Chloroquine resistance in Plasmodium vivax

Antigenic polymorphism and naturally acquired antibodies to Plasmodium vivax merozoite surface protein 1 in rural Amazonians

Merozoite surface protein 1 of Plasmodium vivax (PvMSP-1), a major target for malaria vaccine development, contains six highly polymorphic domains interspersed with conserved sequences. Although there is evidence that the sequence divergence in PvMSP-1 has been maintained over 5 million years by balanced selection exerted by the host's acquired immunity, the variant specificity of naturally acquired antibodies to PvMSP-1 remains poorly investigated. Here, we show that 15 recombinant proteins corresponding to PvMSP-1 variants commonly found in local parasites were poorly recognized by 376 noninfected subjects aged 5 to 90 years exposed to malaria in rural Amazonia; less than one-third of them had detectable immunoglobulin G (IgG) antibodies to at least one variant of blocks 2, 6, and 10 that were expressed, although 54.3% recognized the invariant 19-kDa C-terminal domain PvMSP-1(19). Although the proportion of responders to PvMSP-1 variants increased substantially during subsequent acute P. vivax infections, the specificity of IgG antibodies did not necessarily match the PvMSP-1 variant(s) found in infecting parasites. We discuss the relative contribution of antigenic polymorphism, poor immunogenicity, and original antigenic sin (the skew in the specificity of antibodies elicited by exposure to new antigenic variants due to preexisting variant-specific responses) to the observed patterns of antibody recognition of PvMSP-1. We suggest that antibody responses to the repertoire of variable domains of PvMSP-1 to which subjects are continuously exposed are elicited only after several repeated infections and may require frequent boosting, with clear implications for the development of PvMSP-1-based subunit vaccines.

The chequered history of malaria control: are new and better tools the ultimate answer?

Following Ronald Ross' demonstration in 1897 that mosquitoes transmit malarial parasites, efforts to control malaria were naturally focussed on attacking the mosquito vector by various measures, mainly directed against the aquatic stages. Although the results were spectacular in some areas, there was a growing realisation that effective control of malaria depended on other factors, including the availability of better drugs than quinine and a greater understanding of the epidemiology of the disease under various environmental conditions. With the discovery of DDT, an all-out effort was made to eradicate malaria by attacking adult mosquitoes. Eradication was not achieved in many countries, mainly because of inadequate health infrastructures. With the emergence of chloroquine-resistant parasites, the search for more effective drug regimens intensified, various drugs and drug combinations were assessed, and methods were developed to monitor and assess degrees of resistance. Since resistance to drugs can develop very quickly, the use of drug combinations, especially those containing artemisinin derivatives, is now recommended. Insecticide-impregnated bednets have become the preferred method of vector control. Although the search for better tools must continue, the events of the past century emphasise the need to strengthen health systems to ensure that they are capable of delivering effective interventions and of assessing their effectiveness in controlling malaria.

The genetic diversity of Plasmodium vivax: a review

The genetic diversity of Plasmodium vivax has been investigated in several malaria-endemic areas, including the Brazilian Amazon region, where this is currently the most prevalent species causing malaria in humans. This review summarizes current views on the use of molecular markers to examine P. vivax populations, with a focus on studies performed in Brazilian research laboratories. We emphasize the importance of phylogenetic studies on this parasite and discuss the perspectives created by our increasing understanding of genetic diversity and population structure of this parasite for the development of new control strategies, including vaccines, and more effective drugs for the treatment of P. vivax malaria.

Antifolates can have a role in the treatment of Plasmodium vivax

Plasmodium vivax is a serious health concern in many regions and is sometimes inadvertently treated with sulfadoxine-pyrimethamine (SP). Mutations in the genes that encode dihydrofolate reductase (DHFR) and dihydropteroate synthase (DHPS) confer resistance to pyrimethamine and sulfadoxine, respectively. Numerous studies have examined the prevalence and diversity of mutations in P. vivax dhfr and some have assessed the relationship between dhfr genotype and clinical or in vitro response to pyrimethamine. Other studies have examined the impact of dhps genotype on response to sulfadoxine. These data indicate that, under certain circumstances, SP could be a valuable tool in the fight against P. vivax.

