UMLS:C0024530 - FACTA Search

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

As resistance to chloroquine spreads in sub-Saharan Africa, pyrimethamine plus sulfadoxine (PSD) is increasingly used as a first-line treatment for falciparum malaria. Populations of Plasmodium falciparum (Pf) resistant to PSD have been selected quickly in other regions. The resistance is strongly correlated with point mutations in dihydrofolate reductase (DHFR) and dihydropteroate synthase (DHPS), the two targets of the drug. It is critical to identify drug-resistant Pf-DHFR alleles that are present at a low frequency in these populations since alleles that confer drug resistance will be quickly selected by PSD use. It is difficult to identify these rare alleles by standard molecular techniques. We have designed a yeast expression system that facilitates the identification and rapid analysis of Pf-DHFR alleles that confer PSD resistance, even when they are present at very low frequency in polyclonal patient samples. We analyzed samples from patients in Kilifi, Kenya collected between 1992 and 1995. We determined the prevalence of the drug-sensitive and drug-resistant alleles in patient samples analyzed in parallel by an allele-specific enzyme digestion (ASED) assay. We identified a pyrimethamine-resistant allele (S108N) present at a frequency of < 1% in a sample that was scored as only S108 by ASED. In addition, a novel pyrimethamine-resistant allele (1164M) was isolated twice, once each from two different patient samples. This approach will allow determination of the prevalence of Pf-DHFR alleles that confer pyrimethamine resistance in particular regions, and the rapid identification of novel alleles that confer drug resistance.
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PMID:Identification and analysis of dihydrofolate reductase alleles from Plasmodium falciparum present at low frequency in polyclonal patient samples. 1043 70

Sulfadoxine/pyrimethamine (SP) is considered an alternative treatment for acute uncomplicated malaria caused by Plasmodium falciparum resistant to chloroquine. However, the appearance of resistance to this drug has been reported since its initial use in Colombia. Molecular analysis of the dihydrofolate reductase gene indicates a correlation between in vitro resistance to SP and the Asn-108 point mutation. Little is known about the association of this point mutation and in vivo resistance to SP. We used a mutation-specific polymerase chain reaction strategy to analyze the presence of the Asn-108 point mutation in 48 clinical samples with adequate clinical response (ACR), 2 early treatment failures (ETF), and 1 late treatment failure (LTF). The Asn-108 mutation was detected in 36 of the ACR samples and in all of the ETF and LTF samples. Eleven ACR samples amplified with the wild-type-specific primer and one amplified with the primer for the Thr-108 mutation described for resistance to cycloguanil. These results suggest that the Asn-108 marker may not be useful in predicting SP treatment failure.
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PMID:Lack of an association between the ASN-108 mutation in the dihydrofolate reductase gene and in vivo resistance to sulfadoxine/pyrimethamine in Plasmodium falciparum. 1046 74

To assess pyrimethamine-sulfadoxine (PS) efficacy in Mali, and the role of mutations in Plasmodium falciparum dihydrofolate reductase (DHFR) and dihydropteroate synthase (DHPS) in in vivo PS resistance, 190 patients with uncomplicated P. falciparum malaria were treated with PS and monitored for 56 days. Mutation-specific polymerase chain reactions and digestion with restriction endonucleases were used to detect DHFR and DHPS mutations on filter paper blood samples from pretreatment and post-treatment infections. Only one case each of RI and RII level resistance and no cases of RIII resistance or therapeutic failure were observed. Post-PS treatment infections had significantly higher rates of DHFR mutations at codons 108 and 59. No significant selection for DHPS mutations was seen. Pyrimethamine-sulfadoxine is highly efficacious in Mali, and while the low level of resistance precludes assessing the utility of molecular assays for in vivo PS resistance, rapid selection of DHFR mutations supports their role in PS failure.
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PMID:Pyrimethamine-sulfadoxine efficacy and selection for mutations in Plasmodium falciparum dihydrofolate reductase and dihydropteroate synthase in Mali. 1046 80

