Chlorproguanil/dapsone for uncomplicated Plasmodium falciparum malaria in young children: pharmacokinetics and therapeutic range

Enhancing the antimalarial activity of artesunate

The global challenge to the treatment of malaria is mainly the occurrence of resistance of malaria parasites to conventionally used antimalarials. Artesunate, a semisynthetic artemisinin compound, and other artemisinin derivatives are currently used in combination with selected active antimalarial drugs in order to prevent or delay the emergence of resistance to artemisinin derivatives. Several methods, such as preparation of hybrid compounds, combination therapy, chemical modification and the use of synthetic materials to enhance solubility and delivery of artesunate, have been employed over the years to improve the antimalarial activity of artesunate. Each of these methods has advantages it bestows on the efficacy of artesunate. This review discussed the various methods employed in enhancing the antimalarial activity of artesunate and delaying the emergence of resistance of parasite to it.

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.

Malaria chemoprophylaxis: when should we use it and what are the options?

Malaria chemoprophylaxis concerns prescribing healthy individuals medication for an infection they have an unknown chance of getting. Sensible use of malaria chemoprophylaxis is a balance between the risk of infection and death, and the risk of side effects. The risk of infection can be broken down into the risk of being bitten by a malaria-infected mosquito and the risk of the malaria parasites being resistant to the drug used for prophylaxis. Our knowledge of these parameters is patchy. The risk of infection is not uniform at a given location and the standard of living will greatly influence risk. It is suggested that chemoprophylaxis should not be recommended in areas with less than ten reported cases of P. falciparum malaria per 1000 inhabitants per year. The resistance pattern is known to a certain extent but, for instance, diverging opinion of how much resistance to chloroquine there is in West Africa illustrates the lack of data. There is much debate on rare adverse events, which usually escape Phase III studies prior to registration and are only picked up by passive, postmarketing surveillance. The lessons over the past 20 years with the introduction of amodiaquine, pyrimethamine/dapsone (Maloprim, GlaxoSmithKline) and pyrimethamine/sulfadoxine (Fansidar, Roche), which were all withdrawn for prophylaxis after a few years, show how sensitive drugs for chemoprophylaxis are to side effects. Three levels of chemoprophylaxis are used: chloroquine in areas with sensitive P. falciparum, chloroquine plus proguanil in areas with low level chloroquine resistance, and atovaquone/proguanil (Malarone, GlaxoSmithKline), doxycycline or mefloquine (Lariam, Roche) in areas with extensive resistance against chloroquine and proguanil. Primaquine and the primaquone analog tafenoquine may be future alternatives but otherwise there are few new drugs for chemoprophylaxis on the horizon.

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.

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

Intermittent preventive treatment in infants for the prevention of malaria in rural Western kenya: a randomized, double-blind placebo-controlled trial

Background

Intermittent preventive treatment in infants (IPTi) with sulphadoxine-pyrimethamine (SP) for the prevention of malaria has shown promising results in six trials. However, resistance to SP is rising and alternative drug combinations need to be evaluated to better understand the role of treatment versus prophylactic effects.

Methods

Between March 2004 and March 2008, in an area of western Kenya with year round malaria transmission with high seasonal intensity and high usage of insecticide-treated nets, we conducted a randomized, double-blind placebo-controlled trial with SP plus 3 days of artesunate (SP-AS3), 3 days of amodiaquine-artesunate (AQ3-AS3), or 3 days of short-acting chlorproguanil-dapsone (CD3) administered at routine expanded programme of immunization visits (10 weeks, 14 weeks and 9 months).

Principal Findings

1,365 subjects were included in the analysis. The incidence of first or only episode of clinical malaria during the first year of life (primary endpoint) was 0.98 episodes/person-year in the placebo group, 0.74 in the SP-AS3 group, 0.76 in the AQ3-AS3 group, and 0.82 in the CD3 group. The protective efficacy (PE) and 95% confidence intervals against the primary endpoint were: 25.7% (6.3, 41.1); 25.9% (6.8, 41.0); and 16.3% (−5.2, 33.5) in the SP-AS3, AQ3-AS3, and CD3 groups, respectively. The PEs for moderate-to-severe anaemia were: 27.5% (−6.9, 50.8); 23.1% (−11.9, 47.2); and 11.4% (−28.6, 39.0). The duration of the protective effect remained significant for up to 5 to 8 weeks for SP-AS3 and AQ3-AS3. There was no evidence for a sustained beneficial or rebound effect in the second year of life. All regimens were well tolerated.