Prevention and treatment of vivax malaria

Plasmodium vivax is a significant public health threat throughout most of the tropics and to travelers to these regions. The infection causes a debilitating febrile syndrome that often recurs and in rare cases ends in death. The complex life cycle of the parasite compounds the difficulty of prevention and treatment, principally due to the phenomenon of relapse. Most commonly used drugs for preventing malaria fail to prevent late relapses by this parasite. Treatment requires dealing with both blood and liver stages. Since 1950, primaquine has been the only drug available for treatment of liver stages, and important clinical questions surround its appropriate use (ie, dosing, efficacy, safety, and tolerability). Likewise, chloroquine has been first-line therapy for vivax malaria since 1946, and the emergence of resistance to the drug further complicates therapeutic management decisions.

Therapeutic response of multidrug-resistant Plasmodium falciparum and P. vivax to chloroquine and sulfadoxine-pyrimethamine in southern Papua, Indonesia

To determine the level of antimalarial drug resistance in southern Papua, Indonesia, we assessed the therapeutic efficacy of chloroquine plus sulfadoxine-pyrimethamine (CQ+SP) for Plasmodium falciparum infections as well as CQ monotherapy for P. vivax infections. Patients with P. falciparum failing therapy were re-treated with unsupervised quinine+/-doxycycline therapy and those with P. vivax with either unsupervised quinine+/-doxycycline or amodiaquine. In total, 143 patients were enrolled in the study (103 treated with CQ+SP and 40 with CQ). Early treatment failures occurred in four patients (4%) with P. falciparum and six patients (15%) with P. vivax. The failure rate by Day 28 for P. vivax was 65% (95% CI 49-81). After PCR correction for re-infections, the Day 42 recrudescence rate for P. falciparum infections was 48% (95% CI 31-65). Re-treatment with unsupervised quinine+/-doxycycline resulted in further recurrence of malaria in 48% (95% CI 31-65) of P. falciparum infections and 70% (95% CI 37-100) of P. vivax infections. Eleven patients with recurrent P. vivax were re-treated with amodiaquine; there were no early or late treatment failures. In southern Papua, a high prevalence of drug resistance of P. falciparum and P. vivax exists both to first- and second-line therapies. Preliminary data indicate that amodiaquine retains superior efficacy compared with CQ for CQ-resistant P. vivax.

Improved assessment of plasmodium vivax response to antimalarial drugs by a colorimetric double-site plasmodium lactate dehydrogenase antigen capture enzyme-linked immunosorbent assay

The occurrence of Plasmodium vivax resistance to chloroquine has been reported in several countries of Asia and South America. However, the resistance of P. vivax is insufficiently documented for three reasons: it has received far less attention than P. falciparum; in vivo investigations are handicapped by the existence of hypnozoites, which make it difficult to distinguish between recrudescences due to drug failure and relapses due to dormant forms in the liver; and in vitro studies are greatly limited by the poor growth of P. vivax. We report on the adaptation to P. vivax of a colorimetric double-site Plasmodium lactate dehydrogenase antigen capture enzyme-linked immunosorbent assay previously developed for P. falciparum. The assay proved remarkably sensitive, as under optimal conditions it could detect P. vivax parasitemia levels as low as 10(-8). The technique, which relies on the detection of protein synthesis by the parasite, yielded steep drug-response curves, leading to the precise determination of the 50% inhibitory concentrations for a high proportion of isolates. Chloroquine-resistant parasites were identified in an area where this phenomenon had been documented by in vivo methods. Thus, the results indicate that the in vitro susceptibility of P. vivax can now be monitored easily and efficiently. The data suggest that the threshold of resistance is similar to that of P. falciparum, i.e., in the range of 100 nM for chloroquine and 15 nM for pyronaridine. However, further studies are required to precisely define the cutoff for resistance and the sensitivity to each drug.

