Indolone-N-oxide derivatives: in vitro activity against fresh clinical isolates of Plasmodium falciparum, stage specificity and in vitro interactions with established antimalarial drugs

OBJECTIVES Indolone-N-oxides are characterized by the presence of a highly reactive pharmacophore, the nitrone moiety (C=N(+)-O(-)), which undergoes oxidation-reduction reactions. The aims of the present study were to: (i) evaluate the in vitro activity of the parent compound, designated as compound 1, against 34 fresh clinical isolates of Plasmodium falciparum; (ii) compare the activity of compound 1 with that of chloroquine and dihydroartemisinin to assess the potential for cross-resistance; (iii) investigate drug interactions of indolone-N-oxides with standard antimalarials; and (iv) determine the stage-dependent activity of indolone-N-oxides. METHODS In vitro antimalarial activity was evaluated against clinical isolates collected from Cameroonian patients by the [(3)H]hypoxanthine incorporation assay. In vitro interactions between compound 1 or another analogue, compound 4, and established antimalarial drugs were assessed by the fixed ratio method. Stage specificity was evaluated by light microscopy using highly synchronized P. falciparum cultures. RESULTS The geometric mean 50% inhibitory concentration (IC(50)) of compound 1 was 48.6 nM. Its activity did not differ between the chloroquine-susceptible and the chloroquine-resistant isolates. There was no correlation between chloroquine and compound 1 responses (r = 0.015; P > 0.05), but the in vitro responses of compound 1 and dihydroartemisinin were significantly and positively correlated (r = 0.444; P