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Query: UMLS:C0024530 (malaria)
 44,886 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

n a study parallel to the present one, we conducted a genotype characterization of Plasmodium falciparum based on isolates of patients infected with malaria, who come from a small location in Colombia. The analysis involved extraction of DNA from hematological smears, amplification of the dihydrofolate reductase gene and the dihydropteroate synthase (DHPS) gene for each sample by PCR, and detection through mutation-specific primers nested PCR of mutations associated with resistance to pyrimethamine and sulfadoxine. Given the difficulty in quantifying the DNA extracts due to the type of sample and its heterogeneity, different volumes of the product of the first PCR were tested as template for the nested PCR. Surprisingly, for some samples, we found contradictory results between determinations, which differed only in the amount of template used. This prompted a more general concern that in a natural isolate, where the parasite population is heterogeneous, the nested PCR with mutation-specific primers technique can produce erroneous results that underestimate the complexity of the sample. To test this hypothesis, we designed experiments in this study using position 581 of the DHPS gene as an indicator system and constructed samples simulating the heterogeneity of natural samples. In effect, our data show that the results obtained in the nested PCR can be altered by the amount of template used in the reaction and, therefore, some heterogeneous samples might be classified mistakenly as homogeneous, simple mutant or simple wild type. These observations may explain, at least in part, contradictory results found in the literature. Our data also suggest the need for a more cautious approach to interpretation of the results of nested PCR assays with mutation-specific primers and their implications in the definition of resistance to the pyrimethamine-sulfadoxine combination.
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PMID:Plasmodium falciparum: underestimation of dihydrofolate reductase and dihydropteroate synthase polymorphism in field samples: a technical shortcoming of nested PCR assays with mutation-specific primers. 1184 21

Resistance to antifolates of the malaria parasite Plasmodium falciparum stems from stepwise mutations of the target enzyme dihydrofolate reductase (DHFR). New drugs can be developed against resistant parasites, which are assumed to have limited possibilities in mutations. Mechanisms of resistance other than reduced binding of inhibitors to mutant enzymes may be possible and need to be further explored. New synergistic combinations of drugs targeting DHFR and dihydropteroate synthase may be employed, with new provisions against development of resistance.
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PMID:Basis for antifolate action and resistance in malaria. 1188 49

Yeasts have a justified reputation as one of the world's most versatile organisms. Baker's yeast continues to live up to this recognition by joining the war against malaria. Yeast can now be used to study antifolate drug resistance patterns that depend on the dihydrofolate reductase enzyme (DHFR) from the malaria parasite.
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PMID:Novel approaches to tackling malarial drug resistance using yeast. 1189 77

Every year there are 270 million clinical attacks of malaria and 2 million deaths, caused by the protozoan Plasmodium falciparum. Most of these cases occur in Africa. Chloroquine-resistance has led to reliance on anti-malarial antifolates, in particular the synergistic combination sulfadoxine/pyrimethamine (S/P) which targets enzymatic synthesis of folate co-factors through dihydropteroate synthase (DHPS) and dihydrofolate reductase (DHFR). Resistance to S/P is now increasing and replacement antimalarials are needed. Crystal structures are not yet available for these key enzymes in the folate pathway. This review focuses on the activity of drugs on DHFR in malaria parasites, attempts to interpret differences in activity of pyrimethamine and its related drugs, and to clarify how residue changes due to point mutations determine the development of resistance. In homology-modelled P. falciparum DHFR (PfDHFR), the typical structure of four alpha-helices, 8-stranded beta-sheet, four Loops and eight Turns is clearly seen. Long polar sequences specific for Plasmodium are inserted in Turns 1 and 2. Structures immediately concerned in drug binding are beta-A, L1, alpha-B, alpha-C, T-3, beta-E, alpha-F, and beta-F. The roles of several mutations associated with resistance are discussed. In view of sequence differences in turn 3 in PfDHFR and in the human enzyme, and the marked interaction with residues of T3 of the experimental flexible antifolate WR99210 effective in pyrimethamine and cycloguanil resistance, further drug development in this area is indicated.
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PMID:Resistance to antifolates in Plasmodium falciparum, the causative agent of tropical malaria. 1196 21

