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Antimicrob Agents Chemother. Mar 2009; 53(3): 888–895.
Published online Dec 15, 2008. doi:  10.1128/AAC.00968-08
PMCID: PMC2650543

Selection of Plasmodium falciparum Multidrug Resistance Gene 1 Alleles in Asexual Stages and Gametocytes by Artemether-Lumefantrine in Nigerian Children with Uncomplicated Falciparum Malaria [down-pointing small open triangle]

Abstract

We assessed Plasmodium falciparum mdr1 (Pfmdr1) gene polymorphisms and copy numbers as well as P. falciparum Ca2+ ATPase (PfATPase6) gene polymorphisms in 90 Nigerian children presenting with uncomplicated falciparum malaria and enrolled in a study of the efficacy of artemether-lumefantrine (AL). The nested PCR-restriction fragment length polymorphism and the quantitative real-time PCR methodologies were used to determine the alleles of the Pfmdr1 and PfATPase6 genes and the Pfmdr1 copy number variation, respectively, in patients samples collected prior to treatment and at the reoccurrence of parasites during a 42-day follow-up. The Pfmdr1 haplotype 86N-184F-1246D was significantly associated (P < 0.00001) with treatment failures and was selected for among posttreatment samples obtained from patients with newly acquired or recrudescing infections (P < 0.00001; χ2 = 36.5) and in gametocytes (log rank statistic = 5; P = 0.0253) after treatment with AL. All pre- and posttreatment samples as well as gametocytes harbored a single copy of the Pfmdr1 gene and the wild-type allele (L89) at codon 89 of the PfATPase6 gene. These findings suggest that polymorphisms in the Pfmdr1 gene are under AL selection pressure. Pfmdr1 polymorphisms may result in reduction in the therapeutic efficacy of this newly adopted combination treatment for uncomplicated falciparum malaria in Saharan countries of Africa.

The emergence and spread of parasites resistant to antimalarial drugs continue to be major public health problems in the management of Plasmodium falciparum infections, especially in many countries in Africa where malaria is endemic. The poor clinical efficacies of the older drugs (chloroquine or sulfadoxine-pyrimethamine) due to drug resistance led the World Health Organization (WHO) to recommend the introduction of combination therapy, notably, artemisinin-based combination therapies (ACTs), for the treatment of uncomplicated malaria. Some direct benefits of ACTs include the rapid parasitological cure rates, the potential inhibition of the development of resistance, reductions in gametocyte carriage and malaria transmission rates, and an overall reduction in malaria morbidity and mortality (12, 23, 34-36). Most countries in sub-Saharan Africa where malaria is endemic have now adopted either artemether-lumefantrine (AL) or artesunate-amodiaquine as their first-line ACTs. However, it is not clear whether ACTs will be successful in preventing the selection of resistant parasites in Africa, where Plasmodium falciparum transmission rates and the risk of new infections soon after treatment are generally much higher than they are in Southeast Asia. Recent reports from East Africa (6, 19, 31) and West Africa (39) show some evidence of clinical and parasitological failure after treatment with AL. Thus, the success of treatment with ACTs may largely depend on the parasite's existing level of tolerance to the partner drugs. There is significant interest in describing genetic mutations that are associated with resistance to both the artemisinin derivatives (ARTs) and their partner drugs used in ACTs. Plasmodium falciparum multidrug resistance gene 1 (Pfmdr1) on chromosome 5 encodes a putative ATP-binding cassette transporter similar to the mdr genes that mediate multidrug resistance in mammalian cell lines and the yeast Candida albicans (7). Nonsynonymous point mutations at various positions on the Pfmdr1 gene (N86Y, Y184F, S1034C, N1042D, and D1246Y) or variations in Pfmdr1 copy number have been shown to confer either true resistant phenotypes or increased sensitivity in vitro to several structurally unrelated antimalarial drugs, including chloroquine, quinine, mefloquine, halofantrine, lumefantrine, and ARTs (9, 26-28, 30, 37, 38). In addition, Jambou and colleagues (20) recently described an association between reduced susceptibility to artemether in vitro and point mutations in the P. falciparum Ca2+ ATPase (PfATPase6) gene, although these findings have yet to be confirmed in most areas where malaria is endemic. Thus, molecular markers of drug resistance are potentially useful tools for monitoring of the efficacy, development, and spread of parasites resistant to ACTs in Africa.

Four clinical trials performed in Africa (6, 19, 31, 39) have provided evidence for the selection of particular Pfmdr1 alleles in patients with newly acquired infections or recurrent P. falciparum infections within 28 or 42 days after AL treatment. We present in this report the results of genetic analyses of samples from a previously reported clinical trial on the efficacy of AL (33) in Ibadan in southwest Nigeria. We used molecular tools to demonstrate how AL selects for the Pfmdr1 N86-F184-D1246 haplotype in gametocytes, as well as affects recrudescing and newly acquired infections. In addition, we show a lack of an increase in the Pfmdr1 copy number and the total absence of mutations at codon 89 of the PfATPase6 gene in isolates of Plasmodium falciparum obtained from Nigerian children.

MATERIALS AND METHODS

Study area.

The study was carried out at the Malaria Research Laboratory, College of Medicine, University of Ibadan, Ibadan, Nigeria, in 2006 and 2007. Malaria is hyperendemic in Ibadan, and transmission occurs all year round but is more intense from April to October, during the rainy season.

The study protocol was approved by the Joint University of Ibadan and University College Hospital Institutional Review Committee and the Harvard School of Public Health Human Subject Committee. Documented informed consent was obtained from the parents or guardians of younger children, while assent was obtained from children ages 10 years and older.

Patients, treatment, and follow-up sample collection.

