A method for screening drugs against the liver stages of malaria using Plasmodium gallinaceum and Aedes mosquitos

RESUMO

The radical cure of human malaria caused by Plasmodium vivax requires two drugs, i.e., a blood schizontocide such as chloroquine to clear the circulating parasites, and primaquine aimed at the liver stages (hypnozoites) responsible for the late relapses of this parasite. Primaquine is unique as a radical curative drug but is highly toxic. The only useful model currently availablescreening drugs to replace primaquine is Plasmodium cynomolgi-induced malaria in Rhesus monkeys. Because of the limited availability and cost of these animals, the development of non-primate models for such screening would be of considerable value. We used a drug-screening assay for the liver stage malaria parasite based on the ability of such drugs to stop development of gametocytes in the mosquito vector. The inhibition of the sporogonic cycle of malaria in the mosquito by primaquine (15 mg/kg) was confirmed here and used for re-evaluation of the gametocyte method. We observed that the level of parasitemia in the untreated control chicken used to infect mosquitos was a crucial factor affecting the subsequent development of sporogoniy. Thus, parasitemia was carefully controlled in the studies involving oocyst development. Parasitemias lower than 6% at the beginning of the experiment and increasing were found to be most appropriate for the production of the infectious gametocytes during a period of 8 h. The activity of 4 naphthoquinones and of a natural compund, named "Indian beer", used in the endemic areas of malaria in Brazil as a prophylactic drug, showed that, unlike primaqui9ne, they were unable to inhibit the sporogonic cycle of malaria at the concentrations tested (200 mg/kg and 1000 mg/kg, respectively). We conclude that the P. gallinaceum model was a simple and effective tool for evoluating drugs with radical curative properties, particularly if care was taken to control the level of parasitemia in the chicken host at the time of mosquito feeding