| Research Abstract |
Malaria control become more complexes because causal agent Plasmodium falciparum has developed resistance to nearly all available antimalarial drugs. As malaria parasite are increasingly resistance to more common and effective alkaloid drugs, interest in the antimalarial properties of nonalkaloid compounds such as sesquiterpene artemisinin and the related endoperoxides is rapidly growing. The search for novel antimalarial drugs against specific parasite target is thus an urgent task to pursue.
Malaria parasites invade human erythrocytes in the erythrocytic stage of infection. While residing in erythrocytes, the parasites depend on hemoglobin as a source of food, digesting it with a series of proteases. Two homologous aspartic proteases called plasmepsin I and II initiate the degradative process by cleaving the native hemoglobin molecule in a highly conserved hinge region. A cysteine protease falcipain-2, and a metalloprotease falcilysin, act further downstream in the pathway to degrade
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hemoglobin to small peptides. Inhibition of these proteases kill parasites in culture and animal model, suggesting that hemoglobin degrading proteases are valid targets for chemotherapy.
In this study, we studied development and selection of new antimalarial endopeoxides using about 300 analogs of synthetic endoperoxides. As a results, antimalarial activity of a synthetic endoperoxide, (N-89) was selected using in vitro against Plasmodium falciparum and in vivo against P. bergheii. In vitro N-89 exhibited potent antimalarial activity against chloroquine-sensitive (FCR-3) and chloroquine-resistant (K1) strain, estimated 50% growth inhibitory concentration (IC_<30>) was 28 nM and 26 nM, respectively. In vivo antimalarial activity of the compound was also remarkable, ED_<50> and ED_<90> were 20 mgkg^<-1> and 40 mgkg^<-1>, respectively. Differential effect was found at different developmental stages in vitro. N-89 is more sensitive on early and middle stages parasites and less sensitive on late and schizont stage parasites. The effect was associated with distinct morphological changes. Although late and schizonts stage parasites entered 2^<nd> cycle, their re-invasion was impaired. Consistence with stage-specific effect, inhibition of hemozoin formation is pronounced at mature stage parasites than schizonts. When middle trophozoites stage parasites was incubated with endoperoxide hemoglobin accumulation was found in parasite lysates. An aspartic protease inhibitor of malaria parasite pepstatin A could reverse its hemoglobin accumulation activity, and pepstatin A showed antagonistic effect when combined with N-89. Our results explain antimalarial activity of N-89 may be related to hemoglobin degradation. Less
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