title: Artemether resistance in vitro is linked to mutations in PfATP6 that also interact with mutations in PfMDR1 in travellers returning with Plasmodium falciparum infections.
abstract: BACKGROUND
  Monitoring resistance phenotypes for Plasmodium falciparum, using in vitro growth assays, and relating findings to parasite genotype has proved particularly challenging for the study of resistance to artemisinins.
  
  
  METHODS
  Plasmodium falciparum isolates cultured from 28 returning travellers diagnosed with malaria were assessed for sensitivity to artemisinin, artemether, dihydroartemisinin and artesunate and findings related to mutations in pfatp6 and pfmdr1.
  
  
  RESULTS
  Resistance to artemether in vitro was significantly associated with a pfatp6 haplotype encoding two amino acid substitutions (pfatp6 A623E and S769N; (mean IC50 (95% CI) values of 8.2 (5.7 - 10.7) for A623/S769 versus 623E/769 N 13.5 (9.8 - 17.3) nM with a mean increase of 65%; p = 0.012). Increased copy number of pfmdr1 was not itself associated with increased IC50 values for artemether, but when interactions between the pfatp6 haplotype and increased copy number of pfmdr1 were examined together, a highly significant association was noted with IC50 values for artemether (mean IC50 (95% CI) values of 8.7 (5.9 - 11.6) versus 16.3 (10.7 - 21.8) nM with a mean increase of 87%; p = 0.0068). Previously described SNPs in pfmdr1 are also associated with differences in sensitivity to some artemisinins.
  
  
  CONCLUSIONS
  These findings were further explored in molecular modelling experiments that suggest mutations in pfatp6 are unlikely to affect differential binding of artemisinins at their proposed site, whereas there may be differences in such binding associated with mutations in pfmdr1. Implications for a hypothesis that artemisinin resistance may be exacerbated by interactions between PfATP6 and PfMDR1 and for epidemiological studies to monitor emerging resistance are discussed.
authors.affiliation: Centre for Infection and Immunity, Division of Clinical Sciences, St, George's, University of London, London SW17 0RE, UK.
authors.forename: Dylan R
authors.initials: DR
authors.lastname: Pillai
authors.forename: Rachel
authors.initials: R
authors.lastname: Lau
authors.forename: Krishna
authors.initials: K
authors.lastname: Khairnar
authors.forename: Rosalba
authors.initials: R
authors.lastname: Lepore
authors.forename: Allegra
authors.initials: A
authors.lastname: Via
authors.forename: Henry M
authors.initials: HM
authors.lastname: Staines
authors.forename: Sanjeev
authors.initials: S
authors.lastname: Krishna
mesh_terms: D000818:Animals; D000962:Antimalarials / Q000494:pharmacology*; D000077549:Artemether; D037621:Artemisinins / Q000494:pharmacology*; D000252:Calcium-Transporting ATPases / Q000737:chemistry / Q000235:genetics*; D004351:Drug Resistance; D006801:Humans; D020128:Inhibitory Concentration 50; D016778:Malaria, Falciparum / Q000469:parasitology*; D008958:Models, Molecular; D027425:ATP-Binding Cassette, Sub-Family C Proteins / Q000737:chemistry / Q000235:genetics*; D020125:Mutation, Missense; D021261:Parasitic Sensitivity Tests; D010963:Plasmodium falciparum / Q000187:drug effects*; D011485:Protein Binding; D014195:Travel
chemical_list: D000962:Antimalarials; D000077549:Artemether; D037621:Artemisinins; D000252:Calcium-Transporting ATPases; D027425:ATP-Binding Cassette, Sub-Family C Proteins; C513500:ATP6 protein, Plasmodium falciparum; C513498:Mdr1 protein, Plasmodium falciparum