Chlorproguanil-dapsone: effective treatment for uncomplicated falciparum malaria

Development, Pharmacokinetics and Antimalarial Evaluation of Dose Flexible 3D Printlets of Dapsone for Pediatric Patients

The focus of current studies was to fabricate dose flexible printlets of dapsone (DDS) for pediatric patients by selective laser sintering (SLS) 3D printing method, and evaluate its physicochemical, patient in-use stability, and pharmacokinetic attributes. Eight formulations were fabricated using Kollicoat® IR, Eudragit® L-100-55 and StarCap®as excipients and evaluated for hardness, disintegration, dissolution, amorphous phase by differential scanning calorimetry and X-ray powder diffraction, in-use stability at 30 oC/75% RH for a month, and pharmacokinetic study in Sprague Dawley rats. The hardness, and disintegration of the printlets varied from 2.6±1.0 (F4) to 7.7±0.9 (F3) N and 2.0±0.4 (F2) to 7.6±0.6 (F3) sec, respectively. The drug was partially present as an amorphous form in the printlets. The drug was completely (>85%) dissolved in 20 min. No change in drug form or dissolution extent was observed after storage at in use condition. Pharmacokinetic profiles of both formulations (tablets and printlets) were almost superimposable with no statistical difference in pharmacokinetic parameters (Tmax, Cmax, and AUC0-¥)between formulations (p>0.05). Values of EC50 (half maximal effective concentration) and EC90 (maximal concentration inducing 90% maximal response) were 0.50±0.15 and 1.32±0.26 mM, 0.41±0.06 and 1.11±0.21, and 0.42±0.13 and 1.36±0.19 mM for DDS, printlet and tablet formulations, respectively, and differences were statistically insignificant (p>0.05). In conclusion, tablet and printlet formulations are expected to be clinical similar, thus clinically interchangeable.

Seabra PS, Fonseca ADN, de Azevedo Sousa KM, de Oliveira SBB, Carneiro ADS, Coleman MD, Monteiro MC. Alpha-Lipoic Acid and Its Enantiomers Prevent Methemoglobin Formation and DNA Damage Induced by Dapsone Hydroxylamine: Molecular Mechanism and Antioxidant Action

Dapsone (DDS) therapy can frequently lead to hematological side effects, such as methemoglobinemia and DNA damage. In this study, we aim to evaluate the protective effect of racemic alpha lipoic acid (ALA) and its enantiomers on methemoglobin induction. The pre- and post-treatment of erythrocytes with ALA, ALA isomers, or MB (methylene blue), and treatment with DDS-NOH (apsone hydroxylamine) was performed to assess the protective and inhibiting effect on methemoglobin (MetHb) formation. Methemoglobin percentage and DNA damage caused by dapsone and its metabolites were also determined by the comet assay. We also evaluated oxidative parameters such as SOD, GSH, TEAC (Trolox equivalent antioxidant capacity) and MDA (malondialdehyde). In pretreatment, ALA showed the best protector effect in 2.5 µg/mL of DDS-NOH. ALA (1000 µM) was able to inhibit the induced MetHb formation even at the highest concentrations of DDS-NOH. All ALA tested concentrations (100 and 1000 µM) were able to inhibit ROS and CAT activity, and induced increases in GSH production. ALA also showed an effect on DNA damage induced by DDS-NOH (2.5 µg/mL). Both isomers were able to inhibit MetHb formation and the S-ALA was able to elevate GSH levels by stimulating the production of this antioxidant. In post-treatment with the R-ALA, this enantiomer inhibited MetHb formation and increased GSH levels. The pretreatment with R-ALA or S-ALA prevented the increase in SOD and decrease in TEAC, while R-ALA decreased the levels of MDA; and this pretreatment with R-ALA or S-ALA showed the effect of ALA enantiomers on DNA damage. These data show that ALA can be used in future therapies in patients who use dapsone chronically, including leprosy patients.

Exploring anti-malarial potential of FDA approved drugs: an in silico approach

BACKGROUND

The critically important issue on emergence of drug-resistant malarial parasites is compounded by cross resistance, where resistance to one drug confers resistance to other chemically similar drugs or those that share mode of action. This aspect requires discovery of new anti-malarial compounds or formulation of new combination therapy. The current study attempts to contribute towards accelerating anti-malarial drug development efforts, by exploring the potential of existing FDA-approved drugs to target proteins of Plasmodium falciparum.

METHODS

Using comparative sequence and structure analyses, FDA-approved drugs, originally developed against other pathogens, were identified as potential repurpose-able candidates against P. falciparum. The rationale behind the undertaken approach is the likeliness of small molecules to bind to homologous targets. Such a study of evolutionary relationships between established targets and P. falciparum proteins aided in identification of approved drug candidates that can be explored for their anti-malarial potential.

RESULTS

Seventy-one FDA-approved drugs were identified that could be repurposed against P. falciparum. A total of 89 potential targets were recognized, of which about 70 are known to participate in parasite housekeeping machinery, protein biosynthesis, metabolic pathways and cell growth and differentiation, which can be prioritized for chemotherapeutic interventions. An additional aspect of prioritization of predicted repurpose-able drugs has been explored on the basis of ability of the drugs to permeate cell membranes, i.e., lipophilicity, since the parasite resides within a parasitophorous vacuole, within the erythrocyte, during the blood stages of infection. Based on this consideration, 46 of 71 FDA-approved drugs have been identified as feasible repurpose-able candidates against P. falciparum, and form a first-line for laboratory investigations. At least five of the drugs identified in the current analysis correspond to existing antibacterial agents already under use as repurposed anti-malarial agents.

CONCLUSIONS

The drug-target associations predicted, primarily by taking advantage of evolutionary information, provide a valuable resource of attractive and feasible candidate drugs that can be readily taken through further stages of anti-malarial drug development pipeline.

Role of Plasmodium vivax Dihydropteroate Synthase Polymorphisms in Sulfa Drug Resistance

Dihydropteroate synthase (DHPS) is a known sulfa drug target in malaria treatment, existing as a bifunctional enzyme together with hydroxymethyldihydropterin pyrophosphokinase (HPPK). Polymorphisms in key residues of Plasmodium falciparum DHPS (PfDHPS) have been characterized and linked to sulfa drug resistance in malaria. Genetic sequencing of P. vivax dhps (Pvdhps) from clinical isolates has shown several polymorphisms at the positions equivalent to those in the Pfdhps genes conferring sulfa drug resistance, suggesting a mechanism for sulfa drug resistance in P. vivax similar to that seen in P. falciparum To characterize the role of polymorphisms in the PvDHPS in sulfa drug resistance, various mutants of recombinant PvHPPK-DHPS enzymes were expressed and characterized. Moreover, due to the lack of a continuous in vitro culture system for P. vivax parasites, a surrogate P. berghei model expressing Pvhppk-dhps genes was established to demonstrate the relationship between sequence polymorphisms and sulfa drug susceptibility and to test the activities of PvDHPS inhibitors on the transgenic parasites. Both enzyme activity and transgenic parasite growth were sensitive to sulfadoxine to different degrees, depending on the number of mutations that accumulated in DHPS. Ki values and 50% effective doses were higher for mutant PvDHPS enzymes than the wild-type enzymes. Altogether, the study provides the first evidence of sulfa drug resistance at the molecular level in P. vivax Furthermore, the enzyme inhibition assay and the in vivo screening system can be useful tools for screening new compounds for their activities against PvDHPS.

Factors influencing adverse events reporting within the health care system: the case of artemisinin-based combination treatments in northern Ghana

Background

The use of artemisinin-based combination therapy (ACT) as first-line treatment for uncomplicated malaria was a policy recommended by World Health Organization. In 2004, Ghana changed her first-line anti-malarial drug policy to use ACT. This study examined factors affecting adverse events reporting in northern Ghana after the introduction of ACT.

Methods

This was a qualitative study based on sixty in-depth interviews with health workers, chemical shop owners and patients with malaria who were given ACT at the health facilities. Purposive sampling method was used to select study participants. The interviews were transcribed, coded into themes using Nvivo 9 software. The thematic analysis framework was used to analyse the data.

