Confirmation of emergence of mutations associated with atovaquone-proguanil resistance in unexposed Plasmodium falciparum isolates from Africa

Late clinical failure associated with cytochrome b codon 268 mutation during treatment of falciparum malaria with atovaquone-proguanil in traveller returning from Congo

Background

The drug combination atovaquone–proguanil, is recommended for treatment of uncomplicated falciparum malaria in France. Despite high efficacy, atovaquone–proguanil treatment failures have been reported. Resistance to cycloguanil, the active metabolite of proguanil, is conferred by multiple mutations in the Plasmodium falciparum dihydrofolate reductase (pfdhfr) and resistance to atovaquone by single mutation on codon 268 of the cytochrome b gene (pfcytb).

Case presentation

A 47-year-old female, native from Congo and resident in France, was admitted in hospital for uncomplicated falciparum malaria with parasitaemia of 0.5%, after travelling in Congo (Brazzaville and Pointe Noire). She was treated with atovaquone–proguanil (250 mg/100 mg) 4 tablets daily for 3 consecutive days. On day 5 after admission she was released home. However, many weeks after this episode, without having left France, she again experienced fever and intense weakness. On day 39 after the beginning of treatment, she consulted for fever, arthralgia, myalgia, photophobia, and blurred vision. She was hospitalized for uncomplicated falciparum malaria with a parasitaemia of 0.375% and treated effectively by piperaquine–artenimol (320 mg/40 mg) 3 tablets daily for 3 consecutive days. Resistance to atovaquone–proguanil was suspected. The Y268C mutation was detected in all of the isolates tested (D39, D42, D47). The genotyping of the pfdhfr gene showed a triple mutation (N51I, C59R, S108N) involved in cycloguanil resistance.

Conclusion

This is the first observation of a late clinical failure of atovaquone–proguanil treatment of P. falciparum uncomplicated malaria associated with pfcytb 268 mutation in a traveller returning from Congo. These data confirm that the Y268C mutation is associated with delayed recrudescence 4 weeks or more after initial treatment. Although atovaquone–proguanil treatment failures remain rare, an increased surveillance is required. It is essential to declare and publish all well-documented cases of treatment failures because it is the only way to evaluate the level of resistance to atovaquone.

Alkoxycarbonate Ester Prodrugs of Preclinical Drug Candidate ELQ-300 for Prophylaxis and Treatment of Malaria

ELQ-300 is a preclinical antimalarial drug candidate that is active against liver, blood, and transmission stages of Plasmodium falciparum. While ELQ-300 is highly effective when administered in a low multidose regimen, poor aqueous solubility and high crystallinity have hindered its clinical development. To overcome its challenging physiochemical properties, a number of bioreversible alkoxycarbonate ester prodrugs of ELQ-300 were synthesized. These bioreversible prodrugs are converted to ELQ-300 by host and parasite esterase action in the liver and bloodstream of the host. One such alkoxycarbonate prodrug, ELQ-331, is curative against Plasmodium yoelii with a single low dose of 3 mg/kg in a murine model of patent malaria infection. ELQ-331 is at least as fully protective as ELQ-300 in a murine malaria prophylaxis model when delivered 24 h before sporozoite inoculation at an oral dose of 1 mg/kg. Here, we show that ELQ-331 is a promising prodrug of ELQ-300 with improved physiochemical and metabolic properties and excellent potential for clinical formulation.

Molecular characterization of the cytochrome b gene and in vitro atovaquone susceptibility of Plasmodium falciparum isolates from Kenya

The prevalence of a genetic polymorphism(s) at codon 268 in the cytochrome b gene, which is associated with failure of atovaquone-proguanil treatment, was analyzed in 227 Plasmodium falciparum parasites from western Kenya. The prevalence of the wild-type allele was 63%, and that of the Y268S (denoting a Y-to-S change at position 268) mutant allele was 2%. There were no pure Y268C or Y268N mutant alleles, only mixtures of a mutant allele(s) with the wild type. There was a correlation between parasite 50% inhibitory concentration (IC50) and parasite genetic polymorphism; mutant alleles had higher IC50s than the wild type.

Novel Mutation in Cytochrome B of Plasmodium falciparum in One of Two Atovaquone-Proguanil Treatment Failures in Travelers Returning From Same Site in Nigeria

Background

 Atovaquone-proguanil (AP) is the most commonly used treatment for uncomplicated Plasmodium falciparum malaria in the United States. Apparent AP treatment failures were reported 7 months apart in 2 American travelers who stayed in the same compound for foreign workers in Rivers State, Nigeria.

