Treatment of Plasmodium falciparum malaria with pyrimethamine-sulfadoxine: selective pressure for resistance is a function of long elimination half-life

The influence of biological, epidemiological, and treatment factors on the establishment and spread of drug-resistant Plasmodium falciparum

The effectiveness of artemisinin-based combination therapies (ACTs) to treat Plasmodium falciparum malaria is threatened by resistance. The complex interplay between sources of selective pressure-treatment properties, biological factors, transmission intensity, and access to treatment-obscures understanding how, when, and why resistance establishes and spreads across different locations. We developed a disease modelling approach with emulator-based global sensitivity analysis to systematically quantify which of these factors drive establishment and spread of drug resistance. Drug resistance was more likely to evolve in low transmission settings due to the lower levels of (i) immunity and (ii) within-host competition between genotypes. Spread of parasites resistant to artemisinin partner drugs depended on the period of low drug concentration (known as the selection window). Spread of partial artemisinin resistance was slowed with prolonged parasite exposure to artemisinin derivatives and accelerated when the parasite was also resistant to the partner drug. Thus, to slow the spread of partial artemisinin resistance, molecular surveillance should be supported to detect resistance to partner drugs and to change ACTs accordingly. Furthermore, implementing more sustainable artemisinin-based therapies will require extending parasite exposure to artemisinin derivatives, and mitigating the selection windows of partner drugs, which could be achieved by including an additional long-acting drug.

Investigations of socioeconomic factors associated with follow-up compliance with malaria treatment in Haiti

Objective.

To identify factors affecting compliance with follow-up during treatment in confirmed malaria patients at two health centers in Haiti.

Methods.

A prospective observational study of malaria patients undergoing treatment over a six-week period. Patients’ return visits (follow-up visits) to the health centers for consultation in accordance with the physicians’ requests were recorded and used to determine compliance. Socioeconomic data were obtained from patient enrollment questionnaires and through post-treatment interviews. The management practices and procedures at the health centers to retain patients were also reviewed. Descriptive statistics and Spearman’s rank correlation were used to identify significant factors, which were used as variables in a logistic regression model.

Results.

Sixty-eight percent of the malaria patients completed follow-up, with higher compliance being recorded in the larger, more established health center of Leogane (67%) than Cite Soleil (33%). The patient socioeconomic profiles differed between the two health center locations by level of education, religious diversity, household size, and percentage of married individuals. Crude logistic regression analyses identified health center location (OR = 0.179 [95% CI 0.064, 0.504]) and household size (OR = 1.374 [95% CI 1.056, 1.787]) to be associated with compliance. The adjusted model only identified health center location (OR = 0.226 [95% CI 0.056, 0.918]) as significantly associated with compliance.

Conclusion.

Although patients’ household size may be important according to the crude logistic regression analysis, in the adjusted analysis the site location of the health center where patients receive treatment was identified as the only important factor associated with follow-up compliance in malaria patients during treatment in Haiti. This information might be helpful to improve treatment outcomes and contribute to the monitoring of antimalarial resistance in Haiti.

Exposure to ambient air causes degradation and decreased in vitro potency of buparvaquone and parvaquone

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Buparvaquone and parvaquone are used to treat livestock infected with Theileria spp.

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Air exposure had a significant impact on the stability of buparvaquone and parvaquone.

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Parvaquone was more stable than buparvaquone.

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Drug degradation was related with loss of potency by an in vitro viability assay.

Buparvaquone and parvaquone are hydroxynaphthoquinone compounds commonly used to treat livestock infected with Theileria species such as T. parva and T. annulata. In many (sub)tropical regions, chromatic changes in medicines can result from extreme environmental conditions and improper drug storage or handling, raising the possibility of drug degradation and loss of potency. We evaluated the effects of UV light, elevated temperature, and atmospheric air on the stability and potency of both buparvaquone and parvaquone by using a combination of high performance liquid chromatography (HPLC) and a T. equi based in vitro parasite growth inhibition assay (to measure potency). Aliquots (1 ml; 3 replicates per treatment) of each compound were subjected to a variety of treatments that varied in duration and intensity followed by HPLC and potency assays. Exposure to ambient air for 50 days was correlated with a significant loss of potency for both buparvaquone (4535%, P <  0.05) and parvaquone (247%, P <  0.05), while elevated temperature (37°C) and UV light exposure (24 h) had no significant impact (P >  0.05). The decrease in potency of both buparvaquone and parvaquone correlated with drug degradation (r = -0.74 and -0.88, respectively) as measured by HPLC. In practice, if there is headspace present in the vial, then ambient air will invariably enter the vial and contribute to degradation of these compounds. Such degradation may contribute to increasing drug resistance, economic losses for farmers, and animal welfare concerns for animals that are treated for Theileria infections.

Does finance affect environmental degradation: evidence from One Belt and One Road Initiative region?

This paper explores the effects of finance on environmental degradation and investigates environmental Kuznets curve (EKC) of each country among 52 that participate in the One Belt and One Road Initiative (OBORI) using the latest long panel data span (1980-2016). We utilized panel long run econometric models (fully modified ordinary least square and dynamic ordinary least square) to explore the long-run estimates in full panel and country level. Moreover, the Dumitrescu and Hurlin (2012) causality test is applied to examine the short-run causalities among our considered variables. The empirical findings validate the EKC hypothesis; the long-run estimates point out that finance significantly enhances the environmental degradation (negatively in few cases). The short-run heterogeneous causality confirms the bi-directional causality between finance and environmental degradation. The empirical outcomes suggest that policymakers should consider the environmental degradation issue caused by financial development in the One Belt and One Road region.

The significance of renewable energy use for economic output and environmental protection: evidence from the Next 11 developing economies

Increasing economic activities in developing economies raise demand for energy mainly sourced from conventional sources. The consumption of more conventional energy will have a significant negative impact on the environment. Therefore, attention of policy makers has recently shifted towards the promotion of renewable energy generation and uses across economic activities to ensure low carbon economy. Given the recent scenario, in this paper, we aim to examine the role of renewable energy consumption on the economic output and CO₂ emissions of the next fastest developing economies of the world. The study employs several robust panel econometric models by using annual data from 1990 to 2012. Empirical findings confirm the significant long-run association among the variables. Similarly, results show that renewable energy consumption positively contributes to economic output and has an adverse effect on CO₂ emissions. Given our findings, we suggest policy makers of those economies to initiate further effective policies to promote more renewable energy generation and uses across economic activities to ensure sustainable economic development.

Comprehensive review on various strategies for antimalarial drug discovery

The resistance of malaria parasites to existing drugs carries on growing and progressively limiting our ability to manage this severe disease and finally lead to a massive global health burden. Till now, malaria control has relied upon the traditional quinoline, antifolate and artemisinin compounds. Very few new antimalarials were developed in the past 50 years. Among recent approaches, identification of novel chemotherapeutic targets, exploration of natural products with medicinal significance, covalent bitherapy having a dual mode of action into a single hybrid molecule and malaria vaccine development are explored heavily. The proper execution of these approaches and proper investment from international agencies will accelerate the discovery of drugs that provide new hope for the control or eventual eradication of this global infectious disease. This review explores various strategies for assessment and development of new antimalarial drugs. Current status and scientific value of previous approaches are systematically reviewed and new approaches provide a pragmatic forecast for future developments are introduced as well.