Dihydroartemisinin-piperaquine versus artesunate-amodiaquine: superior efficacy and posttreatment prophylaxis against multidrug-resistant Plasmodium falciparum and Plasmodium vivax malaria

BACKGROUND

Antimalarial drug resistance is now well established in both Plasmodium falciparum and Plasmodium vivax. In southern Papua, Indonesia, where both strains of plasmodia coexist, we have been conducting a series of studies to optimize treatment strategies.

METHODS

We conducted a randomized trial that compared the efficacy and safety of dihydroartemisinin-piperaquine (DHP) with artesunate-amodiaquine (AAQ). The primary end point was the overall cumulative parasitological failure rate at day 42.

RESULTS

Of the 334 patients in the evaluable patient population, 185 were infected with P. falciparum, 80 were infected with P. vivax, and 69 were infected with both species. The overall parasitological failure rate at day 42 was 45% (95% confidence interval [CI], 36%-53%) for AAQ and 13% (95% CI, 7.2%-19%) for DHP (hazard ratio [HR], 4.3; 95% CI, 2.5-7.2; P<.001). Rates of both recrudescence of P. falciparum infection and recurrence of P. vivax infection were significantly higher after receipt of AAQ than after receipt of DHP (HR, 3.4 [95% CI, 1.2-9.4] and 4.3 [95% CI, 2.2-8.2], respectively; P<.001). By the end of the study, AAQ recipients were 2.95-fold (95% CI, 1.2- to 4.9-fold) more likely to be anemic and 14.5-fold (95% CI, 3.4- to 61-fold) more likely to have carried P. vivax gametocytes.

CONCLUSIONS

DHP was more effective and better tolerated than AAQ against multidrug-resistant P. falciparum and P. vivax infections. The prolonged therapeutic effect of piperaquine delayed the time to P. falciparum reinfection, decreased the rate of recurrence of P. vivax infection, and reduced the risk of P. vivax gametocyte carriage and anemia.

Case management of malaria in pregnancy

In all malarious areas, infection by any of the main human plasmodial species during pregnancy is detrimental to the mother and the fetus. These potentially fatal infections must be prevented, but when they develop they require prompt diagnosis and treatment. Current tools to detect malaria parasites in pregnant women are often not used and remain too insensitive to detect a low parasitaemia. The kinetics, safety, and efficacy of available antimalarial drugs are poorly documented because pregnant women are systematically excluded from clinical trials. A considerable effort, involving clinical trials, is urgently required to improve the diagnosis and case management of malaria during pregnancy if the morbidity and mortality of maternal malaria is to be reduced.