Methods

 We analyzed pretreatment (day 0) and day of failure samples from both travelers for mutations in the P falciparum cytochrome B (pfcytb) and dihydrofolate reductase (pfdhfr) genes associated with resistance to atovaquone and cycloguanil, the active metabolite of proguanil, respectively. We genotyped the parasites and sequenced their mitochondrial genomes.

Results

 On day 0, both travelers had proguanil-resistant genotypes but atovaquone-sensitive cytb sequences. Day of failure samples exhibited mutations in cytb for both travelers. One traveler had the common Y268S mutation, whereas the other traveler had a previously unreported mutation, I258M. The travelers had unrelated parasite genotypes and different mitochondrial genomes.

Conclusions

 Despite the infections likely having been contracted in the same site, there is no evidence that the cases were related. The mutations likely arose independently during the acute infection or treatment. Our results highlight the importance of genotyping parasites and sequencing the full cytb and dhfr genes in AP failures to rule out transmission of AP-resistant strains and identify novel mechanisms of AP resistance.

A chemical genomic analysis of decoquinate, a Plasmodium falciparum cytochrome b inhibitor

Decoquinate has single-digit nanomolar activity against in vitro blood stage Plasmodium falciparum parasites, the causative agent of human malaria. In vitro evolution of decoquinate-resistant parasites and subsequent comparative genomic analysis to the drug-sensitive parental strain revealed resistance was conferred by two nonsynonymous single nucleotide polymorphisms in the gene encoding cytochrome b. The resultant amino acid mutations, A122T and Y126C, reside within helix C in the ubiquinol-binding pocket of cytochrome b, an essential subunit of the cytochrome bc₁ complex. As with other cytochrome bc₁ inhibitors, such as atovaquone, decoquinate has low nanomolar activity against in vitro liver stage P. yoelii and provides partial prophylaxis protection when administered to infected mice at 50 mg kg^–1. In addition, transgenic parasites expressing yeast dihydroorotate dehydrogenase are >200-fold less sensitive to decoquinate, which provides additional evidence that this drug inhibits the parasite’s mitochondrial electron transport chain. Importantly, decoquinate exhibits limited cross-resistance to a panel of atovaquone-resistant parasites evolved to harbor various mutations in cytochrome b. The basis for this difference was revealed by molecular docking studies, in which both of these inhibitors were shown to have distinctly different modes of binding within the ubiquinol-binding site of cytochrome b.

Antimalarial drugs: modes of action and mechanisms of parasite resistance

Malaria represents one of the most serious threats to human health worldwide, and preventing and curing this parasitic disease still depends predominantly on the administration of a small number of drugs whose efficacy is continually threatened and eroded by the emergence of drug-resistant parasite populations. This has an enormous impact on the mortality and morbidity resulting from malaria infection, especially in sub-Saharan Africa, where the lethal human parasite species Plasmodium falciparum accounts for approximately 90% of deaths recorded globally. Successful treatment of uncomplicated malaria is now highly dependent on artemisinin-based combination therapies. However, the first cases of artemisinin-resistant field isolates have been reported recently and potential replacement antimalarials are only in the developmental stages. Here, we summarize recent progress in tackling the problem of parasite resistance and discuss the underlying molecular mechanisms that confer resistance to current antimalarial agents as far as they are known, understanding of which should assist in the rational development of new drugs and the more effective deployment of older ones.

Plasmodium chabaudi chabaudi malaria parasites can develop stable resistance to atovaquone with a mutation in the cytochrome b gene

BACKGROUND

Plasmodium falciparum, has developed resistance to many of the drugs in use. The recommended treatment policy is now to use drug combinations. The atovaquone-proguanil (AP) drug combination, is one of the treatment and prophylaxis options. Atovaquone (ATQ) exerts its action by inhibiting plasmodial mitochondria electron transport at the level of the cytochrome bc1 complex. Plasmodium falciparum in vitro resistance to ATQ has been associated with specific point mutations in the region spanning codons 271-284 of the cytochrome b gene. ATQ -resistant Plasmodium yoelii and Plasmodium berghei lines have been obtained and resistant lines have amino acid mutations in their CYT b protein sequences. Plasmodium chabaudi model for studying drug-responses and drug-resistance selection is a very useful rodent malaria model but no ATQ resistant parasites have been reported so far. The aim of this study was to determine the ATQ sensitivity of the P. chabaudi clones, to select a resistant parasite line and to perform genotypic characterization of the cytb gene of these clones.