Patient enrollment, treatment, follow-up, and blood sample collection for molecular analysis have been described elsewhere (33). Briefly, patients were eligible to participate in the study if they were aged 10 years or younger, had symptoms compatible with acute uncomplicated malaria and levels of pure P. falciparum parasitemia of >2,000 asexual forms/μl, a body (axillary) temperature of >37.4°C or a history of fever in the 24 to 48 h preceding presentation, the absence of other concomitant illnesses, no history of antimalarial use in the 2 weeks preceding presentation, and negative urine tests for antimalarial drugs (the Dill-Glazko eosin color and lignin tests). Patients with severe malaria, severe malnutrition, serious underlying diseases (renal, cardiac, or hepatic diseases), and known allergies to the study drugs were excluded from the study.

AL (Coartem) was given orally and according to body weight: patients weighing 5 to 14 kg received one tablet, those weighing 15 to 24 kg received two tablets, those weighing 25 to 34 kg received three tablets, and those weighing >34 kg received four tablets at presentation (0 h) and 8 h later and then at 24, 36, 48, and 60 h after administration of the first dose. Each tablet of AL contains 20 mg of artemether and 120 mg of lumefantrine. Follow-up with clinical and parasitological evaluations was done daily on days 1 to 3 and then on days 7, 14, 21, 28, 35, and 42. Blood was spotted on filter papers on days 0, 3, 7, 14, 21, 28, 35, and 42 and at the time of treatment failures for parasite genotyping.

Patients were retreated whenever they became symptomatic (usually between 18 and 35 days after their initial enrollment). Cure rates were defined as the percentages of patients whose asexual-stage parasitemia cleared from peripheral blood and who were free of patent asexual-stage parasitemia on days 14, 21, 28, 35, and 42 of follow-up. The cure rates on days 21 to 42 were adjusted on the basis of the PCR genotyping results for paired samples from patients with recurrent parasitemia.

Molecular analysis of patients' samples. (i) Determination of P. falciparum population structure in infected patients.

The Plasmodium falciparum population structure was characterized on the basis of the msp-2 (merozoite surface protein 2) polymorphism determined for parasites in all pre- and posttreatment samples from the patients, as described previously (13-16), in order to distinguish true treatment failures from newly acquired infections (reinfections). We chose msp-2 as the only marker because previous reports (13-17) from the same study site have demonstrated that msp-2 is the best and most reliable marker, in comparison to msp-1 or glurp individually or combined, for use in the evaluation of parasite population diversity and the complexity of P. falciparum infections. Briefly, paired parasites from the primary and posttreatment samples were analyzed by the evaluation of parasite loci that exhibit repeated numbers of polymorphisms to distinguish true treatment failures from new infections. Briefly, block 3 of msp-2 was amplified by two rounds of PCR with the primers and amplification conditions described previously (13-17). Ten microliters of the nested PCR products was resolved by electrophoresis on a 2% agarose gel and sized by comparison with a 100-bp molecular size marker (New England Biolabs, Beverly, MA). The banding patterns of the posttreatment parasites were compared with those of the matched primary parasites for each of the patients who had parasitemia after treatment with AL. The treatment was considered a true treatment failure when the posttreatment and primary parasites showed identical bands, while the infection was considered newly acquired if the parasites had nonidentical banding patterns. The complexity of infection was calculated as the average number of distinct fragments of msp-2 per PCR-positive sample. Infections were defined as polyclonal if the parasites in the matched primary and posttreatment samples from the same patient showed more than one allele of msp-2. If an isolate had one allele from each of the three loci, the clone number was taken to be one.

(ii) Molecular identification of gametocytes.

Reverse transcriptase PCR (RT-PCR) and conventional PCR approaches that used primers Pfs25-1, Pfs25-2, Pfs25-3, and Pfs25-4 (Table (Table1)1) and that were performed by the method described previously (1) were used to detect the mRNA of the P. falciparum s25 (Pfs25) gene in patient isolates of Plasmodium falciparum.

TABLE 1.
Primer sequences and thermocycling conditions used for the amplification reactions

(iii) Detection of Pfmdr1 and PfATPase6 alleles.

All pre- and posttreatment samples obtained from patients were analyzed for the detection of mutations in the Pfmdr1 and PfATPase6 genes. Pfmdr1 mutations (N86Y, Y184F, S1034C, N1042D, and D1246Y) were detected by the nested PCR and restriction fragment length polymorphism (RFLP) methodologies, as described previously (4, 8, 14, 17).

A nested PCR and RFLP methodology was developed in order to identify wild-type and mutant alleles at codon 89 of the PfATPase6 gene of the P. falciparum isolates. Primers (Table (Table1)1) in which the XbaI (New England Biosciences) restriction site was engineered were designed to enable the detection of both alleles in field isolates of P. falciparum. The cycling conditions for both the primary and the nested PCRs are shown in Table Table1.1. The secondary amplification product (210 bp) was digested with XbaI, according to the manufacturer's protocol. The enzyme cuts the mutant allele, while the wild type remains uncut.

(iv) Assessment of Pfmdr1 gene copy number variations.

The Pfmdr1 (PFE1150w) copy number in pre- and posttreatment isolates obtained from patients in whom infections reoccurred (either recrudescent infections or reinfections, as determined by msp-2 genotyping) was assessed by real-time PCR amplification with the 7300 real-time PCR system (Applied Biosystems) in the presence of SYBR green (21). Oligonucleotides (Table (Table1)1) were designed to specifically amplify the Pfmdr1 gene and the β-tubulin gene (used as an internal control). Parallel amplification reactions were performed in 96-well plates, with each well containing a final reaction volume of 25 μl (0.5 μM each of the forward and the reverse primers, 5 μl of template DNA from filter paper samples, and 12.5 μl of iQSYBR green PCR master mix [50 mM KCl, 20 mM Tris-HCl, 0.2 mM each deoxynucleoside triphosphate, 0.6 U iTaq DNA polymerase, 3 mM MgCl2, SYBR green I, 10 nM fluorescein]) (Applied Biosystems). Every assay contained reference DNA samples from clones 3D7 (which has one copy of the Pfmdr1 gene) and W2mef (which has two copies of the Pfmdr1 gene). All reactions were performed in triplicate, and the results were rejected if they did not conform to exponential kinetics. At the end of each reaction, the cycle threshold was manually set up at a level that reflected the best kinetic PCR parameters, and melting curves were acquired and analyzed. The assays were repeated, especially if a change in the standard error of the cycle threshold was >0.3 or the copy number fell between 1.3 and 1.6. A copy number of <1.5 was considered a single copy, and a copy number of ≥1.5 was considered multiple copies (10, 26).