Results

Study respondents reported body weakness and dizziness as the most frequent side effects they had experienced from the used of ACT. Other side effects they reported were swollen testes, abdominal pain and shivering. These side effects were mostly associated with the use of artesunate-amodiaquine compared to other artemisinin-based combinations. Patients were not provided information about the side effects of the drugs and so did not report when they experienced them. Also long queues at health facilities and unfriendly health worker attitude were the main factors affecting adverse events reporting. Other factors such as wrong use of ACT at home, farming and commercial activities also affected effective adverse events reporting in the study area.

Conclusion

Patients’ lack of knowledge and health sector drawbacks affected side effect reporting on ACT. Intensive health education on likely side effects of ACT should be provided to patients by health workers. Also, improving health worker attitude toward clients will encourage patients to visit the health facilities when they react negatively to ACT and, subsequently, will improve on adverse events reporting.

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 (PfPR2-10) 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 PfPR2-10 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.

Adherence and uptake of artemisinin-based combination treatments for uncomplicated malaria: a qualitative study in northern Ghana

Background

Based on the recommendations of the World Health Organization in 2004, Ghana changed her antimalarial drug policy from mono-therapy to Artemisinin-based Combination Therapy (ACTs). The country is currently using three first line drugs artesunate-amodiaquine, artemether-lumefantrine and dihydroartemisinin-piperaquine for the treatment of uncomplicated malaria. Despite this policy, little or no qualitative studies have been conducted to establish the factors influencing adherence to the new treatment for malaria. This study explored factors influencing adherence to the use of ACTs in northern Ghana.

Methods

This was a qualitative study comprising forty (40) in-depth interviews with patients with malaria who visited selected public and private health facilities and received ACTs. Systematic sampling technique was used to select participants who were given ACTs for the interviews. Nvivo 9 software was used to code the data into themes for further analysis.

Results

The study revealed very important differences in knowledge about ACTs. As expected, the less or illiterates could not mention the type of ACT they would prefer to use for treating their malaria. The educated ones had a good knowledge on ACTs and preferred artemether-lumefantrinee in treating their malaria. The reason was that the drug was good and it had minimal or no side effects. Individual attitudes toward the use of medications and the side effects associated with the use of these ACTs were found to be the main factors affecting adherence to the use of ACTs. Perceived cure of illness after the initial dose greatly affected adherence. Other factors such as forgetfulness and lack of information also influenced patient adherence to ACTs use.

Conclusion

Individual knowledge, attitudes and behaviors greatly influence patients’ adherence to ACTs use. Since ACTs take a number of days to complete, continuous education by health professionals could improve on adherence to ACTs use by patients with malaria.

The chemotherapy of rodent malaria. LXIII. Drug combinations to impede the selection of drug resistance, part 6: the potential value of chlorproguanil and dapsone in combination, and with the addition of artesunate

Resistance is readily produced in rodent malaria using the single-dose, '2%-relapse technique' (2%RT) against the individual compounds chlorproguanil (CPG), chlorcycloguanil (CCG), cycloguanil, dapsone (DDS) and artesunate (ASN). Using the '4-day test', a low level of synergism or a simple additional action between CPG and DDS was observed with multiple dosing of these two compounds in a combination. Resistance to a 1 : 3 combination of CPG-DDS was selected in each of three parasite lines: Plasmodium berghei NK65, P. yoelii ssp. NS and P. chabaudi AS. Of these lines, P. chabaudi AS was found to be the most sensitive to the 1 : 3 combination in the 2%RT (and was also previously found to be the most sensitive when the compounds were used individually). Plasmodium chabaudi AS was also the line found most sensitive to a 7 : 21 : 300 combination of CPG-DDS-ASN (CDA). In mice infected with P. chabaudi AS, compared with the use of the individual components, the CPG-DDS combination only a gave a modest level of protection (as indicated by the increase in the time required to select resistance in the 2%RT) but the triple CDA combination was totally effective over the duration of the experiment. New pharmacokinetic data to be reported elsewhere indicate, however, that the antimalarial action of CPG in mice is exerted by a mechanism that is not associated with the drug's conversion to the antifolate triazine, CCG. The question thus arises as to how, in the present model, the protective action of CDA was effected. The present results nevertheless reinforce the hypothesis that a CDA combination, appropriately proportioned for human use, should be of practical value, in protecting the individual components, when used for the treatment of multidrug-resistant P. falciparum, and possibly other Plasmodium species, in endemic areas. Clinical trials, both with a CPG-DDS combination (Lapdap) and CDA, are currently under way in tropical Africa. Further studies are now required to determine whether DDS, CPG or an as-yet unidentified metabolite of CPG interact with ASN, and whether a simple double combination of ASN with one or other of these would be as protective, against the selection of resistance, as CDA.

Randomized trial of safety and effectiveness of chlorproguanil-dapsone and lumefantrine-artemether for uncomplicated malaria in children in the Gambia

Background

Chlorproguanil-dapsone (Lapdap), developed as a low-cost antimalarial, was withdrawn in 2008 after concerns about safety in G6PD deficient patients. This trial was conducted in 2004 to evaluate the safety and effectiveness of CD and comparison with artemether-lumefantrine (AL) under conditions of routine use in G6PD normal and G6PD deficient patients with uncomplicated malaria in The Gambia. We also examined the effects of a common genetic variant that affects chlorproguanil metabolism on risk of treatment failure.

Methods

1238 children aged 6 months to 10 years with uncomplicated malaria were randomized to receive CD or artemether-lumefantrine (AL) and followed for 28 days. The first dose was supervised, subsequent doses given unsupervised at home. G6PD genotype was determined to assess the interaction between treatment and G6PD status in their effects on anaemia. The main endpoints were clinical treatment failure by day 28, incidence of severe anaemia (Hb<5 g/dL), and haemoglobin concentration on day 3.

Findings

One third of patients treated with AL, and 6% of patients treated with CD, did not complete their course of medication. 18% (109/595) of children treated with CD and 6.1% (36/587) with AL required rescue medication within 4 weeks, risk difference 12% (95%CI 8.9%–16%). 23 children developed severe anaemia (17 (2.9%) treated with CD and 6 (1.0%) with AL, risk difference 1.8%, 95%CI 0.3%–3.4%, P = 0.02). Haemoglobin concentration on day 3 was lower among children treated with CD than AL (difference 0.43 g/dL, 95% CI 0.24 to 0.62), and within the CD group was lower among those children who had higher parasite density at enrolment. Only 17 out of 1069 children who were typed were G6PD A- deficient, of these 2/9 treated with CD and 1/8 treated with AL developed severe anaemia. 5/9 treated with CD had a fall of 2 g/dL or more in haemoglobin concentration by day 3.

Interpretation

AL was well tolerated and highly effective and when given under operational conditions despite poor adherence to the six-dose regimen. There were more cases of severe malaria and anaemia after CD treatment although G6PD deficiency was uncommon.

Trial Registration

Clinicaltrials.gov NCT00118794

Chloroquine resistance before and after its withdrawal in Kenya

BACKGROUND

The spread of resistance to chloroquine (CQ) led to its withdrawal from use in most countries in sub-Saharan Africa in the 1990s. In Malawi, this withdrawal was followed by a rapid reduction in the frequency of resistance to the point where the drug is now considered to be effective once again, just nine years after its withdrawal. In this report, the polymorphisms of markers associated with CQ-resistance against Plasmodium falciparum isolates from coastal Kenya (Kilifi) were investigated, from 1993, prior to the withdrawal of CQ, to 2006, seven years after its withdrawal. Changes to those that occurred in the dihydrofolate reductase gene (dhfr) that confers resistance to the replacement drug, pyrimethamine/sulphadoxine were also compared.

METHODS

Mutations associated with CQ resistance, at codons 76 of pfcrt, at 86 of pfmdr1, and at codons 51, 59 and 164 of dhfr were analysed using PCR-restriction enzyme methods. In total, 406, 240 and 323 isolates were genotyped for pfcrt-76, pfmdr1-86 and dhfr, respectively.

RESULTS

From 1993 to 2006, the frequency of the pfcrt-76 mutant significantly decreased from around 95% to 60%, while the frequency of pfmdr1-86 did not decline, remaining around 75%. Though the frequency of dhfr mutants was already high (around 80%) at the start of the study, this frequency increased to above 95% during the study period. Mutation at codon 164 of dhfr was analysed in 2006 samples, and none of them had this mutation.

