Mefloquine Exposure Induces Cell Cycle Delay and Reveals Stage-Specific Expression of the pfmdr1 Gene

Drug-resistantPlasmodium falciparum malaria is a major public health problem. An elevatedpfmdr1 gene copy number (CN) is known to decrease parasite sensitivity to the commonly used antimalarial mefloquine (MFQ). To understand the relationship betweenpfmdr1 CN and mefloquine resistance, we evaluatedpfmdr1 transcript levels in threeP. falciparum strains with different CNs in the presence and absence of MFQ. Parasite strains with multiplepfmdr1 gene copies exhibited higher relative transcript levels than single-copy parasites, and MFQ inducedpfmdr1 expression above the levels without treatment in all three strains evaluated. Concomitant morphology analyses of the sampled cultures revealed that MFQ treatment of synchronized ring-stage parasites induced a delay in parasite maturation through the intraerythrocytic cycle.pfmdr1 expression peaks in the ring stage, and MFQ could be causing increased transcription by delaying parasite maturation. However, pretreatment with mefloquine did not affect the artemisininin vitro half-maximal inhibitory concentration (IC50 ). These results suggest that MFQ-induced increases inpfmdr1 expression are the direct result of the maturation delay at the ring stage but that this change in expression does not affect the antimalarial activity of artemisinin.