The mosquito transmission of malaria: the effects of atovaquone-proguanil (Malarone) and chloroquine

Assessment of the transmission blocking activity of antimalarial compounds by membrane feeding assays using natural Plasmodium falciparum gametocyte isolates from West-Africa

Antimalarial drugs that can block the transmission of Plasmodium gametocytes to mosquito vectors would be highly beneficial for malaria elimination efforts. Identifying transmission-blocking drugs currently relies on evaluation of their activity against gametocyte-producing laboratory parasite strains and would benefit from a testing pipeline with genetically diverse field isolates. The aims of this study were to develop a pipeline to test drugs against P. falciparum gametocyte field isolates and to evaluate the transmission-blocking activity of a set of novel compounds. Two assays were designed so they could identify both the overall transmission-blocking activity of a number of marketed and experimental drugs by direct membrane feeding assays (DMFA), and then also discriminate between those that are active against the gametocytes (gametocyte killing or sterilizing) or those that block development in the mosquito (sporontocidal). These DMFA assays used venous blood samples from naturally infected Plasmodium falciparum gametocyte carriers and locally reared Anopheles gambiae s.s. mosquitoes. Overall transmission-blocking activity was assessed following a 24 hour incubation of compound with gametocyte infected blood (TB-DMFA). Sporontocidal activity was evaluated following addition of compound directly prior to feeding, without incubation (SPORO-DMFA); Gametocyte viability was retained during 24-hour incubation at 37°C when gametocyte infected red blood cells were reconstituted in RPMI/serum. Methylene-blue, MMV693183, DDD107498, atovaquone and P218 showed potent transmission-blocking activity in the TB-DMFA, and both atovaquone and the novel antifolate P218 were potent inhibitors of sporogonic development in the SPORO-DMA. This work establishes a pipeline for the integral use of field isolates to assess the transmission-blocking capacity of antimalarial drugs to block transmission that should be validated in future studies.

Transmission Blocking Activity of Low-dose Tafenoquine in Healthy Volunteers Experimentally Infected With Plasmodium falciparum

BACKGROUND

Blocking the transmission of parasites from humans to mosquitoes is a key component of malaria control. Tafenoquine exhibits activity against all stages of the malaria parasite and may have utility as a transmission blocking agent. We aimed to characterize the transmission blocking activity of low-dose tafenoquine.

METHODS

Healthy adults were inoculated with Plasmodium falciparum 3D7-infected erythrocytes on day 0. Piperaquine was administered on days 9 and 11 to clear asexual parasitemia while allowing gametocyte development. A single 50-mg oral dose of tafenoquine was administered on day 25. Transmission was determined by enriched membrane feeding assays predose and at 1, 4, and 7 days postdose. Artemether-lumefantrine was administered following the final assay. Outcomes were the reduction in mosquito infection and gametocytemia after tafenoquine and safety parameters.

RESULTS

Six participants were enrolled, and all were infective to mosquitoes before tafenoquine, with a median 86% (range, 22-98) of mosquitoes positive for oocysts and 57% (range, 4-92) positive for sporozoites. By day 4 after tafenoquine, the oocyst and sporozoite positivity rate had reduced by a median 35% (interquartile range [IQR]: 16-46) and 52% (IQR: 40-62), respectively, and by day 7, 81% (IQR 36-92) and 77% (IQR 52-98), respectively. The decline in gametocyte density after tafenoquine was not significant. No significant participant safety concerns were identified.

CONCLUSIONS

Low-dose tafenoquine (50 mg) reduces P. falciparum transmission to mosquitoes, with a delay in effect.

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.

Abbreviated atovaquone-proguanil prophylaxis regimens in travellers after leaving malaria-endemic areas: A systematic review

BACKGROUND

We evaluated existing data on the prophylactic efficacy of atovaquone-proguanil (AP) in order to determine whether prophylaxis in travellers can be discontinued on the day of return from a malaria-endemic area instead of seven days after return as per currently recommended post-travel schedule.

