title: Laboratory detection of artemisinin-resistant Plasmodium falciparum.
abstract: Conventional 48-h in vitro susceptibility tests have low sensitivity in identifying artemisinin-resistant Plasmodium falciparum, defined phenotypically by low in vivo parasite clearance rates. We hypothesized originally that this discrepancy was explained by a loss of ring-stage susceptibility and so developed a simple field-adapted 24-h trophozoite maturation inhibition (TMI) assay focusing on the ring stage and compared it to the standard 48-h schizont maturation inhibition (WHO) test. In Pailin, western Cambodia, where artemisinin-resistant P. falciparum is prevalent, the TMI test mean (95% confidence interval) 50% inhibitory concentration (IC50) for artesunate was 6.8 (5.2 to 8.3) ng/ml compared with 1.5 (1.2 to 1.8) ng/ml for the standard 48-h WHO test (P = 0.001). TMI IC50s correlated significantly with the in vivo responses to artesunate (parasite clearance time [r = 0.44, P = 0.001] and parasite clearance half-life [r = 0.46, P = 0.001]), whereas the standard 48-h test values did not. On continuous culture of two resistant isolates, the artemisinin-resistant phenotype was lost after 6 weeks (IC50s fell from 10 and 12 ng/ml to 2.7 and 3 ng/ml, respectively). Slow parasite clearance in falciparum malaria in western Cambodia results from reduced ring-stage susceptibility.
authors.affiliation: Department of Clinical Tropical Medicine, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand Mahidol-Oxford University Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
authors.forename: Kesinee
authors.initials: K
authors.lastname: Chotivanich
authors.affiliation: Mahidol-Oxford University Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
authors.forename: Rupam
authors.initials: R
authors.lastname: Tripura
authors.affiliation: Mahidol-Oxford University Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
authors.forename: Debashish
authors.initials: D
authors.lastname: Das
authors.affiliation: The National Center for Parasitology, Entomology, and Malaria Control, Phnom Penh, Cambodia.
authors.forename: Poravuth
authors.initials: P
authors.lastname: Yi
authors.affiliation: Mahidol-Oxford University Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand Centre for Tropical Medicine, Churchill Hospital, Oxford, United Kingdom.
authors.forename: Nicholas P J
authors.initials: NP
authors.lastname: Day
authors.affiliation: Department of Clinical Tropical Medicine, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand Mahidol-Oxford University Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
authors.forename: Sasithon
authors.initials: S
authors.lastname: Pukrittayakamee
authors.affiliation: The National Center for Parasitology, Entomology, and Malaria Control, Phnom Penh, Cambodia.
authors.forename: Char Meng
authors.initials: CM
authors.lastname: Chuor
authors.affiliation: The National Center for Parasitology, Entomology, and Malaria Control, Phnom Penh, Cambodia.
authors.forename: Duong
authors.initials: D
authors.lastname: Socheat
authors.affiliation: Mahidol-Oxford University Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand Centre for Tropical Medicine, Churchill Hospital, Oxford, United Kingdom.
authors.forename: Arjen M
authors.initials: AM
authors.lastname: Dondorp
authors.affiliation: Mahidol-Oxford University Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand Centre for Tropical Medicine, Churchill Hospital, Oxford, United Kingdom nickw@tropmedres.ac.
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: White
mesh_terms: D000818:Animals; D000962:Antimalarials / Q000494:pharmacology*; D037621:Artemisinins / Q000494:pharmacology*; D000077332:Artesunate; D004351:Drug Resistance; D006207:Half-Life; D016778:Malaria, Falciparum / Q000097:blood / Q000469:parasitology; D021261:Parasitic Sensitivity Tests; D010641:Phenotype; D010963:Plasmodium falciparum / Q000187:drug effects* / Q000254:growth & development
chemical_list: D000962:Antimalarials; D037621:Artemisinins; D000077332:Artesunate; C031327:artemisinin