UK malaria treatment guidelines

Malaria is the tropical disease most commonly imported into the UK, with 1500-2000 cases reported each year, and 10-20 deaths. Approximately three-quarters of reported malaria cases in the UK are caused by Plasmodium falciparum, which is capable of invading a high proportion of red blood cells and rapidly leading to severe or life-threatening multi-organ disease. Most non-falciparum malaria cases are caused by Plasmodium vivax; a few cases are caused by the other two species of Plasmodium: Plasmodium ovale or Plasmodium malariae. Mixed infections with more than 1 species of parasite can occur; they commonly involve P. falciparum with the attendant risks of severe malaria. Management of malaria depends on awareness of the diagnosis and on performing the correct diagnostic tests: the diagnosis cannot be excluded until 3 blood specimens have been examined by an experienced microscopist. There are no typical clinical features of malaria, even fever is not invariably present. The optimum diagnostic procedure is examination of thick and thin blood films by an expert to detect and speciate the malarial parasites; P. falciparum malaria can be diagnosed almost as accurately using rapid diagnostic tests (RDTs) which detect plasmodial antigens or enzymes, although RDTs for other Plasmodium species are not as reliable. The treatment of choice for non-falciparum malaria is a 3-day course of oral chloroquine, to which only a limited proportion of P. vivax strains have gained resistance. Dormant parasites (hypnozoites) persist in the liver after treatment of P. vivax or P. ovale infection: the only currently effective drug for eradication of hypnozoites is primaquine. This must be avoided or given with caution under expert supervision in patients with glucose-6-phosphate dehydrogenase deficiency (G6PD), in whom it may cause severe haemolysis. Uncomplicated P. falciparum malaria can be treated orally with quinine, atovaquone plus proguanil (Malarone) or co-artemether (Riamet); quinine is highly effective but poorly tolerated in prolonged dosage and is always supplemented by additional treatment, usually with oral doxycycline. ALL patients treated for P. falciparum malaria should be admitted to hospital for at least 24 h, since patients can deteriorate suddenly, especially early in the course of treatment. Severe falciparum malaria, or infections complicated by a relatively high parasite count (more than 2% of red blood cells parasitized), should be treated with intravenous therapy until the patient is well enough to continue with oral treatment. In the UK, the treatment of choice for severe or complicated malaria is currently an infusion of intravenous quinine. This may exacerbate hypoglycaemia that can occur in malaria; patients treated with intravenous quinine therefore require careful monitoring. Intravenous artesunate reduces high parasite loads more rapidly than quinine and is more effective in treating severe malaria in selected situations. It can also be used in patients with contra-indications to quinine. Intravenous artesunate is unlicensed in the EU. Assistance in obtaining artesunate may be sought from specialist tropical medicine centres, on consultation, for named patients. Patients with severe or complicated malaria should be managed in a high dependency or intensive care environment. They may require haemodynamic support and management of acute respiratory distress syndrome, disseminated intravascular coagulation, renal impairment/failure, seizures, and severe intercurrent infections including gram-negative bacteraemia/septicaemia. Falciparum malaria in pregnancy is more likely to be severe and complicated: the placenta contains high levels of parasites. Stillbirth or early delivery may occur and diagnosis can be difficult if parasites are concentrated in the placenta and scanty in the blood. The treatment of choice for falciparum malaria in pregnancy is quinine; doxycycline is contraindicated in pregnancy but clindamycin can be substituted for it, and is equally effective. Primaquine (for eradication of P. vivax or P. ovale hypnozoites) is contraindicated in pregnancy; after treatment for these infections a pregnant woman should take weekly chloroquine prophylaxis until after delivery when hypnozoite eradication can be considered. Children are over-represented in the incidence of malaria in the UK, probably because completely susceptible UK-born children accompany their overseas-born parents on visits to family and friends in endemic areas. Malaria in children (and sometimes in adults) may present with misleading symptoms such as gastrointestinal features, sore throat or lower respiratory complaints; the diagnosis must always be sought in a feverish or very sick child who has visited malaria-endemic areas. Children can be treated with most of the antimalarial regimens which are effective in adults, with appropriate dosage adjustment. Doxycycline plus quinine should not be given to children under 12 years as doxycycline is contraindicated in this age group, but clindamycin can be substituted for doxycycline, and pyrimethamine-sulfadoxine (Fansidar) may also be an effective substitute. An acute attack of malaria does not confer protection from future attacks: individuals who have had malaria should take effective anti-mosquito precautions and chemoprophylaxis during future visits to endemic areas.

An update on the search for a Plasmodium vivax vaccine

Although Plasmodium falciparum is the leading cause of morbidity and mortality due to malaria worldwide, nearly 2.5 billion people, mostly outside Africa, are also at risk from malaria caused by Plasmodium vivax infection. Currently, almost all efforts to develop a malaria vaccine have focused on P. falciparum. For example, there are 23 P. falciparum vaccine candidates undergoing advanced clinical studies and only two P. vivax vaccine candidates being tested in preliminary (Phase I) clinical trials, with few others being assessed in preclinical studies. More investment and a greater effort toward the development of P. vivax vaccine components for a multi-species vaccine are required. This is mainly because of the wide geographical coexistence of both parasite species but also because of increasing drug resistance, recent observations of severe and lethal P. vivax cases and relapsing parasite behaviour. Availability of the P. vivax genome has contributed to antigen discovery but new means to test vaccines in future trials remain to be designed.