METHODS

PubMed and Embase databases were searched to identify relevant studies. This PROSPERO-registered systematic review followed PRISMA guidelines. The search strategy included terms or synonyms relevant to AP combined with terms to identify articles relating to prophylactic use of AP and inhibitory and half-life properties of AP. Studies considered for inclusion were: randomized controlled trials, cohort studies, quasi-experimental studies, open-label trials, patient-control studies, cross-sectional studies; as well as case-series and non-clinical studies. Data on study design, characteristics of participants, interventions, and outcomes were extracted. Primary outcomes considered relevant were prophylactic efficacy and prolonged inhibitory activity and half-life properties of AP.

RESULTS

The initial search identified 1,482 publications, of which 40 were selected based on screening. Following full text review, 32 studies were included and categorized into two groups, namely studies in support of the current post-travel regimen (with a total of 2,866 subjects) and studies in support of an alternative regimen (with a total of 533 subjects).

CONCLUSION

There is limited direct and indirect evidence to suggest that an abbreviated post-travel regimen for AP may be effective. Proguanil, however, has a short half-life and is essential for the synergistic effect of the combination. Stopping AP early may result in mono-prophylaxis with atovaquone and possibly select for atovaquone-resistant parasites. Furthermore, the quality of the studies in support of the current post-travel regimen outweighs the quality of the studies in support of an alternative short, post-travel regimen, and the total sample size of the studies to support stopping AP early comprises a small percentage of the total sample size of the studies performed to establish the efficacy of the current AP regimen. Additional research is required - especially from studies evaluating impact on malaria parasitaemia and clinical illness and conducted among travellers in high malaria risk settings - before an abbreviated regimen can be recommended in current practice.

PROSPERO REGISTRATION NUMBER

CRD42017055244.

Bioenergetic profiling of Trypanosoma cruzi life stages using Seahorse extracellular flux technology

Energy metabolism is an attractive target for the development of new therapeutics against protozoan pathogens, including Trypanosoma cruzi, the causative agent of human Chagas disease. Despite emerging evidence that mitochondrial electron transport is essential for the growth of intracellular T. cruzi amastigotes in mammalian cells, fundamental knowledge of mitochondrial energy metabolism in this parasite life stage remains incomplete. The Clark-type electrode, which measures the rate of oxygen consumption, has served as the traditional tool to study mitochondrial energetics and has contributed to our understanding of it in T. cruzi. Here, we evaluate the Seahorse XF(e)24 extracellular flux platform as an alternative method to assess mitochondrial bioenergetics in isolated T. cruzi parasites. We report optimized assay conditions used to perform mitochondrial stress tests with replicative life cycle stages of T. cruzi using the XF(e)24 instrument, and discuss the advantages and potential limitations of this methodology, as applied to T. cruzi and other trypanosomatids.

Metabolic Dysregulation Induced in Plasmodium falciparum by Dihydroartemisinin and Other Front-Line Antimalarial Drugs

Detailed information on the mode of action of antimalarial drugs can be used to improve existing drugs, identify new drug targets, and understand the basis of drug resistance. In this study we describe the use of a time-resolved, mass spectrometry (MS)-based metabolite profiling approach to map the metabolic perturbations induced by a panel of clinical antimalarial drugs and inhibitors on Plasmodium falciparum asexual blood stages. Drug-induced changes in metabolite levels in P. falciparum-infected erythrocytes were monitored over time using gas chromatography-MS and liquid chromatography-MS and changes in specific metabolic fluxes confirmed by nonstationary [(13)C]-glucose labeling. Dihydroartemisinin (DHA) was found to disrupt hemoglobin catabolism within 1 hour of exposure, resulting in a transient decrease in hemoglobin-derived peptides. Unexpectedly, it also disrupted pyrimidine biosynthesis, resulting in increased [(13)C]-glucose flux toward malate production, potentially explaining the susceptibility of P. falciparum to DHA during early blood-stage development. Unique metabolic signatures were also found for atovaquone, chloroquine, proguanil, cycloguanil and methylene blue. We also show that this approach can be used to identify the mode of action of novel antimalarials, such as the compound Torin 2, which inhibits hemoglobin catabolism.

