VERAPAMIL REVERSAL OF CHLOROQUINE RESISTANCE IN THE MALARIA PARASITE PLASMODIUM-FALCIPARUM IS SPECIFIC FOR RESISTANT PARASITES AND INDEPENDENT OF THE WEAK BASE EFFECT

Verapamil increases the net uptake and cytotoxicity of structurally di verse hydrophobic molecules in many multidrug-resistant mammalian cell lines, This compound has also been reported to reverse chloroquine re sistance in the human malaria parasite Plasmodium falciparum (Martin, S. K., Oduola, A. M. J., and Milhous, W. K. (1987) Science 235, 899-90 1), Although the mechanism of this reversal is unknown, it apparently involves an increase in the amount of chloroquine present in erythrocy tes infected with the resistant parasites, Chloroquine is a diprotic w eak base that accumulates in acidic organelles as a function of the pH gradient present between the organelle and the external medium, By ch anging the external medium pH, this property of chloroquine was used t o alter the cytotoxicity phenotype of genetically chloroquine-sensitiv e and -resistant trophozoites, Verapamil was also found to be toxic fo r malaria trophozoites, although this toxicity was independent of exte rnal pH and consistently about 3-4-fold higher against resistant strai ns, When verapamil was tested for its effects on chloroquine cytotoxic ity under conditions of phenotypic reversal, it was still found to exe rt only a measurable effect on the genetically resistant trophozoites, In short time incubations, verapamil was found to increase net chloro quine accumulation in erythrocytes infected with both chloroquine-sens itive and -resistant organisms, but only to affect the chloroquine sus ceptibility of the latter, Analysis of our data demonstrates that vera pamil works independently of the overall pH gradient concentrating chl oroquine into a trophozoite's lysosome, Instead, we propose that it in hibits the activity of a membrane ion channel indirectly responsible f or determining chloroquine transit within the parasite's cytoplasm.