Emergence Of Artemisinin Resistance Research Articles

Immunity as a predictor of anti-malarial treatment failure: a systematic review

BackgroundNaturally acquired immunity can reduce parasitaemia and potentially influence anti-malarial treatment outcomes; however, evidence for this in the current literature provides conflicted results. The available evidence was synthesized to determine and quantify the association between host immunity and anti-malarial treatment failure.MethodsFour databases were searched to identify studies investigating malaria antibody levels in patients receiving anti-malarial treatment for symptomatic malaria with treatment failure recorded according to the World Health Organization classification. Odds ratios or hazard ratios were extracted or calculated to quantify the association between malarial antibody levels and treatment failure, and findings from different studies were visualized using forest plots.ResultsEight studies, including patients with falciparum malaria treated with mono- and combination therapy of artemisinin derivatives, sulfadoxine, pyrimethamine and chloroquine, were identified. Reported and calculated effect estimates varied greatly between studies, even those assessing the same antigens and treatments. An association between blood-stage IgG responses and treatment efficacy was observed. The greatest magnitudes of effect were observed for artemisinin [OR/HR (95% CI) range 0.02 (0.00, 0.45)–1.08 (0.57, 2.06)] and chloroquine [0.24 (0.04, 1.37)–0.32 (0.05, 1.96)] treatments, and larger magnitudes of effect were observed for variant surface antigen responses [0.02 (0.00, 0.45)–1.92 (0.94, 3.91)] when compared with merozoite specific responses [0.24 (0.04, 1.37)–2.83 (1.13, 7.09)].ConclusionsNaturally acquired malarial immunity is associated with reduced anti-malarial treatment failure in malaria endemic populations. Anti-malarial IgG effects treatment outcome differently for different anti-malarial drugs and antigen targets, and had the greatest impact during treatment with the current first-line treatments, the artemisinins. This has implications for the assessment of the therapeutic efficacy of anti-malarials, particularly in the context of emerging artemisinin resistance.

Molecular Epidemiology of Malaria in Cameroon and Côte d'Ivoire. XXXI. Kelch 13 Propeller Sequences in Plasmodium falciparum Isolates before and after Implementation of Artemisinin-Based Combination Therapy.

Artemisinin-resistant malaria has not been reported from Africa, but resistance can possibly spread from Asia or arise independently in Africa. The emergence of artemisinin resistance in Africa can be monitored by molecular assay of Kelch 13 (K13) propeller sequences. A total of 251 archived DNA samples of Plasmodium falciparum isolates collected in 2002, 2003, and 2006 in Yaounde, Cameroon, and 47 samples collected in 2006 and 2013 in Abidjan, Côte d'Ivoire, were analyzed for K13-propeller sequence polymorphism. Only one isolate carried a mutant K13-propeller allele (E602D). None of the isolates carried the key mutant alleles (Y493H, R539T, I543T, and C580Y) associated with artemisinin resistance in Cambodia. The presence of the mutant allele was not correlated with in vitro response to dihydroartemisinin determined by the classical hypoxanthine incorporation assay. There was no evidence of K13 mutations associated with artemisinin resistance before and soon after the introduction of artemisinin-based combination therapies in Cameroon and Côte d'Ivoire.

Host immunity to Plasmodium falciparum and the assessment of emerging artemisinin resistance in a multinational cohort

Artemisinin-resistant falciparum malaria, defined by a slow-clearance phenotype and the presence of kelch13 mutants, has emerged in the Greater Mekong Subregion. Naturally acquired immunity to malaria clears parasites independent of antimalarial drugs. We hypothesized that between- and within-population variations in host immunity influence parasite clearance after artemisinin treatment and the interpretation of emerging artemisinin resistance. Antibodies specific to 12 Plasmodium falciparum sporozoite and blood-stage antigens were determined in 959 patients (from 11 sites in Southeast Asia) participating in a multinational cohort study assessing parasite clearance half-life (PCt1/2) after artesunate treatment and kelch13 mutations. Linear mixed-effects modeling of pooled individual patient data assessed the association between antibody responses and PCt1/2.P. falciparum antibodies were lowest in areas where the prevalence of kelch13 mutations and slow PCt1/2 were highest [Spearman ρ = -0.90 (95% confidence interval, -0.97, -0.65), and Spearman ρ = -0.94 (95% confidence interval, -0.98, -0.77), respectively]. P. falciparum antibodies were associated with faster PCt1/2 (mean difference in PCt1/2 according to seropositivity, -0.16 to -0.65 h, depending on antigen); antibodies have a greater effect on the clearance of kelch13 mutant compared with wild-type parasites (mean difference in PCt1/2 according to seropositivity, -0.22 to -0.61 h faster in kelch13 mutants compared with wild-type parasites). Naturally acquired immunity accelerates the clearance of artemisinin-resistant parasites in patients with falciparum malaria and may confound the current working definition of artemisinin resistance. Immunity may also play an important role in the emergence and transmission potential of artemisinin-resistant parasites.