Safety and tolerability of elubaquine (bulaquine, CDRI 80/53) for treatment of Plasmidium vivax malaria in Thailand

We conducted a study to compare the safety and tolerability of anti-relapse drugs elubaquine and primaquine against Plasmodium vivax malaria. After standard therapy with chloroquine, 30 mg/kg given over 3 days, 141 patients with P. vivax infection were randomized to receive primaquine or elubaquine. The 2 treatment regimens were primaquine 30 mg once daily for 7 days (group A, n = 71), and elubaquine 25 mg once daily for 7 days (group B, n = 70). All patients cleared parasitemia within 7 days after chloroquine treatment. Among patients treated with primaquine, one patient relapsed on day 26; no relapse occurred with elubaquine treatement. Both drugs were well tolerated. Adverse effects occurred only in patients with G6PD deficiency who were treated with primaquine (group A, n = 4), whose mean hematocrit fell significantly on days 7, 8 and 9 (P = 0.015, 0.027, and 0.048, respectively). No significant change in hematocrit was observed in patients with G6PD deficiency who were treated with elubaquine (group B, n = 3) or in patients with normal G6PD. In conclusion, elubaquine, as anti-relapse therapy for P. vivax malaria, was as safe and well tolerated as primaquine and did not cause clinically significant hemolysis.

Expression and function of pvcrt-o, a Plasmodium vivax ortholog of pfcrt, in Plasmodium falciparum and Dictyostelium discoideum

Chloroquine resistance in Plasmodium vivax threatens the use of this drug as first-line treatment for millions of people infected each year worldwide. Unlike Plasmodium falciparum, in which chloroquine resistance is associated with mutations in the pfcrt gene encoding a digestive vacuole transmembrane protein, no point mutations have been associated with chloroquine resistance in the P. vivax ortholog gene, pvcrt-o (also called pvcg10). However, the question remains whether pvcrt-o can affect chloroquine response independent of mutations. Since P. vivax cannot be cultured in vitro, we used two heterologous expression systems to address this question. Results from the first system, in which chloroquine sensitive P. falciparum parasites were transformed with pvcrt-o, showed a 2.2-fold increase in chloroquine tolerance with pvcrt-o expression under a strong promoter; this effect was reversed by verapamil. In the second system, wild type pvcrt-o or a mutated form of the gene was expressed in Dictyostelium discoideum. Forms of PvCRT-o engineered to express either lysine or threonine at position 76 produced a verapamil-reversible reduction of chloroquine accumulation in this system to approximately 60% of that in control cells. Our data support an effect of PvCRT-o on chloroquine transport and/or accumulation by P. vivax, independent of the K76T amino acid substitution.

Evaluation of the activities of pyrimethamine analogs against Plasmodium vivax and Plasmodium falciparum dihydrofolate reductase-thymidylate synthase using in vitro enzyme inhibition and bacterial complementation assays

Pyrimethamine analogs were examined as potential agents against vivax malaria using a bacterial surrogate system carrying Plasmodium vivax dihydrofolate reductase-thymidylate synthase (PvDHFR-TS), in which the PvDHFR complemented chemically knocked out host dihydrofolate reductase. The system was initially tested with P. falciparum dihydrofolate reductase-thymidylate synthase and was found to have good correlation with the parasite-based system. The 50% inhibitory concentrations derived from PvDHFR-TS-dependent bacteria were correlated with their corresponding inhibition constants (Ki) from an enzyme inhibition assay, pointing to the likelihood that the potent enzyme inhibitors will also have potent antimalarial activities. Active compounds against both wild-type and S58R S117N (SP21) double-mutant P. vivax include analogs with structures which can avert a steric clash with the asparagine (S117N) side chain of the mutant, similar to those found for homologous Plasmodium falciparum mutants, raising the possibility that the same compounds can be developed against both types of antifolate-resistant malaria. This rapid and convenient drug screening system should be useful for development of new antifolates against P. vivax, for which a continuous culture system is not yet available.

[Is Plasmodium vivax still a paradigm for uncomplicated malaria?]

P. vivax is supposed to be involved in benign tertian fever, responsible for a non-complicated disease that could be easily treated by standard antimalarial drug regimen. This could be considered as a long-standing paradigm of a non-virulent malaria parasite. When a patient exhibits severe malaria with the vivax parasite, the issue is often to find falciparum. However, with the implementation of molecular diagnosis, it has becoming more evident that vivax parasites could be involved in severe disease with probably a different pathogenesis. Mixed infections are frequent in various parts of Southeast Asian endemic areas and it was speculated that drugs used to treat falciparum could be involved in the development of vivax drug resistance. How should primaquine be used today for the treatment and prophylaxis of vivax malaria? Considering the re-emergence of vivax malaria in several areas, improving the treatment for this disease is certainly an important issue to avoid late episodes and transmission potential.

