Binding of drugs to plasma proteins, such as albumin, is a major factor which determines their pharmacokinetics and pharmacological effects. Therefore, the interactions between human serum albumin (HSA) and four antimalarial compounds selected in the 2-aryl-3H-indol-3-one series have been investigated using UV-visible, fluorescence and circular dichroism (CD) spectroscopies. Compounds produced a static quenching of the intrinsic fluorescence of HSA. The thermodynamic parameters have shown that the binding reaction is endothermic for three compounds while exothermic for the 2-phenyl-3H-indol-3-one, 3. The interaction is entropically driven with predominant hydrophobic forces with binding affinities of the order of 10(4) M(-1). The highest binding constant is observed for 3 (Kλ=280nm = 4.53 × 10(4) M(-1)) which is also the less active compound against Plasmodium falciparum. Synchronous fluorescence gave qualitative information on the conformational changes of HSA while quantitative data were obtained with CD. Displacement experiments with site markers indicated that drugs bind to HSA at site I (subdomain IIA). In addition, the apparent binding constant and the binding site number were calculated in the presence of different ions.
Indolone-N-oxides have antiplasmodial properties against Plasmodium falciparum at the erythrocytic stage, with IC50 values in the nanomolar range. The mechanism of action of indolone derivatives involves the production of free radicals, which follows their bioreduction by an unknown mechanism. In this study, we hypothesized that human quinone reductase 2 (hQR2), known to act as a flavin redox switch upon binding to the broadly used antimalarial chloroquine, could be involved in the activity of the redox-active indolone derivatives. Therefore, we investigated the role of hQR2 in the reduction of indolone derivatives. We analyzed the interaction between hQR2 and several indolone-type derivatives by examining enzymatic kinetics, the substrate/protein complex structure with X-ray diffraction analysis, and the production of free radicals with electron paramagnetic resonance. The reduction of each compound in cells overexpressing hQR2 was compared to its reduction in naïve cells. This process could be inhibited by the specific hQR2 inhibitor, S29434. These results confirmed that the anti-malarial activity of indolone-type derivatives was linked to their ability to serve as hQR2 substrates and not as hQR2 inhibitors as reported for chloroquine, leading to the possibility that substrate of hQR2 could be considered as a new avenue for the design of new antimalarial compounds.
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