Mutations in dhfr in Plasmodium falciparum infections selected by chlorproguanil-dapsone treatment

Recent advances in transmission-blocking drugs for malaria elimination

The scientific community worldwide has realized that malaria elimination will not be possible without development of safe and effective transmission-blocking interventions. Primaquine, the only WHO recommended transmission-blocking drug, is not extensively utilized because of the toxicity issues in G6PD deficient individuals. Therefore, there is an urgent need to develop novel therapeutic interventions that can target malaria parasites and effectively block transmission. But at first, it is imperative to unravel the existing portfolio of transmission-blocking drugs. This review highlights transmission-blocking potential of current antimalarial drugs and drugs that are in various stages of clinical development. The collective analysis of the relationships between the structure and the activity of transmission-blocking drugs is expected to help in the design of new transmission-blocking antimalarials.

Sulfadoxine-pyrimethamine resistance in Plasmodium falciparum: a zoomed image at the molecular level within a geographic context

Antimalarial chemotherapy is one of the main pillars in the prevention and control of malaria. Following widespread resistance of Plasmodium falciparum to chloroquine, sulfadoxine-pyrimethamine came to the scene as an alternative to the cheap and well-tolerated chloroquine. However, widespread resistance to sulfadoxine-pyrimethamine has been documented. In vivo efficacy tests are the gold standard for assessing drug resistance and treatment failure. However, they have many disadvantages, such as influence of host immunity and drug pharmacokinetics. In vitro tests of antimalarial drug efficacy also have many technical difficulties. Molecular markers of resistance have emerged as epidemiologic tools to investigate antimalarial drug resistance even before becoming clinically evident. Mutations in P. falciparum dihydrofolate reductase and dihydrofolate synthase have been extensively studied as molecular markers for resistance to pyrimethamine and sulfadoxine, respectively. This review highlights the resistance of P. falciparum at the molecular level presenting both supporting and opposing studies on the utility of molecular markers.

Molecular correlates of high-level antifolate resistance in Rwandan children with Plasmodium falciparum malaria

Antifolate drugs have an important role in the treatment of malaria. Polymorphisms in the genes encoding the dihydrofolate reductase and dihydropteroate synthetase enzymes cause resistance to the antifol and sulfa drugs, respectively. Rwanda has the highest levels of antimalarial drug resistance in Africa. We correlated the efficacy of chlorproguanil-dapsone plus artesunate (CPG-DDS+A) and amodiaquine plus sulfadoxine-pyrimethamine (AQ+SP) in children with uncomplicated malaria caused by Plasmodium falciparum parasites with pfdhfr and pfdhps mutations, which are known to confer reduced drug susceptibility, in two areas of Rwanda. In the eastern province, where the cure rates were low, over 75% of isolates had three or more pfdhfr mutations and two or three pfdhps mutations and 11% had the pfdhfr 164-Leu polymorphism. In the western province, where the cure rates were significantly higher (P < 0.001), the prevalence of multiple resistance mutations was lower and the pfdhfr I164L polymorphism was not found. The risk of treatment failure following the administration of AQ+SP more than doubled for each additional pfdhfr resistance mutation (odds ratio [OR] = 2.4; 95% confidence interval [CI] = 1.01 to 5.55; P = 0.048) and each pfdhps mutation (OR = 2.1; 95% CI = 1.21 to 3.54; P = 0.008). The risk of failure following CPG-DDS+A treatment was 2.2 times higher (95% CI = 1.34 to 3.7) for each additional pfdhfr mutation, whereas there was no association with mutations in the pfdhps gene (P = 0.13). The pfdhfr 164-Leu polymorphism is prevalent in eastern Rwanda. Antimalarial treatments with currently available antifol-sulfa combinations are no longer effective in Rwanda because of high-level resistance.

Molecular surveillance for drug-resistant Plasmodiumfalciparum malaria in Malawi

We assessed the presence of point mutations associated with resistance to chloroquine (CQ) and sulphadoxine-pyrimethamine (SP) in 178 Plasmodiumfalciparum infections from three geographically distinct sites in Malawi. We confirm that CQ-resistance mutations are now rare in Malawi, being detectable at very low frequencies (2-4%) in infections from two of the three study sites. We also show that over 90% of infections from each of the three study sites carry a set of three dihydrofolate reductase (dhfr) and two dihydropteroate synthase (dhps) mutations strongly associated with SP treatment failure. In this short communication, we present these molecular data and discuss their implications for Malawi's first-line antimalarial treatment policy.

Analysis of sulphadoxine/pyrimethamine resistance-conferring mutations of Plasmodium falciparum from Mozambique reveals the absence of the dihydrofolate reductase 164L mutant

Background

Plasmodium falciparum is the predominant human malaria species in Mozambique and a lead cause of mortality among children and pregnant women nationwide. Sulphadoxine/pyrimethamine (S/P) is used as first line antimalarial treatment as a partner drug in combination with artesunate.

