Increased tolerance to artemisinin in Plasmodium falciparum is mediated by a quiescence mechanism

Molecular epidemiology of malaria in Cameroon. XXX. sequence analysis of Plasmodium falciparum ATPase 6, dihydrofolate reductase, and dihydropteroate synthase resistance markers in clinical isolates from children treated with an artesunate-sulfadoxine-pyrimethamine combination

Plasmodium falciparum dihydrofolate reductase (dhfr) and dihydropteroate synthase (dhps) genes are reliable molecular markers for antifolate resistance. The P. falciparum ATPase 6 (pfatp6) gene has been proposed to be a potential marker for artemisinin resistance. In our previous clinical study, we showed that artesunate-sulfadoxine-pyrimethamine is highly effective against uncomplicated malaria in Yaoundé, Cameroon. In the present study, dhfr, dhps, and pfatp6 mutations in P. falciparum isolates obtained from children treated with artesunate-sulfadoxine-pyrimethamine were determined. All 61 isolates had wild-type Pfatp6 263, 623, and 769 alleles, and 11 (18%) had a single E431K substitution. Three additional mutations, E643Q, E432K, and E641Q, were detected. The results did not indicate any warning signal of serious concern (i.e., no parasites were seen with quintuple dhfr-dhps, DHFR Ile164Leu, or pfatp6 mutations), as confirmed by the high clinical efficacy of artesunate-sulfadoxine-pyrimethamine. Further studies are required to identify a molecular marker that reliably predicts artemisinin resistance.

Antimalarial efficacy of piperaquine-based antimalarial combination therapies: facts and uncertainties

Piperaquine is a bisquinoline antimalarial drug extensively used as monotherapy in China in the 1980s and subsequently included as one of the components of the artemisinin-based combination therapies (ACTs) in the 1990s. Among them, dihydroartemisinin-piperaquine (DHA-PQP) represents a new and extremely promising fixed combination. Several clinical trials have repeatedly shown that DHA-PQP is a safe and highly efficacious therapy against uncomplicated Plasmodium falciparum and the asexual stages of Plasmodium vivax malaria. Studies conducted with this drug have reported cure rates consistently above 95%, with the only exception being a study carried out in Papua New Guinea which reported a high rate of treatment failures. Although it has been hypothesized that such treatment failures could be related to cross-resistance mechanisms between piperaquine and other quinolines or to a reduced susceptibility of parasites to artemisinin derivatives, a critical review of the studies published so far seems to exclude both of these possibilities. Overall, there is now sufficient evidence on the safety and efficacy of the DHA-PQP therapy. Accordingly, the use of this ACT for the treatment of P. falciparum malaria has been recently approved in the EU via a centralized procedure by the European Medicines Agency. Moreover, it is now recommended globally by the World Health Organization as an option for the first-line treatment of uncomplicated malaria.

High-throughput multi-parameter flow-cytometric analysis from micro-quantities of plasmodium-infected blood

Despite significant technological and conceptual advances over the last century, evaluation of the efficacy of anti-malarial vaccines or drugs continues to rely principally on direct microscopic visualisation of parasites on thick and/or thin Giemsa-stained blood smears. This requires technical expertise of the microscopist, is highly subjective and error-prone, and does not account for aberrations such as anaemia. Many published methods have shown that flow cytometric analysis of blood is a highly versatile method that can readily detect nucleic acid-stained parasitised red blood cells within cultured cell populations and in ex-vivo samples. However several impediments, including the difficulty in distinguishing reticulocytes from infected red blood cells and the fickle nature of red blood cells, have precluded the development and universal adoption of flow-cytometric based assays for ex-vivo sample analysis. We have developed a novel high-throughput assay for the flow cytometric assessment of blood that overcomes these impediments by utilising the unique properties of the nucleic acid stain DAPI to differentially stain RNA and DNA, combined with novel fixation and analysis protocols. The assay allows the rapid and reliable analysis of multiple parameters from micro-volumes of blood, including: parasitaemia, platelet count, reticulocyte count, normocyte count, white blood cell count and delineation of subsets and phenotypic markers including, but not limited to, CD4(+) and CD8(+) T cells, and the expression of phenotypic markers such as PD-L1 or intracellular cytokines. The assay requires less than one drop of blood and is therefore suitable for short interval time-course experiments and allows the progression of infection and immune responses to be closely monitored in the laboratory or cytometer-equipped field locations. Herein, we describe the technique and demonstrate its application in vaccinology and with a range of rodent and human parasite species including Plasmodium yoelii, Plasmodium chabaudi, Plasmodium berghei and Plasmodium falciparum.

