Effects of antimalarial drugs on movement of Plasmodium falciparum

In vitro antimalarial drug susceptibility is conventionally assessed by the concentration dependent growth inhibition of Plasmodium in an in vitro culture system. Inhibition of the kinetic properties of the parasites could provide an alternative method to assess in vitro antimalarial drugs sensitivity. In this study we used a novel real time microscopic technique, which does not require fixation and staining of the parasite, to study the effects of antimalarial drugs on the intracellular movement of Plasmodium (P.) falciparum trophozoites. Using real time microscopy movement of P. falciparum pigment within erythrocytes was investigated before and after antimalarial drugs exposure (artesunate, quinine, and piperaquine). For artesunate, the 50% inhibition concentration (IC50) at which movement in half of the trophozoites was abolished was estimated by sigmoid curve fitting. Intra- and inter-observer agreements were also assessed. Healthy unexposed P. falciparum trophozoites in culture showed very active movement of malaria pigment. Quinine and piperaquine had no effect but artesunate did reduce pigment movement which started after 2.5 hours exposure to the drug. The mean (SD) IC50 for artesunate regarding abolishment of pigment movement was 54 (14) ng/ml. Assessments of intra- and inter-rater agreement showed good reproducibility of the technique (Kappa value 0.82 to 0.91). Abolishment of active movement of malaria pigment is an alternative approach to assess drug sensitivity for artesunate. Malaria pigment movement is abolished by artesunate early after exposure, but at concentrations higher than those inhibiting growth.