Data analysis and statistics.

For the best assessment of genetically determined parasite phenotypes, only isolates from pre- and posttreatment samples from patients with treatment failures with identical msp-2 banding patterns were defined as AL resistant. For analysis purposes, each isolate was coded on the basis of the presence or the absence of resistance-associated alleles (N86, F86, or 1246D, were N is asparagine, F is phenylalanine, and D is aspartate). Genotyping data at all three Pfmdr1 loci were combined to determine whether the Pfmdr1 86N-184F-1246D (Pfmdr1 N-F-D) haplotype was present in pre- and posttreatment samples as well as in gametocytes. For instance, if a sample contained one mixed allele (e.g., if Pfmdr1 86Y and 86N, 184F, and 1246D were detected), we assumed that the haplotype of interest, the N-F-D haplotype, must have occurred in one parasite genome within this sample, and thus, the N-F-D haplotype was considered for analysis. If two or all three alleles were mixed, we considered them to be the N-F-D haplotype as well, although it should be noted that there is a possibility that the Y-Y-Y (where Y is tyrosine) haplotype did actually occur within a single genome in these samples.

Data were analyzed by using the SPSS statistical programs for Windows (version 10.01), GraphPad Prism software (version 4.0) for Windows (GraphPad Software, San Diego, CA), and the Epi-Info program (version 6.4). Proportions were compared by calculating χ2 with Yates' correction or Fisher's exact tests. Normally distributed, continuous data were compared by Student's t test and analysis of variance. The Wilcoxon signed-rank test was used to compare the frequencies of Pfmdr1 alleles or haplotypes in the pre- and posttreatment samples from patients with true recrudescences or reinfections. Paired and unpaired samples were compared by using t tests. P values of <0.05 (two tailed) were taken to indicate significant differences.

RESULTS

Clinical and parasitological responses.

The demographic parameters and the clinical and parasitological responses of the patients involved in this study have been reported elsewhere (33). Briefly, the previous study reported a PCR-uncorrected parasitological failure rate of 9% (i.e., 8/90 patients), while 4 of 90 patients (4.4%) presented with fever without microscopic detection of parasitemia at the end of the 42 days of follow-up. During the present study, msp-2 genotyping analysis of samples collected at enrollment and throughout the follow-up period showed that a total of 16 of 90 patients (18%) had parasites. However, only 6 of 90 patients (7%) had genuine treatment failures, while the remaining 84 (93%) patients (including 10 patients classified as having reinfections after correction of the results by PCR analysis) were cured by AL.

Determination of clonal profiles of Plasmodium falciparum isolates from patients with malaria.

The msp-2 locus in matched sample pairs collected before and after treatment from all 16 patients in whom infections reoccurred after treatment with AL was successfully analyzed. The presence of different allelic families of msp-2 was often found in parasite DNA derived from a single patient, indicating a polyclonal infection. Ten of 16 (62.5%) paired PCR fragments showed different FC27 and IC1 allelic families of msp-2, indicating newly acquired infections after AL treatment. The posttreatment parasites from the remaining six (37.5%) patients were similar to the pretreatment isolates, and the patients were classified as having genuine treatment failures.

The mean ± standard deviation number of genetically distinct parasite populations, as determined by the analysis of msp-2, was 4.12 ± 0.72 in the enrollment samples, while the numbers were 2.18 ± 1.16 and 3.36 ± 1.07 in the posttreatment samples from patients with recrudescing and newly acquired infections, respectively. There was a significant reduction in the mean number of parasite clones in patients with recrudescing infections (P < 0.00001), suggesting that AL had a strong effect on the initial P. falciparum population. The clonality of the P. falciparum isolates from newly acquired infections was also significantly lower (P = 0.003) than that of the pretreatment isolates.

Gametocyte carriage rate and AL treatment outcome.

Patent and subpatent gametocytes were detected in peripheral blood from 26 (29%) of the 90 children treated with AL (including 16 and 10 children before and after treatment, respectively). The peak gametocyte density occurred 21 days posttreatment (144 gametocytes/μl of blood; n = 5), although it was not significantly different (P = 0.12) from the mean gametocyte density at enrollment (28 gametocytes/μl of blood; n = 16). RT-PCR confirmed subpatent pure gametocyte (without asexual stages) infections in six patients during follow-up (between days 7 and 42), while four other infections consisted of a mixture of both gametocytes and asexual-stage parasitemia. There was an association (P = 0.048) between treatment failure and the presence of gametocytes in patient samples before and after treatment.

Pretreatment baseline prevalence of Pfmdr1 alleles and haplotypes.

Molecular assays were successfully performed with all 90 isolates collected from patients prior to AL treatment. The isolates were evaluated for individual Pfmdr1 alleles present at codons 86 (N86Y), 184 (Y184F), 1034 (S1034C), 1042 (N1042D), and 1246 (D1246Y), as well as the presence of the Pfmdr1 haplotype at codons 86, 184, and 1246, which are known to be associated with AL treatment failure or reinfections (6, 19, 31, 39) in Africa. The frequencies of Pfmdr1 alleles and haplotypes are presented in Table Table2.2. The loci that had the highest prevalence were Pfmdr1 N86 (40%), Pfmdr1 Y184 (88%), and Pfmdr1 1246D (77%). The most predominant Pfmdr1 haplotype in pretreatment samples was N-Y-D (53%) (Table (Table2).2). No sample showed Pfmdr1 mutants with 1034C or 1042D alleles.