Methods

A total of 92 P. falciparum-infected blood samples, from children with uncomplicated malaria attending the Centro de Saude de Bagamoyo in the Province of Maputo-Mozambique, were screened for S/P resistance-conferring mutations in the pfdhfr and pfdhps genes using a nested mutation-specific polymerase chain reaction and restriction digestion (PCR-RFLP). The panel of genetic polymorphisms analysed included the pfdhfr 164L mutation, previously reported to be absent or rare in Africa.

Results

The frequency of the S/P resistance-associated pfdhfr triple mutants (51I/59R/108N) and of pfdhfr/pfdhps quintuple mutants (51I/59R/108N + 437G/540E) was 93% and 47%, respectively. However, no pfdhfr 164L mutants were detected.

Conclusion

The observation that a considerably high percentage of P. falciparum parasites contained S/P resistance-associated mutations raises concerns about the validity of this drug as first-choice treatment in Mozambique. On the other hand, no pfdhfr 164L mutant was disclosed, corroborating the view that that this allele is still rare in Africa.

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.

In vitro metabolism of phenoxypropoxybiguanide analogues in human liver microsomes to potent antimalarial dihydrotriazines

Phenoxypropoxybiguanides, such as 1 (PS-15), are prodrugs analogous to the relationship of proguanil and its active metabolite cycloguanil. Unlike cycloguanil, however, 1a (WR99210), the active metabolite of 1, has retained in vitro potency against newly emerging antifolate-resistant malaria parasites. Unfortunately, manufacturing processes and gastrointestinal intolerance have prevented the clinical development of 1. In vitro antimalarial activity and in vitro metabolism studies have been performed on newly synthesized phenoxypropoxybiguanide analogues. All of the active dihydrotriazine metabolites exhibited potent antimalarial activity with in vitro IC(50) values less than 0.04 ng/mL. In vitro metabolism studies in human liver microsomes identified the production of not only the active dihydrotriazine metabolite, but also a desalkylation on the carbonyl chain, and multiple hydroxylated metabolites. The V(max) for production of the active metabolites ranged from 10.8 to 27.7 pmol/min/mg protein with the K(m) ranging from 44.8 to 221 microM. The results of these studies will be used to guide the selection of a lead candidate.

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.

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.

Drug susceptibility and genetic evaluation of Plasmodium falciparum isolates obtained in four distinct geographical regions of Kenya

The drug resistance profiles of Plasmodium falciparum isolated from four regions in Kenya were analyzed for drug resistance profiles. We observed variability in resistance to a broad range of antimalarial drugs across Kenya as determined from in vitro drug susceptibility screening and genotyping analysis.

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.

Molecular monitoring of the Leu-164 mutation of dihydrofolate reductase in a highly sulfadoxine/pyrimethamine-resistant area in Africa

The selection of point mutation at codon 164 (from isoleucine to leucine) of the dihydrofolate reductase (DHFR) enzyme in Plasmodium falciparum is associated with high sulfadoxine /pyrimethamine (SP) resistance. Using the yeast expression system that allows the detection of dhfr allele present at low level, the presence of this mutation had previously been reported between 1998–1999 in Muheza, Tanzania, an area of high SP resistance. Eighty five P. falciparum isolates, obtained from the same area between 2002 and 2003, were analysed for the presence of Leu-164 mutation, using standard protocol based on PCR-RFLP. None of the isolates had the Leu-164 mutation.

Variation of malaria transmission and morbidity with altitude in Tanzania and with introduction of alphacypermethrin treated nets

Background

Highland areas with naturally less intense malaria transmission may provide models of how lowland areas might become if transmission was permanently reduced by sustained vector control. It has been argued that vector control should not be attempted in areas of intense transmission.

Methods

Mosquitoes were sampled with light traps, pyrethrum spray and window exit traps. They were tested by ELISA for sporozoites. Incidence of malaria infection was measured by clearing existing infections from children with chlorproguanil-dapsone and then taking weekly blood samples. Prevalence of malaria infection and fever, anaemia and splenomegaly were measured in children of different age groups. All these measurements were made in highland and lowland areas of Tanzania before and after provision of bednets treated with alphacypermethrin.

Results

Entomological inoculation rates (EIR) were about 17 times greater in a lowland than a highland area, but incidence of infection only differed by about 2.5 times. Malaria morbidity was significantly less prevalent in the highlands than the lowlands. Treated nets in the highlands and lowlands led to 69–75% reduction in EIR. Malaria morbidity showed significant decline in younger children at both altitudes after introduction of treated nets. In children aged 6–12 the decline was only significant in the highlands

Conclusions

There was no evidence that the health benefits to young children due to the nets in the lowlands were "paid for" by poorer health later in life. Our data support the idea of universal provision of treated nets, not a focus on areas of natural hypo-endemicity.

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.