Strategic use of antimalarial drugs that block falciparum malaria parasite transmission to mosquitoes to achieve local malaria elimination

The ultimate aim of malaria chemotherapy is not only to treat symptomatic infection but also to reduce transmission potential. With the absence of clinically proven vaccines, drug-mediated blocking of malaria transmission gains growing interest in the research agenda for malaria control and elimination. In addition to the limited arsenal of antimalarials available, the situation is further complicated by the fact that most commonly used antimalarials are being extensively resisted by the parasite and do not assist in blocking its transmission to vectors. Most antimalarials do not exhibit gametocytocidal and/ or sporontocidal activity against the sexual stages of Plasmodium falciparum but may even enhance gametocytogenesis and gametocyte transmissibility. Artemisinin derivatives and 8-aminoquinolines are useful transmission-blocking antimalarials whose optimal actions are on different stages of gametocytes. Transmission control interventions that include gametocytocides covering the spectrum of gametocyte development should be used to reduce and, if possible, stop transmission and infectivity of gametocytes to mosquitoes. Potent gametocytocidal drugs could also help deter the spread of antimalarial drug resistance. Novel proof-of-concept compounds with gametocytocidal activity, such as trioxaquines, synthetic endoperoxides, and spiroindolone, should be further tested for possible clinical utility before investigating the possibility of integrating them in transmission-reducing interventions. Strategic use of potent gametocytocides at appropriate timing with artemisinin-based combination therapies should be given attention, at least, in the short run. This review highlights the role that antimalarials could play in blocking gametocyte transmission and infectivity to mosquitoes and, hence, in reducing the potential of falciparum malaria transmissibility and drug resistance spread.

Plasmodium falciparum gametocytes: with a view to a kill

Drugs that kill or inhibit the sexual stages of Plasmodium in order to prevent transmission are important components of malaria control programmes. Reducing gametocyte carriage is central to the control of Plasmodium falciparum transmission as infection can result in extended periods of gametocytaemia. Unfortunately the number of drugs with activity against gametocytes is limited. Primaquine is currently the only licensed drug with activity against the sexual stages of malaria parasites and its use is hampered by safety concerns. This shortcoming is likely the result of the technical challenges associated with gametocyte studies together with the focus of previous drug discovery campaigns on asexual parasite stages. However recent emphasis on malaria eradication has resulted in an upsurge of interest in identifying compounds with activity against gametocytes. This review examines the gametocytocidal properties of currently available drugs as well as those in the development pipeline and examines the prospects for discovery of new anti-gametocyte compounds.

Low infectivity of Plasmodium falciparum gametocytes to Anopheles gambiae following treatment with sulfadoxine-pyrimethamine in Mali

Sulfadoxine-pyrimethamine (SP) treatment increases the rate of gametocyte carriage and selects SP resistance-conferring mutations in Plasmodium falciparum dihydrofolate reductase (DHFR) and dihydropteroate synthase (DHPS), raising concerns of increased malaria transmission and spread of drug resistance. In a setting in Mali where SP was highly efficacious, we measured the prevalence of DHFR and DHPS mutations in P. falciparum infections with microscopy-detected gametocytes following SP treatment, and used direct feeding to assess infectivity to Anopheles gambiae sensu lato. Children and young adults presenting with uncomplicated malaria were treated with SP or chloroquine and followed for 28 days. Gametocyte carriage peaked at 67% 1 week after treatment with a single dose of SP. Those post-SP gametocytes carried significantly more DHFR and DHPS mutations than pre-treatment asexual parasites from the same population. Only 0.5% of 1728 mosquitoes fed on SP-treated gametocyte carriers developed oocysts, while 11% of 198 mosquitoes fed on chloroquine-treated gametocyte carriers were positive for oocysts. This study shows that in an area of high SP efficacy, although SP treatment sharply increased gametocyte carriage, the infectiousness of these gametocytes to the vector may be very low. Accurate and robust methods for measuring infectivity are needed to guide malaria control interventions that affect transmission.

Gametocytaemia after drug treatment of asymptomatic Plasmodium falciparum

OBJECTIVES

Treatment of Plasmodium falciparum malaria with sulfadoxine-pyrimethamine (SP) is followed by a sharp rise in the prevalence and density of gametocytes. We did a randomized trial to determine the effect of treatment of asymptomatic infections with SP or SP plus one dose of artesunate (SP+AS) on gametocyte carriage.

DESIGN

The study was a three-arm open-label randomized trial. We randomized asymptomatic carriers of P. falciparum to receive antimalarial treatment or placebo, and recorded the prevalence and density of gametocytes over the next 2 mo.

