INVITRO AND INVIVO SUSCEPTIBILITY OF PLASMODIUM-FALCIPARUM ISOLATES FROM LIBERIA TO PYRIMETHAMINE, CYCLOGUANIL AND CHLORCYCLOGUANIL

In vivo susceptibility of Plasmodium falciparum to chlorproguanil and in vitro susceptibility to pyrimethamine, cycloguanil and chlorcycloguanil were studied in 38 children from two Liberian villages. Children in one village (Lagbala) had received monthly chemosuppression with chlorproguanil from 1976-1985, and children in the other village (JDF) had received fortnightly chlorproguanil from 1981-1985. The highest and lowest IC100 for pyrimethamine differed by a factor of 105, but they differed only by a factor of 103 for chlorcycloguanil. The mean IC100 for chlorcycloguanil was significantly lower (P<0.0001) than the mean IC100 for pyrimethamine and cycloguanil, and the IC100 for the samples most resistant to chlorycloguanil (10-8 M) was still well below peak blood concentrations after chlorproguanil adminstration. Resistance could be defined as IC100 ≥ 10-8 M for pyrimethamine and IC100 ≥ 10-8 M for chlorcycloguanil. The isolates most resistant or most sensitive to pyrimethamine were also the most resistant or most sensitive to chlorcycloguanil, indicating partial cross-resistance between the two drugs. The in vivo response to chlorproguanil 1.5 mg kg-1 in Lagbala was equal to the response in 1983. Chlorproguanil 1.5 mg kg-1 resulted in lower parasite rates on day 3 and 7, but did not prevent 60% of the children requiring treatment with chloroquine during the four weeks' follow-up after chlorproguanil administration.