Drug combinations with artemisinin as the lead have become the first and second line treatment against malaria caused by Plasmodium falciparum. ACTs helped to bring down the incidence of malaria cases by 18% from 2010 to 2016. However, reduced clinical efficacy of artemisinin due to the emergence and spread of resistance raises the risk of retraction of recent gains achieved in reducing worldwide malaria burden.
In regions where artemisinin resistance is prevalent, ACTs are failing fast. If the trend continues, malaria could become practically untreatable, at least in those parts of the world.
Though there is no consensus on the biochemical targets of artemisinin or on the mechanism of action, the drug is considered a potent inhibitor of P. falciparum phosphatidylinositol-3-kinase (PfPI3K).
Genome-wide association studies (GWAS) have revealed the genetic loci in P. falciparum parasite associated with artemisinin resistance. The resistance is caused by single nucleotide polymorphisms in the parasite›s kelch 13 gene.
Single point mutations in the propeller domain of kelch13 after position 440 were associated with a mean increase in the parasite clearance half-life of 116%, studies revealed. Mutations in kelch13 gene are associated with an upregulated “unfolded protein response” pathway that may antagonize the pro-oxidant activity of artemisinins, and selects for partner drug resistance that rapidly leads to ACT failures.
Increased PfPI3K was linked with the C580Y mutation in P. falciparum kelch13 in resistant clinical strains of the bug. PfKelch13 is a primary marker of artemisinin resistance. Studies found that polyubiquitination of PfPI3K and its binding to PfKelch13 were reduced by the PfKelch13 mutation, which limited proteolysis of PfPI3K and thus increased levels of the kinase, as well as its lipid product phosphatidylinositol-3-phosphate (PI3P).
PI3P levels are predictive of artemisinin resistance. Elevated PI3P induced artemisinin resistance in the absence of PfKelch13 mutations, but remained responsive to regulation by PfKelch13. Studies describe PI3P as the key mediator of artemisinin resistance and the sole PfPI3K as an important target for malaria elimination.
They also identify Kelch13-propeller polymorphism as a useful molecular marker for large-scale surveillance efforts.
The geographic distribution of these SNPs confirm that artemisinin resistance has emerged and spread extensively in mainland Southeast Asia. The widespread availability of artemisinin monotherapies, poor-quality artemisinin-based combination therapies and monotherapies containing subtherapeutic amounts of active ingredients and unregulated use of antimalarial agents, plus the unusual genetic structure of parasites in this region, may have contributed to the emergence of resistant parasites.
Recent studies conducted in West Bengal found 4-5% of patients resistant to artemisinins. Researchers also report some previously unseen mutations in the kelch13 gene, which are critical to decreased artemisinin effectiveness.
Genes other than kelch 13 are also suspected to be involved in P. falciparum resistance. Reduced ART susceptibility can be mediated by PfCoronin gene, a member of the WD40 — an actin binding propeller domain protein family, according to a recent study.
PfCoronin has a seven-bladed propeller domain composed of WD40 repeats and ß-propeller folds in its N terminus, which is structurally similar to the six-bladed propeller domain found in the C terminus of PfKelch13.