Conclusions

These results support the view that IPTi with long-acting regimens provide protection against clinical malaria for up to 8 weeks even in the presence of high ITN coverage, and that the prophylactic rather than the treatment effect of IPTi appears central to its protective efficacy.

Trial Registration

ClinicalTrials.gov NCT00111163

Pharmacokinetics of chlorproguanil, dapsone, artesunate and their major metabolites in patients during treatment of acute uncomplicated Plasmodium falciparum malaria

OBJECTIVE

Chlorproguanil (CPG)-dapsone (DDS)-artesunate was in development for the treatment of uncomplicated Plasmodium falciparum malaria. The pharmacokinetics of CPG, DDS, artesunate and their metabolites chlorcycloguanil (CCG), monoacetyl dapsone (MADDS) and dihydroartemisinin (DHA) were investigated in patients with P. falciparum given CPG-DDS alone or plus artesunate.

METHODS

Adult patients from Malawi and The Gambia taking part in a phase II clinical trial were randomised to receive a 3-day treatment of CPG-DDS alone (2/2.5 mg/kg/day) or plus 1, 2 or 4 mg/kg/day artesunate. Blood samples for pharmacokinetic analysis were collected up to 24 h post-first dose.

RESULTS

The pharmacokinetic analysis included 115 patients. For CPG, there was no significant effect of artesunate on C(max) or AUC(0-24), except the 90% confidence interval (CI) for AUC(0-24) for the 4 mg/kg artesunate dose was slightly below that for the standard bioequivalence range (90% CI 0.78, 1.11); this was not considered clinically relevant. Artesunate increased the CCG AUC(0-24) by 6-17% and C(max) by 0-16%. Artesunate had no significant effect on the rate or extent of absorption of DDS. For MADDS, artesunate increased the AUC(0-24) by 13-47% and C(max) by 8-45%. For 1, 2 and 4 mg/kg artesunate dosing, artesunate AUC(0-infinity) was 64.6, 151 and 400 ng.h/ml and C(max) 48.9, 106 and 224 ng/ml respectively; DHA AUC(0-infinity) was 538, 1,445 and 3,837 ng.h/ml and C(max) 228, 581 and 1,414 ng/ml respectively. Using a power model, the point estimates of slope were greater than 1 for artesunate AUC(0-t) by 16% and C(max) by 5% and for DHA by 39 and 21% respectively.

CONCLUSION

Artesunate did not significantly affect CPG or DDS pharmacokinetics. For CCG and MADDS, small to moderate increases in exposure with artesunate dosing were observed. There was a greater than proportional increase in artesunate and DHA exposure with increasing artesunate dose. These effects are not considered to be clinically relevant. It should be noted that the CPG-DDS-artesunate programme has now been stopped following unacceptable haematological toxicity in patients with glucose-6-phosphate dehydrogenase deficiency during a phase III trial. In addition, the CPG-DDS combination has been withdrawn from clinical use.

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.

CYP2C8 and antimalaria drug efficacy

Malaria is a major infectious disease. In the last 10 years it has killed more than 20 million people, mainly small children in Africa. The highly efficacious artemisinine combination therapy is being launched globally, constituting the main hope for fighting the disease. Amodiaquine is a main partner in these combinations. Amodiaquine is almost entirely metabolized by the polymorphic cytochrome P450 (CYP) isoform 2C8 to the pharmacologically active desethylamodiaquine. The question remains whether the efficacy of amodiaquine is affected by the gene polymorphism. Genotype-inferred low metabolizers are found in 1-4% of African populations, which corresponds to millions of expected exposures to the drug. In vivo pharmacokinetic data on amodiaquine is limited. By combining it with published in vitro pharmacodynamic and drug metabolism information, we review and predict the possible relevance, or lack of, of CYP2C8 polymorphisms in the present and future efficacy of amodiaquine. Chloroquine and dapsone, both substrates of CYP2C8, are also discussed in the same context.

