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 C56 strain of Toxoplasma gondii. Therapy initiated 2 h after infection with 2 X 10(3) or 2 X 10(4) RH or C56 tachyzoites protected more than 80% of the mice, compared with 0% of untreated controls (p less than 0.001). Paradoxically, Toxoplasma gondii was isolated more frequently (greater than 80%) in treated mice surviving infection with the less virulent strain (C56) than those surviving infection with the more virulent RH strain (less than 25%). Furthermore, in vitro studies revealed that roxithromycin at dosages up to 250 micrograms/ml had no effect on the proliferation of Toxoplasma gondii in murine peritoneal macrophage cultures.