Bactericidal activity of ofloxacin versus roxithromycin in the treatment of Streptococcus pneumoniae.

Ofloxacin is highly active against Gram-negative aerobic bacilli, but moderately active against Gram-positive cocci. The minimal inhibitory concentrations (MICs) against Streptococcus pneumoniae range between 1 and 2 mg/L. MICs of roxithromycin (RU 28965) against S. pneumoniae range between 0.004 and 0.03 mg/L, but Gram-negative bacilli are resistant. The bactericidal activities of ofloxacin and roxithromycin were evaluated against 15 strains of S. pneumoniae, which were isolated recently from clinical specimens. Killing activity was evaluated under conditions simulating serum pharmacokinetic parameters. Initial concentrations were 10 mg/L for roxithromycin and 2 mg/L for ofloxacin, and the half-life was 6 hours for both compounds. Under these conditions, roxithromycin was rapidly bactericidal. The speed at which pneumococci were killed was faster with roxithromycin than with ofloxacin. No regrowth was seen with roxithromycin, but regrowth occurred in 8 of 15 strains with ofloxacin.