Does antimalarial mass drug administration increase or decrease the risk of resistance?

All antimalarial drugs developed so far have eventually succumbed to resistance. There is a general belief that the more people that are exposed to an antimalarial drug, the more likely it is that resistance will emerge. Mass drug administration (MDA) is therefore considered a potent cause of antimalarial drug resistance. In this Personal View, I discuss the circumstances under which antimalarial MDA increases or decreases the risk of resistance. It is the total number of parasites exposed and their individual probabilities of survival and spread that determine the risk, not the number of people that contain them. In malaria-endemic areas, a substantial proportion of the community carries malaria parasites in their blood without being ill. Although many more people have asymptomatic than symptomatic malaria at any time, their parasite burdens are several orders of magnitude lower, and their host defence mechanisms are substantially more effective. Symptomatic infections with high parasite numbers are the most likely source of resistance emergence, so effective mass treatment that reduces the number of symptomatic cases of malaria and its transmission can reduce the threat of antimalarial resistance emerging and spreading in treated populations.

Seasonal Malaria Chemoprevention with Sulphadoxine-Pyrimethamine and Amodiaquine Selects Pfdhfr-dhps Quintuple Mutant Genotype in Mali

BACKGROUND

Seasonal malaria chemoprevention (SMC) with sulphadoxine-pyrimethamine (SP) plus amodiaquine (AQ) is being scaled up in Sahelian countries of West Africa. However, the potential development of Plasmodium falciparum resistance to the respective component drugs is a major concern.

METHODS

Two cross-sectional surveys were conducted before (August 2012) and after (June 2014) a pilot implementation of SMC in Koutiala, Mali. Children aged 3-59 months received 7 rounds of curative doses of SP plus AQ over two malaria seasons. Genotypes of P. falciparum Pfdhfr codons 51, 59 and 108; Pfdhps codons 437 and 540, Pfcrt codon 76 and Pfmdr1codon 86 were analyzed by PCR on DNA from samples collected before and after SMC, and in non-SMC patient population as controls (November 2014).

RESULTS

In the SMC population 191/662 (28.9%) and 85/670 (12.7%) of children were P. falciparum positive by microscopy and were included in the molecular analysis before (2012) and after SMC implementation (2014), respectively. In the non-SMC patient population 220/310 (71%) were successfully PCR analyzed. In the SMC children, the prevalence of all molecular markers of SP resistance increased significantly after SMC including the Pfdhfr-dhps quintuple mutant genotype, which was 1.6% before but 7.1% after SMC (p = 0.02). The prevalence of Pfmdr1-86Y significantly decreased from 26.7% to 15.3% (p = 0.04) while no significant change was seen for Pfcrt 76T. In 2014, prevalence of all molecular markers of SP resistance were significantly higher among SMC children compared to the non-SMC population patient (p < 0.01). No Pfdhfr-164 mutation was found neither at baseline nor post SMC.

CONCLUSION

SMC increased the prevalence of molecular markers of P. falciparum resistance to SP in the treated children. However, there was no significant increase of these markers of resistance in the general parasite population after 2 years and 7 rounds of SMC.

High efficacy of artemether-lumefantrine and declining efficacy of artesunate + sulfadoxine-pyrimethamine against Plasmodium falciparum in Sudan (2010-2015): evidence from in vivo and molecular marker studies

Background

The present paper reports on studies that evaluated artesunate + sulfadoxine-pyrimethamine (AS + SP) which is the first-line drug and artemether-lumefantrine (AL) which is a second-line drug against uncomplicated falciparum malaria in Sudan. This evaluation was performed in twenty studies covering six sentinel sites during five successive annual malaria transmission seasons from 2010 to 2015.

Methods

The standard World Health Organization protocol was used for a follow-up period of 28 days. The frequency distribution of molecular markers for antifolate resistance in dihydrofolate reductase (dhfr) and dihydropteroate synthase (dhps) genes was studied in pre-treatment samples in four sites in 2011.

Results

In the nine studies of AL conducted at five sites (n = 595), high PCR-corrected cure rates were found, ranging from 96.8 to 100 %. Among the eleven studies of AS + SP (n = 1013), a decline in the PCR-corrected cure rates was observed in Gedaref in Eastern Sudan: 91.0 % in the 2011–12 season and 86.5 % in the 2014–15 season. In the remaining sites, the AS + SP cure rates ranged between 95.6 and 100 %. The rate of clearance of microscopic gametocytaemia after treatment was not significantly different with AL or AS + SP on days 7, 14, 21 and 28 of follow-up. A total of 371 pre-treatment samples were analysed for molecular markers of SP resistance. The temporal changes and geographical differences in the frequency distribution of SP-resistance genotypes showed evidence of regional differentiation and selection of resistant strains.

Conclusion

The findings of this study call for a need to review the Sudan malaria treatment policy. Epidemiological factors could play a major role in the emergence of drug-resistant malaria in eastern Sudan.

Australian New Zealand Clinical Trials Registry

Trial registration numbers 2011–2012: ACTRN12611001253998, 2013–2015: ACTRN12613000945729

High efficacy of two artemisinin-based combinations: artesunate + sulfadoxine-pyrimethamine and artemether-lumefantrine for falciparum malaria in Yemen

BACKGROUND

Artesunate + sulfadoxine-pyrimethamine (AS + SP) has been the first-line treatment and artemether-lumefantrine (AL) the second-line treatment for uncomplicated falciparum malaria in Yemen since 2005. This paper reports the results of studies conducted to monitor therapeutic efficacy of these two drugs in sentinel sites in Yemen.

METHODS

Eight therapeutic efficacy studies were conducted in six sentinel sites during the period 2009-2013 in Yemen. Five studies were for the evaluation of AS + SP (total of 465 patients) and three studies (total of 268 patients) for the evaluation of AL. The studies were done according to standard WHO protocol 2009 with 28-day follow-up.

RESULTS

In the evaluation of AS + SP, the PCR-corrected cure rate was 98 % (95 % CI 92.2-99.5 %) in one site and 100 % in all of the other four sites. In the sites where AL was evaluated, the PCR-corrected cure rate was 100 % in all the sites. All patients were negative for asexual parasitaemia on day 3 in both the AS + SP and the AL groups. There was a higher rate of clearance of gametocytaemia in the AL-treated group when compared with the AS + SP groups from day 7 onwards.

CONCLUSION

AS + SP remains the effective drug for uncomplicated falciparum malaria in Yemen. AL is also highly effective and can be an appropriate alternative to AS + SP for the treatment of falciparum malaria. AL demonstrated a higher efficacy in clearing microscopic gametocytaemia than AS + SP.

