CLINICAL PHARMACOKINETICS OF AZITHROMYCIN

The bioavailability of azithromycin is approximately 37 %. Concomitant administration of oral azithromycin with food significantly decreases by 50 % drug bioavailability. Following a single oral 500 mg dose, pe ak plasma concentrations of about 0.35-0.45 mg/1 are attained within a pproximately 2 hours. With a 500 mg oral dose on day 1, followed by 25 0 mg daily on days 2 to 5, peak and through plasma concentrations on d ay 5 are around 0.25 and 0.05 mg/1 respectively. These low plasma conc entrations are the consequence of extensive and rapid distribution fro m plasma to tissues. Plasma protein binding is low less than 50 % at p lasma concentrations obtained with the usual dosage regimen. Apparent volume of distribution is very large, 25 to 35 1/kg. Azithromycin is m ainly eliminated unchanged in the faeces via biliary excretion and tra nsintestinal secretion. Urinary excretion is a minor elimination route : about 6 % and an oral dose and 12 % of an intravenous dose are recov ered unchanged in urine. The mean terminal elimination half-life of az ithromycin is 2 to 4 days. The pharmacokinetics of azithromycin is not significantly altered in elderly subjets and in patients with mild to moderate renal or hepatic insufficiency.