SETTING

The trial was conducted during the dry (low malaria transmission) season in four rural villages in Gambia.

PARTICIPANTS

Participants were adults and children aged over 6 mo with asexual P. falciparum infection and confirmed free of clinical symptoms of malaria over a 2-d screening period.

INTERVENTIONS

Participants were randomized to receive a single dose of SP or SP+AS or placebo.

OUTCOME MEASURES

The outcome measures were the presence of gametocytes 7 and 56 d after treatment, and the duration and density of gametocytaemia over 2 mo.

RESULTS

In total, 372 asymptomatic carriers were randomized. Gametocyte prevalence on day 7 was 10.5% in the placebo group, 11.2% in the SP group (risk difference to placebo 0.7%, 95% confidence interval -7.4% to 8.7%, p = 0.87), and 7.1% in the SP+AS group (risk difference to placebo 4.1%, 95% confidence interval -3.3% to 12%, p = 0.28). By day 56, gametocyte prevalence was 13% in the placebo group and 2% in both drug-treated groups. Gametocyte carriage (the area under the curve of gametocyte density versus time), was reduced by 71% in the SP group, and by 74% in the SP+AS group, compared to placebo. Gametocyte carriage varied with age and was greater among children under 15 than among adults.

CONCLUSIONS

Treatment of asymptomatic carriers of P. falciparum with SP does not increase gametocyte carriage or density. Effective treatment of asexual parasitaemia in the dry season reduces gametocyte carriage to very low levels after 4 wk.

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.

Isonicotinic acid hydrazide: an anti-tuberculosis drug inhibits malarial transmission in the mosquito gut

We studied the transmission-blocking effect of isonicotinic acid hydrazide (INH), a widely used anti-tuberculosis drug, against Plasmodium gallinaceum and Plasmodium berghei. INH-treatment of infected animals did not inhibit parasite development in the blood of the vertebrate host, but did inhibit exflagellation, ookinete formation, and oocyst development in the mosquito. Oocyst development was inhibited in a dose-dependent manner. The ED(50) in the P. gallinaceum/chicken/Aedes aegypti model and P. berghei/mouse/Anopheles stephensi model was 72 and 109 mg/kg, respectively. In marked contrast, in vitro exflagellation and ookinete development were not directly affected by physiological concentrations of INH. We suggest that INH exerts its inhibitory effects on the mosquito stages of the malaria parasite by an indirect, and at present undefined mechanism. Further elucidation of the mechanism how INH inhibits parasite development specifically on mosquito stages may allow us to identify new targets for malaria control strategy.

Addition of artesunate to chloroquine for treatment of Plasmodium falciparum malaria in Gambian children causes a significant but short-lived reduction in infectiousness for mosquitoes

OBJECTIVES

Combination therapy using existing anti-malarials together with artesunate (AS) has been advocated as a method to slow the spread of drug resistance. We assessed the effect on Plasmodium falciparum transmissibility of the addition of AS to chloroquine (CQ) in an area of The Gambia where resistance to CQ is increasing.

METHODS

Gambian children with acute uncomplicated P. falciparum malaria were treated with either CQ monotherapy (n=120) or the combination of CQ plus three doses of AS (CQ/AS; n=352). Post-treatment sexual-stage parasitaemia was assessed during a 4-week follow-up period. Experimental infections of Anopheles gambiae s.s. mosquitoes were performed with blood from patients who were carrying gametocytes 7 days after starting treatment (n=69).

RESULTS

The addition of AS significantly reduced post-treatment prevalence and mean density of gametocytes in the first 14 days (day 7: 43.7% vs. 12.4%, 62.4/microl vs. 6.2/microl; day 14: 32.9% vs. 3.7%; 21.9/microl vs. 5.2/microl; CQ vs. CQ/AS), although by day 28 the benefits of the combination were substantially less marked (40.5% vs. 21.8%; 23.0/microl vs. 63.1/microl; CQ vs. CQ/AS). The duration of gametocyte carriage over the study period was significantly lower in the CQ/AS group (5.2 days vs. 1.5 days; CQ vs. CQ/AS). The estimated infectious proportion of children at day 7 was also lower in the combination group (19.2% vs. 3.4%; CQ vs. CQ/AS), as were the proportion of mosquitoes infected and mean oocyst density (11.5% vs. 0.9%; 0.3 vs. 0.01; CQ vs. CQ/AS). Treatment failure was associated with threefold and twofold higher gametocyte carriage rates during follow-up in CQ and CQ/AS groups, respectively (P<0.001 in both cases), and 26-fold and 2.3-fold higher intensity of infection at day 7 among CQ- and CQ/AS-treated children, respectively (P=0.002 and 0.30, respectively).

