Artemisinin is a sesquiterpene lactone isolated from
Artemisia annua. Semi-synthetic artemisinin derivatives are important endoperoxides in artemisinin-based combination therapy (ACT) recommended as first-line treatment of
Plasmodium falciparum malaria in endemic countries. Despite their importance, the precise mechanism of action of this compound class is still under debate. The dominating view for all the peroxides (artemisinin derivatives and synthetic peroxides such as ozonides) is that they undergo reductive activation of the peroxide in the presence of ferrous iron released upon haemoglobin digestion within the parasite food vacuole [
1]. This forms carbon centred radicals, which then alkylate key parasite proteins like haem and membrane-associated parasite proteins [
2], one of which is the translationally controlled tumour protein (PfTCTP) [
3] and another could be PfATP6, a SERCA-type Ca
2+-ATPase [
4]. Studies by Eckstein-Ludwig and co-workers showed that thapsigargin and artemisinin specifically inhibit PfATP6 with Ki values (half-maximum inhibition constants) of 146 and 162 nM, respectively [
4]. In addition, artemisinin was shown to have a similar distribution in parasites when compared with thapsigargin, a specific inhibitor of PfATP6. Also, interaction between thapsigargin and artemisinin resulted in antagonism in cultured parasites. Interestingly, Crespo and co-workers reported that artemisinin and two novel epoxy-endoperoxides showed additive activity when used in combination with thapsigargin [
5]. In the present study the type of interaction between thapsigargin and artemisinin derivatives as well as the ozonide OZ277 (RBx11160 or arterolane) [
6,
7] was evaluated in parasite cultures for the first time. A previous study to understand the mechanism(s) of action of OZ277 revealed that it inhibits PfATP6 with a Ki value of 7,700 nM; thus it is a much weaker inhibitor than artemisinin [
8]. The study further showed that a fluorescent derivative of OZ277 displayed patterns of subcellular distribution different from that of artemisinin [
8]. In order to shed more light on the mechanism of action of peroxidic anti-malarials vis à vis their interaction with PfATP6 in cultured parasites, the interaction of thapsigargin with OZ277 or the semi-synthetic artemisinin derivatives (artesunate and artemether) was evaluated in
Plasmodium falciparum K1 and NF54 strains.