In vitro susceptibility of various genotypic strains of Toxoplasma gondii to pyrimethamine, sulfadiazine, and atovaquone.

Sulfadiazine, pyrimethamine, and atovaquone are widely used for the treatment of severe toxoplasmosis. Their in vitro activities have been almost exclusively demonstrated on laboratory strains belonging to genotype I. We determined the in vitro activities of these drugs against 17 strains of Toxoplasma gondii belonging to various genotypes and examined the correlations among 50% inhibitory concentrations (IC50s), growth kinetics, strain genotypes, and mutations on drug target genes. Growth kinetics were determined in THP-1 cell cultures using real-time PCR. IC50s were determined in MRC-5 cell cultures using a T. gondii-specific enzyme-linked immunosorbent assay performed on cultures. Mutations in dihydrofolate reductase (DHFR), dihydropteroate synthase (DHPS), and cytochrome b genes were determined by sequencing. Pyrimethamine IC50s ranged between 0.07 and 0.39 mg/liter, with no correlation with the strain genotype but a significant correlation with growth kinetics. Several mutations found on the DHFR gene were not linked to lower susceptibility. Atovaquone IC50s were in a narrow range of concentrations (mean, 0.06 +/- 0.02 mg/liter); no mutation was found on the cytochrome b gene. IC50s for sulfadiazine ranged between 3 and 18.9 mg/liter for 13 strains and were >50 mg/liter for three strains. High IC50s were not correlated to strain genotypes or growth kinetics. A new mutation of the DHPS gene was demonstrated in one of these strains. In conclusion, we found variability in the susceptibilities of T. gondii strains to pyrimethamine and atovaquone, with no evidence of drug resistance. A higher variability was found for sulfadiazine, with a possible resistance of three strains. No relationship was found between drug susceptibility and strain genotype.