Longitudinal Pooled Deep Sequencing of the Plasmodium vivax K12 Kelch Gene in Cambodia Reveals a Lack of Selection by Artemisinin

The emergence of artemisinin resistance among Plasmodium falciparum in the Greater Mekong subregion threatens malaria control interventions and is associated with multiple unique mutations in K13 (PF3D7_1343700). The aim of this study was to survey Cambodian Plasmodium vivax for mutations in the K13 ortholog (K12, PVX_083080) that might similarly confer artemisinin resistance. Extracted DNA from Cambodian isolates collected between 2009 and 2012 was pooled by province and year and submitted for next-generation sequencing. Single-nucleotide polymorphisms (SNPs) were identified using a pile-up approach that detected minority SNPs. Among the 14 pools, we found six unique SNPs, including three nonsynonymous SNPs, across six codons in K12 However, none of the SNPs were orthologous to artemisinin resistance-conferring mutations in PF3D7_1343700, and nonsynonymous changes did not persist through time within populations. These results suggest a lack of selection in the P. vivax population in Cambodia due to artemisinin drug pressure.

Endoperoxide Drug Cross-Resistance Patterns for Plasmodium falciparum Exhibiting an Artemisinin Delayed-Clearance Phenotype.

The ring-stage susceptibility assay was modified to quantify the susceptibilities of multiple strains of control and delayed-clearance phenotype (DCP) Plasmodium falciparum strains to seven endoperoxide antimalarial drugs. The susceptibility of all of the DCP lines to six of the drugs was lower than that of the controls. In contrast, DCP parasites did not show reduced susceptibility to the synthetic endoperoxide drug OZ439. These data show that it is possible to circumvent emerging artemisinin resistance with a modified endoperoxide drug.

Selection of N86F184D1246 haplotype of Pfmrd1 gene by artemether-lumefantrine drug pressure on Plasmodium falciparum populations in Senegal.

Background The use of artemisinin as a monotherapy resulted in the emergence of artemisinin resistance in 2005 in Southeast Asia. Monitoring of artemisinin combination therapy (ACT) is critical in order to detect and prevent the spread of resistance in endemic areas. Ex vivo studies and genotyping of molecular markers of resistance can be used as part of this routine monitoring strategy. One gene that has been associated in some ACT partner drug resistance is the Plasmodium falciparum multidrug resistance protein 1 (pfmdr1) gene. The purpose of this study was to assess the drug susceptibility of P. falciparum populations from Thiès, Senegal by ex vivo assay and typing molecular markers of resistance to drug components of ACT currently used for treatment.Methods The ex vivo susceptibility of 170 P. falciparum isolates to chloroquine, amodiaquine, lumefantrine, artesunate, and artemether was determined using the DAPI ex vivo assay. The high resolution melting technique was used to genotype the pfmdr1 gene at codons 86, 184 and 1246.ResultsA significant decrease in IC50 values was observed between 2012 and 2013: from 13.84 to 6.484 for amodiaquine, 173.4 to 113.2 for lumefantrine, and 39.72 to 18.29 for chloroquine, respectively. Increase of the wild haplotype NYD and the decrease of the mutant haplotype NFD (79 and 62.26 %) was also observed. A correlation was observed between the wild type allele Y184 in pfmdr1 and higher IC50 for all drugs, except amodiaquine.ConclusionThis study has shown an increase in sensitivity over the span of two transmission seasons, marked by an increase in the WT alleles at pfmdr1. Continuous the monitoring of the ACT used for treatment of uncomplicated malaria will be helpful.

