Growth curve of Plasmodium falciparum (3D7) after treatment with pyrimethamine.

Synchronous cultures of a P. falciparum 3D7 strain were incubated for 72 hours with doubling concentrations of pyrimethamine. No inhibitory effect could be observed during the first 48 hours at any concentration (A). However, an inhibition was clearly visible at 72 hours (B). This could be explained by the fact that pyrimethamine is a slow acting drug and its effect can only be detected on the second generation, after 48 hours. Interestingly, the growth curves at all concentrations follow the drug free control after the 30 hour peak, while the curve for 200 nM shows a later peak at 36 hours with a higher number of depolarizing events, compared to the drug free control (A). See discussion for possible explanation. Each time point represents the mean value of triplicate samples ± one SD. DF ctrl – drug free control; UI ctrl – uninfected control.