In vivo and in vitro activity of roxithromycin against Toxoplasma gondii in mice

Roxithromycin protected mice against a lethal infection with the extremely virulent RH strain and the CS6 strain of Toxoplasma gondii. Therapy initiated 2 h after infection with 2×10³ or 2×10⁴ RH or C56 tachyzoites protected more than 80% of the mice, compared with 0% of untreated controls (p < 0.001). Paradoxically, Toxoplasma gondii was isolated more frequently (> 80%)in treated mice surviving infection with the less virulent strain (CS6) than those surviving infection with the more virulent RH strain (< 25%). Furthermore, in vitro studies revealed that roxithromycin at dosages up to 250 μg/ml had no effect on the proliferation of Toxoplasma gondii in murine peritoneal macrophage cultures.