Independent evolution of pyrimethamine resistance in Plasmodium falciparum isolates in Melanesia

Pyrimethamine resistance in Plasmodium falciparum has previously been shown to have emerged once in Southeast Asia, from where it spread to Africa. Pyrimethamine resistance in this parasite is known to be conferred by mutations in the gene encoding dihydrofolate reductase (dhfr). We have analyzed polymorphisms in dhfr as well as microsatellite haplotypes flanking this gene in a total of 285 isolates from different regions of Melanesia (Papua New Guinea, Vanuatu, and the Solomon Islands) and Southeast Asia (Thailand and Cambodia). Nearly all isolates (92%) in Melanesia were shown to carry a dhfr double mutation (CNRNI [underlining indicates the mutation]) at positions 50, 51, 59, 108, and 164, whereas 98% of Southeast Asian isolates were either triple (CIRNI) or quadruple (CIRNL) mutants. Microsatellite analysis revealed two distinct lineages of dhfr double mutants in Melanesia. One lineage had the same microsatellite haplotype as that previously reported for Southeast Asia and Africa, suggesting the spread of this allele to Melanesia from Southeast Asia. The other lineage had a unique, previously undescribed microsatellite haplotype, indicative of the de novo emergence of pyrimethamine resistance in Melanesia.

Population pharmacokinetic and pharmacodynamic modelling of the antimalarial chemotherapy chlorproguanil/dapsone

AIMS

To determine the population pharmacokinetics of chlorproguanil, dapsone and the active metabolite of chlorproguanil, chlorcycloguanil; and to estimate the duration of parasitocidal activity for chlorpoguanil/dapsone against Plasmodium falciparum isolates of varying sensitivity.

METHODS

Rich and sparse pharmacokinetic data were collected prospectively from: healthy volunteers (n=48) and adults (n=65) and children (n=68) suffering from P. falciparum malaria. All subjects received 2.0 mg kg-1 of chlorproguanil and 2.5 mg kg-1 of dapsone.

RESULTS

The population pharmacokinetic parameter estimates for chlorproguanil were ka=00.09 h-1 (intersubject variability was 44%), CL/F=51.53 l h-1 (57%), CLD/F=54.67 l h-1, V1/F=234.40 l (50%) and V2/F=1612.75 l; for dapsone were ka=00.93 h-1, CL/F=1.99 l h-1 (72%) and V/F=76.96 l (48%); and for chlorcycloguanil were CLm/Fm=3.72 l h-1 kg-1 (67%) and Vm/Fm=12.76 l kg-1 (64%). For dapsone, CL/F and V/F were both significantly positively correlated with body weight. For a 10-kg child, the mean duration of parasitocidal activity for chlorproguanil/dapsone against the three most susceptible P. falciparum strains was 4.5 days [5th and 95th percentiles 2.4, 7.3] for W282; 5.9 days (3.6, 9.7) for ItG2F6; and 6.1 days (3.7, 10.1) for K39. For an isolate with the ile-164-leu mutation, V1/S, activity ranged from 0.8 days (0.0, 3.3) for a 10-kg child to 1.8 days (0.0, 4.0) for a 60-kg adult.

CONCLUSIONS

Plasmodium falciparum malaria has no effect on the pharmacokinetic parameters for chlorproguanil, dapsone or chlorcycloguanil. Chlorproguanil/dapsone will probably prove to be ineffective against parasite strains with the mutation ile-164-leu, were these to become prevalent in Africa.

Potential impact of intermittent preventive treatment (IPT) on spread of drug-resistant malaria

BACKGROUND

Treatment of asymptomatic individuals, regardless of their malaria infection status, with regularly spaced therapeutic doses of antimalarial drugs has been proposed as a method for reducing malaria morbidity and mortality. This strategy, called intermittent preventive treatment (IPT), is currently employed for pregnant women and is being studied for infants (IPTi) as well. As with any drug-based intervention strategy, it is important to understand how implementation may affect the spread of drug-resistant parasites. This is a difficult issue to address experimentally because of the limited size and duration of IPTi trials as well as the intractability of distinguishing the spread of resistance due to conventional treatment of malaria episodes versus that due to IPTi when the same drug is used in both contexts.