The increasing resistance of falciparum malaria to common antimalarial drugs has renewed interest in the compound proguanil normally metabolized to cycloguanil, a strong dihydrofolate reductase inhibitor, via the cytochrome P450 isozyme CYP2C19. The relationship between CYP2C19 genotypes and proguanil metabolism was therefore studied in 100 uncomplicated malaria patients on Malakula island in Vanuatu, where a CYP2C19-related poor metabolizer genotype status was known to be frequent. The patients (median age, 7 years) with Plasmodium falciparum or P. vivax infections, received proguanil treatment for 3 days in daily doses corresponding to adult doses of 300-500 mg. Capillary blood samples were collected on filter paper for determining both human CYP2C19 mutations by polymerase chain reaction and mutation-specific restriction enzyme digestion and blood concentrations of proguanil and its metabolites by high-performance liquid chromatography. The frequencies of the defective alleles, CYP2C19*2 and CYP2C19*3, were 0.57 and 0.25, respectively. The patients were genotyped as 68 CYP2C19-related poor metabolizers and 32 extensive metabolizers. Proguanil concentrations were higher and cycloguanil and 4-chlorophenylbiguanide concentrations were lower in poor compared to extensive metabolizers. Among the extensive metabolizers, 27 were heterozygous and five were homozygous for unmutated alleles. The tendency of an intermediate degree of proguanil metabolism in heterozygous extensive metabolizers as compared to homozygous extensive metabolizers and poor metabolizers suggests the trend towards the existence of a gene dose effect. Mild adverse events (mainly gastro-intestinal symptoms) were often reported and positively correlated with proguanil concentrations. The incidence was, however, similar in poor and extensive metabolizers. In conclusion, our data demonstrate an association between CYP2C19 mutations and poor metabolism of proguanil.
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PMID:Proguanil disposition and toxicity in malaria patients from Vanuatu with high frequencies of CYP2C19 mutations. 1047 Oct 63

The efficacy of sulfadoxine/pyrimethamine (S/P) in treatment of uncomplicated falciparum malaria in Africa is increasingly compromised by development of resistance. The occurrence of active site mutations in the Plasmodium falciparum gene sequences coding for dihydrofolate reductase (DHFR) and dihydropteroate synthetase (DHPS) is known to confer resistance to pyrimethamine and sulfadoxine. This study investigated the occurrence of these mutations in infected blood samples taken from Ugandan children before treatment with S/P and their relationship to parasite breakthrough by day 7. The results confirm the occurrence of mutations in DHFR and DHPS that were significantly selected under S/P pressure at day 7: a combination of alleles 51-isoleucine and 108-asparagine in DHFR, and 436-serine, 437-alanine, 540-lysine and 581-alanine in DHPS, appears to play a major role in the development of in vivo resistance in P. falciparum strains against S/P. Therefore, earlier results derived from isolates from hyperendemic areas in Tanzania were confirmed by this investigation.
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PMID:Plasmodium falciparum resistance to sulfadoxine/pyrimethamine in Uganda: correlation with polymorphisms in the dihydrofolate reductase and dihydropteroate synthetase genes. 1049 91

The emergence of chloroquine resistance has been associated with a dramatic increase in malaria mortality in some human populations from endemic regions. Plasmodium falciparum drug resistant malaria originates from chromosomal mutations. Analysis using molecular, genetic and biochemical approaches has shown that 1) impaired intake of chloroquine by the parasite vacuole is a common characteristic of resistant strains, the chloroquine-resistance mechanism regulates the access of chloroquine to hematin, this phenotype correlates with Pfmdr1 and Pfcg2 gene mutations; 2) one to four point mutations of dihydrofolate reductase, the enzyme target of antifolinics (pyrimethamine and proguanil), give moderate to high levels of resistance to these drugs but there is a fitness cost to resistance; 3) the mechanism of resistance to sulfonamides and sulfones involves mutations of dihydropteroate synthase, their enzyme target; 4) treatment with sulphadoxine-pyrimethamine (SP) selected for the variants Ile(51), Arg(59) and Asn(108) of DHFR and for the variants Ser(436), Gly(437), and Glu(540) of DHPS; 5)clones that were resistant to some traditional antimalarial agents acquired resistance to new ones at high frequency (accelerated resistance to multiple drugs--ARMD). Amino-alcohol (quinine, mefloquine, halofantrine) mechanisms of resistance are still unclear. Population genetic studies have confirmed that selfing is more frequent in Plasmodium falciparum where the transmission rate is lower in some regions such as Papua-New Guinea, whereas isolates from individuals on the Thai-Burmese border, an area of hypoendemic transmission, revealed a higher number of genotypes per infected person. It has been suggested that intense intra-host competition between co-infecting clones, low numbers of genes required to encode resistance, and high drug usage all encourage the emergence of drug resistance. On the other hand, the greater effective recombination in high transmission areas may breakdown multiple drug resistance when it is coded for by two unlinked loci. Epidemiological studies have established that the frequency of chloroquine resistant mutants varies among parasite isolate populations while resistance to antifolinics is highly prevalent in most malarial endemic countries (more than 92% of Kenyan field isolates have undergone at least one point mutation). Established and strong drug pressure as well as low antiparasitic immunity probably explains the multidrug-resistance encountered in forests of Southeast Asia and South America. In Africa, frequent genetic recombinations in Plasmodium originate from a high level of malaria transmission, and falciparum chloroquine-resistant prevalence seems to stabilise at an equal level as chloroquine-sensitive malaria. Clinical studies demonstrated that control of clinical symptoms is better when chloroquine is used with sulphadoxine-pyrimethamine (SP) than when SP is used alone, and the cure rate also tends to be higher with the triple combination regimen.
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PMID:[Mechanisms and dynamics of drug resistance in Plasmodium falciparum]. 1057 58