TRIAL REGISTRATION

Trial registration number ACTRN12610000696099.

Measuring windows of selection for anti-malarial drug treatments

BACKGROUND

The long half-lives of malaria 'partner' drugs are a potent force selecting for drug resistance. Clinical trials can quantify this effect by estimating a window of selection (WoS), defined as the amount of time post-treatment when drug levels are sufficiently high that resistant parasites can re-establish an infection while preventing drug-sensitive parasites from establishing viable infections.

METHODS

The ability of clinical data to accurately estimate the true WoS was investigated using standard pharmacokinetic-pharmacodynamic models for three widely used malaria drugs: artemether-lumefantrine (AR-LF), artesunate-mefloquine (AS-MQ) and dihydroartemisinin-piperaquine (DHA-PPQ). Estimates of the clinical WoS either (1) ignored all new infections occurring after the 63-day follow-up period, as is currently done in clinical trials, or, (2) recognized that all individuals would eventually be re-infected and arbitrarily assigned them a new infection day.

RESULTS

The results suggest current methods of estimating the clinical WoS underestimate the true WoS by as much as 9 days for AR-LF, 33 days for AS-MQ and 7 days for DHA-PPQ. The new method of estimating clinical WoS (i.e., retaining all individuals in the analysis) was significantly better at estimating the true WoS for AR-LF and AS-MQ.

CONCLUSIONS

Previous studies, based on clinically observed WoS, have probably underestimated the 'true' WoS and hence the role of drugs with long half-lives in driving resistance. This has important policy implications: high levels of drug use are inevitable in mass drug administration programmes and intermittent preventative treatment programmes and the analysis herein suggests these policies will be far more potent drivers of resistance than previously thought.

Changing Malaria Prevalence on the Kenyan Coast since 1974: Climate, Drugs and Vector Control

Background

Progress toward reducing the malaria burden in Africa has been measured, or modeled, using datasets with relatively short time-windows. These restricted temporal analyses may miss the wider context of longer-term cycles of malaria risk and hence may lead to incorrect inferences regarding the impact of intervention.

Methods

1147 age-corrected Plasmodium falciparum parasite prevalence (PfPR_2-10) surveys among rural communities along the Kenyan coast were assembled from 1974 to 2014. A Bayesian conditional autoregressive generalized linear mixed model was used to interpolate to 279 small areas for each of the 41 years since 1974. Best-fit polynomial splined curves of changing PfPR_2-10 were compared to a sequence of plausible explanatory variables related to rainfall, drug resistance and insecticide-treated bed net (ITN) use.

Results

P. falciparum parasite prevalence initially rose from 1974 to 1987, dipped in 1991–92 but remained high until 1998. From 1998 onwards prevalence began to decline until 2011, then began to rise through to 2014. This major decline occurred before ITNs were widely distributed and variation in rainfall coincided with some, but not all, short-term transmission cycles. Emerging resistance to chloroquine and introduction of sulfadoxine/pyrimethamine provided plausible explanations for the rise and fall of malaria transmission along the Kenyan coast.

Conclusions

Progress towards elimination might not be as predictable as we would like, where natural and extrinsic cycles of transmission confound evaluations of the effect of interventions. Deciding where a country lies on an elimination pathway requires careful empiric observation of the long-term epidemiology of malaria transmission.

Inhibition of protein synthesis and malaria parasite development by drug targeting of methionyl-tRNA synthetases

Aminoacyl-tRNA synthetases (aaRSs) are housekeeping enzymes that couple cognate tRNAs with amino acids to transmit genomic information for protein translation. The Plasmodium falciparum nuclear genome encodes two P. falciparum methionyl-tRNA synthetases (PfMRS), termed PfMRS(cyt) and PfMRS(api). Phylogenetic analyses revealed that the two proteins are of primitive origin and are related to heterokonts (PfMRS(cyt)) or proteobacteria/primitive bacteria (PfMRS(api)). We show that PfMRS(cyt) localizes in parasite cytoplasm, while PfMRS(api) localizes to apicoplasts in asexual stages of malaria parasites. Two known bacterial MRS inhibitors, REP3123 and REP8839, hampered Plasmodium growth very effectively in the early and late stages of parasite development. Small-molecule drug-like libraries were screened against modeled PfMRS structures, and several "hit" compounds showed significant effects on parasite growth. We then tested the effects of the hit compounds on protein translation by labeling nascent proteins with (35)S-labeled cysteine and methionine. Three of the tested compounds reduced protein synthesis and also blocked parasite growth progression from the ring stage to the trophozoite stage. Drug docking studies suggested distinct modes of binding for the three compounds, compared with the enzyme product methionyl adenylate. Therefore, this study provides new targets (PfMRSs) and hit compounds that can be explored for development as antimalarial drugs.

Evolutionary biology and the avoidance of antimicrobial resistance

Evolutionary biologists have largely left the search for solutions to the drug resistance crisis to biomedical scientists, physicians, veterinarians and public health specialists. We believe this is because the vast majority of professional evolutionary biologists consider the evolutionary science of drug resistance to be conceptually uninteresting. Using malaria as case study, we argue that it is not. We review examples of evolutionary thinking that challenge various fallacies dominating antimalarial therapy, and discuss open problems that need evolutionary insight. These problems are unlikely to be resolved by biomedical scientists ungrounded in evolutionary biology. Involvement by evolutionary biologists in the science of drug resistance requires no intellectual compromises: the problems are as conceptually challenging as they are important.

Challenges of drug-resistant malaria

Over the past six decades, the drug resistance of Plasmodium falciparum has become an issue of utmost concern. Despite the remarkable progress that has been made in recent years in reducing the mortality rate to about 30% with the scaling-up of vector control, introduction of artemisinin-based combination therapies and other malaria control strategies, the confirmation of artemisinin resistance on the Cambodia–Thailand border threatened all the previous success. This review addresses the global scenario of antimalarial resistance and factors associated with it, with the main emphasis on futuristic approaches like nanotechnology and stem cell therapy that may impede resistant malaria, along with novel medications which are preparing to enter the global antimalarial market. These novel studies are likely to escalate over the coming years and will hopefully help to reduce the burden of malaria.

The effect of intermittent preventive treatment on anti-malarial drug resistance spread in areas with population movement

Background

The use of intermittent preventive treatment in pregnant women (IPTp), children (IPTc) and infant (IPTi) is an increasingly popular preventive strategy aimed at reducing malaria risk in these vulnerable groups. Studies to understand how this preventive intervention can affect the spread of anti-malarial drug resistance are important especially when there is human movement between neighbouring low and high transmission areas. Because the same drug is sometimes utilized for IPTi and for symptomatic malaria treatment, distinguishing their individual roles on accelerating the spread of drug resistant malaria, with or without human movement, may be difficult to isolate experimentally or by analysing data. A theoretical framework, as presented here, is thus relevant as the role of IPTi on accelerating the spread of drug resistance can be isolated in individual populations and when the populations are interconnected and interact.

