Plasmodium falciparum in vitro: determination of chloroquine sensitivity of three new strains by a modified 48-hour test.

Three recently isolated African strains of Plasmodium falciparum were tested in vitro for their response to chloroquine. Both the 48-hour method described earlier and a modified 48-hour test were used, yielding comparable results. Strain FCR-7/Kenya, isolated from a clinically chloroquine-resistant case, was more resistant to the drug in vitro than the two other strains (FCR-8/West Africa and FCN-1/Nigeria, both isolated from chloroquine-sensitive cases). Complete inhibition of parasite growth occurred for strain FCR-7/Kenya in a drug concentration range ten times higher than for strains FCR-8/West Africa and FCN-1/Nigeria. In the modified 48-hour test, a lower erythrocyte suspension (2%) allows continuous growth of the parasites over a 48-hour cycle without necessitating change in medium. It thus offers distinct advantages for routine laboratory work as well as for potential field trials.