Spread of artemisinin resistance in Plasmodium falciparum malaria.

BACKGROUND: Artemisinin resistance in Plasmodium falciparum has emerged in Southeast Asia and now poses a threat to the control and elimination of malaria. Mapping the geographic extent of resistance is essential for planning containment and elimination strategies. METHODS: Between May 2011 and April 2013, we enrolled 1241 adults and children with acute, uncomplicated falciparum malaria in an open-label trial at 15 sites in 10 countries (7 in Asia and 3 in Africa). Patients received artesunate, administered orally at a daily dose of either 2 mg per kilogram of body weight per day or 4 mg per kilogram, for 3 days, followed by a standard 3-day course of artemisinin-based combination therapy. Parasite counts in peripheral-blood samples were measured every 6 hours, and the parasite clearance half-lives were determined. RESULTS: The median parasite clearance half-lives ranged from 1.9 hours in the Democratic Republic of Congo to 7.0 hours at the Thailand-Cambodia border. Slowly clearing infections (parasite clearance half-life >5 hours), strongly associated with single point mutations in the "propeller" region of the P. falciparum kelch protein gene on chromosome 13 (kelch13), were detected throughout mainland Southeast Asia from southern Vietnam to central Myanmar. The incidence of pretreatment and post-treatment gametocytemia was higher among patients with slow parasite clearance, suggesting greater potential for transmission. In western Cambodia, where artemisinin-based combination therapies are failing, the 6-day course of antimalarial therapy was associated with a cure rate of 97.7% (95% confidence interval, 90.9 to 99.4) at 42 days. CONCLUSIONS: Artemisinin resistance to P. falciparum, which is now prevalent across mainland Southeast Asia, is associated with mutations in kelch13. Prolonged courses of artemisinin-based combination therapies are currently efficacious in areas where standard 3-day treatments are failing. (Funded by the U.K. Department of International Development and others; ClinicalTrials.gov number, NCT01350856.).

Laboratory detection of artemisinin-resistant Plasmodium falciparum.

Conventional 48-h in vitro susceptibility tests have low sensitivity in identifying artemisinin-resistant Plasmodium falciparum, defined phenotypically by low in vivo parasite clearance rates. We hypothesized originally that this discrepancy was explained by a loss of ring-stage susceptibility and so developed a simple field-adapted 24-h trophozoite maturation inhibition (TMI) assay focusing on the ring stage and compared it to the standard 48-h schizont maturation inhibition (WHO) test. In Pailin, western Cambodia, where artemisinin-resistant P. falciparum is prevalent, the TMI test mean (95% confidence interval) 50% inhibitory concentration (IC50) for artesunate was 6.8 (5.2 to 8.3) ng/ml compared with 1.5 (1.2 to 1.8) ng/ml for the standard 48-h WHO test (P = 0.001). TMI IC50s correlated significantly with the in vivo responses to artesunate (parasite clearance time [r = 0.44, P = 0.001] and parasite clearance half-life [r = 0.46, P = 0.001]), whereas the standard 48-h test values did not. On continuous culture of two resistant isolates, the artemisinin-resistant phenotype was lost after 6 weeks (IC50s fell from 10 and 12 ng/ml to 2.7 and 3 ng/ml, respectively). Slow parasite clearance in falciparum malaria in western Cambodia results from reduced ring-stage susceptibility.

Effect of high-dose or split-dose artesunate on parasite clearance in artemisinin-resistant falciparum malaria.

BACKGROUND: The emergence of Plasmodium falciparum resistance to artemisinins on the Cambodian and Myanmar-Thai borders poses severe threats to malaria control. We investigated whether increasing or splitting the dose of the short-half-life drug artesunate improves parasite clearance in falciparum malaria in the 2 regions. METHODS: In Pailin, western Cambodia (from 2008 to 2010), and Wang Pha, northwestern Thailand (2009-2010), patients with uncomplicated falciparum malaria were randomized to oral artesunate 6 mg/kg/d as a once-daily or twice-daily dose for 7 days, or artesunate 8 mg/kg/d as a once-daily or twice-daily dose for 3 days, followed by mefloquine. Parasite clearance and recrudescence for up to 63 days of follow-up were assessed. RESULTS: A total of 159 patients were enrolled. Overall median (interquartile range [IQR]) parasitemia half-life (half-life) was 6.03 (4.89-7.28) hours in Pailin versus 3.42 (2.20-4.85) hours in Wang Pha (P = .0001). Splitting or increasing the artesunate dose did not shorten half-life in either site. Pharmacokinetic profiles of artesunate and dihydroartemisinin were similar between sites and did not correlate with half-life. Recrudescent infections occurred in 4 of 79 patients in Pailin and 5 of 80 in Wang Pha and was not different between treatment arms (P = .68). CONCLUSIONS: Increasing the artesunate treatment dose up to 8 mg/kg/d or splitting the dose does not improve parasite clearance in either artemisinin resistant or more sensitive infections with P. falciparum. Clinical Trials Registration. ISRCTN15351875.

