Pharmacokinetics of azithromycin in rats and dogs

After intravenous or oral administration to rats and dogs, azithromycin was rapidly distributed into the tissues, where concentrations frequently exceeded those in serum by 100-fold or more within 24 h of a single dose. Tissue concentrations were proportional to the dose following single administrations of 10 to 40 mg/kg in rats and dogs. Tissue concentrations were higher after multiple dosing and became greater as the dose was increased from 10 to 40 mg/kg. Elimination half-lives were similar in most tissues and were about 40 h in rats after seven doses of 20 mg/kg and about 90 h in dogs after five doses of 30 mg/kg. Serum concentrations declined in a multi-exponential manner, reflecting initial rapid distribution into tissues and then slow return to serum from tissues. Azithromycin had good oral bioavailability in rats and dogs (46% and 97%, respectively). Rapid uptake of azithromycin by tissues from serum and slow redistribution from tissues to serum are apparently factors governing the pharmacokinetics of azithromycin in rats and dogs. Serum concentrations do not reflect the availability of azithromycin in tissues.