CONCLUSION

In accord with the study in Malawi, a reduction in resistance to CQ following official withdrawal in 1999 was found, but unlike Malawi, the decline of resistance to CQ in Kilifi was much slower. It is estimated that, at current rates of decline, it will take 13 more years for the clinical efficacy of CQ to be restored in Kilifi. In addition, CQ resistance was declining before the drug's official withdrawal, suggesting that, prior to the official ban, the use of CQ had decreased, probably due to its poor clinical effectiveness.

Plasmodium berghei: efficacy of 5-fluoroorotate in combination with commonly used antimalarial drugs in a mouse model

Resistance to antimalarial antifolates necessitates a search for new antimetabolites targeting other enzymes of the folate metabolic pathway. In this study, 5-fluoroorotate (FOA), reported to be an inhibitor of thymidylate synthase, was assayed against Plasmodium berghei NK 65 in mice, with(out) an oral uridine supplement. FOA (2.5 and 5.0 mg/kg bw.) was tested alone, or in a double and triple combination with a fixed oral dose of 1.25 and 2.5 mg/kg of pyrimethamine (PYR); 1.0 and 2.0 mg/kg of dapsone (DAP); 1.0 and 2.0 mg/kg of artesunate (ART). FOA achieved high suppression which ranged from 95.7% to aparasitaemic, activity that was dose-dependent. At the highest dosages used, FOA-PYR and FOA-DAP-ART combinations were synergistic with 100% cure rate, while FOA-PYR-ART was antagonistic. Drugs in a synergistic combination may exert less resistance selection pressure, thus FOA-PYR and FOA-DAP-ART warrant further evaluation with an ultimate object of possible clinical use against drug-resistant malaria.

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.

Efficacy of amodiaquine/artesunate combination therapy for uncomplicated malaria in children under five years in ghana

BACKGROUND

In 2005, following several years of declining efficacy of chloroquine, the Ministry of Health recommended the use of Amodiaquine/Artesunate combination therapy for the treatment of uncomplicated malaria. A system of continuous monitoring of therapeutic responses has been established in 10 district hospitals across the country. The data gathered will enable National Malaria Control Programme (NMCP) to respond to changes in the efficacy of the new treatment in a timely manner.

OBJECTIVES

To determine the 28 day therapeutic efficacy of Amodiaquine/Artesunate (AQ/AS) combination treatment in children with uncomplicated malaria in Ghana.

METHODS

Children aged 6 - 59 months attending clinic with signs/symptoms of uncomplicated malaria at 9 district hospitals (3 in each of the 3 eco-epidemiological zones of the country) were eligible for enrolment. Enrolled children were followed up after treatment for a total of 28 days to record the clinical and parasitological resolution of their malaria episode as well as any adverse drug reactions.

RESULTS

Treatment resulted in rapid and complete cure in almost all the children; 99.3% 14 days after treatment and 93.0%, 28 days after treatment. The majority of treatment failures on D28 were seen in the 3 sites located in the forest zones (Sunyani, Bekwai and Begoro). There was no case of Early Treatment Failure at both D14 and D28 assessments. Adverse events (AE's) were minimal, less than 4%, with the most common complaint being vomiting.

CONCLUSION

AQ/AS combination for uncomplicated malaria is efficacious and safe in children less than 5 years.

Multiple synergistic interactions between atovaquone and antifolates against Plasmodium falciparum in vitro: a rational basis for combination therapy

The use of synergistic drug combinations for the treatment of drug-resistant malaria is a major strategy to slow the selection and spread of Plasmodium falciparum resistant strains. In order to investigate synergistic compounds, with different modes of action, as alternative candidates for combination therapy, we used standard in vitro P. falciparum cultures and an established synergy testing method to define interactions among dapsone (DDS), atovaquone (ATQ), chlorproguanil (CPG) and its triazine metabolite chlorcycloguanil (CCG). Strong synergy was observed in the combinations DDS/CCG and ATQ/CPG. Multiple combination of these drugs, DDS/CCG/CPG/ATQ also exhibited high synergy although not higher than that of either of the two drug combinations separately. The use of this triple combination DDS/CPG/ATQ, even without an increase in synergy over their double combinations, ATQ/CPG and DDS/CCG, would contribute towards slowing the selection pressure since these drugs act against different targets and would delay the selection of parasites resistant to the three drugs, extending the useful therapeutic life of these valuable compounds.

MOLECULAR MECHANISMS OF RESISTANCE IN ANTIMALARIAL CHEMOTHERAPY: The Unmet Challenge

▪ Abstract 

The enormous public health problem posed by malaria has been substantially worsened in recent years by the emergence and worldwide spread of drug-resistant parasites. The utility of two major therapies, chloroquine and the synergistic combination of pyrimethamine/sulfadoxine, is now seriously compromised. Although several genetic mechanisms have been described, the major source of drug resistance appears to be point mutations in protein target genes. Clinically significant resistance to these agents requires the accumulation of multiple mutations, which genetic studies of parasite populations suggest arise focally and sweep through the population. Efforts to circumvent resistance range from the use of combination therapy with existing agents to laboratory studies directed toward discovering novel targets and therapies.

The prevention and management of drug resistance are among the most important practical problems of tropical medicine and public health.

Leonard J. Bruce-Chwatt, 1972

Comparative efficacy of antimalarial drugs including ACTs in the treatment of uncomplicated malaria among children under 5 years in Ghana

The emergence and spread of Plasmodium falciparum resistance to commonly used antimalarials such as chloroquine and sulphadoxine/pyrimethamine poses major challenges to malaria control in sub-Saharan Africa. We undertook a study on the efficacy of some antimalarial drugs in 2003 with the view of supporting the National Malaria Control Programme in the review of the antimalarial drug treatment policy in Ghana. Children aged 6-59 months with signs/symptoms of uncomplicated malaria including axillary temperature > or =37.5 degrees C; mono infection with P. falciparum; and parent's willingness to give consent, were randomized into four treatment groups and followed up for a maximum of 28 days. The treatment groups were chloroquine (CHQ), sulphadoxine/pyrimethamine (SP), amodiaquine+artesunate (ADQ+ART) combination, and artemether+lumefantrine (Coartem) combination. Clinical evaluation of 168 children studied showed that cumulative pcr-corrected cure rates on day 28 were 100% for ADQ+ART; 97.5% for coartem, 60% for SP and 25% for CHQ. The artemisinin-based combinations effected rapid fever and parasite clearance. Prevalence of gametocytaemia was highest in the SP group whilst the CHQ group did not show any significant changes in haemoglobin levels during the follow-up period. The findings are in agreement with current recommendations for using artemisinin-based combinations for treating uncomplicated malaria in areas of high CHQ failure such as Ghana.

Why has the dihydrofolate reductase 164 mutation not consistently been found in Africa yet?

Resistance to the antifolate sulfadoxine-pyrimethamine (SP), the current mass-treatment antimalarial drug, is associated with selection of point mutations in dihydrofolate reductase and dihydropteroate synthase. Among these mutations, the leucine 164 dihydrofolate reductase mutation (Leu-164) is associated with higher levels of SP resistance; this mutation is also associated with a decrease in the efficacy of chlorproguanil/dapsone, a newly developed antifolate antimalarial drug. Leu-164 has been detected in Southeast Asia and South America, regions where SP is no longer effective. Surprisingly, this mutation has not yet been detected in Africa, using the standard protocol based on PCR-RFLP, despite high SP resistance. In this paper, we discuss briefly the reasons why Leu-164 has not yet been selected in Africa and we propose a means that may slow down the selection of this mutation.

