Artemisinin-Resistant Plasmodium falciparum Malaria

Abstract

For more than five decades, Southeast Asia (SEA) has been fertile ground for the emergence of drug-resistant Plasmodium falciparum malaria. After generating parasites resistant to chloroquine, sulfadoxine, pyrimethamine, quinine, and mefloquine, this region has now spawned parasites resistant to artemisinins, the world's most potent antimalarial drugs. In areas where artemisinin resistance is prevalent, artemisinin combination therapies (ACTs)-the first-line treatments for malaria-are failing fast. This worrisome development threatens to make malaria practically untreatable in SEA, and threatens to compromise global endeavors to eliminate this disease. A recent series of clinical, in vitro, genomics, and transcriptomics studies in SEA have defined in vivo and in vitro phenotypes of artemisinin resistance, identified its causal genetic determinant, explored its molecular mechanism, and assessed its clinical impact. Specifically, these studies have established that artemisinin resistance manifests as slow parasite clearance in patients and increased survival of early-ring-stage parasites in vitro; is caused by single nucleotide polymorphisms in the parasite's K13 gene, is 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. In SEA, clinical studies are urgently needed to monitor ACT efficacy where K13 mutations are prevalent, test whether new combinations of currently available drugs cure ACT failures, and advance new antimalarial compounds through preclinical pipelines and into clinical trials. Intensifying these efforts should help to forestall the spread of artemisinin and partner drug resistance from SEA to sub-Saharan Africa, where the world's malaria transmission, morbidity, and mortality rates are highest.

Fig 1. Dynamics of parasite clearance by artemisinins and other antimalarial drugs

In sensitive Plasmodium falciparum infections, fast-acting and rapidly-cleared artemisinins reduce the parasite load by a factor of 10,000 per 48-hour asexual-stage parasite cycle. In partially resistant P. falciparum infections, artemisinins reduce the parasite load by only by a factor of 100 per cycle, a parasite clearance rate similar to that of slower-acting drugs such as quinine. Another unique and beneficial feature of artemisinins is their broad stage-specificity, but this seems to be compromised in resistant parasites in Southeast Asia. Parasites that are at the early-ring stage during the brief exposure to rapidly eliminated artemisinins have reduced susceptibility, resulting in delayed parasite clearance following treatment with an artesunate monotherapy or ACT. Reproduced from (77) with permission.

Fig 2. Plasmodium falciparum kelch13 (K13) protein

The parasite K13 protein consists of Plasmodium-specific sequences, a BTB-POZ domain, and six kelch domains that are predicted to form a six-blade propeller. In the structural model, the original M476I mutation discovered by Ariey et al. (29) and six other mutations associated with artemisinin resistance in Southeast Asia are shown. Reproduced from (89) with permission.

Fig 3. Recently proposed mechanisms of artemisinin sensitivity and resistance in Plasmodium falciparum

A. In artemisinin-sensitive parasites, wild-type K13 (green) binds a putative transcription factor and targets it for degradation. In artemisinin-resistant parasites, mutant K13 (red) fails to bind this transcription factor, which translocates to the nucleus and upregulates genes involved in the antioxidant response. In this “pre-prepared” state, parasites are better able to handle the oxidative stress that is exerted by activated artemisinins, for example, by repairing and replenishing oxidant-damaged proteins. B. In artemisinin-sensitive parasites, wild-type K13 (green) binds PI3K and targets it for degradation. In artemisinin-resistant parasites, mutant K13 (red) fails to bind PI3K, leading to increased PI3K activity and PI3P levels. In this “prepared” state, high PI3P levels are presumably able to promote the survival of parasites exposed to artemisinins, for example, by mediating membrane fusion events involved in parasite growth. Reproduced from (89) with permission.