METHODS AND FINDINGS

Using a mathematical model, we evaluated the possible impact of treating individuals with antimalarial drugs at regular intervals regardless of their infection status. We translated individual treatment strategies and drug pharmacokinetics into parasite population dynamic effects and show that immunity, treatment rate, drug decay kinetics, and presumptive treatment rate are important factors in the spread of drug-resistant parasites. Our model predicts that partially resistant parasites are more likely to spread in low-transmission areas, but fully resistant parasites are more likely to spread under conditions of high transmission, which is consistent with some epidemiological observations. We were also able to distinguish between spread of resistance due to treatment of symptomatic infections and that due to IPTi. We showed that IPTi could accelerate the spread of resistant parasites, but this effect was only likely to be significant in areas of low or unstable transmission.

CONCLUSIONS

The results presented here demonstrate the importance of considering both the half-life of a drug and the existing level of resistance when choosing a drug for IPTi. Drugs to which little or no resistance exists are not advisable for IPT in high-transmission areas, but IPTi is not likely to significantly impact the spread of highly resistant parasites in areas where partial resistance is already established. IPTi is more likely to accelerate the spread of resistance in high-transmission areas than is IPT in adults (i.e., pregnant women).

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.

Mechanisms of resistance of malaria parasites to antifolates

Antifolate antimalarial drugs interfere with folate metabolism, a pathway essential to malaria parasite survival. This class of drugs includes effective causal prophylactic and therapeutic agents, some of which act synergistically when used in combination. Unfortunately, the antifolates have proven susceptible to resistance in the malaria parasite. Resistance is caused by point mutations in dihydrofolate reductase and dihydropteroate synthase, the two key enzymes in the folate biosynthetic pathway that are targeted by the antifolates. Resistance to these drugs arises relatively rapidly in response to drug pressure and is now common worldwide. Nevertheless, antifolate drugs remain first-line agents in several sub-Saharan African countries where chloroquine resistance is widespread, at least partially because they remain the only affordable, effective alternative. New antifolate combinations that are more effective against resistant parasites are being developed and in one case, recently introduced into use. Combining these antifolates with drugs that act on different targets in the parasite should greatly enhance their effectiveness as well as deter the development of resistance. Molecular epidemiological techniques for monitoring parasite drug resistance may contribute to development of strategies for prolonging the useful therapeutic life of this important class of drugs.

Artesunate–dapsone–proguanil treatment of falciparum malaria: genotypic determinants of therapeutic response

The combination of chlorproguanil and dapsone is being considered as an alternative antimalarial to sulfadoxine-pyrimethamine in Africa, because of its greater efficacy against resistant parasites, and its shorter half-lives, which exert less selective pressure for the emergence of resistance. A triple artesunate-chlorproguanil-dapsone combination is under development. In a previous study of relatively low-dose chlorproguanil-dapsone in multidrug-resistant falciparum malaria in Thailand failure rates were high. Proguanil is inexpensive, widely available and very similar to chlorproguanil. The safety and efficacy of artesunate-dapsone-proguanil (artesunate 4 mg/kg, dapsone 2.5mg/kg, proguanil 8 mg/kg daily for three days), was studied prospectively in 48 Thai adult patients with acute falciparum malaria followed daily for 28 days. Eleven of these had a recrudescence of their infection. Genotyping of Plasmodium falciparum dihydrofolate reductase (dhfr) and dihydropteroate synthase (dhps) indicated that the Pfdhfr I164L mutation was the main determinant of therapeutic outcome; all 11 failures carried this mutation (failure rate 11/37; 30%) whereas none of the 11 infections with 'wild type' 164 genotypes failed. The addition of artesunate considerably augments the antimalarial activity of the biguanide-dapsone combination, but this is insufficient for infections with parasites carrying the highly antifol-resistant Pfdhfr I164L mutation.