Clinical trials of chemotherapy for falciparum malaria

Plasmodium falciparum remains one of the World's most prevalent and devastating pathogens. Mainly for economic reasons, the parasite's ability to develop resistance to drugs has not been matched by the rate at which new compounds are developed. Even so, there are new drugs (or new combinations of old drugs) currently under investigation, or in the process of development (at the moment): Pyronaridine, a well-tolerated, synthetic drug that may have utility for multi-resistant falciparum malaria in many parts of the world; however,problems remain over the formulation of this drug (which is a major determinant of its bioavailability) and its eventual cost. Chlorproguanil-dapsone (lap dap) is being studied as a possible low-cost'successor' to pyrimethamine-sulfadoxine; the utility of chlorproguanil-dapsone as 'salvage' therapy for clinical cases of pyrimethamine-sulfadoxine failure has yet to be tested in clinical trials. Atovaquone-proguanil (malarone) has utility against multi-resistant parasites; however, it is likely to be expensive (but is currently being provided free-of-charge in certain areas of Africa). Artemether-benflumetol (coartemether) combines the advantages of artemether (a rapid reduction in parasite load) with a second drug that reduces the risk of recrudescence; the cost of this combination is unclear. Rectal artesunate is being studied as an intervention to reduce the proportion of children with falciparum malaria who deteriorate to severe disease; the formulation is appropriate for use in rural health centres.

Novel alleles of Plasmodium falciparum dhfr that confer resistance to chlorcycloguanil

In Plasmodium falciparum, resistance to folate inhibitors like pyrimethamine is mediated by point mutations in the target gene dihydrofolate reductase (dhfr). The resistance to pyrimethamine increases with the accumulation of particular point mutations. These mutations also confer increased resistance to chlorcycloguanil, the active metabolite of chlorproguanil and one component of a newly introduced DHFR inhibitor, LapDap. One genotype (16V/108T) has been previously identified that confers resistance to cycloguanil but not to pyrimethamine. This study was designed to identify novel alleles that might confer resistance to chlorcycloguanil, but escape the surveillance methods currently in place for common pyrimethamine-resistant alleles. Directed mutagenesis was performed using the wild type and the common pyrimethamine-resistant allele, 51I/59R/108N, to determine the effect of the 16V and 108T mutations on enzyme activity and drug resistance. In addition, we randomly mutagenized the 51I/59R/108N allele and identified nine novel alleles that could confer resistance to chlorcycloguanil. These yeast strains were also resistant to pyrimethamine, but retained sensitivity to the experimental DHFR inhibitor, WR99210. None of the alleles generated in this study was as resistant to chlorcycloguanil as the common quadruple mutant, 51I/59R/108N/164L. In addition, selection of high levels of chlorcycloguanil resistance in parasites that carry the 51I/59R/108N allele will require two directed steps, a change from 108N to 108T followed by a mutation from A16 to 16V. The resulting allele, 16V/51I/59R/108T is highly resistant to chlorcycloguanil, but 200-fold more sensitive to pyrimethamine than the 51I/59R/108N allele.

Chlorproguanil-dapsone for treating uncomplicated malaria

BACKGROUND

In Africa, malaria is often resistant to chloroquine and sulfadoxine-pyrimethamine. Chlorproguanil-dapsone is a potential alternative.

OBJECTIVES

To compare chlorproguanil-dapsone with other antimalarial drugs for treating uncomplicated falciparum malaria.

SEARCH STRATEGY

We searched the Cochrane Infectious Diseases Group Specialized Register (May 2004), CENTRAL (The Cochrane Library Issue 2, 2004), MEDLINE (1966 to May 2004), EMBASE (1988 to May 2004), LILACS (May 2004), Biosis Previews (1985 to May 2004), conference proceedings, and reference lists, and contacted researchers working in this field.

SELECTION CRITERIA

Randomized and quasi-randomized controlled trials comparing chlorproguanil-dapsone to other antimalarial drugs.

DATA COLLECTION AND ANALYSIS

Two reviewers independently applied the inclusion criteria, extracted data, and assessed methodological quality. We calculated the relative risk (RR) for dichotomous data and weighted mean difference for continuous data, and presented them with 95% confidence intervals (CI).

MAIN RESULTS

Six trials (n = 3352) met the inclusion criteria. Chlorproguanil-dapsone (with 1.2 mg chlorproguanil) as a single dose had fewer treatment failures than chloroquine (1 trial), but more treatment failures and people with parasitaemia at day 28 than sulfadoxine-pyrimethamine (3 trials). Two trials compared the three-dose chlorproguanil-dapsone (with 2 mg chlorproguanil) regimen with sulfadoxine-pyrimethamine in new attendees. There were fewer treatment failures with chlorproguanil-dapsone by day 7 (RR 0.30, CI 0.19 to 0.49; n = 827, 1 trial) and day 14 (RR 0.36, CI 0.24 to 0.53; n = 1709, 1 trial). Neither trial reported total failures by day 28. A further trial was carried out in participants selected because they had previously failed sulfadoxine-pyrimethamine. Adverse event reporting was inconsistent between trials, but chlorproguanil-dapsone was associated with more adverse events leading to discontinuation of treatment compared with sulfadoxine-pyrimethamine (RR 4.54, CI 1.74 to 11.82; n = 829, 1 trial). It was also associated with more red blood cell disorders (RR 2.86, CI 1.33 to 6.13; n = 1850, 1 trial).

REVIEWERS' CONCLUSIONS

There are insufficient data about the effects of the current standard chlorproguanil-dapsone regimen (three-dose, 2 mg chlorproguanil). Randomized controlled trials that follow up to day 28, record adverse events, and use an intention-to-treat analysis are required to inform any policy decisions.

Comparison of chlorproguanil-dapsone with sulfadoxine-pyrimethamine for the treatment of uncomplicated falciparum malaria in young African children: double-blind randomised controlled trial

BACKGROUND

Increasing resistance to sulfadoxine-pyrimethamine is leading to a decline in its effectiveness. We aimed to assess the safety profile of chlorproguanil-dapsone (CD), and to compare the safety and efficacy of this drug with that of sulfadoxine-pyrimethamine (SP) as treatment for uncomplicated falciparum malaria.

METHODS

We undertook a double-blind, randomised trial in 1850 consecutively recruited children with uncomplicated falciparum malaria, pooling data from five African countries. Analyses were based on all randomised patients with available data.

FINDINGS

CD was significantly more efficacious than SP (odds ratio 3.1 [95% CI 2.0-4.8]); 1313 patients (96%) given CD and 306 (89%) given SP achieved acceptable clinical and parasitological response by day 14. Adverse events were reported in 46% and 50% of patients randomised to CD and SP, respectively (treatment difference -4.4%, [95% CI -10.1 to 1.3]). Haemoglobin in the CD group was significantly lower than in the SP group at day 7, a difference of -4 g/L (95% CI -6 to -2). Mean day 14 haemoglobin (measured only for the small number of patients whose day 7 data caused concern) was 94 g/L (92-96) and 97 g/L (92-102) after CD and SP, respectively. Glucose-6-phosphate dehydrogenase deficient patients on CD had greater odds than those on SP of having a fall of 20 g/dL or more in haemoglobin when baseline temperature was high. Methaemoglobinaemia was seen in the CD group (n=320, mean 0.4% [95% CI 0.4-0.4]) before treatment, 4.2% (95% CI 3.8-4.6) (n=301) at day 3, and 0.6% (0.6-0.7) (n=300) at day 7).

INTERPRETATION

CD had greater efficacy than SP in Africa and was well tolerated. Haematological adverse effects were more common with CD than with SP and were reversible. CD is a useful alternative where SP is failing due to resistance.