Cytotoxic activity of secondary metabolites derived from Artemisia annua L. towards cancer cells in comparison to its designated active constituent artemisinin

Artemisia annua L. (sweet wormwood, qinhao) has traditionally been used in Chinese medicine. The isolation of artemisinin from Artemisia annua and its worldwide accepted application in malaria therapy is one of the showcase success stories of phytomedicine during the past decades. Artemisinin-type compounds are also active towards other protozoal or viral diseases as well as cancer cells in vitro and in vivo. Nowadays, Artemisia annua tea is used as a self-reliant treatment in developing countries. The unsupervised use of Artemisia annua tea has been criticized to foster the development of artemisinin resistance in malaria and cancer due to insufficient artemisinin amounts in the plant as compared to standardized tablets with isolated artemisinin or semisynthetic artemisinin derivatives. However, artemisinin is not the only bioactive compound in Artemisia annua. In the present investigation, we analyzed different Artemisia annua extracts. Dichloromethane extracts were more cytotoxic (range of IC₅₀: 1.8-14.4 μg/ml) than methanol extracts towards Trypanosoma b. brucei (TC221 cells). The range of IC₅₀ values for HeLa cancer cells was 54.1-275.5 μg/ml for dichloromethane extracts and 276.3-1540.8 μg/ml for methanol extracts. Cancer and trypanosomal cells did not reveal cross-resistance among other compounds of Artemisia annua, namely the artemisinin-related artemisitene and arteanuine B as well as the unrelated compounds, scopoletin and 1,8-cineole. This indicates that cells resistant to one compound retained sensitivity to another one. These results were also supported by microarray-based mRNA expression profiling showing that molecular determinants of sensitivity and resistance were different between artemisinin and the other phytochemicals investigated.

Artemisinin resistance in Plasmodium falciparum is associated with an altered temporal pattern of transcription

BACKGROUND

Artemisinin resistance in Plasmodium falciparum malaria has emerged in Western Cambodia. This is a major threat to global plans to control and eliminate malaria as the artemisinins are a key component of antimalarial treatment throughout the world. To identify key features associated with the delayed parasite clearance phenotype, we employed DNA microarrays to profile the physiological gene expression pattern of the resistant isolates.

RESULTS

In the ring and trophozoite stages, we observed reduced expression of many basic metabolic and cellular pathways which suggests a slower growth and maturation of these parasites during the first half of the asexual intraerythrocytic developmental cycle (IDC). In the schizont stage, there is an increased expression of essentially all functionalities associated with protein metabolism which indicates the prolonged and thus increased capacity of protein synthesis during the second half of the resistant parasite IDC. This modulation of the P. falciparum intraerythrocytic transcriptome may result from differential expression of regulatory proteins such as transcription factors or chromatin remodeling associated proteins. In addition, there is a unique and uniform copy number variation pattern in the Cambodian parasites which may represent an underlying genetic background that contributes to the resistance phenotype.

CONCLUSIONS

The decreased metabolic activities in the ring stages are consistent with previous suggestions of higher resilience of the early developmental stages to artemisinin. Moreover, the increased capacity of protein synthesis and protein turnover in the schizont stage may contribute to artemisinin resistance by counteracting the protein damage caused by the oxidative stress and/or protein alkylation effect of this drug. This study reports the first global transcriptional survey of artemisinin resistant parasites and provides insight to the complexities of the molecular basis of pathogens with drug resistance phenotypes in vivo.