Pyrimethamine, in combination with sulfadoxine, is currently one of the major alternative drugs used for the treatment of chloroquine-resistant Plasmodium falciparum malaria infections in Africa. The mechanism of pyrimethamine resistance has been strongly associated with a single, key point mutation in the dihydrofolate reductase-thymidylate synthase gene, resulting in the substitution of the wild-type allele Ser-108 by either Asn-108 or Thr-108. The pyrimethamine-resistant phenotype and/or genotype were determined in 273 Cameroonian clinical isolates obtained in Yaounde by in vitro assays and polymerase chain reaction-restriction fragment length polymorphism over a 5-year period. The in vitro assays showed that 42% (18 of 43) and 63% (69 of 110) of the isolates obtained in 1994-1995 and 1997-1998, respectively, were resistant to pyrimethamine (50% inhibitory concentration [IC50] > 100 nM). The polymerase chain reaction showed that 43% (55 of 127) and 59% (50 of 85) of the isolates in 1994-1995 and 1997-1998, respectively, had the mutant Asn-108 allele. The pyrimethamine-resistant genotype (Asn-108) corresponded with the pyrimethamine-resistant phenotype (IC50 > or = 100 nM) in a large majority (> 95%) of the isolates. The results of our study suggest an increasing prevalence of pyrimethamine resistance in Yaounde. Our study further suggests that pyrimethamine resistance can be monitored by a technique that can be adopted by malaria research centers in Africa.
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PMID:Molecular epidemiology of malaria in Yaounde, Cameroon IV. Evolution of pyrimethamine resistance between 1994 and 1998. 1058 15

In Plasmodium falciparum, dihydrofolate reductase and thymidylate synthase activities are conferred by a single 70-kDa bifunctional polypeptide (DHFR-TS, dihydrofolate reductase-thymidylate synthase) which assembles into a functional 140-kDa homodimer. In mammals, the two enzymes are smaller distinct molecules encoded on different genes. A 27-kDa amino domain of malarial DHFR-TS is sufficient to provide DHFR activity, but the structural requirements for TS function have not been established. Although the 3'-end of DHFR-TS has high homology to TS sequences from other species, expression of this protein fragment failed to yield active TS enzyme, and it failed to complement TS(-) Escherichia coli. Unexpectedly, even partial 5'-deletion of full-length DHFR-TS gene abolished TS function on the 3'-end. Thus, it was hypothesized that the amino end of the bifunctional parasite protein plays an important role in TS function. When the 27-kDa amino domain (DHFR) was provided in trans, a previously inactive 40-kDa carboxyl-domain from malarial DHFR-TS regained its TS function. Physical characterization of the "split enzymes" revealed that the 27- and the 40-kDa fragments of DHFR-TS had reassembled into a 140-kDa hybrid complex. Thus, in malarial DHFR-TS, there are physical interactions between the DHFR domain and the TS domain, and these interactions are necessary to obtain a catalytically active TS. Interference with these essential protein-protein interactions could lead to new selective strategies to treat malaria resistant to traditional DHFR-TS inhibitors.
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PMID:Essential protein-protein interactions between Plasmodium falciparum thymidylate synthase and dihydrofolate reductase domains. 1060 39

