In search of antiparasitic agents, we here identify arylmethylamino steroids as potent compounds and characterize more than 60 derivatives. The lead compound 1o is fast acting and highly active against intraerythrocytic stages of chloroquine-sensitive and resistant Plasmodium falciparum parasites (IC50 1–5 nM) as well as against gametocytes. In P. berghei-infected mice, oral administration of 1o drastically reduces parasitaemia and cures the animals. Furthermore, 1o efficiently blocks parasite transmission from mice to mosquitoes. The steroid compounds show low cytotoxicity in mammalian cells and do not induce acute toxicity symptoms in mice. Moreover, 1o has a remarkable activity against the blood-feeding trematode parasite Schistosoma mansoni. The steroid and the hydroxyarylmethylamino moieties are essential for antimalarial activity supporting a chelate-based quinone methide mechanism involving metal or haem bioactivation. This study identifies chemical scaffolds that are rapidly internalized into blood-feeding parasites.
Previously, we presented the chemical design of a promising series of antimalarial agents, 3-[substituted-benzyl]-menadiones, with potent in vitro and in vivo activities. Ongoing studies on the mode of action of antimalarial 3-[substituted-benzyl]-menadiones revealed that these agents disturb the redox balance of the parasitized erythrocyte by acting as redox cyclers-a strategy that is broadly recognized for the development of new antimalarial agents. Here we report a detailed parasitological characterization of the in vitro activity profile of the lead compound 3-[4-(trifluoromethyl)benzyl]-menadione 1c (henceforth called plasmodione) against intraerythrocytic stages of the human malaria parasite Plasmodium falciparum. We show that plasmodione acts rapidly against asexual blood stages, thereby disrupting the clinically relevant intraerythrocytic life cycle of the parasite, and furthermore has potent activity against early gametocytes. The lead's antiplasmodial activity was unaffected by the most common mechanisms of resistance to clinically used antimalarials. Moreover, plasmodione has a low potential to induce drug resistance and a high killing speed, as observed by culturing parasites under continuous drug pressure. Drug interactions with licensed antimalarial drugs were also established using the fixed-ratio isobologram method. Initial toxicological profiling suggests that plasmodione is a safe agent for possible human use. Our studies identify plasmodione as a promising antimalarial lead compound and strongly support the future development of redox-active benzylmenadiones as antimalarial agents.M alaria remains one of the most severe infectious diseases that disproportionately affects the public health and economic welfare of the world's poorest communities. The causative agents of malaria are protozoan parasites of the genus Plasmodium. Among the five Plasmodium species that can cause malaria in humans (Plasmodium falciparum, P. vivax, P. ovale, P. malariae, and P. knowlesi), P. falciparum is the most virulent and is responsible for severe clinical outcomes and most of the deaths associated with malaria. Until the development of an effective vaccine, chemotherapy remains a major frontline strategy for the control and possible future elimination of malaria. The emergence and rapid spread of drug-resistant parasites undermine efforts at reducing the global disease burden and emphasize the need for drugs with novel chemical entities that exploit new molecular targets while overcoming established drug resistance mechanisms. Ideally, such drugs should quickly kill the pathogenic asexual blood stages of P. falciparum and, in addition, be effective against other developmental stages, in particular the sexual stages that transmit the infection from the human to the mosquito host (1, 2).Previously, we presented the chemical design and synthesis of a novel class of compounds, the 3-[substituted-benzyl]-menadiones, that exhibit potent activities against P. falciparum blood stages in vitro and moderate activities in v...
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