Artemisinin activity against Plasmodium falciparum requires hemoglobin uptake and digestion

Combination regimens that include artemisinin derivatives are recommended as first line antimalarials in most countries where malaria is endemic. However, the mechanism of action of artemisinin is not fully understood and the usefulness of this drug class is threatened by reports of decreased parasite sensitivity. We treated Plasmodium falciparum for periods of a few hours to mimic clinical exposure to the short half-life artemisinins. We found that drug treatment retards parasite growth and inhibits uptake of hemoglobin, even at sublethal concentrations. We show that potent artemisinin activity is dependent on hemoglobin digestion by the parasite. Inhibition of hemoglobinase activity with cysteine protease inhibitors, knockout of the cysteine protease falcipain-2 by gene deletion, or direct deprivation of host cell lysate, significantly decreases artemisinin sensitivity. Hemoglobin digestion is also required for artemisinin-induced exacerbation of oxidative stress in the parasite cytoplasm. Arrest of hemoglobin digestion by early stage parasites provides a mechanism for surviving short-term artemisinin exposure. These insights will help in the design of new drugs and new treatment strategies to circumvent drug resistance.

Expanding the Antimalarial Drug Arsenal-Now, But How?

The number of available and effective antimalarial drugs is quickly dwindling. This is mainly because a number of drug resistance-associated mutations in malaria parasite genes, such as crt, mdr1, dhfr/dhps, and others, have led to widespread resistance to all known classes of antimalarial compounds. Unfortunately, malaria parasites have started to exhibit some level of resistance in Southeast Asia even to the most recently introduced class of drugs, artemisinins. While there is much need, the antimalarial drug development pipeline remains woefully thin, with little chemical diversity, and there is currently no alternative to the precious artemisinins. It is difficult to predict where the next generation of antimalarial drugs will come from; however, there are six major approaches: (i) re-optimizing the use of existing antimalarials by either replacement/rotation or combination approach; (ii) repurposing drugs that are currently used to treat other infections or diseases; (iii) chemically modifying existing antimalarial compounds; (iv) exploring natural sources; (v) large-scale screening of diverse chemical libraries; and (vi) through parasite genome-based ("targeted") discoveries. When any newly discovered effective antimalarial treatment is used by the populus, we must maintain constant vigilance for both parasite-specific and human-related factors that are likely to hamper its success. This article is neither comprehensive nor conclusive. Our purpose is to provide an overview of antimalarial drug resistance, associated parasite genetic factors (1. Introduction; 2. Emergence of artemisinin resistance in P. falciparum), and the antimalarial drug development pipeline (3. Overview of the global pipeline of antimalarial drugs), and highlight some examples of the aforementioned approaches to future antimalarial treatment. These approaches can be categorized into "short term" (4. Feasible options for now) and "long term" (5. Next generation of antimalarial treatment-Approaches and candidates). However, these two categories are interrelated, and the approaches in both should be implemented in parallel with focus on developing a successful, long-lasting antimalarial chemotherapy.

Drug-resistant malaria: molecular mechanisms and implications for public health

Resistance to antimalarial drugs has often threatened malaria elimination efforts and historically has led to the short-term resurgence of malaria incidences and deaths. With concentrated malaria eradication efforts currently underway, monitoring drug resistance in clinical settings complemented by in vitro drug susceptibility assays and analysis of resistance markers, becomes critical to the implementation of an effective antimalarial drug policy. Understanding of the factors, which lead to the development and spread of drug resistance, is necessary to design optimal prevention and treatment strategies. This review attempts to summarize the unique factors presented by malarial parasites that lead to the emergence and spread of drug resistance, and gives an overview of known resistance mechanisms to currently used antimalarial drugs.

