Evolution of Multidrug Resistance in Plasmodium falciparum: a Longitudinal Study of Genetic Resistance Markers in the Greater Mekong Subregion.

Increasing resistance in Plasmodium falciparum to artemisinins and their artemisinin combination therapy (ACT) partner drugs jeopardizes effective antimalarial treatment. Resistance is worst in the Greater Mekong subregion. Monitoring genetic markers of resistance can help to guide antimalarial therapy. Markers of resistance to artemisinins (<i>PfKelch</i> mutations), mefloquine (amplification of P. falciparum multidrug resistance-1 [<i>PfMDR1</i>]), and piperaquine (<i>PfPlasmepsin2/3</i> amplification and specific P. falciparum chloroquine resistance transporter [<i>PfCRT</i>] mutations) were assessed in 6,722 P. falciparum samples from Vietnam, Lao People's Democratic Republic (PDR), Cambodia, Thailand, and Myanmar between 2007 and 2019. Against a high background prevalence of <i>PfKelch</i> mutations, <i>PfMDR1</i> and <i>PfPlasmepsin2/3</i> amplification closely followed regional drug pressures over time. <i>PfPlasmepsin2/3</i> amplification preceded piperaquine resistance-associated <i>PfCRT</i> mutations in Cambodia and reached a peak prevalence of 23/28 (82%) in 2015. This declined to 57/156 (38%) after first-line treatment was changed from dihydroartemisinin-piperaquine to artesunate-mefloquine (ASMQ) between 2014 and 2017. The frequency of <i>PfMDR1</i> amplification increased from 0/293 (0%) between 2012 and 2017 to 12/156 (8%) in 2019. Amplification of <i>PfMDR1</i> and <i>PfPlasmepsin2/3</i> in the same parasites was extremely rare (4/6,722 [0.06%]) and was dispersed over time. The mechanisms conferring mefloquine and piperaquine resistance may be counterbalancing. This supports the development of ASMQ plus piperaquine as a triple artemisinin combination therapy.