Plasmodium falciparum parasite population structure and gene flow associated to anti-malarial drugs resistance in Cambodia.

BackgroundWestern Cambodia is recognized as the epicentre of emergence of Plasmodium falciparum multi-drug resistance. The emergence of artemisinin resistance has been observed in this area since 2008–2009 and molecular signatures associated to artemisinin resistance have been characterized in k13 gene. At present, one of the major threats faced, is the possible spread of Asian artemisinin resistant parasites over the world threatening millions of people and jeopardizing malaria elimination programme efforts. To anticipate the diffusion of artemisinin resistance, the identification of the P. falciparum population structure and the gene flow among the parasite population in Cambodia are essential.MethodsTo this end, a mid-throughput PCR-LDR-FMA approach based on LUMINEX technology was developed to screen for genetic barcode in 533 blood samples collected in 2010–2011 from 16 health centres in malaria endemics areas in Cambodia.ResultsBased on successful typing of 282 samples, subpopulations were characterized along the borders of the country. Each 11-loci barcode provides evidence supporting allele distribution gradient related to subpopulations and gene flow. The 11-loci barcode successfully identifies recently emerging parasite subpopulations in western Cambodia that are associated with the C580Y dominant allele for artemisinin resistance in k13 gene. A subpopulation was identified in northern Cambodia that was associated to artemisinin (R539T resistant allele of k13 gene) and mefloquine resistance.ConclusionsThe gene flow between these subpopulations might have driven the spread of artemisinin resistance over Cambodia.Electronic supplementary materialThe online version of this article (doi:10.1186/s12936-016-1370-y) contains supplementary material, which is available to authorized users.

Using pay for performance incentives (P4P) to improve management of suspected malaria fevers in rural Kenya: a cluster randomized controlled trial.

BackgroundInappropriate treatment of non-malaria fevers with artemisinin-based combination therapies (ACTs) is a growing concern, particularly in light of emerging artemisinin resistance, but it is a behavior that has proven difficult to change. Pay for performance (P4P) programs have generated interest as a mechanism to improve health service delivery and accountability in resource-constrained health systems. However, there has been little experimental evidence to establish the effectiveness of P4P in developing countries. We tested a P4P strategy that emphasized parasitological diagnosis and appropriate treatment of suspected malaria, in particular reduction of unnecessary consumption of ACTs.MethodsA random sample of 18 health centers was selected and received a refresher workshop on malaria case management. Pre-intervention baseline data was collected from August to September 2012. Facilities were subsequently randomized to either the comparison (n = 9) or intervention arm (n = 9). Between October 2012 and November 2013, facilities in the intervention arm received quarterly incentive payments based on seven performance indicators. Incentives were for use by facilities rather than as payments to individual providers. All non-pregnant patients older than 1 year of age who presented to a participating facility and received either a malaria test or artemether-lumefantrine (AL) were eligible to be included in the analysis. Our primary outcome was prescription of AL to patients with a negative malaria diagnostic test (n = 11,953). Our secondary outcomes were prescription of AL to patients with laboratory-confirmed malaria (n = 2,993) and prescription of AL to patients without a malaria diagnostic test (analyzed at the cluster level, n = 178 facility-months).ResultsIn the final quarter of the intervention period, the proportion of malaria-negative patients in the intervention arm who received AL was lower than in the comparison arm (7.3 % versus 10.9 %). The improvement from baseline to quarter 4 in the intervention arm was nearly three times that of the comparison arm (ratio of adjusted odds ratios for baseline to quarter 4 = 0.36, 95 % CI: 0.24–0.57). The rate of prescription of AL to patients without a test was five times lower in the intervention arm (adjusted incidence rate ratio = 0.18, 95 % CI: 0.07–0.48). Prescription of AL to patients with confirmed infection was not significantly different between the groups over the study period.ConclusionsFacility-based incentives coupled with training may be more effective than training alone and could complement other quality improvement approaches.Trial registrationThis study was registered with ClinicalTrials.gov (NCT01809873) on 11 March 2013.Electronic supplementary materialThe online version of this article (doi:10.1186/s12916-015-0497-y) contains supplementary material, which is available to authorized users.

Parasite clearance after malaria therapy: staying a step ahead of drug resistance.