METHODS

To select for ATQ resistance, Plasmodium. chabaudi chabaudi clones were exposed to gradually increasing concentrations of ATQ during several consecutive passages in mice. Plasmodium chabaudi cytb gene was amplified and sequenced.

RESULTS

ATQ resistance was selected from the clone AS-3CQ. In order to confirm whether an heritable genetic mutation underlies the response of AS-ATQ to ATQ, the stability of the drug resistance phenotype in this clone was evaluated by measuring drug responses after (i) multiple blood passages in the absence of the drug, (ii) freeze/thawing of parasites in liquid nitrogen and (iii) transmission through a mosquito host, Anopheles stephensi. ATQ resistance phenotype of the drug-selected parasite clone kept unaltered. Therefore, ATQ resistance in clone AS-ATQ is genetically encoded. The Minimum Curative Dose of AS-ATQ showed a six-fold increase in MCD to ATQ relative to AS-3CQ.

CONCLUSIONS

A mutation was found on the P. chabaudi cytb gene from the AS-ATQ sample a substitution at the residue Tyr268 for an Asn, this mutation is homologous to the one found in P. falciparum isolates resistant to ATQ.

Factors contributing to anaemia after uncomplicated Plasmodiumfalciparum malaria in children

The factors contributing to anaemia in falciparum malaria were characterized in 1261 prospectively studied children in an endemic area of southwestern Nigeria. Of these, 487 (39%) presented with anaemia (haematocrit <30%). The following were found to be independent risk factors for anaemia at presentation: age <5 years, history of illness >3 days before presentation, presence of fever, a palpable liver, >parasitaemia 10,000/microl blood, and gametocytaemia. The mean maximum fractional fall in haematocrit (FFH) after treatment was 13.8% (95% confidence interval [CI] 13-14.6) of the baseline value. This occurred 3 days after treatment began and correlated positively with enrolment haematocrit. In children whose haematocrit was >30% at enrolment, the following were found to be independent risk factors associated with subsequent development of anaemia during follow-up: age <5 years and parasitaemia > or =100,000 parasites/microl. Haematological recovery was usually complete by 4-5 weeks, but was slower in children who were anaemic at enrolment and in those with recrudescence of their infections. Half of the children with recrudescence were still anaemic at 4 weeks. These findings have implications for the control of the burden of malarial anaemia in children in sub-Saharan African countries.

Mutations in the Plasmodium falciparum cytochrome b gene are associated with delayed parasite recrudescence in malaria patients treated with atovaquone-proguanil

Background

Fixed-dose combination antimalarial drugs have played an increasingly important role in the treatment and chemoprophylaxis of falciparum malaria since the worldwide failure of monotherapy with chloroquine. Atovaquone-proguanil is one such combination drug used both for prophylaxis in travellers, and for treatment of acute malaria cases in European hospitals and clinics.

Methods

A series of eight atovaquone-proguanil treatment failures and two prophylaxis breakthroughs from four UK hospitals from 2004–2008 were analysed for evidence of mutations in the pfcyt-b gene, previously found to be associated with failure of the atovaquone component.

Results

Parasites carrying pfcyt-b mutations were found in five falciparum malaria patients with recrudescent parasitaemia occurring weeks after apparently successful treatment of a primary infection with atovaquone-proguanil. Four of these cases carried parasites with the Tyr268Cys mutation in pfcyt-b, previously reported in two French patients with malaria. In contrast, mutations in pfcyt-b were not found in three patients treated with atovaquone-proguanil who exhibited delayed clearance of the primary infection, nor in two returning travellers with malaria who had used the combination for prophylaxis. Using current and previously published data, mean time to recrudescence of parasites carrying pfcytb codon 268 mutations was estimated as 28.0 days after treatment (95% C.I. 23.0 – 33.0 days), whereas treatment failures without codon 268 mutations received rescue treatment an average of 4.71 days after initial AP treatment (95% C.I. 1.76 – 7.67 days).

Conclusion

Genetically-determined parasite resistance to atovaquone is associated with delayed recrudescence of resistant parasites three weeks or more after initial clearance of parasitaemia by atovaquone/proguanil therapy. The 268-Cys allele of pfcyt-b may have been overlooked in previous studies of atovaquone-proguanil treatment failure as it is not detected by current RFLP methods.

