Antimicrobial disposition in pulmonary epithelial lining fluid of horses, part II. Doxycycline.

This study evaluates how the drug doxycycline is absorbed and distributed in horses, particularly in the lung fluid, when given orally. It suggests that doxycycline might be effective for treating lower respiratory infections in horses if its absorption could be improved.

Methodology

• The study investigates the absorption and concentration of doxycycline in horse’s pulmonary epithelial lining fluid (PELF) following two types of oral administration.
• Doxycycline dosage of 10 mg/kg was administered either directly into the stomach or as a food supplement to nonfasted horses.
• Blood samples were collected from the horses before and several times after the drug administration.
• Further samples of PELF were collected using a tampon device.
• All the collected samples were analyzed for doxycycline concentrations.
• The same procedure was then carried out in the case of intravenous administration of doxycycline at a dose of 3 mg/kg.

Findings

• The study found that oral absorption, or bioavailability, of doxycycline was significantly lower than when administered intravenously.
• Specifically, bioavailability was found to be 17% after direct administration into the stomach and dropped to 6% when administered with food.
• The degree to which doxycycline penetrates into PELF, as measured by the AUC(PELF)/AUC(plasma) ratios, was found to be 0.87 after gastric administration.
• This indicates that even though oral bioavailability is low, relevant concentrations of doxycycline can still be maintained in PELF following direct stomach administration.

Conclusion

• This research suggests that, despite its low oral absorption, doxycycline could still be a valuable medicine for treating lower airway infections in horses.
• However, the efficiency of the treatment is likely to be enhanced if the bioavailability of doxycycline could be improved for oral administration.