Artesunate plus sulfadoxine-pyrimethamine for uncomplicated malaria in Kenyan children: A randomized, double-blind, placebo-controlled trial

Plasmodium falciparum has developed resistance to almost all routinely used antimalarial drugs. Sulfadoxine-pyrimethamine (SP) has replaced chloroquine as first-line treatment of uncomplicated malaria infection in Kenya but resistance to SP is already reported. The addition of artemisinin derivatives to SP may delay the development of drug resistance, improve cure rates, and reduce transmission. The efficacy and safety of artesunate plus SP in the treatment of uncomplicated P. falciparum malaria was evaluated in a randomized trial of 600 children at Siaya District Hospital, western Kenya between October 1999 and March 2000. Children aged < 5 years were randomly assigned to receive SP alone (1.25 mg/kg based on pyrimethamine), or in combination with artesunate (4 mg/kg/d) for either 1 or 3 d. Parasitological failure by days 14 and 28 (polymerase chain reaction [PCR]-corrected for new infections) were the primary endpoints. Treatment failure rates by day 14 were 25.5% in the SP alone group, 16.2% (risk difference [delta]-9.3%, 95% CI -17.3 to -1.2%, P= 0.027) in the 1-dose artesunate group, and 9.4% (delta-16.2%, 95% CI -23.6 to -8.7%, P< 0.001) in the 3-dose artesunate group. Corresponding rates by day 28 were 46.0% in the SP alone group, 38.2% (delta-7.8%, 95% CI -17.7 to 2.1%, P= 0.16) in the 1-dose artesunate group, and 26.0% (delta-20.0%, 95% CI -29.4 to -10.6%, P < 0.001) in the 3-dose artesunate group. The artesunate and SP combination was well tolerated. There were no serious drug-related adverse events. Parasite clearance and gametocyte carriage were reduced significantly in both combination groups compared with SP alone. Three days of artesunate were required to reduce significantly the risk of treatment failure by day 28. However, the high background rate of parasitological failure with SP may make this combination unsuitable for widespread use in Kenya.

Safety, efficacy and determinants of effectiveness of antimalarial drugs during pregnancy: implications for prevention programmes in Plasmodium falciparum-endemic sub-Saharan Africa

Plasmodium falciparum malaria in pregnancy poses substantial risk to a pregnant woman and her neonate through anaemia and low birth weight (LBW), respectively, and is responsible for up to 35% of preventable LBW in malaria-endemic areas. Chemoprophylaxis or intermittent preventive treatment (IPT) with an effective antimalarial can ameliorate the adverse effects of malaria during pregnancy. Current guidelines from the WHO recommend that women in highly malarious areas receive IPT with an effective antimalarial. Two central considerations in evaluating drugs for use during pregnancy are safety for the mother and her foetus and effectiveness, which is determined by efficacy, cost, availability, deliverability and acceptability of the drug. These factors may be scored and potential drugs or drug combinations ranked in order of potential effectiveness for use in prevention programmes. The seven most promising regimens are all IPT, primarily because they are more easily delivered and less expensive than chemoprophylaxis. Currently, IPT with sulphadoxine-pyrimethamine (SP) is more likely to have the best overall effectiveness in preventing adverse outcomes associated with malaria in pregnancy. Its low cost, wide availability, easy deliverability and acceptability make it the clear choice in countries where efficacy of the drug remains good. For countries where resistance to SP is rising or already high, amodiaquine (alone or in combination with SP or artesunate) artesunate + SP, chlorproguanil-dapsone (with and without artesunate) and artemether-lumefantrine require urgent evaluation for use in pregnancy.

The development of Lapdap, an affordable new treatment for malaria

There is much discussion on how new drugs can be developed for use in developing countries at a price that makes them accessible to those who need them most. The development of a new antimalarial, chlorproguanil/dapsone (Lapdap), provides an example of a way this can be achieved. The idea of combining chlorproguanil with dapsone came from studies done in east Africa in the 1980s. These studies showed, both in vivo and in vitro, that chlorproguanil/dapsone had advantages over sulphadoxine/pyrimethamine. A public-private partnership was established subsequently to manage a development programme of a fixed ratio tablet of this drug combination. The partnership comprised GlaxoSmithKline (formerly SmithKline Beecham), the World Health Organization (WHO), and the UK's Department for International Development (DFID). All clinical, toxicological, and pharmaceutical chemistry studies are complete and the findings have been submitted for regulatory approval. The question now is how Lapdap might be used safely and appropriately if it receives regulatory approval. A public-health group has been formed by WHO (with funding from DFID and the Gates Foundation) to research into this issue. The Lapdap development team completed its objective of submitting Lapdap for drug registration within a period of 5 years and at a low cost. Experience with the development of Lapdap may provide a model for the introduction of other new drugs developed primarily for use in developing countries.

Genetic diversity of Plasmodium falciparum parasites from Kenya is not affected by antifolate drug selection

The genotypes of merozoite surface protein-1, merozoite surface protein-2 and glutamine rich protein are frequently used to distinguish recrudescence from reinfection when parasitaemia reappears after antimalarial drug treatment. However, none of the previous reports has clearly assessed the change of genetic diversity following drug treatment. In the present study, we have assessed the impact of pyrimethamine/sulfadoxine and chlorproguanil/dapsone on the genetic diversity of isolates and the multiplicity of infection in patient isolates from Kilifi, Kenya. We have analysed the length polymorphism of merozoite surface protein-1, merozoite surface protein-2 and glutamine rich protein and the data clearly show that treatment with pyrimethamine/sulfadoxine and chlorproguanil/dapsone did not change the multiplicity of infection found in patients, in contrast to the selection that these drugs exert on the genes encoded by the target enzymes. In addition, we report that children of less than 2 years tend to have fewer numbers of clones per isolate when compared with older children. Overall, this study shows that the selection for genes that confer drug resistance is not a factor in reducing the genetic diversity of parasite clones in a patient.

Chlorproguanil-dapsone versus sulfadoxine-pyrimethamine for sequential episodes of uncomplicated falciparum malaria in Kenya and Malawi: a randomised clinical trial

BACKGROUND

Chlorproguanil-dapsone exerts lower resistance pressure on Plasmodium falciparum than does sulfadoxine-pyrimethamine, but is rapidly eliminated. We aimed to find out whether chlorproguanil-dapsone results in a higher retreatment rate for malaria than sulfadoxine-pyrimethamine.

METHODS

In a randomised trial of paediatric outpatients with uncomplicated falciparum malaria, patients received either chlorproguanil-dapsone or sulfadoxine-pyrimethamine and were followed up for up to 1 year. Sites were in Kenya (n=410) and Malawi (n=500). We used per-protocol analysis to assess the primary outcome of annual malaria incidence.

FINDINGS

Drop-outs were 117 of 410 (28.5%) in Kenya, and 342 of 500 (68.4%) in Malawi. Follow-up was for a median of 338 days (IQR 128-360) and 342 days (152-359) in Kilifi (chlorproguanil-dapsone and sulfadoxine-pyrimethamine, respectively), and for 120 days (33-281) and 84 days (26-224) in Blantyre. Mean annual malaria incidence was 2.5 versus 2.1 in Kenya (relative risk 1.16, 95% CI 0.98-1.37), and 2.2 versus 2.8 in Malawi (0.77, 0.63-0.94). 4.3% versus 12.8%, and 5.4% versus 20.1%, of patients were withdrawn for treatment failure in Kenya and Malawi, respectively. In Kenya haemoglobin concentration of 50 g/L or less caused exit in 6.9% of chlorproguanil-dapsone patients and 1.5% of sulfadoxine-pyrimethamine patients, but most anaemia occurred before re-treatment. In Malawi only one patient exited because of anaemia.

INTERPRETATION

Despite the rapid elimination of chlorproguanil-dapsone, children treated with this drug did not have a higher incidence of malaria episodes than those treated with sulfadoxine-pyrimethamine. Treatment failure was more common with sulfadoxine-pyrimethamine. Cause of anaemia in Kenya was probably not adverse reaction to chlorproguanil-dapsone, but this observation requires further study.

Pyrimethamine and WR99210 exert opposing selection on dihydrofolate reductase from Plasmodium vivax

Plasmodium vivax is a major public health problem in Asia and South and Central America where it is most prevalent. Until very recently, the parasite has been effectively treated with chloroquine, but resistance to this drug has now been reported in several areas. Affordable alternative treatments for vivax malaria are urgently needed. Pyrimethamine-sulfadoxine is an inhibitor of dihydrofolate reductase (DHFR) that has been widely used to treat chloroquine-resistant Plasmodium falciparum malaria. DHFR inhibitors have not been considered for treatment of vivax malaria, because initial trials showed poor efficacy against P. vivax. P. vivax cannot be grown in culture; the reason for its resistance to DHFR inhibitors is unknown. We show that, like P. falciparum, point mutations in the dhfr gene can cause resistance to pyrimethamine in P. vivax. WR99210 is a novel inhibitor of DHFR, effective even against the most pyrimethamine-resistant P. falciparum strains. We have found that it is also an extremely effective inhibitor of the P. vivax DHFR, and mutations that confer high-level resistance to pyrimethamine render the P. vivax enzyme exquisitely sensitive to WR99210. These data suggest that pyrimethamine and WR99210 would exert opposing selective forces on the P. vivax population. If used in combination, these two drugs could greatly slow the selection of parasites resistant to both drugs. If that is the case, this novel class of DHFR inhibitors could provide effective and affordable treatment for chloroquine- and pyrimethamine-resistant vivax and falciparum malaria for many years to come.