Therapy of falciparum malaria in sub-saharan Africa: from molecule to policy

The burden of falciparum malaria remains as great as ever, and, as has probably always been the case, it is carried mainly by tropical Africa. Of the various means available for the control of malaria, the use of effective drugs remains the most important and is likely to remain so for a considerable time to come. Unfortunately, the extensive development of resistance by the parasite threatens the utility of most of the affordable classes of drug: the development of novel antimalarials has never been more urgently needed. Any attempt to understand the vast complexities of falciparum malaria in Africa requires an ability to think "from molecule to policy." In consequence, the review ambitiously tries to examine the current pharmacopeia, the process by which new drugs are developed and the ways in which drugs are actually used, in both the formal and informal health sectors. The informal sector is particularly important in Africa, where around half of all antimalarial treatments are bought from informal outlets and taken at home without supervision by health care professionals: the potential impact of adherence on clinical outcome is discussed. Given that the full costs are carried by the patient in a large proportion of cases, the importance of drug affordability is explored. The review also discusses the splicing of new drugs into national policy. The various parameters that feed into deliberations on changes in drug policy are discussed.

Monitoring antimalarial drug resistance: making the most of the tools at hand

Most countries in resource-poor, malaria-endemic areas lack current and comprehensive information on antimalarial drug efficacy, resulting in sub-optimal antimalarial treatment policies. Many African countries continue to use chloroquine despite very high rates of resistance, and others have changed policies based on limited data, with mixed success. Methods for measuring antimalarial drug efficacy and resistance include in vivo studies of clinical efficacy and parasitological resistance, in vitro susceptibility assays and molecular markers for resistance to some drugs. These methods have the potential to be used in an integrated fashion to provide timely information that is useful to policy makers, and the combined use of in vivo and molecular surveys could greatly extend the coverage of resistance monitoring. Malawi, the first African country to change from chloroquine to sulfadoxine/pyrimethamine at the national level, serves as a case study for resistance monitoring and evidence-based antimalarial policies. Molecular, in vitro and in vivo studies demonstrate that chloroquine-sensitive parasites reemerged and now predominate in Malawi after it switched from chloroquine to sulfadoxine/pyrimethamine. This raises the intriguing possibility of rotating antimalarial drugs.

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.

Molecular surveillance of the antifolate-resistant mutation I164L in imported African isolates of Plasmodium falciparum in Europe: sentinel data from TropNetEurop

BACKGROUND

Malaria parasites that carry the DHFR-mutation I164L are not only highly resistant to sulfadoxine-pyrimethamine but also to the new antimalarial drug chlorproguanil-dapsone. The spread of this mutation in Africa would result in a public health disaster since there is a lack of effective alternatives that are both affordable and safe. Up to now, this mutation has only been described in Asian and Latin-American countries. The objective of this study was to assess the prevalence of this mutation in African isolates of Plasmodium falciparum that have been imported into Europe through travellers.

METHODS

TropNetEurop is a network for the surveillance of travel-associated diseases and seems to cover approximately 12% of all malaria cases imported into Europe. Within this network we screened 277 imported African isolates of P. falciparum with the help of PCR- and enzyme-digestion-methods for the antifolate-resistant mutation I164L.

RESULTS

The I164L mutation was not detected in any of the isolates tested.

DISCUSSION

Continuous molecular surveillance of mutations in P. falciparum, as it is practised within TropNetEurop, is an essential tool for the understanding and early detection of the spread of antimalarial drug resistance in Africa.

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.

Combination therapy for malaria: the way forward?

Unless new strategies are deployed to combat malaria, the already enormous health and economic burden related to the disease in tropical countries is bound to worsen. The main obstacle to malaria control is the emergence of drug resistant strains of Plasmodium falciparum. As for HIV/AIDS and tuberculosis, the use of combinations of antimalarial drugs reduces the risk of selecting for resistant mutants of the plasmodial parasites. In large field trials, the combination of an artemisinin derivative and a partner drug with an unrelated mode of action (in this case mefloquine), has shown a remarkable double effect: preventing the emergence and spread of drug resistance, and interrupting the transmission of P. falciparum. This has opened the way for a new approach to the deployment of antimalarial drugs. Coupled with early detection and confirmed diagnosis, this strategy represents the only way forward in the chemotherapy of malaria. Massive economic assistance will be needed to detect and treat adequately the estimated 500 million cases of malaria per year, but without radical action there is no prospect of 'Rolling Back' malaria.

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.