Methods

A previously published model is expanded to include human movement between neighbouring high and low transmission areas, with focus placed on the malaria parasites. Parasite fitness functions, determined by how many humans the parasites can infect, are used to investigate how fast resistance can spread within the neighbouring communities linked by movement, when the populations are at endemic equilibrium.

Results

Model simulations indicate that population movement results in resistance spreading fastest in high transmission areas, and the more complete the anti-malarial resistance the faster the resistant parasite will tend to spread through a population. Moreover, the demography of infection in low transmission areas tends to change to reflect the demography of high transmission areas. Additionally, when regions are strongly connected the rate of spread of partially resistant parasites (R1) relative to drug sensitive parasites (RS), and fully resistant parasites (R2) relative to partially resistant parasites (R1) tend to behave the same in both populations, as should be expected.

Conclusions

In fighting anti-malarial drug resistance, different drug resistance monitoring and management policies are needed when the area in question is an isolated high or low transmission area, or when it is close and interacting with a neighbouring high or low transmission area, with human movement between them.

Electronic supplementary material

The online version of this article (doi:10.1186/1475-2875-13-428) contains supplementary material, which is available to authorized users.

A Mathematical Model for the Transmission and Spread of Drug Sensitive and Resistant Malaria Strains within a Human Population

Malaria remains by far the world's most important tropical disease, killing more people than any other communicable disease. A number of preventive and control measures have been put in place and most importantly drug treatment. The emergence of drug resistance against the most common and affordable antimalarials is widespread and poses a key obstacle to malaria control. A mathematical model that incorporates evolution of drug resistance and treatment as a preventive strategy is formulated and analyzed. The qualitative analysis of the model is given in terms of the effective reproduction number, Re. The existence and stability of the disease-free and endemic equilibria of the model are studied. We establish the threshold parameters below which the burden due to malaria can be brought under control. Numerical simulations are done to determine the role played by key parameters in the model. The public health implications of the results are twofold; firstly every effort should be taken to minimize the evolution of drug resistance due to treatment failure and secondly high levels of treatment and development of immunity are essential in reducing the malaria burden.

Prevailing Plasmodium falciparum dihydrofolate reductase 108-asparagine in Hodeidah, Yemen: a questionable sulfadoxine-pyrimethamine partner within the artemisinin-based combination therapy

Given that the evolution and spread of resistance to sulfadoxine-pyrimethamine (SP) have been documented at a quick pace worldwide, the present study investigated the mutant Plasmodium falciparum dihydrofolate reductase 108-asparagine (dhfr 108 N) as a key marker of resistance to the combination among parasite isolates from Hodeidah. The association of parasitologic indices with the dhfr 108 N mutant allele was also studied. Ninety patients with microscopically confirmed P. falciparum infection from Hodeidah were included in the present study. Polymerase chain reaction-restriction fragment length polymorphism approach was adopted for the molecular detection of this marker. The dhfr 108 N was detected among about 61% of P. falciparum isolates, in its pure and mixed-type forms, from Hodeidah. Age, gender and residence of patients were not significant predictors for the presence of the mutant allele among parasite isolates. In contrast, a history of malaria and antimalarial drug intake in the year preceding the study as well as frequent antimalarial drug intake were significantly associated with this mutant allele. The high frequency of dhfr 108 N among parasites isolates makes the role of SP questionable as a partner with outstanding effectiveness within the ACT, at least, in the near future. SP plus artesunate should be monitored for its antimalarial efficacy at regular intervals, preferably through the molecular detection of resistance-associated mutations.

Performance of Paracheck™-Pf, SD Bioline malaria Ag-Pf and SD Bioline malaria Ag-Pf/pan for diagnosis of falciparum malaria in the Central African Republic

Background

Rapid diagnostic tests (RDTs) are the current complement to microscopy for ensuring prompt malaria treatment. We determined the performance of three candidate RDTs (Paracheck™-Pf, SD Bioline malaria Ag-Pf and SD Bioline malaria Ag-Pf/pan) for rapid diagnosis of malaria in the Central African Republic.

Methods

Blood samples from consecutive febrile patients who attended for laboratory analysis of malaria at the three main health centres of Bangui were screened by microscopy and the RDTs. Two reference standards were used to assess the performance of the RDTs: microscopy and, a combination of microscopy plus nested PCR for slides reported as negative, on the assumption that negative results by microscopy were due to sub-patent parasitaemia.

Results

We analysed 436 samples. Using the combined reference standard of microscopy + PCR, the sensitivity of Paracheck™-Pf was 85.7% (95% CI, 80.8–89.8%), that of SD Bioline Ag-Pf was 85.4% (95% CI, 80.5–90.7%), and that of SD Bioline Ag-Pf/pan was 88.2% (95% CI, 83.2–92.0%). The tests performed less well in cases of low parasitaemia; however, the sensitivity was > 95% at > 500 parasites/μl.

Conclusions

Overall, SD Bioline malaria Ag-Pf and SD Bioline malaria Ag-Pf/pan performed slightly better than Paracheck™-Pf. Use of RDTs with reinforced microscopy practice and laboratory quality assurance should improve malaria treatment in the Central African Republic.

The chemotherapy of rodent malaria. LXII. Drug combinations to impede the selection of drug resistance, part 5: rates of development of resistance to some inhibitors of folate metabolism and to artesunate

In recent years infection with chloroquine-resistant Plasmodium falciparum has been combatted with two long-acting antimalarials, pyrimethamine and sulfadoxine, in the combination known as Fansidar that exerts a strong, synergistic action on the asexual stages of the parasite. This second-line regimen, however, is failing increasingly because of the selection of resistant clones in endemic areas, and effective, safe, alternative drugs or drug combinations that are also affordable are urgently needed. Antimalarial drugs with shorter half-lives than those of pyrimethamine or sulfadoxine are likely to be slower to select resistant parasites. In the experiments reported here, the baseline in-vivo responses of rodent malarial parasites to chlorproguanil and proguanil and their active metabolites, chlorcycloguanil and cycloguanil, as well as those to dapsone and artesunate, were explored. In general, the most sensitive parasite to all of these compounds was P. chabaudi. When the drugs were used, individually, to select resistance via the '2%-relapse technique', relatively stable resistance to each was obtained in P. chabaudi as well as in P. berghei and P. yoelii ssp. NS, the last of these being also highly resistant to chloroquine. Of most concern was the rapidity and high level of resistance developed by P. chabaudi to artesunate. The experiments also validated the use of chlorcycloguanil or cycloguanil as surrogates for chlorproguanil or proguanil. Further studies to investigate the possible value of administering chlorproguanil-dapsone, with or without artesunate, are under way and will be reported separately.

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.

