The in vitro susceptibility of five field isolates of Plasmodium falciparum from the region of the Thai-Kampuchean border to pyrimethamine, sulphadoxine, and their combination, was determined using a microtitre test system and media deficient in p-aminobenzoic acid and folic acid. Two-fold serial dilutions of pyrimethamine ranging from concentrations of 8.0 to 0.125 microM and sulphadoxine ranging from 800 to 50 microM were evaluated for antimalarial activity. Viability was based on the maturation of the ring stages to normally-appearing schizonts. Tested singly the parasites had an average ID90 of 3.82 microM for pyrimethamine and greater than 800 microM for sulphadoxine. Analysis of the drugs interaction showed maximum potentiation at approximately 0.8 microM of pyrimethamine and 80 microM of sulphadoxine. These results suggest that resistance to Fansidar is due to the resistance to both components. Although there was a potentiating effect it was probably not sufficient enough for the drugs to be effective in vivo. This may, in part, explain the reduction in clinical cures with the sulphadoxine-pyrimethamine combination in eastern Thailand.
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