In vitro sensitivity of Plasmodium falciparum to artesunate in Thailand

Reported are the in vitro susceptibilities of Plasmodium falciparum to arte sunate, mefloquine, quinine and chloroquine of 86 isolates and to dihydroar temisinin of 45 isolates collected from areas of high resistance to mefloqu ine within Thailand near the borders with Myanmar and Cambodia, and from so uthern Thailand where P, falciparum is generally still sensitive to mefloqu ine. All the isolates were highly sensitive to artesunate, but the geometri c mean IC(50)s were higher in isolates from the Thai-Myanmar and Thai-Cambo dian borders than in those from southern Thailand, The IC(50)s for mefloqui ne and artesunate were strongly correlated (Pearson r = 0.605; n = 86; P< 0 .0001). As expected, the in vitro sensitivities to dihydroartemisinin and a rtesunate were similar and strongly correlated (at IC50, Pearson r = 0.695; n = 45; P< 0.00002). The correlation between the activity of mefloquine an d artesunate requires further investigation in order to determine the poten tial for development of cross-resistance in nature. Our results suggest tha t combination with mefloquine is not the ideal way of protecting the useful ness of artemisinin and its derivatives. A search for more suitable partner drugs to these compounds and careful regulation of their use are necessary in the interest of ensuring their long therapeutic life span.