Iron chelators can deplete cytosolic iron and also interact directly with functional iron in iron-containing enzymes, leading to iron deprivation of intracellular protozoa [
7,
9]. In this study, GTE and CM1 showed iron-chelating property by inhibiting growth and development of
P. falciparum at micromolar level while the reference anti-malarial drugs PYR and DHA are effective at nanomolar level. Their anti-malarial ability seems to be inversely related with their molecular size in potency of DFO > EGCG > CM1 ≃ DFX > DFP. DFO is a fungal hexadentate iron chelator used clinically for treatment of iron overload in β-thalassaemia patients, and EGCG is a natural hexadentate iron chelator used potentially for treatment of iron accumulation in Parkinson′s disease [
33,
34]. Among these chelators, CM1 is the most lipophilic and more efficient than DFP in removing intracellular iron [
19], possibly the compound is a more powerful anti-malarial agent. Interestingly,
N-methylanthranilic desferrioxamine, which is a more lipophilic DFO derivative (termed reversed siderophore) can permeabilize PRBC by bypassing plasma membrane to bind intracellular iron, leading to inhibition of cultured
P. falciparum growth (IC
50 values = 30 ± 8 and 3 ± 1 μM, respectively) [
35‐
37]. While losing their membrane selectivity, PRBC allow ions (e.g. Na
+, K
+, Zn
2+, Fe
2+ and Ca
2+), polar molecules (e.g. amino acids, glucose, purine nucleosides) and even anti-malarial drugs (e.g. mefloquine, chloroquine) to pass into the cells readily [
38]. By this way, influx of DFO and green tea EGCG through parasite-encoded transporters or aqueous leaks and/or pores would have occurred as well. The selectivity can be based either on the selective permeation of the chelators into the parasitized cells or on a higher susceptibility of the latter to iron deprivation or antioxidants. CM1 and DFP are orally active bidentate chelators which possess similar physicochemical and biological properties, including log b value for Fe(III) (37.0 and 37.4, respectively), pFe(III) (20.3 and 20.5, respectively), and uncharged free as well as Fe(III)-bound ligands. Extraordinarily, CM1 is more lipophilic (K
part = 0.53 and 0.11, respectively) and more efficient in chelating hepatic ferritin iron (approximately 18 % and 7 %, respectively) than DFO. The CM1’s log b values at pH 7.4 for Fe
3+, Cu
2+ and Zn
2+ are 20.3, 18.8 and 12.9, respectively; whereas, the selectivity (pM) values for these three metal ions are 20.3, 9.8 and 6.2, respectively, suggesting higher levels in terms of the affinity and selectivity of CM1 for iron ions than other metal ions [
19]. The chelator will chelate the cell irons specifically, leading to iron deprivation of the growing malaria parasites. Since malaria-infected individuals tend to be anemic or hypoeferremic, further reduction of iron stores by a non-specific chelator might be deleterious. Moreover, chelators introduced into cultures continuously for days do not quite represent the in vivo situation, as each agent has its own pharmacokinetic profile and parasite susceptibility to the chelators, as they are highly stage dependent. However, IC
50 values of cell iron scavenging based on single time point measurement is not necessarily indicative of the chelator’s potency since the latter also depends on the permeation kinetics and pharmacokinetics. Scholl and coworkers support the evidence that malaria parasites utilize labile bioavailable iron pool(s) in red cell cytoplasm rather than haemozoin iron in the food vacuoles in order to synthesize their own haem in the mitochondria and apicomplast, while iron chelators can compete the iron utilization process and kill the parasite based on iron deprivation [
6].
GTE contains polyphenols including tannic acid, EGCG, epicatechin, epigallocatechin, gallocatechin-3-gallate, and epicatechin-3-gallate. The compound can chelate irons and inhibit the Fenton reaction-mediated generation of ROS [
22,
25]. GTE is indeed a natural iron chelator. Hellmann and colleagues showed that green tea EGCG inhibited
P. berghei sporozoite motility and liver cell infection efficiently and also synergized with digitonin for these two activities [
39]. ROS and oxidant drugs are toxic to biomolecules of cell components and can cause oxidative cell damage. The oxidants such as
t-butyl hydroperoxide and artemisinin-catalyzed epoxide can damage malaria parasites and PRBC [
40,
41]. Potent antioxidant GTE significantly decreased plasma levels of blood urea nitrogen and creatinine significantly in
P. berghei ANKA-infected mice and consequently reversed their kidney function caused by oxidative stress condition [
23,
42]. Controversially, EGCG and M30 (an iron chelator) did not improve survival of
P. berghei ANKA-infected mice [
43]. Similarly, hot water extract of the tea (
Artemisia annua) containing many polyphenolic acids such as mono-caffeoylquinic acids, tri-caffeoylquinic acid, artemisinic acid, arteannuin B and rosmarinic acid exhibited synergistic anti-plasmodial activity with artemisinin in the CQ- sensitive strain of
P. falciparum [
44].