Activity of clarithromycin and its principal human metabolite against Haemophilus influenzae

Clarithromycin is a new macrolide antibiotic which forms a microbiologically active principal in vivo metabolite, 14-OH-clarithromycin. The in vitro activities of clarithromycin and its metabolite were examined separately and in pharmacokinetically relevant fixed combinations of 4:1 and 2:1 against a group of 50 Haemophilus influenzae isolates. Broth microdilution susceptibility tests indicated that clarithromycin was less active than erythromycin against all but highly erythromycin-susceptible strains, while 14-OH-clarithromycin was generally more active than either antibiotic. An enhancement in activity against the majority of strains was demonstrated when clarithromycin and its metabolite were tested in combination.