Genetic markers of artemisinin resistance in Plasmodium spp. parasites

Abstract

The vast majority of malaria patients worldwide are currently treated with combination therapy comprising one of the artemisinin family of drugs, characterised by rapid action and short plasma half-life, co-formulated with a longer-lasting drug from the amino arylalcohol or quinoline families. There is now a widely perceived threat to treatment efficacy, as reduced susceptibility to rapid artemisinin clearance in vivo has become prevalent among populations of Plasmodium falciparum in the Greater Mekong subregion since 2008. In vitro and in vivo drug selection studies, heterologous cell expression experiments and genetic epidemiology have identified many candidate markers of reduced ring-stage susceptibility to artemisinin. Certain variants of the P. falciparum pfk13 gene, which encodes a kelch domain protein implicated in the unfolded protein response, are strongly associated with slow parasite clearance by artemisinin in the Mekong subregion. However, anomalies in the epidemiological association of pfk13 variants with true treatment failure in vivo and the curious cell-cycle stage specificity of this phenotype in vitro warrant exploration in some depth. Taken together, available data suggest that the emergence of P. falciparum expressing K13 variants has not yet precipitated a public health emergency. Alternative candidate markers of artemisinin susceptibility are also described, as K13-independent treatment failure has been observed in African P. falciparum and in the rodent malaria parasite Plasmodium chabaudi.

Artemisinin susceptibility in the malaria parasitePlasmodium falciparum: propellers, adaptor proteins and the need for cellular healing

“… 2015 ; Hott et al . 2015 ; Sutherland 2017 ), explaining why EC 50 estimates for DHA from standard assays with continuous 48–72 h drug exposure do not correlate with slow clearance in vivo (Fig. 1B ).…”

“…The first is that K13 mutant parasites are completely cleared by extended regimens of artesunate monotherapy and/or ACT (Schallig et al . 2017 ; Sutherland 2017 ). The second is that K13 propeller domain mutations that induce artemisinin tolerance in vitro have not spread widely in African or South American P. falciparum populations (Taylor et al .…”

Daily rhythms of both host and parasite affect antimalarial drug efficacy

“…When uncomplicated malaria cases are diagnosed, information on host internal time as well as parasite IDC stage distribution and synchronicity could be collected by microscopy [ 80 ] or qPCR [ 81 ] to infer the timing of the IDC rhythm and determine when parasites are best targeted. The impact of chronotherapy should be most pronounced in highly synchronous infections, when multi-day treatment regimens repeatedly target the same IDC stage in subsequent replication cycles [ 13 , 82 ], and when parasite and host rhythms are naturally or forcefully misaligned. Finally, if timed treatment is not achievable, then extending artemisinin combination therapy to include not 3 (1.5 IDCs) but 4 days (2 IDCs) would make treatment efficacy less dependent on the sensitivity of the parasite IDC stage that happens to predominate at the start of treatment [ 13 , 20 , 58 , 82 ].…”

Restructured mitochondrial-nuclear interaction inPlasmodium falciparumdormancy and persister survival after artemisinin exposure

“…These findings contrasted with the effects of PfK13 mutations on the parasite survival in Ring Stage Assays (RSAs), wherein early ring-stages (0-3 h post invasion) are exposed for 6 hours to a physiological level of ART (16,17). However, because the RSA phenotype does not include the PfK13-independent susceptibilities of more mature trophozoite-and schizont-stages, it does not correlate with standard halfmaximum inhibitory concentrations (IC50) or clinical treatments that provide continual ART exposures for more than one intraerythrocytic cycle (18). PfK13 status and the ring-stage phenotype are thus divorced from the ability of parasites to become dormant and survive ART monotherapy for periods of up to 10 days (19).…”