Sustained Ex Vivo Susceptibility of Plasmodium falciparum to Artemisinin Derivatives but Increasing Tolerance to Artemisinin Combination Therapy Partner Quinolines in The Gambia

Antimalarial interventions have yielded a significant decline in malaria prevalence in The Gambia, where artemether-lumefantrine (AL) has been used as a first-line antimalarial for a decade. Clinical Plasmodium falciparum isolates collected from 2012 to 2015 were analyzed ex vivo for antimalarial susceptibility and genotyped for drug resistance markers (pfcrt K76T, pfmdr1 codons 86, 184, and 1246, and pfk13) and microsatellite variation. Additionally, allele frequencies of single nucleotide polymorphisms (SNPs) from other drug resistance-associated genes were compared from genomic sequence data sets from 2008 (n = 79) and 2014 (n = 168). No artemisinin resistance-associated pfk13 mutation was found, and only 4% of the isolates tested in 2015 showed significant growth after exposure to dihydroartemisinin. Conversely, the 50% inhibitory concentrations (IC(50)s) of amodiaquine and lumefantrine increased within this period. pfcrt 76T and pfmdr1 184F mutants remained at a prevalence above 80%. pfcrt 76T was positively associated with higher IC(50)s to chloroquine. pfmdr1 NYD increased in frequency between 2012 and 2015 due to lumefantrine selection. The TNYD (pfcrt 76T and pfmdr1 NYD wild-type haplotype) also increased in frequency following AL implementation in 2008. These results suggest selection for pfcrt and pfmdr1 genotypes that enable tolerance to lumefantrine. Increased tolerance to lumefantrine calls for sustained chemotherapeutic monitoring in The Gambia to minimize complete artemisinin combination therapy (ACT) failure in the future.