The discovery and development of the artemisinin class of antimalarial drugs is one of the great recent success stories of global health. However, after at least two decades of successful use, resistance has finally emerged and appears to be spreading rapidly throughout South-East Asia in spite of our best efforts at containment. If this were also to occur in Africa, it would have disastrous implications for the continent subject to the world’s greatest burden of Plasmodium falciparum. The earliest indications of incipient artemisinin resistance may be a slowing of the rate at which parasites are cleared from the blood following treatment. The Worldwide Antimalarial Resistance Network have analysed data from 29,493 patients from 84 clinical trials in order to define the nature and determinants of early parasite clearance following artemisinin-based treatment in African populations. In doing so, they lay the foundation for systems intended to enable the earliest possible detection of emerging artemisinin resistance in Africa.Please see related article: http://www.biomedcentral.com/1741-7015/13/212

Understanding artemisinin-resistant malaria: what a difference a year makes.

The emergence of artemisinin resistance in Southeast Asia (SEA), where artemisinin combination therapies (ACTs) are beginning to fail, threatens global endeavors to control and eliminate Plasmodium falciparum malaria. Future efforts to prevent the spread of this calamity to Africa will benefit from last year's tremendous progress in understanding artemisinin resistance. Multiple international collaborations have established that artemisinin resistance is associated with slow parasite clearance in patients, increased survival of early-ring-stage parasites in vitro, single-nucleotide polymorphisms (SNPs) in the parasite's kelch protein gene (K13), parasite 'founder' populations sharing a genetic background of four additional SNPs, parasite transcriptional profiles reflecting an 'unfolded protein response' and decelerated parasite development, and elevated parasite phosphatidylinositol-3-kinase activity. In Western Cambodia, where the K13 C580Y mutation is approaching fixation, the frontline ACT is failing to cure nearly half of patients, likely due to partner drug resistance. In Africa, where dozens of K13 mutations have been detected at low frequency, there is no evidence yet of artemisinin resistance. In SEA, clinical and epidemiological investigations are urgently needed to stop the further spread of artemisinin resistance, monitor the efficacy of ACTs where K13 mutations are prevalent, identify currently-available drug regimens that cure ACT failures, and rapidly advance new antimalarial compounds through preclinical studies and clinical trials.

The gametocytocidal efficacy of primaquine in malaria asymptomatic carriers treated with dihydroartemisinin-piperaquine in The Gambia (PRINOGAM): study protocol for a randomised controlled trial.

BackgroundFinding efficacious tools to decrease and interrupt malaria transmission is essential to sustain the gains in malaria control and contain the emergence of artemisinin resistance. Primaquine is effective against Plasmodium falciparum gametocytes and recommended for treatment campaigns in (pre-)elimination settings. Safety concerns preclude its use in endemic African countries with variable proportions of glucose-6-phosphate dehydrogenase (G6PD)-deficient individuals. The efficacy of the current recommended dose needs to be evaluated, particularly in individuals with an asymptomatic malaria infection.Methods/designThis is a four-arm, open label, randomized controlled trial that aims to determine and compare the effect of three different single doses of primaquine combined with dihydroartemisinin-piperaquine, an artemisinin-based combination therapy, on gametocyte carriage in asymptomatic, malaria infected, G6PD-normal individuals. Approximately 1,200 participants are enrolled and followed for 42 days, with the primary endpoint being the prevalence of Plasmodium falciparum gametocyte carriage at day 7 of follow-up determined by quantitative nucleic acid sequence based amplification assay. Direct membrane feeding experiments to determine infectiousness to mosquitoes are conducted as a biological secondary endpoint.DiscussionSub-Saharan Africa, with a relatively high but poorly characterized G6PD prevalence, could potentially benefit from the use of primaquine to further reduce or interrupt malaria transmission. However, G6PD screening may not be feasible given the cost and difficulties in interpreting test results in terms of risk of haemolysis. Because the haemolytic effect of primaquine is dose-dependent, determining the minimal gametocytocidal and transmission-blocking dose of primaquine is extremely important to help address public health concerns over its safety and validate the efficacy of lower than recommended dosages. By including infectiousness to mosquitoes, the trial provides complementary evidence for the potential of the drug to interrupt transmission to mosquitoes.Trial registrationClinicalTrials.gov: NCT01838902 (12 April 2013).

