Defining the in vivo phenotype of artemisinin-resistant falciparum malaria: a modelling approach.
BACKGROUND
Artemisinin-resistant falciparum malaria has emerged in Southeast Asia, posing a major threat to malaria control. It is characterised by delayed asexual-stage parasite clearance, which is the reference comparator for the molecular marker 'Kelch 13' and in vitro sensitivity tests. However, current cut-off values denoting slow clearance based on the proportion of individuals remaining parasitaemic on the third day of treatment ('day-3'), or on peripheral blood parasite half-life, are not well supported. We here explore the parasite clearance distributions in an area of artemisinin resistance with the aim refining the in vivo phenotypic definitions.


RESULTS
Data from 1,518 patients on the Thai-Myanmar and Thai-Cambodian borders with parasite half-life assessments after artesunate treatment were analysed. Half-lives followed a bimodal distribution. A statistical approach was developed to infer the characteristics of the component distributions and their relative contribution to the composite mixture. A model representing two parasite subpopulations with geometric mean (IQR) parasite half-lives of 3.0 (2.4-3.9) hours and 6.50 (5.7-7.4) hours was consistent with the data. For individual patients, the parasite half-life provided a predicted likelihood of an artemisinin-resistant infection which depends on the population prevalence of resistance in that area. Consequently, a half-life where the probability is 0.5 varied between 3.5 and 5.5 hours. Using this model, the current 'day-3' cut-off value of 10% predicts the potential presence of artemisinin-resistant infections in most but not all scenarios. These findings are relevant to the low-transmission setting of Southeast Asia. Generalisation to a high transmission setting as in regions of Sub-Saharan Africa will need additional evaluation.


CONCLUSIONS
Characterisation of overlapping distributions of parasite half-lives provides quantitative insight into the relationship between parasite clearance and artemisinin resistance, as well as the predictive value of the 10% cut-off in 'day-3' parasitaemia. The findings are important for the interpretation of in vitro sensitivity tests and molecular markers for artemisinin resistance and for contextualising the 'day 3' threshold to account for initial parasitaemia and sample size.
Centre for Tropical Medicine, Nuffield Department of Medicine, University of Oxford, Oxford, United Kingdom; Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
Lisa J
LJ
White
Worldwide Antimalarial Resistance Network, Oxford University, Oxford, United Kingdom.
Jennifer A
JA
Flegg
Centre for Tropical Medicine, Nuffield Department of Medicine, University of Oxford, Oxford, United Kingdom; Shoklo Malaria Research Unit, Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Mae Sot, Thailand Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
Aung Pyae
AP
Phyo
Shoklo Malaria Research Unit, Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Mae Sot, Thailand Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
Ja Hser
JH
Wiladpai-ngern
Howard Hughes Medical Institute/Center for Vaccine Development, University of Maryland School of Medicine, Baltimore, Maryland, United States of America.
Delia
D
Bethell
Department of Genetics, Texas Biomedical Research Institute, San Antonio, Texas, United States of America.
Christopher
C
Plowe
National Malaria Center, Ministry of Health, Phnom Penh, Cambodia.
Tim
T
Anderson
National Malaria Center, Ministry of Health, Phnom Penh, Cambodia.
Standwell
S
Nkhoma
National Malaria Center, Ministry of Health, Phnom Penh, Cambodia.
Shalini
S
Nair
Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
Rupam
R
Tripura
Worldwide Antimalarial Resistance Network, Oxford University, Oxford, United Kingdom.
Kasia
K
Stepniewska
Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
Wirichada
W
Pan-Ngum
Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
Kamolrat
K
Silamut
Centre for Tropical Medicine, Nuffield Department of Medicine, University of Oxford, Oxford, United Kingdom; Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
Ben S
BS
Cooper
Centre for Tropical Medicine, Nuffield Department of Medicine, University of Oxford, Oxford, United Kingdom; Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
Yoel
Y
Lubell
Centre for Tropical Medicine, Nuffield Department of Medicine, University of Oxford, Oxford, United Kingdom; Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
Elizabeth A
EA
Ashley
National Malaria Center, Ministry of Health, Phnom Penh, Cambodia.
Chea
C
Nguon
Centre for Tropical Medicine, Nuffield Department of Medicine, University of Oxford, Oxford, United Kingdom; Shoklo Malaria Research Unit, Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Mae Sot, Thailand Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
François
F
Nosten
Centre for Tropical Medicine, Nuffield Department of Medicine, University of Oxford, Oxford, United Kingdom; Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
Nicholas J
NJ
White
Centre for Tropical Medicine, Nuffield Department of Medicine, University of Oxford, Oxford, United Kingdom; Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
Arjen M
AM
Dondorp
D000962:Antimalarials / Q000494:pharmacology / Q000627:therapeutic use*; D037621:Artemisinins / Q000494:pharmacology / Q000627:therapeutic use*; D000077332:Artesunate; D002160:Cambodia; D004351:Drug Resistance / Q000235:genetics; D006801:Humans; D008018:Life Cycle Stages; D016778:Malaria, Falciparum / Q000188:drug therapy / Q000469:parasitology*; D002052:Myanmar; D059208:Parasite Load; D018512:Parasitemia / Q000175:diagnosis / Q000188:drug therapy; D010641:Phenotype; D010802:Phylogeny; D010963:Plasmodium falciparum / Q000187:drug effects*; D012016:Reference Values; D013785:Thailand
D000962:Antimalarials; D037621:Artemisinins; D000077332:Artesunate; C031327:artemisinin