In vitro demonstration of pyrimethamine resistance of "wild" Plasmodium falciparum in The Gambia.

The in vitro response of wild Gambian Plasmodium falciparum to pyrimethamine is described. Parasites were grown in 100 microliter cultures through schizogony. The number of rings present after 48 hours in drug-treated cultures was expressed as a percentage of the controls. Neither a medium and drug change after 24 hours nor different starting parasitaemias were found to affect the outcome of the assay, although a medium and drug change did increase the multiplication rate. 60 randomly taken pure P. falciparum infections were studied. 57 were sensitive. The ID50 and ID90 of drug-sensitive infections were 1.7 X 10(-9) M and 4.5 X 10(-9) M pyrimethamine respectively. Three infections were resistant (5%) with individual ID50 values of greater than 10(-6) M, 3 and 4 X 10(-9) M and ID90 values of greater than 10(-6) M, 8 and 9 X 10(-7) M.