CONCLUSION

The benefits of adding AS to CQ monotherapy in lowering gametocyte prevalence and density were transient, suggesting that the addition of AS delayed, but did not prevent, the emergence of gametocytes. This is consistent with our finding that treatment failure, and thus the presence of CQ-resistant parasites, was significantly associated with a higher gametocyte carriage rate in both treatment groups. At day 7, CQ monotherapy significantly favoured transmission of resistant infections, which showed an 11-fold greater intensity of transmission compared with infections that were successfully treated. In contrast, the combination of CQ/AS did not significantly favour resistant infections at day 7. We conclude that significant transmission-reduction is achieved by the combination but is not maintained because of the recrudescence of CQ-resistant parasites.

Infectivity of Plasmodium falciparum gametocytes to Anopheles arabiensis after treatment with sulfadoxine-pyrimethamine

Sulfadoxine-pyrimethamine induces increased gametocytaemia when used for treating Plasmodium falciparum malaria. Laboratory-reared Anopheles arabiensis mosquitoes were fed with blood from patients with post-therapeutic gametocytaemia using a membrane feeder. Fourteen days later the heads and thoraxes of 613 mosquitoes were negative for P. falciparum sporozoites by enzyme-linked immunosorbent assay.

Atovaquone/proguanil: a review of its use for the prophylaxis of Plasmodium falciparum malaria

Atovaquone/proguanil is a fixed-dose combination tablet of two antimalarial agents and is highly effective for the prevention of Plasmodium falciparum malaria. In combination with proguanil, the ability of atovaquone to inhibit parasitic mitochondrial electron transport is markedly enhanced. Both atovaquone and proguanil are active against hepatic (pre-erythrocytic) stages of P. falciparum, thereby providing causal prophylaxis and eliminating the need to continue post-travel treatment beyond 7 days. Both agents are also active against erythrocytic stages of P. falciparum, thereby providing suppressive prophylaxis. Atovaquone/proguanil is highly effective against drug-resistant strains of P. falciparum, and cross-resistance has not been observed between atovaquone and other antimalarial agents. In comparative, randomised clinical trials, there were no cases of P. falciparum malaria in nonimmune adults, adolescents and children (>/=11 kg) visiting malaria-endemic regions for </=28 days and receiving atovaquone/proguanil (250/100 mg in adults and dosage based on bodyweight in children <40 kg) once daily. The efficacy for the prevention of P. falciparum malaria was estimated at 100% for atovaquone/proguanil and for mefloquine, and 70% for chloroquine plus proguanil. In individuals (>/=11 kg) from endemic regions who may carry some immunity to malaria (semi-immune), the prophylactic efficacy rating for atovaquone/proguanil based on placebo-controlled trials was 95-100%. Atovaquone/proguanil is generally well tolerated by both adults and children. The most common treatment-related adverse events in placebo-controlled trials were headache and abdominal pain, which occurred at a rate similar to that observed with placebo. Atovaquone/proguanil therapy was associated with significantly fewer gastrointestinal adverse events than chloroquine plus proguanil, and significantly fewer neuropsychiatric adverse events than mefloquine in nonimmune individuals. Significantly fewer recipients of atovaquone/proguanil discontinued treatment because of adverse events than individuals receiving chloroquine plus proguanil or mefloquine (p < 0.05).

CONCLUSION

Atovaquone/proguanil is a fixed-dose combination antimalarial tablet that provides effective prophylaxis of P. falciparum malaria, including drug-resistant strains. Both atovaquone and proguanil are effective against hepatic stages of P. falciparum, which means that treatment need only continue for 7 days after leaving a malaria-endemic region. Atovaquone/proguanil was generally well tolerated and was associated with fewer gastrointestinal adverse events than chloroquine plus proguanil, and fewer neuropsychiatric adverse events than mefloquine. Thus, atovaquone/proguanil provides effective prophylaxis of P. falciparum malaria and compared with other commonly used antimalarial agents has an improved tolerability profile, and, overall, a more convenient dosage regimen, particularly in the post-travel period.