Artemisinin resistance-associated polymorphisms at the K13-propeller locus are absent in Plasmodium falciparum isolates from Haiti.

Antimalarial drugs are a key tool in malaria elimination programs. With the emergence of artemisinin resistance in southeast Asia, an effort to identify molecular markers for surveillance of resistant malaria parasites is underway. Non-synonymous mutations in the kelch propeller domain (K13-propeller) in Plasmodium falciparum have been associated with artemisinin resistance in samples from southeast Asia, but additional studies are needed to characterize this locus in other P. falciparum populations with different levels of artemisinin use. Here, we sequenced the K13-propeller locus in 82 samples from Haiti, where limited government oversight of non-governmental organizations may have resulted in low-level use of artemisinin-based combination therapies. We detected a single-nucleotide polymorphism (SNP) at nucleotide 1,359 in a single isolate. Our results contribute to our understanding of the global genomic diversity of the K13-propeller locus in P. falciparum populations.

Structural polymorphism in the promoter of pfmrp2 confers Plasmodium falciparum tolerance to quinoline drugs

Drug resistance in Plasmodium falciparum remains a challenge for the malaria eradication programmes around the world. With the emergence of artemisinin resistance, the efficacy of the partner drugs in the artemisinin combination therapies (ACT) that include quinoline-based drugs is becoming critical. So far only few resistance markers have been identified from which only two transmembrane transporters namely PfMDR1 (an ATP-binding cassette transporter) and PfCRT (a drug-metabolite transporter) have been experimentally verified. Another P. falciparum transporter, the ATP-binding cassette containing multidrug resistance-associated protein (PfMRP2) represents an additional possible factor of drug resistance in P. falciparum. In this study, we identified a parasite clone that is derived from the 3D7 P. falciparum strain and shows increased resistance to chloroquine, mefloquine and quinine through the trophozoite and schizont stages. We demonstrate that the resistance phenotype is caused by a 4.1 kb deletion in the 5′ upstream region of the pfmrp2 gene that leads to an alteration in the pfmrp2 transcription and thus increased level of PfMRP2 protein. These results also suggest the importance of putative promoter elements in regulation of gene expression during the P. falciparum intra-erythrocytic developmental cycle and the potential of genetic polymorphisms within these regions to underlie drug resistance.

Evidence for Pyronaridine as a Highly Effective Partner Drug for Treatment of Artemisinin-Resistant Malaria in a Rodent Model

The increasing prevalence in Southeast Asia of Plasmodium falciparum infections with delayed parasite clearance rates, following treatment of malaria patients with the artemisinin derivative artesunate, highlights an urgent need to identify which of the currently available artemisinin-based combination therapies (ACTs) are most suitable to treat populations with emerging artemisinin resistance. Here, we demonstrate that the rodent Plasmodium berghei SANA strain has acquired artemisinin resistance following drug pressure, as defined by reduced parasite clearance and early recrudescence following daily exposure to high doses of artesunate or the active metabolite dihydroartemisinin. Using the SANA strain and the parental drug-sensitive N strain, we have interrogated the antimalarial activity of five ACTs, namely, artemether-lumefantrine, artesunate-amodiaquine, artesunate-mefloquine, dihydroartemisinin-piperaquine, and the newest combination artesunate-pyronaridine. By monitoring parasitemia and outcome for 30 days following initiation of treatment, we found that infections with artemisinin-resistant P. berghei SANA parasites can be successfully treated with artesunate-pyronaridine used at doses that are curative for the parental drug-sensitive N strain. No other partner drug combination was as effective in resolving SANA infections. Of the five partner drugs tested, pyronaridine was also the most effective at suppressing the recrudescence of SANA parasites. These data support the potential benefit of implementing ACTs with pyronaridine in regions affected by artemisinin-resistant malaria.