Artesunate tolerance in transgenic Plasmodium falciparum parasites overexpressing a tryptophan-rich protein

Due to their rapid, potent action on young and mature intraerythrocytic stages, artemisinin derivatives are central to drug combination therapies for Plasmodium falciparum malaria. However, the evidence for emerging parasite resistance/tolerance to artemisinins in southeast Asia is of great concern. A better understanding of artemisinin-related drug activity and resistance mechanisms is urgently needed. A recent transcriptome study of parasites exposed to artesunate led us to identify a series of genes with modified levels of expression in the presence of the drug. The gene presenting the largest mRNA level increase, Pf10_0026 (PArt), encoding a hypothetical protein of unknown function, was chosen for further study. Immunodetection with PArt-specific sera showed that artesunate induced a dose-dependent increase of the protein level. Bioinformatic analysis showed that PArt belongs to a Plasmodium-specific gene family characterized by the presence of a tryptophan-rich domain with a novel hidden Markov model (HMM) profile. Gene disruption could not be achieved, suggesting an essential function. Transgenic parasites overexpressing PArt protein were generated and exhibited tolerance to a spike exposure to high doses of artesunate, with increased survival and reduced growth retardation compared to that of wild-type-treated controls. These data indicate the involvement of PArt in parasite defense mechanisms against artesunate. This is the first report of genetically manipulated parasites displaying a stable and reproducible decreased susceptibility to artesunate, providing new possibilities to investigate the parasite response to artemisinins.

Artemisinin-induced parasite dormancy: a plausible mechanism for treatment failure

BACKGROUND

Artemisinin-combination therapy is a highly effective treatment for uncomplicated falciparum malaria but parasite recrudescence has been commonly reported following artemisinin (ART) monotherapy. The dormancy recovery hypothesis has been proposed to explain this phenomenon, which is different from the slower parasite clearance times reported as the first evidence of the development of ART resistance.

METHODS

In this study, an existing P. falciparum infection model is modified to incorporate the hypothesis of dormancy. Published in vitro data describing the characteristics of dormant parasites is used to explore whether dormancy alone could be responsible for the high recrudescence rates observed in field studies using monotherapy. Several treatment regimens and dormancy rates were simulated to investigate the rate of clinical and parasitological failure following treatment.

RESULTS

The model output indicates that following a single treatment with ART parasitological and clinical failures occur in up to 77% and 67% of simulations, respectively. These rates rapidly decline with repeated treatment and are sensitive to the assumed dormancy rate. The simulated parasitological and clinical treatment failure rates after 3 and 7 days of treatment are comparable to those reported from several field trials.

CONCLUSIONS

Although further studies are required to confirm dormancy in vivo, this theoretical study adds support for the hypothesis, highlighting the potential role of this parasite sub-population in treatment failure following monotherapy and reinforcing the importance of using ART in combination with other anti-malarials.

Fitness costs of disrupting circadian rhythms in malaria parasites

Circadian biology assumes that biological rhythms maximize fitness by enabling organisms to coordinate with their environment. Despite circadian clocks being such a widespread phenomenon, demonstrating the fitness benefits of temporal coordination is challenging and such studies are rare. Here, we tested the consequences--for parasites--of being temporally mismatched to host circadian rhythms using the rodent malaria parasite, Plasmodium chabaudi. The cyclical nature of malaria infections is well known, as the cell cycles across parasite species last a multiple of approximately 24 h, but the evolutionary explanations for periodicity are poorly understood. We demonstrate that perturbation of parasite rhythms results in a twofold cost to the production of replicating and transmission stages. Thus, synchronization with host rhythms influences in-host survival and between-host transmission potential, revealing a role for circadian rhythms in the evolution of host-parasite interactions. More generally, our results provide a demonstration of the adaptive value of circadian rhythms and the utility of using an evolutionary framework to understand parasite traits.

