In vivo and in vitro studies of quinine sensitivity of Plasmodium falciparum in Thailand.

Forty-four patients with falciparum malaria were studied. Nine patients were given quinine orally at a daily dose of 1.5 gm base for a period of 14 days. The mean parasite clearance in all 9 patients was 3.3 days, and none had recrudescence in follow-up examinations for 31 days. The in vivo study of these 9 patients showed sensitivity to quinine which correlated with the in vitro test, with concentration of quinine base 2.5-5.8 microgram/ml of blood that inhibited the maturation of Plasmodium falciparum parasites. The results of the in vitro test of 35 patients showed concentrations of quinine base 2.1-5.4 microgram/ml of blood were able to inhibit the maturation of P. falciparum parasites. Therefore, these studies indicate that Plasmodium falciparum are still sensitive to quinine and quinine remains to be the drug of choice for the treatment of falciparum malaria in Thailand.