Stability of drug resistance of a pyronaridine-resistant line of Plasmodium berghei

Pyronaridine-resistant line (RP line, passage 35, resistance>299 fold) of Plasmodium berghei was successively passed through mice under drug pressure of 600 mg(base)/kg of pyronaridine.When the drug pressure was lifted for 5 passages from Passage 49, the parasites completely lost their resistance to pyronaridine. The drug resistance of RP line dropped slightly when the parasites of Passage 115 were subjected to 21-51 serial passages in the absence of drug pressure. Pyronaridine-resistance was stable in Passage 134, as nodecrease of resistance was detected in the parasites following 31 passages without drug pressure.The parasites under drug pressure for a long period retained not only the high resistance to pyronaridine but also cross resistance to chloroquine.