Drug resistance in vectorborne parasites: multiple actors and scenarios for an evolutionary arms race

Drug-resistant pathogens emerge faster than new drugs come out of drug discovery pipelines. Current and future drug options should therefore be better protected, requiring a clear understanding of the factors that contribute to the natural history of drug resistance. Although many of these factors are relatively well understood for most bacteria, this proves to be more complex for vectorborne parasites. In this review, we discuss considering three key models (Plasmodium, Leishmania and Schistosoma) how drug resistance can emerge, spread and persist. We demonstrate a multiplicity of scenarios, clearly resulting from the biological diversity of the different organisms, but also from the different modes of action of the drugs used, the specific within- and between-host ecology of the parasites, and environmental factors that may have direct or indirect effects. We conclude that integrated control of drug-resistant vectorborne parasites is not dependent upon chemotherapy only, but also requires a better insight into the ecology of these parasites and how their transmission can be impaired.

Improving pharmacokinetic-pharmacodynamic modeling to investigate anti-infective chemotherapy with application to the current generation of antimalarial drugs

Mechanism-based pharmacokinetic-pharmacodynamic (PK/PD) modelling is the standard computational technique for simulating drug treatment of infectious diseases with the potential to enhance our understanding of drug treatment outcomes, drug deployment strategies, and dosing regimens. Standard methodologies assume only a single drug is used, it acts only in its unconverted form, and that oral drugs are instantaneously absorbed across the gut wall to their site of action. For drugs with short half-lives, this absorption period accounts for a significant period of their time in the body. Treatment of infectious diseases often uses combination therapies, so we refined and substantially extended the PK/PD methodologies to incorporate (i) time lags and drug concentration profiles resulting from absorption across the gut wall and, if required, conversion to another active form; (ii) multiple drugs within a treatment combination; (iii) differing modes of action of drugs in the combination: additive, synergistic, antagonistic; (iv) drugs converted to an active metabolite with a similar mode of action. This methodology was applied to a case study of two first-line malaria treatments based on artemisinin combination therapies (ACTs, artemether-lumefantrine and artesunate-mefloquine) where the likelihood of increased artemisinin tolerance/resistance has led to speculation on their continued long-term effectiveness. We note previous estimates of artemisinin kill rate were underestimated by a factor of seven, both the unconverted and converted form of the artemisinins kill parasites and the extended PK/PD methodology produced results consistent with field observations. The simulations predict that a potentially rapid decline in ACT effectiveness is likely to occur as artemisinin resistance spreads, emphasising the importance of containing the spread of artemisinin resistance before it results in widespread drug failure. We found that PK/PD data is generally very poorly reported in the malaria literature, severely reducing its value for subsequent re-application, and we make specific recommendations to improve this situation.

Population pharmacokinetics of mefloquine, piperaquine and artemether-lumefantrine in Cambodian and Tanzanian malaria patients

Background

Inter-individual variability in plasma concentration-time profiles might contribute to differences in anti-malarial treatment response. This study investigated the pharmacokinetics of three different forms of artemisinin combination therapy (ACT) in Tanzania and Cambodia to quantify and identify potential sources of variability.

Methods

Drug concentrations were measured in 143 patients in Tanzania (artemether, dihydroartemisinin, lumefantrine and desbutyl-lumefantrine), and in 63 (artesunate, dihydroartemisinin and mefloquine) and 60 (dihydroartemisinin and piperaquine) patients in Cambodia. Inter- and intra-individual variabilities in the pharmacokinetic parameters were assessed and the contribution of demographic and other covariates was quantified using a nonlinear mixed-effects modelling approach (NONMEM^®).

Results

A one-compartment model with first-order absorption from the gastrointestinal tract fitted the data for all drugs except piperaquine (two-compartment). Inter-individual variability in concentration exposure was about 40% and 12% for mefloquine. From all the covariates tested, only body weight (for all antimalarials) and concomitant treatment (for artemether only) showed a significant influence on these drugs’ pharmacokinetic profiles. Artesunate and dihydroartemisinin could not be studied in the Cambodian patients due to insufficient data-points. Modeled lumefantrine kinetics showed that the target day 7 concentrations may not be achieved in a substantial proportion of patients.

Conclusion

The marked variability in the disposition of different forms of ACT remained largely unexplained by the available covariates. Dosing on body weight appears justified. The concomitance of unregulated drug use (residual levels found on admission) and sub-optimal exposure (variability) could generate low plasma levels that contribute to selecting for drug-resistant parasites.

Antimalarial Drug Therapy: The Role of Parasite Biology and Drug Resistance

Malaria continues to affect public health and economic growth in many regions of the world. The number of infections continues to rise and is associated with increased mortality, despite basic science and public health efforts. Drug therapy remains the mainstay of treatment and prevention of this disease. Plasmodium has a complex life cycle involving an arthropod vector and distinct stages within the human host. Each parasite stage plays a unique role in transmission, disease, and latency. These different stages may vary in their response to the various antimalarial compounds. This article will review antimalarial therapies and drug resistance in the context of the parasites' biology.

Effect of single nucleotide polymorphisms in cytochrome P450 isoenzyme and N-acetyltransferase 2 genes on the metabolism of artemisinin-based combination therapies in malaria patients from Cambodia and Tanzania

The pharmacogenetics of antimalarial agents are poorly known, although the application of pharmacogenetics might be critical in optimizing treatment. This population pharmacokinetic-pharmacogenetic study aimed at assessing the effects of single nucleotide polymorphisms (SNPs) in cytochrome P450 isoenzyme genes (CYP, namely, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP3A4, and CYP3A5) and the N-acetyltransferase 2 gene (NAT2) on the pharmacokinetics of artemisinin-based combination therapies in 150 Tanzanian patients treated with artemether-lumefantrine, 64 Cambodian patients treated with artesunate-mefloquine, and 61 Cambodian patients treated with dihydroartemisinin-piperaquine. The frequency of SNPs varied with the enzyme and the population. Higher frequencies of mutant alleles were found in Cambodians than Tanzanians for CYP2C9*3, CYP2D6*10 (100C → T), CYP3A5*3, NAT2*6, and NAT2*7. In contrast, higher frequencies of mutant alleles were found in Tanzanians for CYP2D6*17 (1023C → T and 2850C → T), CYP3A4*1B, NAT2*5, and NAT2*14. For 8 SNPs, no significant differences in frequencies were observed. In the genetic-based population pharmacokinetic analyses, none of the SNPs improved model fit. This suggests that pharmacogenetic data need not be included in appropriate first-line treatments with the current artemisinin derivatives and quinolines for uncomplicated malaria in specific populations. However, it cannot be ruled out that our results represent isolated findings, and therefore more studies in different populations, ideally with the same artemisinin-based combination therapies, are needed to evaluate the influence of pharmacogenetic factors on the clearance of antimalarials.

