Characterization of Plasmodium Liver Stage Inhibition by Halofuginone

Malaria is a devastating parasitic disease that afflicts one-third of the world’s population. Commonly used malaria drugs address few targets and their efficacy is being undermined by parasite resistance. Most therapeutics target blood stage malaria, while only few compounds are active against malaria’s liver stage, the first stage of the Plasmodium parasite’s life cycle within the human host. The identification of inhibitors active against liver stage malaria would benefit both the development of chemical probes to elucidate the poorly understood biology of this phase of the parasite’s life cycle and potentially provide agents for preventing and eliminating the disease. Here, we report on the development of a live cell parasite traversal assay in 384-well format amenable to high-throughput screening that exploits the wounding of liver cells by the parasite. This method identifies small molecules that may inhibit the parasites actin-myosin motor system. The traversal assay, in addition to established methods, was used to evaluate the activity of halofuginone, a synthetic halogenated derivative of the natural alkaloid febrifugine, against liver stage Plasmodium berghei parasites. Halofuginone was found to inhibit P. berghei sporozoite load in HepG2 cells with an IC50 of 17 nM. While the compound does not affect parasite traversal through human liver cells, an inhibition time course assay indicates that it affects essential processes in both early and late stage parasite development.