A major genome region underlying artemisinin resistance in malaria.

Evolving resistance to artemisinin-based compounds threatens to derail attempts to control malaria. Resistance has been confirmed in western Cambodia and has recently emerged in western Thailand, but is absent from neighboring Laos. Artemisinin resistance results in reduced parasite clearance rates (CRs) after treatment. We used a two-phase strategy to identify genome region(s) underlying this ongoing selective event. Geographical differentiation and haplotype structure at 6969 polymorphic single-nucleotide polymorphisms (SNPs) in 91 parasites from Cambodia, Thailand, and Laos identified 33 genome regions under strong selection. We screened SNPs and microsatellites within these regions in 715 parasites from Thailand, identifying a selective sweep on chromosome 13 that shows strong association (P = 10(-6) to 10(-12)) with slow CRs, illustrating the efficacy of targeted association for identifying the genetic basis of adaptive traits.

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.

Exploring the contribution of candidate genes to artemisinin resistance in Plasmodium falciparum.

The reduced in vivo sensitivity of Plasmodium falciparum has recently been confirmed in western Cambodia. Identifying molecular markers for artemisinin resistance is essential for monitoring the spread of the resistant phenotype and identifying the mechanisms of resistance. Four candidate genes, including the P. falciparum mdr1 (pfmdr1) gene, the P. falciparum ATPase6 (pfATPase6) gene, the 6-kb mitochondrial genome, and ubp-1, encoding a deubiquitinating enzyme, of artemisinin-resistant P. falciparum strains from western Cambodia were examined and compared to those of sensitive strains from northwestern Thailand, where the artemisinins are still very effective. The artemisinin-resistant phenotype did not correlate with pfmdr1 amplification or mutations (full-length sequencing), mutations in pfATPase6 (full-length sequencing) or the 6-kb mitochondrial genome (full-length sequencing), or ubp-1 mutations at positions 739 and 770. The P. falciparum CRT K76T mutation was present in all isolates from both study sites. The pfmdr1 copy numbers in western Cambodia were significantly lower in parasite samples obtained in 2007 than in those obtained in 2005, coinciding with a local change in drug policy replacing artesunate-mefloquine with dihydroartemisinin-piperaquine. Artemisinin resistance in western Cambodia is not linked to candidate genes, as was suggested by earlier studies.

High heritability of malaria parasite clearance rate indicates a genetic basis for artemisinin resistance in western Cambodia.

In western Cambodia, malaria parasites clear slowly from the blood after treatment with artemisinin derivatives, but it is unclear whether this results from parasite, host, or other factors specific to this population. We measured heritability of clearance rate by evaluating patients infected with identical or nonidentical parasite genotypes, using methods analogous to human twin studies. A substantial proportion (56%-58%) of the variation in clearance rate is explained by parasite genetics. This has 2 important implications: (1) selection with artemisinin derivatives will tend to drive resistance spread and (2) because heritability is high, the genes underlying parasite clearance rate may be identified by genome-wide association.

Decreased in vitro susceptibility of Plasmodium falciparum isolates to artesunate, mefloquine, chloroquine, and quinine in Cambodia from 2001 to 2007.

This study describes the results of in vitro antimalarial susceptibility assays and molecular polymorphisms of Plasmodium falciparum isolates from Cambodia. The samples were collected from patients enrolled in therapeutic efficacy studies (TES) conducted by the Cambodian National Malaria Control Program for the routine efficacy monitoring of artemisinin-based combination therapy (ACT) (artesunate-mefloquine and artemether-lumefantrine combinations). The isolates (n = 2,041) were obtained from nine sentinel sites during the years 2001 to 2007. Among these, 1,588 were examined for their in vitro susceptibilities to four antimalarials (artesunate, mefloquine, chloroquine, and quinine), and 851 isolates were genotyped for single nucleotide polymorphisms (SNPs). The geometric means of the 50% inhibitory concentrations (GMIC(50)s) of the four drugs tested were significantly higher for isolates from western Cambodia than for those from eastern Cambodia. GMIC(50)s for isolates from participants who failed artesunate-mefloquine therapy were significantly higher than those for patients who were cured (P, <0.001). In vitro correlation of artesunate with the other drugs was observed. The distributions of the SNPs differed between eastern and western Cambodia, suggesting different genetic backgrounds of the parasite populations in these two parts of the country. The GMIC(50)s of the four drugs tested increased significantly in eastern Cambodia during 2006 to 2007. These results are worrisome, because they may signal deterioration of the efficacy of artesunate-mefloquine beyond the Cambodian-Thai border.