Scaling up of intermittent preventive treatment of malaria in pregnancy using sulphadoxine-pyrimethamine: prospects and challenges

Intermittent preventive treatment of malaria during pregnancy with sulphadoxine-pyrimethamine (IPTpSP) is one of the major strategies of malaria control in most African countries where malaria is endemic. The use of sulphadoxine-pyrimethamine (SP) for intermittent preventive treatment of malaria during pregnancy was adopted when proof of its superiority to weekly prophylactic dosing with either chloroquine or pyrimethamine became evident from studies in different malaria endemic countries. The administration of 2 and 3 treatment doses of SP for HIV-negative and HIV-positive pregnant women respectively, given after quickening and at an interval not less than 4 weeks was recommended. The prospects of this control strategy lies on the efficacy of SP, convenient treatment dose and high compliance rate. However, the implementation of this strategy and the efficacy of SP are faced with challenges such as: timing of SP administration, rising levels of parasite resistance to SP in the general population, effect of folate supplementation, adequacy of the recommended doses with regards to malaria endemicity and HIV status, interactions between SP and antiretroviral drugs and low coverage in the bid to scale-up its use. This review highlights the prospects and challenges of scaling up IPTp-SP.

Anti-malarial drug formulations and novel delivery systems: a review

Artemisinin combination therapies have decreased malaria associated morbidity and mortality in several parts of the world. On the other hand, malaria cases have increased in sub-Saharan Africa largely due to falciparum resistance to the most frequently used drugs (chloroquine and sulphadoxine/pyrimethamine (SP) combination). Therapeutic failure has also been attributed in part to adverse effects of anti-malarial drugs and patients' non-compliance due to inconvenient dosing schedules. We consider that formulation and evaluation of novel drug delivery systems is not only less expensive than developing new drugs, but may also improve delivery of anti-malarials at the desired rates. In this review we evaluate the therapeutic efficacy of existing anti-malarial drugs and assess the feasibility of developing novel formulations and delivery systems.

How artemisinin-containing combination therapies slow the spread of antimalarial drug resistance

Antimalarial drug therapies containing artemisinins, 'ACTs', have become the mainstay for treating uncomplicated malaria in endemic countries. This is a major public health achievement requiring substantial political, financial and scientific input. The most compelling scientific argument for ACT deployment employed a very simple basic rationale that emphasised their role in slowing the origin of drug resistance while largely neglecting the additional role(s) of ACTs in slowing or preventing the spread of resistance once it has arisen. Recent reports suggest that early stages of resistance to artemisinins and/or its partner drugs could be occurring, thus it is timely to briefly review exactly how ACTs slow the origin and spread of resistance and to interpret the threat of resistance within this context.

Population pharmacokinetics of sulfadoxine and pyrimethamine in Malawian children with malaria

In addition to parasite resistance, inadequate levels of exposure to antimalarial drugs may contribute to treatment failure. We developed population pharmacokinetic (PK) models to describe the distribution of sulfadoxine (SDX) and pyrimethamine (PYM) in children with uncomplicated malaria in Malawi. The concentration levels of antimalarial drugs in whole blood were determined using high-performance liquid chromatography. We found no evidence of underdosing in children as compared with adults; the children had drug exposure levels similar to those described in adults. Treatment failure was more likely in children with lower PYM concentrations on day 14 (P = 0.024), and there was a trend for lower SDX concentrations on day 14 (P = 0.061). SDX and PYM concentrations at levels predictive of treatment failure have been identified at day 14. Less than one-third of the children displayed drug concentration levels above these thresholds after receiving the recommended SDX-pyrimethamine (SP) dose. Our findings suggest that PK factors contributed to the observed high rate of treatment failure, and we therefore recommend a higher SP dose for children under the age of 5 years.

A comparison of methods to detect and quantify the markers of antimalarial drug resistance

We compare, contrast, and evaluate methods to quantify genetic markers of antimalarial drug resistance. Frequency estimates should be reported along with crude prevalence. There are four main potential methods to estimate frequencies in blood samples: simple counting of single nucleotide polymorphisms (SNPs) and haplotypes in samples with multiplicity of infection (MOI) = 1; SNP counting in samples with MOI < or = 2; SNP and haplotypes counting in samples with unambiguous genotypes; statistical inference using SNP and MOI data from all samples. Large differences between the methods became apparent when analyzing field data with high MOI. Simple counting dramatically reduced sample size and estimate precision, and we show that analysis of unambiguous samples is biased, leaving maximum likelihood or similar statistical inference as the only practical option. It is essential to account for genotyping missing minor clones; ignoring this phenomenon resulted in a 2-fold underestimation of SNPs and haplotypes present at low frequencies.

Hyperparasitaemia and low dosing are an important source of anti-malarial drug resistance

Background

Preventing the emergence of anti-malarial drug resistance is critical for the success of current malaria elimination efforts. Prevention strategies have focused predominantly on qualitative factors, such as choice of drugs, use of combinations and deployment of multiple first-line treatments. The importance of anti-malarial treatment dosing has been underappreciated. Treatment recommendations are often for the lowest doses that produce "satisfactory" results.

Methods

The probability of de-novo resistant malaria parasites surviving and transmitting depends on the relationship between their degree of resistance and the blood concentration profiles of the anti-malarial drug to which they are exposed. The conditions required for the in-vivo selection of de-novo emergent resistant malaria parasites were examined and relative probabilities assessed.

Results

Recrudescence is essential for the transmission of de-novo resistance. For rapidly eliminated anti-malarials high-grade resistance can arise from a single drug exposure, but low-grade resistance can arise only from repeated inadequate treatments. Resistance to artemisinins is, therefore, unlikely to emerge with single drug exposures. Hyperparasitaemic patients are an important source of de-novo anti-malarial drug resistance. Their parasite populations are larger, their control of the infection insufficient, and their rates of recrudescence following anti-malarial treatment are high. As use of substandard drugs, poor adherence, unusual pharmacokinetics, and inadequate immune responses are host characteristics, likely to pertain to each recurrence of infection, a small subgroup of patients provides the particular circumstances conducive to de-novo resistance selection and transmission.

Conclusion

Current dosing recommendations provide a resistance selection opportunity in those patients with low drug levels and high parasite burdens (often children or pregnant women). Patients with hyperparasitaemia who receive outpatient treatments provide the greatest risk of selecting de-novo resistant parasites. This emphasizes the importance of ensuring that only quality-assured anti-malarial combinations are used, that treatment doses are optimized on the basis of pharmacodynamic and pharmacokinetic assessments in the target populations, and that patients with heavy parasite burdens are identified and receive sufficient treatment to prevent recrudescence.

