STAGE SENSITIVITY OF PLASMODIUM-VINCKEI-PETTERI TO QUININE, MEFLOQUINE, AND PYRIMETHAMINE

The stage-dependent sensitivity of Plasmodium vinckei petteri to the a ntimalarial drugs quinine, mefloquine, and pyrimethamine was investiga ted using single subcurative doses and 2 different tests: a prepatency test evidencing the extension of the prepatent period according to th e stage at which the drug was administered, and a patency test showing the morphological alterations of the parasites and the modifications of the parasitic pattern following drug treatment. Quinine activity wa s maximal when it hit the midterm trophozoite. No pigment clumping was seen and parasites developed normally until they reached the schizogo nic stage when they became morphologically altered and unproductive. M efloquine also acted on the midterm trophozoites. Parasites at this st age were killed immediately by the drug as evidenced by their degenera tive appearance, and dead parasites lingered in the blood smears for a t least 6 hr. However, although mefloquine was active during at least 48 hr (2 cycles), some trophozoites escaped destruction. The schizont stage was found to be the most sensitive to pyrimethamine, as previous ly reported for other parasite species. These results add to our previ ous reports showing that each drug acts preferentially on a specific s tage of parasite development.