TABLE 2.
Baseline frequency and prevalence of Pfmdr1 alleles at codons 86, 184, and 1246 among isolates collected from Nigerian childrena at enrollment, before treatment with AL

Association between Pfmdr1 point mutations or haplotype and treatment outcome.

To test our hypothesis that allelic variations in Pfmdr1 are associated with the outcomes for patients treated with AL, the presence of individual Pfmdr1 alleles at codons 86, 184, and 1246 or the haplotypes present in samples collected prior to treatment were analyzed with respect to the patients' treatment outcomes. The Pfmdr1 N86 (P = 0.332) and Pfmdr1 D1246 (P = 1.00) alleles were not independently associated with AL treatment failure (Table (Table3).3). The Pfmdr1 F184 allele was associated (P < 0.00001) with AL treatment failure. An association (P < 0.00001) between the presence of the Pfmdr1 N-F-D haplotype in Plasmodium falciparum and AL treatment failure was also observed (Table (Table33).

TABLE 3.
Association between Pfmdr1 alleles in P. falciparum isolates collected at enrollment and failure of treatment with ALa

Selection of Pfmdr1 alleles and haplotype by AL in asexual-stage parasites and gametocytes in patients with reoccurring infections.

In order to examine the selection of Pfmdr1 alleles by AL, we compared the prevalence of the Pfmdr1 alleles and haplotypes in baseline samples (samples obtained from all patients) with those in the posttreatment samples obtained from patients with true treatment failures (after correction of the results by PCR) and those with reinfections. The analysis revealed the selection of the Pfmdr1 N86 allele (P = 0.030; χ2 = 6.99), the F184 allele (P < 0.000001; χ2 = 51.14), and the N-F-D haplotype (P < 0.00001; χ2 = 36.5) by AL (Table (Table44).

TABLE 4.
Selection of Pfmdr1 alleles and haplotypes in Plasmodium falciparum infections by AL in Nigerian childrena

Since it has been suggested that the probability that a mosquito will be infective when it has a blood meal is related to gametocyte density and the duration of carriage by the host (32), we sought to characterize the surviving gametocytes after AL treatment by determining the Pfmdr1 alleles and haplotypes in these gametocytes. Analysis of the surviving gametocytes in patient samples after AL treatment showed that gametocytes with the Pfmdr1 N-F-D haplotype survived longer than those without this haplotype. Figure Figure11 is a Kaplan-Meier plot (survival curve) of the cumulative probability that a patient will harbor gametocytes following AL treatment. This probability was significantly higher for patients with gametocytes harboring the Pfmdr1 N-F-D haplotype than for those with gametocytes harboring a different haplotype (log rank statistic = 5; P = 0.025).

FIG. 1.
Kaplan-Meier plot (survival curve) of the cumulative probability that children harbor surviving gametocytes following AL treatment. This probability was significantly higher (log rank statistic = 5; P = 0.025) for patients (n = ...

Impact of age and Pfmdr1 alleles and haplotypes on patient treatment outcome.

The effect of age on the association between the presence of the Pfmdr1 N-F-D haplotype and treatment outcome was evaluated after stratification of the patients into two age groups (<5 years and ≥ 5 years). Forty (44.5%) and 50 (55.5%) patients were <5 years and ≥5 years of age, respectively. The presence of the Pfmdr1 N-F-D haplotype was significantly (P < 0.0001) associated with treatment failures in children <5 years of age but not in children ≥5 years of age (P = 0.155).

Clearance of Pfmdr1 resistance-associated haplotype by the host.

The role of some patients' characteristics on the ability to clear infections with parasites harboring the Pfmdr1 N-F-D haplotype was evaluated by analyzing the potential association between parasite clearance rates and patient characteristics, such as age, parasite density, or packed cell volume at enrollment, as described previously (5, 14). Univariate analysis after age stratification (<5 and ≥5 years) showed that the clearance of parasites harboring the Pfmdr1 N-F-D haplotype by patients was associated with age. Children ≥5 years of age cleared their infections at a significantly greater (P = 0.018) rate than the younger children (Fig. (Fig.2).2). The levels of parasitemia at enrollment (P = 0.0807) and the packed cell volume (P = 0.15) were not associated with the clearance of the Pfmdr1 N-F-D haplotype by children in either age group.

FIG. 2.
Plot of clearance of Plasmodium falciparum infections in children harboring isolates with the Pfmdr1 N-F-D haplotype after stratification by age (<5 and >5 years). Children over age 5 years (n = 50; mean age, 7.52 ± 1.45 ...

Absence of PfATPase6.

Overall, all 90 (100%) samples obtained from the patients at enrollment were successfully analyzed for the presence of the mutation (L89E) at codon 89 of the PfATPase6 gene. No mutant PfATPase6 (E89) allele was detected in the baseline samples. In addition, no mutant E89 allele was found in the 15 successfully tested isolates from the 16 patients in whom infection reoccurred (recrudescing and reinfections) after treatment with AL.

Copy number of Pfmdr1.

The Pfmdr1 gene copy number variation was successfully analyzed in 6 and 10 paired samples (which comprised samples obtained on day 0 and on the day of the reoccurrence of parasites) from patients with recrudescing and newly acquired infections, respectively. No patient isolate showed an increase in the Pfmdr1 copy number either before or after treatment when parasites reoccurred. The estimated gene copy number from all isolates analyzed was close to 1.0 (data not shown).

DISCUSSION

In this study we assessed the parasitological responses of patients to AL treatment after adjustment of the results by genotyping by PCR of the msp-2 gene in posttreatment samples from patients with reoccurring infections. The association between Pfmdr1 mutations and haplotypes and the patients' treatment outcomes, as well as the effect of AL on the gametocyte carriage rate, the variation in the Pfmdr1 copy number, and PfATPase6 gene polymorphisms, was also investigated.

