The in vitro activity of roxithromycin (RU 28965) compared with four oral antibiotics

The in vitro activity of roxithromycin (RU 28965), a new semisynthetic macrolide, was compared with that of amoxycillin, cephradine, doxycycline and erythromycin against 160 respiratory and skin isolates including 10 methicillin-resistant Staph. aureus, 10 beta-lactamase-producing Haemophilus influenzae and 30 anaerobes. The MIC determinations were performed by an agar dilution method using a final inoculum of 10(4)-10(5) c.f.u./ml and results were recorded as the lowest concentration of the drug that inhibited visible growth (MIC). All organisms were incubated aerobically at 37 degrees C for 18 h, except anaerobes which were incubated in an anaerobic chamber at 37 degrees C for 48 h. The MICs of roxithromycin against staphylococci (30 strains, 10 of which were erythromycin-resistant, MIC greater than or equal to 2 mg/l) ranged from 0.03 to greater than or equal to 16 mg/l. The MIC90 against erythromycin-sensitive staphylococci was 1.0 mg/l. The MICs of roxithromycin ranged from 0.03-4 mg/l (MIC90 0.25 mg/l) against streptococci (50 strains), from 1 to 32 mg/l (MIC90 8 mg/l) against Haemophilus spp. (40), and from 0.06 to 0.25 mg/l (MIC90 0.125 mg/l) against B. catarrhalis (10). Roxithromycin was less active than erythromycin against fusobacteria (MIC 0.05 to greater than or equal to 128 mg/l) and against Veillonella spp. (MIC greater than 128 mg/l). Roxithromycin was more active than cephradine, doxycycline and amoxycillin against the aerobic organisms (except for amoxycillin against streptococci, where the activity was similar) but less active against the anaerobes examined. Taken together with reported kinetic advantages, these results suggest that roxithromycin may be a useful antibiotic in selected circumstances and studies to determine its efficacy seem indicated.