Amodiaquine (AQ) is currently being used as a partner drug in combination with artesunate for treatment of uncomplicated malaria in most endemic countries of Africa. In the absence of molecular markers of artemisinin resistance, molecular markers of resistance to AQ may be useful for monitoring the development and spread of parasites resistance to Artesunate-Amodiaquine combination. This study was designed to assess the potential role of polymorphisms on pfcrt and pfmdr1 genes and parasite in vitro susceptibility for epidemiological surveillance of amodiaquine resistance in Plasmodium falciparum. The modified schizont inhibition assay was used to determine in vitro susceptibility profiles of 98 patients' isolates of Plasmodium falciparum to amodiaquine. Polymorphisms on parasites pfcrt and pfmdr1 genes were determined with nested PCR followed by sequencing. The geometric mean (GM) of AQ 50% inhibitory concentration (IC-50) in the 97 P. falciparum isolates was 20.48nM (95% CI 16.53–25.36nM). Based on the cut-off value for AQ in vitro susceptibility, 87% (84) of the P. falciparum isolates were sensitive to AQ (GM IC-50= 16.32nM; 95%CI 13.3–20.04nM) while 13% were resistant to AQ in vitro (GM IC-50= 88.73nM; 95%CI 69.67–113.0nM). Molecular analysis showed presence of mutant CVIET pfcrt haplotype, mutant pfmdr1Tyr86 allele and the double mutant CVIET pfcrt haplotype+pfmdr1Tyr86 in 72%, 49% and 35% respectively. The GM IC-50 of isolates harboring the wild-type pfcrt CVMNK haplotype+ pfmdr1Asn86 allele (3.93nM; 95%CI 1.82–8.46) was significantly lower (p=0.001) than those isolates harboring the double mutant pfcrtCVIET haplotype+pfmdr1Tyr86 allele (50.40nM; 95%CI 40.17–63.24). Results from this study suggest that polymorphisms in pfcrt and pfmdr1 genes are important for AQ resistance and therefore may be useful for epidemiological surveillance of P. falciparum resistance to AQ.
Plasmodium falciparum with reduced sensitivity to artemisinin derivatives has been observed in endemic areas, but the molecular mechanisms for this reduced sensitivity remain unclear. We evaluated the association between in vitro susceptibility of P. falciparum isolates obtained from southwest Nigeria and polymorphisms in selected putative transporter genes (PFE0775C, PF13_0271, pfmrp1, pfcrt, and pfmdr1). Modified schizont inhibition assay was used to determine the in vitro parasite susceptibility to artemether (ATH). Polymorphisms in selected genes were detected by polymerase chain reaction followed by direct DNA sequencing. The half-maximal inhibitory concentration (IC(50)) geometric mean (GM) for all P. falciparum isolates was 1.78 nM (range, 0.03-10.43 nM). Polymorphisms at codons 241, 86, and 76 of PFE0775C, pfmdr1, and pfcrt genes, respectively, were associated with reduced susceptibility to ATH. A new S263P single-nucleotide polymorphism on the PFE0775C gene was also detected in 27% of the isolates. Patient isolates harboring V241L or S263P polymorphisms on the PFE0775C gene showed increased IC(50) (GM: 3.08 nM and 1.79 nM, respectively). Plasmodium falciparum isolates harboring mutant Y86 pfmdr1 and P263 PFE0775C alleles showed a 2.5-5.5-fold increase in ATH IC(50.) This study shows that polymorphisms on the PFE0775C and pfmdr1 genes are associated with reduced sensitivity to ATH in fresh isolates of P. falciparum from Nigeria.
Abstract. The effect of antimalarial drug selection on pfcrt and pfmdr1 polymorphisms in Plasmodium falciparum isolates from two distinct geographical locations was determined in 70 and 18 P. falciparum isolates from Nigeria and Brazil, respectively, using nested polymerase chain reaction and direct DNA sequencing approaches. All isolates from Brazil and 72% from Nigeria harbored the mutant SVMNT and CVIET pfcrt haplotype, respectively. The pfcrt CVMNT haplotype was also observed in (7%) of the Nigerian samples. One hundred percent (100%) and 54% of the parasites from Brazil and Nigeria, respectively, harbored wild-type pfmdr1Asn86. We provide first evidence of emergence of the CVMNT haplotype in West Africa. The high prevalence of pfcrt CVIET and SVMNT haplotypes in Nigeria and Brazil, respectively, is indicative of different selective pressure by chloroquine and amodiaquine. Continuous monitoring of pfcrt SVMNT haplotype is required in endemic areas of Africa, where artesunate-amodiaquine combination is used for treatment of acute uncomplicated malaria.
The development of antimalarial drugs involving novel mechanisms of action is of imminent importance. Several potential drug candidates of synthetic and natural origin as well as their combination therapies are currently being evaluated for their efficacy against drug-resistant strains of the parasite. Various plasmodial targets/pathways, such as the Purine salvage pathway, Pyrimidine biosynthesis pathway and also the processes in the apicoplast, have been identified and are being utilized for the discovery and development of novel antimalarial therapies. This article provides an overview of the latest developments in terms of cell and molecular biology that will improve the knowledge related to drug-resistant malaria and to new molecular targets.
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