Identification of Biochemically Distinct Properties of the Small Ubiquitin-related Modifier (SUMO) Conjugation Pathway in Plasmodium falciparum

Small ubiquitin-related modifiers (SUMOs) are post-translationally conjugated to other proteins and are thereby essential regulators of a wide range of cellular processes. Sumoylation, and enzymes of the sumoylation pathway, are conserved in the malaria causing parasite, Plasmodium falciparum. However, the specific functions of sumoylation in P. falciparum, and the degree of functional conservation between enzymes of the human and P. falciparum sumoylation pathways, have not been characterized. Here, we demonstrate that sumoylation levels peak during midstages of the intra-erythrocyte developmental cycle, concomitant with hemoglobin consumption and elevated oxidative stress. In vitro studies revealed that P. falciparum E1- and E2-conjugating enzymes interact effectively to recognize and modify RanGAP1, a model mammalian SUMO substrate. However, in heterologous reactions, P. falciparum E1 and E2 enzymes failed to interact with cognate human E2 and E1 partners, respectively, to modify RanGAP1. Structural analysis, binding studies, and functional assays revealed divergent amino acid residues within the E1-E2 binding interface that define organism-specific enzyme interactions. Our studies identify sumoylation as a potentially important regulator of oxidative stress response during the P. falciparum intra-erythrocyte developmental cycle, and define E1 and E2 interactions as a promising target for development of parasite-specific inhibitors of sumoylation and parasite replication.

Severe malaria in Battambang Referral Hospital, an area of multidrug resistance in Western-Cambodia: a retrospective analysis of cases from 2006-2009.

BackgroundDespite recent malaria containment and control efforts leading to reduced incidence, Cambodia remains endemic for both Plasmodium vivax and multidrug-resistant Plasmodium falciparum malaria. Little has been reported in the peer-reviewed literature regarding the burden of severe malaria (SM) in Cambodia.MethodsMedical records for all patients admitted to the Battambang Referral Hospital (BRH) with an admitting or discharge diagnosis of SM from 2006 to 2009 (suspected SM cases) were reviewed. Those meeting the case definition of SM according to retrospective chart review and investigator assessment of probable cases, based on published national guidelines available at the time, were analysed for trends in demographics, mortality and referral patterns.ResultsOf the 537 suspected SM cases at BRH during the study period, 393 (73%) met published WHO criteria for SM infection. Despite limited diagnostic and treatment facilities, overall mortality was 14%, with 7% mortality in children 14 and under, but 19% in adults (60% of cases). Cerebral malaria with coma was relatively rare (17%), but mortality was disproportionately high at 35%. Mean time to hospital presentation was five days (range one to 30 days) after onset of symptoms. While patients with delays in presentation had worse outcomes, there was no excess mortality based on treatment referral times, distance travelled or residence in artemisinin-resistance containment (ARC) Zone 1 compared to Zone 2.ConclusionsDespite limitations in diagnosis and treatment, and multiple confounding co-morbidities, mortality rates at BRH were similar to reports from other countries in the region. Interventions to improve access to early diagnosis and effective treatment, combined with modest improvements in intensive care, are likely to reduce mortality further. Patients referred from Zone 1 did not have excess mortality compared to Zone 2 ARC areas. A steep decrease in SM cases and deaths observed in the first half of 2009 has since continued, indicating some success from containment efforts despite the emergence of artemisinin resistance in this area.

Emerging artemisinin resistance in the border areas of Thailand

Emergence of artemisinin resistance has been confirmed in Cambodia and the border areas of Thailand, the well-known hotspots of multidrug resistance Plasmodium falciparum. It appears to be spreading to the western border of Thailand along the Thai–Myanmar border, and will probably spread to other endemic areas of the world in the near future. This raises a serious concern on the long-term efficacy of artemisinin-based combination therapies, as these combination therapies currently constitute the last effective and most tolerable treatment for multidrug-resistant Plasmodium falciparum. Attempts have been made by a diverse array of stakeholders to prevent the emergence of new foci of artemisinin resistance, as well as to limit the spread of resistance to the original foci. The success in achieving this goal depends on effective integration of containment and surveillance programs with other malaria control measures, with support from both basic and operational research.