PCR analysis of all follow-up samples showed a high number (18%) of patients with parasitemia whose parasitemia was not detected by microscopy during follow-up. However, only 7% of these patients had genuine treatment failures. This rate of true treatment failure in the clinical efficacy aspect of the present study is relatively low compared to the 9% uncorrected rate of parasitological failure reported recently (33). The 7% treatment failure rate observed in this study is not dramatically different from the rate of 5.2% reported in Tanzania (31) in a similar 42-day follow-up study. Reports of AL treatment failures in other areas of East Africa (6, 23) or West Africa (39) where malaria is endemic have been lower, although the patients in those studies were followed up for only 28 days. The higher rate of recrudescence of infections after treatment with AL observed in our study could be explained in a number of ways. First, we did not use stepwise genotyping of two highly polymorphic loci (msp-1 and msp-2), as proposed by Mugittu and colleagues (22), to distinguish between treatment failures and new infections. Thus, our use of a single genetic marker (msp-2) to establish the PCR-adjusted cure rate might have resulted in an underestimation of the efficacy of AL. Second, it is also possible that the parasites obtained from the six patients classified as having genuine recrudescences by msp-2 analysis alone were actually resistant to AL, although the blood drug levels were not determined in these patients in order to confirm these findings. A previous report (24) of a study conducted at the same site evaluated in the present study showed that Plasmodium falciparum had reduced in vitro sensitivity to artemisinin, the parent drug of artemether, a component of AL.

Using RT-PCR, we revealed gametocytes that had not been detected microscopically in the clinical component of this study (33). We demonstrated a higher gametocyte carriage rate among AL-treated patients. Our data are consistent with those from previous reports of the submicroscopic detection of gametocytes by real-time quantitative nucleic acid sequence-based amplification or real-time PCR (3, 25, 29). In addition, our findings of the association between the presence of gametocytes at enrollment and during follow-up and AL treatment failures, on the one hand, and a peak gametocyte prevalence 21 days after the initiation of treatment, on the other hand, are of particular interest. Although the reasons for the late (day 21) gametocyte peak observed in this study are unclear, they are in contrast to the findings of previous studies in other areas where malaria is endemic, where the peak gametocyte prevalence occurred 1 week after treatment, irrespective of the antimalarial drug used (36). The gametocytes observed in patients after AL treatment might have been released from sequestration sites or were drug-resistant gametocytes selected by lumefantrine. Evidence from a recent study in West Africa suggests that the persistence of gametocytes in patients following treatment with antimalarial drugs may be an early warning signal for emerging drug resistance (11).

The presence of the Pfmdr1 F184 allele or Pfmdr1 N-F-D haplotype in asexual and gametocyte stages of Plasmodium falciparum was associated with AL treatment failures, although the N86 and D1246 alleles were also quite common in the isolates from the cured patients. These findings are consistent with those of a recent study from Tanzania (19) but are in contrast to those presented in a previous report from Uganda (6), in which no association between the Pfmdr1 N-F-D haplotype and treatment failure was found. The survival advantage of gametocytes harboring the Pfmdr1 N-F-D haplotype (Fig. (Fig.1)1) after AL treatment suggests that the Pfmdr1 N-F-D haplotype may confer some fitness advantage to Plasmodium falciparum in the presence of AL. Recent reports from Uganda (6) and Zanzibar (32) showed a significant accumulation of the Pfmdr1 N86, F184, and D1246 alleles and the Pfmdr1 N86 allele, respectively, among children who had parasites after AL treatment.

To our knowledge, this is the first study in Africa to systematically examine the role of Pfmdr1 haplotype in asexual- and sexual-stage parasites and the involvement of this haplotype in AL treatment failure. There could be several reasons for the increase in the prevalence of the Pfmdr1 N86 and F184 alleles and the Pfmdr1 N-F-D haplotype that we observed after AL treatment compared to the prevalence in the baseline population. First, it is possible that these alleles are selected on a population level but not on an individual level with the recent increase in the use of AL at the study site. Second, it is also possible that in AL-treated patients, lumefantrine might have selected parasites harboring the Pfmdr1 N86 and F184 alleles or the Pfmdr1 N-F-D haplotype in new infections emerging from the liver after clearance of artemether. If this argument is correct, it means that in areas with high rates of malaria parasite transmission in Africa, where reinfection is common and these Pfmdr1 alleles are circulating, the long-term success of AL may be hampered by the emergence of drug-resistant malaria parasites. A third explanation for our findings is that the posttreatment asexual-stage parasitemia or gametocytes observed in this study were the result of parasites that bore signals of artemisinin selection and that survived the artemether concentrations in AL-treated patients. A previous report (24) from our study site described patient isolates of Plasmodium falciparum with innate resistance to new antimalarial drugs, including mefloquine and artemisinin.

It was of particular interest to observe that parasites harboring the Pfmdr1 N-F-D haplotype were associated with AL treatment failure only in children <5 years of age. Children ≥5 years of age cleared their infections, even though they were infected with parasites harboring the Pfmdr1 N-F-D haplotype, suggesting the immunopotentiation of the action of AL in children ≥5 years of age in Ibadan, Nigeria. These findings provide further evidence to our previous observations of the important role of immunity in clearing drug-resistant Plasmodium falciparum infections (13, 14). The findings also further confirm the fact that children <5 years of age are highly vulnerable to malaria, even when treatments are available.

For the first time, the PfATP6 769N allele, which was recently found to be associated with a decrease in artemisinin susceptibility (20), was investigated in a trial of the efficacy of ACTs in Nigeria. This single nucleotide polymorphism was not detected in any patient sample obtained either at enrollment or when infections reoccurred. Thus, this mutation may represent a geographically specific variation, as it has so far been detected only in South America (20).

