Kinetic modelling of the response of Plasmodium falciparum to chloroquine and its experimental testing in vitro. Implications for mechanism of action of and resistance to the drug.

The antimalarial mode of action of chloroquine (CQ) has been investigated in great detail in recent years, but the overall mechanism is still controversial. Instead of further probing the molecular aspects of partial reactions, a model based on the weak base properties of CQ and its delta pH-driven accumulation in acid parasite compartments has been devised, and the integrated response of the parasite to the drug under different experimental conditions has been assayed to verify the validity of the model. Factors such as inoculum size (parasitemia.hematocrit) and medium pH were altered using CQ-sensitive (FCC1) and -resistant (FCR3, VNS) isolates of Plasmodium falciparum. Experimental results were in full agreement with the predictions of the model, implying that therapeutic concentrations of CQ do not raise the pH of the food vacuole, i.e. that alkalinization of the acid parasite compartments is an insufficient explanation for the antimalarial activity of CQ, and that there is no need to invoke an active QC efflux pump to explain drug resistance. Calculations based on the model and the experimental data demonstrate that resistance to CQ is correlated with higher pH and/or higher resistance of the intracellular target to the drug concentration in the parasite food vacuole. The data also have implications for the design and interpretation of in vitro CQ inhibitory tests.