FIG 7.

Stage specificity of artemisinin resistance in P. falciparum. The relative susceptibility of each stage of asexual parasite development was assessed by exposing tightly synchronous parasites to 3-h pulses of 700 nM DHA at 3-h increments throughout one cycle (duplicate experiments). Resistance then was assessed by parasite viability of each stage of development, with viability defined as the ability to grow logarithmically within 14 days of drug exposure. Initially there were no differences in viability of 4G and CB132, and a brief window of hypersusceptibility was observed for both clones. In the presence of drug, the ring-to-trophozoite transition for 4G was accelerated, and in this brief trophozoite window both clones were equally susceptible. Resistance to artemisinin was observed in both ring stages and throughout schizogony in the artemisinin-resistant 4G clone, and this was expressed as an ∼1 log increase in viability.