The fate of ferriprotorphyrin IX in malaria infected erythrocytes in conjunction with the mode of action of antimalarial drugs.

The intraerythrocytic malaria parasite digests considerable amounts of its host cell cytosol, which consists mostly of hemoglobin. In order to avert the toxicity of ferriprotorphyrin IX (FP) thus produced, it is generally accepted that FP is polymerized to the non-toxic hemozoin. Investigating the fate of FP in cultured Plasmodium falciparum -infected human red blood cells, revealed a straight correlation between amounts of digested hemoglobin and hemozoin, but the latter contained less FP than produced. The efficacy of FP polymerization is stage-dependent, increasing with parasite maturation. Different strains display dissimilar efficacy in hemozoin production. Unpolymerized FP possibly exits the food vacuole and is degraded by glutathione, thus accounting for the low levels of free FP found in infected cells. 4-aminoquinoline antimalarials demonstrably form complexes with FP and inhibit hemozoin production in vitro. Chloroquine, amodiaquine, quinine and mefloquine were found to inhibit hemozoin production in intact infected cells, but only the first two drugs caused a dose-dependent accumulation of FP in the membrane fraction of infected cells that correlated well with parasite killing, due to the permeabilization of membranes to ions. This differential effect is explained by the ability of chloroquine and amodiaquine to inhibit the degradation of membrane-associated FP by glutathione and the incapacity of quinine and mefloquine to do so. This discrepancy implies that the antimalarial mode of action of chloroquine and amodiaquine is different in its mechanistic details from that of quinine and mefloquine and is compatible with the diametric sensitivity of most strains to chloroquine and mefloquine and the disparate interaction of these drugs with enhancers of their antimalarial action.