The activity of pyrimethamine and sulphadoxine against Plasmodium falciparum determined by the in vitro microtechnique.

The reported increase of infections of Plasmodium falciparum which are no longer susceptible to a combination of pyrimethamine and sulphadoxine--Fansidar--emphasizes the need for an in vitro test to determine the presence and prevalence of drug-resistant parasites. Studies with the pyrimethamine-sensitive FCB strain and the pyrimethamine-resistant FTA strain showed that the in vitro microtechnique can be used to determine differences in the susceptibility of these two strains to pyrimethamine and to pyrimethamine-sulphadoxine combinations. The FCB strain was almost six times more susceptible to pyrimethamine than the FTA strain. Although relatively high concentrations of sulphadoxine alone exerted no detectable antimalarial effects, the sulphonamide potentiated the activity of pyrimethamine against both strains of P. falciparum. This synergistic activity was relatively more pronounced against the pyrimethamine-resistant strain, particularly at lower concentrations of sulphadoxine. Further studies are indicated to determine to what extent findings obtained with the in vitro microtechnique can be correlated with the response of falciparum infection to treatment with pyrimethamine-sulphadoxine combinations.