Pharmacokinetics of azithromycin in foals after i.v. and oral dose and disposition into phagocytes.

The research paper studied the disposition of azithromycin, a potential alternative to erythromycin for treating Rhodococcus equi pneumonia in foals. The research demonstrated that azithromycin carries important pharmacokinetic properties, with a large volume of distribution, attributing to high concentrations in tissue and intracellular environments.

Study Methodology

• The antibiotics azithromycin was investigated in this study. It was administered to five 2-3 months old foals, both orally and intravenously. The dosages were 10 mg/kg and 5 mg/kg respectively.
• The muscle tissues, along with polymorphonuclear leukocytes (PMN), and alveolar cells in the foals were examined after administering the azithromycin.
• Pharmacokinetic measures including plasma samples collected after oral and intravenous administration, peripheral blood PMN and bronchoalveolar lavage fluid and alveolar cells after oral administration. Azithromycin concentrations were determined using a process known as reverse-phase high-performance liquid chromatography (HPLC).

Key Findings

• The findings reveal that azithromycin absorption was variable with an average systemic availability of 39% (+/- 20%). The half-life (the time taken for the drug concentration in the body to drop by half) of azithromycin in plasma was 16 hours for intravenous administration and 18.3 hours for oral administration.
• The volume of distribution (Vd), indicating the extent of drug dispersion in the body, was notably large. Specifically, Vd of 11.6 L/kg and 12.4 L/kg were recorded. The large Vd highlights high concentrations of azithromycin in tissue and within cells. The PMN half-life was 49.2 hours, again indicating good retention of the drug in the body.

Implications and Conclusion

• The study supports the consideration of azithromycin as an alternative to erythromycin for treating Rhodococcus equi pneumonia in foals, given its helpful pharmacokinetic properties.
• Given the conclusions drawn from azithromycin’s disposition in plasma, and high concentration within PMN and alveolar cells, researchers recommended a dosage of 10mg/kg of azithromycin orally once daily for foals with Rhodococcus equi pneumonia, and suggested further studies to confirm its efficacy.