Decreasing<i>pfmdr1</i>Copy Number in<i>Plasmodium falciparum</i>Malaria Heightens Susceptibility to Mefloquine, Lumefantrine, Halofantrine, Quinine, and Artemisinin

The global dissemination of drug-resistant Plasmodium falciparum is spurring intense efforts to implement artemisinin (ART)-based combination therapies for malaria, including mefloquine (MFQ)-artesunate and lumefantrine (LUM)-artemether. Clinical studies have identified an association between an increased risk of MFQ, MFQ-artesunate, and LUM-artemether treatment failures and pfmdr1 gene amplification. To directly address the contribution that pfmdr1 copy number makes to drug resistance, we genetically disrupted 1 of the 2 pfmdr1 copies in the drug-resistant FCB line, which resulted in reduced pfmdr1 mRNA and protein expression. These knockdown clones manifested a 3-fold decrease in MFQ IC(50) values, compared with that for the FCB line, verifying the role played by pfmdr1 expression levels in mediating resistance to MFQ. These clones also showed increased susceptibility to LUM, halofantrine, quinine, and ART. No change was observed for chloroquine. These results highlight the importance of pfmdr1 copy number in determining P. falciparum susceptibility to multiple agents currently being used to combat malaria caused by multidrug-resistant parasites.