Intrahost modeling of artemisinin resistance in Plasmodium falciparum

Artemisinin-resistant Plasmodium falciparum malaria has emerged in western Cambodia. Resistance is characterized by prolonged in vivo parasite clearance times (PCTs) following artesunate treatment. The biological basis is unclear. The hypothesis that delayed parasite clearance results from a stage-specific reduction in artemisinin sensitivity of the circulating young asexual parasite ring stages was examined. A mathematical model was developed, describing the intrahost parasite stage-specific pharmacokinetic-pharmacodynamic relationships. Model parameters were estimated using detailed pharmacokinetic and parasite clearance data from 39 patients with uncomplicated falciparum malaria treated with artesunate from Pailin (western Cambodia) where artemisinin resistance was evident and 40 patients from Wang Pha (northwestern Thailand) where efficacy was preserved. The mathematical model reproduced the observed parasite clearance for each patient with an accurate goodness of fit (rmsd: 0.03-0.67 in log(10) scale). The parameter sets that provided the best fits with the observed in vivo data consist of a highly conserved concentration-effect relationship for the trophozoite and schizont parasite stages, but a variable relationship for the ring stages. The model-derived assessment suggests that the efficacy of artesunate on ring stage parasites is reduced significantly in Pailin. This result supports the hypothesis that artemisinin resistance mainly reflects reduced ring-stage susceptibility and predicts that doubling the frequency of dosing will accelerate clearance of artemisinin-resistant parasites.

Plasmodium sensitivity to artemisinins: magic bullets hit elusive targets

Artemisinins are efficacious antimalarial drugs widely employed as first-line treatment in endemic countries under the form of combined therapies. Different molecular modes of action have been postulated to explain the parasiticidal effect of these compounds; however, none has been unequivocally accepted, and their physiological relevance is still questioned. Similarly, no definite genetic determinant of Plasmodium sensitivity to artemisinins has been identified so far. A better understanding of the mode of action of artemisinins and the genetic basis of laboratory-induced or field-observed altered susceptibility is crucial for malaria control. In this review different models of artemisinins' molecular action are briefly presented, focusing on recent advances, and the evidence of potential association between various gene polymorphisms and artemisinin resistance is comprehensively reviewed.

Brazilian Plasmodium falciparum isolates: investigation of candidate polymorphisms for artemisinin resistance before introduction of artemisinin-based combination therapy

Background

This study was performed to better understand the genetic diversity of known polymorphisms in pfatpase6 and pfmdr1 genes before the introduction of ACT in Brazil, in order to get a genotypic snapshot of Plasmodium falciparum parasites that may be used as baseline reference for future studies.

Methods

Parasites from P. falciparum samples collected in 2002, 2004 and 2006-2007 were genotyped using PCR and DNA sequencing at codons 86, 130, 184, 1034, 1042, 1109 and 1246 for pfmdr1 gene, and 243, 263, 402, 431, 623, 630, 639, 683, 716, 776, 769 and 771 for pfatpase6 gene.

Results

A pfmdr1 haplotype NEF/CDVY was found in 97% of the samples. In the case of pfatpase6, four haplotypes, wild-type (37%), 630 S (35%), 402 V (5%) and double-mutant 630 S + 402 V (23%), were detected.

Conclusion

Although some polymorphism in pfmdr1 and pfatpase6 were verified, no reported haplotypes in both genes that may mediate altered response to ACT was detected before the introduction of this therapy in Brazil. Thus, the haplotypes herein described can be very useful as a baseline reference of P. falciparum populations without ACT drug pressure.

Artemisone uptake in Plasmodium falciparum-infected erythrocytes

Artemisone is one of the most promising artemisinin derivatives in clinical trials. Previous studies with radiolabeled artemisinin and dihydroartemisinin have measured uptake in Plasmodium falciparum-infected erythrocytes. Uptake is much greater in infected than in uninfected erythrocytes, but the relative contributions of transport, binding, and metabolism to this process still await definition. In this study, we characterized mechanisms by which [(14)C]artemisone is taken up into uninfected and P. falciparum-infected human erythrocytes in vitro. Radiolabeled artemisone rapidly enters uninfected erythrocytes without much exceeding extracellular concentrations. Unlabeled artemisone does not compete in this process. Radiolabeled artemisone is concentrated greatly by a time- and temperature-dependent mechanism in infected erythrocytes. This uptake is abrogated by unlabeled artemisone. In addition, the uptake of artemisone into three subcellular fractions, and its distribution into these fractions, is examined as a function of parasite maturation. These data are relevant to an understanding of the mechanisms of action of this important class of drugs.