Plasmodium vivax genetic diversity: microsatellite length matters

The Plasmodium vivax genome is very diverse but has a relatively low abundance of microsatellites. Leclerc et al. had shown that these di-nucleotide repeats have a low level of polymorphism, suggesting a recent bottleneck event in the evolutionary history of P. vivax. By contrast, in a recent paper, Imwong et al. show that there is a very high level of microsatellite diversity. The difference in these results is probably due to the set array lengths chosen by each group. Longer arrays are more diverse than are shorter ones because slippage mutations become exponentially more common with an increase in array length. These studies highlight the need to consider carefully the application and design of studies involving microsatellites.

Plasmodium vivax: In vitro susceptibility of blood stages to synthetic trioxolane compounds and the diamidine DB75

Plasmodium vivax is an important human pathogen causing malaria in more temperate climates of the world. Similar to Plasmodium falciparum, the causative agent for malaria tropica, drug resistance is beginning to emerge for this parasite species and this hampers adequate treatment of infection. We have used a short-term ex vivo drug assay to monitor activity of OZ277 (RBx-11160), a fully synthetic anti-malarial peroxide, and the diamidine DB75 against P. vivax. For both compounds as well as the anti-malarial reference compounds artesunate, artemether, and chloroquine, the in vitro IC(50) values were determined in one-cycle hypoxanthine incorporation assays. Results from such assays were found to be very similar compared to IC(50) values obtained from one-cycle P. falciparum hypoxanthine assays. We demonstrate the anti-parasite activity of OZ277 and the reference compounds to be faster than that of DB75. These data warrant clinical testing of OZ277 against P. vivax malaria and support recent data on clinical activity against P. vivax for DB75.

A database of antimalarial drug resistance

A large investment is required to develop, license and deploy a new antimalarial drug. Too often, that investment has been rapidly devalued by the selection of parasite populations resistant to the drug action. To understand the mechanisms of selection, detailed information on the patterns of drug use in a variety of environments, and the geographic and temporal patterns of resistance is needed. Currently, there is no publically-accessible central database that contains information on the levels of resistance to antimalaria drugs.

This paper outlines the resources that are available and the steps that might be taken to create a dynamic, open access database that would include current and historical data on clinical efficacy, in vitro responses and molecular markers related to drug resistance in Plasmodium falciparum and Plasmodium vivax. The goal is to include historical and current data on resistance to commonly used drugs, like chloroquine and sulfadoxine-pyrimethamine, and on the many combinations that are now being tested in different settings. The database will be accessible to all on the Web.

The information in such a database will inform optimal utilization of current drugs and sustain the longest possible therapeutic life of newly introduced drugs and combinations. The database will protect the valuable investment represented by the development and deployment of novel therapies for malaria.

Usefulness of the recombinant liver stage antigen-3 for an early serodiagnosis of Plasmodium falciparum infection

In order to develop tools for an early serodiagnosis of Plasmodium falciparum infection, we evaluated the usefulness of P. falciparum liver stage antigen-3 (LSA-3) as a serodiagnostic antigen. A portion of LSA-3 gene was cloned, and its recombinant protein (rLSA-3) was expressed in Escherichia coli and purified by column chromatography. The purified rLSA-3 and 120 test blood/serum samples collected from inhabitants in malaria-endemic areas of Mandalay, Myanmar were used for this study. In microscopic examinations of blood samples, P. falciparum positive rate was 39.1% (47/120) in thin smear trials, and 33.3% (40/120) in thick smear trials. Although the positive rate associated with the rLSA-3 (30.8%) was lower than that of the blood stage antigens (70.8%), rLSA-3 based enzyme-linked immunosorbent assay could detect 12 seropositive cases (10.0%), in which blood stage antigens were not detected. These results indicate that the LSA-3 is a useful antigen for an early serodiagnosis of P. falciparum infection.

