Susceptibility of human malaria parasites to chloroquine is pH dependent

Chloroquine (CQ) accumulates in the acidic food vacuole of intraerythrocytic malaria parasites (Plasmodium falciparum) by virtue of its weak base properties. In the present work, the extent of CQ accumulation was determined by the transvacuolar pH gradient: modification of the latter - either by changing the external pH or by adding the acidotropic agent NH₄Cl - led to a corresponding change in CQ distribution between cells and medium. Changes in pH gradient provoked a change in the susceptibility of parasites to CQ: at external pH values of 8.0, 7.4, and 6.8, the IC₅₀ values for CQ were 0.48 x 10^-7 M, 1.8 x 10^-7 M, and 3.3 x 10^-7 M, respectively. Marked resistance to CQ (IC₅₀ = 9.8 x 10^-7 M) was conferred upon cells by exposing them simultaneously to CQ and 10 mM NH₄Cl, at pH 7.4. The final concentration of CQ attained within the acidic compartment of the parasite was correlated with inhibition of parasite growth. At therapeutic drug levels, CQ accumulation caused minor changes in the food vacuole pH, whereas at higher CQ concentrations substantial alkalinization was observed. The antimalarial activity of CQ is suggested to be exerted by the interference of the high concentrations of accumulated drug with vital functions of the food vacuole.