Plasmodium falciparum: in vitro activity of sulfadoxine and dapsone in field isolates from Kenya: point mutations in dihydropteroate synthase may not be the only determinants in sulfa resistance

We have determined the relationship between point mutations in the gene that encodes the sulfa target, dihydropteroate synthase (DHPS) and the chemosensitivity profile to sulfadoxine and dapsone in 67 isolates from Kilifi, Kenya. We assessed the presence of mutations at codons 436, 437, 540, 581, and 613 of dhps. The results showed that the dhps genotype had a strong influence on the sensitivity to sulfadoxine and dapsone, but that the correlation was far from perfect. Eleven isolates carried a wild-type dhps allele, but were resistant to sulfadoxine (IC(50) values >10 microg/ml), and 4/28 isolates were classed as sensitive to sulfadoxine (IC(50) values <10 microg/ml), but carried a triple mutant (436/437/613) allele of dhps. These data show that in low folate medium in vitro, the dhps genotype alone did not account completely for sulfadoxine or dapsone resistance; other factors such as the utilisation of exogenous folate must also be considered.

Malaria vaccines

Malaria kills one child in Africa every 30 s. After summarising the burden of malaria, the life-cycle of this parasite in humans and female Anopheles mosquitoes is outlined. Important differences between natural immunity and that induced by current candidate vaccines are discussed. In the main part of the review, the recent rapid expansion in evaluation of candidate malaria vaccines in clinical trials across the world is discussed. Subunit vaccine technologies are progressing rapidly with new delivery systems, vectors and antigens under evaluation as well as new polyepitope approaches. Combination vaccination regimens, improved adjuvants and genetic engineering of antigens are all improving the immunogenicity of candidate vaccines. We also discuss particular difficulties in vaccination against malaria, the conduct of field trials of malaria vaccines in non-industrialised countries and the need for even greater co-operation between researchers. Finally, the important concept of iterative vaccine development is raised and the prospects for effective malaria vaccination are discussed.

Pyrimethamine-sulfadoxine resistance in Plasmodium falciparum: what next?

Chemotherapy remains the only practicable tool to control falciparum malaria in sub-Saharan Africa, where >90% of the world's burden of malaria mortality and morbidity occurs. Resistance is rapidly eroding the efficacy of chloroquine, and the combination pyrimethamine-sulfadoxine is the most commonly chosen alternative. Resistant populations of Plasmodium falciparum were selected extremely rapidly in Southeast Asia and South America. If this happens in sub-Saharan Africa, it will be a public health disaster because no inexpensive alternative is currently available. This article reviews the molecular mechanisms of this resistance and discusses how to extend the therapeutic life of antifolate drugs.

Pharmacokinetic interactions of antimalarial agents

Combination of antimalarial agents has been introduced as a response to widespread drug resistance. The higher number of mutations required to express complete resistance against combinations may retard the further development of resistance. Combination of drugs, especially with the artemisinin drugs, may also offer complete and rapid eradication of the parasite load in symptomatic patients and thus reduce the chance of survival of resistant strains. The advantages of combination therapy should be balanced against the increased chance of drug interactions. During the last decade, much of the pharmacokinetics and metabolic pathways of antimalarial drugs have been elucidated, including the role of the cytochrome P450 (CYP) enzyme complex. Change in protein binding is not a significant cause of interactions between antimalarial agents. CYP3A4 and CYP2C19 are frequently involved in the metabolism of antimalarial agents. Quinidine is a potent inhibitor of CYP2D6, but it appears that this enzyme does not mediate the metabolism of any other antimalarial agent. The new combinations proguanil-atovaquone and chlorproguanil-dapsone do not show significant interactions. CYP2B6 and CYP3A4 are involved in the metabolism of artemisinin and derivatives, but further studies may reveal involvement of more enzymes. Artemisinin may induce CYP2C19. Several artemisinin drugs suffer from auto-induction of the first-pass effect, resulting in a decline of bioavailability after repeated doses. The mechanism of this effect is not yet clear, but induction by other agents cannot be excluded. The combination of artemisinin drugs with mefloquine and the fixed combination artemether-lumefantrine have been studied widely, and no significant drug interactions have been found. The artemisinin drugs will be used at an increasing rate, particularly in combination with other agents. Although clinical studies have so far not shown any significant interactions, drug interactions should be given appropriate attention when other combinations are used.

Chlorproguanil-dapsone (LAPDAP) for uncomplicated falciparum malaria

The synergistic antifolate combination of chlorproguanil with dapsone (CPG-DDS; LAPDAP) is being developed by a public-private partnership as a low-cost treatment for uncomplicated falciparum malaria. LAPDAP is rapidly eliminated from the body, giving it low selection pressure for drug resistance. Clinical cases with sulphadoxine-pyrimethamine (SP)-resistant infections acquired in Africa have been predicted to be responsive to LAPDAP, and clinical evidence is available to support this. A regulatory dossier is being prepared for simultaneous submission to the UK Medicines Control Agency and African licencing authorities. The team working on LAPDAP has also started to develop the triple combination of chlorproguanil-dapsone-artesunate (CDA) as a low-cost combination therapy for uncomplicated falciparum malaria. Although LAPDAP does not have regulatory approval (and development of CDA is at an early stage), the development team is keen to communicate with public health scientists to try to anticipate the policy and implementation hurdles that lie ahead. This short paper outlines the current stages that LAPDAP and CDA have reached, and sketches the anticipated public health issues.

Chlorproguanil-dapsone for treatment of drug-resistant falciparum malaria in Tanzania

BACKGROUND

Resistance to the affordable malaria treatments chloroquine and pyrimethamine-sulfadoxine is seriously impeding malaria control through treatment in east Africa. We did an open, alternate drug allocation study to assess the efficacy of chlorproguanil-dapsone in the treatment of falciparum malaria clinically resistant to pyrimethamine-sulfadoxine.

METHODS

Children younger than 5 years with non-severe falciparum malaria, attending Muheza district hospital in Tanzania, were treated with the standard regimen of pyrimethamine-sulfadoxine. Patients whose clinical symptoms resolved but who remained parasitaemic 7 days after pyrimethamine-sulfadoxine were followed up for 1 month. Clinical malaria episodes were retreated with either single dose pyrimethamine-sulfadoxine or a 3-day regimen of chlorproguanil-dapsone. Those with parasitaemia after 7 days were treated with chlorproguanil-dapsone. Parasite DNA was collected on day 7 after first treatment with pyrimethamine-sulfadoxine and we looked for point mutations in the genes encoding dihydrofolate reductase (dhfr) and dyhydropteroate synthetase (dhps).

FINDINGS

360 children were enrolled and treated with pyrimethamine-sulfadoxine. On day 7, 192 (55%) of 348 had cleared parasitaemia. Of the remaining 156 parasitaemic children, 140 (90%) were followed up to day 28, and 92 (66%) of 140 developed clinical malaria. These 92 patients were alternately retreated with either pyrimethamine-sulfadoxine (46) or chlorproguanil-dapsone (46). 28 (61%) of 46 children retreated with pyrimethamine-sulfadoxine were still parasitaemic at day 7, compared with three (7%) of 44 [corrected] children retreated with chlorproguanil-dapsone. Resistance to pyrimethamine-sulfadoxine increased from 45% (156/348) at the first treatment to 61% (28/46) after retreatment. 83 of 85 parasite isolates collected after the first pyrimethamine-sulfadoxine treatment, and before and after the second treatments with pyrimethamine-sulfadoxine and chlorproguanil-dapsone showed triple-mutant dhfr alleles, associated with a variety of dhps mutations.

