In-vitro sensitivity testing of Plasmodium vivax: response to lumefantrine and chloroquine in northwestern Thailand.

The acquisition of resistance to chloroquine by Plasmodium vivax in parts of the Australasian and Malaysian epidemiological zones and hitherto unconfirmed reports of such resistance in neighbouring zones show the need for monitoring the drug response of P. vivax. In this study, a recently developed in-vitro micro-technique for the assessment of the parasite's sensitivity to chloroquine was adapted to and validated for lumefantrine. In 21 P. vivax isolates tested at Mae Hong Son, northwestern Thailand, in 2001, the mean geometric cut-off concentration for full inhibition for lumefantrine was 2080 nM. The EC50 and EC90 were 12 nM and 237 nM, respectively. The response was well within the putative therapeutic range. Sensitivity to chloroquine was assessed in 18 P. vivax isolates, showing a geometric mean cut-off concentration of 1095 nM and a wider variation of the individual cut-off points compared with lumefantrine. The EC50 and EC90 for chloroquine were 16 nM and 511 nM, respectively. One of the isolates, from Myanmar, showed low sensitivity to chloroquine.