We found no Pfmdr1 gene amplification in asexual-stage parasites or gametocytes in samples collected from patients either prior to treatment or after treatment in patients with reoccurring infections, suggesting that the path to resistance to AL may involve a marked selection of the Pfmdr1 N-F-D haplotype rather than an increase in the Pfmdr1 gene copy number in the asexual and gametocytes stages of P. falciparum. Our findings support those presented in previous reports of studies from East Africa (6, 18) where Pfmdr1 amplification in P. falciparum has been shown to be rare but are in contrast to the observations from Southeast Asia, where the level of Pfmdr1 gene amplification can be up to 50% (27). The reasons for this current rarity of Pfmdr1 gene amplification in Africa are unclear, but it may be related to a parasite fitness cost associated with counterselection by the extensive use of chloroquine in areas of Africa where malaria is endemic (2). However, even if Pfmdr1 gene amplification was not observed in the samples analyzed during this study, it may potentially represent an alternative path for the further development toward parasite resistance to AL with increased selection pressure as a result of the current high rate of use of this compound in Africa.

Overall, the Pfmdr1 N-F-D haplotype may be a potential genetic marker of AL resistance, as demonstrated in this study, and there is a need to validate these markers in other areas where the disease is endemic and where AL is currently being used.

Acknowledgments

We thank all the patients and their parents or guardians for volunteering to participate in the study. We thank the Malaria Research and Reference Reagent Resource Centre (MR4) for providing the genomic DNA used as controls for the PCR and RFLP experiments. We also thank Sarah Volkman at the Harvard School of Public Health for providing us with the Plasmodium falciparum W2mef strain for Pfmdr1 copy number analysis.

This study was supported by grants from the Fogarty International Center, the Multilateral Initiative for Malaria in Africa (MIM)/TDR, UNICEF/UNDP/World Bank/WHO/TDR, the Harvard Malaria Initiative, EDCTP, and the International Atomic Energy Agency (IAEA). C. T. Happi is supported by Fogarty International Research Collaboration award no. NIH RO3TW007757-02, IAEA project RAF/0625, UNICEF/UNDP/World Bank/WHO/TDR/PAG/South-South Initiative project ID A50337, and European Union and Developing Countries Clinical Trial Partnership (EDCTP) award no. TA.2007.40200.016 for a senior research fellowship. G. O. Gbotosho is supported by MIM/TDR project ID A20239. A. Sowunmi is supported by a Pfizer Global Pharmaceutical Grant.

None of the authors has a conflict of interest to declare.

Footnotes

[down-pointing small open triangle]Published ahead of print on 15 December 2008.

