Pharmacokinetics of azithromycin and concentration in body fluids and bronchoalveolar cells in foals

Am J Vet Res. 2001 Dec;62(12):1870-5. doi: 10.2460/ajvr.2001.62.1870.


Objective: To determine the pharmacokinetics of azithromycin and its concentration in body fluids and bronchoalveolar lavage cells in foals.

Animals: 6 healthy 6- to 10-week-old foals.

Procedure: Azithromycin (10 mg/kg of body weight) was administered to each foal via i.v. and intragastric (i.g.) routes in a crossover design. After the first i.g. dose, 4 additional i.g. doses were administered at 24-hour intervals. A microbiologic assay was used to measure azithromycin concentrations in serum, peritoneal fluid, synovial fluid, pulmonary epithelial lining fluid (PELF), and bronchoalveolar (BAL) cells.

Results: Azithromycin elimination half-life was 20.3 hours, body clearance was 10.4 ml/min x kg, and apparent volume of distribution at steady state was 18.6 L/kg. After i.g. administration, time to peak serum concentration was 1.8 hours and bioavailability was 56%. After repeated i.g. administration, peak serum concentration was 0.63 +/- 0.10 microg/ml. Peritoneal and synovial fluid concentrations were similar to serum concentrations. Bronchoalveolar cell and PELF concentrations were 15- to 170-fold and 1- to 16-fold higher than concurrent serum concentrations, respectively. No adverse reactions were detected after repeated i.g. administration.

Conclusions and clinical relevance: On the basis of pharmacokinetic values, minimum inhibitory concentrations of Rhodococcus equi isolates, and drug concentrations in PELF and bronchoalveolar cells, a single daily oral dose of 10 mg/kg may be appropriate for treatment of R. equi infections in foals. Persistence of high azithromycin concentrations in PELF and bronchoalveolar cells 48 hours after discontinuation of administration suggests that after 5 daily doses, oral administration at 48-hour intervals may be adequate.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Administration, Oral
  • Animals
  • Anti-Bacterial Agents / administration & dosage
  • Anti-Bacterial Agents / analysis
  • Anti-Bacterial Agents / pharmacokinetics*
  • Area Under Curve
  • Azithromycin / administration & dosage
  • Azithromycin / analysis
  • Azithromycin / pharmacokinetics*
  • Biological Availability
  • Body Fluids / metabolism
  • Cross-Over Studies
  • Female
  • Half-Life
  • Horses / metabolism*
  • Injections, Intravenous / veterinary
  • Male
  • Pulmonary Alveoli / metabolism*
  • Statistics, Nonparametric


  • Anti-Bacterial Agents
  • Azithromycin