Studies on a Strain of Chloroquine-Resistant Plasmodium falciparum from Thailand.

Abstract This report presents the results of chemotherapeutic studies carried out with non-immune volunteers infected with a strain of Plasmodium falciparum from Viet-Nam, termed the Viet-Nam (Sn.) strain, under conditions of study precluding reinfection. Nine volunteers received, during acute clinical attacks, three-day courses of chloroquine, seven receiving 1500 mg of chloroquine base orally, one 3000 mg of chloroquine base orally, and one 2160 mg of chloroquine base intramuscularly. Volunteers also received, again during acute clinical attacks, various regimens of hydroxychloroquine (1500 mg base), amodiaquine (1400 mg base), mepacrine (2198 mg base), proguanil (2610 mg base), or 377-C-54 (1500 or 2500 mg base). These drugs failed to effect radical cure of the infections, both before and after passage of the strain through mosquitos. On the other hand, the administration of either 50 mg of pyrimethamine daily for three days or 1620 mg of quinine base (1935 mg of quinine sulfate) daily for 10 days effected radical cure of the infections. The administration of 1620 mg of quinine base daily for seven days failed to effect radical cure of the infections in two of five volunteers. Under the particular experimental conditions employed, intramuscular injection of CI-501 (a repository preparation of a highly active metabolite of proguanil) did not exert a demonstrable protective effect against the Viet-Nam (Sn.) strain of P. falciparum.