Effect of pH on in vitro potency of amantadine against Plasmodium falciparum

Amantadine is a monoprotic weak base that inhibits intraerythrocytic growth in in vitro cultures of Plasmodium falciparum, specifically chloroquine-resistant strains. Changes in the external pH of the medium are expected to result in a shift in the relative proportion of ionized and unionized species of amantadine by virtue of the weak base characteristic of the drug, influencing passage of the drug through the membrane. The ability of amantadine to alkalinize the food vacuole was determined using the accumulation of acridine orange as a vacuolar probe. Drug sensitivity following alteration of the pH gradient was assessed using the hypoxanthine method. Amantadine was able to alkalinize the food vacuole in the millimolar range; however, since its antimalarial activity is in the micromolar range, alkalinization of the food vacuole is not the primary action of the drug. The pH of the medium profoundly influenced susceptibility to chloroquine; the log of the 50% inhibitory concentration (IC50) values were linearly dependent on the external pH in both chloroquine-resistant and chloroquine-sensitive strains. Log IC50 values of amantadine exhibited a linear dependence on external pH in the chloroquine-sensitive strain, but in the chloroquine-resistant strain, a nonlinear parabolic function was found with the minimum IC50 at pH 7.03. Ammonium chloride did not interfere with the antimalarial activity of amantadine. The presence of the amine group on the hydrocarbon cage is essential for the activity of amantadine in Plasmodium falciparum. These results suggest factors in addition to pH gradient are involved in the effect of amantadine, possibly interactions with membrane phospholipids.