Effects of treatment and drug resistance on the transmission dynamics of malaria in endemic areas

We present a mathematical model for malaria treatment and spread of drug resistance in an endemic population. The model considers treated humans that remain infectious for some time and partially immune humans who are also infectious to mosquitoes although their infectiousness is always less than their non immune counterparts. The model is formulated by considering delays in the latent periods in both mosquito and human populations and in the period within which partial immunity is lost. Qualitative analysis of the model including positivity and boundedness of solutions is performed. Analysis of the reproductive numbers shows that if the treated humans become immediately uninfectious to mosquitoes then treatment will always reduce the number of sensitive infections. If however treated humans are infectious then for treatment to effectively reduce the number of sensitive infections, the ratio of the infectious period of the treated humans to the infectious period of the untreated humans multiplied by the ratio of the transmission rate from a treated human to the transmission rate of an untreated human should be less than one. Our results show that the spread of drug resistance with treatment as a control strategy depends on the ratio of the infectious periods of treated and untreated humans and on the transmission rates from infectious humans with resistant and sensitive infections. Numerical analysis is performed to assess the effects of treatment on the spread of resistance and infection. The study provides insight into the possible intervention strategies to be employed in malaria endemic populations with resistant parasites by identifying important parameters.

In vitro atovaquone/proguanil susceptibility and characterization of the cytochrome b gene of Plasmodium falciparum from different endemic regions of Thailand

BACKGROUND

The emergence of Plasmodium falciparum resistant to most currently used antimalarial drugs is the major problem in malaria control along the Thai-Myanmar and Thai-Cambodia borders. Although artemisinin-based combination therapy has been recommended for the treatment of multidrug-resistant falciparum malaria, these combinations are not available for some people, such as travelers from North America. A fixed-dose combination of atovaquone and proguanil (Malarone) has been proved to be effective for the treatment and prophylaxis of malaria which is already approved by countries in North America and Europe. Determination of the phenotypes and genotypes related to atovaquone/proguanil response in Thai isolates of P. falciparum will be useful for rationale drug use. The main purpose of this study was to explore the in vitro atovaquone/proguanil susceptibility of recently adapted Thai isolates of P. falciparum. Genotypic characterization of the cytb gene of these isolates was also determined since it has been reported that point mutations, particularly codon 268 in the cytochrome b gene (cytb) have been linked to atovaquone/proguanil treatment failure.

METHODS

Eighty three P. falciparum isolates collected during 1998 to 2005 from four different multidrug resistance areas of Thailand were determined for the in vitro atovaquone/proguanil susceptibilities using radioisotopic assay. Mutations in the cytb gene were determined by PCR-RFLP and sequence analysis.

RESULTS

The mean atovaquone and proguanil IC50 was 3.4 nM and 36.5 muM, respectively. All 83 Thai isolates were atovaquone sensitive. None of the 83 isolates contained the mutations at codon 268 of the cytb gene. DNA sequencing of the cytb gene of 20 parasite isolates showed no other mutations.

CONCLUSION

In agreement with a recent efficacy study of atovaquone/proguanil, the present information indicates that atovaquone/proguanil can be one of the drugs of choice for the treatment and prophylaxis of multidrug-resistant falciparum malaria in Thailand.

A drug-selected Plasmodium falciparum lacking the need for conventional electron transport

Mitochondrial electron transport is essential for survival in Plasmodium falciparum, making the cytochrome (cyt) bc(1) complex an attractive target for antimalarial drug development. Here we report that P. falciparum cultivated in the presence of a novel cyt bc(1) inhibitor underwent a fundamental transformation in biochemistry to a phenotype lacking a requirement for electron transport through the cyt bc(1) complex. Growth of the drug-selected parasite clone (SB1-A6) is robust in the presence of diverse cyt bc(1) inhibitors, although electron transport is fully inhibited by these same agents. This transformation defies expected molecular-based concepts of drug resistance, has important implications for the study of cyt bc(1) as an antimalarial drug target, and may offer a glimpse into the evolutionary future of Plasmodium.