The safety of atovaquone/proguanil in long-term malaria prophylaxis of nonimmune adults

BACKGROUND

Data on the long-term safety of atovaquone/proguanil in nonimmune travelers are limited.

METHODS

An open-label study, involving 300 Danish soldiers stationed in Eritrea for 6 months was initiated. The subjects self-reported their symptoms on a post-travel questionnaire. The study compared the symptoms of compliers and noncompliers.

RESULTS

No serious adverse events occurred. Diarrhea, stomach pain, headache, cough, and loss of appetite were the most common symptoms reported. No case of Plasmodium falciparum malaria occurred.

CONCLUSIONS

Atovaquone/proguanil was safe and well tolerated in this group of long-term nonimmune travelers.

Recent Advances and New Challenges in Travel Medicine

Recent advances in travel medicine include the use of computer resources to obtain information on outbreaks and recommendations to travelers, the introduction of atovaquone/proguanil as chemoprophylaxis and treatment for malaria, the use of azithromycin as an alternative in the self-treatment of traveler's diarrhea, and the combination of hepatitis A and hepatitis B vaccines. At the same time, new challenges continue to appear. Shifts in the distribution of infections, such as West Nile virus and dengue fever, underscore the need for up-to-date information. Well-known infectious diseases, such as polio, meningococcal meningitis, and influenza are appearing in unexpected ways and settings. It is increasingly clear that travelers, while at risk for infections, also play a role in the global dispersal of pathogens, such as certain serogroups of Neisseria meningitidis and influenza. Increasing drug resistance affects the choice of drugs for treatment and chemoprophylaxis, and decisions about use of vaccines. Newly identified adverse events associated with yellow fever vaccine have prompted enhanced surveillance after vaccination and careful scrutiny of appropriate indications for the vaccine.

Recent Advances in the Prophylaxis and Treatment of Malaria

Increases in international travel and escalating drug resistance are putting put a growing number of travelers at risk of contracting malaria. Resistance to chloroquine and proguanil and real and perceived intolerance to standard agents, such as mefloquine, has highlighted the need for new antimalarials to prevent and treat malaria. Promising new agents to prevent malaria include the combination of atovaquone and proguanil, primaquine, and a related 8-aminoquinoline, tafenoquine. These agents are active against the liver stage of the malaria parasite, and therefore can be discontinued shortly after the traveler leaves the malaria-endemic area; this offers a clear advantage, in terms of adherence to a treatment regimen. For treatment of multidrug-resistant Plasmodium falciparum malaria, the combination of artemisinin derivatives plus mefloquine, or atovaquone plus proguanil, are the most active drug regimens.

Inhibition of the mosquito transmission of Plasmodium berghei by Malarone (atovaquone-proguanil)

Sera from patients treated with atovaquone-proguanil (Malarone) have previously been shown to inhibit the mosquito transmission of Plasmodium falciparum, though the inhibition was not complete and the effect declined 2 weeks after treatment. In marked contrast, the inhibition of transmission of P. berghei by human sera (fed to mosquitoes, with P. berghei gametocytes, via membrane feeders) from volunteers treated with atovaquone-proguanil was total up to day 28 post-treatment and still very significant at day 56. In view of the short half-lives of atovaquone and proguanil, this was unexpected, and further experiments, reported here, were undertaken. In contrast to the incomplete blockade of infectivity of P. falciparum by serum taken 4 days post-treatment, such serum was totally inhibitory against P. berghei at a 1:10,000 dilution, indicating a remarkable sensitivity of P. berghei and demonstrating an unusual difference between the two Plasmodium species in response to a drug. The inhibitory effect on P. berghei after day 4 was caused by atovaquone and mainly through blockade of development from ookinete to oocyst. Despite previous information on the rapid elimination of atovaquone by patients, the present data indicate that low concentrations of this drug may persist in the plasma for some weeks after treatment.