Ex vivo drug sensitivity profiles of Plasmodium falciparum field isolates from Cambodia and Thailand, 2005 to 2010, determined by a histidine-rich protein-2 assay

Background In vitro drug susceptibility assay of Plasmodium falciparum field isolates processed “immediate ex vivo ” (IEV), without culture adaption, and tested using histidine-rich protein-2 (HRP-2) detection as an assay, is an expedient way to track drug resistance. Methods From 2005 to 2010, a HRP-2 in vitro assay assessed 451 P. falciparum field isolates obtained from subjects with malaria in western and northern Cambodia, and eastern Thailand, processed IEV, for 50% inhibitory concentrations (IC 50 ) against seven anti-malarial drugs, including artesunate (AS), dihydroartemisinin (DHA), and piperaquine. Results In western Cambodia, from 2006 to 2010, geometric mean (GM) IC 50 values for chloroquine, mefloquine, quinine, AS, DHA, and lumefantrine increased. In northern Cambodia, from 2009–2010, GM IC 50 values for most drugs approximated the highest western Cambodia GM IC 50 values in 2009 or 2010. Conclusions Western Cambodia is associated with sustained reductions in anti-malarial drug susceptibility, including the artemisinins, with possible emergence, or spread, to northern Cambodia. This potential public health crisis supports continued in vitro drug IC 50 monitoring of P. falciparum isolates at key locations in the region.