Artemisinin resistance in Plasmodium falciparum malaria.

BACKGROUND: Artemisinin-based combination therapies are the recommended first-line treatments of falciparum malaria in all countries with endemic disease. There are recent concerns that the efficacy of such therapies has declined on the Thai-Cambodian border, historically a site of emerging antimalarial-drug resistance. METHODS: In two open-label, randomized trials, we compared the efficacies of two treatments for uncomplicated falciparum malaria in Pailin, western Cambodia, and Wang Pha, northwestern Thailand: oral artesunate given at a dose of 2 mg per kilogram of body weight per day, for 7 days, and artesunate given at a dose of 4 mg per kilogram per day, for 3 days, followed by mefloquine at two doses totaling 25 mg per kilogram. We assessed in vitro and in vivo Plasmodium falciparum susceptibility, artesunate pharmacokinetics, and molecular markers of resistance. RESULTS: We studied 40 patients in each of the two locations. The overall median parasite clearance times were 84 hours (interquartile range, 60 to 96) in Pailin and 48 hours (interquartile range, 36 to 66) in Wang Pha (P<0.001). Recrudescence confirmed by means of polymerase-chain-reaction assay occurred in 6 of 20 patients (30%) receiving artesunate monotherapy and 1 of 20 (5%) receiving artesunate-mefloquine therapy in Pailin, as compared with 2 of 20 (10%) and 1 of 20 (5%), respectively, in Wang Pha (P=0.31). These markedly different parasitologic responses were not explained by differences in age, artesunate or dihydroartemisinin pharmacokinetics, results of isotopic in vitro sensitivity tests, or putative molecular correlates of P. falciparum drug resistance (mutations or amplifications of the gene encoding a multidrug resistance protein [PfMDR1] or mutations in the gene encoding sarco-endoplasmic reticulum calcium ATPase6 [PfSERCA]). Adverse events were mild and did not differ significantly between the two treatment groups. CONCLUSIONS: P. falciparum has reduced in vivo susceptibility to artesunate in western Cambodia as compared with northwestern Thailand. Resistance is characterized by slow parasite clearance in vivo without corresponding reductions on conventional in vitro susceptibility testing. Containment measures are urgently needed. (ClinicalTrials.gov number, NCT00493363, and Current Controlled Trials number, ISRCTN64835265.)

Pfmdr1 copy number and arteminisin derivatives combination therapy failure in falciparum malaria in Cambodia.

BACKGROUND: The combination of artesunate and mefloquine was introduced as the national first-line treatment for Plasmodium falciparum malaria in Cambodia in 2000. However, recent clinical trials performed at the Thai-Cambodian border have pointed to the declining efficacy of both artesunate-mefloquine and artemether-lumefantrine. Since pfmdr1 modulates susceptibility to mefloquine and artemisinin derivatives, the aim of this study was to assess the link between pfmdr1 copy number, in vitro susceptibility to individual drugs and treatment failure to combination therapy. METHODS: Blood samples were collected from P. falciparum-infected patients enrolled in two in vivo efficacy studies in north-western Cambodia: 135 patients were treated with artemether-lumefantrine (AL group) in Sampovloun in 2002 and 2003, and 140 patients with artesunate-mefloquine (AM group) in Sampovloun and Veal Veng in 2003 and 2004. At enrollment, the in vitro IC50 was tested and the strains were genotyped for pfmdr1 copy number by real-time PCR. RESULTS: The pfmdr1 copy number was analysed for 115 isolates in the AM group, and for 109 isolates in the AL group. Parasites with increased pfmdr1 copy number had significantly reduced in vitro susceptibility to mefloquine, lumefantrine and artesunate. There was no association between pfmdr1 polymorphisms and in vitro susceptibilities. In the patients treated with AM, the mean pfmdr1copy number was lower in subjects with adequate clinical and parasitological response compared to those who experienced late treatment failure (n = 112, p < 0.001). This was not observed in the patients treated with AL (n = 96, p = 0.364). The presence of three or more copies of pfmdr1 were associated with recrudescence in artesunate-mefloquine treated patients (hazard ratio (HR) = 7.80 [95%CI: 2.09-29.10], N = 115), p = 0.002) but not with recrudescence in artemether-lumefantrine treated patients (HR = 1.03 [95%CI: 0.24-4.44], N = 109, p = 0.969). CONCLUSION: This study shows that pfmdr1 copy number is a molecular marker of AM treatment failure in falciparum malaria on the Thai-Cambodian border. However, while it is associated with increased IC50 for lumefantrine, pfmdr1 copy number is not associated with AL treatment failure in the area, suggesting involvement of other molecular mechanisms in AL treatment failures in Cambodia.

