Changes in susceptibility of Plasmodium falciparum to artesunate in vitro in Yunnan Province, China.

We investigated changes in the susceptibility of Plasmodium falciparum to artesunate in vitro using Rieckmann's microtest in Yunnan Province, China, during the period 1988 to 1999. Longitudinal surveillance studies in 1988, 1992 and 1999 revealed that the IC50s were 6.2, 7.2 and 20.7 nmol/L, respectively and mean concentrations completely inhibiting schizont formation (CIMC) were 37.8, 46.1 and 74.0 nmol/L, respectively. The IC50 and CIMC in 1999 were 3.3 and 2 times greater than in 1988. In cross-sectional tests from 1991 to 1993, the susceptibility of P. falciparum isolates from areas in the western and the southern parts of Yunnan Province were similar, but lower than in the south-eastern and central parts of the province. The results suggest that P. falciparum is generally susceptible to artemisinin derivatives but indicate a reduction in susceptibility during the study period in areas where the drugs have been used for a long time.