REFERENCES

1. Babiker, H. A., A. Abdel-Wahab, S. Ahmed, S. Suleiman, L. Ranford-Cartwright, R. Carter, and D. Walliker. 1999. Detection of low level Plasmodium falciparum gametocytes using reverse transcriptase polymerase chain reaction. Mol. Biochem. Parasitol. 99:143-148. [PubMed]
2. Barnes, D. A., S. J. Foote, D. Galatis, D. J. Kemp, and A. F. Cowman. 1992. Selection for high-level chloroquine resistance results in deamplification of the Pfmdr1 gene and increased sensitivity to mefloquine in Plasmodium falciparum. EMBO J. 11:3067-3075. [PMC free article] [PubMed]
3. Bousema, J. T., P. Schneider, L. C. Gouagna, C. J. Drakeley, A. Tostmann, R. Houben, J. I. Githure, R. Ord, C. J. Sutherland, S. A. Omar, and R. W. Sauerwein. 2006. Moderate effect of artemisinin-based combination therapy on transmission of Plasmodium falciparum. J. Infect. Dis. 193:1151-1159. [PubMed]
4. Djimde, A., O. K. Doumbo, J. F. Cortese, K. Kayentao, S. Doumbo, Y. Diourte, A. Dicko, X. Z. Su, T. Nomura, D. A. Fidock, T. E. Wellems, C. V. Plowe, and D. Coulibaly. 2001. A molecular marker for chloroquine-resistant falciparum malaria. N. Engl. J. Med. 344:257-263. [PubMed]
5. Djimde, A. A., O. K. Doumbo, O. Traore, A. B. Guindo, K. Kayentao, Y. Diourte, S. Niare-Doumbo, D. Coulibaly, A. K. Kone, Y. Cissoko, M. Tekete, B. Fofana, A. Dicko, D. A. Diallo, T. E. Wellems, D. Kwiatkowski, and C. V. Plowe. 2003. Clearance of drug-resistant parasites as a model for protective immunity in Plasmodium falciparum malaria. Am. J. Trop. Med. Hyg. 69:558-563. [PubMed]
6. Dokomajilar, C., S. L. Nsobya, B. Greenhouse, P. J. Rosenthal, and G. Dorsey. 2006. Selection of Plasmodium falciparum Pfmdr1 alleles following therapy with artemether-lumefantrine in an area of Uganda where malaria is highly endemic. Antimicrob. Agents Chemother. 50:1893-1895. [PMC free article] [PubMed]
7. Duraisingh, M. T., and A. F. Cowman. 2005. Contribution of the Pfmdr1 gene to antimalarial drug-resistance. Acta Trop. 94:181-190. [PubMed]
8. Duraisingh, M. T., P. Jones, I. Sambou, L. von Seidlein, M. Pinder, and D. C. Warhurst. 2000. The tyrosine-86 allele of the Pfmdr1 gene of Plasmodium falciparum is associated with increased sensitivity to the anti-malarials mefloquine and artemisinin. Mol. Biochem. Parasitol. 108:13-23. [PubMed]
9. Duraisingh, M. T., C. Roper, D. Walliker, and D. C. Warhurst. 2000. Increased sensitivity to the antimalarials mefloquine and artemisinin is conferred by mutations in the Pfmdr1 gene of Plasmodium falciparum. Mol. Microbiol. 36:955-961. [PubMed]
10. Ferreira, I. D., V. E. Rosario, and P. V. Cravo. 2006. Real-time quantitative PCR with SYBR green I detection for estimating copy numbers of nine drug resistance candidate genes in Plasmodium falciparum. Malar. J. 5:1. [PMC free article] [PubMed]
11. Hallett, R. L., S. Dunyo, R. Ord, M. Jawara, M. Pinder, A. Randall, A. Alloueche, G. Walraven, G. A. Targett, N. Alexander, and C. J. Sutherland. 2006. Chloroquine/sulphadoxine-pyrimethamine for Gambian children with malaria: transmission to mosquitoes of multidrug-resistant Plasmodium falciparum. PLoS Clin. Trials 1:e15. [PMC free article] [PubMed]
12. Hallett, R. L., C. J. Sutherland, N. Alexander, R. Ord, M. Jawara, C. J. Drakeley, M. Pinder, G. Walraven, G. A. Targett, and A. Alloueche. 2004. Combination therapy counteracts the enhanced transmission of drug-resistant malaria parasites to mosquitoes. Antimicrob. Agents Chemother. 48:3940-3943. [PMC free article] [PubMed]
13. Happi, C. T., G. O. Gbotosho, O. A. Folarin, D. O. Akinboye, B. O. Yusuf, O. O. Ebong, A. Sowunmi, D. E. Kyle, W. Milhous, D. F. Wirth, and A. M. Oduola. 2005. Polymorphisms in Plasmodium falciparum dhfr and dhps genes and age related in vivo sulfadoxine-pyrimethamine resistance in malaria-infected patients from Nigeria. Acta Trop. 95:183-193. [PubMed]
14. Happi, C. T., G. O. Gbotosho, O. A. Folarin, O. M. Bolaji, A. Sowunmi, D. E. Kyle, W. Milhous, D. F. Wirth, and A. M. Oduola. 2006. Association between mutations in Plasmodium falciparum chloroquine resistance transporter and P. falciparum multidrug resistance 1 genes and in vivo amodiaquine resistance in P. falciparum malaria-infected children in Nigeria. Am. J. Trop. Med. Hyg. 75:155-161. [PubMed]
15. Happi, C. T., G. O. Gbotosho, O. A. Folarin, A. Sowunmi, O. M. Bolaji, B. A. Fateye, D. E. Kyle, W. Milhous, D. F. Wirth, and A. M. Oduola. 2006. Linkage disequilibrium between two distinct loci in chromosomes 5 and 7 of Plasmodium falciparum and in vivo chloroquine resistance in southwest Nigeria. Parasitol. Res. 100:141-148. [PubMed]
16. Happi, C. T., G. O. Gbotosho, A. Sowunmi, C. O. Falade, D. O. Akinboye, L. Gerena, D. E. Kyle, W. Milhous, D. F. Wirth, and A. M. Oduola. 2004. Molecular analysis of Plasmodium falciparum recrudescent malaria infections in children treated with chloroquine in Nigeria. Am. J. Trop. Med. Hyg. 70:20-26. [PubMed]
17. Happi, T. C., S. M. Thomas, G. O. Gbotosho, C. O. Falade, D. O. Akinboye, L. Gerena, T. Hudson, A. Sowunmi, D. E. Kyle, W. Milhous, D. F. Wirth, and A. M. Oduola. 2003. Point mutations in the Pfcrt and Pfmdr-1 genes of Plasmodium falciparum and clinical response to chloroquine, among malaria patients from Nigeria. Ann. Trop. Med. Parasitol. 97:439-451. [PubMed]
18. Holmgren, G., A. Bjorkman, and J. P. Gil. 2006. Amodiaquine resistance is not related to rare findings of Pfmdr1 gene amplifications in Kenya. Trop. Med. Int. Health 11:1808-1812. [PubMed]
19. Humphreys, G. S., I. Merinopoulos, J. Ahmed, C. J. Whitty, T. K. Mutabingwa, C. J. Sutherland, and R. L. Hallett. 2007. Amodiaquine and artemether-lumefantrine select distinct alleles of the Plasmodium falciparum mdr1 gene in Tanzanian children treated for uncomplicated malaria. Antimicrob. Agents Chemother. 51:991-997. [PMC free article] [PubMed]
20. Jambou, R., E. Legrand, M. Niang, N. Khim, P. Lim, B. Volney, M. T. Ekala, C. Bouchier, P. Esterre, T. Fandeur, and O. Mercereau-Puijalon. 2005. Resistance of Plasmodium falciparum field isolates to in-vitro artemether and point mutations of the SERCA-type PfATPase6. Lancet 366:1960-1963. [PubMed]
