Chloroquine susceptibility in malaria: dependence on exposure of parasites to the drug.

Chloroquine-resistant $\text{P.}$$\text{berghei}$ is as susceptible to chloroquine as chloroquine-susceptible $\text{P.}$$\text{berghei}$ when adequately exposed for short periods of time (1 hour) $\text{in}$$\text{vitro}$. In both cases 3.1 mM chloroquine causes a significant decrease in infectivity of the parasites whereas 0.31 mM chloroquine is without effect. Since there is no evidence that chloroquine has a peculiar mechanism of action $\text{in}$$\text{vitro}$, these results support the hypothesis of inadequate exposure of intracellular parasites as the cause of chloroquine resistance.