Copyright © 1994, American Society for Microbiology In Vitro Activity of Dirithromycin against

Dirithromycin is a new macrolide antibiotic with an active metabolite, erythromycylamine. We evaluated the in vitro activities of both drugs against 16 isolates of Chiamydia trachomatis and compared them with that of doxycycline. In vitro susceptibility testing was performed with McCoy cell monolayers. The MIC was defined as the lowest concentration of antibiotic without inclusions. The MBC was defined as the lowest concentration of antibiotic yielding no inclusions after passage onto 24-h-old antibiotic-free McCoy cell monolayers. Dirithromycin and erythromycylamine appeared to be equally effective against these 16 strains of C. trachomatis (MIC for 90%o of strains tested, mg/ml; MBC for 90 % of strains tested, 2 p,g/ml). Both were less active than doxycycline (MIC for 90%o of strains tested, 0.06,g/ml; MBC for 90%o of strains tested, 0.12,ug/ml). The combination of dirithromycin and erythromycylamine appeared to be additive. Dirithromycin is a new macrolide antibiotic which is metab-olized to an active compound, erythromycylamine (2). It has in vitro activity comparable to that of erythromycin (9). Like erythromycin, dirithromycin is concentrated intracellularly (3). Erythromycin and other macrolides have been shown to have both in vitro and in vivo activity against Chlamydia trachomati