Four airport malaria cases have been observed in the vicinity of the Roissy-Charles-de-Gaulle International Airport, Paris, France. These cases were geographically very close to each other and clustered in a short period of time during the summer of 1999. The phenotype and genotype of the Plasmodium falciparum isolates obtained from these patients were determined in order to know whether a single mosquito could have infected more than one subject. The genomic characterisation of isolates was performed using the polymorphic markers merozoite surface protein 1 (Msp 1) and merozoite surface protein 2 (Msp 2) genes, the kappa and omega repeats domains of cg2 and the dihydrofolate reductase (DHFR) genotypes. Results showed identical genotypes for isolates 1, 2 and 4 whereas the genotype of isolate 3 differed at one locus. The molecular analysis was consistent with the hypothesis that all patients could have been bitten by the same mosquito and that patient 3, may have received a different clone and an additional species. In vitro susceptibility data did not confirm or rule out this hypothesis because isolates had the same profile of susceptibility to the tested drugs.
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PMID:Molecular characterisation of airport malaria: four cases in France during summer 1999. 1211 66

Due to increasing trend in chloroquine resistance, the antifolate (sulpha-pyrimethamine combination) drugs are gaining more importance in the treatment of uncomplicated falciparum malaria. The efficacy of sulpha-pyrimethamine combinations in the treatment is compromised by the development of resistance in parasite. The occurrence of mutations at active sites in Plasmodium falciparum gene sequences coding for dihydrofolate reductase (DHFR) and dihydropteroate synthetase (DHPS) confer resistance to pyrimethamine and sulphadoxine. This study presents the characterization of a P. falciparum sample from a patient who did not respond to standard doses of a pyrimethamine/sulpha regimen. Although parasitaemia fell rapidly, the infection had not resolved six days later as because the response to treatment selected resistant sub-population. The in vitro drug sensitivity assays demonstrated resistance to pyrimethamine, sulphadoxine and cycloguanil; while polymerase chain reaction (PCR) and restriction digest based methods indicated that at known drug resistant loci the isolate had a genotype of DHFR Val-16 and Thr-108 previously only associated with cycloguanil resistance. As per the published reports this type of paired mutations in natural isolates are rare. It is of considerable interest to carry out studies on alleles on alleles of this gene in relation to resistance at epidemiological level.
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PMID:Plasmodium falciparum dihydrofolate reductase Val-16 and Thr-108 mutation associated with in vivo resistance to antifolate drug: a case study. 1212 19

Drug resistance is contributing to increasing mortality from malaria worldwide. For assessment of the role of resistance-conferring parasite mutations on treatment responses to sulfadoxine-pyrimethamine (SP) and transmission potential, 120 subjects with uncomplicated falciparum malaria from Buenaventura, Colombia, were treated with SP and followed for 21 days in the period February 1999 to May 2000. Exposures of interest were mutations in Plasmodium falciparum dihydrofolate reductase (DHFR) and dihydropteroate synthase that confer resistance to pyrimethamine and sulfadoxine, respectively. Although SP was highly efficacious (96.7%), the presence together of DHFR mutations at codons 108 and 51 was associated with longer parasite clearance time (relative hazard = 0.24, p = 0.019) more so than the 108 mutation alone (relative hazard = 0.45, p = 0.188). This association remained after controlling for potential confounders. Infections with these mutations were also associated with the presence of gametocytes, the sexual form of the parasite responsible for transmission, 14 and 21 days after treatment (p = 0.016 and p = 0.048, respectively). Higher gametocytemia is probably due to DHFR mutations prolonging parasite survival under drug pressure, resulting in longer parasite clearance time and allowing asexual parasites to differentiate into gametocytes. These results suggest that even when SP efficacy is high, DHFR mutations that are insufficient to cause therapeutic failure may nevertheless increase malaria transmission and promote the spread of drug resistance.
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PMID:Determinants of treatment response to sulfadoxine-pyrimethamine and subsequent transmission potential in falciparum malaria. 1214 57