Current status of malaria chemotherapy and the role of pharmacology in antimalarial drug research and development

Antimalarial drugs have played a mainstream role in controlling the spread of malaria through the treatment of patients infected with the plasmodial parasites and controlling its transmissibility. The inadequate armory of drugs in widespread use for the treatment of malaria, development of strains resistant to currently used antimalarials, and the lack of affordable new drugs are the limiting factors in the fight against malaria. In addition, other problems with some existing agents include unfavorable pharmacokinetic properties and adverse effects/toxicity. These factors underscore the continuing need of research for new classes of antimalarial agents, and a re-examination of the existing antimalarial drugs that may be effective against resistant strains. In recent years, major advances have been made in the pharmacology of several antimalarial drugs both in pharmacokinetics and pharmacodynamics aspects. These include the design, development, and optimization of appropriate dosage regimens of antimalarials, basic knowledge in metabolic pathways of key antimalarials, as well as the elucidation of mechanisms of action and resistance of antimalarials. Pharmacologists have been working in close collaboration with scientists in other disciplines of science/biomedical sciences for more understanding on the biology of the parasite, host, in order to exploit rational design of drugs. Multiple general approaches to the identification of new antimalarials are being pursued at this time. All should be implemented in parallel with focus on the rational development of new agents directed against newly identified parasite targets. With major advances in our understanding of malaria parasite biology coupled with the completion of the malaria genome, has presented exciting opportunities for target-based antimalarial drug discovery.

Resistance to antimalarial drugs: molecular, pharmacologic, and clinical considerations

One of the greatest obstacles to the control of malaria has been the spread of resistance to drugs used on a large scale. This review provides an update of the current understanding of the molecular basis for antimalarial drug resistance. Parasite intrinsic resistance is just one component that determines the in vivo efficacy of a drug. Human immune responses and pharmacologic properties play important roles in determining the clinical outcome of treatment. The emergence and spread of resistance also results from an interplay of these factors. Current efforts to characterize and deter resistance to new combination therapy are also discussed.

Piperaquine pharmacodynamics and parasite viability in a murine malaria model

Piperaquine (PQ) is an important partner drug in antimalarial combination treatments, but the long half-life of PQ raises concerns about drug resistance. Our aim was to investigate the extended antimalarial effect of PQ in a study of drug efficacy, reinoculation outcomes, and parasite viability after the administration of a single dose of PQ in the murine malaria model. Initially, male Swiss mice were inoculated with Plasmodium berghei and at 64 h after parasite inoculation were given PQ phosphate at 90 mg/kg of body weight intraperitoneally. Parasite viability, drug efficacy, reinoculation responses, and parasite resistance were determined at 25, 40, 60, 90, and 130 days after drug administration. At each time point, six mice were reinoculated with 10(7) P. berghei parasites and blood was harvested from another four mice for viability passage into naïve mice (n = 5 for each blood sample) and from another two mice for determination of the plasma PQ concentration. The efficacy study demonstrated that the residual PQ concentrations did not suppress the infection after 25 days. Viable parasites were present up to 90 days after PQ dosing, although only 50% and 25% of the passaged parasites remained viable at 60 and 90 days postdosing, respectively. Viable parasites passaged into the naïve hosts were generally resistant to PQ when they were exposed to the drug for a second time. PQ was found to have a substantial antimalarial effect in this model, and the effect appears to be sufficient for a host immunological response to be established, resulting in the long-term survival of P. berghei-infected mice.

Plasmodium berghei ANKA: selection of resistance to piperaquine and lumefantrine in a mouse model

We have selected piperaquine (PQ) and lumefantrine (LM) resistant Plasmodium berghei ANKA parasite lines in mice by drug pressure. Effective doses that reduce parasitaemia by 90% (ED(90)) of PQ and LM against the parent line were 3.52 and 3.93 mg/kg, respectively. After drug pressure (more than 27 passages), the selected parasite lines had PQ and LM resistance indexes (I(90)) [ED(90) of resistant line/ED(90) of parent line] of 68.86 and 63.55, respectively. After growing them in the absence of drug for 10 passages and cryo-preserving them at -80 degrees C for at least 2 months, the resistance phenotypes remained stable. Cross-resistance studies showed that the PQ-resistant line was highly resistant to LM, while the LM-resistant line remained sensitive to PQ. Thus, if the mechanism of resistance is similar in P. berghei and Plasmodium falciparum, the use of LM (as part of Coartem) should not select for PQ resistance.

Probability of emergence of antimalarial resistance in different stages of the parasite life cycle

Understanding the evolution of drug resistance in malaria is a central area of study at the intersection of evolution and medicine. Antimalarial drug resistance is a major threat to malaria control and directly related to trends in malaria attributable mortality. Artemisinin combination therapies (ACT) are now recommended worldwide as first line treatment for uncomplicated malaria, and losing them to resistance would be a disaster for malaria control. Understanding the emergence and spread of antimalarial drug resistance in the context of different scenarios of antimalarial drug use is essential for the development of strategies protecting ACTs. In this study, we review the basic mechanisms of resistance emergence and describe several simple equations that can be used to estimate the probabilities of de novo resistance mutations at three stages of the parasite life cycle: sporozoite, hepatic merozoite and asexual blood stages; we discuss the factors that affect parasite survival in a single host in the context of different levels of antimalarial drug use, immunity and parasitaemia. We show that in the absence of drug effects, and despite very different parasite numbers, the probability of resistance emerging at each stage is very low and similar in all stages (for example per-infection probability of 10⁻¹⁰–10⁻⁹ if the per-parasite chance of mutation is 10⁻¹⁰ per asexual division). However, under the selective pressure provided by antimalarial treatment and particularly in the presence of hyperparasitaemia, the probability of resistance emerging in the blood stage of the parasite can be approximately five orders of magnitude higher than in the absence of drugs. Detailed models built upon these basic methods should allow us to assess the relative probabilities of resistance emergence in the different phases of the parasite life cycle.

Does the drug sensitivity of malaria parasites depend on their virulence?

Background

Chemotherapy can prompt the evolution of classical drug resistance, but selection can also favour other parasite traits that confer a survival advantage in the presence of drugs. The experiments reported here test the hypothesis that sub-optimal drug treatment of malaria parasites might generate survival and transmission advantages for virulent parasites.

Methods

Two Plasmodium chabaudi lines, one derived from the other by serial passage, were used to establish avirulent and virulent infections in mice. After five days, infections were treated with various doses of pyrimethamine administered over 1 or 4 days. Virulence measures (weight and anaemia), parasite and gametocyte dynamics were followed until day 21.

Results

All treatment regimes reduced parasite and gametocyte densities, but infections with the virulent line always produced more parasites and more gametocytes than infections with the avirulent line. Consistent with our hypothesis, drug treatment was disproportionately effective against the less virulent parasites. Treatment did not affect the relative transmission advantage of the virulent line. Neither of the lines contained known mutations conferring classical drug resistance.

Conclusion

Drug-sensitivity of malaria parasites can be virulence-dependent, with virulent parasites more likely to survive sub-optimal treatment. If this proves to be general for a variety of drugs and parasite species, selection imposed by sub-optimal drug treatment could result in the evolution of more aggressive malaria parasites.