Reduced Susceptibility of Plasmodium falciparum to Artesunate in Southern Myanmar

Background Plasmodium falciparum resistance to artemisinins, the first line treatment for malaria worldwide, has been reported in western Cambodia. Resistance is characterized by significantly delayed clearance of parasites following artemisinin treatment. Artemisinin resistance has not previously been reported in Myanmar, which has the highest falciparum malaria burden among Southeast Asian countries.MethodsA non-randomized, single-arm, open-label clinical trial of artesunate monotherapy (4 mg/kg daily for seven days) was conducted in adults with acute blood-smear positive P. falciparum malaria in Kawthaung, southern Myanmar. Parasite density was measured every 12 hours until two consecutive negative smears were obtained. Participants were followed weekly at the study clinic for three additional weeks. Co-primary endpoints included parasite clearance time (the time required for complete clearance of initial parasitemia), parasite clearance half-life (the time required for parasitemia to decrease by 50% based on the linear portion of the parasite clearance slope), and detectable parasitemia 72 hours after commencement of artesunate treatment. Drug pharmacokinetics were measured to rule out delayed clearance due to suboptimal drug levels.ResultsThe median (range) parasite clearance half-life and time were 4.8 (2.1–9.7) and 60 (24–96) hours, respectively. The frequency distributions of parasite clearance half-life and time were bimodal, with very slow parasite clearance characteristic of the slowest-clearing Cambodian parasites (half-life longer than 6.2 hours) in approximately 1/3 of infections. Fourteen of 52 participants (26.9%) had a measurable parasitemia 72 hours after initiating artesunate treatment. Parasite clearance was not associated with drug pharmacokinetics.ConclusionsA subset of P. falciparum infections in southern Myanmar displayed markedly delayed clearance following artemisinin treatment, suggesting either emergence of artemisinin resistance in southern Myanmar or spread to this location from its site of origin in western Cambodia. Resistance containment efforts are underway in Myanmar.Trial RegistrationAustralian New Zealand Clinical Trials Registry ACTRN12610000896077

Efficacy of Dihydroartemisinin-Piperaquine for Treatment of Uncomplicated Plasmodium falciparum and Plasmodium vivax in Cambodia, 2008 to 2010

We describe here the results of antimalarial therapeutic efficacy studies conducted in Cambodia from 2008 to 2010. A total of 15 studies in four sentinel sites were conducted using dihydroartemisinin-piperaquine (DP) for the treatment of Plasmodium falciparum infection and chloroquine (CQ) and DP for the treatment of P. vivax infection. All studies were performed according to the standard World Health Organization protocol for the assessment of antimalarial treatment efficacy. Among the studies of DP for the treatment of P. falciparum, an increase in treatment failure was observed in the western provinces. In 2010, the PCR-corrected treatment failure rates for DP on day 42 were 25% (95% confidence interval [CI] = 10 to 51%) in Pailin and 10.7% (95% CI = 4 to 23%) in Pursat, while the therapeutic efficacy of DP remained high (100%) in Ratanakiri and Preah Vihear provinces, located in northern and eastern Cambodia. For the studies of P. vivax, the day 28 uncorrected treatment failure rate among patients treated with CQ ranged from 4.4 to 17.4%; DP remained 100% effective in all sites. Further study is required to investigate suspected P. falciparum resistance to piperaquine in western Cambodia; the results of in vitro and molecular studies were not found to support the therapeutic efficacy findings. The emergence of artemisinin resistance in this region has likely put additional pressure on piperaquine. Although DP appears to be an appropriate new first-line treatment for P. vivax in Cambodia, alternative treatments are urgently needed for P. falciparum-infected patients in western Cambodia.

Mitigating the threat of artemisinin resistance in Africa: improvement of drug-resistance surveillance and response systems

Artemisinin-resistant Plasmodium falciparum malaria has emerged in western Cambodia and has been detected in western Thailand. The situation is ominously reminiscent of the emergence of resistance to chloroquine and to sulfadoxine-pyrimethamine several decades ago. Artemisinin resistance is a major threat to global public health, with the most severe potential effects in sub-Saharan Africa, where the disease burden is highest and systems for monitoring and containment of resistance are inadequate. The mechanisms that underlie artemisinin resistance are not fully understood. The main phenotypic trait associated with resistance is a substantial delay in parasite clearance, so far reported in southeast Asia but not in Africa. One of the pillars of the WHO global plan for artemisinin resistance containment is to increase monitoring and surveillance. In this Personal View, we propose strategies that should be adopted by malaria-endemic countries in Africa: resource mobilisation to reactivate regional surveillance networks, establishment of baseline parasite clearance profiles to serve as benchmarks to track emerging artemisinin resistance, improved data sharing to allow pooled analyses to identify rare events, modelling of risk factors for drug resistance, and development and validation of new approaches to monitor resistance.