Abstract
In vitro pyrimethamine response of Plasmodium falciparum isolates and dihydrofolate reductase (dhfr) gene sequences were analyzed in 2004-2005 and compared with our previous data. Most isolates (n = 103, all dhfr mutants) had 50% inhibitory concentrations (IC(50)s) > or = 119 nM, and six isolates had low IC(50)s (five wild-type or mixed dhfr, 0.04-1.37 nM; one triple mutant, 6.4 nM). Of 194 isolates, only 7 had the wild-type dhfr and 187 were mutants. The results of the two methods were highly concordant and indicated a significant increase (P < 0.05) in the prevalence of mutant, pyrimethamine-resistant P. falciparum between 1994 and 2005. The addition of probenecid or sulfinpyrazone to pyrimethamine resulted in a slight-to-moderate decrease in the level of in vitro pyrimethamine resistance without rendering the parasites susceptible to pyrimethamine. Analysis of molecular markers may be useful for the long-term surveillance of antifolate-resistant malaria.
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