INTERPRETATION

Most patients treated with pyrimethamine-sulfadoxine, who remain parasitaemic at day 7, develop new malaria symptoms within 1 month. Chlorproguanil-dapsone was a practicable therapy under these circumstances. Analysis of parasite dhfr and dhps before and after treatment supports the view that pyrimethamine-sulfadoxine resistance in this part of Africa is primarily due to parasites with three mutations in the dhfr domain.

Novel alleles of the Plasmodium falciparum dhfr highly resistant to pyrimethamine and chlorcycloguanil, but not WR99210

We have expressed dhfr alleles of Plasmodium falciparum in the budding yeast, Saccharomyces cerevisiae, and used this yeast model to identify single amino acid substitutions that confer high level pyrimethamine resistance on the background of the triple mutant dhfr (I51+R59+N108). Mutations in three clusters were identified: codons 50-57, 187-193, and 213-214. Several mutations previously identified in field samples were also isolated, including codons 50 and 164. The I164L mutation is of particular interest, because the quadruple mutant genotype (N51I+C59R+S108N+I164L) encodes an enzyme that is no longer inhibited by pyrimethamine, rendering sulfadoxine/pyrimethamine (SP; Fansidar) clinically ineffective. Thirty-six novel alleles were tested to determine their sensitivity to chlorcycloguanil and WR99210, two DHFR inhibitors that are in clinical and pre-clinical development, respectively. Chlorcycloguanil is effective against parasites that carry the triple mutant allele, but in vitro analysis has suggested that chlorcycloguanil will be clinically ineffective against parasites that carry the quadruple mutant allele of dhfr. In our screen, 23 of 36 novel strains were as resistant to chlorcycloguanil as the quadruple mutant, and one strain was 10-fold more resistant. WR99210 is still effective in the nM range against parasites that carry the quadruple mutant allele. In the preliminary screen, 31 of 36 novel alleles were as sensitive to WR99210 as the quadruple mutant. Detailed analysis of the remaining five showed that four of the five had IC(50) values in the same range as the quadruple mutant, and one, N51I+C59R+S108N+E192G, had an IC(50) value about fivefold higher. This result suggests that WR99210 and related compounds will be clinically effective against quadruple mutants currently found in Southeast Asia and South America and against most novel alleles that could be selected on the background of the triple mutant genotype now prevalent in East Africa.

A trial of proguanil-dapsone in comparison with sulfadoxine-pyrimethamine for the clearance of Plasmodium falciparum infections in Tanzania

Considerable levels of resistance to sulfadoxine-pyrimethamine (SP) have been reported in Plasmodium falciparum in north-eastern Tanzania, and the identification of a suitable antimalarial to replace SP is now a high priority. We conducted a trial in July 2000 to determine the efficacy of proguanil (PG) plus dapsone (DS), compared with that of SP, for the treatment of asymptomatic falciparum infection. A total of 220 children with parasitaemia > or = 2000 per microL completed the study; 112 had received a single dose of SP (dosage calculated for pyrimethamine 1.25 mg/kg and sulfadoxine 25 mg/kg) and 108 had taken PG 10 mg/kg with DS 2.5 mg/kg each day for 3 days. Clearance of asexual parasites at day 7 was 14.3% with SP, but 93.5% with PG-DS. The remarkably high failure rate with SP was not associated with occurrence of leucine substitution at position 164 of the dhfr gene. Both treatment regimens were well tolerated. Compared with available data on another antifolate combination, chlorproguanil-dapsone ('Lapdap'), PG-DS was slightly but significantly inferior in achieving parasite clearance (99.5% versus 93.5%). The estimated cost of a 3-day course of PG-DS treatment for a child weighing 18 kg is US $0.15. With the rising incidence of SP-resistant P. falciparum infection, PG-DS could provide an effective, affordable and already available therapeutic alternative for malaria in East Africa at least until chlorproguanil-dapsone is registered.

The comparative efficacy of chloroquine and sulfadoxine-pyrimethamine for the treatment of uncomplicated falciparum malaria in Kampala, Uganda

Chloroquine (CQ) remains the first-line treatment for uncomplicated malaria in much of Africa despite the growing problem of resistance to this drug. Sulfadoxine-pyrimethamine (SP) is often used after CQ treatment failure and has replaced CQ as the first-line treatment in parts of Africa. To compare the efficacy of these 2 regimens, we evaluated, in March-August 1999, clinical and parasitological responses over 28 days in 214 children and adults from Kampala, Uganda, with uncomplicated falciparum malaria. Compared to SP, significantly more patients treated with CQ developed early or late clinical failure (54% vs 11%, P < 0.001) and parasitological failure (72% vs 30%, P < 0.001) during 14 days of follow-up. The risk of treatment failure occurring after day 14 was similar between the 2 treatment groups. Among those treated with CQ, children aged < 5 years were at higher risk of clinical failure than older individuals (76% vs 28%, P < 0.001), an association not seen with SP (11% vs 10%, P = 0.91). Although early parasite clearance was significantly better in the SP group (P = 0.001), fever clearance at day 3 was the same (CQ 85%, SP 86%). These and other recent findings suggest that consideration be given to replacing CQ as the first-line therapy for uncomplicated malaria in Uganda, particularly in young children.

Towards an understanding of the mechanism of pyrimethamine-sulfadoxine resistance in Plasmodium falciparum: genotyping of dihydrofolate reductase and dihydropteroate synthase of Kenyan parasites

The antifolate combination of pyrimethamine (PM) and sulfadoxine (SD) is the last affordable drug combination available for wide-scale treatment of falciparum malaria in Africa. Wherever this combination has been used, drug-resistant parasites have been selected rapidly. A study of PM-SD effectiveness carried out between 1997 and 1999 at Kilifi on the Kenyan coast has shown the emergence of RI and RII resistance to PM-SD (residual parasitemia 7 days after treatment) in 39 out of 240 (16.25%) patients. To understand the mechanism that underlies resistance to PM-SD, we have analyzed the dihydrofolate reductase (DHFR) and dihydropteroate synthase (DHPS) genotypes of 81 patients. Fifty-one samples were obtained, before treatment, from patients who remained parasite free for at least 7 days after treatment. For a further 20 patients, samples were obtained before treatment and again when they returned to the clinic with parasites 7 days after PM-SD treatment. Ten additional isolates were obtained from patients who were parasitemic 7 days after treatment but who were not sampled before treatment. More than 65% of the isolates (30 of 46) in the initial group had wild-type or double mutant DHFR alleles, and all but 7 of the 47 (85%) had wild-type DHPS alleles. In the paired (before and after treatment) samples, the predominant combinations of DHFR and DHPS alleles before treatment were of triple mutant DHFR and double mutant DHPS (41% [7 of 17]) and of double mutant DHFR and double mutant DHPS (29% [5 of 17]). All except one of the posttreatment isolates had triple mutations in DHFR, and most of these were "pure" triple mutants. In these isolates, the combination of a triple mutant DHFR and wild-type DHPS was detected in 6 of 29 cases (20.7%), the combination of a triple mutant DHFR and a single mutant (A437G) DHPS was detected in 4 of 29 cases (13.8%), and the combination of a triple mutant DHFR and a double mutant (A437G, L540E) DHPS was detected in 16 of 29 cases (55.2%). These results demonstrate that the triply mutated allele of DHFR with or without mutant DHPS alleles is associated with RI and RII resistance to PM-SD. The prevalence of the triple mutant DHFR-double mutant DHPS combination may be an operationally useful marker for predicting the effectiveness of PM-SD as a new malaria treatment.

Chemotherapy for falciparum malaria: the armoury, the problems and the prospects

Peter Winstanley here describes the pharmacology and therapeutics of the main drugs used for falciparum malaria in the tropical setting, rather than in the developed world, as an overview for newcomers to the field. He then examines some of the current major problems and prospects for the future.

New therapies and changing patterns of treatment for malaria

The launch of the Roll Back Malaria initiative by the World Health Organization in the period under review confirms malaria's place as one of the great public health priorities worldwide. The period 1998-1999 has seen some advances and some disappointments in the treatment of malaria, against a backdrop of spreading drug resistance. Most encouraging is the clear demonstration that intermittent prospective treatment of asymptomatic pregnant women in endemic areas reduces morbidity. The greatest disappointment has been the result of trials with the artemether-benflumetol fixed-dose combination therapy. Questions have been raised about several widely accepted practices, including measurement of quinine levels, exchange transfusion, and the prophylactic use of anticonvulsants in children with cerebral malaria.