Clinical status and implications of antimalarial drug resistance

Africa carries the greatest burden of disease caused by Plasmodium falciparum, and we can expect this burden to rise in the near future, mainly because of drug resistance. Although effective drugs are available (such as artemether-lumefantrine, mefloquine, atovaquone-proguanil and halofantrine) they are uniformly too expensive for routine use. Affordable options include chloroquine plus sulfadoxine-pyrimethamine (SP), amodiaquine (alone or in combination with SP) and chlorproguanil-dapsone. Artemisinin combination therapy may offer considerable advantages over alternative therapies, but its introduction faces considerable logistic difficulty.

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.

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.

Molecular epidemiology of Plasmodium falciparum antifolate resistance in Vietnam: genotyping for resistance variants of dihydropteroate synthase and dihydrofolate reductase

Using PCR techniques, we analysed the dihydropteroate synthase (DHPS) mutations associated with sulphonamide resistance and the dihydrofolate reductase (DHFR) mutations associated with resistance to pyrimethamine and cycloguanil in samples from Plasmodium falciparum-infected Vietnamese patients. Of the 40 samples analysed, 39 had DHFR mutations associated with high level resistance to pyrimethamine, whereas only three had mutations at position 164, which is linked to cross resistance to both DHFR inhibitors. The DHPS, 437Gly variant associated with very mild resistance to sulphadoxine was found in 38 out of the 40 samples. Of seven samples resistant to Fansidar in vivo, only two were fully explained by the currently documented DHPS mutations. The treatment failure could be due to a high level of pyrimethamine resistance caused by the detected mutations. Most patients, however, were cured with a single dose of Fansidar in spite of the high number of resistance mutations found.

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).

The efficacy of antifolate antimalarial combinations in Africa: a predictive model based on pharmacodynamic and pharmacokinetic analyses

At present, effective treatment for non-severe malaria is the most important malaria control strategy in Africa. Pyrimethamine-sulfadoxine (PSD) is rapidly becoming the first-line treatment in areas of chloroquine resistance, although the parasite chemoresistance factors that dispose towards clinical failure with PSD are still unclear. Here, Bill Watkins and colleagues analyse the relationship between the pharmacokinetic properties of two treatment combinations (PSD and chlorproguanil-dapsone) in vivo and the respective in vitro isobolograms for parasites with specific drug-resistance patterns. From this relationship, they develop a hypothesis that may explain clinical drug failure and differential efficacy between treatments. The deductions can be tested in field studies to validate or refute the model.

Chlorproguanil-dapsone: effective treatment for uncomplicated falciparum malaria

Pyrimethamine-sulfadoxine, the first choice for uncomplicated falciparum malaria in Africa, exerts strong selection pressure for resistance because of its slow elimination. It is likely that resistance will emerge rapidly, and there is no widely affordable replacement. Chlorproguanil-dapsone is cheap, rapidly eliminated, more potent than pyrimethamine-sulfadoxine, and could be introduced in the near future to delay the onset of antifolate resistance and as "salvage therapy" for pyrimethamine-sulfadoxine failure. A total of 448 children were randomly allocated (double blind) to either a single dose of pyrimethamine-sulfadoxine or to one of two chlorproguanil-dapsone regimens: a single dose or three doses at 24-h intervals. Reinfections are clinically indistinguishable from recrudescence and are more likely after treatment with rapidly eliminated drugs; we measured the incidence of parasitemia in 205 initially aparasitemic children to allow comparison with the three treatment groups. The patients and a community surveillance group were followed up for 28 days. At the study end point, 31.2% (95% confidence interval, 24.9-38.0) of the community surveillance group subjects were parasitemic, compared with subjects in the treatment groups, whose rates of parasitemia were 40.8% (32.9-49.0; relative risk [RR], 1.31 [0.99-1.73]) after triple-dose chlorproguanil-dapsone, 19.7% (13.5-27.2; RR, 0.63 [0.43-0.93]) after pyrimethamine-sulfadoxine, and 65.6% (57.5-73.0; RR, 2.10 [1.66-2.65]) after single-dose chlorproguanil-dapsone. Pyrimethamine-sulfadoxine and triple-dose chlorproguanil-dapsone were effective treatments. Pyrimethamine-sulfadoxine provided chemoprophylaxis during follow-up because of its slow elimination. Triple-dose chlorproguanil-dapsone should now be developed in an attempt to reduce the rate of emergence of antifolate resistance in Africa and for affordable salvage therapy in cases of pyrimethamine-sulfadoxine failure.