Serum and WBC pharmacokinetics of 1500 mg of azithromycin when given either as a single dose or over a 3 day period in healthy volunteers

Owing to azithromycin's prolonged half-life, shorter and shorter dosage regimens are being studied for treatment of respiratory tract infections. Previous studies have concluded that the 3 and 5 day (1.5 g total) regimens not only provide at least equal serum and WBC exposures but also equal efficacy rates. An earlier clinical study using the entire 1.5 g dose at once or the current 3 day regimen in patients with atypical pneumonia noted equal efficacy. Similar trials are currently underway In both adult and paediatric populations. The goal of the present study was to investigate whether there were equal serum and WBC exposures when azithromycin was dosed as the current 3 day regimen or as a single large dose. Equal exposures would help validate future clinical trials of single dose regimens. Twelve healthy volunteers received both azithromycin regimens (1.5 g single dose and 500 mg/day for 3 days) in random order. Serum and WBC samples were collected at baseline and repeatedly for 10 days following the first dose of each regimen. Serum samples were assayed via HPLC (CV% < 10) and WBC samples via liquid chromatography/mass spectrometry (CV% < 10). Data were modelled using noncompartmental methods. Statistics were via ANOVA with significance defined as P < 0.05. All subjects completed both regimens with minimal incidence of adverse effects. Serum data [mean (range)] demonstrated no significant difference in exposure between the two regimens [single 13.1 (3.02-20.6) mgh/L versus 3 day 11.2 (2.98-24.5) mg .h/L: P = 0.12], although it favoured the shorter regimen. WBC results demonstrated much higher exposures than seen with serum, but no significant difference between the two regimens was identified. These results suggest that a single oral 1.5 g regimen of azithromycin for respiratory tract infections should provide exposure at least equal to currently approved treatment regimens.