Plasmodium falciparum resistance to ACTs: Emergence, mechanisms, and outlook

Abstract

Emergence and spread of resistance in Plasmodium falciparum to the frontline treatment artemisinin-based combination therapies (ACTs) in the epicenter of multidrug resistance of Southeast Asia threaten global malaria control and elimination. Artemisinin (ART) resistance (or tolerance) is defined clinically as delayed parasite clearance after treatment with an ART drug. The resistance phenotype is restricted to the early ring stage and can be measured in vitro using a ring-stage survival assay. ART resistance is associated with mutations in the propeller domain of the Kelch family protein K13. As a pro-drug, ART is activated primarily by heme, which is mainly derived from hemoglobin digestion in the food vacuole. Activated ARTs can react promiscuously with a wide range of cellular targets, disrupting cellular protein homeostasis. Consistent with this mode of action for ARTs, the molecular mechanisms of K13-mediated ART resistance involve reduced hemoglobin uptake/digestion and increased cellular stress response. Mutations in other genes such as AP-2μ (adaptor protein-2 μ subunit), UBP-1 (ubiquitin-binding protein-1), and Falcipain 2a that interfere with hemoglobin uptake and digestion also increase resistance to ARTs. ART resistance has facilitated the development of resistance to the partner drugs, resulting in rapidly declining ACT efficacies. The molecular markers for resistance to the partner drugs are mostly associated with point mutations in the two food vacuole membrane transporters PfCRT and PfMDR1, and amplification of pfmdr1 and the two aspartic protease genes plasmepsin 2 and 3 . It has been observed that mutations in these genes can have opposing effects on sensitivities to different partner drugs, which serve as the principle for designing triple ACTs and drug rotation. Although clinical ACT resistance is restricted to Southeast Asia, surveillance for drug resistance using in vivo clinical efficacy, in vitro assays, and molecular approaches is required to prevent or slow down the spread of resistant parasites.

Assessment of Plasmodium falciparum Artemisinin Resistance Independent of kelch13 Polymorphisms and with Escalating Malaria in Bangladesh

“…Cox’s Bazar reported a median (range) slope of parasite clearance half-life (PC 1/2 ) of 2.5 (0.7 to 5.4) between 6 and 24 h after administration. Unlike many studies that report K13 polymorphism with either RSA or parasite clearance data ( 30 – 32 ), we established both, because it was important to undertake correlative analyses between RSA values and slope half-lives of <5.5 h and to obtain insight that the RSA value is a better indicator of low levels of AR than PC 1/2 . Notably, RSA values of >1.0% are recognized by WHO to signify resistance and low RSA values ranging from 1 to 4 are consequential.…”

Evidence for the early emergence of piperaquine-resistant Plasmodium falciparum malaria and modeling strategies to mitigate resistance

“…To simulate parasite growth and evolution in the absence and/or presence of drug, we created a more sophisticated version of DARPS (Discrete Asexually Reproducing Population Simulator) [74], dubbed DARPS2. Each simulation was initiated with a monoculture of one of the 11 parasite lines under investigation, which was intended to simulate a population of parasites that was exiting the liver one week after initial sporozoite infection (here, we used an initial population 5×10 5 new asexual blood stage parasites [83]). During each synchronous discrete timestep, the number of parasites for each given line was updated by multiplying the number…”

“…However, delayed parasite clearance after ART treatment is now present throughout SE Asia. Resistance has also emerged to the ACT partner drug piperaquine (PPQ), leading to up to 50% treatment failures with this combination in some areas of the Greater Mekong Subregion [4,5]. Mutations in the transmembrane protein Plasmodium falciparum chloroquine resistance transporter (PfCRT) have been shown to be the major drivers of high-grade CQ and more recently, PPQ resistance [6][7][8][9][10].…”

Effect of Artemisinin on the Redox System of NADPH/FNR/Ferredoxin from Malaria Parasites

“…Concerning the ‘in vivo’ action of artemisinin, artemisinin is thought to be activated (radicalized) primarily by heme, which is mainly derived from hemoglobin digestion in the food vacuole of malaria parasites in red blood cells [ 10 ]. Activated artemisinin and the resulting reactive oxygen species (ROS) can react promiscuously with a wide range of cellular targets, disrupting cellular protein homeostasis [ 22 , 24 ]. Several mutations associated with artemisinin resistance were reported in the P. falciparum genes, including PfFd and ribosomal protein S10 in apicoplasts, autophagy-related protein, purine nucleoside phosphorylase, and chloroquine resistance transporter.…”

“…Several mutations associated with artemisinin resistance were reported in the P. falciparum genes, including PfFd and ribosomal protein S10 in apicoplasts, autophagy-related protein, purine nucleoside phosphorylase, and chloroquine resistance transporter. Among them, the mutations of the kelch family protein K13 (assumed to be either a regulator of polyubiquitination of PfPI3K or a ubiquitin E3 ligase substrate adaptor) are considered as the key determinant of artemisinin resistance [ 24 ]. The molecular mechanisms of K13-mediated artemisinin resistance involve reduced hemoglobin uptake/digestion and an increased cellular stress response against ROS, in which K13 mutations alter multiple aspects of the parasite’s intra-erythrocytic developmental program.…”