The molecular basis of Plasmodium falciparum resistance to the antimalarial lumefantrine
Sammanfattning: Malaria control is compromised by the development and spread of Plasmodium falciparum resistance to antimalarial drugs, which has caused an increase in malaria related morbidity and mortality. Artemisinin based combination therapy (ACT) has been implemented in almost all malaria endemic areas in an attempt to suppress drug resistance. The ACT artemetherlumefantrine (Coartem®; Novartis) is one of the most important and most widely used ACTs, especially in Africa. The aim of this thesis was to investigate a possible molecular basis of P. falciparum resistance to the antimalarial drug lumefantrine, especially the role of the P. falciparum chloroquine resistance transporter gene (pfcrt) and the P. falciparum multidrug resistance gene 1 (pfmdr1). The results showed that treatment with artemether-lumefantrine selects parasites with pfmdr1 N86, 184F, D1246 and pfcrt K76 alleles in high transmission Africa. The observed selection occurred in newly acquired infections after treatment (reinfections). Therefore these alleles were interpreted as markers of tolerance rather than resistance. Selection of pfmdr1 N86 and pfcrt K76 carrying parasites was not observed after treatment with sulfadoxine-pyrimethamine (SP), indicating a drug specific selection. The involvement of pfcrt K76 in lumefantrine response was confirmed in vitro in isogenic pfcrt-modified parasite lines. Pfmdr1 gene amplification has been suggested to increase the risk of artemether-lumefantrine treatment failure. In an in vitro study on field samples from Thailand, we observed that pfmdr1 amplification was associated with a decreased susceptibility to lumefantrine as well as to its metabolite desbutylbenflumetol (DBB) and to artemisinin. Pfmdr1 amplification was notdetected in any of the African study sites in Zanzibar, Tanzania and Uganda, but may potentially represent a further development towards resistance to lumefantrine. Overall, the results indicate that pfmdr1 and pfcrt may be involved in the mechanism of resistance to lumefantrine. Pfmdr1 N86 and pfcrt K76 as markers for tolerance may be of practical use, functioning as an early warning of emerging resistance. Pfcrt 76T, the molecular marker for chloroquine resistance, incurs lumefantrine sensitivity and may increase susceptibility to artemisinin. This suggests that artemether-lumefantrine is ideal in areas of chloroquine resistance. However, as some alleles are associated to both lumefantrine and artemisinin susceptibility, there is a risk for lumefantrine to accelerate the development of resistance to artemisinin derivatives. Although artemisinins may not select for these alleles themselves due to their fast elimination, lumefantrine pressure may provide the main selective force and drive a decrease in susceptibility to both drugs. This shows that an understanding of the molecular basis of drug resistance is crucial.
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