21. Kidgell, C., S. K. Volkman, J. Daily, J. O. Borevitz, D. Plouffe, Y. Zhou, J. R. Johnson, K. Le Roch, O. Sarr, O. Ndir, S. Mboup, S. Batalov, D. F. Wirth, and E. A. Winzeler. 2006. A systematic map of genetic variation in Plasmodium falciparum. PLoS Pathog. 2:e57. [PMC free article] [PubMed]
22. Mugittu, K., M. Adjuik, G. Snounou, F. Ntoumi, W. Taylor, H. Mshinda, P. Olliaro, and H. P. Beck. 2006. Molecular genotyping to distinguish between recrudescents and new infections in treatment trials of Plasmodium falciparum malaria conducted in sub-Saharan Africa: adjustment of parasitological outcomes and assessment of genotyping effectiveness. Trop. Med. Int. Health 11:1350-1359. [PubMed]
23. Mutabingwa, T. K., D. Anthony, A. Heller, R. Hallett, J. Ahmed, C. Drakeley, B. M. Greenwood, and C. J. Whitty. 2005. Amodiaquine alone, amodiaquine + sulfadoxine-pyrimethamine, amodiaquine + artesunate, and artemether-lumefantrine for outpatient treatment of malaria in Tanzanian children: a four-arm randomised effectiveness trial. Lancet 365:1474-1480. [PubMed]
24. Oduola, A. M., A. Sowunmi, W. K. Milhous, D. E. Kyle, R. K. Martin, O. Walker, and L. A. Salako. 1992. Innate resistance to new antimalarial drugs in Plasmodium falciparum from Nigeria. Trans. R. Soc. Trop. Med. Hyg. 86:123-126. [PubMed]
25. Ouedraogo, A. L., P. Schneider, M. de Kruijf, I. Nebie, J. P. Verhave, N. Cuzin-Ouattara, and R. W. Sauerwein. 2007. Age-dependent distribution of Plasmodium falciparum gametocytes quantified by Pfs25 real-time QT-NASBA in a cross-sectional study in Burkina Faso. Am. J. Trop. Med. Hyg. 76:626-630. [PubMed]
26. Price, R. N., A. C. Uhlemann, A. Brockman, R. McGready, E. Ashley, L. Phaipun, R. Patel, K. Laing, S. Looareesuwan, N. J. White, F. Nosten, and S. Krishna. 2004. Mefloquine resistance in Plasmodium falciparum and increased Pfmdr1 gene copy number. Lancet 364:438-447. [PubMed]
27. Price, R. N., A. C. Uhlemann, M. van Vugt, A. Brockman, R. Hutagalung, S. Nair, D. Nash, P. Singhasivanon, T. J. Anderson, S. Krishna, N. J. White, and F. Nosten. 2006. Molecular and pharmacological determinants of the therapeutic response to artemether-lumefantrine in multidrug-resistant Plasmodium falciparum malaria. Clin. Infect. Dis. 42:1570-1577. [PubMed]
28. Reed, M. B., K. J. Saliba, S. R. Caruana, K. Kirk, and A. F. Cowman. 2000. Pgh1 modulates sensitivity and resistance to multiple antimalarials in Plasmodium falciparum. Nature 403:906-909. [PubMed]
29. Schneider, P., L. Wolters, G. Schoone, H. Schallig, P. Sillekens, R. Hermsen, and R. Sauerwein. 2005. Real-time nucleic acid sequence-based amplification is more convenient than real-time PCR for quantification of Plasmodium falciparum. J. Clin. Microbiol. 43:402-405. [PMC free article] [PubMed]
30. Sidhu, A. B., S. G. Valderramos, and D. A. Fidock. 2005. Pfmdr1 mutations contribute to quinine resistance and enhance mefloquine and artemisinin sensitivity in Plasmodium falciparum. Mol. Microbiol. 57:913-926. [PubMed]
31. Sisowath, C., J. Stromberg, A. Martensson, M. Msellem, C. Obondo, A. Bjorkman, and J. P. Gil. 2005. In vivo selection of Plasmodium falciparum Pfmdr1 86N coding alleles by artemether-lumefantrine (Coartem). J. Infect. Dis. 191:1014-1017. [PubMed]
32. Sowunmi, A., T. Balogun, G. O. Gbotosho, C. T. Happi, A. A. Adedeji, and F. A. Fehintola. 2007. Activities of amodiaquine, artesunate, and artesunate-amodiaquine against asexual- and sexual-stage parasites in falciparum malaria in children. Antimicrob. Agents Chemother. 51:1694-1699. [PMC free article] [PubMed]
33. Sowunmi, A., G. O. Gbotosho, C. T. Happi, A. A. Adedeji, F. A. Fehintola, O. A. Folarin, E. Tambo, and B. A. Fateye. 2007. Therapeutic efficacy and effects of artemether-lumefantrine and amodiaquine-sulfalene-pyrimethamine on gametocyte carriage in children with uncomplicated Plasmodium falciparum malaria in southwestern Nigeria. Am. J. Trop. Med. Hyg. 77:235-241. [PubMed]
34. Sutherland, C. J., A. Alloueche, J. Curtis, C. J. Drakeley, R. Ord, M. Duraisingh, B. M. Greenwood, M. Pinder, D. Warhurst, and G. A. Targett. 2002. Gambian children successfully treated with chloroquine can harbor and transmit Plasmodium falciparum gametocytes carrying resistance genes. Am. J. Trop. Med. Hyg. 67:578-585. [PubMed]
35. Sutherland, C. J., R. Ord, S. Dunyo, M. Jawara, C. J. Drakeley, N. Alexander, R. Coleman, M. Pinder, G. Walraven, and G. A. Targett. 2005. Reduction of malaria transmission to Anopheles mosquitoes with a six-dose regimen of co-artemether. PLoS Med. 2:e92. [PMC free article] [PubMed]
36. Targett, G., C. Drakeley, M. Jawara, L. von Seidlein, R. Coleman, J. Deen, M. Pinder, T. Doherty, C. Sutherland, G. Walraven, and P. Milligan. 2001. Artesunate reduces but does not prevent posttreatment transmission of Plasmodium falciparum to Anopheles gambiae. J. Infect. Dis. 183:1254-1259. [PubMed]
37. Uhlemann, A. C., R. McGready, E. A. Ashley, A. Brockman, P. Singhasivanon, S. Krishna, N. J. White, F. Nosten, and R. N. Price. 2007. Intrahost selection of Plasmodium falciparum Pfmdr1 alleles after antimalarial treatment on the northwestern border of Thailand. J. Infect. Dis. 195:134-141. [PubMed]
38. Wilson, C. M., S. K. Volkman, S. Thaithong, R. K. Martin, D. E. Kyle, W. K. Milhous, and D. F. Wirth. 1993. Amplification of Pfmdr 1 associated with mefloquine and halofantrine resistance in Plasmodium falciparum from Thailand. Mol. Biochem. Parasitol. 57:151-160. [PubMed]
39. Zongo, I., G. Dorsey, N. Rouamba, H. Tinto, C. Dokomajilar, R. T. Guiguemde, P. J. Rosenthal, and J. B. Ouedraogo. 2007. Artemether-lumefantrine versus amodiaquine plus sulfadoxine-pyrimethamine for uncomplicated falciparum malaria in Burkina Faso: a randomised non-inferiority trial. Lancet 369:491-498. [PubMed]

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