Amodiaquine remains effective for treating uncomplicated malaria in west and central Africa

Many countries in Africa are now confronted with the dilemma of shifting drug policies for uncomplicated falciparum malaria from chloroquine, which has become largely ineffective, to a new first-line drug and amodiaquine is one of the possible options. A multicentre, open-label randomized controlled trial of amodiaquine 30 mg/kg vs chloroquine 25 mg/kg over 3 days was performed in Senegal, Cameroon, Gabon, and Burkina Faso between 1996 and 1998 and patients were followed-up for 14 days. Sensitivity of isolates in vitro and whole blood levels of chloroquine and amodiaquine were also measured. The primary efficacy parameter was parasitological clearance on day 14 (parasitological success). The secondary efficacy parameter was absence of signs/symptoms of malaria on day 14 (clinical success). Among the 364 patients randomized and receiving the assigned treatment (chloroquine n = 185, amodiaquine n = 179), 137 and 139, respectively, reached the primary endpoint. Amodiaquine proved significantly more effective than chloroquine. The summary odds ratio (95% CI) was 7.79 (4.54-13.35) for parasitological success, and 6.3 (3.4-11.68) for clinical success. Sensitivity in vitro and chloroquine blood levels were good predictors of chloroquine failure. Amodiaquine remains effective for treating uncomplicated falciparum malaria in areas of West and Central Africa where chloroquine resistance is prevalent. However, measures should be taken to protect the lifespan of amodiaquine where the drug is introduced for use.

Host haematological factors influencing the transmission of Plasmodium falciparum gametocytes to Anopheles gambiae s.s. mosquitoes

We investigated the relationship between selected host haematological and parasitological parameters and the density and infectivity of Plasmodium falciparum gametocytes. 143 individuals (age range 1-62 years) attending an outpatient clinic in Farafenni, The Gambia, who had peripheral blood gametocytaemia were recruited (mean gametocyte density 123.7/microl, range 5-17,000/microl). Of the parameters measured, packed cell volume (PCV), reticulocyte count (RetC) and lymphocyte count (LyC) were significantly associated with gametocyte density (r = - 0.17, P < 0.05; r = 0.21, P < 0.01; r = 0.18, P < 0.05, respectively). Data from membrane feeding experiments in which 15 or more mosquitoes were dissected showed that 60.7% (53/87) of gametocyte carriers infected one or more mosquitoes. Gametocyte density was strongly correlated with transmission success (TS) (r = 0.3, P < 0.005) and, in successful infections, with both mosquito prevalence (MP) (r = 0.36, P < 0.005) and mean oocyst burden (MOB) (r = 0.65, P < 0.0001). None of the other factors measured were significantly associated with any of these indices in bivariate analysis. Regression modelling showed that both gametocyte density and PCV were positively associated with gametocyte carrier infectivity to mosquitoes (LRchi2 = 100.7 and 47.2, respectively) and, in successful infections, with MOB (beta = 0.16, t = 4.9, P < 0.001; beta = 0.02, t = 2.3, P < 0.05, respectively). The positive association with PCV suggests that blood meal quality influences infection probably as a nutritional requirement, however, as this effect was most apparent at high gametocyte densities, its epidemiological significance is questionable. Though the haematological parameters associated with gametocyte density are a direct consequence of asexual infection, they may also represent an adaptive mechanism for optimization of sexual stage development.

Comparison of bednets impregnated with different pyrethroids for their impact on mosquitoes and on re-infection with malaria after clearance of pre-existing infections with chlorproguanil-dapsone

Comparisons of bednets treated either with alphacypermethrin or lambdacyhalothrin showed similar effectiveness by various entomological criteria. Lambdacyhalothrin was associated with significantly more reports of nasal irritation than alphacypermethrin. The 2 net treatments were equally effective in reducing incidence of new malaria infections and the treated nets were much more effective than untreated nets. These measurements were made after clearing existing infections with chlorproguanil-dapsone. This drug combination was more than 99% effective in clearing infections 1 week after treatment and a study of children taken to an altitude with no malaria transmission showed that there were very few recrudescences.

Antifolate resistance due to new and known Plasmodium falciparum dihydrofolate reductase mutations expressed in yeast

Two new dihydrofolate reductase (DHFR) mutations were recently discovered in Plasmodium falciparum samples from an area of Bolivia with high rates of in vivo resistance to pyrimethamine-sulfadoxine: a Cys-->Arg point mutation in codon 50 and a five amino acid insertion after codon 30, termed the Bolivia repeat. We used a yeast expression system to screen these new DHFR mutants, as well as all of the other known DHFR mutant genotypes, against four antifolates: pyrimethamine, cycloguanil, chlorcycloguanil, and WR99210. The prodrug proguanil was also evaluated. The primary 108-Asn mutation, the known secondary mutations 51-Ile, 59-Arg and 164-Leu, as well as the 50-Arg mutation, all progressively enhanced pyrimethamine resistance in naturally observed combinations with one another, with the presence of 164-Leu most significantly increasing resistance. Cycloguanil and chlorcycloguanil resistance were most impacted by 164-Leu and the paired 16-Val/108-Thr. Proguanil had no effect on malaria DHFR. All DHFRs analyzed were sensitive to WR99210. The Bolivia repeat did not markedly affect drug sensitivity. We conclude that malaria DHFR can be reliably, rapidly and inexpensively analyzed in yeast for activity against a broad spectrum of antifolates. This system may be useful for initially characterizing newly discovered genotypes before proceeding to P. falciparum transfection; for large-scale geographic surveys of drug resistance; and for screening new antifolates or new antifolate combinations for their effectiveness against a large panel of DHFR mutants.

P. falciparum dihydrofolate reductase and dihydropteroate synthase mutations: epidemiology and role in clinical resistance to antifolates

Plasmodium falciparum resistance to the antifolates has arisen rapidly in Asia and South America, and threatens the usefulness of these drugs in Africa. In vitro resistance to the antifolates is determined by mutations in parasite dihydrofolate reductase (DHFR) and dihydropteroate synthase (DHPS). The role of DHFR and DHPS mutations in therapeutic failure of antifolate antimalarials is less clear. This review summarizes molecular epidemiological surveys, studies of in vivo selection of mutant alleles by drug treatment, and prospective studies of the ability of mutation-specific assays to predict clinical outcomes, and discusses the potential use of these assays for surveillance of resistance.

Kenyan Plasmodium falciparum field isolates: correlation between pyrimethamine and chlorcycloguanil activity in vitro and point mutations in the dihydrofolate reductase domain

Sixty-nine Kenyan Plasmodium falciparum field isolates were tested in vitro against pyrimethamine (PM), chlorcycloguanil (CCG), sulfadoxine (SD), and dapsone (DDS), and their dihydrofolate reductase (DHFR) genotypes were determined. The in vitro data show that CCG is more potent than PM and that DDS is more potent than SD. DHFR genotype is correlated with PM and CCG drug response. Isolates can be classified into three distinct groups based on their 50% inhibitory concentrations (IC50s) for PM and CCG (P < 0.01) and their DHFR genotypes. The first group consists of wild-type isolates with mean PM and CCG IC50s of 3.71 +/- 6.94 and 0.24 +/- 0.21 nM, respectively. The second group includes parasites which all have mutations at codon 108 alone or also at codons 51 or 59 and represents one homogeneous group for which 25- and 6-fold increases in PM and CCG IC50s, respectively, are observed. Parasites with mutations at codons 108, 51, and 59 (triple mutants) form a third distinct group for which nine- and eightfold increases in IC50s, respectively, of PM and CCG compared to the second group are observed. Surprisingly, there is a significant decrease (P < 0.01) of SD and DDS susceptibility in these triple mutants. Our data show that more than 92% of Kenyan field isolates have undergone at least one point mutation associated with a decrease in PM activity. These findings are of great concern because they may indicate imminent PM-SD failure, and there is no affordable antimalarial drug to replace PM-SD (Fansidar).