In vitro activity of dihydroartemisinin against clinical isolates of Plasmodium falciparum in Yaounde, Cameroon

The in vitro activities of dihydroartemisinin (the biologically active meta bolite of artemisinin derivatives), chloroquine, monodesethylamodiaquine (t he biologically active metabolite of amodiaquine), quinine, mefloquine, hal ofantrine, and pyrimethamine were assessed in 65 African isolates of Plasmo dium falciparum from Yaounde, Cameroon using an isotopic microtest. The 50% inhibitory concentration (IC50) values for dihydroartemisinin were within a narrow range from 0.25 to 4.56 nM, with a geometric mean of 1.11 nM (95% confidence interval = 0.96-1.28 nM). Dihydroartemisinin was equally active (P > 0.05) against the chloroquine-sensitive isolates (geometric mean IC50 = 1.25 nM, 95% confidence interval = 0.99-1.57 nM) and the chloroquine-resi stant isolates (geometric mean IC50 = 0.979 nM, 95% confidence interval = 0 .816-1.18 nM). A significant positive correlation was observed between the responses to dihydroartemisinin and mefloquine (r = 0.662) or halofantrine (r = 0.284), suggesting in vitro cross-resistance. There was no correlation between the responses to dihydroartemisinin and other antimalarial drugs.