Pfmdr1 and in vivo resistance to artesunate-mefloquine in falciparum malaria on the Cambodian-Thai border.

Artemisinin combination therapies (ACTs) have recently been adopted as first-line therapy for Plasmodium falciparum infections in most malaria-endemic countries. In this study, we estimated the association between artesunate-mefloquine therapy failure and genetic changes in the putative transporter, pfmdr1. Blood samples were acquired from 80 patients enrolled in an 2004 in vivo efficacy study in Pailin, Cambodia, and genotyped for pfmdr1 copy number and haplotype. Having parasites with three or more copies of pfmdr1 before treatment was strongly associated with recrudescence (hazard ratio [HR] = 8.30; 95% CI: 2.60-26.43). This relationship was maintained when controlling for initial parasite density and hematocrit (HR = 7.91; 95% CI: 2.38-26.29). Artesunate-mefloquine treatment selected for increased pfmdr1 copy number, because isolates from recurrent episodes had higher copy numbers than the paired enrollment samples (Wilcoxon rank test, P = 0.040). pfmdr1 copy number should be evaluated further as a surveillance tool for artesunate-mefloquine resistance in Cambodia.

Efficacy of artemether-lumefantrine for the treatment of uncomplicated falciparum malaria in northwest Cambodia.

OBJECTIVE: To determine the efficacy of artemether-lumefantrine malaria treatment, as an alternative to artesunate + mefloquine, which is becoming ineffective in some areas of the Thai-Cambodian border. METHODS: Two studies were conducted to monitor the efficacy of artemether-lumefantrine in Sampov Lun referral hospital, Battambang Province, in 2002 and 2003, and one study was conducted to assess the efficacy of mefloquine + artesunate in 2003 for comparison. The studies were performed according to the WHO standardized protocol with a follow-up of 28 days. The therapeutic efficacy tests were complemented with in vitro tests and in 2003, with the measurement of lumefantrine plasma concentration at day 7 for the patients treated with artemether-lumefantrine. RESULTS: A total of 190 patients were included: 55 were treated with artemether-lumefantrine in 2002 (AL2002), 80 with artemether-lumefantrine and food supplementation in 2003 (AL2003) and 55 with artesunate + mefloquine in 2003 (AM2003). With the per-protocol analysis, the cure rate was 71.1% in study AL2002, 86.5% in study AL2003 and 92.4% in study AM2003. All the data were PCR corrected. The artemether-lumefantrine cure rate was unexpectedly low in 2002, but it increased with food supplementation in 2003. There was a significant difference (P = 0.02) in lumefantrine plasma concentrations between adequate clinical and parasitological responses and treatment failure cases. In vitro susceptibility to lumefantrine was reduced for isolates sampled from patients presenting with treatment failure, but the difference was not statistically different from isolates sampled from patients who were successfully treated. CONCLUSION: Treatment failure cases of artemether-lumefantrine are most probably because of low levels of lumefantrine blood concentration. Further investigations are necessary to determine whether resistance of Plasmodium falciparum isolates to lumefantrine is present in the region.

Genetic variation of the dihydrofolate reductase gene in Plasmodium vivax in Snoul, northeastern Cambodia.

In Plasmodium vivax, pyrimethamine resistance is associated with amino acid substitutions Ser117Asn and Ser58Arg in dihydrofolate reductase (DHFR), which correspond to Ser108Asn and Cys59Arg in the Plasmodium falciparum homolog, respectively. Sequence variations within the DHFR domain of 32 P. vivax isolates from Snoul, Cambodia, were analyzed by direct sequencing of polymerase chain reaction (PCR) products. Sequence polymorphisms within the entire DHFR domain were limited to codons 58 and 117 and GGDN tandem repeat units. A large majority (30 of 32) of isolates were characterized to be double mutants (Arg-58 and Asn-117) and associated with the presence of two GGDN repeat units. Only one isolate was of wild-type with three GGDN repeat units, and an additional isolate was of mixed type. Our data suggest that most Cambodian P. vivax isolates display double dhfr mutations associated with pyrimethamine resistance, as in the neighboring countries in Southeast Asia. Further molecular characterization of P. vivax isolates from different endemic areas may be a useful alternative approach to establish the epidemiology of drug-resistant malaria.