Characteristics of Plasmodium falciparum dhfr haplotypes that confer pyrimethamine resistance, Kilifi, Kenya, 1987--2006

Resistance to the antimalarial drug sulfadoxine-pyrimethamine (SP) emerged in Plasmodium falciparum from Asia in the 1960s and subsequently spread to Africa. It is not known whether alleles that confer SP resistance also arose independently in Africa. We defined the coding region and microsatellite haplotypes of dhfr alleles in P. falciparum collected in Kilifi, Kenya, during 1987--2006, which spans the period when SP was first introduced. Isolates that carried a double-mutant or triple-mutant dhfr allele were detected at a low frequency, even during 1987--1988. Each of 2 double mutants carried a unique haplotype, and both were related to wild-type haplotypes from the same population. The number of isolates that carried a triple-mutant dhfr allele increased rapidly after introduction of SP and shared the haplotype of the triple mutant derived form Asia. We observed no triple-mutant alleles with haplotypes related to those of the Africa-derived wild-type and double-mutant alleles.

Pharmacokinetic determinants of the window of selection for antimalarial drug resistance

The selection and spread of antimalarial drug resistance pose enormous challenges to the health of people living in tropical countries. Most antimalarial drugs are slowly eliminated and so, following treatment in areas of endemicity, provide a gradient of concentrations to which newly acquired parasites are exposed. There is a variable period during which a new blood-stage infection with resistant malaria parasites can emerge from the liver and subsequently produce gametocyte densities sufficient for transmission while reinfection by sensitive parasites is still suppressed. This "window of selection" drives the spread of resistance. We have examined the factors which determine the duration of this window and, thus, the resistance selection pressure. The duration ranges from zero to several months and is dependent on the degree of parasite resistance, the slope of the concentration-effect relationship, and the elimination kinetics of the antimalarial drug. The time at which the window opens and the duration of opening are both linear functions of the terminal elimination half-life. Because of competition from sibling susceptible parasites, the greater risks of extinction with low starting numbers, and opening of the window only when blood concentrations have fallen below the MIC, the window of selection for de novo resistance is narrower than that for resistance acquired elsewhere. The windows were examined for the currently available antimalarials. Drugs with elimination half-lives of less than 1 day, such as the artemisinins and quinine, do not select for resistance during the elimination phase.

Sulfadoxine-pyrimethamine-based combinations for malaria: a randomised blinded trial to compare efficacy, safety and selection of resistance in Malawi

Background

In Malawi, there has been a return of Plasmodium falciparum sensitivity to chloroquine (CQ) since sulfadoxine-pyrimethamine (SP) replaced CQ as first line treatment for uncomplicated malaria. When used for prophylaxis, Amodiaquine (AQ) was associated with agranulocytosis but is considered safe for treatment and is increasingly being used in Africa. Here we compare the efficacy, safety and selection of resistance using SP or CQ+SP or artesunate (ART)+SP or AQ+SP for the treatment of uncomplicated falciparum malaria.

Methodology and Findings

455 children aged 1–5 years were recruited into a double-blinded randomised trial comparing SP to the three combination therapies. Using intention to treat analysis with missing outcomes treated as successes, and without adjustment to distinguish recrudescence from new infections, the day 28 adequate clinical and parasitological response (ACPR) rate for SP was 25%, inferior to each of the three combination therapies (p<0.001). AQ+SP had an ACPR rate of 97%, higher than CQ+SP (81%) and ART+SP (70%), p<0.001. Nineteen children developed a neutropenia of ≤0.5×10³ cells/µl by day 14, more commonly after AQ+SP (p = 0.03). The mutation pfcrt 76T, associated with CQ resistance, was detected in none of the pre-treatment or post-treatment parasites. The prevalence of the pfmdr1 86Y mutation was higher after treatment with AQ+SP than after SP, p = 0.002.

Conclusions

The combination AQ+SP was highly efficacious, despite the low efficacy of SP alone; however, we found evidence that AQ may exert selective pressure for resistance associated mutations many weeks after treatment. This study confirms the return of CQ sensitivity in Malawi and importantly, shows no evidence of the re-emergence of pfcrt 76T after treatment with CQ or AQ. Given the safety record of AQ when used as a prophylaxis, our observations of marked falls in neutrophil counts in the AQ+SP group requires further scrutiny.

Trial Registration

Controlled-Trials.com ISRCTN22075368

How antimalarial drug resistance affects post-treatment prophylaxis

Slowly eliminated antimalarial drugs suppress malaria reinfections for a period of time determined by the dose, the pharmacokinetic properties of the drug, and the susceptibility of the infecting parasites. This effect is called post-treatment prophylaxis (PTP). The clinical benefits of preventing recrudescence (reflecting treatment efficacy) compared with preventing reinfection (reflecting PTP) need further assessment. Antimalarial drug resistance shortens PTP. While blood concentrations are in the terminal elimination phase, the degree of shortening may be estimated from measurements of in-vitro susceptibility and the terminal elimination half-life. More information is needed on PTP following intermittent preventive treatments, and on the relationship between the duration of PTP and immunity, so that policy recommendations can have a firmer evidence base.

Antimalarial drug quality in Africa

BACKGROUND AND OBJECTIVE

There are several reports of sub-standard and counterfeit antimalarial drugs circulating in the markets of developing countries; we aimed to review the literature for the African continent.

METHODS

A search was conducted in PubMed in English using the medical subject headings (MeSH) terms: 'Antimalarials/analysis'[MeSH] OR 'Antimalarials/standards'[MeSH] AND 'Africa'[MeSH]' to include articles published up to and including 26 February 2007. Data were augmented with reports on the quality of antimalarial drugs in Africa obtained from colleagues in the World Health Organization. We summarized the data under the following themes: content and dissolution; relative bioavailability of antimalarial products; antimalarial stability and shelf life; general tests on pharmaceutical dosage forms; and the presence of degradation or unidentifiable impurities in formulations.

RESULTS AND DISCUSSION

The search yielded 21 relevant peer-reviewed articles and three reports on the quality of antimalarial drugs in Africa. The literature was varied in the quality and breadth of data presented, with most bioavailability studies poorly designed and executed. The review highlights the common finding in drug quality studies that (i) most antimalarial products pass the basic tests for pharmaceutical dosage forms, such as the uniformity of weight for tablets, (ii) most antimalarial drugs pass the content test and (iii) in vitro product dissolution is the main problem area where most drugs fail to meet required pharmacopoeial specifications, especially with regard to sulfadoxine-pyrimethamine products. In addition, there are worryingly high quality failure rates for artemisinin monotherapies such as dihydroartemisinin (DHA); for instance all five DHA sampled products in one study in Nairobi, Kenya, were reported to have failed the requisite tests.

CONCLUSIONS

There is an urgent need to strengthen pharmaceutical management systems such as post-marketing surveillance and the broader health systems in Africa to ensure populations in the continent have access to antimalarial drugs that are safe, of the highest quality standards and that retain their integrity throughout the distribution chain through adequate enforcement of existing legislation and enactment of new ones if necessary, and provision of the necessary resources for drug quality assurance.