Alkalinization of the food vacuole of malaria parasites by quinoline drugs and alkylamines is not correlated with their antimalarial activity

Quinoline-containing antimalarial drugs accumulate inside the acid food vacuole of the parasite where they inhibit the digestion of ingested host cell cytosol, and consequently, parasite growth. In order to verify whether this inhibition is caused by drug-induced alkalinization of the food vacuole, we investigated the accumulation of acridine orange (AO) as a vacuolar pH probe in intact Plasmodium falciparum-infected human erythrocytes as affected by the drugs chloroquine (CQ), 7H-quinoleine (7HQ), quinine (Q) and mefloquine (MQ). It was established by various criteria that AO accumulates primarily in the acid compartment(s) of the parasite as a function of the pH difference between it and the extracellular medium. This pH gradient was dissipated by the drugs in the rank order MQ > CQ > Q > 7HQ. The kinetics of vacuolar alkalinization and the concentration ranges at which it was observed imply that the monoprotic drugs MQ and Q exerted their effect mostly by translocating protons across the vacuolar membrane, i.e. they could cross the membrane as a protonated species, while the diprotic drugs CQ and 7HQ raised the vacuolar pH mostly by proton trapping. Similarly, hydrophobic alkylamines raised the vacuolar pH by proton translocation, while their relatively more polar congeners and ammonia did so by proton titration. However, the alkalinizing effect of each drug was observed at a concentration which was 1-2 orders of magnitude larger than the IC₅₀ of its antimalarial effect. These results mean that vacuolar alkalinization is not the primary effect of antiparasitic action of quinoline antimalarials.