In vitro selection of Plasmodium falciparum lines resistant to dihydrofolate-reductase inhibitors and cross resistance studies.

A cloned Plasmodium falciparum line was subjected to in vitro drug pressure, by employing a relapse protocol, to select progressively resistant falciparum lines to pyrimethamine and cycloguanil, the two dihydrofolate-reductase (DHFR) inhibitor antimalarial drugs. The falciparum lines resistant to pyrimethamine were selected much faster than those resistant to cycloguanil. In 348 days of selection/cultivation, there was 2,400-fold increase in IC50 value to pyrimethamine, whereas only about 75-fold decrease in sensitivity to cycloguanil was registered in 351 days. Pyrimethamine-resistant parasites acquired a degree of cross resistance to cycloguanil and methotrexate, another DHFR inhibitor, but did not show any cross resistance to some other groups of antimalarial drugs. The highly pyrimethamine-resistant line was not predisposed for faster selection to cycloguanil resistance. Resistance acquired to pyrimethamine was stable. The series of resistant lines obtained form a good material to study the 'evolution' of resistance more meaningfully at molecular level.