Facile oxidation of leucomethylene blue and dihydroflavins by artemisinins: relationship with flavoenzyme function and antimalarial mechanism of action

The antimalarial drug methylene blue (MB) affects the redox behaviour of parasite flavin-dependent disulfide reductases such as glutathione reductase (GR) that control oxidative stress in the malaria parasite. The reduced flavin adenine dinucleotide cofactor FADH(2) initiates reduction to leucomethylene blue (LMB), which is oxidised by oxygen to generate reactive oxygen species (ROS) and MB. MB then acts as a subversive substrate for NADPH normally required to regenerate FADH(2) for enzyme function. The synergism between MB and the peroxidic antimalarial artemisinin derivative artesunate suggests that artemisinins have a complementary mode of action. We find that artemisinins are transformed by LMB generated from MB and ascorbic acid (AA) or N-benzyldihydronicotinamide (BNAH) in situ in aqueous buffer at physiological pH into single electron transfer (SET) rearrangement products or two-electron reduction products, the latter of which dominates with BNAH. Neither AA nor BNAH alone affects the artemisinins. The AA-MB SET reactions are enhanced under aerobic conditions, and the major products obtained here are structurally closely related to one such product already reported to form in an intracellular medium. A ketyl arising via SET with the artemisinin is invoked to explain their formation. Dihydroflavins generated from riboflavin (RF) and FAD by pretreatment with sodium dithionite are rapidly oxidised by artemisinin to the parent flavins. When catalytic amounts of RF, FAD, and other flavins are reduced in situ by excess BNAH or NAD(P)H in the presence of the artemisinins in the aqueous buffer, they are rapidly oxidised to the parent flavins with concomitant formation of two-electron reduction products from the artemisinins; regeneration of the reduced flavin by excess reductant maintains a catalytic cycle until the artemisinin is consumed. In preliminary experiments, we show that NADPH consumption in yeast GR with redox behaviour similar to that of parasite GR is enhanced by artemisinins, especially under aerobic conditions. Recombinant human GR is not affected. Artemisinins thus may act as antimalarial drugs by perturbing the redox balance within the malaria parasite, both by oxidising FADH(2) in parasite GR or other parasite flavoenzymes, and by initiating autoxidation of the dihydroflavin by oxygen with generation of ROS. Reduction of the artemisinin is proposed to occur via hydride transfer from LMB or the dihydroflavin to O1 of the peroxide. This hitherto unrecorded reactivity profile conforms with known structure-activity relationships of artemisinins, is consistent with their known ability to generate ROS in vivo, and explains the synergism between artemisinins and redox-active antimalarial drugs such as MB and doxorubicin. As the artemisinins appear to be relatively inert towards human GR, a putative model that accounts for the selective potency of artemisinins towards the malaria parasite also becomes apparent. Decisively, ferrous iron or carbon-centered free radicals cannot be involved, and the reactivity described herein reconciles disparate observations that are incompatible with the ferrous iron-carbon radical hypothesis for antimalarial mechanism of action. Finally, the urgent enquiry into the emerging resistance of the malaria parasite to artemisinins may now in one part address the possibilities either of structural changes taking place in parasite flavoenzymes that render the flavin cofactor less accessible to artemisinins or of an enhancement in the ability to use intra-erythrocytic human disulfide reductases required for maintenance of parasite redox balance.

Genetic predisposition favors the acquisition of stable artemisinin resistance in malaria parasites

The emergence of artemisinin-resistant Plasmodium falciparum malaria jeopardizes efforts to control this infectious disease. To identify factors contributing to reduced artemisinin susceptibility, we have employed a classical genetic approach by analyzing artemisinin responses in the F1 progeny of a genetic cross. Our data show that reduced artemisinin susceptibility is a multifactorial trait, with pfmdr1 and two additional loci (on chromosomes 12 and 13) contributing to it. We further show that the different artemisinin susceptibilities of the progeny strains affect their responses to selection with increasing concentrations of artemisinin. Stable, high-level in vitro artemisinin resistance rapidly arose in those parasites that were the least artemisinin susceptible among the F1 progeny, whereas progeny that were highly artemisinin susceptible did not acquire stable artemisinin resistance. These data suggest that genetic predisposition favors the acquisition of high-level artemisinin resistance. In vitro-induced artemisinin resistance did not result in cross-resistance to artesunate or artemether, suggesting that resistance to one derivative does not necessarily render the entire drug class ineffective.

