Susceptibility of human malaria parasites to chloroquine is pH dependent.

Abstract

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 lattereither 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 the accumulated drug with vital functions of the food vacuole.The development of resistance of malarial parasites to the drug chloroquine (CQ), for many years the most popular and efficient antimalarial drug, has been of growing concern (1). However, the factors responsible for drug resistance have not been identified nor has the mode of CQ action been fully elucidated. The antimalarial effect of the drug has been attributed to the ability of CQ-sensitive parasites to accumulate relatively high levels of the drug (2, 3). It has been proposed that CQ accumulation results from its binding to a putative intraparasitic receptor, ferriprotoporphyrin IX (FeP), with consequent formation of a membrane lytic agent (4). Alternatively, in analogy with the lysosomotropic effects of weak bases on animal cells (5), it was postulated that CQ accumulates to high levels in the acidic food vacuoles of malaria parasites, causing their alkalinization and interfering with vital cellular processes (6).This latter hypothesis rests on the idea that the food vacuole of an intraerythrocytic malaria parasite is an acidic compartment (7,8) whose pH is maintained by metabolic input (9) and in which CQ accumulates by virtue of its weak base properties, following transmembrane proton gradients. This has recently been demonstrated by fluorescence measurements of intravacuolar pH and determination of CQ distribution as a function of established pH gradients and their dissipation by various agents (10). However, at therapeutic levels, the effect of CQ on intravacuolar pH, as determined by CQ and methylamine distribution, was minor, thus casting doubt on the validity of vacuolar alkalinization per se as a major factor in CQ-mediated inhibition of parasite development.The mode of action of CQ and the mechanism of drug resistance in human malaria have been assessed...

RETRACTED: The pfmdr1 gene of Plasmodium falciparum confers cellular resistance to antimalarial drugs in yeast cells.

“…The mechanism(s) underlying resistance to the widely used quinoline-containing drugs remains strongly debated. The antimalarial property of this group of drugs (exemplified by CQ) is believed to be mediated by accumulation/trapping in the acidic food vacuole of the parasite (3,4) and inhibition of polymerization of toxic heme groups generated by degradation of the host hemoglobin (5). Resistance seems pleiotropic as in vivo (6,7) and in vitro (8,9) cross-resistance of MFQ, HA, and QI (but generally not CQ) has been observed.…”

In vitro susceptibility of African isolates of Plasmodium falciparum from Gabon to pyronaridine.

“…23 Chloroquine may interfere with hemoglobin degradation in the food vacuole by raising the vacuolar pH. 24 It was postulated that chloroquine and quinine inhibited hematin polymerization in the parasite.…”

The pH of the digestive vacuole ofPlasmodium falciparumis not associated with chloroquine resistance

“…Previous attempts to measure the pH of the parasite digestive vacuole using the diprotic weak base CQ (Bray et al, 1992;Geary et al, 1990;Geary et al, 1986;Yayon et al, 1984;Yayon et al, 1985) as a probe have proved problematic. Some studies suggested that CQ accumulation by the parasite could be fully accounted for by the diffusion of the uncharged form of the molecule across the various membranes separating the interior of the digestive vacuole from the extracellular medium, with the protonated form accumulating in each compartment in accordance with the Henderson-Hasselbach equation and pH DV~4 .2-4.5 (Geary et al, 1990;Geary et al, 1986;Ginsburg et al, 1989;Hawley et al, 1996;Yayon et al, 1985). However, other studies suggested that some parasites accumulated much more CQ than could be predicted by this model (Krogstad et al, 1992;Krogstad and Schlesinger, 1986;Krogstad and Schlesinger, 1987;Krogstad et al, 1985).…”

“…It has been postulated that an increased pH DV in CQR parasites will result in a reduction in the trapping of CQ in the vacuole (Ferrari and Cutler, 1991;Homewood et al,compatible with the reduction in the apparent haematin binding affinity observed in CQR parasites . Several studies have demonstrated that increasing extracellular pH results in an increase in CQ uptake (Bray et al, 1994;Geary et al, 1990;Hawley et al, 1996;Yayon et al, 1985), an increase in CQ sensitivity of the parasites (Geary et al, 1990;Martiney et al, 1995;Yayon et al, 1985) and an increase in the apparent affinity of CQ-haematin binding . These observations are consistent with the hypothesis that CQ accumulation is a function of transmembrane pH gradients, although the variation in CQ uptake with extracellular pH could also be a result of the pH dependence of the concentration of the membrane-permeant neutral form of the molecule in the external medium, and consequent changes in the concentration and hence extent of binding of the drug in subcellular compartments.…”