Problem pathogens: prevention of malaria in travellers

Human infection with Plasmodium spp leading to clinical episodes of malaria probably began very early in the history of humankind and has persistently inflicted disease among human populations. Malaria is currently considered the world's most important parasitic infection. The global impact of malaria is incalculable and appears to be worsening over the past decades. Although most of this burden of disease is carried by developing tropical countries, cases of imported malaria acquired by international travel are increasingly reported. These numbers are growing because of increased travel to malaria-endemic areas and also due to increased risk of transmission in these areas. Indeed, travel has contributed to the global spread of malaria during the history of humankind. Travellers visiting malaria risk areas should use both personal protective measures and chemoprophylaxis. Non-adherence to chemoprophylactic regimens is frequently secondary to drug side-effects. Therefore, a careful risk-benefit analysis on the use of antimalarial prophylaxis should be carried out in every individual travelling to malaria risk areas. Standby malaria self-treatment represents an alternative in some travellers. However, carefully selected and geographically specific antimalarial drug regimens should be recommended to non-immune people travelling to high-risk areas.

Therapeutic efficacy of chloroquine in Plasmodium vivax from areas with different epidemiological patterns in India and their Pvdhfr gene mutation pattern

Among the four human malaria parasites, drug resistance occurs mainly in Plasmodium falciparum. However, there are some reports of chloroquine (CQ) resistance in P. vivax from different geographical regions. In India, approximately 50% of a total of 2 million cases of malaria reported annually are due to P. vivax. CQ is the drug of choice for treatment. Since few cases of treatment failure have been reported from India, this study was undertaken to generate data systematically on the efficacy of CQ in 287 patients from different epidemiological regions. Cure rates for 28 days were 100% and there was a rapid parasite clearance rate in all age groups from all study sites. Although P. vivax has been reported to be inherently resistant to sulfonamide and pyrimethamine, Indian isolates exhibited only double mutations in dhfr in vitro.

Should chloroquine be laid to rest?

Chloroquine (CQ) has been the front line antimalarial drug due to its efficacy, low cost and scanty side effects, until resistance has evolved. Although its use has been officially discontinued in most malaria-affected countries, it is still widely used. Practical and pharmacological considerations indicate that it could be still used in semi-immune adults and that more efficient treatment protocols could be devised to treat even patients infected with CQ-resistant parasite strains. Since its antimalarial activity is pleiotropic, drug resistance may be due to different mechanisms, each amenable to reversal by drug combination.

Identification of the optimal third generation antifolate against P. falciparum and P. vivax

Inhibitors of dihydrofolate reductase (DHFR) have been mainstays in the treatment of falciparum malaria. Resistance to one of these antifolates, pyrimethamine, is now common in Plasmodium falciparum populations. Antifolates have not traditionally been recommended for treatment of vivax malaria. However, recent studies have suggested that a third-generation antifolate, WR99210, is remarkably effective even against highly pyrimethamine-resistant parasites from both species. Two methods were used to identify a compound that is effective against quadruple mutant alleles from P. falciparum (N51I/C59R/S108N/I164L) and from Plasmodium vivax (57L/111L/117T/173F). The first was simple yeast system used to screen a panel of WR99210 analogs. The biguanide prodrug, JPC-2056, of the 2-chloro-4-trifluoromethoxy analog of WR99210 was effective against both the P. falciparum and P. vivax enzymes, and has been selected for further development. The second method compared the analogs in silico by docking them in the known structure of the P. falciparum DHFR-thymidylate synthase. The program reproduced well the position of the triazine ring, but the calculated energies of ligand binding were very similar for different compounds and therefore did not reproduce the observed trends in biological activity. The WR99210 family of molecules is flexible due to a long bridge between the triazine ring and the substituted benzene. During docking, multiple conformations were observed for the benzene ring part of the molecules in the DHFR active site, making computer-based predictions of binding energy less informative than for more rigid ligands. This flexibility is a key factor in their effectiveness against the highly mutant forms of DHFR.