Pitfalls in new artemisinin-containing antimalarial drug development

Artemisinin combination therapy (ACT) paves the way for new opportunities to eliminate malaria in the tropics. However, the huge increase of ACT consumption raises major concerns about their availability over the next few years. At the same time a decrease in their efficacy has already been reported. Alongside the deployment of multifocal control programs, the process ranging from artemisia crop production to accreditation of new ACT combinations urgently needs to be strengthened to supply sufficient quantities of high-quality drugs. New suppliers will have the opportunity to enter this market to develop new formulations, and bioequivalence studies are required to validate these new formulations. It is thus crucial for national malaria control teams to be able to better scrutinize the dossier of these new formulations.

Artemisinin‐induced dormancy in plasmodium falciparum: duration, recovery rates, and implications in treatment failure

BACKGROUND

Despite the remarkable activity of artemisinin and its derivatives, monotherapy with these agents has been associated with high rates of recrudescence. The temporary arrest of the growth of ring-stage parasites (dormancy) after exposure to artemisinin drugs provides a plausible explanation for this phenomenon.

METHODS

Ring-stage parasites of several Plasmodium falciparum lines were exposed to different doses of dihydroartemisinin (DHA) alone or in combination with mefloquine. For each regime, the proportion of recovering parasites was determined daily for 20 days.

RESULTS

Parasite development was abruptly arrested after a single exposure to DHA, with some parasites being dormant for up to 20 days. Approximately 50% of dormant parasites recovered to resume growth within the first 9 days. The overall proportion of parasites recovering was dose dependent, with recovery rates ranging from 0.044% to 1.313%. Repeated treatment with DHA or with DHA in combination with mefloquine led to a delay in recovery and an approximately 10-fold reduction in total recovery. Strains with different genetic backgrounds appeared to vary in their capacity to recover.

CONCLUSIONS

These results imply that artemisinin-induced arrest of growth occurs readily in laboratory-treated parasites and may be a key factor in P. falciparum malaria treatment failure.

Investigations into the role of the Plasmodium falciparum SERCA (PfATP6) L263E mutation in artemisinin action and resistance

Artemisinin-based combination therapies (ACTs) are highly effective for the treatment of Plasmodium falciparum malaria, yet their sustained efficacy is threatened by the potential spread of parasite resistance. Recent studies have provided evidence that artemisinins can inhibit the function of PfATP6, the P. falciparum ortholog of the ER calcium pump SERCA, when expressed in Xenopus laevis oocytes. Inhibition was significantly reduced in an L263E variant, which introduced the mammalian residue into a putative drug-binding pocket. To test the hypothesis that this single mutation could decrease P. falciparum susceptibility to artemisinins, we implemented an allelic-exchange strategy to replace the wild-type pfatp6 allele by a variant allele encoding L263E. Transfected P. falciparum clones were screened by PCR analysis for disruption of the endogenous locus and introduction of the mutant L263E allele under the transcriptional control of a calmodulin promoter. Expression of the mutant allele was demonstrated by reverse transcriptase (RT) PCR and verified by sequence analysis. Parasite clones expressing wild-type or L263E variant PfATP6 showed no significant difference in 50% inhibitory concentrations (IC(50)s) for artemisinin or its derivatives dihydroartemisinin and artesunate. Nonetheless, hierarchical clustering analysis revealed a trend toward reduced susceptibility that neared significance (artemisinin, P approximately = 0.1; dihydroartemisinin, P = 0.053 and P = 0.085; and artesunate, P = 0.082 and P = 0.162 for the D10 and 7G8 lines, respectively). Notable differences in the distribution of normalized IC(50)s provided evidence of decreased responsiveness to artemisinin and dihydroartemisinin (P = 0.02 for the D10 and 7G8 lines), but not to artesunate in parasites expressing mutant PfATP6.