In vitro and in vivo activities of macrolides against Mycoplasma pneumoniae.

We investigated the in vitro and in vivo activities of macrolides against Mycoplasma pneumoniae. In vitro MICs of azithromycin, erythromycin, clarithromycin, and roxithromycin were determined. Azithromycin was the most potent antimicrobial agent tested in vitro. Its MIC for 90% of the strains was 0.00024 micrograms/ml. MICs for 90% of the strains of erythromycin, clarithromycin, and roxithromycin were 0.0156, 0.0078, and 0.03125 micrograms/ml, respectively. In vivo activities were assessed in a pulmonary infection model with Syrian golden hamsters. We evaluated the in vivo effects on reduction of viable M. pneumoniae cell counts and on reduction of microscopic and macroscopic histopathologies for azithromycin, erythromycin, and clarithromycin given at 10 mg/kg once daily for 1 and 3 days and given at 15 mg/kg twice daily for 2.5 and 5 days. Azithromycin was significantly more effective than erythromycin or clarithromycin in the same regimens. Especially at 10 mg/kg once daily for 1 day, only azithromycin was significantly effective in the reduction of viable M. pneumoniae cells and histopathologies. These results show that azithromycin is more efficacious than the other drugs tested against M. pneumoniae pneumonia in hamsters. These data suggest that clinical studies of macrolides in human patients are warranted.