anti-tuberculosis able to overcome current deficiencies such as resistance and long term treatment.