Genetic transformation of malaria parasites has been limited by the number of selectable markers available. For the rodent malaria parasite, Plasmodium berghei, only a single selection marker has been at hand, utilising the dihydrofolate reductase-thymidylate synthase gene from either P. berghei or Toxoplasma gondii to confer resistance to the anti-malarial drug pyrimethamine. Here we report the use of the human dihydrofolate reductase (hDHFR) gene as a new selectable marker, which confers resistance to the antifolate inhibitor WR99210 upon both pyrimethamine sensitive and resistant isolates of P. berghei. Transfection with circular constructs containing the hDHFR gene resulted in the generation of highly resistant parasites containing multiple copies of episomally-maintained plasmids. These parasites showed around a 1000-fold increase in resistance to WR99210 compared to the parental parasites. We were also able to generate and select transgenic parasites harbouring only a single copy of hDHFR targeted into their genome, despite the fact that these parasites showed only a fivefold increase in resistance to WR99210 compared to the parental parasites. Importantly, and for the first time with malaria parasites, the hDHFR gene could be used in conjunction with the existing pyrimethamine selectable markers. This was demonstrated by reintroducing the circumsporozoite (CS) gene into transgenic CS-knockout mutant parasites that contained the P. berghei DHFR-TS selectable marker. The development of hDHFR as a second selectable marker will greatly expand the use of transformation technology in Plasmodium, enabling more extensive genetic manipulation and thus facilitating more comprehensive studies on the biology of the malaria parasite.
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PMID:The selectable marker human dihydrofolate reductase enables sequential genetic manipulation of the Plasmodium berghei genome. 1069 50

The antifolate combination of pyrimethamine (PM) and sulfadoxine (SD) is the last affordable drug combination available for wide-scale treatment of falciparum malaria in Africa. Wherever this combination has been used, drug-resistant parasites have been selected rapidly. A study of PM-SD effectiveness carried out between 1997 and 1999 at Kilifi on the Kenyan coast has shown the emergence of RI and RII resistance to PM-SD (residual parasitemia 7 days after treatment) in 39 out of 240 (16.25%) patients. To understand the mechanism that underlies resistance to PM-SD, we have analyzed the dihydrofolate reductase (DHFR) and dihydropteroate synthase (DHPS) genotypes of 81 patients. Fifty-one samples were obtained, before treatment, from patients who remained parasite free for at least 7 days after treatment. For a further 20 patients, samples were obtained before treatment and again when they returned to the clinic with parasites 7 days after PM-SD treatment. Ten additional isolates were obtained from patients who were parasitemic 7 days after treatment but who were not sampled before treatment. More than 65% of the isolates (30 of 46) in the initial group had wild-type or double mutant DHFR alleles, and all but 7 of the 47 (85%) had wild-type DHPS alleles. In the paired (before and after treatment) samples, the predominant combinations of DHFR and DHPS alleles before treatment were of triple mutant DHFR and double mutant DHPS (41% [7 of 17]) and of double mutant DHFR and double mutant DHPS (29% [5 of 17]). All except one of the posttreatment isolates had triple mutations in DHFR, and most of these were "pure" triple mutants. In these isolates, the combination of a triple mutant DHFR and wild-type DHPS was detected in 6 of 29 cases (20.7%), the combination of a triple mutant DHFR and a single mutant (A437G) DHPS was detected in 4 of 29 cases (13.8%), and the combination of a triple mutant DHFR and a double mutant (A437G, L540E) DHPS was detected in 16 of 29 cases (55.2%). These results demonstrate that the triply mutated allele of DHFR with or without mutant DHPS alleles is associated with RI and RII resistance to PM-SD. The prevalence of the triple mutant DHFR-double mutant DHPS combination may be an operationally useful marker for predicting the effectiveness of PM-SD as a new malaria treatment.
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PMID:Towards an understanding of the mechanism of pyrimethamine-sulfadoxine resistance in Plasmodium falciparum: genotyping of dihydrofolate reductase and dihydropteroate synthase of Kenyan parasites. 1072 2

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