The binding affinity of human serum albumin (HSA) to three antimalarial indolone-N-oxide derivatives, INODs, was investigated under simulated physiological conditions using fluorescence spectroscopy in combination with UV-vis absorption and circular dichroism (CD) spectroscopy. Analysis of fluorescence quenching data of HSA by these compounds at different temperatures using Stern-Volmer and Lineweaver-Burk methods revealed the formation of a ground state indolone-HSA complex with binding affinities of the order 10(4) M(-1). The thermodynamic parameters ΔG, ΔH, and ΔS, calculated at different temperatures, indicated that the binding reaction was endothermic and hydrophobic interactions play a major role in this association. The conformational changes of HSA were investigated qualitatively using synchronous fluorescence and quantitatively using CD. Site marker competitive experiments showed that the binding process took place primarily at site I (subdomain IIA) of HSA. The number of binding sites and the apparent binding constants were also studied in the presence of different ions.

A series of 66 new indolone-N-oxide derivatives was synthesized with three different methods. Compounds were evaluated for in vitro activity against CQ-sensitive (3D7), CQ-resistant (FcB1), and CQ and pyrimethamine cross-resistant (K1) strains of Plasmodium falciparum (P.f.), as well as for cytotoxic concentration (CC(50)) on MCF7 and KB human tumor cell lines. Compound 26 (5-methoxy-indolone-N-oxide analogue) had the most potent antiplasmodial activity in vitro (<3 nM on FcB1 and = 1.7 nM on 3D7) with a very satisfactory selectivity index (CC(50) MCF7/IC(50) FcB1: 14623; CC(50) KB/IC(50) 3D7: 198823). In in vivo experiments, compound 1 (dioxymethylene derivatives of the indolone-N-oxide) showed the best antiplasmodial activity against Plasmodium berghei, 62% inhibition of the parasitaemia at 30 mg/kg/day.