Decrease in susceptibility of Plasmodium falciparum to mefloquine in continuous culture.

Chloroquine-resistant P. falciparum, line FCK from Thailand, was tested for susceptibility to mefloquine in continuous culture. The drug resulted in 50% schizont inhibition at the level of 8.0 nmol/ml of culture when tested for the first time. After three months of discontinuous exposure to mefloquine at increasing levels, the sensitivity of the parasite decreased. A concentration as high as 128 nmol/ml eventually was required for 50% inhibition of growth.