Antimalarial Drug Susceptibility of Plasmodium Falciparum Isolates from Forest Fringe Dwelling Aborigines (Orang Asli) of Peninsular Malaysia

Chris LambrosMalaria Research Group, U.S. Army Medical Research Unit, Institute for Medical Research, 50588 Kuala Lumpur, Malaysia

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Donald R. DavisMalaria Research Group, U.S. Army Medical Research Unit, Institute for Medical Research, 50588 Kuala Lumpur, Malaysia

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George E. Lewis Jr.Malaria Research Group, U.S. Army Medical Research Unit, Institute for Medical Research, 50588 Kuala Lumpur, Malaysia

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The drug susceptibility of 70 isolates of Plasmodium falciparum to standard and experimental antimalarials was evaluated using a radioisotope microdilution method. All isolates were from forest fringe dwelling Orang Asli, the aborigines of Peninsular Malaysia. The geometric mean IC50 values were: chloroquine, 10 ng/ml; amodiaquine, 4.7 ng/ml; mefloquine, 2.8 ng/ml; quinine, 40.5 ng/ml; halofantrine, 1.5 ng/ml; enpiroline, 3 ng/ml; and pyrimethamine, 21 ng/ml. Four isolates exhibited decreased susceptibility to chloroquine (IC50 > 60 ng/ml), and one exhibited decreased susceptibility to quinine (IC50 = 161 ng/ml). Three isolates showed decreased susceptibility to mefloquine (IC50 = 10–11 ng/ml). The lack of drug pressure may account for the high prevalence of P. falciparum isolates susceptible to chloroquine.

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