Plasmodium vivax is a major public health problem in Asia and South and Central America where it is most prevalent. Until very recently, the parasite has been effectively treated with chloroquine, but resistance to this drug has now been reported in several areas. Affordable alternative treatments for vivax malaria are urgently needed. Pyrimethamine-sulfadoxine is an inhibitor of dihydrofolate reductase (DHFR) that has been widely used to treat chloroquine-resistant Plasmodium falciparum malaria. DHFR inhibitors have not been considered for treatment of vivax malaria, because initial trials showed poor efficacy against P. vivax. P. vivax cannot be grown in culture; the reason for its resistance to DHFR inhibitors is unknown. We show that, like P. falciparum, point mutations in the dhfr gene can cause resistance to pyrimethamine in P. vivax. WR99210 is a novel inhibitor of DHFR, effective even against the most pyrimethamine-resistant P. falciparum strains. We have found that it is also an extremely effective inhibitor of the P. vivax DHFR, and mutations that confer high-level resistance to pyrimethamine render the P. vivax enzyme exquisitely sensitive to WR99210. These data suggest that pyrimethamine and WR99210 would exert opposing selective forces on the P. vivax population. If used in combination, these two drugs could greatly slow the selection of parasites resistant to both drugs. If that is the case, this novel class of DHFR inhibitors could provide effective and affordable treatment for chloroquine- and pyrimethamine-resistant vivax and falciparum malaria for many years to come.
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PMID:Pyrimethamine and WR99210 exert opposing selection on dihydrofolate reductase from Plasmodium vivax. 1237 49

The polymorphism of malaria parasites will greatly influence the efficiency of antimalarial drugs and vaccines. This study determined the genetic diversity of Plasmodium falciparum infections in 107 travelers and estimated the importance of mutations in the parasite dihydrofolate reductase (dhfr) gene for clinical breakthrough during proguanil prophylaxis. Genotyping with regards to the three highly polymorphic antigen-coding regions (merozoite surface protein-1 [msp-1], msp-2, and the glutamate-rich protein [glurp]) revealed multiple genotypes (up to five) in 64% of the patients. Single genotype infections were mainly associated with prior intake of antimalarial drugs, but also with a shorter stay in a malaria-endemic area and low parasite density. Malaria breakthrough despite proguanil prophylaxis was always associated with mutations in the dhfr gene; always the Asn-108 mutation and often the Ile-51 and Arg-59 mutations. The Leu-164 mutation was found in four travelers from Africa. Travelers with limited time in an endemic area were often infected with polyclonal P. falciparum infections, which suggests that single mosquito inoculations are often composed of several genetically diverse parasites. Chemoprophylaxis reduces the number of infecting clones and selects for resistant parasites as shown for proguanil through mutations in the dhfr gene.
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PMID:Polyclonal Plasmodium falciparum malaria in travelers and selection of antifolate mutations after proguanil prophylaxis. 1220 81

Artemisinin-derivative combination therapies (ACT) are highly efficacious against multidrug-resistant Plasmodium falciparum malaria. Few efficacy data, however, are available for vivax malaria. With high rates of chloroquine (CQ) resistance in both vivax and falciparum malaria in Papua Province, Indonesia, new combination therapies are required for both species. We recently found artesunate plus sulfadoxine-pyrimethamine (ART-SP) to be highly effective (96%) in the treatment of falciparum malaria in Papua Province. Following a preliminary study of CQ plus sulfadoxine-pyrimethamine (CQ-SP) for the treatment of Plasmodium vivax infection, we used modified World Health Organization criteria to evaluate the efficacy of ART-SP for the treatment of vivax malaria in Papua. Nineteen of 22 patients treated with ART-SP could be evaluated on day 28, with no early treatment failures. Adequate clinical and parasitological responses were found by day 14 in all 20 (100%) of the patients able to be evaluated and by day 28 in 17 patients (89.5%). Fever and parasite clearance times were short, with hematological improvement observed in 70.6% of the patients. Double (at positions 58 and 117) and quadruple (at positions 57, 58, 61, and 117) mutations in the P. vivax dihydrofolate reductase (PvDHFR) were common in Papuan P. vivax isolates (46 and 18%, respectively). Treatment failure with SP-containing regimens was significantly higher with isolates with this PvDHFR quadruple mutation, which included a novel T-->M mutation at residue 61 linked to an S-->T (but not an S-->N) mutation at residue 117. ART-SP ACT resulted in a high cure rate for both major Plasmodium species in Papua, though progression of DHFR mutations in both species due to the continued use of SP monotherapy for clinically diagnosed malaria threatens the future utility of this combination.
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PMID:Therapeutic efficacies of artesunate-sulfadoxine-pyrimethamine and chloroquine-sulfadoxine-pyrimethamine in vivax malaria pilot studies: relationship to Plasmodium vivax dhfr mutations. 1243


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