Bactericidal activity of clarithromycin and its 14-hydroxy metabolite against Haemophilus influenzae and streptococcal pathogens.

The serum bactericidal activity of clarithromycin in six normal human volunteers was determined after oral doses of 500 mg. The mean plasma levels of clarithromycin plus 14-hydroxy clarithromycin were 2.11 micrograms/mL after the second dose and 4.36 micrograms/mL after the sixth dose. The mean serum bactericidal titer against Haemophilus influenzae after the second dose was 1:8 and after the sixth dose 1:16 when unheated serum was used. Similar values were obtained when serum to which clarithromycin and 14-hydroxy clarithromycin was added was tested. Mean serum bactericidal titers against H. influenzae determined in Haemophilus test broth or heated serum were 1:2 and 1:4, respectively. Against Streptococcus pneumoniae and Streptococcus pyogenes, there was greater than 1:16 serum bactericidal levels at 12 hours after the sixth dose of clarithromycin.