Sequence analysis of Plasmodium falciparum cytochrome b in multiple geographic sites

Background

The antimalarial drug atovaquone specifically targets Plasmodium falciparum cytochrome b (Pfcytb), a mitochondrial gene with uniparental inheritance. Cases of resistance to atovaquone associated with mutant Pfcytb have been reported, justifying efforts to better document the natural polymorphism of this gene. To this end, a large molecular survey was conducted in several malaria endemic areas where atovaquone was not yet in regular use.

Methods

The polymorphism of the Pfcytb was analysed by direct sequencing of PCR products corresponding to the full length coding region. Sequence was generated for 671 isolates originating from three continents: Africa (Senegal, Ivory Coast, Central African Republic and Madagascar), Asia (Cambodia) and South America (French Guiana).

Results

Overall, 11 polymorphic sites were observed, of which eight were novel mutations. There was a large disparity in the geographic distribution of the mutants. All isolates from Senegal, Central African Republic and Madagascar displayed a Camp/3D7 wild type Pfcytb sequence, as did most samples originating from Cambodia and Ivory Coast. One synonymous (t759a at codon V253V) and two non-synonymous (t553g and a581g at codons F185V and H194R, respectively) singletons were detected in Ivory Coast. Likewise, two synonymous (a126t and c793t at codons -T42T and L265L, respectively) singletons were observed in Cambodia. In contrast, seven mutated sites, affecting seven codons and defining four mutant haplotypes were observed in French Guiana. The wild type allele was observed in only 14% of the French Guiana isolates. The synonymous c688t mutation at position L230L was highly prevalent; the most frequent allele was the c688t single mutant, observed in 84% of the isolates. The other alleles were singletons (a126t/a165c, a4g/a20t/a1024c and a20t/t341c/c688t corresponding to T42T/S55S, N2D/N71I/I342L, N71I/L114S/L230L, respectively" please replace with ' corresponding to T42T/S55S, N2D/N71I/I342L and N71I/L114S/L230L, respectively). The codon 268 polymorphisms associated with atovaquone resistance were not observed in the panel the isolates studied. Overall, the wild type PfCYTb protein isoform was highly predominant in all study areas, including French Guiana, suggesting stringent functional constraints.

Conclusion

These data along with previously identified Pfcytb field polymorphisms indicate a clustering of molecular signatures, suggesting different ancestral types in South America and other continents. The absence of mutations associated with most atovaquone-proguanil clinical failures indicates that the atovaquone-proguanil association is an interesting treatment option in the study areas.

Molecular diagnostic and surveillance tools for global malaria control

Malaria is the most devastating parasitic infection in the world, annually causing over 1 million deaths and extensive morbidity. The global burden of malaria has increased over the last several decades, as have rates of imported malaria into non-endemic regions. Rapid and accurate diagnostics are a crucial component of malaria control strategies, and epidemiological surveillance is required to monitor trends in malaria prevalence and antimalarial drug resistance. Conventional malaria diagnostic and surveillance tools can be cumbersome and slow with limitations in both sensitivity and specificity. New molecular techniques have been developed in an attempt to overcome these restrictions. These molecular techniques are discussed with regard to their technical advantages and disadvantages, with an emphasis on the practicality of implementation in malaria-endemic and non-endemic regions.

Modeling the molecular basis of atovaquone resistance in parasites and pathogenic fungi

Atovaquone is a substituted hydroxynaphthoquinone that is used therapeutically for treating Plasmodium falciparum malaria, Pneumocystis jirovecii pneumonia and Toxoplasma gondii toxoplasmosis. It is thought to act on these organisms by inhibiting parasite and fungal respiration by binding to the cytochrome bc1 complex. The recent, growing failure of atovaquone treatment and increased mortality of patients with malaria or Pneumocystis pneumonia has been linked to the appearance of mutations in the cytochrome b gene. To better understand the molecular basis of drug resistance, we have developed the yeast and bovine bc1 complexes as surrogates to model the molecular interaction of atovaquone with human and resistant pathogen enzymes.

Drug-resistant malaria - an insight

Despite intensive research extending back to the 1930s, when the first synthetic antimalarial drugs made their appearance, the repertoire of clinically licensed formulations remains very limited. Moreover, widespread and increasing resistance to these drugs contributes enormously to the difficulties in controlling malaria, posing considerable intellectual, technical and humanitarian challenges. A detailed understanding of the molecular mechanisms underlying resistance to these agents is emerging that should permit new drugs to be rationally developed and older ones to be engineered to regain their efficacy. This review summarizes recent progress in analysing the causes of resistance to the major antimalarial drugs and its spread.