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

Doxycycline concentrations, following two types of oral administration to horses, in pulmonary epithelial lining fluid (PELF) were examined and compared to plasma concentrations. The oral bioavailability was estimated from plasma concentrations achieved after an intravenous study in two horses. Doxycycline (10 mg/kg) was administered either intragastric or as topdressing to nonfasted horses. Blood samples were collected for drug analysis, before and 11 times after administration during 24 h. PELF samples were collected by a tampon device four times after drug administration and analysed for doxycycline concentrations. Another two horses received doxycycline intravenously at a dose of 3 mg/kg and plasma was taken 14 times during a 24- h period. The oral bioavailability of doxycycline was calculated to 17% after intragastric administration and 6% after topdressing administration in nonfasted horses. The degree of penetration of doxycycline into PELF, as described by AUC(PELF) /AUC(plasma) ratios, was 0.87 after intragastric administration. The results indicate that clinically relevant doxycycline concentrations are possible to maintain in PELF after intragastric administration. Furthermore, if bioavailability could be enhanced for per os administration, doxycycline might be a valuable drug for the treatment of lower airway infections in horses.