Cross resistance of pyrimethamine and sulfadoxine to their related compounds in Plasmodium falciparum.

Cross resistance of pyrimethamine and amethopterin, sulfadoxine and the other sulfonamides in Plasmodium falciparum culture lines was studied. Our results indicate some evidence of a cross resistance between pyrimethamine to amethopterin a drug sharing the same mode of action but never been used as an antimalarial before. Studies on sulfonamides revealed that the minimal inhibitory concentration for sulfadoxine was lower than for sulfadiazine and sulfisoxasole, and that a cross resistance between sulfadoxine and the other sulfonamides may not occur.